Langmuir 1999, 15, 1929-1937
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Direct Synthesis and Characterization of Gold and Other Noble Metal Nanodispersions in Sol-Gel-Derived Organically Modified Silicates S. Bharathi, N. Fishelson, and O. Lev* Fredy and Nadine Herrmann School of Applied Science, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received April 28, 1998. In Final Form: November 16, 1998 A general approach to prepare nanodispersions of noble metals in organically modified silicates is presented with particular emphasis on the synthesis of gold nanodispersion and its characterization. Organically modified sol-gel monomers containing amine functional groups are used to stabilize the metal salts before the reduction step and to cap and prevent coagulation of the metal sol after reduction and through gel formation, drying, and aging. Uniform spherical metal nanodispersions of Au, Pt, and Pd with average diameter of 4-6 nm were obtained. The particle size distribution and the average size of silver nanoparticles prepared by the same method were considerably larger. This was attributed to the lower stability of the silver-amine complex and to the lower affinity of amines to silver surfaces. Stable aqueous colloidal solutions and supported metal nanodispersions in porous films and monoliths can be prepared by this route.
Introduction Nanosized dispersions of metals and semiconductors are of current interest because of their size-dependent electrical, chemical, and optical properties.1-5 These materials are promising for practical applications and interesting from the scientific point of view, because the gradual evolution of material properties from molecular level to the solid state can be probed by a change of single parameter. Stabilization of the nanoparticles in silicatebased matrixes provides a promising way to benefit from the advantages of nanocrystalline dispersions for optical, catalytic, and electrochemical applications and still enjoy the convenience of solid handling. The optical properties of the nanocrystalline dispersant are only slightly affected by the transparent encapsulating matrix (e.g., silica or zirconia), and for catalytic and electrocatalytic purposes, fast mass exchange with the surroundings can still be maintained through the porous support. Metallic nanodispersions can be prepared in aqueous and organic solvents using diverse procedures.6-14 Nanodispersions can be stabilized in organic solvents by the (1) Siegel, R. Nanostruct. Mater. 1993, 3, 1. (2) Henglein, A. J. Phys. Chem. 1993, 97, 5457. Hoffman, A. J.; Mills, G.; Yee, H.; Hoffman, M. R. J. Phys. Chem. 1992, 96, 5546. (3) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525. (4) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (5) Akbarain, F.; Dunn, B. S.; Zink, J. I. J. Raman Spectrosc. 1996, 27, 775. (6) Megura, K.; Nakamura, Y.; Hayashi, Y.; Torizuka, M.; Esumi, K. Bull. Chem. Soc. Jpn. 1988, 61, 347. Ishizuki, N.; Torigoe, K.; Esumi, K.; Meguro, K. Colloids Surf. 1991, 55, 15. (7) Cheong Chan, Y. N.; Schrock, R. R.; Cohen, E. R. Chem. Mater, 1992, 4, 24. (8) Turkevich, J. J. Gold. Bull. 1985, 18, 86. (9) Schmid, G. Chem. Rev. 1992, 92, 1709. (10) Torigoe, K.; Esumi, K. Langmuir 1992, 8, 59. (11) Esumi, K.; Shiratori, M.; Ishizuka, M.; Tano, T.; Torigoe, K.; Meguro, K. Langmuir 1991, 7, 457. (12) Toshima, N.; Takahashi, T. Chem. Lett. 1988, 573. (13) Schmid, G.; Lehnert, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 780. (14) (a) Zimmermann, F.; Wokann, A. Mol. Phys. 1990, 73, 959. (b) Fink, J.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. Chem. Mater. 1998, 10, 922.
solvent itself (e.g., naked gold sols in toluene),14b by the addition of long chain cationic surfactants (e.g., quaternary ammonium compounds),10-12 or by specific ligands (e.g. sulfonated phosphines).13,14 Stabilization of metal nanodispersions in aqueous solutions is somewhat more complicated, but still several successful stabilization methods are based on capping by buffer electrolytes (e.g., citrate) 8,9 or polyelectrolytes (e.g., poly(vinylpyrrolidine)).6,7 Removal of the stabilizing agents induces immediate destabilization of the sol. To encapsulate gold and other noble metal nanodispersions in sol-gel-derived silicates it is necessary to avoid aggregation and precipitation of the metal salt in the organic sol-gel precursors and to prevent coalescence or precipitation of the metallic particles in aqueous solutions and during the various stages of sol-gel polymerization, drying, and aging. Several general schemes were reported for the encapsulation of metallic nanoparticles in sol-gel matrixes. Sequential reduction of gold salt and sol-gel processing in inverse micelle solutions15 were used for the preparation of metal-silicate nanodispersions. However, the presence of gel precursors destabilizes the inverse micelles resulting in bigger particles and wider particle size distribution.15 Mulvaney and co-workers16 synthesized gold nanoparticles coated by thin silicate films by a twostep process. First, a gold sol was prepared by the citrate route, and subsequently, the sol was coated by (3aminopropyl)trimethoxysilane-derived aminosilicate. Using a related procedure, Buining et al. 17 reported synthesis of gold nanodispersions stabilized by mercaptosilane. The gold sol was stable for a limited range of S:Au ratio, and the particle size varied with the molar ratio of the silane to gold. Schmidt and co-workers18,19 reported gold nano(15) Martino, A.; Yamanaka, S. A.; Kawola, J. S.; Loy, D. L. Chem. Mater. 1997, 9, 423. (16) Liz-Marzan, M. L.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (17) Buining, P. A.; Humbel, B. M.; Philiose, A. P.; Verkleji, A. J. Langmuir 1997, 13, 3921. (18) Kutsch, B.; Lyon, O.; Schmitt, M.; Mennig, M.; Schmidt, H. J. Non-Cryst. Solids 1997, 217, 143. (19) Kutsch, B.; Lyon, O.; Schmitt, M.; Mennig, M.; Schmidt, H. J. Appl. Crystallogr. 1997, 30, 948.
