Comparison of SERS Performances of Co and Ni Ultrathin Films over

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15348

J. Phys. Chem. C 2008, 112, 15348–15355

Comparison of SERS Performances of Co and Ni Ultrathin Films over Silver to Electrochemically Activated Co and Ni Electrodes Gustavo F. S. Andrade,† Alexandre G. Brolo,†,‡ and Marcia L. A. Temperini*,† Laborato´rio de Espectroscopia Molecular, Instituto de Quı´mica, UniVersidade de Sa˜o Paulo, CP 26.077, CEP 05513-970, Sa˜o Paulo, Brazil, and Department of Chemistry, UniVersity of Victoria, P.O. Box 3055, STN CSC, Victoria, British Columbia V8W 3P6, Canada ReceiVed: February 26, 2008; ReVised Manuscript ReceiVed: May 27, 2008

In this work, the surface-enhanced Raman scattering (SERS) spectra of pyridine (py) on thin films of Co and Ni electrodeposited on an Ag electrode activated by oxidation-reduction cycles (ORC) are presented. The SERS spectra from the thin films were compared to those of py on activated bare transition metal electrodes. It was verified that the SERS spectra of py on 3 monolayers (ML)-thick films of Ni and Co presented only bands assignable to the py adsorbed on transition metal surfaces. It was also observed that even for 50 MLthick transition metal films, the py SERS intensity was ca. 40% of the intensity from the 3 ML-thick films. The relative intensities of the SERS bands depended on the thickness of the films, and for films thicker than 7 ML for Co and 9 ML for Ni they were very similar to those of the bare transition metal electrodes. The transition metal thin films over Ag activated electrodes presented SERS intensities 3 orders of magnitude higher than the ones from bare transition metal electrodes. These films are more suitable to study the adsorption of low Raman cross-section molecules than are ORC-activated transition metal electrodes. 1. Introduction The potential of surface-enhanced Raman scattering (SERS) for monitoring adsorbates on Ag, Au, and Cu electrodes has been widely recognized since its discovery in 1974.1,2 In recent years, the limited applicability of the SERS effect to the study of only coinage metal surfaces was overcome due to several advances: (i) the establishment of efficient protocols to activate transition metal electrodes;3 (ii) the revival interest in the synthesis of transition metals nanoparticle ensembles;4 and (iii) the advent of high throughput Raman spectrometers in the early 1990s. The SERS enhancement factor from transition metals was determined to be in the 102-104 range, and the spectra of a great number of adsorbates on transition metal surfaces have been studied in past years.2 Different approaches have been proposed to enhance the Raman signal from adsorbates on transition metal electrodes. One of them is the deposition of Ag over the transition metal substrate of interest. This procedure has been used, for example, in the study of iron dissolution in borate buffer solutions by Rubim et al.,5 and in the characterization of photochemical and electrochemical decomposition of azo-dyes on TiO2 films by Bonanceˆa et al.6 Another useful approach was idealized by Tian and Fleischmann7,8 and by Leung and Weaver.9,10 It consists of the electrodeposition of a few monolayers of transition metal on electrochemically activated Ag or Au electrodes. The SERS spectra from adsorbates on these thin films of transition metal presented a much larger enhancement factor than those obtained from activated bare transition metal electrodes. One problem in this procedure is the observation of bands due to the direct interaction between the adsorbates and the underlying Ag or Au substrates, because the electrodeposited films had pinhole * Corresponding author. Phone: +(55) 11 3091 3853. Fax: +(55)11 3091 3890. E-mail: [email protected]. † Universidade de Sa ˜ o Paulo. ‡ University of Victoria.

