Comparison of vacuum-annealed and electrochemically cycled

Aug 23, 1988 - Comparison of Vacuum-Annealed and Electrochemically ... changes in packing density observed by Auger spectroscopy. Cycling of the Pt(ll...
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Langmuir 1989,5,819-828

819

Comparison of Vacuum-Annealed and Electrochemically Cycled Electrodes in Adsorption and Electrocatalysis: Aromatic Compounds at Platinum(11 1) and Polycrystalline Platinum John Y. Gui, Bruce E. Kahn, Laarni Laguren-Davidson, Chiu-Hsun Lin, Frank Lu, Ghaleb N. Salaita, Donald A. Stern, and Arthur T. Hubbard* Department of Chemistry] University of Cincinnati, Cincinnati] Ohio 45221 -01 72 Received August 23, 1988. I n Final Form: January 23, 1989 These studies compare adsorbed layer vibrational spectra and packing densities of various aromatic compounds at annealed Pt(111)and polycrystalline Pt surfaces (Pt(po1y))with the behavior of surfaces pretreated by “electrochemical cycling” (the oxidation-reduction procedure commonly employed to pretreat electrodesprior to use). Surface structural changes produced by cycling exert a profound effect upon each of the properties studied. Adsorbates studied represent various types of surface attachment: hydroquinone (HQ), which displays a-bonding to P t surfaces (horizontal orientation) when adsorbed from sufficiently dilute aqueous solutions; 2,2’,5,5’-tetrahydroxybiphenyl(THBP), which adopts a mixture of horizontal and vertical orientations; 3-thiophenecarboxylicacid (3TCA), (3-pyridy1)hydroquinone(BPHQ),and nicotinic acid (NA), which exhibit primarily a-bonding (tilted vertical orientation); and benzyl mercaptan (BM) and 2,5-dihydroxy-4-methylbenzylmercaptan (DMBM),for which attachment occurs through a sulfur atom to form a benzyl pendant. Packing densities (moles adsorbed per unit area) were measured for each compound at each surface by Auger spectroscopy. Surface vibrational spectra were obtained by electron energy loss spectroscopy (EELS) and were assigned by comparison with the IR spectra of the pure compounds. Substrate surfaces were characterized by LEED. Cycling the Pt(ll1) surface affects the adsorbate packing density by up to 50%, while smaller effects are observed for Pt(po1y). Cycling causes the LEED pattern of the Pt(ll1) substrate to become diffuse. Cyclic voltammetry, where applicable, confirms the changes in packing density observed by Auger spectroscopy. Cycling of the Pt(ll1)surface greatly decreases the elastic specular reflection intensity of EELS electrons, while Pt(po1y) exhibits low reflectivity after either pretreatment. EELS spectra of adsorbed HQ and THBP display profound intensity changes as a result of cycling. That is, interaction of the aromatic ring system with the cycled surface is very different from that with the annealed (atomically smooth) Pt(ll1) surface. Spectra of adsorbates at the Pt(po1y) surface are similar to cycled Pt(ll1); cycling of the Pt(po1y) has little additional effect. Adsorbed layers in which the aromatic ring is attached to the surface primarily through a single heteroatom (3TCA, NA, and 3PHQ) are less strongly influenced by the cycling pretreatment. Adsorbates for which the aromatic ring is pendant from the surface through a benzyl mercaptan sulfur atom (BM and DMBM) are affected relatively slightly by surface structure.

Introduction The adsorption of aromatic compounds from solution onto Pt surfaces has revealed that oriented adsorbed layers can be formed on both Pt(ll1) and polycrystalline Pt (Pt(poly)).’ Adsorption of aromatics onto Pt electrodes which were cleaned by electrochemical cycling has also been extensively studied.lalb Those studies have shown (1) (a) Hubbard, A. T.; Stickney, J. L.; Soriaga, M. P.; Chia, V. K. F.; Rosaeco, S. D.; Schardt, B. C.; Solomun, T.; Song, D.; White, J. H.; Wieckoweki, A. J. Electroanal. Chem. 1984,168,43. (b) Hubbard, A. T. Chem. Rev. 1988,88,633. (c) Salaita, G. N.; Lu, F.; Laguren-Davidson, L.; Frank, D. G.; Stern, D. A,; Zapien, D. C.; Batina, N.; Wellner, E.; Walton, N.; Hubbard, A. T. In Chemically Modified Surfaces; Leyden, D., Ed.; Gordon and Breach New York, 1988; Vol. 2. (d) 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. (e) 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. (fJSalaita, G. N.; LagurenDavidson, L.; Lu, F.; Wellner, E.; Stern, D. A.; Batina, N.; Frank, D. G.; Benton, C.S.; Hubbard, A. T. J. ElectroanaL Chem. 1988,245,253. (9) Rank, D. G.; Stern,D. A.; Tarlov, M. J.; Batina, N.; Walton, N.; Hubbard, A. T. Langmuir 1988,4, 711. (h) 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. SOC.1989, 111, 877. (i) Hubbard, A. T.; Frank, D. G.; Stern, D. A,; Tarlov, M. J.; Batina, N.; Walton, N.; Wellner, E. In Redox Chemistry and Interfacial Behauior of Biological Molecules; Dryhurst, G., Niki, K., Eds.: Plenum, New York, 1988. (i)Batina, N.; 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.; Mark, H. B., Jr.; Emmer, H. Langmuir, in press. (k) Batina, N.; McCargar, J. W.; Laguren-Davidson, L.; Lin, C.-H.; Hubbard, A. T. Langmuir 1989,

5, 123.

