【作者】 吕建国；

【导师】 孙兆奇；

【作者基本信息】 安徽大学 ， 材料物理与化学， 2011， 博士

【摘要】 ZnO是一种重要的宽禁带半导体光电材料,同时又具有光催化和光诱导表面浸润性可逆转变,在气敏传感器,太阳能电池,激光二极管、污水处理、空气净化、防雾和自清洁等诸多领域具有广泛的应用前景。所以,通过掺杂和优化制备工艺条件等手段改善ZnO薄膜的光电性能,增强其光催化活性,提高其接触角转变范围以实现超亲水/超疏水可逆转变已成为材料科学和化学领域的重要研究内容。本文采用溶胶凝胶技术在不同衬底(硅片和石英玻璃)上制备了未掺杂和Na掺杂ZnO薄膜,利用X射线衍射仪(XRD)、X射线光电子能谱仪(XPS)、原子力显微镜(AFM)、扫描电镜(SEM)、紫外可见分光光度计(UV-Vis)、荧光光谱仪(FL)和接触角测试仪等测试分析薄膜的微结构、组分、表面形貌、紫外可见透射谱、光致发光谱和表面接触角等,以甲基橙溶液为模拟污染物,研究薄膜的光催化活性。全面系统地研究预处理温度、退火温度和时间和Na掺杂量等对薄膜的微结构、表面化学成分及其价态、表面形貌、紫外可见透射谱及其光学带隙、光致发光带、表面浸润性及其可逆转变、光催化活性等的影响。主要结论如下：利用XRD、AFM、UV-vis和FL等研究预处理温度和退火时间对Na掺杂ZnO (Na:Zn=0.08)微结构、表面形貌、紫外可见透射、光学带隙和光致发光谱等的影响。微结构和表面形貌研究结果显示,预处理温度为150℃和250℃的Na掺杂ZnO薄膜出现明显的c轴择优取向,而预处理温度为200℃的薄膜则未表现出择优取向。在退火温度相同的情况下,随预处理温度的升高,薄膜的平均晶粒尺寸和表面rms粗糙度均逐渐减小。当退火温度为400℃,不同退火时间薄膜的XRD图谱中未观察到钠及其氧化物的衍射峰,随着退火时间的增加,薄膜衍射峰的强度逐渐减小；当退火温度为600℃和800℃时,经90min和120min退火处理的Na掺杂ZnO薄膜中出现了Na02(101)衍射峰。利用紫外可见透射谱计算出薄膜的光学带隙,对于退火温度相同的Na掺杂ZnO薄膜,退火时间越长,Eg值越小；而对于退火时间相同的薄膜,退火温度越高,Eg值越小。PL谱研究结果显示,预处理温度较低时,退火温度对薄膜的PL谱影响较大,高退火温度薄膜中出现了与缺陷有关的黄绿和蓝紫发光峰；预处理温度较高时,退火温度对薄膜的PL谱影响不大。对于相同退火温度的Na掺杂ZnO薄膜,不同退火时间的薄膜均出现相同光致发光带,只是发光强度和峰位有所变化。利用XRD、XPS、SEM、AFM、UV-Vis、FL和接触角测试仪对未掺杂和Na掺杂ZnO薄膜(预处理温度和退火时间分别为150℃和60min)的微结构、组分、表面形貌、光学带隙、光致发光、表面浸润性和光催化活性进行测试分析。XRD结果显示,不同Na掺杂ZnO薄膜的所有衍射峰均与六角纤锌矿结构ZnO的标准谱相对应,当Na:Zn原子比为0.08时,该薄膜出现了明显的c轴择优取向,该结果表明可以通过改变薄膜中的Na含量来调节薄膜的择优取向。表面形貌分析结果显示,对于相同退火温度Na掺杂ZnO薄膜,Na含量较低时,相对较小的颗粒均匀而致密分布在薄膜表面；当Na含量较高时,颗粒之间的间隙增多,致密度下降,平均颗粒尺寸增大。XPS分析结果显示,Na1s峰的半高宽随Na:Zn的增加先增大后减小,中心峰位值逐渐减小,说明在较高Na：Zn原子比薄膜中,Na离子可能替代了晶格Zn而成为受主NaZn。Zn2p3谱和ZnLMM俄歇谱研究结果表明大部分的Zn元素以Zn2+离子形式存在于薄膜中,而也有少量的Zn元素以单质Zn的形式出现在薄膜中。紫外可见透射谱的研究结果表明,所有样品的光学带隙均小于块材的光学带隙(3.37eV),该结果归因于晶格失配和偏离化学计量比等局域缺陷产生的带尾,对于相同退火温度Na掺杂ZnO薄膜,随着Na掺杂量的增加,Eg在一定范围内波动。光致发光谱研究结果显示,经400℃退火处理Na掺杂ZnO薄膜在380-420nm波段范围内出现紫外/紫光发光峰,在可见光波段出现蓝光发射和黄绿发光峰,随着Na掺杂量的增加,紫外/紫光发射峰先红移后蓝移；经800℃退火处理薄膜出现很强的黄绿发射带,随着Na掺杂量的增加,黄绿发光带的峰值在540-570nm之间波动。研究了经600℃和800℃退火处理Na掺杂ZnO薄膜的表面浸润性及其可逆转变,结果显示,光照之前,薄膜的接触角较大,而对薄膜进行紫外光照处理—段时间后,薄膜的接触角逐渐减小,表现为亲水性,归一化接触角减速比与薄膜中的Na掺杂量密切相关；将60min光照后表现为亲水性的薄膜在黑暗处放置7天或者在大气氛围中经200℃热处理60min后,薄膜又恢复到光照前的浸润状态。研究了不同退火温度ZnO薄膜的光催化活性,结果表明,随着退火温度升高,薄膜的光催化降解效率由73.4%增大到88.0%；对于不同的反应时间,光催化降解的表观反应速率分别由0.0056min-1增大到0.0071min-1和从0.0084min-1增大到0.0149min-1。研究了Na掺杂量对600℃退火处理Na掺杂ZnO薄膜光催化活性的影响,结果显示,随着Na:Zn原子比的增加,薄膜的光降解效率先增大后减小,在Na:Zn=0.08时出现最大的光降解效率,为89.1%。本学位论文的创新点在于：1、深入系统地研究Na掺杂量、预处理温度以及退火温度和时间对ZnO薄膜的微结构、表面形貌和光学性质的影响,探讨可能光致发光机理,为ZnO光致发光机理研究提供实验基础。2、研究了未掺杂和Na掺杂ZnO薄膜的表面浸润性及其光诱导表面浸润性可逆转变,并对其形成机理给予合理的解释。3、研究了未掺杂和Na掺杂ZnO薄膜的光催化性能,并对其光催化机理给出合理的解释。4、所制备的未掺杂和Na掺杂ZnO薄膜具有表面浸润性可逆转变特性,可对模拟污染物进行有效降解,在紫外、蓝光和黄绿光区域具有光致发光带,因此,本学位论文的研究工作在光电子器件、污水处理和自清洁等领域具有潜在的应用前景。更多还原

【Abstract】 ZnO is an important semiconductor, with wide direct band gap, which has a broad range of applications in gas sensor, solar cell, laser diode, water treatment, air purification, antifogging materials, and self-cleaning. Therefore, perfecting photoelectric properties, enhancing photocatalytic activity and improving range of reversible transition by means of doping and optimization of preparation, have become important research subject in the areas such as photochemistry and material science.In this paper, undoped and Na-doped ZnO thin films have been prepared by sol-gel method on silicon and quartz substrates. Microstructure, composition, surface topography, transmission spectra, photoluminescence spectra and contact angle have been characterized by X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), scanning electron microscope (SEM), ultraviolet-visible (UV-Vis) spectrophotometer, fluorophotometer (FL) and contact angle