Synthesis of zinc oxide nanostructures during zinc oxidation by sub- and supercritical water

Solid zinc (Zn)(S) and liquid zinc (Zn)(L) are oxidized by water with the formation of zinc oxide (ZnO) nanostructures and the evolution of hydrogen. The maximum rate of this process, called chemical supercondensation by water (CSW), is realized on approaching the melting temperature of zinc from the left and right with increasing density of supercritical water. The CSW process begins with the formation of (ZnO) (n) clusters via the reaction (Zn)(S,L) + nH(2)O = [(Zn)(S,L) center dot (ZnO) (n) ] + nH(2), followed by their subsequent growth at n > 7 in the exothermal process of epitaxy on (Zn)(S) and coagulation of (ZnO) (n) in (Zn)(L). The CSW of (Zn)(S) leads predominantly to the formation of nanowires and nanorods, while the CSW of (Zn)(L) practically always proceeds with the formation of nanoparticles. The rate of (Zn)(S) oxidation increases with the thickness of a layer converted into ZnO. This is related to the self-heating and local melting of (Zn)(S) in the course of CSW. The complete CSR of (Zn)(S) plates and cylinders results in the formation of highly porous nanostructural ceramics.