【作者】 王岩；

【导师】 杨建军；

【作者基本信息】 河南大学 ， 高分子化学与物理， 2012， 博士

【摘要】 对TiO₂进行N掺杂可以有效拓展TiO₂的光谱响应从紫外光区到可见光区，并实现可见光催化活性。但是人们对于N掺杂TiO₂的可见光响应机理以及N的掺杂状态至今仍存在争议，目前关于N掺杂TiO₂的可见光响应机理主要存在四种观点：（1）禁带变窄理论，N2p轨道与O2p轨道重叠；（2）局域能级理论，价带上方形成N2p局域能级；（3）氧空位理论，氧空位产生可见光响应，掺杂的N稳定氧空位；（4）是一种中和理论，氧空位和N2p局域能级协同作用产生可见光响应。四种理论各有其合理的一面，但同时也存在一些争议问题，前两种理论忽略了氧空位的可见光响应作用，后两种理论则没有阐述清楚氧空位及掺杂的N如何作用。在样品制备过程中常常伴随着氧空位的生成，因此有必要进一步弄清楚氧空位在N掺杂TiO₂可见光响应中的作用及氧空位和掺杂元素之间的协同作用，弄清楚N掺杂TiO₂样品的可见光响应机理对于设计、制备高可见光催化活性的光催化剂具有十分重要的意义。在这一研究背景条件下，本文围绕着掺氮TiO₂的制备、物理化学性质表征、可见光催化性能评价及其可见光催化作用机理开展了一系列的工作。针对掺氮TiO2的可见光催化作用机理这一关键问题，以最普遍的P25-TiO₂为前驱体，在氨气气氛中热处理制备得到不同温度处理的N掺杂TiO₂，重点研究了掺氮过程中生成的氧空位、掺杂的氮元素、可见光吸收与掺氮TiO₂的可见光催化活性之间的内在联系，提出了一种与以上4种观点不完全相同的、新的可见光催化作用机理。随后，以novel-TiO₂（一种自制的锐钛矿TiO₂）、P25-TiO₂和纳米管钛酸（Nanotubular Titanic Acid，简写为NTA）为前驱体，同样在氨气气氛中热处理制备得到三类N掺杂TiO₂。通过研究它们的物理化学性质与可见光催化活性的关系，不但验证了我们提出的可见光催化作用机理，还确定了NTA在制备高效可见光催化剂方面具有重要潜力。在这些研究的基础上，进一步研究了金属离子修饰的掺氮TiO₂的制备、表征及其可见光催化作用机理，发现金属离子的修饰可以进一步提高光生电子空穴对的分离效率，从而表现出比单一掺氮或单一修饰金属的TiO₂更好的可见光催化活性性能，并描绘出了金属离子修饰的TiO₂-xNx样品的可见光催化作用机理图。通过这些研究，本文主要得到以下几点结论：1、氨气热处理P25-TiO₂制备N掺杂TiO₂的过程中，生成大量束缚单电子的氧空位，在ESR图上出现一个以g=2.004为中心的三重信号峰。可见光照下，束缚单电子的氧空位浓度增大，表明N掺杂TiO₂含有三种类型的氧空位：束缚单电子的氧空位、束缚两个电子的氧空位和不束缚电子的氧空位，后两种氧空位是反磁性物种不能被ESR检测到。掺杂的N处于间隙位置，直接与晶格氧或氧空位连接，XPS结果得到一个结合能为400eV的信号。研究发现，束缚单电子的氧空位浓度与N掺杂TiO₂的可见光吸收和可见光催化活性具有内在的线性关系。基于此，提出了一种新的可见光催化作用机理，即N掺杂TiO₂的可见光催化活性由束缚单电子的氧空位和掺杂的N元素两个因素共同决定，束缚单电子的氧空位决定N掺杂TiO₂的可见光响应，同时掺杂的N元素在氧空位附近阻止光生电子空穴对的复合，提高光生电子空穴对的分离效率，两者协同作用，共同决定可见光催化活性。在有效N掺杂的前提下，束缚单电子的氧空位浓度与N掺杂TiO₂的可见光催化活性成线性关系，氧空位浓度越高，可见光催化活性越好。2、作为承上启下的一部分，也是为了验证氧空位与掺杂的N元素之间的协同效应，以novel-TiO₂、P25-TiO₂和NTA三种不同结构的物种为前驱体，同样在氨气气氛中热处理制备得到三类N掺杂TiO₂。结果表明，三类N掺杂TiO₂的可见光吸收和可见光催化活性与它们所含的氧空位浓度成线性关系；根据DRS结果，提出并计算了N掺杂TiO₂的能带结构模型，大量束缚单电子的氧空位在TiO₂价带上方2.34eV和导带下方0.57eV处形成一个带隙宽度为0.19eV的氧空位能级，氧空位能级的带隙宽度决定于氧空位浓度的大小；氧空位能级作为中间桥梁使得价带电子可以被可见光激发跃迁到氧空位能级再跃迁到导带，导带上电子再跳回氧空位能级的时候，氧空位和掺杂的N元素与光生电子之间发生了电荷传输反应：V_o+N→V_o+N^-（I）N^-+O₂→N+O₂^-（II）V_o表示束缚两个电子的氧空位，V_o表示束缚单电子的氧空位。从而通过这两步反应切断了光生电子空穴对复合的路径，提高了光生载流子的分离效率，产生可见光催化活性。不同前驱体自身的性能不同，导致相同氮化过程中生成的氧空位浓度不同，使得不同掺氮样品的氧空位中间能级带隙不同，产生不同的可见光催化活性。通过这些研究，不但验证了结论1中提出的氧空位和掺杂的N之间协同作用共同决定可见光催化活性的理论，还发现NTA在制备高效可见光催化剂方面具有重要潜力。3、在对N掺杂TiO₂可见光催化作用机理系统研究的基础上，选择NTA为前驱体，采用一步法制备了Au修饰的N掺杂TiO₂，以期进一步提高光催化剂的可见光催化活性。研究发现，Au修饰的TiO_2-xN_x样品可见光催化活性高于单独Au修饰或单独N掺杂的TiO₂，表明金属离子修饰TiO_2-xN_x样品是获得高可见光催化性能光催化剂的一种方法。单独Au修饰的TiO₂在550⁶50nm出现Au的等离子体共振吸收峰，提高了光催化剂对可见光的吸收，但是光吸收阈值没有变化；Au修饰的TiO_2-xN_x样品光吸收阈值明显红移，并且具有较好的可见光吸收，但是Au的等离子体共振吸收峰弱化；修饰的Au4f_7/2结合能比体相Au的标准结合能（84.0eV）有所降低，单独Au修饰的TiO₂结合能位于83.1eV，主要是由于氧空位上的电子向修饰的Au6s轨道迁移所致；Au修饰的TiO_2-xN_x样品结合能位于83.4eV，比单独Au修饰的TiO₂结合能高，可能是由于Au6s轨道上的电子又向掺杂的N2p轨道迁移所致；掺杂的N、修饰的Au和生成的氧空位三者之间具有协同效应，即掺氮有利于氧空位的生成并提高Au与TiO₂之间的附着力，氧空位和Au的存在则有利于N的掺杂，这种协同作用是Au/TiO2-xNx可见光催化活性提高的根本原因。4、研究了其他几种不同金属离子修饰的M/TiO_2-xN_x（M=Ni, Pt, Pd, Cu）样品的制备及其性能表征，研究发现，不能以金属离子价态或半径大小直接判断TiO2的（锐钛矿/金红石）相变规律，也不能以金属离子价态或半径大小为依据直接比较哪种金属离子修饰的TiO_2-xN_x样品可见光催化活性的高低。提出了M/TiO_2-xN_x样品的可见光催化活性机理图，样品制备过程中生成的氧空位及修饰的金属共同产生可见光吸收，价带电子首先被可见光激发到氧空位能级，再跃迁到导带，XPS结果表明金属元素多以零价态的单质金属分散于TiO₂表面，可以作为电子捕获阱迅速捕获跃迁到导带上的光生电子，提高光生电子空穴对的分离效率；同时，掺杂的N在氧空位附近阻止从导带跳回氧空位能级的光生电子再跳回价带与光生空穴复合，进一步提高了光生载流子的分离效率，两种作用共同提高M/TiO_2-xN_x样品的可见光催化活性。更多还原

