Langmuir 1990,6, 97-105
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Kendall Award Lecture Surface Electrochemistry Arthur T. Hubbard Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 -01 72 Received J u n e 28, 1989
Surface electrochemistry studies are described in which molecular layers formed at Pt(111) surfaces in electrolyte solutions are characterized by surface spectroscopy and voltammetric methods. Types of compounds studied thus far include alcohols, aldehydes, alkenes, alkynes, amides, amines, amino acids, aromatic compounds, electrodeposited metals (Ag, Cu, Bi, Pb, Tl), halides, cyanide, thiocyanate, mercaptans, nitriles, and others. Chemisorbed species are in most cases not removed from Pt by rinsing. Cyclic voltammetry of reversibly electroactive adsorbates before and after evacuation has revealed that chemisorbed layers formed in solution are stable also in vacuum, permitting identification and quantitation by Auger spectroscopy, EELS, LEED, and related methods. Elemental composition and molecular packing density of adsorbed molecular layers are reliably measured by means of Auger spectroscopy, provided that beam currents are kept sufficiently small and the apparatus is directly calibrated. Adsorbate molecular orientation can be deduced from packing density data, supplemented with vibrational spectra, electrochemical reactivity, and other observations. Best progress is made when families of related adsorbates are studied and intercompared. Chemical reactivity and electrochemical reactivity are direct functions of adsorbate molecular orientation. Electrochemical oxidation of oriented, chemisorbed layers begins at the points of attachment to the surface and proceeds further only under special circumstances. Electrode potential influences the nature and mode of attachment of all surface species; those most profoundly affected include carboxylic acids and adsorbates containing pendant redox centers. Surface structure of Pt strongly influences adsorbed layer characteristics, including packing density, mode of attachment, long-range order, orientation, spectroscopic behavior, and reactivity. Surface roughness introduced by the oxidation-reduction cycling procedure commonly employed by electrochemists to activate Pt electrodes has catastrophic effects on adsorbate packing, reactivity, and vibrational spectra, and such cycling procedures are not recommended for fundamental work. Adsorbed molecular, ionic, or atomic layers formed at metal-solution interfaces frequently display long-range order which is closely tied to the electrochemical properties of such layers. The angular distribution of Auger electron emission from metal single crystals and molecular layers proves to be a useful indicator of surface crystal and molecular structure.
Introduction Recent studies of surface electrochemistry illustrate the basic similarity of surface chemistry at interfaces formed from any combination of gas, solid, or liquid phases. For example, adsorption of hydrogen halides and halogens at Pt(ll1) from the gas phase yields many of the same structures as are obtained by immersion of Pt(ll1) into aqueous solutions of the same substances: I, or HI with Pt(ll1) under vacuum forms a Pt(lll)(d3Xd3)R30°-I structure containing 8,= 1/3 I atoms per surface Pt atom and a ( d 7 X ~ ' 7 ) R 1 9 ~ -structure 1 a t 8, = 3/7;'s2 I, vapor a t atomospheric pressure leads to a (3X3)-I structure for which 8,= 4/9 and a (3d3X9d3)R30°-I structure conSimilarly, adsorption from aqueous taining 8,= 0.6.394 I, (or I-) solutions at appropriate pH and electrode potential forms the (d3Xd3)R30°-I, (d7Xd7)R19°-I, and (3x3)-I structure^.^ Thermal desorption mass spectros-
copy of the iodine layer parallels desorption of the I layer by anodic oxidation in regard to the number and energies of the transitions.' Indeed, recent STM pictures of Pt(111)(-\/7xd7)R19O-I and Pt(111)(3X3)-1 formed in I, vapor6 agree with the previously published structures formed in solution, vapor, and an ultrahigh vacuum,'-5 while revealing additional details of domain boundaries, faults, and vacancies in those structures. Other examples of the similarity between gas-solid and liquid-solid surface chemistry a t well-defined surfaces can be found in recent review^.^-^ Adsorbed layers formed a t Pt(ll1) in solution are stable also under vacuum. This is illustrated by the data in Figure 1. On the basis of packing density measurements obtained by use of Auger spectroscopy, surface vibrational spectra obtained by means of electron energy loss spectroscopy (EELS), and electrochemical reactivity observed by use of cyclic voltammetry, compounds such
(1) Felter, T. E.; Hubbard, A. T. J . Electrounul. Chem. 1979, 100,
473.
Garwood, G. A., Jr.; Hubbard, A. T. Surf. Sci. 1980, 92, 617. (3) Wieckowski, A,; Rosasco, S. D.; Schardt, B. C.; Stickney, J. L.; Hubbard, A. T. Inorg. Chem. 1984,23, 565. (4) Wieckowski, A.; Schardt, B. C.; Rosasco, S. D.; Stickney, J. L.; Hubbard, A. T. Surf. Sci. 1984, 146, 115. (5) Lu, F.; Salaita, G. N.; Baltruschat, H.; Hubbard, A. T. J . Electrounal. Chem. 1987, 222, 305. (2)
0743-7463/90/2406-0097$02.50/0
(6) Schardt, B. C.; Yau, S.-L.; Rinandi, F. Science 1989,243, 1050. ( 7 ) Hubbard, A. T.; Stickney, J. L.; Soriaga, M. P.; Chia, V. K. F.; Rosasco, S. D.; Schardt, B. C.; Solomun, T.; Song, D.; White, J. H.; Wieckowski, A. J. Electrounul. Chem. 1984, 168, 43. (8) Hubbard, A. T. In Comprehensive Chemical Kinetics; Bamford, C. H., Tipper, D. F. H., Compton, R. G., Eds.; Elsevier: Amsterdam, 1988; Vol. 28, Chapter 1. (9) Hubbard, A. T. Chem. Reu. 1988, 88, 633.
