Controlled Sn-Doping in TiO₂ Nanowire Photoanodes with Enhanced Photoelectrochemical Conversion

We demonstrate for the first time the controlled Sn-doping in TiO₂ nanowire (NW) arrays for photoelectrochemical (PEC) water splitting. Because of the low lattice mismatch between SnO₂ and TiO₂, Sn dopants are incorporated into TiO₂ NWs by a one-pot hydrothermal synthesis with different ratios of SnCl₄ and tetrabutyl titanate, and a high acidity of the reactant solution is critical to control the SnCl₄ hydrolysis rate. The obtained Sn-doped TiO₂ (Sn/TiO₂) NWs are single crystalline with a rutile structure, and the incorporation of Sn in TiO₂ NWs is well controlled at a low level, that is, 1–2% of Sn/Ti ratio, to avoid phase separation or interface scattering. PEC measurement on Sn/TiO₂ NW photoanodes with different Sn doping ratios shows that the photocurrent increases first with increased Sn doping level to >2.0 mA/cm² at 0 V vs Ag/AgCl under 100 mW/cm² simulated sunlight illumination up to ∼100% enhancement compared to our best pristine TiO₂ NW photoanodes and then decreases at higher Sn doping levels. Subsequent annealing of Sn/TiO₂ NWs in H₂ further improves their photoactivity with an optimized photoconversion efficiency of ∼1.2%. The incident-photon-to-current conversion efficiency shows that the photocurrent increase is mainly ascribed to the enhancement of photoactivity in the UV region, and the electrochemical impedance measurement reveals that the density of n-type charge carriers can be significantly increased by the Sn doping. These Sn/TiO₂ NW photoanodes are highly stable in PEC conversion and thus can serve as a potential candidate for pure TiO₂ materials in a variety of solar energy driven applications