Langmuir 1986,2, 697-700 exposure to oxygen not only causes segregation of tin to the surface but also oxidizes the surface tin to an oxidic form. The fact that features due to both tin metal and tin oxide appear in (a) suggests that metallic tin probably lies just under the tin oxide and above the region containing Pt. Further support for this interpretation is provided by the angle-resolved valence-band spectra shown in parts a and b of Figure 4, corresponding to emission angles of 90° and loo, respectively. The normal emission spectrum shown in (a) is metallic-like with a large density of states near the Fermi level. This is in agreement with the normal-emission spectra shown in Figure 3 parts A and B. The glancing-emission spectrum shown in Figure 4b has more of an oxidic nature and is quite similar to published valence-band spectra of oxidized tin.28,29 From the behavior of the Pt/Sn alloy in the presence of oxygen, it may be possible to deduce information about the Pt/Sn/A1,03 system. The results presented here (28) (29)
Powell, R. A,; Spicer, W. E. Surf. Sci. 1976, 55, 681. Lau, C. L.; Wertheim, G. K. J. Vac. Sci. Technol. 1978,15,622.
697
suggest that the metal crystallites would not be in an alloyed form in the presence of an oxidizing environment. Although assuming that supported crystallites behave like bulk alloys is tenuous, this suggestion agrees with the observation by Davis21that a PtSn alloy is not present in an oxidized Pt/Sn/A1203sample as determined by X-ray diffraction.
Conclusion The oxidation of a clean, platinum-rich Pt3Sn alloy surface during a room temperature exposure to air has been examined by angle-resolved ESCA. It is found that tin segregates to the surface and appears to completely cover the Pt-rich layer. The surface tin is oxidized, but Pt remains in a metallic state. Acknowledgment. We appreciate the efforts of P. Kirszensztejn with regard to sample preparation and the use of the experimental facilities by Perkin-Elmer Physical Electronics. Financial support of this research was provided by the donors of the Petroleum Research Fund, administered by the American Chemical Society. Registry No. PtpSn alloy, 37365-44-9.
Direct Determination of the Mass of an Underpotentially Deposited Monolayer of Lead on Gold 0. Melroy," K. Kanazawa, J. G. Gordon 11, and D. Buttry IBM Research Laboratory, S a n Jose, California 95120 Received February 20, 1986. I n Final Form: J u l y 1, 1986 The mass of an underpotentially deposited monolayer of lead on gold has been measured in situ by using a quartz crystal microbalance. The mass of the monolayer calculated from first principles using the frequency change of the microbalance corresponds to a coverage of 1.3 X mol/cm3. As determined in the present study, at complete coverage, the electrosorption valency for lead deposition onto gold is 2.08 A 0.10, which is in good agreement with literature values.
Introduction Adsorption at the electrode/electrolyte interface plays a central role in electrochemistry. The recognition of its profound influence on the thermodynamic and kinetic properties of the interface is indicated by the enormous number of experimental and theoretical studies relating to this phenomenon. Many methods have been developed for determining the surface excess of a given species. The classical approach of measuring the electrocapillary curve suffers from a reliance on the unique properties of Hg as an electrode material.' The use of differential capacitance data requires a considerable amount of interpretation and assumption and requires separate knowledge of the potential of zero charge.2 Chronocouolometric measurement of adsorption requires that the adsorbate be electr~active.~ Several spectroscopic techniques have been more recently developed, such as surface-enhanced Raman spectroscopy: infrared reflection-absorption spectroscopy,5 and UV-vis (1)Grahame, D. C. Chem. Rev., 1947, 41, 441. Lawrence, J.; Mohilner, D. M. J. Electrochem. SOC.1971,118,259,
(2) 1596. (3) (4)
Anson, F. C. Anal. Chem., 1966, 38, 54. Fleishmann, M.; Hendra, P. J.; McQuillan, A. J. J. Chem. Soc.,
McIntyre, J. D. E.; Kolb, D. M. S y m p . Faraday. SOC.1970,4,99. (a) Nomura, T.; Hattori, 0. Anal. Chem. Acta 1980, 115, 323. (b) Nomura, T. Anal. Chem. Acta 1981,124, 81. (c) Nomura, T.; Iijima, M.
