It was pointed out that the damage which was caused, to highly-dispersed silica, by ion bombardment could be stabilized by carrying the bombardment out in a controlled atmosphere. The native defects which resulted from bombardment were the Si-link vacancy and the O-bridge vacancy. The former, in the absence of relaxation, could be described as a tetra-radical center, (=SiO)4; although it was expected to relax via the formation of peroxide bridges. The other, in the absence of relaxation, could be described as being a di-radical center (=Si)2: the E' center. These radicals reacted with the residual atmosphere via completely new pathways. Thus, bombardment in a CO2 atmosphere resulted in the formation of ester-like and carboxylate groups which were stable up to at least 500C. They were inserted into the SiO2 network at the O-bridge vacancies. Bombardment in a C2H4 atmosphere resulted in more complex configurations. Thus, the O-bridge vacancy reacted with ethylene at room temperature to form a Lewis adduct which, after heating at 500C, apparently reverted to a -CH2-CH2- bridge between Si atoms. The Si-link vacancy probably reacted with C2H4 to form CH3CHO. The present conclusions were supported by the results of density-functional and ab initio molecular dynamics studies.

CO2 and C2H4 Grafting to the Native Defects of Ion-Bombarded Porous Silica. G.F.Cerofolini, L.Meda: Applied Physics A, 1999, 68[1], 29-39