Revival of quantum confinement effect and stabilization of tetragonal phase in TiO2 Nanopowders with annealing temperature

Abstract

This research is concerned with the variations of mechanical, structural, physical, and magnetic properties of titanium dioxide (TiO2) nanopowders produced via sol–gel method with annealing temperatures intervals 300 to 1100 °C. The effects of the phase transitions on the main chracteristics of TiO2 nanopowders are exstensively analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometry (VSM), and Vickers microhardness (Hv) measurements. XRD investigations reveal an amorphous crystal structure at lower annealing temperatures (300–400 °C), followed by the formation of the chracteristic anatase phase (450–500 °C), and finally the transition to the rutile phase (600–1100 °C). All TiO2 nanopowders exhibit a tetragonal crystal structure. The avarage grain size parameter is found to increase depending on the lattice strain relaxation while dislocation density decreases regularly with the increase in the annealing temperature. Further, SEM images show that increasing calcination temperature improves crystallization quality due to increased agglomeration along intergrain boundaries and promotes the formation of a more spherical morphology. Accordingly, the heat energy serving as the driving force for the reordering of skeleton develops the crystallization system and grain boundary couplings in the TiO2 main matrix. Namely, the quantum confinement effect is clearly observed with the temperature gradient in the TiO2 nanometer scale materials produced in this work. Additionally, VSM measurements indicate ferromagnetic behavior in all samples, and magnetic saturation is achieved at 1.5 Tesla. The hardness results indicate that the Hv values are found to enhance regularly with increasing annealing temperature as a consequence of degradation of interparticle bonds, non-uniform grain orientation distributions, reduced contact points between semiconductor particles, and the potential formation of impurity phases. The improvement stems from the phase transformation from anatase-to-rutile. Besides, all the semiconducting nanopowders produced exhibit standard the indentation size effect behavior. The experimental load-indepenendet hardness results in the saturation limit regions are compared with semi-empirical mechanical models. It is observed that the Hays-Kendall (HK) approach demonstrates the most accurate prediction of real microhardness values.