Y-Radiation Induced Synthesis of Metal Oxides

kth royal institute of technology

Doctoral Thesis in Chemistry Y-Radiation Induced Synthesis of Metal Oxides

Control of Particle Size, Composition, and Morphology

ZHUOFENG LI

Stockholm, Sweden 2021 Y-Radiation Induced Synthesis of Metal Oxides

Control of Particle Size, Composition, and Morphology

ZHUOFENG LI

Academic Dissertation which, with due permission of the KTH Royal Institute of Technology, is submitted for public defence for the Degree of Doctor of Philosophy on 17 September 2021 at 10:00 in room Kollegiesalen, KTH, Brinellvägen 8, SE-11428, Stockholm.

Doctoral Thesis in Chemistry KTH Royal Institute of Technology Stockholm, Sweden 2021 © ZHUOFENG LI © RSC, Paper I © Elsevier, Paper II © Elsevier, Paper III

ISBN 978-91-7873-946-2 TRITA-CBH-FOU-2021:30

Printed by: Universitetsservice US-AB, Sweden 2021

“Science and technology constitute a primary productive force.”

Xiaoping Deng

Abstract

Nanomaterials show a significant difference in chemical, mechanical, electronic, magnetic and optical properties compared with bulk counterparts. The synthesis is a key-step to achieve the unique properties of nanomaterials. As an efficient, clean and straightforward approach, the γ- radiation induced synthesis has been extensively applied to fabricate metal nanoparticles. However, with regard to production of metal oxide nanoparticles via γ-radiation induced synthesis, the knowledge is still insufficient. The following metal oxides have been selected as synthesis substances in this thesis: Cu2O, MnO2, Mn3O4 and CeO2. A prerequisite for utilization of the radiation-induced approach to engineer metal oxide nanomaterials is to optimize the reaction conditions for each specific nanofabrication case. The current study aims to understand the effects of different reaction conditions, i.e., pH, scavenger concentration, precursors, dose, and support materials, on the structural, and physico-chemical properties of the fabricated metal oxide nanomaterials. The pH of the reaction plays an important role in determining the thermodynamically stable metal oxide products in radiation-induced synthesis. (Paper I). When using high concentration of solutes, i.e., isopropanol (used as a hydroxyl radical scavenger), the solvent effects must be considered (Paper I). It is found that different metal cation precursors (same element but different oxidation state) can result in metal oxides differing in composition and morphology under different radiation-induced redox conditions (Paper II). By gradually increasing the dose, ceria nanoparticles in different growth stages can be captured, and their morphological development is studied by TEM (Paper IV). Based on this, a mechanism for nucleation of ceria nanoparticles and mesocrystal growth is proposed as a function of the dose. Support materials, i.e., carbon black and PVP nanogel, are used to engineer supported metal oxides. The reactivity of carbon black towards water radiolysis products (Paper III), and the size-controlling effect of nanogel on produced metal/metal oxide nanoparticles (Paper V) are investigated in this thesis. In addition, for the view of application, the primary electrochemical properties of radiation- induced synthesized carbon black supported samples are studied (Paper II).

Keywords: γ-radiation induced synthesis, Metal oxide nanoparticles, Metal oxide nucleation, Mesocrystal formation, Carbon black reactivity towards radical, PVP nanogel.

Sammanfattning

Nanomaterial uppvisar en signifikant skillnad i kemiska, mekaniska, elektroniska, magnetiska och optiska egenskaper jämfört med motsvarande bulkmaterial. Syntesprocessen är ett av de viktigaste stegen för att åstadkomma nanomaterialens unika egenskaper. Som ett effektivt, rent och enkelt tillvägagångssätt har γ-strålningsinducerad syntes använts i stor utsträckning för att tillverka metallnanopartiklar. När det gäller produktion av metalloxid-nanopartiklar via γ- strålningsinducerad syntes är kunskaperna fortfarande otillräckliga. Följande material har valts som syntesämnen i denna avhandling: Cu2O, MnO2, Mn3O4 och CeO2. En förutsättning för att använda strålningsinducerad syntes för att producera metalloxid- nanomaterial är att optimera reaktionsförhållandena för varje specifikt nanofabrikationsfall. Den aktuella studien syftar till att förstå effekterna av olika reaktionsförhållanden, d.v.s. pH, reaktantkoncentration, startmaterial, stråldos och eventuellt templatmaterial, på de tillverkade metalloxid-nanomaterialens strukturella och fysikalisk-kemiska egenskaper. Reaktionens pH spelar en viktig roll för att bestämma de termodynamiskt stabila metalloxidprodukterna vid strålningsinducerad syntes (Artikel I). Vid användning av höga koncentrationer av lösta ämnen, t.ex. isopropanol som används som hydroxylradikalinfångare, måste även lösningsmedelseffekter betraktas (Artikel I). Olika startmaterial (samma grundämne men olika oxidationstal) kan ge produkter som skiljer sig åt i fråga om sammansättning och morfologi under olika strålningsinducerade redoxförhållanden (Artikel II). Genom att gradvis

öka stråldosen kan CeO2-nanopartiklar i olika tillväxtstadier fångas in och deras morfologiska utveckling studeras med TEM (Artikel IV). Utifrån detta föreslås en mekanism för kärnbildning av CeO2-nanopartiklar och mesokristalltillväxt som en funktion av stråldosen. Templatmaterialen kimrök och PVP-nanogel har använts vid syntes av metalloxider. Kimrökens reaktivitet mot vattenradiolysprodukter (Artikel III) och den storleksreglerande effekten av nanogel på syntetiserade metall/metalloxid-nanopartiklar (Artikel V) undersöks i denna avhandling. Dessutom studeras de primära elektrokemiska egenskaperna hos strålningsinducerade syntetiserade kimröksbaserade prover (Artikel II).

Nyckelord: γ-strålningsinducerad syntes, Metalloxidnanopartiklar, Metalloxid kärnbildning, Mesokristallbildning, Kimröks reaktivitet mot radikal, PVP nanogel.

List of Papers

I. pH-Control as a way to fine-tune the Cu/Cu2O ratio in radiation induced synthesis of

Cu2O particles Zhuofeng Li, Inna L. Soroka, Fanyi Min, Mats Jonsson Dalton Transactions, 2018, 47, 16139-16144 DOI: 10.1039/C8DT02916D

II. Tuning morphology, composition and oxygen reduction reaction (ORR) catalytic performance of manganese oxide particles fabricated by γ-radiation induced synthesis Zhuofeng Li, Yi Yang, Axel Relefors, Xiangyang Kong, Gerard Montserrat Siso, Björn Wickman, Yohannes Kiros, Inna L. Soroka Journal of Colloid and Interface Science, 2021, 583, 71-79 DOI: 10.1016/j.jcis.2020.09.011

III. On the reactivity of aqueous radiolysis products towards carbon black used in in- situ radiation-synthesis of catalytic nanoparticles in graphite electrodes Zhuofeng Li, Mats Jonsson Carbon, 2021, 173, 61-68 DOI: 10.1016/j.carbon.2020.10.094

IV. Nanoscopic insights on different stages of CeO2 mesocrystals formation in aqueous solutions explored using radiation chemistry Zhuofeng Li, Diana Piankova, Yi Yang, Yuta Kumagai, Hannes Zschiesche, Mats Jonsson, Markus Antonietti, Nadezda V. Tarakina and Inna L. Soroka Manuscript.

V. Inorganic/organic hybrid nanoparticles synthesized in a two-step radiation- driven process Zhuofeng Li, Inna L. Soroka, Nadezda V. Tarakina, Maria Antonietta Sabatino, Emanuela Muscolino, Marta Walo, Mats Jonsson, Clelia Dispenza Manuscript.

III

Contribution to the papers

I. I participated in designing the experiments, performed all the experiments, wrote the original draft.

II. I participated in part of experimental design, performed part of the experiments, wrote part of the original draft and did the editing work.

III. I participated in the experimental design, performed all the experiments, wrote the original draft and did the editing work.

IV. I participated in the experimental design, performed synthesis, analyzed solutions and did XRD study of the precipitate, wrote a draft of radiation chemistry part, and did the editing work.

V. I participated in the experimental design, performed and wrote the experimental part, and did the editing work.

Contents

Abstract ...... I

Sammanfattning ...... II

List of Papers ...... III

Contribution to the papers ...... IV

1 Introduction ...... 1

1.1 Background ...... 1

1.2 Interactions between ionizing radiation and matter ...... 4

1.3 Water radiolysis ...... 5

1.4 Metal oxide formation in irradiated aqueous solution ...... 6

1.4.1 Radiolytic oxidation of metal ions ...... 7

1.4.2 Radiolytic reduction of metal ions ...... 7

1.5 Hydrolysis and condensation reactions of metal ions in aqueous solution ...... 8

1.6 Supported metal oxide formation in irradiated aqueous system ...... 11

1.7 Research aims and scope ...... 12

2 Experimental details and characterization techniques ...... 13

2.1 The general concept of the γ-radiation induced synthesis method ...... 13

2.2 Characterizations ...... 14

2.2.1 The Hantzsch method ...... 14

2.2.2 The Ghormley triiodide method ...... 14

2.2.3 X-Ray Diffraction (XRD) ...... 14

2.2.4 Scanning electron microscopy (SEM) and Energy dispersive spectroscopy (EDS) .. 15

2.2.5 Transmission electron microscopy (TEM) ...... 15

2.2.6 Ultraviolet-visible spectroscopy (UV-vis) ...... 15

2.2.7 X-ray photoelectron spectroscopy (XPS) ...... 16

2.2.8 Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) ...... 16

2.2.9 Electrochemical characterization ...... 16

2.2.10 Other instruments ...... 17

3 Results and discussion ...... 18

3.1 γ-radiation induced synthesis of metal oxide particles ...... 18

3.1.1 The synthesis of Cu2O ...... 18

3.1.2 The synthesis of MnOx ...... 19

3.1.3 The synthesis of CeO2 ...... 21

3.2 The effect of reaction conditions on metal oxide formation in γ-radiation induced synthesis ...... 22

3.2.1 The effect of pH on the composition and morphology of Cu/Cu2O ...... 22

3.2.2 Solvent effect: the influence of 2-propanol concentration on Cu2O formation ...... 27

3.2.3 MnOx formation via radiolytic oxidation and reduction ...... 28

3.3 CeO2 mesocrystal formation in irradiated aqueous solutions ...... 29

3.3.1 CeO2 formation by γ-radiation induced synthesis at different doses ...... 30

3.3.2 CeO2 mesocrystal evolution as a function of the dose ...... 31

3.3.3 Discussion on the formation and growth of CeO2 mesocrystal in irradiated aqueous solution ...... 34

3.4 The reactivity of carbon black towards aqueous radiolysis products and its impact on γ- radiation induced synthesis of metal oxides ...... 37

3.4.1 Competition kinetics ...... 38

3.4.2 The reactivity of carbon black towards the hydroxyl radical ...... 39

3.4.3 The reactivity of carbon black towards the 2-hydroxy-2-propyl radical ...... 40

3.4.4 The reactivity of carbon black towards the hydrated electron ...... 42

3.4.5 The reaction between carbon black and H2O2 in an unirradiated system ...... 43

3.4.6 Impact of carbon black on the radiolytic formation of H2O2 ...... 46

3.4.7 Discussion about the potential impact of carbon black on γ-radiation induced synthesis of metal and metal oxide ...... 47

3.5 The influence of PVP nanogel on γ-radiation induced synthesis of CeO2 and Ag ...... 49

3.5.1 The synthesis of CeO2 and Ag nanoparticles in the presence of PVP nanogel ...... 49

3.5.2 The morphological change of CeO2 and Ag nanoparticles caused by PVP nanogel. 52

3.6 Application of γ-radiation induced synthesis for electrocatalysts fabrication...... 54

3.6.1 γ-radiation induced synthesis of MnOx/Carbon black electrocatalysts ...... 54

3.6.2 Electrochemical properties of MnOx-Oxi/C and MnOx-Red/C ...... 57

4 Concluding Remarks ...... 60

5 Future Work ...... 62

Acknowledgements ...... 63

References ...... 64

VII

CHAPTER 1. INTRODUCTION

1 Introduction

1.1 Background

Nanomaterials can be defined as materials that possess at least one external dimension below 100 nm. These materials usually show different physico-chemical and mechanical properties from their bulk-form counterparts due to the high ratio of surface area to volume and the possible appearance of quantum effects. Metal oxide nanoparticles constitute an important class of nanomaterials with distinct optical, electronic, magnetic and catalytic properties. Therefore, they have become subjects of intensive studies in recent decades for optics,1 energy storage,2 sensors,3 drug delivery,4,5 catalysis6,7 and many more applications. In order for these metal oxide nanomaterials to become beneficial to human society and economy, process up-scaling is required. The scale-up process must be sustainable and environmental-friendly allowing metal oxide nanoparticles to be fabricated with controlled size, shape, composition, and physico- chemical properties. The synthesis techniques of nanomaterials can be generally categorized into two types of approaches: top-down and bottom-up. For top-down approaches, a bulk form material is degraded into nano-sized objects. For bottom-up approaches, the nanostructures are developed by integrating atomic or molecular building blocks. At present, the most applied strategies for engineering nanomaterials are through the bottom-up growth approach. Within the bottom-up category, wet-chemical synthesis accounts for a major proportion of both laboratory and industrial level methods of fabricating metal oxide nanoparticles. Generally, it involves chemical reactions of hydrolyzed metal precursors with additives to induce nucleation in the aqueous phase followed by condensation to form metal oxide nanomaterials. In some cases, redox reagents are needed to change the oxidation state of the precursors into the desired final valence state. The precursor concentration, pH, heating temperature and reaction time are the basic parameters that one can adjust to control the physico-chemical properties of the fabricated

