Preparation, Structure And Characterization Of Transition Metal Oxide Aerogels

The nanostructured metal oxide aerogels were synthesized by a sol-gel technique. The preparation process and the microstructures of aerogels were studied, and the effects of preparation parameters, as well as the thermal properties, the optical properties, and gas-sensing properties of the prepared aerogels were investigated. Based on the experiments, the formation mechanisms of the metal oxide aerogels were proposed. This study enriched the basic theory and applications of aerogels preparated on the basis of the sol-gel process.1. Preparation of zirconia aerogels with high surface areaZirconia aerogels with high surface areas were successfully prepared by a combined electrolysis/sol-gel method, followed by supercritical extraction or freeze-drying. This provides a facile route to production of ZrO₂ aerogels using inorganic salts as precursor and might be extended to the preparation of other metal oxide aerogels. First, 30.0 mL of 0.3 mol/L ZrOCl₂·8H₂O + YCl₃·6H₂O solution, in which the molar ratio of Y₂O₃ to ZrO₂ was adjusted to 0:100, 3:97, 6:94, and 8:92, was electrolyzed in an electrolytic cell under the same conditions at 25.0℃. After several days, the solution gradually transformed to a transparent sol. Stop the electrolysing and 30.0 mL of isopropyl alcohol was added into the sol under stirring and a wet gel formed. The wet gel obtained was divided into two parts and treated through two processes. One was immersed in absolute ethanol to exchange the solvent (mainly water) in the gel network. Then, the ethanol was extracted with liquid CO₂ in a supercritical extractor to remove solvent from the gel. When the autoclave was depressurized slowly, a lump of aerogel (S-aerogels) was obtained. The other part without exchanging with ethanol was directly put into a flask and quickly frozen with liquid N₂ and then freeze-dried to give the freeze-dried aerogel (F-aerogels). The pure ZrO₂ S-aerogel was a transparent monolith with mesoporous structure (average pore size, 9.7 nm) and surface area of ca. 640 m²/g. However, the pure ZrO₂ F-aerogel had microporous structure with surface area and mean pore size of ca. 400 m²/g and ca. 0.6 nm. After calcination at 500℃, the pure ZrO₂ S-aerogel exhibited as a mixture of m-ZrO₂ and t-ZrO₂, while the pure ZrO₂ F-aerogel showed a single t-ZrO₂ phase. Yittria-stabilized zirconia aerogels showed similar properties including particle size, microstructure, pore size, and surface area, as well as the phase structure of the calcined samples, as the pure zirconia aerogels. Detailed investigation indicated that the Y³⁺ did not enter the zirconia crystal lattice completely, so there had not been obvious effects of the Y₂O₃ contents on the crystalline structure of the calcined zirconia aerogels.2. Synthesis and photoluminescence properties of crystalline TiO₂ aerogels with high-surface- areaTiO₂ wet gels were prepared in acetone by a sol-gel method using tetrabutyl titanate as a precursor and acetylacetone as a stabilizer. Then, the gel was poured into a 15 mL autoclave, and the acetone was added in up to the 80% volume of the autoclave. The autoclave was heated at a designed temperature (120, 140, 160℃) for 2 h to give the crystalline TiO₂ wet gel (anatase). The anatase wet-gels were respectively dried by CO₂ supercritical drying, vacuum drying and atmosphere drying to give the TiO₂ aerogels. A new route came into being for preparation of aerogels by a combined solvothermal/sol-gel method. The properties and microstructures of aerogels obtained under different drying conditions were studied. The surface areas of all aerogels obtained were between 220 and 800 m²/g. The samples solvothermally treated at 140℃had maximum surface area in all the samples obtained with different drying methods. The surface areas were 794.2 m²/g (supercritical drying),662.7 m²/g(vacuum drying),528.9 m²/g(atmosphere drying), respectively. The higher surface area of TiO₂ aerogels dried at atmosphere were over 500 m²/g, which might due to the strengthed network of the gel during the solvothermal process. All TiO₂ aerogels showed obvious PL peaks in spectra, which is due to the small particles and surface vacancy. This was a efficient method for for preparing crystalline TiO₂ aerogels and may be extended to the preparation of othe ??rcrystalline aerogels.3. The preparation and gas-sensing properties of SnO₂ aerogel filmsSnO₂ sol was prepared by a reaction of SnCl₄·5H₂O and 1,2-epoxypropane in ethanol system. Then, the SiO₂ substrates were dipped into the solution and withdrawn from the bath at a constant rate to coat film. After sol geled, the coated substrates were dried by CO₂ supercritical drying to give the SnO₂ aerogel films. SnO₂ aerogel films also were coated on gas-sensing components by the above method to test its gas-sensing properties for ethanol, acetone and gasoline. SnO₂ aerogel film was composed of 4-5 nm crystalline particles. The thickness of film was 250 nm. The surface area was 388 m²/g and average pore size was 8.2 nm(mesoporous structure). The XRD patterns showed its cassiterite structure of the prepared SnO₂ film. After aging at 250℃, the film was stale and no cracks on the film surface can be observed and the particles did not grew up, but the crystalline degree was increased. The gas-sensing components structured by SnO₂ aerogel films had responses for 2-100 ppm ethanol, acetoneand and gasoline in the temperature range of 160℃-440℃with a rapid response/recovery speed.