Journal of Electroanalytical Chemistry 649 (2010) 102–109

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Journal of Electroanalytical Chemistry

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Effect of electrode position on features of electrochemical PolarizationModulation Infrared Reflection Absorption Spectroscopy (PM IRRAS)

Suzanne Blatt, Sharon G. Roscoe, Vlad Zamlynny *

Chemistry Department, Acadia University, 6 University Avenue, Wolfville, Nova Scotia, Canada B4P 2R6

a r t i c l e i n f o a b s t r a c t

Article history:Received 29 October 2009Received in revised form 28 January 2010Accepted 9 February 2010Available online 16 February 2010

This work is dedicated to Professor JacekLipkowski on the occasion of his 65thbirthday in recognition of his manycontributions to the field ofelectrochemistry

Keywords:SpectroelectrochemistryPM IRRASExperiment optimizationInterfacial electrochemistrySpectra simulationsPolarization ModulationReflection Absorption Spectroscopy

1572-6657/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.jelechem.2010.02.012

* Corresponding author. Tel.: +1 902 585 1890; faxE-mail address: Vlad.Zamlynny@AcadiaU.ca (V. Za

Simulation and experimental approaches were utilized to demonstrate that the direction of spectral fea-tures of the electrochemical in situ PM IRRAS depends upon the position of the electrode within the spec-troelectrochemical cell. This behavior is the result of an increasingly stronger interaction of s-polarizedradiation with an aqueous electrolyte at larger window–electrode separations. Thus, the best perfor-mance of the experimental setup is expected for separations between the electrode and window thatare smaller than the crossover point where the reflectivities for p- and s-polarized light are equal. Positivebands, or upright peaks, should be observed for these experimental conditions. Additionally, the correla-tion between simulated and experimental spectra is improved by maintaining the aperture of the opticalbench between 10% and 15%.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

The application of infrared spectroscopy for the investigation ofadsorption of organic molecules at metal surfaces was initiated byseminal works of Greenler [1–3] who was a pioneer of ReflectionAbsorption Spectroscopy (RAS). Later, Bewick and co-workers[4–7] adapted the technique for in situ investigation of electro-chemical systems using the modulation of the electrode potentialto achieve surface sensitivity of RAS. The developed technique ispresently known as Subtractively Normalized Interfacial FourierTransform Infrared Spectroscopy (SNIFTIRS). Concurrently, Goldenet al. [8] and Russel et al. [9] used modulation of the incident beampolarization to achieve the enhancement of the signal at the metalsurface. This approach was later improved with use of Fouriertransformation by Golden and co-workers [10,11] resulting in atechnique known as Polarization Modulation Infrared ReflectionAbsorption Spectroscopy (PM IRRAS). The method has been subse-quently used by Faguy and co-workers [12,13] who demonstrated asuperior signal-to-noise ratio of PM IRRAS and its low susceptibilityto interference from atmospheric CO2 and H2O noise. Further devel-

ll rights reserved.

: +1 902 585 1114.mlynny).

opment of PM IRRAS was also made by Corn and co-workers [14,15]with the addition of the Synchronous Sampling Demodulator (SSD).

The first applications of RAS for determination of orientation ofultrathin organic films such as monolayers and bilayers were madeby Allara and Nuzzo [16]. Further research employed the infraredspectroscopy ex situ [17–21]. Early in situ studies of ultrathin filmswere usually limited to qualitative interpretation of the infraredspectra [22,23]. The first quantitative analysis of ultrathin filmsin situ was advanced by Lipkowski’s group [24–26]. The methodsto process SNIFTIRS [24] and later PM IRRAS [25,26] were devel-oped by the group to determine the orientation of ultrathin filmsof surfactants on Au(1 1 1) surfaces. Developed techniques weredocumented in a Ph.D. Thesis [27] and recently reviewed in a book[28].

