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Voltammetry, Charge Displacement Experiments, and Scanning Tunneling Microscopy of the Pt(100)-Br System J. M. Orts, R. Go´mez, J. M. Feliu,* and A. Aldaz Departamento de Quı´mica Fı´sica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain
J. Clavilier† Universite´ Pierre et Marie Curie, Ecole Nationale Superieure de Chimie de Paris, Laboratoire de Physico-Chimie des Surfaces, 11, Rue P. et M. Curie, 75231 Paris Cedex 05, France Received September 23, 1996. In Final Form: February 19, 1997X Stable, irreversibly adsorbed Pt(100)-Br adlayers composed of discharged bromine atoms can be formed through vapor or electrolyte dosage. They survive emersion, can be studied in electrolytic media (either in the presence or in the absence of bromide anions), and serve as scanning tunneling microscopy samples. The voltammetric profile of Pt(100) in acidic bromide solutions results from both hydrogen and bromide adsorption. Bromine coverages have been measured from charge displacements under different experimental conditions, and they amount to 0.48-0.50 Br/Pt. The hydrogen adsorption contribution to the voltammetric charge in acidic bromide solutions, evaluated by charge displacement, corresponds to hydrogen coverages around 0.90 H/Pt. An ordered Pt(100)-(3×4)-6Br (0.5 Br/Pt) adlayer structure has been imaged, resulting from the coincidence of a distorted hexagonal Br layer (Br-Br distances between 0.38 and 0.42 nm) on the substrate net. However, for the most part of the surface it is difficult to find large, bidimensionally ordered domains. The temperature during bromine vapor dosage strongly affects the surface structure. Low temperature is required to minimize effects on the structure of the Pt(100) substrate.
Introduction Anion specific adsorption plays a key role at the metal/ electrolyte interface, determining some interphasic properties, such as the potential of zero charge,1 or even the electrode surface structure (through reconstruction processes).2 It is well-known that specifically adsorbed anions affect the voltammetric behavior of single crystal electrodes of platinum group metals. A number of surface sensitive techniques have been used to gain information about the anion adlayer composition and structure on these surfaces: radiotracers,3 FTIRS (Fourier transform infrared spectroscopy),4 SXS (surface X-ray scattering),5,6 and STM (scanning tunneling microscopy).2 While radiotracer and FTIRS are well suited for detecting the chemical nature of adsorbed species, SXS and STM can yield structural information, in the latter case in real space, and even in real time, on a local scale. This makes STM an especially interesting technique for the study of adlayer structures, under both in-situ and ex-situ conditions. In the last few years, a number of STM studies have been published on halogen adlayers adsorbed on single crystal electrodes of noble metals, most of them being devoted to gold surfaces. The adsorption of halogen species † Present address: 1, Place A. Briand, 92195 Meudon Cedex, France. X Abstract published in Advance ACS Abstracts, April 15, 1997.
(1) (1) Hamelin, A. In Modern Aspects of Electrochemistry; Conway, B. E., White, R. E., Bockris, J. O’M., Eds.; Plenum Press: New York, 1985; Vol. 16, Chapter 1. (2) Weaver, M. J.; Gao, X. Annu. Rev. Phys. Chem. 1993, 44, 459 and references given therein. (3) Wieckowski, A. In Modern Aspects of Electrochemistry; White, R. E., Bockris J.O’M., Conway, B. E., Eds.; Plenum Press: New York, 1990; Vol. 21, Chapter 3. (4) Iwasita, T., Nart, F. C. In Advances in Electrochemical Science and Engineering; Gerischer, H., Tobias, C. W., Eds.; VCH: Weinheim, 1995; Vol. 4, p 123. (5) Ocko, B. M.; Watson, G. M.; Wang, J. J. Phys. Chem. 1994, 98, 897. (6) Lucas, C. A.; Markovic, N. M.; Ross, P. N. Surf. Sci. Lett. 1995, 340, L949.
