Photocatalysis is a series of advanced light-induced redox reaction processes resulting in the degradation and mineralization of organic pollutants in the presence of oxygen and water. Due to their capability to destroy contaminants under mild conditions, photocatalytic processes have attracted considerable attention in the field of waste-water treatment. However, photocatalytic reactions using the traditional pure TiO2 photocatalyst suffer from low energy efficiencies under solar irradiation. This low efficiency in the utilization of solar energy lies in its incapability in absorbing visible lights and also the high recombination rate of photo-excited species in photocatalysts. In addition, difficulties in the separation of fluids from micro- or nano-scale catalysts in large scale systems substantially impact cost efficiency in practice.
In this thesis, strategies are explored which address these issues in order to improve the feasibility of solar photocatalysis. The preparation photocatalytic transition metal-oxide semiconductor materials are investigated, namely bismuth-based heterogeneous photocatalysts using Deep eutectic solvents (DESs) as green solvents. This research is focused on the design of visible-light-active metal-oxide photocatalysts to increase the absorption of visible light and to decrease the rates of electron-hole recombination, resulting in a high photocatalytic efficiency in regards to the degradation of organic pollutants. In First study deals synthesis of BiOCl/BiVO4 n- p heterojunction photocatalysts was synthesized using DESs reline (Choline chloride: Urea, 1:2) via simple one-pot sol-gel method at room temperature. BiOCl/BiVO4 sheet like structure was characterized and experimentally investigated for the degradation of Methylene blue, rhodamine B under visible light irradiation and also the mechanism was investigated using scavenger experiment. To improve the photocatalytic activity and electron-hole pair recombination time, the silver nanowires combined with BiOCl/BiVO4. Here, we report a one dimensional (1D) AgNWs combined with BiOCl/BiVO4 photocatalysts. The BiOCl/BiVO4@5% Ag NWs photocatalysts exhibited the highest photoactivity, and the degradation efficiency of MB and RhB was 97% and 96% as compared to bare BiVO4 and BiOCl/BiVO4, respectively. The appearance of elemental AgNWs during the photocatalytic reaction would be in favor to enhanced visible light absorption, the facilitated photoinduced electrons transfer, and the enhanced separation of photoinduced electron−hole pairs contributed to the improvement of photocatalytic activities.
Also, BiOCl/BiVO4@AgNWs photocatalysts are attributed to the formation of p-n heterojunctions between BiOCl and BiVO4, leading to an effective separation of photo-generated electron hole pair. The significantly enhanced photocatalytic activity should be ascribed to the


fabrication of a BiOCl/BiVO4 heterojunction, which can result in an efficient interfacial charge transfer, and it can be proved by Photoluminescence, Linear Sweep Voltammetry and Electrochemical Impedance Spectroscopy.
The second study proposed, the preparation of hierarchically nanostructured shuriken like bismuth vanadate (BiVO4) as a bifunctional catalyst for photocatalytic degradation and electrochemical detection of highly toxic hexavalent chromium (Cr(VI)) using the green Deep Eutectic Solvent reline by Solvothermal method, which allows morphology control in one of the less energy-intensive routes. The reline solvents leads the role of a latent supramolecular catalysts where the enhance in reaction rate from solvent driven pre-organization of the reactant is most remarkable. The SEM results showed a good dispersion of BiVO4 catalyst and the HR-TEM revealed an average particle size of ca. 5–10 nm. As a result, the BiVO4 exhibited good photocatalytic activity under UV-light about 95% reduction of Cr(VI) to Cr(III) was observed in 160 min. The recyclability of BiVO4 catalyst exhibited an appreciable reusability and stability of the catalyst towards the photocatalytic reduction of Cr(VI). Also, the BiVO4-modified screen printed carbon electrode (BiVO4/SPCE) displayed an excellent electrochemical performance towards the electrochemical detection of Cr(VI). Besides, the BiVO4/SPCE demonstrated tremendous electrocatalytic activity, lower linear range (0.01–264.5 µM), detection limit (0.0035
µM) and good storage stability towards the detection of Cr(VI). Importantly, the BiVO4 modified electrode was also found to be a good recovery in water samples for practical applications. The shape dependent nanostructured BiVO4 catalyst could also be used an effective electrode material for energy storage and hybrid capacitor in future.

