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New Titanium Hydroxyfluoride Ti0.75(OH)1.5F1.5 as a UV Absorber A. Demourgues,* N. Penin, E. Durand, F. Weill, D. Dambournet, N. Viadere, and A. Tressaud Institut de Chimie de la Matie`re Condense´e de Bordeaux-CNRS, UniVersite´ Bordeaux 1, 87 AVenue du Dr. A. Schweitzer, 33608 Pessac cedex, France ReceiVed NoVember 7, 2008. ReVised Manuscript ReceiVed January 26, 2009
The microwave-assisted route allows the synthesis of a new Ti hydroxyfluoride adopting a derived form of a ReO3-type network. Combined techniques such as (i) powder X-ray and neutron diffraction coupled with electron microscopy; (ii) chemical analysis and density measurements; and (iii) FTIR and TGA analyses support the occurrence of Ti vacancies and the stabilization of hydroxyl groups in the vicinity of Ti4+ cations. A superstructure of the ReO3 network has been proposed with two Ti sites and two anionic (OH/F) positions. This original compound exhibits UV-shielding properties with an optical band gap around 3.2 eV and could be considered for potential applications as protective UV absorbers.The O(2p)-Ti(3d) charge transfer band, responsible for this absorption, implies the OH groups and the nonbonding character of the 2p valence band. By changing the synthesis conditions which become reducing, Ti3+ can be stabilized in this network in an elongated octahedral site as revealed by ESR experiments and UV-visible spectroscopy. This is a second example following the recently prepared Al-based fluoride hydrate with Al vacancies, in which a polarizing cation such as Ti4+ can accommodate cationic vacancies.
Introduction Zinc and titanium oxides are known as inorganic UVabsorber materials; they are characterized by an absorption edge at a wavelength in the 380-400 nm range corresponding to the charge transfer O(2p)-M(4s/3d) (M ) Zn/Ti). However, they present major drawbacks: (i) they both exhibit high photocatalytic activity under UV irradiation that can induce a photodegradation of the organic medium in which they are dispersed (varnish, paint, polymers, etc.) and1,2 (ii) titanium oxide varieties exhibit high refractive indices (n), thus lowering the diffusion of visible light at λ ) 500 nm (rutile: n ) 2.7 and anatase: n ) 2.5).3 As a consequence of these high refractive indices, a whitening of the medium is generally observed, limiting the use of such materials as UV absorbers in the case of wood protection, for example. An increase in transparency in the visible light region can be achieved by decreasing the particle size down to the 50-100 nm range. In the last years, numerous papers have been published concerning cerium oxide CeO2 as a possible alternative (Eg ) 3.2 eV, n ) 2.2-2.4; see for instance ref 4). To overcome the drawbacks of titanium oxide, our approach has been to increase its “anionic electronegativity” focusing our attention on the modification of the nature and the number of anions in the vicinity of the cations and to decrease the electronic polarizability. In this scope, the * Corresponding author. E-mail:
[email protected].
(1) Yu, J. C.; Yu, J.; Ho, W.; Jiang, Z.; Zhang, L. Chem. Mater. 2002, 14, 3808–3816. (2) Hong, R.; Pan, T.; Qian, J.; Li, H. Chem. Eng. J. 2006, 119 (2-3), 71–81. (3) Masui, T.; Yamamoto, M.; Sakata, T.; Mori, H.; Adachi, G. J. Mater. Chem. 2000, 10, 353–357. (4) Masui, T.; Fujiwara, K.; Machida, K.; Adachi, G. Chem. Mater. 1997, 9, 2197–2204.
Ti-O(OH)-F system was investigated. Partial substitution of oxygen atoms by fluorine ones is associated with (i) the reduction of the network electronic polarizability and of the dielectric constant with the consequence that compounds with lower refractive indices should be obtained with a higher transparency in the visible light region and (ii) the occurrence of localized charges inducing the loss of photocatalytic activity. The first reported titanium oxyfluoride, TiOF2, was synthesized and characterized in 1955 by Vorres and Donohue.5 Focusing on this composition, in 2006, Reddy et al.6 investigated the electrochemical behavior of TiOF2 with Li metal under discharge-charge conditions. Topotactic Li reactions with other ReO3-type compounds have been investigated.7 More recently, Mel’nichenko et al.8 have shown the occurrence of a few other titanium oxyfluorides; however, no information about the crystal structure of these compounds was given. In addition other ammonium oxofluorotitanates were prepared and the structures described.9,10 Finally, the optical and electrical properties of Ti oxyfluorides adopting the rutile form were presented.11 Numerous hydrothermal syntheses have been developed to prepare the divided TiO2 phase.12,13 In our group, several (5) Vorres, K.; Donohue, J. Acta Crystallogr. 1955, 8, 25. ]. (6) Reddy, M. V.; Madhavi, S.; Subba Rao, G. V.; Chowdari, B. V. R. J. Power Sources 2006, 162, 1312–1321. (7) Murphy, D. W.; Greenblatt, M.; Cava, R. J.; Zahurak, S. M. Solid State Ionics 1981, 5, 327–330. (8) Mel’nichenko, E. I.; Krysenko, G. F.; Epov, D. G.; Rakovi, E. G. Russ. J. Inorg. Chem. 2001, 46 (12), 1941–1945. (9) Patarin, J.; Marcuccilli-Hoffner, F.; Kessler, H.; Daniels, P. Eur. J. Solid State Inorg. Chem. 1994, 31, 501–511. (10) Laptash, N. M.; Maslennikova, I. G.; Kaidolova, T. A. J. Fluorine Chem. 1999, 99, 133–137. (11) Subbarao, S. N.; Yun, Y. H.; Kershaw, R.; Dwight, K.; Wold., A. Inorg. Chem. 1979, 18 (2), 488–492. (12) Yin, H.; Wada, Y.; Kitamura, T.; Kambe, S.; Murasawa, S.; Mori, H.; Sakata, T.; Ynagida, S. J. Mater. Chem. 2001, 11, 1694–1703.
