Anal. Chem. 2007, 79, 202-207
Simultaneous in Situ Reflectance and Probe Beam Deflection Measurements at Solid Electrode-Aqueous Electrolyte Interfaces Ping Shi, Iosif Fromondi, Qingfang Shi, Zhenghao Wang, and Daniel A. Scherson*
Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106-7078
A method is herein described for performing simultaneous in situ normal incidence reflectance spectroscopy (∆R/R, λ ) 633 nm) and probe beam deflection (PBD) measurements at solid electrodes in aqueous electrolytes, while scanning the potential linearly between two prescribed limits. Results obtained for Au in 0.1 M HClO4 and for Pt in both 0.1 M HClO4 and 0.1 M NaOH were found to be in excellent agreement with those reported in the literature for each individual spectroelectrochemical technique under otherwise similar conditions. Data collected for Pt electrodes in CO-saturated 0.1 M HClO4 revealed rather sudden changes in both ∆R/R and PBD signals in the voltammetric region where the characteristic sharp peak associated with the oxidation of adsorbed CO occurs. This behavior was ascribed, respectively, to oxide formation (∆R/R) and to changes in the electrolyte composition in region neighboring the electrode, involving predominantly the acid concentration (PBD). In contrast, CO oxidation on Pt in 0.1 M NaOH yielded a PBD response consistent with formation of solution-phase carbonate via the reaction of the product, CO2, with hydroxyl ion. The exquisite sensitivity of ∆R/R and PBD to interfacial phenomena was further illustrated using a monolayer of hemin irreversibly adsorbed on glassy carbon surfaces in 0.1 M Na2B4O7 (pH ∼9.2). For this system, ∆R/R was found to be proportional to the relative fractions of hemin and its reduced counterpart, while the PBD signal could be correlated with corresponding variations in the electrolyte concentration induced by the surface-bound redox process. Normalized differential reflectance spectroscopy (∆R/R) and probe beam deflection (PBD) may be regarded as complementary optical probes of a variety of interfacial electrochemical processes. Whereas ∆R/R often displays exceedingly high sensitivity to changes in the electrode surface, such as those induced by changes in the state of charge of the interface, or adsorption of species from the electrolyte,1-3 PBD responds to variations in the * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Kolb, D. M. In Spectroelectrochemistry; Gale, R., Ed.; Plenum Press: New York, 1988; pp 87-188. (2) Conway, B. E.; Angerstein-Kozlowska, H.; Laliberte, L. H. J. Electrochem. Soc. 1974, 121, 1596-1603. (3) Sagara, T.; Murase, H.; Komatsu, M.; Nakashima, N. Appl. Spectrosc. 2000, 54, 316-323.
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index of refraction of the solution, n, along an axis x, normal to the direction of light propagation. Such optical gradients are generally, albeit not always, produced by depletion or generation of species at the electrode surface. In case of a planar electrode embedded in the yz plane, the angular deflection of the probe beam, ψ, may be shown to a good degree of approximation to be given by
ψ)
w dn w dn dc ) n dx n dc dx
( ) ( )( )
(1)
where w is the width of the electrode and c the concentration of species responsible for the changes in n. As originally shown by Ko¨tz et al.,4 PBD can detect minute changes in the optical properties of the solution immediately adjacent to the electrode induced by pseudocapacitive processes down to the equivalent of one to two electrons per surface atom. The sensitivity of this method was eloquently demonstrated using oxide formation/ reduction on Au, Cu underpotential deposition (UPD) on Pt, and redox processes involving functional groups on carbon electrodes. These studies were later extended by Cairns and co-workers to hydrogen adsorption/desorption, oxide formation/reduction, and specific adsorption of ions on Pt in aqueous electrolytes.5 More recently, Hillman and co-workers examined Pb UPD on Au using, in addition to PBD, the electrochemical quartz crystal microbalance technique to monitor simultaneously changes in the mass of the electrode.6 The present contribution describes an experimental arrangement that allows simultaneous in situ PBD and normal incidence ∆R/R signals to be recorded during cyclic voltammetric scans on solid electrodes, using a variety of surface processes as model systems. The latter include oxidation of CO on Pt in aqueous electrolytes and the redox activity of hemin irreversibly adsorbed on a glassy carbon electrode, two systems for which the PBD behavior has not as yet been reported in the literature. EXPERIMENTAL SECTION In situ simultaneous ∆R/R-ψ measurements were carried out in aqueous electrolytes at room temperature using polished (4) Kotz, R.; Barbero, C.; Haas, O. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 427-430. (5) Bidoia, E. D.; McLarnon, F.; Cairns, E. J. J. Electroanal. Chem. 2000, 482, 75-80. (6) Henderson, M. J.; Bitziou, E.; Hillman, A. R.; Vieil, E. J. Electrochem. Soc. 2001, 148, E105-E111. 10.1021/ac061452i CCC: $37.00
© 2007 American Chemical Society Published on Web 11/30/2006
Figure 1. Experimental setup for simultaneous in situ probe beam deflection (PBD) and ∆R/R measurements.