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Figure 1. Cyclic voltammetry of adsorbed 2,5-dihydroxy-4methylbenzyl mercaptan (DMBM)at Pt(111).12 (A) Solid curve: immersion into 0.7 mM DMBM followed by rinsing with 10 mM trifluoroacetic acid. Dotted curve: as above followed by 1 h under vacuum prior to voltammetry. (B) Solid curve: first voltammetric scan. Dotted curve: second scan. Scan rate, 5
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as 2,5-dihydroxy-4-methylbenzyl mercaptan (DMBM) are attached to the surface exclusivelythrough the sulfur atom in a vertical orientation, having a reversibly electroactive hydroquinone pendant (solid curve, Figure 1). Similar voltammetric behavior is observed when the DMBM layer adsorbed from solution is subjected to vacuum prior to reimmersion into electrolyte (Figure 1, dotted curve). Families of compounds belonging to a number of classes have thus far been studied and found to be stable under vacuum: alcohols, aldehydes, alkenes, alkynes, amides, amines, amino acids, aromatic compounds, electrodeposited metals, halides, pseudohalides, mercaptans, nitriles, and others. There are about 5 million known compounds in these classes. Chemisorbed layers stable in contact with electrolyte are ordinarily stable also under vacuum. This stability permits identification, characterization, and quantitation of the chemisorbed layer by use of very effective surface spectroscopies including Auger, EELS, LEED, XPS, SIMS, IR, and others. Although many words have been spoken or written about studying electrode surfaces exclusively in solution, the available evidence indicates that the changes occurring in a chemisorbed layer upon removal from solution are slight and rather predi~table.~ Strong interactions, if they were to occur, would be readily observable by treatment of chemisorbed layers with solvent vapor and other substances under vacuum during surface characterization by spectroscopic methods." In any event, an effective approach to understanding metal surfaces is to employ a balanced combination of electrochemical and spectroscopic methods, "wet" and "dry", "physical" as well as "chemical" approaches, and to incorporate clean singlecrystal surfaces and pure solutions as reliable points of reference into each series of experiments. Metal surfaces immersed into solution usually contain one or more adsorbed substances in amounts approach(10) Katekaru, J. Y.; Hershberger, J.; Garwood, G. A,, Jr.; Hubbard. A. T. Surf. Sci. 1982, 121, 396.
EbIERGY L O S S ( c m - 1
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Figure 2. Vibrational spectra of nicotinic acid (NA).16 (A) EELS spectrum of NA adsorbed at Pt(lll),-0.2 V, pH 3. Lower curve in A and B is mid-IR spectrum of NA vapor at 200 "C (from ref 52). (B) EELS spectrum of NA at Pt(lll),0.6 V, pH 3. (C) EELS spectrum of NA at Pt(lll),-0.3 V, pH 10. Lower curve in c is mid-IR spectrum of solid NA. Experimental conditions: adsorption from 1 mM NA in 10 mM KF, pH 3 (A and B) or pH 7 (C), followed by rinsing with 2 mM HF (A and B, pH 3) or 0.1 mM KOH (pH 10, C); EELS incidence and detection angle, 62' from surface normal; beam energy, 4 eV; beam current, ca. 120 PA; EELS resolution, 10 meV (80 cm-') fwhm; IR resolution, 4 cm-'.
ing saturation. Clean surfaces are very reactive, and the amount of material required to coat such a surface to the point that its chemical properties are profoundly altered is rather small (about 100 pmol/cm2). Accordingly, the most faithful guides to a clean electrode surface are surface analysis techniques capable of detecting and identifying any and all adsorbed species in any practical amount.
Kendall Award Lecture
Langmuir, Vol. 6, No. 1, 1990 99
Potential-Dependent Surface Chemistry Electron energy loss spectroscopy (EELS) has been found to be a valuable tool for investigation of molecular or ionic layers a t electrode ~ u r f a c e s .As ~ illustrated in Figure 2, EELS spectra reveal the vibrational bands of any adsorbed molecule over the entire frequency range with excellent sensitivity and signal-to-noise ratio. Resolution is moderate but adequate a t the present state of the art and will probably improve as a result of future work. As can be seen by comparing the EELS spectrum of adsorbed nicotinic acid (NA) with the gas-phase IR spectrum of NA vapor a t 200 "C (lower curve in Figure 2A), the EELS spectrum of the adsorbed layer is very similar to the IR spectrum of the vapor apart from the difference in resolution. This similarity is usual apart from changes in molecular interaction.''-26 Electron energy loss to vibrational modes is primarily due to impactscattering processes. Accordingly, the vibrational modes of an adsorbed molecule are seen with good intensity regardless of the molecular orientation of the adsorbate, apart from intensity decreases due to intermolecular interaction and c,hanges in adsorbate molecular structure. The influence of electrode potential upon the nature of adsorbed molecular species is illustrated by parts A and B of Figure 2, which show EELS spectra of NA adsorbed from acidic solution a t negative (A) or positive (B) electrode potential.16 Note that the carboxylic acid 0-H stretch at 3566 cm-' vanishes when adsorption is carried out at sufficiently positive potential. Other, less striking changes are observed, as is generally the case. Detailed studies of a series of pyridine carboxylic acids indicate that at positive potentials both the aromatic nitrogen and the carboxylate moiety are bonded to the surface, while adsorption at negative potentials leads to attachment exclusively through the nitrogen, eq 1: (11)Lu, F.; Salaita, G. N.; Laguren-Davidson, L.; Stern, D. A.; Wellner, E.; Frank, D. G.; Batina, N.; Zapien, D. C.; Walton, N.; Hubbard, A. T. Langmuir 1988,4,637. (12) Stern, D. A.;Wellner, E.; Salaita, G. N.; Laguren-Davidson, L.; Lu, F.; Batina, N.; Frank, D. G.; Zapien, D. C.; Walton, N.; Hubbard, A. T. J. Am. Chem. SOC. 1988,110,4885. (13) Salaita, G. N.;Laguren-Davidson, L.; Lu, F.; Walton, N.; Wellner, E.; Stern, D. A.; Batina, N.; Frank, D. G.; Lin, C.