Vibrational Spectroscopy 34 (2004) 269–272
Surface-enhanced Raman scattering of 4,40 -bipyridine on gold nanoparticle surfaces Sang-Woo Joo* Department of Chemistry, Soongsil University, Sangdo-dong 1-1, Seoul 156-743, South Korea Received 6 October 2003; received in revised form 4 December 2003; accepted 10 December 2003
Abstract The adsorption structure and mechanism of 4,40 -bipyridine (BiPy) on gold nanoparticle surfaces has been investigated by means of surfaceenhanced Raman scattering (SERS). The aromatic ring of BiPy appeared to assume a perpendicular orientation with respect to the gold surface from the presence of the n(CH) band at 3060 cm1. The SERS intensities of several vibrational modes of BiPy on Au were found to vary as the bulk concentration. The SERS intensities for BiPy on Au could be ascribed to both the electromagnetic (EM) and charge transfer (CT) enhancement mechanism. # 2004 Elsevier B.V. All rights reserved. Keywords: 4,40 -Bipyridine; Au; Nanoparticles; Adsorption; SERS
1. Introduction Surface-enhanced Raman scattering (SERS) has been one of the most sensitive techniques to monitor the adsorbates on metal substrates at submonolayer coverage limit despite the equivocal selection rule and limited applicability to a few metals [1]. To date, only qualitative arguments concerning molecular orientation have offered for data obtained via SERS, since it has been difficult to model real surfaces due to the fact that both a long range electromagnetic (EM) effect and a short range chemical effect are assumed to simultaneously operate for the overall enhancement [2–4]. On the basis of the electromagnetic surface selection rules [3,4], the interfacial structures of aromatic adsorbates on silver and gold nanoparticles could be explained in a more quantitative way [5,6]. An additional contribution to the SERS phenomenon is the charge transfer (CT) mechanism considered as analogous to a resonance Raman process, although it strongly depends on the nature of the metaladsorbate system without general rules. The band most enhanced by this CT mechanism was reported to be that of the n8a mode for several aromatic molecules [7–9]. The ability of 4,40 -bipyridine (BiPy) to enhance redox reactions of certain molecules at metal surfaces has generated considerable interest [10]. 4,40 -bis-Quaternary compounds, viologens have been widely applied in areas such * Tel.: þ82-2-8200434; fax: þ82-2-8200434. E-mail address:
[email protected] (S.-W. Joo).
0924-2031/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2003.12.006
as electron transfer relays for solar energy systems and a promoter for the redox protein cytochrome C. The preparation of thin films based on BiPy attracted much attention due to their potential application as a molecular switch [11]. Understanding of physicochemical property in selfassembled monolayers prepared by bipyridines on the metal substrates should be important in designing a new nanoscale molecular electronic device. Although Raman studies of BiPy were previously reported [12–14], a complete assignment for the clarification of the adsorption structure on gold has not been performed yet. Recently attenuated total reflection spectroscopy was employed to investigate an orientation of BiPy on a gold electrode [15]. To better understand the adsorption characteristics and the enhancement mechanisms of self-assembled monolayers based on bipyridines, SERS of BiPy was performed on Au nanoparticle surfaces.
2. Experimental BiPy (>98%) was purchased from Tokyo Kasei and used without further purification. The citrate-stabilized gold nanoparticle was synthesized by following the recipes in the literature [16]. A 133.5 mg portion of KAuCl4 (from Aldrich) was initially dissolved in 250 ml of water, and the solution was brought to boiling. A solution of 1% sodium citrate (25 ml) was then added to the KAuCl4 solution under vigorous stirring, and boiling was continued for ca. 20 min.
