二氧化钛光催化课件

Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University课堂讨论二报告人：廖才超1Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University目录目录2.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University能源问题当前的能源结构3.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University环境问题大气污染全球每年排放SO22.9亿吨，NOx约五千万吨，可吸入粉尘等等，导致酸雨，光化学烟雾，呼吸道疾病.4.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University环境问题5.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University环境问题6.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University现在问题来了！新能源技术哪家强？氢能源氢是一种热值很高的清洁能源，其完全燃烧的产物水不会给环境带来任何污染而且放热量是相同质量汽油的2.7倍。因而开发低能耗高效的氢气生产方法，已成为国内外众多科学家关注的问题。光催化光催化技术是有希望的技术，自从日本科学家Fujishima等于1972年首次发现在近紫外光（380nm）的作用下，金红石型TiO2单晶电极能使水在常温下分解为H2和O2以来，从光能量转换的角度出发，光催化 分解水制取氢气领域出现了大量的研究。7.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University什么是光催化光催化剂（一般为半导体材料）再光（可见光或者紫外光）的照射下，通过把光能转化为化学能，从而具有氧化还原能力，使化合物（有机物或无机物）被降解的过程称为催化。8.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University什么是光催化从光合作用这种最简单的光催化反应，总结一下光催化反应发生的三个基本条件：叶绿素-光催化剂光-特定波长范围（400-600nm之间最佳），非所有光都可以反应物-二氧化碳和水光催化反应的三个基本条件：u光催化剂-一般为半导体材料u光-特定波长范围，非所有光都可以u反应物-溶液中的水9.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University光催化技术的发展历史 1972年，Fujishima（藤岛）在N-型半导体TiO2电极上发现了水的光催化分解作用，从而开辟了半导体光催化这一新领域。1977年，Yokota（横田）T等发现了光照条件下，TiO2对环丙烯环氧化具有光催化活性，从而拓宽了光催化反应的应用范围，为有机物的氧化反应提供了一条新思路 近年来，光催化技术在环保，卫生保健，自洁净等方面的应用研究发展迅速，半导体光催化成为国际上最活跃的研究领域之一。10.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University光催化机理二氧化钛光解水制氢的反应历程二氧化钛光催化剂吸收光子，形成电子空穴对电荷分离并转移到表面的反应活性点上在表面进行化学反应，从而析出氢气和氧气 二氧化钛为n型半导体，其价带（VB）和导带（CB）之间的禁带宽度为3.0eV左右。当它受到其能量相当或高于该禁带宽度的光辐射时，半导体内的电子受激发从价带跃迁到导带，从而在导带和价带分别产生自由电子和空穴。水在这种电子-空穴对的作用下发生电离生成H2和O2。11.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University光催化机理n 可以想象一下，在分子的周围，形成了大量的光致电子和光致空穴，在光的照射下，它们不断产生有不断复合，但从宏观的角度看，在某一时刻，总是有大量的来不及复合的电子和空穴的存在，它们不断寻找自己的猎物。n 作为光致电子来说，它们的猎物是电子受体，这样光致电子就可以还原这个电子受体；n 而光致空穴迁移到表面后的猎物时哪些能够提供电子的物质，从而将这些物质氧化。n 在过程中产生的羟基自由基和超氧离子自由基等，这些自由基的氧化能力特别强，强化对污染物的氧化还原能力。n 光照时光子被半导体吸收，这是一个贮能过程。半导体多相光催化研究的主要内容是利用半导体材料的光敏性将太阳能或其他形式的光能，通过光催化反应转化为化学能（如光解水制氢）。12.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University常见半导体的能带结构13.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University常见光催化材料14.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University半导体光催化制氢的条件为了实现太阳光直接驱动水的劈裂，要求光催化材料具有：l 高稳定性、价廉；l 半导体的禁带宽度Eg要大于水的分解电压；l 能带位置要与氢和氧的反应电势相匹配：导带位置要负于氢电极的 反应电势，使光电子能量满足析氢反应要求。价带位置应正于氧电极的反应电势，使光生空穴有效的氧化水。l 高效吸收太阳光谱中的大多数光子。光子的能量还必须大于 半导体禁带宽度Eg：若Eg3v，则入射光波长应小于400 nm，只占太阳光谱很小一部分。15.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong UniversityTiO2光催化剂的优点1.水中所含多种有机污染物可被完全降解成CO2,，H2O等，无机物被氧化或还原为无害物2.不需要另外的电子受体3.具有廉价无毒，稳定及可重复利用等优点4.可以利用太阳能作为光源激活光催化剂5.结构简单，操作容易控制，氧化能力强，无二次污染16.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University光催化材料的改性目前的TiO2光催化剂存在两个问题：1.量子效率低 2.只能吸收紫外光，太阳能利用率低 贵金属沉积复合半导体离子掺杂修饰有机染料光敏化17.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University贵金属沉积沉积Ag后的TiO2光催化性能光生电子在Ag岛上富集，光生空穴向TiO2晶粒表面迁移，这样形成的微电池促进了光生电子和空穴的分离，提高了光催化效率。金属离子可捕获导带中的电子，抑制电子和空穴的复合，但是掺杂浓度过高，金属离子可能成为电子空穴复合中心。金属离子的掺杂浓度对TiO2光催化效果的影响通常呈现抛物线关系。18.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University复合半导体半导体复合的目的在于促进体系光生空穴和电子的分离，以抑制它们的复合，本质上可以看成是一种颗粒对另一种颗粒的修饰，其修饰方法包括简单的组合，掺杂，多层结构和异相组合，插层复合等。当不同半导体的导带和价带分别相连时，若窄禁带半导体的导带具有比TiO2更低的电势时，则在可见光激发时，光生电子向能级更正的导带迁移，而光生空穴迁向能级更负的价带，从而实现光生电子和空穴的分离。19.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University有机染料光敏化有机染料对TiO2的光敏化可以使光催化剂吸收的光波波长红移至可见光范围。染料分子吸收太阳光，电子从基态跃迁至激发态，只要活性物质激发态电势低于半导体的导带电势，光生电子就有可能输送到半导体的导带上，而空穴则留在染料分子中，有效的抑制了电子与空穴的复合，这些光敏化物质在可见光下有较大的激发因子，使光催化反应延伸到可见光范围。常用的光敏化物质有劳氏紫、玫瑰红等。20.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University离子掺杂修饰金属离子掺杂 Fe3+、Co2+、Cr3+非金属离子掺杂 碳、氮、硫及卤素1.掺杂可以形成捕获中心，价态高于Ti4+的金属离子捕获电子，低于Ti4+的金属离子捕获空穴，抑制电子-空穴的复合2.掺杂可以形成掺杂能级，使能量较小的光子能激发掺杂能级上捕获的电子和空穴，提高光子利用率3.掺杂可以导致载流子扩散长度增大，从而延长了电子和空穴的寿命，抑制复合。4.掺杂可以形成晶格缺陷，有利于形成更多的Ti3+氧化中心掺杂离子提高二氧化钛光催化效率的机制可以概括为以下几个方面：21.