Electrochemical and Surfaced-Enhanced Raman Spectroscopic

University, Oxford, Ohio 45056. Received January 2, 2005. In Final Form: April 26, 2005. Corershell AurPt nanoparticles were synthesized by using a se...
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Langmuir 2005, 21, 7449-7455

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Electrochemical and Surfaced-Enhanced Raman Spectroscopic Investigation of CO and SCN- Adsorbed on Aucore-Ptshell Nanoparticles Supported on GC Electrodes Bin Zhang,† Jian-Feng Li,‡ Qi-Ling Zhong,*,† Bin Ren,‡ Zhong-Qun Tian,‡ and Shou-Zhong Zou*,§ Department of Chemistry, Jiangxi Normal University, Nanchang 330027, China, Department of Chemistry, State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, China, and Department of Chemistry & Biochemistry, Miami University, Oxford, Ohio 45056 Received January 2, 2005. In Final Form: April 26, 2005 Core-shell Au-Pt nanoparticles were synthesized by using a seed growth method and characterized by transmission electron microscopy, X-ray diffraction, and UV-vis spectroscopy. Aucore-Ptshell/GC electrodes were prepared by drop-coating the nanoparticles on clean glassy carbon (GC) surfaces, and their electrochemical behavior in 0.5 M H2SO4 revealed that coating of the Au core by the Pt shell is complete. The electrooxidation of carbon monoxide and methanol on the Aucore-Ptshell/GC was also examined, and the results are similar to those obtained on a bulk Pt electrode. High quality surface-enhanced Raman scattering (SERS) spectra of both adsorbed CO and thiocyanate were observed on the Aucore-Ptshell/GC electrodes. The potential-dependent SERS features resemble those obtained on electrochemically roughened bulk Pt or Pt thin films deposited on roughened Au electrodes. For thiocyanate, the C-N stretching frequency increases with the applied potential, yielding two distinctly different dνCN/dE. From -0.8 to -0.2 V, the dνCN/dE is ca. 50 cm-1/V, whereas it is 90 cm-1/V above 0 V. The bandwidth along with the band intensity increases sharply above 0 V. At the low-frequency region, Pt-NCS stretching mode at 350 cm-1 was observed at the potentials from -0.8 to 0 V, whereas the Pt-SCN mode at 280 cm-1 was largely absent until around 0 V and became dominant at more positive potentials. These potential-dependent spectral transitions were attributed to the adsorption orientation switch from N-bound dominant at the negative potential region to S-bound at more positive potentials. The origin of the SERS activity of the particles is briefly discussed. The study demonstrates a new method of obtaining high quality SERS on Pt-group transition metals, with the possibility of tuning SERS activity by varying the core size and the shell thickness.

Introduction Metal nanoparticles have attracted extensive attention because of their potential applications in optical, electronic, and magnetic devices as well as sensors and catalysts.1-5 Great progress concerning the synthesis and characterization of metal nanoparticles has been obtained. Particles with various shapes (including spherical, triangular, cubic, and tadpole) and/or with controlled or modulated sizes have been made.6-14 Among these various systems, core* To whom correspondence should be addressed. (S.-Z.Z.) Tel: 513-529 8084. Fax: 513-529 5715. E-mail: [email protected]. (Q.L.Z.) Tel: 86-791-8507785. E-mail: [email protected]. † Jiangxi Normal University. ‡ Xiamen University. § Miami University. (1) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 1179. (2) Su, X. D.; Li, S. F. Y.; O’Shea, S. J. Chem. Commun. 2001, 755. (3) Nabika, H.; Deki, S. J. Phys. Chem. B 2003, 107, 9161. (4) Yu, A.; Liang, Z. J.; Cho, J. H.; Caruso, F. Nano Lett. 2003, 3, 1203. (5) Stamm, K. L.; Garno, J. C.; Liu, G. Y.; Brock, S. L. J. Am. Chem. Soc. 2003, 125, 4036. (6) Hu, J. Q.; Zhang, Y.; Liu, B.; Liu, J. X.; Zhou, H. H.; Xu, Y. F.; Jiang, Y. X.; Yang, Z. L.; Tian, Z. Q. J. Am. Chem. Soc. 2004, 126, 9470. (7) Mayers, B.; Jiang, X. C.; Sunderland, D.; Cattle, B.; Xia, Y. N. J. Am. Chem. Soc. 2003, 125, 13364. (8) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176. (9) Zhao, Y. B.; Zhang, Z. J.; Dang, H. X. J. Phys. Chem. B 2003, 107, 7574. (10) Stoeva, S. I.; Prasad, B. L. V.; Uma, S.; Stoimenov, P. K.; Zaikovski, V.; Sorensen, C. M.; Klabunde, K. J. J. Phys. Chem. B 2003, 107, 7441. (11) Wang, S. H.; Sato, S.; Kimura, K. Chem. Mater. 2003, 15, 2445. (12) Faure, C.; Derre´, A.; Neri, W. J. Phys. Chem. B 2003, 107, 4038.

