Monodispersed core/shell nanospheres of ZnS/NiO with enhanced H2 generation and quantum efficiency at versatile photocatalytic conditions.

This investigation is 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 varied to 8.2, 12.4, 18.2, and 24.2 nm, while the ZnS-core diameter remained stable about 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 absorption spectrum 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 showed 1.42 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). 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 experiments was also performed without addition of any additives, which showed enhanced H2 gas evolution of 1.6 mmol h-1 g-1cat under the sunlight illumination. Photoelectrochemical measurements revealed the stable photocurrent 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.

Optimization of N doping in TiO2 nanotubes for the enhanced solar light mediated photocatalytic H2 production and dye degradation.

Herein, we report the optimization of nitrogen (N) doping in TiO2 nanotubes to achieve the enhanced photocatalytic efficiencies in degradation of dye and H2 gas evolution under solar light exposure. TiO2 nanotubes have been produced via hydrothermal process and N doping has been tuned by varying the concentration of urea, being the source for N, by solid-state dispersion process. The structural analysis using XRD showed the characteristic occupancy of N into the structure of TiO2 and the XPS studies showed the existence of Ti-N-Ti network in the N-doped TiO2 nanotubes. The obtained TEM images showed the formation of 1D tube-like structure of TiO2. Diffuse reflectance UV-Vis absorption spectra demonstrated that the N-doped TiO2 nanotubes can efficiently absorb the photons of UV-Vis light of the solar light. The optimized N-doped TiO2 nanotubes (TiO2 nanotubes vs urea @ 1:1 ratio) showed the highest degradation efficiency over methyl orange dye (∼91% in 90 min) and showed the highest rate of H2 evolution (∼19,848 µmol h-1.g-1) under solar light irradiation. Further, the recyclability studies indicated the excellent stability of the photocatalyst for the durable use in both the photocatalytic processes. The observed efficiency was ascribed to the optimized doping of N-atoms into the lattices of TiO2, which enhanced the optical properties by forming new energy levels of N atoms near the valence band maximum of TiO2, thereby increased the overall charge separation and recombination resistance in the system. The improved reusability of photocatalyst is attributed to the doping-induced structural stability in N-doped TiO2. From the observed results, it has been recognized that the established strategy could be promising for synthesizing N-doped TiO2 nanotubes with favorable structural, optical and photocatalytic properties towards dye degradation and hydrogen production applications.