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Self-Assembled Monolayers of Organoselenium Compounds on Gold: Surface-Enhanced Raman Scattering Study
Journal of Colloid and Interface Science 240, 492–497 (2001) doi:10.1006/jcis.2001.7702, available online at http://www.idealibrary.com on
Self-Assembled Monolayers of Organoselenium Compounds on Gold: Surface-Enhanced Raman Scattering Study Sang Woo Han and Kwan Kim1 Laboratory of Intelligent Interface, School of Chemistry and Molecular Engineering and Center for Molecular Catalysis, Seoul National University, Seoul 151-742, Korea Received March 26, 2001; accepted May 11, 2001; published online July 12, 2001
To understand the binding nature of organoselenium compounds on gold, we have examined the adsorption behavior of several representative organoselenium compounds, i.e., benzeneselenol (BSe), diphenyl diselenide, dibenzyl diselenide, dioctyl diselenide, and benzyl phenyl selenide (BPSe) on the Au surface by virtue of surfaceenhanced Raman spectroscopy (SERS). BSe chemisorbs on gold as selenolate with a tilted orientation. Upon adsorption, the Se–Se bonds of diselenides are cleaved to form selenolates, analogous to the formation of thiolate monolayers from disulfides. BPSe adsorbs on gold without any C–Se bond scission. The benzyl moiety of BPSe assumes a rather vertical stance while the phenyl moiety is more tilted to the gold surface. °C 2001 Academic Press Key Words: organoselenium; Au; self-assembled monolayer; SERS.
1. INTRODUCTION
Self-assembled monolayers (SAMs) have received a great deal of interest in recent times due to their various applications such as in biomimetic films, chemical sensors, nonlinear optical materials, high-density memory devices, protective coatings, lubricants, and photopatterning methodology (1). Although a wide variety of substrates and functional groups are known to form SAMs, the thiol/disulfide monolayer on Au has received considerable attention due to its simplicity and ease of preparation (2). In contrast, selenol/diselenide monolayers have not received enough attention despite their promising utility for a variety of applications such as in photoresists, photocatalysts, preparation of semiconductor quantum dots, photoinduced electron-transfer systems, and so on (3). There are several reports on organoselenium monolayers on gold surfaces. However, adsorbate structures of SAMs from organoseleniums are somewhat controversial. Dishner et al. (4) revealed the two-dimensional structure of benzeneselenolate monolayers on gold by using scanning tunneling microscopy (STM). They reported that the STM images of the monolayers obtained from benzeneselenol (BSe) and diphenyl dise1 To whom correspondence should be addressed. Fax: +82-2-8743704. E-mail: [email protected].
0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
lenide (DPDSe) are essentially identical. Huang et al. (5) determined the compositions of diphenyl disulfide (DPDS) and DPDSe mixed monolayers formed by displacement and competitive adsorption by surface-enhanced Raman spectroscopy (SERS). They found that the Se–Se bond is cleaved to form benzeneselenolate upon adsorption, analogous to the formation of benzenethiolate monolayers from DPDS (6). On the other hand, in the study of the adsorption properties of DPDS, DPDSe, and naphthalene disulfide on gold films, Bandyopadhyay et al. (7) reported that the Se–Se bond of DPDSe is preserved upon adsorption. For a more detailed understanding of the binding nature of organoselenium compounds, we have examined the adsorption behaviors of several representative organoselenium compounds, namely, BSe, DPDSe, dibenzyl diselenide (DBDSe), dioctyl diselenide (DODSe), and benzyl phenyl selenide (BPSe), on a Au surface by means of SERS. SERS is selected on the basis of its well-proven ability to effectively unravel the molecular-level details of the SAM structure and the degree of organization. For instance, since Raman spectroscopy can give an intense band for Se–Se stretching, valuable information will be obtained about the fate of a diselenide bond during the monolayer formation. 2. MATERIALS AND METHODS
All solvents were purified by standard methods. Reagent grade chemicals were used without further purification. Triply distilled water, of resistivity greater than 18.0 MÄ cm, was used to prepare aqueous solutions. Gold foil (0.05 mm thick), BSe, DPDSe, and DBDSe were purchased from Aldrich. DODSe (8) and BPSe (9) were prepared according to the previously published methods. To generate a SERS-active surface, the Au foil was roughened by oxidation–reduction cycles (ORC) in 0.1 M KCl. The ORC processes were performed following the published procedure (10). SAMs were prepared by immersing the gold surface in 1 mM organoselenium in ethanol solution for at least 1 h. After the substrates were removed, they were rinsed thoroughly with ethanol and then dried with a high-purity N2 gas stream. Raman spectra were obtained by using a Renishaw Raman system model 2000 spectrometer equipped with an integral
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ORGANOSELENIUM COMPOUNDS ON GOLD
FIG. 1. (a) OR spectrum of neat BSe. (b) SER spectrum of BSe adsorbed on Au. The ν(CH) and ν(SeH) regions of the respective systems are shown in the inset.
