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Langmuir 2003, 19, 3974-3980
Coassembly Synthesis of Ordered Mesoporous Silica Materials Containing Au Nanoparticles Haoguo Zhu, Byunghwan Lee, Sheng Dai,* and Steven H. Overbury* Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6201 Received December 18, 2002. In Final Form: February 11, 2003 A coassembly methodology was developed to synthesize ordered mesoporous silica materials containing gold nanoparticles. The essence of this methodology lies in the combination of the complexation-mediated growth of nanoparticles and surfactant-templating synthesis. The formation of nanoparticles was controlled by the complexing ligand and the unique micellar media. Different methods for the removal of the templating surfactants were evaluated. Ordered mesoporous silica materials containing 2-5 nm gold nanoparticles have been successfully synthesized.
Introduction Haruta and co-workers1-3 have demonstrated that Au nanoparticles between 2 and 5 nm are highly active for a number of catalytic reactions at room temperature. Usually, highly dispersed Au nanoparticles are prepared by liquid-phase methods, followed by their incorporation into supported materials via a wet impregnation process.4,5 The isoelectric point (IEP) of the support matrix plays a key role for the successful incorporation and dispersion of gold precursors into the oxide support.6 To deposit Au(OH)3 on the surfaces of oxides, the pH value of HAuCl4 solution is normally adjusted to the range of 6-10. The prerequisite for the incorporation of Au(III) precursors requires the interaction of the anionic Au(OH)xCl4-xcomplexes with a positively charged oxide surface. Accordingly, oxides with high IEPs are favored in the aqueous impregnation synthesis. For example, titanium oxide (IEP ∼ 6.0)7 has been extensively employed as a support for the impregnation synthesis of the gold-nanoparticle precursors. On the other hand, the isoelectric point of silica is relatively low (∼2). This low IEP implies that the surface of silica is highly negatively charged under the impregnation conditions. Therefore, the direct wet impregnation method could not be used to prepare controlled gold nanoparticles on SiO2. This deficiency with silica surfaces prompted Haruta and co-workers8,9 to develop a chemical vapor deposition (CVD) methodology using an organometallic gold precursor. By this method, gold nanoparticles with a narrow distribution were successfully dispersed on mesoporous silica. Other strategies for the immobilization of gold nanoparticles on silica involve surface modification with cationic functional groups to enhance the interaction between anionic gold complex precursors * To whom correspondence should be addressed. E-mail: dais@ ornl.gov,
[email protected]. (1) Haruta, M. Catal. Today 1997, 36, 153. (2) Haruta, M.; Date, M. Appl. Catal., A 2001, 222, 427. (3) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (4) Haruta, M.; Ueda, A.; Tsubota, S.; Torres Sanchez, R. M. Catal. Today 1996, 29, 443. (5) Oh, H. S.; Yang, J. H.; Costello, C. K.; Wang, Y. M.; Bare, S. R.; Kung, H. H.; Kung, M. C. J. Catal. 2002, 210, 375. (6) Zanella, R.; Giorgio, S.; Henry, C. R.; Louis, C. J. Phys. Chem. 2002, 106, 7634. (7) Boehm, H. P. Angew. Chem. 1966, 78, 617. (8) Okumura, M.; Tsubota, S.; Iwamoto, M.; Haruta, M. Chem. Lett. 1998, 315. (9) Okumura, M.; Nakamura, S.; Tsubota, S.; Nakamura, T.; Azuma, M.; Haruta, M. Catal. Lett. 1998, 51, 53.
and oxide surfaces, direct impregnation of gold nanoparticles on porous matrixes, or doping of gold nanoparticles in mesoporous materials.10-14 The gold nanoparticles synthesized by these methodologies have broad size distributions with average diameters larger than 5 nm. For example, Mukherjee et al.11 have developed an interesting method to reduce gold ions with surface silanol groups for the generation of gold nanoparticles on fumed silica. The size of gold nanoparticles obtained via such a method is greater than 10 nm. Both Khushalani et al.13 and Konya et al.14 have recently reported the direct doping of gold nanoparticles inside SBA-15. Commercial gold nanoparticles from Ted Pella, Inc. (Redding, CA) were employed in both syntheses. In contrast to the direct impregnation of ionic gold precursors, the surface structures of oxide hosts are less crucial in direct doping or impregnation of preformed gold nanoparticles. The key drawback in using preformed gold nanoparticles is the difficulty in entrapping a large quantity of gold nanoparticles inside mesoporous hosts without aggregation of nanoparticles and disruption of mesostructures.13 The high-ionic-strength condition used in the synthesis of SBA15 can easily induce the aggregation of gold nanoparticles. Accordingly, the doping concentration of gold nanoparticles is usually less than 1 wt %. Normally, only large gold nanoparticles (diameter > 5 nm) can be doped under such conditions, based on transmission electron microscopy, X-ray diffraction, and UV-vis spectroscopy investigations. Calcination could lead to further aggregation of gold nanoparticles. Herein, we report an efficient synthesis of silica mesoporous materials containing 2-5 nm gold nanoparticles by a coassembly methodology.15 The essence of this method is the use of the bifunctional ligands that can not only complex Au(III) via amine functional groups but also (10) Guari, Y.; Thieuleux, C.; Mehdi, A.; Reye, C.; Corriu, R. J. P.; Gomez-Gallardo, S.; Philippot, K.; Chaudret, B.; Dutartre, R. Chem. Commun. 2001, 1374. (11) (a) Mukherjee, P.; Patra, C. R.; Ghosh, A.; Kumar, R.; Sastry, M. Chem. Mater. 2002, 14, 1678. (b) Mukherjee, P.; Sastry, M.; Kumar, R. Phys. Chem. Commun. 2000, 4. (12) Pietron, J. J.; Stround, R. M.; Rolison, D. R. Nano Lett. 2002, 2, 545. (13) Khushalani, D.; Hasenzahl, S.; Mann, S. J. Nanosci. Nanotechnol. 2001, 1, 129. (14) Konya, Z.; Puntes, V. F.; Kiricsi, I.; Zhu, J.; Alivisatos, A. P.; Somorjai, G. A. Nano Lett. 2002, 2, 907. (15) Zhang, Z. T.; Konduru, M.; Dai, S.; Overbury, S. H. Chem. Commun. 2002, 2406.
10.1021/la027029w CCC: $25.00 © 2003 American Chemical Society Published on Web 03/25/2003
Silica Materials Containing Au Nanoparticles
covalently bond to the porous silica matrix via siloxane groups during the sol-gel surfactant template synthesis. The organosilane functional ligands significantly increase the interaction between Au(III) and the host matrix. This enhanced interaction leads to the successful immobilization of Au(III) on the support surfaces through chemical complexation of Au(III) within the channels of the functionalized mesoporous silica. The discovery of a new family of mesoporous silicon oxides (MCM-41) by scientists at Mobil Oil Research and Development has led to great interest in this area of materials science.16 These materials exhibit large internal surface areas and narrow pore size distributions similar to those found in microporous crystalline zeolites. Unlike the crystalline zeolite materials with maximum pore dimensions of
