【作者】 李海彦；

【导师】 王德军；

【作者基本信息】 吉林大学 ， 物理化学， 2011， 博士

【摘要】 以TiO2为基础的半导体纳米材料具有优异的氧化能力和化学稳定性,能够将太阳能转化为化学能,对于解决目前日益严重的能源危机和环境污染问题极具研究和实用价值。然而将TiO2应用于实际的生产生活中,需要克服材料本身禁带宽度大(锐钛矿3.2eV)、量子效率低的难题。大量研究结果表明离子掺杂是提高TiO2性能的有效方式之一,尤其是具有3d轨道的过渡金属离子,它们的离子半径与Ti4+(61pm)半径相近一些,更容易进入TiO2的晶格取代Ti4+。通过过渡离子掺杂一方面在TiO2禁带内可能引入杂质能级,调节其能带结构,能将TiO2的光响应拓展到可见光区,另一方面可以在TiO2晶格内形成缺陷位,捕获光生电子或空穴,起到分离载流子抑制其复合的作用,因而能在一定程度上提高TiO2的可见光催化活性。而光催化过程本质上都是基于材料光生电荷的微观动力学行为,与光生载流子的产生、分离、复合过程密切相关。因此,研究离子掺杂TiO2光催化剂中光生电荷行为与催化活性之间的关系,理解光催化活性提高的本质原因,有利于设计和制备高活性的光催化剂。本论文的研究重点集中在半导体TiO2光催化纳米材料微结构的调控。探索最佳的合成方法及条件,对TiO2进行变价离子修饰(Bi, Mn, C),拓展其光响应范围,通过促进光生载流子的分离,降低其复合几率,均实现了催化剂紫外和可见光催化活性的提高。利用瞬态光伏技术(TPV)、表面光电压(SPS)、场诱导表面光电压(FISPS)及其相位谱、表面光电流(SPC)技术等其它光电表征手段,研究不同变价离子,掺杂含量及其价态对TiO2材料形貌、晶体结构、热稳定性、光电性质和光催化活性的影响。探讨其光生电荷行为(产生,分离和复合)与光催化活性之间的内在关系,为以后设计、构建高活性的可见光催化剂提供理论依据。具体的工作内容包括以下几个方面：1.用简单、绿色的方法制备一系列单分散的、尺寸均一的Bi-TiO2纳米球,通过改变铋源的加入量,纳米球的尺寸可以从117 nm调节到87nm。XRD、EDX、XPS及UV-vis DRS结果表明样品均为锐钛矿相TiO2,Bi进入TiO2的晶格,部分取代了晶格中的Ti原子,并且以Bi3+和Bi4+的形式共存。在可见光激发下(λ>420nm)光催化降解罗丹明B (RhB)溶液时,Bi-TiO2纳米球表现出较好的光催化活性及稳定性,可以循环使用。利用表面光电压技术并结合相位角分析Bi的引入对TiO2光生电荷行为的影响,探讨其光催化反应机理,结果表明其可见光催化反应机理为RhB染料的光敏化作用,Bi4+/Bi3+在TiO2禁带内形成了杂质能级,能够捕获TiO2导带中的电子,有效地提高TiO2中光生电子-空穴对的分离程度,并抑制其复合,使更多的电子能够参与表面活性物种(O2·-)的生成,从而提高了TiO2的可见光催化氧化能力。2.以四水合氯化锰为锰源,制备Mn3+和Mn4+共存的Mn掺杂Ti02纳米球,通过改变锰源的加入量,纳米球的尺寸在200-300/nm之间可调。利用FESEM、TEM、EDX、XRD、XPS、UV-vis DRS、SPS及SPC技术来研究Mn掺杂量和热处理温度对TiO2催化剂晶体结构、形貌、热稳定性、光生电荷行为及光催化活性的影响。结果表明样品均为锐钛矿相,每个纳米球都是由成百上千个15-20nm的小纳米粒子组成,因而比表面积变化不大。Mn进入了TiO2的晶格,取代了晶格中的部分Ti原子,并且与Ti原子之间有强相互作用,提高了TiO2的相转变温度。虽然将TiO2的光吸收能力拓展到700nm的可见光区,但是SPS及SPC结果表明样品在可见光区并无亚带隙跃迁过程,而且光伏响应强度随着Mn掺杂量的提高不断减弱,这说明Mn的引入并没有在TiO2的禁带内形成杂质能级,而是在TiO2晶格中引入了缺陷位,成为电子的捕获中心,捕获电子的同时降低了载流子的复合几率,提高光生电子-空穴对的分离程度,因而在对罗丹明B的降解过程中样品都表现出较好的光催化活性,但是Mn的掺杂量存在最佳值(摩尔比率,Mn/Ti=0.25%),最佳煅烧温度为500℃。掺杂量过高时,Mn4+/Mn3+位反而会成为光生电子-空穴对的复合中心,从而降低其光催化效率。3.在不引入额外碳源的情况下,通过水热法制备了高度结晶的C掺杂的锐钛矿TiO2纳米粒子,其尺寸约为5-8nm,因而受量子尺寸作用的影响,C-TiO2样品的禁带宽度在3.3eV左右。C的引入成功地将TiO2的光响应拓展到700nm的可见光区,并且在对甲基橙(MO)的降解过程中C-TiO2样品表现出优异的光催化活性。在紫外光激发下,C-TiO2纳米粒子只需要4min就能完全降解MO,其降解效率是商品P25的11倍。XPS及UV-vis DRS结果表明C原子进入了TiO2晶格的间隙位置,以碳酸盐的形式存在,并且与Ti原子形成了强相互作用。利用表面光电压并结合场诱导表面光电压和瞬态光伏技术研究发现：除了在300-380nm的带-带跃迁以外,样品在400-550nm出现了亚带隙跃迁,并且光生电子-空穴对完全复合的时间延长到32ms,而商品P25仅为5.1ms,这说明间隙C原子在TiO2的禁带内引入了一些局域化的杂质能级,杂质能级一方面能够接受TiO2价带中的电子,发生亚带隙跃迁过程,使TiO2的光吸收能力拓展到可见光区；另一方面能接受TiO2导带中的电子,有效提高光生电子-空穴对的分离程度,降低其复合速率,将载流子的寿命延长到32ms,使更多的电子和空穴扩散到样品表面参加光催化反应。更多还原

【Abstract】 TiO2-based nanomaterial which can convert solar energy into chemical energy is considered to be an effective, environmentally friendly means of solving the increasingly serious energy shortages and environmental crises because of their excellent oxidative capacity and chemical stability. However, the high recombination probability of photocarriers and poor solar efficiency have hindered their practical application. To solve these shortcomings, much effort has been directed towards the modification of TiO2 by incorporating ions, especially the transition metal ions with 3d orbital. As the ionic radius of transition metal ions is similar to that of titanium ion (61 pm), they can access the TiO2 lattice and substitute some of the lattice titanium atoms. On the one hand, the introduction of transition metal ions may induce several localized occupied states in the gap, result in modest variations of the band gap, which may account