J. phys. Chem. 1983, 87, 4589-4591
4589
Laser Damage on Silver Electrode Surfaces Exhibiting Intense Raman Scattering from Cyanide Merrlck R. Mahoney and Ralph P. Gooney' Chemistry Department, Unlversliy of Newcastle, New South Wales, Austral& 2308 (Received: December 28, 1982)
Spectroscopic and electrochemical evidence indicates that localized laser-assisted oxidation and dissolution effects occur on silver electrodes exhibiting surface-enhanced Raman scattering (SERS) from cyanide in the system: Ag/O.Ol M KCN, 0.1 M Na$04. The evidence for the laser damage processes emerges from the following investigations: perturbation of steady-state currents and cyclic voltammograms by uniform low-flux surface illumination; laser-interruption effects on SERS intensities and observation of a laser damage-zone (0.13 mm) on vapor-deposited silver electrode surfaces by scanning electron microscopy (SEM). The SEM studies reveal that corrosion of the silver surface is more pronounced within the laser damage zone.
Introduction The role of the laser beam in creating localized composition changes in SERS systems has been the subject of several recent investigations. The present authors detected a laser damage spot on a silver surface exhibiting SERS for an aromatic base.'p2 Subsequent Auger electron spectroscopic (AES) analysis revealed a laser damage spot of pure carbon for the Aglpyridine system where the evidence indicated that the primary source of carbon was ~ y r i d i n e . ~ For the non-carbonizable electrosorbate, cyanide, the AES data revealed a damage zone deficient in carbon and silver but enriched in oxygen, potassium, and sulfur.3 These results supported earlier spectroscopic studies indicating that Aglpyridine SERS was carbon as~ociated.49~ and Aglcyanide S E W was oxide associated.5@ Other recent studies have also provided indications of an active role for the laser beam in SERS systems not involving cyanide. Fleischmann et al.' and Macomber et aL8 have invoked the laser photodecomposition of anodically formed silver halide. Chen et al.9 have observed that the morphology of the silver electrode is modified by laser illumination during the oxidation-reduction cycles in alkali halide electrolytes. In the present study, the formation of soluble cyanosilver(1) complexes (rather than the insoluble silver halide phases) would be expected to lead to reduced surface passivation and greater laser damage. Visible surface black spots with accompanying carbon spectra appeared for anodized illuminated silver electrode surfaces in the presence of each of several carbonizable adsorbates (viz., ethylenediamine, glycine, etc.).1° A laser power threshold had been previously described by Van Duyne." (1)R. P. Cooney, M. W. Howard, and M. R. Mahoney, Electrochemical Soceitv Surine Meetinn. Minneauolis. 1981.Abstract No. 367. (2)R. P Cwne;, M. W. Howard, hi. R. Mahoney, and T. P. Memagh, Chem. Phys. Lett., 79,459 (1981). (3)R. P. Cooney, M. R. Mahoney, T. P. Mernagh, and J. A. Spink, J. Phys. Chem., in press. (4)T. P. Mernagh and R. P. Cooney, J. Raman Spectrosc, 14, 138 (1983). (5)R. P. Cooney, M. R. Mahoney, and A. J. McQuillan in 'Advances in Infrared and Raman Spectroscopy", R. J. Clark and R. E. Hester, Ed., Heyden, London, 1982,Vol. 9,Chapter 4. (6)M. R. Mahoney and R. P. Cooney, J. Raman Spectrosc., 11, 141 (1981). (7)M. Fleischmann and I. R. Hill in "Surface Enhanced Raman Scattering", R. K. Chang and T. E. Furtak, Ed., Plenum, New York, 1982. (8)S. H. Macomber, T. E. Furtak, and T. M. Devine, Chem. Phvs. Lett., SO, 439 (1982). (9)T.T.Chen. K. U. Von Raben. J. F. Owen. R. K. Chane. and B. L. Laube, Chem. Phys. Lett., 91, 494 (1982). (10)R. A. Kydd and R. P. Cooney, J. Chem. Sod., Faraday Trans. 1, in press. I
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QQ22-3654/83/2Q87-4589$Q 1.5010
While the AES study3 established the composition changes caused by the laser, the present study seeks to elucidate the mechanism of damage under working electrode conditions for the silverlcyanide SERS case. I t represents an extension of the pH dependence study which suggested oxide involvement in the cyanide SERS effect! A feature of the present study is the use of "uniform" electrode illumination techniques to bridge the interpretation "gap" created by the contrast in microsampling by Raman spectroscopy and whole-electrode sampling of the electrochemical techniques.
