Synergistic effects of acetic acid and nitric acid in water-based sol–gel synthesis of crystalline TiO2 nanoparticles at 25 °C

Organic and inorganic acid catalysts enable control over the sol–gel synthesis of crystalline TiO2 nanoparticles. While the mechanism of organic acids (e.g., acetic acid) and inorganic acids (e.g., nitric acid) is well documented, the effect of both acid types used simultaneously is still unclear. This work provides more insights into the mechanism of both acetic acid and nitric acid in acid-catalyzed sol–gel synthesis of crystalline TiO2 nanoparticles. Acetic acid and nitric acid were used as examples for organic and inorganic acids, respectively, while keeping the reaction temperature constant at 25 °C. Investigating the particle growth and the polydispersity by dynamic light scattering revealed synergistic effects when both acetic acid and nitric acid were present, resulting in separating the two steps of the sol–gel reaction, namely the hydrolysis and condensation steps. This separation induces a control on the sol–gel reaction toward the formation of crystalline TiO2 as confirmed by wide angle X-ray scattering combined with Rietveld analysis. It also appeared that the anatase-to-brookite phase transition was enhanced by an increased acetic acid and nitric acid content reaching 80% anatase content when the acid to TiO2 precursor molar ratio increased from 1 to 10.6. The results and the insights presented in this work could be useful for optimizing crystalline TiO2 production at low temperature.