【作者】 宋连香；

【导师】 钱逸泰； 占金华；

【作者基本信息】 山东大学 ， 无机化学， 2010， 博士

【摘要】 发光性能随组成、结构变化是零维半导体纳米材料的独特性能之一。与传统有机荧光染料相比,半导体量子点具有独特的优势：荧光产率高,光学稳定性好。表面修饰量子点作为全新的荧光指示器可应用于不同的生物分析和生物过程。表面修饰不同的有机配体用于水溶液中金属离子检测则使量子点成为全新的化学传感器。半导体光电技术可以将太阳光转变为化学能和电能。半导体纳米材料光催化技术在环境净化方面的应用,是纳米半导体材料另一独特性能。半导体材料直接利用太阳光降解空气和水中的污染物,使得半导体材料在环境治理中的充分利用具有巨大的应用前景。光催化技术经过多年的研究已经取得了一定的进展,各国研究者亦一直致力于寻找性能优良的光催化材料,并在光催化理论方面积累了深厚的基础,但是目前仍然存在许多问题需要进一步的探索和研究。本论文是在总结现有文献的基础上致力于运用不同Ⅱ-Ⅵ族半导体固体结构的结晶学相关特性和功能特性,通过溶剂热、微波加热等合成方法制备带隙可调的Ⅱ-Ⅵ族半导体纳米材料,以改进或改善纳米材料的物理、化学、光学性能并探索其在重金属离子检测和光催化方面的应用。主要包括以下四个方面的工作,归纳如下：1.强发光CdSe纳米晶的低温水相合成及其光学性质我们以氯化镉(CdCl2·3／2H2O)和无水亚硒酸钠(Na2SeO3)分别为镉源和硒源,巯基丁二酸(HOOCCH(SH)CH2COOH, MSA)为稳定剂和配体,柠檬酸三钠(C6H5Na3O7·2H2O)为辅助配体,硼氢化钠(NaBH4)为还原剂,低温下合成水溶性、强发光的CdSe量子点,实验中发现反应温度和加热速度对CdSe荧光量子产率具有显著影响。实验中首先通过比较水浴加热和微波加热制备的纳米晶的荧光发光效率高低来确定合适的制备CdSe纳米晶的合成方式。分别在25℃,30℃,35℃,40℃,45℃，50℃和60℃通过水浴加热和微波加热制备了一系列CdSe纳米晶。从实验结果可知,40℃左右水浴加热是获得发光效率高的CdSe纳米晶的适宜方式,得到的CdSe纳米晶的荧光量子产率达到30%。而更低或更高的加热温度则不利于发光效率的提高,当温度升高至80℃以上,不论水浴加热还是微波辐射加热,所得CdSe纳米晶的荧光量子产率迅速降低至3%以下。迅速加热的微波辐射方式不利于获得发强光的CdSe纳米晶,快速加热方式下制备CdSe纳米晶的吸收光谱逐渐失去明显的激子吸收峰,但是吸光度却逐渐增强。实验中得到的CdSe纳米晶是非晶态。25℃至60℃制备的纳米晶的XRD谱图皆无明显的衍射峰,高分辨电镜下亦无其清晰的衍射条纹。虽然高温下(120℃)得到的纳米晶有良好的结晶,但是其荧光发光效率极低。实验结果表明低温下的缓慢生长有利于提高CdSe纳米晶的发光效率。利用微波辐射将加热温度提高至120℃时,CdSe纳米晶的荧光发射峰表现出明显的激子发射和缺陷发射的特征。结合文献报道情况,我们认为制备的CdSe纳米晶的荧光发射属于缺陷发光。2.一次法微波辅助合成荧光发光可调的均匀CdSexTe1-x纳米晶合金以荧光性能优异、广泛研究的CdTe量子点作为研究对象,在合成高质量的二元组成的发光CdTe量子点的基础上,通过CdTe和CdSe的合金化制备CdSe发光量子点。利用硼氢化钠作为还原剂制备了水溶性的CdSexTe1-x量子点,并对量子点的发光性能与组成相关性进行了详细研究。影响半导体纳米晶的荧光效率的因素很多,如纳米晶的结晶性,表面稳定剂,溶液环境,反应生成条件等。我们固定现有反应体系的反应条件,即水溶液，巯基丁二酸作为纳米晶稳定剂且浓度恒定,溶液的pH值控制在在9-10,重点考察两步骤微波加热方式在较低温度下微波加热时间不同对纳米晶成核过程的影响,这种影响的直观结果表现在纳米晶荧光效率的高低。借鉴CdSe、CdTe在有机溶剂中的成核与生长条件,即高温(～300℃)迅速成核、低温(<300℃)快速生长,同时结合CdTe纳米晶水溶液微波合成条件,我们确定微波加热的第一步温度设定在80℃,第二步的反应条件固定在温度120℃,时间10分钟。我们分别考察了组成x在0-0.6之间的CdSexTe1-x纳米晶的荧光产率。实验结果表明,80℃低温下短时间范围内的微波加热,即1-120s,能够显著提高CdSexTe1-x纳米晶的荧光产率。通过比较,确定了不同x值对应不同的最佳成核时间。由CdSevxTe1-x纳米晶合金XRD图(111)峰得到的晶格常数(d)与组成成分段线性关系,数据拟合结果表明二者存在良好的线性光系。当0<x<0.47时,拟合曲线的线性相关系数为0.999。当0.47<x<1.0时,拟合曲线的线性相关系数为0.976。实验结果表明CdSexTe1-x纳米晶合金是均匀组成的,而并非CdTe与CdSe的混合物。3.微波辅助顺序合成Ⅱ型CdTe/CdSe核／壳纳米晶及其在Cu(Ⅱ)检测中的应用利用微波加热方法,使用前述量子点合成方法制备CdTe量子点后,再在其外包覆CdSe层制备Ⅱ型CdTe/CdSe异质复合结构量子点,实现对CdTe发光调节和表面改性。核／壳异质结构的形成通常需要两个条件。一,核层材料能够为壳层材料提供外延生长位点；二,避免壳层材料的均相独立成核与生长。通过比较可知,CdTe和CdSe的晶格常数差别不大于3.5%。二者之间较小的晶格常数之差是CdSe外延生长于CdTe的基础。因此,CdTe/CdSe核／壳纳米晶成功合成的关键是限制CdSe的独立成核与生长。实验中镉源即氯化镉(CdCl2·3／2H2O)的浓度和溶液的pH是影响CdTe／CdSe核／壳纳米晶合成的重要参数。当氯化镉浓度过高或pH≥11时,CdSe几乎完全不能外延生长至CdTe表面上,而是独立成核与生长,制备的纳米晶溶液的吸收光谱中出现CdSe的吸收峰。CdTe／CdSe核／壳异质结构量子点可实现对污水中有害重金属离子Cu(Ⅱ)的选择性检测并具有较宽的线性检测范围。4.