Gold Nanoparticles Supported on Ceria - American Chemical Society

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Spectromicroscopy of a Model Water−Gas Shift Catalyst: Gold Nanoparticles Supported on Ceria David C. Grinter,† Chris Muryn,‡ Benito Santos,§ Bobbie-Jean Shaw,† Tevfik O. Menteş,§ Andrea Locatelli,§ and Geoff Thornton*,† †

Department of Chemistry and London Centre for Nanotechnology, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom ‡ School of Chemistry and Photon Science Institute, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom § Elettra − Sinchrotrone Trieste, Strada Statale 14 - km 163,5 in AREA Science Park, 34149 Basovizza, Trieste, Italy S Supporting Information *

ABSTRACT: Nanometer-sized gold particles supported on ceria are an important catalyst for the low-temperature water−gas shift reaction. In this work, we prepared a model system of epitaxial, ultrathin (1−2 nm thick) CeO2−x(111) crystallites on a Rh(111) substrate. Lowenergy electron microscopy (LEEM) and X-ray photoemission electron microscopy (XPEEM) were employed to characterize the in situ growth and morphology of these films, employing Ce 4f resonant photoemission to probe the oxidation state of the ceria. The deposition of submonolayer amounts of gold at room temperature was studied with scanning tunneling microscopy (STM) and XPEEM. Spatially resolved, energy-selected XPEEM at the Au 4f core level after gold adsorption indicated small shifts to higher binding energy for the nanoparticles, with the magnitude of the shift inversely related to the particle size. Slight reduction of the ceria support was also observed upon increasing Au coverage. The initial oxidation state of the ceria film was shown to influence the Au 4f binding energy; more heavily reduced ceria promoted a larger shift to higher binding energy. Understanding the redox behavior of the gold/ceria system is an important step in elucidating the mechanisms behind its catalytic activity.

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INTRODUCTION Stimulated by their applications in varied fields, including heterogeneous catalysis,1 electronic devices,2 chemical sensing,3 and fuel cells,4 ceria-based (CeO2) materials have been the focus of intensive study for a number of years. As a reducible oxide with facile oxygen vacancy formation and conversion between the Ce3+ and Ce4+ oxidation states, ceria has excellent oxygen storage capacity and therefore displays good characteristics as a catalyst support.1,5 The discoveries by Haruta2,6 and Goodman et al.3,7 that gold nanoparticles display significant catalytic activity when supported on ceria and titania surfaces, respectively, have sparked remarkable interest in gold catalysis. For ceria-supported gold, one special area of interest regards the low-temperature water−gas shift (WGS) reaction, an important process for clean hydrogen production.4,8 Additional systems catalyzed by ceria-supported gold include NO x reduction,9 low-temperature CO oxidation,10 and crosscoupling reactions.11 There appears to be a complex interplay between the gold nanoparticles and the oxide support in the WGS reaction, which depends on the nature and concentration of defects within the oxide and the size and dispersion of the gold. This is the principal reason that a comprehensive mechanistic understanding of the catalytic activity is still not in place despite a great deal of experimental and theoretical work.12−15 Potential explanations for the high activities demonstrated by oxide-supported gold nanoparticles include charge transfer © 2014 American Chemical Society

between the support and gold, oxygen spillover effects, undercoordinated Au atoms, and quantum size effects.12,16−18 One controversial issue that remains unresolved is the oxidation state of the gold nanoparticles within the active catalyst, which has been proposed to be cationic under reaction conditions.19,20 Common experimental methods to interrogate the electronic structure of the gold include X-ray photoelectron spectroscopy (XPS),21,22 extended X-ray absorption fine structure spectroscopy (EXAFS),23 and infrared spectroscopy using CO as a probe molecule.24 Interpretation of photoemission data to identify the gold oxidation state is complicated by final state effects arising from the small size of the gold nanoparticles.25 Ultrathin epitaxial CeO2(111) films prepared on substrates including Au(111),26 Ru(0001),27 Cu(111),28 Pt(111),29 and Rh(111)30 have been studied both as inverse model catalysts and as a means to apply electron-based techniques to an insulating oxide. These films also allow the investigation of novel effects at the interfaces between the ceria and the metal substrates as well as supported nanoparticles. Scanning tunneling microscopy (STM) is one of the key techniques for the study of ceria-supported gold, as demonstrated by a number of research groups.22,24,27,31Recently, low-energy Received: June 4, 2014 Revised: July 9, 2014 Published: July 24, 2014 19194

dx.doi.org/10.1021/jp5055205 | J. Phys. Chem. C 2014, 118, 19194−19204

The Journal of Physical Chemistry C

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Figure 1. (a−h) Bright-field LEEM images (FOV = 6 μm, S.V. = 4.5 eV) that follow the oxidation of a 1 MLE of CeO2−x(111) ultrathin film on Rh(111). The temperature was increased steadily over a period of 2 h from 690 (a) to 1150 K (h) in 1 × 10−7 mbar of O2. To aid in the identification of changes occurring during this process, a number of regions have been highlighted, including a mesa feature (black dashed circle), narrow terraces with a high density of parallel step edges (red lines), and a flat region with a few screw dislocations (black square). (i) A plot of the annealing temperature against time. The times at which the images in a−h were obtained are indicated. (j) An expanded view of the region marked with the black square (900 × 900 nm2) in (a,d,e) that shows the evolution of the smaller ceria islands during the annealing process. Two step edges, terminated by screw dislocations, are highlighted with blue lines.

