Novel strategies to develop efficient titanium dioxide and graphitic … · 2020. 8. 7. · Novel strategies to develop efficient titanium dioxide and graphitic carbon nitride-based

Novel strategies to develop efficient titanium dioxide and graphitic carbon nitride-based photocatalysts

Thèse

Chinh Chien Nguyen

Doctorat en génie chimique

Philosophiae doctor (Ph.D.)

Québec, Canada

© Chinh Chien Nguyen, 2018

Novel strategies to develop efficient titanium dioxide and graphitic carbon nitride-based photocatalysts

Thèse

Chinh Chien Nguyen

Sous la direction de :

Trong- On Do, directeur de recherche

iii

Résumé

Afin de résoudre les problèmes environnementaux et énergétiques modernes, ces

dernières années ont vu le développement de catalyseurs photocataytiques capables d’utiliser

la lumière solaire. En effet, les possibles applications des semiconducteurs présentant des

propriétés photocatalytiques dans les domaines de la production d’hydrogène ou la

dégradation de polluants organiques ont généré un grand intérêt de la part de la communauté

scientifique.

Actuellement, les photocatalyseurs à base de dioxyde de titane (TiO2) et de nitrure

de carbone graphitique (g-C3N4) sont considérés comme les matériaux les plus étudiés pour

leurs faibles coûts et leurs propriétés physico-chimiques exceptionnelles. Cependant, la

performance photocatalytique de ces matériaux reste encore limitée, à cause de la

recombinaison rapide des porteurs de charge et et d'une absorption limitée de la lumière. En

générale, malgré des caractéristiques exceptionnelles, ces matériaux ne contribuent pas

significativement à la séparation de charge et l’absorption de la lumière lorsqu’ils sont

produits par des méthodes conventionnelles. L'objectif de cette thèse est de développer de

nouvelles voies pour la production de matériaux efficaces basés sur TiO2 et g-C3N4.

Nous avons d'abord préparé de la triazine (CxNy) qui fonctionne comme un co-

catalyseur d'oxydation ce qui facilite la séparation des paires «électron-trou» dans le système

du photocatalyseur creux de type Pt-TiO2-CxNy. La présence simultanée de Pt et de CxNy,

qui servent comme co-catalyseurs de réduction et d'oxydation, respectivement, a permis une

amélioration remarquable des performances photocatalytiques du TiO2. De plus, nous avons

développé une nouvelle approche, en utilisant un procédé de combustion de sphère de

carbone assisté par l’air, pour préparer du C/Pt/TiO2 . Ce matériau possède de nombreuses

propriétés uniques qui contribuent de manière significative à augmenter la séparation

« électron-trou », et en conséquence, à améliorer la performance photocatalytique. Dans le

but de développer un matériau qui soit capable de fonctionner sous les rayons du soleil et

dans l'obscurité, nous avons développé un photocatalyseur creux à double enveloppes : le Pt-

WO3/TiO2-Au. Ce matériau a montré non seulement une forte absorption de la lumière

solaire, mais aussi une séparation des charges élevée et une haute capacité de stockage

iv

d'électrons. Par conséquent, ce type de photocatalyseurs a montré une dégradation efficace

des polluants organiques, à la fois sous la lumière visible (λ ≥ 420 nm) et dans l'obscurité.

En ce qui concerne le g-C3N4, nous avons exploité la relation entre les lacunes

d’azote et les propriétés plasmoniques des nanoparticules d’or (Au). Ce type de

photocatalyseur du Au/g-C3N4 a été préparé en présence d’alcali suivi par une post

calcination. En effet, les lacunes d’azote ainsi produites permettent le renforcement des

interactions entre l’or et le g-C3N4 et des propriétés plasmoniques de l’or. Ces caractéristiques

exceptionnelles renforcent l'utilisation efficace de l’énergie solaire ainsi que la séparation des

paires « électron-trou », ce qui contribuent à la performance photocatalytique pour la

production d'hydrogène du photocatalyseur. Afin d’améliorer la capacité d’absorption de la

lumière visible de g-C3N4, une nouvelle voie de synthèse dénommée « poly-alcaline » a été

développée. La possibilité d’ajouter du polyéthylèneimine (PEI) et de l’hydroxyde de

potassium (KOH) pour générer de nombreux centres lacunaires en azote ainsi que des

groupes hydroxyles dans la structure du matériau, a été explorée afin d’optimiser l’efficacité

du matériau. De telles modifications ont démontré leurs capacités à réduire la bande interdite

et à provoquer plus facilement la séparation de charges améliorant ainsi les propriétés

photocatalytiques du photocatalyseur vis-à-vis de la production d’hydrogène. Cette méthode

ouvre donc une nouvelle voie d’avenir pour préparer des photocatalyseurs nanocomposites

efficaces possédant à la fois, une forte d’absorption de la lumière et une bonne séparation de

charges.

v

Abstract

The utilization of solar light-driven photocatalysts has emerged as a potential

approach to deal with the serious current energy and environmental issues. Over the past

decades, semiconductor-based photocatalysis has attracted an increasing attention for diverse

applications including hydrogen production and the decomposition of organic pollutants.

Currently, titanium dioxide (TiO2) and graphitic carbon nitride (g-C3N4)-based

photocatalysts have been considered as the most investigated materials because of their low

cost, outstanding physical and chemical properties. However, their photocatalytic

performances are still moderate owing to the fast charge carrier recombination and limited

light absorption. The main target of the research presented in this thesis is to develop novel

routes to prepare efficient materials based on TiO2 and g-C3N4. These materials possess

prominent features, which contribute to address the fast charge separation and light

absorption problems.

We firstly have prepared triazine (CxNy) acting as an oxidation co-catalyst, which

efficiently facilitates electron-hole separation in a Pt-TiO2-CxNy hollow photocatalyst

system. The co-existence of Pt and CxNy functioning as the reduction and oxidation co-

catalysts, respectively, has remarkably enhanced the photocatalytic performance of TiO2.

Next, we have also developed a new approach employing the air- assisted carbon sphere

combustion process in preparing C/Pt/TiO2. This material possesses many salient properties

that significantly boost the electron-hole separation leading to enhanced photocatalytic

performance. In an attempt to design a material that can operate under sunlight and in

darkness, we have introduced Pt-WO3/TiO2-Au double shell hollow photocatalyst. The

material has shown not only strong solar light absorption but also efficient charge separation

and electron storage capacity. As a result, this type of photocatalyst exhibits a high activity

performance for the degradation of organic pollutants both under visible light (λ ≥ 420 nm)

and in the dark.

Regarding to g-C3N4, we have explored the relationship between nitrogen vacancies

and the plasmonic properties of Au nanoparticles employing alkali associated with the post-

calcination method to prepare Au/g-C3N4. In fact, the produced nitrogen vacancies in the

structure of g-C3N4 essentially enhance the interaction at Au/g-C3N4 interface and the

vi

plasmonic properties of Au nanoparticles. These outstanding features contribute to enhance

the utilization of solar light and electron-hole separation that prompt the photocatalytic

performance towards hydrogen production. Finally, we have employed a novel poly-alkali

route to prepare a strong visible light absorption photocatalyst-based g-C3N4. The co-

existence of PEI and KOH, which induces numerous nitrogen vacancies and incorporated

hydroxyl groups in the structure of the resulted material, has been explored for the first time.

