Nucleation and growth of catalyst-free ZnO nanostructures
  This paper deals with the investigations on the nucleation and growth of ZnO nanostructures in a catalyst free synthesis. The ZnO nanostructures have been formed by evaporation of Zn (99.99%) in O_2 and Ar atmosphere in single zone furnace under two temperature regions, region A (~1173-1073K) and region B (~873-773K). Through application of XRD and TEM techniques, it has been shown that first ZnO is formed which changes to ZnOx through creation of oxygen vacancies. The ZnOx acts as self-catalyst and leads to formation of various nanostructures. Those observed in the present investigation are nanotetrapods (1D, diameter ~ 70-450nm, length ~ 2-4.5mm) nanorods (1D, diameter ~ 45-95nm, length ~ 2.5-4.5mm), nanoflowers(2D,central core diameter ~ 90-185nm, length of petals/nanorod ~ 1.0-3.5mm) and nanoparticles (3D, size ~ 0.85-2.5mm). These nanostructures have been revealed by SEM explorations. Attempts have been made to explain the formation of the various nanostructures in terms of the creation and distribution of the ZnOx, the temperature as well as oxygenation conditions. 
Jai Singh, Anchal Srivastava, R.S. Tiwari and O.N.Srivastava