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Raman and surface Raman spectroscopy with ultraviolet excitation
Spectrochimica Acta Part A 61 (2005) 1991–1995
Raman and surface Raman spectroscopy with ultraviolet excitation Xu Lili, Fang Yan∗ Department of Physics, Beijing Key Lab of Nano-Photonics and Nano-Structure (NPNS), Capital Normal University, Beijing 100037, PR China Received 18 June 2004; accepted 28 July 2004
Abstract We record the accurate and reliable Raman spectra of benzoic acid (BA), p-nitrobenzoic acid (PNBA) and o-nitrobenzoic (ONBA) in aqueous solution with ultraviolet excitation. And we find that the ultraviolet (UV) Raman spectrum of aqueous BA solution has one-to-one correspondence to that of BA solid whereas the others are less resemble to the solid counterparts. We also report surface Raman spectroscopy of them in silver colloid without any enhancement in UV region and call it surface-unenhanced Raman spectroscopy (SUERS) while the surface-enhanced Raman scattering (SERS) effects are perfect in near infrared or visible regions. It demonstrates the SERS effects are strongly dependent on the excitation wavelength. On the basis of the experiments, we discuss the mechanism of SERS excitated in different regions. © 2004 Elsevier B.V. All rights reserved. Keywords: Benzoic acid (BA); p-Nitrobenzoic acid (PNBA); o-Nitrobenzoic acid (ONBA); Ultraviolet (UV) Raman spectroscopy; Surface-unenhanced Raman spectroscopy (SUERS)
1. Introduction Employing photons as probes, Raman spectroscopy is potentially an ideal technique for studying chemical structures and physical interface phenomena [1]. The recent development of ultraviolet (UV) laser sources has made possible experiments in UV Raman spectroscopy with applications to fundamental and applied studies in material science and biology [2–4]. The first major advantage of UV Raman over visible Raman spectroscopy is that fluorescence would not interfere and almost all samples could be measured with UV excitation [5]. The UV Raman technique also benefits by the increased Raman efficiency due to the ∼ν4 dependence of the Raman scattering efficiency. Furthermore, with resonance excitation, the Raman cross-sections could be exceptionally large [6]. And it is possible to observe the Raman bands of typical sample at low concentrations or exiguity. Most likely, surface-enhanced Raman scattering (SERS) spectroscopy is combined force of the chemical effect and the ∗
Corresponding author. Tel.: +86 10 68902965; fax: +86 10 68902965. E-mail address: [email protected] (F. Yan).
1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2004.07.030
electromagnetic effect that gives rise to the total enhancement of ∼106 [7,8]. But in the ultraviolet region, the Raman spectra are focused up on rather than the enhancement effects. However, the researches on the enhancement mechanism in UV region could help to understand the puzzling properties associated with SERS and suggest SERS would have significant application in a variety of problems. Due to the low solubility in the saturated solution, it is difficult to detect the near infrared or visible Raman of benzoic acid (BA) or p-nitrobenzoic acid (PNBA) or o-nitrobenzoic (ONBA) in aqueous solution. To obtain Raman spectra of above-mentioned samples in solution, they are usually dissolved in organic solvents to increase solubility, such as in ethanol [9]. However, it is inevitable that organic solvents would interfere in the Raman signals of the samples. Beyond all doubt, the Raman spectra of the sample in aqueous solution are accurate and credible without disturb from the solvent. We report the high quality Raman spectra of aqueous solution of BA and PNBA and ONBA in UV region. It is worthwhile to note that there is no SERS effects excitated by ultraviolet whereas the prefect SERS occurs in visible
1992
X. Lili, F. Yan / Spectrochimica Acta Part A 61 (2005) 1991–1995
or near infrared region. It maybe accounts for selectivity of SERS to the excitation wavelength and we elucidate the various mechanisms in different regions.
2. Experimental 2.1. Preparation of silver colloid Silver colloid is prepared according to Lee and Meisel’s method [10]. Ninety milligrams of AgNO3 is dissolved in 500 ml deionized water and the solution is heated to boiling. Then, 10 ml of 1% aqueous solution of trisodium citrate is added into the boiling AgNO3 solution drop by drop, accompanying with vigorous stirring. The mixed solution is kept boiling for further 10 min. Finally, a gray-green silver colloid is obtained, which shows a single visible absorbance band at about 430 nm.
Fig. 2. The UV Raman spectra of PNBA solid (a) and PNBA in aqueous solution (b).