-H.; Benton, C. s.; Hubbard, A. T. J. Electroanal. Chem. 1988,245,253. (14) Batina, N.;Frank, D. G.; Gui, J. Y.; Kahn, B. E.; Lin, C.-H.; Lu, F.; McCargar, J. W.; Salaita, G. N.; Stern, D. A.; Zapien, D. C.; Hubbard, A. T. Electrochim. Acta 1989,34,1031. (15) Stern, D. A.;Salaita, G. N.; Lu, F., McCargar, J. W.; Batina, N.; Frank, D. G.; Laguren-Davidson, L.; Lin, C.-H.; Walton, N.; Gui, J. Y.; Hubbard, A. T. Langmuir 1988,4,711. (16) Stern, D. A.; Laguren-Davidson, L.; Frank, D. G.; Gui, J. Y.; Lin, C.-H.; Lu, F.; Salaita, G. N.; Walton, N.; Zapien, D. C.; Hubbard, A. T. J . Am. Chem. Sac. 1989,11,877. (17) Gui, J. Y.;Kahn, B. E.; Lin, C.-H.; Lu, F., Salaita, G. N.; Stern, D. A,; Hubbard, A. T. Langmuir 1989,5,819. (la) Batina, N.;McCargar, J. W.; Laguren-Davidson, L.; Lin, C.-H.; Hubbard, A. T. Langmuir 1989,5,123. (19) Batina, N.;Gui, J. Y.; Kahn, B. E.; Lin, C.-H.; Lu, F., McCargar, J. W.; Salaita, G. N.; Stern, D. A,; Hubbard, A. T.; Mark, H. B., Jr.; Zimmer, H. Langmuir 1989,5,588-600. (20) Gui, J. Y.;Kahn, B. E.; Lin, C.-H.; Lu, F.; Salaita, G. N.; Stern, D. A.; Zapien, D. C.; Hubbard, A. T. J . Electroanal. Chem. 1988,252, 169. (21) Chaffins, S.A.;Gui, J. Y.; Kahn, B. E.; Lin, C.-H.;Lu, F.; Salaita, G. N.; Stern, D. A.; Zapien, D. C.; Hubbard, A. T., submitted to Lungmutr. (22) Batina, N.;Kahn, B. E.; Lin, C.-H.; McCargar, J. W.; Salaita, G. N.; Hubbard, A. T. Electroanalysis, in press. (23) Batina, N.; Chaffins, S. A.; Kahn, B. E.; Lu, F., McCargar, J. W.; Rovang, J.; Stern, D. A.; Hubbard, A. T. Catal. Lett., in press. (24) Chaffins, S. A.;Gui, J. Y.; Lin, C.-H.; Lu, F.; Salaita, G. N.; Stern, D. A.; Hubbard, A. T., submitted to Langmuir. (25) Batina, N.;Chaffins, S. A.; Lu, F., McCargar, J. W.; Rovang, J.; Stern, D. A.; Hubbard, A. T. J. Electround. Chem., in press. (26) Batina, N.; Chaffins, S. A.; Lu, F., McCargar, J. W.; Rovang, J.; Stern, D. A.; Hubbard, A. T. J . Electroanal. Chem., in press.
Similar potential-dependent adsorption behavior has also been reported for families of thiophenecarboxylic acids,lg thiopheneacetic acids,lg pyridineacetic acids, carboxybipyridines," and various alkenoic acidsz3 Electrode potential controls the structure of the layer formed by adsorption of anions a t single-crystal metal surface^.^*^^-^' Figure 3A shows the cyclic voltammogram of a Pt(ll1) surface containing a layer of Br atoms adsorbed from a Br- solution.31 Adsorption of Br is a redox process: Br- Br(ads) + e(2) The sharp peaks (2 and 2') near -0.1 V correspond to a transition between a Pt(111)(3X3)-Br structure containing OB, = 4/9 = 0.4444 Br atoms per surface Pt atom and a Pt(111)(4X4)-Br structure containing e,, = 7/16 = 0.4375, parts B and C of Figure 3, respectively. The variation of e,, from zero to the limiting value versus electrode potential consists of a series of such transition^.^' Iodide adsorption a t Pt(ll1) displays analogous structural transitions with electrode potential5 Similar behavior is observed for ~hloride,~'cyanide," and thiocyanate" at P t ( l l l ) , as well as for chloride, bromide, and iodide a t Ag(lll).30 Electrodeposition of metals onto a substrate composed of a different metal typically commences with formation of metallic monolayers whose structures vary with electrode potential via the amount of metal deposited. Recent studies of metal electrodeposition a t welldefined single-crystal electrode surfaces with subsequent characterization of structure and composition by LEED and Auger spectroscopy have included Ag a t P t ( 1 1 1 ) , 4 ~A~ g ~a ,t ~Pt(100),3936 ~ Cu at Pt(lll),35 P b a t Pt(111),37-39Sn at Pt(lll),40and Cu, Pb, T1, Bi, and Sn a t Ag(lllh4' Metal monolayer structure depends upon the substrate element, deposited element, structure of the substrate surface, and nature of the supporting electrolyte anion.3*4,33-40 About half of the electrodeposited (27) Stickney, J. L.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T. Langmuir 1985,1, 66. (28) Rosasco, S. D.; Stickney, J. L.; Salaita, G. N.; Frank, D. G.; Katekaru, J. Y.;Schardt, B. C.: Soriaga, M. P.; Stern, D. A.; Hubbard, A. T. J . Electroanal. Chem. 1985,I8& 98. (29) Frank. D. G.: Katekaru. J. Y.: Rosasco. S. D.: Salaita. G. N.: Schardt, B. C.;'Soriaga, M. P.; Stern, D. A.; Stickney, J. L.; Hubbard; A. T. Langmuir 1985,1,587. (30) Salaita, G. N.;Lu, F., Laguren-Davidson, L.; Hubbard, A. T. J. Electroanal. Chem. 1987,229,1. (31) 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. (32) Stern, D. A.; Baltruschat, H.; Martinez, M.; Stickney, J. L.; Song, D.; Lewis, S. K.; Frank, D. G.; Hubbard, A. T. J. Electroanal. Chem. 1987,217,101. (33) Hubbard, A. T.;Stickney, J. L.; Rosasaco, S. D.; Soriaga, M. P.; Song, D. J . Electroanal. Chem. 1983,150,165. (34) Stickney, J. L.;Rosasco, S. D.; Song, D.; Soriaga, M. P.; Hubbard, A. T. Surf. Sci. 1983,130,326. (35) Stickney, J. L.; Rosasco, S. D.; Hubbard, A. T. J . Electroanal. SOC. 1984,131,260. (36) Stickney, J. L.; Rosasco, S. D.; Schardt, B. C.; Hubbard, A. T. J.Phys. Chem. 1984,88,251.
