Adsorption and reactivity of nitrogen atoms on silica surface under plasma exposure

Abstract

The kinetics of adsorption, desorption and recombination of nitrogen atoms on a silica surface is investigated. Stable nitrogen atoms are grafted to the inner surface of a fused silica discharge tube by a discharge in N2 at 0.53 mbar. After the pre-treatment, the surface is analysed using x-ray photoelectron spectroscopy and an isotopic exchange technique. The latter consists of the exposure of the pre-treated surface with a discharge in the heavy nitrogen isotope 30N2. Nitrogen isotopologues 29N2 and 28N2 produced on the surface are detected using a mass spectrometer and provide information about the coverage and reactivity of adsorbed 14N atoms. It is found that during the pre-treatment, a silicon oxynitride (SiOxNy) layer is formed on the initially clean SiO2 surface. The coverage of N on the surface increases from 5 × 1013 to 5 × 1015 cm−2 for a pre-treatment duration in the range of 10−2 − 104 s. Atoms on the surface demonstrate a distribution of reactivity, which is attributed to a distribution of their binding energies and configurations on the surface. We demonstrate that stable chemisorbed Nads are not the main recombination sites for N atoms on the surface contrary to previous studies. We conclude that recombination takes place mainly on weakly bonding active sites with the binding energy smaller than 1 eV.