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Langmuir 1997, 13, 6824-6828
Electrochemical Deposition of Polyborate Monolayers at Ag(111) Electrodes Keith J. Stevenson, David W. Hatchett, and Henry S. White* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 Received July 18, 1997. In Final Form: September 30, 1997X Non-Faradaic electrodeposition of polyborate monolayers at Ag(111) in aqueous solutions containing sodium borate (Na2B4O7) is reported. Electrodeposition is characterized by a very sharp voltammetic wave with half-wave potential E1/2 ≈ -1.2 V vs Hg/Hg2SO4 and surface charge density σ ) 20 ( 5 µC/cm2. X-ray photoelectron spectroscopy and electrochemical quartz crystal microbalance measurements demonstrate that monolayer formation occurs at potentials positive of E1/2. The voltammetric behavior suggests that electrodeposition is associated with the polymeric condensation of B(OH)4- and B(OH)3, both species produced by the dissociation of B4O72- in the bulk solution (B4O72- + 7H2O h 2B(OH)4- + 2B(OH)3). The dependence of the voltammetric wave shape on scan rate is consistent with the nucleation and growth of an ordered polyborate monolayer. Deposition of the monolayer is also observed in boric acid/NaOH solutions, strongly supporting the proposed mechanism.
Introduction We wish to report a preliminary investigation of the electrodeposition of polyborate monolayers on Ag(111) from aqueous sodium borate (Na2B4O7) solutions. Specifically, we present evidence for the coadsorption and surface condensation of boric acid, B(OH)3, and hydroxyborate, B(OH)4-, both species being produced by the dissociation of B4O72-. It is well established that rapid and reversible condensation of B(OH)3 and B(OH)4- yields soluble polyborate anions in aqueous solutions at room temperature.18-22 The results presented here suggest that polymeric condensation also occurs at the Ag(111) surface, yielding an ordered molecular film. Molecular monolayers deposited from borate and boric acid solutions are of interest due to their effect in modifying the rates of oxide film growth1 and metal deposition/ dissolution reactions.2 However, very little is known about the molecular structure of these monolayers. Using surface-enhanced Raman spectroscopy, Herne and Garrell3 observed that B(OH)4- is adsorbed onto the surface of Ag colloids. Bruckenstein and co-workers4 reported that expulsion of BO2- from the Ag/solution interface occurs during the underpotential deposition of Pb and Bi monolayers. In recent studies, we have observed that the voltammetric response of highly oriented and atomically smooth Ag(111) electrodes in Na2B4O7 displays an unusually sharp wave. We present evidence that this voltammetric wave is associated with the formation of a polyborate monolayer. Observation of a voltammetric signal that is associated with B4O72- is itself of fundamental interest, since B4O72- is electrochemically inactive and frequently employed as an inert supporting electrolyte/ buffer in electrochemical investigations. The voltammetric wave we observe reflects a change in the interfacial capacitance of the Ag(111)/solution interface that occurs during the potential-dependent formation of the monolayer. Similar non-Faradaic voltammetric behavior has X Abstract published in Advance ACS Abstracts, November 15, 1997.
(1) Li, Y.; Shimada, H.; Sakairi, M.; Shigyo, K.; Takahashi, H.; Seo, M. J. Electrochem. Soc. 1997, 144, 866. (2) (a) Yin, K.-M.; Lin, B.-T. Surf. Coat. Technol. 1995, 78, 205. (b) Kawas, C.; Hepel, T. J. Electrochem. Soc. 1989, 136, 1672. (c) Kawas, C.; Hepel, T. J. Electrochem. Soc. 1988, 135, 839. (3) Herne, T. M.; Garrell, R. L. Anal. Chem. 1991, 63, 2290. (4) (a) Hepel, M.; Kanige, K.; Bruckenstein, S. Langmuir 1990, 6, 1063. (b) Hepel, M.; Bruckenstein, S.; Kanige, K. J. Chem. Soc., Faraday Trans. 1993, 89, 251.
