Anal. Chem. 1995, 67,2791-2799
Sensitivity and Reproducibility in Dnfrared Spectroscopic Measurements at Single-Crystal Electrode Surfaces Peter W. Faguy* and Nebojsa S. Marinkovid
Department of Chemistty, University of Louisville, Louisville, Kentucky 40292
The benefits of optimized in situ infrared measurements performed on single-crystalplatinum electrodes are described in this paper. By using a hemisphericalwindow both as a lens and as the IR-transparent wall of the thinlayer electrochemical cell, several signal-enhancingeffects are achieved: low first-surface reflection losses, nearcritical angle reflection, and beam collimation. In addition, the physical optics of this novel sampling attachment provide interactive tools for positioning the electrode in the cell via the evolving interferogram. Thus, both sensitivity and reproducibility are improved. The effects of poor optical throughput are demonstrated,and a measure of the expected merit for in situ reflection/absorption spectroscopy is presented. Examples are given of adsorption processes from sulfuric acid solutions and for the oxidation of glucose on both Pt(ll1) and Pt(100) electrode surfaces. Over the last Ween years, both electrochemicalinvestigations involving singlecrystal platinum and infrared surface spectroelectrochemistryl-6have evolved into important subdisciplines of physical electrochemistry. Both experimentally intricate areas have gone through generations of technical advances and are now moving from the realm of the dedicated specialist to the arena of the electrochemical surface scientist. This process is, and will continue to be, accelerated by improvements in instrumentation, equipment and methodologies. Studies of electrochemical processes at single-crystal electrodes have benefited greatly from the development of methodologies to prepare well-ordered, clean surfaces outside ultra-highvacuum conditions. This protocol of flame annealing, based substantially on the pioneering efforts of Clavilier,' is used in one form or another by all groups studying singlecrystal electrochemistry with noble metal electrodes; this includes infrared spectroelectrochemical investigations. The powerful appeal of singlecrystal IR spectroelectrochemistry lies in the fact that surface
' Present address: Department of Chemistry, University of Caliiomia, Davis, Davis, CA 95616. (1) Clavilier, J. In Electrochemical Surface Science; Soriaga, M. P., Ed.; American Chemical Society: Washington, 1988; Chapter 14. (2) Ross, P. N. In Structure ofBectrified Interfaces; Lipkowski. J. L., Ross, P. N., Eds.; VCH: New York, 1993; Chapter 2. (3) Adzic, R R. In Modern Aspects ofElectrochemistty;White, R E., Bockris, J., O'M., Conway, B. E., Eds.; Plenum: New York, 1990; Vol. 21, Chapter 5. (4) Nichols, R J. In Adsot$tion ofMolecules at Metal Electrodes; Lipkowski, J. L., Ross, P. N., Eds.; VCH: New York, 1990; Chapter 7. (5) Stole, S. M.; Popenoe, D. P.; Porter, M. D. In Electrochemical Intefaces; Abruna, H. D., Ed.; VCH: New York, 1991; Chapter 7. (6) Seki, H. In Electrochemical Surface Science; Soriaga, M. P., Ed.; American Chemical Society: Washington, 1988; Chapter 22. 0003-270019510367-2791$9,0010 0 1995 American Chemical Society
adsorbate symmetry, as influenced by crystallographic orientation, is experimentally observable in the appearance, position, and strength of IR-active modes, a feature not available in polycrystalline studies. As these transitions can be directly associated with charge density between nuclei, that is, chemical bonds, the importance of in situ infrared studies to molecular structure determination at electrochemical interfaces cannot be overemphasized. With increasing use of IR reflection techniques in electrochemistry there is a need to address concerns of sensitivity and reproducibility. This is an important issue that impacts several problems in the interpretation of in situ data. These include the origin of spectral features in potential difference spectra, the superposition of adsorbate and solution-phasefeatures, and the quantification of species present in the interface, both adsorbed and in the diffuse layer. The goal of this work is to address sensitivity and reproducibility in spectroelectrochemical measurements through analysis of measurements at platinum singlecrystal electrodes and to show that the limiting analytical factor is the design and implementation of the reflection experiment. Furthermore, it will be shown that the in situ infrared reflection experiment can be optimized prior to lengthy data collection and that an expectation of the signal detection limit can be obtained. Finally, an experimental figure of merit for intensity values will be derived. This value is proportional to the intensity of IR bands for a given experiment and is, in fact, a measure of the surfacesensing optical throughput. The experimental basis for this paper is a body of IR data on single-crystal Pt electrodes acquired in our laboratory over the past two years on a variety of electrochemical systems including anion adsorption and organic oxidation studies. EXPERIMENTAL SECTION Chemicals. Sulfuric and perchloric acids and methanol were obtained from Fisher and were all of Optima grade purity. Potassium hydroxide, sodium bicarbonate, and Dglucose were also obtained from Fisher and were of ACS grade purity. All chemicals were used without further purification. All solutions were made from distilled and reverse osmosis filtered water (Barnstead NANOpure). The diagnostic cyclic voltammetry obtained in the spectroelectrochemical cell indicated, for all systems studied, electrolyte solution purity consistent with published electrochemical studies. Electrode Preparation. Pt(ll1) and Pt(100) single-crystals (Cambridge Ltd.) were oriented, cut to better than 0.5", and polished using standard metallurgical procedures, the final polishing step used 0.05 pm alumina. A 10 cm long, 0.7 mm diameter Pt wire was spot welded to the back of each crystal. The wire Analytical Chemktty, Vol. 67,No. 17,September 1, 1995 2791
Figure i. Infrared spectroelectrochemical cell and reflection optics: (a) Teflon cap, (b) NP inlet, (c) glass tube, (d) Teflon cell body, (e) reference electrode port, (I) ceramic lube, (9) R wire counter electrode. (h) single-ciystal working electrode, (i) ZnSe hemisphere, (k) reflection optics focal points, (m) instrument focal point, and (n) folding mirrors.
