Basic Insights Into the Reductive Activation and Covalent

Basic Insights Into the Reductive

Activation and Covalent Functionalization of Graphene and SWCNTs to Yield Novel and Highly Modified Carbon Structures

Grundlegende Einblicke in die Reduktive Aktivierung und Kovalente Funktionalisierung von Graphen und SWCNTs zur Erzeugung neuartiger und hochmodifizierter Kohlenstoffstrukturen

Der Naturwissenschaftlichen Fakultät

der Friedrich-Alexander-Universität Erlangen-Nürnberg

zur

Erlangung des Doktorgrades Dr. rer. nat.

vorgelegt von

Oliver Martin

aus Lindau (am Bodensee)

Als Dissertation genehmigt von der Naturwissenschaftlichen Fakultät

der Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU).

Tag der mündlichen Prüfung: 10.05.2021

Vorsitzender der Prüfungsorgans: Prof. Dr. Wolfgang Achtziger

Gutachter: Prof. Dr. Andreas Hirsch

Prof. Dr. Bernd Meyer I

Hiermit versichere ich, dass die vorliegende Dissertationsarbeit vollständig meinen eigenen Forschungsarbeiten entspringt. Sämtliche Beiträge von Dritten, die der Entstehung dieses Werkes dienlich waren, sowie von mir selbst in wissenschaftlichen Journalen publizierte Artikel, die während der Promotionszeit entstanden sind, sind deutlich gekennzeichnet. Die hier vorliegende Arbeit entstand in der Zeit vom März 2016 bis Juni 2020 am Lehrstuhl für Organische Chemie II der Friedrich-Alexander- Universität Erlangen-Nürnberg sowie am Zentralinstitut für Neue Materialien und Prozesstechnik (ZMP) in Fürth.

“Chemistry begins in the stars. The stars are the source of the chemical elements, which are the building blocks of matter and the core of our subject”

(Peter Atkins)

III

Index of Abbreviations a.u. Arbitrary Units abs absolute

AFM Atomic Force Microscopy

Atm Standard Atmosphere

BLG Bilayer Graphene

CDH Cyclodehydrogenation

CHP N-Cyclohexyl-2-pyrrolidone

CNT Carbon Nanotube

CoMoCat Cobalt-Molybdenum Catalyst

CVD Chemical Vapour Deposition

1D 1-Dimensional

2D 2-Dimensional

3D 3-Dimensional

DME 1,2-Dimethoxyethane

DoF Degree of Functionalization

DOS Density of States

DWCNT Double-walled Carbon Nanotube

EDS Energy dispersive X-ray spectroscopy

Et2O Diethyl Ether

EtOH Ethanol

ETR Electron Transfer Rate

EF Fermi Energy

EF Fermi Level IV

FLG Few Layer Graphene

FWHM Full-Width at Half-Maximum

GIC Graphite Intercalation Compound

GNR Graphene Nanoribbons

GO Graphene Oxide

GQD Graphene Quantum Dots

HBC Hexabenzocoronene

HEM High Energy Modes

HiPco High Pressure Carbon Monoxide Decomposition iLA In Plane Longitudinal Acoustical iLO In Plane Longitudinal Optical

IR Infrared iTA In Plane Transversal Acoustical iTO In Plane Transversal Optical

LD Distance of Defects

MALDI Matrix-Assisted Laser Desorption Ionization

MeOH Methanol

MLG Monolayer Graphene

MS Mass Spectrometry

MWCNT Multi-walled Carbon Nanotube m/z mass to charge ratio nIR Near Infrared

NMP N-Methylpyrilidone

NT Nanotube

PAH Polycyclic Aromatic Hydrocarbon

PES Photo Electron Spectroscopy

PhCN Benzonitrile

PLV Pulsed Laser Vaporization

QED Quantum Electrodynamics

QHE Quantum Hall Effect

RBM Radial Breathing Mode

RDI Raman Defect Index rGO Reduced Graphene Oxide

RT Room Temperature

S Scattering Coefficient

SCA Synthetic Carbon allotrope

SCH Sodium Cholate

SDBS Sodium Lauryl benzenesulfonate

SDS Sodium Dodecyl Sulfate

SET Single Electron Transfer

SLG Single Layer Graphene

SRS Statistical Raman Spectroscopy

SWCNT Single-walled Carbon Nanotube

TBAF Tetra-n-butylammonium Fluoride

TBAP Tetra-n-butylammonium Bromide

TEM Transmission Electron Microscopy

TGA Thermogravimetric Analysis

TG-GC-MS Thermogravimetric Analysis coupled with Mass Spectrometry and Gas

Chromatography

TG-MS Thermogravimetric Analysis coupled with Mass Spectrometry VI

THF Tertrahydrofuran

TM Tangential Modes

ToF Time of flight

UHV Ultra-High Vacuum

US Ultrasonication

UV-vis Ultraviolet-Visible vHS Van Hove Singularities

XPS X-ray Photo Electron Spectroscopy

λE Excitation Wavelength

ѵf Fermi Velocity

VII

Table of Contents

1 Introduction ...... 1

1.1 About Carbon Allotropes ...... 1 1.2 Carbon Nanotubes ...... 3 1.2.1 Structure and Properties ...... 3

1.2.2 Synthesis and Purification ...... 7

1.2.3 Functionalization ...... 11

1.3 Graphene ...... 19 1.3.1 Structure and Properties ...... 19

1.3.2 Production ...... 21

1.3.3 Discharging of Graphenides ...... 27

1.3.4 Functionalization ...... 28

1.4 Characterization Methods ...... 35 1.4.1 Raman Spectroscopy ...... 35

1.4.2 TG-MS/ TG-GC-MS ...... 41

2 Proposal ...... 43

3 Results and Discussion ...... 46

3.1 Solvent-driven Oxidation of Carbon Allotropides by PhCN ...... 46 3.1.1 Reactivity of various Types of Graphite after Activation ...... 46

3.1.2 Influence of PhCN on SWCNTs and Different Activation Routes ...... 60

3.1.3 Impact of PhCN after Covalent Functionalization...... 69

3.2 Mechanistic Investigations of the Reductive Functionalization Process ...... 76 3.2.1 Hydrogenation of Monolayer Graphene Flakes...... 78

3.2.2 Hydrogenation of the Peripheral Regions of CVD Graphene ...... 82

3.2.3 Expansion of Defects ...... 87

3.2.4 Ditopic Hydrogenation Approach of CVD Graphene ...... 90

3.2.5 Dehydrogenation ...... 94

VIII

3.3 Reductive Functionalization of oxo-Graphene ...... 96 3.3.1 Reductive Functionalization of rGO on Substrates ...... 101

3.3.2 Reductive Bulk Functionalization of rGO ...... 102

3.4 Evaluation of the Quantitative Degrees of Functionalization ...... 108 3.4.1 Graphene Functionalization ...... 110

3.4.2 SWCNT Functionalization ...... 118

3.4.3 Comparison of Additions ...... 127

3.5 Halogenation of Carbon Allotropes ...... 130 3.5.1 Reductive Halogenation of Carbon Allotropes ...... 130

3.5.1.1 Halogenation of SWCNTs ...... 131

3.5.1.2 Substitution of halogenated SWCNTs ...... 134

5.5.1.3 Halogenation of Graphene ...... 140

3.5.1.4 Comparison of Reductive Approaches ...... 142

5.5.1.5 Halogenation of Monolayer Graphene ...... 148

3.5.2 In situ Chlorination of Graphite...... 151

4 Conclusion ...... 155

5 Zusammenfassung ...... 160

6 Experimental part ...... 166

6.1 Chemicals and Materials ...... 166 6.2 General Procedures ...... 169 6.3 Instrumentation ...... 182 7 References ...... 185

8 Appendix ...... 200

1 Introduction 1.1 About Carbon Allotropes

Carbon represents one of the earth’s most abundant elements in the periodic table. Due to its ability to bind to itself and almost every other element in various manners, it provides the foundation of life on earth. The resulting diversity of organic compounds and molecules together with a broad range of chemical and physical properties make it a useful tool in modern synthetic chemistry and essential for the usage in various technological applications. When carbon forms bonds solely to itself, it is called a carbon allotrope. While elemental carbon can exist in two natural allotropes with either sp2- or sp3-hybridized carbon atoms, namely diamond and graphite, the discovery of fullerenes in 1985 has changed the situation completely. The coincidental finding marked the start of the era of synthetic carbon allotropes resulting in a Nobel Prize a few years later in 1996.[1] Nowadays, nanometer-sized allotropes exhibit a lot of meaningful members containing only sp2-hybridized carbon atoms such as the mentioned fullerenes, carbon nanotubes, and graphene (Figure 1.1).[1-2]

Figure 1.1: The most prominent synthetic carbon allotropes (SCAs) in the order of discovery from left to right - Fullerenes (C60) - Carbon Nanotubes - Graphene.

The rediscovery of graphene led to a Nobel prize in Physics in 2010 for Novoselov and Geim on the fundamental research of the youngest allotrope.[3] Since that point, a huge amount of researchers has been investigating intensively in that field leading to

numerous publications every year. Theoretically, even further allotropes are conceivable, for instance graphyne, which features mixed hybridized carbon atoms (sp, sp2, sp3) due to its bonding geometry, but being, however, purely hypothetical until now.[4]

The great interest of synthetic carbon allotropes arises from their extraordinary properties which can be related to their structural blueprint. Anyhow, the electronic properties differ significantly between the divergent allotropes. While fullerenes can be added to the electron acceptor category and are applied for instance in photovoltaic devices,[5] graphene has outstanding properties like high electron mobility, high thermal conductivity,[6] tremendous mechanical strength,[7] great flexibility, and an extraordinary transparency[8], to name a few. These properties make it applicable in numerous implementations for instance in sensors,[9] electronic devices[10] as well as in medical goods.[11] Carbon nanotubes, first described in a publication by Iijima in 1991,[2b] can consist of a broad amount of varying structural and physiochemical species leading to novel fascinating characteristics such as an exceptionally high thermal conductivity,[12] extraordinary mechanical properties,[13] and exhibit the highest tensile strength of every material on the planet.[14] Additionally, single-walled carbon nanotubes show an extremely high electron mobility and can be seen as ballistic electron conductors depending on their specific electronic type.[15] Thus, the area of applications is seemingly huge including electronic devices in nanotechnology[16] and electrochemical energy- or hydrogen storage systems.[17]

In this thesis, the main focus lies on the modification of the exciting allotropes graphene and SWCNTs, primarily in a reductive manner. To fully understand the reasons why these materials are so appealing and to comprehend the origin of their properties, an insight into their structures will be given in the following chapters.

1.2 Carbon Nanotubes 1.2.1 Structure and Properties

Carbon nanotubes (CNTs) are commonly divided into two categories: 1) single-walled nanotubes (SWCNTs) consisting of one perfect cylinder and 2) multi-walled nanotubes (MWCNTs) being composed of many concentric walls. MWCNTs bear resemblance to a Russian-doll due to the arrangement of SWCNTs with an increasing diameter being wrapped around the inner SWCNT with the smallest diameter. The number of tubes varies, ranging from two (double-walled CNTs) to multiple ones. The structural blueprint of their framework generally determines the specific electronic properties. The hollow-shaped SWCNTs are built by rolling up a graphene sheet into a cylindrical object in a mental experiment (Figure 1.2.1).

Figure 1.2.1: Hypothetically rolling up of a graphene sheet into a carbon nanotube.

Depending on the roll-up direction of the sheet, three different variations of nanotubes can be created. To be able to distinguish these carbon nanotubes, the (n,m) indices

[18] were established. The chiral or roll-up vector 퐶⃗h, which is a linear combination of the unit vectors and of the graphene layer with the integer parameters n and m (Equation 1.1), defines the direction of the tube, relative to the graphene lattice and the tube scope.

퐶⃗h = n 푎⃑1 + m 푎⃑2 Equation 1.1

With n ≥ m

The CNT being constructed this way is denoted a (n,m)-nanotube corresponding to a cross-section perpendicular to the nanotube axis (the equator). Furthermore, it defines the helicity and the diameter of the tube, as well as the chiral angle Θ, being the angle between 퐶⃗h and the zig-zag direction, which is parallel to 푎⃑1 (Figure 1.2.3). It displays the cut of the sheet along the blue dotted line followed by rolling the tube in a way that the tip is touching the tail forming either armchair, zig-zag or chiral nanotubes.[19]

Figure 1.2.3: Schematic diagram demonstrating the rolling-up of a hexagonal graphene sheet to form a carbon nanotube resulting in three different forms: armchair, zig-zag, and chiral.

The classification of the different cases is exposed in Figure 1.2.3 (right) and expressed in the Equations 1.2, 1.3, and 1.4. Their names arise from the shape of the cross-section.[19-20] The translation vector T is defined to be the unit vector of a 1D nanotube being perpendicular to the chiral vector and parallel to the tube axis.[21] Focusing on the long periodical unit and neglecting the caps, CNTs can be considered as one-dimensional (1D) nanostructures. They are terminated by a hemisphere of a hemispherical endcap, structurally similar to a fullerene consisting of six pentagons and an appropriate number of hexagons.[21]

armchair: (n = m), (n, m ≠ 0), (Θ = 30°) Equation 1.2

zig-zag: (n ≠ 0), (m = 0), (Θ = 0°) Equation 1.3

chiral: (n ≠ m), (n, m ≠ 0), (0°<Θ>30°) Equation 1.4[22]

(Θ = chiral angle, n,m = integer parameters)

The feasibility of the discrimination of nanotubes by the (n,m) indices relies on their either metallic or semiconducting properties depending on their specific helicity. The relation between the helicity and the electronic properties of CNTs is expressed in the following equations (Equation 1.5/1.6).

Metallic properties: n - m = 3q | q ∈ N0 Equation 1.5

Semiconducting properties: n - m ≠ 3q | q ∈ N0 Equation 1.6

(n,m,q = integer parameters)

Accordingly, all armchair (n,n)-CNTs are metallic and zig-zag (n,0)-CNTs are only metallic when n is a multiple of three. For that reason, one third of nanotubes have metallic properties and two thirds behave semiconducting.[23] While the carbon-carbon distance is about 0.14 nm within the tube, this value can vary depending on the respective type of CNT but exhibits approximately a similar value like graphene.[24] According to solid state physics and quantum confinement, sharp distribution functions of the corresponding electron density of states (DOS) exist for SWCNTs, visible in Figure 1.2.4 illustrating the differences between the two diverse electronic types. The distinct energy differences between the valence states and the conductive states define either a metallic or a semiconducting character of the individual nanotubes.[25] In SWCNTs, the density of states becomes singular resulting in the possession of one- dimensional energy bands. Close to the Fermi energy (EF), the value of the metallic tubes is nonzero, whereas semiconducting nanotubes have a zero value. These specific electronic states of carbon nanotubes are called van Hove Singularities (vHS).[25] The delocalization of the π-electrons results in a strong photon absorbance, which causes the deeply black color of the CNTs. Chemical modifications of the framework can influence this specific electron density and can be probed by various kind of spectroscopy (optical or fluorescence), which has been the goal for many researchers recently.[26]

Figure 1.2.4: Illustration of the electronic density of states (EDOS) of SWCNTs; A) Metallic and B) semiconducting SWCNTs.

The diversity of the various morphologies of nanotubes represents also their biggest disadvantage of usage. Their variation in diameter, length, chirality, electronic type, and number of defects are defined by the production process resulting mostly in highly heterogeneous mixtures including residual particles of catalysts or amorphous carbon as impurities. Thus, these complex mixtures of products complicate the comparability of the research results since they depend on their single structure parameters. Therefore, the efficient removal of undesired impurities as well as the separation of these heterogeneous mixtures without harming the structure of the SWCNTs is the challenging task while working with CNTs. In this thesis, exclusively SWCNTs were used as starting material as the stacking of numerous layers in MWCNTs impedes the characterization of the material significantly as some spectroscopic tools cannot be applied anymore.

1.2.2 Synthesis and Purification

A variety of ways to synthesize CNTs is available including high-temperature techniques like arc discharge or laser ablation which used to be the first methods of CNT production.[27] Nowadays, chemical vapor deposition is applied more frequently since some properties like orientation, length, diameter or purity can be controlled by this technique.[27c, 28] Independent of the utilized method, various kinds of impurities cannot be avoided. Besides the desired CNTs, mentioned methods produce for instance nanocrystalline graphite or amorphous carbon, accompanied by the presence of residual catalysts (metals like Fe, Co, Mo or Ni). These impurities can restrict the properties of the CNTs and impede the exact characterization.[27c, 29] Therefore, an efficient and uncomplicated way to purify is the goal of current research.[27c, 30] The most utilized methods for the production of SWCNTs will be described now.

Carbon arc discharge

The carbon arc discharge method represents one of the easiest and most commonly used methods to produce CNTs due to its simplicity to perform. The first SWCNTs growth was described by Iijima and Ichihashi who produced SWCNTs with a diameter of 1 nm.[27d] Initially utilized for the production of fullerenes, the technique consists of two carbon rods placed end to end separated in an enclosure under reduced pressure in an inert gas atmosphere and a voltage of approximately 20 V which creates a high temperature (~4000 K) discharge between the two electrodes (Figure 1.2.5 A). The electrodes are composed of graphite and a certain metal. The production of SWCNTs requires the presence of metal catalysts (Fe, Co, Ni, Y, Mo) which play an important role in the process yield. The discharge leads to the evaporation of the anode and forms a small rod-shaped deposit on the cathode. The yield, composition, and efficiency is depending on several aspects like for instance the varying operating conditions (permanent arcing current, pressure, catalyst, inert gas flow rate, distance of electrodes, etc.). An average tube diameter of 1-3 nm can be reached using this method.[27c, 31]

Figure 1.2.5: Schematic illustration of Carbon Arc Discharge (A), CVD (B) and pulsed Laser Vaporization (C).

Pulsed laser vaporization

Another efficient and superior route for the growth of SWCNTs with a narrow diameter distribution in high yields (70 %) combined with high quality and high purity, is the pulsed laser vaporization. This method was introduced the first time in 1996 by Smalley [27c, 31] et al. In this process, laser pulses (mostly Nd:YAG and CO2 laser) vaporize a graphite target containing small amounts of metal catalysts such as Co and Ni. The target is fixed in a furnace at roughly 1200 °C in an inert gas atmosphere. The nanotubes develop on a water-cooled collector outside the furnace (Figure 1.5 C). The purity of the nanotubes generated by pulsed laser vaporization is relatively high, but the production of high amounts of purified nanotubes is quite costly.[32]

Chemical vapor deposition (CVD)

The chemical deposition method - including plasma-enhanced or thermal - represents an alternative method in which CNTs are grown by the aid of catalysts and is nowadays the standard method for the CNT production. Besides, other CVD methods like water- assisted CVD,[32c] oxygen-assisted CVD,[33] microwave plasma (MPECVD),[34] or radiofrequency CVD[35] have become increasingly relevant recently. In the catalytic CVD process, gaseous carbon sources are decomposed over a transition metal catalyst and the CNT synthesis is initiated by some of the resulting carbon atoms (Figure 1.5 B). Commonly, this growth mechanism implies the dissociation of hydrocarbon molecules and the saturation of carbon atoms in the catalyst metal nanoparticles. The development of carbon from the saturated metal particles induces the formation of carbon nanotubes. Due to the presence of catalysts, the need for high temperatures decreases (600-1200 °C) but the synthesized CNTs exhibit more structural defects since the probability of structural rearrangements is lowered compared to distinctly higher temperatures in laser ablation and arc discharge.[36] The [37] [38] addition of metal precursors like Fe(CO)5 or a metallocene to the gas current leads to the direct preparation of the catalyst in the reactor by decomposition of the complex. Then, the catalyst enhances the nanotube formation in which the precise control of the size represents still an issue. Anyhow, this way of production results in a good yield and is an option for commercial manufacturing.[36] The SWCNTs, which were used predominantly in this thesis, were generated by a special type of CVD method, namely HiPco (High Pressure carbon monoxide decomposition).[39] In this process, high pressure (30-50 atm) and high temperatures (900-1100 °C) are required in order to decompose Fe(CO)5 involving CO as carbon source. The advantages of this process are high yields accompanied by high purity and small diameters (~1.2 nm), which makes it one of the most promising production techniques. Another CVD alternative is the CoMoCat (cobalt-molybdenum catalyst) production technique which describes a heterogeneous process involving growth on supported catalysts. In this process, CNTs are grown by CO disproportionation (decomposition into C and CO2) in the presence of Co and Mo catalyst at 700-950˚C in flow of pure CO at a total pressure that typically ranges from 1 to 10 atm. This method results in high yields, low variation of diameters and selectivity towards SWCNTs better than 80 % as a consequence of the synergistic effect of Co and Mo on a substrate with a low Co:Mo ratio.[40]

Another method was demonstrated recently, considering the conversion of molecular precursors into ultrashort singly capped (6,6) ‘armchair’ nanotube seeds on a platinum surface followed by elongating these during a subsequent growth phase to produce single-chirality and essentially defect-free SWCNTs. This process is illustrated in Figure 1.2.6 consisting of a two-step bottom-up synthesis. Beginning with a cyclodehydrogenation (CDH) of the suitably designed polycyclic precursor C96H54 (P1) which leads to the formation of singly capped ultrashort (6,6) SWCNT seed (S1) followed by the nanotube growth via epitaxial elongation. The entire in situ process involves only low temperatures (~400-500 °C).[41] Thus, this approach achieves defect- free and single-chirality CNTs without further necessary purification steps. With this method, a synthetic strategy was presented for precursor molecules with the aim of the surface-assisted fabrication of isomerically pure SWCNTs until up to 54 chiralities.[41b]

Figure 1.2.6: Two-step bottom-up synthesis of SWCNTs including the formation of a singly capped ultrashort (6,6) SWCNT seed (S1) via cyclodehydrogenation (CDH) of the suitably designed polycyclic hydrocarbon precursor P1 and nanotube growth via epitaxial elongation. Adapted with permission from [41a] (Copyright 2014 Springer).

The big advantage of this method is that further purification steps are no longer necessary, since the other techniques produce various kinds of impurities independently of the production process which interfere with the properties of CNTs. Thus, the removal of these undesired parts and complete purification of the CNTs is required. Moreover, one of the most intrinsic goals concerning CNT research is the

development of efficient and simple purification methods which are currently mainly based on acid treatment of synthesized CNTs.[27c, 30, 42]

1.2.3 Functionalization Reactivity

Considering the modification of CNTs, the reactivity plays an essential role, whereat two parameters have to be taken into account within nonplanar conjugated carbon system. Curvature-induced pyramidalization at the carbon atom, which reflects an unfavorable energetic state, and the π-orbital misalignment between adjacent atoms (Figure 1.2.7) enhancing significantly the reactivity in comparison to the rigid, flat, and therefore rather non-reactive structure of graphene.[43]

Figure 1.2.7: Fullerenes (C60) - Carbon Nanotubes (5,5) - Graphene with their specific σ-π - angle (θ σ π) of carbon-carbon bonds.

Figure 1.2.7 depicts a rather small 5,5-nanotube which exhibits an θP of 6°, calculated out of the equation θP = (θσπ-90°) in comparison to graphene and a fullerene. By increasing the size of the tube, the pyramidalization angle is decreasing, thus a 10,10- nanotube exhibits a smaller pyramidalization angle of 2°. Low diameter SWCNTs are more reactive than ones with larger diameters because of the higher pyramidalization angle at the carbon atoms, which appears as an unfavorable energetic state. The storage of strain energy is released with the binding of an addend. The caps represent the sites of the highest chemical reactivity in the CNT structure exhibiting a 11

semifullerene-like structure. The other parameter, governing the chemical reactivity of CNT sidewalls is depending on the presence of defects. Approximately 2 % of the carbon atoms in SWCNTs are located in non-hexagonal rings. These sidewall defects such as vacancies or pentagon-heptagon pairs (Stone-Wales defects) increases a locally enhanced chemical reactivity of the graphitic nanostructures. Assumingly, covalent sidewall functionalization generates sp3-carbon sites on CNTs intruding on the band-to-band transitions of π-electrons, and result in the loss of the properties of CNTs (e.g. high conductivity or mechanical properties).[44, 45] Furthermore, the π-orbital misalignment in CNTs, which occurs for practically all bonds, is another essential factor influencing the reactivity of CNTs. Despite of all atoms being equivalent, there are two different types of bonds in each (n,m)-CNT. Based on previous calculations of respective torsional strain energies in conjugated organic compounds, the π orbital misalignment can be seen as the main contribution to the strain within CNTs.[43a]

Types of Functionalization

The heterogeneity of the produced CNTs is not the only issue researchers have to fight with while handling CNTs. Due to their enormous surface area and therefore to their huge existent van der Waals forces, they mostly exist as bundles in which they lose most of their relevant electronic properties. As a consequence, CNTs are almost completely insoluble in all aqueous and organic solvents. With respect to these issues, certain ways have to be found to unbundle and separate these CNTs from each other. One of these approaches is the chemical functionalization, in which the surface properties are targeted by the application of reactive molecules. By anchoring specific groups to the carbon lattice, the solubility in certain solvents can be improved, the interplay between specific groups and the surrounding media can be achieved or the separation of CNTs regarding their properties can be enabled. A classification of the different types of functionalization, which can be divided into five various groups depending on the kind of attachment, is given in Figure 1.2.8.[43a, 44-45] As this thesis will mainly focus on the covalent functionalization, the other classes of addition will be merely discussed concisely.

Figure 1.2.8: Types of functionalization; 1) Endohedral functionalization; 2) Non-covalent adsorption; 3) Covalent sidewall functionalization; 4) Covalent defect sidewall functionalization; 5) Wrapping of polymers.

Endohedral functionalization

In the endohedral functionalization, the inner cavity of the nanotubes is used by different molecules (gases, salts, metals, fullerenes) to fill in or to get transported. These changes can affect the physical and chemical properties significantly. Thus, the nanotubes can build a semipermeable membrane which limits the current of liquids and gases.[46] By filling the inner cavity with metals like for instance iron, the CNTs can even exhibit magnetic properties as well.[47]

Wrapping of polymers around CNTs

CNTs can be coated by different polymers that wrap around the tube in an assumedly well-ordered periodic fashion leading to the unraveling of the nanotube bundles.[48] This approach represents a promising solution for the purification issues by “wrapping” discrete macrocycles around the tubes, initiated by Pérez et al. creating an irreversible ring-closing metathesis of macrocycles around the SWCNTs.[49]

Non-covalent adsorption

In this approach, only the weak π-π-stacking interactions of guest molecules on the π- electron-system of the CNTs (van der Waals interactions) are utilized, which act between different molecules and the extended surface of the nanotubes. The amount of defects remains the same as the interaction is only based on surface adsorption.[50] This is interesting for tuning the properties such as increasing the solubility of the nanotubes in aqueous and organic solvents. The application of amphiphilic molecules such as tensides adsorbing with their unpolar ending at the tube surface and with the hydrophilic part transmit solubility in polar solvents like water, can ensure the separation from the bundles.[51] Moreover, CNTs can be bound by different kinds of peptides and proteins (e.g. streptavidin) causing an immobilization of the biomolecules.[52]

Covalent defect sidewall functionalization

The covalent defect sidewall functionalization uses the increased reactivity of the areas of the nanotubes with greater curvature or already existing defects. These areas are opened and oxidized to acid groups by the application of oxidizing acids. This leads to the possibility to amidate or esterify these groups.[53] However, due to the oxidation, the number of defects increases drastically as well.

Covalent Functionalization

The difference to the already listed ways of functionalization (except the covalent defect sidewall functionalization) is that covalent bonds between the attacking molecule and the SWCNT are formed, resulting in a hybridization of the involved sidewall carbon atoms from sp2 to sp3-configuration. Therefore, the properties of the nanotubes are influenced way stronger. Due to their weak reactivity, covalent sidewall functionalization requires highly reactive reagents.[45] This kind of functionalization, especially type-selective reactions are required to enable the separation of metallic and semiconducting CNTs. Anyhow, the difficulty to control the chemo- and regioselectivity, respectively, depends on a specific range of attached moieties like for instance halogens or arynes as well as the prevailing harsh reaction conditions. The pattern or the exact location of the addends is very complex to determine by the common characterization tools.[45, 54] A variety of publications dealing with the covalent attachment of functional moieties is available, a selection of noteworthy and important

reactions is depicted in Scheme 1.2.1.[45, 54] The most important ones will be discussed in the following.

Scheme 1.2.1: Overview about the most common chemical reactions to functionalize SWCNTs covalently.

A common and very interesting type of chemical reactions is the cycloaddition reaction, benefiting from the electronic type selectivity in the majority of the cases, which attracts great interest.[55] During these reactions, predominantly metallic CNTs are reacting which can be traced back to the electronic structure in addition to the electron density concerning the Fermi level of metallic CNTs. A two-step mechanism was proposed including a diradicaloid intermediate which can be stabilized by metallic CNTs leading to this distinct selectivity.[56] These reactions involve the π-electrons of the CNTs as well as the specific electrochemical potential of the reagents, for instance, the azomethine ylide [3+1] addition reaction (“Prato” reaction) or the Diels-Alder [4+2] addition to name the most important ones.[57] Moreover, due to the reactive nature of nitrenes[58] and carbenes,[56a, 59] cycloaddition reactions including these reactants were investigated very intensely recently. Additionally, these reactions have the advantage to be easily controllable.

Furthermore, halogenation of the nanomaterial has been gaining a lot of interest since the beginning of the research related to nanotubes. The first halogenation reaction, which was published in 1998, dealt with the fluorination of the sidewalls of SWCNTs, yielding a high degree of functionalization.[60] Due to the fact that elementary fluorine can be seen as the most reactive element on the planet, the CNTs are fluorinated very rapidly.[61] This reaction was studied extensively during the last years as well as the usage of the functionalized SWCNTs for follow-up chemistry like for substitution approaches with different compounds (Grignard, sulfur compounds, amides).[62] Remarkably, a facile way to brominate SWCNTs by reductive activation to a high extent was published recently by Hirsch et al.[63] The resulting halogenated intermediates are highly sensitive to moisture but are the base for possible substitutions. This topic will be intensified in Chapter 3.5 focusing on chlorination and bromination of the nanomaterial.

An alternative way to functionalize SWCNTs is the reductive approach, which was predominantly applied in this work. Different approaches to form covalent bonds are available generating highly reactive intermediates at first. One of the first methods used to create covalent bonds to the carbon lattice via reduction was the incorporation of buckypaper electrodes. After activation of the SWCNTs by applying a specific potential, diazonium salts were used to functionalize the CNTs.[64] However, this method exhibited various disadvantages, therefore wet chemical experiments gained more interest in which the SWCNTs could be stabilized by Coloumb-Coloumb interactions.[65] Then, it was possible to functionalize the whole surface area as well as to regulate the degree of functionalization by controlling the amount of the reductive agent during the activation step. Following the famous Birch reduction,[66] alkali metals were dissolved in liquid ammonia and mixed with SWCNTs afterwards to generate charged intermediates being highly reactive and able to get trapped with suitable electrophiles like aryl- or alkyl halides.[67] These charged SWCNTs, so-called nanotubides, are in analogy to other carbon allotropes like graphene (which will be discussed in the next chapter) or fullerenes and undergo a single electron transfer (SET) from the reduced SWCNTs to the utilized electrophile resulting in the formation of a radical which reacts subsequently with the lattice of the SWCNTs.[68] The charged intermediates are salts, exhibiting a nanotube anion with alkali metal cations where various stoichiometries of the salts MxCy can be prepared. Dispersed in a suitable solvent, the SWCNTs were individualized as well as stabilized as a consequence of 16

the Coulomb-Coulomb interactions being an extraordinary advantage of this reaction.[69] The conditions were improved and adjusted time by time and many different approaches were carried out for new reagents like heteroatom-centered radicals,[70] monomers for polymerization reactions,[71] carbonyl species,[72] and carbon dioxide.[73] Additionally, many different routes of reduction were investigated and proposed over time, depicted in Scheme 1.2.2. The common element of these ways is the generation of the highly reactive charged nanotubides during a wet chemical process. In addition, reactive agents (alkali metals or earth alkali metals) are typically used accompanied with a promotion agent in two cases like ammonia or naphthalene. After the generation of the nanotubides, subsequent functionalization with trapping agents occurs via the mentioned SET-mechanism.

Scheme 1.2.2: Overview of the most common routes to create nanotubides in a wet chemical way; A) Birch-like reduction; B) Modified Birch/Billups reduction; C) Reduction with Na/K alloy; D) Reduction with naphthalene as promotion agent; E) Reduction with elemental potassium.

One of the reaction sequences (Scheme 1.2.2 A) is based on the classical Birch reduction of aromatic systems published by Billups et al.[66] The metal dissolves in liquid ammonia and represents the reductive agent which is being oxidized. To use components that would react under the Birch conditions with the liquid ammonia, the “modified Birch”/Billups reduction was developed (Scheme 1.2.2 B).[73] This involves the application of an inert co-solvent - THF. After the evaporation of the earlier condensed ammonia, a stable but reactive dispersion of charged nanotubes is formed which avoids potential reactions of functionalization reagents with the promotion agent

ammonia. Anyhow, residual traces of ammonia can impair reactions with specific compounds and complicate the determination of the exact reduction state.

Another reaction using a Na/K alloy as reduction agents is reaction sequence C (Scheme 1.2.2 C) based on commonly used reduction way for graphene chemistry.[74] The alloy is added directly to the mixture of DME and nanotubes resulting in a dispersion of nanotubides. The redox potential of this Na/K alloy accompanied by the high surface area of the CNTs offers a very efficient reducing potential. Afterwards, an electrophile can be added to perform a successful attachment.[108a]

The reduction of SWCNTs can be also achieved utilizing naphthalene as an electron transferring agent between the metal and CNTs (Scheme 1.2.2 D). The formation of a solution of naphthalenide occurs by adding the metal to naphthalene in THF. As the electron transfer from the naphthalenide is not quantitative, various byproducts are formed.[75] As these reaction sequences exhibit many drawbacks, like not quantitatively determinable transfer of electrons or the presence of irremovable promotion agents, another method to generate nanotubides was developed.[76] In this technique, liquid K (Scheme 1.2.2 E) is heated up together with the SWCNTs under inert conditions yielding a brownish powder.[77] After the electron transfer occurs from the K to the nanotubes, an ultrasonication step in THF leads to a dispersion of the nanotubides and therefore represents the basis for the functionalization. The ultrasonication step enables the unbundling of the CNTs and suitable trapping agents can be used for the functionalization. Applying this technique, numerous moieties have been attached to the carbon system of SWCNTs, like for instance alkyl groups,[78] aryl species[79] or recently endohedral fullerenes to form a hybrid material consisting of two different allotropes.[80] In the scope of this work, the main focus lies on the reductive activation of SWCNTs and the following derivatization or incorporation of various useful functionalities.

1.3 Graphene 1.3.1 Structure and Properties

The other carbon allotrope of interest addressed in this thesis is graphene which exhibits numerous extraordinary electronic, optical, and mechanical properties, including high electron mobility, high thermal conductivity,[6] and immense mechanical strength.[7] These properties originate from its electronic structure and have attracted a lot of interest from many researchers in the whole world for years until today. It´s electronic structure will be further elucidated now.

Graphene is a 2-dimensional (2D) carbon lattice consisting of one atom thick sheet of sp2-carbon atoms arranged in a honeycomb structure. The edges of the sheet manifest mainly either zig-zag or armchair structure in analogy to carbon nanotubes (Figure 1.3.1 A).[81]

Figure 1.3.1: A) Graphene sheet with zig-zag and armchair edges; B) Graphite in an ideal AB-stacking.

Naturally, it exists as layers of graphite, in which each sheet sticks together through very pronounced π-π-stacking interactions, which is one reason for its high thermodynamic stability (Figure 1.3.1 B).[81] Natural graphite consists of different 3D stacking orders of the graphene layers, which can be distinguished in either hexagonal (AB), rhombohedral (ABC) or turbostratic within the sheet order.[82] A single layer of graphene can merely be stabilized on a support like a substrate or being suspended in a suitable solvent. The carbon-carbon distance within graphene between the tight σ- bonds is 1.4 Å, which are formed by an s orbital and two p-orbitals. According to the Pauli principle, the σ-bands bear a ﬁlled shell and form a deep valence band. The 19

uninvolved p-orbital can interact with the p-orbitals of the neighboring carbon atoms, resulting in the formation of a π-band perpendicular to the planar structure. Due to the presence of one additional electron, the π-band is half ﬁlled.[83] The hexagonal structure of graphene can be defined as a triangular lattice with a system of two atoms per unit cell with the vectors 풂⃑⃑⃑1 and 풂⃑⃑⃑2. The reciprocal lattice vectors 풃⃑⃑⃑1 and 풃⃑⃑⃑2 form the Brillouin zone which corners K and K´ represent the Dirac points (Figure 1.3.2).