CHAPTER 1. INTRODUCTION

metal oxide nanoparticles. The wet-chemical synthesis strategy has been implemented in a number of ways, including hydro/solvothermal, sol-gel and coprecipitation. The hydro/solvothermal synthesis is a method of fabricating nanomaterials in water or organic solvents. The chemical process is conducted in a stainless-steel autoclave. The solvents can be brought to temperatures (100 to 1000 ℃) above their boiling point by the increase of autogenous pressures (1 to 100 MPa) due to heating. In certain systems, supersaturation of the solute can be achieved at sufficiently high temperatures which induces the crystallization of as-synthesized materials.8 A variety of metal oxide nanoparticles have been fabricated via hydro/solvothermal 9 10,11 12,13 14,15 16,17 18,19 20,21 22,23 methods, e.g., FeO, Fe2O3, Fe3O4, Mn3O4, CoO, Co3O4, Cu2O, NiO and ZnO.24,25 The hydro/solvothermal method is regarded as one of the most promising and practical methods for controlling metal oxide nanoparticle homogeneity, particle size, composition and morphology. In sol-gel synthesis, metal-organic or inorganic salts are used as precursors in water or organic solvents with catalysts, stabilizers and gelling reagents. The process includes the following procedures: formation of a stable precursor solution (sol), polycondensation of the precursor (gel), aging, evaporation and heat treatment to generate crystalline metal oxide (calcination).26 27,28 The method has been widely used to synthesize metal oxide nanoparticles such as TiO2, 29,30 31,32 33,34 ZnO, SnO2, and WO3. The sol-gel method provides several advantages, including low cost, good homogeneity and high purity. Coprecipitation is one of the classical synthesis methods for inorganic nanoparticle fabrication. It involves nucleation, growth and agglomeration when different metal ions are mixed with a rational molar ratio in solution.35 To obtain the corresponding metal oxide, calcination needs to be performed after washing and drying the generated metal hydroxide precipitate. 36 Coprecipitation is the most conventional method for fabricating Fe3O4. In general, the coprecipitation method is a simple and economical technique to produce metal oxide nanoparticles. γ-radiation induced synthesis is one of the wet-chemical methods for fabrication of nanoparticles. The process is based on using radicals (products in water radiolysis) to induce redox reactions to convert solvated precursor metal ions to their desired valence state. The redox conditions of γ-radiation induced synthesis can be tuned by using a proper radical scavenger. Then the metal

CHAPTER 1. INTRODUCTION

ions can transform to a thermodynamically stable metal oxide phase via hydrolysis and condensation at a proper pH. Upon absorption of the radiation energy by water, radiolysis products are generated in situ. The main primary products in γ-radiolysis of water are hydroxyl radicals and solvated electrons and • • in addition H , HO2 as well as H2 and H2O2 are formed with lower yields. Since the hydroxyl radical and the solvated electron are very efficient redox reagents, γ-radiation induced synthesis can avoid additional redox reagents and simplify the production process. The amount of water radiolysis products depends on the absorbed radiation energy (dose). The formation of the water radiolysis products stops immediately when the reaction system is removed from the radiation field. Thus, one can precisely control the conversion yield of the precursor by controlling the absorbed dose. Since γ-radiation induced synthesis is a relatively clean and straightforward method, metal oxide nanoparticle nucleation and growth processes can be proposed without interference from side reactions. Besides producing freestanding metal oxides, γ-radiation induced synthesis is a promising method for incorporating metal oxides on support materials to engineer supported metal oxide catalysts.37 As discussed above, γ-radiation induced synthesis has several advantages in the fabrication of metal oxide nanomaterials compared to other techniques. However, a prerequisite for better utilization of γ-radiation induced synthesis for metal oxide nanomaterials is to optimize the reaction conditions for each specific case. Therefore, the influence of reaction conditions on metal oxide formation by γ-radiation induced synthesis needs to be better understood. The current study aims to understand the effects of different reaction conditions, i.e., pH, scavenger concentration, precursors, dose, and support materials, on the fabricated metal oxide nanomaterials’ structural, physical and chemical properties.

In the present thesis, cuprous oxide (Cu2O), manganese oxides (MnO2 and Mn3O4) and cerium oxide (CeO2) are used as the synthesis subjects due to their potential applications in energy conversion,38 energy storage,39–42 biosensing,43 and catalysis.44–47

CHAPTER 1. INTRODUCTION

1.2 Interactions between ionizing radiation and matter

Ionizing radiation encompasses high-energy particles, such as α and β particles, and electromagnetic waves, such as γ and X rays.48 It has sufficient energy to detach electrons from atoms in matter and thereby creating ions. The α particle is a helium nucleus emitted from a decaying nucleus with kinetic energy of 4-8 MeV.49 A β particle is an electron or positron emitted in the beta decay of a nucleus with kinetic energy ranging from 0.02 to 4 MeV.49 γ rays arise from the radioactive decay of the atomic nucleus with the energy of 0.1-2 MeV.50 From a conventional X-ray tube, the photons are generated in two ways. When a high energy electron collides with an inner shell electron, and both are ejected from atom leaving a vacancy in the inner shell. As the vacancy becomes occupied by an outer shell electron, the energy gain is emitted as a photon with a fixed energy corresponding to the electronic transition. This is referred to as characteristic X-rays in the energy range 0.12-120 keV.51 X-ray photons are also generated as Bremsstrahlung when an electron is slowed-down as it passes near the nucleus and the energy lost is emitted as a bremsstrahlung X-ray photon.52 Besides the ability to ionize matter, the penetration depth is one of the critical parameters of the interaction between ionizing radiation and matter. γ-photons can in general penetrate deep into materials depending on the density of the material.49 Therefore, γ-radiation is useful when processing larger volumes and thicker specimens.48 The absorbed dose is quantified by the radiation energy absorbed per unit weight of the absorber with the SI unit Gy (1 Gy = 1 J kg-1).53 The absorbed dose per unit time is referred to as dose rate with the unit Gy s-1. To quantify radiation-induced chemical changes in a given system, e.g., how much of a specific ion is produced upon absorption of ionizing radiation, the radiation chemical yield or G-value is introduced. The G-value is defined as the number of moles of a given species produced or consumed per absorbed Joule of radiation energy [mol J-1].48 The radiation-induced synthesis of metal oxides is commonly performed in an aqueous solution where the water absorbs the radiation energy. Therefore, it is necessary to understand the radiation chemistry of water. Also, it is essential to understand how the aqueous radiolysis products can act as initiators for the reactions throughout the radiation-induced synthesis of metal oxide materials.

CHAPTER 1. INTRODUCTION

1.3 Water radiolysis

In the 1960s, by applying pulse radiolysis, the nature and reactivity of the aqueous radiolysis products were thoroughly studied.54 With the technical development of the pulse radiolysis facilities, the time resolution of the experiments was increased from microsecond55 to nanosecond56 to picosecond57,58 and thereby the level of mechanistic understanding was improved. At present, the mechanism of water radiolysis is considered well understood after decades of research. Upon absorption of ionizing radiation, water is decomposed into oxidizing and reducing species:48

Ionization radiation - • • • + H2O → e aq, HO , HO2 , H , H3O , H2, H2O2 (1)

Water radiolysis can be separated into three stages by considering different time scales, the physical stage (1 fs), the physical-chemical stage (10-15 - 10-12 s) and the chemical stage (10-12 - 10-6 s). The three primary steps of water radiolysis are depicted in Figure 1.

Figure 1. Water radiolysis products at different stages.

CHAPTER 1. INTRODUCTION

It has been demonstrated that the solution pH influences the G-values of the water radiolysis + - products. For instance, under highly acidic conditions, such as pH = 0.5, H can capture e aq and form H•.59 Table 1 gives the G-values for radiolysis products of water by γ-irradiation at different 60 + pH ranges. As a supplement to Table 1, H3O has a G-value of 0.28 µmol/J in irradiated neutral water.53

Table 1. The G-values (µmol/J) of water radiolysis products in irradiated water at different solution pH range.60

- • • • Radiation e aq HO H2O2 H2 H HO2 γ and fast electrons, 0.28 0.28 0.073 0.047 0.062 0.0027 pH=3-11 γ and fast electrons, 0 0.301 0.081 0.041 0.378 0.0008 pH=0.5

- • As shown in Table 1, among these radiolysis products, e aq and HO are produced with the highest yield at pH range from 3 to 11 in γ-irradiated solutions. The experiments presented in this thesis were mainly performed in this pH range where all primary yields, corrected for solute 61 - 0 - reactivity are pH-independent. e aq is a strong reductant with a standard potential E (H2O/e aq) 62 • 0 • = -2.87 VSHE. HO is a powerful oxidant with a standard potential E (HO /H2O) = 2.59 VSHE at 63,64 • pH 0, 2.18 VSHE at pH 7. In strongly alkaline solution, for example at pH 14, HO is rapidly • - 0 • - - 65 • - transformed to O and E ( O /OH ) = 1.77 VSHE. Since HO and e aq process strong redox potentials and highest G-values, they are the major redox agents used in the radiation-induced synthesis of metal or metal oxide nanomaterials in aqueous solutions.

1.4 Metal oxide formation in irradiated aqueous solution

For decades, the method of γ-radiation induced synthesis has been used for fabricating organic and inorganic nanoparticles in aqueous solutions.66–70 The water radiolysis products range from • - strong oxidizing radical HO to strong reducing species e aq. Therefore, the solution redox condition can be shifted to either oxidizing or reducing condition by adding proper radical scavengers before irradiation. In irradiated aqueous systems, the metal precursor ions are

CHAPTER 1. INTRODUCTION

converted to the corresponding metal oxide or metal through two essential reactions, metal ion valence state change (by reduction or oxidation) followed by precipitation of insoluble products.

1.4.1 Radiolytic oxidation of metal ions

For metal oxides produced via the oxidative radiolytic pathway, the redox condition of the - irradiated system should be adjusted to purely oxidative, which means the reducing radical, e aq, - • should be scavenged. Nitrous oxide (N2O) is commonly used to convert e aq to HO according to reaction (2). As a result, an equivalent amount of HO• is formed:54

- • - 9 -1 -1 N2O + e aq + H2O → HO + OH + N2 k = 9.1 × 10 M s (2)

Therefore, in the oxidative radiolytic pathway, the G-value of HO• is 0.56 µmol/J. HO• can oxidize metal ions via the following reaction:

Mz+ + HO• → M(z+1)+ + OH- (3)

The hydrolysis and condensation of M(z+1)+ happen after the oxidation reaction. After that, the corresponding metal oxide is formed. MnO2 has been successfully synthesized via the oxidative radiolytic path with Mn(II) salt used as a precursor.37,71,72 Moreover, metal oxyhydroxide nanoparticles, i.e., γ-FeOOH73 and CoOOH,74 have been produced by the radiation-induced method. The initial step in these two cases is the oxidation of Fe2+ and Co2+ by HO• to Fe3+ and Co3+, respectively. Subsequently, the trivalent hydroxide (formed by hydrolysis) was converted to solid-state γ-FeOOH and CoOOH.

1.4.2 Radiolytic reduction of metal ions

For metal oxides produced via the reductive radiolytic pathway, the redox conditions of the irradiated system should be adjusted to purely reducing conditions, which means that HO• should • • be scavenged. Isopropanol is commonly used to convert HO to the reducing radical (CH3)2 COH via the following reaction:65

• • HO + (CH3)2CHOH → H2O + (CH3)2 COH (4)

CHAPTER 1. INTRODUCTION

• 0 The 2-hydroxyl-2-propyl radical ((CH3)2 COH) is a potent reducing radical with E • 75 • ((CH3)2CO/(CH3)2 COH) = -1.8 VSHE at pH 7. Thus, by applying isopropanol as HO scavenger, the G-value of reducing species in the system is doubled and equal to 0.56 µmol/J. - Metal ions, in general, are reactive towards e aq. However, the considerations of the redox potential are essential. Therefore, the alkali metal ions (first group) as well as Be, Mg, Ca, Sr - and Ba ions (second group, alkali earth metal), having lower redox potential than e aq, are - 61 - 67 unreactive with e aq. For the metal ions that can be reduced by e aq, the reaction is following:

z+ - (z-1)+ M + e aq → M (5)

(z-1)+ - M can undergo further reduction by e aq or via disproportionation:

2 M(z-1)+ → M(z-2)+ + Mz+ (6)

- • Besides e aq, (CH3)2 COH can also reduce metal ions through the following reaction:

z+ • (z-1)+ + M + (CH3)2 COH → M + CH3COCH3 + H (7)

Manganese oxides, MnO2 and Mn2O3, were fabricated from permanganate ions by the reductive radiolytic pathway.37,70,72,76,77 It was found that solution pH influences the composition of the 70 obtained manganese oxide. Within the pH range from 6.5 to 8.5, MnO2 can be produced, 77 whereas Mn2O3 is the dominant product when the pH is higher than 9.5. Chromium oxide 2- 2 78 (Cr2O3) nanoparticles were synthesized by radiolysis of aqueous CrO4 /Cr2O7 solutions. Besides oxometallate, metal ions are also used as precursors for the production of metal oxide via the reductive radiolytic pathway. For example, Cu2+ has been used as a precursor to fabricate 79,80 81 82 73,80 Cu2O particles. In addition, metallic nanoparticles, i.e., Au, Ag, and Cu, are also produced by reducing metal ions utilizing the reducing radiolytic radicals.

1.5 Hydrolysis and condensation reactions of metal ions in aqueous solution

In addition to the changes in oxidation state induced by the radiolysis products, there are some fundamental steps towards forming the metal oxide. These steps are, as mentioned above, hydrolysis and condensation.

CHAPTER 1. INTRODUCTION

Metal cations Mz+ can act as Lewis acids. The Lewis acid strength of metal cation has a positive correlation with the electronegativity of the metal.83 Water as ligands can coordinate with Mz+ z+ 84 and form metal aquo-complex [M(OH2)N] according to the following reaction:

z+ z+ M + N H2O → [M(OH2)N] (8) where z is the oxidation state of the metal ion Mz+, and N is the hydration number.85

Charge transfer occurs via the M-OH2 σ bond, and the electron density is transferred from coordinated water molecules to the empty orbitals of the metal cation. Thereby, the water molecule deprotonation may take place by the following reaction:86

z+ (z-h)+ + [M(OH2)N] + h H2O ⇌ [M(OH)h(OH2)N-h] + h H3O (9) where h is the hydrolysis ratio. (z-h)+ z+ The product [M(OH)h(OH2)N-h] ranges from aquo-complex [M(OH2)N] to hydroxo- (z-N)+ (z-2N)+ complex [M(OH)N] to oxo-complex [MON] , depending on the degree of deprotonation. (z-h)+ It is necessary to know the exact speciation of [M(OH)h(OH2)N-h] at a given pH, since the metal cations will act as precursors for the following condensation process. According to the 0 Partial Charge Model, when two or more atoms with different initial electronegativity (χi ) combine, they will adjust to the same intermediate electronegativity in the compound.86 Following this principle, the proton exchange between the aqueous-hydroxo complex (z-h)+ [M(OH)h(OH2)N-h] and the aqueous solution will continue until the mean electronegativity of both sides become equal:86

χh = χw = 2.732 – 0.035 pH Eq(1)

(z-h)+ where χh is the mean electronegativity of [M(OH)h(OH2)N-h] , χw is the mean electronegativity of the aqueous solution. The total charge of the aqueous-hydroxo complex +(z-h) can be calculated as the sum of all partial charges (δ) of M, O and H: z-h = δM + N δO + (2 N - h) δH Eq(2) thus, h = [z - δM – N δO – 2 N δH]/[1 - δH] Eq(3)

CHAPTER 1. INTRODUCTION

The partial charge of M, O and H can be calculated as the following equation:

0 0 1/2 δi = (χh – χi )/[1.36 (χi ) ] Eq(4)

Thus, by the equations (1), (3) and (4), and knowing the hydration number N,85 and atom i 0 86 electronegativity χi from the Allred-Rochow scale, one can derive the hydrolysis ratio h of (z-h)+ [M(OH)h(OH2)N-h] at a certain pH. By plotting z as a function of pH, a Charge vs. pH diagram84 can be obtained accounting for the (z-h)+ hydrolysis product [M(OH)h(OH2)N-h] , as shown in Figure 2. In general, hydrolysis of metal cations with z < 4, yields aquo-, aquo-hydroxo-, or hydroxo- complexes. For z > 4, hydrolysis yields hydroxo-, oxo-hydroxo-, or oxo-complexes. For z = 4, the tetravalent metal ions are on the borderline and thereby, depending on pH, they can form any of the possible complexes.

