Page 114 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Chapter 6
6.1 Introduction
In previous chapters we have demonstrated the successful synthesis of metal nanoparticles and nitrides nanoparticles in the gas phase by the microplasma setup. An alternative approach of microdischarge assisted NP production, which is nowadays in focus of considerable research interest, is by means of plasma-liquid interaction. Plasma electrochemistry is an emerging technique for nanomaterial synthesis in the liquid phase, where microdischarges are generated either inside solution or in the gas phase above the solution surface. Since plasma is sustained with aqueous solution as an electrode, it is capable of initiating electrochemical reactions without using additional reducing or hydrolyzing agents.1 To date, plasma-liquid electrochemistry has been successfully applied to produce noble metal nanoparticles such as gold and silver in solutions from their respective bulk metal foils or salts.2–4
The synthesis of yttrium oxide (Y2O3) nanostructures has been of long standing interest, motivated primarily by their fascinating properties such as high refractory performance (melting point ~2450 ˚C), good thermal conductivity (33 W·m-1K-1), superior chemical stability as well as excellent mechanical properties.5,6 Nanoscale Y2O3 powders have been widely used in many areas such as nuclear ceramics,7 superconductors,8 strengthened steels,9 electronic devices 10 and so on. Y2O3 nanoparticles have also been extensively studied as host matrices for luminescent materials that offer high spectral conversion efficiency due to their broad range of optical transparency, large band gaps (5.8 eV) and relatively low phonon energies (~ 500 cm−1).11–13
Currently various physical and chemical methods have been developed for the fabrication of Y2O3 nanostructures, such as combustion,14 pyrolysis,15 evaporation-condensation,16 solvothermal,17 hydrothermal,18 sol-gel19,20 and alkalide reduction.10 While significant progress has been achieved in the synthesis of Y2O3 nanostructures, limitations of these approaches are well known. For example, combustion, pyrolysis and evaporation- condensation are energy consuming and not easy to scale up; furthermore, the obtained products are often non-stoichiometric and characterized by wide size distributions. Compared to physical methods, wet chemistry methods are mostly based on the hydrolysis of yttrium salt solutions, and are relatively energy efficient and facile to scale up. However, due to the low equilibrium solubility of Y(OH)3 (Ks~10-22.1), it is difficult to generate uniform nuclei with tightly controlled sizes and to control the subsequent diffusion-governed nuclei growth.21 Additionally, the processes are highly sensitive to the synthesis conditions and requiring precise control of temperature, pH, concentration and surfactants. In order to obtain high homogeneity products, a crucial step is to have an accurate control of the concentration of the hydrolyzing agents during the precursor hydrolysis to ensure a mild and uniform nucleation. Therefore, surfactants or stabilizers are indispensable, which in turn, require coupled washing and centrifuging procedures to remove impurities and possible byproducts.
The motivation of this chapter is to present the first demonstration of the synthesis of Y2O3 nanoparticles through a simple, low cost and environmental benign microplasma-assisted process by using only an aqueous solution of Y(NO3)3·6H2O. We sought to test a hypothesis that the release of hydroxyl ions from water can also be realized by the impact of electrons
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