Developing New Types of Electrode Materials for Dye-Sensitized Solar Cells (Dsscs)

Developing New Types of Electrode Materials for Dye-Sensitized Solar Cells (DSSCs)

Thesis

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University

Panitat Hasin, M.S.

Graduate Program in Chemistry

The Ohio State University

2009

Thesis Committee:

Assistant Professor Yiying Wu, Advisor

Professor Patrick M. Woodward

Panitat Hasin

2009

Abstract

My research focuses on the development of electrode materials in dye-sensitized solar cells (DSSCs). Exploring new materials used as electrodes in DSSCs and

- - investigating their electrocatalytic activity toward tri-iodide (I 3 )/iodide (I ) redox reaction are of great interest and importance. My studies involve two projects: (1) preparing mesoporous Nb-doped TiO 2 as Pt support for cathode in DSSCs, and (2) investigating the

electrocatalytic activity of graphene films and the effect of polyelectrolyte on their

electrocatalytic property for the development of electrodes in DSSCs.

Mesoporous Nb-doped TiO 2 film was prepared by the sol-gel method on a

transparent conducting FTO glass. Pt nanoparticles were impregnated in the mesoporous

TiO 2 support substrate and tested for the counter electrode in dye-sensitized solar cells

(DSSCs). The mesoporous Pt/Nb-doped TiO 2 was characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The electrocatalytic activity of the Pt nanoparticles on different supports was studied by

- - electrochemical impedance spectroscopy (EIS) for the tri-iodide (I 3 )/iodide (I ) redox

reaction in acetonitrile, a common electrolyte solution for DSSC. By fabricating Pt/Nb-

doped TiO 2 electrode, the charge transfer resistance was reduced and the exchange

current density was increased as the result of a larger active

surface area of Pt in the mesoporous Nb-doped TiO 2. This electrode could be used in

other systems where there is a need to control the amount of Pt surface area and hamper

Pt particle aggregation when operating at high temperatures. The impregnation of Pt in

the mesoporous TiO 2 can also improve the mechanical rigidity and stability of the electrocatalyst against abrasion or generally mechanical contact, a desired property for practical applications.

Graphene films obtained by various reduction conditions were deposited on fluorine-doped tin oxide (FTO) substrates and tested for the electrodes of DSSCs. Cyclic voltammetry (CV) results indicate that graphene electrode obtained by thermal reduction

- - shows a higher activity towards I3 /I redox reaction than that of graphene electrodes

obtained by hydrazine reductions. Electrochemical impedance spectra also reveal that the

charge-transfer resistance of graphene obtained by thermal reduction decreases compared

to that of graphene electrodes obtained by hydrazine reductions due not only to decreased

presence of oxidized carbon species, especially alcohol, epoxides, carbonyl, and

carboxylic groups but also to increased defect density confirmed by x-ray photoelectron

spectroscopy and raman spectroscopy, respectively. Moreover, the electrocatalytic

activity of graphene can be tuned by modifying graphene surface with polyelectrolyte.

Effect of the treatment on these properties was evaluated by CV. It was found that the

reduction peak potential of electrode made from graphene treated with cationic

polyelectrolyte (PDAC) shifted more positive, along with enhanced electrocatalytic

- activity. The enhanced electrocatalytic activity toward reduction of I3 might is due to the attraction between cationic polyelectrolyte (PDAC) on graphene surface carrying positive

- charge and I3 carrying negative charge. This electrode could be used as counter electrode iii

in DSSCs. On the contrary, the reduction peak potential of electrode made from graphene treated with anionic polyelectrolyte (PSS) shifted more negative, along with decreased

- electrocatalytic activity. The decreased electrocatalytic activity toward reduction of I3 might is due to the repulsion between anionic polyelectrolyte (PSS) on graphene surface

- and I3 carrying negative charges. This electrode could be used as anode in DSSCs.

Dedication

Hasin and my family

Acknowledgments

First and foremost, I would like to thank my advisor (research director), Assist.

Prof. Dr. Yiying Wu for giving me with generous guidance, expert advice through all stages of this research and other wide-ranging research project. I am also indebted to Dr.

Mario A. Alpuche-Aviles for sharing his knowledge of all things in the laboratory and his contributions to the manuscript that we submitted for publication in co-authorship, of which a modified version is presented in Chapter 2 of this thesis. Without his assistance, the experiments presented in this thesis would not have been realized. I would also like to thank Prof. Dr. Patrick M. Woodward, member of committee for his valuable suggestion and criticism.

I wish to acknowledge the Strategic Scholarships Fellowships Frontier Research

Networks from the Commission on Higher Education, Thailand for financial assistance that supported my study.

Finally, I would like to thank my parents and my friends, who have given me support and encouragement throughout my graduate career. Their love and companionship have been an essential source of strength to me during the course.

vi Vita

16 January 1981...... Born - Bangkok, Thailand

1999-2003...... B.S. Chemistry Thammasat University

2003-2007...... M.S. Chemistry Kasetsart University

2007-2009...... Graduate Researcher Wu Research group, The Ohio State University

PUBLICATIONS

Hasin, P; Alpuche-Aviles, M. A.; Li, Y; Wu, Y. Mesoporous Nb - Doped TiO 2 as Pt Support for Counter Electrode in Dye - Sensitized Solar Cells. Journal of Physical Chemistry C , 2009, 113 (17), 7456.

FIELDS OF STUDY

Major Field: Chemistry

vii

TABLE OF CONTENTS

Page

Abstract………………………………………………………………………...... ii

Dedication…………………………………………………………………………..v

Acknowledgments…………………………………………………………………..vi

Vita………………………………………………………………………………….vii

List of Figures……………………………………………………………………… xi

List of Tables………………………………………………………………………. xv

List of Abbreviations………………………………………………………………. xvi

Chapters:

1 Introduction………………………………………………………………… 1

2 Mesoporous Nb-doped TiO 2 as Pt Support for Cathode in Dye-Sensitized Solar Cells……...………………...…………………………………………11

2.1 Introduction………………………………………………………… 11

2.1.1 Self-Assembly……………………………………………….13 2.1.2 Mechanism of Mesostructure Formation……………………16 2.1.3 Mesoporous TiO 2…………………………………………....20 2.1.4 Mesoporous titania as catalyst support……………………...23

2.2 Experiments…………………………………………………………26

viii

2.2.1 Reagents…………………………………………………...... 26 2.2.2 Instrumentation……………………………………………...27 2.2.3 Experimental methods……………………………………....27 2.2.3.1 Preparation of mesostructured TiO 2………………. 27 2.2.3.2 Preparation of Nb-doped mesostructured TiO 2 film.28 2.2.3.3 Preparation of platinized electrode………………...28 2.2.4 Characterization of experimental electrodes………………..28 2.2.4.1 Scanning electron microscopy (SEM)……………..28 2.2.4.2 Transmission electron microscopy (TEM)………...28 2.2.4.3 Electrochemical impedance spectroscopy (EIS)…..29 2.2.4.4 Cyclic voltammetry (CV)………………………….29

2.3 Results and Discussion…………………………………………….. 30

2.4 Conclusion…………………………………………………………..40

3 Electrocatalytic Properties of Graphene Films toward Tri-iodide Reduction…………………………………………………………………...42

3.1 Introduction : Graphene……………………………………………..42 3.1.1 Preparation of graphene sheet……………………………….44 3.1.1.1 Thermal expansion route…………………………...44

3.1.1.2 Delamination of intercalate graphite……………….49 3.1.1.3 Chemical reduction of exfoliated graphene oxide…51 3.1.2 Electrochemically carbon electrodes………………………..57

3.2 Experiments………………………………………………………....61 3.2.1 Reagents…………………………………………………….61 3.2.2 Instrumentation……………………………………………..61 3.2.3 Experimental method……………………………………….62 3.2.3.1 Preparation of graphene oxide……………………..62 3.2.3.2 Preparation of graphene oxide film………………..63 3.2.3.3 Preparation of graphene film by hydrazine and thermal reductions………………………………....63 3.2.3.4 Surface modification of graphene by polyelectrolyte coating…………………………….64 3.2.4 Characterization of experimental electrodes………………..65 3.2.4.1 X-ray photoelectron spectroscopy (XPS)…………..65 3.2.4.2 Raman spectroscopy………………………………..65 3.2.4.3 Cyclic voltammetry (CV)…………………………..66 3.2.4.4 Electrochemical impedance spectroscopy (EIS)…...66

3.3 Results and Discussion……………………………………………...66

ix 3.4 Conclusion…………………………………………………………..75

4 Conclusion…………………………………………………………………. 77

References…………………………………………………………………………..79

x Lists of Figures

Figure Page

1.1 A schematic structure of dye-sensitized photoelectrochemical cells………. 2

1.2 Schematic representation of the principle of the nanocrystalline injection cell to indicate the energy level in the different phases. The cell voltage observed under illumination corresponds to the difference in the quasi-Fermi level of the TiO2 under illumination and the electrochemical potential of the electrolyte. The latter is equal to the Nernst potential of the iodide/triiodide redox couple used to mediate charge transfer between the electrodes…………………………………………………………………... 3

2.1 TEM micrographs of mesoporous materials. (a) mesoporous organosilica materials of SBA-15 incorporating Lewis acidic chloroindate(III) ionic liquid moieties (b) calcined Al-MCM-41 (c) mesoporous TiO 2–SBA-15 (d) mesoporous 1.0Fe–N–TiO 2…………………………………………….. 13

2.2 Components of a surfactant molecule………...... 14

2.3 The various surfactant structures formed in aqueous solutions: (a) spherical, (b) ellipsoid, (c) cylindrical, (d) bilayers structures………… 15

2.4 Schematic phase diagram for cetyltrimethylammonium bromide (CTAB) in water. Arrow denotes evaporation-driven pathway during dip-coating, aerosol processing, etc…………………………………16

2.5 Schematic representation of the general mesostructure formation from inorganic precursors and organic surfactants…………………………17

2.6 Steady-state film thinning profile established during dip-coating of a complex fluid………………………………………………………….. 18

xi 2.7 Schematic representation of the procedure for preparation mesoporous materials with surfactant as a template via EISA process………………………………………………………………………20

2.8 Scanning electron microscope (SEM) image of (a) Platinized FTO, (b) Pt/TiO 2/FTO, and (c) Pt/Nb(1.0 mol%)-doped TiO 2 on FTO………….. 32

2.9 TEM images of Pt NPs supported on Nb(1.0 mol%)-doped TiO 2 film. The Pt loading is 25 µg cm -2……………………………………………….. 33

2.10 (a) Nyquist plots of Pt, Pt/TiO 2, Pt/Nb(0.5 mol%)-doped TiO 2 and Pt/Nb 1.0 mol%)-doped TiO 2 cells. (b) Pt/Nb(1.0 mol%)-doped TiO 2 cells showing the fitting to the equivalent circuit shown in Figure 2.11. Electrolyte: 0.1 M I2, 0.1 M LiI, 0.6 M tetrabutylammonium iodide, 0.5 M TBP in MeCN……………………………………………………………... 35

2.11 Equivalent circuit used to fit the high frequency region of the impedance spectra of symmetrical cells in order to obtain charge transfer resistance (Rct ), which contains the uncompensated resistance ( Ru), and constant phase element (CPE)……………………………………………………….. 36

2.12 Cycilc voltammograms of the various electrodes in acetonitrile solution of - I3 (1 mM LiI, 1 mM I 2 and 0.1 M TBAFB, cathodic currents as negative).. 39

2.13 Variation in exchange current density with platinum loading for the platinized and Pt/Nb (1.0 mol%) TiO 2 electrodes on a FTO substrate…….. 40

3.1 Graphene structure………………………………………………………… 42

3.2 Graphene is a 2D building material for carbon materials of all other dimensionalities. It can be wrapped up into 0D buckyballs, rolled into 1D nanotubes or stacked into 3D graphite……………………………………... 43

3.3 SEM image of (a) natural graphite flake (b) expanded graphite (c) expanded graphite surface structure (d) freshly broken graphite worm section (inner surface)……………………………………………………… 46

3.4 TEM of thinner sheets inside the expanded graphite……………………..…47

3.5 Schematic model for the expanded graphite consisting of graphite nanosheets and layer structure………………………………………………48

xii

3.6 Schematic diagram showing the intercalation and exfoliation process to produce graphite nanoplatelets (GNP). Graphite is intercalated with potassium metal to form the first stage compound KC8. Exfoliation in ethanol produces potassium ethoxide and hydrogen gas which aid in separating the graphitic sheets to form graphite nanoplatelets…………….. 50

3.7 Scanning electron micrographs of (a) starting graphite and (b) after intercalation with potassium and exfoliation with ethanol………………… 51

3.8 Structural model for graphene oxide………………………………………..52

3.9 A non-contact mode AFM image of exfoliated GO sheets with three height profiles acquired in different locations……………………….. 53

3.10 (a) An SEM image of aggrega ted reduced GO sheets. (b) A platelet having an upper bound thickness at a fold of ~2 nm………... 55

3.11 Schematic diagram showing chemical reduction of exfoliated graphene oxide………………………………………………………………………... 56

3.12 Schematic diagrams showing the overhead view of a section of the basal plane HOPG surface……………………………………………………….. 57

3.13 Schematic diagram of 4 layer step edge…………………………………… 58

3.14 Cyclic voltammogram recorded at V s-1 for the oxidation of 1 mM ferrocyanide in 1 M KCl solution at (i) a basal plane HOPG electrode and (ii) an edge-plane pyrolytic graphite electrode………………59

