Nanostructured Ti−W Mixed-Metal Oxides ... - ACS Publications

J. Phys. Chem. B 2005, 109, 6075-6083

6075

Nanostructured Ti-W Mixed-Metal Oxides: Structural and Electronic Properties M. Ferna´ ndez-Garcıá,* A. Martıńez-Arias, A. Fuerte, and J. C. Conesa Instituto de Cata´ lisis y Petroleoquı´mica, CSIC, C/ Marie Curie s/n, 28049-Madrid, Spain ReceiVed: July 30, 2004; In Final Form: January 31, 2005

In this article, the structural and electronic properties of Ti-W binary mixed oxide nanoparticles are investigated by using X-ray diffraction, Raman, X-ray absorption spectroscopies (XAS; near edge XANES and extended EXAFS), UV-vis spectroscopy, and density functional calculations. A series of Ti-W binary oxide samples having W content below 20 atom % and with particle size between 8 and 13 nm were prepared by a microemulsion method. The atoms in these nanoparticles adopted the anatase-type structure with a/b lattice constants rather similar to those of the single TiO2 reference and with a c cell parameter showing a noticeable expansion upon doping. Within the anatase structure, W occupies substitutional positions, while Ti atoms only suffer minor structural perturbations. A change in the W local order at first neighboring distance is observed when comparing samples having a W content below and above 15 atom %. Charge neutrality is mostly achieved by formation of cation vacancies located at the first cation distance of W centers. Upon addition of W to the TiO2 structure, the Ti charge is not strongly modified, while changes in the W-O interaction appear to drive a modest modification of the W d-electron density throughout the Ti-W series. A combination of these geometrical and electronic effects produced Ti K- and W LI/LIII-edge XANES/EXAFS spectra with distinctive features. UV-vis spectra show a nonlinear decrease of the band gap in the Ti-W solid solutions with a characteristic turning point at a W content of ca. 15 atom %. The relationship between local/long-range order and electronic parameters is discussed on the basis of these experimental results.

1. Introduction Titanium dioxide (TiO2) is one of the most prominent materials in performing various kinds of industrial applications related to catalysis among which the selective reduction of NOx in stationary sources1,2 and photocatalysis for pollutant elimination3 or organic synthesis4 appear as rather important. Additional applications include its use in photovoltaic devices,5 sensors,6 paintings,7 as a food additive,8 in cosmetics,9 and as a potential tool in cancer treatment.10 The (n-type) semiconductor properties of TiO2 materials are essential in accomplishing these functions. Experimental approaches to scale down the TiO2 primary particle size to the nanometer scale are now actively investigated to improve its current applications and to reach more advanced ones such as its use in electrochromic devices.11 As a general result, the nanostructure induces an increase of surface area with concomitant enhancement of the chemical activity and also of the photochemical and photophysical activities with a potential reduction of light scattering. TiO2 occurs in nature in three different polymorphs, which, in order of abundance, are rutile, anatase, and brookite. An additional synthetic phase is called TiO2(B),12 while several high-pressure polymorphs have been also reported.13 Anatase appears to be the most important for new chemical applications as it is the stable polymorph at working temperatures for size below ca. 15 nm.14 Therefore, the majority of nanostructured materials display this specific structure. The doping of TiO2 structures constitutes an extensive field of research of current interest. Surface and bulk doping have been used to stabilize the anatase or rutile phases, influence the temperature of the anatase f rutile phase transformation, modulate the optical band gap, or alter the ionic/electrical * Corresponding author. E-mail: [email protected].

conductivity by the presence of intrinsic vacancies. The properties of the binary oxide depend primarily on the doping process nature; substitutional mixed-metal oxides have been shown to be formed in the case of Ta, Nb, and W15,16 while V17 and Fe18 appear to occupy (partially in the case of V) interstitial positions. The presence of anion vacancies for substitutional doping with trivalent/divalent ions and cation vacancies for W/Nb substitutional or V/Fe interstitial doping seems to be the main type of defect formed together with, for example, Ti lattice defects related to the presence of hydroxyls in the case of nanostructured Fe-doped TiO2 calcined at T < 673 K.18 Thus, charge neutrality appears to be a rather complex phenomena with elaborated structural/electronic implications, at least with respect to the simplest case of cerium or zirconium oxides.19 The doping process typically decreases primary particle size when compared to TiO2 samples prepared in a similar way, while the presence of the above-mentioned heteroatoms at the surface usually favors coalescence of grains/particles, with a concomitant increase of the secondary particle size and loss of surface area.16-19 On the other hand, doping with Ca, Sr, and Ba,20 or Sn,21 produces blue shifts of the optical band-gap energy which may, at least, partially be due to a decrease in primary particle size and concomitant quantum confinement effect, likely associated with the presence of the heteroatom in the TiO2 structure. In the case of V, Cr, Nb, Mo, and W,22 or rare earth atoms,23 a red shift of the optical band gap appears to be produced, but no detailed physical interpretation has been put forward. Here, we propose the complete analysis of structural and electronic effects of W doping into the anatase structure of nanometer materials. This particular system has found application in photocatalysis as it improves the performance of nanostructured anatase-TiO2 in the elimination of organic volatile compounds under visible-light excitation16,24 and may

10.1021/jp0465884 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/08/2005

6076 J. Phys. Chem. B, Vol. 109, No. 13, 2005

Fernańdez-Garcıá et al.

