4944
Langmuir 1999, 15, 4944-4948
Scanning Tunneling Microscopy Images of Ruthenium Submonolayers Spontaneously Deposited on a Pt(111) Electrode E. Herrero and J. M. Feliu* Departamento de Quı´mica Fı´sica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain
A. Wieckowski* Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Received February 24, 1999. In Final Form: May 19, 1999 Scanning tunneling microscopy (STM) was used to examine spontaneously deposited ruthenium adlayers on the well-defined Pt(111) electrode. Stable and STM discernible structures were obtained after a brief cyclic voltammetric treatment of the ruthenium deposit. As demonstrated previously, the electrochemically stabilized Pt/Ru electrodes are active catalysts in methanol electrooxidation. The STM data indicate that the deposit is arranged in nanometer size islands of which the detailed structural characterization is presented. Maximum ruthenium coverage is no higher than 20%, which confirms our previous results obtained by the use of Auger electron spectroscopy. While most of the islands are monatomic, a fraction of the islands, approximately 10% of the total ruthenium coverage, displays a second monolayer deposit over the first monolayer. This is a new discovery showing an unexpected tendency of the spontaneously deposited ruthenium at such a low coverage to nucleate in a bilayer configuration. The hydrogen underpotential deposition process does not affect the spatial distribution of the islands, but ruthenium is reductively removed from the surface under hydrogen evolution conditions.
Introduction Recently, a major effort has been reported toward producing high activity surfaces made of platinum/ ruthenium composites as a catalyst for methanol electrooxidation.1-3 As recently reviewed in refs 2 and 3, these preparative methods can be arranged into several categories: electrodeposition of ruthenium on polycrystalline platinum, alloying platinum by ruthenium, codeposits of ruthenium with platinum on a platinum substrate, graphite, conducting polymers, as well as on protonconducting membranes. The new trend, dated to the paper by Herrero et al.,4 is to deposit controlled amounts of ruthenium on the well-defined platinum single-crystal substrates of different crystallographic orientations.3-8 This approach allows one to investigate surface structure effects in Pt/Ru methanol oxidation electrocatalysis. Both forced deposition (electrolysis)5,6,12,13 and spontaneous * To whom the correspondence should be sent. (1) Ross, P. N., Jr. In Frontiers of Electrochemistry (Electrocatalysis); Lipkowski, J., Ross, P. N., Jr., Eds.; Wiley-VCH Publishers: New York, 1998; Vol. 4, Chapter 2. (2) Hamnett, A. In Interfacial Electrochemistry: Experimental, Theory and Applications; Wieckowski, A., Ed.; Marcel Dekker: New York, in press. (3) Chrzanowski, W.; Wieckowski, A. In Interfacial Electrochemistry: Experimental, Theory and Applications; Wieckowski, A., Ed.; Marcel Dekker: New York, in press. (4) Herrero, E.; Franaszczuk, K.; Wieckowski, A. J. Electroanal. Chem. 1993, 361, 269. (5) Chrzanowski, W.; Wieckowski, A. Langmuir 1997, 13, 5974. (6) Chrzanowski, W.; Kim, H.; Wieckowski, A. Catal. Lett. 1998, 50, 69. (7) Chrzanowski, W.; Wieckowski, A. Langmuir 1998, 14, 1967. (8) Chrzanowski, W.; Kim, H.; Tremiliosi-Filho, G.; Wieckowski, A.; Grzybowska, B.; Kulesza, P. J. New Mater. Electrochem. Syst. 1998, 1, 31. (9) Attard, G. A.; Banister, A. J. Electroanal. Chem. 1991, 300, 467. (10) Clavilier, J.; Llorca, M. J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1991, 310, 429. (11) Llorca, M. J.; Feliu, J. M.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1993, 351, 299.