apparatus. Methyl orange (C14H14N3NaO3S), a widely used dye, was employed as a representative dye pollutant to evaluate the photocatalytic activity of thin films. The effects of preheating temperature, annealing temperature and Na doping on microstructure, surface chemical composition and valence state, surface topography, transmission spectra and optical band gap, photoluminescence zone, surface wettability and photocatalytic activity were studied in detail. The main results of the study are as follows:The effect of preheating temperature and annealing time on microstructure, surface morphology, transmission spectra, optical band gap and photoluminescence spectra of Na-doped ZnO (Na:Zn=0.08) thin films were studied by XRD, AFM, UV-vis and FL. XRD and AFM results show that the thin films preheated at 150℃and 250℃have a high preferred c-axis orientation, but the thin film preheated at 200℃without preferred c-axis orientation. Surface rms roughness and average grain size of the samples annealed at the same temperature decreases with an increase of preheating temperature. When the annealing temperature is 400℃, no peaks corresponding to either Na metal or any of its oxides are observed in XRD patterns for the thin films with different annealing time. The diffraction peaks intensity of the thin films decreases with increases annealing time. As annealing time increase to 90min and 120min, the thin films annealed at 600℃and 800℃show NaO2(101) peak. UV-Vis results indicate that the optical band gap Eg decreases with increasing annealing time. Eg decreases with increasing annealing temperature. PL spectra results indicate that the annealing temperature has large effect on PL spectra when the preheating temperature is lower. The higher annealing temperature thin films have violet-blue emission and broad green-yellow emission, which are connected with film defects. However, the annealing temperature has little effect on PL spectra when the preheating temperature is higher. The results also indicate that the annealing time have little effect on PL spectra for the Na-doped ZnO thin films with the same annealing temperature, which have the same photoluminescence zone with different intensity and peak position.Microstructure, composition, surface topography, optical band gap, photoluminescence spectra, contact angle and photocatalytic activity of undoped and Na-doped ZnO thin films (preheating temperature and annealing time are 150℃and 60min, respectively)were investigated by XRD, XPS, AFM, SEM, UV-Vis, FL and contact angle apparatus. XRD results show that all the peaks in the pattern correspond to ZnO hexagonal wurtzite structure. The Na-doped ZnO thin film (Na:Zn=0.08) have significant (002) preferential orientation which indicates that moderate Na-doping can enhance the ZnO (002) preferential orientation, but excessive Na-doping will deteriorate it. Surface topography results indicated that as Na content is lower, smaller particles distributed homogeneously and compactly on the surface of the thin films annealed at the same temperature. As Na content increases, average particles size and gap of particles increase and density of the thin film decreases. XPS results show that as Na:Zn increase from 0.0 to 0.10, full width of half maximum of Nals increases at first and then decreases and peak position decreases. These indicate that Na+ions may be occupied substitutional sites and generate acceptor Nazn.Zn2p3 and ZnLMM Auger spectrum indicate that Zn element