Synthesis and Characterization of Nano-Aerogels

1.6. Sol-Gel Synthesis and Aerogel

1.6.1. Sol-Gel Technique and Aerogel

A sol is a colloidal suspension of solid particles in a liquid.^103 When sols attract each other and stick together in such a way to form a network, a gel is formed. A gel is a substance that contains a continuous solid skeleton enclosing a continuous liquid phase.^104 If the smallest dimension of the gel is greater than a few millimeters, the material is called monolith. If gels are in dimension from 1 nm to a few millimeters, it is called particulate gels.^105 Drying a gel by evaporation under ambient pressure gives rise to capillary pressure that causes shrinkage of the gel network. The resulting material is called a xerogel that is a dense material and exhibits a low surface area. In contrast, drying the gel under supercritical conditions eliminates the interface between liquid and vapor, so that there is no capillary pressure and relatively little shrinkage; the resulting materials are called aerogels.^{104, 106} Recently, materials dried by freeze-drying produce materials initially known as cryogels, now also termed aerogels.^95 Aerogels are extremely porous, low-density materials with a larger surface area than the xerogels. A typical aerogel comprises both meso- and micro-pores and exhibits a large surface area of hundreds of square meters per gram. They have extremely low thermal conductivities and fascinating acoustic properties due to their porous structure.^107 The first gel derived from a metal alkoxide was synthesized by Ebelmen, in 1846,^108 while the first aerogel was prepared by Kistler, who invented the supercritical drying method in 1932, which solved the shrinkage problem during gel drying.^{103, 109} Aerogels can be used for advanced applications including electrochemical devices, thin coatings, composite biomaterials, catalysts, ceramics, and heat and electric insulation devices.^110-113 This is because the aerogels have unique morphological and chemical properties which originate from the preparation methods and operational parameters such as the type of alkoxide, the H₂O/alkoxide ratio, the catalyst used for hydrolysis and condensation, the calcination temperature, and the SCF used for drying.^113

1.6.2. Conventional Method of Aerogel Synthesis

The conventional method for producing aerogels by the wet sol-gel route was developed after Kistlers pioneering work. This method involves hydrolysis of the metal alkoxide with water and a catalyst, i.e. an acid or a base, condensation into macromolecules, forming a colloidal sol and three-dimensional network wet gel, solvent exchange to remove water by alcohol, then drying the wet gel into an aerogel using a supercritical fluid (Figure 1.3).^114 The mechanism of hydrolysis and condensation of transition metal alkoxides is relatively well-understood.^115



Figure 1.3. Flow chart of conventional sol-gel route.

Hydrolysis and condensation of metal alkoxide precursors.

When a metal alkoxide reacts with water, nucleophilic substitutions occur as shown in Figure 1.4:^104 For the coordinatively saturated metals such as the metal alkoxides, hydrolysis and condensation both occur by nucleophilic substitution (S_N) mechanisms, in which coordination and proton transfer are involved and followed by removal of either alcohol or water.

Figure 1.4. Mechanisms of hydrolysis and condensation of metal alkoxides.^105

1.6.3. Direct Synthesis of Aerogel via a Sol-Gel Route in SCFs

A modified non-aqueous sol-gel route, was first reported by Loy et al. in 1997, in which the direct synthesis of SiO₂ aerogel monolith in CO₂ was studied by using formic acid as the polycondensation agent.^116 The direct synthesis of silica spherical particles in micron-size and nanofibers in supercritical acetone was reported by M. Moner-Girona et al. in 2003.^78 The new method simplified the aerogel process by completing both the reaction process and the drying process in one step. The success of this work on the direct sol-gel process in SCFs shed light and inspired this research.

Chapter 2. Direct Sol-Gel Process in SCF: A Review

2.1. Overview

“The 1980s were the decade of extraction, the 1990s have been the decade of materials, and the start of the next century will be the decade of reactions.”

------ Michel Perrut, in 1997^94
Since the discovery of supercritical drying by Kistle,^109 the combination of sol-gel processing and supercritical fluid (SCF) techniques provides significant potential for material science and nanotechnology.

The conventional method of synthesizing inorganic aerogels comprises a wet sol-gel process, in which alkoxides are hydrolyzed and condensed into a wet network, followed by SCF drying in order to maintain the micro-architecture. The inorganic aerogels exhibit microporous and mesoporous structures and significantly higher surface area. Their applications include catalysis, acoustic and thermal insulation, optics, and electrics. The conventional sol-gel method for aerogel synthesis has been well reviewed by Gesser et al. in 1989 and Pierre et al. in 2002.^{54, 113}

In the last decade, a new approach for synthesizing oxides directly in SCFs has been developed, and nanoarchitectures with a variety of morphologies, i.e., aerogel monolith, films, monolayer, nanospheres, and pillared structure, have been prepared.

In the present review, the direct sol-gel process in SCFs designates a one-pot synthesis for inorganic materials through either hydrolysis-condensation or polycondensation of alkoxides or salts in subcritical or supercritical fluids.