microscope (Olympus BH2-UMA). The 632.8 nm radiation from a 17-mW air-cooled He/Ne laser (Spectra Physics Model 127) was used as an excitation source. Raman scattering was detected with 180◦ geometry using a Peltier-cooled CCD detector. The Raman band of a silicon wafer at 520 cm−1 was referenced in calibrating the spectrometer, and the accuracy of the spectral measurement was estimated to be ±1 cm−1 . The Raman spectrometer was interfaced with an IBM-compatible PC, and the spectral data were analyzed using Renishaw WiRE v. 1.2 software based on the GRAMS/32C suite program (Galactic). 3. RESULTS AND DISCUSSION
3.1. Selenols: BSe Figures 1a and 1b show the ordinary Raman (OR) spectrum of neat BSe and the SER spectrum of BSe self-assembled on a Au surface, respectively. The observed peaks in Fig. 1 are collectively summarized in Table 1. Referring to the published data in the literature (5, 11–14), most of the peaks in Figure 1 can be assigned without difficulty. Nevertheless, for a more reliable analysis of the Raman spectra of BSe, we have performed ab initio quantum mechanical calculations on BSe in free and deprotonated, i.e., benzeneselenolate, states. [The ab initio calculations were performed at the RHF/LANL2DZ level with the Gaussian 98 program (15) running on an IBM-SP2 computer. Based on the optimized geometries, the harmonic vibrational frequencies were computed. The calculations yielded no imaginary frequency for the optimized structures, which means that the structures are ground state (16). To correlate with the observed frequencies, the calculated harmonic frequencies were scaled by
TABLE 1 Raman Spectral Data and Vibrational Assignments of BSe and DPDSe BSe OR
(cm−1 )
3054 2301 1580 1475 1438 1179 1156 1078 1068 1022 999 987 903 796 670
DPDSe
SERS
(cm−1 )
OR
(cm−1 )
Assignmenta 2(a1 ), ν(CH) ν(SeH) 8a(a1 ), ν(CC) 19a(a1 ), ν(CC) 19b(b2 ), ν(CC) 9a(a1 ), β(CH) 15(b2 ), β(CH) 1(a1 ), β(CCC) + ν(CSe) 18b(b2 ), β(CH) 18a(a1 ), β(CH) 12(a1 ), β(CCC) 5(b1 ), γ (CH) 17b(b1 ), γ (CH) δ(CSeH) 4(b1 ), γ (CCC) 6a(a1 ), β(CCC) + ν(CSe) 6b(b2 ), β(CCC) 16b(b1 ), γ (CCC) ν(SeSe) 7a(a1 ), β(CCC) + ν(CSe) δ(CSeSe) δ(CSeSe)
3055
3053
3053
1570 1470 1434 1176 1156 1061
1573
1569 1470 1435 1176 1156 1060
1018 997 985 904 686 663
614
613 456
304
301
1178 1154 1062 1057 1020 997 984
663 613 313 305 263 213
a Assigned
SERS (cm−1 )
1018 997 986 906 685 663 613 457 301
based on Refs. (5, 11–14) and ab initio calculations.
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0.9.] On the basis of the calculated vibrational frequencies, the observed frequencies of BSe could be properly assigned. In the OR spectrum of BSe, the Se–H stretching peak (ν(SeH)) appeared at 2301 cm−1 , but its counterpart was completely absent in the SER spectrum (see the inset in Fig. 1). The CSeH bending band (δ(CSeH)) identified at 796 cm−1 in the OR spectrum of BSe was also completely missing upon adsorption. The ring 1, 6a, and 7a modes, which have a contribution from the C–Se stretching vibration (ν(CSe)), are red-shifted by 17, 7, and 3 cm−1 , respectively, upon adsorption. These observations indicate that BSe chemisorbs on gold as selenolate forming a Au–Se bond. The adsorption mechanism of an adsorbate can be deduced from its SER spectrum through a detailed analysis of the peak shift and band broadening caused by the surface adsorption (17). According to the SERS investigation of benzene derivatives by Gao and Weaver (18), the ring modes have to redshift by around 10 cm−1 along with an increase in their widths in so far as the surface-ring π orbital interaction is the driving force of the surface adsorption; the red shift can be attributed to the bond weakening caused by the electron back donation from metal to the antibonding π ∗ orbital of the benzene moiety (18, 19). It is worthwhile on this basis to note that the bandwidths of the benzene ring modes are not so different between the OR and the SER spectra. In addition, the peak positions of ring modes other than the ones having contributions from the ν(CSe) mode are also similar to those in the OR spectra. (In fact, the 8a mode of BSe is red-shifted by 10 cm−1 . Using the downshifts alone as evidence that the ring plane lies flat on the metal surface, how-
ever, neglects the effect that bonding to the metal through the head group can have on the ring-mode vibrational frequencies. Complexation of benzenethiol (BT) with Ag(I) (12) leads to a downshift comparable to that observed when BT adsorbs on Au or Ag, for example.) These observations indicate that the molecular plane of benzeneselenolate never lies flat on the gold surface, even though it is still undetermined whether the adsorbate takes a perpendicular stance or a tilted orientation with respect to the substrate surface. According to the electromagnetic (EM) surface selection rule (20), vibrational modes whose polarizability tensor elements are perpendicular to a metal surface should be strongly en0 hanced in a SER spectrum, namely, those corresponding to αzz where z is along the surface normal. Vibrations derived from 0 0 and αyz should be the next most intense modes, and those αxz 0 0 0 , αyy , and αxy should be least enhanced. corresponding to αxx Based on the EM theory, Creighton (21) reported that for aromatic C2v molecules, relative enhancement factors for different modes should be a1 :a2 :b1 :b2 = 1–16:4:4:1 for face-on adsorption of benzene ring and a1 :a2 :b1 :b2 = 1–16:1:4:4 for perpendicular adsorption. On these grounds, it is noticeable that the most enhanced bands in Fig. 1b arise from the a1 mode. Although the a2 band could not be identified clearly, the amounts of enhancement of b1 and b2 modes of BSe are comparable with each other. These suggest that benzeneselenolate should take at least a tilted orientation on the gold surface. The noticeable appearance of the C–H stretching band at 3055 cm−1 in the inset of Fig. 1 is also indicative of such an orientation on gold (21, 22).