structural and chemical sensitivity.39 The primary operational modes of the microscope are imaging (LEEM and XPEEM with spatial resolutions of 10 and 30 nm, respectively, and better than 300 meV energy resolution in XPEEM), diffraction (microprobe angle-resolved photoemission spectroscopy and microprobe-LEED), and spectroscopy (microprobe-XPS).40 The experiments described in this paper have employed LEEM in both bright- and dark-field modes, respectively, employing primary or secondary diffracted beams for imaging. STM measurements were carried out at UCL using an Omicron variable-temperature scanning tunneling microscope (VTSTM). The Rh(111) single crystal (Surface Prep. Lab.) was prepared by repeated cycles of argon ion sputtering and annealing to 1150 K in UHV and 800 K in 1 × 10−6 mbar of O2, followed by a final flash up to 1500 K in UHV. This procedure yielded a well-ordered surface according to LEED,

electron microscopy (LEEM) and X-ray photoemission electron microscopy (XPEEM) have been applied to ceria films on Pt(111),32 Re(0001),33 Ru(0001),34−36 W(110),37 and Cu(111).38 These techniques combine high-quality spectroscopy with excellent lateral resolution and have provided useful insights into the topographic and chemical structure of such films. In this paper, we describe a study of Au nanoparticles supported on ultrathin ceria films prepared on Rh(111), using STM and spectroscopic XPEEM.

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EXPERIMENTAL METHODS Synchrotron radiation measurements were carried out at the Nanospectroscopy beamline (1.2L) at Elettra (Sincrotrone Trieste, Italy) using the Elmitec Spectroscopic Photoemission and Low-Energy Electron Microscope (SPELEEM). This instrument combines LEEM and energy-filtered XPEEM using electrons or soft X-rays as a probe and provides both 19195



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closely spaced steps of the Rh(111) substrate that has a clear clockwise screw dislocation at its top. This is marked with a dashed circle, which acts as a reference point in subsequent frames. The contrast in Figure 1a is dominated by the Rh(111) substrate, with a number of steps and step-bunches being visible as well as other common features of similar metal surfaces, including the mesa feature mentioned previously and large, atomically flat terraces. As the temperature is increased, the contrast from the substrate is first of all lost (Figure 1b, 800 K) and then reappears with the addition of small dark spots decorating the step edges of the substrate (Figure 1c, 980 K). Increasing the temperature further (Figure 1d, 1010 K) leads to much sharper contrast between the dark islands, which will be later identified as CeO2−x(111), and the bright Rh(111) substrate. It is observed that as well as the larger ceria islands, which are located primarily at the step edges of the Rh(111), a number of small ceria islands are located in the middle of the terraces on the substrate. These smaller islands can be most clearly seen in the middle of the region of Figure 1d, marked with the yellow square. Further increasing the temperature leads to a ripening of the larger ceria islands and an overall reduction in their number and increase in the interisland separation (Figure 1e, 1040 K). We also observe that the islands on the heavily stepped regions (those bounded by the red lines and also around the base of the mesa) are in general smaller than those nucleated at step edges where the terraces are larger. As we increase the temperature further, there is a distinct change in this red-bounded region, and the smaller islands appear to coalesce into larger, less densely spaced islands (Figure 1f, 1080 K, and Figure 1g, 1120 K). This behavior suggests that below this temperature, the transport of ceria (either partially or fully oxidized) is predominantly across the same terrace, whereas above this temperature, it is also enabled across step edges, leading to the observed morphological changes. In Figure 1h (1150 K, the maximum temperature reached), large open patches of the substrate are clearly visible along with an overall lower density of ceria islands, some of which are large (up to 300 nm in diameter). The ceria islands have clearer geometric shapes at this point as well, displaying quasi-hexagonal and triangular profiles. There is some anisotropy evident in their shapes, with the islands in the heavily stepped region (red highlight) displaying a clear preference for growth parallel to the substrate step edges. Figure 1j displays the evolution of the region marked with a black square (900 × 900 nm2) in the movie sequence, allowing us to examine the changes in more detail as the sample is heated over a period of 20 min with only a moderate temperature rise (20°). The first frame (obtained prior to the annealing process) permits the study of the substrate structure. A number of steps are clearly distinct (two are marked in dark blue), some of which are also terminated in screw dislocations. The subsequent images indicate that nucleation of the ceria islands is initially favored at these step edges and dislocations. As the annealing process continues, the smaller ceria islands are observed to disappear, resulting in larger bare areas of the substrate becoming exposed. By the end of the sequence, the majority of islands less than 50 nm in diameter are no longer present, with the majority of the remainder having diameters of 80−100 nm. Analyzing the LEEM images in Figure 1 in terms of the statistics of island growth leads to a number of other observations about the annealing process. Figure 2a shows a