2.2. Instrumentation
We respectively record the UV Raman spectra of the sample mentioned above. From Fig. 1, we have obtained precise indications that the UV Raman spectrum of aqueous BA solution has strong similarity to that of BA solid, shown for comparison in Fig. 1b. The UV Raman spectrum of aqueous
PNBA solution resembles to the corresponding solid Raman spectrum in Fig. 2, while the Raman spectrum of ONBA in aqueous solution has distinct difference to the counterpart in Fig. 3. The NIR or visible Raman spectrum of BA in aqueous solution is hardly obtained due to the low concentration of the saturated aqueous BA solution. In contrast to the NIR and visible case, the UV Raman spectrum of aqueous BA solution is perfect and shown in Fig. 1b. There are few difference between Fig. 1a and b, mainly in the 1600–1700 cm−1 region. And there is one to one correspondence between the Fig. 1a and 1b below 1600 cm−1 apart from a general small increase in linewidth with respect to the solution. In the UV Raman spectrum of aqueous BA solution, there is a weak band at 1689 cm−1 due to C O stretching mode while the spectrum of solid BA has no counterpart. In the solid
Fig. 1. The UV Raman spectra of BA solid (a) and BA in aqueous solution (b).
Fig. 3. The UV Raman spectra of ONBA solid (a) and ONBA in aqueous solution (b).
The UV–vis Raman spectra are recorded by a RENISHAW H13325 spectrophotometer, and the UV laser line is at 325 nm, the visible line at 514.5 nm.
3. Results and discussion 3.1. UV Raman spectra of BA and PNBA and ONBA in aqueous solution
X. Lili, F. Yan / Spectrochimica Acta Part A 61 (2005) 1991–1995
state, benzoic acid molecules occur as nearly planar, centrosymmertic dimers of symmertry D2h , with hydrogen bonds ˚ between adjacent carboxyl groups [11]. And in the (2.64 A) aqueous solution, hydrogen bond established between phenolic groups is destroyed and the dimer turns into molecule or ion, and the band located at 1689 cm−1 appears due to decomposition of dimer. In the cases of analyzing the UV Raman spectra of aqueous PNBA and ONBA solutions, we concern for the assignments to the bands because there are notable differences between the counterparts. PNBA is usually dissolved to organic solvent rather than to aqueous solvent to obtain the Raman spectrum because of the low solubility in aqueous solution. The early spectra of PNBA in silver colloid consist of two different types. One type of the spectra is features of the strong bands located at 1600, 1355, 1115, and 875 cm−1 , resemble to the normal Raman of p-nitrobenzoate salt in ethanol or aqueous solution [9,12]. The other is characteristic of two weak bands at 1355 and 1115 cm−1 , a strong band at 1460 cm−1 and a strong doublet near 1150 cm−1 . But some authors have argued that those bands of the latter type are due to azodibenzoate [13]. The UV Raman spectrum we have recorded is in agreement with the former, and the two prominent bands occur at 1353 and 1604 cm−1 , respectively due to NO2 and ring stretching vibration. Two medium intensity bands appear at 867 and 1111 cm−1 , respectively attributed to C N antisymmetry stretching and C H vibration. Another relative weak band emerges at 626 cm−1 . The UV Raman spectrum of aqueous PNBA solution is similar to that of PNBA in ethanol, whose concentration is higher than that in aqueous solution, about 3 mg/ml. But the former gives more information than the latter. It is evident that an additional weak band appears at 1178 cm−1 . And there are several weak bands between 1350 and 1600 cm−1 , respectively at 1442, 1461, and 1492 cm−1 . And the bands of 1442 and 1461 cm−1 are too weak to distinguish. All thought, the UV spectrum of aqueous PNBA solution correlates much better with the ordinary Raman spectrum of solid PNBA than the spectrum of PNBA in ethanol. The UV Raman spectrum of aqueous ONBA solution exhibits remarkable difference from that of it is solid. The UV Raman spectrum of solid ONBA is dominated by three bands: one broad and strong at 1607 cm−1 , and two weak bands at about 1477 and 1257 cm−1 whereas the Raman of aqueous ONBA solution gives much more information. The most prominent band at 1353 cm−1 is ascribed to NO2 symmetry stretching vibration. And a strong doublet at 1616 and 1578 cm−1 is attributed to ring vibration. There is a band at 1146 cm−1 to the ring breath vibration. Several bands at 1480, 1442, 1073 and 1038 cm−1 are all assigned to C H bending vibration. And the bands at 861 and 699 cm−1 might be correlated with NO2 bending and symmetry rocking, respectively [14]. It is worthwhile to note that the strong and medium intensity of bands are all ascribed to C C or C H vibration while NO2 and COO− show the weak intensity or absence.
