Solar-Driven Photocatalytic CO2 Reduction in Water Utilizing a Ruthenium Complex Catalyst on p-Type Fe2O3 with a Multiheterojunction

A hybrid photocathode that consists of a ruthenium complex catalyst and a p-type semiconductor composed of earth-abundant elements, N,Zn-codoped Fe 2 O 3, with a multiheterojunction structure (TiO 2 /N,Zn-Fe 2 O 3 /Cr 2 O 3 ) was developed for the reduction of CO 2 in aqueous solution with application of an electrical bias under simulated solar light irradiation. The TiO 2 layer prevents contact between N,Zn-Fe 2 O 3 and the electrolyte, so that dissolution of N,Zn-Fe 2 O 3 by photoelectrochemical (PEC) self-reduction cannot occur. Both TiO 2 and Cr 2 O 3 significantly enhanced the cathodic photocurrent by tuning the band bending in N,Zn-Fe 2 O 3 . The use of a Ru complex with an electronic network provided by polypyrrole improved the performance and resulted in a stable photocurrent of 150 μA cm –2 for the production of HCOOH, CO, and a small amount of H 2 under 1 sun irradiation with application of 0.1 V vs the reversible hydrogen electrode (RHE). The total amount of generated HCOOH, CO, and H 2, two-electron-reduction products, was equal to half the amount of photogenerated electrons. The functional combination of the hybrid iron-based photocathode with a reduced SrTiO 3 (SrTiO 3– x ) photoanode realized stoichiometric solar CO 2 reduction coupled with the water oxidation reaction without an external electrical bias. The solar to chemical energy conversion efficiency was 0.15%, which is comparable to that of a reported tandem system using a Ru complex/single-crystalline InP photocathode.