Synthesis and Characterization of Nano-Aerogels

4.4. Conclusions

This study of the particle formation process for aerogels in CO₂ demonstrated that in situ ATR-FTIR spectroscopy is a valuable technique for studying this high-pressure process. Both anhydrous formic and acetic acid were found to be mild and controllable agents for sol-gel route preparation of SiO₂ aerogel particles in CO₂. By modifying the ratio of silicon alkoxide, acetic acid, and water, the polymerization rate is tunable and precipitation can be prevented, and therefore agglomeration minimized. Increasing the reaction temperature led to higher rates of reaction and increasing the pressure decreased the rate of reaction. Submicron particle sizes were obtained when the sol-gel solution was destabilized by pressure reduction. Particles as small as 100 nm were formed using the RESS process.

Chapter 5. Synthesis and Characterization of Titania Nanofibers and Nanospheres

This chapter is reproduced from a letter and an article by the author: Formation of Titania Nanofibers: A Direct Sol-Gel Route in Supercritical CO₂, and FTIR Study on the Formation of TiO₂ Nanostructures in Supercritical CO₂, with permission from Langmuir, 21 (14) 6150-6153, and the Journal of Physical Chemistry B, 110 (33), 16212-16218, respectively, Copyright [2005-6] American Chemical Society.

This chapter describes a new method to synthesize titania aerogel composed of nanofibers or nanospheres via a sol-gel route in CO₂. The aerogel was formed by polycondensation of titanium alkoxides using acetic acid as the polymerization agent in CO₂ at 40-70 °C and 2500-8000 psig. The TiO₂ morphology was characterized by means of SEM and HRTEM. Depending on experimental conditions, TiO₂ anatase nanospheres with a diameter of 20 nm, or TiO₂ anatase/rutile nanofibers with a diameter of 10 to 100 nm were obtained. N₂ physisorption and powder XRD showed that the nanofibers exhibited high surface areas up to 400 m²/g, and anatase and/or rutile nanocrystallites were formed after calcination.

It was found that fiber formation was enhanced by a higher HOAc/Ti ratio, and the use of the titanium isopropoxide (TIP) precursor. The mechanism of the microstructure formation was studied using in situ FTIR analysis in CO₂. The FTIR results indicated that the formation of nanofibers was favored by a titanium hexamer that leads to one-dimensional condensation, while nanospheres were favored by a hexamer that permits three-dimension condensation.

5.1. Introduction

5.1.1. Applications of TiO₂ Aerogel and TiO₂ Nanomaterials

TiO₂ aerogel is a material with significant potential for many applications. As examples, it can be used as a ceramic material,^182 an opacifier in dental filler,^{183, 184} a bone filler,^185 a construction ceramic in nanofiltration membranes,^186 an anion- and cation-exchange packing material for ion chromatography,^187 a catalyst support for oxides and group VIII metals,^{20, 188-190} a photocatalyst,^{25, 191-195} a semiconductor for dye-sensitized solar cells (DSSCs) and other applications.^196-202 Particularly, TiO₂ has been of great interest for energy conversion and photocatalysis since the discovery of its ability for photoelectrolyzing water to produce hydrogen.^203 Many efforts were focused on decreasing the band gap energy of TiO₂ (3.2 eV) into the sunlight spectrum (0.5 ~2.9 eV) by synthesizing new nanoarchitectures and transition-metal-doped, oxide-coated or nitrogen-doped TiO₂.^204-207 TiO₂ and hybrid TiO₂ nanoarchitectures have been extensively prepared by using hydrolytic^{208, 209} or nonhydrolytic^{210, 211} sol-gel routes.