9 8 7 Oxo-complex [MO ](z-2N)+ 6 N 5

Hydroxo-complex z+ 4 (z-N)+ [M(OH)N] 3

2 Aquo-complex z+ 1 [M(OH2)N] 0 02468 101214 pH Figure 2. The charge of metal ion (z+) vs. pH diagram.84 The red line corresponds to h = 1, and

- the blue line corresponds to h = 2N -1. The two lines separate three domains in which H2O, OH or O2- ligands are formed.

The formed hydrolyzed metal cations are unstable and could condense via olation or oxolation reaction to produce the corresponding thermodynamically stable metal oxides. Olation corresponds to the nucleophilic addition of a negatively charged OH- group onto a positively (z-h)+ charged metal cation (z < 4), [M(OH)h(OH2)N-h] . In this case, there must be at least one water

CHAPTER 1. INTRODUCTION

molecule in the coordinated metal cation sphere. Olation leads to the OH bridges formation as follows:86–89

Figure 3. Olation mechanism for condensation reaction where a water molecule is eliminated from metal aquo-hydroxo-complexes.

For high valence metal cations z  4, the metal complexes mainly exist as oxo-hydroxo anions (z-N-x)+ [MOx(OH)N-x] in solutions. Since there are no coordinated water molecules, the olation reaction cannot happen, and the condensation only undergoes an oxolation reaction. The nucleophilic addition of a negatively charged OH- group is followed by a proton transfer to the leaving ligand:86,88,89

Figure 4. Oxolation mechanism for condensation reaction where a water molecule is eliminated from metal oxo-hydroxo-complexes.

In addition, the thermodynamically stable metal oxide product from condensation reaction is dependent on the reaction condition, i.e., pH. This information can be easily obtained from Pourbaix diagrams. Therefore, a Pourbaix diagram is constructed for each study throughout this thesis to establish the experimental conditions for engineering metal oxides with the desired composition.

1.6 Supported metal oxide formation in irradiated aqueous system

A catalyst supporting material is a material with a high surface area to which the active components are affixed.90 As a result, more active ingredients can be loaded on the accessible surface of the supporting materials. Thus, the activity of the catalysts can be promoted. Besides

CHAPTER 1. INTRODUCTION

a large surface area, the catalyst support should also possess high chemical stability and the ability to disperse active components on its surface. From an application point of view, the synthesized metal oxide particles can be incorporated into a supporting material to achieve high catalytic activity. The radiation-synthesis method is a promising approach for producing metal oxide nanoparticles on support and in confined spaces. However, to design and produce metal oxide nanoparticles on a solid support, the impact of supporting materials on the radiation chemistry of water and the interaction between support materials and precursors must be known.

1.7 Research aims and scope

This thesis is based on experimental studies of using the γ-radiation induced synthesis method to fabricate various metal oxide nanomaterials. By controlling variables, the effects of reaction conditions on the composition, morphology and structural properties of fabricated metal oxide are investigated. Additionally, the growth process of metal oxide nanoparticles is discussed. The thesis is divided into sections to address the following sub-topics.

1) Cu2O, MnO2, Mn3O4 and CeO2 are fabricated by the γ-radiation induced synthesis method. The Pourbaix diagram is constructed in each case to determine the fundamental reaction conditions, i.e., precursor type, solution pH. 2) The effects of pH, scavenger concentration and precursor on the composition and morphology of fabricated metal oxide are studied.

3) The different growth stages of CeO2 mesocrystal in aqueous solutions is studied by using radiation chemistry. 4) The influence of support materials on the yield and size of synthesized metal oxides are investigated. The selected substrates are carbon black and PVP-nanogel. 5) Carbon black supported manganese oxide is fabricated by γ-radiation induced synthesis method. The primary electrochemical properties of the samples are investigated. The results presented in the current work contribute to the knowledge about the influence of the experimental conditions on metal oxide nanomaterials formation in irradiated aqueous solutions.

CHAPTER 2. EXPERIMENTAL DETAILS AND CHARATERIZATION TECHNIQUES

2 Experimental details and characterization techniques

Throughout all the present experiments, Milli-Q (18.2 MΩ.cm) was used as the solvent. All chemicals were of reagent grade or higher otherwise state.

2.1 The general concept of the γ-radiation induced synthesis method

For the γ-radiation induced synthesis experiments, an MDS Nordion 1000 Elite Gamma-cell with a 137Cs γ-source was used. The dose rate of the γ-source was determined to 0.12 Gy s-1 using Fricke dosimetry.91 In a typical preparation of metal oxide nanoparticles using the γ-radiation induced synthesis method, an aqueous solution containing the metal salt precursor and a radical scavenger to obtain the desired redox conditions is irradiated. In most cases, oxidizing conditions are obtained by purging the solution with N2O and reducing conditions are obtained by using isopropanol as a scavenger for hydroxyl radicals. In some cases, a buffer was used to adjust solution pH. To control the particle size, surfactants were used in some experiments. A schematic illustration of a reaction system is shown in Figure 5.

Figure 5. Schematic illustration of a reaction system for the metal oxides production by the γ- radiation induced synthesis.

CHAPTER 2. EXPERIMENTAL DETAILS AND CHARATERIZATION TECHNIQUES

2.2 Characterizations

2.2.1 The Hantzsch method

The accumulated concentration of produced hydroxyl radical was determined by using the modified Hantzsch method.92 The hydroxyl radicals were scavenged by Tris(hydroxymethyl) aminomethane or methanol to generate formaldehyde according to reaction (10). 1 mL 0.2 M acetoacetanilide (dissolved in ethanol), 2.5 mL 4 M ammonium acetate and 1.5 mL sample solution were mixed and kept at 40 ℃ for 15 min in a water bath. The formed formaldehyde reacts with acetoacetanilide and ammonium acetate to produce a pyridyl derivative (via reaction (11)), which can be quantified using UV-vis spectrophotometry at λ = 368 nm.

• • (CH2OH)3CNH2 + OH → CH2O + (CH2OH)2 CNH2 + H2O (10)

CH2O + NH3 + 2 C10H11NO2 → C21H20N3O2 (11)

2.2.2 The Ghormley triiodide method

93 The concentration of H2O2 was determined by the Ghormley triiodide method. 100 μL 1M sodium acetate/acetic acid buffer mixed with ammonium molybdate was used as a catalyst. Then 100 μL catalyst mixed with 100 μL 1M potassium iodide and 1.8 mL sample solutions which - - contains H2O2 in a 1 cm cuvette. The I can be oxidized by H2O2 to I3 through reactions below:

- - 2 I + H2O2 → I2 + 2 OH (12)

- - I2 + I → I3 (13)

- - The formed I3 absorbs light at λ = 350 nm. Therefore, the concentration of I3 can be measured by a UV-vis spectrophotometer at this wavelength.

2.2.3 X-Ray Diffraction (XRD)

The structural characterization was performed by powder X-ray diffraction (XRD) and the patterns were recorded by a PANalytical X’Pert PRO diffraction system using Cu Kα radiation

CHAPTER 2. EXPERIMENTAL DETAILS AND CHARATERIZATION TECHNIQUES

(λ =1.54 Å) in Bragg-Brentano geometry. XRD scans were recorded in 2θ range from 15° to 90° with a step of 0.01°. The full width at half maximum caused by the instrument (βi) is 0.05°. The collected XRD patterns were analyzed and matched with a database, i.e., ICSD,94 to find the corresponding crystalline phase. In the case of nanoparticle sizes below 100 nm, the size of the nanoparticles can be calculated from the XRD patterns via the Scherrer formula.95 Typically, the particle size was calculated by the mean of the values from the three strongest peaks. Thus, from the XRD characterization, we can obtain the composition and particle size information.

2.2.4 Scanning electron microscopy (SEM) and Energy dispersive spectroscopy (EDS)

Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) was performed using a VP-SEM S3700N setup. SEM is used for observing the surface morphology of produced metal oxide materials. The EDS is usually cooperated with Mapping (EDS-Mapping) and can give the information of particles surface chemistry, i.e., elemental density and the dispersion of specific elements over the surface. It should be noted, EDS-Mapping has a resolution of 1 μm, thus it was used for micro-meter sized particles.

2.2.5 Transmission electron microscopy (TEM)

The fine morphologies of the samples were investigated by transmission electron microscopy (TEM) operated at 200 kV using JEOL2100F at Max Planck Institute of Colloids and Interfaces, Potsdam, Germany. From TEM images, we can obtain clear and precise structure information of the produced nanomaterials. Size and size distribution can be calculated from the TEM images. Moreover, crystal structure analysis was performed in some cases.

2.2.6 Ultraviolet-visible spectroscopy (UV-vis)

Optical absorption spectra were acquired using a JASCO V-730 UV-vis spectrophotometer. It is used for measuring the metal ion concentration in most cases. A calibration curve (Concentration

CHAPTER 2. EXPERIMENTAL DETAILS AND CHARATERIZATION TECHNIQUES

vs. Absorbance) was made before the experiments for every specific reagent to be measured by UV-vis.

2.2.7 X-ray photoelectron spectroscopy (XPS)

The surface elemental analysis was done by using X-ray photoelectron spectroscopy (XPS) at Chalmers University of Technology, Gothenburg, and Umeå Universitet, Umeå, Sweden. XPS spectra of the MnOx-Red were recorded with a Kratos Axis Ultra electron spectrometer with a delay line detector. A monochromatic Al Kα source operated at 150 W, and a charge neutralizer was used for the measurements. The chamber pressure was below 3 × 10-9 Torr. XPS spectra of the MnOx-Oxi were recorded with a PerkinElmer PHI 5000C ESCA system. A monochromatic Al Kα source (1486.7 eV) operated at 50 W, and a charge neutralizer was used for the measurements. The chamber pressure was below 6 × 10-10 Torr.

2.2.8 Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES)

The manganese wt.% on carbon was measured with ICP-OES (Inductively Coupled Plasma- Optical Emission Spectroscopy) using a Thermo Scientific iCAP 600 series instrument. Before the ICP-OES measurements, 0.2 g of dry sample was immersed into 20 ml of 5% HNO3 with high-speed stirring for 24 h. The analysis of Mn was performed at wavelengths of 279 nm and 285.2 nm.

2.2.9 Electrochemical characterization

The oxygen reduction reaction (ORR) activity of the samples was characterized by rotating disc electrode (RDE) measurements. It was performed on mirror polished glassy carbon electrodes with a geometric area of 0.196 cm2. The electrodes were modified by drop-casting specific volumes of catalyst ink and dried in an ambient environment. All electrochemical measurements were performed in a three-electrode system using a potentiostat (Bio-Logic SP-300) with a graphite rod as a counter electrode and an Ag/AgCl in saturated KCl solution as a reference electrode. All the potentials reported here were iR-corrected (85% automatic iR compensation)

CHAPTER 2. EXPERIMENTAL DETAILS AND CHARATERIZATION TECHNIQUES

and calibrated to reversible hydrogen electrode (RHE) according to E(RHE) = E(Ag/AgCl) + 0.964 V in 0.1 M KOH.

2.2.10 Other instruments

The chemical reagents and samples were weighed by a Mettler Toledo AT261 Delta Range microbalance. The pH of the sample solution was measured by a Thermo Scientific Orion Star Series pH Meter with an accuracy of ± 0.01. Standard pH buffers calibrated the pH meter at 4.01, 7.00 and 10.01.

CHAPTER 3. RESULTS AND DISCUSSION

3 Results and discussion

3.1 γ-radiation induced synthesis of metal oxide particles

3.1.1 The synthesis of Cu2O

The Pourbaix diagram of the possible thermodynamically stable phases for the Cu-H2O system was constructed using the software Medusa96 and is shown in Figure 6 (a). As seen in the diagram, in the pH range from 3.5 to 5, there is a potential interval where Cu2O(s) can be formed directly from reducing Cu2+ in solution. The standard reduction potential E0(Cu2+/Cu+) is +0.16 97 0 - 62 0 • 75 VSHE while E (H2O/e aq) = -2.87 VSHE and E ((CH3)2CO/(CH3)2 COH) = -1.8 VSHE at pH 7. - • 2+ + Therefore, e aq and (CH3)2 COH are capable of reducing Cu to Cu . To stabilize and control the size and morphology of synthesized Cu2O particles, various surfactants and capping agents, such as sodium dodecyl sulfate (SDS),98,99 polyvinylpyrrolidone (PVP),100 cetrimonium bromide (CTAB)101, and polyethylene glycol (PEG)102 were used in numerous studies. In the present work, SDS was used as the surfactant. Acetate buffer was used to adjust pH. The acetate concentration - 103 8 -1 -1 2+ was 0.05 M. Acetate reacts with e aq with a rate constant of of 2.2 × 10 M s whereas Cu - 104 10 -1 -1 (0.01 M) can reacts with e aq with a rate constant of 3.8 × 10 M s . Taking the rate constants and concentrations into account we can conclude that the rate of the reaction between Cu2+ and - - e aq is at least 34 times faster than the reaction between acetate and e aq. Thus, the contribution - from the reaction between acetate and e aq can be neglected. Cu2O was synthesized at pH 4.3. The obtained precipitate was characterized by XRD, and the result is shown in Figure 6 (b). The peaks of the XRD pattern can be indexed to crystalline Cu2O with a cubic cuprite structure 2+ (JCPDS #05-0667). The formation of Cu2O from hydrated Cu in aqueous solution by γ- radiation induced synthesis can be accounted for by the following reactions:105–107

2+ - + 10 -1 -1 104 Cu + e aq → Cu k = 3.8 × 10 M s (14)

2+ • + + 7 -1 -1 108 Cu + (CH3)2 COH → Cu + (CH3)2CO + H k = 5.0 × 10 M s (15)

Then the Cu+ undergoes hydrolysis and condensation,

CHAPTER 3. RESULTS AND DISCUSSION

+ + Cu + H2O → Cu(OH) + H (16)

2 Cu(OH) → Cu2O + H2O (17)

1.0 (a) (b) Cu2O Cu2+ CuO

0.5 (111)

/V Cu O

0.0 2

SHE

Intensity (a. u.) (200)

-0.5 Cu (220) (110)

-1.0 2468 1012 253035404550556065 pH at 25 °C 2Theta (degrees) 2+ Figure 6. Pourbaix diagram for the Cu–H2O system. [Cu ] = 0.01 M and T = 25 °C (a). The XRD pattern of the Cu2O precipitate prepared by γ-radiation induced synthesis from 10 mM CuSO4,

− in the presence of 2 M 2-propanol, 8 mM SDS, and 0.05 M CH3COOH/ CH3COO buffer. The pH was 4.8. The solution was irradiated with a dose of 8.9 kGy. Miller indices correspond to

Cu2O (JCPDS #05-0667) (b).