3.15 The structures of (a) poly(diallyldimethylammonium chloride) (PDAC) (b) poly(4-styrenesulfonic acid) (PSS)……………………………………...65

3.16 High-resolution XPS analysis of the effect of different reduction treatments on the graphene oxide films. Deconvolution revealed the presence of C-C (~284.7 eV), C-O (~285.7 eV), C=O (~288.0 eV) species in the film. (a) Hydrazine reduction at 200 °C for 2.5 h (b) Hydrazine reduction at 400 °C for 5 h…………………………………. 68

3.17 (a) The Raman spectra of graphene obtained from hydrazine reduction at 200 °C for 2.5 h (b) hydrazine reduction at 400 °C for 5 h (c) thermal reduction at 400 °C for 5 h…………………………………………………..69

3.18 The recorded cyclic voltammograms of graphene films reduced by - various conditions in acetonitrile solution of I 3 (1 mM LiI, 1 mM I 2 and 0.1 M TBAFB, cathodic currents as negative). The scan rate is

xiii 0.1 V s−1 and the expose area of the graphene electrodes is 0.28 cm 2……... 71

3.19 Nyquist plots of graphene obtained by hydrazine reduction at 200 °C for 2.5 h, at 400 °C for 5 h, and thermal reduction at 400 °C for 5 h cells. Electrolyte: 0.1 M I 2, 0.1 M LiI, 0.6 M tetrabutylammonium iodide, 0.5 M TBP in MeCN………………………...72

3.20 Cyclic voltammograms of graphene films with and without - polyelectrolyte coating in acetonitrile solution of I 3 (1 mM LiI, 1 mM I 2 and 0.1 M TBAFB, cathodic currents as negative). The scan rate is 0.1 V s−1 and the expose area of the graphene electrodes is 0.28 cm 2……………………………………………………….75

xiv Lists of Tables

Table Page

2.1 Pore-size regimes and representative porous inorganic materials…………. 13

2.2 Results of the impedance measurements……………………………………36

3.1 Results of the impedance measurements……………………………………72

xv Lists of Abbreviations

PB 1-pyrenebutyric acid

TBP 4-tertbutylpyridine

MeCN Acetonitrile

Ǻ Angstrom

AFM Atomic Force Microscopy

BET Brunauer-Emmett-Teller cm Centimeter cm 3 Cubic centimeter

CTAB Cetyltrimethylammonium bromide

CTAC Cetyltrimethylammonium chloride

Rct Charge transfer resistance

CVD Chemical vapor deposition

CPE Constant phase element

xvi cmc Critical micelle concentration

CV Cyclic voltammetry

°C degrees Celsius

°C/min degrees Celsius/minute

DSSCs Dye sensitized solar cells eppg Edge-plane pyrolytic graphite

EIS Electrochemical impedance spectroscopy

EHE Electrochemically heterogeneous electrode eV Electron volt

EISA Evaporation-Induced Self Assembly

Jo Exchange current density

FTO Fluorine-doped tin oxide g Gram

G Graphene

GO Graphene oxide

GNP Graphite nanoplatelets

HOPG Highly ordered pyrolytic graphite

xvii h Hour

HDS Hydrodesulfurization

IPCE Incident photon-to-current conversion efficiency

IUPAC International Union of Pure and Applied Chemistry kV Kilovolt

MC Mesoporous carbon m Meter

µg cm -2 Microgram/square centimeter

µm Micrometer mA/cm 2 Milliamp/square centimeter mg l -1 Milligram/liter mg ml -1 Milligram/milliliter ml Milliliter mm/s Millimeter/second mM Millimolar mΩ cm 2 Milliohm- square centimeter mV s -1 Millivolt/second

xviii min minute nm Nanometer

NPs Nanoparticles

N Normality

Ω Ohm

Ω cm 2 Ohm-square centimeter

1 D One-dimensional

PProDOT-Et 2 Poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine)

PEDOT Poly (3,4-ethylenedioxythiophene)

PANI Polyaniline

PDAC Poly(diallyldimethylammonium chloride)

PPy Polypyrrole

PSS Poly(4-styrenesulfonic acid)

Pt/AB Pt-loading acethylene-black cm -1 Raman shift

RF Roughness factor

SEM Scanning electron microscopy

xix STM Scanning tunneling microscopy cm 2 Square centimeter m2/g Square meter/gram

TBAFB Tetrabutylammonium fluoroborate

3 D Three-dimensional

TEM Transmission electron microscopy

TMA + Trimethylammonium

2 D Two-dimensional

Ru Uncompensated resistance

V Volt

V s -1 Volt/second w.d. Working distance

XPS X-ray photoelectron spectroscopy

0 D Zero-dimensional

ZP ZnO nanoparticles

ZW ZnO nanowire

ZWTP ZnO nanowire-covered TiO 2 nanoparticle

xx Chapter 1

Introduction

The use of fossil fuels causes several problems such as the air pollution and green house effect. These problems are worsening owing to the rising energy demands throughout the world, especially the necessity for energy happening in the remote areas. In addition to the pollution caused by their combustion, the increasing price of fossil fuels as well as their rapid exhaustion is giving impetus to the development of renewable energy sources. Currently, solar energy is the most promising form of sustainable energy. Dye sensitized solar cells (DSSCs) have currently attracted widespread commercial and academic interest due to their relatively high efficiency and low production cost 1-7. A DSSC consists of three layers: a photoanode, an iodide/tri-iodide redox electrolyte layer and a platinized counter electrode. The photoanode is a thin porous layer of annealed nanocrystalline semiconductor electrode such as TiO 2 supported on transparent conducting glass. Dye molecules such as ruthenium bipyridine derivatives, which are sensitive to the visible light region in the solar spectrum, are attached by chemisorption onto the semiconductor electrode. The three layers are sandwiched together as shown in

Figure 1.1 8.

Figure 1.1 A schematic structure of dye-sensitized photoelectrochemical cells 9.

The photoexcitation of dye molecules leads to an electron injection from the excited state of the dye molecule into the conduction band of the semiconductor.

Subsequently, the injected electrons are collected by the substrate and pass through the external circuit via electronic load. Simultaneous, the oxidized dye is reduced and turns back to the ground state by the electrolyte. The electrons that pass through the external circuit come to the counter electrode and take part in the reformation of triiodide as shown in the following equation 1.1 9-11 :

- - - I3 + 2e 3I (1.1)

The operation principle of a DSSC is shown schematically in Figure 1.2.

Figure 1.2 Schematic representation of the principle of the nanocrystalline injection cell to indicate the energy level in the different phases. The cell voltage observed under illumination corresponds to the difference in the quasi-Fermi level of the TiO 2 under illumination and the electrochemical potential of the electrolyte. The latter is equal to the Nernst potential of the iodide/triiodide redox couple used to mediate charge transfer between the electrodes 9.

Continuous efforts have been invested around the world to enhance the performance and longevity for instance the minimization of charge recombination between semiconductor and oxidized sensitizer by using two semiconductors with different energy levels, 12-17 the improvement of new dye sensitizers which show their strong absorption in

the visible and near infrared regions, 18-20 and preparation of solid-state dye solar cells by assembling with a polymer electrolyte, 10,21-24 etc. However, not much work has been done on counter electrodes in DSSCs 25-33 .

Cathode or counter electrode is one of the most critical component in DSSCs,

- which reduces tri-iodide (I 3 ) back to

iodide (I -) used as a redox charge mediator in regenerating the light absorbing sensitizer after electron injection, according to equation 1.1 34 . In order to achieve higher

- - electrocatalytic activity, the eligible requirement of cathode is that the I /I 3 redox reaction

- - rate on cathode should be fast. In other words, electrocatalytic activity in I /I 3 redox

reaction should be high.

Several studies have shown that platinum is the best candidate for use as counter electrode however, its limitations including no controlling of Pt size and high cost material are not in accordance with the favorable characters of counter electrode.

Currently, many groups have demonstrated improvement of the electrocatalytic activity of counter electrode in DSSC. For example, Qinghua Li et al. reported on using microporous polyaniline (PANI) as counter electrode for DSSC 35 . The overall energy conversion efficiency of the DSSC with PANI counter electrode reaches 7.15%, which is higher than that of the DSSC with Pt counter electrode (6.90%) under the same conditions. The improvement of overall energy conversion of DSSC with PANI counter electrode mainly comes from its unique properties, such as high-conductivity, good

- stability, and catalytic activity for I 3 reduction. PANI counter electrode shows excellent photoelectric properties, easy synthesis as well as inexpensiveness and as such is now sometimes used in DSSCs practical applications. Jihuai Wu et al. synthesized the polypyrrole (PPy) nanoparticle and coated on a conducting fluorine-doped tin oxide

(FTO) glass to construct PPy counter electrode used in DSSC 36 . Under the same conditions, overall energy conversion efficiency of the DSSC with PPy counter electrode is 7.66%, which is 11% higher than that of the DSSC with Pt counter electrode (6.90%) due to its larger surface area, small charge-transfer resistance and high electrocatalytic

− activity for the I 2/I redox reaction. The PPy electrode can be an alternative to conventional Pt electrode because of its mechanical and chemical stability, high conductivity, and simple preparation procedure. Pinjiang Li et al. prepared the platinum counter electrode for application in DSSCs by electrodepositing platinum nanoparticle on a FTO conductive glass sheet 37 . Overall energy conversion efficiency of 6.40% was achieved for DSSC with such a counter electrode under one sun illumination (AM1.5,

100 mW cm −2 ). Using platinum electrode prepared by electrodeposition, the light absorption of the solar cells could be improved because light may be irradiated from both sides of the TiO 2 electrode and the counter electrode resulted into improving the photovoltaic performances of DSSCs. Moreover, the platinum loading can also be reduced to reduce the production cost of DSSCs. Fengshi Cai et al. reported on the preparation of acethylene-black (Pt/AB) with low Pt loading of 2.0 µg cm −2 and their high electrocatalytic activity in DSSCs 38 . The light-to-electricity energy conversion efficiency of the DSSCs using Pt/AB electrode with low Pt loading reached 8.6% due to its excellent electric conductivity, large specific surface area and strong adsorptive ability. This study should be commercially realistic for reducing the cost i of DSSCs manufacturing. Pinjiang Li et al. investigated the electrocatalytic activity of Pt/Carbon black counter electrode 39 . They concluded that the Pt/Carbon black counter electrode showed energy conversion efficiency of 6.72% under one sun illumination indicating that the Pt/Carbon black electrode had a high electrocatalytic activity for the reduction of triiodide because of a great amount of edges which are the catalytic active sites for iodide/triiodide redox reaction. Since Pt/Carbon black electrode is inexpensive and shows energy conversion efficiency identical to Pt electrode, it is practical to use it as the

5 counter electrode in DSSCs. Guiqiang Wang and Yuan Lin analyzed the photovoltaic performance of DSSCs with the counter electrode based on NiP-plated glass and titanium plate 40 . Owing to high light reflectivity and low sheet resistance of Pt/NiP and Pt/TiP electrodes, the overall energy efficiency of dye-sensitized solar cells with Pt/NiP and

Pt/TiP electrodes was increased by 32% and 27%, respectively when compared with the cell using Pt/FTO electrode. Guiqiang Wang et al. analyzed the photovoltaic performances of DSSCs with mesoporous carbon (MC) counter electrode 41 . Because of accessible porosity, high surface area, high electrical conductivity, and high chemical, thermal as well as mechanical stability of MC, the photovoltaic performances of DSSCs with MC counter electrode were improved (6.18%) by increasing the carbon loading on counter electrode which is comparable to that of DSSCs with conventional platinum counter electrode (6.26%). Kun-Mu Lee et al. prepared several poly(3,4- alkylenedioxythiophene)-derived films by electrochemical polymerization used as counter electrodes for DSSCs 42 . Cells fabricated with a poly(3,3-diethyl-3,4-dihydro-2H- thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et 2) counter electrode had the best conversion efficiency of 7.88% amongst all the derivatives. This conversion efficiency is comparable to that of cells fabricated with sputtered-Pt (7.77%) electrodes owing to its

− high effective surface area and good catalytic properties for I 3 reduction.

Another essential component in DSSCs is the anode, which accepts electrons injected from the photoexcited sensitizers. Contrary to the requirement of cathode, the

- - desirable criteria of anode is that the electrocatalytic activity toward I /I 3 redox reaction

- should be weak in order to prevent the recombination of e anode with oxidized redox agent

- (I 3 ) causing the reduction of energy conversion efficiency of the cell

- - - 2e anode + I3 3I (1.2)

Traditionally, nanocrystalline TiO 2, particularly in the anatase phase is usually

employed as the photoanode of DSSCs 1,43 . In theory, the maximum efficiency of a single-

absorber solar cell is approximately 33%; however, DSSCs currently reach an efficiency

of 10%. It is still far away from the theoretical efficiency value 44 . In order to make breakthroughs in progress, many research groups have made efforts to enhance performance of some aspects, for instance maximization of light absorption efficiency by increasing the surface area of photoanode materials. Yang-Qin Wang et al. investigated photoelectrochemical properties in DSSCs utilizing mesoporous TiO 2 under different

45 conditions synthesized by ultrasonic waves . Cell fabricated with mesoporous TiO 2 sintered at 450 °C had the best performance. The improvement of performance is attributed to the more ordered structures required for high solar cell conversion efficiencies. Mingdeng Wei et al. introduced mesoporous TiO 2 to a DSSC and obtained a remarkably high efficiency of 10.0% under illumination of simulated AM 1.5 solar light

(100 mW cm -2) due to its high surface area, uniform nanochannels and a homogeneous

44 nanocrystalline TiO 2 several nm in size arranged along the framework . Mesoporous

2 -1 TiO 2 having high surface area (approx. 200 m g ) can adsorb dye in a large volume,

resulting in a higher photocurrent density. Sheng Xianliang et al. introduced high surface

area ZnO nanotube photoanodes templated by anodic aluminum oxide for use in dye-

sensitized solar cells (DSSCs) 46 . When ZnO nanotube photoanode was used in the cell,

the power conversion efficiency reached 1.6% due to its relatively high area

photoelectrodes (RF ~ 350-450) resulting in reasonable light-harvesting efficiency,

excellent photovoltage, and good fill factors in addition to moderate power efficiency.