These modifications have been proved to narrow the bandgap and facilitate the charge

separation leading to enhance the solar light-driven hydrogen production. This method also

opens up a new approach to prepare efficient nanocomposite photocatalysts possessing both

strong light absorption and good charge separation.

vii

Table of Content

Résumé .................................................................................................................................. iii

Abstract ................................................................................................................................. v

Table of Content ................................................................................................................. vii

List of Figures .................................................................................................................... xiii

List of Tables ....................................................................................................................... xx

List Schemes ....................................................................................................................... xxi

List of Abbreviations ........................................................................................................ xxii

Acknowledgments ............................................................................................................ xxiv

Foreword ......................................................................................................................... xxvii

Chapter 1: Introduction ....................................................................................................... 1

1.1. Using photocatalysts as the potential solution to address energy and environment

issues ................................................................................................................................... 2

1.2. Fundamental of photocatalysis based on semiconductors ........................................... 3

1.2. Current advances in semiconductor photocatalysis ..................................................... 5

1.3. Scope of the thesis ..................................................................................................... 10

1.4. Organization of the thesis .......................................................................................... 10

Chapter 2: State- of- the- art development of photocatalysts ......................................... 14

2.1. TiO2- based photocatalysts ........................................................................................ 15

2.1.1. Visible light- active TiO2 .................................................................................... 15

2.1.2. Anatase-rutile homojunction .............................................................................. 17

2.1.3. TiO2 nanocomposite ........................................................................................... 18

2.2. Graphitic carbon nitride- based photocatalysts ......................................................... 19

2.2.1. Structural engineering ........................................................................................ 20

2.2.2 g-C3N4- based heterostructure photocatalysts ..................................................... 22

2.3. Development of co-catalysts for efficient photocatalysts.......................................... 27

2.3.1 Mxene-based co-catalysts .................................................................................... 28

viii

2.3.2. Phosphorene-based co-catalysts ......................................................................... 29

2.3.3. Ni-based co-catalysts .......................................................................................... 31

2.3.4. Pt-based co-catalysts .......................................................................................... 35

2.3.5. Carbon-based co-catalysts .................................................................................. 35

2.4. Surface- Plasmon- Driven Photocatalysts ................................................................. 37

2.4.1. Fundamental of surface plasmon resonance ....................................................... 37

2.4.2. Indirect electron transfer ..................................................................................... 38

2.4.3. Direct electron transfer ....................................................................................... 39

2.4.4. Current advances of surface plasmon resonance-based photocatalysts.............. 40

2.5. Hollow structure photocatalysts ................................................................................ 43

2.5.1. Advantages of hollow structure photocatalytic materials................................... 43

2.5.2. General strategies for constructing hollow structure photocatalysts .................. 44

2.5.3. State-of-the-art in the development of hollow photocatalysts ............................ 46

2. 6. Conclusions and outlook for future development .................................................... 64

Chapter 3: Characterization Techniques ......................................................................... 66

3.1. Electron microscopy .................................................................................................. 67

3.1.1. Transmission electron microscope ..................................................................... 68

3.1.2. Scanning electron microscope ............................................................................ 69

3.2. X-ray diffraction ........................................................................................................ 71

3.3. X-ray photoelectron spectroscopy ............................................................................. 73

3.4. Nitrogen physisorption .............................................................................................. 74

3.5. Inductively coupled plasma mass spectrometry (ICP-Ms) ........................................ 77

3.6. Fourier Transform Infrared Spectroscopy ................................................................. 77

3.7. UV-Visible spectroscopy ........................................................................................... 80

3.8. Photoluminescence spectroscopy .............................................................................. 82

ix

3.9. Photoelectrochemical ................................................................................................ 84

3.10. Gas chromatography ................................................................................................ 85

Chapter 4: Role of CxNy-Triazine in Photocatalysis for Efficient Hydrogen Generation

and Organic Pollutant Degradation Under Solar Light Irradiation ............................. 88

Résumé ............................................................................................................................. 90

Abstract ............................................................................................................................. 91

4.1. Introduction ............................................................................................................... 92

4.2. Results and Discussion .............................................................................................. 93

4.3. Conclusion ............................................................................................................... 101

4.5. Experimental ............................................................................................................ 102

4.5.1. Chemicals ......................................................................................................... 102

4.5.2. Synthesis of carbon colloidal spheres@Pt ........................................................ 102

4.5.3. Synthesis of titanate nanodisks (TNDs) ........................................................... 102

4.5.4. Synthesis of carbon colloidal spheres@Pt-TNDs ............................................ 102

4.5.5. Synthesis of Pt-TiO2-CxNy................................................................................ 103

4.5.6. Preparation of Pt-TiO2 and Pt-g-C3N4 .............................................................. 103

4.5.7. Characterizations .............................................................................................. 103

4.5.8. Photocatalysis activity tests .............................................................................. 104

4.5.9 Photocatalytic efficiency (PE) calculation ........................................................ 104

4.6. Supporting information ........................................................................................... 107

Chapter 5: A Novel Route to Synthesize C/Pt/TiO2 Phase Tunable Anatase–Rutile TiO2

for Efficient Sunlight-Driven Photocatalytic Applications ........................................... 118

Résumé ........................................................................................................................... 119

Abstract ........................................................................................................................... 120

5.1. Introduction ............................................................................................................. 122

5.2. Results and discussion ............................................................................................. 123

x

5.3. Conclusions ............................................................................................................. 132

5.4. Experimental ............................................................................................................ 133

5.4.1 Chemicals .......................................................................................................... 133

5.4.2. Synthesis of carbon colloidal spheres@Pt ........................................................ 133

5.4.3. Synthesis of phase- tunable anatase–rutile C/Pt/TiO2 (A/R) ............................ 133

5.4.4 Preparation of Pt-TiO2 by conventional method ............................................... 133

5.4.5. Preparation of Pt-TiO2-P25 .............................................................................. 133

5.4.6. Characterization ................................................................................................ 134

5.4.7. Photocatalytic tests ........................................................................................... 134


Chapter 6: Efficient hollow double-shell photocatalysts for the degradation of organic

pollutants under visible light and in darkness ............................................................... 151

Résumé ........................................................................................................................... 153

Abtract ............................................................................................................................ 154

6.1. Introduction ............................................................................................................. 155


6.3. Conclusion ............................................................................................................... 166

6.4. Experimental ............................................................................................................ 167

6.4.1. Chemicals ......................................................................................................... 167

6.4.2. Synthesis of carbon colloidal spheres@Pt-WO3: ............................................. 167

6.4.3. Synthesis of titanate nanodisks (TNDs): .......................................................... 167

6.4.4. Synthesis of Carbon colloidal spheres@Pt-WO3/TNDs-AuCl4- ...................... 167

6.4.5. Hydrogen treatment .......................................................................................... 168

6.4.6. Characterization ................................................................................................ 168

6.4.7. Photocatalytic test ............................................................................................. 168

xi


Chapter 7: The role of nitrogen vacancies for the enhanced surface plasmonic

resonance of Au/g-C3N4 crumpled nanolayers as an efficient solar light-driven

photocatalyst ..................................................................................................................... 181

Résumé ........................................................................................................................... 183

Abstract ........................................................................................................................... 184

7.1. Introduction ............................................................................................................. 185


7.3. Conclusion ............................................................................................................... 197

7.4. Experimental ............................................................................................................ 197

7.4.1 Materials ............................................................................................................ 197

7.4.2 Alkali-assisted post-calcination synthesis of Au/g-C3N4 .................................. 197

7.4.3 Conventional synthesis of Au/g-C3N4 ............................................................... 198

7.4.4 Characterizations ............................................................................................... 198

7.4.5 Photocatalytic activity test ................................................................................. 198

7.4.6 Photo-electrochemical Measurements ............................................................... 199


Chapter 8: Engineering the high concentration and efficiency of N3C nitrogen vacancies

to prepare strong solar light-driven photocatalysts-based g-C3N4 .............................. 209

Résumé ........................................................................................................................... 211

Abstract ........................................................................................................................... 212

8.1. Introduction ............................................................................................................. 213


8.3. Conclusion ............................................................................................................... 222

8.4. Experimental ............................................................................................................ 223

8.4.1. Materials ........................................................................................................... 223

xii

8.4.2. Preparation of g-C3N4-PA ................................................................................ 223

8.4.3. Preparation g-C3N4, g-C3N4-P and g-C3N4-A .................................................. 223

8.4.4. Characterizations .............................................................................................. 223

8.4.5. Photocatalytic activity tests .............................................................................. 224

8.4.6. Photo-electrochemical Measurements .............................................................. 224

8.5. Suporting information ............................................................................................. 226

Chapter 9: Conclusions and future outlook ................................................................... 235

9.1. General conclusion .................................................................................................. 236

9.2. Future outlook ......................................................................................................... 237

References.......................................................................................................................... 240

List of publications ........................................................................................................... 272

xiii

List of Figures

Figure 1.1. Electron-hole generation under illumination in an inorganic photosystem; A:

electron acceptor; D: electron donor. ..................................................................................... 3

Figure 1.2. General processes in a semiconductor photocatalyst: 1) bandgap excitation; 2)

charge diffusion; 3) charge recombination; 4) chemical conversion. .................................... 4

Figure 1.3. Typical example of the current developments in photo-composite materials; A)

two separated co-catalysts; B) plasmonic photocatalysts. ...................................................... 7


two separated co-catalysts; B) plasmonic photocatalysts. ...................................................... 8

Figure 2. 1. A) HR-TEM image of black TiO2; B) UV-Vis spectra of back and white TiO2;

C) Schematic illustration of the density of state (DOS) and D) solar-driven photocatalytic

activity of disordered- engineered black TiO2 ...................................................................... 16

Figure 2. 2. The schematic illustration of the charge separation between anatase and rutile

phases ................................................................................................................................... 17

Figure 2. 3. Illustration of conventional WO3/TiO2 nanocomposite under UV illumination

and in darkness. .................................................................................................................... 19

Figure 2.4. Crystal structure and optical property of g-C3N4 prepared from dicyanamide.

