Abstract
Two main approaches for nanomaterials fabrication are the top-down and the bottom-up methods. The first is limited to mechanical grinding, thermal evaporation, ion sputtering, arc discharge, pulsed laser ablation, and other physical and chemical vapor deposition. These routes are costly, consume higher energy, and require complex technology such as ultrahigh vacuum. The bottom-up methods refer to the production of complex nanostructured materials from atoms and molecules. This approach is relatively simple and low in cost. However, it requires a good knowledge of the optical properties of the particles and their modifications when the particles are integrated with nanostructures. One of the widest bottom-up methods is the sol-gel. It involves a solution or sol (single-phase liquid) that undergoes a sol-gel transition (stable suspension of colloidal particles). In this chapter, we throw light on the history of sol-gel, its advantages, and limitations, operating this method for the production of different types of nanomaterials in the form of powders or thin films. In addition, some applications of the sol-gel-derived nanosized materials will be discussed.
Keywords
- sol-gel preparation
- metal oxide nanomaterials
- characterization
- oxides
- sol-gel chemistry
1. Introduction
Nanosized material, a material with at least one dimension limited to ˂100 nm (A nanometer is 10−9 of a meter.), displays unique and unexpected physicochemical properties. This behavior of nanomaterials arises from the large surface area to volume ratio and the quantum confinement effect that can be defined as the reduction of the band structure of the material into discrete quantum levels and the emerging of new energies for the electrons, resulting from the limited size of its particle, also known as the “size-effect.” Figure 1 shows that the surface atoms/volume ratio increases exponentially with decreasing particle size. Increasing the surface of the material increases its reactivity and photoelectrochemical performance. The accumulation of information on nanosized materials resulted in or emerged two branches, “Nanoscience” and “Nanotechnology.” The former focuses on the preparation and characterization of the nanomaterials and the fundamental study of their properties, whereas the latter is related to designing and using structures and devices based on these nanosized materials in different applications [2, 3].
The literature survey revealed that the physical and chemical properties of nanosized materials as well as the particles’ morphology (0D, 1D, 2D, ...), also depend on the preparation method and preparative parameters and conditions. With the continuous headway of nanotechnology, there are several methods or techniques for preparing nanosized materials which can be classified into two main branches; top-down and bottom-up; the top-down methods are based on breaking down large pieces/particles of the material to convert it to the required nanostructures. The “bottom-up” methods are based on assembling single atoms/molecules (in solutions or gas phase) into larger nanostructures. We will discuss the details of one of the bottom-up methods in this chapter, named the sol-gel.
2. Sol-gel chemistry
As a phenomenon, the sol-gel transition was discovered and explored by Ebelmen in 1846 by observing the slow transformation of silicic esters, in the presence of moisture, to hydrated silica and the spontaneous gelation when the alkoxide was placed in contact with the atmosphere [4]. However, the interest in the sol-gel method began in 1980 and received a continuous and increased interest exponentially until today, and we expect a growing interest during the current decade, as shown in Figure 2.
A sol is defined as a colloidal system in which the dispersion medium is a liquid, and the dispersed phase is a polymerized molecule or fine particles, where the particle/molecular size should be in the range of 1 nm – 1 μm. A gel is a continuous solid network that supports the continuous liquid phase [5]. In the typical sol-gel process, consecutive steps are the sol formation through hydrolysis, the sol-gel transition (gel state), the gel drying, and conversion into a calcined material, as shown in Figure 3. The chelating agent binds tightly with the metal ions to prevent the formation of aggregations. The sol-gel chemistry begins with mixing the precursor (acetate, nitrate, or chloride) with the solvent. If water is the solvent, the sol-gel is hydrolytic but named nonhydrolytic sol-gel in the case of using an organic solvent such as ethanol [6]. The solution prepared by the sol-gel chemistry is used cooperatively with coating techniques such as spray, dip, and spin coating. For thin film deposition, the chelating agent has the role of stabilizer to prevent the metal ions to be precipitated or agglomerated. The spin-coated films will form in nanoparticulate layers, as will be discussed.