Bewick,A.; Kunimatau, K.; Pons, B. S. Electrochim. Acta 1980,25,
Anal. Chem. Acta 1981, 131, 97. (8) Bruckenstein, S.; Shay, M. J. Electroanal. Chem. Interfacial Electrochem. 1985, 188, 131.
Chem. Commun. 1973, 80. (5) 465.
reflection spectroscopy! However, these techniques give only limited information regarding the number, identity, and structure of interfacial species and in some cases require very special conditions for the measurement. It is the purpose of this paper to describe the use of an oscillating quartz crystal microbalance (QCM) to measure extremely small changes in mass of the electrode surface. Quartz crystal microbalances are used extensively in vacuum and the theory for their operation has been well developed. Nomura and his co-workers' first reported the use of a QCM in solution. In his later studies, the microbalance was used as an analytical tool for trace metal determination. This was accomplished by electrodeposition of the trace metals onto the QCM electrode in tandem with a measurement of the resulting frequency change. More recently, Bruckenstein and Shay8 have reported on the use of the QCM as part of an electrochemical cell, so that one of the QCM electrodes was also the working electrode in the cell. By simultaneously measuring the frequency-potential and current-potential curves in cyclic (6) (7)
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h F i g u r e 1. Oscillator and potentiostat circuit for operation in solution.
voltammetric experiments, they were able to determine the amount of silver electrodeposited onto the electrode from the solution. The reported mass sensitivity was 1.1x 10." g/Hz. In their study, the relationship between the oscillation frequency and the mass on the electrode was determined empirically, by calibration of the QCM response to a known amount of electrodeposited silver. However, using their experimental parameters (Le., f = 1 x lo7 Hz, A = 0.25 cm2)in eq 1 gives a theoretical sensitivity of 1.1 X g/Hz, in excellent agreement with experiment. It has been shown that many of the same ideas apply in the liquid environmentgand that the mass sensitivity is the same in liquid and in vacuum. Since this technique directly measures mass changes, it is not subject to the experimental limitations and interpretive difficulties inherent in those previously mentioned. As a demonstration of the technique, the mass of an underpotentially deposited monolayer of P b on a Au electrode is measured simultaneously with the charge required for its deposition.
Experimental Section Five-megahertz AT-cut quartz crystals (Valpey-Fisher) were used in all experiments. Gold electrodes were deposited onto both sides of the crystals by standard thermal evaporation techniques. A thin Cr layer was used to enhance the adhesion between the Au and the quartz. A P d layer was used to block the diffusion of Cr through the Au to avoid the possibility of observing electrochemistry and the accompanying mass changes from the underlying Cr layer.'O Connections were made to the electrodes with either indium solder or a conducting silver paste. The crystals were mounted onto the ends of glass tubes using RTV silicone adhesive, taking care to completely mask off the connection to the electrode so that only the Au surface was exposed to the electrolyte solution. The crystals were operated at their third harmonic, 15 MHz, which provided greater mass sensitivity than at the fundamental (vide infra). The oscillator circuitry and its accompanying electrochemical instrumentation are presented in Figure 1. The oscillator circuit uses a high gain differential amplifier with current feedback to ensure oscillation in the series resonant mode. The parallel capacitor and inductor in the feedback loop is tuned to inhibit oscillation a t 5 MHz so that the crystals operate a t their (9) Kanazawa, K.; Gordon, J. G. Anal. Chem., in press. (10) Kanazawa, K., unpublished results.
08
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F i g u r e 2. (a) Current-potential and (b) frequency-potential curves of Au quartz crystal microbalance electrode in 1 M HC104 and 0.5 mM PbO. Scan rate, 10 mV/s; 5-MHz crystal operated at third harmonic (15 MHz). third harmonic. The Schottky diodes are present to clip the sinusoidal output of the difference amplifier and to limit the potential applied across the crystal. One of the key features of this circuit is that one side of the crystal is grounded. This allows that side of the crystal to be used as a working electrode in a Wenking-type potentiostat and also prevents the electrolyte from seeing any of the rf fields. Electrolyte solutions (1M perchloric acid) were prepared from J. T. Baker Ultrex-grade perchloric acid and PbO (Johnson Matthey, Puratonic) which were used as received. All solutions were prepared by using water from a Barnstead Nanopure purification train which included photolysis to remove organics. A standard two-compartment electrochemical cell was used for all experiments. A Pt wire quasi-reference electrode was used to eliminate the possibility of leakage of adsorbable ions from other reference electrodes (e.g., Ag/AgCl or calomel). All potentials were corrected and are reported with respect to the saturated calomel electrode. High purity Ar was used to maintain an inert atmosphere within the electrochemical cell. The polycrystalline Au electrodes were electrochemically cleaned by repeated cycling into the hydrogen evolution and oxide growth regions. The electrolyte solution was changed several times during this process so as to discard any contaminants which might have been desorbed into the solution.