Keywords: Deep eutectic solvents, Bismuth based photocatalysts, p-n heterojunction, organic pollutant removal, Cr(VI) removal, electrochemical sensing

Table of the contents
Abstract I
Acknowledgement III
Table of the contents VII
List of Tables IX
List of Figures X
List of Publication in Ph.D program XIV
Chapter 1: General Introduction 1
1.1 Background 3
1.2 Water Treatment 4
1.2.1 Conventional wastewater treatment technology and their drawbacks ……5 1.2.2 Advanced oxidation process 6
1.2.3 Photocatalysts 7
1.2.4 Photocatalytic phenomena and applications 8
1.2.5 Photocatalysis: reactors and operational parameters 11
1.2.5.1 Reactors 11
1.2.5.2 Operational parameters 12
1.3 Objectives .13
Chapter 2: Literature review 15
2.1 Introduction 17
2.2 Bismuth Vanadate crystal structures and electronic properties 17
2.2.1 Bismuth Vanadate (BiVO4) 17
2.2.2 Crystal Structure and Phase Transition 18
2.2.3 Electronic Structure 20
2.3 Photocatalytic Applications of BiVO4 under Visible Light 22
2.3.1 Photocatalysis 22
2.3.2 Basic Principles of Photocatalytic Reactions 24
2.3.3 Organic degradation 27
2.4 Limitations of BiVO4 and Overcome the Problems 30
2.5 Preparation Hierarchical nanostructures of BiVO4 and Challenges 32
2.5.1 What is Deep Eutectic Solvents? 32

2.5.2 DESs in Nanostructure synthesis 35
2.6 BiVO4 in Water Splitting 36
2.6.1 Powder Suspension System 36
2.7 Conclusion 42
Chapter 3: Materials and Characterization 45
3.1 Synthesis Routes 47
3.1.1 Sol–Gel Method 47
3.1.2 Solvothermal/hydrothermal method 48
3.2 Characterization techniques 49
3.2.1 Structural and morphological studies 50
3.2.2 X-ray diffraction (XRD) 50
3.2.3 Scanning electron microscopy (SEM) 51
3.2.4 Transmission electron microscopy (TEM) 52
3.2.5 UV/Vis. Spectroscopy 53
3.2.6 Fourier transform infrared (FTIR) 54
3.2.7 X-ray photoelectron spectroscopy (XPS) 55
3.2.8 Photoluminescence (PL) 56
3.3 Photoelectrochemical measurements 57
3.4. Photocatalytic activity 58
3.5 Photocatalytic degradation kinetics 59
Chapter 4: Facile synthesis of deep eutectic solvent assisted BiOCl/BiVO4@AgNWs plasmonic photocatalysts under visible light enhanced catalytic performance .61
4.1 Experimental 63
4.1.1 Synthesis of BiVO4 and BiOCl/BiVO4 by sol-gel method 63
4.1.2 Preparation of silver nanowires (AgNWs) 63
4.1.3 Deposition of BiVO4@AgNWS, BiOCl/BiVO4@AgNWs 64

4.2 Materials characterization 64
4.3 Photocatalytic activity 64
4.4 Photoelectrochemical Measurement 65
4.5 Results and discussion 65

4.5.1 XRD characterization 65
4.5.2 Band gap 66
4.5.3 SEM images 67
4.5.4 Photoluminescence property 68
4.5.5 TEM images 69
4.6 Photocatalytic MB dye degradation 69
4.6.1 The photocatalytic mechanism 72
4.7 Photoelectrochemical studies 75
4.8 Conclusion 78
Chapter 5: Facile Synthesis of Hierarchically Nanostructured Bismuth Vanadate: An …. 79 Efficient Photocatalyst for Degradation and Detection of Hexavalent Chromium
5.1 Experimental 81
5.1.1 Solvothermal preparation of BiVO4 81
5.1.2 Materials characterization 82
5.1.3 Photocatalytic experiments 83
5.1.4 Electrochemical sensing of Cr(VI) at BiVO4/SPCE 83
5.2 Results and discussion 84
5.2.1 Hydrothermal method prepared BiVO4 characterization 84
5.3 Photocatalytic chromium (VI) reduction 93
5.4 Reduction mechanism of Cr (Ⅵ) in presence of BiVO4 94
5.5 Electrocatalytic activity of BiVO4/SPCE 98
5.5.1 Effect of scan rate 99
5.5.2 Electrochemical determination of Cr(VI) by chronoamperometry ………101
5.5.3 Selectivity, stability and reproducibility of the catalyst 102
5.6 Conclusion 103
Chapter 6: Overall Conclusion 105
6.2 Introduction 107
6.2 Discussion 107
6.3 Suggestions for future work 109
Bibliography 111

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