【作者】 张世英；

【导师】 陈振华；

【作者基本信息】 湖南大学 ， 材料学， 2008， 博士

【摘要】 纳米TiO₂作为光催化剂在水处理和气体净化等环保及光解制氢新材料领域具有广阔的应用前景,受到人们的广泛关注。但目前广泛研究的纳米TiO₂悬浮相体系存在着催化剂易凝聚、易失活、难回收及光能利用率低等弊端,从而限制了该项技术的实际应用。本文在全面综述TiO₂光催化技术、改性途径、TiO₂纤维合成方法及应用现状的基础上,制备并系统研究了纳米TiO₂纤维（TiO₂ nanofibers, TF）,以适应TiO₂光催化技术在水处理和环境治理中实用化的迫切要求。首先采用溶胶-乳化-凝胶技术合成了TiO₂纳米粉体,通过XRD、TEM、FT-IR、Raman、XPS等手段对其进行表征,相比溶胶-凝胶法制备的TiO₂纳米粉体颗粒尺寸减小、比表面积增大、团聚程度下降,带隙能增大、紫外吸收带边发生蓝移。当溶胶pH达到8.5,表面活性剂与蒸馏水摩尔比W=1.08时,TiO₂颗粒尺寸达到最小为7nm,比表面积达到最大为273m2/g,且粒度分布较均匀。随着热处理温度的升高和热处理时间的延长,晶粒尺寸增大,TiO₂粉体晶型从无定型,经过锐钛矿最终转变成金红石。TiO₂晶粒生长服从Eastman晶粒生长动力学模型。以上述纳米TiO₂粉体为前驱体,采用简单水热法和热处理成功地制备了TF。探讨了反应温度、时间、碱液浓度、洗涤条件、热处理温度等对产物结构和形貌的影响。研究发现,随着反应时间的延长,TF的长度和产率逐渐增加,72h以后,前驱体完全转变为纤维,长度达到微米级;反应温度低于130℃时,TiO₂粉体转化不完全,150℃是较合适的反应温度,高于170℃时,产物的形貌发生变化,成为带状;KOH溶液浓度低于8mol/L时,只有少量纤维生成,超过12mol/L后,产物为片状或短杆状,不能成长为纤维;洗涤条件基本不影响TF的形貌,但对TF的结构和化学成分有较大影响,当酸洗不够,离子交换不完全时,TF中有较多的K2Ti6O13成分;煅烧最佳温度为400℃,产物形貌均匀,结晶完善,随着煅烧温度的升高,TF的烧结现象逐渐加重,达到550℃时,TF明显被烧塌。样品在热处理温度低于550℃时为锐钛矿相,当热处理温度超过650℃时,为锐钛矿和金红石的混合晶型,当热处理温度达到850℃时,样品中锐钛矿完全转变成金红石相。HRTEM、XPS、DRS等分析表明:TF为实心层状结构,由Ti、O、C元素组成,相对于TiO₂原料粉体,紫外吸收带出现红移现象。其形成机理是锐钛矿型TiO₂纳米颗粒在强碱作用下生成K2Ti6O13晶核,这些晶核遵循溶解-结晶-生长机理,沿（010）晶面方向生长,逐渐长成为纳米K2Ti6O13纤维。通过离子交换和热处理,转化成锐钛矿TiO₂纳米纤维。从光催化反应的基本特点出发,研究了光催化反应条件对亚甲基蓝（MB）光催化降解的影响。TF对MB（3mmol/L）光催化降解的最佳反应条件是:通气速率为56mL·s-1,催化剂浓度为2g/L,紫外灯功率为40W,pH值为6.0。建立了包含有机物初始浓度、紫外灯功率、催化剂浓度等在内的动力学模型,即在相同的实验条件下,TF对大肠杆菌（E.Coli）的灭活性能高于P25。TF光催化灭菌机理是:由于TiO₂在光照射下生成的活性组分（·O2-,·OH,h+）对细胞外膜的破坏,使其产生孔洞、从而导致细胞外膜对活性组分渗透性改变,使活性组分通过孔洞达到细胞质膜发生过氧化反应,以致细胞死亡。分别采用原位法和浸渍法制备了Ag-TF。以TEM、XRD、DRS及XPS手段对不同工艺条件下获得的产物晶型结构、微观形貌以及化学组成进行了表征,探讨了掺杂方式对TF结构和性质的影响。结果表明:两种方法制备的纤维表面形貌有明显差别,采用原位法制备的Ag-TF表面平滑,采用浸渍法制备的纤维表面有粒径2～5nm的球状颗粒存在,且浸渍法制备纤维的XRD谱出现Ag2O的特征衍射峰,而原位法制备的纤维谱线没有出现Ag2O衍射峰,说明Ag+嵌入了晶格,而非沉积在纤维表面形成颗粒。与原位法制备的Ag-TF谱线相比,浸渍法制备的纤维DRS谱线发生红移。当TF中掺杂适量Ce4+时,TF的光催化活性增大,最佳掺杂率为0.5wt%。在适当的热处理温度和掺杂浓度下,制备的Ce-TF对亚甲基蓝的光催化活性比P25好。利用相关分析方法,对掺杂0.5wt%Ce4+的TF样品[Ce-TF（0.5）]进行了表征,结果表明:相对于未掺杂TF,Ce-TF（0.5）中TiO₂纳米颗粒粒径减小,同时Ce4+掺杂对锐钛矿向金红石的相转变具有阻碍作用,Ce-TF（0.5）的荧光强度减弱,光量子产率增大。原因是掺杂Ce4+可作为电子—空穴对的捕获位,降低了电子和空穴的复合率。通过深入研究TF合成的影响因素和生长机理,为规模制备形态和尺寸可控的一维钛基纳米结构材料奠定了基础,研究结构表明:TF对微生物和有机物具有很高的光催化降解活性和很长的连续使用寿命,可在污水处理中发挥重要作用,具有非常广阔的应用前景。更多还原

【Abstract】 As one kind of photocatalyst, TiO₂ （nanomaterials） has a vast range of prospects for waste water treatment, gas purification and hydrogen preparation, thus a great deal of attention has aroused. But the TiO₂ nanoparticles suspension, which is widely studied now, has many disadvantages: it is easily to flocculate and deactivate and it is difficult to be recovered, the efficiency of using photoenergy is low, therefore the practical application of this photocatalytic technique is restricted. On the bases of comprehensive review of the photocatalysis technology and the approaches for improving photoactivity, TiO₂ nanofibers （TF） were prepared and investigated systematically to meet obliged requirements for practical applications of TiO₂ in waste water treatments and environmental management.At first, TiO₂ nanopowder was prepared by sol-microemulsion-gel technique and was characterized by X-ray diffraction spectra （XRD）, transmission electron microscope images （TEM）, fourier transform infrared spectra （FT-IR）, Raman spectra