Real-Time Multiscale Imaging of Heterogeneous Multistage Reactions: Insights into Nanoscale TiO₂ Synthesis

Hydrothermal methods are widely used to synthesize functional inorganic materials. The interplay between the reactive species, solution chemistry, and the nanoscale product makes it challenging to control the reaction pathway to achieve a uniform product. Here, we resolve the heterogeneity that arises during hydrothermal synthesis across different length scales. We combine spatially resolved in situ X-ray pair distribution function (PDF) and small-angle X-ray scattering analysis, which are sensitive to structure on the atomic and nanoscale, with a novel time-lapse optical imaging strategy that reveals heterogeneity and phase separations across the entire reaction. For TiO₂ synthesis via hydrothermal hydrolysis of TiCl₄, we identify multiple cycles of TiO₂ formation and separation that contribute to nonuniformity in the polymorphic product. The PDF data show that the characteristics of TiO₂ formed during each formation-separation cycle differ, contributing to the ongoing challenge of precisely identifying reaction controls. The imaging strategy pioneered here provides an efficient in situ means to systematically compare how the reaction evolves under different chemical conditions, thereby advancing our understanding of functional inorganic material synthesis.