J. Phys. Chem. 1994,98, 11163-11168
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Microstructural and Spectroscopic Studies of Metal Liquidlike Films of Silver and Gold Ala H. R. AI-Obaidi,?Stephanie J. Rigby2 John J. McGarvey;’? D. G. Walmsley,’ Ken W. Smith: Louis Hellemam,$ and Johann Snauwaerts School of Chemistry, The Queen’s University of Belfast, Belfast BT9 5AG, Northem Ireland, Department of Pure & Applied Physics, The Queen’s University of Belfast, Belfast BT7 INN, Northem Ireland, and Department of Chemistry, University of Leuven, Leuven 3030, Belgium Received: March IO, 1994; In Final Form: June 9, 1994@
Studies are described by several techniques including scanning tunneling, atomic force, and transmission electron microscopy (STM, AFM, TEM) and Raman spectroscopy of deposited metal liquidlike films (MELLFs) of silver and gold. Comparative studies using the same range of techniques have been carried out on the silver and gold sols used for MELLF preparation. TEM images clearly show that spheroidal particles feature in both types of MELLF, with diameters in the range 50-70 nm for silver and a factor of 2-3 smaller for gold MELLFs. In both cases comparative studies on the corresponding sols showed a very similar range of particle diameters but with a significantly lower particle density. For silver MELLFs and sols, but not for gold, rodlike particles are also evident. The STM technique, more sensitive to dimensions perpendicular to film surfaces, indicates flat facets on both the spheroidal and rodlike MELLF particles, and this is confirmed by AFM studies. Surface-enhanced Raman scattering has been used to study the MELLFs both in their natural liquid state and after deposition. Virtually identical spectra are observed, suggesting that drying and deposition of MELLFs does not radically alter the surface structure. The findings regarding particle dimensions and shapes are discussed in relation to the plasmon resonance mechanism for surface-enhanced Raman scattering. The formal similarities between MELLFs and reverse micelles are briefly considered.
Introduction The unusual liquidlike interfacial metal films (MELLFs) formed between aqueous sols and organic solutions were first reported for silver sols by Yogev and Efrima1s2and subsequently by McGarvey et aL3who also described4the preparation of gold MELLFs. Copper MELLFs have also recently been ~ r e p a r e d . ~ One of the useful features of these films is the ability to enhance6 Raman scattering from the wide variety of stabilizer molecules which can be used in their f ~ r m a t i o n .These ~ vary from small organic species such as pyridine to metal complexes with polypyridyl or porphyrin ligands.’ There is an ongoing interest8 from a catalytic standpoint in formation of small metal particles such as are formed in colloidal systems and of which MELLFs provide novel examples. MELLFs also bear interesting chemical resemblances to reverse micelles, i.e., water-in-oil micro-
emulsion^.^ The use of electron microscopy to examine metal colloids is well-established. Efrima and Bradley and co-workers have explored the use of scanning and transmission electron microscopy (SEM, TEM) to examine the structure of the MELLFS prepared with anisic acid as stabilizer.12 We carried out a preliminary study4 by scanning tunneling microscopy (STM) of the deposited films prepared in the presence of metal complexes and other stabilizers. The exploratory studies by STM have now been extended and have been augmented by some investigations using the related technique of atomic force microscopy (AFM). This work is presented here together with parallel studies by TEM on both the MELLFs and the sols used in their production. Results of further investigations of Raman scattering from the films prepared from various stabilizers are also reported. School of Chemistry, The Queen’s University of Belfast. Department of Pure & Applied Physics, The Queen’s University of Belfast. 8 University of Leuven. Abstract published in Advance ACS Absrrucrs, September 15, 1994. t
