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Anal. Chem. 1905, 57, 1276-1280
Spectroelectrochemical Cell Optimized for Raman Spectrometry and Its Application to the Study of Pyridine Adsorbed onto Silver Electrodes E. Steven Brandt Research Laboratories, Eastman Kodak Company, Rochester, New York 14650
A spectrOeiectrochemicai cell designed for observing eiectrode surface and interfacial phenomena in conjunction with Raman spectroscopy Is described. The many novel features of the cell allow experimental flexlbility and ease of operatlon substantially above those of other cells. With this cell, the effect of laser illumination and excitation wavelength on the Intensity of the Raman spectrum from pyridine adsorbed to sliver was studied. The largest enhancements were found for electrochemically roughened sllver with Illumhatlon of the surface by 6824-A light during the roughening. A model is proposed to explain the phenomenon.
Since Fleischmann and co-workers first detected a Raman signal from pyridine adsorbed to an electrochemically roughened silver electrode, enormous interest has developed in using Raman spectroscopy to study surface and interfacial phenomena (1-13). This interest is a result of the realization by other workers that the Raman signal observed by Fleischmann’s group was many orders of magnitude greater than predicted by conventional Raman scattering theory (2, 3). With enhancements typically in the range of 105-106, reports of the vibrational spectra of monolayers of adsorbed species have now become common (4). Our interest in “surface-enhanced Raman scattering” (SERS) stems from its potential utility in studying the interfacial behavior of silver in a photographic environment. Although SERS has been reported for a variety of metals including copper (14), gold (14,15), platinum (16),and nickel (15), most of the work has been on silver that has been roughened electrochemically (16). The roughening procedure, using an oxidation-reduction cycle (ORC) in the presence of an alkali halide, produces a surface that can be reasonably compared to developed silver in a photographic image, and as such it provides a good electrode surface to study electrochemical processes associated with photographic chemistry including development and image stability ( I 7). During our literature survey before the start of our photographically oriented studies, we were surprised to find that experimental procedures were seldom described well enough for duplication of the experiments. Of particular concern was the lack of documentation of the design of spectroelectrochemical cells optimized for SERS, taking into account both the electrochemical and spectroscopic requirements of the experiment. The cells described also do not generally allow for rapid replacement of the working electrode, and thus it is often unclear whether results were obtained with a fresh electrode or one that had been subjected to multiple ORC roughening (18-21). We describe here the construction of a spectroelectrochemical cell optimized for observing SERS from electrode surfaces. Among the unique features of the cell are (1)easily replaceable working and reference electrodes, (2) workingelectrode geometry and fabrication that maximize the area 0003-2700/85/0357-1276$0 1.50/0
sampled for Raman scattering and minimize the total electrode area, (3) low uncompensated resistance, (4)variable positioning of the working-electrode surface with respect to the optical window, (6) chemically inert engineering materials, (7) adjustable internal volume, (8) versatile plumbing, and (9) mechanical compatibility with commercial Raman instrumentation. Using this cell, we investigated the effect of laser illumination during the ORC roughening on the SERS intensity from pyridine adsorbed to silver as a function of wavelength. We propose a model to explain the enhanced signal when the electrode surface is illuminated with long-wavelength laser light during the ORC roughening. The model considers the similarities and differences between the SERS/ORC procedure and photographic image formation.