溶剂热合成ZnxCd1-xS纳米材料及其光催化性能研究将总量为1mmol按照一定的化学计量比x混合的Zn(CH3COO)2与Cd(CH3COO)2和2mmol的铜试剂分别溶于水中,利用共沉淀法制备ZnxCd1-xS前躯体。取ZnxCd1-xS前躯体置于乙醇或水或乙二胺的溶剂内,在180℃或210℃的恒温条件下反应,即获得ZnxCd1-xS纳米颗粒。XRD结构分析表明乙醇溶液中制备的ZnxCd1-xS纳米颗粒是六方相(纤锌矿)结构。ZnxCd1-xS的所有衍射峰皆位于六方相CdS和ZnS二者之间。随着Zn含量的增加,ZnxCd1-xS的主要衍射峰逐渐向更大的衍射角(20),即向ZnS移动。ZnxCd-1-xS纳米颗粒的衍射峰(100),(002)和(101)具有相近的衍射强度。而衍射峰(110),(103),和(112)明显变宽,说明形成的Zn-Cd1-xS颗粒是小至纳米尺度的。制备温度在180-210℃之间对Zn-Cd1-xS纳米颗粒的晶体结构没有明显的影响,不同组成的纳米颗粒仍旧是六方相晶体。而由相同的前驱体在水溶液中制备的ZnxCd1-xS纳米颗粒则表现出与在乙醇溶液中不完全相同的晶体结构。整体上ZnxCd1-xS纳米颗粒还是六方相晶体。但是XRD衍射峰(103)的强度随x降低,即随着ZnxCd1-xS纳米颗粒中Cd的增加而降低,并在高温时逐渐消失。同时在衍射峰(101)附近出现了新的衍射峰。通过实验结果比较可以断定,溶剂效应,即乙醇与水的不同是ZnxCd1-xS晶体结构不同的主要原因。溶剂效应使得ZnxCd1-xS的生长具有特异性并导致所得晶体结构不同。不同组成x的对ZnxCd1-xS晶体结构也有选择性。只有Zn组份较高的ZnxCd1-xS(x=0.83)纳米颗粒对有机污染物对氯苯酚(4-CP)具有较高的光催化降解效率。不同溶剂条件下制备的相同组成的ZnxCd1-xS纳米颗粒对有机污染物4-CP具有不同的催化降解效率。乙醇溶液中制备的ZnxCd1-xS的催化效率显著高于水中制备的,乙醇中高温下制备的ZnxCd1-xS的催化效率明显高于低温,但是水溶液中低温制备的ZnxCd1-xS反而具有更高的催化效率。不同的溶剂,乙醇,水和乙二胺对ZnxCd1-xS的光催化性能影响显著,乙醇溶液中高温下制备的ZnxCd1-xS晶体的结晶性良好,具有较强的抗光腐蚀性能。研究结果表明,对ZnxCd1-xS半导体纳米颗粒组份进行调节,通过不同溶剂对其表面修饰、改性,可以提高ZnxCd1-xS半导体纳米颗粒对有机污染物的可见光催化性能,在纳米材料可见光催化领域具有潜在的应用价值。更多还原

【Abstract】 In this dissertation, many efforts had been devoted to the synthesis of II-VI group semiconductor nanocrystals based on the correlations of separate nanocrystal constituents between structural and functional speciality. A series of II-VI group semiconductor nanocrystals are successfully synthesized, and their physical and chemical properties are systematically studied, such as optical properties and photocatalytic activities. Further study shows that the chemical, physical or optical properties of theses semiconductor nanocrystals have been largely enhanced or modified, or even resulting in novel functionality.1. Aqueous synthesis of highly luminescent amorphous CdSe nanocrystals at low temperatureWe utilize CdC12·3/2H2O and Na2SeO3 as Cd and Se sources, repectively. MSA was stabilzers and ligands, NaBH4 was reducing agent. Aqueous CdSe nanocrystals was prepared at low temperature (low than 100℃) with high PL QYs. The reaction temperature and heating speed had distinctive effect on QYs of CdSe NCs.We prepared a series of CdSe NCs at temperature between 25 and 60℃through water bath and microwave irradiation. The experimental results showed that water bath at 40℃was the suitable method for preparing highly fluorescent CdSe NCs. The PL QYs of prepared CdSe NCs were as high as 30%. But more high temperature is not suitable for preparing highly fluorescent CdSe NCs. Microwave irradiation that