shell nanoparticles have become a focus because they can have properties that do not exist in single-component metal nanoparticles and some of these properties can be tuned by varying the core size or shell thickness.14-18 For example, it is found that Fe2O3-coated calcium or magnesium oxide core-shell nanoparticles have higher activities than the alkaline earth metal oxides alone for SO2 adsorption17 and H2S removal.18 The magnetic properties of the bimagnetic FePt/MFe2O4 (M ) Fe, Co) core/shell nanoparticles were tunable by changing the thickness of the coating oxide.19 Pt-based nanoparticles are one of the best catalysts for methanol fuel cells. Although impressive progress has been made on the synthesis of platinum nanoparticles, the majority of the work was focused on the investigation of Pt clusters with a diameter less than 20 nm.20,21 To synthesize larger nanoparticles, the seed induced growth method is often used.22-25 In this method, a metal nanoparticle serves as the seed (core), and the salt of a (13) Zhang, Z. P.; Han, M. Y. J. Mater. Chem. 2003, 13, 641. (14) Teng, X. W.; Black, D.; Watkins, N. J.; Gao, Y. L.; Yang, H. Nano Lett. 2003, 3, 261. (15) Zhong, C. J.; Maye, M. M. Adv. Mater. 2001, 13, 1507. (16) Henglein, A. J. Phys. Chem. B 2000, 104, 2201. (17) Decker, S.; Klabunde, K. J. J. Am. Chem. Soc. 1996, 118, 12465. (18) Jiang, Y.; Decker, S.; Mohs, C.; Klabunde, K. J. J. Catal. 1998, 180, 24. (19) Zeng, H.; Sun, S.; Li, J.; Wang, Z. L.; Liu, J. P. Appl. Phys. Lett. 2004, 85, 792. (20) Petroski, J. M.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. A 2001, 105, 5542. (21) Wahl, R.; Mertig, M.; Raff, J.; Selenska-Pobell, S.; Pompe, W. Adv. Mater. 2001, 13, 736.

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second metal is chemically reduced on the core surface to form the shell layer.22-25 The growth of the shell layer can occur either with the core in a solution22-24 or assembled on a substrate.25 The size of the core-shell particles can be modulated by controlling the ratio of the amount of the seed and the metal ion.22-24 The successful preparations of monodispersed Au-Ag and Au-Pd core-shell nanoparticles by Zhang’s group pave a way for making platinum-coated nanoparticles with controlled size.22,23 Recently Lu et al. reported the preparation of Aucore-Ptshell nanoparticles by using ascorbic acid to reduce H2PtCl6 with the presence of a Au nanoparticle seed.24 The Pt-coated Au nanoparticles when assembled on a silicon surface show a relatively high surfaceenhanced Raman scattering (SERS) activity as demonstrated by a strong peak around 2155 cm-1 in the Raman spectrum from adsorbed SCN-.24 This shows a new way of obtaining SERS spectra on Pt surfaces. Previously, there are two effective methods in fabricating SERS active Pt surfaces: One is to deposit an ultrathin (3-5 monolayers) Pt film on a SERS active Au electrode26-29 or nanoparticles.30 The other is to electrochemically roughen a Pt electrode by applying square wave potential steps.31-36 Compared to these two methods, the assembled coreshell Au/Pt nanoparticle films have the advantage of controlled particle size by varying the core size and shell thickness and therefore provide a means to tune the SERS activity. The preliminary results reported in ref 24 were obtained in air. It would be interesting to examine whether the SERS activity is attainable in the electrochemical environment and to study the electrochemical properties of these particles. Given that the Pt based nanoparticles are extensively used in methanol fuel cells and other catalytic reactions, such endeavors are worthwhile. In this paper, Pt-coated Au nanoparticles were synthesized and assembled on glassy carbon (GC) electrodes by drop-coating. The electrochemical behavior of the new electrode was examined in acidic solutions, along with their electrocatalytic activities toward carbon monoxide and methanol oxidation. The electrochemical in situ SERS investigations of these Pt-coated Au nanoparticles supported on GC were also performed by using CO and SCNas the probe molecules. These two adsorbates were selected partly because there are plenty of data available for comparison33,37-41 and, more importantly, because their intramolecular vibration modes are sensitive to the applied (22) Lu, L. H.; Wang, H. S.; Xi, S. Q.; Zhang, H. J. J. Mater. Chem. 2002, 12, 156. (23) Lu, L. H.; Wang, H. S.; Xi, S. Q.; Zhang, H.; Hu, J. W.; Zhao, B. Chem. Commun. 2002, 144. (24) Lu, L. H.; Sun, G. Y.; Zhang, H. J.; Wang, H. S.; Xi, S. H.; Hu, J. Q.; Tian, Z. Q.; Chen, R. J. Mater. Chem. 2004, 14, 1005. (25) Cao, L. Y.; Tong, L. M.; Diao, P.; Zhu, T.; Liu, Z. F. Chem. Mater. 2004, 16, 3239. (26) Zou, S.; Williams, C. T.; Chen, E. K. Y.; Weaver, M. J. J. Am. Chem. Soc. 1998, 120, 3811. (27) Zou, S.; Weaver, M. J. Anal. Chem. 1998, 70, 2387. (28) Zou, S.; Go´mez, R.; Weaver, M. J. Langmuir 1997, 13, 6713. (29) Mrozek, M. F.; Xie, Y.; Weaver, M. J. Anal. Chem. 2001, 73, 5953. (30) Park, S.; Yang, P. X.; Corredor, P.; Weaver, M. J. J. Am. Chem. Soc. 2002, 124, 2428. (31) Bilmes, S. A.; Rubim, J. C.; Otto, A.; Arvı´a, A. J. Chem. Phys. Lett. 1989, 159, 89. (32) Tian, Z. Q.; Ren, B.; Wu, D. Y. J. Phys. Chem. B 2002, 106, 37. (33) Tian, Z. Q.; Ren, B.; Mao, B. W. J. Phys. Chem. B 1997, 101, 1338. (34) Yao, J. L.; Ren, B.; Huang, Z. F.; Cao, P. G.; Gu, R. G.; Tian, Z. Q. Electrochim. Acta 2003, 48, 1263. (35) Ren, B.; Lin, X. F.; Yang, Z. L.; Liu, G. K.; Aroca, R. F.; Mao, B. W.; Tian, Z. Q. J. Am. Chem. Soc. 2003, 125, 9598. (36) Ren, B.; Wu, D. Y.; Mao, B. W.; Tian, Z. Q. J. Phys. Chem. B 2003, 107, 2752.