deposition (electroless process)5,6 from sulfuric acid12,13 and perchloric acid5,6 media were used. In the spontaneous deposition category, we have found that ruthenium (similarly to palladium9-11) can be added to platinum effectively and that the depositsafter brief voltammetric treatmentsis very reactive in electrooxidation of methanol to CO2.6 Since Ru films do not dissolve from platinum in the electrode potential range that precedes platinum oxidation, such “Pt/Ru” surfaces are ideal candidates for interrogating surface structure effects in Pt/Ru heterogeneous electrocatalysis. There is a clear significance in such investigations since the assemblies of platinum/ ruthenium nanoparticles are considered to be the viable anode catalyst in practical methanol and hydrogen fuel cells.1-3 Research in the fuel cell area has attracted much attention both abroad and in the US. Despite a large body of research on Pt/Ru methanol oxidation electrocatalysis, the nature of the surface site for the oxidation reaction is still an outstanding issue that needs to be addressed. The question about the surface site is particularly relevant in the investigations of the spontaneously deposited Ru adlayers since the spontaneous (electroless) deposition process on platinum per se has been under scrutiny but since quite recently.5,6 Since the finding is new, there is not much known either about the morphology of the deposits or about how the morphology changes, for instance, with ruthenium coverage. These are the topics we are addressing in this paper. Earlier, Stimming et al. had produced ruthenium deposits by electrodeposition from sulfuric acid media and investigated the Ru surface structure by scanning tunneling (12) Cramm, S.; Friedrich, K. A.; Geyzers, K.-P.; Stimming, U.; Vogel, R.; Fresenius J. Anal. Chem. 1997, 358, 189. (13) Friedrich, K. A.; Geyzers, K.-P.; Henglein, F.; Marmann, A.; Stimming, U.; Unkauf W.; Vogel, R. In Electrode Processes VI; The ECS Proceedings; Wieckowski, A., Itaya, K., Eds.; 1996; Pennington, Vol. 96-8, p 119.
10.1021/la990219v CCC: $18.00 © 1999 American Chemical Society Published on Web 06/23/1999
Letters
Langmuir, Vol. 15, No. 15, 1999 4945
microscopy (STM).12,13 It was found that up to 0.7 monolayer (ML) coverage of ruthenium was deposited as mainly monatomic islands with a tendency to create threedimensional deposits as the coverage increased. In this communication, we have obtained STM imaging data of spontaneously deposited ruthenium on a Pt(111) electrode. We confirm the tendency of ruthenium to become deposited as an array of surface islands.12,13 However, when the spontaneous deposition rather than electrolysis is used, a fraction of the islands is no longer monatomic. Instead, such islands have a bilayer character, displaying a second monolayer deposit over the first monolayer. The significance of these data is in the theory of noble metal on noble-metal deposition processes5 and in increasing our understanding of methanol oxidation mechanisms on mixed-metal, catalytic electrodes.3 For the reported research, only the Pt(111) surface (out of other Pt(hkl) surfaces that have already been considered5) was used because, when covered by ruthenium, such a Pt(111) surface is the most active catalyst for methanol electrooxidation known to date.6 Experimental Section A Pt(111) single-crystal surface of ca. 0.2 cm in diameter was used as the working electrode. The crystal was flame-annealed, cooled in an argon/hydrogen atmosphere,14 and positioned in an electrochemical cell via a meniscus configuration. The state of the Pt(111) surface was controlled by cyclic voltammetry (CV) applied to the electrode at 50 mV s-1 in 0.1 M HClO4. Stable CV profiles were examined to confirm the surface cleanness and order. A surface with the appropriate CV characteristic (Figure 1, dashed line) was ready to be used as a substrate for spontaneous deposition of ruthenium, as described below. STM experiments were carried out in air,15,16 using a Nanoscope III system (Digital Instruments) equipped with a molecular imaging scanning head and using electrochemically etched cut Pt80Ir20 tips. STM drift rates were evaluated by comparison of a series of consecutive upward and downward scanned images and were found negligible. Voltage bias is given in each figure presenting the STM images. The chemicals were Millipore water (18 MΩ cm), perchloric acid (Merk Suprapur), hydrated ruthenium trichloride (Merk), and potassium iodide (Merk Suprapur). All experiments were carried out at room temperature and all potentials have been measured against a reversible hydrogen electrode (RHE). Ruthenium coverage obtained from STM represents the average of at least five images obtained from different regions of the electrode surface. When the experiment with a given Pt/Ru surface ended, ruthenium was removed from platinum by several negative- and positive-going scans at 50 mV s-1 in the electrode potential range terminated by the onsets of hydrogen and oxygen evolution.
Results and Discussion Spontaneous deposition of ruthenium was carried out by contacting the Pt(111) surface with a 0.5 mM solution of RuCl3 in 0.1 M HClO4 at open circuit. The deposition time was one of the variables of this research and was set from 10 to 60 s. The electrode was rinsed with Millipore water, and the Pt(111)/Ru surface was subjected to a brief voltammetric treatment in a clean supporting electrolyte in the potential range from 0.06 to 0.80 V, as reported before.5-8 In most of the experiments the electrode, covered by a thick film of solution adhering to its working plane, was transferred to another electrochemical cell containing 1 mM KI solution, and it was treated by iodide for 2 min.15,16 The adlayer of adsorbed iodine protects the electrode from (14) Clavilier, J. J. Electroanal. Chem. 1980, 107, 211. (15) Orts, J. M.; Go´mez, R.; Feliu, J. M.; Aldaz, A.; Clavilier, J. J. Phys. Chem. 1996, 100, 2334. (16) Orts, J. M.; Go´mez, R.; Feliu, J. M.; Aldaz, A.; Clavilier, J. Langmuir 1997, 13, 3016.