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interface thickness tends to be small (on the order of 10
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the potential difference can be appreciable (fl V is the common range), the electric field can be quite large (about lo7 V/cm, compared with fields in the vicinity of an Na+ ion estimated to be about lo8 V/cm). Measurement of the electrode potential of the metal relative to a reference electrode half-cell such as Ag/AgCl (KC1) at open circuit (that is, without imposing a current) is advisable in order to gauge the state of oxidation/reduction of the metal-solution interface. Addition of a third electrode (“auxiliary”)to the cell allows the electrode potential of the metal surface of interest (“indicator”) to be controlled by application of a current, without the necessity of passing current through the reference electrode. The current flowing per unit area of the indicator electrode is also a convenient measure of the various rate processes occurring at the indicator surface including transport of the reactants, rates of activated processes, and charging of the interfa~e.~’Experiments in which the indicator electrode potential is scanned linearly with time while the current is measured (cyclic voltammetry) yield sensitive indications as to reaction rates, energies, quantitative aspects, and surface characteristics of metals in Surface voltammetric data are somewhat ambiguous in the absence of other evidence, and thus the voltammetric and surface spectroscopic approaches make a useful combination. The balance between oxidation and reduction can be controlled in the gas phase by introducing redox agents. For example, traces of H, and proton donors such as H,O can efficiently poise the reaction system in the reducing (electron-rich) range: H, 2H’ + 2e(3) The position of the redox equilibrium depends upon the proportions of H, and H+ at the surface. Oxidizing conditions are achieved by addition of a species such as oxygen:
Figure 3. Cyclic voltammetry and structure of Br adsorbed a t Pt(lll).31. (A) Cyclic voltammogram of Pt(ll1) in 5 mM CaBr, ( p H 4.1); scan rate, 2 m V / S ; temperature, 23 f 1 OC. (b) Model structure of Pt(111)(3x3)-Br; rBr= 4 / 9 = 0.4444 Br atoms per surface Pt atom. (C) Model structure of Pt(111)(4x4)Br; 0,, = 7/16 = 0.4375.
layer structures encountered thus far consist of domains or “islands” having a uniform shape and structure separated by a shift in registry with the substrate. Deposition onto hexagonal substrates has been found to form a variety of hexagonal, oblique, or rectangular overlayers, some having quite large unit cells and complex structures. Immersion of a metal surface into any solution produces an interface which is electrified to at least some extent due to the differing electron affinities of the metal, solvent, electrolyte, adsorbate, and other species. As t,he (37) Stickney,J. L.; Stern, D. A,; Schardt, B. C.; Zapien, D. C.; Wieckowski, A,; Hubbard, A. T. J . Electroanal. Chem. 1986,213, 293. (38) Schardt, B. C.; Stickney, J. L.; Stern, D. A.; Wieckowski, A.; Zapien, D. C.; Hubbard, A. T. Langmuir 1987, 3, 239. (39) Schardt, B. C.; Stickney, J. L.; Stern, D. A,; Wieckowski, A,; Zapien, D. C.; Hubbard, A. T. Surf. Sci. 1986, 175, 520. (40) Stickney, J. L.; Schardt, B. C.; Stern, D. A,; Wieckowski, A.; Hubbard, A. T. ,I. Electrochem. Soc. 1986, 133, 648. (41) Delahay, P. Double Layer and Electrode Kinetics; Wiley-Interscience: New York, 1966.
0, + 4H’ + 4e- 2H20 (4) The position of the equilibrium depends upon the proportions of O,, H+, and H,O at the surface. For example, the stability of oxidizable adsorbates such as alcohols can be improved by introduction of traces of H,.25 pH Dependence of Electrode Surface Chemistry Figure 2C illustrates one aspect of the influence of solution pH upon molecular adsorption: the carboxylic acid 0-H stretch observed a t pH 3 (3566 cm-’) vanishes at alkaline pH, as does the C=O vibration at 1748 cm-’, and the EELS spectrum displays the prominent carboxylate C-0 stretch at 1612 cm-’ and the other bands characteristic of aromatic carboxylate, in keeping with eq 1, above. Auger spectra reveal the presence of potassium in amounts appropriate for neutralization of adsorbed NA by KOH, as described by eq 1, but only when NA is adsorbed at potentials sufficiently negative to produce noncoordinated carboxylic acid moieties. Other chemisorbed carboxylic acids also retain K+ ions provided that coordination between the acidic moiety and the surface is prevented by electrode potential and/or molecular ~ t r u c t u r e . ’ ~ ~ ~Adsorbed ’ ~ ~ ’ * ~ cyanide ~ and thiocyanate also retain cations at the Pt(ll1) surface.,’ The amount of cation retained and the relative affinity of the surface layer for cations are controlled by the size and strength of solvation of the cation, as well as by the pH. Halides and sulfide form an adsorbed layer which behaves as a
Langmuir, Vol. 6, No. I, 1990 101
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Figure 4. Vibrational spectra of hydroquinone (HQ) absorbed at annealed or oxidation-reduction-cycled surface^.'^ Solid curve: annealed Pt(ll1) surface. Dotted curve: cycled Pt(ll1) surface. Experimental conditions: adsorption was from 1mM HQ in 10 mM KF/HF (pH 4), followed by rinsing with 0.1 m M KF (pH 4); electrode potential, -0.1 V; other conditions as in Figure 2.
