Effect of Fluorination on the Surface Properties of Titania P25 Powder

Oct 30, 2009 - Influence of fluorine on the synthesis of anatase TiO 2 for photocatalytic partial oxidation: are exposed facets the main actors? Maria...
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Effect of Fluorination on the Surface Properties of Titania P25 Powder: An FTIR Study Marco Minella,† Maria Giulia Faga,‡,§ Valter Maurino,† Claudio Minero,† Ezio Pelizzetti,† Salvatore Coluccia,‡ and Gianmario Martra*,‡ † Department of Analytical Chemistry and NIS Center of Excellence, University of Torino, Via P. Giuria 5, Torino 10125, Italy, ‡Department of IPM Chemistry and NIS Center of Excellence, University of Torino, Via P. Giuria 7, Torino 10125, Italy, and §CNR-ISTEC, Strada delle Cacce 73, Torino 10135, Italy

Received July 30, 2009. Revised Manuscript Received October 7, 2009 A study was carried out on the consequences of the -OHsurf/F- exchange occurring at the surface of TiO2 P25 when suspended in HF/F- solutions. The maximum extent of fluorination was reached at pH 3.0, resulting in the fixation on the surface of ca. 2.5 F-/nm2. The surface features of fluorinated samples under two selected conditions were investigated by IR spectroscopy, in comparison with pristine TiO2. The collected data suggested that bridged -OHsurf, likely located on regular facets, was more resistant to exchange with F-. Combined high resolution transmission electron microscopy (HRTEM), inductively coupled plasma mass spectrometry (ICP-MS) and IR measurements indicated that the fluorination performed in the adopted condition did not induce any etching of TiO2 particles, and the -OHsurf/Fexchange appeared reversible by treatment in concentrated basic solutions. Furthermore, fluorination resulted in an increase of the Lewis acid strength of surface Ti4þ sites, which, as a consequence, retained adsorbed water molecules even after outgassing at 423 K. Such an effect involved the overwhelming majority of cations exposed on regular facets.

1. Introduction Photoeffects at the TiO2 interface have potential for a wide variety of environmentally friendly applications, ranging from photocatalytic water purification to hydrogen generation through water photosplitting, to functional coatings with self-cleaning and superhydrophilic properties.1-5 The main limit to the practical application of heterogeneous photocatalysis has so far been the low photonic efficiency of current photocatalysts. Crucial issues are the efficiency in photogenerated charge carriers separation, the rate of interfacial charge transfer reactions,6 and the low absorption in the visible spectrum. It is widely recognized that the nature of the surface influences the processes of charge separation and electron transfer to organic substrates or water. Intrinsic surface properties such as the plane exposed, reconstruction and point defects, as well as extrinsic properties such as hydroxylation and the specific adsorption can affect the nature and the energetics of charge carrier surface traps. The TiO2 Degussa P25 is a landmark for photocatalytic applications and in many instances shows unsurpassed photocatalytic activity,7 attributed to the presence of nanoheterojunctions between rutile (20%) and anatase (80%) particles8 that, on one hand, improve the efficiency of the separation of the photogenerated charges and, on the other hand, favor the suppression *Corresponding author. E-mail: [email protected]. (1) Hoffman, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69–96. (2) Thompson, T. L.; Yates, J. T., Jr. Chem. Rev. 2006, 106, 4428–4453. (3) Minero, C.; Maurino, V. In Catalysis for Renewables; Centi, G., van Santen, R., Eds.; Wiley-VCH: Weinheim, 2007; pp 351-385. (4) Fujishima, A.; Tryk, D. A.; Rao, T. N. J. Photochem. Photobiol., C 2000, 1, 1–21. (5) Minero, C.; Maurino, V.; Pelizzetti, E. In Molecular and Supramolecular Photochemistry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker: New York, 2003; Vol. 10, pp 211-229. (6) Minero, C. Catal. Today 1999, 54, 205–216. (7) Ryu, J.; Choi, W. Environ. Sci. Technol. 2008, 42, 294–300. (8) Bickley, R. I.; Gonzalez-Carreno, T.; Lees, J. S.; Palmisano, L.; Tilley, R. D. J. Solid State Chem. 1991, 92, 178–190.

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of their recombination.9 An overall consensus on the relative location of the two phases has not been reached yet, as Bickley et al.8 proposed that some anatase particles are covered by a thin overlayer of rutile while Datye et al.10 and Ohno et al.11 suggested that the peculiar performances of TiO2 P25 powder result from the contact between anatase and rutile particles. However, anatase contributes the overwhelming part of the exposed surface, as reported in papers based on high resolution transmission electron microscopy (HRTEM) and IR spectroscopy of adsorbed probe molecules.12-14 A recent comparison between 10 different TiO2 powders, with relevant differences in surface properties, showed how the selectivity toward photocatalytic oxidation of different organic substrates can change dramatically, confirming TiO2 P25 as one of the most active, but with a comparatively low activity toward carboxylic acids.7 Furthermore, the specific adsorption of ions can play an important role in TiO2 photocatalytic activity. The anatase/water interface and, more generally, the metal oxide/water interface are characterized by a charge buildup, the entity and sign of which is dependent not only on pH but also on the specific adsorption of ions,15-18 which can also affect the position of the flat band (9) Sun, B.; Vorontsov, A. V.; Smirniotis, P. G. Langmuir 2003, 19, 3151–3156. (10) Datye, A. K.; Riegel, G.; Bolton, J. R.; Huang, M.; Prairie, M. R. J. Solid State Chem. 1995, 115, 236–239. (11) Ohno, T.; Sarukawa, K.; Tokieda, K.; Matsumura, M. J. Catal. 2001, 203, 82–86. (12) Spoto, G.; Morterra, C.; Marchese, L.; Orio, L.; Zecchina, A. Vacuum 1990, 41, 37–39. (13) Cerrato, G.; Marchese, L.; Morterra, C. Appl. Surf. Sci. 1993, 70/71, 200– 205. (14) Su, W.; Zhang, J.; Feng, Z.; Chen, T.; Ying, P.; Li, C. J. Phys. Chem. C 2008, 112, 7710–7716. (15) Stumm, W. Chemistry of the solid-water interface; Wiley Interscience: New York, 1995. (16) Dzombak, D. A.; Morel, F. M. M. Surface complexation modeling; Wiley Interscience: New York, 1990. (17) James, R. O.; Healy, T. W. J. Colloid Interface Sci. 1972, 40, 53–64. (18) Nelson, B. P.; Candal, R.; Corn, R. M.; Anderson, M. A. Langmuir 2000, 16, 6094–6101.

