Interaction of Zr with CeO2(111) Thin Film and Its Influence on

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The Journal of Physical Chemistry

Interaction of Zr with CeO 2 (111) Thin Film and Its Influence on Supported Ag Nanoparticles Shanwei Hu, Weijia Wang, Yan Wang, Qian Xu, Junfa Zhu* National Synchrotron Radiation Laboratory and Collaborative Innovation Center of Suzhou Nano Science and Technology , University of Science and Technology of China, Hefei, Anhui 230029, People’s Republic of China Email: [email protected] . ABSTRACT: Metal-support interaction plays a crucial role in modulating the activity and selectivity of oxidesupported metal nanoparticles in heterogonous catalysis. Zr-doped ceria is a critical component in automotive three-way catalysts. In order to understand the doping effect and Zr-CeO 2 interaction at the atomic level, here we employed X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) to comprehensively investigate the structures, morphologies and interfacial electronic properties of submonolayer coverage of Zr deposited onto 2 nm-thick well-ordered CeO 2 (111) thin films grown on Cu(111) at different temperatures and its effect on Ag growth under ultrahigh vacuum conditions. The strong Zr-CeO 2 interaction leads to a two-dimensional (2D) growth of Zr on the CeO 2 (111) surface and the formation of a homogeneous Zr-O-Ce mixed oxide layer at room temperature, accompanied by partial reduction of Ce from 4+ to 3+ state. At high temperature, both XPS and STM results indicate that the reverse oxygen spillover from ceria substrate to Zr clusters occurs, leading to the formation of reconstructed zirconia and further reduction of ceria. Deposition of Ag onto the Zr pre-deposited CeO 2 (111) surface results in a three-dimensional growth of Ag nanoparticles with a higher particle density and smaller particle size compared to those on the pure CeO 2 (111) surface under same conditions. In addition, an enhanced thermal stability of Ag particles upon annealing was observed on such mixed oxide surface. 1. INTRODUCTION Ceria has been widely used as catalysts in many redox catalytic reactions owning to its excellent oxygen storage/release capacity (OSC) – the ability to release or store oxygen in the

ACS Paragon Plus Environment


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reaction environments because of the high redox potential of the Ce4+ – Ce3+ couple.1,2 Especially, such property plays an important role in maintaining the activity of three-way catalysts in a large range of air/fuel ratio. However, the OSC of pure ceria is known to degrade with time in the automotive exhaust environment.3 In order to improve the thermal stability, metal ions are normally introduced into ceria.4-7 In particular, the insertion of zirconium ions into ceria was found to be an effective approach to enhance the thermal stability and oxygen storage capacity of ceria.8-11 For a long time, the degradation of ceria has been ascribed to its surface area decrease, which in turn leads to the decrease of its OSC.12 Doping ceria with zirconia was believed to stabilize the crystal structure and maintain the surface area of ceria.13,14 However, many studies on pure ceria demonstrated that the surface area might not be the only factor that determined the efficiency of ceria.15,16 Mamontov et al. suggested that the enhanced stability of oxygen defects after incorporating smaller Zr4+ ions into ceria might account for the improved oxygen storage capacity and thermal stability compared to pure ceria.12 In contrast, Kaspar et al. believed that the distortion of the metal–oxygen bonds resulted in a highly mobile lattice oxygen species, such that the reduced zone was not conﬁned to the surface layers but extended deeply into the bulk.17 Some EXAFS studies showed the presence of Zr4+ ions in ceria resulted in two type of oxygen species in Ce 1-x Zr x O 2 : strongly and weakly bound oxygen. Such weakly bound oxygen was believed to be the source of OSC.9 Despite of these studies, the exact origin of the enhanced redox properties of the ZrO 2 -CeO 2 mixed oxides and, particularly, the mechanism of the reduction process are still a matter of debate, which requires further detailed studies. In order to better understand the enhanced OSC and the improved thermal stability of zirconia-ceria mixed oxide system as compared to pure ceria, it is crucial to gain a fundamental insight of the chemistry of zirconia-ceria system in terms of the interactions between the zirconia and ceria as well as their interface structures.