According to Brinker and others, the sol-gel method is a technology where the solution containing the precursor solid materials evolves gradually to form a networked gel comprising both the liquid and solid phases. The precursors react with each other in the common solvent to form a colloidal suspension (sol). This sol undergoes a hydrolysis reaction that could be represented as
The sol-gel approach became one of the key technologies of the twenty-first century owing to low-energy consumption, reproducibility, eco-friendly, simplicity, low-cost, and pollution-free. In addition, it allows the combination of inorganic/organic materials in a single-phase and yields an organic/inorganic hybrid coating which attracted great attention owing to their high compatibility, good adhesion to the substrate, and corrosion resistance. Moreover, the sol-gel technology is represented in low-temperature requirements, repeatability, and controllability. In addition, it is possible to tune the intrinsic properties and the elemental chemical composition of the material. The final product of the sol-gel reaction can be controlled by precursors, pH, processing time, and molar ratios between the reacting agents. Löbmann revealed that the sol-gel route could yield various topologies; porous λ/4 films, dense interference layers, and arrays of antireflective structures (called moth-eye). These topologies can be used for antireflective coatings for architectural glazing, the display industry, solar energy conversion, and ophthalmic lenses [8]. Controlling the structure of sol-gel prepared film could yield highly selective gas sensors [9]. Chen et al. [10] studied the effect of pH value (1–10) on the corrosion protection ability of the sol-gel coatings. The highest condensation degree occurred at pH 4, resulting in a compact and stable 3D sol-gel network of high crosslinking density, and this provided highly effective corrosion protection.
It was also found that the photocatalytic properties of the sol-gel prepared TiO2 nanopowder depend mainly on the sol composition, where the addition of water, HCl, and diethanolamine as well as the type of alcohol as solvent (ethanol, propanol, and butanol) were found to greatly affect the photocatalytic activity of the powder toward bromophenol blue dye removal [11]. Luo et al. [12] studied some of the variables related to the sol-gel preparation of CaO as a high-performance sorbent and they concluded that the molar ratio of H2O:Ca2+ had a minor effect on the CO2 sorption performance of the CaO, and the optimal molar ratio was 80:1. The optimal molar ratio of citric acid: calcium nitrate optimal molar ratio is 1:1, and adding an excess of citric acid led to more gaseous products. In addition, when the pH 3, the sol-gel structure was destroyed, and the optimal pH value was 2, where the best performance of CaO sorbent was achieved. A. C.-Soria et al. [13] fabricated Fe3C/few-layered graphene core/shell nanoparticles, with potential magnetic properties, embedded in a carbon matrix by a modified two-step surfactant sol-gel method, where the hydrolysis, polycondensation, and drying took place in a one-pot. Hashjin et al. [14] tuned up the sol-gel technique for preparing high-durable superhydrophobic coatings. The prepared layers are useful for anti-icing, self-cleaning, and anti-bacterial applications, in the energy and photovoltaic devices, textile and coating industry, construction, and aerospace industry.
Sol-gel technique, among various solution methods, is found to be more suitable for metal oxide thin films and nanopowder. Controlling the conditions of preparation, nanoparticles of controlled shape/morphology, control stoichiometry, size, textural, surface characteristics, purity, and high quality can be obtained. Besides, uncomplicated ideas can be executed via this technique for more recent and advanced technological applications [6]. In the following section, some examples of the sol-gel derived nanostructures will be mentioned with their characterization and some related applications. The data presented here are based on our experimental results. It would be better to throw light on some selected materials that were prepared using the sol-gel method.
3. Sol-gel preparation of NiO, CdO, SnO2, and PbO and their nanocomposites
3.1 Experimental (preparation and characterization techniques)
The precursor materials used for NiO, CdO, SnO2, and PbO preparation were: NiCl2·6H2O of molecular weight (MW = 237.7), supplied by Schorlau, Spin, Cd(NO3)2 MW = 236.42, supplied by Nova Oleochem Limited, SnCl2.2H2O, MW = 225.63, from Merck, and CH3COO)2Pb.3H2O, MW = 279.33, Adwik, Egypt, were used to prepare 0.7 M solutions by dissolving the required mass of each salt in 100 ml double-distilled water. To each solution, 8.825 g of oxalic acid (C2H2O4), as a chelating agent, was added under stirring at 60°C for 1 h. The obtained solutions were maintained in an oven at 80–90°C for 20 h to evaporate the excess water above the precipitate. The solutions were then cooled to room temperature and aged for 24 at room temperature (RT). Finally, the four gel was calcined at 400°C for 2 h to obtain the nanopowders: NiO, CdO, SnO2, and PbO nanoparticles (NP). The characterization of these nanometal oxides will be discussed.