EXPERIMENTAL SECTION Apparatus: Cell Design and Fabrication. Figure 1 is an exploded diagram showing the major components of our spectroelectrochemical cell. The body of the cell (Figure 1, F) is a 5-cm-diameter X 4-cm-long long cylinder machined from KEL-F polymer. The cell cavity is a 12.5-mm concentric hole drilled through the body of the cylinder. The faces of the cell body were machined with 8-mm recesses threaded to accept retaining rings (Figure 1,D). Both faces are grooved to accept a No. 017 0 ring set into a 25.4 X 1.6 mm concentric recess. On one end, a 25.4 X 1.6 mm thick optical-quality quartz disk window (Suprasil 11)is clamped into place to provide an entrance for the incident laser beam and an exit for the scattered light signal (Figure 1, G). On the opposite end, an adapter and a gland nut (Figure 1, E and B, respectively) are mounted in an identical arrangement. The inner diameters of the adapter, the gland nut, and the smaller 0 ring contained therein are determined by the diameter of the electrode used. The electrode (Figure 1,A) is held in place by first sliding it through the gland nut and the 0 ring and then tightening the knurled nut to compress the 0 ring around the body of the electrode rod. With appropriate indexing of the electrode rod, this arrangement allows reproducible positioning of the electrode face with respect to the quartz window, a factor that appears to influence the intensity of the SERS signal (22). As demonstrated by Fleischmann and co-workers, the ability to slide the electrode face up to the inner surface of the cell window can also be used to discriminate between signals originating from species adsorbed to the surface of the electrode and those from solution species (23).
All parts of the cell in contact with either the electrolyte or the electrode were machined from KEL-F. A version of the cell was also made from Macor, a high-density machinable glass ceramic with excellent chemical properties (Duramic Products, Inc., Palisades Park, NJ) (24). When Kalrez 0 rings are used throughout, the cell can be cleaned with a 1:l mixture of concentrated HN03 and H2S04(25). After several months of use, the cell was not degraded by this cleaning procedure, as measured by physical appearance, electrochemical behavior, and spectrophotometric response. Spectroelectrochemical Cell Configuration for Raman Spectroscopy. Figure 2 shows a cutaway view of the cell in its operational configuration as a reflection spectroelectrochemical cell. Plumbing and electrical hookups are made through the eight 0 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7. JUNE 1985
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B
C
D
E
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1. S m m i c a i cel exploded view: (A) working electrcdw (E) ccmpmdm (g!and) nut; (C) Kaher clrh~: (0) theaded retainer: (E) adapter: (F) call body: (0) Suprasil 11 optical window. n
F~QUIO3. SpeckwWoctmmicai d l with reference cmnprbnent munted to Spex banslatbn stage.
Fbum 2. S p i &I cutaway view: (A) working slecbode lead (E) Luggin probe to reference mparbnent: (C) getterlngelectrode lead (D)mnIerdleCtmde lead (E) vent pat: (F) drainlfiii pat.
regularly spaced annular ports, which are drilled and tapped to accept Omnitit fitAingn. Some of the porta were drilled at an angle sloping toward the front of the cell, and the rest were drilled perpendicular to the axis of the cell. Two of the angled ports are used to bring connections for the auxiliary and reference electrodes near the face of the working electrode (Figure 2, D and B, respectively). A 1.6-mm PTFE Teflon tube is used as a Luggin probe to connect an external reference electrode compartment to the cell. The reference electrode itself incorporates an integral salt bridge, which is separated from the reference compartment elearolyte by a porous Vycor plug held in place by heatshrinkable Teflon tubing. For disk-shaped planar working electrodes, a platinum wire mil is used as the auxiliary electrode. Thii configuration is similar to that used by Fleischmann and eo-workers in an all-glass cell of similar design (18). However, as Creighton and co-workers noted, during the roughening necessary to produce the SERS effect, this design creates a nonuniform electrode surface, owing to the uneven current flux between working and auxiliary electrodes (26). This effect can he minimized by making the working-electrode area small with respect to the auxiliary electrode, which posed no problems if the laser beam is prefocused to a spot before it strikes the electrode surface. However, for minimum damage by high laser