Study of the Transformations of Elemental Nanowires to Nanotubes of Metal Oxides and Chalcogenides through the Kirkendall Effect

We have employed the Kirkendall effect to transform elemental nanowires of metals and silicon to nanotubes of the corresponding oxides and chalcogenides, having prepared the metal nanowires by nebulized spray pyrolysis of metal acetates in an inert atmosphere and silicon nanowires by carbon-assisted synthesis. The formation of ZnO nanotubes by the oxidation of Zn nanowires has been studied as a function of time observing the intermediate structures during the oxidation. Nucleation of Kirkendall voids in the nanowires during the oxidation leads to the formation of the ZnO nanotubes. The kinetics of the Zn nanowire−ZnO nanotube transformation has been studied and the activation energy for the transformation found to be 12.2 kcal/mol, a value smaller than that for bulk metal oxidation. ZnCr2O4 nanotubes are formed by the reaction of Zn nanowires with CrO2Cl2 in an oxygen atmosphere. We have obtained nanotubes of Co3O4, starting from Co nanowires and SiO2 (cristobalite) nanotubes starting from Si nanowires. Nanotubes of ZnS, CdS, and CdSe have been obtained by the reaction of the metal nanowires with the chalcogens. The activation energy for the formation of CdS nanotubes from Cd nanowires is found to be only 8.5 kcal/mol. The present study establishes the Kirkendall effect as a novel means of preparing nanotube structures of several inorganic materials.