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 Fe2O3, with a multiheterojunction structure (TiO2/N,Zn-Fe2O3/Cr2O3) was developed for the reduction of CO2 in aqueous solution with application of an electrical bias under simulated solar light irradiation. The TiO2 layer prevents contact between N,Zn-Fe2O3 and the electrolyte, so that dissolution of N,Zn-Fe2O3 by photoelectrochemical (PEC) self-reduction cannot occur. Both TiO2 and Cr2O3 significantly enhanced the cathodic photocurrent by tuning the band bending in N,Zn-Fe2O3. The use of a Ru complex with an electronic network provided by polypyrrole improved the performance and resulted in a stable photocurrent of 150 mu A 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 SrTiO3 (SrTiO3-x) photoanode realized stoichiometric solar CO2 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.