Rational synthesis of MnCdS for enhanced photocatalytic H evolution: Effects of S precursors and the feed ratio of Mn/Cd on its structure and performance.

Rational synthesis of photocatalytic materials is an effective way to improve their performance. In this work, to optimize the S precursors, a series of MnCdS (MCS) were first hydrothermally synthesized with the prevalent thiourea (TA), thioacetamide (TAA) and L-cysteine (L-Cys) as the S sources. The optimum feed ratio of Mn/Cd was then determined based on the optimized S precursor. The effects of S precursors and the feed ratio of Mn/Cd on the phase structure, absorption, morphology, band structure, and the photocatalytic hydrogen evolution reaction (HER) performance of MCS were investigated systematically. The hexagonal phase structures of MnS, CdS, and MCS are favored by TA and L-Cys as the S sources, while their cubic phases are benefited by TAA. TAA is the preferred S source for the preparation of highly active MCS and the solid solution is formed through the consolidation of cubic α-MnS into cubic CdS. The activity of MCS can be improved with the increase of Mn content from x = 0-0.6. The sample with x = 0.6 shows the highest HER activity (2253 μmol·h·g) and the performance is almost 6 times higher than CdS (416 μmol·h·g). The enhanced activity can be attributed to the improved separation efficiency of photo-induced charge carriers and the negative-shifts of E, which are induced by the introduction of Mn. A segregation of inert α-MnS from MCS is occurred when Mn content is >0.6, resulting in a decay of the HER activity. A change of the semiconductivity from n-type to bipolar type is occurred in MCS due to the uneven sulfidation of Mn in MCS.