10.1021/la980490x CCC: $18.00 © 1999 American Chemical Society Published on Web 02/19/1999
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dispersion in glycidoxypropylsilane-aminosilane copolymer using thermal reduction of gold chloride in alcohol solutions. Up to 80% reduction of the gold chloride was achieved by appropriate thermal treatment. Several other methods to prepare highly dispersed gold particles in porous silica supports were devised by circumventing the gold sol stage. Hydrogen reduction or thermal decomposition of supported metal oxides or salts that were previously deposited on the support is often used.20,21 The method is simple to perform but gives a rather wide particle size distribution. Kuzuka and Sakka22 have used acid-catalyzed hydrolysis of tetraethoxysilane, TEOS solutions containing HAuCl4 without additional stabilizing agents. Films were deposited on a glass slide from this sol and fired at 500 °C to obtain nanocrystalline gold dispersion in silicate matrix. The particle size distribution was dependent on H2O:TEOS ratio. Treatment of the gels with monoethanolamine prevented the escape of gold particles from the gel during heat treatment and reduced the average particle size. Schubert and coworkers23 developed a general methodology for the preparation of supported nanodispersions in sol-gel-derived ceramics. The metal ions are first chelated by organofunctional silane groups. After sol-gel polymerization the gel is oxidized at elevated temperature giving a dispersion of metal oxide grains in porous ceramic xerogel. At a final stage the metal oxide is reduced by a chemical treatment (e.g., hydrogen reduction at elevated temperature) giving highly homogeneous nanodispersions in sol-gel materials. We are interested in the catalytic and electrochemical activity of highly dispersed metals in silicates, and for this we were looking for a simple and robust method to produce monodispersed metal particles. For practical reasons it is highly desirable that a single component will stabilize both the metal ions in the organic solvent as well as the metal nanoparticles throughout sol-gel polymerization. Additional prerequisites were that the aqueous sol will be stable for at least several weeks and there will be no leaching of organic moieties or chelating agents from the supported catalysts. It was also desirable that the resulting films will be stable during the aging stage and that the configuration of nanodispersion will be indifferent to minor changes in the composition of the starting solution. This is necessary in order to provide common denominator for comparative performance evaluation of different sol-gel-derived electro- and biocatalysts.24 Additionally, to prevent degradation of heat sensitive compounds and biochemicals it was desirable to employ only moderate temperatures throughout sol formation and xerogel synthesis.25,26 Sol-gel monomers containing amine functional groups can exhibit dual functionality. The amines can complex the metal ions and cap the metal sol. Amines have high affinity toward noble metals, and they are often used to anchor metallic colloids onto different substrates that are precoated with aminosilanes.27-29 Alkylamines form stable monolayers on bare gold substrates in the vapor phase,30 (20) Ueno, A.; Suzuki, H.; Kotera, Y. J. Chem. Soc., Faraday Trans. 1 1983, 79, 127. (21) Lopez, T.; Bosch, P.; Moran, M.; Gomez, R. J. Phys. Chem. 1993, 97, 1671. (22) Kozuka, H.; Sakka, S. Chem. Mater. 1993, 5, 222. (23) Breitscheidel, B.; Zieder, J.; Schubert, U. Chem. Mater. 1991, 3, 559. Schubert, U.; Breitscheidel, B.; Buhler, H.; Egger, C.; Urbaniak, W. Mater. Res. Soc. Symp. Proc. 1992, 271, 621. Morke, W.; Lamber, R.; Schubert, U.; Brietscheidel, B. Chem. Mater. 1994, 6, 1659. (24) Bharathi, S.; Lev, O. Anal. Commun. 1998, 35, 29. (25) Lev, O.; Wu, Z.; Bharathi, S.; Modestov, A.; Glezer, V.; Gun, J.; Rabinovich, L.; Sampath, S. Chem. Mater. 1997, 9, 2354. (26) Avnir, D.; Braun. S.; Lev, O.; Ottolenghi, M. Chem. Mater. 1994, 6, 1605.
