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Synthesis of Au@Pd core–shell nanoparticles with controllable size and their application in surface-enhanced Raman spectroscopy
Chemical Physics Letters 408 (2005) 354–359 www.elsevier.com/locate/cplett
Synthesis of Au@Pd core–shell nanoparticles with controllable size and their application in surface-enhanced Raman spectroscopy Jia-Wen Hu
b
a,b
, Yong Zhang b, Jian-Feng Li b, Zheng Liu b, Bin Ren b, Shi-Gang Sun b, Zhong-Qun Tian b,*, Tim Lian c
a Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, China State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, China c Department of Chemistry, Emory University, Atlanta, GA 30322, USA
Received 22 February 2005; in final form 20 April 2005 Available online 13 May 2005
Abstract Au core Pd shell (Au@Pd) nanoparticles with controllable size from 35 to 100 nm were prepared by chemical deposition of Pd over pre-formed 12 nm Au seeds. Both transmission electron microscopy and UV–visible spectroscopy studies confirmed the core– shell structure of the synthesized nanoparticles. The Au@Pd nanoparticles dispersed on a polished Pt electrode surface exhibit high surface-enhanced Raman scattering (SERS) effect for the adsorbed pyridine and SCN. With the aid of the long-range effect of the electromagnetic (EM) enhancement created by the SERS-active Au core underneath the Pd shell, the good quality SERS spectra of adsorbates on the palladium metal overlayer can be obtained. Such kind of SERS-active substrate can be used as an alternative substrate to massive metals for investigating adsorption and reactions occurring on the Pd metal catalyst. 2005 Elsevier B.V. All rights reserved.
1. Introduction The field of surface enhanced Raman spectroscopy (SERS) was inaugurated largely through contribution of Fleischmann, Van Duyne, Creighton and their coworkers in the mid-1970s [1–3]. Because of the large enhancement of SERS, the intrinsically low detection sensitivity is no longer a fatal disadvantage for surface Raman spectroscopy. The unique surface sensitivity makes SERS an in situ diagnostic tool, widely applicable to electrochemical, biological and other ambient interfaces, for probing the detailed molecular structure and orientation of surface species [4–8]. Unfortunately, it turned out that only a few metals, mainly roughened Ag, Au and Cu, provide the highest enhancement (106–1013) of the Raman effect, which severely limited *
Corresponding author. Fax: +86 592 2085349. E-mail address: [email protected] (Z.-Q. Tian).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.04.071
the breadth of practical applications of SERS involving other metallic materials [9]. Therefore, the strategy of extending the SERS substrate from the noble metals of Au, Ag and Cu to a wide variety of metallic and nonmetallic materials is crucially important for developing SERS as a versatile and powerful tool in surface science and materials science. A long goal of our group is to directly generate SERS from massive transition metals. Since 1996, by developing various surface roughening procedures and optimizing the performance of confocal Raman microscope, we have demonstrated that most transition (VIIIB) metals (e.g., Fe, Co, Ni, Ru, Rh, Pd, Pt) can directly generate weak SERS activity with typical enhancement factors ranging from 10 to 103, and in some systems the enhancement can even reach 104 [9,10]. The applications of SERS in surface adsorption, electro-catalysis and corrosion of transition metal-based systems have been realized [11–14]. Because SERS activity critically depends
J.-W. Hu et al. / Chemical Physics Letters 408 (2005) 354–359
not only on the nature of the metal but also on the size, shape, and spacing of metal nanostructures, the preparation of substrate is always of central concern to gain a higher SERS activity. The ever-developing techniques of nanoscience have provided opportunity to obtain surface nanostructures with controllable size and shape. Accordingly, it is of special interest to extend the SERS substrate from randomly rough surfaces to nanostructures of transition metal with tailored elements. The pioneering works along this avenue can be traced back to Parker et al. and Srnova´ et al. [15,16], who obtained SER spectra from Rh and Pd colloidal particles, respectively. Later, Guo et al. [17] reported SER spectra from Fe nanoparticles dispersed at the electrode surfaces. Recently, Go´mez et al. [18] obtained high quality SER spectra of the adsorbed CO from Pt and Pd nanoparticles dispersed on electrode surfaces, showing an enhancement of 550 for cyanide on aggregated Pt nanoparticles. More recently, Kim et al. [19] also obtained the SERS of benzenethiol from the aggregates of Pt nanoparticles with a maximum enhancement of 200. It should be noted that these works are limited to monometallic particles with particle size generally smaller than 20 nm, not emphasizing the size effect. Since the SERS effect is a size-dependent optical phenomenon, it seems necessary to further develop a method capable of controllably synthesizing a series of the nanoparticles with various sizes and find some ways to further improve the SERS activity. Lu et al. [20] developed an interesting Au seeding method, by which bimetallic core–shell nanoparticles can be easily synthesized with controlled size. They reported a preliminary result on the good SERS activity of Au@Pt nanoparticles [21]. In the present Letter, we prepared a series of Au@Pd nanoparticles based on this method, and dispersed them on a polished Pt electrode. The reason to synthesize nanoparticles with Pd shell is that Pd is a very important electrode material in electro-catalysis but showed the weakest SERS activity among the transition metals already exploited [22]. With the aid of the longrange effect of the electromagnetic (EM) enhancement created by the SERS-active Au core underneath the Pd shell, the high SERS activity of Au@Pd nanoparticles could be reached. The detailed structure and optical properties of these nanoparticles were characterized by TEM and UV–visible spectroscopy, and the SERS activity was investigated by using pyridine and SCN as probe molecules.
