Water pollution occurs when harmful substances such as chemicals contaminate a stream, river, lake, ocean, aquifer, or other body of water, degrading water quality and rendering it toxic to humans or the environment. As per a study published in “The Lancet”, water pollution had killed 1.8 million people in 2015. Xenobiotics such as nitrobenzene, nitrophenols and di-nitrotoluenes are a major chunk of water pollutants nowadays. Especially nitrophenols, used in synthesis of industrial products viz; pesticides, herbicides, petrochemicals, explosives etc. are highly stable and soluble in water. They are powerful carcinogens and also can cause liver and kidney failures in humans and other diseases in animals. In this regard, for nitrophenol pollutant remediation, catalysts are an important class of materials. Catalysts are the backbone of chemical reactions which elevate the progress of a reaction by many folds without mostly undergoing any structural or chemical changes on themselves. This unique property is utilized by us in reactions such as photocatalysis, catalytic reduction of pollutants and electrochemical analysis.
Our objective is to use the same catalyst for multiple applications of Photocatalysis, electrochemical sensor, electrochemical hydrogen and oxygen evolution and catalytic reduction. For this purpose, we chose copper-based oxides and its hybridization with carbonaceous materials since they are earth abundant (50 ppm in earth’s crust) and inexpensive. These catalysts, must be able to perform efficient photocatalysis and electrocatalysis via increasing their carrier lifetime through effective channeling of electrons throughout their composite phases.
Herein, in our first study, we synthesized CuO/g-C3N4 hybrid to mineralize 4-nitrophenol (4-NP) pollutant into non-harmful products via photocatalysis by tuning the band alignments to channel the electrons and holes appropriately. Electron lifetimes were calculated via time resolved fluorescence spectroscopy and scavenger tests were conducted to understand the mechanism of photodegradation. 92.8% of 20 ppm 4-NP could be degraded within 150 min. no reduction in degradation efficiency even up to 5 cycles. Electrochemical oxygen evolution was also demonstrated to showcase the multifunctional nature of these photocatalysts.
In our second work, we were able to synthesize copper oxides in two phases viz; CuO (Cu2+) and Cu2O (Cu+) and hybridize it with g-C3N4, simultaneously doping CuO with Ag in order to obtain better catalytic activity. Presence of binary oxidation state of copper was proved via XRD and XPS analysis results. Finally, the catalyst was utilized to convert 100 ppm 4-NP to 4-aminophenol (4-AP) in a duration of 4 minutes. Effects of catalyst loading, 4-NP concentration and external chemicals were also explored in this work. Post 4-NP reduction XRD revealed the formation of metallic copper which was converted back to Cu2O after reannealing the catalyst. The synthesized catalyst was also applied as an electrochemical sensor for neurotransmitter dopamine showcasing its effective bi-functional nature.
In our final work, Cu-cuprous/cupric oxide-based family of catalysts were synthesized via simple hydrothermal technique. These catalysts were applied towards dual application namely 4-NP conversion and electrochemical hydrogen evolution reaction (HER). Sample properties were analyzed via intensive characterizations such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction of hydrogen (H2-TPR) analysis. XRD revealed the presence of varying copper oxide phases across all the synthesized catalysts. XPS discovered the disparities between catalysts annealed at various temperatures. Reducibility of catalysts was studied via H2-TPR and they disclosed phase conversions taking place in the material thereby directly affecting the application. Cu-Cu2O (150℃) could convert 98.6 % of 50 ppm 4-NP into 4-AP with recycle tests up to 10 cycles yielding conversion values between 96.3–98.9% which are high enough for any catalyst to reach its maximum potential. CuO-Cu2O (300℃) exhibits best electrochemical hydrogen evolution activity with the least onset potential of -0.28 V vs. RHE at 1 mA cm-2 possessing a maximum current density of 366 mA cm-2. With the momentum of these works, in the future, it could be possible to use these copper-based catalysts for photocatalytic nitrogen fixation i.e., to produce ammonia from nitrogen through photocatalysis which is considered to be much cleaner than the current commercial (Haber Bosch) process.