FIG. 2. (a) OR spectrum of neat DPDSe. (b) SER spectrum of DPDSe adsorbed on Au. The inset shows an expanded illustration of the band at ∼310 cm−1 in (a); the band has been fitted with a Gaussian function.
ORGANOSELENIUM COMPOUNDS ON GOLD
495
3.2. Diselenides: DPDSe, DBDSe, and DODSe Figures 2a and 2b show the OR spectrum of neat DPDSe and the SER spectrum of DPDSe self-assembled on an Au surface, respectively. The observed peaks in Fig. 2 are collectively summarized in Table 1, along with appropriate assignments. For a more reliable analysis of the Raman spectra of DPDSe, we have also performed ab initio vibrational frequency calculations. As mentioned under Introduction, the fate of the Se–Se bond upon adsorption onto gold is somewhat controversial. Huang et al. (5) assigned the band near 260 cm−1 to the ν(SeSe) mode of DPDSe and argued that the Se–Se bond cleaves upon adsorption onto gold based on the disappearance of this band in the SER spectrum. On the other hand, Bandyopadhyay et al. (7) reported that the band near 300 cm−1 can be assigned to the ν(SeSe) mode, and its presence in the SER spectrum indicates the possibility of the undissociated Se–Se bond. This disagreement between the two groups may be ascribed to the inappropriate vibrational assignment presented in the previous reports. However, we could obtain reasonable assignments of vibrational modes of DPDSe through a comparison with the adsorption behavior of BSe on gold and ab initio calculations. On the basis of this assignment, we could estimate the adsorption behavior of DPDSe on gold more reasonably. For instance, it is noticeable that the SER spectral features of DPDSe adsorbed on Au are nearly the same as those of the BSe adsorbed on Au (see Figs. 1b and 2b and Table 1). This means that the adsorbate responsible for the SER spectrum of DPDSe on Au should be same as that for the spectrum of BSe, i.e., benzeneselenolate. This is further confirmed by the fact that the bands related to the Se–Se bond are completely missing upon adsorption. In the OR spectrum of DPDSe, the CSeSe bending peaks (δ(CSeSe)) appeared at 263 and 213 cm−1 , but their counterparts were completely absent in the SER spectrum. The Se–Se stretching band (ν(SeSe)) identified at 313 cm−1 in the OR spectrum was also completely missing upon adsorption. Since the ν(SeSe) mode of DPDSe overlapped with the 7a + ν(CSe) mode, the band near 310 cm−1 in the OR spectrum of DPDSe was fitted with a Gaussian function to separate each component (see the inset in Figure 2). The fitting gave two well-resolved bands with peak positions at 313 and 305 cm−1 , which can be assigned to ν(SeSe) and 7a + ν(CSe) modes, respectively. For more evidence of the dissociation of the Se–Se bond upon adsorption onto gold, we also measured the SER spectra of other diselenides, i.e., DBDSe and DODSe. Figures 3a and 3b show the OR spectrum of neat DBDSe and the SER spectrum of DBDSe self-assembled on an Au surface, respectively. Figures 3c and 3d show the OR spectrum of neat DODSe and the SER spectrum of DODSe on Au, respectively. As shown in Fig. 3, the ν(SeSe) bands identified at 283 and 288 cm−1 in the OR spectra of DBDSe and DODSe (23), respectively, were completely missing upon adsorption onto gold. From this fact, it can be concluded that the Se–Se bond cleavage surely occurs when the diselenide adsorbs onto gold, resulting in the formation of a selenolate monolayer. The dissociation of the Se–Se bond on gold is analogous to the behaviors of aliphatic and aromatic
FIG. 3. (a) OR spectrum of neat DBDSe. (b) SER spectrum of DBDSe adsorbed on Au. (c) OR spectrum of neat DODSe. (d) SER spectrum of DODSe adsorbed on Au.
disulfides, in which the S–S bond cleaves upon adsorption to form thiolate (2, 6). Since Se–Se cleavage is known to be more facile than S–S cleavage in several chemical systems (24), the dissociation of the diselenide bond upon adsorption is obvious. 3.3. Monoselenides: BPSe Figures 4a and 4b show the OR spectrum of neat BPSe and the SER spectrum of BPSe self-assembled on an Au surface, respectively. The observed peaks in Fig. 4 are collectively summarized in Table 2, along with appropriate assignments. All of the SER peaks shown in Fig. 4b can be correlated with the OR peaks in Fig. 4a. As summarized in Table 2, the SER bands can be attributed to either the benzylseleno or phenylseleno moieties of BPSe. The SER spectral pattern of BPSe in Fig. 4b is obviously different from those of BSe and DBDSe shown in Figs. 1b and 3b, respectively. This implies that BPSe should adsorb on gold without any C–Se bond scission. The main driving force for the adsorption of BPSe on gold must be the formation of the Au–Se bond. This can be evidenced from the peak shift in the modes having contributions from the
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FIG. 4. (a) OR spectrum of neat BPSe. (b) SER spectrum of BPSe adsorbed on Au.
ν(CSe) vibration occurring through the surface adsorption of BPSe on gold. Namely, 1(P), 7a(P), and ν(CSe)(B) modes are red-shifted by 15, 9, and 39 cm−1 , respectively, upon adsorption (see Fig. 4 and Table 2). The electron donation from selenium TABLE 2 Raman Spectral Data and Vibrational Assignments of BPSe BPSe OR (cm−1 )
SERS (cm−1 )
Assignmenta
1600 1578 1215 1178 1163 1074 1021 1000 984 805 761 670 631 619 558 466 312 258
1599 1569 1215 1179 1156 1059 1019 1000 987 803 760 670 592 620 547 454 303
8a (B) 8a (P) CH2 wag. (B) 13 (B), 9a (P) 15 (P) 1 + ν(CSe) (P) 18a (B,P) 12 (B,P) 5 (P) 1 (B) 11 (B) 6a + ν(CSe) (P) ν(CSe) (B) 4(B), 6b(P) 6b(B) 6a(B), 16b(P) 7a + ν(CSe) (P) δ(CCSe) (B)
242(br) 227 a Assigned
δ(CSeC) based on Refs. (5, 11–14) and ab initio calculations.
