【作者】 肖信；

【导师】 张伟德；

【作者基本信息】 华南理工大学 ， 应用化学， 2011， 博士

【摘要】 能源短缺和环境污染是当前人类面临的重大挑战。光催化技术可以将低密度的太阳能转化为高密度的化学能和电能,直接利用太阳光降解和矿化水与空气中的各类污染物,并且能够实现光解水制氢。因此,光催化技术在环境净化和新能源开发方面具有广阔的发展前景。光催化技术的核心是光催化剂,目前发现的具有光催化活性的物质主要是金属氧化物和硫化物等半导体化合物。为了拓展光催化剂的光响应范围以便充分利用太阳光和进一步提高光催化效率,开发新型和高效的可见光光催化材料已经成为了当前的研究重点和热点。卤氧化铋BiOX （ X= F, Cl, Br, I）作为一类新型的光催化剂,由于具有特殊的层状结构和内部电场,能够有效地实现光生电子-空穴对的分离,从而显示出优异的光催化性能。BiOX的带隙随X从F到I逐渐减小,BiOI具有最小的带隙,呈现出极佳的可见光光催化活性。更有意思的是,除了BiOX外,Bi、O和X三种元素还能够以不同配比组成一系列更加复杂的化合物,由于BiOX系列的价带位置主要由X和O元素所决定,因此可以通过调配BiOX中非金属元素的配比来设计新型的具有合适能带结构的新材料。另一方面,光催化剂的微观结构,特别是尺寸、三维结构、表面形貌、晶面比例、孔道等对材料的性能、效率和应用具有极大的影响。本文的工作致力于新型碘氧化铋系列光催化剂的合成、表征和可见光光催化降解水中环境污染物的研究,特别注意通过使用不同的合成方法和实验参数对材料形貌和结构的控制合成,研究其形成机理,以及材料微结构对光催化活性的影响等。本论文的主要研究工作如下:1.使用乙醇-水混合溶剂,以氨水调节体系的pH值,在低温条件下合成了均匀的具有3D结构的BiOI微球。分别通过XRD、SEM、TEM、DRS、BET、TOC和光催化降解等手段对所合成材料的组成、结构、形貌、光学性质、比表面积和光催化性能等性质的表征和检测,探讨了BiOI 3D微球的形成机理。结果表明,3D BiOI微球由纳米片通过自组装形成,乙醇-水混合溶剂和体系pH值对3D结构的形成起重要作用。以苯酚作目标降解物,在可见光照射下,3D BiOI微球的光催化活性明显优于纯水体系合成的随机片状BiOI。这是由于3D BiOI微球具有小的带隙、高的比表面积和表面/体积比率,因而呈现出更高的可见光光催化活性。对降解产物的TOC分析表明,所合成材料在可见光条件下能够有效地矿化苯酚。更重要的是,所合成的材料可以很容易地回收和重复使用,因而具有良好的实际应用价值。2.使用多元醇法,在没有添加模板或表面活性剂的条件下成功地合成了一种具有分级微/纳米结构的新型碘氧铋材料。使用多种手段和技术对所合成材料进行了表征,XRD、XPS和TG的表征结果显示所合成的材料为Bi₇O₉I₃;SEM结果揭示了这种材料为平行四边形的微米级片状结构,这一微米片表面存在大量的高密度纳米片;DRS结果显示新材料的带隙略高于BiOI,具有强烈的可见光吸收。通过时间序列实验研究推测了Bi₇O₉I₃的结构和形貌的形成机理,结果显示其形成机理为Ostwald熟化过程。在可见光条件下,以苯酚为降解物研究Bi₇O₉I₃的光催化性能,结果显示材料具有良好的可见光光催化活性,这是由于所合成Bi₇O₉I₃具有合适的带隙、更强的氧化能力、高的比表面积、高的表面/体积比率和三维孔道等特性所决定的。回收实验表明,所合成的Bi₇O₉I₃材料可以方便地回收,具有良好的性能稳定性,具备实际应用前景。本实验首次通过简单的合成方法,一步地获得了具有复杂组成的、分级微/纳米结构的Bi₇O₉I₃材料,对于进一步理解和设计新型高效的卤氧化铋系光催化剂,具有重要的意义。3.使用一锅微波化学法,以乙二醇为溶剂,以Bi（NO₃）₃作铋源,分别加入KI和CTAB,在极短的反应时间（3分钟）内,成功地合成了结晶良好的,具有三维分级微纳结构的Bi₇O₉I₃片、BiOI微球和BiOBr花状结构。使用XRD、SEM等技术对所合成材料的组成、结构和形貌进行了相应的表征,讨论了其可能的形成机理,以及不同实验参数对产物的组成和形貌的影响。在可见光照射下,以苯酚作为目标模拟降解物研究了所合成材料的光催化活性,结果发现Bi₇O₉I₃存在最佳可见光光催化性能,其次为BiOI,BiOBr的性能最差。材料表现出来的这种光催化活性的差异,与其能带结构密切相关,合适的光催化剂应该具有相对较强的氧化能力和较强的光吸收能力,本实验表明,Bi₇O₉I₃和BiOI即属于这类材料,而又以Bi₇O₉I₃更佳。另一方面,三维分级微纳结构具有较大的比表面积、高的表面/体积比和抗团聚能力,以及大量的三维多孔通道等优势也是材料呈示出来的高的可见光活性的原因。所合成Bi₇O₉I₃材料易于回收,在多次重复使用后,其结构和性能基本不变,具有良好的实际应用价值。本实验对于合成复杂的分级微纳材料、通过调节非金属原子的配比从而调节半导体材料的能带结构和合成复杂的卤氧化物体系,具有极好的启发作用。更多还原