power densities and to average out effects caused by uneven electrode pretreatment, the laser can be prefocused to a line by a cylindrical lens before it is impinged onto the working electrode (27). The experimental constraints outlined above suggest that the most efficient working electrode is rectangular. For electrodes with this geometry, the platinum spiral was replaced with a platinum gauze mask with a rectangular opening slightly larger than the working-electrode surface (see Figure 2). (For experiments in which the electrode is pushed up against the quartz window (see above), this arrangement can be easily replaced with the disk/spiral configuration.) A connection is provided to a platinum spiral electrode a t the end of the cell opposite the quartz window (Figure 2, C). This electrode cnn be used 89 a ‘gettering” or preelectrolysis electrode to clean up residual electroactive contaminants before an experiment. If the working electrode and adapter are replaced with a quartz window, the cell can be used in the transmission mode for spectroelectrochemical experiments other than Raman. In this configuration, the second platinum spiral serves as a counterelectrode for a hulk electrolysis to generate spectrochemidy active species. When the cell is fully assembled with an Emm-diameter working electrode, the electrolyte volume, excluding tubing and reference compartment, is - 2 mL. The cell volume can be
manipulated without remachining the cell body by choice of electrode diameters or by using an adapter that extends into the cell cavity (see Figure 2. E). Fabrication of Working Electrode. As mentioned above, the shape (and size) of the working electrode is dictated by spectroscopic and electrochemical criteria. Empirically, we determined dimensions of 2 X 4 mm for a planar rectangular working-electrode surface as providing the best compromise between the design constraints of uniform current distribution and surface area illuminated by the laser beam. Sealing a bar of silver into an Emm glass tube with epoxy resin (Torr Seal) provided an acceptable working electrode for initial experiments. However, special care was needed to ensure that no Raman signal was o b e e ~ e dfrom the organic matrix that might be confused with the signal of interest. A more satisfactory electrode was made by totally encapsulating a bar of silver and an electrical lead in a TFE Teflon matrix in the shape of an Emm rod, exposing only the planar surface of one end of the bar (fabrication details are available from J. R. Fluoro-Plastics, Inc., Palmyra, NY).The integrity of the TFE/silver seal was checked by electroehemical behavior during the oxidation and reduction cycle in halidecontaining electrolytes, by frequency dependence of the differential capacitance behavior in 0.05 M NaF and by disassembly of several such electrodes that had been soaked for several days in a dye-containing solution. About 80% of the electrodes produced by this procedure have been found acceptable for use. No electrode that passed initial performance checks haa later shown behavior that degraded with time, The front edge of the electrode rod was trimmed to bring the tip of the Luggin capillary close to the surface of the working electrode, thereby minimizing the uncompensated cell resistance (see Figure 2). When the working electrode is positioned, the thickness of the Luggin probe tube can be used to gauge the distance between the electrode face and the quartz window. Interfacing and Operating t h e Cell within a Spectrophotometer. Figure 3 shows the assembled cell and separate reference compartment mounted on a kinematic platform equipped with a three-dimensional translation stage (Spex Model No. 1456). To accommodate the cell,the vertical stage was backset 4 cm from the lower stages by use of a machined aluminum adapter. A second aluminum adapter provides a mount for attaching the cell and reference compartment to the stage. This arrangement allows the cell to be mounted reproducibly into the spectrophotometer sample illuminator (Spex Model No. 1459) and provides for fme-positioning the electrode surface with ream to the laser beam and spectrophotometer optia. A front surface illuminator designed and built in-house directs the laser beam onto the working-electrode surface. Once m place, the cell can be fded by opening the vent stapcnck in the line attached to the top of the cell (Figure 2, E) and filling through the bottom fill/drain tube (Figure 2, F) with an all-glass and Teflon syringe attached to a three-way Teflon valve, The electrolyte is typically degassed with purified argon for at least 15 min before it is put into the cell. The reference-compartment
