Oxidation-Induced Dispersion of Gold on Ru(0001): A Scanning

Article pubs.acs.org/JPCC

Oxidation-Induced Dispersion of Gold on Ru(0001): A Scanning Tunneling Microscopy Study Daniel Langsdorf,† Benjamin Herd,† Yunbin He,†,‡ and Herbert Over*,† †

Department of Physical Chemistry, Justus-Liebig-University, Heinrich-Buff-Ring 58, D-35392 Gießen, Germany Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials; Key Laboratory of Green Preparation and Application for Functional Materials, Ministry of Education; Faculty of Materials Science & Engineering, Hubei University, Wuhan 430062, China

‡

S Supporting Information *

ABSTRACT: With scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) we studied the redox properties of Au islands supported on Ru(0001) as a function of the island thickness. Both the size and the height of Au islands on Ru(0001) can be controlled by the density of the oxygen precoverage on ruthenium and the sample temperature during the deposition of gold. The oxidation of the Au islands at 300 K was accomplished by exposing atomic oxygen produced from a thermal gas cracker. Regardless of the lateral size of the three monolayer (ML) thick Au islands, the oxidation leads to a fragmentation into a number of small particles (3−5 nm) whose arrangement reflects the shape of the former intact Au islands. This oxygen-induced dispersion of Au on Ru(0001) is explained by a shoveling process. Quite in contrast, no fragmentation of the 4−5 ML thick Au islands into smaller entities is observed. Rather, the entire Au island transforms into one big particle. From Au 4f core level spectroscopy we provide evidence that the nanoparticles consist of Au oxide and metallic Au. The Au oxide/Au particles can be reduced by thermal annealing to 670 K under vacuum or by chemical reduction via CO exposure at 670 K, forming again extended Au islands. However, reduction of Au oxide/Au metal particles by CO exposure at room temperature retains the high dispersion of the prior formed nanoparticles. intensively addressed the oxidation behavior of Au.4,17−22 The oxidation of gold has shown to lead to a higher number of undercoordinated gold atoms that are considered to be responsible for the high catalytic activity.19,23−31 Even if gold is not in a nanoparticular form, Au can be activated by oxidation, thus inducing a transient activity of the surface. For instance, Friend and co-workers17,25,26 studied the oxidation of the Au(111) single crystal surface, employing atomic oxygen as the oxidizing agent since exposure to molecular oxygen is hardly able to oxidize bulk gold.29,32−35 Although an enhanced O2 dissociation probability has been proposed, it is not clear whether the oxidized gold surface can sustain a catalytic cycle in oxidation reactions (e.g., CO oxidation) or the surface is simply reduced by the reducing agent. Besides the oxidation of gold single crystal surfaces, other groups have studied the redox behavior and activity of ultrathin Au films on dissimilar metal substrates.10,36,37 In this paper we will focus on the redox properties of Au islands supported on oxygen precovered Ru(0001) surfaces. Au deposition on Ru(0001) and O precovered Ru(0001) has previously been studied by various methods including scanning

1. INTRODUCTION Bulk gold is the most noble metal in the periodic table that is associated with limited reactivity and hence a poor catalyst material.1 However, as soon as gold is nanoparticular, such as Au particles with a typical size of about 2 nm, the catalytic activity of gold is extraordinarily high.2 Beyond 5 nm the activity of Au particles decreases dramatically. Gold particles need to be supported on a carrier to stabilize the dispersion,3 and it has been shown that Au particles supported on reducible oxides reveal a significantly higher activity than Au particles supported on nonreducible oxide supports.4−8 For the simple CO oxidation reaction, which is prototypical in surface chemistry, Au supported on reducible TiO2(110) has shown to be very active when the size of the Au particles is around 2 nm.9 These Au particles are usually anchored at oxygen vacancies on TiO2(110). For ultrathin TiO2(110) films grown on Mo(112) a Au double layer structure is formed and has shown to be active for the CO oxidation.10 Chen et al. concluded that the morphology of the gold nanostructure is the main reason for the high reactivity and not simply the particle size.11 Besides the size and morphology of the Au particles, the presence of undercoordinated Au atoms and most notably the redox properties of Au particles are important for the observed catalytic activity.12−16 Several research groups have therefore © 2015 American Chemical Society

Received: April 14, 2015 Revised: June 16, 2015 Published: June 16, 2015 16046

DOI: 10.1021/acs.jpcc.5b03583 J. Phys. Chem. C 2015, 119, 16046−16057

Article

The Journal of Physical Chemistry C tunneling microscopy (STM),38 X-ray photoelectron spectroscopy (XPS), 39 and thermal desorption spectroscopy (TDS).37,39 Similar results have been reported for Ru(10− 10) by Lambert et al.40 However, the oxidation of such supported Au islands has not been studied systematically. We show with STM and XPS that the oxidation of Au islands on Ru(0001) depends critically on the thickness of the Au islands but not on their lateral size. Most notably, the oxidation of 2−3 ML thick islands leads to fragmentation into particles that consist of gold oxide and metallic Au. Quite in contrast, thicker Au islands of 4−5 ML are less prone to fragment. The entire Au island is transformed into one big particle that also consists of Au oxide and metallic Au. The oxidation of thicker Au islands of about 100 ML leads only to a roughening of the surface similar to that observed for bulk Au(111).

and oxygen exposure, using atomic oxygen as the oxidizing agent. In order to learn more about the redox behavior of Au on Ru(0001), we chemically and thermally reduced the previously oxidized Au islands. 3.1. Au Deposition on Oxygen-Precovered Ru(0001). During Au deposition the Ru(0001) sample was kept at 700 K, while the O precoverage was either 0.5 or 1 ML. Depositing 0.5 ML of Au on a (2 × 1)O precovered Ru(0001) surface leads to large hexagonal Au islands (lateral size ranging from 15 to 100 nm), which are preferentially located at the step edges of Ru(0001) (cf. Figure 1a). The Au islands are oriented along the

2. EXPERIMENTAL DETAILS The experiments were conducted in a custom-built threechamber ultrahigh-vacuum (UHV) system; details have been described elsewhere.41 Briefly, the main chamber is furnished with a mass spectrometer and a dual X-ray source together with a hemispherical analyzer (PSP Vacuum Technology) to perform X-ray photoelectron spectroscopy (XPS) experiments. In addition, the analysis chamber contains an electron beam evaporator (EMF 3, Omicron). The sample can be transferred from the analysis chamber to the STM chamber (VT-STM, Omicron). For the STM experiments we used homemade tungsten tips. All STM images presented in this paper were taken in the constant current mode at 298 K. Typical sample voltage and tunneling current used for scanning were 0.5−4.5 V and 0.8−1.5 nA, respectively. The Ru(0001) sample was cleaned by Ar ion sputtering for 10 min (p(Ar) = 10−5 mbar, 1.5 kV, 20 mA) and roasting the sample at 950 K in 10−7 mbar of oxygen in order to remove carbon contamination segregating from the bulk Ru. The highest sample temperature achieved with the present sample holder is limited to 1100 K. The sample temperature was measured with an infrared (IR) pyrometer, which was precalibrated with a K type thermocouple. Gold was deposited on oxygen precovered Ru(0001) by physical vapor deposition using a well-outgassed electron beam evaporator. With STM and XPS the deposition rate of gold was calibrated to be approximately one monolayer (ML) per 20 min. The Au-covered Ru(0001) sample was oxidized by exposing atomic oxygen at room temperature from a thermal gas cracker (Oxford Applied Research: TC50 - Universal Thermal Cracker). The quoted exposures of atomic oxygen O are determined by the actual exposure of molecular oxygen times the cracking efficiency determined by mass spectrometry. For an O2 pressure of 2 × 10−8 mbar the cracking efficiency is about 30%, so that the partial pressure of atomic oxygen O is estimated to be 0.6 × 10−8 mbar. The given exposure of atomic oxygen O is based on this partial pressure estimate times the exposure time. One langmuir corresponds to an exposure of 1.33 × 10−6 mbar·s.