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though monolayers that were obtained from solvents were rather unstable.31 Furthermore, Leff and co-workers32 demonstrated preparation of alkylamine-capped gold nanoparticles in a way that is similar to the well-accepted preparation of thiol-capped gold particles.33,34 These sols were, however, stable only in hydrophobic solvents. Brief reports on the direct synthesis of gold nanodispersions in silicate sols35 and their potential bioanalytical24 applications were recently communicated. This paper describes the detailed preparation and characterization of the gold nanodispersions, and it further demonstrates that the method is generic and can be applied for the preparation of other metallic monodispersions including platinum and palladium and for the preparation of wide size distribution silver nanodispersions as well. Experimental Section Chemicals. N-[3-(trimethoxysilyl)propyl]ethylenediamine (EDAS), (aminopropyl)trimethoxysilane (APS), N-((trimethoxysilyl)propyl)-N,N,N-trimethylammonium chloride, and tetraethoxysilane (TEOS) were obtained from Aldrich. Hydrogen tetrachloroaurate trihydrate, hexachloroplatinate, palladium nitrate, silver nitrate, sodium borohydride, and all other chemicals used in this work were of analytical grade. Triply distilled water was used. Apparatus. UV-visible spectra of the different sols were recorded using a Cary E1 Varian double-beam UV-visible spectrophotometer. FT-IR spectra were recorded in the region between 600 and 4000 cm-1 using a Bruker FT-IR spectrophotometer. Analysis was performed on thin films formed by evaporation of several drops of the reduced sol on a AgCl window. Transmission electron microscope (TEM) images of the different sols were obtained using a JEOL electron microscope at 80 keV. Samples were prepared by evaporating a drop of the sol on an amorphous carbon-coated copper TEM grid. Considerable solgel polymerization is expected during the evaporation stage. So, interparticle distances in the TEM images do not really reflect the interparticle distance in the sol, and unfortunately, there is no reason to believe that they reflect the structure of dense thick films and monoliths either. ζ potential measurements were conducted on a Malvern pore size analyzer. Electrophoretic mobility was determined by an Ortec 4100-EG&G gel electrophoretic unit on an agarose layer. Preparation of Au-Silicate Sol. Typically, 10 mL of sol with a molar ratio of 100:1 (EDAS:HAuCl4) was prepared by dissolving 1 mL of 1 M EDAS solution in 7 mL of methanol (or water) and 1 mL of 0.01 M HAuCl4. A clear yellow solution was obtained and sonicated for 10 min. The hydrolysis and condensation were then initiated by addition of 0.05-0.1 mL of 0.1 M HCl and ca. 1.0 mL of water to the homogeneous solution. The solution was then reduced using 0.05 mL of freshly prepared 5% NaBH4. The same protocol was used to prepare sols with different EDAS:HAuCl4 ratios. Complete reduction of HAuCl4 was verified by the absence of an absorbance peak at ca. 310 nm corresponding to gold ions.36 Sols were also prepared with different ratios of (27) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (28) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J.; Langmuir 1996, 12, 2353. (29) Musick, M. D.; Keating, C. D.; Keefe, M. H.; Natan, M. J. Chem. Mater. 1997, 9, 1499. (30) Xu, C.; Sun, L.; Kepley, L. J.; Crooks, R. M. Anal. Chem. 1993, 65, 2102. (31) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc., 1989, 111, 7155. (32) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (33) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (34) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J.; Adv. Mater. 1995, 7, 795. (35) Bharathi, S.; Lev, O. J. Chem. Soc., Chem. Commun. 1997, 2303. (36) Matsuoka, J.; Mizutani, R.; Kaneko, S.; Nasu, H.; Kamiya, K.; Kadono, K.; Sakauchi, T.; Miya, M. J. Ceram. Soc. Jpn. 1993, 101, 53.
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Langmuir, Vol. 15, No. 6, 1999 1931
Table 1. Characteristic Composition of Gold, Platinum, Palladium, and Silver Sols Prepared with Different Precursors in Aqueous Solution no.
composn (mM)
pH
stability and appearance
1 2 3
2-12 7 7
4 5
TEOS:Au ) 10:1 RNH2:Au ) 10:1 (C2H5O)SiRNMe3+: Au ) 100:1 EDAS:Au ) 0.1:1 EDAS:Au ) 1: 0.1
6 7 8 9 10
EDAS:Au ) 3:1 EDAS:Au ) 10:1 EDAS:Au ) 30:1 EDAS:Au ) 30:1 EDAS:Au ) 100:1
7 7 2 5-12 7
black metallic aggregates metallic aggregates wine red color sol, stable for 1 day metallic aggregates wine red sol as well as metallic aggregates stable wine red sol wine red color reddish color wine red color deep wine red color
11 12 13
APS:Au ) 0.1:1 APS:Au ) 10:1 APS:Au ) 1000:1
2-12 7 7
black metallic aggregates wine red color deep wine red color
14 15 16
EDAS:Pt ) 100:1 EDAS:Pd ) 100:1 EDAS:Ag )100:1
7 7 7
brown sol brown sol deep yellow sol
2-12 7
aminosilane-TEOS monomers by adding required amount of TEOS along with the aminosilane precursors. Gold sols at different pH, viz., 2, 5, 7, 9.5, and 12, were prepared by adjusting the pH of the sol by dropwise addition of concentrated HCl, and the metal was then added and reduced by borohydride. Solutions were sonicated for 5-10 min between each step to ensure homogeneity. Care should be however taken not to add buffer phosphate to the aminosilane solution in order to avoid precipitation. In cases that buffer phosphate is required-e.g., for biocatalytic applications-the phosphate salts can be safely added after the reduction step. Apparently, the surface-bound amines have higher affinity to the gold surfaces and they do not protonate and complex the phosphates. Preparation of Platinum, Palladium, and Silver Sols. The method used to prepare other noble metal colloids was similar to that of the gold sols. Typically, a 100:1 NH2:Me sol was prepared by dissolving 1 mL of 1 M EDAS or APS in 7 mL of methanol, followed by the addition of 1 mL of 0.01 M H2PtCl6, Pd(NO3)2, or AgNO3, and the solution was then sonicated for 10-15 min. A volume of 0.05-0.1 mL of 0.1 M HCl (except for the silver nitrate sol) and an appropriate amount of water were added to this clear and transparent solution which was sonicated again for 10 min. The contents were reduced by the addition of 0.05 mL of 5% NaBH4. Preparation of Nanodispersed Films and Gels. All noble metal sols could be gelled to give monoliths of various shapes by allowing them to gel in a container of desired shape. Gelation time followed the general trends for aminosilicates and will not be discussed here. The 1-10 µm thick films were also prepared by repeated dip coating or spin coating of microscope glass slides.