Lei Yang et al. presented a strategy to improve the performance of DSSC when the as-

47 synthesized TiO 2 rough spheres were used as photoanode . The overall conversion

efficiency of the photo-anode made from rough TiO 2 spheres reached 7.36%, which is

over 1.11% higher than that of smooth ones because of their high light scattering effect

2 −1 and large BET surface area (80.7 m g ). TiO 2 rough spheres can greatly enhance the light harvesting efficiency of the photoanode, and improve Jsc with a little decrease of the dye loading.

In addition, various studies also show an improvement in the power conversion efficiency by suppressing the charge recombination between the electrons injected in the conduction band of the semiconductor and the oxidized sensitizer. Weon-Pil Tai reported

the photoelectrochemical properties of RuL 2(NCS) 2 dye-sensitized nanocrystalline

16 SnO 2:TiO 2 coupled (bilayer) system . The high incident photon-to-current conversion

efficiency (IPCE) value of 82.4% at 530 nm was obtained by the DSSC composed of

SnO 2/TiO 2 coupled (bilayer) system. This has been ascribed to a better charge separation by fast electron transfer process between the conduction band of TiO 2 and the low lying

conduction band of SnO 2. P. K. M. Bandaranayake et al. constructed DSSCs with

nanocrystalline films CdS and [CdS]MgO (i.e., the films where the CdS crystallites are

coated with an outer shell of MgO) 14 . A photoelectric conversion efficiency of

approximately 2.1% were achieved for cell composed of [CdS]MgO. From this result, it

is possible that electron injection from excited dye molecules on the outer magnesium

oxide shell to the conduction band of cadmium sulfide isvery fast thus suppressing the

relaxed electron leakage to the interface by the magnesium oxide barrier, where

recombination takes place. Some researchers had introduced an array of oriented single-

8 crystalline nanowires replacing the nanoparticle TiO 2 films in order to increase the

electron diffusion length in the anode resulting in increasing the efficiency of DSSCS.

Matt Law et al. substituted the nanoparticle film by introducing a dense array of oriented,

crystalline ZnO nanowires utilized as the anode 48 . Better electron transport can be

observed when ZnO nanowires are used as the anode due to their higher crystallinity and

internal electric fields that can separate injected electrons from the surrounding redox

mediator and drive them towards the transparent conducting electrode resulting in

improving carrier collection. Yuqiao Wang et al. improved the charge transfer of anode

by fabrication of oriented ZnO nanowire-covered TiO2 nanoparticle (ZWTP) electrode

for a high efficient DSSC 49 . It was found that ZWTP composite film electrodes exhibited

higher power conversion efficiency of 2.15% than those of ZnO nanoparticles (ZP) cell

(0.44%) and ZnO nanowire (ZW) cell (1.46%). The enhancement of the power

conversion efficiency can be explained in terms of rapid charge transfer.

In my studies presented in this thesis, Pt nanoparticles in mesoporous Nb-doped

TiO 2 counter electrodes were prepared to promote the interfacial electron transfer

- - properties between Pt/Nb-doped TiO 2 on FTO and the tri-iodide (I 3 )/iodide (I ) redox electrolyte. The use of Nb doping and mesoporous TiO 2 is necessary to increase the

conductivity of the film and the active surface area of Pt nanoparticles, respectively. The

experiments presented in Chapter 2 probed their morphology and microstructure by

Scanning electron microscopy (SEM) as well as Transmission electron microscopy

- - (TEM) and analyzeed the electrocatalytic activity toward I /I 3 redox reaction of Pt nanoparticles in mesoporous Nb-doped TiO 2 by Electrochemical impedance spectroscopy

(EIS) inclding Cyclic voltammetry (CV).

In addition, graphene obtained from reduced graphene oxide with various reduction conditions was prepared to study the possibility to utilize graphene as the electrodes in DSSCs. The expereiments presented in Chapter 3 studied the process parameters of graphene oxide reduction in two categories including the effect of reduction methods and temperatures. The comprehensive structural study of graphene was investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy.

- - The electrocatalytic properties of graphene for the tri-iodide (I 3 )/iodide (I ) redox

reaction were indicated by EIS and CV. Moreover, we also tuned the electrocatalytic

activity of graphene films by modifying graphene surface with polyelectrolytes and

studied their electrocatalytic activity by CV.

Chapter 2

Mesoporous Nb-doped TiO 2 as Pt Support for Counter Electrode in Dye-

Sensitized Solar Cells

2.1 Introduction

Mesoporous materials have prominent properties such as large surface area and uniform pore sizes with molecular dimensions 50-54 . The possibility of

incorporating guest molecules into the structure and the ion exchange capacities make

mesoporous materials unique in several processes (catalysis, adsorbents, membrane

separations, molecular sieves and catalyst supports etc.) 55-57 . Therefore, mesoporous

materials with well-controlled structures and pore sizes are of great interest 58 .

After the discovery of mesoporous molecular sieves, in particular, the M41S

family of ordered mesoporous silicas at the beginning of the 1990s by the Mobil oil

researchers, the synthesis of advanced mesoporous materials has attracted much

research attention 59-60 . Several national and international research programs have investigated the novel architecture and composition of mesoporous materials in view of specific applications in areas as diverse as catalysis 61 , sorption 62 , separations 63 , sensing 64 , optics 65 , drug delivery 66 , etc. According to International Union of Pure and

Applied Chemistry (IUPAC) definition, inorganic solids comprising pores with diameters

between 20-500 Å are classified mesoporous materials. Examples of mesoporous

materials include M41S, aerogels, and pillared layered structures, as listed in Table 2.150,

67 .

Table 2.1 Pore-size regimes and representative porous inorganic materials 50 .

Pore-size regimes Definition Examples Actual size range

Macroporous > 500 Å Glasses > 500 Å

Mesoporous 20-500 Å Aerogels > 100 Å

Pillared layered clays 10 Å, 100 Å [a]

M41S 16-100 Å

Microporous < 20 Å Zeolites, zeotypes < 14.2 Å

Activated carbon 6 Å

[a] Bimodal pore-size distribution.

Figure 2.1 shows the examples of mesoporous material images.

(a) (b)

Figure 2.1 TEM micrographs of mesoporous materials. (a) mesoporous organosilica materials of SBA-15 incorporating Lewis acidic chloroindate(III) ionic liquid moieties 68 69 70 (b) calcined Al-MCM-41 (c) mesoporous TiO 2–SBA-15 (d) mesoporous 1.0Fe–N– 71 TiO 2 .

2.1.1 Self-Assembly

During the last decade, the novel family of mesostructured materials synthesized by evaporation-induced self-assembly with surfactant-templating has played an important role in materials chemistry 72 . The first observation began with the discovery of ordered mesoporous silicate, designated M41S and FSM-16 materials, by Mobil Corp 59, 73 . and

Kuroda et al. 74-75 ,respectively. These materials have been of growing interest because of

13 the extremely high surface areas (>1000 m 2g-1)76 and the well-defined pore sizes 77 . Most eminently, their pore sizes could be adjusted in the range of nanometer by employing self-assembled molecular aggregates or supramolecular assemblies as the structure- directing agents 78 . Therefore, novel sorbents 62 , sensors 62 , catalysts 79-80 , and host molecules for large guest materials 81 were conceived.

A general definition of self-assembly is a processes in which the spontaneous and reversible organization of disordered system of pre-existing molecular units into an organized pattern or structure as a consequence of specific, intermolecular interactions

(e.g. Van der Waals, capillary, π − π, hydrogen bonds), without external intervention.

Self-assembly usually utilizes supramolecules to organize into well-defined the surfactant

molecular self-assemblies 82 . Most common are amphiphilic surfactant molecules

containing both hydrophilic groups (a polar head) that are compatible with water and

hydrophobic groups (a nonpolar tail) that are compatible with oil as shown in the Figure

2.2.

Figure 2.2 Components of a surfactant molecule 83 .

In aqueous solution above the critical micelle concentration (cmc), the surfactants start aggregating into micelles, may be spherical, ellipsoid, cylindrical or bilayers structures maintaining the hydrophilic head regions of the surfactant in contact with

surrounding water, sequestering the hydrophobic single tail regions in the micelle interior

as shown in the Figure 2.3.

(a) (b)

(c)

(d)

Figure 2.3 The various surfactant structures formed in aqueous solutions: (a) spherical, (b) ellipsoid, (c) cylindrical, (d) bilayers structures 84-85 .

Increasing concentration of surfactant results into the self-organization of micelles into periodic hexagonal, cubic, or lamellar mesophases as shown in Figure 2.4.

Figure 2.4 Schematic phase diagram for cetyltrimethylammonium bromide (CTAB) in water. Arrow denotes evaporation-driven pathway during dip-coating, aerosol processing, etc 82, 86 .

2.1.2 Mechanism of Mesostructure Formation

A variety of models have been proposed to describe the mesoporous material formation and to provide a fundamental explanation of the several synthesis methods.

Commonly, these models are based on the existence of surfactants in a solution in order to suggest the inorganic mesostructure formation from the dissolved inorganic precursors as shown in the Figure 2.5.

Figure 2.5 Schematic representation of the general mesostructure formation from inorganic precursors and organic surfactants 50 .

The sol-gel dip coating condition in Figure 2.6 suggests an alternative method to

prepare continuous films with attainable porosity promising applications of

mesostructured materials. The process begins with a homogeneous solution of soluble

inorganic precursor and surfactant prepared in ethanol/water solvent with an initial

surfactant concentration co « the critical micelle concentration cmc. Ethanol evaporates

concentrating the film in water and inorganic species including nonvolatile surfactant

(Figure 2.6). By ethanol evaporation, the progressive exceeding the cmc of a bulk

inorganic-surfactant solution induces self-assembly of inorganic-surfactant and their

further developing into liquid-crystalline mesophases (Figure 2.6) 87-89 . The developing

inorganic-surfactant mesostructures present at solid-liquid and liquid-vapor interfaces at c

< cmc utilized to nucleate and arrange developing mesophase 90-91 . Consequently, the highly oriented thin film mesophases with respect to the substrate surface are rapidly formed. By varying co, it is enable to select the films at different final morphologies

occurring at the drying process. For example, the ordered mesophase structures, 1-D

hexagonal, cubic, 3-D hexagonal and lamellar silica-surfactant mesophases were formed

under acid conditions with substoichiometric amounts of water and

17 cetyltrimethylammonium chloride (CTAC) were employed in the synthesis route 87-88, 92 .

Such mesophases showed well ordered porosity are essential for low dielectric-constant

applications.

Figure 2.6 Steady-state film thinning profile established during dip-coating of a complex fluid 82, 87 .

A rapidly dynamic self-assembly process carried out in a concentration gradient by dip-coating operation was depicted in Figure 2.6. Its steady, perhaps liquid-crystalline or micellar species are continuously gradual growth onto organized mesostructure interfaces. With increasing distance above the reservoir surface, the micellar domains become advancingly larger ingoing from the interfaces of solid-liquid and liquid-vapor

(Figure 2.6). Deposited films exhibit optically transparency and absolutely characterless on the micrometer-length scale.

To rapidly organize thin film mesophases, it is crucial to restrain polymerization of inorganic precursor during the dip-coating process. To achieve this purpose, operation

under acidic conditions at a concentration of hydronium ion corresponding nearly to the

isoelectric point of colloidal inorganic species is acquired 93 . The hydrolysis reaction

happens at this stage, a hydroxyl ion becomes attached to the metal atom as illustrated in

following equation 94 :

M(OR) 4 + 4 H 2O —> M(OH) 4 + 4 R-OH (2.1)

Thus, impeding condensation of inorganic precursor is required in order to facilitate inorganic-surfactant self assembly cooperation to carry out. The resulting as- deposited films present liquid-crystalline (semi-solid) behavior. Consequently, drying or aging process is needed to remove the remaining solvent (liquid) phase normally followed by a significant amount of densification and shrinkage. At this process, the condensation of metal hydroxide molecules takes place to form a 1, 2, or 3- dimensional network [M–O–M] bonds followed by the production of H-O-H and R-O-H species as shown in the following equations 93 .

M-OH + HO-M —> M-O-M + H2O (water condensation) (2.2)

M-O-R + HO-M —> M-O-M + R-OH (alcohol condensation) (2.3)

This can solidify the inorganic skeleton locking in the desired mesostructure. The

distribution of porosity in the mesostructure can be ultimately controlled by the rate at

which the solvent can be removed. Apparently, the changes of the surfactant or template

structure during this processing phase have strongly influenced the final microstructure of

the final component. Subsequently, a thermal treatment, or firing process, is often

compulsory in order to proceed further polycondensation and improve the structural

stability and mechanical properties via final annealing, solidification, and particle growth.

The densification is often accomplished at a much lower temperature. This is one of the

distinguished advantages of Evaporation-Induced Self Assembly (EISA) process as

opposed to the more conventional processing routes 82, 87, 95.

The formation of the mesostructure materials with surfactant as a template via

EISA process can be shown in Figure 2.7.

