Structure of the amorphous titania precursor phase of N-doped photocatalysts

Abstract

Amorphous titania samples prepared by ammonia solution neutralization of titanyl sulphate have been characterized by chemical and thermal analyses, and with reciprocal-space and real-space fitting of wide-angle synchrotron X-ray scattering data. A model that fits both the chemical and structural data comprises small segments of lepidocrocite-type layer that are offset by corner-sharing as in the monoclinic titanic acids H₂Ti_nO_2n+1·mH₂O. The amorphous phase composition that best fits the combined chemical and scattering data is [(NH₄)₃H₂₁Ti₂₀O₅₂]·14H₂O, where the formula within the brackets is the cluster composition and the H₂O outside the brackets is physically adsorbed. The NH₄⁺ cations are an integral part of the clusters and are bonded to layer anions at the corners of the offset layers, as occurs in the alkali metal stepped-layer titanates. The stepped-layer model is shown to give a consistent mechanism for the reaction of aqueous ammonia with solid hydrated titanyl sulphate, in which the amorphous product retains the exact size and shape of the reacting titanyl sulphate crystals.