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
stopcock is opened only after all bubbles have been expelled from the cell. After the reference compartment is opened, filling continues until enough electrolyte has been forced into the compartment to touch the Vycor tip of the reference electrode. The vent and fill/drain stopcocks are now closed, and the spectrometer and laser input optics are adjusted with solution in the cell. Electrodes are usually changed while the cell is still on the spectrophotometer. The cell is first drained by reversing the filling procedure, with a syringe used to aspirate electrolyte. Then the electrode is removed along with the rear retaining ring, adapter, and gland nut (Figure 2, A-E). Instrumentation. A Spex Model 1403 Ramalog spectrometer equipped with a cryocooled RCA C31034A photomultiplier tube and standard Spex photon counting and wavelength drive instrumentation is used to monitor the SERS signal from the spectroelectrochemical cell. Raman excitation is provided by a Spectra Physics Model 164 argon ion laser operating at 5145 A. For other wavelengths, the argon ion laser is used to pump a Coherent Radiation Model 590 dye laser. A presample monochromator is used to remove stray laser lines. The spectrometer is fitted with a polarization scrambler to remove the wavelength-dependent sensitivity of the depolarized surface Raman bands. Raman spectra are recorded at 0.5-2 cm-'/s, depending upon the signal-to-noiseratio, typically with a band-pass of 2-4 cm-'. Electrode potentials are controlled with conventional EG&G Princeton Applied Research electrochemical instrumentation. A comparator circuit interface controls the accumulation of charge during the anodization pretreatment necessary to produce the SERS effect on silver. All potentials are referenced to a silver/silver chloride reference electrode filled with a saturated solution of NaCl, which had a measured potential of -0.043 V vs. an SCE in 0.1 M KC1 electrolyte. Reagents. Laboratory distilled water was purified in a Milli-Q purification unit and then passed through a photolysis UV chamber to remove all traces of organic and inorganic impurities. Analysis by high-performance liquid chromatography and by published electrochemical procedures (28) revealed no measurable impurities. For reference, water thus purified compared favorably with water purified by pyrocatalytic distillation over a platinum rhodium alloy at 900 "C (28). All other reagents were analytical reagent grade, except for pyridine, which was Kodak Spectro Grade. No differences were observed in results obtained with reagents that had been purified by recrystallization and distillation and those used as received. Electrodes were made from 99.999% (metals) silver bars. Procedure. Polycrystalline silver electrode surfaces were first mechanically polished to an approximate roughness of 0.25 pm with Buehler Metadi diamond pastes and extender. The electrodes were then chemically polished, to remove residual imbedded and a cyanide diamond particles, with a 2:l mixture of 30% H202 solution described by Setty (29). The polished electrodes were soaked in 20% perchloric acid for at least 30 min to remove insoluble silver cyanide complexes produced by the chemical polishing. Just before use, the electrode was electropolished by slow stirring in a similar cyanide bath, but without hydrogen peroxide, and then soaked for 30 min in perchloric acid. After the electrode was mounted and positioned within the cell, electrolyte was introduced and potential control was established at the rest potential of the solution. The electrode was preconditioned by several scans between the cathodic limit, defined by the onset of hydrogen evolution, and a potential 50 mV cathodic of the rest potential. The cell electrolyte was then aspirated and replaced with the electrode at open circuit. Roughening to achieve the SERS effect was done as outlined earlier.
RESULTS AND DISCUSSION Electrochemical Characterization of the Spectroelectrochemical Cell. Numerous experiments have demonstrated that the SERS effect originates from species whose interaction with the electrode surface can be manipulated by potential control of the interfacial region between electrode and bulk electrolyte (16). Therefore, the geometry of the spectroelectrochemical cell is designed to minimize the path length of the incident and scattered radiation through the bulk
L 3 1
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20 24 1/ 1/2 mV l/2 s- 1/2 12
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.