Figure 1.3.2: A) Honeycomb structure of graphene consisting of two interpenetrating triangular lattices with the vectors 풂⃑⃑⃑1 and 풂⃑⃑⃑2. and δi, I = 1,2,3 as the nearest-neighbor vectors; B) Brillouin zone in which the Dirac points are placed at the K and K´ corners.

The Dirac points can be seen as the intersections of the two energy bands of graphene, the conduction and the valence band. Figure 1.3.3 A illustrates the 3D Brillouin zone with the linear dispersion relation of graphene around the six Dirac points. With respect to the crystal and with it also the band structure, the low energy quasi particles of graphene can be described with a Dirac equation instead of the Schrödinger equation.[84] One of the explanations why quantum effects are stable in graphene at 6 [83, 85] RT is the Fermi velocity of the massless Dirac fermions of ѵf = 10 m/s. Moreover, an unconventional Quantum Hall effect (QHE) can be measured in graphene and represents an additional proof for the Dirac like behavior, donating to the Quantum Electrodynamics (QED)-like quantization of graphene´s electronic spectrum in the magnetic field.[86]

Figure 1.3.3: A) Electronic dispersion in the honeycomb lattice. Energy spectrum in units of t for ﬁnite values of t and t´, with t = 2.7 eV and t´ = −0.2 t, including zoom-in of the energy bands close to one of the Dirac points, adapted with permission from [83] (Copyright 2009 Elsevier); B) Scheme depicting the band structure and the resulting ambipolar field effect in graphene.

Another meaningful aspect about the charge transport in graphene is ambipolarity due to the disappearance of the electronic density of states at the Dirac points. Undoped graphene, however, shows almost zero quantum conductivity.[84, 86a, 87] The ambipolarity implies that charge carriers can be tuned continuously between the holes and electrons by providing the essential gate bias. The Fermi level (EF) declines below the Diarc point in the case of a negative gate bias, whereas the Fermi level increases above the Dirac point under a positive gate bias which supports a significant amount of electrons into the conduction band (Figure 1.3.3 B). The access to an ambipolar semiconductor offers a broad range of opportunities for the production of novel devices.[87a]

1.3.2 Production

Graphene, as a single layer of graphite, can be produced by various methods, which determine significantly its structure, properties, and general quality. The result can be defined as “graphene” when both its dimensions are larger than 100 nm discriminating it from nanographenes including graphene quantum dots (GQDs), graphene nanoribbons (GNRs), and nanographene molecules.[88] The most common

approaches can be basically distinguished between two techniques: bottom-up and top-down methods (Figure 1.3.4).

Figure 1.3.4: Top-down and bottom-up approaches for the graphene production; A) Synthesis of nanographenes; B) Annealing of SiC; C) CVD growth in a CVD reactor; D) Exfoliation via intercalation; E) Exfoliation via mechanical force; F) Exfoliation via scotch tape from graphite.

Bottom-up methods can yield higher-quality graphene mostly deposited on substrates exhibiting increased charge carrier mobilities, whereas the top-down techniques sustain bulk scale graphene with rather low qualities generated as powders or in dispersions, except from the scotch tape method.[88]

Bottom-up

One bottom-up method is to synthesize graphene starting from small organic molecules as precursors. This provides extremely defined but small graphene-like structures such as GQDs or GNRs. The solubility of these molecules is decreasing with increasing size.[88-89]

Nevertheless, the epitaxial growth of graphene on SiC and chemical vapor deposition (CVD) are the most frequently used fabrication methods. The growth on SiC is started by the sublimation of Si atoms from the SiC surface followed by the graphitization of

the residual C atoms by annealing at temperatures above 1000 °C under ultra-high vacuum. The graphene can be yielded as crystalline domains with the size of several hundreds of micrometers, just limited by the size and quality of the substrate and its high costs.[88, 90] During this process, a chemically inert carbon buffer layer is formed between the SiC and the graphene layer which presents a lateral layer-by-layer growth on a terrace.

CVD can be understood as the most promising method for graphene fabrication with respect to the growth size of uniform graphene flakes. In this technique, a suitable substrate, mostly transition metals like Cu[91] or Ni[92] foil, are placed in a CVD reactor consisting of an oven, gas inlets, and a sample holder (Figure 1.3.5 A). The usage of Cu foil is well-established due to its low solubility of C atoms within it and its moderate catalytic activity.[93]

Figure 1.3.5: A) CVD chamber including gas inlet and quartz glass sample holder with a Cu substrate; B) Optical microscope image of graphene structures grown on a Cu foil.

The formation of graphene is started by a thermal decomposition of a carbon source like CH4 or EtOH at temperatures around 1000 °C after annealing the surface of the substrate with hydrogen gas. Subsequently, carbon atoms are assembled into the graphitic pattern.[88] The grown graphene (Figure 1.3.5 B) is transferred afterwards on an optional substrate, commonly Si/SiO2, with the aid of a polymer which sticks to the graphene layer and can be removed later on. Prior to that, the Cu foil is etched away with for instance FeBr3 followed by the “fishing” of the PMMA/graphene layer and the dissolving of the PMMA by acetone vapor. This roll-to-roll process is very time- consuming and residual chemicals on the grown graphene sheet cannot be completely

avoided.[94] The quality of the graphene significantly depends on the crystallinity of the [95] used Cu-catalyst, the H2 concentration, and the used carbon source.

Top-down

One of the most common methods is the micromechanical cleavage of graphite using adhesive tape since it yields high-quality graphene sheets exhibiting properties close to the intrinsic values predicted by theoretical studies even though this technique lacks in yield and is very time-consuming.[96] In this technique, scotch tape is used to overcome the van der Waals interactions in order to exfoliate the single sheets until [3, 86b, 97] the sheet is individualized followed by the transfer on a Si/SiO2-substrate. Anyway, this method can be perfectly applied for fundamental research and proof-of- concept reactions, which are typically performed in the laboratories of natural scientists and electrical engineers.[88]

Another possibility to produce graphene is the usage of graphite oxide (GO) as precursor. GO is highly defect-rich graphene bearing mostly oxygen-containing functionalities such as hydroxyl or epoxy groups originating from the way of production.[98] Commonly, the oxidation of graphite to GO is performed via the Hummers method using sodium nitrate, sulfuric acid, and potassium permanganate followed by the exfoliation to single sheets in high quantities.[98] Different ways can be applied then to reduce the defect-rich graphite oxide to reduced graphene oxide (rGO) either in bulk or on a substrate for instance via thermal annealing. On the one hand, this method results in high quantities of manufactured rGO but on the other hand, the products contain high amounts of structural defects that compromises its electronic properties.[99] This topic will be discussed in Chapter 3.3 in more detail.

Another relatively new method has recently emerged: the electrochemical exfoliation of graphite, which results efficiently and low-costly in high-quality graphene.[100] There are many different approaches but in general the similarity is to use graphite as working electrode in a solution of electrolytes (e.g. TBAF, Li, (NH4)2SO4, TBAP) which intercalate into the layers after the application of an electrical potential followed by the subsequent expansion enabling the exfoliation.[101]

A liquid-phase exfoliation represents another approach to enable a large scale production of graphene with lower density of defects compared to for instance the GO approach. This top-down principle relies on the application of external forces (e.g. sonication and shear mixing in solution, ball milling in either a suspension or of a solid sample) which weaken the van der Waals interactions between the adjacent graphene sheets in various organic solvents or in aqueous dispersions.[102] With the aid of polymers or surfactants or different intercalants (K, Cs, H2SO4, or FeCl3) the exfoliation process can be improved. Hence, for instance, intercalants such as alkali metals like potassium are commonly used to exfoliate graphene followed by a possible functionalization (which will be discussed later in the text) yielding an assortment of single-layer and few-layer graphene.[103]

Figure 1.3.6: A/B) GIC (Graphite Intercalation Compound) with the carbon/potassium ratio of 1:8 illustrating the distance between the K+-ions in stage 1 of intercalation in comparison to C) graphite. This technique is based on the formation of the so-called graphite intercalation compound (GIC) in which a reductive activation of the sheets takes place.[104] The graphene sheets get reduced in a specific stoichiometry (maximum up to K:C8) simultaneously to the enrichment of K+ ions in between the charged sheets resulting in an increased layer-by-layer distance (3.35 Å to 5.3 Å) (Figure 1.3.6 B/C). When every interlayer spot is filled, a metal-metal distance of 4.9 Å is reached (Figure 1.3.6 A).[105] The intercalation process proceeds in different stages, whereby stage I represents a fully charged species. Thus, only mild mechanical forces are sufficient to exfoliate the layers. Various factors like the size of graphite, intercalant or chemical affinities determine the course of intercalation. During that process, an electron is transferred from the intercalating guest molecule to the graphene lattice which induces electrostatic forces within the layers replacing the π-π-interactions.[105- 106] The addition of a suitable inert solvent enables the formation of a dispersion consisting of the solvent and the negatively charged graphene layers (graphenides) 25

based on their negative energy of mixing.[107] To create GICs, a variety of different routes have been established. As mentioned already in Chapter 1.2.2, the methods for graphene bear resemblance or are identical compared to the reductive activation approaches for CNTs, respectively. For graphene, merely three of these techniques are usually applied, all following the same principle, including graphite as starting material and a reductive agent which intercalates as an activation step to form the graphite intercalation compounds for the exfoliation of the individual layers.

1) First, the usage of Na/K alloy in DME which was developed by Englert et al. in 2011.[74] The presence of the metal alloy enables the exfoliation of the respective graphene sheets and supports the further functionalization with for instance diazonium compounds or alkyl reagents.[108] 2) Second, the modified Birch reduction keeping harsh conditions at -78 °C as well as the presence of ammonia as promotion agent and THF as inert co-solvent aside Li or K as reductive agents can be applied to SCAs to create GICs.[67b, 72, 109] 3) Third, the mentioned activation route via elemental potassium and the further dissolution in a suitable solvent like THF or DMSO is the most recently [104b, c, 107] developed approach. This resulting dispersion, especially KC8, is the base for covalent graphene functionalization, which will be intensified in the next chapter.

Applying these methods to create GICs results in defect-free graphene without the usage of harsh mechanical forces but exhibits one major drawback, the low yield of monolayer graphene. Asides from monolayer graphene, mostly few and multilayer graphene will be produced in high quantities. Additionally, the GICs are very reactive towards water and oxygen which undergo undesired side reactions and impair possible future goals. The reaction of charges on the graphene or CNTs surface with oxygen to hydroxyl-groups was already described.[78] Moreover, it is known that water reacts with these charges to form hydrogen-bonds to the lattice of the respective SCA.[110]

To overcome the issue of undesired side reactions, an additional step was introduced for a complete and defect-free exfoliation as well as for a progressed discrimination of attached groups in further functionalization sequences.

1.3.3 Discharging of Graphenides

As mentioned above, the control of the occurring side reactions caused by remaining negative charges on the graphene surface is essential to characterize the material completely and gives the opportunity to fully understand possible following addition patterns. Hirsch et al. discovered that the addition of an oxidative agent, namely benzonitrile (PhCN), exhibiting a rather low reduction potential, leads to a facile, effective, and quantitative electron trapping from the respective negatively charged graphene sheets independent on the existing form in a heterogeneous solid/liquid phase reaction.[103]

Scheme 1.3.1: Reduction of graphite to GIC (KC8) and quantitatively discharging of GIC by PhCN.

Thus, graphenide dispersions, solid-state GICs or graphene deposited on a substrate can be easily discharged by the exposure to PhCN which forms the red-colored radical anion PhCN•- (Scheme 1.3.1). This perfectly enables the quantitative determination of the corresponding amount of oxidized electrons via absorption spectroscopy. Hence, it impedes the addition of undesired moieties due to the avoidance of feasible side reactions with moisture and oxygen.[78] Moreover, this technique simplifies the wet- chemical exfoliation of GICs in a suitable solvent. After ultrasonication of the respective dispersion and the resulting exfoliation, the graphene sheets get oxidized by the added PhCN and can be obtained afterwards as individualized, defect-free layers.[103] Accompanied by the oxidation of the corresponding carbon material, a quantitative migration of the positively charged interlayer cations takes indeed place as well. This mass transport derives from the electrostatic potential of the formed PhCN•- anions in the dispersion and their diffusion in the liquid phase.[103]

Now, the question arises if this procedure can also be applied to other carbon allotropes, namely SWCNTs, fullerenes or the recently synthesized nano-onions.[111] Additionally, the oxidative effectiveness of PhCN after functionalization with divergent 27

organic moieties can be queried. Certain topics, as well as the investigation of the discharging behavior of different types of graphite, will be addressed in Chapter 3.1.

1.3.4 Functionalization Reactivity

The reactivity of graphene and its derivatives is determined by the nanomaterial’s π- orbital-composition. In a defect-free, fully relaxed, and pristine graphene section the specific σ-π-angle (θ σ π) equalizes zero which causes no curvature of the graphene plane minimizing its reactivity due to the strain-free structure and leads to a zero band gap.[112]

Figure 1.3.7: Fullerenes (C60) - Graphene - Carbon Nanotubes (5,5) - with their specific σ-π - angle (θ σ π) of carbon-carbon bonds.

This derives from the 90° angle between the p-orbitals and the sp2 -hybridized orbitals (Figure 1.3.7, middle). Furthermore, according to theoretical investigations, different areas of the graphene structure are more reactive than other regions, namely the edges and areas in vicinity to present defects. These regions exhibit various electronic structures introducing an increased affinity towards covalent chemical functionalization which was predicted by theoretical investigations but will be discussed intensively in Chapter 3.2.[113]

Covalent Functionalization

The tailoring of the band gap has been a big aim of most researchers dealing with SCAs as well as chemical functionalization in order to improve solubility or stabilization of monolayer graphene (MLG). These goals can be reached by chemical doping or covalent functionalization which opens the band gap of graphene and enhances the solubility depending on the nature of the attached moieties. Covalent functionalization of graphene causes a conversion of sp2 to sp3 hybridization but introduces additional strain to the carbon system, therefore an intrinsically decreased reactivity can be proposed in contrast to CNTs and fullerenes.[114] This imposed strain can be released by further addition from the other side of the graphene plane, which is merely feasible in bulk reactions but not in monotopic approaches like on MLG. Strain release is therefore achieved by binding of the dangling bonds to the used substrate.[115] Additionally, the conversion of differently hybridized orbitals increases the reactivity of specific areas in proximity to the attached molecule but this will be intensified in Chapter 3.2.[113c] Moreover, it improves the solubility in specific solvents and in general enhances the handling in various applications. The covalent graphene functionalization can be distinguished between two kinds: deposited on a substrate or in bulk (Figure 1.3.8).

Figure 1.3.8: A) Monotopic functionalized graphene sheet with phenyl-moieties (cis-1,4-addition); B) Ditopic functionalized bulk graphene with hexyl-chains (exemplary trans-1,4-addition).

There are favored addition patterns of functionalization depending on the specific type, like 1,4 and 1,6-addition pattern for the monotopic approaches, as well as 1,2-addition for ditopic approaches.[81] Anyhow, theoretical investigations claim that an ideal defect- free graphene surface is highly unlikely to be functionalized.[113c, 116] It was demonstrated that alkylation of reductively activated graphene can merely occur at

preexisting defects or close to the edges in the favored trans-1,2- or 1,4-addition in a ditopic manner using hexabenzocoronene (HBC) as a model system for graphene.[113e] The general reductive functionalization process will be further addressed and discussed in Chapter 3.2. Monolayer functionalization can be achieved in various ways including the reductive approach. Here, the graphenide is generated by the addition of Na/K alloy in DME, followed by the treatment with a suitable electrophile leading to the functionalized nanomaterial.[117] The functionalization with divergent diazonium compounds represents another common approach for the monolayer modification.[118] The exact mechanisms will be discussed later in this chapter. Many different ways to functionalize the graphene lattice covalently are already available and published. Few of the most important examples are depicted in Scheme 1.3.2.

Scheme 1.3.2: Exemplary covalent functionalization pathways of graphene yielding the functionalized products; A) [2+1]-cycloaddition; B) Hexylation; C) Fluorination; D) Hydrogenation; E) Arylation.

Cycloadditions on graphene represent one of the fundamental covalent functionalization routes with commonly a low degree of addends.[77] These reactions were already known in nanotube and fullerene chemistry and then adapted for graphene as well.[119] The so-called pericyclic reactions involve a circular move of electrons during bond cleavage and formation.[120] Various cycloadditions are feasible including the [2+1]-reaction which can be divided in two approaches, either implying carbenes or specific aziridines which lead either to the formation of cyclopropane or to 30

aziridine adducts.[120] The approach involving carbenes is depicted in Scheme 1.3.3 combined with the formation of the specific product.

Scheme 1.3.3: Exemplary [2+1]-cycloadditon mechanism; (top) Formation of dichlorocarbene with chloroform and base; (bottom) Cyclopropanation of graphene with dichlorocarbene.

Another important reaction yields hydrogenated graphene, officially defined as graphane, which has been theoretically studied for the first time in 2007 by Sofo et al.[121] They predicted graphane having two different configurations (chair or boat), in which the fused cyclohexane rings can be existent (Figure 1.3.9). As in the chair configuration the CH staggering is completely impeded, it is more stable showing a band gap of 3.5 eV (0.2 eV less than in the boat configuration).[121-122] In general, the hydrogenation of graphene provides several advantages, as it accentuates in the field of hydrogen storage as well as improved electronic properties.[121-123] The development of a considerable band gap in the advance of the hydrogenation reaction highlights another interesting aspect of this reaction which was theoretically proven first and later confirmed by experiments later independent of mechanism or degree of hydrogenation.[113a, 124] Additionally, the band gap results in photoluminescence of the hydrogenated graphene giving rise to many useful applications.[110b]

Figure 1.3.9: Chair (left) and boat (right) configuration of graphene.

Considering the mechanism, an “island-type” hydrogenation was proposed via computational predictions due to the deformation caused by the hydrogen addition.[125] More information about the experimental procedures, as well as the exploration of the corresponding mechanism, will be elaborated in Chapter 3.2.

Among other compounds, halides are of great interest for graphene modification intending to tailor the electronic properties of the material. Additionally, halogenated graphene can be used for all kinds of follow-up chemistry due to possible substitution reactions with various nucleophiles. During the last years, graphene was successfully functionalized with all accessible halides but in various extents and implying different methods. The most stable halogenated graphene is fluorographene (C1F1)n which resembles hydrogenated graphene in its possible existing configurations (chair and boat).[126] This fully fluorinated material was introduced in 2010 by chemical or mechanical exfoliation of graphite fluoride[127] or by direct fluorination of graphene applying various temperatures.[127a] The synthesis of partially fluorinated material was achieved for instance by the application of cold plasma to the allotrope preserving its typical 2D symmetry but still influencing the electronic properties effectively.[126b, 127a, b]

Partially iodinated graphene could be prepared by exfoliation of GO with HI/I2 keeping high pressure and temperature resulting in around 30 % functionalization.[128] Different [129] [130] approaches using Cl2/N2 plasma or applying photo-chemistry yield partially chlorinated graphene, whereas 100 % functionalized graphene could not be synthesized yet, even though it was theoretically confirmed.[131] Moreover, precise edge chlorination of nanographenes was published in 2013.[132] The thermal reduction of graphene in gaseous bromine atmosphere resulted in brominated graphene.[129] Both, the incorporation of bromine and chlorine will be discussed intensively in Chapter 3.5. 32

Another functionalization route is the free radical addition. On the one hand, this mechanism takes place in prior activated carbon systems.[74] As already discussed in Chapter 1.3.2, various reductive activation methods are accessible. The activated graphenides are able to undergo single electron transfer (SET) reactions with the attacking electrophile, commonly alkyl/aryl halides (hexyl iodide in the Scheme 1.3.4).[79a, 108a, 133] This mechanism allows for the formation of free radicals exhibiting an extremely high reactivity, which forces the addition to the graphene layer accompanied by the formation of a halide anion as a side product (Scheme 1.3.4).

Scheme 1.3.4: Reductive activation of graphene and following SET mechanism of the formed hexyl radical with the graphene lattice to yield the hexylated product.

On the other hand, this kind of radical addition can be achieved by either electrolysis[134] or photochemical[135] and thermal treatments. Most frequently used reactions of graphene are performed with divergent substituted diazonium tetrafluoroborate derivatives.[120, 136] These reactions lead to an aryl functionalization of the graphene sheet due to the prior release of nitrogen followed by the formation of a free radical which attacks the graphene lattice in analogy to the reductive hexylation with alkyl halides.[120] Anyhow, the feasible degree of functionalization for mentioned methods is rather low (normally <1 %), therefore the structure of graphene does not alter considerably.

Using the reductive approach, multi-functional carbon-carbon architectures were successfully created by Hirsch et al. including divergent graphene-fullerene hybrids

(endohedral fullerene Sc3N@C80 and C80). Those were synthesized for the first time 33

avoiding the application of defect-rich graphene oxide.[137] These new carbon hybrids are the basis for future research with various allotropes and the potential discovery of combined improved properties.

Non-covalent Functionalization

As mentioned, an essential object for researchers dealing with graphene is to increase the solubility to improve the processability of the material for future applications. Aiming to increase the solubility, non-covalent functionalization plays a crucial role in graphene chemistry aside from the already described covalent pathway.[77] A major advantage of this type of functionalization is the preservation of the conjugated π-system and the intrinsic electronic properties since merely weak π-interactions with the utilized reagent are formed. A variety of π-complexes are known including nonpolar gas−π interactions, H−π interactions, π−π interactions, cation−π interactions, and anion−π interactions.[77, 138] Non-covalently functionalized graphene has been produced by various ways involving the interaction of graphite with surface-active compounds for instance pyrene derivatives[139], triphenylene[140], sodium cholate[141], polyvinylpyrolidone[142], etc. Moreover, the stabilization of graphene received from graphene oxide (GO) was achieved with different surfactants as well.[143] However, in this thesis the main focus is placed on covalent carbon networks.

1.4 Characterization Methods 1.4.1 Raman Spectroscopy

Due to the polydispersibility of functionalized graphene and SWCNTs, common characterization tools known from organic chemistry, like NMR spectroscopy, mass spectrometry (MS) or infrared spectroscopy (IR), are unsuitable to use for analysis. Hence, Raman spectroscopy is the most common and frequently used method to analyze SCAs being fast, non-destructive and it provides information about the respective structure and electronic properties by probing the carbon lattice.[144]

SWCNTs

A typical Raman spectrum of pristine SWCNTs (HiPco) is depicted in Figure 1.4.1 emphasizing the important features like G-band, D-band, and RBMs.

Figure 1.4.1: Schematic Raman spectrum of SWCNTs (HiPco) using a laser with the excitation wavelength of 633 nm emphasizing the most important modes: RBMs, D-band, and G-band.

The so-called G-band (~1582 cm-1) origins from the transversal lattice vibration of the sp2-atoms in contrast to the D-mode (~1350 cm-1) which is defect-introduced by a disturbance of the carbon lattice.[145] The origin of the disturbance can be of physical or chemical nature. A missing carbon atom within the lattice (“hole”) or a seven- membered ring (Stone-Wales-defect) can be reasons for the activation of the D-band. But the most important and analytically most useful reason is the formation of sp3-

centers which derive from the covalent functionalization of the framework of CNTs and therefore allow us to determine the ratio of sp2 to sp3 centers qualitatively.[146] The so- called radial breathing modes (RBMs) are unique for SWCNTs and shed light into the composition of the analyzed sample in relation to the quantity of specific electronic types of the CNTs. They are exclusively dependent on the diameter of the respective SWCNTs and based on the perpendicular in-phase vibration.[145, 147] Thus, the analysis of these modes including peak positions and intensities enables the identification of certain electronic SWCNT types in an analyzed sample.[148] By measuring a sample with a specific excitation wavelength, a scattering event takes place which results in an electron hole pair formation, namely virtual state.[149] In the case if an allowed electronic transition to the specific DOS of a certain SWCNT coincides with the energy of a virtual state, the nanotube is in resonance leading to an intensified Raman signal.[150] Due to this fact, by the application of distinct excitation wavelengths, solely nanotubes can be analyzed which bear electronic states corresponding to the chosen laser energies.[151] The “Kataura-Plot” illustrating electronic transition energies of individual SWCNTs as a function of diameter, gives information about which CNTs are in resonance for a specific laser excitation wavelength.[152] Thus, using different laser wavelengths helps to analyze the polydisperse SWCNTs greatly.

Graphene

Raman spectroscopy is one of the most important characterization methods for graphene as well. It can provide necessary information about structural damages,[153] doping effects,[154] strain,[155] number of layers[156] or chemical modification.[157] Figure 1.4.2 A depicts a Raman spectrum of pristine monolayer graphene grown by CVD process illustrating the main Raman modes of the allotrope: D-band, G-band, and 2D-band. The unit cell of MLG consists of two carbon atoms resulting in six phonon dispersion bands which are distributed in three optical and three acoustic branches (Figure 1.4.2 B).[158]

Figure 1.4.2: A) Schematic Raman spectrum of CVD graphene using a laser with the excitation wavelength of 532 nm emphasizing the most important modes: D-band, G-band and 2D-band; B) Calculated phonon dispersion relation depicting iLO, iTO, oTO, iLA, iTA, and oTA phonon branches (Adapted with permission from [158] (Copyright 2016 Elsevier)); C) Representation of phonon vibrations contributing to G-band iTO at Γ; D) and iLO at Γ. There are two out-of-plane phonon modes (oTA and oTO) involving one acoustic and one optical branch in which the vibrations are perpendicular to the graphene lattice. For the other four branches the vibrations are in-plane and can be categorized with respect to the direction to the nearest carbon-carbon bond as longitudinal or as transvers (iTA, LA, iTO, and LO), which can be seen in Figure 1.4.2 C/D.[158] The G- band is a first order Raman process and origins from the doubly generated iTO and LO phonon modes in the Brillouin zone center (Figure 1.4.3, left), while the D-mode can be related to a second-order process which implicates one iTO phonon and one defect phonon visible in Figure 1.4.3 center and right.

Fig. 1.4.3: Left) First-order G-band process; center) one-phonon second-order double resonance process for the D-band (inter-valley process) (top) and for the D´-band (intravalley process) (bottom); right) two-phonon second-order resonance Raman spectral processes for the double resonance G´ process (top), and for the triple resonance G´ band process (TR) for monolayer graphene (bottom). For one-phonon, second-order transitions, one of the two scattering events is an elastic scattering event. Resonance points are depicted as open circles near the K point (left) and the K´ point (right); adapted with the permission from [158] (Copyright 2016 Elsevier).

Due to the double resonance process of the D-band, starting with an electron which absorbs a photon of the laser energy, the position of the D-mode is significantly dispersive with the utilized excitation energy.[158-159] The electron being inelastically scattered recombines with the hole at the starting point while emitting a phonon. Since two inelastic scattering events occur for the 2D-band, no defect is necessary for the mode activation. Whereas, for the activation of the D-band, one elastic scattering event by a defect and one inelastic scattering event by emitting or absorbing a phonon takes place. Due to functionalization or sp3-center generation, respectively, an additional band D‟ (~1610 cm-1) appears originating from another double resonant process which derives from an intra-valley process where two points of the equal Dirac cone are joined. The overtone of this mode (2D‟-mode) can be observed at ~3250 cm-1.[144b] As mentioned above, Raman spectroscopy can also provide information about the amount of layers by analyzing the shape and intensity of the 2D-band.The ratio between the G-band and the 2D-band in MLG can be up to 1:4 due to a triple resonant process (Figure 1.4.3 right).[158] Furthermore, MLG exhibits D-modes which can be

approximated by a single Lorentzian function, whereas for Bilayer graphene four Lorentzian Curves are necessary to be fitted and this number increases as well with a rising amount of layers.[156] As for the SWCNTs the D-band intensity can be used to measure the introduction of sp3-defects in the graphene lattice by setting it in relation [156] to the G-band intensity (ID/IG).

Figure 1.4.4: A) Illustration of ID/IG in correlation to the distance of defects (LD) depicting the curves for three different laser energies; B) Raman spectra of five ion bombarded SLG measured [160] at EL =2.41 eV (λE=514.5 nm), both adapted with the permission from (Copyright 2016 American Chemical Society).

The so-called Tuinstra-Koenig equation is the consequence of the separation in two stages of the amount of defects and disorder of graphite.[161] In 1970, they analyzed the correlation of the Raman spectra on the distance of defects with respect to the laser excitation wavelength in single crystals of graphite. In the first stage, the ID/IG- ratio increases in addition to a broadening of all modes and the appearance of the D‟-band. Furthermore, the G-band and the D’-band merge accompanied by the rise of the D+D’’ peak (normally ~2450 cm-1). This leads to a correlation of the G-band 2 intensity to the sample area (La ) which are proportional to each other. The same [153, 162] proportionality applies for the ID to the overall length of the edges (La).

The mentioned equation predicates that in a sample with a deficient number of defects the ID is proportional to the amount of defects, but it is not valid in the case that the number of defects increases combined with a distortion and diminishment of sp2- [158] areas. This leads to a decrease of the ID/IG ratio directing towards zero afterwards, which is the beginning of stage two. At this stage, all modes and their overtones

broaden including the increase of the FWHM of the G-mode, which enables the discrimination of these two areas.[158] A semi-empirical model for the quantification of defects was suggested by Lucchese[153] in 2010 and Cançado[160] one year later. This model was based on the ion bombardment of graphene and correlated microscopic and spectroscopic data. They stated that the distance of defects (LD) decreases during stage one while the ID/IG-value is increasing until the maximum of this value is reached when LD=3 nm followed by a subsequent decrease to zero for values of LD<3 nm 2 (Figure 1.4.4). The equation ID/IG~1/LD is valid for an average distance of defects and a specific laser spot size with an average amount of defects in the area measured by the laser but does not apply anymore in case of an increasing number of defects. Further information, for instance about the chemical identity of potential addends can be provided by an additional characterization method, TG-MS, which will be discussed in the following chapter.

1.4.2 TG-MS/ TG-GC-MS

Another important and established characterization tool for functionalized SCAs is the thermogravimetric analysis coupled with mass spectrometry (TG-MS), which is able to deliver proof for the chemical nature of incorporated addends in contrast to Raman spectroscopy.[108b, 117],[108a, 163] Additionally, gas chromatographic separation has been included in advance of the mass spectrometric analysis as a supplementary characterization method (TG-GC-MS).[79a, 133b]

A certain amount of the respective SCAs (commonly 0.5-1.5 mg for TG-GC-MS,

4-5 mg for TG-MS) is annealed under a constant inert gas flow (He, Ar or N2) and heated up from RT to a desired temperature in a TG furnace. Due to the heating process, covalently-bound molecules in addition to physisorbed moieties[164] are cleaved from the carbon lattice at certain temperatures and can be analyzed by either (1) electron ionization mass spectrometry (TG-MS) or (2) gas chromatographic separation followed by mass spectrometry (TG-GC-MS).

Figure 1.4.5.: Exemplary TG-MS profiles (A) and GC elugram (B) including mass spectrum (hexane) of hexyl-functionalized graphene; A) TG curve of functionalized graphene (red curve) in comparison to pristine graphite (dashed grey line) in addition to ion currents of mass traces of the specific mass fragments of the detached hexane (m/z 29, 42, 57); B) GC elugram depicting various peaks including the hexane peak at 4.1 min and demonstrating the mass spectrum of hexane.

Figure 1.4.5 demonstrates exemplary spectra of TG-MS (A) and GC-MS (B) of a graphene sample functionalized with hexyl chains. The detected mass loss in comparison with the mass loss of the pristine material can be correlated to the removal of molecules from the carbon structure at various temperatures and the additional

analysis is therefore able to provide information about the chemical nature of the detached groups. The cleavage of the groups depends on the binding enthalpy, degree of functionalization, and sample morphology. In this example, the fragmentation pattern of the cleaved hexane chain can be observed starting at around 180 °C accompanied by a certain mass loss confirming the successful functionalization sequence. In the TG-GC-MS analysis, a certain fraction of gases can be injected into the GC column at a distinct temperature creating an elution chromatogram including the specific coupled MS signals at respective retention times. This technique enables an improved discrimination of fragments which develop during the ionization process. In Figure 1.4.5 B, an elugram of hexyl-functionalized graphene is depicted showing various peaks like for instance hexane (4.1 min) with different retention times and their respective mass spectra. The disadvantage of that system is that water, hydrogen or in general molecules with m/z values below 40 can hardly be detected due to technical limitations of the column. Deriving from the specific mass loss of the sample, the degree of functionalization can be calculated. Anticipating that the respective mass loss merely origins from the attached molecule, the degree of addends can be approximately determined. This topic will be intensified in Chapter 3.4.

Furthermore, several other characterization tools are available to gain information about the complete process or mechanisms of reaction sequences that are to be investigated. Hence, methods like energy dispersive X-ray spectroscopy (EDS)[165], UV-vis spectroscopy[103] or MALDI-ToF (matrix-assisted laser-desorption ionization - time of flight)[166] are available to clarify unanswered questions arising after the analysis with the mentioned characterization techniques.

2 Proposal

Both nanomaterials, SWCNTs and graphene, exhibit outstanding properties with a huge potential for applications in various fields. In the last decades, excellent science has been pursued with the synthetic carbon allotropes leading to fascinating, illuminating, and beneficial insights into their chemical reactivity and behavior. Nevertheless, certain issues remain to be unresolved and have to be further elucidated to fully comprehend their chemistry in order to implement the nanomaterials at their best for future areas of applications. By applying fundamental research, this work was intended to shine light into specific unanswered questions for a better understanding of these carbon allotropes. The idea was to predominately address questions with regard to the reductive activation and covalent functionalization, which opens the door to solving the most essential impeding factors for the application of the two nanomaterials like overcoming insolubility or simplifying exfoliation/unbundling, respectively. The specific research targets were:

1) The full understanding of the recently discovered, effective, and facile electron trapping of negatively charged graphene sheets by benzonitrile is still in its infancy. A substantial and extensive investigation of the discharging behavior concerning the quantification of charge uptake and intrinsic chemical reactivity is inevitable to exploit the maximum potential of this crucial finding. Hence, the effect of benzonitrile on intercalated graphite of different types and of different origins should be tested with regard to possible differences in the mentioned subjects, with the purpose to yield exclusively-functionalized products by controlling side reactions efficiently. Furthermore, the question if this process can be adapted for nanotubides as well and whether the variation of the activation pathway may affect the outcome of the reaction, with and without prior covalent functionalization, should be addressed and thoroughly be explored.

2) The exact mechanistic pathway of functionalization of carbon allotropes has been mainly investigated in theory but was barely conducted experimentally yet. Based on a reductive modification of monolayer graphene, the covalent addition with respect to the enhanced reactivity of various areas, the regiochemistry, and the detailed process of the propagation should be precisely elaborated and the idea was to propose a

simplified model for the general reductive functionalization of graphene. Thus, applying reductive conditions accompanied by selective hydrogenation with the focus on specific areas of graphene flakes or CVD-grown graphene followed by an exhaustive Raman spectroscopic study should shine light into the way and the mechanism of this complex topic.

3) A well-known problem of reduction-based liquid-phase exfoliation and functionalization of graphene is the low yield of resulting monolayer material. Due to occurring restacking processes during established work-up procedures, reductive activation pathways lack quantitative yield of the desired functionalized or pristine monolayer sheets impeding further usage. To overcome this hurdle, prior delaminated oxo-graphene should be applied as starting material with the goal to increase the amount of functionalized monolayer graphene implying a more facile and more effective redispersion of the products in common solvents. Limitations of the Raman spectroscopic analysis of the products should be compensated by supplementary characterization with mass spectrometric and gas chromatographic investigations.