was pushed through a ceramic tube, 0.6 cm in diameter. After polishing and cleaning for 30 min in an ultrasonic cleaner, the single crystal was heated in a hydrogen/air flame for 30 s and quicldy transferred into a quartz tube filled with ultrapure (99.99%) hydrogen. After cooling for 60 s, a drop of water was placed on the oriented surface of the electrode, thus preventing contamina tion during the transfer to the spectroelectrochemical cell. Before every measurement run, identified in this study with a sixcharacter alphanumeric string, the Pt(hk0 electrode was flameannealed following a final polishing step. Spectroelectrochemical Cell. The cell is shown schematically in Figure 1. The cell body was milled from virgin Teflon stock. It has four ports for solution inlet, solution outlet, the reference electrode and the connection to the counter electrode. All of these ports were bored to press fit standard h e r tip fittings (only one port is shown in Figure 1). The Ag/AgCI reference electrode was placed in an external compartment separated from the h e r tip fitting by a glass frit. The counter electrode F i r e lg) was a Pt wire wound in a loop and placed in a groove in the cell body. The working electrode, backed with a ceramic tube Figure 14 through which rnns the Pt wire connection,was held against the flat polished face of a 1in. diameter hemispherical ZnSe window (Harrick) by a rubber band attached to the top of the ceramic tube. The tube was press fit into a Teflon cap which was loosely fit into a glass tubing ( F i i lc) coupling the working electrode assembly to the cell and allowing for Nz purging and some alignment adjustment Electrochemical Control. Solutions were deaerated with NZ gas for 15 min prior to spectral data collection, whereas during the data collection the solution was covered with Nz. Cyclic voltammehy was used to check the structural order and the cleanliness of the solution before and after each spectroelectre chemical data collection, which, depending on the number of sample potentials, could last several hours. A custom-made potentiostat was used to control the applied potential. Both the spectrometer and the potentiostat were driven by a 80386 PC platform personal computer. The digital-teanalog converter in 2792 Analytical Chemistry, Vol. 67, No. 17, September 1, 1995
the €42 was interfaced to the potentiostat to provide an applied potential resolution of f1.2 mV; the actual reference electrode potential was monitored throughout the experiment The com puter program was written in such a way to set the potential of the working electrode alternately to sample or reference potential prior to data acquisition. Ag/AgCI reference electrodes were used in all studies. IR Measurements. A commercially available (An-Mattson Galaxy 8020) rapid-scanning FT-IR spectrometer equipped with a 45" Michelson interferometer,a water-cooled Sic globar source and a narrowband-pass MCT detector, D' = 4 x 1O'O mHz"~W-', was used to collect the spectra at a laser modulation (interferogram sampling) rate of 100 kHz. Digitization of the detector voltage directly into onboard memory was achieved with an 18 bit A/D converter operating at a clock speed of 100 kHz. An Al metal mesh (r 3 shown with filled circles. 0.25
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Figure 6. Peak height as a function of potential for the strongly perturbed HS04- mode from nine different measurements (see Table 3). Lines are cubic fits to individual measurement runs. Symbols have the same signifcance as in Figure 5.
