Theoretical Study of Electronic Properties of X-Doped (X = F, Cl, Br, I) VO2 Nanoparticles for Thermochromic Energy-Saving Foils

First-principles density functional theory (DFT) electronic structure calculations were carried out for the model halogen-doped VO2 (M1 phase) to evaluate the effect of halogen (X = F, Cl, Br, I) doping on the band edges. The model structures of X-doped VO2 with X at V site or O site were constructed on the basis of 96-atom 2 x 2 x 2 supercell of monoclinic M1 phase of VO2. Our results showed that the band gap Eg(2) for Cl-doped VO2 at O1 site (0.51 eV) is smaller than that of F-doped VO2 at O1 site (0.61 eV) and that of pure VO2 (0.78 eV). We also investigated the substitution of chlorine, bromine, and iodine for vanadium in VO2, where the band gaps Eg(2) are 0.40, 0.45, and 0.37 eV for Cl-, Br-, and I-doped VO2 at V site, respectively. The Cl-doped VO2 at V site is the best one for achieving good VO2 thermochromic energy-saving foils.