Using TiO₂ as a Conductive Protective Layer for Photocathodic H₂ Evolution

Abstract

Surface passivation is a general issue for Si-based photoelectrodes because it progressively hinders electron conduction at the semiconductor/electrolyte interface. In this work, we show that a sputtered 100 nm TiO₂ layer on top of a thin Ti metal layer may be used to protect an n⁺p Si photocathode during photocatalytic H₂ evolution. Although TiO₂ is a semiconductor, we show that it behaves like a metallic conductor would under photocathodic H₂ evolution conditions. This behavior is due to the fortunate alignment of the TiO₂ conduction band with respect to the hydrogen evolution potential, which allows it to conduct electrons from the Si while simultaneously protecting the Si from surface passivation. By using a Pt catalyst the electrode achieves an H₂ evolution onset of 520 mV vs NHE and a Tafel slope of 30 mV when illuminated by the red part (λ > 635 nm) of the AM 1.5 spectrum. The saturation photocurrent (H₂ evolution) was also significantly enhanced by the antireflective properties of the TiO₂ layer. It was shown that with proper annealing conditions these electrodes could run 72 h without significant degradation. An Fe²⁺/Fe³⁺ redox couple was used to help elucidate details of the band diagram