Type-II Heterojunction CdIn₂S₄/BiVO₄ Coupling with CQDs to Improve PEC Water Splitting Performance Synergistically

Bismuth vanadate (BiVO₄) has been considered as a promising photoelectrocatalytic (PEC) semiconductor, but suffers from severe hole recombination, attributed to the short hole-diffusion length and the low carrier mobility. Herein, a type-II heterojunction CdIn₂S₄/BiVO₄ is designed to improve the photocurrent density from 1.22 (pristine BiVO₄) to 2.68 mA cm^-2 at 1.23 V vs the reversible hydrogen electrode (RHE), accelerating the bulk separation of photogenerated carriers by the built-in field from the matched energy band. With the introduction of CQDs, CQDs/CdIn₂S₄/BiVO₄ increases the photocurrent density to 4.84 mA cm^-2, enhancing the light absorption and cathodically shifting its onset potential, due to the synergetic effect of the heterojunction and CQDs. Compared with BiVO₄, CQDs/CdIn₂S₄/BiVO₄ promotes the bulk separation efficiency to 94.6% and the surface injection efficiency to 72.2%. Additionally, spin-coating of FeOOH on CQDs/CdIn₂S₄/BiVO₄ could further improve the PEC performance and keep a long stability for water splitting. The density function theory (DFT) calculations illustrated that the type-II heterojunction CdIn₂S₄/BiVO₄ could decrease the oxygen evolution reaction (OER) overpotential and accelerate bulk charge separation for the built-in field of the aligned band structure.