Blue and gray spheres represent nitrogen and carbon atoms, respectively .......................... 20

Figure 2.5. A) FTIR spectra of g-C3N4 and g-C3Nx using the different amount of KOH; B)

Structure model of C3Nx with C≡N group and N vacancy; C) photocurrent density; D) time

course hydrogen production of g-C3N4 and g-C3Nx under visible-light illumination .......... 21

Figure 2.6. A) side; B) top view of the atomic structure of layered carbon nitride with

hydrogen bonds; C) X-ray diffraction pattern of g-C3N4 heating at different temperatures; D

and E) hydrogen produced under visible region ................................................................... 22

Figure 2. 7. Charge transfer in type II g-C3N4- based photocatalysts.................................. 23

Figure 2.8. A) Schematic illustration of the preparation of g-C3N4/ K+Ca2Nb3O10

−

(CN/K+CNO−) nanosheet heterojunctions; B and C) the degradation efficiency of tetracycline

and electron-hole charge separation of g-C3N4/ K+Ca2Nb3O10

−, respectively ..................... 24

Figure 2. 9. Two type of all- solid Z-scheme; A) SC-C-g-C3N4; B) SC-g-C3N4 ................ 25

Figure 2. 10. A) Schematic illustration for the synthesis of α-Fe2O3/2D g-C3N4; B) HR-TEM

image α-Fe2O3/2D g-C3N4 heterojunction; C,D) Photocatalytic performance; E) Z-scheme

mechanism in α-Fe2O3/2D g-C3N4 ....................................................................................... 26

Figure 2. 11. The illustration of the MAX and corresponding Mxenes structures. ............. 28

Figure 2. 12. Illustration photocatalysis mechanism of CdS/phosphorene hybrid, in which

phosphorene functions a co-catalysts, under visible- light irradiation ................................. 30

Figure 2. 13. H2 production- the photocatalytic activity of various materials under visible

light ....................................................................................................................................... 32

xiv

Figure 2. 14. HRTEM and corresponding FFT images of the (A, D) as-prepared, (B, E)

illuminated, and (C, F) regenerated Ni-NiOx/SrTiO3 .......................................................... 33

Figure 2. 15. Schematic illustration of electron-hole separation in the r-GO/C3N4

nanocomposite for the reduction of CO2 to CH4 under visible light .................................... 36

Figure 2. 16. A) Oscillation of localized SPR on spherical plasmonic nanoparticles; B) hot

electron-hole pairs generation .............................................................................................. 37

Figure 2. 17. A) Hot electron transfer; B) schematic illustration of the presence of Schottky

barrier at the interface of metal/semiconductor. ................................................................... 39

Figure 2. 18. Schematic illustration of called the plasmon-induced interfacial charge-transfer

transition from Au to CdSe nanorods. .................................................................................. 40

Figure 2. 19. A, B) Absorption spectrum and imaginary part of permittivity (ε″) for c) Au

NPs (50 nm) and d) TiN nanocubes (50 nm) in solution; C,D) Band diagram of a plasmonic

Schottky interface (PSI) and plasmonic Ohmic interface (POI), respectively. .................... 42

Figure 2. 20. Schematic illustration of the formation the hollow structure; A) template

strategy; B) Ostwald ripening;44* C) Kirkendall effect;45** D) ion exchange. ..................... 44

Figure 2. 21. A) Schematic illustration of the procedure for synthesis of C3N4 hollow

spheres; B) TEM image of C3N4 hollow spheres; C) photoactivity for hydrogen evolution of

C3N4 hollow spheres. ............................................................................................................ 47

Figure 2. 22. A) Schematic illustration of the synthesis of Ta3N5 hollow spheres; B) TEM

image of hollow Ta3N5; C) nitrogen adsorption-desorption isotherm and pore size

distribution of the Ta3N5 hollow microspheres; D) photocatalytic evaluation of Ta3N5 hollow

spheres. ................................................................................................................................. 50

Figure 2. 23. A) Schematic illustration of GaN:ZnO hollow photocatalyst fabrication; B)

SEM image of as-prepared carbon colloidal spheres; C) TEM image of GaN:ZnO hollow

spheres; D) photoactivity of GaN:ZnO hollow spheres in water splitting under visible light.

.............................................................................................................................................. 51

Figure 2. 24. Formation of TiO2 hollow spheres by Ostwald ripening; A). Formation of TiO2

hollow spheres from amorphous titania; B,C,D,E) SEM images of each step ..................... 52

Figure 2. 25. A) The formation of CdS spheres through the Kirkendall effect: a) Cd particles;

b) CdS hollow spheres.; B) The formation of Bi2WO6 through the anionic exchange

mechanism. ........................................................................................................................... 53

Figure 2. 26. Multiple light absorption in a multi-shell hollow structure; B) schematic

illustration of CeO2 triple-hollow spheres; C) TEM image of triple-hollow CeO2; D)

photoactivity for oxygen evolution of triple-hollow CeO2 spheres. ..................................... 54

Figure 2. 27. A) Illustration of the photoactivity improvement in a separated co-catalyst with

a hollow structure; B) SEM image of hollow Pt/Ta3N5; C) photoactivity for oxygen

generation under visible light of hollow Pt/Ta3N5/CoOx. .................................................... 58

Figure 2. 28. A) Schematic illustration of Fe2O3-TiO2-PtOx preparation from MIL-88B; B)

TEM images of hollow Fe2O3-TiO2-PtOx; C) amount of hydrogen generated under visible

light irradiation. .................................................................................................................... 59

xv

Figure 2. 29. A) Schematic illustration of the fabrication of Au/TiO2-3DHNSs; B) SEM, and

C,D) TEM and STEM of Au/TiO2-3DHNSs; E) UV-Vis spectroscopy of Au/TiO2-3DHNSs;

F) amount of CO2 generated under visible light illumination, 1: Au-TiO2 (P25); 2: Au/TiO2-

3DHNSs; 3: crushed Au/TiO2-3DHNSs; 4: disordered Au/TiO2-HNSs. ............................. 60

Figure 2. 30. A) Schematic illustration of the production of a Ta3N5/TaON hollow composite

photocatalyst; B, C) TEM images of TaON and Ta3N5/TaON, respectively. ...................... 61

Figure 2. 31. A) formation of hollow structure ZnFe2O4/ZnO; B,C) SEM and TEM images,

respectively, of hollow structure ZnFe2O4/ZnO; C) amount of hydrogen generated on hollow

structure ZnFe2O4/ZnO; E) schematic diagram of energy band structures and the expected

transfer direction of electron–hole pairs in the ZnFe2O4/ZnO heterostructures under visible

light irradiation. .................................................................................................................... 62

Figure 2. 32. A) Schematic illustration of TiO2/GO hollow composite formation: 1: PEI, 2:

Ti0.91O2, 3: PEI, 4: GO nanosheet, 5: five repeats of steps 1–4, 6: microwave treatment;

B,C,D) SEM images of bare PMMA spheres, PEI/Ti0.91O2/PEI/GO@PMMA, and (G-

Ti0.91O2)5 hollow spheres, respectively; E) TEM image of (G-Ti0.91O2)5 hollow spheres; F)

CO2 reduction by (G-Ti0.91O2)5 hollow spheres. .................................................................. 63

Figure 3. 1.The interaction between the electron beam and the sample. ............................. 67

Figure 3. 2. Basic components of a TEM ............................................................................ 68

Figure 3. 3. Schematic illustration of a scanning electron microscope. .............................. 70

Figure 3. 4. Schematic diagram of the diffraction of X-rays by a crystal (Bragg condition)

.............................................................................................................................................. 72

Figure 3. 5. Schematic illustration of the photoemission process. ...................................... 73

Figure 3. 6. Six types of sorption isotherms. ....................................................................... 75

Figure 3. 7. Essential components of a typical ICP-Ms. ..................................................... 77

Figure 3. 8. The typical FTIR spectrum of graphitic carbon nitride (g-C3N4) structure. .... 78

Figure 3. 9. Basic components of a FTIR system consisting the Michelson interferometer.

