Synthesis and Characterization of Nano-Aerogels

5.4. Conclusion.

As described in the introduction, the reaction between TBO and acetic acid is rather mild, such that these two ingredients could be placed together in the view cell before addition of CO₂. It was found that the morphology of the TiO₂ aerogel formed in CO₂ depends on the starting concentration of the precursor and the molar ratio of acetic acid and TA, while temperature and pressure had little effect. Very low concentration of the precursor facilitates the formation of precipitate. When the concentrations of TBO and acetic acid were as low as 0.031mol/L and 0.26 mol/L, respectively, a precipitate with a surface area as low as 5 m²/g was formed. Very high molar ratios of the acid to the alkoxide and high temperature also facilitate formation of the precipitates. If the concentrations of TBO and acetic acid were 1.10 mol/L and 5.50 mol/L, respectively, the gel time was less than two days under the specified conditions, and mesoporous fiber clusters were obtained. When concentrations of TBO and acetic acid are 1.48 mol/L and 6.12 mol/L, respectively, the gel time is much longer (3-4 days) and TiO₂ aerogel monolith was produced.

Comparing with TBO, TIP is much more active with acetic acid. The gelation time was shorter and only a fragile monolith was obtained (as described in Table 5.2), even with a temperature as low as 40 °C.

Different from the conventional sol-gel process, from which the TiO₂ aerogel monolith is difficult to be prepared due to the rapid hydrolysis reaction,^20 the modified non-aqueous sol-gel method makes it feasible to form a TiO₂ monolith. The morphology and the pore size of the aerogel could be tailored by changing the concentration of the starting materials. Aerogels with a higher surface area and crystalline phases could be obtained by increasing the process temperature.

Chapter 6. Synthesis and Characterization of ZrO₂ Nanoarchitectures

This chapter is reproduced from the published article by the author: Direct Synthesis of Zirconia Aerogel Nanoarchitecture in ScCO₂, with permission from Langmuir, 22 (9), 4390-4396, Copyright [2006] American Chemical Society.

Similar to the approach of synthesizing titania, ZrO₂ nanomaterials were synthesized at 40-50 °C and 6000 psig. Either a translucent or opaque monolith was obtained. The resulting materials were characterized by electron microscopy, X-ray diffraction, thermal analysis, N₂ physisorption and infrared spectroscopy analysis. The electron microscopy results showed that the translucent monolithic ZrO₂ exhibited a well-defined mesoporous structure, while the opaque monolith formed using added alcohol as a co-solvent was composed of loosely compacted nanospherical particles with a diameter of ca. 20 nm. After calcination at 400 and 500 C, X-ray diffraction results indicated that the ZrO₂ exhibited tetragonal and/or monoclinic phases. In situ infrared spectrum results showed the formation of Zr-acetate coordinate complex at the initial stage of the polycondensation, followed by further condensation of the complex into macromolecules.

6.1. Introduction

Porous zirconia (ZrO₂) is an insulation material that has both acidic and basic properties that are desirable for several current applications of interest, including catalyst supports^{30, 31} and electrodes in dye sensitized solar cells^32 and solid oxide fuel cells.^33 For catalyst supports, CuO/ZrO₂ was used for synthesis of methanol from hydrogen and carbon dioxide,^{243, 244} Pt/ZrO₂ was studied for hydrogenation of formate species,^245 and noble-metal/ZrO₂ was chosen for NOx removal.^246 Zirconia is also a very useful ceramic hardening material, and is often used as a component in composite biomaterials for joint prostheses.^34 Due to the interest in zirconia’s unusual properties and wide-spread applications, well-defined mesoporous ZrO₂ has been prepared by several techniques including templating^{213, 247} and evaporation-induced self-assembly (EISA) methods.^248 Sol-gel processing followed by supercritical fluid drying has also been used to synthesize ZrO₂ aerogels. This technique normally provides a very porous material with irregular patterned mesopores having a large pore-size-distribution.^{95, 104} Recently, a well-developed mesoporous structure in the ZrO₂ aerogel was reported having a large surface area by calcination in flowing helium at 300 °C and then in flowing oxygen at 500 °C.^249

Like several other materials, zirconia nanoparticles are of significant current interest in preparing piezoelectric, electro-optic, dielectric and nanocomposite materials,^250-252 and hybrid materials for solid oxide fuel cells.^253 Sub-micron and nano spherical particles of ZrO₂ have been prepared by several methods including a sol-gel technique utilizing hydroxypropyl cellulose as a polymeric steric stabilizer^254 and miniemulsification of molten salts.^255 Using a so-called flame spray pyrolysis (ESP) method, spraying of combustible droplets into a CH₄/O₂ flame, formed ZrO₂ particles with a diameter ranging from 6 to 90 nm.^{48, 256} Thermal decomposition of ZrO(OH)₂·xH₂O polymer precursor generated ZrO₂ particles as small as 12 nm.^257 A biosynthesis process using the fungus Fusarium oxysporum developed ZrO₂ particles less than 10 nm.^219

This research was motivated by usage of the green solvent, scCO₂, and developing a new synthesis method for ZrO₂ that allows for material properties appropriate for catalyst preparation, inorganic/organic hybrid nanomaterials, and porous ceramics for biocomposites. The properties of the resulting material that were of interest for these applications were studied, including morphology, particle size, crystal structure, and mesoporous structure.