The role of titanium in TiO2:SiO2 mixed sol-gels: An x-ray and neutron diffraction study

Conventional transmission and shallow-angle x-ray diffraction, together with complementary high real-space resolution neutron diffraction data, are used to reveal details of the effect of adding titania to a silica-based sol-gel. A direct observation of the Ti-O correlation has been possible. At Ti contents low enough to ensure atomic homogeneity, the data are consistent with the fourfold oxygen coordination around the metal site derived from NMR spectra. Even at these low concentrations the nature of the silica network is shown to have been affected significantly, with bondlength distributions being narrowed, and with an increased level of Si-OH indicating a qualitative change in the mesoscopic structure associated with interfacial surfaces. Data from a sample having a Ti content high enough to bring about phase separation is also presented. The shallow-angle-of-incidence x-ray diffraction method is used to provide information on the differences in structure between 'bulk' materials and those deposited as spun thin films; the data suggest an increased level of disorder in the thin films.