polycrystalline Pt(poly) and Au(poly) and bare or hemin-modified glassy carbon (GC) disks (Pine Instruments) as working electrodes (A ) 0.164 cm2). The main body of the spectroelectrochemical cell designed specifically for these measurements was an elongated solid Teflon cube with orifices drilled along each of its three main axes, equipped with individual quartz disk windows mounted along five of its six faces (see Figure 1). This arrangement allows light to be transmitted through the solution parallel to the electrode surface for PBD or be reflected at normal incidence from the electrode surface for ∆R/R. The additional set of windows are used to visually assist alignment of the beams for both PBD and reflectance measurements. A saturated calomel electrode placed in a separate compartment connected to the main cell via a Teflon tubing and a Au wire were used as reference and counter electrodes, respectively. For the PBD measurements, a He-Ne CW laser (Aerotech OEM05P, 633 nm, 1 mW) was focused with a plano-convex lens (Newport, 62.9-mm focal length) to achieve a very narrow beam parallel and close to the working electrode mounted on a threeaxis translation stage for PBD. The deflected beam was detected using a position-sensitive photodiode detector (UDT SPOT 9DMI position-sensitive detector) incorporating four identical and independent sector-type photodiodes forming a circle, which was placed ∼44 cm behind the center of the working electrode. Beam displacements were determined electronically by adding and subtracting the photocurrent generated by each of the sectors. Normal incidence ∆R/R signals were recorded simultaneously with a second He-Ne laser (JDS Uniphase 1144P, ∼15-17 mW) as the light source, using a system similar to that described in detail elsewhere that includes a beam splitter (CVI Laser Corp., Rs/Ts ) 50/50, 45°, 630 nm).7 The applied potential, current, and the signals from the PBD and battery-biased Si PIN detector (∆R/R) were fed to the inputs (7) Fromondi, L.; Shi, P.; Mineshige, A.; Scherson, D. A. J. Phys. Chem. B 2005, 109, 36-39.
