Enhanced Bonding of Silver Nanoparticles on ... - ACS Publications

May 21, 2010 - Jonas Ø. Hansen,‡ Estephania Lira,‡ Patrick Galliker,‡ Jian-Guo Wang,§ Phillip T. Sprunger,|. Zheshen Li,‡ Erik Lægsgaard,‡ Stefan Wend...
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J. Phys. Chem. C 2010, 114, 16964–16972

Enhanced Bonding of Silver Nanoparticles on Oxidized TiO2(110)† Jonas Ø. Hansen,‡ Estephania Lira,‡ Patrick Galliker,‡ Jian-Guo Wang,§ Phillip T. Sprunger,| Zheshen Li,‡ Erik Lægsgaard,‡ Stefan Wendt,*,‡ Bjørk Hammer,‡ and Flemming Besenbacher*,‡ Interdisciplinary Nanoscience Center (iNANO), Department of Physics and Astronomy, and Institute for Storage Ring Facilities (ISA), Aarhus UniVersity, DK-8000 Aarhus C, Denmark, College of Chemical Engineering and Materials Science, Zhejiang UniVersity of Technology, Hangzhou, 310032, People’s Republic of China, and Department of Physics and Astronomy, Louisiana State UniVersity, Baton Rouge, Louisiana 70808 ReceiVed: February 25, 2010; ReVised Manuscript ReceiVed: May 4, 2010

The nucleation and growth of silver nanoclusters on TiO2(110) surfaces with on-top O adatoms (oxidized TiO2), surface O vacancies and H adatoms (reduced TiO2) have been studied. From the interplay of scanning tunneling microscopy/photoelectron spectroscopy experiments and density functional theory calculations, it is found that silver clusters are much more strongly bonded to oxidized TiO2(110) surfaces than to reduced TiO2(110) model supports. It is shown that electronic charge can be transferred from silver clusters to the oxidized TiO2(110) surface, as evidenced by the reappearance of the Ti3d defect state upon silver exposure. Furthermore, from both scanning tunneling microscopy data and density functional theory calculations the most favorable adsorption site of silver monomers on oxidized TiO2(110) is one that bridges between on-top O adatoms and regular surface O atoms nearby. 1. Introduction Supported transition metal nanoclusters have a wide range of technological applications within diverse areas including heterogeneous catalysis, photocatalysis, solar cells, and gas sensors.1-5 Intense research has been carried out to unravel the structure-reactivity relationship(s) of metal nanoclusters, but many questions are still unanswered, particularly in relation to supported gold (Au) nanoclusters.6-16 Whereas supported Au nanoclusters appear to be very promising and viable in future applications in heterogeneous catalysis,6-16 supported silver (Ag) nanoclusters are often used for applications in photocatalysis.2,3,5,17-23 Ag clusters supported on titania (TiO2) are of practical relevance since Ag/TiO2 nanocomposites are promising photocatalysts for areas such as self-cleaning, disinfecting, and water splitting as well as air and water purification. Since other noble metals such as Pt, Pd, Rh, and Au are very expensive, Ag nanoparticles may serve as a much more cost-efficient solution. In addition, Ag nanoparticles dispersed on metal oxide supports are of interest because they are good catalysts for the oxidative dehydrogenation of methanol to formaldehyde24,25 and are efficient epoxidation catalysts.26-28 For studies performed under ultrahigh vacuum (UHV) conditions, the titania surface that has attracted the most attention is rutile TiO2(110)-(1 × 1).29-33 The nucleation of Ag nanoclusters has also been investigated on this model support. In previous surface science studies, scanning tunneling microscopy (STM),34-39 photoelectron spectroscopy (PES),36 and low-energy ion scattering (LEIS)36 have been used to characterize the Ag/TiO2(110) model catalytic surface. On the basis of these studies, it has †

Part of the “D. Wayne Goodman Festschrift”. * To whom correspondence should be addressed. E-mail: (S.W.) [email protected]; (F.B.) [email protected]. ‡ Aarhus University. § Zhejiang University of Technology. | Louisiana State University.

been reported that Ag clusters grow on the reduced TiO2(110) surface as uniform, three-dimensional clusters, indicating a weak interaction between Ag and the support. However, exclusively reduced TiO2(110) surfaces were used as model supports in previous surface science studies, and the nucleation of Ag clusters on oxidized TiO2(110) surfaces has not been addressed. Recent studies on the intensely studied Au/TiO2(110) model system have revealed that the adhesion of Au nanoclusters is greatly enhanced on oxidized TiO2(110) supports with oxygen adatoms (Oot) as compared to reduced TiO2(110) surfaces with bridging oxygen (Obr) vacancies and H adatoms.12,16,40-42 Here we show from the interplay of high-resolution STM, synchrotronbased PES and density functional theory (DFT) studies that also the adhesion of Ag nanoclusters is greatly enhanced on oxidized TiO2(110) surfaces. Together with our previous report on the Au/TiO2(110) model catalyst12 and recent observations for Pt supported on oxidized TiO2(110),43 the present results for Ag/ TiO2(110) suggest that transition metal nanoclusters are generally more strongly attached on oxide supports that allow the formation of O-rich metal-support interfaces. Moreover, the obtained PES data for Ag/TiO2(110)-(1 × 1) confirm that the defect state in the band gap of reduced TiO2 is correlated with charge donors in the near-surface region.44 2. Experimental and Computational Details The STM and PES experiments were performed in two separate UHV chambers which both had base pressures in the 10-11 mbar range. The STM studies were conducted using a home-built, temperature-variable Aarhus STM.45,46 The STM images presented in this work were all acquired in the constant current mode using a tunneling voltage of ∼+1.25 V and a tunneling current of ∼0.1 nA at sample temperatures between 110-140 K. For the deduction of the Ag cluster height histograms from the STM data, total areas of ∼8000 nm2 on the Ag exposed TiO2(110) surfaces were scanned and analyzed.