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potential and the surface structure, which may provide a hint about the nature of the SERS active sites. In addition, thiocyanate shows interesting potential-dependent adsorption orientation.33,37-41 On most metal surfaces, at negative potentials, the dominant species is the SCNadsorbed through the N terminal, whereas at the more positive potentials, the S-bound species is more favorable. At even more positive potentials, a bridge-bound SCNwith both S and N terminals bonding to the surface has also been reported on Pt.33 It is of fundamental interest to see if such an orientation change also exists on the Pt-Au particles. Experimental Section Chemical. All of the chemicals were of analytical grade and were used as received. Aqueous solutions were prepared by using Milli-Q water. Preparation and Characterization of Aucore-Ptshell Nanoparticles. A 12 nm Au sol was synthesized by sodium citrate reduction of AuCl4- according to the Frens method.42 Pt-coated Au nanoparticles were prepared following the method reported in refs 22-24 with slight modifications. Briefly, 10 mL of 0.5 mM H2PtCl6 were mixed with various amounts (15, 25, and 40 mL) of 12 nm Au seed solution. To this mixture was slowly added 7.5 mL of 100 mM ascorbic acid with stirring. The reaction was controlled at 80 °C, and the size of the Aucore-Ptshell particle was regulated by changing the volume of Au sol while keeping the amount of H2PtCl6 constant. X-ray diffraction (XRD) was obtained with a Rigaku Dmax Diffraction System (Tokyo, Japan) using a Cu KR source (λ) 0.15418 nm) at a scan rate of 4° min-1. The nanoparticle size was examined using a JEM-200-CX-II transmission electron microscope (TEM). The UV-Vis absorption spectra were recorded using a Cary 5000 spectrophotometer. Preparation of Glassy Carbon Supported Aucore-Ptshell Nanoparticle Electrodes. A glassy carbon (GC) disk electrode (d ) 5 mm) was mechanically polished successively with fine emery paper and alumina powder (Buehler Ltd., Lake Bluff, IL) down to 0.05 µm to make a mirror finish surface, followed by sonication in Milli-Q water for 5 min. The Aucore-Ptshell nanoparticle sol was washed with water to remove excess reactants before being deposited on the GC electrode using the following procedure: 3 mL of the as-prepared Aucore-Ptshell sol were centrifuged and the supernatant was discarded. The residue containing particles was redispersed in 2 mL of Milli-Q water with sonication and the washing was repeated twice. After being washed, 25 µL aliquot of sol was cast on the pretreated GC electrode and dried in a vacuum for 6 h. This deposition method was utilized to prepare GC supported Pt-coated Au nanoparticle electrodes (hereafter referred to as Aucore-Ptshell/GC electrodes) with different Pt shell thickness. The surface morphology of the electrode was characterized by atomic force microscopy (AFM, P4-SPM-MDT, Nanotechnology MDT. Inc., Moscow, Russia). Electrochemical Measurements. All electrochemical measurements were performed in a conventional three-compartment glass cell by using an electrochemical workstation (CHI 631A, CH Instruments, Austin, TX). The Aucore-Ptshell/GC electrode was used as the working electrode. A large platinum ring served as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. The working electrode was cleaned in 0.5 M H2SO4 by scanning the applied potential between -0.3 and +1.25 V at 100 mV/s for 15 cycles before every measurement. Raman Measurements. All Raman spectra were acquired using a confocal microprobe Raman system (LabRam I, Dilor, France). Detailed description of the instrument has been given elsewhere.32,43 Briefly, a 50× long working distance (8 mm) (37) Cao, P. G.; Yao, J. L.; Ren, B.; Gu, R. A.; Tian, Z. Q. J. Phys. Chem. B 2001, 106, 7283. (38) Li, X.; Gewirth, A. A. J. Am. Chem. Soc. 2003, 125, 11674. (39) Luo, H.; Weaver, M. J. Langmuir 1999, 15, 8743. (40) Ashley, K.; Weinert, F.; Feldheim, D. L. Electrochim. Acta 1991, 36, 1863. (41) Parry, D. B.; Harris, J. M.; Ashley, K. Langmuir 1990, 6, 209. (42) Frens, G. Nature 1973, 241, 20.

