NANO LETTERS
Nanocrystal Templating of Silica Mesopores with Tunable Pore Sizes
2002 Vol. 2, No. 8 907-910
Zoltan Ko´nya,† Victor F. Puntes,† Imre Kiricsi,‡ Ji Zhu, A. Paul Alivisatos, and Gabor A. Somorjai* Department of Chemistry, UniVersity of California, Berkeley, Berkeley, California, 94720, and Materials Science DiVision, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California, 94720 Received June 24, 2002
ABSTRACT Metallic nanoparticles (platinum and gold) encapsulated in mesoporous silica (SBA-15) were prepared in the same solution by a novel twostep method. Characterization by X-ray scattering and electron microscopy consistently shows that the metal nanoparticles were homogeneously incorporated in the mesopores (retaining their size and morphology), even when the nanocrystal diameter exceeds the normal mesopore diameter. The nanoparticles nucleated the expansion of the mesopore channels in the 92−116 Å range so they could accommodate the metal particles. This expansion occurs in the concentration range of 1−103 nanoparticles per 103 mesopore channels. This effect can be used to tune the pore size.
Mesoporous silica frameworks provide a very robust, open, and tunable periodic scaffold on the nanometer scale. These properties have led to extensive work to create composites with active components embedded inside the pores. Nanoscale building blocks with interesting electrical, optical, magnetic, and catalytic4 functionality have all been embedded inside mesoporous silica frameworks. From the catalytic point of view, inorganic nanoparticle-mesopore composites have the potential to allow three-dimensional designer catalysts with separate tuning of the microscopic catalytic rates on the nanoparticle surfaces, as well as control over material transport by changes in the pore size. The dominant strategy for creation of these types of composites has been to grow the inorganic nanocrystals inside the pores after the mesopore synthesis.2 In this paper, we have investigated a different sequence of events, in which the nanoparticles are formed first, and then the mesopores are grown around them. This strategy is not necessarily equivalent. The nanoparticles can be larger than the pores that the surfactant structure-directing agents would dictate, so that the nanoparticle can actively perturb the structure that is formed. Further, the distribution of the inorganic additive in the mesopore should be extremely different in the two approaches, since the growth of an inorganic nanostructure inside a given part of a mesopore necessarily restricts further transport of feedstock materials for growth. †
These authors contributed equally to this work. Department of Applied and Environmental Chemistry, University of Szeged, Hungary. * Correspondence author; E-mail:
[email protected]; Fax: (510) 643-9668. ‡
10.1021/nl0256661 CCC: $22.00 Published on Web 07/12/2002
© 2002 American Chemical Society
We have investigated the influence of two different nanocrystal systems on the growth of mesoporous silica. Cubic Pt nanoparticles were prepared by the method of Rampino and Nord.3 Linear polymers have the potential to control not only the size but also the shape of the metal nanoparticles.4 Instead of using sodium polyacrylate, the capping material was tri-block copolymer poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) EO20PO70EO20 (Pluronic P-123, BASF). EO20PO70EO20 was dissolved in a 1 × 10-4 M freshly prepared K2PtCl4 solution to form Pt/polymer ) 1:1 molar ratio, the pH was set to 7.5 and Ar gas was bubbled in this solution for 30 min. Afterward, the reduction of platinum ions to form metal nanoparticles was carried out by bubbling hydrogen gas into the solution for 5 min. The reaction vessel was left under H2 atmosphere overnight, and after 12 h golden yellow color appeared showing the formation of Pt nanoparticles. The resulting Pt nanoparticles show a broad size and shape distribution, which was narrowed with centrifugation. Gold nanoparticles5 of 2, 5, and 20 nm in water were also used. We have used these particle sizes since (i) the 2 nm particles can be easily accommodated in the interior of the channels, (ii) the 5 nm particles are at the border they may enter and may not the channels, and (iii) the 20 nm particles are surely not accommodated in the channels due to their size. The mesopore synthesis was performed in the presence of the nanoparticles as follows. To meet SBA-15 optimal synthesis conditions,6 an excess of P-123 was added to the nanoparticle solutions. The solution was acidified using
Figure 1. Small angle XRD of mesoporous silica containing metallic nanoparticles: (a) different size and (b) as a function of concentration. In this (b) experiment, features below 1.5° could not be measured.
