Fabrication And Properties Of TiO₂ Nanotube Arrays Prepared By Anodic Oxidation

Nano titania, an important inorganic functional material, finds wild use in the field of photocatalysis, dye-sensitized solar cells (DSSC), gas sensing, water photolysis, and so on. Compared with any other morphologic form of titania, TiO₂ nanotube arrays are expected to exhibit novel and improved functional characteristics, due to their higher specific surface area, stronger adsorbability and efficient electron transfer path. Thus the systematical studies on the preparation and application of TiO₂ nanotube arrays have both scientific and engineering significances.TiO₂ nanotube arrays were fabricated by anodic oxidation at a constant potential in this paper. The morphology and the structure of TiO₂ nanotubes were characterized respectively by means of FE-SEM, XRD, TEM, and so on. The effects of anodic potential, anodic time and electrolyte composition on the nanotube morphology were investigated. The results indicate that TiO₂ nanotube arrays can be regularly formed at the specific anodic voltage and anodic time. Furthermore, the pore size, wall thickness and length of the nanotube become bigger with the increasing anodic voltage, while anodic time only obviously influences the nanotube length. The appropriate electrolyte is the key aspect to achieve high-aspect-ratio TiO₂ nanotube arrays. Longer nanotubes can be formed in organic electrolytes because of their large dielectric constants and coefficient of viscosities.The anatase and the rutile crystallites from amorphous as-fabricated TiO₂ nanotube arrays can be formed inside the tube walls at elevated temperatures. The crystallization process of nanotubes is different to the other morphologic form of titania. Anatase crystallite in the tubes does not transform to rutile and its size keeps unchanged at elevated temperatures after reaching a special size of anatase crystallite. The small anatase crystallites can disappear, only following the collapse of nanotube structure.Moreover, the properties of such novel nano-architecture, used for methyl orange photolysis, the anode electrocatalyst of direct methanol fuel cells, and the photoelectrode of DSSC, were investigated individually. The TiO₂ nanotube arrays were used as photocatalyst to degrade the methyl orange. The effects of anodic potential, anodic time, annealing temperature on the photocatalysis of TiO₂ nanotube arrays were also studied. The results show that TiO₂ nanotube arrays, fabricated with anodic voltage of 20V for 20 minutes in 0.5wt%HF electrolyte and then annealed at 400℃, possess a better photo-catalytic activity, i.e. 99.6% decolourisation of 10mg/L methyl orange after irradiation with high-pressure Hg-lamp for 40 minutes.Furthermore, Pt/TiO₂ nanotube arrays/Ti composite electrodes were fabricated by direct current electrodeposition with or without ultrasonic vibration. The results show that both ultrasonic vibration and nanotube array itself can promote the dispersion of Pt particles onto the electrode. The Pt/TiO₂ nanotube arrays/Ti composite electrode with ultrasonic vibration is of the best electrocatalytic property, while the Pt/TiO₂ nanotube arrays/Ti composite electrode without ultrasonic vibration is better than the Pt/Ti composite electrode. Anyway, the three kinds of composite electrodes have better electrocatalytic properties than Pt electrode with the same working area. Their high electrocatalytic activities for methanol oxidation can be attributed to the high dispersion of Pt particles.Finally, TiO₂ nanotube arrays were successfully used as photoelectrodes to build DSSC. The specific surface area of TiO₂ nanotube arrays has an important influence on the photovoltaic properties of the cells. It is found that the dye-sensitized solar cell (1cm×1cm), composed with TiO₂ composite nanostructures prepared in 1vol% water/0.25wt% NH₄F/glycol electrolyte with an anodic voltage of 20V for 2h and annealed at 450℃, exhibits the best performance with the short-circuit photocurrent density (J_sc) 3.53mA/cm², the open-circuit photovoltage (V_oc) 0.81V, and the photo-current conversion (η) 1.508%.