Page 67 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Iron Oxide Nanoparticles Synthesis and Mechanism Study - a Proof-of-Concept Model Study
One can see that the ferrocene dissociation via the neutral pathway requires considerably lower energy than via ionic states. In the neutral dissociation-cycle, the direct dissociation of Fe(C5H5)2 molecule to Fe and C5H5 free radicals requires energy of 6.17 eV (pathway 6), where the energy for first Cp ring scission is 3.96 eV (pathway 1), while scission of second ligand requires less energy (2.21 eV, pathway 3). By contrast, in the ionic dissociation-cycle, the supplied energy has to exceed 6.9 eV for the ionization of Fe(C5H5)2 molecules (pathway 2). Once Fe(C5H5)2+ ions are formed, they will furtherly be dissociated to yield fragments such as Fe(C5H5)+, C5H5 and Fe+ by the ligands breaking processes.
On the other hand, according to the Bolsig+ result, the bulk electrons of argon microplasma are in the energy region below 10 eV, and their densities decreased with electron energies.42 By combining the electron energy distribution function (EEDF) with the threshold energy of possible reaction pathways, the dissociation pathway of bulk Fe(C5H5)2 can be predicted.
Overall, among those competing dissociation channels, the first step in sequential dissociation of Cp rings from ferrocene molecule through the neutral dissociation-cycle requires the minimum threshold energy (3.96 eV). Therefore, it is supposed that the preferential pathway for Fe(C5H5)2 molecules dissociation starts with the reaction 1 to form Fe(C5H5) and C5H5 free radicals by breaking the first ligand, followed by the scission of the second Cp ring to form Fe atoms. The continuous generation of Fe atoms in aerosol phase eventually leads to a supersaturated Fe concentration, which is energetically unstable and tends to form atom-atom bonds to release energy. As a consequence, Fe atoms begin to assemble together spontaneously to form clusters. Once the nucleation seed forms, it will act as a convergence point for Fe nanoparticle growth. However, due to the high reactivity of Fe nanoparticles, they can be easily oxidized when exposed to air, as revealed by the complementary characterization results.
3.4.3 Cyclopentadienyl Ring Dissociation Stage
In addition to Fe atoms, the cleavage of Cp-Fe bonds from Fe(C5H5)2 vapors also results in organic residues in the gas phase. The decomposition of these residues may lead to carbon incorporation in the products. According to OES measurements in Figure 3.3, hydrocarbon fragments (CH, C2 and C3) and hydrogen radicals were detected in our experiment, which reprove the dissociation of cyclopentadienyl radicals. Besides, their intensities were found to be increased significantly with discharge power, indicating an enhanced dissociation of cyclopentadienyl radicals. Therefore, based on experimental results and information from literature, it is assumed that in the second stage, some of the formed cyclopentadienyl radicals will firstly undergo a ring opening process to produce linear C5H5 radicals. Afterwards, they are broken into smaller fragments under the impact of electrons or excitation and charge transfer process. Meanwhile, recombination reactions among those radicals may occur in the plasma, giving rise to more complex hydrocarbon fragments.
The dissociation processes in microplasma take place very fast and by different pathways. Therefore, all kinds of reactions could happen in the same time. However, based on the detected fragments from OES measurement, experimental modelling and literature information, possible mechanisms for ferrocene molecule dissociation in microplasma are assumed in Figure 3.13.
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