Properties of two-dimensional insulators: A DFT study of bimetallic oxide CrW2O9 clusters adsorption on MgO ultrathin films

Applied Surface Science 379 (2016) 213–222

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Properties of two-dimensional insulators: A DFT study of bimetallic oxide CrW2 O9 clusters adsorption on MgO ultrathin films Jia Zhu a,∗ , Hui Zhang a , Ling Zhao a , Wei Xiong a , Xin Huang b , Bin Wang b , Yongfan Zhang b,c,∗ a

College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, Jiangxi 330022, China Department of Chemistry, Fuzhou University, Fuzhou, Fujian, 350108, China c State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou, Fujian, 350002, China b

a r t i c l e

i n f o

Article history: Received 22 December 2015 Received in revised form 25 March 2016 Accepted 3 April 2016 Available online 6 April 2016 Keywords: DFT Bimetallic oxide CrW2 O9 clusters MgO ultrathin films Adsorption

a b s t r a c t Periodic density functional theory calculations have been performed to study the electronic properties of bimetallic oxide CrW2 O9 clusters adsorbed on MgO/Ag(001) ultrathin films (<1 nm). Our results show that after deposition completely different structures, electronic properties and chemical reactivity of dispersed CrW2 O9 clusters on ultrathin films are observed compared with that on the thick MgO surface. On the thick MgO(001) surface, adsorbed CrW2 O9 clusters are distorted significantly and just a little electron transfer occurs from oxide surface to clusters, which originates from the formation of adsorption dative bonds at interface. Whereas on the MgO/Ag(001) ultrathin films, the resulting CrW2 O9 clusters keep the cyclic structures and the geometries are similar to that of gas-phase [CrW2 O9 ]− . Interestingly, we predicted the occurrence of a net transfer of one electron by direct electron tunneling from the MgO/Ag(001) films to CrW2 O9 clusters through the thin MgO dielectric barrier. Furthermore, our work reveals a progressive Lewis acid site where spin density preferentially localizes around the Cr atom not the W atoms for CrW2 O9 /MgO/Ag(001) system, indicating a potentially good bimetallic oxide for better catalytic activities with respect to that of pure W3 O9 clusters. As a consequence, present results reveal that the adsorption of bimetallic oxide CrW2 O9 clusters on the MgO/Ag(001) ultrathin films provide a new perspective to tune and modify the properties and chemical reactivity of bimetallic oxide adsorbates as a function of the thickness of the oxide films. © 2016 Published by Elsevier B.V.

1. Introduction Supported early transition metal oxides are of great technological importance in heterogeneous catalysis both as supports and as active materials directly involved in the reaction processes. Among them, tungsten trioxide is of particular interest which has been found to accelerate many types of reactions [1], such as alkanes isomerization [2], alkenes metathesis [3], alcohols oxidation [4], and selective NO reduction [5]. In recent years, an increasing interest has been devoted on the bimetallic oxides which incorporate different transition metals at varying levels for controlling and improving the catalyst properties. Specially, bimetallic involving group six transition metal (Cr, Mo, W) oxides are frequently encountered in studies on new catalysts. Extensive evidence indicates that bimetallic oxides often exhibit novel properties that are not presented on

∗ Corresponding authors. E-mail addresses: jia [email protected] (J. Zhu), [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.apsusc.2016.04.006 0169-4332/© 2016 Published by Elsevier B.V.

their parent metal oxides [6–8]. Now, alloying the tungsten trioxide clusters with group six transition metals manipulates the geometric and electronic structure of the resulting clusters to give the enhanced selectivity, activity and stability in surface-catalysed reactions, such as oxidative dehydrogenation of alkanes [9–15] and dehydration of alcohols [16,17]. Recently, the bimetallic oxide clusters with pentavalent and hexavalent cations in gas phase, such as Mo(3-x) Wx Oy − (x = 0 − 3, y = 3 − 9) [6,7], MW2 O9 −/0 (M = V, Nb, Ta) [8] clusters, have been determined using a combination of photoelectron spectroscopy (PES) and density functional calculations. It has been found that the cyclic structure predicted to be the most favorable for the neutral cluster. While in fact such bimetallic oxide clusters are commonly as active components supported on metal oxide surfaces as model catalyst, due to the combination of cluster-substrates interactions often inducing more interesting catalytic properties compared to the clusters in gas phase. In this respect, a general understanding of the adsorption behavior on supported bimetallic oxides can help us

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discover more efficient catalysts for a given reaction and may open new scenarios for the rational design of new catalytic materials. In the past few years a continuously increasing interest has been devoted to ultrathin oxide films grown on a metal substrate [18–23]. These systems exhibit unusual chemical and electronic properties with respect to thicker films or single crystal oxide surfaces, due to the presence of the dielectric boundary and the reduced dimensionality of the insulating films, offering new opportunities for the design of new functional materials [24–27]. One specific and crucial property of ultrathin oxide films is the occurrence of a spontaneous charge transfer from the metal support to an adsorbed species through the thin insulating layer (or vice versa) [22,26,28–32]. This has been done recently for various systems, including Au atoms [33,34], Au clusters [35,36], W3 O9 cluster [31], or simple molecules like O2 [37], NO2 [32,38] when adsorbed on MgO/Ag(001) films becoming negatively charged. Among them, we have reported that the monometal oxide W3 O9 clusters deposition on MgO ultrathin films have formed stable, doubly charged [W3 O9 ]2− clusters, and the charge state is of importance for the structural and catalytic properties of adsorbates [31]. However, up to now very little information is obtained for the atomic structure and electronic properties of supported bimetallic oxides which would represent a great advancement in the design of clusterbased catalysts. And the situation appears to be more complicated for the supported bimetallic oxide cluster where the interaction between the culster and support materials need to be considered. Therefore, it is interesting and appealing to further investigate the effect on the bimetallic oxide CrW2 O9 clusters adsorbing on the MgO/Ag(001) ultrathin films whether will exihibit a similar sensitivity to charge states with respect to pure W3 O9 cluster, and show different catalytic properties compared with that on the crystal MgO(001) surface. In this work we extend our theoretical research to the deposition of the bimetallic oxide CrW2 O9 clusters on the MgO/Ag(001) ultrathin films and the crystal MgO(001) surface. The article is organized as follows: We first study the gas-phase CrW2 O9 clusters in its neutral and charged states. Next, we investigate the most stable configurations of the deposited CrW2 O9 clusters based on the results of first-principles molecular dynamics (MD) simulations. Furthermore, we compare the electronic properties of CrW2 O9 clusters supported on the MgO(001) surface and MgO/Ag(001) ultrathin films with particular attention to cluster-support interaction effects in terms of electronic modifications of the clusters, as well as variation of the properties of the supports. Finally, we discuss some observable properties, including STM images and the vibrational spectra. We report for the first time that, for bimetallic oxide clusters deposited on oxide ultrathin films, opening new perspectives for the design and study of model catalysts and their activity as a function of structure and charge state.

