【作者】 冯亚娟；

【导师】 潘国强； 潘志云；

【作者基本信息】 中国科学技术大学 ， 核科学与技术， 2014， 博士

【摘要】 人类社会的进步过程即是不断使用新型材料满足自身需求的过程。半导体材料、光伏材料、催化剂、热电材料等功能材料在当代人类的经济生活和科学研究中都具有重要的价值。为使种类日益增长的新型材料能够物尽其用,我们首先需要深入了解这些材料的基本结构。随着同步辐射实验技术的快速发展,材料科学家获得了在原子尺度上研究物质精细结构的有力工具。X射线吸收精细结构谱学(XAFS)使精确确定物质中的原子结构成为可能。在本文中,我们利用基于同步辐射的XAFS技术并结合X射线衍射(XRD)和紫外可见吸收光谱(UV-Vis)等多种常规技术,重点采用同步辐射原位XAFS方法,从原子结构出发,更为深入地研究各种功能材料的构效关系。对半导体材料(Se纳米管)、热电材料(Ag2Se纳米颗粒)、光电极材料(TiO2)和光电极反应催化剂(CoxO)的原子结构进行了详细的研究：在原子尺度上,给出了Se纳米管生长过程的精确描述；确定了超晶格Ag2Se的原子局域结构和相变机理；详细研究了高性能光降解催化剂Mo掺杂TiO2的原子结构；证实了CoxO系列光电极催化剂具有活性中间体：为上述功能材料的理论研究和实际应用提供了参考信息。本论文的主要研究内容如下：1. Se纳米管液相生长过程的原位XAFS研究Se是一种被广泛研究的非本征半导体材料,其间接带隙为1.6eV。除了Ge和Si之外,Se是另一种在半导体工业中获得广泛应用的材料。常见的Se单质具有特殊的晶体结构六方晶系(三方硒,t-Se),是一种理想的产生一维纳米结构的前驱物。在已报道的关于一维Se纳米结构生长过程的研究中,研究者大多是基于中间产物的阶段性形貌变化来推断Se纳米结构的生长机理,并不能在原子尺度上精确描述Se纳米管或纳米线的生长机理。为了明确一维Se纳米结构生长过程中的结构变化信息,尤其是初期成核阶段的机理,我们利用具有原子尺度结构敏感性的XAFS技术,原位研究了一维硒纳米管的生长过程,并使用多重散射理论模拟进一步分析发现Se纳米颗粒前驱物和Se纳米管的基本组成单元都是Se单原子链。在Se纳米颗粒中,Se单原子链无规排布形成无定型结构；经超声处理,Se单原子链分散后按照六棱柱形组装为Se纳米管。本项研究从原子尺度给出了Se纳米管的成核和生长机理,为一维Se纳米材料的生长机理和可控制备研究提供了重要的参考信息。2. Ag2Se超晶格结构的XAFS研究及相变动力学模拟β-Ag2Se具有低的热导率、高的Seebeck系数和低的电阻率,是一种有着重要应用前景的热电材料。近年来,很多研究报道了非计量比的超晶格β-Ag2Se且有特殊的优良性能。然而对超晶格β-Ag2Se性质的深入研究受限于其原子结构仍不明确。为了在原子尺度上探测超晶格β-Ag2Se的结构信息,我们采用了同步辐射XAFS实验技术。示差量热法(DSC)研究表明样品显示出了除常规正交-立方相变之外的新相变过程。我们采用模拟退火方法来辅助拟合Se K边的扩展X射线吸收精细结构谱(EXAFS)的x(k)函数,获得超晶格β-Ag2Se的精确结构参数。拟合结果显示在超晶格β-Ag2Se中存在一个间隙位Ag原子。根据这一结构模型进行的第一性原理分子动力学研究发现,新的相变过程来自于间隙位Ag原子周围的结构扭曲。这些结果为超晶格β-Ag2Se的相变机理研究提供了理论基础。3.光电材料Mo掺杂Ti02的结构解析及其性能研究二氧化钛是半导体中最具有应用前景的光电催化剂之一,具有高催化活性、长载流子寿命、高化学稳定性和无毒等优点。但由于带隙宽度的限制,二氧化钛只能够吸收太阳光光谱中的紫外区(<380nm),因此增大二氧化钛的可见光利用率对提高其光催化性能有着非常重要的意义。我们研究了用凝胶溶胶方法制备的高性能Mo掺杂二氧化钛光催化剂的局域结构和性能。EXAFS分析结果表明正六价的Mo原子替代了Ti原子的位置掺入锐钛矿相中,并且存在Mo-O键长收缩现象。结合第一性原理计算,我们发现键长收缩来源于Mo原子周围的高电子密度从而导致Mo-O键共价性增强。光催化降解亚甲基蓝实验证实了Mo掺杂提高了二氧化钛的光催化性能。这些结果为提高光电材料性能的研究提供了实验基础。4.产氧催化剂构效关系的原位XAFS和UV-Vis研究太阳光驱动电化学水分解反应产生氢气和氧气是一种有效的生产清洁能源的手段。目前广泛使用的半导体光电极(如Ti02)的能量转化率普遍受到电极产氧反应的高过电势影响。而过渡金属氧化物(TMO,如CoOx)产氧催化剂可以有效降低产氧反应过电势,提高能量转化效率。我们通过在电解质中引入适当的质子受体,实现了COOx产氧催化剂在较宽pH范围内的反应活性。我们还利用电化学方法结合原位紫外可见光谱和原位XAFS技术研究了TMO催化剂的活性起源以及催化剂活性与工作电解质之间的关系。更多还原