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that the interaction and orientation of the adsorbate can be profoundly influenced by factors such as molecular structure, concentration, and surface roughness. The effect of surface roughness on the adsorption and orientation of aromatic compounds on Pt(po1y) has been studied previously.2 Adsorbate orientation and mode of surface bonding are of fundamental as well as practical importance, as has been shown by their influence on the nature of electrochemical oxidation and reduction products and surface chemical reactions. Several factors are known to influence the interaction, reactivity, and orientation of molecular adsorbates at metal electrode surfaces including molecular structure, concentration, and extent of surface roughness. The voltammetric manifestations and the resulting surface structural implications of different electrode surfaces and methods of preparation have been examined by a variety of author^.^ Wagner and Ross have reported several surface structural studies of a variety of Pt electrode surfaces in UHV using LEED and Auger spe~troscopy.~Their results demon(2) (a) White, J. H.; Soriaga, M. P.; Hubbard, A. T. J. Electroanal. Chem. 1984, 177, 89. (b) Chia, V. K. F.; White, J. H.; Soriaga, M. P.; Hubbard, A. T. J. Electroanal. Chem. 1987,217, 121. (3) (a) Ishikawa, R. M.; Katekaru, J. Y.; Hubbard, A. T. J. Electroanal. Chem. 1978, 271, 86. (b) Yamamoto, K.; Kolb, D. M.; Kotz, R.; Lehmpful, G. J. Electroanal. Chem. 1979,96,233. ( c ) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (d) Clavilier, J.; Durand, R.; Guinet, G.; Faure, R. J. Electroanul. Chem. 1981, 127, 281. (e) Clavilier, J.; Armand, D.; Wu,B. L. J.Electroanal. Chem. 1982, 135, 159.

0 1989 American Chemical Society

820 Langmuir, Vol. 5, No. 3, 1989

strated that electrochemical cycling of Pt electrode surfaces induces randomly spaced monoatomic steps on the surface. Scanning tunneling microscopy has been very recently employed to show the effects of electrochemical cycling on platinum surface morphology.6 These studies indicate that the form of pretreatment causes marked changes in adsorbate orientation and electrochemical reactivity, as evidenced by various UHV electron spectroscopic as well as electrochemical techniques. In the present work, adsorbed layers formed from aqueous solutions on Pt(ll1) and Pt(po1y) surfaces pretreated by annealing in UHV or electrochemical oxidation and reduction cycles are characteiized by electron spectroscopic techniques in UHV and by cyclic voltammetry at atmospheric pressure in argon. The combination of these techniques makes determination of the effects of polycrystallinity and electrochemical cycling on aromatic adsorption and reactivity possible. Cycling procedures are commonly employed to clean and pretreat platinum electrodes. The compounds studied in this work (various phenols, mercaptans, pyridines, and thiophenes) are chosen to be representative of a variety of aromatic adsorbates, including examples of horizontal and vertical adsorbate orientations. Molecular packing densities are measured by use of Auger spectroscopic methods: Layer stoichiometry and surface cleanliness are also monitored by means of Auger spectroscopy. Long-range order of the substrate and adsorbed layer is characterized by means of LEED. Vibrational spectra of adsorbed molecules are obtained by use of high-resolution electron energy loss spectroscopy (EELS). These results reveal that electrochemical oxidation-reduction cycling of the Pt(ll1) surface using a variation of the procedure commonly employed to prepare Pt electrodes for electrochemical studies leads to scrambling of the surface layer, which results in major changes in the vibrational spectra of the surface layer, substantial increases in packing density, and striking changes in adsorbed layer electrode reaction behavior. Polycrystalline Pt surfaces behave similarly to cycled Pt(ll1) surfaces.

Experimental Section Reported here are experiments in which an electrode surface containing an adsorbed layer is investigated by means of specially constructed instrumentation:’ surface structure is examined by means of low-energy electron diffraction (LEED), surface elemental composition and molecular packing density are determined by using Auger spectroscopy, adsorbed layer vibrational bands are observed by electron energy loss spectroscopy (EELS), and electrochemical reactivity of the surface is explored by using voltammetry and coulometry. The Pt(ll1) surfaces employed for this work are oriented and polished such that all six faces are crystallographically equivalent. All faces are cleaned simultaneously by bombardment with Ar+ ions at 700 eV and are annealed at about lo00 K in ultrahigh vacuum. “Electrochemicallycycled” surfaces are prepared by stepping the electrode potential of the annealed surface from -0.05 V (vs Ag/AgCl) to 1.45 V (vs Ag/ AgCl) at a frequency of 0.1 Hz for 8 min in a 1M HC104 solution. Cleaning and annealing of the Pt surface are continued until Auger spectroscopy shows that the surface is free from detectable impurities. LEED patterns of Pt(ll1) have the usual hexagonal appearance. LEED patterns of Pt(po1y) after cleaning and annealing in UHV show a series of sharply defined LEED patterns, indicating that the Pt(po1y) surface is a patchwork of locally ordered facets having various square, oblique, or hexagonal

(4)(a) Ross, P. N., Jr. Surf. Sci. 1981, 102, 463. (b) Wagner, F. T.; Ross, P. N., Jr. J. ElectroanaL Chem. 1983,150,141. ( c ) Wagner, F. T.; Ross, P. N., Jr. Surf. Sci. 1985, 160, 305. (d) Wagner, F. T.; Ross,P. N., Jr. J. Electround. Chem. 1988,250, 301. ( 5 ) Fan, F. F.; Bard, A. J. Anal. Chem. 1988, 60, 751.

