Induced phase transition from ZnO to Co3O4 through Co substitution

Physical properties of Co substituted ZnO nanoparticle systems were investigated by varying the ratio of cobalt from 1 to 95 at.%. The 1 at.% Co substituted ZnO system acquired ZnO's wurtzite structure, however the crystallinity gradually reduced with further increase in ratio of Co up to 55 at.%. Upon 65 at.% Co substitution, the system contained ZnO (100), Co3O4(311) and Co (111) phases. Further increase in Co ratio up to 95 at.% lead to the formation of Co3O4(311) phase. Shape of the particles were cylindrical, fractal and hexagonal/rectangular, for the systems with 1, 15 and 85 at.% Co, respectively. The UV-visible absorption peaks corresponding to ZnO in 1 at.% Co substituted sample blue shifted to 376 nm, and Co3O4 in 95 at.% Co substituted sample blue shifted to 494 nm. The magnetization studies on highly crystalline samples revealed the paramagnetic nature of the systems. An increase in magnetization of the system was observed with increase in Co from 1 at.% to 15 at.%, however above 75 at.% the magnetization reduced with an increase in Co and eventually became super-paramagnetic at 85 at.% and anti-ferromagnetic at 95 at.%, due to the formation of Co3O4 phase.