= Surfactant

= Inorganic precursor

Thermal

Evaporation treatment

Figure 2.7 Schematic representation of the procedure for preparation mesoporous materials with surfactant as a template via EISA process 96.

2.1.3 Mesoporous TiO 2

Only six years have passed, mesoporous TiO 2 materials are widespread interest in

the use of ordered mesoporous materials for photocatalysts 97 , highly specific chemical sensors 98 , solar cells 99 and catalysts support 100 . Since the kinetics of hydrolysis and

condensation reactions of the titanium precursor are very large resulting in non-porous

101 and even poorly structured materials . Owing to its difficulty to form TiO 2 mesophase,

such problem confines their applications. When mesoporous TiO 2 materials are prepared by sol-gel methods, the crystallization of TiO 2 tends to induce the collapse of the mesoporous strcture 80, 102-103 . Thus, it is difficult to definitely manipulate the material's strong tendency to precipitate and crystallize. In the last few years, many of these

disadvantages have been addressed and overcome to develop structures of TiO 2 materials

on a nanometer scale in a controlled method. In 1995, Antonelli and Ying first prepared a

thermally stable mesoporous (templates were removed) TiO 2 material with a hexagonal pore structure, but phosphorous from the molecular sieve used was difficult to remove causing their application was restricted when used as a catalyst 80 . Ulagappan and Rao

synthesized mesostructured (templates were not removed) TiO 2 materials with neutral amine surfactants; however, they could not form mesoporous TiO 2 because the

mesostructure was destroyed during amine removal process 104 . A variety of types of mesostructured TiO 2 materials using different phosphate surfactants were synthesized by

Fröba et al; however, after the templates were removed, the mesostructures also collapsed 105 . Yang et al. first succeeded in synthesizing thermally stable phosphorous- free mesoporous TiO 2 materials with hexagonal and cubic pore structures from a non-

102-103 aqueous solution by restraining the crystallization reaction of TiO 2 . Dai et al.

accomplished in removing alkylphosphate and alkylamine surfactants utilized as a

template in synthesizing hexagonally ordered TiO 2 materials without collapsing of the mesoporous structure by refluxing 106 . Trong on had prepared lamellar and hexagonal

+ + TiO 2 from solutions containing trimethylammonium (TMA ) and Na , respectively;

107 however the obtained materials had low thermal stability . Wang et al. prepared TiO 2 materials by sonochemical synthesis to reduced the reaction time but the obtained materials only were a wormhole-like structure 108 .

An alternative approach to modify the physical and chemical properties of

108 mesoporous TiO 2 is a sol-gel method using a triblock copolymer as template . The

precise control over the pore sizes and surface properties of the mesoporous TiO 2 can be

achieved by utilizing a triblock copolymer as template 109 . In addition, a triblock copolymer can also stabilize the materials towards hydrolysis 110 . Bulk properties of

mesoporous TiO 2 can be influenced by combining organic and inorganic molecules in the mesostructures 111 . The organic aspects can contribute flexibility into the framework, or alter the optical properties of the solid while inorganic compounds can offer thermal, mechanical, or structural stability 112. In the last few years, much advancement of mesoporous TiO 2 development has been made towards applications of mesostructured

materials in a various fields. The functionalized mesoporous TiO 2 at specific site presents

enhanced selectivity, stability, and activity in a plenty of sorption process and catalytic

reactions.

To obtain maximum utilization from the materials in their applications, it is

necessary to synthesize mesoporous TiO 2 materials in a thin film form. This is

specifically essential for photonic and electronic devices 89, 113-115. Although a wide variety of new methods to prepare mesostructured TiO 2 thin film have been reported, such as the spray-on process 116, chemical vapor deposition (CVD) process 117,

evaporation-induced self-assembly (EISA) process 118, etc., the EISA process would be considered as the most practical one for mesostructured TiO 2 thin film synthesis because it has several eminent advantages. First, EISA enables the rapid production of thin film porous materials 119. Second, EISA process can tune the thickness of the mesostructured films by the multiple dipping process 120 . Third, EISA process provides thin film with

nano-size pore diameter and narrow pore size distribution 121 .

2.1.4 Mesoporous titania as catalyst support

During the last few years, the mesoporous titania has attracted considerable

attention for a wide range of catalyst support 122-124 . The mesoporous titania can improve the overall efficiency of the catalytic processes because: 1) the mesoporous titania has remarkably large surface areas and narrow pore size distributions 102-103, 125 ; 2) mesopores can accommodate molecules of various sizes 77 ; 3) a high dispersion of chemically active species can easily be achieved 76 ; and 4) the mesoporous titania support would give rise to

well dispersed and stable metal particles on the surface upon calcination and reduction 78 .

Usually, mesoporous titania supports are initially synthesized using relatively

inexpensive titanium tetraalkoxide templated with surfactant such as polyamines,

polyalkyl halides, polyalkenes, polynitriles, or polythiols. These moieties can be further

developed by EISA in order to obtain mesoporous titania supports with a narrow

mesopore size distribution and controlled pore structure as mentioned earlier. Reactions

that have been studied using mesoporous titania supports include solid acid catalysis 126 ,

heterogeneous base catalysis 127 , oxidations 128 , reductions 122 , and enantioselective catalysis 129 .

It has been observed that the performance of catalysts supported by mesoporous titania is dissimilar to that of catalysts without mesoporous titania support. In several studies, the mesoporous titania limits the catalyst agglomeration enhancing the performance compared to the catalysts supported by amorphous or non-porous materials.

Improving their activity is attributed from improved selectivity in a sterically homogeneous catalysis or superior catalyst turnover caused by the catalyst stabilization within the channels. In other examples, confined accessibility of the active sites in the

mesopores reduced the catalytic activity of the catalysts supported by mesoporous titania

compared to the catalysts supported on amorphous or non-porous materials. In the latter

investigations, the pores were generally less than 4.0 nm. To overcome such problem, the

pores with larger mesoporous hosts would be expected.

Many groups have demonstrated the improvement of catalytic activities by utilizing ordered mesoporous titania as the support. For example, Tatiana Klimova et al. prepared and characterized a series of hydrotreating Mo catalysts supported on mesoporous titania-modified MCM-41 130 . They observed the higher intrinsic hydrodesulfurization (HDS) activity. This high activity has been ascribed to the good Mo dispersion and high surface area arising from the relatively high density of reactive hydroxyl groups on the TiO 2 surface. Additionally, it was found that mesoporous titania enhances the reduction and sulfidation of Mo 6+ oxide species providing easier the catalytically active MoS 2 formation. Juan Carlos Amezcua et al. prepared NiMo catalysts supported on titania modified SBA-16 for 4,6-dimethyldibenzothiophene hydrodesulfurization (HDS) 131 . They stated that the interaction of Ni and Mo species with the support becomes stronger with TiO 2 incorporation in the SBA-16 support contributing increased Ni and Mo dispersion to the sulfided metal species and therefore elevated HDS activity. L. Ilieva et al. studied the reduction behavior of nanostructured gold catalysts supported on mesoporous titania. They concluded that the nature of the mesoporous titania support plays a decisive role in the structure as well as morphology of supported gold nanoparticles, and the reactivity of the gold nanoparticles in particular for their redox properties due to highly dispersed on metal oxide support. In addition, they also demonstrated that the mesoporous titania does not simply act as an inert carrier, but

24 intervenes in the catalytic process and there is a synergetic effect between the mesoporous titania support and Au particles 122 . Yoshitake et al. synthesized Mo supported on mesoporous titania for ethanol dehydrogenation/dehydration reactions and attributed the higher degree of conversion with respect to Mo/P-25 to a possible higher percentage of sixfold-coordinated Ti which would offer a unique coordination environment for chemically active molecules 77 . Idakiev et al. reported the use of gold nanoparticles (NPs) supported on ceria-modified mesoporous titania as highly active catalysts for low-temperature water-gas shift reaction 100 . They reported the formation of well-dispersed and stable gold particles on the oxide support upon calcination and reduction facilitated by the mesoporous structure. Eguchi et al. prepared cerium- incorporated mesoporous titania catalyst for the oxidation/dehydration of ethanol 132 . They concluded that Ce-meso TiO 2 was superior to Ce/P-25 catalyst because most Ce(III) ions were exposed at the surface due to the extremely high specific surface area of the catalyst.

However, to the best of our knowledge, no study has been reported on the use of

- mesoporous TiO 2 as Pt support for the electrocatalytical reduction of tri-iodide (I 3 ), the reaction occurring at the counter electrode in dye-sensitized solar cell (DSSC). In this study, we prepared counter electrodes consisting of Pt nanoparticles in mesoporous Nb- doped TiO 2. The use of Nb doping is necessary to increases the conductivity of the

film 133-134 to allow its use in electrocatalytic reactions. We measured the charge transfer

- - resistance and exchange current density for the tri-iodide (I 3 )/iodide (I ) redox reaction.

As the result of a larger active surface area of Pt supported in the mesoporous Nb-doped

TiO 2, we observed the reduction of the charge transfer resistance and the increase of the

25 exchange current density in comparison with the platinized electrode supported on bare

FTO substrate under the same Pt loading.

2.2 Experiments

2.2.1 Reagents

All listed of the following chemicals were used as purchased without further purification

- Pluronic F127 [OH(CH 2CH 2O) 106 (CH 2CHCH 3O) 70 (CH 2CH 2O) 106 H

Aldrich,]

--- Titanium ethoxide [Ti(OCH 2CH 3)4, Aldrich]

--- Anhydrous niobium(V) chloride (NbCl 5, Aldrich, 99.999%)

--- Chloroplatinic acid hydrate (H2PtCl 6.xH2O, Aldrich, 99.9%)

--- Iodine (I2, Aldrich, 99.99%)

--- Lithium iodide (LiI, Aldrich, 99.9%)

--- Tetrabutylammonium iodide (C 16 H36 NI, Aldrich, 99%)

--- 4-tertbutylpyridine (C 9H13 N, Aldrich, 99%)

--- Tetrabutylammonium fluoroborate (C16 H36 BF 4N, Strem Chemicals, 99%)

- Anhydrous acetonitrile (CH 3CN, Puriss, Sigma-Aldrich, 99.5%)

--- Ethanol 200 Proof (CH 3CH 2OH, Decon Labs, 99.9%)

--- Iso-propanol [(CH 3)2CHOH, Fisher Scientific, 99.9%)

--- Hydrochloric acid (HCl, Fisher Scientific, 37.5%)

--- F-doped SnO 2 glass slides (8 Ω/, TEC 8, Hartford Glass Co, IN, 2.5 cm x

2.5 cm,)

--- 60 µm thick spacer (Surlyn, from Solaronix, Switzerland).

2.2.2 Instrumentation

- Scanning electron microscopy (SEM, Serion Electron Microscope)

- Transmission electron microscopy (TEM, Tecnai F20)

- Electrochemical impedance spectroscopy (EIS, Gamry Instruments)

- Cyclic voltammetry (CV, Bioanalytical Systems)

- Dip coater (Velmex, , Inc.)

- Horizontal tube furnance (Fisher Scientific)

2.2.3 Experimental methods

2.2.3.1 Preparation of mesostructured TiO 2 films

The films were prepared by supramolecular templating with amphiphilic

135 triblock copolymers (Pluronic F127) . Titanium ethoxide (Ti(OCH 2CH 3)4, Aldrich, 0.37 ml) was dissolved in HCl conc. (2.7 ml of 12.1 N, Aldrich) by slow addition under vigorous stirring at room temperature. After 5 min, a previously prepared solution of block copolymer Pluronic F127 [OH(CH 2CH 2O) 106 (CH 2CHCH 3O) 70 (CH 2CH 2O) 106 H

from Aldrich, 0.13 g) in ethanol (5 ml) was added to the HCl/Ti(OCH 2CH 3)4 solution , stirred for 15 min, and 20 ml of ethanol were added into the final mixture before dip- coating (1 mm/s) onto F-doped SnO 2 glass slides (8 Ω/, TEC 8, Hartford Glass Co, IN,

2.5 cm x 2.5 cm,). The films were dried at room temperature overnight and calcined in a furnace under stagnant air (2 h at 400 °C, heating rate: 1 °C/min).

2.2.3.2 Preparation of Nb-doped mesostructured TiO 2 films

The films were doped with Nb by adding anhydrous niobium(V) chloride

(NbCl 5, 0.0025 g, 0.5% or 0.0050 g, 1.0%) into the titanium ethoxide before addition of

HCl; after drying overnight, the films were heat treated at 550 °C for 2 h.

2.2.3.3 Preparation of platinized electrodes

Electrodes were platinized by thermal decomposition of H 2PtCl 6.xH 2O

(Aldrich, 5 mM in 2-propanol) at 385 °C for 20 min with a constant Pt loading of 5 µg

cm -2.

All electrodes obtained were characterized by SEM, EIS and CV. TEM was used to characterize only Nb-doped mesostructure TiO 2 film.

2.2.4. Characterization of experimental electrodes

2.2.4.1 Scanning electron microscopy (SEM )

The Scanning Electron Microscope (Serion Electron Microscope) operating at an acceleration voltage of 10-15 kV, a working distance (w.d.) of 5 mm and magnification values in 100,000x – 150,000x was employed to investigate the microstructure of electrodes. SEM was performed on the films as prepared.

2.2.4.2 Transmission electron microscopy (TEM)

Transmission electron microscopy (TEM, Tecnai F20, at 200 kV) was

done on a dispersion of the film prepared on Si, removed with a blade and transferred

with iso-propanol to a Cu grid. The Pt loading was increased by 5 times for the TEM

studies in order to investigate the distribution of the Pt particles on Nb(1.0 mol%)-doped

TiO 2 film.

