Preparation Of TiO₂ Nanoring/nanotube Hierarchical Structure And Their Photoelectrochemcial Properties

The development and utilization of clean and renewable energy is the essential way to solve energy and environmental problems.Hydrogen is considered to be an ideal energy.Solar water splitting for hydrogen evolution is a pollution-free process,which will become the dominant way of hydrogen production in the future.It is clear that TiO₂is one of the most promising candidates for photoelectrochemical hydrogen production due to its less-expensive,innoxious and stable chemical properties.Among TiO₂ based nanostructures,TiO₂ nanotube arrays possess high electron mobility with the existence of its unique one-dimensional tubular structure,which can effectively improve the separation of electron hole pairs.In this work,based on the advantage of one-dimensional tubular structure,TiO₂ nanoring/nanotube hierarchical structure is successfully prepared.By tuning the structure of nanorings and nanotubes,the growth mechanism is discussed,and the effects of the structure parameters of this special structure on its photoelectrochemcial properties are mainly investigated.During anodic oxidation process,nanograss structure may be generated on the top of TiO₂ nanotube arrays.This is because the electrolyte can accelerate the growth rate of nanotube,leading to the splitting of the top of nanotubes,and eventually resulting in the nanograss structure attached to the surface.TiO₂ hierarchical structure with nanorings and nanotubes can be fabricated by two step anodic oxidation process.By tuning the oxidation time and oxidation voltage in the second step anodization,TiO₂nanoring/nanotube hierarchical structure with different surface morphologies can be prepared.The formation of TiO₂ nanoring/nanotube hierarchical structure is significantly affected by the surface nanostructure of titanium substrate.After the TiO₂nanotube arrays grown in the first step anodization are removed by ultrasonic treatment,the growth of nanotubes is limited by the regular hexagonal nanocaves on the surface of Ti substrate during the second step of anodization,resulting in TiO₂nanoring/nanotube hierarchical structure.Meanwhile,the nanorings can provide the support of their internal nanotubes,avoiding the formation of the nanograss structure.TiO₂ nanoring/nanotube hierarchical structure shows oscillating peaks in visible light region due to the uniform refractive index and smooth surface.The oscillating peaks are derived from the interference of light which is reflected from the top nanorings and the bottom Ti substrate through the film of nanotubes.The oscillation frequency of the multiabsorption peaks increases and their amplitude decreases with increasing film thickness,and the film thickness where the oscillating peaks disappeared can be considered as the optical penetration depth.Based on the relationship between the oscillation peaks shape and the film thickness,the maximum optical penetration depth of TiO₂ nanoring/nanotube hierarchical structure is estimated to be about 2μm.There exist some Ti³⁺,oxygen vacancies and C,N elements in the TiO₂ lattice,and all samples show enhanced light absorption after annealing.During the process of photoelectrochemical hydrogen production,the light absorption performance and the generation and transport process of photogenerated electrons for TiO₂ electrode are mainly affected by its surface morphology and structural characteristics.When the diameter of nanorings is 100 nm,the optimum tube length of TiO₂ nanoring/nanotube hierarchical structure is about 1.5μm,and the optimum diameter of nanotubes is 20⁴0 nm.At the bias of 0 V（vs.Ag/AgCl）,the photocurrent density of the sample with optimum structural parameters is 0.85 mA/cm².The photocurrent densities of TiO₂ nanoring/nanotube hierarchical structure generated by visible light are in the region of 0.015⁰.025 mA/cm².The visible light activity is more related to the visible light absorption which induced by Ti³⁺,oxygen vacancies and C,N elements in TiO₂ materials.