S0743-7463(96)00932-8 CCC: $14.00
has been studied on Au,7-9 Pt,10-15 Ag,16,17 and Rh18 electrodes. Other studies include coadsorption of halide anions and underpotentially deposited copper on platinum basal orientations.19 Iodine adlayers on a number of metals and orientations are the most widely studied systems. Other halides have been less studied. Bromide anions have recently received some attention. STM has been carried out on the bromine adlayers for the three basal planes of platinum,20-22 especially for Pt(111). A remarkable fact concerning Br and I adsorbed on platinum is that these surface adlayers are stable under a wide range of environmental conditions and may be formed by exposure either to the respective halide solutions or to the corresponding halogen vapor phase. Their electrochemical behavior in a supporting electrolyte free of the halide salt is the same irrespective of the procedure used for their formation. Because of the similar behavior (7) Gao, X.; Weaver, M. J. J. Am. Chem. Soc. 1992, 114, 8544. (8) Batina, N.; Yamada, T.; Itaya, K. Langmuir 1995, 11, 4568. (9) Magnussen, O. M.; Wang, J. W.; Adzic, R. R.; Ocko, B. M. J. Phys. Chem. 1996, 100, 5500. (10) Itaya, K. In The Electrochemical Society Proceedings; Vol. 96-1, Abstract 1086. (11) Bittner, A. M.; Wintterlin, J.; Ertl, G. J. Electroanal. Chem. 1995, 388, 225. (12) Schardt, B. C.; Yau, S. L.; Rinaldi, F. Science 1989, 243, 1050. (13) Gao, X.; Weaver, M. J. J. Am. Chem. Soc. 1992, 114, 8544. (14) Baltruschat, H.; Bringemeier, U.; Vogel, R. Faraday Discuss. 1992, 94, 317. (15) De Simone, W. L.; Breen, J. J. Langmuir 1995, 11, 4428. (16) Schott, J. H.; White, H. S. Langmuir 1994, 10, 48. (17) Aloisi, G.; Funtidkov, A. M.; Will, T. J. Electroanal. Chem. 1994, 370, 297. (18) Wan, L.-J.; Yau, S.-L.; Swain, G. M.; Itaya, K. J. Electroanal. Chem. 1995, 381, 105. (19) Matsumoto, H.; Inukai, J.; Ito, M. J. Electroanal. Chem. 1994, 379, 223. (20) Bittner, A. M.; Wintterlin, J.; Beran, B.; Ertl, G. Surf. Sci. 1995, 335, 291. (21) Tanaka, S.; Yau, S. L.; Itaya, K. J. Electroanal. Chem. 1995, 396, 125. (22) Orts, J. M.; Go´mez, R.; Feliu, J. M.; Aldaz, A.; Clavilier, J. J. Phys. Chem. 1996, 100, 2334.
© 1997 American Chemical Society
Pt(100)-Br Adlayers
of the surface compound formed from halides compared to that formed in the vapor phase from neutral species, it may be expected that adsorbed halides are chemisorbed discharged species in the wide range of potentials where they are stable on the platinum surface in the absence of halide salt in solution. This remark accounts for the irreversibility of halide anion adsorption above a threshold potential comparable to halogen adsorption as well as the reversible adsorption/desorption of halides observed below this potential with halide-containing solutions with adequate pH.23 The study of halogen adsorbate behavior on Pt(100) electrodes, although more difficult to accomplish, is a necessary step in the pursuit of a comprehensive picture of anion electrochemisorption. Compared to Pt(111), fewer structural studies have been reported for Pt(100), whose sensitivity to the surface preparation conditions (namely the cooling of the flame-treated surface) is well-known from voltammetric studies.24 The samples prepared by following procedures that give well-defined voltammograms have been shown to present different surface structures at the atomic level, by means of ex-situ STM.25 STM work on Pt(100) has been reported for CO adlayers26 and iodine adlayers under both ex-situ27 and in-situ28 conditions. With regard to Br adsorption, in a recent work20 the existence of partially ordered structures has been indicated. The only study with ultrahigh vacuum (UHV) techniques, to our knowledge, of the Pt(100)-Br