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Intermetallic Ni-Zn nanoparticles (NPs) were synthesized via the chemical conversion of nickel NPs using a zerovalent organometallic zinc precursor. After the injection of a diethylzinc solution, Ni NPs progressively transformed from a solid to a hollow Ni-Zn intermetallic structure with time. During the transformation of Ni NPs to intermetallic structures, they retained their overall spherical morphology. The growth mechanism for the solid-to-hollow nanoparticle transformation is ascribed to the nanoscale Kirkendall effect due to unequal diffusion rates of Ni and Zn. We develop a diffusion model for nonreactive, homogeneous, diffusion-controlled intermetallic hollow NP formation including moving boundaries at the interfaces of void-solid and solid-bulk solutions. Apparent diffusion coefficients for both metals and vacancy were evaluated from modeling the time-dependent growth of the void. The apparent diffusion coefficients obtained in this system compared favorably with results from measurement at grain boundaries in bulk Ni-Zn. This study represents the first combined experimental modeling of the formation of hollow nanostructures by the nanoscale Kirkendall effect.

The thermodynamic adsorption profile at a solvated organic-inorganic interface is probed by following the binding and organization of carboxylic acid-terminated alkanethiols of varying chain lengths (C2, C3, and C6) to the surface of gold nanoparticles (NPs) (5.4 ± 0.7, 9.5 ± 0.6, and 19.4 ± 1.1 nm diameter) using isothermal titration calorimetry (ITC). We discuss the effect of alkyl chain length, temperature, and Au NP size on the energetics at an organic-inorganic interface. ITC allows for the quantification of the adsorption constant, enthalpy of adsorption, entropy of adsorption, and the binding stoichiometry in a single experiment. The thermodynamic parameters support a mechanism of stepwise adsorption of thiols to the surface of Au NPs and secondary ordering of the thiols at the organic-inorganic interface. The adsorption enthalpies are chain-length dependent; enthalpy becomes more exothermic as longer chains are confined, compensating for greater decreases in entropy with increasing chain length. We observe an apparent compensation effect: the negative ΔH is compensated by a negative ΔS as the thiols self-assemble on the Au NP surface. A comparison of the thermodynamic parameters indicates thiol-Au NP association is enthalpy-driven because of the large, exothermic enthalpies accompanied by an unfavorable entropic contribution associated with confinement of alkyl chains, reduced trans-gauche interconversion, and the apparent ordering of solvent molecules around the hydrophobic organic thiols (hydrophobic effect). Understanding the thermodynamics of adsorption at NP surfaces will provide critical insight into the role of ligands in directing size and shape during NP synthesis since thiols are a common ligand choice (i.e., Brust method). The ITC technique is applicable to a larger number of structure-directing ligands and solvent combinations and therefore should become an important tool for understanding reaction mechanisms in nanostructure synthesis.

Replica molding with elastomeric polymers has been used routinely to replicate features less than 10 nm in size. Because the theoretical limit of this technique is set by polymer-surface interactions, atomic radii, and accessible volumes, replication at subnanometer length scales should be possible. Using polydimethylsiloxane to create a mold and polyurethane to form the replica, we demonstrate replication of elementary steps 3-5 A in height that define the minimum separation between molecular layers in the lattices of the ionic crystals potassium dihydrogen phosphate and calcite. This work establishes the operation of replica molding at the molecular scale.