The pore wall structure of porous semi-crystalline anatase TiO₂

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 TiO₂ 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 TiO₂ 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 TiO₂ matrix and the pores filled with liquid H₂O/D₂O mixtures was 51/49%(v/v) H₂O/D₂O, 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 m² cm⁻³ (283–302 m² g⁻¹).