J. Phys. Chem. C 2008, 112, 191-200
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Gold Nanostructures on TiOx/Mo(112) Thin Films Yongfan Zhang,† Livia Giordano, and Gianfranco Pacchioni* Dipartimento di Scienza dei Materiali, UniVersita` di Milano-Bicocca, Via Cozzi, 53-20125 Milano, Italy ReceiVed: June 21, 2007; In Final Form: October 10, 2007
The structure and composition of Au nanostructures deposited on TiOx/Mo(112) films has been investigated based on first principles DFT calculations. The aim of the calculations is to identify plausible candidates for the structures generated under experimental conditions in the growth of Au/TiOx/Mo(112) films. To this end, both static and molecular dynamics (MD) calculations have been performed. Various Au coverages have been considered, low (below monolayer), medium (monolayer), and high (bilayer) coverages. An ab initio thermodynamic analysis shows that, at the experimental temperature and oxygen partial pressure, the Au monolayer forms a (1 × 1) Au/TiO3/Mo(112) structure. On the basis of the comparison of the computed and experimental valence bands, phonon spectra, and STM images, we propose a tentative assignment to a structure where Au chains are formed at the interface between the Mo support and the TiO3 film. The additional deposition of Au results in the formation of a bilayer (1 × 3) Au/TiO3/Mo(112) structure. Here, we observe the tendency for gold to aggregate and form nanowires with half of the Au atoms in contact with the TiO3/ Mo(112) support and the rest forming a second layer. The top and bottom Au layers of the nanowire exhibit a different chemical nature.
1. Introduction The unusual catalytic activity of nanodispersed gold on oxide surfaces has stimulated a constantly increasing number of model studies based on surface science approaches.1-3 Despite the large set of data and information available, the real nature of the active site of the gold-based catalysts remains under debate.4 If one restricts the discussion to the special case of CO oxidation, two main factors have been identified as being responsible for the catalytic activity,3 the structure and topology of the Au nanostructures, including size and shape, and the chemical interaction with the oxide substrate. Since the size and the shape of Au nanoparticles are generally not well-defined, the study of their effect is a difficult task. Also, the identification of the role of the oxide substrate presents several problems.5-9 As a result of the interaction between gold and the oxide surface, charge transfers can occur in both directions, and depending on the preparation of the sample, oxidized gold (Au+δ), metallic gold (Au0), or negatively charged gold (Au-δ) can be obtained.10 In order to better understand the mechanism of activation of gold catalysts, one has to prepare well-defined supports and to investigate their chemical activity under controlled conditions. Recently, Chen and Goodman have prepared well-ordered gold monolayers and bilayers on titania ultrathin films supported on the Mo(112) substrate.11-13 The results for the catalytic oxidation of CO show that the gold bilayer structure is significantly more active than the monolayer one. Theoretical calculations have been reported with the aim to explain the unique behavior of dispersed gold on TiOx/Mo(112).14-16 However, both stoichiometry and structure of the TiOx supporting film have not been completely elucidated. Thus, it is no surprise that also the atomic structure of supported Au * To whom correspondence should be addressed. E-mail:
[email protected]. † Permanent address: Department of Chemistry, Fuzhou University, Fuzhou, Fujian 350002, China.
nanostructures on TiOx thin films is largely undetermined. Very recently, we carried out a detailed study of the geometry and composition of TiOx/Mo(112) films17 based on ab initio thermodynamics. The results indicate that at the experimental conditions used to grow the titania films, a TiO3 phase with no analogue in the bulk phases of titania is preferred. Starting from this result, in this paper, we consider the structural and electronic properties of Au atoms and Au nanowires deposited at various coverages on TiOx/Mo(112) thin films. The paper is organized as follows. In section 2, we briefly outline the details of the calculations. In section 3.1, we describe the adsorption properties of low coverages of Au on the (1 × 2) TiO3/Mo(112) surface. Section 3.2 is dedicated to the Au monolayer. We discuss, on the basis of the results of ab initio thermodynamics, molecular dynamics, and total energy calculations, the composition and configuration of the (1 × 1) Au/ TiOx/Mo(112) film (1 e x e 4); in section 3.3, we analyze the electronic structure of the most stable (1 × 1) Au/TiO3/Mo(112) monolayer phase. The comparison of UPS, STM, and EELS data with the computed properties allows a tentative assignment of the structure. In section 3.4, we consider the bilayer (1 × 3) Au/TiO3/Mo(112) structure; we show that there are more stable phases compared to those originally proposed by Chen and Goodman,11-13 and we discuss the electronic properties of these films. Some conclusions are summarized in the last section. 2. Computational Details First-principles density functional theory (DFT) calculations were carried out using the Vienna ab initio simulation package18,19 and the projected augmented wave (PAW) method.20 The Perdew-Wang (PW91) exchange-correlation functional was employed.21 The kinetic cutoff energy for the plane-wave expansion was set to 400 eV, and a Monkhorst-Pack k-point mesh with a size of (21 × 13 × 1) was adopted for the (1 × 1) unit cell, while (11 × 7 × 1), (11 × 13 × 1), and (21 × 3 × 1)
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Figure 1. Structures of low-coverage Au adsorption on the reconstructed (1 × 2) TiO3/Mo(112) film. Some selected distances (in Å) are shown. The Mo, Ti, O, and Au atoms are indicated by blue, black, red, and yellow spheres, respectively. The supercells employed in the calculations are denoted by dark lines.
