|Place of Conferral||北京|
|Keyword||P型半导体 氧化铜 石墨相氮化碳 可见光催化氧化 异质结结构|
随着经济的快速发展，人们对化石能源的依赖程度也越来越高，在这种情形下，两大问题变得日益严重，一方面石油、天然气作为不可再生能源已日益枯竭；另一方面化石燃料的燃烧使得环境状况也越来越恶劣。自从1972年Fujishima和Honda发现被太阳光照射的TiO2表面可以析出氧气以来，太阳能作为一种取之不尽的清洁能源，利用光催化技术降解环境中的有机污染物是解决环境污染问题的有效途径之一。研究表明，在环境污染物的光催化降解中，光催化氧化反应起主要作用，光生空穴在光催化氧化过程中起到很重要的作用，p型半导体以空穴为主体进行导电。因此，p型半导体更适合光催化氧化反应。本论文首先选取CuO这种廉价、易制备、性能稳定的窄带隙p型半导体，通过形貌调控增强氧化能力；通过简单搅拌的方法，使化学稳定性及热稳定性良好的普通n型石墨相氮化碳的导电类型转变为p型，以增强其可见光催化氧化性能。由于本征TiO2半导体光催化剂只能吸收在太阳光中约4%的紫外光，对太阳光的利用率较低。纯的非金属有机聚合物半导体光催化剂g-C3N4虽然可以吸收可见光，但是只能吸收450 nm处的蓝光，对可见光的利用效率低；而且光生电子-空穴对容易复合，导致有效光生载流子的数量比较少。而通过半导体复合可以提高系统的电荷分离效率，拓展光谱响应的范围。本研究以CuO这种窄带隙p型半导体为基础，通过和TiO2以及g-C3N4复合构筑异质结结构材料，增强可见光催化氧化性能。具体工作分两个部分：第一部分：可见光催化氧化性能增强为导向的p型半导体的设计 （1）利用微波水热法快速制备了纳米结构CuO，并且通过控制柠檬酸钠的加入量分别得到纳米棒、万寿菊状、牡丹花状CuO，并阐述其形成机理。通过在可见光条件下光催化降解亚甲蓝（Methylene blue，MB）活性测试发现，牡丹花状CuO光催化性能最好，主要原因是牡丹花状CuO的特殊层状结构增强了对光的吸收。（2）通过将HCl与超薄纳米片状剥层g-C3N4简单搅拌的方法成功地将Cl-引入g-C3N4中形成中间能级，g-C3N4的半导体类型从n型转变为p型。通过可见光条件下NO的去除测试，比较了这两种类型的g-C3N4的活性。实验结果表明，p型g-C3N4对NO的光催化降解速率是n型石墨相氮化碳的3.5倍。活性的提高主要是由两方面引起的：一方面，g-C3N4从n型转变为p型后，导电主体由电子变为空穴，这有利于NO的氧化反应；另一方面，Cl-引入g-C3N4中，在价带上方形成的中间能级，增强了对光的吸收，同时使电子更容易被激发、抑制了光生载流子的复合。第二部分：可见光催化氧化性能增强为导向的异质结的构筑（1）利用微波水热法成功将CuO负载到TiO2上，得到p-n异质结结构CuO/TiO2材料。采用透射电镜、扫描电镜对CuO/TiO2复合光催化材料的形貌、尺寸进行了分析，结果表明所合成的CuO/TiO2复合光催化材料由直径为40 nm左右颗粒组成。通过可见光条件下降解MB来考察加入不同量TiO2所得到的CuO/TiO2异质结材料的光催化性能。实验结果发现，当加入0.25 g TiO2时，CuO/TiO2异质结材料的光催化性能最优，在可见光条件下照射70 min后，MB的降解率达到93.2%，TiO2和CuO的复合拓宽了TiO2的光谱响应的范围，增强了CuO的光催化性能。进一步通过光电化学试验及紫外可见漫反射光谱，探讨了光催化活性增强的机理。（2）利用简单的搅拌浸渍以及后煅烧的方法，将CuO与超薄纳米片状g-C3N4复合，形成p-n型、有机-金属半导体混合的CuO/g-C3N4异质结结构复合材料。与纯g-C3N4相比，CuO/g-C3N4异质结光催化材料的光催化活性明显提高，在可见光条件下照射30 min，对罗丹明B（Rhodamine B，Rh B）的降解率为90.1%，降解速率常数是纯g-C3N4的4倍。性能增强的原因是CuO与g-C3N4的复合，提高了对可见光的吸收，抑制光生电子-空穴对的复合，增强了导电性能。我们还做了CuO与g-C3N4物理混合后得到的复合材料与CuO/g-C3N4异质结光催化材料在同样条件下降解Rh B的对比，推测CuO/g-C3N4光催化剂活性增强的另一个原因是在CuO/g-C3N4的p-n型异质结界面相上，形成一个由n型g-C3N4到p型CuO方向的内建电场，使光生电子的转移方式由传统的双转移机制变为Z型机制，从而增强了光催化活性。
The rapid development in economy leads to increasing demand for fossil fuels in our society. In this situation, two important issues have become increasingly important. One is energy shortage since fossil fuels are finite and can only be exploited for a limited number of years. The other is air pollution as a result of the combustion of fossil fuels. Since Fujishima and Honda found that the TiO2 electrode produce oxygen under the irradiation of light in 1972, as solar energy is inexhaustible and clean, the utilization of photocatalytic technologies to remove pollutants has attracted interest worldwide. According to previous reports, most pollutants are removed through photocatalytic oxidation reactions. Therefore, photogenerated holes are preferred over electrons in these processes. Holes are known to prevail over electrons as photogenerated carriers in p-type semiconductors. Thus, it is reasonable to speculate that p-type semiconductors are more suitable for pollutant removal. Copper oxide (CuO), as a cost-effective p-type semiconductor with a narrow band gap, was designed to control the morphology in order to enhance the photocataystic oxidation property, meanwhile, the chemical and thermal stable pristine n-type g-C3N4 was designed to switch the semiconductor behavior to p-type for the improvement of photocataystic oxidation property.The bulk TiO2 is only respond to the UV light which only occupy 4% of the sunlight. As a non-metal organic polymer semiconductor, the pure g-C3N4 can absorb the visible light, however, it can only activated by the blue light up to 450 nm, meanwhile, the effective photogenerated carrier is very low because of the fast recombination of photogenerated electrons and holes. Coupling with other semiconductors can improve the separation of photogenerated electrons and holes, broaden spectral response range. In this dissertation, CuO was selected as the fundamental research object, coupling with TiO2 or g-C3N4 to fabricate heterogeneous structure oriented to enhance the visible light phtocatalystic