Anal. Chem. 1995, 67,2415-2418
Simultaneous Normalized Optical Reflectivity and Microgravimetric Measurements at Electrode/ Electrolyte Interfaces: The Adsorption of Bromide on Gold in Aqueous Media Yibo Mo, Euijin Hwang, and Daniel A. &herson*
Department of Chemistty, Case Westem Reserve Univetsiiy, Cleveland. Ohio 44 106
A system is herein described for the simultaneous acquisition of in situ microgravimetric (QCM) and UV-visible reflectance data (ARIR)of metal film electrodes sputtered on the surface of optically polished quartz crystals. 'Ihis dual-technique approach makes it possible to establish correlations between the changes in weight and relative reflectivities associated with the adsorption of ions (and other species) on the electrode surface as a function of the applied potential without uncertainties derived from electrode surface preparation. Simultaneous in situ QCM-ARIR measurementsfor the adsorption of bromide on gold from aqueous electrolytes yielded results in excellent agreementwith those reported by other authors for each independent technique. In particular, plots of the change in the resonantfrequency of the QCM vs ARIR were found to be linear in the potential region in which bromide adsorption is expected to occur for concentrations of bromide spanning more than 2 orders of magnitude. The optical reflectivity of metals in electrochemical environments is often modified by t h e detailed nature of the surface microtopography and by the state of charge and composition of the metal/electrolyte interface.' This effect provides a basis for monitoring in situ processes of fundamental and technological importance, such as surface reconstruction2 and atomic and molecular adsorption? as a function of the applied potential. In the case of interfacial adsorption, the normalized reflectivity change, defined as W R = (Rads- R,)/R,, where Radsand R, are the reflectivitieswith and without the adsorbate, respectively, may amount to a few percent, even for coverages on the order of a single monolayer.1 Despite the relative ease by which AR/R can be detected with rather conventionalinstrumentation,a theoretical basis for the quantitative understanding of reflectance spectroscopy in the UV-visible range (includingellipsometry),particularly for adsorbed layers of a thickness on the order of a single atom or molecule, has not as yet been fully developed. Much of the difficultiesstem from the fact that the formation of the adsorbate/ substrate interface gives rise to changes in the intrinsic properties ~~~~
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(1) Kolb, D. M. In Spectroelectrochemisty; Gale, R., Ed.; Plenum Press: New
York 1988;Chapter 4. (2) (a) Kolb, D. M.; Schneider, J. Surf: Sci. 1985, 162, 764. (b) Kolb. D.M.; Schneider, J. Electrochim. Acta 1986,31, 929. (3) See, for example: (a) Kim, S.; Xu,X.; Bae, I. T.; Wang, 2.; Scherson. D. A Anal. Chem. 1990,62,2647.(b) Kim, S.;Scherson. D. A Anal. Chem. 1992, 64,3091 and references therein.
0003-2700/95/0367-2415$9.00/0 0 1995 American Chemical Society
of the two phases; therefore, the optical properties of the adsorbed phase cannot be extracted from a simple threelayer model by assuming that the optical constants of the substrate remain invariant upon adsorption. Efforts have been made to establish experimental correlations between AR/R and the surface coverage using simple species as model systems. Adzic et a1.,4 for example, provided evidence that, in the case of bromide adsorption on gold, AR/R varies linearly with the surface coverage, without making any assumptions regarding the extent of charge transfer involved in the adsorption step. Such conclusions were made on the basis of experiments performed in very dilute bromide solutions in which W R was monitored as a function of time following a potential step from a value sufficiently negative to desorb any adsorbed bromide from the surface to a value positive enough for adsorption of the species to ensue. Under these conditions, the adsorption of bromide is controlled purely by diffusion, and therefore the coverage, at least during the initial stage, can be assumed to follow a square root dependence with respect to time. As shown by these workers, plots of AR/R vs t1I2were found to be linear, lending strong support to their claims. This paper describes an instrument capable of providing simultaneously in situ electrochemical, AR/R, and microgravimetric data using a quartz crystal microbalance (QChl).5 This multi-technique approach offers a significant advantage, as it enables correlations between three physical properties to be explored without uncertainties derived from electrode surface preparation. A similar strategy was exploited by Gottesfeld and co-workers,6who combined in situ spectroscopicellipsometry and QCM to monitor the optical properties of an electrogenerated polymer as a function of its thickness (or mass). More recently, Xie et coupled reflectance spectroscopy and QCM to examine the reduction of an ammonium copper complex in the solution phase. Particularly noteworthy, however, is the work of Shimazu et al.,swho succeeded in producing sufficientlythin electrodes to investigate in the transmission mode the optical and gravimetric changes accompanying the redox transition of a species adsorbed from the solution phase. (4) Adzic, R.; Yeager, E. B.; Cahan, B. D. J. Elecfroanal. Chem. 1977,85,267. (5) Butby. D. In Electrochemical Interfaces;Abmna, H. D., Ed.; VCH: New York, 1991;Chapter 10. (6) Rishpon, J.; Redondo, A; Derouin, C.; Gottesfeld, S. /. Electroanal. Chem.
