Anal. Chem. 1999, 71, 1967-1974
Surface-Enhanced Infrared Absorption of CO on Platinized Platinum Amy E. Bjerke and Peter R. Griffiths*
Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343 Wolfgang Theiss
Hard and Software, Dr. Bernhard Klein Strasse, 110D-52078 Aachen, Germany
We report here the first observation of surface-enhanced infrared spectroscopy on platinized platinum surfaces, as well as a thorough explanation of the resulting spectra. Smooth platinum electrodes were electrochemically platinized to produce regular metal island surfaces that led to enhanced absorption of the infrared spectrum of adsorbed carbon monoxide. The infrared spectrum of CO adsorbed from an aqueous electrolyte onto the electrode surface was measured in situ by external reflection spectrometry. The amount of adsorbed CO was estimated from the difference spectrum before and after the CO was oxidized to CO2 by finding the ratio of the absorbance of adsorbed CO prior to oxidation to the absorbance of dissolved carbon dioxide formed when the adsorbed CO was oxidized. By varying the platinization conditions, platinized Pt surfaces that yielded IR band enhancements of up to 20 times that of CO adsorbed on smooth Pt electrodes were prepared. When CO was adsorbed on a smooth Pt electrode, the shape of the band due to the CO stretching mode was quite symmetrical. As the degree of platinization was increased, the band became asymmetrical, then bipolar, and finally appeared as a reflection maximum. This behavior was simulated using the Bergman representation of effective dielectric function. Surface-enhanced infrared absorption (SEIRA) is a well-known phenomenon characterized by the enhancement of infrared (IR) absorption bands of thin layers of analytes deposited on island films of the coinage metals (Au, Ag, Cu). Trace amounts of substances applied either over or under metal films can show signal enhancement of 5-1000 times that observed for a sample without the metal film.1 Since its discovery by Hartstein through an internal reflection experiment utilizing metal under- and overlayers,2 the SEIRA effect has been studied in many different configurations. For example, Hatta et al. applied SEIRA to the detection of species at the metal/water interface3 and Terui and Hirokawa applied it to the study of thin polymer films.4 Osawa and Nishikawa have studied the phenomenon of SEIRA exten(1) Kang, S. Y.; Jeon, I. C.; Kim, K. Appl. Spectrosc. 1997, 52, 278-283. (2) Hartstein, A.; Kirtley, J. R.; Tsang, J. C. Phys. Rev. Lett. 1980, 45, 201-204. (3) Hatta, A.; Chiba, Y.; Suetaka, W. Surf. Sci. 1985, 158, 616-623. (4) Terui, Y.; Hirokawa, K. Vib. Spectrosc. 1994, 6, 309-314. 10.1021/ac981093u CCC: $18.00 Published on Web 04/17/1999
© 1999 American Chemical Society
sively, measuring transmission, external reflection, and internal reflection spectra of thin organic films on islands of Au, Ag, and Cu.5-8 Although SEIRA enhancement has mainly been observed using thin films of the coinage metals, Osawa has predicted theoretically that enhancement would be observed using other transition metals.9 The SEIRA effect is especially appealing because of its practical applications. It has been successfully applied to the analysis of trace quantities of organic molecules on a variety of dry surfaces, as well as to in situ spectroelectrochemistry.10 Although many infrared spectroelectrochemical measurements have been reported that did not need surface enhancement for the electroactive species to be observed, these measurements usually require strongly absorbing analytes in order to yield spectra of adequate signal-to-noise ratio (SNR). Because the SEIRA effect can enhance the absorbance of bands associated with surface species adsorbed on gold or silver electrodes with an island structure by at least 1 order of magnitude over the corresponding signal obtained from a smooth metal surface, it is capable of expanding the number of electrochemical systems that may be explored spectroscopically.10 Osawa and co-workers have reported extensively on applications of SEIRA to infrared spectroelectrochemistry. In 1992, they used attenuated total reflection (ATR) to study monolayers of p-nitrothiophenol on a silver electrode and reported an enhancement of 20 times over conventional IR reflection/absorption spectroscopy.11 More recently, this group published a surfaceenhanced infrared spectroelectrochemical study of heptoviologen, also on a silver electrode in an ATR configuration.12 In our laboratory, we previously used an external reflection configuration for spectroelectrochemistry. Here we report the results of SEIRA experiments using the same external reflection configuration described in our previous reports.13-15 (5) Osawa, M.; Ikeda, M. J. Phys. Chem. 1991, 95, 9914-9919. (6) Nishikawa, Y.; Fujiwara, K.; Shima, T. Appl. Spectrosc. 1991, 45, 747-751. (7) Osawa, M.; Yoshii, K., unpublished work, 1996. (8) Nishikawa, Y.; Nagasawa, T.; Fujiwara, K. Vib. Spectrosc. 1993, 6, 43-53. (9) Osawa, M.; Ataka, K.; Yoshii, Katsumasa, Y.; Yotsuyanagi, T. J. Electron Spectrosc. Relat. Phenom. 1993, 64/65, 371-379. (10) Osawa, M. Bull. Chem. Soc. Jpn. 1997, 70, 2861-2880. (11) Matsuda, N.; Katsuma, Y.; Ataka, K.; Osawa, M.; Matsue, T.; Uchida, I. Chem. Lett., 1992, 1385-1388. (12) Osawa, M.; Yoshii, K.; Hibino, Y.; Nakano, T.; Noda, I. Electroanal. Chem. 1997, 11-16. (13) Budevska, B. O.; Griffiths, P. R. Anal. Chem. 1993, 65, 2963.
