Spontaneous Transformations in the Solid State: Towards Porous and Biphasic Materials
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Spontaneous Transformations in the Solid State: Towards Porous and Biphasic Materials Eric S. Toberer and Ram Seshadri Materials Dept., University of California, Santa Barbara, Santa Barbara, CA, 93106
ABSTRACT Epitaxial thin films of Mn3O4 and ZnMn2O4 have been grown hydrothermally on (100) and (111) MgAl2O4 substrates. Film growth was characterized as a function of pH, concentration, and time and thin film X-ray diffraction revealed that the resulting films are an epitaxial continuation of the underlying spinel lattice. Reduction of these films to MnO occurred topotactically and in the case of ZnMn2O4, resulted in mesopores aligned along the directions. As the films maintain an epitaxial relationship with the substrate, the mesopores are aligned macroscopically within a single crystal lattice. INTRODUCTION Solids which have undergone spontaneous transformations frequently exhibit local patterning. Such patterns can arise from chemical gradients, as in the formation of lamellae during the solidification of a eutectic or from the interplay of attractive and repulsive forces, as seen in block copolymers. Structures which order spontaneously are highly attractive, as they achieve microscopic order without requiring control on fine length scales. In the formation of porous materials, alignment has been achieved through the use of structure-directing agents such as surfactants [1] and through spontaneous methods such as directional dissolution or solidification [2,3]. In our previous work, we found the reduction and vapor phase leaching of Mn3O4 and ZnMn2O4 to MnO resulted in a volume loss (16% and 46%, respectively) which induced aligned rectangular mesopores 30-50 nm in diameter perforating through the grains [4,5]. Transmission electron microscopy studies suggested that the pores were within the resulting MnO grains and oriented along the directions of the grain. The rectangular shape arises due to the low free energy of the {100} surface. The resulting MnO grains appeared to transform topotactically from the original spinel grains, suggesting that controlling the starting spinel orientation would equate to control of the resulting pore orientation [6]. The hydrothermal growth of epitaxial thin films has been pioneered by Lange and coworkers over the last decade. The technique relies up moving from a soluble to an insoluble state through the application of temperature and/or pressure while providing a site for oriented nucleation, namely a substrate. Using this technique, perovskites such as BaTiO3 [7], PbTiO3 [8], KTaO3 [9], and KNbO3 [10] have been grown on SrTiO3 substrates. Likewise, ZnO films have been grown on the (111) face of MgAl2O4 substrates [11,12]. In this work, we employ hydrothermal conditions to precipitate the spinel oxides Mn3O4 and ZnMn2O4 from cations in solution while providing MgAl2O4 substrates for heterogeneous nucleation. The resulting epitaxial thin films provide ideal materials for reductive transformations to mesoporous MnO. During these transformations the crystallography d
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