Studies of thiophene and substituted thiophenes at platinum(111

Nikola Batina, John Y. Gui, Bruce E. Kahn, Chiu Hsun Lin, Frank Lu, James W. McCargar, Ghaleb N. Salaita, Donald A. Stern, Arthur T. Hubbard, and. Lan...
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Langmuir 1989,5, 588-600

on the Cu(ll0) and the Cu(ll1) surfaces has been observed at T = 80 K. The order-disorder phase transitions of these overlayers are continuous, and the transition temperatures are dependent on the alkali-metal coverage. In addition, three more ordered structures of Cs on Cu(ll1) have been observed. The Cu(ll0) surface reconstruction induced by alkalimetal overlayers occurs a t T > 150 K. The surface re-

construction is a well-ordered (1x2) structure at temperature about 320 K and coverage 6 ii: 0.2. Phase diagrams detail the relation of the surface phases for K and Cs on the Cu(ll0) and Cu(ll1) surfaces.

Acknowledgment. Partial support for this work has been supplied by the R.A. Welch Foundation and NASA. Registry No. K, 7440-09-7; Cs, 7440-46-2; Cu, 1440-50-8.

Studies of Thiophene and Substituted Thiophenes at Platinum(11 1) Electrodes by Vibrational Spectroscopy and Auger Spectroscopy: Monomers, Dimers, and Polymerst Nikola Batina, John Y. Gui, Bruce E. Kahn, Chiu-Hsun Lin, Frank Lu, James W. McCargar, Ghaleb N. Salaita, Donald A. Stern, Arthur T. Hubbard,* Harry B. Mark, Jr., and Hans Zimmer Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 -01 72 Received October 27, 1988. I n Final Form: January 23, 1989 The adsorption behavior of various thiophenes from organic as well as aqueous solutions at well-defined Pt(111)surfaces is examined in this study. The adsorbates studied include 3-thiophenecarboxylic acid (3TCA), 2-thiophenecarboxylic acid (XTCA),3-thiopheneaceticacid (STAA), 2-thiopheneaceticacid (ZTAA), thiophene (TPE), 3-methylthiophene (3MT), 3,3’-dimethyL2,2’-bithiophene(33’DMBT), and 4,4’-dimethyL2,2’-bithiophene (44’DMBT). Packing densities (moles adsorbed per unit area) were measured for each compound 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. The Pt(ll1) surfaces used in this study were characterized by LEED. All the thiophenes studied are adsorbed with the ring plane nearly perpendicular to the platinum surface. Vibrational spectra of thiophenecarboxylic acids, and the dependence of adsorption on electrode potential, give valuable information about the adsorbate structure. The compounds 3TCA and 2TAA adsorb with pendant carboxylic acid moieties and give vibrational spectra that are noticeably dependent on the electrode potential. Adsorption at relatively positive potentials (+0.4 V vs Ag/AgCl) shows increased interaction of the carboxylic acid moieties with the metal surface compared with relatively negative potentials (-0.1 V vs Ag/AgCl), as evidenced by diminution of the intensities of bands due to 0-H and C=O stretching as well as shifts in the frequency and intensity of aromatic CC modes. Further evidence for the pendant nature of the carboxylic acid moieties in 3TCA and 2TAA is their reactivity with KOH. Auger spectroscopy shows that the pendant carboxylic acid functionality in 3TCA and 2TAA reacts with KOH to a larger extent than does STCA or 3TAA. The geometry of STCA and 3TAA allows the carboxylic acid to interact with the metal surface, even at relatively negative (-0.1 V vs Ag/AgCl) electrode potentials. This is borne out by low-intensity 0-H and C=O stretching vibrations, as well as relatively little reactivity with KOH (compared with STCA and 2TAA). The C/S elemental ratios based on Auger spectra are smaller than those expected on the basis of molecular formulas for all the thiophene derivatives studied. This result is due to desulfurization of the thiophenes at the Pt surface, occurring while the Pt is immersed in the thiophene solution, and is more pronounced on surfaces immersed at relatively positive potentials. The amount of desulfurization increases continuously with thiophene concentration in acetonitrile but reaches a maximum at about lo4 M in water. This study includes the first reported surface vibrational spectra of poly(3-methylthiophene) (P-3MT), as well as the first electropolymerizedpolymer f i i s on well-characterizedsingle-crystalmetal surfaces. P - 3 W vibrational spectra are compared with the EELS spectra of the corresponding monomer and dimers (33’DMBT and 44’DMBT), and spectral evidence is presented for oxidative C-C bond formation in the electropolymerization of 3MT.

Introduction The adsorption of aromatic compounds from solution onto well-defined Pt surfaces produces oriented adsorbed layers.’ Adsorbed layers can also be formed by vaporphase adsorption, if the vapor pressure of the adsorbate is sufficient. Adsorption from solution provides access to a much larger variety of adsorbates, in addition to giving valuable information about the liquid/solid interface and effects due to electrode potential, pH, nature of the solvent, +Presenteda t the symposium on “Adsorption on Solid Surfaces”, 62nd Colloid and Surface Science Symposium, Pennsylvania State University, State College, PA, June 19-22, 1988; W. A. Steele, Chairman.

0743-7463/89/2405-0588$01.50/0

and solubility. The molecular orientation of the adsorbate has profound fundamental as well as practical implications, (1) (a) 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. (b) 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. (c) 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. (d) 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. (e) 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. (0Batina, 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, in press.