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potential of the semiconductor.19 The adsorption of ions can thus change the driving force of electron transfer, introduce surface states that could act as carrier trapping sites and recombination centers, and inhibit the adsorption of other species. The influence of inorganic anions on photocatalytic processes over TiO2 has been known for a long time.20 Among such species are the redox inactive fluoride anions,21,22 which have received attention as surface modifiers of titania powders3,23-28 and films,29-31 and also as etching agents.32-34 Interestingly, both density functional theory (DFT) calculations and hydrothermal synthesis experiments in the presence of fluoride recently showed that fluoride termination stabilizes the high energy surfaces of anatase, reversing the relative stability of the (001) and (101) facets.35 As for the effects on molecular pathways involved in degradation processes photoassisted by titania, previous studies carried out by some of the authors21-23 indicated that surface complexation of TiO2 P25 by F- (ligand exchange reaction between the surface hydroxy groups and the fluoride ions) leads to (i) an increase in the degradation rate of organic substrates that react mainly through an OH• radical mediated pathway (e.g., phenol), with a bell shaped dependence on pH, reflecting the amount of Ti-F surface groups;21 (ii) an almost absolute prevalence of free versus adsorbed OH• as an oxidative species;22 and (iii) a sustained photoproduction of H2O2 in the presence of oxygen and formate as hole scavenger.23 The H2O2 photoformation rate follows the TiO2 surface coverage by fluoride ions, increasing as the extent of surface fluorination increases, suggesting that Fions inhibit the surface complexation of superoxide/peroxide species, derived from the e-CB reduction of O2, thus in turn inhibiting H2O2 degradation. The surface fluorination can also affect the photoelectrochemical behavior of polycrystalline TiO2 films, enhancing anodic photocurrents30 and slowing down both surface recombination and electron transfer to molecular oxygen.31 As an investigation complementary to that research, a study of the surface modifications of TiO2 P25 induced by fluorination was carried out by IR spectroscopy. This technique is well suited to monitoring the evolution of surface species such as hydroxy groups and water molecules, as well as of Ti4þ centers, by using CO as a probe molecule. (19) Graetzel, M. In Photocatalysis: Fundamentals and Applications; Pelizzetti, E., Serpone, N., Eds.; Wiley Interscience: New York; 1989, pp 123-157. (20) Abdullah, M.; Low, G. K. C.; Matthews, R. W. J. Phys. Chem. 1990, 94, 6820–6825. (21) Minero, C.; Mariella, G.; Maurino, V.; Pelizzetti, E. Langmuir 2000, 16, 2632–2641. (22) Minero, C.; Mariella, G.; Maurino, V.; Vione, D; Pelizzetti, E. Langmuir 2000, 16, 8964–8972. (23) Maurino, V.; Minero, C.; Mariella, G.; Pelizzetti, E. Chem. Commun. 2005, 2627–2629. (24) Vohra, M. S.; Kim, S.; Choi, W. J. Photochem. Photobiol., A 2003, 160, 55– 60. (25) Park, H.; Choi, W. J. Phys. Chem. B 2004, 108, 4086–4093. (26) Mrowetz, M.; Selli, E. New J. Chem. 2006, 30, 108–114. (27) Lv, K.; Xu, Y. J. Phys. Chem. B 2006, 110, 6204–6212. (28) Li, D.; Haneda, H.; Hishita, S.; Ohashi, N.; Labhsetwar, N. K. J. Fluorine Chem. 2005, 126, 69–77. (29) Tang, J.; Quan, H.; Ye, J. Chem. Mater. 2007, 19, 116–122. (30) Cheng, X. F.; Leng, W. H.; Liu, D. P.; Xu, Y. M.; Zhang, J. Q.; Cao, C. N. J. Phys. Chem. C 2008, 112, 8725–8734. (31) Monllor-Satoca, D.; Gomez, R. J. Phys. Chem. C 2008, 112, 139–147. (32) Ohno, T.; Sarukawa, K.; Matsumura, M. J. Phys. Chem. B 2001, 105, 2417– 2420. (33) Kitano, M.; Iyatani, K.; Tsujimaru, K.; Matsuoka, M.; Takeuchi, M.; Ueshima, M.; Thomas, J. M.; Anpo, M. Top. Catal. 2008, 49, 24–31. (34) Taguchi, T.; Saito, Y.; Sarukawa, K.; Ohno, T.; Matsumura, M. New J. Chem. 2003, 27, 1304–1306. (35) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Nature 2008, 453, 638–642.