355
citrate. Then five solutions (samples 1–5) with 50 ml of 1.0 mM H2PdCl4 and a varied amount of 12-nm Au seeds (20, 10, 5, 2.5 or 0.5 ml of the gold sols) were prepared and cooled to ca. 4 C in an ice bath. Next, into each of the above solution, 6 ml of 100 mM ascorbic acid, also cooled in an ice bath, was slowly dropped while stirring. The color of the mixture turned to black brown within minutes. The stirring was continued for 30 min after finishing the addition of ascorbic acid. To clean the nanoparticles and remove particles smaller than 10 nm, 4 ml of the resulting sols were subjected to centrifugation at 4000 rpm for 15 min. After the supernatant was removed, ultrapure water was added. These procedures were normally repeated for another two times. At last, concentrated Au@Pd sols were obtained at the bottom of the centrifuging tube. A drop of the concentrated sols was then dispersed on the mirror-like surface of a polished Pt rod (2 mm in diameter) sealed in a teflon sheath and dried in a desiccator. With water evaporating, nanoparticles aggregated together and closely packed, forming thin films of nanoparticles which attach tightly on the Pt surface, as they show no Ôfall offÕ visible to the naked eyes even after rinsed with water or electrochemically cleaned in a wide potential range. The substrate denoted as nm-Au@Pd/Pt was then mounted on a homemade spectroelectrochemical cell to acquire Raman spectra [23]. A large Pt ring and a saturated calomel electrode (SCE) served as the counter electrode and the reference electrode, respectively. All the potentials were quoted versus SCE. UV–visible spectra of the sols of gold seeds and the Au@Pd nanoparticles that were not further treated were both recorded on a Shimadzu UV-2100 spectrometer using a 1 cm quartz cell from their reaction mixtures. TEM and HRTEM images of the nanoparticles were acquired on JEM-100 CXII and Tecnai F30, respectively, after dipping the carbon-coated copper grid in the concentrated sols for 1 min. In situ SERS measurements were carried out on a Labram I confocal microprobe Raman system (Dilor, France). The excitation line is the 632.8-nm line from a He–Ne laser. A long-working length (8 mm) objective with 50 magnifications was used for directing the laser to the sample surface and collecting the Raman signal in a backscattering geometry. A more detailed introduction of the Raman system can be found in [12].
3. Results and discussion 2. Experimental Au@Pd nanoparticles were synthesized by the Auseed mediated method with a slight modification [20]. In brief, a 12-nm Au sol (used as gold seeds) was first prepared by the reduction of AuCl 4 ions using sodium
Fig. 1a–d is the TEM images of the Au seeds, samples 1, 3 and 5, respectively. As evidenced by the figure, nanoparticles show well-defined spherical shape and fair uniformity. With the decrease in the concentration of the gold seeds, the averaged size of the nanoparticles, measured from more than 200 nanoparticles, increases
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Fig. 1. TEM images: (a) the gold seeds; (b) sample 1; (c) sample 3; (d) sample 5.
from 31 (sample 1), 40 (sample 2), 52 (sample 3), 63 (sample 4) to 105 nm (sample 5). Figs. 2a and b are the HRTEM and scanning TEM (STEM) images of sample 1, respectively, both of which show clearly a core–shell structure. The central concept of the seeding method is that the purposely introduced small nanoparticles serve as the nucleation centers to grow nanoparticles to a desired size [24]. This idea originates from the well-known growing mechanism of nanoparticles: a burst nucleation process and a growth stage of the nucleation centers [25]. Ideally, if the two steps are separated in time and the growth would occur only on the surface of the nucleation centers, the final nanoparticles would be highly monodisperse. Nevertheless, in many cases, the introduced seeds do not guarantee such a separation as successful as they could be. Instead, during the growth stage further nucleation occurs [26], which may be the reason why in some batches of the Au@Pd products, the size of the final nanoparticles are not as
monodisperse as expected. We found that the monodispersity of the Au@Pd nanoparticles can be improved largely by controlling the dropping rate of ascorbic acid and the reaction temperature. Under our experimental condition, the forming rate of the Pd monomers generated from the reduction reaction was slowed down. Therefore, the concentration of the Pd monomers is sufficient for the diffusion or the reaction-controlled surface growth of the seeds, but is low for nucleation to occur. As a result, highly monodispersed Au@Pd nanoparticles can be prepared. Fig. 3 shows the UV–visible spectra of the sols of gold seeds and Au@Pd nanoparticles (sample 1). Since excess ascorbic acid is present in the Au@Pd sols and has a strong absorption peak at about 250 nm, the UV absorption below 300 nm is not shown in Fig. 3. A noticeable change that the plasmon band of the Au seeds is completely damped, can be seen from Fig. 3 upon the formation of the Au@Pd nanoparticle. On
Fig. 2. TEM images of sample 1: (a) HRTEM, (b) scanning TEM in a dark-field mode.
