Fabrication Of 3C-SiC Nanocrystal Composites And Applications In Photocatalysis

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.