【作者】 刘杨；

【导师】 方鹏飞；

【作者基本信息】 武汉大学 ， 材料物理与化学， 2014， 博士

【摘要】 Ti02基光催化技术由于能利用太阳能将有机污染物氧化分解成无毒无害的有机小分子,以及水和二氧化碳,受到了人们广泛的关注。TiO2基光催化剂虽然有着光催化性能稳定、价格便宜、无毒安全等优势,但在实际应用中依然面临着两个主要的瓶颈。首先是低的光吸收能力,Ti02基纳米材料有着相对较大的能隙,导致了其只能吸收紫外光的辐射,无法利用可见光进行有效的光化学作用,而在实际应用中往往需要利用自然光(紫外仅仅约占其中的4%)来进行光催化分解污染物；另一个问题是其电子-空穴对的分离效率低,即光电转换利用效率有待进一步提高。为了解决上述问题,不同形貌Ti02基纳米光催化剂被制备出来。在Ti02基光催化材料的多种形貌中,纳米片层结构有着优秀的催化性能,得到了许多关注与研究。其中,Ti02颗粒与浓碱的水热反应因有着操作简便、产物光催化活性高等优势,被许多研究者用来制备Ti02基纳米片层(TNS)以及其它形貌的纳米材料。TNS优秀的催化性能主要归功于：1、仅为3-5m的厚度使得光生电子-空穴对能迅速迁移到催化剂表面参与化学反应；2、TNS具有高达300m2/g以上的比表面积,能够接触到和吸附到更多的污染物,同时大比表面积的片层结构暴露在光辐射下,提高了光利用率；3、钛酸片层是阳离子型层状化合物,层面具有负电性,所以该催化剂对许多有机物有着强的亲和力和吸附能力,比如工业中常用的各种阳离子染料。然而,对TNS来说,光响应能力窄与电子-空穴对分离效率低两方面的瓶颈依然存在并制约着其实际应用。如果能通过对片层进行设计与改性,突破以上两方面的制约,就将能够获得更优质、高效的纳米光催化剂。根据片层结构的特殊性,基于水热法制备纳米片层机理,本工作对Ti02基纳米片层作了如下的改性：1)改进水热法合成TNS及其结构与其光催化性能研究。通过在水热法制备纳米片层的过程中添加十六烷基三甲基溴化铵(CTAB)作为助剂,在不改变其它水热反应参数的情况下,制备出了比表面积为TNS1.23倍的Ti02基纳米片层(CT-TNS)。在CTAB的诱导下,水热反应中片层的生长过程得到了引导和促进。CT1-TNS有着比TNS更高的光电转换效率。对结晶紫(CV)的吸附研究表明：CT1-TNS的吸附能力相比TNS有明显的提高。依据Langmuir吸附计算出二者对CV的最大单层吸附量ηmax分别为60.9mg/g与49.1mg/g,说明CT1-TNS的单层吸附能力为TNS的1.24倍。催化效率最高的样品CT1-TNS,其催化降解CV和罗丹明B (RhB)的效率分别为TNS的1.61和1.62倍。2)碳沉积TNS的制备、结构及其可见光光催化性能研究。采用两步水热法成功制备了碳沉积的Ti02基纳米片层,得到了形貌均一的C-TNS,并提出了C-TNS的形成机理。沉积的碳主要以含碳物质的形式存在,包含有Cn, C-OH和C=O。碳沉积使TNS表面的Ti-O振动加强了,羟基氧的含量降低了。C-TNS对可见光的吸收能力也大大增强。C-TNS对RhB的吸附能力相比TNS有很大的提高。最高可见光光催化速率的C2-TNS,其光催化降解RhB的反应速率k值为TNS的3倍,为P25的12倍。结果表明,适量的碳沉积能够显著改善TNS的吸附能力与可见光光催化效率。3)Ce02复合TNS的制备、结构及其光催化性能研究。铈元素是变价稀土元素的代表,本文以硝酸铈为铈源,采用一锅水热法制备了氧化铈复合的Ti02基纳米片层光催化剂(CeO2@TNS). CeO2@TNS具有良好的片层结构,结晶度较低,Ce离子以三价和四价的形式共存。氧化铈复合在Ti02表面形成异质结构,引入了少量的Ti-O-Ce键,同时导致了Ti3+的增加。适量的复合(0.5%摩尔比)能显著降低荧光光谱的强度,提高光生电子-空穴对的分离效率。氧化铈复合能显著改善纳米片层的紫外-可见光催化活性,但对可见光光催化活性有抑制现象。0.5%CeO2@TNS有着最高的紫外-可见光催化效率,其降解液相RhB的效率是P25的5倍、TNS的1.56倍；降解气相苯的光催化效率为TNS的1.36倍。4)Ho203复合TNS与Gd203复合TNS的制备、结构及其光催化性能研究。稀土元素是一组既相似又有不同之处的元素,不同稀土元素电子结构、价态、原子半径等因素对复合光催化剂影响不同。选择了不可变价的稀土金属氧化物Ho203与Gd203,来制备复合型的TNS光催化剂(Ho2O3@TNS、Gd2O3@TNS)。 Ho2O3@TNS与Gd2O3@TNS具有良好的片层结构,复合在TNS表面形成异质结构,引入了少量的Ti-O-Ho/Gd键。与Ce02复合不同,Ho203与Gd203的复合一定程度阻碍了水热过程中P25颗粒向片层的转变,导致了催化剂样品的比表面积降低与吸附能力下降。虽然Ho203与Gd203影响电子-空穴对分离效率的规律不尽相同,但都能显著改善纳米片层的光催化活性。1.0%Ho2O3@TNS有着最高的光催化效率,其紫外-可见光光催化效率是TNS的1.67倍,是P25的5.35倍；可见光光催化效率是TNS的1.90倍,是P25的12.38倍。0.5%Gd2O3@TNS的紫外-可见光光催化效率是TNS的1.60倍,是P25的5.13倍；可见光光催化效率是TNS的1.98倍,是P25的12.90倍。更多还原