FIG. 5. Plausible adsorption structures of (a) BSe, (b) DPDSe, and (c) BPSe adsorbed on the Au surfaces.
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ORGANOSELENIUM COMPOUNDS ON GOLD
to gold should induce a weakening of the C–Se bond(s), resulting in a red-shift of the above bands (12). It is noteworthy that the ring 8a and 1 vibrational modes of the phenylseleno moiety of BPSe are substantially broadened and their peak positions are red-shifted by 9 and 15 cm−1 , respectively, upon adsorption, while their counterparts arising from the benzyl moiety are redshifted at best by 1 and 2 cm−1 , respectively. As mentioned previously, such considerable red-shifts and band broadening have to be ascribed to a rather stronger surface–aromatic ring interaction between the gold and the phenylseleno moiety of BPSe (17, 18). It is also noticeable that the in-plane ring 8a mode of the benzyl moiety at 1600 cm−1 is more enhanced than that of the phenyl moiety at 1578 cm−1 when the BPSe is adsorbed on Au. Invoking the aforementioned EM surface selection rule (20), it can be assumed that the benzyl moiety assumes a rather vertical stance while the phenyl moiety of BPSe is more tilted with respect to the gold surface. The relatively strong enhancement of the ν(CSe) mode of the benzylseleno moiety can also be understood based on such a structure. A plausible adsorbate structure of BPSe on the gold surface is drawn in Fig. 5c, along with those of (a) BSe and (b) DPDSe adsorbed on the gold. The adsorption behavior of BPSe on Au is very similar to that of benzyl phenyl sulfide on Au (25). 4. SUMMARY
We have investigated the adsorption behaviors of several representative organoselenium compounds, viz., BSe, DPDSe, DBDSe, DODSe, and BPSe, on a Au surface by means of SERS. From careful analyses of the SER spectral features, we can estimate the adsorbate structures of SAMs made from the organoselenium compounds. First, selenol chemisorbs on gold as selenolate. Second, the Se–Se bond cleavage surely occurs when the diselenide adsorbs onto gold, resulting in the formation of the selenolate monolayer. The dissociation of the Se–Se bond on gold is analogous to the behaviors of aliphatic and aromatic disulfides, in which the S–S bond cleaves upon adsorption to form thiolate. Finally, monoselenide adsorbs on gold without any C–Se bond scission. From this work, we could obtain a clearer understanding of the binding nature of organoselenium compounds on gold. ACKNOWLEDGMENTS K.K. was supported by the Korea Research Foundation (KRF, 042-D00073) and by the Korean Science and Engineering Foundation (KOSEF, 1999-2-121001-5). S.W.H. was supported by KOSEF through the Center for Molecular Catalysis at Seoul National University and by KRF through the Brain Korea 21 program.
REFERENCES 1. (a) Hickman, J. J., Ofer, D., Laibinis, P. E., and Whitesides, G. M., Science 252, 688 (1991); (b) Mirkin, C. A., and Ratner, M. A., Annu. Rev. Phys. Chem. 43, 719 (1992); (c) Wollman, E. W., Kang, D., Frisbie. C. D., Larcovic, T. M., and Wrighton, M. S., J. Am. Chem. Soc. 116, 4395 (1994). 2. (a) Ulman, A., “An Introduction to Ultrathin Organic Films from LangmuirBlodgett to Self-Assembly.” Academic Press, San Diego, CA, 1991; (b) Ulman, A., Chem. Rev. 96, 1533 (1996). 3. Pandey, G., and Poleshwar Rao, K. S. S., Angew. Chem., Int. Ed. Engl. 34, 2669 (1995). 4. Dishner, M. H., Hemminger, J. G., and Feher, F. J., Langmuir 13, 4788 (1997). 5. Huang, F. K., Horton, R. C., Jr., Myles, D. C., and Garrell, R. L., Langmuir 14, 4802 (1998). 6. Szafranski, C. A., Tanner, W., Laibinis, P. E., and Garrell, R. L., Langmuir 14, 3570 (1998). 7. Bandyopadhyay, K., Vijayamohanan, K., Venkataramanan, M., and Pradeep, T., Langmuir 15, 5314 (1999). 8. Higuchi, H., Otsubo, T., Ogura, F., Yamaguchi, H., Sakata, Y., and Misumi, S., Bull. Chem. Soc. Jpn. 55, 182 (1982). 9. Reich, H. J., and Shah, S. K., J. Org. Chem. 42, 1773 (1977). 10. Brolo, A. G., Irish, D. E., Szymanski, G., and Lipkowski, J., Langmuir 14, 517 (1998). 11. Varsanyi, G., “Vibrational Spectra of Benzene Derivatives.” Academic Press, New York, 1969. 12. Joo, T. H., Kim, M. S., and Kim, K., J. Raman Spectrosc. 18, 57 (1987). 13. Whiffen, D. H., J. Chem. Soc. (London) 1350 (1956). 14. Rasheed, F. S., and Kimmel, H. S., Spectrosc. Lett. 10, 791 (1977). 