Preparation,Modification And Photoelectrochemical Hydrogen Generation Performance Of TiO₂ Nanowire Arrays

Vertically aligned TiO2 nanowire array is one of the most promising semiconductor materials for solar water splitting. However, the quantum efficiency of TiO2 extremely depressed by its large band gap and high electron-hole recombination. Focusing on these disadvantages, we prepared a serious TiO2 nanowire array with different topography parameters by varying the preparation condition to optimize its morphology for photoelectrochemical performance. The optimized TiO2 nanowire arrays are subsequently modified by various methods including the semiconductor composition, catalyst surface modification and the coupling of semiconductor composition and catalyst surface modification. The detail content includes the following four aspects:(1) TiO2 nanowire arrays with different topography parameters are synthesis by varying the preparation condition including the hydrothermal temperature,hydrothermal time, concentration of Ti4+ and additive in hydrothermal process. The photoelectrochemical performance of the as-synthesized TiO2 nanowire arrays are investigated to optimized the morphology of TiO2.(2) Type-II TiO2/CdS, TiO2/CdTe and TiO2/CdSTe hetero-structured core/shell nanowire arrays on FTO substrates are synthesized via physical vapor deposition of CdS, CdTe and the alloyed CdSTe layer onto the hydrothermally pre-grown TiO2 nanowire arrays. Their morphologies, microstructures, and optical properties are characterized in detail. As photoanode for photoelectrochemical hydrogen generation, the ternary CdSTe alloy sensitized TiO2 nanowire arrays exhibits a photocurrent density of 4.52 mA cm-2 under 1 sun illumination, much higher than that of the TiO2/CdS(2.97 mA cm-2) and TiO2/CdTe(0.46 mA cm-2) electrodes. This highest photocurrent density level of the alloy TiO2/Cd STe electrode is attributed to the broadened light absorption range and enhanced charge separation efficiency according to the optical and electrochemical impedance spectra investigation.(3) Cobalt phosphate(Co-Pi) is photo-electrodeposited on TiO2 nanowire arrays in Co2+ containing phosphate buffer. The resulting composite photoanode shows a generally enhanced photocurrent near the flat band potential region, and represents a 2.3 times improved photo-conversion efficiency compared to that of pristine TiO2 in a neutral electrolyte. A negative effect on the photocurrent generation is also observed when loading TiO2 with a relatively thick Co-Pi layer, which is demonstrated to be due to the poor photo-hole transfer kinetics in the Co-Pi layer. Moreover, we find that Co-Pi can facilitate the photoelectrochemical performance of TiO2 over a wide pH range from 1-14. This improved activity is studied in detail by optical and electrochemical analyses. It is suggested that the mechanism of the over potential demanding water oxidation reaction is changed to a facile pathway by the Co-based catalyst.(4) The Co-Pi catalyst stabilized, CdS sensitized TiO2 nanowire arrays for non-sacrificial solar water splitting are reported. This triple TiO2/CdS/Co-Pi hybrid photoanode exhibits much enhanced photocurrent density and negatively shifts in onset potential, resulting in 1.5 and 3.4 times improved photo-conversion efficiency compared to the TiO2/CdS and TiO2 photoanode, respectively. More importantly, the TiO2/CdS/Co-Pi shows significantly improved photoelectrochemical stability compared to the TiO2/CdS electrode, with about 72% of the initial photocurrent retained after 2 hours irradiation.