Investigation On The Synthesis And Properties Of ZnO Nanomaterials

Zinc oxide (ZnO) is a kind of direct band gap semiconductor with wide band gap of3.37eV at room temperature and large exciton binding energy of60meV. It may find wide potential applications in many fields, such as light-emitting diodes, UV light detectors, gas sensors, piezoelectrics, photocatalysis, biological labeling, and solar cell et al. The studies on the basic problems of controllable synthesis, properties tunning, photoluminescence mechanism and origin of room temperature ferromagnetism of ZnO nanostructures possess very important theoretical value and practical significance. In this article, systematic investigation have been done on the controllable synthesis, doping, composite and surface modification, applications in photocatalysis and solar cell of ZnO nanomaterials. The main contents are summarized as follows:1. The hexagonal faceted ZnO quantum dots (QDs) about3-4nm were prepared via a sol-gel route by using oleic acid (OA) as the capping agent. Controlling the concentration of reaction solution and synthesis time, the structure of ZnO was successfully transformed from quantum dots to quantum rods. Compared with spherical ZnO QDs, the hexagonal faceted ZnO QDs show enhanced photocatalytic activity. Besides small size of ZnO QDs, the enhanced photocatalytic activity can mainly be ascribed to the special hexagonal morphology. This structure contains more Zn-terminated (001) faces, which is considered to play a key role in photocatalytic performance of ZnO.2. A sol-gel route was developed to synthesize ZnO QDs with tunable diameters in a range of2.2-7.8nm, using self-made zinc-oleate complex as a precursor. To get a real understanding on the mechiansm of visible light emission, we systematically study the origin and property of visible light emisison of ZnO QDs. Based on analysis, two important points can be obtained:one is that single ioned oxygen vacancies determine the origin and intensity of visible emission of ZnO QDs; another is that the visible emission peak position of ZnO QDs is decided by their size, and a transition of holes from the preexisting deep donor energy level to the valence band is responsible for the visible emission of the ZnO QDs.3. Room temperature ferromagnetism (RT FM) was observed in synthesized ZnO quantum dots (QDs). The results of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) indicate that the ZnO QDs are pure and no other phase or impurities exist. It was also found that the saturation magnetization decreased with size increasing of the ZnO QDs. The results of photoluminescence (PL) show that the intensities of visible light emission have the same changing trend as that of FM, and both of them are directly associated with an electron paramagnetic resonance (EPR) signal with g=2.0056. This EPR signal is triggered by singly ionized oxygen vacancies. Therefore, it is reasonable to consider that singly ionized oxygen vacancy is the origin of RT FM in pure ZnO QDs. Experimental data on the annealed ZnO QDs in air further confirm this conclusion.4. Platium (Pt) nanoparticles with controllable loading content are modified on ZnO nanorods (NRs) using chloroplatinic acid (H2PtCl6) as Pt precursor via a chemical reduction process in ethylene glycol. It is indicated from x-ray photoelectron spectroscopy (XPS), PL and UV-vis spectra that the deposited Pt atoms can act as electron acceptors and facilitate the electron-hole separation. Therefore, the Pt-ZnO heterostructures display obviously enhanced photocatalytic activity with an optimal loading value of1.0atom Pt%. Meanwhile, in order to resolve the reclclable issue of photocatalysts, nickel (Ni) and Pt was simultaneously deposited to form PtNi19alloy on the ZnO NRs. This is mainly considering that nickel is also the metal like Pt and it is magnetic. It is found that when Ni nanoparticles were modified on ZnO NRs, although they are magnetic and can be seperated by magnetic force, the photocatalytic activity becomes poorer than that of pure ZnO NRs. When alloyed PtNi nanoparticles were modified on ZnO NRs, the composites are magnetic, and show almost the same photocatalytic activity as that of pure ZnO NRs.5. Via sol-gel method, ZnO QDs with uniform size were synthesized. PbS QDs with more uniform size were synthesized via a chemical precipitation. Using these two kinds of QDs, we prepared QDs P-N junction heterostructure solar cell and tested their performance. This preparation method is simple and low-cost, and may find potential application in next generation solar cells.