defects. The presence of pinholes severely limits the use of these substrates in the characterization of organic adsorbates. Nevertheless, the Weaver group proposed in 1998 a low-current density protocol for obtaining pinhole-free ultrathin films of Pt, Pd, Rh, and Ir.11 The protocol involves the use of low concentration of the metallic ions (0.5-5.0 mmol L-1), and current densities that range from 20 to 200 µA cm-2. The SERS spectra of carbon monoxide on the ultra thin films synthesized using this protocol presented only bands of CO adsorbed on the Pt-group metals and not on the underlying Au substrate. This protocol has been used for the preparation of transition metal thin films SERS substrates for the adsorption of benzene and derivatives,12,13 ethylene,14 1,4-phenylene diisocyanide,15 methanol,16 hydrogen,17 and corrosion inhibitors.18 Recently, Tian et al. presented a feature article with the most up-to-date trends in this branch of SERS research.19 In the present work, the use of ultrathin films of Ni and Co over electrochemically activated Ag electrode is presented as substrates for SERS, and their performance is evaluated. The probe molecule for the SERS experiments was pyridine (py), and the results were compared to those obtained in activated bare Ag, Ni, and Co electrodes. The comparative parameters were the total intensity of the SERS signal, wavenumbers, and relative intensities of the bands. 2. Experimental Section KCl (Fluka), NiCl2 · 6H2O (Synth), and CoCl2 · 6H2O (Synth) were used as received. Py was distilled from CaCl2 before use. Deionized water (USF Elga, Maxima, model Scientific MK3, with Fwater ) 18.2 MΩ cm) was used to prepare all aqueous solutions. The electrochemical measurements were done in a potentiostat-galvanostat EG&G model PAR-273A. The auxiliary andreferenceelectrodeswereaplatinumwireandAg|AgCl|KCl(sat.), respectively. The working electrodes were polycrystalline Ag

10.1021/jp8016858 CCC: $40.75  2008 American Chemical Society Published on Web 09/05/2008

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Figure 1. SERS spectra of py on Ag electrode as compared to thin films of Co (A) and Ni (B) with 1, 2, or 3 ML thickness and to Ni and Co bare electrodes for the indicated applied potentials. Solution: 0.1 mol L-1 KCl, [py] ) 50 mmol L-1. The spectra were separated in three different spectral wavenumber ranges for clarity.

(99.99%, Aldrich), Co (99%, Votorantim Metais), and Ni (99.8%, Votorantim Metais) rods inserted in Teflon sleeves and plates inserted in resin sleeves. Prior to each experiment, the working electrodes were mechanically polished initially with 800-mesh sandpaper and subsequently with 1200-mesh sandpaper and were thoroughly rinsed with deionized water. The activation procedures of bare metal electrodes were done in the absence of py. The procedure for Ag was described in a previous work.20 The protocols for transition metals are detailed below, and for all procedures the final step was thoroughly rinsing the activated electrode with deionized water followed by a quick immersion in the working solution for SERS measurements. The activation of the cobalt electrode consisted of the following steps:21,22 (1) chemical etching: 1 min immersion in 1.0 mol L-1 HNO3 solution at a sonication bath, and thoroughly rinsing with deionized water; (2) the Co electrode was held at -1.1 V for 60 s in 0.1 mol L-1 KCl solution; (3) one oxidation-reduction cycle in 0.1 mol L-1 KCl solution was performed from -1.0 to +1.0 V at V ) 100 mV s-1 (at E ) 1.0 V, current density was between -15 and -28 mA cm-2) and back to -1.0 V at V ) 200 mV s-1; and (4) the applied potential was held at -1.2 V for 30 min for the reduction of surface oxides. The activation procedure for the nickel electrode was as follows:22,23 (1) chemical etching: 3 min immersion in 1.0 mol L-1 HNO3 solution at a sonication bath, followed by a thorough rinse with deionized water; and (2) the applied potential was held at -0.4 V for 3 s, followed by a potential step to 0.55 V for 3 s (current density was between -10 and -15 mA cm-2)