【Abstract】 Nitrogen doping is one of the most efficient methods to extend light response of TiO₂into the visiblelight region in association with an expected enhancement of visible-light-responded photocatalytic activity.However, the origin of visible light photoactivity of N-doped TiO₂is still in debate. Four differentmechanisms have been proposed as below:（1） Band gap narrowing caused by mixing of N2p and O2porbitals, resulting in visible light response;（2） Localized midgap induced by doped-N, whose electrons canbe excited to jump to the conduction band by visible light;（3） Oxygen vacancy states below conductionband formed during nitrogen doping process;（4） A neutralization theory that suggests the synergistic effectbetween N2p localized midgap and oxygen vacancy states is responsible for the visible light photoactivity.Unfortunately, the role of the oxygen vacancy is usually not considered in the former two theories, whilehow the oxygen vacancy and doped-N work is not well elucidated in the latter two theories. Therefore, it isimperative to clarify the role of oxygen vacancy in enhancing visible light photoactivity and themechanisms by which the oxygen vacancy and doped-N work. The solution of these problems facilitates todesign and prepare highly visible-light-active photocatalyst.Therefore, we pay special attention to the effects of oxygen vacancy and dopants on the visible lightphotocatalytic activity, aiming at revealing the origin of visible light sensitization of N-doped TiO₂. Firstly,N-doped TiO₂is prepared by heat treatment of commercial P25-TiO₂in flowing NH3and the relationshipamong the doped-N, single-electron-trapped oxygen vacancy, optical absorbance and enhanced visible lightphotocatalytic activity is systematically studied. It is proposed that the origin of visible light sensitization ofN-doped TiO₂can be ascribed to the synergistic effect between oxygen vacancy and dopant nitrogen.Subsequently, three kinds of precursors, nanotubular titanic acid （denoted as NTA）, raw P25-TiO₂andnovel-TiO₂, are separately used to prepare visible-light-active N-doped TiO₂samples by annealing inflowing NH3, aiming to reveal the determinative factors on visible light response. By comparing thephysicochemical properties of the three kinds of N-doped TiO₂samples with their visible lightphotocatalytic activity, the synergistic effect between oxygen vacancy and dopant nitrogen is proved andNTA precursor is pronounced to be one of the most promising materials for designing visible light activephotocatalyst. Furthermore, the characterization and mechanism of metal ion decorated TiO₂-xNxsamples hat based on the theory of N-doped TiO₂is systematically studied. In summary, four