neutral metal complex and does not retain cat i o n ~ . ~ * ’ Hydroxide ~ * ~ ‘ ~ ~ ~tends to form a chemisorbed layer at metal surfaces, particularly a t positive potentials, and thus strongly influences surface behavior under alkaline condition^.^^^*.*^-^^ Hydroxide/oxide chemisorbed layers at Pt tend to be disordered as judged by LEED. The above studies illustrate the facility with which surface electrochemistry investigates adsorbates which are not sufficiently volatile or stable for convenient study by vacuum techniques alone: salts, polar organic compounds, solid metallic films, biomolecules, organic reagents and other reactive compounds, strained rings, unusual oxidation states or otherwise high-energy unstable molecules, multifunctional molecules, and compounds which undergo side reactions when exposed to a surface at open circuit.’ Formation of a surface layer from solution affords relatively mild conditions compared with high-energy evaporation procedures and offers the additional advantages of controlled pH, redox balance, solution, adsorbate activity, and other factors. Recent results also illustrate the comparative effectiveness with which surface electrochemistry deciphers the complexities of interfacial structure, composition, chemical bonding, reaction dynamics, and other complexities intractable to electrochemical methods alone.’
Illustrations of the Importance of Electrode Surface Structure EELS spectra in Figure 4 illustrate the importance of using annealed single-crystal surfaces for studies of molecules a t electrode surfaces.17 EELS spectra of adsorbed hydroquinone (HQ) a t an annealed Pt(ll1) surface (Figure 4, upper solid curve) are clearly delineated and resemble the IR spectrum of solid HQ, apart from the absence of an 0-H stretch. In contrast, adsorption of HQ a t the same surface after it had been roughened by the oxidation-reduction cycling (orc) procedure commonly employed by electrochemists to activate Pt electrodes led to a relatively indistinct EELS spectrum, qualitatively different from the smooth surface EELS spectrum and the IR spectrum of pure HQ (Figure 4, dotted curve). The “structural impurities” introduced by even a single cycle of the
orc procedure disordered the surface as viewed by LEED, increased the packing density of HQ by as much as 30%, and qualitatively altered the voltammetric oxidation behavior of adsorbed HQ. Annealed polycrystalline Pt electrodes displayed this same complex HQ adsorption behavior, although to a lesser extent than Pt(ll1) electrodes subjected to orc without subsequent annealing. Clearly, annealed single-crystal surfaces are preferable to orctreated surfaces in view of their superior vibrational spectroscopic behavior, as well as their inherently simpler adsorption and reactivity behavior, as discussed later. Results to date indicate that electrode surface structure has a very important influence upon every aspect of surface physical and chemical behavior.’ Surface structure affects adsorbate packing density, long-range order, molecular (or ionic) orientation, spectroscopic properties, and intermolecular interactions, which in turn control the surface reaction rates and energies. Single-crystal surfaces carefully oriented, cleaned, annealed, and analyzed contribute surface area of a single predominant type and therefore will tend to yield inherently simpler results in most fundamental studies and practical applications. Formation of single-crystal surfaces is energetically favorable for a variety of materials7* and is rather easily accomplished once the proper conditions have been f o ~ n d . ~ , ~ Furthermore, the techniques employed to guide the preparation process and provide verification of surface quality are then available for use in characterization of the changes in surface and in identification of surface species resulting from subsequent treatment and use of the initially clean, ordered, reproducible, “well-defined” surface. Adsorption of unsaturated hydrocarbons from solution or from vapor near atmospheric pressure at Pt(ll1) produces adsorbed species different from those obtained by adsorption at room temperature under ultrahigh v a c u ~ m . ~Species ~ , ~ ~adsorbed from solution resemble the parent compounds much more closely than those adsorbed from vacuum.23,25*26 The details and underlying causes of these differences are being sought. Immersion of the metal surface into electrolyte solution prior to contact between the surface and the adsorbate tends to moderate the reactivity of the surface toward the adsorbate. The lesser tendency of various hydrocarbons and hydrocarbon derivatives to dehydrogenate when adsorbed from solution at negative Pt electrode potentials than when adsorbed at Pt surfaces under v a c u ~ mmay ~ ~ be ,~~ related to the presence of adsorbed hydrogen atoms, anions, and other species at the Pt electrode surface. That is, the Pt electrode surface is probably closer to equilibrium with respect to hydrogen, redox balance, and acidity/ basicity than the clean Pt surface under vacuum. This prior equilibration of the metal surface with an electrolyte evidently provides an increased degree of control over the chemical reactions as well as electrochemical process occurring between an adsorbate and a metal surface. Surface electrochemistry is a logical starting point (42) Lumsden, J. B.; Garwood, G. A., Jr.; Hubbard, A. T. Surf.Sci. 1982,121, 1524. (43) Harrington, D. A.; Wieckowski, A.; Rosasco, S. D.; Salaita, G.
N.; Hubbard, A. T.; Lumsden, J. B. Proceedings of the Pourbaix Symposium;The Electrochemical Society: Princeton, NJ, 1984. (44) Harrindon. D. A.: Wieckowski. A.: Rosasco. S. D.: Schardt. B. C.; Salaita, G. N ;L k s d e n , J. B.; Hubbard, A. T. Corros. h i . 1985,’25, 849. (45) Harrington, D. A.; Wieckowski, A.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T. Langmuir 1985,1,232. (46) Frank, D. G.; Chia, V. K. F.; Schneider, M.; Hubbard, A. T. Langmuir 1987,3,860. (47) Wieckowski, A.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T.; Bent, B.; Zaera, F.; Somorjai, G. A. J. Am. Chem. SOC.1986, 107, 5910.