10.1021/jp101714r  2010 American Chemical Society Published on Web 05/21/2010

Enhanced Bonding of Ag Nanoparticles on Oxidized TiO2(110) A threshold of 1.2 Å above the terraces was chosen when counting the Ag clusters to construct the STM height histograms. STM heights of the Ag clusters are given with respect to the lower terrace if located on a step edge, whereas the full width at half-maximum (fwhm) of the clusters were used as measure of the particle width. The PES experiments were carried out in an UHV end-station, on the SX700 plane grating monochromator beamline47 at the ASTRID synchrotron-radiation facility at Aarhus University. The PES data were acquired using a VG CLAM II spectrometer working at 30 eV pass energy. Binding energy (BE) determination was facilitated by a Fermi edge cutoff from multilayer Au and Ag films. Valence spectra were acquired with photon energy of 47.5 eV corresponding to the Ti 3p-3d resonance in order to maximize the intensity of the Ti3d defect state in the band gap.48,49 All PES valence and core-level spectra were acquired in normal emission geometry and normalized to the incident photon flux. Two TiO2(110)-(1 × 1) crystals were used for the experiments; one for the STM experiments and one for the PES studies. The crystals were identically cleaned by cycles of Ar+ ion sputtering at room temperature (RT) and vacuum annealing at 900-1050 K. Reduced TiO2(110) surfaces with Obr vacancies [r-TiO2(110)] were prepared by applying short flashes to 600 K when the samples reached RT after vacuum annealing.50 From STM measurements on both crystals, we estimated Obr vacancy densities of 0.09-0.12 ML with 1 ML (monolayer) being the density of the (1 × 1) units, 5.2 × 1014 cm-2. Ag was evaporated onto the TiO2(110) surfaces of interest using an e-beam evaporator (Oxford instruments). Ag coverage is given in ML corresponding to the TiO2(110) substrate and gas exposures are given in Langmuir (L) with 1 L ) 1 × 10-6 Torr · s. In the DFT calculations, the TiO2(110) surface was modeled using periodic slabs of four tri-layers with a c(4 × 2) surface unit cell, the theoretically derived lattice constants (a ) 4.69 Å, b ) 2.99 Å, u ) 0.305 Å) and with 2 × 1 k-points. All of the calculations were performed using the Dacapo package51,52 with a plane-wave basis set (Ecut ) 25 Ry), and ultrasoft pseudopotentials.53 The generalized gradient approximation (GGA) with the revised Perdew-Burke-Ernzerhof (RPBE) functional was used to describe the exchange-correlation (XC) effects.51 In the DFT calculations, all four trilayers including the Ag monomers were fully relaxed until the total residual force was less than 0.05 eV/Å. Metastable configurations were calculated using the nudged elastic band method.54 For most of the calculations a Ti interstitial was introduced in the super cell on an octahedral site between the second and third TiO2 trilayer. This was done because charge donation from the bulk appears to be essential in order to describe surface phenomena correctly. In a recent paper, we proposed that the Ti3d defect state in the band gap arises due to the extra charge originating from Ti interstitials in the near surface region rather than from Obr vacancies.44 We emphasize, however, that the exact nature of the defects that provide excess electrons is of secondary importance for the present DFT calculations, because the bond strength and other properties of supported metal nanoclusters depend essentially on the available excess charge irrespective of its origin.55 3. Results 3.1. STM Studies. The TiO2(110) surface consists of alternating rows of 5-fold coordinated Ti (5f-Ti) atoms and protruding, 2-fold coordinated Obr atoms. The Ti atoms underneath the Obr atoms are 6-fold coordinated as any other Ti atom in stoichiometric rutile. An Ar+ sputtered and vacuum-annealed

