Evidence of Si–OH species at the surface of aged silica

Synchrotron radiation (SR) photoemission and x-ray photoelectron spectroscopy (XPS) are used to reveal the presence of Si–OH species at the surface of aged silica. SR Si 2p spectra, recorded in the photon energy range 130 eV≤hν≤240 eV, show the coexistence of Si–O bonding states and Si–OH bonding states, the latter being marked by a component at 105.2 eV in binding energy. The intensity ratio I[Si–OH]/I[Si–O] varies monotonically from 0.24 to 0.77 with increasing photoelectron kinetic energy from 15 to 135 eV, respectively. On the contrary, no sign of silanol groups is evident in the XPS spectra, although some excess oxygen is measured in the nO/nSi atomic ratios. Combination of SR and XPS results suggest that the silanol groups lie in a region which extends approximately 16 to 21 Å from the surface, whereas the composition of the material is virtually that of silica for depths≥30 Å. Possible sources of uncertainty in the analysis of Si–OH species in silica are discussed.