exist mainly in form of Zn2+ ions but slightly in form of Zn elementary substance. The optical band gaps of all the thin films are smaller than the direct band gap of ZnO bulk (3.37eV) at room temperature. This result may be explained by the band tail which is composed of defect localized states both at the bottom of conduction band and the top of the valance band. For Na-doped ZnO thin films annealed at the same temperature, with the increasing of Na doping, Eg fluctuate in a certain range. For the Na-doped ZnO thin films annealed at 400℃, UV-violet emission band around 380-420nm, blue emission band and green emission band are observed. The UV-violet peak redshifts first, and then blueshifts with the increases of Na content. For the thin films annealed at 800℃, a strong green-yellow emission band fluctuate in a range of 540-570nm with increasing Na content.The effect of Na content on surface wettability and reversible transition of undoped and Na-doped ZnO thin films annealed at 600℃and 800℃was studied in detail. The results show that the contact angel of the un-irradiated thin films is larger. Under UV irradiation all samples exhibit a light induced transition. The contact angle reduction rate is closely connected with Na content. It must be pointed out that the changes in wettability of the thin films are reversible. After the UV irradiated films were placed in dark for seven days (or annealed at 200℃for 60min) in ambient conditions, a new water droplet was used to measure the surface wettability, and the initial wetting state was obtained again. The effect of annealing temperature on photocatalytic activity of ZnO thin films was investigated. The results show that photocatalytic degradation efficiency of the films increases from 73.4% to 88.0% with increasing annealing temperature. When the annealing temperature increase from 400 to 800℃, the apparent reaction rate increase from 0.0056min-1 to 0.0071min-1 for the reacting process in the range of 0-60min and increase from 0.0084 min-1 to 0.0149 min-1 for the reacting process in the range of 80-180min. The effect of Na content on photocatalytic activity of undoped and Na-doped ZnO thin films annealed at 600℃was studied in detail. The results indicate that photocatalytic degradation efficiency of the thin films increases first and then decrease with increasing Na:Zn. The maximum value is 89.1% for the thin film with Na:Zn=0.08.The main innovations of this paper are as follows:1. The effect of Na doping, preheating temperature, annealing temperature and time on microstructure, surface topography and optical properties are studied in detail. The possible mechanisms of photoluminescence of the thin films have been investigated. This research provides experimental basis for mechanism study of photoluminescence of ZnO.2. Surface wettability and reversible transition of undoped and Na-doped ZnO thin films have been studied, and the corresponding mechanisms have been analyzed.3. For the photocatalytic activity of undoped and Na-doped ZnO thin films have been investigated, and the corresponding mechanisms have been suggested.4. For the undoped and Na-doped ZnO thin films, reversible transition of surface wettability is obvious. Photocatalytic degradation of contaminated water is effective. UV emission, blue emission and green-yellow emission are observed. Therefore, the researches of this paper have potential applications in optoelectronic device, water treatment, and self-cleaning.更多还原