10.1021/cm8030297 CCC: $40.75 2009 American Chemical Society Published on Web 03/18/2009
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Al-based hydroxyfluorides have been prepared by an original synthesis route, namely, the microwave-assisted route,14-16 and the structural features have been determined and correlated to the acidic properties. The same method has been adapted to prepare original Ti(IV)-hydroxyfluorinated compounds highly divided.17,18 The peculiar heating mode of the microwave irradiation19 leads to a rapid and homogeneous heating that gives rise to numerous advantages as compared to conventional routes. Such advantages can be summarized as follows: increase of the kinetics of reaction, phase selectivity, homogeneous precipitation, and fast evaluation of the relevant synthesis parameters. Additionally, because of the interest of such a method, microwave oven technology dedicated to the chemistry has been developed over the past decade offering safe equipment and very reproducible experiments owing to the possibility to accurately control the temperature, pressure, and power. During the synthesis investigation, the modification of the R ) [HF]/[Ti] ratio has enabled the preparation of several compounds that adopt different structural types. For increasing R values, compounds deriving from the anatase, the hexagonal tungsten bronze, and the ReO3 types have been thus synthesized.17 Among them, the structural properties of the ReO3-derived compound obtained for R ) 3 that is, for the highest fluorine content, are detailed in this paper. The effect of the substitution of fluoride ions F- for O2ones in the vicinity of Ti4+ cations has been investigated as a first step on the basis of electronic structure calculation, by determining the variation of dielectric function and optical constants, refractive index, and attenuation coefficient in the UV-visible range.18 These calculations showed that the light scattering properties of fluorine-substituted phases can be controlled by modifying the fluorine amount and consequently the cell volume. Chemical analysis, TGA, and density measurements allow determining accurately the chemical formulas of Ti hydroxyfluorides. The structural features of the compounds will be correlated to their UV-absorption properties. Experimental Section Titanium hydroxyfluorides were synthesized using the microwaveassisted process. Experiments were conducted in a microwaveaccelerated reaction system MARS5 (CEM Corporation, Matthews, (13) Baldassari, S.; Komarneni, S.; Mariani, E.; Villa, C. Mater. Res. Bull. 2005, 40, 2014–2020. (14) Dambournet, D.; Demourgues, A.; Martineau, C.; Durand, E.; Majimel, J.; Vimont, A.; Leclerc, H.; Lavalley, J. C.; Daturi, M.; Legein, C.; Buzare´, J. Y.; Fayon, F.; Tressaud, A. J. Mater. Chem. 2008, 18 (21), 2483–2492. (15) Dambournet, D.; Demourgues, A.; Martineau, C.; Pechev, S.; Lhoste, J.; Majimel, J.; Vimont, A.; Lavalley, J. C.; Legein, C.; Buzare´, J. Y.; Fayon, F.; Tressaud, A. Chem. Mater. 2008, 20 (4), 1459–1469. (16) Dambournet, D.; Eltanamy, G.; Vimont, A.; Lavalley, J. C.; Goupil, J. M.; Demourgues, A.; Durand, E.; Majimel, J.; Rudiger, S.; Kemnitz, E.; Winfield, J. M.; Tressaud, A. Chem. Eur. J. 2008, 14 (20), 6205– 6212. (17) Penin, N.; Viadere, N.; Dambournet, D.; Tressaud, A.; Demourgues, A. Mater. Res. Soc. Symp. Proc. 2006, 891. (18) Rocquefelte, X.; Goubin, F.; Montardi, Y.; Viadere, N.; Demourgues, A.; Tressaud, A.; Whangbo, M. H.; Jobic, S. Inorg. Chem. 2005, 44 (10), 3589–3593. (19) Rao, K. J.; Vaidhyanathan, B.; Ganguli, M.; Ramakrishnan, P. A. Chem. Mater. 1999, 11, 882–895.
