MoS2/CQDs obtained by photoreduction for assembly of a ternary MoS2/CQDs/ZnIn2S4 nanocomposite for efficient photocatalytic hydrogen evolution under visible light

Abstract

MoS₂/CQDs (CQDs stands for carbon quantum dots) with a mass ratio of MoS₂/CQDs of 1 : 5 were obtained via photoreduction of (NH₄)₂MoS₄ over irradiated CQDs in the presence of methanol as a sacrificial agent. Self-assembly of ZnIn₂S₄ microspheres on the surface of MoS₂/CQDs was realized under microwave conditions to obtain ternary MoS₂/CQDs/ZnIn₂S₄ microspheres. The ternary MoS₂/CQDs/ZnIn₂S₄ nanocomposite shows significantly superior photocatalytic activity for hydrogen evolution to both MoS₂/ZnIn₂S₄ and CQDs/ZnIn₂S₄, indicating the existence of a synergistic effect of MoS₂ and CQDs in promoting the hydrogen evolution. It was proposed that the transfer of the photo-generated electrons from the semiconductor-based photocatalyst (ZnIn₂S₄) to the hydrogen evolution cocatalyst (MoS₂) is promoted by CQDs acting as a mediator in the ternary MoS₂/CQDs/ZnIn₂S₄ nanocomposite. An optimum performance was observed over a 3.0 wt% MoS₂/CQDs/ZnIn₂S₄ nanocomposite in the presence of Na₂S/Na₂SO₃ as a sacrificial agent, where 750 μmol of hydrogen was evolved in 5 h. The optimum apparent quantum efficiency (AQE) was determined to be 25.6% at a wavelength of 420 nm, which is among the highest AQEs ever reported for ZnIn₂S₄-based systems. This study provides a strategy for developing highly efficient photocatalysts for hydrogen evolution via using CQDs as a bridge to promote charge transfer in nanocomposites.