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A few molecules surface-enhanced Raman scattering studies on nickel-modified silver substrates
Chemical Physics Letters 457 (2008) 434–438
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
A few molecules surface-enhanced Raman scattering studies on nickel-modified silver substrates Andrzej Kudelski * Department of Chemistry, University of Warsaw, Pasteur 1, PL-02-093 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 6 March 2008 In final form 10 April 2008 Available online 13 April 2008
a b s t r a c t A few clusters surface-enhanced Raman scattering (SERS) measurements on a substrate covered with a layer of metal with negligible SERS activity were realized. We measured SERS spectra dominated by the contribution from only a few carbon clusters for hydrogenated carbon deposited on the nickel-modified silver substrate. The contributions characteristic for various clusters could be identified in the recorded spectra (and hence presence of these clusters in the analysed mixture could be proved), even when contribution of many types of carbon clusters to the standard (averaged) SERS spectrum cannot be reliably identified. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction When metal nano-resonators are illuminated with the electromagnetic wave the electric field can be locally significantly enhanced (even by three orders of magnitude) [1,2]. Since for Raman bands with a small Raman shift the increase in the efficiency of Raman scattering is roughly proportional to the fourth power of the field enhancement, the field enhancement of 103 may lead to the increase in the efficiency of Raman scattering by a factor of 1012 [1,2]. Moreover, the efficiency of Raman scattering for molecules adsorbed on metal surfaces can be further increased by the resonance process similar to the ordinary resonance Raman process occurring in the metal–ligand complexes [3,4]. The increase of the intensity of Raman signal for molecules adsorbed on resonator-shaped metal substrates is called surface-enhanced Raman scattering – SERS. The efficiency of SERS scattering is sometimes sufficient to observe a good quality Raman spectrum dominated by the contribution even from a single molecule [5–10]. For highly SERS-active substrates large SERS enhancement factors are only achieved at very small parts of its surface (so called ‘hot spots’) and hence only very small part of molecules adsorbed on highly SERS-active substrates actually contributes to the measured SERS signal. For example, for some dyes adsorbed on silver nanoparticles only 0.01% of the adsorbates are responsible for almost all measured SERS signal [11]. Due to the Abbe’s diffraction limit the standard optical Raman microscope collects signal from the area not smaller than ca . 0.5 lm2 [1]. However, since for highly SERS-active substrate only very small part of this area is actually very SERS-active, when the average lifetime of molecules in the
* Fax: +48 22 8225996. E-mail address: [email protected] 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.04.035
‘hot spots’ is not significantly shorter than the accumulation time of the spectrum, using standard confocal Raman microscope we can measure Raman spectrum dominated by the contribution from a relatively small number of molecules [12–18]. Using this technique in 2005 Itoh et al. measured SERS spectra dominated by the contribution from only a few carbon clusters [16]. After carefully analysed of many spectra Itoh et al. showed that it is possible to identify spectra characteristic for various clusters (and hence prove their presence in the analysed mixture), even when their contribution to the averaged spectrum of the analysed sample cannot be reliably identified [16]. Similar ‘a few molecules’ SERS measurements, which allowed for the identification of some fluctuating Raman bands, have been recently carried out by Szeghalmi et al. [19] and Neugebauer et al. [20]. Very large SERS enhancement factors are achieved for molecules adsorbed at rough surfaces of only a few metals (e.g., Ag, Au and Cu) [1,3,4]. The SERS enhancement factors at other metals are significantly smaller (however, after proper roughening, reliable SERS spectra may be also recorded on some other metals) [3,21,22]. To extend intensive SERS effect on other metals one can deposit (usually electro-deposit) these metals on highly SERS-active substrates as ultrathin films [23,24]. This procedure changes the surface chemical properties to those of the deposited film while the underlying substrate maintains SERS enhancement [23,24]. Despite the thickness of deposited layers is usually only five atoms [23,24], deposition of SERS non-active (or practically SERS non-active) metals causes some decrease of the SERS enhancement factor [23]. Since many species are often preferentially deposited at SERS ‘hot spots’, deposition of even submonolayer of SERS non-active metal can cause large reduction of SERS enhancement factor. For example, Pettinger and Moerl reported observable quenching of SERS effect after deposition on the silver surface of 0.3% of the monolayer of Tl [25,26].