S.-W. Joo / Vibrational Spectroscopy 34 (2004) 269–272
of conjecture whether the pyridine rings of BiPy should twist with respect to one another without lying on the same plane. In the OR spectrum, the major bands associated with the ring vibrational modes can be divided into four symmetry species assuming C2n symmetry [17]. The A1 ring modes appeared at 572 cm1 (n20a), 660 cm1 (n6a), 999 cm1 (n1 ), 1085 cm1 (n18a), 1217 cm1 (n9a), 1509 cm1 (n19a), 1607 cm1 (n8a), and 3051 cm1 (n2 ). The bands at 324 cm1 (n16a) and 853 cm1 (n10a) could be ascribed to the A2 mode. The bands at 383 cm1 (n16b) and 755 cm1 (n11 ) could belong to the B1 mode. The B2 ring modes appeared at 1297 cm1 (n3 ), 1621 cm1 (n8b), and 3027 cm1 (n7b). Their peak positions are listed in Table 1 along with the appropriate vibrational assignments. Fig. 1b shows the Au nanoparticle SERS spectrum. According to the TEM measurement, the average diameter of gold particles was 35 nm. The surface area of the single particle is calculated to be 3850 nm2. A single particle is composed of 1:8 106 Au atoms and the concentration of the Au nanoparticles is estimated to be approximately 7:2 1010 M. The concentration of 1:3 105 M is required for a full coverage of monolayers. The concentration of BiPy at 103 M in Fig. 1b was higher than that required for its monolayer coverage. The geometry of aromatic compounds on metal surfaces has been inferred from the presence of the n2 band in the previous SERS studies [19,20]. The ring CH stretching n2 band was clearly observed at 3060 cm1 in the SERS spectrum at higher concentration. On the other hand, the n2 mode was not detected for a lower concentration of BiPy. The spectra in this region were examined at least several times to ensure the reproducibility. Since the other bipyridine ring modes
The resulting Au nanoparticle solution was stable for several weeks. All the chemicals otherwise specified were reagentgrade and triply distilled water, of resistivity greater than 18.0 MO cm, was used in making aqueous solutions. To estimate the sizes of gold particles, their transmission electron microscope (TEM) images were obtained after placing sequentially a drop of Au colloidal solution onto carbon coated Cu grids. Their TEM images were obtained with a Tecnai F20 Philips transmission electron microscope after placing a drop of colloidal solution on a carbon-coated copper grid. The statistical analysis revealed a size distribution of standard deviations in diameters, s 14:7% for 34.8 (5.1) nm particles, respectively. The mean particle diameters and their relative standard deviations were obtained for each sample by counting at least 150 gold particles. UVVis absorption spectra of the colloidal solutions were obtained with a Shimadzu UV-1601PC spectrophotometer. Raman detection methods are described in the previous literatures [5,6]. The lmax values appeared at 535 nm. A 0.05 M ethanolic solution of BiPy was added dropwise to 0.1–2 ml of Au nanoparticle solution to a final concentration of 103 M using a micropipet. The purple gold nanoparticles became bluish green by the addition of BiPy.
3. Results and discussion 3.1. Raman spectra
3200
3000
1500
770
657 625
769
656 626
1000
384 353
531
383 324
572
660
755
853
1217
999
1000
1076
1019
383 353
854 866
1074 1019 1002
1295 1293 1228 1297 1509
X5
1621 1607
(b)
(a)
1228
1511 1511
X5
1609
3059
(c)
3051 3027
Intensity (Arbitrary Unit)
1610
Fig. 1a shows the ordinary Raman (OR) spectrum of BiPy in the solid state. Consulting the earlier vibrational assignments [17,18], the Raman spectrum is analyzed. It is a matter
1085
270
500 -1
Raman shift (cm ) Fig. 1. (a) Ordinary Raman spectrum of BiPy in the solid state and SERS spectrum at (b) low (105 M) and (c) high concentration (103 M) of BiPy in aqueous gold nanoparticles. The spectral region between 2900 and 1750 cm1 was omitted due to a lack of any information.