【Abstract】 Photocatalysis is one of the most promising processes in controlling of organic pollutants in recent years, it is a clean technology in water system treatment and the final decomposition products of organic compounds can be only CO2and water. TiO2-based photocatalysts have many advantages such as stable photocatalytic activity, high physical and chemical stability, and non-toxicity, so they are widely used for pollution control both in water and air system. However, the traditional TiO2photocatalysts have some shortcomings, i.e., the narrow range of light absorbing, and the low efficient use of photogenerated electron-hole pairs. The further application of TiO2-based photocatalysts must face and solve these problems.Nanostructures of TiO2have been actively exploited to overcome these drawbacks. Among many kinds of TiO2-based nanomaterials, TiO2-based nanosheets (TNS) have attracted many attentions because of high photoactivities. Hydrothermal preparation of TiO2-based nanosheets under strongly alkaline condition is widely employed by the researchers for its high productive rate and convenience. The high photocatalytic performance is mainly due to three superiorities. The first, because of the sheet-like structure, the photoexcited electrons and holes can easily transport to the surface of TNS to participate in the photoreactions; The second, the high surface area (8times of that of P25), so that TNS have more opportunities to contact with the pollution molecules in water system; The third, TNS have a negative charged surface, and strong adsorption capacity for many organic, i.e., cationic dyes used in industries. However, TNS still have the similar shortcomings with regular TiO2-based materials. The narrow range of light absorbing and the low efficient use of photogenerated electron-hole pairs are limiting the application of TNS. The modification of TNS may provide breakthroughs to solve these problems and obtain photocatalysts with higher photocatavities. In this research, we prepared different kinds of modified TNS:1) TNS with higher BET surface area (1.23times of that of P25) was abtained by adding CTAB in the process of hydrothermal reaction, providing nanosheets with larger size, higher photocurrent, and lower FL intensity. The adsorption test suggests that the CT1-TNS have higher adsorption of crystal violet (CV) than that of TNS. According to Langmuir model, the highest adsorption amount by monolayer (ηmax) of CT1-TNS (60.9mg/g) is1.24times of that of TNS (49.1mg/g). The photoactivities of CT1-TNS in degrading CV and Rhodamine B (RhB) are1.61and1.62times of that of TNS, respectively.2) Carbon deposited TiO2-based nanosheets (C-TNS) photocatalysts were prepared by a two-step hydrothermal treatment of Degussa P25using glucose as the carbon precursor. The carbon deposition has significantly enhanced the visible light absorbing ability and dye molecules adsorption ability of TNS. The XPS result suggests that, the carbon deposition results in the decrease of hydroxyl oxygen, and the deposited carbon mainly exists in the form of carbonaceous species. When the starting glucose solution concentration is2g/L, the obtained C-TNS possesses the highest visible light photoactivity in degrading RhB, which is12times and3times of that of P25and the pure TNS, respectively. The enhancement of visible light photoactivity is due to the strong visible light absorbing ability of carbon species and the dye absorption enhancement by carbon deposition. It is suggested that carbon deposition is a promising way to enhance the visible light photoactivity of TiO2-based nanosheets.3) CeO2@TNS photocatalysts were prepared by a one-pot hydrothermal method using cerium nitrate as the cerium precursor. Cerium ions co-exist in two forms of Ce3+and Ce4+. The result shows that the binding energy of Ti and O elements changed, and the content of Ti3+increased obviously. Appropriate amount of CeO2can significantly inhibit the recombination of electron-hole pairs, which is proved by the decline in the intensity of the fluorescence spectra. CeO2@TNS with cerium ratio of0.5%(molar ratio) possesses the highest photocataytic activity when degrading Rhodamine B (RhB), which is5times and1.6times of P25and TNS, respectively. It is suggested that cerium ions are efficient electron trappers to improve the separation of electrons and holes.4) Ho2O3@TNS and Gd2O3@TNS were prepared by hydrothermal method. Ho2O3/Gd2O3@TNS photocatalysts have well developed nanosheet structure and relativiely low degree of crystallinity. The deposited HO2O3and Gd2O3have changed the surface structure of TNS, and Ti-O-Ho/Gd bonds were formed. The compositions of HO2O3and Gd2O3have inhibited transformation of P25to nanosheets during the hydrothermal reaction, and cause the decrease in the BET surface and the adsorption ability. Ho2O3@TNS with cerium ratio of1.0%(molar ratio) possesses the highest UV-Vis photocataytic activity when degrading Rhodamine B (RhB), which is3.94 times and1.67times of P25and TNS, and its visible photoactiviy is1.90times and12.38times of P25and TNS. Gd2O3@TNS with cerium ratio of0.5%(molar ratio) possesses the highest UV-Vis photocataytic activity when degrading RhB, which is1.60times and5.13times of P25and TNS, and its visible photoactiviy is1.98times and12.90times of P25and TNS.更多还原