Orientation-dependent structural and photocatalytic properties of LaCoO₃ epitaxial nano-thin films

LaCoO₃ epitaxial films were grown on (100), (110) and (111) oriented LaAlO₃ substrates by the polymer-assisted deposition method. Crystal structure measurement and cross-section observation indicate that all the LaCoO₃ films are epitaxially grown in accordance with the orientation of LaAlO₃ substrates, with biaxial compressive strain in the ab plane. Owing to the different strain directions of CoO₆ octahedron, the mean Co-O bond length increases by different amounts in (100), (110) and (111) oriented films compared with that of bulk LaCoO₃, and the (100) oriented LaCoO₃ 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 LaCoO₃ films, it is found that the change of the photocatalytic activity is closely related to the crystal field splitting energy of Co³⁺ and Co-O binding energy. The increase in the mean Co-O bond length will decrease the crystal field splitting energy of Co³⁺ 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 LaCoO₃.