Experimental Section Silver electrodes were prepared from Matthey Garret specpure polycrystalline silver foil or wire. Vapor-deposited silver electrode surfaces were prepared on 8 mm diameter glass disks. Such surfaces appear smooth at magnifications of X60000 when viewed by SEM.4 The electrolyte salts were analytical reagents and distilled fractionated water was used.12 The electrolyte prior to adding the KCN was deoxygenated by using an extended (12 h) N2 purge. After KCN addition the natural pH of the medium was 10.3. The spectroscopic and electrochemical equipment has been previously described.12J3 A cell of design similar to that of Pettinger et al.14 was used. The electrode surfaces were pretreated in three stages: (a) mechanical polishing; (b) etching (30 s) in 1:l mixture (v/v) of 70% aqueous ammonia and 27% aqueous H202;and (c) the surface was fully reduced (i.e., rendered silver(1) and carbon free) by standing at -1.4 V for 60 m.5J2 All potentials were measured relative to a saturated calomel reference electrode (SCE). The oxidation-reduction cycle (ORC) or anodization pretreatment followed that of Billmann et al.,15 viz. a step to +0.5 V, a pause for 5 s, and a step back to the original potential. The original potential was usually either -0.95 or -0.8 V. Unless stated, all ORC were performed with the working electrode exposed to the laser. The SERS exciting line was 514.5-nm Ar+ (100 mW) and was spot focussed. To achieve "uniform" illumination a l-mm silver wire section surface was used as the working electrode. The laser was used in an unfocused condition (to fill the 1mm (11)R. P. Van Duyne, Chem. Biochem. Appl. Lasers, 4, 101 (1979). (12) M. W. Howard, R. P. Cooney, and A. J. McQuillan, J. Raman Spectrosc., 9, 273 (1980). (13) M. W. Howard and R. P. Cooney, Chem. Phys. Lett., 87, 299 (1982). (14)B. Pettinger, U.Wenning, and D. M. Kolb, Ber. Bunsenges. Phys. Chem., 82, 1326 (1978). (15)J. Billmann, G.Kovacs, and A. Otto, Surf. Sci., 92, 153 (1980).
0 1983 American Chemical Soclety
4590
Mahoney and Cooney
The Journal of Physical Chemistry, Vol. 87, No. 23, 1983
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1 Figure 1. The effect of low flux uniform laser illumination on steadystate currents for the system: Ag/O.Ol M KCN, 0.1 M Na2S0,. Solid curve, laser illumination; dotted curve, no laser. Current densities calculated by assuming a roughness factor of 10.
diameter surface), and the output power was increased to partly compensate for the reduction in flux. The “percent of SERS flux” quoted later assumes a focused laser spot dimension of 0.03 mm and an unfocused laser beam diameter of 1.4 mm. The “percent of SERS flux” parameter is 100% for a conventional SERS spectrum recorded by using 100 mW of laser power. Clearly given variations in flux across the gain profile of the unfocused beam, the term “uniform illumination” is intended to have relative not absolute significance. It is intended to contrast with the usual SERS condition of nonuniform surface illumination by using a focused laser. Results and Discussion Laser-Assisted Oxidation (12% of SERS Flux). The electrochemical evidence for laser involvement in the formation of intensely scattering species emerges from the sensitivity of the steady-state currents (Figure 1)and the cyclic voltammogram (Figure 2) to low-flux uniform laser illumination (see Experimental Section) of the electrode surface. The scanning Auger micrographs and AES data3 revealing a silver-deficient laser damage zone is direct compositional evidence of a laser-assisted oxidation zone. Uniform illumination of a small electrode leads to an increase in current flowing for potentials more anodic than the potential of onset of surface oxidation (ca. -0.7 V, Figure 1). For potentials more negative than this, Le., in the region in which essentially no electrochemical reaction is occurring, the illuminated and nonilluminated currents
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Flgwe 2. The effect of low flux uniform laser illuminatbn on the cyclic voltammetry of the system: AglO.01 M KCN, 0.1 M NalSO,. S o l i curve, no laser; dotted curve, laser Illumination. Current densities calculated by assuming a roughness factor of 10.
