Preparation and Electrochemical Behavior of Ordered Rh Adlayers on

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Preparation and Electrochemical Behavior of Ordered Rh Adlayers on Pt(100) Electrodes F. Javier Gutie´rrez de Dios, Roberto Go´mez,* and Juan M. Feliu Departament de Quı´mica Fı´sica i Institut Universitari d’Electroquı´mica, Universitat d’Alacant, Apartat 99, E-03080 Alicante, Spain Received April 1, 2005. In Final Form: May 18, 2005 Rhodium adlayers on Pt(100) substrates have been prepared by electrodeposition from dilute Rh(III) acidic solutions. The initially disordered layer is electrochemically annealed by applying a polarization program consisting of high-sweep-rate multicycle sequences between 0.05 and 0.78 VRHE in 0.1 M H2SO4. In this way, a pseudomorphic Rh monolayer can be prepared on Pt(100) substrates. The degree of order of the electrochemically annealed layer has been evidenced not only through voltammetric experiments but also by means of scanning tunneling microscopy with atomic resolution for iodine-protected adlayers, which show a c(2 × 2) structure. The electrochemically induced ordering of the Rh adlayer appears to be a consequence of the repeated cycles of adsorption/desorption of H and, especially, oxygenated species. Voltammetry in sulfuric acid solutions permits examination of the energetics of H/anions and OH/O adsorption as a function of the Rh coverage. The first monolayer adsorbs both hydrogen and oxygenated species more strongly than the second one. This can be explained through an electronic effect caused by the underlying Pt(100) substrate.

1. Introduction Among the different aspects of the electrochemical Surface Science, the modification of surface properties by introduction of metallic heteroatoms occupies a paramount place. Within this wide topic, the preparation and properties of noble-metal-on-platinum electrodes has a particular importance because of the unparalleled electrocatalytic properties of platinum samples, which can be further improved by the introduction of adatoms of another Ptgroup metal. Most of these studies have dealt with Pd adlayers on Pt(111)1-12 and Pt(100) electrodes.2,13-17 The adlayers of Rh on Pt(111) electrodes have also been studied a number of times.6,18-20 Because of their interest as * Corresponding author. Tel.: +34-965903536. Fax: +34965903537. E-mail: [email protected]. (1) Clavilier, J.; Llorca, M. J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1991, 310, 429. (2) Clavilier, J.; Llorca, M. J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1993, 351, 299. (3) A Ä lvarez, B.; Climent, V.; Rodes, A.; Feliu, J. M. Phys. Chem. Chem. Phys. 2001, 3, 3269. (4) Feliu, J. M.; A Ä lvarez, B.; Climent, V.; Rodes, A. In Thin Films: Preparation, Characterization and Applications; Soriaga, M. P., Ed.; Kubler/Plenum: New York, 2002; p 37. (5) A Ä lvarez, B.; Rodes, A.; Pe´rez, J. M.; Feliu, J. M. J. Phys. Chem. B 2003, 107, 2018. (6) Inukai, J.; Ito, M. J. Electroanal. Chem. 1993, 358, 307. (7) Attard, G. A.; Al-Akl, A. Electrochim. Acta 1994, 39, 1525. (8) Climent, V.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. B 2000, 104, 3116. (9) Schmidt, T. J.; Markovic, N. M.; Stamenkovic, V.; Ross, P. N.; Attard, G. A.; Watson, D. J. Langmuir 2002, 18, 6969. (10) Arenz, M.; Stamenkovic, V.; Schmidt, T. J.; Wandelt, K.; Ross, P. N.; Markovic, N. M. Surf. Sci. 2003, 523, 199. (11) Hoyer, R.; Kibler, L. A.; Kolb, D. M. Electrochim. Acta 2003, 49, 183. (12) Arenz, M.; Stamenkovic, V.; Schmidt, T. J.; Wandelt, K.; Ross, P. N.; Markovic, N. M. Phys. Chem. Chem. Phys. 2003, 5, 4242. (13) Llorca, M. J.; Feliu, J. M.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1994, 376, 151. (14) Attard, G. A.; Price, R. Surf. Sci. 1995, 335, 63. (15) Ball, M. J.; Lucas, C. A.; Markovic, N. M.; Stamenkovic, V.; Ross, P. N. Surf. Sci. 2003, 540, 295. (16) Arenz, M.; Stamenkovic, V.; Ross, P. N.; Markovic, N. M. Electrochem. Commun. 2003, 5, 809. (17) A Ä lvarez, B.; Berna´, A.; Rodes, A.; Feliu, J. M. Surf. Sci. 2004, 573, 32.