The main challenge of in situ quantitative RAS stems from thestrong absorbance of infrared radiation by the aqueous electrolyte.To overcome this problem one can use either attenuated totalreflection (ATR) spectroscopy in a Kretschmann cell configurationdescribed by Osawa et al. [29] or an external reflection cell wherethe electrode and optical window are separated by a thin layerof aqueous electrolyte that can span several micrometers. Sekiet al. [30] were the first to note that optimization of separationbetween the electrode and cell window is necessary for the best

S. Blatt et al. / Journal of Electroanalytical Chemistry 649 (2010) 102–109 103

performance of the in situ RAS. Following their work, a series ofstudies contributed further developments towards optimizationof experimental methods [31–37].

The information contained in these contributions was frag-mented and sometimes contradictory which prompted us to per-form a more detailed simulation study aimed at optimizing theexperimental conditions such as the angle of incidence at the spec-troelectrochemical cell and the separation between the cell win-dow and metal electrode [38]. During our experimental workwith water soluble organic surfactants we noted that, dependingupon the experimental conditions, PM IRRAS spectra displayed fea-tures which were unexpected, i.e. peaks due to adsorbed moleculeswere directed upward or downward depending on experiment. Thepresent contribution provides an explanation for this observation.It also improves the understanding of advantages and limitationsof the application of PM IRRAS for in situ investigations of adsorbedultrathin organic films, particularly water-soluble surfactants, atmetal surfaces.

2. Methods

2.1. Fresnel simulations

Fresnel equations in matrix form described by Hansen [39]were utilized to simulate PM IRRAS spectra as well as to determinethe electric field magnitudes within a spectroelectrochemical cell.Fresnel1 software (available upon request from the correspondingauthor) was used to generate all simulated data as described in[27,28].

Reflectivities for p- and s-polarized radiation were generated byFresnel1 directly. They were used to calculate the expected PM IR-RAS spectra using the following equation:

jDRjhRi ¼

jRp � RsjRpþRs

2

ð1Þ

Fresnel1 was also used to calculate the time averaged magni-tude of the electric field, often denoted as Mean Squared ElectricField Strength (MSEFS) which is proportional to the irradiance,i.e. power of radiation arriving at a unit area per unit of time[39]. The MSEFS was used to evaluate the intensity of the infraredlight interacting with an organic film adsorbed at the metal elec-trode surface. Using Fresnel1 the (time averaged) MSEFS was alsoaveraged over the entire space between the window and the elec-trode. The obtained values, denoted as average electric fieldstrength, are proportional to the intensity of the infrared lightinteracting with organic surfactant molecules evenly distributedbetween the window and electrode as an aqueous solution. The(space) averaged electric fields for the p-polarized beam (AverageEp) and s-polarized beam (Average Es) were calculated for a rangeof electrode–window separations from 0 to 10 lm in increments of0.1 lm. To highlight the difference between the p- and s-polarizedlight, the electric field difference (Average Ep � Average Es) andthe absolute value of the difference |Average Ep � Average Es| werealso calculated and plotted.

Simulations were performed for a cell comprised of CaF2/D2O/Au as the optical window, electrolyte solvent and working elec-trode, respectively. Optical constants for these materials are avail-able in the literature [40–43]. Pyridine was assumed to adsorb as acompact monolayer of 0.5 nm thickness. Optical constants and thethickness of pyridine were taken from Li et al. [24] where the ori-ginal method for their determination is also described. Calculationswere done for an angle of incidence of 65� (the optimum angle forpyridine band at 1600 cm�1 in the CaF2/D2O/Au cell [38]) and abeam convergence of ±5� which is close to the actual angle usedin the experiments. The wavelength was either set at 1600 cm�1,

the wavenumber of the a1 peak of pyridine, or was varied from1000 to 4000 cm�1, i.e. essentially the whole mid-infrared rangeaccessible for PM IRRAS.

2.2. Experimental procedures

A 0.05 mM solution of pyridine (>99.9%, HPLC grade, Sigma–Al-drich) was prepared by dissolving pyridine in a 0.05 M KClO4 (dou-bly recrystallized, calcinated at 300 �C) solution in D2O (99.9% D,Cambridge Isotope Laboratories, Inc.). The Au(1 1 1) single crystalelectrode (15 mm diameter, grown, cut and polished in our labora-tory) was prepared by flame annealing followed by quenching withMilli-Q water. The cell was assembled as described in [27].

