In situ photodeposition of cobalt nanoparticles onto ZnIn2S4 enhancing H2 production under visible light

Abstract

Hydrogen production via semiconductor photocatalysis has been the subject of extensive research over the past four decades. Recently, much work has focused on ZnIn2S4 (ZIS), due to its ability to initiate water splitting under visible light. In this work, we report a 11-fold increase of H2 production via Co2+ addition (equivalent to 4 wt% Co on ZIS). Reaction was conducted under a 420 nm LED lamp in presence of triethanolamine. During the photocatalytic reaction, Co2+ ions were quickly reduced into metallic Co. A five-repeat test showed that the in situ Co0 nanoparticles was relatively stable. By contrast, 4 wt% Co0ex situ deposited ZIS via a (photo)chemical or physical route had a low activity, due to partial oxidation of Co0 into less active CoOx in air. Impressively, all Co-deposited ZIS samples were at least 5 times more active than 0.5 wt% Pt-loaded g-C3N4, CdS, ZnS, and In2S3, respectively. Through photoluminescence and (photo)electrochemical measurement, a plausible mechanism is proposed, involving electron transfer from ZIS to Co species, followed by proton reduction. This work shows that cobalt nanoparticles are excellent cocatalysts for H2 production on ZIS under visible light, but they are better produced in situ without involvement of O2.