【作者】 黄垒；

【导师】 李忠； 彭峰；

【作者基本信息】 华南理工大学 ， 化学工程， 2009， 博士

【摘要】 TiO2具有无毒、稳定、廉价等突出优点,是光催化中应用最为广泛的一种半导体,但由于不能吸收可见光而制约了其对太阳能的有效利用。本论文采用窄带隙半导体Cu2O来敏化TiO2形成异质结构,提高TiO2在紫外光和可见光下的光催化活性;深入研究了Cu2O光催化失活的原因,考察了影响异质结构光催化效率的因素,并用原位X射线光电子能谱(XPS)技术对异质结构的界面能带结构进行了研究。采用湿化学法制备了不同形貌和颗粒大小的Cu2O,发现不同形貌Cu2O纳米颗粒对水中阴离子染料具有非常好的吸附效果,其吸附机理是静电吸附。对不同尺寸Cu2O颗粒的光催化降解行为研究则表明Cu2O纳米颗粒容易发生光腐蚀,其原因是Cu2O纳米颗粒表面Cu主要以+1价存在而容易被自身的光生空穴氧化成CuO,而Cu2O微米颗粒表面Cu主要以+2价存在而相对稳定;通过在反应液中加入适量空穴捕获剂(如甲醇)可以一定程度抑制Cu2O纳米颗粒的自腐蚀,从而同时提高光催化反应的效率和稳定性。采用研磨法和化学沉淀法分别制备了Cu2O/TiO2混合物和Cu2O/TiO2纳米异质结构,考察了影响它们光催化效率的因素。发现异质结构的界面对光催化效率有重要影响,由粒径为~3nm的Cu2O颗粒负载在P25(商品TiO2,粒径为20~30nm)上构成的纳米异质结构由于具有较好的接触界面而比Cu2O/TiO2混合物显现出更好的催化效果。Cu2O的含量对Cu2O/TiO2纳米异质结构(或混合物)的催化效果也有较大影响,紫外-可见光下负载少量Cu2O有利于TiO2活性的提高,Cu2O质量含量为30%时比同等条件下P25的催化效率提高了5倍多;而可见光下负载大量Cu2O则有利于TiO2催化活性的提高,Cu2O质量含量为70%时比P25的催化效率高出26倍。采用电化学沉淀法可控制备了不同形貌Cu2O与TiO2纳米管(TNT)阵列形成的异质结构光催化剂,考察了不同制备条件对Cu2O沉积的影响。发现电化学沉积过程中可以得到四种不同形貌的Cu2O,并可以通过改变TNT预处理条件、沉积电压和电解液条件对四种形貌的Cu2O进行调控;分析表明Cu2O的生长过程受动力学控制,八面双锥Cu2O只有在沉积电流密度大于-0.09 mA/cm2时才出现。具有晶型的纳米颗粒和八面双锥形Cu2O颗粒沉积进入TNT管结构中有利于光催化活性的提高,尤其是八面双锥形的Cu2O由于暴露有大量{111}晶面,在和TiO2纳米管结合后保持了非常好的吸附和可见光催化效果,而无定形Cu2O的沉积对TNT阵列的催化活性有抑制作用。采用“蒸发-氧化法”在具有不同缺陷浓度的TiO2表面上沉积薄层Cu2O,制备出不同厚度的Cu2O/TiO2异质结构界面,并同时用XPS对界面的电子结构进行了原位表征。发现Ar+溅射和不同温度下氧化可以得到洁净而含有不同缺陷浓度的TiO2表面,350℃以下表面氧缺陷和间隙Ti缺陷共存,600℃时可以得到近似化学计量比的TiO2表面;缺陷浓度对Cu在其表面的氧化过程有重要影响:缺陷浓度越高,沉积在表面的Cu越容易被氧化,Cu2O在350℃下热处理的TiO2表面上会发生岐化反应生成Cu和CuO;TiO2中缺陷浓度对界面能带结构也有重要影响:缺陷浓度越小,界面能带弯曲越多,从而产生更小的导带不连续。以上结论对于设计高效的异质结构具有非常重要的指导意义。更多还原