.............................................................................................................................................. 79

Figure 3. 10. A) General energy-level diagram for electronic excitation and B) singlet-singlet

transitions and their assignment to the absorption spectrum. ............................................... 81

Figure 3. 11. The illustration of A) conventional UV- visible; B) diffuse reflectance UV-

visible spectrophotometers. .................................................................................................. 82

Figure 3. 12. A) Main photophysical processes of a semiconductor excited by light with

equal to or higher than band gap energy (I: photo-excited process; II: band–band PL process;

III: excitonic PL process; IV: non-radiative transition process); B) Schematic illustration of

static PL spectroscopy. ......................................................................................................... 83

Figure 3. 13. Two typical three-electrode- configuration PEC cell. .................................... 84

Figure 3. 14. Basic components of a typical gas chromatograph. ....................................... 85

Figure 4. 1. A) Powder XRD spectrum of Pt-TiO2-CxNy and B) UV-Vis spectra of Pt-TiO2-

CxNy (red) and Pt-TiO2-550 (black). Inset: plot of (αhν) versus photon energy to determine

xvi

band gap energies. C, D) C1s and N1s XPS spectrum of Pt-TiO2-CxNy. E) O1s XPS spectra

of Pt-TiO2-CxNy and Pt-TiO2-550. ...................................................................................... 94

Figure 4. 2. SEM images of A) carbon colloidal spheres@Pt/TND and B) Pt-TiO2-CxNy; C)

TEM, and D) high-resolution TEM images of the interior of the broken Pt-TiO2-CxNy sphere,

respectively. .......................................................................................................................... 96

Figure 4. 3. A) STEM image and EDS elemental mapping of B) platinum, C) carbon, D)

oxygen, and E) nitrogen and titanium in the selected region of Pt-TiO2-CxNy. ................... 97

Figure 4. 4. Production of (A) H2 from water and (B) CO2 from methanol degradation under

simulated solar light (AM 1.5; 100 mW cm−2) using a) Pt-TiO2-CxNy, b) Pt-TiO2-550, c) Pt-

C3N4, and d) Pt-TiO2 (P25) photocatalysts. .......................................................................... 98

Figure 4.S1. SEM image of as–prepared carbon colloidal spheres@Pt; B,C) TEM image and

photograph of water soluble Titanate nanodisks (TNDs). .................................................. 107

Figure 4.S2. Fourier transform infrared (FT-IR) spectra of (a): Pt-TiO2-CxNy; b) graphic

carbon nitride (g-C3N4); c) Pt-TiO2-550. ........................................................................... 107

Figure 4.S3. XPS survey spectrum of Pt-TiO2-CxNy; ▲: unidentified peaks caused by

contaminants during the sample preparation. ..................................................................... 108

Figure 4.S4. Deconvoluted Ti2p XPS spectrum of Pt-TiO2-CxNy. ................................... 108

Figure 4.S5. Deconvoluted Pt4f XPS spectrum of Pt-TiO2-CxNy. .................................... 109

Figure 4.S6. General molecular structure of triazines. ...................................................... 109

Figure 4.S7. TEM image of hollow Pt-TiO2-CxNy. ........................................................... 110

Figure 4.S8. Energy-dispersive X-Ray spectroscopy (EDS) data from the HR-TEM image

of Pt-TiO2-CxNy in Figure 2D, indicating the presence of Pt on TiO2. .............................. 110

Figure 4.S9. Hydrogen production of Pt-TiO2-CxNy with various catalyst weights. ...... 111

Figure 4.S10. Photo-stability for H2 generation with Pt-TiO2-CxNy. ................................ 111

Figure 4.S11. Nitrogen adsorption–desorption isotherm and pore size distribution of Pt-

TiO2-CxNy. .......................................................................................................................... 113

Figure 4.S12. Hydrogen production under simulated solar light in pure water with various

photocatalysts: a) Pt-TiO2-CxNy, b) Pt-TiO2-CxNy-L, c) Pt-TiO2-550, and d) Pt-TiO2 -P25.

ND: not detected. ................................................................................................................ 114

Figure 4.S13. Hydrogen production under simulated solar light with various photocatalysts

in the presence of a sacrificial agent: a) TiO2-CxNy, b) TiO2-CxNy-L , c) TiO2-550, and d)

TiO2 (P25). ND: not detected. ............................................................................................ 115

Figure 4.S14. Photoluminescence (PL) spectra of various samples excited at 270 nm .... 116

Figure 4.S15. Schematic illustration of Pt-TiO2-CxNy operating under solar illumination.

............................................................................................................................................ 116

Figure 5.1. A) Schematic representation of the air-assisted carbon sphere combustion

(ACSC) process using carbon colloidal spheres as a solid fuel; B, C, and D) photographs of

carbon colloidal spheres@Pt/TiO2 before, during, and after ACSC process, respectively,

xvii

corresponding to the preparation of C/Pt-TiO2 (58/42); E and F) SEM images of colloidal

carbon core/shell structures before and after the combustion process. .............................. 123

Figure 5.2. A) XRD patterns of and B) photocatalytic hydrogen production by C/Pt-TiO2

(A/R): A/R = a) 0/0, b) 79/21, c) 58/42, d) 40/60, and e) 0/100. ND: not determined. ..... 125

Figure 5.3. A) XRD pattern, B) Raman spectrum, C) C1s XPS spectrum, and D) Pt4f XPS

spectrum of C/Pt-TiO2 (58/42). .......................................................................................... 127

Figure 5. 4. A) TEM image with inset SAED pattern, B) HR-TEM image with inset FFT

image, and C and D) HR-TEM-EDX analysis of C/Pt-TiO2 (58/42). ................................ 129

Figure 5.5. (A) Hydrogen produced from water and (B) CO2 produced by methanol

degradation using a) C/Pt-TiO2 (58/42), b) Pt-TiO2 (58/42)-550, c) Pt-TiO2-P25 and d) Pt-

TiO2-CV. ............................................................................................................................ 130

Figure 5.S1. XRD patterns of the sample Pt-TiO2-CV prepared by conventional calcination

method. ............................................................................................................................... 139

Figure 5.S2. UV-vis spectra of (a) C/Pt/TiO2 (58/42) and (b) Pt/TiO2 (58/42)-550. ........ 140

Figure 5.S3. XRD pattern of (a) C/Pt/TiO2 (58/42) and (b) Pt/TiO2 (58/42)-550. ............ 140

Figure 5.S4. Survey spectrum of C/Pt-TiO2 (58/42). ........................................................ 141

Figure 5.S5. Ti2p XPS spectrum of C/Pt/TiO2 (58/42). ................................................... 141

Figure 5.S6. TEM images of C/Pt/TiO2 (58/42) showing the presence of Pt nanoparticles on

anatase TiO2. ....................................................................................................................... 142

Figure 5.S7. The power spectrum of simulated solar simulator. ....................................... 143

Figure 5.S8. TEM images and corresponding Pt XPS spectra of Pt-TiO2 (58/42)-550 (A, B),

Pt-TiO2-P25 (C, D) and Pt-TiO2-CV (E,F). ...................................................................... 144

Figure 5.S9. A, B) produced H2 and CO2 as the function of reaction time; C,D ) the stability

of C/Pt-TiO2(52/48) for hydrogen production from water and methanol degradation. ..... 145

Figure 5.S10. (A) Hydrogen produced from water and (B) CO2 produced by methanol

degradation under UV light (254 nm) using a) C/Pt-TiO2 (58/42), b) Pt-TiO2 (58/42)-550, c)

Pt-TiO2-P25 and d) Pt-TiO2-CV. ........................................................................................ 146

Figure 5.S11. A) Nitrogen adsorption–desorption isotherm and B) pore size distribution of

C/Pt-TiO2 (58/42). .............................................................................................................. 146

Figure 5.S12. Steady-state photoluminescence (PL) spectra (excitation at 340 nm) of Pt-

TiO2-CV, Pt-TiO2-P25, Pt-TiO2 (58/42)-550, and C/Pt-TiO2 (58/42). .............................. 147

Figure 5.S13. Proposed photocatalytic mechanism in C-Pt/TiO2 (A/R) under simulated solar

light (AM 1.5). .................................................................................................................... 149

Figure 6. 1. Schematic illustration for the synthesis of hollow double-shell H:Pt-WO3/TiO2-

Au nanospheres: 1) one-pot synthesis of Pt-WO3@carbon colloidal spheres, 2) coating with

TNDs using a layer-by-layer strategy followed by Au precursor loading, 3) calcination at 550

°C for 3 h, and 4) hydrogen treatment at 350 °C for 1 h. ................................................... 156

xviii

Figure 6. 2. A, B) SEM images of Pt-WO3/TiO2-Au before and after calcination followed by

hydrogen treatment; C, D) TEM image of the hollow H:Pt-WO3/TiO2-Au; E) High resolution

