Enhanced Ultraviolet Luminescence of ZnO Nanorods Treated by High-Pressure Water Vapor Annealing (HWA)

Zinc oxide nanorods (ZnO NRs), synthesized by a low temperature chemical method, were postannealed at 260 degrees C in air and under high pressure water vapor (HWA) at 1.3-3.9 MPa. We found that the UV luminescence intensity increased by a factor of 2-3 after HWA annealing compared to that observed after annealing in air. Structural analysis of the nanorods in relation with their optical properties by means of Raman and XPS spectroscopies, transmission and scanning electron microscopies allow to conclude that the origin of the UV luminescence enhancement is due to the transformation of the Zn(OH)(2) surface-layer into ZnO, but also to the growth of a new thin ZnO layer at the surface of the rods. This layer is 1-2 rim thick and its presence leads to surface reconstruction of the nanorods. In addition, we show that the size and the density of the nanopores within the ZnO NRs are reduced upon HWA annealing with respect to air annealing.