Twinned particle

An examination of a dark-field micrograph of a field such as that of Fig. 1 confirms the presence of an appreciable proportion of multiply twinned particles, and also shows that most of the particles are orientated with a 111 parallel to the substrate (29, 30). 

A further point is that for a multiply-twinned particle of diameter 1 nm, for example, the constituent single crystal regions are half of this size or less and so contain only two or three planes of atoms. One can not expect, under these circumstances, that the diffraction pattern will be made up merely by addition of the intensities of the single crystal regions. Coherence interference effects from atoms in adjacent regions will become important. It is then necessary to compare the experimental patterns with patterns calculated for various model structures. 

Figure 2. Nanodiffraction patterns from small gold particles for an incident beam diameter of 1-2 nm (a) Observed for a particle of 2-3 nm diameter showing twinning on two planes (b) Observed for a multiply twinned particle of 1.5 nm diameter. (c) Calculated for a model multiply twinned particle. The black spots in (a) and (b) are the small mirrors in the optical analyser system used as detectors for imaging.

Single crystal particles and multiply twinned particles (MTP)... 

There has been considerable work in the literature on the structure of very small particles and clusters. Interest in this field has been primarily due to Ino s (1966) early experimental studies of normally fee metals prepared by vapour condensation which showed that a sizable portion of the particles exhibited non-crystallographic structures. These non-crystallographic atomic clusters or polycrystalline nuclei have been observed to consist of pentagonal bi-pyramid or icosahedra form of twinned structures and are known as multiply twinned particles (MTPs). EM studies of supported transition metal catalyst systems have indicated that MTPs sinter faster in catalytic reactions leading to the loss of surface area and are not beneficial to catalysis (Gai 1992). We describe the structure and the role of MTPs in catalysis in the following sections. 

Figure 5.12. Multiply twinned particles (MTPs) (a) decahedral MTP (top) and icosahedral MTP (b) an HRTEM image of Au MTP. The decahedral MTP is at the top.

Figure 5.17. TEM image of (a) fresh Pd/alumina catalyst and (b) used catalyst in the H2O2 process, showing sintered twinned particles.

Figure 6.11. Schematic diagram showing defects in a twinned particle. Here, m means misfit dislocation, and g.b. stands for grain boundary [102].

Figure 3. High-resolution TEM micrograph of the Ag/Si02-SG1/A catalyst prepared under acidic sol-gel conditions (STP singly-twinned particles, MTP multiply-twinned particles).

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