with no contamination observed in XPS and large, atomically flat terraces in LEEM. Epitaxial CeO2−x(111) films were prepared by depositing Ce metal (99.9%, Alfa-Aesar) onto the Rh(111) surface at room temperature from an electron beam evaporator (Focus EFM-3) at a deposition rate of 0.04 ML min−1. This was followed by an oxidation process of annealing stepwise up to ∼1150 K in 1 × 10−7 mbar of O2 while monitoring the changes in surface morphology in LEEM and LEED. The evaporation rate of cerium was calibrated by monitoring the attenuation of the Rh(111)(1 × 1) LEED pattern during deposition according to the known growth mode exhibited on Rh(111).41 Ceria coverages are given in monolayer equivalents (MLEs), where one monolayer is defined as the complete coverage of the surface by a single CeO2 trilayer unit, which has a height of 0.31 nm.29 Gold was evaporated onto the CeO2−x(111)/Rh(111) system from an electron beam evaporator at a rate of 0.05 ML min−1, calibrated by monitoring the layer-by-layer growth behavior on Re(0001) in LEEM. For synchrotron measurements, temperatures were monitored with a thermocouple attached close to the sample. The ceria film investigated with STM in London was prepared under the same conditions as the films used for synchrotron measurements; however, there is a slightly larger uncertainty in the final annealing temperature (1150 ± 50 K) due to the measurement method (optical pyrometer). Photon energies employed to record core-level spectra were as follows: Au 4f, 200 eV; Rh 3d, 420 eV; and O 1s, 650 eV. For Ce 4f resonant spectra, the photon energies used were 115, 120.8, and 122.5 eV, corresponding to off-resonance, onresonance for Ce3+, and on-resonance for Ce4+, respectively.42 Binding energies are referenced to the Fermi level of the Rh(111) substrate. The electron kinetic energy (K.E.) in the SPELEEM instrument is controlled by applying a potential to bias the sample, namely, the start voltage (S.V.). The kinetic energy of the emitted/scattered electrons is given by K.E. = S.V. − Δφ, where Δφ is the difference in work function between the sample and the instrument. To ensure precise calibration for the Au 4f spectra, a Fermi level scan was performed immediately after each Au 4f spectrum was acquired, using the same photon energy (200 eV). Images and spectra were acquired in a background pressure of 1 × 10−7 mbar of O2 to prevent X-ray or electron-beam-induced reduction of the ceria, except where indicated.43

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RESULTS AND DISCUSSION CeO2−x(111) Film Growth and Structure. One of the strengths of the SPELEEM instrument is its ability to study in situ processes; in this case, the LEEM mode is employed to investigate the changes that the ceria film undergoes upon increasing temperature during oxidation. This is detailed in Figure 1a−h where a number of frames have been extracted from a bright-field LEEM movie (full movie available as Supporting Information) obtained during a 2 h annealing process as the sample temperature was ramped (see Figure 1i) from 690 to 1150 K in an O2 pressure of 1 × 10−7 mbar. The long time scale was employed to minimize the effects of thermal drift, which are considerable and require constant small corrections of the sample position to keep the same region within the field of view. The gentle rate of heating is also necessary to enable easier separation of time and temperature dependence during the annealing process. A number of features are common to all of the frames, allowing identification of the regions of interest. One such is a large mesa consisting of 19196



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2b shows an image obtained after annealing to 1030 K with a total of 1110 (±30) ceria islands within the FOV, which together cover 28 (±3)% of the surface. Figure 2c shows an image after annealing to 1150 K with a total of 510 (±20) islands within the FOV, covering 19 (±2)% of the substrate. The histograms of the island areas show broadly similar distributions for both frames below 10000 nm2 (note the logarithmic scale on the y axis); however, in Figure 2c, there are significantly more islands with areas > 10000 nm2. This is reflected in the mean ceria island area, which increases from 3200 nm2 for Figure 2b to 4800 nm2 for Figure 2c. It is important to note that the areas sampled are slightly different due to thermal drift during the course of the measurement, which will undoubtedly induce some error due to inhomogeneity in the substrate morphology. However, we do not expect this to affect the general trend observed. The reduction in the surface area covered by ceria islands (28−19%) implies that there is also a change in their heights; using the calibrated initial dose of cerium (1.1 ML), this results in an increase in island height from ∼1.2 (four CeO2 trilayers) to ∼1.8 nm (six CeO2 trilayers), consistent with the trend observed in STM experiments. Figure 3a displays a typical LEED pattern (S.V. = 35 eV) from an epitaxial CeO2−x(111) film on Rh(111), with the reflexes highlighted according to their origin. In red is the Rh(111)(1 × 1) substrate, in blue is the CeO2−x(111) at (1.4 × 1.4) relative to this, and in green is an O-(2 × 2) overlayer. No rotational domains of ceria are observed, in contrast to some other results in the literature,34,35,43 likely due to the high oxidation temperature used (1050 K) and lengthy annealing time. Figure 3b and c shows dark-field LEEM images (FOV = 4 μm) from the ceria diffracted beams labeled “A” and “B” on the LEED pattern in Figure 3a. As expected, the islands appear bright on a dark background, and their morphology is consistent with LEEM investigations of ceria on Re(0001)43 and Ru(0001)35 as well as STM studies on Rh(111).44 The islands in this area of the film have a relatively uniform size distribution with an average width (assuming a triangular shape) of 140 nm. A number have visible holes in their centers, another common motif for these low-coverage films.29 The ceria islands observed in the dark-field images in Figure 3b and c have two levels of brightness (depicted as white and gray), with the brighter islands appearing dark (and vice versa) when the other spot is used for illumination. Examining the bright islands from each image in detail (see the orange expanded region in Figure 3b and the yellow region in Figure 3c), we can see that the ceria takes on near-triangular shapes that are rotated by 60° with respect to each other. The ratio of the number of islands that appear bright/dark in each dark-field image is 1:3.4 (±0.5). The ratio of the area covered by the two island orientations is 1:6.2 (±0.5), consistent with the observation that the minority bright islands in Figure 3b are generally smaller than the majority dark islands. To minimize the influence of the substrate step morphology on these number and area ratios, a large number of regions across the film were sampled, leading to a total probed area of over 900 μm2. A ceria film of the same coverage was prepared under the same conditions, with the exception of a slightly higher final oxidation temperature (1150 versus 1050 K). The dark-field LEEM was collected in the same manner; for this film, the number ratio of bright/dark islands was 1:1.9 (±0.5), and their area ratio was also 1:1.9 (±0.4), suggesting that small variations in the preparation procedure may influence the structure of the