are very similar. Therefore the presence of laser flux on the electrode surface at potentials more anodic than -0.7 V (for example, during an ORC) would therefore result in some laser-assisted oxidation. The laser-sensitivity of the cyclic voltammogram under uniform laser illumination conditions were detectable at very low laser flux (0.5% of SERS flux). Peak currents changed and at higher laser flux (2% of SERS flux) the cyclic voltammogram changed in the +0.3- to +0.4-V region. Under conditions which favored greater electrode surface warming under the laser (see later) inversion of cathodic and anodic sweeps was observed in that potential region. Generally, under uniform laser illumination the currents flowing at potentials more anodic than -0.6 V are significantly increased (Figure 2). Also the effect of the uniform laser illumination on the cyclic voltammetry continued for a few minutes after the laser beam was blocked from the surface. Such a “memory” effect argues in favor of laser flux warming the silver electrode surface, rather than a photochemical effect. The relatively large dimensions of the laser damage zone revealed by SEM (0.13 mm) and previously by SAM? compared to the expected dimensions of the laser spot (ca. 0.01 mm), also suggest that a warm laser zone exists around the point of laser focus.3 Such laser warming may increase the rate of silver dissolution by cyanide within the warm zone. Significantly, uniform laser illumination of the surface for a Na2S04electrolyte containing no cyanide did not reveal laser-voltammetric changes comparable to those observed with cyanide present. It is clear that the chemistry of silver corrosion (leaching)by cyanide5J6is of central importance to the Ag/CN- SERS effect. Laser-Assisted Dissolution (100% of SERS Flux). The intensity of the 2113-cm-’ (uCN) SERS line was followed with time (Figure 3) until it attained a maximum (B) and had started to decline (B to C). Once the trend of intensity loss (B to C)was established, the laser beam was blocked for a significantly long interval of time (C to D). The laser beam was then restored to the electrode surface and the intensity monitored once again (Dto E). If the decline in intensity B to C was not associated with the laser, then after the interval of laser interruption the original trend (C to C’)should have continued. However, if the decline in intensity is laser induced (Le., a laser-damage effect has occurred) no intensity loss should have occurred during the interval of laser interruption (C to D)but the rate of (16) F.A.Cotton and G. Wilkimon, “Advanced Inorganic Chemistry”, 4th ed, Wiley-Interscience, New York, 1980, pp 968-9.
The Journel of Physh1 chemistry, Vol. 87, No. 23, 1983 4591
Laser Damage on Slhrer Electrode Surfaces
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Flgure 3. Cyanide SERS lntenstty-time m e showing the effect of laser interruption. The time refers to time after the ORC.
decline should recommence (at D) on the reintroduction of laser flux. The experimental data (Figure 3) therefore clearly indicate that the loss of cyanide intensity is laser induced. The involvement of the laser in the cyanide line intensity decline would appear to result from simple thermal diffusion/dissolution phenomena. This was indicated by the changes in the cyclic voltammogram when the electrolyte was stirred. These changes were of the same form as, but of greater magnitude than, the changes caused by uniform laser illumination. The uninterrupted intensity-time curves recorded by using 514.5-nm Ar+ and 647.1-nm Kr+ excitation were similar in appearance. This suggests that photon-energy-sensitive phenomena are of secondary importance. Laser Morphological Damage (100% of SERS Flux). To confirm the presence of a laser damage zone, we examined the surface of a vapor-deposited silver electrode (after anodization-illumination) by SEM. A laser damage zone of 0.13 mm diameter was detected. The observation of the damage zone on vapor-deposited silver was consistent with the SAM/AES data3 for solid silver (wire) electrodes. However, the magnitude of the laser damage zone was somewhat larger for solid silver electrodes than for vapor-deposited silver electrodes. This difference presumably arises from the contrasting thermal conduction properties of silver metal and the glass substrate used for vapor-deposited surfaces. Examination of scanning electron micrographs of the laser damage zone reveal substantial differences in morphology compared to the nonilluminated surface (Figure 4). The laser damage zone incorporatescorrcwion channels and residual metal islands which are almost absent on the nonilluminated surface (Figure 4). Electron micrographs a t higher magnification than in Figure 4 confirm the complex nature of the modified surface morphology within the laser-damage zone. This evidence supports the earlier conclusion that the laser beam increases the rate of surface oxidation (corrosion). The corrosion channels are presumably the result of the laser-assisted oxidation and dissolution phenomena The morphological change arising from laser damage would appear to be more pronounced in the present study of the Ag/CN- system than in the case
II.I,!IMlNATE~-~rlR~A AFTER ~~ THE OR$
Flglm 4. s c a mtihctmn mlaogaphs (X3000) showing the change in morphology r fo m the nonRlwninated surface to the laser-illuminated damage zone.
of Ag/halide as reported by Chen et al?
Conclusion The evidence presented here suggests that localized laser warming of the silver electrode surface accelerates the cyanide corrosion reactions and assists in the dissolution of the silver(1) complexes formed. These two processes result in the formation of the silver-depleted laser damage zone characterized by SEM/AES/SAM.3 Relative to the nonilluminated surface, the laser damage zone is substantially rougher (Figure 4) and presumably richer in cyanosilver(1) complexes. Therefore it is clear from the present data that the status of enhancement factors calculated from parameters for the nonilluminated surfaces should be revised. However, such a revision is complicated by the difficulty of selectively monitoring the increase in surface coverage for a miniscule laser zone (1mm). Acknowledgment. The authors are grateful to A.R.G.S. for providing the equipment for this project. Registry No. KCN, 151-50-8; Na2S0,, 7757-82-6;silver, 7440-22-4.