oxidation in this dissertation. Specific content was divided into two parts:PartⅠ Design of p type semiconductor oriented on improvement of visible light photocatalystic oxidation：(1) The technique of microwave-assisted hydrothermal method was applied to synthesize nanorods、chrysanthemum-like and peony-like CuO respectively through changing the addition of sodium citrate (SC). The plausible growth mechanism for the formation of CuO nanostructure is proposed. Moreover, The photocatalytic activity of the as-prepared CuO for degradation of methylene blue (MB) were investigated under visible light irradiation, which indicates the CuO with peony-like morphology exhibited the highest activity because of the presence of nanosheets with special microstructures improve the absorbtion of visible light. (2) We have experimentally and theoretically demonstrated that the incorporation of HCl in g-C3N4 nanosheets can switch the semiconducting behavior of g-C3N4 nanosheets from n-type to p-type. In this study, the incorporated Cl- formed an acceptor level above the VB via facile stirring method. The photocatalytic activities of different samples were investigated by nitric oxide removal under visible light, the result showed that p-type can efficiently improve the photocatalytic activity of g-C3N4 for nitric oxide (NO) removal about 3.5 times. There are two main reasons for the switching of the semiconductor improves the activity in photooxidation NO removal, One is in p-type semiconductors, photogenerated holes prevail over electron carriers, the other is this new formed intermediate level improves the visible light absorption efficiency while favors electron excitation and suppresses the recombination of the photogenerated carriers of g-C3N4, therefore, p-type g-C3N4 produces photogenerated carriers to a larger extent for subsequent redox reaction. PartⅡ Design of heterogeneous structure oriented on improvement of visible light photocatalystic oxidation：(1) CuO was coupled with TiO2 successfully via the microwave-assisted hydrothermal method form the p-n heterogeneous structure. SEM and HRTEM results showed the synthesized samples were consisting of small particles with average size of 40 nm. Moreover, the photocatalytic activities of different heterogeneous photocatalysts were investigated by degradation of methylene blue (MB) in the presence of a small amount of H2O2 under the visible light irradiation, results demonstrated that the composite added 0.25g TiO2 possesses the highest activity, degradation rate of MB is 93.2% after 70 min irradiation. TiO2 recombined with CuO broaden the optical absorption range and enhance the CuO photocatalytic activity. We proposed the photocatalytic mechanism of the CuO/TiO2 composite by the photoelectrochemical experiments and UV-vis DRS test.(2) p-n heterogeneous structure, organic-metal hybrid CuO/g-C3N4semiconductor was fabricated by facile impregnation-calcine method. CuO/g-C3N4 exhibit higher photocatalytic activity than the pure g-C3N4, the degradation rate of RhB up to 90.1% after 30 min visible light, the degradation rate constant is 4 times compared to the pure g-C3N4. The reason for the photocatalytic improvement is the recombination of CuO with g-C3N4 improves the visible light absorption, increase the separation of photogenerated electrons and holes, and enhance conductivity. A sample prepared by CuO mixed with g-C3N4 was also tested the photocatalytic property at the same condition as the heterogeneous CuO/g-C3N4, from the result we speculated the other reasonfor the improvement of CuO/g-C3N4 is that the p-n junctions were formed at the interfacial phases, resulting in the establishment of inner electrical fields in the direction from the n-type g-C3N4 to p-type CuO, switch the electron transfer mechanism from double-charge transfer mechanism to Z-scheme mechanism.
|罗建民. 可见光氧化性能增强为导向的p型半导体的设计及其异质结的构筑[D]. 北京. 中国科学院大学,2017.|
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