- Email: [email protected]
A SERS study of 1-methyl-2-pyrrolidinone adsorbed on active silver surfaces
20 March 1998
Chemical Physics Letters 285 Ž1998. 266–270
A SERS study of 1-methyl-2-pyrrolidinone adsorbed on active silver surfaces G. Compagnini, B. Pelligra, B. Pignataro Dipartimento di Scienze Chimiche, UniÕersita` di Catania, V. le A. Doria 6, 95125 Catania, Italy Received 24 October 1997; in final form 31 December 1997
Abstract Surface-enhanced Raman scattering ŽSERS. spectra of 1-methyl-2-pyrrolidinone molecules adsorbed on rough silver surfaces have been obtained. A strong signal due to AgN vibrations has been found at 240 cmy1 from molecules chemically bonded at the silver surface. A band in the CH-stretching region has been used to quantify molecules present on top of the first layer as a consequence of weaker interactions. These latter molecules have been removed with distilled water, allowing evaluation of the ratio between the SERS cross-section for ‘physisorption’ and ‘chemisorption’. A difference in the selection rules between normal Raman and SERS has also been found and briefly discussed. q 1998 Published by Elsevier Science B.V.
1. Introduction Surface-enhanced Raman scattering ŽSERS. by molecules adsorbed on metal surfaces has now been reported for a relatively large number of molecules and for a growing number of metals w1–5x. It appears that most of the papers found in literature can be categorised into two groups. One group deals with the nature of SERS phenomena sharing its origin between the so-called chemical and electromagnetic mechanisms. In particular, several works have been devoted to studying changes in the morphology of the metal surfaces in order to account for one of the processes w6,7x. The second category consists of all those works which use SERS in order to gain spectroscopic information about the vibrational behaviour of molecules adsorbed on metal surfaces. This is an important challenge in modern spectroscopy, both for fundamental and technological reasons. New emerging
technologies are expected to further advance the sensitivity and selectivity of existing instrumentation, making it possible to reach the goal of characterizing monomolecular arrangements. In this case, using SERS can significantly improve band assignment, identification of structure and structure–property relationships for both small Žsimple diatomic w8x or small organic w3,9x. and larger Žpolymers w10x or biomolecules w11x. molecules. However, studies in both of the above-mentioned categories are involved with several problems that remain open to further scientific research. The aim of this Letter is intended to contribute, in particular, a simple method that is proposed to distinguish between SERS due to molecules at the immediate contact with the silver solid surface and to those lying at least one molecular diameter above but not chemically bonded to the surface itself. The study has been centered on 1-methyl-2-pyrrolidinone, also called N-methyl pyrrolidinone ŽNMP.. This has been
0009-2614r98r$19.00 q 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 0 2 9 - 3
G. Compagnini et al.r Chemical Physics Letters 285 (1998) 266–270
chosen because of the presence of the nitrogen lone pair suitable for dative bonding with metallic Ag. Moreover its high solubility with water ensures the possibility to wash out all the molecules not chemically bonded to the metal surface and therefore to evaluate their contribution to the observed surfaceenhanced effect. At the same time, this Letter will also give a contribution to the problem of the already reported w1,2,8x difference in the selection rules between normal Raman ŽNR. and SERS, by comparing both NR and SERS with infrared ŽIR. spectra.
267
ratio with 0.5 s integration time. Different scans have been performed in order to test the stability of the adsorbate systems. We also obtained the same results after 1 day. Integrated total IR reflectance measurements were performed with a Perkin Elmer l 1000 Fourier transform spectrometer in the range 400–4400 cmy1 . All IR spectra obtained on SERS-active surfaces have been taken with a resolution of 4 cmy1 and 100 scan accumulations.
