Open access peer-reviewed chapter
Abstract
Sol-gel method is a novel technology of producing new materials in a convenient and cost-effective way. This method allows a highly ordered and well-connected network structure to be developed and better controlled. It is a simple procedure to produce homogenous multi-component systems. Homogenous mixed oxides can be developed by combining different molecular precursor solutions. The advantages of sol-gel method include its simplicity, affordability, controllability, and ability to mass production of nano-sized particles with large surface areas. Due to this simplicity and versatility, sol-gel technology has higher admiration and industrial application compared to many prevailing methods and is widely being used in various fields. Sol-gel procedure has been comprehensively used as a common and practical way for the development of nano-structured materials for a wide range of applications. This chapter primarily concentrates on the fundamentals of sol-gel science, particularly with respect to the development of nanoparticles, and their numerous applications, with a focus on more recent, sophisticated, and advanced applications.
Keywords
- sol-gel science
- novel technology
- hydrolysis
- condensation
- network structure
1. Introduction
The sol-gel method initially appeared in the realm of nanoscience and technology in the year 1921. Its development happened in the 1960s as a consequence of the nuclear industry’s obligation to new synthesis techniques. This development gained admiration around 1984 and had substantial development up to the year 2011. An essential guide for the development of the sol-gel method was provided by Dr. Jeffrey Brinker, a pioneer in the synthesis of materials and sol-gel science [1]. Dongyuan Zhao and David Avnir were two distinguished researchers who have advanced their studies in the field of sol-gel science and made noteworthy contributions in this field [2]. The process of sol-gel synthesis is so widely [3, 4, 5, 6, 7, 8, 9, 10, 11] used that it has become the most attractive technology for versatile applications.
The sol-gel chemistry is based on poly-condensation and hydrolysis processes. Due to their propensity to create homogenous solutions in a wide range of solvents, as well as due to their reactivity toward nucleophilic reagents, such as water and metal alkoxides [M(OR)3], it is a useful process to generate oxides. Metal oxides, nitrides, and carbides are just a few examples of ceramic materials that are commonly prepared using sol-gel techniques [12, 13, 14, 15, 16, 17]. This method has a number of benefits over traditional processing technologies [18, 19, 20, 21], including low reaction temperatures [5, 22, 23, 24, 25, 26, 27], precise composition control [28, 29], high levels of purity [30, 31, 32], and the capacity to create processes for large-area applications [33, 34, 35, 36, 37, 38, 39, 40, 41]. The key steps of the reaction are the hydrolysis and condensation of an inorganic or metal-organic precursor, which produces a sol that, following a series of chemical reactions and/or mild thermal treatments, transforms into a gel, which is then calcined to produce the end product.
The final product and its applications were the main goals of the majority of studies that were reported, with little regard for the conditions of synthesis or the reaction mechanisms used to produce gels, even though the characteristics of a gel and how it responds to heat treatment may be highly dependent on the structure that was already established at the sol stage [42, 43, 44, 45, 46]. The primary characteristics of the resultant powder are thus determined by the generation of colloidal aggregates. The structure and shape of samples generated are significantly changed by altering the chemical conditions under which materials are polymerized. A few applications of sol-gel-based nanomaterials are shown in Figure 1.
Figure 1.
Applications of sol-gel-based nanomaterials.
Sols are thermodynamically unstable, due to their large surface area to volume ratio. However, they can be stabilized by introducing an energy barrier on the solid particles present in the sol. This stabilization process can be done either by electrostatic stabilization or steric stabilization.
Although it was researched earlier, sol-gel chemistry has been the subject of substantial research since the mid-1970s, when it was discovered that metal alkoxide solutions can be used as starting materials for sol-gel processes that build a variety of inorganic networks [33, 34, 35, 36]. Sol-gel processing enables the production of homogeneous, high-purity inorganic oxide glasses at room temperature as opposed to the extremely high temperatures necessary for traditional glass creation. Products have been developed for use in a variety of fields, including gas separations, elastomers, coatings, and laminates [34, 37, 38, 39, 40, 41, 42]. These products include molded gels, spun fibers, thin films, molecular cages, and xerogels. Numerous property adjustments can be made by incorporating inorganic compounds into organic polymers.