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2. Experimental Section 2.1. Materials. The titania used was TiO2 P25 by Degusssa (SSABET = 50 m2 g-1; ca. 80% anatase, 20% rutile). To clean the surface of possibly adsorbed organic compounds and inorganic impurities, the TiO2 powder was suspended in water (Milli-Q purity), UV irradiated by using a mercury vapor lamp (Osram Ultravitalux 300 W, Germany) for 8 h, and then carefully washed with Milli-Q water and dried for 8 h at 50 °C. Highly pure chemicals, HF and NaOH from Sigma-Aldrich and NH4OH from Fluka, were used for the “wet” treatment of TiO2. High purity CO (Praxair) was used as a probe molecule in IR measurements, without any other purification except liquid nitrogen trapping. 2.2. Fluorination Procedure. The fluorination of the titania surface was obtained by suspending TiO2 in HF/F- solutions (TiO2 concentration 10 g dm-3; total fluoride concentration 4.0  10-3 M; F-/TiO2 ratio = 4  10-4 mol g-1) at different pHs (in the 2.0-8.0 range) adjusted by addition of NH4OH(aq), stirring for 8 h at 298 K in the dark. The suspensions were filtered through 0.45 μm cellulose acetate membranes (Millipore HA), and the liquid was analyzed by ion chromatography (Dionex DX 500 equipped with an AS9-HC column, K2CO3 9 mM as mobile phase, eluent flow rate 1 mL min-1). The amount of adsorbed fluoride ions was obtained from the difference between the measured concentration in the liquid phase of the equilibrated suspension and the nominal total fluoride concentration. On the basis of the obtained results, two fluorination conditions, at pH 3.0 and 6.0, hereafter referred to as TiO2-F/3.0 and TiO2-F/6.0, respectively, were selected for the preparation of the samples for the IR investigation. After filtering, the fluorinated powders were dried for 8 h at 50 °C. Tests on the reversibility of fluorination were performed by stirring 500 mg of TiO2-F/3.0 in NaOH solutions at pH = 10.8 or 12.0 (8 h, in the dark). The powder was recovered by filtration (Millipore HA 0.45 μm), washed with HCl solution (1.0  10-3 M) and then with Milli-Q water, and finally dried for 8 h at 50 °C. The same treatment was carried out on a TiO2 P25 sample to evaluate possible surface modifications induced by the alkaline treatment. 2.3. Methods. Surface features of pristine and fluorinated TiO2 were investigated by IR spectroscopy. The samples were pressed in self-supporting pellets (“optical thickness” of ca. 20 mg cm-2) and placed in an IR cell with KBr windows, permanently attached to a vacuum line (residual pressure = 1.0  10-6 Torr; 1 Torr = 133.33 Pa), allowing all adsorption/desorption and thermal treatments to be carried out in situ. The spectra were collected with a Bruker Vector 22 instrument (DTGS detector, resolution: 4 cm-1) and reported as absorbance. In the case of the spectra of adsorbed CO, the spectrum of the sample before CO admission was subtracted as a background. In all cases, the intensity of each spectrum was normalized to the optical thickness of the pellets (absorbance per mg cm-2). In principle, this should render the differences in intensity among the spectra related to different samples independent of the differences in thickness of the pellets; in practice, because of the error in the measurement of the area of the pellets, deviations of up to 15% of the expected results could have occurred. Possible etching of TiO2 particles as a consequence of treatments in HF or NaOH solutions was checked by ultrahigh resolution transmission electron microscopy (Jeol JEM 3010UHR operating at 300 kV). Furthermore, titanium was searched for in the supernatants by inductively coupled plasma mass spectrometry (ICP-MS, Varian, model 820).

3. Results and Discussion 3.1. Quantitative Aspects of Fluorination. In the presence of F-, a pH dependent exchange equilibrium between the surface hydroxy groups of the TiO2 and the fluoride ions is reached Langmuir 2010, 26(4), 2521–2527