Figure 2. Evolution of the ceria film morphology with temperature. (a) A plot of the number of islands in the 4 μm FOV LEEM images versus the oxidation temperature. At 1090 K, the island number density drops off due to ceria island coalescence, as observed in the image sequence in Figure 1a−h. (b) A histogram of the island areas in the inset LEEM image obtained after annealing to 1030 K. (c) A histogram of the island areas in the inset LEEM image obtained after annealing to 1150 K. The change in film morphology after annealing to one with fewer, larger islands as seen in the images is reflected in the increased number of islands with areas > 10000 nm2.

plot of the number of ceria islands versus the final annealing temperature. Above ∼1090 K, the number of islands drops away quickly to approximately half of its initial value when the final annealing temperature is reached. This transition correlates with the changes observed between the frames in Figure 1e and g, where the ceria islands in the heavily stepped areas coalesce. From the plot of temperature versus time (Figure 1i), around this transition point, we observe that there is no significant change in the rate of heating at this point, although the origin of the increased coalescence is not clear. Figure 2 also displays two frames from the annealing sequence with associated histograms of island area versus number. Figure 19197



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Figure 3. (a) LEED (S.V. = 35 eV) and (b,c) dark-field (DF) LEEM (S.V. = 30 eV, FOV = 4 μm) obtained from a 1.1 MLE of CeO1.80(111) ultrathin film on Rh(111). The LEED contains reflexes from the Rh(111)(1 × 1) substrate with epitaxial ceria spots at (1.4 × 1.4) relative to these, as well as an O-(2 × 2) overlayer. The DF-LEEM images in (b) and (c) are obtained from the ceria diffracted spots labeled “A” and “B”, respectively. Expanded views of selected ceria islands from (b) and (c) are shown beneath the LEEM images, along with schematic representations of the top ceria trilayer (red: oxygen; yellow: cerium). The island highlighted in yellow allows identification of the areas subsequently monitored during gold deposition (see Figure 6).

final film. These observations can be explained by the breaking of the three-fold symmetry of the Rh(111) substrate, which leads to a preferred orientation of the CeO2(111) islands. The minority rotated ceria islands are therefore due to the presence of stacking faults in the substrate. The difference in island size between the rotated phases (the minority islands tend to be smaller) suggests that the interface between the Rh(111) substrate and the ceria may be different for those islands formed on stacking faults. Rh 3d core-level XPS (see Supporting Information Figure S1) indicates the presence of a Rh oxide species shifted to 0.74 eV higher binding energy (B.E.). The dark-field LEEM images (not shown) obtained from the diffracted beams corresponding to the Rh(111) substrate and the O-(2 × 2) overlayer show the expected brightest contrast from the regions in between the ceria, with the islands imaged as dark patches. CeO2−x(111) Film Characterization. Confirmation that the islands observed in LEEM are composed of ceria is provided by spectroscopic imaging using XPEEM, the results of which are displayed in Figure 4. Figure 4a is an energy-filtered XPEEM image at the maximum of the Rh 3d5/2 peak (307.1 eV B.E.; see Supporting Information Figure S1 for the full Rh 3d XPS spectrum). The islands are characterized by dark features on a bright background due to a screening of the Rh signal through the relatively thick island. A number of the islands have been highlighted in yellow to allow comparison between the various images presented in Figure 4. Figure 4b is an energyfiltered XPEEM image taken at the maximum of the O 1s peak (532 eV B.E.). The islands appear brightest, but due to the presence of the oxygen overlayer extending over the Rh(111) substrate, there is a significant contribution across the whole image and therefore a reduced contrast. Figure 4c is a secondary-electron XPEEM image obtained at a photon energy of 902 eV, corresponding to the maximum of the Ce M4 absorption edge. The islands appear bright, their shapes and positions matching perfectly with those in Figure 4a and b. The composite dark-field LEEM image in Figure 4d was obtained by

