Influence of SnO2 coating and thermal annealing on the structure and luminescence properties of CuO nanorods

CuO-core/ SnO₂-shell one-dimensional nanostructures have been fabricated by thermal oxidation of a copper foil and then atomic layer deposition of SnO₂. The structure and optical properties of the nanostructures have been investigated by using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, photoluminescence (PL) spectroscopy, and energy-dispersive X-ray analysis techniques. The nanostructures are found to have the form of nanorods, with the diameter of the CuO cores being in the range from a few tens to a few hundreds of nanometers, the thickness of the SnO₂ shells being ～15 nm, and with a length of a few tens of micrometers. The CuO cores and the SnO₂ shells of the as-synthesized nanorods have crystalline monoclinic CuO and amorphous SnO₂ structures, respectively, but the SnO₂ shells are found to crystallize to tetragonal SnO₂ on thermal annealing. The PL emission intensity of the CuO nanorods has been slightly increased by SnO₂ coating. The PL emission of the SnO₂-coated CuO nanorods is somewhat increased and the emission peak position is red-shifted from 550 to 580 nm by annealing in a reducing atmosphere. On the other hand, the PL emission is significantly increased and the emission peak position is shifted from 550 nm further to around 595 nm by annealing in an oxidative atmosphere. In addition, the origins of the PL enhancements in the nanorods by coating and annealing are discussed.