Nanomaterials Employing Ultrasound

Solids made from nanometer-sized components often exhibit properties distinct from those of the bulk, in part because clusters that small have an electronic structure with a high density of states, but not continuous bands [14, 15]. Such nonstructural materials are of intense current interest, and several preparative methods have been developed for their synthesis. As a general approach to this synthesis of nanophase materials (Fig. 5), extreme conditions of cavitation produce a variety of nanostructured and often-amorphous metals, alloys, and carbides [16-19]. Volatile organometallic compounds decompose inside a collapsing bubble, and the resulting metal atoms agglomerate to form nanostructured materials. The sonochemical synthesis of nanostructured materials is also extremely versatile; simply changing the reaction medium can generate various forms of nanophase materials. When precursors are sonicated in low-volatile alkanes, nanostructured metal powders are formed. If sonication occurs in the presence of a bulky or polymeric surfaced ligand, stable nanophase metal colloids are created. Sonication of the precursor in the presence of an inorganic support (silica or alumina) provides an alternative means of trapping the nanometer clusters. The nanoparticles, once fixed on the surface of these supports, are very active supported heterogeneous catalysts.

Figure 5. Sonochemical synthesis of nanostructured materials is highlighted. Given the unusual conditions created during cavitation, various potential applications for generating amorphous and nanoscale materials is given, including nanometals, alloys, nanometal colloids and catalysts, and nanometal chalcogenides. Reprinted with permission from [16], K. S. Suslick, MRS Bull. 20, 29 (1995). © 1995, Materials Research Society.

Figure 5. Sonochemical synthesis of nanostructured materials is highlighted. Given the unusual conditions created during cavitation, various potential applications for generating amorphous and nanoscale materials is given, including nanometals, alloys, nanometal colloids and catalysts, and nanometal chalcogenides. Reprinted with permission from [16], K. S. Suslick, MRS Bull. 20, 29 (1995). © 1995, Materials Research Society.

4.1. Nanometals

Suslick and Grinstaff were the first to demonstrate the use of ultrasound to prepare amorphous metals. The ultrasonic irradiation of volatile solutions of transition-metal carbonyls (e.g., Fe[CO]5, Co[CO]3NO) produces highly porous aggregates of nanometer-sized clusters of amorphous metals [20]. Typically, sonication of 1 ¡m penta-carbonyl in decane at 0 0C under a flow of argon yielded a dull black powder. Scanning electron micrographs (SEMs) revealed that the powder is an agglomerate of 20 nm particles; transmission electron micrographs (TEMs) further indicated that these 20 nm particles consist of smaller 4-6 nm particles. The amorphous nature of the iron powder was confirmed by several different techniques, including SEM, differential scanning calorimetry (DSC), electron diffraction, X-ray powder diffraction, and neutron diffraction. Initial X-ray powder diffraction showed no diffraction peak; with heat treatment under helium at 350 0C, the diffraction lines characteristic of bcc iron metal are observed. DSC also shows one exothermic irreversible disorder-order transition temperature at 308 0 C. The amorphous metal formation appears to result from the extremely high cooling rate during acoustic cavitation.

There had been a long-standing controversy concerning the magnetic properties of amorphous iron, which had not been previously available without a substantial amount of added alloying elements (e.g., boron). Magnetic studies of the sonochemically prepared amorphous iron showed that amorphous iron is a very soft ferromagnet with a saturation magnetization of ca. 173 emu • g-1 and a Curie temperature in excess of 580 K. The effective magnetic moment is 1.7¡B, with an effective exchange constant of only ca. 30% of crystalline Fe [21-23]. The magnetic properties fall close to those of liquid iron. The neutron-diffraction data confirmed these measurements, and are consistent with a random packing model, as observed for many thin amorphous metal films. The magnetic moment ¡i of the amorphous iron determined by neutron diffraction data is significantly below that of crystalline iron, and good agreement with other iron-based alloys on some extrapolation is also observed. Sonochemically synthesized chromium amorphous powders [24] showed interesting particulate characteristics unlike Fe nanopowders. The SEM micrograph (Fig. 6) showed the particles to be monosized and necked to each other in an ordered fashion. Such an arrangement reflects room-temperature sintering effects due to the surface energy of the highly reactive powders. A method from Katabi and co-workers reported the preparation of amorphous Ni powder with a particle size of about 10 nm by sonolysis of neat Ni(CO)4 with decane as a solvent [25].