system, deals with adlayers formed from HBr29 and reports the formation of an ordered Pt(100)[c(2x2×x2)]-(Br + HBr) adlayer, with a total Br coverage of 0.5 Br/Pt, on the (1×1) substrate structure. In this paper we present our electrochemical and STM results on the Pt(100)-Br system. The electrochemical experiments give information on the stability and coverage of the bromine adlayer. Ex-situ STM experiments have produced topographic as well as atomically-resolved images of the surface. Experimental Section The electrochemical techniques used in this work were cyclic voltammetry and potentiostatic charge displacement by CO adsorption. The Pt(100) single crystal surfaces used in this work (both for STM and electrochemical experiments) were prepared from monocrystalline platinum spherical beads following the technique described in ref 30. Before each electrochemical experiment, the electrode was flame-annealed and cooled in a H2 + Ar atmosphere.24 In some cases, the electrode was cooled in a bromine vapor atmosphere and directly transferred to the experimental setup. In this manner, a cyclic voltammogram was recorded which resembles that resulting from cooling in the hydrogen + argon mixture after desorption of the preadsorbed bromine layer. In order to minimize the disordering effect on the surface structure caused by bromine dosage at high temperature (that is, to obtain a final voltammogram as close as possible to that obtained after hydrogen cooling), the temperature of the crystal during exposure to bromine vapor must be as low as possible. (23) Salaita, G. N.; Stern, D. A.; Lu, F.; Baltruschat, H.; Schardt, B. C.; Stickney, J. L.; Soriaga, M. P.; Frank, D. G.; Hubbard, A. T. Langmuir 1986, 2, 828. (24) Rodes, A.; Zamakhchari, M. A.; El Achi, K.; Clavilier, J. J. Electroanal. Chem. 1991, 305, 115. (25) Clavilier, J.; Orts, J. M.; Feliu, J. M. J. Phys. IV 1994, 4 C1, 303. (26) Vitus, C. M.; Chang, S. C.; Schardt, B. C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 7559. (27) Vogel, R.; Kamphausen, I.; Baltruschat, H. Ber. Bunsenges. Phys. Chem. 1992, 96, 525. (28) Vogel, R.; Baltruschat, H. Surf. Sci. Lett. 1991, 259, L739. (29) Garwood, G. A.; Hubbard, A. T. Surf. Sci. 1981, 112, 281. (30) Clavilier, J.; Armand, D.; Sun, S. G.; Petit, M. J. Electroanal. Chem. 1986, 205, 267.
Langmuir, Vol. 13, No. 11, 1997 3017 Working solutions were prepared by dissolving HClO4 and either KBr or NaBr (Merck Suprapur) in ultrapure water (from Millipore MilliQ and SuperQ systems). Liquid bromine for vapor dosage was Merck pro analysi. Electrode potentials were measured against and are quoted versus a reversible hydrogen electrode (RHE). All experiments were done at room temperature. STM experiments were carried out in air with a commercial Nanoscope III system (Digital Instruments) and using mechanically cut Pt90Ir10 or Pt80Ir20 tips. Two different procedures were used for preparing irreversibly adsorbed Pt(100)-Br adlayers, stable ex-situ: (a) The flame-treated Pt(100) electrode was placed, after a short cooling period (from a few to 30 s) in air, in an atmosphere of bromine vapors (in equilibrium with liquid bromine in a flask open to the air) for 20-90 s. (b) After cooling the Pt(100) surface in a H2 + Ar mixture, the electrode, covered with a droplet of ultrapure water in equilibrium with this atmosphere, was immersed in either a KBr or a NaBr solution (0.001-0.01 M in water or in 0.1 M HClO4). The electrodes prepared by immersion in bromide solutions were thoroughly rinsed with ultrapure water to remove any excess of electrolyte and carefully dried before being mounted for STM. Charge displacement experiments by potentiostatic CO adsorption were carried out following the procedure described in ref 31.