2.2.4.3 Electrochemical impedance spectroscopy (EIS)

Electrochemical impedance spectroscopy (EIS) was carried out with an

EIS600 potentiostat (Gamry Instruments, Warminster, PA) in the range of 10 -2 to 10 6 Hz to study the Pt electrocatalytic activity with the various supports. A circuit of the form in

Figure 4 was used to fit to the high frequency end of the spectra using the simplex method (Gamry software). The charge transfer resistance ( Rct ) presented here was

calculated as half the value obtained from the fitting (the cells used were symmetric) and

then multiplied by the geometric area of the electrode. The measurements were done

simulating open circuit conditions (0 V applied across the dummy cell) for the electrolyte

of the dye-sensitized solar cell. The cells were built by sealing two identically-prepared

electrodes with a 60 µm thick spacer (Surlyn, from Solaronix, Switzerland). The spacer

2 had an opening of 0.28 cm filled with a liquid electrolyte containing 0.1 M iodine (I 2),

0.1 M lithium iodide (LiI), 0.6 M tetrabutylammonium iodide, 0.5 M 4-tertbutylpyridine

(TBP) in anhydrous acetonitrile 136 .

2.2.4.4 Cyclic voltammetry (CV)

The electrocatalytic activity of various counter electrodes was verified by Cyclic voltammetry (CV). CV experiments were done with a 50W potentiostat

(Bioanalytical Systems, West Lafayette, IN) at 100 mV/s in a three-electrode one-

- - compartment cell with a graphite rod as the counter electrode and an I /I 3 reference

- - electrode. The reference electrode was a Pt wire immersed in 10 mM I , 10 mM I 3 , 0.2 M tetrabutylammonium fluoroborate (TBAFB) solution in anhydrous acetonitrile (MeCN).

This solution was prepared from 20 mM LiI, 10 mM I 2. The electrolyte solution for the

- CVs was an Ar-saturated solution of 1 mM I 3 prepared from 1 mM LiI, 1 mM I 2, and 0.1

M TBAFB in MeCN.

2.3 Results and Discussion

As previously described, mesoporous titania with uniformly sized pores and large surface area can be obtained by using template-direct method. The surfactant templating procedure can be used to synthesize the mesoporous titania with a narrow pore size distribution and controlled pore structure. It is generally anticipated that the utilization of a high surface area mesoporous titania as the catalyst support rather than a commercially low surface area for supporting the catalyst such as noble metals or transition metals has some advantageous effects on the catalytic performance.

Due to the well dispersed and stable metal particles on the surface of mesoporous titania catalyst support upon thermal treatments, therefore, the catalytic performance of the catalyst could be improved by mesoporous titania.

In this work, we prepared counter electrodes consisting of Pt nanoparticles in mesoporous Nb-doped TiO 2. We measured the charge transfer resistance and exchange

- - current density for the tri-iodide (I 3 )/iodide (I ) redox reaction. As the result of a larger active surface area of Pt supported in the mesoporous Nb-doped TiO 2, we observed the reduction of the charge transfer resistance and the increase of the exchange current density in comparison with the platinized electrode supported on bare FTO substrate under the same Pt loading.

Figure 2.8 shows SEM images of the electrodes used in this work. Figure 2.8 (a) is a thermally platinized F-doped SnO 2 electrode (Pt/FTO) prepared as reported by

87 Papageorgiou et al . Fig. 2.8 (b) and (c) display the SEM images of Pt/TiO 2 (2.8 b) and

Pt/Nb(1.0 mol%)-doped TiO 2 (2.8c). As it can be seen from Fig. 2.8 (b) and (c), the TiO 2 films clearly show the mesoporous character with a pore size of approximately 5-10 nm.

The same Pt precursor solution was used to load Pt onto the various supports. On the bare

FTO substrate, the Pt NPs were concentrated at the valleys formed between the SnO 2 grains, resulting in inhomogeneous distribution (Fig. 2.8a). In contrast, no apparent Pt

NPs could be identified in Fig. 2.8b and 2.8c, presumably because the NPs were impregnated into the mesopores and thus invisible under SEM imaging.

(a)

(b)

(c)

Figure 2.8 Scanning electron microscope (SEM) image of (a) Platinized FTO, (b) Pt/TiO 2/FTO, and (c) Pt/Nb(1.0 mol%)-doped TiO 2 on FTO.

The detailed morphology and microstructure of the Pt/Nb(1.0 mol%)-doped TiO 2 film were further characterized by transmission electron microscopy (TEM). As shown in

Fig. 2.9 (a and b), Pt nanoparticles with the diameter of 2-4 nm were homogeneously dispersed on the Nb(1.0 mol%)-doped TiO 2 surface.

Figure 2.9 TEM images of Pt NPs supported on Nb(1.0 mol%)-doped TiO 2 film. The Pt loading is 25 µg cm -2.

The Pt electrocatalytic activity with the various supports was studied by

electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The

electrochemical reaction at the Pt/electrolyte interface is:

- - - I3 + 2e 3I (2.4)

For EIS experiments, the electrolyte composition used corresponds to the electrolyte composition for the DSSC. Thus, our measurements were performed under conditions that closely simulate the DSSC since adsorption of the electrolyte components

- - on Pt could modify the kinetics of the I /I 3 reaction.

The charge transfer resistance ( Rct ) associated with the equilibrium of eq. 2.4 is a

- - measurement of the electrocatalytic activity for the tri-iodide (I 3 )/iodide (I ) redox reaction. The exchange current density, Jo, i.e., the equal cathodic and anodic currents

normalized to the projected electrode area at equilibrium is calculated from Rct by the following equation: 137

= RT J o nFR ct (2.5) where Rct is the kinetic component of the resistance determined by EIS multiplied by the projected area of the electrode.

Figure 2.10 shows the Nyquist plots obtained from various cells, while Table 2.2 summarizes the charge transfer resistance ( Rct ) and exchange current densities ( Jo) for

different cells. Rct was measured by fitting the arc observed at highest frequency in

Nyquist plots (leftmost semicircle) to the circuit shown in Fig. 2.11, which contained the

138 uncompensated resistance ( Ru), and constant phase element (CPE) .

(a)

(b)

Figure 2.10 (a) Nyquist plots of Pt, Pt/TiO 2, Pt/Nb(0.5 mol%)-doped TiO 2 and Pt/Nb(1.0 mol%)-doped TiO 2 cells. (b) Pt/Nb(1.0 mol%)-doped TiO 2 cells showing the fitting to the equivalent circuit shown in Figure 2.11. Electrolyte: 0.1 M I2, 0.1 M LiI, 0.6 M tetrabutylammonium iodide, 0.5 M TBP in MeCN.

Figure 2.11 Equivalent circuit used to fit the high frequency region of the impedance spectra of symmetrical cells in order to obtain charge transfer resistance ( Rct ), which contains the uncompensated resistance ( Ru), and constant phase element (CPE).

Table 2.2 Results of the impedance measurements

2 2 Samples N Rct (Ω cm ) Jo (mA/cm )

FTO 3 (3 ± 1) x 10 5 (5 ± 2) x 10 -5

Platinized cell 5 0.66 ± 0.08 20 ± 2

Pt/TiO 2 cell 3 3.5 ± 0.5 3.6 ± 0.5

Pt/Nb(0.5 mol%)-doped TiO 2 cell 5 0.58 ± 0.05 22 ± 2

Pt/Nb(1.0 mol%)-doped TiO 2 cell 5 0.53 ± 0.07 25 ± 3

Note: The experiment error is the standard deviation of Rct measured for N independent

cells and the correspondent calculated error of Jo. N = number of measurements.

Electrolyte composition : 0.1 M iodine (I 2), 0.1 M lithium iodide (LiI), 0.6 M

tetrabutylammonium iodide, 0.5 M 4-tertbutylpyridine (TBP) in acetonitrile (MeCN).

The impedance results are summarized in Table 2.2 showing the average of N cells

and the corresponding sample standard deviation. The charge transfer resistance of the

Pt/Nb-doped TiO 2 electrodes was decreased from that of the platinized electrodes on bare

FTO, and thus the exchange current density was increased, indicating an increase in the

active surface area of the Pt in the Pt/Nb-doped TiO 2 electrode due to the mesoporous

TiO 2 support. When the Nb loading increases from 0.5 to 1.0 mol%, the charge transfer resistance decreases and the exchange current density slightly increases, which is consistent with an increase in conductivity of the TiO 2 support with higher Nb content.

When the Nb loading is higher than 1.0 mol%, we observed a decrease in the exchange

current density, probably due to the Nb segregation at the TiO 2 surface and poisoning the

Pt catalysts. Also, we note that when the Pt/Nb-doped TiO 2 films were prepared on a glass substrate a higher charge transfer resistance (> 250 M Ω cm 2) was observed because the conductivity of Pt/Nb-doped TiO 2 film is not high enough to replace the FTO transparent conducting oxide.

The electrocatalytic activity of various counter electrodes was also independently

- verified with CV in an Ar-purged acetonitrile solution of 1 mM I 3 (from 1 mM LiI, 1

- - mM I 2 and 0.1 M TBAFB as the supporting electrolyte). The CVs of I /I 3 redox mediator

on the three types of counter electrodes (Scan rate of 100 mV s -1) are shown in Fig. 2.12.

The cathodic peak (negative current) is assigned to the reduction of triiodide and the

anodic peak (positive current) is assigned to the reverse of this reaction (eq. 2.4)139 . As shown in Figure 2.12, the positions of the oxidation and reduction peaks on the three types of electrodes were identical. The current density was increased (30 %) when the Nb doped TiO 2/Pt electrodes were utilized as the working electrode. This indicates that the triiodide reduction rate on the Pt/Nb-doped TiO 2 electrode is higher than that on the

- - platinized electrode on bare FTO. In other words, the Rct for the I /I 3 redox reaction is

lower on the Pt/Nb-doped TiO 2 electrode compared with the platinized electrode on bare

FTO under the same conditions in accordance with our impedance measurements. The slightly higher current density is consistent with a higher electroactive area of Pt for the electrode prepared on the Nb-doped TiO 2 support, with respect to the conventional platinized electrode on FTO. A larger peak-to-peak separation and a decreased current density were observed when the Pt/TiO 2 electrode was used as the working electrode.

This indicates a slower reaction rate on the Pt/TiO2 electrode than that on Pt/Nb-doped

- - TiO 2 electrode. This is consistent with the charge transfer resistance ( Rct ) for the I /I 3 redox reaction on the Pt/TiO 2 electrode being larger as compared with the Pt/Nb-doped

TiO 2 electrode under identical conditions as shown in Table 1.We have attempted to

+ measure the changes in the electroactive area by using the H /H 2 (not shown); while we

were able to detect the underpotential desorption of H 2, these currents are barely above

background. Thus, we were not able to systematically study the changes in surface area

since it is below the level of quantification of this electrochemical technique.

Nevertheless, we can estimate them to be in the order of 1-2 m 2/g. We are unaware of any

- - study of the electrocatalytic activity vs. specific surface area of Pt for the I / I3 couple at the loading level of the DSSC counter electrode. While this is of interest to our group, a complete study is beyond the scope of this paper.

The use of Nb doping is necessary to increase the conductivity of TiO 2 to get a good

conductive path to the electroactive particles; however, increasing the precursor

concentration of Nb above 1.0 mol% decreases the electrocatalytic activity of the Pt

- probably due to surface segregation. The I 3 ions have to penetrate into the mesopores of

- the TiO 2 films and must be reduced to the I at the particles surface in the mesopores.

Also, although one may expect that the use of the mesoporous support to be detrimental

- - for the electrocatalysis because the pores could hamper the diffusion of the I 3 (or I ) to

(or from) the Pt surface inside the pores and the support may block the surface of the Pt

particles; we consistently observed the opposite effect. When the Nb content was

optimized to 1.0 mol%; the exchange current density for this couple slightly increases as

measured by both EIS and CV, consistent with an increase in surface area due to the good

dispersion of the Pt precursor in the mesoporous support.

0.6

0.4

0.2 Platinized electrode 0 Pt/Nb(0.5 mol%)-doped TiO2 electrode -0.2 Pt/Nb(1.0 mol%)-doped TiO2 electrode -0.4 Pt/TiO2 electrode

Current density (mA/cm2) -0.6

-0.8

-1 -0.6 -0.4 -0.2 0 0.2 0.4 Voltage [V vs I-/I3- (10 mM)]

- Figure 2.12 Cycilc voltammograms of the various electrodes in acetonitrile solution of I3 (1 mM LiI, 1 mM I 2 and 0.1 M TBAFB, cathodic currents as negative).

In addition, we also investigated the variation in exchange current density with Pt loading for the conventionally platinized FTO electrodes and the Pt/Nb (1.0 mol%)- doped TiO 2 electrodes as shown in Figure 2.13. We observed that when we doubled the

Pt loading from 5.0 to 10.0 µg cm -2, the exchange current density of Pt/Nb (1.0 mol%)-

doped TiO 2 electrodes was also doubled. In contrast, the exchange current density of the platinized electrodes on bare FTO only increased by 1.5 times. These results clearly indicate an increase in the specific surface area of the Pt nanoparticles in the Pt/Nb (1.0 mol%)-doped TiO 2 electrodes owing to the high dispersion of Pt nanoparticles giving rise

- - to promising electrocatalytic activity for the tri-iodide (I 3 )/iodide (I ) redox reaction.

Figure 2.13 Variation in exchange current density with platinum loading for the platinized and Pt/Nb (1.0 mol%) TiO 2 electrodes on a FTO substrate.