electrocatalysts for organic fuel oxidation, a considerable effort has been devoted to Ru adlayers on both Pt(111)21-24 and Pt(100) electrodes.21,25 Finally, some work has been devoted to Os/Pt(111).26,27 Both the structural and electrocatalytic aspects of these systems have been thoroughly reviewed very recently.28 The preparation of Pt electrodes modified by another Pt-group metal entails in most cases the electrodeposition of the adlayer from a solution containing a suitable aquocation or complex of the metal to be deposited. The electrodeposition has been done either under potential control or by exposing the Pt electrode with a droplet of the metal solution to a flow of a mixture H2 + Ar.1 This is tantamount to the application of a low-enough potential (actually that corresponding to the H+/H2 couple). The latter procedure is known sometimes as the forced deposition method. These simple experimental procedures do not provide, in general, a control over the deposition rate and sometimes lead to a growth in three-dimensional clusters. Such an adlayer growth method, although useful in screening applications, is far from desirable for performing fundamental studies in electrocatalysis. There are several effects, electronic and geometric, which can be better studied in ordered bimetallic alloy surfaces and in one-atom-height adatom islands. In addition, the (18) Go´mez, R.; Feliu, J. M. Electrochim. Acta 1998, 44, 1191. (19) Attard, G. A.; Price, R.; Al-Akl, A. Surf. Sci. 1995, 335, 52. (20) Go´mez, R.; Gutie´rrez de Dios, F. J.; Feliu, J. M. Electrochim. Acta 2004, 49, 1195. (21) Chrzanowski, W.; Wieckowski, A. Langmuir 1997, 13, 5979. (22) Lu, G.-Q.; Waszczuk, P.; Wieckowski, A. J. Electroanal. Chem. 2002, 532, 49. (23) Koper, M. T. M.; Levedeva, N. P.; Hermse, C. G. M. Faraday Discuss. 2002, 121, 301. (24) Spendelow, J. S.; Lu, G. Q.; Kenis, P. J. A.; Wieckowski, A. J. Electroanal. Chem. 2004, 568, 215. (25) Crown, A.; Johnston, C.; Wieckowski, A. Surf. Sci. 2002, 506, L268. (26) Crown, A.; Moraes, I. R.; Wieckowski, A. J. Electroanal. Chem. 2001, 500, 333. (27) Rhee, C. K.; Wasikawa, M.; Tolmachev, Y. V.; Johnston, C. M.; Haasch, R.; Attenkofer, K.; Lu, G. Q.; You, H.; Wieckowski, A. J. Electroanal. Chem. 2003, 554-555, 367. (28) Spendelow, J. S.; Wieckowski, A. Phys. Chem. Chem. Phys. 2004, 6, 5094.

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possibility of preparing pseudomorphic films opens up the exploration of electrodes with intrinsic properties different from both those of the substrate and those of the metal forming the monolayer (ML). The preparation of complete pseudomorphic layers has been demonstrated in the case of Pd and Rh on Pt(111).2,18 The formation of such layers on Pt(100), a more open surface, is more difficult. For instance, the deposition of the second layer begins prior to the completion of the first one in the case of Pd.2,17 In the case of Rh, trials to obtain a full monolayer or beyond have rendered highly disordered adlayers.29-32 In fact, several procedures were attempted for preparing ordered Rh adlayers on Pt(100) substrates. The so-called forced deposition method, employed early for preparing thick adlayers (multilayers) on Pt(100),31 did not yield samples with well-defined, reproducible-enough voltammetric profiles. Electrodeposition from dilute rhodium solutions in sulfuric acid improved the quality of the deposit to some extent, but best results were obtained through electrodeposition at 0 V RHE from a Rh(III) acidic solution containing chloride (10-3 M NaCl + 0.1 M HClO4).33 The voltammetric profiles obtained by Tanaka and co-workers32,34 by means of a combined electrochemical-ultrahigh vacuum procedure rendering ordered adlayers were akin to those obtained by us, thus supporting the formation of well-ordered adlayers also for our simple electrochemical procedure. However, this electrodeposition procedure seems to be less suitable for attaining high Rh coverage values (close to 1 ML or higher). In this work, we present a new method for preparing Rh monolayers (and bilayers) on Pt(100) electrodes. It combines electrodeposition from dilute Rh(III) acidic solutions and subsequent electrochemical annealing of the adlayer by means of high-sweep multicycle sequences applied to this electrode in a sulfuric acid solution. A precedent of the electrochemical annealing presented here and reported by Armand and Clavilier consisted of the application of voltammetric multicycle sequences to a Pt(100) electrode to tailor the voltammetric profile of this basal plane.35 Finally, this procedure also resembles that presented in the 1980s by the Arvia group concerning the electrochemical preferential faceting of fcc metals such as Pt, Rh, Au, or Pd.36 2. Experimental Section Pt(100) single-crystal electrodes were prepared, cut, and polished according to a procedure described previously.37 Prior to each experiment, the electrode was flame-annealed and cooled in a mixture of hydrogen + argon. Once protected with a droplet of ultrapure water in equilibrium with the cooling atmosphere, the electrode was transferred to a conventional electrochemical cell. All of the experiments were done at room temperature. Potentials were measured against and are referred to a reversible (29) Taniguchi, M.; Kuzembaev, E. Z.; Tanaka, K. Surf. Sci. 1993, 290, L711. (30) Tamura, H.; Tanaka, K. Langmuir 1994, 10, 4530. (31) Go´mez, R.; Rodes, A.; Pe´rez, J. M.; Feliu, J. M.; Aldaz, A. Surf. Sci. 1995, 344, 85. (32) Sasahara, A.; Tamura, H.; Tanaka, K. J. Phys. Chem. 1996, 100, 15229. (33) Gutie´rrez de Dios, F. J.; Go´mez, R.; Feliu, J. M. Electrochem. Commun. 2001, 3, 659. (34) Tanaka, K.; Okawa, Y.; Sasahara, A.; Matsumoto, Y. In SolidLiquid Electrochemical Interfaces; Jerkiewicz, G., Soriaga, M. P., Uosaki, K., Wieckowski, A., Eds.; ACS Symposium Series: Washington, 1997; p 245. (35) Clavilier, J.; Armand, D. J. Electroanal. Chem. 1986, 199, 187. (36) Arvıá, A. J.; Canullo, J. C.; Custidiano, E.; Perdriel, C. L.; Triaca, W. E. Electrochim. Acta 1986, 31, 1359. (37) Clavilier, J.; Armand, D.; Sun, S.-G.; Petit, M. J. Electroanal. Chem. 1986, 205, 267.