k-meshes were used for (2 × 2), (2 × 1), and (1 × 3) supercells, respectively. The Mo(112) substrate was modeled by a sevenlayer slab, in which the bottom three Mo layers were fixed at their bulk positions; the spacing between the adjacent slabs was about 17 Å. The structures of the TiOx/Mo(112) films with different compositions were taken from our previous work.17 Furthermore, because there are many possible arrangements of the configurations after the deposition of Au, molecular dynamics (MD) calculations using the Nose´ algorithm22 were also performed to determine the most stable structure of the Au/ TiOx/Mo(112) system. In the MD simulation, a low cutoff energy (300 eV) and a small k-mesh were used, and the simulation lengths ranged from 1 to 3 ps, with time steps of 1
fs at the simulation temperature of 900 K. Details about the MD simulations can be found in ref 17. The absence of spin polarization has been checked by performing spinpolarized calculations and looking for the best solution. In all cases considered, non-spin-polarized solutions are preferred, except for the cases where the TiOx film is not fully oxidized (x < 3). 3. Results and Discussion 3.1. Low-Coverage Au Adsorption on (1 × 2) TiOx/Mo(112). In Chen and Goodman’s experiments,11 the TiOx film on the Mo(112) substrate, before the deposition of gold, exhibits
Gold Nanostructures on TiOx/Mo(112) Thin Films an (8 × 2) structure. When Au is deposited on this surface at room temperature and submonolayer coverage, the surface is disordered, but after annealing at 900 K, the formation of a (1 × 1) Au/TiOx/Mo(112) structure is observed. Very little information exists on the initial interaction of isolated Au atoms with the TiOx/Mo(112) support. The stability of various configurations of TiOx overlayers has been studied in our previous work,17 and the results indicate that, at the experimental conditions, the film with a Ti/O ratio of 1:3 is the most stable one, and the reconstructed (1 × 2) configuration, Figure 1a, is more favorable than the (1 × 1) structure. Notice that the TiO3 formula does not reflect the complex structure and oxidation state of the Ti ions in the film where Ti is connected to the Mo metal by Ti-O-Mo bonds and forms other Ti-O-Ti linkages; furthermore, there are oxygen atoms bound at the Mo surface, Figure 1a. In this respect, the film can be seen as a TiO2 layer on an O-covered Mo surface or as a TiMoO3 compound, with formal Ti oxidation states between +IV and +III; experimentally, there is evidence from EELS and XPS that Ti is in a +III oxidation state.11 The analysis of the phonon spectra and of the core level shifts has shown some analogies with those reported experimentally for the TiOx/Mo(112) film but also some differences which remain to be explained.17 In this respect, the (1 × 2) TiO3/Mo(112) structure used here to study the Au deposition at low coverage (1.5 eV) than structure 7a, clearly indicating the preferential formation of Au-Au bonds with respect to AuTiOx bonds. The analysis of the Bader charges shows that, except for the structure 7b where some Au atoms bound to oxygen atoms are partly oxidized, in the other structures, the gold atoms can be classified as metallic (Au0) or negatively charged (Au-δ). These latters are those in direct contact with the Ti ions of the film. The different nature of the Au atoms at the interface and in the second layer of the nanowire is also clear from the DOS curves; see Figure 4e. The 5d states of the interface Au atoms are slightly shifted to higher binding energies. Since they carry a partial negative charge, the position of the 5d band should be at lower binding energies compared to that of the second layer Au0 atoms. Clearly, other effects contribute to the position of the 5d band, like, for instance, the different coordination, the hybridization with the TiOx states, and so forth.29 The fact that two different kinds of Au atoms are present in the supported film can have important consequences for chemistry. Since both of these kinds of gold atoms are accessible to CO and O2 molecules during the catalytic oxidation of CO, one can speculate that this can be the reason why the (1 × 3) surface is significantly more active (by about a factor of ∼45) than the (1 × 1) Au/TiOx/Mo(112) surface.11 In fact, it has been proposed by several authors6,30,31 that the formation of a layer of negatively charged gold is crucial for the activation of the O2 molecule. On the other hand, CO has been shown to bind more strongly on the Au bilayer structure, differently from the (1 × 1) Au/TiO3/Mo(113) where the interaction is rather weak. 4. Conclusions Au nanostructures deposited on TiOx/Mo(112) thin films have been investigated with first-principles DFT calculations for various Au coverages and TiOx compositions. The problem is complicated by