Ultrathin CoO_x-modified hematite with low onset potential for solar water oxidation

Photoelectrochemical water splitting holds great potential for solar energy conversion and storage with zero greenhouse gas emission. Integration of a suitable co-catalyst with an absorber material enables the realization of highly efficient photocleavage of water. Herein, nanostructured hematite film was coated with an ultrathin and conformal CoO_x overlayer through atomic layer deposition (ALD). The best performing hybrid hematite with a 2-3 nm ALD CoO_x overlayer yielded a remarkable turn on potential of 0.6 V_RHE for the water oxidation reaction. Moreover, material analyses revealed that the surface amorphous CoO_x/Co(OH)₂ component exhibited good optical transparency and hydrophilic properties, which were beneficial for the formation of an ideal hematite/electrolyte interface. In addition to the presence of the CoO_x overlayer, a negative shift of flat band potential (V_FB) as well as suppression of surface recombination helped to significantly promote the charge separation and collection properties, contributing to the overall solar conversion efficiency. As a result, the external quantum efficiency (IPCE) obtained on hematite increases by 66% at 1.23 V_RHE.