【Abstract】 Energy shortage and environmental pollution are two major challenges currently faced by human beings. Semiconductor photocatalytic materials show great potential for solar energy conversion and environmental protection, and have been widely applied in air and water purification, H₂ production by water splitting as well as dye-sensitized solar cells, etc., which have attracted increasing attention over the past two decades. However, the performance of most photocatalysts requires to be further improved due to their narrow light-response range as well as low quantum efficiency. Therefore, design and development of new and highly efficient visible light photocatalytic materials have become the focus of current research.Recently, bismuth oxyhalides （BiOX, X = F, Cl, Br and I） have drawn much attention because of their possessing unique structure of alternate [Bi₂O₂]²⁺ sheets with the X- slabs and the internal electric fields between positive slabs and anionic slabs, which are more effective in improving the separation of photoinduced electron-hole pairs, and therefore have demonstrated excellent photocatalytic activities and are offering a new family of promising photocatalysts. Among them, BiOI has the strongest absorption in the visible light region because it has the smallest band gap （¹.8 eV）. Except for the BiOI, there are other complex bismuth oxyiodide compounds, which have a different ratio of Bi to I, including Bi₂O₄I₅, Bi₇O₉I₃, -Bi₅O₇I andβ-Bi₅O₇I. Since the valence band in BiOX is mainly composed of Bi 6s, O 2p and X 5p orbits, while the conduction band is based on Bi 6s and Bi 6p orbits, the bandgap energy of bismuth oxyiodides may gradually increase by substitution of I with O in their structures, resulting in a lower bandgap than that of Bi₂O₃ （².8 eV）. Consequently, all these kinds of bismuth oxyiodides are expected to demonstrate visible light photocatalytic activity and it is also interesting to obtain the different oxidation-reduction ability by tuning the ratio of Bi/I.In addition to its electronic and band structure of the semiconductor, microstructures of the materials, especially in size, three-dimensional structures, surface morphology, the ratio of crystal planes, porous and internal channels, are obviously influencing the performance, efficiency and applications of a specific photocatalyst. Generally, photocatalysts with three-dimensional （3D） micro/nano-structures based on the assembly of low-dimensional building blocks are expected to have enhanced photocatalytic performance although the synthesis of such complex superstructure remains a challenge for nanotechnology.In this dissertation, we focus on design and development of new and highly efficient bismuth oxyiodides as photocatalysts with visible light response and 3D hierarchical micro/nano-structures. The resulting materials were characterized carefully by various techniques, their structural formation mechanism was discussed in detail, and their photocatalytic activities were tested under the visible light irradiation.The main contents were shown as the following:1. Self-assembled three dimensional （3D） BiOI microspheres consisting of nanoplatelets were synthesized at low temperature using ethanol-water mixed solvent as reaction media and NH3·H2O as pH an adjustment. The as-prepared BiOI was characterized by XRD, SEM, TEM, UV-vis DRS, and BET. The possible formation mechanism for the architectures was discussed. It was found that the mixed solvents and alkali play key roles in the formation of the BiOI microspheres. The photocatalytic activity of the as-prepared sample was evaluated by degradation of phenol in water under visible light irradiation. The 3D BiOI microspheres show much higher photocatalytic activity than the random BiOI platelets. The TOC measurement after the degradation process indicated that phenol was effectively mineralized over the BiOI microspheres. In addition, the BiOI microspheres are stable during the reaction and can be used repeatedly. The high catalytic performance of the 3D BiOI microspheres comes from their narrow band gap, high surface area and high surface-to-volume ratio.2. A novel Bi₇O₉I₃ material with hierarchical nano/micro-architecture is successfully synthesized by a one-step, template and surfactant-free solution method. The as-prepared product was characterized by various techniques. XRD, XPS and TG measurements confirm that the composition of the as-fabricated sample is Bi₇O₉I₃. SEM and TEM observation reveals that the as-synthesized sample is microsized plate-like structure with dense nanosheets standing on their surfaces. The time-dependent morphology of the Bi₇O₉I₃ sample was investigated, and a possible formation mechanism of the hierarchical structure is proposed. More importantly, the Bi₇O₉I₃ exhibits an excellent photocatalytic activity than that of BiOI towards degradation of phenol under visible light irradiation. The high catalytic performance of the Bi₇O₉I₃ hierarchical structure comes from its electronic band structure, high surface area and high surface-to-volume ratio. In addition, the Bi₇O₉I₃ hierarchical architecture is stable during the reaction and can be used repeatedly. The present work not only gives insight into understanding the hierarchical growth behaviour of complex bismuth oxide iodides in a solution-phase synthetic system, but also provides a new way to improve the photocatalytic performance by designing desirable structures and morphologies.3. The hierarchical Bi₇O₉I₃ microplates, BiOI microspheres and BiOBr flowers were successfully synthesized via a facile, rapid and reliable microwave-assisted one-pot method, employing Bi（NO₃）₃·5H₂O, KI and CTAB as starting reagents without using any other additives. The as-prepared powders were characterized by XRD, SEM, and UV-vis DRS. The compositions, structures, morphologies and influential factors of the products were discussed based on the additional experiments. The photocatalytic activities of the as-prepared samples were evaluated towards the degradation of phenol solution under visible light irradiation. It was found that under the same conditions, Bi₇O₉I₃ microplates exhibited the best activity, BiOI microspheres showed the second and BiOBr flowers presented the worst. The high catalytic performance of the Bi₇O₉I₃ microplates comes from its relatively strong oxidative capacity, wider light-response range as well as hierarchical nano/micro-architecture. And its structure and activities remain unchanged after repeated four times. This work provides a facile, rapid, low-energy consumption route to prepare novel BiOX hierarchical architecture.更多还原