and returning to -0.4 V for 30 s in 0.1 mol L-1 KCl solution. This procedure was repeated five times. In the electrodeposition of the thin films over activated Ag (a disk of 0.13 cm of diameter), the electrolytic solution was 0.1 mol L-1 KCl and 5 mmol L-1 MCl2 (where M ) Ni(II) or Co(II)). The cathodic current was 160 µA cm-2, and the time for one monolayer (ML) deposition was 3.18 s for Ni and 3.20 s for Co. These values were obtained considering the following standard assumptions used in the literature:11 (i) The number of transition metal atoms in one ML was estimated considering a compact layer having atoms separated by the sum of atomic radii, taken from ref 24. (ii) The current efficiency for the two electron reduction of Co(II) and Ni(II) was considered as 100%. The number of ML electrodeposited throughout the text refers to multiples of the time for deposition of one ML. SERS spectra were obtained using a Renishaw Raman imaging microscope system 3000 equipped with a Peltier cooled CCD detector and an Olympus metallurgical microscope with 50× objective lens. As exciting radiation, the line at 632.8 nm from a He/Ne laser (Spectra Physics mod. 127-35) was used. The laser power at the sample was ca. 1.6 mW, and the spot focus of the laser beam was 1 µm. A PVC film was used to protect the objective lens. The resolution of the spectrometer is ca. 3 cm-1, allowing one to identify the bands at 619 and 625 cm-1 used as a diagnostic tool for the adsorption of py on Ag and Co, respectively. The scanning electron microscopy (SEM) was from a JEOL JSM-7401 F field emission gun scanning electron microscope (FEG-SEM), acquired in the LEI mode, with a gun tension of 5 kV. The energy dispersive X-ray spectroscopy (EDS) was

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Figure 2. SEM images: (1) Ag bare electrode; (2) 6 ML Ni thin film over Ag; (3) 6 ML Co thin film over Ag; (a-c) increasing magnification, from 5000× to 80 000×, as indicated.

also from the JEOL JSM-7401 F, with a gun tension of 10 kV, and the L-line of Ag, Ni, and Co was monitored. 3. Results and Discussion Figure 1 presents the SERS spectra of py on activated Ag, Ni, and Co bulk electrodes and those of py on ultrathin films (from 1 to 3 ML) of Co and Ni deposited over activated Ag electrodes. The applied potentials for each spectrum are indicated in Figure 1. The tremendous change in the total SERS intensities from the films as compared to the bulk electrodes is one outstanding parameter. The total intensity of the band at ca. 1000 cm-1 in the py/bulk Co electrode spectrum is 14 cps, but in the py/3 ML Co film spectra the intensity is 10 500 cps. For Ni surface the total SERS intensities changed from 5 cps in the py/bare Ni electrode spectrum to 13 000 cps in the py/3 ML Ni film spectra. The SERS performance of the py/transition metal film substrate can be evaluated by comparing to the total SERS intensity of py on Ag electrode, which is one of the best surfaces for SERS. The intensity ratio (py/Ag)/(py/3 ML transition metal film) is 1.3 for Co films and 1.1 for Ni films. These results show that the electrodeposited transition metal films over activated Ag surface preserve the enhancing qualities of the activated Ag electrode. One of the problems that has been found with this technique is the presence of pinholes in the electrodeposited transition

metal thin films.7-10 The most sensitive py SERS band to the nature of the surface is the ring deformation mode at 619 cm-1 (6a in Wilson’s nomenclature) in the py/Ag spectrum. This band is shifted to 625 in the py/Co and to 631 cm-1 in the spectra of py/Ni electrode; consequently, it is suitable for monitoring the presence of pinholes. Other strong bands at ca. 1000, 1215, and 1595 cm-1 did not present wavenumber shifts when the metallic surface was changed. Wu et al.25 used the 6a mode to follow the periodical trends in the interaction between py and metallic electrodes. In the SERS spectra of py on Co thin films, shown in Figure 1A, two bands in the 6a mode region, at 621 and 625 cm-1, are present for a film with 1 ML thickness, indicating the presence of py on Ag (pinholes) and py on Co. Only one band at 626 cm-1 can be observed for 2 and 3 ML. The wavenumber of this band is very similar to that of py adsorbed on bulk Co electrode, and its value does not change as the film thickness increases. The presence of only one band at 626 cm-1 is strong evidence that only bands of py on a Co surface are observed in thin films thicker than 2 ML. In Figure 1B, the SERS spectrum of py on 1 ML Ni film presents only one band at 631 cm-1; nevertheless, this band is broader than the equivalent band in the py/Ag and in the py/Ni electrodes. The broadness of this band occurs due to the presence of pinholes in the Ni film, exposing the Ag surface to the

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Figure 3. (A) SERS spectra of py on different thickness thin films of Co, for Vapp ) -0.9 V; (B) SERS spectra of py on different thickness thin films of Ni, for Vapp ) -1.0 V. The SERS spectra were vertically dislocated for clarity.