0 1989 American Chemical Society

Studies of Thiophene and Substituted Thiophenes

Langmuir, Vol. 5, No. 3, 1989 589

parent thiophene, are investigated. The carboxylic acid as has been shown by the effect on electrochemical oxiderivatives are of interest for many reasons. A carboxylic dation/reduction products, surface chemical reactions, and acid functionality on an aromatic ring can be used as a very heterogeneous catalysis. sensitive adsorbate structural probe.Ie The amount of In the present work, adsorbed layers formed by adinteraction between the carboxylic acid and the metal sorption from aqueous, acetonitrile, and hexane solutions surface is a function of adsorbate geometry and adsorption onto well-defined Pt(111)electrode surfaces are characpotential and is easily determined from EELS spectra and terized by electron spectroscopic techniques in ultrahigh from Auger spectra of KOH-rinsed adsorbed layers. vacuum (UHV) and by cyclic voltammetry at atmospheric Furthermore, the presence of a carboxylic acid functionpressure in argon. ality enhances water solubility and permits the comparison The interaction of sulfur-containing molecules with of adsorption isotherms and reactivity from organic and metal surfaces has commanded intense research interest. aqueous solutions, which is reported for the first time in Since sulfur compounds frequently act as catalyst poisons, this work. Thiophenecarboxylic acid derivatives are also their removal is of great importance. This removal is of interest for the preparation of thiophene polymers that typically done by catalytic hydrodesulfurization.2 Decontain easily functionalized substituents. The influence veloping an understanding of the adsorption of organoof polar substituents as well as electrode adsorption besulfur compounds is essential for the optimization of havior on electrooxidized polymer film morphology is also catalytic C-S bond cleavage processes. Heteroaromatic of interest. The polymerization process of 3MT is also thiophene compounds are relatively difficult to hydrodeexamined by comparing EELS spectra of monomeric 3MT, sulfurize and are the principal sulfur-containing constitdimeric 33'DMBT and 44'DMBT, and P-3MT. uents in petroleum feedstocks. Thiophene has been investigated extensively as a model for heteroaromatic deExperimental Section sulfurization over metallic and metal-containing surfaces. Reported here are experiments in which an electrode surface Single-crystal metal surfaces have been shown to possess catalytic activity for hydrodesulfurization p r ~ c e s s e s . ~ ~ - g ' ~containing an adsorbed layer is investigated by using specially constructed instrumentati~n:~ surface structure is examined with The adsorption of thiophene vapor on Pt(100)," Pt(111); low-energy electron diffraction (LEED),surface elemental comPt(210),4C Mo(~OO),~ M 0 ( l l 0 ) , ~Ni(100),5 ~ Ni(111),68 Cuposition and molecular packing density are determined by using Cu(111),68 W(2ll),' and Si(111)2XlS has been Auger spectroscopy, adsorbed layer vibrational bands are observed studied. by electron energy loss spectroscopy (EELS),and electrochemical The orientation of thiophene on metal surfaces has been reactivity of the surface is explored by using voltammetry and investigated and shown to influence the stereospecificity coulometry. The Pt(ll1)surfaces used in this work are orientedl0 and polished" such that all six faces are crystallographically of surface r e a ~ t i o n s . ~ ~The - ~ * 'adsorbate orientation has equivalent. All faces are cleaned simultaneously by bombardment been shown to vary with t e m p e r a t ~ r e ~as~well v ~ as coverwith Ar+ ions at 700 eV and are annealed at about 1000 K in A variety of adsorbate geometries have been age,3e~f~6b ultrahigh vacuum. Cleaning and annealing of the Pt surface is proposed, including: continued until Auger spectroscopy and LEED show that the surface is free from detectable impurities and disorder. The Pt surfaces are isolated in an argon-fiied antechamber for immersion into buffered aqueous electrolytes or acetonitrile solutions, which contain the subject adsorbates. Electrode potentials are measured and controlled by using i I three-electrode electrochemical circuitry based on operational amplifiers. The electrochemical cell is made 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 containingthe reference electrode (Ag/AgCl prepared with 1mM KCl) and Pt auxiliary electrode is introduced into the antechamber by using a bellows assembly and gate valve; there are no sliding seals or other sources of contamination in the (2) (a) Mitchell, P. C. H. In Catalysis;Kemball, C., Ed.; Specialist apparatus. All potentials are referred to Ag/AgCl (1 M KCl). Periodical Report; The Chemical Society: London, 1977; Vol. 1, p 204. Aqueous solutions used for adsorption and voltammetric or (b) Mitchell, P. C. H. In Catalysis;Kemball, C., Ed.; Specialist Periodical Report; The Chemical Society: London, 1981; Vol. 4, p 175. (c) Grange, coulometric measurements contain 10 mM KF pH-adjusted with P. Cat.Rev.-Sci. Eng. 1980,21,135. (d) Gates, B. C.; Katzer, J. R.; Schuit, HF as indicated to provide adequate conductivity and buffer G. C. A. In Chemistry of Catalytic Processes; McGraw-Hill: New York, capacity. Water used in the experiments is pyrolytically distilled 1979. in pure O2 through a Pt gauze catalyst at 800 "C and distilled (3) (a) Salmeron, M.; Somorjai, G. A. Surf. Sci. 1983, 126, 410. (b) again. Gellman, A. J.; Farias, M. H.; Salmeron, M.; Somorjai, G. A. Surf. Sci. 1984,136,217. (c) Gellman, A. J.; Farias, M. H.; Somorjai, G. A. J.Catal. Acetonitrile solutions used for adsorption measurements con1984,88,546. (d) Kelly, D. G.; Salmeron, M.; Somorjai, G. A. Surf. Sci. tained 10 mM NaC104(GFSChemical Co., Columbus, OH 43223) 1986,175,465. (e) Zaera, F.; Kollin, E. B.; Gland, J. L. Surf. Sci. 1987, as a supporting electrolyte. The acetonitrile (high-puritygrade, 184, 75. (f) Roberta, J. T.; Friend, C. M. Surf. Sci. 1987, 186, 201. (9) American Burdick and Jackson, Muskegon, MI 49442) was used Gellman, A. J.; Neiman, D.; Somorjai, G. A. J. Catal. 1987,107,92. (h) as received. n-Hexane (99+%)was used as received from Aldrich Gellman, A. J.; Bussell, M. E.; Somorjai, G. A. J. Catal. 1987,107, 103. Chemical Co., Inc., Milwaukee, WI 53201. (i) Fulmer, J. P.; Zaera, F.; Tysoe, W. T. J.Phys. Chem. 1988,92,4147.

c

(4) (a) Sette, J.; Gland, J. L.; Kollin, E. B.; Koestner, R. J.; Johnson, A. J.; Muetterties, E. L.; Sette, F. Phys.Rev. Lett. 1984,53(22), 2161. (b) Hitchcock, A. P.; Horsley, J. A,; Setta, J. J. Chem. Phys. 1986,85(9), 4835. (c) Lang, J. F.; Masel, R. I. Surf. Sci. 1987, 183, 44. (5) (a) Schoofs, G. R.; Preston, R. E.; Benziger, J. B. Langmuir 1985, 1,313. (b) Sette, J.; Kollin, E. B.; Fischer, D. A.; Hastings, J. B.; Zaera, F.; Sette, F. Phys. Reo. Lett. 1985,55(14), 1468. (c) Zaera, F.; Kollin, E. B.; Gland, J. L. Langmuir 1987,3, 555. (6) (a) Richardson, N. V.; Campuzano, J. C. Vacuum 1981,31(10-12), 449. (b) Sexton, B. A. Surf. Sci. 1985,163,99. (7) Preston, R. E.; Benziger, J. B. J. Phys. Chem. 1985, 89, 5010. (8) Piancastelli, M. N.; Kelly, M. K.; Margaritondo, G.; Frankel, D. J.; Lapeyre, G. J. Phys. Reu. B 1986, 34(6), 3988.

(9) (a) Hubbard, A. T. J. Vac.Sci. Technol. 1980,17,49. (b) 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. Electroanal.Chem. 1984,16%,43. (c) Hubbard, A. T. In Comprehensiue Chemical Kinetics;Bamford, C. H., Tipper, D. F. H., Compton, R. G., Eds.; Elsevier: Amsterdam, 1988; Vol. 28, Chapter 1. (d) Hubbard, A. T. Chem. Reu. 1988,88,633. (10) Wood, E. A. Crystal Orientation Manual; Columbia University Press: New York, 1963. (11) Samuels, L. E. Metallographic Polishing by Mechanical Methods; Pittman: London, 1967.

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

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3TCA, OTCA, BTAA, BTAA, TPE, and 3MT were used as received from Aldrich Chemical Co. 33'DMBT and 44'DMBT were prepared according to published procedures.12

TPE

3TCA

2TCA

H2COoH

3TAA

2TAA

3MT

,C"

CH 3

CH3

4 4' DMBT

33'DMBT

P-3MT was prepared by immersing annealed clean Pt(ll1) or 3MT monomer covered Pt(ll1)into 10 mM 3MT at 0.035 V, prior to stepping the electrode potential to 1.635 V for 50 s and then back to 0.035 V. The resulting polymer film was rinsed 4 times with water. Electron energy loss spectra (EELS) were obtained with 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 at a resolution of about 10 meV (80 cm-') in these experiments. Vapor-phase infrared spectra (2TCA,TPE)'" and FTTR spectra (solid 3TAA and STAA, neat 3MT)13bwere obtained from published compilations. The FTIR spectrum of P-3MT was obtained from ref 13c. The infrared spectrum of 3TCA in Nujol on ZnS was obtained by using a Perkin-Elmer Model 1420 spectrometer. Auger electron spectra were collected by using a cylindrical mirror analyzer equipped with an integral electron gun (Model 981-2707, Varian Associates, Inc., Palo Alto, CA 94303; or model model 15-255G, Perkin Elmer, Eden Prairie, MN 55344). A lock-in amplifier was used to acquire the first derivative of the spectrum; modulation amplitude was 5 V peak-to-peak at 1000 Hz. The equipment was interfaced to a computer so that the data could be collected and stored on disk for later manipulation. The incident beam current was only 0.1 pA at 2000 eV to minimize the effect of beam damage and was controlled to within 1% to limit scatter in the resulting data. Standard deviations of the Auger measurements were about *5%. Packing densities rx (moles of adsorbed atoms/centimeters squared) or r (moles of adsorbed molecules/centimeten squared) were obtained as described previously.1f Auger signals, ZX,due to each element X were measured and normalized by the Auger signal at 235 eV due to the clean Pt surface ZRo (Figure 2). Packing density was obtained from (Z,/ZRo) with equations of the following form: B ~ C L J ~ i-1