1993
Fig. 4. The UV Raman spectra of BA before (a) and after (b) being mixed with silver colloid.
3.2. UV surface Raman spectra of BA and PNBA and ONBA in silver colloid The SERS spectra of BA and PNBA and ONBA are expected to resemble to those perfect Raman excited with ultraviolet radiation. But the behavior of the samples in silver colloid proves spectra without giant field enhancement, shown in Figs. 4–6. And there is no notable difference between curves a and b of these figures. The fine difference between curves a and b in Fig. 4 is because the aqueous BA solution is diluted after adding an aliquot silver colloid. If no SERS effect occurs when molecules adsorb on the metal surface, it is usually called surface-unenhanced Raman scattering (SUERS). And we observe the SUERS of BA and PNBA and ONBA in silver colloid with UV excitation. When an aliquot BA or PNBA or ONBA solution is added into silver colloid, the interaction between them takes place
Fig. 5. The UV Raman spectra of PNBA before (a) and after (b) being mixed with silver colloid.
1994
X. Lili, F. Yan / Spectrochimica Acta Part A 61 (2005) 1991–1995
Fig. 6. The UV Raman spectra of ONBA before (a) and after (b) being mixed with silver colloid.
Fig. 8. The Vis Raman spectra of PNBA before (a) and after (b) being mixed with silver colloid.
immediately. We not only observe the color change of colloid after adding aqueous BA or PNBA or ONBA solution, but also obtain the visible SERS, which could elucidate the interaction between the molecules and the silver particles. Namely, there are few Raman bands except 1356 cm−1 in the visible Raman spectrum of ONBA in aqueous solution, while the abundant bands occur after the silver colloid is added, shown in Figs. 7–9. Until recently, most of people seem to accept that the classical electromagnetic effect and a charge-transfer (CT) mechanism in which the excitation can take place by a charge transfer between the metal and the charge transfer complex [15]. The former reflects optical properties and nano structure of metal. And the latter is involved with geometrical and electronic configuration of molecules, interaction with substrate and so on. But both of them can operate simultaneously and it is difficult to separate one effect alone from SERS.
SERS in the visible may due to electromagnetic mechanism, which occurs in the vicinity of small, interacting metal particles that are illuminated with light resonant or near resonant with the localized surface-plasmon frequency of the metal structure [16]. On one hand, the surface-plasmon resonance condition is that the real part of dielectric constant must be negative and the imaginary part should approach to zero as much as possible. Silver works best because its dielectric constant is almost real and negative in the visible region [17]. On the other hand, some literature note peak Raman enhancement at 540 nm (2.3 ev) for Ag [18]. The poor performance of SERS in UV region can be explained by the absence of both surface-plasmon resonance and charge transfer excitation. There is no surface-plasmon resonance because the real part of dielectric constant for Ag is positive while the imaginary part increases rapidly for transition between energy bands.
Fig. 7. The Vis Raman spectra of BA before (a) and after (b) being mixed with silver colloid.
Fig. 9. The Vis Raman spectra of ONBA before (a) and after (b) being mixed with silver colloid.
X. Lili, F. Yan / Spectrochimica Acta Part A 61 (2005) 1991–1995
The charge transfer excitation occurs from the highest occupied state of the metal to the electron affinity level of the molecule. The energy required for this CT excitation is found to become resonance with the laser frequencies used in SERS experiments [19]. The photon energy of ultraviolet used in experiments is about 3.8 ev so the CT excitation condition cannot be satisfied in this region. In a brief, the SUERS in UV region maybe explained in terms of the absence of electromagnetic and CT mechanism. The dependence on the excitation of SERS can account for the mechanism in near infrared and visible regions. 4. Conclusions We report the high quality Raman spectra of BA and PNBA and ONBA in aqueous solution (≤0.01 M) while almost no Raman signals could be detected in visible region because of the low concentration. And we reveal the SUERS in UV region. We discuss the possible reasons and it is likely that there is neither electromagnetic nor chemical enhancement. Acknowledgments The authors are grateful for the support to this research by the National Natural Science Foundation of China and the Natural Science Foundation of Beijing.