TABLE 1: Main Characterization Results of Ti-W Mixed-Metal Oxide and Reference Systems

a

sample

W atomic percentage (%)a

BET surface area (m2 g-1)

TiO2 W2 W4 W11 W14 W19

1.8 3.7 10.9 13.8 19.3

106 116 111 106 108 122

Cation basis (100 × W/(W + Ti)) by ICP-AAs.

be of potential interest in electrochromic devices because the W(VI)/W(V) redox pair may play an important role in modified TiO2 structures to tune color changes under potential.11 We have explored the doping of TiO2 with W in a large range, 1-20 atom %, by using a microemulsion preparation method. In this range, the presence of W-only phases is totally discarded.16 The structural characterization of such materials will be done via XRD, Raman, and EXAFS spectroscopies, while electronic characterization will be mainly done via XANES and UV-vis spectroscopies. DFT quantum mechanical calculations have been performed to interpret structural and electronic properties. 2. Experimental Section Materials were prepared using a microemulsion preparation method by addition of titanium tetraisopropoxide to an inverse emulsion containing an aqueous solution (0.5 M) of ammonium tungsten oxide dispersed in n-heptane, using Triton X-100 (Aldrich) as surfactant and hexanol as cosurfactant. The resulting mixture was stirred for 24 h, centrifuged, decanted, rinsed with methanol, and dried at 298 K for 12 h. Following the microemulsion preparation method, the amorphous Ti-W material was calcined under air for 2 h at 773 K. Samples are named Wn, n being the W atomic content. Table 1 shows the main characterization results for the synthesized materials; Ti:W composition was analyzed via inductively coupled plasma and atomic absorption (ICP-AAS), while BET surface areas were measured via nitrogen physisorption (Micromeritics ASAP 2010). XRD patterns were obtained in a Siemens D-500 apparatus using nickel-filtered Cu KR radiation operating at 40 kV and 30 mA (1200 W) with a 0.02° step size and accumulating a total of 10 s per point. Particle size and strain were extracted from XRD measurements by using the (101) and (200) peaks appearing, respectively, at ca. 25° and 48° and WilliamsonHall plots.19 Lattice parameters were calculated by a leastsquares fitting using Pearson VII functions and the Winfit Program (http://www.geol.unerlanger.de). Errors for XRDderived parameters were estimated from 10% position (cell volume and parameters) and fwhm (particle size and strain) deviations. UV-visible transmission experiments were performed with a UV-vis Varian 2300 apparatus. Raman data were acquired using a Renishaw dispersive system 1000, equipped with a single monochromator, a holographic Notch filter, and a cooled TCD. Samples were excited using the 514 nm Ar line. XAS (X-ray absorption spectroscopy) data were recorded at beamline BM-29 of the ESRF synchrotron (Grenoble, France) using a Si(111) monochromator detuned at 50% intensity. Three ionization chambers filled with N2/O2 and a Ti/W foil located between the second and the third ionization chamber were also used. A He cryostat was utilized to obtain EXAFS spectra from samples at 35 K. W-O phase and amplitude functions were extracted from a home-prepared Cr2WO6 compound (with purity checking by XRD), which displays a nearly octahedral W-O

coordination for the first shell with an average distance of 1.92 Å. The Cr2WO6 reference was prepared by following the method of Krustev.25 W-Ti phase and amplitude functions were extracted from FEFF8.0 calculations modeling an anatase-type structure by using a Ti8W2O14 cluster, having two W ions bridged by one cation vacancy to achieve electroneutrality. The model includes all Ti/W/vacancy first neighbor atoms of the W/vacancy centers. The goodness of FEFF8.0 calculations for analyzing phase and amplitude functions was tested by simulating the Cr2WO6 experimental spectrum. Fitting results were obtained by using the VIPER program (www.dessy.de/klmn/ viper.html),26 and error bars were estimated with k1/k3 weighted fittings. Computer models of the mixed oxide were built forming superlattices of the anatase structure and considering, on the basis of the other results (of this work; see below), that W is in the W(VI) state located in Ti sites, and that charge compensation occurs via the presence of cation vacancies. One of the superlattices had primitive cell vectors a ) aa + ba, b ) (3ba - 3aa + ca)/2, c ) ca, where aa, ba, and ca are the unit vectors of the tetragonal anatase cell; the second one had a′ ) 2aa, b′ ) 2ba, c′ ) ca, and a third one was derived from this latter by a′′ ) a′ + b′, b′′ ) b′ + c′, c′′ ) c′ + a′. These models have, respectively, 12, 16, and 32 cation sites/cell. The structures studied were formed by substituting in each of these superlattice cells one Ti by a vacancy, and two of the Ti ions neighboring the latter by W ions (these latter placed so that the distance between them is maximized). Thus, structures with cell contents Ti9W2O24, Ti13W2O32, and Ti29W2O64 (I, II, and III), respectively, are obtained, in which all W atoms are symmetryequivalent and defect clusters formed by one cation vacancy and two substitutional W ions (these having thus the three nearest Ti neighbors) are maximally separated from one another. An additional structure IIb was made also in which only one of the W atoms is located close to the Ti vacancy, the other one being located as far apart as possible from the W-cation vacancy pair. On these models, DFT calculations were carried out with the total energy CASTEP code (as implemented in the simulation package Cerius2),27 which uses a plane wave expansion of the periodic single-electron wave functions,28 the atomic cores (1s for O and up to 5s5p and 3s3p shells for W and Ti, respectively; nonlinear core corrections were included in the latter) being represented by ultrasoft Vanderbilt-type pseudopotentials29 derived from relativistic atomic computations. Geometry optimizations (both atomic positions and cell dimensions) and density-of-states (DOS) calculations were carried out within the LDA approximation30 and using a plane wave cutoff of 340 eV. Full use of crystal symmetry was made to speed calculations. k space grid sampling according to a Monkhorst-Plank (MP) scheme was used to give a sampling interval not higher than 0.075 Å-1. A Mulliken-type population analysis was achieved following a method31,32 in which the periodic singleelectron orbitals are projected into atomic orbitals generated independently, so that those periodic orbitals can be represented approximately as combinations of atomic contributions. 3. Results 3.1. Structural Characterization. XRD/Raman. X-ray diffraction (Figure 1) and Raman spectroscopy (Figure 2) give evidence that all samples contain titania in mainly the anatasetype structure with crystallinity generally decreasing with the W content. The absence of Raman frequencies attributable to W presence in interstitial positions33 provides evidence of the

Nanostructured Ti-W Mixed-Metal Oxides

J. Phys. Chem. B, Vol. 109, No. 13, 2005 6077

Figure 1. XRD patterns of Ti-W mixed-metal oxides.

