RamanMonitoringof Electrochemical Events Richard L. McCreery Department of Chemistry The Ohio State University Columbus, OH 43210
Richard T. Packard E. I. du Pont de Nemours and Co. Deepwater, NJ 08023
Most electrochemical p h e n o m e n a and techniques are based on electron transfer between a conductor and a redox system, usually dissolved in a solution. E l e c t r o s y n t h e t i c processes s u c h as metals production, electroplating, electroanalysis, and electrochemical energy conversion account annually for more t h a n $30 billion worth of products ( ~ 1 % of t h e U.S. G N P ) a n d contribute to a range of industries worth much more (2). All of these processes rely on t h e direct interconversion of electrical current and chemical energy in the form of a redox system. For problems in analytical chemistry, the heterogeneous electron transfer phenomenon results in certain imp o r t a n t consequences (2). First, electroanalytical measurements can be extremely sensitive, with measurable currents corresponding to minute amounts of analyte. For example, easily measured currents of 1 0 - 1 2 a m p equal 10~ 17 mol/s of a one-electron redox system, and state-of-the-art techniques are approaching detection limits c o r r e s p o n d i n g t o single redox events. Even small currents can b e measured with high precision and accuracy, resulting in electroanalytical techniques with similarly high quantitative accuracy. A second consequence of particular relevance to this article is the continuously tunable driving force available for electron transfer, in the form of the 0003-2700/89/0361-775A/$01.50/0 © 1989 American Chemical Society
electrode potential. Variations in potential correspond t o changes in elect r o n energy, a n d t h e free-energy change associated with a redox reaction can therefore be controlled. T h e electrode behaves like a tunable redox reagent t h a t can be made, at will, as oxidizing as fluorine or as reducing as lithium. T h e ability t o impose extreme redox potentials is t h e reason t h a t aluminum, alkali metals, fluorine, chlorine, a n d so forth are produced electrochemically. Furthermore, changes in electrochemical driving force can be executed rapidly (i.e., electrode potential changes can be made on a submicrosecond time scale). An analogous
it may undergo homogeneous reactions to form new products, some of which may themselves be electroactive. Various electrochemical methods have long been used t o monitor such electroinitiated reactions. This is usually done by monitoring the perturbation of t h e observed current caused by the homogeneous reaction (3,4). An example of t h e E C E mechanism, in which a homogeneous chemical reaction occurs between two heterogeneous electron transfer reactions, is shown in Figure 1 and involves the oxidation of tbe neurotransmitter dopamine in H B r solution (5-7). T h e additional current resulting from the reaction in Equation 3
INSTRUMENTATION approach is stopped-flow kinetics, except the time scale is longer ( ~ milliseconds) and one is constrained to redox potentials available with known chemical reagents.
Electrochemical initiation of a reaction in solution Consider a n electrode immersed in a solution containing the stable, reduced form of a redox couple, denoted Red. If the applied potential, Eapp, is negative relative to the standard potential, E°, for t h e redox system, electrons are more stable in Red t h a n in the electrode, and no current flows. If £ a p p is changed to a value t h a t is positive relative to E° (rapidly if desired), electrons will be more stable in the now positive electrode, and heterogeneous electron transfer will occur from Red to the electrode, thereby generating t h e oxidized form, Ox. If Ox is unstable in solution,
is directly related to the rate constant, kc. N o t only has the electrode reaction (Equation 1) provided a convenient means t o produce the reactive orthoquinone, it has done so on a fast time scale, submilliseconds if necessary. Because the whole process occurs within the diffusion layer, extremely small amounts of dopamine are consumed (~30 ng for typical conditions and a 1-s experiment). T h e excellent time resolution a n d tunable driving force inherent in electrochemical generation of reactive species have resulted in widespread application. However, electrochemical monitoring of the homogeneous reaction, usually via t h e current, has two drawbacks. First, current measurements are rather nonselective compared with analogous spectroscopic observations, leading t o difficulties with complex reaction sequences. F o r some mecha-
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INSTRUMENTATION amounts of material in the electro chemical diffusion layer. The majority of techniques common ly used for structure determination, such as NMR spectroscopy, MS, EXAFS, and so forth, lack either the sensitivity or the time resolution re quired for SEC applications. Recall that the amount of material generated in a typical diffusion layer is small, on the order of 10~10 mol/cm 2 for a 1-ms experiment. In the case of FT-IR re flectance, the required sensitivity is available with sufficient signal averag ing, but it is difficult to mate the in strumental requirements of an F T in strument with the transient nature of electrochemical reactant generation. Raman spectroscopy
Figure 1. Reaction mechanism for the oxidation of dopamine, H3DA, in 1 M HBr solution. kQ is the pseudo first-order rate constant for HBr addition; K^ is the equilibrium constant for the redox cross reaction in Equation 4. HDOQ = dopamine orthoquinone, H2DABr = 6-bromodopamine, DOQBr = 6-bromodopamine orthoquinone.