Results and Discussion 1. Electrochemical Results. 1a. Cyclic Voltammetry and Charge Displacements in Acidic Bromide Solutions. The voltammetric behavior of a well-ordered (cooled in a hydrogen + argon mixture) Pt(100) electrode in 10-2 KBr + 0.1 M HClO4 is characterized by a couple of sharp reversible peaks located at around 0.20 V RHE (Figure 1A). This potential value is significantly lower than those observed in other acidic electrolytes containing specifically adsorbed anions (0.380 V in 0.5 M H2SO4, 0.30 V for 10-3 M chloride in 0.1 M HClO432 ). This indicates that bromide anions adsorb more strongly than bisulfate and chloride. The reversible adsorption/desorption is probably coupled with the desorption/adsorption of hydrogen. This coupling, which has been previously pointed out by Hubbard on Pt(111),23 Bittner,20 and other authors,33 modifies the adsorption isotherm of Had, as shown for Pt(111).34 At higher potentials, a less pronounced peak is observed at 0.22 V, which is more evident at lower sweep rates (Figure1B). The total voltammetric charge density measured in Figure 1 between 0.06 and 0.30 V RHE amounts to 300 µC‚cm-2. This charge density is significantly higher than that expected theoretically for a full hydrogen monolayer (defined as 1 H/Pt) on an ideal Pt(100)-(1×1) surface (209 µC‚cm-2). This suggested the possibility of a reconstruction of the Pt(100) surface in contact with acidic halide solutions, giving a denser, hexagonal topmost platinum layer, as a tentative explanation for this high charge density value.35 Even in this case, the total voltammetric charge density is too high, which indicates the existence of other processes in addition to hydrogen adsorption/ desorption. Those contribute to the apparent excess of voltammetric charge. Therefore, the observed behavior would correspond to two coupled adsorption-desorption processes, both involving charge transfer. The most likely process linked to the voltammetric charge density in excess (31) Clavilier, J.; Albalat, R.; Go´mez, R.; Orts, J. M.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1992, 330, 489. (32) Clavilier, J.; Rodes, A.; El Achi, K.; Zamakhchari, M. A. J. Chim. Phys. 1991, 88, 1291. (33) Orts, J. M.; Go´mez, R.; Feliu, J. M.; Aldaz, A.; Clavilier, J. Electrochim. Acta 1994, 39, 1519. (34) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N. Langmuir 1996, 12, 1414. (35) Armand, D.; Clavilier, J. J. Electroanal. Chem. 1989, 270, 331.
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Figure 2. Cyclic voltammograms showing the desorption of the Pt(100)-Br adlayer with cycling in a pure 0.1 M HClO4 test electrolyte: initial potential, 0.79 V; sweep rate ) 50 mV‚s-1; (A) adlayer dosed by immersion in 10-3 M KBr solution; (B) adlayer prepared by dosing Br2 vapor at the end of the cooling stage during the surface preparation procedure.
Figure 1. Stationary cyclic voltammograms of a Pt(100) electrode in a 0.1 M HClO4 + 10-2 M KBr electrolyte. Sweep rates were (A) 10 mV‚s-1 and (B) 5 mV‚s-1.
is the specific adsorption of anions with electron transfer from the halide to the metal. The technique of charge displacement by potentiostatic CO adsorption31,33 has been shown to be a convenient means of separating the charge contribution due to the adsorption/desorption of underpotentially deposited hydrogen from that due to desorption/adsorption of specif-
ically adsorbed anions (or another surface species formed in an anodic process). The maximum coverage for underpotentially deposited (u.p.d.) hydrogen on the Pt(100) surface (cooled in H2 + Ar atmosphere) can be evaluated from charge displacement at potentials negative to the corresponding adsorption states and positive to the threshold of the hydrogen evolution reaction. For a CO adsorption potential of 0.10 V, mean values of +181, +196, and +191 µC‚cm-2 ((6 µC‚cm-2) have been found respectively for 10-2, 10-3, and 10-4 M KBr in 0.1 M HClO4. These values correspond to hydrogen coverages of 0.87, 0.94, and 0.92 H/Pt (all coverages in this paper are referred to the atomic surface density of an ideal Pt(100)-(1×1) surface). The value obtained in the transient at 0.10 V RHE for a hydrogencooled Pt(100) electrode surface amounts to 197 µC‚cm-2 in 0.5 M sulfuric acid electrolyte.36 All these values are close to the theoretical one for a full monolayer of hydrogen on a Pt(100) (1 × 1) surface. This similarity does not rule out the possibility of a reconstruction, which, in turn, is no longer needed for interpreting the voltammetric charge density values. In this regard, it must be noted that surface X-ray scattering experiments37 indicate that the Pt(100) electrode (cooled in hydrogen or nitrogen atmosphere) is not reconstructed in the usual voltammetric range in 0.1 N sulfuric acid electrolyte, neither in 0.1 M NaOH nor in 0.1M HClO4 solutions. However, some insitu STM results point to the possibility of observing the existence of surface reconstruction,38 requiring especially careful procedures. The charge displacement at a potential immediately positive to the voltammetric peaks (E ) 0.40 V) yielded negative transient currents, with mean values of displaced charge density of -99, -101, and -105 µC‚cm- 2 ((6 µC‚cm-2) for 10-2, 10-3, and 10-4 M KBr in 0.1 M HClO4, respectively. At this potential, no adsorbed hydrogen species exist. The negative sign of the transient current indicates that the desorption of the adsorbates existing (36) Clavilier, J.; Orts, J. M.; Go´mez, R.; Feliu, J. M.; Aldaz, A. In Proceedings of the Symposium on Electrochemistry and Materials Science of Cathodic Hydrogen Absorption and Adsorption; Conway, B. E., Jerkiewicz, G., Eds.; The Electrochemical Society: Pennington, NJ, 1994; Proc. Vol. 94-21, p 167. (37) Tidswell, I. M.; Markovic, N. M.; Ross, P. N. Phys. Rev. Lett. 1993, 71, 1601. (38) Zei, M. S.; Batina, N.; Kolb, D. M. Surf. Sci. Lett. 1994, 306, L519.
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Figure 3. Constant-current STM image showing the topography of a Pt(100)-Br sample obtained by bromine vapor dosage at the end of surface cooling.
at this potential involves their reduction. This suggests that the adsorbed form of bromide is discharged, i.e., this form is neutral bromine atoms. (It must be borne in mind that the displacement of discharged iodine monolayers on Pt(111) and Pt(100) by CO potentiostatic adsorption yielded charge densities consistent with one electron exchanged per adsorbed iodine atom, desorbed as iodide.39) Previous results obtained with Pt(111) are consistent with one electron exchanged per desorbed Brad,22 and with an electrosorption valency of 0.99.34 The same electrosorption value has been recently reported for Pt(100).40 If one accepts an electronic exchange of 1 electron per desorbed bromide anion, the values of displaced charge density, referred to the (1×1) substrate structure, are equivalent to coverages of 0.48, 0.49, and 0.50 ( 0.03 Br/ Pt. These values are in good agreement with that for the Pt(100)([c(2x2×x2)]R45-(Br + HBr) structure (0.50 Br/ Pt), prepared by HBr dosage in UHV and detected by lowenergy electron diffraction (LEED).29 It should be said that, as in the case of the transients at low potential, a small contribution from purely capacitive charging exists. 1b. Cyclic Voltammetry and Charge Displacements for Irreversibly Adsorbed Pt(100)-Br Adlayers in 0.1 M HClO4. As in the case of Pt(111),22 stable, irreversibly adsorbed Br adlayers can be prepared by dosing from bromine vapor after thermal treatment or by immersing the Pt(100) surface (previously checked by cyclic voltammetry), in aqueous KBr solutions (in the latter case the surface is subsequently rinsed to remove the excess of bromide electrolyte). The adlayers stability domain is defined by their voltammetric behavior in 0.1 (39) Clavilier, J.; Albalat, R.; Go´mez, R.; Orts, J. M.; Feliu, J. M. J. Electroanal. Chem. 1993, 360, 325. (40) Markovic, N. M.; Lucas, C. A.; Gasteiger, H. A.; Ross, P. N. Surf. Sci. 1996, 365, 229.