3. Results and discussion 2. Experimental 3.1. A-band assignment and selection rules SERS active surfaces were prepared through a plasma oxidation-reduction cycle as reported in detail elsewhere w12x. This method essentially consists of a mild oxidation using CO 2 plasma treatment followed by reduction using a H 2 –N2 mixture. The SERS quality of the samples has been optimised by choosing the right oxidation and reduction times. The surface morphology was further checked by atomic force microscopy. In this case, we used a Nanoscope IIIA instrument in either contact or tapping mode. Etched silicon tips were used in tapping mode while Si 3 N4 tips were used in contact mode. We generally obtained a highly rough surface with a microscopic hills structure of about 100–300 nm in height due to coalescence of grains about 10–50 nm in size w12x. NMP was adsorbed onto the prepared silver samples by dipping the SERS active surface into different NMP–H 2 O solutions at different concentrations. Each immersion lasted about 15 min and was followed by drying the surface with a flux of argon gas. In order to obtain samples with only chemisorbed molecules, some samples were immersed into distilled water for 15 min to wash out the NMP molecules that were not chemically bonded. All Raman spectra were performed through the use of an Ar ion laser working at 514.5 nm and a Jobin Yvon U1000 double monochromator equipped with two holographic gratings and connected to an Hamamatsu photomultiplier with a photon-counting chain. Laser power was always maintained at 10 mW on the sample, and the estimated resolution was at about 5 cmy1 . This ensures a good signal-to-noise
Fig. 1 shows the comparison between a bulk Žliquid. NMP spectrum and that obtained on top of the silver-active surface after dipping it into pure NMP ŽSERS.. Immediately one can recognise several differences. The most prominent 240 cmy1 signal observed in the SERS spectrum can be assigned to an AgN stretching as already discussed by several other authors with other systems w13x. This indicates
Fig. 1. NR and SERS spectra of NMP. SERS has been obtained through adsorption onto a rough silver surface.
268
G. Compagnini et al.r Chemical Physics Letters 285 (1998) 266–270
Fig. 2. NMP spectra obtained in the CH-stretching region. Here are reported Raman ŽSERS, top. and IR Žtotal diffuse reflectance, bottom. spectra of the adsorbate together with a spectra obtained from pure liquid NMP ŽNR, Bulk..
the chemical bond between the NMP molecules and silver surface and can be accounted for as a signal coming from chemisorption through the free nitrogen electron pair. In going through higher wave numbers, several other signals are found with positions and intensities similar in both NR and SERS spectra. Among the others we observed C5O stretching at around 1700 cmy1 and some CH 2 and CH 3 deformation signals in the region around 1400–1500 cmy1 . The other most prominent structure found in both spectra is located in the CH-stretching region, that is between 2800 and 3200 cmy1 . It is a highly structured spectral region where all the modes ŽRaman active. involving CH stretching can be found. Careful investigation of these signals in both NR and SERS configurations leads to the detection of some non-trivial differences that can be better seen in Fig. 2. Here in particular we reported the CH-stretching region from 2700 to 3200 cmy1 for both an NR and SERS spectrum. Unfortunately, in the literature there are no data
regarding interpretation of the CH spectra for this compound; therefore it remains difficult to use our experimental result as a guide to understanding the molecular dynamics on the surface. However, if we take into account general classifications for compounds similar to NMP w14x, we can assign the frequencies found in the NR spectrum in agreement with those reported in Table 1. Thus we observe two antisymmetric stretching and two symmetric stretching modes belonging to CH 2 and CH 3 groups together with a small N–CH 3 signal found at around 2800 cmy1 . From Fig. 2 it appears that the mode at 2930 cmy1 , assigned to CH 2 antisymmetric stretching, is strongly enhanced in SERS measurements, that is when the molecule is adsorbed on rough silver surfaces. This experimental observation can be explained in two different ways: Ža. a strong difference in the selection rules between NR and SERS, as was already reported for other SERS experiments on a wide class of molecules w1,2,8x, or Žb. a change in the vibrational spectrum due to adsorption at the surface which inhibits