High-temperature solid-state rutile-to-anatase phase transformation in TiO2

Abstract

TiO2 has many polymorphs, among which rutile is the most thermodynamically stable. Bulk anatase TiO2 is usually a metastable phase at room temperature. Hence, phase transformation of bulk rutile TiO2 to anatase TiO2 by heat treatment is thermodynamically forbidden. However, the stability of TiO2 polymorphs is dependent on particle size and doping level. In this work, we harnessed this characteristic to develop a solid-state high-temperature route of sequential NH3 and O2 treatment that transforms rutile TiO2 to anatase phase via grain fracture and N-doping. As an additional advantage, this technique produces a mixture of anatase and rutile phases, with anatase being a major phase. Biphasic TiO2 is advantageous from a charge separation perspective for photocatalytic applications. Furthermore, the bandgap of anatase TiO2 can be manipulated by controlling the O2 treatment time to tune it for specific applications.