2. Computational details For the CrW2 O9 cluster, we carried out spin-polarized density functional calculations employing the hybrid B3LYP [39–41] scheme to describe the exchange-correlation contribution to the electron-electron interaction, as implemented in GAUSSIAN03 program [42]. All calculations have been performed employing the following basis sets: aug-cc-Pvtz for O [43,44]; Stuttgart relativistic small core basis set and efficient core potential augmented with two f-type ( W = 0.256, 0.825;  Cr = 2.555, 0.640) and one g-type ( W = 0.627;  Cr = 1.712) [45]. Vibrational frequency calculations were performed at the same level of theory to verify the nature of the stationary points. For the CrW2 O9 cluster adsorbed on the MgO/Ag(001) ultrathin films and crystal MgO(001) surface, the periodic calculations

have been performed with density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP) [46–48] and the projector augmented wave method (PAW) method [49,50]. The generalized gradient approximation of the exchangecorrelation functional proposed by Perdew-Wang (PW91) [51] was employed and the kinetic energy cutoff for the plane-wave expansion was set to 400 eV. The choice of the PW91 functional was suggested by the possibility to directly compare results of this study with those of similar systems investigated previously [22,28,29,31]. Spin polarization has been considered. The MgO/Ag(001) substrate was modeled by four metal layers (two layers for molecular dynamics (MD) simulations), and a two-layer (2ML) MgO film was deposited on top of it. We studied the deposition of CrW2 O9 clusters on the MgO/Ag(001) thin films as well as on the bare MgO(001) surface represented by a three-layer MgO slab. The results of test calculations showed that the three-layer slab model was thick enough to obtain the properties focused on the present work. The experimental lattice constant of Ag (4.09 Å) is 3% smaller than that of MgO (4.21 Å). In the calculations, the optimized Ag and MgO lattice parameters are 4.16 and 4.25 Å, respectively, and the lattice mismatch is reduced to about 2%. Therefore, the MgO layers are slightly contracted with respect to their bulk distance after they were supported on the Ag metal. During the geometry optimization of MgO/Ag(001) interface, all atoms in the MgO films and in the two outmost Ag layers were relaxed while the remaining two Ag layers were fixed to their bulk position (in the MD simulations only the atoms of the top Ag layer were relaxed). For the bare MgO(001) surface, the atoms of the two upper layers of MgO(001) surface were relaxed while the bottom MgO layer were immobilized. To avoid interactions between neighboring CrW2 O9 clusters, a (6 × 6) supercell consisting of 288 atoms(144 Ag, 72 Mg, and 72 O atoms) was employed, and a (3 × 3 × 1) Monkhorst-Pack was used for the k-point sampling. Compared with metal clusters [35,36] or monometal oxide (WO3 )3 clusters [31,52–54] which have been studied, the deposition of bimetallic tungsten oxide CrW2 O9 clusters on the surface or films is more complicated. In the CrW2 O9 clusters, there are two kinds of the terminal oxygen (Ot ) atoms connected with W or Cr atoms, respectively. Besides, there are also two kinds of bridging oxygen (Ob ) atoms which are bound to one W atom and one Cr atom or bound to two W atoms. Therefore, various O Mg bonds can be formed when the CrW2 O9 clusters deposited on the MgO(001) surface or the MgO/Ag(001) films. Beyond that, both the acid W6+ sites and Cr6+ sites of the CrW2 O9 clusters can be bound to the surface oxide anions Os . Hence, there are many possible arrangements of the CrW2 O9 clusters on the MgO(001) surface or MgO/Ag(001) films. In order to find the most stable configurations of the deposited CrW2 O9 clusters as much as possible, and reduce the influence of artificial factors, first-principles molecular dynamics (MD) simulations using the Nosé algorithm [55] were put into effect to explore possible configurations of CrW2 O9 /MgO(001) and CrW2 O9 /MgO/Ag(001) systems using a low-energy cutoff (200 eV). Besides, for the molecular dynamics simulations, the initial configurations of the clusters placed horizontally or vertically all were considered. The simulation time was 10 ps with a time step of 1 fs at the temperature of 600 K. Then we extracted samples of possible adsorption configurations from the results of the molecular dynamics (MD) simulation every 50 steps, resulting in 200 initial structures for each MD process. Further structural optimizations were carried out to determine the most stable adsorption configurations using more accurate settings (see above). The results of previous studies [52,56,57] demonstrate that adopting the method of molecular dynamics combining with quantum mechanics to determine the configuration of complex systems is feasible.

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Fig. 1. The optimized structures of (a) CrW2 O9 , (b) [CrW2 O9 ]− and (c) [CrW2 O9 ]2– in gas phase. The Cr, W, O atoms are denoted by green, dark blue and red spheres, respectively. Selected bond lengths are given in angstroms and bond angles in degrees. The results obtained by PW91 functional with plane-wave basis set are also listed in parentheses for comparisons. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3. Results and discussion 3.1. Structures of the gas phase CrW2 O9 clusters, CrW2 O9 /MgO(001) and CrW2 O9 /MgO/Ag(001) 3.1.1. Structures of [CrW2 O9 ], [CrW2 O9 ]− , and [CrW2 O9 ]2− clusters in gas phase Before considering the cases of supported clusters, firstly the configurations of CrW2 O9 and its anions have been discussed. The configurations of the ground state at the B3LYP level of theory of the [CrW2 O9 ], [CrW2 O9 ]− , and [CrW2 O9 ]2− clusters are presented in Fig. 1, as well as the corresponding structural parameters and relative energies. For the CrW2 O9 neutral cluster, it exhibits a coplanar CrW2 O3 cyclic conformation with C2v symmetry (Fig. 1a). Each metal atom is tetrahedrally coordinated with two terminal (Ot ) and two bridging O atoms (Ob ), and the lengths of W Ob , W Ot , Cr Ob , and Cr Ot bonds are 1.90 Å, 1.71 Å, 1.76 Å and 1.55 Å, respectively. The W W and W Cr distances are 3.50 Å and 3.41 Å, respectively. In addition, the geometry of the isolated neutral CrW2 O9 (placed in a 20 × 20 × 20 Å cubic box) cluster is also optimized by PW91 functional with plane-wave basis set for comparison, and the corresponding structural parameters are labeled in Fig. 1a in parentheses. The configurations predicted by the two approaches are similar, and the differences among the bond lengths and bond angles are smaller than 0.04 Å and 1.5◦ , respectively. The ground state of [CrW2 O9 ]− anion remains C2v symmetry as displayed in Fig. 1b. The Cr Ot and W Ot distance (1.58 Å and 1.73 Å, respectively) are slightly elongated with respect to the neutral cluster. On the other hand, the lengths of Cr Ob and W Ob bonds in the six-member ring increase obviously about 0.13 Å and 0.07 Å, respectively, implying the weakening of the Cr Ob and W Ob bonds. The above expansion of the six-member ring also results in the increase of the metal-metal distances between the Cr and W atoms (3.52 Å). For [CrW2 O9 ]2− clusters (Fig. 1c), the six-remember rings observed in neutral and [CrW2 O9 ]− anion are not found, and a four-member ring is formed, which is similar to the results reported for TaV2 O9 − [8]. Obviously, it is interesting that series of the structures of the neutral and anion bimetallic (CrW2 O9 ) oxides are strongly influenced by different charge states, whereas these phenomenons are not observed in the results reported for pure tungsten oxide (W3 O9 ) [58].

