Raman characterization of metal-alkanethiolates

Transcription

1 Spectrochimica Acta Part A 55 (1999) Raman characterization of metal-alkanethiolates F. Bensebaa a, *, Y. Zhou b,c, A.G. Brolo d, D.E. Irish d, Y. Deslandes a, E. Kruus b, T.H. Ellis c a ICPET, NRC of Canada, Building M-12, Ottawa Ont. K1A OR6, Canada b Département de Chimie et de Biochimie, UQAM, Montréal Quc. H3C 3P8, Canada c Département de Chimie, Uni ersité de Montréal, Montréal Quc. HC3 3J7, Canada d Department of Chemistry, Uni ersity of Waterloo, Waterloo Ont. N2L 3G1, Canada Received 27 April 1998; received in revised form 9 September 1998; accepted 9 September 1998 Abstract Raman spectroscopy has been used to characterize neat alkanethiol and various metal-alkanethiolate materials. Neat alkanethiol gives rise to two C S stretching peaks at 662 and 735 cm 1, assigned to gauche and trans rotamers respectively. Only one C S stretching peak positioned at 725 cm 1 was found from CuC 12 and AgC 12 layered compounds, implying the absence of gauche rotamer near the thiolate group. An all-trans conformation of the chain is inferred from the peak position values of the C C stretching modes of CuC 12 and AgC 12 layered compounds. Gauche rotamers were observed in silver colloids capped with alkanethiolate Elsevier Science B.V. All rights reserved. 1. Introduction A new approach to the stabilization of gold nanoparticles using alkanethiol chains has been recently reported [1,2]. The possibility of controlling the size of the core metal [3], the length of the capping chain [4,5], the end group [2,6,7], and the nature of the metal [8 12] has opened a new range of fundamental studies and potential applications [13,14]. Nanoparticles with sizes ranging from 20 to 200 Å have been stabilized with HS(CH 2 ) n 1 CH 3 (abbreviated to C n ) chains [3]. * Corresponding author. Tel.: ; Fax: address: faird.bensebaa@nrc.ca (F. Bensebaa) The high curvature of these materials gives rise to substantial free inter-chain volume, allowing the possibility of the occurrence of the gauche conformation. However, as for the case of the densely packed self-assembled monolayers (SAMs) on flat substrate [15], very few gauche rotamers are found [4]. For example, the position of the infrared CH 2 anti-symmetric stretch ( as ) peak from gold nanoparticles capped with C n alkanethiolate was found at about 2918 cm 1 for n 11 [4]. This implies the prevalence of all-trans chains [5,16]. Interdigitation [4] and the faceted core metal [17] could play a critical role in attaining the conformational order in these nanoparticles capped alkanethiolate /99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S (98)

2 1230 F. Bensebaa et al. / Spectrochimica Acta Part A 55 (1999) Some structural differences between layered compound and nanoparticle in one hand and alkanethiolate self-assembled on flat substrate in the other hand have been suggested [3,4,9,11,12]. In particular, the difference in the metal sulfur ratio would lead to conformational difference around the thiolate group. Raman spectroscopy has been used in the past to characterize the conformation and structure of various alkanethiolates adsorbed on Ag and Au flat substrates [18 24]. Vibrational modes of S Au, S Ag, C S, and C C bonds give rise to characteristic Raman bands with features sensitive to the structural and conformational details. Sandroff et al. (1983) identified two different C S stretching peaks in the Raman spectra of an alkanethiolate monolayer [18]. Based on the C S peak position, they attributed these two peaks to gauche and to trans conformation of the C C bond near the C S bond [18]. Bryant and Pemberton (1991) [20] used the C S and C C peak positions to follow the conformational and structural changes of the alkanethiolate molecule as a function of the chain length and the degree of roughness of the metal substrate. An alternative approach to characterize metalalkanethiolate monolayers is to study related materials, which contain metal-thiolate bonds. Layered compounds composed of thiolate-metalthiolate sheets have been prepared and characterized [9,11,12,23]. Their planar structure and high degree of order make them excellent analogs for alkanethiolate monolayers. There are effectively no bulk metal atoms, as there is a 1:1 metal:sulfur stoichiometry [9,12]. An obvious advantage in studying theses new materials over the SAMs on flat substrate is the increase in surface area. Very few studies comparing directly the structural and conformational properties of nanoparticles and layered metal-alkanethiolate have been so far reported [11]. As schematically shown in Fig. 1, better packing is expected in layered compounds than with the corresponding nanoparticle material. The overall structure of these layered compounds is suggested to be similar to the flat substrate based alkanethiolate SAMs [9,23]. In the present study, Raman spectroscopy is used to characterize the conformational and structural properties of different metal-based alkanethiolate samples. Preliminary Raman results of various metal-alkanethiolate materials are reported in this paper. Particular attention will be paid to the following peculiarities; (i) the difference between Raman features of the neat alkanethiol molecule and the metal capped alkanethiolate; (ii) the influence of the metal nature (Cu, Ag, and Au) on the Raman features of these metal-alkanethiolate materials; (iii) the difference between Raman features of layered compounds and the corresponding colloid alkanethiolates. Fig. 1. Schematic representation of: (a) a layered silveralkanethiolate 9 ; and (b) colloidal gold alkanethiolate. 5 Open rectangle and open oval correspond to the metal layer sheet and the metal colloid respectively. Thick lines correspond to the alkanethiolate chains. Dimensions are not in scale.