J.-W. Hu et al. / Chemical Physics Letters 408 (2005) 354–359
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Fig. 3. UV–visible absorption spectra of (a) 12-nm gold seeds and (b) Au@Pd nanoparticles illustrated by sample 1.
the other hand, for the Au@Pd sols, no characteristic absorption peaks can be observed in the region of 300–800 nm, but only a gradual increase in absorption toward the blue is observed. This absorption behavior is very similar to the case of Pd sols with an averaged particle size of 4 nm [27], with increased absorption below 300 nm and the appearance of a clear peak at 218 nm. From this phenomenon we may infer that, Pd shell dominates the optical properties of the Au@Pd nanoparticles. It is not surprising because the thinnest thickness of the Pd shell is already about 10 nm as determined from the TEM images of sample 1 and the Au seeds. No obvious difference in the UV absorption among these samples has been observed with the increase of the shell thickness. To test the SERS activity of the as-prepared Au@Pd nanoparticles, pyridine was selected as the probe molecule, which is a widely used Raman probe molecule because it has a large Raman cross-section. Fig. 4 is the potential-dependent Raman spectra of pyridine on the nm-Au@Pd/Pt electrode. As is evidenced, with the increase of the electrode potential from 0.8 to 0.4 V, the overall Raman intensity increases and reaches a maximum at 0.6 V. Above this potential, the band intensity gradually decreases. Such potential dependent spectral behavior strongly indicates that the Raman signals are from surface species. One fact that no Raman signals are observed from the polished Pt electrode under the same experimental conditions should be mentioned prior to the following discussions. This fact indicates that the spectra shown in Fig. 4 are essentially from the pyridine adsorbed on the supported nanoparticles, being independence of the conducting substrate Pt. Moreover, the nanoparticles have a continuous thick Pd shell around the Au core (see the HRTEM images). Therefore, the spectral interference from the encapsulated core can be effectively ruled out and the spectra only report the fruitful structural information of species on the Pd metal. To evidence the existence of the SERS effect unambiguously, we calculated the enhancement factor (G),
-0.6 V
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0
300
600
900
1200 1500 1800
Raman shift / cm-1 Fig. 4. Potential-dependent Raman spectra of pyridine adsorbed at the nm-Au@Pd/Pt electrode (prepared from sample 1) in 0.1 M NaClO4 + 0.01 M pyridine; acquisition time: 10 s.
which compares the integrated Raman intensity obtained from the surface with that from the solution phase. On the basis of a confocal Raman system, the G can be expressed as G¼
hcN 0 rI surf ; RI bulk
where h is a characteristic value of the confocal Raman system [28,29]. c, N0, r are the concentration of bulk pyridine solution, the Avogadro constant and surface area occupied by each pyridine molecule, respectively. Isurf and Ibulk are the integrated Raman intensities of the m1 mode (ca. 1006 cm1) of pyridine adsorbed on surface and in solution, respectively, and can be measured from experiment. The surface roughness factor (R) is determined as a ratio of the real area of a roughened surface to that of a smooth surface. We assumed that the monodispersed Au@Pd nanoparticles adopt hexagonal arrays on the supporting Pt electrode; R was then calculated to be 1.8. r for a vertically adsorbed pyridine is 0.254 nm2 [28]. The G for pyridine on the nmAu@Pd/Pt electrode (prepared from sample 1) at potential of 0.8 V is then estimated to be as large as about 2.7 · 103. This calculation clearly indicates a SERS effect in the present system. The SERS intensity of the m1 mode of pyridine in Fig. 4 is as strong as 200 cps, which is significantly stronger than that (ca. 5 cps) reported for pyridine on Pd films chemically deposited on a glassy carbon substrate reached [22]. The core–shell Au@Pd nanoparticles also exhibit higher SERS activity than monometallic Pd nanoparticles and roughed net Pd electrode. Furthermore, when the potential was stepped
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from 0.8 to 0.4 V and repeated again to measure one and another series of spectra, the SERS intensity of pyridine in the second measurement decreases only by about 10% at each potential. From above, we can conclude that the Au@Pd nanoparticles exhibit high SERS activity, good stability and reversibility for practical SERS applications, which may result from their unique core–shell structure and monodispersed particle size. The general application of such type of nanoparticles for SERS was further attested by SCN, which has a smaller Raman cross-section than that of pyridine. Shown in Fig. 5 are the potential-dependent SER spectra of SCN adsorbed on the nm-Au@Pd/Pt electrode. A strong peak at ca. 2075 cm1 was observed, which is due to the C–N stretching (mC–N) of SCN. The peak exhibits two features. First, when the potential moves from the negative to the positive limit, the full-width at half-maximum (FWHM) of the mC–N band gradually increases from 29 to 69 cm1, implying an increasing discreteness of the vibration energy states. This feature may indicate a re-orientation of the adsorbed SCN on the Pd surface in the potential range investigated. Second, the applied potential has a strong influence on the band frequency of the mC–N stretching, which results from the electrochemical Stark effect [30]. The Stark tuning rate, dm/dE, where m is the band frequency and E is the applied potential, is as high as 87 cm1/V. Surprisingly, when we changed the size of the Au@Pd nanoparticles from 31 nm (sample 1) to 105 nm (sample 5), neither the frequency nor the band intensity of CN stretching vibration exhibits the size dependency. This result is consistent with the UV absorption results,
which also exhibit no size-dependent characteristics. However, when the shell thickness is thin enough, viz. in a range between ca. one monolayer and a few nanometers, both the plasmon band of gold and the SERS intensity of CO exhibit core size and shell thickness dependent behaviors (results will be published elsewhere). Such experimental results clearly indicate that the EM mechanism operates in the core–shell nanoparticles with a thin skin. Because the present used nanoparticles exhibit higher SERS activity than monometallic Pd nanoparticles and roughed Pd electrode, the long-range EM enhancement may further enhance the Raman signals even when their shell is thicker than 10 nm. In conclusion, in the present Letter, core–shell Au@Pd nanoparticles with controllable size were prepared by the Au seed-mediated method. With the aid of the long-range effect of the electromagnetic (EM) enhancement created by the SERS-active Au core underneath the Pd shell, the high SERS activity of Au@Pd nanoparticles has been reached. Their high SERS activity makes possible the in situ spectral investigation of the catalytic reactions occurring on a Pd surface. Moreover, their easy preparation and characterization may provide deep insight into the relationship between the SERS behaviors and the detailed nanostructures.