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Although the factors involved in the enhancement of infrared spectral absorption are still the subject of active investigation,16,17 most workers agree that the phenomenon is caused by plasmon resonance at the surface of very small metal islands or particles of Au, Ag, In, and Cu that are in contact with the substrate. Although SEIRA experiments are generally performed using the same metals that are used for surface-enhanced Raman spectra, Aroca and Price have demonstrated that tin surfaces are also capable of causing SEIRA.18 In 1986, Nakao and Yamada reported the results of some ATR measurements designed to prove the possibility of SEIRA enhancement on Pt.19 Bulk polymers were pressed against a platinum-coated internal reflection element (IRE). Their spectra revealed enhancement factors of less than 2, and the difference in absorbance between the uncoated and platinum-coated substrates could as readily be attributed to a variation in the contact of their sample with the surface of the IRE than any effect caused by SEIRA. The size of the metal islands, and their proximity to each other, is known to be critical to SEIRA enhancement. It has been previously reported that an island size significantly smaller than the wavelength of light being absorbed is necessary to produce enhancement. Several workers have noted that the islands must be close but not touching for optimum enhancement.8,10 Other factors that affect the enhancement include the substrate material, the chemistry of the sample being observed, and the average thickness of the metal layer.10 Osawa proposed a theory for this phenomenon that suggested that SEIRA enhancement results from a combination of electromagnetic and chemical effects in a way analogous to surfaceenhanced Raman scattering (SERS).16,17 He utilized the MaxwellGarnett and Bruggeman effective medium models; these models are frequently used for metal films and assume a first-order interaction between the metal islands. Osawa has also utilized the Fresnel equations to analyze SEIRA spectra, including spectra featuring inverted and bipolar IR bands.20 In this paper, we are utilizing the Bergman representation, which we believe is a more accurate and complete model for the enhancement effect produced by metal island substrates than either the Maxwell-Garnett or Bruggeman model. The most common method used to form metal islands for SEIRA has been physical vapor deposition of the metal of interest on a smooth substrate. This method of island formation requires specialized equipment and is not yet reproducible enough for use as a quantitative analytical technique. Kang et al. recently showed that similar enhancement could be achieved on colloidal silver particles.1 Another method for forming metal islands is through electrodeposition. The surface roughness, or island size, may be controlled by changing the concentration of the platinum solution used, the voltage or current applied, and the time of platinization.21 (14) Pharr C. M.; Griffiths, P. R. Anal. Chem. 1997, 69, 4665-4672. (15) Pharr C. M.; Griffiths, P. R. Anal. Chem. 1997, 69, 4673-4679. (16) Osawa, M.; Ataka, K.; Yoshii, K.; Nishikawa, Y. Appl. Spectrosc. 1993, 47, 1497-1502. (17) Osawa, M.; Ataka, K. Surf. Sci. Lett. 1991, 262, L118-L122. (18) Aroca, R.; Price, B. J. Phys. Chem. 1997, 101, 6537-6540. (19) Nakao, Y.; Yamada, H. Surf. Sci. 1986, 176, 578-591. (20) Nishikawa, Y.; Fujiwara, K.; Ataka, K.; Osawa, M. Anal. Chem. 1993, 65, 556-562. (21) Feltham, A. M.; Spiro, M. Chem. Rev. 1971, 71, 177-193.
