Titanium dioxide nanoparticles: Effect of sol-gel pH on phase composition, particle size, and particle growth mechanism

This work investigates the effect of pH during sol-gel synthesis on the brookite content and average anatase and brookite particle sizes. In general, anatase is the primary product of such sol-gel syntheses; however, significant amounts of brookite are frequently observed. Rietveld refinements of powder X-ray diffraction (XRD) patterns enabled tracking of phase composition and the Scherrer equation was used to determine average anatase and brookite sizes from the full widths at half-maximum of XRD peaks. Furthermore, two nanoparticle growth models are employed to fit nanoparticle growth data in order to elucidate growth mechanisms operating during hydrothermal aging. In general, an increase in pH during sol-gel synthesis results in an increase in brookite content and an increase in average anatase particle size. Results from hydrothermal aging of dialyzed sol-gel products show that the average particle size and dominant particle growth mechanism depend strongly on the pH employed during synthesis (pH -0.5 to pH 3), despite dialysis prior to aging, which increased the pH of all suspensions from -0.5-3 to 5.4-5.7. Interestingly, the dominant anatase particle growth mechanism during hydrothermal aging also depends on the sol-gel pH, with growth dominated by coarsening for particles synthesized at pH -0.5 and by simultaneous oriented aggregation and coarsening for particles synthesized at pH 3. This difference is most likely due to the differences in average particle sizes and phase composition of the products of each sol-gel synthesis. Substantial control over the brookite content, particle size, and particle growth mechanism can be achieved by varying the pH of the sol-gel synthesis.