J. Phys. Chem. C 2008, 112, 8951–8956
8951
The Nature of Defects in Fluorine-Doped TiO2 A. M. Czoska,† S. Livraghi,† M. Chiesa,† E. Giamello,*,† S. Agnoli,‡ G. Granozzi,‡ E. Finazzi,§ C. Di Valentin,§ and G. Pacchioni§ Dipartimento di Chimica IFM, UniVersita` di Torino and NIS, Nanostructured Interfaces and Surfaces Centre of Excellence, Via P. Giuria 7, I-10125 Torino, Italy, Dipartimento di Scienze Chimiche, Unita` di Ricerca CNR-INFM INSTM, UniVersita` di PadoVa, Via Marzolo 1, I-35131 PadoVa, Italy, and Dipartimento di Scienza dei Materiali, UniVersita` di Milano-Bicocca, Via R. Cozzi, 53, 20125, Milano, Italy ReceiVed: January 16, 2008; ReVised Manuscript ReceiVed: March 18, 2008
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. 1. Introduction The discovery, some forty years ago, of the applications of titanium dioxide (titania) in the areas of photovoltaic and photocatalysis have made this cheap, stable, and nontoxic solid a strategic material and an object of an exponentially growing number of studies. One of the limits in these applications stems from the wide band gap (3.2 eV) of this oxide, which requires the use of UV photons for the promotion of valence band electrons in the conduction band. The modification of the electronic structure of the solid to allow the use of visible light in its applications is therefore of paramount importance and, as recently pointed out in a thorough review paper,1 a third generation of chemically modified titanium dioxide nanomaterials has followed systems based on the pure and metal iondoped TiO2. The third generation, according to Chen and Mao,1 is that of titania doped with nonmetal elements, in particular N, C, S, and F. An intense debate is emerging in the literature aimed at identifying the nature of the doping centers and the reason for the photoactivity in the visible light. However, only a blurred picture is presently available caused by interpretations based either on conjectures or on weak experimental grounds. As for the case of fluorine-doped titania, TiO(2-x)Fx (hereafter F-TiO2), which is increasingly attracting the attention of researchers, it has been shown that the inclusion of this element in the oxide matrix improves the photocatalytic performance of the bare oxide in the mineralization of various organic pollutants using UV2 but also visible3 light. This latter fact is somehow surprising as, differently from the case of N containing TiO2, which is the most investigated doped system and shows nitrogen-based intraband gap localized impurity states,4 no modification of the optical absorption in the visible region was reported for F-TiO2. Various interpretations of the electronic structure are available in the literature for F-TiO2 with the most frequent one being that involving the presence of both doubly * To whom correspondence should be addressed. † Universita ` di Torino and NIS. ‡ Universita ` di Padova. § Universita ` di Milano-Bicocca.
occupied and singly occupied anion (O2-) vacancies in the solid5 whose presence is conjectured on the basis of photoluminescence spectra.6 A second fact has to be discussed when F-doped TiO2 is considered: a few years ago, investigating the photocatalytic degradation of phenol in aqueous suspensions of bare TiO2 in UV light, Minero et al.7 observed that the presence of fluoride ions in solution increases of about three times the catalytic activity. According to these authors, F- ions coordinate to surface Ti4+ ions, displacing OH- groups and consequently modifying the surface reaction mechanism. In the case of F-TiO2 prepared by wet chemistry, it is therefore necessary to take into account the possible formation of both surface (hereafter FsurfTiO2) and bulk F- dopants (F-TiO2) as observed for instance by Yu et al.2 The present paper aims to investigate the features of fluorinedoped TiO2 prepared via sol-gel, which is found to be photocatalytically active in visible light. The main goal of the present paper is to discuss the influence of fluorine bulk doping on the electronic structure of the oxide. We show by experimental spectroscopic techniques (electron paramagnetic resonance, EPR) coupled with state of the art theoretical calculations that the effect of fluorine insertion in the TiO2 matrix is the formation of Ti3+reduced centers, which localize one electron in the t2g orbitals of the metal without generating oxygen vacancies. However, the preparation procedure in liquid phase via sol-gel involves also diamagnetic Fsurf-TiO2 centers whose presence has been demonstrated by using the X-ray photoemission spectroscopy technique (XPS). 