Figure 1. STM images for the deposition of 0.5 ML of gold on (a) Ru(0001)-(2 × 1)O at 700 K and (b) Ru(0001)-(1 × 1)O at 700 K (a, b: 250 nm × 250 nm). The corresponding height distributions of the hexagonal gold islands are presented in (c) and (d), respectively. The height distributions in (e) and (f) show the influence of the applied deposition temperature on the resulting gold island thickness.

(111) direction that is also the surface orientation of Au with the lowest surface energy.42,43 The height of the Au islands is 3 ML with a quite narrow thickness distribution (cf. Figure 1c). For comparison, Au has also been deposited on O-free Ru(0001) (cf. Figure 2), which leads to a wetting gold film on the Ru(0001) surface. Based on the well-known Young equation, the epitaxial growth behavior can be described by the interface energy and the surface free energies of the involved species, if the system is assumed to be in thermodynamic equilibrium. Generally, if the sum of the interface energy and the surface free energy of the deposited material is higher than the surface free energy of the substrate, a three-dimensional (Volmer−Weber-like) growth can be expected.

3. RESULTS AND DISCUSSION All presented experiments employ a combination of STM and XPS investigations. We started with a comparing study of Au deposition on Ru(0001), depending on its oxygen precoverage from chemisorbed O up to RuO2(110). Next, we investigated the oxidation of Au/Ru(0001) depending on the Au coverage 16047


Article

The Journal of Physical Chemistry C

The height of the Au islands can also be controlled by varying the substrate temperature during Au deposition, while keeping the O precoverage constant. From various STM images, the height distributions for Au deposition on Ru(0001)-(2 × 1)O at 670, 700, and 800 K are derived and compiled in Figure 1c,e,f. If Au deposition on Ru(0001)-(2 × 1)O takes place at 670 K, most of the islands exhibit a height of 6.5 Å (corresponding to 3 ML), while some are around 4.3 Å (2 ML). Increasing the deposition temperature just by 30 to 700 K leads to a significant narrowing of height distribution around 3 ML. Practically no island with 2 ML is observed with STM. When increasing the deposition temperature to 800 K, the height distribution becomes broader with the maximum at 8.6 Å, i.e., 4 ML. For the case of 700 K we extracted also from STM the height distribution for 0.5 ML Au deposited on Ru(0001)-(1 × 1)O (cf. Figure 1d). Clearly, a trimodal distribution with maxima at 8.2, 11.8, and 14 Å is apparent. Altogether, the influence of the oxygen precoverage and the deposition temperature on the growth of gold on Ru(0001) has been elaborated. The height distribution can be more easily adjusted by the O precoverage than by the deposition temperature. 3.2. Au Film Growth on O-Free Ru(0001). The STM images of the growth of 2 ML Au on the oxygen-free Ru(0001) surface are depicted in Figure 2. The Au forms a layer, which is almost completely covering the Ru(0001) surface. The wetting behavior is reconciled with the surface energy of Au (Au(111): 1.3 J/m2) which is significantly smaller than that of Ru(0001) (3.9 J/m2).39,42,51 In a previous STM study47 it was shown that Au deposition at room temperature and subsequent annealing to 650 K results in a flat Au film wetting the Ru(0001) surface. Beyond two layers additional Au forms three-dimensional islands on the second Au layer (Stranski−Krastanov growth). The internal structure of these wetting Au films has been shown to be herringbone-like48 as known from bulk Au(111).52 The Au film in Figure 2 reveals defects which are likely to be strain induced. The holes in the Au film are due to residual oxygen on the Ru(0001) (cf. Figure 2b, blue circle). In these regions the residual oxygen is compressed into a dense O overlayer. 3.3. Au Deposition of RuO2(110). Wu and Hrbek37 studied Au deposition on the oxidized Ru(0001) surface with thermal desorption spectroscopy. They found out that this heterostructure was more active in the CO oxidation than pure RuO2(110), so that they suggested a synergistic effect between Au and RuO2. Structural details of this system have not been reported in the literature but are shown in Figure 3. The RuO2(110) film was prepared by exposing the Ru(0001) surface to 1 × 10−4 mbar of molecular oxygen at a sample temperature of 720 K. As shown previously, this preparation leads to a RuO2(110) film which completely covers the Ru(0001) surface with an averaged film thickness of 2−5 ML.53 On this surface 0.5 ML of gold was deposited at 700 K. From STM (cf. Figure 3) three differently shaped Au islands are recognizable. These are cuboid islands, hexagonal islands, and flat islands. The cuboid islands are preferentially formed on the flat regions of RuO2(110) (cf. Figure 3b) with a broad distribution of apparent height of more than 22 Å (cf. Figure 3e, red distribution). From their cuboid shape we can assign these islands to gold with (100) orientation, which has a slightly higher surface energy than Au(111) (Au(111): 1.3 J/m2; Au(100): 1.6 J/m2).42 Since gold has a higher surface energy

Figure 2. Two ML of Au are deposited at 700 K on an oxygen-free Ru(0001) surface. (a) The large-scale STM image shows the formation of a wetting gold layer on Ru(0001) (500 nm × 500 nm). (b) STM image with smaller scan area (300 nm × 300 nm) evidently reveals voids in the film (blue ovals) due to residual oxygen on the Ru(0001) substrate.

Vice versa, if the sum of interface energy and surface free energy of the deposited material is lower than the surface free energy of the substrate, a two-dimensional growth (Frank−van der Merwe or Stranski−Krastanov-like growth) would be preferred for the first several layers. With this simple model, the growth behavior of gold on oxygen free and oxygen precovered ruthenium surfaces can be explained: The two-dimensional growth of a gold film on bare Ru(0001) evidently displays that the sum of the surface free energy of gold and the interface energy is lower than the surface free energy of ruthenium. In the case of an oxygen overlayer on Ru(0001) the surface free energy of the substrate changes. Now the sum of the interface energy and the surface free energy of gold is higher than the surface free energy of the oxygen covered Ru(0001) surface, which leads to a three-dimensional growth of gold islands. The here observed growth of gold on bare and oxygen covered Ru(0001) surfaces is fully consistent with previous STM studies performed by Hwang and co-workers.44−48 As Malik and Hrbek proposed first39 and Hwang et al.44 corroborated with STM, Au deposition induces a compression of the (2 × 1)O layer on Ru(0001) into denser O overlayers such as the (2 × 2)3O or (1 × 1)O followed by the formation of Au islands on the oxygen-free Ru(0001) surface regions. The reason behind this compression is determined by thermodynamics. Au can bind much stronger to the bare Ru(0001) surface than to oxygen precovered Ru(0001) so that the energy penalty, due to compression of the oxygen overlayer by about 0.5 eV per O atom, is overcompensated.49 The maximum compression of the O overlayer is limited to the formation of a (1 × 1)O overlayer which is considered to be the saturation O overlayer on Ru(0001).50 Therefore, we deposited 0.5 ML Au also on a nominally (1 × 1)O precovered Ru(0001) surface (cf. Figure 1b). The resulting hexagonal Au islands reveal a quite narrow lateral size distribution of about 10 nm and a thickness of 5−7 ML (cf. Figure 1d). From this STM image we infer that the (1 × 1)O was not fully developed and that the size of the Au islands is defined by the bare Ru regions which were formed by locally compressing the (2 × 2)3O to (1 × 1)O. XPS indicates a constant O 1s signal during deposition of 0.5 ML Au (cf. Figure S1). This may imply that no oxygen is buried under the Au islands, and the oxygen overlayer on the Au-free regions of Ru(0001) is only compressed. From these experiments we conclude that the height of the Au islands can be readily controlled by the O precoverage on Ru(0001). 16048