Results Synthesis and Stability of Gold and Other Noble Metal Nanodispersions in Silicate Sols and Films. The procedure described herein is generic for all the noble metals addressed in this report. Though, for simplicity, we focus on the preparation of gold sols. Sols with different compositions were prepared in order to elucidate the stability region and to understand the mechanism of sol stabilization. Table 1 presents details of the stability and other characteristics of exemplary sols of different compositions. When HAuCl4 is added to the silane precursors, a clear yellow colored solution results. The intensity of this yellow color increases with increasing silane:Au ratio, due to formation of mixed amine, chloride complexes. Upon addition of NaBH4, a gold sol with wine red color appears immediately for sols with large EDAS:Au ratio. For low EDAS:Au molar ratio ( 4 months stability > 4 months aggregates obsd by TEM
536 515
4-6 4-6
450
5-7 5-7 2-20
530 534
10% rodlike particles with aspect ratio 2-3 stable for 3 weeks 10% rodlike particles with aspect ratio 2-3 wide particle size distribution
Figure 1. Phase diagram of HAuCl4, EDAS, and H2O based on weight fraction (pH 7). Shaded area depicts the stable sol region.
particles appear after reduction. The triangular phase diagram in Figure 1 depicts the region of stable sols at pH 7. It can be noted that stable sols containing up to 40 g/L HAuCl4‚3H2O (corresponding to 19.6 g/L gold) could be prepared. For comparison, conventional Au(CN)2- electroplating baths contain approximately 8.3 g/L gold.37 The stability of the sols was very good, and no visible aggregation was noticed in several months for sols that were prepared with high EDAS:Au ratio (as per Figure 1). Between the two silanes studied, the long-term stability of the EDAS sols was much better than the APS sols, where often a black precipitate was observed after 2-3 week storage. Change of the Si:H2O ratio only affected the gelation time and did not influence the nanoparticle size or the stability of the sol. Changing the pH of 100:1 EDAS:Au precursorssin the pH range 2-12shad no significant effect on the visual appearance of the sol or its stability, though other characteristics such as the UVvis spectra and the average particle size were altered. Repeating the preparation procedure with TEOS instead of EDAS resulted in precipitation of dark-brown gold particles immediately after the addition of NaBH4. Mixed TEOSsEDAS precursors could stabilize the gold sols (see Table 1), but we did not map the stability domain of these sols. The stability of the other 100:1 EDAS-metal sols (37) Visco, R. E. In Modern Electroplating, 3rd ed.; 1974. Lowenheim, F. A., Ed.; Wiley-Interscience Pub.: New York, 1974: p 224.
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Figure 3. Particle size distributions for EDAS:Au 100:1, EDAS: Pt 100:1, EDAS:Pd 100:1, and EDAS:Ag 100:1 sols, calculated from TEM micrographs.
Figure 2. TEM micrographs of EDAS-Au sols with different EDAS:Au molar ratio: (a) EDAS:Au ) 10:1; (b) EDAS:Au ) 100:1; (c) EDAS:Au ) 1000:1. Bar ) 10 nm (a, b) and 8 nm (c).
