Ultrathin CoO-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 overlayer through atomic layer deposition (ALD). The best performing hybrid hematite with a 2-3 nm ALD CoO overlayer yielded a remarkable turn on potential of 0.6 V for the water oxidation reaction. Moreover, material analyses revealed that the surface amorphous CoO/Co(OH) 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 overlayer, a negative shift of flat band potential (V) 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.