Synthesis and Characterization of Nano-Aerogels

Chapter 1. Introduction

1.1. Overview

This thesis is focused on developing a new sol-gel route for synthesizing oxide nanomaterials with various morphologies, i.e. nanofibers, nanospheres, and mesoporous architectures in CO₂. In the new sol-gel route, carboxylic acids are used for polycondensation of metal alkoxides. Fundamental studies on the synthetic process and resulting materials were carried out using in situ FTIR, N₂ physisorption, X-ray diffraction, electron microscopy, and thermal analysis.

In this chapter, the motivation of the research and the potential applications of the synthesized nanomaterials are addressed. Also, the methodologies for synthesizing nanomaterials, especially the sol-gel process and supercritical fluid techniques, are introduced.

1.2. Motivation of the Research

The scientific and industrial demand for oxide nanomaterials with desirable morphologies has increased significantly in recent years. As a result, tremendous work has been performed in synthesizing the oxide nanomaterials on both the laboratory and industrial scales.^{1, 2} Also, concerning human health and the environment, chemical engineers are seeking methodologies for producing nanomaterials in a green and sustainable manner with a low cost.^3

The purpose of this research was to develop an inexpensive, environmental friendly, and easy-to-scaleup method for synthesizing oxide nanomaterials with desired morphology, i.e., nanofibers, nanospheres and mesoporous material.

1.3. Potential Applications of Oxide Nanomaterials

An oxide is a compound containing oxygen atom(s) and another element. When the other element is electropositive such as a metal atom, the oxides tend to be basic; when the other element is electronegative such as a non-metal atom, the oxides tend to be acidic. Some oxides such as aluminum and zirconium oxides act as both an acid and a base.^4

In the oxide family, SiO₂, TiO₂ and ZrO₂ nanoparticles have attracted significant attention in recent years due to their interesting electrical,^5 optical,^6 magnetic properties and applications for catalysis,^7 energy conversion,^8 biomedical applications,^9 functionalized hybrid materials,^10 and nanocomposites.^{11, 12}

Specifically, because of the high-temperature stability, biocompatibility, and metal-oxide-semiconductive properties, SiO₂ nanomaterials have many potential applications. For instance, SiO₂ nanomaterials can be used as catalyst supports where a metal such as Pt is combined onto SiO₂,^{13, 14} as inorganic fillers for SiO₂/polymer nanocomposites where the SiO₂ strengthens the polymer composites,^{15, 16} as nanosized biosensors where SiO₂ core-Au shell nanoparticles can be tested at visible wavelength,^{17, 18} and as a drug capsule where the bioactive molecules are encapsulated in the SiO₂ particles.^19

Because of its semiconductivity, photoelectrical and photochemical activity under UV light and opacity under X-rays,^20 TiO₂ nanomaterials can be used as dye-sensitized solar cells (DSSC)^{21, 22} and photoelectrochemical cells (PEC),^23 photocatalysts, chemical sensors,^24 self-cleaning coating,^25 and TiO₂/polymer nanocomposites.^{16, 26-28}

Porous and particulate ZrO₂ has outstanding electrical, chemical and mechanical properties.^29 Hence it can be used as catalyst supports,^{30, 31} and as a component of core-shell nanoporous electrodes to prevent recombination of an electron and a hole in dye sensitized solar cells and solid oxide fuel cells (SOFCs).^{32, 33} ZrO₂ is also a very useful ceramic hardening material and is often used as a filler in composite biomaterials for joint prostheses.^34

1.4. Methodology of Synthesizing Oxide Nanomaterials

Currently, there are many synthetic methods to produce high quality oxides, including sol-gel, hydrothermal or solvothermal, microemulsions, electrospinning, chemical vapor deposition (CVD), thermal decomposition, pulsed laser ablation, templating, and self-assembly techniques.^35 However, most of the current methods suffer from environmental pollution and scale-up problems. For example, the conventional sol-gel process and templating methods involve volatile organic compounds (VOCs); hydrothermal or solvothermal methods involve high temperatures in an autoclave that generates corrosion and safety problems; while electrospinning or CVD processes are difficult to scale-up. These methods are briefly outlined below.