S0743-7463(97)00811-1 CCC: $14.00
been associated with the deposition of two-dimensional organic adlayers5 and with the potential-induced conformational changes of preadsorbed molecules6 at both liquid and solid metal electrodes. Experimental Section Ag(111) Film Preparation. A detailed description of the preparation and characterization of the highly oriented Ag(111) films has been previously reported.7 A brief description is provided here because the electrochemical response of the Ag(111) electrodes in Na2B4O7 solutions is very sensitive to electrode preparation. Ag films were deposited on mica in a Veeco CVC CVE-20 filament evaporator (TFS Technologies, Albuquerque, NM). A base pressure between 8 × 10-7 and 2 × 10-6 Torr was maintained during deposition using sorption and turbomolecular pumps. Freshly cleaved muscovite mica substrates (thickness ∼ 0.25 mm and area ∼ 3 cm2) were positioned 23 cm above the Ag source using a stainless steel substrate holder. A movable shutter separated the source and the mica substrates. Prior to deposition of Ag, the mica was heated, using backside illumination from two quartz lamps, to 250 °C for 1 h to desorb surface impurities. The sample temperature was monitored using a type K thermocouple sandwiched between two pieces of mica that were attached to the sample holder. Ag shot (99.999%, Alpha/ Aesar) was resistively heated in a Mo boat for 2 min with the shutter closed to outgas impurities in the metal. The shutter was then opened, and Ag was allowed to deposit onto the mica at a rate of 0.2 nm/s, until a film thickness of 300 nm was achieved. Deposition rates and film thicknesses were monitored using a Kronos QM-311 thickness monitor. After the desired film thickness was achieved, the Ag films were annealed at 250 °C for 6 h using the quartz lamp heaters. The lamps were turned off, and the samples were allowed to return to room temperature (5) (a) Scharfe, M.; Hamelin, A.; Buess-Herman, C. Electrochim. Acta 1995, 40, 61. (b) Popov, A.; Naneva, R.; Dimitrov, N.; Vitanov, T.; Bostanov, V.; de Levie, R. Electrochim. Acta 1992, 37, 2369. (c) Holzle, M. H.; Kolb. D. M. Ber. Bunsen-Ges. Phys. Chem. 1994, 98, 330. (d) Buess-Herman, C. Prog. Surf. Sci. 1994, 46, 335. (e) Popov, A. Electrochim. Acta 1992, 40, 551. (f) Wandlowski, T. J. Electroanal. Chem. 1995, 395, 83. (g) Holzle, M. H.; Krznaric, D.; Kolb, D. M. J. Electroanal. Chem. 1995, 386, 235. (h) Wandlowski, T.; Jameson, G. B.; de Levie, R. J. Phys. Chem. 1993, 97, 10119. (i) Wadlowski, T.; de Levie, R. J. Electroanal. Chem. 1995, 380, 201. (j) Wandlowski, T.; Jameson, G. B.; de Levie, R. J. Electroanal. Chem. 1994, 379, 215. (6) (a) Gao, X.; White, H. S.; Chen, S.; Abruna, H. D. Langmuir 1995, 11, 4554. (b) Pagano, R. E.; Miller, I. R. J. Colloid Interface Sci. 1973, 45, 126. (c) Lecompte, M. F.; Miller, I. R. Bioelectrochem. Bioenerg. 1988, 20, 99. (d) Lecompte, M. F.; Miller, I. R. J. Colloid Interface Sci. 1988, 123, 259. (e) Lecompte, M. F.; Miller, I. R. Biochemistry 1980, 19, 3439. (f) Lecompte, M. F.; Clavillier, J.; Dode, C.; Elion, J.; Miller, I. R. Bioelectrochem. Bioenerg. 1984, 13, 211. (7) Stevenson, K. J.; Hatchett, D. W.; White, H. S. Langmuir 1996, 12, 494.
