Hydrogen-Treated TiO₂ Nanowire Arrays for Photoelectrochemical Water Splitting

Abstract

We report the first demonstration of hydrogen treatment as a simple and effective strategy to fundamentally improve the performance of TiO₂ nanowires for photoelectrochemical (PEC) water splitting. Hydrogen-treated rutile TiO₂ (H:TiO₂) nanowires were prepared by annealing the pristine TiO₂ nanowires in hydrogen atmosphere at various temperatures in a range of 200–550 °C. In comparison to pristine TiO₂ nanowires, H:TiO₂ samples show substantially enhanced photocurrent in the entire potential window. More importantly, H:TiO₂ samples have exceptionally low photocurrent saturation potentials of −0.6 V vs Ag/AgCl (0.4 V vs RHE), indicating very efficient charge separation and transportation. The optimized H:TiO₂ nanowire sample yields a photocurrent density of ∼1.97 mA/cm² at −0.6 V vs Ag/AgCl, in 1 M NaOH solution under the illumination of simulated solar light (100 mW/cm² from 150 W xenon lamp coupled with an AM 1.5G filter). This photocurrent density corresponds to a solar-to-hydrogen (STH) efficiency of ∼1.63%. After eliminating the discrepancy between the irradiance of the xenon lamp and solar light, by integrating the incident-photon-to-current-conversion efficiency (IPCE) spectrum of the H:TiO₂ nanowire sample with a standard AM 1.5G solar spectrum, the STH efficiency is calculated to be ∼1.1%, which is the best value for a TiO₂ photoanode. IPCE analyses confirm the photocurrent enhancement is mainly due to the improved photoactivity of TiO₂ in the UV region. Hydrogen treatment increases the donor density of TiO₂ nanowires by 3 orders of magnitudes, via creating a high density of oxygen vacancies that serve as electron donors. Similar enhancements in photocurrent were also observed in anatase H:TiO₂ nanotubes. The capability of making highly photoactive H:TiO₂ nanowires and nanotubes opens up new opportunities in various areas, including PEC water splitting, dye-sensitized solar cells, and photocatalysis.