Synthesis and Characterization of Nano-Aerogels

3.3. Synthesis Procedures

Synthesis of SiO₂, TiO₂ and ZrO₂ aerogels. The experimental procedures for synthesizing SiO₂ TiO₂ and ZrO₂ included addition of a carboxylic acid and an alkoxide into the reactor (a view cell or an autoclave), followed by addition of CO₂ to a desired temperature and pressure. In the case of synthesis of SiO₂ nanoparticles, two methods have been used: (1) quick depressurizing with the particles being collected directly in the autoclave, and (2) using the RESS process, in which the SCF was passed through a nozzle and the particles were generated in the collection vessel as described in Figure 3.5. In the case of synthesizing TiO₂ and ZrO₂ monolithic aerogels, typically 2-3 days were allowed for aging after the gel was formed. After CO₂ extraction and venting, the resulting aerogels were subject to further drying and calcination in air. More details of the synthetic procedures for SiO₂, TiO₂ and ZrO₂ are described in Chapter 4, 5 and 6, respectively. The synthetic procedures for Ti-acetate single crystals are provided in Appendix 8.

3. 4. Characterization Methods

3.4.1. Online FTIR

IR spectrometry is a type of absorption spectrometry that uses the infrared region of the electromagnetic spectrum, at which frequency chemical bonds vibrate in either a stretching or bending mode. The absorption frequency is determined by the molecular structure. FTIR is an improved technique to make the IR measurements easier and faster, in which the IR beam is guided through an interferometer. A FTIR spectrum is obtained from performing a mathematical Fourier Transform on the interferegram.

Recent literature has shown that ATR-FTIR spectroscopy is a powerful tool for in situ measurement of concentration, solubility, and supersaturation in crystallization processes for the chemical and pharmaceutical industries.^{149, 150} ATR spectroscopy uses the phenomenon of total internal reflection. In this technique, the radiation beam is directed onto an angled crystal and reflected within the crystal until it emerges from the other “end”, where it is collected. The depth of penetration, as the beam of radiation penetrates a fraction of a wavelength beyond the reflecting surface, distinctively produces: (1) little to no disturbing interference from the solid particles already present in the solution, or generated by crystallization, and (2) a less intense solvent contribution to the overall infrared spectrum so the solvent spectra can be easily subtracted from the sample spectrum. These characteristics of a small depth of penetration with no sensitivity to solid particles constitute the technical motivation for the employment of ATR-FTIR technology as an online monitoring tool for supercritical crystallization and chemical reactions. Figure 3.6 shows a schematic of the ATR-probe cell used in this thesis. In the ReactIR-4000 main box, there are instrument computer boards, a laser power supply, an IR source, and a detector. A mirrored optical conduit with 25 mm interior diameter is used to transmit the light between the IR source/detector and the diamond probe, which is located at the bottom of the reactor. The diamond probe, with a commercial name Dicomp^TM composition probe, is robust and pressure-rated to 5000 psig (Figure 3.7).

Figure 3.10. Schematic of the reactor with the ATR-FTIR probe assembly.


Figure 3.11. Schematic of the Dicomp^TM probe. Infrared radiation is reflected into the chemically resistant ATR disk that is in contact with the reaction mixture. The drawing is adapted from ASI.

Beer-Lambert law: The Beer-Lambert law is an empirical relationship that relates the absorption of light to the properties of the material through which the light is traveling.

A = α×l×c (3.10)

where, A is the absorbance, α is the absorption coefficient, l is the light path length, and c is the concentration of absorbing species in the solution.^151

3.4.2. Electron Microscopy

Electron microscopes, e.g., TEM and SEM, are instruments that use a beam of electrons to examine solid materials and provide the surface and internal imaging on a nanometer scale. Since their commercialization, TEM and SEM have made crucial contributions to the science and engineering of microelectronic devices, and made them indispensable tools for nanomaterials research.^152

TEM. More than 70 years ago, Knoll and Ruska invented the TEM to overcome the limitations of the light microscope.^152 However, the TEM had not been used for material studies until 40 years ago, when the thin-foil preparation technique was developed. More improvements of the TEM technique in the 1990s provided 0.1 nm of resolution,^153 which made TEM an indispensable analysis technique for studying materials in the micron or nano size region.

When incident electrons interact with a specimen, what could be generated are X-ray, cathode luminescence and six type of electrons: (1) transmission, (2) back-scattered, (3) reflected, (4) secondary, (5) Auger and (6) trapped electrons (Figure 3.8).^154 The transmission and scattered electrons are collected for TEM imaging.



Figure 3.12. Schematic: interactions of a specimen with incident electrons (redrawn from Ref. ^154).

TEM is the only technique allowing a 3D observation and quantitative characterization of most kinds of lattice defects. Its high spatial resolution for orientation determination and high resolution of TEM images can be advantageous over SEM for nanomaterial analysis.^155

A TEM such as Philips CM10 has a resolution of 0.5 nm, while a high resolution TEM (HRTEM), e.g., JOEL 2010, is capable of a spatial resolution of 0.194 nm and can provide images with 10241024 pixel resolution by using a multiscan digital camera. The diffraction patterns of HRTEM images are also very useful for studying the nanocrystalline phases of metals and nanoceramics.

