Synthesis of gamma and theta alumina phases complemented with first principles calculation

Abstract

The synthesis of aluminium oxide (alumina) Al2O3 nanopowders has attracted much attention because of its high specific surface area and a large number of defects in its crystalline structure, which make it widely applicable in ceramic applications. In this study, co-precipitation technique was used to synthesize single-phase alumina nanopowders under various annealing temperatures. The crystalline phase, purity, morphology, chemical bonds and optical properties of the prepared powders were characterized by different spectroscopy techniques. To realize a real understanding of phenomena regarding nanoparticles growth, the material on an atomic scale must be studied. In this case, electronic and optical properties of the alumina at atomic scale have also been studied by the first principles within the framework of density functional theory (DFT). The computational approach is based on a full-potential linearized augmented plane wave method (FP-LAPW) within the generalized gradient approximation (GGA), local density approximation (LDA), and modified Becke–Johnson (mBJ) potential. The experimental results show the direct phase transitional behavior of ?-Al2O3 into ?-Al2O3 at annealing temperature of 900ºC. X-ray diffraction (XRD) and Brunauer–Emmett–Teller analysis confirm the existence of alumina nanopowders with particle diameters of < 5 nm, which also can be classified as ultrafine powder. The surface areas of prepared nanopowders were 366.67 m2/g (200ºC) and 100 m2/g (900ºC) for ?-Al2O3 and ?-Al2O3, respectively. The optical results indicate that ?-Al2O3 possesses a lower band gap (5.5 eV), compared to the ?-Al2O3 (5.8 eV). Theoretical results show that these compounds have a direct band gap (G-G) of 5.375 eV and 4.716 eV for ?-Al2O3 and ?-Al2O3, respectively. Several optical parameters of these materials were also investigated. The values of the real part of dielectric constant ( ) are found to be 3.259 and 3.694 for ?-Al2O3 and ?-Al2O3, respectively, while the refractive indices ( ) are found to be 1.806 for ?-Al2O3 and 1.922 for ?-Al2O3. These GGA findings are consistent with the experimental results and are better than the other approximations. There are no salient differences between GGA and LDA results. The present results advocate the use of this material as transparent conducting layer in solar cell structure, which can be operated in a wide energy range.