Interparticle growth

As the plug sits for a longer period of time, the masses anneal and the plug becomes more solid-like, with less flexibility. That is, both intra-and interparticle growth occurs. This annealing process is not shown in Figure 8.8. 

Once nanoparticles have been formed, whether in an early state of growth or in a more or less final size, their fate depends on the forces between the individual particles and between particles and solid surfaces in the solution. While particles initially approach each other by transport in solution due to Brownian motion, convection, or sedimentation, when close enough, interparticle forces will determine their final state. If the dominant forces are repulsive, the particles will remain separate in colloidal form. If attractive, they will aggregate and eventually precipitate. In addition, they may adsorb onto a solid surface (the substrate or the walls of the vessel in which the reaction is carried out). For CD, both attractive particle-sur-... 

In all the models discussed so far, the support surface was assumed to be flat. In fact, very many case studies, particularly of supported metals, have used flat, low-surface-area substrates. However, Wynblatt and Ahn [36] have demonstrated that surface curvature does affect the surface free energy, the growth of particles (sintering) via particle migration and interparticle transport. Therefore, the sintering process of practical supported catalysts which frequently use high-surface-area, porous supports must be significantly more complex than described by the simple models. 

The effect of temperature on the size distribution of the Au nanocrystals can be readily seen from the TEM images in Figure 2. The mean diameters of the nanocrystals formed at 30, 45, 60, and 75 °C are 7, 10, 12, and 15 nm, respectively, but the interparticle separation remains nearly the same at 1 nm. X-ray diffraction measurements show that with increase in temperature, the crystallinity of the film increases (Figure 3). The films obtained at 45 and 60 °C exhibit prominent (111) peaks (d = 2.33 A), while those obtained at 30 °C show weak reflections, probably due to the small particle size. The growth of the (111) peak with temperature indicates an increase in the particle size. 

The neck growth of the initial stage will be accompanied by interparticle shrinkage of only several percent. After the neck has become... 

In the model catalysts described so far, the interparticle distances were more or less random, governed by the separation of substrate defects which control the nucleation and growth process of the metal particles (74,101). The position and separation of metal particles can be controlled accurately by electron beam lithography (EBL) (which has also been used to fabricate model catalysts), but the minimum size of the metal aggregates is currently still approximately lOnm. Figure 3g shows an example of a platinum nanoparticle array on Si02 (mean size 28 nm interparticle separation 200 nm) (53,106,107). 

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