of a 0.5-GHz oscilloscope (Tektronix TDS 744A) for data monitoring and data storage. Low-pass electronic filters for both deflection and reflectance were applied. Experiments involving Pt were performed in Ar gas purged or CO (Matheson Tri-Gas)-saturated 0.1 M HClO4 (Ultrex) or 0.1 M NaOH (Fisher, certified ACS) solutions, whereas those involving Au and GC (bare or hemin-modified, see below) electrodes were carried out in Ar gas purged 0.1 M HClO4 (Ultrex) and 0.1 M Na2B4O7 (pH 9.2), respectively. All solutions were prepared with 18.3-MΩ water (Barnstead). Glassy carbon was modified with hemin by immersing the polished clean GC electrode in a 0.1 M Na2B4O7 (Fisher, certified ACS) aqueous solution (18.3-MΩ water, Barnstead water purifier) containing 0.1 mM hemin (Porphyrin Products, Inc.) overnight. The electrode was then rinsed with pure water to remove nonadsorbed hemin and then transferred to neat 0.1 M Na2B4O7 deaerated solution for subsequent measurements. A gas-phase CO sensor placed ∼50 cm directly above the cell was used to monitor possible leaks into the atmosphere while the solutions were purged with CO. The CO bubbling rates employed, however, were very low, and not surprisingly, at no time was the detector triggered. RESULTS AND DISCUSSION I. Au and Pt Electrodes in Neat Dearated Electrolytes. Shown in Figure 2 are the results of voltammetric (scan rate: 100 mV/s, upper panel) and in situ PBD (center panel) and normal incidence ∆R/R (lower panel) measurements acquired simultaneously for a Au(poly) electrode in dearated 0.1 M HClO4. Similar data collected for a Pt(poly) electrode in dearated 0.1 M HClO4 and 0.1 M NaOH solutions are given in thick curves in Figures 3 and 4, respectively. Both the PBD and ∆R/R results obtained for Au and Pt in the neat electrolytes are in very good agreement with data published in the literature for each of the two techniques.2,4,5 As discussed elsewhere,2,8 changes in ∆R/R for both Analytical Chemistry, Vol. 79, No. 1, January 1, 2007
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Figure 2. Cyclic voltammogram (upper panel), ψ vs E (center panel) and ∆R/R vs E (lower panel) recorded simultaneously for a polycrystalline Au disk electrode (A ) 0.164 cm2) in neat 0.1 M HClO4 at a scan rate of 100 mV/s. The optical data represent the average of 63 scans. The solid line in the lower panel was obtained by 10 AA smoothing of the data in scattered points.
of these metals, denoted as M in eqs 2 and 3 below, track quantitatively oxide formation and its subsequent reduction, i.e.
M + H2O f MO + 2H+ + 2eM + 2OH- f MO + H2O + 2e-
(acid media)
(2)
(alkaline media)
(3)
where, for simplicity, the formation of a generic metal oxide MO, not necessarily of that stoichiometry, is shown. According to reaction 2 in this scheme, surface oxidation in acid media generates protons, which, in turn, will lead to an increase in the concentration of ClO4- in the immediate vicinity of the electrode surface to preserve electroneutrality. Under these conditions, (dc/ dx)(HClO4) < 0, and since (dn/dc)(HClO4) > 0, as reported for other mineral acids in the literature,9 the beam deflection, ψ, should be negative (toward the electrode), in agreement with the experimental observations. Yet another contributing factor to a negative beam deflection is the increase in the concentration of the acid due to water consumption. Also in agreement with eq 3 are the results obtained in basic media; formation of the oxide, in this case, not only consumes hydroxyl ions forcing Na+ to migrate into the bulk solution but also dilutes the neighboring solution by generating water; hence, (dc/dx)(NaOH) > 0 and since (dn/dc)(NaOH) > 0, ψ > 0. As the scan is reversed at the most positive limit, and regardless of the pH of the solution, the current drops to negligible (8) Shi, P.; Fromondi, I.; Scherson, D. A. Anal. Chem. To be submitted. (9) Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 64th ed.; CRC Press,: Boca Raton, FL, 1984.
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Figure 3. Cyclic voltammogram (upper panel), ψ vs E (center panel) and ∆R/R vs E (lower panel) of a Pt polycrystalline disk electrode (A ) 0.164 cm2) in 0.1 M HClO4 without CO (thick curve) and saturated with CO (thin curve) recorded simultaneously at a scan rate of 100 mV/s. Acquisitions for data: 75 (without CO) and 147 (with CO). Rref ) R0.1V.
