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Chem. Mater. 1996, 8, 2172-2179
Nanostructured Materials Generated by High-Intensity Ultrasound: Sonochemical Synthesis and Catalytic Studies Kenneth S. Suslick,* Taeghwan Hyeon, and Mingming Fang School of Chemical Sciences, University of Illinois at Urbana-Champaign, 505 S. Mathews Ave., Urbana, Illinois 61801 Received January 29, 1996. Revised Manuscript Received May 22, 1996X
The sonochemical decomposition of volatile organometallic compounds produces high surface area solids that consist of agglomerates of nanometer clusters. For iron pentacarbonyl and tricarbonylnitrosylcobalt, nanostructured metals and alloys are formed; for molybdenum hexacarbonyl, the metal carbide is produced. These sonochemically produced nanostructured solids are active heterogeneous catalysts for hydrocarbon re-forming and CO hydrogenation. The sonochemical synthesis, characterization, and catalytic studies will be discussed in this review.
Nanostructured materials have been intensively studied in recent years because the physical properties of these materials are often quite different from those of the bulk.1-4 A variety of chemical and physical preparative methods have been developed to produce materials with nanometer structure. Gas-phase techniques for the production of nanostructured metals include molten metal evaporation to produce a supersaturated metal vapor and subsequent condensation with inert gas5-7 and metal vapor synthesis with trapping of solvated metal clusters in alkane solvents at low temperature.8-10 A variety of solution chemical reduction methods has also been applied to make nanostructured materials; alkali-metal borohydrides,11-13 alkalides, and electrides14,15 have each been used to reduce metal halides to produce nanoscale metals. Thermal decomposition and laser pyrolysis of organometallic compounds have been recently used to generate nanostructured metals.16,17 In addition, colloid chemical methods generating nanostructured materials were reviewed recently by Fendler and Meldrum.18 And of course, the synthesis of conventional heterogeneous
catalysts on oxide supports can also produce high dispersions.19 To this range of techniques, we have added the sonochemical reactions of volatile organometallics as a general approach to the synthesis of nanophase materials. The chemical effects of ultrasound do not come from a direct interaction between the sound field and molecular species. Sonochemistry arises from acoustic cavitation: the formation, growth, and implosive collapse of bubbles within a liquid.20 The collapsing bubbles generate localized hot spots. This local heating produces a wide range of high-temperature and highpressure chemistry as well as the emission of light (i.e., sonoluminescence). The conditions formed in these hot spots have been experimentally determined to have transient temperatures of ∼5000 K, pressures of ∼1800 atm, and cooling rates in excess of 1010 K/s.21,22 Using these extreme conditions, we have produced a variety of nanostructured and often amorphous metals, alloys, and carbides and examined their catalytic activity.23-31 Volatile organometallic compounds decompose inside a collapsing bubble, and the resulting metal atoms agglomerate to form nanostructured materials.
Abstract published in Advance ACS Abstracts, July 15, 1996. (1) Weller, H. Adv. Mater. 1993, 5, 88. (2) Ozin, G. A. Adv. Mater. 1992, 4, 612. (3) Gleiter, H. Adv. Mater. 1992, 4, 474. (4) Chorley, R. W.; Lednor, P. W. Adv. Mater. 1991, 3, 474. (5) Agata, M.; Kurase, H.; Hayashi, S.; Yamamoto, K. Solid State Comm. 1990, 76, 1061. (6) Paperfthymiou, V.; Kosticas, A.; Simopoulos, A. J. Appl. Phys. 1990, 67, 4487. (7) Hahn, H.; Averback, R. S. J. Appl. Phys. 1990, 67, 1113. (8) Davis, S. C. ; Klabunde, K. J. Chem. Rev. 1982, 82, 152. (9) Klabunde, K. J.; Li, Y.-X.; Tan, B.-J. Chem. Mater. 1991, 3, 30. (10) Klabunde, K. J.; Zhang, D.; Glavee, G. N.; Sorensen, C. M. Chem. Mater. 1994, 6, 784. (11) Bo¨nnemann, H.; Brijoux, W.; Brinkmann, R.; Joussen, T. Angew. Chem., Int. Ed. Engl. 1990, 29, 273. (12) Bo¨nnemann, H.; Brijoux, W.; Brinkmann; Dinjus, E.; Joussen, T.; Korall, B. Angew. Chem., Int. Ed. Engl. 1991, 30, 1312. (13) Zeng, D.; Hampden-Smith, M. J. Chem. Mater. 1993, 5, 681. (14) Tsai, K.-L.; Dye, J. L. J. Am. Chem. Soc. 1991, 113, 1650. (15) Tsai, K.-L.; Dye, J. L. Chem. Mater. 1993, 5, 540. (16) Lisitsyn, A. S.; Golovin, A. V.; Chuvilin, A. L.; Kuznetsov, V. L.; Romanenko, A. V.; Danilyuk, A. F.; Yermakov, Y. I. Appl. Catal. 1989, 55, 235. (17) (a) Rice, G. W. ACS Symp. Ser. 1993, 530, 273. (b) Chaiken, J. Chem. Ind. 1993, 751.
