Title : Monodispersed core/shell nanospheres of ZnS/NiO with enhanced H2 generation and quantum efficiency at versatile photocatalytic conditions
Hydrogen gas is a cleaner fuel and generates electrical energy in proton exchange membrane fuel cells with high efficiency and the best alternative energy for alternative 1. The present investigation is the first to elucidate the synthesis of mono-dispersed ZnS/NiO-core/shell nanostructures with a uniform thin layer of NiO-shell on the ZnS-nanospheres as a core under controlled thermal treatments. NiO-shell thickness was varied to be 8.2, 12.4, 18.2, and 24.2 nm, while the ZnS-core diameter was fixed as 96±6 nm. The crystalline phase and core/shell structure of the materials were confirmed using XRD and HRTEM techniques, respectively. Optical properties through UV-vis spectroscopy analysis revealed the manifestation of red shift in the optical properties of core/shell materials, while the XPS analysis of elements elucidated their stable oxidation states in ZnS/NiO core/shell structure. The optimized ZnS/NiO-core/shell showed1.42 times higher H2 generation (162.1 mmol.h-1.g-1cat) than the pristine ZnS-core (113.2 mmol.h-1.g-1cat) and 64.5 times higher than the pristine NiO-shell (2.5 mmol.h-1.g-1cat) owning to the many reasons (a) uniform core-shell structure (b) monodispersed nanospheres, (c) high surface area and more number of active sites and (d) Vacant 3d orbital. The quantum efficiency at wavelengths of 420, 365 nm, and 1.5 G air mass filters was found to be 13.5, 25.0, and 45.3%, respectively. Water splitting was also performed without adding any additives, which resulted in an enhanced H2 gas evolution of 1.6 mmol.h-1.g-1cat under sunlight illumination. Photo-electrochemical measurements revealed a stable current density and minimized charge recombination in the system. The performed recyclability and reusability tests for five recycles demonstrated the excellent stability of the developed photocatalysts. Based on the characterization properties we proposed the plausible photocatalytic water splitting mechanism for superior photocatalytic hydrogen generation under solar light irradiation as shown in Figure, here majorly three steps occur i.e, (i) photon energy absorption (ii) charge carrier migration, and (iii) redox reaction NiO shell thickness protects from the photo-corrosion and charge carrier recombination, finally give good yield hydrogen fuel conversion efficiency.
Keywords: Hetero junction, Nanohybrid, photocatalyst, Wurtzite, and cubic.