Assembly of nonstoichiometric molybdenum oxide on Si as p-n junction photocathode for enhanced hydrogen evolution

•Compositionally tuned MoOX was successfully deposited on Si photocathode as catalyst.•Oxygen-deficient MoOx catalyst on p-Si form p-n junction with appropriate band edge alignment for water splitting.•Proposed catalyst induces a higher carrier density and superior charge separation.•MoOX/Si exhibits enhanced photocurrent density of ∼ -30 mA cm−2 at -0.5 V vs RHE.