435
A. Kudelski / Chemical Physics Letters 457 (2008) 434–438
d 1575
c 1370
Raman intensity
Raman intensity
1290
640
970
b a 2000
1500
1000
Raman shift / cm
500
−1
Fig. 1. SERS spectrum of hydrogenated carbon formed on the surface of the nickelmodified silver electrode during electrochemical reduction of CO2. Spectrum was accumulated for 100 s.
In this contribution, we analyse SERS spectra of hydrogenated carbon deposited on the nickel-modified silver substrate. Nickel is practically SERS non-active metal and only small SERS enhancement factors may be obtained on pure nickel substrates [22]. We show, that when a five atoms thick layer of nickel is deposited on the highly SERS-active silver substrate (five atoms thick layers are typically used as modifying layers [23,24]), the achievable SERS enhancement factors are still sufficient to record SERS spectra dominated by the contribution from only a few carbon clusters. Since nickel is an important catalyst used for formation of many carbon materials (e.g., carbon nanotubes) [27–29], SERS measurements on nickel-modified surfaces may be of significant practical importance. According to our knowledge the spectra presented in
2000
1500
1000
Raman shift / cm
500
−1
Fig. 2. Four successively recorded SERS spectra of hydrogenated carbon formed on the surface of the nickel-modified silver electrode during electrochemical reduction of CO2. Each spectrum was accumulated for 1 s. Spectra are presented at the same scale, vertically shifted for the sake of clarity.
this contribution are the first examples of a few molecules (clusters) SERS spectra recorded on the substrate covered with a layer of practically SERS non-active metal. 2. Experimental details All reagents were of analytical reagent grade and were used as received from commercial companies. Solutions were prepared with water of resistivity of 18.0 MX cm (purification was carried out with a Millipore ultrapure water system). To obtain an efficient system of nano-resonators, the silver substrate was roughened electrochemically before deposition of nickel
Fig. 3. 2D map showing 120 subsequently recorded SERS spectra of hydrogenated carbon formed on the surface of the nickel-modified silver electrode during electrochemical reduction of CO2. Each row shows a single colour-coded spectrum. The accumulation time of a single spectrum was 1 s. The intensity of the spectra is given in counts per second.
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A. Kudelski / Chemical Physics Letters 457 (2008) 434–438
3. Results and discussion Fig. 1 shows SERS spectrum of the hydrogenated carbon created on the nickel-modified silver electrode during the electrochemical reduction of CO2. The accumulation time of this spectrum was
1330 1240
1590
1510 s
1580 1370
i
corresponding to the average thickness of Ni layer of about five atoms. Raman spectra have been recorded with an ISA T64000 (Jobin Yvon) Raman spectrometer equipped with an Olympus BX40 microscope with a 50 long distance objective. A Laser-Tech model LJ-800 mixed argon/krypton laser provided excitation radiation of 514.5 nm. The temporal instability of intensity of the laser radiation was less than 5%.
1370 1300 1240
1570 1500
layer. Roughening was carried out in a conventional three-electrode cell with a large platinum sheet as the counter-electrode and an Ag/AgCl (1 M KCl) electrode as the reference one (all potentials are quoted versus this electrode). Silver was roughened by three successive negativepositive negative cycles in a 0.1 M KCl aqueous solution from 0.3 to 0.3 V at a sweep rate of 5 mV s1 [30]. The cycling was finished at 0.3 V and then the silver electrode was kept for 5 min at 0.4 V. After that the working electrode was removed at an open circuit potential and very carefully rinsed with water. On such activated silver substrate the layer of nickel was electrochemically deposited. Nickel deposition was carried out in a two-electrode cell (with one electrode made of pure nickel) from a 0.84 M NiCl2 solution saturated with H3BO3. The reduction charge density was set to about 3 mC cm2
1190
1550
r
1590
1190
h
1370
680
p 1500
660
1510
1300 o
1590 1240
1670 f
1520
n 1520
1270 1160
1630
1630
Raman intensity
g
680
1590 e
m 1330 d
1250
1270
830
1540 l
640 c
1550
1190
k
b
1580
1370
1580
890
680 a
j 1600
1200
Raman shift / cm
800 −1
400
1600
1200
800
400
−1
Raman shift / cm
Fig. 4. Eighteen example SERS spectra of hydrogenated carbon formed on the surface of the nickel-modified silver electrode during the electrochemical reduction of CO2 that reveal relatively intensive narrow Raman bands. The accumulation time of a single spectrum was 1 s.