© 1997 American Chemical Society
Polyborate Monolayers at Ag(111) Electrodes (∼3 h). The evaporation chamber was vented to the atmosphere, and the samples were immediately placed in a desiccator filled with N2. Surface Characterization. Scanning tunneling microscopy and glancing angle X-ray crystallographic diffraction measurements indicate that the Ag films have a high degree of (111) orientation and are atomically smooth with terrace widths of the order of ∼100 nm.7 In contrast, films that are not annealed displayed a lower degree of (111) orientation and a significantly rougher surface. The electrochemical response of electrodes prepared from unannealed films is also significantly less well resolved, as demonstrated below in the discussion of borate adsorption. The XPS data were obtained using a VG ESCA lab 220i multianalyzing instrument with Al KR radiation of 1486.7 eV and an electron take off angle of 90° with respect to the sample surface. The X-ray gun (200-µm spot size) was positioned at 45° with respect to the entrance axis of the hemispherical kinetic energy analyzer. The pass energy for the survey scans was set to 100 eV while that for the high resolution scans was set to 30 eV. The binding energy scale was calibrated with respect to the C 1s binding energy of 284.6 ( 0.1 eV. Materials and Electrochemical Apparatus. Solutions were prepared using water obtained from a Barnstead water purification system (“E-pure”) with the feed water inlet connected to an in-house deionized water line. Na2B4O7 (Aldrich, 99.999%), B(OH)3 (Malinckrodt, 99.9%), and NaOH (Malinckrodt, 99.6%) were used as received. A one-compartment, three-electrode glass cell was used for the electrochemical measurements. Pt wire and Hg/Hg2SO4, K2SO4 (saturated) electrodes were employed as the counter and reference electrodes, respectively. All electrode potentials are reported with respect to Hg/Hg2SO4, K2SO4 (saturated), which is ca. 0.44 V positive of Ag/AgCl, KCl (saturated). Ag(111) electrodes were prepared by cutting the mica/Ag(111) substrates into approximately 1 cm2 pieces. Electrical contact to the Ag(111) film was made using a spring-loaded Cu clip. Geometrical electrode areas were determined, after removal of the electrode from the solution, by measuring, using a micrometer, the length of the electrode wetted by the solution. Electrochemically active electrode areas were determined by measuring the integrated anodic charge, Qelec, associated with the underpotential deposition of Pb on the Ag(111) film. The surface roughness factor (RF) was found by dividing Qelec by the theoretical charge expected for a closed packed monolayer of Pb on a Ag(111) film (Qtheo ) 318 µC/cm2).7,8 A RF of 1.04 ( 0.04 was determined for the Ag(111) films used in this study; all charge density data reported herein are corrected using this value. Specific values of the geometrical electrode areas are listed in figure captions. Solutions were bubbled for ∼20 min with N2 before electrochemical analysis to remove O2 from the solution, and a positive pressure of N2 was maintained over the solution during the measurements. The cell temperature was 25 ( 2 °C. Voltammetric measurements were performed using an EG&G Princeton Applied Research model 173 potentiostat/galvanostat, a model 175 universal programmer, and a Kipp & Zonen model BD 90 XY recorder. pH was measured using an Orion Model 910600 combination electrode connected to a Corning Model 320 pH meter. The electrochemical quartz crystal microbalance (EQCM) has been previously described.9,10 The electrode areas for the voltammetric and frequency responses were 0.399 and 0.178 cm2, respectively. A home-made oscillator circuit was used to drive the crystal at its fundamental resonant frequency (5 MHz). The frequency and current were monitored at 1.0 s intervals with a Hewlett Packard 5384A frequency counter and a Keithley 195A digital multimeter, interfaced with a Macintosh Centris 650 computer.
Results and Discussion The voltammetric behavior of a Ag(111) electrode in an aqueous solution containing 0.05 M Na2B4O7 is shown in (8) Toney, M. F.; Gordon, J. G.; Samant, M. G.; Borges, G. L.; Melroy, O. R.; Yee, D.; Sorensen, L. B. J. Phys. Chem. 1995, 99, 4733. (9) Ward, M. D. J. Phys. Chem. 1988, 92, 2049. (10) Hatchett, D. W.; Gao, X.; Catron, S. C.; White, H. S. J. Phys. Chem. 1996, 100, 331.