was also good, and only in the case of the silver metals we observed some brown-black precipitate after 2 weeks, though the sol remained clear. Transmission Electron Microscopy. Gold. Figure 2a-c shows typical TEM micrographs of EDAS-gold sols with molar ratios of 10:1, 100:1, and 1000:1. The sols constitute predominantly of spherical particles with rather narrow size distributionsaround 5-6 nm. Figure 3 presents the particle size distribution based on the TEM images for 100:1 EDAS-Au sols (pH 7). The particles size histograms of the sols with molar ratio 10:1 and 100:1 were similar. Further, the particle size was not (measurably) affected either by the ratio of Si:Au or by the aminosilane precursor (APS vs EDAS). The spacing between the particles is of the order of 2-3 nm. The size of two (aminopropyl)silane molecules is about 2.5 nm, which again supports the view of amine groups stabilizing the Au colloids. At higher amine:Au ratio (>300:1), the presence of considerable amounts of nonspherical particles with aspect ratio ranging between 1 and 3 was also noticed (Figure 2c). The rodlike particles seem to be formed by coalescence of two to three spherical particles, but
unexpectedly their formation was induced by higher N:Au ratio and not by higher Au concentrations. For example, rod shaped grains were formed even for EDAS:Au ) 1:0.001 M and they were absent from the EDAS:Au ) 0.1:0.1 M composition. Figure 4a-e shows the TEM images of gold-EDAS sols (30:1) at different pHs ranging between 2 and 12, and Figure 5 depicts the average particle size as a function of pH. The average particle size remained almost the same, ca. 5-6 nm, except for pH 2 where the average particle size was much larger (ca. 14 nm) due to the apparent aggregation of the metal particles (Figure 4a). Small increase in the average particle size can be observed for pH 5-12 (Figure 5). One can also notice that for pH 5 a bimodal size distribution (particles of about 1 and 5 nm) is observed in the TEM image (Figure 4b). The average sizes of the two populations are also shown in Figure 5. Palladium, Platinum and Silver Nanodispersions. Figure 6a-c shows TEM micrographs of Pt, Pd, and Ag nanodispersions in the silicate matrix for 100:1 EDASmetal sols. In all cases, electron diffraction confirmed the crystalline nature of the metal clusters. The characteristics and particle size distribution of Pd and Pt are almost identical to the gold nanodispersion. The average particle size distribution of these colloids was 5-7 nm, which is almost identical to that of gold. In the case of Ag, the particle size varied between 2 and 20 nm. Figure 3 gives the particle size distributions for Pd, Pt, and Ag. The similarity of the particle size distribution of the Pd, Pt, and Au nanodispersions is striking. It is as if the type of noble metal had no effect on the stabilization phenomenon. UV-Visible Absorption Spectra. Gold. The absorbance spectra of EDAS-Au and APS-Au sols with different molar ratios are presented in Figures 7 and 8, respectively. The gold concentration was fixed at 1 mM. The absorbance maxima of the two sols were found to be similar for the given molar ratio. Further, the absence of an absorbance peak around 320 nm characteristic of Au(II) species (not shown in the figure) indicates complete reduction of gold. The absorbance maximum of EDAS sol was shifted from 540 to 514 nm as the Si:Au ratio was increased from 5:1 to 100:1. A similar trend was observed for APS as well. The surface plasmon resonance should shift slightly to higher energies and should broaden somewhat as particle size is decreased.38,39 The surface plasmon absorbance of colloidal gold particles is also affected by the electronic or dielectric properties of the interface region, and thus, it is impossible to determine the average particle size on the basis of the optical
Gold and Other Noble Metal Nanodispersions
Langmuir, Vol. 15, No. 6, 1999 1933
Figure 4. TEM micrographs of a EDAS-gold deposits from a sol containing EDAS:Au ) 30:1 and Au ) 1 mM at pH (a) 2, (b) 5, (c) 7, (d) 9.5, and (e) 12. Bar ) 8 nm (a, b) and 6 nm (c-e).
Figure 5. Average metal nanoparticle size as a function of pH for EDAS-gold sol (EDAS:Au ) 30:1, 1 mM Au) (bars represent the standard deviation).
characteristics alone.38,39 However, the TEM images have shown that particle size distribution remained almost the (38) Kerker, M. The Scattering of Light, and Other Electromagnetic Radiations; Academic Press: New York, 1969.
same (5-6 nm). Hence, the small absorbance shift observed in the electronic spectra is probably caused by changes in the colloid interface by alteration of the refractive index of the sol33,34 or it can be attributed to the rather crude way by which we derived the particle size distribution from the TEM pictures. The stability of the sols was further examined by recording absorbance spectra at different time intervals. Figures 7c and 8c show the absorbance spectra recorded after 1 month for EDAS and APS sols, respectively. The spectra were similar to the ones recorded immediately after the preparation of the sol, indicating the absence of coarsening of the sol. Despite of the fact that a precipitate was observed for the APS sols after 2 weeks, the clear sol had the same absorbance spectrum as the initial one, though the peak intensity was obviously somewhat diminished. The pH dependence of the absorbance spectra for 30:1 EDAS-Au sol containing 1 mM Au is depicted in Figure 9. The curves correspond to the TEM micrographs (39) Mulvaney, P.; Underwood, S. Langmuir 1994, 10, 3427.
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Figure 8. Visible spectra of APS-stabilized gold sols with different molar ratio (a) APS:Au ratio 5:1 and (b) 100:1. (c) Sol of (b) after 1 month with Au concentration 1 mM.
Figure 9. Visible spectra of EDAS-stabilized gold sols at different pH (a) 2, (b) 5, (c) 7, (d) 9.5, and (e) 12 (EDAS:Au ) 30:1 and Au ) 1 mM).