values, allowing both the acid and base concentration profiles to relax, leading to a net decrease in ψ. Once the oxide begins to reduce, the reactions in eqs 2 and 3 above proceed in the reverse direction, and not surprisingly, ψ becomes precisely opposite in sign compared to those found during the scan in the positive direction. In the hydrogen adsorption region, (dc/dx)(HClO4) > 0, and since (dn/dc)(HClO4) > 0, ψ > 0. II. Pt Electrodes in CO-saturated Aqueous Electrolytes. Acid Media. As has been amply documented in the literature,10-13 oxidation of adsorbed CO, COads, in 0.1 M HClO4 is characterized by a rather sharp peak centered at ∼0.6 V according to
Pt-CO + 2H2O f PtO + CO2 + 4H+ + 4e-
(4)
As indicated by the thin curve in the center panel, Figure 3, this peak is accompanied by a sudden beam deflection in the negative direction, which correlates very well with the sudden drop in reflectance (due to oxide formation) observed in the lower panel (see thin curve) in this figure. These results underscore the complementary character of these two techniques. Oxidation of COadssand to a much smaller extent bulk COsinduces an increase in the concentration of HClO4 beyond that expected from the simultaneous Pt oxidation and, thus, elicits a negative beam (10) Jambunathan, K.; Hillier, A. C. J. Electroanal. Chem. 2002, 524, 144-156. (11) Jiang, J. H.; Kucernak, A. J. Electroanal. Chem. 2002, 533, 153-165. (12) Couto, A.; Rincon, A.; Perez, M. C.; Gutierrez, C. Electrochim. Acta 2001, 46, 1285-1296. (13) Caram, J. A.; Gutierrez, C. J. Electroanal. Chem. 1991, 305, 259-274.
Figure 4. Cyclic voltammogram (upper panel), ψ vs E (center panel) and ∆R/R vs E (lower panel) of a Pt polycrystalline disk electrode (A ) 0.164 cm2) in 0.1 M NaOH without CO (thick curve) and saturated with CO (thin curve) recorded simultaneously at a scan rate of 100 mV/s. Acquisitions for data: 96 (without CO) and 106 (with CO). Rref ) R-0.55V.
deflection of a magnitude larger than that observed in the absence of CO. According to eq 4, oxidation of COads generates solutionphase CO2; hence, (dc/dx)(CO2) < 0, and since (dn/dc)(CO2) < 0, ψ > 0.14 The fact that the experimental data show ψ < 0 implies that the effect due to variations in the acid concentration is predominant. This is not surprising, because the oxidation of one CO molecule generates four hydronium ions but only one CO2 molecule. At more positive potentials, i.e., E > 0.7 V, both the oxidation of bulk CO and of the Pt surface will proceed according to
deflection should be negative. This effect, however, will be counterbalanced, at least in part, by the formation of CO2, which, as discussed above, should yield a positive deflection contribution. When the potential is scanned back from 1.2 to 0.6 V, i.e., prior to platinum oxide reduction, oxidation of bulk CO is the only (faradic) process that accounts for the very small current leading to a further decrease (upward shift) in ψ, as shown in the middle panel, Figure 3. Not surprisingly, as the oxide is reduced, the behavior of ψ will be similar to that found in the absence of CO in the solution. At more negative potentials, CO readsorption takes place; hence, (dc/dx)(CO) > 0, and assuming as before (dn/dc)(CO) < 0, ψ < 0 (see deeper negative wide valley around 0.1 V). Once the Pt surface is saturated with COads, the current drops to a very small value allowing for the concentration profiles of all species to relax leading to a gradual decrease in ψ toward negligible values. Alkaline Media. Unlike the behavior found in 0.1 M HClO4, oxidation of COads in 0.1 M NaOH occurs over a much wider potential region and gives rise to at least three voltammetric peaks in the scan in the positive direction (see the thin curve in upper panel, Figure 4). As evident from the data shown in the middle panel in this figure, the voltammetric peaks correlate well with the corresponding stepwise drop in the deflectogram. Furthermore, based on the voltammetric results, oxidation of COads occurs predominantly over a potential region negative to the onset of oxide formation. The marked decrease in the ∆R/R found in acid media cannot be discerned in this case. In alkaline electrolytes, the oxidation of both adsorbed CO (see eqs 6 and 7 below) and bulk CO (eq 8) can be written formally as follows:15,16
Pt-CO + 3OH- f PtOH + CO2 + H2O + 3e-
(6)
Pt-CO + 4OH- f PtO + CO2 + 2H2O + 4e-
(7)
Pt + CO(sol) + 2H2O f CO2(sol) + 4H+ +4e- + PtO (5)
CO + 2OH- f CO2 + H2O + 2e-
(8)
Since the net rate of oxidation of bulk CO is much smaller than that of COads (smaller currents), the rate at which HClO4 will be generated at the interface will be much smaller than the initial burst brought about by the corresponding oxidation of COads. On this basis, the flux of HClO4 to the surface will decrease, and hence, the absolute magnitude of ψ will also decrease (upward shift) as the experiment shows. Despite a thorough search, no values for (dn/dc)(CO) were found in the literature; however, since dn/dc for dissolved gases, such as O2 and CO2, are negative,14 we may also assume this would be the case for CO. Consumption of bulk CO at the interface is equivalent to (dc/ dx)(CO) > 0, and since by assumption (dn/dc)(CO) < 0, the contribution due to the depletion of solution-phase CO to the
At high pH, CO2 will react further with OH- to produce HCO3and CO32-; however, since for H2CO3, pKa1 ) 6.3, pKa2 ) 10.3, the dominant species at pH 13 will be CO32-. On this basis, the net reaction can be written as
(14) Kertesz, V.; Inzelt, G.; Barbero, C.; Kotz, R.; Haas, O. J. Electroanal. Chem. 1995, 392, 91-95.
