Orientation-dependent structural and photocatalytic properties of LaCoO(3) epitaxial nano-thin films

LaCoO(3) epitaxial films were grown on (100), (110) and (111) oriented LaAlO(3) substrates by the polymer-assisted deposition method. Crystal structure measurement and cross-section observation indicate that all the LaCoO(3) films are epitaxially grown in accordance with the orientation of LaAlO(3) substrates, with biaxial compressive strain in the ab plane. Owing to the different strain directions of CoO(6) octahedron, the mean Co–O bond length increases by different amounts in (100), (110) and (111) oriented films compared with that of bulk LaCoO(3), and the (100) oriented LaCoO(3) has the largest increase. Photocatalytic degradation of methyl orange indicates that the order of photocatalytic activity of the three oriented films is (100) > (111) > (110). Combined with analysis of electronic nature and band structure for LaCoO(3) films, it is found that the change of the photocatalytic activity is closely related to the crystal field splitting energy of Co(3+) and Co–O binding energy. The increase in the mean Co–O bond length will decrease the crystal field splitting energy of Co(3+) and Co–O binding energy and further reduce the value of band gap energy, thus improving the photocatalytic activity. This may also provide a clue for expanding the visible-light-induced photocatalytic application of LaCoO(3).