Gui et al. symmetries, separated by disordered regions. Since these facets give sharp LEED patterns, they must be at least 100 8, wide. Optical microempy reveals that typical facets are 10-100 pm wide. After ion bombardment and annealing, the Pt surfaces are isolated in an argon-filled antechamber for immersion into buffered aqueous electrolytes containing the subject adsorbates. Electrode potentials are measured and controlled by means of three-electrode electrochemicalcircuitry based upon operational amplifiers. The electrochemicalcell is constructed of Pyrex glass. Solutions and gases are transferred through Teflon-jacketed tubing. The jacket is purged with argon to minimize diffusion of air into the tubes conveying the solutions and inert gases. The electrochemical cell containing the reference electrode (Ag/AgCl) and Pt auxiliary electrode is introduced into the antechamber by means of a bellows assembly and gate valve; there are no sliding seals or other sources of contamination in the apparatus. Solutions employed for adsorption and voltammetric or coulometric measurements contained 10 mM KF, p.H adjusted with HF as indicated to provide adequate conductivity and buffer capacity. Water used in the experiments is pyrolytically distilled in pure O2 through a Pt gauze catalyst a t 800 OC and distilled again. The adsorbates were obtained aa follows: Hydroquinone (HQ), nicotinic acid (NA), benzyl mercaptan (BM), and 3-thiophenecarboxylic acid (3TCA) were used as received from Aldrich Chemical Co. (Milwaukee, WI 53223). 3-Pyridylhydroquinone (3PHQ),lh2,2’,5,5’-tetrahydroxybiphenyl(THBP): and 2,5-dihydroxy-4-methylbenzyl mercaptan (DMBM)’ were synthesized as described previously.

k

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BM

MA

p” n

nd imo

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Electron energy loss spectra (EELS) were obtained by means of an LK Technologies EELS spectrometer (Bloomington, IN 47405). Beam current at the sample was approximately 200 PA; beam energy was 4 eV. The spectrometer was operated a t a resolution of about 10 meV (80 cm-I) in these experiments. Infrared spectra of solid compounds in KBr or in Nujol on ZnS were obtained by using a Perkin-Elmer Model 1420 spectrometer. Packing densities, rX (moles of adsorbed atoms/centimeters squared) or r (moles of adsorbed moldes/centimetem squared) were obtained as follows: Auger signals, Zx, due to each element X were measured and normalized by the Auger signal at 161 or 235 eV due to the clean Pt surface ZR0. Packing density was obtained from (Zx/Z,O) by means of equations of the following form:

rx = ( Z ~ / Z ~ O ) / [ B ~ ( L J ~ ~ ~+ ,52fXMa + ... + L , ~ ~ ~ N ) ] where Bx was calibrated by means of hydroquinone1dand [email protected] Li is the fraction of element X located in level i (i = 1is adjacent to the Pt surface, and N is the outermost layer); fx is the attenuation factor for Auger electrons of element X by light atoms such as C, N, or 0;fx = 0.70 for X = C, N, or 0, based upon the observed attenuation of Pt Auger electrons (235 eV) by a (3 X 3) layer of horizontally oriented hydroquinone; and Mi is the number of non-hydrogen atoms located on the average path from the emitting atom to the detector. (6) Ullman,F. Justus Liebigs Ann. Chem. 1904,332, 68. (7) Fields, D. L.; Miller, J. B.; Reynolds, D. D. J . Org. Chem. 1965,30, 3962.

Langmuir, Vol. 5, No. 3, 1989 821

Aromatics at Pt(ll1) and Ptboly) 0.5

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Figure 1. Packing density of HQ versus concentration. (A) Pt(ll1). (B) Pt(po1y). Solid curves: annealed surfaces. Dotted curves: cycled surfaces. Experimental conditions: electrode potential during adsorption,-0.1 V vs Ag/AgCl reference (1M KC1); electrolyte, 10 mM KF, adjusted to pH 4 with HF; rinsed with H20 adjusted to pH 4 with H F temperature, 23 1 O C ; cycling procedure, 48 cycles between 1.45 and -0.05 V, 5 s at each potential in 1 M HClO&

*

Results and Discussion 1. Hydroquinone (HQ).HQ is an interesting compound in this context because it adsorbs at Pt(ll1) in a horizontal orientation with the phenyl ring parallel to the Pt surface.ld Auger data for HQ are summarized in Table I. Packing densities obtained from the Auger data by use of equations given in Table I11 are given in Table 11. For simplicity and consistency, packing density is calculated in the same manner for all surfaces. Molecular packing densities of HQ at Pt(ll1) and Pt(poly) surfaces cleaned by argon ion-bombardment and annealed in ultrahigh vacuum (UHV) are graphed in Figure 1. The isotherms of HQ a t Pt surfaces are profoundy affected both by cycling and by polycrystallinity, Figure 1. Evidently, adsorption of HQ at annealed Pt(ll1) leads to close packing in the horizontal orientation over a wide range of HQ concentrations (calculated by using covalent and van der Waals radii tabulated by Pauling? 0.293 nmol/cm2; found 0.31-0.33 nmol/cm2), while adsorption at annealed Pt(po1y) results in packing densities (0.41-0.47 nmol/cm2) greater than would be expected for horizontal close packing at a smooth surface. High packing densities of HQ (that is, large adsorbed amounts) could (8) P a u l i , L. C. The Nature of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, New York, 1960.