15. Frisch, M. J., Trucks, G. W., Schlegel, H. B., Scuseria, G. E., Robb, M. A., Cheeseman, J. R., Zakrzewski, V. G., Montgomery, J. A., Jr., Stratmann, R. E., Burant, J. C., Dapprich, S., Millam, J. M., Daniels, A. D., Kudin, K. N., Strain, M. C., Farkas, O., Tomasi, J., Barone, V., Cossi, M., Cammi, R., Mennucci, B., Pomelli, C., Adamo, C., Clifford, S., Ochterski, J., Petersson, G. A., Ayala, P. Y., Cui, Q., Morokuma, K., Malick, D. K., Rabuck, A. D., Raghavachari, K., Foresman, J. B., Cioslowski, J., Ortiz, J. V., Stefanov, B. B., Liu, G., Liashenko, A., Piskorz, P., Komaromi, I., Gomperts, R., Martin, R. L., Fox, D. J., Keith, T., Al-Laham, M. A., Peng, C. Y., Nanayakkara, A., Gonzalez, C., Challacombe, M., Gill, P. M. W., Johnson, B., Chen, W., Wong, M. W., Andres, J. L., Gonzalez, C., Head-Gordon, M., Replogle, E. S., and Pople, J. A., Gaussian 98, Gaussian, Inc., Pittsburgh, PA, 1998. 16. Foresman, J. B., and Frisch, A., “Exploring Chemistry with Electronic Structure Methods.” Gaussian, Inc., Pittsburgh, PA, 1996. 17. (a) Joo, T. H., Kim, K., and Kim, M. S., J. Phys. Chem. 90, 5816 (1986); (b) Kwon, Y. J., Son, D. H., Ahn, S. J., Kim, M. S., and Kim, K., J. Phys. Chem. 98, 8481 (1994); (c) Cho, S. H., Han, H. S., Jang, D.-J., Kim, K., and Kim, M. S., J. Phys. Chem. 99, 10594 (1995); (d) Kim, S. H., Ahn, S. J., and Kim, K., J. Phys. Chem. 100, 7174 (1996). 18. Gao, P., and Weaver, M. J., J. Phys. Chem. 89, 5040 (1985). 19. (a) Moskovits, M., and Dillela, D. P., J. Chem. Phys. 73, 6068 (1980); (b) Avouris, P., and Demuth, J. E., J. Chem. Phys. 75, 4783 (1981). 20. Moskovits, M., J. Chem. Phys. 77, 4408 (1982). 21. Creighton, J. A., Surf. Sci. 124, 209 (1983). 22. Moskovits, M., and Suh, J. S., J. Am. Chem. Soc. 108, 4711 (1986). 23. Green, W. H., and Harvey, A. B., J. Chem. Phys. 49, 3586 (1968). 24. Patai, S., and Rappoport, Z., “Organic Selenium and Tellurium Compounds.” Vol. 1. Wiley, New York, 1986. 25. Joo, S. W., Han, S. W., and Kim, K., Appl. Spectrosc. 54, 378 (2000).
Self-Assembled Monolayers of Organoselenium Compounds on Gold: Surface-Enhanced Raman Scattering Study Sang Woo Han and Kwan Kim1 Laboratory of Intelligent Interface, School of Chemistry and Molecular Engineering and Center for Molecular Catalysis, Seoul National University, Seoul 151-742, Korea Received March 26, 2001; accepted May 11, 2001; published online July 12, 2001
To understand the binding nature of organoselenium compounds on gold, we have examined the adsorption behavior of several representative organoselenium compounds, i.e., benzeneselenol (BSe), diphenyl diselenide, dibenzyl diselenide, dioctyl diselenide, and benzyl phenyl selenide (BPSe) on the Au surface by virtue of surfaceenhanced Raman spectroscopy (SERS). BSe chemisorbs on gold as selenolate with a tilted orientation. Upon adsorption, the Se–Se bonds of diselenides are cleaved to form selenolates, analogous to the formation of thiolate monolayers from disulfides. BPSe adsorbs on gold without any C–Se bond scission. The benzyl moiety of BPSe assumes a rather vertical stance while the phenyl moiety is more tilted to the gold surface. °C 2001 Academic Press Key Words: organoselenium; Au; self-assembled monolayer; SERS.
1. INTRODUCTION
Self-assembled monolayers (SAMs) have received a great deal of interest in recent times due to their various applications such as in biomimetic films, chemical sensors, nonlinear optical materials, high-density memory devices, protective coatings, lubricants, and photopatterning methodology (1). Although a wide variety of substrates and functional groups are known to form SAMs, the thiol/disulfide monolayer on Au has received considerable attention due to its simplicity and ease of preparation (2). In contrast, selenol/diselenide monolayers have not received enough attention despite their promising utility for a variety of applications such as in photoresists, photocatalysts, preparation of semiconductor quantum dots, photoinduced electron-transfer systems, and so on (3). There are several reports on organoselenium monolayers on gold surfaces. However, adsorbate structures of SAMs from organoseleniums are somewhat controversial. Dishner et al. (4) revealed the two-dimensional structure of benzeneselenolate monolayers on gold by using scanning tunneling microscopy (STM). They reported that the STM images of the monolayers obtained from benzeneselenol (BSe) and diphenyl dise1 To whom correspondence should be addressed. Fax: +82-2-8743704. E-mail: [email protected].
