Nanophase Materials
SCHEME 1 Minimum mechanism for the formation of Ir(0) nanoclusters, consisting of (a) slow, continuous nucleation (steps 1–3), rate constant k 1 for the pseudoelementary step,
A→ B, followed by (b) fast autocatalytic surface growth (step 4), rate constant k2for
the pseudoelementary step A⫹ B → 2B. Nucleation and growth are separated in time because k1 ⬍⬍ k2[B], which, in turn, is a key to the observed formation of a near-
FIG. 9 Idealized, roughly-to-scale representation of a P2W15Nb3O629⫺polyoxoanion and Bu4N⫹stabi-
lized Ir(0)⬃300nanocluster, [Ir(0)⬃300(P4W30Nb6O12316⫺)⬃33](Bu4N)⬃300Na⬃228. The Ir(0) atoms are
known (by electron diffraction) to be cubic close packed as shown. For the sake of clarity, only 17 polyoxoanions are shown, in their monomeric form, and the⬃300 Bu4N⫹and⬃228
Na⫹cations have been deliberately omitted. (From Ref. 29.)
number’’ theory of nanoclusters, which states that because closed-shell structures are stable, they will be more common in the size distribution of completed na- nocluster formations [29].
Further exploration of the catalysis properties of transition metal colloids indi- cates that their behavior is extremely reaction-specific and dependent on many factors. For example, in the synthesis of silicone polymers, the catalytic perfor- mance of bimetallic colloids of Au(core)/Pt(ligand) and Pd(core)/Pt(ligand) was compared to the performance of Pt nanoclusters formed during the induction period of typical industrial reactions [30]. Previously, Au–Pt had been shown to provide marked improvement over Pt alone in the semihydrogenation of 2-hexyne into cis- 2-hexene [31]. However, in the synthesis of bis(trimethylsiloxyl)octamethylsilane (BTMOS), the Au–Pt showed no improvement over Pt alone, whereas Pd–Pt showed marked improvement [30]. These experiments were designed in order to study the behavior of a bimetallic colloid with a more electronegative core than the ligand (Au–Pt) and compare it to that of one with a more electropositive core than ligand (Pd–Pt) [30]. For this particular commercial synthesis, the more elec- tropositive core seemed to yield superior results. However, in light of the Au–Pt semihydrogenation mentioned, experimental verification is required before such findings can be extended to other catalytic systems [30].
Nanocluster agglomeration is prevented through surface stabilization using li- gands, polymers, or surfactants [32]. Using a combination of STM and transmis-
FIG. 10 A schematic diagram showing how STM and TEM can be used in concert to determine surfactant stabilizer thickness. Thicknesses observed in this manner are consistent with stan- dard MM2 force-field calculation theoretical values. (From Ref. 32.)
sion electron microscopy (TEM) to observe and examine the size relationships between surfactant stabilizers and metal cores, it has been shown that shell thick- ness is independent of core size and is directly dependent on the size of the surfac- tant ion, as shown in Fig. 10 [32].
Synthesis
Any discussion of nanophase materials in chemical processes must address the topic of catalysis. The high surface area to volume ratios associated with nanophase materials makes them exceptional catalysts as compared to bulk materials. Al- though nanophase metal catalysts supported on substrata have been industry stan- dards for high-throughput reactions, in recent years particle size control has strived to produce stoichiometrically defined particles. Such a well-defined nanophase cat- alyst offers the prospect of engineering the ratio of edge atomic surface sites on a particle surface to alter reaction product efficiencies and mixtures. To this end, transition metal nanocluster systems have been synthesized which behave as isola- ble and compositionally well-defined soluble heterogeneous catalysts [33]. Further, new methodologies and a better understanding of nanoclusters in general are call- ing into question previously identified catalysts in previously explored reactions [33].
One such example is a hydrogenation of benzene, in which the accepted ion- pair catalyst, [(C8H17)3NCH3]⫹[RhCl4]⫺, has been discredited, with the catalyst shown more likely to be Cl⫺- and [(C8H17)3NCH3]⫹-stabilized Rh(0) nanoclusters [33]. This indicates that in the cases of other putative homogenous catalysts where a facile heterogeneous M(0) catalyst is well established, catalysis by even trace amounts of possibly highly active nanocluster catalysts cannot be ruled out [33].