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(surface exchange constant, pKF = 6.2,36 see scheme inserted in Figure 1). The amount of fluoride ions fixed at the surface can be modulated by changing the pH of the HF/F- solution. The fluoride surface exchange on P25 titania was measured as a function of pH, as described in the Experimental Section. A maximum fluoride coverage of 2.1  10-4 mol g-1 was measured, which, given the surface area of TiO2 P25 (50 m2 g-1), corresponds to 2.5 fluorinated surface sites per nm2 (Figure 1). Such experimental data are in agreement with the surface distribution of TiOH/TiF species calculated from the reported surface acidity and ligand exchange constants,21 which predicts a maximum of surface fluorination at pH 3.0 for TiO2 P25. The maximum is the result of the interplay among the acidity of the surface hydroxy groups, the complexing properties of fluoride ion toward surface Ti(IV) ions, and the acidity of hydrofluoric acid (pKa = 3.15). Due to F-/OH- surface exchange equilibrium, a decrease higher than 50% in the surface coverage of fluoride was observed when varying the pH from 3.0 to 6.0. The evaluation of the fraction of surface hydroxy groups exchanged with F- is relevant but not straightforward. Actually, we found a wide discrepancy among the values reported in the literature for the concentration of such species, ranging from 9.0 -OH nm-2, measured by DTGA/IR,37 to 3.3 and 3.0 obtained via XPS38 and acid/base titration,24 respectively. The similarity between the two last values, obtained with different methods, might suggest a closer correspondence to the actual amount of surface -OH. If this would be the case, in the absence of other phenomena (as we checked), ca. 75-80% of hydroxy groups should be substituted by F- at pH = 3.0. This aspect will be further discussed on the basis of the IR results. Furthermore, the possible action of HF as etching agent was checked, and no Ti was found by ICP-MS in the supernatant resulting from fluorination at pH = 3.0. Similarly, no changes in the surface topography were observed in high resolution TEM images (Supporting Information, Figure S1A,B). 3.2. Effect of Fluorination on Surface Hydroxy Groups and Adsorbed Water. 3.2.1. IR Spectra of Samples Outgassed at Room Temperature. IR spectroscopy in a controlled atmosphere was employed to investigate the effect of fluorination on the surface properties of titania. Figure 2 shows the spectra of TiO2, TiO2-F/3.0, and TiO2-F/6.0 after outgassing at room temperature (rt), which was expected to leave on the surface all the hydroxy groups and most of the water molecules initially adsorbed on the surface. In all cases, a series of quite narrow, partly resolved components between 3750 and 3550 cm-1, a broad absorption spread over the 3550-2500 cm-1 range, and a band at ca. 1620 cm-1, with minor features at lower frequency, were observed. The last two sets of signals are simple in nature, as they are due to the δH2O of adsorbed water molecules39 and to carbonate-like species,40 respectively. Conversely, both components at higher frequency are due to the -OH stretching modes of surface hydroxy groups [in both the linear (Ti-OH) and bridged (Ti-OH-Ti) forms] and adsorbed H2O, which, depending on their behavior as “free” or “H-bonded” oscillators, can (36) Herrmann, M.; Kaluza, U.; Bohem, H. P. Z. Anorg. Chem. 1970, 372, 308– 313. (37) Munuera, G.; Rives-Arnau, V.; Saucedo, A. J. Chem. Soc., Faraday Trans. 1 1979, 75, 736–747. (38) Erdem, B.; Hunsicker, R. A.; Simmons, G. W.; Sudol, E. D.; Dimonie, V. L.; El-Aasser, M. S. Langmuir 2001, 17, 2664–2669. (39) Morterra, C. J. Chem. Soc., Faraday Trans. 1 1988, 84, 1617–1637 and references therein. . (40) Morterra, C.; Chiorino, A.; Boccuzzi, F.; Fisicaro, E. Z. Phys. Chem. Neue Folge 1982, 124, 211–222.

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Figure 1. Fluoride content of TiO2 P25 treated at 298 K in HF solutions at different pHs (total fluoride concentrations = 4.0  10-3 M; TiO2 = 10 g dm-3). Solid symbols indicate the fluorination conditions of the samples studied by IR.

Figure 2. IR spectra, in the 3900-1200 cm-1 range, of (a) TiO2, (b) TiO2 fluorinated at pH = 6.0, and (c) TiO2 fluorinated at pH = 3.0, all outgassed at rt for 45 min. The spectra are limited on the low frequency side by the onset of the absorption due to lattice modes (maximum out of scale).

contribute to the 3750-3550 cm-1 pattern (free νOH oscillators) or to the 3550-2500 cm-1 broad absorption (H-bonded νOH oscillators)39,41-44 (see Scheme 1). The presence of both hydration species on the titania surface is well-known, and the ratio between them depends on the types of facets exposed and on the presence of surface sites in low coordination. Actually, previous HRTEM investigations of TiO2 P2512,44 indicated the presence of different types of facets, including the (001) one, on which theoretical calculations showed that, at high coverage, dissociated and undissociated H2O molecules can coexist.45-47 In addition, titania particles exhibited rough borders, with the consequent presence of sites in low coordination that both theoretical48 and experimental39,49 studies indicated as centers for the dissociation of water molecules. The comparison between the spectra in Figure 2 indicated that fluorination resulted in an increase in the intensity of the δH2O band, indicating the presence of a larger amount of adsorbed (41) Primet, M.; Pichat, P. P.; Mathieu, M. V. J. Phys. Chem. 1971, 75, 1216– 1220. (42) Tsyganenko, A. A.; Filimonov, V. N. J. Mol. Struct. 1973, 19, 579–589 and references therein. . (43) Busca, G.; Sausey, H.; Saur, O.; Lavalley, J. C.; Lorenzelli, V. Appl. Catal. 1985, 14, 245–260 and references therein. . (44) Martra, G. Appl. Catal., A 2000, 200, 275–285. (45) Arrouvel, C.; Digne, M.; Breysse, M.; Toulhoat, H.; Raybaud, P. J. Catal. 2004, 222, 152–166. (46) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Graetzel, M. Phys. Rev. Lett. 1998, 81, 2954–2957. (47) Barnard, A. S.; Zapol, P.; Curtiss, L. A. Surf. Sci. 2005, 582, 173–188. (48) Gong, X.-Q.; Selloni, A.; Batzill, M.; Diebold, U. Nat. Mater. 2006, 5, 665– 670. (49) Henderson, M. A. Langmuir 1996, 12, 5093–5098.