summing images acquired from two ceria diffracted beams in the LEED pattern, and the positions of the ceria islands coincide exactly with those identified from the XPEEM images in Figure 4. There is a small amount of thermal drift between the images in Figure 4 due to the gradual cooling of the sample after the oxidation step as part of the film growth process, resulting in the slightly shifted positions of the highlighted islands relative to the field of view. The combination of the XPEEM imaging at various core levels confirms the assignment from the dark-field LEEM that the islands are composed of ceria. Gold Nanoparticle Deposition: Coverage Effects. Gold particles deposited at room temperature at submonolayer coverages will be below the resolution limits of LEEM and XPEEM. As a result, STM was used to obtain detailed information on the particle size and nucleation behavior. The results of this are presented in Figure 5 for 0.1 MLE of Au (a− c) and 1.0 MLE (d−f) evaporated onto the CeO2−x(111)/ Rh(111) system at room temperature. This film was prepared under the same conditions as those used in the synchrotron experiments, annealing up to 1150 K in 1 × 10−7 mbar of O2 with a gradual temperature increase over 2 h as for the film shown in Figure 1. The LEED pattern from this film was identical to that shown in Figure 3a. In the overview image for 0.1 MLE of Au on CeO2−x(111)/Rh(111) shown in Figure 5a, the ceria islands are clearly defined with bright contrast and are covered with brighter spots corresponding to the Au clusters. These clusters are evenly distributed across the ceria, with no nucleation at the edges of the islands, consistent with earlier studies for reduced ceria films prepared on Ru(0001).24,27,31,45 A zoomed-in image from the region of Figure 5a highlighted with a blue square is displayed in Figure 5b, showing in more detail the Au clusters on the ceria. The clusters are measured to have a density of 0.06 nm−2, with widths in the range of 1−2 nm (uncorrected for tip-convolution effects). The Au particles have a wide distribution of heights ranging from ∼0.2 up to ∼0.9 nm, corresponding to single Au atoms/dimers and 19198



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the formation of an Au(111)-type top facet. Although a precise characterization of their shapes is not possible due to tipconvolution effects, the cluster density suggests that, on average, they contain ∼100 atoms, consistent with their heights, derived assuming a pyramidal cluster shape. The 2D growth of Au on the substrate in the gaps between the ceria islands is clear, with a near-complete layer of Au observed in Figure 5f. The diffuse Rh(111)-(2 × 2)-O overlayer spots in the LEED pattern (Figure 3a) are no longer present after the deposition of 1 MLE of Au. Although the individual Au particles shown in Figure 5 cannot be resolved in XPEEM or LEEM, spectroscopic information can still be obtained using the SPELEEM instrument, as shown in Figure 6. Figure 6a displays the Au 4f region for 0.1 MLE of Au using the energy-filtered XPEEM mode. Here, a representative image obtained at the maximum of the Au 4f7/2 peak is displayed next to XPS Au 4f spectra obtained by selecting areas of the Rh substrate (blue) and the ceria islands (red). There is a small shift to higher B.E. (+0.10 ± 0.02 eV) for the gold particles located on the ceria (Au 4f7/2 B.E. = 83.65 eV) relative to those on the Rh substrate (Au 4f7/2 B.E. = 83.55 eV). Upon increasing the gold coverage to 1.0 MLE, this shift decreases to +0.05 (±0.02) eV, as can be seen in Figure 6b. The trend in the decreased shift with increasing gold particle size is consistent with the literature for ceria films prepared on other substrates.21,22,24 It is not possible, however, to attribute the B.E. shift of the ceria-supported gold to either an initial or final-state effect on the basis of this spectroscopic data alone. The energy-filtered XPEEM images displayed in Figure 6 are obtained at binding energies corresponding to the maximum of the Au 4f7/2 peak (∼83.6 eV). The ceria islands appear as dark patches on a bright background (one distinctively shaped island is highlighted in yellow and can also be observed in the LEEM images in Figure 3). This contrast is reflected in the spectra, where the intensity of the Au 4f for the ceria-supported gold is approximately 70% of that of the gold on the substrate. This is a result of the different growth modes for the Au on the ceria and on the substrate, as evidenced in Figure 5. The 3D Au clusters on the ceria are larger and more sparsely distributed than the 2D Au on the Rh(111) substrate, leading to greater blocking of the XPS signal due to the limited inelastic mean free path (IMFP) of the photoelectrons (∼0.5 nm for a K.E. of 120 eV).47 An advantage of the high lateral resolution of the XPEEM instrument is illustrated in this case as it allows us to clearly distinguish small B.E. shifts between gold species within the same sample, in contrast with area-averaged XPS. Islands with different rotations of their ceria lattices, as identified from the DF-LEEM in Figure 3 (see the yellow highlighted island), do not display any variations between their Au 4f spectra. The B.E. of the Au 4f on the Rh(111) substrate is 0.45 eV lower than that of bulk metallic Au (4f7/2 at 83.55 versus 84.0 eV), a shift attributed to the 2D growth mode of Au(111) on Rh(111) that is also demonstrated in the STM images in Figure 5.46 A sensitive measure of small changes in the cerium oxidation state is provided by resonant XPS of the valence band (V.B.) region, which contains contributions from Ce 4f.33,42,48 Figure 7 presents V.B. spectra obtained via the μ-XPS mode of the SPELEEM microscope for the as-prepared film (a), the film with 0.1 MLE of Au (b), and the film with 1 MLE of Au (c). In this mode of operation, the dispersive plane of the microscope energy analyzer is imaged, and a region of the film with ∼2 μm diameter is sampled, which will contain both ceria islands and