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Figure 1. TEM images of Au seed (A) and Aucore-Ptshell nanoparticles synthesized by reduction of H2PtCl6 with ascorbic acid in the presence of different volumes of gold seed: (B) 25 mL, (C) 15 mL. objective was used for laser illumination and scattered light collection. The excitation line at 632.8 nm was provided by an internal He-Ne laser with 3 mW power on the sample. The diameter of the laser spot on the electrode surface was about 3 µm. The potential dependent Raman spectra were acquired by stepping the potential to the positive direction. All experiments were performed at ambient temperature (26 ( 3 °C).

Results and Discussion TEM, UV-Vis Spectroscopy and XRD Characterization of Aucore-Ptshell Nanoparticles. TEM images of Au seeds and core-shell Au-Pt nanoparticles are shown in Figure 1. Clearly, these nanoparticles are nearly monodispersed spheres. The diameter of Au-Pt nanoparticles is larger than that of 12 nm Au and can be increased by decreasing the volume of Au seed as shown in Figure 1, panels B and C. No seed-sized or smaller particles were observed in these TEM images, indicating that the nucleation growth only took place on the surface of the Au seed. In the UV-vis spectra (see Figure S1 in the Supporting Information), a characteristic absorption peak from Au particle was seen at 520 nm, and the H2PtCl6 solution showed a strong peak at ∼260 nm which can be attributed to the absorption of aqueous Pt complex.16 However, for the Pt-Au particles, the absorption profile does not feature any well-defined surface plasmon bands, which is consistent with those reported for smaller Pt and Pd nanoparticles.16,44 The Aucore-Ptshell nanoparticles are polycrystalline, as indicated by multiple bands in the X-ray diffraction pattern (see Figure S2 in the Supporting Information). In the following sections, unless otherwise stated Aucore-Ptshell particles with a nominal diameter of 50-60 nm made by using 25 mL of gold seed are used to demonstrate the electrochemical and SERS characteristics of the particles assembled on GC electrodes. AFM Characterization of Aucore-Ptshell/GC. To characterize the surface morphology of the Aucore-Ptshell/ GC, tapping-mode AFM was used. A representative image of the Aucore-Ptshell/GC prepared by deposition of Pt-coated Au nanoparticles (the same sample in Figure 1B) on the GC surface is displayed in Figure 2. Its surface morphology is akin to that of a platinum electrode subjected to an electrochemical roughening treatment developed in our lab, which yields a strong Raman enhancement effect.43 This suggests that the particle electrode may also be SERS active. Electrochemical Behavior. Cyclic voltammograms (CVs) of a typical Aucore-Ptshell/GC electrode and a similarly prepared 12 nm Au modified GC electrode in 0.5 M H2SO4 are shown in Figure 3. The upper potential limit for AucorePtshell/GC was selected to avoid significant oxygen evolution (43) Tian, Z. Q.; Ren, B. In Encyclopedia of Electrochemistry; Bard, A. J., Stratmann, M., Eds.; Wiley&VCH: Weinheim, Germany, 2003; Vol. 3, p 572. (44) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881.

