Transition-Metal\nDoping of Oxide Nanocrystals for\nEnhanced Catalytic Oxygen Evolution

Abstract

Catalysts for the oxygen reduction\nand evolution reactions are\ncentral to key renewable-energy technologies including fuel cells\nand water splitting. Despite tremendous effort, the development of\noxygen electrode catalysts with high activity at low cost remains\na great challenge. In this study, we report a generalized sol–gel\nmethod for the synthesis of various oxide nanocrystals (TiO₂, ZnO, Nb₂O₅, In₂O₃,\nSnO₂, and Ta₂O₅) with appropriate\ntransition metal dopants for an efficient electrocatalytic oxygen\nevolution reaction (OER). Although TiO₂ and ZnO nanocrystals\nalone have little activity, all the Mn-, Fe-, Co-, and Ni-doped nanocrystals\nexhibit greatly enhanced OER activity. A remarkable finding is that\nCo dopant produces higher OER activity than the other doped metals.\nX-ray photoelectron and X-ray absorption spectroscopies revealed the\nhighly oxidized metal ions that are responsible for the enhanced catalytic\nreactivity. The excellent OER activity of the Co-doped nanocrystals\nwas explained by a synergistic effect in which the oxide matrix effectively\nguards the most active Co dopants at higher oxidation states by withdrawing\nthe electrons from the metal dopants. The metal-doped NCs exhibit\nenhanced catalytic activity under visible light irradiation, suggesting\ntheir potential as efficient solar-driven OER photoelectrocatalysts.