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Scheme 1. Schematic Diagram Showing the Location/Range, on a Wavenumber Axis, of the IR Signals Related to Surface Hydroxy Groups and Adsorbed H2O and CO

Figure 3. IR spectra, in the 3900-2500 cm-1 range of (a) TiO2 fluorinated at pH = 3.0 outgassed at rt for 45 min and (b) TiO2 outgassed at rt for 10 min. The inset shows the δH2O region.

water molecules, which should also be responsible for the increase in intensity of the signals at higher frequency. Such a larger amount of adsorbed water molecules may be ascribed to an increase in the number of Ti4þ surface centers with a Lewis acid strength high enough to render the coordinative interaction with adsorbed water molecules resistant to the outgassing at rt. As treatment with the HF solutions did not result in any etching of the titania surface (see below), the increase in the Lewis acid strength should be ascribed to the inductive effect on surface Ti4þ centers originating from adjoining electronegative F- ions, as proposed in the case of fluorine-doped TiO2 powders,28 TiO2 fluorinated films,29,50 and AlF3.51 Moreover, the differences in shape of the broad adsorption in the 3550-2550 cm-1 range indicated differences in the H-bonding of the additionally adsorbed molecules, as H2O stretching modes exhibit a higher sensitivity than the bending one to this kind of feature.52 As a consequence of the exchange with F-, the components related to surface hydroxy groups were expected to decrease in intensity, but, because of the superposition with the components related to the stretching modes of water, it was difficult to identify such a behavior. Nevertheless, changes in the relative intensity among the components in the 3850-3550 cm-1 range occurred, indicating that surface hydroxy groups were actually involved in the fluorination, likely to an extent depending on their type (linear, bridged) and location (facets, borders). Indirect insights on the decrease of the amount of surface -OH by exchange with F- were obtained by comparing the spectra of TiO2-F/3.0 after full outgassing at rt (Figure 3a) and of TiO2 only partially outgassed at rt (Figure 3b), which contained equivalent amounts of adsorbed water, as assessed by comparing the intensity of the δH2O band (Figure 3, inset). Besides the difference in shape, the patterns in both the “free” and H-bonded -OH/H2O regions appeared less intense in the case of the fluorinated materials, indicating the presence of a lower amount of surface hydroxy groups. Unfortunately, the overlap between the components due to hydroxy groups and water molecules prevented us from measuring the extent of the relative decrease of the -OH population. However, the decrease in intensity of the 3850-3550 cm-1 pattern (where the contribution of hydroxy (50) Chen, Y.; Chen, F.; Zhang, J. Appl. Surf. Sci. 2009, 255, 6290–6296. (51) Morterra, C.; Cerrato, G.; Cuzzato, P.; Masiero, A.; Padovan, M. J. Chem. Soc., Faraday Trans. 1992, 88, 2239–2250. (52) Takeuchi, M.; Bertinetti, L.; Martra, G.; Coluccia, S.; Anpo, M. Appl. Catal., A 2006, 307, 13–20.

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Figure 4. IR spectra of (a) TiO2, (b) TiO2 fluorinated at pH = 6.0, and (c) TiO2 fluorinated at pH = 3.0, all outgassed at 423 K for 45 min. The spectra have been divided into three sections for proper scaling in each range.

groups should be prevalent) appeared to be less than the 75-80% expected by combining the quantitative data in Figure 1 and the reported amount of 3.0-3.3 -OH nm-2 for TiO2. 3.2.2. IR Spectra of Samples Outgassed at 423 K. To remove (at least partially) the interference between the νOH signals of adsorbed water and the surface hydroxy groups, the samples were outgassed at 423 K, as this treatment removes almost all H2O molecules but only a minor part of the hydroxy groups.37,39 Furthermore, the removal of F- was expected to be very limited, as temperatures on the order of 873 K are required for the complete desorption of such species from titania.35 The spectra recorded after such outgassing are reported in Figure 4, divided into three sections in order to allow use of the most appropriate scales for each range of interest. As a feature common to all samples, the pattern in the 3850-3550 cm-1 range (Figure 4C) appeared more intense in the corresponding spectral profile after outgassing at rt (see Figure 2), indicating the conversion of -OH species previously interacting via H-bonds with H2O molecules (and other -OH groups) into “free” hydroxy groups. Conversely, in the other two ranges, the spectral intensity appeared to decrease because of the desorption of water. In the case of the pristine TiO2, the δH2O band appeared extremely weak (Figure 4A, a), indicating the presence of only a residual amount of adsorbed water, accompanied at lower frequency by similarly weak components due to carbonate-like species. The stretching modes of adsorbed H2O should contribute to the broad component in the 3550-2500 cm-1 range for the -OH moieties involved in H-bonds (Figure 4B, a) and to the 3750-3550 cm-1 range for those pointing outward from the surface (Figure 4C, a), such as, for example, the pair at Langmuir 2010, 26(4), 2521–2527

