Page 64 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin
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Chapter 3
According to the results above, it is confirmed that in our products Fe was mainly in the form of iron oxides. This can be considered as a consequence of high chemical activity of Fe nanoparticles. It is usually very difficult to avoid oxygen and water impurities in the atmospheric pressure system. Therefore, iron nanoparticles can be partly oxidized already in the reactor. Moreover, oxidization is also unavoidable in sample transfer and characterization process.
3.4 Mechanism Study
Generally, in non-equilibrium plasma, free electrons gain energy of several eV from the applied electric field, which is sufficient to initiate a broad range of non-elastic processes leading to dissociation, excitation and ionization, thus, enabling plasma-chemical reactions. This way the metal organic compounds are dissociated in plasma, resulting in supersaturated metal vapors as well as other radicals or fragments. Afterwards, they will condense and/or coagulate to form nanoparticles.29,30
Recently microplasma has been applied to produce nanoparticles from metallocene precursors, including ferrocene. However, because of the complexity of ferrocene decomposition in plasma, almost all studies only focused on process optimization and material characterization, instead of going deep into mechanism study. Fortunately, OES can provide us valuable information about intermediate state of radicals existing in plasma process, which can hardly be obtained from other in situ or ex situ analysis. Therefore, it may give us some clues about how ferrocene vapors are dissociated in microplasma.
In this section, first, the plasma parameters such as gas temperature and mean electron energy are estimated. Next, based on limited data obtained from literature and information gained from our experiment, we discuss a possible mechanism of ferrocene dissociation in microplasma. In this research, we divide ferrocene dissociation process into two stages: metal-cyclopentadienyl dissociation stage (first stage) and cyclopentadienyl ring dissociation stage (second stage).
3.4.1 Microplasma Parameters
The recorded optical emission spectra were used to estimate gas temperature of discharges at low (1.05 W) and high power (2.27 W) dissipated in plasma. Commonly the second positive system (SPS) emission band of molecular nitrogen at 337 nm is employed to derive gas temperature in plasma. However, in the studied microplasma nitrogen is present only as an impurity and the observed N2 peaks were too weak to get accurate results. Instead a strong emission of C2 molecules was observed, with the highest intensity, attributed to swan system, in the range of 510-520 nm. Therefore, the peak at 516 nm, corresponding to ∆v=0 of C2 swan bands, was used for temperature estimation.31,32 By fitting experimental spectra with simulated ones with SPECAIR software, the rotational temperature can be obtained. According to previous reports, in atmospheric pressure plasma the rotational temperature can be roughly regarded as an approximation of the gas temperature.33,34 Figure 3.11 shows experimental spectra as well as simulated ones of microplasma containing 98.5 ppm ferrocene at 1.05 W (a) and 2.27 W (b). The gas temperatures are estimated to be around 900
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