3.1.2. Structures of CrW2 O9 clusters supported on the bare MgO(001) surface and MgO/Ag(001) films The most stable structures obtained from the ab initio MD simulations of CrW2 O9 adsorption on the MgO(001) surface and on the MgO/Ag(001) ultrathin films are shown in Fig. 2a and b, respectively. On the bare MgO(001) surface, Fig. 2a, in this structure three terminal (Ot ) and one bridging (Ob ) oxygen atoms associated with two W atoms in the CrW2 O9 clusters are bonded to surface Mg atoms, and at the same time, two W atoms of the cluster bind

to two surface oxygen atoms (Os ) with bond length of 2.04 Å and 2.06 Å, respectively. This leads to a total of six adsorption bonds formed at the interface, and the corresponding adsorption energy is about 3.07 eV (see Table 1). It can be seen that the Cr atom and its surrounding Ot and Ob atoms are not bonded with the perfect MgO(001) surface and prefer the positions far from the substrate. Not surprisingly, the deposition results in a strong distortion of the cluster and the obvious relaxation of the MgO surface. In particular, the two Os atoms relax outwards by 0.36 Å and 0.45 Å, in order to maximize the interaction with the W atom, and the movements of surface Mg atoms are also observed due to the formation of new Mg O bonds with the Ot and the Ob atoms. After depositing, the cyclic conformations of CrW2 O9 clusters are strongly distorted with respect to the configuration in the gas phase. The W W distance is elongated to 3.83 Å than that in gas-phase (3.46 Å). The most stable configuration for the adsorption of CrW2 O9 on the MgO/Ag(001) ultrathin films is completely different from that found on MgO(001) surface, Fig. 2b. In this structure, three terminal (Ot ) oxygen atoms associated with three different metal atoms are bonded to surface Mg atoms forming bonds of about 2.1 Å length, respectively. This is shorter than the analogous distance on the bare MgO(001) surface, suggesting a stronger Ot Mg interaction on the MgO film. At the same time, one bridging (Ob ) oxygen atom between the Cr atom and one W atom of CrW2 O9 cluster bind to one surface Mg atom, Fig. 2b. Thus, there are four new bonds at the interface, and the corresponding adsorption energy is 3.64 eV, which is 0.57 eV obviously higher than that on the bare MgO(001) surface, despite the fact that here only four bonds are formed. Here we discussed the surface rumpling(SR) for the dielectric layer at the interface, SR(Dint ) and for the top layer of the dielectric film, SR(Dext ). The rumpling is defined as the difference in height of Mg2+ and O2− of the dielectric. The interface distance (2.71 Å) and the surface rumpling (−0.01 Å and −0.04 Å for SR(Dint ) and SR(Dext ), respectively) of the clean MgO(2ML)/Ag films are agree well with the previous results carried by Prada et al. [59,60]. While for the CrW2 O9 cluster adsorption on the MgO/Ag(001) film, the CrW2 O9 /MgO/Ag(001) system exhibit reduced interface distances about 2.6 Å, but significantly larger surface rumpling including the SR(Dint ) (0.06–0.22 Å) and SR(Dext ) (0.19–0.37 Å), which indicated the cations Mg2+ is strong relaxation. The obvious relaxation of Mg2+ results in the strengthening of Mg O bonds. This is more than twice the analogous distortion found on MgO(001) surface, and provides a first indication of the occurrence of a net charge transfer from the MgO/Ag(001) interface to the deposited oxide cluster. In fact, similar strong surface rumpling have been predicted theoretically and observed experimentally in connection with the formation of negatively charged species on the insulating MgO thin film [28,31,37]. In addition, we also consider properties of CrW2 O9 cluster supported on 3ML of MgO film compared with that of CrW2 O9 /MgO(2ML)/Ag(001). The CrW2 O9 /MgO(3ML)/Ag(001) system exhibit similar interface

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Fig. 2. Optimized ground state structures for (a) CrW2 O9 /MgO(001) (top view and side views); (b) CrW2 O9 /MgO/Ag(001) (top view and side views). W, O, Mg, Ag and Cr atoms are represented by dark blue, red, gray, light blue and green spheres, respectively. The bond lengths are in international angstrom unit. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1 Adsorption Energy (Eads ), the Net Bader Charge (q), Work Function Change (␾), One W W Distance and Two W Cr Distances for CrW2 O9 Clusters Deposited on MgO(001) Surface and on MgO/Ag(001) Ultrathin Films. For Eads and q, the values in the parentheses are obtained by using DFT-D2 method. System

Eads (eV)a

q(e)b

␾ (eV)

W Wdistances (Å)

W Crdistances (Å)

CrW2 O9 /MgO(001) CrW2 O9 /MgO/Ag(001)

3.07(4.67) 3.64(5.21)

−0.73(−0.71) −1.72(−1.73)

0.30 1.18

3.83 3.33

3.42, 3.41 3.33, 3.59

a The Eads in Table 1 is defined as Eads = Esurface + ECrW2O9 − ECrW2O9/surface , where Esurface , ECrW2O9 and ECrW2O9/surface represent the total energies of the clean MgO(001) surface or MgO/Ag(001) surfaces, the ground state of CrW2 O9 cluster in the gas phase and the whole system after depositing CrW2 O9 on MgO(001) surface and MgO/Ag(100) films, respectively. b The negative values indicate that the electrons are transferred from the surface to the CrW2 O9 cluster.

distance about 2.6 Å, but significantly smaller surface rumpling including the SR(Dint ) (0.06–0.22 for CrW2 O9 /MgO(2ML)/Ag(001) and 0.03–0.06 for CrW2 O9 /MgO(3ML)/Ag(001), respectively.) and SR(Dext ) (0.19–0.37 for CrW2 O9 /MgO (2ML)/Ag(001) and 0.12–0.30 for CrW2 O9 /MgO(3ML)/Ag(001), respectively.). The displacement of the Mg cations is indeed the signature of a polaronic distortion to screen the negative charge accumulated on the adsorbed clusters. After depositing, the configuration of the CrW2 O9 fragment changes significantly with respect to the case in the gas phase, in which the W Cr distances, about 3.59 Å, are shorter than that in the neutral gas-phase cluster (3.41 Å), while W W distance slightly decreases from 3.50 Å to 3.33 Å. This structure of CrW2 O9 unit is closer to that found in the gas-phase anion, [CrW2 O9 ]− . Thus this is another signature of the occurrence of a charge transfer on MgO/Ag(001) ultrathin films comes from the geometrical parame-

ters analysis within the CrW2 O9 unit. In the following section we will analyze in detail the electronic origin of this difference. It is known that MgO/Ag is a weakly bound interface, and the inclusion of van der Waals (vdW) correction in the DFT functional affects the geometries and therefore electronic properties of the interface [60]. Here we consider the effect of van der Waals on the adsorption energies of the CrW2 O9 cluster on MgO(001) surface and MgO/Ag(001) ultrathin films using Grimme’s DFT-D2 method. As expected, the adsorption energies are increased with DFT-D2 approach, as shown in Table 1. However, the effect is not the same for CrW2 O9 /MgO(001) and CrW2 O9 /MgO/Ag(001). For the CrW2 O9 cluster on thin MgO film supported on Ag, because of more electron transfer by direct electron tunneling, the binding is likely to be strong and therefore the effect of inclusion of vdW correction is smaller than the CrW2 O9 cluster on perfect MgO(001) surface, with an increase in the adsorption energy of around 43%. While for