nisms, the homogeneous reaction does not perturb the current at all, making measurements of current uninformative. A second problem is the paucity of structural information available from the current response. Although E° is sensitive to structure in many cases, there is no systematic procedure to de duce structure from electrochemical currents or observed E°. Chemists who study homogeneous solutions are used to deducing structures from NMR spectroscopy, mass spectrometry, vi brational spectroscopy, and the like, but electrochemists cannot obtain this type of structural information if they are constrained solely to electrochemi cal measurements. Spectroelectrochemistry
The desire for greater selectivity and molecular information led to the devel opment of hybrids of spectroscopic and electrochemical techniques under the general label of spectroelectrochemis try (SEC) (2, 8-12). A common SEC experiment might involve reflection of a spectrophotometer beam off a planar electrode, thus permitting electro chemical reactant generation and spec troscopic monitoring. UV-vis absorp tion techniques have been developed extensively and are characterized by excellent quantitative accuracy and time resolution in both transmission and reflection modes. Although UVvis techniques have been applied to many problems with valuable results,
they are still constrained by the nature of the spectroscopic transition. Vibra tional spectroscopy, magnetic reso nance, or other techniques commonly used to probe homogeneous reactions provide structural information that UV-vis absorption does not. Further more, electronic absorption spectra of solution species have very broad linewidths, often making it difficult to monitor several species in a possibly complex reaction path. Thus the driving force for developing vibrational spectroscopy of electrode reactions is the need for high-resolu tion, narrow-linewidth spectra to im prove selectivity and provide structur al information. Given t h a t electro chemistry permits easy generation of a reactive species by a charge transfer reaction, vibrational spectroscopy is an informative means to monitor the fate of an electrogenerated material. Spe cifically, the spectroscopic probe must • provide information about molecular structure, • have sufficient resolution to permit selective monitoring of more than one species, • have time resolution comparable to or better than the electrode process that generates the reactive species (spectral information must be ob tained on a rapid time scale, to ex ploit the advantages of electrochemi cal generation), and • possess sufficient sensitivity to de tect and monitor the very small
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Starting in the mid-1970s, Van Duyne reported the use of resonance Raman spectroscopy for SEC applications (1315). Raman spectroscopy involves scat tering of incident light from a laser beam (ηνη) reflected from the elec trode. The vast majority of laser light is scattered without a change in frequen cy and carries little chemical informa tion. Such Rayleigh scattering is re sponsible for the blue sky, because it is more efficient at shorter wavelengths. Raman scattered light has lost energy to the sample molecule through vibra tional excitation and is shifted in ener gy from the input laser light by a vibra tional quantum (hvi). If the incident light wavelength is close to an electron ic absorption band of the sample mole cule, the scattering is more intense (by factors of 10-10 6 ) and the process is called resonance Raman scattering. Being a vibrational spectroscopy, Raman produces high-resolution spec tra with significant structural informa tion. Because there are usually many Raman lines, comparisons to known spectra may allow structure identifica tion in "fingerprint" mode. Changes in normal modes or bond strengths may be inferred from changes in the Raman spectrum accompanying a redox reac tion. Raman peaks are narrow com pared with UV-vis absorptions, so it may be possible to selectively monitor one component in a reaction sequence. For the case of resonance Raman, one species may have a much higher scat tering cross section than others, thus enhancing its Raman spectrum. With suitable choice of input laser wave length, the enhanced species may be monitored with high sensitivity and se lectivity. Several aspects of resonance enhancement will be discussed later. To exploit the significant advantages in selectivity and information content of Raman SEC over UV-vis SEC, the instrumentation must address a funda-
INSTRUMENTATION mental issue regarding sensitivity. Ra man scattering is a weak effect, and cross sections are much lower than those common to UV-vis absorption. Consider the experiment depicted in Figure 2, in which a laser beam reflects off the electrode surface and the Ra man scattering is collected normal to the electrode plane. Given the small amount of scatterer generated by a transient electrochemical experiment, instrument sensitivity is crucial to suc cessful acquisition of spectra. The usual case of a species generated at a diffusion-controlled rate by a po tential step is depicted in Figure 3. Al though the current exhibits a transient response with a t - 1 / 2 decay, the Raman scattering results from the integrated concentration of electrogenerated scatterers in the laser beam. For a stable species, this integral is proportional to t1/2, yielding a parabolic shape of Ra man intensity versus time (13,16,17). The predicted Raman signal is given by Equation 5 S = 4P0/Wb(Di)1/2AD-^