The identifying of the crystalline phase and samples purity was done by recording XRD spectra using the PANalytical X’Pert PRO diffractometer, with Cu K
3.2 Results and discussion
The crystallite size and phase identification of NiO, CdO, SnO2, and PbO were examined by XRD, shown in Figure 4, and the shape and particle morphology was studied by HR-TEM, as shown in Figure 5. Figure 4 shows the XRD pattern of NiO; the sharp peaks indicate that a good crystallite material was grown by the sol-gel technique. The diffraction peaks at 2θ = 37.14, 43.13, and 62.89° are indexed for the crystal planes (111), (200), and (220) of NiO of rhombohedral [
In the XRD pattern of the sol-gel prepared CdO nanoparticles, all of the detected diffraction peaks are indexed to the cubic phase of CdO with a lattice parameter a = 4.69483 Å. The peaks at
The HR-TEM image of NiO shows an average particle size of NiO in the range of 24.85–34.10 nm, which is smaller than that reported for NiO prepared from the thermal decomposition of Ni(OH)2 at 600°C [16]. TEM image of the CdO shows that CdO nanoparticles are well-defined and their size is in the range of 52–116 nm with an average particle size of 72 nm. Besides, the image for the SnO2 shows that SnO2 grains are segregated together and form agglomerates or clusters of primary crystallites. Most of the observed particles are tetragonal in shape. The average particle size measured by HR-TEM is ∼41 nm. Finally, the TEM image of the PbO formed as nanoparticles of sizes from tens of nm to <100 nm, with an average of about 59 nm, which is consistent with the XRD results.
Nickel oxide (NiO) is an interesting ceramic material with reasonable photostability and thermal stability, high melting at 1955°C, and a refractive index of ≈ 2.2. In addition, NiO is a
Cadmium oxide (CdO) is a promising II–VI compound that has
Tin oxide (SnO2) is also a transparent conducting oxide that exhibits outstanding electrical and optical properties. Its wide band gap (≈3.68 eV), high exciton binding energy (130 meV), high transmittance in the visible region of the spectra, high
As shown in Figure 8, both the direct and indirect transitions for the polymeric films are possible. This is evidenced by the linear relationship of both (
Lead oxides exist with a variety of oxidation states;
4. Preparation of nanosized hematite with different sizes and morphology
Controlling the morphology of the material at the nanosize is the key to broadening its industrial applications. Here we will describe tuning the microstructure and morphology of the nanosized hematite (
4.1 The preparation and measurements
A nanopowder of
4.2 Results and discussion
Figure 10 shows XRD patterns of the prepared materials (M = 0–2.0). All the diffraction peaks belong to the hexagonal structure of hematite according to JCPDS card no. 04–015-7029. No peaks related to any other FeO phases not detected. The strong peaks indicate the good crystallization of samples. The lattice parameters
This indicates increasing the
Figure 11 shows the XRD patterns of M = 1.0 sample calcined at 350–750°C. At 350°C, the thermal energy delivered to the material is sufficient to remove all of the organic molecules and the full oxidation of Fe into Fe2O3. Calcination temperatures higher than 350°C lead to an increase in the cell parameters and the diffraction peaks intensity. The
5. Ferrites and hexaferrites
The sol-gel method was also used to prepare different ceramics, for instance, as reported earlier [42, 43]. In which, M-type hexagonal ferrites Ba1-
6. Rare earth oxides and titanium oxide-based perovskites
Lanthanum oxide (La2O3) is a rare earth sesquioxide that is optically active with a wide bandgap energy range of 4.3–5.4 eV. The ultrafine La2O3 NP has attractive properties for automobile exhaust-gas convectors, optical filters, and catalysis, as a strengthening agent in structural and ceramic materials, in high κ gate dielectric materials [44]. On the other hand, the Y2O3 is an interesting host material for solid-state lasers, high-temperature refractories (melting point of 2430°C), and infrared ceramics, owing to the distinctive
6.1 Experimental
To prepare La2O3 nanoparticles (NP), 27.85 g of LaCl3.7H2O (
6.2 Results and discussion
Figure 15(a) displays the XRD pattern of the sol-gel-prepared La2O3. The sharp XRD peaks indicate the good crystallinity of the materials. The peaks at 2θ = 13.17, 26.48, 29.57, 40.11, 46.40, 51.55, 54.66, and 61.42° arise from the (001), (100), (011), (012), (110), (103), (112), and (202) crystal planes of La2O3 of hexagonal structure, consistent with the JCPDS no. 04–006-5083. The
La2O3 has spherical NP morphology with a particle size of 30 nm, which is consistent with XRD results. Dal et al. [51] fabricated La2O3 NP of size 12.4 nm by sintering the La2O3 microparticles at 1250°C for 48 hrs with grinding for more than 3 hrs. This illustrates that our sol-gel process is low-cost in time and energy. Figure 15(b) displays the XRD peaks of the Y2O3, which are indexed as (211), (222), (400), (411), (332), (431), (440), (532), (622), (444), (552), and (800), corresponding to yttria (Y2O3) of body-centered cubic structured, according to JCPDS 043–1036. No other phases are detected in this spectrum, indicating that all the Y(NO3)3 entirely transformed into Y2O3 of single-phase after calcination at 400°C. The crystallite size of Y2O3 is 24.7 nm. The inset of Figure 15(b) shows the powder morphology, where the particle sizes of Y2O3 look like nanosheets (Ns), which are allocated with each other to be bigger than the calculated crystallite size from XRD. Similarly, Y2O3 Np of