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for future studies of reactions a t well-defined surfaces under high-pressure conditions. Quantitative Analysis of Electrode Surfaces Auger spectroscopy will probably always be a mainstay of electrode surface characterization in view of the efficacy of the technique and its wide range of applicability!."," Auger spectroscopy sensitively detects all of the elements except H and He, making it suitable for checking the purity of a surface following cleaning. The incident electron beam is readily focused, regulated, calibrated, and rastered for microscopy, quantitative analysis, and other purposes. Beam current is sufficiently small that damage to the adsorbed layer is negligible. A primary method for calibration of an Auger spectrometer for quantitative purposes such as measurement of packing density (r,nmol cm2) is to compare the derivative Auger signal (d21/dE , A/eV2) with a coulometric measurement for the same surface. The coulometric surface reaction can be an irreversible oxidation in cases where the reaction stoichiometry is known5,11.15.20.23.25.26 or a reversible redox reaction of a pendant hydroquinone or other redox center.12J3J6 A secondary approach to calibration of Auger spectrometers is to obtain spectra of an adsorbed molecular layer which gives a definitive LEED pattern in which the numher of molecules in the unit cell is already known.11J2J6 Normalization of Auger signals from the adsorbed layer by an Auger signal from the clean substrate minimizes the influence of characteristics of an individual spectrometer. Normalized signals due to NA a t various packing densitieP are graphed in Figure 5A. Note that the Pt Auger signals decrease as the carbon Auger signal from NA increases. Two independent values of r are obtained from the data by use of eq 5-7
1
r = (Ic/lRo)/[Bc(1/3 + 2fc/3)1 r = r,/3 r = (1-
(5) (6)
(7) where B, = 0.314 nmol/cm2, fc = 0.70, and K = 0.16 cm2/nmol. Values of r are in good agreement, Figure 5B. Comparison of the plateau value of r with molecular models indicates a tilted vertical orientation in which the angle between the ring and the Pt(ll1) surface is about 75'. Angular Distribution Auger Microscopy (ADAM) Observations of the angular distribution of Auger electrons from single-crystal surfaces and overlayers, Figure 6, show promise of increasing the understanding of electron-scattering phenomena while at the same time yielding direct structural information concerning the adsorbed layer and the first 1C-15 atomic layers of the ~ u b s t r a t e . ~ The Auger electron angular distributions from Pt(ll1) contain a series of prominent features due to scattering of Auger electrons by overlying atoms, Figure 7A. The geometric relationships among these features are in agreement with the known structure (fcc) and orientation of P t ( l l l ) , Figure 7B. The data set shown in Figure 7A consists of 183 300 measurements of Auger signal (Pt, 51 (48) Schwffel. J. A,; Hubbard. A. T. Anal. Chem. 19'/8,86.271. (49) Hubbard, A. T. Eur. Speetrose. News 1988, 78,28. (50) Frsnk.D. G.:Batina.N.:MeCanar. J. W1Hubbard.A.T. Lanem w , in press. (51) Stickney, J. L.; Soriaga, M. P.; Hubbard, A. T.; Anderson, S. E.J . Eleetroonol. Chem. 1981, 125.73. (52) Pouchert, C. J. The Aldrieh Library of FTIR Spectra; Aldrich
-
Chemical Co.: Milwaukee,
1985.
I
A
5 J
a
5 v)
-LOG CMA(mol/L)
pH 33. -0.2V. Pt(ll1)
-6
-5
-4
-3
-2
-I
0
LOG C (MI Figure 5. Auger data and packing densities for nicotinic acid adsorbed at Pt(111).'6 (A) Auger signal ratios. (B)Molecular packing density. Experimental conditions: adsorption from NA solutions containing 10 mM KF adjusted to pH 7, followed by rinsing with 1 mM HF (pH 3.3); temperature, 23 1 "C;incident beam, 100 nA, Zoo0 eV at normal incidence; cylindrical mirror analyzer modulated at 5 V p-p; I, clean surface Pt Auger signal at 161 eV; I,,, coated surface $;signal; I,, carbon Auger signal due to NA.
Figure 6. Schematic diagram of the angular distribution Auger microscopy (ADAM) expermentm eV) collected at 0.1-deg intervals. The regions of lowest intensity are shown in black, followed by dark blue, magenta, red,and yellow, with the highest intensities shown in white. The green spots in Figure 7B represent linear projections from each emitting atom through the various scattering atoms to the detector as it scans the hemisphere above the sample. Projections for seven Pt overlayers are shown. The largest dots represent scattering of Auger electrons by atoms in the layer immediately above the emitter. The next largest dots represent scattering by atoms one layer removed from the emitter, and so
Kendall Award Lecture
Langmuir, Vol. 6, No. I , 1990 103
Figure 7. Angular distribution of platinum Auger intensity (71 eV) from the Pt(ll1) surface." (A, Left) Angular distribution (darker colors represent lower Auger signals. (B, Right) Angular distribution and geometric projection (green). Experimental conditions: incident beam, 4 MA,500 eV, 1l0from (111)surface plane.