M HClO4 test solutions (Figure 2). In these experiments, the electrode was put in contact with the test solution at 0.79 V (in order to avoid Br desorption), and the potential was swept between 0.06 and 0.80 V for several cycles. In both cases the first negative-going sweep shows a peak similar to those observed in the presence of bromide salts in the working solution, attributable to Br reductive desorption, which is complete at the lower potential limit. (Several cycles are needed to restore the characteristic voltammogram of the Pt(100) surface in 0.1 M HClO4 because of readsorption of diffusing bromide, which takes place at sufficiently high potentials during cycling). These irreversibly adsorbed layers can serve as samples for charge displacement experiments, provided that the potential of CO dosage is chosen high enough to prevent potential-driven desorption of Br. Experimental charge densities for charge displacement at E ) 0.50 V give a mean value of -102 µC‚cm-2 for KBr-dosed samples. The values obtained with Br2-dosed samples were found to depend strongly on the temperature during exposure to the halogen. For surfaces exposed to bromine after quasicomplete cooling, the same value of displaced charge density as for aqueous KBr dosed samples was obtained. However, for samples put in the bromine atmosphere at higher temperatures (orange or red-hot crystal) the dispersion of displaced charge values was high, with minimum values as low as -60 µC‚cm-2 (at the higher temperatures). This dramatic variation in Br coverage is rationalized as a consequence of surface modifications due to bromine chemical attack at high temperatures, confirming the suggestions by Wintterlin et al.20 Hence, samples for STM work were always prepared by dosing Br2 on an almost cool Pt(100) sample. The charge density value of -102 µC‚cm-2 corresponds to 0.49 Br/Pt, in good agreement with those obtained in
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Figure 4. Atomic resolution, constant height STM image of an ordered domain of the Br adlayer, showing a Pt(100)(3×4)-6Br structure: left, raw data; right, filtered image.
the presence of aqueous bromide species. This means that the irreversible adlayers formed by these ex-situ procedures have coverages similar to those obtained under in-situ conditions. 2. STM Results. Figure 3 shows a constant current STM image of a Pt(100) surface after thermal treatment, partial cooling, and subsequent exposure to a bromine atmosphere. These surfaces are characterized by wide terraces usually separated by monoatomic steps running preferentially along the dense rows of the (100) surface (in the [110] direction). The direction of these dense rows is known from the position of the natural (111) facets which are clearly visible on the spherical part of the single crystal metal bead. These steps are mostly rather straight, but curved steps were also observed, which could be originated by a chemical attack of the platinum surface by the halogen gas at high temperature. Relatively away from the steps, we can distinguish some isolated holes in the terraces, with rectangular or squared shapes and delimited by monoatomic steps parallel to the dense atomic rows of the fcc(100) (face centered cubic) surface. Similar features, characteristic of long-range ordered surfaces at the atomic level, have been described for Pt(100) samples cooled in iodine vapor26 and in hydrogen + argon25 atmospheres. In contrast with the Pt(100) samples obtained after cooling in a hydrogen-containing atmosphere,25 we never observed mesae on the bromine- or iodine-cooled surfaces. The presence of the halogen adlayer protects the surface against contamination, and impedes the incipient formation of surface oxides, which are known to induce the formation of monodimensionally ordered step defects, as suggested by voltammetry30 and shown by STM.25 When images were recorded with a higher resolution and a smaller sweep range (20-40 nm), a pattern of rows attributable to the bromine adlayer could be distinguished.
Figure 5. Cross section profile along a row of maxima in the [110] direction, and its corresponding FFT frequency spectrum.
The rows run parallel to the (100) dense rows, being usually separated by three or four Pt-Pt distances, and were also visible in atomic resolution images. Figure 4 shows a constant height atomic resolution image (raw and filtered data) of the Pt(100)-Br surface, showing maxima aligned along the directions parallel to the dense rows. The distance values between neighboring spots, measured along the rows of main maxima running from upper left to down right corners of the image are of
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Langmuir, Vol. 13, No. 11, 1997 3021
Figure 6. Virtual STM image of the distorted hexagonal Br adlayer corresponding to the image in Figure 4.