Figure 2. Geometry of typical Raman SEC experiment for monitoring solution spe cies generated at a planar electrode. Incident beam (hi>0) enters the cell and reflects off the electrode, and scattered light is collected normal to the electrode surface. The electrochemical diffusion layer, which is typically 1-100 μνη thick, contains a solution scatterer.
(5)
where S = Raman signal, counts s - 1 Po = incident laser power, pho tons s _ 1 β = differential Raman scatter ing cross section, cm 2 s r - 1 molecule - 1 (often denoted άσ/άΩ); sr indicates steradian, unit of solid angle iVb = bulk number density of starting material (Red in the discussion so far), mole cules c m - 3 D = diffusion coefficient of Red, cm2 s _ 1 t = time from beginning of dif fusion-controlled oxidation, s Ω = collection efficiency of spec trometer and collection op tics, sr Τ = spectrometer and optics transmission, unitless Q = detector quantum yield, counts photon - 1 Arj = electrode area sampled by the spectrometer, cm2 α = laser beam radius at elec trode, cm Equation 5 assumes the laser spot on the electrode overfills the spectrometer and detector. The many variables in Equation 5 can be classified into four subgroups. First, the laser power P 0 is limited by the laser employed and the damage threshold of the electrode sur face or the diffusion layer. Second, the cross section β varies greatly (at least a factor of 106) for different molecules and ultimately controls signal-to-noise
Figure 3. Electrochemical generation of a Raman scatterer by oxidation of a stable reduced material. (a) Potential, (b) current, and (c) Raman scattering waveforms.
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INSTRUMENTATION (S/N) ratio and sensitivity. Third, the product iVb(i)i)1/2AD equals the num ber of electrogenerated molecules ob served by the spectrometer. As the dif fusion layer thickens with time, this quantity increases with t 1/2 , yielding the parabolic response of Raman signal versus time. Fourth, the product QTQ/ 1Γ3/2α2 ; s a n indication of the spectrome ter's performance and permits compar ison of different instruments. Several conclusions relevant to the problem of detecting electrogenerated species can be drawn from Equation 5. First, a high β permits the observation of low concentrations or short-lived species. As β decreases, the experimen tal time scale or the concentration will have to increase to maintain the same signal. Second, improvements in the instrument (ilTQ) will permit experi ments on weaker scatters, shorter times, or lower concentrations. Third, the ability to monitor a larger AD is important, particularly for SEC appli cations. Surface damage or thermal disturbance of the diffusion layer lim its the acceptable power density, so a larger AD yields a larger signal before the damage threshold is reached. Fi nally, spectral monitoring of normal Raman scatterers will be much more difficult than observing resonance Ra man scatterers because of the 10-10 6 fold difference in β. As with any spectroscopic method, the quality of the spectra and detection limits are determined by S/N ratio, not just signal. Noise sources include shot noise from both sample and back ground scattering, detector dark noise, and detector readout noise. In most cases, the background scatter from an electrochemical solution is large com
pared with detector shot or readout noise. In this limiting but often valid case, the S/N ratio is proportional to the square root of the signal given in Equation 5. If a multichannel detector having Ν channels is employed (as de scribed below), a factor οΐΝ1/2 is added to the S/N ratio expression. After re ducing the S/N ratio equation to in clude only terms involving the spec trometer and detector, Equation 6 is obtained S/N = K(AOTQN)m
(6)
for the case of a photon shot noise lim ited experiment and a fixed total mea surement time (18). Κ is a constant, derived from concentration, laser pow er, and other factors and is related to Raman scattering intensity. The N1^ enhancement in S/N ratio is predicted for multichannel Raman detectors and will be beneficial under certain condi tions, as discussed below. Instrumentation
Now that the variables of Raman SEC have been defined, the instrument nec essary to obtain spectral information about electrogenerated species, as well as two data acquisition modes, can be described. First, it is usually desirable to obtain complete spectra after elec trolysis begins (preferably time re solved) to produce a 3D data set of Ra man intensity versus Raman shift and versus time. Such data will reveal vi brational spectral changes following electrochemical generation and will provide the most structural informa tion. Once time-dependent spectral changes are observed, it is often desir able to conduct fixed-wavelength ex periments in which a particular Raman
Figure 4. Block diagram of scanning Raman spectrometer. PMT is a single-photon photomultiplier tube. Working, reference, and auxiliary electrodes (W, R, and A, respectively) are immersed in the solution of interest.