-0.2 0.0 0.2 0.4 0.6 0.8

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POTENTIAL. VOLT vs. Ag/AgCI Figure 2. Cyclic voltammetry at Pt electrode surfaces. (A) Pt(ll1). (B) Pt(po1y). Solid curves: annealed surfaces. Dotted curves: cycled surfaces. Experimental conditions: scan rate, 5 mV/s; electrolyte, 1M HClO,; temperature, 23 f 1O C ; reference electrode, Ag/AgCl (1 M KCl).

be due to tilted or vertical adsorbate orientationsla*b*d and/or to increases in surface area. Since the hydrogen adsorption peaks increase due to cycling for the Pt(ll1) surface, Figure 2A, an increase in surface area is at least partly responsible for the observed increase in the amount of adsorbed HQ. In the experiments pertaining to electrode pretreatment by cycling, Pt(ll1) and Pt(poly) annealed electrodes were subjected to 48 cycles of electrochemical oxidation and reduction prior to adsorption. Electrochemical cycling of the Pt(111)surface results in a 29% larger packing density of HQ (0.40 nmol/cm2) than at annealed Pt(ll1) (0.31 nmol/cm2). However, cycling has little influence onIkQ packing density at Pt(poly), for example, 0.42 nmol/cm2 a t the cycled surface, compared with 0.45 nmol/cm2 at the annealed surface when the HQ concentration is 1mM. The increase in packing density at Pt(ll1)as a result of the electrochemical cycling pretreatment is consistent with the increase in hydrogen and oxide adsorption peaks of the cyclic voltammogram of clean Pt(lll),Figure 2, and with the disordering of the surface resulting from cycling, as observed by use of LEED.la* A striking contrast is seen between the EELS spectra of HQ at Pt(ll1) surfaces prepared by annealing in UHV

822 Langmuir, Vol. 5,No. 3, 1989

compd HQ

HQ

HQ

THBP THBP DMBM DMBM 3PHQ 3PHQ NA NA 3TCA 3TCA

Table I. Auger and Electrochemical Data electrode electrode surface -log C (M) potential, V annealed Pt(ll1) 6.00 0.00 5.00 0.00 4.00 0.00 3.00 0.00 2.00 0.00 1.00 0.00 cycled Pt(ll1) 0.00 6.00 5.00 0.00 4.00 0.00 3.00 0.00 2.00 0.00 0.00 1.00 6.00 0.00 annealed Pt(po1y) 5.00 0.00 4.00 0.00 3.00 0.00 2.00 0.00 1.00 0.00 cycled Pt(po1y) 0.00 6.00 5.00 0.00 4.00 0.00 3.00 0.00 2.00 0.00 1.00 0.00 3.00 0.00 annealed Pt(ll1) 3.00 0.00 cycled Pt(ll1) 0.00 3.00 annealed Pt(po1y) 3.00 0.00 cycled Pt(po1y) 3.00 -0.12 annealed Pt(ll1) 3.00 -0.12 cycled Pt(ll1) 3.00 -0.12 annealed Pt(po1y) 3.00 -0.12 cycled Pt(po1y) 3.30 -0.10 annealed Pt(ll1) 3.30 4.10 cycled Pt(ll1) 3.30 -0.10 annealed Pt(po1y) 3.30 -0.10 cycled Pt(po1y) 0.00 3.00 annealed Pt(ll1) 3.00 0.00 cycled Pt(ll1) 0.00 3.00 annealed Pt(po1y) 3.00 0.00 cycled Pt(po1y) 3.00 0.00 annealed Pt(ll1) 3.00 0.00 cycled Pt(ll1) 3.00 0.00 annealed Pt(po1y) 3.00 0.00 cycled Pt(po1y)

Gui et al. for Molecules Adsorbed at Pt Electrodes” ~~

pH 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0

ZClZp, 0.405 0.506 0.509 0.601 0.559 0.576 0.407 0.698 0.727 0.745 0.760 0.780 0.464 0.611 0.659 0.743 0.817 0.851 0.428 0.730 0.751 0.790 0.809 0.794 0.656 0.611 0.696 0.880 0.752 0.947 1.092 1.136 0.785 0.861 0.972 1.204 0.526 0.594 0.521 0.786 0.364 0.571 0.544 0.590

Zo/Zp, 0.238 0.377 0.341 0.343 0.361 0.342 0.365 0.447 0.445 0.451 0.474 0.505 0.449 0.459 0.500 0.468 0.488 0.495 0.463 0.479 0.445 0.475 0.549 0.558 0.478 0.382 0.463 0.554 0.113 0.391 0.443 0.471 0.310 0.386 0.382 0.542 0.342 0.394 0.504 0.534 0.260 0.411 0.296 0.233

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0.111 0.100 0.112 0.135 0.155 0.224 0.287 0.272 1.110 1.400 1.376 1.460

“Experimental conditions: beam current 100 nA at 2000 eV, normal inci--,nce; mo-dation 5 V p.-p. at 1 k__-; reference r._ctrode Ag/ AgCl (1M KC1); adsorption from 10 mM KF solutions adjusted to the pH indicated with HF; rinsing with 0.1 mM KF adjusted to the pH indicated with HF.

