【作者】 张伟伟；

【导师】 尹衍升；

【作者基本信息】 中国海洋大学 ， 海洋化学工程与技术， 2008， 博士

【摘要】 氧化钛（TiO₂）是一种非牺牲性的光阴极保护材料,它的保护作用源于其光电化学性能。在能量大于其能隙的光子的照射下,TiO₂价带电子激发至导带,形成电子-空穴对。正电空穴与TiO₂表面的水发生氧化反应,而负电电子则由导带进入金属基体,使金属的电极电位降低至不发生腐蚀的阴极保护区。TiO₂的光阴极保护性能取决于其光电效率,非金属掺杂是近年来发展起来的提高TiO₂光电效率的改性方法,具有稳定性高、可见光响应好和成本低的优点。利用非金属掺杂改性TiO₂,使其在可见光下即能够产生较好的光阴极保护效果,具有十分重要理论研究和实际应用意义。本论文针对TiO₂纳米材料的制备、非金属掺杂改性及光阴极保护性能,从实验和理论两个方面进行了较为系统的研究,主要研究内容及成果如下:采用溶胶-凝胶工艺制备纳米TiO₂粉体,研究了TiO₂的溶胶-凝胶过程及温度、pH值等因素对TiO₂晶相的影响。水与钛醇盐的体积比、乙醇添加量及水解温度决定了溶胶体系的胶凝过程;前驱体溶胶的pH值和热处理温度决定了TiO₂晶体的晶相结构和晶型转变。在实验的基础上,利用密度泛函理论计算了优选的不同pH值溶液中前驱体的结构模型,模型的键长和Mulliken电荷分析表明,在低pH值的溶胶中,前驱体易发生共顶点连接,形成类金红石结构,从而在随后的热处理过程中形成金红石相或者作为相变诱导剂,减小锐钛矿向金红石的相变温度和临界粒径。溶胶pH值增大,配位多面体之间易发生共边连接,形成锐钛矿相。通过理论和实验结果的对比分析,初步建立了前驱体溶胶工艺参数与TiO₂晶型的联系。在TiO₂溶胶中掺杂少量SiO₂溶胶可使TiO₂晶粒减小、晶型转变温度提高,掺杂粉体的红外光谱在960cm-1处出现了不同于TiO₂和SiO₂的吸收峰。利用密度泛函理论优化计算了SiO₂/TiO₂二元氧化物可能的生长基元及两相界面的团簇结构,能量分析及红外光谱的理论与实验对照证实了二元氧化物界面环状Si-O-Ti键的形成。界面结构的形成同时引起TiO₂团簇原子Mulliken电荷减小,最终导致SiO₂掺杂抑制TiO₂的晶体生长和晶型转变。利用密度泛函理论优化计算了（TiO₂）n （n =1-9）团簇各种可能的结构和红外光谱,提出了（TiO₂）n （n =1-9）团簇的最稳定构型,为掺杂团簇模型的建立提供了依据。通过对团簇结构及能量的分析发现,在n=1⁹的范围内,结构紧凑、原子间达到最大程度交联的团簇稳定性较高,过高的钛配位数（≥5）和终端Ti-O键的存在是TiO₂团簇的能量不稳定因素。团簇结构的平均结合能、能隙大小及二次能量差分的结构表明, n=3、5、7时,团簇的相对稳定性较高。理论红外光谱表明,所有稳定构型均在1070cm-1出现了终端Ti-O键的伸缩振动引起的红外吸收,800 cm-1⁹00 cm-1范围内的吸收峰对应着团簇六元环的振动,600～700 cm-1附近的峰对应着团簇Ti-O2-Ti四元环的振动吸收,团簇结构的理论振动吸收与体相氧化钛的红外光谱取得了较好的一致。探索了不锈钢表面制备纳米TiO₂薄膜的工艺参数,在粘度为3.0的TiO₂溶胶中浸渍提拉可在不锈钢表面获得均匀致密的TiO₂薄膜,一次膜厚约为40nm。在溶胶中添加N,N-二甲基甲酰胺能够有效地减少TiO₂薄膜表面的裂纹分布。紫外光照的条件下,500℃热处理获得的TiO₂薄膜使不锈钢的电位负移、失重减小,对不锈钢起到了光阴极保护作用,在模拟海水中浸泡15天的平均保护度可达85.7%。薄膜的热处理温度大于500℃时,不锈钢中的铁元素向薄膜中大量渗透,在TiO₂晶体的价带和导带之间形成杂质能级,杂质能级可成为电子-空穴的复合中心,降低TiO₂的光阴极保护效率。分别以三乙胺、氨水和氯化铵为N前驱体,硫脲为S前驱体,制备了非金属N、S掺杂的纳米TiO₂,三乙胺由于较小的N-C键能,能够在TiO₂晶体中引入更大的N掺杂量,而硫脲则由于较高的C=S键能,导致同样条件下S在TiO₂中的最大掺杂量低于N的掺杂量。XRD结果表明,非金属N、S掺杂能够抑制TiO₂晶体生长和晶相转变,并引起TiO₂晶体的晶型畸变。在兼顾掺杂位和计算量的条件下选择TiO₂团簇的一个最稳定构型,在不同掺杂位分别引入N原子和S原子。能量分析证明N原子在TiO₂团簇中容易以替代O原子的形式掺杂,而S原子则容易在替代Ti原子位形成掺杂。非金属掺杂降低了TiO₂团簇原子间的作用势,提高了团簇稳定性,表现在宏观上就是抑制了TiO₂的晶体生长和晶相转变。非金属N、S掺杂使TiO₂晶体的光吸收限红移,从而产生可见光响应。在TiO₂周期性体系中分别引入不同浓度的N原子和S原子,利用密度泛函理论计算非金属N、S掺杂TiO₂晶体的能带结构及态密度,发现N掺杂能够使TiO₂晶体的电子平均自由程变大,有利于电子在光电反应过程中的输运。掺杂N原子的2p态与O 2p态发生耦合,使TiO₂晶体价带变宽、能隙减小,提高其在可见光区的响应;而掺杂S原子的3p态在TiO₂晶体的价带和导带之间形成杂质能级,是S掺杂TiO₂晶体能隙减小,产生可见光响应的原因。非金属掺杂引起TiO₂晶体的可见光响应,从而使掺杂TiO₂在白光条件下的光阴极保护效果显著增强。与N掺杂相比,虽然S掺杂TiO₂晶体的能隙较小,但由于S掺杂引入的杂质能级的电子态密度较高,使杂质能级成为电子-空穴对的复合中心,部分减少了迁移到TiO₂表面的光生载流子的量,从而使S掺杂TiO₂薄膜的光阴极保护效果低于N掺杂的TiO₂薄膜。更多还原