is a fast heating method was not suitable for preparing CdSe NCs with high PL QYs. CdSe NCs obtained through fast heating method had unobvious exciton absorption peak. CdSe NCs were not crystalloid. XRD patterns of CdSe NCs prepared at 20-60℃had no obvious diffraction peaks. HRTEM pictures did not show clear crystal stripes. CdSe NCs prepared at high temperature had good crystalline but with poor fluorescent quality. The PL of CdSe NCs should be defect related emission.In summary, synthesis of CdSe NCs at low temperature through water bath resulting in high fluorescence.2. One-pot microwave assisted synthesis of homogeneously alloyed CdSexTe1-x nanocrystals with tunable photoluminescenceMany factors affect the photoluminescence quantum yields (PL QYs) of nanocrystals (NCs), such as crystalline, surface state, solution surroundings, reaction conditions, et al. In our reaction system, we focus on the microwave heating methods which had obvious influence on nucleation of CdSexTe1-x NCs. As a result, the NCs had distinctly different PL QYs output. Conventionally, synthesis of CdSe or CdTe NCs in organic solvents is realized always at high temperature through fast nucleation and fast growth. Microwave can heat rapidly and uniformly, which could accelerate fast nucleation of CdSexTe1-x NCs similarly. The temperature and time of the first process were 80℃and 15s-120s, respectively. We fixed 120℃as the temperature of the second process, which was regulated as the optimum condition according to the QYs of obtained CdSexTe1-x NCs. Microwave heating at 80℃for a few seconds can evidently improve the PL QYs of CdSexTe1-x NCs. Different x had special corresponding heating time.The lattice distance (d) of (111) peaks from XRD patterns of CdSexTe1-x NCs show segmental linear relationship with x When 0<x<0.47, the linear constant was 0.999. When 0.47<x< 1.0, the linear constant was 0.976. These results illustrated that CdSexTe1-x NCs were homogenous and were not a mixture of CdSe and CdTe NCs.3. Microwave assisted sequential synthesis of Type II colloidal CdTe/CdSe core/shell nanocrystals and detection on Cu(II)After CdTe NCs were prepared through method metioned in 2.2 under microwave irradiation, epitaxial deposition of CdSe shell on the surface of CdTe NCs can also be realized.Two preconditions should be met for synthesis of core/shell NCs. One is that the core materials can supply epitaxial sites for the shell materials. The other is that avoidance of separate nucleation of the shell