A. Kudelski / Chemical Physics Letters 457 (2008) 434–438
100 s. The electrochemical reduction of CO2 was carried out for 10 min at the potential of 2.1 V in a 0.1 M KHCO3 solution saturated with CO2 at pCO2 ¼ 1 bar. As can be seen in Fig. 1, the measured spectrum is dominated by two broad overlapping ‘background’ bands centred at about 1370 and 1575 cm1. The spectrum composed of such two bands is typical of amorphous carbon [17,31]. In addition to these two broad ‘background’ bands, one can also see some narrow features, for example, at ca. 640, 970 and 1290 cm1. The band at 640 cm1 is probably due to the cage deformations of the carbon clusters, whereas the bands at higher wavenumbers are due to the CC stretching and CH bending vibrations [31]. The relative intensities of all narrow Raman bands shown in the ‘averaged’ SERS spectrum presented in Fig. 1 are relatively low. As has been already shown by Itoh et al. [16], Szeghalmi et al . [19], Neugebauer et al . [20] and our group [12,13,17], when SERS substrate is highly active, one can identify well visible narrow Raman bands characteristic for the individual carbon clusters in the SERS spectra measured using short accumulation time. As mentioned in the Introduction, deposition of a layer of SERS non-active (or practically SERS non-active) metal on the highly SERS-active substrate causes significant decrease of the SERS enhancement factor [23]. In this contribution, we decided to verify whether it is possible to carry out a few molecules SERS measurements on substrates covered with a layer of metal with negligible SERS activity. We focus on SERS substrates modified with nickel, since nickel has strong catalytic properties and therefore, SERS measurements on Ni-modified substrates may be of large practical significance. The average thickness of deposited nickel layers was only five atoms, which is a typical thickness of layers applied for the modification of highly SERS-active substrates [23,24]. Even though the modifying layer is very thin and we could not guarantee its integrity, we found that a modification of the silver substrate may induce significant changes of the measured spectra. For example, the intensity of SERS spectrum of pyridine (a model molecule for SERS measurements) adsorbed from 0.1 M KCl + 0.05 M pyridine solution was about 101 times higher for the unmodified substrates than for the nickel-modified substrates (the value of intensity decrease was calculated from the results of 20 measurements). Figs. 2 and 3 show series of SERS spectra of the carbon material produced during electrochemical reduction of CO2 on the nickelmodified silver electrode. The accumulation time of the spectra presented in Figs. 2 and 3 was 1 s. Fig. 2 presents four successively recorded spectra plotted in the traditional manner (Raman intensity vs. Raman shift). The longer series of 120 subsequently measured spectra is presented in Fig. 3 as the two-dimensional (2D) map in which each spectrum is colour-coded in a single row (a series of 120 spectra presented in the traditional way is very difficult to analyse). As can be seen in Figs. 2 and 3, some measured SERS spectra are dominated by a few strong narrow Raman bands. Moreover, successively recorded spectra are significantly different – for example, compare spectrum b and c in Fig. 2. It is not conceivable that carbon clusters in the whole illuminated area of about 1 lm2 adopt similar structure, and therefore, when in the series of subsequently measured spectra, spectra dominated by some narrow Raman bands are observed, one can conclude that the measured spectra are dominated by the contribution from only a few carbon clusters adsorbed in so called ‘hot spots’. Due to the diffusion of clusters (molecules) in and out of the ‘hot spots’, which changes the number of effectively scattering species, subsequently measured SERS spectra reveal significantly different intensities. Since the energy density in the ‘hot spot’ is large, further thermally activated diffusion of the carbon cluster from the ‘hot spot’ is very likely, and therefore, in a ‘few molecules’ SERS measurements the signal-to-noise ratio cannot be improved by using a longer accumulation time.
