Page 96 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Chapter 5
condition 8, particles show irregular shapes and adhere together to form hydrangea-like shapes instead of rod-structures. Meanwhile, as shown in Supplementary Material Figure S3 c-d, the particles are smaller and more uniform compared to the particles synthesized at a 35 ppm Ni(cp)2 concentration. Therefore, SEM imaging reveals that the precursor concentration also affects the product morphology and CNTs formation.
The XRD patterns plotted in Figure 5.4 correspond to products synthesized at condition 1-4. All results indicate a crystalline nature of the as-prepared particles, revealed by distinctive diffraction peaks. The XRD pattern of the condition 1 exhibits three prominent peaks at 2θ values of 44.5◦, 51.8◦ and 76.4◦, which are indexed to the (111), (200) and (220) planes of fcc Ni nanoparticles (JCPDS PDF card #04-0850). Meanwhile, less intensive peaks of the hcp Ni (JCPDS PDF card #45-1027) at 39.1◦(010), 41.5◦(002), 58.4◦(012), 71.0◦(110), 78.0◦(103), 85.8◦(112), 87.9◦(201) as well as characteristic carbon peaks at 43.25◦ and 43.49◦ are also detected. However, with increasing plasma powers, one can observe that hcp Ni peaks and the relative intensity ratio (RIR) of hcp to fcc Ni phase decrease apparently, indicating the inhibition of hcp Ni nanoparticles. For products obtained at condition 4, negligible hcp Ni are present, with the absence of any characteristic carbon peaks, suggesting the formation of CNTs is totally suppressed. This is in agreement with the SEM results. Furthermore, the narrowing of the peaks with increasing plasma power reflects increase in grain size, and the crystal size and phase composition is listed in Table 1. The XRD results confirm our hypothesis, where a structural transition of Ni nanoparticles from hcp to the fcc phase is observed at high plasma powers. This is because hcp Ni nanoparticles are stable at low temperature, but undergo a transition to the fcc phase above 673 K.28 The XRD pattern of products synthesized at condition 8 is also shown in Supplementary Material Figure S4. It turns out that all hcp Ni peaks vanish, leaving three significant peaks of fcc Ni nanoparticles. Therefore, it is concluded that the structure of products can be tuned by adjusting the precursor concentration.
84