Hydrothermal shape controllable synthesis of La0.5Sr0.5MnO3 crystals and facet effect on electron transfer of oxygen reduction

Abstract

Controllable growth of perovskite structure oxide crystals with well-defined facets is challenging, especially in the mixed-valence state manganites with both rare-earth and alkaline-earth cations in their A-site. In this paper, we systematically studied the effects of the synthetic conditions of mineralizer concentration, reaction temperature and facet directing agent on the crystal shape formation of La_0.5Sr_0.5MnO₃. The shapes of single crystal La_0.5Sr_0.5MnO₃ have been controllably grown as cube, octahedron, rhombicuboctahedron and sphere by a high concentration KOH mineralized mild hydrothermal method with urea (CO(NH₂)₂) as crystal facet directing agent. The crystal growth mechanism for different facets has been analyzed according to Bravais−Friedel−Donnay−Harker theory. The as-grown facets for the different shapes of crystals indicate that the surfaces of La_0.5Sr_0.5MnO₃ are composed of {100}, {110} and {111} facets. Electrochemical cyclic and linear sweeping voltammetry measurement by rotating ring-disk electrode of these crystals indicates a facet-dependent process for oxygen reduction catalysis. This work provides a useful strategy for growing high index crystal facets of complex oxides and materials for crystal-facet dependent physico-chemical applications.