J. Zhu et al. / Applied Surface Science 379 (2016) 213–222

the CrW2 O9 /MgO(001) the inclusion of vdW correction has a bigger effect because of the weaker of the interaction, with an increase in the adsorption energy by about 52%. 3.2. Electronic structures of CrW2 O9 /MgO(001) and CrW2 O9 /MgO/Ag(001) 3.2.1. Density of states (DOSs) Fig. 3 displays the partial density of states (DOSs) of the most stable configuration for CrW2 O9 cluster adsorption on bare MgO(001) surface. It shows that the occupied O 2p states of CrW2 O9 clusters are in an energy region below the Fermi level, EF , which here coincides with the top of the MgO valence band. While some empty states dominated by Cr 3d states can be observed within the band gap of MgO conduction. There is no new state near or above the top of valence band. While the partial density of states (DOSs) of the most stable structure for CrW2 O9 cluster deposited on the MgO/Ag(001) films are given in Fig. 3c. In this structure, the Fermi level is determined by the Ag support, and falls in the gap of the MgO insulating film. An obvious and crucial distinction with respect to the CrW2 O9 /MgO system is that now a new sharp peak originated from the states of CrW2 O9 appears at the Fermi level, which is significantly different from the case on the bare MgO(001) surface. According to the atomic partial DOSs of Cr, W, O atoms of cluster after adsorption (Fig. 3d), it is clear that the above peak is mainly derived from Cr 3d state, and also contains a slight contribution of some O 2p states surrounding the Cr atom in deposited CrW2 O9 clusters, Fig. 3d. Moreover, the analysis of spin density, Fig. 4, further proves that the charge states are mainly localized around the Cr atom, and slightly around some O atoms surrounding the Cr atom. And this phenomenon is significantly different from that of W3 O9 /MgO/Ag(001) system, in which the charge state is evenly distributed [31]. Therefore, it can be expected that incorporating Cr atom to tungsten oxide can controlling and improving electronic structure and catalyst properties.The doped Cr acting as a strong Lewis acid site, is chemically active and may act as a favorable catalytic center. This phenomenon has been reported in previous works carried by Valentin et al. for the W3 O9 supported on TiO2 (110) [54]. 3.2.2. Bader charges At first, the direction of charge transfer can be predicted by examining the relative positions of the Fermi level (EF ) of MgO surface, Ag, and the lowest unoccupied molecular orbital (LUMO) of the CrW2 O9 cluster. The value of EF with respect to the vacuum level of MgO surface and Ag can be determined by the corresponding work function, which is calculated as the deviation between the potential at the middle of the vacuum region and EF . For the MgO surface, a value of 4.85 eV is predicted, agreeing well with previous results of 4.8 eV [52] and 4.91 eV [61], while values of 4.22 eV is obtained for metal Ag, which is consistent with previous computational value (4.23 eV) [59] and experimental value (4.22 eV) [62]. For the CrW2 O9 cluster, the position of the LUMO can be deduced from the adiabatic electron affinity (EA) of the CrW2 O9 cluster and the value is 5.1 eV. Therefore, the relative energy levels of MgO surface, the metal Ag, and CrW2 O9 cluster can be arranged. It is clear that the CrW2 O9 cluster tends to obtain electrons from MgO surface and Ag surface. And the electron transfer becomes rather easier to occur for the metal Ag. To access the charge transfer more accurately, Table 1 shows the values of charge transfer obtained by analyzing the Bader charges distribution for CrW2 O9 /MgO(001) and CrW2 O9 /MgO/Ag(001). In here the negative value means that the CrW2 O9 cluster obtains electrons. It can be seen clearly that, for the systems of CrW2 O9 /MgO(001) and CrW2 O9 /MgO/Ag(001), the directions of charge transfer at the interface are both from the substrate to the

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cluster. However, the magnitudes of charge transfer, sources and distribution of electrons are completely different on the two systems. For the system of CrW2 O9 /MgO(001), just a small electrons (0.73 e) are transferred from the bare MgO(001) surface to the CrW2 O9 cluster (Table 1). As mentioned above (Section 3.1.2), the CrW2 O9 cluster interacts with the surface via formation of six adsorption dative bonds (three Ot Mg, one Ob Mg and two Os W bonds). Among them, formation of the O Mg dative bonds implies that the electrons transfer from MgO surface to CrW2 O9 cluster, on the contrary, formation of the W O dative bonds leads to the opposite direction of charge transfer, namely, from the CrW2 O9 cluster to substrate. Thus, there appears to exist a competition between formation of two kinds of adsorption dative bonds, and the final magnitude and direction of the charge transfer are determined by the number and strength of the O Mg and W O dative bonds. Because the number of O Mg bonds are more than that of W O bonds, the CrW2 O9 cluster tends to obtain electrons from the MgO(001) surface. This illustrates that the CrW2 O9 cluster is negatively charged, which is the consequence of the formation of adsorption dative bonds at the interface. Further analyses of the charge distribution for each atom in the deposited cluster show that transfer charges are mainly shared by oxygen atoms, while the charge of Cr or W atoms have hardly any changed (<0.07e). Additionally, the electrons transferred from substrate to CrW2 O9 cluster also lead to a moderate increase of work function (about 0.30 eV). While for the system of CrW2 O9 /MgO/Ag(001), we found a net charge transfer of 1.72 e (Table 1) occur at the interface. This is more than twice the charge found on the bare MgO(001) surface, and the origin of the negative charge on the cluster is also completely different with respect to the CrW2 O9 /MgO(001) system. The analysis of Bader charge shows that the charge transfer occurs directly from the metal Ag support to the deposited CrW2 O9 clusters by an electron tunneling mechanism through the MgO thin dielectric barrier. This charge comes mostly from the MgO Ag interface, similar phenomenons have been predicted in other cases [28,31]. Then, further charge analysis for each atom in the deposited CrW2 O9 clusters shows that the electrons are mainly distributed on the specific Cr atom. Additionally, the more electrons transferred from substrate to CrW2 O9 also lead to increase of work function (about 1.18 eV). In addition, for CrW2 O9 /MgO(3ML)/Ag(001) system, the charge transfer are also large (1.59 e) but slightly less than that in CrW2 O9 /MgO(2ML)/Ag(001) (1.72 e). Meanwhile, the analysis of the Bader charge using Grimme’s DFT-D2 method is also displayed in Table 1 in parentheses. It can be seen clearly that the net Bader charges did not change in a significant way when the vdW interactions were included. 3.3. Observable properties 3.3.1. STM simulations Using the Tersoff-Hamann approach, we have simulated STM images for the most stable configurations of the CrW2 O9 clusters supported on bare MgO(001) surface and MgO/Ag(001) films as shown in Fig. 5. Due to the well known DFT underestimate of the band gap in insulating materials, these values do not necessarily correspond to regions where tunneling occurs via empty or occupied states in direct measurements; these can, in fact, be typically shifted by 1–2 V compared to the computed regions [63]. We have simulated STM images of the CrW2 O9 /MgO(001) and CrW2 O9 /MgO/Ag(001) systems at positive and negative bias, V = +5.0 V, +3.5 V, −5.0 V, −3.5 V, respectively. The best topographic images reflecting the cluster morphology are obtained at a positive bias voltage (V = +5.0 V) when tunneling into the Cr and W empty states is taken place. Not surprisingly, the shape and brightness