2. Experimental Section Fluorine-doped TiO2 powders were prepared by sol-gel synthesis. Titanium (IV) isopropoxide was mixed with a solution of isopropyl alcohol in water also containing hydrofluoric acid. This mixture was stirred at ambient temperature to complete the hydrolysis with formation of a gel. The gel was left aging for 20 h at room temperature, then dried at 340 K, and eventually
10.1021/jp8004184 CCC: $40.75 2008 American Chemical Society Published on Web 05/27/2008
8952 J. Phys. Chem. C, Vol. 112, No. 24, 2008 calcined in air at 770 K for 1 h. The final sample is a white powder, having an anatase structure and a surface area of about 50 m2 g-1. A sample of pure anatase (A-TiO2) was prepared by the same procedure without addition of HF and used for comparison. A portion of the A-TiO2 material was used to prepare a sample of titania containing fluoride surface-substituted ions (Fsurf-TiO2). This was done by a classic impregnation technique suspending 1 g of pure sol-gel A-TiO2 powder in 1 L of aqueous HF solution [0.01M] with continuous stirring for 1 h. The material was then filtered and washed with water until the pH was approximately 6 to remove the excess of HF. The material was finally dried at 40 °C for 4 h. The structure of the prepared materials were determined by X-ray diffraction (XRD) using a Philips 1830 diffractometer with KR(Co) source. A X’Peret High-Score software was adopted for data handling. Diffuse reflectance UV-visible spectra (DR UV-vis) were recorded by a Varian Cary 5 spectrometer using a Cary win-UV/scan software. EPR spectra were run on a X-band CW Bruker EMX spectrometer equipped with a cylindrical cavity and operating at 100 KHz field modulation. The EPR computer simulations were performed using the EPRSIM32 program developed by Profesor Z. Sojka (Jagiellonian University, Cracow).8 Sample pellets were analyzed with XPS (VG ESCALAB chamber, equipped with a sputter gun, a hemispherical energy analyzer and an X-ray double anode source). The XPS data reported in the present study were obtained using Mg-KR lines (1253.6 eV) as an excitation source and a charge neutralizer. All the binding energies (BEs) were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. The F 1s spectra reported in the present paper were taken with a pass energy of 20 eV. 3. Computational Details The calculations were performed using spin-polarized density function theory (DFT), with either the hybrid B3LYP9 or H&HLYP functionals and the PBE10 functional in the generalizedgradientapproximation.ThepercentageofexactHartree-Fock exchange in the B3LYP and H&HLYP functionals is 20 and 50%, respectively. In the hybrid functional calculations, the Kohn-Sham orbitals are expanded in Gaussian type Orbitals (GTO), as implemented in the CRYSTAL06 code11,29 (the allelectron basis-sets are Ti 86411(d41),12 O 8411(d1)13). In the PBE calculations, a plane wave basis set is used. The cutoffs for the smooth part of the wave function and the augmented density were 25 and 200 Ry, respectively, as implemented in the Quantum ESPRESSO code.14 We considered a nearly cubic 22 × 22 × 1 supercell to model anatase phase. The optimized bulk lattice parameters are taken from previous B3LYP (a ) 3.776 Å, and c ) 9.866 Å)b and PBE (a ) 3.786 Å, and c ) 9.737 Å)15 calculations. F-doping was modeled by replacing one oxygen atom in the 96-atoms supercell. The resulting stoichiometry is TiO2-xFx with x ) 0.031. For each method used, full geometry optimization was performed until the largest component of the ionic forces was less than 5 × 10-4 au; k-space sampling was restricted to the Γ point. Projected densities of states (PDOS) have been obtained with a 36 k-points mesh. To compute the g-tensors we also performed some hybrid functional calculations (B3LYP and H&HLYP) on a cluster model using the GAUSSIAN03 code. The model is a Ti5O22 bulk fragment centered on a Ti atom and surrounded by 24 H
Czoska et al.