Nexus 8700 FTIR (Thermo Fischer Scientific) equipped withPhotoelastic Modulator (PEM) (Hinds Instruments) set to half waveretardation at 1600 cm�1 and MCT-HighD� detector (ThermoFischer Scientific) was used to collect PM IRRAS spectra. Synchro-nous Sampling Demodulator (SSD) (GWC) was utilized to demod-ulate the spectra. Two sets of 8000 scans with the resolution of2 cm�1 were collected at electrode separations of 4.0 and 6.5 lmwith respect to the cell window. The separation between the elec-trode and the window were set and determined as described ear-lier [24,27,28]. The spectra from the difference and averagechannels of SSD were subsequently co-added and saved separately.They were later processed by taking the ratio of the difference sig-nal over the average signal as shown by Eq. (1) followed by manualbaseline correction in OMNIC (v.7.1 Thermo Fischer Scientific,1992–2004). Final plots were generated in Origin (v.7.5 OriginLabInc., 1991–2004).

3. Results and discussion

3.1. Simulations

The main objective of this work was to explain the change indirection of experimental peaks observed at different separationsbetween the electrode and the infrared window. Simulations wereused to explore this observation, followed by a set of experimentsto confirm conclusions derived from the Fresnel1 simulationresults.

To achieve the set objective it was important to understandhow the infrared light interacts with the aqueous electrolyte solu-tion within the space between the window and the electrode. Forthis purpose the space-averaged Mean Squared Electric FieldStrength (MSEFS) acting across the window–electrode gap as wellas the reflectivity of p- and s-polarized light were calculated.Fig. 1A, shows the Average MSEFS for p- and s-radiation, whileFig. 1B highlights the difference and absolute magnitude of the dif-ference of the Average MSEFS and Fig. 1C shows the reflectivity ofthe CaF2/D2O/Au cell for p- and s-polarized light. Fig. 1A and B indi-cates that for small separations, the Average Ep magnitude is great-er than the Average Es due to the enhancement of p-polarized lightin the proximity of a metal surface. Because of the stronger magni-tude, the p-polarized light is more strongly absorbed by aqueouselectrolyte, resulting in its lower reflectivity with respect tos-polarized light for separations up to 5 lm (Fig. 1C). With an in-crease in separation, the space-averaged MSEFS and reflectivitiesbecome identical at ca. 5 lm and after this point the Average Epbecomes smaller than Average Es. This happens because the MSEFSof p-polarized light decreases at larger distances from the electrodewhile the reverse is true for s-polarized light. This property ofpolarized light is well known as the surface selection ruledescribed by Moskovits [44]. Consequently, the interaction ofp-polarized radiation with the aqueous electrolyte is compara-tively weaker in this range of longer distances while its reflectivity

Fig. 1. Average electric field strength (A), electric field strength (B) and reflectivity(C) of s-polarized (dashed line) and p-polarized (solid line) radiation as a function ofwindow–electrode separation. B shows electric field strength of p-polarizedradiation at the metal surface (dash-dot), difference in average electric filedstrength within electrolyte (dash double dot) and the absolute value of thedifference of the average electric filed strength (dot) within the cell. Data weregenerated using Fresnel equations with CaF2/D2O/Au cell, a wavelength of1600 cm�1, an incidence angle of 65� and the beam convergence of ±5�. Arrowsdenote separations used experimentally. �, space-averaged values within the thincavity.

Fig. 2. Calculated spectra of reflectivity of s- (solid line) and p- (dashed line)polarized light for a Au electrode positioned at 4.0 lm (A) and 6.5 lm (B) distancefrom the CaF2 prism. Data were generated using Fresnel equations with an angle ofincidence of 65� and incident beam convergence of ±5� for CaF2/D2O/Au cell. Theposition of the a1 absorption band of pyridine, at 1600 cm�1, is indicated by thearrow.

