Capping and Passivation of Aluminum Nanoparticles Using Alkyl-Substituted Epoxides

Abstract

We report here on the synthesis and passivation of small (20-30 nm) aluminum nanoparticles using alkyl-substituted epoxides as capping agents. FTIR and 13C NMR spectroscopy indicate that the epoxides polymerize to form a polyether cap on the surfaces of the aluminum nanoparticles. Nanoparticles capped with epoxyhexane and epoxydodecane are stable in air, but particles capped with epoxyisobutane are pyrophoric. TEM images show spherical Al particles. Powder X-ray diffraction shows the presence of crystalline Al. Titrimetric analysis of the core-shell nanostructures in air reveals that 96% of the total aluminum present is active (unoxidized) aluminum.

Structure, Thermodynamics, and Energy Content of Aluminum–Cyclopentadienyl Clusters

“…This is consistent with the idea that AlCp or AlCp* removal is the initial decomposition step and that steric hindrance from the ligand is an important limiting mechanism for monomer detachment. We also note that the tetramer Al 4 (C 5 Me 4 H) 4 , in which each Cp ligand has four methyl groups instead of five as in Cp*, has also been experimentally observed in solid state form, further suggesting the necessity of strong steric hindrance for cluster stability. 14 Combustion Properties.…”

“…This implies that the MÀL bond is much stronger than the AlÀAl bonds, as would be expected. The same is true for Al 4 Cp 4 *, where the MÀL BDE of 44.89 kcal/mol is much higher than the tetramerization energy of 9.09 kcal/mol. Previous calculations by Huber and Schn€ ockel using BP86 and an SVP basis set also showed that Al 4 Cp 4 is more stable against decomposition into monomers than Al 4 Cp 4 *.…”

“…In fact, the BDE associated with breaking the η 5 bond in AlCp* is 76.81 kcal/mol, which is almost equal to the MÀL BDE in the largest cluster, at 79.72 kcal/mol. Thus, while the MÀL bond is slightly weaker in the smaller Al 4 and Al 8 clusters, it has a strength in the largest cluster comparable to that of the monomer. The MÀL bond is generally stronger than other aluminum bonding, and it is likely that for all clusters larger than the tetramer the initial unimolecular thermal decomposition step is the removal of AlCp or AlCp* units.…”

“…Their study of larger aluminum clusters builds on a large body of work on low-valence aluminum coordination complexes. 9À13 A number of these clusters, such as the tetramers Al 4 Cp 4 * and Al 4 (C 5 Me 4 H) 4 , crystallize into lowsymmetry solid state structures and have been observed via X-ray crystallographic analysis and NMR spectroscopy. 14,15 Attempts to isolate similar compounds with unmethylated Cp (C 5 H 5 ) ligands have generally been unsuccessful due to their tendency to disproportionate to aluminum metal and higher-valence compounds such as AlCp 3 .…”

“…Early HartreeÀFock calculations by Ahlrichs and co-workers 16 examined basic AlCp as well as Al 4 Cl 4 , Al 4 F 4 , and Al 4 Cp 4 . No correlation effects were included, and with their methodology, Al 4 Cp 4 was found to be unstable with respect to decomposition into monomers. A number of later works considered the simple Al metallocenes in the context of examining trends in a broader range of main-group metallocenes or similar organometallics.…”