Page 101 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Solvent-Free Nickel Nanoparticles Synthesis and Engineering ‒Controllable Magnetic Properties
In addition to plasma power, M-H measurements of products obtained at condition 5-8 are also conducted to study the effect of precursor concentration, as illustrated in Supplementary Material Figure S6. It is clearly shown that all samples have ferromagnetic behavior, and their Ms increased with plasma power. Compared to products obtained with double Ni(cp)2 concentration, the increasing trend is “milder”, from 22.8 mAm2/g at 1.3 W to 38.4 mAm2/g at 3.4 W.
Based on experimental results, here we have verified the hypothesis that magnetic properties of Ni nanoparticles can be controlled and tuned either by the plasma power or by the precursor concentration, through which the governing parameters such as composition, particle size and structure are influenced. As reflected by the measured M-H loops, at discharge power of 1.3 W an increase of precursor concentration from 17.5 ppm to 35 ppm results in lower Ms value for the products. By contrast, at higher powers the same variation in precursor concentration leads to rise in Ms value. This can be ascribed to overall impact from interrelated and competing parameters that together determine the magnetic properties: 1) At low plasma powers the influence of carbon impurities is dominant. Ultrafine Ni nanoparticles (5-15 nm), generated at 1.3 W, are highly active in catalyze CxHy species to form carbon atoms. For the lowest studied Ni(cp)2 vapors concentration value of 17.5 ppm, the relatively insufficient carbon supply as well as reduced interactions over hydrocarbon fragments and catalytic surface limit the formation of CNTs impurities in the products. However, as revealed by the OES, the densities of CxHy species increase considerably with the Ni(cp)2 concentration. Once the Ni(cp)2 vapors are increased to 35 ppm, the density of dissociated species and the probability of interface contact between hydrocarbon radicals and Ni nanoparticle increase as well, leading to faster carbon acquisition from the gas phase. As a result, the products have higher carbon content, which in turn, resulting in a lower Ms value compared to the nanostructures synthesized at a 17.5 ppm Ni(cp)2 concentration. This is consistent with experimental results, where abundant CNTs were observed at condition 1, but were suppressed at other conditions. 2) When the plasma power is enhanced, the effects of particle size become dominant. Ni nanoparticles generated at high plasma powers are larger and less active in catalyzing hydrocarbon fragments to form CNTs. Therefore, much less CNTs are formed, and their influence is considerably reduced. In this situation, the increase of Ni(cp)2 concentration mainly promotes the nucleation and growth of Ni nanoparticles instead of carbon inclusion in the products. On the other hand, larger-sized Ni nanoparticles have better magnetic properties, contributing to a further increase of Ms. This hypothesis is supported by the M-H result, where the optimum Ms is achieved at condition 4.
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