【作者】 亓丽芳；

【导师】 余家国；

【作者基本信息】 武汉理工大学 ， 材料物理与化学， 2012， 博士

【摘要】 环境污染与能源危机是当今世界困扰人类的两大难题。出于对化石燃料即将枯竭以及因化石燃料燃烧引发的一系列环境污染问题的担忧使得氢气作为-种新的可替代能源载体吸引了越来越多研究者们的注意。近年来,光催化分解水产氢凭借其清洁、低成本以及环境友好的特点,已经成为一种具有很大发展前景的能够利用太阳能来获取氢气的新途径。与其他半导体光催化材料相比较,Ti02具有很多优点,诸如生物及化学惰性,强氧化力,无毒及不易发生光与化学腐蚀等,因此得到了最为广泛的应用。然而,TiO2光催化分解水产氢的效率受到光生电子与空穴高复合率的限制。而且,锐钛矿相TiO2本身具有较大的带隙能(3.2eV),只能被紫外光(λ<380nm)激发。通常太阳光中紫外光的含量不足5%。因此开发新的光催化剂使其在紫外光和可见光照射下都具备良好的光催化分解水产氢性能具有重大意义。近来,暴露高反应活性(001)晶面的锐钛矿相TiO2单晶纳米片引起了大量研究者的注意。理论与实验研究皆表明表面能更高的(001)晶面比热力学更加稳定的(101)晶面在反应物分子的解离吸附方面有着更高的效率。本论文主要围绕暴露(001)晶面TiO2纳米片的合成、修饰(包括贵金属Pt与/或CdS纳米颗粒沉积),表征和光催化性能等方面开展了一系列工作。我们的主要想法是从暴露(001)晶面TiO2纳米片出发,通过对其进行修饰来提高Ti02在紫外光区与可见光区的光催化产氢活性。本论文的主要内容涉及以下几个方面：1)Pt负载暴露(001)晶面TiO2纳米片(Pt/TiO2纳米片)光催化产氢性能研究。催化剂的形貌结构与其光催化活性的高低密切相关。TiO2的(001)晶面比(101)晶面在反应物分子的解离吸附方面表现出更高的效率,水分子在(001)晶面上能够发生化学解离,而在(101)晶面上只能进行物理吸附,由此推断(001)晶面比(101)晶面更有利于产氢。而且,TiO2纳米片的缺陷密度低,使得光生电子与空穴的复合速率也较低,这也对光催化产氢有利。因此,我们希望利用暴露(001)晶面TiO2纳米片的这种特殊结构,来得到较高光催化产氢活性的光催化剂。在第二章中,我们首先通过水热处理钛酸丁酯(Ti(OC4H9)4)-氢氟酸(HF)-水(H2O)混合溶液得到TiO2纳米片,接着采用光化学还原法将Pt纳米颗粒沉积在纳米片表面。研究并讨论了Pt含量对样品在乙醇水溶液中的光催化产氢性能的影响。结果表明Pt的负载量对TiO2纳米片的光催化产氢活性有着显著影响,并且在Pt含量为2wt%活性达到最佳。当Pt含量相同时,在表面氟化与暴露在外的(001)晶面的共同作用下,所有氟化TiO2纳米片比Degussa P-25TiO2和在纯水溶液中制得的TiO2纳米颗粒有着更高的光催化产氢活性。表面氟化TiO2纳米片在太阳能电池、光电器件、传感器、催化、生物工程与纳米技术等领域都具有重大利用价值。2) CdS敏化Pt/TiO2纳米片的制备及其增强的可见光光催化产氢活性。TiO2的光催化产氢性能受到TiO2内部光生电子-空穴对快速复合的限制,而且因为带隙能较大使得TiO2只对紫外光有响应。半导体量子点敏化是解决光响应范围窄的方法之一。因此,我们希望结合CdS量子点敏化与暴露(001)晶面TiO2纳米片的这两个优势,制备出具有较高可见光光催化产氢活性的光催化材料。在第三章中,首先通过简单地水热处理Ti(OC4H9)4-HF-H2O的混合溶液制备出Ti02纳米片,接着利用光还原法将Pt纳米颗粒沉积在Ti02纳米片表面,最后利用化学浴沉积法将CdS纳米颗粒沉积在Pt/TiO2纳米片表面。新制样品的紫外光与可见光光催化活性通过在乳酸水溶液中在紫外光与可见光(λ≥420nm)照射下样品的光催化产氢速率来评价。结果表明纯的TiO2纳米片样品不论在紫外光还是可见光的照射下都观察不到光催化产氢活性。在经过CdS纳米颗粒的沉积处理后Pt/TiO2纳米片样品在紫外光与可见光照射下的光催化产氢速率都得到显著增强。TiO2样品的形貌对其可见光光催化产氢活性有着重大影响。在TiO2纳米片,商业P25与TiO2纳米颗粒三种形貌中,CdS敏化的Pt/TiO2纳米片样品展现出最高的可见光光催化产氢活性,这个样品在420nm处的单色光表观量子效率可达到13.9%,超出CdS敏化的Pt负载P25样品的10.3%与Pt负载TiO2纳米颗粒样品的1.21%表观量子效率,这是TiO2纳米片暴露高比例(001)晶面,表面氟化以及高比表面积三个因素共同作用的结果。在以乳酸作为牺牲剂,进行多次重复光催化产氢的循环实验后,CdS敏化的Pt/TiO2纳米片的光催化活性并未表现出任何损失,表明CdS敏化的Pt/TiO2纳米片体系比较稳定。CdS敏化Pt/TiO2纳米片在许多其他领域,比如太阳能电池、光电子与光电器件、传感器、催化、生物医学工程与技术领域等,也都有着重大意义。3)离子液体对TiO2光催化性能及机理的影响。为满足产业化要求,TiO2的光催化性能需要进一步提高。TiO2表面的基团种类及数量影响着TiO2的光催化性能。离子液体凭借其诸多优点不但可以用于形貌调控,而且也可以用于表面修饰以改善材料性能。我们预计在光催化反应体系中添加离子液体也会对TiO2的光催化性能造成影响。在第四章中,我们研究了离子液体对TiO2光催化降解活性的影响。具体实验是通过研究在TiO2光催化降解体系中添加室温离子液体1-丁基-3-甲基咪唑四氟硼酸盐前后TiO2光催化降解甲基橙与罗丹明B的速率变化情况来实现。通过设计不同的活性物种捕获实验,我们对TiO2的光催化机理进行了系统研究。结果表明,光生电子是光催化降解甲基橙反应中起主要作用的活性物种,而羟基自由基与光生空穴在罗丹明B的光催化降解过程中扮演着重要角色。1-丁基-3-甲基咪唑四氟硼酸盐的添加会促进TiO2光催化降解甲基橙,因为离子液体有利于光生电子的捕获与传输,抑制光生电子与空穴的复合,而添加离子液体后TiO2光催化降解罗丹明B会受到抑制,这是由于离子液体与罗丹明B在TiO2的表面会产生竞争吸附,从而阻碍罗丹明B与TiO2表面光生空穴、羟基自由基的接触,不利于罗丹明B的光催化降解。离子液体对TiO2的光催化降解活性所带来的双面影响取决于待降解污染物的种类以及特定的光催化反应类型。这个研究为我们理解光催化机理提出了一种新的见解,有助于我们开发设计高性能的光催化材料。更多还原

【Abstract】 Environmental pollutions and energy crisis are the two problems confronted with the world nowadays. Anxiety about depletion of fossil fuel reserves and pollution resulted from the combustion of fossil fuels make hydrogen an attractive alternative energy source. Recently, photocatalytic water-splitting has become a promising way for a clean, low-cost and environmentally friendly production of hydrogen by using solar energy. In comparison to other semiconductor photocatalysts, TiO2has been widely used because of its biological and chemical inertness, nontoxicity, low cost, availability and long-term stability against photo and chemical corrosion. However, the photocatalytic efficiency of TiO2for water-splitting is limited due to the high recombination rate of photogenerated electron-hole pairs. Furthermore, anatase TiO2is only effective under ultraviolet irradiation (λ<387nm) due to its large