1995
References [1] L. Can, C.S. Peter, Catal. Today 33 (1997) 353. [2] X. Guang, Y. Yi, F. Zhaochi, X. Qin, X. Fengshou, L. Can, Microporous Mesoporous Mater. 42 (2001) 317. [3] C. John, D.S. Alastair, FEBS Lett. 503 (2001) 30. [4] C. Zhenhuan, X.G. Chen, J.S.W. Holtz, S.A. Asher, Biochemistry 37 (1998) 2854. [5] Y. Yi, X. Guang, L. Can, X. Fengshou, J. Catal. 194 (2000) 487. [6] S.A. Asher, C.H. Munro, C. Zhenhuan, Laser Focus World (1997) 99. [7] M. Moskovits, J. Chem. Phys. 69 (9) (1978) 4159. [8] C.K. Chen, A.R.B. de Castro, Y.R. Shen, Phys. Rev. Lett. 46 (1981) 14. [9] J.C. Tsang, P. Avouris, J.R. Kirtiey, J. Chem. Phys. 79 (1) (1983) 493. [10] P.C. Lee, D. Meisel, J. Phys. Chem. 86 (1982) 3391. [11] G.A. Sun, J.M. Robertson, T.H. Goodwin, Acta Crystallorgr. 8 (1955) 157. [12] H. Ratinen, M. Kiviharju, Spectrochim. Acta A 45 (7) (1989) 729. [13] R.S. Venkatachalam, F.J. Boerio, P.G. Roth, J. Raman Spectrosc. 19 (1988) 281. [14] G. Varsanyi, Assignments for Vibrtional Spectra of Seven Hundred Benzene Derivations, Wiley, New York, 1974. [15] A. Otto, I. Mrozek, H. Grabhorn, W. Akemann, J. Phys. Condens. Matter 4 (1992) 1143. [16] M. Moskovits, Rev. Mod. Phys. 57 (1985) 783. [17] S.L. McCall, P.M. Platzman, Phys. Rev. 22 (4) (1980) 1660. [18] E.J. Zeman, G.C. Schatz, J. Phys. Chem. 91 (1987) 634. [19] H. Ueba, Surf. Sci. 131 (1983) 347.
Raman and surface Raman spectroscopy with ultraviolet excitation Xu Lili, Fang Yan∗ Department of Physics, Beijing Key Lab of Nano-Photonics and Nano-Structure (NPNS), Capital Normal University, Beijing 100037, PR China Received 18 June 2004; accepted 28 July 2004
Abstract We record the accurate and reliable Raman spectra of benzoic acid (BA), p-nitrobenzoic acid (PNBA) and o-nitrobenzoic (ONBA) in aqueous solution with ultraviolet excitation. And we find that the ultraviolet (UV) Raman spectrum of aqueous BA solution has one-to-one correspondence to that of BA solid whereas the others are less resemble to the solid counterparts. We also report surface Raman spectroscopy of them in silver colloid without any enhancement in UV region and call it surface-unenhanced Raman spectroscopy (SUERS) while the surface-enhanced Raman scattering (SERS) effects are perfect in near infrared or visible regions. It demonstrates the SERS effects are strongly dependent on the excitation wavelength. On the basis of the experiments, we discuss the mechanism of SERS excitated in different regions. © 2004 Elsevier B.V. All rights reserved. Keywords: Benzoic acid (BA); p-Nitrobenzoic acid (PNBA); o-Nitrobenzoic acid (ONBA); Ultraviolet (UV) Raman spectroscopy; Surface-unenhanced Raman spectroscopy (SUERS)
1. Introduction Employing photons as probes, Raman spectroscopy is potentially an ideal technique for studying chemical structures and physical interface phenomena [1]. The recent development of ultraviolet (UV) laser sources has made possible experiments in UV Raman spectroscopy with applications to fundamental and applied studies in material science and biology [2–4]. The first major advantage of UV Raman over visible Raman spectroscopy is that fluorescence would not interfere and almost all samples could be measured with UV excitation [5]. The UV Raman technique also benefits by the increased Raman efficiency due to the ∼ν4 dependence of the Raman scattering efficiency. Furthermore, with resonance excitation, the Raman cross-sections could be exceptionally large [6]. And it is possible to observe the Raman bands of typical sample at low concentrations or exiguity. Most likely, surface-enhanced Raman scattering (SERS) spectroscopy is combined force of the chemical effect and the ∗
Corresponding author. Tel.: +86 10 68902965; fax: +86 10 68902965. E-mail address: [email protected] (F. Yan).