3 F. Bensebaa et al. / Spectrochimica Acta Part A 55 (1999) Experimental Previously published recipes have been used to prepare gold colloids capped with C 12 and SH(CH 2 ) 15 CO 2 H (abbreviated to C 15 CO 2 H) [1], silver colloid capped with C 12 chain, and also 1:1 C 2 and C 12 silver layered compounds [12]. A new recipe has been devised to prepare the copper alkanethiolate layered compound [12]. In the following the 1:1 metal-alkanethiolate layered compound will be abbreviated by MC n, where M is either Ag or Cu, and n stands for the total number of carbon atoms within the alkanethiolate chain. These samples present some characteristic color, the AgC 2 layered compound is yellow, AgC 12 is pale yellow, and the CuC 12 compound is green. Silver nanoparticles capped with C 12 are dark grey, and gold colloids capped with C 12 or C 15 CO 2 H look black and shiny. Morphologically, the CuC 12 and AgC 12 layered compounds are also different. The silver layered compounds are powder-like. The copper layered compound is composed of sheets of about l mm thick and up to 1 cm long. More details on preparation and characterization of these materials can be found in a forthcoming paper [12]. For Raman analysis in the backscattering mode, samples were deposited on a glass microscope slide, and covered with a very thin glass slide. Raman spectra were measured with a Renishaw-1000 Raman microscope system equipped with a thermoelectrically cooled CCD array detector. Raman spectra were measured in two ranges, ( cm 1 ) and (0 800 cm 1 ). Raman peak positions are reproducible within 1 2 cm 1. A 50 times microscope objective lens with ca. 6 mm focal length was used. The spot size was about 1 m. A Melles Griot He Ne laser (wavelength of nm) was used as an excitation source. The maximum power at the laser head was 35 mw. At this power most of the silver layered compound burns and turns blackish at and around the laser spot. Neutral density filters, with optical density ranging from 0.3 (50% transmission) to 2 (1% transmission), were used to attenuate the laser power and consequently avoid decomposition. 3. Results and discussion Raman spectra from 50 to 3300 cm 1 of a neat alkanethiol powder and four metal alkanethiolate materials measured under similar conditions are displayed in Fig. 2. Also displayed in Fig. 3 are the Raman spectra of three of the samples recorded from 50 to 800 cm 1. The Raman spectrum of neat C 15 CO 2 H alkanethiol shows some characteristic lines (Fig. 2a). Lines attributed to; C H stretching (2921, 2883, and 2852 cm 1 ); S H stretching (2575 cm 1 ); C H deformation (1466, 1442 and 1411 cm 1 ); CH 2 Wagging (1370, 1296 and 1276 cm 1 ); C C stretching (1118 and 1061 cm 1 ); CH 2 rocking (708 and 905 cm 1 ) and; C S stretching (662 and 735 cm 1 ) modes have been detected. Most of these (and the following) assignments are based on previously reported investigations of neat alkanethiol and alkanethiolate self-assembled on flat metal substrates [18 24]. A summary of peak assignments of the different samples is given in Table 1. The Raman spectra of the different metal alkanethiolate materials show also some characteristic features. In contrast to the neat alkanethiol, no S H stretching peak is observed. This confirms the cleavage of the S H bond upon formation of the thiolate bond. The relative intensities of the different Raman peaks seem to vary quite drastically from one alkanethiolate material to another. Given the fact that the same volume of sample (defined by the laser spot size) is probed, the sample density (or the number of bonds per unit volume) is obviously a factor that could influence the Raman signal intensity. This could contribute to the overall more intense Raman signal observed on neat alkanethiol and on CuC 12 layered compound as well as to a weaker signal from colloids in comparison with 1:1 compounds. Besides sample density, the type of metal (Ag versus Cu) and the microscopic shape (nanoparticles versus. layered compound) may affect the Raman signal intensity. The copper layered compound contains some interesting Raman features. Besides the stretching, deformation and rocking peaks from CH 2 and CH 3 groups (see Table 1), sharp peaks assigned to C S stretching, C C stretching, and to wagging