Figure 3. Energy position and fwhm of the Eg anatase-type band located at 144 cm-1 as a function of the W content (top). Comparison with theoretical expectations (taken from ref 37) from the phonon confinement model (bottom). Figure 2. Raman spectra of Ti-W mixed-metal oxides; the spectrum of a TiO2-rutile sample is shown as a reference.

TABLE 2: XRD-Derived Parameters for the Anatase Phase of the Samplesa sample

size (nm)

microstrain 〈ξ2〉1/2

TiO2 W2 W4 W11 W14 W19

13.3 11.6 12.0 9.4 10.7 7.9

0.0011 0.0026 0.0029 0.0038 0.0032 0.0035

cell parameters (Å) a)b c 3.78 3.78 3.79 3.79 3.79 3.79

9.38 9.34 9.425 9.42 9.42 9.45

cell volume (Å3) 134.2 133.6 135.5 135.3 135.5 135.9

Maximum errors: 12% (size), 10% (strain), (0.01 Å (cell parameters), 0.5% (cell volume). a

formation of Ti-W substitutional mixed-metal oxides. As detailed in Table 2, the analysis of the XRD profiles shows that the presence of W induces varying levels of microstrain in the anatase lattice that must be mainly related to dopant-induced rather than particle size effects, considering the linear relationship between size and microstrain observed for pure and V-doped nanosized anatase-type samples.34 In addition, a small lattice expansion (affecting both parameters of the tetragonal lattice) appears to be generally produced with increasing W content, although a small contraction is observed for the least doped sample. The presence of W also affects the crystal size of the anatase phase in the samples; this parameter generally

decreases with increasing W content, sample W19 displaying the most pronounced effect. The presence of small amounts of the brookite phase of titania, which is apparently favored by W presence, is revealed by close analysis of the Raman spectra, displaying the presence of a weak peak at 246 cm-1, along with other weaker peaks or shoulders at ca. 320, 366, and 452 cm-1.35 The presence of such a phase is proposed to be responsible of the broad unresolved feature in the 22-35° 2θ region of the X-ray diffractograms onto which the anatase (101) reflection at 2θ ≈ 25° appears superimposed, taking into account that the most intense peaks of the titania brookite phase appear in that zone. As mentioned, the Raman spectra (Figure 2) show mainly peaks due to the titania anatase phase at ca. 144, 195, 399, 517, and 639 cm-1, corresponding, respectively, to the Eg(1), Eg(2), B1g(1), B1g(2), A1g(1), and Eg(3) modes expected for such a phase.36 Other details observed in the Raman spectra concern the presence, for W content g10.9%, of a line at ca. 970 cm-1 related to the WdO stretching of surface wolframyl entities.33 This Raman frequency can be correlated with a W-O distance of ca. 1.75 Å,33 which, as will be detailed, is not detected by EXAFS, indicating the small contribution of such species to the W distribution on the solids. Regarding Raman results, the main difference between the samples concerns the position and width of the first anatase Eg line at ca. 144 cm-1 (Figure 3; top); no important changes in the frequencies of the other anatase-related bands are observed upon changing the W doping level. Simultaneous variations in



Figure 5. W LI-edge XANES spectra of Ti-W mixed-metal oxides and reference compounds.

Figure 4. Module and imaginary part Fourier transforms of Ti K-edge EXAFS spectra of Ti-W mixed-metal oxides.

peak position and width on oxide systems are typical of effects related to phonon confinement rather than other structural effects (for instance, related to lattice contraction/expansion). This is supported by the fact that the change in the line shape accompanying the increase of line width, Figure 2, is mainly related to increasing the line asymmetry (higher right halfwidth), taking into account that such line characteristic is the most affected by detection of phonons outside of the Brillouin zonecenter.37,38 Additionally, the shift detected for this band goes in the same direction (blue shift) as that reported for the pure oxide subjected to hydrostatic pressure,39 further confirming that the detected increase in cell parameters and volume is not the main parameter driving the behavior observed for such an Eg mode. A quantitative analysis of both Raman observables is presented at the bottom of Figure 3; according to comparison with expectations from the phonon confinement model applied to pure anatase-type titania samples, the evolution in both width and position of the line does not appear to correlate solely with confinement effects related to decreasing the size of the anatase domains in the Ti-W samples. This would indicate that the presence of W somewhat induces further phonon confinement. In particular, sample W19 deviates most from expected values. This can be likely related to the presence of some type of W-associated defect; a similar effect has been reported to occur for some pure nanosized titania samples in the presence of nonstoichiometry attributed to vacancy presence,38 and this may thus be the primary source in our case. Of course, additionally to this, the effect of W in the effective mass and force constant of the out-of plane Eg Raman mode must be also considered to account for the observed frequency shift. XAS. Characterization of Ti-W binary oxides provides evidence also of the anatase-type structure of all materials. Ti K-edge XANES and EXAFS data indicate that the local anatasetype structure around Ti atoms is essentially the same throughout the series, without significant changes. Figure 4 displays the Fourier transform of EXAFS data for selected samples which, as mentioned, are rather similar, indicating the relatively low effect of W on the local structure of neighboring Ti cations. The only appreciable change is the moderate growth of the first coordination number with W content of the material, although

