Band Gap Engineering of Spin-Coated α-Fe2O3 for Solar Water Splitting Utilizing a Facile Zinc Doping Route

Abstract

Iron oxide (hematite, α-Fe₂O₃) is an earth-abundant material having a favorable band gap for solar energy harvesting by producing Hydrogen (H₂) by photoelectrochemical (PEC) water splitting. Although Fe₂O₃ has a valence band maximum (VBM) more positive than the water oxidation potential in the normalized hydrogen energy scale (NHE), it can not be used as a photocathode because of the more positive conduction band minima (CBM) than the reduction potential of the electrons required for the hydrogen evolution reaction (HER). This paper reports the band gap engineering of Fe₂O₃ by doping it with Zinc (Zn) through a facile way to modify its CBM and VBM to enable it to work as an ideal and efficient photocatalyst. One-pot synthesis of α-Fe₂O₃ doped with Zn was achieved by spin-coating a precursor solution prepared with FeCl₂ and Zinc acetate (Zn(CH₃COO)₂.2H₂O) on fluorine-doped tin oxide (FTO) or glass substrates. The films were then annealed in air at around 450 °C. The surface morphology and the crystal structure of the Fe₂O₃ were studied using SEM (Scanning Electron Microscope) and XRD (X-ray diffractometer). The UV-VIS spectrophotometry and spectroelectrochemistry (SEC) determined the band energies of α-Fe₂O₃ and Zn-doped α-Fe₂O₃. The band gap and CBM of pristine α-Fe₂O₃ were found to be -2.09 eV and -5.31 eV vs. E_VAC, whereas the band gap and CBM of Zn-doped α-Fe₂O₃ were -1.96 eV and -4.3 eV vs. E_VAC respectively. The outcome of the photoelectrochemical study shows that α-Fe₂O₃ doped with Zn can have a reduced band gap with more negative CBM than the H⁺/H₂ reduction potential that efficiently ensures both oxygen and hydrogen evolution reaction.