ИСТИНА

Converting the solar energy into the hydrogen via photoelectrochemical (PEC) water splitting on the semiconductor materials appears to be an effective method to solve the energy problems in the future. TiO2, Fe 2O3, and WO3, have been studied as potential candidates for photoanode materials in PEC cells most extensively due to their good stability in aqueous electrolyte under continuous solar radiation and their relative abundance in nature. Despite titania has a big bandgap and adsorbs only UV fraction of the sunlight it is still promising material for photoanodes due to its high stability in aqueous solutions and low cost. In recent years considerable efforts have been made to synthesize vertically-oriented, highly-ordered titania nanorods and nanotubes. Among various methods, electrochemical anodization is considered as relatively simple technique for preparation of titania films consisted of highly oriented nanotubes with tunable structural parameters. It is very important that one-dimensional highly ordered nanostructure provides a unidirectional electrical channel for charge transfer, so that photoinduced electron-hole pairs can be effectively separated by external electric field. It is worth noting, that an amorphous structure of as-prepared anodic titanium oxide films contains large number of defects that leads to high electron-hole recombination rates. In the same time the photocurrent is determined by the efficiency of photogenerated electron/hole transfer at the semiconductor/electrolyte and semiconductor/surface interface. Therefore crystallization of as-prepared amorphous film is required for producing high effective photoanodes for PEC. The crystallization of titania film by heat treatment leads to curling up and breaking the oxide film. This can be explained by different crystallization rates of the barrier and the porous layers. Thus crystallization of titania films on metallic substrates demands soft methods. In this case hydrothermal treatment is one of the most appropriate method to solve crystallization problems. Here we report on a soft crystallization method for preparation crystalline titania nanotubes array by hydrothermal treatment of as-prepared amorphous titania film on Ti substrate without curling up and breaking of oxide film. The hydrothermal treatment leads to structural transformation from amorphous titania film consisted of nanotubes with smooth external and internal walls to tubular like structure composed of anatase nanoparticles with average size of 21 nm ± 5 nm. Photoelectrochemical measurements were performed without adding of redox species and showed pure real-life efficiency of the material for water splitting. The maximum incident photon to current efficiency was observed at 250 nm and to 6.5 % at 1.2 V bias potential applied vs. Ag/AgCl reference electrode, while as-prepared anodized Ti substrate exhibits no photoactivity.