The use of 1,2-epoxyhexane as a passivating agent for core–shell aluminum nanoparticles with very high active aluminum content

Abstract

Aluminum nanoparticles synthesized by titanium (IV) isopropoxide-initiated decomposition of alane have been passivated and capped using oligomerization of 1,2-epoxyhexane. Preliminary synthetic protocols with this capping agent, where the nanoparticle formation reaction and passivation processes were both conducted at ambient temperatures, had resulted in nanoparticles that were highly unstable and that either oxidized rapidly upon exposure to air or were pyrophoric. Use of 1,2-epoxydodecane, on the other hand, had produced stable nanoparticles that were successfully characterized and reported. A modification of the procedure whereby the epoxyhexane passivation process is carried out at 85 °C for 30 min, has afforded surprisingly stable aluminum nanoparticles. Powder X-ray analysis and transmission electron microscopy reveal nanoparticle diameters on the order of 30 nm with 19 nm crystalline aluminum cores. The passivation process yields an extraordinarily high active aluminum (Al0) content of 83%, with degradation of the core to 52% active aluminum after 9 days exposure in a dry air chamber. Differential scanning calorimetry coupled with thermogravimetric analysis reveals distinct cap combustion and metal ignition exotherms, though they are not as well-defined as those found with their epoxydodecane-capped congener. With the additional observation of a metal melting endotherm, it is suggested that while carrying out the passivation process at an elevated temperature affords a higher degree of kinetic stabilization of the aluminum core, the passivation shell is inhomogeneous, possibly as a result of the polydisperse nature of the oligomerized epoxyhexane.