Figure 2. AFM image of Aucore-Ptshell nanoparticles assembled on a GC electrode. These Aucore-Ptshell particles with a diameter of 50-60 nm were synthesized in the presence of 25 mL of Au seed. Image size: 5 × 5 µm.

Figure 3. Cyclic voltammograms of different nanoparticles assembled on a GC electrode surface in 0.5 M H2SO4. Dotteddashed line: 12 nm Au; Solid line: 50-60 nm Aucore-Ptshell. Aucore-Ptshell nanoparticles were prepared using 25 mL of Au seed. Scan rate: 50 mV/s.

which is catalyzed by the Pt. At a high oxygen evolution rate, the oxygen bubbles may detach the loosely bound particles from the surface. When Au nanoparticles are assembled on the GC surface, the CV shows typical surface oxidation and reduction current peaks similar to those of a bulk gold electrode. However, when the working electrode is replaced by the Aucore-Ptshell/GC, the CV changes to resemble that of a pure platinum electrode, showing the characteristic hydrogen adsorption/desorption peaks between -0.3 and +0.1 V as well as the formation and removal of platinum oxide at higher potentials.27,30 The absence of the Au oxide reduction peak at ca. 0.9 V indicates that the Au core is completely covered by the Pt shell. This further supports that the particles have a Aucore-Ptshell structure, even after being assembled on a

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Figure 4. Cyclic voltammograms of Aucore-Ptshell/GC surface in 0.5 M H2SO4 after the electrode was held at -0.2 V in COsaturated 0.1 M H2SO4 for 5 min (A) and 30 min (B). The size of Aucore-Ptshell nanoparticles used here is ca. 50-60 nm. Scan rate: 50 mV/s.

surface. It is interesting to see if these nanoparticles show similar electrochemical catalytic activities to bulk platinum. In the following, we use carbon monoxide and methanol oxidation as probe systems to examine this point. Figure 4 displays anodic segments of CVs of AucorePtshell/GC electrodes with irreversibly adsorbed CO in 0.5 M H2SO4. The CO adlayer was formed by holding the electrode at -0.2 V in CO-saturated 0.1 M H2SO4 for different lengths of time. The solution was replaced with deareated 0.5 M H2SO4 before recording the CVs. When the CO adsorption time is 5 min (Figure 4A), the hydrogen desorption peaks between -0.3 and +0.1 V decrease, indicating the surface is partially covered by CO. The sharp peak at ca. 0.49 V disappeared upon the second potential sweep (not shown here); therefore, it can be safely attributed to the oxidation of irreversibly adsorbed CO. When the CO adsorption time was increased to 30 min, the hydrogen desorption peak further decreases, indicating that a higher CO coverage was obtained. Two oxidation peaks appeared at ca. 0.24 and 0.54 V, which arise from the oxidation of the weakly and strongly adsorbed CO, respectively. To further explore the catalytic behavior of these AucorePtshell nanoparticles, we examined the oxidation of methanol in view of the reaction’s important role in methanol fuel cells. The cyclic voltammogram recorded in 0.1 M CH3OH + 0.1 M H2SO4 solution shows an oxidation peak at around 0.65 V in the anodic scan direction, which is similar to that commonly observed on a bulk Pt electrode (Figure 5A).45,46 If the Aucore-Ptshell/GC electrode was immersed in 0.1 M CH3OH + 0.1 M H2SO4 solution at the open circuit potential and then transferred to 0.5 M H2SO4, the CV obtained in the latter solution is similar to that from the electrode covered by irreversibly adsorbed CO, suggesting CO is produced during methanol dissociation (Figure 5B). These results show that, like bulk platinum electrodes, the platinum-coated gold nanoparticles have a strong electrocatalytic activity toward methanol oxidation and methanol dissociates spontaneously to produce CO on the particle surface. So the AucorePtshell nanoparticles could be used as the anode materials of methanol fuel cells, especially after being modified with other metals, such as Ru or Sn, which are known to (45) Iwasita, T. Electrochim. Acta 2002, 47, 3663. (46) Spendelow, J. S.; Wieckowski, A. Phys. Chem. Chem. Phys. 2004, 6, 5094.

Figure 5. Cyclic voltammograms of a Aucore-Ptshell/GC electrode in (A) 0.1 M CH3OH + 0.1 M H2SO4, and (B) in 0.5 M H2SO4 after immersion in 0.1 M CH3OH + 0.1 M H2SO4 for 5 min at open circuit potential. The Aucore-Ptshell nanoparticles were prepared in the presence of 25 mL of Au seed. Scan rate: 50 mV/s.

