Figure 1 A) Schemes of growth of metal phase at different positions of semiconductor nanoparticles (top) and growth of semiconductor on the metal nanoparticle (bottom). B) Transmission electron microscopy (TEM) image of Pt-CdS heterostructure, the scale bar is 20 nm, C) The number of Pt nanocrystals on a CdS nanorod as a function of its length. t = 120 min. D-G) Au nanocrystals growth on CdS nanorods for 1 h: D) 313 K, dark E) 313K, 473 nm laser irradiation, F) 273 K, dark and G) 273 K, 473 nm laser irradiation, the scale bar is 20 nm. A) Reproduced with permission.[5e] Copyright 2010, Wiley-VCH. B-C) Reproduced with permission.[6c] Copyright 2008, Wiley-VCH. D-G) Reproduced with permission.[6d] Copyright 2009, American Chemical Society.
2.2. Materials diffusion
The driving force of Ostwald ripening comes from the interface energy which tends to reach its lowest value under thermodynamic conditions. As the larger particles grow at the expense of the smaller particles, the specific interface energy per unit mass decreases, while the total free energy of the system is reduced.[5e] This mechanism can be used in the design and fabrication of metal-semiconductor hybrid nanostructures. Bala et al. used this mechanism for surface diffusion growth. The phase transfer of AuCl4- and subsequent metal reduction were carried out using a water-soluble HAuCl4 precursor and octylamine, and Au was grown to the tip of the CdSe nanorod. Initially, as shown in Figure 2B, several small Au particles were found on CdSe nanorods like gold islands, not only at the tip but also on the body of the rod. After 4 hours, it was found that all the small gold islands on the CdSe were aggregated at one end of the nanorods and became bigger (Figure 2C). The metal tip size can be adjusted through controlling the nucleation time and growth rate of the gold islands at different locations during the growth process. Under the Ostwald ripening mechanism, when the size of each Au nanoparticle reaches thermodynamic stability, the smaller tip will gradually be oxidized and dissolved into the solvent, and electrons will be transferred from the surface of the nanorod to the Au-tip, thereby continuing to produce Au and makes the nanoparticles grow at one end.[7]Therefore, the metal-semiconductor hybrid structure with diverse metal domain size and growth location can be achieved.