Langmuir, Vol. 13, No. 25, 1997 6825
Figure 1. Voltammetric response of a 6-h annealed Ag(111) electrode (area ) 0.42 cm2) in a 0.05 M Na2B4O7 solution. Scan rate ) 0.1 V/s. Inset: Comparison of the voltammetric responses of a thermally annealed Ag(111) electrode (solid line) and an unannealed electrode (dashed line) of equal area (0.25 cm2) in a 0.05 M Na2B4O7 solution.
Figure 1. The voltammetric response is characterized by a single sharp wave, with a half-wave potential, E1/2, of -1.17 V vs Hg/Hg2SO4, K2SO4 (saturated). The charge density, σ, obtained by integration of the anodic peak, is equal to 22.1 µC/cm2. The symmetry and sharpness of the wave indicate that the voltammetric response corresponds to a reaction involving an adsorbed species. Since B4O72- is electrochemically inactive within the potential range of the voltammetric scan,11 we attribute the sharp voltammetric peak to the adsorption of B4O72- or to the adsorption of a chemical species derived from B4O72-. Thus, the voltammetric currents are associated with the rapid variation in interfacial capacitance at electrode potentials at which adsorption occurs rather than with a conventional redox process.6a X-ray photoelectron spectroscopy (XPS) and electrochemical quartz crystal microbalance measurements presented below demonstrate that reversible adsorption of a borate monolayer occurs at electrode potentials positive of E1/2. No additional voltammetric peaks are observed between E1/2 and the onset of Ag dissolution (∼ -0.1 V vs Hg/Hg2SO4). The observed E1/2 is located near the potential-of-zero charge (pzc) of Ag(111) in weakly adsorbing electrolytes, e.g., ∼ -1.1 V vs Hg/Hg2SO4 in NaF. This finding, coupled with the knowledge that adsorption occurs at electrode potentials positive of E1/2, is consistent with adsorption of an anionic species. The larger capacitive currents at potentials positive of E1/2 suggest that adsorption results in an increase in the capacitance of the Ag(111)/solution interface. The voltammetric wave is very sensitive to the preparation of the Ag(111) electrode. Specifically, the inset of Figure 1 shows that a poorly defined voltammogram is obtained at Ag electrodes that have not been thermally annealed, in contrast to the sharp voltammetric peaks observed at the 6-h thermally annealed Ag(111) films. Since thermal annealing of the Ag(111) film results in a significantly smoother electrode surface, the difference in these voltammetric responses suggests that the sharp voltammetric wave is associated with long-range struc(11) Bellavance, M. I.; Miller, B. In Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker, Inc.: New York, 1974; Vol. II, Chapter 1, p 1. (12) Hamelin, A.; Stoicoviciu, L.; Doubova, L.; Trasatti, S. Surf. Sci. 1988, 201, L498.
6826 Langmuir, Vol. 13, No. 25, 1997
Figure 2. XPS data obtained for two Ag(111) electrodes poised at (I) -1.09 V and (II) -1.25 V vs Hg/Hg2SO4. Inset: Voltammetric response of a Ag(111) electrode (area ) 0.88 cm2) in a 0.1 M Na2B4O7 solution indicating points I and II. Scan rate ) 0.1 V/s.