Acknowledgment This work is support by NSFC 20228020, 20021002 and 90206039.
2132 5 cps
References
0.0 V 2116
- 0.2 V 2094 - 0.4 V 2084 - 0.6 V 2075 - 0.8 V
1600
1800
2000
2200
2400
Raman shift / cm-1 Fig. 5. Potential-dependent SERS spectra of SCN adsorbed at the nm-Au@Pd/Pt electrode (prepared from sample 1) in 0.1 M NaClO4 + 0.01 M NaSCN; acquisition time: 100 s.
[1] M. Fleischmann, P.J. Hendra, A.J. McQuillan, Chem. Phys. Lett. 26 (1974) 163. [2] D.L. Jeanmaire, R.P. Van Duyne, J. Electroanal. Chem. 84 (1977) 1. [3] M.G. Albrecht, J.A. Creighton, J. Am. Chem. Soc. 99 (1977) 5215. [4] Z.Q. Tian, B. Ren, Annu. Rev. Phys. Chem. 55 (2004) 197. [5] X.M. Dou, Y. Ozaki, Rev. Anal. Chem. 18 (1999) 285. [6] Y.C. Cao, R. Jin, C.A. Mirkin, Science 297 (2002) 1536. [7] M.F. Mrozek, M.J. Weaver, Anal. Chem. 74 (2002) 4069. [8] K. Kneipp, H. Kneipp, I. Itzkan, R.R. Dasari, M.S. Feld, J. Phys.: Condens. Matter. 14 (2002) R597. [9] Z.Q. Tian, B. Ren, D.Y. Wu, J. Phys. Chem. B 106 (2002) 9463. [10] Z.Q. Tian, B. Ren, in: R.A. Meyers (Ed.), Encyclopedia of Analytical Chemistry, Wiley, Chichester, 2000, p. 9162. [11] Z.Q. Tian, B. Ren, Y.X. Chen, S.Z. Zou, B.W. Mao, J. Chem. Soc., Faraday Trans. 92 (1996) 3829. [12] B. Ren, X.Q. Li, C.X. She, D.Y. Wu, Z.Q. Tian, Electrochim. Acta 46 (2000) 193. [13] X.F. Lin, B. Ren, Z.Q. Tian, J. Phys. Chem. B 108 (2004) 981. [14] P. Cao, R. Gu, Z. Tian, Langmuir 18 (2002) 7609.
J.-W. Hu et al. / Chemical Physics Letters 408 (2005) 354–359 [15] W.L. Parker, R.M. Hexter, A.R. Siedle, Chem. Phys. Lett. 107 (1984) 96. [16] I. Srnova´, B. Vlcˇkova´, V. Baumruk, J. Mol. Struct. 410–411 (1997) 201. [17] L. Guo, Q.J. Huang, X.Y. Li, S.H. Yang, Phys. Chem. Chem. Phys. 3 (2001) 1661. [18] R. Go´mez, J.M. Pe´rez, J. Solla-Gullo´n, V. Montiel, A. Aldaz, J. Phys. Chem. B 108 (2004) 9943. [19] N.H. Kim, K. Kim, Chem. Phys. Lett. 393 (2004) 478. [20] L.H. Lu, H.S. Wang, S.Q. Xi, H.J. Zhang, J. Mater. Chem. 12 (2002) 156. [21] L.H. Lu, G.Y. Sun, H.J. Zhang, H.S. Wang, S.Q. Xi, J.Q. Hu, Z.Q. Tian, R. Chen, J. Mater. Chem. 14 (2004) 1005.
359
[22] J.S. Gao, Z.Q. Tian, Spectrochim. Acta Part A 53 (1997) 1595. [23] B. Ren, X.F. Lin, Y.X. Jiang, P.G. Cao, Y. Xie, Q.J. Huang, Z.Q. Tian, Appl. Spectrosc. 57 (2003) 419. [24] N.R. Jana, L. Gearheart, C.J. Murphy, Chem. Mater. 13 (2001) 2313. [25] J. Belloni, Curr. Opin. Colloid Interf. Sci. 1 (1996) 184. [26] M.A. Watzky, R.G. Finke, Chem. Mater. 9 (1997) 3083. [27] A. Henglein, J. Phys. Chem. B 104 (2000) 6683. [28] W.B. Cai, B. Ren, X.Q. Li, C.X. She, F.M. Liu, X.W. Cai, Z.Q. Tian, Surf. Sci. 406 (1998) 9. [29] B. Ren, D.Y. Wu, B.W. Mao, Z.Q. Tian, J. Phys. Chem. B 107 (2003) 2752. [30] D.K. Lambert, Phys. Rev. Lett. 50 (1983) 2106.
