【作者】 张向超；

【导师】 杨华明；

【作者基本信息】 中南大学 ， 材料学， 2010， 博士

【摘要】 二氧化钛(TiO2)是一种重要的无机功能材料,具有优异的光催化活性和光电特性。自1972年Fujishima发现了在TiO2电极上光分解水制氢的作用以来,人们对TiO2的光催化特性进行了大量的研究,其在光电转换、污染物降解、自洁净、传感器以及潜在的癌症治疗等高新技术领域显示出广阔的应用前景。这些应用领域都可以归结为“环境”和“能源”两个方面,即围绕着光催化环境污染物的消除和控制以及洁净新能源材料的研制和开发等展开广泛研究。TiO2的应用不仅受其物理化学特性和改性客体的影响,还依赖于TiO2与环境的相互作用。研制具有可见光活性的TiO2薄膜,是提高太阳光利用率、推进光催化技术实用化进程的关键,也成为光催化领域具有挑战性的重要课题之一。本论文采用溶胶-凝胶法,结合浸渍-提拉技术,在载玻片或铟锡氧化物(ITO)导电玻璃基片上,合成了单一锐钛矿型TiO2薄膜和(101)晶面择优取向TiO2薄膜；不同金属离子和非金属(共)掺杂TiO2薄膜；金属氧化物／Ti02薄膜。利用热分析(TG/DSC)、X-射线衍射(XRD)、傅立叶红外光谱(FTIR)、原子力显微镜(AFM)、扫描电镜(SEM)、X-射线光电子能谱(XPS)、紫外-可见吸收光谱(UV-vis)、交流阻抗(EIS)、线性扫描伏安(LSV)、循环伏安(CV)及润湿角等多种现代测试手段对制备的系列产物进行了系统的表征,并讨论了制备TiO2薄膜过程中溶胶的物理化学变化及镀膜工艺,着重研究了优化制备工艺、(共)掺杂不同离子和复合半导体氧化物等不同改性手段对合成系列TiO2薄膜的结构和可见光条件下电化学、光诱导亲水性能的影响,并结合基于密度泛函理论的第一性原理对TiO2纳米材料进行了模拟计算,旨在分析不同改性手段提高TiO2可见光活性的内在机制。(1)优化制备工艺对TiO2薄膜结构和可见光活性的影响热处理对TiO2薄膜结构和性能的影响已有报道,本论文在分析TiO2薄膜溶胶-凝胶制备过程中物理化学性能变化的基础上,着重研究了100℃、500℃、冷冻干燥和微波加热不同预处理方式对TiO2薄膜结构和光学性能的影响。由于4种预处理方式在干燥过程中加热机理不同,TiO2薄膜表面的颗粒形貌、晶粒尺寸和表面粗糙度产生了明显变化,其中100℃预处理制备的薄膜表面由大小均匀(15.5 nm)的球形颗粒紧密排列组成,粗糙度仅为4.1 nm,由于“量子尺寸效应”,微波加热和冷冻干燥相对100℃预处理制备的薄膜产生了“红移”。通过优化工艺制备了具有(101)晶面择优取向TiO2薄膜,其UV-vis吸收光谱的吸收边向可见光范围明显偏移,禁带宽度为2.9 eV,表现出良好的可见光活性,电化学测试结果表明,其电极对应的电容值随着外加电压的增大而减小,平带电位Vfb为-0.493V,空间电荷浓度为6.28×1021cm-3。这些研究结果为制备可见光活性TiO2薄膜提供了有效途径,为实现TiO2薄膜的调控合成提供新思路。(2)离子(共)掺杂改性对TiO2薄膜结构和可见光活性的影响系统研究了不同金属离子Li+、Ni2+、Bi3+、Ce4+、V5+和W6+和非金属F、S和N掺杂以及N-Bi、Ni-Ce和S-Ce共掺杂对TiO2薄膜结构和光学性能的影响。研究结果发现,由于掺杂离子的半径、价态及电子轨道构型等改性客体的差异,使得TiO2薄膜表面化学组成、氧化态、颗粒形貌和晶粒尺寸产生了明显变化,从而对TiO2薄膜的光学性能产生了不同的影响,不同价态金属离子掺杂后(Li+除外),吸收边产生了不同程度的“红移”,其中Ni2+和Ce4+的效果更明显,禁带宽度分别为2.42 eV和2.51 eV,高价态的V5+和W6+并不明显,而掺杂Li+反而增加了TiO2的禁带宽度。非金属F掺杂后,吸收边产生了“蓝移”,S和N掺杂后,TiO2薄膜的禁带宽度明显减小,可见光活性明显增强。通过研究两种离子共掺杂对TiO2薄膜结构和光学、电化学、亲水性能的影响,分析了N-Bi、Ni-Ce和S-Ce共掺杂产生的有效协同效应。这些研究查明了不同掺杂离子的存在状态,发现了N-Bi等共掺杂对提高TiO2薄膜的可见光活性具有协同效应,为探索掺杂不同离子改善TiO2薄膜可见光活性的影响规律提供了新思路。(3)金属氧化物对TiO2薄膜结构和可见光活性的影响金属氧化物复合是制备具有可见光响应TiO2的重要技术途径。硫化物复合TiO2已有大量研究,论文着重研究了Y2O3/Ti02和CuO／TiO2薄膜结构、光学、电化学和光诱导亲水性能,并与单一TiO2薄膜进行比较。研究发现Y2O3／TiO2薄膜产生了额外的氧空位Vo¨,同时O 1s和Ti 2p的原子轨道结合能产生偏移,相对较多的Y和Ti元素富集表面,使得Y2O3／TiO2薄膜的表面活性明显提高,吸收边向长波方向“红移”,禁带宽度为2.68 eV,可见光照射一定时间后,与水的接触角仅为8°,表现良好的亲水性能,Y2O3／TiO2薄膜电极的空间电荷浓度为1.05×1020cm-3,比TiO2薄膜电极的空间电荷浓度(1.57×1019cm-3)提高了1个数量级,平带电位和空间电荷层厚度也明显降低。利用混合半导体系统的禁带宽度是其组成函数的原理,结合溶胶-凝胶制备薄膜的技术特点,成功制备了的CuO／TiO2梯度化薄膜,从薄膜表面至基片的深度方向,Cu的含量逐渐降低,O的含量逐渐增多,Ti的含量先缓慢增加再急剧下降,在沿薄膜厚度方向化学组成梯度化的基础上,实现了禁带宽度的梯度化,其薄膜电极产生的阳极光电流明显增多,光电响应范围相对TiO2薄膜电极明显扩大,CuO／TiO2梯度化薄膜不仅拓宽了光电响应范围和提高了可见光活性,还实现了TiO2薄膜光电特性稳定化和无突变势垒,有利于光生载流子有效分离,提高了光电化学稳定性能。这些研究结果为TiO2薄膜在自洁净建筑材料和薄膜太阳能电池的应用提供了基础理论支持,对开发新颖结构的TiO2薄膜及拓宽其应用领域具有重要的指导意义。(4)第一性原理计算模拟TiO2可见光活性机理结合密度泛函理论第一性原理,采用赝势平面波方法,通过对掺杂TiO2建立超晶胞模型,对三种晶型TiO2和F、S、N、Li+、Ni2+、Bi3+、Ce4+、V5+、W6+掺杂以及N-Bi共掺杂TiO2的能带结构、态密度和光学性能进行了系统的计算模拟。研究发现金红石、锐钛矿和板钛矿三种晶型TiO2的导带和价带结构的差异,是其物理化学性能不同的主要原因；掺杂离子的F 2p、N 2P、Ni 3d、Bi 6p、Ce 5d和Ce 5p、V 3d和V 3p以及W 5d轨道分别混合了O 2p和Ti 3d轨道,引起了TiO2的导带和价带的构成的变化,从而使TiO2的禁带宽度产生了变化；计算得到掺杂TiO2光学性质的变化趋势与实验测试的结果一致。研究结果查明了不同掺杂离子对TiO2导带和价带的影响,揭示了不同离子掺杂TiO2可见光活性提高的内在机制,为归纳不同离子掺杂TiO2改善可见光活性的影响规律和掺杂效应提供了理论依据,同时也为建立材料组成—结构—性能之间的关系提供了崭新的思路。更多还原

