Biomedical subjects

Raymond E Schaak

Publications and source records attributed to Raymond E Schaak.

9 recordsLinked to original sources

PubMed2006

Multistep solution-mediated formation of AuCuSn2: mechanistic insights for the guided design of intermetallic solid-state materials and complex multimetal nanocrystals.

Understanding how solids form is a challenging task, and few strategies allow for the elucidation of reaction pathways that are useful for designing new solids. Here, we describe an unusual multistep reaction pathway that leads to the formation of AuCuSn(2), a new ternary intermetallic compound that was discovered as nanocrystals using a low-temperature solution route. The formation of AuCuSn(2) using a modified polyol process occurs through a multistep pathway that was elucidated by taking aliquots throughout the course of the reaction and studying the products using a variety of techniques. The reaction proceeds through four distinct steps: (a) formation of Au nanoparticles at or near room temperature, mediated by a galvanic reaction between Au(3+) and Sn(2+) (forming Au(0) and Sn(4+), precipitated as SnO(2) that forms a shell around the nanoparticles), (b) formation of NiAs-type AuSn nanoparticles, along with Cu and Sn, upon addition of NaBH(4), (c) aggregation and thermal interdiffusion to form AuCu(x)Sn(y) alloy nanoparticles, and (d) nucleation of intermetallic AuCuSn(2), which has an ordered NiAs-derived structure. The proposed mechanism was tested by starting the reaction with the AuSn intermediate. AuSn nanoparticles were synthesized separately and reacted with Cu and Sn nanoparticles, and ordered AuCuSn(2) formed as expected. Elucidation of this reaction pathway has important implications for guiding the design of new intermetallic solids, as well as for controlling the synthesis of complex multimetal nanocrystals.

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PubMed2006

Reacting the unreactive: a toolbox of low-temperature solution-mediated reactions for the facile interconversion of nanocrystalline intermetallic compounds.

Metallurgical materials, including intermetallic compounds, are notoriously inert toward low-temperature reactivity. However, as nanocrystals, their reactivity is significantly enhanced. Here we show that intermetallic PtSn and AuCu nanocrystals can be converted, in solution at low temperatures, into derivative intermetallics. For example, PtSn can be converted into PtSn2 and Pt3Sn by reaction with SnCl2 and K2PtCl6, respectively. The reactions are also reversible, for example, the sequences PtSn --> PtSn2 --> PtSn and PtSn --> Pt3Sn --> PtSn are all readily achievable. The strategy also allows nanocrystalline AuCu to be successfully converted into AuCu3 via reaction with Cu(C2H3O2)2.H2O, suggesting that this approach may be general.

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PubMed2006

One-pot synthesis of bi-disperse FePt nanoparticles and size-selective self-assembly into AB2, AB5, and AB13 superlattices.

By chemically modifying the nucleation burst that generates monodisperse FePt nanocrystals, a mixture of Pt and Fe(x)Pt(1-x) nanoparticles forms during a one-pot reaction that includes a small amount of Cu as a catalyst; size-selective precipitation yields a bi-disperse population of Fe(x)Pt(1-x) nanoparticles, which can assemble into high-quality AB2, AB5, and AB13 superlattice structures.

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PubMed2005

One-pot synthesis of hollow superparamagnetic CoPt nanospheres.

Hollow metal nanospheres are of interest for a variety of academic and technological applications, including drug delivery, catalysis, plasmonics, and lightweight structural composites. Despite recent advances in synthesizing metal nanostructures with controlled morphologies, there are very few reports of hollow bimetallic nanospheres, although such systems promise to offer advantages over single-metal systems. Here, were report a one-pot synthetic strategy for accessing hollow CoPt nanospheres with a Co-Pt alloy structure. The approach utilizes an in situ Co template and exploits galvanic displacement reactions to selectively dissolve the Co core while depositing a Pt shell. The combination of reducing conditions and a polymer stabilizer appears to allow the Co and Pt to co-reduce and form a Co-Pt fcc alloy phase with a morphology that is templated by the sacrificial Co core. The hollow CoPt nanospheres, which show magnetic hysteresis at low temperatures, are thermally stable up to 300 degrees C. The approach, which adds to a growing toolbox of reactions that yield morphologically controlled magnetic CoPt and FePt nanomaterials, is likely to be general for a variety of alloy systems.

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PubMed2005

Low-temperature polyol synthesis of AuCuSn2 and AuNiSn2: using solution chemistry to access ternary intermetallic compounds as nanocrystals.

