Titanium dioxide (TiO2) is one of the most used oxides in renewable energy applications, such as hydrogen production, photovoltaics, and light-emitting diodes. To further improve the efficiency of the devices, doping strategies are used to modify their fundamental properties. Here, we used density functional theory (DFT) simulations to explore the effect of all the halogen dopants on the structural, electronic, and optical properties of TiO2. We investigated both the interstitial and the oxygen substitutional positions, and for the optimized structures, we used hybrid DFT calculations to predict the electronic and optical properties. In all cases, we found that halogen dopants reduce the bandgap of the pristine TiO2 while gap states also arise. The halogen dopants constitute a single acceptor when they occupy interstitial sites, while when they are inserted in oxygen sites, they act as donors. This can be established by the states that form above the valence band. It is proposed that these states contribute to the significant changes in the optical and electronic properties of TiO2 and can be beneficial to the photovoltaic and photocatalytic applications of TiO2. Importantly, the iodine doping of TiO2 significantly reduces the bandgap of TiO2 while increasing its dielectric constant, making it suitable for light-harvesting applications.
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