【作者】 娄在祝；

【导师】 黄柏标；

【作者基本信息】 山东大学 ， 材料学， 2014， 博士

【摘要】 能源短缺和环境污染是目前人类社会可持续发展所面临的两个重大威胁。寻找可替代化石能源的可持续清洁能源是解决两个威胁的关键。新兴发展的半导体光催化剂技术,能利用太阳光分解水产生清洁能源氢和氧,也可降解去除有机污染物和无机重金属。因此半导体光催化技术已经成为解决能源短缺和环境污染的可能途径。自1972年,日本科学家首次发现二氧化钛电极在光照下可以分解水产氢,半导体光催化研究迅速发展。经过四十多年的研究探索,大量的半导体材料被报道具有光催化活性。但是目前光催化材料的研究仍然面临限制其实际应用的两个重要问题。1)光响应范围窄,具有高活性的传统半导体,例如二氧化钛,其能带较宽,只能吸收占太阳能4%左右的紫外光,无法利用占太阳能大部分的可见光,造成太阳能利用率较低；2)量子效率低,光照下产生的光生电子和空穴在半导体内部或表面迅速发生复合,无法有效的参与光催化过程,造成量子效率较低。针对上述的两个问题,亟待发展新型高效的可见光催化材料,提高材料的光催化效率,对半导体光催化技术在环境和能源中的实际应用具有重要意义。贵金属表面等离子体增强效应可以有效的拓展宽禁带半导体光催化材料的可见光响应范围,提高材料的可见光催化活性；材料的晶体工程对其光催化活性有重要的影响,包括微结构调控,晶面工程、晶体内建电场及晶体内部不同配位体间的协同作用,能促进光生电子和空穴的有效分离,提高材料的量子效率。本论文的研究内容主要分三部份：1)针对具有可见光响应的等离子体光催化材料Ag@AgX,通过晶面工程,利用离子液水热法、湿化学氧化法、电化学氧化法制备了具有类球形、凹面立方块、三维枝状、立方块等不同晶面结构和微结构的Ag@AgX,并研究微结构与晶面活性对其可见光催化活性的影响；2)研究量子尺寸效应对窄带隙半导体纳米颗粒能带结构的调控,及其对材料光吸收和光催化活性的影响：3)探索新型光催化材料,研究其内建电场和不同电子结构配位多面体的协同效应促进光生电子和空穴分离,提高光催化效率的机理。具体的研究内容如下：在第一章中,首先简介了半导体光催化技术的发展、应用及其光催化机理。并总结了光催化材料研究面临的主要问题,及目前光催化材料的研究现状,提高光催化活性的新途径,及存在的问题。引出本论文的选题意义及主要研究内容。在第二章中,利用含Cl-/Br-离子液体辅助水热的方法制备具有弯曲表面的类球形AgCl/AgBr,发现离子液体中有机阳离子在晶核表面的空间位阻效应对AgCl/AgBr晶体的表面生长具有重要的影响。研究了不同碳链长度离子液体CxMimBr(x=4,8,12,16)对AgBr晶体表面结构的影响,并利用经典的台阶生长机理解释了弯曲表面的形成机理。弯曲表面存在大量原子台阶,具有很高的表面活性。由类球形AgCl/AgBr转化成的表面等离子体光催化剂Ag@AgCl/Ag@AgBr后,在降解有机污染物方面表现出高效的光催化活性。在第三章中,利用湿化学氧化法制备了具有凹面立方块结构的AgCl微米晶体。在制备过程中利用Na2C102作为氧化剂控制Ag片产生Ag+的速度,使立方晶核在生长过程中,表现出高活性方向上优先生长的习性。立方晶核在<111>和<110>方向的生长速度迅速,而<100>方向上的生长缓慢,形成具有<111>和<110>方向优先过渡生长的凹面立方块结构。凹面立方块结构的表面存在大量的高指数面,具有很高的表面活性,因此转化成凹面立方块催化剂Ag@AgCl后,紫外和可见光照射下,在光催化裂解水产氧和降解有机污染物中表现出较高活性。同时通过理论计算发现,当AgCl晶体表面吸附大量的C1-后,{111}、{110}和{100}晶面的表面能依次升高,其中{111}晶面的表面能最低。通过调节晶体生长的条件,可以控制生成的Ag+扩散到晶核的表面优先与吸附在晶核{111}晶面的C1-反应,使晶体沿<111>快速生长,而<110>和<100>方向上的生长缓慢,形成具有三维分等级枝状结构AgCl微米晶体,其具有较高的表面活性,在光催化产氧中表现出较高的活性。在第四章中,利用电化学阳极氧化的方法,在NaCl溶液中进行银片的阳极氧化,快速合成具有{100}晶面暴露的AgCl立方块结构。AgCl纳米立方块的生长机理可以通过界面动力学扩散生长的机理来解释。通过改变银片的反应位置和反应条件,也可以控制AgCl生长形成具有凹面结构的立方块。与无规则AgCl颗粒相比,立方块在光催化降解有机污染物中表现出较好的活性。同时控制Ag片在Na3PO4溶液中的电化学氧化反应,快速合成具有凹面结构的Ag3PO4微米晶体。通过晶体形貌的晶体学分析,晶核的生长表现出高活性方向优先生长的习性。Ag3PO4凹面微米结构中具有沿<111>和<110>方向的优先生长。凹面微米晶体的表面存在大量的高指数晶面暴露,具有较高的表面活性,与无规则Ag3PO4颗粒相比,具有较高的光催化活性。在第五章中,通过简单的一步共沉淀的方法将Ag2O纳米颗粒镶嵌到非晶SiO2中,利用SiO2来限制和保护量子尺寸的Ag2O纳米颗粒。样品的光吸收和能带结构可以通过量子尺寸来调节。当Ag2O纳米颗粒的尺寸为1.5nm时,Ag2O纳米颗粒具有较宽的可见光吸收范围和合适的能带结构,表现出最佳的光催化效率。同时,在以上工作的基础上,制备了新型高效光催化材料Ag6Si207。研究发现Ag6Si2O7具有很宽的可见光吸收范围,几乎包括了整个可见光区(λ<740nm)。并证明Ag6Si2O7几乎在整个可见光区都具有光催化活性。在Ag6Si2O7内部存在12种不等价的AgO配位多面体分别为二配位Ag(a)O2,三配位Ag(b)O3,和四配位Ag(c)O4。计算其偶极矩发现,在b-轴方向上存在剩余偶极矩,在晶体的内部存在沿b-轴方向上的极化电场。研究其电子结构发现,四配位Ag(c)O4与三配位Ag(b)O3基团间,以及三配位Ag(b)O3与二配位Ag(a)O2基团间存在电子转移。这些因素都有利于光生载流子的分离,提高Ag6Si2O7的光催化效率。在第六章中,对本论文的内容进行总结,并提出了本论文的创新点,并根据当前的研究现状提出了存在的问题及下一步工作的计划。总之,晶体的晶面结构和微结构具有不同的表面活性,对光催化材料的催化反应进行有重要的影响；量子尺寸效应能有效的调控光催化材料的能带结构,调节其光吸收范围和光生电子和空穴的氧化还原能力；晶体内建电场的构建,有利于光生电子和空穴的有效分离；晶体内部不同电子结构配位体之间存在电荷转移,抑制光生载流子的复合速率,提高光催化效率。更多还原

【Abstract】 Energy shortage and environment pollution are the two serious issues which greatly threat the development of human society. The key for solving the issues is to explore new clean and sustainable energy to alter the dependence on fossil energy. The development of novel semiconductor photocatalysis which can utilize the solar energy to split water for hydrogen evolution, and to decompose the organic contaminants and toxic heavy metal ions, has been the most possible path for solving energy shortage and environment pollution. Since1972, Fujishima and Honda firstly reported that the TiO2electrode can split water for hydrogen evolution under solar light irradiation, the semiconductor photocatalysis had been a hot research spot. After more than forty years’ development, various materials were found to have photocatalytic activities under solar light irradiation. However, there are still two issues existed in photocatalytic materials, which limit their practical applications in water splitting and environmental conservation. One is the narrow light absorption region; generally, the traditional photocatalysts with high efficiency always have a wide band