4) A variety of covalent functionalization sequences for SWCNTs and graphene have been discovered and published during the last years including numerous methods to determine the amount of the attached moieties quantitatively. However, most of the approaches resulted in the introduction of organic entities hampering the evaluation of an exact degree of addition for both allotropes. Thus, the possibility of implementing marker atoms to improve the characterization should be evaluated. The direct comparison of the two allotropes functionalized with identical molecules bearing ferrocene as heavy heteroatom marker should help to improve the general evaluation of the degree of functionalization. Using established characterization methods, the addition sequences should be compared and opposed with the purpose to determine a significant value or range of the addition extent.

5) The covalent attachment of halides to the carbon framework of SCAs is a suitable choice to be intensively conducted since this modification provides extraordinary possibilities and advantages. Besides the formation of efficient halogen storage systems and accordingly improved electronic properties, the halogenation enables a variety of versatile, facile, and general applicable substitution reactions forming an ideal basis for follow-up chemistry. Different ways to incorporate halogens to the

carbon systems should be investigated and compared in addition to potential substitution and derivatization reactions.

3 Results and Discussion 3.1 Solvent-driven Oxidation of Carbon Allotropides by PhCN 3.1.1 Reactivity of various Types of Graphite after Activation

The reductive activation of carbon allotropes opens the door for the simple and fast formation of covalent bonds to applied electrophiles due to the high reactivity of the introduced charges. Nevertheless, the reactivity of potential remaining charges leads to diverse undesired side reactions, which need to be controlled to avoid a major amount of additional oxygen- and hydrogen functionalities besides the desired functionalization. As stated in the introduction, the application of benzonitrile (PhCN) as oxidative agent forces a facile, effective, and quantitative electron trapping from the respective negatively charged GICs, independent from the existing form in a heterogeneous solid/liquid phase reaction.[103] Besides the simplified exfoliation of single graphene sheets, this additional step facilitates the synthesis of selectively- functionalized materials preventing a high extent of side reactions by quenching residual free charges. Along with this reaction, the red colored radical anion PhCN•- is formed, which enables a quantification of the oxidized charges via absorption spectroscopy. In general, alkali metals dissolve entirely in the solvent forming the mentioned radical anion. Anyhow, this procedure was merely applied for SGN18 graphite during the reductive approach using elemental potassium until now. Numerous different types of graphite types are available varying in their chemical, physical and morphological properties. Principally, these types of graphite can be distinguished by several characteristics like grain size, bulk density, pH, carbon content or surface area, which influence conceivably their behavior.[167] Since differences regarding the alkylation of the diverse graphite sources could be shown applying reductive conditions, a possibly altered charge uptake or discharging behavior due to the morphological features of every individual carbon source is worth to be considered.[108b] Therefore, the oxidation behavior of common graphenides of different origins was relevant to be investigated in particular with regard to speed, extent, and intrinsic chemical reactivity towards occurring side reactions.[103]

For this purpose, four different types of graphite (SGN18, Natural Passau, Asbury3772, and PEX10) were chosen as starting material and thoroughly compared in order to gain more insights into their oxidation processes during the reductive approach using elemental potassium as intercalant. The first graphite type is SGN18, which is spherical and of synthetic origin, being practically pure (99.9 % C) and exhibiting a relatively low average grain size of 20 μm along with an averaged surface area of 6.2 m2/g. This type of graphite was predominantly used as starting material in this thesis and serves for comparison in this chapter. In contrast, Natural Passau with a flaky and platy morphology can be found in nature as it started its geological life as either an organic [167] or inorganic carbon (CO2). Untreated material of Natural Passau has a high degree of crystallinity and an averaged grain size of 1000 μm making a prior treatment indispensable. Thus, a certain amount of the flakes was comminuted with 5 times the amount of NaCl in a mortar for around 20 min resulting in flakes with a significantly smaller size which enhances their dispersibility. The graphite type Asbury3772 contains sulfuric acid as intercalant (1.4 %) and is surface-enhanced with an average surface area of 20 m2/g. The grain size of this source varies considerably exhibiting values between 300 and 850 μm. The last used kind of graphite is PEX10, which represents a prior intercalated (H2SO4) and further delaminated and expanded material. Thus, this treatment leads to the smallest average grain size (3-5 μm) of all used graphite types in the study. A summary about the properties of the different types of graphite can be found in Table 6.1 in the experimental part. Using these different types of graphite as starting material, conditions in compliance with the established reduction route using elemental potassium were applied.[103, 104b, c, 107]

Scheme 3.1.1: Reaction scheme of the formation of various types of GICs including Natural Passau, PEX10, Asburry3772, and SGN18 followed by two different work-up procedures.

Therefore, all graphitic sources were mixed with a stoichiometric amount of potassium and heated together under strictly inert conditions (glovebox) to yield in the respective 47

GICs (KC8). Prior to that, all graphitic samples were heated for three days at 300 °C under vacuum for annealing. Having the negatively charged sheets in hand, these were characterized under air exclusion (in situ) via Raman spectroscopy in order to confirm the successful intercalation. Furthermore, a dilution series of graphenides in

PhCN was performed varying in the amount of the respective KC8 leading to three different descending concentrations by ultrasonication. These solutions were subsequently analyzed employing UV-vis spectroscopy. Moreover, two dispersions of each GIC type in THF were produced via ultrasonication. One of two duplicate samples of specific graphenides was allowed to be exposed to ambient conditions followed by an aqueous work-up process, whereas the other sample was pretreated with an excess of PhCN followed by the identical work-up procedure (Scheme 3.1.1). In the end, these samples were qualitatively analyzed via Raman spectroscopy and TG-MS measurements.

In situ characterization of graphenides

After producing intercalated graphenides (KC8) of all mentioned origins by mixing a stoichiometric amount of potassium with graphite while heating at 150 °C for three days under complete inert conditions in a glovebox, a little amount was transferred to glass ampoules which were subsequently sealed under ultra-high vacuum (p<10-7 mbar). The duration of the heating time was increased from one night up to three days in order to ensure that completely intercalated and fully charged carbon systems could be received and the success was controlled by in situ Raman measurements. Concerning the respective intercalation compound, they differ noticeably in each specific color from light-brown until bright brownish, besides their diverse visual morphological appearance (Figure 3.1.1 B). The graphite sources exhibiting the smallest grain sizes (SGN18, PEX10) have a homogenously dark brown color, whereas the flaky Natural Passau or the Asbury3772 appear as bright and shiny colored featuring an inhomogeneous distribution, which refers to potential differences in the intercalation process. Of each sample, in situ single point Raman spectra were recorded with regard to the comparison of the individual intercalation processes. In Figure 3.1.1 A, the recorded single point spectra of the used graphite sources are

depicted. In this case, the main focus lies on characteristic changes of specific indicators, namely the most prominent Raman modes (D-, G-, 2D-modes) in addition to the pronounced CZ-mode. A thorough determination of the accurate intercalation stage of the sample is decisive due to the dependence of the properties of the GICs on the concentration of intercalants.

Figure 3.1.1: A) In situ single point Raman spectra (λE=532 nm) of the analyzed GIC types including Natural Passau (red line), Asbury3772 (orange line), PEX10 (blue line), and SGN18 (green line). The curves are vertically shifted along the y-axis for an improved overview. The

D-mode, G-mode and Cz-mode are marked by #, *, and Cz, respectively. B) Optical image of graphite intercalation compounds (KC8) of various graphite types sealed under vacuum in glass ampoules.

The Fano-line shaped signature of the curves of SGN18 and PEX10 due to the coupling and the interference with the conduction electrons accompanied with the disappearance of every single Raman mode including G- and D-band indicate the complete intercalation reaching the highest intercalation stage I (Figure 3.1.1 A).[168] This finding is in accordance with the visual homogeneity of the GICs. The analysis of Natural Passau and Asbury3772 gives evidence about the uncompleted intercalation or attaining a diverse stage, respectively, indicated by the missing Fano-line shaped curve and a discernible unshifted G-mode, which can be assigned to the graphitic E2g G-mode (Figure 3.1.1 A). According to literature, four main modes show a Raman response within the region between D-band and G-band involving the mentioned E2g1 [169] and E2g2 (G-mode), A1g (D-mode) and graphitic KC24 mode. The intercalation of graphite proceeds through several stages (I, II, III, etc.) dependent on the quantity of applied intercalant. The first stage or configuration, which represents the highest 49

doping (KC8), has one single layer enclosed by potassium intercalants. Figure 3.1.2 illustrates two configurations (stage I and III) as examples for possible intercalation stages after forming the GIC. While increasing the single layers in between the positively charged potassium ions, the configuration changes as the intercalant [168b, 169-170] concentration decreases. For stage I, regularly a characteristic Cz-mode along with a distinctly broadened and red-shifted Fano line-shape independent from the nature of the intercalant can be obtained. This Cz-mode derives from the intercalation architecture in the bulk crystal.[168b, 169-170] In the case of the different GICs, a substantial line-shape analysis of the spectra enables the identification of various modes.

Figure 3.1.2: Exemplary configurations of GIC stages, A) stage I and B) stage III with the + respective distances between the K -cations; C) Top view of KC8 in an orthorhombic cell; Brown layers represent the charged sheets, violet spheres stand for K+-cations.

Therefore, different stages of intercalation were reached during the reduction due to the morphological variety dependent on the respective source. In general, local defects, different laser wavelengths or laser-induced deintercalation represent reasons for changing positions of present modes or varying line shapes.[171] To overcome this issue, longer heating times up to 14 days of duration of the mixture potassium/graphite, in particular Natural Passau and Asbury3772, were applied in order to yield in fully intercalated species. The corresponding Raman analysis revealed Raman predominantly spectra containing higher stages of intercalation. This observation is in accordance with the Daumas-Hérold model which predicts the transitional dynamics of intercalation between stages. In this model, partially filled sheets are imaginable while the existence of unfilled layers is inconceivable.[168b, 172] These findings let conclude that the morphology has a significant impact on the charging behavior during the

oxidation of the alkali metal. The varying grain sizes in addition to different sized surface-areas hinder the complete reduction to yield the fully intercalated stage in case of Natural Passau and Asbury3772. For the latter, residual intercalant which surpasses the washing procedure may also affect the oxidation, probably due to feasible reactions of the unremoved water with the alkali metal. The splitting of the G-mode in the Natural Passau sample at ~1500 cm-1 can be explained by the Nearest Layer model (NL), which governs the structure and arrangement of atoms in GICs.[173] This critical study distinguishes between highly charged graphene layers bounded by intercalants in the outside parts, and “inner” graphene layers with little charge and enclosed by other graphene sheets. This enables an assignment of the stages through the relative intensities of the G-lines of inner and outer layers.[168b]

Dilution series

The varying behavior can be observed in the further analysis as well, e.g. in the UV- vis spectroscopic investigation. With the purpose to monitor the amount of the formed benzonitrile radical anion quantitatively aiming to discover differences in the oxidation process, a dilution series of the specific graphite type in PhCN was performed by creating three different descending concentrations of K in PhCN via sonication of the mixture for 5 min under complete inert conditions in a glovebox. These were analyzed in sealed UV-vis cuvettes under air exclusion. Figure 3.1.4 demonstrates that the resulting red solutions of the PhCN unveil a relatively broad absorption mode at around 390 nm accompanied by an additional feature at 500 nm, which is in perfect agreement with recent findings.[103]

Figure 3.1.3: UV-vis analysis of Natural Passau and SGN18 (A) and Asbury3772 and PEX10 (B) in PhCN with descending concentrations under air exclusion; C) Corresponding absorbance values at 390 nm plotted against the specific concentrations of the respective graphite type.

While the Natural Passau graphite sample, as well as Asbury3772, reveal the highest absorption peaks for the highest concentrations, SGN18 exhibits the lowest absorption values independent on the respective concentrations (Figure 3.1.3 A/B). The PEX10 sample merely shows minor differences for all three concentrations compared to the Asbury3772 sample. Nonetheless, it is an undeniable fact that each sample contains an identical amount of potassium leading to the assumption that non-intercalated potassium is present and dissolves in PhCN. Thus, this observation may indicate a faster oxidation due to the presence of residual elemental potassium left in the reference system, which is responsible for the accelerated charge transfer to the reductive agent (PhCN) being merely plausible for non-fully intercalated systems like Natural Passau or Asbury3772 graphite. As a consequence, this interpretation would imply uncompleted oxidation of the graphenides PEX10 and SGN18. Actually, recent results could corroborate that all charges of the system (SGN18) are transferred to

PhCN and the graphite is normally oxidized entirely.[103] A possible reason for the result of this study could be that the sonication time was insufficient to oxidize the charged sheets of SGN18 in full extent. By correlation of the respective concentration of potassium related to specific extinction value at 390 nm, the extinction coefficient can be determined and therefore the electron transfer rate (ETR). A linear correlation would be expected according to previous results. Herein, the extinction coefficient of the PhCN.- has been defined with the Lambert-Beer equation to be -1 -1 [103] ε390nm = 4000 L x mol x cm . Having this information at hand, it generally allows to quantify the oxidation process implying the calculation of the ETR for the electron donor by the ratio between the extinction coefficients obtained for the charged intermediates and the extinction coefficient of solid potassium dissolved in PhCN, which reflects an ETR of 100 %. However, the obtained inconsistent correlation impairs the exact calculation of the specific extinction coefficient in this case and complicates the quantitative analysis of the electron transfers. Nevertheless, unambiguous differences in the oxidation behavior are not arguable. Besides the oxidation, another process occurs during that reaction, namely the mass transport of K+-ions from the solid graphitic phase to the liquid phase of PhCN driven by electrostatic forces between species bearing opposite charges.[103]

Reactivity towards side reactions

For the systematic investigation of the chemical reactivity of the diverse graphite sources after reductive activation, a comparison between intercalated graphite samples with and without prior treatment with PhCN was performed. For that purpose, two alike GIC samples of each graphite type were produced by sonication of the respective GIC in absolute THF to yield a perfect dispersion. The utilized solvent was degassed and dried completely beforehand. Subsequently, an excess of PhCN was added to one of the matching dispersions and further sonicated for 15 min. As mentioned in the introduction, remaining charges on the graphene lattice can undergo reactions with oxygen in the air to form hydroxylic groups or with moisture/water to result in hydrogenation of the surface.[78, 103] The determination of the extent of occurring side reactions can be carried out via a substantial Raman and TG-MS

analysis. Figure 3.1.5 illustrates the Raman comparison of all graphite samples after the described work-up procedures by means of SRS. Since the non-destructive SRS enables the detection of alterations in hybridization of carbon atoms in carbon allotropes, it is crucial for the determination of newly introduced covalent bonds. In particular, in these experiments, it is more decisive than other tools, as it delivers the most important message with regard to covalent interactions and therefore to the chemical reactivity.

Figure 3.1.5: SRS/mean Raman spectra (λE=532 nm) of various GICs after work-up with and without prior treatment with PhCN: Asbury3772 (A), PEX10 (B), Natural Passau (C) and SGN18 (D) in comparison to pristine material (black). The respective color refers to the kind of work-up: blue (without PhCN), red (with PhCN), and black (pristine), spectra y-shifted for clarity.

Considering the Raman analysis of the investigated samples, in case of PEX10 and SGN18 obtained without the addition of PhCN, a pronounced D-mode at around (1337 cm-1 for PEX10/1325 cm-1 for SGN18) can be observed indicating a high amount of newly incorporated addends (Figure 3.1.5 B/C). These addends originate from the recently elucidated reaction of the charges with oxygen in the air and in the work-up chemicals, as well as from the reaction of charges with water. Apart from that, the

appearance of a supplementary Raman feature at 1478 cm-1 (PEX10) or at 1480 cm-1 (SGN18) suggests present C-O-bonds, additionally confirming the side reaction with [168a] oxygen. These side reactions cause high ID/IG ratio values of 0.92 for PEX10 and even 1.21 for SGN18, respectively. In contrast, the respective samples treated with PhCN beforehand, reveal unambiguously lower intensities of the D-bands, being similar to pristine graphite, in particular in the case of SGN18. This observation let draw the conclusion that almost all charges are quenched by the added PhCN, leading to the restoration of the entire carbon lattice, which is in excellent agreement with earlier findings regarding SGN18.[103] Notably, the pristine PEX10 already exhibits a prominent D-mode (ID/IG = 0.29) in contrast to the other pristine materials. The presence of such high addition patterns correlates to the in situ Raman monitoring, which clearly corroborated the complete and homogeneous intercalation of both graphite types (SGN18/PEX10), implying raised chemical reactivity. With regard to the other used types of graphite, namely Natural Passau and Asbury3772, both showed an inhomogeneously intercalated nature of their GICs according to the in situ Raman analysis, depicted above. After the work-up procedures of the samples, a distinct increase of the ID/IG-ratios could be found for both types in comparison with the respective pristine material. The obtained values range from 0.62 for Asbury3772 and to 0.81 for Natural Passau, which are still below the achieved values of the other sources, implying lower reactivity towards side reactions. A Raman response for C-O-bonds can be detected located between the D- and G-mode (~1480 cm-1) corroborating present covalently bonded hydroxyl groups for Natural Passau graphite. While the samples with prior treatment with PhCN reveal relatively high degrees of addition suggesting the presence of residual charges after the treatment which underwent reactions during the work-up procedure. The lower extent of functionalization is probably caused by the inhomogeneously intercalated material proven by the experiments before resulting in diminished repulsive Coulombic interactions. As a result of this unequal intercalation, the efficiency of the exfoliation is reduced, leading to a decreased reactivity and thus to lower degrees of functionalization. Therefore, the morphology plays a decisive role with respect to charging and the subsequent occurring side reactions.

In order to corroborate the results, the samples were analyzed via TG-MS measurements, which findings were correlated to the obtained Raman results.

Applying this method, an overview of the extent of side reactions as well as information about the nature of bound addends can be provided.

The TG results from the usage of the homogeneously intercalated SGN18 and PEX10 as starting material are presented in Figure 3.1.6 B and D. The mass losses of the samples mainly derive from the side reactions of free charges to form covalently attached OH- and H-groups at the carbon lattice, as described above.

Figure 3.1.6: TG-MS analysis of various GICs after work-up with and without prior treatment with PhCN: Asbury3772 (A), PEX10 (B), Natural Passau (C) and SGN18 (D) in comparison to pristine material. The respective mass losses are depicted in blue (without PhCN), red (with PhCN) and black dotted (pristine).

Regarding Asbury3772, the highest total mass loss of all samples can be obtained (~11.6 %, Figure 3.1.6 A). This observation can be explained by the already high mass loss present in the pristine material (~6 %), which derives from embedded intercalated compounds (sulfuric acid) within the graphitic layers. The mass loss difference between both work-up procedures exhibits a relatively high value of ~4.6 % and is contradictory to the Raman results which proposed a higher density of defects for the

sample worked-up with PhCN. Anyhow, the mass loss does not merely reflect the degree of addition and is significantly influenced by adsorbed molecules like PhCN or the cointercalated solvent THF within the layers. The trends of the graphite sources PEX10 and SGN18 bear resemblance, whereas PEX10 shows a significantly higher total mass loss (9.2 % to 5.5 %). Opposed to the other sources, both work-up procedures result in similar mass loss values, being contradictory to the Raman findings, depicted above. Cointercalated THF in addition to adsorbed PhCN represent reasons for the low variation of both mass losses leading to resembling values as these interactions cannot be detected by Raman spectroscopy. Considering Natural Passau, the biggest differences between both work-up procedures can be stated with a mass loss value of 6.7 %. The sample with prior treatment with PhCN is nearly identical with the pristine material suggesting complete oxidation of the system yielding in pristine graphite. The analysis of the respective ion currents detected in the TG-MS measurements provides perceptible evidence about the nature of the introduced addends.

Figure 3.1.7 illustrates the mass currents for the expected addends during the TG-MS measurements, specifically hydrogen (m/z 2), hydroxyl groups (m/z 17) and water (m/z 18). This presentation focusses on the relation of the samples of equal types of graphite with each other, having applied two different work-up procedures. For all samples, water and hydroxyl groups can be detected being a result of the described reaction of charges with oxygen as well as from remaining adsorbed water on the carbon surface. Hydrogen is formed after cleaving from the carbon lattice due to the prior hydrogenation followed by the recombination of two proton radicals to hydrogen molecules. Besides for PEX10, the ion currents for water and OH-groups (Figure 3.1.7 D) and for Natural Passau (Figure 3.1.7 C), all intensities of the respective ion currents are significantly higher for the samples without prior addition with PhCN as a result of the reactions with unquenched charges. While, water and OH- groups can be detected during the whole temperature frame, the detachment of hydrogen starts to occur at around 300 °C. Remarkably, the Natural Passau graphite sample without the treatment with PhCN shows nearly no traces of evolving hydrogen despite of the obtained high mass loss in the TG measurement. The extent of the respective cointercalation with THF or adsorption of PhCN can vary as well hampering the direct comparison of detected mass loss values.

Figure 3.1.7: Ion currents of respective TG-MS analysis depicting A) hydrogen (m/z 2) and

B) OH (m/z 17) and H2O (m/z 18) of SGN18 (a/e), PEX10 (b/f), Asbury3772 (c/g) and Natural Passau (d/h) with (red/dotted blue) and without treatment with PhCN (blue/straight blue).

To sum up, the varying morphological properties of the different graphite sources unambiguously influence the charging behavior via elemental potassium, resulting in inhomogenously intercalated species. This condition affects the chemical reactivity due to reduced repulsive Coulombic interactions impairing the affinity to exfoliate and therefore the extent of the side reactions in addition. This could be unambiguously

corroborated by the substantial analysis of the occurring side reactions. Due to misleading and contradictory results, in particular using TG-MS and the absorption spectroscopy, the results cannot be adopted one-to-one and have to put in relation with recent findings. Nevertheless, it can be unambiguously stated that the type of graphite has a fundamental and decisive impact on the reaction outcome applying reductive chemistry. Having compared four kinds, SGN18 graphite proved to be the best alternative being used as starting material for reductive functionalization. The resolved beneficial charge uptake and discharging behavior accompanied with the high purity, excluding a pretreatment of the material, and therefore enable simple characterization, are major advantages for subsequent functionalization reactions. Nevertheless, the prior intercalated, expanded, and delaminated PEX10 represents also a possible option for further usage but residual intercalants can impede the applied characterization and can therefore falsify the results. Since the application of Natural Passau graphite requires a pretreatment due to the present grain sizes of the graphite flakes and its morphology significantly affects the charge uptake and oxidation behavior, the experimental implementation is more complex compared to the other graphite sources mentioned. The same applies for the usage of Asbury3772 graphite as starting material which major impeding factors derive from the intercalated species affecting the subsequent intercalation with potassium and may influence further characterization steps. Hence, the comparison of the types of graphite clearly revealed advantages and disadvantages of all used types. Now, the question arises whether and in which extent the application of PhCN affects the discharging of other carbon allotropes, namely SWCNTs. Additionally, the effect of alternative reductive activation methods has to be further elucidated.

3.1.2 Influence of PhCN on SWCNTs and different Activation Routes

The upcoming results have been carried out in collaboration with Konstantin Edelthalhammer, Isabell Wabra, and Maria-Eugenia Pérez-Ojeda Rodríguez. Herein, Konstantin Edelthalhammer focused on the treatment of different kinds of SWCNTs, Isabell Wabra dealt with the discharging behavior of fullerenes, whereas Maria-Eugenia Pérez-Ojeda Rodríguez handled the characterization of the nano- onions upon PhCN treatment, and I mainly focused on the investigation of different reductive activation routes for graphene and HiPco SWCNTs. Due to the orientation of this thesis and a better understanding, the results concerning SWCNTs and the different activation routes are presented in this chapter. The results evolving the carbon nano-onions and fullerenes are omitted since it would exceed the scope of this work.

Figure 3.1.8: Intercalated synthetic carbon allotropides: A) graphite intercalation compound (GIC) and B) bundled nanotubides; The violet spheres stand for K+-cations.

Since a difference in charging/discharging could be observed for different kinds of graphite, this chapter conducts the behavior of different SCAs experimentally as well as the application of additional reductive activation methods. In fact, the different carbon allotropes like fullerenes, nanotubes or graphene vary particularly in terms of charge uptake, geometry and reactivity, therefore, a comparison between the SWCNTS and graphene was made. (Figure 3.1.8).

SWCNTs

At first, carbon nanotubes were investigated using three different origins of these materials (HiPco, Arc Discharge, CoMoCat). Due to varying production techniques, they differ in multiple aspects including geometry, length, and diameter. In addition, the amount or nature of catalysts and impurities can be different, respectively, dependent on the applied synthetic route, which may have an impact on the specific reduction or oxidation behavior by interfering with their own redox potentials. Thus, SWCNTs originating from Arc Discharge production techniques are the longest compared to the other CNT types reaching lengths up to 5 µm, whereat exhibiting the largest diameters as well (1.2-1.7 nm). The smallest diameters (0.7-0.9 nm) accompanied by an average length of 1.5 µm is present in CoMoCat CNTs.

Figure 3.1.9: A) UV-vis analysis, B) corresponding molar coefficients in PhCN at 390 nm for a) HiPco, b) CoMoCat, and c) Arc Discharge CNTs.

However, the largest amount of catalyst impurities can be found in HiPco SWCNTs. A detailed description of the properties is listed in Table 6.2 in the experimental part.

After the reduction of the specific CNT types to yield intercalated KC8, a dilution series 61

of KC8 dispersions in PhCN was performed followed by inert UV-vis measurements of these samples (in analogy to Chapter 3.1.1) displaying the characteristic absorption maxima at 390 nm and 500 nm (Figure 3.1.9 A). These results were obtained by Konstantin Edelthalhammer. This analysis enables the determination of the molar coefficients in PhCN (at 390 nm, Figure 3.1.9 A) and therefore allows for the calculation of the respective ETR for each individual CNT type. Anticipating the calculated values, a declining trend is observable in descending order starting from HiPco CNTs (58 %) to CoMoCat (43 %) and Arc Discharge CNTs (17.5 %) exhibiting the lowest oxidization potential. Plenty of reasons for the varying ETRs can be numerated involving the influence of the mentioned catalysts onto the redox potentials of the respective CNT samples. Furthermore, the CNT length is decisive since the accessibility of charges for PhCN is reduced with increasing length of the tubes. This is in good agreement with the results for the relatively long Arc Discharge CNTs exhibiting an average length of 5 µm.

Different activation routes

Various reductive activation routes for carbon allotropes have been established varying in the reaction conditions and nature of reductive agents, as described in the introduction. Two of the most prominent and most frequently used methods aside from the already presented method via solid-state reduction with elemental potassium,[103] have been applied for SGN18 graphite and HiPco SWCNTs to yield fully intercalated carbon species. The reduction was then followed by the oxidation of the respective materials by PhCN with the aim to investigate the discharging process of the allotropes depending on the specific activation route in a qualitative manner (Scheme 3.1.2).

Scheme 3.1.2: Reductive activation of SWCNTs/ graphene via modified Birch/Billups conditions (1) or using Na/K alloy in DME (2) followed by the comparison of two work-up procedures.

On the one hand, the method using Na/K alloy in the solvent DME was applied to SWCNTs and graphite. On the other hand, an activation was accomplished by applying modified Birch-type/ Billups conditions, which include the promotion agent NH3 in the presence of an inert co-solvent (THF). For both allotropes, duplicate samples were produced in order to compare the different behavior of discharging. To one set of the activated CNTs and graphite dispersions 5 ml of PhCN were added in order to oxidize the activated carbon materials. The duplicate set of reduced samples were directly exposed to ambient conditions without prior addition of PhCN and subsequently worked up aqueously. The samples were analyzed subsequently via Raman spectroscopy and TG-MS in order to explore differences in the oxidation behavior.

Figure 3.1.10: SRS/mean Raman spectra of graphene (λE=532 nm) or SWCNTs (λE=633 nm); activated via Na/K alloy in DME and under modified Birch/Billups conditions comparing work-up process with (red line) and without (blue) prior treatment with PhCN in contrast to graphite (black line), spectra y-shifted for clarity.

Figure 3.1.10 illustrates the SRS (survey spectra) of the different allotropes using different reductive methods and comparing the work-up procedure with PhCN

pretreatment or direct exposure to air. Under modified Birch conditions, the discharging behavior of graphenides varies considerably from that of nanotubides. A prominent -1 D-mode (~1320 cm ) and therefore ID/IG-ratio (ID/IG = 1.19) can be observed for the graphene sample without prior treatment with PhCN, whereas the addition of PhCN leads to the complete removal of the residual charges on the carbon lattice preventing undesired side reaction indicated by the decreased ID/IG-ratio (ID/IG = 0.20, Figure 3.1.10 A). This observation is in good agreement with the previously calculated ETR of 100 % using elemental potassium as intercalant. Generally, the increase of the D-mode can be traced back to an increased density of defects due to the side reactions of the negatives charges on the reduced material with water and oxygen forming OH- and H-groups and therefore the introduction of sp3-defects. The appearing mode at ~1466 cm-1 (Figure 3.1.10 A) originates from the vibrations of C-O-groups proving the presence of newly introduced hydroxyl moieties which is missing for the graphene sample after the oxidation with PhCN.[168a] Regarding the CNT samples activated under modified Birch conditions, both samples reveal relatively high ID/IG-ratios

(ID/IG = 0.62/1.10) suggesting that the addition of PhCN does not lead to a removal of charges, which implies a lower uptake of charges by the utilized PhCN for the CNTs (Figure 3.1.10 C). In the case of the activation route using Na/K alloy in DME, the addition of PhCN results in a diminished extent of side reactions for CNTs indicated by the decrease of the respective ID/IG ratios (ID/IG = 0.75/0.52, Figure 3.1.10 D). For the graphene samples, however, no pronounced variation of the ID/IG-values for the material pretreated with PhCN in comparison to normally worked up samples can be obtained (Figure 3.1.10 B). Anyhow, the side reactions are completely suppressed by the addition of PhCN due to the complete removal of the charges. The low increase of the density of defects after the aqueous work-up possibly stems from an insufficient exfoliation of the graphene layers and therefore less accessible surface area during that approach.

The samples were then analyzed via TG-MS measurements and the results were correlated to the obtained Raman findings. Applying this technique, an overview about the extent of side reactions as well as information about the nature of bound addends can be provided. Figure 3.1.11 depicts the TG-MS analysis of the corresponding samples. A higher uptake of charges by the PhCN in the graphenides could be also corroborated by a significant difference of the mass losses (10.5 %) visible in the corresponding TG-MS measurements (Figure 3.1.11 A), while the respective mass 64

losses of the graphene samples activated via Na/K alloy in DME are virtually alike (~1 %, Figure 3.1.11 B).

Figure 3.1.11: TGA analysis of SWCNTs and graphene activated via Na/K alloy in DME (B/D) and under modified Birch/Billups conditions (A/C) comparing work-up process with (red line) and without (blue) prior treatment with PhCN in contrast to the respective graphite source (dotted line). Thus, both graphenide samples got nearly completely oxidized, corresponding to the findings for graphene using elemental potassium for reduction. Anyhow, an increased mass loss in graphene samples can be influenced by several undesired reasons including conintercalated solvent (THF) or physisorbed PhCN. Considering the CNT samples, both activation routes results in considerably high addition patterns indicated by the significantly high mass losses for both, treated and untreated, materials (Figure 3.1.11 C/D). Interestingly, the mass loss of the sample treated with PhCN is remarkably higher than the mass loss without the addition of PhCN, which is in good agreement with the corresponding Raman results. Moreover, the CNT samples activated via Na/K alloy in DME exhibit both relatively high mass loss values, but the difference of the values for both work-up procedures (6.5 %) confirm the removal of 65

charges. The chemical identity of the newly introduced addends can be corroborated by the respective mass spectroscopic analysis of the samples (Figure 3.1.12/3.1.13).

Figure 3.1.12: Ion currents of respective TG-MS analysis of samples activated via modified Birch/Billups conditions comparing both work-up procedures depicting hydrogen (m/z 2);

OH (m/z 17); H2O (m/z 18) and mass fragments of PhCN (m/z 50, 76, 103) of HiPco SWCNTs (A/B/E) and SGN18 (C/D/F) with (red/ dotted blue) and without treatment with PhCN (blue/ straight blue).

As mentioned before, the reaction between free charges and water and oxygen leads to hydrogenation and hydroxylation of the respective carbon materials, respectively, which can be seen as emerging fragments (H2, OH, H2O) during to the temperature 66

treatment in the TG measurements. Considering the respective ion currents, the intensities of the cleaved molecules are higher for the samples without prior treatment with PhCN as expected (Figure 3.1.12/3.1.13).

Figure 3.1.13: Ion currents of respective TG-MS analysis of samples activated via Na/K alloy in

DME comparing both work-up procedures depicting hydrogen (m/z 2); OH (m/z 17); H2O (m/z 18) and mass fragments of PhCN (m/z 50, 76, 103) of HiPco SWCNTs (A/B/E) and SGN18 (C/D/F) with (red/ dotted blue) and without treatment with PhCN (blue/ straight blue). Anyhow, the mass fragments deriving from water and OH-groups extend to the whole temperature area for almost all samples, possibly due to additional adsorbed water on 67

the respective surfaces. However, the graphene sample activated via Na/K alloy in DME showed the comparably lowest difference for all ion currents (Figure 3.1.12/13), which is in accordance with the merely weakly varying Raman modes for these samples. The presence of physisorbed PhCN can be observed in particular for the graphene samples, indicated by the specific mass fragments emerging at around 150 °C for graphene activated under modified Birch conditions (Figure 3.1.12 F) and at around 280 °C for graphene activated via Na/K alloy in DME (Figure 3.1.13 F).

Comparing both activation routes with each other, the method using Na/K alloy benefits from the inertness of the solvent and the possibility to work at RT, whereas the Birch- like conditions are harsh (-78 °C) and remaining traces of NH3 in THF after the evaporation impair reactions with specific reagents and the determination the exact reduction state is rather complex. This generates differences in the reaction behavior like for instance in the efficiency of the proton sources during the hydrogenation of graphene.[110c] In this case, however, the discharging behavior of SWCNTs or graphene is merely dependent on the respective allotrope but varies in the reduction process in comparison to the activation using elemental potassium as intercalant, which is in excellent agreement with previously discovered findings. The next chapter relates to the efficiency of the discharging process by PhCN after the application of successful functionalization sequences onto SWCNTs and graphene.

3.1.3 Impact of PhCN after Covalent Functionalization

Having elucidated the behavior of CNTs and various graphite sources, another fundamental question arises whether the addition of PhCN of SCAs is indispensably necessary concerning undesired side reactions after the application of successful functionalization sequences. Now, this question was addressed to SWCNTs and graphene by applying different activation routes followed by covalent hexylation and a substantial comparison of both work-up procedures. A complete control about the side reactions is desired to yield in solely selectively-hexylated or generally selectively- functionalized products. To conduct this, SGN18 graphite and HiPco SWCNTs were used as starting materials and two samples of each allotrope were first activated via three different reduction pathways, in analogy to the methods used in the previous chapter.

Scheme 3.1.3: Reductive activation of SWCNTs/ graphene via 3 different routes followed by hexylation and comparison of two work-up procedures.

Thus, the samples of both allotropes were reduced either via 1) elemental potassium and dispersed in THF, 2) under modified Birch/Billups conditions or 3) via Na/K alloy in DME while prevailing completely inert conditions. Having the charged intermediates in dispersion at hand, an excess of hexyl iodide as functionalization reagent was added in order to graft hexyl chains to the specific carbon lattice. Consequently, an excess of PhCN was added to one of the two alike samples and stirred for 15 min in order to quench remaining charges on the allotropes, followed by the identical work-up procedures (Scheme 3.1.3).

Figure 3.1.14: SRS/mean spectra (λE=532 nm) and TGA analysis of graphene activated via different reductive routes including reduction via Na/K alloy in DME (A/B); reduction under modified Birch conditions (C/D) and via elemental potassium in THF (E/F) followed by the functionalization with hexyl iodide. The blue line represents the sample without prior treatment with PhCN, the red line refers to the respective sample evolving the addition of PhCN, whereas the black and dotted line reflect the pristine material, Raman spectra y-shifted for clarity.

