【作者】 张俊；

【导师】 吴兴龙；

【作者基本信息】 南京大学 ， 物理学， 2015， 博士

【摘要】 碳化硅是碳-硅键合的Ⅳ-Ⅳ族半导体,具有优异的物理性质,已被广泛应用于制造高温、高压和高频等极端条件下的半导体器件。此外,由于碳化硅具有非常高的化学惰性,因此常被用于催化和催化剂的载体。本文选择立方相碳化硅(3C-SiC)作为主要研究对象,设计、制备了具有高催化活性的与3C-SiC相关的新型复合光催化材料,并对其纳米结构及复合物的催化性能进行了深入、系统的研究,获得的主要结果如下：1、以微米级商用大颗粒3C-SiC为前驱物,使用硝酸与氢氟酸对其进行化学腐蚀并超声剥离,制备出平均粒径在2-7nm之间的近圆形3C-SiC纳米颗粒。这些纳米颗粒具有良好的发光性质,发光峰位在400-600 nm范围,且随激发波长增大而红移。根据傅里叶变换红外光谱(FTIR)和X射线光电子能谱(XPS)等谱学检测手段对3C-SiC纳米颗粒进行分析研究,发现所制备的SiC纳米颗粒表面有大量的Si-OH键。理论计算和实验均表明,这是由于3C-SiC在纳米尺度时,颗粒表面的Si-Si二聚物能够有效地解离水分子,形成Si终端的Si-H和Si-OH键,这些-OH键捕获光生空穴后,能够形成具有高氧化活性的羟基自由基(OH·),从而提高复合反应体系的降解能力。2、TiO2半导体材料具有很高的催化活性,并且相对廉价,无毒无害,化学稳定性高,是一种理想的光催化材料。但由于TiO2能隙较大(锐钛矿相约3.2eV),只有约占太阳光5%的紫外光部分才能激发,光响应范围较窄,限制了其实际应用。我们通过改进相关实验方法,在阳极氧化法制备的TiO2纳米管阵列中均匀地涂覆上3C-SiC纳米颗粒,经过退火后,制备出3C-SiC与TiO2复合材料。复合材料的降解效率明显高于单纯的TiO2纳米管阵列,通过对比实验研究,我们认为除3C-SiC与TiO2之间良好的能带匹配外,在SiC与TiO2之间还形成了大量的p-n结,这些p-n结能够有效地分离光生电子和空穴,而且复合物中3C-SiC纳米颗粒的自催化性能也能够提供更多的-OH,从而提高光催化活性。3、ZnS是宽禁带、具有多种晶体形态的直接带隙半导体,在光电以及催化等方面有重要的应用。我们利用3C-SiC与ZnS的能带特点,设计并制备了3C-SiC/ZnS复合光催化材料,有效地拓展ZnS的光响应范围,增强其光催化活性。我们研究了ZnS与3C-SiC的自组装生长行为,并提出了其生长模型和机理,由于化学腐蚀法制备的3C-SiC纳米颗粒表面具有丰富的化学基团,这些基团能够与L-半胱氨酸的硫醇键和Zn2+所生成的络合物(Cys/Zn)结合,在随后的水热过程中,生长成为ZnS与SiC纳米复合物。光电化学测试显示,ZnS为n型,而3C-SiC为p型,因此两者的密切接触形成了p-n结,能够有效驱动光生电子和空穴的分离。3C-SiC/ZnS复合材料与单纯的ZnS相比,其光催化活性大大提高。本方法材料制备方便,能耗低,可以推广到其它3C-SiC基复合催化材料的生长。更多还原

