Determination of the surface silanol concentration of amorphous silica surfaces using static secondary ion mass spectroscopy

Abstract

A novel method has been developed to determine the concentration of silanol (Si–OH) groups on planar, amorphous silica glass surfaces. This method involves neutral beam static secondary ion mass spectroscopy (SIMS) analysis as a function of sample temperature. The calibration necessary to achieve absolute quantification of the silanol concentrations with static SIMS was obtained using a combination of Fourier-transform infrared (FTIR) spectroscopy and x-ray photoemission spectroscopy (XPS). High purity, amorphous silica glass samples with varying silanol and molecular water concentrations were prepared ex situ, and in vacuo by fracturing and dosing silica rods with water in the sample preparation chamber of a UHV system. Static SIMS spectra collected as a function of sample temperature show a decreasing SiOH+/Si+ peak area ratio with increasing temperature due to desorption of physisorbed, molecular water. It is shown that the equilibrium SiOH+ signal intensity obtained after the heat treatment is proportional to the silanol concentration of the silica surface. XPS analysis of derivatized samples was used to confirm the results of the static SIMS experiments. In order to achieve absolute quantification of the silanol concentration in OH/nm2, FTIR was used to calibrate the SiOH+/Si+ peak area ratios measured from the static SIMS spectra.