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Figure 1. (A) Series of successive voltammetric profiles obtained during electrodeposition of rhodium onto a Pt(100) electrode in contact with a 0.1 M H2SO4 solution containing 5 × 10-5 M Rh3+. (B) Voltammetric profile for a Pt(100) electrode in a Rh-free 0.1 M H2SO4 solution. Sweep rate: 50 mV s-1. hydrogen electrode (RHE). Working solutions were prepared with ultrapure water (Millipore MilliQ) and Merck Suprapur H2SO4. Rhodium(III) solutions were prepared by dissolving Rh powder (Merck) in hot concentrated sulfuric acid. Initial rhodium adlayers were prepared by electrodeposition, carried out during voltammetric cycles from 0.05 to 0.7 or 0.8 V at a sweep rate of 50 mV s-1 in 5 × 10-5 M Rh(III) + 0.1 M H2SO4 solution. Scanning tunneling microscopy (STM) experiments were carried out in air using a Nanoscope III system (Digital Instruments). Tips were prepared from 0.25-mm diameter PtIr wires (80:20 alloy) by an electrochemical AC melt etching procedure in a mixture of NaOH and NaNO3 (6:7 mass ratio, mp 270-300 °C). Thermal drift was found to be the main difficulty for obtaining correct measurements of interatomic spacings. Thus, we used scan rates from 15 to 60 Hz (corresponding to a y-direction scan rate of around 1 nm/s) to minimize the influence of drift. The images presented in the following were stable and virtually identical for consecutive upward and downward sweeps.

3. Results and Discussion 3.1. Deposition and Characterization of the First Monolayer of Rh on Pt(100) Electrodes. Figure 1A shows successive voltammetric profiles corresponding to a complete electrodeposition experiment, consisting in this case of 19 voltammetric cycles. The voltammetric profile for a Pt(100) electrode in a Rh-free 0.1 M H2SO4 solution is also given for the sake of comparison (Figure 1B). The progressive deposition of Rh adatoms is reflected in the appearance of a couple of peaks at 0.15 V whose reversibility (estimated from the Ep,a - Ep,c values) increases as the deposition process takes place. There is a concurrent and progressive diminution in the signals associated to free platinum sites [peaks at 0.38 V corresponding to adsorption/ desorption processes at (100) terrace sites], but no significant changes in either shape or potential of

Ordered Rh Adlayers on Pt(100) Electrodes

the Pt voltammetric features are detected during the deposition process. Because of the low concentration of Rh(III) being employed (50 µM), the relatively high sweep rate (50 mV s-1), and the sluggishness of the process, the electroreduction of Rh(III) does not give rise to well-defined voltammetric features. Instead, a small deviation of the apparent current baseline toward negative values (reduction) can be observed. This process is continued until the voltammetric signals corresponding to the bare Pt sites are virtually suppressed. The broadness of the voltammetric peaks associated with the Rh adatoms points to the fact that rhodium disordered islands or clusters are formed. This growth mode is favored for this system since the calculated average surface energy for Rh(100) is higher than that for Pt(100), 2.90 vs 2.17 J m-2, respectively.38 Ideally, the Bauer relation, which takes into account not only the surface energies but also the interfacial energies, could be employed to predict accurately the adlayer growth mode.39 Unfortunately, this criterion is very difficult to use in practice, especially if we take into account that adsorption of solvent and electrolyte species alters the values of the vacuum surface energies. In any case, the initial formation of clusters occurs although the lattice mismatch between these two metals is quite low, which should favor a Frank-van der Merwe growth.39,40 The actual deposition mode leaves large platinum regions unaffected, which show a behavior similar to that of bare Pt(100) electrodes. The deposition of Rh adatoms is also witnessed by the appearance of a couple of peaks at around 0.55 V, ascribed to the adsorption-desorption of oxygenated species at rhodium adatoms. Tanaka et al. have reported a similar general behavior32,34 and suggested an initial random growth of the Rh adlayer on the basis of the high background of the corresponding low-energy electron diffraction pattern. Once the Rh adlayer is deposited, the electrode is rinsed with ultrapure water and transferred to another cell containing a Rh-free 0.1 M H2SO4 solution. The dashed line in Figure 2 shows the stationary voltammogram corresponding to the Rh disordered layer (as deposited). It is virtually identical to that attained in the last voltammetric cycle of the electrodeposition process (Figure 1A). This indicates that, as expected, the very low concentration of Rh(III) employed in the deposition solution (5 × 10-5 M) does not have any noticeable further effect on the voltammetric profile. Nevertheless, we can still discern in this voltammogram a tiny contribution corresponding to free platinum sites (small peak at 0.38 V corresponding to Pt(100) terrace sites). Immediately after recording this stationary voltammogram, we proceeded to apply a polarization program consisting of high-sweep-rate multicycle sequences for approximately 4 min. The potential limits chosen for this program were 0.05 and 0.78 V. The sweep rate used was 50 V s-1. The solid line in Figure 2 shows the stationary voltammetric profile recorded at 50 mV s-1 and corresponding to the Rh/Pt(100) electrode once submitted to the above-mentioned procedure. As observed, the rather featureless initial voltammogram is transformed into a well-defined one. The development of a couple of sharp reversible peaks located at 0.18 V and a pair of sharp quasi-reversible peaks situated at 0.52 V is an indication of a narrower range of adsorption energies, expected for a higher degree of surface order. Between these two couples of peaks there is a wide region with no apparent process (38) Baskes, M. I. Phys. Rev. B 1992, 46, 2727. (39) Chang, T.-M.; Carter, E. A. Surf. Sci. 1994, 318, 187. (40) Chang, T.-M.; Carter, E. A. J. Phys. Chem. 1995, 99, 7637.

