Factors impacting the aggregation/agglomeration and photocatalytic activity of highly crystalline spheroid- and rod-shaped TiO2 nanoparticles in aqueous solutions.

We investigate the characteristics, fate and photocatalytic activity of spheroid- and rod-shaped TiO2 nano-crystals in aqueous solutions to better understand their behaviour in media of biological and environmental interest. For this purpose, the potential of a solvothermal method in synthesizing highly crystalline nanoparticles and tuning their sizes/shapes is explored. Spheroid- and rod-shaped nanoparticles are successfully obtained with different aspect ratios, while keeping their structures as well as their cross-sectional areas identical. The aggregation/agglomeration of these nanostructures in aqueous solutions shows an obvious shape effect, revealing critical coagulation concentrations (CCCs) significantly lower for the rods compared to the spheroids (aspect ratio ∼ 2-3). This trend is observed in both NaCl and CaCl2 electrolytes at pH values above and below the pHPZC of TiO2 nanoparticles. The photocatalytic activity of the spheroids is unexpectedly superior to that of the rods at NaCl and CaCl2 concentrations over a range of 2 to 100 and 1 to 50 mM, respectively. Our results show that an increase in the chloride concentration leads to an inhibition of the photocatalytic activity rate, with a more pronounced impact for the rods. In contrast, the size of aggregates/agglomerates has only a little effect on the photocatalytic properties of both nano-crystals.