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peak intensity is monitored as a func tion of time. The first Raman SEC experiments employed an instrument similar to that in Figure 4 (14, 16). In addition to the considerations of designing a Raman spectrometer, the cell design and optics are crucial to assure that the region sampled by the spectrometer coincides optically with the electrode surface and diffusion layer. The first reported Ra man SEC experiments were conducted at steady state, with a square wave po tential perturbation (14, 19). Eaj>p was oscillated between values above and below E°, thus establishing a concen tration of electrogenerated products. In this condition the Raman spectrom eter could be scanned slowly relative to the potential perturbation frequency, producing a spectrum of intensity ver sus Raman shift. As far as the spectrometer is con cerned, the experiment is conventional, whereas in fact the advantages of elec trochemical generation are being ex ploited to produce a possibly reactive sample. Because of the steady-state na ture of sample production, the steadystate scatterer concentration will de crease for shorter lived species, thus degrading the S/N ratio. For this rea son and because of certain instrumen tal factors, the initial experiments ex amined resonance-enhanced scatterers with relatively large cross sections. For stable electrogenerated products, this relatively simple, static acquisition mode and related procedures proved particularly valuable for identifying molecular structures, and excellent ex amples are available in the literature (14, 20, 21). A sample spectrum ob tained in our lab with this acquisition mode appears in Figure 5. To obtain time-resolved spectra fol lowing electrochemical generation, an important change in spectrometer de sign is required. Not only is scanning impractical on a short time scale (a few seconds or less), but transient signals are weak as a result of the t112 factor of Equation 5, and high sensitivity is re quired. Consider the apparatus of Fig ure 4, after replacement of the photomultiplier tube (PMT) with a multi channel detector. The detector consists of many (500-1000) photosensitive ele ments, each monitoring a different wavelength at the focal plane (22). The most important benefit of such a change in spectrometer design is the operation of the spectrometer as a spectrograph rather than a scanning monochromator. Instead of scanning wavelength (and Raman shift) across the exit slit, the dispersed spectrum is focused onto a multichannel detector at the focal plane. An intensified pho-
Figure 5. Resonance Raman spectrum of chlorpromazine cation radical (CPZ+) gen erated electrochemically in steady-state mode from reduced CPZ. Laser wavelength was 514.5 nm. CPZ is not resonance-enhanced and scatters too weakly to interfere with the CPZ+' spectrum. · denotes solvent peaks. (Adapted from Reference 16.)