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Recently Lu et al. published a paper featuring their work with glassy carbon electrodes that had been electroplated in platinum solutions.22 These electrodes showed inversion and enhancement of analyte peaks, when compared to smooth platinum electrodes. These authors were unable to fully account for the dramatic change in the spectra. By varying the platinization times for deposition on a platinum electrode surface, we were able to examine this effect in detail and offer a theoretical explanation for it. Presumably Lu et al. used a glassy carbon electrode as a substrate in order to avoid using a metal substrate, which has been predicted theoretically to give no SEIRA enhancement.20 Recently, Wanzenbock et al. showed the usefulness of smooth metal surfaces prepared on glass slides as a base for depositing metal island SEIRA surfaces.23 Our studies confirm the viability of using a metal electrode surface as a substrate for SEIRA experiments. EXPERIMENTAL SECTION Instrumentation. All spectra were collected on a Bio-Rad FTS60A FT-IR spectrometer with an external optical bench equipped with a Specac (Orpington, Kent, U.K.) external reflection accessory. Interferograms were measured using a narrow-band mercury cadmium telluride (MCT) detector (Infrared Associates Inc., Cranbury, NJ). All spectra were measured at a resolution of 2 cm-1 by coadding 2048 scans. The beam (unpolarized) entered the cell at an incidence angle of 60°. The spectroelectrochemical cell, which was described previously,12-14 consisted of a Pyrex main compartment containing the working and auxiliary electrodes, which were connected to the reference electrode through a Luggin capillary. The working electrode was an 8-mm-diameter platinum disk, and the auxiliary electrode was a circle of platinum wire equidistant from the working electrode. The reference electrode was a saturated calomel electrode, and all potentials are reported versus SCE. The electrode potential was controlled with a CV-27 potentiostat (BioAnalytical Systems, West Lafayette, IN). Experimental Conditions. The electrochemical cell was prepared by soaking overnight in a 1:1 solution of concentrated HNO3/H2SO4 solution, followed by rinsing in Milli-Q water. The working electrode was prepared by polishing with 1- and 0.25µm Metadi (Buehler, Lake Bluff, IL) diamond polish and then buffed on a bare polishing cloth to remove any residue. This procedure was followed by a Milli-Q water rinse, a 10-min soak in the concentrated acid bath described above, and concluded with more rinsing with Milli-Q water. The electrode was then platinized at a constant voltage or a constant current in either in a 5% chloroplatinic acid solution or a 0.072 M chloroplatinic acid/2.6 × 10-4 M lead acetate solution. The extent of the platinum black coverage was controlled by the time of platinization. In general, longer platinization times resulted in darker depositions, which indicated the formation of larger platinum islands. After platinization, the electrode was rinsed with Milli-Q water and transferred to the spectroelectrochemical cell, which contained 1.0 M HClO4. The HClO4 solution was purged for 10 min with 99.99% pure nitrogen. The electrode was then electrochemically cleaned by cycling the electrode voltage from -0.4 to +1.5 (22) Lu, G.; Sun, S.; Chen, S.; Cai, L. J. Electroanal. Chem. 1997, 421, 19-23. (23) Wanzenbock, H. D.; Mizaikoff, B.; Weissenbacher, N.; Kellner, R. Fresenius J. Anal. Chem. 1998, 362, 15-20.
Figure 3. Difference spectrum for CO on a Pt electrode platinized with a 5% chloroplatinic acid solution at a constant voltage of 2.75 V for times of 5, 10, and 15 s. (Same experimental conditions as Figure 2.)
Figure 1. (a) Cyclic voltammogram demonstrating a clean Pt electrode surface after 2 h of cycling from -0.4 to +1.5 V and (b) voltammogram showing the oxidation of adsorbed CO to CO2.
Figure 2. Difference spectrum for CO on a Pt electrode platinized with 0.072 and 2.6 × 10-4 M Pt and 2.6 × 10-4 M lead acetate with constant current before 0.0 V and after 1.0 V oxidation. Constantcurrent platinization at 10 mA for 3 min.