【Abstract】 Silicon carbide, an important Ⅳ-Ⅳ group non-metallic semiconductor, has widespread applications including biosensors, photocatalysts and super capacitors due to its excellent physical properties, especially in extreme conditions, such as high temperature, high pressure and high frequency of semiconductors. In addition, silicon carbide is also chemically inert and can be used as catalyst or catalyst carrier. In this thesis, we choose the 3C-SiC as our research subjects to design and synthesize new composite photocatalytic materials with high catalytic activity related to it. Based on the obtained experimental results, we carry out systematical research on the surface of the nanostructures and the corresponding catalytic properties of hybrid nanocomposites to explore the role in the catalytic applications of 3C-SiC. The main results obtained are described as follows:1. We use commercial micronscale 3C-SiC powder as precursor to prepare nanoparticles by chemical etching in nitric acid and hydrofluoric acid, followed ultrasonic vibration peeling off the SiC nanoparticles in water or ethanol. The sizes of the 3C-SiC nanoparticles are between 2 and 7 nanometer with strong and stable photoluminescence (PL), and the peaks of PL locate in 400-600 nm range which changes with excitation wavelength. By studying the PL spectra of 3C-SiC nanoparticles in solutions, we find that the 440 nm emission is related to recombination of carriers caused by the quantum confinement effect, and the 510 nm emission peak is related to Si-OH bonds on the surface of SiC nanoparticle, and the results are consistent to Fourier Transform Infrared Spectrometry (FTIR) and X-ray photoelectronic spectroscopy (XPS). Based on these spectral characterization and theoretic calculation from first principles, we deduce that the 3C-SiC nanoparticle may dissociate water to form H+ and OH- which attach to Si dimers on the modified Si-terminated portion of the nanoparticles. Once these OH’connections on the surface of nanoparticles trap the photogenerated holes and it will form hydroxyl radicals (OH·), and these hydroxyl radicals are of high oxidation activity, which can improve the degradation ability of the whole complex reaction system.2. TiO2 is an ideal photocatalytic material because it has many advantages such as high activity and high chemical durability, relatively low cost, innocuous and unpoisonous and no subsequent pollution. But TiO2 posseses broad band gap (anatase with a band gap of 3.2 eV), so only ultraviolet part of sunlight, wich is no more than 5% of the solar spectrum that can be utilized to generate electrons and holes. Moreover, the fast recombination of photogenerated electrons and holes results in low quantum efficiency, which limits its practical application performance. Here we use evaporation aided ultrasonic oscillation method to coat the 3C-SiC nanoparticles evenly onto the TiO2 nanotube arrays that are made by anodic oxidation. After annealed, the 3C-SiC/TiO2 composite material was synthesized. The degradation rate of as-synthesized material is significantly higher than the pure TiO2 nanotube arrays, indicating that the introduction of 3C-SiC improves the TiO2 photocatalytic ability. By compared experiments, we think that a large number of p-n junctions are formed between SiC and TiO2, which can more effectively separate the photogenerated holes and electrons besides the proper band structure of 3C-SiC and TiO2.3C-SiC nanoparticles can also provide more -OH for the self-catalytic properbility, thus providing more hydroxyl radicals (OH) and improving the photocatalytic activity of the compounds.3. ZnS is a direct wide bandgap semiconductor, which is found to have wurtzite structure (hexagonal a-ZnS structure) and zinc blende structure (cubic β-ZnS structure). ZnS has important application in optoelectronic and catalytic. We use the hydrothermal method to synthesize 3C-SiC/ZnS compound material to enhance the protocatalytic activity of ZnS. We investigate the assembly of SiC/ZnS composite, and the growth model and mechanism were proposed as a plausible interpretation for the formation of the nanocomposites. It is well known that 3C-SiC nanoparticles have complicated surface chemical groups, and these chemical groups help L-cysteine-Zn2+ complexes (Cys/Zn) to attach on the SiC surface. Subsequent hydrothermal process decomposites Cys/Zn and leads to attendant growth of ZnS nanospheres that are surrounded by SiC nanoparticles. By adjusting the amount of 3C-SiC, we get ZnS nanospheres surrounded by different amount of the SiC nanocrystals. Compared with pure as-synthesized ZnS, the nanocomposites improve the photocatalytic activity greatly. The main reason is that SiC nanocrystals introduced here can not only separate the photogenerated electrons and holes due to proper band structure and built-in electric field between the p-type SiC and n-type ZnS, but also inhance the concentration of hydroxyl radicals (OH’) so as to improve the photocatalytic properties. The adopted method can be extended to fabricate other 3C-SiC based composite catalyst conveniently.更多还原