0.83 ( 0.03 nm, which correspond to three Pt-Pt distances. In the direction normal to the previous one, adjacent main maxima are separated by 1.08 ( 0.03 nm (close to four Pt diameters). These periodicities define the patterns that appear in the center of the image, which could be described as (3×4). These features are evident in the raw image. It is unlikely that the main maxima were the only Br adspecies. In that case, the discrepancy between the theoretical coverage for a Pt(100)(3×4)Br adlayer (1/12 Br/Pt) and that obtained by charge displacement (0.49 ( 0.03 Br/Pt) would be too high. The relatively high noise level could be the responsible of the concealing of other secondary current maxima in the raw data. These can be observed in the filtered image which has been obtained by selecting all the groups of frequencies which are significantly different from the background in the 2D frequency spectrum obtained by fast Fourier transform (FFT). Other Br adatoms are contained in this (3×4) unit cell as evidenced by the filtered images. This strongly suggests that the observed periodicity is the manifestation of the coincidence of a denser bromine adlayer with the square substrate net of Pt(100). Inside the (3×4) unit cell, the minimum distance separating different maxima is around 0.39 nm, a value very close to the van der Waals diameter of bromine (0.392 nm41 ). These distances are usually measured in directions rotated by about 60° with respect to the platinum dense rows. Figure 5 shows a cross section of the unfiltered image along the [110] direction. Between each pair of neighboring main maxima, a secondary, much less intense current maximum is observed. The distances between main and secondary peaks are around 0.39 ( 0.04 nm. In the corresponding frequency spectrum (obtained after FFT), peaks are obtained at 0.42 and 0.84 (41) Jones, R. G. Prog. Surf. Sci. 1988, 27, 25.
Figure 7. Tentative ball model for the Pt(100)(3×4)-6Br structure.
nm. Moreover, the measured distances of 0.83 and 1.08 nm along the rows of main maxima defining the (3×4) unit cell are very similar to twice and (3x3)/2 times the van der Waals diameter of bromine. These facts strongly suggest the existence of a dense, almost close-packed, hexagonal bromine adlayer. Careful inspection of the 2D frequency spectrum after FFT reveals the presence of a hexagonal set of spots corresponding to this dense adlayer. Figure 6 shows the virtual image obtained by reversing the FFT but selecting only this latter set of frequencies. It depicts a distorted hexagonal adlayer, with maxima corresponding to bromine adatoms separated by distances
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Figure 8. Atomic resolution, constant height STM image of a region with small (3×3)
(
)( )
3 0 3 0 ; -1 3 1 3
ordered domains on the Pt(100)-Br sample.
around 0.4 nm (0.39 and 0.46 in the main direction and 0.39 and 0.42 in those rotated by +60° and -60°, respectively). These distances agree well with the values of the van der Waals diameter of Br and are significantly higher than the Br-Br distance in the diatomic molecule (0.227 nm41). The distortion of the adlayer can arise from the influence of its interaction with the underlying Pt(100) substrate surface. (A set of spots in agreement with the square substrate surface lattice are also visible in the 2D frequency space of some images, although with amplitudes just slightly higher than the background level.) The comparison of figures 4 and 6 allows us to conclude that the (3×4) cell contains six bromine atoms. This yields a coverage of 0.5 Br/Pt, in good agreement with that obtained from charge displacement experiments (0.49 ( 0.03). Consequently, the Pt(100)(3×4)-6Br structure imaged in Figure 4 arises basically from the coincidence of a hexagonal bromine adlayer with the (1×1) structure of the Pt(100) substrate. A tentative, simplified ball model is given in Figure 7. The degree of order of the Br adlayer on Pt(100) was not high. Most of the surface was covered by complex patterns, with clearly distinguishable lines separated by distances of around three or four Pt atoms, these being also the prevalent separations between main tunneling maxima in a line. Although these features were easily found, it was difficult to image large two-dimensionally ordered domains such as those in Figure 4. This indicates that the adsorbate layer is mostly ordered in a local scale, in essential agreement with the STM observations of Bittner et al.20 and the results of SXRS by Markovic et al.40 We have attempted the analysis of some of these partially ordered domains. Figure 8 shows an image obtained on