Synthesis of Au@Pd core–shell nanoparticles with controllable size and their application in surface-enhanced Raman spectroscopy Jia-Wen Hu
b
a,b
, Yong Zhang b, Jian-Feng Li b, Zheng Liu b, Bin Ren b, Shi-Gang Sun b, Zhong-Qun Tian b,*, Tim Lian c
a Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, China State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, China c Department of Chemistry, Emory University, Atlanta, GA 30322, USA
Received 22 February 2005; in final form 20 April 2005 Available online 13 May 2005
Abstract Au core Pd shell (Au@Pd) nanoparticles with controllable size from 35 to 100 nm were prepared by chemical deposition of Pd over pre-formed 12 nm Au seeds. Both transmission electron microscopy and UV–visible spectroscopy studies confirmed the core– shell structure of the synthesized nanoparticles. The Au@Pd nanoparticles dispersed on a polished Pt electrode surface exhibit high surface-enhanced Raman scattering (SERS) effect for the adsorbed pyridine and SCN. With the aid of the long-range effect of the electromagnetic (EM) enhancement created by the SERS-active Au core underneath the Pd shell, the good quality SERS spectra of adsorbates on the palladium metal overlayer can be obtained. Such kind of SERS-active substrate can be used as an alternative substrate to massive metals for investigating adsorption and reactions occurring on the Pd metal catalyst. 2005 Elsevier B.V. All rights reserved.
1. Introduction The field of surface enhanced Raman spectroscopy (SERS) was inaugurated largely through contribution of Fleischmann, Van Duyne, Creighton and their coworkers in the mid-1970s [1–3]. Because of the large enhancement of SERS, the intrinsically low detection sensitivity is no longer a fatal disadvantage for surface Raman spectroscopy. The unique surface sensitivity makes SERS an in situ diagnostic tool, widely applicable to electrochemical, biological and other ambient interfaces, for probing the detailed molecular structure and orientation of surface species [4–8]. Unfortunately, it turned out that only a few metals, mainly roughened Ag, Au and Cu, provide the highest enhancement (106–1013) of the Raman effect, which severely limited *
Corresponding author. Fax: +86 592 2085349. E-mail address: [email protected] (Z.-Q. Tian).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.04.071
the breadth of practical applications of SERS involving other metallic materials [9]. Therefore, the strategy of extending the SERS substrate from the noble metals of Au, Ag and Cu to a wide variety of metallic and nonmetallic materials is crucially important for developing SERS as a versatile and powerful tool in surface science and materials science. A long goal of our group is to directly generate SERS from massive transition metals. Since 1996, by developing various surface roughening procedures and optimizing the performance of confocal Raman microscope, we have demonstrated that most transition (VIIIB) metals (e.g., Fe, Co, Ni, Ru, Rh, Pd, Pt) can directly generate weak SERS activity with typical enhancement factors ranging from 10 to 103, and in some systems the enhancement can even reach 104 [9,10]. The applications of SERS in surface adsorption, electro-catalysis and corrosion of transition metal-based systems have been realized [11–14]. Because SERS activity critically depends
J.-W. Hu et al. / Chemical Physics Letters 408 (2005) 354–359
not only on the nature of the metal but also on the size, shape, and spacing of metal nanostructures, the preparation of substrate is always of central concern to gain a higher SERS activity. The ever-developing techniques of nanoscience have provided opportunity to obtain surface nanostructures with controllable size and shape. Accordingly, it is of special interest to extend the SERS substrate from randomly rough surfaces to nanostructures of transition metal with tailored elements. The pioneering works along this avenue can be traced back to Parker et al. and Srnova´ et al. [15,16], who obtained SER spectra from Rh and Pd colloidal particles, respectively. Later, Guo et al. [17] reported SER spectra from Fe nanoparticles dispersed at the electrode surfaces. Recently, Go´mez et al. [18] obtained high quality SER spectra of the adsorbed CO from Pt and Pd nanoparticles dispersed on electrode surfaces, showing an enhancement of 550 for cyanide on aggregated Pt nanoparticles. More recently, Kim et al. [19] also obtained the SERS of benzenethiol from the aggregates of Pt nanoparticles with a maximum enhancement of 200. It should be noted that these works are limited to monometallic particles with particle size generally smaller than 20 nm, not emphasizing the size effect. Since the SERS effect is a size-dependent optical phenomenon, it seems necessary to further develop a method capable of controllably synthesizing a series of the nanoparticles with various sizes and find some ways to further improve the SERS activity. Lu et al. [20] developed an interesting Au seeding method, by which bimetallic core–shell nanoparticles can be easily synthesized with controlled size. They reported a preliminary result on the good SERS activity of Au@Pt nanoparticles [21]. In the present Letter, we prepared a series of Au@Pd nanoparticles based on this method, and dispersed them on a polished Pt electrode. The reason to synthesize nanoparticles with Pd shell is that Pd is a very important electrode material in electro-catalysis but showed the weakest SERS activity among the transition metals already exploited [22]. With the aid of the longrange effect of the electromagnetic (EM) enhancement created by the SERS-active Au core underneath the Pd shell, the high SERS activity of Au@Pd nanoparticles could be reached. The detailed structure and optical properties of these nanoparticles were characterized by TEM and UV–visible spectroscopy, and the SERS activity was investigated by using pyridine and SCN as probe molecules.