The SRS and complete TG-MS analysis of the graphene samples is depicted in Figure 3.1.14 and enables qualitative conclusions about the potential discharging after successful addition sequences. The addition of PhCN did not lead to a color change of the dispersion indeed impairing any optical spectroscopy. Concerning the alkylation,

the entire reductive routes yield highly functionalized material according to the respective Raman responses. In theory, the reactants undergo a SET mechanism including the formation of a highly reactive hexyl radical which forces the covalent attachment of hexyl groups to the graphene sheets at first. In all samples, the characteristic mass fragments could be observed confirming the successful hexylation (Figure 8.1, supplementary spectra). Figure 3.1.15 depicts an exemplary spectrum of functionalized graphene using Na/K alloy in DME for activation, which confirms the presence of the covalently bound hexyl chains. The detachment of the hexyl moiety starts at around 120 °C for the graphene sample, implied by the appearance of + + + + characteristic mass fragments (m/z 29 (C2H5 ), 41 (C3H5 ), 43 (C3H7 ) and 57 (C4H9 )) at this temperature.

Figure 3.1.15: Exemplary TG-MS spectrum of functionalized graphene activated via Na/K alloy in DME followed by the aqueous work-up including the characteristic mass fragments of a hexyl chain (colored) besides the TG curve (black).

The highest degrees of functionalization are achieved using Na/K alloy in DME indicated by the high values for the ID/IG-ratios (ID/IG = 2.88/2.19, Figure 3.1.14 A/B), whereas the activation under modified Birch conditions reveals comparable low amounts of newly introduced defects (ID/IG = 0.64/0.69, Figure 3.1.14 C/D). Regarding the Raman results, a significant difference can be seen for the samples activated via Na/K alloy and via elemental K, which disagrees with the corresponding TG profiles. These spectra are not altering to a pronounced extent exhibiting either no difference (elemental K/THF) or merely 3.2 % for the method using Na/K alloy. For the sample activated under modified Birch conditions and treated with PhCN, even a slightly higher degree of functionalization can be proposed by the corresponding ID/IG-ratio value.

Even though it is a statistical analysis, e.g. several hundreds of spectra, it is still an extremely low area compared to the whole sample size, whereas TG-MS measurements do not depend on specific areas. Consequently, while coalescing both characterization tools, a significant influence of PhCN after successful additions can be excluded. The remaining charges after the hexylation were quenched by PhCN, but since this amount is minor, the effect of the side reactions is negligible as a consequence.

The behavior of HiPco SWCNTs bears resemblance to the results for graphene to a certain extent. Figure 3.1.16 illustrates the respective Raman and TG-MS results for the functionalized SWCNTs without and with the addition of PhCN during the work-up procedures.

Figure 3.1.16: SRS/mean spectra (λE=633 nm) and TGA analysis of SWCNTs activated via different reductive routes including reduction via Na/K alloy in DME (A/B); reduction under modified Birch conditions (C/D) and via elemental potassium in THF (E/F) followed by the functionalization with hexyl iodide. The blue line represents the sample without prior treatment with PhCN, the red line refers to the respective sample evolving the addition of PhCN, whereas the black and dotted line reflect the pristine material, Raman spectra y-shifted for clarity.

According to the SRS and the TG-MS analysis, the attachment of hexyl groups onto the SWCNTs indeed occurred (Figure 8.2, supplementary spectra/Figure 3.1.16). An exemplary TG-MS spectrum can be seen in Figure 3.1.17 depicting the mass

fragments of hexyl groups cleaved from SWCNTs using Na/K alloy in DME for activation. The detachment of the hexyl moiety starts at around 170 °C and ends at around 350 °C, implied by the appearance of characteristic mass fragments of the + + + + hexyl chain (m/z 29 (C2H5 ), 41 (C3H5 ), 43 (C3H7 ) and 57 (C4H9 )) at these temperatures, in analogy to the graphene results. The second mass slope at around 480 °C can be referred to structure decomposition.

Figure 3.1.17: Exemplary TG-MS spectra of functionalized SWCNTs after activation via Na/K alloy in DME followed by the aqueous work-up including the characteristic mass fragments of a hexyl chain (colored) besides the TG curve (black).

For the tube samples activated under Birch-like conditions and via elemental potassium, pronounced D-modes arise for both duplicate tube samples, exhibiting even higher ID/IG-ratio values for the ones treated with PhCN beforehand

(ID/IG (Birch)= 0.66/0.48 and ID/IG(K/ THF)) = 0.59/0.33, Figure 3.1.16 C/E). Along with this, the respective mass loss values do not differ significantly by any means featuring low values of 8.7 % and 0.6 % (Figure 3.1.16 D/F). This observation implies a rather low influence of PhCN on the remaining charges of the SWCNTs, which is good agreement with previously demonstrated findings in the last chapter. For HiPco CNTs, an ETR of 58 % was calculated in contrast to the 100 % for graphene both using elemental potassium, which is reflected in the obtained results. Even though the Raman results of the samples activated via Na/K alloy pronounce a higher density of defects compared with the treated material implied by the higher ID/IG-ratio values, the mass loss values resemble with a difference of only 1.4 %. These findings are in perfect agreement with the results for alkylated graphene using elemental potassium as intercalant.[133b]

Summarizing, the effect of PhCN is negligible after the application of successful covalent functionalization sequences onto carbon allotropes independent on the reductive activation route or the starting material, CNTs or graphene, respectively. The quantity of the remaining charges onto the surface of the allotropes after the reductive functionalization could be too small with respect to potential occurring side reactions. Therefore, merely a small amount of new defects is introduced to the system, which does not alter the outcome of the characterization in a significant extent.

After having a deeper look into the precise process of the intercalation and the discharging behavior of diverse synthetic carbon allotropes, the next topic relates to the detailed exploration of the functionalization route of graphene as model system after prior reductive activation. The exact way of addition has been barely conducted with experiments until now and remains to be investigated. Thus, the precise pathway of the introduction of new sp3-centers onto the surface of SCAs will be addressed in the following chapter.

3.2 Mechanistic Investigations of the Reductive Functionalization Process

In general, the exact and detailed mechanism of synthetic carbon allotropes functionalization is still rather unexplored due to the complexity of the experimental implementation. Since functionalized graphene or CNTs are considered as polydisperse macromolecules, they can merely be analyzed by indirect tools, for instance Raman spectroscopy, TG-MS or TEM. Using these techniques, a fundamental characteristic of reductive functionalization patterns of graphene has been discovered already: the island formation of modified areas, which are inhomogeneously distributed all around the graphene sheet, alongside unimpaired carbon networks.[74, 110c] Nevertheless, many questions with respect to the enhanced reactivity of various areas (periphery, basal plane or defects), the regiochemistry of covalent functionalization, the detailed process of the propagation or in general the mechanism are still unresolved. On the one hand, the functionalization from the periphery to the interior of the graphene sheet is an often assumed, theoretically predicted pathway but rarely investigated experimentally.[113, 174] On the other hand, different publications claim homogeneously occurring functionalization of monolayer material to a specific degree of functionalization.[175] Hence, the goal of the current research is to follow the starting steps of functionalization in order to obtain deeper information about the precise functionalization pattern. With respect to this aim, monolayer graphene (MLG) was used as model system for the selective functionalization with the smallest possible molecule, namely hydrogen. On that level, a better understanding of the basics of the functionalization process can be gained by means of Raman spectroscopy. Hydrogenation of graphene was already achieved by various approaches. One of the most common methods to create completely hydrogenated graphene (graphyne or graphane) is the hydrogenation via hydrogen plasma.[85, 112, 124, 176] Furthermore, wet chemical methods were performed to yield polyhydrogenated graphene, for instance applying the Birch-type conditions[110c, 125, 175] or the reaction of GICs in THF with a suitable proton source.[110a] The issue is that most of these approaches either treated the material in bulk or could not identify the exact way of functionalization, which impedes the investigation about the exact mechanism. To gain more information about the functionalization nature, it is necessary to draw attention to the behavior of the monolayer material.[115, 177] Regarding the reaction, on 76

the one hand, hydrogenation occurs on one side as hydrogen radicals are formed via SET from the charged graphene sheet to the applied water.[178] On the other hand, these intermediately generated radicals can also recombine to form elementary hydrogen instead of attacking the extended carbon lattice. The successful hydrogenation is the result of the applied Birch-like conditions and the formation of ammonium hydroxide (NH4OH) due to the reaction of water and liquid ammonia. As a consequence of the low temperatures, the added water instantly freezes and is removed from the equilibrium ammonia/water and ammonium hydroxide. Therefore, water is slowly released from the equilibrium after the cooling bath is removed. The low concentration of radicals favors the addition to the π-system of the graphene instead of the recombination to hydrogen.[110c] Intending to resolve the precise addition behavior, a wet-chemical approach for the hydrogenation of the monolayer graphene on a substrate was performed and the corresponding reaction was tracked by applying short reduction and reaction times followed by Raman analysis. For this purpose, different kinds of MLGs were prepared, specifically CVD graphene and mechanically exfoliated graphene on Si/SiO2-wafers as substrate and the focus lied on either small graphene flakes and peripheral areas of MLG. Moreover, the monotopic functionalization was compared with the ditopic functionalization by applying scratches to the carbon material in order to create free space below the graphene sheet. At these locations, small parts of graphene are basically free-standing due to the introduced space, which should increase the accessibility to the surface drastically from two possible sides.

The theoretical part related to the graphene functionalization patterns, being the basis for these experiments, was published in the journal “Carbon” in 2019 in collaboration with Konstantin Amsharov as first-author among others: K. Amsharov, D.I. Sharapa, O.A. Vasilyev, O. Martin, F. Hauke, A. Goerling, H. Soni, A. Hirsch, fractal-seaweeds type functionalization of graphene, Carbon 2019, 158, 435-448.[179]

3.2.1 Hydrogenation of Monolayer Graphene Flakes

At first, graphite was mechanically exfoliated and transferred to a Si/SiO2-wafer yielding in a ~20x40 μm sized flake (flake 1), which was characterized subsequently confirming the monolayer character of the sample. The carbon material on the substrate was moved to a flask under argon atmosphere exhibiting two gas inlets in order to perform the Birch reduction. After condensing ~25 mL ammonia by cooling it down to -78 °C, 15 mmol of freshly cut lithium pieces were added resulting in a deep blue solution which was kept stirring for 30 min. The simplified procedure of the upcoming reactions is depicted in Scheme 3.2.1.

Scheme 3.2.1: Birch-type Reduction of MLG deposited on a Si/SiO2- wafer and the subsequent hydrogenation with a proton-source.

In the following, the sample was immersed into the blue solution for 20 s in order to reach a complete reduction of the graphene sheet. Subsequently, the proton source (water) was added to hydrogenate the material. Water in combination with lithium were used since this combination showed the highest efficiency for this specific reaction.[110c] After the disappearance of the blue color of the solution (~1 min), the wafer was removed from the flask and subsequently washed several times with methanol (MeOH) to avoid residuals from the reaction, which could interfere with the characterization method. The sample was dried under a constant argon flow and characterized via Raman spectroscopy afterwards. The same procedure was repeated twice maintaining the same reduction times, with a total reduction time of 60 s. After each step, the sample was characterized via Raman spectroscopy in order to follow the addition process precisely.

Figure 3.2.1: A) Optical image of mechanically exfoliated pristine MLG flake 1; B) Raman map depicting G-band intensity with scale in counts; Raman map depicting the ID/IG-ratio values within the flake after 20 s (C), 40 s (D), 60 s (E) of reduction time and addition of water.

Figure 3.2.1 depicts the obtained results in terms of Raman intensity maps prior to and after each new reduction and hydrogenation step. Figure 3.2.1 A and B illustrate the pristine MLG flake (~30x40 μm) on the substrate by depicting, on the one hand, the optical image accompanied by the measuring area (green dotted square) (A) and, on the other hand, a map of the respective G-band intensities within the flake (B). Both presentations enable the determination of the shape of the graphene flake pointing out the boundary between the flake and the respective substrate (Si/SiO2). The varying G-mode intensities do not play a decisive role as these heavily depend on the morphology of the flake, which affects the signal intensity. After each reduction and addition of the water step, the recorded Raman maps demonstrate the ID/IG-ratio values on the whole flake size with a step distance of 1 μm in comparison with the results of each other with increasing ratio values from the color blue over green to red as highest. As higher ID/IG-ratio values reflect a higher density of defects, the reaction course over a certain time frame can be followed. In Figure 3.2.1 C, the whole flake is already covered with hydrogen indicated by the presence of high values ranging between 0.9 to 1.45. A certain functionalization pattern can be obtained as the outer

part close to the periphery and edges exhibits higher values than in the inner part of the flake. The starting point of the hydrogenation is nearby the edges, which is in agreement with recent findings.[113d, e] The very same course of functionalization continues during the next reduction and addition steps, which can be observed in

Figure 3.2.1 D/E. The ID/IG-ratio values increase significantly reaching values over 1.85 (depicted as white color) and becoming more homogeneously distributed over the whole flake covering the inner space as well. Nevertheless, the second and third hydrogenation steps have to be considered differently as after the first functionalization new sp3-centers or defects are introduced and therefore create more reactive areas on the flake but this will be discussed later in this chapter in detail.[113c, e]

Another experiment could support the course of functionalization and visualizes the pathway more obvious. In this approach, a bigger sized MLG flake (~50x50 μm/ flake 2) was identified after the mechanical exfoliation of graphite onto the Si/SiO2- wafer and reduced under the Birch-type conditions for 2 min first while using less amount of the reductive agent lithium. In this approach, 5 mmol instead of 15 mmol of lithium were used to in order to get more information about the course of hydrogenation due to a potentially decelerated propagation behavior. Subsequently, the charged sheet was hydrogenated using water as proton source followed by the characterization via Raman spectroscopy. This step was repeated once with an extended reduction time of 8 min and the subsequent addition of water. The results of the functionalization are depicted in Figure 3.2.2 showing the pristine flake in addition to the Raman analysis of the material and the extent of hydrogenation demonstrated by the ID/IG-ratio values. It is quite obvious that the functionalized sheet exhibited lower values of the degrees of functionalization indicated by the low ID/IG-ratio values ranging from 0.1 (basically unfunctionalized) to 1.1 for the first reaction reaching values of 1.6 for the second addition (Figure 3.2.2 C).

Figure 3.2.2: A) Optical image of the mechanically exfoliated pristine MLG flake 2; B) Raman map depicting G-band intensity of the pristine MLG flake 2 with scale in counts; C) SRS depicting average ID/IG-ratio of the flake before the reaction (1) and after 2 min (2) and 10 min (3) of reduction time and addition of water, respectively; D) Raman map depicting ID/IG-ratio within the flake after

2 min of reduction time and addition of water; E) Raman map depicting ID/IG-ratio within the flake after 10 min of reduction time and addition of water.

Even though the degree of functionalization reaches lower dimensions, the course of addition proceeds in analogy to that example presented above using lower reduction times. The corresponding proton addition increases implied by the raising ID/IG-ratio values illustrated in ascending order by the color green to yellow to orange/red. The functionalization starts at the periphery and continues to proceed its way to the interior of the flake. After the second hydrogenation, the outer areas in vicinity to the edges exhibit a homogeneously distributed value of around ~1.35, whereas, the values in the center of the flake remain to be lower (Figure 3.2.2 C/E). Thus, the complete surface of the flake is indeed covered by hydrogen in various extent, decreasing from the periphery to the inner sphere with ID/IG-ratio values ranging from 0.35 until 1.6, which corroborates the pathway of hydrogenation/functionalization likewise. However, the applied reduction times were not sufficient to reach a homogeneous coverage yet due to the reduced amount of Li in this approach. Additionally, the introduction of sp3-

defects leads to new reactive locations on the flake creating new addition possibilities for the hydrogen radicals.[113c, e]

3.2.2 Hydrogenation of the Peripheral Regions of CVD Graphene

Another attempt to verify the course of functionalization was performed using CVD graphene as starting material with the focus on the peripheral regions. The purchased

CVD graphene was transferred from the original substrate to a Si/SiO2-wafer followed by the removal of PMMA with acetone vapor. Hereupon, CVD-grown graphene was treated with Birch-type conditions and a specific spot focusing on the edge of the sheet was chosen to be analyzed. To avoid rapid homogeneous hydrogenation of the whole sheet, short reduction times were selected. After a reduction time of 5 s and subsequent hydrogenation, the sample was removed, washed, and characterized. The same procedure was carried out twice maintaining the same reduction times of 5 s each followed by the characterization via Raman spectroscopy. The chosen spot of the graphene layer shows a small part of the whole sheet at the periphery. In Figure 3.2.3 A, the chosen part can be seen depicting the 2D/G-band ratio values for this certain area which confirms the monolayer character of the graphene sheet. Besides, this kind of presentation enables the visualization of the start of the graphene edges, intensified by the addition of the black line which simplifies the regard of observation, respectively. Figure 3.2.3 C/D depicts the development of the hydrogenation in an equal presentation as in the experiment above in order to track the course of the addition. According to the observations presented above, the hydrogenation starts at the edges/periphery of the graphene sheet, further continuing to the inner parts of the carbon lattice. After the first hydrogenation step (Figure 3.2.3 C), the whole visible part is functionalized but to a different extent. Analogue to the results above, the edge area reveals significantly high values of the

ID/IG-ratio. Interestingly, some areas closer to the inner parts are strongly hydrogenated as well indicated by the high values of the ID/IG-ratio presented by red color.

Figure 3.2.3 A) Raman map depicting 2D/G-ratio values before the reaction, the black line illustrates the border of the graphene sheet; B) Overview about the Raman mean spectra after the iterative reduction and hydrogenation steps: (1: pristine, 2: 5 s, 3: 10 s, 4: 15 s reduction time); C) Raman map depicting the ID/IG-ratios after 5 s of reduction time and addition of water;

D) Raman map depicting the ID/IG-ratios after 10 s of reduction time and addition of water; E)

Raman map depicting the ID/IG-ratios after 15 s of reduction time and addition of water; the black line illustrates the border of the peripheral graphene layer.

From that point on, the additions are spreading from the periphery (Figure 3.2.3 D) until almost the whole sheet is covered until a certain range of values is reached, illustrated by the homogenous red color (Figure 3.2.3 E). Figure 3.2.3 B illustrates the average Raman maps of the visualized areas compared to each other suggesting that a certain constant value of ID/IG-ratio is reached (ID/IG = ~1.2) after three times of activation and addition. Moreover, this experiment emphasizes the additional functionalization in the inner part of the sheet. Different explanations are probable why the hydrogenation took place in the interior of the sheet as well. As the whole sheet is hydrogenated to a certain extent, the functionalization could have progressed very accelerated with the start from the periphery. Another possibility is the presence of already present defects within the pristine material, from which the hydrogenation could have advanced. This seems very reasonable as graphene without defects is extremely unreactive even with activation by reduction but has been further elaborated in the following chapter.[113e] Despite the low reduction times, the coverage of the 83

graphene by hydrogen occurs very rapidly, thus, another approach was performed using a less potent reducing agent and a less effective hydrogenation agent.[110c] The amount of lithium was reduced from 15 mmol to 5 mmol and MeOH was utilized to quench the charges instead of water. Anyhow, merely the same observations could be made, the speed of the reaction did not decelerate and the course of functionalization remained identical.

All these experiments represent a crucial and excellent prove for the assumed functionalization behavior starting from the periphery followed by distribution to the interior of the graphene sheet. This behavior stems from a variety of probable reasons. The carbon atoms at the graphene edges exhibit a different electronic structure and are therefore activated for further derivatization. Theoretical predictions, for instance regarding the arylation of graphene, confirm the observations as the preferred functionalization occurs at the edges (mainly 1,2-addition at armchair edges, isolated phenyl group at zig-zag edges due to a unique localized state near the Fermi level).[113b] Moreover, the results of HBC as a model system for graphene functionalization conducting reductive alkylation are in good agreement with the findings as well.[113e] The increased reactivity at the periphery can be traced back to the formation of local energy minima at the edges, which impede the migration of chemisorbed atoms and is the base of the preferred addition at these areas.[113c] The introduced strain by the first addition of hydrogen is significantly less on carbon atoms located at the edges in contrast to the carbon atoms in the basal plane of the sheet, which represents another corroboration of the observations.[113a] As already described above, the various kind of edges, namely zig-zag and armchair edges, seem to show a different reactivity towards the functionalization. Due to considerations about the specific molecular orbitals, zig-zag edges have an enhanced reactivity in comparison to armchair edges.[174, 180] For this specific type of functionalization, namely the reductive hydrogenation, a certain mechanistic scenario regarding the regiochemistry based on Clar´s concept for the start of the hydrogenation can be suggested.[179] The most representative formula (Clar formula) for a given molecule is the representation with a maximum number of sextets of electrons. This formula gives information of properties of polycyclic aromatic hydrocarbons (PAHs) including bond lengths alternations or the local density of π-states as well as the reactivity. Scheme 3.2.2 depicts the beginning of the hydrogenation after activation with an electropositive metal. The reducing agent provides the electrons for the reduction and the applied 84

water represents the proton donator. The start of the suggested route is the reduction of pristine graphene (1) to the radical anion (2) by the electron transfer from the metal. The charges are delocalized over the whole π-system of the graphene which formally activated every carbon atom for a possible addition but the presented one can be seen as one of the most energetically favorable since both centers are located in the same hexagon and therefore obey the Clar´s rule.

Scheme 3.2.2: Schematic illustration of the possible mechanism of the start of the reductive Birch-type hydrogenation beginning at the periphery (highlighted in blue), adapted and modified with permission from [179] (Copyright 2020 Elsevier).

The protonation of the radical anion (2) is the next step resulting in the formation of a cyclohexadienyl-type radical (4) and the fixation of the radical location on three specific positions according to Clar´s rule. These reactive intermediates (4) are able to undergo rearrangements via a hydrogen shift in order to yield the most stable product (6).[181]

These rearrangements appear to play a crucial role to determine the regioselectivity of the hydrogen addition process. The second reduction of 6 results in a cyclohexadienyl carbanion (7), which reacts with a proton to yield three isomeric products (8a/b/c). These isomers represent the 1,2-addition (8b/c) and 1,4-addition (8a) and are formed due to the presence of three available nucleophilic centers, all possessing the maximum number of Clar´s sextets with only one involved aromatic hexagon. Very large carbon systems like graphene or nanographenes are capable to accept large numbers of electrons yielding in multiple charged systems. Anyhow, the influence of Coulomb interactions within the system can prevent the charge localization. Accordingly, the mono- or di-anionic systems revealed to be correct models for the hydrogenation. Therefore, a second electron can be transferred from the metal to the radical anion (2) to yield compound 3. After further protonation, the product 5 is formed generating the most stable monoanion.[181a] This anion cannot undergo 1,2-hydrogen shifts as this reaction would require a four-electron anti-aromatic transition state which impedes the reaction of 5 to 7. This mechanism presents the start of the graphene hydrogenation, while the further functionalization continues presumably with the addition of the protons at the non-aromatic hexagon in accordance with the regioselectivity of small PAHs like naphthalene or anthracene.[181a] This specific selectivity results in the formation of thermodynamically more stable products exhibiting most Clar´s sextets possible. Thus, according to the findings, the all-trans- hexa-additions to aromatic sextets in vicinity to the edges can be seen as a predominant process which leads to a progressed expansion of newly introduced defects starting from the periphery going to the interior of the graphene plane until a homogeneous coverage is reached.[179] The hydrogen radical addition can be seen as a simplified model for describing of the complex reductive addition, independently on the exact mechanism of the reaction.

3.2.3 Expansion of Defects

After the previous experiments comprehensibly demonstrated that the functionalization process starts from the more reactive periphery and propagates to the interior of the sheet, the focus is now on the functionalization behavior in vacancy to already present or newly introduced defects. The edges as well as already pre-existing basal imperfections like Stone-Wales defects, vacancies, holes, substitution atoms or merely sp3-hybridized carbon atoms can be all considered as basal plane defects exhibiting modified electronic, mechanical, and optical properties in comparison to the residual carbon network.[113c, e] The attachment of a new moiety to graphene equalizes an introduction of a new defect into the carbon lattice since the orbital configuration alters (sp2 -> sp3). Therefore, these centers modify the chemical properties as well by enhancing the intrinsic reactivity at these spots which was primarily topic of theoretical considerations until nowadays.[113b, c, 182] Anyhow, this aspect was barely explored or verified by experiments yet waiting to be validated. Thus, herein the focus lies on a specific spot on a MLG flake (flake 1) after prior activation and hydrogenation, therefore, a prior introduction of new defect areas on the graphene flake displaying a specific growth of defects. From this spot, it is possible to follow the functionalization process in order to shed light on the respective functionalization pathway. This can be impressively seen in the hydrogenation attempt of the small MLG flake 1 which was presented in the beginning of this chapter (Figure 3.4.1). Thus, the MLG flake after 20 s of reduction in the liquid ammonia solution followed by the hydrogenation is depicted in Figure 3.2.4 again emphasizing an M-shaped functionalization pattern. This M-shaped pattern, colored in brighter green, indicates a higher hydrogenation extent in this specific area of the flake. Originating from this certain shape, the pattern seems to be expanding in every direction forming a bigger sized pattern of the M- shape. The functionalization propagates in every direction from the preexisting defects introduced by the first hydrogenation step. Therefore, the newly introduced sp3-centers behave as catalysts for the additional attachment of addends. A second reduction accompanied by the addition of water resulted in an expansion of the defects starting from this area as well (Figure 3.2.4 C). The expansion advanced in every direction as a consequence of the third reduction and addition step leading to a new pattern

(Figure 3.2.4 D). These steps can be repeated until a certain extent of hydrogenation is reached.

Figure 3.2.4: A) Raman map depicting the ID/IG-ratios within the flake after 20 s of reduction time and addition of water; B) Raman map of the flake depicting the ID/IG-ratios emphasizing the M shaped functionalization pattern after 20 s; C) Raman map of the flake depicting the ID/IG-ratios after 40 s emphasizing the growth of the M shaped functionalization pattern; D) Raman map of the flake depicting the ID/IG-ratios after 60 s emphasizing the growth of the M-shaped functionalization pattern.

It was predicted that direct covalent functionalization of the pristine basal plane of graphene is rather improbable, which was confirmed experimentally by results of the Hirsch group[113e] or the group of Choi claiming exclusively edge functionalization.[116] As a consequence, it can be stated that graphene seems to be finite and exhibits imperfections, which results in an adequate activation of these areas for further additions due to the perturbation of the π-system. Therefore, this can be seen as the reason why covalent binding can take place at the unreactive basal plane as well, which can be observed in the experiments. Due to the defect insertion by the addition of hydrogen, a subsequent defect expansion occurs during the functionalization in close proximity to these areas (Scheme 3.2.3). For this purpose, a model was suggested in order to explain the general defect expansion correctly.[179] According to that, a defined patterning with Clar sextets close to the defect is formed, which is in good agreement with recent theoretical considerations on graphene aromaticity at 88

boundaries and/or close to the defects.[183] In this model, the addition can merely take place close to defects.

Scheme 3.2.3: Suggested model of the formation of perhydro-benzene-holes leading to the generation of active sites around the hole (red circles), adapted and modified with permission from [179] (Copyright 2020 Elsevier).

Thus, graphene can be considered as an all-benzoid system with localized aromatic sextets (Scheme 3.2.3), according to the empirical Clar theory and the kinetic and thermodynamic analysis of the graphene reactivity.[179] The hexagon in which the addition took place, determines the localization of the double bonds as well. The formation of this hole/defect also determines the subsequent addition of the functionalization agent. After the full addition of one hexagon, the surrounding hexagons are activated for further covalent bonding (Scheme 3.2.3/ red circles). After another full addition to one of these activated hexagons, three additional hexagons are initiated to be functionalized and this process continues leading to expanded growth of defects.[179]

To sum up, with respect to the graphene reactivity like strain or conjugation energy, an addition to the basal plane graphene is less favored to functionalization of the respective periphery or defect areas. However, a “defected” surface will undergo the mentioned continuous defect expansion resulting in highly functionalized materials. The expansion process is promoted by the formation of the thermodynamically most preferred substructures and leads to the generation of highly modified islands incorporated into the sp2-carbon lattice of intact carbon network. Based on Monte- Carlo-simulations, the functionalization pattern can be explained as a fractal-seaweed defect expansion.[179] Furthermore, the introduction of desired defects can be a goal for scientists to modify the electronic and chemical properties in addition to the

possibility to detect imperfections in graphene surface by selective functionalization.[113c]

3.2.4 Ditopic Hydrogenation Approach of CVD Graphene

After exploring the functionalization pathway of MLG, the question arises how the functionalization occurs with respect to the precise mechanism during this Birch-type reaction in general. Since the MLG is located on a substrate, one side of the plane is protected or shielded towards the attack of the protons, respectively. Due to the functionalization from one side, dangling bonds start to appear accompanied by a significant increase of the molecular strain in the graphene lattice. To overcome this strain, it is assumed that the substrate, SiO2 in this case, binds covalently to the free dangling bonds in order to release the mentioned strain.[115] As mentioned above, hydrogenation takes place on one side as hydrogen radicals are formed via SET from the charged graphene sheet to the applied water.[178] The yielded radicals favor to recombine forming elementary hydrogen instead of attacking the extended carbon lattice when the concentration of radicals is too high. Due to the low temperatures, the added water instantly freezes and is removed from the equilibrium ammonia/water and ammonium hydroxide. Therefore, water is slowly released from the equilibrium after the cooling bath is removed which ensures a low concentration of radicals followed by the addition to the π-system of the graphene instead of the recombination to hydrogen.[110c] In order to investigate the hydrogenation behavior in more detail, we tried to compare various areas of MLG deposited on a substrate after the introduction of trenches (scratches) under the present graphene resulting in partly free-standing material.

Scheme 3.2.4: Schematic presentation of substrates bearing scratches in the vicinity to the periphery (A) and under the interior of the graphene sheet (B).

Hence, several scratches (deepness: ~0.1-0.5 mm) were introduced on the surface of the SiO2/Si-substrate with the aid of a very thin tip of a tweezer, implying free space underneath definite locations of the graphene sheet (Scheme 3.2.4). These modified surfaces served then as substrates for the transfer of CVD-graphene. After the transfer, the MLG on the substrate was hydrogenated as described above applying a reduction time of 20 s and the usage of water as proton source followed by the Raman analysis. Additional 20 s of reduction time was applied followed by subsequent hydrogenation and the SRS analysis.

Figure 3.2.5: A) Optical microscope picture of the measured area including the scratch on the left-hand part of the spot close to an edge; B) Raman map depicting the G-band intensity of the area with scale in counts defining the start of the graphene sheet (colored); C) Raman map depicting the ID/IG-ratios after a reduction time of 20 s and addition of water, the black line illustrates the begin of the scratch to the left-hand side; D) Raman map depicting the ID/IG-ratios after a total reduction time of 40 s and addition of water.

Figure 3.2.5 A and B illustrate the measured area focusing on a scratch region in the vicinity to the periphery, pointing out the border of substrate/MLG with varying colors (black/colored). The applied black line separates the picture in two areas and the

region on the left-hand side of the line displays the area under which space/scratch was introduced. In Figure 3.2.5 C and D, the development of the hydrogenation process can be obtained suggesting an inhomogeneous functionalization for the area without the scratch starting from the periphery and progressing then further like in the way explained above. Besides, some areas in the inner plane are functionalized as well forming “island-like” addition patterns which continuously expand in every direction, as described in the previous chapter. In the area under which the dangling bonds of the graphene cannot bind to the substrate, hydrogenation takes place as well indicated by the respective raised ID/IG-ratios (visible in Figure 3.2.5 C in the area on the left-hand bottom corner), but to a lower extent, especially in vicinity of the periphery. After additional hydrogenation, the functionalization appears to be more homogeneous, especially for the “free-standing” graphene area due to the expansion of defects in every direction. On the one hand, this can be explained by the passing of hydrogen underneath the graphene layer at the periphery and the formation of bonds to the present dangling bonds in order to release the introduced strain. In this case, the reaction with the substrate seems to be more efficient and faster indicated by the higher degree of functionalization in the area without a present space under the graphene sheet. On the other hand, considering that protons can move under the present graphene sheet, the amount of the protons is considerably higher but also more distributed in the deep scratch area. Therefore, the concentration of protons is locally higher in the other parts resulting in an increased extent of the hydrogenation. Nevertheless, this explanation requires relatively high concentrations of hydrogen but as mentioned before, the concentration of the hydrogen radicals is rather low due to the equilibrium with NH4OH avoiding a possible recombination to form elemental hydrogen. In order to evaluate the mobility of the protons, a specific area was focused bearing a scratch in the interior of the sheet. This part excludes the possibility of rapidly approaching protons from the periphery. Figure 3.2.6 illustrates the development of the hydrogenation in that specific area involving three reduction and functionalization steps. The scratch area is surrounded by the black rectangle (dashed black line).

Figure 3.2.6:A) Optical microscope picture of the measured area including the scratch in the middle part of the spot before exposure to Birch-type reduction conditions; B) Raman map depicting the G-band intensity with scale in counts; the inside of dotted area presents the area with the scratch or free space underneath the sheet; C) Raman map depicting the ID/IG-ratio within the CVD graphene after 5 s of reduction time and addition of water; D) Raman map depicting the

ID/IG-ratio within the CVD graphene after 10 s of reduction time and addition of water; E) Raman map depicting ID/IG-ratio within the CVD graphene after 15 s of reduction time and addition of water; the area inside of the dotted square represents the scratch or the free space underneath the layer.

The hydrogenation starts to take place homogeneously over the whole observed area of the MLG (Figure 3.2.6), whereas the values do not increase significantly and rather remain similar after the second and third hydrogenation step (ID/IG ratios ranging from 0.7 until 1.15). According to the SRS, the scratch area exhibits lower degrees of functionalization in comparison with the residual parts. Anyhow, functionalization takes place indeed in these areas suggesting a ditopic addition from both sides of the sheet. As the dangling bonds of the graphene layer miss the capability to bind to the substrate, a formation of covalent bonds to captured air, moisture or moving protons is feasible. As the reaction of graphene from both sides is preferred in order to release the strain, we cannot exclude bonding to any on the surface adsorbed impurities or to moving protons from the applied water.

3.2.5 Dehydrogenation

The reversibility of the hydrogenation reaction was conducted via thermal treatment of the used MLG flakes. Thus, the hydrogenated material of the corresponding MLG flakes 1 and 2 was stepwise annealed at certain temperatures under air exclusion followed by SRS of the resulting material. Both flakes exhibit similar degrees of hydrogenation indicated by the resembling values of the respective ID/IG-ratios (1.41 for flake 1/ 1.15 for flake 2).

Figure 3.3.7: Development of the mean Raman spectra of MLG flake 1 (A) and flake 2 (B) during heating from 300 °C, 500 °C and 700 °C demonstrating the thermal dehydrogenation under constant argon flow; C) Development of the ID/IG-ratio values of the corresponding flakes dependent on the respective temperature.

Figure 3.3.7 illustrates the Raman analysis of the thermal annealing of the graphene flakes 1 and 2 for one hour at 300 °C, 500 °C, and 700 °C under a constant flow of

argon. The first increase of the temperature from RT to 300 °C has already an impact on the density of defects of the respective flakes in both cases but to a minor extent. The thermal treatment gives rise to a rather sharp D-band accompanied with a significantly decreasing FWHM of the 2D-mode suggesting the partial reversion to pristine material. Further dehydrogenation occurs, when the temperature is increased to 500 °C indicated by a strong decrease of the D-mode intensity for both flakes. The lowest difference can be obtained for the last temperature step with a minor drop of the D-band for flake 1, whereas a major drop of the D-mode intensity for flake 2 can be observed. Concurrently, the residual modes of the spectra remain unchanged. The decrease can be related to a completely diminished density of defects within the flakes due to the dehydrogenation process, while the relatively high remaining D-mode intensity in both cases derives from structural damages potentially caused by the harsh reaction conditions of the Birch-type reaction and the annealing process applying high temperatures. Thus, after the annealing step at 700 °C the dehydrogenation is virtually finished leaving damages in the lattice behind.

Up to now, we gained insights into the different discharging behavior of various graphite types and different carbon allotropes. Furthermore, the addition pattern of the hydrogenation of MLG was highlighted and could shed light on the general addition patterns of carbon allotropes. The question arises at this point, how it would be possible to scale up the yield of functionalized monolayer graphene, which represents an ongoing issue among scientists. The next chapter applies to this topic using graphene oxide or reduced graphene oxide, respectively, as an alternative starting material in order to increase the amount of resulting monolayer material.