Sol-gel-based nanomaterials have been considered as being of high technological impotence compared to many other materials [43, 44, 45, 46]. In order to have strong control over the properties of the finished material, the goal of this effort is to better understand the chemistry involved in the creation of nanomaterials
2. Importance of sol-gel technology
Years after its discovery, sol-gel science and technology still continue to draw attraction of researchers from around the globe. Sol-gel method is a bottom-up synthesis technique. Sol-gel surface modification of substrates is very successful and in high demand [47, 48, 49, 50, 51, 52, 53, 54]. The main benefit of the sol-gel process is that it produces stable surfaces with a high surface area. A novel method of creating new materials is offered by the sol-gel technique. This approach differs from others in a few key ways. Low processing temperature, great homogeneity, and simplicity characterize this process. Its compatibility with polymers and polymerization allows it to produce nanoparticles in an organic environment and is one of its key advantages. This process results in high-purity products. Through this method, the entire sequence of processes involved in the synthesis of solids can be better controlled. It is simple to create homogenous multi-component systems; in particular, homogenously mixed oxides can be created by combining molecular precursor solutions. Although all of the technologies can produce large amounts of nanomaterial, the sol-gel process is more widespread and has more industrial uses than other existing technologies [55, 56, 57, 58, 59, 60]. This technology can create high-quality identical nanoparticles in commercial quantities due to their unique traits and properties [61, 62, 63, 64, 65]. Functional materials like photocatalysts [66, 67, 68, 69, 70, 71], ferroelectrics [72, 73, 74, 75, 76, 77, 78, 79], nonlinear optical materials [80, 81, 82, 83, 84], and superconductors [85, 86, 87, 88, 89, 90, 91, 92, 93] can be made using the sol-gel process. Therefore, significant contribution is continuing throughout the world still today. Well-cited articles on sol-gel science and technology published in 2020 (Source: SCOPUS) are shown in Figure 2.
Figure 2.
Well-cited articles on sol-gel science and technology published in 2020 (source: SCOPUS).
Metal NPs-doped inorganic-organic hybrid films, such as organically modified silica, are created using the sol-gel method. Metal oxides, nitrides, and carbides are just a few examples of ceramic materials that are commonly prepared using sol-gel techniques. The sol-gel method can be used to create ceramics as a molding substance and as a transitional layer between thin metal oxide coatings in a variety of applications. Numerous optical, electrical, energy, surface engineering, biosensor, medicinal, and separation technologies utilize the materials produced by the sol-gel process [55, 94, 95, 96, 97, 98]. To connect researchers in the field of science and technology all over the world International Sol-Gel Society (ISGS) has been formed. It is an international, interdisciplinary, not-for-profit organization whose primary purpose and objective are the advancement of sol-gel science and technology. International Sol-Gel Society member/subscriber affiliation in 2020 is shown in Figure 3 [97].
Figure 3.
International sol-gel society member/subscriber affiliation in 2020 [
Sol-gel method has a number of benefits over traditional processing technologies, including low reaction temperatures, precise composition control, high levels of purity, and the capacity to create processes for large-area applications. It is a well-known technique for a variety of materials, including inorganic membranes, monolithic glasses and ceramics, thin films, and ultra-fine powders. Even for the synthesis of 1D nanomaterials the process is used today. The numbers of merits of this method have made it highly attractive, which can be carried out at room temperature with the potential for simple chemical doping. The aforementioned examples show that sol-gel methods are simple processes to modify the final morphological characteristics of nanoparticles. Alloy products can be made in a single step by combining two or more metal (or metal oxide) precursors in a precise ratio since this technique can simultaneously make two or more different types of nanoparticles. There are alternative single-step procedures that can produce alloy products, such as the plasma method and electrochemical processes, but the key feature that sets the sol-gel method apart from them is its industrial scale. The sol-gel process can also be used to produce highly homogenous composites with a 99.99% purity level. Metal and ceramic nanoparticles can be produced with this technology at temperatures between 70 and 320°C because the process temperature is lower than with current methods. Different ways of sol-gel synthesis are shown in Figure 4.
