二硫化钼纳米片及量子点的制备与性能研究

发布时间：2018-05-30 23:09

本文选题：二硫化钼 + 纳米片　；参考：《南京大学》2016年博士论文

【摘要】：过渡金属二硫化物二维层状材料因具有丰富的物理、化学性质引起了人们广泛的研究,该类材料的多样性、多功能性和可调性使其可以应用到能量存储、析氢催化、光电子器件等众多领域。在众多二维半导体材料中,单层MoS2纳米片具有1.9 eV的直接带隙以及1×108的高电流开/关比,引起了人们极大的研究兴趣。若能在单层MoS2中引入局域的未配对的自旋,便可获得二维的磁性半导体,有望在纳米自旋电子器件中发挥重要应用。此外,MoS2量子点由于其显著的量子限域效应和边界效应而具有独特的性质,受到越来越多的关注。本文中,我们利用液相剥离法和Li离子插层化学剥离法成功制备了MoS2纳米片及量子点。这两种方法都可以大规模地制备尺寸可控的单层或几层的MoS2纳米片。通过改变初始实验条件和后续的退火处理,对纳米片及量子点的形貌及性能进行了研究,全文的主要结论如下：1.由于制备工艺简单且易于批量生产,在实验室中液相剥离法已被广泛应用于二维层状材料的制备。本文中,以N-甲基-2-吡咯烷酮(NMP)为溶剂,通过改变超声功率来改变超声空化的强度和类型,研究了不同的超声功率对块体MoS2剥离效率、形貌的影响。找到了该溶剂中纳米片悬浮液浓度较高,纳米片尺寸较小,且分布较均匀的最佳功率范围。研究结果表明,随着超声功率的增加,悬浮液的浓度不是单调的增加,而是先增加,在320 W功率下浓度最高,超过320 W,浓度显著地下降,这是由于空化屏蔽效应导致的。从100 W至250 W,纳米片的尺寸减小,200 W和250 W情况下,尺寸大多小于60 nm,且尺寸分布较均匀,285 W以后略有增加；350 W后,纳米片的平均横向尺寸急剧的增加。文中分析了超声空化效应对样品悬浮液浓度及样品的形貌的影响机制。2.采用Li离子插层法制备了大量单层MoS2纳米片,纳米片的尺寸大多在100-800 nm之间。经过Li离子插层,MoS2的晶体结构发生明显的变化,大部分从2H相转变为1T相。通过在氩气中不同温度下退火可以改变MoS2纳米片中1T相含量,随着退火温度增加,1T相逐渐减少,2H相逐渐增多,在240℃温度下退火,1T相全部转变为2H相。磁性研究表明块体MoS2经过剥离后,磁性发生显著的变化,由完全抗磁性转变为抗磁与顺磁的混合态。顺磁强弱与1T相含量成正比,抗磁的强弱与2H相含量成正比,表明顺磁来源于1T相的MoS2。第一性原理计算的自旋密度分布及能态密度进一步证实顺磁主要来源于1T相中的Mo4d电子。根据晶体场理论分析,1T相中的Mo4d电子可以提供2μB的磁矩,而2H相的Mo4d电子的磁矩为零,与实验结果一致。3.利用多次Li离子插层的方法成功制备了平均尺寸在3 nm左右的MoS2量子点,量子点尺寸较为均匀,且大部分为单层。随着剥离次数的增加,纳米片的尺寸呈十倍左右减小,三次剥离后可以得到大量单层的尺寸为几纳米的MoS2量子点。量子点的形成机理可能为多次的Li离子插层使MoS2纳米片产生许多缺陷而变脆,在LixMoS2与水反应和随后的超声过程中易于被破碎。刚获得的MoS2量子点由于具有大量1T相没有光致发光现象。超声加热后,在300 nm紫外光的激发下光致发光强度显著增强。对于新制备出的样品,随着激发波长的改变,410 nm左右的发光峰位位移较小,说明量子点尺寸均匀。放置一段时间之后,量子点存在团聚现象,吸收带边出现红移,量子点的发光表现出与激发波长相关的特性,发光峰位随着激发波长的增加向高波长区移动。MoS2量子点的发光现象可以用量子尺寸效应来解释。4.利用多次Li离子插层化学剥离的方法成功制备了平均尺寸为3 nm的单层MoS2量子点,研究了多次剥离对样品的形貌及作为析氢催化剂的催化活性的影响。随着剥离次数的增加,纳米片的尺寸迅速减小。相比于块体及纳米片MoS2, MoS2量子点表现出更佳的析氢催化性能,具有较低的起始过电位(120 mV)和较小塔菲尔斜率(69 mV dec-1),表现出更快的析氢催化速率。经过分析对比,我们认为MoS2量子点较好的析氢催化性能来自于大量的活性边界。
[Abstract]:The two-dimensional layered material of transition metal two sulfide has been widely studied because of its rich physical and chemical properties. The diversity, multifunction and tunability of this kind of material can be applied to many fields, such as energy storage, hydrogen evolution catalysis, optoelectronic devices and so on. In the multidimensional semiconductor materials, the single layer MoS2 nanoscale has 1.9 The direct band gap of eV and the high current / off ratio of 1 x 108 have caused great interest in research. If the local unpaired spin is introduced into a single layer MoS2, a two-dimensional magnetic semiconductor can be obtained. It is expected to play an important role in the nano spintronic devices. In addition, the MoS2 quantum dots have a significant quantum confinement effect and a significant quantum confinement effect. In this paper, we have successfully prepared MoS2 nanoscale and quantum dots by liquid phase stripping and Li ion intercalation chemical stripping in this paper. These two methods can be used to produce single or several layers of MoS2 nanoscale on a large scale. After the subsequent annealing treatment, the morphology and properties of the nanoscale and quantum dots are studied. The main conclusions are as follows: 1. because the preparation process is simple and easy to be produced in batch, the liquid phase stripping method has been widely used in the preparation of two-dimensional layered materials in the laboratory. In this paper, the N- methyl -2- pyrrolidone (NMP) is used as the solvent. The ultrasonic power is changed to change the intensity and type of ultrasonic cavitation. The effect of different ultrasonic power on the peeling efficiency and morphology of block MoS2 is studied. The optimum power range is found in this solvent, which is high in concentration, smaller in size, and more uniform in distribution. The results show that the suspension liquid is increased with the increase of ultrasonic power. The concentration is not monotonous, but the first increase, the highest concentration at 320 W power and more than 320 W, the concentration decreases significantly, which is due to the cavitation shielding