Tuning the Electronic Structure of Anatase Through Fluorination

A highly fluorinated anatase lattice has been recently reported, providing a new class of materials whose general chemical formula is $\rm Ti_{1- \mathit x}\square_{\mathit x}X_{4\mathit x}O_{2- 4\mathit x}$ (X$^-$ = F$^-$ or OH$^-$). To characterise the complex structural features of the material and the different F environments, we here apply a computational screening procedure. After deriving a polarisable force--field from DFT simulations, we screen in a step-wise fashion a large number of possible configurations differing in the positioning of the titanium vacancies ($\square$) and of the fluorine atoms. At each step only 10% of the configurations are retained. At the end of the screening procedure, a configuration is selected and simulated using DFT-based molecular dynamics. This allows us to analyse the atomic structure of the material, which is strongly disordered, leading to a strong decrease (by 0.8~eV) of the band gap compared to conventional anatase.