and those prepared by electrochemical cycling, Figure 3. Elastic (specular angle) reflection from Pt(ll1) is sharply decreased (about 40-fold) by cycling. This is further evidence that surface disorder results from cycling. The intensity of all vibrations, with the exception of the C-H stretch, is much smaller on the cycled surface than on the annealed surface. Vibrational frequencies as well as amplitudes are affected. Asymmetric CC streching is shifted to a higher frequency, from 1591cm-’ at annealed Pt(ll1) to 1639 cm-’ at cycled Pt(lll), suggesting that the aromatic ring is perturbed to a lesser extent at the cycled surface. Symmetric CC stretching is shifted to a lower frequency, indicating a change in adsorbate molecular symmetry. Phenolic C-0 stretching, 1157 cm-’ at annealed Pt(lll), increases in frequency 88 a result of cycling (1242 cm-’), close to that in the IR spectrum of solid HQ (1210 cm-9. A small O-H stretching peak, 3421 cm-’, seen only at the cycled surface, is assignable to O-H bending, compared with 1097 cm-l in the IR spectrum of solid HQ. Shifts in rings deformation frequencies and amplitudes are also seen. Similar changes may be observed in the EELS spectra of HQ adsorbed from more concentrated (0.1 M) solutions. However, the effect is much more pronounced. A t this

concentration, the presence of a broad O-H stretch, 3350 cm-‘, is clearly visible on the cycled surface but not on the annealed surface. As was observed for the 1 mM concentration, the asymmetric CC stretch, 1600 cm-’, shifts to a higher frequency, 1655 cm-’, while the symmetric CC stretch, 1450 cm-’, shifts to a lower frequency, 1432 cm-’, upon cycling. The phenolic C-0 stretching frequency, 1223 cm-l, for annealed Pt(ll1) is very close to that observed for solid HQ, 1210 cm-’. Evidently, the “roughening”of the surface resulting from electrochemical cycling leads to reorientation of a substantial fraction of the adsorbate from horizontal to tilted or vertical orientation, with increased retention of phenolic hydrogens. The changes to the surface caused by cycling may disrupt the surface structure to such an extent that horizontal adsorption of HQ on the surface is less favorable.

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EELS spectra taken of HQ adsorbed on annealed polycrystalline Pt surfaces resemble those obtained on cycled

Langmuir, Vol. 5, No. 3, 1989 823

Aromatics at P t ( l l 1 ) and Pt(po1y)

compd HQ

HQ

HQ

THBP, THBP DMBM DMBM 3PHQ 3PHQ

NA NA 3TCA 3TCA

Table 11. Packing Densities for Molecules Adsorbed at Pt Electrodes electrode electrode surface -log C (M) potential, V pH rc ro rN 4.0 1.29 0.41 6.00 0.00 annealed Pt(ll1) 4.0 1.61 0.59 0.00 5.00 1.62 4.0 0.59 4.00 0.00 1.92 4.0 0.60 3.00 0.00 4.0 1.78 0.63 2.00 0.00 4.0 1.83 0.60 1.00 0.00 4.0 1.30 0.64 0.00 6.00 cycled Pt(ll1) 2.22 0.78 5.00 0.00 4.0 4.0 2.42 0.79 4.00 0.00 2.37 0.79 3.00 0.00 4.0 2.42 2.00 0.00 4.0 0.83 4.0 2.48 0.88 1.00 0.00 1.48 0.78 6.00 0.00 4.0 annealed Pt(po1y) 4.0 1.94 0.80 5.00 0.00 4.0 2.10 0.87 4.00 0.00 4.0 2.43 0.82 3.00 0.00 4.0 2.60 0.85 2.00 0.00 2.62 0.00 4.0 1.00 0.86 6.00 0.00 4.0 1.36 0.81 cycled Pt(po1y) 4.0 2.32 0.83 5.00 0.00 4.00 0.00 4.0 2.39 0.78 4.0 2.52 0.83 3.00 0.00 2.00 0.00 4.0 2.58 0.96 1.00 0.00 4.0 2.53 0.97 4.0 2.09 0.83 3.00 annealed Pt(ll1) 0.00 4.0 1.95 0.67 3.00 0.00 cycled Pt(ll1) 2.21 3.00 0.00 4.0 0.81 annealed Pt(po1y) 2.80 4.0 0.97 3.00 0.00 cycled Pt(po1y) 3.00 -0.12 2.71 4.0 0.20 annealed Pt(ll1) 3.41 3.00 -0.12 4.0 0.65 cycled Pt(ll1) 3.00 -0.12 4.0 3.93 0.76 annealed Pt(po1y) 3.00 -0.12 4.0 4.15 0.81 cycled Pt(po1y) 3.30 -0.10 4.0 3.20 0.58 0.19 annealed Pt(ll1) 4.0 3.30 -0.10 3.51 0.73 0.17 cycled Pt(ll1) 4.0 3.97 0.72 0.19 annealed Pt(po1y) 3.30 -0.10 4.91 1.02 0.23 4.0 3.30 -0.10 cycled Pt(po1y) 3.0 3.00 0.00 2.16 0.64 0.27 annealed Pt(ll1) 3.0 2.44 0.74 0.39 0.00 cycled Pt(ll1) 3.00 2.14 3.00 0.00 3.0 0.95 0.24 annealed Pt(po1y) 3.00 0.00 3.0 3.23 1.01 0.27 cycled Pt(po1y) 3.00 -0.10 3.0 1.36 0.45 amealed Pt(ll1) 3.00 -0.10 3.0 2.14 0.72 cycled Pt(ll1) 3.00 -0.10 3.0 2.04 0.52 annealed Pt(po1y) 3.0 2.21 0.41 3.00 -0.10 cycled Pt(po1y)