main conclusions aredrawn as below:1. N-doped TiO₂catalysts are prepared by nitridation of P25in NH3flowing under various temperaturesand the synergistic effect between the single-electron-trapped oxygen vacancy and doped-N in associationwith the origin of visible light sensitization of N-doped TiO₂is systematically studied. It is found that alarge amount of single-electron-trapped oxygen vacancies （denoted as SETOVs） generate during the dopingprocess, giving rise to a triplet ESR signal centered at g=2.004. The concentration of SETOVs is enhancedunder visible light irradiation as compared with that obtained in the dark, possibly because many freshadditional SETOVs generated under visible light irradiation. Therefore, it can be inferred that the oxygenvacancies can be divided into three categories: SETOVs with triplet ESR signal, dual-electrons-trappedoxygen vacancy and oxygen vacancy without electron, while the latter two will not produce ESR signals.The doped-N at interstitial site is directly combined with lattice oxygen or oxygen vacancy, showing abinding energy at400eV in XPS spectra. The origin of visible light photocatalytic activity is ascribed tothe synergetic effect between the formation of SETOVs in TiO₂matrix and the existence of doped-N on thesurface. Namlely, the formation of SETOVs results in visible light reponse, while doped-N plays a role inpreventing photoinduced electrons and holes from recombination. In other words, in the absence of eitherSETOVs in TiO₂matrix or doped-N on the surface, N-doped TiO₂will not show visible light photocatalyticactivity; and the higher the SETOVs concentration is, the better the visible light photocatalytic activity willbe.2. Not only as a connecting link between the preceding and the following but also in order to prove thesynergistic effect between oxygen vacancy and dopant nitrogen, three kinds of precursors are separatelyused to prepare visible light active N-doped TiO₂samples. Both the visible light absorption andphotocatalytic activity of the three kinds of N-doped TiO₂samples are found to be proportional to theconcentration of SETOVs, well conforming to what are summarized above. According to the DRS results,the band gap energy structure model of N-doped TiO₂is proposed. It is calculated that an intra-bandinduced by SETOVs is located at0.57eV below conduction band and2.34eV above valence band, with aband gap Eg=0.19eV resulting in visible light response which varies with varying concentration ofSETOVs. Given that the formation of SETOVs induces an intra-band, it is proposed that electrons are initially excited from valence band to intra-band and then jumped to the conduction band under visible lightirradiation. As a result, the photoinduced electrons may jump back from conduction band to intra-band,accompanying with a predicted charge transfer state that contributes to the visible