some particular vibrational motion. Chemisorption could also contribute to the change in the vibrational spectrum because of modification in the electronic configuration when strong interactions between NMP and silver take place. In order to distinguish between these two hypothesis, we performed some IR spectra Žin the total diffuse reflectance configuration. on some samples we used to account for the SERS phenomenon, as reported in the bottom of Fig. 2. The spectrum has been plotted in a way suitable for comparison with Raman. It is to be noted that all the modes found in the 2700–3200 cmy1 region resemble those obtained in the IR spectrum of bulk NMP Žnot shown.. Moreover, there is a certain correspondence in the stretching features already classified in Table 1. This correspondence is lost in SERS experiments. As a consequence, the observed improvement in the importance of the 2930 cmy1 structure found in SERS is in large part attributable to the first hypothesis, that is to a strong difference in the selection rules between NR and SERS. This finding is not surprising since, as was already observed, similar results have been reported in the literature. However, we would like to remark
G. Compagnini et al.r Chemical Physics Letters 285 (1998) 266–270
that, in the present experiment, comparison with an IR investigation of the adsorbate rules out a possibility which in principle may be operative and gives us more confidence with the Raman data. Enhancement of the 2930 cmy1 CH 2 antisymmetric stretching does not find correspondence in other antisymmetric modes and its origin remains an open question. 3.2. B-adsorption at the surface As already mentioned previously, the strong 240 cmy1 structure found in SERS spectra ŽAgN stretching. of NMP leads us to confirm that a chemical bond occurs between the silver surface and nitrogen atom of the molecule through its lone pair. However, we observed a difference in the relative AgN versus CH intensities when adsorption is made through immersion of the substrate in a ŽNMP, H 2 O. solution rather than in pure NMP. For this reason, we performed a detailed experiment as function of the concentration. As was already mentioned in Section
269
2, adsorption was made by dipping a SERS-active sample into the solution for some minutes. In Fig. 3 we report, as function of NMP concentration, the ratio Ž R . between the integrated signal intensities of AgN Ž AAgN . and CH stretchings Ž A CH .. It must be noted that, while the AgN signal is certainly due to chemisorption of the molecule on top of the SERS active sample, CH signals could be related to both chemisorption of the first layer andror to adsorption of further molecules on top of the first layer and therefore not chemically bonded to the Ag surface. We will call these last molecules ‘physisorbed’ for the sake of simplicity. This is a crude model that we tried to use in order to account for the results plotted in Fig. 3. In particular, in this hypothesis, we can write: AAgN s sAgN Nc X A CH s sCH Nc q sCH Np
where sAgN and sCH are the scattering cross-sections for AgN and CH stretchings in the case of X chemisorption; sCH is the scattering cross-section for CH stretching in the case of physisorption; Nc and Np are the surface molecular concentrations in the case of chemisorption and physisorption, respectively. From these considerations we can write: 1 s R
AAgN
y1
ž /
s
A CH
sCH sAgN
q
X sCH
Np
sAgN
Nc
ž /ž /
Ž 1.
We define R 0 s sAgN rsCH the value of R at a complete chemisorption without physisorbed molecules Ž Np s 0.. Experimentally, this can be obtained by rinsing the adsorbed sample into distilled water in order to remove all those molecules with weak interactions with the substrate. The corresponding results are shown in Fig. 3 at zero NMP concentration. Once again: 1
1 s
R
q R0
1
Np
R1
Nc
ž /ž /
Ž 2.
X where R 1 s sAgN rsCH . Using a Langmuir isotherm for NprNc s Ž1 q 1rkc .y1 we obtain:
Fig. 3. AgNrCH integrated signal ratios reported as function of NMP concentration of the solutions used for the adsorption. Points at zero NMP concentration have been obtained by washing the samples in distilled water after adsorption. Solid line: a data fit discussed in the text.
Rs
1 q kc Ry1 0 q
Ž R 0 q R1 . rR 0 R1 kc
Ž 3.
where c is the NMP concentration. This relation has
270
G. Compagnini et al.r Chemical Physics Letters 285 (1998) 266–270
been used to fit the experimental points as reported in Fig. 3, thus obtaining the following results:
tions. A different SERS cross-section for these last two cases has been obtained.