4 1232 F. Bensebaa et al. / Spectrochimica Acta Part A 55 (1999) Fig. 2. Raman spectra of: (a) HSC 15 H 28 CO 2 H, (b) CuC 12 layered compound; (c) AgC 12 layered compound; (d) silver colloid capped with C 12 (or Ag*C 12 ); and (e) gold colloid capped with C 15 CO 2 H (or Au*C 15 CO 2 H). modes are also observed (Fig. 2b) at 725, 1062 (and 1128) and 1213 (1240, 1265, 1297, and 1320) cm 1, respectively. The presence of distinct and sharp peaks assigned to the wagging progression modes is a strong indication of high crystallinity [25]. In contrast, the Raman spectra of the two silver-based alkanethiolate samples are relatively different. Signal intensities are weaker especially in the case of the colloid samples. Except for the wagging progression bands, all other characteristic bands are not observed on the silver-alkanethiolate colloids. The absence of wagging bands may suggest that both the morphology and the type of the supporting metal affect the Raman signal. The trend in the signal-to-noise ratio (S/N) of the Raman spectra from the different metal-alkanethiolate materials is difficult to explain. The eventual occurrence of Raman signal enhancement in these layered compounds is a very interesting phenomenon. The intensity ratios I(C S)/I(C H) and I(C S)/I(C C) involving C S, C H and C C stretching peaks are estimated in the case of neat alkanethiol (Fig. 2a) and the copper layered compound alkanethiolate (Fig. 2b). Both ratios are found to be about five times larger in the CuC 12 layered compound. A similar Raman signal enhancement was also observed on the silver layered compound. In both cases, the signal enhancement is probably related to the charge transfer involving the sulfur and metal atoms, following the thiolate bond formation. This could explain why the signal enhancement is limited to the C S stretching peak. Furthermore, it has been shown in the past that the Raman intensity decreases with the distance between the bond and the interface [24]. Given that only a single atomic layer is present, we may speculate that this effect is probably more extreme in the layered compounds. However it is difficult to attribute this signal enhancement to a SERS (Surface enhanced Raman spectroscopy) effect [26 28] on these layered compounds. Firstly, the SERS effect has been coined as a surface phenomenon involving bulk metals or relatively large clusters. In the present study, only a single layer of atoms is involved [9,12,23]. Secondly, the enhancement factor is much weaker than what was observed on bulky metal surfaces. In the following, we will compare the prominent Raman bands of the different samples of alkanethiol and metal alkanethiolates.

5 F. Bensebaa et al. / Spectrochimica Acta Part A 55 (1999) Fig. 3. Raman spectra of AgC 2 layered compound, CuC 12 layered compound and gold colloid capped with C 12 (or Au*C 12 ) M S stretch region The metal sulfur (M S) stretching mode is critical, since the observation of its corresponding Raman peak is a unique direct spectroscopic proof of the thiolate bond formation. Vibrational peaks attributed to the M S stretching mode have been observed in the region between 200 and 235 cm 1. The relatively wide variation in the peak position of the M S stretching peak is not yet understood. Sexton and Nyberg (1986) reported an EELS (electron energy loss spectroscopy) vibrational peak at about 200 cm 1 from dimethyl sulfide adsorbed on Cu (100) [29]. They ascribed this peak to the Cu S stretching mode. Nuzzo et al. (1987) [30] also observed a strong band at 220 and 235 cm 1 on methanethiol and dimethyl disulfide adsorbed on Au (111), respectively. Using FT-SERS (Fourier transform surface enhanced Raman spectroscopy) Dai et al. (1995) [22] observed a peak at 218 cm 1 from phenyl disulfide adsorbed on a silver surface. Two factors could influence the M S peak position; (i) the nature of the metal and; (ii) the composition of the adsorbed molecule, particularly the type of chemical group near the sulfur. As shown in Fig. 3 very different Raman spectra features have been observed on three different metal alkanethiolates. Of the two peaks observed from the AgC 2 layered compound at 218 and 250 cm 1, only the former one is straightforwardly assigned to an S Ag stretching mode. The second peak is probably due to the C C S deformation [23,31 33]. It is worth noting that no Raman peak is observed between 200 and 220 cm 1 in the neat alkanethiol sample. At least two peaks (200 and 214 cm 1 ) are also observed in the M S stretching region on the CuC 12 layered compound. We could not presently explain the presence of these two peaks. The absence of a Raman peak from the S Au stretching mode is probably due to a weak Raman scattering for gold colloids. This could not be due to the absence of alkanethiolate. Indeed, a Raman peak is observed in the C H stretching region. Furthermore, XPS measurements of the same sample in the S2p spectral region have given binding energy values characteristics of thiolate species [11] C S stretch region Two C S stretching peaks positioned at 735 and 662 cm 1 were observed from the neat alka-