1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2004.07.030
electromagnetic effect that gives rise to the total enhancement of ∼106 [7,8]. But in the ultraviolet region, the Raman spectra are focused up on rather than the enhancement effects. However, the researches on the enhancement mechanism in UV region could help to understand the puzzling properties associated with SERS and suggest SERS would have significant application in a variety of problems. Due to the low solubility in the saturated solution, it is difficult to detect the near infrared or visible Raman of benzoic acid (BA) or p-nitrobenzoic acid (PNBA) or o-nitrobenzoic (ONBA) in aqueous solution. To obtain Raman spectra of above-mentioned samples in solution, they are usually dissolved in organic solvents to increase solubility, such as in ethanol [9]. However, it is inevitable that organic solvents would interfere in the Raman signals of the samples. Beyond all doubt, the Raman spectra of the sample in aqueous solution are accurate and credible without disturb from the solvent. We report the high quality Raman spectra of aqueous solution of BA and PNBA and ONBA in UV region. It is worthwhile to note that there is no SERS effects excitated by ultraviolet whereas the prefect SERS occurs in visible
1992
X. Lili, F. Yan / Spectrochimica Acta Part A 61 (2005) 1991–1995
or near infrared region. It maybe accounts for selectivity of SERS to the excitation wavelength and we elucidate the various mechanisms in different regions.
2. Experimental 2.1. Preparation of silver colloid Silver colloid is prepared according to Lee and Meisel’s method [10]. Ninety milligrams of AgNO3 is dissolved in 500 ml deionized water and the solution is heated to boiling. Then, 10 ml of 1% aqueous solution of trisodium citrate is added into the boiling AgNO3 solution drop by drop, accompanying with vigorous stirring. The mixed solution is kept boiling for further 10 min. Finally, a gray-green silver colloid is obtained, which shows a single visible absorbance band at about 430 nm.
Fig. 2. The UV Raman spectra of PNBA solid (a) and PNBA in aqueous solution (b).
2.2. Instrumentation
We respectively record the UV Raman spectra of the sample mentioned above. From Fig. 1, we have obtained precise indications that the UV Raman spectrum of aqueous BA solution has strong similarity to that of BA solid, shown for comparison in Fig. 1b. The UV Raman spectrum of aqueous
PNBA solution resembles to the corresponding solid Raman spectrum in Fig. 2, while the Raman spectrum of ONBA in aqueous solution has distinct difference to the counterpart in Fig. 3. The NIR or visible Raman spectrum of BA in aqueous solution is hardly obtained due to the low concentration of the saturated aqueous BA solution. In contrast to the NIR and visible case, the UV Raman spectrum of aqueous BA solution is perfect and shown in Fig. 1b. There are few difference between Fig. 1a and b, mainly in the 1600–1700 cm−1 region. And there is one to one correspondence between the Fig. 1a and 1b below 1600 cm−1 apart from a general small increase in linewidth with respect to the solution. In the UV Raman spectrum of aqueous BA solution, there is a weak band at 1689 cm−1 due to C O stretching mode while the spectrum of solid BA has no counterpart. In the solid
Fig. 1. The UV Raman spectra of BA solid (a) and BA in aqueous solution (b).
Fig. 3. The UV Raman spectra of ONBA solid (a) and ONBA in aqueous solution (b).
The UV–vis Raman spectra are recorded by a RENISHAW H13325 spectrophotometer, and the UV laser line is at 325 nm, the visible line at 514.5 nm.
