Silicon nanowire/TiO2 heterojunction arrays for effective photoelectrocatalysis under simulated solar light irradiation

The silicon nanowire (SiNW)/TiO2 heterojunction arrays have been prepared by chemical vapor depositing a TiO2 layer on SiNW arrays which are obtained by chemical etching of Si wafers. The types of SiNW/TiO2 heterojunctions can be modulated simply by using p-type or n-type Si wafers. Compared with samples of depositing TiO2 on p-type or n-type Si wafers (p-Si/TiO2 or p-Si/TiO2), both n-SiNW/TiO2 and p-SiNW/TiO2 heterojunctions can improve the ultraviolet photoresponse under a bias potential, and in particular, a remarkable visible photoresponse of n-SiNW/TiO2 heterojunction is observed when the bias potential is higher than 1.7 V (vs. SCE). Cyclic voltammetry (CV) curves illuminate that n-SiNW/TiO2 heterojunctions have steady visible photoresponse in aqueous solution. According to the surface photovoltage (SPV) measurements, it is found that n-SiNW/TiO2 heterojunctions possess window effect, namely, n-SiNW/TiO2 heterojunctions can utilize the superposition photoresponse in the wavelength ranges from 300 to 400 nm (contributed by TiO2) and from 400 to 800 nm (contributed by n-SiNW). The mechanism of this phenomenon is explained based on the energy band model. The photoelectrocatalytic activity of the n-SiNW/TiO2 heterojunction arrays is evaluated using phenol as a test substance under Xe lamp irradiation. The kinetic constant and total organic carbon (TOC) removal are 1.7 times and 2 times as great as those of n-Si/TiO2, respectively.