Figure 4. XPEEM and LEEM identification (FOV = 4 μm) of the CeO1.80(111) islands on Rh(111). A number of islands have been highlighted in each image as a visual aid. (a) Energy-filtered XPEEM image at the Rh 3d5/2 peak (hν = 420 eV, B.E. = 307.1 eV); the ceria islands appear as dark patches. (b) Energy-filtered XPEEM image at the O 1s peak (hν = 650 eV, B.E. = 532 eV); the ceria appears bright. (c) Secondary-electron (S.V. = 4 eV) XPEEM image at the Ce M4 edge (hν = 902 eV); the ceria appears bright. (d) A composite darkfield LEEM image (S.V. = 30 eV) from the ceria diffracted spots; the islands appear bright.

clusters containing up to ∼40 Au atoms (assuming pyramidal cluster shapes).45 For the structure of 0.1 MLE of Au on the substrate in between the ceria islands, we turn to Figure 5c, which shows the area in Figure 5a bounded with a black square. The structure of the Au is more difficult to resolve here, although it appears that in preference to forming 3D particles, the gold is present as isolated single atoms and 2D islands with heights of ∼0.3 nm that cover ∼10% of the substrate surface area. This 2D growth is consistent with previous investigations of Au deposition on the bare Rh(111) surface.46 It therefore appears that the presence of the Rh(111)-(2 × 2)-O overlayer in this case does not significantly alter the Au adsorption behavior. Figure 5d shows an overview image as the gold coverage is increased to 1 MLE. On the surface of the ceria, the clusters are still readily visible (Figure 5e), and there are still gaps between them. The density of clusters is measured at 0.07 nm−2, only a small increase from that measured for 0.1 MLE of Au (0.06 nm−2), suggesting that there is no significant extra nucleation with increasing Au coverage, merely an increase in the cluster size. The clusters in Figure 5e are measured to be 1−1.5 nm tall (∼4−6 atomic layers), with widths of 3−5 nm, although the lateral tip convolution will significantly enhance this latter value. Some of the clusters appear to have more clearly defined quasi-hexagonal/triangular shapes, which are consistent with 19199



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Figure 5. STM images (Vs = +2.1 V, It = 0.05 nA) of Au deposited at room temperature onto a CeO2−x(111) film supported on Rh(111) prepared under the same conditions as the films for the synchrotron experiments, with a final oxidation temperature of 1150 K. (a) Overview image of 0.1 MLE of Au on the ceria film; ceria islands appear bright and are decorated with small, bright Au clusters evenly distributed across their tops (160 × 80 nm2). (b) The 0.1 MLE of Au clusters on the surface of the ceria islands (blue square) (23 × 23 nm2). (c) The 0.1 MLE of Au clusters (2D) on the Rh(111) surface in between the islands (black square) (23 × 23 nm2). (d) Overview image of 1 MLE of Au on the ceria film (160 × 80 nm2). (e) The 1 MLE of Au clusters on the surface of the ceria island (blue square) (23 × 23 nm2). (f) The 1 MLE of Au on the Rh(111) surface in between the ceria islands (black square) demonstrating the 2D growth on the substrate (23 × 23 nm2).

Figure 6. Au 4f XPS spectra and energy-filtered XPEEM images from (a) 0.1 and (b) 1.0 MLE of Au deposited on 1.1 MLE of CeO1.80(111)/ Rh(111) (hν =200 eV, FOV = 6 μm). Red spectra are obtained by selecting regions exclusively from the ceria islands (dark patches in the XPEEM images), and blue spectra are from the substrate in between the islands. A distinctively shaped ceria island has been highlighted in yellow. This island is also seen in the LEEM images in Figure 3

areas of the Rh(111) substrate. This leads to the presence of a clear Fermi edge in all of the V.B. spectra, resulting from the

metallic substrate. For each scenario, a spectrum is obtained at a photon energy corresponding to off-resonance (hν =115 eV, 19200



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Figure 7. Resonant XPS of the CeO1.80(111) V.B. with increasing Au coverage (a−c) and associated difference spectra (d−f). Spectra are obtained at photon energies corresponding to on-resonance for Ce4+ (red), on-resonance for Ce3+ (blue), and off-resonance (black). Difference spectra are obtained by subtracting the off-resonance curves from the Ce3+ and Ce4+ on-resonance curves. The calculated RER and stoichiometries are also displayed for the respective films.