band gap (3.2eV). Usually, sunlight contains about4%ultraviolet light only. Therefore, it is highly desirable to develop a photocatalyst with enhanced photocatalytic activity for water splitting under both UV and visible-litht irradiation. Recently, anatase TiO2single-crystalline nanosheets with high percentage of reactive (001) facets have attracted lots of attention. Both theoretical and experimental studies indicate that higher surface energy of (001) facets are more effective for dissociative adsorption of reactant molecules compared with the thermodynamically more stable (101) facets. In this dissertation, valuable explorations have been carried out on the synthesis, modification (including noble metal Pt and/or CdS nanoparticles deposition), characterization and photocatalytic activity of the TiO2nanosheets with exposed (001) facets. Our main ideas are to improve the photocatalytic activity of TiO2for water-splitting under both UV and visible-litht irradiation by using anatase TiO2single-crystalline nanosheets with high percentage of reactive (001) facets as photocatalytst. The main points could be summarized as follows:1) Hydrogen production by photocatalytic water splitting over Pt/TiO2nanosheets with exposed (001) facets (Pt/TiO2NS). The photocatalytic activity of TiO2is strongly dependent on its morphology and structure. Compared with (101) facets,(001) facets are more effective for dissociative adsorption of reactant molemules. Furthermore, water molecules can chemically dissociate on the (001) surface but, contrarily, only physically adsorb on the (101) surface. Therefore, it is reasonable to infer that these (001) facets should be much more effective for water splitting than the (101) facets. Moreover, the single-crystalline structure of TiO2nanosheets with a low density of defects can reduce the recombination rate of photogenerated electron-hole pairs on grain boundaries and crystalline defects, thus improve the photocatalytic efficiency. So, we hope that TiO2nanosheets with exposed (001) facets will exhibit high photocatalytic activity for water splitting. In chapter2, Pt/TiO2NS were fabricated by a simple hydrothermal route in a Ti(OC4H9)4-HF-H2O mixed solution, followed by a photochemical reduction deposition of Pt nanoparticles on TiO2nanosheets under xenon lamp irradiation. The effects of Pt loading on the rates of photocatalytic hydrogen production of the as-prepared samples in ethanol aqueous solution were investigated and discussed. The results showed that the photocatalytic hydrogen production rates of TiO2nanosheets from the ethanol aqueous solutions were significantly enhanced by loaded Pt on the TiO2nanosheets, and the latter with a2wt%of deposited Pt exhibited the highest photocatalytic activity. All fluorinated TiO2nanosheets exhibited much higher photocatalytic activity than Degussa P25TiO2and pure TiO2nanoparticles prepared in pure water due to the synergistic effect of surface fluorination and exposed (001) facets. The surface-fluorinated Pt/TiO2NS are of great interest for solar cell, photonic and optoelectronic devices, sensors, catalysis, biomedical engineering and nanotechnology.2) Preparation and enhanced visible-light photocatalytic H2-production activity of CdS-sensibized Pt/TiO2NS. The photocatalytic activity of TiO2is limited by the high recombination rate of photogenerated electrons and holes. Moreover, TiO2is only effective under ultraviolet irradiation due to its large band gap. Semiconductor quantum dots sensitization is one of the methods to resolve this problem. So, we hope that the prepared CdS-sensitized Pt/TiO2NS can exhibit high