Fluorine-free synthesis of well-dispersed hollow TiO₂ spheres via ostwald ripening: Process, mechanism, and photocatalytic performance

Well-dispersed hollow TiO₂ spheres were synthesized via Ostwald ripening through a fluorine-free solvothermal process in a n-PrOH/H₂O mixed solvent. Several commonly used acids, such as HNO₃, HCl, and H₂SO₄, were found to be effective as the ripening-directing agent to replace the highly corrosive HF, and the hollow TiO₂ sphere size could be modulated by varying the reactant concentrations. The effects of the solvents and reactants were explored in details, which demonstrated that four important criteria existed in this fluorine-free process to create well-dispersed hollow TiO₂ spheres, including the utilization of n-PrOH/H₂O mixed solvent, certain degree of acidity, coexistence of different acids, and the existence of SO ₄^2- in the reaction solution. After calcination for a better crystallization, these hollow TiO₂ spheres were composed of pure anatase phase, and had a good photocatalytic degradation performance on RhB under UV illumination.