الفهرس 
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Abstract
Student Name: Heba Elsayed Ali Hassan
Title of the thesis: Synthesis and characterization of nanostructured modified TiO2 arrays as photoanodes for solar energy conversion
Degree: Ph.D. in Science (Chemistry)
In recent years, the increase in world population and industrial activities has led to accelerated energy consumption and unabated release of hazardous pollutants release to the air and water sources, which lead to pollution-related diseases and global warming. These aroused our awareness of the urgency of initiating an alternative renewable energy sources that would deliver the least environmental negative impacts.
Sun planet radiation is the immense source of renewable energy and can potentially be used to satisfy the world’s increasing energy demand. However, intensive efforts are being perused to develop new and high performance solar energy conversion systems. Such conversion systems require novel photosensitive materials to assist proper function of the photovoltaic cells, or chemical energy processes in the form of hydrogen using photoelectrochemical (PEC) cells.
The present work focuses on developing materials used for the solar energy conversion to hydrogen through the process of PEC water splitting. PEC water splitting process is aimed to produce hydrogen and oxygen through the use of solar irradiation of a semiconductor material that present in water is considered as an ideal method for renewable energy production and storage. Hydrogen can be efficiently used to produce electric power using the proper fuel cell or it can be directly driven to an internal combustion engine. Advantageously, hydrogen gas produces only water upon oxidation which in turn has no hazardous exhaust of greenhouse gasses.
The key component for the PEC water splitting system is the proper and efficient semiconductor photocatalyst. TiO2-based photosensitive materials are considered as promising candidate for the photoelectrochemical generation system. However, most of the TiO2 photocatalysts are presented in powdered form, thus limits their practical or industrial applications due to the ease of hydrogen flammability unless the system is made more complex by using compressors and separating units to effectively separate hydrogen and oxygen mixtures away from reactors. Therefore, in recent years, considerable research were adopted for the replacement of the powder material with a 1D crystalline semiconductor photoanodes. Photoanodes are characterized by enhanced electron transport, higher surface area, in addition to the safety aspect of producing hydrogen and oxygen at an opposite electrodes.
To further improve the solar energy conversion efficiency, developing of TiO2 thin films can operate not only under UV radiation, but also under visible light, where the band gap of TiO2 lies in the UV regime at 3 eV for rutile, and 3.2 eV for anatase. Therefore, could be activated by a small fraction of the energy of the Sun (4%). Thus, efforts were presented by scientists to modify TiO2 in order to narrow its band gap. Consequently, the light in the visible spectrum, which represents 45% of the solar spectrum, can also be adsorbed to activate the splitting process. Thereby, allowing more efficient conversion of sunlight to chemical fuels. To this end, extensive efforts are directed towards various modification strategies of photoanodes, such as preparation of photoanode which consists of several semiconductors that vary in their band gaps so as to utilize the advantage of the entire solar spectrum.
With these objectives in mind, we prepared different TiO2 arrays (nanowires and nanotubes) and modified them with various materials, including tungsten oxide, cobalt oxide, rutile TiO2, and cadmium sulfide. After that, these modified and unmodified TiO2 arrays were examined as photoanodes in PEC water splitting system.
The thesis is structured into three chapters. Chapter 1 provides a brief introduction and literature review in solar hydrogen production; such as pathways for conversion of solar energy to hydrogen, the basic principles of PEC cell, properties of high performance semiconductor photoanodes, efficiency of PEC device, and synthesis of one dimensional TiO2. Chapter 2 deals with materials, synthesis methods and conditions, experimental setup, and characterization tools applied in our work. The results and discussion of the synthesized materials and their performance as photoanodes in PEC cells are given in Chapter 3, which is divided into 6 parts dealing with the following aspects:
Part (1): TiO2 nanorod arrays modified with tungsten oxide
TiO2-WO3 nanorod arrays were synthesized on fluorine doped tin oxide (FTO) substrates via a template-free process, hydrothermal procedure combined with electrodeposition. The designed photoelectrodes were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), and photoelectrochemical measurements. The study demonstrated that the WO3 deposition interval time (0, 10, 20, 40, 60 min) can significantly affect the photoelectrochemical performance and the amount of hydrogen generated. The optimal deposition period was 20 min, which is sufficient for homogeneous coating the TiO2 nanorods that enhanced the photoconversion efficiency of the TiO2-WO3 array by 60% compared to the pure TiO2 array. The enhanced electrode efficiency was attributed to the efficient charge separation and reduction of the electron-hole pair recombination rate.
Part (2): Vertically aligned TiO2 nanotube arrays modified with tungsten oxide & synthesized in a two-step process
Titanium nanotube arrays modified with tungsten oxide (W-oxide TNTAs) were synthesized via a two-step process, namely, electrochemical oxidation of titanium foil and electrodeposition of W-oxide followed by annealing. The designed photoelectrodes were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), UV-Vis diffuse reflectance spectroscopy, and photoelectrochemical measurements. W-oxide deposition interval time of 1, 2, 3, 5, 20 min was varied to improve the photoelectrochemical performance and the amount of hydrogen produced. The optimal deposition time was 2 min, which improved the photoconversion efficiency by 30% compared to the pure TNTA. In addition, it was noted that pure TNTA acquire higher photoconversion efficiency than pure TiO2 nanorod array; this may be attributed to its higher surface, highly ordered vertically aligned tubular structure, and anatase phase is catalytically more active than rutile phase. For these reasons, other modifications will be carried out on TNTAs only.