【Abstract】 TiO₂ is an n-type semiconductor with chemical and physical stability. In addition, if illuminated TiO₂ is in contact with a metal, electrons are injected from the semiconductor to the metal via the conduction band. As a result, the potential of the metal will be shifted in the negative direction to the flatland potential of TiO₂. If the potential is more negative than the potential at which the metal beings to oxidize, the metal can be protected from corrosion. The counterreaction by photogenerated holes （h+） is not the decomposition of TiO₂ but the oxidation of water. This behavior means that TiO₂ can act as a nonsacrificial anode.The photocathodic protection effect of TiO₂ is dependent on the photoelectrochemical efficiency, which has been improved in two ways: the first is to extend visible light response of TiO₂; the second is to decrease the undesirable hole-electron recombination. Nonmetal dopants, such as N, S, may be more appropriate for the improvement of photoelectrochemical activity of TiO₂ because of it extending the visible light response effectively with thermal stability and cost efficiency.In this work, TiO₂ and nonmetal （N, S mainly） doped TiO₂ materials were prepared by a simple sol-gel method, and their photocathodic protection effects were analyzed from experimental and theoretical approaches:The sol-gel process is an important step in the preparation of TiO₂ material. In our experiment, Tetrabutyl titanate was used as TiO₂ source. When the ratio of tetrabutyl titanate to water by volume is in the range of 2³, with less absolute ethanol and higher temperature, the system get gel point acceleratedly. The resulting crystal phases were determined by the structures of the precursor cation, which are affected by the exact pH of the precursor solutions. Through the analysis of the geometric parameters and Mulliken charge populations of the titanium complexes, theoretical correlation was set up between the precursor solution conditions and the resulting crystal structures.The interfacial structure of Ti/Si binary oxide, which will retard the crystal growth, is proved to be energetically feasible. When the binary clusters are small enough, the formation of edge-shared structures has the largest trend while corner-shared cluster can not form through the analysis of Gibbs energy. As the growth of the binary oxide clusters, the edge-shared structures are disrupted and ring-like clusters form.The structures and the stability of （TiO₂）n clusters with n = 1⁹ have been investigated using the density functional B3LYP/6-31G（d） method. The lowest-lying singlet clusters were put forward and some structure-stability correlation factors, such as coordination number and bond distance, were generalized. Infrared absorption spectra for the cluster structures are comparable with the spectra of rutile and anatase.In the solution with N, N-dimethylformamide, uniform and compact nanosized TiO₂ thin films were prepared on 303 stainless steels. The film shows good photocathodic protection effect as the electrode potential shifted in the negative direction and the weight loss of the steel decreased markedly. With excessively high thermal treating temperature, Fe atoms penetrated into the films and acted as the hole-electron recombination center, deceasing the photoelectrochemical activity and photocathodic protection effect of TiO₂ films.Triethylamine can introduce more nitrogen content into titania crystal than ammonia and ammonium chloride with small bonding energy of N-C bond. Our calculations gave smaller formation energies for substitutional N to O model which indicates the relatively easier synthesis. The doping of nitrogen atoms suppressed the growth of the titania crystal and the phase transformation. The substitutional-type doping was effective for the band gap narrowing of TiO₂ due to the mixing of N 2p with O 2p states, as a result, the photocathodic efficiency increased under the irradiation of white light.Less sulfur content can be introduced into TiO₂ as S=C in thiourea has a more bonding energy. Our calculations verified that S atoms to replace Ti atoms has the largest trend. The substitution results in the suppression of the growth of the titania crystal and the phase transformation. The doped TiO₂ materials have visible response due to the mixing of S 3p with O 2p states. Compared with N doped TiO₂, S doped TiO₂ has a narrower energy gap while a lower photocathodic protection efficiency. The impurity energy level introduced by S dopant act as the electron-hole recombination center, offset some charge carrier induced by the visible light response in a certain extent.更多还原