J. Phys. Chem. C, Vol. 114, No. 40, 2010 16965 TiO2(110) crystal is an n-type semiconductor, and its surface has a number of Obr vacancies.30,32,33 In the STM images acquired on r-TiO2(110) surfaces [cf. Figure 1a] these Obr vacancies (white rectangle) are imaged as faint protrusions on the dark Obr rows.50,56,57 Note that the empty-state STM images of the TiO2(110) surface are dominated by electronic effects, which leads to a reversed contrast, that is, bright rows correspond to the Ti troughs and geometrically protruding Obr atoms appear dark.30,32,50,58 Starting with a clean r-TiO2(110) surface, we prepared two additional well-defined TiO2(110) surfaces under ultrahigh vacuum conditions.12,44,50,59,60 First, we produced a hydrated TiO2(110) [h-TiO2(110)] surface with H adatoms, capping some of the Obr atoms by letting water dissociate in Obr vacancies [cf. Figure 1b]. The capping H adatoms (Hcap) form bridging hydroxyls (OHbr) on the surface48,59-61 that appear brighter in the STM images than the Obr vacancies.32,50,59 Second, we prepared an oxidized TiO2(110) [o-TiO2(110)] surface by letting O2 dissociate in Obr vacancies.32,44,50,62,63 The o-TiO2(110) surfaces were prepared either via O2 exposure at 120 K followed by a flash up to RT or by O2 exposure at RT. The resulting o-TiO2(110) surfaces are characterized by perfect Obr rows and by a number of Oot adatoms in the Ti troughs (white circle) that appear as bright spots in the STM images. In addition to the dissociation of O2 in the Obr vacancies, a second, nonvacancy related O2 dissociation channel also exists, resulting in the formation of paired Oot adatoms on the surface (white ellipse).44 We note that some of the Obr vacancies remained unfilled in spite of the oxidation, which can be explained by a lack of electronic charge in the near-surface region.44,64,65 The oTiO2(110) surface represented by the STM image depicted in Figure 1c is characterized by ∼0.078 ML Oot adatoms and ∼0.012 ML Obr vacancies. Ag exposure corresponding to ∼0.041 ML onto these three different TiO2(110) surfaces at RT led to Ag cluster morphologies as shown in Figure 1d-i. On the r- and the h-TiO2(110) surfaces, we observed fairly large Agn clusters (0.5-1.1 nm high and ∼1.5-3.0 nm wide) that preferentially decorate the step edges of the substrate. Only ∼30% of the Agn clusters were found on the terraces, in agreement with a previous STM study by Tong et al..34 In contrast, after exposing the o-TiO2(110) surface to Ag at RT we found minimal size Agn clusters that are homogeneously distributed on the terraces [cf. Figure 1f, i]. Most of the Agn particles, identified as round bright protrusions, are characterized by an apparent height of ∼1.8 Å. From these STM results we can conclude that the adhesion of the Agn clusters is fairly low on the r- and the h-TiO2(110) surfaces and substantial on the o-TiO2(110) surface. The results obtained on r- and the h-TiO2(110) surfaces are in accord with previous STM studies addressing Ag/TiO2(110),34-37 and the result obtained for Ag/o-TiO2(110) resembles the situation as found for small Aun clusters on o-TiO2(110).12 For Ag/o-TiO2(110), a comparison of the total density of Agn clusters (∼0.026 ML) and the evaporated amount of Ag (∼0.041 ML) suggests that the majority of the Agn clusters are Ag monomers (Ag1), which, as will be shown below, bind to the Oot adatoms. That the majority of the Agn clusters are indeed Ag monomers trapped at Oot sites is supported by the fact that after the Ag exposure the density of Oot adatoms was decreased by ∼0.025 ML, which is close to the density of Agn clusters in this experiment. We also evaporated Ag onto the o-TiO2(110) surface at 110 K, which resulted in Agn cluster morphologies very similar to those obtained in the RT case [cf. Figure 1f, i].

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Hansen et al.

Figure 1. (a-c) STM images (all 200 Å × 200 Å) showing clean r- (a), h- (b) and o-TiO2(110) surfaces (c). Insets (40 Å × 40 Å) show the point defects of interest enlarged. Symbols indicate Obr vacancies (rectangle), H adatoms (hexagon), and Oot adatoms (circle), respectively. The densities of point defects are ∼0.091, ∼0.140, and ∼0.078 ML for Obr vacancies (a), H adatoms (b), and Oot adatoms (c), respectively. (d-i) Corresponding STM images after ∼0.041 ML Ag exposure at RT. The images are 200 Å × 200 Å in (d-f) and 242 Å × 242 Å in (g-i). The size of the Agn clusters is indicated by contour lines at 1.2, 3.2, 5.2, and 7.2 Å STM heights; for (d-e) and (g,h) with respect to the upper terrace. A black-redyellow color scale was used for the z contrast, and the individually chosen z contrasts are given directly in the STM images.

To examine the stability of Ag1 and the propensity of Ag1 and other small Agn clusters to sinter together into larger Agn clusters, an Ag/o-TiO2(110) sample was heated to 340 K for 120 s. As is evident from the STM image depicted in Figure 2a and the STM height histograms shown in Figure 2b, heating of the Ag/o-TiO2(110) sample did not lead to a different Agn cluster size distribution. In Figure 2b, the Agn cluster heights obtained on o-TiO2(110) after Ag exposure at 110 K are compared to those on o-TiO2(110) after Ag exposure at RT followed by heating at 340 K for 120 s. Within experimental error, the STM height histograms of the Ag-exposed o-TiO2(110) surfaces in the temperature range studied are identical, indicating that the Agn clusters do not sinter at temperatures up to 340 K. STM images acquired on the r-TiO2(110) surface after Ag exposure at RT [cf. Figure 1d, g] reveal that small Agn clusters are not trapped at the Obr vacancies, as opposed to the case of Aun where the Obr vacancies have been identified as trapping sites for Au1 (monomers) and Au3 (trimers).12,66 To investigate whether small Agn clusters can be stabilized by Obr vacancies at low temperatures, we also studied the nucleation of Agn

clusters on r- and h-TiO2(110) surfaces cooled to 110 K [cf. Figure 3]. However, also after Ag deposition onto r- and h-TiO2(110) at 110 K, no preference for small Agn clusters centered about the Obr rows was seen. Rather, we observed small and medium-sized Agn clusters randomly distributed on these two surfaces. On the cooled r- and h-TiO2(110) surfaces, the majority of the small Agn clusters is only one layer thick (corresponding to STM heights of ∼2-2.5 Å), but we also found a few Agn clusters of two layer height (STM height ∼5.3 Å) on these surfaces [cf. Figure 3a]. The density of the two layer high Agn clusters increased considerably when Ag was evaporated with the r-TiO2(110) held at 160 K (not shown) and 200 K, respectively. Comparing the Ag height histograms corresponding to Ag/r-TiO2(110) and Ag/h-TiO2(110) for Ag exposure at the low temperatures [cf. Figure 3a,b], it appears that there is no clear difference between Ag nucleation on cooled r- and h-TiO2(110) surfaces, a result that is further supported by the low-temperature PES data presented below. The similarity found for Ag/r- and Ag/h-TiO2(110) is somewhat surprising because it indicates that Obr vacancies are not attractive