Figure 4.
Different ways of sol-gel synthesis.
The main advantages of the sol-gel method include superior product purity, narrow particle size distribution, and production of a homogenous nanostructure at low temperatures. Metal nano-oxides are frequently created using this technique [29, 99, 100, 101]. The sol-gel process, as mentioned, entails changing a sol into a gel using a variety of methods, the majority of which call for mild drying to get rid of the solvent. The ability to prevent cracks from forming is one of the most important properties of this technology. It can produce integrated components by molding and curing the gel that is produced. Products made by molding or casting are utilized as filters and membranes.
Thin films with a thickness of 50–500 nm can be created using the sol-gel method. Sol-gel thin films are used in both the chemical and electronic sectors for a range of applications. Coatings created using the sol-gel technique also affect the material’s optical properties. Materials that are composite or nanocomposite can be created using the sol-gel technique. This is accomplished by loading secondary components into continuous porosity at the nanoscale. To create denser parts, the produced or synthesized pieces are put through sintering operations. Because the large specific surface area accelerates the rate of compaction or compaction of the structure, the nanoporous gels compact better and more quickly during the sintering process. But it is crucial to remember that increasing the temperature during this process encourages grain growth and results in a microstructure with coarse grains.
3. What is sol and gel?
3.1 What is sol?
In a liquid phase, a sol is a stable suspension of colloidal solid particles. These solid particles are small enough and denser than the liquid they are in, so the forces of dispersion outweigh the forces of gravity. Lyophilic and lyophobic sols are sub-classified. A lyophobic sol is one in which the solvent-particle interaction is relatively weak, whereas a lyophilic sol is one in which this interaction is quite strong. When the forces between two particles are repellent and the surface’s assisting forces keep the particles from agglomerating and coagulating, sols become stable. When another additive removes the particle’s charge, flocculation occurs and the system disintegrates, which causes gel to develop.
3.2 What is sol-gel?
A gel is a colloidal dispersion of tiny particles in a liquid. It is a translucent, porous, three-dimensional solid network that is interconnected and porous. In other terms, gel is a two-part, liquid system with a semi-solid character. A sol or solution turns into a gel during the process of gelation, and the gel’s continuous solid structure provides it flexibility. These solid particles that are present in the gel can be macromolecules, amorphous solids, or crystalline solids. The gel is recognized as colloidal when the solid network is composed of colloidal sol particles and as polymeric when the solid network is composed of sub-colloidal chemical units. Colloidal gels and polymeric gels differ primarily in that the former’s sol-gel transition is brought on by a physicochemical effect, while the latter is brought on by chemical bonding as opposed to a chemical reaction like poly-condensation. Sols have a high surface area-to-volume ratio, which makes them thermodynamically unstable.
4. Sol-gel processes
For the development of various types of nanostructures, particularly metal oxide nanoparticles, sol-gel method is one of the most attractive and widely used wet chemical methods. The hydrolysis of a precursor-containing solution yields suspended colloidal particles, which is the basis of this procedure. The creation of a homogenous sol from the precursors and its transformation into a gel is the foundation of the sol-gel process. In the conventional procedure of sol-gel procedure, the molecular precursor mainly a metal alkoxide is dissolved in water or alcohol and stirred until it gels. Since the gel created during the hydrolysis process is wet in nature, it needs to be dried appropriately depending on its use and desired qualities. For instance, burning alcohol is used to complete the drying process if the solution is alcoholic. The generated gels are pulverized and then calcined after the drying stage. Due to the low reaction temperature and cost-effectiveness of the sol-gel process, the chemical composition may be controlled well. The leftover gel is then dried after the solvent in the gel is removed from the gel structure. The dried gel’s characteristics are highly dependent on the drying method. In other words, the “removing solvent method” is chosen in line with the intended use of the gel. Industries including surface coating, building insulation, and the creation of specialty garments all use dried gels in various ways. It is important to note that nanoparticles can be produced by crushing gel in specialized mills.
