Mechanochemical synthesis of ternary heterojunctions TiO2(A)/TiO2(R)/ZnO and TiO2(A)/TiO2(R)/SnO2 for effective charge separation in semiconductor photocatalysis: A comparative study.

TiO2, ZnO, and SnO2 metal oxides were synthesized by the sol-gel method and heterojunctions were fabricated by combining TiO2 with either ZnO or SnO2 in a 1:1 ratio using mechanochemical ball milling process. The ball milling process promotes phase transition of TiO2 from anatase to rutile and yields ternary heterojunction of the type TiO2(A)/TiO2(R)/ZnO and TiO2(A)/TiO2(R)/SnO2 (A-anatase and R-rutile). These ternary heterojunctions were characterized by various analytical techniques and its photocatalytic efficiency is evaluated using 4-Chloro Phenol as a model compound under UV and solar light. The enhanced catalytic activity of TiO2(A)/TiO2(R)/ZnO heterojunction is attributed to the formation of Ti3+-Vo defect states which leads to the efficient charge carrier separation. During the ball milling process severe crystal deformation takes place in TiO2 and ZnO lattices by creating crystal lattice distortion which leads to the formation of defects due to valency mismatch between Ti4+ and Zn2+. A mechanistic pathway is proposed for the enhanced photocatalytic activity of the ternary heterojunctions.