Figure 6. Potential-dependent SERS spectra of a Aucore-Ptshell/ GC electrode in 0.1 M H2SO4 after being held at -0.2 V for 30 min in CO-saturated 0.1 M H2SO4. Aucore-Ptshell nanoparticles were prepared in the presence of 25 mL of Au seed. Excitation line: 632.8 nm. Acquisition time: 100 s.

improve the electrocatalytic activities of Pt to the oxidation of methanol.45,46 Synthesis of Aucore-alloy(Pt-Ru)shell nanoparticles is underway in our laboratory. The results and detailed discussions will be reported in due course. Surface-Enhanced Raman Scattering (SERS) Investigation. As revealed by the AFM image, the surface morphology of the Aucore-Ptshell/GC electrode is similar to that of an electrochemically roughened Pt, which showed strong SERS activity. It is interesting to see if these nanoparticles also show the Raman enhancement effect. In this section, CO and SCN- are used as model systems to assess the SERS activity of the core-shell Au-Pt nanoparticles. Potential Dependent SERS Spectra of CO. Representative potential-dependent SERS spectra in the frequency range of 1850-2350 cm-1 obtained from a Aucore-Ptshell/ GC electrode in 0.1 M H2SO4 solution after the electrode being held at -0.2 V in CO saturated 0.1 M H2SO4 for 30 min are displayed in Figure 6. A broad band appearing at ca. 2050 cm-1 is clearly seen when the electrode was polarized at -0.3 V. The band frequency increases with the electrode potential, and the intensity remains constant until at 0.3 V where both the frequency and the intensity

Investigation of Adsorbed CO and SCN-

Figure 7. Peak frequency of the C-O stretching mode for CO adsorbed on a Aucore-Ptshell/GC surface obtained in 0.1 M H2SO4 plotted against electrode potential. Data extracted from Figure 6.

decrease. The peak is almost indiscernible when the potential is held at 0.4 V, at which the second CO oxidation current peak starts to take off (Figure 4). In comparison with previous SERS results of CO at Pt films and roughened Pt electrodes,33,47 this band can be confidently assigned to the C-O stretching vibration of the linearlyadsorbed CO. The νCO varies linearly with the applied potential in the region where CO is stable, yielding a slope dνCO/dE ca. 40 cm-1/V (Figure 7). This so-called electrochemical Stark tuning effect is similar to the reported values on various Pt electrodes.47-50 Although the noise level of the spectra shown in Figure 6 is relatively high due to the performance deterioration of our Raman setup, the peaks are more intense than those reported on roughened bulk Pt electrodes after taking the five times lower laser power used in the current study into account.33 Clearly good quality SERS spectra can be obtained on these Aucore-Ptshell nanoparticles supported on GC electrodes, suggesting that these kinds of nanoscale metal particles can be used as a SERS active substrate for other studies. Potential-Dependent Orientation of SCN- Adsorbed at the Aucore-Ptshell/GC Electrodes. As demonstrated above, we have developed a new method to fabricate a SERS substrate by deposition of Au-Pt core-shell nanoparticles on GC surfaces. To further explore the applicability of this approach to other adsorbates, we studied the adsorption of SCN- on the nanoparticles in view of its simple structure and interesting potential-dependent adsorption orientation. Figure 8 shows a typical set of potential-dependent SERS spectra of SCN- obtained in 0.01 M NaSCN + 0.1 M NaClO4 in the frequency regions of 1850-2350 cm-1 and 200-800 cm-1, covering most of the SCN- vibration modes. The spectra were recorded sequentially from -0.8 to +0.8 V. This potential window was limited by hydrogen evolution and SCN- oxidation (see Figure S3 in the Supporting Information). The overall quality of the spectra is comparable to that obtained on roughened bulk Pt electrodes.33 Given that a much lower laser power is used in the current study, the SERS activity shown by Figure 8 is much higher than those reported in ref 33. Starting from the higher frequency region, a peak at 2080 cm-1 is clearly evident at -0.8 V. The center frequency increases with the applied potential, indicating that the signal is from surface species rather than from those in the bulk solution. Compared with previous SERS (47) Zou, S.; Weaver, M. J. J. Phys. Chem. 1996, 100, 4237. (48) Kim, C. S.; Tornquist, W. J.; Korzeniewski, C. J. Phys. Chem. 1993, 97, 6484. (49) Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 5391. (50) Kunimatsu, K.; Seki, H.; Golden, W. G.; Gordon, J. G., II; Philpott, M. R. Langmuir 1986, 2, 464.