355
citrate. Then five solutions (samples 1–5) with 50 ml of 1.0 mM H2PdCl4 and a varied amount of 12-nm Au seeds (20, 10, 5, 2.5 or 0.5 ml of the gold sols) were prepared and cooled to ca. 4 C in an ice bath. Next, into each of the above solution, 6 ml of 100 mM ascorbic acid, also cooled in an ice bath, was slowly dropped while stirring. The color of the mixture turned to black brown within minutes. The stirring was continued for 30 min after finishing the addition of ascorbic acid. To clean the nanoparticles and remove particles smaller than 10 nm, 4 ml of the resulting sols were subjected to centrifugation at 4000 rpm for 15 min. After the supernatant was removed, ultrapure water was added. These procedures were normally repeated for another two times. At last, concentrated Au@Pd sols were obtained at the bottom of the centrifuging tube. A drop of the concentrated sols was then dispersed on the mirror-like surface of a polished Pt rod (2 mm in diameter) sealed in a teflon sheath and dried in a desiccator. With water evaporating, nanoparticles aggregated together and closely packed, forming thin films of nanoparticles which attach tightly on the Pt surface, as they show no Ôfall offÕ visible to the naked eyes even after rinsed with water or electrochemically cleaned in a wide potential range. The substrate denoted as nm-Au@Pd/Pt was then mounted on a homemade spectroelectrochemical cell to acquire Raman spectra [23]. A large Pt ring and a saturated calomel electrode (SCE) served as the counter electrode and the reference electrode, respectively. All the potentials were quoted versus SCE. UV–visible spectra of the sols of gold seeds and the Au@Pd nanoparticles that were not further treated were both recorded on a Shimadzu UV-2100 spectrometer using a 1 cm quartz cell from their reaction mixtures. TEM and HRTEM images of the nanoparticles were acquired on JEM-100 CXII and Tecnai F30, respectively, after dipping the carbon-coated copper grid in the concentrated sols for 1 min. In situ SERS measurements were carried out on a Labram I confocal microprobe Raman system (Dilor, France). The excitation line is the 632.8-nm line from a He–Ne laser. A long-working length (8 mm) objective with 50 magnifications was used for directing the laser to the sample surface and collecting the Raman signal in a backscattering geometry. A more detailed introduction of the Raman system can be found in [12].
3. Results and discussion 2. Experimental Au@Pd nanoparticles were synthesized by the Auseed mediated method with a slight modification [20]. In brief, a 12-nm Au sol (used as gold seeds) was first prepared by the reduction of AuCl 4 ions using sodium
Fig. 1a–d is the TEM images of the Au seeds, samples 1, 3 and 5, respectively. As evidenced by the figure, nanoparticles show well-defined spherical shape and fair uniformity. With the decrease in the concentration of the gold seeds, the averaged size of the nanoparticles, measured from more than 200 nanoparticles, increases
356
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Fig. 1. TEM images: (a) the gold seeds; (b) sample 1; (c) sample 3; (d) sample 5.
from 31 (sample 1), 40 (sample 2), 52 (sample 3), 63 (sample 4) to 105 nm (sample 5). Figs. 2a and b are the HRTEM and scanning TEM (STEM) images of sample 1, respectively, both of which show clearly a core–shell structure. The central concept of the seeding method is that the purposely introduced small nanoparticles serve as the nucleation centers to grow nanoparticles to a desired size [24]. This idea originates from the well-known growing mechanism of nanoparticles: a burst nucleation process and a growth stage of the nucleation centers [25]. Ideally, if the two steps are separated in time and the growth would occur only on the surface of the nucleation centers, the final nanoparticles would be highly monodisperse. Nevertheless, in many cases, the introduced seeds do not guarantee such a separation as successful as they could be. Instead, during the growth stage further nucleation occurs [26], which may be the reason why in some batches of the Au@Pd products, the size of the final nanoparticles are not as
monodisperse as expected. We found that the monodispersity of the Au@Pd nanoparticles can be improved largely by controlling the dropping rate of ascorbic acid and the reaction temperature. Under our experimental condition, the forming rate of the Pd monomers generated from the reduction reaction was slowed down. Therefore, the concentration of the Pd monomers is sufficient for the diffusion or the reaction-controlled surface growth of the seeds, but is low for nucleation to occur. As a result, highly monodispersed Au@Pd nanoparticles can be prepared. Fig. 3 shows the UV–visible spectra of the sols of gold seeds and Au@Pd nanoparticles (sample 1). Since excess ascorbic acid is present in the Au@Pd sols and has a strong absorption peak at about 250 nm, the UV absorption below 300 nm is not shown in Fig. 3. A noticeable change that the plasmon band of the Au seeds is completely damped, can be seen from Fig. 3 upon the formation of the Au@Pd nanoparticle. On
Fig. 2. TEM images of sample 1: (a) HRTEM, (b) scanning TEM in a dark-field mode.
