Photo-oxidation of Water on the 3C-SiC Surface: A First-principles Study

Abstract

Oxidation of water on the 3C-SiC(111) surface is studied using density functional theory calculations. A four-step oxidation reaction is investigated and a reaction energy landscape is modeled from the free energy of the reaction intermediates. The reaction is studied on the unreconstructed Si surface of the 3C-SiC, which in the (111) cleaving is composed of alternating layers of carbon and silicon atoms. Convergence tests were carried out, first on bulk structure and then on the slab, to obtain a suitable computational model. The results indicates that the photo-induced water oxidation reaction requires an additional potential of 0.47 V, which, if true would prevent the use of SiC as photoanode for this reaction. The limiting reaction intermediate is the formation of a bare surface site by desorption of HOO. The calculations suggests that this overpotential is caused by the high surface energy of the bare surface. However, by covering all sites that does not take part in the reaction with hydroxyl groups the overpotential was lowered with about 1 V. As the photoanode is in contact with water, a hydroxyl covered surface is actually a possible structure. Hence, the last result provides a possibility that the overpotential can come down to reasonably values. Future studies should focus on this finding.