Flgure 4. Electrochemical performance of the spectroelectrochemical cell. Curve a: sweep-rate dependence of the cathodic peak current for the reductlon of 1.0 mM K,Fe(CN), in 0.1 M KCI. Numbers in parentheses are ai,. Curve b: typical cyclic voltammogram obtained with the system. For these experiments a 0.079-cm2platinum working electrode was used.
solution by positioning the working electrode as close as possible to the optical window. A simple calculation, assuming a diffusion coefficient of cm2/s, predicts that even at slow sweep rates (5 mV/s) this distance, -1 mm in our cell, is still too great for the cell configuration to create thin-layer behavior of the type described by Hubbard (30). The predicted electrochemical behavior of the cell was confirmed by using a 1 mM solution of ferricyanide in 0.1 M KC1 supporting electrolyte. For this experiment, the silver working electrode was replaced with a platinum electrode of similar area. The peak current i, for both cathodic and anodic processes varied linearly with the square root of the sweep rate v1j2, confirming that the growth of the diffusion layer is unaffected by the cell geometry (Figure 4). The separation of the anodic and cathodic current peaks 4ip,theoretically 59 mV for this couple at room temperature, is given in parentheses beside each point in Figure 4a. Considering the compromises made in the placement of the reference electrode and the counter electrode to allow optical accessibility of the working-electrode surface, Aip falls within an acceptable range up to sweep rates of -100 mV/s. In practice, v rarely exceeds 20 mV/s. Figure 4b shows a representative cyclic voltammogram taken from the set used to produce the results in Figure 4a. SERS from Pyridine. Figure 5 shows the Raman signal obtained at two excitation wavelengths from (a) a freshly polished silver electrode in neat pyridine, (b) a silver electrode roughened by ORC in an aqueous electrolyte containing 0.05 M pyridine and 0.1 M NaCl with the laser beam blocked, and (c) a silver electrode in 0.1 M NaCl and 0.05 M pyridine roughened with the laser beam incident on the electrode surface during the entire cycle. In the experiments of Figure 5b and c, ORC was done on freshly prepared electrodes (see Experimental Section) while the potential was swept a t 5 mV/s. During this cycle, 50 mC/cm2 of charge was passed on the anodic sweep, which is equivalent to converting -240 monolayers of silver surface to AgCl. Charge recovery was quantitative during the cathodic reduction sweep before -0.1 V was reached, the potential at which the spectra in Figure 5 were recorded.
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985 5145 H
6824 B
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(x 2.51
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1000
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1500
500
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t s kin-') Figure 5. Raman intensities for pyridine with a silver electrode in the spectroelectrochemical cell: (a)neat, Spectro Grade pyridine, no ORC; (b) 0.05 M pyridine 0.1 M NaCI, 50 mC/cm2 ORC with laser light blocked; (c) 0.05 M pyridine 0.1 M NaCI, 50 mC/cm2 ORC with laser illumination. Spectra recorded at -0.1 V vs. Ag/AgCI.
+
+
In the two spectra of Figure 5a, the Raman intensities of the strong pyridine ring vibrations at 8 = 991 and 1031 cm-l follow the predicted fourth-power dependence on excitation frequency, with the signals obtained for 5145-A laser illumination being -3 times larger than those obtained from 6824-A radiation (31). Within experimental error, the signal is derived exclusively from solution pyridine and is enhanced only by the relatively long path length through the solution produced by specular reflection of the incident beam off the mirrored surface of the working electrode. When a silver electrode is subjected to ORC with the laser beam blocked in the presence of chloride and pyridine, the Raman signal from pyridine increases by a factor of -250 for red illumination and by -40 for green illumination, thus effectively reversing the relative intensity ratio observed with neat pyridine and smooth electrodes. However, these enhancement factors were calculated by using differences in bulk concentrations. Jeanmaire and VanDuyne were the first to recognize that the spectrum obtained after the electrode is