Structural and Electronic Properties of a W3O9 Cluster Supported on

Sep 15, 2009 - Department of Chemistry, Fuzhou UniVersity, Fuzhou, Fujian, 350108, ... Base of Photocatalysis, Research Institute of Photocatalysis, F...
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J. Phys. Chem. C 2009, 113, 17509–17517

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Structural and Electronic Properties of a W3O9 Cluster Supported on the TiO2(110) Surface Jia Zhu,† Hua Jin,† Wenjie Chen,† Yi Li,†,‡ Yongfan Zhang,*,† Lixin Ning,§ Xin Huang,*,†,| Kaining Ding,† and Wenkai Chen† Department of Chemistry, Fuzhou UniVersity, Fuzhou, Fujian, 350108, China, State Key Laboratory Breeding Base of Photocatalysis, Research Institute of Photocatalysis, Fuzhou, Fujian 350002, China, Department of Physics, Anhui Normal UniVersity, Wuhu, Anhui, 241000, China, and State Key Laboratory of Structural Chemistry, Fuzhou, Fujian, 350002, China ReceiVed: July 1, 2009; ReVised Manuscript ReceiVed: August 9, 2009

The stoichiometric tritungsten oxide cluster (W3O9) deposited on the TiO2(110) surface has been investigated using first-principles density functional theory calculations. Various possible configurations mainly derived from molecular dynamics simulations have been considered. On the basis of the comparisons of thermodynamic stability and STM images, two isoenergetic structures with chirality relationship between the geometries of W3O9 clusters can be tentatively assigned to be the most likely configurations under the experimental conditions, in which five new bonds including three Ti-O and two W-O bonds are formed at the interface. Due to the loss of three WdO groups, the low chemical reactivity of the W3O9 cluster can be expected. After deposition of W3O9, the TiO2(110) surface still retains the semiconducting character and the charge transfer occurring between the cluster and the substrate is small. The variation of surface dipole moment is sensitive to the orientation of the six-membered ring structure of W3O9. In addition, our results indicate that the properties of the TiO2(110) surface may be adjusted by deposition of nonstoichiometric tungsten oxides. 1. Introduction The supported early transition metal oxides (TMOs) with pentavalent and hexavalent cations are an important family of catalysts used in the petrochemical and refining industry to catalyze many reactions, such as oxidative dehydrogenation of hydrocarbons, partial oxidation of alcohols, and selective reduction of NOx.1 Among these TMOs, tungsten oxide (WOx) has been focused on in many studies because it possesses the strongest Brønsted acid sites.2-5 Various substrates have been used for the preparation of supported tungsten oxide species including Si,6 SiO2/Mo(112),7 TiO2,8-12 Al2O3,13 SiO2,14 and ZrO2;15-20 the experimental results indicate that the structure and catalytic properties of WOx are strongly influenced by the support.21 To understand the nature of the interaction between the support and WOx, which is directly related to the catalytic activity, the preparation of monodispersed WOx clusters with well-defined structure is a key step since they are the most suitable models for investigating the structure-reactivity relationships of this particularly important class of TMOs. Recently, using the direct evaporation of WO3 solid method, Bondarchuk et al. successfully prepared monodisperse WO3 clusters on the rutile TiO2(110) surface after deposition of tungsten oxide at 300 K followed by annealing at 600 K.11,12 On the basis of the results from scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), temperatureprogrammed desorption (TPD), quartz crystal microbalance (QCM) mass measurement, and theoretical calculation of the gas-phase molecule, they concluded that the composition of the clusters is W3O9 with a cyclic structure. A possible configuration * To whom correspondence should be addressed. E-mail: zhangyf@ fzu.edu.cn (Y.Z.); [email protected] (X.H.). † Fuzhou University. ‡ Research Institute of Photocatalysis. § Anhui Normal University. | State Key Laboratory of Structural Chemistry.

of the W3O9 cluster supported on TiO2(110) (see Figure S1f in the Supporting Information) has also been proposed to explain the dark triangular feature in the STM image.11,12 In this structure, the plane of the W3O3 six-membered ring is tilted away from the surface normal with two W atoms pointing into the 5-fold Ti row at the substrate and the third W atom tilted toward the neighboring row of the bridging oxygen, resulting in the formation of two new Ti-O bonds between 5-fold Ti and terminal oxygen atoms of the W3O9 cluster. However, according to this model, one edge of the dark triangle must be parallel to the row of bridging oxygens (or along the [001] direction of the TiO2(110) surface), which is not observed in the measured STM images. Furthermore, since STM probes only the electronic structure near the Fermi energy, the relative contributions of the electronic and geometric effects are hardly distinguished in constant current STM images, especially for the oxide and semiconductor surfaces.22 Therefore, STM does not always show the atomic positions, and additional information, such as that provided by theoretical calculations, is required to inspect the experimental STM data of this complex surface. In a recent theoretical calculation on the W3O9/TiO2(110) system, Kim et al. employed the above model as the configuration of as-deposited W3O9 cluster (prepared at 100 K and may be poorly defined11) to explain the special catalytic dehydration of 2-propanol on W3O9 clusters supported on TiO2(110).23 It was noted that in their experiments the 600 K preannealed W3O9 clusters show a lower catalytic activity with respect to the asdeposited clusters. The origin of this low activity is not clear, and they speculated that it may be due to the partial loss of active WdO groups. Therefore, the atomic structure of this welldefined supported model TMO catalyst is still under debate. In this paper, we present a detailed study on the configurations and electronic structures of the W3O9/TiO2(110) system. We first use the ab initio molecular dynamic (MD) method to sample possible configurations of this complicated multidimensional