Article


oxide (2-fold symmetry) the lattice misfit is by far too large (Au(100): 2.88 Å × 2.88 Å; RuO2(110): 6.38 Å × 3.11 Å).55,56 However, if the bridging O atoms of RuO2(110) are removed, Au can bind to Ru atoms in on-top and in bridge positions, forming a quasi-unit cell of 3.19 Å × 3.11 Å. This lattice misfit induces a horizontal strain that limits the lateral expansion of the gold island on the oxide surface. The flat and slender islands are mostly found on the flat regions of RuO2(110). Their apparent thickness ranges from 3.5 to 10 Å. These islands look somehow similar to the Au double layers on TiO2(110),10 but their chemical nature is unclear. The hexagonal Au islands are mostly found at the intersections of rotational domains of RuO2(110);57 the rotational RuO2(110) domains are due to the 3-fold symmetry of the supporting substrate and the 2-fold symmetry of the coating oxide. The hexagonal islands reveal a broad height distribution ranging from 12 to 27 Å. Very likely the orientation of these Au islands is (111). Evidently, the symmetry of the RuO2(110) substrate at these intersections favors the formation of gold islands with a similar symmetry. Therefore, the energy balance of surface energy, interface energy, and strain energy is assumed to determine the crystallographic orientation of gold on the RuO2(110) surface. In the intersection regions of RuO2(110) gold islands with (111) orientation are formed, while on flat regions of RuO2(110) gold islands with (100) orientation are favored. 3.4. Oxidation of Au Islands on Ru(0001)-(2 × 1)O. In a next set of experiments we studied the oxidation behavior of Au islands on oxygen precovered Ru(0001) by exposing atomic oxygen. The Au islands were prepared by depositing 0.5 ML of Au at 670 K on a previously prepared Ru(0001)-(2 × 1)O surface (cf. Figure 4a). Exposing the 0.5 ML Au−Ru(0001)-(2 × 1)O surface to 40 langmuirs of atomic oxygen at room temperature leads to dramatic changes in the morphology of the Au islands. The Au islands break up into a number of small particles whose arrangement nicely reflects the former Au island shape (cf. Figure 4b). The height of these particles is about 18 Å (cf. Figure 4d), i.e., much larger than the height of the original Au islands (3 ML corresponds to 6.5 Å, cf. Figure 4c). Accompanying XPS experiments (cf. Figure 4e) indicate that these particles consist of Au oxide (very likely Au2O3 according to the literature27,29,58−62) and metallic Au. Additionally, an inhomogeneous broadening of the Au 4f signals is observable. The width of the gold oxide signals is broader than the width of the corresponding metallic gold signals. This broadening of the Au 4f peak is induced by several effects. At first, due to the fragmentation of the gold island into smaller oxidized clusters final state effects may occur, thus broadening the peaks. This size-dependent shift of the Au 4f signals has been reported in the literature.61,63,64 Additionally the observed broadening may be rationalized by different gold−oxygen species formed during the oxidation and fragmentation process. Besides the formed Au(III) species also metallic gold atoms directly bound to the oxide as well as gold atoms with chemisorbed oxygen contribute to the broader XPS signal. A similar interpretation was proposed by Gottfried et al., who also observed an inhomogeneous broadening of the Au 4f signal upon the oxidation of small gold nanoparticles.22 Since the surface energy of Au oxide is lower than that of metallic Au,59 the occurrence of both species in the formed nanoparticles may indicate a core−shell structure, with the Au oxide shell covering the metallic Au core. Such a core−shell nanoparticle structure is also consistent with current interpretations in the literature.3,60

Figure 3. STM images of 0.5 ML Au deposited at 700 K on a previously prepared ultrathin RuO2(110) film: (a) Three kinds of islands on RuO2(110) are discernible, i.e., flat and slender islands (green), cuboid-like (red), and hexagonal islands (blue) (200 nm × 200 nm). (b) The cuboid islands are located on the flat RuO2(110) regions (60 nm × 60 nm). (c) At the intersection of differently rotated domains of RuO2(110) the hexagonal gold islands are visible (70 nm × 70 nm). (d) From the three different island types the flat and slender islands are found to be the minority on RuO2(110) (50 nm × 50 nm). The height distributions of these islands are summarized in (e).

than RuO2(110) (0.7 J/m2),54 its tendency to grow threedimensional on the oxide substrate is reasonable. However, to describe the formation and the crystallographic orientation of the gold islands the interface energy and also the occurring strain energy have to be accounted for. Au in (100) orientation forms laterally small but high islands. Evidently, the sum of the interface energy and the surface energy of Au(100) are higher than the surface energy of RuO2(110). On the flat regions of RuO2(110) Au grows in (100) and not in (111) orientation due to a better adoption of the substrate symmetry. This would indicate a lower interface energy, which in turn would promote a better covering on the RuO2(110). Although the (100) oriented gold islands have the same symmetry as the underlying 16049


Article


Figure 4. STM images summarizing the oxidation behavior of thin gold islands grown on Ru(0001)-(2 × 1)O (300 nm × 300 nm). (a) 0.5 ML of Au are deposited on Ru(0001)-(2 × 1)O at 670 K, leading to the formation of three layered gold islands as indicated by the line scan (c). The STM image (b) and a line scan (d) after the oxidation of the three layered gold islands by exposing 40 langmuirs of atomic oxygen at 300 K reveals the fragmentation of the gold islands into several significantly thicker gold clusters. (e) Corresponding Au 4f spectra of prepared Au islands (a, red) and oxidized Au islands (b, gray) including a peak deconvolution of the Au 4f signal area into the metallic Au (red) and the Au oxide (light blue) signals to elucidate the oxidation of gold.

In addition, upon atomic O exposure, the Au-free Ru(0001)(2 × 1)O surface regions transform from a (2 × 1)O into a (1 × 1)O. We need to recall that atomic oxygen is able to oxidize the Ru(0001) substrate even at room temperature, starting from corrosion of the step edges.65 Under these mild conditions, i.e., 40 langmuirs of O′ at 300 K, Ru oxide particles with a size of 2−4 nm are formed on the bare Ru(0001) surface. However, careful inspection of the STM images in Figure 5 does not indicate any corrosion of the Ru(0001) step edges or the formation of Ru oxide particles. We presume therefore that only Au islands are oxidized and the step edges of Ru(0001) are less active toward the atomic oxygen than the gold islands. The oxidation of Au islands on Ru(0001) depends critically on the thickness of the Au islands. In Figure 5 we show STM images for the oxidation of double, triple, and quadruple layered Au islands by exposing 40 langmuirs of atomic O at 300 K. The 2 ML thick Au islands are shown to readily fragment into small particles (cf. Figure 5a,b), while 3 ML thick Au islands fragment into a number of particles that are not fully separated yet (cf. Figure 5c,d). The lateral size of these still connected Au oxide particles is 2−6 nm across. Last, the 4 ML Au islands do not break up into Au oxide/Au particles at these low exposures of atomic oxygen (cf. Figure 5e,f). Rather, most of the Au islands transform into a single big oxidized gold particle during oxidation (cf. Figure 5e). Even thicker Au islands do not fragment at all even if very high amounts of atomic oxygen are dosed (not shown). In order to deepen our atomic scale understanding, the oxidation process was studied by STM as a function of the exposure of atomic O (cf. Figure 6). We started from threelayer-thick Au islands prepared on Ru(0001)-(2 × 1)O by