todiode array (IPDA) positioned at the focal plane can monitor 1000 wave lengths simultaneously, in principle producing a complete spectrum in as little as 10 ns. Each channel of the IPDA is physically a 25 μπι Χ 2.5 mm rectangle and acts as a detector analo gous to the P M T used for the scanning experiment. A range of wavelengths is monitored at all times, thus lengthen ing the time monitored for each wave length and providing S/N ratio en hancement. The width of the spectrum typically monitored by our spectrome ter is 600 cm - 1 . Although the PMTs normally em ployed for Raman spectroscopy oper ate in a single-photon counting mode, an IPDA is inherently an integrating detector. Each element of the IPDA stores charge proportional to the inte gral of the number of photons striking the element. At intervals of 10 ms or more, the stored charge from all pixels is read out, and the pixel is reset. With repetitive integration and readout, it is possible to generate a series of spectra, each of which represents Raman scat tering during a 10-ms segment of time after electrogeneration begins. The se quence of events for the on-the-fly ac-
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INSTRUMENTATION quisition of a series of Raman spectra is shown in Figure 6, and a series of spectra obtained in this manner is shown in Figure 7. The 3D data set of Raman scattering versus Raman shift and time contains several types of information of signifi-
cant value for diagnosing the fate of electrogenerated species. The most obvious is the sensitivity of the spectra to structural changes evolving with time. Because certain Raman peaks correspond to particular normal modes of the intermediates of interest, changes
Figure 6. Relationship between Eapp, Raman signal, and timing of detector electronics for time-resolved spectral acquisition and single-wavelength monitoring. The third waveform from the top shows sampling by the intensified photodiode array (IPDA) detector, wherein each readout produces a spectrum over some wavelength interval. The fourth waveform is the multichannel scaler (MCS) trigger, and the fifth trace shows which MCS channels are active after the trigger.
Figure 7. Time-resolved spectra obtained with a multichannel detector (third trace of Figure 6) during the oxidation of CPZ to CPZ+'. Each spectrum represents a 10-ms period after the beginning of the potential step. OMA denotes the multichannel detector.
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in peak intensities with time reflect concentration versus time behavior and provide implications about reaction mechanism. An informative example is the oxidation of dopamine (H3DA) in the presence of HBr, shown in Figure 8. H 3 DA is a relatively weak Raman scatterer, so the initial spectrum (Figure 8a) shows no features in the 1300-1800 cm" 1 region. After the initial spectrum was acquired, the electrode potential was stepped to a value that oxidized H3DA at a diffusion-controlled rate. Raman peaks at 1572 c m - 1 and 1672 c m - 1 appear rapidly and are assignable to the dopamine orthoquinone (HDOQ). If HDOQ were stable (e.g., in HCIO4), these peaks would grow with the t 1/2 dependence expected from Equation 5. However, it is obvious that the 1572 c m - 1 band is weakening with time as a strong new band at 1540 c m - 1 appears. At least two products are formed, one an intermediate and the other a more stable product. Less obvious but similar changes also occur in the 1600-1750 c m - 1 range. These spectral changes have been assigned to particular species in the reaction mechanism, and the mechanism of Figure 1 has been confirmed (17). The time-resolved spectra from the multichannel spectrometer provide valuable qualitative information about reactive systems, on a millisecond to several second time scale. In principle, one could construct quantitative intensity versus time transients for particular peaks from the 3D data set, thus permitting quantitative kinetic analysis. In practice, better S/N ratio and time resolution are possible with a single-channel detector ( P M T in this case), because the integrative nature of the IPDA and readout requirement are avoided. In single-wavelength mode, one is sacrificing the structural information available from complete spectra such as those of Figure 8, but one is still exploiting the high resolution of Raman spectroscopy to monitor individual species. As an indication of the resolution and narrow linewidth of Raman features compared with UV-vis absorption spectra, consider the 1540 c m - 1 and 1572 c m - 1 peaks in Figure 8. These two features and the species to which they correspond are easily resolved on the Raman spectrum, yet the difference in Raman shift expressed in wavelength is only 1 nm. The UV-vis spectra of the two quinones are severely overlapped, and independent quantitative monitoring of them would be difficult. The timing for single-wavelength transient Raman spectroelectrochemistry is shown in the bottom two traces in Figure 6. With the exception of the
counting electronics and potential waveform, the spectrometer is the same as the scanning system of Figure 4. The spectrometer is set and held at the wavelength of interest (e.g., 1572 cm -1 ) for the H3DA oxidation. Photons detected by the PMT are counted, as they were for a scanning system, except the counts are stored in a multichannel
scaler (MCS). The MCS consists of many (~1000) channels, each of which is assigned to a different time interval following a trigger. As a shown in Fig ure 6, photon counts arriving after the trigger are stored in MCS channels, ac cording to arrival time. The width of a channel varies from a microsecond up to several seconds. If the potential step
Figure 8. Time-resolved spectra of dopamine oxidation in 1 M HBr solution obtained with a multichannel spectrometer. Times after initiation of oxidation are (a) 0 ms, (b) 0-50 ms, (c) 100-150 ms, (d) 200-250 ms, and (e) 400-450 ms. Integration period was 50 ms; successive spectra are displaced upward for clarity. (Adapted from Reference 17.)