0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
lenide (DPDSe) are essentially identical. Huang et al. (5) determined the compositions of diphenyl disulfide (DPDS) and DPDSe mixed monolayers formed by displacement and competitive adsorption by surface-enhanced Raman spectroscopy (SERS). They found that the Se–Se bond is cleaved to form benzeneselenolate upon adsorption, analogous to the formation of benzenethiolate monolayers from DPDS (6). On the other hand, in the study of the adsorption properties of DPDS, DPDSe, and naphthalene disulfide on gold films, Bandyopadhyay et al. (7) reported that the Se–Se bond of DPDSe is preserved upon adsorption. For a more detailed understanding of the binding nature of organoselenium compounds, we have examined the adsorption behaviors of several representative organoselenium compounds, namely, BSe, DPDSe, dibenzyl diselenide (DBDSe), dioctyl diselenide (DODSe), and benzyl phenyl selenide (BPSe), on a Au surface by means of SERS. SERS is selected on the basis of its well-proven ability to effectively unravel the molecular-level details of the SAM structure and the degree of organization. For instance, since Raman spectroscopy can give an intense band for Se–Se stretching, valuable information will be obtained about the fate of a diselenide bond during the monolayer formation. 2. MATERIALS AND METHODS
All solvents were purified by standard methods. Reagent grade chemicals were used without further purification. Triply distilled water, of resistivity greater than 18.0 MÄ cm, was used to prepare aqueous solutions. Gold foil (0.05 mm thick), BSe, DPDSe, and DBDSe were purchased from Aldrich. DODSe (8) and BPSe (9) were prepared according to the previously published methods. To generate a SERS-active surface, the Au foil was roughened by oxidation–reduction cycles (ORC) in 0.1 M KCl. The ORC processes were performed following the published procedure (10). SAMs were prepared by immersing the gold surface in 1 mM organoselenium in ethanol solution for at least 1 h. After the substrates were removed, they were rinsed thoroughly with ethanol and then dried with a high-purity N2 gas stream. Raman spectra were obtained by using a Renishaw Raman system model 2000 spectrometer equipped with an integral
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ORGANOSELENIUM COMPOUNDS ON GOLD
FIG. 1. (a) OR spectrum of neat BSe. (b) SER spectrum of BSe adsorbed on Au. The ν(CH) and ν(SeH) regions of the respective systems are shown in the inset.
microscope (Olympus BH2-UMA). The 632.8 nm radiation from a 17-mW air-cooled He/Ne laser (Spectra Physics Model 127) was used as an excitation source. Raman scattering was detected with 180◦ geometry using a Peltier-cooled CCD detector. The Raman band of a silicon wafer at 520 cm−1 was referenced in calibrating the spectrometer, and the accuracy of the spectral measurement was estimated to be ±1 cm−1 . The Raman spectrometer was interfaced with an IBM-compatible PC, and the spectral data were analyzed using Renishaw WiRE v. 1.2 software based on the GRAMS/32C suite program (Galactic). 3. RESULTS AND DISCUSSION
3.1. Selenols: BSe Figures 1a and 1b show the ordinary Raman (OR) spectrum of neat BSe and the SER spectrum of BSe self-assembled on a Au surface, respectively. The observed peaks in Fig. 1 are collectively summarized in Table 1. Referring to the published data in the literature (5, 11–14), most of the peaks in Figure 1 can be assigned without difficulty. Nevertheless, for a more reliable analysis of the Raman spectra of BSe, we have performed ab initio quantum mechanical calculations on BSe in free and deprotonated, i.e., benzeneselenolate, states. [The ab initio calculations were performed at the RHF/LANL2DZ level with the Gaussian 98 program (15) running on an IBM-SP2 computer. Based on the optimized geometries, the harmonic vibrational frequencies were computed. The calculations yielded no imaginary frequency for the optimized structures, which means that the structures are ground state (16). To correlate with the observed frequencies, the calculated harmonic frequencies were scaled by
TABLE 1 Raman Spectral Data and Vibrational Assignments of BSe and DPDSe BSe OR
(cm−1 )
3054 2301 1580 1475 1438 1179 1156 1078 1068 1022 999 987 903 796 670
DPDSe
SERS
(cm−1 )
OR
(cm−1 )
Assignmenta 2(a1 ), ν(CH) ν(SeH) 8a(a1 ), ν(CC) 19a(a1 ), ν(CC) 19b(b2 ), ν(CC) 9a(a1 ), β(CH) 15(b2 ), β(CH) 1(a1 ), β(CCC) + ν(CSe) 18b(b2 ), β(CH) 18a(a1 ), β(CH) 12(a1 ), β(CCC) 5(b1 ), γ (CH) 17b(b1 ), γ (CH) δ(CSeH) 4(b1 ), γ (CCC) 6a(a1 ), β(CCC) + ν(CSe) 6b(b2 ), β(CCC) 16b(b1 ), γ (CCC) ν(SeSe) 7a(a1 ), β(CCC) + ν(CSe) δ(CSeSe) δ(CSeSe)
3055
3053
3053
1570 1470 1434 1176 1156 1061
1573
1569 1470 1435 1176 1156 1060
1018 997 985 904 686 663
614
613 456
304
301
1178 1154 1062 1057 1020 997 984
663 613 313 305 263 213
a Assigned
SERS (cm−1 )
1018 997 986 906 685 663 613 457 301
based on Refs. (5, 11–14) and ab initio calculations.