437
Fig. 4 presents some example spectra of carbon materials formed during the electrochemical reduction of CO2, in which narrow Raman bands can be clearly identified. A strong band at about 1590 cm1 (see spectra d, g, n, s) is characteristic for the in-plane stretching mode of the monolayer of hexagonally ordered carbon atoms [16]. The bands at slightly lower frequencies (1500–1580 cm1) are due to the smaller graphite-like ring structures or C@C stretching vibrations in various carbon chain clusters (see spectra: b, c, e, f, h, i, j, k, m, o, p, r) [15,16]. The band at 1370 cm1 (see spectra a, g, h and i) may be ascribed to the D mode in the microcrystalline graphite [15]. Strong narrow band at 1330 cm1 (see spectra l and s) is probably due to the diamond like carbon cluster(s) [15,31]. The bands in the region 13001050 cm1 (e.g., the band at 1190 cm1 in spectra b, g and h, the band at 1270 cm1 in spectra c and m, the band at 1300 cm1 in spectra i and o, the band at 1240 cm1 in spectra i, n and s, the band at 1250 cm1 in spectrum l and the band at 1160 cm1 in spectrum m) are mainly associated with the CC stretching and CH bending vibrations [31]. The bands in the region 550850 cm1 (e.g., the band at 640 cm1 in spectrum c, the band at 660 cm1 in spectrum g, the band at 680 cm1 in spectra h, j and m and the band at 830 cm1 in spectrum l) are probably associated with the deformation vibrations of various carbon networks [31]. As can be seen in Fig. 4, the quality of a few clusters SERS spectra measured on nickel-modified silver substrate is sufficient for the identification of bands characteristic for an individual carbon clusters. 4. Conclusions For the first time a few clusters SERS measurements on a substrate covered with a layer of practically SERS non-active metal were realized. It means that a few molecules SERS spectra of species adsorbed on many catalytically important metals can be measured. In recorded a few clusters (molecules) SERS spectra the contributions characteristic for various clusters could be identified (and hence presence of these clusters in the analysed mixture could be proved) even when the contribution of many clusters to the standard (averaged) SERS spectrum of the analysed sample cannot be reliably identified. Acknowledgement This work was supported financially by Ministry of Science and Higher Education (Poland) funds through the Department of Chemistry, the University of Warsaw. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
B. Pettinger, Top. Appl. Phys. 103 (2006) 217. F. Demming, J. Jersch, K. Dickmann, P.I. Geshev, Appl. Phys. B 66 (1998) 593. Z.Q. Tian, B. Ren, D.Y. Wu, J. Phys. Chem. B 106 (2002) 9463. A. Kudelski, Vib. Spectrosc. 39 (2005) 200. K. Kneipp, Y. Wang, H. Kneipp, L.T. Perelman, I. Itzkan, R.R. Dasari, M.S. Feld, Phys. Rev. Lett. 78 (1997) 1667. S. Nie, S.R. Emory, Science 275 (1997) 1102. H.X. Xu, E.J. Bjerneld, M. Kall, L. Borjesson, Phys. Rev. Lett. 83 (1999) 4357. Z. Wang, L.J. Rothberg, J. Phys. Chem. B 109 (2005) 3387. A.J. Meixner, T. Vosgrone, M. Sackrow, J. Lumin. 94 (2001) 147. C.C. Neacsu, J. Dreyer, N. Behr, M.B. Raschke, Phys. Rev. B 73 (2006) 193406. K. Kneipp, Y. Wang, R.R. Dasari, M.S. Feld, Appl. Spectrosc. 49 (1995) 780. A. Kudelski, B. Pettinger, Chem. Phys. Lett. 321 (2000) 356. A. Kudelski, J. Phys. Chem. B 110 (2006) 12610. A. Kudelski, Chem. Phys. Lett. 414 (2005) 271. A. Hu, Q.B. Lu, W.W. Duley, M. Rybachuk, J. Chem. Phys. 126 (2007) 154705. K. Itoh, I. Kudryashov, J. Yamagata, T. Nishizawa, M. Fujii, N. Osaka, J. Phys. Chem. B 109 (2005) 271. A. Kudelski, J. Raman Spectrosc. 38 (2007) 1494. A. Kudelski, B. Pettinger, Chem. Phys. Lett. 383 (2004) 76. A. Szeghalmi, S. Kaminskyj, P. Rosch, J. Popp, K.M. Gough, J. Phys. Chem. B 111 (2007) 12916. U. Neugebauer, et al. , Chem. Phys. Chem. 8 (2007) 124.