(12) Cunningham, D. D.; Laguren-Davidson, L.; Mark, H. B., Jr.; Pham, C. V.; Zimmer, H. J. Chem. Soc., Chem. Commun. 1987, 1021. (13) (a) Sadtler Standard Spectra, IR Vapor Phose; Sadtler Research Laboratories: Philadelphia, PA, 1987. (b) Pouchert, C. J. The Aldrich Library of FTIR Spectra; Aldrich Chemical Co., Inc.: Milwaukee, WI, 1985. IC) Hotta. S.: Shimatsuma. W.: Taketani.. M.:. Kohiki., S. Swath. Met. 1985, 11, 139.' I

,

where Bx was calibrated using hydroquinone? L-DOPA,'~or adsorbed ~u1fur.l~Bs = 4.53 cm2 nmol, Bc = 0.35 cm2/nmol, Bo = 0.69 cm2/nmol using Ipt,253 normalization, BN = 0.747 cm2/nmol using ZR16: normalization,Li is the fraction of element X located in level i (i = 1is adjacent to the Pt surface and N is the outermost layer), and 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 on the observed attenuation of Pt Auger electrons (235 eV) by a (3x3) 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. The calculation of sulfur and potassium packing densities digitally subtracts the contribution from platinum, as described in ref l b and le.

6

Results and Discussion 1. Adsorption of Monomers from Solution. Monomer adsorption before polymerization, which might in principle affect the nature of adhesion of a polymer film, was studied. The adsorption behavior of aqueous solutions is simpler than the behavior of acetonitrile solutions, because water adsorbs less strongly than acetonitrile%lk and, therefore, interferes to a lesser extent with the adsorption of thiophenes. To identify the adsorbed species, measurements were made of the elemental Auger signals, leading to the elemental composition of the adsorbed layers, and of the EELS spectra of the adsorbed layers to obtain clues about adsorbate molecular structure and mode of surface attachment. Cyclic voltammetry was also carried out for the adsorbed layers in aqueous fluoride electrolytes. a. 3-Thiophenecarboxylic Acid (3TCA). i. Adsorption from Aqueous Solutions. EELS spectra of 3-thiophenecarboxylic acid (3TCA) adsorbed from dilute aqueous solutions are shown in Figure 1A. By analogy with ref le, the prominent carboxylic acid 0-H stretching mode at 3570 cm-' in Figure 1A is evidence that the 3TCA molecule is present a t the surface in a tilted or vertical orientation with a pendant carboxylic acid moiety. Assignments of other EELS peaks by analogy with the IR spectra are summarized in Table I. Previous examples of vertically oriented adsorbed molecules that display prominent 0-H stretching bands due to pendant carboxylic acid moieties when adsorbed from acid solutions a t relatively negative electrode potentials are 3-pyridinecarboxylic acid (niacin); 4-pyridinecarboxylic acid, and the various pyridinedicarboxylic acids having carboxylate groups at the 3- and/or 4-positions.le Conversely, several horizontally oriented aromatic acids (phenylalanine, 3,4dihydroxyphenylacetic acid, and benzoic acid) have been studied, in which the close proximity of the carboxylate moiety to the surface leads to coordination with the surface.ldre Other evidence that 3TCA adsorbs in a vertical (or tilted) orientation with a pendant carboxylate moiety is the observed dependence of the 0-H stretching vibration of the electrode potential during adsorption. Figure 1A (solid curve) shows the EELS spectrum recorded following adsorption of 3TCA at a relatively negative electrode potential (-0.1 V vs Ag/AgCl), while the dotted curve shows the spectrum after adsorption a t a relatively positive electrode potential (+0.4 V vs Ag/AgCl). The principle influences of the positive potential are a twofold decrease in the intensity of the acid 0-H stretch (3570 cm-'1, a substantial decrease in the intensity of the C = O stretching band (1726 cm-l), and noticeable shifts in frequency and (14) Batina, N.; McCargar, J. W.; Salaita, G. N.; Lu, F.; LagurenDavidson, L.; Lin, C.-H.; Hubbard, A. T. Langmuir 1989,5, 123. (15) (a) Garwood, G. A., Jr.; Hubbard, A. T. Surf. Sci. 1982,118,223. (b) Sexton, B. A.; Avery, N. R. Surf.Sci. 1983,129,21. (c) Sexton, B. A,; Hughes, A. E. Surf.Sci. 1984, 140, 227. (d) Song, D.; Soriaga, M. P.; Hubbard, A. T. J. Electrochem. SOC.1987, 134(4), 874.

Langmuir, Vol. 5, No. 3, 1989 591

Studies of Thiophene and Substituted Thiophenes , " " I , " '

A

D

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.

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2000 3000 ENERGY LOSS ( c m - I )

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4000

W \ n

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U

Z

B

B

73

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2000

3000

Figure 2. Measurement of Auger signals. (A) Spectrum of clean Pt(ll1). (B) Spectrum of 3TCA adsorbed on Pt(ll1) (1 mM aqueous solution, -0.1 V, adsorbed and rinsed at pH 3). (C) Spectrum of BTCA and KOH adsorbed on R(111)(1mM aqueous solution, -0.1V, adsorbed at pH 3, rinsed at pH 10). (D)Spectrum of P-3MT on clean annealed Pt(ll1). Experimental conditions: incident beam, 100 nA at 2000 eV, normal to the surface; modulation amplitude, 5 V peak to peak at 1000 Hz; electrolyte, 10 mM KF (pH 3).

r " " ~ " P ' ~ " ' ~ ' " ' " " ' ~ ' ' " " " ~ " ' ' ~ " ' ' 1

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.

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moieties in the meta positions of adsorbed pyridine carboxylic acids (3-pyridinecarboxylic acid, 3,5-pyridinedicarboxylic acid, and 3,4-pyridinedicarboxylicacid).'* Interconversion of these adsorbed species at the surface has not been investigated.

.COOH

n

4000

ENERGY LOSS ( c m - I )

Figure 1. Vibrational spectra of 3TCA. (A) Upper solid curve: EELS spectrum of BTCA at Pt(ll1) adsorbed from 1mM 3TCA in 10 mM KF (adjusted to pH 3 with HF) at -0.1V (w Ag/AgCl), followed by rinsing with HF (pH 3). Dotted curve: EELS spectrum of BTCA at Pt(ll1) adsorbed from 1mM 3TCA in 10 mM KF (pH 3) at +0.4 V, followed by rinsing with HF (pH 3). Lower solid curve: IR spectrum of BTCA (Nujol mull). (B) Solid curve: EELS spectrum of BTCA at Pt(ll1) adsorbed from 1mM BTCA in 10 mM KF (pH 3) at -0.1 V, followed by rinsing with KOH (pH 10). Dotted curve: EELS spectrum of 3TCA at Pt(ll1) adsorbed from 1 mM 3TCA in 10 mM KF (pH 3) at -0.1 V, followed by evacuation (lob Torr, 21 min) and rinsing with KOH (pH 10). Lower solid curve: IR spectrum of [Na][3TCA](KBr). (C) Upper solid curve: EELS spectrum of 3TCA at Pt(ll1) adsorbed from 1mM 3TCA in CH&N containing 10 mM NaC104 at -0.1 V, followed by rinsing with aqueous HF (pH 3). Dotted curve: EELS spectrum of BTCA at Pt(ll1) adsorbed from 100 mM BTCA in CH&N containing 10 mM NaClO, at -0.1 V, followed by rinsing with aqueous HF (pH 3). Lower solid curve: IR spectrum of 3TCA (Nujol mull). Experimental conditions: EELS incidence and detection angle, 62" from surface normal; beam energy, 4 eV; beam current, about 120 PA; EELS resolution, 10 meV (80 ern-') fwhm; IR resolution, 4 cm-l. intensity of the aromatic CC stretching bands (1161 and 1356 cm-9. That is, the potential dependence of adsorbed 3TCA closely resembles that of the pendant carboxylic acid

I

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+

0.4

V

i E

--

0.1 V

Further indications of the presence of a pendant carboxylic acid moiety in adsorbed 3TCA are the changes in Auger and EELS spectra when the 3TCA layer is rinsed with dilute aqueous KOH (pH 10) at relatively negative electrode potentials. The 3TCA layer retains K+ ions very efficiently under those conditions, as evidenced by a prominent potassium Auger signal, Figure 2C and Table 11. The acidic proton is removed by rinsing with KOH, shown by the dramatic reduction of the 0-H stretching vibration (3570 cm-', Figure 1A) and the emergence of a C-0 stretching vibration at 1595 cm-' characteristic of the carboxylate anion, Figure 1B. Retention of K+ ions from KOH solutions by adsorbed pyridinecarboxylic acids has been reported;le conversely, the layer of horizontally oriented benzoic acid does not retain K+ ions to a significant extent.le Elemental packing densities and the Auger data from which they are obtained are given in Table 11. The equations used for the calculations are given in Table 111.