10.1021/jp906194t CCC: $40.75  2009 American Chemical Society Published on Web 09/15/2009

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system, the most stable structure is determined by further geometrical optimizations, and the interactions between the oxide cluster and the TiO2 substrate are discussed by examining the formation of different kinds of adsorption bonds. Furthermore, we describe the changes introduced by deposition of the W3O9 cluster on the electronic properties of the TiO2(110) substrate. Finally, we simulate the STM images of some typical configurations. Our theoretical works can provide deep insight into the properties of this well-defined model system. 2. Computational Details First-principles calculations based on density functional theory (DFT) were carried out utilizing the Vienna ab initio simulation package (VASP) and the projected augmented wave (PAW) method.24-26 The generalized gradient approximation PerdewWang (PW91) exchange- correlation functional was employed,27 and the kinetic cutoff energy for the plane-wave expansion was set to 400 eV. The TiO2(110) substrate was modeled by a ninelayer slab in which the bottom two atomic layers were kept at their bulk positions, and a (4 × 2) supercell (containing 48 Ti and 96 O atoms) with dimensions of 11.82 Å × 12.99 Å was adopted to avoid the obvious interactions between neighboring W3O9 clusters. The spacing between the adjacent slabs was set to about 13 Å. Considering the large size of the supercell, only Γ point was considered for Brillouin zone integration. Because there are many possible arrangements of the W3O9 cluster on the TiO2(110) surface, ab initio molecular dynamics(MD) simulations using the Nose´ algorithm28 were performed to explore possible configurations of the W3O9/TiO2 (110) system. In the MD simulations, a low cutoff energy of 250 eV was used, and the simulation length was 8 ps with a time step of 1 fs at a temperature of 600 K. In this stage, a (3 × 2) slab with six-layer thickness was employed to model the substrate. All possible adsorption configurations were sampled from the results of the MD simulations every 100 steps, resulting in more than 160 initial configurations explored in this work. The extensive calculations were carried out to optimize these structures using a cutoff energy of 250 eV, and then further structural optimizations using highly accurate settings were performed to determine the most stable configuration. Similar approaches have been employed to determine the configurations of other complicated systems, such as TiOx/Mo(112) and Au/TiOx/Mo(112).29,30 In order to facilitate the subsequent discussion, we use symbols Ob and Ot to represent the bridging and terminal oxygen atoms of W3O9 clusters, respectively, while the 5-foldcoordinated Ti and bridging oxygen atoms on the TiO2(110) surface are denoted by Ti(5f) and Ob′, respectively. 3. Results and Discussion 3.1. Structure of the W3O9 Cluster in the Gas Phase. The configuration of the W3O9 cluster in the gas phase has been determined by photoelectron spectroscopy (PES) experiments combined with DFT calculations.31,32 As shown in Figure 1, the ground state of the W3O9 cluster is found to be closed shell with D3h symmetry. Each tungsten atom is tetrahedrally coordinated with two terminal (Ot) and two bridging oxygen (Ob) atoms. The W-Ob bond is a single bond with a length of 1.9 Å, whereas the distance between W and Ot is relatively small (1.7 Å), implying formation of a WdOt double bond. The main feature of the W3O9 structure is the coplanar cyclic conformation, which consists of three W and three Ob atoms. In this work, the geometry of the isolated W3O9 cluster (placed in a 20 × 20 × 20 Å cubic box) is optimized by PW91 functional with plane-

Figure 1. Optimized structural parameters of the gas-phase W3O9 cluster. The tungsten and oxygen atoms are denoted by blue and red spheres, respectively. The results obtained by the B3LYP method with an atomic orbital basis set are also listed in parentheses for comparisons.