Demourgues et al. NC, U.S.A.) operating at 2.45 GHz with a power supply varying in the 300-1200 W range, using HP500 vessels. The multimode instrument was equipped with temperature and pressure monitoring device (Pmax ∼ 30 bar; Tmax ) 200 °C). To accurately investigate the Ti-O(OH)-F system, several syntheses were performed with a ratio R ) [HF]/[Ti] ranging from 0 to 3; TiOCl2 · (HCl)x (Ti ∼ 15%; HCl ∼ 38 - 42%) and HF aqueous solution (40%) were used as precursors. Two other new compounds will be detailed in a forthcoming paper. In the case of Ti hydroxyfluorides containing a low Ti3+ content, identified by ESR experiments, isopropanol was preferred to water as solvent, as well as Ti isopropoxide instead of TiOCl2 as a precursor, and the R) [HF]/[Ti] precursor ratio varied from 3 to 4. The microwave-assisted synthesis could be summarized as a twostep process. The first one, corresponding to an activation period, consisted of three steps: heating up to 90 °C at a rate of 12 °C/ min, annealing at 90 °C for 30 min, and cooling down to room temperature. It should be noted that for synthesis temperatures higher than 90 °C and annealing durations longer than 30 min, TiO2 anatase-type structure was obtained as an impurity. At the end of the first step, a limpid solution was obtained. The second step consisted of a microwave-assisted precipitation under drying conditions with an argon flow and a primary vacuum at 90 °C; the resulting white powder was recovered, further washed with alcohol, and filtrated using a Millipore Amicon Stirred Ultrafiltration Cell. Finally, the powder was outgassed at 100 °C under primary vacuum for 4 h. Structural properties of titanium hydroxyfluorides were characterized by X-ray and neutron powder diffraction. For X-ray characterization, diffraction data were recorded on a PANalytical X’Pert PRO diffractometer in the Bragg-Brentano geometry using a Ge(111) incident beam monochromator (Cu KR1 radiation; λ ) 1.54056 Å) and equipped with X’Celerator dectector. For neutron characterization, data collection was realized at ILL (Grenoble) on the CRG-D1B line using the wavelength (λ ) 1.287 Å). To minimize inelastic scattering of hydrogen atoms, isotope exchange was performed using D2O as solvent. The diffraction data were analyzed using the Rietveld technique as implemented in the Fullprof Suite Program.20 Peak shape was described by a pseudo-Voigt function, and the background level was fitted with linear interpolation between a set of given points. Transmission electron microscopy (TEM) experiments were performed with a JEOL 2000 FX microscope operating at 200 kV. Chemical analysis was carried out to accurately determine the chemical composition of the compound and to confirm the structural model refined using both combined X-ray and neutron diffraction data. F/Ti atomic ratio, as well as the homogeneity of the sample, was checked by wavelength dispersive spectrometry (WDS) using a CAMECA SX100 electron probe microanalysis (EPMA) on samples pelletized to get a planar surface. The quantitative determination was performed on the basis of the intensity measurements of the more energetic Ti, O, and F X-ray emission lines using TiO2 and TiF3 as reference compounds. The fluorine content was also quantified by F- titration with specific electrode at Analysis Central Service (SCA - CNRS, Vernaison, France). Density measurements were performed using either a liquid medium (bromobenzene)21 or Micromeritics pycnometer operating under helium flow (Micromeritics AccuPyc 1330 Instrument). Thermal analyses were conducted in the temperature range 25-800 °C using two instruments: (i) a Setaram SETSYS Evolution thermoanalyser and (ii) a simultaneously coupled TGA-MS device, (20) Roisnel, T.; Rodriguez-Carvajal, J. Mater. Sci. Forum 2001, 378, 118. (21) Rabardel, L.; Pouchard, M.; Hagenmuller, P. Mater. Res. Bull. 1971, 6, 1325–1336.
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a Netzsch STA 409C Skimmer, equipped with a Balzers QMG 421 and a Pulse TA unit.22 Samples of 5-40 mg were measured versus an empty reference crucible. A constant purge gas flow of 70 mL/ min N2 and a constant heating rate of 10 °C/min were applied. Electronic paramagnetic resonance (ESR) spectra were recorded at T ) 5 K and at room temperature on an ESR 500 Bruker spectrophotometer working at ν ) 9.449 GHz to identify the local environments of Ti3+. The UV-visible diffuse reflectance spectra were recorded with a Varian Cary-5 SE spectrophotometer equipped with an integrating sphere. PTFE was used as reference with optimum thickness and density to ensure maximum reflectivity and therefore the best absolute accuracy. The reflectance versus wavelength measurements were made in the 300-600 nm range (i.e., 4.14-2.07 eV), step size 1 nm, with signal integration taking place in a 2 nm range.
Results and Discussion Microwave Synthesis. As already mentioned, the [HF] content is a relevant parameter since it allows the stabilization of several phases in an aqueous medium. Focusing on the R ) 3 molar ratio, the synthesis operated at T ) 90 °C with Ti oxychloride as precursor enables the preparation of a single phase related to TiOF2 type structure as revealed by X-ray diffraction analysis with crystallite size around 80 nm. Interestingly, the increase of the reaction temperature and/ or the duration time leads to the occurrence of TiO2 anatase as an impurity while an excess of fluoride ions is present. A lower synthesis temperature, that is, 90 °C, thus favors the coexistence of F- and OH-/O2- species in the vicinity of Ti4+, whereas additional energy favors the formation of the oxide through oxolation reaction. The presence of hydroxyl groups has been confirmed by FTIR spectroscopy at various temperatures. (ν + δ) bands around 4530 cm-1 as well as large bands around 3400 cm-1 (OH) or 2500 cm-1 (OD) corroboratetheoccurrenceofOH(D)groupsandOH(D)-OH(D) interactions. The effect of the nature of the synthesis medium has also been considered through the replacement of both titanium precursor and solvent. An organic medium has therefore been used containing titanium isopropoxide as precursor and isopropanol as solvent. In these conditions, two different R ) [HF]/[Ti] molar ratios have been studied, that is, 3 and 4. The corresponding X-ray diffraction powder patterns (not shown) are similar to those obtained from the aqueous medium with a broadening of the X-ray peaks suggesting smaller particle size. Here, the salient point is that the recovered powders are colored and are consistent with mixedvalency Ti. Such a point will be discussed later. Structural Characterization. In the X-ray diffraction powder pattern of the R ) 3 compound (in aqueous medium) most of main lines fall at similar 2θ values as those of TiOF2 published by Vorres and Donohue.5 These authors successfully synthesized this compound using two different routes: hydrolysis of titanium tetrafluoride or titanium trifluorochloride or reaction of aqueous or anhydrous HF with TiO2. The oxyfluoride crystallizes in the Pm3jm space group with a lattice parameter of 3.80 Å and a powder density of 3.06 g · cm-3 with one TiOF2 molecule per unit cell. The crystal (22) Feist, M.; Kemnitz, E. Thermochim. Acta 2006, 446, 84.