for the experimentally observed red shift of the absorption edge of TiO2 toward the visible region. On the other hand, it may favor the formation of lattice defects which enable to trap photogenerated electron or hole, enhance the separation extent and restrain the recombination of the photogenerated electron and hole carriers in TiO2, this has been reported to improve its catalytic activity to a certain extent. It is now commonly recognized that visible-light absorption does not always result in visible-light photocatalytic activity, and the key factor to enhancing the photocatalytic activity lies in effectively combining the photon absorption, bulk diffusion, and surface transfer of photogenerated charge-carriers in the photocatalyst. A detailed explanation of the photovoltaic properties of the charge carriers (including generation, separation and recombination) in photocatalyst is meaningful, and may provide useful information for understanding the catalytic mechanism, the information is important for designing new photocatalyst active under visible light.In this thesis, we study focused on the control of semiconductor TiO2 microstructure, developed the best synthetic method and conditions. The modification of TiO2 by incorporating ions to extend photoresponding range to the visible region, promote the separation of charge and inhibite their recombination. Using transient photovoltaic technology (TPV), surface photovoltage (SPS), field induced surface photovoltage (FISPS) and its phase spectrum, the surface photocurrent (SPC) technology to study the influence of different ions, doping content and valence on morphology, crystal structure, thermal stability, photoelectric properties and photocatalytic activity of TiO2 nanomaterial. The relationship between behavior of photogenerated charges (including separation, transport and recombination) and photocatalytic reactions is also discussed. Specific work includes the following aspects:1. Near-monodisperse Bi-doped anatase TiO2 nanospheres with almost uniform diameters in the range of 117 to 87 nm were prepared simply by introducing different amounts of bismuth nitrate pentahydrate into the reaction system and subsequent calcinations. X-ray diffraction, UV-visible diffuse reflectance spectra, and X-ray photoelectron spectroscopy confirm that the doped ions substitute some of the lattice titanium atoms, and furthermore, Bi3+ and Bi4+ ions coexist. All the Bi-doped TiO2 samples show much better photocatalytic activity than pure TiO2 in the degradation of rhodamine B (RhB) under the irradiation of visible light (λ>420 nm), after detailed studies of the results of surface photovoltage spectroscopy (SPS), and UV-visible diffuse reflectance spectroscopy, we propose that two aspects respond to the visible photocatalytic activity of Bi-doped anatase TiO2 nanospheres. One is the photosensitization of the dye with a compatible energy band, the other is the increase of the separation probability of the electrons transferred from dye in the samples by introducing bismuth atoms.2. Multivalent Mn-TiO2 nanospheres with controllable sizes were prepared with manganous chloride as manganese sources, the diameter of nanospehers can be controlled in the range of 200-300 nm by simply varying the amout of manganous chloride. The