Part (3): Vertically aligned TiO2 nanotube arrays modified with tungsten oxide & synthesized in a one-step process
TiO2 nanotube arrays modified with tungsten oxide (W-oxide TNTAs) were synthesized via one-step process; one pot-anodic oxidation of pure titanium substrate in the presence of ammonium fluoride and tungsten salt subsequently calcined in air. The designed arrays were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), UV-Vis diffuse reflectance spectroscopy, and photoelectrochemical measurements. The results of the present study concluded that the concentration of tungsten salt at 3, 5, 10, 15, and 25 mM significantly affect the photoelectrochemical performance of TNTAs. The optimal amount was 5 mM, which is sufficient to increase the photoconversion efficiency by 30% relative to pure TNTA. It is noticed that using a one-step process for the preparation of W-oxide TNTAs achieves the same enhancement in the photoconversion efficiency as that of using a two-step process. Moreover, it provides a novel and a more economic pathway to prepare photoanode material for the large scale and can also be extended to the synthesis of TiO2 nanotube arrays modified with other metal oxides, as it will be illustrated in the next part.
Part (4): Vertically aligned TiO2 nanotube arrays modified with cobalt oxide & synthesized in a one-step process
Vertically aligned TiO2 nanotube arrays modified with cobalt oxide (Co-oxide TNTAs) were synthesized via one-pot anodic oxidation of pure titanium substrate in the presence of ammonium fluoride and cobalt salt. After a subsequent step of annealing in air, the produced arrays acquired a tubular structure doped with Co-oxide that was confirmed by Energy dispersive X-ray (EDX) spectroscopy. The Tauc plot, estimated from UV–Vis diffuse reflectance spectra, reveals that the insertion of the optimum amount of Co-oxide led to a decrease in the band gap of TiO2 from 3.2 to 2.9 eV. The influence of various cobalt salt concentrations in the electrolyte solution (5, 10, 15, 20 and 25 mM) on the TNTA morphology was studied. The evaluation of the photocurrent and photoconversion efficiency was performed for all the fabricated electrodes. Morphological studies illustrated that the addition of cobalt salt with small concentration had no an obvious effect on the ordered tubular structure of TNTA, whereas, at higher concentrations the tubular structure was partially distorted. Incorporation of Co-oxide was recognized to enhance the photoconversion efficiency of TNTA electrode by 30% at its optimum concentration.
Part (5): Vertically aligned anatase TiO2 nanotube arrays modified with rutile TiO2
Efficient photoanodes were designed of aligned anatase TiO2 nanotube arrays (anatase TNTAs) decorated with different shaped rutile TiO2 (particles, 1D nanorods, 3D microflowers) to improve the photoelectrochemical water oxidation performance of pristine TNTAs. Anatase TNTAs were prepared by anodic oxidation of Ti substrate in NH4F electrolyte, and the rutile weight percent and shape were controlled by tuning the TiCl4 treatment time (20-120 min). The influence of treatment time on the morphology, crystal structure, band gap, photocurrent, and photoconversion efficiency was characterized by FESEM, HRTEM, XRD, UV-Vis diffuse reflectance spectroscopy, and photoelectrochemical measurements. XRD data confirmed that TiCl4 treatment induced the formation of the rutile phase of TiO2 over anatase TNTAs and the rutile amount in the TNTAs was increased upon increasing the treatment time. Also, the band gap of TNTAs was gradually decreased by increasing the rutile percent. The optimum treatment time was 80 min, which produces TiO2 photoanode array that possess the following characteristics; rutile/anatase mixed phase, nanorods/nanotubes mixed morphology, 3.09 eV band gap, two times increase in photocurrent, and 125% enhancement in the photoconversion efficiency relative to pure TNTA.
Part (6): Vertically aligned TiO2 nanotube arrays modified with cadmium sulfide
TiO2 nanotube arrays modified with cadmium sulfide (TNTAs/CdS) were synthesized, via a combination of anodic oxidation of Ti substrate and chemical bath deposition (CBD) of CdS, to improve the photoelectrochemical properties of TNTAs. The prepared electrodes were characterized by FESEM, HRTEM, EDX, XRD, UV-Vis diffuse reflectance, and photoelectrochemical measurements. EDX examination confirmed the incorporation of CdS in the modified TNTAs. The UV-Vis diffuse reflectance spectra showed that the deposition of CdS led to a decrease in the band gap of TiO2, respectively, from 3.2 eV to 2.16 eV and 2.57 eV with increasing the deposition cycles from 0 to 2 and 6 cycles. The influences of changing the deposition cycles of CdS (2, 4, 6, 8 and 10 cycles) on the morphology, the photocurrent, and the photoconversion efficiency of TNTAs were investigated. Morphological analysis illustrated that 2 deposition cycles cause the formation of CdS particles distributed over the ordered tubular structure of TNTAs, whereas, 6 deposition cycles resulted in a progressive thick growth of CdS. Photoelectrochemical measurements demonstrated that the 6 cycles deposited CdS layer provided the optimal sample with the highest photocurrent and photoconversion efficiency, which reached about four times improvement as compared to that of pure TNTA.
Thus, the current thesis provides a clear vision on how the morphology, selection of modifier, and its amount aid the generation of highly efficient photoanodes for photoelectrochemical water splitting. The appropriate dopant and its amount have two rules inhibit the recombination of electron-hole pairs and facilitates the absorption of visible light by narrowing the absorption band gap.
Furthermore, the prepared arrays at the optimum conditions can also potentially be used as photoanodes in other solar energy conversion technology (solar cells) as well as in photocatalytic systems.
Keywords: Titanium dioxide; nanotube arrays; rutile phase; potentiostatic anodization; WO3; TiCl4 treatment; CdS; water splitting; hydrogen energy; photoelectrochemical properties; optical properties; XRD