roughened is due almost exclusively to pyridine adsorbed to the silver electrode ( 2 ) . Indeed, even a cursory comparison of the spectra in Figure 5a and b reveals differences in the number and relative intensity of the predominant spectral bands, suggesting different chemical environments for pyridine in the two sets of results. Without ORC roughening, spectra from pyridine in chloride electrolyte compare more closely to those for pure pyridine than to the SERS spectra of pyridine (16). Recalculation of the enhancement factor considering only adsorbed pyridine as the source of the Raman signal produces numbers lo4-lo5, in rough agreement with results obtained by VanDuyne and others ( 2 ) . It is surprising that for the chloride-pyridine system, the enhancement is larger for redlight-illuminated silver compared to silver illuminated by green light, even though the ORC was done in the dark and the analyses were done with comparable laser power densities. Fleischmann and Hill have noted a tendency for pyridine adsorbed to roughened silver to photolyze when illuminated with green laser light, a factor that might account for some of the change in relative intensities (32). Using 6824-A excitation, we have observed stable SERS from pyridine from up to 1 h with the electrode held a t -0.1 V. Recently Barz et al. (33) and Macomber et al. (34)showed that laser illumination during roughening substantially increased the SERS signal compared to that from a silver electrode roughened in the dark. Using the i j m metal-ligand stretching mode for chloride adsorbed to silver, Barz et al. also
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observed that the enhancement was stronger for electrodes that had been illuminated with red light from a krypton laser (6470 A) than with green (5309 A) argon laser light. These results agreed with earlier studies by Creighton and co-workers (26) and by Wetzel and co-workers (35),who also observed a wavelength dependence for SERS opposite to that predicted by normal Raman scattering theory. Figure 5c shows the enhancement of the Raman pyridine spectrum when the silver electrode is illuminated during ORC (see Figure 5b). With red illumination, the enhancement is 10 times greater than with green illumination at a similar power density. All bands, including the large band at 242 nm, which is absent in the Raman spectrum of pore pyridine, are substantially enhanced compared to the intensities observed during dark ORC roughening. The strong enhancement of the SERS effect by red illumination is unexpected, considering that the onset of the adsorption edge of pure AgCl is near 420 nm a t room temperature (36). According to accepted models, the SERS effect is derived from microscopic surface roughness produced during ORC pretreatment of the electrode (37). On the basis of these considerations, the enhancement due to the formation of these morphological features must be expected to be enhanced more by green illumination than by red illumination, since more potential nucleation sites (latent images in photographic terminology) should be created with shorter-wavelength radiation. Barz et al. have suggested that spectral sensitization by impurities, silver complexes, or silver atom clusters causes a shift to longer wavelengths for photon absorption during ORC roughening, which, in turn, creates more latent-image sites (33). This mechanism might contribute to the anomalous wavelength dependence of the component of SERS due to the ORC pretreatment, although a red shift in the onset of light absorption should increase the sensitivity to longer-wavelength radiation without greatly decreasing the sensitivity of the silver halide to shorter wavelengths. Alternatively, one can explain the phenomenon by considering the opposite point of view, that is, that the enhancement by red illumination during ORC is due to the transparency of the silver chloride layer at long wavelengths. During normal ORC roughening, several tens to several hundred layers of silver halide are produced; therefore, the interface between silver metal and silver halide lies buried on an atomic scale with respect to the surface illuminated by the laser. When the layers are reduced, the silver/silver halide interface moves toward the illuminated surface, i.e., the silver halide/solution interface. Photographically, the opposite