wave basis set, and the corresponding structural parameters are labeled in Figure 1. The results obtained by Huang et al. using the B3LYP method with an atomic orbital basis set31,32 are also shown in parentheses for comparison. The configurations predicted by the two approaches are similar, and the differences among the W-O bond lengths and O-W-O bond angles are smaller than 0.03 Å and 3°, respectively. 3.2. Structures of the W3O9/TiO2(110) System. Examining the structures of the W3O9 and TiO2(110) surface, the cluster may be anchored on the substrate through formation of two types of adsorption bonds. One is the bond between the Ot and the 5-fold Ti atoms (namely, the Ti(5f)-Ot bond), and the other is the bond between the tungsten and the bridging oxygen of the substrate (namely, the W-Ob′ bond). Due to several W/Ot and Ob′/Ti(5f) pairs existing in the system, many possible configurations can be deduced by the different assignments of the above bonds at the interface. In the present work, MD simulations were carried out to explore some typical structures of the W3O9/TiO2(110) system. Two different initial configurations displayed in Figure 2 were employed in the MD simulations, corresponding to the weak and strong interactions between the cluster and the substrate, respectively. In the weak interaction model (Figure 2a), the cyclic W3O3 ring of the cluster is nearly parallel to the TiO2(110) surface and no visible adsorption bonds are formed in this structure. On the contrary, the geometry of Figure 2b is artificial and represents the strong interaction case, in which the coplanar cyclic conformation of the cluster is deformed obviously to form as many adsorption bonds as possible. Although the initial configurations of the MD simulations differ significantly, our results indicate that they tend to produce very similar structures when the simulation time is long enough. In addition, MD simulation for the structure of Figure 2a at high temperature (1500 K) is also considered. By using the above strategy (MD to sample the configuration space plus energy minimization of low-energy structures), eight possible configurations, M1-M8 (see Figure 3 and Figure S1 in the Supporting Information), are obtained. Among them, M1 and M2 models come from the structure of Figure 2b and M3-M7 are derived from the results of Figure 2a while the simulation at 1500 K of this structure results in the M8 model. Furthermore, the configuration model M9, proposed by Bondarchuk et al. based on STM experiments,11,12 is also investigated for comparison. As it will be shown that the M1, M2, and M7 models are energetically favorable, these three configurations

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Figure 2. Two initial configurations correspond to (a) weak interaction and (b) strong interaction of the W3O9/TiO2(110) system considered in the MD simulations. The W, O, and Ti atoms are denoted by blue, red, and gray spheres, respectively.

Figure 3. Optimized structures of three thermodynamically preferred models for the W3O9/TiO2(110) system. The configurations of the W3O9 fragment are also shown in the insets. The W, O, and Ti atoms are denoted by blue, red, and gray spheres, respectively. Only part of the TiO2(110) surface is displayed, and to see the structural parameters of the cluster more clearly, the orientation of W3O9 shown in the inset is adjusted and not a top view of the cluster. The bond lengths and bond angles labeled in the figures are in the Angstroms and degrees, respectively.

will be the focus of the following sections. The description of the other structures is provided in the Supporting Information. 3.2.1. Structure of Model M1. Model M1 is only observed in the initial stage of the MD simulation of structure Figure 2b. As shown in Figure 3a, in this configuration three Ot atoms of the W3O9 cluster are bonded to the Ti(5f) atoms on the surface, and two additional bonds between the W and the Ob′ atoms are also formed. Therefore, five new bonds exist at the interface, and as a result, three Ti(5f) atoms on the substrate become 6-fold coordinated and two W atoms (numbered W(2) and W(3) in Figure 3a) change to 5-fold coordination. The lengths of three Ti(5f)-Ot bonds vary in the range between 1.85 and 1.91 Å, which are smaller than that of the Ti-O bond (>1.95 Å) in TiO2 bulk, indicating a stronger Ti-O interaction for these adsorption bonds. Consequently, the Ti(5f) atoms involved in the interaction with Ot are relaxed significantly toward the side of cluster. Furthermore, the short W-Ob′ bond with a length of 1.89 Å also implies strong interactions between the W and the Ob′ atoms. Due to the obvious interactions between the cluster and the substrate, in model M1 the configuration of the W3O9 fragment changes significantly with respect to the case in the gas phase. In fact, now three W and three Ob atoms are not coplanar, and the Ob atom between W(2) and W(3) deviates significantly from the cyclic plane to reduce the repulsions from

