Page 15 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Introduction - Plasma and Microplasma-assisted Nanofabrication
1.2 Plasma-assisted Nanomaterials Synthesis
Plasma is considered as the forth state of matter which is generated when gases are partially or fully ionized, being characterized by complex collective behavior in the external or self- induced electromagnetic field.25,26 It contains charged particles such as electrons, positive and negative ions as well as neutral atoms and molecules, which can be chemically radicalized by the dissociation processes sand exist in various excited electronic and rovibrational states. Plasma is electrically conductive as well as plasma bulk tends to be electrically quasi-neutral, however at certain conditions a volumetric charge is formed.27
Generally, plasma is categorized into two main groups: the high-temperature plasma (HTP, e.g. fusion plasmas) and the low-temperature plasma (LTP, e.g. gas discharges). In HTP gas is fully ionized with the characteristic temperature in the order of ~107 K while in LTP the temperatures are considerably lower and the typical degree of ionization is below 1 %. The LTP can be furtherly subdivided into thermal plasmas (the temperature of all species is the same, ~104 K) and non-thermal plasmas (the temperatures of different species are different, in which electrons have sufficiently high temperature to maintain ionization balance while heavy particles such as ions and neutral species are at the temperature of few hundreds K or lower), as shown in Table 1.1. Compared to HTP and thermal plasmas, non-thermal (or non- equilibrium) plasmas exhibit higher selectivity and are the most widely used plasmas for various applications. This is because high energy electrons give rise to inelastic collisions, leading to a ‘chemically-rich’ environment, while the bulk temperature remains relatively low, making it attractive for various practical applications.25
Table 1.1 Plasma classification in terms of radical temperatures28
Low temperature plasma (LTP) High temperature plasma (HTP)
Thermal plasma Non-thermal plasma
e.g. arc plasma at normal pressure
Te≈Ti≈T≤~104 K
e.g. low pressure glow discharge, corona, DBD plasma, plasma jets
Ti≈T≤~1000K Ti<<Te≤105 K
e.g. fusion plasmas Te≈Ti≥~107 K
Starting from the traditional applications such as ozone generation, light sources, thin film deposition and etching with the development of plasma technology, it has been expanded to a wide range fields such as methane reforming, VOC decomposition, CO2 conversion, surface functionalization, medical treatment etc.29,30 There are also a large number of researches focusing on the plasma-assisted nanomaterial synthesis, primarily driven by their fascinating properties. Plasma-enhanced chemistries, in which charged particles, excited states and radicals are expected to play a role in the nanofabrication process, differ essentially from traditional solution-based media. In the non-thermal plasma the electrons gain energies in the range of ~ eV from the electric field, which are sufficient to initiate plasma-chemical reactions by collision with precursor compounds.31 Therefore, these processes are inherently solvent and ligand-free, enabling the synthesis of high purity nanomaterials.32 Meanwhile, the energetic electrons also make it possible to produce composition-adjustable metal alloys 3