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studies on the same system, the observed band can be confidently assigned to the C-N stretch from the adsorbed SCN-.33,51 Close inspection of the potential-dependent νCN reveals that there are two significantly different Stark tuning slopes, dνCN/dE, over the examined potential region (Figure 9). In the potential range of -0.8 to -0.2 V, the dνCN/dE is about 49 cm-1/V. From -0.2 to 0.8 V, it increases to 87 cm-1/V. These dνCN/dE values vary somewhat from sample to sample, but the difference is typically within 10%. As pointed out in the outset, the adsorption orientation of SCN- depends on the applied potential. The C-N stretching frequency at different potentials has been used as an indicator for the adsorption orientation change on most metals, such as Au, Ag, Fe, and Pt.33,37-41,51 In an SNIFTIR study, Ashley et al. assigned a bipolar peak at ca. 2050 cm-1 to N-bound SCN- and a positive peak at about 2135 cm-1 to S-bound SCN- on Pt.51 The former is favored at far negative potentials, whereas the latter predominates at more positive potentials.51 A sum frequency generation spectroscopic study also revealed that N-bound adsorbed SCN- was the major species at the relatively negative potential region, and a band at 2140 cm-1 at -0.4 V during the positive potential scan was assigned to S-bound SCN-.52 Tian et al. recently reported SERS spectra of SCN- at electrochemically roughened bulk Pt surfaces.33 At -1.2 V, a sharp strong peak appeared at 2073 cm-1. The frequency of this peak increased linearly with potential and reached 2110 cm-1 at -0.2 V. At 0.2 V, a higher frequency shoulder at around 2145 cm-1 started to show up and gradually became dominant. Very similar potential-dependent spectral changes were observed here. Thus at -0.8 V, the νCN lies at 2080 cm-1. With increasing applied potential, the νCN shifts linearly to larger values, reaching 2110 cm-1 at -0.2 V. The Stark tuning slope in this potential range agrees reasonably well with those reported on Pt thin films or roughened bulk Pt surfaces.33,39 Further increasing the potential, a higher frequency shoulder appears and gradually becomes dominant, yielding a much larger dνCN/dE. This large increase in dνCN/dE signals the orientation change of adsorbed SCN- from N-bound to S-bound, in agreement with the previous studies.33,37-41,51 It should be emphasized that the νCN and dνCN/dE values depend on the experimental conditions, such as the applied potential history and the supporting electrolyte, presumably due to the coadsorption of other ions in the solution.39,51 The full width at half-maximum (fwhm, circles, left axis) and the integrated band intensity (triangles, right axis) of the νCN band decrease slightly from -0.8 to -0.2 V and increase rather sharply above 0 V (Figure 10). These spectral changes agree with the emergence of a new band above 0 V, which can be assigned to the C-N stretch of the S-bound SCN-.33,51 Unfortunately, deconvolution of the bands by peak fitting did not yield meaningful results. Nonetheless, the growth of the C-N stretch bandwidth and intensity above 0 V strongly supports the SCNorientation change. Direct evidence of the orientation change comes from the lower frequency spectra (Figure 8). At -0.8 V, two peaks at ca. 462 and 350 cm-1 were observed, which can be assigned to the bending mode and the Pt-NCS stretching mode from N-bound SCN-, respectively.33,39 When the potential is increased to about 0.1 V, a band at ca. 280 cm-1 assigned to the Pt-SCN stretching mode33,39 (51) Ashley, K.; Samant, M. G.; Seki, H.; Philpott, M. R. J. Electroanal. Chem. 1989, 270, 349. (52) Tadjeddine, A.; Guyot-Sionnest, P. Electrochim. Acta 1991, 36, 1849.

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Figure 8. Potential-dependent SERS spectra of SCN- adsorbed at Aucore-Ptshell/GC electrode in 0.01 M NaSCN + 0.1 M NaClO4. Aucore-Ptshell nanoparticles with a diameter of 50-60 nm were prepared in the presence of 25 mL of Au seed. Excitation line: 632.8 nm. Acquisition time: 100 s.

Figure 9. Peak frequency of the C-N stretching mode for SCN- adsorbed on a Aucore-Ptshell/GC surface obtained in 0.01 M NaSCN + 0.1 M NaClO4 plotted against electrode potential. The diameter of Aucore-Ptshell nanoparticles is 50-60 nm.

Figure 10. Bandwidth (fwhm, circles, left axis) and integrated band intensity (triangles, right axis) of νCN mode plotted against the applied potential. Data extracted from the spectra shown in Figure 8.

