An electrochemical study of the adsorption of pyridine and chloride ions on smooth and roughened silver surfaces

J. Electroanal. Chem., 117 (1981) 257--266

257

Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

AN ELECTROCHEMICAL STUDY OF THE ADSORPTION OF PYRIDINE AND CHLORIDE IONS ON SMOOTH AND ROUGHENED SILVER SURFACES

M, FLEISCHMANN,J. ROBINSON and R. WASER* Chemistry Department, University of Southampton, Southampton S09 5NH (England

(Received 30th May 1980)

INTRODUCTION The potential dependence of the double-layer capacitance of single-crystal and polycrystalline silver electrodes in the absence of specific adsorption (sodium fluoride and sodium sulphate electrolytes) has been extensively investigated in recent years [ 1,2] (and references quoted therein). There have also been a number of double-layer studies of the specific adsorption of halide ions on single-crystal [3] (and references quoted therein) and polycrystalline electrodes [4] while the adsorption of pyridine has been investigated by spectrop h o t o m e t r y [5] and by a radiotracer m e t h o d [6]. On the other hand, there have been extensive investigations of the Raman spectra o f pyridine adsorbed on silver electrodes in the presence of chloride ions (for reviews covering a substantial part of this field, see refs. 7 and 8; the major effort in the Raman spectroscopy of electrode--solution interfaces has been devoted to this system). It has been well established that the Ramanscattering cross-section of the adsorbed species is m a n y orders of magnitude higher than t h a t of the species in solution [9,10] provided measurements are preceded by at least one anodic--cathodic polarisation sequence to roughen the electrodes; these roughening procedures have been used for both singlecrystal and polycrystalline silver electrodes. The major aim of most of the Raman spectroscopic studies has been to find the causes o f the enhanced Raman scattering, and there has been little a t t e m p t so far to relate the measurements to the electrochemical behaviour of the system. The main objective of the work reported in this paper has been the definition of the characteristics of the double-layer capacitance of smooth and roughened single~rystal and polycrystalline silver electrodes in sodium fluoride electrolytes containing chloride or pyridine alone, as well as pyridine in the presence of chloride ions and to relate these characteristics to the Raman spectra.

* Present address: Institut fiir Physikalische Chemie, Technische Hochschule Darmstadt, 6100 Darmstadt, F.R.G. 0022-0728/81/0000--0000/$02.50 © 1981 Elsevier Sequoia S.A.

258 EXPERIMENTAL Measurements were carried out using a standard three-compartment cell. The working electrodes were sealed into close-fitting glass tubes using epoxy resin and were polished as described below; a saturated calomel electrode was used as the reference electrode and all potentials are reported on this potential scale. A Hitek 2001 potentiostat was used to control the potential of the working electrode, the chosen voltage waveforms being generated by a Chemical Electronics waveform generator Type RB1 (voltage sweeps normally at 4 mV s-1 ) in conjunction with a Levell RC oscillator, Type TG 2000 (sine waves). The ac o u t p u t from the potentiostat was analysed into the in-phase and quadrature components using a Brookdeal Ortec phase-sensitive detector Type 9501 and ac voltammograms were recorded using a Bryans X--Y recorder, Type 2600 A4. Phase adjustments and calibrations were made by using appropriate resistance/capacitance networks. A sine-wave frequency of 40 Hz of 1.4 mV rms amplitude was used for all the measurements reported in this paper. The differential capacitances have been derived directly from the quadrature component; under the conditions used here, the ratio of the in-phase to the out-of-phase c o m p o n e n t of the cell current was usually between 5 and 30, giving a maxim u m error of ~4%. In practice, the relative deviation of the capacitance vs. potential curves is smaller than this error as the curves of the in-phase component vs. potential has approximately the same shape as that for the quadrature c o m p o n e n t (as would be predicted for the equivalent circuit of a capacitor in series with a small resistor). All solutions were prepared using AnalaR grade chemicals and triply distilled water. Electrolyte solutions were cleaned further by treating with granular active charcoal for at least 24 h. Oxygen was removed by passing nitrogen, both before filling the cell and in t h e c e l l itself. Glassware was cleaned with chromicsulphuric acid, rinsed with distilled water and finally boiled twice for 3 h in fresh triply distilled water. Silver electrodes were mechanically polished with alumina down to 0.3 pm and then chemically polished in a cyanide bath [11]. Before recording capacitance curves electrodes were normally polarised to hydrogen evolution for 1 min and this was repeated until there was no change in the capacitance curves [ 1 ] (variations from this procedure in certain experiments are described below). Results obtained with the cyanide polishing procedure were found to be identical to those obtained with electrochemical polishing [ 12]. However, polishing with a chromic chloride acid mixture [13] was found to lead to poisoning of the electrodes, the adsorption of chloride and or organic substances (including pyridine) being inhibited almost completely. RESULTS Figures 1A--D show the differential capacitance of silver (100), (110), {111) and polycrystalline surfaces as a function of the chloride ion concentration in mixed sodium fluoride--sodium chloride electrolytes. The data shown in these and the subsequent figures have always been taken with the sweep in the anodic direction. The results agree closely with published data [3]. Chloride

