In situ total X-ray scattering study of the formation mechanism and structural defects in anatase TiO2 nanoparticles under hydrothermal conditions

Abstract

Polymorphism, morphology, particle size, and defects play key roles in the physical and chemical properties of nanoparticles. In hydrothermal synthesis of metal oxide nanoparticles, it is important to understand the influence of the specific precursor on these characteristics. Here, the formation mechanism of anatase TiO₂ nanoparticles by hydrolysis of titanium isopropoxide under hydrothermal conditions is studied by in situ total X-ray scattering and pair distribution function (PDF) analysis. It is shown that the amorphous precursor structure has short-range order up to ~6.5 Å consisting of titanium hydroxide clusters made from TiO₆/TiO₅ units in an arrangement related to anatase. Insight into the structural disorder of the anatase TiO₂ nanocrystals is obtained from both PDF and powder X-ray diffraction (PXRD) analyses. Defects of OH species are present on the surface of the nanocrystals, and their concentration correlates strongly with the particle size. Even though the formation of anatase TiO₂ under hydrothermal conditions resembles a solid-state phase transition from amorphous titania, the crystallization and grain growth kinetics of the nanocrystals are different due to the effects of the solvent.