The tetraoctylammonium bromide (TOAB)-stabilized gold nanoparticles have been successfully fabricated. After an annealing of the as-synthesized nanoparticles at 300 °C for 30 min, the coalescence behavior of gold nanoparticles has been investigated using high-resolution transmission electron microscopy in detail. Two types of coalescence, one being an ordered combination of two or more particles in appropriate orientations through twinning, and the other being an ordered combination of two small particles with facets through a common lattice plane, have been observed.
KeywordsGold nanoparticles Coalescence Faceting
Low-dimensional quantum structures have shown to have unique optical and electronic properties. In particular, the shape and size of low-dimensional structures are crucial parameters that determine those physical properties [1, 2]. The characterization of these parameters is an important issue either in fundamental research or in technological applications, covering from fabrication and characterization to device processing. Among nanostructures, metallic nanoparticles such as gold and silver particles are also important because some of their main physical properties might be completely different from the corresponding ones in either molecules or bulk solids. Therefore, it is essential to investigate the size and shape of metallic nanoparticles, especially for gold nanoparticles.
In our earlier paper , we reported the fabrication and microstructure of Au–Cu2O nanocube heterostructures. Transmission electron microscopy (TEM) observations show that there are also intact gold nanoparticles apart from these nanocube heterostructures. The coarsening of gold nanoparticles during the annealing is usually attributed to Ostwald ripening , in which the crystal growth takes place by diffusion of atoms between neighboring nanoparticles. Recent studies [5–7] of TiO2 and ZnS nanocrystals growing under hydrothermal conditions have shown that the oriented attachment or coalescence plays an important role in the coarsening of nanocrystals. In addition, the coalescence of small particles by twinning was also reported in FePt and Si nanocrystals [8–10]. In the process of the oriented attachment or coalescence, the nanoparticles can themselves act as the building blocks for crystal growth. For gold nanoparticles, no detailed investigation on the coalescence behavior has been carried out using high-resolution transmission electron microscopy (HRTEM).
Gold nanoparticles were synthesized via a modification of a literature protocol [11, 12]. Briefly, an aqueous solution of HAuCl4 · 3H2O (0.03 M, 6 mL) was added to a solution of TOAB in toluene (0.15 M, 6 mL). The yellow aqueous phase became colorless, and the toluene phase turned orange as a result of phase transfer and complexing of [AuCl4− with tetraoctylammonium cations. After stirring for 10 min at room temperature, a freshly prepared aqueous solution of sodium borohydride, NaBH4 (0.26 M, 6 mL) was added dropwise into the reaction mixture over a period of 30 min, after which the mixture was vigorously stirred for additional 30 min. Subsequently, the organic phase was separated and was washed with 1% H2SO4 once and then with distilled-deionised water five times. Finally, the organic phase was dried using MgSO4 and filtered through a filter paper. The as-synthesized gold nanoparticles were characterized using conventional TEM and HRTEM. The specimen for TEM observation was prepared by evaporating a drop (5 μL) of the nanoparticle dispersion onto a carbon-film-coated copper grid.
In order to investigate coalescence behavior of the gold nanoparticles, a copper grid covered with gold nanoparticles was placed in an oven and the temperature was raised to and kept constant at 300 °C for 30 min. After the annealing, the gold nanoparticles were extensively examined using HRTEM. The bright-field (BF) imaging, selected-area electron diffraction (SAED) and HRTEM were carried out using a field emission gun (FEG) transmission electron microscope operating at 200 kV.
Results and Discussion
Extensive HRTEM observations demonstrate that most gold particles have an elongated shape and there are two types of coalescence. One type is that two or more small particles (without facets) coalesce into bigger ones through twinning, and the other type is that two particles (with facets) combine together through a common lattice plane. The statistical analysis of more than 200 gold nanoparticles showed that the volume fraction of the nanoparticles with characteristics of the first-type coalescence is around 40%, while the volume fraction for those with characteristics of second-type coalescence is around 20%.
As temperature increases, the TOAB stabilizing ligands melt and serve as solvent, and those gold nanoparticles which are situated close to each other can move around and start the coalescence process. As for the gold particles adopting which type of coalescence, it depends on the shapes of the initial particles, the concentration of particles put on the copper grid, their positions on the copper grid, and their crystal orientations.
In summary, the coalescence behavior of gold nanoparticles has been investigated using HRTEM. Two types of coalescence, one being an ordered combination of two or more particles through twinning and the other being combination of two particles through a common lattice plane, have been observed.
The authors would like to thank the financial support from Qingdao University. The project was supported by The Scientific Research Funding for the Introduced Talents (No. 06300701).