Results Figure 2a shows the cyclic voltammogram resulting from the underpotential deposition (UPD) of P b onto a Au electrode. The shapes and relative potentials of the waves are in agreement with the results of other groups who have studied this system on polycrystalline and single-crystal Au electrode^.^'-*^ The sharpness of the most negative of the two waves and the small peak separation between the anodic and cathodic branches of this wave (approximately 16 mV) have been observed by many other workers and are taken to indicate that the active area of the electrode is relatively free of adsorbed impurities.l' Curve b shows the oscillation frequency of the quartz microbalance, which was measured concurrently with the cyclic voltammogram. The UPD of Pb, as measured by the deposition corresponding to both cathodic peaks, causes a frequency decrease of 46 Hz. It is important to note that the mass change resulting from the UPD process is entirely reversible. Also, note that it is possible to distinguish inflection points in the frequency vs. potential (11) Adzic, R.; Yeager, E.; Cahan, B. D. J. Electrochem. SOC.1974,121, 414. (12) Schultze, J. W.; Dickertmann, D. Surf. Sci. 1976, 54, 489. (13) Kolb, D. M. In Advances in Electrochemistry;Gerischer, H., Tobias, C. W., Eds., Voi. 11, Wiley: New York, 1978; Vol. 11, p 125. (14) Engelsmann, K.; Long, W. J.; Schmidt, E. J . Electroanal. Chem. 1970, 114, 1. (15) Hamelin, A. J . Electroanal. Chem., 1984, 165, 167.
Langmuir, Vol. 2, No. 6, 1986 699
Mass Determination of a Monolayer of Lead I
I
The crystals may also be operated at the frequencies of the odd harmonics, in which case the relation between the added mass and the frequency change is given by (2)
20 Hz
OB
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Figure 3. (a) Current-potential and (b) frequency-potential curves as in Figure 1 except the solution contained no PbO.
curve which correspond to the two waves in the cyclic voltammogram. Figure 3 shows a background scan taken by using nearly the entire potential window available for this system. The negative limit is dictated by the reduction of hydrogen ions and the positive limit by the initial stages of gold oxide formation; the gold films are not stable to formation of thick oxide films. There is no detectable frequency change at either limit. Over the entire potential range the frequency is stable to within 1 Hz. This limit is determined by the accuracy of the frequency counter and is not a reflection of the stability of the oscillator circuitry.
Discussion It has only recently been demonstrated that the QCM, commonly employed for mass measurements in vacuum, will operate in sol~tion.'-~J~ These piezoelectric devices are very sensitive to mass changes at the surface of the crystal and provide the capability of detecting extremely small amounts of deposited material. The operation of the device relies on the excitation of a normal mode of oscillation by the application of a radio frequency field across a thin disk of the material." For AT-cut crystals, the resonant condition corresponds to a shear oscillation in which the shear wave propagates through the bulk of the material, perpendicular to the faces of the disk. The atomic displacements correspondingto this shear motion are thus parallel to the disk faces. If material is deposited on either or both faces of the crystal, the resonant frequency decreases. The first quantitative investigation of this effect was by Sauerbrey,ls who derived the following relationship for the frequency change (Af) caused by the added mass (m):
where f is the fundamental resonant frequency of the unloaded quartz, N (=1670 kHz mm for AT-cut crystals) is the frequency constant for the particular cut employed, p (=2.648 g/cm3) is the density of the quartz, and A is the surface area of the deposit. This equation has been found to be an accurate description of the system for cases in which the added mass amounts to less than a few percent of the total mass of the res0nat0r.l~ (16)Komash, P.L.;Bastians, G. J. Anal. Chem. 1980,52, 1929. (17) See, for example; Byrne, R. J.; Lloyd, P.; Spencer, W. J. J. Acoust. SOC.Am. 1968,43, 232. (18) Sauerbrey, G . 2. Phys. 1959, 155, 206. (19)Lu, C.;Lewis, 0. J . Appl. Phys., 1972, 43, 4385.