3.1.2 The synthesis of MnOx

The Pourbaix diagram of the possible thermodynamically stable phase for the Mn-H2O system was plotted and is shown in Figure 7 (a). As seen in the diagram, MnO2 and Mn2O3 are the thermodynamically stable manganese oxide phases at pH < 6 and 6 < pH < 6.4, respectively. - 2+ They both can be produced via either reduction of MnO4 or oxidation of Mn . At an even higher pH, Mn3O4 is the predominant phase. In the present work, the manganese oxides were fabricated by the γ-radiation induced synthesis using either MnCl2 or KMnO4 as precursors. However, in both synthesis routes, it is problematic to control the pH due to the complexation of Mn ions with the buffer and the potential competition reactions between Mn ions and buffer agents towards the active radicals. For example, under oxidative conditions, citric acid (in using the citric acid-citrate buffer, pH 3.0-6.2) can compete with Mn2+ to consume HO• with a fairly high 7 -1 -1 109 2- rate constant, 5 × 10 M s . HPO4 in using phosphate buffers (pH 5.8-8.0) can complex Mn + ions. Irradiated aqueous solutions become more acidic due to the accumulated H3O (with a G-

CHAPTER 3. RESULTS AND DISCUSSION value of 0.28 µmol/J in neutral water).53 In a set of exploratory experiments, the maximum acidity of the irradiated aqueous solution was explored. 50 mL of 5 mM MnCl2 solution was irradiated for a dose of 8.9 kGy. The initial pH (pH0) was adjusted by adding HCl. The pH after irradiation was noted as pH1. The results indicated that the pH was not changed notably when the pH0 is 1.50 (pH0/pH1: 2.25/2.05, 1.96/1.90, 1.75/1.71, 1.65/1.62, 1.50/1.50). 2+ To synthesize manganese oxide via the oxidation of Mn , the initial MnCl2 solution pH was adjusted to 1.50 by HCl to ensure the pH will not change significantly during irradiation. To - synthesize manganese oxide via the reduction of MnO4 , the pH was initially 11.4 and it decreased to 8.5 after the irradiation. Figure 7 (a) shows that Mn3O4 is the stable product in this pH range (pH 8.5 to 11.4). Therefore, in both cases, the sample solutions were not buffered. The XRD patterns of the synthesized manganese oxides are shown in Figure 7 (b). The peaks were noted with Miller indices corresponding to γ-MnO2 (JCPDS 14–0644) for the sample 2+ - produced from Mn and to Mn3O4 (JPCDS 24–0734) for the sample produced from MnO4 .

(a) 2.0 (b)

MnO - (103) 1.5 (211)

4 (101)

1.0 MnO (112)

2 (224)

(200)

(004)

(220)

(321) (105)

0.5 (314)

/V 2+ Mn2O3 0.0 Mn SHE Mn O ( ) 3 4 (131) MnOx-Red Mn3O4 E +

-0.5 Mn2(OH)3

(300) (160)

-1.0 Mn(OH)2 u.) (a. Intensity

(120) (421) -1.5 Mn MnOx-Oxi (-MnO2) -2.0 0 2 4 6 8 10 12 14 15 20 25 30 35 40 45 50 55 60 65 70 pH at 25 °C 2Theta (degrees) 2+ Figure 7. Pourbaix diagram for the Mn–H2O system. [Mn ] = 0.01 M and T = 25 °C (a). XRD patterns of the fabricated samples. γ-MnO2 (black) was produced from 5 mM MnCl2, N2O saturation solution, and pH was adjusted to 1.50 by HCl. Mn3O4 (red) was produced from 5 mM

KMnO4, 1 M 2-propanol solution. The solution was irradiated with a dose of 8.9 KGy (b). Miller indices correspond to -MnO2 (JCPDS 14–0644) and Mn3O4 (JPCDS 24–0734).

CHAPTER 3. RESULTS AND DISCUSSION

3.1.3 The synthesis of CeO2

The Pourbaix diagram of the possible thermodynamically stable phases for the Ce-H2O system was plotted and is shown in Figure 8 (a). According to the diagram, CeO2 can be produced by oxidation of Ce3+ in a pH range from 0.3 to 7.5. In addition, the reduction potential E(Ce4+/Ce3+) 95 • = 1.13 VSHE at pH 3.0. Therefore, the oxidizing radical HO is thermodynamically capable of 3+ 4+ oxidizing Ce to Ce . CeO2 nanoparticles were produced by γ-radiation induced synthesis from a 5 mM CeCl3 aqueous solution saturated with N2O in the present study. As in the synthesis of 3+ • MnOx discussed above, most available pH buffers can compete with Ce for HO or possibly form precipitates with Ce3+ or Ce4+. For this reason, the solutions were not buffered during radiation. The pH before irradiation was 5.9 and it was found to be 2.8 after irradiation, which is still in the stability range for CeO2. The resulting solid was characterized by XRD, and the results are shown in Figure 8 (b). All diffraction peaks have been indexed to the cubic fluorite structure of CeO2 (JCPDS 34-0394). The produced CeO2 nanoparticles have an average size of 3.5 nm calculated from the XRD pattern by Scherrer formula.

(a) 3+ (b) 2 Ce(OH) CeO2

CeO2(s)

1 (111)

/V 0

SHE

(200)

E (220)

-1 Ce3+

Intensity (a. u.) (311)

Ce(OH)3(s)

(222) (400)

-2 (311) (420) Ce -3 -2 0 2 4 6 8 10 12 14 20 30 40 50 60 70 80 90 pH at 25 °C 2Theta (degrees) 3+ Figure 8. Pourbaix diagram for the Ce–H2O system. [Ce ] = 0.01 M and T = 25 °C (a). The XRD pattern of a sample of CeO2 precipitate prepared by γ-radiation induced synthesis from 5 mM

CeCl3 aqueous solution. The solution was saturated with N2O and then irradiated with a dose of 8.9 kGy. Miller indices correspond to CeO2 (JCPDS 34-0394) (b).

CHAPTER 3. RESULTS AND DISCUSSION

3.2 The effect of reaction conditions on metal oxide formation in γ- radiation induced synthesis

3.2.1 The effect of pH on the composition and morphology of Cu/Cu2O

A set of Cu2O samples was prepared from aqueous solutions of varying pH. The pH before (pH0) 2+ and after (pH1) irradiation as well as the concentration of Cu before (C0) and after (C1) 2+ irradiation were determined. The conversion yield of Cu was calculated as (C0 – C1)/C0 × 100%. The obtained precipitate was analyzed by XRD. The XRD patterns were recorded from the products synthesized at different pH are selected as representatives and shown in Figure 9. These results are summarized in Table 2.

2+ Table 2. The pH before (pH0) and after (pH1) the irradiation. The conversion yield of Cu ((C0 –

C1)/C0 × 100%) in different samples. The composition of the obtained precipitate. 10 mM CuSO4 as the precursor, 2 M 2-propanol as the scavenger, 8 mM SDS as surfactant and 0.5 M

− CH3COOH/ CH3COO buffer to adjust the pH. The solutions were irradiated with a dose of 8.9 kGy.

Sample pH0/pH1 (C0 – C1)/C0 × 100% Composition 1 3.0/3.0 15.1% Cu 2 3.75/3.77 17.8% Cu

3 3.85/3.85 17.7% Cu and Cu2O

4 3.9/3.9 18.1% Cu and Cu2O

5 4.0/4.0 26.0% Cu and Cu2O

6 4.2/4.2 33.3% Cu and Cu2O

7 4.1/4.1 34.2% Cu and Cu2O

8 4.25/4.25 30.4% Cu and Cu2O

9 4.3/4.3 37.3% Cu and Cu2O

10 4.4/4.4 41.4% Cu and Cu2O

11 4.6/4.6 41% Cu2O

12 4.8/4.8 43.1% Cu2O

CHAPTER 3. RESULTS AND DISCUSSION

As shown in Figure 9, for the sample synthesized at pH 3.75, only diffraction peaks that belong to metallic Cu were detected.110–112 For the samples obtained at pH = 3.85, the XRD pattern 113 exhibited the characteristic peaks of Cu2O in addition to the metallic Cu peaks. As the pH was further increased, the intensities of metallic Cu diffraction peaks gradually decreased while the peaks corresponding to Cu2O increased.

¨ Cu © © Cu O 2 ©¨ © ¨ © pH = 4.8

pH = 4.3 pH = 4.1

pH = 3.9

Intensity (a. u.) (a. Intensity pH = 3.85 pH = 3.75

253035404550556065 2Theta (degrees)

Figure 9. XRD patterns of the Cu-Cu2O powders produced by γ-radiation induced method at different pH values.

To quantify the composition of the obtained precipitate, the ratio ICu2O/(ICu + ICu2O) was calculated (shown in Table 3) from the XRD results. ICu2O and ICu were the sum of integral intensities of XRD peaks for Cu2O ((110), (111), (200) and (220) planes) and for Cu ((111) and

(200) planes), respectively. Thus, the ratio represented the relative content of crystalline Cu2O in the Cu/Cu2O precipitate. The results indicate that one can vary the composition of the obtained precipitate from metallic copper to cuprous oxide just by varying solution pH and while keeping the other parameters unchanged. The transformation of Cu/Cu2O composition (as a function of pH) is consistent with the Pourbaix diagram of the Cu-H2O system shown in Figure 6 (a).

CHAPTER 3. RESULTS AND DISCUSSION

Table 3. The ratio ICu2O/(ICu + ICu2O) of samples synthesized at different pH. Samples are selected as representatives from all samples.

Sample pH0/pH1 ICu2O/(ICu + ICu2O)

2 3.75/3.77 0 3 3.85/3.85 0.54 4 3.9/3.9 0.82 4-1 3.9/3.9 0.25 5 4.0/4.0 0.97 5-1 4.0/4.0 0.69 6 4.1 0.98 7 4.2/4.2 1 7-1 4.2/4.2 0.64 7-2 4.2/4.2 0.97 8 4.25/4.25 0.95 9 4.3/4.3 0.99 10 4.4/4.4 0.98

To further understand the Cu2O formation in irradiated aqueous solutions, the ratio ICu2O/(ICu + 2+ 2+ ICu2O) was plotted as a function of pH in Figure 10 (a). The consumption of Cu (∆Cu = C0 –

C1) is plotted as a function of pH in Figure 10 (b). The ICu2O/(ICu + ICu2O) ratio increased from 0 to almost 1 within the pH range from 3.75 to 4.0. It is also observed that the ∆Cu2+ increases as the pH increases, and the value is twice as large when pure Cu2O was formed as compared to that when metallic Cu was generated.

CHAPTER 3. RESULTS AND DISCUSSION

(a) (b) 1.0 0.006

) 0.8

) 0.005 -1

Cu2O 0.6

0.004

+ I

(mol L

Cu 2+

/(I 0.4

0.003

Cu2O D I 0.2 0.002 0.0 0.001 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5 3.63.84.04.24.4 pH pH 2+ 2+ Figure 10. The ratio ICu2O/(ICu + ICu2O) as a function of pH (a), the consumption of Cu (∆Cu ) as a function of pH (b).

The formation of Cu is attributed to the following reactions:105–107

+ - 0 10 -1 -1 114 Cu + e aq → Cu k = 2.7 × 10 M s (18)

2 Cu+ → Cu0 + Cu2+ (19)

2+ whereas the formation of Cu2O from Cu aqueous solution by γ-radiation induced synthesis can be accounted for by reactions (14)-(17). The metallic Cu particles can be produced by two consecutive one-electron transfer processes from Cu2+ via Cu+ to Cu0 (reactions (14) and (18)),105,106 or by disproportionation of Cu+ 107 (reaction (19)). The ratio of ICu2O/(ICu + ICu2O) changes with pH as shown in Figure 10 (a), which indicates a pH-dependent competition between the reactions to form Cu2O and Cu. Given the results presented in Figure 10 (a), it can be concluded that the reaction of Cu2O formation is favored at higher pH. Furthermore, the increased consumption of Cu2+ at pH 4.4, as shown in Figure 10 (b), is fully in line with the fact that two electrons are needed to reduce Cu2+ to Cu 2+ while only one electron per Cu is needed to produce Cu2O. The morphologies of the obtained precipitates were characterized by SEM and are shown in Figure 11 (a)-(e). It can be observed that the synthesis solutions pH influences the morphologies of Cu-Cu2O powders. The metallic Cu particles (pH 3.75, Figure 11 (a)) represent quasi- spherical agglomerates with an average diameter of 2.9 µm. As the pH increased from 3.75 to

3.85, the octahedron-shaped particles corresponding to the Cu2O phase are formed. The fraction

CHAPTER 3. RESULTS AND DISCUSSION

of Cu2O octahedrons is gradually increased with the further increase of pH. The obtained octahedron Cu2O is (111) oriented due to the increased stability of the (111) surface in the 2- 115,116 117 presence of SO4 and SDS. The particle size distributions of Cu2O obtained at different pH are shown in Figure 11 (f). The average size of the produced Cu2O particles was decreased from 13.5 µm to 7 µm when the solution pH was increased during synthesis from 3.85 to 4.4.

This could be due to the nucleation of Cu2O being favored at higher pH, and consequently, more

Cu2O nucleation centers are generated. It results in smaller particle size and narrower size distribution than those in the samples obtained at lower pH, i.e., at pH 3.85.

Figure 11. SEM image of the synthesized Cu/Cu2O particles by γ-radiation induced method at pH 3.75 (a), 3.85 (b), 4.0 (c), 4.25 (d), 4.4 (e) and the Cu2O particle size distribution at different pH (f).

SEM-EDS Mapping was performed, and composition-morphology information of the sample (produced at pH 3.85) could be obtained. The results are shown in Figure 12. The four octahedron structured particles are labeled on the SEM image in Figure 12 (a). A visible difference can be observed in Cu Kα SEM-EDS mapping Figure 12 (c), showing that there is less Cu on the surface of the labeled octahedron-shaped particles area than the other areas. This

CHAPTER 3. RESULTS AND DISCUSSION indicated that the octahedron-shaped particles may have a lower Cu content on the surface than the unlabeled spherical particles. This may prove that the labeled octahedron-shaped particles with low Cu content and unlabeled particles correspond to Cu2O and Cu, respectively.

