Co nanoparticles

Lee et al. [145] succeeded in preparation of Co-based bimetallic nanoparticles with core/shell structure via transmetalation reaction (Figure 3). The Co-core/Au-shell nanoparticles, e.g., were confirmed to be almost the same in particle size with the seeded Co nanoparticle, as shown in Figure 4. 

Figure 4. TEM and HRTEM images of (a) 6.5 nm Co nanoparticles and (b) Co-core/Au-shell nanoparticles using Co nanoparticles as the seed material. Lattice distances measured by HRTEM as well-matched to known Au lattice parameters for the (111) plane (inset). The average size of the Co-core/Au-shell nanoparticles is ca. 6.4 nm, which is similar to the initial size of the Co nanoparticles because the atom exchange process is the only operative reaction. (Reprinted from Ref [145], 2005, with permission from American Chemical...

Figure 3. Valence band spectra of Co/Si(100). Upper curve UPS spectra for 100 nm thick Co/Si(l 1 1) film middle curve thinned 4-5 nm Co/Si(l 1 1) film after ion etching (Co nanoparticles) lower curve clean silicon substrate after removing the Co layer by in situ sputtering. The photoemission data were obtained by He(I) excitation. (Reprinted from Ref [78], 1994, with permission from Springer.)...

Delpeux S., Szostak K., Frackowiak E., Bonnamy S., Beguin F. High yield carbon nanotubes from the catalytic decomposition of acetylene on in-situ formed Co nanoparticles. JNanosci Nanotech 2002 2 481-4. 

A Dual Catalytic Role of Co Nanoparticles in Bulk Synthesis of Si-Based Nanowires... 

Nanoparticles were prepared on the basis of our previous report. In brief, Co nanoparticles were synthesized via a high-temperature thermo-decomposition method.Three-, five-, and twelve-nanometer Co nanoparticles in dichlorobenzene (DCB) solutions were made and used to make self-aligned nanostructures (SAN) and SiNW. 

Synthesis of SiNW. SiNW were made by reacting Co nanoparticles with Si wafers in H2 and Ar between 1000 and 1100°C. " After nanoparticle deposition, the... 

The optimal densities of nanoparticle deposition for the formation of SAN and SiNW were different. For SAN formation, low-density deposition of Co nanoparticles was preferred. In contrast, high-density deposition was needed to produce high yield SiNW. 

Figure 10.3 shows a Co nanoparticle deposition prior to SiNW growth. The average size of the nanoparticles used in this deposition was 12 nm. The standard deviation was 5 nm. As shown in Fig. 10.3, no individual nanoparticles were detected by SEM. The nanoparticles were embedded in large amounts of organic solvents or hydrocarbons such as oleic acid (OA) and tri-octylphosphine oxide (TOPO). 

FIGURE 10.10. SiNW grown on a Si wafer coated with Co nanoparticles at 1100°C, with H2. Several short SiNW are visible under SEM. 

Oxidized Si wafers were prepared by placing them in a furnace in the presence of air at 1100°C for 45 min. They were then coated with Co nanoparticles for SiNW growth as discussed above. It was found that almost no SiNW were made, and the Si wafers were covered with either cracks or short wires with Co nanoparticles at their tips. 

XAS. In order to better understand the catal)4ic processes, we investigated Co nanoparticle catalysts in SAN and SiNW with grazing incidence X-ray absorption spectroscopy (GI-XAS). Figure 10.16 shows the results of EXAFS measurements. FT of the EXAFS of both SAN and SiNW samples are shown. 

However, it is unclear what the growth mechanisms are because no Si feedstock was fed in the gas form, which is required for the tip-growth model to work. In the following, we will discuss the composition and growth mechanisms of the nanowires made in this work, and show that the Co nanoparticles may play a dual-catalytic role by helping form gaseous silicon species and catalyze the growth of Si-based nanowires. 

We also attempted to produce SiNW from oxidized Si wafers. However, as shown earlier, only a small number of SiNW were produced. This indicates that a thick Si02 layer may not allow the formation of SiNW, even in the presence of H2 and Co nanoparticles. Since Si is oxidized and becomes amorphous, no SAN would form under this condition. This result perhaps implies that SAN may contribute to SiNW growth (e.g., via the production of silane gas). Also, since H2 can reduce Si02 to Si or SiO, and SiO can be evaporated at 1200°C, oxidized Si in the presence of H2 may, in principle, produce SiNW without the use of catalysts. However, the negative result shown here implies that Co nanoparticles may not react well with Si02 to form CoSi2 and thus H2 cannot affect the growth of SiNW. 

SCHEME 10.1. Formation of silane and SiNW by a series of reactions involving Co nanoparticles, hydrogen, and silicon wafers. 

Proposed Growth Models. Summarizing the results given above, the growth mechanisms are proposed as follows. Because of the location of nanoparticles at the tips of SiNW, we conclude that growth must occur in the gas phase. More precisely, we think that Co silicide SAN play a role here. They help convert Si and H2 into SiH4 (gas). Unreacted Co nanoparticles left on the surface, possibly due to the... 

FIGURE 10.23. Proposed catalytic processes to make SiNW from Co nanoparticles and hydrogen. Si wafers act as the source of silicon. Silane is produced, which then reacts with Co or Co silicide catalysts to make SiNW. [Chem Comm 2005]—Reproduced by permission of The Royal Society of Chemistry, (ref 54)... 

A schematic of the proposed growth model is shown in Fig. 10.23. In this model, Co nanoparticles play a dual catalytic role. On the one hand, they catalyze silane formation by reacting first with silicon to form Co silicides, and then react with hydrogen to form silane while being reduced to Co metal. The second role of Co nanoparticles is their classic catalytic ability of making nanowires by first dissolving the silane and precipitating out Si nanowires. 

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