Figure 4. Experimental SERS intensity of the band at 1004 cm-1 as a function of Ni film thickness, assuming the intensity for 3 ML as 100%, considering a (10% uncertainty. The continuous lines are plots of relative intensity, with calculations based on the literature for different diameters of isolated Ag spheres.29 The broader black line was calculated considering a distribution of Ag sphere diameters, with the following contributions: 7% d ) 10 nm; 7% d ) 20 nm; 35% d ) 30 nm; 7% d ) 80 nm; 45% d ) 120 nm.

adsorption of py. On the other hand, in the spectra from the 2 and 3 ML-thick films, the band at 631 cm-1 is narrower than that for 1 ML film. The wavenumber of the 6a mode does not show any further change with film thickness, and its value is equal to that in the py/bulk Ni electrode spectrum. Similarly to the observed for Co electrodes, only SERS bands of py adsorbed on Ni are present for Ni films thicker than 2 ML. The possibility of acquiring SERS spectra of py on Ni and Co films as thin as 2 ML without interference of the underlying Ag substrate shows that the films synthesized in this work have the same quality as those prepared by Weaver et al. for Pt, Pd, Rh, and Ir.11 The growth mechanism of Ni and Co films on coinage metal surfaces for low overpotentials has been studied in the literature by in situ scanning tunneling microscopy (STM). 26 The in situ STM technique allowed one to follow the growth of the first

Figure 5. Experimental SERS intensity of the band at 1004 cm-1 as a function of Co film thickness, assuming the intensity for 3 ML as 100%, considering a (10% uncertainty (A and B). (A) Continuous lines are plots of relative intensity, with calculations based on the literature for different diameters of isolated Ag spheres.29 (B) Continuous lines were calculated considering a Co3O4 layer as a second dielectric for the indicated relative thickness (percentage of total Co layer); the broader black line was calculated considering a distribution of oxide layer thickness over the particle surface, with distribution: 65% of a 2.5% Co3O4 layer + 10% of a 2.0% Co3O4 layer + 25% of a 0.5% Co3O4 layer.

monolayers of Ni26a-d and Co,26e-g and the results indicate that the growth of the thin films of both metals follows the

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Figure 6. Relative intensity of SERS bands of py assigned to modes 1 (1004 cm-1) and 8a (1590 cm-1) as a function of film thickness. (A) Co; (B) Ni. The red hachured rectangles indicate the relative intensity of the two bands in the respective massive electrodes, considering an uncertainty of (10%. The intensity for the pure Ag electrode is presented in the figure.