A stable colloidal solution termed sol is first produced during the sol-gel process.
The sol is a liquid suspension of 1 nm to 1 micron-sized solid particles.
It can be made by partial condensation and hydrolyzation of precursors of an inorganic salt or a metal alkoxide.
A gel is developed when sol particles are further condensed into a three-dimensional network.
In the gel, which is a diphasic substance, the solids enclose the solvent.
The generated oxide species have a steadily rising molecular weight. When using water as a solvent, the substances are known as an aqua sol or aqua gels, and when using alcohol, they are known as aquosol or alcogel.
The product that results from evaporating gels is known as xerogel.
Aerogels are the dried gels produced by supercritical drying of gels. High porosity and high pore volume are retained by the aerogel.
Different forms of gel formation are shown in Figure 5.
Figure 5.
Different forms of gel formation through the sol-gel method.
The sol-gel method is distinguished from other routes of material preparation from solutions or melts such as precipitation and crystallization by two main characteristics:
Formation of clear colloidal solution due to primary condensation of dissolved molecular precursors.
These colloidal particles merge during subsequent gelation stage into polymeric chain by chemical bonding between local reactive groups at their surface.
Formation of a metal oxide involves connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metal-hydroxo polymers in solution.
The sol-gel process allows synthesizing ceramic materials of high purity and homogeneity by a process that occurs in a liquid solution of organometallic precursors (TMOS, TEOS, Zr (IV)—Propoxide, Ti(IV)—Butoxide, etc.), which, by means of hydrolysis and condensation reactions, lead to the formation of a new phase (SOL).
Effect of catalyst on hydrolysis and condensation in sol-gel method is shown in Figure 6.
Figure 6.
Effect of catalyst on hydrolysis and condensation in sol-gel method.
A summary of the sol-gel method’s steps is shown in Figure 7.
Figure 7.
Different forms of materials formation through sol-gel method.
4.1 Hydrolysis step
The sol-gel method is a chemical process used to produce solid materials from small molecules, typically metal alkoxides. The hydrolysis step of the sol-gel process involves the reaction of a metal alkoxide with water to produce metal hydroxide and alcohol.
The hydrolysis reaction occurs in the presence of a catalyst, typically an acid or base, which promotes the reaction between the metal alkoxide and water. The reaction can be represented by the following equation:
Where M represents a metal atom, R represents an organic group, and n is the number of organic groups attached to the metal atom.
During the hydrolysis step, the metal hydroxide and alcohol formed can further react to form a gel-like network of metal oxide particles. This gel network can then be dried and calcined to produce a solid material.
The hydrolysis step is crucial in the sol-gel process as it determines the size and morphology of the resulting metal oxide particles and the properties of the final product. Control of the reaction conditions, such as the pH and temperature, is important to achieve the desired product properties.
4.2 Condensation step
The condensation step of the sol-gel method involves the reaction of metal hydroxide species formed during the hydrolysis step to form metal oxide particles. The condensation reaction occurs in the presence of a catalyst or under controlled conditions such as temperature, pH, and solvent choice.
The condensation reaction can be represented by the following equation:
Where M represents a metal atom and n is the number of hydroxyl groups attached to the metal atom.
The reaction involves the elimination of water molecules and the formation of metal-oxygen-metal bonds, leading to the formation of a network of metal oxide particles. The resulting structure can range from discrete particles to a continuous gel network depending on the reaction conditions.
The properties of the resulting metal oxide particles are influenced by the reaction conditions during the condensation step. Factors such as temperature, pH, and catalyst concentration can affect the particle size, morphology, and crystallinity of the final product.