Figure 12. SEM image of the synthesized particles at pH 3.85 (a) SEM-EDS Mapping for O Kα (b) and SEM-EDS Mapping for Cu Kα (c).

3.2.2 Solvent effect: the influence of 2-propanol concentration on Cu2O formation

The influence of 2-propanol concentration on Cu2O formation was also investigated, and the results are shown in Figure 13. The ratio of ICu2O/(ICu + ICu2O) increases as the 2-propanol concentration increases from 0.5 to 1.0 M. This could partly be attributed to two competing reactions: between the buffer (acetate) and the 2-propanol (reaction (4)) with hydroxyl radicals. 9 -1 -1 However, based on the relative rate constants of these reactions (ki = 2.3 × 10 M s for hydroxyl 6 -1 -1 radicals react with 2-propanol, ka = 9.2 × 10 M s for hydroxyl radicals react with acetate),118,119 the contribution from the reaction of acetate with hydroxyl radicals could be neglected. Given the magnitude of the 2-propanol concentration, it is more likely that it can be attributed to solvent effects on the relative rates of the competing reactions between the Cu and

Cu2O formation. Therefore, 2.00 M of 2-propanol was selected for the synthesis of Cu2O to minimize these solvent effects.

CHAPTER 3. RESULTS AND DISCUSSION

1.0 )

0.8 Cu2O

0.6

+ I + Cu

/(I 0.4

Cu2O I 0.2

0.0 0.50 0.75 1.00 1.25 1.50 1.75 2.00 Concentration (M)

Figure 13. The ratios ICu2O/(ICu + ICu2O) as a function of the 2-propanol concentration. Cu/Cu2O was prepared at pH 4.40.

3.2.3 MnOx formation via radiolytic oxidation and reduction

Manganese oxides were prepared by γ-radiation induced synthesis through both the oxidative and the reductive pathways. Hereafter, the samples are denoted as MnOx-Oxi and MnOx-Red for obtained by oxidative and reductive routes, respectively. The redox conditions were adjusted by adding a proper radical scavenger. For the oxidative pathway, the solution was purged with N2O - • to scavenge e aq while for the reductive pathway, 2-propanol was used to scavenge HO . The synthesis parameters are summarized in Table 4.

Table 4. Synthesis parameters of manganese oxide by γ-radiation induced method. pH before/after Sample Precursor Radical scavenger Dose irradiation 1.50/1.46, HCl MnO -Oxi MnCl •4H O, 5 mM ~ 2.5 × 10-2 M N O 8.9 kGy x 2 2 adjust 2

MnOx-Red KMnO4, 5 mM 11.4/8.5 1 M 2-propanol 8.9 kGy

2+ 2+ 85 In the radiolytic oxidative pathway, the oxidation of hydrated Mn ([Mn(OH2)6] ) is achieved through the reaction with HO•:

2+ • (2+n)+ - [Mn(OH2)6] + n HO → [Mn(OH2)6] + n OH (20)

(2+n)+ [Mn(OH2)6] can hydrolyze and undergo condensation reactions to generate corresponding manganese oxide.86

CHAPTER 3. RESULTS AND DISCUSSION

- Under the radiolytic reductive pathway, the precursor Mn(VII), (MnO4 ), can be reduced by the - • reducing radicals (e aq and (CH3)2 COH) to lower valence state Mn cations. The corresponding manganese oxides can be formed via the condensation reactions of the hydrolyzed lower valence state Mn cations.86 According to the XRD patterns shown in Figure 14 (a), the compositions of the fabricated

MnOx-Oxi and MnOx-Red samples are γ-MnO2 and Mn3O4, respectively. The morphology of γ-

MnO2 was studied by SEM and shown in Figure 14 (b). The γ-MnO2 particles have a spherical shape with nano-flakes composed on their surface. Mn3O4 was characterized by TEM and showed the morphology of nanorods (Figure 14 (c)).

Figure 14. XRD patterns of the fabricated samples. MnOx-Oxi (black) produced from 5 mM

MnCl2, N2O saturation and HCl to adjust pH to 1.50. MnOx-Red (red) produced from 5 mM

KMnO4, 1 M 2-propanol. The solution was irradiated with a dose of 8.9 kGy. Miller indices correspond to -MnO2 and Mn3O4 (a). SEM image of MnOx-Oxi (b). TEM image of MnOx-Red (c).

3.3 CeO2 mesocrystal formation in irradiated aqueous solutions

The reaction condition of γ-radiation induced synthesis of CeO2 is relatively clean and straightforward with Ce3+ and HO• as the primary reactants. Interestingly, the results indicated we obtained CeO2 mesocrystals that encompassed well-aligned primary particles.

CHAPTER 3. RESULTS AND DISCUSSION

3.3.1 CeO2 formation by γ-radiation induced synthesis at different doses

3+ Nanocrystalline CeO2 was synthesized by radiation-induced oxidation of Ce in aqueous solution. The solution contained 5 mM CeCl3 and was saturated with N2O. The doses varied from 0.04 to 10.7 kGy. The irradiated solutions gradually became light yellow suspensions as the dose increased. The solutions were centrifuged, and the obtained yellow precipitate was washed and characterized by XRD. The XRD patterns are selected as representatives and shown in Figure 15.

As seen in the figure, only the diffraction peaks of crystallographic planes in CeO2 are present.

XRD patterns confirm that the obtained CeO2 has the fluorite type (CaF2) structure with a space group Fm-3m. 120 The average lattice parameter for all studied samples was calculated based on XRD patterns, and the value is 5.458  0.005 Å. This is larger than the lattice parameter of bulk 121,122 CeO2, a = 5.412 Å measured under ambient conditions. As shown in the literature, for metal oxide nanoparticles with diameters below 10 nm, the surface strain may cause expansion of the outermost layers, leading to significant modification of the lattice parameters measured by X- ray diffraction.123

(111) (220) (311) (200) (311) (400) (222) 10.7 kGy

8.9 kGy

2.7 kGy Intensity (a. u.)

1.3 kGy

20 30 40 50 60 70 80 90 2Theta (degrees)

Figure 15. XRD patterns of CeO2 produced via γ-radiation induced synthesis with different doses. The solution contains 5 mM CeCl3 and is saturated with N2O. Miller indices labelled in the figure correspond to CeO2.

The TEM images and Selected Area Electron Diffraction (SAED) pattern of fully developed

CeO2 mesocrystals are presented in Figure 16. As shown in Figure 16 (a), the precipitate

CHAPTER 3. RESULTS AND DISCUSSION comprises particles, which are in fact mesocrystals consisting of nano-blocks (primary particles), see Figure 16 (b), where the preferable orientation of the primary particles can be clearly seen. Moreover, the SAED pattern from the whole mesocrystal (Figure 16 (c)) consists of diffraction rings with non-uniform intensity, i.e., there are high and low-intensity regions. Such pattern indicates the presence of texture in the studied material. The average diameter of the primary particles calculated using TEM images is about 3 nm. Thus, the yellow precipitate synthesized by -radiation induced method consists of primary CeO2 particles which are self-organized into the larger agglomerates, mesocrystals.

Figure 16. HRTEM image of CeO2 agglomerates formed from 15 mM CeCl3 solution at a dose of 29 kGy (a) and (b), the yellow lines are the primary particles alignment direction. Selected area electron diffraction pattern obtained from a single agglomerate. The variation of the intensities within the diffraction rings indicates the presence of a preferential orientation (c).

3.3.2 CeO2 mesocrystal evolution as a function of the dose

The formation and growth of CeO2 mesocrystal are studied using TEM on the samples obtained at different doses. The initial nucleation of CeO2 and the aggregation of primary particles could arise at very low doses, TEM images are shown in Figure 17. As seen in the Figure 17 (a) and

(d), at a dose of 0.04 kGy, the precipitate is composed of CeO2 primary particles agglomerates that form a network. Upon increase of the dose, the primary particles gradually become compacted within the network and form initial mesocrystals where the primary particles are randomly arranged (Figure 17 (b) and (e)). At an absorbed dose of 0.22 kGy, the mesocrystals

CHAPTER 3. RESULTS AND DISCUSSION

are detached from each other as well as the CeO2 primary particles start to align within the mesocrystals as the dose increased to 0.22 kGy (Figure 17 (c) and (f)).

Figure 17. TEM images of CeO2 mesocrystals produced via γ-radiation induced synthesis with different doses. 0.04 kGy (a) and (d), 0.11 kGy (b) and (e), 0.22 kGy (c) and (f).

Figure 18 shows the TEM images of CeO2 mesocrystals obtained at doses above 0.4 kGy. It can be observed that the CeO2 primary particles are primarily aligned in the generated mesocrystals at a dose of 0.45 kGy (Figure 18 (d). The average mesocrystal sizes were calculated and presented in Figure 19. From a dose of 0.45 to 2.7 kGy, the average agglomerate size increases from 15 to 25 nm. Meanwhile, at higher doses, up to 10.7 kGy it remains unchanged at about 27 nm.

CHAPTER 3. RESULTS AND DISCUSSION

Figure 18. TEM images of CeO2 mesocrystals produced via γ-radiation induced synthesis with different doses. 0.45 kGy (a) and (d), 1.3 kGy (b) and (e), 8.9 kGy (c) and (f).

Average mesocrystal size (nm) size Average mesocrystal 0 0 1 2 8 9 10 11 12 Dose (kGy)

Figure 19. The average size of the CeO2 mesocrystals which were synthesized at different doses, from 0.45 to 10.7 kGy.

CHAPTER 3. RESULTS AND DISCUSSION

3.3.3 Discussion on the formation and growth of CeO2 mesocrystal in irradiated aqueous solution

3+ 3+ 85 When Ce is solvated by dipolar water molecules, it will form aquo-cations [Ce(OH2)6] . The aquo-cations can be oxidized by the water radiolysis product HO• as the aqueous solution is irradiated via the following reaction:

3+ • 4+ - [Ce(OH2)6] + HO → [Ce(OH2)N] + OH (21) where N is the hydration number of Ce4+ During irradiation the pH of the aqueous solution continuously decreased. For example, the pH can decrease from 5.88 to around 2.82 when the dose was 8.9 kGy and the concentration of CeCl3 was 5 mM. Therefore, the hydrolysis ratio (h) of the tetravalent cerium cation changed during the irradiation. The exact h was calculated via the partial charge model and shown in Table 5.

Table 5. The calculated mean electronegativity of water (χw), partial charge of H (δH), O (δO)

(4-h)+ and Ce (δCe), and corresponds hydrolysis ratio (h) of [Ce(OH)h(OH2)N-h] at different pH via the partial charge model. The hydration number (N) was set as 9 according to literature.124

pH χw/χh δH δO δCe h

6 2.52 0.21 -0.38 0.92 3.4 5.5 2.54 0.22 -0.38 0.93 3.2 5 2.56 0.23 -0.37 0.94 2.5 4.5 2.57 0.24 -0.36 0.95 2.6 4 2.60 0.25 -0.36 0.97 2.3 3.5 2.61 0.26 -0.35 0.98 2.0 3 2.63 0.27 -0.34 0.99 1.8 2.5 2.64 0.28 -0.34 1.00 1.5

The deprotonation takes place as follows:

4+ (4-h)+ + [Ce(OH2)N] + h H2O → [Ce(OH)h(OH2)N-h] + h H3O (22) where h is in a range from 3.4 to 1.5 as pH from 6 to 2.5.

CHAPTER 3. RESULTS AND DISCUSSION

As the calculation results indicates, there are coordinated water remain in the complex after hydrolysis, thus both olation and oxolation can proceed during the condensation and leading to (4-h)+ CeO2 formation. The olation reaction of [Ce(OH)h(OH2)N-h] can occur according to:

(4-h)+ Scheme 1. The olation reaction of [Ce(OH)h(OH2)N-h] .

Since the OH bridges formed in the olation reaction, the oxolation reaction can proceed simultaneously to eliminate water molecules during the condensation process:

Scheme 2. The oxolation reaction of the tetravalent cerium species.

As the condensation proceeds, the nucleation and growth of the fluorite structure CeO2 lead to the formation of primary particles from the aquo-hydroxo complex domains. Thereafter, the primary particles tend to aggregate and orient along the same crystallographic direction to minimize the surface energy, in other words, forming the mesocrystals. As the TEM study reveals: (i) the agglomeration of the primary particles occurs at an early stage of the synthesis processes; (ii) the primary particle formation and their self-assembly into mesocrystals occur simultaneously; (iii) the crystalline order of the primary particles formed in the samples synthesized at low doses have smaller extension as compared to those synthesized at higher doses (iv) the non-agglomerated crystalline primary particles detected by TEM is in trace amount. The 3+ possible procedures of the CeO2 mesocrystals formation from irradiated Ce aqueous solution are shown in the following scheme.

CHAPTER 3. RESULTS AND DISCUSSION

3+ Scheme 3. The possible formation process of CeO2 mesocrystals in irradiated Ce aqueous solution.

While being in contact with the solution, the CeO2 mesocrystals are randomly oriented relative to each other. However, upon drying at ambient conditions, these particles assemble to form an ordered structure at a microscopic scale, a supra-crystal (see Figure 20). As seen in the figure, uniform units with an average diameter of 25 nm are arranged into a hexagonal structure. The sizes of these units are close to those of the mesocrystals. Interestingly, the presented supra- crystal can easily fall apart into separate units when immersed into water and placed in an ultrasonic bath.

CHAPTER 3. RESULTS AND DISCUSSION

Figure 20. SEM images (a)-(c) and optical image (d) of the crystal formed upon drying at ambient conditions the CeO2 precipitate. The precipitate is obtained by γ-radiation induced synthesis of CeO2 from 5 mM CeCl3 precursor solution at 6.1 kGy dose. SEM images are taken at different magnifications.

3.4 The reactivity of carbon black towards aqueous radiolysis products and its impact on γ-radiation induced synthesis of metal oxides

During recent decades, extensive research has been devoted to the fabrication metal125–127 and metal oxide nanoparticles37 on different support materials via radiation-induced synthesis methods. When adding support material into the water, a solid-liquid interface is introduced, and interfacial processes must also be considered. Therefore, it is necessary to understand how support materials react with the water radiolysis products and thereby affect metal oxide formation via γ-radiation induced synthesis. One important support material for electrocatalytic applications is carbon black. It has good electrical conductivity and low cost.128–131 However, its reactivity towards the aqueous radiolysis products and products formed upon reaction with common radical scavengers is still unexplored. Therefore, we investigated the reactivity of carbon black towards the hydroxyl radical, the solvated electron and the 2-hydroxy-2-propyl radical. As described in detail above, these radicals

CHAPTER 3. RESULTS AND DISCUSSION are utilized in producing metal and metal oxide nanoparticles by γ-radiation induced synthesis.