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Experimental Section MELLFs were prepared by previously described methods3g4 by using sols prepared by the method of Lee and Mei~e1s.l~ For the series of experiments presented here, the stabilizer used in most cases was the Cu(1) complex [Cu(DMCH)zI[BF4] (DMCH = 6,7-dihydro-5,8-dimethyldibenzo[bjl[ 1,IOIphenanthroline). In some instances pyridine or the porphyrin mesotetrakis(4-N-methylpyridylporphyrin)or the Fe(II) complex [Fe(tp[ 10]aneN3)I2+ (tp[ 101aneN3 = N,N‘,N”-tris(2-pyridylmethyl)-l,4,7-triazacyclodecane)were also used. For the STM work samples of either gold or silver MELLFs were deposited onto substrates consisting of either a 40-nm layer of Au on mica or a 400-nmlayer of Au on an 800-nm layer of Ag overlaying the mica. The STM equipment used in this work is described elsewhere.l4 Briefly, it consists of a homemade, ‘‘pocket-size&’ tripod STM with a maximum scan range of (1.5 pm).* The image acquisition, display, and processing software was also written “in-ho~se”.’~Imaging by the AFM technique was carried out at the University of Leuven on a Nanoscope I1 (Digital Instruments Inc., Santa Barbara) equipped with a 15pm range piezoelectric scanner. Commercial silicon nitride cantilevers with integrated tip were utilized. For gold and silver MELLFs spring constants of 0.06 and 0.12 N m-l, respectively, were employed. The applied force used in the experiments was estimated at N. Studies at the University of Burgundy used a NanoScope I11 instrument with an 8-pm piezoscanner. V-shaped, 100-pm-long silicon nitride cantilevers were employed, having a spring constant of 0.36 N m-l and a radius of curvature of 20 nm. Both STM and AFM imaging were conducted under ambient laboratory conditions. For the TEM studies the MELLF samples were deposited on fine copper grids. Comparative TEM studies of metal sols were also carried out by placing a few drops of sol onto Formvar film which was previously coated onto the copper grid. All
0022-3654/94/2098-11163$04.50/00 1994 American Chemical Society
11164 J. Phys. Chem., VoI. 98, No. 43, 1994
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Figure 1. Transmission spectra of silver sols: (i) no additive; (ii) mol dm-' pyridine; (iii) mol dm-' pyridine. Inset: transmission spectrum of freshly deposited silver MELLF prepared from silver sol used to record trace (iii). E M scans were recorded by using a Phillips EM400 electron microscope with magnifications up to 220 000. The equipment and procedures for recording Raman spectra of MELLFs either in their natural (liquidlike) state or as deposits have been described el~ewhere.~.' Absorption spectra of sols were recorded by using a Hewlett Packard 8452A diode m y spectrophotometer. Owing to the interfacial nature of the MELLFs, in situ recording of the absorption spectra of the liquid films, without interference from adjacent sol, proved difficult. Following preparation, the MELLF was scooped onto a glass slide and the transmission spectrum of the wet film recorded by using the diode array instrument. The spectrum remained practically unchanged after the film had dried.