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

Batina et al.

Table I. Assignments (cm-’) of EELS Bands for Adsorbed Layers H,O HzO + KOH CHqCN comnd c3TCA

-0.1 v 3570 3040 1726 1466 1356 1161

-

2TCA

3TAA

819 579 3598 3030 1755 1388 1265 1144 873 600 3593 3032 1730

-

2TAA

1347 1135 885 585 3608 3039 1754 1640 1390 1287 1162 905 606

-

compd

TPE

-0.1

v

3585 2951 1403 1228 1042

+0.4 V 3562 3064 1708 1458 1165 806 548 3596 3083 1645 1505 1222 810

before vacuum 3583 2971

-

1595

-

1400 1128

-

782 542

P-3MT

33’DMBT

44’DMBT

1 mM

3546 2979 2052 1692 1373 1156

-

703 543

100 mM 3565 2993 2026 1703 1368 1161 707 557

-

3584 3034 1738 1608 1386 1165 951 676 518 470 3579 3054 1731 1619 1384

-

1168 840 634 474

descriDtion

0-H stretch C-H stretch (CN stretch) C=O stretch CC stretch CC stretch C-H bend C-0 stretch C-0 bend ring bend ring bend 0-H stretch C-H stretch C=O stretch CC stretch C-H bend C-0 stretch C-0 stretch ring bend ring bend 0-H stretch C-H stretch C=O stretch CC stretch C-H bend C-0 stretch ring bend ring bend ring bend Pt-S stretch 0-H stretch C-H stretch C=O stretch CC stretch C-0 stretch C-0 stretch C-0 stretch ring bend ring bend Pt-S stretch

H20 +0.4 V

+1.5 V

CH3CN-0.1 V

hexane open circuit

2987 1404 1201 998

3548 3364 2972 1686 1384 1195 1019

563

613

-

-

description

(0-H stretch) C-H stretch CC stretch CC stretch C-H bend C-H bend (0-H stretch) (0-H stretch) C-H stretch (C=O stretch) CC stretch CH, bend C-H bend C-H bend

3597 2985 1645 1411 1213 988

3MT 2991 1405 1164 lo08 703

after vacuum 3584 2996 1682 1572 1378 1146 981 823 553

3475 2996 1440 1119 977 748 538 3010 1439 1199 1014 884 707 557

ring bend

C-H stretch CC stretch

CH3bend C-H bend C-H bend C-H bend 2987 1428 1311 1214 1084 989 2979 1436 1126

ring bend C-H stretch CC stretch CH3bend CC stretch C-H stretch C-H bend C-H stretch CC stretch CH3 bend

Langmuir, Vol. 5, No. 3, 1989 593

Studies of Thiophene and Substituted Thiophenes

Table 11. Auger Data for Molecules Adsorbed at Pt Electrodes compd

-log C

3TCA 3TCA BTCA 3TCA 3TCA 3TCA BTCA BTCA BTCA 3TCA BTCA 3TCA 3TCA 3TCA BTCA BTCA STCA 3TCA BTCA BTCA BTCA 3TAA 3TAA 3TAA 2TAA BTAA 2TAA TPE TPE 3MT 3MT P-3MT 33'DMBT

60 5.5

44'DMBT

2.7

solvent

~~

4.5 3.5 3.0 2.0 3.0 3.0 3.0 6.0 5.0 4.5 4.0 3.5 3.0 2.0 1.0 0.35 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 1.7 2.7

H20

electrode potential rinse pH Zs/ZRo ZK/Zp? 0.641 -0.1 3 0.827 -0.1 3 3 1.560 -0.1 -0.1 3 1.668 -0.1 3 1.704 1.818 -0.1 3 1.412 +0.4 3 1.377 1.504 -0.1 10 -0.1 10' 1.521 1.549 -0.1 0.159 3 -0.1 3 0.399 -0.1 3 0.312 0.962 -0.1 3 1.411 -0.1 3 -0.1 1.232 3 1.708 -0.1 3 1.628 -0.1 3 -0.1 3 2.039 -0.1 1.290 3 +0.4 3 1.385 1.324 1.152 -0.1 10 -0.1 3 1.167 +0.4 1.286 3 -0.1 10 1.408 0.851 -0.1 3 1.180 +0.4 3 1.307 -0.1 10 1.161 1.230 -0.1 3 2.206 +0.4 1.901 3 -0.1 3 1.368 +0.4 3 1.764 +0.4 3 3.742 open 3 0.958 circuit 3 0.976 open circuit

Rinsed after evacuation-see

Zc/ZptO

ZN/Zpto Zo/ZptO

0.368 0.466 0.471 0.507 0.542 0.591 0.466 0.486 0.565 0.262 0.355 0.501 0.481 0.522 0.470 0.558 0.473 0.581 0.510 0.419 0.503 0.602 0.573 0.651 0.580 0.517 0.580 0.522 0.538 0.636 0.695 2.290 1.052

0.171 0.237 0.148 0.112 0.153 0.200 0.319 0.333 0.289 0.320 0.350 0.569 0.327 0.338 0.293 0.340 0.247 0.281 0.204 0.339 0.279 0.244 0.324 0.288 0.250 0.350 0.262

0.155 0.132 0.092 0.091 0.095 0.090 0.065 0.027 0.000

0.773

rs

rc rc/rs rN

ro

r

0.292 0.404 0.252 0.191 0.260 0.341 0.462 0.483 0.418 0.464 0.507 0.825 0.473 0.490 0.424 0.492 0.358 0.407 0.296 0.491 0.475 0.416 0.552 0.491 0.362 0.507 0.380

0.266 0.337 0.341 0.367 0.392 0.428 0.314 0.352 0.408 0.152 0.224 0.340 0.286 0.313 0.279 0.360 0.323 0.420 0.342 0.282 0.350 0.328 0.312 0.375 0.325 0.290 0.334 0.403 0.415 0.400 0.436

1.33 1.69 1.70 1.83 1.96 2.14 1.57 1.76 2.04 0.76 1.12 1.70 1.43 1.57 1.40 1.80 1.62 2.10 1.71 1.41 1.75 1.97 1.87 2.25 1.95 1.74 2.01 1.61 1.66 2.00 2.18

4.61 4.53 2.42 2.44 2.55 2.61 2.47 2.84 2.98

0.432 3.30

7.65

0.330

0.440 2.43

5.52

0.243

0.289 0.372 0.703 0.752 0.768 0.819 0.636 0.620 0.685 0.072 0.180 0.141 0.433 0.636 0.555 0.769 0.733 0.919 0.581 0.624 0.596 0.526 0.579 0.634 0.532 0.589 0.523 0.994 0.856 0.616 0.795

3.31 2.47 2.52 2.34 2.21 2.29 3.11 2.39 2.94 3.94 3.40 3.56 3.82 3.11 3.84 1.62 1.94 3.25 2.74

0.207 0.177 0.123 0.122 0.127 0.120 0.087 0.037 0.000

text.