S.-W. Joo / Vibrational Spectroscopy 34 (2004) 269–272 Table 1 Spectral data and vibrational assignment of BiPy (week)
3.2. Electromagnetic surface selection rule
Au SERS (103 M)
Assignmenta
3059 1639 1610 1511 1295 1228
2 (A1) 7b (B2) 8b (B2) 8a (A1) 19a (A1) 3 (B2) 9a (A1)
1085
1074 1019
18a (A1) 12 (A1)
999
1002
1 (A1)
660
657 625
6a (A1) 6b (B2)
Ordinary Raman In-plane 3051 3027 1621 1607 1509 1297 1217
20a (A1)
572 Out-of-plane 853 755 383 324
854 770 383 353
271
10a (A2) 11 (B1) 16b (B1) 16a (A2)
a
Based on [17] in Wilson notation with symmetries based on C2n point group.
appeared nicely at almost identical positions, BiPy is also assumed to have a standing geometry at the low concentration even without the absence of the n2 mode. As shown in Fig. 1b and c, the n2 mode at 3060 cm1 looked prominent at a high bulk concentration of BiPy (103 M), but disappeared at low concentration (105 M). The position of the n8a band of BiPy in gold nanoparticles was found at 1610 cm1 at the high concentration while it appeared at 1609 cm1 at the low concentration. Considering the resolution of our Raman spectrometer is 1 cm1, the ring modes were found to remain almost unchanged. Except for the n18a band weakly found at 1080 cm1, most inplane ring modes have not shown a red shift after adsorption on Au. These results implied a rather weak ring p interaction of BiPy on Au [5,6]. In particular, neither a substantial red shift nor a significant band broadening of the ring breathing n1 and n12 modes implied that a direct ring p orbital interaction with gold substrates should be quite low. These results may support a rather vertical orientation of the bipyridine ring in BiPy on gold. Considering that the citrate anion weakly bind on gold nanoparticle surfaces in comparison with that of the anions on silver [6], it is not absolutely certain whether BiPy would form an ion pair with the citrate anions in adsorbing on Au surfaces. Also, from our SERS spectra, it seems difficult to discern whether BiPy would bridge two different gold particles at low concentrations as in the case of 4,40 -biphenyldiisocyanide [21]. Due to a relatively poor signal to noise ratio SERS spectra at other concentrations were not presented here.
It is unfortunate that the structural change cannot be analyzed more precisely from the SERS spectra since the set of SERS selection rules established so far is highly dependent on the specific enhancement mechanism. The analysis was made herein for a few selected peaks on the basis of the prediction of the electromagnetic selection rule. From the EM surface selection rule [3,4], the vibrational mode perpendicular to the surface is more enhanced than the parallel mode. For BiPy, most ring modes were found to belong to those of in-plane modes except for the weak features at 850 cm1 (n10a), 750 cm1 (n11), 380 cm1 (n16b), and 324 cm1 (n16a). The relative weakness of the out-of-plane bands supported that the adsorbate should have a rather vertical structure. Moskovits proposed that the relative enhancement should be in the order of A1 > B1 A2 > B2 for flat orientation and of A1 > B1 B2 > A2 for vertical orientation [3]. For most bands of BiPy, the relative band enhancements were found to be in the order of A1 > B2 > B1 > A2 on gold as shown in Table 2. Although not definite, this result may suggest a rather vertical orientation of BiPy with respect to the surfaces. According to the EM surface selection rule [4], relative enhancements for normal modes can be classified into three groups in terms of the polarizability tensor elements. Creighton has shown on the grounds of the EM surface selection rule that for molecules with C2n symmetry, the relative enhancement factors for A1, A2, B1, B2 modes, Table 2 Relative enhancement factors of SERS bands of BiPy in Au SERS Symmetry typea
Tensor elementb
Normal modec
Relative enhancement factor (ISERS/IOR)d Au SERS (103 M)
A1
axx ayy azz
1 2 6a 8a 9a 18a 19a
0.623 1.00 0.304 20.2 4.74 6.05 7.36
A2
axy
10a 16a
1.81 2.87
B1
axz
11 16b
2.89 2.50
B2
ayz
3 8b
5.46 2.65
Subtracted from the intensity of the n2 band. a Symmetry types corresponding to the C2n point group. b Subscripts, i.e., x, y, and z, correspond to the conventional molecular axes. The x-axis lies perpendicular to the ring, and the z-axis passes through the two nitrogen atoms. c See Table 1 for the vibrational assignment. d Normalized to 1.00 for the n2 band at ca. 3060 cm1 in the SERS and OR spectra.
