Particle multiply twinned

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 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).

An alternative method of preparing cobalt nanopartides is based on the thermal decomposition of Co2(CO)g in the presence of a suitable surfactant [27]. A typical recipe involves the injection of a solution of Co2(CO)g in diphenylether (solution A) into the hot (200 °C) mixture of oleic acid and TOP or TBP, dissolved also in diphenylether (solution B). The mixture is then heated at 200 °C for 15-20 min. Decomposition of the cobalt carbonyl, under the conditions described above, results in Co nanopartides with a so-called multiply-twinned face-centered cubic (mt-fee) lattice (Figures 3.115b and (3.116)d). The multiply-twinned particles are composed of domains with a distorted fee lattice, this structure being similar to that observed in multiply-twinned, icosahedral gold particles [27]. Post-preparative size-selection... 

Powder X-ray di action of the gold partides dispersed on alumina support produced a very broad peak at 20 = 40.0° after subtradion of the background pattern from the alumina (see Fig. 3). Recent Debye Function Analysis (DFA) shows this pattern to be consistent with decahedra (decahedral multiply twinned particles, MTPs) of about 2.2 nm mass-mean diameter [7]. X-ray diffraction from a... 

In another variation on the polyol process, Zhang et al. recently described a steel-assisted method for the production of silver nanowires [92]. It was proposed that the in situ acidic etching of multiply twinned particles by nitric acid was one of the most harmful factors affecting the preparation of silver nanowires by the polyol process. As shown in Figure 3.13, by including the steel pieces in the synthesis, the nitric acid generated in situ is decomposed consequently, the multiply twinned seed particles are not etched and the yield of nanowires is increased. 

Figure 3.13 Schematic detailing the proposed steel-assisted polyol mechanism. MTPs = multiply twinned particles. Reprinted with permission from Ref [92] 2008 The American Chemical Society.

Hall, C. R., Fawzi, S. A. H. (1986). On the occnrrence of multiply twinned particles in electro-deposited nickel films. Philosophical Magazine A, 54, 805-820. doi 10.1080/01418618608244439. 

Takeguchi, M., Tanaka, M., Yasuda, H., Furuya, K. (2001). Real-time high-resolution transmission electron microscopy observation of the growth process of (001) surfaces on a nanometer-sized Si multiply twinned particle. Surface Science, 493, 414-419. doi 10.1016/S0039-6028(01)01247-X. 

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