the same Pt(100)-Br sample. Parallel rows of main STM maxima are observed in the direction of the platinum dense rows, rotated about -45° with respect to the horizontal scan axis. Within a row, neighboring maxima are separated by 0.42 nm ( 0.02 nm, a distance equivalent to 1.5 Pt-Pt (dense rows), and slightly higher than the van der Waals bromine diameter (0.39 nm). The prevalent distance between the parallel rows is 0.83 ( 0.04 nm (aproximately three Pt-Pt distances). However, in this relatively disordered domain, other distances between the rows or fragments containing the highly bright spots have been measured. They are always multiples of 0.27 nm (0.27, 0.54, 1.08 nm). These latter distances always separate rows pertaining to different domains. A poorer correlation is observed in the direction normal to the previous one. Coming back to the small ordered domains, an additional row with lower current signal is distinguished halfway between each two rows of main maxima. The patterns imaged in Figure 8 could be described as (3×3)-4Br and
(
)( )
3 0 3 0 ; -1 3 1 3
all with a coverage of 4/9 ) 0.44 Br/Pt. The observation of different local arrangements is in agreement with the conclusions by Markovic et al.40 Conclusions Charge displacement experiments have given evidence that the charge under the voltammetric peak observed for well-defined Pt(100) electrodes in acidic solutions of bromide anions corresponds to two different processes:
Pt(100)-Br Adlayers
hydrogen adsorption/desorption, and bromide desorption/ adsorption. The evaluation of the maximum coverages of these species yields values of around 0.90 H/Pt at 0.10 V and 0.48-0.50 Br/Pt at 0.40 V (assuming one electron transferred per desorbed species). The Br adlayers are strongly adsorbed on Pt(100) and survive emersion, remaining irreversibly adsorbed even in solutions free of bromide anion. The coverage agrees well with the value found by Hubbard in UHV for HBr-dosed samples29 and are slightly higher that those reported by Markovic40 for experiments in very diluted bromide solutions. Adlayer preparation can be carried out either by exposure to bromine vapor or by dosage in aqueous bromide solutions (the adsorption is spontaneous at open circuit). It can be concluded that the adlayer is formed by totally discharged Br adspecies, in agreement with rotating ring-disk electrode results.40 Charge displacements for the isolated adlayers agree in both cases with a reductive desorption step for Br with calculated coverages in agreement with those obtained in bromide solutions. The adlayer structure is strongly influenced by the temperature at which the vapor dosage is carried out. The higher the temperature, the greater the discrepancy of the coverages with the values obtained for electrolytically-dosed adlayers (either emersed or nonemersed). This is probably due to a strong perturbation of the surface structure caused by a chemical attack, as evidenced by the significant differences of the voltammetric profiles recorded after the charge displacement experiment. STM of samples dosed at low temperature show wide flat terraces. Ordered structures, which can be described
Langmuir, Vol. 13, No. 11, 1997 3023
as Pt(100)-(3×4)-6Br (0.5 Br/Pt), have been imaged with atomic resolution. Bromine seems to be forming a dense, slightly distorted, hexagonal adlayer, with Br-Br distances close to the van der Waals diameter of bromine. The observed structures arise from the coincidence of the hexagonal adlayer with the substrate net. These 3-fold and 4-fold periodicities were observed all over the surface, separating rows of tunneling maxima. However, in most areas it was difficult to obtain large bidimensionally ordered domains, the images showing locally ordered arrangements. The sample could be described as being mostly unidimensionally ordered. This constrats with the long-range ordered structure revealed by LEED for HBrdosed Pt(100).29 This could be considered as surprising, as the total Br concentrations in this work compare well with those of ref 29. It must be taken into account, however, the significantly different experimental conditions between both works, both for dosage (HBr at room temperature, in vacuum, with a UHV-prepared substrate vs bromine vapor dosage at the end of the cooling period after thermal treatment at atmospheric pressure) and for structural analysis. Acknowledgment. Financial support from CICYT through Projects PB93-0944 and UE-94-0031 is greatly acknowledged. Funding for the STM facility was provided by Conselleria de Educacio´ i Cie`ncia de la Generalitat Valenciana. J.C. is grateful to Fundacio´n Banco BilbaoVizcaya for the award of a Visiting Professor stay at the University of Alicante. LA960932+
