Photocarrier Transport Mechanisms in Amorphous and Epitaxial TiO₂/SrRuO₃ Heterojunction Photocatalysts

Heterojunction photocatalysts in thin-film form offer the possibility of improved optical absorption of solar radiation but have found limited use due to various material challenges. Epitaxial structures based on strontium ruthenate (SRO) and TiO₂ have demonstrated unexpectedly high activity under visible-light-only illumination because of strong absorption by the SRO, high electrical conductivity, and the ability to inject hot electrons into the active TiO₂ photocatalyst. The role of photoholes, the mechanisms of carrier transport to the TiO₂ surface, and the necessity of an epitaxial structure remain unclear. The present work helps to fill these gaps through rate measurements of methylene blue (MB) photooxidation on SRO-TiO₂ under visible light, together with photoemission measurements of interfacial and free surface band edges of the TiO₂. Diffusive transport of thermalized holes appears sufficient to explain the results, and surprisingly, a heterojunction based on amorphous SRO and TiO₂ retains a great deal of the metallic properties of crystalline SRO and provides MB degradation rates comparable to an equivalent heteroepitaxial structure reported previously. These findings relax considerably the constraints on translating heterojunctions based on correlated metal oxides into photocatalytic applications.