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[21] Z.Q. Tian, B. Ren, B.W. Mao, J. Phys. Chem. B 101 (1997) 1338. [22] B. Ren, Q.J. Huang, W.B. Cai, B.W. Mao, F.M. Liu, Z.Q. Tian, J. Electroanal. Chem. 415 (1996) 175. [23] M.F. Mrozek, Y. Xie, M.J. Weaver, Anal. Chem. 73 (2001) 5953. [24] S. Zou, M.J. Weaver, Anal. Chem. 70 (1998) 2387. [25] L. Moerl, B. Pettinger, Solid State Commun. 43 (1982) 315. [26] B. Pettinger, L. Moerl, J. Electron. Spectrosc. Relat. Phenom. 29 (1983) 383.
[27] Y.D. Li, J.L. Chen, Y.N. Qin, L. Chang, Energy Fuels 14 (2000) 1188. [28] S. Hofmann, C. Ducati, J. Robertson, B. Kleinsorge, Appl. Phys. Lett. 83 (2003) 135. [29] C. Ducati, I. Alexandrou, M. Chhowalla, J. Robertson, G.A.J. Amaratunga, J. Appl. Phys. 95 (2004) 6387. [30] A. Kudelski, Langmuir 18 (2002) 4741. [31] J. Filik, Spectrosc. Eur. 17 (2005) 10.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
A few molecules surface-enhanced Raman scattering studies on nickel-modified silver substrates Andrzej Kudelski * Department of Chemistry, University of Warsaw, Pasteur 1, PL-02-093 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 6 March 2008 In final form 10 April 2008 Available online 13 April 2008
a b s t r a c t A few clusters surface-enhanced Raman scattering (SERS) measurements on a substrate covered with a layer of metal with negligible SERS activity were realized. We measured SERS spectra dominated by the contribution from only a few carbon clusters for hydrogenated carbon deposited on the nickel-modified silver substrate. The contributions characteristic for various clusters could be identified in the recorded spectra (and hence presence of these clusters in the analysed mixture could be proved), even when contribution of many types of carbon clusters to the standard (averaged) SERS spectrum cannot be reliably identified. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction When metal nano-resonators are illuminated with the electromagnetic wave the electric field can be locally significantly enhanced (even by three orders of magnitude) [1,2]. Since for Raman bands with a small Raman shift the increase in the efficiency of Raman scattering is roughly proportional to the fourth power of the field enhancement, the field enhancement of 103 may lead to the increase in the efficiency of Raman scattering by a factor of 1012 [1,2]. Moreover, the efficiency of Raman scattering for molecules adsorbed on metal surfaces can be further increased by the resonance process similar to the ordinary resonance Raman process occurring in the metal–ligand complexes [3,4]. The increase of the intensity of Raman signal for molecules adsorbed on resonator-shaped metal substrates is called surface-enhanced Raman scattering – SERS. The efficiency of SERS scattering is sometimes sufficient to observe a good quality Raman spectrum dominated by the contribution even from a single molecule [5–10]. For highly SERS-active substrates large SERS enhancement factors are only achieved at very small parts of its surface (so called ‘hot spots’) and hence only very small part of molecules adsorbed on highly SERS-active substrates actually contributes to the measured SERS signal. For example, for some dyes adsorbed on silver nanoparticles only 0.01% of the adsorbates are responsible for almost all measured SERS signal [11]. Due to the Abbe’s diffraction limit the standard optical Raman microscope collects signal from the area not smaller than ca . 0.5 lm2 [1]. However, since for highly SERS-active substrate only very small part of this area is actually very SERS-active, when the average lifetime of molecules in the
* Fax: +48 22 8225996. E-mail address: [email protected] 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.04.035