Stranski-Krastanov growth mechanism. This mechanism involves the layer-by-layer growth of the film for the first monolayers, resulting in a continuous film; this initial growth is followed by the growth of tri-dimensional islands of the deposited metals.27 A similar behavior is expected in the deposition procedure used in the present work, because of the low deposition rate for the thin films on the Ag electrode. Cyclic voltammetry has been established by Weaver as an important diagnostic tool for the presence on pinholes on Pt films deposited on gold.11 However, it was not possible to use cyclic voltammetry as a diagnostic tool in this work because both Ni and Co dissolve for potentials less positive than the Ag electrode.26a,e Figure 2 presents the SEM micrographs of the SERS-active Ag electrode and of the Ni and Co thin films over Ag. One can observe that the SEM image of the SERS-active Ag electrode presents structures with diameters ranging from ∼5 µm (in the image with 5000× magnification) to ∼20 nm (in the image with 80 000× magnification). The SEM images of the 6 ML Ni and Co covered Ag electrode present a similar distribution of metallic structures of the bare Ag electrode. This indicates that the deposition of the transition metal thin films preserves the main morphological features of the SERS-active Ag electrode. The EDS spectra of the surfaces presented strong peaks assigned to the Ag surface, but due to the low thickness of the transition metal films, it was not possible to identify by EDS neither the Ni nor the Co peaks in the respective films. To assess the SERS intensity dependence on the distance of the py species to the activated Ag surface, Co and Ni films with different thickness were deposited, and the SERS spectra of py were obtained. Figure 3 shows typical py SERS spectra for three films of both metals with thicknesses of 2, 7, and 50 ML. As expected, there was a decrease in SERS intensity with the increase in the film thickness for both transition metals. It is interesting to notice that the SERS intensity of the band at 1004 cm-1 is ca. 2500 and 2000 cps for 50 ML films of Ni and Co, respectively. This result demonstrates that, even for 50 ML film thickness, the total SERS intensity is 2 orders of magnitude higher than the 5 and 14 cps obtained from the bulk Ni and Co electrodes, respectively. This result may look surprising at first glance, because the well-established exponential dependence of the SERS intensity with the distance from the surface suggests that the surface-enhanced signal should decrease fast toward the bare electrode value. Zou et al.28 also observed that the SERS intensity of carbon monoxide decays fast for the thinnest Rh films and reaches a

plateau for thicker films. The SERS intensity from the 50 ML Rh film was only ca. 30% less than from the 4 ML. These results are similar to those presented in this study. Zou et al. proposed that for the thicker films there is a significant contribution of the transition metal surface to the total SERS enhancement factor. Wasileski et al.29 showed that the behavior of the thin transition metal films over SERS-active gold surface can be reasonably modeled considering a modified electrostatic approach. The proposed model resulted in a theoretical profile similar to the experimental ones obtained for thin films of Ptgroup metals. It would be interesting to compare the theoretical results obtained by the model in ref 29 to the experimental values for Co and Ni films presented in Figures 1 and 2. Figure 4 presents the results of calculations based on the model proposed in the literature,29 considering an isolated Ag sphere immersed in water, with increasing Ni film thickness. The Ag and Ni optical constants for λ0 ) 632.8 nm were obtained from refs 30 and 31, respectively. The dashed lines correspond to the calculations for individual silver spheres of different diameters. It can be observed that none of the dashed lines provides a reasonable fit to the experimental points. However, a reasonable fit (solid line in Figure 4) was obtained when a distribution of Ag sphere of different diameters was considered. The solid line in Figure 4 resulted from the following distribution of Ag sphere diameters: 7% 10 nm, 7% 20 nm, 35% 30 nm, 7% 80 nm, and 45% 120 nm (the diameters chosen for the fitting were arbitrary and are the same as those used to calculate the dashed curves presented in Figure 4). A wide distribution of particle size on an ORC-activated Ag electrode is very reasonable and justifies the use of this procedure. The solid line in Figure 4 presents a good agreement with the experimental points, which shows that the very simple electrostatic model used in this work can semiquantitatively describe the experimental behavior of the Ni thin films. Figure 5A shows the same calculation as Figure 4, but for the Co thin films instead. Again, the dashed lines correspond to Ag spheres with varying diameters coated with different thickness of Co. The Co optical constants for λ0 ) 632.8 nm were obtained from the literature.31 One can observe in Figure 5A that the SERS intensity of py on the Co thin films decreases more rapidly with the film thickness than what was observed for the Ni films (Figure 4). Consequently, all of the calculated thickness-dependent SERS indicated a slower decrease in intensity than what was observed experimentally.

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Figure 7. (A) SERS spectra of py adsorbed on 7 ML Co thin films and the intensity of the bands at 630, 1209, and 1591 cm-1 relative to the band at 1004 cm-1 as a function of applied potential for the thin film of Co on Ag and for the bare Co electrode; (B) SERS spectra of py adsorbed on 7 ML Ni thin films; (D) same as (B), for the thin film of Ni on Ag electrode, and for the bare Ni electrode.

