© Copyright 2000 American Chemical Society
OCTOBER 31, 2000 VOLUME 16, NUMBER 22
Letters Metal Nanoparticles Grown in the Nanostructured Matrix of Poly(octadecylsiloxane) Lyudmila M. Bronstein,*,†,‡ Dmitri M. Chernyshov,† Peter M. Valetsky,† Elizabeth A. Wilder,§ and Richard J. Spontak*,§ Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov St. 28, 117813 Moscow, Russia, Department of Chemistry, Indiana University, Bloomington, Indiana 47405, and Department of Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695 Received July 3, 2000. In Final Form: August 28, 2000 Metal nanoparticles grown within the nanostructured matrix of the amphiphilic polymer poly(octadecylsiloxane) (PODS) are investigated here by transmission electron microscopy. Due to its silanol groups and alkyl chain, PODS forms a bilayered nanostructure containing an intercalated layer of water within an aqueous environment. Replacement of water molecules with metal ions within the siloxy bilayers, followed by reduction, results in the formation of metal nanoparticles. The increase in electron density upon nanoparticle formation permits direct visualization of these bilayers, as well as the individual nanoparticles residing within them. These nanoparticles measure about 1-2 nm in diameter and possess a relatively narrow size distribution due presumably to volume availability within the ordered bilayers of PODS.
Introduction Due to their huge surface areas, nanosized metal particles possess unique chemical and physical properties that can be exploited in a wide variety of technological applications, including catalysis, nonlinear optics, ultrapurification, and microelectronics.1-4 It has been established that the properties of such nanoparticles depend sensitively on factors such as size, size distribution, shape, and chemical environment. One design strategy that shows tremendous potential as a viable route by which to * To whom correspondence should be addressed. † Russian Academy of Sciences. ‡ Indiana University. § North Carolina State University. (1) Henglein, A. Chem. Rev. 1989, 89, 1861. (2) Oggawa, S.; Hayashi, Y.; Kobayashi, N.; Tokizaki, T.; Nakamura, A. Jpn. J. Appl. Phys. 1994, 33, L331. (3) Fendler, J. H., Ed., Nanoparticles and Nanostructured Films; Wiley-VCH: New York, 1998. (4) Bradley, J. In Clusters and Colloids; Schmid, G., Ed.; WileyVCH: New York, 1994.
produce metal nanoparticles with improved size control and colloidal stability employs polymeric matrixes as the growth media. In this manner, nanoparticles can be grown to not only satisfy specific application requirements but also endow the polymer with new properties. Recent studies5-9 have demonstrated that if the polymer matrix is nanostructured, control over nanoparticle development and stabilization can be further enhanced. Examples of nanostructured polymer matrices (NPMs) include amphiphilic block copolymers either in the bulk or as micelles. While well-defined block copolymers constitute the most efficient NPMs in which to produce nanoparticles with (5) Antonietti, M.; Wenz, E.; Bronstein, L.; Seregina, M. Adv. Mater. 1995, 7, 1000. (6) Chan, Y. N. C.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 24. (7) Saito, H.; Okamura, S.; Ishizu, K. Polymer 1992, 33, 1099. (8) Moffit, M.; McMahon, L.; Pessel, V.; Eisenberg, A. Chem. Mater. 1995, 7, 1185. (9) Spatz, J. P.; Roescher, A.; Mo¨ller, M. Adv. Mater. 1996, 8, 337.
10.1021/la0009341 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/29/2000
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well as of reaction and reduction media, polymers containing nanoparticles of metals such as Pt, Pd, and Au may be produced. In this work, we confirm the existence of such nanoparticles through direct visualization by transmission electron microscopy (TEM). The size ranges of the nanoparticles, as well as of the lamellar PODS nanostructure upon nanoparticle formation, are presented.
