Hydrothermally grown ZnO nanorod based heterostructure: synthesis and application in gas sensor and photocatalyst

Abstract

The one-dimensional (1D) semiconductor nanostructures have attracted considerable research activities because of their great potential for diverse application. Various fabrication method and properties change of the materials, and numerous applications of 1D nanomaterials have been reseached in recent. One of famous semiconducting material, 1D nanostructured ZnO with 3.2eV of direct wide band gap also have been evaluated for infinite their application and studied to improve the potential of ZnO. The formation of heterostructures on ZnO with desired composition and/or morphology could modulate the properties of materials, improve the efficiency or performance in application, and find a new potential application. In this thesis, the new and facile synthesis method and enhance performance in application as a gas sensor and photocatalyst of the hydrothermally grown ZnO with some advantage like easy and cheap in process based metal/metal oxide heterostructured nanorod was described. In chapter 1, the literature survey part introduced the basic properties of ZnO, role of researched heterostructure, and trend and principle of application as gas sensor and photocalyst. In chapter 2, the synthesis method of hydrothermally grown ZnO nanorod bundle and the ZnO based CuO/ZnO heterostructured nanorod by photochemical process. A one-dimensional ZnO nanostructure was synthesized using the hydrothermal method

scanning electron microscopy (SEM) and x-ray diffraction (XRD) spectra confirmed that the structures were crystalline ZnO of hexagonal structure. Through the control growth of ZnO at various conditions as different pH and synthesis time, the shape change of ZnO nanorod bundle was confirmed. Using these ZnO, the CuO/ZnO heterostructured nanorod was synthesized by photochemical method. The formation of monoclinic CuO on surface of ZnO was confirmed by scanning electron microscopy (SEM), x-ray diffraction (XRD) and transmission electron microscopy (TEM). I suggested the synthesis mechanism at different growth condition as UV irradiation time and concentration of copper salt in precursor by observation of morphological change at the conditions. In chapter 3, the sensing properties and sensing mechanism of hydrothermally grown ZnO nanorod bundle were investigated. A furnace type gas sensing system was used to characterize the nanorod bundles’ sensing properties in air containing dilute H2S gas (50 ppm) at sensing temperatures Ts ≤ 500 oC. The response of ZnO nanorod bundle sensors to H2S gas increased with Ts

this trend may be due to chemical reaction of nanorods with gas molecules. X-ray photoelectron spectroscopy(XPS) results indicated that the sensing mechanism of ZnO nanorod bundle sensor was explained by both the well-known surface reaction between H2S and adsorbed oxygen on ZnO, and the formation of zinc sulfur bonding in ZnO nanorods, which becomes a dominant sensing mechanism at high Ts above 300 oC.In chapter 4, the results of CuO/ZnO’s improved sensing properties and their sensing mechanism with role of CuO were described. The response of CuO/ZnO nanorod bundle sensors to H2S gas improved than ZnO without CuO, and increased with same exponential tendency of ZnO depending on Ts

XPS and SEM results indicated that the reason of response enhancement is of CuO/ZnO nanorod bundle sensor was explained by role of CuO as electrical gates by conversion to metallic Cu2S on surface of ZnO. The trade-off effect of CuO on surface ZnO between to enhance the sensing perpormance and to screen of acrive sensing aria of ZnO was also suggested. In chapter 5, the synthesis of Au/ZnO heterostructured nanorods by a facile sonochemical method and their improved photocatalytic activity was investigaed. A simple and fast deposition of gold nanoparticles on ZnO nanorods was performed using a mixed solvent of ethanol and deionized water (D.I. water) with no need for any surfactants or additives. Through various analyses (SEM, XRD, TEM, and EELS), we confirmed the formation of a highly crystalline Au/ZnO nanostructure

several tens of nanometer-sized polycrystalline Au nanoparticles were uniformly, densely deposited on the surface of ZnO nanorods. The size and the density of Au nanoparticles could be controlled by the modulation of the concentration of the gold ion precursor and the solvent compositions. Formation of smaller Au nanoparticles with a lower density was induced at a lower concentration of the gold ion source with a ratio of 1:4 of D.I. water to ethanol. Compared with bare ZnO nanorods, Au/ZnO heterostructures exhibited enhanced photocatalytic activity in the photodegradation of an organic dye (rhodamine B, RhB), which is caused by the Au nanoparticles acting as an electron sink and lowering the local work function.