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Figure 2. (Solid line) Stationary voltammetric profile corresponding to a well-ordered full monolayer of Rh on Pt(100) after electrochemical annealing. (Dashed line) Corresponding profile immediately after electrodeposition. Working solution: 0.1 M H2SO4. Sweep rate: 50 mV s-1.

of adsorption/desorption and basically corresponding to double-layer charging currents. Interestingly, the small voltammetric signal associated with bare Pt(100) disappears. This means that initially the deposit contains a fraction of adatoms corresponding to second and perhaps successive Rh monolayers. Upon electrochemical annealing these adatoms relocate, covering the free Pt sites, which leads to the completion of the first layer. Palladium adlayers on Pt(100) show a similar behavior.2,17 In the case of high Rh coverages, the voltammetric profiles recorded after application of the electrochemical annealing procedure are better defined than those obtained by means of former methods: by electrodeposition at 0 V RHE from dilute Rh(III) solution in the presence of chloride (10-3 M NaCl + 0.1 M HClO4)33 and by following a combined electrochemical-ultrahigh vacuum procedure.32,34 This, on the whole, supports the formation of highly ordered adlayers. Determining the curve of total charge vs potential is helpful for ascribing the voltammetric features to the different interfacial processes. This requires the knowledge of the total charge at least at one potential and the integration of the voltammogram. The current transient recorded during the adsorption of CO at that potential, once integrated, corresponds to minus the total charge borne by the electrode at that potential (charge displacement experiment).3,17,18,31 Concretely, in Figure 3(inset), the current transient recorded upon CO adsorption at 0.25 V is shown. Its integration gives a value of -71 µC cm-2. With this value, it is possible to plot the total charge vs potential curve (see Figure 3) together with the corresponding cyclic voltammogram for the sake of comparison. As observed, the potential of zero total charge is located at 0.175-0.18 V, that is, at the main peak potential in the hydrogen adsorption region. This indicates that for potentials lower than 0.18 V, the hydrogen adsorption/ desorption process accounts for most of the current flowing, whereas for more positive potentials (within the hydrogen region) most of the current is due to the reversible

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Figure 3. Stationary voltammogram (dashed line) for an ordered Rh monolayer on Pt(100) together with the corresponding total charge density vs potential curve (solid line). Inset: CO charge displacement current transient recorded at 0.25 V. The arrow indicates admission of CO.

adsorption/desorption of (bi)sulfate anions. On the other hand, the voltammetric process occurring at potentials more positive than 0.5 V should be ascribed to the adsorption of oxygenated species, probably OH, since bisulfate adsorption already takes place in the low potential region. A total charge of 227 µC cm-2 is found at 0.10 V (current minimum near the onset of the hydrogen evolution reaction). If we correct for the diffuse layer contribution by assuming in a crude approximation that the double layer capacity is constant and that the potential of zero free charge coincides with the potential of zero total charge, the charge corresponding to the adsorption of hydrogen would amount to 217 µC cm-2. This value agrees well with that expected for a full hydrogen monolayer (one H atom per Rh adatom). The reversible currents appearing at potentials less positive than 0.10 V could be tentatively ascribed to subsurface hydrogen adsorption, since the corresponding coverage values would surpass unity. On the other hand and on the basis of the total charge curve, the coverage of oxygenated species at 0.7 V is quite high, approaching one on the basis of one electron exchanged per adspecies. The choice of the potential limits as well as the sweep rate for the electrochemical annealing was made according to previous reports. In particular, Clavilier and Armand35 devised a similar treatment for tailoring the voltammetric response and, therefore, the surface structure of air-cooled Pt(100) electrodes. They observed that the relative weight of the different voltammetric adsorption states in the hydrogen adsorption/desorption region could be reversibly changed by submitting the electrode to fast cycling (50200 V s-1) between 0.07 and 1.00 V. It is noteworthy that in our case the positive limit during the electrochemical annealing was set at 0.78 V, a value significantly lower than that used for bare Pt(100) electrodes. Remarkably, the procedure employed by us did not trigger any change in the voltammetric profile of the hydrogen-cooled Pt(100) electrode. Therefore, the ordering of the Rh adlayer should not be attributed to a restructuring of the Pt(100) substrate. As mentioned in the Introduction section, a similar annealing procedure has been proposed for the faceting of spherical polyoriented single-crystal electrodes of