Results Prior to the studies by the various techniques of electron microscopy, the absorption spectra of silver and gold sols and of MELLFs deposited on glass slides were recorded. Representative spectra for a silver sol with varying amounts of pyridine added are displayed in Figure 1. The spectrum shown in the inset was recorded for a MELLF p r e p q d by using pyridine as stabilizer. (a) TEM Studies. TEM images for both silver and gold MELLFs and sols are shown in Figures 2 and 3 at magnifications up to 220 000. In the case of silver MELLFs, Figure 2a shows particles with average diameters in the range 50-100 nm. Some of the particles are spherical but are in general more accurately described as spheroidal. Many of the particles exhibit well-defined crystallographic shapes and it is possible to single out trigonal, tetrahedral, and pentagonal particles. Rod-shaped particles with faceted surfaces are also evident. Several examples of the latter can be seen, apparently at various stages of growth, with sizes ranging from ca. 70 nm up to 200 tun in length and 30-50 nm in width. TEM images of silver sols are shown in Figure 2b. Although the particle density in these images is obviously mncb less than in the case of MELLFs, the particle size ranges and shapes are very similar. Both spheroidal and rod-shaped particles are again evident, the former
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Figure 2. (a) High-magnification TEM image of a silver MELLF deposited on a fine copper grid, showing spheroidal and rod-shaped particles; (b) TEM image of a silver sol deposited onto a Fomvarcoated fine copper grid. with diameters in the 50-100-nm range and the latter up to 150 nm in length. Figure 3 shows the TEM images obtained for gold MELLFs, which are seen to consist of densely packed spherical and spheroidal shapes. The particle size in the gold MELLFs, 2030-nm diameter, is significantly smaller than for silver, with particles of smaller (5-10 nm) and larger (up to 50 nm) dimensions also apparent. Facets are evident on many of the "grains" as in the case of silver. In contrast to silver, however, no rodlike s t ~ c h u e could s be discerned in any of a large number of images examined. Most of the particles appear to retain their integrity with a small number of contact points between them, thongb some of the spheroids are fused to form larger, more irregularly shaped entities. TEM images of gold sols (Figure 4a) exhibit characteristics similar to those of the gold MELLFs, with faceted, spheroidal particles having an average diameter of 20-30 nm. The influence of aging on gold sols is shown by the images in Figure 4b recorded for sols prepared some weeks prior to imaging. The formation of larger units is evident, with particles
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Figure 3. TEM image of a gold MELLF deposited onto a fine copper grid, showing densely-packed spheroidal particles with some trigonal strucNres interspersed.
fusing to form irregularly shaped chains, which combine further to produce a loose network of colloidal material. As has been noted previously,1osuch open structures as opposed to tighter clusters seem to he a characteristic of colloidal systems. (b) STM Studies. Figure 5 shows STM images recorded for silver MELLFs at a tunneling current and voltage of (610) nA and 400 mV, respectively. These provide evidence for spheroidal particles with minor axis diameters of ca. 40-50 nm (Figure 5a) as well as more cuboidal or rectilinear features (Figure 5h). For these and subsequent STM studies the instrument was held in the scanning mode for periods of up to 20 min before images were recorded. This settling down procedure consistently yielded well-resolved images. Lower apparent resolution in STM images, which we described in an earlier preliminruy report? may have been the result of the scanning tip having acquired some “debris” before or during the first approach to the MELLF surface; this would cause smearing of the image. The higher resolution STM images reported in the present work are essentially corroborated by images recorded by the AFM technique, vide infra. STM images for a gold MELLF are displayed in Figure 6. They show very closely packed particles. Facets are evident in the cross-sectional display (not shown) recorded along the direction indicated by the line in Figure 6h. Consistent with the TEM pictures just presented, the particle size, with typical diameter of 20-30 tun, is significantly smaller for gold MELLFs than for silver. (c) AFM Studies. As a complement to the STM studies, investigations by the AFM technique were also canied out for several MELLF samples of both silver and gold. The results of measurements from two laboratories (Universities of Burgundy and Leuven, see Experimental Section) are presented in Figures 7 and 8. In Figure 7 the traces shown were recorded for MELLFs deposited on glass slides, while for those in Figure 8 the films were deposited on mica. In Figures 7a and 8a the images for silver MELLFs show particles with typical diameters of 50- 100 nm with some rod-shaped particles evident in Figure 7a, in agreement with TEM images (e.g., Figure 2a). The spheroidal shapes which are evident in Figure 8a bear a marked resemblance in overall appearance and dimensions to the STM images, e.g., as displayed in Figure 5a. In the case of gold
