Microwave-assisted hydrothermal synthesis of transition-metal doped ZnIn2S4 and its photocatalytic activity for hydrogen evolution under visible light

Aiming at the enhancement of photocatalytic activity for hydrogen evolution over ZnIn2S4, different transition metals (Cr, Mn, Fe, Co) are doped into the lattices of ZnIn2S4 to narrow the band gap. The doped ZnIn2S4 is characterized by XRD, Raman, UV-vis spectra, photoluminescence spectra, SEM and XPS techniques. The photocatalytic evaluation shows that Mn-doped ZnIn2S4 performs photocatalytic activity 20% higher than undoped ZnIn2S4, while Cr-, Fe-, and Co-doped ZnIn2S4 perform poorer activities in an order of Cr > Fe > Co. Based on the combined characterization results, the band structures of doped ZnIn2S4 are schematically depicted, which illustrates the different effects of transition-metal doping on the photocatalytic activity for hydrogen evolution. For Mn-doped ZnIn2S4, the enhancement of photocatalytic activity could be due to narrowed band gap induced by Mn doping. However, for Cr-, Fe-, and Co-doped ZnIn2S4, the suppressed photocatalytic activities should be attributed to the dopant-related impurity energy levels localizing the charge carriers or acting as non-radiative recombination centers for photoexcited electrons and holes. Hence, this study indicates that it is of great importance to make the in-depth investigation on the effects of band structures on the photocatalytic activity, especially for the doped semiconducting photocatalysts. (C) 2011 Elsevier B.V. All rights reserved.