5.1.2. TiO₂ Synthesis Method

The formation of mesoporous materials as thin films, fibers, spheres or monoliths is of great interest with regard to various applications.^212 Titania materials with a variety of morphologies have been reported. Mesoporous powder or film was synthesized via a templating route^213-216 or a non-templating route;^217 nano-aerogel particles were obtained by stabilizing with surfactants;^53 micron size TiO₂ fibers were manufactured using electrostatic deposition;^218 hollow fibers were produced by electrospinning two immiscible liquids through a coaxial, two-capillary spinneret;^219 TiO₂ pillared clay structure was developed using scCO₂;^139 unsupported large-area membranes were also fabricated.^220 However, the formation of sol-gel-derived pure TiO₂ monolith is rarely reported.^221 The reason is that pure TiO₂ aerogel is extremely brittle and fluffy. This makes it unsuitable for catalyst supports.^{95, 222} To solve this problem, TiO₂ was either grafted onto other oxides, i.e., SiO₂ or Al₂O₃, to form a composite monolith,^223 or was mixed with water into a paste, extruded and calcined to form a hardened extruded catalyst support.^224 Suh et al. believed that the brittleness of TiO₂ aerogel was due to the fast hydrolysis of titanium precursor and the destabilization of the alcogel.^225

The conventional sol-gel route of TiO₂ synthesis involves wet-chemistry: dissolving of titanium alkoxide in alcohol and a stabilization agent, controlled hydrolysis of the metal alkoxide with a limited amount of water and a catalyst, i.e. an acid or a base, condensation into polymers, forming colloidal particles sol and three-dimension network wet gel, aging, extraction of alcohol by supercritical fluid, and calcination.^226

Since titanium alkoxides are very active with water and tend to form precipitate, they were modified by acetic acid before addition of water to control the reaction rate in the conventional sol-gel route.^{227, 228} It was believed that a coordination bond was formed between the titanium and the acetate group, so as to decrease the hydrolysis activity.

5.1.3. Ti-Carboxylate Complex

Ti(VI) is known to be able to form acetate complexes. In the complex, the acetate can potentially coordinate with the metal as a chelating, bridging bidentate or monodentate (Figure 5.1 a, b, and c respectively).^229 FTIR is an established technique for analyzing the complexes of metal carboxylate species.^230 During in situ FTIR interpretation, we took many benefits from the reports of the synthesis and analysis of Ti-carboxylate complexes in conventional media using single-crystal XRD and IR techniques. Nakamoto and Deacon et al. summarized the IR studies of a variety of metal-acetate complexes having known X-ray crystal structures.^{229, 230} Figure 5.1 d and e show two examples of titanium hexamer complex structures determined by single crystal XRD.^231 The bridging acetate exhibited peaks at 1600, 1580, 1555 and 1445 cm^-1 from rutilane shape Ti₆O₄(OBuⁿ)₈(OAc)₈ 1,^232 and at 1603 and 1548 cm^-1 from hexaprismane shape Ti₆O₆(OPrⁱ)₆(OAc)₆ 2.^233 By reacting titanium isopropoxide with a lower amount of acetic acid (with the molar ratio of HOAc/alkoxide = 1.33:1) in CO₂, an additional two crystals were also synthesized: 2 and rutilane shape Ti₆O₄(OPrⁱ)₈(OAc)₈ 3.^234 The single crystal 2 exhibited peaks of bridging acetate at 1604, 1543 and 1458 cm^-1, and 3 showed peaks at 1581, 1560, 1452 and 1410 cm^-1. The IR results of the crystals, as summarized in Table 5.1, are important for understanding the formation of TiO₂ nanoarchitecture as described later.


Figure 5.26. Reported structures of TA oligomers with acetate ligands: (a) chelating bidentate; (b) bridging bidentate; (c) monodentate; (d) Ti₆O₄(OBuⁿ)₈(OAc)₈, rutilane shape; (e) Ti₆O₆(OPrⁱ)₆(OAc)₆, hexaprismane shape. (a) ~ (c), Ref.^229; (d) and (e) images were modified from Ref.^231. The black balls stand for Carbon, the red ones stand for Oxygen, Titanium is in the middle of the octahedrons.

Table 5.4. IR absorption peaks corresponding to COO^-1 stretching vibration in various acetates.

Ti-acetate coordination	νsym(COO) cm^-1	νasym(COO) cm^-1	Ref.
1, Ti6O4(OBu^n)8(OAc)8, rutilane shape	1445	1600, 1580, 1555	^232
2. Ti6O6(OPr^i)6(OAc)6, hexaprismane shape		1603, 1548	^233
	1458	1604, 1543	^234
3. Ti6O4(OPr^i)8(OAc)8, rutilane shape	1452, 1410	1581, 1560	^234