Sol-Gel Method. Metal alkoxides or salts are widely used as precursors for synthesizing metal oxides with various morphologies via a sol-gel route. Since the precursors react with water quickly and tend to precipitate, many methods have been used to control the reaction rate in order to obtain desired nanostructures. For instance, TiO₂ nanoparticles with a diameter of 4.3 nm were prepared by controlled hydrolysis of titanium alkoxide in reverse micelles with the aid of a surfactant in a hydrocarbon solvent;^36 while mesoporous anatase TiO₂ nanoparticles with a mean diameter of 25.5 nm and a surface area of 189 m²/g were synthesized recently using the precursor of Ti(SO₄)₂ and laurylamine hydrochloride;^37 TiO₂ nanoparticles with a diameter of ca. 14 nm were prepared using a sol-gel process of isopropoxide with the aid of ultrasonication.^38 Hansen et al. states that the sol-gel method is becoming the standard method for synthesizing oxide materials in Chemical Reviews, 2004.^29

Hydrothermal/Solvothermal. Under subcritical or supercritical water/solvent conditions, TiO₂ and ZrO₂ nanoparticles have been produced from a variety of precursors. More details about this method are provided in Chapter 2.

Microemulsions. The synthesis of oxides from reverse micelles relies on the precipitation of metal ions in aqueous droplets.^39 For example, 20-300 nm TiO₂ gels were produced by hydrolysis of tetraisopropyltitanate in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles.^40

Electrospinning. This technique involves the use of a high voltage to charge the surface of the metal alkoxide solution that passes through a nozzle (or coaxial capillaries). Using this method, TiO₂ and ZrO₂ nanofibers and nanotubes have been produced.^41-43 However, the resulting one-dimensional materials usually exhibited a diameter in the micron size.

CVD. This method involves the chemical vapor deposition of alkoxides or salts. The particle size was found to be a function of temperature, reaction rate, and concentration of the precursors, and TiO₂ particles as small as 2 nm were obtained.^{44, 45} The advantages of this method include the uniform coating of the nanoparticles or nanofilm, and the disadvantages include the higher temperatures being used and difficulties of scaleup.^46

Thermal Decomposition and Pulsed Laser Ablation. Metal alkoxides and salts can be decomposed and nanoparticles formed when high energy from heat or electricity is applied, if the process is well controlled. TiO₂ nanoparticles with a diameter less than 30 nm were produced by the thermal decomposition of titanium alkoxide or TiCl₄ at 1200 C. The particle size was found to be a function of precursor concentration and the flow rate of the precursors.^47 Tetragonal ZrO₂ particles from 6 to 35 nm were also produced using this method.^48 TiO₂ nanoparticles with a diameter ranging from 3 to 8 nm were synthesized by pulsed laser ablation of a titanium target immersed in an aqueous solution of surfactant or deionized water.^49 However, the drawbacks of these methods are high cost and low yield, and difficulty in controlling the morphology of the particles.^29

Templating. A template is a well-defined architecture onto which another material can form a nanostructure. A variety of templates have been studied for synthesizing oxide nanomaterials. For instance, cationic colloidal particles have been used to prepare the hollow TiO₂ nanospheres by thermal condensation of the TiO₂ shell on the template, followed by calcination to remove the polystyrene core;^50 scaffolds of poly (styrene-block-2-vinylpyridiane) diblock copolymers have been used to synthesize highly dense arrays in a regular pattern of TiO₂ nanoparticles through a sol-gel process;^51 starch gel templates have been used to synthesize meso/macroporous TiO₂ monoliths.^52 The advantage of this method is that the template allows nanomaterials with a variety of morphologies to be prepared, while the disadvantage is the complicated synthetic procedures and the consumption of the nanotemplates, which need to be removed, normally by calcination.

Direct Sol-gel Process in subcritical CO₂. By using a surfactant to disperse water, TiO₂ nanoparticles were produced by the hydrolysis and condensation of titanium alkoxides using subcritical CO₂ as a solvent.^53 More details about this method are described in Chapter 2.