The condensation step is a critical step in the sol-gel process as it determines the structure and properties of the final product. Careful control of the reaction conditions is necessary to produce materials with desired properties.
4.3 Olation step
The olation step in the sol-gel method involves the reaction of metal-organic complexes or metal carboxylates with a base to form a stable metal-oxide precursor. This step is also referred to as deprotonation, and it is typically used to improve the reactivity and stability of the precursor.
The reaction can be represented by the following equation:
Where M represents a metal atom, R represents an organic group, and n is the number of organic groups attached to the metal atom.
The reaction occurs in the presence of a solvent, typically water or alcohol, and a catalyst such as a base. The metal-oxide precursor formed during the olation step is often more reactive and stable than the metal-organic complex and can be used to produce a sol.
The olation step is critical in the sol-gel method as it helps to improve the reactivity and stability of the precursor, which in turn affects the properties of the final product. The properties of the metal-oxide precursor are dependent on the nature of the metal precursor, the type of organic acid used, and the reaction conditions such as temperature and pH.
Overall, the sol-gel method is a versatile approach for synthesizing metal oxide materials with unique properties, and the olation step is an essential component of this process.
4.4 Oxolation step
The oxolation step in the sol-gel method involves the reaction of metal alkoxide or metal salt with organic acid or carboxylic acid to form a stable metal-organic complex. This step is also known as chelation, and it is typically used to improve the stability of the sol by preventing the hydrolysis of the metal precursor.
The reaction can be represented by the following equation:
Where M represents a metal atom, R represents an organic group, and n is the number of organic groups attached to the metal atom.
The reaction occurs in the presence of a solvent, typically alcohol, and a catalyst such as a base. The metal-organic complex formed during the oxolation step is soluble in the solvent and can be used as a precursor to form a sol. The oxolation step is often followed by the hydrolysis and condensation steps to produce a gel-like network of metal oxide particles.
The oxolation step is critical in the sol-gel process as it helps to improve the stability of the sol, which in turn affects the properties of the final product. The stability of the sol is dependent on the nature of the metal precursor, the type of organic acid used, and the reaction conditions such as temperature and pH.
Overall, the sol-gel method is a versatile approach for synthesizing metal oxide materials with unique properties, and the oxolation step is an essential component of this process.
4.5 Sol formation
The hydrolysis reaction forms the foundation of this phase. A small amount of water is introduced to the reaction medium to start the hydrolysis reaction of precursor material. Large molecules are broken down into smaller parts by the consumption of water in the hydrolysis reaction. When a homogenous solution is developed in a water-free solvent, water is added. When water is present, the precursor undergoes a hydrolysis reaction that somehow activates it, causing the metal oxide particles to form into small, solid particles that are disseminated in the solvent resulting in the formation of sol. In a true sol, the solute is equally dispersed as an atom, molecule, or ion in the solvent, and the particle size is less than 1 nm. When the size of the particles ranges between 1 and 100 nanometers, the mixture is referred to as a colloid, and they typically remain dispersed. The sol is actually a solution or, more correctly, a colloidal mixture since it is made up of extremely small particles (less than 100 nm) scattered in the solvent phase.
4.6 Gel formation
It just takes a small amount of stimulation to the solution to cause the distributed tiny particles, each of which contains many molecular or atomic units of the respective precursors, to start forming a gel [66]. A suitable reagent can be used to perform this stimulation (pure water or water with NaCl and NaOH). Units of tens of thousands of molecules line up together to form an infinitely large, three-dimensional molecule that fills the entire volume of the reaction vessel by causing physical and chemical interactions between suspended particles and the solution. Wet gel is a massive molecule that holds all the solvent within its numerous holes. Sol-to-gel conversion is an inorganic polymerization reaction whose ultimate product is an oxide network including MOM metal oxide clusters, controlled by reactions referred to as condensation. When two simple molecules join to create a more complex molecule in the compaction reaction, a tiny molecule, like water, is released developing a long-range well-ordered three-dimensional network structure. Depending on the solvent employed or the drying technique, several types of gels are formed, and they have distinct properties and uses. Colloidal network formation in sol-gel materials is shown in Figure 8.
