Fabrication of Hierarchical Two-Dimensional MoS2 Nanoflowers Decorated upon Cubic CaIn2S4 Microflowers: Facile Approach to Construct Novel Metal-Free p-n Heterojunction Semiconductors with Superior Charge Separation Efficiency

Due to the enormous demand for effective conversion of solar energy and large-scale hydrogen production, cost-effective and long-lasting photocatalysts are believed to be necessary for global production of sustainable and clean hydrogen fuel. Robust and highly efficient p-n heterojunction photocatalysts have a striking ability to enhance light-harvesting capacity and retard the recombination of photoexcitons. A series of p-MoS2/n-CaIn2S4 heterojunction composites with different MoS2 contents have been synthesized via a facile two-step hydrothermal technique in which rose-like p-MoS2 nanoflowers are decorated upon n-type cubic CIS microflowers. In the synthesis protocol highly dispersed MoS2 nanoflowers provided more active edge sites for the growth of c-CIS nuclei, leading to a hierarchical architecture with intimate interfacial contact. The formation of a hierarchical flower-like morphology of the photocatalyst has been established by an HRTEM and FESEM study. Electrochemical characterization, especially the slope of the curve from Mott-Schottky analysis and nature of the current from LSV, reveals the p-n heterojunction nature of the composite photocatalyst. The fabricated heterojunction photocatalysts were further examined for visible light photocatalytic H2 evolution. Far exceeding those for the neat c-CIS and MoS2, it is seen that the p-MoS2/n-CIS heterojunction photocatalyst with an optimum content of MoS2 exhibited enhanced H2 evolution using a 0.025 M Na2S/Na2SO3 solution as hole quenching agent under visible light illumination. The 0.5 wt % p-MoS2/n-CIS photocatalyst presents a higher H2 production rate of 602.35 μmol h-1 with 0.743 mA cm-2 photocurrent density, 19 times and 8 times higher than those of neat c-CIS, respectively. This superior photocatalyic activity is due to the efficient separation of electron-hole charge carriers at the interface, as supported by a photoluminescence study and EIS measurements.