neighboring Ot atoms. Some structural parameters of the W3O9 fragment are also shown in the inset of Figure 3a. Compared to the gas phase (Figure 1), variations of the W-Ob bond length are observed, especially for those bonds involving the Ob atom connecting W(1) and other W atoms. Since the W(2) and W(3) atoms are coordinated strongly with the Ob′ atoms of the substrate, one W-Ob bond of these W atoms is weakened and elongated obviously to around 2.1 Å. On the other hand, strengthening of two W-Ob bonds around the W(1) atom is observed, as indicated by a shorter W-Ob distance (about 1.8 Å). Therefore, in this model, it seems that the ring structure of W3O9 can be more easily opened, which most likely happens at the sites near the W(2) or W(3) atom. 3.2.2. Structure of Model M2. When the time of the MD simulation for Figure 2b is longer than 2.5 ps, a configuration similar to the M2 model is often observed. In this structure (Figure 3b), the W3O9 cluster is attached to the substrate by five adsorption bonds, including three Ti(5f)-Ot and two W-Ob′ bonds. The configuration of M2 can be obtained by replacing the W(3) with the W(1) atom to coordinate with one Ob′ atom in model M1, and now W(3) becomes the highest tungsten atom. Although the numbers of the Ti(5f)-Ot and W-Ob′ adsorption bonds of the M1 and M2 models are the same, the binding strength between W3O9 and the substrate in

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Figure 4. Top view for the chirality relationship between the geometries of the W3O9 cluster in M2 (left) and M7 (right) configurations. The W, O, and Ti atoms are denoted by blue, red, and gray spheres, respectively.

M2 is slightly weaker than that in M1, as reflected by the optimized structural parameters. It was shown that the newly formed W(1)-Ob′ bond (1.957 Å) and the Ti(5f)-Ot bond (1.958 Å) just under the W(3) atom are obviously longer than the corresponding bonds (1.885 and 1.848 Å) in model M1. Like M1, deposition introduces significant variation of the configuration of the W3O9 cluster with respect to the gas phase, and deformation of the ring structure of W3O9 is also observed in M2. However, since the longest W-Ob bond is shorter (about 0.1 Å) than that in the M1 model, the cyclic structure of W3O9 in M2 may be more stable relative to the case in M1. 3.2.3. Structure of Model M7. The M7 model (Figure 3c) is the most stable configuration found in the MD results of Figure 2a. This structure can be obtained by moving the W(2) atom in the M6 model (Figure S1d in the Supporting Information) toward the substrate, and one Ot of the W(2) atom is bonded to Ti(5f) just below it; at the same time, the decreasing distance between W(1) and one Ob′ of the substrate results in the formation of a new W-Ob′ bond. Consequently, this model is similar to models M1 and M2, in which there are five adsorption bonds, namely, three Ti(5f)-Ot bonds and two W-Ob′ bonds, at the interface between W3O9 and the TiO2 surface. Actually, carefully examing the structural parameters of M2 and M7 indicate that they correspond to the same adsorption pattern with a chirality relationship between the geometries of the W3O9 fragments (see Figure 4), and the total energies of two models are almost the same. According to the descriptions of the structures of M3-M6 (see Supporting Information for details), we know that the M7 model is derived from MD simulation of the structure in Figure 2a when the plane of the W3O3 ring is tilted first to make the W(3) atom move toward the substrate; on the other hand, the reversed declination of the W3O3 plane is also possible; in this case it is the W(2) atom shifted toward the substrate first, and the M2 structure

can be produced following the same process from M3 to M7. Therefore, the M2 and M7 configurations can be observed with equal probability. Although different initial configurations of MD simulations are employed in this work, the structures with the same adsorption pattern are obtained when the simulation time is long enough. 3.3. Thermodynamic Stability of the W3O9/TiO2(110) Surface. To compare the thermodynamic stability of the different arrangement of W3O9 cluster, the adsorption energy of each configuration is also calculated and listed in Table 1. The adsorption energy reported here is defined as Eads ) Esubstrate + Ecluster - Etot, where Etot, Esubstrate, and Ecluster represent the total energies of the W3O9/TiO2(110) system, the clean TiO2(110) surface, and the ground state of W3O9 in the gas phase, respectively. According to the calculated adsorption energies, the M1 model is energetically most favorable among the nine configurations considered in this work. The next stable structure is the M2 (or M7) model, which is only slightly higher (about 0.22 eV) in energy than model M1. Since models M1 and M2 (or M7) have similar thermodynamic stability and also structures similar to M2 (or M7) are often observed after MD simulations for certain steps, it seems that the M2 (and M7) model is the most likely configuration in the experimental conditions. For M4, M5, and M6 models, they are metastable and the corresponding adsorption energies range from 3.0 to 3.5 eV. The small adsorption energy (Eads < 2.0 eV) of M3 and M9 indicates the poor thermodynamic stability of these two structures, especially for the M9 model. It is obvious that the thermodynamic stability of the W3O9/ TiO2(110) system is mainly dependent on the strength of the interactions between the cluster and the substrate, which can be roughly determined by the number of the bond at the interface and the length of the corresponding bond. Table 1 lists the number and average length of two kinds of adsorption bonds, namely, the W-Ob′ and Ti(5f)-Ot bonds for each configuration. It can be seen clearly that in all these structures the number of the W-Ob′ bond is no more than that of Ti(5f)-Ot bond, implying the higher possibility to form adsorption bonds involving a terminal oxygen of the cluster at the TiO2(110) surface. For M1 and M2 (or M7) models, the maximum number of the adsorption bond is found and good stability can be expected for these structures. Moreover, since the average lengths of the W-Ob′ and Ti(5f)-Ot bonds of the M1 model are slightly smaller than those of M2 (or M7), M1 is the lowest energy configuration in accord with prior interpretation of the adsorption energy. On the contrary, only two adsorption bonds are formed in the M3 and M9 models, and of course, their adsorption energies are relatively small, especially for M9 with the longest adsorption bonds. Besides M5 that has four

