Size- and Temperature-Dependent Structural Transitions in Gold Nanoparticles

Size- and temperature-dependent structural transitions in gold nanoparticles were revealed with morphology statistics obtained by high-resolution electron microscopic observations for thousands of particles annealed in a helium heat bath. We found that gold nanoparticles over a wide size range, 3–14 nm, undergo a structural transformation from icosahedral to decahedral morphology just below the melting points. It was also clarified that the formation of bulk crystalline structures from the decahedral morphology requires the melt-freeze process due to an insurmountable high free-energy barrier.

Figure 1

HREM images of gold nanoparticles. As grown (a) Ih and (b) Dh (faceted pentagonal bipyramid) particles. (c) Dh particle formed by annealing at 1273 K. (d) Single-crystalline particle formed by the melt-freeze process. Scale bar, 5 nm.

Figure 2

Populations of different morphologies of gold nanoparticles as a function of size. Populations of Ih (–●–), Dh (--▲--), and fcc ($\cdots$■$\cdots$) particles, (a) before annealing and (b) after annealing at 1173 K, (c) 1223 K, (d) 1273 K, (e) 1373 K (cf. bulk melting point 1337 K). The volume-mean size is applied for Dh particles with low sphericity [10].

Figure 3

A size-temperature structural stability diagram of Ih gold nanoparticles. Melting points ($T_m$) and Ih-to-Dh transition points ($T_{\mathrm{Ih}\mathrm{\text{−}}\mathrm{Dh}}$) determined in this study are denoted by ● and $\cdots$■$\cdots$, respectively. The solid and dashed curves are melting temperatures predicted by Pawlow’s theory [16] and the liquid shell model [17], respectively. Melting points by previous simulation works are plotted by ○ [17], ◇ [18], and △ [19], where each value is normalized by the bulk melting temperature.

Figure 4

A cooperative slip-dislocation mechanism proposed for the Ih-to-Dh transition. Ih model (a) can convert to Dh structure (b) without any atomic diffusion processes, when each of ten central tetrahedral segments within the Ih model cooperatively deforms into a right-triangular pyramid shape. The slip dislocation of each {111} plane on its underlying plane in the tetrahedron (c), with a Burgers vector $\mathit{b}=a/6⟨211⟩$ (white arrow), brings about conversion into the right-triangular pyramid (d).