Manoharan and Rao [26] reported that nanoscale particles of metallic cobalt clusters are obtained from Co2(CO)8 in decalin in a flow of Ar gas. Ultrasonic irradiation of the Co-carbonyl yields a dull black powder, highly pyrophoric when exposed to air. The most striking feature of sonochem-ically prepared Co is that it exhibits a negative magnetoresistance of the order of ~1.5% at room temperature (Fig. 7). The temperature and magnetic-field dependencies of the conductivity for the nanosized cobalt follow a simple activated behavior. These observations indicate that conduction is by spin-polarized electron hopping between the metallic nanoparticles.

Pol and co-workers [27] reported the synthesis of silver nanoparticles with an average particle size of nm on the surface of preformed silica submicrospheres with the aid of power ultrasound (Fig. 8). Ultrasound irradiation of slurry of silica submicrospheres, silver nitrate, and ammonia in an aqueous medium for 90 min under an atmosphere of argon to hydrogen (95:5) yielded a silver-silica nanocomposite. Control over the size and shape of the silver particles was achieved by a modified pulse sonoelec-trochemical method. Such a process yields spherical Ag nanoparticles, Ag nanorods, and elegant, highly ordered dendritic nanostructured silver [28]. It was found that the concentration of both silver nitrate and nitrilotriactete plays a key role in the formation growth of different shaped nanoparticles. The mechanism of formation of silver nanoparticles from a solution of silver nitrate in an Ar/H2 (95:5) atmosphere takes into consideration the radical species generated from water molecules by the absorption of

Figure 6. Scanning electron micrograph of sonochemically prepared chromium metal powder shows particles to be monosized and necked to each other in an ordered fashion arising out of high surface energy. Reprinted with permission from [24], M. L. Rao, Ph.D. Dissertation, IIT Kanpur, 2003.

Figure 7. Negative magnetoresistance in sonochemically prepared cobalt metal. Reprinted with permission from [26], S. Sundar Manoha-ran and M. L. Rao (private communication).

Figure 7. Negative magnetoresistance in sonochemically prepared cobalt metal. Reprinted with permission from [26], S. Sundar Manoha-ran and M. L. Rao (private communication).

water molecules. An argon-hydrogen atmosphere produces more H radicals than in air, thereby enhancing the reduction of Ag+ ions under sonochemical conditions [29].

Nanoscale particles of metallic copper clusters have been prepared by the sonochemical reduction of copper (II) hydrazine carboxylate complex (CHC) in an aqueous medium. Ultrasonic irradiation of the CHC under a mixture of argon and hydrogen yielded pure copper metallic nanoclusters, which also explains the scavenging action of hydrogen toward the OH- radicals, generated from water molecules on the absorption of ultrasound [30].

Arul Dhas and co-workers [31] reported the in-situ preparation of amorphous carbon-activated palladium nano-particles. Such palladium metallic clusters have been prepared (in-situ) at room temperature by ultrasound irradiation of an organometallic precursor, tris-mu-[dibenzyl ideneacetone]- dipalladium [CH=CH-CO-CH =CH3Pd2] in mesitylene [32]. Mizukoshi and co-workers [33] demonstrated the sonochemical reduction processes of Pt

Figure 8. TEM image of silver nanoparticle coated on silica surface. Reprinted with permission from [27], V. G. Pol et al., Langmuir 18, 3352 (2002). © 2002, American Chemical Society.

(IV) ions in water in the presence of various kinds of surfactants, such as sodium dodecylsulfate (SDS) and sodium dodecylbenzenesulfonate (DBS) as anionic surfactants, polyethylene glycol monostearate (PEG-MS) as nonionic, and dodecyltrimethylammonium chloride (DTAC) and bromide (DTAB) as cationic surfactants. An improved colorimetric determination reveals that the Pt (IV) ion is reduced to zero valent metal in two steps: step (1) Pt (IV) ion to Pt (II) ion, and step (2) Pt (II) ion to Pt (0), and after the completion of step (l), step (2) sets in. The average diameter (1.0 nm) of platinum particles prepared from the system of PEG-MS is smaller than those from the aqueous solution of the anionic surfactant SDS (3.0 nm) and DBS (3.0 nm). Diodati and coworkers [34] have reported the non-crystalline phase of palladium by cavitation technique starting from a solution of palladium acetylacetonate and toluene. The microscopic structure of the sample, a very fine powder, was investigated by X-ray diffraction and it showed the characteristic features of a disordered system.