Figure 8.
Colloidal network formation in sol-gel materials.
4.7 Electrostatic stabilization of the sol
The electrostatic repulsion caused by charges adsorbed on the particles and attractive Van der Waals forces are combined to provide the net force between the particles in a sol. In a liquid medium, two microscopic particles develop a Van der Waals interaction. Some particular ions are frequently preferentially adsorbed on the surface of solid particles when disseminated in a liquid medium that also contains an electrolyte. Near the charged surface, a surface charge forms in the solution. These dispersed layers that overlap and encircle two sol particles repel one another. If the van der Waals force of attraction is greater than the repelling forces caused by the surface charges, the sol will be stable. The two types of ions that make up the double layer—charge-determining ions, which regulate the charge on the surface of the particles, and counter ions, which are present in the solution close to the particle—are what create the repulsive barrier. The potential difference between the two layers is known as “zeta potential,” and the counter ions serve as screen charges for the determining ions’ potential. The size of the zeta potential is also connected to a colloid’s stability. The point of zero charge is the pH at which a particle is neutrally charged, and the isoelectric point is the potential at which the value of zeta potential is zero (PZC).
4.8 Electro steric stabilization of the sol
By adhering sterically packed organic molecules to the surface of the colloidal particles, steric stabilization can be accomplished. This organic coating that has been adsorbed creates a steric barrier that prevents the two particles from coming too close to one another in terms of both enthalpy and entropy. The adsorbed layer must fulfill the following criteria in order to function as a barrier effectively:
The particle’s surface must be entirely covered.
The organic molecules should be chosen so that the resulting energy barrier is greater than or substantial enough to defeat the alluring van der Waals forces.
Stabilization by electrostatic effects can result from the combination of steric stabilization and electrostatic stabilization.
5. Factors influencing the sol-gel reaction
The hydrolysis and condensation reactions can be affected by a variety of factors. The water-to-alkoxide ratio, the kind and quantity of the catalyst, the kind of organic group(s), pH, and solvent effects are among those that are most important. These elements will now be taken into account.
5.1 Water-to-alkoxide ratio
The water-to-alkoxide ratio has an impact on the sol-gel process such that as the ratio rises, the amount of oxide formation increases. As a result, each alkoxide group requires at least one mole of water for full hydrolysis. Some studies have gone even further, asserting that re-esterification, which is the reverse process, will happen more quickly than the forward reaction if more than one mole of water is utilized for each alkoxide group. The water/alkoxide ratio did not correlate with the accomplishment of complete hydrolysis in a recent publication by Mc Cormick et al. Because water is produced in the process
5.2 Type of network modifier
Organically modified oxides are produced when the precursor is combined with a capping agent that is chemically linked directly to the metal atom. Co-ordination centers are caused by organic groups and condensation functionality of less than four and affect the connectivity of the sol-gel network in two ways by altering the reactivity of the alkoxy groups. The diffusion of partly hydrolyzed molecules is necessary for the formation of oxide linkages. The rate of diffusion is slowed down and, as a result, there is less interconnection within the network. It is also claimed that polymers with greater surface areas are produced by alkyl groups with a larger size through this network formation. Higher concentrations of unreacted oxide are possible in gels with larger surface areas. The sol-gel network experiences a branching effect as a result. Along with these two aspects, it has also been previously mentioned that steric factors, such as the presence of bulky and/or lengthy alkyl substituents, may slow down the condensation process impacting the network formation.
5.3 Solvent effect
The regulation of hydrolysis and condensation rates can be significantly impacted by the addition of exogenous solvents. Tetrahydrofuran, formamide, dimethylformamide, and oxalic acid are among the solvents that fall under the category of drying chemical control additives, which exclude the typical cosolvents of water and alcohol. Solvent type may have a significant impact on condensation rates. Protic solvents appear to have little to no impact on the rate of condensation. By placing the negative end of the polar aprotic around the cation and adding unshared electron pairs to the cation’s empty orbitals, the cation is solvated. The transition state is stabilized by this mechanism, which also speeds up condensation. For sol-gel reactions inside a membrane template, the use of DCCAs may be advantageous.