3. Results and discussion 3.1. UV Raman spectra of BA and PNBA and ONBA in aqueous solution
X. Lili, F. Yan / Spectrochimica Acta Part A 61 (2005) 1991–1995
state, benzoic acid molecules occur as nearly planar, centrosymmertic dimers of symmertry D2h , with hydrogen bonds ˚ between adjacent carboxyl groups [11]. And in the (2.64 A) aqueous solution, hydrogen bond established between phenolic groups is destroyed and the dimer turns into molecule or ion, and the band located at 1689 cm−1 appears due to decomposition of dimer. In the cases of analyzing the UV Raman spectra of aqueous PNBA and ONBA solutions, we concern for the assignments to the bands because there are notable differences between the counterparts. PNBA is usually dissolved to organic solvent rather than to aqueous solvent to obtain the Raman spectrum because of the low solubility in aqueous solution. The early spectra of PNBA in silver colloid consist of two different types. One type of the spectra is features of the strong bands located at 1600, 1355, 1115, and 875 cm−1 , resemble to the normal Raman of p-nitrobenzoate salt in ethanol or aqueous solution [9,12]. The other is characteristic of two weak bands at 1355 and 1115 cm−1 , a strong band at 1460 cm−1 and a strong doublet near 1150 cm−1 . But some authors have argued that those bands of the latter type are due to azodibenzoate [13]. The UV Raman spectrum we have recorded is in agreement with the former, and the two prominent bands occur at 1353 and 1604 cm−1 , respectively due to NO2 and ring stretching vibration. Two medium intensity bands appear at 867 and 1111 cm−1 , respectively attributed to C N antisymmetry stretching and C H vibration. Another relative weak band emerges at 626 cm−1 . The UV Raman spectrum of aqueous PNBA solution is similar to that of PNBA in ethanol, whose concentration is higher than that in aqueous solution, about 3 mg/ml. But the former gives more information than the latter. It is evident that an additional weak band appears at 1178 cm−1 . And there are several weak bands between 1350 and 1600 cm−1 , respectively at 1442, 1461, and 1492 cm−1 . And the bands of 1442 and 1461 cm−1 are too weak to distinguish. All thought, the UV spectrum of aqueous PNBA solution correlates much better with the ordinary Raman spectrum of solid PNBA than the spectrum of PNBA in ethanol. The UV Raman spectrum of aqueous ONBA solution exhibits remarkable difference from that of it is solid. The UV Raman spectrum of solid ONBA is dominated by three bands: one broad and strong at 1607 cm−1 , and two weak bands at about 1477 and 1257 cm−1 whereas the Raman of aqueous ONBA solution gives much more information. The most prominent band at 1353 cm−1 is ascribed to NO2 symmetry stretching vibration. And a strong doublet at 1616 and 1578 cm−1 is attributed to ring vibration. There is a band at 1146 cm−1 to the ring breath vibration. Several bands at 1480, 1442, 1073 and 1038 cm−1 are all assigned to C H bending vibration. And the bands at 861 and 699 cm−1 might be correlated with NO2 bending and symmetry rocking, respectively [14]. It is worthwhile to note that the strong and medium intensity of bands are all ascribed to C C or C H vibration while NO2 and COO− show the weak intensity or absence.
1993
Fig. 4. The UV Raman spectra of BA before (a) and after (b) being mixed with silver colloid.
3.2. UV surface Raman spectra of BA and PNBA and ONBA in silver colloid The SERS spectra of BA and PNBA and ONBA are expected to resemble to those perfect Raman excited with ultraviolet radiation. But the behavior of the samples in silver colloid proves spectra without giant field enhancement, shown in Figs. 4–6. And there is no notable difference between curves a and b of these figures. The fine difference between curves a and b in Fig. 4 is because the aqueous BA solution is diluted after adding an aliquot silver colloid. If no SERS effect occurs when molecules adsorb on the metal surface, it is usually called surface-unenhanced Raman scattering (SUERS). And we observe the SUERS of BA and PNBA and ONBA in silver colloid with UV excitation. When an aliquot BA or PNBA or ONBA solution is added into silver colloid, the interaction between them takes place
Fig. 5. The UV Raman spectra of PNBA before (a) and after (b) being mixed with silver colloid.
1994
X. Lili, F. Yan / Spectrochimica Acta Part A 61 (2005) 1991–1995
Fig. 6. The UV Raman spectra of ONBA before (a) and after (b) being mixed with silver colloid.
Fig. 8. The Vis Raman spectra of PNBA before (a) and after (b) being mixed with silver colloid.
immediately. We not only observe the color change of colloid after adding aqueous BA or PNBA or ONBA solution, but also obtain the visible SERS, which could elucidate the interaction between the molecules and the silver particles. Namely, there are few Raman bands except 1356 cm−1 in the visible Raman spectrum of ONBA in aqueous solution, while the abundant bands occur after the silver colloid is added, shown in Figs. 7–9. Until recently, most of people seem to accept that the classical electromagnetic effect and a charge-transfer (CT) mechanism in which the excitation can take place by a charge transfer between the metal and the charge transfer complex [15]. The former reflects optical properties and nano structure of metal. And the latter is involved with geometrical and electronic configuration of molecules, interaction with substrate and so on. But both of them can operate simultaneously and it is difficult to separate one effect alone from SERS.