Enhanced Bonding of Ag Nanoparticles on Oxidized TiO2(110)

Figure 2. (a) STM image (200 Å × 200 Å) showing an o-TiO2(110) surface that was exposed to ∼0.041 ML Ag at RT and subsequently heated to 341 K. (b) STM Height histograms of Agn clusters obtained on the o-TiO2(110) surface after Ag exposure at 110 K (red) and on o-TiO2(110) after Ag exposure at RT followed by heating to 340 K (blue). The heights of the Agn clusters are given by contour lines at 1.2 Å above the terrace.

nucleation sites for small Agn clusters, in contrast to the general belief that missing O defects on oxide surfaces act as trapping sites.4 3.2. PES Studies. In addition to the STM studies, we also performed PES measurements to better understand the nucleation of Ag on the three TiO2(110) surfaces. Figure 4 shows the PES Ag3d core level spectra obtained after Ag exposure on r-, h- and o-TiO2(110) surfaces at 100 K. The Ag coverage was ∼0.053 ML in each of the three systems and is nearly the same as the coverage used in the STM studies discussed above. On r- and h-TiO2(110) and this Ag coverage, the Ag3d peaks are shifted to a higher binding energy (BE) by ∼0.9 eV compared to the BE of “bulk Ag”, as obtained after dosing ∼10 ML of Ag. On o-TiO2(110), a slightly larger BE shift of ∼1.0 eV was observed. The shift to higher BE obtained after Ag evaporation on all three TiO2(110) surfaces is caused, to a large extent, by the very small Agn cluster size (final state effect).36,67 Note that in the PES experiments, the Agn clusters are small on all three TiO2(110) surfaces, because the Ag adatoms and small Agn clusters can not diffuse at 100 K. In addition to the final state effect, initial state effects (e.g., charge transfer) also contribute to the peak shift, as discussed below. For Ag/oTiO2(110) one may expect a small shift to lower BE,68 but we found a shift to higher BE irrespective of the TiO2(110) support used. It is likely that the changed oxidation state of the TiO2(110) supports leads only to very small shifts, since the density of point defects on the three TiO2(110) supports is low and the BE shifts due to Ag oxide formation are generally small.68 Despite the fact that the peak positions of the Ag3d core levels for Ag/r-TiO2(110), Ag/h-TiO2(110) and Ag/o-TiO2(110) are almost identical, the Ag3d core level spectra contain valuable information, because their integrated areas are markedly different. Whereas the integrated areas are small and of about the same size for Ag/r-TiO2(110) and Ag/h-TiO2(110), the integrated area of the Ag3d core level peaks obtained for Ag/oTiO2(110) is a factor of ∼1.3 larger than on the other two surfaces [Figure 4]. Because PES at the energies employed essentially probes only the outermost layers of the surfaces considered, larger Agn clusters composed of several Ag layers will give rise to less intense core level peaks than many small Agn clusters that cover a larger portion of the TiO2(110) support. Thus, for identical Ag sticking coefficients on the three TiO2(110) surfaces compared, the larger integrated area of the Ag3d peaks found for Ag/o-TiO2(110) is in accord with our STM results that revealed the Agn clusters to be much more

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Figure 3. STM height histograms corresponding to Ag/r-TiO2(110) (a) and Ag/h-TiO2(110) (b), respectively, for Ag exposures at the given temperatures. Since the histograms are not normalized with respect to the area, the total numbers of counts are plotted instead. For an easier comparison, two of the histograms are scaled with a factor 3. One layer thick Ag clusters are prevailing for 110 K, whereas thicker (3D) clusters are observed for 200 K.

Figure 4. PES Ag3d core level spectra acquired on r-, h-, and o-TiO2(110) surfaces exposed to ∼0.053 ML Ag at 100 K. The dotted lines indicate the peak positions found for “bulk Ag” as obtained after exposure of ∼10 ML Ag.

dispersed on the o-TiO2(110) surface than on the two reduced surfaces, r- and h-TiO2(110), respectively. For higher Ag exposures at 100 K (not shown) a similar trend is observed in the PES spectra. For Ag exposures ranging from ∼0.027 to ∼5.1 ML, the integrated Ag3d areas are largest on the o-TiO2(110) surface. For Ag coverage ranging from ∼0.027 to ∼0.053 ML, the integrated Ag3d area found for Ag/rTiO2(110) and Ag/h-TiO2(110) were almost identical, but smaller than for o-TiO2(110). This indicates that on the r- and h-TiO2(110) surfaces 3D growth has already commenced even at the low temperature, which is consistent with the STM data discussed above. The stronger adhesion of Ag on the o-TiO2(110) support inferred from the higher dispersion of Ag is also evident in the PES valence band spectra shown in Figure 5a,b. Before the Ag exposure, the spectra are dominated by the TiO2-related valence band peaks between ∼3 eV and ∼9.5 eV that are predominantly O2p-derived.30,48,69 Upon Ag exposure at 100 K, the O2p peaks attenuate and concomitantly new peaks appear in the same energy range that are ascribed to Ag4d/5s bands.68,70 This transition from O2p-derived bands to Ag4d/5s bands occurs in case of Ag/o-TiO2(110) surface at lower Ag coverage compared with the situation obtained for Ag/r-TiO2(110). For example, after depositing ∼2.46 ML Ag, the peak at ∼4.8 eV that is