【Abstract】 Titanium dioxide (TiO2) is one of important functional inorganic materials. Since Fujishima discovered the photocatalytic splitting of water on a TiO2 electrode under ultraviolet light in 1972, enormous efforts have been devoted to the research of TiO2 materials due to its excellent optical, electrical, photocatalytic and thermal properties, which led to promising applications in the fields of photovoltaics, photocatalysis, sensors, medicine et al. These applications can be roughly divided into "energy" and "environmental" categories. For example, the control of environmental contamination and the investigation of new energy sources depend not only on the properties of the TiO2 material itself but also on the modifications of the TiO2 and on the interactions of TiO2 materials with the environment. The development of photocatalysts under visible light irradiation is one of the major goals for enhancing the efficient utilization of solar energy and realizing the practical industrialization, which is also one of the challenging tasks in the field of photocatalysis.In this thesis, series of TiO2 thin films have been synthesized deposited on microscope slides or indium tin oxides (ITO) glass substrate using sol-gel dip-coating method, including pure anatase TiO2 thin films and (101) crystalline plane oriented TiO2 thin films; nonmetallic ions (N, S and F) doped TiO2 thin films; metallic ions (Li+, Ni2+, Bi3+, Ce4+, V5+ and W6+) doped TiO2 thin films; Ce-Ni, Bi-N and Ce-S co-doped TiO2 thin films; Y2O3/TiO2 nanocomposite thin films and CuO/TiO2 up-graded nanocomposite thin films with gradient bandgap. The as-synthesized samples are characterized using thermo-gravimetric /differential scanning calorimetric (TG/DSC), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-vis), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), cyclic voltametry (CV) and contact angle analysis technologies. The synthesis process of TiO2 thin films and the physicochemical changes of sol have been briefly investigated. The effects of different treatments on structure, optical, electrochemical and photo-induced hydrophilic properties of TiO2 based thin films have been systematically emphasized. The as-synthesized TiO2 based materials have been simulated using the first-principle calculation based on the Density Functional Theory (DFT). The aim is to investigate the intrinsic factor of enhancing visible light activation of TiO2 based thin films from different modified methods. The main works and results are as following.(1) Effects of processes optimization on the structure and properties of TiO2 thin filmsBased on the analysis on the physicochemical changes of sol-gel preparation processes, the effects of 100℃,500℃, freeze drying and microwave heating pretreatment on crystalline structure, surface morphology and optical properties of TiO2 thin films have been primarily investigated. The pretreatment process is important during preparation of thin films and shows obvious effect on the morphology, crystalline size and roughness of the surface of TiO2 thin films due to the different heating mechanisms. The absorption edge of TiO2 thin films after freeze drying and microwave heating pretreatment, whose