Normal Raman bands assigned to cobalt oxides were observed during the experiments with Co thin films, but no equivalent oxide bands were observed in the experiments with nickel thin films. In addition, it is well established that Co surfaces present greater coverage of oxide layers than Ni surfaces.32,33 These observations may help to shed light on the discrepancy between the experimental data and the theory observed in Figure 5A. A thin layer of cobalt oxides was certainly present in the electrodeposited films, and this should be responsible for the fast decrease in SERS intensities

observed for Co films. To verify this possibility, a Co3O4 layer was added to the model as a dielectric layer covering the Co film over the Ag sphere. Co3O4 was used instead of CoO because results in the literature indicate that CoO is converted to Co3O4 under laser irradiation.34 The optical properties of the Co3O4 layer were estimated from Figure 7 of ref 35. The calculated decrease in SERS intensity considering oxide layers with different percentages of thickness of the total Co thin film is shown in Figure 5B for an Ag sphere with 20 nm diameter.

15354 J. Phys. Chem. C, Vol. 112, No. 39, 2008 It can be seen in Figure 5B that the SERS intensity decreases more rapidly with the film thickness as the oxide layer increases. The calculated SERS enhancement goes to zero after 15 ML for the film with 2.5% oxide layer; on the other hand, the decrease in intensity for the 0.5% oxide layer is much slower than the experimental points. A distribution of oxide layer thickness was considered, and the result is plotted in Figure 5B as a solid black line. The calculated curve considering different thicknesses of oxide layer presents reasonable agreement with the experimental points, indicating that the influence of the oxide layer may be responsible for the different behavior of the Co films as compared to that of Ni. Another important parameter in SERS is the relative intensities of the bands. This parameter gives information about the adsorption configuration of the molecule on the surface, according to the surface selection rules derived using the electromagnetic mechanism.36 In the chemical mechanism, the changes in the relative intensities of the bands are related to changes in the bond distances and angles of the adsorbed molecule after a CT transition.37 Thus, it is of great importance that the relative intensities of the SERS bands of the adsorbates on transition metals thin films are equivalent to those on the bulk electrodes. In other words, it is important to determine how many ML provide surface properties similar to those of the bulk electrodes. This issue comes out because it is known that several surface properties extended for 5-10 ML into the material bulk. For instance, the interlayer distances are shorter close to the surface, and the bulk distances are only reached after 5 ML deep inside the metal.27 To preserve the intrinsic characteristic of the surface, the use of thicker films in SERS experiments is recommended. Figure 6 shows the change in relative intensities of the 1004 and 1594 cm-1 bands (I1/I8a, py breathing ring (1) and CC stretching (8a) modes, respectively) when the film thickness is increased. These bands were chosen as probes because this intensity ratio strongly depends on the substrate material.38 The I1/I8a ratios for bulk Ag and for different thickness of Co and Ni films are also presented in Figure 6. The results for bulk Co and bulk Ni are presented in a hachured interval considering an error of (10% in the relative intensities, which is the same amount considered for drawing the error bar for the data on thin films. For Co and Ni films with small thickness, the I1/I8a ratio is different from that of bulk transition metal electrodes. For films with thicknesses of 7 ML for Co and 9 ML for Ni, the I1/I8a ratio reaches values similar to those of the bulk electrodes, although for the Ni films some dispersion could be seen up to 15 ML. The total SERS intensities for films with these thicknesses are still very high. The results indicate that thin films of Co and Ni with approximately 7-10 ML should be used for SERS. Figure 7 presents the SERS spectra at several applied potentials (SERS potential profiles) of 50 mmol L-1 py in 0.1 mol L-1 KCl on 7 ML Co film and py on 9 ML Ni film together with the SERS profile of three bands normalized to the intensity of the 1004 cm-1 band. The SERS potential profiles of the same normalized bands from the activated bulk transition metal electrodes are also presented. The SERS potential profiles in Figure 7A and B present a good correlation between the films and the bare activated electrodes, although the signal-to-noise ratio of the SERS from films is much higher than those of activated electrodes.38 The enhancement of the bands at 625, 1211, and 1593 cm-1 relative to the band at 1004 cm-1 at more negative potential was also