TEM image of hollow H:Pt-WO3/TiO2-Au; F) Survey spectrum of hollow H:Pt-WO3/TiO2-

Au, inset XPS of Au and Pt. ............................................................................................... 159

Figure 6. 3. A: Powder XRD spectrum of hollow H:Pt-WO3/TiO2-Au; B: UV-vis spectra of

different samples: (a) hollow Pt-WO3/TiO2, (b) hollow Pt-WO3/TiO2-Au before H2

treatment; (c) hollow H:Pt-WO3/TiO2, (d) hollow H:Pt-WO3/TiO2-Au, and (e) hollow H:Pt-

WO3 after hydrogen treatment; C, D: XPS Ti2p and W4f spectra of Pt-WO3/TiO2-Au before

and after H2 treatment. ........................................................................................................ 160

Figure 6. 4. A) Degradation of HCHO as a function of reaction time under visible light

illumination (λ≥420 nm) and in the dark; B,C) Amount of CO2 generated over 6 h in visible

light and 18 h in the dark; (a) hollow H:Pt-WO3/TiO2-Au; (b) hollow H:Pt-WO3/TiO2; (c)

hollow H:Pt-WO3; (d) conventional H:Pt -WO3 (prepared from commercial WO3 powders).

Reaction conditions: catalysts: 50 mg; irradiated area: 4 cm2. Light source: simulated solar

light with 420 nm cut-off filter. .......................................................................................... 163

Figure 6. 5. Schematic illustration of the catalytic mechanism of the hollow double-shell

H:Pt-WO3/TiO2-Au photocatalyst under visible light irradiation and in darkness. ........... 166

Figure 6.S1. SEM of the hollow H:Pt-WO3 nanospheres. ................................................. 173

Figure 6.S2. EDS of hollow H:Pt-WO3/TiO2-Au spheres confirms the presence of Pt, W, Ti,

and Au in the sample. ......................................................................................................... 174

Figure 6.S3. UV-Visible spectra of different samples in the absence of Pt before and after

H2 treatment; It should be noted that no significant improvement in light absorption occurs

before and after H2 treatment.............................................................................................. 175

Figure 6.S4. Illustration of oxygen vacancies in the WO3 structure after H2 treatment

enhancing light absorption and electron storage capacity. ................................................. 176

Figure 6.S5. CO2 generation rates of the samples under visible illumination (red colour) and

in darkness (black colour) before hydrogen treatment (A and B) (1) hollow Pt-WO3/TiO2-

Au spheres; (2) hollow Pt-WO3/TiO2; (3) hollow Pt-WO3; (4) conventional Pt-WO3/TiO2,

prepared from commercial WO3 and TiO2-P25, (5) conventional Pt-WO3 prepared from

commercial WO3. ............................................................................................................... 176

Figure 6.S6. Catalytic stability of hollow H2-treated Pt-WO3/TiO2-Au nanospheres for

degradation of formaldehyde over five cycles under visible light and in darkness. .......... 177

Figure 6.S7. Schematic of hollow H:Pt-WO3/TiO2-Au with strong sunlight absorption, high

surface area, and high concentration of oxygen vacancies. ................................................ 179

Figure 7.1. Illustration for the preparation of Au/g-C3N4-AAPC; B, C) TEM images of Au/g-

C3N4-AAS and Au/g-C3N4-AAPC, respectively; D) the photograph of g-C3N4 (yellow) and

as-prepared Au/C3N4-AAPC (green). ................................................................................. 187

Figure 7. 2. A) produced hydrogen under full solar irradiation; B) X-ray diffraction; C) UV-

Vis; and D) FT-IR spectra of a) g-C3N4-C; b) g-C3N4-K, c) Au/g-C3N4-AAS and d) Au/g-

C3N4- AAPC. ...................................................................................................................... 189

xix

Figure 7.3. XPS spectra of C1s, N1s, and Au4f of Au/C3N4-AAS (line a) and Au/C3N4-AAPC

(line b) ................................................................................................................................ 192

Figure 7.4 A) Produced hydrogen; B) photocurrent; C) electrochemical impedance

spectroscopy of a) bare g-C3N4, b) Au/g-C3N4-CV, c) Au/g-C3N4-AAPC under solar

irradiation;D,E, and F) HR-TEM image and corresponding HR-TEM-energy-dispersive X-

ray (EDX) analysis of Au/g-C3N4-AAPC. ......................................................................... 195

Figure 7.5. Proposed schematic illustration of the photocatalytic process in the Au/g-C3N4-

AAPC ................................................................................................................................. 196

Figure 7.S1. The transmission electron microscopy (TEM) image of g-C3N4-K .............. 201

Figure 7.S2. XRD pattern of Au/g-C3N4-CV ................................................................... 202

Figure 7.S3. UV-Vis spectra of Au/g-C3N4 prepared by conventional route before and after

2nd step calcination.............................................................................................................. 203

Figure 7.S4. FT-IR spectrum of Au/g-C3N4-CV .............................................................. 204

Figure 7.S5. C1S XPS spectra of Au/g-C3N4-CV ............................................................. 204

Figure 7.S6. N 1s XPS spectra of Au/g-C3N4-CV ............................................................. 205

Figure 7.S7. O 1s XPS spectra of Au/g-C3N4-CV ............................................................. 205

Figure 7.S8. A) HR-TEM images; B) corresponding HR-TEM-energy-dispersive X-ray

(EDX) analysis of Pt-photo-deposited Au/g-C3N4-AAPC, respectively. ........................... 207

Figure 8.1. TEM images of A) g-C3N4; B) g-C3N4-PA. .................................................... 215

Figure 8. 2. A) XRD pattern; B) FT-IR; C) UV-Vis spectra and D) Bandgap Energy of g-

C3N4 and g-C3N4-PA, respectively. .................................................................................... 216

Figure 8. 3. XPS spectra of A) C1s; B) N1s; C) survey of bare g-C3N4 (line a) and g-C3N4-

PA (line b); D) XPS O1S spectrum of g-C3N4-PA. ............................................................ 219

Figure 8. 4. A) Produced hydrogen; B) linear sweep voltammetry (LSV) measurements, C)

photocurrent density employing light-chopping D) electrochemical impedance spectra (EIS)

of g-C3N4 (red) and g-C3N4-PA (pink), respectively, under solar irradiation. ................... 221

Figure 8.S1. TEM images of A) g-C3N4-P; B) g-C3N4-A. ................................................ 226

Figure 8.S2. The Solid-state 13C NMR spectra of a) g-C3N4; b) g-C3N4-PA. .................. 226

Figure 8.S3. XRD pattern of g-C3N4-P and g-C3N4-A, repsectively . ............................... 227

Figure 8.S4. FT-IR spectra of g-C3N4-P and g-C3N4-A, repsectively. .............................. 228

Figure 8.S5. UV-Vis spectra of g-C3N4, g-C3N4-P, g-C3N4-A and g-C3N4-PA using high

amount of PEI. .................................................................................................................... 229

Figure 8.S6. XPS spectra of g-C3N4-A (A,B,C) and g-C3N4-P (D,E,F) ............................ 230

Figure 8.S7. Typical structure of bare g-C3N4 ................................................................... 233

Figure 8.S8. Proposed structure of g-C3N4-PA ................................................................. 234

xx

List of Tables

Table 2. 1. List of single shell hollow photocatalysts .......................................................... 48

Table 2. 2. List of hollow composite photocatalysts ........................................................... 55

Table 3. 1. Selected functional group absorption ................................................................ 78

Table 4.S1. Hydrogen production under simulated solar light (AM 1.5G) for various TiO2-

based photocatalysts. .......................................................................................................... 112

Table 4.S2. Specific surface area of various photocatalysts. ............................................. 114

Table 5.S1. Elemental chemical analysis of different samples .......................................... 139

Table 5.S2. Pt content of different samples ....................................................................... 142

Table 5.S3. Hydrogen generated under solar light irradiation (AM 1.5; 100 mW.cm2) using

various previously reported TiO2-based photocatalysts and those developed in this study

............................................................................................................................................ 143

Table 5.S4. Physicochemical properties of various anatase/rutile TiO2-base photocatalysts.