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Figure 5. PES valence band spectra (Ekin ) 47.5 eV) acquired on r-TiO2(110) (a) and o-TiO2(110) (b) surfaces before (black line) and after Ag exposure at 100 K (red, blue, and green lines). In both plots, a BE of 5.7 eV [most intense O2p feature on r-TiO2(110)] are marked by a dashed line. (c) Enlargement of the spectral region around the defect state of the spectra shown in (b). Note that (c) also shows the spectrum corresponding to ∼0.67 ML Ag that was omitted in (b).

attributed to metallic Ag starts to grow in the case of oTiO2(110), whereas this peak did not yet appear for Ag/rTiO2(110) for the same Ag coverage. For Ag coverage larger than ∼5.1 ML the measured valence band spectra (green lines) are very similar and resemble the spectrum obtained for bulk Ag (∼10.15 ML). In an analogous experiment, where we started with an h-TiO2(110) surface, we obtained valence band spectra that were very much like those observed for Ag/r-TiO2(110). To better elucidate the overlapping O2p peaks from the Ag4d/ 5s bands, a curve fitting procedure was applied to the valence PES spectra of each of the three different Ag/TiO2(110) composites. As a function of increasing Ag coverage, the low coverage Ag/r-TiO2(110), Ag/h-TiO2(110), and Ag/o-TiO2(110) spectra were fitted by a combination of two distinct spectra, namely an original clean, non-Ag coverage valence PES and a corresponding thick Ag-covered valence PES (∼10 ML) of the respective surface. Employing the binding energy and intensities of the clean and Ag-covered spectra as parameters to maximize the goodness of the fit, this analysis procedure allows us to qualitatively decipher important questions regarding charge transfer and accompanied induced screening. The fitting result indicates that there is markedly different behavior for the lowest coverage Ag valence band structure between the preoxidized and reduced TiO2(110) surfaces. At low Ag coverage (∼0.35 ML) we found very large positive BE shift (∼0.7 eV) of the Ag valence bands measured for Ag/o-TiO2(110) compared to the commensurate valence bands obtained for the Ag/rTiO2(110) and Ag/h-TiO2(110) surfaces. In combination with the STM and the Ag core level data, this large BE shift at initial Ag coverage on the o-TiO2(110) surface is attributed to a charge transfer from the Ag monomers to the underlying TiO2 support. The positive BE shift is a consequence of a minimized screening, and subsequent lowered kinetic energy, due to the charge transfer. From our PES valence band studies very important information can be obtained from the Ti3d-derived defect state at ∼0.85 eV BE below the Fermi level (EF). In the case of Ag/r-TiO2(110) the Ti3d defect state is superimposed on the Ag5sp states in the low BE region from -0.5 to 2 eV BE [Figure 5a]. Because an increasingly larger area of the r-TiO2(110) surface becomes covered by Ag, the weight of the defect state decreases with increasing Ag coverage. Note that a quite high Ag coverage is required to extinguish the defect state. An analogous set of experiments was carried out for the h-TiO2(110) surface (not

Hansen et al. shown), and the obtained results were very similar to those obtained for Ag/r-TiO2(110). In contrast, when the o-TiO2(110) surface was exposed to Ag we found markedly different behavior of the defect state [Figure 5b,c]. Before the Ag exposure, the Ti3d defect state was almost completely attenuated because of the high O2 exposure used to oxidize the TiO2(110) surface (400 L O2 at RT) [Figure 5c, black curve]. Upon Ag exposure, the Ti3d defect state reappeared [Figure 5c, red curve] and for a Ag coverage between ∼0.05 and ∼0.12 ML its weight increased to around a quarter of the weight obtained on the clean r-TiO2(110) surface. With further increasing Ag coverage, the weight of the defect state decreased, and finally at ∼5.1 ML Ag coverage the Ti3d defect state was replaced by Ag5sp states. The reappearance of the Ti3d defect state for small Ag coverage further supports that charge is transferred from Agn clusters to the o-TiO2(110) surface. Upon O2 exposure to the r-TiO2(110) surface, when preparing an o-TiO2(110) surface, the accumulation of electronegative adsorbates on the surface causes a withdrawal of electronic charge from the donor sites in the near-surface region. As a consequence, the Ti3d defect state is quenched,44 as has been also observed in previous studies.30,48,71 When Ag is exposed to this electron deficient o-TiO2(110) surface, small Agn clusters nucleate on the terraces, as observed by STM, and electrons are transferred from the clusters to the empty Ti3d donor states in the near-surface region, causing the defect state to reappear. Thus, in agreement to the fitting analysis of the valence band spectra described above, the Agn clusters are positively charged in the low coverage range. To ensure that the appeared peak in the band gap upon Ag exposure is indeed Ti3d-derived and not Agderived, we exposed a Ag covered o-TiO2(110) surface (∼0.45 ML Ag) to 1000 L O2 at RT. This reoxidation again led to the depopulation of the peak in the band gap, which confirms our assignment. Because the Agn nanoclusters act as charge donors, the results presented in Figure 5b,c thus indicate that more electronegative species can be adsorbed on the surfaces of Ag/ TiO2 composites than on pure TiO2 materials. 3.3. DFT Calculations. To rationalize the enhanced bonding of Ag on the o-TiO2(110) surface, we have performed firstprinciples DFT calculations. In the calculations, we considered the adsorption and diffusion of Ag1 on three different surfaces, r-, h-, and o-TiO2(110), respectively [Figure 6]. The results are presented in Figures 7 and 8 and summarized in Tables 1 and 2. The r-TiO2(110) surface was modeled as a stoichiometric slab containing one Obr vacancy on the surface [Figure 6a]. The h-TiO2(110) surface was modeled as a slab with an H adatom, Hcap, capping an Obr atom, and finally the o-TiO2(110) surface was described by including an O adatom, Oot, adsorbed on-top on a 5f-Ti site in the Ti trough. To account for the bulk reduction in the various models, we added a capping H in addition to the Obr vacancy [Figure 6a] for the r-TiO2(110) system, while for modeling the h-, and o-TiO2(110) surface a Ti interstitial was introduced between the second and third trilayer [Figure 6b,c].44 With these model systems, we found the Ag1 adatoms to adsorb very weakly on the two reduced surfaces, r- and h-TiO2(110), respectively [Figure 7a,b]. On the r- and the h-TiO2(110) surfaces the preferred binding sites are on-top of the Obr atoms, that is, not in the Obr vacancy or adjacent to the capping H atom (cf. Table 1). The calculated adsorption bond strength of Ag1 is 0.36 eV on r-TiO2(110) and 0.25 eV on h-TiO2(110), respectively. On r-TiO2(110), adsorption of the Ag1 in the Obr vacancy is weaker by 0.08 eV than adsorption on-top the Obr

