Study On Controlled Synthesis And Photocatalytic Performance Of Nanometer-sized Zinc Oxide

It has been demonstrated that the properties of nanometer-sized ZnO were related to its surface characteristics closely, such as size, shape, deficiency and so on. Thus, to tailor ZnO with novel electrical, optical and chemical properties, the growth of nanometer-sized ZnO should be controlled well. At the same time, the application of nanometer-sized ZnO in the treatment and protection of environmental pollution has been attracted the public concern for semiconductor-assisted photocatalysis can convert the pollutants into harmless substances directly. ZnO is one of the most widely used effective photocatalyst for its high efficiency, non-toxic nature and low cost. In this paper, firstly, several preparation methods of nanometer-sized ZnO were introduced in brief. The microcosmic mechanisms of the nanometer-sized ZnO photodegradation and the factors that can affect the catalysis efficiency were discussed in detail as well as the methods that can improve the efficiency so far. Subsequently, the controlled synthesis methods of nanometer-sized ZnO and its photocatalytic performance have been studied in detail.In the chapter 2, several kinds of ZnO nanostructure with special morphologies and structures have been fabricated through the low temperature solution route. The key factors in the synthesis process have been found and discussed. 1) In the methanol solvent ( CH3OH ), the hydrolysis of zinc acetate dihydrate ( Zn(CH3COO)2.2H2O ) was controlled well through adding H2O to obtain ZnO with different morphologies. The results of field emission scanning electron microscopy (FESEM) and X-ray diffraction ( XRD ) showed the morphology transition process of ZnO from irregular shape to regular cone-like, to standard hexagonal plate with the increasing volume ratio of H2O to methanol solvent from 2:15 to 1:2, which demonstrated the role of H2O on tailoring the morphology of ZnO. 2) In this section, the novel ZnO microspheres were synthesized by the complex system in which starting reagents, Zn(CH3COO)2, H2O, and hexamethylenetetramine ( (CH2)6N4,HMTA ), were kept in methanol solution at 60℃for 12 h. The FESEM, transmission electron microscopy ( TEM ) images and XRD pattern results showed that the microspheres were formed by ZnO nanoplates that align in a regular periodicity via self-assembly. The formation process of this novel nanostructure was a slow and complicated one and the chemical reagent HMTA was the key factor. 3) In the solution with distilled water as solvent and zinc nitrate hexahydrate ( Zn(NO3)2.6H2O ), HMTA as reactants, a novel microsphere with nanometer holes separated by thin flakes and ZnO branch rodlike structure and a kind of thin solid film were formed after 24 h at 60℃. Through controlling the deposition time and the concentration of solution, the pure porous thin solid films can be prepared. When the citric acid as capping molecules was added into the solution, the precipitates were the pure microspheres. As a contrast, the pure ZnO branch structure was obtained through changing the solvent from H2O to methanol. The results of FESEM,XRD and thermogravimetry-differential scanning calorimetry ( TG-DSC ) showed that after annealing at 250℃for 1h, the thin flakes of novel microspheres have been torn off and consisted of the small ZnO nanoparticles with the average size 10 nm. However, the porous surface of microspheres was maintained.In the chapter 3, the relation between the surface characteristics of ZnO nanoparticles and the photocatalytic performance was studied. 1) Through the evaporation-condensation method, ZnO nanoparticles with three different morphologies including irregular, tetrapod and plate were prepared by controlling the evaporation temperature at a constant oxygen partial pressure 2000Pa. The photodegradation efficiency of Methyl Orange showed that the effect of morphology on the photoactivity of ZnO nanoparticles can be ascribed to the surface area and the composition of surface crystal planes; 2) ZnO nanoparticles with different amounts of oxygen deficiency were prepared through controlling the oxygen partial pressure at a constant evaporation temperature. The X-ray photoelectron spectroscopy ( XPS ) results demonstrated the amount of surface oxygen deficiency as well as the chemisorbed oxygen decreased with the increase of oxygen partial pressure from 700-800Pa to 2200-2300 Pa. However, ZnO nanoparticles that prepared at 1500-1900Pa displayed the highest photoactivity by decomposing Methyl Orange, which showed the appropriate amount of oxygen deficiency was beneficial to the separation of photoinduced carriers;3) ZnO nanoparticles were prepared by chemical deposition method. Its size, morphology, surface characteristic was presented using different testing techniques such as TEM, Ultraviolet-visible ( UV-Vis ) absorption spectrum and Fourier transform-infrared spectrometry ( FT-IR ). The size of ZnO nanoparticles was 10nm, which showed the quantum size effect. However, its surface was surrounded by organic substances (acetate ion). The results of photodegradation showed that the acetate ions suppressed the transfer of photoinduced carriers, which resulted in the lower photoactivity of ZnO nanoparticles than the ZnO (50nm) nanoparticles prepared by evaporation-condensation.In the chapter 4, the ZnO solid films were fabricated through the liquid deposition method. The results of FESEM and XRD demonstrated ZnO solid films with different size and morphologies were obtained under different annealing temperature (300℃,500℃,800℃). The photocatalytic experimental results showed ZnO solid films prepared using 0.05mol/L Zn(CH3COO)2.2H2O in methanol solvent under annealing temperature 500℃had the highest photoactivity. Furthermore, the ZnO nanorod array was prepared by two-stage method via modifying the fabrication of ZnO seeds particle film. The ZnO nanorod array showed the higher photoactivity than the ZnO particle film for the separated nanorods surrounded by the Methyl Orange solution would enhance the charge separation and thus reduced the recombination of the photogenerated electrons and holes. Furthermore, with the prolonging of deposition time, the photoactivity of ZnO nanorod array decreased greatly.