depositing 0.5 ML of Au at 700 K. Most the Au islands are topflatted (cf. Figure 6a). Already an exposure of 5 langmuirs of atomic O modifies the morphology of the Au islands (cf. Figure 6b) in that the topmost Au layer is now corrugated. Exposure of an additional 5 langmuirs of atomic oxygen leads to further structuring of the Au islands (cf. Figure 6c). For 20 langmuirs of atomic oxygen exposure (cf. Figure 6d), the Au islands start to fragment, a process which becomes more pronounced at higher exposures of atomic oxygen (cf. Figure 6e,f). The former shape of the Au islands is still recognizable in the arrangement of these oxidized Au particles. At 80 langmuirs of atomic oxygen exposure the Au oxide/Au particles separate. The corresponding Au 4f XPS data (not shown) are similar to the previously presented spectra (cf. Figure 4e); i.e., these separated Au oxide particles also consist of Au oxide (e.g., Au2O3) and metallic Au. The height distribution of oxidized gold particles (cf. Figure S2 in the Supporting Information) reveals that their height increases significantly from about 10 Å for 5 langmuirs of O to 22 Å for 80 langmuirs of O and then saturates at about 25 Å for 250 langmuirs of O with a relatively broad height distribution of ±5 Å. The steady increase in height of the oxidized nanoparticles is accompanied by a decrease of the lateral extension of Au on the Ru(0001) surface. This observation indicates a shovel process where Au atoms are shoveled from the fragmentation areas to the top of the Au island, thus progressively exposing more of the Ru(0001) surface. Below an exposure of 40 langmuirs of atomic O, only the Au islands are attacked, while for exposures above 80 langmuirs the oxidation of the Ru(0001) surface is visible. At higher exposures of atomic O the steps on Ru(0001) are corroded, 16050


Article


Figure 5. STM images depicting the oxidation of gold islands after exposure of 40 langmuirs of atomic oxygen at 300 K. Larger (a, c, e: 300 nm × 300 nm) and magnified (b, d, f: 60 nm × 60 nm) STM image areas evidently show a different degree of fragmentation, depending on the former gold island thickness: Complete fragmentation after oxidation of two layered gold islands (a, b), network of connected clusters after oxidation of three layered gold islands (c, d), and no fragmentation after oxidation of four layered gold islands (e, f).

and the formation of Ru oxide clusters are discernible (cf. Figure 6g,h). Increasing the averaged thickness of the Au islands to 4 ML (by depositing Au at 800 K instead of 700 K), the oxidation process by atomic O at room temperature is slowed down significantly compared to the oxidation of 3 ML thick Au islands (cf. Figure 7). Line scan analysis revealed that after an exposure of 5 langmuirs of O at room temperature, a new layer of gold is formed ontop of the Au island (cf. Figure 7c,d). With ongoing oxidation additional Au atoms are shoveled to the rim of Au islands (cf. Figure 7e−g), where they accumulate (see line scan Figure 7h). Instead of fragmentation the Au islands are transformed into a single big particle (cf. Figure 7k). Again, the corresponding Au 4f XPS data indicate that these bigger particles consist of Au oxide and metallic Au (not shown). The lateral size of these Au oxide/Au particles is significantly smaller than the size of the starting Au islands (cf. line scan Figure 7l). The height distribution indicates (cf. Figure S2) that Au oxide particles grow steadily in thickness upon atomic O exposure

Figure 6. STM images (100 nm × 100 nm) depicting the oxidation of three-layered gold islands with increasing amounts of atomic oxygen at room temperature. (a) 0.5 ML of Au deposited on Ru(0001)-(2 × 1) O at 700 K; (b) 5, (c) 10, (d) 20, (e) 40, (f) 80, (g) 150, and (h) 250 langmuirs of O′. Until 80 langmuirs of O oxidation and fragmentation of the gold islands is visible (b−f), whereas higher dosages of atomic oxygen (g, h) leads to the oxidation of the Ru(0001) substrate (highlighted in h).

and saturates similar to the oxidation of 3 ML Au around 2.6 nm with a quite broad height distribution. The lateral size distribution of the Au oxide/Au particle is centered around 8 nm (cf. Figure 7j−l) and therefore much smaller than the mean lateral Au island dimension of 20 nm (cf. Figure 7d). By considering that the amount of gold is constant during the transformation of a single four-layered gold island to an oxidized gold cluster, a shovel process becomes evident where Au atoms are transported to the top of the Au islands. 16051


Article


Figure 7. Oxidation of four layered gold islands by increasing amounts of atomic oxygen at room temperature is depicted by a series of STM images (100 nm × 100 nm): (a) clean Ru(0001)-(2 × 1)O; (b) 0.4 ML of Au deposited at 800 K; (c) 5, (e) 10, (f) 20, (g) 40, (i) 80, (j) 150, and (k) 250 langmuirs of O′. During oxidation no fragmentation into several clusters was observed, more likely the transformation of single gold islands into one bigger cluster occurred at higher dosages of atomic oxygen (j, k, l) as well as the oxidation of the Ru(0001) substrate. From line scans (d, h) the shovelling of Au atoms to the rim of the islands becomes evident.

3.5. Oxidation of Au Islands on RuO2(110). The data for oxidation of Au islands on RuO2(110) are shown in the Supporting Information (Figure S4). 40 langmuirs of atomic O were exposed to 0.5 ML of Au/RuO2(110). The underlying RuO2(110) substrate was not affected by the oxidation process. However, the morphology of the Au islands (both cuboid and hexagonal) has slightly changed. While the former hexagonal islands are now round-shaped, the cuboid/rectangular islands keep their overall shape, although their corners seem to be rounded. The corresponding Au 4f XPS data (Au 4f spectra in Figure S4) confirm that the Au islands are partially oxidized. 3.6. Oxidation of Closed Au Films on Ru(0001). Without precovering the Ru(0001) surface by oxygen, Au forms uniform and closed films. The oxidation of these Au films by 40 langmuirs of atomic oxygen depends also on their thickness (cf. Figure 8). While the 2 ML Au film breaks up completely into small Au oxide/Au particles (cf. Figure 8a), the 3 ML Au film does not fragment completely. Instead, a network of connected particles is formed (cf. Figure 8c). The morphologies of these oxidized films are similar to fragmentation structures of gold islands with similar thickness

(cf. Figure 5b,d). In particular, the height of 1.65 nm (cf. Figure 8d) for the particles of the fragmented 2 ML Au film (cf. Figure S2) fits nicely to the height of the nanoparticles (1.7 nm) which are formed after fragmentation of the 2 ML thick Au islands by exposure of 40 langmuirs of atomic O. The experiments in Figure 8 indicate that the rims of ultrathin Au islands are not required for their oxidation. Rather, the shoveling process can even proceed on the terraces of closed Au films. The corresponding Au 4f XPS data of the oxidized 2 ML Au film (cf. Figure S3a) confirm that the formed nanoparticles consist of Au oxide and metallic Au. For Au films with a thickness of more than 100 ML (cf. Figure 8e), the oxidation is similar to the oxidation of bulk Au(111).23,66 No particle formation is observed. Instead, the thick Au films starts to roughen by 2−3 Å (cf. Figure 8f) consistent with bulk Au(111) data from the literature.66 In particular, the step edges of 100 ML Au(111) are not corroded. We may therefore surmise that thick Au films do not need heterogeneities (such as steps) for the oxidation. This is quite in contrast to the oxidation of Ru(0001): Under similar 16052


Article


Figure 8. STM images depicting the oxidation of gold films after exposure of 40 langmuirs of atomic oxygen at 300 K. STM images (150 nm × 150 nm) evidently show a different degree of fragmentation, depending on the former gold film thickness: For a 2 ML film the complete fragmentation into clusters is visible (a, b) while for a 3 ML film a network of connected particles is formed (c, d). The oxidation of very thick gold islands (>100 layers thick) is similar to Au(111) single crystal oxidation (e, f).

