Fluorine-Doped TiO₂ Materials: Photocatalytic Activity vs Time-Resolved Photoluminescence

Abstract

Aiming at discerning the role of fluorine from that of nitrogen as a dopant in N,F-codoped TiO 2 , a series of HF-doped TiO 2 photocatalysts were investigated in the decomposition of formic and acetic acid in aqueous suspensions, also as a function of the irradiation wavelength (action spectra analysis), in comparison with recent results obtained with an analogous series of NH 4 F-doped TiO 2 photocatalysts. Visible light absorption around 420 nm, which was found to be inactive in acetic acid decomposition, is definitely shown to be associated with nitrogen doping, whereas the enhanced absorption at ca. 365 nm, increasing with increasing calcination temperature, can be unambiguously attributed to structural modifications induced by fluorine doping. Action spectra analysis confirms that this absorption is active in acetic acid decomposition, in both HF-and NH 4 F-doped TiO 2 photocatalysts. From time-resolved photoluminescence (PL) spectroscopy analysis, a clear correlation is outlined between the photoactivity of the materials and the long-lasting component of the PL signal, which increases with the calcination temperature and is related to the formation of surface defects. Thus, fluorine doping, followed by calcination at high temperature, increases the amount of surface traps originating the long-lasting PL signal, which are beneficial in photoactivity by ensuring long-living photoproduced charge couples.

Enhancement of the intrinsic photocatalytic activity of TiO2 in the degradation of 1,3,5-triazine herbicides by doping with N,F

“…5a). E g within the range of 684.3 -685.5 eV is consistently associated with F -ions physically adsorbed on the catalyst surface [33,45] promoting the formation of TiOF 2 [45,48]. Wang et al [49] reported an E g of 684.5 eV for substitutional F-atoms in TiO 2 (Ti-F bonds), while Pelaez et al [29] indicated that E g at higher eV (688.3 -688.6 eV) is designated to Ti-F bonds.…”

“…The increment of exposed {001} facets due to the change of morphology contributed a positive effect towards an efficient electron/hole charge transfer, and facilitates enhanced photocatalytic activity [33,59]. PL spectra thus indicate that in undoped TiO 2 there is a higher tendency of radiative recombination in comparison to N,F-TiO 2 , with thus a reduced formation of radicals on the catalyst surface.…”

“…Although debate exists on nitrogen-incorporation in TiO 2 , a reasonable interpretation of this band is the creation of Nstate about 0.14 eV above the valence band, with thus reduced photon energy to populate the conduction band [50]. The enhancement of {001} facets also played an important role in extending the light absorption towards the visible region [33] coupled with formation of oxygen vacancies due to N and F doping respectively. TiO 2 is an indirect band gap semiconductor.…”

“…Previous research has shown improvement of TiO 2 optical characteristic in degrading organic pollutant by the incorporation of nitrogen and fluorine atoms into its crystal lattice [20,[27][28][29][30][31][32][33][34][35][36][37][38]. In addition, the foreign atom incorporation has proven to improved TiO 2 particle dispersion, inhibit particle size agglomeration and retard phase transformation [15].…”

“…In addition, the foreign atom incorporation has proven to improved TiO 2 particle dispersion, inhibit particle size agglomeration and retard phase transformation [15]. Recent results demonstrates that fluorine doping, followed by calcination at high temperature, increases the amount of surface traps originating a longlasting photoluminescence (PL) signal and long-living photoproduced charge couples [33]. Limited research has been conducted for the degradation of atrazine using N,F-TiO 2 nanoparticles or thin films.…”