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0.9.] On the basis of the calculated vibrational frequencies, the observed frequencies of BSe could be properly assigned. In the OR spectrum of BSe, the Se–H stretching peak (ν(SeH)) appeared at 2301 cm−1 , but its counterpart was completely absent in the SER spectrum (see the inset in Fig. 1). The CSeH bending band (δ(CSeH)) identified at 796 cm−1 in the OR spectrum of BSe was also completely missing upon adsorption. The ring 1, 6a, and 7a modes, which have a contribution from the C–Se stretching vibration (ν(CSe)), are red-shifted by 17, 7, and 3 cm−1 , respectively, upon adsorption. These observations indicate that BSe chemisorbs on gold as selenolate forming a Au–Se bond. The adsorption mechanism of an adsorbate can be deduced from its SER spectrum through a detailed analysis of the peak shift and band broadening caused by the surface adsorption (17). According to the SERS investigation of benzene derivatives by Gao and Weaver (18), the ring modes have to redshift by around 10 cm−1 along with an increase in their widths in so far as the surface-ring π orbital interaction is the driving force of the surface adsorption; the red shift can be attributed to the bond weakening caused by the electron back donation from metal to the antibonding π ∗ orbital of the benzene moiety (18, 19). It is worthwhile on this basis to note that the bandwidths of the benzene ring modes are not so different between the OR and the SER spectra. In addition, the peak positions of ring modes other than the ones having contributions from the ν(CSe) mode are also similar to those in the OR spectra. (In fact, the 8a mode of BSe is red-shifted by 10 cm−1 . Using the downshifts alone as evidence that the ring plane lies flat on the metal surface, how-
ever, neglects the effect that bonding to the metal through the head group can have on the ring-mode vibrational frequencies. Complexation of benzenethiol (BT) with Ag(I) (12) leads to a downshift comparable to that observed when BT adsorbs on Au or Ag, for example.) These observations indicate that the molecular plane of benzeneselenolate never lies flat on the gold surface, even though it is still undetermined whether the adsorbate takes a perpendicular stance or a tilted orientation with respect to the substrate surface. According to the electromagnetic (EM) surface selection rule (20), vibrational modes whose polarizability tensor elements are perpendicular to a metal surface should be strongly en0 hanced in a SER spectrum, namely, those corresponding to αzz where z is along the surface normal. Vibrations derived from 0 0 and αyz should be the next most intense modes, and those αxz 0 0 0 , αyy , and αxy should be least enhanced. corresponding to αxx Based on the EM theory, Creighton (21) reported that for aromatic C2v molecules, relative enhancement factors for different modes should be a1 :a2 :b1 :b2 = 1–16:4:4:1 for face-on adsorption of benzene ring and a1 :a2 :b1 :b2 = 1–16:1:4:4 for perpendicular adsorption. On these grounds, it is noticeable that the most enhanced bands in Fig. 1b arise from the a1 mode. Although the a2 band could not be identified clearly, the amounts of enhancement of b1 and b2 modes of BSe are comparable with each other. These suggest that benzeneselenolate should take at least a tilted orientation on the gold surface. The noticeable appearance of the C–H stretching band at 3055 cm−1 in the inset of Fig. 1 is also indicative of such an orientation on gold (21, 22).
FIG. 2. (a) OR spectrum of neat DPDSe. (b) SER spectrum of DPDSe adsorbed on Au. The inset shows an expanded illustration of the band at ∼310 cm−1 in (a); the band has been fitted with a Gaussian function.
ORGANOSELENIUM COMPOUNDS ON GOLD
495
3.2. Diselenides: DPDSe, DBDSe, and DODSe Figures 2a and 2b show the OR spectrum of neat DPDSe and the SER spectrum of DPDSe self-assembled on an Au surface, respectively. The observed peaks in Fig. 2 are collectively summarized in Table 1, along with appropriate assignments. For a more reliable analysis of the Raman spectra of DPDSe, we have also performed ab initio vibrational frequency calculations. As mentioned under Introduction, the fate of the Se–Se bond upon adsorption onto gold is somewhat controversial. Huang et al. (5) assigned the band near 260 cm−1 to the ν(SeSe) mode of DPDSe and argued that the Se–Se bond cleaves upon adsorption onto gold based on the disappearance of this band in the SER spectrum. On the other hand, Bandyopadhyay et al. (7) reported that the band near 300 cm−1 can be assigned to the ν(SeSe) mode, and its presence in the SER spectrum indicates the possibility of the undissociated Se–Se bond. This disagreement between the two groups may be ascribed to the inappropriate vibrational assignment presented in the previous reports. However, we could obtain reasonable assignments of vibrational modes of DPDSe through a comparison with the adsorption behavior of BSe on gold and ab initio calculations. On the basis of this assignment, we could estimate the adsorption behavior of DPDSe on gold more reasonably. For instance, it is noticeable that the SER spectral features of DPDSe adsorbed on Au are nearly the same as those of the BSe adsorbed on Au (see Figs. 1b and 2b and Table 1). This means that the adsorbate responsible for the SER spectrum of DPDSe on Au should be same as that for the spectrum of BSe, i.e., benzeneselenolate. This is further confirmed by the fact that the bands related to the Se–Se bond are completely missing upon adsorption. In the OR spectrum of DPDSe, the CSeSe bending peaks (δ(CSeSe)) appeared at 263 and 213 cm−1 , but their counterparts were completely absent in the SER spectrum. The Se–Se stretching band (ν(SeSe)) identified at 313 cm−1 in the OR spectrum was also completely missing upon adsorption. Since the ν(SeSe) mode of DPDSe overlapped with the 7a + ν(CSe) mode, the band near 310 cm−1 in the OR spectrum of DPDSe was fitted with a Gaussian function to separate each component (see the inset in Figure 2). The fitting gave two well-resolved bands with peak positions at 313 and 305 cm−1 , which can be assigned to ν(SeSe) and 7a + ν(CSe) modes, respectively. For more evidence of the dissociation of the Se–Se bond upon adsorption onto gold, we also measured the SER spectra of other diselenides, i.e., DBDSe and DODSe. Figures 3a and 3b show the OR spectrum of neat DBDSe and the SER spectrum of DBDSe self-assembled on an Au surface, respectively. Figures 3c and 3d show the OR spectrum of neat DODSe and the SER spectrum of DODSe on Au, respectively. As shown in Fig. 3, the ν(SeSe) bands identified at 283 and 288 cm−1 in the OR spectra of DBDSe and DODSe (23), respectively, were completely missing upon adsorption onto gold. From this fact, it can be concluded that the Se–Se bond cleavage surely occurs when the diselenide adsorbs onto gold, resulting in the formation of a selenolate monolayer. The dissociation of the Se–Se bond on gold is analogous to the behaviors of aliphatic and aromatic
FIG. 3. (a) OR spectrum of neat DBDSe. (b) SER spectrum of DBDSe adsorbed on Au. (c) OR spectrum of neat DODSe. (d) SER spectrum of DODSe adsorbed on Au.