effect. From 100 W to 250 W, the size of the nanoscale is reduced, the size is less than 60 nm, and the size distribution is more uniform, and the 285 W is slightly increased after 285 W. After 350 W The influence of the effect of ultrasonic cavitation on the concentration of sample suspension and the morphology of the sample.2. was analyzed. A large number of monolayer MoS2 nanoscale films were prepared by Li ion intercalation, and the size of the nanoscale was mostly between 100-800 nm. The crystal structure of MoS2 was obvious after the Li ion intercalation. The change is mostly from 2H phase to 1T phase. The 1T phase content in MoS2 nanoscale can be changed by annealing at different temperatures in argon. As the annealing temperature increases, the 1T phase decreases gradually and the 2H phase increases gradually. The 1T phase is all transformed into 2H phase at the temperature of 240 C. Magnetic studies show that the magnetic properties change significantly after the bulk MoS2 is stripped. Transformation from complete diamagnetic to magnetic and paramagnetic states. The paramagnetic strength is proportional to the content of the 1T phase, and the strength of the diamagnetic is proportional to the content of the 2H phase. It shows that the spin density distribution and the state density of the MoS2. first principle derived from the 1T phase further confirm that the paramagnetic main to be derived from the Mo4d electron in the 1T phase. Theoretically, the Mo4d electron in the 1T phase can provide a magnetic moment of 2 mu B, while the magnetic moment of the Mo4d electron of the 2H phase is zero. In accordance with the experimental results, the MoS2 quantum dots with an average size of about 3 nm have been successfully prepared by the method of multiple Li ions intercalation. The size of the quantum dots is more uniform and most of them are monolayer. With the increase of the number of dissection, nanoscale The size of the film decreases about ten times, and a large number of single-layer MoS2 quantum dots with a few nanoscale size can be obtained after three peeling. The formation mechanism of the quantum dots may be a number of Li ion intercalation to make many defects of MoS2 nanoscale and become brittle. It is easy to be broken during the reaction of LixMoS2 with water and in the subsequent ultrasonic process. The MoS2 quantum obtained has been obtained. There is no photoluminescence in a large number of 1T phases. After ultrasonic heating, the photoluminescence intensity of the 300 nm ultraviolet light is significantly enhanced. For the newly prepared samples, with the change of the excitation wavelength, the luminescence peak displacement around 410 nm is smaller, indicating that the quantum dots are uniform in size. After a period of placement, the quantum dots have reunion. The luminescence of the quantum dots appears to be related to the excitation wavelength. The luminescence of the.MoS2 quantum dots moving to the high wave length region with the increase of the excitation wavelength can be explained by the quantum size effect. The mean size of.4. is 3 nm by the method of multiple Li ion intercalation dissection. The single layer MoS2 quantum dots have studied the effect of multiple peeling on the morphology of the sample and its catalytic activity as a catalyst for hydrogen evolution. With the increase of the number of dissection, the size of the nanoscale decreases rapidly. Compared with the bulk and MoS2, the MoS2 quantum dots show a better catalytic energy for hydrogen evolution, with a lower initial overpotential (120 mV) and a comparison. The small Tafel slope (69 mV dec-1) shows a faster catalytic rate for hydrogen evolution. Through analysis and comparison, we think that the better catalytic performance of MoS2 quantum dots is from a large number of active boundaries.
【学位授予单位】：南京大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O471.1

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