Investigations of Interactions Between Thin Metal Catalyst Films and a-TiO₂ Photoelectrode Protection Layers Through Synchrotron

Photoelectrochemical cells based on semiconductor-liquid interfaces provide a solution of converting solar energy to electricity or fuels. Heterojunctions between TiO₂ and small-band-gap semiconductors have been shown to be stable against photocorrosion while in contact with 1.0 M KOH(aq) and under simulated solar illumination. However, conduction through the amorphous TiO₂ (a-TiO₂) films has been shown to be strongly dependent on the top contact. While metallic Ir and Ni have mutually similar overpotentials in alkaline media towards oxygen evolution reaction (OER), a-TiO₂/Ir requires higher overpotential than a-TiO₂/Ni to achieve similar current densities. We describe herein a detailed investigation of the a-TiO₂/M junctions (M=Ni, Ir, Au) to study the interaction of buried protection layer with OER catalysts using XPS and resonant Photoemission techniques. Upon deposition of Ni, Ir, and Au the electronic structure of the TiO₂ support is modified. The band-energy diagrams and interfacial hole conduction mechanism through a-TiO₂ to the deposited metal catalysts is realized. Whereas both Ni and Ir produce band bending in the a-TiO₂ favoring hole conduction, only Ni creates multiple states within the a-TiO₂ band gap at the a-TiO₂/Ni interface, which produces a quasi-metallic interface at the a-TiO₂/Ni junction. Au, however, produces a flat-band interface that limits hole conduction without any new band gap states. The interfacial chemistry, device physics, and photoelectron spectroscopic insights provide directions for improving the energy-conversion performance.