‘hot spots’ is not significantly shorter than the accumulation time of the spectrum, using standard confocal Raman microscope we can measure Raman spectrum dominated by the contribution from a relatively small number of molecules [12–18]. Using this technique in 2005 Itoh et al. measured SERS spectra dominated by the contribution from only a few carbon clusters [16]. After carefully analysed of many spectra Itoh et al. showed that it is possible to identify spectra characteristic for various clusters (and hence prove their presence in the analysed mixture), even when their contribution to the averaged spectrum of the analysed sample cannot be reliably identified [16]. Similar ‘a few molecules’ SERS measurements, which allowed for the identification of some fluctuating Raman bands, have been recently carried out by Szeghalmi et al. [19] and Neugebauer et al. [20]. Very large SERS enhancement factors are achieved for molecules adsorbed at rough surfaces of only a few metals (e.g., Ag, Au and Cu) [1,3,4]. The SERS enhancement factors at other metals are significantly smaller (however, after proper roughening, reliable SERS spectra may be also recorded on some other metals) [3,21,22]. To extend intensive SERS effect on other metals one can deposit (usually electro-deposit) these metals on highly SERS-active substrates as ultrathin films [23,24]. This procedure changes the surface chemical properties to those of the deposited film while the underlying substrate maintains SERS enhancement [23,24]. Despite the thickness of deposited layers is usually only five atoms [23,24], deposition of SERS non-active (or practically SERS non-active) metals causes some decrease of the SERS enhancement factor [23]. Since many species are often preferentially deposited at SERS ‘hot spots’, deposition of even submonolayer of SERS non-active metal can cause large reduction of SERS enhancement factor. For example, Pettinger and Moerl reported observable quenching of SERS effect after deposition on the silver surface of 0.3% of the monolayer of Tl [25,26].
435
A. Kudelski / Chemical Physics Letters 457 (2008) 434–438
d 1575
c 1370
Raman intensity
Raman intensity
1290
640
970
b a 2000
1500
1000
Raman shift / cm
500
−1
Fig. 1. SERS spectrum of hydrogenated carbon formed on the surface of the nickelmodified silver electrode during electrochemical reduction of CO2. Spectrum was accumulated for 100 s.
In this contribution, we analyse SERS spectra of hydrogenated carbon deposited on the nickel-modified silver substrate. Nickel is practically SERS non-active metal and only small SERS enhancement factors may be obtained on pure nickel substrates [22]. We show, that when a five atoms thick layer of nickel is deposited on the highly SERS-active silver substrate (five atoms thick layers are typically used as modifying layers [23,24]), the achievable SERS enhancement factors are still sufficient to record SERS spectra dominated by the contribution from only a few carbon clusters. Since nickel is an important catalyst used for formation of many carbon materials (e.g., carbon nanotubes) [27–29], SERS measurements on nickel-modified surfaces may be of significant practical importance. According to our knowledge the spectra presented in
2000
1500
1000
Raman shift / cm
500
−1
Fig. 2. Four successively recorded SERS spectra of hydrogenated carbon formed on the surface of the nickel-modified silver electrode during electrochemical reduction of CO2. Each spectrum was accumulated for 1 s. Spectra are presented at the same scale, vertically shifted for the sake of clarity.
this contribution are the first examples of a few molecules (clusters) SERS spectra recorded on the substrate covered with a layer of practically SERS non-active metal. 2. Experimental details All reagents were of analytical reagent grade and were used as received from commercial companies. Solutions were prepared with water of resistivity of 18.0 MX cm (purification was carried out with a Millipore ultrapure water system). To obtain an efficient system of nano-resonators, the silver substrate was roughened electrochemically before deposition of nickel
Fig. 3. 2D map showing 120 subsequently recorded SERS spectra of hydrogenated carbon formed on the surface of the nickel-modified silver electrode during electrochemical reduction of CO2. Each row shows a single colour-coded spectrum. The accumulation time of a single spectrum was 1 s. The intensity of the spectra is given in counts per second.