So far, only a few surface science studies

addressing on the electronic structure and chemical properties of zirconia-ceria mixed oxide can be found.18-21 For example, by utilizing synchrotron-based photoemission, XPS, X-ray adsorption near edge spectroscopy (XANES), and first-principles density-functional calculations, Rodriguez et al. found that the doping of zirconia in CeO 2 reduced the band gap of CeO 2 , the Zr atoms in the mixed-metal oxide showed smaller positive charges than the cations in ZrO 2 or CeO 2 .19 Moreover, three different types of O atoms presented in the zirconia-ceria mixed oxide, resulting in different surface chemistry towards SO 2 adsorption compared to pure ceria and


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zirconia.18,19 However, regarding the atomic level acquaintance to the interfacial structures between zirconia and ceria, it still remains largely unclear. In this paper, we report a detailed study of interactions between Zr and well-ordered CeO 2 (111) thin films prepared on Cu(111). Considering Zr as a dopant metal to ceria, only submonolayer coverages of Zr were physicalvapor-deposited onto the well-ordered CeO 2 (111) surface to establish the mixed oxide system. XPS, STM and low energy electron diffraction (LEED) have been employed to investigate the growth, structure and interfacial electronic properties of Zr on CeO 2 (111). Except for zirconium, transition metals like palladium, platinum, rhodium, gold or silver were also frequently added onto ceria surface to improve the catalytic performance of ceria.22-29 These catalytically active metals formed small particles with high surface area-to-volume ratio on ceria surface, providing adsorption sites for reactants. For example, catalysts composed of Ag/ceria system have been wildly used in CO and hydrocarbon oxidation30 for automotive exhaust treatment reactions, where CO molecules adsorbed on the surface of Ag particles react with oxygen provided by ceria to form CO 2 . In addition, Ag/ceria system has exhibited excellent catalytic properties in water-gas shift reaction, formaldehyde oxidative decomposition and methane oxidation.5,24,31 However, these catalysts experienced serious sintering at elevated temperatures.27,32 Therefore, how to improve the Ag catalysts supported on ceria is of great importance for industrial applications. It has been often reported that noble metals supported on mixed oxides exhibit enhanced catalytic activities compared to conventional metal/monoxide catalysts.33-37 For instance, catalysts made up of Au supported on CeO x /TiO 2 (110) displayed an water-gas-shift reaction (WGS) activity that was 2.5-3.8 times higher than Au supported on plain TiO 2 (110) or CeO 2 (111).34 In order to investigate how the zirconia doping affects the growth and thermal stability of Ag particles on CeO 2 , we studied the surface structure of Ag deposition onto submonolayer zirconium pre-deposited CeO 2 (111) surface and its changes due to subsequent thermal annealing. It has been found that both the dispersivity and thermal stability of Ag particles have been improved on Zr-pre-deposited CeO 2 (111) surface compared to the case for Ag on pure CeO 2 (111) under the same conditions.

2. EXPERIMENTAL SECTION The experiments were performed in two separate ultrahigh vacuum (UHV) multi-chamber



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systems. LEED and STM measurements were performed in a system that comprises three chambers with base pressures all below 8 × 10-11 Torr. Detailed description of this system could be found in our previously published paper.38 The XPS measurements were mainly performed in the Catalysis and Surface Science Endstation in the National Synchrotron Radiation Laboratory, where a monochromatic Al Kα X-ray source and a VG Scienta R4000 electron analyzer are equipped to gain a high resolution data. If not mentioned, the XPS spectra were collected at an emission angle of 0º with respect to the surface normal. The Cu(111) single crystalline disc (Mateck, 8 mm diameter, 2 mm thickness) was cleaned using several cycles of Ar+ ion sputtering and annealing until no impurity could be detected by XPS, and LEED gave a sharp (1×1) pattern with low background. 2 nm-thick well-ordered CeO 2 (111) thin films were grown on the Cu(111) substrate at gradually increased substrate temperature by depositing Ce from a water-cooled e-beam evaporator in the 2 × 10-7 Torr of oxygen environment followed by annealing at 1000 K for 5 min.39 Our previous study32 has proved that the ceria films grown by this recipe are fully oxidized and well-ordered, exhibiting large and flat terraces with few surface defects. Zr was deposited onto the CeO 2 (111) thin film surface from an e-beam heated Mo crucible at 300 K. The deposition rate was determined by means of a quartz crystal microbalance (QCM) and by STM measurements of the island volume of clean Zr. Due to the low vapor pressure of Zr, the deposition rate was controlled at about 0.05 ML/min. Consider that Zr was used as a dopant metal to ceria, only submonolayer coverage of Zr was deposited onto the CeO 2 surfaces. One monolayer (ML) of Zr is defined throughout as 7.9 × 1014 atoms/cm2, which is the number of oxygen atoms exposed to the vacuum per unit area for CeO 2 (111).40 Such definition results in a monolayer thickness of 0.18 nm for Zr. Ag was physical vapor evaporated onto the Zr/CeO 2 surface at 300 K by evaporating Ag metal (99.999%, Alfa Aesar) from a tantalum basket. The deposition rate of Ag was kept at ~0.1 ML /min. One ML of Ag is defined as the number of Ag atoms per area in a closed-packed layer (111) of bulk Ag atoms (1 ML = 1.4 × 1015 atoms/cm2), resulting in a monolayer thickness of 0.239 nm/ML. Adopting Zhou’s method,29 the coverage of Ag was calculated based on the mean particle volume and particle density of Ag particles deposited on pure ceria obtained from the STM image. All the STM images were collected using an etched tungsten tip at room temperature in a constant current mode (0.01-0.05 nA, 3-4 V). During the STM experiments, great efforts were made to minimize the tip convolution effect.