Okitsu and co-workers [35] found that sonochemically prepared metal particles such as Ag, Pd, Au, Pt, and Rh are of nanometer size with a fairly narrow distribution (e.g., about 5 nm for Pd particles obtained from a 1.0 mM Pd (II) in polyethylene glycol monostearate solution). Three different reduction pathways under sonication: (1) reduction by H atoms, (2) reduction by secondary reducing radicals formed by hydrogen abstraction from organic additives with OH radicals and H atoms, and (3) reduction by radicals formed from the pyrolysis of the additives have been identified at the interfacial region between cavitation bubbles and the bulk solution. The reduction of Ag (I) and Pt (II) mainly proceeds through reaction pathway (2). In the cases of Pd (II) and Au (III), the reductions mainly proceed through reaction pathway (3). Similar reports on the synthesis of Ag and Au, and bimetallic particles such as Au/Pd and Pt/Pd were also presented [36-39]. The effect of atmospheric gas on the particle size and size distribution of nanoparticles has been reported [39].

4.2. Nanometal Composites, Ferrofluids, Colloids, and Catalysts

Composite materials containing amorphous iron embedded in poly(methylacrylate), PMA, or poly(methylmethacrylate), PMMA, and amorphous cobalt embedded in PMA were reported by Wizel and co-workers [30, 41], using a sono-chemical method. The physical and thermal properties of the composite materials were probed. A significant difference in the solubility of the iron-PMA and cobalt-PMA in various solvents was observed. This difference is accounted for by the stronger interaction existing between the cobalt and the surrounding polymer. For iron-PMA, this interaction is weakened due to the formation of an iron complex. Composites made of PMA and amorphous iron nanoparticles reveal a superparamagnetic behavior.

The existence of aggregates of nanometer clusters in the sonochemically prepared materials suggests the possibility of trapping these particles before they aggregate. The colloids of ferromagnetic materials are of special interest due to their many important technological applications as ferro fluid [42]. Such magnetic fluids find uses in information media, magnetic refrigeration, audio reproduction, and magnetic sealing. Exhaustive grinding of magnetite [Fe3O4] in ball or vibratory mills for several weeks in the presence of surfactants generally produces commercial magnetic fluids, which produces a very broad particle-size distribution.

Suslick and co-workers [43] developed a new method for the preparation of stable ferromagnetic colloids of iron. High-intensity ultrasound is used to sonochemically decompose volatile organometallic compounds in the presence of colloid-stabilizing ligands such as poly-vinylpyrrolidone (PVP) or oleic acid. A TEM micrograph show that the iron particles have a relatively narrow range in size from 3 to 8 nm for polyvinylpyrrolidone, while oleic acid gives a more uniform distribution at 8 nm. Electron microdiffraction revealed that the particles are amorphous on the nanometer scale as formed, and that, during in-situ electron-beam heating, these particles crystallize to bcc iron. Magnetic studies indicate that these colloidal iron particles are superpara-magnetic, with a respectable saturation magnetization of 101 emu • g-1 per iron at 290 K. High saturation magnetization is desirable for magnetic fluid applications, and is highly sensitive to the method of preparation. Bulk amorphous Fe saturates at 156 emu • g-1 per iron. In comparison, the saturation magnetization of a commercial magnetic fluid is 123 emu • g-1 [Fe] (Ferrofluids Corp. cat. no. APG-047) [21, 22].

Self-assembled monolayer coatings of amorphous iron nanoparticles binding the thiol chromophore were recently reported by Gedanken and co-workers [44-46]. The coverage of the iron surface by a long alkyl-chain thiol was as good as the coating of the short alkyl-chain thiol. Magnetic proteinaceous microspheres composed of iron oxide filled and coated with globular bovine serum albumin (BSA) were been reported by Levi and co-workers [47]. The magnetic microspheres are prepared from BSA and iron penta-carbonyl, or from BSA and iron acetate.

Suslick et al. [48] probed the catalytic activity of the amorphous iron powder for two commercially important reactions: the Fischer-Tropsch process (hydrogenation of CO), and the hydrogenolysis and dehydrogenation of saturated hydrocarbons. The amorphous iron powder was roughly ten times more reactive than 5 ¡m diameter crystalline powder. Also, the overall activity for cyclohexane dehydogenation (to benzene) and hydrogenolysis (predominantly to methane) was 30 times greater for the sonochemically produced amorphous iron compared to its crystalline counterpart.

The synthesis and characterization of molybdenum colloidal particles were evaluated using sonochemistry, and starting from different metal precursors, Mo(CO)6 and (NH4)2MoS4. Using Mo(CO)6 as the metal source, particle sizes with an average diameter of 1.5 nm can be obtained using tert-amyl alcohol as a solvent and tetrahydrothio-phene as a sulfurating ligand. The characterization of these particles showed that they are composed of molybdenum oxide MoO3. Using (NH4)2MoS4 as a metal precursor, particles with average diameters of 2.5 nm were synthesized using high-intensity ultrasound. The characterization of these particles showed them to be composed of molybdenum sulfide, MoS2 [49].

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