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Figure 4. (a) TEM image of a gold sol deposited onto a Formvarcoated copper grid, showing spheroidal particles with surface structure; (b) TEM image of an “aged” (ca. 2 weeks) gold sal deposited onto a
Formvar-coated copper grid. MELLFs, for which scans are shown in Figures 7b and 8h, closely packed, rounded entities are the primary feature, with no indication of the rod-shaped particles seen for silver MELLFs. The rounded structures which are a feature of both gold and silver MELLFs have also been seen by Bradley and co-workers12bin SEM studies of silver MELLFs deposited on silica gel substrates. Some images of both silver and gold films showed clusters with sharply rising walls which reflected more the sloping sides of the pyramidal-shaped scanning tip than the real structures. This is one of several artifacts which can appear in both AFM and STM images, necessitating caution in their interpretation. (d) Surface-Enhanced Raman Studies. All of the images described above for MELLFs were recorded on dried films, after deposition onto the appropriate substrates as described in the Experimental Section. Such procedures could well be expectedl2 to have altered the films, compared to the natural, liquidlike state. In order to provide some data which might hear more directly on this point, a series of surface-enhanced Raman (SER) spectra were recorded for both the “natural”
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b Figure 5. STM scans of a silver MELLF showing 3-D shaded images of (a) “spheroidal” particles; hmneling current IO nA, potential 400 mV, (b) “cuboidal” particles; tunneling current 6 nA, potential 400 mV. The scanned area in both images is (96 nm)2. (Note: The 3-D images of STM scans presented here and in subsequent figures are to scale, i.e., X K Z = 1:l:l).
(liquidlike) and dried, deposited films. Representative spectra are shown in Figure 9. The spectra displayed in traces a and b, for liquidlike and deposited films, respectively, were recorded at an excitation wavelength of 457.9 nm for a MELLF prepared by using a complex15 of Fe(I1) as stabilizer. It is clear that high-quality, essentially identical spectra, with equivalent levels of surface enhancement, are being observed in both instances, suggesting that the deposition and drying procedures do not significantly alter the film surface. Trace c was recorded for the same MELLF as used for recording trace a but at an excitation wavelength of 514.5 nm. The spectra, normalized and recorded at the same excitation laser power, show a significant increase in enhancement as the excitation wavelength shifts toward the red. Analogous comparison at the same two excitation wavelengths were also made for MELLFs prepared from pyridine as stabilizer.
Discussion Spheroidal particles are the primary feature of the ‘EM images observed for both silver and gold sols and MELLFs. The major distinction between the sols and the MELLFs prepared from them is one of particle density rather than particle size or shape. This is clear, for instance, from an examination of Figure 2 for silver and of Figures 3 and 4 for gold. It should not be implied, however, that these images represent the
b Figure 6. STM image of a gold MELLF (a) 3-D shaded image; (b) 2-D grey scale image. Tunneling m m n t and potential 10 nA and 400 mV. The area scanned in both images is (96 nm)’.
unaggregated ‘basic” particles of which the sols and MELLFs are composed. The fact that some aggregation must occur in these types of systems is already clear from earlier electron microscopy studies of MELLFs by Efrima et al.? which suggest that the basic particles are some 2-3 nm across and that aggregation into larger groups and clusters can occur. Other reports’O of E M studies on silver and gold sols have described patterns of more spherical panicles for freshly prepared sols, which subsequently evolved into short strings of particles, with a maximum in the extinction spectrum characteristic of the longitudinal resonance associated with linear aggregates. Similar spectral behavior is expected for the spheroidal shapes seen in the TEM images of MELLFs and sols reported here and indeed comparison of the transmission spectra of sols and MELLFs (Figure 1) shows the development of an absorption toward the red for MELLFs and aggregated sols. As stated earlier, satisfactory transmission spectra of MELLFs can only be obtained by depositing the films onto glass slides. However, the spectra of wet or dried films are virtually identical. Comparison of the sol and MELLF spectra shows a marked increase in the extinction of the peak in the red for the MELLF, athibutable* to the larger particle density in the latter rather than to any significant change in particle aggregation, just as the TEM images suggest. The pattern of enhanced Raman scattering from MELLFs is also consistent with the TEM results. The intensity of the enhanced Raman spectra (Figure 9) from the MELLF prepared by using an Fell complex as stabilizer increases as lacis altered
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Figure 7. AFM images of (a) silver and @) gold MELLFs deposited on a glass slide. Scanned area is (775 nm)’ in both images.