and/or
and
Pt-CO + 5OH- f PtOH + CO32- + 2H2O + 3e-
(9)
and/or
Pt-CO + 6OH- f PtO + CO32- + 3H2O + 4e- (10) and
CO + 4OH- f CO32- + 2H2O + 2e-
(11)
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Figure 5. Cyclic voltammogram (upper subpanels), ψ vs E (center subpanels) and ∆R/R vs E (lower subpanels) of a bare (left panel) and a hemin-modified (right panel) GC electrode (A ) 0.164 cm2) in deoxygenated 0.1 M Na2B4O7 (pH ∼9.2). Scan rate: 100 mV/s. Number of acquisitions: 199 (left) and 207 (right).
In the potential region from -0.45 to -0.2 V (positive-going scan), adsorbed OH (OHad) forms easily on the bare Pt surface because of the high concentration of OH-. This promotes oxidation of COads giving rise to very high concentrations of CO32- near the electrode surface. Rather interestingly, OH- consumption and CO32- formation elicit a positive and a negative deflection, respectively; however, since (dn/dc)(CO32- ) ) 0.023 M-1 is about twice that for (dn/ dc)(OH- ) ) 0.011 M-1,9 and (dc/dx)(CO32- ) is larger than that for OH- near the electrode surface, the net deflection is largely governed by CO32- formation, and thus responsible for the dramatic decrease observed in the deflectogram. In the region -0.2 < E < 0.1 V, most of COads has been oxidized, and hence, the carbonate produced arises solely from the oxidation of bulk CO, which is expected to occur under diffusion control. In view of the much smaller rates of this process, carbonate formation would markedly decrease, and consequently, the deflection signal will, in absolute magnitude, decrease (i.e., upward shift), as the experiment shows. When the potential is scanned from 0.1 to 0.55 V and then back from 0.55 to 0.1 V, electrooxidation of CO (bulk) is inhibited by the formation of platinum oxide, as evident from the current drop in the voltammogram, which is consistent with the continuous upward shift in the deflection. Once the potential reaches 0.1 V, electrooxidation of bulk CO starts to recover, which shifts the deflection downward due to the CO32- formation. At more negative potentials, i.e., -0.2 to -0.5 V, three processes are expected to proceed: reduction of platinum oxide, (15) Santos, E.; Giordano, M. C. J. Electroanal. Chem. 1984, 172, 201-210. (16) Caram, J. A.; Gutierrez, C. J. Electroanal. Chem. 1991, 305, 275-288.
