Band Structure Engineering of Metal Oxynitride Semiconductors for Enhanced Photoelectrochemical Water Splitting

Abstract

In this work, we reported quantum chemical computations on some metal oxides and their oxynitrides counter parts in quest for engineering their bandgaps and band edges positions. Our goal was to fine-tune the bandgap for driving the water splitting reactions (1.23: 2.2 eV) and bandgap edges to be proper with water splitting potentials. The studies have been carried out performing DFT calculations using the standard CASTEP package implemented in Material Studio version 6. One of the group 5 metal oxide and its metal oxynitride counterpart (B-Nb2O5 and NbON) have been targeted for this work. In the first part of the thesis, the electronic and optical properties of monoclinic Niobium pentoxide (B-Nb2O5) belongs to 2/m (B112/B) space group have been studied through studying the doping effects. Two types of dopant been implemented to achieve the targets: (i): The first dopant was Tungsten (W), the bandgap of B-Nb2O5 (3.45 eV) can be significantly tuned into 1.39 eV. The W 5d orbitals affected the position of CBM with negligible effect on VBM. The calculated bandgaps of the B-Nb2O5: W showed a bowing phenomenon. Further, the optical dielectric function showed an increased electronic contribution of the dielectric constant at high W content, while the absorption spectrum and refractive index pointes out a red-shift and cladding behaviors, respectively. (ii): The second dopant was Fluorine (F), the band calculations revealed that B-Nb2O5:F is indirect bandgap semiconductor 2.28 eV, the Fermi-level shifts towards the conduction band, allowing optical absorption in the visible region with enhanced transmittance in the wavelength range 400-1000 nm. The effective mass of free charge carriers increased upon F-incorporation.