concentrated HCl, and then tetramethoxysilane (TMOS) was added to reach the mass ratio of EO20PO70EO20:cc. HCl/ TMOS/Pt-sol ) 2:15:3.6:60. The solution was stirred for 24 h at 30 °C and then heated at 80 °C for 1 day. The product was filtered, washed, dried and calcined at 550 °C for 12 h to remove the template. Small-angle X-ray diffraction (SAXS) and transmission electron microscopy studies showed that the samples have well-ordered hexagonal mesoporous structure with diameter around 10 nm. The product is a fine powder with different coloration depending on the type of particle and particle concentration. All of the structural experiments consistently show that the nanoparticles influence the structure of the mesopore in an unexpected way. In Figure 1a, we observe the SAXS of a series of samples prepared with similar concentrations [5 × 1014 particle/mL] of different Au and Pt particles.7 The intensity of reflections remained almost unchanged for template-free (burned) samples. The presence of the nanocrystals influences only slightly the crystal quality but changes the lattice spacing of the silicate. It can be observed that as a function of the particle size, peaks are shifted toward small angles, suggesting that the structure expands when particles are included. The peak broadness of the sample remains similar suggesting that, for this concentrations, the template channels increase regularly to accommodate the structure, due to its strong tendency of forming regular channels,8 instead of becoming a bumpy structure with similar lattice spacing but with lower crystal quality. This is supported by the experiments varying the concentration from 1012 to 1016 particles per 1 g silica (representing roughly from 1:2000 to 2:1 particle/channel ratio and/or 23-350 nm average interparticle distance) keeping the particle size constant (Figure 1b). For very low concentrations, the SiO2 diffraction peaks are at the same position as without particles. Apparently, for low concentrations the system accommodates the particles and relaxes. For a higher particle concentration, it becomes more favorable to expand the whole channel, rather than 908
relaxing it for every inclusion. In addition it seems that those expanded channels determine the lattice spacing of the selfassembly, i.e., the hexagonal structure would be strained if the tubes had a distribution of diameters. Thus, a progressive shift toward lower angles, large lattice parameters (∼92 Å to ∼116 Å) is observed, indicating the Au intake by the tubular micelles. This fact could suggest that it is preferable for the soft micellar structure to expand rather than protrude in order to accommodate the metallic particles, and so does the mesopore structure. From the lowest concentration at which this effect is first observed, we can conclude that influence of a single nanoparticle inclusion within the soft micellar structure extends over a distance of several hundred nm. It could be argued that a similar effect would be obtained by modifying the salt concentration in the solution.9 However, the main ion source is from the HCl concentration, which has been carefully maintained constant in all the experiments. When a high amount of nanoparticles or gold solution containing 20 nm particles was used, the crystallinity of the samples decreased. The silica structure stayed porous, but the hexagonal arrangement was often lost and worm like channels appeared. It could be that at high concentrations, the presence of the particles in the channels impedes the longrange order to be set due to nanoparticle-nanoparticle interaction, as well as the damaging effects of impurities in the crystallization process. Thus, final metal content in these samples is low (0.5% in volume) but reasonable. Other studies confirm that the nanoparticles are inside the pores and act to expand the mesopore spacing. From TEM (Figure 2) we see that (i) the silica has hexagonal mesoporous structure, (ii) particles are not agglomerated, (iii) the metallic particles’ spatial distribution in the perpendicular direction to the channels is consistent with the pore lattice spacing; similarly, separated particles in a line parallel to the pore are often observed, and (iv) that cubic particles remained cubic. Wide-angle XRD studies of the nanoparticles show Nano Lett., Vol. 2, No. 8, 2002
Figure 2. TEM pictures of the silica containing metallic nanoparticles.