Enhanced Bonding of Ag Nanoparticles on Oxidized TiO2(110)

Figure 6. Ball models of the systems used in the DFT calculations to describe r- (a), h- (b), and o-TiO2(110) (c) surfaces. Alternative descriptions of an r-TiO2(110) surface are shown in (d-f). In the ball models, old rose balls represent O atoms and gray balls Ti atoms. Capping H atoms are shown in light gray, on-top bonded O species (Oot) is shown in red, and Ti interstitials (Ti int.) are shown in yellow.

Figure 7. Adhesion and diffusion of Ag1 on TiO2(110) surfaces in different oxidation states. (a) r-TiO2(110), (b) h-TiO2(110), and (c) o-TiO2(110). Blue balls represent Ag1 in the starting configurations, ii, whereas white dotted open circles are used to mark alternative adsorption sites. Identities of the atoms of the TiO2 supports are given directly in the ball models. The white square marks the Obr vacancy on r-TiO2(110). Corresponding DFT results are summarized in Table 1 and 2.

atoms. The weak binding of Ag1 on r- and h-TiO2(110) surfaces is accompanied by small diffusion energy barriers in the order of 0.2 eV. In contrast to these results for Ag1 on r- and h-TiO2(110), the Ag1 adatom binds much more strongly on the o-TiO2(110) surface, and on this surface the Ag1 adatom also experiences high diffusion energy barriers. On the o-TiO2(110) surface, the Ag1 adatom binds in a modified geometry in which it is displaced from the Obr site and additionally coordinated to the Oot adatom [Figure 7c]. For this bridging adsorption site, we calculated a bond strength of 1.35 eV, and the smallest energy barrier for diffusion out of this adsorption site was found to be 0.95 eV. If on the o-TiO2(110) surface Ag1 is placed on-top of the Obr atom, a relatively weak binding energy of 0.49 eV results that is comparable to the binding energies calculated for Ag1

