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.

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.

A modified polyol process has been used to synthesize intermetallic nanocrystals and nanowire networks directly in solution using a one-pot reaction. The synthesis of AuCu nanocrystals in tetraethylene glycol shows that atomically ordered intermetallic nanocrystals form above 250 °C, while atomically disordered alloy nanocrystals form at lower temperatures. The particle size increases with increasing solvent temperature, and there is a gradual shift from spherical to ellipsoidal morphology. Fully ordered intermetallic AuCu nanocrystals synthesized at 310 °C have an average particle width and height of 10 ( 3 and 8 ( 2 nm, respectively, and exist with faceted ellipsoidal, hexagonal, and cubic shapes. Replacing tetraethylene glycol with ethylene glycol, diethylene glycol, triethylene glycol, and glycerol yields highly branched nanowire networks. The morphology of the nanowire networks remains the same for all of the solvents, but the structure can be tuned from fully disordered alloy to fully ordered intermetallic AuCu, based on the boiling point of the solvent. The nanowire networks synthesized in ethylene glycol show that they likely form through a nanoparticle coalescence mechanism. By changing the stoichiometry of Au and Cu in solution, intermetallic AuCu 3 nanocrystals and nanowire networks can also be synthesized using tetraethylene glycol and glycerol, respectively. These results establish that it is possible to simultaneously control the structure, size, shape, and composition of intermetallic nanocrystals using solution chemistry, which has important implications for both fundamental scientific studies and future technological applications.

Silicon anodes based on an alloy reaction with lithium have a large theoretical specific capacity making them an appealing candidate for use in lithium-ion batteries. A major factor influencing the power cyclability and cycle life of the battery is the formation of the solid electrolyte interphase (SEI) layer. In this work, the progression of SEI formation on hydrogenated amorphous Si (a-Si:H) anodes is determined as a function of applied electrochemical potential during the first charging cycle by combining cyclic voltammetry measurements with detailed surface chemical analysis. During this first lithiation cycle, the SEI layer begins to form at 1.8 V by decomposition of the LiPF6 electrolyte to LiF, Li x PF y , and PF y . The SEI layer, with LiF as the major species, continues to form upon further charging and forms a nonuniform layer on the surface of the electrode. At 0.4 V the Li atoms begin to penetrate the a-Si:H network, and upon full charging at 0.0 V, the anode itself is comprised in part by Si–Li, Si–F, and a network of F–Si–Li n . During the second lithiation cycle, Li causes significant scission of the Si–Si bonds resulting in the formation of high concentrations of Li x Si.

Reduction of aqueous RhCl 3 with NaBH 4 in the presence of poly(vinyl pyrrolidone) (PVP) yields dense spherical nanostructures. The spherical aggregates, which generally have diameters between 10 and 100 nm, are built from smaller 1-3 nm Rh particles. The dense nanostructures are thermally stable beyond 100 °C, and they have a tendency to form ordered superstructures upon drying. Combining sodium n-dodecyl sulfate (SDS) with PVP modifies the size and morphology of the primary 1-3 nm particles, but does not change the spherical shape of the aggregates except at high concentrations of SDS. Smallangle X-ray scattering measurements show that the large aggregates are formed directly in solution from small Rh particles, consistent with TEM and AFM results. Magnetic measurements indicate that the Rh nanoparticle aggregates are Pauli paramagnetic.