k s 2.1, R 0 s 2.6 and R 1 s 0.8 X Interestingly, R 0rR1 s sCH rsCH s 3.3 represents the ratio of the SERS scattering cross-sections for the chemisorbed NMP molecules with respect to those physisorbed as seen by the CH 2 antisymmetric stretching mode. The reader should be careful to consider terms such as physisorption and chemisorption as they stand. In this case, a chemisorbed molecule is intended as a molecule in direct contact with the Ag surface with a strong chemical interaction while physisorption is related to all those molecules which are not in intimate contact with Ag but are still involved in the SERS effect for their distances from the surface. In this respect, the results found can also be seen as a comparison between chemical and electromagnetic SERS effects because they are concerned with molecules adsorbed at different distances from the silver surface. The obtained value of 3.3 in the scattering cross-section ratio is consistent with the data available in literature where ‘first-layer effects’ have been evaluated through the use of nanometric spacers w15x. In conclusion, we have presented a detailed study of the surface enhanced Raman spectra related to NMP molecules adsorbed on a rough silver surface. Here we have obtained some interesting information about the SERS selection rules for CH-stretching modes and also have proposed a way to distinguish molecules at different distances from the substrate and therefore with different chemical bonding situa-
Acknowledgements We would like to thank A. Raudino for his useful discussions. CNR and MURST are acknowledged for their partial financial support. References w1x A. Otto, I. Mrozek, H. Grabhorn, W. Akemann, J. Phys.: Condens. Matter 4 Ž1992. 1143. w2x M. Moskovits, Rev. Mod. Phys. 57 Ž1985. 783. w3x M. Fleischmann, P.J. Hendra, A.J. MsQuillan, Chem. Phys. Lett. 26 Ž1974. 163. w4x G. Niaura, A. Malinauskas, Chem. Phys. Lett. 207 Ž1993. 455. w5x P.B. Dorain, K.U. Von Raben, R.K. Chang, B.L. Laube, Chem. Phys. Lett. 84 Ž1981. 405. w6x S.A. Lyin, J.M. Worlock, Phys. Rev. Lett. 51 Ž1983. 593. w7x C. Pettenkofer, A. Otto, Surf. Sci. 151 Ž1985. 37. w8x C. Pettenkofer, I. Mrozek, T. Bornemann, A. Otto, Surf. Sci. 188 Ž1987. 519. w9x W. Akemann, A. Otto, Langmuir 11 Ž1995. 1196. w10x P.P. Hong, F.J. Boerio, S.J. Clarson, D.H. Smith, Macromolecules 24 Ž1991. 4770. w11x R.L. Garrel, Anal. Chem. 61 Ž1989. 401A. w12x G. Compagnini, B. Pignataro, B. Pelligra, Chem. Phys. Lett. 272 Ž1997. 453. w13x K.U. VonRaben, P.B. Dorain, T.T. Chen, R.K. Chang, Chem. Phys. Lett. 95 Ž1983. 269. w14x D. Lin-Vien, N.B. Colthup, W.G. Fateley, J.G. Grasselli, The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic Press, San Diego, CA, 1991. w15x I. Mrozek, A. Otto, Appl. Phys. A49 Ž1989. 389.
Chemical Physics Letters 285 Ž1998. 266–270
A SERS study of 1-methyl-2-pyrrolidinone adsorbed on active silver surfaces G. Compagnini, B. Pelligra, B. Pignataro Dipartimento di Scienze Chimiche, UniÕersita` di Catania, V. le A. Doria 6, 95125 Catania, Italy Received 24 October 1997; in final form 31 December 1997
Abstract Surface-enhanced Raman scattering ŽSERS. spectra of 1-methyl-2-pyrrolidinone molecules adsorbed on rough silver surfaces have been obtained. A strong signal due to AgN vibrations has been found at 240 cmy1 from molecules chemically bonded at the silver surface. A band in the CH-stretching region has been used to quantify molecules present on top of the first layer as a consequence of weaker interactions. These latter molecules have been removed with distilled water, allowing evaluation of the ratio between the SERS cross-section for ‘physisorption’ and ‘chemisorption’. A difference in the selection rules between normal Raman and SERS has also been found and briefly discussed. q 1998 Published by Elsevier Science B.V.