While essentially the same information can be achieved in terms of peak position irrespective of the optical throughput, the situation is dramatically different if a similar analysis is performed on the peak height variation with applied potential. Figure 6 compares the peak height of the vl(ads) absorption as a function of sample potential among all experiments listed in Table 3. The two different symbols are used in the same sense as in Figure 5, and now the importance of optical throughput becomes obvious. From these experiments, a measure of the analytical signal can be determined. Due to the differencesin potentials and potential ranges studied, the intensity used for comparison purposes is the interpolated value at a potential of +0.5 V using a cubic fit to the data shown in Figure 6. This value is listed in the second to last column of Table 3. How this intensity depends on the optical throughput is shown in Figure 7. There is good correlation between two different indirect measures of the MSEFS at the electrode surface, F m and Arefl, and the vl(ads) band. The calculated optical throughput explains 61.2%of variation in 10.5 while the water-bending absorp tion accounts for 65.6%of the variation. This is in strong, and understandable, contrast to the noise dependency on optical throughput. If the normalized peak height is plotted vs applied potential, Figure 8, using the same conventions as Figures 5 and 6, the difference between unoptimized and optimized experiments Analytical Chernistty, Vol. 67,No. 17,September 1, 1995
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Figure 7. Figures of merit for nine Pt(ll1) 0.05 M H2S04 measurements detailed in Figures 5 and 6 and Table 3. Interpolated peak height at f0.5 V as a function of (a) an optical throughput parameter, F , , and (b) the single-beam relative absorption for the H20bending mode.
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Figure 9. -AWR spectra, 1300-900 cm-I, for Pt(l00) in 0.1 M HC104 and 0.01 M 0-glucose: (a) multiple potential alterations, 128 x 32 scans coadded into each single-beam spectra; (b) single potential alteration, 4096 scans coadded. Spectra in (a) have been multiplied by 1.24 to normalize for optical throughput. Sample potentials range from -0.1 (bottom) to 0.6 V (top) in 0.1 V increments. The background spectra were acquired at -0.25 V.
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Figure 8. Normalized peak height as a function of potential for the strongly perturbed HS04- mode, from data shown in Figure 6. Peak heights were normalized to an interpolated peak height at +0.5 V. The line is the best quadratic fit to the data. Symbols have the same signifcance as in Figure 5
becomes even more important. The quadratic fit for optimized experiments, four independent experiments, has a correlation coefficient of 0.9634, while five independent unoptimied experiments leads to a quadratric fit with an r value of 0.8386. Statisticallythere is 32%more correlation to the expected applied electrode potential dependency when the MSEFS has been optimized than when it has not. Faradaic Reactions. When electrode reactions involving the formal transfer of charge are studied, the diffusion, production, and consumption of IR-absorbing species complicate the difference spectra. Depending on the sample potential perturbation protocol, the difference spectra may show an averaging effect or a total change. This is in contrast to the study of non-Faradaic electrochemical processes, where such diflerences in sampling/perturbation schemes generally result in only differences in long-term stability of the spectroscopic experiment. What this means is that infrared studies of oxidation and reduction processes should be performed with both single-potential alteration potential control and multiple-potential alteration control.24Figure 9 demonstrates typical differences between the two sampling schemes. When multiple potential alterations are used in PDIR experiments on the oxidation of glucose on Pt(100) in perchloric acid (Figure 9a), only one spectral feature between 1300 and 900cm-l is found. The 1100 cm-l band indicates increasing C104concentration in the diffuse layer with applied positive potential. While also showing the same perchlorate feature, the analogous 2798 Analytical Chemistry, Vol. 67, No. 77, September I , 1995
single-potential alteration experiment (Figure 9b) also shows several bands indicating the depletion of glucose from the IRsampled thin-layer electrochemical cell. Increasingly larger negative-going bands are superimposed on the positive-going C104band at frequencies of 991,1034,1078, and 1211cm-l. In addition, a broad shoulder at higher wavenumbers due to changes in adsorbed water structure is evident in the single alteration data but not in the multiple alteration data. The v3 Clod- stretch, even when normalized to constant optical throughput (Figure 9a), F,,, = 4.22 and Figure 9b, Fm=5.25, is still much larger for the consecutive coaddition case as compared to multiple alterations. This is due to the overall difference in diffuse layer anion concentration as sampled by the two different methods. With multiple alterations, more scans are coadded into the interferogram with the diffuse layer composition still changing; this is true for both the sample and background spectra. Thus, concentration differences between the two potentials are somewhat smeared. For the same reasons, the small changes in glucose concentration, evident in Figure 9b, are not seen in Figure 9a. While it is not expected that differentpotential excitation wave forms will yield similar results, it is assumed that, from run to run using the same potential alteration scheme, the experiments would show agreement. A comparison of four sets of data, two replicate runs for both Pt(ll1) and Pt(100) electrodes in perchloric acid/glucose solutions, over the mid-IR region is shown in Figure 10. The replicate runs are overlaid to demonstrate the similarity in features within each stack plot and to show the differences in reproducibility between the two stack plots. All four of these experiments were performed in identical manners on flameannealed surfaces with the same electrolyte solution. Optical throughputs were determined in all cases: Pt(ll1) (Figure loa), F,,, = 3.58 and 5.02; Pt(100) (Figure lob), Fm = 2.78 and 4.22.
