【作者】 李辉；

【导师】 王金淑；

【作者基本信息】 北京工业大学 ， 材料学， 2009， 博士

【摘要】 TiO2基半导体纳米材料在光催化,光电转换等领域具有广泛的应用前景。介孔结构TiO2具有诸多优点,如规整的孔道结构、高的比表面积、孔径和孔道结构可调、高的光电转换性能等。目前对于介孔TiO2材料的研究大多集中于孔道微观结构的控制,而对于介孔TiO2结构热稳定性的研究还有限。片层状H2Ti4O9是一种带结晶水的特殊的TiO2材料,与介孔TiO2相似,其也具有高的比表面积,可作为催化剂和载体使用,其通常采用软化学法进行层间剥离制备,此制备过程往往比较费时,而且需要添加大量的有机物辅助层间剥离。以三嵌段共聚物F127(EO106PO70EO106)为模板剂,以Ti(OBun)4为前驱体,采用蒸发致自组装法和溶胶-凝胶法首次制备了大孔径(7.4nm)、厚孔壁(10-13nm),热稳定性高达600℃的介孔TiO2薄膜。模板剂F127具有的长亲水PEO嵌段和长疏水PPO嵌段是形成厚孔壁和大孔径介孔TiO2结构的前提;采用无水乙醇强酸性介质并且加入乙酰丙酮来控制前驱体的水解/缩聚过程有利于其水解形成体积较小的Ti-O低聚物;而控制薄膜的陈化条件,如相对湿度(45%RH)和陈化时间(24h)使得前驱体的水解产物Ti-O低聚物与F127胶束进行充分的组装,几种因素共同作用形成了厚壁的介孔TiO2结构,从而使得其具有高的热稳定性。首次系统研究了焙烧温度对介孔TiO2薄膜的结构、光催化及亲水性能的影响。450~600℃温度范围内制备介孔TiO2的孔径可保持在7nm以上,显示了TiO2介孔结构好的热稳定性,700℃时介孔结构塌陷。研究发现,焙烧过程中介孔TiO2框架结构是在系统动力学和热力学共同作用下,引起结构中晶粒的长大、重排及介孔框架结构的收缩,最后导致了介孔TiO2结构的塌陷。亲水性能测试表明,制备的介孔TiO2薄膜即使在无光照时也显示了较好的亲水性能,其与水的接触角最低可达22.25°,这源于介孔TiO2薄膜表面的粗糙度和由多孔结构引起的渗透效应或灯芯效应。光致性能实验表明,500℃制备的样品显示了最优的光催化和光致亲水性能,其接触角最小为9.5°,其具有的10.2nm的平均晶粒尺寸更有利于光生电子-空穴对的分离和传输。通过改变模板剂的添加量,而前驱体的质量不变,经500℃焙烧制备了孔径从5.4nm到9.1nm的介孔TiO2薄膜材料。具有9.1nm孔径的介孔TiO2是目前报道的可以稳定到500℃的具有最大孔径的介孔TiO2材料。当溶液中模板剂含量较高时,相对较少的Ti-O低聚物与F127胶束组装从而形成薄孔壁的介孔结构,相应的可以得到较大孔径的介孔TiO2薄膜,反之,则形成厚孔壁、小孔径的介孔TiO2结构,因而可以通过改变模板剂与前驱体的比例对介孔结构进行调控。光催化实验表明具有7.4nm和9.1nm孔径的介孔TiO2材料显示了高于商业P25 TiO2的光催化活性。以硫脲为掺杂源,首次制备了N、S共掺杂的介孔TiO2薄膜材料,提高了介孔TiO2的可见光吸收性能。研究发现添加的硫脲具有双重作用,一方面,硫脲反应产生的NH4+起到扩孔的作用,孔径最大可达到12.4nm;另一方面,实现了对介孔TiO2结构的N、S共掺杂。紫外-可见反射光谱表明,样品的吸收边最大可扩展至550nm左右,其禁带宽度降低到2.25eV。光催化降解甲基橙实验表明,制备的样品在紫外光区和可见光区的光催化性能均有明显的提高,某些样品显示了比商业P25 TiO2更高的光催化活性。以层状K2Ti4O9粉体为原料,首次采用机械球磨结合离子交换法制备了纳米片层状H2Ti4O9,其径向尺寸低于50nm,比表面积超过240 m2·g-1。球磨过程中磨球与罐壁对K2Ti4O9的高速碰撞和挤压作用使得其层状结构发生断裂、破碎并且层间结合变得松散,从而导致其在离子交换过程中容易发生层间剥离。通过TiO2纳米粉体和H2Ti4O9纳米晶片之间的静电相互作用制备了TiO2/H2Ti4O9的纳米复合材料。光催化降解甲基橙的实验表明,TiO2/H2Ti4O9复合材料的光催化活性高于商业P25 TiO2,而H2Ti4O9的光催化活性较低,这是由于TiO2与H2Ti4O9之间的复合避免了TiO2及H2Ti4O9在溶液中的团聚,从而显示了较高的光催化活性。更多还原