1. Introduction Surface-enhanced Raman scattering ŽSERS. by molecules adsorbed on metal surfaces has now been reported for a relatively large number of molecules and for a growing number of metals w1–5x. It appears that most of the papers found in literature can be categorised into two groups. One group deals with the nature of SERS phenomena sharing its origin between the so-called chemical and electromagnetic mechanisms. In particular, several works have been devoted to studying changes in the morphology of the metal surfaces in order to account for one of the processes w6,7x. The second category consists of all those works which use SERS in order to gain spectroscopic information about the vibrational behaviour of molecules adsorbed on metal surfaces. This is an important challenge in modern spectroscopy, both for fundamental and technological reasons. New emerging
technologies are expected to further advance the sensitivity and selectivity of existing instrumentation, making it possible to reach the goal of characterizing monomolecular arrangements. In this case, using SERS can significantly improve band assignment, identification of structure and structure–property relationships for both small Žsimple diatomic w8x or small organic w3,9x. and larger Žpolymers w10x or biomolecules w11x. molecules. However, studies in both of the above-mentioned categories are involved with several problems that remain open to further scientific research. The aim of this Letter is intended to contribute, in particular, a simple method that is proposed to distinguish between SERS due to molecules at the immediate contact with the silver solid surface and to those lying at least one molecular diameter above but not chemically bonded to the surface itself. The study has been centered on 1-methyl-2-pyrrolidinone, also called N-methyl pyrrolidinone ŽNMP.. This has been
0009-2614r98r$19.00 q 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 0 2 9 - 3
G. Compagnini et al.r Chemical Physics Letters 285 (1998) 266–270
chosen because of the presence of the nitrogen lone pair suitable for dative bonding with metallic Ag. Moreover its high solubility with water ensures the possibility to wash out all the molecules not chemically bonded to the metal surface and therefore to evaluate their contribution to the observed surfaceenhanced effect. At the same time, this Letter will also give a contribution to the problem of the already reported w1,2,8x difference in the selection rules between normal Raman ŽNR. and SERS, by comparing both NR and SERS with infrared ŽIR. spectra.
267
ratio with 0.5 s integration time. Different scans have been performed in order to test the stability of the adsorbate systems. We also obtained the same results after 1 day. Integrated total IR reflectance measurements were performed with a Perkin Elmer l 1000 Fourier transform spectrometer in the range 400–4400 cmy1 . All IR spectra obtained on SERS-active surfaces have been taken with a resolution of 4 cmy1 and 100 scan accumulations.
3. Results and discussion 2. Experimental 3.1. A-band assignment and selection rules SERS active surfaces were prepared through a plasma oxidation-reduction cycle as reported in detail elsewhere w12x. This method essentially consists of a mild oxidation using CO 2 plasma treatment followed by reduction using a H 2 –N2 mixture. The SERS quality of the samples has been optimised by choosing the right oxidation and reduction times. The surface morphology was further checked by atomic force microscopy. In this case, we used a Nanoscope IIIA instrument in either contact or tapping mode. Etched silicon tips were used in tapping mode while Si 3 N4 tips were used in contact mode. We generally obtained a highly rough surface with a microscopic hills structure of about 100–300 nm in height due to coalescence of grains about 10–50 nm in size w12x. NMP was adsorbed onto the prepared silver samples by dipping the SERS active surface into different NMP–H 2 O solutions at different concentrations. Each immersion lasted about 15 min and was followed by drying the surface with a flux of argon gas. In order to obtain samples with only chemisorbed molecules, some samples were immersed into distilled water for 15 min to wash out the NMP molecules that were not chemically bonded. All Raman spectra were performed through the use of an Ar ion laser working at 514.5 nm and a Jobin Yvon U1000 double monochromator equipped with two holographic gratings and connected to an Hamamatsu photomultiplier with a photon-counting chain. Laser power was always maintained at 10 mW on the sample, and the estimated resolution was at about 5 cmy1 . This ensures a good signal-to-noise
Fig. 1 shows the comparison between a bulk Žliquid. NMP spectrum and that obtained on top of the silver-active surface after dipping it into pure NMP ŽSERS.. Immediately one can recognise several differences. The most prominent 240 cmy1 signal observed in the SERS spectrum can be assigned to an AgN stretching as already discussed by several other authors with other systems w13x. This indicates
Fig. 1. NR and SERS spectra of NMP. SERS has been obtained through adsorption onto a rough silver surface.
268
G. Compagnini et al.r Chemical Physics Letters 285 (1998) 266–270
Fig. 2. NMP spectra obtained in the CH-stretching region. Here are reported Raman ŽSERS, top. and IR Žtotal diffuse reflectance, bottom. spectra of the adsorbate together with a spectra obtained from pure liquid NMP ŽNR, Bulk..