SEM. Following the invention of the TEM, Knoll went on to develop the first SEM.^152 Even though the SEM was invented after the TEM, the maximum resolution of SEM is never higher than that of the TEM.

In the SEM, a very fine beam of electrons with energies up to several tens keV is focused on the surface of the specimen and scanned across it in a parallel pattern. The intensity of emission of secondary and backscattered electrons is very sensitive to the angle at which the electron beam strikes the surface of the sample. The emitted electron current is collected and amplified. The magnification produced by SEM is the ratio between the dimension of the final image display and the field scanning on the specimen. Usually, the magnification range of SEM is between 10 to 222,000 times, and the resolution is between 4 to 10 nm. The advantages of SEM include that a relatively large sample can be examined, while TEM can only examine the nanoparticles or thin layers with a thickness in nano size.

It should be mentioned that the specimen might be damaged by the electron-beam sputtering, and focusing for a long time on a small spot of the specimen should be avoided.^156

Both TEM and SEM were widely used in this thesis for studying the nanostructures of metal and oxide nanomaterials.

3.4.3. Crystals and Powder X-Ray Diffraction

By definition, a crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. An amorphous material does not have the ordered repeating pattern; however, it may change into a crystalline material upon heat treatment. For instance, when amorphous TiO₂ was heated up to 400 °C, either a rutile or an anatase crystalline phase can be formed depending on the particle size.^157

Single Crystal. In a single crystal, all atoms in the crystal are related either through translational symmetry or point symmetry.

Polycrystalline Materials. Polycrystalline materials are made up of a great number of tiny (m or nm) single crystals.

Unit cell, Tetragonal and Monoclinic

By definition, a unit cell is the smallest repeat unit of a crystal structure, in 3D, which shows the full symmetry of the structure. The unit cell is a box with three sides (a, b, c) and three angles (α, β, γ) (Figure 3.9).

Figure 3.13. Schematic of a unit cell of crystals.^158

There are seven unit cell shapes: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal and rhombohedral.

Tetragonal: a = b ≠ c, α = β = γ = 90°

Monoclinic: a ≠ b ≠ c, α = γ = 90°, β ≠ 90°

The perpendicular distance between pairs of adjacent planes in the crystal is the d-spacing.

For orthogonal crystal systems, i.e., α = β = γ = 90°:

(3.11)

Where, d is d-spacing; a, b, c are sides of the unit cell; h, k, and l are Miller indices, which are used to describe lattice planes and directions in a crystal. Table 3.1^159 shows the crystallographic data of the rutile and anatase TiO₂.

Table 3.3. Crystallographic data for rutile and anatase.

Substance	Structure Type	Lattice Parameters (nm)	Space Group
Rutile	Tetragonal	a = b = 4.593 c = 2.959	P42/mnm
Anatase	Tetragonal	a = b = 3.758 c = 9.514	I 42/amd

Powder X-ray Diffraction (XRD) is one of the primary techniques used to characterize solid-state materials such as metal oxides. Powder XRD can provide information about the crystalline structure in a sample even when the crystallite size is too small for single crystal x-ray diffraction.

Bragg’s Law

The X-ray diffraction of a crystal can be formulated by means of Bragg’s law (Figure 3.10):

(3.12)

where d is d-spacing, is the incident angle, n is the layer of planes, and is the wave length of the X-rays.^160

Figure 3.14. Schematic of X-ray reflection on the crystal planes.

3.4.4. N₂ Adsorption/Desorption

N₂ adsorption/desorption (physisorption) is an important method for characterization of solid-state materials to determine the surface area and pore structure.

BET Surface Area. When N₂ molecules are introduced into a vacuumed system filled with the solid-state sample, the N₂ molecules tend to form a thin layer that covers the entire surface of the solid. Using the Brunauer, Emmett and Teller (BET) theory, one can calculate the sample's surface area by means of multiplying the number of monolayer N₂ molecules required to cover the solid surface by the cross-sectional area of the N₂ molecule.

The BET equation is written as:

(3.13)

where, P and P₀ are the equilibrium and the saturation pressures of adsorbates at the temperature of adsorption, υ is the adsorbed gas quantity (for example, in volume units), and υ_m is the monolayer adsorbed gas quantity, c is the BET constant and can be expressed as:

c = exp (3.14)

where E₁ is the heat of adsorption of the first layer, and E_L the heat for liquefaction.

Equation 3.4 is an adsorption isotherm and can be plotted as a straight line, called a BET plot, with P/v(P₀-P) on the ordinate and P/P₀ on the abscissa according to experimental results. The value of the slope and the y-intercept of the line are used to calculate the monolayer adsorbed gas quantity, v_m, and the BET constant, c, respectively.^161 An example of a BET plot of N₂ adsorption in silica gel at 91 K is shown in Figure 3.11, from which v_m and the BET constant c can be determined as 116.2 ml/g and 80.6, respectively.^161



Figure 3.15. BET plot of N₂ adsorption on silica gel at 91 K. The data was obtained from Ref. ^161

A total surface area (S_total) and a specific surface area (S) can be calculated as:

(3.15)

(3.16)

where, S_total = total surface area, N = Avogadro’s number, s = adsorption cross section, M = molecule weight of adsorbate, S = specific surface area, and a = weight of sample solid.