Figure 1. X-ray powder diffraction analysis (Rietveld) of Ti hydroxyfluoride considering two hypotheses in the Pm3jm space group: (a) without Ti vacancies and (b) with Ti vacancies.
structure is built up of corner-sharing Ti(X)6 (X ) O, F) octahedra, where the O and F atoms are randomly distributed. Therefore, in a first attempt, the structure of the R ) 3 compound was refined using a similar model, corresponding to a full occupancy of each site and a TiOF2 formula. However, such a hypothesis led to significant differences between calculated and observed XRD peak intensities, especially for the (100) and (110) reflections (Figure 1). This mismatch could be correlated with a high value of isotropic thermal displacement parameter for the Ti site (B ) 2.17(3) Å2) and a negative value for the X one (X ) O, F; B ) -0.14(4) Å2) (Tables 1 and 2). The strong dependence between site occupancy and thermal displacement parameters led us to improve the structural model by adding titanium vacancies and fixing B factors to acceptable values. Finally, both these parameters were refined simultaneously. The best fit is achieved for a refined titanium occupancy value of 0.74(1) and thermal displacement parameters of 1.06(2) and 2.55(6), for titanium and oxygen/fluorine, respectively.
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Table 1. Experimental Conditions and Refined Parameters of the Powder X-ray Diffractogram of Ti Hydroxyfluoride (Deuterated Phase) with Pm3jm Space Group (a ) 3.8101 Å) model diffraction data collection temperature diffractometer
TiOF2 X-ray T ) 293 K (deuterated) PANalytical X’Pert MPD PW 3040 Bragg-Bentano geometry, θ-θ radiation λ1 ) 1.54051 Å; λ2 ) 1.54433 Å (Cu) 2θ angular range 10-120° step, time per step 0.008°, 30 s symmetry cubic space group Pm3jm parameter (Å) a ) 3.81017(4) volume (Å3); Z 55.314(2); 1 calculated density (g/cm-3) 3.06 number of reflections 16 number of structural parameters 2 number of profile parameters 5 number of atoms 2 reliability factors Rp ) 0.212 Rwp ) 0.201 RBragg ) 0.100 Table 2. Atomic Positions, Isotropic Thermal Displacements, Occupancies, and Interatomic Distances Determined by Powder XRD (T ) 293 K) Data Analysis for Ti Hydroxyfluoride (Deuterated Phase) with Pm3jm Space Group (a ) 3.8101 Å) atom
site
x
Empirical Formula TiOF2 Ti 1a 0 O/F 3d 0
y
z
ai
B (Å2)
0 0
0 1/2
1 1
2.17(3) -0.14(4)
Distances: Ti-O/F: 1.9051(2) Å O/F-O/F: 2.6942(2) Å Table 3. Experimental Conditions and Refined Parameters of the Powder X-ray Diffractogram of Ti Hydroxyfluoride (Deuterated Phase) with Pm3jm Space Group (a ) 3.8101 Å) Taking into Account Ti Vacancies chemical formula diffraction data collection temperature diffractometer
Ti0.7500.25(OH,D)1.5F1.5 X-ray T ) 293 K (deuterated) PANalytical X’Pert MPD PW 3040 Bragg-Bentano geometry, θ-θ radiation λ1 ) 1.54051 Å; λ2 ) 1.54433 Å (Cu) 2θ angular range 10-120° step, time per step 0.008°, 30 s symmetry cubic space group Pm3jm parameter (Å) a ) 3.81012(4) 55.312(2); 1 volume (Å3); Z calculated density (g/cm-3) 2.69 number of reflections 16 number of structural parameters 3 number of profile parameters 5 number of atoms 2 reliability factors Rp ) 0.181 Rwp ) 0.164 RBragg ) 0.0372
The structure refinement using the titanium vacancies model corresponds to conventional R-factors, Rwp ) 16.4%, Rexp ) 13.1%, and RBragg ) 3.72% (Table 3). The observed and calculated diffraction profiles are drawn in Figure 3. The atomic positions are gathered in Table 4; refined bond distances are equal to 1.9051(2)Å and 2.6942(2) for Ti-O/F and O/F-O/F, respectively. These Ti-X distances are in good agreement with those generally encountered for the Ti atom. A representation of the structure is given in Figure 4. The unit cell is built of corner-shared Ti-X6 octahedra.
