Page 74 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Chapter 4
4.2.2 Characterization
The optical emission spectra (OES) of the discharge were recorded by a spectrometer (HR4000, Ocean Optics, Inc.) to confirm the dissociation of TiCl4 vapor. The emitted light was collected by an optical fiber fixed at 20 mm distance from the electrodes axis. All review spectra were recorded in the wavelength range from 200 nm to 1000 nm, with spectral resolution of 0.91 nm. Morphology and chemical composition of synthesized products was studied by scanning electron microscopy (SEM), which was performed on a Phenom ProX (Phenom World) operated at 15 kV, with a silicon drift energy dispersive X-ray spectrometer (EDX) detector. The particles shape and size distribution were characterized by a FEI Tecnai 20 (type Sphera) transmission electron microscopy (TEM), operating with a 200 kV LaB6 filament. The present phase was analyzed by X-ray diffraction (XRD), performed with a Rigaku Geigerflex Bragg-Brentano Powder Diffractometer using Cu radiation (λ=1.54056 Å). The chemical compositions and bonds information were furtherly studied by X-ray photoelectron spectroscopy (XPS) and carried out with a Thermo Scientific Kα, and spectra were recorded by an aluminium anode (Al Kα=1486.6 eV). A glass fibre was used to deposit prepared nanoparticles. Meanwhile, a pristine bare glass fiber was also scanned to determine the background signals.
4.3 Results and Discussion
4.3.1 Electrical Characterization of the Microplasma during TiN Synthesis Process
A typical V-I characteristic of the discharge during TiN nanoparticle synthesis is shown in Figure 4.2. Since relatively low voltage is needed for breakdown and sustaining discharge in argon (see typical Paschen curves in Ref. [23]), in every experiment the plasma was first ignited in pure argon flow. After the argon discharge became stable, a certain amount of N2 (25 sccm) was introduced into the plasma. One can observe an obvious increase of plasma voltage from 240 V to 430 V, while the current reduced slightly from 14.4 mA to 13.6 mA. The plasma was maintained stably for a while before adding the TiCl4 vapors. With the admixture of the precursor, the plasma voltage after a small transitional fluctuation stabilizes at approximately 440 V. While, the discharge current shows practically no variation, leading to a plasma power change from 5.9 W to 6.1 W (3.4%). The V-I characteristics indicates that the introduction of TiCl4 to the plasma has only a slight influence on the discharge ionization and thermal balance, which can be attributed to the relatively low concentration of the TiCl4 vapor in the gas flow. In time, as microplasma operates, there was an increased fluctuation trend in the I-V characteristics, which can be explained by the accumulation of generated nanoparticles in the electrode mesh. The plasma voltage and current were influenced by filament length and position. After the desired process time, the precursor flow was stopped before switching off the plasma to prevent any residues in the pipelines. As a consequence, the plasma voltage dropped gradually from 450 V to 430 V, while the current showed a minor increase.
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