Derivation of the four numerical constants is described in ref If. The elemental isotherms (elemental packing density vs concentration) for carbon and sulfur are shown in Figure 3A. Molecular packing densities are obtained from r, by use of the stoichiometric ratios. At very low concentration, the elemental carbon and sulfur packing densities are indicative of horizontally oriented 3TCA (lo4 M, r c = 1.33 nmol/cm2, rs = 0.29 nmol/cm2 (calculated from molecular models);I6rC = 1.29 nmol/cm2, rS = 0.258 nmol/cm2). As the concentration increases, both the sulfur and carbon packing density increase, indicating an orientational transition of the adsorbed BTCA from a horizontal to tilted vertical orientation. A similar concentration-dependent orientation for TPE adsorbed from the gas phase on Mo(ll0) has been This tilted vertical orientation agrees closely with the geometry suggested by EELS. Lang and Masel have proposed a similar geometry for TPE on Pt(ll1) between 170 and 350 K." Sulfur packing densities that exceed that calculated for vertically oriented 3TCA (0.608 nmol/cm2) are due to desulfurization (see below). Carbon/sulfur elemental packing density ratios based on Auger spectra for STCA adsorbed from dilute aqueous solutions are given in Table 11. The concentration dependence of the C/S ratio is shown in Figure 4. The observed C/S ratios are less than that expected from the molecular formula, 5.0. At very low concentrations, the observed C/S ratio is close to the stoichiometric value and decreases with increasing concentration. C/S ratios less than 5 are an indication that 3TCA decomposes a t some (16)Pauling, L. C . The Nature of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960.

Table 111. Formulas for Obtaining Packing Densities from Auger Data compd BTCA

formulaa

2TCA

3TAA

2TAA

TPE 3MT 33'DMBT 44'DMBT

aConstants: Bc = 0.35 cm2/nmol; Bo = 0.69 cm2/nmol; Bs = 4.53 cm2/nmol;f = 0.70.

stage during adsorption. Apparently, a vertically oriented layer of adsorbed STCA is formed that then decomposes

Batina et al.

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

0 IN

(-jCOOH

6

5

4

3

-LOG I

B

I

2

1

H,0

IN CH,CN

0

C (MI I

,

,

-LOG C (MI Figure 4. Carbon/sulfur packing density ratio for BTCA vs concentration. Experimental conditions: adsorption at -0.1 V, adsorbed and rinsed at pH 3, other conditions as in Figures 1and 2.

0 6

5

4

3

2

1

0

-LOG C (MI Figure 3. Elemental packing densities for BTCA at Pt(ll1) vs concentration. (A) Aqueous solution. (B) ACN solution. Experimental conditions: adsorption at -0.1 V, adsorbed and rinsed at pH 3, other conditions as in Figures 1 and 2.

to an adsorbed layer (consisting of sulfur atoms and vertically oriented STCA molecules) and a hydrocarbon fragment that desorbs. The total sulfur signal corresponds to initially vertically oriented 3TCA. Presence of the molecular adsorbed species is indicated by the EELS spectra and potassium retention data (see above). Desulfurization of T P E a t gas/solid interfaces is well documented.3-5*7s8 To determine the stage at which decomposition occurs, experiments were performed in which the adsorbed layer was rinsed with dilute aqueous KOH (pH lo), followed by surface analysis in vacuum (Figure lB, solid curve). In a variation of this experiment, the adsorbed layer was also exposed to vacuum before rinsing with KOH (Figure lB, dotted curve). The reasoning behind these experiments is that decomposition occurring before evacuation will reduce retention of K+ counterions, due to decomposition of adsorbed 3TCA- molecular anions, while decomposition occurring during evacuation or spectroscopy will not reduce K+ retention, because K+ ions do not leave the surface under vacuum with the present conditions. Analogous experiments have been reported with the pyridinecarboxylic acids.'* The results of the present experiments clearly show that the decomposition of the adsorbed layer occurs in solution: the EELS spectra (Figure 1B) as well as the I K / I p tratios (Table 11) of the adsorbed layers that were rinsed with K+ before and after exposure to vacuum are nearly identical.

The concentration-dependent behavior of the sulfur packing density and CIS packing density ratio (Figures 3 and 4) is consistent with desulfurization in solution. The CIS ratio is largest at low concentrations of 3TCA, suggesting that 3TCA and the hydrocarbon products of desulfurization are most efficiently retained by the Pt(ll1) surface a t low 3TCA concentrations. On the basis of the increased rate of desulfurization for saturated compounds (compared with TPE and other aromatic derivatives), hydrogenation of T P E before desulfurization has been suggested (a bimolecular process).2 The importance of hydride additionlelimination for desulfurization reactions has been shown in many s t u d i e ~ . ~ Intermolecular -~,~ reactions giving rise to a trimer have been found for TPE on Ni(ll1) surfaces between 300 and 350 K.58 ii. Adsorption from Acetonitrile (ACN) Solutions. To study the effect of solvent on the adsorption and surface reactivity behavior of thiophenes, as well as to study adsorbates that are not appreciably soluble in water, adsorption of thiophenes from ACN solutions was carried out. Adsorption from ACN solutions is more complex than adsorption from aqueous solutions because ACN chemisorbs at Pt surfaces.*15 For instance, competition between ACN and STCA for adsorption at the Pt surface is evident in the EELS spectra obtained from 1 mM 3TCA (Figure lC, solid curve) or 100 mM 3TCA (Figure lC, dotted curve): the CN stretch a t 2052 cm-' is much larger for 1 mM BTCA than for 100 mM, due to adsorption of a greater proportion of the solvent at the lower 3TCA concentration. This frequency is shifted from that in the IR spectrum of neat ACN (2253 cm-') due to lowering of the CN bond order by interaction with the Pt surface. There is a slight increase in the frequency of the C-H stretching band (from 2979 to 2993 cm-') with increasing 3TCA concentration. That is, the proportion of 3TCA a t the surface increases, and 3TCA has the higher C-H stretching frequency (3003 cm-' for ACN and 3098 cm-' for 3TCA). The intensity of the 0-H stretch (3546 cm-') and ring-bending modes (500-800 cm-') from 3TCA is also more apparent in the EELS spectrum obtained at the higher BTCA concentration.