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Figure 1. STM images of Zr deposited on 2 nm CeO2 films with different coverages: (a) 0.04; (b) 0.06; (c) 0.08; (d) 0.12; (e) 0.25; and (f) 0.50 ML. Image size: 130 nm × 130 nm. (g) and (h) show the diagrams of cluster sizes and cluster densities as a function of zirconium coverage, respectively.

The STM images were processed using the WSXM program.41 Particle size distribution (PSD) analysis was achieved by commercial software SPIP.

3. RESULTS



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3.1. Zr Growth on CeO 2 (111) at 300 K. Figure 1 displays the STM images acquired after depositing different amount of metallic Zr up to 0.5 ML onto 2 nm-thick fully oxidized CeO 2 (111) at 300 K. For the lowest coverage of 0.04 ML, relatively small clusters are observed with an average height of 0.14 ± 0.08 nm and average diameter of 1.08 ± 0.03 nm. Zr clusters grow on top of the ceria surface dispersedly without showing any preferential nucleation sites, resulting in a large cluster density of (8.1 ± 0.3) × 1012/cm2 (Figure 1a) . This result suggests a strong interaction between the Zr clusters and the CeO 2 support. As the coverage is increased to 0.06 and 0.08 ML, the number of clusters on the surface increases while the cluster size keeps almost the same, which is about 1.30 ± 0.40 nm in diameter and 0.14 ± 0.05 nm in height (Figure 1b and 1c). The aspect ratio (height/diameter) is only about 0.11, suggesting that the deposited Zr tends to wet the CeO 2 (111) surface. The same trend continues as the Zr coverage is increased to 0.12 ML. This phenomenon implies that the Zr adatoms have short diffusion length and do not migrate on the CeO 2 (111) surface to form relatively large Zr clusters. Further increasing the Zr coverage leads to the increase in clusters size as the chance of an incoming Zr atom joining an existing cluster becomes high. As shown in Figure 1f, at 0.5 ML, the average size of Zr clusters increases to 2.01 ± 0.43 nm in diameter and 0.19 ± 0.05 nm in height. On the other hand, Zr clusters exhibit a monotonic increase in cluster density during the whole deposition (Figure 1h) with a relatively fast rate below 0.12 ML followed by a slow increase. This is probably because

Figure 2. (a) Zr 3d spectra (hν = 1486.6 eV) for different amounts of Zr deposited on CeO2(111) surface

at 300 K; (b) Ce 3d spectra for CeO2(111) upon the deposition of different amounts of Zr. The coverages of deposited Zr are indicated in the Figure.


