Reactivity and durability of TiO2 photoanodes with dominant high-energy (211) facets

Abstract

Owing to a large band gap and high surface energy determined via theoretical calculation, anatase TiO₂ with dominant (211) facets can be expected to serve as one of the candidates for inorganic photoanode materials. Unfortunately, high-energy (211) surfaces containing four- and five-coordinated Ti atoms usually diminish rapidly during the crystal growth by a thermal process as a result of the minimization of surface energy. Here we report a successful preparation of an anatase TiO₂ photoanode with dominant (211) facets on various substrates by magnetron sputtering technology. The proposed formation mechanism of (211) facets is derived from the competition between high-energy ion impinging and thermodynamics in the deposition process. The TiO₂ photoanode with over 1 μm thickness yields a record charge-separation efficiency of ∼54.4% at 0.46 V versus the reversible hydrogen electrode, corresponding to a photocurrent density of ∼0.32 mA cm⁻². The photogenerated charge carriers are directionally separated within the crystal grains by the internal magnetic field and quickly transported on the (211) surfaces with unsaturated coordination sites. Most importantly, we find that the photoanode simultaneously integrates flexible, hard and lubricant effects into photoelectrochemical behaviors, leading to long-term stable operation (over 1100 h), good durability with a low wear rate, and facile bendability (similar performance before and after bending). We also fabricate a large-area (10 × 10 cm²) photoanode and use it in an unbiased photoelectrochemical device, further suggesting its scaling and practical potential.