Figure 1. Diffuse reflectance spectra of (a)TiO2, (b) F-TiO2, and (c) Fsurf-TiO2.
Figure 2. (a) Experimental and (a′) computer simulated X band EPR spectra of F-TiO2. (b) EPR spectrum of surface impregnated Fsurf-TiO2. All spectra were recorded at 77 K.
atoms. This is a common technique to generate clusters of covalent materials and works also for defective TiO2.16 The O-H distance was fixed at 0.96 Å; the Ti-O distances have been fixed at bulk values. Only the central Ti atom and the 6 surrounding O atoms have been allowed to relax in the presence of the excess electron. 4. Results and Discussion The diffuse reflectance UV-vis spectroscopy of the three solids recorded at room temperature (RT) are reported in Figure 1. The figure confirms previously reported findings,17 that is, that no significant shift of the TiO2 adsorption edge nor absorption in the visible region are observed for F-TiO2. The same applies for Fsurf-TiO2. However, we should mention that in a previous study a weak red shift in the absorption has been reported.18 The X-band EPR spectrum of F-TiO2 recorded at 77 K (Figure 2, spectrum a) shows an intense powder pattern dominated by an axial feature with g⊥ ) 1.991 and g| )1.961 as assessed by computer simulation (Figure 2, spectrum a′). No resolved hyperfine interaction is observed with F nuclei (I ) 1/2). The above values are those expected for a d ion in an axially distorted octahedral symmetry,19 and the spectrum is easily assigned to Ti3+ ions. The short relaxation times, typical of Ti3+species, lead to a broad and unresolved line at room
The Nature of Defects in Fluorine-Doped TiO2
J. Phys. Chem. C, Vol. 112, No. 24, 2008 8953
Figure 3. (a) X band EPR spectra of F-TiO2 after thermovacuum annealing at 570 K and (b) subsequent contact with oxygen at RT. All spectra recorded at 77 K.
temperature. Similar spectra were reported in various cases for partially reduced titania or for titania under irradiation.20 The surface impregnated Fsurf-TiO2 sample in the same conditions is characterized by a flat EPR baseline without any evidence of paramagnetic centers (Figure 2, spectrum b) exactly like the A-TiO2 sample. When pure TiO2 is thermally reduced by annealing in vacuo, molecular oxygen is depleted leaving anion vacancies and excess electrons in the solid. A fraction of these electrons become localized on Ti4+ ions giving rise to Ti3+ ions, which are easily detected by the EPR technique.20 A complex debate exists in the literature about the nature of the centers created by reduction, either Ti3+ in substitutional or in interstitial positions.21 Under these circumstances, the reduced solid reacts with molecular oxygen at RT22 forming surface adsorbed superoxide ions according to the equation
Ti3+ + O2 f Ti4+ + O2-(surf)
(1)
This is not the case of F-TiO2. The signal in Figure 2, spectrum a, which is typical of the as-prepared sample, is not affected by interaction with oxygen at RT and at higher temperatures, so that it can be concluded that no charge transfer toward oxygen occurs in these conditions. The line width of the Ti3+ signal in F-TiO2 is very narrow (about 0.15 mT) indicating that the paramagnetic centers are isolated and the spectral line width is not modified by adsorption of oxygen at 77 K, proving the bulk nature of the species. Thermal reduction of F-TiO2 in vacuo at 570 K leads to the broad signal reported in Figure 3, spectrum a, which is typical of reduced titania. The signal, which overlaps with the narrow spectrum discussed above, is due to magnetically interacting Ti3+ ions. In the reduced solid, oxygen vacancies are likely to coexist with regular and interstitial Ti3+ ions. The solid in this case reacts with oxygen at RT forming superoxide O2- ions adsorbed on Ti4+(reaction 1, Figure 3, spectrum b), whose typical orthorhombic EPR signal with g1 ) 2.020, g2 ) 2.090, and g3 ) 2.002 is visible in the low-field portion of spectrum b in Figure 3. The electron transfer is observed immediately after the contact with O2 at room temperature. Full reoxidation at higher temperature causes recovering of the initial oxidation state of F-TiO2, which still shows the Ti3+ signal originally present on the as-prepared material (Figure 2, spectrum a). The XPS spectra in the F 1s region of both F-TiO2 and FsurfTiO2 samples are shown in Figure 4. A single, rather asymmetric, peak is observed; the peak maximum is practically the
Figure 4. F 1s XPS spectra (Mg KR) of the F-TiO2 and Fsurf-TiO2 samples. In the inset, a deconvoluted profile is reported where the higher binding energy component is outlined.
