Fabrication And Photo Water Splitting Performance Optimization Of TiO₂Nanotube Arrays

Solar water splitting is an ideal method to generate renewable clean energy. TiO2nanotube arrays have great potential in photo water cleavage. However, difficulties infabricating highly regular top-open TiO2nanotube arrays still exist, the effect of phasestructure on the performance of TiO2nanotube arrays is controversial, there is little reporton the control of the thickness of barrier layer at the oxide/substrate interface. Moreover,the report solar conversion efficiencies are~1%, which is far less than the target10%. Toimprove the solar conversion efficiency, we modified the anodization method of TiO2nanotube arrays and investigated the effects of heat treatment on thephotoelectrochemical properties.The modified fabrication route of TiO2nanotube arrays is a two step anodizationmethod: a rutile protective layer is firstly formed on the Ti surface via anodization andannealing, then the TiO2nanotube arrays grew through the porous of the rutile layer inthe second anodization. The tube top is separated by the rutile layer and the growth ofnanowires due to chemical etching is avoided. The wall thickness, outer diameter andlength of the fabricated TiO2nanotube arrays are about20nm,135nm and21μm,respectively.We studied the effect of heat treatment on the phase structure of TiO2nanotubearrays. Experiment results show that the crystallinity and photo current density of TiO2nanotube arrays enhanced when the annealing temperature increased. But when theannealing temperature is higher than650℃, the content of anatase phase and the photocurrent density decreased due to anatase-to-rutile transformation. With faster cooling rate,the anatase-to-rutile transformation temperature shift to higher temperature, indicating thenucleation and growth of the rutile could be suppressed by quenching. Furthermore, fastcooling rate could also inhibit the increase of barrier layer thickness, which is benefit to the enhancement of photo current density.Element analysis by XPS showed that C could incorporate nto the lattice of TiO2nanotube arrays during anodization in organic electrolyte. UV-vis absorption test provedthat our TiO2nanotube arrays had strong absorption in both UV and visible region. TheTiO2nanotube arrays annealed at650℃for30min with oil quenching showed thehighest solar conversion efficiency of1.8%.