2.4 Conclusion

Mesoporous Nb-doped TiO 2 film was prepared by the sol-gel method on a

transparent conducting FTO glass. This material has been tested as a Pt support for the

DSSC counter electrode. The use of Nb is necessary to increase the conductivity of the

film to allow its use in electrocatalytic reactions. Pt nanoparticles were prepared in the

mesoporous TiO 2 support substrate and tested for the DSSC counter electrode. The formation of nanosized Pt supported on mesoporous Nb-doped TiO 2 was confirmed by

SEM and HRTEM images. By fabricating Pt/Nb-doped TiO 2 electrode, the charge

transfer resistance was reduced and the exchange current density was increased as the

result of a larger active surface area of Pt in the Pt/Nb-doped TiO 2. While this works

focused on demonstrating the use of this support for the DSSC counter electrode reaction,

this electrode could be used to other systems where there is a need to control the amount

of Pt surface area and hamper Pt particle aggregation when operating at high

temperatures. The impregnation of Pt in the mesoporous TiO 2 can also improve the mechanical rigidity and stability of the electrocatalyst against abrasion or generally mechanical contact, a desired property for practical applications.

Chapter 3

Electrocatalytic Properties of Graphene Films toward Tri-iodide Reduction

3.1 Introduction: Graphene

Graphene has attracted extensive attention in condensed-matter physics and

materials science 140 . Graphene is composed of a flat two-dimensional (2D) sheet of

hexagonally arrayed sp 2-covalently bonded carbon atoms arranged into a honeycomb

lattice as shown in Figure 3.1 141.

Figure 3.1 Graphene structure 141.

Graphene is a basic building block of all graphitic carbon forms. Since graphene is a 2D carbon material, it can be wrapped up, rolled or stacked into buckyballs or

42 fullerenes (0D), carbon nanotubes (1D) or graphite (3D), respectively as shown in Figure

3.2142.

Figure 3.2 Graphene is a 2D building material for carbon materials of all other dimensionalities. It can be wrapped up into 0D buckyballs, rolled into 1D nanotubes or stacked into 3D graphite 142.

While graphene has been studied theoretically for the last sixty years 143-145, it is only recently that experimental evidence has shown that graphene exhibits exceptionally high crystallinity 146-147, electronic quality 142, 148, and strong catalytic activity 149-150.

3.1.1 Preparation of graphene sheet

3.1.1.1 Thermal expansion route

Graphite consists of stacked graphene sheets, which are held together by Van der Waals forces. The weak interlayer forces allow appropriate atoms, molecules, or ions, known as the intercalating agent, to enter or intercalate into the interplanar spaces of the graphite, thereby resulting in the increased distance between layers of the graphite 151-153 .

For example the intercalation is performed by mixing graphite flakes with concentrated sulfuric acid and nitric acid (4:1, v/v) at room temperature. Then, the interplanar spacing of graphite flakes will be expanded to some extent. The following equation describes the reaction occurring between graphite and a mixture of concentrated sulfuric acid and nitric acid.

n (Graphite) + n H 2SO 4 + n/2 (O) n (Graphite.HSO 4) + n/2 H 2O (3.1)

where O = Oxidant

The obtained product, known as graphite intercalation compound (GIC) consists of carbon layers and intercalated layers periodically stacked on top of each other. The stacking can be explained in the form of C-C-I-C-C-I-C-C-I-C-C where C and I represent carbon and intercalated layers, respectively. The number of carbon layers between the intercalated layer pair is called the stage. The structure previously written represents a stage 2 GIC. The stage structure of intercalated graphite greatly depends upon the intercalation condition. GICs resulted from chemical intercalation are usually stage 1 to stage 5 154.

Exfoliation takes place as intercalated graphite flakes are heated at an elevated temperature causing the dimension perpendicular to the carbon layers of the GIC rapidly increase. Consequently, the vermicular graphite will be formed, also called expanded graphite.

The exfoliated graphite flakes are extended up to 100 times along the c-axis of the graphite. For instance, the original graphite flakes with thickness of 0.4-0.6 µm and diameter of 500 µm (Figure 3.3(a)) might expand up to 200-300 times (2-20,000 µm in length) as shown in Figure 3.3(b). The expanded graphite structure which is basically parallel boards can loose or collapse and deform desultorily forming porous vermicular or warmlike structure with different sizes of many pores varying from 100 µm to 10 nm 155-

156 . The graphene sheets with thickness varying 400-100 nm in the exfoliated graphene surface can be observed (Figure 3.3(c)). Whereas the graphene sheets of thickness less than 100 nm are barely found on the expanded graphite surface. As shown in Figure

3.3(d), several graphene sheets with a thickness 50-80 nm are also discovered inside a newly broken graphite worm section.

(a) (b) )

Figure 3.3 SEM image of (a) natural graphite flake (b) expanded graphite (c) expanded graphite surface structure (d) freshly broken graphite worm section (inner surface) 153 .

If the precursor GIC is stage 1, the thickness of graphene sheets in exfoliated graphite could be identical to that of a layer of single carbon in the precursor GIC. As previously described, the precursor GICs stage 1 to stage 5 are generally obtained when chemical oxide intercalation used to prepare GICs, hence the vermicular graphene sheets

46 with thickness of 0.5-2.5 nm will be observed, presuming the single carbon layer has the thickness of 0.5 nm which can be confirmed by TEM. Figure 3.4 shows a TEM micrograph of the thinner sheets inside the expanded graphite. It is clear that the thicker sheets comprised of the thinner sheets of thickness less than 5 nm and the interplanar spacing between sheets is approximately 10 nm. It also signify that thicker sheets reveal via SEM consist of thinner lamellae, nanosheets, of thickness as thin as 5 nm. Possibly, the fusing or stacking of nanosheets on the surface at the high temperature causes the

SEM reveals few sheets thinner than 100 nm on the surface of the expanded graphite.

Figure 3.4 TEM of thinner sheets inside the expanded graphite 153 .

The structure of a graphite could be demonstrated by the model in Figure 3.5.

Figure 3.5 Schematic model for the expanded graphite consisting of graphite nanosheets and layer structure 157 .

Even though this conventional method has been produced the graphene sheets on the large scale and concentrated sulfuric and nitric acids are commercially available, the extent of exfoliation is negligible resulting in incomplete exfoliation of graphite in the scale of individual graphene sheets. The degree of thermal expansion depends upon the graphite type utilized and on the intercalation process 155, 158 . Typically, this procedure

provides the graphene sheets comprised hundreds of stacked graphene layers (presuming

that one layer of graphene sheet has the identical thickness to the interlayer separation

found in graphite, 0.34 nm) and the average thickness is approximately between 30-100

nm.

3.1.1.2 Delamination of intercalated graphite

Since graphite has layered structure, it can readily be separated via intercalation with alkaline metals followed by exfoliation in aqueous conditions. Alkaline metals are common to prepare graphite intercalation compounds. First, second or higher stage compounds can be synthesized by varying the ratio between alkaline metal and graphite. The first stage intercalation compound, KC 8 is produced by mixing

stoichiometric amounts of potassium and high-purity graphite in a Pyrex tube under an

inert helium environment. Then, the reactant filled tube was evacuated, sealed and

incubated at 200 °C for 24 h until a homogeneous, bright gold powder is formed.

The obtained potassium intercalated graphite product is highly sensitive to air and vigorously reacts with alcohols or water. Ethanol is observed to be a sufficient exfoliating agent. An extremely exothermic reaction between potassium intercalated graphite and aqueous solvents causes exfoliation as described in equation 3.2 159 .

KC 8 + CH 3CH 2OH 8C + KOCH 2CH 3 + 1/2H2 (3.2)

When the sheets separate to form graphite nanoplatelets the color of intercalated graphite changes from gold to black. Solvation of potassium ions, together with hydrogen evolution, allows the graphitic layers to exfoliate easily (Figure 3.6) 160 . The obtained

dispersion is basic owing to the existence of potassium ethoxide in the solution, which is

neutralized by washing with ethanol until resulting pH of the solution is 7.

Figure 3.6 Schematic diagram showing the intercalation and exfoliation process to produce graphite nanoplatelets (GNP). Graphite is intercalated with potassium metal to form the first stage compound KC8. Exfoliation in ethanol produces potassium ethoxide and hydrogen gas which aid in separating the graphitic sheets to form graphite nanoplatelets 160 .

Figure 3.7 shows the morphologies of the starting graphite powder and the

graphite nanoplatelets obtained after intercalation/exfoliation process which can be

observed by SEM. SEM images revealed that after the intercalation and exfoliation, the

graphite turns from a compactly layered morphology (Figure 3.7(a)) to an expanded

layered structure (Figure 3.7(b)). However, this process still can not form individual

sheets because exfoliation is incomplete 160 .

Figure 3.7 Scanning electron micrographs of (a) starting graphite and (b) after intercalation with potassium and exfoliation with ethanol 160 .

3.1.1.3 Chemical reduction of exfoliated graphene oxide

The chemical reduction of exfoliated graphene oxide has recently attracted attention as a promising technique to prepare graphene in large scales 161-163 . In this route,

graphene oxide (GO) is utilized as a medium to produce stable dispersions of graphene

nanosheets in a solvent: graphene oxide (GO) is a resulting product from the oxidative

treatment of graphite which still retains the original layered structure of graphite 164 , its

color is much lighter than graphite because of the breakdown of electronic conjugation

occurring during the oxidation process. Due to the exsistance of large amounts of epoxide

51 and hydroxide functional groups decorated onto the basal planes, as well as the carbonyl and carboxyl groups attached pressumably at the edges 164-169 as shown in Figure 3.8, GO is highly hydrophilic and readily intercalated as well as exfoliated in aqueous media to form stable colloidal dispersion. On the other hand, GO can be also considered of as a graphite-type intercalation compound with both covalently bound oxygen and non- covalently bound water between the graphene sheets. Actually, the mechanism of exfoliation is largely the expansion and delamination of gases evolved into the interspacing between the carbon layers during rapid heating by pyrolysis of the oxygen- containing functional groups 170 . Whereas the vaporization of water is endothermic, the decomposition of functional groups of GO producing CO 2 is exothermic resulting in

lessening the vigorousness of heating stage. To obtain the individual graphene sheets, the

elimination of the interstices of graphene sheets associated with the original graphene

during the oxidation process need to be completely succeeded. Furthermore, it is essential

to decrease the deleterious role of water vaporization as well. Such thermal process has

currently been recommended to be accomplished of producing individual functionalized

graphene sheets 170 .

Figure 3.8 Structural model for graphene oxide 165 .

AFM images revealed that the GO exfoliated by the ultrasonic treatment revealed the uniformity in thickness of GO sheets (~1 nm as shown in Figure 3.9). AFM images did not show the GO consisted of sheets either thinner or thicker than 1 nm indicating that individual GO sheets was obtained or complete exfoliation was achieved under this method. Whereas a pristine graphene sheet is atomically flat with van der Waals thickness of 0.34 nm, such thickness is significantly larger than that of single-layer pristine graphene normally ascribed to the presence of covalently bound oxygen attached on both sides of the graphene sheet and to the atomic roughness arising from the displacement of the sp 3-hydridized carbon atoms generated from the original graphene

plane 163 .

Figure 3.9 A non-contact mode AFM image of exfoliated GO sheets with three height profiles acquired in different locations 163 .

By nature, GO is an electrical insulator because of the increasing in hydrophilicity

during the oxidation process. During oxidation process, the carbon atom is changed

completely from a planar sp 2-hybridized geometry to a distorted sp 3-hybridized geometry causing in gradual losing excellent electrical properties of graphite 170 . Moreover, GO is

also thermally unstable due to the existence of the oxygen functional groups during the

pyrolysis undergoing at elevated temperatures 171 . To restore the electrical conductivity

and thermal stability of GO, GO has to be converted back to conducting graphene by

chemical deoxygenation, for example, using hydrazine in order to eliminate most of

oxygen-containing functional groups with partial restoration unoxidized aromatic regions

(sp 2-carbon atoms) 172-173 . Applying graphite oxidation and a consequent solution-based chemical reduction, single graphite sheets (i.e., finite-sized graphene sheets) can be obtained.

As is shown in Figure 3.10a and b, the reduced GO material showed by unorganized agglomeration consisting of thin and crumpled sheets closely associated with each other, which developed into a disordered solid (Figure 3.10a). By high- resolution SEM, the folded regions of the sheets with average widths of ~2 nm were found as shown in Figure 3.10b. The individual sheets in reduced GO materials were not presented because of the resolution limit of instrument; however, the charging during the

SEM operating was absent suggesting that the network of graphene-based sheets and the individual sheets are electrically conductive. Generally, chemical reduction of exfoliated graphene oxide could be illustrated by the following schematic diagram as shown in

Figure 3.11.

(a)

(b)

Figure 3.10 (a) An SEM image of aggregated reduced GO sheets. (b) A platelet having an upper bound thickness at a fold of ~2 nm 163 .

Graphite

Oxidation/Exfoliation processes

Exfoliated graphene oxide (GO)

Reduction process

Graphene (G)

Figure 3.11 Schematic diagram showing chemical reduction of exfoliated graphene oxide.

3.1.2 Electrochemically carbon electrodes

We can identify two kinds of graphitic plane in the graphite structure: (1) the basal plane consisting of all the carbon atoms in particular graphite layer (2) the edge plane consisting of all the carbon atoms perpendicular to the graphite layer as shown in

Figure 3.12.

Figure 3.12 Schematic diagrams showing the overhead view of a section of the basal plane Highly ordered pyrolytic graphite (HOPG) surface 174 .