J. Phys. Chem. C, Vol. 114, No. 40, 2010 16969 on-top of the Obr atom on the two reduced TiO2(110) surfaces, r- and h-TiO2(110), respectively. As a comment on how to choose the model system for TiO2(110) in a given oxidation state, we note that the adsorption behavior of Ag1 on the r-TiO2(110) surface is highly dependent on the availability of charge in the model system (Table 2). Previously, a similar situation has been described for both the associative and the dissociative adsorption of O2 on r- and h-TiO2(110) surfaces.44,65,72 In the present case, we tested the following three additional situations as complementary models for an r-TiO2(110) surface: (i) one Obr vacancy in the surface of an otherwise perfect slab (i.e., without Hcap present) [Figure 6d], (ii) two surface Obr vacancies in the slab, one of which is kept free of adsorbates [Figure 6e], and (iii) one Obr vacancy and one Ti interstitial in the slab [Figure 6f]. In these models, formally two, four, and six excess electrons exist, whereas in our reference model depicted in Figure 6a three electrons are available, two from the Obr vacancy and one from the capping H atom. The consequence of the altered abundance of electrons in the slabs is that the Ag1 binds in the Obr vacancy with different strength, cf. Table 2. On the r-TiO2(110) system with one capping H atom and one Obr vacancy [Figure 6a], the adsorption binding energy of Ag1 is 0.28 eV. For the three alternative models for an r-TiO2(110) surface, we found either weaker adsorption binding energies of 0.16 eV [Figure 6d] with fewer electrons or stronger adsorption binding energies, 0.33 [Figure 6e] and 0.74 eV [Figure 6f], respectively, with more electrons in the slab (cf. Table 2). Furthermore, we found a strong tendency toward larger diffusion barriers on the electron rich slabs. The unusually large adsorption binding energy for Ag1 corresponding to the slab depicted in Figure 6f leads us to conclude that electronic charge is too abundant in this model. The assertion that the abundance of electronic charge is determining the stability of an Ag1 adatom in the Obr vacancy is supported by the evaluated Bader charges, Q, given in Table 2. On the (reference) r-TiO2(110) surface [Figure 6a], the Ag1 adsorbed in the Obr vacancy site is anionic with Q ) -0.15 e, whereas it has Q ) -0.02, -0.21, and -0.36 e, respectively, on the three alternative models for an r-TiO2(110) surface with fewer or more electrons in the slab. At this point it is instructive to compare the charge states of Ag1 on the three TiO2(110) surfaces considered and how the charge state correlates with the adsorption strength. As discussed above, Ag1 is negatively charged in the Obr vacancy, and for realistic models the adsorption strength in the Obr vacancy is low. By contrast, Ag1 attains a cationic charge state when it is attached to Obr sites on all TiO2(110) surfaces considered (cf. Table 1). Comparing the charge states and the adsorption strength separately for each model TiO2(110) surface, it can be generally stated that Ag1 binds the stronger the more positive its charge state is. Thus, regardless of the TiO2 oxidation state, Ag1 shows a strong tendency to bind at sites on the TiO2(110) surface where it is in a cationic charge state. For example, Ag1 is most positively charged (Q ) +0.75 e) on the particularly stable Obr-Oot bridge site on the o-TiO2(110) surface [Figure 8]. We propose that this pronounced tendency toward positively charged Ag1 explains the repopulation of the Ti3d gap state on the o-TiO2(110) surface upon Ag exposure. Ag is best dispersed on the o-TiO2(110) surface where it becomes most cationic, that is, where charge is transferred from the Agn clusters to the TiO2(110) support. The same argument can also be used to explain why Ag1 binds only weakly at the Obr vacancy sites on the r-TiO2(110) surface. When placing the Ag1 in the Obr vacancy, the Ag1 is forced to become anionic and, hence, its

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TABLE 1: Adhesion Potential Energy (Eadh), Bader Charge and Diffusion Activation Barrier for Ag1 Bound on Different Adsorption Sites on the Three TiO2(110) Surfaces Considereda adsorption site

a

Eadh (eV)

Bader charge (e)

Ediff (eV)

r-TiO2(110) (H cap)

5f-Ti Obr vac. Obr

i/i* ii iii/iii*

-0.16/-0.13 -0.28 -0.30/-0.36

-0.03/+0.05 -0.15 +0.48/+0.52

0.29 (iifi) 0.15 (iifiii)

h-TiO2(110) (Ti int.)

5f-Ti Obr Obr

i/i* ii iii/iii*

-0.02/-0.14 -0.25 -0.22/-0.16

+0.10/-0.04 +0.47 +0.48/+0.43

0.25 (iifi) 0.12 (iifiii)

o-TiO2(110) (Ti int.)

5f-Ti Obr-Oot br. Obr

i/i* ii/ii* iii

-0.14/-0.12 -1.35/-1.23 -0.49

+0.37/+0.28 +0.75/+0.52 +0.59

1.33 (iifi) 0.95 (iifiii)

The used model systems are shown in Figure 6a-c, and the adsorption sites are indicated in Figure 7.

TABLE 2: Adhesion Potential Energies (Eadh) and Bader Charges for Ag1 Bound in an Obr Vacancy on Various r-TiO2(110) Surfaces That Are Characterized by Different Excess Charge According to the Introduced Defects r-TiO2(110) support 1 1 2 1

Obr Obr Obr Obr

vac vac + 1 H cap vac vac + 1 Ti int.

Eadh (eV)

Bader charge (e)

-0.16 -0.28 -0.33 -0.74

-0.02 -0.15 -0.21 -0.36

The corresponding model systems are shown in Figure 6a,d,e,f.

stability is low. Thus, at most of the temperatures covered in the experiments Ag sinters instantly after adsorption on the r-TiO2(110) surface, since Ag1 is also rather weakly bound on the Obr atoms. Comparing the calculated behavior of Ag1 on the three different TiO2(110) surfaces with the measured STM data, the agreement is apparent. On the two reduced TiO2(110) surfaces, the weak Ag bonding and the low diffusion barriers directly rationalize the formation of the large Agn clusters that we found for these surfaces in our STM studies, cf. Figure 1. On the o-TiO2(110) surface, the calculated energetics of Ag1, high adhesion energy and high diffusion energy barrier, offer a rationale for the measured nucleation behavior involving a high degree of dispersion. In Figure 8, we compare the energetically most-stable Ag1 configuration identified in our DFT calculations on the oTiO2(110) surface with the most-abundant small Agn clusters in high-resolution STM images, which we ascribe to Ag1 adatoms. In the most-stable configuration found (-1.35 eV) Ag1 binds closely to an Oot adatom and forms a bridge to one of the