Figure 7. Visible spectra of EDAS stabilized gold sols with different molar ratios (a) EDAS:Au ) 5:1 and (b) 100:1. (c) Sol of (a) after 1 month. (d) Thin film deposited from the sol of (a).
addition, the absorption peak for pH 2 is somewhat narrower. These observations agree well with the TEM observations, i.e., redshifting of the absorption maxima and sharpening of the plasmon peak which accompany the increased particle size at pH 2.38,39 A typical spectrum of a thin aminosilicate film containing gold dispersion (from 100:1 EDAS-Au sol) is depicted in Figure 7d. The resemblance between Figure 7b,d spectra points on the relative stability of the dispersion during sol-gel processing. Monoliths obtained by allowing the sol to gel at room temperature were also of wine red color and transparent. There was no significant change in the absorbance maxima of the monoliths from that observed in the sol stage, indicating that there was no coalescence of the gold nanoparticles during gelation and drying. Addition of TEOS decreased the gelation time drastically but did not change the absorbance maxima. Monoliths and films were heated to 500 °C with no change in their absorbance maxima. Pd, Pt, and Ag. The gold, silver, platinum and palladium metal nanopdispersions exhibit characteristic surface plasmon spectra. The absorbance spectra of gold and silver were characterized by surface plasmon resonance at ca. 540 and 450 nm, respectively. The absorption peak of the Pt dispersion was very broad ranging between 400 and 800 nm. The absorption spectrum of the Pd dispersion showed a continuous rise toward higher energies, which is attributed to light scattering from Pd nanoparticles. The absorption maxima obtained in this work compare well with reported spectra for the metal sols.40-42
presented in Figure 4. The absorption maxima of all the sols remained similar at around 513 nm except for the pH 2 sols where particle aggregation (Figure 4a) is manifested in a shift of the absorbance peak to around 525 nm. In
(40) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Fararaday Trans. 1979, 21, 790. (41) Teranishi, T.; Hosoe, M.; Miyake, M.; Adv. Mater. 1997, 9, 65. (42) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881.
Figure 6. TEM micrographs of Pt (a), Pd (b), and Ag (c) nanoparticles prepared as described in the Experimental Section with EDAS:metals ) 100:1. Bars in plates a and b equal 10 nm. The bar of plate c equals 50 nm.
Gold and Other Noble Metal Nanodispersions
Langmuir, Vol. 15, No. 6, 1999 1935 Table 2. Tentative Assignments of FT-IR Bands (in cm-1) of EDAS, APS, EDAS-Au, and APS-Au Sols17,43-47 EDAS
EDAS-Au sol
APS
3380 3296 2939 2840 1600 1460
3334.3 3256.3 2929
3380 3296 2941 2841 1600 1465
1578 1471 1114
1085
Figure 11. FT-IR spectra of neat (a) APS and (b) APS-Au gold sol (APS:Au ) 100:1). Curve a is arbitrarily shifted upward.
ζ Potential Measurements. ζ potential measurements of the 1:100 Au-aminosilane sols were conducted using a Malvern zetameter. The EDAS and APS derived sols gave similar, very low potential readings (-0.5, -2.6, -4.3) and (-0.7, -2.3, -4.6) for pH 4.5, 7, and 10, respectively. The small absolute ζ potential values are attributed to the small size of the particles and the high ionic strength that compress the double layer. The negative sign of the ζ potentials supports the view that the particles are stabilized by negatively charged silanols and that the amine groups are oriented toward the gold nanoparticles. Thus, protonation of the amine is prohibited and the particles remain negatively charged even at relatively low pH (ca. 4). In contradistinction, the point of zero charge (pzc) of aminosilane films lies above pH 7 due to the positive charges of the ammonium groups. These conclusions are also supported by agarose electrophoresis tests which show that the aminosilicate-coated gold nanoparticles exhibited more negative electrophoretic mobility as compared to aminosilane sols at the same pH. In fact, the stabilized gold particles were always (down to pH 2) negatively charged while the pzc of the aminosilane sol was around pH 4.5. The low pzc of the aminosilane so compared to reported pzc of films is probably due to lower degree of condensation and, thus, larger concentration of silanols. FT-IR Spectroscopy. To understand the structure and orientation of the gold-silicate sols, FT-IR spectra of EDAS and APS stabilized sols and unhydrolyzed EDAS and APS were recorded and presented in Figures 10 and 11, respectively. It should be noted that the FTIR spectra reflect gelled material that has undergone considerable
3370 3270 2932 1589.2 1471.5 1115
1084 937.5
Figure 10. FT-IR spectra of (a) EDAS and (b) EDAS-Au gold sol (EDAS:Au ) 100:1). Curve a is arbitrarily shifted upward.