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3630-3415 cm-1.39 However, these latter, if present, must account for only a minor part of the complex pattern observed in this region, which now exhibits a much higher relative intensity with respect to the δH2O band (the “marker” of water molecules) than that after outgassing at rt (Figure 2a). A similar type of evaluation for the H-bonded hydroxy groups turned out to be more difficult, and it can be only stated that, if present, they should contribute along with water molecules to the broad absorption over the 3550-2500 cm-1 range. Focusing on the 3750-3550 cm-1 region, it can be observed that the spectral profile exhibited several components (Figure 4C, a), suggesting a wide heterogeneity of surface hydroxy groups. Besides the seminal work of Busca et al.43 dealing with general assignment, to the best of our knowledge, a precise interpretation of the stretching bands of hydroxy groups of TiO2 P25 has not yet been proposed, despite the broad use of this material as a kind of benchmark in the field of heterogeneous photocatalysis. Nevertheless, the investigation of the IR features of the surface -OH of titania powders has been the subject of many papers,38,41-43,53-57 which based the assignment of various νOH bands on the pioneering works by Primet et al.41 and Tsyganenko and Filimonov.42 In these papers, νOH bands at frequencies higher than 3680 cm-1 were assigned to linear hydroxy groups, while bands at lower frequency were related to bridged -OH, with the two species resulting from the dissociation of H2O on pairs of coordinatively unsaturated Ti4þ and O2- surface sites. However, more recent theoretical works proposed that the dissociation of water Ti-O pairs exposed on reactive anatase TiO2 faces such as the (001) one also resulted in the breaking of the Ti-O bond,45,46 with the production of two linear Ti-OH, one pointing out from the surface and the other involved as the donor in a hydrogen bond with the previous one. Also, these papers accounted for the dependence of the Ti-OH frequency on the surface morphology, but calculations indicated that only νOH bands related to the stretching modes of molecularly adsorbed water should fall in the 3680-3600 cm-1 range. Conversely, experimental data indicated the presence in that range of νOH bands even in the absence of adsorbed H2O molecules.39,42,43,53,54 In addition, both experimental39,49 and theoretical48,58 investigations have pointed out the dissociative character of the adsorption of H2O on coordinatively defect sites such as steps and corners, the amount of which should increase as the size of the TiO2 particles decreases. It is worthy of note that modeling indicated that the dissociation of water on this kind of sites did not produce the breaking of the Ti-O bond at the surface defect,48,58 in which case both linear and bridged hydroxy groups should be produced. Unfortunately, to the best of our knowledge, no vibrational analyses were performed as part of such modeling. In summary, by combining the overview of the literature data with the previous HRTEM investigations of TiO2 P25 cited above, indicating the presence of different types of facets reactive toward waters, such as anatase (001), and rough borders, which should expose a significant amount of coordinatively defect sites, it is possible to conclude that the multiplicity of components present in the spectra of the pristine material outgassed at 423 K (Figure 4C, a) can be related to the presence of hydroxy groups on (53) Hadjiivanov, K. I.; Klissurski, D. G. Chem. Soc. Rev. 1996, 25, 61–69. (54) Szczepankiewicz, S.; Colussi, A. J.; Hoffman, M. R. J. Phys. Chem. B 2000, 104, 9842–9850. (55) Du, P.; Bueno-Lopez, A.; Verbaas, M.; Almeida, A. R.; Makkee, M.; Moulijn, J.-A.; Mul, G. J. Catal. 2008, 260, 75–80. (56) Finnie, K. S.; Cassidy, D. J.; Bartlett, J. R.; Woolfrey, J. L. Langmuir 2001, 17, 816–820. (57) Khadzhiivanov, K. I.; Davydov, A. A.; Klisurski, D. G. Kinet. Catal. 1986, 29, 161–167. (58) Gong, X.-Q.; Selloni, A. J. Catal. 2007, 249, 134–139.

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sites with a different local structure, exposed on both reactive facets and borders, and that the subbands located at ν > 3680 cm-1 might be ascribed to linear Ti-OH species while those at lower frequency might be ascribed to Ti-OH-Ti bridged species. Moving on to the TiO2-F/6.0 sample, the amount of water molecules left adsorbed after outgassing at 423 K (Figure 4A, b), as indicated by the intensity of the δH2O band, was somewhat higher than that for pristine TiO2, indicating that the Lewis acid strength of some of the surface cationic sites was increased enough to render the coordinative interaction of H2O molecules resistant to desorption at higher temperature. It is worth noting that the higher intensity of the δH2O band in Figure 4A, b is accompanied by a broadening toward the low frequency side, as expected for water molecules experiencing a stronger coordinative interaction with cationic sites without significant changes in H-bonding.52 The weak component at ca. 1550 cm-1, related to carbonate-like species, disappeared, while a signal at about 1445 cm-1 was still present. Such a behavior can be interpreted as a change in the type/structure of the carbonate-like species40,59 or as a removal of such groups during fluorination, accompanied by the fixation of other species onto the surface. In this case, the signal at 1445 cm-1 might be assigned to the antisymmetric deformation mode of adsorbed ammonium ions, with the partner stretching modes producing the additional components overlapping the broad νOH absorption related to H-bonded hydroxy groups and/or water molecules in the 3550-2550 cm-1 range (Figure 4B, b).60 However, the main effect of fluorination was the overall decrease in intensity, accompanied by a significant simplification of the νOH pattern related to “free” hydroxy groups (Figure 4C, b), which after fluorination exhibited only three components: at 3675 (main), 3648 (medium), and 3730 (weak) cm-1. Such a behavior clearly indicates the substitution of hydroxy groups by fluoride anions, and the larger impact on the νOH components at frequency g 3690 cm-1 indicates that the OH-/F- exchange preferentially involved linear hydroxy species. Moreover, the well-defined shape of the signals at 3675 and 3648 cm-1 suggests that each of the two families of bridged hydroxy groups that resisted the exchange with F- at pH = 6.0 (and the outgassing at 423 K) should be characterized by a quite homogeneous structure of the hydroxy sites, such as those exposed on facets. Finally, in the case of TiO2-F/3.0, an even simpler pattern was observed in the 3750-3550 cm-1 range, where only the component at 3675 cm-1, asymmetric toward the low frequency side, was still present (Figure 4C, c), indicating the resistance of one type of bridged hydroxy groups to the exchange with F-. The spectral features at lower frequency, related to adsorbed water molecules (Figure 4B and C, curves c), appeared significantly more intense than those of TiO2-F/6.0, indicating the presence of a larger amount of surface Ti4þ sites with Lewis acidity increased enough to render the coordinative interaction with H2O resistant to outgassing at 423 K. Actually, the integrated intensity of this band was ca. 75% of that observed after outgassing at rt (Figure 2c). Moreover, the absorption in the 3550-2550 cm-1 range also appeared broadened toward the low frequency side, indicating that water molecules were also involved in stronger H-bonds (Figure 4B, c). This feature could be also responsible for the upward shift of the δH2O band with respect to the previous case (Figure 4A, c).52 The extent of the decrease in intensity of the spectral pattern in the 3850-3550 cm-1 range deserves some comment. Assuming (59) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7, 89–126. (60) Rajadhyaksha, R. A.; Kn€ozinger, H. Appl. Catal. 1989, 51, 81–92.