104 S. Blatt et al. / Journal of Electroanalytical Chemistry 649 (2010) 102–109

is stronger. Following the surface selection rule, an organic film ad-sorbed at a metal surface can only interact with p-polarized lightsince s-polarized light diminishes to zero at the surface. PM IRRASutilizes these properties by employing p-polarized radiation as thesurface enhanced probe that carries the signal about the adsorbedmolecules and s-polarization as the background reference. The dif-

ference between the reflectivity of p- and s-polarized radiation iscalculated to discriminate the signal from the background. This dif-ference must then be normalized by the average reflectivity of p-and s-radiation to remove the contribution of the whole opticalbench. Eq. (1) describes the procedure which results in a signalproportional to the absorbance of the surface film. It is importantto note that because the decay of the MSEFS of the p-polarizedradiation spans to distances comparable to the wavelength of theincident beam, i.e. several micrometers, in situ PM IRRAS is sensi-tive not only to the organic film directly adsorbed at the metal sur-face but also to several micrometers of aqueous solvent utilized forin situ experiments. Contribution of aqueous electrolyte typicallycauses gradual variation of the baseline, which can be removedvia background correction. It must be noted that the demodulationof experimental PM IRRAS signals produces not the difference butthe absolute value (i.e. magnitude) of the difference in reflectivitybetween p- and s-polarized radiation which is always positive.

It is apparent from Fig. 1 and the above discussion that the dif-ference between the reflectivity of p- and s-polarized radiation(Rp � Rs) could be either negative (e.g. at 4 lm) or positive (e.g.at 6.5 lm). It is always opposite in sign relative to the differenceof the Average Ep with respect to the Average Es (Fig. 1B). Thisobservation is not surprising because low absorption of the radia-tion by aqueous electrolyte should result in high reflectivity.Experimentally, only the absolute value of the reflectivity differ-ence is measured by Synchronous Sampling Demodulator (SSD),therefore the difference signal remains positive at all times.Consequently, for the negative difference between the reflectivitiesof p- and s-polarized light (e.g. at 4 lm), inversion of the reflectiv-ity of p-polarized light occurs due to taking the absolute value,

S. Blatt et al. / Journal of Electroanalytical Chemistry 649 (2010) 102–109 105

yielding positive direction of features in the PM IRRAS spectra.However, for the positive difference between the reflectivities ofp- and s-polarized light (e.g. at 6.5 lm), no inversion of p-polarizedreflectivity takes place due to taking the absolute value and thespectral features remain unchanged, i.e. negatively directed as inthe original reflectivity of p-polarized light [1–3].

The dotted line in Fig. 1B shows also the MSEFS of the p-polarizedradiation at a metal surface. It is important to note that this linerepresents the time-averaged MSEFS which is localized, i.e. notspace-averaged over the entire window–electrode gap. Themaximum displayed by this line indicates the best separationbetween the electrode and window where the greatest surfaceenhancement of p-polarized radiation takes place. It is convenientto choose this electrode position for PM IRRAS experiment becauseit yields the best signal-to-noise ratio. Hence this separation waschosen to confirm the positive direction of PM IRRAS peaks aspredicted by simulation. On the other hand, the negative bands areexpected to be best observed at a separation where the greatestnegative difference between the Average Ep and the Average Es isobserved, i.e. where absorption of s-polarized radiation by aqueouselectrolyte is highest with respect to p-polarized radiation. Hence,electrode separations of 4 and 6.5 lm, indicated by the arrows inFig. 1, were chosen for experimental tests as well as furthersimulations.

Fig. 2A and B shows the theoretically simulated mid-infraredreflectivity spectra of p- and s-polarized light for the electrodepositions of 4 and 6.5 lm, respectively. The spectra display charac-teristic transmission bands of D2O [43]. Note that the magnitude ofthe reflectivity of p-polarized light (Rp) relative to s-polarized light(Rs) is dependent on the wavenumber and displays several inver-sions of relative position for both electrode separations. Particu-larly, in the vicinity of 1600 cm�1, the region of interest shown

Fig. 3. Simulated PM IRRAS spectra generated using Fresnel equations for the system: Cthin cavities of 4 lm (A and B) and 6.5 lm (C and D). Arrows indicate pyridine a1 peak p|DR|/hRi.

with an arrow, the magnitude of the reflectivities are inverted asthe electrode position changes from 4 to 6.5 lm. This behavior is ex-pected as explained above and shown in Fig. 1C. Spectra shown inFig. 2 can be utilized to obtain the simulated PM IRRAS spectra withthe use of Eq. (1). Similar behavior was also observed for H2O as theelectrolyte solvent except that characteristic absorption bands dueto stretching and bending vibrations were located in other regionsas expected from the optical constants of this solvent.

