Fabrication and characterization of abrupt TiO2–SiOx core-shell nanowires by a simple heat treatment

Three dimensional hierarchical metal oxide nanostructures, like TiO2 nanowire arrays, have attracted great attention for electrochemical energy conversion and storage applications. The functionality of such devices can be further enhanced by adding a nanowire shell with a different stoichiometry or composition compared to the core. Here, we report an approach with a facile heat treatment at 1050 °C, which allows the fabrication of rutile TiO2–SiOx core-shell nanowire arrays on silicon substrates. Our detailed electron microscopic investigation shows that this method is able to cover hydrothermally grown rutile TiO2 nanowires with a uniform shell of several nanometers in thickness. Moreover, the treatment improves the quality of the rutile TiO2 core by removing lattice defects, introduced from the hydrothermal growth. Electron energy loss spectroscopy reveals that the homogeneous shell around the TiO2 core consists of amorphous SiOx and does not form any intermediate phase with TiO2 at the interface. Thus, the properties of the TiO2 core are not affected by the shell, while the shell suppresses undesired electron back transfer. Latter leads to performance losses in many applications, e.g., dye sensitized solar cells, and is the main reason for a fast degradation of devices incorporating organic materials and TiO2.