Figure 1. Cyclic voltammetric curves for a Pt(111) electrode with a ruthenium coverage of 0.19 ML obtained by spontaneous deposition (solid line) and for the clean Pt(111) electrode before the deposition (dashed line). Arrows refer to the rutheniumcovered surface and indicate enhancement (arrows up) and suppression (the arrow down) of the voltammetric current. (Since the CV profile is symmetric, arrows on the negativegoing scan are not given.) Data were taken in 0.1 M HClO4 solution, at the scan rate of 50 mV s-1. Other details pertinent to this figure are given in the text (see also ref 5).
contamination during the STM imaging, allowing longer working periods.15,16 After the iodine treatment, the electrode was thoroughly rinsed with the Millipore water to remove excess of the iodide electrolyte and was mounted on the STM probe-head. Notably, in the absence of the iodine treatment, the images showed the same topographic characteristics as with the protective iodine films. This observation attests that the iodine adlayer, except for the protection that allows for longer STM imaging times, plays no role in the topography of the deposit (e.g., via compression of the Ru adlayer or other I-Ru coadsorption consequences). By use of the spontaneous deposition procedure, the coverage of ruthenium on Pt(111) was adjusted by the length of platinum-solution contact time period, from 10 to 120 s. A characteristic cyclic voltammetric (CV) response of the Pt(111) surface before and after deposition of ruthenium (contact time 2 min) is shown in Figure 1 (solid line). Note that arrows shown in Figure 1 indicate the modifications made to the CV after ruthenium addition. These modifications are5 (i) the increase of the “double layer” charging, (ii) different CV morphology in the anion adsorption range, from 0.6 to 0.85 V, and (iii) some, although not clearly defined, effect on the hydrogen adsorption/desorption processes. On a 50 × 50 nm STM image (Figure 2A) obtained after 10 s of deposition, there is already some indication that ruthenium was added to the surface (see white spots or spots agglomerations, especially in the lower right corner of the image). However, the image is too noisy to make a firm conclusion possible. Clear-cut data are obtained when a 30 × 30 nm image is examined (Figure 2B). Basic features
4946 Langmuir, Vol. 15, No. 15, 1999
Figure 2. STM image of the Pt(111) electrode (at 16 mV bias) after spontaneous deposition of ruthenium for 10 s. After the deposition, the electrode was dipped into iodide-containing solution (see text), and the surface was covered by a Pt(111)(3 × 3)-I adlattice. Ruthenium coverage is 0.01 ML; see Table 1. The 50 × 50 nm and 30 × 30 nm images are shown in parts A and B, respectively.
to focus on in this better-resolved image are the presence of a surface step and the molecularly resolved iodine structure on the wide Pt(111) terraces, perturbed by the presence of the ruthenium features (clear white spots in the image), all approximately 0.5-1 nm in diameter. As originally reported in the STM literature,17 the iodine structure on the (111) terraces is Pt(111)(3 × 3), and it corresponds to the iodine coverage 0.44 ML. The appearance of the white spots is undoubtedly due to the presence of ruthenium since, otherwise, there would be no perturbation of the crystallographically perfect, I-covered Pt(111) surface at our STM resolution.17 The spots are not uniformly distributed over the surface but concentrate in some areas of the image. This is clear evidence that ruthenium is deposited as surface islands. The right-hand inset in Figure 2 demonstrates that a specific height can be assigned to the islands. The height bars18 (Figures 2 and 3, Table 1) indicate that all spots correspond to monatomic ruthenium islands, as in the case of ruthenium deposition from sulfuric acid media on the Pt(111) electrode obtained from electrolysis.12,13 Figures 3-5 depict the morphological response of the surface along the increase in the deposition time. Initial inspection of the images shows an increase in the uptake (17) Schardt, B. C.; Yau, S.-L.; Rinaldi, F. Science 1989, 243, 981.
