Size dependence of nanoparticle dissolution in a matrix: Gold in bismuth

Abstract

We discuss the dissolution of Au nanoparticles in a Bi matrix. The particles were produced by vapor deposition at room temperature, and they burrowed to minimize their free energy. Using in situ transmission electron microscopy, we measured the rate of dissolution as a function of particle size and deduced the activation energy for this process. We also followed the structural changes in the system by high-resolution electron microscopy and nanobeam diffraction. This study develops an important understanding of the dependence of size on the stability and reactivity of nanoparticles.The dissolution of an unstable second phase ͑precipitate͒ in a stable matrix dictated by bulk thermodynamics has been studied extensively. 1-3 As has been demonstrated, precipitate sizes, compositions, densities, and distributions affect the dissolution kinetics. Dissolution cannot be modeled simply as the antithesis of growth due to differences in time evolution of the solute concentration profile across the precipitatematrix interface. 1 Recent theoretical studies have shown deviations from the bulk phase behavior for small particles, and this affects precipitate formation and dissolution. 4,5 Size affects phase transformations by changing the solid solubility of components, 6,7 introducing metastable phases, 8 and/or lowering the transformation temperature. 9 Transformation kinetics are also enhanced due to modified diffusion channels. 10,11 Size enters through the decreased coordination of atoms at the curved interface which leads to an increased solute concentration given by the Gibbs-Thompson equation. 12 Size effects have been observed for supported nanoparticles, where depressions in melting and evaporation temperatures reflect increased vapor pressures around a curved surface. 13 The dependence of reactivity on size can be used to obtain their free energies, 14 and this has applications in variety of areas including catalysis. 15 Recently, we showed that kinetic processes associated with growth and burrowing make it possible to assess the significance of the latter on the size distribution of Au particles formed during the deposition of Au on Bi. 16 During atom deposition at room temperature, Au clusters nucleate and grow but-because of energy considerations-they burrow into the film after reaching sizes at which concepts such as surface and interface energies become meaningful. Because this different pathway leads to metastable nanoparticles in a matrix, we are now able to follow the second step in the thermodynamic pathway, namely, the dissolution of these particles. In this paper, we examine the size and temperature dependencies of that dissolution using in situ transmission electron microscopy ͑TEM͒. From the rate of dissolution, we are able to deduce the energetics of this process. We also examined the phase transformations using nanobeam electron diffraction ͑NBED͒ and followed structural changes in individual particles using high-resolution TEM ͑HRTEM͒.Sample preparation was performed in an ultrahigh vacuum chamber with...

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