Page 72 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Chapter 4
4.1 Introduction
In the previous chapter we have proven the feasibility of microplasma based technological platform for the gas phase synthesis of iron nanoparticles using ferrocene as the model case precursor. Then questions may rise, if this concept of microplasma reactor can be extended to produce other nanoparticles such as metal alloys or nitrides by means of precursor gas chemistry adjustment? While microplasma-assisted synthesis of metal nanoparticles was recently recognized as a promising approach, the applicability of this method to nitrides production was not investigated yet, despite serious practical appeal.
Due to the superior thermal stability, high wear resistance associated with extreme hardness and good corrosion resistance, TiN nanostructures are widely used as coating material,1,2 cermet,3 cutting tools,4 refractory material 5 and electrodes of electrochemical capacitors.6,7 In recent years TiN nanoparticles were also found to be a promising catalyst support in battery application for their excellent conductivity.8 Moreover, an emerging application of nanostructured TiN lies in the field of plasmonics, where transition metal nitrides are expected to bring technological breakthrough.9
Currently, TiN nanoparticles are produced either by plasma assisted methods, such as thermal plasma approach,10–13 plasma spray,14 RF plasma 15,16 and microwave plasma;17 or by non- plasma methods, such as direct nitridation of TiO2,18 chemical vapor deposition,19 benzene- thermal route,20 hydrazide sol-gel process,21 carbon thermal reduction etc.22 Although significant progress has been achieved in TiN nanostructure synthesis and its application, some issues remain to be addressed. For technologies without plasma employment, they are generally multi-steps methods and need complex manipulation, involving catalysts, toxic stabilizers or surfactants in most cases. Moreover, post treatments such as separation, washing, heating or annealing to improve particle purity and crystallinity are required, being time and energy consuming. As to the plasma assisted process, they often operate at low pressures and need high power consumption, which limit their industrial application. The produced nanoparticles are usually relatively large in average and characterized by the broad size distribution, for it is difficult to sustain a uniform temperature and to avoid coagulation in bulk plasma process.
Fortunately, those problems might be addressed by microplasma. As compared to bulk plasma methods, one notable advantage is that microdischarges can be generated and maintained stably at atmospheric pressure, thus reducing the costs by omitting need for expensive vacuum equipment. On the other hand, high pressure operation also contributes to increase in radical densities and non-equilibrium chemistry enhancement, allowing for efficient, non-thermal dissociation of molecular gases or precursor vapors at higher reaction rates. Another advantage is short residence time and a narrow residence time distribution (RTD) of precursors in the microplasma, limiting particle nucleation and growth. Meanwhile, it also leads to high energy density, making it possible to initiate reactions even at very low power.
The focus of this chapter is to show that the microplasma process can be extended to the synthesis of high quality titanium nitride nanoparticles at relatively low energy consumption.
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