6 1234 F. Bensebaa et al. / Spectrochimica Acta Part A 55 (1999) Table 1 Position and assignment of the Raman peaks from neat alkanethiol and different alkanethiolate samples a HSC 15 CO 2 H CuC 12 AgC 12 Ag*C 12 Au*C 15 CO 2 H a M S 200 (215) CCC/CCS deformation 326; 424 (435) C S T d C S G 662 c 700 b 642 d CC T 1061; ; C C G CH 2 rocking 708; l4 c CH 2 wagging band pro- 1296; 1276 c ; 1213; 1240; 1265; 1297; 1320 c ; gression 1370 c 1342 CH deformation 1411 c ; 1442; ; 1450; ; 1441; 1429; 1442 l43l c 1464 SH stretch 2575 CH stretch 2852; 2883; ; 2884; ; ; ; 2910 a Silver or gold colloid capped with alkanethiolate. b Weak shoulder. c Weak peak. d Broad peak. nethiol sample. The high frequency mode (735 cm 1 ) is assigned to alkyl chains with the C C bond in the trans conformation [18,20]. The low frequency peak is assigned to the C S bond from the alkyl chain with the C C bond in the gauche conformation [18,20]. C and C correspond to the two nearest carbon atoms nearest to the sulfur atom. The relatively small peak at 708 cm 1 is probably due to the CH 2 rocking mode [20,34]. In most samples, the high frequency C S stretching peak is downshifted by at least 10 cm 1, upon thiolate formation. This observation confirms that a change occurred to the thiol group, as suggested by the disappearance of the S H Raman peak and the appearance of new Raman peaks in the M S stretching region. A similar observation has been reported in the case of alkanethiolate self-assembled on flat Ag and Au substrate [18 20]. This shift is probably due to a weakening of the C S bond following the charge transfer following the thiolate bond formation. The single C S peak observed on CuC 12 and AgC 12 layered compounds positioned at 725 cm 1 is attributed to the trans rotamer. This is in line with the geometrical model of the layered compound [9] showing ample similarity to SAMs [15]. As shown in Fig. 3, the AgC 2 layered compound contains at least two rotamers. Indeed two C S stretching peaks positioned at 644 and 720 cm 1 are observed. This observation contrasts with AgC 12 data showing only one band in this region. The low frequency band is observed only in small chain alkanethiolate. A similar feature has been observed in colloid alkanethiolate [34] and in alkanethiolate SAMs on flat substrate [15,16]. Most of the silver layered compounds based on short alkanethiolate chains (with n 7) look yellow or bright yellow in color. Based on the work of Fijolek et al. (1997) [23], this suggests that they contain gauche defects, and this seems consistent with the present work C C stretch region The Raman spectrum of neat alkanethiol in the C C stretching region shows the presence of two prominent peaks at 1061 and 1118 cm 1. They are attributed to trans rotamers. Two strong peaks (at 1062 an d 1128 cm 1 ), also attributed to the trans C C rotamers, are observed on the CuC 12 layered compound. In both samples a relatively weak peak is observed at 1084 cm 1. This peak is attributed to the gauche C C rotamers. Given the value of the C S stretching peak, it is

