Page 24 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Chapter 1
Figure 1.5 The experiment setup and schematic diagram of the indirect contact plasma-liquid system for preparing Au nanoparticles. Reprinted with permission from [93], copyright 2014 Springer
Direct contact plasma-liquid systems refer to configurations where plasmas are formed within the liquid instead of the gas phase above the liquid. Generally plasma gas is not necessitated, but a high yield of solvent-split radicals is generated in the solution. The precursor can be either salts that being dissolved in the water or other solvents to form the electrolyte, or metal wires functioning as the consumable electrodes. Moreover, due to the pressure originating from the liquid, plasma usually operates at pressures a slightly higher than atmospheric pressure. In light of the configuration, the electrodes are mostly in the form of pin-to-pin or pin-to-plate structure, with a characteristic distance of ~mm. The discharges are commonly produced by high voltage pulses and referred as streamers or corona discharges. A common excitation method is by means of discharging a capacitor where a short-rise-time switch (e.g. a spark gap) produces microsecond pulsed discharges. Recently, more excitation methods are applied, including nanosecond pulses, radio frequency powers or microwave plasmas.
One representative example is demonstrated in Figure 1.6, where Sn nanoparticles were synthesized by a direct contact plasma-liquid system. In this study the tin chloride dehydrates (SnCl2·2H2O) were used as the precursor, and the cetyltrimethylammonium bromide (CH3(CH2)15N(CH3)3Br) was used as the surfactant. Both were dissolved in pure water to prepare the electrolyte. During each operation a 300 ml electrolyte with tin chloride concentration of 2-4 mM was added in the reactor. Tungsten capillaries (Diameter= 2 mm) coated by ceramic insulator were applied as the electrodes and fixed in the “needle-to-needle” form, with an electrode gap of 0.2 mm. A high-frequency bipolar pulse power supply was applied to generate the plasmas directly in the liquid phase in a double annular tube type reactor (I.D.= 50 mm, O.D.= 80 mm, Height= 150 mm), and the applied voltage, pulse width and frequency were fixed at 250 V, 5 μs and 30 kHz, respectively. Under the plasma treatment the Sn2+ was reduced to form the Sn nanoparticles.
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