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after 0.12 ML Zr deposited, the exposed ceria sites for newly coming Zr atoms to nucleate become less and less. Consequently, the incoming Zr atoms are likely to incorporate into the existing clusters, resulting in the increase in both diameter and height of clusters. The cluster density is eventually increased to (25.1 ± 0.7) × 1012/cm2 for 0.5 ML Zr on ceria. About 70% of the CeO 2 surface is covered by these Zr clusters. Note that the surface area covered by Zr could be overestimated due to the tip convolution effect. Based on these observations, we conclude that Zr follows a 2D growth mode on CeO 2 (111) surface at the submonolayer coverage. However, the final mean height of Zr clusters (0.19 nm) is slightly higher than the thickness of monolayer Zr layer (0.18 nm), which will be discussed later. It is necessary to point out that there are some dark patches/features present in the STM images shown in Figure 1a and 1d. These might be attributed to the presence of a very small amount of oxygen vacancies on the surface and/or the tip convolution with sample. Upon deposition of submonolayer coverage of Zr on ceria at 300 K, XPS was also employed to investigate the interaction between Zr and ceria. Figure 2 shows Zr 3d and Ce 3d core level photoemission spectra as a function of Zr coverage on the CeO 2 (111) thin film surface at room temperature. As expected, the Zr 3d peak intensities increase linearly with increasing Zr coverage (Figure 2a). None of the Zr 3d spin-orbit split doublet (3d 5/2 and 3d 3/2 ) shows any resolved structure and can be easily fitted with two single peaks. After the first deposition step (0.04 ML), the Zr 3d 5/2 exhibits a binding energy (BE) of 182.2 eV, higher than the BE of metallic Zr (179.0 eV).42 This BE shift (3.2 eV) is too large to be contributed to the initial state effects due to the low coordination of Zr atoms in very small clusters or final state relaxation effects.43 Further deposition of Zr leads to slightly increase the BE of Zr 3d 5/2 . At 0.56 ML, it shifts to 182.7 eV. This BE value of Zr 3d 5/2 is closed to that for ZrO 2 thin film (182.8 eV).44 Therefore, we expect the formation of oxidized Zr4+ species on ceria upon Zr deposition. The 0.6 eV BE shift of Zr 3d from very low coverage to higher coverage can be ascribed to the change in coordination of Zr atom with O atoms on ceria surface compared to that in ZrO 2 thin film. Such phenomenon was also observed on Ti/CeO x (111) by Zhou et al.45 The oxidation of Zr upon deposition is in good agreement with our Ce 3d XPS study, as will be shown below. XPS spectra of the Ce 3d after each deposition step are plotted in Figure 2b. As shown in Figure 2b, the bottom spectrum marked as CeO 2 can be labelled as u, uʹʹ, uʹʹʹ, v, vʹʹ, and vʹʹʹ, as described previously in the literatures.40,46 The u and v refer to the 3d 3/2 and 3d 5/2 , respectively.



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These six satellite peaks correspond to three pairs of the spin-orbit splitting doublets which are associated with the Ce4+ cations.47 Upon the deposition of Zr at 300 K, the intensity of Ce4+ related peaks decreases while four more peaks start to show up. These extra new peaks originate from different Ce 4f configurations in both the initial and final states related with Ce3+ ions and can be similarly labelled as u0, uˈ, v0 and vˈ, suggesting that the ceria film gets reduced upon Zr deposition. Further deposition of Zr leads to the continuous increase in the peak intensities associated with Ce3+ cations, indicating the further reduction of ceria. By fitting the Ce 3d spectra,48 we can calculate that 24.3% of the as-grown ceria film is reduced after depositing 0.56 ML Zr, resulting in an average stoichiometry of CeO 1.88 . The above XPS data suggest a strong interaction between Zr and CeO 2 , which agrees well with our STM result. Upon deposition of Zr, the CeO 2 thin film experiences partial reduction. Regarding the mechanism of metal induced reduction of CeO 2 films, there are several explanations can be found in the literatures: (1) metal to oxide charge transfer; (2) reverse spillover of lattice oxygen from the ceria surface to the metal; (3) metal-catalyzed desorption of surface oxygen atoms; (4) metal-catalyzed reactions of surface O atoms with background CO gas to make CO 2 ; and (5) metal-induced accumulation of subsurface oxygen vacancies from deep in ceria to the bottom of metal clusters.25,28,49-52 In the present case, it is evident that the reduction of CeO 2 surface is attributed to the oxidation of deposited Zr. The process can be described as Zr + 4CeO 2 → 2Ce 2 O 3 + ZrO 2 , accompanied by the charge transfer from Zr to unoccupied 4f

orbitals of the Ce-O complex. Based on ∆Hf298 from the bulk energetics of oxide formation,53 this

process is favorable by 362.65 kJ/mol, indicating the partial reduction of CeO 2 with Zr to form ZrO 2 is energetically expected. Such interfacial oxidation/reaction also occurs upon deposition of metals such as Ti, W, Ga, Mn and V on CeO 2. 45,54-57 It is worth to mention that the zirconia clusters on ceria possess an average height of 0.19 ±

0.05 nm, which is not a multiple of a complete O-Zr-O layer (~0.3 nm).58 Such phenomenon could be ascribed to the different surface termination of ZrO 2 , which has been also observed on the epitaxially grown submonolayer ZrO 2 thin film on Pt(111).58 Based on the stacking sequence of the O and Zr layers for ZrO 2 , we propose a Zr termination for the present 0.19 nm-thick zirconia clusters, which is Zr-O-Ce mixed oxide layer at the Zr/ceria interface. Similar observations are also found on the Sn-CeO 2 , Ca-CeO 2 and Al-CeO 2 systems,59-61 where the formation of a mixed metal-O-Ce oxides was found upon the deposition of submonolayer


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coverage of these metals onto the CeO 2 (111) surface.