Experimental evidence for such a structure containing titanium vacancies can be correlated to an experimental density value smaller than the one of pure TiOF2 (i.e., 2.92 g · cm-3). Indeed, using two different density measurement methods, bromobenzene pycnometry and helium pycnometry, similar values were obtained: 2.628(3) and 2.631(2), respectively. To take into account 25% of titanium vacancies, a formula close to Ti0.75(OH)1.5F1.5 should be considered. This formula was confirmed by EPMA measurements showing the presence of 39.6% of titanium atoms and 30.6% of fluorine atoms, which is in good agreement with the 39.9% and 31.6% calculated values. From the above observations, it is clear that the compound stoichiometry is not TiOF2. However, to conclude about such a chemical formula, TEM investigations have been carried out (Figure 2). The accurate observation of the electronic diffraction patterns along the [100], [211j], and [111j] zone axes (4-fold and 6-fold axes characteristic of cubic symmetry) shows the presence of extra spots in between the most intense ones, suggesting the existence of a superstructure with a doubled a parameter (a ) 7.60 Å). Assuming this cell, all visible spots can be indexed, and two reflection conditions were deduced: 0kl, k + l ) 2n, and h00, h ) 2n. Therefore, only one space group belonging to the m3j Laue class, Pn3jm, can be considered. Regarding this last space group (Pn3jm), four atomic positions were found: 2a (0,0,0) and 6d (0,1/2,1/2) for Ti4+ and 12d (1/4,0,1/2) and 24k (x,x,z) with x ∼ 0.027 and z ∼ 0.246 for O2-/F-. The 24k position is half-occupied instead of a full occupancy for all other positions. Both regular and distorted octahedra are observed too. One should note that trials with 12g (0,0,x) atomic positions instead of 24k (x,x,z) ones did not bring any improvement. A random distribution of fluorine atoms in 24k positions (50% occupied) corresponds to a better representation of the structure of Ti0.75(OH)1.5F1.5. The Ti vacancy rate deduced from the Ti occupancy in 2a and 6d sites is equal to 0.15 instead of 0.25 as deduced from chemical analysis. Such a difference remains high, but taking into account the number of structural parameters, one should have to consider that the Ti occupancy spread into two sites cannot be determined accurately. Furthermore, the Debye-Waller factors are correct: around 0.9 Å2 for Ti and 1.38/1.22 Å2 for O/F atoms. The final reliability factors considering the Pn3jm space group are Rp ) 9.69%, Rwp ) 13.00% for the X-ray counterpart and Rp ) 1.36%, Rwp ) 1.76% for the neutron diffractogram. Experimental conditions and refined parameters of powder X-ray and neutron diffractograms of Ti hydroxyfluorides (deuterated phase, Pn3jm space group, a ) 7.6177(2) Å) are gathered in Table 5 as well as atomic positions and displacements in Table 6. Experimental and calculated X-ray and neutron diffraction patterns with this new hypothesis are compared in Figure 3. The relationships between ReO3-type structure (SG: Pm3jm, a ) 3.80 Å) and the supercell (SG: Pn3jm, a ) 7.61 Å) are represented in Figure 4. The supercell exhibits two Ti and O/F atomic positions characterizing the ordering of both Ti4+ cations and OH-/F- anions, in the as-prepared Ti hydroxyfluoride. The structure contains two regular octahedra Ti1(2a) (d(Ti1-O2/ F2) ) 1.905 Å) and six distorded octahedra Ti2(6d)
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Figure 2. TEM study of the Ti0.7500.25(OH,D)1.5F1.5 phase considering a supercell (a ) 7.6177 Å) in the Pn3jm space group with reflection conditions 0kl, k + l ) 2n, and h00, h ) 2n. A TEM micrograph of Ti hydroxyfluoride showing the size particle is given. Table 4. Atomic Positions, Isotropic Thermal Displacements, Occupancies, and Interatomic Distances Determined by Powder XRD (T ) 293 K) Data Analysis for Ti0.7500.25(OH,D)1.5F1.5 (Deuterated Phase) with Pm3jm Space Group (a ) 3.8101 Å) Taking into Account Ti Vacancies atom
site
x
y
z
Empirical Formula Ti0.7500.25(OH,D)1.5F1.5 Ti 1a 0 0 0 O/F 3d 0 0 1/2
ai
B (Å2)
0.74(1) 1
1.06(2) 2.55(6)
Distances: Ti-O/F: 1.9051(2) Å O/F-O/F: 2.6942(2) Å
(d(Ti2-O1/F1) ) 1.904Å [×4] and d(Ti2-O2/F2) ) 1.951 Å [×2]). Considering these last octahedra, a pseudo-sequence of [Ti2(O1/F1)4(O2/F2)2] elongated octahedron with [Ti1(O2/ F2)6] regular octahedron can be easily visualized. On the basis of the “Ti0.75(OH)1.5F1.5” formula deduced from chemical analysis and this polyhedron sequence, two extreme formulations can be proposed: [TiF6/2]6[Ti2/301/3(OH)4/2F2/2]6 [Ti(OH)6/2]2[Ti2/301/3(OH)1/3F8/3]6
Figure 3. X-ray (above) and neutron (bottom) data refinements of Ti hydroxyfluoride in the Pn3jm space group (a ) 7.6177 Å) taking into account vacancies in Ti sites.
To probe the anionic environment of Ti atoms, the valence bond model has been used. On the basis of the Ti-F and Ti-O average bond distances in TiF4 and TiO2 rutile-type structure, two r0 values (r0(F) ) 1.723 Å, r0(O) ) 1.815 Å) can be calculated from the Brown and Altermatt model23 Vi ) ∑j exp[(r0 - rij)/0.37], where Vi corresponds to the valence of i atoms. The first hypothesis with [TiF6/2]2[Ti2/301/3(OH)4/2F2/2]6 distribution should be considered. In this case the valence of the first Ti atom surrounded by six fluorine atoms in F2 positions is equal to +3.67, whereas (23) Brown, I. D.; Altermatt, D. Acta Crystallogr. 1985, B41, 244–247.