samples are further characterized by FESEM, TEM, EDX, XRD, XPS, UV-vis DRS, SPS and SPC techniques to study the influence of Mn doping content and annealing temperature on crystal structure, morphology, thermal stability of the photoinduced charge behavior and photocatalytic activity of TiO2. The results indicate that all the samples exhibit similar XRD patterns that can be indexed to anatase, each Mn-TiO2 nanosphere is assembled by hundreds of nanoparticles, and the average crystallites size is about 15-20 nm, This is why there is no significant change in the BET surface area (SBET) with an increase of Mn/Ti molar ratio. The manganese exists in multivalency (Mn4+/Mn3+) and substitutes for some Ti4+ in the anatase TiO2 lattice, enhance the thermal stability of TiO2 against the structural transformation. Although the absorption edge is extended to the visible-light region up to 700 nm for all the Mn-TiO2 materials, SPS and SPC measurements reveal that the sub-band-gap transition of Mn-TiO2 is not observed, moreover, the intensity of SPV gradually decreased with a increase of Mn contet, which indicate that the presence of Mn formed lattice defects in TiO2, Mn served as electron acceptors and effectively inhibited the charge recombination in TiO2, resulting in a high efficient photocatalytic activity in the degradation of rhodamine B (RhB) under visible-light irradiation (λ＞420 nm). There is an optimal doping amout and calcination temperature(Mn/Ti=0.25%,500℃), while an excessive amount of Mn will act as recombination center of electron-hole pair, reducing the photocatalytic efficiency.3. Carbon-doped anatase TiO2 nanoparticle was prepared by a facile hydrothermal process without adding additional carbon source, the average grain size is 5-8 nm, thus give a wider band gap (3.3 eV) than TiO2 bulk due to quantum size effects. The as-prepared sample shows highly efficient photocatalytic activity, which only requires 4 minutes and is about 11 times higher than that of Degussa P25 TiO2 in degradation of methyl orange (MO) dye under UV light irradiation. Moreover, a highly visible-light activity is also observed. UV-visible diffuse reflectance spectra and X-ray photoelectron spectroscopy confirm that the carbon atoms are incorporated into the interstitial positions of TiO2 lattice and form a strong interaction with titanium atoms and extend photoresponding range to 700 nm. According to surface photovoltage spectra (SPS) and transient photovoltage (TPV) results, the photovoltaic responses of C-TiO2 in the range of 400-500 nm which correspond to the sub-band-gap transition are observed, and TPV response peaks move towards the longer timescale for C-TiO2 (32 ms), while the response time of P25 TiO2 is only 5.1 ms. The above results suggest that the presence of interstitial carbons induce several localized occupied states in the gap, which can accept electron from the TiO2 valence and occur the sub-band-gap transition, thus extend the photoresponding range. On the other hand, the localized occupied states can trap electrons in TiO2 conduction, enhance the separation extent and restrain the recombination of the photo-induced electron and hole carriers in TiO2, thus more electrons and holes can spread to the TiO2 surface to participate in the photocatalytic reaction.更多还原