In addition, H2O2 can be radiolytically produced and accumulated during irradiation. Therefore, the impact of carbon black on H2O2 decomposition in unirradiated aqueous solution and on the radiolytic production of H2O2 was studied.

3.4.1 Competition kinetics

In a continuously irradiated system, a steady-state of radicals is rapidly reached.65 At the steady- state, the production rate of the radical (given by the dose rate and the radiation chemical yield) is balanced by its consumption rate.65 Competition kinetics method was used to determine the reactivity of carbon black towards the radicals. In a competition kinetics system, the solution contains a reference solute A of concentration [A] and a test solute B of concentration [B]. During the irradiation, the generated radical R can react with either solute A or B in proportion to the reactant concentration and the rate constant (ka and kb) of the following reactions:

R + A → product a ka (23)

R + B → product b kb (24)

푑[푝푟표푑푢푐푡 푎] If ( ) is the formation rate of the product a when solute B is absent and 푑푡 푟푒푓 푑[푝푟표푑푢푐푡 푎] ( ) is the formation rate of the product a when solute B is present, then it follows 푑푡 퐵 that:

푑[푝푟표푑푢푐푡 푎] 푑[푝푟표푑푢푐푡 푎] 푘 [퐵] ( ) /( ) = 1+ 푏 Eq(5) 푑푡 푟푒푓 푑푡 퐵 푘푎[퐴] where [product a] is the concentration of product a. 푑[푝푟표푑푢푐푡 푎] 푑[푝푟표푑푢푐푡 푎] By knowing the value of ( ) /( ) , the rate constant ka, [B] and [A], then 푑푡 푟푒푓 푑푡 퐵 65 the rate constant kb can be derived.

CHAPTER 3. RESULTS AND DISCUSSION

3.4.2 The reactivity of carbon black towards the hydroxyl radical

4- 2 mM Fe(CN)6 was used as a reference solute in a carbon black (Vulcan XC-72, Cabot Carbon Corporation. USA) containing solution to investigate the kinetics of the reaction between HO• 4- • 3- 132 and carbon black. Fe(CN)6 can be rapidly oxidized by HO to Fe(CN)6 via reaction (25). 3- The concentration of Fe(CN)6 was measured by absorption of light at the wavelength of 420 - • nm. e aq was converted to HO by N2O in the system via reaction (2). The carbon black concentration was set to 0, 0.4, 1.6 and 4.0 g/L, and the results are shown in Figure 21. The 3- figure shows that the changes in concentration of Fe(CN)6 with irradiation time is reduced when the concentration of carbon black is increased. This effect is significant at high carbon black concentration, as expected from competition kinetics:

4- • 3- - 10 -1 -1 Fe(CN)6 + HO → Fe(CN)6 + OH k1 = 1.0 × 10 M s (25)

• Carbon Black + HO → Product(s) k2 (26)

0.5 0.0 g/L 0.4 0.4 g/L 1.6 g/L 4.0 g/L 0.3

0.2 concentration (mM) concentration

3- 0.1 6

0.0 Fe(CN) 0.0 0.5 1.0 1.5 2.0 Irradiation time (h) 3- Figure 21. The concentration of Fe(CN)6 as a function of irradiation time in solutions initially containing 2 mM K4Fe(CN)6 and 0, 0.40, 1.60, 4.00 g/L carbon black with N2O purging.

The [Carbon black] is expressed as the solid surface area to solution volume (SA/V, m2/m3). The results are shown in Table 6. By using competition kinetics, the ratio (Ratio) between the 3- Fe(CN)6 formation rates in reference case and in systems containing carbon black was derived. The Ratio as a function of SA/V was plotted in Figure 22. The fitted line has a slope of (1.0 ± -6 -1 0.2) ×10 m. Thus, we can determine k2 to be 20 ± 5 m s considering that k1 is known as (1.0

CHAPTER 3. RESULTS AND DISCUSSION

10 -1 -1 132 4- -3 × 10 M s ) and [Fe(CN)6 ] (2 × 10 M). Thus, carbon black can potentially compete with metal ions to consume hydroxyl radicals.

3- 3- Table 6. Rates of Fe(CN)6 production, ratios between rates of Fe(CN)6 production in the presence and absence of carbon black and carbon black surface area to aqueous solution volume ratio (SA/V) for irradiations performed under continuous N2O purging.

Sample Slope (M s-1) Ratio SA/V (m2/m3)

0.00 g/L 6.25 × 10-8 1.00 0 0.40 g/L 4.60 × 10-8 1.36 8.60 × 104 1.60 g/L 3.80 × 10-8 1.64 3.44 × 105 4.00 g/L 3.15 × 10-8 1.98 8.60 × 105

3.0

2.5

2.0

1.5 Ratio 1.0

-6 0.5 y = (1.01 ± 0.25) ´ 10 x + (1.17 ± 0.12) R2 = 0.888 0.0 0 200000 400000 600000 800000 2 3 SA/V (m /m ) 3- Figure 22. Ratios between Fe(CN)6 production rates in the presence and absence of carbon black as a function of SA/V.

3.4.3 The reactivity of carbon black towards the 2-hydroxy-2-propyl radical

• 3- To investigate the reaction between carbon black and (CH3)2 COH, 5 mM Fe(CN)6 was used as a reference solute in 2 M 2-propanol and carbon black containing solution with N2O purging - • • during irradiation. In this case, both e aq and HO were converted to (CH3)2 COH in equimolar • 3- 4- quantities. The formed (CH3)2 COH can reduce Fe(CN)6 to Fe(CN)6 with a rate constant of k3 = 4.7 × 109 M-1 s-1 via the following reaction:133

CHAPTER 3. RESULTS AND DISCUSSION

• 3- 4- + 9 -1 -1 (CH3)2 COH + Fe(CN)6 → CH3COCH3 + Fe(CN)6 + H k3 = 4.7 × 10 M s (27)

• The reaction between carbon black and (CH3)2 COH follows the reaction:

• (CH3)2 COH + Carbon Black → Product(s) k4 (28)

The results are shown in Figure 23, and the calculated data are shown in Table 7.

0.6

0.5

0.4

0.3

0.2

concentration (mM) 0.0 g/L 3- 6 0.4 g/L 0.1 1.6 g/L 4.0 g/L

Fe(CN) 0.0 0.00.51.01.52.0 Irradiation time (h) 3- Figure 23. The Concentration of Fe(CN)6 as a function of irradiation time. The solutions initially contain 0.5 mM K3Fe(CN)6, 2 M 2-propanol and 0, 0.4, 1.6 and 4.0 g/L carbon black irradiated under continuous N2O purging.

3- 3- Table 7. Rates of Fe(CN)6 consumption, ratios between rates of Fe(CN)6 consumption in the presence and absence of carbon black and carbon black surface area to aqueous solution volume ratio (SA/V) for irradiations performed under continuous N2O purging.

Sample Slope (M s-1) Ratio’ SA/V (m-1)

0.00 g/L -6.56 × 10-8 1 0 0.40 g/L -6.07 × 10-8 1.08 8.60 × 104 1.60 g/L -5.59 × 10-8 1.17 3.44 × 105 4.00 g/L -4.99 × 10-8 1.31 8.60 × 105

3- Using competition kinetics, the ratio (Ratio’) between the Fe(CN)6 consumption rates in reference case and in systems containing carbon black was derived. The Ratio’ as a function of -7 SA/V was plotted in Figure 24. The fitted slope is (3.4 ± 0.5) × 10 m. Thus, we can derive k4

CHAPTER 3. RESULTS AND DISCUSSION

-1 9 -1 -1 133 3- - as 0.8 ± 0.1 m s considering that k3 is known as (4.7 × 10 M s ) and [Fe(CN)6 ] (5.3 × 10 4 M). Thus, carbon black shows lower reactivity towards the 2-hydroxy-2-propyl radical than towards the hydroxyl radical.

2.0 1.8 1.6 1.4 1.2 1.0

Ratio' 0.8 0.6 y = (3.37 ± 0.47) ´ 10-7 x + (1.03 ± 0.02) 0.4 R2 = 0.962 0.2 0.0 0 200000400000600000800000 -1 SA/V (m ) 3- Figure 24. Ratio’ between rates of Fe(CN)6 consumption in the presence and absence of carbon black as a function of SA/V.

3.4.4 The reactivity of carbon black towards the hydrated electron

- 3- To study the reaction between carbon black and e aq, experiments were performed in 0.5 mM Fe(CN)6

- and 2 M 2-propanol solution with continuous N2 purging during irradiation. In this case, e aq and

• 3- 9 9 -1 -1 (CH3)2 COH have the same G-value and can reduce Fe(CN)6 rapidly (3.0 × 10 and 4.7 × 10 M s , respectively).133,134 The results are shown in Figure 25, and the corresponding data are summarized in Table 8. The impact of increasing the carbon black concentration is smaller than the case where 2- propanol was the only reductant. The only conclusion that we can draw from this is that the reactivity of

- • e aq towards carbon black is negligible compared to the reactivity of (CH3)2 COH.

CHAPTER 3. RESULTS AND DISCUSSION

0.6

0.5

0.4

0.3

0.2

concentration (mM) 0.0 g/L 3- 6 0.4 g/L 0.1 1.6 g/L 4.0 g/L

Fe(CN) 0.0 0.00.51.01.52.0 Irradiation time (h) 3- Figure 25. Concentration of Fe(CN)6 as a function of irradiation time in solutions initially containing 0.5 mM K3Fe(CN)6, 2 M 2-propanol and 0, 0.4, 1.6 and 4.0 g/L carbon black with continuous N2 purging during irradiation.

3- 3- Table 8. Rates of Fe(CN)6 consumption, ratios between rates of Fe(CN)6 consumption in the presence and absence of carbon black and carbon black surface area to aqueous solution volume ratio (SA/V) for irradiations performed under continuous N2 purging.

Sample Slope (M s-1) Ratio’’ SA/V (m-1)

0.00 g/L -6.59 × 10-8 1 0

0.40 g/L -6.44 × 10-8 1.02 8.60 × 104

1.60 g/L -6.06 × 10-8 1.09 3.44 × 105

4.00 g/L -5.91 × 10-8 1.12 8.60 × 105

3.4.5 The reaction between carbon black and H2O2 in an unirradiated system

Before investigating the impact of carbon black on the radiolytic formation of H2O2, one needs to understand how carbon black influences the H2O2 decomposition in a non-irradiated aqueous solution. Therefore, a set of experiments were performed in aqueous carbon black suspensions with 0.04 mM H2O2 under the non-irradiated condition. The kinetics of solute consumption on a solid surface is usually pseudo first order when the solid surface area to solution volume is excessive. Therefore, we plotted ln([H2O2]) vs. reaction time in Figure 26. The results indicate

CHAPTER 3. RESULTS AND DISCUSSION

that H2O2 is stable in the absence of carbon. However, the consumption rate of H2O2 increases with increasing carbon black concentration. The pseudo first order rate constants are plotted vs. the carbon black SA/V in Figure 27. The derived second order rate constant is (6.7 ± 0.4) × 10- 10 m s-1.

-10

-5 -1

-11 8.95 × 10 s

)

(M) ]

2 -12

O -4 -1

2 2.88 × 10 s

5.90 × 10-4 s-1

H ( [ -13

ln 0.0 g/L 0.4 g/L -14 1.6 g/L 4.0 g/L -15 012345 Time (h)

Figure 26. ln([H2O2]) as a function of time in the presence of different concentrations of carbon black 0, 0.40, 1.60 and 4.00 g/L. Points represent experimental data, while the dashed line represents the linear fits to the data sets. The corresponding pseudo first order rate constants are shown near the linear fits.

0.0006

) 0.0005 -1 0.0004

0.0003

0.0002 y = (6.72 ± 0.45) ´ 10-10 x Rate constant (s 0.0001 + (2.50 ± 2.11) ´ 10-5 0.0000 R2 = 0.991

0 200000 400000 600000 800000 SA/V (m-1)

Figure 27. Pseudo first order rate constant for H2O2 consumption as a function of carbon black SA/V.

CHAPTER 3. RESULTS AND DISCUSSION

It has previously been demonstrated that active carbon (AC) could catalytically decompose H2O2 into HO• on its surface through a Fenton-like reaction:135–137

+ - • AC + H2O2 → AC + HO + HO (29)

+ + • AC + H2O2 → AC + H + HO2 (30)

To understand the H2O2 consumption mechanism in the carbon black containing system, an experiment was performed in an aqueous suspension containing 4 mM H2O2 and 4.0 g/L carbon black. The results shown in Figure 28 reveal a trend with initially rapid H2O2 consumption followed by slower consumption. After refilling H2O2 to ~ 4 mM before all the H2O2 has been consumed, the H2O2 consumption rate remains as same as before the refill. However, if the refill takes place after all H2O2 in the previous experiment has been consumed (second refill in Figure

28), the rate of H2O2 consumption is the same as the initial rate in the original experiment. This indicates a fast initial adsorption step that follows first order kinetics is followed by decomposition of adsorbed H2O2 on the surface. Since the surface is saturated with H2O2 after the initial adsorption step, we approach zeroth order kinetics of H2O2 decomposition.

5 Refill H2O2

-8 -1 Refill H2O2 4 6.28 ´ 10 M s 1.28 ´ 10-7 M s-1 1.10 ´ 10-7 M s-1 3 6.28 ´ 10-8 M s-1 5.97 ´ 10-8 M s-1

2 concentration (mM)

2 1

2 H 0

0 20406080 100 120 Time (h)

Figure 28. H2O2 concentration as a function of time. The initial H2O2 concentration is 4 mM, and the suspension contains 4.0 g/L carbon black. The suspension is continuously purged with N2.

The H2O2 consumption rate was calculated based on the change of H2O2 concentration, ∆C (in mole)/reaction time, ∆t (in second) in a specific reaction period and shown nearby the red dash lines.

CHAPTER 3. RESULTS AND DISCUSSION

An experiment of H2O2 consumption was performed in a 16 mM H2O2 and 4.0 g/L carbon black suspension. In addition, the solution contained 100 mM tris (hydroxymethyl) aminomethane to • scavenge HO that could be formed during H2O2 decomposition. Formaldehyde (CH2O) is • produced from the reaction between Tris and HO . The concentration of CH2O was measured by the Hantzsch method.92 The results are presented in Figure 29. The results indicate that HO• is formed simultaneously with the decomposition of H2O2. The plot also reveals a fast initial consumption of H2O2 followed by a considerably slower process that almost follows zeroth order kinetics. This is consistent with the previous experimental results where the initial H2O2 concentration was 4 mM, which showed in Figure 28. Hence, the carbon black used in this work • can catalytically decompose H2O2 via a mechanism that involves the formation of HO .