Figure 3. STM images of 0.5 ML Zr deposited on CeO2 at 300 K (a) and then sequentially annealed to 600 K (b), 800 K (c), 900 K (d) and 1000 K (e). Image sizes are 130 nm × 130 nm.

3.2. Annealing of the Zr/CeO 2 Layer. To investigate the effect of temperature on the stability of the zirconia clusters, the CeO 2 (111) surface covered with 0.5 ML Zr was annealed in four steps up to 1000 K. The morphologies of the Zr/CeO 2 surface upon heating are displayed in Figure 3. As mentioned, after 0.5 ML Zr deposited on CeO 2 (111) at 300 K, mixed Zr-O-Ce oxide clusters with an average size of 2.01 ± 0.43 nm in diameter and 0.19 ± 0.05 nm in height are formed (Figure 3a). After the surface was heated to 600 K, no significant changes can be found, implying that Zr clusters do not undergo sintering process at this temperature. Upon heating the surface to 800 K, larger clusters emerge accompanied by the decrease of cluster density (Figure 3c). This observation indicates that at this temperature zirconia clusters undergo Ostwald ripening process to form larger islands at the expense of smaller ones.62 Further annealing to 900 K drives the zirconia clusters to continuously diffuse and aggregate into larger ones, along with



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further decrease in cluster density, as seen in Figure 3d. However, although the zirconia clusters incorporate into larger ones upon heating, the cluster heights are not larger than one monolayer height of pure zirconia. Meanwhile, the area of uncovered CeO 2 surface increases dramatically. Eventually, upon heating to 1000 K, much few zirconia clusters could be found on ceria surface (Figure 3e), which exhibit an average diameter of 4.11 ± 0.91 nm and an average height of 0.25 ± 0.05 nm. In addition, zirconia clusters also experience structural change during the annealing process. As can be seen in Figure 3d and 3e, after heating to high temperatures, small protrusions can be seen on top of the zirconia clusters. This phenomenon can be better observed in Figure 4a, which is magnified image of Figure 3d. These little protrusions behaving as white spots sit on top of zirconia islands with an average diameter of about 0.7 nm, as indicated in the line scan profile (Figure 4b). Basically, these local formed protrusions can be attributed to a distorted (2×2) structure of ZrO 2 ,63 as will be further discussed latter. In conjunction with the structural change of zirconia clusters upon high temperature annealing, an army of black holes can be found on ceria terraces, which are identified as surface defects.32 These defects are associated with oxygen vacancies on the surface,64,65 implying that ceria film is further reduced upon heating at high temperatures.

Figure 4. (a) Magnified STM image of 0.5 ML Zr on CeO2 upon heating to 900 K. Image size: 65 nm × 65 nm. (b) Line scan of zirconia clusters on top of the CeO2 surface.

In order to further understand the structural evolution at the Zr/CeO 2 interface during annealing, we performed XPS measurements to monitor the evolution of Zr 3d and Ce 3d spectra


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Figure 5. (a) Ce 3d spectra of 0.5 ML zirconium deposited on CeO2(111) upon annealing to different temperatures. The spectra were collected at 300 K. For comparison, the Ce 3d spectra of pure CeO2 is also displayed as bottom curve. (b) Diagram of Ce3+ concentration change in ceria film upon the deposition of Zr and gradual annealing.

with annealing temperature for 0.5 ML Zr deposited on CeO 2 (111). Upon annealing, the Zr 3d spectra collected at 0° with respect to surface normal do not show obvious changes in peak intensity up to 1000 K (spectra not shown). However, when the spectra were collected at 45°, the Zr 3d peak intensity appears obvious decrease with increasing the annealing temperature. This result suggests that upon annealing, except for the aggregation of zirconia into larger particles, the diffusion of Zr into subsurface also happens. The Ce 3d spectra and the evolution of Ce3+ concentration at each annealing temperature are shown Figure 5. For comparison, the spectrum from clean CeO 2 (111) is also shown. As can be seen, at 300 K, the features related to Ce3+ immediately show up after Zr deposition. As mentioned above, 24.3% of the Ce4+ is reduced to Ce3+ based on the XPS data analysis. At the initial stage of annealing (< 600 K), the percentage of Ce3+ on the ceria surface keeps on decreasing (Figure 5b). When the surface was heated at 600 K, the Ce3+ concentration reaches minimum of 16.5%. Similar phenomenon was also found on the Pt-CeO 2 interface upon heating, where the decrease of Ce3+ concentration was ascribed to the migration of Ce3+ ions into deeper layers.49 Moreover, since the presence of zirconia improves the mobile ability of oxygen atoms in ceria,17,66 the decrease of Ce3+ concentration might be attributed to the compensation of surface oxygen vacancies by deeper oxygen atoms, whose diffusion process is facilitated by heating. In fact, the angle dependent XPS results (spectra now