where M is the shear modulus of quartz, f' is the frequency, and n is the order of the harmonic. Substituting f = nf and noting that n enters into the denominator of eq 2 shows that the mass sensitivity for operation of a given crystal increases linearly with the harmonic number. In contrast, the sensitivity of different crystals operated at the fundamental frequency increases as the square of this frequency. The present study uses crystals which are operated at their third harmonic, therefore we use eq 2 to calculate the mass of P b deposited in the UPD process. A frequency decrease of 46 Hz corresponds to a mass gain of 2.8 x g/cm2. This gives a coverage of 1.3 X lo4 mol/cm2, taking the electrode area to be 0.34 cm2. Manual integration of the cyclic voltammogram gives a Pb coverage of 1.4 X mol/cm2, a value which is in excellent agreement with the mass change since Engelsmann et al.14 have shown the electrosorption valency for Pb deposition onto Au to be 2. Thus, the frequency change measured with the QCM may be used to calculate the mass of a deposted monolayer from first principles with a high degree of accuracy. It has previously been demonstrated that at potentials sufficiently close to the bulk deposition potential, the UPD of Pb on Au results in the formation of a close-packed hcp monolayer of Pb.12 The atomic coverage of Pb in such a structure is 9.4 X 1014atoms/cm2. This corresponds to 1.6 X mol/cm2. Thus, a mass change and charge consumption corresponding to approximately 85 % of monolayer coverage is observed in our experiments (the actual discrepancy may be somewhat larger since a surface roughness of 1 was assumed). The cause of this discrepancy is unclear at the present time. However, the extreme sensitivity of UPD processes to the presence of adsorbed impurities is well-known." Hence, the present results may be an indication that, despite the pretreatment procedure employed, approximately 15% of the electrode area is insufficiently clean to allow for the UPD of Pb. The work of Schultze and Dickertmannl* and others15 has shown that the response observed at polycrystalline electrodes is essentially the sum of the responses for the various single-crystal faces exposed at the polycrystalline surface. It is worth noting that the deposition of Pb onto the different crystal faces is easily detected in the frequency response of the QCM, being especially pronounced in the desorption curves. One of the most interesting aspects of the use of the QCM in the study of UPD systems is the possibility of extracting the electrosorption valency20,21 directly from the mass gain and the number of Coulombs consumed in the deposition. The data in the present work show that at complete coverage the electrosorption valency for P b deposition onto Au is 2.08 f 0.10, in agreement with the earlier work of Engelsmann et al.'* Experiments to determine the electrosorption valency as a function of coverage are currently under way. Conclusions The ability to accurately determine the mass of adsorbed species at the electrode, and especially the mass changes (20) Schultze, J. W.; Vetter, K. J . J . Electroanal. Chem. 1973,44, 63. (21) Swathirajan,S.;Bruckenstein, S. Electrochim. Acta 1983,28,865.
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Langmuir 1986, 2, 700-704
accompanying electrochemical reactions, should greatly enhance our understanding of adsorption processes at electrode surfaces. The excellent agreement between the present results and those of other workers is very encouraging and demonstrates both the accuracy and sensitivity of the technique. Future studies will be directed at systems which have received less attention than Pb on AU.
Many possible uses for the QCM in electrochemistry can be imagined. For example, it could be used in the study of modified electrodes as an aid in the elucidation of the role of the supporting electrolyte in charge propagation or in the study of specific adsorption and the electrical double layer, to name but a few. It is our hope that this new technique will prove to be a powerful tool in the study of electrochemical systems.