V until the cyclic voltammogram (CV) showed hydrogen oxidation and reduction peaks characteristic of a clean Pt electrode in HClO4 solution (Figure 1A). After cleaning, the electrode was held at 0 V for 40 min while CO was bubbled through the cell to ensure saturation of the solution and the adsorption of a monolayer of CO on the electrode surface. A single-beam spectrum was then measured. The adsorbed CO was then oxidized from the electrode surface by increasing the voltage from 0 to 1 V at a rate of 0.1 mV/s. The voltage of the working electrode was then held at 1 V until all the CO had been removed, and a second single-beam spectrum was recorded. The ratio of spectrum measured when CO was present on the electrode surface and the spectrum taken after the CO had been completely oxidized was measured. A typical result is shown in Figure 2. The upward-going band centered at ∼2060 cm-1 is caused by the removal of adsorbed CO, while the downward-going band at ∼2350 cm-1 is due to solution-phase CO2 formed by the oxidation of the adsorbed CO. The enhancement of the band due to adsorbed CO was determined in two ways. The first was by direct comparison of the area of the CO peak to that of the solution-phase CO2 that was formed when the CO was oxidized. Because the CO2 is in
solution, and hence undergoes no enhancement, the area of this peak is directly proportional to the quantity of CO that had been adsorbed on the electrode. The second way in which the amount of adsorbed CO was determined was by comparison of the area under the CO oxidation peaks on the CVs (see Figure 1B). RESULTS Constant-Voltage Platinization. In our initial experiments, a 5 wt % solution of chloroplatinic acid was used with a constant cathodic voltage of 2.75 V. If the current was passed for less than ∼20 s, the deposited platinum was dull gray and compact by nature. Spectra in the region of adsorbed CO measured after platinization times of 5, 10, and 15s are shown in Figure 3. Although there were no noticeable changes in the appearance of the electrode between these three platinization times, there did appear to be a subtle change in the CO band shape observed for each platinization. The spectrum measured after 5 s of platinization shows almost no difference in shape from that of a spectrum of CO taken on a smooth platinum electrode, but the asymmetry of the band increases with increasing platinization times. There appeared to be little enhancement in these spectra. To achieve a different sort of deposit, platinizations were also performed using a solution of 0.072 M chloroplatinic acid and 2.6 × 10-4 M lead acetate. These depositions were also made using a cathodic voltage of 2.75 V for similar time periods. This chloroplatinic acid solution containing a small amount of lead acetate produced fluffier platinum black deposits (Figure 4). Examination of the platinized platinum electrode by an energydispersive X-ray analysis system (EDX) showed the deposits to be nearly lead free. The physical appearance of the deposits obtained using this solution varied from dull gray at 10 s to dense, sooty black at 26 s. For the shorter platinization times, there was little change in the shape or intensity of the CO band, as shown in Figure 5. With longer platinization times, however, the band shape became increasingly distorted, and a bipolar band shape was observed in the spectrum taken with the electrode prepared with 20-s platinization. For electrodes platinized between 20 and 26 s, the CO band changed from a bipolar shape to a completely inverted version of the CO peak. To find the enhancement factor for bipolar bands, the spectrum in the region between 2500 and 2000 cm-1 was baseline-corrected, squared to make all features positive, and then the square root of Analytical Chemistry, Vol. 71, No. 10, May 15, 1999
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Table 1. Ratio of the Integrated Area of the Band Due to Adsorbed CO (measured at 0.0 V) and That of the Band Due to Dissolved CO2 (measured at 1.0 V), before and after Normalization to the Corresponding Parameter Measured on Smooth Platinum platinization technique
platinization time (t), s
ratio of CO/CO2 at time t to CO/CO2 at time 0
none 2.75 V 2.75 V 2.75 V 2.75 V 2.75 V 60 mA 60 mA 60 mA 10 mA
0 10 15 20 23 26 5 10 15 3 min
1 3.2 7.0 7.4 15 0.96 1.2 3.9 19 1.2
Figure 4. Difference spectrum for CO after platinization with 0.072 M chloroplatinic acid and 2.6 × 10-4 M lead acetate at 2.75 V for times listed. (Same experimental conditions as Figure 2.)
Figure 6. Difference spectra for CO on platinum electrode platinized using a solution of 0.072M chloroplatinic acid and 2.6 × 10-4 M lead acetate at a constant current of 60 mA for times listed. (Same experimental conditions as Figure 2.)
