Effect of heat treatment on the surface chemical structure of glass: Oxygen speciation from in situ XPS analysis

Compositional changes in unleached and acid-leached soda-lime silicate surfaces were tracked with in-vacuo heating and X-ray Photoelectron Spectroscopy. Surface oxygen speciation was determined using a stoichiometry-based algorithm via elemental composition, instead of the typical O 1s peak-fitting approach. Accurate surface hydroxyl quantification is shown to require dehydration at temperatures near 200°C. On the unleached surface, no change in surface hydroxyl density (~2.5 OH/nm²) is observed in the temperature range of 200°C-500°C after the initial dehydration. However, repolymerization in the network (non-bridging oxygen→bridging oxygen) is observed due to volatilization of sodium. The acid-leached surface undergoes sodium out-diffusion from the bulk at sub-T_g temperatures with laterally resolved inhomogeneity and shows a reduction in the concentration of hydroxyls from 4.5 OH/nm² (200°C) to 3.2 OH/nm² (500°C) accompanied by an increase in bridging oxygen. These results suggest that when [OH] > 2.5~3/nm², vicinal OH undergo dehydroxylation with evolution of water, whereas when [OH] < 2.5/nm², most OHs are non-interacting and isolated (at temperatures below T_g). Furthermore, at temperatures exceeding 300°C, sodium has enough thermal energy to desorb in vacuum and diffuse from the bulk (depending on the abundance & local structure).