The XRD pattern of TiO2, Figure 15(c), consists of the following peaks; 2θ = 25.32°, 36.96°, 37.78°, 48.08°, 53.9°, 55.05°, 62.68°, 68.77°, 70.26°, and 75.05°. These reflections correspond to Miller’s indices of (101), (103), (004), (200), (105), (211), (204), (116), (220), and (215), respectively, as mentioned above their XRD peaks. This result confirms the formation of anatase TiO2 of lattice parameter
When Y2O3 or TiO2 are introduced to PVAc/PMMA blend to make polymer nanocomposites, the morphology of this sol-gel prepared nanopowder did not change. Figure 16(a–c) shows the SEM image of PVAc/PMMA blend loaded with 1.0 wt% nanofillers. The un-doped blend surface exhibits a networked structure or wavy-like like the pool surface, Figure 16(a). The fillers are distributed homogeneously and maintain their morphology, where Y2O3 distribute as nanosheets, Figure 16(b), while TiO2 is like small spheres of agglomerated particles, Figure 16(c).
Moreover, differently prepared nanofillers were prepared to load with polymers and to get nanocomposites for suitable applications [24, 53, 54, 55]. The dielectric permittivity of poly(methyl methacrylate, PMMA, significantly increased while the dielectric loss remained almost low due to the doping with CuO/Co3O4 nanoparticles [53]. The semiconducting properties of poly(vinyl acetate)/poly(methyl methacrylate), P(VAc/MMA), were enhanced by adding TiO2 nanoparticles and Y2O3 nanosheets to be used for some device applications [54]. The Zn0.95N0.05O (ZNO) nanoparticles loaded with polyvinyl chloride (PVC) affected the optical as well as the dielectric properties of the pristine sample [55]. Figure 17 represents the pronounced change in the absorbance of the polymeric materials by adding the ZNO and TiO2 nanoparticles that were prepared using the sol-gel method.
7. Limitations exist for sol-Gel processing
Sol-gel method process needs special attention at times of drying and aging.
The reduction in the volume through the densification may cause shrinkage and cracking of the sample surface.
Precursors used may be sensitive to moisture, and thus reduce the possibility of using the sol-gel materials on a large scale in optical coatings.
The involved chemical reactions may yield undesired byproducts, which could affect the material’s properties.
The grade of the using precurses should be analytical in type to get pure materials.
8. Conclusion and outlook
The simplicity of the synthesizing process makes the sol-gel one of the most popular options in the coating industry and is projected to have a wide range of applications, with continued expansion. Recent progress in several applications was made by using the sol-gel technology, including the anti-reflection for solar cells, coating protection of aircraft, and cotton fabrics for flame retardant. In addition, the sol-gel process offers the possibility of large-area deposition, compared to vacuum-based-deposition processes, with the possible control of the microstructure and density of the films for improving the coloring efficiency and storage capacity of electrochromic films. Other advantages can be listed as:
The low-temperature applied in all stages, except the densification, inhibits the thermal degradation of the material. Also, the development of amorphous, porous, and nanocrystalline materials is easily possible.
Most of the precursors, solvents, and chelating agents are volatile, so the obtained materials are of high purity.
Using the miscible precursors permits a homogeneous control over doping.
The synthesis procedure requires only kind of chemical conditions.
The easy casting of the materials into different shapes, such as monoliths, films, fibers, etc. However,
More research on the effect of several preparative parameters in the sol-gel synthesis on the final structure of products and their physicochemical properties should be carried out. These parameters (solution molarities and concentration, pH, temperature, aging and preannealing/annealing times, and drying conditions).
Utilizing the sol-gel technology for CO2 capture within the developed materials (carbon-capture materials) will be a promising research area for a safe environment.
Acknowledgments
This work is funded by the Deputyship of Research & Innovation, Ministry of Education in Saudi Arabia, through project number) 904/1443). In addition, the authors would like to express their appreciation for the support provided by the Islamic University of Madinah.
Declaration (conflict of interest)
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Therefore, there are no interests to declare toward any financial interests/personal relationships that may be considered potential competing interests.
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