black), on-resonance for Ce4+ (hν =122.5 eV, red), and onresonance for Ce3+ (hν =120.8 eV, blue). The methodology for identification of the resonant photon energies for the Ce3+ and Ce4+ components is described in Figure S3 of the Supporting Information. For the Ce3+ peak at 1.5 eV B.E., we observe an enhancement at all photon energies above 120.8 eV, an effect that we attribute to the heavily reduced nature of this ceria film. The spectra have been normalized to the point at a B.E. of 2.8 eV that is independent of photon energy. The difference spectra calculated by subtracting the off-resonance curve from the respective on-resonance curves are presented in Figure 7d− f along with the resonant enhancement ratios (RERs). From the measured RER for each Au coverage in our experiment, the stoichiometry is estimated (assuming a proportionality constant of 1) as follows: (d) as-prepared film, CeO1.80; (e) 0.1 MLE of Au, CeO1.77; (f) 1.0 MLE of Au, CeO1.75. It is important to note that due to difficulties in normalizing the V.B. spectra and obtaining the difference spectra, there is a relatively large error in these stoichiometries, estimated as ±0.02. There are significant changes in the appearance of the V.B. upon increasing Au coverage. The peak at 6 eV B.E. is observed to become progressively sharper in Figure 7a−c. On the as-prepared film (Figure 7a), this peak originates mainly from O 2p states,28 and as the Au coverage is increased, it tends toward the appearance of the V.B. of Au, which has a sharp peak at 6 eV B.E.49 The increasing Au coverage is also reflected in the changes to the V.B. at a B.E. of ∼4 eV, where a broad, rounded feature is observed to develop

(Figure 7c, black curve). In their resonant photoemission study of Au adsorption on CeO2(111) ultrathin films on Cu(111),28 Škoda et al. observe reduction of the ceria upon increasing gold coverage up to 3 MLE of Au, possibly explained by the formation of cationic gold species after charge transfer to the ceria film. This trend of increased reduction of the ceria with increased gold coverage is also observed in our data. The detection of significant concentrations of Ce3+ using the resonant XPS contrasts with the results from Ce M5 XAS. Spectra obtained from the clean and Au-covered surfaces (Figure S2, Supporting Information) are all consistent with only Ce4+ being present. This can be rationalized by considering the high sensitivity of resonant V.B. spectroscopy to the Ce oxidation state and highlights the strength of the technique in the investigation of small changes in oxidation state within the top few atomic layers. XAS probes deeper into the film compared with the V.B. XPS measurements due to the longer IMFP, suggesting that the majority of the cerium in the bulk of the ceria islands is in the Ce4+ oxidation state, with only the surface layers having a significant Ce3+ concentration. To examine any lateral variations in oxidation state across the ceria islands, the approach pioneered in our study of ceria films on Re(0001)33 was repeated; high-resolution XPEEM was carried out using a photon energy corresponding to Ce3+ onresonance, with energy-filtered images obtained at 1.2 and 3.5 eV binding energies, corresponding to the locations of the resonantly enhanced Ce3+ and Ce4+4f contributions to the V.B. These images were then divided, and any variation in the 19201



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Figure 8. XPEEM/LEEM and XPS of 0.1 MLE of Au on CeO1.67(111)/Rh(111) showing the effects of beam-induced reduction of the ceria: (a−c) obtained in a background of 5 × 10−7 mbar of O2 and (d−f) obtained in UHV. (a) Energy-filtered XPEEM at a B.E. of 83.5 eV (Au 4f7/2 maximum on the Rh(111) substrate). (b) Dark-field LEEM (S.V. = 16 eV) from the ceria showing the film structure and island locations. (c) Au 4f7/2 XPS derived from energy-filtered XPEEM; the spectra obtained from the substrate and the ceria islands are shown in blue and red, respectively. (d) Energy-filtered XPEEM at a B.E. of 84.7 eV. (e) Energy-filtered XPEEM at a B.E. of 83.3 eV. (f) Au 4f7/2 XPS derived from energy-filtered XPEEM; the spectra obtained from the substrate and the ceria islands are shown in blue and red, respectively. The largest ceria islands have been highlighted in green in all of the images as a visual aid (hν =200 eV, FOV = 2 μm).

contrast should therefore correlate with a change in the Ce3+/ Ce4+ ratio. No such lateral variation was observed for any of the films in this study, either before or after gold deposition. Gold Nanoparticle Deposition: Influence of Ceria Oxidation State. In order to investigate the influence of the ceria oxidation state on the behavior of the Au nanoparticles, a more reduced ceria film was prepared using a shorter oxidation process in the film growth procedure. This film displayed a very similar structure in LEED and LEEM to the one used for the previous experiments (as in Figure 3). The results of resonant XPS of the V.B. are shown in the Supporting Information Figure S4. The RER is measured to be 1.9, corresponding to 66% Ce 3+ and therefore an overall stoichiometry of CeO1.67±0.02. Figure 8 shows the results of evaporating 0.1 MLE of Au onto the more reduced ceria film. Initially, there is very similar behavior to that of the more oxidized film examined in the previous section; a comparison of the XPEEM images (Au 4f7/2) in Figure 8a and Figure 5a again shows a larger signal from Au on the Rh(111) substrate compared with Au on the ceria due to blocking from the 3D clusters present on the islands. The locations and identity of the ceria islands are confirmed by the dark-field LEEM shown in Figure 8b; the largest islands, which are also seen in the XPEEM images in Figure 8, are highlighted. The associated XPS spectra presented in Figure 8c show maxima for Au 4f7/2 at binding energies of 83.73 and 83.55 eV on the ceria and substrate, respectively, a shift of +0.18 eV. This is a larger shift than what was observed for the more oxidized ceria film in Figure 5a (+0.10 eV), a