same (centered at a BE of 684.1 eV) in both samples, while small changes in the peak shape are observed in the two cases. This peak is to be associated with the surface fluorination (i.e., to a terminal Ti-F bond) on the basis of the comparison with literature data (684.3, 684.4-684.8 eV).2,23 Yu et al.2 also observed the presence on F-doped TiO2 of a higher BE peak (688.3-688.6 eV) assigned to substitutional lattice fluorine ions. No evidence of such a resolved peak in this region was obtained in our XPS investigations. The asymmetrical shape of the observed peak requires further discussion. Because it relates to a photoionization process from a 1s orbital in a nonmetallic sample, the observed asymmetry strongly suggests the presence of a minor component at a higher BE. So we have deconvoluted the observed experimental line by using a standard fitting procedure (reported in the inset in Figure 4). We have used a Shirley background to correct for the inelastic losses, and the photoemission line has been fitted using two almost pure Gaussian peaks with just a minor Lorentzian component (about 92% Gaussian and 8% Lorentzian). Even if no constrains have been imposed, a robust minimum has been found as well as a good consistency between the two samples. The BE shift of the minor component is about +1.2 eV with respect to the dominant feature. This shift is lower with respect to the one expected for a bulk substitutional F- ion (a Ti-F-Ti bridging species) which amounts, according to Yu et al.2 to about 4 eV. We suggest that this minor component can be assigned to a fraction (ca. 20%) of fluorine atoms that have replaced two OHgroups coordinated to the same Ti atom. The same deconvolution procedure gives a semiquantitative assessment of the intensity decrease (ca. 50% reduction) of the higher BE component in the Fsurf-TiO2 sample with respect to the F-TiO2 one (Figure 4). An interesting experimental evidence is provided by the analysis of the XPS spectra of F-TiO2 and Fsurf-TiO2 samples
8954 J. Phys. Chem. C, Vol. 112, No. 24, 2008
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Figure 5. (Top) Ball and stick models for F-doped anatase TiO2 reporting the spin density of the Ti3+ excess electron: (a) Ti3+ cation first neighbor to the F-dopant; (b) Ti3+ cation at ∼7 Å distance from the F-dopant. (Bottom) Bond lengths in angstroms. The data refer to the H&HLYP calculations.