16 0.18

14 0.16 0.14 12 0.12 10 0.10 8 0.08 6

0.06

concentration (mM) 2

4 O concentration (mM) 2

O 0.04

2 H 2 0.02 CH 0 0.00 0 20406080100120140 Time (h)

Figure 29. H2O2 concentration as a function of reaction time (black squares) and CH2O concentration as a function of irradiation time (red dots). The initial H2O2 concentration is 16 mM, the tris concentration is 100 mM, and the suspension contains 4.0 g/L carbon black. The suspension is continuously purged with N2.

3.4.6 Impact of carbon black on the radiolytic formation of H2O2

A set of experiments were performed in de-oxygenated aqueous solutions containing 0, 0.40,

1.60 and 4.00 g/L carbon black, and the H2O2 concentration was measured as a function of irradiation time. The results are shown in Figure 30. It was found that the H2O2 formation rate is strongly dependent on the concentration of carbon black, which is opposite to the previous

CHAPTER 3. RESULTS AND DISCUSSION

stability studies of H2O2 in carbon black suspensions discussed in section 3.4.5 where carbon • black can decompose H2O2. This could be explained if we consider the reaction between HO and carbon black. Carbon black is an efficient scavenger of HO•. The carbon black appears to • catalytically decompose H2O2 to form surface-bound HO , similar to what was observed on surfaces of metals and metal oxides in the literature.138–141 Furthermore, since the surface-bound • HO can be produced not only from the decomposition of H2O2 but also from the reaction with free HO•, it is possible that the carbon black could also catalyze the recombination of HO• to produce H2O2 and thus, effectively increase the yield of H2O2 compared to the condition with absence of carbon black.

0.06 0.0 g/L 0.05 0.4 g/L 1.6 g/L 0.04 4.0 g/L

0.03

0.02

Concentration (mM) 2

O 0.01

2 H

0.00

012345 Irradiation time (h)

Figure 30. H2O2 concentration in an aqueous solution containing 0, 0.40, 1.60 and 4.00 g/L carbon black as a function of irradiation time.

3.4.7 Discussion about the potential impact of carbon black on γ-radiation induced synthesis of metal and metal oxide

To assess the impact of carbon black on the γ-radiation synthesis of metal and metal oxide nanoparticles, we must look at the competition kinetics under relevant conditions. In a recent work, Soroka et al. synthesized a Co-based nanoscale carbon black supported electrocatalyst by γ-radiation synthesis.142 The mass loading of Co was 3.4 wt.% which corresponds to less than 50 % of the theoretical loading. The experimental aqueous solution containing 0.1 M Co2+, 12 g L-1 carbon black, and 3 M 2-propanol were irradiated by a dose of

CHAPTER 3. RESULTS AND DISCUSSION

67 kGy. The accumulated amount of one-electron reductants produced via radiolysis can then be calculated to 6.7 × 104 (J kg-1) × 1 kg L-1 × 5.6 × 10-7 mol J-1 = 0.038 M. To produce metallic Co, two one-electron reductants are needed per Co2+. Consequently, there should be 1.1 g Co L-1 produced by the irradiation. Since the carbon black concentration is 12 g L-1, the mass loading of Co can be calculated as 1.1/(1.1+12) × 100 wt.% = 8.4 wt.%. To rationalize this discrepancy, one needs to consider the competing reactions involving • • 2+ 6 -1 (CH3)2 COH. The rate constant of the reaction between (CH3)2 COH and Co is < 1 × 10 M -1 143 • s and the rate constant for the reaction between (CH3)2 COH and carbon black is 0.8 ± 0.1 m s-1. The rates for the two reaction pathways are given by: 2+ 6 2+ • -1 r(Co ) < 1 × 10 × [Co ] × [(CH3)2 COH] M s • -1 r(Carbon black) = 0.8 × SA/V(Carbon black) × [(CH3)2 COH] M s • Since both expressions contain [(CH3)2 COH], one can compare the rate constants multiplied with the respective reactant concentration. Thereby, the reaction with carbon black is at least 21 2+ • times faster than the reaction with Co . Therefore, (CH3)2 COH does not contribute to the - formation of metallic Co in this system. As shown above, the reaction between e aq and carbon - black can be neglected. Hence, the formation of Co is driven solely by e aq, which explains why the reported Co loading corresponds to less than 50 % of the theoretical loading. To control the synthesis of metal and metal oxide in carbon black suspension, one needs to keep in mind the competition reactions between cations and carbon black towards radicals. For -1 • example, with 12 g L carbon black and 0.1 M of the cationic precursor, (CH3)2 COH can only contribute to metal/metal oxide formation if the rate constant for reduction of the cations by • 8 -1 -1 (CH3)2 COH is in the order of 2 × 10 M s (corresponding to a rate 10 times higher than the reaction with carbon black) or higher. On the other hand, for the synthesis of metal oxide according to the oxidative synthesis route, the rate constant for the oxidation of the cations by HO• must be 5 × 109 M-1 s-1 (corresponding to a rate 10 times higher than the reaction with carbon black) or higher.

CHAPTER 3. RESULTS AND DISCUSSION

3.5 The influence of PVP nanogel on γ-radiation induced synthesis

of CeO2 and Ag

The detailed synthesis of PVP nanogel has been described elsewhere.144 In short, the PVP nanogel used in the present work was synthesized from an aqueous solution containing 0.2 wt.% polyvinylpyrrolidone (PVP, Mw = 6.64 MDa) that has been irradiated by an electron beam with a dose of 80 kGy. After irradiation, the obtained PVP nanogel was dialyzed against Milli-Q water for 48 h using dialysis tubes of 100 kDa cut-off (Sigma Aldrich).

CeO2 and Ag nanoparticles were synthesized by γ-irradiation in the presence of PVP nanogel, denoted as CeO2/NG and Ag/NG. The freestanding CeO2 and Ag particles were used as references to investigate the influence of the PVP nanogel on the morphologies of the fabricated particles. The formation of CeO2 nanoparticles has been described in section 3.3. The synthesis 145 - • of Ag clusters via γ-irradiation has been reported in the literature where e aq and (CH3)2 COH were used to reduce Ag+ to Ag and Ag-nanoparticles are formed through the coalescence of Ag- atoms.

3.5.1 The synthesis of CeO2 and Ag nanoparticles in the presence of PVP nanogel

The reaction conditions of -radiation induced synthesis CeO2/NG, Ag/NG and the freestanding samples are summarized in Table 9. It should be noted that when Ce3+ salt was added to the nanogel solution, the pH of the nanogel solution decreased to around 1.2. To ensure that the pH 3+ is the same as in the synthesis of freestanding CeO2, the pH of Ce -PVP nanogel solution was adjusted by NaOH to 5 before the irradiation. It was observed that a light-yellow floc phase was formed when metal cations were mixed into the nanogel solution. This indicates there are strong interactions between the inorganic precursors and the nanogel.

CHAPTER 3. RESULTS AND DISCUSSION

Table 9. γ-radiation induced synthesis conditions: precursor, solvent, radical scavenger and the pH before and after irradiation. The dose was 8.9 kGy. pH Radical Sample Precursor Solvent before/after scavenger irradiation 0.2 wt.% gel ~ 2.5 × 10-2 M CeO2/NG CeCl3, 4.28 mM 5/3 solution N2O 0.2 wt.% gel Ag/NG AgNO , 4.88 mM 2 M 2-propanol 5/4.5 3 solution Freestanding ~ 2.5 × 10-2 M CeCl3, 5.06 mM Milli-Q water 4.4/2.7 CeO2 N2O Freestanding AgNO , 5.00 mM Milli-Q water 2 M 2-propanol 4.7/4.0 Ag 3

To quantify the interaction between metal cations and nanogel, the concentrations of Ce and Ag species in the homogenized systems and in the clear solution that after removing the precipitate via centrifugation was measured by ICP-OES. The same analysis was performed on the irradiated solution as well as the two freestanding cases. The results are shown in Table 10. As the results indicate, the Ce3+ ions mainly attached to the nanogel before irradiation, whereas half of the Ag+ ions formed the floc phase prior to irradiation. In addition, the presence of nanogel has decreased the efficiency in reducing Ag+. Since the nanogel was synthesized under oxidizing conditions, it is possible that the nanogel could react with reducing radicals and thereby may compete with Ag+ to consume reducing radicals. For Ce, it appears that the presence of nanogel has increased the efficiency in oxidizing Ce3+. However, given that fact that 88% Ce3+ was attached to nanogel before irradiation, the major oxidation of Ce3+ was taken place in the nanogel rather than in solution. The Ce3+ is probably bound to the functional groups of nanogel, and when 3+ it is converted to CeO2 the functional groups are liberated and more Ce could be taken by the nanogel from solution. The accumulation of Ce3+ in the nanogel results in a high concentration locally and as the lifetime of HO• in general is very short, only radicals produced in the vicinity of nanogels will oxidize Ce3+. Therefore, the low Ce-concentration in solution (after irradiation) cannot be automatically interpreted as high yield of Ce3+ conversion (see further discussion below).

CHAPTER 3. RESULTS AND DISCUSSION

Table 10. The concentration of Ce and Ag species in the systems with/without containing nanogel. Homogenized Solution after Solution after Sample system before centrifugation before irradiation and irradiation irradiation centrifugation

CeO2/NG 4.3 mM 0.5 mM 0.1 mM Ag/NG 4.9 mM 2.6 mM 2.3 mM Freestanding 5.1 mM 3.5 mM CeO2 Freestanding 5.0 mM 0 mM Ag

The samples were characterized by XRD, and the obtained patterns are shown in Figure 31. In addition, it should be noted that the pure dialyzed nanogel and initial floc phase (in the two cases) possess an amorphous phase. XRD patterns confirm that the freestanding CeO2 and the CeO2/NG 120,146,147 have the fluorite CaF2 structure of CeO2. The lattice constants calculated for both types of particles have similar values and equal to a = 5.46 Å, that is larger than the corresponding 122 bulk value for CeO2, a = 5.41 Å. Both silver samples show the face-centered cubic (fcc) Ag phase.148 The lattice parameters for both types of Ag particles are found to be the same as a = 4.10 Å. This value is slightly higher than the lattice constant for a bulk Ag, a = 4.09 Å.149 In 150 addition, there is some amount of Ag2O covered on freestanding Ag, while no traces of crystalline Ag2O can be observed in Ag/NG.

(a) (b) O 2 O

O (111) O 2 2

(200) (220) (111) Ag

(200) Ag (311)

(200) Ag (222) (220) Ag (111) Ag (110) Ag

(211) Ag (400)

Freestanding CeO2 Freestanding Ag Intensity (a. u.) Intensity (a. u.) Ag/NG

CeO2/NG 203040506070 203040506070 2Theta (degrees) 2Theta (degrees) Figure 31. XRD patterns of freestanding samples (red patterns) and the nanogel supported samples (black patterns). Freestanding CeO2 and CeO2/NG (a); Freestanding Ag and Ag/NG

150 (b). Miller indices correspond to CeO2 (JCPDS 34-0394) (a), and Ag (b).

CHAPTER 3. RESULTS AND DISCUSSION

3.5.2 The morphological change of CeO2 and Ag nanoparticles caused by PVP nanogel

To study the morphology of the obtained samples, TEM was performed on nanogel supported samples. The freestanding Ag was characterized by SEM. The results are shown in Figure 32.

The TEM images of freestanding CeO2 are shown as previous, see Figure 18 (c) and (f). An amorphous phase (nanogel structure) can be observed in both CeO2/NG and Ag/NG samples, where the particles exist. Furthermore, the CeO2 primary particles (as described in section 3.3) and Ag nanoparticles are well dispersed relatively evenly within the nanogel structure.

Figure 32. TEM images of the obtained samples. CeO2/NG (a) and (d), Ag/NG (b) and (e). SEM images of freestanding Ag (c) and (f).

The lognormal particle size distributions of CeO2/NG and Ag/NG are presented in Figures 33 (a) and (b), respectively. The average particle sizes of the four obtained samples are summarized in Table 11. The average size of the CeO2 primary particles in CeO2/NG sample is slightly smaller than that of the freestanding CeO2 sample. The Ag particles transformed from micro- size to 5 nm size when Ag was synthesized in the presence of nanogel.

CHAPTER 3. RESULTS AND DISCUSSION

80 80 (a) (b) 70 70

60 60

50 50

40 40

Count Count 30 30

20 20

10 10

0 0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Particle size (nm) Particle size (nm)

Figure 33. Lognormal particle size distributions of CeO2 (a) and Ag (b) particles in PVP nanogel supported samples.

Table 11. The average particle size of freestanding samples and CeO2/NG and Ag/NG. Size data collected from the TEM (CeO2/NG, Ag/NG, and freestanding CeO2) and SEM (freestanding Ag) images.

Sample Average particle size (nm)

CeO2/NG 2.7 ± 0.4

Ag/NG 5.5 ± 1.8

Freestanding CeO2 3.2 ± 0.5

Freestanding Ag 150-500

In addition, TEM images were also used to estimate the particle density in the nanogel, see

Figure 34. The nanoparticle density for CeO2 and Ag are estimated to 0.015 and 0.0094 particles 2 per nm , respectively. Considering that the Ag particles are larger than the CeO2 particles and 2 that Ag has a higher density than CeO2, the amount of Ag per nm of nanogel is still higher than the amount of CeO2. This implies that the CeO2 yield is much lower than indicated by the measured changes in Ce-concentration in solution (as indicated above).

CHAPTER 3. RESULTS AND DISCUSSION

2 Figure 34. Particle counts of per nm for the CeO2/NG (a) and Ag/NG (b) sample. The blue squares represent the counting area. The blue dots in the small insert image represent particles.

2 2 There are 0.015 CeO2 particles per nm and 0.0094 Ag particles per nm .

The size decrease and good dispersion of the fabricated nanoparticles within nanogel could be explained as the precursor cations were adsorbed in the PVP nanogel (forming the initial floc phase), and the motion of the cations was relatively limited compared with freestanding samples. Thus, the PVP nanogel can provide excellent size-control of the inorganic inclusions.

3.6 Application of γ-radiation induced synthesis for electrocatalysts fabrication

3.6.1 γ-radiation induced synthesis of MnOx/Carbon black electrocatalysts

MnOx/Carbon black electrocatalysts were fabricated by -radiation induced synthesis. The electrocatalysts were produced via the oxidative and the reductive pathways (denoted as MnOx-

Oxi/C and MnOx-Red/C, respectively). The synthesis parameters are summarized in Table 12.

CHAPTER 3. RESULTS AND DISCUSSION

Table 12. γ-radiation induced synthesis conditions: precursor, carbon black concentration

(Ccarbon) and radical scavenger, dose, mass loading of Mn on carbon black.