result consistent with that observed by Weststrate et al. in their study of defective ceria films on Ru(0001).22 More reduced films, with their higher defect densities, have been shown by STM to stabilize a higher density of smaller Au clusters relative to more oxidized films.18,24,31 Removing the background oxygen (5 × 10−7 mbar) from the microscope chamber during measurement leads to immediate reduction of the top few layers of the ceria.43 Figure 8d−f presents the results of this on the ceria-supported gold nanoparticles. There are dramatic changes to the Au 4f7/2 XPS spectrum (Figure 8f) for the gold adsorbed on the ceria islands (red curves); three species can be identified at binding energies of 83.55 (Au-i), 84.36 (Au-ii), and 85.17 (Au-iii). The Au 4f7/2 XPS spectrum from gold located on the Rh(111) substrate is unchanged (blue), with a B.E. of 83.55 eV. Energyfiltered XPEEM images obtained at binding energies of 84.7 and 83.3 eV are shown in Figure 8d and e, respectively. As reflected in the spectra in Figure 8f, the highest intensity at the B.E. at which the image in Figure 8d is obtained at 84.7 eV is from gold supported on the ceria islands. As a result, the ceria islands appear as bright shapes on a dark background. Figure 8e displays the opposite contrast; the ceria islands are dark, as in Figure 8a, due to the greater contribution from the gold on the substrate at this B.E. (83.3 eV). Large shifts to higher binding energies have been attributed to the presence of positively charged gold species,24 but under the strongly reducing conditions that we encounter during photon irradiation, this is not a likely occurrence. We propose therefore that the intense reduction of the ceria support by the X-rays leads to a 19202



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very reactive surface that spontaneously forms Ce−Au alloys at the surface of the film. The observed B.E. shifts in Figure 8f are consistent with those observed for Au−Al alloy systems where the shifts to higher binding energies for the Au 4f are dependent on the Au/Al ratio within the alloy.50,51 To investigate the reversibility of the photon-induced changes, O2 was reintroduced into the chamber up to a pressure of 5 × 10−7 mbar while obtaining energy-filtered XPEEM images at the Au 4f7/2 peak corresponding to oxidized Au (∼85 eV B.E.) (see Supporting Information Figure S5). The contrast in the energy-filtered XPEEM images reverted to that of Figure 8a from that observed in Figure 8d. The XPS spectrum returned to that shown in Figure 8c as the higher B.E. peaks due to the Au−Ce alloys disappeared, indicating full reversal of the beaminduced effects at room temperature.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +44 (0)20 7679 7979. Fax: +44 (0)20 7679 0595. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Dr. Chi Pang for helpful discussions. This work was supported by the European Research Council Advanced Grant ENERGYSURF (G.T.), the EU COST Action CM1104, the EPSRC (U.K.), and the Royal Society (U.K.).

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CONCLUSIONS A spectromicroscopic study of the preparation of low-coverage CeO2−x(111) films on Rh(111) and their interaction with gold nanoparticles was carried out using high-resolution low-energy electron microscopy (LEEM), X-ray photoemission electron microscopy (XPEEM), and scanning tunneling miscroscopy (STM). The island structure was found to be highly dependent on the final annealing temperature, and dark-field imaging indicated the presence of an anisotropic distribution of islands having 60° rotations with respect to each other. The assignment of these islands as ceria was confirmed with energy-filtered and secondary-electron XPEEM, and their oxidation state was monitored with resonant photoemission of the Ce 4f contribution to the V.B. The morphology and nucleation of gold nanoparticles deposited onto the films at room temperature was probed with STM for coverages of 0.1 and 1 MLE of Au. At a coverage of 0.1 MLE, gold was present as single atoms and clusters containing up to a few tens of atoms (widths of 1−2 nm, heights up to 0.9 nm). These displayed binding energy shifts of the Au 4f of +0.10 eV with respect to Au on the Rh(111) substrate, as measured with energy-selected XPEEM. This energy shift was reduced to +0.05 eV when the Au coverage was increased to 1.0 MLE (particle widths of ∼5 nm, heights of 1−1.5 nm). Adsorption also affected the CeO2−x support. As the Au coverage was increased, the ceria was reduced, CeO1.80 (as-prepared) → CeO1.77 (0.1 MLE of Au) → CeO1.75 (1.0 MLE of Au), an observation consistent with charge transfer from the gold particles to the oxide support. The Au 4f binding energy was also dependent on the initial ceria oxidation state, with morereduced ceria displaying larger shifts to higher binding energy (CeO1.80: +0.1 eV; CeO1.67: +0.18 eV) due to the stabilization of smaller gold clusters on the more reduced ceria surface. Gold nanoparticle adsorption on heavily reduced ceria, generated by removal of the background oxygen used to mitigate any X-ray photon induced reduction, resulted in the identification of a number of additional species with even higher Au 4f binding energies, which are attributed to a Au−Ce alloy phase.

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ASSOCIATED CONTENT

S Supporting Information *

Rh 3d XPS spectrum, Ce M5 XAS spectra, waterfall plot showing the methodology for finding the on- and off-resonance photon energies, resonant XPS and difference spectra, plot of the reversal of X-ray damage with the reintroduction of O2, and a bright-field LEEM movie. This material is available free of charge via the Internet at http://pubs.acs.org. 19203



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Gold Nanoparticles Supported on Ceria - American Chemical Society

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