it is always very close to 6 ((15%) for all cases, as indicated by the fitting data (not shown). More apparent changes can be visualized in the W LI-, LIIIedges. The W LI-edge XANES spectra (Figure 5) indicate that the edge position of all samples is rather close to that of the W(VI) references, suggesting that W is in the VI oxidation state throughout the series.40,41 The s f d pre-edge transition, located at ca. 12 107 eV, has significant intensity as compared to the nearly octahedral (Cr2WO6 reference) and strongly distorted Cslike (WO3-orthorombic) hexacoordinated local symmetries, indicating a significant distortion of the oxygen first coordination shell of the W ions. The energy position and intensity of the W LI-edge s f d pre-edge transition are functions of the W-O average distance and local symmetry.41-43 In systems with a W(VI) oxidation state, tetrahedral local environments displace the pre-edge at lower energy and with higher intensity than octahedral ones. For samples with similar local symmetry, small to moderate intensity changes are related to the (inverse of the) W-O average distance. Although here the intensity is higher than in both references, the fact that the position is essentially the same suggests a 6-fold coordination rather than a tetrahedral one. The higher intensity would mainly indicate a higher distortion from the octahedral symmetry than those characteristic of both reference compounds. Apart from the pre-edge region, Figure 5 only displays a small shift in the continuum resonances (CRs) positions (located at ca. 12 121 and 12 138 eV) for the W19 sample to a somewhat higher energy than those observed for the remaining samples; this shift is totally consistent with the 1/R2 rule43 and would be thus attributed to a decrease in the average W-O distance, as detected by EXAFS (see below). To gain further insight into the W local structure, the W LIIIedge EXAFS spectra of the sample were obtained (Figure 6). The EXAFS spectra are somewhat similar for the Ti-W series except for the W19 case, where a different shape is manifested, particularly near k ) 7 Å-1. The signal-to-noise ratio is optimum in the W4 and W11 samples as there is a compensation-type effect between the improvement of the signal with W quantity and its worsening due to an increasing disorder around and/or at the cation positions. The presence of double excitations (2d4d f 5d5d),44 characteristic of W absorption centers in single oxide matrixes, is not detected. The analysis of the EXAFS signals quantifies all of the above-mentioned points; the corresponding fitting results are summarized in Table 3 and graphically displayed in Figure 7, which includes the Fourier transform of the experimental and simulated signals. K- and R-ranges of


J. Phys. Chem. B, Vol. 109, No. 13, 2005 6079

Figure 6. W LIII-edge EXAFS spectra of Ti-W mixed-metal oxides.

TABLE 3: W LIII-Edge EXAFS Fitting Results for Ti-W Mixed-Metal Oxides ∆σ/103 × Å2

∆E0/eV

0.7-3.3 Å 10.3 ( 0.4 11.4 ( 0.9 9.1 ( 0.6

-4.9 ( 1 -7.9 ( 1 -8.0 ( 1.5

W-O W-O W-Ti

W4;a 2.52-12.16 Å-1; 0.7-3.35 Å 4.6 ( 0.5 1.83 ( 0.01 12.0 ( 0.4 1.8 ( 0.2 2.62 ( 0.01 10.5 ( 1.2 3.2 ( 0.25 3.395 ( 0.025 5.5 ( 0.6

-4.9 ( 1 -4.7 ( 1 -11.0 ( 1.5

W-O W-O W-Ti

W11;a 2.58-11.86 Å-1; 0.7-3.35 Å 4.35 ( 0.4 1.82 ( 0.01 12.0 ( 0.4 1.55 ( 0.3 2.615 ( 0.01 7.5 ( 1.5 3.1 ( 0.2 3.415 ( 0.02 8.0 ( 0.9

-5.0 ( 1 -7.9 ( 1 -8.5 ( 1

W-O W-O W-Ti

W14;a 2.64-11.25 Å-1; 0.7-3.35 Å 4.1 ( 0.35 1.82 ( 0.01 10.4 ( 0.4 1.6 ( 0.1 2.60 ( 0.01 7.7 ( 0.6 3.3 ( 0.3 3.425 ( 0.01 8.0 ( 0.5

-4.8 ( 1 -7.1 ( 1 -7.2 ( 1.5

W-O W-O W-Ti

W19;a 2.58-11.35 Å-1; 0.7-3.35 Å 3.25 ( 0.35 1.81 ( 0.01 6.1 ( 0.4 1.7 ( 0.2 2.54 ( 0.01 9.1 ( 0.4 2.6 ( 0.3 3.40 ( 0.01 8.7 ( 1.5

-5.0 ( 1 -10.0 ( 1 -8.5 ( 1.5

shell

N

R/Å W2;a

W-O W-O W-Ti

a

4.55 ( 0.35 1.6 ( 0.2 3.2 ( 0.3

Å-1;

2.63-10.80 1.82 ( 0.01 2.61 ( 0.01 3.425 ( 0.01

Sample label followed by K and R fitting ranges.