photocatalytic activity for water splitting. In chapter3, CdS-sensitized Pt/TiO2NS were prepared by hydrothermal treatment of a Ti(OC4H9)4-HF-H2O mixed solution followed by photochemical reduction deposition of Pt nanoparticles on TiO2nanosheets and chemical bath deposition of CdS NPs on Pt/TiO2NS, successively. The UV and visible-light driven photocatalytic activity of the as-prepared samples was evaluated by photocatalytic H2production from lactic acid aqueous solution under UV and visible-light (λ>420nm) irradiation. It was shown that no photocatalytic H2-production activity was observed on the pure TiO2nanosheets under UV and/or visible-light irradiation. Deposition of CdS nanoparticles on Pt/TiO2NS caused a significant enhancement of the UV and visible-light photocatalytic H2-production rates. The morphology of TiO2particles had also a significant influence on the visible-light H2-production activity. Among TiO2nanosheets, P25and the nanoparticles studied, the CdS-sensitized Pt/TiO2NS show the highest photocatalytic activity (a13.9%apparent quantum efficiency obtained at420nm), exceeding that of CdS-sensitized Pt/P25by10.3%and that of Pt/NPs by1.21%, which can be attributed to the combined effect of several factors including the presence of exposed (001) facets, surface fluorination and high specific surface area. After many replication experiments of the photocatalytic hydrogen production in the presence of lactic acid, the CdS-sensitized Pt/TiO2NS did not show a great loss in the photocatalytic activity, confirming that the CdS/Pt/TiO2NS system is stable. The as-prepared CdS/Pt/TiO2NS are of great interest in solar cells, catalysis, photonic and optoelectronic devices, sensors, biomedical engineering and nanotechnology.3) From the aspect of practice application, the photocatalytic activity of TiO2needs to be further improved. The kinds and number of surface functional groups on the surface of TiO2have effects on the photocatalytic performance of TiO2. Ionic liquids have been widely used to control the morphology and modify the surface of inorganic materials. It is expected that the addition of ionic liquid affects the photocatalytic performance of TiO2. In chapter4, the effects of ionic liquid on the photocatalytic performance of TiO2were investigated by adding a water immiscible room temperature ionic liquid (1-butyl-3-methylimidazolium terafluoroborate) into the photocatalytic reaction of TiO2, and the photocatalysis mechanism of TiO2was also systematically investigated by designing different reactive radicals trapping experiments. The results show that photogenerated electrons are the main reactive species involved in the photocatalytic degradation of methyl organe, while·OH radicals and photogenerated holes play an important role in the photocatalytic degradation of rhodamine B. The addition of ionic liquid can enhance the photocatalytic degradation of methyl organe due to the enhanced separation of photogenerated electron-hole pairs through the trap and transfer of photogenerated electrons by ionic liquid, on the contrary, supress that of rhodamine B because of the competitive adsorption of ionic liquid and rhodamine B, which hinders the access of rhodamine B to the photogenerated holes and·OH radicals on the surface of TiO2. Ionic liquid exhibits ambilateral effects on the photocatalytic performance of TiO2and the effects are related to the kinds of contaminants and the different mechanisms in specific photocatalytic reaction. This report may provide new insight into the understanding of photocatalysis mechanism and the design of novel photocatalytic materials.更多还原