Pt(ll1) surfaces. The surface pc.jcrystallinity reduces the amount of elastic (specular angle) reflection in a manner similar to electrochemical cycling. The C-H stretching amplitude at annealed Pt(po1y) is much larger, compared with the other vibrational modes, than on the annealed Pt(ll1) surface. This increased C-H stretching intensity is also seen for the cycled Pt(ll1) surface. The C-0 stretching frequency is 1245 cm-', a shift to higher frequency by 100 cm-' from that found for annealed Pt(ll1); the value found for the IR spectrum of solid HQ is 1210 cm-'. The effect of electrochemical cycling on the EELS spectrum of HQ on Pt(po1y) is much less dramatic than that observed for Pt(ll1). This concurs with the results obtained both electrochemically and by Auger spectroscopy. A slight 0-H stretch can be seen for the cycled Pt(po1y) surface, which was not present on the annealed surface. The frequency of the asymmetric CC stretch, 1588 cm-', shifts to a higher frequency, 1645 cm-', upon cycling, as was observed on the Pt(ll1) surface. There is also a sharpening and high-frequency shift of the C-H bending mode from 830 cm-' on the annealed surface to 887 cm-' on the cycled surface. The EELS spectrum of HQ adsorbed on annealed Pt(poly) from concentrated HQ solutions (0.1 M) is similar to that obtained for HQ adsorbed from dilute (1 mM) solutions. The C-H stretching band is proportionately

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0.80 0.55 0.95 0.97

0.41 0.52 0.51 0.54

r 0.21 0.27 0.27 0.32 0.30 0.31 0.22 0.37 0.40 0.40 0.40 0.41 0.25 0.32 0.35 0.39 0.43 0.45 0.23 0.39 0.40 0.42 0.43 0.42 0.17 0.16 0.18 0.23 0.34 0.43 0.49 0.52 0.29 0.32 0.36 0.45 0.36 0.41 0.36 0.54 0.27 0.43 0.41 0.44

rel

0.07 0.06 0.07 0.06 0.35 0.52 0.41 0.42 0.22 0.26 0.23 0.18

larger than that obtained for L e corresponding Pt(ll1) surface. The effect of electrochemical cycling of the Pt(po1y) surface is most noticeable in the packing densities and EELS spectra of HQ adsorbed from concentrated solutions, Figure 3D,although the changes are less dramatic than those at Pt(ll1). A small 0-H stretch is seen. There is an increase in frequency of the asymmetric CC stretch. In addition, a new peak, probably a ring bending mode,ld appears at 644 cm-' on the cycled surface; this is not apparent for the annealed surface. 2. 2,2',5,5'-Tetrahydroxybiphenyl (THBP). Electrochemical cycling has no noticeable effect on the packing density of the THBP adsorbed from 1.0 mM THBP solution at Pt(lll), although cycling of Pt(po1y) causes a slight increase in packing density of THBP, Table I. There is a 14% decrease in packing density of reversibly electroactive THBP, rei:

,g;; Ho,g -

t 2H* + 2 d

(2)

HO

Evidently, the additional surface area created by the cycling pretreatment is not efficiently occupied by the com-

824 Langmuir, Vol. 5, No. 3, 1989

Gui et al.

A " ~ ' ' " " ' " ' " l " " l " " " " ' " ' ' ' " ' " ~

I

'

"-

ANNEALED

. ........:. , .. .....'... .....: " ' ......... .,..

......_.. .......

.........

CYCLED

000

2000

3000

4 000

L L c

I000

D

ENERGY LOSS (crn-1)

2000

3000

4000

3000

4000

ENERGY LOSS ( c m - 1 )

0 w

0

1000 2000 ENERGY LOSS ( c m - 1 )

3000

4 000

0

IO00

2000

ENERGY LOSS (cm-1)

Figure 3. Vibrational spectra of HQ adsorbed at Pt electrode surfaces. (A) 1 mM HQ at Pt(ll1). (B)0.1 M HQ at Pt(ll1). (C) 1 mM HQ at Pt(po1y). (D) 0.1 M HQ at Pt(po1y). Solid curves: annealed surfaces. Dotted curves: cycled surfaces. Experimental conditions: adsorption was from 10 mM KF/HF (pH 4), followed by rinsing with 0.1 mM KF (pH 4); EELS incidence and detection angle, 6 2 O from surface normal; beam energy, 4 eV; beam current, about 200 PA; EELS resolution, 10 meV (80cm-I); IR resolution, 4 cm-'.

paratively bulky THBP adsorbed molecule. The fraction of electroactive adsorbed material decreases due to cycling. EELS spectra of THBP adsorbed at annealed and cycled Pt(ll1) surfaces contain the same vibrations at nearly the same frequencies but with noticeably different intensities, Figure 4. Cycled Pt(ll1) yields a smaller elastic peak (3 kHz) than the annealed surface (100 kHz), due to increased surface roughness. Cycling accentuates the peaks due to 0-H stretching (3400 and 3600 cm-') and C-H stretching (3076 cm-') relative to the other modes of the adsorbate, Table IV. The decrease in elastic peak intensity and the changes in the proportions of the various energy loss peaks as a result of electrochemical cycling of the surface suggest that cycling disorders the surface in such a way that the EELS spectrum takes on the characteristics of spectra recorded at off-specular angle^.^ EELS spectra of THBP adsorbed at annealed Pt(po1y) are similar to those obtained for cycled Pt(po1y) and for cycled Pt(lll), Figure 4. The C-H stretching amplitude is large in relation to the CC and C-0 features when compared with spectra for THBP at annealed Pt(ll1). Evidently, cycling the Pt(111)surface prior to adsorption of THBP disrupts the layer and alters the vibrational spectrum. However, cycling the Pt(po1y) surface appears

to have little additional influence on the vibrational spectra of the layer, Figure 4. 3. 2,5-Dihydroxy-4-methylbenzylMercaptan (DMBM). Electrochemical cycling of Pt(ll1) increases the packing density of DMBM by 26%, Table 11. Evidently, the thiol "anchor group" of DMBM makes efficient use of the additional surface area created by electrochemical cycling of Pt(ll1). A somewhat smaller increase (6%) occurs due to cycling of Pt(po1y) prior to adsorption of DMBM. Apparently, the area increase resulting from cycling of Pt(po1y) is relatively small. Cycling the Pt(ll1) surface increases the electroactive fraction by 49% (compared with 12%), indicating that the roughened Pt(ll1) surface leads to formation of multiple adsorbed states, some of which are not reversibly electroactive. The packing density of DMBM at annealed Pt(lll), 0.34 nmol/cm2, corresponds to a sulfur-bonded adsorbate having a pendant hydroquinone moiety'* (calculated 0.399 nmol/cm2):