different noble metals, and, in particular, of Rh.41,42 However, in the latter case, the positive potential limit was either 1.20 or 1.55 VRHE, much higher than that employed here. Such positive potential limits are not applicable in our case because they would give rise to a disordering of the Pt(100) substrate. In our case, the adlayer ordering is probably caused by the potentialinduced repeated cycles of adsorption/desorption of hydrogen adatoms, on one hand, and oxygenated species on the other. The adsorption of the latter is deemed to be important as it could lead to an increased mobility of the Rh adatoms. In any case, the adsorption of oxygenated species seems to play a crucial role in the annealing process. Indeed, the positive potential limit employed both for Pt(100) and Rh/Pt(100) electrodes allows the adsorption/desorption of a significant coverage of oxygenated species. In the case of polyoriented Rh surfaces,41,42 repetitive electrodissolution and electrocrystallization processes were suggested to be responsible for the faceting. In our case, this contribution does not seem to be so important because most of the Rh adatoms remain at the surface after the treatment. The positive potential limit employed by us is relatively low (0.78 V), but as the standard reversible potential for the couple Rh3+/Rh is 0.758 VSHE,43 electrodissolution from three-dimensional clusters could also have a role in the annealing process. The sensitivity of the annealing process to the negative potential limit is not so high, which means that the role of hydrogen adsorption on the smoothing of the Rh adlayer is not as decisive as the role of oxygen adsorption and Rh dissolution/deposition. It is noteworthy that a related procedure has been employed for annealing a Rh layer on Pt(100) in a UHV environment. Tanaka and co-workers,32,34 after electrochemical deposition of a Rh adlayer onto a Pt(100) electrode, transferred it to a UHV chamber and exposed it to a low-pressure atmosphere of O2 and, subsequently, of H2 at different temperatures, which led to an ordered Rh adlayer. Our procedure is related to the latter in the sense that also in our case the (electrochemical) adsorption of hydrogen and oxygen seems to play a fundamental role in the annealing process. Although the evolution of the voltammograms clearly indicates that the fast cycling procedure leads to an ordering of the adlayer, more direct evidence can be achieved by means of STM images of the ordered adlayer, if possible, with atomic resolution. Once the annealed monolayer of Rh on a Pt(100) electrode was attained, the electrode was transferred to a 0.01 M KI solution for about 1 min to obtain a saturated compact overlayer of iodine. Covering the electrode with iodine has a 2-fold purpose: first, the iodine adlayer protects the electrode surface from contaminants and, second, it facilitates the obtainment of STM images with atomic resolution. After contact with the iodide-containing solution, the electrode was removed from solution, rinsed with ultrapure water, and transferred to a cell containing a 0.1 M H2SO4 test electrolyte. The corresponding voltammogram (solid line in Figure 4) shows a virtually complete blockage of the electrode adsorption capability in the low potential region. Only a capacitative current appears without a trace of either hydrogen or anion adsorption (compare with the voltammogram before immersion in the iodine-containing (41) Canullo, J.; Custidiano, E.; Salvarezza, R. C.; Arvıá, A. J. Electrochim. Acta 1987, 32, 1649. (42) Meńdez, E.; Castro Luna, A. M.; Cerda´, M. F.; Mombru´, A. W.; Zinola, C. F.; Martins, M. E. J. Solid State Electrochem. 2003, 7, 208. (43) CRC Handbook of Chemistry and Physics, 74th ed.; Lide, D. R., Ed.; CRC: Boca Raton, 1993.


Figure 4. (Dashed line) Stationary voltammetric profile corresponding to a full monolayer of Rh on Pt(100) after electrochemical annealing. (Solid line) Voltammetric profile corresponding to the same electrode after immersion into a 10-2 M KI aqueous solution for about 1 min. Working solution: 0.1 M H2SO4. Sweep rate: 50 mV s-1.

Figure 5. STM image (constant-height) for a Rh monolayer on Pt(100) covered in turn by a monolayer of iodine. (Inset) Corresponding FFT spectrum.

solution and corresponding to the dashed line in Figure 4). Finally, we proceeded to the STM characterization of our sample. Figure 5 corresponds to a constant-height image of the Pt(100)-Rh-I surface. We can clearly distinguish a long-range ordered square adlayer of iodine. The inset in Figure 5 shows the corresponding Fourier transform (FT)-STM image. The FT-STM image looks directly into the reciprocal space and allows the identification of features which could not be distinguished in the original, real-space STM image because they are

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Figure 6. STM image (constant-height) for a well-ordered Rh(100) substrate covered by a monolayer of iodine. (Inset) Corresponding FFT spectrum.

obscured by the main periodicity due to the iodine overlayer. The information on the main periodicities is contained in the reciprocal space image, where spots forming two squares of different sizes and orientation clearly indicate the existence in the real space of two square lattices. The main contribution (spots labeled A, forming a square and corresponding to an atomic periodicity of 0.38 ( 0.01 nm) is associated with a (x2 × x2) lattice linked to the iodine overlayer. Interestingly, weaker spots forming a larger square rotated 45° (marked with an arrow and corresponding to a periodicity of 0.27 ( 0.01 nm) also appear. They correspond to the (1 × 1) structure of the Rh adatoms. This experiment indicates that, once submitted to the electrochemical annealing, the Rh adlayer on Pt(100) is pseudomorphic. The iodine adatoms on the Rh/ Pt(100) surface form an ordered c(2 × 2) adlayer ((x2 × x2)R45° - I adlayer). It is interesting to compare the ordered overlayer formed by iodine on the pseudomorphic Rh adlayer with that formed on a Rh(100) surface. A Rh(100) sample was prepared following a method similar to that used for Pt single-crystal electrodes.44 It was flame-annealed and cooled in a H2 + Ar atmosphere. Next, the electrode was placed in contact with a 0.1 M H2SO4 solution to check that its voltammogram coincided with that of a wellordered surface. Then the electrode was immersed, as in the case of the Pt(100)-Rh electrode, in a 0.01 M KI solution, which led to the virtual suppression of its adsorption capabilities in the hydrogen adsorption/desorption region. Figure 6 shows a constant-height STM image with atomic resolution corresponding to the iodineprotected Rh(100) sample. Again, an ordered square array appears, attributable to an adlayer of iodine on Rh(100). The corresponding FT image shows anew four squarearranged spots due to the I adlayer (labeled with A) together with four weaker spots (see arrow) arranged in a square rotated 45° with respect to that formed by spots A. As in the case of the Rh pseudomorphic layer, the iodine adatoms (44) Clavilier, J.; Wasberg, M.; Petit, M.; Klein, L. H. J. Electroanal. Chem. 1994, 374, 123.