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Figure 10. Overlaid spectra, 2100-900 cm-l, for two independent runs for (a) Pt(ll1) and (b) Pt(l00) electrodes in 0.1 M HC104 and 0.01 M o-glucose. The background potential was -0.25 V; sample potentials are shown on the figure. Fm values for the four runs are given in the text.
Despite the higher overall F, values, the Pt(ll1) data show much poorer reproducibility than do the Pt(100) data where the spectra are essentially identical. Two of the strongest features in these PDIR spectra are the Clod- stretch at 1100 cm-I and the positive lobe of the end-on Pt-CO stretch at -2030 The latter band position blue shifts with applied positive potential. The ratios of integrated intensities for these two modes as a function of potential are shown in Figure 11. If the replicate experiments are reproducible and if the source of any difference in signal is due primarily to optical throughput, then the ratio of intensities should be potential independent and should be equal to the relative optical throughputs. This is precisely what is seen in Figure llb,d. The extremely good agreement between the optical throughput ratio and the ratios of integrated intensities in these experients indicates that, despite the Faradaic processes occurring, the throughput analysis presented here can still be implemented. In the Pt(ll1) data (Figure lla,c), sources of error other than optical throughput are probably operative. These could include electrode fouling, transient loss of potential control, changes in the electrochemical cell physical configuration, and contributions from water vapor. CONCLUSIONS By using a ZnSe hemisphere as the IR spectroelectrochemical cell window positioned as a lens to collimate the light and using a ZnSe/electrolyte solution angle of incidence a few degrees higher than the critical angle it is possible to (i) achieve an interactivemeasure of electrode alignment, (ii) quantify the optical throughput of the reflection experiment, and (iii) obtain PDIR (28) Bae, I. T.;Xing, X;Liu, C. C.; Yeager, E. B. J. Electroanal. Chem. 1990, 284, 335-349. (29) Bae, I. T.;Yeager, E. B; Xing, X.; Liu, C. C. J. Electroanal. Chem. 1991, 309, 131-145. (30) Marinkovic, N. S.; Adzic, R R; Faguy, P. W., submitted for publication in Electrochim. Acta.
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Figure 11. Ratio of integrated intensities for two glucose oxidation experiments (Figure 10) as a function of sample potential for Pt(ll1) (a, c) and Pt(l00) (b, d). Intensities were integrated over vco f 2 5 (a, b) and 1177-1011 cm-I (c, d). Lines indicate relative optical throughputs for P t ( l l l ) , Fm(ljFm(2)= 0.713, and for Pt(l00), Fm(lj Fm(2)=
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spectra with excellent signal-to-noise ratios on any sensitive and stable center-focused FT-IR spectrometer. For the spectrometric experimental conditionsused here (4096 scans, 16 cm-' resolution, the above-mentioned spectroelectrochemical cell, aqueous electrolyte solutions, and platinum mirror electrodes), the smallest discernible analytical signal is 75 ppm. However, this limit does not depend on the optical throughput and should not be used as a detection limit. The optical throughput quantiiied from the preexperimental interferograms as F, provides a measure of the analytical signal response. For the systems and conditions of this study, acceptable or optimized experiments are defined as those with F, values equal to or greater than 3, while those with F, < 2 will result in unacceptable results. Reproducibility among PDIR experiments under identical conditions is found to depend strongly on optical throughput. This is true for both non-Faradaic and Faradaic processes; although with the presence of solution-phasereactants and products, the dominant source of error may not be throughput. The absorption peak for the liquid water bending mode in the singlebeam scales with F,, and its band center and size reflect the angle of incidence and the thickness of the electrolyte solution layer. ACKNOWLEDGMENT The financial support of the National Science Foundation and the Commonwealth of Kentucky through NSF-Kentucky EPSCoR Advanced Development Program (Grant EHR-9108764) and the Petroleum Research fund through a !ype G grant is gratefully acknowledged. The authors also thank Dr. Radoslav Adzic for the loan of the Pt singlecrystal electrodes. Received for review March 15, 1995. Accepted June 15,
1995.a AC9502615 @
Abstract published in Advance ACS Abstracts, August 1, 1995.
Analytical Chemistry, Vol. 67, No. 17, September 1, 1995
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