light photocatalyticactivity:V_o+N→V_o+N^-（I）N^-+O₂→N+O₂^-（II）Where V_o represents for dual-electrons-trapped oxygen vacancy and V_o equivalents to SETOV. Thedoped-N plays a role in preventing photoinduced electrons and holes from recombination by cutting off theroute that the electrons jump back to valence band, resulting in enhanced visible light photocatalyticactivity. The N-doped TiO₂samples obtained by annealing of NTA as the precursor in flowing NH3exhibited the highest visible light photocatalytic activity, implying that NTA is one of the most promisingmaterials for designing visible light active photocatalyst.3. Au/TiO_2-xN_xsamples are prepared via a facile one-pot route and they showed much better visible lightperformance than Au/TiO₂or TiO_2-xN_xsamples. An additional absorption band from550⁶50nm index togold surface plasmon resonance is observed in DRS spectra for Au/TiO₂, while this peak for Au/TiO₂-xNxsamples is very weak. The binding energy values of Au4f7/2are83.1and83.4eV for Au/TiO₂and TiO₂-xNxsamples, respectively, lower than84.0eV of metallic Au. As to Au/TiO₂samples, the electrons can transferfrom SETOVs to Au6s obitals, resulting in lower Au4f binding energy. Accompanying with the Nincorporation, an electron transfer from Au6s obital toward the N2p level can be expected, resulting in thebinding energy shifting from83.1eV for Au/TiO₂to83.4eV for Au/TiO₂-xNxsamples. The higher visiblelight photocatalytic activity of Au/TiO₂-xNxsamples is attributed to the synergetic effect among doped-N,modified-Au and oxygen vacancy. Namely, nitrogen doping favors the formation of oxygen vacancy andincreases the Au-surface adhesion energy, while the existence of both Au and oxygen vacancy results ineasily nitrogen doping into TiO₂.4. Noble metal ions Pd2+and Pt4+or transition metal ions Cu2+and Ni2+modified TiO₂-xNxsamples（denoted as M/TiO₂-xNx） are also prepared separately according to the same method mentioned in the thirdportion. It is found that there has no direct relationship between the valence state or ionic radius of metalion and Anatase/Rutile phase transformation or visible light photocatalytic activity with respect to M/TiO₂-xNxsamples. The visible light response of M/TiO₂-xNxis attributed to the formation of SETOVsduring the preparation process, while dopant nitrogen plus modified metal contribute not only to improvethe visible light absorption but also suppress the recombination of photogenerated electrons and holes. Inone word, the enhanced separation efficiency of photogenerated electron–hole pairs attributed to thesynergistic effect among oxygen vacancies and N dopant as well as modified metal particulates on thesurface of TiO₂jointly account for the increased visible light photocatalytic performance of M/TiO₂-xNxsamples. The visible light photocatalytic mechanism of M/TiO₂-xNxis proposed and discussed in detail.更多还原