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shown) further confirm the diffusion of surface Ce3+ into deep layers upon annealing to 600 K. Further annealing the surface to high temperatures above 600 K lead to the increase of Ce3+ concentration, which reaches 23.0% at 1000 K eventually. This pronounced ceria reduction is consistent with the increase of surface defects observed in our STM result. Such ceria reduction upon heating above 600 K could be explained by the oxygen reverse spillover from ceria to zirconia, as will be discussed in detail in the Discussion section. In a word, our results provide clear evidences that thermal treatment influences both the morphology and electronic structure of

Figure 6. (a) STM image of 0.1 ML Zr deposited on CeO2(111) surface at 900 K. (b) Magnified STM image of (a), showing the detailed structure of zirconia islands on top of ceria surface. (c) Line scan along one of the zirconia island on the CeO2(111) surface. Image size: 65 nm × 65 nm (a), 26 nm × 26 nm (b).

zirconia/ceria interface. 3.3 Zr Growth on CeO 2 (111) at 900 K. In order to further investigate the effect of temperature on the morphology and structure of zirconia clusters, Zr was deposited onto the CeO 2 (111) surface at 900 K. In contrast to the randomly dispersed Zr on the CeO 2 (111) surface at room temperature, at 900 K a number of zirconia islands are found to disperse on top of the ceria terraces with similar sizes, as displayed in Figure 6a. These islands have a shape of triangular or quasi triangular. Besides the CeO 2 terraces, zirconia islands can be also found along the step edges of ceria terraces. Shown in Figure 6b is a magnified STM image which displays the detailed structure of these zirconia islands. Interestingly, most of these zirconia islands exhibit an ordered structure, which can be identified as hexagonal superstructure of protrusions. A line scan profile along one of the islands shown in Figure 6b demonstrates that the height of the islands is about 0.3 nm (Figure 6c), which agrees well with that of O-Zr-O layer (~0.3 nm),58


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suggesting the formation of a complete ZrO 2 layer. Moreover, the hexagonal protrusion patterns possess a periodicity of 0.67 nm, which fits nicely to the twice of the ZrO 2 (111) lattice constant (0.35 nm), indicating the (2×2) reconstruction.63 Such (2×2) unit cell can be perfectly registered with a �√3 × √3� cell of the underlying CeO 2 (111) surface, whose unit cell size is 0.66 nm.

Hence, we can ascribe the hexagonal protrusion pattern to the formation of ZrO 2 (111)(2×2)/�√3 × √3� superstructure at the Zr/CeO 2 interface, which reduces the effective lattice mismatch to only 1.5%. This ZrO 2 (2×2) structure has also been observed on continuous ZrO 2

thin film epitaxially grown on Pt(111) at elevated temperatures.44,58,63 In addition, the formation of this hexagonal corrugation pattern demonstrates that the zirconia islands exhibit a complete OZr-O layer, consistent with the measured island height of 0.3 nm from the line scan profile.58



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3.4 Deposition of Ag Nanoparticles on Zr pre-deposited CeO 2 (111) Surface. Our previous studies demonstrated that the growth of Ag on pure CeO 2 (111) surface forms three dimensional nanoparticles at room temperature.27,32 Due to the weak interaction between Ag and ideal CeO 2 terraces, Ag nanoparticles mainly bind to the step edge sites of the CeO 2 (111) surface. Here we investigated how small amount of pre-deposited Zr on the CeO 2 (111) surface would affect the growth and stability of post-deposited Ag particles. The Zr/CeO 2 (111) surface was prepared by depositing 0.25 ML Zr onto CeO 2 (111) at 300 K, as shown in Figure 7a. As mentioned above, Zr randomly distributes on the CeO 2 terraces with a large cluster density. The average height of zirconia clusters is 0.19 ± 0.05 nm. When Ag was deposited on this Zr predeposited CeO 2 (111) surface, distinct different growth behavior of Ag was observed as compared

Figure 7. STM images of Ag deposited on 0.25 ML Zr pre-deposited CeO2(111) surface at 300 K with different coverages: (a) 0.0 ML; (b) 0.3 ML; (c) 0.6 ML. Image size: 130 nm × 130 nm. (d) Density of particles on 0.25 ML Zr pre-deposited CeO2(111) surface as a function of Ag coverage(f) Histogram of height of Ag-Zr particles in Figure 7c.


