Nanoscale formation of new solid-state compounds by topochemical effects: The interfacial reactions ZnO with Al2O3 as a model system

The chemical reactivity of thin layers (ca. 10 nm thick) of ZnO deposited onto differently oriented Al₂O₃ single crystals has been investigated by means of atomic force microscopy inspections and X-ray absorption spectroscopy at the Zn-K edge. The interface yields the ZnAl₂O₄ spinel and a quite stable film morphology. Instead, the and (0001)_ZnO∥(0001)_sapphire interfaces give origin to a new compound (or, possibly, even two new compounds), whose chemical nature is most likely that of a ZnO/Al₂O₃ phase, with still unknown composition and crystal structure. In addition, in the last two cases, films collapse into prismatic twins of ca. 1 μm in dimension. These experimental findings demonstrate that in a solid-state reaction, the topotactical relationships between the reacting solids are of crucial importance not only in determining the kinetic and mechanisms of the process in its early stages, but even the chemical nature of the product.