after a mild sputtering (Ar+ at 0.5 KeV for few minutes); in both cases the peak shape is maintained, even though, a larger intensity decrease is observed in the impregnated Fsurf-TiO2 sample demonstrating that the adopted preparative procedure (impregnation) essentially involves the surface of the solid. In addition, in the case of the F-TiO2 sample the main F 1s peak is still observed after a prolonged sputtering (beyond the complete loss of the C 1s signal), thus demonstrating that the terminal Ti-F bonds are also present at the internal surfaces of microcavities, which are always present in systems prepared via sol-gel procedures. A general description of the F-TiO2-doped system can thus be attempted based on the above-reported experiments. The hydrolysis of titanium isopropoxide during the sol-gel synthesis and the subsequent calcinations at high temperature lead to both the insertion in the bulk and the adsorption on the surface of fluoride ions, that is, Ti-F-Ti and Ti-F bonding situations. In the former case, the insertion of a F- ion in the O2- sites of the TiO2 lattice needs one extra electron for charge compensation. This electron localizes on a lattice cation causing its reduction from Ti4+ to Ti3+ according to the polaron theory.26 The observed EPR signal is thus typical of a reduced Ti3+ in anatase where titanium ions are in octahedral coordination with a slight axial elongation.24 No oxygen vacancy is actually needed to allow the fluorine insertion. The Ti3+ EPR signal is therefore the fingerprint of the occurred fluorine insertion in the TiO2 lattice. The surface fluorination of the solid is a consequence of the preparation procedure in liquid phase. As clearly proved by XPS, F-TiO2 holds at the surface the same type of fluoride ions that are found when the bare oxide sample is impregnated with a F--containing solution (Fsurf-TiO2 sample). In both cases, each surface F- ion substitutes an isoelectronic OH- group with no
alteration of the charge balance. This is the reason why FsurfTiO2 is EPR silent. The fact that the substitutional ions in the lattice of F-TiO2 are not observed by XPS (like in the case of J. Yu et al.2) might be due to a smaller concentration in our case, below the detection limit of the XPS technique. On the other hand, the higher intrinsic sensitivity of EPR is sufficient to detect the observed signal whose intensity roughly corresponds to about 1017 spins/g of Ti3+, (hence bulk substitutional F). The formation of lattice Ti3+ as a consequence of the insertion of fluorine in the TiO2 structure is fully supported by theoretical calculations. The correct description of Ti3+ states requires the use of hybrid DFT functionals.25 This holds also for F-TiO2 where the presence of F substitutional of O introduces one excess electron that is expected to reduce Ti4+ to Ti3+ with consequent polaronic distortion.26 A first set of standard DFT periodic plane-wave calculations using the PBE functional10 indicated that the excess electron introduced by F is distributed over several Ti ions in the supercell; the resulting occupied state lies at the bottom of the conduction band. The same delocalized picture has been reported also in previous studies.17,27 However, this is not the correct description due to the underestimation of the titania band gap by standard DFT. To obtain a physically correct picture, we have repeated the calculations with the CRYSTAL06 code,11 using a localized basis-set and hybrid functionals, in particular H&HLYP and B3LYP.9 Two different models have been considered with the excess electron localized on (1) a Ti atom adjacent to the F atom (Figure 5a) and (2) on a Ti atom well separated from the F atom (∼7 Å) (Figure 5b). In both cases, the H&HLYP functional shows a full spin localization on a Ti 3dxz state (Figure 5) with structure (1) 0.15 eV more stable than structure (2). This energy difference is small, and both configurations are in principle possible. How-
The Nature of Defects in Fluorine-Doped TiO2
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TABLE 1: Band-Gap Values (Eg) at Γ Point and Relative Position (∆E) of the Defect State with Respect to the Bottom of the Conduction Band (CRYSTAL06 Calculations with Different Hybrid Functionals) configuration 3+
Ti adjacent to F (Figure 2, spectrum a) Ti3+ at ∼7 Å from F (Figure 2 spectrum b)
Ega
∆E
B3LYP
3.92b
-0.77
H&HLYP B3LYPc
6.88 4.01b
-4.75 -0.92
H&HLYP
6.89
-4.56
method
a
Only band gap values at Γ are reported because they are the smallest direct band gaps computed on the grid of k-points considered. b B3LYP band gap value for pure anatase TiO2 is 3.90 eV. c Single point calculation on the H&HLYP optimized geometry.