TABLE 1: Summary of the Structural Parameters, Adsorption Energy (Eads), Charge Transfer Occurring between W3O9 and Substrate, and Change of Dipole Moment along the Surface Normal Direction (∆D) with Respect to the Clean TiO2(110) Surface of Nine Possible Models of W3O9/TiO2(110) Systems models

total number of adsorption bond

number and average length (Å) of the W-Ob′ bond

number and average length (Å) of the Ti(5f)-Ot bond

Eads (eV)

charge (e)

∆D (eV × Å)

M1 M2(M7) M3 M4 M5 M6 M8 M9

5 5 2 3 4 3 3 2

2(1.885) 2(1.903) 1(1.914) 1(1.878) 2(1.911) 1(1.839) 1(1.828) 0

3(1.871) 3(1.892) 1(1.783) 2(1.880) 2(1.773) 2(1.880) 2(1.901) 2(2.274)

4.34 4.12 1.76 3.00 3.15 3.45 2.83 0.76

-0.06 -0.08 -0.10 -0.05 0.05 -0.01 -0.09 -0.09

0.76 0.59 2.32 1.16 2.27 0.54 0.59 -0.91

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Figure 5. (a) Total density of state (DOS) of the M1 model, and the contribution of W3O9 is denoted by the light gray area after enlarging three times, (b) partial DOSs of TiO2(110) substrate of the M1 model (solid line) and clean surface (dashed line), (c) partial DOSs of three tungsten atoms of the M1 model, and (d) partial DOSs of three types of oxygen atoms surrounding the W(1) atom. The vertical dashed line indicates the position of the Fermi level, taken as zero energy. The oxygen labeled as Ot′ is the terminal oxygen connected with the Ti(5f) atom of the substrate.

adsorption bonds, W3O9 in other models interacts with TiO2(110) via three adsorption bonds, corresponding to configurations with intermediate strength of the interaction between the cluster and the surface. To accurately predict the relative stability of these metastable systems, the destabilization influences originating from the structural deformation of the cluster and substrate also need to be considered. For example, although in M5 there are four adsorption bonds (including the shortest Ti(5f)-Ot bond), the M5 structure is 0.3 eV less stable than the M6 one that contains three adsorption bonds. This is partly due to the fact that the shortest Ti(5f)-Ot bond in M5 introduces obvious relaxation of the substrate around the Ti(5f) atoms. 3.4. Electronic Structures of the W3O9/TiO2(110) Surface. It is well known that the perfect TiO2(110) clean surface is a typical semiconductor surface in which the valence band (VB) and conduction band (CB) are dominated by O 2p and Ti 3d states, respectively.22 The properties of the TiO2(110) surface can be modified by introducing new states within the band gap or by changing the filling situation of the energy bands through the charge transfer occurring between the adsorbate and the surface. To investigate the effects introduced by adsorption of the W3O9 cluster, in this section we will focus on the electronic structures of two stable configurations, namely, the M1 and M2 models. For M1, according to the total density of state (DOS) shown in Figure 5a, the TiO2 (110) surface still retains semiconductor

behavior after deposition of W3O9. Figure 5a also presents the partial DOS of the W3O9 fragment which, when compared to that of TiO2(110), indicates that states of the W3O9 cluster are distributed in the regions further away from the Fermi level. It is noted that after depositing on the TiO2(110) surface, the Fermi level of the system is located within the energy gap of W3O9, implying a small charge transfer occurring between W3O9 and the substrate. Moreover, as displayed in Figure 5b, the DOS of substrate in M1 is similar to that of the clean TiO2(110) surface. Therefore, it seems that adsorption of the W3O9 cluster has a small influence on the electronic structure of the TiO2 surface in this configuration. Figure 5c and 5d displays some partial DOSs of the W and O atoms of the W3O9 cluster in the M1 structure. Like other metal oxides, the occupied and unoccupied states of W3O9 close to the Fermi level are mainly derived from oxygen 2p and metal d states. Since the coordination environment of W(2) is analogous to that of the W(3) atom (both of them are 5-fold coordinated, see Figure 3a), the DOSs of the W(2) and W(3) atoms are somewhat similar. However, for the 4-fold-coordinated W(1) atom, a distinct distribution of the DOS is observed, which shows a tendency to centralize in the regions close to the Fermi level with respect to the other two W atoms. After depositing on surface, there are three types of oxygen atoms in the W3O9 cluster, including the original bridging and terminal O and the new kind of terminal oxygen connected to the

