Optical Properties of Hydrothermally Grown ZnO Nanoflowers

Zinc oxide (ZnO) is a well-known multifunctional material possessing unique structural, electrical and optical properties that are very useful in various device applications. Being a high and direct band gap semiconductor, its is potentially being used in various UV light sources and detectors fabrication. However, the emission and absorption properties strongly depend on the size of the ZnO nanoparticles which in turn depends on the morphology of the nanostructure. Therefore, it is very much important to understand to structure-property relationship for predictable device performance. Our objective of this work is to synthesize flower-like ZnO nanostructures using simple hydrothermal method. The flower-like ZnO morphology offers large surface area that will be very suitable for designing gas and chemical sensor devices. Other objective of this work is to study the crystallography of ZnO. Next the optical properties (emission and absorption) have been investigated to understand the defect related photoluminescence mechanism. A simple hydrothermal method has been deployed to synthesized flower-like ZnO nanostructures. A chloride decomposition scheme has been used to produce zinc hydroxide ions that will produce ZnO nuclide. At the onset of saturation, ZnO nanocrystals start to grow. The entire reaction was performed inside a teflon cell stainless steel autoclave. The autoclave was placed in a horizontal tube furnace and maintained at 150 °C for 2 hr resulting the formation of white powder-like material. The X-ray diffraction data confirms the formation of polycrystalline ZnO having wurtzite structure. Flower-like morphology was clearly observed in FESEM images. The EDS data confirms the composition of ZnO with proper stoichiometry. Gibb’s free energy calculation favours the reaction under the experimental condition. The absorption spectrum was used to calculate the band gap of the synthesized ZnO nanoflowers. The Tauc plot revealed the band gap of the synthesized ZnO to be ~ 3.69 eV. This enhancement of band gap compared to bulk ZnO occurs due to quantum confinement effect. The synthesized ZnO nanoflowers exhibit broad photoluminescence peaked at 429 nm owing to the presence of interstitial zinc. A hydrothermal method has been successfully used to synthesize well-crystalline ZnO nanoflowers of proper stoichiometry. The flower-like nanostructure exhibits band gap enhancement due to quantum confinement effect. Room temperature visible photoluminescence observed from the ZnO nanoflowers with a board emission peak at 429 nm. This emission arises due to the presence of deep level zinc interstitial states. This finding will be very useful in understanding the role of defects in the visible emission from ZnO nanostructures.