crystalline size is 30.1 nm and 61.45 nm respectively, has a red shift compared with that of 100℃pretreated sample (15.45 nm) due to quantum size effect. On the other hand, the researches on the different crystalline plane orientation of TiO2 thin films have drawn greater attention gradually. The oriented TiO2 thin films synthesized by modified preparation process have obviously absorbed property in range of visible spectra and the band gap is 2.9 eV for indirect transmission. The value of capacitance for (101) oriented TiO2 thin film electrode decreases with increasing the potential, the flat-band potential is-0.493 V and the donor concentration (ND) is 6.28×1021 cm-3. These results can provide technical support for the development of visible light activity of TiO2 thin films and novel ideas for the realization of the controlled synthesis of TiO2 thin films.(2) Effects of different ions doping on the structure and properties of TiO2 thin filmsThe effects of doping of different ions, including Li+, Ni2+, Bi3+, Ce4+, V5+, W6+ metallic ions and N, S, F non-metallic ions, on the structure, optical and hydrophilic properties of TiO2 thin films have been primarily investigated. The results indicate that the morphology and crystalline size is different on the surface of thin films; the absorption edge produces a red shift to some degree, except for Li+. The band gap of Ni2+ and Ce4+ doped TiO2 thin films are 2.42 eV and 2.51 eV, respectively, and that of doped with V5+ and W6+ are not obvious, while the band gap increases by doping Li+. The absorption edge of S and N doped TiO2 also has red shifts because the band gap of TiO2 thin film decreased obviously and the visible light activity is remarkably improved, while that of F doped sample produces blue shift. The N-Bi、Ni-Ce and S-Ce co-doped TiO2 thin films have been successfully synthesized for the first time. The effective synergetic effects resulted from co-doping have been discussed through analyzing the effects on the structure, optical electrochemical and photo-induced hydrophilic properties of TiO2 thin films doped with the two different dopants. The results shows that the properties of doped TiO2 thin films with different ions have close relations with the intrinsical properties of the doped ions, the effective methods of modification by doping are the conduction band (CB) and valence band (VB) of TiO2 influenced by doping ions and the band gap obviously narrows to enhance the response in visible light region.(3) Effect of composite metal oxides on the structure and properties of TiO2 thin filmsIt is an important technological method to prepare visible-light irradiated TiO2 thin films through compositing with semiconductors. Many researches have focused on sulfide compositing with TiO2. In this thesis, the optical, electrochemical and photo-induced hydrophilic properties of Y2O3/TiO2 (YTF) and CuO/TiO2 nanocomposite thin films in visible light region have been obviously improved compared with that of the pure TiO2 thin film. The binding energy appears chemical shift and relatively more Y and Ti species