Energy and environmental issues have been the most concern of the worldwide in this recent decade. Due to extremely utilization of fossil energy have been causing the environmental issues like global warming and energy crisis. In the near future, the global energy needs must be obtained from the renewable and sustainable resources. Hydrogen is as a great candidate fuel to replace fossil fuels in the future since it has high energy density, zero-emission, and renewability. Unfortunately, the major hydrogen production still obtained from fossil by using steam reforming and gasification techniques. These techniques not only use unrenewable resource but also emit the carbon dioxide as the waste product. Therefore, in this particular research the green and sustainable hydrogen production is performed using photocatalytic method. This research also provides green chemical conversion of 4-nitrophenol to 4-aminophenol without using any sacrificial reagents. The first chapter of this dissertation briefly gives the introduction including background of study, hydrogen production techniques and 4-nitrophenol reduction. Then, the next chapter provides the basic theory and clear literature review of present progress in this particular research topic. This dissertation includes four works as follows.
In the first work, we apply the doping concept to enhance the hydrogen production rate of Zn(O,S) with nickel as a dopant. High efficient hydrogen evolved Ni-doped Zn(O,S) photocatalyst with different Ni amounts had been successfully synthesized with a simple method at low temperature of 90C. Our Ni-doped Zn(O,S) catalyst reached the highest hydrogen generation rate of 14,800 µmole g-1·h-1, which was 2.3 times higher compared to the TiO2/Pt used as a control in this work. It was found that a small amount of Ni doped into Zn(O,S) nanoparticles could increase the optical absorbance, lower the charge transfer resistance, accordingly decrease the electron-hole recombination rate, and significantly enhance the photocatalytic hydrogen evolution reaction (HER). The as-prepared catalyst has the characteristics of low cost, low power consumption for activating the catalytic HER, abundant and environmental friendly constituents, and low surface oxygen bonding for forming oxygen vacancies. The photocatalytic performance of Ni-doped Zn(O,S) was demonstrated with a proposed kinetic mechanism in this work.
In the second work, we design surface modification of Zn(O,S) by coating thin layers of graphene oxide (GO). This work demonstrates a feasible synthesis method of Zn(O,S)/GO nanocomposite with graphene oxide (GO) to serve as an inexpensive cocatalyst. Raman spectra and transmission electron microscopy (TEM) images clearly verified that GO was successfully loaded on the surface of Zn(O,S). This GO layer could effectively decrease the charge transfer resistance and promote the charge carrier separation for enhancing hydrogen production rate. By optimizing the GO content, the best hydrogen production rate of 6,400 µmol/g·h was achieved under 16W 352 nm UV lamp with Zn(O,S)/0.5 wt% GO catalyst, which showed about two times higher than GO-free Zn(O,S). The effect of sacrificial reagent on the hydrogen production rate of Zn(O,S)/0.5 wt% GO catalyst was also evaluated. The sacrificial reagent showed the efficiency with the following trend: ethanol > methanol > isopropanol > ethylene glycol. We consider the simple synthesis method of Zn(O,S)/GO and its low cost have a great potential for practical application.
In the third work, the p-type Ag2O and n-type Zn(O,S) were loaded on mesoporous silica to form SiO2/Ag2O/Zn(O,S) with the nano p-n heterojunction to improve the efficiency of photocatalytic hydrogen evolution reaction (HER). The photocatalysts were systematically characterized to identify their properties. Through the optimization of the Zn(O,S)-loaded amount and position of p-Ag2O, the highest hydrogen production rate of 9,200 µmol. g-1cat.h-1 was achieved by SiO2/Ag2O/Zn(O,S)-0.6 catalyst, which was about 2.7 times higher than pure Zn(O,S). By placing n-Zn(O,S) of diodes outwards was proposed for the electron-rich part to enhance the reduction reaction, while placing p-Ag2O inwards for the hole-rich part to modulate the electron concentration and establish the built-in electrical nano field. Our design reveals that p-n heterojunction was superior and efficient for enhancing its properties and generating hydrogen.
In the fourth work, the conversion of 4-nitrophenol as toxic pollutant and hazardous waste to be 4-aminophenol as non-toxic and useful compound by photocatalytic reduction was conducted. The solid solution and doping concept was combined to synthesis earth- abundant and green material Mn-doped Zn(O,S) by a simple and facile method. Different Mn contents doped Zn(O,S) was easily synthesized at low temperature of 90C for 4-NP reduction without using the reducing agent NaBH4. The Mn-doped Zn(O,S) catalyst exhibited the enhancement optical and electrochemical properties of un-doped Zn(O,S). It was found that 10% Mn doped Zn(O,S) had the best properties and it could totally reduce 4-NP after 2h photoreaction under low UV illumination. The hydrogen ion was proposed for reduction 4-NP to involved for the 4-NP reduction to 4-AP which is the hydrogen ion and electron replaced the oxygen in amino group to form the nitro group. We proposed the incorporation of Mn in Zn side in the Zn(O,S) host lattice could make the oxygen surface bonding weak to easily form the oxygen vacancy. As more oxygen vacancy, more hydrogen ion would be generated to consume for 4-NP reduction.

