Electrochemical analysis of thermally treated two dimensional zinc sulphide hexagonal nano-sheets with reduced band gap

Abstract

Significantly reduced band gap (2.0 eV) zinc sulphide (ZnS) nanoparticles are synthesized using microwave/ultraviolet/ultrasonic assisted hydrothermal route in a two step process. Initially, ZnS nanospheres are synthesized, showing a cubic structural phase with a band gap of 3.2 eV, which is further annealed at 1000 °C to get the final product. Following annealing, the cubic ZnS undergoes a phase change to wurtzite ZnS, in the form of hexagonal nanosheets showing polymorphism phenomena, along with a reduced band gap of 2 eV. The optical analysis reveals a red shift in the absorbance region, transitioning from the absorption of UV radiations in cubic ZnS to visible radiations in wurtzite ZnS. Thermogravimetric (TGA) measurements and its analysis also revealed the phase change of ZnS (cubic) to ZnS (Wurtzite) when heated at 1000 °C. Microstructural analysis reveals the formation of sheets oriented along (100) plane, which is evidenced by the interplanar spacing and lattice fringes. The photoluminscence spectra highlights quantum energy states present between the highest occupied molecular orbital (HOMO), which is 2.36 eV for cubic phase and 1.76 eV for the hexagonal phase, and the lowest unoccupied molecular orbital (LUMO), with values of −0.84 eV for cubic while −0.24 for hexagonal ZnS. The CIE coordinates for wurtzite ZnS, at X= 0.55 and Y= 0.23, corresponds to red light emission. The suitability of wurtzite phase ZnS for solar cell applications has been demonstrated through electrochemical studies using Nyquist plot and cyclic voltrammetry (CV) techniques. CV demonstrates the presence of redox peaks and reversibility of the material during the redox process. The diffusive behaviour is also confirmed by observing the variation of peak current with scan rate, following Rendle Sevick equation. The presence of Warburg diffusion in Nyquist plot indicates the efficient charge transfer dynamics of the material, suggesting a high potential for exciton formation in energy production. Consequently, this material stands as a promising candidate for efficient solar cells.