Langmuir, Vol. 5, No. 3, 1989 595

Studies of Thiophene and Substituted Thiophenes

ENERGY LOSS (cm-1) Figure 5. Vibrational spectra of BTCA. Upper solid curve: EELS spectrum of 2TCA at Pt(ll1) adsorbed at -0.1 V. Dotted curve: EELS spectrum of 2TCA at Pt(ll1) adsorbed at +0.4 V. Lower solid curve: vapor-phase IR spectrum of BTCA. Experimental conditions as in Figure 1A. served. Accordingly, a tilted or vertical orientation is Several distinct differences between the EELS spectra suggested. of STCA adsorbed from ACN (Figure 1C) and those from water (Figure 1A) are evident, which suggests that the The variation of rs with STCA concentration for adunbuffered ACN solutions used in this study are less acidic sorption from ACN solutions (Figure 3B) is similar to that for adsorption from aqueous solutions (Figure 3A). For than the pH 3 aqueous solutions. The 0-H stretch is concentrations less than 10"' M, rs increases rapidly with much larger for 3TCA adsorbed from water than from concentration but more slowly a t concentrations greater ACN. In particular, the intensity of the C-0 stretching mode (1161 cm-') is greater for 3TCA adsorbed from ACN than 10"' M. Above M 3TCA, rs greatly exceeds the than for BTCA adsorbed from water, indicative of fewer value calculated for vertical 3TCA orientations (0.608 nmol/cm2). This is further evidence that the adsorbed protonated carboxylate moieties. The C=O stretching frequency for 3TCA adsorbed from ACN (1692 cm-') is layer contains a greater proportion of atomic sulfur when lower than that from water as solvent (1726 cm-l) for the formed from less dilute solutions. same reason. That is, the solvent acidity difference is most The dependence of desulfurization on the nature of the apparent in the EELS vibrations related to the acidic solvent can be seen by comparing the concentration decarboxylic acid functionality (0-H and other carboxylic pendence of the C/S ratio in ACN or water, Figure 4. Two important differences are noted. First, a t low concentrastretches). There is a considerable shift in the frequency of the C-H stretching band (from 3040 cm-' in water to tions, the extent of desulfurization for aqueous solutions 2979 cm-' in ACN). This could be due to the greater extent of 3TCA is much larger than for ACN solutions. Secondly, the extent of desulfurization in ACN increases monotonof desulfurization in ACN (see below) giving rise to more ically (decreasing C/S ratio) with concentration, while a abundant aliphatic C-H fragments (from 3TCA) than in slight maximum is observed in water. water, as well as from ACN. Likewise, the ring-bending modes (500-900 cm-') are less noticeable after adsorption b. 2-Thiophenecarboxylic Acid (2TCA). The EELS from ACN. spectrum of BTCA adsorbed from dilute acidic solutions at relatively negative potential (-0.1 V) is shown in Figure The competitive adsorption of ACN and 3TCA is also 5 (solid curve). The carboxylic acid 0-H stretching vireadily apparent from the concentration dependence (3TCA) of the nitrogen packing density, as measured by bration (3598 cm-') is smaller than that observed for 3TCA, Auger spectroscopy, Figure 3B and Table 11. It is readue to greater interaction of the carboxylate with the metal surface. In addition, the C=O stretch, prominent for sonable to suppose that the amount of ACN adsorbed is STCA, is weak for 2TCA. Evidently, the proximity of the equal to rN. From Figure 3B, it can be seen that the a-carboxylate group to the Pt surface promotes strong amount of ACN adsorbed on the Pt surface decreases approximately linearly with the log of the STCA conceninteraction with the surface even a t relatively negative tration. electrode potentials. The carbon packing densities, rc,shown in Figure 3B represent the carbon contribution from STCA, corrected for that due to ACN. The carbon contribution from ACN can be determined from the nitrogen packing density and is relatively small (Table 11). The carbon isotherm shows a packing density plateau near 1.80 nmol/cm2 a t a conE - 0.1 V E = + 0.4 V centration of 10 mM, corresponding to a molecular packing density of 0.36 nmol/cm2, which is larger than the calcuThe EELS spectrum of 2TCA adsorbed at a relatively lated value for horizontally oriented BTCA (0.258 nmol/ positive potential, Figure 5 (dotted curve), shows a virtual cm2); this difference is in spite of carbon loss due to deabsence of a C=O stretching vibration and further desulfurization. At concentrations greater than about 10 crease in the 0-H stretching frequency (3598 cm-') relative mM, a slight increase in carbon packing density is obto the negative potential. This is evidence that both

-

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

Batina et al.

ENERGY LOSS ( c m - 1 ) Figure 6. Vibrational spectra of 3TAA. Upper solid curve: EELS spectrum of 3TAA at Pt(ll1) adsorbed at -0.1 V. Dotted curve: EELS spectrum of BTAA at Pt(ll1) adsorbed at +0.4 V. Lower solid curve: FTIR spectrum of BTCA (Nujol mull). Experimental conditions as in Figure 1A.

carboxylate oxygens are coordinated to the Pt surface when adsorption is carried out a t positive potentials. These observations correspond closely to those reported for picolinic acid and related compounds, which are examples of vertically oriented heteroaromatics displaying strong interaction between o-carboxylate substituents and the Pt surface.1e Additional evidence for the interaction of the BTCA carboxylate with the metal surface can be seen from the Auger data of the 2TCA layer following rinsing with dilute aqueous KOH (pH 10) at -0.1 V (Table 11). The BTCA layer does not retain K+ ions as efficiently ( I K / I p ? = 1.151) as does 3TCA ( I K / I p , O = 1.505). The C/S elemental packing density ratio (3.11) is smaller than that expected from the molecular formula, 5.0, an indication that desulfurization occurs. However, the elemental packing density of sulfur, 0.581-0.624 nmol/cm2, is close to the theoretical prediction based on molecular models of BTCA, 0.530 nmol/cm2. As for 3TCA, these observations suggest that desulfurization of adsorbed 2TCA occurs at the Pt/solution interface, giving adsorbed sulfur atoms. c. 3-Thiopheneacetic Acid (3TAA). The EELS spectrum of 3TAA adsorbed from dilute acidic aqueous solution a t a relatively negative electrode potential (-0.1 V) is shown in Figure 6. The intensity of the carboxylate 0-H stretching vibration a t 3393 cm-' is considerably smaller than the corresponding vibration of 3TCA. Evidently, the insertion of a CH2 group gives the molecule enough flexibility so that the carboxylate is able to coordinate to the surface, while such interaction is sterically difficult for 3TCA.

The C=O and 0-H stretching vibrations are virtually absent for 3TAA adsorbed at relatively positive potentials, Figure 6 (dotted curve), indicative of a very strong met-

al-carboxylate interaction, while the other vibrations are relatively independent of adsorption electrode potential. The strong interaction of the 3TAA carboxylate with the metal surface is supported by the KOH rinsing data. The potassium Auger signal (IK/IRo= 0.850), Table 11, shows that only a small amount of potassium is incorporated. Decomposition of the layer formed by adsorption of 3TAA is shown by the C/S elemental packing density ratios, 3.40 (+0.4 V) and 3.94 (-0.1 V), which are smaller than that expected from the molecular formula, 6.0. The sulfur elemental packing density, 0.526-0.579, is close to that calculated for molecular models of 3TAA, 0.55, indicating that adsorption occurs before desulfurization. d. 2-Thiopheneacetic Acid (2TAA). In contrast to the behavior of 3TAA, some or all the carboxylic acid groups in the 2TAA adsorbed layer are pendant and relatively unperturbed by the surface regardless of the adsorption electrode potential, as evidenced by the presence of an 0-H stretching peak at 3608 cm-', Figure 7. Similarly, a relatively large Auger signal due to K+ ions (IK/IR0 = 1.230) is observed following rinsing with KOH, Table 11) indicative of pendant carboxylate moieties. Possibly, the structure of the adsorbed BTAA isomer hinders coordination of the carboxylate functionality to the Pt surface.

The C/S packing density ratios, 3.11 (+0.4 V) and 3.82 (-0.1 V), are lower than expected, 6.0, indicating desulfurization. Once again, the sulfur elemental packing density, 0.532-0.589 nmol/cm2, agrees with that predicted on the basis of molecular models of vertically oriented 2TAA,0.507 nmol/ cm2,indicating that adsorption occurs before desulfurization. e. Thiophene (TPE). The EELS spectra of TPE adsorbed from dilute aqueous solutions, Figure 8 and Table I, are similar to those reported for T P E adsorbed a t Pt(100) in vacuum a t 125 K and annealed to 350 K.4c That

Langmuir, Vol. 5, No. 3, 1989 597

Studies of Thiophene and Substituted Thiophenes

ENERGY LOSS

(cm-1)

Figure 7. Vibrational spectra of 2TAA. Upper solid curve: EELS spectrum of 2TAA at Pt(ll1) adsorbed at -0.1V. Dotted curve: EELS spectrum of BTAA at Pt(ll1) adsorbed at +0.4 V. Lower solid curve: FTIR spectrum of 2TCA (Nujol mull). Experimental conditions as in Figure 1A.

ENERGY LOSS ( c m - 1 )

Figure 8. Vibrational spectra of TPE. Upper solid curve: EELS spectrum of TPE at Pt(ll1) adsorbed at -0.1 V. Dotted curve: EELS spectrum of TPE at Pt(ll1) adsorbed at +0.4 V. Lower solid curve: FTIR spectrum of neat TPE. Experimental conditions as in Figure 1A. is, the frequencies and amplitudes are qualitatively the same. There is a remarkable correspondence between the EELS spectrum of adsorbed TPE and the IR spectrum of T P E vapor, Figure 8, with the exception that the EELS peak at 765 cm-' due to the out-of-plane C-H bending modes is much smaller than in the IR spectrum. This out-of-plane bending mode would be expected to be small for TPE adsorbed in a nearly vertical orientation, for which the component of this mode that is perpendicular to the surface is small. Therefore, TPE is most probably present in a vertical or nearly vertical orientation. The C-H stretching frequency, 2957 cm-', is lower by approximately 100 cm-' than that observed for gas-phase TPE or for the

EELS spectra of 3TCA, BTCA, 3TAA, and 2TAA discussed previously. This shift suggests a rehybridization of TPE in which the platinum-sulfur u interaction is enhanced a t the expense of the aromatic ?r coordination. At this point, we cannot rule out hydrogenation of the TPE molecule, which has been postulated by several authors to accompany or enable desulfurization;2that would affect the C-H stretching frequency. Auger data show that extensive adsorbate desulfurization occurs (Table 11). That is, the C/S ratio is 1.6, compared with 4 in the T P E molecule. Evidently, the hydrocarbon fragment resulting from desulfurization rinses away, in contrast to the room temperature behavior of

Batina et al.

