Synthesis and characterization of zinc oxide/copper oxide/core-shell heterojunction nanowires grown by vapor deposition

Abstract

This thesis investigates the controlled growth and vertically aligned ZnO/CuO core-shell heterojunction nanowires (NWs) formation by vapor deposition and oxidation approach. ZnO/CuO heterostructure nanowires were grown on n-type Si substrate using modified thermal chemical vapor deposition (TCVD) assisted by sputtering deposition followed by thermal oxidation under controlled growth conditions. The effects of fabrication parameters on structure, growth mechanism, optical and electrical properties of the ZnO/CuO core-shell heterojunction were thoroughly investigated. Structural characterization by field emission scanning electron microscope (FESEM), high resolution transmission electron microscope (HR-TEM), scanning transmission electron microscope (STEM), X-ray photoelectron spectroscope (XPS), X-ray diffractometer (XRD) and energy dispersive X-ray (EDX) reveals that a highly pure crystalline ZnO core and polycrystalline CuO shell were successfully fabricated in which ZnO and CuO are of hexagonal wurtzite and monoclinic structures, respectively. The growth of ZnO nanowires is along the c-axis [002] direction and the nanowires have relatively smooth surfaces with diameters in the range of 35-45 nm and lengths in the range of 700-1300 nm. The CuO nanoshell with thickness of around 8-10 nm is constructed of nanocrystals with sizes in the range of 3–10 nm. EDX spectrum, elemental mapping and high angle annular dark field (HAADF) STEM confirmed that the NW compositions were Zn, Cu and O. Photoluminescence (PL) study shows the enhancement of intensity ratio and decrease in the energy band of ZnO/CuO coreshell heterojunction NW arrays that might be very useful in photocatalysis, light emission devices and solar energy conversion applications. Similarly, UV-VIS-NIR spectroscopy study shows that the grown ZnO NW arrays have a maximum reflectance of approximately 42% in the 200 to 800 nm range while the ZnO/CuO core-shell heterojunction NW arrays have a decreased value of 24%. This means that the absorption efficiency of ZnO/CuO core-shell heterojunction nanowire arrays clearly shows a higher absorption compared to pure ZnO nanowire arrays. Besides, the good rectifying behavior of ZnO/CuO core-shall NW by conductive AFM (CAFM) showed that p-n junction was successfully fabricated. Furthermore, from the XPS analysis, the measured values for valence band offset (VBO) and conduction band offset (CBO) were found to be 2.4 eV and 0.23 eV, respectively for the fabrication of ZnO/CuO core-shell heterojunction NWs. It was observed that ZnO/CuO core-shell heterojunction NWs have type-II band alignment. This study obviously suggests that using the controlled growth mechanism, it is possible to control crystal structure, surface morphologies and orientation of the core-shell NW arrays.