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Cr

CrW2O9x3

DOS(arb.units)

DOS(arb.units)

MgO

-6

-4

-2

0

2

4

Energy(eV)

6

8

W

O

-6

10

-4

-2

0

(a)

-2

0

6

8

10

Cr

2

4

6

8

DOS(arb.units)

DOS(arb.units)

-4

4

(b) Ag MgO CrW2O9x3

-6

2

Energy(eV)

10

Energy(eV)

(c)

W

O

-6

-4

-2

0

2

Energy(eV)

4

6

8

10

(d)

Fig. 3. Partial DOSs, and Partial DOSs projected on the Cr, W and O atoms of the cluster for the most stable configuration of CrW2 O9 /MgO(001) (a and b, respectively), and CrW2 O9 /MgO/Ag(001) (c and d, respectively). The vertical dashed line indicates the position of the Fermi level, taken as zero energy.

Fig. 4. Spin density spots of the top (left) and side (right) views of the CrW2 O9 cluster deposited on the MgO/Ag(001) films.

appeared in the STM-topographic images are quite different when CrW2 O9 is supported on bare MgO(001) surface and MgO/Ag(001) films. Since the STM contrast is governed by the electronic properties and topographic position of the deposited CrW2 O9 clusters. At the positive bias voltage, the brightest spots in the simulated STM

images are mainly associated to the conductance spectra of the Cr and W atom with the higher position. The variation of the height of the Cr and W atoms in the deposited structures produces different distributions of the bright spots which reflect the irregular shape of the clusters. As shown in Fig. 5a, for the CrW2 O9 clusters deposited on the MgO(001) surface, one brightest spot correspond-

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Fig. 5. Simulated STM images of the most stable structures of CrW2 O9 /MgO(001) and CrW2 O9 /MgO/Ag(001) system taken at different bias. (Top panel) CrW2 O9 /MgO(001): (a) V = +5.0 V, (b) V = +3.5 V, (c) V = −5.0 V, (d) V = −3.5 V. (Bottom panel) CrW2 O9 /MgO/Ag(001): (e) V = +5.0 V, (f) V = +3.5 V, (g) V = −5.0 V, (h) V = −3.5 V, respectively.

ing to the Cr atom at the highest position and three less brighter spots even distribution around the brightest spots are observed. At negative bias, one brightest spot and another brighter spot occur corresponding to the two Ot atoms connected with the Cr atom at higher position. Very different STM images are expected for the CrW2 O9 /MgO/Ag(100) system (Fig. 5e–5g). The structure is symmetric and shows three bright spots at +5.0 V corresponding to the metal atoms (Fig. 5e); these images are similar to those reported for (WO3 )3 clusters deposited on MgO/Ag(001) [31], Cu(110) [64] and Pt(111) metals [65]. At negative bias the bridging and the terminal oxygens also become visible in the simulated images (Fig. 5g,h). Different from those reported for (WO3 )3 clusters [31], the spots in images of adsorbed CrW2 O9 clusters are almost independent due to the breaking of the multi-center multi-electron bond. 3.3.2. Vibrational spectra The vibrational properties of the gas-phase CrW2 O9 cluster, the CrW2 O9 /MgO(001) and CrW2 O9 /MgO/Ag(001) systems have been determined by using a central finite difference method, respectively; the corresponding high resolution electron energy loss specotroscopy (HREELS) intensities have been obtained from the derivatives of the dipole moment, considering only the component perpendicular to the surface (surface selection rule) for the deposited clusters. In addition, for the deposited CrW2 O9 clusters, according to the coordination of the original six Ot atoms (Fig. 2), they can be divided into two groups, the first group is Ot atoms that are still at the terminal positions, the second group is Ot atoms that are connected with surface Mg atoms. In the following discussions, the later kind of oxygen atoms are denoted by Ot Mg . While the Ob atoms that are connected with surface Mg atoms are denoted by Ob Mg . For the gas-phase cluster, Fig. 6a, the Cr Ot double bonds produce peaks at highest frequencies centered at 1012 and 1021 cm−1 , while W Ot vibrational modes are located at higher frequencies centered at 941, 992 cm−1 . Another intense Cr Ob W stretching is found around 866 cm−1 . The peak at 846 cm−1 can be assigned to W Ob W and Cr Ob W stretchings. For the CrW2 O9 clusters deposited on MgO(001) surface, the HREELS (Fig. 6b) becomes considerably more complex compared

to gas-phase CrW2 O9 , due to the distortion of the CrW2 O9 clusters deposited on MgO(001) surface. Peak related to the Cr Ot and W Ot bonds not involved in a bond with the surface is almost unchanged (1031 cm−1 and 938, 943 cm−1 , respectively). For the W Ot Mg bonds, the peaks position (878 cm−1 ) is sligthly redshifted, because the W Ot Mg bonds are somewhat longer (about 0.03 Å, see Fig. 2) than that in the gas phase. The Cr Ob vibrational modes are found at lower energies, typically 833, 770 cm−1 , which are redshifted by about 15–100 cm−1 compared with the gas-phase cluster (846, 866 cm−1 ), indicating the weaking of the Cr Ob bond interaction. As discussed above (Section 3.1.2), on the perfect MgO(001) surface one Ob atom surrounding the W atom is bonded to one surface Mg atom, result in corresponding peak related to W Ob Mg modes, redshifted and appeared at 614 cm−1 . We now consider the vibrational spectrum of the most stable configuration of CrW2 O9 on the MgO/Ag(001) films, as displayed in Fig. 6c. First of all, the peaks located at 1023 cm−1 and 968 cm−1 can be assigned to the modes of Cr Ot and W Ot bonds, respectively. However, the contributions related to the W Ot Mg bonds are found at the lowest frequencies (905, 855 cm−1 ), and appear relatively substantial redshift (>80 cm−1 ) with respect to the gas phase CrW2 O9 due to the elongation of W Ot bonds which increase about 0.05–0.06 Å, see Fig. 2. And the intensive peak at 870 cm−1 is attributed to the Cr Ot Mg and W Ot Mg modes, showing a red-shift due to the elongation of some Cr Ot Mg and W Ot Mg modes. The mode of Cr Ob Mg bond gives rise to another peak at 671 cm−1 .

