Nanostructured Ti-W Mixed-Metal Oxides: Structural and Electronic Properties M. Ferna ´ ndez-Garcı ´a,* A. Martı ´nez-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 TiO 2 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 TiO 2 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 L I /L III -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 (TiO 2 ) 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 sources 1,2 and photocatalysis for pollutant elimina- tion 3 or organic synthesis 4 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 TiO 2 materials are essential in accomplishing these functions. Experimental approaches to scale down the TiO 2 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. TiO 2 occurs in nature in three different polymorphs, which, in order of abundance, are rutile, anatase, and brookite. An additional synthetic phase is called TiO 2 (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 TiO 2 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 conductivity by the presence of intrinsic vacancies. The proper- ties 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 W 15,16 while V 17 and Fe 18 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 substi- tutional 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 TiO 2 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 TiO 2 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 TiO 2 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 applica- tion in photocatalysis as it improves the performance of nanostructured anatase-TiO 2 in the elimination of organic volatile compounds under visible-light excitation 16,24 and may * Corresponding author. E-mail: mfernandez@icp.csic.es. 6075 J. Phys. Chem. B 2005, 109, 6075-6083 10.1021/jp0465884 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/08/2005