1990,294, 73. (7) Xie, Q.; Shen, D.; Nie, L.; Yao, S. Electrochim. Acta 1993,38, 2277. (8)Shimazu, K.; Yanagida, M.; Uosaki. K. J. Electroanal. Chem. 1993,350,
321.
Analytical Chemistty, Vol. 67, No. 14,July 15,1995 2415
The capabilities of this new apparatus were assessed using, in addition to bromide adsorption on Au from aqueous solutions, selected interfacial adsorption processes on Au electrodes for which both AR/R and QCM measurements have been reported independently by other research groups. EXPERIMENTAL SECTION
Preparation and Electrochemical Characterization of Quartz Crystal Microbalances for SimultaneousMicrogravimehic/Spectroelectrochemical Measurements. Quartz crys
tal microbalances were prepared by sputtering sequentially 5 nm of Ni and 520 um Au for the working electrode and 20 nm Ni and 100 nm Au for the back electrode onto optically polished 6 MHz ATcut quartz crystals Walpey-Fisher). Prior to their transfer to the dual-target, metal sputtering apparatus (Materials Research Corp.. SEM-8620). the bare crystals were cleaned using a mixture of nitric and hydrochloric acids, degreased with Alconox under ultrasonic agitation, rinsed with ultrapure water, and finally dried in a nitrogen stream. Once the two sides of the QCM had been sputtered and prior to the microgravimehic/spectroelectrochemical experiments, the working electrodeswere annealed in the open laboratory onto a preheated graphite plate with a hydrogen microtorch for about 1 min. This procedure was found to yield high quality gold surfaces with no evidence for the presence of pinholes? displaying, in certain cases, voltammetric features in perchloric and sulfuric acids characteristic of Au(ll1) surfaces. Spectroelectmchemical CeU. A schematic diagram of the spectroelectrochemical cell for simultaneous W-visible reklectance spectroscopy and microgravimetric measurements is shown in Figure 1. The main body of the cell (A) is a Teflon cylinder with three oriIices drilled on its side, which houses a loopshaped gold counterelectrode (CE) around the working electrode and away from the path of the optical beam, and the inlet and outlet press-fit Teflon tubes (a and b) for the electrolyte and an external reference electrode compartment. The Au/Ni coated QCM is mounted in between two O-rings in a special threepiece assembly consisting of a Kel-F threaded block (B). a brass insert (C) (which prevents the back @ringfrom rotating while the assembly is being tightened), and a W C threaded block. The O-rings are compressed by screwing the W C block into the Kel-F Teflon piece until the pressure is sufficient to avoid electrolyte leakage. This assembly fits into a cylindrical cavity machined in the main Teflon cell body (A) and is compressed against an O-ring resting on a circular ledge inside (A) using an aluminum plate E. Electrical contact to the front face of the QCM or working electrode is made with a Cu-Be wire in the form of a loop through a small hole drilled in the Kel-F piece, which touches areas outside the circular region defined by the O-ring. The same approach was used to connect the back side of the QCM through the large opening in the W C block. The main compartment of the cell is formed by compressing a flat optical quartz window (Qw) against a Won O-ring placed in a groove drilled on the front of (A). using a beveled aluminum plate 0 in the front and (E) in the back of the cell. One of the Teflon tubes, acting as a Luggin capillary. is attached to a separate
1
r 1
Quartz Window
Light in
,
F
Light out
Figure 1. Schematicdiagram of the spectroelectrochemical cell for SimultaneOuS UV-visible reflectance spectroscopy and microgravimetric measurements.