Scheme 1
Experimental Section
low polydispersity, they are typically prepared by anionic polymerization, which relies on precise synthetic procedures. A robust alternative by which to prepare metal nanoparticles in a hydrated NPM is reported here. Rather than use block copolymers, we employ poly(octadecylsiloxane) (PODS), a nanostructured polyamphiphile that is prepared through the hydrolytic polycondensation of n-octadecyltrichlorosilane in water. In its hydrated form, PODS forms bilayers composed of hydrophilic silanol groups and hydrophobic alkyl chains. According to previous reports,10 the siloxy bilayer consists of (-SiO)x(OH)y moieties that stabilize a layer of intercalated water through hydrogen bonds. We expect that, in the presence of an aqueous solution of metal salt, the siloxy bilayer will absorb metal anions due to competitive hydrogen bonding. Subsequent reduction of the metal ions would result in a polymeric material with regularly spaced metal nanoparticles (Scheme 1). Through judicious choice of metal salt, as
Reagent-grade acetone and ethanol were purchased from Aldrich and were used after distillation, whereas NaBH4 (Riedelde-Haen), H2PtCl6‚6H2O (Reakhim), K2PtCl4, AuCl3, and Na2PdCl4 (Aldrich) were all used as-received. Water was purified with a Milli-Q water purification system, and the PODS was synthesized according to the protocol detailed elsewhere.10 Metal ions were incorporated into PODS at ambient temperature by first mixing PODS powder (0.1 g) in a specific metal salt solution (10 mL at 10-3 M) of either water/acetone (1/1 v/v) or water/ ethanol (1/1 v/v) for 24 h. Reaction and reduction media, as well as the metal salts employed and sample designations, are listed in Table 1. Each specimen was prepared according to the specific chemical nature of the metal compound employed. After 12 h of agitation in the presence of Na2PdCl4, the initially white PODS-Pd suspension transformed from brown to dark gray, signaling the onset of metal-ion reduction. Additional ion reduction was not performed. An analogous color change occurred after only 4 h for PODS-Pt1 in the presence of K2PtCl4. Both products were isolated, washed with the reaction medium (water/ethanol or water/acetone), and dried overnight in a vacuum desiccator. Reduction of Na2PdCl4 and K2PtCl4 occurred after they were incorporated within PODS due to the silanol groups comprising the siloxy bilayers. This was not the case for AuCl3, which generated the monovalent AuCl3(OH)- ion upon dissolution in water and underwent reduction within the reaction medium after only 15 min. The PODS-Au product was isolated after 4 h of agitation and treated in the same manner as above. In marked contrast, PODS-Pt2 was reduced neither by PODS nor by the
Table 1. Characteristics of the Metalated PODS Systems Investigated in This Study
a
designation
metal salt
reaction medium
reducing agent
metal contenta (wt %)
PODS-Pd PODS-Pt1 PODS-Pt2 PODS-Au
Na2PdCl4 K2PtCl4 H2PtCl6‚6H2O AuCl3
water/ethanol water/acetone water/acetone water/ethanol
PODS silanol groups PODS silanol groups NaBH4 reaction medium
0.80 1.20 0.48 0.60
Measured by elemental analysis.
Figure 1. Representative TEM micrograph of PODS-Pd, which consists of Pd nanoparticles derived from Na2PdCl4 and embedded within the siloxy bilayers of the PODS lamellar nanostructure. The Pd and Si atoms present in this system provide sufficient electron contrast relative to the alkyl bilayers to permit differentiation. The 3× enlargement more clearly displays the nanoparticles within their host medium.
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reaction medium. Instead, it was isolated after 24 h of agitation in the presence of platinic acid, washed in water/acetone to remove unreacted salt, and dried overnight under vacuum. Reduction was performed with NaBH4 after redispersion in water/acetone. Metal compositions of the specimens produced here were measured by elemental analysis. Powders of metalated PODS were embedded in epoxy resin, cured at 60 °C for 24 h, and microtomed at ambient temperature to yield thin sections measuring about 50-70 nm in thickness. Zero-loss images11,12 were recorded with nonscattered and elastically scattered electrons on plate film in a Zeiss EM902 electron spectroscopic microscope operated at an accelerating voltage of 80 kV and an energy-loss setting of nominally 0 eV (the width of the window was 30 eV). Prints prepared from the negatives were digitized to permit further analysis using the Digitalmicroscope (Gatan, Pleasanton, CA) and Image (National Institutes of Health, Washington, DC) software packages. Twodimensional Fourier transforms of selected areas of the digital images yielded the lamellar period of the PODS nanostructure. Nanoparticle sizes were also measured here, but a statistical assessment of the nanoparticle size distribution could not be performed due to the background lamellar texture of the PODS matrix. The error in the measurements reported here is expected to be less than 0.4 nm.
