ARTICLE pubs.acs.org/JPCC
Structural Defects in W-Doped TiO2 (101) Anatase Surface: Density Functional Study Antonio M. Marquez,* Jose J. Plata, Yanaris Ortega, and Javier Fdez. Sanz Departamento de Química Física, Universidad de Sevilla, Facultad de Química, 41012 Sevilla, Spain
bS Supporting Information ABSTRACT: In this Article, the structural and electronic properties of the W-doped anatase (101) surface are investigated by first-principles density functional theory calculations. Several surface and subsurface substitutional positions are examined as well as the interaction of the W-dopant atoms with structural defects: cation vacant sites and additional oxygen atoms that are required to compensate the extra charge of the W6+ cations. It is found that the preferred configurations are those on which one W6+ cation and one Ti vacant site are first cationic neighbors with simultaneous formation of a wolframyl entity. The main mechanism of system stabilization is found to be based on the formation of wolframyl species that result from the close proximity, as first cationic neighbors of the W-dopant atom and the Ti vacant site. A second factor for system stabilization seems to be the separation of W6+ cations to reduce the energetic cost of the structural distortions introduced by the doping process. Results from molecular dynamics calculations indicate that W6+ cations have a 5 + 1 coordination with two W O distances at 1.8 to 2.0 and 2.5 to 2.6 Å. All of these structural results are used to understand the experimental information available for W Ti nanostructured oxides. The modifications introduced in the electronic structure of the anatase (101) surface by the doping process are discussed and rationalized. A comparative analysis of the density of states of doped and undoped slab models of this surface and a Bader charge analysis will be used to understand the electronic redistribution that takes place around the impurity atoms.
1. INTRODUCTION Titanium dioxide (TiO2) is a material that has received considerable attention in the literature over the years because of its versatility and multitude of its applications in catalysis, photocatalysis, solar cells, waste remediation, and biocompatible materials.1 9 In particular, nanocrystalline TiO2 has been extensively studied because of its notable catalytic and photocatalytic activity. Various experimental approaches are used to scale the TiO2 particle size down to the nanometer range, resulting in an increased surface area that goes parallel with an enhancement of the chemical and photochemical activity. Of the different titania polymorphs, anatase appears to be the most important for these applications because the majority of nanostructured materials displays this specific structure,10,11 with the (101) face dominating >94% of the crystal surface.12 An obvious problem hindering the photochemical applications of TiO2 is its poor visible light response due to its wide band gap (∼3.2 eV on the anatase polymorph). Different approaches have been pursued to narrow this band gap and increase visible light absorption. One of the most common methods is the introduction of impurities in the TiO2 structure, either nonmetals or metals.13,14 Nanosized W-doped TiO2 photocatalysts have been synthesized and characterized by a few research teams.15 18 Tian et al.16 have used a simple hydrothermal process for the synthesis and evaluated the photocatalytic activity of the prepared material by r 2011 American Chemical Society
examining its performance in the photocatalytic degradation of methyl orange. However, in their procedure, the W-dopant was incorporated to only ∼4 mol %, and the limited effect of doping in the photocatalytic activity of the solid was related to the surface presence of W6+ acid centers. A more detailed account of the structural and electronic effects of W doping into the anatase structure of nanosized materials is the work of Fernandez-García et al.17,18 They have synthesized nanostructured Ti W mixed metal oxides using a microemulsion preparation technique. The structural and electronic properties of the materials resulting after calcination were characterized by X-ray diffraction, Raman, EXAFS, XANES, and UV vis spectroscopies. From their data, these authors concluded that no phase segregation appears up to a W content of ∼20 atom %, with cation vacancies being the main mechanism to achieve charge compensation in the nanostructured mixed W Ti oxides. The Raman spectra data indicate the presence of wolframyl species (WdO) that, however, were not detected by EXAFS spectroscopy. Besides, two kinds of W O distances, distinctive of the anatase-type structure, are detected, indicating that some distortion of the anatase network is required to accommodate the W cations. The analysis of the XANES and Received: April 7, 2011 Revised: July 26, 2011 Published: July 26, 2011 16970
dx.doi.org/10.1021/jp203223f | J. Phys. Chem. C 2011, 115, 16970–16976
The Journal of Physical Chemistry C UV vis results seems to conclude that electronically Ti and W are in +4 and +6 oxidation states, respectively, although some variation of the W d-population is detected. The second main feature obtained from the UV vis data is a moderate (∼0.2 eV) reduction in the band gap of these binary solids with respect to that of bare anatase that is related to an enhanced photocatalytic activity upon sunlight excitation in the gas-phase photo-oxidation of toluene. These authors have also conducted some LDA-density functional theory (DFT) calculations in bulk models of the anatase structure considering that W dopants are located in Ti sites and that charge compensation occurs exclusively by the presence of Ti cation vacancies. These calculations offered an explanation for the band gap reduction in terms of the presence of W 5d states at the lowenergy edge of the conduction band but failed to account for the structure distortions experimentally observed and also did not reproduce the presence of WdO entities detected in Raman spectroscopy. Other theoretical studies have been focused on the effect of titania W-doping on the catalytic (thermal) oxidation of CO,19 electrical conductivity properties,20,21 and effects of N/Wcodoping22,23 in the electronic structure of bulk TiO2. However, none of these works has investigated, to any extent, the structural implications on the presence of Ti vacancies and WdO entities evidenced by the experiment,18 and only one19 has taken into account the presence of surface wolframyl species but in the (110) surface of the rutile polymorph. In the present work, we use DFT calculations to address the issue of the interaction between the W-dopant and the structural charge-compensating point defect structures: Ti-vacancies and wolframyl species, experimentally manifested. In particular, we focus on the anatase (101) surface because, as previously indicated, anatase is the main structural polymorph found in nanostructured TiO2 materials, and the (101) surface is the thermodynamically most stable crystal face. Therefore, its structure is expected to play a fundamental role in any chemical or photochemical process that may take place on it. Moreover, the activity of anatase in photocatalytic reactions is found to be much higher than that of the rutile polymorph (see, e.g., refs 10, 24, and 25 and references therein). After briefly presenting the necessary computational details about the models and the methodology used, we will start by examining the energetics and geometries of surface defects on W-doped anatase TiO2. Experimental facts, like the observation by EXAFS spectroscopy of a single local arrangement around the W-dopant, the formation of surface WdO species, and the interaction among W-dopant atoms, Ti-vacant sites, and surface wolframyl species will be discussed and rationalized. In a later section, the implications on the electronic structure of these materials and the electronic redistribution that take place around the position of the impurity atom will be explored.
2. MODEL AND COMPUTATIONAL DETAILS Periodic 3D calculations were carried out using the VASP 4.6 code26 28 with the projector-augmented wave method (PAW).29,30 In these calculations, the energy was computed using the generalized gradient approximation (GGA) of DFT proposed by Perdew, Burke, and Ernzerhof (PBE),31 and the electronic states were expanded using plane wave basis set with a cutoff of 500 eV. Forces on the ions were calculated through the Hellmann Feyman theorem, including the Harris Foulkes correction to forces.32 This calculation of the force allows a geometry optimization using the conjugated gradient scheme.
ARTICLE
Figure 1. One 2 supercell model of a TiO2 (101) anatase surface with three O Ti O layers. Atom labeling for the substitutional W positions: 5s, pentacoordinated surface site; 6s, hexacoordinated surface site; 6u, subsurface hexacoordinated (up) site; and 6d, hexacoordinated subsurface (down) site. Atom colors: Ti, gray; O, red.
Iterative relaxation of the atomic positions was stopped when the forces on the atoms were
