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.