3.3 Reductive Functionalization of oxo-Graphene

The production of monolayer graphene remains a challenge with respect to the scalability of the production. The liquid-phase exfoliation with its aim to overcome the van der Waals interactions between the graphene layers while preserving the size of the flakes or layers, respectively, lacks of quantitative yield. Due to restacking processes in the used solvents and during the work-up, the yield of single layers is rather little and the distribution of the number of the layers is broad and therefore too complex to realize for mass production.[103, 184] To control the amount of layers, the suspensions have to be exposed to harsh conditions for separation, like for instance, with the addition of surfactants or functionalization with suitable electrophiles after reductive activation.[184] Therefore, few-layer functionalization is a competing process and thus, the yield of the functionalized monolayer material is limited.[133, 185]

As already mentioned in the introduction, an additional established approach to produce high quantities of monolayer graphene is the oxidative intercalation with oxidizing agents, namely nitric and sulfuric acids and potassium permanganate.[186] The resulting product is called graphite oxide bearing a numerous amount of oxygen functionalities and therefore sp3-hybridized carbon centers in the individual layers of the respective graphite.

Figure 3.3.1: Exemplary structures of oxo-graphene/ GO (left) and reduced GO (right).

Due to the increased interlayer distance, which originates from the hydrophilic functionalities like epoxides or hydroxyl groups bonding to adsorbed water within the layers, the interaction can be easily broken by stirring or ultrasonication. This treatment can yield in individual sheets of oxidized graphene without a significant fraction of few-

layer material, called graphene oxide (GO), visible in Figure 3.3.1 (left).[184, 186] The subsequent reduction with various methods (thermal/microwave heating, electrochemical, irradiation, plasma/ion bombardment, reduction with chemical reagents)[187] leads to the elimination of most of the oxo-functionalities yielding reduced graphene oxide (rGO, Figure 3.3.1 right), which can be simply produced in masses and with little effort. The major drawback of this approach is that rGO exhibits a huge amount of structural defects due to the harsh oxidation process and remaining oxo- addends alongside a largely disturbed carbon framework that may not even exhibit long-range order of the hexagonal lattice as confirmed by Raman spectroscopy or transmission electron microscopy (TEM), which may impair the properties to a certain extent.[188] In collaboration with the Eigler group, established reductive chemistry was applied on oxidatively delaminated materials in order to compare the reactivity of functionalization and outcome of the reactions with lab-scale produced graphene. The following results were published in 2018. C.E. Halbig was focusing on the synthesis of GO/rGO, while I performed the reductive functionalization sequences. The results were plotted and evaluated together.

C. Halbig,* O. Martin,* F. Hauke, S. Eigler, and A. Hirsch, Oxo-Functionalized Graphene - A Versatile Precursor for Alkylated Graphene Sheets by Reductive Functionalization, Chem. Eur. J. 2018, 24, 13348-13354 (*both authors contributed equally to the manuscript).

This project was based on oxo-graphene (oxo-G1) as starting material, which was prepared under mild conditions as single layers analogous to GO.[189] This material has covalently bound epoxy and hydroxyl groups in addition to organosulfate entities with a total degree of functionalization of around 60 %. The low density of in-plane lattice defects (θVD = 0.7%) and thus, the preservation of the long-range order of the hexagonal carbon lattice as recently shown by TEM presents the major benefit of oxo-G in comparison with GO.[190] These properties enable almost solely on-plane functionalization and minimize undesirable in-plane functionalization. A major issue of the covalent functionalization of this material is that the degree of functionalization

(DoF, θFD) is generally much lower than the density of intrinsic in-plane lattice defects

(θLD) hampering the analysis of the functionalized material. Moreover, no statistical Raman analysis of the functionalized material based on a large area Raman screening could be achieved in the past.[191] The combination of the benefits of both

functionalization techniques to yield functionalized graphene in high quantities is on the focus (Scheme 3.1.1).

Scheme 3.3.1: Reaction scheme illustrating the reductive functionalization of oxo-G1 on surfaces 0.7% and in bulk. G1 obtained after deposition and reduction of oxo-G1 on Si/SiO2 wafer (above) and following functionalization to an exemplary cis-1,4-adduct. Bulk functionalization of sodium cholate stabilized graphene in water obtained by the reduction of aqueous oxo-G1 with sodium borohydride and subsequent functionalization to an exemplary trans-1,4-adduct (below), adapted with permission from[187b] (Copyright 2018 WILEY-VCH Verlag GmbH).

Furthermore, the reductive functionalization approaches were transferred to wet- chemically exfoliated graphene (which will be named as G1) prepared from easily accessible oxo-G1. Oxo-G1 was prepared by mild oxidation of graphite with two mass equivalents of potassium permanganate in sulfuric acid (98%) at temperatures below [186a, 189b] 10 °C. The process leads to an aqueous dispersion of oxo-G1 with a larger flake size than 5 µm (Figure 3.3.2 A). The TG-MS analysis (Figure 3.3.2 B,

Δm = 38.2 %) of the freeze-dried oxo-G1 dispersion reveals a high degree of functionalization of about 60 %.[189a]

0.7% Figure 3.3.2: A) AFM (intermittent mode) of G1 with an average height of 0.9 nm after deposition on a Si/SiO2 wafer; inset: height profile along the black line; B) TGA mass-loss traces 0.7% of oxo-G1 (red) and G1/SCH (black); C) Representative Raman spectra of oxo-G1, G1/SCH, G1 , CVD G1 and pristine graphite representing the mean quality of all materials used for functionalization, spectra y-shifted for clarity. The depicted numbers in the picture correspond to the average ID/IG-ratio; D) Normalized absorbance spectra of oxo-G1 and G1/SCH, adapted with the permission of [187b] (Copyright 2018 WILEY-VCH Verlag GmbH).

Subsequently, sodium cholate stabilized graphene (G1/SCH) was synthesized by the reaction of aqueous oxo-G1 with 30 mass equivalents of sodium borohydride in the presence of an excess of sodium cholate (SCH) as a stabilizer to avoid aggregation of the material. The reduction was performed at 4 °C to minimize the lattice degradation of oxo-G1 by hydroxyl anions formed in a side reaction of sodium borohydride with water.[192] After the reduction step, the product was centrifuged and thoroughly washed with sodium cholate solution. Finally, the last washing step was performed with water to remove excess of sodium cholate but preserving the state of individual suspended

G1 (Figure 3.3.2 B). Additional purification with pure water would lead to less stable graphene dispersions tending to agglomerate presumably because the stabilizing surfactant is removed. Successful reduction can be observed in the absorption spectra

of the initial aqueous oxo-G1 dispersion with a maximum at 234 nm (π→π*) and an additional shoulder at ~300 nm (n→π*) and the corresponding spectrum of the reduced and surfactant stabilized derivative with the red-shifted maximum at 270 nm and an increased absorbance in the offset region (>400 nm, Figure 3.2.2 D).[193]

TG-MS measurements of freeze-dried oxo-G1 and G1/SCH present the mass loss in the temperature range from 25 °C to 325 °C, which is reduced from 38.2 % for the initial oxo-G1 to 2.1% for G1/SCH. The decomposition of cholate is the reason for the high mass loss of G1/SCH between 325 °C and 550 °C.

Figure 3.3.3: Combined TGA profiles of sodium cholate (blue) and G1/SCH (black). Individual mass losses in the temperature range between 200 °C and 600 °C are given.

The difference in the mass loss can be observed in Figure 3.3.3 and the value was determined to 11.7 %, which can be taken as a rough measure of the amount of graphene in G1/SCH. The small shoulder at around 100 °C can be attributed to water.

The mass ratio of SCH to G1 was determined to about seven and one sodium cholate on five carbon atoms, respectively (Figure 3.3.3). To further study the quality of the carbon lattice, statistical Raman spectroscopy (SRS) was used.

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3.3.1 Reductive Functionalization of rGO on Substrates

Therefore, oxo-G1 was deposited on a Si/SiO2 wafer and analyzed before and after the chemical reduction by hot vapor of hydrogen iodine and trifluoroacetic acid (HI/TFA).

With this procedure the amount of functionalization defects (θFD) and vacancy defects [191, 194] (θVD) can be certified and discriminated. Considering earlier studies of Cançado [153, and Lucchese et al., the measured ID/IG-ratio can be used to quantify θFD and θVD. 160] Both, the pristine graphite source (ID/IG = 0.06±0.04) and commercially purchased CVD CVD-graphene (G1 , ID/IG = 0.10±0.03) show a neglectable D-mode, as expected, and therefore θVD could be determined to be <0.001 % with respect to carbon atoms [160] for both materials. In contrast, reduced oxo-G1 contains a lattice with θVD=0.7 % 0.7% (ID/IG = 2.73±0.23, Figure 3.3.4 B). Therefore, it will be termed as G1 to clearly CVD discriminate the quality from G1 with almost zero vacancy defects. The materials 0.7% CVD oxo-G1, G1 and G1 were deposited on a Si/SiO2 substrate to be activated by Na/K alloy in DME and to further study the functionalization by hexyl iodide.

Figure 3.3.4 A) SRS of samples deposited on a 300 nm Si/SiO2 wafer. This plot shows the CVD 0.7% functionalization of G1 (left arrow) and reduction of oxo-G1 to G1 by hydrogen iodine and trifluoroacetic acid (HI/TFA, right arrow), the x-axis represents the FWHM (Γ2D) of the 2D-mode. B) Representative spectra extracted from the maps obtained by SRS, spectra y-shifted for clarity, depicted with the permission of [187b] (Copyright 2018 WILEY-VCH Verlag GmbH).

After treating of the graphenide sheets with hexyl iodide, residual unreacted electrons were removed with PhCN. After the reaction, SRS was used to measure changes of CVD θFD. As illustrated in Figure 3.3.4 A and B, the ID/IG-ratio of G1 increased from average 0.10±0.03 to 1.44±0.74 accompanied by a decrease of the full-width at half- maximum of the 2D band (Γ2D/FWHM) by the functionalization process. The average degree of functionalization could be calculated to about 0.03 %.[160] Interestingly, the 101

0.7% SRS of charged and functionalized G1 did not indicate any statistically relevant change as the obtained ID/IG ratios are within standard deviation (initial: 2.73±0.23; CVD functionalized: 2.85±0.44), although over 99 % of the material is similar to G1 and should therefore own similar reactivity. Charging of graphite by alkali metals saturates at best for 1 charge on six for (lithium) or eight carbon atoms (potassium).[195] However, CVD θFD detected for G1 is 0.03 %, suggesting that additional activation of the carbon 0.7% framework is necessary or side reactions occur. In G1 , there are about 0.7 % of lattice defects and functional groups can be assumed to be located at the edges, such as hydroxyl, carbonyl, and carboxyl groups.[81, 196] Such moieties may accumulate an unknown amount of charge and therefore, surface alkylation by alkyl radicals is presumably to be less effective, however, the functionalization may not be impossible. CVD Assuming a similar degree of hexylation, as found for G1 (θFD = 0.03 %), would not increase the overall density of defects significantly (θVD+FD = 0.73 %).

3.3.2 Reductive Bulk Functionalization of rGO

The subsequent step included the covalent bulk functionalization of rGO with hexyl iodide in a reductive manner. This kind of addition, however, can be verified without

SRS but with the aid of TG-MS and TG-GC-MS. Therefore, G1/SCH and pristine spherical graphite (SGN18) were used as a reference system for comparison and analyzed cleaved off mass fragments before and after functionalization (Figure 3.3.5 C/D). Both materials were added to glass vials and stirred with Na/K alloy in DME to form charged individual layers of graphene, in the case of G1/SCH, or the corresponding GIC from pristine graphite. After both charged materials were treated with hexyl iodide, the products were purified in a separation funnel and filtered under vacuum. The direct bulk-functionalization of graphite or corresponding KC8, respectively, results in only minor amounts of single layers and mainly fractions of GIC [133a] restacked multilayer graphene sheets (hexylG≥1 ). The as-obtained bulk materials were then used for further TG-MS analysis. The specific mass fragments for + + covalently bound hexyl groups with their m/z values of 29 (C2H5 ), 41 (C3H5 ), 42 + + + (C3H6 ), 43 (C3H7 ), and 57 (C4H9 ) appear within the temperature range of 200-300 °C GIC for hexyl-G1/SCH and 150-200 °C for hexyl-G≥1 (Figure 3.3.5 A/D). Same signals

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can be detected in the range of 325-550 °C stem from SCH decomposition (Figure 3.3.5 C). Due to the more complex and oxygen-containing structure of SCH, these signals can also be attributed to oxygen-containing fragments like CHO+ (m/z 29) + and CH3CO (m/z 43).

Figure 3.3.5: A) TG-MS profile of initially prepared hexyl-G1/SCH; B) TG-MS profile of G1/SCH; GIC C) Hexyl-G≥1 obtained by the reaction of bulk graphite with Na/K alloy and hexyl iodide; The red curve illustrates the mass loss, whereas the different colored lines represent specific ion currents; Adapted with permission from [187b] (Copyright 2018 WILEY-VCH Verlag GmbH).

GIC Temperature-dependent SRS of hexyl-G≥1 clarifies that the intensity of the defect -1 GIC induced D-mode at 1334 cm in hexyl-G≥1 is continuously decreasing from 2.52 to

0.27 (ID/IG-values) in the temperature range of 100-200 °C and therefore, the detected mass traces by TG-MS in the same range are related to the regeneration of the π- system (Figure 3.3.6 A). The same temperature-dependent SRS analysis was performed for hexyl-G1/SCH (Figure 3.3.6 B). However, the thermal treatment results in two competing processes.[187b] First, a detachment of covalent bound hexyl-groups from the carbon lattice occurs and second, additional vacancy defects are formed. The results indicate that the formation of lattice defects is the major process for thermally treated hexyl-G1/SCH, both in the temperature range of 100-200 °C and up to 450 °C.

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GIC Figure 3.3.6: A) Temperature-dependent SRS of hexyl-G≥1 in the temperature range of 50 °C to

200 °C; B) of hexyl-G1/SCH; (C) ID/IG-ratios of the Raman spectra at distinct temperatures extracted from A) and B); D) Pathways of the defunctionalization and decomposition processes GIC of hexyl-G1/SCH and hexyl-G≥1 at the beginning and end of the thermal treatment; The dotted line follows the formula published by Lucchese et al[153]. Adapted with permission from[187b] (Copyright 2018 WILEY-VCH Verlag GmbH).

This conclusion can be drawn from the ID/IG ratio of 1.25 and an accompanied broadening of all modes in the Raman spectra (Figure 3.3.6).[160] Although the initial

θVD and further temperature-induced generation of defects in the graphene lattice of 0.7% hexyl-G1/SCH and hexyl-G1 disturbs the proper analysis by statistical and temperature dependent Raman spectroscopy, clear evidence for the cleavage of alkyl species upon thermal treatment is found by combining thermogravimetric analysis coupled to gas chromatography and mass spectrometry (TG-GC-MS). In addition to the high sensitivity of mass spectrometry towards detached addends, the additional combination with gas chromatography allows the discrimination of cleaved fragments with similar m/z values and fragmentation patterns by their retention time. The optimal temperature was identified for the injection of a gas sample of the cleaved and gaseous decomposition products to the GC column by TG-MS (Figure 3.3.5 A, hexyl-G1/SCH: 250 °C).

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Figure 3.3.7: Gas chromatogram of the hexyl functionalized G1/SCH presenting the peaks for n-hexane and the dimerization product dodecane from cleaved hexyl radicals; The injection temperature was 250 °C and the corresponding mass spectrometric characterization of dodecane and hexane with its specific fragmentation pattern are shown on the right-hand side.

The resulting elugram features two dominant peaks with varying intensities (Figure 3.3.7). The analysis of the fragmentation pattern of the peaks at 4.1 min and 12.3 min indicates that they originate from n-hexane and n-dodecane, respectively.[133b] The appearance of n-dodecane can be explained by the cleavage of covalently bound n-hexyl radicals from the carbon lattice and their recombination.

Accordingly, these two signals are absent in samples of G1/SCH or graphite prepared under similar conditions, but without the essential activation by Na/K alloy, as it is inevitable for a successful covalent functionalization. These results are in excellent agreement with those recently published by our work group on the functionalization of graphene sheets with activated bulk graphite under similar conditions.[133b]

Accordingly, the hexylation reaction of G1/SCH, which was prepared from oxo-G1 becomes analytically accessible. A partial individualization of hexyl-G1/SCH was possible by the use of o-dichlorobenzene and mild ultrasonication (Figure 3.3.7), but not in sodium cholate solution or with other common organic solvents such as NMP, CHP, cyclohexane or THF. This represents a major advantage in comparison to the established functionalization route with the precursor graphite and the following reductive treatment.

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Figure 3.3.7: AFM images of single-layered (A) and partially agglomerated hexyl-G1/SCH (B); The corresponding height profiles are depicted in the right column, adapted with permission from [187b] (Copyright 2018 WILEY-VCH Verlag GmbH).

Summarizing this chapter, alkylated graphene sheets on surfaces and in bulk by the combination of oxidative exfoliation and reductive functionalization could be obtained. Functionalization was performed with different graphene derivatives and the results were compared to each other. It was demonstrated, that the introduction of 0.03 % on- 0.7% plane alkyl moieties onto G1 and G1/SCH generated by reductive functionalization cannot be detected by Raman spectroscopy as the number of introduced hexyl chains is about 23.3 times lower than the number of vacancy defects in the lattice structure

(θVD = 0.7 %; θVD+FD = 0.73 %). However, by the use of TG-MS and temperature- dependent Raman spectroscopy, reasonable mass fragments in the expected range could be detected for hexyl-G1/SCH. With TG-GC-MS, it could be shown that covalent hexylation occurs by the detection of dodecane – the dimerization product of two hexyl [133b] 0.7% radicals cleaved from the π-system. Thus, alkylation of G1 and G1/SCH to their hexylated derivatives is possible, but not more efficient as reported for pristine graphene, although it could be expected that vacancy defect may activate the carbon lattice.[113e] However, the here presented results demonstrate the advantage of

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combining TGA with MS and GC as an analytical tool to characterize functionalized graphene materials. Moreover, the combination of oxidative delamination and reductive functionalization allows the functionalization of the basal plane of graphene, excluding the preparation of major amounts of functionalized few-layers and therefore a more efficient way to yield high amounts of functionalized monolayer material. An enhanced dispersibility in specific solvents simplifies the rapid discovery and generation of monolayer material as well.

Since we gained better knowledge about the course of functionalization and the possibility to scale up the yields of functionalized monolayer graphene, an exact evaluation of the degree of functionalization for graphene as well as for SWCNTs and the relation between each other is rather unresolved. The next chapter deals with the determination of the degrees of addition using several established tools for characterization accompanied with the comparison of both allotropes with regard to the amount of newly introduced moieties using an organometallic marker molecule.

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3.4 Evaluation of the Quantitative Degrees of Functionalization

Covalent functionalization of graphene and SWCNTs was demonstrated for many different molecules including for instance alkyl- and aryl-halides,[108a, 133a] iodonium[79a] or diazonium salts.[74, 136b] However, these literature-known addends consist out of carbon structures and are hence, difficult to distinguish from the allotrope carbon lattice with the established characterization tools. Therefore, a heteroatom or marker would be ideal to confirm the success of the covalent attachment and would give raise to the possibility to determine the degree of functionalization. Without such a marker, the comparison of the extent of the functionalization would be rather complex, since both allotropes differ in their reactivity and equal reaction conditions are needed. Ferrocene with one Fe atom as marker per introduced defect presents a suitable functional entity. Additionally, ferrocene exhibits extraordinary properties like a high electron transfer rate, a high stability of two redox states, and a low oxidation potential, which all enable the usage as mediators in various areas.[197] Typically, such systems are linked via oxygenated groups on the respective carbon allotrope in order to form amide or ester bonds.[197c, 198] In the case of graphene, this strategy has been only applied with graphene oxide until now.[197c, 199] Herein, the successful bulk ferrocene functionalization was demonstrated without exploiting oxygen functionalities on the carbon material and therefore one of the first functionalization reactions of defect-free graphene with ferrocene. Furthermore, the functionalization of CNTs and graphene were correlated with two ferrocene derivatives bearing different leaving groups and varying spacer length (Figure 3.4.1). The following results were obtained in collaboration with Daniela Dasler and the respective publication is already prepared.

Figure 3.4.1: Synthesized ferrocene derivatives; (1) Ferrocenyl-5-bromopentylketone; (2) 4-Iodobutylferrocene.

Besides the illustrated molecules, a variety of similar compounds bearing the ferrocene moiety (bromomethylferrocene, ferrocenium hexafluorophosphate) were investigated 108

in order to reach a successful attachment to the lattice of the respective allotrope. Nevertheless, the two presented molecules proved to yield in the best outcome for the covalent addition of both materials. Therefore, these two ferrocene derivatives provide the ideal base to evaluate and compare the degree of functionalization deriving from the results of various established characterization methods.

Scheme 3.4.1: Reaction sequence illustrating the reductive functionalization of CNTs and graphene in bulk with two different ferrocene derivatives 1 and 2.

For that purpose, pristine HiPco SWCNTs and spherical graphite (SGN18) were reduced by a stoichiometric amount of potassium, yielding the respective intercalation compounds (KC4 for SWCNTs, KC8 for graphene). These highly reactive intermediates were dispersed in absolute THF and sonicated in order to generate delaminated allotropide dispersions. After the successful synthesis of the compounds, one equivalent of ferrocenyl-5-bromopentylketone (1) or 4-iodobutylferrocene (2) was added to the dispersions, respectively and stirred under inert gas atmosphere for 1 d. The benefit of the wet chemical functionalization approach is that the generated radicals can attack in a ditopic fashion to reduce the lattice strain originating from the covalent addition (Scheme 3.4.1).[177] Unreacted residual charges on the ferrocene- functionalized CNTs or graphene are quenched with benzonitrile as described in the previous chapters. The charge uptake by benzonitrile inhibits side reactions with

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oxygen and water during the work-up of the respective carbon allotrope, which could be proven in Chapter 3.1. All samples were thoroughly washed with water and several organic solvents in order to remove byproducts and non-covalently attached molecules. The resulting functionalized materials were first analyzed by SRS. All CNT samples were measured with laser excitation wavelengths of 532 nm, 633 nm and 785 nm and all graphene samples with 532 nm. Additionally, as mentioned, several other methods including TG-MS, TG-GC-MS, and EDS were considered. Independently from SRS, it is possible to confirm a successful functionalization by the use of TG-MS and TG-GC-MS measurements. With the aid of these methods, we can observe different mass fragment patterns of the molecules, which are cleaved at certain temperatures by MS. Additionally the GC can clarify the origin of the cleaved fragments. Furthermore, EDS measurements can provide useful information about the chemical/elemental composition of the hybrid material.

3.4.1 Graphene Functionalization

First, the Raman results of the functionalized graphene will be discussed now. In the case of graphene, a prominent D-band at ~1327 cm-1 arises for the ferrocenyl-5- bromopentylketone (1)-functionalized G-1 indicating a successful sp3-defect introduction (Figure 3.4.2 A/B). Therefore, G-1 exhibits an increase of the mean

ID/IG-ratio of 0.61 with respect to 0.17 for the starting material graphite. The 4-iodobutylferrocene (2) functionalized sample G-2 shows a remarkable D-band at ~1328 cm-1 (Figure 3.4.2 C/D).

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Figure 3.4.2: SRS/mean spectra (λE=532 nm) of G-1 (A) and G-2 (C) in comparison to pristine graphite (SGN18, black lines); Histogram presentation of G-1 (B) and G-2 (D) in comparison to pristine graphite (SGN18), average spectra y-shifted for clarity.

The increase of the ID/IG ratio from 0.17 in pristine graphite to 0.96 for G-2 confirms the presence of a conversion of sp2 to sp3 carbon atoms suggesting an eminently high addition pattern as well. Furthermore, the histogram presentation reflects a wider distribution of defects in a Gaussian-like shape in case for G-1, in contrast to G-2. In both samples, a significant shift of the histogram from the starting material to the functionalized samples is existent (Figure 3.4.2 B/D). Additionally, the broadening of the first order Raman modes, emphasizing especially the D- and G-band, originate from the decline of individual modes (e.g. D*-mode) as a result of the occurred chemical modification in both experiments. A viable approach to determine the degree of functionalization (DoF) is given by correlating the mean defect distance of densely packed point defects and the Raman ID/IG ratio directly acquired from the SRS using the Cançado-curve, as mentioned in the previous chapter.[117] This is true for remarkably low degrees of functionalization exhibiting high distances of defects (LD), which results in a primary activation of D- and D*-modes. These modes reveal a narrow width and therefore enable the determination of the DoF. In this case, however, the 111

respective D-band pictures itself very broad for both samples involving underlying Raman modes hindering the exact evaluation of the specific degree of functionalization.[168a] As a consequence, an unambiguous statement about the amount of newly introduced defects cannot be made. Additionally, Raman spectroscopy is a more macroscopic characterization tool, hence, further analysis was performed with supplementary characterization methods to obtain additional qualitative and quantitative information.

Therefore, the produced graphene samples were analyzed with TG-MS and TG-GC-MS and Figure 3.4.3 shows the results of the corresponding reactions with the diverse ferrocene compounds in bulk. In both cases, a relatively high mass loss accompanied by the appearance of specific mass fragments can be clearly observed, which are related to the cleaved ferrocene derivatives (Figure 3.4.3 A/B). The functionalized graphene samples exhibit a total mass loss of ~10.5 % for G-1 and ~11 % for G-2, respectively. Specific mass fragments for covalently bound ferrocenyl- + + + 5-bromopentylketone with their m/z values of 39 (C3H3 ), 40 (C3H4 ), 41 (C3H5 ), + + 56 (Fe ), and 66 (C5H6 ) are mainly deriving from the fundamental ferrocene structure. The total compound including the spacer group could not be detected as structure decomposition occurs too accelerated. The graph (Figure 3.4.3 A) shows two main slopes, which can be explained by an alongside occurring process, precisely the conintercalation of THF within the graphene layers while restacking during the work- up procedure. Therefore, the first slope merely refers to the detachment of the solvent indicated by two characteristic mass traces of THF (m/z 39, 41) appearing in the low temperature region from 200-300 °C which, however, resemble fragments of the ferrocene structure as well. The additional slope at 400-550 °C derives from the cleavage of the functional group implied by the appearance of the mass fragments of the respective ferrocene addend. For G-2 the specific mass fragments of the basic + + + ferrocene structure could be detected (m/z 56 (Fe ), 65 (C5H5 ), 66 (C5H6 ) and 121 + (FeC5H6 )) arising from 200-550 °C indicating the successful functionalization.

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Figure 3.4.3: TG-MS profiles of G-1 (A) and G-2 (B) including the characteristic mass traces of the respective ferrocene addends in comparison to pristine graphite (gray dotted line). The blue square highlights the temperature area, in which the respective functional group is cleaved. GC elugrams of G-1 (C) at an injection temperature of 450 °C, and G-2 (D) at a injection temperature of 230 °C revealing cointercalated THF as main peaks.

Concerning the cointercalation, Figure 3.4.4 highlights the definite mass fragments of THF appearing in the temperature frame form 200-500 °C proving the ubiquitous presence of the solvent. The cleavage of the attached ferrocene moiety starts at low temperatures (~200 °C) for G-2 but lasts even until higher temperatures are reached (~550 °C) implied by the appearance of the specific mass traces at these temperatures. Anyway, to be precise, the main removal of the cointercalated solvents occurs in the temperature frame from 200 until 400 °C indicated by the highest ion currents of the respective mass fragments.

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Figure 3.4.4: TG-MS profile of G-2 including mass traces of THF (m/z 39, 41, 42, 72) in comparison to pristine graphite (grey dotted line).

Considering that a specific mass loss value correlates with the detachment of the molecules, the determination of the degree of functionalization is theoretically feasible. Using the Equation 3.4.1 by inserting the corresponding values of G-1 and G-2, following degrees of functionalization could be calculated.

∆푚(퐹퐺)∙푀(퐶) 퐷푒푔푟푒푒 표푓 퐹푢푛푡푖표푛푎푙푖푧푎푡푖표푛 (퐷표퐹) = x 100 Equation 3.4.1 푚(퐶)∙푀(퐹퐺)

In this equation, FG stands for functional group which corresponding mass loss is utilized to determine the desired values, whereas M (C or FG) equalizes the molar mass of carbon (12.01 g/mol) or of the respective addend. The value of m(C) concludes from the residual mass of carbon less the present value of m(FG) and the pristine mass loss. Inserting the distinct values of G-1 and G-2, we can determine a DoF(G-1) = 0.27 % and DoF(G-2) = 0.54 %, respectively. Nonetheless, the presence of THF falsifies the exact evaluation of m(FG) as the mass traces of both components overlap in both samples, and therefore, contributes significantly to the obtained mass loss.

To deliver proof about the successful addition of the addends, an additional feature of analysis was used to provide further information, namely gas chromatographic analysis. As shown in the previous chapter, this supporting characterization tool enables the determination of the nature of cleaved fragments emerging from the corresponding samples. At a certain temperature (G-1: 450 °C, G-2: 230 °C) at which the signature mass fragments of the addends start to cleave, the injection of a gas sample of the gaseous decomposition products to the GC column occurs followed by 114

the chromatographic separation. In case of the graphene samples, both GC elugrams (Figure 3.4.3 C/D) merely show one dominant peak deriving from the solvent THF between the restacked layers confirming the conintercalation. This undesired cointercalation process indeed takes place during the work-up procedure but cannot be avoided, and clearly impedes the detailed characterization of the products. For both samples, the peaks of THF can be observed at a similar retention time as a main fragment of the ferrocene structure, specifically cyclopentadiene, should appear (around 4 min) making the observation of cyclopentadiene in the spectrum impossible in either case. This technique also confirms the presence of THF in the mass fragment pattern of the second slope at around 450 °C for the sample G-1. Nonetheless, this method represents an additional qualitative proof for the successful functionalization but lacks in quantitative exploration and is significantly depending on the proportions of the containing compound mixture hindering concrete proof. Since one component of the sample is much higher on a quantity basis (THF>FG), the separation is extraordinarily complex in this case.

As a matter of fact, we need extra methods to gain more knowledge about the degree of addition, therefore, several EDS measurements were performed. Energy dispersive X-ray spectroscopy (EDS) represents a useful tool to determine the chemical composition of the samples. Moreover, particularly heavy hetero atoms can be probed perfectly, which enables the determination of specific non-carbon atoms as a consequence. Table 3.4.1 shows the resulting values (Wt%) after measuring a statistical map of 250x250 µm size of G-1 and G-2 providing quantitative details. Regarding the graphene samples G-1 and G-2 a significant increase of the amount of Fe in comparison with the pristine graphite (0.03 %) can be stated (G-1: 2.05 %, G-2: 0.55 %) confirming the presence of Fe containing groups.

Table 3.4.1: EDS-measurements containing the chemical composition of all samples.

element [Wt%] C O K Fe

graphite 95.9 3.9 - 0.03

G-1 86.7 8.12 2.55 2.05

G-2 92.2 5.76 1.25 0.55

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Hence, this method enables another qualitative confirmation as a consequence of the covalent addition of the specific ferrocene molecules. As a result of the usage of carbon tape as sample background the amount of carbon can be neglected. Besides this, oxygen and potassium values are enhanced deriving from the prior reduction step with the metal and moreover, the reaction of possible residual charges on the graphene lattice with oxygen in the air forming hydroxyl-groups or simply adsorbed water on the surface.[78, 103] Figure 3.4.5 and Figure 3.4.6 illustrate the complete atomic distribution of the various elements on the measured spot for G-1 and G-2, respectively, accompanied with the resolution of every individual element that has to be considered.

Figure 3.4.5: SEM picture (A) illustrating the atomic distribution of G-FcOHexBr (G-1), including K (B), O (E), C (D), and Fe (C).

Figure 3.4.6: SEM picture (A) illustrating the atomic distribution of G-FcButI (G-2), including K (B), O (E), C (D), and Fe (C).

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Considering the iron distribution for both samples (Figure 3.4.3 C; Figure 3.4.4 C) a homogenously distributed arrangement with few island-like addition patterns can be observed, whereas, the intensities for G-2 are significantly higher, corresponding to the results of the comparison via Raman spectroscopy. Having all these data in hand, the covalent addition of both individual ferrocene derivatives to the surface of graphene in dissimilar extent is evidently proved but merely two of these characterization methods can give evidence about the amount of attached groups, namely the degree of functionalization. Before summarizing the findings, the functionalization of SWCNTs will be shown in order to compare all results together conclusively.

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3.4.2 SWCNT Functionalization

First, the resulting functionalized SWCNTs were analyzed by SRS. All CNT samples were measured with three different laser excitation wavelengths (532 nm, 633 nm, 785 nm) in order to characterize all kinds of CNTs including semiconducting and metallic species. Metallic CNTs are in resonance using the excitation wavelength of 532 nm, whereas, semiconducting are selectively resonant with the laser of 785 nm. In this case, a remarkable D-band emerges at around 1320 cm-1 for the ferrocenyl-5- bromopentylketone-functionalized CNT-1 for all kinds of laser excitation wavelengths indicating a high functionalization accompanied by a homogenous reaction outcome (Figure 3.4.7 A/C/E). The reaction occurs for both species in similar extent implied by the marginal deviating ID/IG-ratio values. The ID/IG-values merely range from 0.46 for semiconducting CNTs over 0.48 for both species to 0.50 for metallic CNTs. The highest

ID/IG-ratio for metallic tubes (0.50) accompanied with a diminishment of the intensities of the corresponding RBMs suggest a slightly selective reaction (Figure 3.4.7 A). All spectra show a significant shift of the histograms of the functionalized material in contrast to pristine graphite (Figure 3.4.7 B/D/F). The semiconducting CNT-1 shows the widest distribution in the histogram presentation but all samples do not vary significantly and show a similar behavior. In Figure 3.4.8 the Raman results for the analysis of the 4-iodobutylferrocene-functionalized CNT-2 are illustrated. A prominent D-mode arises for all measured samples, in particular for CNT-2 measured with an excitation wavelength of 532 nm exhibiting the highest ratio of D- to G-band

(ID/IG = 0.74) combined with a significant decrease of the intensities of the RBMs pointing to a slight selectivity towards metallic CNTs.[145] The histogram presentation confirms this observation depicting the largest shift in comparison with the other spectra. This diminishment of RBM intensities can be observed for the semiconducting CNTs as well. However, the altering of absolute Raman intensities is difficult to quantify and depends mainly on experimental conditions (sample morphology, focusing conditions).[145] Generally, the specific histograms of the different laser wavelengths display a Gaussian-like distribution of the functionalized nanomaterial (Figure 3.4.8 B/D/E). Hence, the results point to a successful and slightly electronic type selective reaction of functionalized material in high yields.

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Figure 3.4.7: SRS/mean spectra and histogram presentation of ferrocenyl-5-bromopentylketone- functionalized CNT-1 using three different laser excitation wavelengths: 532 nm (A/B), 633 nm

(C/D), and 785 nm (E/F); Spectra show the increasing ID/IG ratio indicating the successful functionalization, in comparison to pristine SWCNTs and graphite (SGN18), average spectra y- shifted for clarity.

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Figure 3.4.8: SRS/mean spectra and histogram presentation of 4-iodobutylferrocene- functionalized CNT-2 using three different laser excitation wavelengths: 532 nm (A/B), 633 nm

(C/D), and 785 nm (E,F); Spectra show the increasing ID/IG ratio indicating the successful functionalization, in comparison to pristine SWCNTs and graphite (SGN18), average spectra y- shifted for clarity.

To confirm the effective addition, TG-MS and TG-GC-MS measurements were performed. The TG-MS analysis of CNT-1 and CNT-2 is demonstrated in Figure 3.4.9 A and B, respectively. A similar total mass loss for both samples can be detected (CNT-1: 27.5 %, CNT-2: 25.5 %).

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Figure 3.4.9: TG-MS profiles of CNT-1 (A) and CNT-2 (B) including the characteristic mass fragments of the respective ferrocene addends depicting the mass losses. The blue square highlights the temperature area in which the respective functional group is cleaved. GC elugrams of G-1 (C) containing the corresponding mass spectrometric characterization of cyclopentadiene with its specific fragmentation pattern (red square), and CNT-2 (D). The peaks can be assigned to cyclopentadiene and benzonitrile. The observed low-intensity peaks (*) can be traced back to fragments of different siloxanes being components of the polysiloxane-coated columns of the GC-Clarus system.