the chemical bond between the NMP molecules and silver surface and can be accounted for as a signal coming from chemisorption through the free nitrogen electron pair. In going through higher wave numbers, several other signals are found with positions and intensities similar in both NR and SERS spectra. Among the others we observed C5O stretching at around 1700 cmy1 and some CH 2 and CH 3 deformation signals in the region around 1400–1500 cmy1 . The other most prominent structure found in both spectra is located in the CH-stretching region, that is between 2800 and 3200 cmy1 . It is a highly structured spectral region where all the modes ŽRaman active. involving CH stretching can be found. Careful investigation of these signals in both NR and SERS configurations leads to the detection of some non-trivial differences that can be better seen in Fig. 2. Here in particular we reported the CH-stretching region from 2700 to 3200 cmy1 for both an NR and SERS spectrum. Unfortunately, in the literature there are no data
regarding interpretation of the CH spectra for this compound; therefore it remains difficult to use our experimental result as a guide to understanding the molecular dynamics on the surface. However, if we take into account general classifications for compounds similar to NMP w14x, we can assign the frequencies found in the NR spectrum in agreement with those reported in Table 1. Thus we observe two antisymmetric stretching and two symmetric stretching modes belonging to CH 2 and CH 3 groups together with a small N–CH 3 signal found at around 2800 cmy1 . From Fig. 2 it appears that the mode at 2930 cmy1 , assigned to CH 2 antisymmetric stretching, is strongly enhanced in SERS measurements, that is when the molecule is adsorbed on rough silver surfaces. This experimental observation can be explained in two different ways: Ža. a strong difference in the selection rules between NR and SERS, as was already reported for other SERS experiments on a wide class of molecules w1,2,8x, or Žb. a change in the vibrational spectrum due to adsorption at the surface which inhibits some particular vibrational motion. Chemisorption could also contribute to the change in the vibrational spectrum because of modification in the electronic configuration when strong interactions between NMP and silver take place. In order to distinguish between these two hypothesis, we performed some IR spectra Žin the total diffuse reflectance configuration. on some samples we used to account for the SERS phenomenon, as reported in the bottom of Fig. 2. The spectrum has been plotted in a way suitable for comparison with Raman. It is to be noted that all the modes found in the 2700–3200 cmy1 region resemble those obtained in the IR spectrum of bulk NMP Žnot shown.. Moreover, there is a certain correspondence in the stretching features already classified in Table 1. This correspondence is lost in SERS experiments. As a consequence, the observed improvement in the importance of the 2930 cmy1 structure found in SERS is in large part attributable to the first hypothesis, that is to a strong difference in the selection rules between NR and SERS. This finding is not surprising since, as was already observed, similar results have been reported in the literature. However, we would like to remark
G. Compagnini et al.r Chemical Physics Letters 285 (1998) 266–270
that, in the present experiment, comparison with an IR investigation of the adsorbate rules out a possibility which in principle may be operative and gives us more confidence with the Raman data. Enhancement of the 2930 cmy1 CH 2 antisymmetric stretching does not find correspondence in other antisymmetric modes and its origin remains an open question. 3.2. B-adsorption at the surface As already mentioned previously, the strong 240 cmy1 structure found in SERS spectra ŽAgN stretching. of NMP leads us to confirm that a chemical bond occurs between the silver surface and nitrogen atom of the molecule through its lone pair. However, we observed a difference in the relative AgN versus CH intensities when adsorption is made through immersion of the substrate in a ŽNMP, H 2 O. solution rather than in pure NMP. For this reason, we performed a detailed experiment as function of the concentration. As was already mentioned in Section
269
2, adsorption was made by dipping a SERS-active sample into the solution for some minutes. In Fig. 3 we report, as function of NMP concentration, the ratio Ž R . between the integrated signal intensities of AgN Ž AAgN . and CH stretchings Ž A CH .. It must be noted that, while the AgN signal is certainly due to chemisorption of the molecule on top of the SERS active sample, CH signals could be related to both chemisorption of the first layer andror to adsorption of further molecules on top of the first layer and therefore not chemically bonded to the Ag surface. We will call these last molecules ‘physisorbed’ for the sake of simplicity. This is a crude model that we tried to use in order to account for the results plotted in Fig. 3. In particular, in this hypothesis, we can write: AAgN s sAgN Nc X A CH s sCH Nc q sCH Np
where sAgN and sCH are the scattering cross-sections for AgN and CH stretchings in the case of X chemisorption; sCH is the scattering cross-section for CH stretching in the case of physisorption; Nc and Np are the surface molecular concentrations in the case of chemisorption and physisorption, respectively. From these considerations we can write: 1 s R
AAgN
y1
ž /
s
A CH
sCH sAgN
q
X sCH
Np
sAgN
Nc
ž /ž /
Ž 1.