436
A. Kudelski / Chemical Physics Letters 457 (2008) 434–438
3. Results and discussion Fig. 1 shows SERS spectrum of the hydrogenated carbon created on the nickel-modified silver electrode during the electrochemical reduction of CO2. The accumulation time of this spectrum was
1330 1240
1590
1510 s
1580 1370
i
corresponding to the average thickness of Ni layer of about five atoms. Raman spectra have been recorded with an ISA T64000 (Jobin Yvon) Raman spectrometer equipped with an Olympus BX40 microscope with a 50 long distance objective. A Laser-Tech model LJ-800 mixed argon/krypton laser provided excitation radiation of 514.5 nm. The temporal instability of intensity of the laser radiation was less than 5%.
1370 1300 1240
1570 1500
layer. Roughening was carried out in a conventional three-electrode cell with a large platinum sheet as the counter-electrode and an Ag/AgCl (1 M KCl) electrode as the reference one (all potentials are quoted versus this electrode). Silver was roughened by three successive negativepositive negative cycles in a 0.1 M KCl aqueous solution from 0.3 to 0.3 V at a sweep rate of 5 mV s1 [30]. The cycling was finished at 0.3 V and then the silver electrode was kept for 5 min at 0.4 V. After that the working electrode was removed at an open circuit potential and very carefully rinsed with water. On such activated silver substrate the layer of nickel was electrochemically deposited. Nickel deposition was carried out in a two-electrode cell (with one electrode made of pure nickel) from a 0.84 M NiCl2 solution saturated with H3BO3. The reduction charge density was set to about 3 mC cm2
1190
1550
r
1590
1190
h
1370
680
p 1500
660
1510
1300 o
1590 1240
1670 f
1520
n 1520
1270 1160
1630
1630
Raman intensity
g
680
1590 e
m 1330 d
1250
1270
830
1540 l
640 c
1550
1190
k
b
1580
1370
1580
890
680 a
j 1600
1200
Raman shift / cm
800 −1
400
1600
1200
800
400
−1
Raman shift / cm
Fig. 4. Eighteen example SERS spectra of hydrogenated carbon formed on the surface of the nickel-modified silver electrode during the electrochemical reduction of CO2 that reveal relatively intensive narrow Raman bands. The accumulation time of a single spectrum was 1 s.
A. Kudelski / Chemical Physics Letters 457 (2008) 434–438
100 s. The electrochemical reduction of CO2 was carried out for 10 min at the potential of 2.1 V in a 0.1 M KHCO3 solution saturated with CO2 at pCO2 ¼ 1 bar. As can be seen in Fig. 1, the measured spectrum is dominated by two broad overlapping ‘background’ bands centred at about 1370 and 1575 cm1. The spectrum composed of such two bands is typical of amorphous carbon [17,31]. In addition to these two broad ‘background’ bands, one can also see some narrow features, for example, at ca. 640, 970 and 1290 cm1. The band at 640 cm1 is probably due to the cage deformations of the carbon clusters, whereas the bands at higher wavenumbers are due to the CC stretching and CH bending vibrations [31]. The relative intensities of all narrow Raman bands shown in the ‘averaged’ SERS spectrum presented in Fig. 1 are relatively low. As has been already shown by Itoh et al. [16], Szeghalmi et al . [19], Neugebauer et al . [20] and our group [12,13,17], when SERS substrate is highly active, one can identify well visible narrow Raman bands characteristic for the individual carbon clusters in the SERS spectra measured using short accumulation time. As mentioned in the Introduction, deposition of a layer of SERS non-active (or practically SERS non-active) metal on the highly SERS-active substrate causes significant decrease of the SERS enhancement factor [23]. In this contribution, we decided to verify whether it is possible to carry out a few molecules SERS measurements on substrates covered with a layer of metal with negligible SERS activity. We focus on SERS substrates modified with nickel, since nickel has strong catalytic properties and therefore, SERS measurements on Ni-modified substrates may be of large practical significance. The average thickness of deposited nickel layers was only five atoms, which is a typical thickness of layers applied for the modification of highly SERS-active substrates [23,24]. Even though the modifying layer is very thin and we could not guarantee its integrity, we found that a modification of the silver substrate may induce significant changes of the measured spectra. For example, the intensity of SERS spectrum of pyridine (a model molecule for SERS measurements) adsorbed from 0.1 