Development, characterization and evaluation of selected transition metal doped zinc sulphide nanostructure surface layers decorated with graphene for water splitting

Abstract

Water splitting (WS) is the dissociation of Water (H2O) into Hydrogen (H2) and Oxygen (O2). Zinc Sulphide (ZnS) provides an excellent option for the hydrogen reduction cathode in photo electrochemical (PEC) cells for WS. However, its low sensitivity to visible range in electromagnetic spectrum limits its practical appli cability. Few comprehensive studies consider a wide range of transition metals as potential dopants to meet future energy requirements for greater PEC WS. The main objective of this research was to develop, characterize and evaluate the selected Transitional metal (TM) doped ZnS nanostructure (NS) surface layers decorated with graphene (rGO) for WS. The specific objectives were to: simu late the optimal dosage of TM dopants for ZnS nanostructure layers, synthesize TM doped ZnS NS layers decorated with graphene, characterize TM doped ZnS NS layers decorated with graphene and to evaluate the photocatalytic hydrogen production of TM doped ZnS NS layers decorated with graphene. Theoretical f irst principles Ab-Initio calculations based on Density functional theory (DFT) method was employed to examine the electronic structure of ZnS nanostructures (NSs) doped with selected TM dopants including; manganese (Mn), copper (Cu), cobalt (Co) and iron (Fe) in order to modify the structural properties of ZnS NSs. Highly distributed cobalt doped ZnS NSs were effectively fabricated on the surfaces of graphene sheets via simple hydrothermal technique. The structural, electronic and optical properties of the cobalt doped ZnS decorated with graphene (Co-ZnS-rGO-NS’s) were examined using X-ray diffraction (XRD), X-ray pho tocurrent spectroscopy (XPS), Raman spectroscopic (RS), Fourier transmission infrared spectroscopy (FTIR), Scanning electron microscopy (SEM) and Ultra violet visible absorbance spectroscopy (UV-vis). The photocatalytic activity of CoxZn1−xSrGO NS’s at (x = 0, 1, 2, 4 and 6) atomic percentage (atm.%) was determined in lab experiments using water and visible light. The stability of 3d orbital transitional metal dopant (TMD’s)’s in ZnS NSs were shown to be depen dent both on the dopant concentrations and the d orbital character of the TMD’s. Evidently, the 3d orbital TMD’s’s (Cu, Co,Mn and Fe) showed low formation energies and appropriate band edge states due to their low lattice strain, hence absorbed into ZnS NSs. ZnS doped with 4 atm.% of Cu and Co was shown to be optimal for photocatalytic hydrogen generation based on theoretical studies. The f indings of XRD, FTIR, RS, XPS and SEM investigation suggest that graphene oxide (GO) was successfully transformed into graphene sheets, CoxZn1−xSrGO NS’s possessed a crystalline, cuboidal and spheroidal form of structure displaying a paper like appearance. UV-vis spectrophotometric analysis verified a notable rapid increase in transmittance and high transparency (≈ 90%) within (180-800) vi nm wavelength range. Calculations of transmittance spectra revealed a direct allowable band gap range of (1.26-5.46) eV, demonstrating a band gap decrease as cobalt content increased, consistent with theoretical predictions. Furthermore, the optimal cobalt loading of 0.04 atm.% generated a maximum hydrogen yield of 7649µmolh−1 after 720 minutes of Ultra Violet (UV) light exposure, indicating that the ZnS NSs’s electronic and optical characteristics were influenced by their stability with respect to dopant concentration. In conclusion, the results show that improved transfer of photo-generated electrons, increased surface area and better dispersion-absorption properties all contributed to higher photocatalytic hydrogen generation activity. The study recommended synthesis optimization for commercially viable technology.