Formation of monodispersed anatase TiO2 spheres from TiOSO4 for enhanced hydrogen evolution of TiO2/g-C3N4 photocatalysts

Monodispersed anatase TiO2 spheres were synthesized from TiOSO4 solution made from Ti-bearing blast furnace slags (TBFS) via the hydrothermal hydrolysis-calcination route. By simply increasing the solution acidity before the hydrolysis, 2-4 mu m TiO2 aggregates formed by 30-50 nm primary particles convert to 200-400 nm monodispersed spheres. The key to dispersion is investigated: As the acidity rises, SO4 2- in solution is converted to HSO4- , which changes the SO42-/HSO4- species adsorbed on the primary particle surface. DFT calculations show that Ti-SO4-Ti bridges between the primary particles are more easily broken than Ti-HSO4-Ti, allowing for the dispersion of the primary particles. The reasons for the narrow size distribution were investigated: The high acidity and hydrothermal treatment enhance the dissolution-deposition of TiO2. The small primary particles dissolve quickly and increase the monomer concentration, which promotes the growth of monodispersed TiO2 spheres following the "size focusing" mode, leading to a narrow size distribution. Finally, TiO2/g-C3N4 photocatalysts were prepared based on the aggregated TiO2 and monodispersed TiO2 spheres. Due to the more surface oxygen vacancies and higher dispersion, the photocatalyst made from the monodispersed TiO2 spheres shows enhanced hydrogen evolution than the aggregated TiO2. The present results may provide insights into TiO2 sphere synthesis and TBFS recycling.