Fig. 3 shows the resulting PM IRRAS simulations. Since PM IR-RAS represents the difference spectra, one can easily see the con-nection between the reflectivities of p- and s-polarized lightshown in Fig. 2 and the resulting PM IRRAS spectra. Fig. 3A and Cshows the PM IRRAS spectra without taking the absolute magni-tude of the reflectivity difference Rp � Rs while Fig. 3B and D showscorresponding spectra with the absolute magnitude of the differ-ence |Rp � Rs|. Note that spectra 3A and 3C cannot be measuredexperimentally while 3B and 3D are similar to the experimentalPM IRRAS shown in Fig. 6 below. Thus, comparing 3A and 3C with3B and 3D helps to understand what happens to PM IRRAS spectradue to taking the absolute magnitude of Rp � Rs. Depending on therelative magnitude of the reflectivity of p- with respect to s-polar-ized light, an inversion of the reflectivity difference spectra takesplace as highlighted by dashed lines. It is clear from the graph thatthe region of interest surrounding 1600 cm�1, highlighted with anarrow, is inverted or remains unchanged depending upon the elec-trode position of 4.0 lm (Fig. 3A and B) or 6.5 lm (Fig. 3C and D).PM IRRAS spectra display broad positive absorption bands charac-teristic of D2O vibrations [43] which represent the contribution ofthe aqueous electrolyte in the proximity to the electrode surfacediscussed above. This broad background must be removed in orderto observe much smaller features arising from the organic film ad-sorbed at the metal surface.

aF2/D2O/Au with an angle of incidence of 65�, incident beam convergence ±5�, andosition. Fig. 3A and C shows the DR/hRi while 3B and 3D show absolute magnitudes

106 S. Blatt et al. / Journal of Electroanalytical Chemistry 649 (2010) 102–109

Using optical constants of pyridine and assuming monolayercoverage with the thickness of 0.5 nm, the PM IRRAS spectrum ofthe ultrathin organic film of pyridine adsorbed at the metal elec-trode within CaF2/D2O/Au cell can be simulated using Eq. (1). Thesame equation can be used to simulate PM IRRAS of the cell with-out a pyridine monolayer at the metal. This latter spectrum can beused to subtract the contribution of the aqueous electrolyte fromthe PM IRRAS spectrum of the film at the metal surface. Fig. 4Aand B shows the resulting background-corrected spectra simulatedat 4.0 and 6.5 lm window–electrode separations, respectively. Asexpected, a positive peak is observed for an electrode position of4.0 lm where Rp < Rs and hence |Rp � Rs| = Rs � Rp, i.e. Rp is inverted(Fig. 4A) while a negative peak is observed if the electrode positionis 6.5 lm where Rp > Rs and |Rp � Rs| = Rp – Rs, i.e. Rp is not inverted(Fig. 4B). Thus one can conclude that the precise positioning of theelectrode within a spectroelectrochemical cell is expected to changethe direction of the PM IRRAS features in this spectral range. It is alsoworth noting that the magnitude of the peak shown in Fig. 4B issmaller than that in Fig. 4A. This difference can be explained as theresult of a decrease of the MSEFS magnitude at the metal electrode(shown as a dotted line in Fig. 1B) from its maximum value of ca.2.2 at 4 lm to ca. 1.5 at 6.5 lm electrode separation.

3.2. Experimental approach

To verify the validity of the above simulations, experimentalmeasurements were made with the electrode separation set at4.0 and 6.5 lm from the window. The raw experimental signals

Fig. 4. Simulated spectra of a 5 Å pyridine film at the Au surface in CaF2/D2O/Aucell. The a1 peak is observed at ca. 1600 cm�1 for electrode–window separations of4 lm (A) and 6.5 lm (B). Spectra were calculated using Fresnel equations withangle of incidence of 65� and the incident beam convergence ±5�.