Results and Discussion Figure 1 shows a representative large-area TEM image acquired from the PODS-Pd specimen and verifies the existence of a highly ordered PODS nanostructure. As previously deduced from scattering data,10 PODS constitutes an example of a polygranular material in which each grain consists of alternating siloxy and alkyl bilayers. Upon reduction of PdCl42- ions by the silanol groups of PODS (12 h after metal salt incorporation), Pd nanoparticles form within the PODS matrix. The location of the “reducing agent” in this case ensures growth of Pd nanoparticles within the siloxy bilayers. Inclusion of an organic solvent in the reduction medium is necessary to compatibilize the highly hydrophobic alkyl layers of PODS (which reside on the outside of the PODS powder).10 Due to the presence of Si and Pd atoms within the PODS bilayers, the lamellae that are oriented parallel to the electron beam possess sufficient contrast relative to the carbonaceous (alkyl) background to be visible in this and subsequent images. An enlargement of one of these regions is provided in the inset for closer examination and reveals the presence of discrete Pd nanoparticles measuring only 0.5-1.0 nm in diameter. These nanoparticles are seen to reside within the siloxy bilayers (which appear dark), which is consistent with the molecular packing arrangement envisaged earlier. This result indicates that the PdCl42- ions replace some of the water molecules within the hydrophilic siloxy bilayer and form hydrogen bonds with the silanol groups of PODS. The lamellar period measured from images such as the one displayed in Figure 1 is about 3.1 nm, which is slightly smaller than the smallangle X-ray scattering (SAXS) result13 of 5.24 nm. The images presented in Figures 2 and 3 have been collected from PODS-Pt1 and PODS-Pt2, in which Pt nanoparticles have been grown within PODS. In the case of PODS-Pt1, PtCl42- and water/acetone are used as the metal ion precursor and reaction medium, respectively, whereas the PODS silanol groups constitute the reducing agent. The PODS2-Pt2 specimen, on the other hand, is produced by reducing PtCl62- ions with NaBH4 in water/ acetone. On the basis of geometrical and hydrogen-bonding considerations,14 incorporation of the planar PtCl42- ion (10) Parikh, A. N.; Schivley, M. A.; Koo, E.; Seshadri, K.; Aurentz, D.; Mueller, K.; Allara, D. L. J. Am. Chem. Soc. 1997, 119, 3135 and references therein. (11) Reimer, L., Ed. Energy-Filtering Transmission Electron Microscopy; Springer-Verlag: Berlin, 1995. (12) Du Chesne, A. Macromol. Chem. Phys. 1999, 200, 1813.
Figure 2. (a) A TEM image of PODS-Pt1, derived from K2PtCl4 and reduced by the silanol groups of PODS (see Table 1). According to elemental analysis, this system contains 1.20 wt % Pt. (b) An enlargement of (a) to permit closer examination of the Pt nanoparticles (arrowheads) within the PODS nanostructure.
is expected to disturb the ordered siloxy bilayer less than the octahedral PtCl62- ion, resulting in a higher metal content (confirmed in Table 1). According to the micrographs shown in Figures 2a and 3a, both materials display a lamellar nanostructure similar to the one evident in Figure 1. Lamellar periods measured from images such as these are about 3.1-3.4 nm for PODS-Pt1 and 3.0 nm for PODS-Pt2. These periods are also smaller than the period discerned from SAXS, most likely due to the orientation of the PODS lamellae relative to the electron beam: if they are tilted, the observed period is smaller (13) Svergun, D. I.; Kozin, M. B.; Konarev, P. V.; Shtykova, E. V.; Volkov, V. V.; Chernyshov, D. M.; Valetsky, P. M.; Goerigk, G.; Bronstein, L. M. Submitted for publication in Chem. Mater. (14) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; Wiley-Interscience: New York, 1962.