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Figure 7. Series of successive voltammetric profiles obtained during electrodeposition of rhodium onto an ordered RhML/Pt(100) electrode in contact with a 0.1 M H2SO4 solution containing 5 × 10-5 M Rh3+. Sweep rate: 50 mV s-1.

form a c(2 × 2) adlayer on Rh(100). This fact suggests that the Rh monolayer on Pt(100) has an atomic arrangement and degree of order similar to those of a Rh(100) surface. As far as we know only one previous study on the structure of Rh(100)-I has been published.45 Yang and co-workers dosed iodine from gas phase and obtained the same structure for the saturation iodine adlayer on Rh(100), which demonstrates that the same adlayer is formed regardless of the dosing procedure employed. Although the iodine adlattice presents frequently more vacancies when prepared from iodide solutions, this dosage method avoids the formation of multilayers and, more importantly in the present context, does not alter significantly the structure of the substrate, which is crucial for characterization purposes. 3.2. Deposition and Characterization of Rh Adlayers with Coverage Higher Than One. After growing an ordered Rh monolayer on Pt(100) in the way described above, the electrode was again immersed in a 5 × 10-5 M Rh3+ + 0.1 M H2SO4 solution at a potential of 0.78 V. Then, the electrodeposition of further Rh was carried out as in the first monolayer case, that is, under cyclic voltammetric conditions, between 0.05 and 0.78 V at 50 mV s-1. Figure 7 shows the successive profiles corresponding to a typical deposition experiment, consisting of 14 cycles in this case. As observed, from the very first negative-going scan, the voltammetric profile is significantly different from that of Figure 2 (solid line). There is a remarkable broadening of both the couple of peaks at 0.18 V, linked to the adsorption/desorption of H/anions on the Rh adlayer and the couple at 0.52 V, due to the adsorption/desorption of oxygenated species on Rh. During subsequent cycles, the peaks located at around 0.18 V diminish; meanwhile, a new contribution (as a shoulder in the positive-going sweep and as a peak in the negativegoing one) at 0.17 V appears and grows. The broadness of the different voltammetric contributions points to the fact that rhodium disordered islands are again formed on the ordered Rh/Pt(100) surface. The electrode was then transferred to a Rh-free 0.1 M H2SO4 solution, and the electrochemical annealing procedure was performed as in the case of the first monolayer. (45) Yang, C.-H.; Yau, S.-L.; Fan, L.-J.; Yang, Y.-W. Surf. Sci. 2003, 540, 274.

Figure 8. (Solid line) Stationary voltammetric profile corresponding to a well-ordered Rh adlayer (about 1.5 ML) on Pt(100) after electrochemical annealing. (Dashed line) Profile resulting from electrodeposition. Working solution: 0.1 M H2SO4. Sweep rate: 50 mV s-1

The dashed line in Figure 8 represents the stationary voltammogram of the as-deposited adlayer of Rh on an ordered RhML/Pt(100) electrode. It coincides with the last voltammetric profile obtained in the electrodeposition solution (Figure 7). The solid line in Figure 8 corresponds to the stationary voltammetric profile obtained after submitting the electrode to the electrochemical annealing procedure. As in the case of the Rh monolayer, the change in shape of the voltammogram is remarkable. Again one notices the pair of sharp reversible peaks situated at 0.18 V and corresponding to the adsorption/ desorption of hydrogen/anions on Rh adatoms in contact with Pt substrate sites (first monolayer). Another pair of sharp quasi-reversible peaks at 0.52 V is due to the process of adsorption-desorption of oxygenated species on the first monolayer of Rh. In comparison with the voltammogram for the first complete monolayer (dashed line in Figure 2), both contributions show a clear diminution, as expected, because the Rh adatoms in the growing second layer block the adsorption states characteristic of the underlying first monolayer. In turn, the partial formation of a second layer is revealed by the appearance of two new pairs of peaks located now at 0.17 V (adsorption/desorption of hydrogen/anions) and at 0.62 V (reversible adsorption/ desorption of oxygenated species). A crude comparison of the charge involved in both types of features allows us to estimate a total Rh coverage around 1.5 ML. If the growth occurs according to the Frank-van der Merwe model, two different levels (uncovered portions of the first monolayer and islands corresponding to the growing second layer) should be distinguished in a STM image. As in the previous cases, prior to acquiring the STM images, the electrode was immersed in a 0.01 M KI solution in order to protect the surface with an iodine adlayer. A constant-height image corresponding to this sample is shown in Figure 9. Clearly, two levels are found at the surface, revealing that the second layer also grows pseudomorphically onto the first one. It is remarkable


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and shape of the different peaks, especially for couples b1/b1′ and a2/a2′, do not depend on the surface Rh amount, we can estimate its coverage (θi) by taking into account that, in a first approximation

j(E) ) j1ML(E)θ1ML + j2ML(E)θ2ML

(1)

θ1ML + θ2ML ) 1

(2)

where j(E) is the total current recorded at any potential and, in particular, at the potential of the different peaks (corrected for the dl contribution); j1ML(E) and j2ML(E) are the current densities for the voltammograms corresponding to 1 ML and 2 ML, respectively (Figure 10A,F). We have calculated the total Rh coverage (θRh) on the basis of peaks b1 and b1′ according to

θRh ) 1 + θ2ML ) 1 +

j - j1ML j2ML - j1ML

(3)

A calculation has also been done on the basis of currents for peaks a2 and a2′ according to Figure 9. STM image (constant-height) for an iodine overlayer on a Rh/Pt(100) electrode with a rhodium coverage of 1.5.