disulfides, in which the S–S bond cleaves upon adsorption to form thiolate (2, 6). Since Se–Se cleavage is known to be more facile than S–S cleavage in several chemical systems (24), the dissociation of the diselenide bond upon adsorption is obvious. 3.3. Monoselenides: BPSe Figures 4a and 4b show the OR spectrum of neat BPSe and the SER spectrum of BPSe self-assembled on an Au surface, respectively. The observed peaks in Fig. 4 are collectively summarized in Table 2, along with appropriate assignments. All of the SER peaks shown in Fig. 4b can be correlated with the OR peaks in Fig. 4a. As summarized in Table 2, the SER bands can be attributed to either the benzylseleno or phenylseleno moieties of BPSe. The SER spectral pattern of BPSe in Fig. 4b is obviously different from those of BSe and DBDSe shown in Figs. 1b and 3b, respectively. This implies that BPSe should adsorb on gold without any C–Se bond scission. The main driving force for the adsorption of BPSe on gold must be the formation of the Au–Se bond. This can be evidenced from the peak shift in the modes having contributions from the
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FIG. 4. (a) OR spectrum of neat BPSe. (b) SER spectrum of BPSe adsorbed on Au.
ν(CSe) vibration occurring through the surface adsorption of BPSe on gold. Namely, 1(P), 7a(P), and ν(CSe)(B) modes are red-shifted by 15, 9, and 39 cm−1 , respectively, upon adsorption (see Fig. 4 and Table 2). The electron donation from selenium TABLE 2 Raman Spectral Data and Vibrational Assignments of BPSe BPSe OR (cm−1 )
SERS (cm−1 )
Assignmenta
1600 1578 1215 1178 1163 1074 1021 1000 984 805 761 670 631 619 558 466 312 258
1599 1569 1215 1179 1156 1059 1019 1000 987 803 760 670 592 620 547 454 303
8a (B) 8a (P) CH2 wag. (B) 13 (B), 9a (P) 15 (P) 1 + ν(CSe) (P) 18a (B,P) 12 (B,P) 5 (P) 1 (B) 11 (B) 6a + ν(CSe) (P) ν(CSe) (B) 4(B), 6b(P) 6b(B) 6a(B), 16b(P) 7a + ν(CSe) (P) δ(CCSe) (B)
242(br) 227 a Assigned
δ(CSeC) based on Refs. (5, 11–14) and ab initio calculations.
FIG. 5. Plausible adsorption structures of (a) BSe, (b) DPDSe, and (c) BPSe adsorbed on the Au surfaces.
497
ORGANOSELENIUM COMPOUNDS ON GOLD
to gold should induce a weakening of the C–Se bond(s), resulting in a red-shift of the above bands (12). It is noteworthy that the ring 8a and 1 vibrational modes of the phenylseleno moiety of BPSe are substantially broadened and their peak positions are red-shifted by 9 and 15 cm−1 , respectively, upon adsorption, while their counterparts arising from the benzyl moiety are redshifted at best by 1 and 2 cm−1 , respectively. As mentioned previously, such considerable red-shifts and band broadening have to be ascribed to a rather stronger surface–aromatic ring interaction between the gold and the phenylseleno moiety of BPSe (17, 18). It is also noticeable that the in-plane ring 8a mode of the benzyl moiety at 1600 cm−1 is more enhanced than that of the phenyl moiety at 1578 cm−1 when the BPSe is adsorbed on Au. Invoking the aforementioned EM surface selection rule (20), it can be assumed that the benzyl moiety assumes a rather vertical stance while the phenyl moiety of BPSe is more tilted with respect to the gold surface. The relatively strong enhancement of the ν(CSe) mode of the benzylseleno moiety can also be understood based on such a structure. A plausible adsorbate structure of BPSe on the gold surface is drawn in Fig. 5c, along with those of (a) BSe and (b) DPDSe adsorbed on the gold. The adsorption behavior of BPSe on Au is very similar to that of benzyl phenyl sulfide on Au (25). 4. SUMMARY
We have investigated the adsorption behaviors of several representative organoselenium compounds, viz., BSe, DPDSe, DBDSe, DODSe, and BPSe, on a Au surface by means of SERS. From careful analyses of the SER spectral features, we can estimate the adsorbate structures of SAMs made from the organoselenium compounds. First, selenol chemisorbs on gold as selenolate. Second, the Se–Se bond cleavage surely occurs when the diselenide adsorbs onto gold, resulting in the formation of the selenolate monolayer. The dissociation of the Se–Se bond on gold is analogous to the behaviors of aliphatic and aromatic disulfides, in which the S–S bond cleaves upon adsorption to form thiolate. Finally, monoselenide adsorbs on gold without any C–Se bond scission. From this work, we could obtain a clearer understanding of the binding nature of organoselenium compounds on gold. ACKNOWLEDGMENTS K.K. was supported by the Korea Research Foundation (KRF, 042-D00073) and by the Korean Science and Engineering Foundation (KOSEF, 1999-2-121001-5). S.W.H. was supported by KOSEF through the Center for Molecular Catalysis at Seoul National University and by KRF through the Brain Korea 21 program.
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