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Demourgues et al. Table 6. Atomic Positions, Isotropic Thermal Displacement, Occupancies, and Interatomic Distances Determined by Neutron Diffraction (T ) 293 K) Data Analysis for Ti0.7500.25(OH,D)1.5F1.5 (Deuterated Phase) with Pn3jm Space Group (a ) 7.6177 Å) Taking into Account Ti Vacancies atom
site
x
y
Ti0.7500.25(OH,D)1.5F1.5(Neutron) Ti1 2a 0 0 Ti2 6d 0 1/2 O1/F1 12f 1/4 0 O2/F2 24k 0.027(1) 0.027(1) Distances: Ti1-O2/F2: 1.9044(3) Å [×6] Ti2-O1/F1: 1.9095 Å [×4] Ti2-O2/F2: 1.9503(3) Å [×2]
Figure 4. Relationships between ReO3-type structure (SG: Pm3jm, a ) 3.8088 Å) and Ti0.7500.25(OH,D)1.5F1.5 with the supercell (SG: Pn3jm, a ) 7.6177 Å). The two Ti sites and the two O/F atomic positions characterizing the occurrence of Ti4+ cations and O2-/F- anions ordering are pointed out. Table 5. Experimental Conditions and Refined Parameters of Powder X-ray and Neutron Diffractograms of Ti0.7500.25(OH,D)1.5F1.5 (Deuterated Phase) with Pn3jm Space Group (a ) 7.6177 Å) chemical formula diffraction data collection temperature diffractometer radiation 2θ angular range step, time per step symmetry space group parameter (Å) volume (Å3); Z calculated density (g/cm-3) number of reflections number of structural parameters number of profile parameters number of atoms reliability factors
Ti0.7500.25(OH,D)1.5F1.5 X-ray Neutrons T ) 293 K T ) 293 K (deuterated) (deuterated) PANalytical X’Pert MPD CRG-D1B PW 3040 Bragg-Bentano geometry, θ-θ λ1 ) 1.54051 Å; λ ) 1.287 Å λ2 ) 1.54433 Å (Cu) 5-120° 10.5-90.5° 0.008°, 30 s 0.2° cubic Pn3jm a ) 7.6177(2) 442.05(3); 8 2.70 16 8 5 4 Rp ) 0.121 Rwp ) 0.162 RBragg ) 0.0313
Rp ) 0.015 Rwp ) 0.021 RBragg ) 0.0278
the second Ti atom surrounded by four hydroxyls in OH1 atomic positions and two fluorine atoms in F2 positions is equal to +4.23. Then the deviation from the +4 theoretical value for both valence bonds leads to considering the occurrence of OH groups in the F2 position as well as Fions in OH1 atomic position. As far as the valence bonds of F1/OH1 and F2/OH2 atomic positions are concerned, taking into account Ti vacancies in the Ti2 atomic position and F atoms in F2/OH2 position as well as OH groups in OH1/F1 atomic position, the F2 valence is equal to -0.97 and the valence of OH1 is equal to -1.04, both close to the
z
ai
B (Å2)
0 1/2 1/2 0.246(3)
0.87(3) 0.80(3) 1 0.50
0.9(2) 0.9(3) 1.38(3) 1.22(4)
O1/F1-O1/F1: 2.7004 Å O1/F1-O2/F2: 2.59(2)-2.88(2) Å O2/F2-O2/F2: 2.37(3)-2.95(3) Å
theoretical value -1. Then, the Ti vacancies seems to be exclusively distributed on Ti2 sites despite the results of XRD and neutron diffraction data refinement showing the distribution of Ti vacancies on both Ti1 and Ti2 sites. Furthermore, one should have to consider the occurrence of distribution of F/OH in two anionic sites. Thus, the electronegativity difference between the two anions OH- and F- induces a local distortion of Ti4+ cations, creating an ordering. In terms of driving force of this new atomic arrangement, the conflict between the small ionic radius of fluoride ions and its lower ligand field leads Ti4+ ions to be surrounded by various distributions of OH/F anions. Off-centered Ti4+ ions in regular octahedra will be surrounded mainly by F- ions, even if a OH/F distribution can occur on these sites, whereas elongated octahedra contain OH-/F- mixed anions, the two ligand fields creating anisotropy with Ti-vacancies stabilized in these distorted octahedral sites. Although it is rather unusual to observe a segregation between fluoride and oxide octahedra in oxide fluorides, this property is found rather often in hydrated fluorides in which MF6 octahedra alternate with [M(OH2)]6 octahedra, as in R AlF3 · 3H2O, in chain compounds RbMnF4 · H2O and CuSiF6 · 4H2O, or in inverse weberite Fe2F5 · 2H2O. Concerning the cationic vacancies, one should note that the K2NiF6 structure30 can also be described as deriving from the perovskite (elpasolite) or ReO3-type networks by an ordering between cationic vacancies and NiIV ions. It has been pointed out that new compositions containing small amounts of Ti3+ can be prepared using microwaveassisted synthesis and a large HF/Ti precursor ratio (3 < HF/ Ti < 4). In these conditions, Ti hydroxyfluorides with ordered ReO3-type structure exhibit blue or violet coloration associated with Ti4+-Ti3+ d-d intervalence bands around 600 nm (Figure 5). Additionally, the ESR signal at 4 K and room temperature (Figure 6) can be attributed to Ti3+ ions stabilized in an elongated octahedral site [g ) 1.90],24,25 thus confirming the octahedral distortion observed in the Ti0.75(OH)1.5F1.5 compound. Thermal Behavior. The thermal behavior of Ti0.75(OH)1.5F1.5 was evaluated by simultaneously coupled thermogravimetric and mass spectrometry analysis (TGA-MS). The thermogravimetric curve displayed in Figure 7 shows two distinct weight losses: a first one of 13 wt %, starting (24) Howe, R.; Gra¨tzel, M. J. Phys. Chem. 1985, 89, 4495–4499. (25) Purcell, T.; Weeks, R. A. J. Chem. Phys. 1971, 54 (7), 2800–2810.