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Figure 6. (a) Total density of state (DOS) of the M2 model, and the contribution of W3O9 is denoted by the light gray area after enlarging three times, (b) partial DOSs of three tungsten atoms of the M2 model, and (d) partial DOSs of three types of oxygen atoms surrounding the W(3) atom. The vertical dashed line indicates the position of the Fermi level, taken as zero energy. The oxygen labeled as Ot′ is the terminal oxygen connected with the Ti(5f) atom of the substrate.

substrate. For instance, Figure 5d displays partial DOSs of three types of oxygens around the W(1) atom. The DOSs of the Ob and Ot atoms bound to the substrate show some similarities, while the occupied state of the unreacted Ot atom distributes more closely to the Fermi level. This difference can be seen more clearly by comparing the distribution of the O 2s state localized in the region from -19 to -16 eV. The electronic structure of the M2 model is analogous to the M1 model due to the thermodynamic stability and bonding of the two configurations being quite similar. In this case, the TiO2(110) surface still exhibits semiconducting character (Figure 6), and the VB and CB are also mainly originated from O 2p and Ti 3d of the substrate, respectively. In addition, similar distributions of the states of the W and O atoms of the W3O9 cluster are also observed in the M2 model. Now let us consider the charge transfers occurring between W3O9 and the TiO2(110) surface. As mentioned above, the W3O9 cluster interacts with the surface via formation of W-Ob′ and Ti(5f)-Ot bonds. Among them, formation of the W-Ob′ bond implies that the electrons transfer from W3O9 to the TiO2 (110) surface, whereas formation of the Ti(5f)-Ot bond leads to the opposite direction of charge transfer, namely, from the substrate to W3O9. Therefore, there appears to exist a competition between formation of two kinds of adsorption bonds, and the final magnitude and direction of the charge transfer are determined by the number and strength of the W-Ob′ and Ti(5f)-Ot bonds. The charge transfer occurring at the interface for nine adsorption

configurations is determined by analyzing the Bader charge distribution,33 and the results are reported in Table 1. Here, the negative and positive values mean receiving and losing electrons for the W3O9 cluster, respectively. As pointed out previously, in all structures the number of W-Ob′ bonds is not larger than that of Ti(5f)-Ot bond, so the W3O9 cluster tends to obtain electrons from the substrate if the strength difference between two types of adsorption bonds is not taken into account. Correspondingly, the negative value of the charge transfer of the cluster may be predicted for all adsorption models, and the results listed in Table 1 also indicate that except for M5, the electrons are transferred from the substrate to the cluster. However, it is noted that for all structures the magnitude of the charge transfer occurring between W3O9 and the TiO2(110) surface is small, which is not larger than 0.1 e. For example, only 0.06 e electrons are transferred from the TiO2(110) surface to W3O9 for M1, and a similar value of 0.08 e is obtained for the M2 structure. Therefore, it seems that when the stoichiometric tungsten oxide clusters are deposited on the perfect TiO2(110) surface, the charge transfer between the cluster and the substrate nearly keeps balanced. This conclusion is in accordance with the prior result that the TiO2(110) surface still maintains the semiconducting property. The charge distributions within the W3O9 cluster were also investigated. In the gas phase, the calculated Bader charges of the W, Ob, and Ot atoms are +2.70, -1.08, and -0.81 e, respectively. For the M1 model, the magnitudes of the charges of three W atoms (+2.74, +2.76,

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Figure 7. Simulated STM images for the (a) M1, (b) M2, (c) M9, and (d) M7 models taken at a bias of V ) +1.7 V. The values shown in the panels are distances in Angstroms. In the figures, the positions of the three tungsten atoms of the W3O9 cluster are denoted by the vertexes of the triangle, and the edge opposite to the highest W atom is indicated by a thick line. The deviation angle between this edge and the [001] direction of the TiO2(110) surface for each model is also given in the picture.