arriving at the detector are shown in Fig. 5. The symbols |DR| rep-resent the absolute magnitude of the difference signal, |Rp � Rs|; hRiis the average reflectivity signal (Rp + Rs)/2; and Rp is the reflectivityof p-polarized light. Note that |DR| and hRi are the experimentalsignals normally measured during PM IRRAS experiments whileRp is determined in an independent calibration experiment. SignalRs� was calculated using experimental hRi and Rp signals as Rs

� = 2hRi � Rp. All the signals shown have been normalized by the inci-dent beam intensity, i.e. each signal has been divided by the inci-dent beam signal obtained by measuring the radiation reflectedfrom the prism attached to an empty cell. Fig. 5A and B showsthe signals recorded at 4 and 6.5 lm separations, respectively.Fig. 5C and D shows the corresponding signals magnified withinthe region shown by the rectangles in Fig. 5A and B. The shapesof hRi and Rp in Fig. 5A and C are similar to each other and are char-acteristic of D2O transmittance, i.e. similar in shape to Rp and Rs

shown in Fig. 2. The shape of |DR| in both cases is quite similarto the expected PM IRRAS spectra shown in Fig. 3B and D. The po-sitive peaks at ca. 1250 cm�1 as well as the broad-band shouldersin the vicinity of 1600 cm�1 arising due to D2O are clearly visible.The regions of interest of PM IRRAS spectra shown in Fig. 5B andD clearly indicate that the average experimental reflectivity hRiand consequently the calculated Rs

� is greater than Rp if the separa-tion is 4 lm and smaller than Rp if the separation is 6.5 lm. Thisobservation is consistent with simulated reflectivity spectra shownin Fig. 2.

Following the discussion of the simulations, one can concludethat positive PM IRRAS features are expected to be observed exper-imentally for the 4 lm separation and negative features should beobserved for 6.5 lm. Another useful feature presented in Fig. 5Aand C is the shape of |DR| signal in the region of interest close to1600 cm�1. Closer inspection of this region in Fig. 5B (4 lm) showshigh positive values of |DR| due to Rp being much lower in magni-tude than Rs (see Fig. 1C) which should result in an inversion of Rp.However, the |DR| signal in Fig. 5C (6.5 lm) is close to zero, and thepart shown in boxed region corresponds to the region where Rp isslightly higher than Rs, as expected from Fig. 1C and hence the |DR|signal should not be inverted.

The collected |DR| and hRi spectra were processed as prescribedby Eq. (1) to obtain raw PM IRRAS spectra (|DR|/hRi) followed bybaseline correction to remove the features arising due to aqueouselectrolyte in the vicinity of the metal electrode. Fig. 6A and Cshows the obtained raw PM IRRAS spectra while Fig. 6B and D de-notes the spectra after baseline correction. The spectra in Fig. 6Aand B display a broad band due to D2O absorption along with asmall peak due to pyridine film at the metal surface highlightedby a rectangle. It is visible from these raw PM IRRAS spectra thatthe direction of the pyridine peak is opposite in two figures: posi-tive in Fig. 6B (4 lm separation) and negative in Fig. 6D (6.5 lmseparation). The trends are better seen on the baseline-correctedPM IRRAS spectra shown in Fig. 6A and C which display the fea-tures without contribution of an aqueous electrolyte. Note that inaddition to the peak direction showing consistency with the simu-lated predictions, the peak magnitude is commensurate to themagnitude of the MSEFS at the metal electrode. Hence, it can beconcluded that experimental observations confirm the argumentsand conclusions advanced using simulations.

At this point it is important to make a note of a technical obser-vation. Specifically, we had difficulty achieving inverted peakswhen using apertures wider that 15%. Narrower apertures allowedexperimental spectra to more closely resemble their simulatedcounterparts. The explanation for this behavior is that theAu(1 1 1) single crystal electrode is not flat on a microscopic scalebut has a slight convex shape. As a result, larger apertures providea greater variation of window–electrode separations as opposed toa nearly parallel placement for small apertures. Fresnel1, used in

Fig. 5. Experimental spectra of 0.05 mM pyridine in a CaF2/D2O/Au cell showing relative intensities of the average (solid) and difference (dashed) PM IRRAS spectra.Reflectivity of p-polarized light (dotted) and calculated s-polarized reflectivity (dash-dot) signals with a window–electrode separation of 4.0 lm (5A and 5B) and 6.5 lm (5Cand 5D). Fig. 5B and D shows close-up views of the region of interest, shown as boxes in 5A and 5D. Arrow denotes wavelength of interest of 1600 cm�1 for the a1 pyridinepeak. Rs

� – denotes s-polarized reflectivity signal calculated using the experimental Rp and hRi.

