Boosting Visible Light Harvesting in p-Type Ternary Oxides for Solar-to-Hydrogen Conversion Using Inverse Opal Structure

P-type semiconductors based on ternary oxides have attracted wide interest owing to their earth-crust abundance and favorable optoelectronic properties. Among the p-type ternary oxides, delafossite-phase CuFeO2 has received considerable attention because it has the potential to fully harness visible light (<800 nm) owing to its narrow bandgap (1.4-1.6 eV). Despite the favorable optoelectronic properties predicted by theoretical studies, CuFeO2 photocathodes have low quantum efficiency under visible light near the bandgap edge, which is a major bottleneck for efficient solar-to-hydrogen conversion. Herein, a novel method is presented for boosting visible-light harvesting in the CuFeO2 photocathode by employing an inverse opal structure as a periodic macrostructure. The periodic macroporous structure allows exceptional near-bandgap photon harvesting, particularly within the range of 600-700 nm, owing to the enhanced light absorption due to multiple scattering together with the short diffusion distance for minority carriers toward the electrolyte. After surface modification with a low-cost double hydroxide electrocatalyst, our CuFeO2-based photocathode exhibits a record-breaking photocurrent density of 5.2 mA cm(-2) at -0.1 V with respect to the reversible hydrogen electrode for water reduction among p-type ternary oxide-based photocathodes.