7 F. Bensebaa et al. / Spectrochimica Acta Part A 55 (1999) more likely that gauche rotamers are located at the chain end. Silver colloid capped with C 12 is the only sample with relatively high concentration of gauche rotamers. Indeed the Raman peak positioned at about 1080 cm 1 is more intense than the one at 1125 cm C H stretch and deformation region C H stretching and C H deformation modes give rise to very strong Raman peaks. The Raman bands are better defined for the copper and silver layered compounds than for the silver colloid sample. In the case of gold colloids, only weak C H stretching peaks have been detected using Raman. Particularly interesting is the difference between the C H stretching peak shape of layered and colloid silver alkanethiolate. Indeed, C H stretching peaks are better defined in the former sample. This is in agreement with IR data showing that higher order is obtained with the layered compounds [4,12,34]. 5. Conclusions We have studied a series of metal alkanethiolates using Raman spectroscopy. The results indicate that following the thiolate bond formation, the S H bond cleaves and new bands appear in the metal sulphur stretching region. Based on the value of the peak position of the C S stretching mode, we concluded that the rotamer near the thiolate group adopts the trans conformation, for both C 12 alkanethiolate colloidal and layered compound. However contrarily to the layered compound, the colloid sample contain some gauche rotamer. The relative intensity of the C S Raman peak increased by a factor of five following the thiolate bond formation in the case of CuC 12 and AgC 12 layered compounds. Such signal enhancement is still not well understood. Acknowledgements This work was supported by the National Research Council of Canada and by the Natural Sciences and Engineering Research Council of Canada. References [1] M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, J. Chem. Soc. Chem. Commun. (1994) [2] M. Brust, J. Fink, D. Bethel, D.J. Schiffrin, C. Kiely, J. Chem. Soc. Chem. Soc. Commun. (1995) [3] D.V. Leff, P.C. Ohara, J.R. Heath, W.M. Gelbart, J. Phys. Chem. 99 (1995) [4] A. Badia, L. Cuccia, L. Demers, F. Morin, R.B. Lennox, J. Am. Chem. Soc. 119 (1997) [5] R.H. Terrill, T.A. Postlethwaite, C. Chen, J. Am. Chem. Soc. 117 (1997) [6] D.V. Leff, L. Brandt, J.R. Heath, Langmuir 12 (1996) [7] K.S. Mayya, V. Partil, M. Sastry, Langmuir 13 (1997) [8] S. Murthy, T.P. Bigioni, Z.L. Wang, J.T. Khoury, R.L. Whetten, Mater. Lett. 30 (1997) [9] I.G. Dance, K.J. Fisher, R.M. Herath Banda, M.L. Scudder, Inorg. Chem. 30 (1992) [10] K.V. Sarathy, G.U. Kulkarni, C.N.R. Rao, Chem. Commun. (1997) [11] F. Bensebaa, Y. Zhou, Y. Deslandes, E. Kruus, T.H. Ellis, Surf. Sci. 405 (1998) [12] F. Bensebaa, T.H. Ellis, E. Kruus, R. Voicu, Y. Zhou, Can J. Chem. (in press). [13] A.P. Alivatos, Endeavou 21 (1997) [14] J.J. Storhoff, R.C. Mucic, C.A. Mirkin, J. Cluster Sci. 8 (1997) [15] A. Ulman, Chem. Rev. 96 (1996) [16] F. Bensebaa, C. Bakoyannis, T.H. Ellis, Mikrochim. Acta 14 (1997) [17] R.L. Whetten, J.T. Khoury, M.M. Alvarez, et al., Adv. Mater. 8 (1996) [18] C.S. Sandroff, S. Garoff, K.P. Leung, Chem. Phys. Lett. 96 (1983) [19] T.H. Joo, K. Kim, M.S. Kim, J. Mol. Struct. 158 (1987) [20] M.A. Byant, J.E. Pemberton, J. Am. Chem. Soc. 113 (1991) [21] S.D. Evans, T.L. Freeman, T.M. Flynn, D.N. Batchelder, A. Ulman, Thin Solid Films 244 (1994) [22] Q. Dai, C. Xue, G. Xue, L. Jiang, J. Adhes. Sci. Technol. 9 (1995) [23] H.G. Fijolek, J.R. Grohal, J.L. Sample, M.J. Natan, Inorg. Chem. 36 (1997) [24] M. Tsen, L. Sun, Anal. Chim. Acta 307 (1995) [25] L. Senak, D. Moore, R. Mendelsohn, J. Phys. Chem. 96 (1992)

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