| Nowadays, one of man's endeavors is to cope with the energy crisis and environmental pollution for the sustainable development of human society, in which the energy problem is vital. Solar and hydrogen energy are considered as the'Green Energy'which is most likely to gradually replace fossil fuels. Currently, the conversion of solar directly to electrical energy and the hydrogen production from water splitting by solar are the two most promising research directions. It is absolutely vital to develop the efficient, low-cost and practical photoelectrode materials.In the past several decades, TiO2-based nanostructure semiconductors have been extensively studied due to their excellent chemical stability, nontoxicity, low-cost, and high photocatalytic activities. They were considered as particularly versatile materials with technological application prospects in solar cell and hydrogen production from water photolysis, and more. Especially over the past few years, highly ordered, vertically oriented TiO2 nanotube arrays fabricated by electrochemical anodization constitute a material architecture that offers a large specific surface area, favorable surface chemistry and narrow distribution of diffusion path not only for entering the tubular depth but also for species to be transported through the tube wall, have attracted tremendous efforts. In the present work, TiO2-based nanotubular oxide layers have been fabricated on the surfaces of metal Ti, TC1 (Ti-2Al-1.5Mn) alloy and TC4 (Ti-6Al-4V) alloy by a two-electrode electrochemical anodic oxidation technique. The preparation and properties were investigated carefully. At the same time, S-doped TiO2 nanotubular films were achieved by a facile thermal vulcanization process, in which the photoelectric properties were significantly improved. Under optimum conditions, furthermore, nanotubular films fabricated on the surface of TC1 alloy and the S-doped TiO2 nanotube arrays have been found to have the ability of water photolysis. The main contents of the paper were listed as follows:1. Self-organized TiO2 nanotube arrays were prepared by a facile two-electrode anodization in a 0.5wt% NH4F aqueous electrolyte at 20V for 3h with the average inner diameter of about 91nm and the tube length about 720nm. In the current system, the best polar distance (between anode and cathode) was about 3cm and the optimal temperature of the electrolyte was the normal room temperature (～18℃). The as-formed nanotubes under the optimized conditions were annealing at different temperature to obtain different crystalline structures. The tests of optical absorption and photoelectrochemical properties showed that the nanotubes had best properties after annealed at 600℃.The photoconversion efficiency was about 1.07% under 100 mW/cm2 illuminations. TiO2 nanotube arrays can also be fabricated in a 0.5wt%NH4F + 0.1MH3PO4 aqueous electrolyte at 20V for 1h with the average inner diameter of about 100nm and the tube length about 830nm. Having been annealed at different temperatures, the nanotubular structure retained intact when the temperature was lower than 600℃. When the annealing temperature was 700℃, the local damage could be found. And the nanotubular structure would be completely destroyed into a porous granule membrane when the annealing temperature was higher than 800℃. The tests of optical absorption and photoelectrochemical properties showed that the nanotubes had best properties after annealed at 600℃.The photoconversion efficiency was about 0.95%. Under their respective optimum conditions, as compared with the acidic electrolyte system, the TiO2 nanotube arrays prepared in the neutral system had better photoelectrochemical properties. This was because that the nanotubes prepared in the neutral electrolyte system have better uniformity and ordered character. In addition, the electrochemical oxidation of Ti was faster in the acidic electrolyte system, which would increase the thickness of'barrier layer'at the bottom of nanotubes in some degree, thus inhibiting the transmission of photo-induced carriers. In the present system, the optimal conditions for the TiO2 nanotube arrays were in the 0.5wt% NH4F electrolyte, anodizing voltage of 20V, oxidation time of 3h, polar distance of 3cm, and electrolyte temperature of room temperature, and the annealing temperature of 600℃.2. TiO2-based nanotube arrays were prepared on TC1 alloy for the first time by a facile two-electrode anodization in a 0.5wt% NH4F aqueous electrolyte at 20V for 3h, with the average inner diameter of about 90nm, the tube length about 650nm and the tube wall thickness of 21nm.The main growth mechanism governing the formation of nanotubes on TC1 alloy was suggested to be consistent with those of tubes synthesized on pure Ti in F– containing solutions on the whole. Nanotubes formation in fluoride ion bearing electrolytes occurred as a result of the interplay between three simultaneously occurring processes, namely the field assisted oxidation of alloy to form oxides, the field assisted dissolution of metal ions in the electrolyte, and the