Mixed-Phase ZnIn2S4 Nanosheets Grown on TiO2 Nanotrees for the Visible-Light Photocatalytic Degradation of Organic Dyes

A broad-band, environmentally friendly multijunction photocatalyst, composed of mixed-phase ZnIn2S4 in situ grown on TiO2 nanotree powders (NtPs), was prepared via a facile hydrothermal method. The unique structure of the TiO2 NtPs provided a large surface area for the easy growth of ZnIn2S4 and for the efficient adsorption of reactants. The introduced ZnIn2S4, possessing a homojunction structure enabled by two closely interlaced crystalline phases, endowed the ZnIn2S4/TiO2 composite with an enhanced visible-light response and boosted charge separation between the two phases. Moreover, the in situ growth of ZnIn2S4 on TiO2, forming a heterojunction at the interface, rendered an intimate contact and strong interaction, which was favorable for the efficient charge transfer between the two components. These advantages all together resulted in the significantly enhanced visible-light photocatalytic activity in the degradation of methyl orange. The charge-transfer dynamics and pathways were studied by performing photoluminescence measurements and in situ irradiated X-ray photoelectron spectroscopy analysis. The active species involved in photocatalysis were also explored by carrying out trapping experiments. Based on these results, the possible mechanism for the enhanced photocatalytic activity was proposed and discussed. This work highlights the great potential of developing broad-band and efficient ZnIn2S4- and TiO2-based photocatalysts through simultaneously constructing both a homojunction and a heterojunction for remediating environmental pollution and provides a mechanistic understanding of photocatalysis in a type-II, multijunction hybrid.