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

1 m

0 al M

\

I

aJ v

W

l-

U QL

I-

z

3 0 V

\ I . . . . I , , , . I . . . . I . , . . I , . , , I . , , , , , , ,

1000 2000 3000 4 ( IO ENERGY LOSS ( c m - 1 ) Figure 9. Vibrational spectra of 3MT. Upper solid curve: EELS spectrum of 3MT at Pt(ll1) adsorbed at -0.1 V. Dotted curve: EELS spectrum of 3MT at Pt(ll1) adsorbed at +0.4 V. Lower solid curve: FTIR spectrum of neat 3MT. Experimental conditions as in Figure 1A.

gas/solid interfaces where the hydrocarbon fragment was retained a t the ~ ~ r f a ~ e . ~ ~ * ~ ~ ~ ~ * ~ ~ Voltammetric scans of adsorbed TPE in aqueous electrolyte are virtually identical with those shown for 3MT in Figure 10. Combining the voltammetry with packing densities obtained by Auger spectroscopy gives the number of electrons required to oxidize adsorbed T P E and adsorbed sulfur completely: Qox - Qb FA[nox(TPE)W'PE) + nox(S)W)l (2) where Q,, and Q b are the coulometric charges for the oxidation and background (first and second scans), respectively, F is Faraday's constant, A is the electrode area, and I'(TPE) is the molecular packing density of TPE obtained from rcby Auger spectroscopy. r(S) is the packing density of sulfur atoms not present as T P E molecules: r(s)= rs - rc/4 (3) where rS is the total sulfur packing density from Is/Ip?, Table 11. The observed (Qox - Q b ) / A is 651 pC/cm2, compared with a calculated value of 723 pC/cm2 for complete oxidation of the adsorbed layer to C 0 2 and Sod2-:

0+

12H20

S S

+

-

2-

so4

4H20

-

+ 4co2 + S O : -

+

8H*

28H'

+

26e6e-

(4) (5)

Evidently, the hydrocarbon fragments are almost completely oxidized; similar results have been reported for a variety of other adsorbates? although sulfur oxidation is probably complete under these condition^.'^ The charges passed before (651 pC/cm2) and after exposure to vacuum (658 pC/cm2) are nearly identical, indicating that there is no loss of electroactive material on evacuation. f. 3-Methylthiophene (3MT). The EELS spectrum of 3MT adsorbed from dilute aqueous solutions, Figure 9, follows the envelope of the IR spectrum of neat 3MT closely above 900 cm-', but the C-H bending modes are weak in the EELS spectra, as is seen also for TPE. Little

1

I

I

I

I

0.0

-04

04

I

I

POTENTIAL, VOLT

I

12

08 vs. Ag/AgCI

Figure 10. Cyclic voltammetry of adsorbed 3MT at Pt(ll1). Solid curve: f i t scan. Dotted curve: second scan. Experimental conditions: immersion into 2 mM 3MT at -0.1 V and pH 3, followed by rinsing with 0.1 mM HF; scan rate 5 mV/s; electrolyte 10 mM KF adjusted to pH 3 with HF; temperature 23 i 1 "C.

dependence of the EELS spectrum on the adsorption potential was noted. Voltammetric scans of adsorbed 3MT in aqueous electrolyte are shown in Figure 10. The charge passed (745 pC/cm2) is similar to that expected for complete oxidation, eq 5 and 6 (726 FC/cm2):

dCH3 +

14H20

-:OS

+

5C02

+

34H+

+

324

(6)

2. Polymerization of Thiophene Derivatives. The vibrational structure of polymer surfaces has been investigated by using EELS only recently. A variety of polymers have been studied, including polyethylene,l' poly-

Studies of Thiophene and Substituted Thiophenes

Langmuir, Vol. 5, No. 3, 1989 599

(methyl methacrylate),l8tZ1poly~tyrene,’~ polyimide,20and

poly[N,N’-bis(phenoxyphenyl)pyromellitimide].2’The EELS spectrum of the conducting polymer polypyrrole has been reported.22 The vibrational structure of Langmuir-Blodgett films of fatty acids has also been investigated recently with EELS.23 There have been no reports of vibrational spectra of polymer films prepared by electrogeneration at single-crystalmetal surfaces; however, the nature of the substrate on which the polymer or fatty acid is supported has been shown to have marked effects on the EELS s p e ~ t r u m .We ~ ~report ~ ~ ~ here ~ the first surface vibrational spectra of P-3MT and the first examples of EELS spectra of electropolymerized polymer films on well-characterized single-crystal metal surfaces. EELS spectra of 3MT monomer, the dimeric 33’DMBT and 44’DMBT, and polymer on Pt(ll1) are shown in Figure 11. The EELS features at 1164 and 1008 cm-’, due to methyl-bending and -wagging modes, are prominent in the spectrum recorded for 3MT adsorbed on Pt(ll1) a t -0.1 V (Figure 11A,B). When an adsorbed layer of 3MT is oxidized a t +1.5 V in ACN, the features due to methyl group modes at 1195 and 1019 cm-’ are diminished in intensity, compared with the CC stretch at 1384 cm-l. This is consistent with the formation of CC bonds (between the ring CY positions) on electrochemical oxidation. The EELS spectrum of 3MT adsorbed at Pt(ll1) from ACN solution a t -0.1 V (Figure 11B) is similar to that obtained for the corresponding aqueous solution (Figure 1lA, solid curve). The EELS spectra of the two dimers, 33’DMBT and 44’DMBT, adsorbed from hexane solution, are instructive for comparison of the features of the monomer with those of the polymer. The orientation of the methyl groups in 33’DMBT hinders adsorption of the compound a t the electrode surface, relative to 44’DMBT. This steric effect is manifested in the reduced EELS intensity of 33’DMBT relative to 44’DMBT. The EELS spectra of 33’DMBT and 44’DMBT are otherwise virtually identical. Comparison of the EELS spectra of the dimers (Figure 1lC) with that of 3MT (Figure 11A,B) shows that the presence of the 2,2’ C-C bond has an effect similar to that achieved by oxidation of adsorbed 3MT: the features from 1000 to 1200 cm-’ are broadened, and their intensity is less than that of the CC stretch a t 1436 cm-’. EELS spectra of neutral, polymerized 3MT (P-3MT) at Pt(ll1) are shown in Figure 11D, along with the FTIR spectrum reported previously.lk The increase in intensity of the CC stretch at 1439 cm-’, compared with the intensity of the features below 1200 cm-l, is pronounced and larger than that observed for the dimeric compounds 33’DMBT and 44’DMBT. The C-H out-of-plane bending modes at 617 and 819 cm-l, distinct in the FTIR spectrum, are not present in the EELS spectrum. This lack of C-H bending modes is also observed for the EELS spectra of the monomer. EELS spectra of the oxidized (doped) form of the (17)Pireaux, J. J.; Thiry, P. A.; Caudano, R.; Pfluger, P. J . Chem. Phvs. 1986.84(11).6452. 118)Pireaux, J:’J.; Gregoire, C.; Vermeersch, M.; Thiry, P. A.; Caudano, R. Surf. Sei. 1987,189/190, 903. (19)War, M. R.;Schott, M.; Pireaux, J. J.: Greaoire, C.: Thirv, P. A.: Caudano, R.; Lapp, A,; Botelho do Rego, A. M.; Lopes da Silva,>. Surf. Sci. 1987,189/190,927. (20)Dinardo. N.J.;Demuth, J. E.: Clarke, T. C. Chem. Phvs. Lett. 1985,121(3),239. 121)Dinardo,. N. J.:. Demuth.. J. E.:, Clarke. T. C. J.Chem. Phvs. 1986. 85(11),6739. (22)Jennings, W. D.; Chottiner, G. S.; Natarajan, C.; Melo, A. V.; Hoffman, R. W.; O’Grady, W. E.; Lundstrom, I.; Salaneck, W. R. Appl. Surf. Sci. 1985,21, 80. (23)(a) Wandass, J. H.; Gardella, J. A., Jr. Surf. Sci. 1985,150,L107. (b) Wandass, J. H.; Gardella, J. A., Jr. Langmuir 1986,2,543.(c) Wandass, J. H.; Gardella, J. A., Jr. Langmuir 1986,3, 183.