SERS in the visible may due to electromagnetic mechanism, which occurs in the vicinity of small, interacting metal particles that are illuminated with light resonant or near resonant with the localized surface-plasmon frequency of the metal structure [16]. On one hand, the surface-plasmon resonance condition is that the real part of dielectric constant must be negative and the imaginary part should approach to zero as much as possible. Silver works best because its dielectric constant is almost real and negative in the visible region [17]. On the other hand, some literature note peak Raman enhancement at 540 nm (2.3 ev) for Ag [18]. The poor performance of SERS in UV region can be explained by the absence of both surface-plasmon resonance and charge transfer excitation. There is no surface-plasmon resonance because the real part of dielectric constant for Ag is positive while the imaginary part increases rapidly for transition between energy bands.
Fig. 7. The Vis Raman spectra of BA before (a) and after (b) being mixed with silver colloid.
Fig. 9. The Vis Raman spectra of ONBA before (a) and after (b) being mixed with silver colloid.
X. Lili, F. Yan / Spectrochimica Acta Part A 61 (2005) 1991–1995
The charge transfer excitation occurs from the highest occupied state of the metal to the electron affinity level of the molecule. The energy required for this CT excitation is found to become resonance with the laser frequencies used in SERS experiments [19]. The photon energy of ultraviolet used in experiments is about 3.8 ev so the CT excitation condition cannot be satisfied in this region. In a brief, the SUERS in UV region maybe explained in terms of the absence of electromagnetic and CT mechanism. The dependence on the excitation of SERS can account for the mechanism in near infrared and visible regions. 4. Conclusions We report the high quality Raman spectra of BA and PNBA and ONBA in aqueous solution (≤0.01 M) while almost no Raman signals could be detected in visible region because of the low concentration. And we reveal the SUERS in UV region. We discuss the possible reasons and it is likely that there is neither electromagnetic nor chemical enhancement. Acknowledgments The authors are grateful for the support to this research by the National Natural Science Foundation of China and the Natural Science Foundation of Beijing.
1995
References [1] L. Can, C.S. Peter, Catal. Today 33 (1997) 353. [2] X. Guang, Y. Yi, F. Zhaochi, X. Qin, X. Fengshou, L. Can, Microporous Mesoporous Mater. 42 (2001) 317. [3] C. John, D.S. Alastair, FEBS Lett. 503 (2001) 30. [4] C. Zhenhuan, X.G. Chen, J.S.W. Holtz, S.A. Asher, Biochemistry 37 (1998) 2854. [5] Y. Yi, X. Guang, L. Can, X. Fengshou, J. Catal. 194 (2000) 487. [6] S.A. Asher, C.H. Munro, C. Zhenhuan, Laser Focus World (1997) 99. [7] M. Moskovits, J. Chem. Phys. 69 (9) (1978) 4159. [8] C.K. Chen, A.R.B. de Castro, Y.R. Shen, Phys. Rev. Lett. 46 (1981) 14. [9] J.C. Tsang, P. Avouris, J.R. Kirtiey, J. Chem. Phys. 79 (1) (1983) 493. [10] P.C. Lee, D. Meisel, J. Phys. Chem. 86 (1982) 3391. [11] G.A. Sun, J.M. Robertson, T.H. Goodwin, Acta Crystallorgr. 8 (1955) 157. [12] H. Ratinen, M. Kiviharju, Spectrochim. Acta A 45 (7) (1989) 729. [13] R.S. Venkatachalam, F.J. Boerio, P.G. Roth, J. Raman Spectrosc. 19 (1988) 281. [14] G. Varsanyi, Assignments for Vibrtional Spectra of Seven Hundred Benzene Derivations, Wiley, New York, 1974. [15] A. Otto, I. Mrozek, H. Grabhorn, W. Akemann, J. Phys. Condens. Matter 4 (1992) 1143. [16] M. Moskovits, Rev. Mod. Phys. 57 (1985) 783. [17] S.L. McCall, P.M. Platzman, Phys. Rev. 22 (4) (1980) 1660. [18] E.J. Zeman, G.C. Schatz, J. Phys. Chem. 91 (1987) 634. [19] H. Ueba, Surf. Sci. 131 (1983) 347.