that their diffraction patterns are unchanged by the encapsulation. Nitrogen porosimetry data showed a BET surface area around 650 cm2/g. The shape of the N2 adsorptiondesorption isotherm reveals that the sample possesses wellordered structure and a narrow pore size distribution.10 FTIR spectroscopy studies of thiols (ethyl-mercaptan, cyclohexylmercaptan) bonded to the Au nanoparticles indicated that the Au nanoparticles are accessible. In summary, it appears that the described simple two-step synthesis of encapsulated nanoparticles in mesoporous silica alters neither the size and morphology of the metal clusters nor the symmetry of the pore structure of the mesoporous silica. However, the incorporation of a low concentration of the nanoparticles that are larger than the structure directing agent pore size acts to expand the entire mesopore structure uniformly. This is most probably due to the prohibitive energy cost of bending the surfactant assembly. Thus, we believe that the approach described here of using previously grown nanoparticles as templates from which to grow mesoporous structures provides an interesting system for the study of surfactant mediated growth, as well as a new route for creating advanced catalysts. Further studies of the kinetics of growth and the physical properties are under way. Nano Lett., Vol. 2, No. 8, 2002
Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Science, Divisions of Materials and Chemical Sciences of the U.S. Department of Energy under contract no. DE-AC0376SF00098 and NIH National Center for Research Resources, grant no. 1 R01 RR-14891-01. References (1) Zhou, W. Z.; Thomas, J. M.; Shephard, D. S.; Johnson, B. F. G.; Ozkaya, D.; Maschmeyer, T.; Bell R. G.; Ge, Q. F. Science 1998, 280, 5364. (2) (a) Han, Y.-J.; Kim, J. M.; Stucky, G. D. Chem. Mater. 2000, 12, 2068. (b) Lee, K.-B.; Lee, S.-M.; Cheon J. AdV. Mater. 2001, 13, 517. (c) Raja, R.; Sankar, G.; Hermann, S.; Shephard, D. S.; Bromley, S.; Thomas, J. M.; Johnson, B. F. G. Chem. Commun. 1999, 1571. (3) Rampino L. D.; Nord, F. F. J. Am. Chem. Soc. 1942, 62, 2745. (4) (a) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein A.; El-Sayed, M. A. Science 1996, 272, 1924. (b) Teranishi, T.; Kurita R.; Miyake, M. J. Inorg. Organomet. Polym. 2000, 10, 145. (5) Colloidal gold nanoparticles were obtained from Ted Pella Inc. (Redding, CA). These solutions were used either as received or concentrated by simultaneous filtering and centrifugation in some cases. (6) (a) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka B. F.; Stucky, G. D. Science 1998, 279, 548. (b) Zhao, D.; Huo, Q.; Feng, J.; Chmelka B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. 909
(7) Powder X-ray diffraction was performed on a Bruker-AXS D8 general area detector diffraction system (GADDS), using Co KR radiation (1.79026 Å). The instrument resolution is 0.05° in 2θ. (8) As a control, reference samples were prepared by mechanical mixture of gold particles and the respective mesoporous silicate. A calculated amount of gold solution (containing nanoparticles of 2, 5, and 20 nm) was mixed with the silicate. After stirring for 30 min, the mechanical mixture was dried and powdered. TEM pictures showed agglomerated particles definitely on the outer surface, providing
910
further indirect evidence that the synthetic insertion of the nanoparticles leads to embedded particles. (9) Yu, C. Z.; Tian, B. Z.; Fan, B.; Stucky, G. D.; Zhao, D. Y. Chem. Commun. 2001, 2726. (10) (a) Sing, K. S. W.; Everett, D. H.; Haul, R. A.; Moscou, L.; Pierottu, R. A.; Rouquerol J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (b) Gregg S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic: London, 1982.
NL0256661
Nano Lett., Vol. 2, No. 8, 2002
