Page 90 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Chapter 5
5.1 Introduction
Owing to their unique magnetic properties, iron group nanoparticles (Fe, Co and Ni) enable promising applications in high density recording, medical imaging, ferrofluids, drug delivery and magnetic thermal therapy.1–3 Presently there is an increasing interest to Ni nanoparticles as they show promise in the field of magnetic hyperthermia for cancer treatment. This is induced by the synergistic combination of characteristics such as high saturation magnetization (Ms) value, excellent catalytic acivity, slow oxidation rate compared to Fe and Co as well as cytotoxicity against cancerous cells.4,5
It is well known that material properties depend critically on particle size, structure and morphology. For example, the surface plasmon resonance of gold nanoparticles show a red shift with increasing particle size, resulting in a considerable variation in optical properties.6 Iron oxide nanoparticles of maghemite structure (γ-Fe2O3) show excellent superparamagnetic behavior, in contrast to the antiferromagnetic nature of hematite (α-Fe2O3).7 Also, the hyperthermia therapy efficacy of Ni nanoparticles is affected by several parameters: (1) Composition. The existence of impurities will significantly reduce their magnetic performance. (2) Particle size. The saturation magnetization of Ni nanoparticles is size- dependent, which decreases with decreasing particle size.8 (3) Structure. The face-centered- cubic (fcc) phase Ni nanoparticles show better hyperthermia properties and have much higher Ms values (~48.5 mAm2/g) compared with the hexagonal-close-packed (hcp) phase (< 1 mAm2/g).9 Since magnetic properties of Ni nanoparticles are closely related to those parameters, the tuning of particle size, morphology and crystalline phase will allow us to have a precise control of their properties that can be optimized for specific applications.
The synthesis of Ni nanoparticles has been widely studied over the past decades, including a variety of novel or well-established methods such as microemulsion, sputtering, pyrolysis, ultrasound-assisted, solution combustion, chemical reduction and sol-gel process.10,11 However, these approaches are generally multi-steps and time consuming, requiring pre/post treatments such as separation, washing or annealing to improve purity and crystallinity. Flexible control over composition, structure and particle size during these processes is difficult to achieve, with the mechanisms poorly understood. Moreover, a great concern for bio-application is toxic chemicals involvement (e.g., reducing agents, solvents, catalysts or surfactants), leading to undesirable hazardous interactions with biological systems and the environment. Therefore, the controllable synthesis of Ni nanoparticles with desired properties in a simple, continuous, non-toxic and efficient way is still a challenge.
As shown in the chapter 3 and the chapter 4, the microplasma-assisted approach not only provides a rapid, cost-effective and continuous route to prepare high quality nanoparticles, but also simplifies the preparation process by omitting complex pre/post treatments as well as expensive vacuum equipment.12–14 With the motivation to synthesize Ni nanoparticles of controllable magnetic properties in a simple, continuous and environmental friendly way, this chapter firstly sought to test a hypothesis that the properties of Ni nanoparticles can be controlled and tuned by adjusting the power dissipated in the plasma. This is because Ni nanoparticles of fcc phase are more stable than hcp phase at high temperatures, increasing the power density should lead to an increased gas temperature, inducing a structure transition
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