from 451.9 to 514.5 nm. Analogous effects are also seen (not displayed here) for the enhanced Raman spectra recorded at the same two excitation wavelengths for MELLFs prepared with pyridine as stabilizer. These findings for both systems are as would be expected if the plasmon resonance is shifted to the red for a MELLF as compared to an unaggregated sol. They are substantially in line with the marked increases in enhancement of Raman scattering from silver and gold sols which have been observed1° as the excitation is moved to longer wavelengths. The appearance of rod-shaped particles in the case of silver sols is notable. Similar shapes have been reponed previously for sols prepared by citrate reduction.16 The shapes suggest anisotropic growth at some crystal faces such as could occur if atoms aniving at the particle surface became immobilized, hence encouraging faster growth at those faces, hut it is not easy to explain why this should be the case for silver but not for gold. Interestingly, the growth rate of colloidal particles of gold in aqueous solution is slower1’ by orders of magnitude than for silver particles.lS The interpretation of the STM images receives independent confirmation from the TEM pictures, which support the conclusions regarding size and shape of the precipitated films. The STM technique is intrinsically more sensitive than TEM to feature dimensions perpendicular to the plane of the object observed because of the shon-range exponential nature of the electron tunnel current between probe and sample. The AFM data which result from short-range interatomic forces confirm the STM results and consolidate the insight that tip contamination can reduce resolution in STM imaging. The STM and AFM results do suggest that there are fairly (nearly atomically) flat facets displayed by both the spheroid and rodlike MELLF particulates. This may be a pointer toward the mechanism of their growth. The close resemblance between the TEM images of sols and
Figure 8. AFM scans for (a) silver and (b) gold MELLFs deposited on mica.
MELLFs, aside from the obvious and expected differences in particle densities, clearly demonstrates the colloidal nature of MELLFs and supports the earlier conclusions of Efrima and Bradley and co-workers.1.2J2 Closer scrutiny of the TEM pictures shows a “web” of more transparent material (barely discemible in images such as Figure 2a) between the spheroidal particles. Very similar features, possibly a further pointer2 to the o c c m n c e of aggregation in these systems, have been noted by Bradley,lZ6for MELLFs on gel surfaces, and by Efrima2 and in that case appeared to be composed of the surfactant materials which stabilize the MELLFs. This is very likely to be so in the present instance also, where the role of “surfactant” is fulfilled by a metal complex or other stabilizer such as pyridine, which envelops the MELLF. Further aggregation of the panicles does not appear to occur once they are stabilized in the MELLF environment. This has significant practical advantages for recording SER spectra of stabilizers used in MELLF preparation, since the spectra of both deposited and “natural” (i.e., liquid) films (provided the latter have been protected from the atmosphere) remain unchanged over a significant period. Independent evidence from other spectroscopic (including Raman) and microscopic studies for the similar nature of deposited and natural (liquid) MELLFs has been provided by Bradleylzb and Efrima? In terms of chemical constitution MELLFs have features in common with colloidal microparticles synthesized in a so-called reverse micelle! A reverse micelle or water-in-oil microemulsion is a thermodynamicallystable dispersion of two immiscible liquids which are stabilized by a surfactant.19 In essence the
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Al-Obaidi et al. the method employed for Ag+ ion reduction in sol preparation may have on the properties of the MELLFs produced.
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Acknowledgment. We thank the Engineering & Physical Sciences Research Council for support (Grants GR/F 82672 and GR/H 62381) and the award of an earmarked studentship to S.J.R. We also thank E. Lesniewska and the staff of the AFM Laboratory at the University of Burgundy for providing access to their Nanoscope I11 AFM.