are present on the surface, indicating that active surfaces of the nanocomposite films have been enhanced with more oxygen vacancies Vo due to doping of Y2O3 to TiO2. The absorption edge has a red shift with the band gap of 2.68 eV. The water contact angle is about 8°after daylight lamp irradiation for 60 min. Based on the Mott-Schottky equation, the donor concentration (ND) for YTF is 1.05×1020cm-3, which enhances one order of magnitude than that for pure TiO2 film (TF), but the flat-band potential (Vfb) and the space charge layer(dsc) obviously decreased. The CuO/TiO2 nanocomposite thin film has successfully been synthesized on the basis of the relation between the chemical composition and band gap in mixed semiconductor systems. From the surface of the up-graded thin films to substrate glass in the direction of depth, the percentage of Cu decreases, that of O increases, that of Ti is slowly enhancing and then quickly decreasing. It can be concluded that the band gap of the as-synthesized film decreases alone the direction of depth due to the graded chemical composition. The anode photocurrent of up-graded nanocomposite thin film electrode obviously increases and the photo-response region is greater than that of TiO2 thin film electrode. These results should provide basic theories for the TiO2 thin film as solar cells and as a novel self-cleaning material in the field of in construction industry, which also have important instructional functions for the development and application of novel structured TiO2 thin films.(4) Computational simulation of visible light activities for nano-TiO2 using the first-principle calculation based on the Density Functional Theory.The structure of energy level, density of states and optical properties of three crystalline models TiO2, and F、S、N、Li+、Ni2+、Bi3+、Ce4+、V5+、W6+ doping TiO2, and N-Bi co-doping TiO2 have theoretically been calculated using the first-principle methods based on plane-wave pseudo-potential theory in detail. The results indicate that:(1) the CB and VB of TiO2 consist of both the Ti 3d and O 2p orbital, the CB distributes is mainly attributed to Ti 3d orbital with a smaller contribution from O 2p orbital, and the VB consists of the O 2p orbital (the main contribution) hybridized with the Ti 3d orbital, the physicochemical properties of rutile, anatase and brookite TiO2 are different due to the difference in the structure of CB and VB; (2) the O 2p and Ti 3d orbital, mixed with F 2p, S 3p, N 2p, Ni 3d, Bi 6p, Ce 5d、Ce5p, V 3d、V 3p and W 5d orbital of doping F, S, N, Ni, Bi, Ce, V and W ions, respectively, causes the changes of constitute of CB and VB and narrows the band gap; (3) the optical properties obtained from calculation are in good agreement with that of experimental results. These computational results discover the intrinsic nature of improving visible light activities of TiO2 through doping ions, which can provide basic theory for developing novel catalyst with visible light irradiation and significative directions to build the relation of composition, structure and property of TiO2 materials.更多还原