and +2.77 e, respectively) and three unreacted Ot atoms (-0.75, -0.79, and -0.81 e, respectively) slightly increase and decrease with respect to the values for the gas phase. Because of the deformation of the ring structure, the charges of the Ob atoms (-0.93, -1.00, and -1.03 e, respectively) vary somewhat obviously, and the largest decreasing of the magnitude of the charge is about 0.15 e. For those Ot atoms attached to the substrate, they carry more negative charges (-0.98, -0.99, and -1.02 e, respectively). Thus, after supporting the TiO2(110) surface, the charge redistribution of the W3O9 cluster mainly appears on the Ob and Ot bound to Ti(5f) atoms. Similar results can be observed for the next stable M2 (or M7) structure. The slight change of the charges of the tungsten atoms in these configurations is consistent with the XPS result that the dominant formal oxidation state of tungsten still remains 6+ after depositing on the TiO2(110) surface.12 Although the charge transfer is small and the W3O9 molecule is nonpolar in the gas phase, deposition of W3O9 has a large effect on the surface dipole moment. Table 1 lists the change of the dipole moment along the surface normal direction (∆D) with respect to the clean surface. It is interesting to note that the magnitude of ∆D is dependent on the orientation of ring structure of W3O9. Overall, the magnitude of ∆D is increased with the increasing of the inclination angle of the W3O3 plane of the six-membered ring. For instance, in the configurations of M3 and M5 the planes of the W3O3 ring are significantly

tilted away from the horizontal direction (Figures S1a and S1c in Supporting Information), resulting in the obvious changes of the surface dipole moment. Therefore, our results indicate that the experimental measurement of the surface dipole moment may be a valuable method to determine the orientation of the ring structure of W3O9 supported on the TiO2(110) surface. 3.5. STM Simulations. Using the Tersoff-Hamann approximation,34 we determined the STM images for two stable structures M1 and M2, and the M9 model proposed by Bondarchuk et al. is also considered. The images displayed in Figure 7 are obtained at a positive basis V ) 1.7 V, the same voltage used in the experiment.11,12 To compare with the experimental image,11,12 the triangle formed by three W atoms is marked in the pictures. It should be mentioned that the experimental STM images could not be reproduced exactly by this method because the effects of the STM tip, including the topological and electronic effects, are not taken into account in the simulation.35,36 As a result, the dark triangular feature formed by three Ob atoms in the experiments is not observed in the simulated images for all models (Figure 7). Actually, due to unknown variations in the details of the tip,12 the experimental results show that the STM images of 600 K annealed samples exhibit strong tunnelling current variation, and in some cases, the dark triangular feature is also not distinguishable. Although there are differences between the measured and simulated STM images, the qualitative comparison between the

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Figure 8. Schematic illustrations for the charge transfer occurring between the tungsten oxide cluster and the TiO2(110) surface when the nonstoichiometric W3Ox clusters, including (a) oxygen enriched (x > 9) and (b) oxygen deficient (x < 9), are deposited on the surface. The positions of the Fermi level (EF) before and after the depositions are given in the pictures, and the arrows indicate the directions of charge transfer and the shift of the Fermi level. The area filled by dense grids denotes the occupied states before charge transfer.

experimental and the theoretical results can provide some information about the configuration of W3O9. Since the heights of three W atoms in these structures are different (see Figure 2 and Figure S1f in the Supporting Information), the brightest features in the simulated STM images are associated to the 5d state of the W atom with the highest position, namely, the W(1), W(3), and W(1) atoms for M1, M2, and M9, respectively. The different brightness above three W atoms is also observed in the experimental high-resolution empty-state STM image.11,12 As a common character, these structures cannot be distinguished solely on the brightness shown in the STM image. However, the orientation of the brightest feature is different, which can be seen more clearly by examining one edge of the triangle opposite to the highest W atom (see the thick line in Figure 7). In the M9 structure this edge is just parallel to the Ti(5f) rows or [001] direction, while it deviates slightly and obviously from the [001] direction in the M1 and M2 structures, respectively. The calculated deviation angles between this edge and the [001] direction for the M1, M2, and M9 structures are 8°, 35°, and 0°, respectively. We also estimated the corresponding angle from the experimental STM image, and the result is about 38°. Therefore, it seems that only for the M2 configuration is the alignment of the brightest feature close to that observed experimentally. Additionally, by considering the coexistence of another isoenergetic structure, namely, the M7 model (Figure 7e), the brightest features in the STM image can be tilted with equal probability to the right or left away from the Ti(5f) row. This result is also consistent with the experimental observation.11,12 Therefore, the consistency between the measured and simulated STM images for the M2 (or M7) model provides support in favor of the corresponding arrangement of W3O9 on the TiO2(110) surface. 3.6. Prospection for Nonstoichiometric the W3Ox/TiO2(110) Surface. According to previous discussion about the electronic structure, we know that the energy gap of W3O9 is larger than the gap of the TiO2(110) surface, and the position of the HOMO of the cluster (mainly derived from O 2p) is under the VB top of the substrate, while the energy level of the LUMO (dominated by W 5d) is above the CB edge of TiO2. Consequently, deposition has a small influence on the electron occupation of W3O9, and as mentioned above, the charge transfer occurring between the stoichiometric W3O9 cluster and the