Letters
Figure 3. STM image of the Pt(111) electrode (at 100 mV bias) obtained after ruthenium deposition for 20 s (top). See also results of the grain size analysis of ruthenium island distributions (bottom left) and the histogram of the grain sizes (bottom right). Ruthenium coverage is 0.08 ML, Table 1. Other conditions are as in Figure 2. Table 1. Ruthenium Coverage and the Ru Island Heights as Determined by STM as a Function of Deposition Time deposition time (s)
Ru coverage (ML)
relative height in the island center (nm)
10 20 40 90 120
0.01 0.08 0.14 0.19 0.19
0.22 0.22 0.22 and 0.45 0.22 and 0.45 0.22 and 0.45
of ruthenium from Figure 3 to 5, as expected.5 Ruthenium coverage in monolayers is defined as the ratio between the area covered by the islands to the total area, and the coverage data are tabulated in Table 1. For the low coverage, i.e., 0.08 ML, obtained after 20 s of deposition, the island distribution is not uniform, with some areas of the electrode displaying higher coverage. There is no significant enhancement in the island density at the steps indicating that the spontaneous deposition is not nucleated (18) The quality of determination of the island heights (see the righthand side insets in Figures 2-5) was assessed using the grain size option available on the Nanoscope software, version 4.22. The Ru monatomic island border was set at the island height over the Pt(111) terrace of 0.11 nm (0.11 nm corresponds to a half-distance between the Ru layer and Pt(111) surface layer in a closed-packed Ru configuration). This is well above any noise level displayed by the STM system. Therefore, the center of the island gave the expected height of ca. 0.22 nm, which fits well to the theoretical interlayer distance of 0.226 nm. Throughout this study, the 0.22 nm height was used as a marker of the STM piezoelectric device. The atoms at the border of the second layer (see further) were considered to be those that yielded the height of 0.33 nm: the Pt-Ru distance plus half the expected Ru-Ru distance for the second layer.
Letters
Figure 4. STM image of the Pt(111) electrode (at 100 mV bias) obtained after ruthenium deposition for 40 s (top). Shown also are results of grain size analysis of ruthenium island distributions for all islands (bottom left) and those from the analysis of bilayer ruthenium islands (bottom right). Ruthenium coverage is 0.14 ML, Table 1. Other conditions are as in Figure 2.
Figure 5. Same as Figure 4 but after the deposition time of 90 s. Ruthenium coverage is 0.19 ML, Table 1.
by the crystallographic defects at the surface. Similarly, the histogram for the island size distribution at low coverage (Figure 3, bottom right) demonstrates thatsin
Langmuir, Vol. 15, No. 15, 1999 4947
a broad size range from ca. 0.3 to 8 nm2sno preferential island size is obtained. The islands appear to be rounded without any particular orientation with respect to the surface (Figure 3). Moreover, there seems to be no exclusion zone for the island growth. The formation of a new island takes place at any distance from a preexisting island, as there is no minimum distance between the islands. In fact, the islands can collapse while growing forming a bigger island, which no longer has a rounded shape. The key observation made in this project is however that at and above 0.14 ML (Figures 4 and 5), in addition to the monolayer islands in the image, there is also a second layer deposit on top of the inner layer, although the inner monatomic layer has not yet been completed (and is far from approaching a monolayer coverage). The formation of the bilayer islands at such a low total coverage is surprising and is in contrast to the data from previous STM studies on ruthenium electrodeposition on Pt(111).12,13 This is a new result that may provide a more detailed description of the surface properties of the deposit. For instance, if Ru2O3 oxide is formed as one of the island components, as shown by X-ray photoelectron spectroscopy (XPS) (ruthenium oxidation state 3+19), the Ru2O3 surface molecule may assume the orientation requiring the RuRu axis to be parallel to the surface normal. Systematic data concerning the island heights as a function of ruthenium coverage (Figures 3-5, Table 1) discriminate even more unambiguously between the monatomic and bilayer islands. As already indicated, only monatomic islands are found at the Ru coverage lower than 0.14 ML. At the coverage of 0.14 and 0.19 ML, the bilayer character of the islands is quite clear, especially at 0.19 ML. The height-resolved data at 0.19 ML indicate that the area of the surface covered by the bilayer islands corresponds approximately to 10% of the overall island population. Since this is only a small fraction of the relatively small total ruthenium coverage, we believe that the bilayer islands escape detection at the Ru coverage lower that 0.14 ML. Nevertheless, from the STM grain size analysis it appears that the development of the second Ru layer is restricted to islands that have a surface area of at least 2 nm2. Interestingly, extending the exposure time to the values higher that 90 s does not result in a higher ruthenium uptake (Table 1). This confirms previous data indicating that surmounting the Ru coverage of ca. 0.20 ML in the spontaneous deposition process is practically impossible.5,6 In some experiments, the stability of the ruthenium deposits with respect to hydrogen coadsorption and evolution was also examined. In the first group of experiments, the spontaneous deposit was formed and voltammetrically stabilized in 0.1 M HClO4. In this series, while keeping the electrode in the same solution, the electrode potential was set at +0.050 V for 1 min, that is, in the hydrogen underpotential deposition region. The electrode was subsequently removed from the cell, iodine treated, and examined by STM. The second experiment was similar but the end potential was in the hydrogen evolution range, -0.050 V. (The electrode exposure to hydrogen was again for 1 min.) We found that hydrogen coadsorption in the underpotential deposition region introduced no discernible changes to the STM image. In contrast, the images obtained for the electrode subjected to hydrogen evolution showed a clear diminution of the Ru coverage, for instance, from 0.19 to 0.11 ML, but still preserving the island (19) Kim, H.; Tremiliosi-Filho, G.; Haasch, R.; Wieckowski, A. In preparation.