tural ordering of the monolayer. Similar voltammetric responses are observed for the underpotential deposition (upd) of foreign metal monolayers on metal electrodes, where strong attractive interactions dominate the adsorption process, and monolayer formation follows a nucleation and growth mechanism.13 XPS was used to determine if borate adsorption occurs at potentials negative or positive of E1/2. Figure 2 shows the XPS spectra for two Ag(111) electrodes that were voltammetrically cycled in a 0.1 M B4O72- solution and then removed at potentials negative and positive of the voltammetric peak (see potentials labeled I and II on the voltammogram shown in the inset of Figure 2). The electrodes were removed while under potential control, rinsed twice with distilled water, dried under a stream of N2, and analyzed by XPS within 30 min. No detectable adsorption of a boron-containing species is observed at potentials negative of the voltammetric wave (-1.25 V, potential II), as evidenced by the absence of a B 1s signal in the XPS spectrum. However, a strong XPS signal at 192.3 eV is observed for the electrode removed at potentials positive of the voltammetric peak (-1.09 V, potential I). The 192.3 eV binding energy is within the range of B 1s binding energies (192-193 eV) reported for Na2B4O7, B(OH)3, and B2O3.14 Thus, the XPS data clearly demonstrate that the sharp voltammetric wave is associated with reversible adsorption of a boron-containing species at potentials positive of E1/2. Quantitation of the XPS spectra yields an elemental boron-to-oxygen ratio of ∼0.42, consistent with the adsorption of a borate species. The potential-dependent adsorption isotherm of the borate species was determined using a 5 MHz electrochemical quartz crystal microbalance (EQCM). Measurements were made using a quartz crystal onto which Ag electrodes were thermally deposited and annealed in vacuum.10 Figure 3 shows the voltammetric and frequency responses, recorded simultaneously in an EQCM experiment. The voltammetric response for the Ag-coated quartz crystal in contact with a 0.1 M Na2B4O7 solution is not as well resolved as that observed for the annealed mica/Ag(13) (a) Juttner, K.; Lorenz, W. J. Z. Phys. Chem. Neue Folge 1980, 122, 163. (b) Kolb, D. In Advances in Electrochemistry and Electrochemical Engineering; Gericher, H., Tobais, C. W., Eds.; John Wiley and Sons: New York; 1978; Vol. II, p 125. (c) Kolb, D.; Przasnyski, M.; Gericher, H. J. Electroanal. Chem. 1974, 54, 25. (d) Harrison, J. A.; Thirsk, H. R. In Electroanalytical Chemistry; Bard, A. J., Ed.; Academic Press: New York, 1971; Vol. 5, p 67. (e) Fletcher, S. J. Electroanal. Chem. 1981, 118, 419. (14) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., King, R. C., Eds.; Physical Electronics, Inc.: Eden Prairie, MN, 1995; p 39.
Stevenson et al.
Figure 3. Voltammetric and frequency response of a Ag-coated quartz crystal in a 0.1 M Na2B4O7 solution. Scan rate ) 0.1 V/s.
(111) electrodes, reflecting the fact that the Ag films deposited on quartz substrates are significantly rougher and less crystallographically oriented. However, a reversible 3.7 ( 0.4 Hz decrease in the frequency response is observed upon scanning the electrode potential in the positive direction, indicating, in agreement with the XPS data, that adsorption occurs at potentials positive of E1/2. The Sauerbrey equation15 (eq 1) allows for the estimation
∆f ) -C∆m
(1)
of the interfacial mass change, ∆m (µg cm-2), from the measured frequency change, ∆f (Hz). In eq 1, C is the proportionality constant for the 5 MHz quartz crystal employed in this study ()56.6 Hz cm2 µg-1). After correcting for the surface roughness of the Ag film electrode deposited on quartz (RF ) 1.3), the observed 3.7 ( 0.4 Hz frequency change corresponds to the deposition of 5.0 ( 1.2 × 10-2 µg/cm2. Coulometric integration of the voltammetric wave measured during the EQCM experiment yields ∼20 µC/cm2, consistent with the data obtained for the thermally annealed mica/Ag(111) electrodes. Thus, the mass deposited per unit area on the Ag electrode in the EQCM experiment is approximately equal to that for the more highly oriented mica/Ag(111) electrodes. Assuming that the adsorbed species has a molecular weight that is approximately equal to that of B4O72- (155.2 g/mol), the mass deposited per unit area corresponds to a surface coverage of 3.2 × 10-10 mol/cm2. This value is typical of monolayer coverages of small inorganic and organic molecules adsorbed on metal surfaces. The dependencies of the voltammetric peak current (ip), peak full-width at half-height (Efwhh), and peak splitting (∆Ep ) Epa - Epc, where Epa and Epc are anodic and cathodic peak potentials) on scan rate, ν, provide evidence for longrange ordering within the electrodeposited molecular film. Figure 4 shows that ip is proportional to ν2/3 (both anodic and cathodic waves) and that both Efwhh and ∆Ep are proportional to ν1/3. The negative intercepts for these plots reflect the small error (∼20%) associated with subtraction of the voltammetric peaks from the baseline charging current. The ν2/3 and ν1/3 dependencies are predicted on the basis of theoretical models of voltammetric behavior associated with the nucleation and growth of twodimensional molecular films.16 In contrast, for simple adsorption of non-interacting redox active molecules, ip is (15) Sauerbrey, G. Z. Phys. 1959, 155, 206. (16) (a) Maestre, M. S.; Rodriguez-Amaro, R.; Munoz, E.; Ruiz, J. J.; Camacho, L. J. Electroanal. Chem. 1994, 373, 31. (b) Prieto, I.; Martin, M. T.; Munoz, E.; Ruiz, J. J.; Camacho, L. J. Electroanal. Chem. 1997, 424, 113.
