The Nature of Defects in Fluorine-Doped TiO₂

Abstract

Fluorine-doped titanium dioxide was prepared via sol−gel synthesis and subsequent calcination in air. The presence of fluorine in the lattice induces the formation of reduced Ti3+ centers that localize the extra electron needed for charge compensation and are observed by electron paramagnetic resonance. Density functional theory calculations using hybrid functionals are in full agreement with such description. The extra electron is highly localized in a 3d orbital of titanium and lies a few tenths of an electron volt below the bottom of the conduction band. The preparation via sol−gel synthesis using aqueous solutions of fluorides also causes the formation of surface F− ions that substitute surface hydroxyl groups (OH−) without generating reduced centers.

Influence of the chemical synthesis on the physicochemical properties of N-TiO2 nanoparticles

“…This different behaviour can be ascribed to the presence, on the surface, of F -groups replacing the OH -groups. It has been reported in fact that the use of fluorine containing compound in the preparation of doped TiO 2 always leads not only to bulk doping but also to the simultaneous fluorination of the surface [3,21]. This fact is confirmed in the present case by XPS analysis (not reported for sake of brevity) showing the typical peaks of surface F -ions.…”

“…The presence of Ti 3+ in the solid is a direct consequence of the substitution of O by F in some lattice site of the structure. The extra electron introduced by F in the system is thus stabilized by Ti 4+ ions [3]. The solid can thus be written with regard to the fluorine modification, in the following chemical terms:…”

“…Different strategies have been followed until now to reach this target such as doping with transition metal ions or surface sensitization with organic and organometallic dyes. More recently, doping TiO 2 with p-block (N, F, C, S) elements and in particular with nitrogen, has attracted the attention of the photocatalytic community as the introduction of such elements in the crystalline network produces materials capable of absorbing photons corresponding to visible radiation [2][3][4][5].…”

Nature of Reduced States in Titanium Dioxide as Monitored by Electron Paramagnetic Resonance. II: Rutile and Brookite Cases

“…This was done using a material doped with an aliovalent element bearing an extra electron (F and/or Nb). [31,32] The same convenient approach could not be extended to rutile since the extra electron carried by Nb, in Nb-doped rutile, is not transferred to Ti ions. [33,34] We tried also to obtain Fdoped rutile by high temperature treatments of doped anatase.…”

Charge compensation in trivalent cation doped bulk rutile TiO2

“…The most widely used approach for what has been called "band gap engineering" has been to dope TiO 2 with another species, which could be a metal cation on the Ti site [8][9][10][11][12][13] or C/N/P on the anion site [14][15][16][17] or co-doping at cation and anion sites [18][19][20]. A single dopant introduces new states into the energy gap, in principle leading to a reduction of the band gap of the host oxide; whether this occurs at the valence band or at the conduction band depends on the dopant.…”