【Abstract】 TiO2-based semiconductor nano-materials have a broad application prospects in such fields as photocatalysis, photoelectric conversion. The mesoporous TiO2 materials have many benefits, for instance, well-defined pore structure, high specific surface area, tunable pore size and mesostructure, high photo-electronic conversion ability. At present, much work is focused on the controlling of microstructure of the mesoporous TiO2 materials. However, few efforts are devoted in the thermal stability of the mesostructured TiO2 materials. The layered H2Ti4O9 is a special kind of TiO2 material with crystal waters between its layers. Like the mesoporous TiO2 materials, the layered H2Ti4O9 has high specific surface area, which permits it to apply in catalysts and carriers. Generally, the preparation of layered H2Ti4O9 is carried out through soft-chemical methods. These methods have some drawbacks, for instance, an excessively long reaction time is required, additionally, much organic materials are needed to help to exfoliate the layers.Using triblock copolymer Pluronic F127 (EO106PO70EO106) as the templating agent, Ti(OBun)4 as the inorganic precusror, mesostructured TiO2 thin films with large mesopores (7.4nm), thick pore walls (10-13nm), and high thermal stability (up to 600oC) were constructed through sol-gel combined with evaporation-induced self-assembly (EISA) methods for the first time. The long hydrophilic PEO block and hydrophobic PPO block of the F127 are the premises for the formation of thick pore wall and large pore size in the TiO2 network. In the synthesis process, anhydrous and strong acidic media, and acetylacetone were taken to controll the hydrolysis/condensation of the precursor, which favored the the formation of small Ti-O oligomers. The controlling of aging conditions, such as relative humidity (about 45%RH), aging time (about 24 h) can make the Ti-O oligomers and F127 micells assembly adequately. The cooperation of three factors above contributed to the formation of mesoporous TiO2 with thick walls, which deliveres the high thermal stability of the TiO2 mesostructure.We have investigated systematically the effect of different calcination temperatures on the mesostructure, photocatalytic activity, and photo-induced hydrophilicity of the mesoporous TiO2 thin films. The results showed that when the calcination temperatures increased from 450℃to 600℃, the synthesized samples have the pore diameters more than 7 nm, indicated that the excellent thermal stability of the obtained TiO2 mesostructure. At 700℃, the mesostructure was destructed. The analysis results indicated that the destruction of the mesoporous TiO2 thin films was induced by the cooperation of dynamics and thermodynamics effect, which resulted in the grain’s growth and rearrangement and the contraction of the mesostrcuture. Hydrophilic performance measurement indicated that the synthesized samples had good hydrophilicity in spite of the absence of light irradiation, the smallest contact angle with water was 22.5 o, which was induced by the roughness and the infiltration (nanowicking) of the mesoporous TiO2 thin film surface. Photo-induced experiment indicated that the sample calcined at 500℃had the best photocatalytic activity and photo-induced hydrophilicity, the contact angle was 9.5 o, due to the suitable grain size of 10.2 nm favored the separation and transmission of electron-hole pairs.The mesoporous TiO2 materials with different pore sizes range from 5.4 to 9.1nm (calcined at 500℃) were prepared through changing the mass of templating agent but the precursor was kept constant. The obtained mesoporous TiO2 materials has the pore size of 9.1 nm was the largest pore size for the mesostructured TiO2 materials calcined at 500℃reported up to now. When more templating agent F127 is used, relatively less Ti-O oligomers assembly with F127 micelles in the solution, so the mesoporous TiO2 with thin walls can be formed, accordingly a larger pore mesostructure will be fabricated. Contrarily, if fewer F127 is adopted, much Ti-O oligomers react with the micelles, so the mesoporous TiO2 with thick walls and small pores can be constructed. In such a way, the TiO2 mesostructure can be adjusted through changing the ratio of templating agent and inorganic precursor. Photocatalytic experiments indicated that the synthesized mesoporous TiO2 with pore size of 7.4 nm and 9.1nm exhibited the better photocatalytic activity than the commercial P25 TiO2.Taking the thiourea as the doping resource, the N/S codoped mesoporous TiO2 were prepared for the first time. It can be found that the addition of the thiourea had twofold roles. Alternatively, changing the mesostrcuture, the NH4+ ion yielded by the thiourea can swell the pore size of obtained mesoporous TiO2, the largest pore size of the obtained mesostructured TiO2 is 12.4 nm, on the other hand, doping N and S to the mesoporous TiO2 materials. The UV-vis reflection spectra showed that the absorption edges of the samples were expanded to about 550 nm at most and the band gap can be decreased to 2.25 eV. The photocatalytic degradation of methyl orange (MO) results indicated that the (N,S)-codoped samples had good photocatalytic performance both in visible light (λ>400nm) and UV light (λ<400nm). Some samples showed the better photocatalytic ability than commercial P25 TiO2. Taking layered K2Ti4O9 as the raw materials, the H2Ti4O9 crystallite nanosheets were prepared by using the cooperation of ball milling and ionic exchange strategies. The obtained sample is mainly composed of single layered nanosheete H2Ti4O9, which has the lateral size less than 50 nm, and specific surface area more than 240 m2·g-1. Additionally, the TiO2/H2Ti4O9 composite was prepared through electrostatic interplay between the TiO2 nanopowder and H2Ti4O9 nanosheets. The photocatalytic results indicated that the TiO2/H2Ti4O9 composition had a higher activity than the commercial P25 TiO2 in the photocatalysis degradating methyl orange (MO). But the H2Ti4O9 exhibited a lower photocatalytic activity. The compound effect of TiO2 and H2Ti4O9 avoids agglomerating of TiO2 and H2Ti4O9 in the solution, which benefited the good photocatalytic activity.更多还原