【Abstract】 As one of the most famous and widely used semiconductors, TiO2 owns certain outstanding advantages, such as non-toxic, stable and low-cost, etc. However, its efficiency of utilizing solar energy is heavily limited due to the restricted absorbance of visible light. In this thesis, Cu2O with narrower band gap was used as a sensitizing semiconductor to couple with TiO2 to form Cu2O/TiO2 heterostructure, which exhibited highly enhanced photocatalytic efficiency (PE) both under UV and visible light irradiation. The photocorrosion of Cu2O and the factors that affect the activity of the heterostructures were studied, and the band structure at the interface of the Cu2O/TiO2 heterostructure was also characterized by“in situ”X-ray photoelectronic spectra (XPS).Cu2O particles with different morphologies and sizes were successfully prepared by wet chemical method. It was found that the Cu2O particles with different morphologies owned highly selective adsorption of anionic dyes due to electrostatic adsorption. While the Cu2O particles with different sizes were found to exhibit different stabilities during the photocatalytic process. The results suggested that Cu2O nanoparticles were easy to be photo-oxidized to CuO by photo-induced holes since Cu1+ existed on the surface. Comparatively, Cu2O micro-particles were more stable in photocatlaytic process due to existing Cu2+ on the surface. It was also suggested that the addition of hole scavenger (eg. methanol) in the pollutant solution could restrain the photocorrosion of Cu2O nanoparticles and promote the PE meanwhile.Cu2O/TiO2 hybrid and Cu2O/TiO2 nano-nano heterostructures were successfully prepared by physical milling and chemical precipitation method, respectively; the factors that affect PE were studied and summarized as follows. Firstly, the interfaces between the two semiconductors were very important to the heterostructure, Cu2O/TiO2 nano-nano heterostructure composed of Cu2O nanoparticles (~3nm) and P25(commercial TiO2, 20~30nm ) had much better PE than that of Cu2O/TiO2 hybrid due to higher contact area between the two semiconductors. Secondly, the content of Cu2O had extraordinary influence on PE of Cu2O/TiO2 nano-nano heterostructure (or hybrid), less Cu2O content was better in UV-visible light, while higher Cu2O content was better in visible light; the heterostructures with Cu2O contents of 30wt.% and 70wt.% possessed of the highest photocatalytic activities under UV-vis light and visible light irradiation, which were almost 5 times and 26 times higher than that of P25, respectively. Cu2O/TiO2 nanotube (TNT) arrays heterostructures were successfully prepared controllably by electrochemical precipitation, and the preparing conditions influencing on the precipitation process were studied. Four different types of Cu2O could be obtained by adjusting the pretreatment conditions of TNT arrays, the reduction potentials and the electrolyte conditions. The results indicated that the growth of Cu2O was controlled by kinetics, octahedral Cu2O just can be obtained at the current density higher than 0.09mA/cm2. Depositing crystallized Cu2O nanoparticles and octahedral particles into TiO2 nanotube structures favored the promoting of PE. Eespecially, Cu2O octahedral particles had significant photocatalytic and adsorptive activities due to the exposure of {111} facets; whereas, amorphous Cu2O decreased PE of the heterostructure.Thin Cu2O layers were successively deposited on rutile TiO2 (001) single crystal surfaces that had different levels of defect concentrations to form Cu2O/TiO2 heterojunctions, and the resultant band bending and offset characteristics were studied by“in situ”X-ray photoelectron spectroscopy (XPS). It was found that the concentrations of defects could be controlled by the oxidizing temperature. Oxygen vacancies and interstitial Ti3+ defects co-existed in the 350℃treated surface, while near TiO2 stoichiometry surface was obtained at the temperature of 600℃. The defect concentrations were found very important to the oxidization process of Cu on the TiO2 surface. The higher the defect concentration was, the easier the oxidization of Cu was. Cu2O was found to disproportionately decompose to CuO and Cu on the 350℃treated surface. Furthermore, the presence of the defects in TiO2 surface dramatically influenced on the band bending and band offset at the interface; the less defects in TiO2 substrate was, the larger band bending values on the TiO2 side were observed, inducing smaller conduction band offset. This conclusion will help to design more effective Cu2O/TiO2 heterojunctions for solar energy conversion.更多还原