Hierarchical 3D ZnIn2S4/graphene nano-heterostructures: their in situ fabrication with dual functionality in solar hydrogen production and as anodes for lithium ion batteries

Abstract

Hierarchical 3D ZnIn₂S₄/graphene (ZnIn₂S₄/Gr) nano-heterostructures were successfully synthesized using an in-situ hydrothermal method. The dual functionality of these nano-heterostructures i.e. for solar hydrogen production and lithium ion batteries has been demonstrated for the first time. The ZnIn₂S₄/Gr nano-heterostructures were optimized by varying the concentrations of graphene for utmost hydrogen production. An inspection of the structure shows the existence of layered hexagonal ZnIn₂S₄ wrapped in graphene. The reduction of graphene oxide (GO) to graphene was confirmed by Raman and XPS analyses. The morphological analysis demonstrated that ultrathin ZnIn₂S₄ nanopetals are dispersed on graphene sheets. The optical study reveals the extended absorption edge to the visible region due to the presence of graphene and hence is used as a photocatalyst to transform H₂S into eco-friendly hydrogen using solar light. The ZnIn₂S₄/Gr nano-heterostructure that is comprised of graphene and ZnIn₂S₄ in a weight ratio of 1 : 99 exhibits enhanced photocatalytically stable hydrogen production i.e. ∼6365 μmole h⁻¹ under visible light irradiation using just 0.2 g of nano-heterostructure, which is much higher as compared to bare hierarchical 3D ZnIn₂S₄. The heightened photocatalytic activity is attributed to the enhanced charge carrier separation due to graphene which acts as an excellent electron collector and transporter. Furthermore, the usage of nano-heterostructures and pristine ZnIn₂S₄ as anodes in lithium ion batteries confers the charge capacities of 590 and 320 mA h g⁻¹ after 220 cycles as compared to their initial reversible capacities of 645 and 523 mA h g⁻¹, respectively. These nano-heterostructures show high reversible capacity, excellent cycling stability, and high-rate capability indicating their potential as promising anode materials for LIBs. The excellent performance is due to the nanostructuring of ZnIn₂S₄ and the presence of a graphene layer, which works as a channel for the supply of electrons during the charge–discharge process. More significantly, their dual functionality in energy generation and storage is quite unique and commendable.