5.4 Type and amount of catalysis
The presence of a catalyst can speed up a chemical reaction. This is extremely pH sensitive in a lot of sol-gel chemistry. This is due to the fact that whereas bases (OH) and acids (H+) are both catalysts, they do so through various processes. Whether a specific concentration is required should be taken into account when choosing the catalyst concentration. Water is produced
5.5 pH
pH has a significant impact on any colloidal chemistry involving water. As sol-gel chemistry is catalyzed by the presence of acid, the presence of H+ catalyzes comprehensively affects hydrolysis and condensation reaction rates and sol-gel chemistry is extremely pH sensitive. The increase in pH rapidizes the gel formation process. This is due to the fact that whereas bases (OH−) and acids (H+) are both catalysts, they do so through various processes. It has been discovered that reaction rates are heavily influenced by pH, a number of pH-dependent rate profiles have been described in different literatures.
5.6 Solvent
The solvent has two crucial roles in the polymerization process as molecules are formed into nanoparticles. First, it must be able to hold the dissolved nanoparticles in the liquid so that they do not precipitate out of it. Secondly, it must play a role in aiding nanoparticle connectivity.
5.7 Temperature
Temperature impacts the gel duration because it accelerates the chemical kinetics of the many events involved in nanoparticle synthesis and the assembly of the nanoparticles in a gel network. Gelation is a gradual process that can take weeks or months at very low temperatures. The reactions that bind the nanoparticles to the gel network, on the other hand, happen so quickly at high temperatures that lumps form in their place, and a solid precipitates out of the liquid. To maximize the reaction time, the gelation temperature needs to be adjusted.
5.8 Time
The various phases in the gel formation process operate differently at various time scales depending on the sort of gel that is desired. In order to create a more homogeneous structure and a stronger gel, it is generally advised that the gel-forming process be delayed. Accelerating processes over a short period of time result in the formation of precipitates rather than a gel network, which can make a gel weak and hazy or prevent it from forming altogether.
5.9 Agitation
The mixing of the sol during gelation at this point should guarantee that the chemical reactions in the solution are produced equally, allowing all molecules to obtain an appropriate supply of the chemicals they need for these reactions to be carried out properly. In most cases, microscopic and macroscopic gel network domains are partially created throughout the liquid; however, agitation can occasionally disrupt the creation of these domains, causing the network fragments to reassemble into a larger network.
6. Advantages of the sol-gel method
One of the top methods for material synthesis so far is sol-gel technology. The use of the sol-gel process is quite beneficial for creating superior materials. Since the materials’ introduction in 1960, the less expensive sol-gel processing method has been the most effective means to prepare target materials for both extensive and intensive study. By using the sol-gel process, metallic, inorganic, organic, and hybrid materials can be created. The sol-gel process is essential for producing a wide range of materials, from those needed for everyday usage to those used in extremely complex applications such as photonics, electronics, mechanics, biology, and medicine. The result of the sol-gel method is an improvement in the processing of traditional materials and their properties, as well as the synthesis of new materials. Due to its low-temperature nature, the organic-inorganic hybrids sol-gel technique is highly helpful for creating high-performance liquid chromatography. The following are some benefits of the sol-gel technique:
Easy procedure.
The creation of extremely pure products.
The efficiency of synthesis is very high.
Synthetic optical components have intricate shapes.
Consistent synthesis of composite oxides.
Customizing composition design and control for the synthesis of homogeneous materials.
The use of fibers, aerogels, and materials with unique shapes.
More thorough surface coverage.
Synthesis of thin-layer amorphous materials.
Synthesis of materials with specialized physical properties, including low thermal expansion coefficient, low UV absorption, and high optical transparency.
