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

Epitaxial thin films of ZnMn_2O_4, grown from an aqueous solution on (100) and (111) spinel MgAl_2O_4 substrates, have been topotactically reduced and leached to produce epitaxial rock salt MnO on the same substrates. The volume change accompanying the conversion of ZnMn_2O_4 to 2MnO associated with a loss of Zn and O_2 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) MgAl_2O_4 substrate, pores with rectangular cross-sections are aligned parallel to the surface normal. On the (111) MgAl_2O_4 substrate, the pores are aligned at constant, fixed angles with respect to the substrate normal.