that atomic resolution is achieved, especially for I adsorbed on the uppermost Rh layer, showing again the typical square array of the c(2 × 2) surface lattice. During the growth of the second adlayer, Rh forms small islands (a few nanometers in size) on the Pt(100)-RhML surface. This adlayer structure is typical of noble metal adatoms having high cohesive energies, which favors the formation of compact islands rather than ordered adlayers of low coverage. From Figure 9 the average size of the islands in the second layer can be evaluated. It happens to be as low as 2-3 nm. The size of these islands is significantly smaller than that of the mesas46 formed upon flame annealing and cooling in the hydrogen + argon atmosphere. For our electrodes, their average size is 10-15 nm and, in addition, are located only in the center of the terraces. The image shown in Figure 9 was acquired near a boundary of a terrace where no mesas could be distinguished. Finally, Figure 10 shows voltammetric profiles for Pt(100) substrates with deposits of rhodium from 1 ordered ML to 2 ordered ML in contact with a 0.1 M H2SO4 solution. They were prepared as described above, but for progressively higher times (A to F) in the electrodeposition solution. In all cases, the electrochemical annealing procedure was applied. As already mentioned, the first Rh monolayer, in direct contact with the Pt(100) substrate, is characterized by the (quasi)-reversible pairs of peaks labeled a1 and a2. The second layer gives rise to the voltammetric features labeled b1 and b2. It is worth noting that neither the potential nor the shape of the different voltammetric peaks changes for increasing Rh coverage values. This is a reflection of the growth mode of the adlayer, characterized by the formation of compact islands whose electrosorptive properties would not change significantly as a function of coverage. A similar behavior has been found for other cases of Rh and Pd growth on Pt(hkl) electrodes.2,4-7,14,17-20. Given that the voltammetric contributions from the first and second layers overlap, the evaluation of the Rh coverage on the basis of the integrated charge is not straightforward. If we take into account that the position (46) Villegas, I.; Weaver, M. J. J. Electroanal. Chem. 1994, 373, 245.

θRh ) 2 - θ1ML ) 2 -

j - j2ML j1ML - j2ML

(4)

The values of coverage given by eqs 3 and 4 agree within (0.10. They are given in the caption for Figure 10. It is worth noting that thicker adlayers can be grown and subsequently annealed following the procedure described above. In fact, preliminary results indicate that the third monolayer gives a voltammetric response similar to that of the second monolayer. This behavior is analogous to that found for Rh deposited on Pt(111) electrodes.18,19 3.3. Adsorption Properties of the First and Second Rh Monolayers. Apart from its inherent fundamental interest, an understanding of the adsorptive properties of different bimetallic surfaces is essential for attaining a solid basis allowing a rational design of electrocatalysts or, more generally, of heterogeneous catalysts. Cyclic voltammograms in an inert electrolyte for both Rh and Pt electrodes furnish valuable information on the thermodynamics and kinetics of the hydrogen and OH adsorption processes. In the case of Rh electrodes, the application of potentials between 0.05 and 0.70 V in 0.1 M H2SO4 solutions only gives rise to (pseudo)capacitive currents. In addition, the surface structure is stable in this potential window. It is of interest to compare the voltammetric profiles corresponding to the first and second Rh monolayers on Pt(100). Figure 11 shows stationary cyclic voltammograms for a full Rh monolayer (dashed line) and for a complete second Rh monolayer (solid line) on Pt(100). Interestingly, this behavior is similar to that found in our recent study of Rh adlayers on Pt(111) electrodes.20 In fact, the adsorption of hydrogen (together with that of bisulfate anions on the first Rh monolayer) occurs at more positive potentials than on the second monolayer. Conversely, the adsorption of oxygenated species occurs at less positive potentials at the first Rh monolayer than at the second. Obviously, this behavior leads to a significantly wider purely capacitive (double layer) potential region in the case of the second monolayer. Taking into account that the electrosorption of hydrogen is a reductive process, whereas that of OH/O is an oxidative one, the previous observation can be summarized in one statement: the adsorption of hydrogen and oxygen is, on the first Rh monolayer, energetically stronger than on the second. In other words, these

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Figure 10. Series of stationary voltammetric profiles for annealed Rh/Pt(100) electrodes with different rhodium coverages from 1 to 2 ML. Working solution: 0.1 M H2SO4. Sweep rate: 50 mV s-1. Coverage values: (A) 1; (B) 1.25; (C) 1.43; (D) 1.68; (E) 1.85; (F) 2.

adsorption processes occur at higher underpotentials at the first monolayer. Basically, two complementary physical effects could underlie this behavior: the so-called strain47-50 and ligand50,51 effects, which would affect the atoms in the (47) Ruban, A.; Hammer, B.; Stoltze, P.; Shriver, H. L.; Norskov, J. K. J. Mol. Catal. 1997, 115, 421. (48) Mavrikakis, M.; Hammer, B.; Norskov, J. K. Phys. Rev. Lett. 1998, 81, 2819. (49) Meier, J.; Schiotz, J.; Liu, P.; Norskov, J. K.; Stimming, U. Chem. Phys. Lett. 2004, 390, 440. (50) Liu, P.; Logadottir, A.; Norskov, J. K. Electrochim. Acta 2003, 48, 3731. (51) Gauthier, Y.; Schmidt, M.; Padovani, S.; Lundgren, E.; Bus, V.; Kresse, G.; Redinger, J.; Varga, P. Phys. Rev. Lett. 2001, 87, 036103.

adlayer. The strain is due to the fact that the distances between adatoms in the monolayer are different from those corresponding to the surface atoms of the bulk metal: larger for atoms subjected to tensile strain and smaller for atoms subjected to compressive strain. This distortion in the interatomic distances causes a modification of the electronic structure through modification in the overlap between orbitals of neighboring atoms. The d-band sharpens as the overlap between the d electrons in neighboring atoms decreases, which is the situation occurring in the case of adlayers submitted to tensile strain. Together with this diminution of the d-bandwidth, there is an upshift in the average energy of the d-band


Figure 11. Stationary cyclic voltammograms for a full Rh monolayer (dashed line) and for a second Rh monolayer (solid line) on Pt(100). Working solution: 0.1 M H2SO4. Sweep rate: 50 mV s-1.