【Abstract】 In the developing process of the human society, exploiting functional material plays a crucial role. Various functional materials such as semiconductors, photovoltaic materials, catalysts, and thermoelectric materials, are of vital importance in the industrial and scientific fields. To optimize this utilization, deep understanding in the atomic structures of functional materials is required. As the advanced synchrotron radiation technique, X-ray absorption fine structure spectroscopy (XAFS) has become a powerful tool to precisely determine atomic structure of samples, by which the properties of maters could be reviewed from a basic aspect. In the thesis, we detected the atomic structures of semiconductors (Se nanotubes), thermoelectric materials (Ag2Se nanoparticles), photoelectrode catalyst (CoxO) and photoelectrochemical materials (TiO2). Based on the obtained structure information, we discovered their growth mechanism, phase-transition process and activities. The main contents in this thesis are as follow:(1) Growth mechanism of Se nanotube revealed by in situ XAFS techniqueAs a kind of extrinsic semiconductor with an indirect gap of1.6eV, Se is extensively studied and widely used in semiconductor industry. The common Se, t-Se with a hexagonal crystal structure, is an ideal precursor for1-dimension nanostructures. The previous study on the growth mechanism of1-dimension Se nanostructures based on morphology could not provide precise growth process of Se nanotubes or nanowires, especially in the initial stage. We adopted XAFS with high sensitivity of local atomic structure to explore the growth mechanism of Se nanotubes. The Se K-edge XANES spectra reveal that both of the basic units for the precursors (Se nanoparticles) and Se nanotubes are Se monatomic chains. In Se nanoparticles, monatomic chains are irregularly packed whereas they are regularly packed in a hexagonal prism pattern in Se nanotubes. This study provided important information about growth mechanism of Se nanotube in atomic scale.(2) Superlattice structure of Ag2Se investigated by XAFS and dynamic simulationP-Ag2Se is a promising thermoelectric material due to its low thermal conductivity, high Seebeck factor and low resistivity. Recently, non-stoichiometric (3-Ag2Se with superlattice structure was reported to be with excellent performance. However, deep understanding of non-stoichiometric β-Ag2Se is hindered by its unclear atomic structure. We prepared β-Ag2Se with superlattice structure, which exhibits a new phase-transition. We fitted the XAFS results with simulated annealing method to obtain the structure of P-Ag2Se, which reveals that an interstitial Ag atom is in the lattice of β-Ag2Se. We also performed the first-principle molecular dynamic simulation based on the obtained structure parameters. The results demonstrate that the new phase-transition arise form local distortion around the interstitial Ag atom.(3) Investigation on the local structure and degradation property of Mo-doped TiO2photocatalystsTiO2is a promising semiconductor for photochemical catalyst due to its high activity, long carrier lifetime, high chemical stability and biosafty. However, visible light cannot be effectively absorbed by TiO2because of its wide band gap. Using XAFS technique, we studied the Mo-doped TiO2with enhanced photochemical activity. The detailed XANES and EXAFS analysis reveal that all of the Mo atoms substitute Ti atoms in the anatase lattice with shrinkage of Mo-O bond length. Density functional theory (DFT) calculation reveals that such shrinkage arises from higher co-valency of Mo-O bond than that of Ti-O bond, which is also the origin of decrease of band gap. The results in degradation of methylene blue experiments confirm the enhanced photochemical activity of Mo-doped TiO2.(4) Local structure and optical characteristic of water oxidation catalysts studied by in-situ XAFS and UV-Vis SpectroscopySolar-powered water splitting for generating oxygen and hydrogen is a promising way to store clean energy. The energy conversion efficiency of semiconductor oxides (for instance, TiO2) suffers from the high overpotential of water oxidation in the surface of photoelectrodes. Transition metal oxide (for instance, CoxO) is an efficient catalyst to reduce the overpotential. We introduced proton acceptor in the electrolyte to realize high activity of CoxO catalyst in a wide pH range. We also explored the active sites of CoxO catalyst with in-situ XAFS and UV-Vis spectroscopy to investigate the origin of catalyst activity and the relationship between catalyst activity and electrolyte.更多还原