oxidation conditions steps of the Ru(0001) surface are required for the oxidation process.67 3.7. Reduction of Au Oxide on Ru(0001). We started from oxidized Au islands, which were prepared by depositing 0.5 ML of Au at 670 K on a Ru(0001)-(2 × 1)O surface and the subsequent exposure of 40 langmuirs of atomic oxygen at room temperature (cf. Figure 9a). These Au oxide/Au particles have been reduced, both chemically and thermally. Experimentally, we followed the reduction process by XPS and STM as summarized in Figure 9. Exposing 100 langmuirs of CO at 670 K to the oxidized Au/Ru(0001)-(2 × 1)O surface leads to the formation of metallic gold islands decorating the steps of Ru(0001) in a similar way as in the as-prepared Au/Ru(0001)(2 × 1)O situation (cf. Figure 9b). From XPS the reduction of oxidized gold particles to its metallic state is evident (cf. Figure 9f, green spectrum b). The chemical reduction by CO can also be accomplished at lower sample temperatures (cf. Figure S5). In this way, the nanoparticular dispersion of Au is retained. Starting from islands or films, Au can be dispersed by oxidation with atomic

Figure 9. Reduction of oxidized 3 ML thick Au islands on Ru(0001)(2 × 1)O. (a) STM image of the oxidized islands. (b) Chemical reduction of Au oxide clusters by exposing 100 langmuirs of CO at 670 K. (c) Thermal reduction by annealing to 670 K in a vacuum for 15 min. Both line scan analysis (d, e) and XPS data of the Au 4f signals (f) evidently show the reduction of the clusters as well as the gold island reformation and their lateral expansion on the surface depending on the oxygen overlayer.

oxygen and chemical reduction at 300 K. Even sintered bigger gold particles can be transformed to smaller particles by 16053


Article


Figure 10. Schematic illustration of the oxidation mechanism of thin gold islands (or films) by using atomic oxygen at room temperature (shoveling process). (a) By adsorption of atomic oxygen at room temperature single gold atoms are released, thus forming the mobile AuOx precursor species (here depicted as AuO2). (b) Several AuOx precursors form a Au oxide thus covering metallic gold atoms. Newly formed AuOx precursors from deeper Au layers are expelled from the Au/Ru(0001) interface, leading to the fragmentation of the island. (c) With ongoing oxidation of thin gold islands (or films) the degree of fragmentation is high because the formation of AuOx precursors at the Au/Ru(0001) interface is more probable compared to thicker gold islands (or films) (>4 layers). With increasing thickness of the gold islands (or films) the probability of formed AuOx precursors at the interface decreases, prohibiting the fragmentation.

4. DISCUSSION OF RESULTS The influence of preadsorbed species on film growth can be rationalized by the underlying thermodynamics. The mobility of the deposited gold atoms is high at the chosen conditions, and kinetic limitations are negligible. As a result, the growth can be well described by the energy contributions according to Bauer.72 For instance, the adsorption of Au on Ru(0001) is stronger than Au on Au and the surface energy of Au is lower than that of Ru(0001). Consequently, Au will wet the Ru(0001) surface as observed in STM (cf. Figure 2).44 Preadsorbing oxygen on Ru(0001) will, however, drastically reduce the adsorption energy of Au. Therefore, Au does not tend to wet O/Ru(0001). Oxygen buried under a Au island at the Au/Ru(0001) interface is energetically so unfavorable that adsorption of Au leads rather to a compression of the oxygen overlayer on Ru(0001) in order to provide O-free Ru(0001) regions where Au islands can grow. The energy penalty due to compression of the O overlayer by about 0.5 eV per O atom is overcompensated by the energy gained by stronger adsorption of Au.50 With varying O precoverage on Ru(0001) the lateral size and height of the Au islands can be tuned (cf. Figure 1). Gold adsorption on RuO2(110) leads to Au island growth with varying crystalline orientations: cuboid, hexagonal, and rectangular shape of Au islands (cf. Figure 3). The cuboid shape of the islands may be explained by the growth of (100) oriented islands of Au, while the hexagonal islands are ascribed to gold with (111) orientation. The crystalline orientation of the gold island depends on the symmetry of the underlying RuO2(110). The Au(100) islands are found to grow preferentially on the flat regions of RuO2(110), while the Au(111) islands are located at the interaction areas of the rotational RuO2 domains. This indicates that a complex interplay between the interface energy, the surface free energy, and the strain energy that determines the actual orientation of the gold island depending on the area where the gold island is formed. Since Au is not wetting the RuO2(110) surface, we infer that the surface energy of Au is higher than that of RuO2(110), which actually is reconciled with surface energies (Au(111): 1.3 J/m2; Au(100), 1.6 J/m2; Au(110): 1.7 J/m2 versus RuO2(110): 0.7 J/m2).42,54 The Au islands and layers were oxidized at room temperature by exposing atomic oxygen from a thermal cracker. The oxidation of such Au islands on O precovered Ru(0001) by atomic oxygen depends critically on the thickness of the islands. The thicker the Au islands the more reluctant it is to fragmentation in the course of oxidation. The 2 and 3 ML thick islands and films of Au can readily be oxidized,

utilizing their wetting behavior at higher temperatures in oxidizing or reducing environments (depending on the substrate) and the subsequent oxidation of the formed ultrathin gold islands or films by atomic oxygen. This recipe may be applicable for the redispersion of Au particles on more familiar supports such as TiO2. Simple annealing of the oxidized Au/Ru(0001)-(2 × 1)O surface at 670 K in a vacuum for 15 min also leads to the reduction of Au oxide/Au particles. This is consistent with previous studies where decomposition of Au oxide and the desorption of chemisorbed oxygen from gold surfaces have been observed when heated to 423−473 K68 or to 520−590 K,20,24,33,35,69,70 respectively. The thermal reduction of the Au oxide/Au particles results in laterally smaller but significantly thicker Au islands, compared to chemically reduced islands by CO exposure at the same sample temperature (cf. Figure 9d,e). XPS also confirms that the Au islands are metallic after thermal reduction (cf. Figure 9f, orange spectrum c). From these experiments we conclude that either the mobility of Au is higher under a CO atmosphere or the oxygen precoverage of Ru(0001) is reduced so that larger Au islands are able to form. Both arguments are confirmed by related observations in the literature.44−47,71 However, we favor the second interpretation since the strong wetting behavior of gold on oxygen-free Ru(0001) surfaces has been evidenced in various experiments.39,44−47 Annealing to 670 K alone will not change the O coverage on Ru(0001); however, CO exposure at 670 K may remove chemisorbed oxygen on Ru(0001) reacting to CO2 so that more gold atoms can bind directly to bare ruthenium and as a consequence larger Au islands are formed. The oxidized 2 ML Au films in Figure 8a were thermally reduced by annealing the sample to 700 K for 15 min in vacuum. As indicated by STM (Supporting Information, Figure S6) the reduced Au particles arrange into thicker (6−9 layers) hexagonally shaped islands that are indicative of metallic gold in (111) orientation. Finally, we studied the stability of Au oxide particles under CO oxidation reaction conditions (e.g., p(CO) = 10−7 mbar and p(O2) = 10−6 mbar) at room temperature. The most important question concerns whether molecular oxygen is able to maintain the oxidation state of Au when a reducing gas is coexposed. Unfortunately, the oxidized Au particles turned out to be not stable under CO oxidation conditions even if the reaction mixture is very oxidizing. The Au oxide particles have always been reduced but remained their spherical shape due to the low temperature. 16054


