Evolution of the optical band gap in Titanium-based Oxy-(Hydroxy)-Fluorides series
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Evolution of the optical band gap in Titanium-based Oxy-(Hydroxy)-Fluorides series Nicolas Penin, Nicolas Viadere, Damien Dambournet, Alain Tressaud, and Alain Demourgues Institute of Condensed Matter Chemistry of Bordeaux (ICMCB-CNRS), 87, Avenue du Dr. A. Schweitzer, 33608 PESSAC Cedex, FRANCE ABSTRACT The optical band gap related to the electronic structures as well as the refractive index function of the electronic polarizability of the network can be tailored by changing the nature and the number of anions into the vicinity of cations. New routes have been developed in order to prepare new divided Ti(IV)-based oxyfluorinated compounds. In these compounds, the charge transfer band (O→Ti) appears at the UV-Vis frontier and the refractive index is always smaller than the one of equivalent oxides. The chemical bonding, the hybridation and the density of the network play key roles in the variation of the optical band gap and the refractive index. For this family of titanium-based oxyfluorides containing mixed anions, chemical compositions and structural features have been correlated to the optical band gap and the refractive index, i.e. the complex index of materials n(λ) + ik(λ). Several examples will be given in order to illustrate the potentialities of these new inorganic compounds by changing the F/Ti ratio and the cationic substitution. INTRODUCTION In the past ten years, numerous studies have been focused on the enhancement or the reduction of the photocatalytic activity of TiO2 [1]. In this scope, the Ti-O-F system has been investigated. A partial substitution of oxygen atoms by fluorine ones, owing to an increase of anionic electronegativity leading to a reduction of the electronic polarizability of the network and the dielectric constant, would result in a decrease of various key parameters. Thus, the refractive index, as well as a loss of the photocatalytic activity and also band gap change which appear at the UV-Visible frontier in the case of titanium dioxide can evolve. Titanium oxy-(hydroxy)fluorides were synthesized using microwave-assisted synthesis route. By changing the HF/Ti molar ratio, two separate compositions, which adopt on one hand a derived ReO3-type structure (SG: Pn-3m) and on the other hand a derived Hexagonal Tungsten Bronze-type structure (SG: P63/mmc), were identified. In order to optimize the UV shielding and refractive index, cationic substitutions, which are strongly dependent on the O/F ratio, were carried out. The Ti-Al-O-F system was especially investigated. Its structural properties were characterized by X-ray and neutron diffractions, chemical and microprobe analyses were carried out to accurately determine the chemical compositions. Herein, the relationship between the structural properties and composition of Ti-Al-O-F system, with optical UV-Visible absorption performances will be discussed. In the first part of this paper will be reported a comparison between well-known titanium oxides such as anatase-type and rutile-type structure, and the titanium oxy-(hyd
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