Gold particle agglomerate

Processes in nature often result in fractal-like forms that differ from the mathematical fractals such as the Koch curve in two ways (a) the self-similarity is not exact but is a congruence in a statistical sense and (b) the number of repeated splittings is finite and random fractals have an upper and a lower cutoff length. A spatial example of a random fractal is the colloidal gold particle agglomerate shown earlier in Figure 7.4. 

Surface-functionalized gold particles were obtained by a phase-transfer reaction of gold ions with dodecanethiol. Figure 16.19 shows the structure of such particles. These particles are of interest because they have nonlinear optical polarizability. The particles were mixed with MMA and initiator and polymerized. No agglomeration of particles was detected, but the polymerization rate in the presence of nanoparticles was inhibited. 

After about 200 s the activity dropped sharply, owing to agglomeration of gold particles, the effect can be shown by determining the mean particle size during the kinetic test, as shown in Fig. 13.5. 

In contrast, TEM and PXRD data of Au MOE-5 show polydispersed Au particles in a size range of 5-20 nm (see Fig. 17), with a metal loading in Au MOF-5 determined to be 48 wt%. The gold particles appear to interact more weakly with the host matrix than the Pd, Ru and Cu particles and thus larger agglomerates are formed possibly by diffusion of the particles to the outer surface. 

Two synthesis methods were applied to prepare the TS-1 supports (TS-1 a and TS-lb). It was found that, after deposition of Au by precipitation method HDP-II, TS-1 a was a good catalyst, whereas catalyst TS-lb was totally inactive. In Figs. 1.15(a) and (b), TEM pictures of the Au-TS-1 samples are shown. The morphology of the two samples is quite different. TS-1 a consisted of very small crystallites (10 nm), agglomerated to form 150 nm particles, whereas TS-lb consisted of 35 pm crystals. The size of the gold particles in the TS-1 a sample with the small crystallites, was estimated to be 3-15 nm, which is not significantly smaller than those observed on the edges... 

The decrease in the gold particle size with the time of deposition-precipitation (Table 14.5) was attributed to a phenomenon of peptization (redispersion), as observed, for instance, when nanocolloids of titania, formed by hydrolysis of alkox-ides, agglomerate rapidly to produce large precipitates, but can be slowly redispersed through the action of nitric acid. 

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