Monolayer Studies of a New Surfactant Carboxyporphyrin Michel Ringuet* and Jacques Gagnon Ddpartement de Chimie-Biologie, Universitg d u Qudbec ci Trois-Rivisres, Trois-Rivisres, Qugbec, Canada G9A 5H7
Roger M. Leblanc Centre de Recherche e n Photobiophysique, Universitg d u Qudbec ci Trois-Riuidres, Trois-Rivicres, Qugbec, Canada G9A 5H7 Received March 11, 1986 The surface chemistry of a new surfactant carboxyporphyrin is described. This molecule forms wellbehaved monomolecular layers at the air-water interface. The surface pressure-area isotherms are given for subphases at various pH's and temperatures. Significant pH and temperature effects are obtained. The monolayer films are transferred onto quartz slides by using the Langmuir-Blodgett technique. Spectrophotometric studies of the LB films show an acid-base equilibrationto occur within the monolayer leading to mixtures of free base and dicationic porphyrin. Free baseldication ratios in the various LB films are presented. Introduction Chemists have already produced an array of efficient chemical systems based on the role played by porphyrins in some biological processes. Few examples are the use of porphyrins as photosensitizers for the reduction of water,')2as synthetic complexing agents of molecular oxygeq3 and, more generally, as photocatalysts for organic redox reaction^.^-^ However, it is generally accepted that, in some natural systems like the ones involved in photosynthesis, the proximity and the relative orientation of the porphyrins in the cellular membrane are of critical importance.8 Therefore it is highly possible that a similar organization of the synthetic mimetic systems might lead t,o a greater efficiency. Since organized assemblies like the molecular monolayer at the air-water interface can be taken as a model of the environment of the chromophore in a natural membrane,g many teams, starting with Alexander'O some 50 years ago, (1) Darwent, J. R.; Douglas, P.; Harriman, A.; Porter, G.; Richoux, M.-C. Coord. Chem. Rev. 1982,44, 83-126. (2) Kalyanasundaram, K. J. Chem. SOC.,Faraday Trans. 2 1983, 79, 65-74. (3) Jones, R. D.; Summerville, D. A.; Basolo, F. Chem. Reu. 1979, 79, 139-179. (4) (a) Groves, J. T.; Nemo, T. E. J. Am. Chem. SOC.1983, 105, 6243- 6248. (b) Groves, J. T.; Myers, R. S. Ibid. 1983, 105, 5791-5796. ( 5 ) Irie, R.; Li, X.; Saito, Y. J . Mol. Catal. 1983, 28, 263-265. (6) Callot, H. J.; Metz, F.; Piechocki, C . Tetrahedron 1982, 38, 2365-2369. (7) Mansuy, D.; Fontecave, M.; Bartoli, J. F. J. Chem. SOC.,Chem. Commun. 1983,253-254. (8) Katz, J. J.; Norris, J. R.; Shipman, L. L In Chlorophyll-Proteins, Reaction Centers and Photosynthetic Membranes; Olson, J. M., Hind, G., Eds.; Brookhaven National Lab.: Upton, NY, 1977; pp 16-55. (9) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982.
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worked on the characterization of the molecular interactions of porphyrins in monolayers."-'* Until now, these studies have focused on monolayer films of two types of substrates, the derivatives of 5,10,15,20-tetraphenylporphyrin and of some natural porphyrins. All the tetraphenylporphyrins studied were substituted with ionizable polar groups and some of them were also substituted with long alkyl chains to enhance their surfactant properties and improve their film-forming ability. Even then, only the most recent studies l6-I8 have shown films homogeneous enough to be easily transferred onto glass slides. On the other hand, the natural carboxyporphyrins had to be esterified with a long-chain alcohol to allow for a good film formation."J2J4J7 Unfortunately, this esterification prevented the determination of the effect of the ionized carboxyl groups on the monolayer properties of those natural products. We recently reported the synthesis and a protonation study in solution of a new surfactant porphyrin,lg i.e., (10) Alexander, A. E. J . Chem. SOC.1937, 1813-1816. (11) Bergeron, J. A.; Gaines, G. L., Jr.; Bellamy, W. D. J. Colloid Interface Sci. 1967, 25, 97-106. (12) Hopf, F. R.; Whitten, D. G. J . A m . Chem. Soc. 1976, 98, 7422-7424. (13) Mercer-Smith,J. A,; Whitten, D. G. J. A m . Chem. SOC.1979,101, 6620-6625. (14) Adler, G. J. Colloid Interface Sci. 1979, 72, 164-169. (15) Bull, A. A.; Bulkowski, J. E. J . Colloid Interface Sci. 1979, 92, 1-12. (16) Ruaudel-Teixier,A.; Barraud, A,; Belbeoch, B.;Roulliay, M. Thin Solid Films 1983, 99, 33-40. (17) Jones, R.; Tredgold, R. H.; Hodge, P. Thin Solid Films 1983,99, ~
~~
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(18) Bardwell, J.; Bolton, J. R. Photochem. Photobiol. 1984, 39, 735-746.
0 1986 American Chemical Society