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Langmuir, Vol. 13, No. 25, 1997 6827
Figure 4. Plot of (A) the anodic peak currents (ip) vs ν2/3 and (B) the peak full-width at half-height (Efwhh) and peak splitting (∆Ep) vs ν1/3 for a Ag(111) electrode (electrode area ) 0.72 cm2) in a 0.05 M Na2B4O7 solution.
expected to be proportional to ν, and both Efwhh and ∆Ep are expected to be independent of ν.17 In order to determine the nature of the adsorbed borate species, it is useful to consider the various chemical equilibria associated with aqueous B4O72- solutions. It is well-known that Na2B4O7 is an excellent buffer (pH ∼ 9), dissociating completely into equal concentrations of neutral boric acid, B(OH)3, and hydroxyborate, B(OH)4-, (eq 2).18
B4O72- + 7H2O h 2B(OH)4- + 2B(OH)3
(2)
In turn, B(OH)4- and B(OH)3 undergo condensation reactions to form anionic polyborates. For instance, the trimeric borate species, B3O3(OH)4-, is produced (eq 3) in
2B(OH)3 + B(OH)4- h B3O3(OH)4- + 3H2O (3) solutions containing a total boron concentration greater than 0.025 M.19 The formation of higher molecular weight polyborates, e.g., B4O5(OH)42- and B5O6(OH)4-, occurs in more concentrated borate solutions. The kinetics of polyborate formation have also been investigated and found to be relatively fast.20 For instance, the rate constant for the formation of B3O3(OH)4- (eq 3) at pH ∼9 is reported to be 3.2 × 103 M-2 s-1.21 (17) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley and Sons: New York, 1980; p 522. (18) Nies, N. P.; Campbell, G. W. In Boron, Metallo-Boron Compounds and Boranes; Adams, R. M., Ed.; Interscience Publishers: New York, 1964; p 85. (19) Nies, N. P.; Campbell, G. W. In Boron, Metallo-Boron Compounds and Boranes; Adams, R. M., Ed.; Interscience Publishers: New York, 1964; p 87. (20) (a) Momii, R. K.; Nachtrieb, N. H. Inorg. Chem. 1967, 6, 1189. (b) Salentine, C. G. Inorg. Chem. 1983, 22, 3920. (c) Smith, D. H.; Wiersema, R. J. Inorg. Chem. 1972, 11, 1152. (d) Maya, L. Inorg. Chem. 1976, 15, 2179. (21) Anderson, J. L.; Eyring, E. M.; Whittaker, M. P. J. Phys. Chem. 1964, 68, 1128.
Figure 5. Voltammetric response of a Ag(111) electrode (area ) 0.43 cm2) as a function of Na2B4O7 concentration. Solution concentrations (from top to bottom): 0.2, 0.1, 0.05, 0.035, 0.025, and 0.010 M Na2B4O7. Scan rate ) 0.1 V/s.