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ions are most strongly adsorbed on (111), less strongly on (100) and least on (110) faces [3]; the data for all the faces show a capacitance hump at potentials near the point of zero charge (pzc) [in sodium fluoride solutions (0.1 M) at - - 0 . 6 9 , - 0.91 and --1.0 V for (111), (100) and (110) faces], while the peaks at the positive end of the potential range give a particularly clear indication for the reconstruction of the surface layer. The data for polycrystalline silver can

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be reasonably interpreted as a superposition of appropriately weighted curves for the single-crystal faces. Figures 2A--D show a comparable set of data for the adsorption of pyridine on the same set of surfaces in sodium fluoride solutions. The single-crystal faces all show a strong adsorption-~desorption peak at negative potentials, the potential at a given concentration depending on the nature of the face. The data for polycrystalline silver can again be interpreted as a superposition of curves for different crystal faces. The pyridine remains adsorbed over a wide potential range, while at still more positive potentials the data become highly irreproducible. Figure 2E indicates the spread of the results which have been obtained in this study: the magnitude of the capacitance depends on the previous history of the electrode, including the cathodic and anodic limits of earlier sweeps and the time elapsed from the start of the experiment. It is likely that the surface restructures in an irreproducible manner in this potential region in the presence of pyridine and fluoride. The further structure in the middle potential region [e.g. - 0.55 to --0.9 V on (110) electrodes ] will be referred to below. While the general features of the adsorption of pyridine in fluoride solutions are in accord with currently accepted views, the detailed character is clearly complex. Thus, for the (111) electrodes, Fig. 2C shows that in addition to the well-known adsorption--desorption peak and the general reduction of the differential capacitance in the region of the pzc, a new h u m p appears at negative potentials while a new capacitance peak develops a t ~ - O . 6 5 V whose position is insensitive to concentration. Figures 3A--D show the data for the adsorption of pyridine at a fixed chloride ion concentration on the same collection of smooth surfaces. On the (100) and (111) faces where C1- ions are relatively strongly adsorbed (see Fig. 1A and ref. 3) the differential capacitance curves show clearly that pyridine is adsorbed only at relatively high concentrations; a major effect of the adsorption of pyridine is the depression of the peak due to the restructuring of adsorbed C1- ions at positive potentials, the relevant capacitance peak being

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virtually eliminated at pyridine concentrations > 0 . 3 M. It is especially noteworthy that the adsorption of pyridine leads to a broad capacitance maximum in the middle potential range [e.g. --0.6 to --1.0 V on (100) electrodes]. This is most readily interpreted as being due to a coadsorption of C1-ions which can become so marked that the reorientation is thereby inhibited. The fur-

