Effect of defect-rich Co-CeOx OER cocatalyst on the photocarrier dynamics and electronic structure of Sb-doped TiO2 nanorods photoanode

Abstract

This work demonstrates the in-depth mechanism of enhanced photoelectrochemical (PEC) water oxidation of Sb-doped rutile TiO2 nanorods (NRs) photoanode coupled with oxygen vacancy defect-rich Co-doped CeOx (Co-CeOx) oxygen evolution reaction (OER) cocatalyst. The defect-rich Co-CeOx cocatalyst modification improves the conductivity, light absorption, charge transfer efficiency, and surface photovoltage generation of the Co-CeOx/Sb-TiO2 hybrid NRs photoanode. The Co-CeOx cocatalyst also serves as the surface passivating overlayer for the Sb-TiO2 photoanode, which suppresses the surface states mediated recombination of electron-hole pairs in the NRs. The PEC studies further indicate that Co-CeOx cocatalyst induces remarkably large band bending at the semiconductor/electrolyte interface and shortens the carrier diffusion length and depletion layer width, facilitating the rapid separation and transportation of the photocarriers for the surface PEC reactions. The experimental and theoretical studies confirm that the Co-doping in CeOx cocatalyst enhances the surface oxygen vacancy defects, which provides active catalytic sites for OH− adsorption and charge transportation for enhanced OER kinetics. The density functional theory (DFT) calculations demonstrate a higher conductivity of the Co-CeOx cocatalyst, advantageous for rapid charge transfer capability during PEC reactions. The synergy between all these merits helps the optimized Co-CeOx/Sb-TiO2 photoanode to deliver a maximum photocurrent density of 1.41 mA cm−2 at 1.23 V vs. reversible hydrogen electrode (VRHE) and an ultra-low turn on potential (Von) of 0.1 VRHE under AM 1.5G solar illumination compared to the Sb-TiO2 NRs (0.96 mA cm−2 at 1.23 VRHE and Von = 0.42 VRHE). This work demonstrates the design of an efficient defect-rich cocatalyst modified photoanode for solar energy harvesting.