Characterization of silica surfaces by free radical spin labels

Abstract

Silanol groups on the surface of various silicon oxide materials have been labeled for esr study by bonding or adsorption of free radicals. The number of bonded radicals was correlated with the relative concentrations of vicinal and isolated silanols. When the vicinal silanols have been removed by treatment above 600°C, the labeling was found to be quantitative and complete. The improved label attachment in the latter case indicates that steric crowding is not a consideration, but that the resistance of vicinal silanols to labeling reagents is responsible for limited labeling at low temperatures. The ratio of trichlorosilane di-labels to dichlorosilane mono-labels was observed to approach the theoretical limiting value 2.0 above 600°C, and the variation of this ratio may be a basis for quantitative determinations under conditions of incomplete labeling. In contrast to covalently bonded labels, adsorbed labels were found to attach to both vicinal and isolated silanols, and observed silanol concentrations are closely comparable to the bonded label values above 600°C. Labeling susceptibility of the silica surface was found to be drastically modified by pretreatment with inorganic and organometallic reagents. The potential usefulness of spin-labeling in electronics was illustrated by labeling of silanols in quartz surfaces (piezoelectric resonators), on oxidized silicon (MOS integrated circuits) and in evaluation of various surface passivation techniques.