APS-Au sol
938.7
assgnt νNH2 νNH νCH2 νCH3 of Si-OCH3 δNH2 δCH2 νSi-O-Si νC-O νSi-OH
degree of polymerization during sample preparation. Thus, these data give a rather biased picture of the sol state. Table 2 gives the observed bands and the proposed peak assignments. The assignments are by and large based on a previous work of Buining et al.17 and refs 43-47. The following are the salient features: (1) The absence or presence of only very weak bands at 2960, 2880 and 1190, 1087 cm-1 in Figures 10b and 11b characteristic of methoxy groups confirmed that the hydrolysis is nearly complete. The amount of unhydrolyzed methoxy group is negligibly small. Silicates that are produced at pH 7 usually have a large remaining concentration of unhydrolyzed methoxy groups. This was not observed for the EDAS and APS samples probably due to the catalysis of the hydrolysis step by the amine groups. The presence of predominant bands at 1117 and 1003 cm-1 characteristic of a siloxane group suggests condensation of hydrolyzed methoxy group and polymer formation. The very low intensity of Si-O stretching mode at 937 cm-1 suggests that the remaining unreacted surface silanol group is not large. (2) The band near 1600 cm-1 related to primary amine groups in the neat EDAS shifts to 1579 cm-1 with concomitant reduction in the intensity and band-broadening. Further the N-H stretching band at 3263 and 3282 cm-1 shift to 3257 (EDAS sol) and 3270 cm-1 (APS sol) respectively with concomitant reduction of its intensity. This reduction and band broadening, can be attributed to the intramolecular hydrogen bonding with the hydrogen of the silanol groups or to the adsorption of amine groups to gold colloids. Similar reductions in the intensity and band broadening of the N-H bands were observed for amine-capped gold colloids.32 (3) The high intensity of the siloxane bands (1115 cm-1) along with the very weak Si-OH at 938 cm-1 suggests that the polymerization is extensive and a three-dimensional network is formed. Buining and co-workers17 demonstrated that aminosilane coating of mercaptosilanemodified gold nanocrystals exhibited sharp FT-IR bands at 1113 cm-1, very similar to that of linear dimethylsiloxane. The authors attributed these sharp peaks to the formation of linear networks. On the basis of Buining’s consideration, the gold nanodispersion in the silicate (43) Lin-Vein, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The hand book of infrared and Raman characteristic frequencies of organic molecules; Academic Press Ltd.: London, 1991. Vansant, E. F.; Van Der Voort, P.; Vrancket, K. C. Characterisation and Chemical Modification of the Silica Surface. Studies in Surface Science and Catalysis Vol. 93; Elsevier: Amsterdam, 1995. Plueddemann, E. P. Silane Coupling Agents; Plenum Press: New York, 1991. (44) Pouchert, C. J. The Aldrich Library of FT-IR spectra; Aldrich Chemical Co., Inc.: Milwaukee, WI, 1985. (45) Vanderberg, E. T.; Bertilsson, L.; Liedberg, B.; Uvdal, K.; Erlandsson, R.; Elwing, H.; Lundstrom, I. J. Colloid Interface Sci. 1991, 147, 103. (46) Furukawa, T.; Eib, N. K.; Mittal, K. L.; Anderson, H. R., Jr. J. Colloid Interface Sci. 1983, 96, 322. (47) Boerio, F. J.; Armogan, L.; Cheng, S. Y. J. Colloid interface Sci. 1980, 73, 416.
1936 Langmuir, Vol. 15, No. 6, 1999
matrix can be visualized as a three-dimensional network of organically modified silicates. We do not know how many such nanoparticles are tied together in the sol stage since the electronic spectra reflect only the configuration of the metal phase.
Bharathi et al. Chart 1. Aminosilicate-Stabilized Gold Particles
Discussion Two related observations should be reemphasized before explaining the results that are presented here. AuCl4-TEOS mixtures are immediately destabilized upon reduction with BH4-, and so do aqueous propylamine-AuCl4sols (Table 1). These observations bring about the dual role played by the aminosilane in the stabilization of the nanoparticles in aqueous solutions. Only the combination of both the Si-OH and the -NH2 groups can stabilize the metal particles, while the presence of only one of these functional groups is not sufficient to accomplish this task. We postulate that the aminosilanes complex the Au(III), forming the respective amine-metal complexes or mixed chloride, amine-gold complexes, depending on the chloride and amine concentrations. The amine-gold bond is very stable (the reported stability constant of Pt(ethylenediamine)22+ is 1036.5, and the EDAS-Au bond should be even more stable48). So, the stability of this complex slows down the rate of reduction of the ion metals except for the silver amine complexes that are labile and are known to be easily reduced.49 Upon reduction, the amine groups immediately cap the surface of the nucleating gold moieties. Although for pH < ca. 10.5 the amines are present in the solution in their protonated ammonium form, the affinity to the metal is sufficient to deprotonate the ammonium compound and to induce adsorption even at low pH. The decrease in the bare surface of the metal particles due to the amine capping slows down the rate of particle growth even further. Previous reports on the formation of alkylamine self-assembled monolayers in aqueous solutions show that the amine-metal affinity is low or that the lability of the gold-amine bond is rather high.30 However, the N-Au affinity is apparently sufficient to increase the local concentration of the aminosilane groups on the gold surface and to induce condensation and cross-linking of adjacent silanols or methoxy groups, thus creating stable silicate shells. The particles are stabilized in aqueous solutions by the negative charges endowed by the silanol groups of the silicate shell (Chart 1). The model presented in Chart 1 is supported by the fact that the gold-EDAS sol is negatively charged even at pH 2, while the pKa of aminosilicate films lies around pH 7. Amine groups adjacent to the metal do not protonate and thus do not contribute positive charge. This picture is commensurate with the view presented by Leff et al.32 that attributed the formation of monodispersion of alkylmercaptan-stabilized gold particles to thermodynamic stability, while the alkylamine stabilization of gold particles was considered to be purely kinetic. The different stabilization mechanisms were attributed to the larger affinity of the thiols to the gold surfaces as compared to the amines. This simple model explains why the alkylamines (for lack of negative charge barrier) and the tetraethoxysilanes (for lack of affinity to the metal particles) cannot stabilize the gold nanoparticles by themselves. This mechanism is also commensurate with the fact that the alkylamine can stabilize the gold nanoparticles in apolar solvents as (48) Smith, R. M.; Martell, A. E. Critical Stability Constants, Volume 2: Amines; Plenum Press: New York, 1975. (49) Kortum, G. Treatise on Electrochemistry; Elsevier Pub. Co.: Amsterdam, 1965; p 497.