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References and Notes
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Wavenumber/ cm Figure 9. SER spectra recorded for a silver MELLF prepared with an Fe(II) complex15 as stabilizer: (a) liquid MELLF in situ; (b) same MELLF as in (a), after deposition onto a glass slide; excitation wavelength 457.9 nm for both; (c) fresh sample of the same MELLF as in (a), excitation wavelength 514.5 nm. Laser power as the sample is 20 mW in all cases.
system consists of water droplets surrounded by a surfactant “skin” and dispersed in an excess oil phase. In situ chemical reduction of a metal salt can be carried out in the reverse micellar medium, leading to the formation of colloidal microparticles within the surfactant-protected water droplets.20 In a somewhat analogous fashion, the MELLF system consists of small colloidal metal particles each surrounded by a layer of a stabilizing chemical species, generally in our case a metal complex. The analogies between micelles and MELLFs have been pointed out by Efrima et aLZ1 Our studies in this direction are continuing, including an investigation of the influence which
(1) Yogev, D.; Efrima, S. J . Phys. Chem. 1988, 92, 5754. (2) Efrima, S. Crit. Rev. S U Chem. ~ 1991, 1, 167. (3) Gordon, K. C.; McGarvey, J. J.; Taylor, K. P. J . Phys. Chem. 1989, 93, 6814. (4)Bell, S. E. J.; McGarvey, J. J.; Rigby, S. J.; Walmsley, D. G. J . Raman Spectrosc. 1991, 22, 763. (5) Rigby, S. J.; McGarvey, J. J., to be published. (6) Cotton, T. M. In Advances in Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley: New York, 1988; Vol. 15, p 91. (7) Al-Obaidi, A. H. R.; Rigby, S. J.; Bell, S. E. J.; McGarvey, J. J. J. Phys. Chem. 1992, 96, 10960. (8) Lisiecki, I.; Pileni, M. P. J . Am. Chem. SOC.1993, 115, 3887. (9) Fendler, J. H. Chem. Rev. 1987, 87, 877. (10) Blatchford, C. G.;Campbell, J. R.; Creighton, J. A. S u 6 Sci. 1982, 120, 435. (11) Duff, D. G.; Curtis, A. C.; Edwards, P. P.; Jefferson, D. A,; Johnson, B. F. G.; Kirkland, A. I.; Logan, D. E. Angew. Chem. Int. Ed. Engl. 1987, 26, 676. (12) (a) Yogev, D.; Deutsch, M.; Efrima, S. J. Phys. Chem. 1989, 93, 4174. (b) Krech, J.; Lorenc, C.; Bradley, M. J . Colloid Interface Sci. 1992, 153, 437. (13) Lee, P. C.; Meisel, D. J . Phys. Chem. 1982, 86, 3391. (14) Smith, K. W. Ph.D. Thesis, Queen’s University of Belfast, 1993. (15) AI-Obaidi, A. H. R.; McGarvey, J. J.; Taylor, K. P.; Bell, S. E. J.; Jensen, K. B.; Toftlund, H. J . Chem. Soc., Chem. Commun. 1993, 536. (16) Yaping, X.; Yongxi, Z. Anal. Chim. Acta 1989, 225, 227. (17) Kurihara, K.; Kizling, J.; Stenins, P.; Fendler, J. H. J. Am. Chem. SOC.1983, 105, 2574. (18) Tausch-Treml, R.; Henglein, A. J . Colloid Interface Sci. 1981, 80, 84. (19) Robinson, B. H. R.; Khan-Lodhi, A. N.; Towey, T. In Structure and Reactivity in Reverse Micelles; Pileni, M. P., Ed.; Elsevier: Amsterdam, 1989; p 198. (20) Barnickel, P.; Wokaun, A.; Sager, W.; Heicke, H.-F. J . Colloid Interface Sci. 1992, 148, 80. (21) Yogev, D.; Rostier-Edelstein, D.; Efrima, S. J . Colloid Interface Sci. 1991, 147, 78.
