Charge Dynamics at Surface-Modified, Nanostructured Hematite Photoelectrodes for Solar Water Splitting

The balance of the charge transfer and recombination kinetics of photoelectrodes governs the device efficiency for solar water splitting. Hematite (alpha-Fe2O3) is a photoanode typically used because of advantages such as its abundance, low cost, multiple convenient deposition methods, and an attractive bandgap energy; however, poor electrical properties prevent high solar energy to hydrogen conversion efficiencies. In this work, we evaluate and compare several strategies to address this issue, using a nanorod array morphology and incorporation of overlayers of one or more materials that favor the charge carrier transfer kinetics and reduce surface recombination. We use intensity-modulated photocurrent spectroscopy (IMPS) to evaluate these systems, and demonstrate that the presence of TiO2 and MoO x overlayers successfully suppresses surface recombination through passivation of hematite interfacial recombination sites. However, the hole transfer process at the overlayers occurs at more positive potentials due to the location of the new surface states at the overlayer-electrolyte interface. We show that the deposition of the CoPi oxygen evolution reaction co-catalyst partially addresses this disadvantage. The best efficiencies were obtained for the CoPi-TiO2/alpha-Fe2O3 and CoPi-MoO x /TiO2/alpha-Fe2O3 photoelectrodes, with internal quantum efficiencies of 0.42-0.44 under 455 nm irradiation.