Figure 8. Fourier transforms of angular distribution data.% (A) e = 0 0 , =~ 900, ~ B = 2(t160. (B) e = OD; a = 109.50; B = 20-160" ((111)zone). Experimental conditions as in Figure 7. forth. Unblocked paths from emitter to detector give rise to white "butterflies" near the (110) and equivalent
normals, or 'circles" in Figure 7A, while trajectories hampered by rows or planes of atoms result in clusters of dark spots. These sharp distributions contain an ahundance of intriguing features: work is continuing on the theoretical as well as experimental aspects. Analysis of angular distribution data by Fourier transformation is simple to implement and offers advantages for extraction of periodic signals (crystallinity), for location of the densest planes of the structure, and for placement of the atoms within those planes. Prior to transformation, the angular distribution is linearized by replacing the angle p in Figure 6 with its tangent (Figure 8). The linearized data are multipled by an isosceles-triangular function peaked at the middle of the distribution (apodized) as is customary in Fourier transformation of finite data sets. Figure 8A shows the results of a scan in which the analyzer followed an arbitrary path including the (111)surface normal. The transform reveals prominent periodicities correspondingto layers of Pt atoms parallel to the (111) plane t o a depth of about 10 layers, although the first three layers predominate. Figure 8B shows the results of a scan in which the analyzer path lies entirely within the (111) plane. The depth/width ratios ( p ) correspond to interatomic distances within the (iii)plane. Interfacial Structure and Electrochemical Reactivity Adsorbate structure and mode of bonding control electrochemical reactivity, as illustrated by recent studies of terminal alkenoic acids, alkenols, and alkenes at Pt(ll1) electrode s u r f a ~ e s Aromatic . ~ ~ ~ compound^^^"^'^ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ and amino acidsL5have also been studied. Most adsorbed alkenes, alkynes, and aromatic compounds a t Pt(ll1) surfaces undergo oxidation at Pt(ll1) electrode surfaces in acidic, aqueous electrolytes at electrode potentials near 0.9 V (vs Ag/AgCI Measurements of the number of electrons, no,, transferred during oxidation of each adsorbed molecule provide a first indication of the stoichiometry of the oxidation process: no, = (Q- Q , ) / ( F A r ) (8) where Q is the faradaic charge to oxidize the Pt surface
104 Langmuir, Vol. 6, No. 1, 1990
Hubbard
Table I. Packing Densities of Adsorbed Alkenoic Acids at Pt(ll1). molecular F (nmol/cm2 based on -log electrode adsorption rinse rc, rot rK, compd C (M) potential, V pH pH nmol/cm2 nmol/cm2 nmol/cm2 PPA 3.00 -0.1 3 3 1.33 0.67 PPA 3.00 0.3 3 3 1.23 0.74 -0.1 3 10 1.44 0.74 0.53 PPA 3.00 PPA neat open 1.56 0.52 -0.1 3 3 2.20 0.61 BBTA 4.00 3 3 2.20 0.63 BBTA 4.00 0.35 -0.1 3 3 2.05 0.59 3BTA 3.00 3BTA 3.00 0.35 3 3 2.13 0.83 -0.1 3 10 2.12 0.48 0.44 3BTA 3.00 BBTA neat open 2.33 0.70 -0.1 3 3 2.34 0.52 4PTA 4.00 4PTA 4.00 0.2 3 3 2.34 0.58 -0.1 3 3 2.33 0.57 4PTA 3.00 -0.1 3 10 2.80 0.51 0.21 4PTA 3.00 -0.1 3 3 3.62 0.30 GHPA 4.00 GHPA 4.00 0.2 3 3 3.27 0.38 -0.1 3 10 3.90 0.23 0.21 GHPA 3.00 -0.1 3 3 5.20 0.19 lOUDA sat. lOUDA sat. 0.2 3 3 5.13 0.29 -0.1 3 10 5.35 0.16 0.26 lOUDA sat.
I C 1
IPtl
0.44 0.41 0.48 0.51 0.50 0.54 0.52 0.53 0.53 0.58 0.47 0.44 0.47 0.51 0.52 0.47 0.56 0.47 0.47 0.49
IPtl Iptz 0.40 0.39 0.45 0.39 0.43 0.49 0.50 0.53 0.57 0.46 0.47 0.42 0.42 0.54 0.49 0.42 0.45 0.42 0.42 0.53
no,
before after vacuum vacuum 16.50 14.5
19.2
18.3
22.0
18.1
15.0
15.9
15.7
16.9
18.7
20.2
Experimental conditions: P t ( l l 1 ) surface, Ar+ bombarded and annealed (1000 K) under ultrahigh vacuum; electrolyte, 10 mM KF adjusted to pH 3 with HF; surface was rinsed with 2 mM HF (pH 3) or 0.1 mM KOH (pH 10) prior t o evacuation; incident electron beam, 300 nA at 2000 eV (cylindrical mirror analyzer); temperature, 23 i 1 "C.
and adsorbed layer, Qbis the charge to oxidize the clean Pt surface, F is the Faraday constant, A is the geometric area of the Pt(111)electrode, and I' is the molecular packing density (nmol/cm2) measured by means of Auger spectroscopy. Measured values of no, are given in Table I for a series of terminal alkenoic acids (c3-c6, C,, and C1,),23alkenols (c& and C,,),25 and alkenes (Cz-X,, C,, and C,,).26 The alkenoic acids and alkenols were adsorbed from dilute aqueous solutions, while the alkenes were adsorbed from the vapor at atmospheric pressure (C,-C,) or a t the vapor pressure. The packing densities of all of the alkenoic acids were similar (near 0.5 nmol/cm2),close to the theoretical limiting packing density of propenoic acid based upon molecular models of the horizontal orientation (0.430 nmol/cm2), Figure 9. Evidently, the aliphatic chain extends away from the surface such that the surface area occupied is only that of the CH,=CHCH, moiety. Also, the magnitude of nox for C,, acid (18.7) is similar to that for the C, acid (18.11, indicating that oxidation is confined to the unsaturated carbons and one adjacent carbon regardless of chain length: CH,=CH(CH,),CO,H
+ 6H,O
-
2C0, + HO,(CH,),CO,H + 16H'
-0.1 v
Q35V
h
+ 16e- (9)
When adsorbed a t negative electrode potentials (less than -0.1 V vs Ag/AgCl), 3-butenoic acid exhibits a carboxylic acid 0-H stretching band (EELS)at 3571 cm-l which does not completely disappear when adsorption is carried out a t 0.35 V. Evidently, coordination of the carboxylate moiety to the Pt surface occurs to some extent at positive potentials. Propenoic acid is bonded to the Pt surface through the carboxylate and the C=C moieties a t all potentials, while the C,, C,, and C , , acids display intermolecular hydrogen bonding, but their carboxylate moieties do not interact appreciably with the Pt surface. Terminal alkenes and alkenols are likewise attached to the Pt(ll1) surface with the C=C moiety parallel to the surface plane, with the aliphatic chain pendant from the surface, and undergo electrochemical oxidation which is localized to the C=C moiety and one or two adjacent carbons, Table I.25926
Figure 9. Structural models of adsorbed alkenoic acids a t Pt(111).23 (A) P P A (adsorbed at -0.1 or 0.30 V). (B) BBTA (adsorbed a t -0.1 or 0.35 V). (C) 4 P T A (adsorbed a t -0.1 or 0.20 V). (D)GHPA (potential dependence is negligible). (E) 10 UDA (potential dependence is negligible).