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Figure 3. (a) A TEM image of PODS-Pt2, produced from H2PtCl6‚6H2O and reduced by NaBH4 (see Table 1). According to elemental analysis, this system contains only 0.48 wt % Pt. (b) An enlargement of (a) to facilitate visualization of the Pt nanoparticles within the PODS nanostructure.
than the actual period. Another possibility is that PODS shrinks either during sample preparation (in epoxy) or upon exposure to the beam. Enlargements of the nanostructure are provided in Figures 2b (PODS-Pt1) and 3b (PODS-Pt2) and reveal the existence of discrete Pt nanoparticles in each material. Those in PODS-Pt1 measure from about 0.8 to 1.8 nm in diameter, while those in PODS-Pt2 exhibit a slightly narrower size distribution, ranging from about 1.1 to 1.4 nm across. Incorporation of Au nanoparticles within PODS is verified in the image of PODS-Au (Figure 4a). To achieve Au metalation, the PODS matrix is first impregnated with AuCl3(OH)- ions in a solution of water/ethanol. Unassisted reduction occurs within this reaction medium in only 15 min. According to elemental analysis, the specimen displayed in Figure 4 contains about 0.60 wt % Au (see Table 1). The lamellar period measured from this and
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Figure 4. (a) A TEM image of the PODS-Au system, which contains Au nanoparticles. This system, prepared from AuCl3 in water/ethanol and reduced by the reaction medium, contains 0.60 wt % Au (see Table 1). (b) An enlargement of (a) to permit closer scrutinization of the Au nanoparticles residing within their host siloxy bilayers of the PODS nanostructure.
other images of PODS-Au is about 3.0-3.1 nm, in good agreement with the periods discerned from PODS-Pt1 and PODS-Pt2 provided in Figures 2 and 3, respectively. Close examination of the enlargement of PODS-Au included in Figure 4b reveals the presence of Au nanoparticles ranging in size from about 1.0 to 1.4 nm in diameter, in excellent agreement with the Pt nanoparticles seen in Figure 3b. In addition to these nearly monodisperse Au nanoparticles residing in the siloxy bilayers of PODS, larger particles varying in size also exist in the PODSAu specimen due to uncontrolled particle growth within the reaction medium. Comparison of the images displayed in Figures 1-4 indicates that the PODS nanostructure is retained even after being subjected to various reaction and reduction media, as well as the various metal salts used to imbibe PODS. This observation suggests that
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PODS constitutes a robust NPM in which to grow metal nanoparticles. Moreover, insofar as particle growth is restricted to the siloxy bilayers of PODS, the resultant metal nanoparticles possess a relatively narrow size distribution centered around 1.0 nm, confirming that this NPM affords tremendous control over nanoparticle growth. Another system that we previously examined15 in which the NPM regulates metal nanoparticle formation is hypercrosslinked polystyrene (HPS), composed of pores measuring on the order of 2-3 nm. Cobalt nanoparticles grown in HPS from either Co2(CO)8 in 2-propanol or [Co(DMF)6]2+[Co(CO)4]2- in dimethylformamide (DMF) have been found to measure about 2-3 nm in diameter up to about 8 wt % Co. Conclusions In summary, we have demonstrated that nanoparticles of metals such as Pd and Pt can be controllably grown (with relatively low polydispersity) exclusively within the (15) Sidorov, S. N.; Bronstein, L. M.; Davankov, V. A.; Tsyurupa, M. P.; Solodovnikov, S. P.; Valetsky, P. M.; Wilder, E. A.; Spontak, R. J. Chem. Mater. 1999, 11, 3210.
siloxy bilayers of the lamellar nanostructure of PODS. Nanoparticles of Au can likewise be grown within the siloxy bilayers, but control over particle size is partially compromised due to rapid AuCl3(OH)- reduction in the reaction medium. Evidence of nanoparticle formation, acquired from TEM, confirms that the metal-ion precursors employed here possess sufficient mobility and hydrogen-bonding capability to replace a fraction of the intercalated water molecules that are hydrogen-bonded to the silanol groups comprising the siloxy bilayers. Since it remains virtually unaffected by the reaction and reduction media used to incorporate these metal ions, the PODS nanostructure appears to be sufficiently robust to warrant further investigation as a viable route by which to produce not only highly stable metal nanoparticles but also a nanostructured polymer containing regularly spaced nanoparticles. Acknowledgment. Financial support for D.M.C. and P.M.V. has been provided by the Russian Foundation for Basic Research (Grant No. 98-03-33372). E.A.W. and R.J.S. are supported by Milliken Chemicals. LA0009341