The two planes exhibit clearly different electrochemical properties owing to the feature of the chemical bonding in graphite. For examples, the reaction rates of electrode at edge-plane graphite are significantly higher than those at basal plane graphite for most redox couples. Thus, a carbon electrode surface can be considered as an electrochemically heterogeneous electrode (EHE) because it contains edge and basal plane graphite, where the redox reaction is explained by two sets of electrode kinetics:

Edge plane: k°edge , αedge A + e - B (3.3)

Basal plane:

k°basal , αbasal A + e - B (3.4)

where k°i and αi are the standard electron transfer rate constant and symmetry coefficient for electron materials i (i = edge and basal)

Within the basal plane surface of an HOPG electrode, the interplane distance of each layer comprising of cleavage steps which lie parallel to the surface are usually multiples of 3.35 Å 175 . Figure 3.13 illustrates surface defects occurring in the form of steps exposing the edges of the graphite layers.

Figure 3.13 Schematic diagram of 4 layer step edge 174 .

The distance between these defects is less than 1-10 µm which is the lateral grain

size of HOPG. Usually, HOPG contains 1-20 layers and its layers are separated from

each other by 3.35 Å deep resulting in negligible total surface coverage of edge-plane graphite.

Mark T. McDermottt and Richard L. McCreery have shown that the average surface coverage on edge-plane defects was 0.01±0.004 confirmed by STM. However, these edge-plane defects have high adsorption and electronic perturbation depending on an electronic effect such as an electrostatic attraction between the adsorbate and partial surface charges, rather than a specific chemical effect which are responsible for electrochemical performance 176 .

Figure 3.14 shows a cyclic voltammogram for the ferrocyanide oxidation at a basal plane HOPG electrode and an edge-plane pyrolytic graphite (eppg) electrode. The cyclic voltammograms exhibit the lower peak to peak separation for edge-plane defect apparently signifying that the edge-plane defect is essential for fast electron transfer resulting in higher effective electron transfer rate constant 177-178 . This result clearly indicates the relationship between carbon electrode microstructure and heterogeneous electron transfer activity.

Figure 3.14 Cyclic voltammogram recorded at V s-1 for the oxidation of 1 mM ferrocyanide in 1 M KCl solution at (i) a basal plane HOPG electrode and (ii) an edge- plane pyrolytic graphite electrode 174 .

Ryan R. Moore et al. reported on the first electrocatalytic detection of thiols using

an edge plane pyrolytic graphite electrode 179. Compared to glassy carbon and basal plane

pyrolytic graphite electrodes, the enhancing signal-to-noise characteristics of reduction

potential peak and the response of the oxidation of thiol moieties were found when

utilizing an edge plane pyrolytic graphite electrode, such graphite electrode confers low

detection limits identical to current sensing methodologies. The effectiveness of the edge

plane graphite electrode obtains from a high density of states providing excellent

electrical conductivity and electron transfer kinetics; probably allied to active sites for

adsorption

Graphene has recently been utilized in DSSCs as transparent conductive

electrodes or as counter electrodes. For example, Wang et al. demonstrated transparent,

conductive, and ultrathin graphene films, as an alternative to the ubiquitously employed

metal oxide window electrodes for solid-state dye-sensitized solar cells 180 . They

concluded that the obtained films exhibited an excellent conductivity and a good

transparency in both the visible and near-infrared regions. In addition, they showed an

ultrasmooth surface with tunable wettability including high thermal and chemical

stabilities. Hong et al. reported the fabrication of transparent graphene/PEDOT–PSS

composite films by simply spin coating the aqueous mixture of PB-stabilized graphene

and PEDOT–PSS on ITO substrates at room temperature for used as counter electrodes

for DSSCs 181 . The energy conversion efficiency of the cell with this film as counter electrode reached 4.5%. This is mainly due to that the high specific surface area and many chemical defects of ultrathin graphene sheets provide them with high catalyzation

- activity toward reduction of tri-iodide (I 3 ). Despite these applications, there is no reported investigation on the fundamental electrochemical properties of graphene films

- - towards the I3 /I redox reaction. This motivates my research presented in this chapter.

3.2 Experiments

3.2.1. Reagents

All listed of the following chemicals were used as purchased without further

purification

- Flake graphite (Bay Carbon, Inc., SP-1, 99%)

- Sodium nitrate (NaNO 3, Mallinckrodt chemicals, 99%)

--- Potassium permanganate (KMnO 4, Sigma-Aldrich, 99%)

--- Sodium chloride (Fisher Scientific, 99.9%)

--- Hydrazine hydrate (H 4N2.xH 2O, Aldrich, 99.9%)

--- Poly(diallyldimethylammonium chloride) (C 8H18 ClN, Aldrich, 20wt% in

water)

--- Poly(4-styrenesulfonic acid) (C 8H8O3S, Aldrich, 18wt% solution in water)

--- Sulfuric acid (H 2SO 4, Fisher Scientific, 97.2%)

--- Hydrogen peroxide (H 2O2, Fisher Scientific, 31.7%)

--- Chloroform (CHCl 3, Mallinckrodt chemicals, 99%)

3.2.2 Instrumentation

- X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra XPS)

- Raman spectroscopy (Renishaw-Smiths Raman Microprobe)

- Cyclic voltammetry (CV, Bioanalytical Systems)

- Electrochemical impedance spectroscopy (EIS, Gamry Instruments)

- Horizontal tube furnace (Fisher Scientific)

3.2.3 Experimental methods

3.2.3.1 Preparation of graphene oxide

Graphene oxide was prepared by a modified Hummers method 182-183 . Ten

grams of flake graphite and 7.5 g of NaNO 3 (purity 99%) were placed in an Erlenmeyer

flask. Then, 621 g of concentrated H2SO 4 (purity 96%) was added with stirring while the mixture was then cooled in an ice water bath. Forty five grams of KMnO 4 (purity 99%) was gentle added over about 1 h and allowed to dissolve with the aid of stirring. The mixture was completely chilled to 20 °C using an ice bath and allowed to stand for five days with slow stirring, to acquire a highly viscous liquid.

3 The 5 wt% H 2SO 4 aqueous solution (1000 cm ) (water having specific

resistivity of 18 M Ω·cm was used for dilution (and the same was done consistently

hereinafter)), was added, initially in 20-30 ml aliquots, into the obtained mixture over

about 1 h with stirring. The temperature of the resultant mixture will increase rapidly

during H2SO 4 aqueous solution addition; the addition of such solution was performed in an ice bath until the temperature does not exceed 50 °C. The more H2SO 4 aqueous

solution was poured, the less mixture became reactive until the remaining such solution

(~ 700 ml) can be added in with no resulting climb in temperature observation. After all

3 of 5 wt% H 2SO 4 aqueous solution (1000 cm ) addition, the mixture was further stirred for

2 h. Then 30 wt% H2O2 (30 g) was added to the above mixture with stirring for 2 h to complete the oxidation, and the mixture resulted in a brilliant yellow color along with bubbling.

In order to eliminate inorganic anions, un-reacted oxidant ions, particularly ions of manganese, and other impurities, the resultant mixture was purified by

centrifugation followed by discarding supernatant liquid. 0.5 wt% H2O2/3 wt% H2SO 4 aqueous solution was added to re-disperse the remaining solid. This washing cycle was repeated 15 times.

The remaining mixture was purified by repeating above purification process cycle further three times except for the addition of water instead of mixed aqueous solution to remove remaining acid. Finally the resultant mixture was allowed to settle for a day, to precipitate thick particles after which the clear supernatant was removed resulting in dark brown slurry of graphene oxide which can be stored in sealed dark bottles.

3.2.3.2 Preparation of graphene oxide film

The concentration of the obtained graphene oxide slurry was determined by

drying graphene oxide slurry in a vacuum oven for a day and was found to be 47 mg ml -1.

To prepare the graphene oxide solution (2.64 mg l -1), the slurry was diluted in water and

sonicated to accomplish graphene oxide sheet exfoliation. Then a 50 ml of graphene

oxide solution mixed with 50 ml of water was vacuum-filtrated using polycarbonate

memebrane (PCT) with 50 nm pores (Nuclepore). The membrane with graphene oxide

was cut into sizes of choice, wetted with deionized water, and pressed against the FTO

surface with the graphene oxide side in contact with the FTO used as substrate. After

that, the membrane was dissolved using chloroform to leave a thin film of graphene oxide

on the FTO. The film was dried at room temperature.

3.2.3.3 Preparation of graphene film by hydrazine and thermal reductions

The hydrazine reduction of the graphene oxide films was performed by

exposing the films to hydrazine hydrate (Aldrich) vapor generated using a bubbler of

63 argon connected to a tube furnace in which the graphene oxide films were placed.

Hydrazine was passed through the tube furnace for 1 h to let the tube furnace overwhelm with hydrazine vapor and remove any ambient moisture. Then the temperature of the tube furnace was increased to 200 or 400 °C (Heating rate: 15 °C/min) with hydrazine bubbling at approximate 110 cc/min flow rate and maintained for 2.5 or 5 h. The hydrazine flow was maintained until along the furnace was cooled down. In order to flush out any remained hydrazine traces from the films, argon was steadily passed over the samples for another 1 h.

For the thermal reduction of the graphene oxide films, the procedure was similar to hydrazine reduction except that only argon gas was flowed through the tube furnace without hydrazine bubbling and flowing argon gas after the reaction completed does not require.

3.2.3.4 Surface modification of graphene by polyelectrolyte coating

The obtained graphene films were dipped into 0.02 M solutions of positively charged poly(diallyldimethylammonium chloride) (PDAC, Aldrich, Mw ~ 400,000-

500,000) as shown in Figure 3.15(a) or negatively charged poly(4-styrenesulfonic acid)

(PSS, Aldrich, Mw ~ 75,000) as shown in Figure 3.15(b) for 20 min followed by washing creating one layer. Both polyelectrolyte concentrations are based on the repeating unit of the polymer and the solutions of PDAC as well as PSS contained 0.1 M NaCl and water, respectively

(a) (b)

Figure 3.15 The structures of (a) poly(diallyldimethylammonium chloride) (PDAC) (b) poly(4-styrenesulfonic acid) (PSS)

3.2.4 Characterization of experimental electrodes

3.2.4.1 X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectrophotometer (Kratos Axis Ultra XPS) was used to record the XPS spectra of graphene electrodes. Photoelectron processes were excited using an Al K α source with photon energy of 1486.6 eV. The vacuum in the analysis

chamber was maintained at 10 −9 Torr. Kratos Vision 2 software was used to perform curve fitting.

3.2.4.2 Raman spectroscopy

The Raman microprobe (Renishaw-Smiths Raman Microprobe) operating with 514 nm wavelength incident argon ion laser light and a 50× objective was employed to investigate the significant structural changes occurring for each stage of the chemical processing. The incident laser power was carefully tuned to avoid sample damage or laser induced heating and measurements were thus performed at around 10 mW incident laser power. Raman spectra were recorded from 100-3200 cm-1.

3.2.4.3 Cyclic voltammetry (CV)

The electrochemical behavior of graphene electrodes were determined by

Cyclic voltammetry (CV) (50W potentiostat, Bioanalytical Systems, West Lafayette, IN).

The experiments were done in the same procedure as previously described in chapter 2.

3.2.4.4 Electrochemical impedance spectroscopy (EIS)

The kinetics of graphene electrodes was verified by Electrochemical impedance spectroscopy (EIS600 potentiostat, Gamry Instruments, Warminster, PA). The

EIS was carried out in the same condition as mentioned above in chapter 2.

3.3 Results and Discussion

As previously described, in the development of electrodes in DSSCs, graphene is considered as a “rising-star” carbon material, attracting enormous interests for use as electrodes in DSSCs. Recently, chemical reduction of graphene oxide in solution is considered to be the facile chemical method for the synthesis of graphene. The obtained graphene exhibits unique nanostructure and extraordinarily electrical properties 142.

In this work, graphene films were prepared by chemical and thermal reductions of graphene oxide obtained from the oxidation of graphite. The goal of this work was to study the appropriate reduction conditions for obtaining the graphene films having the

- - highest electrocatalytic activity towards I /I 3 redox reaction. Additionally, the influence of graphene surface modification by polyelectrolyte coating on electrocatalytic properties of graphene was also investigated. In addition, the electrocatalytic activity of graphene after surface modification was also compared to that of graphene without surface modification.

The graphene films obtained from various reduction conditions were analyzed by

employing X-Ray photoelectron spectroscopy (XPS). Curve fitting of the C1s spectra

was performed using a Gaussian-Lorentzian peak shape after performing a Shirley

background correction. The C 1s XPS spectrum of graphene film obtained from hydrazine reduction at 200 °C for 2.5 h (Fig. 3.15a) apparently displays an oxidation extent with three components typically assigned for carbon atoms in different functional groups. The binding energies of 284.7, 285.7, and 288.0 eV are corresponded to the non- oxygenated ring C (C-C and C-H), the C in C-O bond (C-OH), and the carbonyl C (C=O) functional groups, respectively 162-163 . Most structural models of graphene also contain an

epoxide group (C-O-C), which have a C1s binding energy similar to C-OH. Even though the C 1s XPS spectra of the graphene films obtained from hydrazine and thermal reductions at 400 °C for 5 h (Fig. 3.15b-c) also present oxygen functionalities except for the C=O groups that have been ascribed in Fig. 3.15a, the peak intensities of these components in the graphene films from hydrazine and thermal reductions at 400 °C for 5 h (Fig. 3.15b-c) are much smaller than those in the graphene film obtained from

hydrazine reduction at 200 °C for 2.5 h (Fig. 3.15a), signifying extensive de-oxygenation by the reduction process at higher temperature.