Andrade et al. observed in the SERS of py on activated bare electrodes. The SERS profiles of py on films are very similar to the corresponding ones from the activated electrodes for all modes. Indeed, the difference is of about 10%. It should be noted that the measured values of relative intensities of SERS bands for Ni and Co bulk electrodes in Figure 7 are significantly different, but the same order of values is obtained for the thin films, which reinforces that the thin films present a very similar SERS behavior as compared to the bulk electrodes. These are relevant results because they show that information of the adsorption behavior of a molecular species on transition metal surfaces can be obtained faster and with high total SERS intensity when thin films, prepared using the techniques described, are used as substrates. 4. Conclusions In this work, we have shown that thin films of Co and Ni over activated silver electrode could replace the activated bulk transition metal electrodes in SERS experiments, when some experimental conditions are fulfilled. Using py as a molecular probe, it was determined that 2 ML transition metal films over SERS-active Ag present no pinhole, as detected by the position of py bands sensitive to the metal substrate. The SERS intensity dependence with film thickness was calculated using a modified electrostatic model,29 and it was demonstrated that, considering reasonable assumptions, this model agrees well with the experimental data. For the Ni thin films, a size distribution of Ag spheres needed to be considered for a semiquantitative agreement between calculation and experiment. In the case of the Co thin films, the presence of a very thin Co3O4 layer was required to account for the fast initial decrease in the SERS intensity versus Co film thickness plot. The dependence of relative SERS intensities on the transition metal film thickness was measured for the first time. The films thicker than 7 ML for py/Co film and 9 ML for py/Ni film present behavior similar to that of the correspondent bulk electrodes. Acknowledgment. G.F.S.A. and A.G.B. thank FAPESP for fellowships, and M.L.A.T. thanks CNPq for a research fellowship. We thank FAPESP for financial support, Votorantim Metais S.A. (Brazil) for the kind gift of the Co and Ni samples, and Central Analı´tica-IQ-USP for the FEG-SEM facilities. References and Notes (1) Brolo, A. G.; Irish, D. E.; Smith, B. D. J. Mol. Struct. 1997, 405, 29–44, and references therein. (2) Tian, Z. Q.; Ren, B. In Encyclopedia of Electrochemistry; Bard, A. J., Stratmann, M., Eds.; Wiley-VCH: Weinheim, 2003; Vol. 3, pp 572659; and several references therein. (3) Tian, Z. Q.; Ren, B.; Wu, D. Y. J. Phys. Chem. B 2002, 106, 9463– 9483. (4) Tian, Z. Q.; Yang, Z. L.; Ren, B.; Li, J. F.; Zhang, Y.; Lin, X. F.; Hu, J. W.; Wu, D. Y. Faraday Discuss. 2006, 132, 159–170. (5) Rubim, J. C.; Dunnwald, J. J. Electroanal. Chem. 1989, 258, 327– 344. (6) Bonanceˆa, C. E.; do Nascimento, G. M.; de Souza, M. L.; Temperini, M. L. A.; Corio, P. Appl. Catal., B 2006, 69, 34–42. (7) Fleischmann, M.; Tian, Z. Q.; Li, L. J. J. Electroanal. Chem. 1987, 217, 397–410. (8) Fleischmann, M.; Tian, Z. Q. J. Electroanal. Chem. 1987, 217, 385–395. (9) Leung, L. W. H.; Weaver, M. J. J. Electroanal. Chem. 1987, 217, 367–384. (10) Leung, L. W. H.; Weaver, M. J. J. Am. Chem. Soc. 1987, 109, 5113–5119. (11) Zou, S. Z.; Weaver, M. J. Anal. Chem. 1998, 70, 2387–2395. (12) Zou, S. Z.; Williams, C. T.; Chen, E. K. Y.; Weaver, M. J. J. Phys. Chem. B 1998, 102, 9039–9049.

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