BJH Pore-Size Distribution and Pore Volume. According to IUPAC’s definition, the microporous, mesoporous and macroporous materials exhibit pore diameters of less than 2 nm, in the range of 2-50 nm and 50 nm, respectively.^162

Addition of more N₂ molecules beyond a monolayer of N₂ on the surface leads to the formation of multiple layers and capillary condensation. The capillary condensation is believed to be proportional to the equilibrium gas pressure and the size of the pores inside the solid. The Barrett, Joyner and Halenda (BJH) computational method allows the determination of pore sizes from equilibrium gas pressures. The BJH method for calculation of pore size distribution is based on a model of the adsorbent exhibiting cylindrical pores.^163 During desorption, a pore loses its condensed liquid adsorbates, known as the core of the pore, at a particular relative pressure related to the core radius by the Kelvin equation (Equation 3.8). After the core evaporated, a layer of adsorbate remains on the wall of the pore. The thickness of this layer can be calculated for a particular relative pressure from a thickness equation. Using this approach, the pore size distribution and pore volume of solid with various pore sizes can be determined.^163

(3.17)

where, σ is the surface tension of liquid nitrogen, V is the liquid molar volume of nitrogen, r_k is radius of capillary in cm, T is the absolute temperature in K, and 8.31610⁷ is the gas constant in ergs per degree.

Hysteresis Loops. After generation of adsorption isotherm and fulfillment of saturation, one can also generate desorption isotherms by withdrawing a known amount of gas from the system in steps. Adsorption and desorption isotherms rarely overlay each other. The resulting hysteresis leads to isotherm shapes that can be mechanistically related to those expected from particular pore shapes. Figure 3.12 shows the classification of hysteresis loops denoted as H1, H2, H3 and H4 by IUPAC. Type H1 and H2 loops were obtained from agglomerated spherical particles and corpuscular systems, respectively, while type H3 and H4 were obtained from slit-shaped pores or plate-like particles.^164


Figure 3.16. Classification of hysteresis loops as recommended by the IUPAC.^164

It is widely recognized that two mechanisms contribute to the sorption hysteresis that is observed in experimental measurements on mesoporous solids.^165 The first mechanism, the single-pore mechanism, is purely thermodynamic in origin and does not relate to the interconnectivity of the pores. In the case of adsorption, the gas phase includes the adsorbed film on the pore wall at a certain pressure P^*. However, it is possible for a metastable gas phase to persist beyond P* during the adsorption process, thus a higher pressure is required to reach the adsorbed volume. Similarly, a metastable liquid phase may persist below P^* during desorption, thus a lower pressure is required to reach the desorbed volume.^165 Due to the existence of the metastable gas or liquid, the adsorption and desorption curves cannot overlay one another, and hysteresis loops are generated. The second hysteresis mechanism is a result of the interconnectivity of the pores. During the desorption process, nitrogen can vaporize only if the pore is in direct contact with the vapor phase. If the pore is not at the surface of the adsorbent particle, nitrogen can only vaporize if an adjacent pore contains vapor.^165 The more the pores connect, the smaller the hysteresis loops, and vise versa.

The relationship between the morphology of the pores and the hysteresis profile has been studied using computational simulations.^166-168 Although gas adsorption techniques and the data analysis methods appear to be well established, it is still difficult to evaluate the pore structures accurately, due to the surface heterogeneity and structural heterogeneity of the solid materials.^169

3.4.5. Thermal Analysis

Thermal analysis is an important technique for the characterization of solid-state materials, as it provides information on the physical and chemical changes involving endothermic and exothermic processes, temperatures for phase transitions, melting points and crystallization, and the weight loss when the temperature is increased.

Differential Scanning Calorimetry (DSC). DSC is a thermal analysis technique that is used to measure the heat flows associated with transitions in materials as a function of time or temperature. Such measurements provide qualitative and quantitative information about physical and chemical changes that involve endothermic processes such as melting, and exothermic processes such as crystallization, or changes in heat capacity such as at the glass transition temperature (Figure 3.13).^170 DSC can determine the glass transition temperature of polymers and the phase transition temperatures of solids. This makes DSC an indispensable technique for characterization of polymer and oxide materials.

Figure 3.17. A schematic DSC curve demonstrating the appearance of glass transition, crystallization and melting.

Thermogravimetric Analysis (TGA). TGA measures any changes of the sample weight with increasing temperature. It can determine: (1) moisture/liquid content and the presence of volatile species, (2) decomposition temperatures, and (3) the rate of degradation. TGA is widely used for characterization of solid materials including polymer, organic, inorganic and composite materials. Especially, TGA can be used to determine the percentage of inorganic fillers in inorganic/polymer composites.