materials. There is only little difference less than 3.5% between CdTe and CdSe NCs in crystal lattices, which is the basis of deposition of CdSe shell on CdTe surface. Therefore, the key point is the avoidance of the separate nucleation of CdSe NCs in synthesis of CdTe/CdSe core/shell NCs. The Cd concentration and pH are important reaction parameters which should be well controlled in the preparation of core/shell NCs. Too high concentration of Cd or pH would impede the CdSe deposition onto CdTe NCs, and separate nucleation of CdSe can not be avoided.CdTe/CdSe core/shell NCs can be applied to selective detection of Cu(II) in waste water and the detection limit was as low as 1.7×10-9mol/L.4. Synthesis of solvent dependent ZnxCd1-xS nanoparticles for photodegradation of organic pullutantsZnxCd1-xS precursors were prepared by co-precipitating Zn(CH3COO)2 and Cd(CH3COO)2 with sodium diethyldithiocarbamate (DDTC) under certain x in water. Then the precursors were put into different solvents, such as ethanol, water or ethylenediamine at 180℃or 210℃and ZnxCd1-xS nanoparticles were obtained.XRD characteristic of ZnxCd1-xS nanoparticles in ethanol showed that hexagonal phase (wurtzite) were formed. As Zn content increased, the diffraction peaks of ZnxCd1-xS nanoparticles moved toward ZnS. Diffraction peaks of (100), (002) and (101) had similar intensity. The (110), (103) and (112) had broad peaks, which illustrated the appearance of nanocrystals.ZnxCd1-xS nanoparticles prepared in water showed different crystal structures. In general the ZnxCd1-xS nanoparticles were hexagonal phase. But intensity of diffraction peak (103) decreased gradually with the x increased at high reaction temperature. At the same time new peak appeared around (101). The difference between ZnxCd1-xS nanoparticles should attribute to solvent effect.Only ZnxCd1-xS (x=0.83) showed effective photocatalytic activity on 4-CP. ZnxCd1-xS (x=0.83) prepared in different solvents also showed different photocatalytic activity on organic pollutants. ZnxCd1-xS (x=0.83) prepared in ethanol at high temperature (210℃) showed the highest photocatalytic efficiency, and had strong anti-corrosion properties.Our results showed that both modulation on composition of ZnxCd1-xS nanoparicles and surface modification through different solvents could effectively improved the photocatalytic activity of ZnxCd1-xS nanoparicles on organic pollutants, which should find potential applications of semiconductor ZnxCd1-xS nanoparicles in photocatalysis.更多还原