Synthesis of TiO2 nanoparticles by a combined sol-gel ball milling method and investigation of nanoparticle size effect on their photocatalytic activities

► Different sized TiO2 nanoparticles were synthesized by a combined sol-gel and ball milling method. ► The different sized TiO2 nanoparticles had different photocatalytic activities. ► The 55 nm nanoparticles showed a maximum photocatalytic degradation effect on the Congo red dye. ► These particles were a mixture of anatase and rutile phases. ► These optimal sized nanoparticles did not show the same effect on the different dyes.