J.-W. Hu et al. / Chemical Physics Letters 408 (2005) 354–359
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(b)
400
500 600 700 Wavelength (nm)
-0.2 V
800
-0.4 V
Fig. 3. UV–visible absorption spectra of (a) 12-nm gold seeds and (b) Au@Pd nanoparticles illustrated by sample 1.
the other hand, for the Au@Pd sols, no characteristic absorption peaks can be observed in the region of 300–800 nm, but only a gradual increase in absorption toward the blue is observed. This absorption behavior is very similar to the case of Pd sols with an averaged particle size of 4 nm [27], with increased absorption below 300 nm and the appearance of a clear peak at 218 nm. From this phenomenon we may infer that, Pd shell dominates the optical properties of the Au@Pd nanoparticles. It is not surprising because the thinnest thickness of the Pd shell is already about 10 nm as determined from the TEM images of sample 1 and the Au seeds. No obvious difference in the UV absorption among these samples has been observed with the increase of the shell thickness. To test the SERS activity of the as-prepared Au@Pd nanoparticles, pyridine was selected as the probe molecule, which is a widely used Raman probe molecule because it has a large Raman cross-section. Fig. 4 is the potential-dependent Raman spectra of pyridine on the nm-Au@Pd/Pt electrode. As is evidenced, with the increase of the electrode potential from 0.8 to 0.4 V, the overall Raman intensity increases and reaches a maximum at 0.6 V. Above this potential, the band intensity gradually decreases. Such potential dependent spectral behavior strongly indicates that the Raman signals are from surface species. One fact that no Raman signals are observed from the polished Pt electrode under the same experimental conditions should be mentioned prior to the following discussions. This fact indicates that the spectra shown in Fig. 4 are essentially from the pyridine adsorbed on the supported nanoparticles, being independence of the conducting substrate Pt. Moreover, the nanoparticles have a continuous thick Pd shell around the Au core (see the HRTEM images). Therefore, the spectral interference from the encapsulated core can be effectively ruled out and the spectra only report the fruitful structural information of species on the Pd metal. To evidence the existence of the SERS effect unambiguously, we calculated the enhancement factor (G),
-0.6 V
-0.8 V
0
300
600
900
1200 1500 1800
Raman shift / cm-1 Fig. 4. Potential-dependent Raman spectra of pyridine adsorbed at the nm-Au@Pd/Pt electrode (prepared from sample 1) in 0.1 M NaClO4 + 0.01 M pyridine; acquisition time: 10 s.
which compares the integrated Raman intensity obtained from the surface with that from the solution phase. On the basis of a confocal Raman system, the G can be expressed as G¼
hcN 0 rI surf ; RI bulk
where h is a characteristic value of the confocal Raman system [28,29]. c, N0, r are the concentration of bulk pyridine solution, the Avogadro constant and surface area occupied by each pyridine molecule, respectively. Isurf and Ibulk are the integrated Raman intensities of the m1 mode (ca. 1006 cm1) of pyridine adsorbed on surface and in solution, respectively, and can be measured from experiment. The surface roughness factor (R) is determined as a ratio of the real area of a roughened surface to that of a smooth surface. We assumed that the monodispersed Au@Pd nanoparticles adopt hexagonal arrays on the supporting Pt electrode; R was then calculated to be 1.8. r for a vertically adsorbed pyridine is 0.254 nm2 [28]. The G for pyridine on the nmAu@Pd/Pt electrode (prepared from sample 1) at potential of 0.8 V is then estimated to be as large as about 2.7 · 103. This calculation clearly indicates a SERS effect in the present system. The SERS intensity of the m1 mode of pyridine in Fig. 4 is as strong as 200 cps, which is significantly stronger than that (ca. 5 cps) reported for pyridine on Pd films chemically deposited on a glassy carbon substrate reached [22]. The core–shell Au@Pd nanoparticles also exhibit higher SERS activity than monometallic Pd nanoparticles and roughed net Pd electrode. Furthermore, when the potential was stepped
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from 0.8 to 0.4 V and repeated again to measure one and another series of spectra, the SERS intensity of pyridine in the second measurement decreases only by about 10% at each potential. From above, we can conclude that the Au@Pd nanoparticles exhibit high SERS activity, good stability and reversibility for practical SERS applications, which may result from their unique core–shell structure and monodispersed particle size. The general application of such type of nanoparticles for SERS was further attested by SCN, which has a smaller Raman cross-section than that of pyridine. Shown in Fig. 5 are the potential-dependent SER spectra of SCN adsorbed on the nm-Au@Pd/Pt electrode. A strong peak at ca. 2075 cm1 was observed, which is due to the C–N stretching (mC–N) of SCN. The peak exhibits two features. First, when the potential moves from the negative to the positive limit, the full-width at half-maximum (FWHM) of the mC–N band gradually increases from 29 to 69 cm1, implying an increasing discreteness of the vibration energy states. This feature may indicate a re-orientation of the adsorbed SCN on the Pd surface in the potential range investigated. Second, the applied potential has a strong influence on the band frequency of the mC–N stretching, which results from the electrochemical Stark effect [30]. The Stark tuning rate, dm/dE, where m is the band frequency and E is the applied potential, is as high as 87 cm1/V. Surprisingly, when we changed the size of the Au@Pd nanoparticles from 31 nm (sample 1) to 105 nm (sample 5), neither the frequency nor the band intensity of CN stretching vibration exhibits the size dependency. This result is consistent with the UV absorption results,
which also exhibit no size-dependent characteristics. However, when the shell thickness is thin enough, viz. in a range between ca. one monolayer and a few nanometers, both the plasmon band of gold and the SERS intensity of CO exhibit core size and shell thickness dependent behaviors (results will be published elsewhere). Such experimental results clearly indicate that the EM mechanism operates in the core–shell nanoparticles with a thin skin. Because the present used nanoparticles exhibit higher SERS activity than monometallic Pd nanoparticles and roughed Pd electrode, the long-range EM enhancement may further enhance the Raman signals even when their shell is thicker than 10 nm. In conclusion, in the present Letter, core–shell Au@Pd nanoparticles with controllable size were prepared by the Au seed-mediated method. With the aid of the long-range effect of the electromagnetic (EM) enhancement created by the SERS-active Au core underneath the Pd shell, the high SERS activity of Au@Pd nanoparticles has been reached. Their high SERS activity makes possible the in situ spectral investigation of the catalytic reactions occurring on a Pd surface. Moreover, their easy preparation and characterization may provide deep insight into the relationship between the SERS behaviors and the detailed nanostructures.