and X-ray photoelectron spectroscopy spectra （XPS）. Comparing with TiO₂ nanopowder prepared by sol-gel method, TiO₂ nanopowder synthesized by sol-microemulsion-gel method shows smaller particle size, larger surface area, lower aggregation degree, higher binding energy and blue shift of ultraviolet absorption. When pH value is 8.5 and the mole ratio of surfactant to distilled water （W） is 1.08, the TiO₂ particle size reaches the least （7nm）, the granularity distribution is more uniform and the surface area is the largest （273m2/g）. Increasing heat-treatment temperature and prolonging the heat-treatment time, TiO₂ powders transform from amorphous, anatase and finally transform to rutile, the crystallite size increases. The process of TiO₂ crystalline growth accords with the model of crystalline growth kinetics which was suggested by Eastman.Taking the above TiO₂ powder as precursor, TF was prepared successfully with the simple hydrothermal method and heat treatment. The influences of reaction temperature, reaction time, alkali content, washing conditions and heat treatment temperature on the structure and morphology of the product were investigated. Prolonging the reaction time, the length of nanofibers and its yield increase gradually, after 72h all the precursors transform into fibers, the length of them reaches micron degree, when the reaction temperature is lower then 130℃, the transformation of TiO₂ powder is incomplete, the appropriate temperature is 150℃. If the temperature is higher than 170℃, the morphology of the product changes into band shapes; when the KOH concentration is lower than 8mol/L, the yield of fibers is little. When the concentration is over 12mol/L, the shape of the products becomes flakes or short poles and cannot turns to be fibers. Washing conditions do not affect the morphology of the fibers, but make great influences on the structure and chemical component of TF.When acid washing is not enough and the ion exchange is incomplete, there is more K2Ti6O13 component in TF. The optimum calcination temperature is 400℃, at this temperature, the morphology of the product is uniform, the crystallite is complete. With increasing the calcination temperature, the agglomeration degree of fibers increases gradually. When the temperature reaches 550℃, fibers collapse evidently. When the heat treatment temperature is less than 550℃, the sample phase is anatase. When the heat treatment temperature is over 650℃, the sample phase is the mixture of anatase and rutile. When the heat treatment temperature reaches 850℃, the anatase in the sample turns to be rutile completely.High-resolution transmission electron microscope images （HRTEM）, X-ray photoelectron spectra （XPS） and diffuse reflection spectra （DRS） show that TiO₂ fibers have solid-layer structure and are consisted of Ti, O and C elements comparing with TiO₂ powder. Their crystalline sizes are small and the red shift appears in the ultraviolet absorption band. The synthesis mechanism of TF can be described as follows: under the effect of strong alkali, TiO₂ nanoparticles of anatase turn to be the nuclei of K2Ti6O13, which may grow along the