M KCl + 0.05 M pyridine solution was about 101 times higher for the unmodified substrates than for the nickel-modified substrates (the value of intensity decrease was calculated from the results of 20 measurements). Figs. 2 and 3 show series of SERS spectra of the carbon material produced during electrochemical reduction of CO2 on the nickelmodified silver electrode. The accumulation time of the spectra presented in Figs. 2 and 3 was 1 s. Fig. 2 presents four successively recorded spectra plotted in the traditional manner (Raman intensity vs. Raman shift). The longer series of 120 subsequently measured spectra is presented in Fig. 3 as the two-dimensional (2D) map in which each spectrum is colour-coded in a single row (a series of 120 spectra presented in the traditional way is very difficult to analyse). As can be seen in Figs. 2 and 3, some measured SERS spectra are dominated by a few strong narrow Raman bands. Moreover, successively recorded spectra are significantly different – for example, compare spectrum b and c in Fig. 2. It is not conceivable that carbon clusters in the whole illuminated area of about 1 lm2 adopt similar structure, and therefore, when in the series of subsequently measured spectra, spectra dominated by some narrow Raman bands are observed, one can conclude that the measured spectra are dominated by the contribution from only a few carbon clusters adsorbed in so called ‘hot spots’. Due to the diffusion of clusters (molecules) in and out of the ‘hot spots’, which changes the number of effectively scattering species, subsequently measured SERS spectra reveal significantly different intensities. Since the energy density in the ‘hot spot’ is large, further thermally activated diffusion of the carbon cluster from the ‘hot spot’ is very likely, and therefore, in a ‘few molecules’ SERS measurements the signal-to-noise ratio cannot be improved by using a longer accumulation time.
437
Fig. 4 presents some example spectra of carbon materials formed during the electrochemical reduction of CO2, in which narrow Raman bands can be clearly identified. A strong band at about 1590 cm1 (see spectra d, g, n, s) is characteristic for the in-plane stretching mode of the monolayer of hexagonally ordered carbon atoms [16]. The bands at slightly lower frequencies (1500–1580 cm1) are due to the smaller graphite-like ring structures or C@C stretching vibrations in various carbon chain clusters (see spectra: b, c, e, f, h, i, j, k, m, o, p, r) [15,16]. The band at 1370 cm1 (see spectra a, g, h and i) may be ascribed to the D mode in the microcrystalline graphite [15]. Strong narrow band at 1330 cm1 (see spectra l and s) is probably due to the diamond like carbon cluster(s) [15,31]. The bands in the region 13001050 cm1 (e.g., the band at 1190 cm1 in spectra b, g and h, the band at 1270 cm1 in spectra c and m, the band at 1300 cm1 in spectra i and o, the band at 1240 cm1 in spectra i, n and s, the band at 1250 cm1 in spectrum l and the band at 1160 cm1 in spectrum m) are mainly associated with the CC stretching and CH bending vibrations [31]. The bands in the region 550850 cm1 (e.g., the band at 640 cm1 in spectrum c, the band at 660 cm1 in spectrum g, the band at 680 cm1 in spectra h, j and m and the band at 830 cm1 in spectrum l) are probably associated with the deformation vibrations of various carbon networks [31]. As can be seen in Fig. 4, the quality of a few clusters SERS spectra measured on nickel-modified silver substrate is sufficient for the identification of bands characteristic for an individual carbon clusters. 4. Conclusions For the first time a few clusters SERS measurements on a substrate covered with a layer of practically SERS non-active metal were realized. It means that a few molecules SERS spectra of species adsorbed on many catalytically important metals can be measured. In recorded a few clusters (molecules) SERS spectra the contributions characteristic for various clusters could be identified (and hence presence of these clusters in the analysed mixture could be proved) even when the contribution of many clusters to the standard (averaged) SERS spectrum of the analysed sample cannot be reliably identified. Acknowledgement This work was supported financially by Ministry of Science and Higher Education (Poland) funds through the Department of Chemistry, the University of Warsaw. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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