Page 66 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Chapter 3
3.4.2 Metal-cyclopentadienyl Dissociation Stage
The π-system cyclopentadienyl anion (Cp-) of ferrocene has 6 electrons (5 come from the 5 sp2 hybridized C-atoms, 1 comes from negative charge). According to Hückel's rule-a cyclic, planar molecule which has 4n+2 electrons is aromatic- cyclopentadienyl anion is aromatic and quite stable for its mesomeric nature. Therefore, it is widely accepted that the ferrocene dissociation starts with the breaking of metal-Cp ligand bond instead of splitting the Cp rings.38 The following equation was proposed to describe the metallocene thermal decomposition process.39
Me(C5H5)2(g)→Me(s)+H2 +CH4 +C5H6 +...
The situation becomes even more complicated when it comes to the plasma phase, since the plasma-enhanced gas chemistry differs essentially from traditional solution-based media. In argon plasma electrons, electronically excited metastable argon atoms Ar* as well as ions Ar+ can play a role in the dissociation process. Table 3.4 and Figure 3.12 give a summary of relevant reactions and threshold energy data of this complex process.40,41
Table 3.4 Summary of reactions related to ferrocene dissociation process Energy (eV)
Reaction
(1) Fe(C5H5)2→Fe(C5H5) • + C5H5•
(2) Fe(C5H5)2→Fe(C5H5)2+ + e
(3) Fe(C5H5) •→Fe• + C5H5•
(4) Fe(C5H5)2+→Fe(C5H5) + + C5H5• (5) Fe(C5H5) +→Fe+ + C5H5•
(6) Fe(C5H5)2→Fe• + 2C5H5•
(7) Fe(C5H5)2→Fe(C5H5)+ + C5H5• (8) Fe(C5H5)2→Fe+ + 2C5H5•
(9) Fe(C5H5)2+→Fe + + 2C5H5•
(10) e + Ar →Ar*+ e
(11) e + Ar →Ar++ 2e
3.96 6.9 2.21 6.1 1.04 6.17 13.0 14.04 7.14 11.5 15.8
Figure 3.12 Summary of reactions and the energy data corresponded to ferrocene dissociation process 54