TiO2(110) substrate is small. However, this situation may be changed when the nonstoichiometric W3Ox (x * 9) cluster is supported on the TiO2(110) surface. The obvious charge transfers can be expected when the orbitals of the cluster mainly originating from the O 2p or W 5d states are partly occupied, resulting in the variations of the Fermi level of the system. The different changes of the Fermi level can be seen more clearly in Figure 8, and the downward and upward shifts of the Fermi level are observed for deposition of oxygen-enriched (x > 9) and oxygen-deficient (x < 9) tungsten oxide clusters, respectively. To verify the above mechanisms, we choose here the M1 model as an example by adding one Ot atom to simulate adsorption of the W3O10 cluster. The preliminary results (see Figures S2 and S3 in the Supporting Information) agree with the schematic illustrations shown in Figure 8, and there are obvious (0.86 e) electrons transferred from the substrate to the cluster. With this charge transfer, the work function of the TiO2(110) surface is increased significantly (about 1.0 eV) with respect to the clean surface. Because of the shift of the Fermi level, the TiO2(110) surface becomes metallic after depositing nonstoichiometric W3Ox (x * 9) cluster on the substrate. Therefore, it seems that the properties of the TiO2(110) surface can be controlled by depositing tungsten oxide clusters with different compositions. 4. Conclusions In this paper, the atomic structures and electronic properties of the W3O9 cluster supported on the perfect TiO2 (110) surface have been systematically investigated by means of firstprinciples DFT calculations. Eight possible configurations derived from MD simulations and the hypothetical model proposed by Bondarchuk et al. according to experimental STM images11,12 are considered. Our results indicate that the thermodynamic stability of the W3O9/TiO2(110) system is mainly dependent on the number and strength of the bonds at the interface. The maximum number of adsorption bonds between the cluster and the TiO2 surface is five, which are observed in three configurations, namely, M1, M2, and M7. In these structures, the W3O9 cluster is adsorbed on the surface via three Ti(5f)-Ot bonds and two W-Ob′ bonds. Although the M1 configuration is thermodynamically most favorable, it only

W3O9 Cluster Supported on the TiO2(110) Surface appears in the beginning stage of the MD simulation for the artificial initial structure with strong interaction between the cluster and the substrate (Figure 2b). The M2 and M7 structures come from MD simulations for different initial structures; however, two structures are isoenergetic and correspond to the same adsorption pattern with a chirality relationship between the geometries of the W3O9 cluster (Figure 4). Since the configurations of M2 and M7 are often observed in the MD calculations and slightly higher (0.22 eV) in energy with respect to M1, we tentatively assign the M2 and M7 configurations to the most likely structure of the W3O9 supported on the TiO2(110) surface under the experimental conditions. This assignment is further confirmed by comparing the simulated and experimental STM images. Compared to the structure of the W3O9 cluster in the gas phase, in the M2 and M7 models, one-half of the WdO groups take part in the interactions with the TiO2 surface, and this loss of active WdO groups may be the reason why the 600 K preannealed W3O9 cluster shows low catalytic activity for the dehydration of 2-propanol.23 Furthermore, it is noted that two of the three tungsten atoms are also involved in formation of bonds with the bridging oxygen on the substrate, and the reaction process may be blocked by five oxygen atoms surrounding these W atoms. Although deposition has a significant influence on the chemical activities of the W3O9 cluster, it has a small effect on the electronic structure of the substrate. The TiO2(110) surface still retains the semiconducting character, and the VB and CB of the system near the Fermi level are mainly derived from the O 2p and Ti 3d states of the substrate, respectively. The results of the Bader charge analysis show that the charge transfer occurring between W3O9 and the TiO2(110) surface is small. Our results reveal that the change of the surface dipole moment is sensitive to the orientation of the ring structure of W3O9. Correspondingly, the experimental measurement for the surface dipole moment may provide us some useful information to determine the arrangement of the W3O9 ring structure supported on the TiO2(110) surface. Acknowledgment. This work was supported by the National Natural Science Foundation of China (grant nos. 20773024, 20771026, 10804001, and 10676007), the Specialized Research Fund for the Doctoral Program of Higher Education of China (grant no. 20060386001), and the Natural Science Foundation of Fujian Province (grant no. 2008J0151). Y.Z. and W.C. also thank the programs for New Century Excellent Talents at the University of Fujian Province (grant nos. HX2006-97 and HX2006-103). Supporting Information Available: Configurations of some unstable models (M3-M6, M8, and M9) for the W3O9/ TiO2(110) systems, possible structure of W3O10 supported on the TiO2(110) surface, and the corresponding DOSs. This material is available free of charge via the Internet at http:// pubs.acs.org.

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