Synthesis of MoS2 from [Mo3S7(S2CNEt2)3]I for enhancing photoelectrochemical performance and stability of Cu2O photocathode toward efficient solar water splitting

Abstract

Cu₂O is a typical p-type semiconductor that can efficiently absorb visible light and has a high absorption coefficient due to its narrow forbidden band. Thus, it finds potential applications in solar energy conversion and photocatalysis. However, Cu₂O photocathodes suffer from a major issue of chemical stability and sluggish proton reduction for splitting water using sunlight. We present here a facile method of coating a thin MoS₂ layer onto Cu₂O to significantly improve its stability and proton reduction efficiency. MoS₂ coating on top of Cu₂O is achieved by spin coating a [Mo₃S₇(S₂CNEt₂)₃]I precursor combined with a thermal annealing process to obtain the optimal stoichiometry. MoS₂ thin films synthesized using this method show good prospects as both a protection layer and an electrocatalyst for hydrogen evolution reactions (HER) due to excellent stability and high electrocatalytic activity. The proton reduction performance of spin-coated MoS₂/FTO electrodes is studied to determine the optimal synthesis conditions using various derivatives of MoS₂ precursors. Our study suggests that the rate-limiting kinetic step of MoS₂ synthesized in this method is the desorption of adsorbed hydrogen atoms to form molecular hydrogen, and that nanocrystalline MoS₂ with copiously exposed S edges are more active for HER. Photoelectrochemical measurements demonstrate the highest activity for 3-layered (<40 nm thick) MoS₂/Cu₂O photocathode fabricated at 450 °C with a photocurrent density of ∼6.5 mA cm⁻² at −0.2 V vs. RHE. Additionally, the MoS₂ coating helps minimize the dark current of the Cu₂O photocathode.