ever, in structure (1) (F near the Ti3+) a hyperfine coupling of the unpaired electron with the F nucleus is expected on the basis of the theoretical analysis, which is in contrast with what experimentally is observed. The electron localization induces only very small structural deformations resulting in an elongation of the Ti3+-O bonds compared to the Ti4+-O ones (Figure 5). One drawback of the H&HLYP functional is that it gives a band gap for TiO2 which is largely overestimated (about 6.9 eV). Therefore, the position of the localized impurity states associated to Ti3+ (Table 1) is not reliable. B3LYP calculations provide a better estimate of the band gap (Eg ) 3.90 eV).28 The position of the Ti3+ defect states is 0.77 and 0.92 eV (Table 1), respectively, below the bottom of the conduction band for case (1) and (2).29 These values are slightly smaller than those computed for lattice Ti3+ species generated by creation of oxygen vacancies on the rutile (110) TiO2 surface,25 which compare well with UPS30 and EELS31 measurements on reduced TiO2 samples. The total and projected DOS obtained at the B3LYP level for the two cases analyzed (Figure 6) clearly show that the defect state in the band gap is associated to a 3d1 level of the Ti3+ cation, thus providing a further proof of the localization of the spin density on a single Ti atom also at the B3LYP level. The computational findings, therefore, provide a clear indication that the introduction of substitutional fluorine results in the formation of a lattice Ti3+ species where the excess unpaired electron is highly localized in the 3d shell. A comparison with the experimental EPR data indicates that the Ti3+ species is not in the proximity of the F impurity (absence of hyperfine interaction). On this basis it is possible to conclude that a fluorine-free TiO2 cluster model, centered on an octahedrally coordinated Ti3+ ion, provides a reliable representation of the experimentally observed species and can be used to estimate the g-factor for this center using the GAUSSIAN03 code.32 Cluster calculations performed with the two hybrid functionals, B3LYP and H&HLYP, show a different degree of spin localization: the spin density on Ti3+ is 0.58 (B3LYP) and 0.87 (H&HLYP). The H&HLYP value is closer to the periodic calculations where the spin density on Ti3+ is between 0.90 (B3LYP) and 0.96 (H&HLYP). The g-tensor computed at the H&HLYP level, Table 2, is in excellent agreement with the experimental one measured for the F-doped sample providing further support to the nature of the paramagnetic center. The nature of F-doped TiO2 is based on the mechanism of valence induction and the stoichiometry of the solid can be represented as Ti4+(1-x)Ti3+xO2-(2-x)F-x. This idea was already advanced in purely hypothetical terms by Yu et al.2 and is now confirmed by our EPR measurements and theoretical calcula-
Figure 6. Total and projected (on the Ti3+ cation) density of states from B3LYP calculations: (a) Ti3+ cation first neighbor to the F-dopant; (b) Ti3+ cation at ∼7 Å distance from the F-dopant. The zero energy value is set at the top of the valence band. States below and above the dotted vertical line are occupied and unoccupied, respectively.
TABLE 2: Spin Density and EPR g-Tensor for the Cluster Models Computed with GAUSSIAN03 and Different Hybrid Functionals method
spin (on Ti3+)
gxx
gyy
gzz
B3LYP H&HLYP Exp.
0.58 0.87
1.997 1.998 1.991
1.996 1.992 1.991
1.989 1.971 1.961
tions. The signal reported in Figure 2 can thus be considered the prototype of a Ti3+in a regular lattice site of anatase. The reduced state is relatively close in energy to the bottom of the conduction band (∼ 0.8 eV from theoretical calculations with a band gap for anatase of 3.9 eV; if one considers that the experimental band gap is 3.2 eV, this would correspond to a defect state about 0.6 eV below the conduction band). The position of this Ti3+ state is higher in energy than that of Ti3+ states associated to oxygen vacancies at the (110) surface of rutile;25 preliminary calculations indicate that also Ti3+ states due to oxygen vacancies in bulk anatase lie deeper in the gap. This may be related to the different coordination of Ti3+centers in F-doped TiO2 and in reduced TiO2: six- versus fivefold, an effect that could induce a different stability of the defect state. This may also be the reason why F-doped TiO2 does not absorb in the vis-region (Figure 1) in contrast to the reduced sample. In fact, we expect that the excitations from the F-derived Ti3+ states to the conduction band fall in the IR region in accord with the polaron theory.26