reservoir filled with electrolyte that houses a saturated calomel reference electrode (SCE. not shown in the figure). Electronics. The circuit involved in the QCM/electrochemical part of the measurements was similar in design to that described by Melroy et al.,'" except that the single QCM was changed into a dual QCM by incorporating an additional crystal into the board box" to reduce long-term drifts derived from temperature changes. The difference in the frequency between the two oscillators was measured with an HP5313lA universal counter. Microgravimetricdata are reported in this work in terms of mass change, Am,using the frequency observed at a judiciously selected potential as a reference and assuming that the observed 4fis due entirely to Am." Standardization of the QCM. Prior to performing simultaneous spectroelec~ochemical/microgravimehic experiments. the proper operation of the QCM was tested by inserting a holder arrangement essentially identical to that used for the optical cell described above into a simple (nonoptical) &Teflon electrochemical cell. Experiments involving the electrodeposition of Ag on Au from 0.50 mM AgCIOl in 0.10 M HCIOl solutions'" yielded plots of mass, as measured by the QCM. vs charge in agreement with Faraday's law, indicating that within the range of masses examined the QCM displays ideal behavior. Optical System and Other Considerations. Reflectance spectroscopy measurements were performed with an instrument (IO) Melmy. 0.:Kanwawa. K Gordon. J. G..II:Buttry. D. Longnuir 1986.2. w7 I".. (11) A dual QCM system far in situ measurements in which both crystals are
immersed in the electrolyte has been described recently by Bruckenstein. S.: Michalski. M.: Fensore. k: Li. 2.: Hillman. A R. Ami. C h m . 1994.66.
1847.
(9) A series of cyclic voltammetly measurements performed at very slow scan rates in =id media yielded "x in excellentagreement with t h e b e l i e d 10 be characteristic of Au(poly) orAu(t11).On this basis. the possibility of pinholes being present in thio films (which would have exposed Ni to the media) can
2416
be safely excluded.
AnalyiicalChemisVy, Vol. 67. No. 14, July 15, 1995
(12) Effects due to changes in the viscosity and density 01 the solution of the type reported In Lee. W.-W.: White. H. S.: Ward. M. D. And. Chmm. 1993. 65.3232. are negkded in this work. (13) Kana.mwa. K. Borgepes. G . L: Doss. S.: Hildebrand. C. Pmc,h l . Coni Surf Finish. 1994.
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Potential (V vs. SCE) Figure 3. Cyclic voltammetry and Am vs potential curves for a carefully annealed crystal displaying A u ( l l 1 ) character in 0.1 M HCIO.+ Scan rate, 20 mV/s. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Potential (V vs. SCE) Figure 2. Cyclic voltammetry, Am and AR/R vs potential obtained simultaneously for a Au(poly) electrode in 0.1 M HClOl solutions. Scan rate, 20 mV/s. Wavelength, 540 nm (nonpolarized light); Ro = 0.1 V vs SCE. All the curves represent the average of nine consecutive scans.
described in detail elsewhere14 at an angle of incidence of 45" with either unpolarized or ppolarized light. Microgravimetric/ spectroelectrochemical experiments involving bromide were performed at a wavelength of 500 nm in ultrapure water 0.1 M NaF solutions containing bromide in the range 50 pM to 2.5 mM. For the few experiments conducted in 0.1 M HC104,the wavelength of the unpolarized light was set at 540 nm. The operation of the entire system was controlled by a personal computer which enabled the simultaneous collection of electrochemical, microgravimetric, and optical reflectivity data. In addition to measurements involving Au in an aqueous perchloric acid solution, for which data are presented in the next section, the performance of the complete instrumental array was assessed quantitatively, using the underpotential deposition of copper on Au(ll1) in sulfuric acid electrolytes as a model system. Although not shown in this work, the dual-technique results obtained for the latter system were in excellent agreement with those reported in the literature.15J6 RESULTS AND DISCUSSION
Oxide Formation and Reduction on Gold Electrodes in Acid Media. The cyclic voltammogram, Am and AR/R (540 nm), (14) Zhao, M.; Scherson, D. A. J. Electrochem. SOC.1993, 140, 729. (15) Borges, G. L.; Kanazawa, K. IC; Gordon, J. G.; Ashley, K; Richer, J. J. Electroanul. Chem. 1994, 364, 281. (16) Kolb, D.M.; Leutoff, D.; Przasnyski M. Sulf: Sci. 1975, 47, 622.