fitting are presented in Table 3 and ensure, according to Nyquist theorem (Nf ) 2∆k∆R/π + 1),45 a number of free parameters above 15. The quality of the fittings can be visualized in Figure 7, which essentially shows a reasonable to good match of the simulated and experimental signals between 0.5 and 3.3 Å. Some shells located at larger distances can be present in the experimental spectra, but the number of free parameters does not allow a conclusive analysis of such contributions. All spectra are therefore adequately simulated by using three shells corresponding to two O distances and a third one, characteristic of the first cationic distance, populated exclusively by Ti (clearly worse fits are obtained by locating W atoms at this position). This latter clearly indicates the formation of a binary solid solution and the absence of W-only phases, confirming the XRD/Raman results. The W-O first coordination shell displays two welldifferentiated distances at 1.81-1.83 and 2.54-2.61 Å. It can be noted that the distances obtained by the fitting results can be considered accurate enough; the inclusion of cumulants up to fourth order gives maximum deviation of (0.03 Å. The W-O distance at about 2.6 Å implies a significant displacement of certain anion positions from the regular ones in anatase-TiO2. A total number of 6 oxygen neighbors is detected around W for all samples except W19, which gives a somewhat lower

Figure 7. Module and imaginary part Fourier transforms of LIII-edge EXAFS (solid line) and simulated (doted line) signals of Ti-W mixedmetal oxides.

number (5). The W19 sample also displays lower W-O distances, particularly in the case of the longer one, with respect to the other samples. This distance decrease seems parallel to a decrease in the disorder in the W-O shell, as measured by the Debye-Waller factors (Table 3), suggesting the idea that a particular local order is achieved for the W19 sample. The coordination numbers of the W-O shells would thus indicate that the first oxygen coordination is a mixture of 4+2 and 5+1 local structures with predominance of the former in samples with W content below 15 atom %, while W19 may have a dominant 3+2 coordination. The W-Ti shell appears at 3.40 ( 0.02 Å, a distance significantly longer than the Ti-Ti distance of 3.05-3.10 Å characteristic of the nanostructured anatase single oxides.46 The coordination number is roughly 3, while in nanostructured anatase-TiO2 materials with an average size similar to that here encountered it is 4,45 supporting a model in which charge compensation of W substitution is mainly achieved by presence of cationic vacancies in the series. The detection of a smaller number of oxygen and titanium neighbors in W19 would again indicate that a different situation is encountered for samples with W content above 15 atom %. DFT. The geometry optimizations of the models gave structures in which atomic distances relaxed to produce relatively heterogeneous W-O coordinations. Typically, two short distances were observed around 1.85 Å (thus longer than in WO3, which has two W-O distances of ca. 1.75 Å) and a long one around 2.15 Å (as in WO3), while another three were distributed in a more varying pattern in the range 1.9-2.1 around 1.95 Å. One exception to this behavior was found in case IIb, in which the W ion located away from the cation vacancy had a more regular coordination (all W-O bonds were in that case within the 1.90-2.005 Å range), showing the influence of defect clustering in achieving important W-O distance distortions. No W-O distances were ever found within the 2.2-3.5 Å range. W-Ti distances, on the other hand, were always in the 3.103.30 Å range, indicating the presence of Ti-Ti distances larger than in the TiO2 reference but somewhat smaller than those given by the EXAFS data. In respect to the lattice dimensions, an increase in volume per MO2 lattice motif (where M includes also any vacancy present) was generally observed in the



Figure 8. W LIII-edge XANES spectra of Ti-W mixed-metal oxides.

calculation results, amounting to 3.2%, 1.8%, 3.0%, and 1.5% for models I, II, Iib, and III, respectively. These changes are in the same direction (although significantly larger) as that found in the experimental diffraction data. Because these models are of low symmetry (monoclinic or triclinic), comparison of the cell dimensions is not straightforward, but the periodicity constant in the direction corresponding to the ca axis of the parent anatase structure increased (respect to that in pure anatase) in general in magnitudes around 1-2%, this increase being (except for model I) significantly smaller in the directions corresponding to aa and ba. The trends agree with those observed experimentally, although the computed changes tend to be larger. A side result of these calculations is that the total energy of model II is lower by 0.18 eV than that of model IIb. Although LDA is not a good method for computing accurate energy values, the result indicates, as expected, that the neutral W-vacancy-W complex is stable against its dissociation into two formally charged defects (one W and one W-vacancy pair). This justifies the local arrangements used for the Ti-W models used in the FEFF8.0 calculations. 3.2. Electronic Characterization. XAS. As mentioned in section 3.1, only modest differences are encountered by XAS around Ti centers, while larger differences can be detected for W centers. Concerning the W LI-edge, Figure 5, only a small shift can be observed in the continuum resonances (CRs) positions of the W19 sample with respect to the rest of samples; this was previously ascribed to the shortening of the average W-O distance as detected by EXAFS (Table 3). Electronic effects are, however, detected in the W LIII-edge XANES spectra, as can be visualized in Figure 8. The continuous decrease of the white line intensity with W content increase cannot be ascribed in this case to a shortening of the W-O average distance; in fact, the decrease of the W-O distance should increase the white line intensity.40,41 This clearly states that W in Ti-W binary oxides undergoes a continuous increase in d electronic density while the mixed oxide is enriched in W; at the same time, the sp sub-band (LI-edge) appears significantly less affected. A rough estimation, considering the white line intensity proportional to the d-hole count, would indicate a 10% decrease in d-hole when going from W2 to W19. Note, however, that the effect of the d-count increase is partially offset in the W19 XANES spectrum by effect of the W-O distance shortening. Differential (final state) effects of particle size, which goes from 8 to 13 nm, in the relaxation of the core-hole may also contribute to make this estimation of just semiquantitative character. In any case, the mentioned W electronic enrichment

Figure 9. Band-gap energy of Ti-W mixed-metal oxides as a function of W content.