4948 Langmuir, Vol. 15, No. 15, 1999
structure of the deposit. These results strongly suggest that the ruthenium oxide reduction process19 at such a negative potential is accompanied by a partial ruthenium dissolution from the surface. Finally, it was also interesting to investigate the ruthenium deposit removed from the cell at an open circuit potential where the spontaneous deposit actually forms, i.e., without the voltammetric treatment and without iodine protection. The deposition time was 90 s, which is expected to yield 0.19 ML coverage; see above. Under such conditions, the images were noisy and essentially featureless, displaying some structural features that disappear with time. After 2 min of imaging the noise was significantly reduced and the image was clear, characteristic of a clean platinum surface perturbed only by a highly reduced number of the islands in the zooming area, which corresponds roughly to ca. 0.01 ML of Ru coverage. Since the original (and noisy) signal was obtained while imaging other areas of the electrode, we conclude that the Ru species can easily be rearranged or removed from the surface by the STM tip. This is in clear contrast to the STM behavior of the voltammetrically stabilized ruthenium islands, which are invariant to imaging in any practical experimental times. On the basis of these observations, we conclude that the Ru species formed spontaneously at the open circuit potential5 are a precursor of the Ru islands that are formed in the process of the voltammetric treatment of the deposit. Conclusions Exposure of the Pt(111) electrode surface under open circuit conditions to ruthenium chloride solution in perchloric acid causes spontaneous deposition of ruthenium species, which do not develop a surface structure accessible to STM. However, when such species are subjected to a brief voltammetric treatment, the surface displays an array of Ru islands that are nanometer in size, and largely monatomic. Despite this dominating monatomic character of the islands, STM data indicate that ca. 10% of the deposit is made of bilayer islands. The
Letters
result that such bilayer islands are formed at low coverage is a new observation that is related to the composition and morphology of the ruthenium deposits formed under a variety of electrochemical conditions. XPS data toward chemical identification of the chemical state of the oxidized ruthenium species as the island components are already available.19 Since the electrode structure in solution is poised by iodine surrounding the ruthenium islands (Results and Discussion, Figure 2B), the STM images obtained in air are a replica of the in situ distributions of the islands. (The islands appear on the STM image even without iodine, a hallmark of their inherent stability.) To appreciate the significance of this result, one needs to recall that as required by the bifunctional mechanism of methanol oxidation on Pt/Ru,20 only the Pt component of the surface is capable of activating methanol toward its decomposition to CO, while the ruthenium coadditive is effective in transformation of CO to CO2.3 Therefore, the data and the mechanistic entries identify the active surface site involved in platinum/ruthenium methanol electrooxidation electrocatalysis. This active site is at the edge of a ruthenium island deposited on the platinum substrate. The extent to which this conclusion will lead to better control of smallparticle fuel cell catalysis involved in the methanoloxidation process will be addressed in further work by this group. Acknowledgment. This work was supported by the National Science Foundation under Grant CHE 97-000963 (U.S.A.), by the Department of Energy Grant DEFG0296ER45439 administered by the Frederick Seitz Materials Research Laboratory at the University of Illinois, and by the DGES Grant, PB96-0409 (Spain). A.W. acknowledges helpful discussions with Hongsun Kim during preparation of this paper for publication as well as his help with the paper editing. LA990219V (20) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267.
