Nanostructured TiO2 synthesized by different methods: Relationship between TiO2 porosity and crystalline-amorphous structure

The present manuscript focuses on important aspects regarding TiO₂, which is typically used in many applications. On one hand, a thorough review of methods for synthesizing nanostructured TiO₂ is presented. Using these methods, and different post-synthesis heat-treatment conditions, close to thirty TiO₂ materials have been prepared, and their porous texture and crystalline and amorphous structure have been characterized. In this large number of samples, the porous texture characterization has revealed a high contribution of mesoporosity in a large number of the synthesized materials, being in many materials around 60–70 %. Moreover, different percentages of crystalline phases, anatase, brookite, rutile, and amorphous titania are developed depending on the TiO₂ preparation conditions. These fractions have been determined from X-ray diffraction data using the “simple” characterization method reported by Cano-Casanova et al., inspired by Jensen et al., and Bellardita et al., also exploring its limitations. This analysis has allowed assessing a linear relationship between porosity, particularly between the surface area, and the amorphous phase content in titanium dioxide. A relationship had been previously suggested in the literature, but never correlated in such a way. In summary, the present study constitutes a tool to help other researchers select the most suitable method for synthesizing nanostructured TiO₂ materials with target properties and, also, to characterize them, and have a very easy and straightforward estimation of TiO₂ amorphous phase content after the determination of their surface areas.