crystalline faces to be K2Ti6O13 nanofibers gradually according the mechanism of dissolution-crystallization -growth. By ionic exchange and heat treatment, K2Ti6O13 nanofibers are changed into TiO₂ nanofibers with anatase.From the fundamental characteristics of photocatalytic reaction, the influences of photocatalytic reaction conditions on photocatalytic degradation of MB were investigated. The optimum conditions are 56mL/s of aerating rate, 2.0g/L of photocatalyst concentration, 40W of UV-lamp power and 6.0 pH value. The kinetics model is established which contains initial concentration of organic compound, UV-lamp power, photocatalyst concentration etc, i.e.With the same experimental condition, TF show better disinfection performance than P25. The disinfection mechanism can be described as follows: in the course of TF disinfection, the outer membrane of cells are destroyed and holes are made by the reactive species （·O2-、·OH and h+） produced by TiO₂ photocatalysis, which results in the change of the permeability membrane and enables the reactive species to reach the cytoplasmic membrane easily. The cytoplasmic membrane is attacked by reactive species, which leads the peroxidation of membrane lipid and the death of cells.Ag-TF fibers were prepared by in-situ method and immersion method. Their phase structure, microscopic morphology and chemical component were characterized by TEM, XRD, DRS, and XPS in order to analyze the effects of different-doping methods on the structures and performances of the products, The results show that the morphology of the fibers prepared by two methods is evidently different. The surface of Ag-TF fibers prepared by in-situ method is smooth, but on the surface of Ag-TF fibers prepared by immersion method, there are spheric grains with size range of 2 to 5nm which have Ag2O characteristic X-radiation peak. However, fibers prepared by an in-situ method do not show diffraction peak Ag2O, which suggest Ag+ implant into the crystal lattice of TiO₂ and do not exist on the fibers surface to form grains. Comparing with the Ag-TF fibers prepared by in-situ method, the fibers prepared by immersion method show red shift of DRS.Doping with certain amount of Ce4+, the photoactivity of TF increases. The optimum content is 0.5wt% Ce4+. With the adequate heat-treatment temperature and doping concentration, the photocatalytic rate of prepared Ce-TF to MB degradation is higher than that of P25. Ce-TF（0.5） was characterized by correlation analysis methods. The results show that TiO₂ nanoparticle size of Ce-TF（0.5） fibers is less than that of TF, and Ce4+ has barrier effect on the phase transition from anatase to rutile in fibers. Ce-TF（0.5） fibers show low fluorescence intensity and high photoquantum yield. It is attributed to the fact that the doping Ce4+ is the capturing places for electron/hole pairs, which decreases the recombination rate of electron/hole pairs.Through the research of TF, it is showed that TF has very high photocatalytic degradation activity and a very long continuously-utilizing life. It has a very important role in the treatment of waste water and a vast range of prospects.更多还原