Sample Precursor Ccarbon Scavenger Dose Mn wt.% on carbon

18 mM -2 MnOx-Oxi/C 2.2 g/L ~2.5 ´10 M N2O 44.6 kGy 21 wt.% MnCl2

15.4 mM MnOx-Red/C 2.2 g/L 1 M 2-propanol 44.6 kGy 5.4 wt.% KMnO4

15.4 mM * MnOx-Red/C 0.6 g/L 1 M 2-propanol 44.6 kGy 21 wt.% KMnO4

The elemental composition of the carbon supported manganese oxide was analyzed by XPS. Mn 2p and 3s spectra are shown in Figure 35, and the corresponding binding energies are summarized in Table 13. Three binding energies for Mn 2p3/2 are distinguished in both samples corresponding to different valence states of manganese: Mn2+, 151–153 Mn3+ 152 Mn4+.152,154,155 To estimate the average Mn valence state in MnOx-Oxi/C and MnOx-Red/C, we considered the binding energies of Mn 3s core-shell. The 3s line for the 3d transition metals has a splitting that depends on the number of 3d electrons.156 Thus, one can determine the valence state of Mn ions considering the splitting magnitude of the Mn 3s line. Figure 35 (b) shows that the splitting of 3.6+ 157 Mn 3s lines is 4.7 eV for MnOx-Oxi/C and MnOx-Red/C, which corresponds to Mn . Thus, both of the supported manganese oxides may have a mixed-valence state. 3+ 4+ As shown in Table 13, the Mn /Mn ratio of MnOx-Oxi/C and MnOx-Red/C samples is 2.01 and 1.56, respectively, as calculated from XPS spectra. The oxygen reduction reaction (ORR) activity of different types of manganese oxide is dependent on the amount of Mn3+ on the surface due to Jahn-Teller distortion effects158 and the ease of oxidation to Mn4+, thus, the ratio of Mn3+/Mn4+ may be considered as an essential parameter for the evaluation of the catalytic efficiency of manganese compounds.

CHAPTER 3. RESULTS AND DISCUSSION

(a) Mn 2p (b) 4.7 eV

MnOx-Oxi/C 2p3/2 4.7 eV MnO -Oxi/C 2p1/2 x

Intensity (a. u.) (a. Intensity Intensity (a. u.) (a. Intensity

MnOx-Red/C MnOx-Red/C 660 657 654 651 648 645 642 639 96 93 90 87 84 81 78 Binding Energy (eV) Binding Energy (eV) Figure 35. Mn 2p (a) and Mn 3s (b) XPS spectra of carbon supported manganese oxide particles synthesized by both radiolytic oxidation (upper curves) and radiolytic reduction (lower curves) routes.

Table 13. Results of XPS studies of carbon supported manganese oxides synthesized by both radiolytic oxidation and radiolytic reduction.

MnOx-Oxi/C MnOx-Red/C

Binding Energy Atomic ratio Binding Energy Atomic ratio Compound (eV) (at.%) (eV) (at.%)

C 1s (C-C) 284.8 81.45 284.3 92.53

O 1s (Mn-O) 529.1 6.9 529.8 2.3

3+ Mn 2p3/2 (Mn ) 641.9 2.98 642.1 0.78

2+ Mn 2p3/2 (Mn ) 640.5 1.89 640.8 0.14

4+ Mn 2p3/2 (Mn ) 643.5 1.48 643.6 0.49

Mn 2p1/2 652.7 652.0

XRD was performed to further elucidate the composition of the synthesized material, and the results are presented in Figure 36. The pattern of MnOx-Oxi/C indicates that -MnO2 is the only crystalline phase formed upon radiolytic oxidation. Meanwhile, the XPS measurements reveal that Mn2+ and Mn3+ are also present in the samples. Thus, we assume the obtained particles 2+ 3+ consist of -MnO2 as a bulk crystalline phase and contain amorphous phases of Mn and Mn

CHAPTER 3. RESULTS AND DISCUSSION

formed on the surface. For MnOx-Red/C, the tiny sharp peaks on the XRD pattern can be matched to Mn3O4. The low intensity of the peaks is probably due to the low Mn wt.% in the

MnOx-Red/C (5.7 wt.%) sample. Therefore, the intensity of diffraction peaks is dampened by the signal from carbon black.

(131) (300) (160) (421)

MnOx-Oxi/C (21 wt.%) Intensity (a. u.) (004) (103) (101)

MnOx-Red/C (5.7 wt.%) 203040506070 2Theta (degrees) Figure 36. X-ray diffraction patterns of carbon black supported manganese oxide synthesized by radiolytic oxidation (red line) and radiolytic reduction (black line). The Miller indices correspond to γ-MnO2 (JCPDS 14–0644) for MnOx-Oxi/C and Mn3O4 (JPCDS 24–0734) for

MnOx-Red/C.

3.6.2 Electrochemical properties of MnOx-Oxi/C and MnOx-Red/C

The oxygen reduction reaction (ORR) measurements were implemented with a rotating disk electrode (RDE) setup in 0.1 M KOH. Cyclic voltammetry (CV) of MnOx-Oxi/C, MnOx-Red/C, and commercial Pt/C (20 wt.%, ETEK) were obtained at a scan rate of 50 mV s−1 (Figure 37

(a)). The CV curves of MnOx-Red/C and MnOx-Oxi/C samples showed reduction peaks at potentials around 0.7 V and oxidation peaks around 1.05 V, consistent with the result in the literature.163 Pt/C showed the standard CV curve with a shape characteristic of polycrystalline Pt in an alkaline medium.164 Table 14 summarizes the key parameters for ORR performance obtained from the polarization curves (Figure 37 (b)). As seen in the table, the diffusion limiting -2 current density (Jd) of MnOx-Oxi/C is close to the value of Pt/C, 5.0 mA cm , and is much higher -2 than that of MnOx-Red/C (2.8 mA cm ). Both of the onset potentials (Eonset) and half-wave

CHAPTER 3. RESULTS AND DISCUSSION

potential (E1/2) of the synthesized carbon black supported manganese oxide catalysts were lower than those of Pt/C. On the other hand, the potentials determined for MnOx-Oxi/C were much more positive than those for MnOx-Red/C, which indicates the MnOx-Oxi/C possesses a higher ORR activity at low overpotentials. To eliminate the effect of carbon black support on * electrochemical activity of samples, the ORR activity of MnOx-Red/C (21 wt. % of Mn) was compared to the other samples. The results are shown in Figure 37 (b). Although, the overall

ORR activity of MnOx-red/C catalysts only slightly increases when the concentration of Mn in the sample is increased by 3.7 times (from 5.7 wt. % to 21 wt. %). Still, this ORR activity remains significantly lower than that of the MnOx-Oxi/C sample.

(b) (a)0.4 0 @1600 rpm Pt/C (20 wt.%)

0.3 -1

)

-2 )

-2 -2 0.2 MnOx-Red/C (5.7 wt.%) MnOx-Oxi/C (21 wt.%) -3

mA cm 0.1 (mA cm

J MnOx-Red/C (21 wt.%)

J ( MnOx-Red/C (5.7 wt.%) -4 MnO -Oxi/C (21 wt.%) 0.0 x Jd Oxidation -5 Reduction Pt/C (20 wt.%Pt) E1/2 Eoneset -0.1 0.00.20.40.60.81.01.2 0.20.40.60.81.0 E (V vs RHE) E (V vs RHE)

Figure 37. Cyclic Voltammetry curves recorded for MnOx-Oxi/C, MnOx-Red/C, and commercial

-1 Pt/C (20 wt.%) catalysts in N2 saturated 0.1 M KOH solution with a scan rate at 50 mV s (a). Polarization curves of the samples were measured at a rotation speed of 1600 rpm (d). All the polarization curves were recorded at the scan rate of 10 mV s-1. The key factors for ORR performance, Jd, E1/2 and Eoneset are marked with MnOx-Oxi/C for instances.

CHAPTER 3. RESULTS AND DISCUSSION

Table 14. Diffusion limiting current density (Jd), onset potential (Eonset), half-wave potential (E1/2), and specific activity @ 0.7 V of the three samples. These ORR performance parameters obtained from polarization curves are shown in Figure 36 (b).

Sample Jd Eoneset E1/2

MnOx-Oxi/C 4.8 mA cm-2 0.78 V 0.71 V

MnOx-Red/C 2.8 mA cm-2 0.76 V 0.66 V

MnOx-Red/C* 3.1 mA cm-2 0.75 V 0.65 V

Pt/C 5 mA cm-2 0.94 V 0.82 V

High ORR activity of the MnOx-Oxi/C can be attributed to its high electrochemical surface area originating from the nano-flakes composed spherical nanostructure, as shown in Figure 14 (b). 3+ 4+ Moreover, the ratio of Mn /Mn in the MnOx-Oxi/C sample is higher than that of the MnOx- Red/C sample (as confirmed by the XPS analysis), which may contribute to a higher electrocatalytic activity. Thus, the -radiation induced synthesis method can be applied as a structural and composition adjustable approach to fabricate manganese oxide electrocatalysts.

CHAPTER 4. CONCLUDING REMARKS

4 Concluding Remarks

The work in this thesis is focused on using the γ-irradiation as a versatile tool to synthesize metal oxides. Cuprous oxide, manganese oxide and cerium oxide are used as the synthesis subjects. To precisely tune the size, composition and morphologies of the produced metal oxide nanomaterials, the effects of different reaction conditions on metal oxides engineering were investigated.

In summary, the main achievements and findings in this thesis include:

1. The solution pH plays a critical role in metal oxide formation via γ-radiation induced synthesis. It determines the thermodynamically stable metal oxide products formed via

the redox reaction between metal cations and radicals. In producing Cu2O, the reaction

of yielding Cu2O and the reaction of yielding Cu show pH-dependent competition. At

pH higher than 3.85, the reactions of yielding Cu2O are favored. Therefore, more Cu2O nucleus can be generated during the irradiation, resulting in a smaller size and narrower

size distribution of fabricated Cu2O octahedron particles at high pH. In addition, a high concentration of isopropanol (> 1 M) can lead to a solvent effect, which favors the

reactions of yielding Cu2O. 2+ - 2. Manganese oxides can be produced with Mn and MnO4 as precursors via radiation- induced oxidation and reduction, respectively. The samples prepared via the oxidation route have a hollow spherical shape composed with nano-flakes, which structure

corresponds to γ-MnO2. On the other hand, the samples produced by the reduction route

have a rod-like morphology and have Mn3O4 as the main crystalline phase.

3. CeO2 mesocrystals are produced with different doses. The CeO2 nanoparticles were captured at different growth stages and characterized by TEM. It is observed that at the

initial stage (dose of 0.04 kGy), CeO2 primary particles arise in a network structure. As the dose increasing, the primary particles start to aggregate and align to form the

corresponding mesocrystals. As the dose increased to 10.6 kGy, the average size of CeO2

CHAPTER 4. CONCLUDING REMARKS

mesocrystals remains unchanged at about 27 nm which the primary particles are closely compacted and fully aligned. 4. Carbon black was used as support material and its reactivities toward the hydroxyl radical, the solvated electron, and the 2-hydroxy-2-propyl radical were derived by the competition kinetics. HO• reacts with carbon black with a rate constant of 20 ± 5 m s-1, • -1 and (CH3)2C OH reacts with carbon black with a rate constant of 0.8 ± 0.1 m s . The - • reactivity of e aq towards carbon black is negligible compared to that of (CH3)2C OH.

Furthermore, the radiolytic formation of H2O2 is found to be favored by carbon black.

5. PVP nanogel is used as a support to fabricate CeO2 and Ag nanoparticles. It is found that

the average sizes of the supported CeO2 and Ag nanoparticles are smaller than that of their corresponding freestanding particles, especially for Ag. Moreover, the supported samples also achieve an excellent dispersion of the nanoparticles. These could be explained as the precursor cations were adsorbed in the PVP nanogel, and the motion of the cations is relatively limited compared with freestanding samples. Thus, the PVP nanogel can hinder the growth and aggregation of the fabricated nanoparticles. 6. The γ-radiation induced method can be applied as a structural and composition adjustable approach to fabricate manganese oxide electrocatalysts.

The reaction conditions’ impact on the composition, morphology and physico-chemical properties of the engineered metal oxides particles is studied to a large extent. Meanwhile, the presented results can provide a better understanding of the generation and growth of metal oxide particles via radiation-induced method. Thus, the optimization of reaction conditions for γ- radiation induced synthesis of metal oxides can be further improved.

CHAPTER 5. FUTURE WORK

5 Future Work

There are also challenges to be solved in using γ-radiation induced method to fabricate metal oxides. As discussed in the present thesis, the pH plays a very important role in determining the product species. However, controlling the pH by conventional buffer is difficult and sometimes impossible because the buffer agent can complex with metal cations or compete with precursor to consume radicals. Therefore, to find a proper way to control the reaction pH needs to be resolved. In addition, most the particles in the present thesis are synthesized without surfactant. To achieve a more controllable synthesis condition, the role of surfactants should be investigated. Hence, more work is required to increase our understanding of surfactant’s behavior in water radiolysis condition, for example, the reactions between surfactant and radicals.

ACKNOLEDGEMENTS

Acknowledgements

Time flies quickly, like a white horse crossing a gap. Those enthusiastic people who encouraged and helped me gave this four years journey a special meaning. Here, I would like to express my most sincere thanks to all of you. First of all, I want to thank my supervisor, Prof. Mats Jonsson. I am very grateful for your endless patience, profound knowledge and thoughtful guidance throughout my PhD. And your constructive advice will always be kept in my mind for my future work. I also would like to thank my co-supervisor, Dr. Inna Soroka. Thank you so much for your warm- hearted help not only in academia but also in daily life. Your enthusiastic and brave attitude encouraged me to pass through the toughest period of my PhD. Further, I want to thank my colleagues in my group. Annika, thank you for your patient help that made me became familiar with everything in the lab. Yi, thank you for opening the door of electrochemistry for me, you are the master. Ghada, thank you for your encouragement all the way. Junyi, thank you for your great advice on the carbon work. Sawsan, thank you for your kind accompany to Stockholm University for the measurements. More importantly, all of you bring color to my PhD life, and I am very proud of having my four years-work with you. Nadezda and Diana are greatly acknowledged for the TEM characterization. Prof. Kong, great thanks for your nice collaboration. Pavel, Sergey, Gerard and Andrey, great thanks for your kind help in the characterizations. I also would like to thank Wei Zhang for the collaboration in the course and the extensive help in my daily life. Great thanks for Lijuan. Though we are separated by thousands of mountains and rivers, you are my most solid back-up. Thank you for your endless love and unhesitating support all the way. Thank you, mum and dad, for trusting me and let me chase my dreams freely. China Scholarship Council (CSC) is gratefully acknowledged for financial support.

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