Synthesis And Characterization Of Aerogels And Nanoparticles Using Supercritical Fluids

Aerogel has great potential for wide application in catalytic and material sciences, because of its unique properties of low density, large surface area and high porosity. However, at current stage, lot of research has to be done to enhance their performance, including to improve the synthesis technology and develope new varieties and improve their properties such as transparency and monoliths. In this thesis, monolithic aerogels of CdS, Gd₂O₃ and ZnO were prepared by Sol-Gel and supercritical fluids drying method; and their respective synthesis routes and properties were studied.Supercritical fluid can not only be applied to obtain aerogel, but to act as reaction medium to synthesize material. It can control the particle size, morphology, composition and structure, as a result, it can meet the various demand for material property and enrich the synthesis methods. With supercritical water as reaction medium, we successfully obtained FeIn₂S₄, a multicomponent sulfide.The major research work is as follows:1. Synthesis of CdS nanocrystal aerogel and study on its photocatalytic propertyNanoparticles of CdS with a particle size of 5-10nm were prepared with reverse micellar technique, CdS wet gel was obtained by using these particles as building blocks and fluorophenylthiolate as complexing agent. The CdS aerogels which was acquired after CO₂ supercritical drying has large surface areas (202m²/g) and big porosity (0.47 ml/g) with average pore diameters of 6.3nm. The photocatalytic experiment was performed on self-made equipment, and the result showed that CdS aerogel has better photocatalytic property than those of nanoparticle and bulk material. Photocatalytic experiments were also carried out on different crystal types of CdS material that prepared by heat treatment. Result showed that CdS aerogel as prepared has the best photocatalytic efficiency, and cubic CdS has better photocatalytic property than its hexagonal one. The mechanism was studied by UV absorption, BET analysis and FT-IR spectrum.2. Synthesis and characterization of monolithic Gd₂O₃ aerogel Both transparent and translucent monolithic Gd₂O₃ aerogel were prepared from inorganic gadolinium salts, including gadolinium nitrate and gadolinium chloride, through sol-gel polymerization in ethanol, using epoxide as gelation initiator. Its relatively high surface area (226 m²/g), average pore diameters (42 nm) and large pore volume (2.45 ml/g) were determined by nitrogen adsorption/desorption analyses. The experiment explored the most optimal reaction conditions, and found out how the solvent property, quantity and additives impacted on the reaction process and product properties (i.e. transparency, monoliths, etc.). The wet gels were also dried by supercritical ethanol, and the XRD, TEM and luminescent spectrum of the products were compared with samples dried by CO₂. The results showed that although its luminescent intensity was enhanced, its network crashed. We investigated the luminescent property of Eu doped Gd₂O₃ aerogels, which showed that it is a good luminescent material and its luminescent intensity increased with the concentration of Eu in a certain range.3. Synthesis and characterization of ZnO aerogelSince the usual epoxide addition method can only be applied to producing monolithic metal oxide gel whose formal charge is not less than +3, we made some improvement in this method and finally produced the ZnO areogel. The detaild process is as follows: ZnO wet gel was produced by the sol-gel method and with the inorganic salt of Zn as precursor, epoxide as gelation initiator and citric acid as additives. After being dried by CO₂ supercritical fluid, the ZnO wet gel turned into monolithic ZnO aerogel.The ZnO aerogels were characterized by XRD, TEM, FT-IR, Nitrogen adsorption/desorption and luminescent spectrum. Results showed that it had open porosity, its surface area was 202m²/g and porosity was 1.973 ml/g. The experimental parameter, the sol-gel mechanism and the function of citric acid were also studied.4. Synthesize nanoparticles of multicomponent compound with supercritical water as reaction mediumFeIn₂S₄ nanoparticles were successfully synthesized with supercritical water as reaction medium. Compared with the conventional method, this product has high purity and more regular morphology, moreover, its particle has more uniformed size and better crystallinity. Compared with solid-state reaction, the reaction with supercritical water as reaction medium was conducted at lower temperature, in a simplified process, and with shortened time. In the paper, the emphasis is put on the reaction mechanism and the difference between the new and conventional methods. The experiment results show that the new method can not only be applied to synthesizing multicomponent oxides but multicomponent sulfids. Moreover, it can effectively control the properties such as crystallinity and morphology.