throughout the series would be viewed as an increase in covalency on the W-O bond; formally W is in the (VI) oxidation state as detected here by using the edge position of the W LI-edge XANES spectra (Figure 5) and in previous papers for similar samples by using XPS and EPR.16 UV-Vis. The influence of W on the electronic density of the Ti-W mixed oxides can be also viewed in the band-gap energy. While bulk TiO2 is an indirect semiconductor, nanostructured materials can be direct ones.47,48 The confinement of carriers in a limited space causes their wave functions to spread out in momentum space, in turn increasing the likehood of radiative transition with respect to bulk indirect semiconductors. Band-gap values were estimated from the absorbance spectra using both types of analysis (i.e., for direct and indirect transitions);47 the trends observed were the same in both cases. In Figure 9, we display the calculated indirect band-gap energy. The ca. 3.1 eV value obtained for nanostructured anatase-TiO2 agrees well with data previously reported.47,48 As it can be seen in Figure 9, the band-gap energy decreases from pure anatase with the increase of W content, but two roughly linear regions with different slopes are displayed for samples with W content above and below 15 atom %. An effect of particle size on bandgap energy can be essentially discarded as titania shows rather small changes in the 5-10 nm range.48,49 Although this last fact is at odds with EMA (effective mass approximation) theory, a satisfactory interpretation has not been put forward.19,48 DFT. Although, as it is well known, the LDA-DFT method is not valid to obtain absolute band-gap values, but rather tends to underestimate them, still it is worth using it for observing trends, and indeed the density of states (DOS) plots obtained for these models, some of which are shown in Figure 10, indicate a decrease in band gap upon incorporation of W to the TiO2 lattice, in qualitative agreement with experimental observations. These plots have been mutually aligned using the main features of the DOS corresponding to the O2s levels (located at energies around -17 eV, not presented) and show that the valence band (VB) upper edge position is hardly modified, although in some cases some O-centered levels are slightly pulled up from it. The bottom part of the VB does change, reflecting probably the larger stabilizing effect of the W ions (with higher formal positive charge than Ti) on the neighbor oxygens; indeed, these


J. Phys. Chem. B, Vol. 109, No. 13, 2005 6081 4. Discussion

Figure 10. Total and atom-projected density of states for two Ti-W mixed oxide models as compared to that of pure anatase-TiO2.

lower levels are those in the VB which have the highest component (mixing) of W-derived orbitals, as shown by the W-projected partial DOS curves. On the other hand, the position of the conduction band (CB) bottom does decrease in energy. The metal atom-projected DOS curves (displayed also in Figure 10) evidence that this decrease is not due merely to the apparition of new W-derived levels near the CB edge position; there is as well a decrease in energy for the lowest d levels of some of the Ti atoms, and detailed inspection of the data shows that the Ti ions more distant to the vacancy are those undergoing this level lowering (e.g., a long-range effect). The electron density over the W ions was also examined through the Mulliken-type analysis, which yielded a significant d level population (around 3.8 electrons, i.e., showing a high degree of covalency in the W-O bonds); an increase in the latter population with W/Ti ratio was observed, in line with the above presented W LIII-edge XANES results, although the increase found (from 3.82 d-electrons in III to 3.87 in I and II), which was accompanied by a noticeable decrease in the sp level population in W (from 1.09 sp-electrons in III to 0.88 in I), is clearly larger than that expected from the W LI-edge XANES data (see below). Of course, this Mulliken-type charge distribution analysis cannot give correctly absolute electron population values, but can be convenient for comparative purposes.

The XAS study gives a clear picture of the local order around the Ti and W cations implying that a solid solution with anatasetype structure is formed by our preparation method. This structure would be anticipated on the basis of Raman results, which do not give evidence of the occupation of any interstitial position by W.33 In the Ti-W binary oxide samples, Ti appears to have a geometry rather close to that characteristic of the bare anatase oxide, without significant changes at a local level. W cations show two well-defined W-O first coordination distances. This could be an effect of the anatase structure as pure W oxides with the same oxidation state (W(VI)) appear to have a significantly less homogeneous distances in the first coordination shell.40 Apart form that, the W-O distance at ca. 1.80 Å is typical of a WdO double bond40-44 and implies a shortening of anion-cation distances with respect to those characteristic of bare anatase (1.93-1.98 Å).46 The W-O distance at ca. 2.6 Å, however, cannot be easily reconciled with the occupied anionic positions in bare anatase. In view that the DFT results lack to predict this W-O distance of ca. 2.6 Å, one may infer that the materials examined here have defect structures more complex than those assumed in the computed models. To explain this latter fact, calculations were made with O atoms displaced to interstitial positions (Frenkel defects) which could correspond to such a distance, but after geometry relaxation they returned to the regular anion sites. This suggests that the substitution of Ti by W is accompanied by some large lattice distortion or rearrangement, maybe connected with some specific defect interaction or association, which leads to a significant modification of the weakest W-O bond and which is not captured by the present modeling. The nanostructured nature of the materials may have a significant role in this, complicating the theoretical analysis. In any case, an additional, important point for this study is that a shortening of W-O first shell distances is detected by EXAFS for samples with W atom content above 15%. This is not an effect of an increasing contribution by surface species (with different coordination) due to particle size (which decreases from ca. 13 to 8 nm) because the percentage contribution from W-O(H) surface bonds is expected to be rather low for all of the Ti-W specimens.46 The analysis of local order in Ti-W samples will be completed below in connection with charge electroneutrality of Ti-W mixed oxides. The insertion of W into the anatase structure not only distorts the structure at a local level but also changes the cell parameters and volume. Because the radii of both cations are rather similar (0.605 Å for Ti(IV) and 0.600 Å for W(VI)),50 one would expect that the shorter cation-O bonds in the anatase structure, which are nearly parallel to the (001) lattice plane, would induce moderate changes in the a parameter, while the longer cation-O bonds perpendicular to that plane would accommodate the W-O contribution at ca. 2.6 Å, producing a measurable increase of the c parameter. This is roughly what can be seen in Table 2, with the near invariance of the a/b parameters (0.27%) and a noticeable increase of c (0.8%). It can be mentioned that changes in cell parameters in Ti-M mixed oxides would be dependent on the crystallization degree of the nanostructured materials; still, in the literature for the anatase-type materials, the general trends observed indicate a relative constancy of a/b parameters, while both positive and negative shifts have been detected in the c parameter.16,34,51 Loosely, this would justify the decrease in c parameter for sample W2, but further analysis of the results would be needed to interpret the macroscopic effect of the local order on the cell parameters. As a result of the cell parameter changes detected in Table 2, the cell volume slightly increases.

