Snapshots of Zinc Oxide Formation in Molten Salt: Hollow Microtubules Generated by Oriented Attachment and the Kirkendall Effect
Résumé
Formation of anisotropic structures by oriented attachment of inorganic nanoparticles in ionic melts has been evidenced for the first time for the case study example of ZnO growth. Hollow microtubules of ZnO were prepared from the reaction of zinc nitrate in alkali metal nitrate-chloride (or nitrate bromide) mixed fluxes. A series of snapshots of the morphology evolution versus temperature and reaction duration straightforwardly demonstrate oriented attachment of ZnO nanocrystallites and progressive formation of voids. The reactivity in the melts was studied by mass spectrometry of evolved gases, X-ray diffraction (XRD), and extended X-ray absorption fine structure (EXAFS) of solidified melts. The presence of chloride or bromide in the flux is essential for the manifestation of oriented attachment. Chloride decreases the reactivity of Zn species toward nitrate due to formation of ZnCl42- ions. Primary nanoparticles of the precipitate are nanosheets of ZnO exposing (0001) planes and containing residual halogenide on the surface. Further stacking of primary nanosheets might occur in the melts as illustrated by molecular dynamics simulations. External diffusion of Zn from the interior of stacks toward the reactive melt creates hollow tubules. The resulting ZnO microtubules are oriented along the [0001] axis and absorb visible light due to abundant defects, formed probably because of a stacking misfit of the primary nanosheets.