| Semiconductor photocatalysts, especially anatase titanium dioxide(TiO2), have huge potential in solving environmental pollution and energy shortage. However, anatase TiO2 only responses to ultraviolet(UV) light, because of its large band gap of 3.2 eV. Since UV light only has 5% content in sun spectrum and UV light may bring certain damages to human, practice application of TiO2 is limited. Recently, researchers have tried many methods, such as elemental doping and nano engineering, to decrease and tune the band gap of TiO2, in order to shift its optical absorption edge from UV region to visible-light region. Therefore, fabrication of TiO2 with visible-light response and study of its related physical properties are hot topics around the world.Based on the investigation and analysis previous work, in this thesis, we have found that anatase Ti O2 with narrow band gap,black colour and visible-light response can be fabricated by using nonactive atomosphere, e.g., high-purity nitrogen(N2),as calcination atomosphere for the precursor of TiO2, This method is explored for the first time by us. We have successfully applied this method to a series of TiO2 micro/nano particles, such as nanoparticles, submicron solid spheres, submicron hollow spheres, as well as magnetically recyclable particles. We have systematically studied visible-light responsed properties of these particles, including optical absporption, photocatalytic activity, and photoelectric performance. The main contents and innovation points are summarized as below.1. We have successfully fabricated monodisperse submicron solid spheres of TiO2 precursor using sol-gel method, followed by high-temperature calcination to obtain submicron solid spheres of anatase TiO2. We have found that calcination atomosphere and temperature have significant effects on sample’s color and optical properties. Experimental results show that the samples calcined in air display usual white color and only have strong absorption in UV region, and this property is almost independent of calcination temperature. However, the color of the samples calcined in high-purity nitrogen shows calcination-temperature dependence, i.e., it turns from light gray at 400°C to greyish black at 500-600°C, and finally becomes black at 800°C. The samples calcined in N2 have strong absorption both in UV region and visible-light region, and its visible light absorbance increases with calcination temperature. Further experiment indicates that the variation of color reflects its band-gap narrowing. The band gap of the samples calcined in N2 is narrower than that of those calcined in air(3.19 eV), and it decreases with calcination temperature, i.e., it decreases from 2.81 eV at 400°C to 1.63 eV at 800°C. Characterizations of X-ray diffraction, Raman spectrum, and X-ray photoelectron spectrum show that all the samples calcined both in air and nitrogen have pure anatase phase without the impurity trace of carbon and nitrogen. Experiment on photocatalytically decomposing organic dyes of RhB shows that the black samples exhibit poor photocatalytic activity in both visible light and UV light. Even for the white samples, its UV photocatalytic acivity is inferior to that of standard P25 which has nanoscale size. The poor photocatalytic acivity of the submicron solid spheres may be ascribed to high probability of bulk combination of photon-generated carries because of its submicron size.2. In order to improve photocatalytic activity in visible light, we have fabricated nanosized TiO2 precursor using sol-gel methods in nonaqueous solvents. After calcination in high purity N2, we have successfully obtained TiO2 nanoparticles with size of 22 nm, displaying black color and visible-light absoption. The band gap of the black nanoparticles is 2.05 eV, which is much smaller than that of the nanoparticles calcined in air(3.13 eV). Since the black nanoparicles have nanoscle size and large surface-volume ratio, they exhibit high visible-light photocatalytic activity, resulting from the effective transportation of photon-generated carriers to paricles’ surfaces and hence effective decomposition of organic dyes. Therefore, the black Ti O2 nanoparticles may also have potential applications in the fields of photocatlytically splitting of water into hydrogen and solar cells under visible light.3. In order to verify whether it is valid to apply the high-purity nitrogen calcination to other micro/nano-structures to obtain visible-light response, we have fabricated TiO2 submicron hollow spheres using template method. After calcination in high-purity nitrogen, we have successfully obtained TiO2 submicron hollow spheres with black color and visible-light response, which futher indicates that the high-purity nitrogen calcination method is an effective way to realize visible-light response in such system. The black TiO2 submicron hollow spheres show prior photocatalytic acivity to that of black TiO2 submicron solid spheres, may be due to its unique hollow structures and nanoscale particles constructed in the nanoshell. Base on this, we have further fabricated composite particles consisted of both the visible-ligh response TiO2 and ferromagnetic Fe3O4. Experimental results show that the composite particles exhibit rapid magnetic reponse under magnetic field. Therefore, people may realize magnetically recycling and reusage of photocatalysts in photocatallytic decomposition of pollutions by using such kind of composite particles. |
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