Article


decoration has been demonstrated first by Chorkendorff and co-workers for the dissociation of N2 on the Ru(0001) surface.75

fragmenting into small oxidized Au particles (cf. Figures 4 and 5). These particles consist of an Au oxide (XPS: presumably Au2O3) and metallic Au. These XPS spectra and a comparison to similar interpretations in the literature are consistent with a core−shell structure for the oxidized particles with the oxide (shell) covering the metallic Au (core).3,60 The oxidation process starts from the top layer of the Au islands penetrating into the Au islands until the Ru(0001) substrate is reached. An oxygen-induced high mobility of Au atoms at room temperature is required to explain the fragmentation of the Au islands into particles. Since metallic Au atoms are not mobile enough at room temperature, a mobile molecular AuOx precursor needs to be introduced in the oxidation process of the Au islands. Theoretical studies proposed a molecular AuO2 species to be this mobile precursor species.73,74 In the oxidation process atomic oxygen penetrates the Au layer and shovels Au atoms via mobile AuOx precursors onto the surface. Obviously, the formed AuOx species is expelled from the Au/Ru(0001) particle interface. Oxygen can attack the Au islands from the rims. However, also closed 2−3 ML thick Au films are readily fragmented by the oxidation, in that atomic oxygen penetrated these thin layers also from the densely packed terraces. For thin Au islands (2 and 3 ML thick) oxygen can readily reach the Au/Ru interface, where the formed AuOx species are not stable and transported (shoveled) to the top surface layer of Au. In this way holes can be corroded in the Au islands/films so that the Au islands/films are fragmented into Au oxide/Au particles as illustrated in Figure 10. The oxidation of 2−3 ML thick Au islands shows therefore some kind of nucleation and growth behavior. When the thickness of Au islands or films is beyond 3−4 ML fragmentation is suppressed. Rather, the whole Au island is transformed into a single big particle. Atomic oxygen can hardly attack the Au islands from the terrace of the thicker Au islands, but rather the aforementioned shovel process is limited to the rims of the islands (cf. Figure 7). Much thicker Au islands (>100 ML) are mostly stable under exposure of atomic oxygen. Oxygen may still penetrate the topmost layer as indicated by the observed roughening of thicker Au films (averaged roughness of 2−3 Å; cf. Figure 8e). Quite in contrast, the oxidation of Ru(0001) under similar reaction conditions has shown to proceed heterogeneously in that the steps of Ru(0001) are corroded and Ru oxide particles are formed.41,67 For the thicker Au islands/films, however, the corrosion of step edges and the formation of clusters have not been observed. We should note that atomic oxygen is able to oxidize the underlying Ru(0001) surface.65,67 However, we have no experimental indication that this happens on a Au-covered Ru(0001) surface below an exposure of 40 langmuirs of atomic O. At higher exposures of atomic oxygen corrosion of the steps on Ru(0001) and the formation of Ru oxide clusters are discernible. Therefore, we assume that Au passivates partly through decorating the steps of the Ru(0001) substrate. Or alternatively, the oxidation of Au is kinetically more favored than the oxidation of Ru(0001) at room temperature. We checked this view by depositing small amounts of Au on the Ru(0001) and then exposed this surface to 10−5 mbar of molecular oxygen at 670 K. In general, with this recipe RuO2(110) is formed on the bare Ru(0001).41 However, the Au-covered Ru(0001) surface shows no indication of oxide formation, neither in STM nor in XPS. This experiment corroborates the passivating role of Au for the oxidation of Ru(0001). Actually, the passivation of step edges by Au

5. CONCLUSIONS The oxidation of Au islands at room temperature needs the exposure of atomic oxygen. Oxidation of Au islands already proceeds at 300 K and has found to depend critically on the island thickness, but not on the lateral size of the Au islands. The 3 ML thick Au islands can readily be oxidized leading to fragmentation of the Au islands into small clusters whose arrangement reflects the shapes of the former Au islands. From Au 4f core level spectroscopy these particles consist of Au oxide and metallic Au. The oxidation of 2 and 3 ML thick Au islands is explained invoking a shoveling process (cf. Figure 10). Quite in contrast, thicker Au islands are less prone to be oxidized. The oxidation of 4−5 ML thick Au islands starts also from the top layer at the rim of the islands. However, no fragmentation of the Au islands into smaller particles is observed. Instead, the entire Au island is transformed into one big particle that also consists of Au oxide and metallic Au. The oxidized Au islands can be reduced by thermal annealing to 670 K or by chemical reduction via CO exposure at 670 K, leading to the reformation of thin metallic gold islands from the Au oxide/Au particles. Exposing the oxidized Au particles to a mixture of CO and O2 at room temperature with various feed compositions indicates that the Au oxide particles are chemically reduced and are accordingly not stable. If the chemical reduction of the oxidized Au particles by CO is carried out at 300 K, the particle structure of Au persists. These redox processes may be of utility to redisperse Au particles after sintering.

■

ASSOCIATED CONTENT

S Supporting Information *

XPS data for the oxygen overlayer compression in the case of Au island formation, the overgrowth of the oxygen overlayer for covering gold films, statistics of the gold island or Au oxide cluster thickness depending on the O exposure, XPS data for the oxidation of covering gold films and Au islands on RuO2(110), the reduction of the oxidized films by CO at room temperature as well as their thermal reduction at 700 K in vacuum. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcc.5b03583.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax ++49641-9934559 (H.O.). Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS We thank fruitful discussions with Franziska Hess and financial support from the German Science foundation (Ov21-9/1). REFERENCES

(1) Hammer, B.; Norskov, J. K. Why Gold is the Noblest of All the Metals. Nature 1995, 376, 238−240. (2) Haruta, M. Size- and Support-Dependency in the Catalysis of Gold. Catal. Today 1997, 36, 153−166.