The development of industrial catalysts,many of which are composed of metal particles on oxide substrata, is a topic of great commercial interest [34]. Fabrication and engineered control in order to deposit tailored metal clusters on oxide surface with an ordered structure remain a goal of catalyst research [34]. Lithographic
nanostructure fabrication technologies of the semiconductor industry can be ap- plied to create such engineered clusters and to provide viable model systems for industrial-supported catalysts [34]. This model of catalytic structure and behavior has been tested against ethylene hydrogenation on a platinum nanocluster [34].
In a study of the thermal stability of supported silver catalysts, these model systems and fabrication techniques were used to demonstrate that the presence of oxygen is the key factor influencing the thermal stability of the silver nanoclusters [34]. No migration of silver clusters was observed at⬎700°C in the absence of oxygen [34]. Yet, with increasing annealing temperature in oxygen, silver cluster surface oxidation occurs at⬍200°C [34].
Methods of nanostructure lithographic fabrication also can be applied to other surface science studies. When combined with AFM, these developments are used to determine surface mechanical properties, such as the elastic modulus, on nano- meter-scaled samples [34]. Methodologies also have been developed to measure the yield of an ion-sputtering process [34].
In addition to the synthesis and use of ligand-stabilized metal clusters, signifi- cant benefits are realized by polymer stabilization of nanocluster catalysts [35]. The platinum nanoclusters’ catalytic performance on polymer-based supports has been compared to performance on oxide supports [35]. Polymeric supports show a marked increase in the stability of the catalysts, especially at room temperatures [35].
Although stabilizers control particle size and prevent agglomeration, cluster surface passivation is often the result, with the net effect of reducing catalytic performance [36]. One proposed solution to this dilemma is the use of dendrimers to act simultaneously as monodisperse synthesis templates and stabilizers [36]. Researchers have partitioned transition metal ions, such as Pt, into the interior of polyamidoamine (PAMAM) and have achieved an encouraging combination of particle size control, particle stability, and electrocatalytic behavior and perfor- mance [36]. (See Fig. 11.)
Nanophase catalysts also function in decomposition reactions of harmful green- house gases,such as CO2[37]. Experiments with different ferrites (XFe2O4) have shown that with appropriate selection of X and particle size, CO2is broken down into carbon and oxygen with virtually no CO by-product [37]. The decomposition of CO2also produces quantities of methane. Comparisons of Zn, Ni, and Co ferrites and of varying particle sizes yielded dramatic results, with ZnFe2O4showing the most promise [37].
In addition to purely chemical catalysis, nanocluster materials also function as electrocatalysts. In the synthesis of sodium chlorate, the Dimensionally Stable Anode (DSA) has realized extensive energy savings on one side of the oxidation reaction, but high activation overpotentials on the cathodic side still contribute to large energy losses [38]. The experimental production of a solid Ti2RuFe nanocrys- talline cathode has shown promising results. In fact, once the nanocrystalline mate- rial was pressed into a usable cathode, the activation overpotential was reduced dramatically when compared to a standard iron electrode [38]. However, this method of electrode fabrication failed to produce anticipated current density in- creases expected from the very small crystal size [38].
Template methods, combined with chemical vapor deposition (CVD), also can be used in the preparation of carbon nanotubules, with diameters ranging from 20
FIG. 11 Schematic illustration of fourth-generation (G4) PAMAM dendrimers having EOH and NH2terminal groups, synthesis of Pt nanoparticles within the hydroxyl-terminated den-
drimer template, and attachment of the composite to an electrode surface. Between 12 and 60 Pt2⫹ions can be loaded into a single dendrimer and, upon reduction with BH
4⫺, an en-
trapped cluster containing the same number of atoms results. The dendrimer-encapsulated Pt nanoparticles are electrocatalytically active. (From Ref. 36.)
to 200 nm [39]. Such tubules can be filled with metal catalyst nanoclusters of the types previously discussed to display interesting and potentially useful electrocata- lytic properties [39]. A basic method used to synthesize metal-nanocluster-filled carbon nanotubes includes carbon deposition by CVD onto an alumina template membrane, followed by immersion in a metal ion solution. Air-drying and reduc- tion by hydrogen gas completes the formation of the metal nanoclusters within the nanotubules. Alumina is removed by HF immersion [39]. The mechanical and electrochemical properties displayed by metal-filled nanotubules hold promise for the fuel cell industry [39].