Energy and environmental issues have been the most concern of the worldwide in this recent decade. Due to extremely utilization of fossil energy have been causing the environmental issues like global warming and energy crisis. In the near future, the global energy needs must be obtained from the renewable and sustainable resources. Hydrogen is as a great candidate fuel to replace fossil fuels in the future since it has high energy density, zero-emission, and renewability. Unfortunately, the major hydrogen production still obtained from fossil by using steam reforming and gasification techniques. These techniques not only use unrenewable resource but also emit the carbon dioxide as the waste product. Therefore, in this particular research the green and sustainable hydrogen production is performed using photocatalytic method. This research also provides green chemical conversion of 4-nitrophenol to 4-aminophenol without using any sacrificial reagents. The first chapter of this dissertation briefly gives the introduction including background of study, hydrogen production techniques and 4-nitrophenol reduction. Then, the next chapter provides the basic theory and clear literature review of present progress in this particular research topic. This dissertation includes four works as follows.
In the first work, we apply the doping concept to enhance the hydrogen production rate of Zn(O,S) with nickel as a dopant. High efficient hydrogen evolved Ni-doped Zn(O,S) photocatalyst with different Ni amounts had been successfully synthesized with a simple method at low temperature of 90C. Our Ni-doped Zn(O,S) catalyst reached the highest hydrogen generation rate of 14,800 µmole g-1·h-1, which was 2.3 times higher compared to the TiO2/Pt used as a control in this work. It was found that a small amount of Ni doped into Zn(O,S) nanoparticles could increase the optical absorbance, lower the charge transfer resistance, accordingly decrease the electron-hole recombination rate, and significantly enhance the photocatalytic hydrogen evolution reaction (HER). The as-prepared catalyst has the characteristics of low cost, low power consumption for activating the catalytic HER, abundant and environmental friendly constituents, and low surface oxygen bonding for forming oxygen vacancies. The photocatalytic performance of Ni-doped Zn(O,S) was demonstrated with a proposed kinetic mechanism in this work.
In the second work, we design surface modification of Zn(O,S) by coating thin layers of graphene oxide (GO). This work demonstrates a feasible synthesis method of Zn(O,S)/GO nanocomposite with graphene oxide (GO) to serve as an inexpensive cocatalyst. Raman spectra and transmission electron microscopy (TEM) images clearly verified that GO was successfully loaded on the surface of Zn(O,S). This GO layer could effectively decrease the charge transfer resistance and promote the charge carrier separation for enhancing hydrogen production rate. By optimizing the GO content, the best hydrogen production rate of 6,400 µmol/g·h was achieved under 16W 352 nm UV lamp with Zn(O,S)/0.5 wt% GO catalyst, which showed about two times higher than GO-free Zn(O,S). The effect of sacrificial reagent on the hydrogen production rate of Zn(O,S)/0.5 wt% GO catalyst was also evaluated. The sacrificial reagent showed the efficiency with the following trend: ethanol > methanol > isopropanol > ethylene glycol. We consider the simple synthesis method of Zn(O,S)/GO and its low cost have a great potential for practical application.
In the third work, the p-type Ag2O and n-type Zn(O,S) were loaded on mesoporous silica to form SiO2/Ag2O/Zn(O,S) with the nano p-n heterojunction to improve the efficiency of photocatalytic hydrogen evolution reaction (HER). The photocatalysts were systematically characterized to identify their properties. Through the optimization of the Zn(O,S)-loaded amount and position of p-Ag2O, the highest hydrogen production rate of 9,200 µmol. g-1cat.h-1 was achieved by SiO2/Ag2O/Zn(O,S)-0.6 catalyst, which was about 2.7 times higher than pure Zn(O,S). By placing n-Zn(O,S) of diodes outwards was proposed for the electron-rich part to enhance the reduction reaction, while placing p-Ag2O inwards for the hole-rich part to modulate the electron concentration and establish the built-in electrical nano field. Our design reveals that p-n heterojunction was superior and efficient for enhancing its properties and generating hydrogen.
In the fourth work, the conversion of 4-nitrophenol as toxic pollutant and hazardous waste to be 4-aminophenol as non-toxic and useful compound by photocatalytic reduction was conducted. The solid solution and doping concept was combined to synthesis earth- abundant and green material Mn-doped Zn(O,S) by a simple and facile method. Different Mn contents doped Zn(O,S) was easily synthesized at low temperature of 90C for 4-NP reduction without using the reducing agent NaBH4. The Mn-doped Zn(O,S) catalyst exhibited the enhancement optical and electrochemical properties of un-doped Zn(O,S). It was found that 10% Mn doped Zn(O,S) had the best properties and it could totally reduce 4-NP after 2h photoreaction under low UV illumination. The hydrogen ion was proposed for reduction 4-NP to involved for the 4-NP reduction to 4-AP which is the hydrogen ion and electron replaced the oxygen in amino group to form the nitro group. We proposed the incorporation of Mn in Zn side in the Zn(O,S) host lattice could make the oxygen surface bonding weak to easily form the oxygen vacancy. As more oxygen vacancy, more hydrogen ion would be generated to consume for 4-NP reduction.

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