PH3-Treated TiO2 Nanorods with Dual-Doping Effect for Photoelectrochemical Oxidation of Water

Elemental doping is an effective way to suppress charge recombination and modify the band gap of TiO2, therefore enhancing its photocatalytic activity. Here, we report a dual-doping method induced by a one-step novel low temperature (300 degrees C) PH3 annealing method for improving the photoelectrochemical performance of TiO2 nanorods grown on transparent conducting substrates. X-ray photoelectron spectroscopy (XPS) indicates that Ti4+ is completely converted to Ti3+ within a surface layer of TiO2 to a depth of about 20 nm following this treatment. In addition to Ti3+ self-doping, phosphorus ions in two different oxidation states (P5+ and P3-) are observed. Incorporation of these ions into TiO2 leads to an increase of 1 order of magnitude in the carrier density, resulting in faster transport and longer lifetimes of photogenerated electrons. Additionally, the valence band maximums of the PH3-treated ruffle and anatase TiO2 shift toward the direction of the Fermi level by 0.92 and 0.42 eV, respectively, together with absorption change indicating successful band gap narrowing. This doping effect gives rise to extension of absorption to the longer wavelength and enhancement of the photoactivity of TiO2 photoelectrodes under visible light. Although the PH3 treatment increases the density of surface states and thus leads to a positive shift in the photocurrent onset potential and a lower charge injection efficiency, a greatly improved photocurrent of 1.8 mA.cm(-2) at 1.23 V vs RHE (AM 1.5G, 100 mW.cm(-2)) for ruffle TiO2 is seen. This is five times higher than the photocurrent observed for undoped control samples (as compared with 0.35 mA.cm(-2) of the undoped control samples). The PH3 annealing strategy seems to be quite general and should have applications in improving the visible light photon absorptivity of other oxide semiconductors, especially those with wide band gaps.