............................................................................................................................................ 148

Table 6.S1. Summaries of specific surface area and CO2 generation rate of different samples

before and after hydrogen treatment for formaldehyde (HCHO) decomposition under visible-

light and in darkness. .......................................................................................................... 178

Table 7.S1. Calculation of d spacing of various samples .................................................. 202

Table 7.S2. Measured specific surface area of various synthesized samples .................... 203

Table 7.S3. XPS analysis of N 1s of different samples ..................................................... 206

Table 8S.1. Calculation of d spacing of various samples .................................................. 228

Table 8S.2. XPS analysis of C 1s of various samples ....................................................... 231

Table 8.S3. XPS analysis of N 1s of different samples ..................................................... 232

xxi

List Schemes

Scheme 4. 1. Schematic illustration for the synthesis of Pt-TiO2-CxNy: 1) one-pot synthesis

of carbon@Pt, followed by coating titanate nanodisks (TNDs) using a layer-by-layer

technique to obtain core-shell carbon spheres@Pt-TNDs, as previously described; 2) loading

of cyanamide at 70 °C; and 3) calcination in air at 550 °C for 5 h to obtain Pt-TiO2-CxNy..93

Scheme 5.S 1. Illustration of the utilized test system ........................................................ 138

Scheme 5.S 2. A) Schematic for the preparation of C/Pt/TiO2 (A/R): 1- one-pot synthesis of

carbon colloidal spheres@Pt; 2- loading of TiO2 using titanium isopropoxide; 3 cavitation

combustion process assisted with air flow to obtain C/Pt/TiO2 (A/R) with phase-tunable

Anatase-Rutile. ................................................................................................................... 138

Scheme 6.S1. Illustration of conventional WO3/TiO2 nanocomposite under UV illumination

and in darkness; the catalytic activity depends on UV irradiation and interfacial contact

between WO3 and TiO2, whereas the activity in darkness is effected by electron transfer from

TiO2 and oxygen vacancies in the WO3 structure. For these reasons, this conventional

nanocomposite shows low activity under visible irradiation and in the dark. .................... 172

Scheme 6.S2. Illustration of TNDs coated on carbon spheres@ Pt-WO3 using layer-by-layer

technique. ............................................................................................................................ 173

Scheme 8.1. A) Schematic illustration of g-C3N4-PA; (1) the interaction between

dicyanamide, KOH, and PEI during calcination; (2) two-step calcination. ....................... 214

xxii

List of Abbreviations

AAPC Alkali-assisted post-calcination

BA Benzyl alcohol

BET Brunauer−Emmett−Teller

CB Conduction band

EDS Energy dispersive X-ray spectroscopy

FTIR Fourier transform infrared spectroscopy

GC Gas chromatography

HNS Hollow nanosphere

HRTEM High resolution transmission electron micoscopy

IUPAC International Union of Pure and Applied Chemistry

NP Nanoparticle

OA Oleic Acid

OM Oleylamine

PA Poly-alkaline

PEI Poly(ethyleneimine)

SAED Selected area electron diffraction

SEM Scanning electron microscopy

SI Supporting information

SHE Standard hydrogen electrode

SPR Surface plasmon resonance

STEM Scanning transmission electron microscopy

TB Titanium butoxide

TBA Tetrabutyl ammonium

xxiii

TEA Tetraethyl ammonium

TEABH Tetraethylammonium borohydride

TEM Transmission electron microscopy

TND Titanate nanodisk

UV Ultraviolet

UV-vis Ultraviolet-visible spectroscopy

VB Valence band

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction

xxiv

Acknowledgments

First of all, I would like to thank my supervisor, Prof. Trong- On Do, for giving me

the great opportunity to perform the research at Université Laval. His tremendous support

and guidance significantly motivated me. Moreover, Prof. Do provided me freedom and

valuable pieces of advices to pursue my interest research.

The work represented in this thesis would not have been possible without the

assistance of many other people. I would like to give special thanks to Richard Janvier for

their help with electron microscopes, Alain Adnot for XPS analysis, Jean Frenette for XRD

measurement, Jean-Nicolas Ouellet, Jérôme Noël, and Marc Lavoie for their help with

laboratory safety and reactor set up. Thanks to Yann Giroux for his great help with Autosorb

instrument and spin coating technique. I would also like to thank the Chemical Engineering

department staffs for all the technical and administrative assistance that I received during my

study period.

I would like to express my acknowledgment and appreciation to all past and current

members of the Do Research Group. I have learned a lot from them since I started the

program. Dr. Cao-Thang Dinh taught me many great ideas about synthesis methods and

photocatalysts. Dr. Minh-Hao Pham and Dr. Ving- Thang Hoang showed me to handle

various experimental setups and organize the lab. I have enjoyed discussing with them so

much. Moreover, I would like to thank the students in our group, who are very kind people:

Dr. Mohammad Reza Gholipour, Mathieu St-Jean, Amir Enferadi Kerenkan, Nhu- Nang Vu,

Manh- Hiep Vu, Duc- Trung Nguyen, Arnaud Gandon, Rokesh Karuppannan. Additionally,

I would like to thank Dr. Van- Re Bui, Manh-Duy Phan and Arnaud Gandon for their great

help in completing the abstracts in French of the thesis.

I gratefully acknowledge the Natural Science and Engineering Research Council of

Canada (NSERC) through Collaborative Research and Development (CRD), Strategic

Project (SP), and Discovery Grants. I also would like to thank Exp Inc for their support.

From a personal perspective, all friends in Université Laval who have made my

graduate school experience in Canada memorable. Particularly, I would like to thank Cao-

Thang Dinh for his tremendous help from the first days I came to Université Laval.

xxv

Finally, I would like to thank my parents and parents-in-law for their encouragement

and support during my education. I would like to be grateful to Ann, Dieter, and Betty who

always beside me since my first steps in Canada. Especially, I would like to appreciate my

wife, Thi Bach Hac Nguyen, for being the amazing friend, and partner. Her endless love and

support have been the best flower of my life.This thesis would not have been possible without

their love and support. Thank you all from the bottom of my heart.

xxvi

To my dear parents, Vietnamese and Canadian families, and my loving and amazing wife,

Nguyen Thi Bach Hac

xxvii

Foreword

This thesis is composed of nine chapters. Six of them were prepared in the form of scientific

papers that have been published and submitted. The candidate is the primary author for these

papers.

A part of chapter 1 and 2 has been published as Chinh-Chien Nguyen, Nhu-Nang Vu, and

Trong-On Do “Recent Advances in the Development of Sunlight-Driven Hollow Structure

Photocatalysts and their Applications” Journal of Materials Chemistry A 3.36 (2015): 18345-

18359.

Chapter 4 has been published as Chinh-Chien Nguyen, Nhu-Nang Vu, Stéphane Chabot,

Serge Kaliaguine and Trong-On Do, “Role of CxNy-Triazine in Photocatalysis for Efficient

Hydrogen Generation and Organic Pollutant Degradation Under Solar Light Irradiation.”

Solar RRL, 2017, 1(5).

Chapter 5 has been published as Chinh-Chien Nguyen, Duc- Trung Nguyen, and Trong-On

Do. "A novel route to synthesize C/Pt/TiO2 phase tunable anatase–Rutile TiO2 for efficient

sunlight-driven photocatalytic applications." Applied Catalysis B: Environmental 226

(2018): 46-52.

Chapter 6 has been published as Chinh-Chien Nguyen, Nhu-Nang Vu, and Trong-On Do.

"Efficient hollow double-shell photocatalysts for the degradation of organic pollutants under

visible light and in darkness." Journal of Materials Chemistry A 4.12 (2016): 4413-4419.

Chapter 7 has been submitted as Chinh-Chien Nguyen, Mohan Sakar, Manh-Hiep Vu and

Trong- On Do. " The role of nitrogen vacancies for the enhanced surface plasmonic resonance

of Au/g-C3N4 crumpled nanolayers as an efficient solar light-driven photocatalyst.”

Chapter 8 has been submitted as Chinh- Chien Nguyen and Trong-On Do.” Engineering the

high concentration and efficiency of N3C nitrogen vacancies to prepare strong solar light-

driven photocatalysts-based g-C3N4 ”

In these works, the candidate designed and performed all of the experiments under the

supervision of Prof. Trong-On Do and help from other co-authors. The candidate collected

the data and wrote the first drafts of all manuscripts. All the authors revised the manuscripts

prior to publication.

1

Chapter 1: Introduction

In this chapter, solar- driven photocatalysis, which has been considered as the most potential

approach to deal with the global energy and environmental issues, is determined. The

fundamental and current advances of semiconductor-based photocatalysis are discussed.

Additionally, the scope and organization of the thesis are also stated.