Figure 5. SEM images of the surface of the platinum electrode after constant-voltage platinization at 2.75 V for times of (clockwise from top left) 5, 10, 15, and 20 s.
the spectrum was calculated. The actual amount of CO adsorbed on the surface varied significantly from run to run. To compensate for these variations, we made use of the fact that the area of the CO2 band in each spectrum is directly proportional to the quantity of CO in the same spectrum adsorbed on the electrode surface. Comparing the ratio of the area of the band due to adsorbed CO to the area of the band due to solution-phase CO2, the ratio of this parameter measured after platinization time t to the same parameter measured from the unplatinized electrode gives the SEIRA enhancement factor. These data are shown in Table 1. It can be seen that the enhancement factor can be increased to as much as 19 for a 60-s platinization at a constant current of 60 mA. When the surface is fully covered, however, the enhancement factor drops back to approximately unity, suggesting that the islands should not be in full contact for a significant enhancement 1970 Analytical Chemistry, Vol. 71, No. 10, May 15, 1999
to be observed. In practice, it was quite difficult to find when this point occurred as the reflectance of the platinized electrode rapidly dropped to the point that the SNR was too low to allow the area of the band due to adsorbed CO to be observed above the noise. The effect is best seen by comparing the data in Table 1 for constant-voltage platinization at t ) 23 and 26 s. The scanning electron microscope images of several of these platinized platinum surfaces shown in Figure 4 allow the physical changes associated with these spectra to be understood. There appears to be a significant increase in the platinum island size between the 5- and 10-s platinizations, even though very little change in the CO band shape is seen between the 0- and 10-s platinization times. Between 15 and 20 s, there is no noticeable increase in the Pt island diameter as seen in the SEM images (see Table 1) but the height of the islands may increase. Constant-Current Platinization. Constant-current platinizations were effected in the same chloroplatinic acid/lead acetate solution described above. Initial attempts were made to platinize the electrode using a current of 10 mA. Platinization under these conditions resulted in a compact, dark gray deposit similar to those samples platinized with constant voltage and 5% chloroplatinic acid
Figure 7. SEM micrographs of the platinum electrode surface after platinization for 15 s at a constant current of 60 mA: (left) same magnification as Figure 4; (right) 2.5× magnification for clarity. Notice the difference in the connection between the metal islands from that observed in the micrographs of the surface after platinization at constant voltage.
solution. The spectra measured from electrodes platinized with a current of 10 mA showed little change from spectra of CO on bare platinum. No signal enhancement was achieved at this current, even when platinization times were increased to 3 min (Figure 6). When the platinization current was increased to 60 mA, the resulting platinum black deposits were very black, but looked more compact than those achieved using constant voltage and the chloroplatinic acid/lead acetate solution. As in the case of the spectra measured from electrodes prepared at constant voltage, spectra on electrodes prepared with a current of 60 mA showed the CO band becoming bipolar and larger with increased platinization time (Figure 6). No spectra obtained from electrodes prepared with constant current demonstrated complete peak inversion. When the platinization time was increased beyond 15 s, the electrode became so black that its reflectance was too low for usable infrared spectra to be measured. DISCUSSION In the spectra represented in Figures 4 and 6, we see changes in band shape and enhancement with an increase in platinization time. The amount of enhancement of the CO band was found by comparing the height of the CO peak to that of the CO2 peak in each spectrum. The CO band is enhanced to 15 times its original size in the 23-s constant-voltage experiment. Enhancement by a factor of 19 was seen in the 60-mA/15-s constant-current spectrum exhibiting the greatest band distortion. Although both sets of data seemed to show an enhancement effect, SEM micrographs of the surfaces prepared at constant current show a slightly different platinum island structure from that seen in the SEM images of electrodes platinized at constant voltage (Figure 7). To support the interpretation of the experimental results, we have performed spectrum simulations for comparison. The model system is the platinum substrate covered by platinum islands that are coated by a harmonic oscillator absorbing at 2000 cm-1. A sketch of the system is shown in Figure 8. Effective Medium Considerations. Since typical dimensions of the island film are much smaller than wavelengths of infrared radiation, we have applied an effective medium approach to describe the response of the heterogeneous platinum/molecule system to electric fields of the incident light. The island structure of the metal film has been replaced by a homogeneous layer
Figure 8. Simulation model. The platinized platinum system in the left cartoon is treated as an effective medium layer (a mixture of platinum, water, and test molecules) on top of solid platinum as shown in the right cartoon. The infrared radiation is incident at 60°, equal fractions of s- and p- polarized contributions are assumed.