situation exists. Latent-image centers are formed predominantly near the surface of a silver halide grain by absorption of incident radiation. Upon subsequent contact with a reducing agent or developer, only silver nucleation sites close to the surface of the grain are readily developed into filaments or clusters. In effect, the silver filament grows inward away from the solution, moving the silver/silver halide interface deeper into the silver halide matrix, which is itself consumed during the development process. In ordinary spectral sensitization, the surface of a silver halide grain is coated with about a monolayer of sensitizer, to increase the number of these developable surface nucleation sites (38). In the formation of a silver surface that exhibits the SERS effect, the area most likely to be influenced by absorption of radiation is the silver/silver halide interface, since the morphological features necessary to produce SERS are formed there. Red light can illuminate this interface with a higher quantum efficiency than green light, which is more strongly absorbed by intervening silver halide layers. There is insufficient information concerning the morphological features responsible for the SERS effect to determine
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LITERATURE CITED m a . P. J.; l.+C€umn. A. J. Unn. Rp.Len. 1974.26,123. (2) Jsanmirs. 0. L.: Va-. R. P. J . Ekmanal. Unn. 1977. 84. (1) Fb!schnnn. M.;
(3) ~ ~ . M . G . : ~ . J . A . J . A m . U n n . S o c . l O n , g s . 5 2 1 5 . (4) COOMY. R. P.: Mahoney. M. R.: McOAbn. A. J. In '"AdVeh Inhared and Raman SpecbosCopq": Claa. R. J. H.. Hesler. R. E., E*.: Heyden Rem: phibdelphia. 1982; Voi. 9. (5) Furtak. 1. E.: Reyes. J. S u l . S d . 1980. 93. 382. (6) Birke. R. L.: Lombardi. J. R.: SBndwz. L. A. A&. Unn. Sa. 1982. No. 201, 89. (7) Lee. T. K.: Wman. J. L. In "Sutace Enhanced Raman Scanedng"; Chang. R. K.. Fulsk. 1. E.. Ea.: wnum Ress: New Y a k . 1982. (8) Mahoney. M. R.: Haward. M. W.: Caoney. R. P. chem. mys. Len. 1980. 71. 59. (9) Cwney. R. P.: Mahonsy. M. R.: Howard. M. W. chan.mp.Len. 1980. 76. 448. (10) Vitko. J.. C.: Banner. R. E.: Shelby. J. E. sol. EnwgvnrSta. 1983.9 , 51. (11) 1ShW. H.: IsMsni. A. A+?#. $wcbmc. 19113. 37. 450. (12) Malendres. C. A,; Xu. S.;Tani. 0. J . E~@o.%%QI.Chem. 1984, 162.
.. ,.'f,,
'
A Flgm 6. RintOut of laser beam on e!echOde surlaw afla ORC t r e a M h 0.1 M M U with ebctrcde M n a l e d Lam wa-. 5 1 4 5 A. Laser power at sampk position. 40 mW phot raph was obtained a n a two ORC roughening cycles ot 50 mC/cm each
"B
whether the enhancement by illumination during ORC is due to a photochemical effect or is simply a manifestation of convective currents created by localized heating of the interfacial region (39). Both mechanisms could influence the morphology of the developed silver as well as the strength of this silver attachment to the bulk silver lattice and could lead to the laser 'printout effect" that we and others observed (e.& Figure 6 ) . In any event, this model explains why different workers observe illumination effects that cannot be duplicated outside of a particular laboratory. That is. the thickness of the silver halide layer produced during ORC roughening and the wavelength (as well as the power density) of the laser illumination will influence the formation of the silver surface morphology. This mechanism accounts only for the relative enhancements observed while the sample is illuminated during the ORC. As evidenced by the results in Figure 5. these are by far the largest signals recorded for the pyridine/chloride system. It does not explain why samples roughened in the dark show larger relative enhancements when the sample surface is analyzed with long-wavelength illumination after the silver halide layer has been reduced (cf. Figure 5b). To explain this intrinaic sensitivity of the SERS surface to long-wavelength excitation. some additional component such as the excitation of rnughness-dependent plasmon resonances, as suggested by Creighton and co-workers, should be considered in the overall mechanism (26, 40).
ACKNOWLEDGMENT My thanks to J. L. Lippert and especially to W.J. Brattlie for their cooperation and help in making the Raman measurements and to R. P. VanDuyne for useful discussions. Registry No. K,Fe(CN)B, 13943-583; KCI, 7447-40-1; silver, 7440.22.4; pyidine. 110-86-1.
ndn
-.