(band center). On the other hand, the energetics of adsorption for simple adsorbates such as H, O, or CO can be linked to the location of the d-band center of the metal surface, which in turn would be related to its bandwidth. Concretely, it has been shown47,48 that the higher the d-band center, the stronger the interaction adsorbatesurface (adsorption energy). Therefore, in the case of tensile-strained adlayers, there should be an increase in adsorption energy for simple adsorbates. The ligand effect would be caused by the electronic interaction between metals. The electronic structure and thus the chemical properties of the adatom would change according to its electronic environment, different from that of the surface atom of the parent metal. This effect, compatible with the previous one, would predominate for adlayers submitted to negligible strain. Taking into account that Rh has a significantly smaller atomic diameter than platinum (by 3.2%), we expect that a significant strain exists in the Rh overlayer on Pt(100) in the case of both the first and the second monolayers, as they grow pseudomorphically. If we are comparing the behavior of these adlayers with that of the bulk Rh(100), it is appropriate to invoke the strain effect. However, in comparing the adsorptive properties of both monolayers, the electronic interaction of the adlayer with the substrate should be considered as prevalent because both monolayers are equally strained. In a related study, it was found that for a Pt adlayer (up to 3-ML thick) on Ru(0001) a strong Pt-Ru interface coupling exerted an important effect on the CO adsorption energy.52 A similar mechanism would be operative in our case. It is worth noting that the underlying substrate could also affect through charge transfer the position of the d-band level, which, in turn, could underlie the observed behavior. As mentioned above, the adsorption energy for both hydrogen and oxygenated species is higher for the first Rh monolayer than for the second one. This result would be rationalized as the result of electron transfer from the Pt substrate to the Rh adlayer, which would cause an upshift of the surface d-band level and therefore an increase in adsorption energy for simple adsorbates. (52) Schlapka, A.; Lischka, M.; Gross, A.; Ka¨sberger, U.; Jakob, P. Phys. Rev. Lett. 2003, 91, 016101.

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It is also interesting to compare the adsorptive behavior of the Rh monolayer with that of the bulk Rh(100) substrate.31 On the basis of solely electrochemical data, it is difficult to decide unambiguously which of the effects (either strain or ligand) is more important. However, the fact that the adsorption voltammetric peaks for the second monolayer are similar to those for the bulk Rh(100) electrode, especially in the hydrogen adsorption region, points anew to a prevalence of the chemical (ligand) effects. Such tendency occurs even in this case where the comparison is established between a strained surface layer and an unstrained one. Finally, it is interesting to compare the difference in the adsorptive behavior found for Rh/Pt(100) with that of related systems. Incidentally, a Pd monolayer on Pt(100)2,17 shows a behavior contrary to that reported here for the Rh monolayer, at least for the adsorption of hydrogen. The shift direction for the hydrogen adsorption voltammetric peak on going from the first to the second Pd monolayer is opposite to that found here for Rh/Pt(100) electrodes. As in the case of Rh adlayers, the growth is again pseudomorphic,15 and the observed effect should be again the result of localized chemical effects. 4. Conclusions The possibility of preparing ordered Rh adlayers on Pt(100) electrodes (up to 2 ML) has been demonstrated. The procedure consists of two steps. First, Rh is electrodeposited onto Pt(100) samples during voltammetric cycles from 0.05 to 0.7 V RHE from dilute Rh(III) solutions in 0.1 M H2SO4. Although the platinum surface can be fully covered in this way, the resulting adlayer does not seem to be well-ordered. Second, and with the purpose of inducing a higher degree of order, a polarization program consisting of high-sweep-rate (50 V s-1) repetitive voltammetric cycles is applied to this electrode for 4 min (electrochemical annealing). The potential limits chosen for this program were 0.05 and 0.78 V. This reversible modification of the surface seems to be a consequence of the combined effect of OH/O and H adsorption, the former being more important. The increase of order in the Rh adlayer is evidenced by the substantial change triggered by the annealing procedure in the voltammetric profile corresponding to a 0.1 M H2SO4 solution. In fact, both in the hydrogen and oxygen potential ranges, well-defined adsorption peaks appear as opposed to the rather featureless profile resulting from the deposition process. More direct information on the morphology of the deposit is extracted from STM experiments for iodine-protected Rh adlayers. In the case of a full monolayer, a compact Rh layer is formed with a low concentration of defects and exhibiting a c(2 × 2) structure for the iodine uppermost layer, analogous to that found for a bulk Rh(100) sample. For coverage values exceeding 1 ML, the iodine-protected Rh adlayer shows two atomic levels, forming small islands and exhibiting again the c(2 × 2) lattice arrangement. This result confirms that a layerby-layer pseudomorphic growth prevails for our deposition procedure. The voltammetric profiles for Rh adlayers with different coverages in sulfuric acid solutions allow examination of the energetics of the H and OH/O adsorption processes. Interestingly, the electrosorption energetics for the first Rh monolayer differs significantly from that of the second one. The first monolayer adsorbs both hydrogen and oxygen more strongly than the second. This should be

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explained through ligand effects (modification of the electronic properties of the Rh adlayer because of underlying Pt) because both adlayers are equally strained (pseudomorphic growth). In any case, this study shows that Rh adlayers on platinum substrates have particular adsorptive capabilities. Taking into account the close relation existing between the thermodynamics of adsorption and the kinetics of electrocatalyzed (heterogeneously catalyzed) reactions, it is expected that fundamental research along


these lines could contribute to a more rational design of bimetallic electrocatalysts. Acknowledgment. This work was supported by the Spanish Ministry of Education and Science through Projects No. BQU2003-04029, BQU2003-03737 (Fondos FEDER) and the EU Framework V Growth Program, CLETEPEG project, Contract No. G5RD-CT-2001-00463. LA050863B

Preparation and Electrochemical Behavior of Ordered Rh Adlayers on

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