New photocatalytic materials based on alumina with reduced band gap: A DFT approach to study the band structure and optical properties

In this study, first-principles calculations using Density Functional Theory (DFT) have been conducted, which were carried out using the Vienna Ab initio Simulation Package (VASP) to examine the effect of Tl insertion on electronic and optical properties of the alpha-Al2O3. Alumina materials are abundant and the main shortcoming of alumina for photocatalyst applications is their large energy band gap and little absorption in the visible region of electromagnetic (EM) radiation. Insertion of transition metals (TM) into semiconductor or insulating materials is a hot approach to improve the absorption behavior of these materials using DFT assessment. In the current work an analysis of the band structure (BS) and the density of states (DOS); comprising both the total density of states (TDOS) as well as the partial density of states (PDOS) were carried out. The BS diagram revealed that various concentrations of Tl insertion into the alpha-Al2O3 reduced the band gap to 2.38 eV. In the density of state diagram, the band gap energy shifted to lower photon energies with increasing Tl concentrations which supports the BS results. The band gap obtained from the first peak in the imaginary part of dielectric function is close enough to those established from the BS diagram. Distinguished shifting of absorption coefficient to lower photon energy (2.27 eV) reveals the suitability of the doped alpha-Al2O3 for various applications. The increase of refractive index (n) with increasing of Tl into the alpha-Al2O3 structure is evidence for the increase of charge, which is a source for polarization and attenuates the velocity of light in a medium. The increase of optical conductivity with photon energy started after band gap values. The reflectance, absorbance and transmittance results indicate that the doped alpha-Al2O3 is responsive to the visible region of EM radiation while in pure state almost transparent.