8956 J. Phys. Chem. C, Vol. 112, No. 24, 2008 Even though it is outside the scopes of this paper, it should be noted that the reasons of the photocatalytic activity of F-TiO2 in visible light2 are not easy to understand considering the lack of appreciable optical absorption in the visible range. However, it is important to mention the possible role of surface fluorination,7 which is known to influence the photocatalytic activity under ultraviolet irradiation.33,34 The combined action of the two types of fluorine doping could be the driving force of the photocatalytic properties of F-TiO2 in visible light. Another crucial point concerns the reactivity of the F-doped sample toward oxygen. The different behavior toward O2 of reduced (Figure 3) and as-prepared F-doped TiO2 can be explained considering that in the latter case the defect centers are in the bulk of the material and the electron transfer to adsorbed species is an activated process, which requires higher energies than for the reduced samples. Evidence has been reported that Ti3+ ions in TiO2 may be incapable of transferring electrons to adsorbed oxygen molecules.20c,34 It is very interesting to note that in both the mentioned publications, the EPR signal of the Ti3+ center essentially coincides with that reported here for as-prepared F-TiO2 samples. 5. Conclusions In summary, we have shown that fluorine doping of TiO2 using a wet chemistry technique (sol-gel) causes the appearance of two types of fluorine species in the solid. The former species consists of fluoride F- ions that substitute O2- ions in the solid lattice yielding a bridging Ti-F-Ti bond. This induces the formation of bulk Ti3+ species in an octahedral environment that introduce localized states just below the conduction band. The doped samples do not show absorption in the visible region and do not react with adsorbed O2 to form superoxide anions, suggesting that the defect centers are located inside the material and not on the external layers. The second species consists of F- ions at the solid surface (both external and internal) where they substitute OH-hydroxyl groups yielding a terminal Ti-F bond without generating reduced centers. Acknowledgment. The work has been supported by the COST Action D41 “Inorganic oxides: surfaces and interfaces”. Part of the computing time was provided by the Barcelona Supercomputing Center - Centro Nacional de Supercomputacio´n (BSC-CNS). References and Notes (1) Chen, X.; Mao, S. S. Chem. ReV. 2007, 107, 2891. (2) Yu, J. C.; Yu, J.; Ho, W.; Jiang, Z.; Zhang, L. Chem. Mater. 2002, 14, 3808. (3) Li, D.; Haneda, H.; Labhsetwar, N.; Hishita, S.; Ohashi, N. Chem. Phys. Lett. 2005, 401, 579. (4) (a) Hirie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483. (b) Sakthivel, S.; Janczarek, M.; Kisch, H. J. Phys. Chem. B 2004, 108, 19384. (c) Sato, S.; Nakamura, R.; Abe, S. Appl. Catal. 2005, 284, 131. (d) Di Valentin, C.; Pacchioni, G.; Selloni, A.; Livraghi, S.; Giamello, E. J. Phys Chem. B. 2005, 109, 11414. (e) Livraghi, S.; Paganini, M. C.; Giamello, E.; Selloni, A.; Di Valentin, C.; Pacchioni, G. J. Am. Chem. Soc. 2006, 128, 15666. (5) (a) Li, D.; Haneda, H.; Hishita, S.; Ohashi, N.; Labhsetwar, N. K. J. Fluorine. Chem. 2005, 126, 69. (b) Li, D.; Haneda, H.; Hishita, S.; Ohashi, N. Chem. Mater. 2005, 17, 2596. (c) Ho, W.; Yu, J. C.; Lee, S. Chem. Commun. 2006, 1115. (6) In the mentioned papers, these centers are actually indicated as F centers. We prefer to avoid this nomenclature as it contains some ambiguity and some element of confusion with the color centers in ionic insulators that are traditionally called F centers but have different features.