(called the effective medium), the optical constants of which are obtained from the corresponding quantities of the individual phases by a suitable averaging procedure. It was shown by Theiss24 and Sturm et al.25 that the effective optical constants of metal-insulator composites depend very strongly on details of the microtopology. Simple effective medium approaches commonly used (such as the Maxwell-Garnett26 or Bruggeman27 formulas) are not sufficient in most cases. A more successful general framework was given by Bergman.28 This representation is usually called the Bergman representation of effective dielectric functions. The so-called effective dielectric function eff of a two-phase composite (particles with dielectric function embedded in a matrix material with dielectric function M) is given by
(
eeff ) M 1 - f
g(u,f) du M/(M - )) - u
∫ ( 1
0
)
(1)
where f is the volume fraction of the embedded particles. The central quantity of the Bergman representation is the spectral density g(u,f), which is a distribution function of so-called “geometric resonances”24 characteristic of the microtopology of the system, where u is the integration variable. (24) Theiss, W. Festko¨rperprobleme/Advances in Solid State Physics 1994, 33, 149-176. (25) Sturm, J.; Grosse, P.; Morley, S.; Theiss, W. Z. Phys. D.-Atoms, Mol. Clusters 1993, 26, 195-197. (26) Maxwell-Garnett, J. C. Philos. Trans. R. Soc. London 1904, 203, 385. (27) Bruggeman, D. A. G. Ann. Phys. 1935, 24, 636. (28) Bergman, D. Phys. Rep. 1978, C43, 377.
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Figure 9. Spectral simulations for various stages of the platinization. The spectral density of the effective medium treatment is shown in the left row. The center row shows the (effective) complex refractive index of the effective medium layer; the real index, n, is shown as a broken line and the imaginary index, k, as a solid line. The right row shows the computed reflectance spectrum. (a) No platinum islands with a 2-nm layer of water/test molecules on top of the platinum substrate; (b) 10% volume fraction of platinum islands and a layer thickness of 10 nm, no percolation (note the different scale of the refractive index); 10% volume fraction platinum, layer thickness 10 nm, no percolation; (c) 30% volume fraction of platinum islands with a layer thickness of 20 nm, no percolation; (d) 35% volume fraction of platinum islands with a layer thickness of 20 nm and a percolation strength of 0.05.
The complex refractive index n + ik of the effective medium is simply given by the square root of the effective dielectric constant:
n + ik )xeff
(2)
Unfortunately, little is known about the relation between the microgeometry and spectral density of a system. Using the dipole approximation, small dilute spheres give rise to a single, sharp peak in the spectral density at u ) 1/3, which splits into many resonances with increasing particle density due to electrical dipole/dipole interaction.24 Densely packed systems of separated, irregularly shaped particles are characterized by a very broad peak in the spectral density whose maximum shifts toward u ) 0 for increasing volume fraction. If there is no percolation (i.e., no particle network), the value of the spectral density at u ) 0 remains zero. If particle interconnections are built up, the spectral density at u ) 0 increases 1972 Analytical Chemistry, Vol. 71, No. 10, May 15, 1999
and, for a percolated system, a δ-function at u ) 0 appears in the spectral density.29 These general features of spectral densities have been verified in a number of different cases.24,25,29,30 It has been found that details of the spectral density vary from case to case, however, and that the shape of g(u,f) must be adjusted by parameter fits to reproduce the measured optical spectra. It was shown24,25 that the infrared properties of metal/insulator composites as studied in this work are determined by the shape of the spectral density very close to u ) 0, whereas the optical appearance in the visible is given by g(u,f) in the range of approximately 0.01 < u < 0.3. An overview on the background of the Bergman representation and its use in optical spectroscopy has been discussed by Theiss.24 A discussion of its application to describe surface-enhanced infrared absorption has been presented by Theiss et al.30 (29) Theiss, W.; Henkel, S.; Arntzen, M. Thin Solid Films 1995, 255, 177-180. (30) Theiss, W.; Detemple, R.; Ozanam, F. Am. Inst. Phys. Conf. Proc. 430 1998, 586-589 (Proc. 11th Int. Conf. on Fourier Transform Spectrosc.).
