DFT analysis of the role of the number of layers: accounting for the multiple energy gaps in iron oxychloride

Abstract

Iron oxychloride (FeOCl), synthesized by the thermolysis of FeCl₃·6H₂O, is a photoassisted self-Fenton-like catalyst, exhibiting two distinct absorption peaks in the UV-visible spectra. The Kubelka–Munk transform of a carefully recorded UV-visible diffuse reflectance spectrum of this reddish-brown colored – thus strongly absorbing – sample, diluted with a non-absorbing BaSO₄ matrix (mass ratio of 1 : 40), confirms the onset of these transitions at 1.68 and 2.38 eV. Although the presence of two optical energy gaps has been reported for this layered n-type semiconductor, its origin has not yet been explained. A scrutiny of the previous reports reveals a possible correlation between the number of layers and the presence of multiple energy gaps. DFT analysis of the band structures of bulk, monolayer and few-layered FeOCl validates this hypothesis. Furthermore, the analysis reveals the indirect nature and a progressive decrease in the energy gap with an increase in the number of layers. The energy gaps for monolayer, five-layered and bulk FeOCl are calculated to be 2.25, 2.16 and 1.86 eV, respectively. Electron microscopy reveals the presence of both bulk and few-layered FeOCl in the specimen, supporting the DFT analysis. The energy gap estimated from the combined results of valence band X-ray photoelectron spectroscopy and Mott–Schottky analysis is 2.2 eV. Due to the surface sensitive nature of these two techniques, the value is closer to both the higher energy gap estimated from UV-visible spectroscopy and the value obtained for few-layered FeOCl from DFT analysis.