16055


Article

The Journal of Physical Chemistry C (3) Ono, L. K.; Cuenya, B. R. Formation and Thermal Stability of Au2O3 on Gold Nanoparticles: Size and Support Effects. J. Phys. Chem. C 2008, 112, 4676−4686. (4) Park, E. D.; Lee, J. S. Effects of Pretreatment Conditions on CO Oxidation over Supported Au Catalysts. J. Catal. 1999, 186, 1−11. (5) Delannoy, L.; Weiher, N.; Tsapatsaris, N.; Beesley, A. M.; Nchari, L.; Schroeder, S. L. M.; Louis, C. Reducibility of Supported Gold (III) Precursors: Influence of the Metal Oxide Support and Consequences for CO Oxidation Activity. Top. Catal. 2007, 44, 263−273. (6) Herranz, T.; Deng, X.; Cabot, A.; Alivisatos, P.; Liu, Z.; Soler-Illia, G.; Salmeron, M. Reactivity of Au Nanoparticles Supported over SiO2 and TiO2 Studied by Ambient Pressure Photoelectron Spectroscopy. Catal. Today 2009, 143, 158−166. (7) Widmann, D.; Liu, Y.; Schüth, F.; Behm, R. J. Support Effects in the Au-catalyzed CO Oxidation − Correlation between Activity, Oxygen Storage Capacity, and Support Reducibility. J. Catal. 2010, 276, 292−305. (8) Weiher, N.; Beesley, A. M.; Tsapatsaris, N.; Delannoy, L.; Louis, C.; van Bokhoven, J. A.; Schroeder, S. Activation of Oxygen by Metallic Gold in Au/TiO2 Catalysts. J. Am. Chem. Soc. 2007, 129, 2240−2241. (9) Valden, M.; Lai, X.; Goodman, D. W. Onset of Catalytic Activity of Gold Clusters on Titania with the Appearance of Nonmetallic Properties. Science 1998, 281, 1647−1650. (10) Chen, M. S.; Goodman, D. W. The Structure of Catalytically Active Gold on Titania. Science 2004, 306, 252−255. (11) Chen, M.; Goodman, D. W. Catalytically Active Gold: From Nanoparticles to Ultrathin Films. Acc. Chem. Res. 2006, 39, 739−746. (12) Bond, G. C.; Thompson, D. T. Gold-Catalysed Oxidation of Carbon Monoxide. Gold Bull. 2000, 37, 41−51. (13) Meyer, R.; Lemire, C.; Shaikhutdinov, S. K.; Freund, H. Surface Chemistry of Catalysis by Gold. Gold Bull. 2004, 37, 72−124. (14) Stephen, A.; Hashmi, K.; Hutchings, G. J. Gold Catalysis. Angew. Chem., Int. Ed. 2006, 45, 7896−7936. (15) Gong, J. Structure and Surface Chemistry of Gold-Based Model Catalysts. Chem. Rev. 2012, 112, 2987−3054. (16) Xu, B.; Madix, R. J.; Friend, C. M. Predicting Gold-Mediated Catalytic Oxidative-Coupling Reactions from Single Crystal Studies. Acc. Chem. Res. 2014, 47, 761−772. (17) Min, B. K.; Friend, C. M. Heterogeneous Gold-Based Catalysis for Green Chemistry: Low-Temperature CO Oxidation and Propene Oxidation. Chem. Rev. 2007, 107, 2709−27024. (18) Ojifinni, R. A.; Gong, J.; Flaherty, D. W.; Kim, T. S.; Mullins, C. B. Annealing Effect on Reactivity of Oxygen-Covered Au(111). J. Phys. Chem. C 2009, 113, 9820−9825. (19) Gong, J.; Mullins, C. B. Surface Science Investigations of Oxidative Chemistry on Gold. Acc. Chem. Res. 2009, 42, 1063−1073. (20) Nakamura, I.; Takahashi, A.; Fujitani, T. Selective Dissociation of O3 and Adsorption of CO on Various Au Single Crystal Surfaces. Catal. Lett. 2009, 129, 400−403. (21) Gottfried, J. M.; Christmann, K. Oxidation of Carbon Monoxide over Au(1 1 0)-(1 × 2). Surf. Sci. 2004, 566−568, 1112−1117. (22) Dumbuya, K.; Cabailh, G.; Lazzari, R.; Jupille, J.; Ringel, L.; Pistor, M.; Lytken, O.; Steinrück, H.-P.; Gottfried, J. M. Evidence for an Active Oxygen Species on Au/TiO2(11) Model Catalysts during Investigation with in Situ X-Ray Photoelectron Spectrosocopy. Catal. Today 2012, 181, 20−25. (23) Min, B. K.; Alemozafar, A. R.; Biener, M. M.; Biener, J.; Friend, C. M. Reaction of Au(111) with Sulfur and Oxygen: Scanning Tunnelling Microscopic Study. Top. Catal. 2005, 36, 77−90. (24) Deng, X.; Min, B. K.; Guloy, A.; Friend, C. M. Enhancement of O2 Dissociation on Au(111) by Adsorbed Oxygen: Implications for Oxidation Catalysis. J. Am. Chem. Soc. 2005, 127, 9267−9270. (25) Baker, T. A.; Xu, B.; Liu, X.; Kaxiras, E.; Friend, C. M. Nature of Oxidation of the Au(111) Surface: Experimental and Theoretical Investigation. J. Phys. Chem. C 2009, 113, 16561−16564. (26) Baker, T. A.; Liu, X.; Friend, C. M. The Mystery of Gold’s Chemical Activity: Local Bonding, Morphology and Reactivity of Atomic Oxygen. Phys. Chem. Chem. Phys. 2011, 13, 34−46.

(27) Koslowski, B.; Boyen, H.-G.; Wilderotter, C.; Kästle, G.; Ziemann, P.; Wahrenberg, R.; Oelhafen, P. Oxidation of Preferentially (111)-Oriented Au Films in an Oxygen Plasma Investigated by Scanning Tunneling Microscopy and Photoelectron Spectroscopy. Surf. Sci. 2001, 475, 1−10. (28) Pireaux, J. J.; Liehr, M.; Thiry, P. A.; Delrue, J. P.; Gaudano, R. Electron Spectroscopic Characterization of Oxygen Adsorption on Gold Surfaces I. Substrate Impurity Effects on Molecular Oxygen Adsorption in Ultra High Vacuum. Surf. Sci. 1984, 141, 211−220. (29) Pireaux, J. J.; Liehr, M.; Thiry, P. A.; Delrue, J. P.; Gaudano, R. Electron Spectroscopic Characterization of Oxygen Adsorption on Gold Surfaces II. Production of Gold Oxide in Oxygen DC Reactive Sputtering. Surf. Sci. 1984, 141, 221−232. (30) Baber, A. E.; Torres, D.; Müller, K.; Nazzarro, M.; Liu, P.; Starr, D. E.; Stacchiola, D. J. Reactivity and Morphology of Oxygen-Modified Au Surfaces. J. Phys. Chem. C 2012, 116, 18292−18299. (31) Biener, J.; Biener, M. M.; Nowitzki, T.; Hamza, A. V.; Friend, C. M.; Zielasek, V.; Bäumer, M. On the Role of Oxygen in Stabilizing Low-Coordinated Au Atoms. ChemPhysChem 2006, 7, 1906−1908. (32) Canning, N. D. S.; Outka, D.; Madix, R. J. The Adsorption of Oxygen on Gold. Surf. Sci. 1984, 141, 240−254. (33) Sault, A. G.; Madix, R. J.; Campbell, C. T. Adsorption of Oxygen and Hydrogen on Au(110)-(1 × 2). Surf. Sci. 1986, 169, 347−356. (34) Huang, L.; Zeppenfeld, P.; Chevrier, J.; Comsa, G. Surface Morphology of Au(111) after Exposure to Oxygen at High Temperature and Pressure. Surf. Sci. 1996, 352−354, 285−289. (35) Kim, J.; Samano, E.; Koel, B. E. Oxygen Adsorption and Oxidation Reactions on Au(211) Surfaces: Exposures Using O2 at High Pressures and Ozone (O3) in UHV. Surf. Sci. 2006, 600, 4622− 4632. (36) Pauls, C.; Christmann, K. Growth and Structure of Gold Films on a Re(1010) Surface. J. Phys.: Condens. Matter 2009, 21, 134012. (37) Wu, Q.; Hrbek, J. Coadsorption of Oxygen, Gold and Carbon Monoxide on Ru(0001) and CO2 Formation: A Thermal Desorption Study. Surf. Sci. 2005, 588, 117−126. (38) Hwang, R. Q.; Bartelt, M. C. Scanning Tunneling Microscopy Studies of Metal on Metal Epitaxy. Chem. Rev. 1997, 97, 1063−1082. (39) Malik, I. J.; Hrbek, J. Coadsorption Induced Effects at Surfaces: Thermal Desorption Spectroscopy and X-Ray Photoelectron Spectroscopy Study of Oxygen Compression by Au on Ru(001). J. Vac. Sci. Technol., A 1991, 9, 1806−1809. (40) Poulston, S.; Tikhov, M.; Lambert, R. M. Coadsorption of Gold and Oxygen on Ruthenium(100). Langmuir 1997, 13, 5356−5361. (41) Herd, B.; Knapp, M.; Over, H. Atomic-Scale Insights into the Initial Oxidation of Ru(0001) Using Molecular Oxygen: A Scanning Tunneling Microscopy Study. J. Phys. Chem. C 2012, 116, 24649− 24660. (42) Vitos, L.; Ruban, A. V.; Skriver, H. L.; Kollár, J. The Surface Energy of Metals. Surf. Sci. 1998, 411, 186−202. (43) Barmparis, G. D.; Remediakis, I. N. Dependence on CO Adsorption of the Shapes of Multifaceted Gold Nanoparticles: A Density Functional Theory. Phys. Rev. B 2012, 86, 085457. (44) Hwang, R. Q.; Günther, C.; Schröder, J.; Günther, S.; Kopatzki, E.; Behm, R. J. Nucleation and Growth of Thin Metal Films on Clean and Modified Metal Substrates Studied by Scanning Tunneling Microscopy. J. Vac. Sci. Technol., A 1992, 10, 1970−1980. (45) Hwang, R. Q.; Schröder, J.; Günther, C.; Behm, R. J. Fractal Growth of Two-Dimensional Islands: Au on Ru(0001). Phys. Rev. Lett. 1991, 67, 3279−3282. (46) Pötschke, G.; Schröder, J.; Günther, C.; Hwang, R. Q.; Behm, R. J. A STM Investigation of the Nucleation and Growth of Thin Cu and Au Films on Ru(0001). Surf. Sci. 1991, 251/252, 592−596. (47) Schröder, J.; Günther, C.; Hwang, R. Q.; Behm, R. J. A Comparative STM Study of the Growth of Thin Au Films on Clean and Oxygen-Precovered Ru(0001) Surfaces. Ultramicroscopy 1992, 42−44, 475−482. (48) Ling, W. L.; Hamilton, J. C.; Thürmer, K.; Thayer, G. E.; de la Figuera, J.; Hwang, R. Q.; Carter, C. B.; Bartelt, N. C.; McCarty, K. F. 16056


