Epitaxial Manganese Oxide Thin Films with Connected Porosity: Topotactic Induction of Crystallographic Pore Alignment

Epitaxial thin films of ZnMn2O4, grown from an aqueous solution on (100) and (111) spinel MgAl2O4 substrates, have been topotactically reduced and leached to produce epitaxial rock salt MnO on the same substrates. The volume change accompanying the conversion of ZnMn2O4 to 2MnO associated with a loss of Zn and O2 manifests as a network of interconnected, 50-100 nm pores. The topotactic nature of the transformation ensures that the pores are crystallographically aligned with the substrate: on the (100) MgAl2O4 substrate, pores with rectangular cross-sections are aligned parallel to the surface normal. On the (111) MgAl2O4 substrate, the pores are aligned at constant, fixed angles with respect to the substrate normal.