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong UniversityPhotocatalysis of Dye-Sensitized TiOPhotocatalysis of Dye-Sensitized TiO2 2 Nanoparticles with Nanoparticles with Thin Overcoat of AlThin Overcoat of Al2 2O O3 3:Enhanced Activity for H:Enhanced Activity for H2 2 Production Production and Dechlorination of CCland Dechlorination of CCl4 4Wooyul Kim,Takashi Tachikawa,Tetsuro Majima,and Wonyong Choi*,School of Environmental Science and Engineering,Pohang UniVersity of Science and Technology(POSTECH),Pohang 790-784,Korea,and The Institute of Scientific and Industrial Research(SANKEN),Osaka University,Mihogaoka 8-1,Ibaraki,Osaka 567-0047,JapanReceived:January 27,2009;Revised Manuscript Received:March 28,201122.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong UniversityIntroduction23.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University24.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong UniversityFigure 3 compares the XPS spectra of Al2O3/TiO2/Pt and TiO2/Pt in the Al 2p and Ti 2p bands.The Al 2p band of Al2O3/TiO2/Pt was positioned at 74.4 eV,which exactly matches that of Al 2p in pure Al2O3(74.4 eV).The intensity of the Ti 2p band in Al2O3/TiO2/Pt was reduced because of the presence of Al2O3 overlayer,but the Ti 2p binding energy(464.34,458.8 eV)in both Al2O3/TiO2/Pt and TiO2/Pt was identical to that of pure TiO2.25.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong University26.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong UniversityThis confirms that the presence of the barrier layer enhances the steady-state concentration of CB electrons in suspended TiO2 particles under irradiation.The higher concentration of CB electrons due to the hindered recombination should lead to the enhanced interfacial electron transfer.27.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong UniversityThe overall sensitization efficiency rapidly increases with the alumina loading and then reaches a maximum beyond which the efficiency decreases with increasing the barrier layer loading.The optimal loading was found at Al/Ti=0.009-0.018,and the activity was insignificantly low at Al/Ti=0.18.28.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong Universitywhich suggests that the presence of the alumina overlayer does not introduce defect sites thataffect the major charge recombination dynamics in TiO2 lattice.29.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong UniversityAn increase in by about 3 times in the presence of Al2O3 layer supports the idea that the barrier layer slows down the recombination.A slight increase in might be attributed to a shift from electron transport limited charge recombination to more interfacial electron transfer limited dynamics as suggested elsewhere.The initial absorbance of the dye cations thatwere generated within the laser pulse(at t=0)was not reduced at all by the presence of the alumina layer.This indicates that the electron injection yield was not affected at all by the thin layer of Al2O3(with Al/Ti=0.018).On the other hand,when the barrier layer was much thicker with Al/Ti=0.18,the dye cations were not generated at all,which indicates that the thick barrier layer prohibited the electron injection from the excited dye30.Xian Jiaotong UniversityXian Jiaotong UniversityXian Jiaotong Universityu Introducing a thin Al2O3 overlayer on the dye-sensitized TiO2increased activity for hydrogen production and the degradation of CCl4.u By measuring TDR spectra of bare TiO2 and Al2O3/TiO2 in slurry,the main role of the alumina barrier layer is proven to retard the charge recombination between the electron injected from the excited dye and the oxidized dye.u The yield of the electron injection from the excited dye to TiO2 CB was little affected by the presence of the alumina thin layer.u The proposed Al2O3/TiO2/Pt photocatalyst should serve as a model of more practical dye-sensitized photocatalysts.Conclusions31.