We define R 0 s sAgN rsCH the value of R at a complete chemisorption without physisorbed molecules Ž Np s 0.. Experimentally, this can be obtained by rinsing the adsorbed sample into distilled water in order to remove all those molecules with weak interactions with the substrate. The corresponding results are shown in Fig. 3 at zero NMP concentration. Once again: 1
1 s
R
q R0
1
Np
R1
Nc
ž /ž /
Ž 2.
X where R 1 s sAgN rsCH . Using a Langmuir isotherm for NprNc s Ž1 q 1rkc .y1 we obtain:
Fig. 3. AgNrCH integrated signal ratios reported as function of NMP concentration of the solutions used for the adsorption. Points at zero NMP concentration have been obtained by washing the samples in distilled water after adsorption. Solid line: a data fit discussed in the text.
Rs
1 q kc Ry1 0 q
Ž R 0 q R1 . rR 0 R1 kc
Ž 3.
where c is the NMP concentration. This relation has
270
G. Compagnini et al.r Chemical Physics Letters 285 (1998) 266–270
been used to fit the experimental points as reported in Fig. 3, thus obtaining the following results:
tions. A different SERS cross-section for these last two cases has been obtained.
k s 2.1, R 0 s 2.6 and R 1 s 0.8 X Interestingly, R 0rR1 s sCH rsCH s 3.3 represents the ratio of the SERS scattering cross-sections for the chemisorbed NMP molecules with respect to those physisorbed as seen by the CH 2 antisymmetric stretching mode. The reader should be careful to consider terms such as physisorption and chemisorption as they stand. In this case, a chemisorbed molecule is intended as a molecule in direct contact with the Ag surface with a strong chemical interaction while physisorption is related to all those molecules which are not in intimate contact with Ag but are still involved in the SERS effect for their distances from the surface. In this respect, the results found can also be seen as a comparison between chemical and electromagnetic SERS effects because they are concerned with molecules adsorbed at different distances from the silver surface. The obtained value of 3.3 in the scattering cross-section ratio is consistent with the data available in literature where ‘first-layer effects’ have been evaluated through the use of nanometric spacers w15x. In conclusion, we have presented a detailed study of the surface enhanced Raman spectra related to NMP molecules adsorbed on a rough silver surface. Here we have obtained some interesting information about the SERS selection rules for CH-stretching modes and also have proposed a way to distinguish molecules at different distances from the substrate and therefore with different chemical bonding situa-
Acknowledgements We would like to thank A. Raudino for his useful discussions. CNR and MURST are acknowledged for their partial financial support. References w1x A. Otto, I. Mrozek, H. Grabhorn, W. Akemann, J. Phys.: Condens. Matter 4 Ž1992. 1143. w2x M. Moskovits, Rev. Mod. Phys. 57 Ž1985. 783. w3x M. Fleischmann, P.J. Hendra, A.J. MsQuillan, Chem. Phys. Lett. 26 Ž1974. 163. w4x G. Niaura, A. Malinauskas, Chem. Phys. Lett. 207 Ž1993. 455. w5x P.B. Dorain, K.U. Von Raben, R.K. Chang, B.L. Laube, Chem. Phys. Lett. 84 Ž1981. 405. w6x S.A. Lyin, J.M. Worlock, Phys. Rev. Lett. 51 Ž1983. 593. w7x C. Pettenkofer, A. Otto, Surf. Sci. 151 Ž1985. 37. w8x C. Pettenkofer, I. Mrozek, T. Bornemann, A. Otto, Surf. Sci. 188 Ž1987. 519. w9x W. Akemann, A. Otto, Langmuir 11 Ž1995. 1196. w10x P.P. Hong, F.J. Boerio, S.J. Clarson, D.H. Smith, Macromolecules 24 Ž1991. 4770. w11x R.L. Garrel, Anal. Chem. 61 Ž1989. 401A. w12x G. Compagnini, B. Pignataro, B. Pelligra, Chem. Phys. Lett. 272 Ž1997. 453. w13x K.U. VonRaben, P.B. Dorain, T.T. Chen, R.K. Chang, Chem. Phys. Lett. 95 Ž1983. 269. w14x D. Lin-Vien, N.B. Colthup, W.G. Fateley, J.G. Grasselli, The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic Press, San Diego, CA, 1991. w15x I. Mrozek, A. Otto, Appl. Phys. A49 Ž1989. 389.