C-C

C-O (a) C=O

C-C

C-O (b)

C-C

C-O

(c)

Figure.3.15 High-resolution XPS analysis of the effect of different reduction treatments on the graphene oxide films. Deconvolution revealed the presence of C-C (~284.7 eV), C-O (~285.7 eV), C=O (~288.0 eV) species in the film. (a) Hydrazine reduction at 200 °C for 2.5 h (b) Hydrazine reduction at 400 °C for 5.

Raman spectra of graphene reduced with various reduction conditions reflecting

the significant structural changes occurring for each stage of the chemical processing are

shown in Fig. 3.16. The Raman spectra of the all graphene display a broad G-band

-1 170, 184- around 1570-1580 cm corresponding to the first-order scattering of the E 2g mode

187 . In addition, the Raman spectrum of graphene also exhibits the prominent D-band at

1350-1360 cm -1 resulting from the structural defects produced by the bonding of

hydroxyl and epoxide groups on the carbon basal plane and the possible decrease in the

average size of the sp 2 domains, possibly owing to considerable reduction of the graphene

oxide. The Raman spectra of the all graphene films also contain both G and D bands;

however, an increased D/G peak area ratio can be observed when graphene obtained from

thermal reduction (Fig. 3.16(c)). This change suggests an increase in the quantity of

unorganized carbon or defect in the graphene structure and a reduction in the graphitic

crystal size. The latter suggestion can be described if new “graphene-like” domains were

constructed that are tinier in size than the graphitic domains exist in the graphene oxide

films before hydrazine or thermal reduction, but more infinite in number.

A(D)/A(G) = 1.97±0.11 (a)

A(D)/A(G) = 1.67±0.15 (b)

A(D)/A(G) = 2.52±0.17 (c)

Figure 3.16 (a) The Raman spectra of graphene obtained from hydrazine reduction at 200 °C for 2.5 h (b) hydrazine reduction at 400 °C for 5 h (c) thermal reduction at 400 °C for 5 h. To investigate the electrochemical behavior of graphene films reduced by various conditions, we have carried out cyclic voltammetry experiment from -0.7 V - +0.425 V at a scan rate 0.1 V s -1. Fig. shows their cyclic voltammograms in an Ar-purged acetonitrile

- solution of 1 mM I 3 (from 1 mM LiI, 1 mM I 2 and 0.1 M TBAFB as the supporting

electrolyte). In Fig. 3.17, it is observed that there are positive oxidation and negative

reduction current peaks for all electrodes. These peaks are assigned to formation of

triiodide from iodide and to the reverse of this reaction, respectively (eq. 2.4). The

oxidation current peaks appear at the potential of +0.391 V and +0.264 V and the

reduction current peaks appear at the potential of -0.542 V and -0.533 V for graphene

films reduced by hydrazine reduction at 200 °C for 2.5 h and 400 °C for 5 h. Meanwhile,

the oxidation and reduction current peaks for graphene film reduced by thermal reduction

at 400 °C for 5 h appear at the potential of +0.232 V and -0.498 V, respectively.

Interestingly, the oxidation and reduction current peaks observed in graphene film

reduced by thermal reduction at 400 °C for 5 h show about 0.16 V – 0.3 V negative shift

and 0.4 V – 0.30 V positive shift in peak positions, respectively when compared to those

observed in graphene films reduced by hydrazine reduction at 200 °C for 2.5 h and 400

°C for 5 h. It indicates that a relatively higher electrocatalytic activity of graphene

reduced by thermal reduction at 400 °C for 5 h than that of graphene reduced by

hydrazine reduction at 200 °C for 2.5 h and 400 °C for 5 h. This is due to many chemical

defects of graphene sheets reduced by thermal reduction at 400 °C for 5 h as confirmed

- - by Raman spectra providing them with high catalyzation activity toward redox of I /I 3 reaction.

Figure 3.17 The recorded cyclic voltammograms of graphene films reduced by various - conditions in acetonitrile solution of I 3 (1 mM LiI, 1 mM I 2 and 0.1 M TBAFB, cathodic currents as negative). The scan rate is 0.1 V s−1 and the expose area of the graphene electrodes is 0.28 cm 2.

To verify the kinetics of graphene electrodes, their Nyquist complex plane

impedance plots are shown in Fig. 3.18. Fig. 3.18. shows the EIS analysis of the

FTO/graphene/electrolyte/graphene/FTO device associated with different reduction

conditions. In general, the high-frequency semicircle is corresponded to the charge-

transfer process happening at the electrode ( Rct ) associated with the equilibrium of eq.

2.4. Table 3.1 summarizes the charge transfer resistance ( Rct ) and exchange current densities ( Jo) for different cells.

Figure 3.18 Nyquist plots of graphene obtained by hydrazine reduction at 200 °C for 2.5 h, at 400 °C for 5 h, and thermal reduction at 400 °C for 5 h cells. Electrolyte: 0.1 M I 2, 0.1 M LiI, 0.6 M tetrabutylammonium iodide, 0.5 M TBP in MeCN 188 .

Table 3.1 Results of the impedance measurements.

2 2 Reduction conditions N Rct (Ω cm ) Jo (mA/cm )

Hydrazine reduction at 200 °C for 2.5 h 3 (5.1 ± 0.9) x 10 2 0.025 ± 0.004

Hydrazine reduction at 400 °C for 5 h 3 (1.8 ± 0.1) x 10 2 0.072 ± 0.006

Thermal reduction at 400 °C for 5 h 3 48 ± 5 0.27 ± 0.03

Note: The experiment error is the standard deviation of Rct measured for N independent

cells and the correspondent calculated error of Jo. N = number of measurements.

Electrolyte composition : 0.1 M iodine (I 2), 0.1 M lithium iodide (LiI), 0.6 M

tetrabutylammonium iodide, 0.5 M 4-tertbutylpyridine (TBP) in acetonitrile (MeCN).

The impedance results are summarized in Table 1 showing the average of N cells

and the corresponding sample standard deviation. The value of Rct for graphene electrode

obtained by thermal reduction decreases to the minimum value 48.40 Ω·cm -2 from that of graphene electrode obtained by hydrazine reduction, implying that graphene electrode

- - obtained by thermal reduction is more favorable for the tri-iodide (I 3 )/iodided (I ) redox

reaction at the electrolyte/electrode interface. Furthermore, the exchange current density

of graphene electrode obtained by thermal reduction (~ 0.27 mA·cm -2) is higher than that

of graphene electrode obtained by hydrazine reduction (~ 0.03-0.07 mA·cm -2), indicating

- - that the electrochemical activity for the tri-iodide (I 3 )/iodided (I ) redox reaction of

graphene is enhanced. When the graphene films obtained from the hydrazine or thermal

reductions at 400 °C for 5 h were used as the working electrodes, the charge current

density was decreased and the exchange current density was increased, as the result of the

decreased oxidized carbons on the graphene surfaces confirmed by XPS spectra.

Specially, when the graphene film obtained from thermal reductions at 400 °C for 5 h,

which has high defect sites compared to the graphene film obtained by hydrazine

reduction confirm by Raman spectra and similar percentages of oxidized carbons with

graphene film obtained by hydrazine reduction at 400 °C for 5 h, was used as the working

electrode, the charge transfer resistance was decreased to 89.55%-72.70% and the

exchange current density was increased up to 800.00%-285.71% when compared to

graphene films obtained by hydrazine reductions at 200 °C for 2.5 h and 400 °C for 5 h,

respectively. This was due, not only to decreased presence of oxidized carbon species,

especially alcohol, epoxides, carbonyl, and carboxylic groups but also to increased defect

density.

Since the highest electrocatalytic activity was obtained when graphene film

reduced by thermal reduction at 400 °C for 5 h was utilized as an electrode, so graphene

film reduced by such condition was selected in order to study the possibility to tune the

electrocatalytic activity of graphene via surface modification by polyelectrolyte.

Fig. 3.19. shows the cyclic voltammetry of graphene with and without

- polyelectrolyte coating in an Ar-purged acetonitrile solution of 1 mM I 3 . When PDAC

used as cationic polyelectrolyte is applied, the positive shift of cathodic peak potential

can be seen when compared to the cathodic peak potential of graphene without

polyelectrolyte coating, indicating that the triiodide reduction rate on the graphene

electrode modified with PDAC is higher than that on the graphene without

polyelectrolyte coating owing to the attraction between PDAC coated on graphene

- surface which carries positive charge and I3 in the electrolyte solution carrying negative

charge. This electrode could be used as counter electrode in DSSCs. On the other hands,

when graphene coated with PSS used as anionic polyelectrolyte, the cathodic peak

potential can not be seen obviously when compared to that of graphene without polyelectrolyte coating, indicating that the triiodide reduction rate on the graphene

electrode modified with PSS is lower than that on the graphene without polyelectrolyte

- coating due to the repulsion between PSS coated on graphene surface and I3 which both carries negative charges. This electrode could be used as anode in DSSCs.

Figure 3.19 Cyclic voltammograms of graphene films with and without polyelectrolyte - coating in acetonitrile solution of I 3 (1 mM LiI, 1 mM I 2 and 0.1 M TBAFB, cathodic currents as negative). The scan rate is 0.1 V s−1 and the expose area of the graphene electrodes is 0.28 cm 2.

3.4 Conclusion

The graphene electrodes have been prepared by hydrazine and thermal reductions

of graphene oxide films with various conditions. The graphene electrode obtained by

thermal reduction has the advantage over the graphene electrodes obtained by hydrazine

reductions in reducing charge transfer resistance and increasing exchange current density

- - resulting in higher electrocatalytic activity towards I3 /I redox reaction. The improvement

of electrocatalytic activity of graphene electrode obtained by thermal reduction due to

decreased presence of oxidized carbon species, especially alcohol, epoxides, carbonyl,

and carboxylic groups and increased defect density leads to the decrease of charge

transfer resistance by 89.55%-72.70% and increase of exchange current density by

800.00%-285.71% as compared with that obtained by hydrazine reductions at 200 °C for

2.5 h and 400 °C for 5 h, respectively. As the results of the reduced charge transfer

resistance and increased exchange current density of graphene electrode obtained by

thermal reduction, the electrocatalytic activity of graphene electrode was improved

effectively by modifying graphene surface with polyelectrolyte. The reduction peak

potential of graphene modified with cationic polyelectrolyte (PDAC) shifted more

- positive resulting in enhancing electrocatalytic activity toward reduction of I3 . The

- attraction between cationic polyelectrolyte (PDAC) and I3 occurring on graphene surface leads to the improvement of electrocatalytic activity. This electrode could be used as counter electrode in DSSCs. On the other hands, the reduction peak potential of electrode made from graphene coated with anionic polyelectrolyte (PSS) shifted more negative

- resulting in decreasing electrocatalytic activity toward reduction of I3 . The

electrocatalytic activity was decreased owing to the repulsion between anionic

- polyelectrolyte (PSS) on graphene surface and I3 . This electrode could be used as anode

in DSSCs.

Chapter 4

Conclusion

This thesis describes the development of new types of electrode materials for

DSSCs. Alternative platinized counter electrode which consisted of mesoporous Nb-

doped TiO 2 as the support substrate for DSSC was applied to increase the electrocatalytic activity. The formation of a nanosized Pt polycrystalline phase mixed with Nb doped

TiO 2 was confirmed by SEM and HRTEM images. The charge transfer resistance was slightly reduced and the exchange current density was slightly increased as the result of the increased active surface of nanosized Pt in the Pt/Nb-doped TiO 2. Specially, when

Pt/Nb(1.0 mol%)-doped TiO 2 electrode was used as counter electrode, the exchange

current density was slightly higher than that of conventional platinized counter electrode.

This was due to, not only an increase in active area but also an increase in conductivity of

the counter electrode, which resulted in an enhancement of electrocatalytic activity.

The graphene electrode obtained by thermal reduction has higher electrocatalytic

- - activity towards I3 /I redox reaction than that obtained by hydrazine reductions. This is mainly due to the decreased presence of oxidized carbon species and increased defect density leading to the decrease in charge transfer resistance and increase in exchange current density. In addition to this, the electrocatalytic activity of graphene electrode was

77 tuned effectively by modifying graphene surface with polyelectrolyte. The electrocatalytic activity of graphene can be enhanced and reduced by modifying with

PDAC and PSS, respectively. In my opinion this study will contribute to open up research to develop of electrode in DSSCs, which in terms of enhancing electrocatalytic

- - properties of graphene electrode for the tri-iodide (I 3 )/iodide (I ) redox reaction. Finally,

we are confident that the presented surface modification by polyelectrolyte is also

amenable for tuning their electrocatalytic property.

In the future, I plan to attach nanoparticles of transition metals onto graphene films and investigate their effects on the electrocatalytic properties of the electrodes.

Generally, it is realized that transition metals such as Fe, Co, Ni, as well as their mixtures can be used as catalytic sites with high activity. Moreover, they can catalyze the formation carbon nanotubes and carbon “onions”. Therefore, in my future project, graphene oxide immersed in transition metal salt solutions will be prepared to enhance the thermal reduction of graphene oxide. Thus, higher degree of reduction may be observed because those transition metals might deposit at oxygenated functional group of graphene oxide and serve as reduction catalytic sites. Moreover, transition metals might change the structure of graphene after the reduction process resulting in improved electrocatalytic activity and stability of graphene. Since transition metal oxides have

- - reduction activity, they might show electrocatalytic activity towards I3 /I redox reaction as well, which will increase the catalytic sites on graphene sheets besides edge plane defects.

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