Fabrication And Formation Mechanism And Photoelectrochemical Performance Of TiO₂ Nanotubes

Due to the unique physicochemical properties, large specific surface areas and direct pathways for charge transport, TiO2 nanotube arrays have been widely used in photoelectrochemical water splitting system. However, the photoelectrochemical water splitting performance of TiO2 nanotube is significantly affected by its diameter size and morphology. The reported diameter size of TiO2 nanotube is between 20nm and 300nm, the comprehensive conclusion of the diameter effect on photoelectrochemical water splitting performance of TiO2 nanotube is unavailable. Herein, the formation mechanism of TiO2 nanotube was discussed, and the factors affecting the nanostructures of TiO2 nanotube arrays have been investigated in detail. The TiO2 nanotube arrays obtained at high voltage have a fluctuation diameter sizes and disordered surface, thus, the UV nanoimprinting technology assisted anodization was used to improve the shortcomings.Firstly, the triple-layered TiO2 nanotube arrays were fabricated by changing the electrolytes during the electrochemical anodizing processes. The current-time curves of the second time anodization and the fourth time anodization are hardly clarified by the field-assisted dissolution or chemical dissolution model. However, the relationships between the special triple-layered TiO2 nanotube arrays and the anodizing current-time curves can be clarified by the electronic current and oxygen bubble model. We conclude that the formation of nanopores during the anodizing process is related to the generation of electronic current and oxygen bubble mold effect; this conclusion will provide theoretical guidance for the fabrication of large diameter TiO2 nanotube arrays.Secondly, Fabrication and photoelectrochemical water splitting performance of large diameter TiO2 nanotube arrays. The factors affecting TiO2 nanotubes’diameter and morphology were researched in detail, including fluoride concentration, water content and voltage. The results show that the diameter of TiO2 nanotube arrays did not always increase with the water content, when the water content increase to 13 vol%, the self-ordered nanotubes cannot be obtained by anodization; the diameter of TiO2 nanotube reach their maximum at a voltage of 170 V, and the increase of TiO2 nanotubes’diameter is beneficial to their photoelectrochemical water splitting performance; fluoride concentration has little effect on diameter of TiO2 nanotubes, however, properly increase fluoride concentration will improve the photoelectrochemical water splitting performance of the TiO2 nanotubes.Moreover, a simple method was used to enlarge the pore size of the TiO2 nanotubes, the as-prepared TiO2 nanotubes were dipped in ethylene glycol solution containing 3 vol%HF, the inner layer of TiO2 nanotubes is easy to be dissolved, so the pore size of TiO2 nanotube arrays were enlarged, the photoelectrochemical water splitting results show that the increase of TiO2 nanotubes’ pore size is beneficial to its photoelectrochemical water splitting performance.Finally, we first report the fabrication of TiO2 nanotube arrays by UV imprinting assisted anodizaion. The results indicate that the patternings on Ti foil will guide the growth of TiO2 nanotube. In comparison with the TiO2 nanotube arrays obtained by anodizing the smooth Ti foil, the TiO2 nanotube arrays prepared by anodizing the patterned Ti foil have ordered surface and uniform diameter size.