the fact that the structure of the TiOx support is still under debate and is certainly not similar to any of the known phases of bulk titanium oxide. In a previous study,17 we found that a film with a Ti/O stoichiometry of 1:3 is most likely formed under the experimental conditions. Thus, the attention here is concentrated on this phase. However, we have also considered other Ti/O compositions in the case of a supported Au monolayer. We first analyzed the initial stages of the Au deposition on the (1 × 2) TiO3/Mo(112) film (submonolayer coverage); this substrate represents a model of the experimentally observed (8 × 2) TiOx/Mo(112) unit cell. Au atoms prefer to bind on top of the Ti ions, forming a Au-Ti bond of about 1 eV. As a consequence, the Ti‚‚‚O-Mo bond is broken and the Ti atoms are displaced from the interface toward the surface of the oxide film. This process is favored by the formation of strong Mod O bonds at the interface. This, and similar results for other adsorption sites and structures, clearly indicates that by adsorb-
J. Phys. Chem. C, Vol. 112, No. 1, 2008 199 ing Au, one favors a reconstruction of the oxide film, a phenomenon observed experimentally. In fact, while the reconstruction of the “clean” (8 × 2) TiOx/Mo(112) film does not occur even at temperatures of 1400 K, upon Au adsorption, the reconstruction is observed at 900 K.11 In the next step, we considered the thermodynamic stability of various structures and compositions of a Au monolayer forming the (1 × 1) pattern observed experimentally. To this end, several (1 × 1) Au/TiOx/Mo(112) structures with x from 1 to 4 have been considered. On the basis of static calculations and the thermodynamic analysis, we came to the conclusion that the most stable phase consists of a TiO3/Mo(112) film where one-dimensional Au chains are formed. Three structures with similar stability have been identified, where the Au chain is deposited above the oxide film, at the interface between the oxide and the Mo metal, or in between. A molecular dynamics ab initio simulation shows that the structure where Au is at the interface is definitely more stable. To corroborate this finding, we considered a series of observable properties, so as to compare the computational findings with available experimental data. In particular, we considered the density of states (compared with UPS spectra), the vibrational properties (compared with phonon spectra), and the simulated STM images (compared with the experimental ones). On the basis of this comparison, we tentatively assigned the structure of the (1 × 1) Au monolayer on the TiO3/Mo(112) films to the structure where the Au chains are in between the oxide film and the metal support (Figure 2e). In the last section, we considered the formation of a Au bilayer. Experimentally, this results in a (1 × 3) superstructure, and this is the cell adopted in the calculations. In this case, the configuration space becomes very complex, and the number of possible arrangements of the atoms of the TiO3 film and of the Au deposit is very large. In this respect, what we have considered here are some of the possibilities, but certainly, we cannot exclude that other more stable configurations exist. The best configuration found consists of a Au nanowire, two-layers thick and two-atoms wide, which partially covers the TiO3 support. This structure is definitely more stable, by more than 1 eV, than others proposed in the literature consisting of two separate Au nanostructures, a one-dimensional chain, and a nanowire with a section of three Au atoms. In the preferred structure, two kinds of Au atoms can be distinguished, those at the interface with the oxide support, which carry a partial negative charge, and those in the second layer, with a zero oxidation number. It is likely that the presence of these chemically different Au atoms is the reason for the strongly enhanced chemical reactivity of the Au bilayer compared to that of the monolayer observed experimentally. Acknowledgment. This work has been supported by the European Project GSOMEN. Y.Z. also acknowledges funds from Fujian Province (Z0513005) and the Ministry of Education of China (SRFDP-20060386001). References and Notes (1) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896. (2) Arenz, M.; Landman, U.; Heiz U. ChemPhysChem 2006, 7, 1871. (3) Chen, M. S.; Goodman, D. W. Acc. Chem. Res. 2006, 39, 739. (4) Matthey, D.; Wang, J. G.; Wendt, S.; Matthiesen, J.; Schaub, R.; Lægsgaard, E.; Hammer, B.; Besenbacher, F. Science 2007, 315, 1692. (5) Laursen, S.; Linic, S. Phys. ReV. Lett. 2006, 97, 026101. (6) Molina, L. M.; Hammer, B. Phys. ReV. Lett. 2003, 90, 206102. (7) Rodriguez, J. A.; Perez, M.; Jirsak, T.; Evans, J.; Hrbek, J.; Gonzalez, L. Chem. Phys. Lett. 2003, 378, 525.
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