vs potential (R, at +0.1 V vs SCE) curves for an annealed gold electrode recorded simultaneously in ultrapure 0.1 M HClOd are shown in Figure 2. As reported by other authors, the formation of the oxide is accompanied by a gain in m a d 7 and a marked decrease in refle~tivity.~J~J~ The charge to mass-change ratio (&,,/Am& associated with oxide formation, as calculated over the potential range 0.91-1.35 V, vs SCE in the scan in the positive direction yielded for these experiments a value of (524 pC/cmz)/ (43.8 ng/cm2) = 12.0pC/ng. This latter parameter is independent of the actual area of the electrode and nearly identical to that reported by Bruckenstein et al.li for Au electrodes in 0.2 M HClOI, i.e., 12.2 pC/ng. Furthermore, the magnitude of Q,, is consistent with a surface roughness of about 1.2. The gain in mass observed in the scan in the positive direction between ca. 0.15-0.8 Vvs SCE, Le., doublelayer region, is similar to that reported by Bruckenstein et al.li and more recently by Gordon et al.?O who ascribed the phenomenon to specific adsorp tion of anions and an increase in surface hydration, respectively. This peculiar effect appears to be correlated with the point of zero charge (pzc) , as evidenced by the results obtained with a carefully annealed Au surface displaying (111) character (Figure 3, pzc ca. 0.3 V vs SCE),21for which the onset of the mass increase occurs about 0.2-0.3 V more positive than for the Au(po1y) substrate in Figure 2 (pzc ca. 0.0 V vs SCE).21 The magnitude of AR/R associated with gold oxide formation (see lower panel in Figure 2) was ca. 4.5% and thus intermediate between values reported (17) Bruckenstein, S.; Shay, M. J. Electrounul. Chem. 1985, 188, 131. (18) Rath. D. L.; Hansen, W. N. Sulf: Sci. 1984, 136, 195. (19) Sedlmaier, H. D.; Plieth, W. J. J. Electroanal. Chem. 1984, 180,219. (20) Gordon, J. S.; Johnson, D. C. J Electrounul. Chem. 1994, 365, 267. (21) Lecour, J.; Andro, J.; Parsons, R. Croat. Chim. Acta 1980, 53, 1980 and references therein.
Analytical Chemistry, Vol. 67,No. 14, July 15, 1995
2417
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Potential ( V vs. SCE ) Figure 4. Plots of Am (panel A) and AR/R (panel 6)vs potential data for a Au(poly) crystal acquired simultaneously in 0.1 M NaF solutions containing different amounts of bromide. Scan rate, 10 mV/ s. All curves shown in this figure were recorded while the potential was being scanned in the negative direction.
by Kolb for Au(100) in 0.01 M HClOl at this wavelength for p and s-polarized light,' Le., 2 and 5%,respectively. Adsorption of Bromide on Au(poiy). Panels A and B in Figure 4 show mass change and AR/R data acquired simultaneously in 0.1 M NaF solutions containing three different amounts of bromide. With the exception of the most dilute bromide solution examined, for which diffusion effects cannot be ignored, all curves are remarkably similar in shape. This may not be surprising, since an increase in the (bulk) concentration of the adsorbate gives rise to a shift of the entire adsorption curve toward more negative potentials. These results are in excellent agreement with those reported independently by other authors. In particular, Deakin et aLZ2obtained a net gain of 108 ng/cm2 for a saturation coverage of bromide on gold in 1 mM NaBr/50 mM NaC104 solutions, which compares very well with the value of 103 ng/cm2 found in this work. Also in fairly good agreement with literature data is the change in normalized reflectivity associated with the adsorption of a full bromide monolayer on gold, 2.4%, compared to 2.9%observed by Sedlmaier and Plieth.lg The direct proportionality between the coverage of bromide, as measured by the QCM, and the relative change in optical reflectivity is in
agreement with the earlier findings for the same system of Adzic et al.,4 who employed much lower bromide concentrations, and can be best illustrated by combining the results shown in panels A and B in Figure 4 as shown in Figure 5. As clearly indicated, the slopes of the AR/R vs mass change are linear with essentially identical slopes, Le., -0.27 i 0.01, over an order of magnitude in bromide concentration. SUMMARY
The spectroelectrochemical cell for simultaneous electrochemical, optical, and microgravimetric measurements described in this work has been found to yield data in quantitative agreement with those obtained independently by other authors for all of the systems selected for this study. In the case of bromide adsorption of gold, a direct proportionality was found between the change in normalized reflectivity AR/R and the change in mass Am in a concentration range of bromide spanning over an order of magnitude. Although not presented in this work, the overall design of the spectroelectrochemical cell is sufficiently versatile to enable, with only a few minor modifications, simultaneous electrochemical Am and AR/R measurements in nonaqueous electrolytes. ACKNOWLEDGMENT
This work was supported by the Department of Energy, Basic Energy Sciences, and Eveready Battery Co., Westlake, OH. Received for review November 7, 1994. Accepted April 12, 1995.@ AC941082A
(22) Deakin, M. R.; Li, T.T.;Melroy, 0. R. J. Electround. Chem. 1988,243, 343.
2418 Analytical Chemistry, Vol. 67, No. 14, July 75, 7995
Abstract published in Advance ACS Abstracts, June 1. 1995.
