Study of the optical properties of BiFeO3 under different heat treatment temperatures

Abstract

It is known that the optical absorption property of a material is linked to its crystal structure. Therefore, it can be modified or displaced when the material is subjected to thermal treatments, generating displacements of the unit cell, atomic positions and bond angles of its atoms. In this work, the analysis of three samples of BiFeO3 powders subjected to different heat treatment temperatures was carried out to study their optical properties by analyzing the crystal structure, diffuse reflectance, Raman scattering spectroscopy, density of state and band structure. In the results, the pure phase and the hexagonal structure of the samples were identified, showing an increase in the angles between the Fe-O-Fe atoms when the material is thermally treated. The band gap (Egexperimental) of the BiFeO3 samples without and with treatment temperature showed direct and indirect transitions according to the Tauc equation, these values were 2.36, 2.11 and 2.19 eV for direct band gaps and 2.10, 1.58 and 1.85 eV for indirect band gaps. In the Raman scattering spectroscopy analysis, overtones were observed in the samples when a green laser with a wavelength of 532nm (2.33 eV) was applied. In the density of states, the maximum valence band formed by the O(2p) state and the minimum of the conduction band formed by the hybridization of the Fe(3d) and O(2p) states were identified. In the electronic band structure, indirect band gap values (Egtheoretical) of 2.274, 2.216 (500 °C) and 2.217 eV (600 °C) were found for the samples without and with heat treatment. It is concluded that BiFeO3 absorbs visible light and that its band gap can decrease by increasing the bond angle of Fe-O-Fe atoms when the sample is subjected to heat treatment. These results demonstrate the semiconductor property of BiFeO3 and can be applied to solar cells, batteries, hydrogen production, among other applications.