reported by Leff et al.32 It also explains the relative lack of sensitivity to the Au:aminosilane ratio. The stability constant of the gold-amine is sufficient to compensate for changes in pH, and virtually all the surface of the gold nanoparticles is covered by aminosilanes when sufficient aminosilane is initially present in the solution. For aminosilane:Au ratios . 1 virtually all the gold ions and gold surface atoms are complexed. When the ratio of aminosilane:Au is low, there is a competition (in the early stages of reduction) between the metal surface and the Au(III) ions over the amine groups. This competition is largely biased in favor of the solution species. Aminosilane moieties that are released by the reduction step do not adsorb and cap the gold nuclei but instead are immediately captured by the mixed complexes of Au(III)-chlorideamine due to the larger affinity of the Au(III) to the amine as compared to the chloride. In light of the model of Chart 1, the increased stability of EDAS-goldsas compared to the APS-gold solsscan be attributed to the two bonds that are formed between the diamine moieties and the gold surface, which hinder the release of segments of the protective cross-linked aminosilane shell from the metal nanoparticles. Once a fragment of the shell is released, then metal coalescence can take place. The stability of the sol prepared at pH 2 was rather surprising because protonation of the amine group should pose a substantial competition to the amine-gold bonds. The explanation for the stability of the low pH sols is that the particles are capped by ammonium moieties that stabilize the sol and the silicate shell stabilizes this network. Alkylammonium moieties were reported to stabilize gold nanparticles in organic solvents,14b but this is not expected for aqueous solutions, because the hydrophilic ammonium groups will orient toward the water side and the alkyl groups will not bond the metal surface. However, silyl-alkylammonium moieties have two functional groups and thus the ammonium end can orient toward the metal surface and the silicate shell can still be formed around the metal particles (similar to Chart 1).
Gold and Other Noble Metal Nanodispersions
Indeed, replacing the EDAS precursor with N-((trimethoxysilyl)propyl)-N,N,N-trimethylammonium chloride gave wine-red gold sol after mixing with gold chloride and subsequent reduction (Table 1, row 3). This sol lost stability (only) after approximately 12 h due to the lower affinity of the quaternary ammonium groups for the gold surfaces (as compared to primary alkylammonium groups and alkylamines). The lower affinity of the ammonium groups to the gold surfaces compared to amine groups explains also the formation of gold aggregates for the low pH EDAS-Au sols (Figure 4a). Pd, Pt, and Ag. This mechanism also explains the insensitivity of the configuration of the metal nanoparticles to the noble metal used, provided that both the stability constant of the metal ion-amine and the metal surfaceamine are high. The geometry of the particles is determined by the configuration of the cross-linked silicate shell. This is certainly the case for Au, Pd, and Pt but evidently not for silver ions which exhibit much lower affinity to alkylamines (e.g., the stability constant of Ag(ethylenediamine)2 is 107.7 as compared to 1036.5 for the Pt(ethylenediamine)2. This constant should be similar for palladium(II) and even higher for Au(III)). Conclusions We have demonstrated a protocol for direct synthesis of metal nanodispersions and concentrated in particular on the synthesis of gold nanodispersions in silicate sol,
Langmuir, Vol. 15, No. 6, 1999 1937
gels, and films. The gold nanodispersion was characterized using UV-visible, FT-IR, and TEM. On the basis of these results, we attributed the stabilization of gold nanodispersions in aqueous solution to amine-gold complex formation in the solution and to amine-gold surface interactions which form stable negatively charged silicate shells on the gold nanoparticles. Of the two aminosilanes studied, EDAS sol was more stable due to the stronger binding (by two amino groups) to the gold surfaces as compared to APS. The gold particles were predominantly spherical with an average size of 5-6 nm. The particle size was neither affected by the type of aminosilane or by the Si:Au molar ratio. Further, we have demonstrated that this method is generic and can be used to prepare other metal nanodispersions such as platinum, palladium, and silver. Platinum and palladium nanodispersions were of uniform size, similar to the ones observed for gold, while Ag nanodispersions exhibited wide particle size distributions. The different behavior is explained by the lower affinity of amine groups to silver ions and surfaces as compared to Pt, Pd, and Au. Acknowledgment. The authors acknowledge gratefully the funding from MOS, Israel, and S.B thanks the Valazzi-Pikovsky Fellowship Fund of The Hebrew University of Jerusalem for a postdoctoral fellowship. LA980490X
