Estudo ab initio das propriedades eletrônicas, ópticas e fotocatalíticas dos compostos InNbO4 e InTaO4 e o efeito da vacância de oxigênio no InTaO4

Abstract

Ab initio calculations based on density functional theory were performed to study the electronic, optical, and photocatalytic properties of InNbO4 and InTaO4 compounds in the monoclinic crystal phase, as well as to investigate the impact of oxygen vacancies on these properties for the InTaO4 compound. These materials have been extensively investigated in the literature, both experimentally and through first-principles calculations. However, divergent conclusions remain regarding the bandgap energy and optical properties of these pristine (defect-free) materials. These materials are of significant interest for photocatalysis applications; therefore, accurately describing their properties is highly relevant. In this study, various approximations for the exchange and correlation potentials were employed to investigate the pristine InNbO4 and InTaO4 systems. It was concluded that the Becke-Johnson potential was the most successful, resulting in bandgap values of 3.71 eV and 4.20 eV for InNbO4 and InTaO4, respectively, in agreement with the most recent experimental studies. By employing this potential, the optical and photocatalytic properties of both compounds were calculated and analyzed. A good agreement of the calculated reflectivity and optical absorption spectra with the experimental data available in the literature was also obtained. Their photocatalytic efficiencies were analyzed by calculating the band edge positions relative to the H+/H2 reduction and O2/H2O oxidation potentials. It was concluded that both compounds present suitable band alignments for efficient water splitting photocatalysis. However, we conclude that the pure materials do not absorb in the visible spectrum region as some experimental predictions indicated. To further investigate this issue raised by some experimental studies, InTaO4 with oxygen vacancy was also investigated using the supercell method. Two types of oxygen vacancies were simulated, i.e., an oxygen atom that occupies two different crystallographic positions in the crystal lattice of the material was removed. Based on the optimized geometry of the system with oxygen vacancy, the electronic structure and optical absorption were calculated using the Becke-Johnson potential. Defect states (6s of In and 2p of O) were obtained within the bandgap of the pure material. We verified that these states contribute to the optical absorption process of the material in the visible spectrum range. The calculated results were compared with the experimental ones. Based on these results, it was concluded that the photocatalytic activity of InTaO4 that occurs in the visible spectrum range can be attributed to oxygen vacancies in the material.