Langmuir 1990, 6, 105-113
Acknowledgment. This work is supported by the Air Force Office of Scientific Research, the U S . Department of Energy, the Edison Program of Ohio, the Gas
105
Research Institute, Lehr Precision Inc., the National Institutes of Health, the National Science Foundation, and Pratt-Whitney, Inc.
Articles XPS Studies of Gold Films Prepared from Nonaqueous Gold Colloids Beng Jit Tan, Peter M. A. Sherwood,* and Kenneth J. Klabunde* Department of Chemistry, Willard Hall, Kansas State University, Manhattan, Kansas 66506 Received January 3,1989. I n Final Form: June 23, 1989 XPS studies on gold and gold-palladium films prepared from nonaqueous colloidal solution show evidence of an unusual surface species. This "Au-carbon" species exhibits a positive shift in the core level binding energy. Applying a biasing potential to the sample enabled the separation of the Au 4f peaks of the surface species from those of the bulklike metal particles. The results of the biasing experiment are consistent with the model of the surface metal cluster being negatively charged and coated with a carbonaceous film of solvent fragments. The "Au-carbon" species are not stable to prolonged exposure to ambient conditions, agglomerating to form larger bulklike metal particles. The nature of the substrate on which the film is grown plays an important role in determining the formation of the surface species.
Introduction In recent years, we have been interested in clusters/ particles formed from metal atom accretion in low-temperature Unusual shapes and high reactivities for such particles have been realized, and this is probably due to the formation of surface structural features not usually encountered, since under these conditions particle growth will be kinetically controlled. These studies have led to a new preparative procedure for zero-valent metal heterogeneous catalysts, which we have termed solvated metal atom dispersed (SMAD).'v5 Both monometallic and bimetallic SMAD catalysts have exhibited unusual proper tie^.^" Their behavior can be explained by structural considerations mentioned above (defect sites, etc.) and by the fact that some carbonaceous fragments from the solvent end up incorporated in the particles, which can affect magnetics and electricalg properties as well as catalytic metal-support interactions, and the particle size of the metal c l u ~ t e r s . ~ J ~ (1)Klabunde, K. J.; Efner, H. F.; Murdock, T. D.; Ropple, R. J. Am. Chem. SOC.1976.98. 1021. (2) Klabunde, K: J. In Chemistry of Free Atoms and Particles; Academic: New York, 1980.
(3) Davis, S. C.; Klabunde, K. J. Chem. Reu. 1982,82,153. (4) Tan, B. J.; Klabunde, K. J. 'Solvated Metal Atom Dispersed Catalysts-A Comprehensive Review", to be published. (5) Imizu, Y.; Klabunde, K. J. In Catalysis of Organic Reactions; Augustine, R. L., Ed.; Marcel Dekker: New York, 1985. (6) (a) Klabunde, K. J.; Davis, C.; Hattori, H.; Tanaka, Y. J . Catal. 1978,54,254. (b) Klabunde, K. J.; Tanaka, Y. J . Mol. Catal. 1983,21, 57. (7) (a) Kanai, H.; Tan, B. J.; Klabunde, K. J. Langmuir 1986,2, 760. (b) Li, X.Y.; Klabunde, K. J. Langmuir 1987,3,558.
0743-7463/90/2406-0105$02.50/0
More recently, we have reported on a modification of the SMAD procedure which has allowed the synthesis of stable, nonaqueous metal colloids. In particular, noble metal atoms solvated in polar organics can lead to stable colloidal dispersions of indefinite stability.13-ls The method seems to be wide in scope and allows entry into a series of hitherto unavailable nonaqueous colloidal systems. An additional interesting feature is the ability of some of these dispersions to form metallic films simply by solvent removal. In order to understand these materials better, such as particle formation and film growth, we have chosen a colloidal suspension of gold in acetone as a model. Gold is very suitable for XPS studies and one that we have studied as part of a program to investigate electrode surface oxidation.16 By studying films grown from goldacetone solutions, we hope to learn something about par(8) Davis, s. C.; Severson, s.;Klabunde, K. J. J. Am. Chem. SOC. 1981,103,3024. (9) Davis, S. C.; Klabunde, K. J. J. Am. Chem. SOC.1978,ZOO,5973. (10) Klabunde, K. J.; Ralston, D.; Zoellner, R.; Hattori, H.; Tanaka, Y.J. Catal. 1978,55,213. (11) Matsuo, K.; Klabunde, K. J. J. Org. Chem. 1982,47,843. (12) Ralston, D. H.;Klabunde, K. J. Appl. Catal. 1982,47,843. (13) Lin, S.-T.;Franklin, M. T.; Klabunde, K. J. Langmuir 1986,2, 259. (14) Franklin, M. T.; Klabunde, K. J. In "Living Colloidal Metal Particles From Solvated Metal Atoms: Clustering of Metal Atoms in Organic Media"; High Energy Processes in Organometallic Chemistry; ACS Symposium Series 33, American Chemical Society: Washington, D.C., 1987;pp 246-259. (15) Cardenas-Trivino, G.;Klabunde, K. J.; Dale, E. B. Langmuir 1987,3,986. (16) Sherwood, P.M. A. Chem. SOC.Reu. 1985,14,1.
0 1990 American Chemical Society
