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

WO₃ Nanoflakes for Enhanced Photoelectrochemical Conversion

We developed a postgrowth modification method of two-dimensional WO₃ nanoflakes by a simultaneous solution etching and reducing process in a weakly acidic condition. The obtained dual etched and reduced WO₃ nanoflakes have a much rougher surface, in which oxygen vacancies are created during the simultaneous etching/reducing process for optimized photoelectrochemical performance. The obtained photoanodes show an enhanced photocurrent density of ∼1.10 mA/cm² at 1.0 V <i>vs</i> Ag/AgCl (∼1.23 V <i>vs</i> reversible hydrogen electrode), compared to 0.62 mA/cm² of pristine WO₃ nanoflakes. The electrochemical impedance spectroscopy measurement and the density functional theory calculation demonstrate that this improved performance of dual etched and reduced WO₃ nanoflakes is attributed to the increase of charge carrier density as a result of the synergetic effect of etching and reducing