Effective Optical Constants of Rough Metal Surfaces Covered with an Adsorbate. The expression for the effective dielectric function (eq 1) is restricted to two-phase composites. However, the samples considered here consist of three phases (platinum islands covered with molecules and surrounded by an aqueous solution). Since there is no satisfactory three-phase effective medium theory for metal/insulator systems we continue with the approximation that the space between the platinum islands is filled by a mixture of water and “test molecules”. For this example, the molecules are represented by a single vibrational mode described by a harmonic oscillator at 2000 cm-1. Figure 9a shows the complex refractive index chosen for the water/molecule mix and the reflectivity of a 2-nm water/molecule layer on a clean metal surface without any metal islands. The spectrum is normalized to the reflectivity computed for the same system but without the oscillator at 2000 cm-1. As indicated in Figure 9, spectra are calculated for an angle of incidence of 60° and unpolarized radiation. Three stages of “platinization” are considered starting with low coverage of the metal surface by unconnected Pt islands (see Figure 4, top right). The effective medium layer is taken to be 10 nm thick and the volume fraction of the metal is 10%. Once the spectral density is chosen for this topology, the refractive index is obtained, and the resulting reflectance spectra are shown in Figure 9b. The only effect of the presence of the metal islands seems to be the increase of the real part of the refractive index with respect to the pure system shown in Figure 9a. By increasing the metal volume fraction to 30% and the layer thickness to 20 nm and assuming a spectral density that describes a more densely packed topology (but still does not contain connections of the Pt islands), a much larger refractive index than before is obtained; see Figure 9c. An example of such conditions can been seen in the bottom picture on the left side of Figure 4. With continuing platinization, ultimately connections are built up between the Pt islands. This is called percolation, and its effect may be seen by comparing the pictures from left to the right at the bottom of Figure 4. The effect of percolation is described by selecting a characteristic increase of the spectral density toward u ) 0 and also a contribution to the spectral density in the form of a δ-function at u ) 0. This term with coefficient g0 (called the percolation strength) leads to an effective dc conductivity of a metal/insulator composite of σeff ) f g0 σ, where σ is the conductivity of bulk Pt.30 As shown in Figure 9d, the real part of the refractive index decreases as a direct consequence of the metal connections while the imaginary part of the refractive index increases significantly. This increase in the imaginary part of the refractive index is what causes the surface enhancement phenomenon. Band Inversion With Increasing Percolation Strength. Using spectral densities such as the one shown in Figure 9d, we have shown that the experimentally observed band inversion can readily be reproduced. Without changing the shape of the spectral density, the percolation strength of the system is increased from a starting value of 0.05-0.15. The complex refractive index and corresponding reflectance spectra at various stages are compared in Figure 10. With increasing percolation strength, the real part of the complex index decreases and the imaginary part increases. In the
Figure 10. Calculated reflection optical constants (left row) and reflectance (right row) for percolation strengths, g0, from 0.05 to 0.15. For all cases, the continuous part of the spectral density is the same as the one shown in Figure 9d.
extreme cases (where the real part is much larger than the imaginary part or vice versa), the absorption band appears as a dip in the reflectance spectrum. The intermediate stages, however, show a dispersion-like appearance and, for g0 ) 0.1, even an inverted dip. CONCLUSIONS The results reported here indicate that platinized platinum is indeed a substrate that can produce surface enhancement. Although a variety of platinization parameters were studied, it appeared that the method of platinization was not very useful for predicting what type of absorption band changes would be seen. Instead, the physical appearance of the platinum black deposit as determined by the naked eye was a surprisingly accurate indicator of the enhancement effect that would be encountered. As the thickness of the platinum black deposits increased with platinization time, the intensity enhancement of the band due to CO increased concomitantly. The extent of band Analytical Chemistry, Vol. 71, No. 10, May 15, 1999
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inversion also progressed with increased platinization time. Brittle, compact, gray deposits were incapable of giving the enhancement seen using softer, darker platinum deposits. As in all surface-enhanced infrared spectra, the proximity of the metal islands to each other is a very significant factor in the enhancement achieved. In the discussion based on the Bergman representation, we have shown that both the enhancement and the distortion of the CO peaks on the platinized platinum substrate could be easily explained.
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ACKNOWLEDGMENT Partial support for this work was obtained through the National Science Foundation’s Experimental Program to Stimulate Competitive Research. Received for review October 6, 1998. Accepted March 2, 1999. AC981093U