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to Ag on pure CeO 2 (111) surface. Figure 7b and 7c show the STM results of 0.3 and 0.6 ML Ag deposited onto the 0.25 ML Zr pre-covered CeO 2 (111) at 300 K, respectively. It can be seen that Ag particles are largely distributed on the terraces of CeO 2 with smaller particle size and higher density as compared to those on pure ceria surface under same conditions. Moreover, they mainly nucleate on the existing zirconia clusters, as evidenced by the almost unchanged particle density after deposition of Ag (shown in Figure 7d). This is understandable since the interaction of Ag with ceria is rather weak; the incoming Ag atoms landing on the Zr pre-covered ceria surface would diffuse around on the surface to find a stable adsorption site. If they land on the exposed ceria surface, the most stable sites for them are the step edge sites on the pure ceria surface.32 However, since Zr forms 2D islands on ceria, the pathway for Ag atoms diffusion to

Figure 8. STM images of 0.6 ML Ag deposited on (a) pure CeO2 and (b) 0.25 ML Zr pre-deposited ceria surfaces at 300 K followed by heating to 800 K. Image sizes are 130 nm × 130 nm. (c) Histograms of particle heights of Ag nanoparticles on the Zr/ceria surface derived from (b). Plots of mean particle height (d) and particle density (e) of 0.6 ML Ag deposited on these two ceria surfaces as a function of annealing temperatures.



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step edges will be blocked. As a result, it is plausible that Ag atoms incorporate into the existing Zr clusters to form larger clusters. The particle size distribution of 0.6 ML Ag on Zr/CeO 2 surface is displayed in Figure 7e, where the histogram of the particle heights exhibits a typical bimodal distribution. It can be roughly estimated that Ag-Zr clusters represent 70% of the total cluster density, with an average height of 0.38 ± 0.06 nm. Apparently, on the Zr pre-deposited ceria surface, silver are trapped at Zr sites instead of migrating on the oxide surface to form relatively large Ag particles. The next step is to check the thermal stability of Ag nanoparticles on such Zr pre-deposited ceria surface by stepwise annealing to 800 K. As comparison, Ag deposited on pure CeO 2 surface at 300 K followed by heating to 800 K was first given in Figure 8a. It is obvious that Ag particles undergo severe sintering and desorbing at this temperature on pure CeO 2 (111) thin films.32 Only several large Ag particles can be found on the surface at end. While on the Zr predeposited ceria surface, much more silver particles with relatively smaller size can be observed on the surface after annealing to 800 K, indicating that the Ag particles sinter much slowly on the Zr pre-deposited ceria surface than on the pure ceria surface. Except for silver nanoparticles, Zr clusters with uniform size are found to disperse on ceria surface. These Zr clusters maintain the same size and shape as they were initially deposited on the CeO 2 (111) surface, indicative of high thermal stability upon heating. From the histogram shown in Figure 8c, we can find that Zr clusters are the majority of the clusters remained on the ceria surface at 800 K. While the small peak with large particle height represents the residual Ag particles after annealing at 800 K. Figure 8d and 8e display the mean particle height and particle density of Ag particles deposited on pure ceria and Zr/ceria surfaces as a function of annealing temperatures, respectively. Apparently, silver particles exhibit smaller size and larger particle density on the Zr predeposited ceria surface at the same annealing temperature compared to those on pure CeO 2 . Hence, our results suggest that the pre-deposited Zr clusters on ceria can act as nucleation sites for the silver particles and hinder their diffusion on the surface to form larger particles upon heating to higher temperatures, which would be beneficial to promote their catalytic activity.7,34 4. DISCUSSION Our results indicate that at 300 K the deposition of Zr on CeO 2 (111) forms Zr-O-Ce structure with an average height of 0.19 nm. However, when Zr was deposited on CeO 2 at 900 K, a hexagonal protrusion pattern with a (2×2) reconstruction was observed. XPS results suggest