W i & . M. J.: ~ u p p J. . T.: earn. F.: J. G.. 11: philpon. M. R. J . EkbUanal. Unn. 1984. 160, 321. P e n i w . 0.: Wetrel. H. In "Stmiace Enhanced Raman Scanedng"; Chang. R. K.. F w k . . T . E., Ed%: wnum Ress: New Y a k . 1982. yamads. H.: ~smamoto.Y. chem. mp. ~ s n1981. . 77. 520. VanDuyne. R. P. In "Chemical and BlOchernical Appncauons of Lasers": Mme. C. 0.. Ed.: Academic R e s : New Y a k . 1979; Vol. 4. James. 1. H.. Ed. "The Thsav 01 me Photopraphlc ROCBSS". 4th ed.: Macmilbn: New Y a k . 1977. k(luilbn. A. J.: m a . P. J.: Fbischmann. M. J . E1BcbOanaI. chan. 1975.65. 933. Jeanmire. D. L.; SudrsnrLi. M. R.; VanDwne. R. P. J. Am. Unn. Soc. 1975. 97. 1899. Peninger. 0.: Waning. U.: Kolb. 0. M. Sa. Buusngss. phys. Unn. 1978. 82. 1328. Billman. J.: Koua~s.0.;Otto. A. suf. Sd. 19110,9 2 , 153. VanDqne. R. P.. ROCIYIsIer. NY. @ale uwnnnnicsUm. M a y 1984. Fbischmann. M.: Hendra. P. J.: Hill. 1. R.: Pembb. M. E. J. E k b o e M I . Chsm. 1981. 117, 243. ~awkridge.F. M.: ~embertm.J. E.: ~lwnt. H. w. AMI. ion.
am.
(28) Crelghlon. J. A.: Albrsdt. M. G.: Hestsr. R. E.: Malthew. J. A. 0. Chem. Pmn. Lett. 1978. 55. 55. (27) Kunz. R. E.: Gm2-m. J. 0 , II;philpon. M. R.; W n d o . A. J . ElY3manal. Chsm. 1980. 112, 391.
(28) Mnway. B. E.; Angerstrs(nXo2lowska. H.: Sherp. W. 0. A,: ClWL9. E. E. Anal. Unn. 1973. 45. 1331. 1291 sew. T. H. v. rnd. J . cham. 1987. 5. 5. Hubbard. A. T. &H. Rev. Anal. Uuwn. 1973. 2 . 201. Long. P. A. "Raman Specboscopy": Mcc+awUiii InImalkmal: New Ynrk. 1977; Chapter 2. Fleischmann. M.: Hill. 1. R. J . E k m a m I . chan. 1083. 146. 353. Bar& F.: Oadon. J. 0.. II: RlpatI.M. R.: Weaver. M. J. Unn.phys. Len. 1982. 9 1 . 291. Macomber. S. H.: F w k . T. E.: Devine, 1. M. Unn. mp.Leu. 1982. 90.439. W e u d . H.: G&sher. H.; P e H l w . 0. Unn. phyr. Len. 1081. 78. -011
(36) Okamoto. Y . Mdr. Akad. Wprr. B m n g S n . Mem--phys. K l . 2A: Ms~l--phys.-Unn. Abt. 1958. No. 14, 275. (37) Devine. 1. M.; F w k . 1. E.; Macomber. S. H. J . Dscmranal. Unn. 1984. 164. 299. (38) west. w.: ah". P. 0.. I" "The Thaay of PhOt~pMc Process". 4th ed.; J a m . 1.H.. Ed.: Macmilbn: New Y a k . 1977. (39) vOn(iutteld, R. J.: Tynan. E. E.: M a W . R. L.: Blm. S. E. Appf. Ulp. Len. 1878.35.651. (40) Bhtchlad. C. 0.: Campbsil. J. R.; O&$ltWl. J. A. suf. Sd. 1981. 108. 411.
*.
me
RFZEMKI for review October 3,1984. Accepted February 8, 1985.