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i.e.,
PtO + H2O + 2e- f Pt + 2OH-
(12)
oxidation of adsorbed or bulk CO, i.e.,
(Pt)CO + 4OH- f CO32- + 2H2O + 2e-
(13)
and readsorption of CO, i.e.,
Pt + CO f Pt-CO
(14)
Each of these processes will cause a net negative deflection and thus account for the large negative peak observed experimentally. III. Adsorbed Hemin in Acidic and Alkaline Solutions. An eloquent demonstration of the sensitivity of PBD was found using a layer of hemin (Hm), a well-known redox-active iron porphyrin irreversibly adsorbed on a GC electrode. In analogy with a host of axially unprotected iron porphyrins, hemin forms a µ-oxo dimer in alkaline solution in which a single oxygen atom is bonded to the Fe iron centers of two different hemin molecules.17 Coulometric analysis of the voltammogram obtained for heminmodified GC (see upper right panel, Figure 5) corrected by the featureless capacitance of bare GC (see upper left panel in this figure) yielded a surface concentration of 6.2 × 10-10 mol/cm2. As evident from the data in the middle right panel, Figure 5, the (17) Falk, J. E. Porphyrins and metalloporphyrins: a new edition based on the original volume; Smith, K. M., Ed.; Elsevier Scientific Pub. Co.: Amsterdam, 1975.
surface-bound redox process elicits clear changes in ψ over the potential region in which hemin changes oxidation state. This behavior is in striking contrast with that found for the bare GC electrode (see middle left panel, Figure 5) for which no detectable deflection was observed over the same potential range. These results clearly indicate that changes in the redox state of adsorbed hemin are accompanied by the flow of ions from the solution, creating gradients in the concentration of the supporting electrolyte large enough to be detected. The process most likely responsible for the PBD response involves changes in the composition of the solution in the neighborhood of the electrode surface induced by the reduction of the µ-oxo form of hemin to yield its corresponding monomeric (reduced) form, namely,
Hm(FeIII)-O-(FeIII)Hm + 2e- + H2O f 2Hm(FeII) + 2OH- (15) As indicated, reduction and subsequent oxidation of hemin will generate and consume hydroxyl ion, respectively, a behavior consistent with the experimentally observed results. More specifically, reduction of µ-oxo hemin will generate hydroxyl ions in solution forcing migration of Na+ toward the electrode, as well as water consumption due to the reduction process. These processes will contribute to negative values for dc/dx in the diffusion layer next to the electrode leading to a decrease in ψ. During the scan in the positive direction, the reduced form of Hm is oxidized, forming the original µ-oxo derivative evoking an opposite response both in PBD and ∆R/R. It is noteworthy, that the peaks in the ψ versus E curves appear to lag behind the corresponding voltammetric peaks. Preliminary theoretical calculations in this laboratory have shown that the shape of the ψ versus E curve is a very sensitive function of the distance between the electrode surface and the PBD beam, xo. In fact, a match (18) Vieil, E.; Lopez, C. J. Electroanal. Chem. 1999, 466, 218-233. (19) Shi, P.; Geraldo, D.; Fromondi, I.; Zagal, J. H.; Scherson, D. A. Anal. Chem. 2005, 77, 6942-6946.
between the theoretical and experimental shape of the curve in the middle panel of Figure 5, could only be found for a very narrow range of xo values around 100 µm, which appears very reasonable. The lack of a PBD response for the bare GC electrode (see middle left panel, Figure 5) clearly demonstrates that the charging of the interfacial double layer does not create observable gradients in the concentration of the electrolyte neighboring the electrode. Most importantly, it indicates that the current contributions due to double layer charging must be excluded when using convolution techniques of the type described by Veiel and Lopez for the theoretical analysis of PBD data.18 Normalized differential reflectance data recorded simultaneously during the acquisition of cyclic voltammetry and results yielded a sigmoidal-type curve similar to those described in a recent publication for a series of Co phthalocyanines adsorbed on carbon electrodes.19 As discussed therein, analysis of the ∆R/R data allows quantitative correlations to be made between the fraction of two redox forms of hemin on the surface and the applied potential. CONCLUSIONS Probe beam deflection and normalized reflectance spectroscopy provide highly sensitive complementary information about the interface and the region immediately adjacent to the electrode, as has been illustrated for bare Au and Pt, the oxidation of adsorbed CO on Pt, and hemin adsorbed on glassy carbon electrodes. The ability to conduct these measurements simultaneously removes ambiguities derived from the analysis of separate experiments and affords excellent means to gain better insight into the nature of a variety of interfacial processes of both fundamental and technological interest. ACKNOWLEDGMENT This work was supported by NSF. Received for review August 4, 2006. Accepted September 25, 2006. AC061452I
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