chemical dissolution of metal and oxides due to etching by fluoride ions, which was substantially enhanced by the presence of H+ ions. And the formation of nanotube arrays were ultimately determined by the dynamic equilibrium between the growth and dissolution processes. As the alloy contained two different phases, it was important to restrain the selective dissolution of the phases, which could result in non-uniform surface layer. It was profitable to select subacid electrolyte and to optimize anodization voltage and time. In the current system, the best polar distance was about 3cm and the optimal temperature of the electrolyte was the normal room temperature. The tests of optical absorption and photoelectrochemical properties showed that the nanotubes had best properties after annealed at 600℃. The photoconversion efficiency was about 1.2%. Nanotubular layers can also be fabricated on TC1 alloy in a 0.5wt%NH4F + 0.1MH3PO4 aqueous electrolyte at 20V for 1h with the average inner diameter of about 85nm. By annealing the initially amorphous films at different temperatures, the nanotubular structure retained intact when the temperature was lower than 650℃. And the nanotubular structure would be completely destroyed into a rod-like film when the annealing temperature was higher than 800℃. The tests of optical absorption and photoelectrochemical properties showed that the nanotubes had best properties after annealed at 650℃. The photoconversion efficiency was about 0.79 %. Compared with the neutral electrolyte, nanotube arrays prepared in the acidic system had poor performance. This was because that the nanotubular layers prepared in the acidic electrolyte system were relatively lack of uniformity and the'barrier layer'may thicker. In addition, since the more serious selective dissolution of the two phase of TC1 alloy, some holes would be formed in the wall of the nanotubes, which might seriously affect the transmission process of photo-induced carriers.3. Nanotubular films were fabricated on TC4 alloy in a 0.5wt%NH4F + 0.1MH3PO4 aqueous electrolyte. The nanotubes anodiced at 20V for 1h were in highly ordered with the average inner diameter of about 120nm, the wall thickness of 17nm and the tube length about 300nm. EDX and XPS analysis showed that metal ion doping could not be achieved. In the current system, the optimal electrolyte temperature was the normal room temperature. The thermal stability studies showed that the annealing temperature should be lower than 650℃. When the temperature was higher than 700℃, the nanotubular structure was completely transformed into rod-like granule membrane. The photoelectrochemical properties of as-prepared nanotubular films were rather poor. However, given that as-prepared nanotubes were in highly ordered, it was expected to have broad application prospects in the traditional application area of TC4 alloy.4. S-doped TiO2 nanotubular films were achieved by a facile thermal vulcanization process. The best pre-annealing temperature was 600℃and the optimum S-doped temperature was 550℃. The studies had shown that the main way of the present sulfur doping was S atoms to replace the O vacancies in the TiO2 Crystal lattices. The possible doping mechanism was that the surface of TiO2 nanotube arrays were partly reduced by H2 resulting in oxygen vacancy in the first under the appropriate temperatures, and then, S atoms occupied these oxygen vacancies. This processes finally achieved the sulfur-doped structures. Having been doped, the nanotubular structure had not been seriously affected and the optical absorption in the visible area had been markedly increased. The photoconversion efficiency was about 1.59%. The photoelectrochemical properties of S-doped TiO2 nanotube arrays were significantly improved. It was because that the range of effective optical absorption was largely expanded and the photoconversion efficiency was also improved remarkably.5. The results of open-circuit potential and photocurrent response tests of the TiO2-based nanotube arrays prepared at different conditions were consistent with their respective photoelectrochemical properties. The fast photocurrent response could be obtained for the nanotube arrays prepared on pure Ti and TC1 alloy. Having been doped by sulfur, the photocurrent response performance was further enhanced. The fast photoelectric response indicated less recombination of photogenerated electrons and holes and higher transmission efficiency of photo-induced carriers, which were vital to the application in the field of photovoltaic conversion.6. The initial experimental studies have shown that the nanotubular films prepared on TC1 alloy in the neutral electrolyte under optimal conditions have some capacity of photoelectrochemical hydrogen production from water splitting. Furthermore, the S-doped TiO2 nanotube arrays prepared at optimal conditions have been found to have remarkable ability of water photolysis, which are expected to become the excellent photoelectrode materials for water splitting by solar.
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