Additionally, in both spectra specific mass fragments of the basic ferrocene structure + + + + + are detected (CNT-1: 39 (C3H3 ), 40 (C3H4 ), 41 (C3H5 ), 56 (Fe ), and 66 (C5H6 ) and + + + + CNT-2: 56 (Fe ), 65 (C5H5 ), 66 (C5H6 ), and 121 (FeC5H6 )) which appear in the temperature frame of 300-500 °C for CNT-1 and at 200-550 °C for CNT-2 indicating the successful covalent functionalization for both derivatives. As already demonstrated for the graphene samples, the whole ferrocene moiety decomposes and therefore only fragments of the specific structures could be observed. Considering the ion currents of both allotropes, the values for modified CNTs range in 10-11 A in contrast to the graphene samples exhibiting values in pikoampere regions, which allows the postulation of a higher functionalization rate for SWCNTs. In consideration of the analyzed amount, the respective molar masses and if the mass loss is referred only at cleaving temperatures accompanied with the mass fragments of the desired molecule, 121

the degree of functionalization could be determined as DoF(CNT-1) = 0.83 % and DoF(CNT-2) = 1.13 % using the Equation 3.4.1 to calculate the respective DoF. Remaining non-covalently bound or adsorbed molecules on the carbon lattice can be still included, which has to be considered. Due to the weaker bond strength of non- covalently bound molecules like PhCN for SWCNTs in this case, the cleavage should normally start at lower temperatures. PhCN traces could be clearly detected in both reactions and is impossible to remove by washing or drying due to the strong interactions of the molecule with the π-system of the CNTs. However, it can falsify the estimation of the degree of functionalization in a significant extent. Figure 3.4.10 illustrates the mass fragmentation patterns of the desired cyclopentadiene and benzonitrile evolving from the GC-MS results.

Figure 3.4.10: Mass fragmentation pattern of benzonitrile (A) and cyclopentadiene (B) taken from GC results of CNT-1.

In analogy to the functionalized graphene samples, TG-GC-MS measurements were recorded to find clear evidence for the cleavage of the ferrocene derivatives upon thermal treatment. The additional combination with gas chromatography allows the discrimination of cleaved fragments with similar m/z values and fragmentation pattern by their retention times. For both samples the injection temperature of 300 °C was chosen and the resulting GC elugrams of the cleaved and gaseous decomposition products are depicted in Figure 3.4.7 C and D for CNT-1 and CNT-2, respectively. The elugrams for the CNT samples feature two dominant peaks with varying intensities besides less intense peaks being siloxanes deriving from the column marked with a asterisk (*). The main peaks at 4 min originate from the cyclopentadiene ring of the ferrocene which corresponding mass spectrometric characterization as well as its specific fragmentation pattern is additionally shown in Figure 3.4.7 C. Non-covalently

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attached PhCN from the work-up procedure represents the second distinct peak in the spectra. Nonetheless, this method represents an additional qualitative proof for the successful functionalization.

Furthermore, these samples were also investigated using EDS to gain quantitative and qualitative knowledge about the amount of iron in the CNT samples. Table 3.4.4 illustrates the mass percentage (Wt%) of the samples presenting a distinct increase of iron in both samples in comparison to pristine HiPco SWCNTs (CNT-1 = 20.4 %; CNT-2 = 22.4 %).

Table 3.4.1: EDS-measurements containing the chemical composition of all samples.

element [Wt%] C O K Fe

HiPco CNT 75.7 6.53 - 13.0

HiPco CNT-1 63.6 14.3 1.22 20.4

HiPco CNT-2 67.8 14.8 1.39 22.4

This increase can be related to the newly attached ferrocene moieties on the surface of the specific SWCNTs. The SWCNTs already exhibit a relative high value for iron due to the usage of iron as a catalyst in the production process of the HiPco nanotubes but the increase is still significant, anyhow.

Figure 3.4.8: SEM picture (A) illustrating the atomic distribution of CNT-FcButI (CNT-2), including K (B), O (E), C (D) and Fe (C).

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Figure 3.4.9: SEM picture (A) illustrating the atomic distribution of CNT-FcOHexBr (CNT-1), including K (B), O (E), C (D), and Fe (C).

As already observed for the functionalized graphene samples, the increased amount of potassium can be referred to alkaline metal traces from the prior reduction steps, which cannot be completely removed by washing. The water content derives from on the surface adsorbed water or due to formed OH-groups as described above. In Figure 3.4.8 and Figure 3.4.9 the distribution of the mass percentages across the whole measured sample can be seen including the mentioned mass traces listed in Table 3.4.1. In particular, the Fe content (Figure 3.4.8 C, Figure 3.4.8 C) can be explicitly observed over the whole focussed spot being very intense.

Additionally, pristine CoMoCat nanotubes (CoMoCat-CNT) were functionalized with ferrocenyl-5-bromopentylketone to yield in modified CoMoCat nanotubes CoMoCat-CNT-1. The advantage of the synthesis of CoMoCat-CNTs is that the presence of Fe catalysts is avoided and replaced by cobalt and molybdenum catalysts. To prove the successful attachment of the ferrocene derivative 1 to the carbon lattice of the CoMoCat-CNTs, Raman spectroscopy, TG-MS, and TG-GC-MS were applied at first. The complete analysis of the specific sample is depicted in Figure 3.4.10 containing SRS (A/B), TG-MS (C), and the respective GC elugram (D).

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Figure 3.4.10: A) SRS/mean spectra (λE=633nm) of functionalized CoMoCat-CNT-1 with respect to pristine CoMoCat CNTs, spectra y-shifted for clarity; B) histogram presentation of respective

ID/IG-ratios; C) TG-MS profile of functionalized CoMoCat-CNT-1 including characteristic mass fragments of ferrocenyl-5-bromopentylketone; (C) GC elugram of the functionalized CoMoCat- CNT-1 exhibiting cyclopentadiene at 4.4 min as main fragment of the ferrocene structure, whereas the two main peaks (*) can be traced back to fragments of different siloxanes being components of the polysiloxane-coated columns of the GC-Clarus system.

The sample was measured with a laser excitation wavelength of 633 nm in which both species of CNTs are in resonance. The appearance of a prominent D-mode yielding an ID/IG-ratio of 0.81 indicates the functionalization with the ferrocene derivative in high extent. Moreover, the histogram presentation shifts clearly to higher values in comparison to pristine material. These aspects are verified by the TG-MS and the TG- GC-MS results (Figure 3.4.10 C/D). A total mass loss of around 29.5 % accompanied + by the presence of the desired mass fragments of the ferrocene unit (39 (C3H3 ), + + + + 40 (C3H4 ), 41 (C3H5 ), 56 (Fe ), and 66 (C5H6 )) ranging from 250-500 °C in the covalent bond area, draws the conclusion of the successful covalent addition. The specific mass loss deriving from the functional group has a value of around 20.4 % illustrated by the blue area in Figure 3.4.10 C. Subsequently, the degree of

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functionalization could be determined as 1.17 % using the Equation 3.4.1. Furthermore, the fundamental ferrocene substructure cyclopentadiene could be found in the specific GC elugrams in accordance to the findings in CNT-1 and CNT-2. These data provide comprehensible evidence for the covalent functionalization. Furthermore, EDS measurements could confirm the successful addition, besides the presence of Mo and Co, visible in their considerably raised values in the pristine material as well as in the modified sample (Table 3.4.2).

Table 3.4.2: EDS-measurements containing the chemical composition of all samples.

element [Wt%] C O K Fe Co Mo

CoMoCat CNT 87.1 6.29 0.01 0.03 2.02 4.53

CoMoCat CNT-1 65.8 13.3 1.53 16.7 1.18 1.44

In case of the functionalized CoMoCat-CNT-1, a remarkable increase from 0.03 % to 16.7 % of Fe amount in comparison to pristine material can be stated suggesting the effective covalent addition to the CNT lattice. The coverage with Fe can be observed for the CoMoCat-CNTs functionalized with the FcOHex-moiety in Figure 3.4.10 as well displaying the distribution of the respective elements, like the catalysts Mo and Co in addition to the newly introduced marker Fe.

Figure 3.4.10: SEM picture (A) illustrating the atomic distribution of CoMoCat CNT-FcOHexBr including Mo (B), O (E), Co (D), and Fe (C).

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3.4.3 Comparison of Additions

Having all these data in hand, the successful functionalization of graphene and SWCNTs with two diverse ferrocene marker molecules could be proven using a simple, rapid, and efficient route. This type of functionalization paves the way for various application fields taking advantage of the extraordinary properties of the organometallic ferrocene.[197c] Thus, promising areas of application like the usage of the functionalized allotropes for instance as electrochemical sensors are available in large extent.[197c, 200] A comparison of the degrees of functionalization of these samples was carried out with established tools for characterization and could therefore determine a specific extent of functionalization for each method. Anyhow, in contrast to the initial intention exact values cannot be clearly defined for the modified allotropes due to the different findings of each method. Even though the values differ significantly for each characterization tools, it is possible to establish a reliable functionalization range. In the case of CNT- 1, a range of 0.83-7.40 %, for G-1 0.27-2.02 % and for CNT-2 1.13-9.40 % could be stated, additionally, the values for the functionalized CoMoCat CNTs-1 range between 1.17-16.7 %. Only for G-2, the received values vary merely weakly (0.53-0.54 %). Remarkably, it is evident that SWCNTs were prone to have a higher degree of functionalization for both reactions. On the one hand, this can be elucidated by the present curvature of CNTs consistently increasing the reactivity in contrast to the planar structure of graphene. On the other hand, an insufficient exfoliation of the graphene sheets during the sonication leading to not exclusively individualized single sheets may have hindered the attack of the ferrocene derivatives to the corresponding surface of graphene. Additionally, as the physiosorbed THF could be identified as the largest part in both graphene samples, this minimized the accessible carbon surface for the attack of the resulted electrophiles in these reactions. Furthermore, all utilized characterization methods have advantages and drawbacks, which have to be considered while evaluating the exact degree of functionalization. Raman spectroscopy is a macroscopic tool and provides valuable information about the introduction of defects due to the detection of alterations in hybridization for SWCNTs and graphene. However, the comparison of both Raman results is rather complex as the Raman scattering occurs dissimilarly impeding the exact quantitative determination. As described in the introduction, in case of functionalized graphene, it exhibits unambiguously a non-linear ”Cancado-like” behavior, implying higher ID/IG- 127

ratios symbolize higher functionalization degrees until a certain extent is reached [160] (~1%) and proceeds vice versa for higher additions. This correlation between LD and the corresponding ID/IG-ratio values enables a quantitative determination of the functionalization degree but merely for functionalized graphene samples revealing high [117] values for LD, connoting graphene in the low-defect area. For higher addition patterns alongside for broad natures of the D-bands implying additional underlying modes, the evaluation is impossible.[168a] According to recent literature, SWCNTs act similar to graphene with respect to the correlation of the increasing ID/IG ratio with merging degree of functionalization values up to a resembling value of 0.9 %, in the case of the addition of diazonium compounds.[201] Several reports in literature demonstrate a similar behavior for SWCNTs.[145, 202] Problematically, the respective

ID/IG-ratio values significantly depend on the applied laser wavelengths impeding an exact determination of the DoF in addition. This happens due to a present electronic type selectivity by different chirality, metallicity, and diameters, which are probed with varying laser wavelengths, strongly complicating a comparison of the diverse tube types. While TG-MS profiles yield an insight into quantitative issues, this can be intensely influenced by non-covalent interactions or adsorbed compounds falsifying the exact values. Therefore, it is a limited analytical method, particularly when several molecules are cleaved from the sample and detected concurrently at the same temperature, resulting in an undesired overlay of the MS signals. Anticipating that only one specific molecule is cleaved and the detected fragments merely derive from that, it is possible to determine a value for the degree of addition. The supporting gas chromatographic analysis can confirm qualitative issues by an exact discrimination of the cleaved organic molecules, which were questioned in TG-MS profiles, but lacks in quantitative evidence. On the one hand, the determination of a certain value is theoretically feasible since the peak areas of resulting elugrams correlate with the amount of evolved gas.[79a] On the other hand, however, this requires a series of reference experiments, which have to be conducted at first, demanding a sufficient amount of educts at hand. These are a key prerequisite for a suitable calibration curve in order to perform a possible quantification of the attached moieties. Nevertheless, the specific peak area can provide useful information about the dimensions of additions while comparing identical samples with varying injection temperatures. In this case, this possibility was excluded as the applied ferrocene precursors were liquids of limited yields. Thus, this technique provides the possibility for an ambiguous identification of

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investigated samples. Regarding EDS, the corresponding values are significantly depending on the measured sample spot and the utilized accelerating voltage. Furthermore, the results of this technique can also be falsified by possible adsorbed residues, which does not allow the quantitative determination of the functionalization. Taking all advantages and drawbacks of the used tools for characterization into consideration, the determination of the exact functionalization degree remains a challenging task due to tremendous differences in the operating methods. This is the reason why the calculated degrees of functionalization can differ significantly in literature generally. Therefore, it is indispensable to specify a reliable range of the respective values for CNTs and graphene after the characterization with several methods as well as weighing the informative value of each method.

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3.5 Halogenation of Carbon Allotropes 3.5.1 Reductive Halogenation of Carbon Allotropes

The results concerning the reductive halogenation of SWCNTs presented in this chapter have been carried out in collaboration with Milan Schirowski. In the large field of the covalent grafting onto the surface of carbon allotropes, specifically SWCNTs and graphene, electron-withdrawing groups like halogens are of great interest alongside various organic compounds. In order to improve the electronic properties accompanied with feasible reversible halogen storage or introducing the possibility of follow-up chemistry with substituted new moieties, halogenation of these materials became a crucial goal for researchers starting with the fluorination of CNTs in 1998.[60] Besides, the grafting of halides assists in the conversion from metallic to semiconducting properties of SWCNTs, allowing for the regulation of their electronic behavior. SWCNTs and graphene modified with chlorine, bromine, iodine, and fluorine are available by different methods but, however, the majority of these methods employs the usage of gaseous compounds in complicated and elaborate experimental settings. The focus of this work lies on the incorporation of bromine and chlorine in a simple and easily reproducible fashion. Even though fully chlorinated graphene was theoretically investigated, it could not be proven experimentally yet.[131b] At this point, graphene was chlorinated by either exfoliation of graphite oxide in the quartz reactor using Cl2/N2 plasma or applying photochemistry.[129-130] Partial chlorination could be achieved by [203] treatment with liquid Cl2 and CCl4 irradiated by UV light. Moreover, atomically precise edge functionalization of nanographenes by chlorine was reported in 2013.[132]

Chlorination of SWCNTs was reached by UV photolysis of Cl2 leading to a covalent addition.[204] Bromination of graphene was accomplished for instance by thermal reduction in gaseous bromine atmosphere.[129, 203] UV dissociation of gaseous HBr resulted in the bromination of SWCNTs to a minor extent.[204] In contrast, a promising and wet-chemical approach has been published using initial reductive activation followed by in situ addition of the halogen and possible follow-up substitutions by suitable nucleophiles.[63] These reactions enable the synthesis of “global” precursors which afford an enormous potential for every kind of follow-up chemistry and therefore provides plenty of new possibilities. Another advantage stems from the simplicity and efficiency of the applied method.

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3.5.1.1 Halogenation of SWCNTs

In this chapter, a facile and fast method to chlorinate SWCNTs under reductive conditions will be investigated and compared to the already existing reductive bromination route[63] including the exploration of further substitution attempts. Additionally, two different reductive activation routes will be opposed and compared, namely the Billups reaction under modified Birch conditions[72, 109] and the solid-state reduction with elemental potassium.[78] Furthermore, the method was adapted to another carbon allotrope, graphene followed by a substantial characterization with several methods.

Scheme 3.5.1: Modified Birch/ Billups-type reduction of SWCNTs and subsequent halogenation

with either Br2 or ICl.

As already shown in one of our previous publications, a reductive route in order to brominate SWCNTs is available.[63] The conditions of the reaction were altered leading to successful substitutions with different kinds of strong nucleophiles. Using this approach with SWCNTs and another interhalogen compound, namely iodine monochloride, results in another novel halogenated nanomaterial, chlorinated SWCNTs (Scheme 3.5.1). To achieve this, pristine HiPco SWCNTs with potassium dissolved in liquid ammonia dispersed in the desiccated and degassed solvent THF yielding in highly reactive nanotubides. In this reaction, the ratio of 5 mmol of potassium or lithium to 1 mmol of carbon were utilized to obtain the negatively charged

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intermediate. Subsequently, iodine monochloride was added to the dispersion SWCNTs in THF yielding reactive chlorinated intermediates after a reaction time of 24 h. A subsequent washing step under inert conditions with THF removed the residual halide enabling the following substitution. A small portion of the reactive, halogenated intermediate was retained for TG-MS and Raman experiments to confirm the addition of the halide. In order to perform the Raman characterization, the powder was dried under vacuum and subsequently transferred to a glass ampoule, which was sealed under high vacuum afterwards. Figure 3.5.1 is depicting the recorded TG-MS spectra of the brominated and chlorinated SWCNTs alongside the statistical Raman analysis of the samples.

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Figure 3.5.1: TG-MS profiles and SRS/mean Raman spectra (λE=633nm) of the halogenated SWCNTs; A) TG-MS profile of the brominated SWCNTs including the mass fragments of the bromine isotopes and the respective HBr in comparison to the pristine mass loss; B) TG-MS profile of the chlorinated SWCNTs including the mass fragments of the chlorine isotopes and the respective HCl in comparison to the mass loss of the pristine material; SRS using the excitation wavelength 633 nm of the brominated SWCNTs (C) and the chlorinated SWCNTs (D) in comparison to pristine material (black). The temperature area in which the functional group is cleaved is highlighted in blue.

The spectra clearly demonstrate a significant mass loss (~20 % for SWCNTs-Br, ~16 % for SWCNTs-Cl) in the temperature range from 200 to 300 °C in both cases accompanied with the appearance of the MS ion currents for the halides and their protonated form (HBr, HCl) in the characteristic isotopic distribution. In the case of bromine, the natural isotopic pattern of m/z 79 to m/z 81 is 1:1, which correlates exactly to the mass traces detected experimentally. Their protonated form HBr+, m/z 80 and m/z 82, behaves alike, as the peaks are virtually superimposed. The ratio of the chlorine isotopes would be expected to be 3:1 (m/z 35 to m/z 37), which is in good agreement with the present results. In contrast, no iodine traces (m/z 127) could be detected at all, which indicates that an iodination of SWCNTs did not occur. This is 133

also in accordance with the higher C-Cl bond strength (84 kcal/mol) compared to C-I (56 kcal/mol).[205] This fact also refutes the possibility of merely physisorbed ICl on the SWCNT sidewall. Anticipating that the entire mass loss derives from the covalently bonded halogen, the corresponding degree of functionalization amounts for 3.8 % and 11.3 % for sample SWCNT-Br and SWCNT-Cl, respectively, using the Equation 3.4.1 from the previous chapter. The increase of the pristine mass is deriving from fluctuations of the ultra-high scale in the instrument. Furthermore, the Raman spectra yield insight into the success of the reaction. Generally, the addition of sidewall moieties leads to an increased D-band compared to the G-band (~1580 cm-1). The present spectra (Figure 3.5.1) confirm a successful functionalization with the respective halide showing a significant increase of the D-band (~1310 cm-1) in both cases (ID/IG=0.48 for SWCNT-Br, ID/IG=0.36 for SWCNT-Cl). Most likely, the presence of residual potassium bromide or iodide is the reason for the uneven shape of the Raman spectra. More thorough washing steps including water, however, were not feasible due to the reactivity of the halogenated compounds. The samples were analyzed under air exclusion without washing steps with water to prevent further substitutions. The obtained powders exhibit a grey color suggesting the formation of the side product KBr/KCl, respectively.

3.5.1.2 Substitution of halogenated SWCNTs

As previously shown the brominated CNTs are able to undergo substitution reactions with nucleophiles like water and 1,1,1-trifluoroethanolate. A halogenated SWCNT compound, which is generally prone to substitution with all kinds of nucleophiles, could be of tremendous interest for future nanomaterial functionalization sequences as “global” precursor. Therefore, further experiments to screen the reactivity towards common nucleophilic compounds were conducted. Hence, halogenated CNTs were produced as described above and then transferred to a new flask with absolute THF and readily dissolved nucleophiles. After 16 h of reaction time, the SWCNTs were filtered, washed and dried under vacuum. In Scheme 3.5.2 an overview of the successful substitution reactions of the brominated SWCNTs is given.

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Scheme 3.5.2: Successful substitution reactions of brominated SWCNTs (SWCNTs-Br) with various nucleophiles: a) morpholine, b) sodium trifluoroethanlolate[63], c) sodium decanthiolate, d) sodium octanolate.

Thus, the usage of rather strong nucleophiles like alcoholates and thiolates (n-octanolate, n-dodecanthiolate, 1,1,1-trifluoroethanolate) led to a substitution as well as aliphatic amines like morpholine, whereas the respective alcohols, anilines, and thiols did not react, possibly due to a lower nucleophilicity. Thus, amines (tert-butylphenylanline), phosphines (triphenylphosphin) as well as alcohols and thiols (dodecanethiol, octanole, ethanole) were used as substitution reagents but with no success. Besides the substitution with the mentioned nucleophiles another attempt was to perform the lithium-halogen-exchange in order to replace the halide with Li leading to the corresponding CNT-Li derivative. A Grignard-like reaction using Mg with the goal to change the polarity of the carbon in order to make it nucleophilic was also possible but both attempts did not lead to a success and only resulted in hydroxylated

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products due to the substitution with water during the aqueous work-up instead of the desired reactants.

Figure 3.5.2: SRS/mean spectra of the halogenated SWCNTs and the respective products (SWCNTs-2: dodecanthiolate; SWCNTs-3: octanolate, SWCNTS-4: morpholine) after substitution in comparison to pristine SWCNTs with three different excitation wavelengths: 532nm (A). 633nm (B), 785nm (C), spectra y-shifted for clarity.

The highest degree of functionalization indicated by the highest ID/IG-ratio can be observed for the semiconducting SWCNTs, which are in resonance using a laser with 785 nm excitation wavelength (Figure 3.5.2 C). Anyhow, the D-mode of the brominated semiconducting SWCNTs pictures itself very broad complicating the determination of an exact ID/IG-value. The same is true for the RBM area of the brominated semiconducting CNTs which complicates the analysis of the corresponding RBM intensities. However, applying the 633 nm laser in which both nanotube metallicities are in resonance, the ID/IG-ratio value (ID/IG=0.48) increased in comparison to the value (ID/IG=0.35) of the metallic tubes. Remarkably, it is evident that the intensities of the RBMs of brominated metallic SWCNTs significantly decrease, in

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particular for small diameters (i.e. high wavenumber) breathing modes, pointing to an electronic type selectivity towards metallic tubes in a minor extent (Figure 3.5.2 A). Considering the shape of the spectrum of brominated semiconducting tubes, a clear statement about the selectivity is difficult to make since a substantial analysis of the respective Raman modes is rather impossible. After substitution, the ID/IG-ratio values remain similar for metallic and differ weakly for the CNTs excited by higher wavelengths, which can be explained by rearomatization of the π-system by an uncompleted substitution. Due to this reason, the intensities of the RBM increase as well for metallic tubes after substitution. The presence of the substituted moieties groups was confirmed by TG-MS (Figure 3.5.3).

Figure 3.5.3: TG-MS profiles of the products after substitution and further work-up procedure in comparison to the spectrum of pristine SWCNTs (dotted line); A) SWCNTs-4 depicting the mass traces of morpholine, B) SWCNTs-2 depicting the mass traces of dodecanethiolate, C) SWCNTs- 3 depicting mass traces of octanolate.

In the TG spectra, the respective mass fragments of the specific utilized nucleophiles can be obtained appearing at around 350 °C with the main cleavage occurring in the

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+ + + + case of the morpholine (m/z 29 (C2H5 ), 42 (C3H6 ), 57 (C4H9 ), 87 (C4H9NO )), and at + + around 270 °C until 500 °C for dodecanethiolate (m/z 43 (C3H7 ), 55 (C4H7 ), + 56 (C4H9 ), 69 (C5H9)), visible in Figure 3.5.3 A/B. The highest mass loss (~22 %) can be observed for the SWCNTs substituted with octanolate while its detachment occurs slightly later from 350 °C until 500 °C, implied by the appearance of the characteristic + + + mass fragments (m/z 42 (C3H6 ), 56 (C4H9 ), 84 (C6H12 )) int this temperature frame. These observations suggest the successful substitution of the halogenated CNTs and therefore the newly introduced covalent bonds. Merely physisorbed volatile compounds are known to detach at much lower temperatures, further corroborating that the attached compounds are bonded covalently.[79a]

Figure 3.5.4: SRS/mean spectra of the chlorinated SWCNTs and the products after the aqueous work-up in comparison to pristine SWCNTs with three different excitation wavelengths: 532nm (A), 633nm (B), 785nm (C); spectra y-shifted for clarity.

Figure 3.5.4 depicts the Raman analysis of the chlorinated SWCNTs and of the substituted products in comparison with the pristine material. In the case of the 138

chlorinated SWCNTs, a low selectivity towards semiconducting tubes can be also observed exhibiting an ID/IG-ratio of 0.57 in contrast to metallic SWCNTs with an

ID/IG-ratio value of 0.27 recorded using a laser with the excitation wavelength of 532 nm (Figure 3.5.4). With respect to the respective RBMs, the chlorinated intermediates show a relative decrease of the RBM intensities of small diameters breathing modes in all cases, which recover after substitution or rearomatization, respectively. Subsequently, the same substitution reactions were attempted on SWCNT-Cl. Surprisingly none of the performed reactions resulted in the desired sidewall- functionalized nanotubes. This different behavior can be explained by the varying binding strength of the halides as bromide is a better leaving group.[206] However, during the aqueous work-up step the chlorinated CNTs also react with the water as nucleophile yielding in hydroxylated material in similarity to the brominated CNTs.[63]

However, the significantly decreasing ID/IG-ratio values of the substituted samples compared to the values of the chlorinated intermediates can be referred to the rearomatization of the π-system by an uncompleted substitution to a large extent.

The precise reaction mechanism behind the substitution reactions has not been elucidated yet. Due to the difficulty of replacement by weak nucleophiles (anilines, alcohols) a SN1 mechanism seems unlikely. To elaborate a possible radical mechanism, an excess of a radical scavenger (ascorbic acid) was added to the reaction prior to the addition of the halides. In every reaction, the outcome remained identical with and without the radical scavenger, excluding a possible radical mechanism.

Scheme 3.5.3: Probable reaction mechanism of the halogenation of negatively charged carbon allotrope (A) and the subsequent nucleophilic substitution of the halogenated nanomaterial (B).

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Therefore, an SN2 mechanism seems very likely. However, especially in the case of SWCNTs, a backside attack is impossible, as the nucleophile would need to attack from the interior of the tube, resulting in an endohedral functionalization. Hence, by exclusion, the underlying reaction mechanism is likely to be a vinylogous Michael-type nucleophilic substitution. A proposed mechanism for the halogenation and subsequent nucleophilic substitution can be found in Scheme 3.5.3.

5.5.1.3 Halogenation of Graphene

Having elucidated the reactions around halogenated SWCNTs, these attempts were expanded onto graphene. Possible substitution reactions and altered electronic properties are extremely appealing for graphene as well. For this purpose, graphite was dispersed in absolute THF and mixed with potassium in liquid ammonia to yield reactive negatively charged graphenides. With these in hand bromine or iodine monochloride was added to the dispersion under completely inert conditions. Scheme 3.5.4 illustrates the corresponding halogenation sequence, including the unsuccessful chlorination in addition to the efficient bromination of the carbon system.

Scheme 3.5.4: Modified Birch/ Billups-type reduction of graphite to dispersed graphenides and

subsequent halogenation with Br2.

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In Figure 5.3.5, the TG-MS and SRS analysis of the graphenides treated with bromine is given. The mean Raman spectrum (Fig. 5.5.5A) unambiguously demonstrates the newly introduced defects indicated by the increase of the D-band (~1582 cm-1) up to an ID/IG-ratio of 0.60. Since the SRS analysis was performed directly after the reaction to avoid atmospheric degradation, merely washed with absolute THF, the D-band pictures itself very broad complicating an accurate determination of the ID/IG-ratio, probably due to additionally underlying Raman modes in the area between D- and G- band (1300-1500 cm-1). However, a decrease of the FWHM of the 2D-band (~2703 cm-1) is evident indicating the newly introduced functionalities.

Figure 3.5.5: SRS/Raman mean spectrum (λE=532 nm) (A) and TG-MS profile (B) of brominated graphene in comparison to graphite (black/ dotted line). The red line in spectrum B represents the mass loss of the functionalized material accompanied by the mass traces of bromine isotopes m/z 79 and 81 and HBr m/z 80 and 82, respectively.

The TG-MS profile of graphene-Br (Figure 3.5.5 B) gives a very similar picture as the halogenated SWCNTs. A mass loss of around 27.5 %, corresponding to a degree of functionalization of 5 %, can be observed accompanied by the appearance of the characteristic masses of bromine and HBr with the isotopic pattern 1:1. This suggests a successful reductive bromination of the graphene. Considering the fact that THF was also evolved in the reaction as solvent, the mass loss value can be also affected. Therefore, an exact evaluation of the degree of addition is rather complex. Surprisingly, the respective chlorination using ICl as functionalization agent failed. Substitution attempts with morpholine, thiolates, and alkoholates failed as well since their nucleophilicity seems to be too low. In order to find another suitable way to brominate/chlorinate graphene in bulk, graphenides were generated using Na/K alloy

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in DME as well followed by the addition of the halogen compound. Anyhow, this attempt failed probably due to the preferred reaction of bromine with the present metal alloy in the reaction mixture. Therefore, the bromination of the graphene is suppressed and side reactions with the charged intermediates predominate.

3.5.1.4 Comparison of Reductive Approaches

In order to get a better insight into the mechanism of the reaction, a comparison was carried out involving the most frequently used reductive route using elemental potassium and the route under modified Birch/ Billups conditions, whereas the method using Na/K alloy in DME failed to yield the halogenated allotropes. The decisive difference of these activation methods, aside from the additionally applied promotion agent ammonia is the used amount of the reductive agent. The Billups conditions give best results when 5 eq of Li or K with respect to nanotube carbon are applied, which leads to an uncontrollably high charging of the material in contrast to the stoichiometrically controlled charging of graphite or SWCNTs to KC8 or KC4. For the solid-state reaction, pristine SWCNTs and graphite were reduced by a stoichiometric amount of potassium (KC4 for SWCNTs, KC8 for graphite) in an argon-filled glovebox under strictly inert conditions yielding the respective intercalation compounds.

Figure 3.5.6: Vacuum set-up to perform in situ Raman measurements under strictly inert conditions emphasizing the sampler holder (red square) containing the carbon material and the liquid inlet (blue square).

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The absence of any co-reducing agents or solvent enables a real-time tracking of the reaction. Hence, these highly reactive intermediates were placed in an in situ setup (Figure 3.5.6) bearing a flat optical window of borosilicate glass, a liquid inlet for potential functionalization reagents connected to a turbopump to keep constant vacuum conditions (p<10-7 mbar) in order to perform in situ Raman investigations.[168a] The liquid inlet was filled with either bromine or ICl, respectively, for each allotrope. When opening the valve, the halogen compound can evaporate and the reaction between the intercalation compounds and the halogen vapor could be investigated. Thus, the respective experiments were conducted and the reaction was followed via Raman spectroscopy. Figure 3.5.7 depicts the obtained single-point spectra of pristine SWCNTs (Figure 3.5.7 a/e) and the with potassium intercalated nanotubides. (Figure 3.5.7 b/f), which exhibits a Fano-shaped curve (~1400-1600 cm-1) and the disappearance of the G-band confirming the successful intercalation of the CNTs.[168a]

Figure 3.5.7: In situ Raman measurements/single spectra of pristine SWCNTs (a/e), nanotubides (b/f), spectra of the nanotubides after the reaction with bromine (c) and ICl (g) before the exposition to air and after exposition to air (d/h).

After opening the valve to the functionalization agent, spectra were recorded for both SWCNT-Br (Figure 3.5.7 c) and SWCNT-Cl (Figure 3.5.7 g). Both spectra reveal diminished intensity for all signals, but for the nanotubides treated with ICl vapor a significant increase of the D-band (~1320 cm-1) can be obtained suggesting a high functionalization. A potential mass transport involving the potassium ions and the chlorine could have taken place forming the Cl-bonds to the carbon lattice and the respective KI, which is formed as grey powder as side product. The proof of Cl moieties in the bulk reactions with SWCNTs suggests that a chlorination occurred in this case as well. During the reaction of the negatively charged CNTs with the bromine vapor, a 143

strong reaction took place expressing in a shiny explosion, probably due to the exothermic reaction of potassium with bromine to the formation of KBr. Due to the low surface area of both allotropes in addition to the stacked form of the graphite or the bundled system of the CNTs, merely low functionalization would have been expected. The missing unbundling or exfoliation in a suitable solvent decreases the reactivity significantly and the presence of the positively charged potassium ion in between the layers or the bundles could have influenced the reaction in a critical way. The same reactions were repeated with GICs as starting material in order to investigate the behavior of this allotrope. Figure 3.5.8 illustrates the results for the halogenation reactions using GIC.

Figure 3.5.8: In situ Raman measurements/single spectra of pristine of graphite (a/d), graphite intercalation compounds (GIC) (b/e) and spectra of GIC after the treatment with bromine (c) and ICl (f) after exposition to air. The Raman spectra show the development of the intercalation from the pristine graphite (Figure 3.5.8 a/d) followed by the spectra of the intercalated species (Figure 3.5.8 b/e) confirmed by the Fano-shaped signature of the curve in addition to the disappearance of the characteristic D- and G-band [168a] resulting in the spectra after the addition of bromine (Figure 3.5.8 c) and ICl (Figure 3.5.8 f) under ambient conditions. The same observation like for the CNTs can be made. The treatment with ICl led to a high functionalization indicated by the intense rise of the D-band (~1350 cm-1) and the broadening of the FWHM of the 2D-band (~2700 cm-1). Therefore, the results suggest the formation of bonds to the respective halide in case of iodine monochloride accompanied by the mass transport of interlayer K+ counterions from the interior of the solid GIC outside the layers. Thus, the in situ measurements demonstrated that both allotropes activated via elemental potassium reacted with ICl

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to form chlorinated materials, whereas bromination could not be observed. Susequently, a consequent EDS analysis was performed in order to provide more useful information and to shed light onto this complex topic. The EDS analysis of both the materials, which were carefully washed beforehand to remove residual halide and salts, corroborate the presence of the halides on the sample, in particular for the samples treated with ICl (Table 3.5.1). Interestingly, bromine traces were also detected.

Table 3.5.1: EDS measurements of halogenated material using solid-state reduction in atomic distribution (Wt%).

element CNTs CNTs- CNTs- graphite G-Cl G-Br Cl Br [Wt%]

C 77.5 84.0 84.8 95.7 93.6 89.1

O 8.30 13.1 13.7 4.0 1.89 3.30

K - 2.82 1.47 - 2.25 6.71

Cl - 0.08 - - 1.14 -

Br - - 0.10 0.10 - 0.89

The residual potassium derives from the prior activation and could not be washed away completely during the work-up procedure. The oxygen traces are due to adsorbed water on the surface in addition to side reactions of the intercalated and charged materials with oxygen to form hydroxyl groups. Anyway, this method lacks in significance since it is surface-limited and fails to detect functionalities located deep within the layers or bundles. Additionally, a quantitative determination of the degree of functionalization is rather complex due to the reasons mentioned in the previous chapter, as the detected values are strongly depending on the measured spot, its size and the applied accelerating voltage. Furthermore, this method cannot distinguish between adsorbed molecules and covalent bonds but gives an additional hint for a successful functionalization. Corresponding SEM pictures of the pretreated materials are depicted in Figure 3.5.9 displaying the different halogenated structures.

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Figure 3.5.9: SEM images of brominated (A) and chlorinated (B) Graphene/FLG; brominated (C) and chlorinated (D) SWCNTs. The recorded pictures depict an area of 60 μm x 60 μm.