2

1.1. Using photocatalysts as the potential solution to address energy and

environment issues

Global energy demand continues to increase due to population growth and economic

expansion. Worldwide energy consumption reached 16.2 terawatts (TW) in 2008 and is

expected to nearly triple by 2100.1 Consequently, global energy shortages and the

environmental damage caused by the combustion of fossil fuels will be huge challenges

facing civilization over the next few decades. Research and development of renewable, clean,

and carbon-neutral alternative energy resources is, thus, urgently required to reduce our

dependence on fossil fuels.

Among the renewable energy resources, solar energy is the most abundant.

Moreover, solar light, being free and green, is an ideal energy source for overcoming current

environmental challenges. Considering that, in a single hour, the sun delivers energy

sufficient for all human activities on the planet for an entire year, the harvesting of sunlight

by artificial photocatalysts, and it’s conversion into solar fuels, is both viable and highly

attractive.1

Since Fujishima and Honda first reported the generation of H2 through the

photoelectrochemical splitting of water on TiO2 electrodes under ultraviolet (UV) light in the

early 1970s, the conversion of solar light to chemical energy using semiconductors has been

explored as a key solution for energy production and pollutant degradation. However, a major

challenge lies in designing an efficient sunlight-driven photocatalyst system.1d

Solar fuels can take the form of hydrogen and hydrocarbons such as methane and

methanol. These products, which are considered as next-generation energy carriers, may be

formed by photocatalytic water splitting and by photoreduction of CO2 with water,

respectively. Also, the photocatalytic process allows direct use of sunlight to decompose a

wide range of organic pollutants, as photocatalysts can be excited by light to generate

electron-hole pairs which can drive a variety of redox reactions.2

3

Much effort has been focused on the development of sunlight-driven photocatalysts

in recent years; however, these materials still suffer from two fundamental efficiency

bottlenecks: weak photon absorption and poor electron-hole pair separation. The

development of highly efficient photocatalysts that absorb a large amount of solar energy and

exhibit high charge separation is a key requirement for the conversion of solar radiation into

chemical energy. Such a system could have a revolutionary impact on supplying our energy

needs in a sustainable manner.

1.2. Fundamental of photocatalysis based on semiconductors

A photocatalytic system based on a semiconductor can be described by the bandgap

model, in which the valence band (VB), the highest occupied band, and the conduction band

(CB), the lowest empty band, are separated by a band gap, a region of forbidden energies in

a perfect crystal. An electron is excited to the CB and leaves a hole (h+) in the VB when the

incident energy is equal to or larger than the band gap of the semiconductor, as depicted in

Figure 1.1.

Figure 1.1. Electron-hole generation under illumination in an inorganic photosystem; A:

electron acceptor; D: electron donor.

The photoexcited electron becomes utilized in a reduction reaction with an electron

acceptor; for example, the reduction of protons to hydrogen, generation of an O2.- ion radical,

or CO2 reduction, but only if the CB minimum is located at a more negative potential than

the electrochemical potential of the desired reaction. In the VB of the semiconductor, the

4

photo-generated hole can also perform an oxidative reaction with an electron donor with

oxidation potentials more negative than the VB maximum. Thus, the semiconductor is the

most essential component in systems intended for the production of green fuel and other

environmental applications.3

Figure 1.2. General processes in a semiconductor photocatalyst: 1) bandgap excitation; 2)

charge diffusion; 3) charge recombination; 4) chemical conversion.

Photocatalysis occurs in the semiconductor via multiple steps, as illustrated in

Figure 1.2. When the semiconductor is illuminated by a light source with higher photom

energy than the band gap, an electron is excited to the CB, leaving behind a hole (Figure 1.2-

step1). Then, the thus-produced charge carriers migrate to the photocatalyst surface (Figure

1.2-step 2). The electron and hole recombine on the surface or in the bulk material while

diffusing to the photocatalytic surface within a few nanoseconds (Figure 1.2- step 3).

Simultaneously, the charge carriers that reach the semiconductor surface participate in the

chemical conversion of the adsorbed reactants (Figure 1.2-step 4).4 Therefore, it is widely

accepted that the greater the number of photo-generated carriers that are generated and reach

the surface, the more efficiently the photoactive material will perform, and thus,

photocatalytic performance is significantly influenced by the incident light absorption ability

and charge separation efficiency of the catalyst. Particularly, a photocatalyst that exhibits a

+-

Surface

recombination

-- ++

-+

Band gap

excitation

A

A-Oxidation

D

D+Bulk

recombination

Reducing

(1)

(2) (2)(3) (3)

(4) (4)

(3)

5

band gap around or larger than 3 eV will show restricted performance in the UV-region, as

less than 5% of sunlight can be harvested by materials with a 3 eV band gap. Therefore, it

should be noted that the number of electron-hole pairs generated is greater if the light

absorption of semiconductor expands to the long wavelength region, i.e., a photocatalyst

working under visible light will produce a higher quantity of photo-induced carriers in

comparison with one irradiated by UV light.

In addition, the structure of the photosystem that utilizes the incident light is also an

important factor in generating charge carriers effectively. For instance, nanotube structures

reduce light loss due to the photon being trapped inside the structure by multiple light

reflections with the wall. The number of photogenerated charge carriers, therefore, is

significantly enhanced in comparison with the conventional morphology.5,6 The second

factor that has a strong influence on photoactivity is the number of the charge carriers

reaching the surface to take part in chemical conversion. Recombination is often caused by

a scavenger or crystalline defects which can trap the electron or the hole. Unfortunately, the

vast majority of charge carriers produced recombine immediately after the bandgap

excitation event.7 Based on the fundamental principles of semiconductor photocatalysis, an

efficient photocatalyst must satisfy the light absorption and charge separation criteria

simultaneously in order to exhibit high photoactivity.

1.2. Current advances in semiconductor photocatalysis

Light absorption ability and charge carrier separation are arguably the primary areas

that need development in photocatalysis, and much research concerned with furthering this

development has been recently reported. Band gap engineering, using nano-sized materials

and utilization of nanocomposites are the major approaches to improving photocatalysts.

Electronic band structure is the key to solar chemical conversions. Doping is a method

commonly used to extend the light absorption of wide band gap semiconductors to the longer-

wavelength region. An absorption shift toward the red region is easy to realize in most doped

semiconductors.8 The dopants can introduce localized electronic states, such as a donor level

above the VB or an acceptor level below the CB in the forbidden band of wide band gap

photocatalysts, which can narrow their band gaps. However, it is worth noting that the

dopants may or may not cause enhancement to photocatalytic performance, depending on the

6

doping-induced change in the electronic band structure, as doped element(s) can function as

charge recombination sites and reduce photoactivity.9 Design and morphological control of

the crystal facets of semiconductor photocatalysts with highly exposed active planes has been

proved to be useful for the development of an efficient material.10 Employing different facets

with different surface atomic structures can narrow the band structure, as well as drive the

photogenerated electron-hole pair in two distinct directions, leading to enhanced charge

separation.11 An alternative approach using solid solution photocatalysts has emerged as a

viable solution for the production of effective materials. GaN:ZnO, for instance, has shown

potential for application in overall water splitting that originates from the strong visible light

absorption, despite the wide band gap of both GaN (3.4 eV) and ZnO (3.2 eV).

In nano-sized materials, the smaller particle size exposes more active sites as well

as lowers the travel path of the charge carrier to the surface. Quantum dots of metal

chalcogens, such as CdS, CdSe, CdTe, and CuInS2, have already attracted a great attention

due to their unique optical and electronic properties. Photogeneration of electron–hole pairs

and their subsequent split into free carriers are the two key elements that directly determine

the light response efficiency of a quantum dot material. However, it is not always accurate

that the smaller the particle size, the higher the efficiency. A strong quantum confinement

effect appears to increase the recombination probability of photogenerated electron-hole

pairs. Although, small particle sizes indicate that charge carriers travelling to the surface have

favourably short distances, this process requires a suitable concentration gradient or potential

gradient (internal electric field) from the core of the particle to the surface, which has a close

association with the morphology, structure and surface properties of nanostructured

materials. In other words the internal electric field that helps separate electron-hole pairs in

nanoscale photocatalysts is not sufficient to drive charge carriers in different directions and

therefore lead to increased charge recombination .4, 12

Recently, the development of hybrid nanostructured photocatalysts has been shown

to be the most efficient method to separate charge carriers and improve light absorption. A

large number of excellent reviews have been published on the progress of composite

photocatalysts that readers can refer to for more detailed information 13-24. In general, the

development of photo-composites can be categorized into three primary approaches: co-

7

catalysts, plasmonic photocatalysts, and heterojunction structures, as shown in Figures 1.3

and 1.4.


two separated co-catalysts; B) plasmonic photocatalysts.