4. Conclusions In the present work, the first-principles DFT calculations have been carried out to study the atomic structures and electronic properties of bimetallic oxide CrW2 O9 clusters supported on MgO/Ag(001) ultrathin films with the thickness smaller than 1 nm. The corresponding characteristics on the bulk MgO(001) surface are also investigated for comparison. Our results show that the CrW2 O9 clusters adsorbed on the two different substrates exhibit completely different electronic properties and chemical reactivity. On the bulk MgO(001) surface, adsorbed CrW2 O9 clusters are

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Intensity (arb.units)

Intensity(arb.units)

833(Cr-Ob)

1012,1021(Cr-Ot)

846(W-Ob,Cr-Ob)

866(Cr-Ob) 941,992 (W-Ot)

1031(Cr-Ot) 770(Cr-Ob) Mg

878(W-Ot ) 614(W-ObMg)

650

700

750

800

850

900

950

1000

1050

600

-1

Frequency(cm )

650

700

938,943(W-Ot) 750

800

850

900

-1

950

1000 1050

Frequency (cm )

(b)

(a) Mg

671(Cr- Ob ) Mg

Mg

Intensity(arb.units)

870(Cr- Ot ,W- Ot ) 1023(Cr- O ) t

Mg

905(W- Ot ) 968(W- Ot)

Mg

855(W- O ) t 650

700

750

800

850

900 -1

950

1000

1050

Frequency (cm )

(c) Fig. 6. The simulated vibrational spectra of (a) the gas-phase CrW2 O9 cluster, (b) CrW2 O9 /MgO(001), and (c) CrW2 O9 /MgO/Ag(001). A schematic assignment to specific vibrational modes is given in the spectra. The terminal oxygens and the bridging oxygens in CrW2 O9 which are bonded with surface Mg atoms are denoted as Ot Mg and Ob Mg , respectively.

bound via direct Mg O and W O covalent bonds and are distorted significantly. A little electron transfer occurs from oxide surface to clusters, which originates from the formation of adsorption dative bonds at the interface, indicating the electron tunneling is not possible. Whereas on the MgO/Ag(001) ultrathin films, we predicted completely different properties and chemical reactivity are expected as function of the thickness of the films. When the CrW2 O9 clusters are adsorbed on the MgO/Ag(001) ultrathin films, more than twice electrons found on the deposited cluster, despite the fact that there are only four new bonds formed at the interface. This is the consequence of a net transfer of one electron occuring from the MgO/Ag(001) films to the oxide cluster via electron tunneling through the thin MgO dielectric barrier. The resulting CrW2 O9 cluster keeps the cyclic structure and is similar to that of gas-phase [CrW2 O9 ]− cluster. Furthermore, our work reveals a progressive Lewis acid site where spin density preferentially localizes around the Cr atom not the W atoms for CrW2 O9 /MgO/Ag(001) system, indicating better catalytic activities than pure W3 O9 which

are closer to those found in the gas-phase dianion, [(W3 O9 ]2− . And we will make further study to verify in our later work. In conclusion, both results reveal that the bimetallic oxide CrW2 O9 clusters supported on the MgO/Ag(001) ultrathin films exhibit completely different structure and charge states with respect thicker films or single crystal MgO surfaces. Therefore, studying the reactivity of bimetallic oxide CrW2 O9 clusters supported on the MgO/Ag(001) ultrathin films as a function of film thickness provides a new perspective to tune and modify the properties and chemical reactivity of heterogeneous catalyst on the basis of their structures and charge states. Finally, it must be mentioned that, the presence of lowcoordinated sites (such as steps, edges, corners and kinks) and point defects (such as F◦ , F+ , F2+ ) in MgO may also affect the electronic properties of the MgO/Ag [66] and therefore may further affect the electron transfer to adsorbed CrW2 O9 cluster. Further works are necessary to study the changes of the structure and the electron transfer when CrW2 O9 cluster is adsorbed on the MgO

