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
Novel strategies to develop efficient titanium dioxide and
graphitic carbon nitride-based photocatalysts
Thèse
Chinh Chien Nguyen
Sous la direction de :
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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
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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
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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.
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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
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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
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
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
5.5. Supporting information ... 138
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
6.2. Results and discussion ... 157
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
xi
6.5. Supporting information ... 172
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
7.2. Results and discussion ... 187
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
7.5. Supporting information ... 201
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
8.2. Results and discussion ... 214
8.3. Conclusion ... 222
8.4. Experimental ... 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
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
Figure 1.4. Typical example of the current developments in photo-composite materials; A)
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
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
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
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-TiO 2-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)
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-C 3N4; 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
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
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
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
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
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 excitationA
A -Oxidation D D+ Bulk recombination Reducing (1) (2) (3) (3) (2) (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.
Figure 1.3. Typical example of the current developments in photo-composite materials; A)
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
Figure 1.4. Typical example of the current developments in photo-composite materials; A)
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.
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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 under visible light (λ ≥ 420 nm) and in darkness with high photocatalytic efficiency.
In Chapter 7, we present a novel alkali-assisted post-calcination (AAPC) route to prepare an efficient Au/g-C3N4 nanocomposite for hydrogen production under sunlight. We
explore that nitrogen vacancies in the structure of g-C3N4 contribute a crucial role for the
enhanced surface plasmon resonance of Au nanoparticles. Consequently, these outstanding properties in the resulted Au/g-C3N4 prompt electron-hole generation and separation, which
significantly boost the photocatalytic performance for solar-driven hydrogen production in comparison to both g-C3N4 and Au/g-C3N4 prepared by the conventional method.
In Chapter 8, we introduce a novel poly-alkaline method to prepare enhanced sunlight-driven photocatalysts based on g-C3N4. The constructive modifications in the
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containing groups that contribute to enhance solar light absorption and photocatalytic performance in comparison to conventional g-C3N4. This study provides a new understanding
of the important role of N3C nitrogen vacancies in the structure of g-C3N4. Moreover, it also
opens a new approach to prepare efficient sunlight-driven nanocomposite photocatalysts. In Chapter 9, the relevant conclusions drawn from the works in this thesis are highlighted. Additionally, the application of this work in the development of the next generation of photocatalysts is also mentioned.
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Chapter 2: State- of- the- art development of photocatalysts
This chapter aims at providing the latest research in the field of heterogeneous photocatalysis. The content is focused on the strategies to improve the photocatalytic performance of TiO2
and g-C3N4- based photocatalysts. Additionally, the current advances in the development of
co-catalysts, plasmonic, and hollow photocatalysts are also detailed. These three approaches have attracted considerable attention due to their significant contributions towards enhancing charge separation and light absorption, respectively.