Page 95 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Solvent-Free Nickel Nanoparticles Synthesis and Engineering ‒Controllable Magnetic Properties
particles to form coralline-structure morphology. These coralline structures are typically 2-3 μm in diameter and covered with a high density of entangled fibril/felt-like branches. According to previous studies, such branches are attributed to carbon nanotubes (CNTs) growing on the surface of the catalytic nanoparticles.26 For products synthesized at 2.0 W (Figure S2 a-b), small particles of 20-50 nm are clustered together to form cauliflower-like bunches, containing significantly less fibril/felt-like carbon fibers. The further increase of the plasma power to 2.7 W results in well dispersed spherical agglomerations and the absence of entangled fibril/felt-like CNTs (Figure S2 c-d). As to products synthesized at 3.4 W, typical bi-dimensional rod structures are formed by randomly oriented nanoribbons, demonstrating an entirely different morphology, and CNTs totally disappear. The morphology transition from coralline-structure to nanoribbon-structure with increasing temperatures was also observed by Chinnasamy et al., where a polyol process was used to prepare fcc/hcp phase Ni nanoparticles.27 Based on the above analysis, the plasma power is shown to have a significant effect on the sample morphology. At high powers nanoparticles tend to agglomerate in rod- like structures, without the formation of CNTs, suggesting that heating contributes to the specific agglomeration of nanoparticles and the inhibition of CNTs.
Figure 5.3 SEM images at different magnification of materials synthesized at plasma power of (a-c) 1.3 W, and (d-f) 3.4 W, with fixed Ni(cp)2 concentration of 35 ppm.
The influence of precursor concentration on the particle morphology was also investigated using SEM analysis. Representative comparisons are carried out by keeping the plasma powers at 1.3 W and 3.4 W while halving the Ni(cp)2 concentration to 17.5 ppm (condition 5 and condition 8). Supplementary Material Figure S3 a-b shows that products synthesized at condition 5 form a characteristic hydrangea-like morphology. The magnified image reflects the existence of CNTs covering the particle surface. However, their density is much lower compared to products obtained at the double Ni(cp)2 concentration. For products collected at 83