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Figure 7. Thermogravimetry analysis coupled with mass spectrometry of the Ti0.75(OH)1.5F1.5 compound showing the water and TiF4 evolutions. Figure 5. UV-visible spectra of Ti hydroxyfluorides prepared by microwave assisted synthesis using either water, R ) HF/Ti ) 3, Ti oxychloride as precursor (red curve), or isopropanol as solvent, Ti isopropoxide as precursor, R ) 3 (green curve), or isopropanol as solvent, Ti isopropoxide as precursor, R ) 4 (blue curve).
gas is condensed at the entrance of mass spectrometer and a small amount of gas is detected with m/z ) 19 corresponding to fluorine. Based on the results of thermogravimetric analysis coupled with mass spectrometry, one can summarize the scheme of Ti0.75(OH)1.5F1.5 decomposition as follows: temperature domain (°C) 25-140 140-350
400-600
Figure 6. ESR spectra (T ) 5 K and room temperature) of Ti hydroxyfluoride prepared by microwave assisted synthesis using isopropanol as solvent, Ti isopropoxide as precursor, and R ) HF/Ti ) 3 precursor ratio.
around 140 °C and ending at 350 °C, and a second one of 45 wt %, starting around 400 °C and ending at 600 °C. These two separate weight losses are associated on the differential thermogravimetric curve with a broad peak extending from 100 to 320 °C and a sharp one centered at 570 °C. The absence of weight loss below 140 °C excludes the existence of free water (H2O). Analyses of the evolved gases by mass spectrometry indicate that the first and second weight losses are mainly related to the departure of water vapor (m/z ) 18 signal) and of fluorine (m/z ) 19 signal), respectively. Besides, to understand the huge second weight loss (around 45%), a departure of Ti- based fluoro-species coupled with that of fluorine should be considered. It would result from the high vapor pressure of TiF4, which melts at a temperature slightly higher than 400 °C.26 Unfortunately, the m/z signals relating to TiF4 species can not be detected because most of
released species stable H2O (13 wt %)
F and TiF4 (45 wt %)
resulting products Ti0.75(OH)1.5F1.5 Ti hydroxyfluorides or “TiOF2” + ε TiO2 anatase TiO2 anatase
Taking into account this scheme of decomposition, we can note that the observed weight losses are in agreement with the theoretical ones. Indeed, in a first step, a complete loss of OH groups (as 0.75H2O) corresponds to a theoretical weight loss of 15 wt %. This theoretical value is in fair agreement with the observed 13 wt % weight loss; in a second step, the decomposition of Ti-based fluoro-species (as 0.375TiF4) is associated with a theoretical weight loss of 52 wt %, slightly higher than the observed 45 wt % weight loss. The small discrepancies reported previously could be explained by a more complex decomposition scheme, assuming a simultaneous departure of OH groups and fluorine as HF. This decomposition scheme is also in agreement with the ex situ annealing of Ti0.75(OH)1.5F1.5 during 4 h under N2 atmosphere. The evolution of the X-ray powder pattern is reported in Figure 8. After the first weight loss, the XRD pattern of the sample annealed at 350 °C exhibits mainly Bragg peaks corresponding to TiOF2 (ICDD 08-0060).5 Bragg peaks corresponding to TiO2 anatase can also be noted (ICDD 21-1272).27 Finally, after the second weight loss, the XRD pattern of the sample annealed at 600 °C shows only the presence of Bragg peaks indexed with the TiO2 anatase structure. In conclusion, the thermogravimetric analysis (26) CRC Handbook of Chemistry and Physics, 74th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, 1993-1994.
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Figure 8. XRD patterns of ReO3-derived Ti hydroxyfluoride annealed under N2 during 4 h at various temperatures (T ) 350 °C, T ) 600 °C).
Figure 9. Comparison of the UV-visible reflectance of TiO2 and Ti hydroxyfluoride.
confirms the Ti0.75(OH)1.5F1.5 formula deduced from structural refinements and chemical analysis. UV-Shielding Properties. To evaluate the UV-shielding properties of Ti0.75(OH)1.5F1.5 with the one of anatase- and rutile-type forms of TiO2, the respective diffuse reflectance is compared in Figure 9. These spectra exhibit an absorption edge at a wavelength in the 380-400 nm range corresponding to the charge transfer O(2p)-Ti(3d). In the visible part at 500 nm, the lower reflective intensity for Ti0.75(OH)1.5F1.5 with ReO3-type structure (around 85%) could be related to the texture and morphology of the powder which exhibits larger size particles (around 80 nm) with respect to the 50 nm size for nanoanatase and nanorutile. To get a better understanding of the differences between these compounds, the UV-visible absorption properties have been discussed on the basis of a schematic band diagram. The electronic band structures of titanium oxides and titanium hydroxyfluorides exhibit similar features. The conduction (27) Natl. Bur. Stand., Monogr. (U.S.) 1969, 25 (7), 82.
Demourgues et al.