D

C

A

0

1000 2000 3000 4000

ENERGY LOSS (cm-’1 Figure 11. Vibrational spectra of 3MT monomers, dimers, and polymers. (A) Upper solid curve: EELS spectrum of 3MT at Pt(ll1) adsorbed from 1 mM 3MT in H20 at -0.1 V. Dotted curve: EELS spectrum of 3MT at Pt(ll1) adsorbed from 1 mM 3MT in H20 at -0.1 V and oxidized to +1.5 V in CH3CN. Lower solid curve: FTIR spectrum of neat 3MT. (B) Upper curve: EELS spectrum of 3MT at Pt(ll1) adsorbed from 16 mM 3MT in CH3CNat -0.1 V. Lower curve: FTIR spectrum of neat 3MT. (C)Solid curve: EELS spectrum of 44’DMBT at Pt(ll1) adsorbed from 2 mM 44’DMBT in n-hexane. Dotted curve: EELS spectrum of 33’DMBT at Pt(ll1) adsorbed from 2 mM 33’DMBT in n-hexane. Lower curve: IR spectrum of neat 33’DMBT. (D) Upper solid curve: EELS spectrum of P-3MT at clean annealed Pt(ll1). Dotted curve: EELS spectrum of P-3MT at 3MT monomer covered Pt(ll1). Experimental conditions as in Figure 1A.

polymers studied showed extensive peak broadening and less signal intensity than the corresponding EELS spectra of the reduced (undoped) polymers. This could be due to changes in the physical nature of the polymer film (polymer swelling or decrease in reflectivity) or to excitation of phonon modes in the polymer. The EELS spectrum of P-3MT prepared on a Pt(ll1) surface containing preadsorbed 3MT (Figure 11D, dotted curve) is different from that obtained for 3MT polymerized on a clean, annealed Pt(ll1) surface (Figure 11D, solid curve). Most of the

Langmuir 1989,5,600-607

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features are comparable, but the EELS intensity of P-3MT on the 3MT preadsorbed Pt surface is less than that observed on the clean, annealed Pt(ll1) surface. EELS spectra of polyimide films have also been shown to be very dependent on the nature of the underlying s u p p ~ r t .This ~ is not surprising considering our previous studies, which have shown that the nature of the electrode surface can have profound effects on the voltammetric and UHV electron spectroscopic behavior of adsorbed layers,%as well as on the electrochemistry of unadsorbed reagents.26 It is apparent that electrode surface structure and compo(24) (a) Gui, J. Y.; Kahn,B. E.; Lin, C.-H.; Lu, F.; Salaita, G. N.; Stern, D. A,; Hubbard, A. T. Langmuir, in press. (b) 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. (25) Batina, N.; Kahn, B. E.; Lin, C.-H.; McCargar, J. W.; Salaita, G. N.; Hubbard, A. T. Electroanalysis 1989, 1, in press.

sition affect polymerization reactions conducted a t electrodes. The polymers studied on Pt(ll1) certainly contain more than a monomolecular layer of P-3MT. The Auger spectra of P-3MT show no peaks attributable to Pt, indicating that the polymer film is thick enough (and sufficiently uniform) to attenuate the Pt Auger signal completely.

Acknowledgment. This work was supported by the Edison Sensors Technology Center, administered by the Cleveland Advanced Manufacturing Program. Instrumentation was provided by the National Science Foundation, the Air Force Office of Scientific Research, and the University of Cincinnati. Registry No. 3TCA, 88-13-1; BTCA, 527-72-0; BTAA, 696421-2; ZTAA, 1918-77-0;WE, 110-02-1;3MT, 616-44-4;33'DMBT, 67984-20-7; 44'DMBT, 111372-97-5; P-3MT, 84928-92-7; Pt, 7440-06-4.

Computer Simulations of Mono- and Trilayer Films of Argon on Graphite? Ai-Lan Cheng* and William A. Steele*Ps Department of Physics and Department of Chemistry, The Pennsylvania State University, 152 Davey Laboratory, University Park, Pennsylvania 16802 Received October 27, 1988. In Final Form: February 1 , 1989 The structures and thermodynamic energies of tri- and monolayer films of argon adsorbed on the basal plane of graphite have been studied at temperatures ranging from 50 to 110 K. Within this range, layer-by-layer melting was found for each of the three layers and for the single layer simulated. We surmise that these processes are first-order transitions observed under conditions such that the process is continuous. Energies and areas of melting are estimated. Roughening of the simulated trilayer film occurs gradually as the temperature increases. 1. Introduction

Computer simulation studies of model physisorption systems are proving to be a valuable addition to experiment and theory,'P2 especially if the simulation employs realistic models for the adsorbate-adsorbate and the adsorbate-solid interactions. For example, one can generate information about the temperature and coverage dependence of the orientational behavior of nonspherical adsorbate molecule^,^ which is difficult if not impossible to obtain experimentally. Similarly, computer simulation of multilayer films can give local densities and energies, which are essentially inaccessible to experimental measurements (of course, careful analysis of diffraction studies of solid multilayer films could yield local crystal structures,4 but this technique loses its power as the layer liquefies). Furthermore, questions concerning the nature of the thermodynamic phases and phase transitions within the layers of a multilayer film are hard to answer accurately by either experiment or theory. In this paper, we report a molecular dynamics computer simulation study of argon monolayer and multilayer films Presented at the symposium on "Adsorption on Solid Surfaces", 62nd Colloid and Surface Science Symposium, Pennsylvania State University, State College, PA, June 19-22, 1988; W. A. Steele, Chairman. Department of Physics. 8 Department of Chemistry.

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0743-7463/89/2405-0600$01.50/0

adsorbed on a graphite substrate. The temperature of these simulations ranged from -50 K, where the films were completely solid, up to 110 K, where the films are completely liquid. We will show that melting occurs layer-by-layer for a trilayer film. The thermodynamic melting parameters were estimated for each layer in these films. In addition, the melting of a single monolayer of adsorbed argon was studied. We should note a t the outset that the solidified Argraphite system presents a particularly difficult problem for computer simulation. It is well-known that solid argon layers form an incommensurate lattice on graphite.6 Diffraction data indicate that the monolayer a t very low temperature is triangular with a nearest-neighbor spacing of -3.8 A,compared to 4.26 A for a commensurate film.6 This spacing changes with the thermal expansion of the layer, and furthermore, one might guess that the spacing in a low-temperature trilayer film might not be exactly the same as that in the monolayer. In our simulation, the periodic variation in the Ar-graphite interaction energy was included in an attempt to be as realistic as possible.

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(1) Nicholson, D.;Parsonage, N. G. Computer Simulation and the Statistical Mechanics of Adsorption; Academic Press: New York, 1982. (2) Abraham, F. F. Adu. Phys. 1986,36, 1-111. (3) Talbot, 3.; Tildesley, D. J.; Steele, W. A. Surf. Sei. 1986, I69,71. (4) Larese, J.; Passell, L., private communication. ( 5 ) Shaw, C. G.; Fain, S. C., Jr.; Chinn, M.D. Phys. Reu. Lett. 1978, 41,955. (6) Hanson, F.; McTague, J. P. J. Chem. Phys. 1980, 72, 6363.

0 1989 American Chemical Society