Czoska et al. (7) Minero, C.; Mariella, G.; Maurino, V.; Pelizzetti, E. Langmuir 2000, 16, 2632. (8) Spalek, T.; Pietrzyk, P.; Sojka, Z. J. Chem. Inf. Model 2005, 45, 18. (9) (a) Becke, A.; D, J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (10) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (11) Saunders, V. R.; Dovesi, R.; Roetti, C.; Orlando, R.; ZicovichWilson, C. M.; Harrison, N. M.; Doll, K.; Civalleri, B.; Bush, I. J.; D’Arco, Ph.; Llunell, M. CRYSTAL06 User’s Manual; University of Torino: Torino, Italy,2006. (12) Zicovich-Wilson, C. M.; Dovesi, R. J. Phys. Chem. B 1998, 102, 1411. (13) Ruiz, E.; Llunlell, M.; Alemany, P. J. Solid State Chem. 2003, 176, 400. (14) Baroni, S.; et al. http://www.pwscf.org. (15) Lazzeri, M.; Cittadini, A.; Selloni, A. Phys. ReV. B 2001, 63, 155409. (16) Bredow, T.; Pacchioni, G. Chem. Phys. Lett. 2002, 355, 417. (17) Li, D.; Ohashi, N.; Hishita, S.; Haneda, H.; Kolodiazhnyi, T.; Haneda, H. J. Solid State Chem. 2005, 178, 3293. (18) Yu, J. C.; Yu, J.; Ho, W.; Jiang, Z.; Zhang, L. Chem. Mater. 2002, 14, 3808. (19) Mabbs, F.; Collison, D. EPR of Transition Metal Compounds; Elsevier, New York. (20) (a) Howe, R.; Graetzel, M. J. Phys. Chem. 1985, 89, 4495. (b) Micic, O. I.; Zhang, Y.; Cromack, K. R.; Trifunac, A. D.; Turnhauer, M. C. J. Phys. Chem. 1993, 97, 13284. (c) Nakaota, Y.; Nosaka, Y. J. Photochem. Photobiol A, Chem 1997, 110, 299. (21) Li, M.; Hebenstreit, W.; Diebold, U.; Tryshkin, A. M.; Bowman, M. K.; Dunhan, G. G.; Henderson, M. J. Chem. Phys. B 2000, 104, 4944. (22) Anpo, M.; Che, M.; Fubini, B.; Garrone, E.; Giamello, E. Top. Catal. 1999, 8, 189. (23) Park, H.; Choi, W. J. Phys. Chem. B 2004, 108, 4086. (24) Meriaudeau, P.; Che, M.; Jorgensen, C. K. Chem. Phys. Lett. 1970, 5, 131. (25) Di Valentin, C.; Pacchioni, G.; Selloni, A. Phys. ReV. Lett. 2006, 97, 166803. (26) Austin, I. G.; Mott, N. F. AdV. Phys. 2001, 50, 757. (27) Yamaki, T.; Umebayashi, T.; Sumita, T.; Yamamoto, S.; Maekawa, M.; Kawasuso, A.; Itoh, H. Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 206, 254. (28) (a) This value for the anatase bulk pure TiO2 band-gap is large (with respect to the 3.2 eV exp. value but is consistent with previously reported values for the direct band gap of the same system: Calatayud, M.; Mori-Sanchez, P.; Beltran, A.; Pendans, A. M.; Francisco, E.; Andrei, J.; Recio, J. M. Phys. ReV. B 2001, 64, 184113. (b) Zhang, Y.; Lin, W.; Ding, K.; Li, J. J. Phys. Chem. B 2005, 109, 19270. (29) For case (1), we have performed a full geometry optimization, although for case (2) we have simply performed a single point B3LYP calculation on the H&HLYP geometry. (30) Henderson, M. A.; Epling, W. S.; Peden, C. H. F.; Perkins, C. L. J. Phys. Chem. B 2003, 107, 534. (31) Kurtz, R. L. Surf. Sci. 1989, 218, 178. (32) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision A.7; Gaussian Inc.: Pittsburg, PA, 2003. (33) Maurino, V.; Minero, C.; Mariella, G.; Pelizzetti, E. Chem. Commun. 2005, 2627. (34) Ke, S.; Wang, T.; Wong, M.; Gopal, N. O. J. Phys Chem B. 2006, 110, 11628.
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