Due to the limitation of our TG-MS system, which requires quite high amounts of products, Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF-MS) was applied to the halogenated graphene and SWCNT samples. This method employs laser activation for desorption and ionization of the materials demanding merely low amounts of products. Taking the MALDI spectra in consideration, the characteristic isotopic distribution of chlorine and bromine can be detected for the respective samples (Figure 3.5.10). The chlorine signals appear as most demonstrative revealing relatively high intensities compared to the bromine signals of brominated species (Figure 3.5.10 B/D), pointing to the presence of covalently connected halides. Additionally, carbon clusters accompanied by a distinct signal of iodine (m/z 129) can be observed in every spectrum. The appearance of the iodine peak can be traced back to KI3, which was used as a calibrant for all measurements. The carbon clusters can be identified as every signal adds up 12 or a multiple of 12 on the mass of the current signal value and are formed during the measurement. interestingly, the formation of these clusters does not occur during the measurement of the chlorinated SWCNTs.

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Figure 3.5.10: MALDI-ToF-MS spectra of nanotubides treated with bromine (A) and iodine monochloride (B); graphenides treated with bromine (C) and iodine monochloride (D); The graphs on the right-hand side focus on specific mass ranges of the expected halides.

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This method is an alternative approach for the qualitative determination of the samples, but lacks of quantitative information. The distinction between covalent bound groups or simply adsorbed molecules cannot be made either. But taking into account the respective in situ Raman results, this represents additional convincing evidence for the successful halogenation via solid-state reduction.

5.5.1.5 Halogenation of Monolayer Graphene

Moreover, the halogenation of monolayer material in case of graphene was elucidated and therefore, an established method of reductive monolayer functionalization was applied. For this purpose, CVD-grown monolayer graphene on a Si/SiO2-wafer was activated by Na/K alloy in DME followed by the addition of few drops of a concentrated solution of ICl or Br2 in DME, respectively, and after few washing steps and the subsequent exposition to ambient conditions, the material was analyzed via Raman spectroscopy (Scheme 3.5.5).

Scheme 3.5.5: Reductive functionalization of CVD-graphene on Si/SiO2 substrate with Br2 and ICl.

To avoid side reactions with ambient moisture or oxygen, the reactions were carried out under inert conditions in a glovebox with desiccated and degassed DME. After treating of the negatively charged sheet with the respective halogen compound, possible residual electrons were removed with PhCN.[103] The wafer was subsequently analyzed by SRS, which is presented in Figure 3.5.11. The pronounced increase of the D-band (~1350 cm-1) in the case of the reaction with iodine monochloride accompanied with the appearance of the defect-induced D´-band (~1615 cm-1), and the decrease of the FWHM of the 2D-band (~2680 cm-1) indicates a successful 3 introduction of sp -defects by the utilized reagent. The calculated ID/IG-ratio of 0.95 148

suggests a high degree of functionalization for the assumed chlorination. In this case, the histogram presentation provides beneficial information about the chlorination pattern as well. The inhomogeneously distributed addition pattern, indicated by the wide range of the ID/IG-values (Figure 3.5.11 B), suggests the presence of high- functionalized areas besides considerable low-functionalized areas on the carbon lattice. The addends are determined as chlorine indicated by the halogenation results in bulk, shown above.

Figure 3.5.11: A) SRS/mean spectra (λE=532 nm) of CVD-graphene (purple line) after activation with Na/K alloy in DME and reaction with iodine monochloride (purple line) or bromine (orange line) in comparison to pristine CVD-graphene (black line), spectra y-shifted for clarity;

B) histogram presentation of the ID/IG-ratios of the chlorinated CVD graphene sample (red) in comparison with pristine material (black).

However, the appearing band at 1450 cm-1 can be traced back to C-O-vibrations and therefore gives a hint for the reaction of remaining charges with oxygen to form hydroxyl groups, despite the removal of remaining charges by the prior addition of PhCN.[103, 168a] Whereas, the bromination reaction did not succeed as no new defects could be detected after activation of the monolayer and subsequent addition of the bromine dissolved in DME, according to the recorded Raman spectra (Figure 3.5.11 A, orange line). This behavior is contradictory to the presented results for bulk graphene. Since in bulk the halogenation occurs in a ditopic fashion forming strain-free architectures, this could have major impact on the reactivity compared to the MLG functionalization. For MLG, solely monotopic halogenation of the halide is probable alongside simultaneous backbonding to the substrate.[115] Therefore, minor reactivity for this kind of addition can be proposed in contrast to bulk functionalization.

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Summarizing, an efficient, simple, and fast way to yield halogenated SWCNTs and graphene was reported. The applied Billups conditions led to the best results to chlorinate and brominate SWCNTs including successful substitutions of the brominated intermediates by various nucleophiles. Merely the usage of strong nucleophiles like for instance thilolates and alcoholates was successful, probably underlying a vinylogous Michael-type nucleophilic substitution mechanism. The chlorination of both allotropes using solid-state reduction conditions was possible but to a minor extent. Brominated graphene in bulk could be obtained by applying the mentioned conditions as well. However, monolayer graphene did not react with Br2 but with ICl to form stable C-Cl-bonds to the lattice after prior activation. Additionally, the bonds between graphene and the halides prove to be more stable towards substitutions compared to the respective CNT samples. The different behavior of monolayer graphene in contrast to multilayer graphene can be referred to their different reactivity. In bulk, the halides can attack from both sides leading to a strain-free architecture, whereas for the monolayer deposited on a wafer only a monotopic halogenation accompanied by backbonding to the substrate is feasible, which is theoretically proven and has been part of investigation in Chapter 3.2.[115, 177] Comparing the two allotropes with each other, SWCNTs generally exhibit a higher susceptibility to react due to the present curvature in its basic structure, which could be one explanation for the greater success of halogenation via the applied methods in comparison to graphene. Nevertheless, the easy access and high reactivity of halogenated nanomaterials opens the door for a broad range of beneficial opportunities regarding conceivable follow-up chemistry and halogen storage. Moreover, the formation of these versatile starting materials enable the generation of multifunctional carbon architectures for future applications.

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3.5.2 In situ Chlorination of Graphite

Another approach to halogenate SCAs was carried out applying conditions in close resemblance to recently published findings for azafullerenes.[207] Scheme 3.5.6 illustrates the reaction sequence of the halogenation attempt for graphite.

Scheme 3.5.6: Reaction scheme of the in situ chlorination of graphite.

For this purpose, graphite was chosen to replace the azafullerenes in this reaction in order to yield the respective halogenated materials. Thus, a low amount of the specific allotrope was placed in heat-dried glass ampoule followed by the addition of one drop of elemental bromine and few mL titanium tetrachloride. Subsequently, this mixture was frozen with the aid of liquid nitrogen and the ampoule was sealed under ultra-high vacuum (p<10-7 mbar). In the case of graphite, the mixture showed a clear red color but after a sonication time of 15 min and heating at 180°C for 3 days, the red color changed to an orange appearance (Figure 3.5.12). This alteration indicated a possible reaction as TiCl4 regularly is colorless and the red color probably derives from elemental bromine, whereas titanium bromide compounds commonly appear as yellow/orange. Afterwards, the ampoule was carefully opened and the resulting powder was filtered and washed with a sufficient amount of EtOH under air exclusion. The products were dried under argon flow at RT and were then characterized by Raman spectroscopy and TG-GC-MS.

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Figure 3.5.12: Illustration of the color change of the reaction mixture graphite/Br2/TiCl4 before (left) and after the heating step (right).

Due to the technical limitation of the vacuum pump which merely allows the attachment of small glass ampoules with low volumes, this approach yields quite low amounts and therefore limits the subsequent characterization using TG-MS. However, MALDi-ToF and TG-GC-MS measurements were performed instead in order to characterize the resulting powder in addition to SRS. These methods require merely low amounts of the material to be analyzed. Figure 3.5.13 illustrates the Raman, the TG-GC-MS and the MALDI-ToF analysis of the graphite sample treated with the halogen compounds. Mass trace values below 40 cannot be detected by the TG-GC-MS system due to technical limitations, therefore chlorine (MCl= 35,453 u) could not be observed. However, a relatively high mass loss can be obtained (~16.5 %), exhibiting two major mass slopes (Figure 3.5.13 B). One mass slope appears from 50 °C until around 300 °C and the second emerges from 460 °C until 600 °C. Considering the obtained result of Chapter 3.5, the first mass slope relates to the detachment of the covalently bonded halide according to the cleavage temperature. Anticipating the fact that sample loss at temperatures above 450 °C additionally reflects mostly structure decomposition, merely the first mass loss (~10 %) can be used to calculate a potential degree of addition. Using the Equation 3.4.1 from the previous chapter, the degree of functionalization can be determined as 3.89 %. The Raman spectrum of the sample right after the opening and washing steps shows a prominent D-mode in comparison with the pristine graphite. However, the D-band pictures itself very broad, probably due to the impurities on the sample which could not be washed away completely.

Therefore, an exact determination of the ID/IG-ratio is rather complex and not

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conclusive. At least two underlying Raman modes in the region from 1200 until 1500 cm-1 are present determining the uneven shape of the D-mode. Moreover, the shape of the 2D-band (~2700 cm-1) is affected as well. These effects could originate from impurities that are generated during the reaction and could not be completely removed by the thorough washing steps impeding the Raman analysis. The formation of the side product TiO2 cannot be completely prevented as the reaction of TiCl4 with moisture to TiO2 and HCl occurs quite rapidly. However, the influence of the formed

TiO2 is neglectable since the Raman features of this compound appear in early wavenumber areas from 150 to 800 cm-1.[208]

Figure 3.5.1: A) SRS/mean spectra (λE=532 nm) of the chlorinated sample (blue) in comparison with graphite (black); B) TG-GC-MS profile of the chlorinated sample compared to graphite (dashed line) showing the respective mass loss, C) Zoom into specific mass range of the MALDI- ToF spectrum showing the range of m/z 30 to 40 to depict chlorine isotopes with the characteristic m/z of 35, 36 and 37; D) MALDI-ToF spectrum of chlorinated SWCNTs showing the complete mass range from 30 up to 150.

For further information about the nature of the addends, MALDI-ToF investigations shed light into this question. The MALDI-ToF spectrum is depicted in Figure 3.5.13 C/D demonstrating plenty of appearing peaks including formed carbon 153

clusters indicated by mass steps of 12. Additionally, three peaks deriving from the common isotopical distribution of chlorine can be obtained confirming the presence of chlorine at the graphite lattice. Altering the reaction conditions like increasing the amounts of the nanomaterial or the other reactants led to the same outcome varying weakly. Longer reaction times as well as higher temperatures did not lead to higher functionalization but ended alike. Instead of graphite, CVD graphene as starting material was investigated as well in order to gain more insight into the mechanism and process of this reaction but did not lead to a success. Interestingly, the same attempt was performed with the more reactive SWCNTs but did not lead to a successful outcome. Herein, neither a significant mass loss or modified Raman features could be seen excluding the success of the halogenation. The question arises now, on which mechanism this reaction is based on. Assumingly, these conditions provide an ideal base for the chlorination of graphite. As proposed in literature, the graphite could undergo a selective bromination by the elementary bromine in the first place, followed by the quantitative exchange of bromine by chlorine originating from the applied TiCl4 due to the higher strength of the C-Cl bonds.[207] Anyhow, the reaction outcome is quite surprising as well since no exfoliation of the graphite occurred decreasing the reactivity drastically, also due to an extremely diminished accessible carbon surface. Having calculated the degree of functionalization, the value (3.89 %) is similar compared to the reductive route which was described in Chapter 3.5.1 for brominated graphene and represents therefore a worth considering alternative. As the Raman spectra remain identical over time, a quantitative exchange with moisture cannot be excluded but is rather unlikely. Problematically, this approach demands quite low amounts of starting material due to the experimental implementation which restricts the production of larger quantities of modified material. In the future, the process could be more investigated, optionally using an additional selection of allotropes like different types of fullerenes for instance. Moreover, full potentialities could be exploited by substitution- based chemistry, in analogy to the previous chapter.

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4 Conclusion

The covalent functionalization of graphene and single-walled carbon nanotubes represents one of the most important and most efficient ways to improve the properties of carbon-based nanomaterials including band gap tuning and increase of solubility. Among other routes, especially the reductive activation provides inimitable and unprecedented prospects regarding the facile, versatile, and selective covalent functionalization of the respective carbon frameworks. The present thesis is dedicated to tackle fundamental aspects in particular with respect to the reductive activation of the SCAs in order to overcome the hurdles and the difficulties to successfully functionalize these allotropes and to generally improve our knowledge about the elementary chemistry of the nanomaterials.

With the purpose to yield exclusively-functionalized products with side-reaction suppression, the addition of benzonitrile to the dispersion of graphene sheets bearing free charges enables the quantitative oxidation of graphene preventing undesired side reactions accompanied by the generation of the colored and quantifiable benzonitrile radical anion. In an in-depth study, this procedure was applied to different types of graphite varying primarily in their morphology and is discussed in Chapter 3.1.1. The study has shown that the contrasting properties of the different graphite sources have an unambiguous impact on the charge uptake behavior via elemental potassium, leading to inhomogeneously intercalated species which reduces the chemical reactivity due to diminished repulsive Coulombic interactions. Thus, the affinity of graphene sheets to exfoliate is impaired and therefore the extent of the side reactions is diminished as a consequence, which could be revealed performing in situ Raman measurements of the intercalated species and by further analysis of the side reactions. As described in Chapter 3.1.2, charged CNTs, so-called nanotubides, are prone to undergo liquid phase oxidation with benzonitrile as well, but to a minor extent in comparison to graphenides, essentially depending on the respective type of production and therefore on the resulting properties. The efficiency of the discharging by benzonitrile is primarily predicated on the used allotrope and not depending on the specific activation route, which could be clearly proven by established characterization methods like TG-MS and statistical Raman analysis. The very same observation could be made in Chapter 3.1.3 while investigating the impact of benzonitrile on prior

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covalently functionalized products of the respective carbon allotropes. Consequently, the effect of benzonitrile on functionalized species of both materials is negligible, since besides the desired alkylation merely a very small amount of new defects is introduced into the π-system which does not alter the outcome of the reaction in a significant extent, yielding predominantly selectively-alkylated products.

After the extensive analysis of the discharging behavior of graphene and SWCNTs by benzonitrile, the process of functionalization was explored in detail in Chapter 3.2, focusing on the precise propagation of newly introduced defects and enhanced reactivity of certain areas. Based on the reductive hydrogenation of monolayer graphene, which serves as a simplified model for the description of the general reductive functionalization, the course of addition was tracked via Raman spectroscopy and the respective results provide excellent evidence for the assumed functionalization behavior. The investigations were carried out using graphene with varying dimensions, small monolayer flakes and larger CVD-grown graphene sheets, as starting material for the subsequent reductive hydrogenation and the corresponding results are provided in Chapter 3.2.1 and Chapter 3.2.2, respectively. Consequently, the reductive hydrogenation occurs via radical addition processes involving predominantly all-trans-hexa-additions to aromatic sextets in vicinity to the edges leading to a progressed expansion of newly introduced defects starting from the periphery propagating to the interior of the graphene plane until a homogeneous coverage is reached. The periphery of graphene can be seen as defect region and it´s functionalization with respect to graphene reactivity like strain or conjugation energy is more favored than the basal plane of the sheet. Moreover, as reported in Chapter 3.2.3, it was demonstrated experimentally that newly introduced defects act as a catalyst for further functionalization and lead to the generation of highly modified islands incorporated into the intact sp2-carbon lattice. From the locations of these defects, the expansion of defects is promoted and proceeds continuously in every direction, clearly visible in Figure 3.2.4 (Chapter 3.2.3, p. 88). Besides, detailed investigations of the ditopic functionalization process during the reductive hydrogenation are provided in Chapter 3.2.4. By selectively introducing trenches to the corresponding substrate prior to the attachment of graphene sheets to the top surface, the ditopic attack is enabled, while concurrently preventing backbonding to the substrate in these specific areas. However, the hydrogenation occurs homogenously in all regions independent on the observed spot indicating either the 156

reaction between the partly “free-standing” graphene and present hydrogen radicals or undesired residuals in these regions of the substrate. Furthermore, the dehydrogenation process of the functionalized monolayer graphene flakes was considered in Chapter 3.2.5. Accordingly, the thermal dehydrogenation of the modified materials is feasible but leaving a significant amount of structural damages behind within the carbon lattice as a consequence of the thermal treatment and the harsh conditions of the Birch-like reaction.

Moreover, the application of oxo-graphite as alternative starting material for the reductive approach was probed in Chapter 3.3 in order to increase the amount of modified monolayer graphene sheets. The alkylation of graphene sheets on surfaces and in bulk by the combination of oxidative exfoliation and reductive functionalization could be obtained. However, as reported in Chapter 3.3.1, the introduction of a minor amount of on-plane alkyl moieties onto the materials generated by reductive functionalization could not be detected by Raman spectroscopy as the number of introduced hexyl chains is significantly lower than the number of vacancy defects in the lattice structure. Anyhow, as revealed in Chapter 3.3.2, by the use of TG-MS and temperature-dependent Raman spectroscopy, the hexylation of reduced GO could be unambiguously be verified. Additionally, the GC analysis demonstrated that the covalent hexylation indeed occurs proven by the detection of the dimerization product of two hexyl radicals, dodecane, cleaved from the π-system. Thus, alkylation to their hexylated derivatives is possible, but not more efficient as reported for pristine graphene. Nevertheless, an enhanced dispersibility in specific solvents could be pronounced due to a higher density of defects within the carbon lattice simplifying a rapid discovery of the monolayer material in contrast to approaches without prior oxidative delamination. Moreover, the results demonstrate the advantage of combining TGA with MS and GC as an analytical tool to characterize the functionalized graphene materials. Thus, the combination of oxidative delamination and reductive functionalization allows for the modification of the basal plane of graphene, excluding the preparation of major amounts of functionalized few-layers and therefore a more efficient way to yield high amounts of functionalized monolayer material.

The analysis of the altered density of defects of SWCNTs and graphene, especially regarding the quantification of newly introduced addends within the carbon frameworks is still an ongoing challenge for scientists due to the high quantity of covalent

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functionalization routes which incorporate primarily organic moieties. In Chapter 3.4, the successful covalent bulk functionalization of graphene (Chapter 3.4.1) and two different types of SWCNTs (Chapter 3.4.2) with two organometallic ferrocene compounds has been demonstrated varying in their leaving group and spacer lengths, being one of the first ferrocene-based functionalization reactions of defect-free graphene. Ferrocene with one iron atom as marker per introduced defect presents a perfectly suitable functional entity to be quantified and which could unambiguously verify the success of the reactions according to a variety of used characterization methods, e.g. Raman spectroscopy, TG-MS, TG-GC-MS, and EDS. Additionally, this type of functionalization paves the way for various fields of application taking advantage of the extraordinary properties of ferrocene linked with the respective allotrope. As described in Chapter 3.4.3, the determination and the corresponding comparison of the degrees of functionalization prevented the specification of a certain degree of functionalization since the values for the respective data differ considerably depending on the analysis method used. Anyhow, significantly higher addition rates for the CNTs compared to graphene could be revealed. Nevertheless, a reliable range of the respective values for each functionalized product could be assigned by all findings of the established characterization techniques.

Moreover, fundamental insights into the reductive halogenation of SWCNTs and graphene could be received in Chapter 3.5.1 demonstrating a simple, fast, and efficient way to yield halogenated nanomaterials in an easily reproducible way. On the one hand, in Chapter 3.5.1.1 the conditions of the recently developed bromination route of SWCNTs were thoroughly adjusted and on the other hand, a simple and novel chlorination route of SWCNTs was established prevailing analogous reductive conditions in combination with iodo monochloride as halogenation reagent. In the case of the bromination, the applied conditions lead to the formation of highly-reactive intermediates, which are prone to react with a variety of strong nucleophiles including thiloates, amines, and alcoholates in substitution reactions, as reported in Chapter 3.5.1.2. Hence, these brominated intermediates represent potential global precursors for various kinds of conceivable follow-up chemistry in order to generate multifunctional SWCNT architectures for future applications. The chlorinated intermediates, however, are less reactive and have a much higher stability towards substitution reactions. In Chapter 3.5.1.3 and Chapter 3.5.1.5, these conditions were also applied to graphene in bulk and to graphene on a substrate, respectively. Due to 158

graphene´s lower susceptibility to react, this procedure leads to either brominated or chlorinated mono- and multilayer graphene to a minor extent compared to the SWCNTs, but, however, with enhanced stability towards substitutions. In Chapter 3.5.1.4, the successful halogenation in solid-state has been further proven by in situ Raman monitoring of the reactions under strictly inert conditions. The comparison of reductive approaches, however, revealed the best results for the applied Billups conditions for both allotropes. Anyhow, large differences regarding the reactivity can be obtained, which derives from altering fundamental characteristics for both allotropes influencing essentially the outcome of the halogenation and further substitutions. The latest Chapter 3.5.2 presents another alternative approach using graphite and the combination of TiCl4 and Br2 followed by a thermal treatment under ultra-high vacuum prevailing strictly inert conditions. The extensive characterization of the resulting products indicated the formation of chlorinated graphite in low yields and thus provides the basis for future investigations.

Shining light into these complex topics, this thesis could help to broaden the understanding of the reductive covalent chemistry of carbon allotropes. Fundamental aspects of the synthesis of novel carbon architectures, distinct addition patterns, quantization of covalently attached groups and the reduction/oxidation behavior could be remarkably revealed by combining spectroscopic analysis with the application of other established characterization techniques. This knowledge can lead to crucial progress and to a better understanding in the field of 1D and 2D materials in general.

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5 Zusammenfassung

Die kovalente Funktionalisierung von Graphen und einwandigen Kohlenstoffnanoröhren stellt eine der wichtigsten und effizientesten Möglichkeiten dar, die Eigenschaften der Nanomaterialien zu verbessern, einschließlich der Öffnung der Bandlücke und der Erhöhung der Löslichkeit. Neben anderen Wegen, bietet insbesondere die reduktive Aktivierung unnachahmliche und beispiellose Möglichkeiten in Bezug auf die einfache, vielseitige und selektive kovalente Funktionalisierung der jeweiligen Kohlenstoffgerüste. Diese Arbeit widmete sich grundlegenden Aspekten, insbesondere hinsichtlich der reduktiven Aktivierung der SCAs, um Hürden und Schwierigkeiten bei der erfolgreichen Funktionalisierung dieser Allotrope zu überwinden und unser Wissen über die elementare Chemie der Nanomaterialien allgemein zu verbessern.

Um selektiv-funktionalisierte Produkte zu erhalten, ermöglicht die Zugabe von Benzonitril zu einer Dispersion von geladenen Graphenlagen die quantitative Oxidation von Graphen, wodurch unerwünschte Nebenreaktionen verhindert werden und gleichzeitig zur Bildung eines farbigen und quantifizierbaren Radikalanions führt. In einer umfangreichen Studie wurde dieses Verfahren an verschiedenen Graphitarten, welche sich hauptsächlich in ihrer Morphologie unterscheiden, angewendet und in Kapitel 3.1.1 erörtert. Die unterschiedlichen Eigenschaften der verschiedenen Graphitquellen haben einen eindeutigen Einfluss auf das Reduktionsverhalten mit elementarem Kalium, was in der Entstehung von inhomogen interkalierten Spezies resultiert. Die aufgrund verminderter abstoßender Coulomb- Wechselwirkungen reduzierte chemische Reaktivität beeinträchtigt den Exfolierungsgrad und damit das Ausmaß der Nebenreaktionen, was durch in situ Raman Messungen der interkalierten Spezies und die Analyse der Nebenreaktionen gezeigt werden konnte. Wie in Kapitel 3.1.2 beschrieben sind positiv geladene CNTs, sogenannte Nanotubide, ebenfalls anfällig für die Oxidation durch Benzonitril in der Flüssigphase, aber in geringerem Ausmaß im Vergleich zu Grapheniden, was im Wesentlichen von der jeweiligen Produktionsart und der daraus resultierenden Eigenschaften abhängt. Die Effizienz der Entladung durch Benzonitril beruht in erster Linie auf dem verwendeten Allotrop und ist nicht von der spezifischen

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Aktivierungsroute abhängig, wie durch etablierte Charakterisierungsmethoden wie TG-MS und statistische Raman Spektroskopie eindeutig nachgewiesen werden konnte. Die gleiche Beobachtung konnte in Kapitel 3.1.3 bei der Untersuchung des Einflusses von Benzonitril auf bereits kovalent funktionalisierte Produkte der jeweiligen Kohlenstoffallotrope gemacht werden. Demzufolge ist die Wirkung von Benzonitril auf alkylierte Spezies beider Materialien vernachlässigbar, da neben der gewünschten Alkylierung nur eine sehr geringe Menge neuer Defekte in das System eingeführt wird, die das Ergebnis nicht wesentlich verändern, was zu überwiegend selektiv-alkylierten Produkten führt.

Nach der umfassenden Analyse des Entladungsverhaltens von Graphen und SWCNTs durch Benzonitril wurde der Funktionalisierungsprozess in Kapitel 3.2 detailliert untersucht, wobei der Fokus auf der präzisen Ausbreitung neu eingeführter Defekte und der erhöhten Reaktivität bestimmter Bereiche lag. Anhand der reduktiven Hydrierung von einlagigem Graphen, welches als vereinfachtes Modell für die allgemeine reduktive Funktionalisierung dient, wurde der Additionsverlauf mittels Raman-Spektroskopie verfolgt, was einen hervorragenden Beweis für den angenommenen Funktionalisierungsverlauf erbrachte. Die Untersuchungen wurden unter Verwendung von Graphen mit unterschiedlichen Größen, kleinen einlagigen Flocken und größeren CVD-beschichteten Graphenlagen, als Ausgangsmaterial für die anschließende reduktive Hydrierung durchgeführt, und die entsprechenden Ergebnisse sind in Kapitel 3.2.1 bzw. Kapitel 3.2.2 erläutert. Demnach erfolgt die Hydrierung durch radikalische Additionsprozesse, in denen sich vorwiegend trans- Hexa-Additionen an aromatischen Sextetten in der Nähe der Ränder ereignen, welche zu einer fortschreitenden Ausdehnung neu eingeführter Defekte führt, die sich von der Peripherie aus bis ins Innere der Graphenebene ausbreiten, bis eine homogene Funktionalisierung erreicht ist. Die Peripherie von Graphen kann als Defektregion angesehen werden, deren Funktionalisierung hinsichtlich der Reaktivität von Graphen aufgrund von Spannung oder Konjugationsenergie bevorzugter ist als die planare Ebene der Graphenlage. Darüber hinaus konnte, wie in Kapitel 3.2.3 beschrieben, experimentell nachwiesen werden, dass neu eingeführte Defekte als Katalysator für weitere Funktionalisierungen wirken und zur Inselbildung von stark modifizierten Bereichen führen, die in das intakte sp2-hybridisierte Kohlenstoffgitter integriert sind. Von den Additionsbereichen wird die Ausdehnung der Defekte vorangetrieben und kontinuierlich weitergeführt, was sich klar in Abbildung 3.2.4 (Kapitel 3.2.3, S. 88) 161

zeigt. Außerdem wurden in Kapitel 3.2.4 detaillierte Untersuchungen des ditopen Funktionalisierungsprozesses während der reduktiven Hydrierung durchgeführt. Durch das gezielte Anbringen von Einkerbungen auf dem entsprechenden Substrat vor dem Aufbringen der Graphenlagen darauf wird der ditope Angriff ermöglicht und gleichzeitig eine Rückbindung an das Substrat in diesen bestimmten Bereichen verhindert. Die reduktive Hydrierung erfolgt jedoch homogen in allen Bereichen unabhängig von der betrachteten Stelle, was entweder auf die Reaktion zwischen dem teilweise „freiliegenden“ Graphen und vorhandenen Wasserstoffradikalen oder mit unerwünschten Rückständen hinweist. Darüber hinaus wurde in Kapitel 3.2.5 der Dehydrierungsprozess der funktionalisierten einschichtigen Graphenflocken betrachtet. Demzufolge ist die thermische Dehydrierung der modifizierten Materialien möglich, hinterlässt aber als Folge der thermischen Behandlung und der extremen Bedingungen der Birch-Reaktion eine erhebliche Menge an strukturellen Schäden innerhalb der Kohlenstoffgitters.

Außerdem wurde in Kapitel 3.3 die Anwendung von reduziertem Graphenoxid als alternatives Ausgangsmaterial für den reduktiven Ansatz untersucht, mit der Intention, die Ausbeute an modifizierten Monolagengraphen zu erhöhen. Die Alkylierung von Substrat-gebundenem und dispergiertem Graphen konnte durch die Kombination von oxidativer Exfolierung und reduktiver Funktionalisierung erreicht werden. Wie in Kapitel 3.3.1 beschrieben, konnte die Einführung von der durch reduktive Funktionalisierung erzeugten geringen Menge an Alkyleinheiten auf der Graphenebene durch Raman-Spektroskopie nicht eindeutig nachgewiesen werden, da die Anzahl der eingeführten Hexylketten deutlich geringer ist als die Anzahl der vorhandenen Defekte in der Gitterstruktur. Wie in Kapitel 3.3.2 ausgeführt, konnte die Hexylierung von rGO jedoch durch den Einsatz von TG-MS und temperaturabhängiger Raman-Spektroskopie eindeutig belegt werden. Außerdem zeigte eine weitere GC-Analyse durch den Nachweis des Dimerisierungsprodukts von zwei Hexylradikalen, Dodecan, dass die kovalente Hexylierung tatsächlich erfolgt. Somit ist eine Alkylierung zu ihren hexylierten Derivaten zwar möglich, aber nicht effizienter als für reines Graphen. Durch die höhere Defektdichte innerhalb des Kohlenstoffgitters, konnte jedoch eine verbesserte Dispersibilität in bestimmten Lösungsmitteln festgestellt werden, wodurch eine schnelle Entdeckung von Monolagen im Gegensatz zu Ansätzen ohne vorherige oxidative Delaminierung stark vereinfacht wird. Im Übrigen zeigen die Ergebnisse den Vorteil der Kombination von TGA mit MS und GC 162

als Analysewerkzeug zur Charakterisierung funktionalisierter Graphenmaterialien. Also ermöglicht die Kombination von oxidativer Delaminierung und reduktiver Funktionalisierung, die Modifizierung der planaren Ebene von Graphen, da sie die Herstellung großer Mengen funktionalisierter Mehrlagengraphen verhindert und damit einen effizienteren Weg darstellt, um große Mengen funktionalisierte Monolagen zu erhalten.

Die Analyse der veränderten Defektdichte von SWCNTs und Graphen, insbesondere im Hinblick auf die Quantifizierung neu eingeführter Gruppen in den Kohlenstoffgerüsten der Allotrope, stellt aufgrund der hohen Anzahl von bereits bekannten kovalenten Funktionalisierungswegen eine ständige Herausforderung für Wissenschaftler dar, da diese Varianten hauptsächlich organische Gruppen in die jeweiligen Kohlenstoffsysteme einführen. In Kapitel 3.4 konnte die erfolgreiche kovalente Funktionalisierung von dispergiertem Graphen (Kapitel 3.4.1) und zwei verschiedener Arten von SWCNTs (Kapitel 3.4.2) mit organometallischen Ferrocenverbindungen gezeigt werden, welche sich in ihren jeweiligen Abgangsgruppen und deren Abstandslängen unterscheiden, und gleichzeitig eine der ersten ferrocenbasierten Funktionalisierungsreaktionen von defektfreiem Graphen darstellt. Das Ferrocen-Molekül mit einem Eisenatom als Marker pro eingeführtem Defekt stellt eine besonders gut zu quantifizierende funktionelle Einheit dar, durch die der Erfolg der Reaktionen nach einer Vielzahl von verwendeten Charakterisierungsmethoden, wie zum Beispiel Raman-Spektroskopie, TG-MS, TG- GC-MS und EDS, eindeutig bewiesen werden konnte. Wie in Kapitel 3.4.3 beschrieben verhinderte die Bestimmung und der entsprechende Vergleich der Funktionalisierungsgrade die Angabe eines exakten Werts, da sich die Ergebnisse für die jeweiligen Daten je nach verwendeter Analysemethode erheblich unterscheiden. Jedoch konnten deutlich höhere Werte für SWCNTs im Vergleich zu Graphen festgestellt werden. Nichtsdestotrotz, konnte ein zuverlässiger Bereich der jeweiligen Werte für jedes funktionalisierte Produkt durch die Erkenntnisse aller etablierten Methoden zugewiesen werden.

Zudem konnten in Kapitel 3.5.1 grundlegende Einblicke in die reduktive Halogenierung von SWCNTs und Graphen gewonnen werden, welche eine einfache, schnelle und effiziente Art und Weise darstellt, halogenierte Nanomaterialien zu generieren. Zum einen wurden in Kapitel 3.5.1.1 die Bedingungen der kürzlich

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entwickelten Bromierungsroute von SWCNTs gründlich verbessert, zum anderen wurde eine schnelle und einfache Möglichkeit zur Chlorierung von SWCNTs unter Verwendung von Iodmonochlorid als Halogenierungsreagenz und analogen reduktiven Reaktionsbedingungen etabliert. Im Fall der Bromierung führen die gewählten Bedingungen zur Entstehung von hochreaktiven bromierten Zwischenprodukten, welche anfällig sind, mit einer Vielzahl starker Nucleophile Substitutionsreaktionen einzugehen, einschließlich mit Thiloaten, Aminen und Alkoholaten, wie in Kapitel 3.5.1.2 beschrieben wird. Die bromierten SWCNTs stellen somit potenzielle globale Vorläufer für verschiedene Arten von denkbarer Folgechemie dar, um für zukünftige Anwendungen multifunktionale SWCNT-Architekturen zu generieren. Die chlorierten Zwischenprodukte jedoch sind weniger reaktiv und weisen eine deutlich höhere Stabilität gegenüber Substitutionsreaktionen auf. Darüber hinaus wurden diese Bedingungen in Kapitel 3.5.1.3 und Kapitel 3.5.1.5 auch auf dispergiertes Graphen bzw. auf substrat-gebundenes Graphen angewendet. Dies führte jedoch aufgrund der geringeren Reaktivität von Graphen entweder zu bromiertem bzw. zu chloriertem Mono- und Multilagengraphen in geringerem Ausmaß im Vergleich zu den SWCNTs, aber zu einer erhöhten Stabilität gegenüber Substitutionen. Die erfolgreiche Halogenierung in der Festphase konnte außerdem durch in situ Raman-Messungen unter streng inerten Bedingungen nachgewiesen werden. Ein Vergleich der reduktiven Ansätze in Kapitel 3.5.1.4 ergab allerdings die besten Ergebnisse für die angewandten Billups-Bedingungen für beide Allotrope. Dennoch sind große Unterschiede in Bezug auf die Reaktivität sichtbar, die sich aus den Unterschieden grundlegender Merkmale beider Allotrope ergeben, die im Wesentlichen das Ergebnis der Halogenierung und weiterer Substitutionen beeinflussen. Ein weiterer alternativer Ansatz wurde in Kapitel 3.5.2 unter

Verwendung von Graphit mit der Kombination von TiCl4 und Br2 durchgeführt, gefolgt von einer thermischen Behandlung im Ultrahochvakuum unter inerten Bedingungen. Die umfangreiche Charakterisierung des Produkts wies auf die Bildung von chloriertem Graphit mit geringen Ausbeuten hin und stellt damit die Grundlage für zukünftige Untersuchungen dar.

Indem die Arbeit diese komplexen Themen beleuchtet, konnte sie dazu beitragen, das Verständnis der reduktiven kovalenten Chemie der Kohlenstoffallotrope zu erweitern. Grundlegende Aspekte der Synthese neuartiger Kohlenstoffarchitekturen, präziser Additionsmuster, der Quantifizierung kovalent gebundener Gruppen und des 164

Reduktions-/Oxidationsverhaltens konnten durch die Kombination der spektroskopischen Analyse mit anderen etablierten Charakterisierungstechniken bemerkenswert aufgedeckt werden. Dieses Wissen kann zu entscheidenden Fortschritten und zu einem besseren Verständnis auf dem Gebiet der 1D- und 2D- Materialien im Allgemeinen führen.

