Synergistic Correlation in the Colloidal Properties of TiO2 Nanoparticles and Its Impact on the Photocatalytic Activity

In this work, the relationship between the photodegradation rate of methylene blue (MB) and the effective surface charge of titania nanoparticles (TiO2 NPs) in an aqueous solution is addressed. Colloidal dispersions were prepared from TiO2 NPs (4–10 nm) for the heterogenous photocatalysis test. The dispersion properties such as pH, hydrodynamic diameter, zeta potential, and isoelectric point were studied. Acidic TiO2 dispersions (pH = 3.6–4.0) with a positive zeta potential and smaller hydrodynamic diameter exhibit larger colloidal stability and pseudo-first-order kinetics for the degradation of MB. The largest rate constant (5 × 10−2 min−1) corresponded to a conversion of 98% within 75 min under UV light. This enhanced rate is a synergic effect between the surface area, charge, and optimal hydrodynamic diameter of TiO2 NPs. A linear correlation between the calculated values for the absorption cross-section and normalized rate constant was found for the systems under study. It was observed that an eventual increase in the pH (4–5.5) reduces the effective surface charge and dispersion stability, causing a decrease in the rate constants of one order of magnitude (10−3 min−1) for TiO2 agglomerates with a larger hydrodynamic diameter (300–850 nm).