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264

ther minor structures are reproducible but cannot be interpreted at this stage. Figure 3B shows that for (110) electrodes, where C1- ions are weakly adsorbed, pyridine is strongly adsorbed at concentrations much lower than those for the (100) face. The adsorption of pyridine is accompanied by a weak coadsorption of chloride ions while the pyridine desorbs (or reorientates) at positive potentials. Figure 3C shows that pyridine is comparatively weakly adsorbed on (111) faces; the adsorption of pyridine leads to a small reduction in the adsorption of chloride ions, but the major features (adsorption and restructuring at high positive potentials) remain unchanged. As for the case of the adsorption of chloride ions and of pyridine alone, the data for the polycrystalline electrodes can be interpreted as an ensemble of superimposed plots for different crystal faces (Fig. 3D). Figure 4A shows the effect of two roughening cycles on the differential capacitance of a polycrystalline silver electrode in 0.1 M NaC1 (sweep at 0.005 V s-1 to +1.150 V followed b y a return sweep to --0.200 V; the ac voltammogram was recorded after 10 min). Comparison with Fig. 1D or Fig. 3D shows that the surface area has increased by a factor of a b o u t 10---20 and this is in agreement with other observations [ 14]. However, it is evident that apart from the change in area there is no major change in the double-layer characteristics. On the other hand, roughening in the presence of pyridine (Fig. 4B) leads t o the development of distinct features as compared to the smooth electrode (Fig. 3D), and these features can be explained in terms of a superposition of plots for individual crystal faces (Fig. 3A--C). It would appear, therefore, that the roughened electrodes develop a faceted microstructure containing a distribution of low-index faces. In line with this explanation the roughening of single-crystal electrodes (Figs. 4C--E) leads to a decrease in amplitude of the individual features and, indeed, the differential capacitance curves of all roughened surfaces resemble each other. Addition of pyridine to chloride-containing solutions after roughening leads to closely similar capacitance plots; it should be noted, however, that the Raman spectra of adsorbed pyridine are highly

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265 sensitive to the manner of addition. Equally, the Raman spectra are sensitive to the exact potential history during the roughening cycle (e.g. sweep or pulse) as well as the potential history after roughening: the intensity of the spectral lines decreases after a single potential excursion to the negative limit, whereas the differential capacitance plots are not sensitive to these factors except on a long time scale. Figure 5 illustrates this progressive decrease in the differential capacitance in chloride-ion solutions at --0.9 V. DISCUSSION The results for the adsorption of chloride ions reported in this paper are in close accord with published data [3] : adsorption on (111) and (100) faces is strong while it is relatively weak on (110) electrodes. The hump in the vicinity of the pzc has been attributed to the behaviour of water [3]. The data give good evidence for the restructuring of strongly adsorbed chloride ions at positive potentials. Kinetic measurements using pulse methods show that this restructuring takes place by a nucleative process [15]. The adsorption of pyridine in sodium fluoride solutions is much as expected. As the potential is swept positive, adsorption takes place near the pzc; at highly positive potentials the differential capacitance becomes markedly irreproducible and dependent on the previous history of the electrodes, presumably because of the restructuring of the surfaces. The adsorption of pyridine in the presence of chloride ions is partly in accord with that which would be predicted for pyridine and chloride alone. Thus, the adsorption of pyridine is strong on the (110) faces and weaker on the (100) and (111) faces in the presence of chloride ions. A surprising feature of the results is the capacitance hump for the (100) and (110) electrodes [which is especially marked for the (100) system], which indicates a marked coadsorption of pyridine and chloride ions. For the (111) electrodes the adsorption of chloride ions is only slightly affected, even by high concentrations of pyridine. The results of the electrochemical measurements are in good accord with the Raman spectroscopic data [ 16,17 ] on roughened electrodes. In the positive potential region corresponding to restructured chloride layers the Raman spectra show intense peaks due to Ag--C1- vibrations and librations which must correspond to the formation of a discrete structure. These spectral features are accompanied b y spectra due to strongly perturbed water; the spectra due to chloride adsorption and water adsorption become weak in the middle potential range, i.e. in the region negative to that where the chloride-ion layer restructures. It is likely, therefore, that chloride ions and water coadsorb in a discrete structure: by contrast, electrochemical measurements cannot give any evidence as to water adsorption. The Raman spectroscopy of pyridine in the presence of chloride ions also indicates coadsorption of the species [16] which is in accord with the differential capacitance measurements. Although the Raman spectra have always been taken on roughened electrodes, the electrochemical measurements show that these roughened surfaces behave as though they consist of an assembly of lowindex facets. While the two independent techniques lead to similar conclusions it is n o w clearly important to make Raman spectroscopic measurements on

266

unroughened single-crystal surfaces whose electrochemical behaviour is well characterised. REFERENCES

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

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