two adjacent Obr atoms [Figure 8a]. This DFT result agrees very well with the STM data, since in high-resolution STM images the majority of Ag1 adatoms are centered between the Ti and the Obr rows [cf. Figure 8b,c]. In the STM images we found equal densities of Ag1 adatoms aligned to the left (red circles) and right (blue dots), respectively, of the Ti troughs. Therefore we can rule out that tip effects account for the Ag1 adatoms that are centered between the Obr rows and the Ti troughs. The different alignments of the Ag1 adatoms with respect to the Ti troughs are illustrated in Figure 8c. The broken pink line is a profile through an Ag1 adatom that is aligned to the left, and the light blue line a profile through an Ag1 adatom aligned to the right of the Ti trough. In addition to the asymmetric species assigned to Ag1 adatoms on Obr-Oot bridge sites, few examples where found in the STM images where protrusions appeared in central positions above the Ti troughs (black cross). However, the precise nature of these Ag-related species remained unclear. 4. Concluding Remarks In light of the STM, PES, and DFT results regarding the Ag/ TiO2(110) system discussed here and of recent results obtained for the nucleation of Au12,16,40-42 and Pt43 on oxidized TiO2(110) surfaces we propose that noble metals are generally stronger attached to oxidized TiO2(110) surfaces than to reduced surfaces with Obr vacancies and/or H adatoms, respectively. Compared to the adhesion of Au, where small clusters can be stabilized on TiO2(110) surfaces with Obr vacancies,12 small Agn clusters appear not to be stabilized at Obr vacancy sites, since even at low temperatures we found only very few examples for Ag1 adatoms centered about the Obr rows. Instead, Agn nanoclusters adhere strongly on the TiO2(110) surface with Oot adatoms, thus

Figure 8. (a) Ball model (side and top view) of the most-stable configuration of Ag1 on o-TiO2(110) surface with the Ag1 in bridging Obr-Oot position as revealed by DFT. Colors were chosen as in Figure 7. (b) Zoom-in STM image (46 Å × 46 Å) acquired on the o-TiO2(110) surface after 0.041 ML Ag exposure at RT. Ag1 adatoms that are shifted away from the Ti troughs to the right and left side are indicated by filled blue and open red circles, respectively. The black cross denotes an Ag-species (presumably Ag1) that is centered exactly in the Ti trough. (c) STM profiles along the lines drawn in (b).

Enhanced Bonding of Ag Nanoparticles on Oxidized TiO2(110) rendering the o-TiO2(110) surface a much more robust support for Ag nanoclusters than the r- and the h-TiO2(110) surfaces. We consider the o-TiO2(110) surface, that is, bulk-reduced TiO2(110) crystals with an oxidized surface, to mimic the situation in “real” (photo-) catalysts better than the r- and the h-TiO2(110) surfaces do. Note that the stable attachment of the metal nanoclusters on the oxide support is a precondition for catalytic activity of the nanoclusters and that the nanoclusters have to be stable against sintering to keep them catalytically active.6-8,10,11,13-16 It is also worth noting that recent results obtained for supported Au clusters on FeOx,15,73 CeO2,74,75 and Mn2O376 likewise point to the view that O-rich metal cluster/ support interfaces are advantageous for active and sustainable catalysts. Thus, the hitherto often invoked schemes wherein surface O vacancies are proposed to be the nucleation centers of metal particles need to be revised, if not in general, then at least for TiO2 supports. In summary, from the interplay of STM and PES experiments and DFT calculations we have shown that Agn clusters are more strongly attached to TiO2(110) surfaces with Oot adatoms than to TiO2(110) surfaces with Obr vacancies or H adatoms. This result resembles the situation as previously observed for the nucleation of Au on TiO2(110) surfaces in different oxidation states.12 Furthermore, we have shown that charge is transferred from Agn clusters to the o-TiO2(110) surface, as evidenced by the reappearance of the Ti3d defect state upon Ag exposure on oxidized TiO2(110). This observation is consistent with our previous assertion that the Ti3d defect state originates essentially from bulk defects such as Ti interstitials rather than from Obr vacancies.44 Finally, we identified the most favorable adsorption site of Ag monomers on oxidized TiO2(110). Ag monomers bind close to Oot adatoms and form bridges to Obr atoms nearby. These data addressing the nucleation of Ag on model TiO2(110) surfaces are additional indication that oxidized surfaces of bulkreduced TiO2(110) are suitable model supports that are of higher relevance than TiO2(110) surfaces with H adatoms or with Obr vacancies. Acknowledgment. We are grateful to D. Wayne Goodman for many stimulating and enlightening discussions throughout the years. We acknowledge financial support from the Danish Ministry of Science, Technology, and Innovation through iNANO, the Danish Research Councils, the Danish Center for Scientific Computing, the Carlsberg Foundation, ERC for an Advanced Grant (F.B.) and Key Projects of Science and Technology Research of the Chinese Ministry of Education (No. 209055). P.T.S. acknowledges partial support from NSF-CHE0615606 and from the EFRC “Center for Atomic Level Catalyst Design” (DOE-DE-SC0001058). References and Notes (1) Ba¨umer, M.; Freund, H. J. Prog. Surf. Sci. 1999, 61, 127. (2) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (3) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33. (4) Fu, Q.; Wagner, T. Surf. Sci. Rep. 2007, 62, 431. (5) Fujishima, A.; Zhang, X. T.; Tryk, D. A. Surf. Sci. Rep. 2008, 63, 515. (6) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (7) Haruta, M. Gold Bull. 2004, 37, 27. (8) Meyer, R.; Lemire, C.; Shaikhutdinov, S. K.; Freund, H.-J. Gold Bull. 2004, 37, 72. (9) Date´, M.; Okumura, M.; Tsubota, S.; Haruta, M. Angew. Chem., Int. Ed. 2004, 43, 2129. (10) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896. (11) Chen, M.; Cai, Y.; Yan, Z.; Goodman, D. W. J. Am. Chem. Soc. 2006, 128, 6341.

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