Fabrication and characterisation of ZnO@TiO2 core/shell nanowires using a versatile kinetics-controlled coating growth method

Abstract

This study aims to provide a strategy for fabricating a scalable and reproducible ZnO@TiO2 heterostructure with precisely engineered TiO2 layer thickness by employing a simple wet chemical method to fabricate Zinc oxide nanowires (ZnO NWs) with a titanium oxide (TiO2) shell. It is also directed at investigating the impact of the reaction parameters: temperature, time and ammonia concentration, on the heterostructure, as well as the influence of photonic annealing on both ZnO NWs and ZnO@TiO2 heterostructure. Transmission, scanning electron microscopy, X-ray mapping and X-ray diffraction confirmed the ZnO@TiO2 heterostructure morphology, its shell thickness and presence of its elements. They also indicated that one hour was sufficient to produce the TiO2 shell at room temperature. In addition, both the reaction time and temperature did not impact on the TiO2 shell thickness. However, changing ammonia content from 300 µl to 400 µl significantly increased the TiO2 shell thickness from ∼2.5 nm up to ∼90 nm. The 2.5 nm shell showed an anatase crystalline structure whilst the thicker shells were mostly amorphous. X-ray photoelectron spectroscopy confirmed that the oxidation state of Ti was consistent with TiO2. Photoluminescence measurements showed a large improvement in the optical properties of ZnO NWs after forming the TiO2 shell, increasing the intensity of the UV band relative to the visible band by a factor of 4.5. The photonic annealing process showed a greater improvement by a factor of 7.1 for the ZnO@TiO2 heterostructure and tenfold for bare ZnO nanowires.