Theoretical Insights into the Experimental Observation of Stable p-Type Conductivity and Ferromagnetic Ordering in Vacuum-Hydrogenated TiO2

Abstract

Tuning of electrical and magnetic properties to achieve stable p-type conductivity and room temperature ferromagnetism in undoped TiO2 is quite challenging. Here both are attained simultaneously through a facile method of vacuum-hydrogenation, wherein vacuum annealing as well as hydrogenation play crucial roles. The p-type conductivity in hydrogenated TiO2 is investigated through the Hall measurement studies, which show considerable enhancement in Hall mobility and electrical conductivity. The high and low pressures of hydrogenation show strong and weak ferromagnetic ordering, respectively, whereas the pristine TiO2 NPs manifest paramagnetic behavior. In order to understand the mechanism of these characteristic changes, density functional theory (DFT) calculations are performed. DFT calculations reveal that the smaller amount of hydrogenation leads to gap-states above valence band maximum (VBM) due to the effect of hydrogen atoms 1s orbitals and by the formation of ∼Ti–H and ∼O–H bonds. Further increase in the hydrogenation changes the ∼O–H bond to the ∼H2O bond, and these H2O molecules will be easily detached during the next vacuum annealing step. These processes will lead to the formation of excess oxygen vacancies and cause the localization of excess electrons on Ti atoms. This results in emergence of well pronounced midgap states in the forbidden bandgap. These midgap states are mostly contributed by the 3d orbitals of Ti atoms. DFT studies also disclose that the higher spin polarization for the high hydrogen concentration, which is reflected as the ferromagnetic ordering in the experimental results.