Crystallographic biography on nanocrystalline phase of polymorphs titanium dioxide (TiO2): A perspective static review

Titanium dioxide (TiO₂) nanocrystals with dimensions below 100.0 nm exhibit a high crystal growth polymorphic behaviour in at least three distinct phases anatase, rutile and brookite. The phase composition and structural differences of these TiO₂ polymorphs profoundly influence their physicochemical properties, leading to variations in performance for various applications. Particular emphasis is placed on quantifying the structure-property relationships that govern the distinct behaviours of anatase (bandgap ∼3.20 eV), rutile (bandgap ∼3.0 eV) and brookite (bandgap ∼3.4 eV) nanocrystals which exhibit variations in photocatalytic activity. This review provides a comprehensive crystallographic analysis of the polymorphic phases of TiO₂ nanocrystals, focusing on their structural characteristics, phase transitions and stability. Crystalline TiO₂ phases show anatase (101), brookite (121) and rutile (110) diffraction and anatase and rutile are tetragonal, while brookite shows an orthorhombic structure. The review provides a systematic compilation of the phase biographs of TiO₂ polymorphs at the nanoscale through a detailed examination of X-ray diffraction patterns, electron microscopy images, and spectroscopic data. The effects of synthesis conditions such as temperature, precursors, and additives, on the phase composition and structural evolution are thoroughly discussed. Overall, this review provides a timely and comprehensive understanding of the crystallographic phase biographs of TiO₂ polymorphs at the nanoscale, paving the way for the rational design of high-performance TiO₂-based materials with tailored properties for diverse applications.