(18) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (19) (a) Satterfield, C. N. Heterogeneous Catalysis in Industrial Practice, 2nd ed.; McGraw-Hill: New York, 1991. (b) Twigg, M. V. Catalyst Handbook, 2nd ed.; Wolfe Publishing: London, 1989. (c) Stiles, A. B. Catalyst Manufacture: Laboratory and Commercial Preparations; Dekker: New York, 1983. (20) Suslick, K. S. In Ultrasound: Its Chemical, Physical, and Biological Effects; Suslick, K. S., Ed.; VCH Press: New York, 1988; p 123. (21) Suslick, K. S. Science 1990, 247, 1439. (22) Flint, E. B; Suslick, K. S. Science 1991, 253, 1397. (23) Suslick, K. S. MRS Bull. 1995, 20, 29. (24) Suslick, K. S. Ultrasound: Applications to Materials Chemistry. In Encyclopedia of Materials Science and Engineering; Cahn, R. W., Ed.; Pergamon Press: Oxford, 1993; 3rd Suppl., pp 2093-2098. (25) Suslick, K. S.; Hyeon, T.; Fang, M; Cichowlas, A. A. Mater. Sci. Eng. A 1995, 204, 186. (26) Fang, M. Catalytic and Magnetic Properties of Nanostructured Materials Generated by Ultrasound. Ph.D. Thesis, University of Illinois at Urbana-Champaign, 1995. (27) (a) Suslick, K. S.; Choe, S. B.; Cichowlas, A. A.; Grinstaff, M. W. Nature 1991, 353, 414. (b) Grinstaff, M. W.; Cichowlas, A. A.; Choe, S. B.; Suslick, K. S. Ultrasonics 1992, 30, 168. (28) Grinstaff, M. W.; Salamon, M. B.; Suslick, K. S. Phys. Rev. B 1993, 48, 269.
Introduction
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Nanostructured Materials
Chem. Mater., Vol. 8, No. 8, 1996 2173
Figure 1. Sonochemical synthesis of various forms of nanostructured materials.
Sonochemical decomposition rates for volatile organometallic compounds depend on a variety of experimental parameters such as vapor pressure of precursors, solvent vapor pressure, and ambient gas. To achieve high sonochemical yields, the precursors should be highly volatile since the primary sonochemical reaction site is the vapor inside the cavitating bubbles.32 So that decomposition takes place only during cavitation, thermal stability is also important. In addition, the solvent vapor pressure should be low at the sonication temperature, because significant solvent vapor inside the bubble reduces the bubble collapse efficiency.20 Our sonochemical synthesis of nanostructured materials is also extremely versatile: various forms of nanophase materials can be generated simply by changing the reaction medium (Figure 1). When precursors are sonicated in high-boiling alkanes, nanostructured metal powders are formed. If sonication occurs in the presence of a bulky or polymeric ligand (e.g., poly(vinylpyrrolidone) (PVP)), stable nanophase metal colloids are created. A transmission electron micrograph of the nanocolloid Fe/PVP is shown in Figure 2. Sonication of the precursor in the presence of an inorganic support (silica or alumina) provides an alternative means of trapping the nanometer clusters. These nanoparticles trapped upon these supports produce active supported heterogeneous catalysts. To demonstrate the utility of sonochemistry for nanostructured materials preparation, we will examine here the sonochemical synthesis and heterogeneous catalytic studies of nanostructured amorphous iron, nanostructured Fe on silica, nanophase Fe-Co alloys, and nanostructured Mo2C. Experimental Details General Procedures. All manipulations for the preparation of samples were performed using Schlenk vacuum line and inert-atmosphere box (Vacuum Atmospheres, 96% iron by weight, with trace amounts of carbon (