Article

The Journal of Physical Chemistry C Herringbone and Triangular Patterns of Dislocations in Ag, Au, and AgAu Alloy Films on Ru(0001). Surf. Sci. 2006, 600, 1735−1757. (49) Stampfl, C.; Scheffler, M. Theoretical Study of O Adlayers on Ru(0001). Phys. Rev. B 1996, 54, 2868−2872. (50) Stampfl, C.; Schwegmann, S.; Over, H.; Scheffler, M.; Ertl, G. Structure and Stability of a High-Coverage (1 × 1) Oxygen Phase on Ru(0001). Phys. Rev. Lett. 1996, 77, 3371−3374. (51) Aqra, F.; Ayyad, A. Surface Energies of Metals in Both Liquid and Solid States. Appl. Surf. Sci. 2011, 257, 6372−6379. (52) Barth, J. V.; Brune, H.; Ertl, G.; Behm, R. J. Scanning Tunneling Microscopy Observations on the Reconstructed Au(111) Surface − Atomic Structure, Long-Range Superstructure, Rotational Domains, and Surface Defects. Phys. Rev. B 1990, 42, 9307−9318. (53) Kim, S. H.; Wintterlin, J. Morphology of RuO2(110) Oxide Films on Ru(0001) Studied by Scanning Tunneling Microscopy. J. Chem. Phys. 2009, 131, 064705. (54) Seitsonen, A. P.; Over, H. Intimate Interplay of Theory and Experiment in Model Catalysis. Surf. Sci. 2009, 603, 1717−1723. (55) Aschoff, M.; Speller, S.; Kuntze, J.; Heiland, W.; Platzgummer, E.; Schmid, M.; Varga, P.; Baretzky, E. Unreconstructed Au(100) Monolayers on a Au3Pd(100) Single-Crystal Surface. Surf. Sci. 1998, 415, L1051−L1054. (56) Over, H.; Kim, Y. D.; Seitsonen, A. P.; Wendt, S.; Lundgren, E.; Schmid, M.; Varga, P.; Morgante, A.; Ertl, G. Atomic-Scale Structure and Catalytic Reactivity of the RuO2(110) Surface. Science 2000, 287, 1474−1476. (57) Over, H.; Seitsonen, A. P.; Lundgren, E.; Schmid, M.; Varga, P. Experimental and Simulated STM Images of the Stoichiometric and Partially Reduced RuO2(110) Surfaces Including Adsorbates. Surf. Sci. 2002, 515, 143−156. (58) Shi, H.; Asahi, R.; Stampfl, C. Properties of the Gold Oxides Au2O3 and Au2O: First-Principles Investigation. Phys. Rev. B 2007, 75, 205125. (59) Shi, H.; Stampfl, C. First-Principles Investigations of the Structure and Stability of Oxygen Adsorption and Surface Oxide Formation at Au(111). Phys. Rev. B 2007, 76, 075327. (60) Boyen, H.-G.; Kästle, G.; Weigl, F.; Koslowski, B.; Dietrich, C.; Ziemann, P.; Spatz, J. P.; Riethmüller, S.; Hartmann, C.; Möller, M.; et al. Oxidation-Resistant Gold-55 Clusters. Science 2002, 297, 1533− 1536. (61) Lim, D. C.; Lopez-Salido, I.; Dietsche, R.; Bubek, M.; Kim, Y. D. Electronic and Chemical Properties of Supported Au Nanoparticles. Chem. Phys. 2006, 330, 441−448. (62) Lim, D. C.; Lopez-Salido, I.; Dietsche, R.; Bubek, M.; Kim, Y. D. Oxidation of Au Nanoparticles on HOPG Using Atomic Oxygen. Surf. Sci. 2006, 600, 507−513. (63) Wertheim, G. K.; DiCenzo, S. B.; Youngquist, S. E. Unit Charge on Supported Gold Clusters in Photoemission Final State. Phys. Rev. Lett. 1983, 51, 2310−2313. (64) Chusuei, C. C.; Lai, X.; Luo, K.; Goodman, D. W. Modeling Heterogeneous Catalysts: Metal Clusters on Planar Oxide Supports. Top. Catal. 2001, 14, 71−83. (65) Herd, B.; Goritzka, J. C.; Over, H. Room Temperature Oxidation of Ruthenium. J. Phys. Chem. C 2013, 117, 15148−15154. (66) Min, B. K.; Alemozafar, A. R.; Pinnaduwage, D.; Deng, X.; Friend, C. M. Efficient CO Oxidation at Low Temperature on Au(111). J. Phys. Chem. B 2006, 110, 19833−19838. (67) Herd, B.; Over, H. Atomic Scale Insights into the Initial Oxidation of Ru(0001) Using Atomic Oxygen: A Scanning Tunnelling Microscopy Study. Surf. Sci. 2014, 622, 24−34. (68) Krozer, A.; Rodahl, M. X-Ray Photoemission Spectroscopy Study of UV/Ozone Oxidation of Au under Ultrahigh Vacuum Conditions. J. Vac. Sci. Technol., A 1997, 15, 1704−1709. (69) Saliba, N.; Parker, D. H.; Koel, B. E. Adsorption of Oxygen on Au(111) by Exposure to Ozone. Surf. Sci. 1998, 410, 270−282. (70) Gottfried, J. M.; Elghobashi, N.; Schroeder, S. L. M.; Christmann, K. Oxidation of Gold by Oxygen-Ion Sputtering. Surf. Sci. 2003, 523, 89−102.

(71) Willneff, E. A.; Klanner, C.; Schroeder, S. L. M. Gas Phase Transport of Gold with Gold(III) Oxide and Carbon Monoxide. Chem. Commun. 2003, 258−259. (72) Bauer, E. Epitaxy of Metals on Metals. Appl. Surf. Sci. 1982, 11/ 12, 479−494. (73) Baker, T. A.; Friend, C. M.; Kaxiras, E. Local Bonding Effects in the Oxidation of CO on Oxygen-Covered Au(111) from Ab Initio Molecular Dynamics Simulations. J. Chem. Theory Comput. 2010, 6, 279−287. (74) Sun, K.; Kohyama, M.; Tanaka, S.; Takeda, S. Theoretical Study of Atomic Oxygen on Gold Surface by Hückel Theory and DFT Calculations. J. Phys. Chem. A 2012, 116, 9568−9573. (75) Dahl, S.; Törnqvist, E.; Chorkendorff, I. Dissociative Adsorption of N2 on Ru(0001): A Surface Reaction Totally Dominated by Steps. J. Catal. 2000, 192, 381−390.

16057


Oxidation-Induced Dispersion of Gold on Ru(0001): A Scanning

Recommend Documents