that initiates the electrochemical reac tion is used as the trigger, the MCS will store the Raman intensity versus time at a given wavelength into the series of successive channels. Thus an intensity versus time transient is obtained at fixed wavelength, with time resolution equal to the MCS channel width. Sev eral intensity versus time transients are shown in Figure 9 for the dopamine oxidation monitored at 1540 and 1572 cm -1 . The lines were calculated for the reactions of Figure 1; details of the ap proach appear elsewhere (17,18). The sensitivity constraints of normal Raman SEC are more fundamental than merely instrumental limitations. Although instruments, cross sections, and laser powers vary greatly, a crude estimate of signal strength may be cal culated from Equation 5. For the case of electrogenerated benzoquinone, a Raman signal of ~10 kHz is predicted at Is after a potential step. At t = 1 ms, this value is 300 Hz, or 3 photons in a 10-ms MCS channel. Even for ideal de tectors and spectrometers (T = 1, Ω = large), the count rate might be in creased by a factor of 10-100, but one would still be dealing with a few counts in each interval at short times. Signal averaging, digital filtering, and the like can be used to enhance S/N ratio, but eventually one is always limited by the weakness of Raman scattering. The performance of the instrument deter mines how closely this fundamental limit is approached. As noted earlier, any increase in scat tering cross section (β in Equation 5) results in improved S/N ratio or faster time resolution. Partly for this reason,
Figure 9. Raman intensity versus time curves for double potential step experiment on the H3DA system. Solid curves are simulated results, (a) 8.0 mM H3DA in 0.8 M HCI, simulated ^ = 4.0 s - 1 , K^ = 20 for reactions 1-4. (b) 8.0 mM H3DA in 0.8 M HBr, simulated kc = 5.0 s \ K., = 0.5. Transients obtained with PMT detection at 5 c m - 1 resolution. Incident laser power was 40 mW at 488 nm. Width of each MCS channel was 20 ms. HDOQ was monitored at 1572 cm 1 (circles), DOQCI at 1548 c m - 1 (squares in a), and DOQBr at 1539 c m - 1 (squares in b). (Adapted from Reference
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INSTRUMENTATION
Figure 10. Transient intensity of the 1126 cm
1
band of CPZ+- generated by potential pulses from 0.4 to 0.8 V versus SCE.
The transient was recorded by a 1024 multichannel scaler with a dwell time of 30 μδ. Potential pulse duration was 1.5 ms before returning to 0.4 V and was re peated every 30 ms to allow signal averaging for a total of 50,000 runs. Curve a shows raw data, and curve b is a plot of intensity versus f1'2 for the 1.5 ms dura tion of the potential pulse. (Adapted from Reference 16.)
resonance Raman SEC (RRSE) was developed before normal Raman SEC. Not only is β much higher (by factors of 10-10 6 ), but the resonance enhance ment permits improved selectivity, be cause only one or a few components of a mixture will exhibit strong resonance enhancement. RRSE was presented initially in the mid-1970s (13), and mi crosecond time resolution was demon strated in the mid-1980s (16, 22). As shown in Figure 10, a resonance-en hanced electrogenerated scatterer can be monitored quantitatively, and the expected i 1/2 dependence at short elec trolysis times is down to tens of micro seconds. Unfortunately, strong resonance en hancement occurs only near an absorp tion band of the scatterer of interest, and the input laser power is necessarily attenuated as the beam passes through the diffusion layer. Because Equation 5 assumes constant laser power (P0), any attenuation will lead to a negative devi ation from a t 1/2 dependence (17). This effect is shown in Figure 11 for the case of electrogenerated chlorpromazine cation radical (CPZ + ) monitored with a 515-nm laser. It is obvious that atten uation is severe, at times greater than a few milliseconds, and quantitative de ductions about kinetics or mechanisms would be invalid if attenuation were ignored. A nontrivial correction involv ing some mathematical complexity can be made to extend the useful time scale of RRSE from 50 μβ to 5 s, but this correction must be applied with care for any but the simplest systems (17, 18). Thus RRSE pays significant bene fits in sensitivity, time resolution, and selectivity, but at the cost of mathe matical complexity and loss of quanti tative accuracy.
