Synthesis and Characterization of Some Core-Shell Oxide Materials

Abstract

Core-shell oxide thin films of TiO2/MnO, TiO2/CoO, TiO2/NiO and TiO2/CuO have been successfully synthesized and deposited on glass substrates, using the chemical bath deposition [CDB] technique. To ensure uniform coating, surface pre-treatment was carried out by cleaning slides with detergent, deionised water, ammonia acid and rinsed with acetones. The core of the films was synthesized from TiCl3, NaOH and PVA while the shells were synthesized from MnCl2.4H2O, CoCl2.6H2O, NiSO4 and CuCl2 respectively. Four samples of each core-shell thin film were each annealed in a given temperature range for 1 hour per sample. X-ray diffraction [XRD], scanning electron microscopy [SEM], Rutherford Backscattering [RBS], Energy Dispersive X-ray [EDX] and the four points probe were used in characterizing and analysing the films’ crystalline structures, optical and solid properties, elemental compositions, crystalline quality and electrical properties. Graphs of the films’ properties such as absorbance, and transmittance etc were plotted against wavelengths while graphs of absorption coefficient and versus photon energy [ were plotted. From the experimental results obtained, the chemical bath deposition technique has proved to be highly specific, convenient and easily adaptable to large area processing, without the presence of an externally applied electric field. Range of deposition time was 3 to 4 hours. It was found that: annealing has no significant effects on the crystalline size which is approximately 39nm. Thermal treatment of the studied core-shell films affected their solid state and electrical properties significantly, while optical properties depended on the wavelengths. All the films are nanocrystalline with more or less the same average grain size but different film thicknesses. These films have good photoluminescence and there were no crystalline defects in any sample. The band gaps determined for the core-shell films range from 1.9-3.8eV while the range of 1.7-2.3eV was determined for the conventional oxide films. The resistivity of the studied thin films decreased with increase in temperature and this further indicated the semiconducting nature of these films. Only TiO2/CuO core-shell thin film has both direct and indirect transitions while other films have direct transitions gaps. The main applications of these films include: TiO2/MnO showed potential for n-type window layer for thin film heterojunction solar cells; TiO2/CoO for ideal material for PV applications, TiO2/NiO was identified to be a good material for electroluminescent devices and photocatalysis. TiO2/CuO for UV sensors and coating for poultry houses, TiO2 for electric and optoelectronic devices such as effective window layer materials for solar cells and CoO for light emitting diodes. This work has shown that for the first time, some transition metal oxides can be used as the cores and shells of core-shell thin films, for various applications.