becomes apparent and overlaps with the 350 cm-1 band, indicating the coexistence of N-bound and S-bound SCN-. With the potential going beyond 0.3 V, the 280 cm-1 band becomes dominant, signaling that S-bound thiocyanate is prevalent above this potential. The increasing intensity of the Pt-SCN stretching mode provides direct, solid evidence for the potential-dependent SCN- orientation change. Note that the νPt-SCN appears at about the same potential as the higher frequency νCN. This self-consistency provides further evidence for the SCN- adsorption orientation change around 0 V. In agreement with these experimental observations, recent density functional theory calculations by Li and Gewirth also showed that the adsorption energy for both orientations on Au is negative, with the S-bound species more favorable at the positive potentials.38 It remains to discuss the origin of the SERS activity on these nanoparticles. Previous SERS studies performed on these Aucore-Ptshell nanoparticles showed that dispersed particles in the colloidal solution do not yield a discernible SERS signal.24 However, when deposited on a 3-aminopropyltrimethoxysilane modified silicon wafer, these particles show a strong SERS spectrum of adsorbed SCN-. This presumably arises from particle aggregations on the substrate, forming SERS “hot spots” which are believed

to be at the junction between two particles.53 It is possible that the Pt shell is thinner at these sites and the SERS effect arises from the coupling between neighboring Au cores. Alternatively, it is also possible that the SERS effect could just come from Pt itself. In fact, strong SERS spectra of CO, CN-, and adsorbed H from aggregates of 4 nm Pt and Pd nanoparticles were reported very recently.54 To clarify this, it is important to see whether the Au core plays a role in the observed SERS. This can be achieved by examining SERS activities of Aucore-Ptshell nanoparticles with various core sizes and/or shell thicknesses. One advantage of the present approach is that the nominal shell thickness can be controlled by varying the amount of the gold seed used in the particle fabrication (vide supra). If the Raman enhancement effect is mainly from the Au core, the SERS activity can be tuned by varying the core size and the intercore distance. The coupling between neighboring particles can be controlled by the platinum shell thickness. The ability to engineer the substrate with tunable SERS activity is of both fundamental and practical interest. Studies along this line are underway. (53) Xu, H. X.; Ka¨ll, M. Phys. Rev. Lett. 2002, 89, 246802. (54) Gomez, R.; Perez, J. M.; Salla-Gullon, J.; Montiel, V.; Aldaz, A. J. Phys. Chem. B 2004, 108, 9943.

Investigation of Adsorbed CO and SCN-

Conclusions Large size Pt nanoparticles with a core-shell structure were synthesized by using a Au seed growth method. TEM images showed these particles are nearly monodispersed and their size can be modulated by changing the concentration of the Au seed. These core-shell Au-Pt nanoparticles were successfully deposited on clean GC electrodes, and they showed characteristic hydrogen adsorption/desorption as well as surface oxidation/reduction cyclic voltametric current features of platinum in acidic solutions. The electrocatalytic activities of the nanoparticles toward CO oxidation and methanol dissociation/oxidation were also similar to those of a bulk Pt electrode. High quality SERS spectra of adsorbed CO and thiocyanate were obtained on these Aucore-Ptshell/GC electrodes. The potential dependent SERS features resemble those obtained on electrochemically roughened bulk Pt or Pt thin films deposited on roughened Au electrodes. The Stark tuning slope of the C-N stretching mode of SCN- showed two distinctly different values, 49 cm-1/V from -0.8 to -0.2 V and 87 cm-1/V from 0 to 0.8 V. The νCN bandwidth and intensity increase with the applied potential at the latter potential range. At the lower frequency spectral region, a broad band centered at 350 cm-1 assigned to the Pt-NCS stretching mode was observed at the lower potential regions. Above 0 V, a second band centered at 280 cm-1 appears and gradually becomes dominant as the potential moved to more positive values. These spectral transitions were attributed to the SCNadsorption orientation switching from N-bound dominant

Langmuir, Vol. 21, No. 16, 2005 7455

at negative potential region to S-bound at more positive potentials. The observation of good quality SERS spectra on the Aucore-Ptshell/GC points to a new method of obtaining SERS activity on Pt-group transition metals, which may find applications in catalysis and chemical sensors. It would be of both fundamental and practical interest to find factors affecting the SERS activity, such as the size and chemical nature of the core, the shell thickness, etc. Research along these lines would help to optimize the enhancement effect and is currently underway. Acknowledgment. This work is financially supported by the Natural Science Foundation of China (90206039), the Science and Technology Key Programs of the Ministry of Education of China (02080), and State Key Laboratory for Physical Chemistry of Solid Surfaces at Xiamen University under the Contract No. 9905. S.-Z.Z. thanks Miami University for Start-up funds. B.Z. thanks Prof. Y. Xie at University of Science and Technology of China for valuable discussions. We thank an anonymous reviewer for many helpful editorial and technical suggestions. Supporting Information Available: UV-Vis spectra of 12 nm Au particle, H2PtCl6 and Aucore-Ptshell particle solutions, XRD patterns of Aucore-Ptshell nanoparticles and CVs of AucorePtshell/GC obtained in 0.01 M NaSCN + 0.1 M NaClO4 and 0.1 M NaClO4 are provided. This material is available free of charge via the Internet at http://pubs.acs.org. LA050004N