6082 J. Phys. Chem. B, Vol. 109, No. 13, 2005 Variations of the cell parameters and volume are, however, qualitatively consistent with the DFT calculations, suggesting that the W/Ti substitution and the presence of cationic vacancies associated with W are main parameters to rationalize the structural effects at long-range order on the anatase-TiO2 lattice. As noted in the previous section, XANES shows that Ti and W are, respectively, in the (IV) and (VI) formal oxidation states. The W LIII-edge results, Figure 8, modulate this picture, giving indications that a moderate electronic effect exists as a function of the W content of the materials. This determines a moderate gain in 5d electronic density which cannot be accounted for by a W internal (sp/d) rehybridation (W LI-results; Figure 5); in such case, the corresponding sp electron loss would produce an increase of ca. 10-15% on the intensity of the 1s f 4p features (located between the edge and the first CR at ca. 12 121 eV) of the W LI-edge through the Ti-W series, while experimentally this increase would be well below 5%. Unfortunately, DFT-Mulliken population analyses give results for W that are not in agreement with the experimental XANES results. This may again point toward the known limitations of the Mulliken analysis of charge population. Thus, the W 5d electron density change is mainly related to a concomitant change in the oxygen and/or Ti electron density, although the latter seems to be less disturbed in its occupied electronic density with respect to the bare anatase reference. It should be mentioned that a complete analysis of sp/d Ti population must be performed to confirm this hypothesis; however, the somewhat “low” limit for W insertion into the anatase structure (which gives a Ti/W lower ratio of 5 for the maximum W content) will complicate the analysis as changes in the Ti electron population must be of significantly lower magnitude that those described here for W. According to the experimental results just outlined, it would seem that Ti-W mixed oxides would not obey the “Barr model” for charge redistribution,52 which states that in the mixing of two oxides, the cation of the more ionic oxide is expected to become even more ionic, while the cation of the more covalent oxide should experience a corresponding increase in covalency. It therefore would appear that the charge rearrangement induced by the presence of W is connected with the anion sublattice properties; changes in the W-O coordination distance and numbers seem thus related to the electronic modification of W, displaying smooth variations for samples with W content below ca. 15 atom % and an abrupt one beyond this point. On the other hand, the formal oxidation states of the cations indicate that the stoichiometry of the mixed oxide must be Ti1-xWxO2+x. Such an O/(Ti + W) ratio higher than 2 could be accommodated into the anatase lattice in several ways. EXAFS results displayed in Table 3 indicated that this depends on the W content of the material. As a general result, the presence of cation vacancies is evidenced by EXAFS and inferred from the analysis of Raman, indicating that it is the main mechanism for achieving charge neutrality in Ti-W substitutional oxides. The EXAFS/DFT study reveals that such vacancies are located at the first cation-cation distance around W centers, presuming the existence of some kind of local- to medium-range order in all Ti-W mixed oxides. Additionally, the strong enhancement of confinement effects detected in Raman for the W19 sample (Figure 3) would indicate the establishment of domain frontiers for samples having a W content above 15 atom %. This happens concomitantly with an abrupt decrease of the W-Ti coordination number (Table 3), which suggests that W tends to locate in those frontiers leading to a change in its environment. All of these facts suggest that local- to medium-range order changes above and below this turning point, with a variation of the local

Fernańdez-Garcıá et al. order mainly associated with the W-O coordination shell and the formation of domain frontiers by, possibly, some kind of defect clustering. It is worth recalling here that the existence of such complex structural phenomena to satisfy electroneutrality requirements could be typical of Ti-based mixed oxides, as previously suggested by results reported for the Ti-Fe system.18 This phenomenology contrasts with other cases, like fluoritetype Ce-based binary oxides, where charge neutrality is accommodated by the presence of anion/cation vacancies and a rather limited reorganization of the anion lattice positions.19 The presence of a turning point with W content in Ti-W mixed oxides has been discussed in connection with the local/ long-range order but has additional structural and electronic implications. A first point is the effect on the (root squared) microstrain behavior with W content (Table 2). The presence of W in W2 already increases the microstrain in the solid, mainly as an effect of the local distortion of the geometry and presence of cationic vacancies.34,53 However, this observable increases up to a W content in the solid of ca. 11%, having a nearly constant behavior for higher W content. The limitation in the strain growth as the W content of the material rises cannot be accounted for by particle size effects which should increase as size decreases34,53 and may thus be related to the “ordering” effect already mentioned in the defect distribution. A somewhat similar behavior can be observed for the primary particle size of the material. This observable shows a slow decrease with W up to the W11 sample and a more or less erratic behavior after it. As mentioned in the Introduction, the presence of vacancies in titania samples usually favors a decrease in primary particle size,16-19 probably ascribable to a limited stability of the mixed oxide solid nanoparticle with respect to the single oxide nanoparticle. In this way of thinking, it would appear that the change in local/long-range order on the material is having an impact on the primary particle size for samples located near the turning point of 15 atom %. However, the exact nature of this effect is unknown at the moment. Finally, a strong influence is detected in the band-gap energy. The decrease of this observable is, as deduced by the DFT calculations, primarily associated with the contribution of W and, particularly, Ti d-like states at the bottom of the conduction band. It is important to note that W/defect presence appears to stabilize Ti d unoccupied orbitals. Additionally, DFT calculations indicate that the W electronic effect on Ti unoccupied electronic density is of longrange character. The higher density of states appearing at the bottom of the conduction band with respect to that of pure TiO2 seems to increase with the W content (Figure 10); however, the strong effect detected by using UV-vis for samples having W content above 15 atom % is not well understood at the present. A detailed theoretical analysis of several Ti-W models having more complex defect associations than here described, probably related to changes in the anion-sublattice and maybe stabilized by the effect of the nanostructure, would be needed to solve the problem. 5. Conclusions The structural and electronic characteristics of Ti-W mixedmetal oxides have been analyzed. W occupies substitutional positions in the anatase-type structure with the presence of cation vacancies at their first cation-cation distance. Electronically, Ti and W are in +4 and +6 oxidation states, respectively, in all Ti-W mixed-metal oxides investigated. However, a mild variation in the W d-electron population has been detected. Cation vacancies are shown to be the main mechanism to achieve charge neutrality in these Ti-W mixed oxides solid