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Figure 5. IR spectra, in the 3900-3550 cm-1 range, of (a) TiO2, (b) TiO2 fluorinated at pH = 3.0, (c) TiO2 fluorinated at pH = 3.0 and then treated in NaOH solution at pH= 12.0, and (d) TiO2 treated in NaOH solution at pH = 12.0. All samples were outgassed at 423 K for 45 min.

that the -OH stretching mode of linear and bridged hydroxy groups should exhibit similar extinction coefficients, the ratio between the integrated areas of those patterns indicated that ca. 50% of the hydroxy groups that resisted the outgassing at 423 K were substituted by F- at pH = 6.0. This value increases to ca. 60% for fluorination at pH = 3.0. This trend does not correspond to the quantitative data reported in Figure 1, where in passing from pH = 3.0 to 6.0 the extent of fluorination decreased by ca. 50%. However, it can be considered that these values are related to the overall amount of hydroxy groups and that fluorination could have affected their resistance to condensation under outgassing at 423 K. Finally, as in the case of the samples outgassed at rt (Figure 3), we carried out a check that the described spectral behavior was actually due to the replacement of OH- by F-, and not to the conversion of hydroxy groups from the “free” to the H-bonded form because of an interaction with additional water molecules left adsorbed on the surface. Toward this aim, the spectrum of TiO2-F/3.0 outgassed at 423 K (i.e., the sample that retained the highest amount of H2O after such treatment) was compared with the spectrum of TiO2 simply outgassed at rt (i.e., the pristine material with the highest amount of adsorbed water) (Supporting Information, Figure S2). It was clearly observed that, in the latter case, a much richer pattern was present in the 3850-3550 cm-1 range, despite the much higher amount of adsorbed H2O (see the relative intensity of the δH2O band in the inset). 3.3. Reversibility of Fluorination. The back-exchange of F- with OH- was checked by treating aliquots of the TiO2-F/3.0 sample with NaOH solutions at increasing pH. The effect of such treatments was monitored by IR spectroscopy (on pelletized samples outgassed at 423 K), and no significant back-exchange was obtained until pH = 12.0 was reached. As shown in Figure 5, where the spectra of TiO2 (curve a) and TiO2-F/3.0 (curve b) are also reported for comparison, several components appeared again in the 3750-3550 cm-1 range after such treatment (Figure 5c) and the δH2O band exhibited an intensity as weak as that for TiO2 (inset, curve c). The positions of the νOH components were similar to those observed for TiO2, while the relative intensity was significantly different. However, a similar pattern was obtained by treating an aliquot of TiO2 with NaOH solution at pH = 12.0 (Figure 5d), confirming that fluorination should not have induced significant modification 2526 DOI: 10.1021/la902807g

Minella et al.

Figure 6. IR spectra of CO adsorbed at 100 K on (A) TiO2 and (B) TiO2 fluorinated at pH = 3.0, both outgassed at 423 K for 45 min. The CO pressure ranged from 15 to 1.0  10-4 Torr, in the sense of the lettering. The insets show the spectra of the samples: (a0 ) before CO adsorption, (b0 ) after admission of 15 Torr CO, and (c0 ) after decreasing the CO pressure to 2 Torr.

of the TiO2 surface structure. As no evidence for the etching of TiO2 particles was found either by ICP analysis of the supernatant or by HRTEM inspection of the powder (Supporting Information, Figure S1A,C), the modification of the spectral pattern observed after the treatment with NaOH might be the result of the deprotonation of surface hydroxy groups, producing negative surface centers where Naþ ions could have been fixed by electrostatic interaction. 3.4. Extent of the Effect of Fluorination on the Lewis Surface Acidity. The data reported above indicated that the presence of F- on the TiO2 surface increases the Lewis acidity of Ti4þ centers that, due to their ability to adsorb water in a molecular form, should be exposed on facets. To confirm this location, and to estimate the extent of the surface involved in this effect of fluorination, the IR spectra of CO adsorbed on TiO2 and TiO2-F/3.0 have been recorded. The adsorption of CO was carried out at low temperature (ca. 100 K) in order to allow the interaction of the probe molecules with coordinatively unsaturated Ti4þ on facets.12,44,53 The samples were pre-outgassed at 423 K, as such treatment resulted in an almost complete removal of water molecules from the pristine TiO2 (see above). The spectra obtained for this sample are reported in Figure 6A. At high CO coverage, an intense peak at ca. 2180 cm-1 (maximum out of scale in the figure), a very weak band at 2212 cm-1, and several components in the 2170-2100 cm-1 range were observed. Except for the minor feature at 2212 cm-1, the various components can be assigned based on the data in refs 12, 43, and 53. Thus, the main peak is due to CO molecules adsorbed on surface five-coordinated Ti4þ ions [as those exposed on (010) planes], the weaker component at 2164 cm-1 to CO stabilized onto Ti4þ surface centers with a lower Lewis acidity, located on less energetic planes [e.g., the (001) ones], while the absorption at 2154 cm-1 is related to CO interacting with surface OH groups (see Scheme 1). As a consequence of this interaction, the νOH pattern in the 3800-3550 cm-1 range was shifted to lower frequency (Figure 6, inset, curves a0 ,b0 ). However, the possibility cannot be excluded that CO molecules adsorbed on weak Lewis acid Ti4þ sites also contributed to the signal peak at 2154 cm-1, as found for a different type of anatase particles.43 Finally, the broad Langmuir 2010, 26(4), 2521–2527