Production of porous and dense materials using organic and polymeric compounds.
Precursors have high chemical reactivity as a result of the solution phase process.
The creation of low-cost, high-quality materials. Glass can be fabricated using the low-temperature sol-gel technique in a variety of shapes, from the most basic to the most complex.
Synthesis at low temperature.
Preparation of high-purity products.
Very high production efficiency.
Production of optical components with complex shapes.
Synthesis of uniform compounds in the form of composite oxides.
Can produce thin bond coating to provide excellent adhesion between the metallic substrate and the top coat.
Can produce a thick coating to provide corrosion protection performance.
Can easily shape materials into complex geometries in a gel state.
Can produce high-purity products because the organometallic precursor of the desired ceramic oxides can be mixed, dissolved in a specified solvent, and hydrolyzed into a sol, and subsequently a gel, the composition can be highly controllable.
Can have low-temperature sintering capability, usually 200–600°C.
Can provide a simple, economic, and effective method to produce high-quality coatings.
7. Applications of sol-gel technology
Thermal insulation.
Acoustic insulation.
Protective optical coatings.
Lightweight materials.
Tough ceramics.
Membranes and microfilters.
Nuclear waste storage.
Ultra-fine powder abrasives.
Encapsulation of biomolecules for controlled drug release.
It can be used in ceramics manufacturing processes, as an investment casting material, or as a means of producing very thin films of metal oxides for various purposes.
Sol-gel-derived materials have diverse applications in optics, electronics, energy, space, (bio) sensors, medicine (e.g., controlled drug release), and separation (e.g., chromatography) technology. One of the more important applications of sol-gel processing is to carry out zeolite synthesis.
Other elements (metals, metal oxides) can be easily incorporated into the final product and the silicalite sol formed by this method is very stable.
Other products fabricated with this process include various ceramic membranes for microfiltration, ultrafiltration, nanofiltration, preevaporation, and reverse osmosis.
Different applications of sol-gel technology is shown in Figure 9.
Figure 9.
Applications of sol-gel technology.
8. Disadvantages of sol-gel processing
The loose dispersion of particle proportions and particle clustering are two drawbacks of this method. Despite all the advantages, sol-gel technology has certain restrictions.
sluggish kinetics, poor wear resistance, and weak bond formation.
The sol-gel method is a laborious and drawn-out process that requires extra care, particularly during times of drying and aging.
Monoliths may shrink during the densification step, and this can lead to surface cracking brought on by capillary forces.
Sol-gel materials cannot be employed in large-scale optical coatings since the required precursors are pricey and moisture-sensitive.
The property of the material will be diminished by the development of undesirable byproducts because the process involves numerous chemical reactions. Materials formed from sol-gel can be produced in a variety of shapes, including monoliths, powders, fibers, and films. Sol-gel materials can be categorized according to their uses as catalysts, inorganic pigments, pharmaceuticals, and magnetic and metallic nanoparticles, as well as materials used to encase biological molecules like proteins and enzymes. Films are crucial from a technological perspective, and in this review, we concentrate primarily on sol-gel-generated films that are produced
9. Summary
The science and technology of sol-gel have the ability to significantly alter the characteristics of materials. The fact that sol-gel science operates at room temperature and is insensitive to the atmosphere is one of its most notable advantages. These properties enable it to be used with a variety of substances that cannot withstand high temperatures and do not require the researcher to take extra. Any sol-gel system can have its water content, modifiers, and solvent changed, which changes the network’s connectivity and impacts the characteristics. Sol-gel chemistry is used in a number of ongoing research. The sol-gel method is a popular and useful technique for producing nanoparticles with different chemical compositions. One of the widely utilized, adaptable materials in the field of nanotechnology is sol-gel-based nanoparticles, thin films, and fibers. A wide variety of applications have been extensively employed for the sol-gel-derived nanomaterials. This chapter largely focuses on the principles of sol-gel science with an emphasis on the fundamentals of creating nanomaterials and their many applications.
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