Table of Contents
Abstract i
Acknowledgement iii
List of Publications iv
List of Figures x
List of Tables xv
List of Schemes xvi
Chapter 1: Introduction 1
1.1 Copper oxides, their properties and applications 3
1.2 Water treatment and current technology 6
1.3 Advanced oxidation process (AOP) and photocatalysis 7
1.4 Metal oxide semiconductors in photocatalysis 11
1.5 4-NP pollutant, electrochemical sensor, Oxygen evolution reaction (OER) and Hydrogen evolution reaction (HER) 12
1.6 Objectives 15
Chapter 2: Literature survey 18
2.1 Copper oxides and its composite in photocatalysis 20
2.1.1 Heterojunction 21
2.1.2 Doping 23
2.1.3 Carbonaceous 24
2.2 Copper oxides for 4-NP reduction and electrochemical dopamine sensors 27
2.3 Copper oxides in electrochemical OER and HER 29
2.4 Synthesis strategies 30
2.4.1 Sol-gel method 30
2.4.2 Hydrothermal method 31
2.5 Characterization instruments 34
2.5.1 X-ray diffractometer (XRD) 34
2.5.2 Field emission scanning electron microscope (FE-SEM) 34
2.5.3 Transmission electron microscope (TEM) 34
2.5.4 X-ray photoelectron spectroscopy (XPS) 35
2.5.5 UV-Visible Diffuse reflectance spectroscopy (UV-Vis DRS) 35
2.5.6 Photoluminescence (PL) and lifetime PL 35

Chapter 3: Photocatalytic 4-nitrophenol degradation and oxygen evolution reaction in Cl-CuO/g-C3N4 hybrids synthesized via simple sol-gel technique 37
3.1 Experimental 40
3.1.1 Synthesis of Cl-CuO by wet chemical method 40
3.1.2 Synthesis of g-C3N4 and Cl-CuO/g-C3N4 composite 41
3.1.3 Materials characterization 41
3.1.4 Photocatalytic degradation of 4-NP and electrochemical OER 42
3.2 Results and discussion 43
3.2.1 Crystalline, structural and Morphologies Studies 43
3.2.2 Optical analyses 48
3.2.3 BET (Brunauer-Emmett-Teller) surface area 52
3.2.4 XPS analysis 53
3.2.5 Photocatalytic degradation of 4-NP and mechanism 56
3.2.6 Electrochemical oxygen evolution reaction 65
3.3 Conclusion 67
Chapter 4: Ag-CuxO/g-C3N4 hybrid catalysts for the reduction of 4-nitrophenol and for electrochemical detection of dopamine 68
4.1 Experimental 71
4.1.1 Synthesis of g-C3N4 71
4.1.2 Synthesis of Cu-based oxide composites 71
4.1.3 Formation of Cu2O and CuO 71
4.1.4 Materials Characterization 73
4.1.5 Catalytic reduction of 4-NP 74
4.1.6 Preparation of electrodes and electrochemical dopamine detection 74
4.2 Results and discussion 74
4.2.1 XRD, FE-SEM and TEM 74
4.2.2 BET surface area and FTIR 79
4.2.3 X-ray photoelectron spectroscopy (XPS) 82
4.2.4 Catalytic Reduction of 4-NP 84
4.2.5 Mechanism of 4-NP catalytic reduction 88
4.2.6 Electrochemical Dopamine Sensor 91
4.2.7 Chronoamperometric response of GCE/ACCG electrode 94
4.3 Conclusion 95
Chapter 5: Cu-cuprous/cupric oxide nanoparticles towards dual application for efficient nitrophenol conversion and electrochemical hydrogen evolution 96
5.1 Experimental 99
5.1.1 Synthesis of copper/cuprous oxides 99
5.1.2 Materials Characterization 100
5.1.3 Catalytic reduction of 4-NP and electrochemical HER 101
5.2 Results and discussion 102
5.2.1 XRD and TEM 102
5.2.2 XPS analysis 105
5.2.3 BET, FTIR and H2-TPR analysis 107
5.2.4 Catalytic reduction of 4-NP 110
5.2.5 Mechanism of 4-NP reduction by Cu-150 115
5.2.6 Electrochemical HER 117
5.3 Conclusion 119
Chapter 6: Future work and final conclusions 121
6.1 Future work 123
6.2 Final conclusions 124
References 127

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