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band at 2140 cm-1 is assignable to physically adsorbed CO in a liquidlike form, while the minor signal at 2127 cm-1 corresponds to the stretching of 13CO molecules (natural abundance: ca. 1%) adsorbed on Ti4þ ions on (010) type faces. In decreasing the CO coverage (Figure 6A, a-m), the components due to CO in liquidlike form and CO adsorbed on hydroxy groups were the first to disappear, and the original νOH pattern in the 3800-3550 cm-1 range was restored by decreasing the CO pressure to 2 Torr (curve c0 in the inset), followed by the 2165, 2180, and 2212 cm-1 bands, due to CO adsorbed on stronger sites. The shift of the 2180 cm-1 peak toward higher frequency following the decrease of the CO coverage was due to the progressive fading of adsorbate-adsorbate interaction as the amount of adsorbed CO decreased. As for the origin of the weak band at 2212 cm-1, it must be considered that the sample was outgassed at 423 K, leaving on the surface hydroxy groups that should also occupy edges, steps, and corners, and the dependence on the CO coverage exhibited by this weak component appeared similar to that of the main peak at 2180 cm-1. Both of these features make it possible to exclude assignment to CO adsorbed on Ti4þ ions different from those of the facets. A detailed discussion is outside the scope of the present paper, but we can anticipate that, on the basis of additional experiments, carried out at temperatures lower than 100 K, it will be possible to propose an assignment to a combination mode involving the νCO and a very low frequency mode of CO molecules adsorbed on facets.61 Moving on now to the TiO2-F/3.0 sample, a significantly different spectral pattern was obtained (Figure 6B), where the band related to CO on Ti4þ sites on “energetic” facets exhibited a rather weak intensity and a position of the maximum (at 2181 cm-1) almost insensitive to CO coverage. Such features clearly indicated that only a minor fraction of the cationic sites exposed on facets of TiO2 at the maximum level of fluorination were available for interaction with CO, while the overwhelming majority of them were still occupied by water molecules, retained more firmly because of the increased Lewis acid strength. The second interesting feature is the main spectral component at lower frequency, resulting from the overlapping of at least three subbands, at ca. 2165 (very weak), 2158, and 2150 cm-1. The first and the third ones can be assigned to CO adsorbed on Ti4þ sites on facets of the (001) type and on hydroxy groups (40% of those present on the pristine TiO2 outgassed at the same temperature), respectively. The effect of CO adsorption and desorption on the νOH band of such species is shown in the inset of Figure 6B. Conversely, the assignment of the main component at 2158 cm-1 is not straightforward. The possibility that this band is due to CO interacting with the protons of H2O molecules left adsorbed on (61) Martra, G.; Deiana, C.; Davit, P.; Coluccia, S. Phys. Chem. Chem. Phys., to be submitted.

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the surface, as occurred in the case of mesoporous silicoaluminas,62 was excluded because of the absence of νOH bands arising from the -OH moieties of water pointing out from the surface. Thus, on the basis of position, reversibility under decreasing CO pressures, and intensity, it is proposed that the 2158 cm-1 signal is due to CO molecules adsorbed on very weak Lewis acid Ti4þ sites. Some such sites could be present on the pristine TiO2 (adsorbing CO molecules contributing to the broad with a maximum at 2154 cm-1), and others could result from the increase in adsorbing strength of Ti4þ ions originally characterized by a very weak Lewis acidity.

4. Conclusions The investigation provided evidence of two noteworthy features of the exchange of surface hydroxy groups of TiO2 P25 with fluoride anions: (i) among the numerous types of -OH, not all are substituted, and in particular one type of bridged hydroxy species (characterized by an IR band at 3675 cm-1) is less prone to exchange with F-; (ii) fluorination should occur on both borders and facets of the TiO2 particles, and the overwhelming majority of Ti4þ sites exposed on the latter exhibited an increased Lewis acid strength, which resulted in wider and stronger adsorption of water. Notably, this last phenomenon is responsible for free •OH generation in solution21 and can also play an interesting role in applications of TiO2 other than heterogeneous photocatalysis, in particularly those related to surface hydrophilicity. Similarly, the insight at the molecular level into the local structure of surface hydroxy groups and the changes induced by fluoride exchange may be relevant not only for a better understanding of the photocatalytic process, but also for other applications of the photoeffects at the TiO2 surface, such as wettability phenomena and the charge carrier dynamic in dye-sensitized solar cells (DSSCs). Acknowledgment. The authors are grateful to Universita di Torino (Ricerca Locale) and Regione Piemonte (Project D34: Nanostructured polymeric materials for the fabrication of functional coatings) for financial support. Furthermore, the Compagnia di San Paolo is acknowledged for the support to the NIS Center of Excellence. Supporting Information Available: TEM images of pristine TiO2 particles and of TiO2 particles after treatment in HF or NaOH solutions; comparison between the IR spectra of TiO2 outgassed at rt and TiO2-F/3.0 outgassed at 423 K. This material is available free of charge via the Internet at http://pubs.acs.org. (62) Garrone, E.; Onida, B.; Bonelli, B.; Busco, C.; Ugliengo, P. J. Phys. Chem. B 2006, 110, 19087–19092.

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