gap leading to their only UV light absorption which is a small part of solar energy. The other one is the low quantum yield; when the electron-hole carriers are exited by incident light, they are easy to recombine rapidly, resulting in low percent of generated carriers which can transfer to surface of materials for photocatalytic reactions. For solving above two issues, one path is to explore novel photocatalysts with wide visible light absorption and high quantum yield leading to their useful practical applications in energy and environment.Surface plasmon resonance (SPR) of noble metal nanoparticles (NPs) is an efficient approach to enhance the visible light driven photocatalytic acitivity of wide band semiconductors, and the microstructures of plasmonic photocatalysts play a critical role on determining their photocatalytic efficiency. Under the action of electrical field, the recombination of photo-generated electron-hole carriers would be restricted efficiently. So, to construct an electrical field in internal structures of crystal is an efficient path to enhance the separation of carriers and to improve the quantum yield of photocatalysts. There are three main parts in this thesis:1) Fabrication and modulation of plasmonic photocatalysts Ag@AgX by using ionic liquid assisted hydrothermal, wet chemical oxidization, electrochemical oxidization methods, and various microstructures including near-spherical, concave cubic,3D hierarchical, cubic structures are obtained; to study the relationships between microstructures and their photocatalytic activities.2) To study the modulation of quantum sized effect on the band structures, optical properties and photocatalytic performance of narrow band gap semiconductors.3) Exploring novel highly active photocatalysts, to study the synergistic effect of the internal electrical field and coordination polyhedron on enhancement of carriers’ separations for improvement of photocatalysts’efficiency. Specific research contents are given as below:In chapter one, firstly we introduced the development, the mechanism and applications of semiconductor photocatalysis. Then, we summarized the main issues in study of photocatalytic materials and the latest progress of highly active photocatalysts including some paths for improving the efficiency of photocatalysts by means of SPR enhancement of metallic nanoparticles, construction of internal electrical field, and transfer of electrons between different coordination polyhedrons, and modulation of band structures by quantum sized effect. Finally, the significance of topic selection, the research thought and outline of the thesis were summarized.In chapter two, using ionic liquid containing Cl" or Br-as the stabilizer and X-resource to synthesize the AgCl microcrystal which had a near-spherical structures with convex surface. The organic cations of ionic liquid have a steric effect on the diffusion of Ag+ions which play a critical role on surface structures of AgX microcrystals. Ionic liquids with different length of carbon chains were used to synthesize AgBr microcrystals, with the increase of carbon chains, the surface of microcrystal became convex gradually. The classic atomic growth mechanism was used to illustrate the growth process of near-spherical microcrystals. With more active sites existed on convex surface, the plasmonic photocatalysts Ag@AgX with near-spherical structures exhibited higher activity than those of cubic ones.In chapter three, a novel wet chemical oxidization method was used to control the generation of Ag+by oxidization of Ag sheet in NaCl solution. The growth of AgCl cubic seeds exhibited a preferential overgrowth along<111> and<110> directions. Fast growth along<111> and<110> directions and slight growth along <100> directions resulted in the formation of cubes with concave structures. With more active sites existed on concave surface, the concave cubic Ag@AgCl expressed higher