(9) (e) Sexton, B.A. Appl. Phys. 1981, A26. (b) Kesmodel, L. L.J. Vac. Sci. Technol. A 1983, 1466.

EELS spectra of DMBM adsorbed at annealed and cycled Pt(ll1) are quite similar, Figure 5. The primary

"7

4- 4

&

-

"7

+2H'+Zd

(3)

i

826 Langmuir, Vol. 5, No. 3, 1989

Gui et al.

A

A

U

1000 2000 ENERGY LOSS ( c m - 1 )

0

B,,

, , ,

, , , , , , . I

3000

4000

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1000

2000

3000

4000

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2000 3000 ENERGY LOSS ( c m - I )

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.',

.4,010 0

Figure 6. Vibrational spectra of 3PHQ. (A) Pt(ll1). (B)Pt-

A l . :,: ENERGY LOSS ( c m - 1 )

Figure 7. Vibrational spectra of NA. (A) Pt(ll1). (B) Pt(p01y).

(poly). Solid curves: annealed surfaces. Dotted curves: cycled surfaces. Experimental conditions: adsorption from 0.5 mM 3PHQ in 10 mM K F (pH 4); other conditions as in Figure 3.

Solid curves: annealed surfaces. Dotted curves: cycled surfaces. Experimental conditions: adsorption from 1mM NA in 10 mM KF (pH 4);other conditions as in Figure 3.

sponds to a tilted vertical orientation with a 77O angle between the pyridine ring and the Pt(ll1) surface:lh

The EELS spectra of adsorbed 3PHQ at cycled Pt(ll1) are distinctly different (Figure 6A, dotted curve) from those for the annealed Pt(ll1) surface (Figure 6A, upper solid curve). Peak heights are considerablysmaller for the cycled surface, and peaks are less well resolved. The peak at 804 cm-l, attributable to C-H bending, shifts to 908 cm-l at the cycled surface, and ring bending at 621 cm-I grows at the expense of the bend at 500 cm-l. Thus, the EELS spectra support the findings from cyclic voltammetry that cycling leads to formation of multiple adsorbed states of 3PHQ. 6. Nicotinic Acid (NA). NA has the interesting property that coordination of its carboxylic acid moiety to the Pt(ll1) surface is a function of the electrode potential during adsorption.lh It is thus of interest to examine the extent to which cycling of the surface prior to adsorption affects the mode of bonding of NA to the surface. Cycling of the Pt(ll1) surface prior to adsorption leads to a 14% increase in packing density of NA, Table 11. Similarly, cycling of the Pt(po1y) surface leads to a 18% increase. The packing density of NA at the annealed Pt(ll1) surface, 0.36 nmol/cm2, corresponds to a tilted vertical orientation in which the average angle between the pyridine ring and the Pt(ll1) surface is 72O. Cycling the Pt(ll1) surface also leads to major changes in the vibrational spectra of the NA adsorbed layer, Figure 7A. The 0-H stretching peak (3581 cm-') height decreases

c!

0 -?LA

The electroactive component, rei, of 3PHQ at Pt(ll1) increases by 18%as a result of cycling. Evidently, cycling the Pt(ll1) surface leads to formation of a mixture of 3PHQ adsorbed states, some of which are not electroactive. Packing density of BPHQ at annealed Pt(po1y) is 24% larger than at annealed P t ( l l l ) , Table 11. Cycling the Pt(po1y) surface further increases the 3PHQ packing density by 25%. That is, the BPHQ packing density at cycled Pt(po1y) is 55% larger than at annealed Pt(ll1). However, the electroactive packing density of 3PHQ, rei, decreases by 22% as a result of cycling the Pt(po1y) surface. The electroactive packing density is essentially the same at the annealed Pt(ll1) and Pt(po1y) surfaces. Evidently, surface disorder leads to increased total amounts adsorbed while decreasing the fraction of reversible electroactivity.

Langmuir, Vol. 5, No. 3, 1989 827

Aromatics at P t ( l l 1 ) and Pt(po1y)

Table IV. Assignments of EELS Bands for Adsorbed Layers compd HQ (1mM)

E 0.0

pH

Pt(ll1) annealed 3008 1591 1452 1157 882 623 494 339

HQ (0.1 M)

-0.1

THBP (1mM)

0.0

DMBM (0.7 mM)

0.0

BM (1mM)

0.0

3PHQ (0.5 mM)

NA (1 mM)

3TCA (1 mM)

-0.1

0.0

0.0

3000 1600 1450 1223 894 605 477 3600 3400 3076 1645 1443 1178 840 611 456 3550 3005 1600 1415 1204 863 660 530 292

