Insights into the Characteristic Gap Level and n-Type Conductivity of Rutile TiO2 from the Hybrid Functional Method

Using the Heyd-Scyseria-Ernzerhof (HSE) hybrid functional in the framework of the density functional theory (DFT), we probe the insight into the characteristic gap level and n-type conductivity of the intrinsic rutile TiO2. Thermodynamic and kinetic investigations have been conducted to elaborate the favorability for the formation of the possible n-type defects and unintentional impurities in rutile TiO2. Results show that oxygen vacancy is clearly identified to induce a deep localized state inside the forbidden energy region through localizing two excess electrons at two Ti4+ ions along the [001] direction and reducing them into Ti3+ ions, accounting for the characteristic gap level observed experimentally. The e(g) orbital composition of this gap level offers an accountable explanation of the experimentally measured ferromagnetism in TiO2-x, while the electron transition from this characteristic level is contributable to the photocatalytic behaviors and visible photoluminescence of slightly reduced TiO2. Also, unintentional incorporation of hydrogen substitution for oxygen acts as a shallow donor, providing a consistent explanation of the n-type conductivity in TiO2. The fundamental understanding of these characteristic properties and the associated functionalities would be essential to improving and expanding the practical applications of TiO(2-)based materials and devices.