activity in oxygen evolution of water splitting and decomposition of organic containments. Based on calculations, when much Cl" absorbed on surface of AgCl crystals, the surface energy of{111} and{110} facets would be reduced rapidly to be lower than those of{100} facets. Due to the lowest surface energy of{111} facets, the growth of{111} facets will be faster than those of{110} and{100} facets. By controlling the reaction conditions, AgCl cubic seeds could grow fast along<111> directions, resulting in formation of3D hierarchical superstructures which exhibited more activity than those of concave cubic ones in oxygen evolution of water splitting under visible light irradiation.In chapter four, electrochemical oxidization method was used to oxidize Ag sheet in NaCl solution, and AgCl nanocubes were obtained in fast and continuous fashion. The interface kinetic diffusion mechanism was used to illustrate the growth process of nanocubes; the morphologies of AgCl could be tuned by change the reaction position of Ag sheets. Concave cubes could be obtained when the Ag sheet was put on the bottom of reactor. Compared with the irregular-shaped particles, the nanocubes exhibited higher active in decomposition of organic containments under visible light irradiation. When the reaction was carried in Na3PO4solution, the Ag3PO4concave microcrystals with preferential overgrowth along<111> and<110> directions were obtained. With high-index facets exposed on surface, the concave microcrystals exhibited more high efficiency in decomposition of organic pollutions under irradiation of visible light.In chapter five, we prepared quantum sized Ag2O nanoparticle embedded in amorphous silicate and novel highly active photocatalyst Ag6Si2O7and studied their applications in photocatalysis.Firstly, quantum sized Ag2O nanoparticles were embedded in amorphous silicates by using one-step co-precipitation method in solution. The silicate was used to restrict the growth and aggregation of Ag2O nanoparticles. Due to the quantum sized effect of Ag2O nanoparticles, the visible light absorption of photocatalyst Ag2O-SiO2and redox ability of photo-generated carriers could be tuned. When the size of Ag2O was1.5nm, the photocatalyst Ag2O-SiO2have a wide visible light absorption and proper band structures, leading to its optimal efficiency in photocatalysis.Secondly, a highly active photocatalyst Ag6Si2O7was obtained by the di-hydrolysis and ion-exchange process. Ag6Si2O7had a wide absorption regions (<740nm) and exhibited photocatalytic activity in nearly whole visible light region. There were12nonequivalence Ag-O coordination polyhedrons existed in the crystal structure of Ag6Si2O7, they are two-coordinated Ag(a)02, three-coordinated Ag(b)O3, and four-coordinated Ag(c)O4. Based on calculation, an internal polar electric field was constructed in Ag6Si2O7along b-axis which faciliate the separation of photo-generated electron-hole carriers. By the calculation of electronic structures, it was possible for the electrons to transfer between Ag(c)O4and Ag(b)O3, Ag(b)O3and Ag(a)O2. Both of those leaded to highly active of Ag6Si2O7in decomposition of organic containments under visible light irradiation.In chapter six, we summarized the research work, proposed the innovative points, and discussed the problems of this thesis. Finally, we proposed the research plan in the future work.In summary, microstructure of crystals had different surface activity which play a critical role on improvement of photocatalytic reaction; construction of internal electric field was advantageous for the separation of photo-generated electron-hole; transfer of electrons between coordination polyhedrons with different electronic structures also enhanced the photo-efficiency of photocatalysts; modulation of quantum sized effects on the band structures of narrow band gap semiconductors affected their optical absoption and redox abilities greatly.更多还原