Acknowledgment This work is support by NSFC 20228020, 20021002 and 90206039.
2132 5 cps
References
0.0 V 2116
- 0.2 V 2094 - 0.4 V 2084 - 0.6 V 2075 - 0.8 V
1600
1800
2000
2200
2400
Raman shift / cm-1 Fig. 5. Potential-dependent SERS spectra of SCN adsorbed at the nm-Au@Pd/Pt electrode (prepared from sample 1) in 0.1 M NaClO4 + 0.01 M NaSCN; acquisition time: 100 s.
[1] M. Fleischmann, P.J. Hendra, A.J. McQuillan, Chem. Phys. Lett. 26 (1974) 163. [2] D.L. Jeanmaire, R.P. Van Duyne, J. Electroanal. Chem. 84 (1977) 1. [3] M.G. Albrecht, J.A. Creighton, J. Am. Chem. Soc. 99 (1977) 5215. [4] Z.Q. Tian, B. Ren, Annu. Rev. Phys. Chem. 55 (2004) 197. [5] X.M. Dou, Y. Ozaki, Rev. Anal. Chem. 18 (1999) 285. [6] Y.C. Cao, R. Jin, C.A. Mirkin, Science 297 (2002) 1536. [7] M.F. Mrozek, M.J. Weaver, Anal. Chem. 74 (2002) 4069. [8] K. Kneipp, H. Kneipp, I. Itzkan, R.R. Dasari, M.S. Feld, J. Phys.: Condens. Matter. 14 (2002) R597. [9] Z.Q. Tian, B. Ren, D.Y. Wu, J. Phys. Chem. B 106 (2002) 9463. [10] Z.Q. Tian, B. Ren, in: R.A. Meyers (Ed.), Encyclopedia of Analytical Chemistry, Wiley, Chichester, 2000, p. 9162. [11] Z.Q. Tian, B. Ren, Y.X. Chen, S.Z. Zou, B.W. Mao, J. Chem. Soc., Faraday Trans. 92 (1996) 3829. [12] B. Ren, X.Q. Li, C.X. She, D.Y. Wu, Z.Q. Tian, Electrochim. Acta 46 (2000) 193. [13] X.F. Lin, B. Ren, Z.Q. Tian, J. Phys. Chem. B 108 (2004) 981. [14] P. Cao, R. Gu, Z. Tian, Langmuir 18 (2002) 7609.
J.-W. Hu et al. / Chemical Physics Letters 408 (2005) 354–359 [15] W.L. Parker, R.M. Hexter, A.R. Siedle, Chem. Phys. Lett. 107 (1984) 96. [16] I. Srnova´, B. Vlcˇkova´, V. Baumruk, J. Mol. Struct. 410–411 (1997) 201. [17] L. Guo, Q.J. Huang, X.Y. Li, S.H. Yang, Phys. Chem. Chem. Phys. 3 (2001) 1661. [18] R. Go´mez, J.M. Pe´rez, J. Solla-Gullo´n, V. Montiel, A. Aldaz, J. Phys. Chem. B 108 (2004) 9943. [19] N.H. Kim, K. Kim, Chem. Phys. Lett. 393 (2004) 478. [20] L.H. Lu, H.S. Wang, S.Q. Xi, H.J. Zhang, J. Mater. Chem. 12 (2002) 156. [21] L.H. Lu, G.Y. Sun, H.J. Zhang, H.S. Wang, S.Q. Xi, J.Q. Hu, Z.Q. Tian, R. Chen, J. Mater. Chem. 14 (2004) 1005.
359
[22] J.S. Gao, Z.Q. Tian, Spectrochim. Acta Part A 53 (1997) 1595. [23] B. Ren, X.F. Lin, Y.X. Jiang, P.G. Cao, Y. Xie, Q.J. Huang, Z.Q. Tian, Appl. Spectrosc. 57 (2003) 419. [24] N.R. Jana, L. Gearheart, C.J. Murphy, Chem. Mater. 13 (2001) 2313. [25] J. Belloni, Curr. Opin. Colloid Interf. Sci. 1 (1996) 184. [26] M.A. Watzky, R.G. Finke, Chem. Mater. 9 (1997) 3083. [27] A. Henglein, J. Phys. Chem. B 104 (2000) 6683. [28] W.B. Cai, B. Ren, X.Q. Li, C.X. She, F.M. Liu, X.W. Cai, Z.Q. Tian, Surf. Sci. 406 (1998) 9. [29] B. Ren, D.Y. Wu, B.W. Mao, Z.Q. Tian, J. Phys. Chem. B 107 (2003) 2752. [30] D.K. Lambert, Phys. Rev. Lett. 50 (1983) 2106.