272
S.-W. Joo / Vibrational Spectroscopy 34 (2004) 269–272
respectively, would be 1–16, 1, 4, and 4 for edge-on orientation whereas these would be 1–16, 4, 4, and 1 for face-on orientation [4]. On these grounds, the relative intensities have been evaluated for the normal modes of BiPy. Table 2 lists the intensity ratios with the relevant molecular symmetry elements. The ring mode intensities were normalized to the intensity of the ring n2 mode at 3060 cm1 in each SERS spectrum. For most observed bands of BiPy, the relative band enhancements were found to be in the order of A1:A2: B1:B2 ¼ 0:6–20:2–3:3:3–5 on gold as shown in Table 2. Although not definite, this result may support a rather vertical orientation of BiPy with respect to the surface.
4. Summary and conclusion The adsorption behavior of 4,40 -bipyridine on gold nanoparticle surfaces was studied by means of surface-enhanced Raman scattering. It appeared to be problematic to explain the origin of the band intensities by a single SERS mechanism for BiPy on Au. A combination of the electromagnetic and charge transfer effects should be the cause of the SERS intensities for BiPy on Au. By determining the relative vibrational intensity factors in the Au SERS spectrum, the adsorption characteristics and enhancement mechanism of BiPy on Au were examined in a more quantitative way on the basis of both the EM and CT mechanisms.
3.3. Charge transfer mechanism It was reported that the charge transfer mechanism could also significantly contribute the enhancement of the SERS intensities [7–9]. Although the SERS spectra feature could be roughly described by the EM mechanism, it is admitted that the CT mechanism may also contribute the SERS intensities of several vibrational bands of BiPy on Au. In fact, the different enhancement values for the same symmetry as listed in Table 2 could be better explained with a combination of the EM and CT mechanisms. For molecules such as pyrazine [7,8] and phenylacetic acid [9] on silver surfaces, the vibrational band most enhanced by the charge transfer mechanism was calculated to be that of the n8a mode. As listed in Table 2, it is intriguing that the n8a mode is most enhanced for the present SERS spectrum of BiPy on Au. The CT mechanism also appeared to influence the enhancement of several vibrational bands for BiPy. For BiPy, except the most enhanced feature of the n8a mode, the SERS enhancements of other ring modes were found to be quite different from those of pyrazine. Although the electronic states or the changes in the equilibrium geometry for BiPy in the CT process would be dissimilar from those of pyrazine, the SERS enhancements appeared to be better explained by both the EM and CT mechanism. Considering that the possibility of the interaction between the bipyridine group and gold surfaces is low, the pyridine rings of BiPy are not assumed to lie on the same plane. It is however a matter of conjecture whether the pyridine rings of BiPy should twist with respect to one another without lying on the same plane. It is not absolutely certain as to how polycrystalline surfaces of gold nanoparticles would affect the adsorbate orientation. Only average orientation could be observed for BiPy on polycrystalline gold nanoparticle surfaces [5]. Since SERS selection rules do not provide a precise interpretation of band intensities, other spectroscopic techniques have been currently applied to reach a more consistent conclusion. Thermodynamic, electrochemical, and theoretical studies shall also be useful to explain the adsorption characteristics of BiPy on Au surfaces.
Acknowledgements S.-W.J. would like to thank Prof. Kwan Kim for supporting the Raman facilities and introducing SERS studies of aromatic adsorbates. This work was supported by the Soongsil University Research Fund.
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