Glass Silicates
A physicochemical analysis of nanophase crystalline silicalite-1 shows that it has many properties in common with micrometer-sized crystalline silicalite-1 [40]. These common properties include a refined structure and concentrations of tetra- propylammonium species incorporated during the synthesis [40]. Yet, nanophase silicates have nondegenerate spectroscopic features such as the characteristic framework infrared (IR) vibration (550 cm⫺1) of the micrometer-sized crystal split- ting into a doublet (at 555 cm⫺1and 570 cm⫺1) in the nanophase material [40]. In addition, the nanophase material exhibits a high concentration of defect sites, a
FIG. 12 Log-log plot of the bulk (K), shear (G), and Young’s (E) moduli for bulk and nanophase
a-SiO2, as a function of the density relative to the bulk density. The solid lines are the best
least-squares fits for each of the moduli. (From Ref. 41.)
strain in the crystallites along the a crystallographic direction, as well as a two- stage dinitrogen physisorption in the low-pressure region [40].
The pore morphologies and mechanical behaviors of nanophase amorphous SiO2, investigated by molecular-dynamics (MD) simulations, show that the bulk amorphous densities of the various nanophase a-SiO2glasses are characterized by different pore sizes and distributions, yet the morphology of the pores, defined in terms of the fractal dimension of pores and the roughness exponents of pore–silica interfaces, is similar across various densities [41]. This consistent short-range order (SRO) of nanophase silica glass of various densities differs little from the SRO of bulk glass, with both structures consisting of corner-sharing Si(O1/2)4tetrahedra [41]. However, analysis of the first sharp diffraction peak (FSDP) shows a signifi- cant difference in the intermediate-range order (IRO), dependent on the density, which is controlled synthetically in the nanophase silica glass [41].
In terms of mechanical properties, the elastic moduli of both the bulk and the nanophase a-SiO2 clearly are density dependent [41], as shown in Fig. 12. The moduli (M) scale as M ⬀ (ρ-bar)3.5⫾ 0.2, where ρ-bar is the ratio of the sample density to the density of bulk silica glass [41]. An understanding of this power- law dependence can lead to very specific tailoring of physical and mechanical properties for various uses.
Glass Ceramics
Nanophase glass–ceramic technology also is a rapidly growing field with a wide variety of commercial applications. Nanophase microstructures composed of crys- tals⬍100 nm in size are achieved through efficient nucleation and slow crystal growth, providing an impressive uniformity of microstructure and the controlled mechanical properties not available in glass–ceramics of larger crystalline micro- structures [42].
Transparent nanophase glass–ceramics achieve near-zero coefficients of ther- mal expansion, along with high thermal stability, high thermal shock resistance, and, of course, transparency [42]. The requirements to achieve transparency have previously been recited [42]. Glass–ceramics with these properties typically utilize lithium-stuffedβ-quartz crystals to satisfy scientific and commercial applications,
such as telescope mirror blanks, stove cook tops, cookware, woodstove windows, fire doors, and other technical devices [42].
Near-zero coefficients of thermal expansion result from adjusting the various composition components and percentages in stuffed lattices [42]. ‘‘Stuffed’’ β- quartz crystals are so named because Al3⫹replaces Si4⫹in theβ-quartz framework of interlinked helixes of SiO2tetrahedra, with the charge balance being maintained by ions that stuff the interstitial tetrahedral cavities [42].
Other interesting subclasses of transparent glass–ceramics, many of which also have a ‘‘stuffed’’ microstructure, consist of materials with coefficients of thermal expansion near that of silicon [42], materials such as transparent mullite or spinel glass–ceramics that can serve as superior-performance host media for luminescent transition metal ions such as CR3⫹ [42], and oxyfluoride glass-ceramics used as hosts for optically active rare earth (RE) cations, because of their low phonon energies and broad transparency in the IR region of the spectrum, used for the amplification of light in telecommunications systems [42].
Conclusions
As we have seen, nanophase materials show great promise for many areas of chem- ical process. Their unique properties, brought about by a high surface area to vol- ume ratio and incomplete electronic band structures, give them extraordinary flex- ibility with regard to the desired tailoring of mechanical and chemical properties. Composition and configuration options offer exciting opportunities to take nano- phase materials out of the laboratory and into innumerable industrial applications.
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AVERY N. GOLDSTEIN DAVID M. FISHBACH