The formation of a polyborate species at the Ag(111) surface by similar condensation reactions appears consistent with the electrochemical data. In particular, the voltammetric wave shape and scan rate dependencies of ip, ∆Ep, and Efwhh are all strongly suggestive of attractive intermolecular interactions between the adsorbed species, consistent with the deposition of oligomeric or polymeric borates. To explore this possibility, we measured the voltammetric response of Ag(111) electrodes as a function of the bulk B4O72- concentration. The results are shown in Figure 5. At [B4O72-] < 0.01 M, the voltammetric wave is relatively broad (∼150 mV), suggesting that attractive intermolecular interactions are greatly reduced at low B4O72- concentrations, conditions that favor the dissociation of a polyborate species. The voltammetric peak becomes very sharp at [B4O72-] ∼ 0.035 M, consistent with the range of concentrations in which polyborates are reported to first appear in aqueous solutions. Figure 5 also shows that the voltammetric wave shifts toward negative potentials with increasing [B4O72-], in agreement with the expected shift in the electrode pzc resulting from adsorption of anionic molecules. A mechanism involving adsorption of polyborates is also supported by the finding that the voltammetric response at Ag(111) in boric acid/NaOH solutions is essentially identical to that shown in Figure 1. The reaction of boric acid, B(OH)3, with strong bases in aqueous solution produces B(OH)4- (eq 4).
B(OH)3 + OH- h B(OH)4-
(4)
Thus, B(OH)3 and B(OH)4- coexist at pH’s near the pKa of B(OH)3, producing the same polyborate species that are formed following the dissociation of B4O72-. (The formation of anionic polyborates in aqueous boric acid/
6828 Langmuir, Vol. 13, No. 25, 1997
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NaOH solutions has been documented by Ingri22 and others.21,23) Assuming that the voltammetric behavior is associated with deposition of a polyborate monolayer, the voltammetric curves recorded in B(OH)3/NaOH and B4O72solutions should be identical. Figure 6 shows the voltammetric response of a Ag(111) electrode in a 0.2 M B(OH)3 solution as a function of pH. At low pH (∼4), the voltammetric response exhibits Ohmic behavior, reflecting the fact that boric acid is a weak acid (pKa ) 9.2),24 resulting in relatively few free ions in solution. As the pH is increased, B(OH)4- is generated by eq 4, and the voltammetric wave associated with borate adsorption sharpens and shifts to negative potentials. The voltammetric response recorded at pH ) 9.3 is essentially identical to that obtained in the 0.1 M B4O72- solution, demonstrating that electrodeposition in these solutions must involve the same chemical species, i.e., B(OH)4- and B(OH)3. Conclusion The adsorption of borate on highly oriented Ag(111) electrodes is characterized by a single sharp voltammetric wave, suggestive of strong lateral interactions between the adsorbed species. XPS and EQCM data demonstrate that adsorption occurs at potentials positive of the voltammetric wave, resulting in the deposition of ca. one monolayer. A mechanism based on the solution chemical equilibria of B4O72- has been presented which suggests that monolayer formation results from codeposition and condensation of B(OH)4- and B(OH)3. Spectroscopic (22) (a) Ingri, N.; Lagerstrom, G.; Frydman, M.; Sillen, L. G. Acta Chem. Scand. 1957, 11, 1034. (b) Ingri, N. Acta Chem. Scand. 1962, 16, 439. (c) Ingri, N. Acta Chem. Scand. 1963, 17, 573. (d) Ingri, N. Acta Chem. Scand. 1963, 17, 581. (23) (a) Edwards, J. O. J. Am. Chem. Soc. 1953, 75, 6154. (b) Mesmer, R. E.; Baes, C. F.; Sweeton, F. H. Inorg. Chem. 1972, 11, 537. (c) Waton, G.; Mallo, P.; Candau, S. J. J. Phys. Chem. 1984, 88, 3301. (d) Maeda, M. J. Inorg. Nucl. Chem. 1979, 41, 1217. (24) Owen, B. B. J. Am. Chem. Soc. 1934, 56, 1695.
Figure 6. Voltammetric response of a Ag(111) electrode (area ) 0.72 cm2) in a 0.2 M B(OH)3 solution as a function of solution pH. Scan rate ) 0.1 V/s.
studies are currently underway to determine the precise molecular structure of the monolayer. Acknowledgment. This research was supported by the Office of Naval Research. The assistance of Mr. Andrew D. Vogt with the collection of the XPS data is greatly appreciated. The authors gratefully acknowledge informative discussions with Dr. Robert Parry regarding the chemistry of borates. LA970811P