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ultra-thin film with the presence of low-coordinated sites and point defects. Acknowledgements This work was supported by National Natural Science Foundation of China (grant nos. 21403094, 21373048, 21371034, and 21301030), Natural Science Foundation of Jiangxi Province (20142BAB216031), Natural Science Foundation of Jiangxi Provincial Educational Committee (GJJ14261), Scientific Research Foundation of Graduate School of Jiangxi Normal University (YJS2015018), and the Independent Research Project of State Key Laboratory of Photocatalysis on Energy and Environment (2014A02). We are grateful for the generous allocation of computer time on the National Supercomputer Center in Guangzhou houses Tianhe-2, Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase). References [1] R. Rousseau, D.A. Dixon, B.D. Kaya, Z. Dohnáleka, Dehydration, dehydrogenation, and condensation of alcohols on supported oxide catalysts based on cyclic (WO3 )3 , (MoO3 )3 clusters, Chem. Soc. Rev. 43 (2014) 7664–7680. [2] R.D. Wilson, D.G. Barton, C.D. Baertsch, E. Iglesia, Reaction and deactivation pathways in xylene isomerization on zirconia modified by tungsten oxide, J. Catal. 194 (2000) 175–187. [3] W. Grunert, R. Feldhaus, K. Anders, E.S. Sphiro, K.M. Minachev, Reduction behavior and metathesis activity of WO3 /Al2 O3 catalysts: III. The activation of WO3 /Al2 O3 catalysts, J. Catal. 120 (1989) 444–456. [4] C. Martin, G. Solana, P. Malet, V. Rives, Nb2 O5 -supported WO3 : a comparative study with WO3 /Al2 O3 , Catal. Today 78 (2003) 365–376. [5] L. Lietti, J.L. Alemany, P. Forzatti, G. Busca, G. Ramis, E. Giamello, F. Bregani, Reactivity of V2 O5 -WO3 /TiO2 catalysts in the selective catalytic reduction of nitric oxide by ammonia, Catal. Today 29 (1996) 143–148. [6] D.W. Rothgeb, E. Hossain, J.E. Mann, C.C. Jarrold, Disparate product distributions observed in Mo(3-x) Wx Oy − y = 3 − 9) reactions with D2 O and CO2 , J. Chem. Phys. 132 (2010) 064302. [7] D.W. Rothgeb, E. Hossain, A.T. Kuo, J.L. Troyer, C.C. Jarrolda, Structures of Mox W(3-x) O6 (x = 0−3) anion and neutral clusters determined by anion photoelectron spectroscopy and density functional theory calculations, J. Chem. Phys. 131 (2009) 044310. [8] W.-J. Chen, C.-F. Zhang, X.-H. Zhang, Y.-F. Zhang, X. Huang, Computational study on the molecular structures and photoelectron spectra of bimetallic oxide clusters MW2 O9 (−/0) (M = V, Nb Ta), Spectrochim. Acta A 109 (2013) 125–132. [9] B.Y. Jibril, Catalytic performances and correlations with metal oxide band gaps of metal-tungsten mixed oxide catalysts in propane oxydehydrogenation, React. Kinet. Catal. Lett. 86 (2005) 171–177. [10] B.Y. Jibril, S.M. Al-Zaharani, A.E. Abaseed, R. Hughes, Effects of reducibility on propane oxidative dehydrogenation over ␥-Al2 O3 —supported chromium oxide-based catalysts, Catal. Lett. 87 (2003) 121–132. [11] M.O. Guerrero-Pérez, M.C. Herrera, I. Malpartida, M.A. Larrubia, L.J. Alemany, M.A. Banares, Operando Raman study of propane oxidation over alumina-supported V-Mo-W-O catalysts, Catal. Today 126 (2007) 177–183. [12] S. Yang, E. Iglesia, A.T. Bell, Nature density, and catalytic role of exposed species on dispersed VOx /CrOx /Al2 O3 catalysts, J. Phys. Chem. B 110 (2006) 2732–2739. [13] N.O. Elbashir, S.M. Al-Zahrani, A.E. Abasaeed, M. Abdulwahed, Alumina-supported chromium-based mixed-oxide catalysts in oxidative dehydrogenation of isobutane to isobutene, Chem. Eng. Prog. 42 (2003) 817–823. [14] X. Li, E. Iglesia, Support and promoter effects in the selective oxidation of ethane to acetic acid catalyzed by Mo-V-Nb oxides, App. Catal. A 334 (2008) 339–347. [15] G. Mestl, MoVW mixed metal oxides catalysts for acrylic acid production: from industrial catalysts to model studies, Top. Catal. 38 (2006) 69–82. [16] C.D. Baertsch, K.T. Komala, Y.-H. Chua, E. Iglesia, Genesis of brønsted acid sites during dehydration of 2-butanol on tungsten oxide catalysts, J. Catal. 205 (2002) 44–57. [17] Y.K. Kim, R. Rousseau, B.D. Kay, J.M. White, Z. Dohnálek, Catalytic dehydration of 2-propanol on (WO3 )3 clusters on TiO2 (110), J. Am. Chem. Soc. 130 (2008) 5059–5061. [18] H.-Y.T. Chen, G. Pacchioni, Properties of two-dimensional insulators: a DFT study of Co adsorption on NaCl and MgO ultrathin films, Phys. Chem. Chem. Phys. 16 (2014) 21838–21845. [19] F. Ringleb, Y. Fijimori, M.A. Brown, W.E. Kaden, F. Calaza, H. Kuhlenbeck, M. Sterrer, H.-J. Freund, The role of exposed silver in CO oxidation over MgO (001)/Ag (001) thin films, Catal. Today 240 (2015) 206–213.

221

[20] H.-J. Freund, D.W. Goodman, Ultrathin oxide films, in: Handbook of Heterogeneous Catalysis, Wiley-VCH Verlag GmbH & Co., KGaA, 2008, pp. 1309–1338. [21] A.K. Santra, D.W. Goodman, Oxide-supported metal clusters: models for heterogeneous catalysts, J. Phys.: Condens. Matter 15 (2003) R31–R62. [22] H.-J. Freund, G. Pacchioni, Oxide ultra-thin films on metals: new materials for the design of supported metal catalysts, Chem. Soc. Rev. 37 (2008) 2224–2242. [23] S.M. McClure, D.W. Goodman, Simulating the complexities of heterogeneous catalysis with model systems: case studies of SiO2 supported Pt-group metals, Top. Catal. 54 (2011) 349–362. [24] G. Pacchioni, Two-dimensional oxides: multifunctional materials for advanced technologies, Chem. Eur. J. 18 (2012) 10144–10158. [25] J. Pal, M. Smerieri, E. Celasco, L. Savio, L. Vattuone, R. Ferrando, S. Tosoni, L. Giordano, G. Pacchioni, M. Rocca, How growing conditions and interfacial oxygen affect the final morphology of MgO/Ag(100) films, J. Phys. Chem. C 118 (2014) 26091–26102. [26] S. Sicolo, L. Giordano, G. Pacchioni, Adsorption of late transition metal atoms on MgO/Mo(100) and MgO/Ag(100) ultrathin films: a comparative DFT study, J. Phys. Chem. C 113 (2009) 16694–16701. [27] G. Pacchioni, Electronic interactions and charge transfers of metal atoms and clusters on oxide surfaces, Phys. Chem. Chem. Phys. 15 (2013) 1737–1757. [28] L. Giordano, G. Pacchioni, Oxide films at the nanoscale: new structures, new functions, and new materials, Acc. Chem. Res. 44 (2011) 1244–1252. [29] G. Pacchioni, L. Giordano, M. Baistrocchi, Charging of metal atoms on ultrathin MgO/Mo(100) films, Phys. Rev. Lett. 94 (2005) 226104. [30] S. Sicolo, L. Giordano, G. Pacchioni, CO adsorption on one- two-, and three-dimensional Au clusters supported on MgO/Ag(001) ultrathin films, J. Phys. Chem. C 113 (2009) 10256–10263. [31] J. Zhu, L. Giordano, S.J. Lin, Z.X. Fang, Y. Li, X. Huang, Y.F. Zhang, G. Pacchioni, Tuning the charge state of (WO3 ) 3 nanoclusters deposited on MgO/Ag (001) films, J. Phys. Chem. C 116 (2012) 17668–17675. [32] H. Grönbeck, Mechanism for NO2 charging on metal supported MgO, J. Phys. Chem. B 110 (2006) 11977–11981. [33] M. Sterrer, T. Risse, U.M. Pozzoni, L. Giordano, M. Heyde, H.-P. Rust, G. Pacchioni, H.-J. Freund, Control of the charge state of metal atoms on thin MgO films, Phys. Rev. Lett. 98 (2007) 096107. [34] M. Yulikov, M. Sterrer, M. Heyde, H.-P. Rust, T. Risse, H.-J. Freund, G. Pacchioni, A. Scagnelli, Binding of single gold atoms on thin MgO(001) films, Phys. Rev. Lett. 96 (2006) 146804. [35] V. Simic-Milosevic, M. Heyde, N. Nilius, T. König, H.-P. Rust, M. Sterrer, T. Risse, H.-J. Freund, L. Giordano, G. Pacchioni, Au dimers on thin MgO(001) films: flat and charged or upright and neutral? J. Am. Chem. Soc. 130 (2008) 7814–7815. [36] X. Lin, N. Nilius, H.-J. Freund, M. Walter, P. Frondelius, K. Honkala, H. Häkkinen, Quantum well states in two-dimensional gold clusters on MgO thin films, Phys. Rev. Lett. 102 (2009) 206801. [37] A. Gonchar, T. Risse, H.-J. Freund, L. Giordano, C. Di Valentin, G. Pacchioni, Activation of oxygen on MgO: O2 − radical ion formation on thin metal supported MgO(001) films, Angew. Chem. Int. Ed. 50 (2011) 2635–2638. [38] P. Frondelius, A. Hellman, K. Honkala, H. Häkkinen, H. Grönbeck, Charging of atoms, clusters, and molecules on metal-supported oxides: a general and long-ranged phenomenon, Phys. Rev. B 78 (2008) 085426. [39] A.D. Becke, A new mixing of hartree-fock and local density-functional theories, J. Chem. Phys. 98 (1993) 1372–1377. [40] C. Lee, W. Yang, R.G. Parr, Development of the colle-salvetti correlation-energy formula into a functional of the electron density, Phys. Rev. B 37 (1988) 785–789. [41] P.J. Stephens, F.J. Devlin, C.F. Chabalowski, M.J. Frisch, Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields, J. Phys. Chem. 98 (1994) 11623–11627. [42] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian 03, Revision D. 01, Gaussian, Inc., Pittsburgh, PA, 2005. [43] J.T.H. Dunning, Gaussian basis sets for use in correlated molecular calculations, I. The atoms boron through neon and hydrogen, J. Chem. Phys. 90 (1989) 1007–1023. [44] R.A. Kendall, T.H. Dunning Jr, R.J. Harrison, Electron affinities of the first-row atoms revisited. Systematic basis sets and wave functions, J. Chem. Phys. 96 (1992) 6796–6806. [45] J.M.L. Martin, A. Sundermann, Correlation consistent valence basis sets for use with the Stuttgart- dresden-bonn relativistic effective core potentials: the atoms Ga-Kr and In-Xe, J. Chem. Phys. 114 (2001) 3408–3420. [46] G. Kresse, J. Hafner, Ab initio molecular dynamics for liquid metals, Phys. Rev. B 47 (1993) 558–561. [47] G. Kresse, J. Hafner, Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium, Phys. Rev. B 49 (1994) 14251–14269.