A co-catalyst is a component that can only work together with a photocatalyst

semiconductor. It is worth noting that co-catalysts play two main roles in the enhancement

of photocatalytic performance: they promote charge separation and serve as reaction sites.

Upon light irradiation, electrons migrate to the reduction sites to promote the reduction

reaction, while the hole migrates to the oxidation co-catalyst to take part in the oxidation

reaction, suppressing recombination and significantly enhancing the photoactivity, as shown

in Figure 1.3-A.25 Noble metals are used as the reduction co-catalyst.26 The different

properties of the noble metal and n-type semiconductor cause a barrier (Schottky barrier) and

space charge region (also called the depletion layer) as a result of the electron transfer process

from the semiconductor near the metal-semiconductor interface to the metal when they come

into contact. Moreover, the charge redistribution creates an internal electric field which

drives the photogenerated electron and hole to the bulk semiconductor and metal,

respectively. Under the successive photoexcitation of the semiconductor, a large number of

electrons accumulate in the semiconductor, making them hot enough to transfer to the metal.

Using noble metal-free co-catalysts has also received much attention over recent

years. Inexpensive and abundant nano-sized materials formed from transition metals (Ni, Co,

8

Cu) and transition metal compounds, such as transition metal oxides, metal hydroxides, and

metal sulfides, have been shown to be efficient co-catalysts for reduction reactions. These

co-catalysts show activities comparable to that of Pt, which is attributed to the effective

charge separation caused by efficient electron transfer from the semiconductor to the co-

catalyst. 27, 28 Moreover, carbonaceous nanomaterials, such as carbon nanotubes and

graphene, have been shown to promote charge carrier separation. Because of their high

electrical conductivity, which is caused by sp2-hybrided carbon atoms, they can function as

co-catalysts that accept photogenerated electrons from the semiconductor photocatalyst,

leading to significantly enhanced charge separation.29,30 In the same manner, photogenerated

holes are also attracted by oxidation co-catalysts to enhance the oxidation reaction. To extract

the holes, their band levels should be higher than that of the light-harvesting semiconductor.

Generally, metal oxides like MnOx, FeOx, CoOx, NiOx, CuOx, RuO2 and IrO2 are selected for

oxidation co-catalysts.31 Very recently, carbon quantum dots, a novel class of carbon

nanomaterials, have been shown to promote the rate of water oxidation in the decomposition

of pure water under solar light. Because of their unique photo-induced electron transfer,

photoluminescence, and electron reservoir properties, photocatalyst-based carbon quantum

dots not only facilitate charge separation, but are also promising as efficient and full sunlight

absorption materials.32


two separated co-catalysts; B) plasmonic photocatalysts.

9

Localized surface plasmon resonance (LSPR) has been applied in the photocatalytic

field and has attracted considerable attention over the past few years. The presence of

plasmon metal nanoparticles shifts the light absorption to the long wavelength region, caused

by free electron oscillation on the metal particle surface when the frequency of photons

matches the natural frequency of these electrons. Through the LSPR excitation of plasmonic

metals, energetic electrons are produced at the metal surface. These energetic electrons

remain in the excited “hot” state for up to 0.5–1 ps; they gain enough energy under visible

light illumination to facilitate the transfer to the conduction band of a semiconductor and

participate in the chemical conversion, as shown in Figure 1.3B. Cu, Ag, and Au

nanoparticles generally reveal a strong photoabsorption of visible light because their surface

plasmon exhibits the absorbance at approx. 580, 400 and 530 nm, respectively. However,

nanostructured copper and silver are easily oxidized, whereas Au nanoparticles display the

chemical stability. Furthermore, it is noted that the photocatalytic performance of plasmonic

photocatalysts is influenced by many factors such as the nanoparticle size, the shape and the

surrounding environment. 31, 33

Figures 1.4-A,B show the typical semiconductor alignment in composite materials

that have been shown to cause a remarkable improvement in photoactivity. Not only is light

absorption improved in the composite photosystem, but also the electron-hole separation,

which is enhanced by electron and hole transfer at the semiconductor-semiconductor junction

(type II semiconductor) or semiconductor-conductor-semiconductor (all solid Z-scheme).31

In type II semiconductors, the hole-electron separation highly depends on the electric field at

the interface. In the other words, the strong internal electric field promotes efficient charge

separation (see Figure 1.4A). However, the redox ability of the charge carriers usually

decreases after the charge transfer processes. The Z-scheme is proposed to address the

problem through mimicking the natural photosynthesis system. Thus, using the Z-scheme

has some advantages, such as maintaining charge carrier energy levels and harvesting visible-

light to archive the overall reaction. For example, an all solid Z-scheme has been developed

by inserting a conductor between two semiconductors to form ohmic contact with low contact

resistance. As a consequence, electrons from the CB of semiconductor A can directly

recombine with holes from semiconductor B (see Figure 1.4B).31

10

1.3. Scope of the thesis

Currently, titanium dioxide (TiO2) and graphitic carbon nitride (g-C3N4) have been

attracting the increasing attention due to their low cost and high chemical stability. 34

However, these materials are suffering low photoactivity owing to the charge recombination

and light absorption issues. Therefore, many efforts have been devoted to enhance the

photocatalytic performances, which are focused on improving three crucial factors: i) charge

separation by employing co-catalysts; ii) surface area; and iii) light absorption by coupling

with a surface plasmon resonance component. Although numerous reports have been

describing the various strategies to enhance these three parameters, the photocatalytic

performance has been still moderated. Therefore, exploring novel routes to prepare TiO2 and

g-C3N4-based photocatalysts, which possess salient properties, is the critical point to address

the above mentioned problems. In this thesis, we aim to prepare efficient materials based on

TiO2 and g-C3N4 for hydrogen production and the degradation of organic pollutants. Using

carbon spheres as the platform, we provide three different strategies to prepare TiO2-based

photocatalysts, which exhibit great photocatalytic performances towards hydrogen

production and organic pollutant degradation under sunlight. For g-C3N4, we also explore

two novel and facile approaches, which could not be achieved by the previous preparation

methods, to produce constructive modifications in the structure of g-C3N4 and exert surface

plasmonic resonance efficiently.

1.4. Organization of the thesis

Chapter 2 provides the state-of-the-art development of photocatalysts. In this

chapter, the latest strategies employed to improve the activity performance of TiO2 and g-

C3N4-based photocatalysts are discussed. Additionally, the current advances in the

development of co-catalysts, surface plasmon resonance (SPR) and hollow structure-

enhanced photocatalysis are also mentioned in detail. Through these discussions, we aim to

provide a brief overview of essential pathways in the field of semiconductor-based materials

for the development of the next generation of solar-driven photocatalysts.

In Chapter 3, the characterization techniques employed in the thesis are described.

The fundamental and information that could be extracted from each method are detailed.

11

In Chapter 4, we report the first synthesis of Pt/TiO2/CxNy-triazine nanocomposite,

in which Pt and CxNy-triazine located on the two opposite sides of a hollow sphere function

as the reduction and oxidation co-catalysts, respectively. We found that CxNy-based triazine

species act as an efficient oxidation co-catalyst and stabilize the surface area. This

nanocomposite shows one of the best TiO2-based photocatalysts working under solar light

irradiation up to date, which is 125 and 62 times higher than that of Pt/TiO2–P25 for hydrogen

generation and methanol decomposition, respectively.

In Chapter 5, we introduce a novel air-assisted carbon sphere combustion process to

prepare C/Pt/TiO2 photocatalysts with tunable phases. The as-prepared material possesses

the anatase/rutile (A/R) homojunction, high surface area, C and Pt/PtO co-catalysts. These

features contribute to a synergistic effect to boost the photocatalytic performance for

hydrogen production and organic pollutant degradation in comparison to Pt/TiO2-P25 and

photocatalysts prepared by the conventional method.

In Chapter 6, we report the synthesis of hollow double-shell H:Pt-WO3/TiO2-Au

nanospheres as the material that can work both under light irradiation and in darkness (day-

night photocatalysis). Possessing high specific surface area, large TiO2/WO3 interfacial

contact and strong visible light absorption, the resulted hollow double-shell H:Pt-WO3/TiO2-

Au exhibits high charge separation and electron storage capacity driving the efficient

degradation of organic pollutants both und

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Novel strategies to develop efficient titanium dioxide and graphitic … · 2020. 8. 7. · Novel strategies to develop efficient titanium dioxide and graphitic carbon nitride-based