3627 3069 1611 1463 1213 804 676 500 304 3581 3101 1752 1568 1375 1182 807 651 475 3566 3037 1735 1550 1336 1150 610 274

cycled 3040 1639 1355 1244 1029 890 644 418 3350 3043 1655 1423 1119 934 637 443 3600 3400 3054 1607 1438 1207 868 609 431 3559 3001 1611 1427 1206 873 615 476 274

Pt(P01Y) annealed cycled 3041 3022 1645 1588 1406 1422 1210 1245 1100 1062 830 886 461 340 3031 1601 1418 1261 866 423

410 3368 3034 1632 1411 1236 877 644 441 3619

3071 1645 1440 1254 836

3063 1591 1372 1224 876

427

461

3056 1496 1169 992 757 439 3585

3062 1470 1169

3082 1619 1461 1237 811 402

3074 1664 1497 1183 869 647 444

3579 3075 1740 1562 1340 1158

3586 3080 1765 1550 1353 1159

623

656

3568

3569

3016 1723

3029 1712

1386 1147 933

1348 1182

1364 1172 898

583

761 577

3592 3450 3078 1568 1466 1160 908 621 463 306 3584 3090 1745 1588 1375 1154 832 638 452 3556 3313 3010 1703

2-fold in relation to the C-H stretching peak (3101 cm-'), indicating increased coordination between the carboxylic acid moiety and the Pt surface due to cycling. Cycling also

713 411 3556

614

description C-H stretch asym CC stretch s y m CC stretch C-0 stretch 0-H bend C-H bend ring bend ring bend ring bend, Pt-C stretch 0-H stretch C-H stretch asym CC stretch s y m CC stretch C-0 stretch C-H bend ring bend ring bend 0-H stretch 0-H stretch C-H stretch CC stretch CC stretch C-O stretch C-H bend ring bend ring bend 0-H stretch C-H stretch CC stretch CC stretch C-0 stretch C-H bend C-S stretch ring bend Pt-S stretch C-H stretch CC stretch C-H bend (ring) C-H bend (CH,) ring bend ring bend 0-H stretch 0-H stretch C-H stretch CC stretch CC, CN stretch C-H bend ring bend, C-H bend ring bend ring bend Pt-N stretch 0-H stretch C-H stretch C=O stretch CC stretch CC, CN stretch C-0 stretch C-H, 0-H bend OCO, C-H bend CC, OCO bend Pt-N stretch, ring bend 0-H stretch 0-H stretch C-H stretch C=O stretch CC stretch CC stretch C-0 stretch, C-H bend 0-H bend ring bend ring bend Pt-S stretch

results in 10-fold decreases in heights of peaks due to C=O stretching (1742 cm-'), CC symmetric stretching (1375 cm-'), C-0 stretching/C-H bending (1182 cm-'), and the

828 Langmuir, Vol. 5, No. 3, 1989

Gui et al. significantly affect the EELS spectra. Evidently, cycling the Pt(ll1) surface leads to scrambling of NA adsorption states, including a mixture of pendant and coordinated carboxylate states, similar to that occurring at the Pt(po1y) surface, Figure 7B: 0

7. (3-Thiophene)carboxylic Acid (3TCA). STCA resembles NA in having a carboxylic acid substituent that undergoes potential-dependent coordination to the Pt surface.'j Cycling the Pt(ll1) surface prior to adsorption increased the packing density of 3TCA by 59%, Table 11. The packing density of 3TCA on Pt(po1y) increases only slightly (7%) upon cycling, however. EELS spectra of 3TCA adsorbed at annealed P t ( l l l ) , Figure 8A (upper solid curve), can be assigned by analogy with the IR spectra of the pure solid,'" Table IV. Cycling the Pt(ll1) surface decreases the peak height of the carboxylic acid 0-H stretch (3566 cm-'). The C=O peak (1735 cm-') also decreases. All of the peaks due to the thiophene moiety are decreases in height at the cycled Pt(ll1) surface (610, 1008, and 1150 cm-l). Evidently, cycling alters or mixes the adsorption states of 3TCA, including increased interaction of the ring and the carboxylic acid moiety with the Pt surface.

13kHz

u

W

t-

U

w

+ Z

3 0

"

~

. '

.:'

I . , , , , . . , . , , , . . , . . . I I . , . . I . . . . I . .

1

1000 2000 ENERGY LOSS ( c r n - I )

3000

, ,

4

0

Acknowledgment. We are grateful to the Gas Research Institute for support of this research. Instrumentation was provided by the Air Force Office of Scientific Research.

Figure 8. Vibrational spectra of m A . (A) Pt(ll1). (B) Pt(p0ly). Solid curves: annealed surfaces. Dotted curves: cycled surfaces. Experimental conditions: adsorption from 1mM STCA in 10 mM KF (pH 4); other conditions as in Figure 3.

€&&try NO.HQ, 123-31-9;THBP, 4371-32-8; STCA, 8813-1; 3PHQ, 79445-47-9; NA, 59-67-6; BM, 100-53-8; DMBM, 8175311-9; Pt, 7440-06-4.

several ring deformation modes (475,651, and 807 cm-'). There is a high degree of similarity between EELS spectra of NA adsorbed at cycled Pt(ll1) and at Pt(po1y). Cycling the Pt(po1y) surface prior to adsorption of NA does not

(10) (a) Angelelli, J. M.; Katritzky, A. B.; Pinzelli, R. F.; Topson, R. D. Tetrahedron 1972,B(7) 2037. (b) Peron, J. J.; Saumagne, P.; Lebas, J. M.Spectrochim. Acta A 1970,26,2037. (c) Akiyama, M.; Katiuti, Y. J. Mol. Spectrosc. 1970, 34(2), 288.