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

Abstract

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 pH_PZC 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.