222

J. Zhu et al. / Applied Surface Science 379 (2016) 213–222

[48] G. Kresse, J. Furthmüller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Phys. Rev. B 54 (1996) 11169–11189. [49] P.E. Blöchl, Projector augmented-wave method, Phys. Rev. B 50 (1994) 17953–17979. [50] J. Hafner, Ab-initio simulations of materials using VASP: density-functional theory and beyond, J. Comput. Chem. 29 (2008) 2044–2078. [51] J.P. Perdew, Y. Wang, Accurate and simple analytic representation of the electron-gas correlation energy, Phys. Rev. B 45 (1992) 13244–13249. [52] J. Zhu, S.J. Lin, X.W. Wen, Z.X. Fang, Y. Li, Y.F. Zhang, X. Huang, L.X. Ning, K.N. Ding, W.K. Chen, Deposition of (WO3 )3 nanoclusters on the MgO(001) surface: a possible way to identify the charge states of the defect centers, J. Chem. Phys. 138 (2013) 034711. [53] Y.-Q. Luo, M. Qiu, W. Yang, J. Zhu, Y. Li, X. Huang, Y.-F. Zhang, Configuration and electronic structure of W3 O9 clusters supported on Li- and Al-doped MgO(001) surface, Acta Phys. -Chim. Sin. 30 (2014) 2224–2232. [54] C. Di Valentin, M. Rosa, G. Pacchioni, Radical versus nucleophilic mechanism of formaldehyde polymerization catalyzed by (WO3 )3 clusters on reduced or stoichiometric TiO2 (110), J. Am. Chem. Soc. 134 (2012) 14086–14098. [55] S. Nosé, A unified formulation of the constant temperature molecular dynamics methods, J. Chem. Phys. 81 (1984) 511–519. [56] J. Zhu, H. Jin, L.L. Zang, Y. Li, Y.F. Zhang, K.N. Ding, X. Huang, L.X. Ning, W.K. Chen, Deposition of nonstoichiometric tritungsten oxides on the TiO2 (110) surface: a possible way to stabilize the unstable clusters in the gas phase, J. Phys. Chem. C 115 (2011) 15335–15344. [57] J. Zhu, H. Jin, W.J. Chen, Y. Li, Y.F. Zhang, L.X. Ning, X. Huang, K.N. Ding, W.K. Chen, Structural and electronic properties of a W3 O9 cluster supported on the TiO2 (110) surface, J. Phys. Chem. C 113 (2009) 17509–17517.

[58] X. Huang, H.J. Zhai, B. Kiran, L.S. Wang, Observation of d-orbital aromaticity, Angew. Chem. Int. Ed. 44 (2005) 7251–7254. [59] S. Prada, U. Martinez, G. Pacchioni, Work function changes induced by deposition of ultrathin dielectric films on metals: a theoretical analysis, Phys. Rev. B 78 (2008) 235423. [60] S.L. Ling, M.B. Watkins, A.L. Shluger, Effects of atomic scale roughness at metal/insulator interfaces on metal work function, Phys. Chem. Chem. Phys. 15 (2013) 19615–19624. [61] L. Giordano, J. Goniakowski, G. Pacchioni, Properties of MgO(100) ultrathin layers on Pd(100): Influence of the metal support, Phys. Rev. B 67 (2003) 045410. [62] M. Chelvayohan, C.H.B. Mee, Work function measurements on (110), (100) and (111) surfaces of silver, J. Phys. C 15 (1982) 2305–2312. [63] N. Nilius, Properties of oxide thin films and their adsorption behavior studied by scanning tunneling microscopy and conductance spectroscopy, Surf. Sci. Rep. 64 (2009) 595–659. [64] M. Wagner, S. Surnev, M.G. Ramsey, G. Barcaro, L. Sementa, F.R. Negreiros, A. Fortunelli, Z. Dohnálek, F.P. Netzer, Structure and bonding of tungsten oxide clusters on nanostructured Cu-O surfaces, J. Phys. Chem. C 115 (2011) 23480–23487. [65] Z.J. Li, Z.R. Zhang, Y.K. Kim, R.S. Smith, F. Netzer, B.D. Kay, R. Rousseau, Z. Dohnálek, Growth of ordered ultrathin tungsten oxide films on Pt(111), J. Phys. Chem. C 115 (2011) 5773–5783. [66] S.L. Ling, M.B. Watkins, A.L. Shluger, Effects of oxide roughness at metal oxide interface: MgO on Ag(001), J. Phys. Chem. C 117 (2013) 5075–5083.