Nanostructured Ti-W Mixed-Metal Oxides solutions. Two types of oxygen distances around W centers, distinctive of the anatase-type structure, are detected at about 1.8 and 2.6 Å. The 2.6 Å distance is not easily correlated with any simple distortion of the anatase-type anion sublattice positions. It thus appears that W presence into an anatase-type network requires, besides the presence of cationic vacancies, a significant short-range reorganization of the O ions located at the first shell distance of the heteroatom. In addition, the W-O shell parameters appear both sensitive to the W content of the material, showing decreasing bond distance(s) and coordination number for samples having W content above 15 atom %. Changes in W-O and W-Ti coordination detected by EXAFS would suggest that W tends to accumulate near/in domain frontiers for the W19 sample. As justified by the DFT calculations, the presence of W and cation vacancies affects lattice cell dimensions and is a main factor in rationalizing the significant increase of the c cell parameter and the lower effect in the a/b ones. The above-described, main structural and electronic properties of the Ti-W mixed oxides allow to qualitatively interpret the behavior of the strain and particle size as well as the band gap as a function of the W content of the material. DFT (qualitatively) justifies also the decrease in band-gap energy as due to a lowering of the bottom of the conduction band states, which have contributions from both W and Ti. Acknowledgment. Financial support by the CYCIT project CTQ2004-03409/BQU is fully acknowledged. Thanks are given to the BM-29 staff (Dr. S. Ramos; project number ME-605) of the ESRF synchrotron for technical support during XAS measurements. Financial support by the European Large Scale program for XAS was fully appreciated. Thanks are given to Dr. C. Belver for recording XRD patterns. Thanks are also due to Ms. A. Iglesias-Juez for recording Raman spectra and for the help given during the recording of XAS spectra. References and Notes (1) Bosh, H.; Janssen, F. Catal. Today 1988, 2, 369. (2) Forzatti, P. Catal. Today 2000, 62, 51. (3) Hoffman, M. R.; Martin, S. T.; Choi, W.; Bahneman, D. W. Chem. ReV. 1995, 95, 69. (4) Maldoti, A.; Molinari, A.; Amadeni, R. Chem. ReV. 2002, 102, 3811. (5) Kalyanasendevan, K.; Gratzel, M. In Optoelectronics Properties of Inorganic Compounds; Roundhill, D. M., Fackler, J. P., Eds.; Plenum: New York, 1999; pp 169-194. (6) Sheveglieri, G., Ed. Gas sensors; Kluwer: Dordrecht, 1992. (7) Johnson, R. W.; Thieles, E. S.; French, R. H. Tappi J. 1997, 80, 233. (8) Phillips, L. G.; Barbeno, D. M. J. Dairy Sci. 1997, 80, 2726. (9) Selhofer, H. Vacuum Thin Films 1999, August, 15. (10) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (11) Bonhole, P.; Gogniat, E.; Gratzel, M.; Ashrit, P. V. Thin Solid Films 1999, 350, 269. (12) Marchand, R.; Broham, L.; Tournoux, M. Mater. Res. Bull. 1980, 15, 1129. (13) Muscat, J.; Swany, V.; Harrison, N. M. Phys. ReV. B 2002, 65, 224112. (14) Zhang, H.; Bandfield, J. F. J. Mater. Chem. 1998, 8, 2073. (15) Guidi, V.; Carotta, M. C.; Ferroni, M.; Martinelli, G.; Sacerdoti, M. J. Phys. Chem. B 2003, 107, 120. (16) Fuerte, A.; Hernańdez-Alonso, M. D.; Maira, A. J.; Martıńez-Arias, A.; Fernańdez-Garcıá, M.; Conesa, J. C.; Soria, J.; Munuera, G. J. Catal. 2002, 212, 1. (17) Luca, V.; Thomson, S.; Howe, R. F. J. Chem. Soc., Faraday Trans. 1997, 93, 2195. (18) Wang, J. A.; Limas-Ballesteros, R.; Lo´pez, T.; Moreno, A.; Go´mez, R.; Novaro, O.; Bokhim, X. J. Phys. Chem. B 2001, 105, 9692. (19) Fernańdez-Garcıá, M.; Martıńez-Arias, A.; Hanson, J. C.; Rodriguez, J. A. Chem. ReV. 2004, 104, 4063.

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