Ternary intermetallic compounds, which possess a wide variety of important properties with both academic and technological relevance, are typically synthesized using traditional high-temperature methods. Here, we demonstrate that the polyol method, which is used extensively to synthesize nanocrystals and nanocrystalline powders of metals and simple binary compounds, serves as an effective low-temperature exploratory medium for synthesizing new ordered ternary intermetallics as nanocrystals. Accordingly, we describe the synthesis and structural characterization of AuCuSn2 and AuNiSn2, which adopt an ordered NiAs-type superstructure that is not observed using equilibrium synthetic methods. AuCuSn2 forms in solution of 120 degrees C as well-formed nanocrystals, and the ordered phase is stable up to 450 degrees C. AuNiSn2 behaves similarly to AuCuSn2.

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PubMed2005

Metallurgy in a beaker: nanoparticle toolkit for the rapid low-temperature solution synthesis of functional multimetallic solid-state materials.

Intermetallic compounds and alloys are traditionally synthesized by heating mixtures of metal powders to high temperatures for long periods of time. A low-temperature solution-based alternative has been developed, and this strategy exploits the enhanced reactivity of nanoparticles and the nanometer diffusion distances afforded by binary nanocomposite precursors. Prereduced metal nanoparticles are combined in known ratios, and they form nanomodulated composites that rapidly transform into intermetallics and alloys upon heating at low temperatures. The approach is general in terms of accessible compositions, structures, and morphologies. Multiple compounds in the same binary system can be readily accessed; e.g., AuCu, AuCu3, Au3Cu, and the AuCu-II superlattice are all accessible in the Au-Cu system. This concept can be extended to other binary systems, including the intermetallics FePt3, CoPt, CuPt, and Cu3Pt and the alloys Ag-Pt, Au-Pd, and Ni-Pt. The ternary intermetallic Ag2Pd3S can also be rapidly synthesized at low temperatures from a nanocomposite precursor comprised of Ag2S and Pd nanoparticles. Using this low-temperature solution-based approach, a variety of morphologically diverse nanomaterials are accessible: surface-confined thin films (planar and nonplanar supports), free-standing monoliths, nanomesh materials, inverse opals, and dense gram-scale nanocrystalline powders of intermetallic AuCu. Importantly, the multimetallic materials synthesized using this approach are functional, yielding a room-temperature Fe-Pt ferromagnet, a superconducting sample of Ag2Pd3S (Tc = 1.10 K), and a AuPd4 alloy that selectively catalyzes the formation of H2O2 from H2 and O2. Such flexibility in the synthesis and processing of functional intermetallic and alloy materials is unprecedented.

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PubMed2004

Colloidal crystal microarrays and two-dimensional superstructures: a versatile approach for patterned surface assembly.

A simple, fast, and robust approach to colloidal assembly on patterned surfaces was developed. The approach involves the rapid settling and dewetting of suspensions of spherical colloids on lithographically templated surfaces. Using this method, we can quickly and easily fabricate close-packed colloidal crystal microarrays of both silica and polystyrene spheres that range in size from 500 nm to 4.5 microm. The microarrays tend to induce the formation of monolayer colloidal crystals, which can be interconnected and removed from the templates as free-standing colloidal crystal slabs. The same approach can also be used to assemble two-dimensional colloidal crystal superlattices that can adopt a variety of structures. Graphite, kagome, body-centered cubic, open hexagonal, tetragonal, and linear chain structures can all be quickly accessed by adjusting the ratio of the sphere diameter to the template diameter.

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PubMed2004

Synthesis of atomically ordered AuCu and AuCu(3) nanocrystals from bimetallic nanoparticle precursors.

A new multistep approach was developed to synthesize atomically ordered intermetallic nanocrystals, using AuCu and AuCu(3) as model systems. Bimetallic nanoparticle aggregates are used as precursors to atomically ordered nanocrystals, both to precisely define the stoichiometry of the final product and to ensure that atomic-scale diffusion distances lower the reaction temperatures to prevent sintering. In a typical synthesis, PVP-stabilized Au-Cu nanoparticle aggregates synthesized by borohydride reduction are collected by centrifugation and annealed in powder form. At temperatures below 175 degrees C, diffusion of Cu into Au occurs, and the atomically disordered solid solution Cu(x)Au(1)(-)(x) exists. For AuCu, nucleation occurs by 200 degrees C, and atomically ordered AuCu exists between 200 and 400 degrees C. For AuCu(3), an AuCu intermediate nucleates at 200 degrees C, and further diffusion of Cu into the AuCu intermediate at 300 degrees C nucleates AuCu(3). Atomically ordered AuCu and AuCu(3) nanocrystals can be redispersed as discrete colloids in solution after annealing between 200 and 300 degrees C.

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PubMed2002

Exfoliation of layered rutile and perovskite tungstates.

The layered trirutile phases HMWO6 (M = Nb, Ta) and the layered perovskite H2W2O7 (synthesized by acid leaching of Bi2W2O9) were exfoliated into nanoscale colloids by reaction with quaternary ammonium hydroxides.

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