band is characterized by the 3d states of titanium, and the valence band is composed of the 2p orbitals of O and/or F atoms. In all these compounds, titanium is surrounded by six oxygen or fluorine atoms in an octahedral environment. The band gap is mainly dependent on three parameters: (1) First, the Ti-O bond distances play a key role; it is well-known that longer Ti-O bond distances lead to smaller band gap energy associated with the oxygen-metal transfer, and this trend has been quoted in the literature as the “redshift phenomenon” in semiconductor band gap. In perovskitetype compounds Sr(Ba)TiO3 which adopt the same octahedra framework as the ReO3-type structure, a Ti-O bond distance around 2.00 Å (BaTiO3) is related to a charge transfer band around 3.5 eV.28 Then, a higher band gap energy should be normally expected in the case of the Ti0.75(OH)1.5F1.5 compound where the Ti-O is around 1.90 Å. (2) In these compounds, three types of octahedra frameworks are found: edge-sharing, corner-sharing, and simultaneously edge and corner-sharing; the presence of edgesharing leads to strong Ti(3d)-Ti(3d) interactions, with short Ti-Ti bond distances of 3.05 Å and 2.96 Å for anatase and rutile forms, respectively. By taking into account competitive bonds, this effect may induce a larger energetic O(2p)-Ti(3d) charge transfer in the anatase form. This phenomenon explains the higher band gap of 3.20 eV for anatase-type structure TiO2, as compared to 3.1 eV for the rutile-type one. (3) The third parameter corresponds to the presence of mixed anions such as, for example, F-, O2-, and OH-. The presence of fluoride ions with a higher electronegativity compared to that of oxygen may lead to a decrease of the network polarizability and a reduction of the refractive index, and finally a more energetic band gap is expected. Taking into account the above points, the occurrence of a charge transfer band around 3.2 eV in the case of Ti hydroxyfluoride could be explained by the presence of OH groups, the reduction of electronic density around O atoms being due to H atoms and the nonbonding character of 2p O orbitals at the top of the valence band. However, the occurrence of Ti vacancies because of the reduction of O-Ti interactions leads to a decrease of bandwith of the valence band as well as the stabilization of the O(2p) valence band. Then only the stabilization of OH groups in the vicinity of Ti4+ cations can explain the evolution of absorption edge in the case of Ti hydroxyfluoride. Conclusion A new Ti hydroxyfluoride has been prepared by microwaveassisted synthesis. All performed characterizations, chemical analysis, density and TGA measurements, FTIR, and powder XRD, allow a conclusion to be made about the occurrence of Ti vacancies in a ReO3-type structure as well as fluorine substituting hydroxyl groups. All experimental data account for the chemical formula Ti0.7500.25(OH)1.5F1.5, that is, the low density value, around 2.63 g · cm-3, as well as the low electronic density on the Ti atomic position identified by (28) van Benthem, K.; Elsa¨sser, C.; French, R. H. J. Appl. Phys. 2001, 90 (12), 6156–6164.
Ti0.75(OH)1.5F1.5 as a UV Absorber
powder XRD data refinement, the F/Ti atomic ratio determined by chemical analysis, and the occurrence of OH groups specified by FTIR analysis as well as the thermal stability of this compound with the water evolution at low temperature (140 °C < T < 350 °C) and the TiF4 gas evolution at higher temperature (T > 400 °C) leading to the formation of anatase form. In a second step, TEM studies reveal the occurrence of the superstructure of the ReO3 network with two Ti atomic positions as well as two anionic (OH/F) ones. Refinements of the powder XRD and neutron diffraction patterns in the Pn3jm space group support the presence of isotropic TiX6/2 octahedra and elongated TiY4/2X2/2 (X, Y ) OH/F) octahedra. On the basis of valence bond calculations, the Ti vacancies seem mainly located in the latter distorted site, whereas F-/OH- are randomly distributed on the two anionic sites. Moreover, Ti3+ can be stabilized in this network by changing the synthesis conditions: the precursor Ti isopropoxide instead of Ti oxychloride, the R ) [HF]/Ti precursor ratio, and the solvent, isopropanol instead of water. ESR measurements confirm the occurrence of elongated octahedra taking into account the stabilization of mixed F/OH anions around Ti3+ cations. Finally, Ti0.75(OH)1.5F1.5 exhibits interesting UV-shielding properties with an absorption edge around 3.2 eV as TiO2 anatase forms but with a lower (29) Dambournet, D.; Demourgues, A.; Martineau, C.; Durand, E.; Majimel, J.; Legein, C.; Buzare´, J. Y.; Fayon, F.; Vimont, A.; Leclerc, H.; Tressaud, A. Chem. Mater., in press. (30) Taylor, J. C. Z. Kristallogr. 1987, 181, 151–160. (31) Demourgues, A.; Clabau, F.; Penin, N.; Dambournet, D.; Viadere, N.; Tressaud, A. To be submitted. (32) Demourgues, A.; Dambournet, D.; Masson, R.; Duttine, M.; Chemin, N. To be submitted.
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refractive index (n ) 1.9 in the visible range).18 The optical band gap remains low despite the low Ti-O/F bond distance and can be explained considering the stabilization of OH groups in the vicinity of Ti4+ cations which allow destabilizing the 2p (O) valence band with nonbonding character because of the presence of protons and Ti vacancies. A new Al fluoride hydrate with Al vacancies has been recently prepared by the microwave-assisted route and characterized by powder XRD, FTIR and NMR spectroscopies, and thermal analysis.29 These results mean that the polarizing cations such as Ti4+ or Al3+ can accepted OH or H2O molecules in their vicinity, forcing cationic vacancies. Thus, microwave synthesis allows preparing metastable phases exhibiting unusual chemical formulas with acidic properties in the case of Al-based compounds and UV shielding properties in the case of Ti4+ hydroxyfluoride. It can be added that, using the same microwave-assisted route with lower R ) F/Ti ratios, two other Ti-based frameworks [hexagonal tungsten bronze17 and anatase forms] have been synthesized. Their characterization by X-ray, neutron, and electron diffraction will be presented in forthcoming papers.31,32 A new generation of Ti-based UV absorbers with low refractive index due to the presence of fluorine has been thus prepared and characterized. Moreover, one should note that the titanium hydroxyfluoride Ti0.75(OH)1.5F1.5 does not exhibit any photocatalytic activity because of the stabilization of Fions in the vicinity of Ti4+ cations and the limitation of charge carriers in such ionic compounds, which is an important feature for applications in UV protection. CM8030297