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6 Experimental part 6.1 Chemicals and Materials

Graphite

The following listed types of graphite were included in this thesis while mainly graphite type SGN18 from Future Carbon® was used if not stated otherwise within the text.

Table 6.1: : Properties of different types of graphite.

material description grain surface carbon content pH size [m2/g] [%] [μm]

SGN18 spherical, natural 20 6.2 99.9 7.12

Natural flaky, untreated, 1000 0.2 100 7.33

Passau natural

Asbury surface- 300-850 20 98.6 7.95

3772 enhanced, intercalated

(H2SO4)

PEX10 intercalated 3-5 6 98.5 6.02

(H2SO4), expanded and delaminated

The use of Natural Passau graphite (Kropfmühl AG, Passau) required a pretreatment including comminution with 5 times the amount of NaCl in a mortar for 15 min, followed by the elution of NaCl with distilled water and drying in vacuum. This treatment results

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in smaller and easier dispersible graphite flakes. All graphite sources were annealed for 3 d at 300 °C under vacuum for further reactions.

Carbon Nanotubes

The purified HiPco SWCNTs from the batch P2772 were obtained from Unidym Inc. (Sunnyvale, CA) and used as starting material in Chapter 3.1.2/3.1.3/3.5.1. HiPco SWCNTs (Nano) from the batch #HP28-141 were purchased from NanoIntegris and used as starting material for Chapter 3.4.2. Arc Discharge SCWNTs were obtained from Sigma Aldrich (Lot #MKBR0066V). CoMoCat SWCNTs SWeNT SG65 were bought from Sigma Aldrich as freeze-dried powder. All SWCNTs were annealed at 300 °C for 3 days under vacuum for further reactions.

Table 6.2: Properties of different types of SWCNTs. material diameter length carbon content metal catalysts

[nm] [nm] [%] [%]

HiPco 0.8-1.4 100-1000 >85 <10

HiPco 0.8-1.2 100-1000 >85 <10 (Nano)

CoMoCat 0.7-0.9 ~1500 >90 <5

Arc 1.2-1.7 300-5000 <3.5 Discharge

Chemicals:

All used chemicals were purchased from Sigma Aldrich Co. (Germany) in HPLC quality and used as-received if not stated otherwise.

Water (H2O) for hydrogenation reactions was received from Sigma-Aldrich Co. (Germany) purified, deionized, and bidistilled.

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Solvents and gases

The solvents were purchased from Sigma Aldrich Co. (Germany) and were used as received if not stated otherwise. Argon was received from Rießner-Gase GmbH (Germany), purity 5.0 and was led through a drying column (filled with molecular sieve, blue gel, KOH, and P4O10) to dry the gas. Ammonia was obtained from Rießner-Gase GmbH (Germany), purity 4.5. The solvent used for reductive functionalization reactions was distilled with a rotary evaporator to get rid of the stabilizer. The organic solvents were dried then over molecular sieve (3 Å) and afterwards the oxygen was removed by the pump-freeze technique (5-6 cycles). In the case of THF and PhCN, the compounds were purchased inhibitor-free and anhydrous (<50 ppm H2O). The solvents were distilled again over Na/K alloy under argon atmosphere inside a glovebox (<1 ppm H2O, <1 ppm O2) and then utilized for the reactions. Karl-Fischer titration was carried out to proof the absolute purity to assure the complete removal of water.

Equipment

Silicon wafer with a 300 nm thick SiO2 layer (SiO2/Si) were obtained from Fraunhofer- Institut für Integrierte Systeme und Bauelementetechnologie IISB, Schottkystraße 10, 91058 Erlangen.

CVD graphene was purchased from Graphene Supermarket either on a PMMA layer, 2 which was subsequently transferred on a 0.5 cm Si/SiO2 substrate (300 nm thick oxide layer) or directly on a SiO2/Si wafer.

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6.2 General Procedures

0.7 % [98, 189b] Preparation of oxo-G1 and G1

2 g of graphite (SGN18) (166 mmol) were stirred in 50 mL sulfuric acid (97.5%) using a Teflon reactor combined with an external refrigerating coil to keep a low temperature of <10 °C to avoid carbon loss by overoxidation. Afterwards, 2 mass equivalents of

KMnO4 (4 g) were continuously added over a time of 4 h and stirred for further 16 h. Subsequently, cold diluted sulfuric acid (50 mL, 20 %) and cold bidistilled water (60 mL) were carefully added by a dropping funnel over 4 h and 16 h, respectively. Additionally, a hydrogen peroxide solution (5 %, 40 mL) was added then to reduce any insoluble manganese species. The resulting oxidized carbon material was purified by centrifugation (six times) and redispersion with pure water. The application of pulsed tip sonication (4 min, 2 s on/off 40 W) led to the efficient exfoliation of the material. By the aid of repetitive centrifugation at 1000 RCF any non-monolayer-material was removed and the supernatant containing the smallest graphene oxide particles was removed by a single centrifugation step at 15000 RCF for 45 min, respectively. The concentration of the aqueous oxo-G1 dispersion was achieved by lyophilization and further dilution to reach a final concentration of 0.25 mg/mL. (C = 41.10 %, H = 2.50 %,

N = 0.06 %, S = 5.96 %). After deposition of oxo-G1 on a SiO2/Si wafer the substrate can be placed for 10 min at 80 °C on a fibreglass in a glass vial containing a few drops 0.7%. of hydrogen iodide and trifluoroacetic acid for reduction to yield G1

Aqueous graphene suspension (G1/SCH)

The addition of solid sodium cholate (SCH) to the prepared oxo-G1-dispersion led to the formation of a suspension of graphene in water with a concentration of 2 mg/mL.

Subsequently, 30 mass equivalents of sodium borohydride relative to oxo-G1 were added inside the cold dispersion to reduce side reactions with H2O. After storing it cold overnight, the material was purified by repetitive centrifugation steps (5 times with SCH solution and once with water). The fluffy product G1/SCH could be obtained after lyophilization until complete dryness.

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General synthesis of graphene intercalation compounds (GIC) and graphenide solution

For the synthesis of a solid-state GICs with the specific potassium/carbon ratio of 1:8 which was mainly used for all reactions if not stated otherwise, 12 mg (1 mmol carbon) graphite and 4.88 mg (0.125 mmol) potassium were heated to 150 °C in a glass vial in the glovebox (Figure 6.1) overnight. The formation of the brown GIC could be verified by in situ Raman spectroscopy under inert conditions. After the complete formation of the GIC (~12 h for SGN18), the respective brown powder was allowed to cool to room temperature for further usage.

To obtain a graphenide solution, 1.0 mmol of freshly produced KC8 (16.8 mg) was dissolved in 25 mL of THF (0.67 mg/mL) by temporary ultrasonication (5 minutes, 20 kJ, 1 s pulse). This step leads to the exfoliation of the graphenide sheets resulting in a brownish dispersion (Figure 6.2 B).

Figure 6.1: GIC (KC8) of Natural Passau (A), Asbury3772 (B), PEX10 (C), and SGN18 (D).

General synthesis of nanotubides and nanotubide solutions

For the synthesis of solid-state nanotubides with the specific potassium/carbon ratio of 1:4, which was mainly used for all respective reactions if not stated otherwise, 12 mg (1 mmol carbon) SWCNTs and 9.77 mg (0.25 mmol) potassium were heated to 150 °C in a glass vial in the glovebox. The formation of the inhomogeneous brown colored nanotubides could be verified by in situ Raman spectroscopy under inert conditions. 170

After the complete formation of the nanotubides (~12 h), the respective brown powder was allowed to cool to room temperature for further usage.

To obtain a nanotubide dispersion 1.0 mmol of freshly produced KC4 (21.7 mg) was dissolved in 25 mL of THF (0.87 mg/mL) by temporary ultrasonication (5 min, 20 kJ, 1 s pulse). This step leads to the unbundling of the nanotube bundles yielding a black dispersion (Figure 6.2 A).

Figure 6.2: A) Nanotubide dispersion in THF; B) Graphenide dispersion in THF.

General bulk functionalization route via elemental potassium

For the synthesis of functionalized graphene or SWCNT samples, 1 mmol of GIC/nanotubides in the respective THF dispersion was mixed with a certain amount of a functionalization reagent (Table 6.3) in a heat-dried Schlenk flask and the reaction mixture was stirred at RT overnight. Afterwards, 5 mL of PhCN (0.05 mmol) was added to quench residual charges on the charged material to prevent possible side reactions with water and oxygen. Subsequently, the reaction mixture was transferred from the glovebox and 50 mL of H2O were added followed by the transfer to a separation funnel filled with 50 mL of cyclohexane. After separating the phases, the organic layer including the functionalized material was purged three times with distilled water (each 50 mL). The organic layer is then filtrated through a 0.2 μm cellulose membrane filter

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(Sartorius) and washed with 100 mL THF, isopropanol, water, and acetone. The sample was dried at RT under vacuum.

General bulk functionalization via Na/K alloy in DME

The reductive activation of carbon nanomaterials (graphite, SWCNTs, and G1/SCH) was achieved by the addition of 7.5 mL dry DME to 6 mg of graphite or G1/SCH (0.5 mmol) under inert conditions in the glovebox. Then, this mixture was slowly dropped into a vigorously stirred mixture of 7.5 mL DME and a 1:3 mixture of sodium and potassium alloy (50 mg = 2.17 mmol + 150 mg = 3.85 mmol) and stirred overnight, whereby a deep-blue solution is formed. Afterwards, in case of G1/SCH further 50 mg sodium potassium alloy were added due to the possible residual adsorbed water on the lattice, and the reaction mixture was stirred for another three days. Subsequently, a certain amount of a functionalization reagent (Table 6.3) was added and stirred for 1 d. Subsequently, benzonitrile (5 mL) was added to remove any residual charges on the reductively activated carbon materials.[103] Afterwards, the airtightly sealed flask was unloaded from the glove box and transferred carefully to a separation funnel with 5 mL water and 5 mL cyclohexane. The water/DME layer was discarded and the organic phase with the nanomaterial was purged for three times with water (50 mL). The organic layer was filtered through a 0.2 μm cellulose membrane filter (Sartorius) and washed with 100 mL THF, isopropanol, water, and acetone, respectively. The respective sample was dried at RT under vacuum.

General bulk functionalization route via modified Birch/Billups reduction

In a heat-dried three-necked round-bottomed flask (250 mL) equipped with two gas inlets 12 mg of SWCNTs (1 mmol carbon) were dispersed in 100 mL THF. The flask was cooled down to -78 °C by an acetone/dry ice bath and purging ammonia was condensed. Immediately after the addition of 5 eq. of small cut pieces of potassium (195 mg, 5 mmol) or lithium (34.7 mg, 5 mmol), respectively, a black dispersion with a blue glow was created which maintained its color after 1 hour of stirring. The cooling bath was removed and the mixture kept stirring until all ammonia was evaporated. The dispersion was stirred overnight at RT. Afterwards, a certain amount of the specific functionalization reagent (Table 6.3) was added to the dispersion and stirred overnight.

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Subsequently, the reaction product was transferred carefully to a separation funnel with 50 mL water and 50 mL cyclohexane. The water/THF layer was discarded and the organic phase with the carbon material was purged three times with water (50 mL) following by filtration of the organic layer through a 0.2 μm cellulose membrane filter (Sartorius) and washed with 100 mL THF, isopropanol, water, and acetone. The sample was dried at RT under vacuum.

Table 6.3: Amounts of functionalization reagents.

reagent n [mmol] m [mg] V [mL] M [g/mol]

FcButI 1 368 368.04

FcOHexBr 1 363 363.08

Hexyl Iodide 1, 10 0.21; 2.12 0.15; 1.47 212.07

General functionalization on substrates

Monolayer carbon material (CVD graphene, mechanically exfoliated graphene, oxo-G1 0.7% or G1 ) deposited on Si/SiO2 wafers were transferred to a glovebox. 150 mg Na/K (50 mg = 2.17 mmol + 150 mg = 3.85 mmol) were dissolved in 5 mL of DME in a vial and subsequently stirred for 2 h. Two drops of the so formed deep-blue Na/K solution were utilized to activate the deposited material on the substrate. The contact time was about 30 s. Afterwards, two drops of a functionalization reagent (dissolved in DME in case of a solid reagent) were dropped on the wafers. After a reaction time of 15 min, two drops of PhCN were added followed by a few drops of DME to remove the PhCN and residual reagent. The wafers were washed with 50 mL acetone and water and subsequently characterized by SRS outside of the glove box.

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General hydrogenation reaction of monolayer graphene

A heat-dried three-necked round-bottomed flask (250 mL) equipped with two gas inlets was cooled down to -78 °C by an acetone/dry ice bath and purging ammonia (~25 mL) was condensed. Immediately after the addition of small cut pieces of Li (Table 6.4), a blue solution was formed and stirred for 30 min. Subsequently, the graphene sample

(CVD or mechanically exfoliated graphene on a Si/SiO2) was immersed into the solution for prescribed reduction times (Table 6.4) followed by the addition of 5 mL of an H+-source (water or methanol) via a syringe. The cooling bath was removed and the blue color disappeared ~1 min after the addition of the proton donor. Afterwards, the sample was washed with ~15 mL of methanol and dried under a constant flow of argon.

Table 6.4: Reduction times and amounts of used reagents.

attempts prior wafer reduction n (Li) [mmol] proton source reduction time time [min] (time steps) [s]

30 60 (20) 15 H2O

30 600 (120/480) 5 H2O

30 15 (5) 15 H2O/ MeOH

30 40 (20) 15 H2O

30 15 (5) 5 H2O/ MeOH

Thermal dehydrogenation

The dehydrogenation was performed in a CVD reactor by locating the sample in the reaction chamber surrounded by a furnace and ramping it to different temperature steps (300, 500, 700 °C) for 30 min each under a constant argon flow. Afterwards, the sample was cooled down to RT and characterized with the aid of the described methods.

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Modified reductive halogenation via modified Birch/ Billups conditions

In a heat-dried three-necked round-bottomed flask (250 mL) equipped with two gas inlets 12 mg of HiPco SWCNTs (1 mmol carbon) were dispersed in 100 mL THF(abs). The flask was cooled down to -78 °C by an acetone/dry ice bath and purging ammonia was condensed. Immediately after adding of 5 eq. of small cut pieces of lithium (34.7 mg, 5 mmol)/ potassium (95 mg, 5 mmol), a black dispersion with a blue glow was already created which maintained its color after 1 h of stirring. The cooling was removed and the mixture kept stirring until all ammonia was evaporated. The dispersion was stirred overnight at RT. Afterwards, the dispersion was cooled down to 0 °C and subsequently 0.25 mL of liquid bromine (10 mmol) for bromination or 1.62 g (1 mmol) iodo monochloride for chlorination was carefully added via a syringe and the dispersion was stirred at RT overnight. To remove the excess of bromine the sample was filtered through an argon purged and heat-dried frit and washed 5 times with

10 mL THF. For substitution reactions the carbon material was redispersed in 25 mL THF.

Substitution reactions

The respective dispersion was purged for 10 min with argon and subsequently 5 mmol of the respective substitution reagents were added (Table 6.4). After stirring overnight, the reaction mixture was carefully transferred to a separation funnel with 50 mL water and 50 mL cyclohexane. The water/THF layer was discarded and the organic phase was purged 3 times with water (50 mL) following by filtration of the organic layer through a 0.2 μm cellulose membrane filter (Sartorius) and washed with 100 mL THF, isopropanol, and water. The resulting functionalized material was dried at 75 °C under vacuum.

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Table 6.4: Amount of inserted substitution reagents.

reagent n [mmol] m [g] V [mL] M [g/ mol]

sodium octanolate 5 0.76 152.21

(in THF)

dodecanthiolate 5 1.12 224.38

(in THF)

morpholine 5 0.44 0.43 87.10

In situ chlorination of graphite

In a heat-dried glass ampoule, 1 mg/4 mg graphite (0.08 mmol, 0.5 mmol) was mixed with 1 drop of bromine and 5 mL TiCl4 (0.03 mmol). Subsequently, the ampoule was connected to an ultra-high-vacuum machine (Figure 6.3), carefully evacuated and sealed after the mixture was frozen with liquid nitrogen. The sealed ampoule was sonicated in an ultrasonic bath for 15 min and then placed in an oven and heated at 180 °C for 3 days. After that time, the ampule was opened and the resulting powder was filtered and washed with 10 mL EtOH under argon and dried under argon at RT.

Figure 6.3: Ultra-high-vacuum construction for sealing of glass ampoules.

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Dilution series of different graphite/ CNT types with PhCN

GICs of four different types of graphite (Asbury3772, PEX10, SGN18, Natural Passau) were produced. Therefore, 6 mg of graphite (0.5 mmol) and 2.44 mg potassium (0.0625 mmol) were heated to 150 °C for 3 d in a glass vial in a glovebox. The same was done for three different types of CNTs (HiPco, Arc Discharge, CoMoCat). Therefore, 6 mg of CNTs (0.5 mmol) and 4.88 mg potassium (0.125 mmol) were heated to 150 °C overnight in a glass vial in a glovebox. The cooled down powders were diluted in PhCN and sonicated (5 min, 20 kJ, 1 s pulse) for 5 min to yield the concentrations (K in PhCN) of 0.75 mmol/L (graphite) and 0.86 mmol/L (CNTs), respectively, followed by the transfer in a sealed UV-vis cuvette to exclude air contact. The solutions were turning red immediately due to the formation of the radical anion (Figure 6.4). After the UV-vis spectroscopic characterization in order to quantify the residual charges, the sample was subjected back to the main solution and the procedure was repeated to reach a descending dilution series (Table 6.5).

Table 6.5: Dilution series of PhCN for optical absorption measurements under inert conditions with the respective concentrations of K in PhCN.

material c [mmol/L] (K)

KC4 0.86 0.58 0.43 0.035 0.026

(CNTs)

KC8 0.75 0.25 0.12

(graphite)

Figure 6.4: Sealed UV-vis cuvette containing the red-colored benzonitrile radical anion.

177

Investigation of the discharging process with PhCN of different reductive pathways via elemental potassium in THF:

To compare the chemical reactivity of GICs/nanotubides and their respective dispersions with and without prior PhCN treatment, two duplicate KC8 (graphite)/KC4 (SWCNTs) dispersions were produced (each 17 mg in 25 mL THF for GIC/ 21.7 mg in 25 mL THF for SWCNTs, 1 mmol) like described before in the text. One of the dispersion was treated with an excess of PhCN (5 mL, 48.5 mmol) which led to the formation of a red colored solution, an indication for the formation of the PhCN•- in THF followed by an aqueous work-up under ambient conditions. In contrast to the first dispersion, the second sample was subjected to an aqueous work-up without the addition of PhCN. After the filtration of both samples through a 0.2 μm cellulose membrane filter (Sartorius) and washing steps with 100 mL THF, isopropanol, water, and acetone the samples were dried at RT under vacuum. via Na/K alloy in DME

In an argon atmosphere a freshly prepared, liquid Na/K alloy (50 mg = 2.17 mmol + 150 mg = 3.85 mmol) was added to 75 mL DME in a flame-dried round bottom flask at RT. Subsequently, 12 mg (1.0 mmol C) of graphite or HiPco SWCNTs, respectively, was added to the solution after the formation of a deep blue color and the mixture was further stirred for 3 d. After the reaction time, one of each allotrope sample was transferred out of the glove box. The other two samples of each allotrope were mixed with PhCN (5 mL, 48.5 mmol) and stirred for 15 min. Subsequently, these samples were taken out of the glove box as well. All samples were treated now with 50 mL cyclohexane and 50 mL distilled water for work-up. After washing of the materials with distilled water (50 mL) for three times, the organic layer was filtered through a 0.2 μm cellulose membrane filter (Sartorius) and washed with 100 mL THF, isopropanol, and acetone. The resulted powders were dried under vacuum overnight.

178

via modified Birch reduction/Billups conditions

In two heat-dried three-necked round-bottomed flask (250 mL) equipped with two gas inlets 12 mg of HiPco SWCNTs/graphite (1 mmol carbon) were dispersed in 100 mL THF. The flasks were cooled down to -78 °C by an acetone/dry ice bath and purging ammonia was condensed. Immediately after the addition of 5 eq. of small cut pieces of lithium (34.7 mg, 5 mmol) to both flasks, two black dispersions with a blue glow could be obtained, which were kept stirring for 1 h. The cooling was removed and the mixture kept stirring until all ammonia was evaporated. The dispersions were stirred overnight at RT and afterwards an excess of PhCN (5 mL, 48.5 mmol) was added to one dispersion resulting in the formation of the PhCN•-, which caused the formation of red color. After the aqueous work-up under ambient conditions, the other dispersion was also worked up analogously. Both samples were filtrated through a 0.2 μm cellulose membrane filter (Sartorius) and washed with 100 mL THF, isopropanol, water, and acetone. The samples were dried at RT under vacuum.

Investigation of the discharging process with PhCN of different reductive pathways after functionalization

The respective carbon allotrope (1 mmol C) was activated via the specific reductive route as described above. Having the charged allotropides in hand, 1.47 mL (10 mmol) hexyl iodide was added to each allotrope sample bearing 1 mmol of carbon. After a reaction time of 24 hours, PhCN (5 mL, 48.5 mmol) was added to one of the allotrope samples and was continuously stirred for 15 min. The other samples without prior treatment of PhCN were directly transferred to a separation funnel with 50 mL cyclohexane and 50 mL distilled water. After washing the sample with water for thee times, the organic layer was filtrated through a 0.2 μm cellulose membrane filter (Sartorius) and washed with 100 mL THF, isopropanol, water, and acetone. The samples were dried at RT under vacuum.

179

Synthesis of 4-Chlorobutylferrocene[209]

7 mL (6.6 mmol) of a 1.7 M solution of tert-butyllithium in pentane was added dropwise to a solution of ferrocene (1 g,

5.5 mmol) in THFabs (18 mL) at 0 °C. and the resulting mixture was stirred for 40 min. Subsequently, 0.83 mL 1-bromo-4- chlororbutan (7.2 mmol) was added, and the solution was stirred for 3 h. The reaction mixture was quenched with 20 mL saturated aqueous solution of NaCl and extracted three times with 20 mL Et2O. The organic layer was dried over Na2SO4, the solvent was removed on a rotary evaporator and the crude product was separated on a SiO2 column (hexane) resulting in the desired product in the second fraction.

Yield: 21 % (1.2 mmol, 331 mg)

1 H-NMR (400 MHz, CDCl3, rt): δ (ppm) = 1.63 (m, 2H, CH2), 1.79 (m, 2H, CH2),

2.32 (t, J = 7.6 Hz, 2H, CH2), 3.53 (t, J = 6.4 Hz, 2H, CH2), 4.09 (m, 9H, cp-H).

13 C-NMR (100 MHz, CDCl3, rt): δ (ppm) = 28.2 (s, 1C, CH2), 28.8 (s, 1C, CH2), 32.4

(s, 1C, CH2), 44.9 (s, 1C, C-Cl), 67.4 (s, 2C, cp-C), 68.2 (s, 2C, cp-C), 68.7 (s, 5C, cp-C), 88.8 (s, 1C, Cipso).

Synthesis of 4-Iodobutylferrocene[210]

300 mg (1.09 mmol) 4-Chlorobutyl-ferrocene was refluxed over night with 996 mg (6.0 mmol) potassium iodide, in 50 mL dry acetone. After the potassium chloride precipitate was filtered off through a Celite filter, the solvent was evaporated.

Yield: 93 % (1.01 mmol, 371 mg)

1 H-NMR (400 MHz, CDCl3, rt): δ (ppm) = 1.63 (m, 2H, CH2), 1.79 (m, 2H, CH2), 2.32

(t, J = 7.6 Hz, 2H, CH2), 3.53 (t, J = 6.4 Hz, 2H, CH2), 4.09 (m, 9H, cp-H).

180

13 C-NMR (100 MHz, CDCl3, rt): δ (ppm) = 28.2 (s, 1C, CH2), 28.8 (s, 1C, CH2), 32.4 (s,

1C, CH2), 44.9 (s, 1C, C-I), 68.3 (s, 2C, cp-C), 67.5 (s, 2C, cp-C), 68.8 (s, 5C, cp-C),

89.0 (s, 1C, Cipso).

Synthesis of ferrocenyl-5-bromopentylketone[211]

In a 50 mL flask 1.4 mL (7.5 mmol) 1-bromo-6-

chlorohexane and 1.0 g (7.5 mmol) AlCl3 in 10 mL

CH2Cl2 were stirred under cooling in an argon atmosphere for 20 min. The light yellow solution was transferred to a dropping funnel and slowly added to a solution of 1.39 g (7.5 mmol) ferrocene in 15 mL

CH2Cl2. The solution was stirred for 3 h under an argon atmosphere. Afterwards, 50 mL H2O was slowly added via a dropping funnel. After the separation of the phases, the organic layer was washed as long as the pH was confirmed as neutral and subsequently dried over MgSO4 followed by filtration and the removal of the solvent. The resulting product was dried and purified via column chromatography with CHCl3 as eluent.

Yield: 74 % (5.51 mmol, 2.0 g)

1 H-NMR (400 MHz, CDCl3, rt): δ (ppm) = 1.51 (m, 2H, CH2), 1.71 (m, 2H, CH2), 1.90

(m, 2H, CH2), 2.70 (t, J = 7.0 Hz, 2H, CH2), 3.42 (t, J = 6.6 Hz, 2H, CH2), 4.18 (s, 5H, cp-H), 4.48 (s, 2H, cp-H), 4.76 (s, 2H, cp-H).

13 C-NMR (100 MHz, CDCl3, rt): δ (ppm) = 23.5 (s, 1C, CH2), 28.0 (s, 1C, CH2),

32.6 (s, 1C, CH2), 33.7 (s, 1C, CH2), 39.4 (s, 1C, C-Br), 69.3 (s, 5C, cp-C), 69.8 (s, 2C, cp-C), 72.2 (s, 2C, cp-C), 79.2 (s, 1C, Cipso)., 204.2 (s, 1C, C-O).

181

6.3 Instrumentation

Raman spectroscopy:

Statistical Raman spectroscopic characterization was carried out on a LabRam Aramis and HR Evolution confocal Raman microscope (Horiba) with a laser spot size of about 1 µm (Olympus LMPlanFl 50x LWD, NA 0.50) in backscattering geometry. As excitation source the lasers with the excitation wavelength λexc = 532 nm, 633 nm and 785 nm were used depending on the nature of the sample and the incident laser power was kept as low as possible (1.35 mW) to prevent any structural sample damage or possible detachment of functional groups. Spectra were recorded with a CCD array at -70 °C and the chosen grating had 600 grooves/mm. Exact sample movement was provided by an automated xy scanning table. Calibration in frequency was carried out with a diamond as reference. A map size depending of the respective sample was recorded (consisting of 81, 498 or 784 single spectra depending on the accessible sample area) were recorded and statistically evaluated via Labspec 5 software. All given spectra are the respective mean spectra.

Temperature-dependent Raman spectroscopy:

Temperature-dependent Raman measurements were performed in a Linkam Stage THMS 600, using a liquid nitrogen pump TMS94. The measurements were performed on a Si/SiO2 substrate (300 nm oxide layer) with a heating rate of 20 K/min in several stages/steps depending on the kind of sample until the maximum temperature of 450 °C under a constant flow of nitrogen.

In situ Raman spectroscopy

In situ Raman Spectroscopy characterization was carried out in Vienna on a HORIBA JOBIN YVON, model: LabRAM HR (HR800) with a 514 nm excitation wavelength at 0.5 mW. To record in situ measurements, a specific in situ set-up consisting of an inert sample chamber bearing a flat (0.7 mm thick) optical window of borosilicate glass

(PGO GmbH, Germany), which has been filled before in a glovebox (<1 ppm H2O, <1 ppm O2), a liquid inlet for potential functionalization reagents as well as connection to a turbo pump to keep constant vacuum conditions (p< 10-7 mbar).

182

A different way of in situ measurements were carried out as well in our institute. The samples were prepared in a glovebox (<1 ppm H2O, <1 ppm O2) and subsequently transferred to glass ampoule attached to a closable valve. Before, the glass ampoule was heated for 1 hour at high vacuum conditions to avoid traces of water. After the application of high vacuum at a Pfeiffer Ultra High Vacuum System (p~10-7 mbar) the glass ampoules were fused at the end. The resulting ampoules were measured now in the Horiba Raman system under inert conditions.

TGA-MS

The TGA-MS measurements were performed with a Netzsch STA 409 CD Skimmer equipped with a QMS 422 mass spectrometer (MS/EI). All measurements were carried out under helium atmosphere with a gas flow of 80 mL/min with a heating rate of 20 K/min until the maximum temperature of 600 °C or 700 °C was reached.

TG-GC-MS

For TGA/GC-MS measurements, a PerkinElmer Pyris 1 TGA instrument was used. The heating rate was set to 20 K/min at a constant nitrogen flow (70 mL/min) and about 0.5-1.5 mg of the prepared sample was used for analysis. The emerged gases were transferred to the GC unit through a TL9000 TG-GC interface at a constant temperature of 200 °C. The gas-chromatographic separation was achieved by a GC- Clarus 680 with a polysiloxane-coated Elte-5MS capillary column (length: 30 m; diameter: 0.25 mm; film thickness: 0.25 μm). At a selected TG temperature, a certain injection fraction (150 µl) was collected with following parameters: flow rate (He) = 1 mL/min, dynamic ramp = 24 min (10 K/min gradient followed by an isothermal step of 10 min at the final temperature). MS measurements were performed on a MS Clarus SQ8C quadrupole in EI+ (electronic ionization) mode (multiplier: 1300 V) and the obtained data was processed with the TurboMass Software and bibliographic searches were performed with NIST MS Search 2.0.

UV-vis absorption spectroscopy

The measurements were carried out on a Perkin Elmer Lambda 1050 cells with a path length of 10 mm. Quartz cuvettes were used to contain the samples for the measurements.

183

Optical Microscopy

The optical microscopic images were captured with a Zeiss Axio Imager M1m with white light illumination (100 W halogen lamp, HAL100) utilizing bright imaging modes and different 0.80 NA objectives. A Zeiss AxioCam MRc5 has been used to record the images.

EDS/SEM

The energy dispersive X-ray spectroscopy (EDS) was carried out on an Oxford Instruments X-Max detector integrated with a focused ion beam/scanning electron microscope (FIB/SEM) FEI-elios NanoLab 600i FIB Workstation) to determine the chemical composition.

MALDI-ToF

The Laser Desorption/Ionization Time of Flight Mass Spectrometry was carried out on a reflectron time-of-flight mass spectrometer (Reflex IV, Bruker), equipped with a nitrogen laser (λ=337 nm). The target was a ground steel microtiter plate (MTP 384, Bruker).

184

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8 Appendix

Supplementary Spectra

Figure 8.1: TG-MS spectra of hexyl-functionalized graphene activated via Na/K alloy in DME (A/B), under modified Birch conditions (C/D) and using elemental K in THF (E/F) with (right-hand side) and without (left-hand side) prior addition of PhCN including the characteristic mass fragments of a hexyl chain (colored) besides the TG curve (black).

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Figure 8.2: TG-MS spectra of hexyl-functionalized SWCNTs activated via Na/K alloy in DME (A/B), under modified Birch conditions (C/D) and using elemental K in THF (E/F) with (right-hand side) and without (left-hand side) prior addition of PhCN including the characteristic mass fragments of a hexyl chain (colored) besides the TG curve (black).

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Publications

• L. Bao, O. Martin, T. Wei, M. E. Pérez-Ojeda Rodríguez, F. Hauke, and A. Hirsch, A Straightforward Reductive Approach for the Deoxygenation, Activation and Functionalization of Ultrashort Single-Walled Carbon Nanotubes, Carbon 2020, accepted.

• B. Balakrishna, A. Menon, K. Cao, S. Gsänger, S. B. Beil, J. Villalva, O. Shyshov, O. Martin, A. Hirsch, B. Meyer, U. Kaiser; D. M. Guldi, M. von Delius, Dynamic Covalent Formation of Concave Disulfide Macrocycles Mechanically Interlocked with Single-Walled Carbon Nanotubes, Angew. Chem. Int. Ed. 2020, 132, 2-15.

• O. Martin et al., Production and Processing of Graphene and Related Materials, 2D Mater. 2020, 7, 022001.

• K. Amsharov, D.I. Sharapa, O.A. Vasilyev, O. Martin, F. Hauke, A. Goerling, H. Soni, A. Hirsch, Fractal-Seaweeds Type Functionalization of Graphene, Carbon 2019, 158, 435-448.

• T. Wei, O. Martin, S. Yang, F. Hauke, and A. Hirsch, Modular Covalent Graphene Functionalization with C60 and the Endohedral Fullerene Sc3N@C80: A Facile Entry to Synthetic-Carbon-Allotrope Hybrids, Angew. Chem. Int. Ed. 2019, 58, 816-820.

• T. Wei, O. Martin, M. Chen, S. Yang, F. Hauke, and A. Hirsch, Covalent Inter-Carbon-Allotrope Architectures Consisting of the Endohedral Fullerene Sc3N@C80 and Single Walled Carbon Nanotubes, Angew. Chem. Int. Ed. 2019, 0, 1-5.

• C. Halbig,* O. Martin,* F. Hauke, S. Eigler, and A. Hirsch, Oxo-Functionalized Graphene - A Versatile Precursor for Alkylated Graphene Sheets by Reductive Functionalization, Chem. Eur. J. 2018, 24, 13348-13354.

*: These Authors contributed equally to this manuscript.

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Danksagung

Mein besonderer Dank gilt an dieser Stelle meinem Doktorvater Prof. Dr. Andreas Hirsch für die Vergabe des Themas und der Möglichkeit, in seinem Arbeitskreis vorwiegend selbstständig Forschung betreiben zu können.

Des Weiteren richtet sich mein Dank an eine Vielzahl von Menschen, die mich auf diesem Weg begleitet und zudem kontinuierlich unterstützt haben. Darunter sind vor allem meine Kollaborationspartner, Christian E. Halbig, Daniela Dasler, Milan Schirowski, Konstantin Edethalhammer, Tao Wei und Lipiao Bao, mit dessen Zusammenarbeit viele komplexe Themen in Angriff genommen wurden. Intensive Diskussionen und die Planung gemeinsamer Projekte konnte sich in einigen bedeutenden Veröffentlichungen auszahlen.

Außerdem möchte ich mich auch besonders bei Prof. Dr. Konstantin Amsharov bedanken, welcher mit seiner umfangreichen Kompetenz und Kooperationsbereitschaft, das Thema der Monolagenhydrierung mit voranbrachte. Mein Dank gilt weiterhin auch den Kollegen im AK von Prof. Dr. Pichler in Wien, welche uns bei den in situ Raman Messungen stets hilfreich mit Rat und Tat an unserer Seite standen.

Nicht zuletzt geht mein Dank an meine Arbeitskollegen in Erlangen sowie besonders am ZMP in Fürth, durch die die Stimmung und Arbeitsbereitschaft stets sehr hoch gehalten wurde. Der kontinuierliche Einsatz unserer Techniker Franz Pislcajt, Michael Florian und Thomas Neubauer soll auch hervorgehoben werden. Danke an meinen Bürokollegen und Kumpel Andreas Meyer und an alle anderen Doktoranden vom ZMP: Vicent Lloret Segura, Stefan Wild, Michael Fickert, Claudia Kröckel, Udo Mundloch und Lisa Jurkiewicz sowie Katharina Kotz und allen Bachelor-, Master- und Nano- Studenten und HiWis, die unsere Arbeit unterstützt haben. Ein besonderer Dank gilt unserem Team in Erlangen: zum einen den Akademischen Räten, Dr. Michael Brettreich, Dr. Marcus Speck und Dr. Kathrin Knirsch, zum anderen dem Charakterisierungsteam um Dr. Harald Maid und Wolfgang Donaubauer, sowie Prof. Dr. Norbert Jux. Zum anderen danke ich auch allen Mitarbeitern und Doktoranden aus den AKs Hirsch, Jux und Amsharov. 203

Abschließend bedanke ich mich auch herzlich bei Dr. Frank Hauke, den wohl denkbar besten Gruppenleiter, welcher mir durch seine hervorragende Betreuung und Kompetenz wissenschaftlich und menschlich immer mit Rat und Tat zur Seite stand.

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