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that ZrO 2 clusters was formed. STM results demonstrate that these zirconia clusters have an average height of ~0.3 nm, corresponding to a complete O-Zr-O layer. Similar ZrO 2 -(2×2) structure was reported on the epitaxially grown ZrO 2 thin films on Pt(111) by Meinal.58,63,67 In those studies, ZrO 2 thin films were prepared by first depositing Zr onto Pt(111) at 470 K followed by post-annealing the sample at 1050 K in an O 2 atmosphere of 1 × 10-6 mbar. However, in the present study, despite of the similar substrate temperature, the UHV condition was maintained during Zr deposition or annealing. Therefore, the oxygen in the top layer of the zirconia islands could only come from the lattice oxygen in the underneath ceria film, which migrates through the Zr/ceria interface to zirconia clusters during the deposition at 900 K. Moreover, the experiment of annealing the room-temperature deposited Zr on CeO 2 (111) to 900 K provides a further evidence for the oxygen migration from ceria to zirconia at high temperature. As stated above, the room temperature deposited Zr forms Zr-O-Ce layers with a Zr termination. After annealing above 900 K, distorted ZrO 2 (2×2) structures emerge on the zirconia clusters with the cluster heights increasing to ~ 0.25 nm. Meanwhile, the XPS results suggest a continuous reduction of ceria at high temperature range (600 K-1000 K). Based on these selfconsistent observations, we propose that at the zirconia/ceria interface, an oxygen reverse spillover process from ceria to Zr occurs at high temperatures, accompanied by massive restructuring of both zirconia and ceria. Oxygen reverse spillover process has been found in many model catalyst systems such as Pt-CeO 2 , Rh-CeO 2 and Ag-CeO 2 .27,32,49,50,68 In these studies, metal particles were found mainly anchored on the ceria step edges or ceria with nanoscale sizes. The formation of oxygen vacancies in nanostructured ceria is strongly favored with respect to bulk ceria,69 which has a pivotal consequence for the oxygen migration from the ceria support to the supported metal nanoparticles. For example, the energy for oxygen vacancy formation (E vac ) on the Ce 40 O 80 particle is only 0.80 eV, which is much lower than the value of 2.25 eV calculated for a regular CeO 2 (111) slab.49 Similarly, the E vac value on ceria steps were calculated to be up to 0.7 eV lower than that on the regular CeO 2 (111) surface.70 In the present case, Zr atoms are mainly dispersed on the regular terraces of ceria surface, where the oxygen spillover from ceria to Zr, in principle, is not easy because the energy cost for moving one oxygen atom out of regular ceria surface is too high. However, recent theoretical studies demonstrated that the doped ceria with Zr could change the charge distribution and weaken the Ce-O bond, resulting in a decreased E vac of



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ceria.9,71 Indeed, our experimental results provide clear evidences that oxygen reverse spillover is plausible at the Zr/CeO 2 interface, suggesting that the surface-doped zirconium lowers the E vac of regular CeO 2 (111) surface. Such result could be explained by the strong interaction between Zr and CeO 2 , which alternates the electronic structure of ceria and further increases the OSC by decreasing E vac .

5. CONCLUSION Zr randomly distributes on top of CeO 2 (111) surface with large cluster density upon room temperature deposition. Metallic Zr strongly interacts with CeO 2 (111) thin films and forms Zr-OCe mixed oxides at submonolayer coverage, leading to a partial reduction of ceria surface. These room temperature deposited Zr particles prefer to remain almost unchanged in both size and shape rather than undergo sintering below 800 K on the ceria surface, indicating a good thermal stability of Zr on CeO 2 . Higher temperature deposition or annealing facilitates the oxygen reverse spillover at zirconia/ceria interface, which results in the formation of reconstructed ZrO 2 (2×2) clusters and further reduction of ceria. Sequential deposition of Ag on the Zr pre-deposited CeO 2 surface exhibits distinct different growth behavior of Ag compared to that on pure CeO 2 surface. Ag particles are found to randomly distribute on the Zr pre-deposited CeO 2 surface with a large particle density from the initial deposition, in contrast to Ag mainly populates the step edge sites on pure CeO 2 . The presence of Zr on CeO 2 (111) surface significantly enhances the thermal stability of Ag particles upon annealing.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the National Basic Research Program of China (2013CB834605), Natural Science Foundation of China (Grant Nos. U1232102 and 21403205), and Scientific Research Grant of Hefei Science Center of CAS (SRG-HSC) (No. 2015SRGHSC031) for the financial support of this work.

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Interaction of Zr with CeO2(111) Thin Film and Its Influence on

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