The pore wall structure of porous semi-crystalline anatase TiO ₂

The structure of porous TiO2 prepared by electrochemical anodization in a fluoride-containing ethylene glycol electrolyte solution was quantitatively studied using small-angle neutron scattering (SANS) and ultra-small-angle neutron scattering (USANS). The cylindrical pores along the coaxial direction were somewhat irregular in shape, were widely distributed in diameter, and seemed to have a broadly pseudo-hexagonal arrangement. The scattering from the pore wall showed a negative deviation from Porod scattering, indicating that the interface between TiO ₂ and the pore was not sharp. A density gradient of around 40-60 Å at the pore wall (i.e. the interface between the pore and the TiO ₂ matrix) was estimated using both constant and semi-sigmoidal interface models. This gradient may be due to the presence of fluorine and carbon partially absorbed by the pore wall from the fluoride-containing electrolyte or to sorbed water molecules on the wall. The neutron contrast-matching point between the TiO ₂ matrix and the pores filled with liquid H2O/D2O mixtures was 51/49%(v/v) H ₂O/D ₂O, yielding an estimated mass density of 3.32 g cm ^-3. The specific surface area of the sample derived from the (U)SANS data was around 939-1003 m ² cm ^-3 (283-302 m ² g ^-1).