Synergistic hydrazine-driven regulation and Mo/S co-doping to endow BiOBr with heterovalent molybdenum states and abundant oxygen vacancy defects for photocatalytic hydrogen evolution†

Herein, we demonstrate a Mo/S co-doped BiOBr-based bimetal bismuth sulfur-oxybromide (Mo/S-BiOBr) catalyst with heterovalent molybdenum states and abundant oxygen vacancy defects for photocatalytic hydrogen evolution (PHER) via a facile method. Mo/S co-doping adjusts the energy band structure of BiOBr and expands its visible light absorption. Hydrazine regulates the molybdenum with heterovalent states while endowing Mo/S-BiOBr with oxygen vacancy defects to balance the valence-charge deviations from electrical neutrality induced by Mo⁶⁺ → Mo⁴⁺. These oxygen-vacancy defects act as active sites for capturing water molecules and activating the H–O–H bond to produce protons for hydrogen generation. The heterovalent Mo⁶⁺/Mo⁴⁺ states act as photogenerated electron hosts to hop fast between Mo⁶⁺ and Mo⁴⁺, facilitating efficient electron transfer for the PHER. The hybridization between S 3p and O 2p orbitals improves the stability of continuous PHER. The hydrazine-regulated Mo/S-BiOBr-3 with an optimal n(Mo⁴⁺)/n(Mo⁴⁺ + Mo⁶⁺) ratio and abundant oxygen vacancy defects exhibit an excellent PHER activity of 710.5 μmol h⁻¹ at a catalyst weight of 50 mg and an apparent quantum efficiency (AQE) of 13.9% at 420 nm. After six recycles, the H₂ yield of Mo/S-BiOBr-3 decreased by only about 3.5%, indicating its good stability and durability. This work provides a practical approach to using bismuth-based oxyhalides in the PHER.