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 CuFeO ₂ 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, CuFeO ₂ 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 CuFeO ₂ 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 CuFeO ₂ -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.