S. Blatt et al. / Journal of Electroanalytical Chemistry 649 (2010) 102–109 107

simulations, assumes a single uniform gap between the windowand electrode. A variation in gap that occurs when larger aperturesare used results in greater discrepancies between simulated andexperimental results. Hence, smaller apertures aid closer correla-tion between experimental and simulated spectra. However, toosmall apertures can degrade signal-to-noise ratio due to the de-crease in the optical throughput. Apertures between 10% and 15%were experimentally found to have the best overall performance.

Another important conclusion relevant to PM IRRAS of water-soluble surfactants is that it is desirable to set the window–elec-trode separation as close to the value where the reflectivity ofp- and s-polarized light are equal, such as 5 lm in Fig. 1 for pyri-dine. At this point the contribution of organic molecules dissolvedin electrolyte is approaching zero (since Rp = Rs, |DR| = 0). Enhancedsurface sensitivity is thus expected. From a practical point of viewhowever, this point should be avoided because the surface signalcan also be diminished due to the inversion of the peak directionfor separations slightly less and slightly more than the crossoverpoint. As mentioned earlier, the slightly convex surface of theelectrode and a finite aperture of the infrared beam result in arange of gaps instead of a single well defined value. Thereforeinternal cancellation of fractions of positive and negative featuresof the PM IRRAS spectrum can take place. Hence, the separation

distance between the electrode and the window should be set toa value slightly smaller than the crossover point. This approachshould avoid internal peak cancellation while improving signal-to-noise ratio due to better surface enhancement of p-polarizedradiation.

4. Conclusions

The main conclusion of this research is that the direction ofpeaks observed during in situ PM IRRAS depends on the positionof the electrode with respect to the optical window. This behavioris the result of an increasing interaction of s-polarized radiationwith aqueous electrolyte at larger window–electrode separations.This is the first work aimed at examining the connection betweenelectrode–window separation and peak direction. Other contribu-tions considering optimization of RAS have focused on the angleof incidence or thin cavity thickness [30–38] in an attempt toachieve the greatest possible signal. The possibility that a reducedsignal could occur for thin cavity thicknesses of 4–6 lm, or that thespectral response in this region could change due to solvent effects,has not been previously reported. As a result of this work, it is nowknown that the electrode position should be adjusted to smallerseparations than the crossover point where the reflectivities, or

Fig. 6. Raw PM IRRAS spectra of CaF2/D2O/Au cell with angle of incidence 65� and the incident beam convergence of ±5� for window–electrode distance of 4.0 lm (A) and6.5 lm (C). Boxes indicate the region of the peak. Fig. 6B and D shows baseline-corrected spectra for 4 lm and 6.5 lm, respectively. Note the scale difference of y-axis.

108 S. Blatt et al. / Journal of Electroanalytical Chemistry 649 (2010) 102–109

p- and s-polarizations, are equal. For water insoluble organic films,the best conditions are at the point where the MSEFS of the p-polarized light is at maximum (consistent with our early publica-tion [38]). For the water-soluble molecules, a position closer tothe crossover point could be desirable to aid in offsetting the con-tribution of the solution species. It is also important to set the aper-ture between 10% and 15% to achieve a better correlation betweensimulated and experimental spectra. These apertures ensure thatthe region of the convex shaped electrode is relatively flat, as isconsistent with the assumptions of the calculations based on theFresnel equations.

Acknowledgments

This work is an invited contribution dedicated to Professor Ja-cek Lipkowski on the occasion of his 65th birthday. The corre-sponding author was fortunate to work with and learn fromProfessor Lipkowski as a graduate student.

The authors gratefully acknowledge support for this researchfrom the Natural Science and Engineering Research Council of Can-ada (NSERC), Canadian Foundation for Innovation (CFI) and theNova Scotia Research and Innovation Trust (NSRIT).

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