Electrode surface structure The discussion presented thus far has dealt with solution species generated electrochemically, which may be stable or reactive. A more difficult but equally important problem is the spectroscopic observation of electrode surface struc ture and dynamics. Because heteroge neous events are the basis of all electro chemical phenomena, it is particularly important to probe the electrode/solu
tion interface to determine its molecu lar structure, both statically and dy namically. To determine the relation ship between interfacial structure and electrochemical behavior, it is essential for researchers to have informative structural probes. As has already been noted, Raman spectroscopy can provide such infor mation on a millisecond time scale for electrogenerated species in solution. Unfortunately, the sensitivity problem
Figure 11. Transient Raman intensity of the 1126 cm" 1 band of CPZ+' generated by potential pulses from 0.2 to 0.85 V versus Ag/AgCI. The solid line is the simulated result assuming no input light absorption by CPZ+'. Points are the experi mental values. CPZ concentration was 5 mM in 1.0 M HCI in 43% (w/w) MeOH/H20. Input laser beam was 20 mW at 458 nm. Ten transients were averaged; spectral resolution was 7 c m - 1 . (Adapted from Reference 17.)
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becomes even more severe for thin lay ers (perhaps submonolayer) of species on the electrode surface. For the case of an adsorbed layer of analyte or the in terfacial layers of the electrode materi al, Equation 5 can be modified to pro duce Equation 7 (16,23,24) Ρ0βΝίναίίΐΤ(}ΑΏ ^surf
Ô
cos a I ''
7TCT
where Ssurf = Raman signal from surface species, counts s _ 1 .iVsurf = surface number density, molecules c m - 2 ο = incident beam angle, rel ative to surface normal NSUI{ need not be restricted to a mono layer case; for multilayer film or finite laser penetration depth into the elec trode material, iVsurf will equal the total density of molecules sampled by the laser beam and spectrometer. An important difference between the surface and solution experiments is ap parent after comparing Equations 5 and 7. Ssurf is not time-dependent, be cause the number of scattering mole cules on the surface does not usually increase with diffusional time. All else being equal, a monolayer of a normal Raman scatterer will result in an Ssurf roughly comparable to solution scat terer generated by a 1-ms potential step. Thus the surface signal is very small because so few molecules are sampled (~10~ 13 mol) and the Raman effect is so weak. For a normal scatterer with no enhancement mechanisms, a monolayer will produce a few counts per second of signal, and the experi ment is very demanding. The conse quence of this weak signal has been in vestigation of a variety of enhancement mechanisms and a comparative dearth of surface dynamic experiments. The current state of the art in instru mentation has resulted in examples of unenhanced surface Raman spectra in an ultra-high-vacuum environment (24). However, the vast majority of sur face Raman results have exploited sur face enhancement (SERS), which pri marily involves an increase in Po of Equation 7. Local electromagnetic ef fects on certain roughened electrode materials (e.g., Ag, Au, Cu) enhance the Raman signal by factors of up to 106 and make detection and spectral char acterization of even submonolayers straightforward (23-26). (SERS was the subject of a recent A-page article [27] and will not be discussed exten sively here.) Surface enhancement may also involve an increase in β through chemical interactions of the adsorbate and metal. SERS has proven to be a very valuable technique for examining Ag, Au, and Cu electrodes in vacuum,
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989 · 785 A
INSTRUMENTATION solution, and so forth and has revealed significant new information about elec trochemical interfaces. Less explored but still valuable ap proaches include surface resonance Ra man spectroscopy (SRRS), which ex ploits the high β of adsorbed RR mole cules to enhance the signal, and surface enhanced RRS (SERRS), which com bines resonance and surface enhance ment (13, 28). SERS, SRRS, and SERRS can exhibit strong signals that are easily measured, but SERS and
SERRS have the added advantage of providing surface selectivity. Because both local field and cross section en hancements are short-range (
