Introductory Chapter: Sol-Gel Synthesis

1. Introduction

Sol-gel technology is by far one of the best techniques for synthesis of materials for both extensive and intensive research since the inception of the materials way back in 1960 [1, 2, 3, 4, 5]. An improvement in the processing of conventional materials and their properties as well as novel materials synthesis are the outcomes of the sol-gel method. For the preparation of high-performance liquid chromatography, organic-inorganic hybrids’ sol-gel method is very useful due to its low-temperature nature [6, 7, 8]. A schematic showing the formation of nanoparticles is depicted in Figure 1. Nanoparticles (magnesium ferrite (MgFe₂O₄₎, in this case) can be transformed to bulk MgFe₂O₄. This reflects the ability of this method to synthesise both nanoparticles and bulk particles in a very easy process [9].

Figure 1.

Synthesis of magnesium ferrite (MgFe₂O₄) nanoparticles and their transformation to bulk.

In this synthesis, metal nitrates are used as starting materials; however, sol-gel processing can also be carried out using other salts of metals (M). Figure 2 depicts the mechanism of nanoparticle formation, either using acetate (molybdenum acetate (M(OAc)) or using halides (metal halides (MX)). Dimethyl formamide (DMF) was used as host matrix in this synthesis [10].

Figure 2.

Sol-gel chemical process followed by self-assembly approach for the formation of either metal oxides or metal hydroxide nanostructures. Reprinted with permission from [10].

In brief, advantages of the sol-gel technique are as follows:

• Simple process.

• Synthesis of highly pure products.

• Synthesis efficiency is very high.

• Complex shapes of synthesis of optical components.

• Uniform composite oxides synthesis.

• Tailoring of composition design and control of homogeneous materials synthesis.

• Use of specially shaped materials fibres and aerogels.

• Better surface coverage.

• Thin-layer amorphous materials synthesis.

• Low thermal expansion coefficient, low ultraviolet (UV) absorption and high optical transparency materials synthesis with tailored physical properties.

• Porous and rich materials production with organic and polymeric compounds.

• High chemical reactivity of precursors due to processing in solution phase.

• Less expensive and high-quality materials synthesis.

2. Application of sol-gel to nanoparticles

Considering these benefits, sol-gel processing is considered as a favourable processing technique for growing nanostructures. The technique is effectively used to grow metal nanoparticles [11], metal oxide nanoparticles [12, 13] and non-metallic oxide nanoparticles [14]. In the cases where certain doped elements are believed to induce impurity phases, sol-gel processing is considered very effective. For example, sol-gel processing allows higher concentration of rare earth ions inside ferrite nanoparticles with pure spinel phase [15, 16]. Figure 3 shows the X-ray diffraction patterns (XRD) of Dy^3 + −doped cobalt ferrite nanoparticles using sol-gel processing [17].

Figure 3.

Synthesis of dysprosium (Dy)-doped cobalt ferrite (CoFe₂O₄). No impurity phase is observed up to a Dy concentration (x) of 0.15. Reprinted with permission from [17].

Some complex oxides such as lithium lanthanum zirconic oxide (LLZO), a solid state electrolyte for Li ion battery [18], can also be effectively grown using this method (Figure 4) [19]. This method is considered suitable to optimise phase, composition and ionic conductivity of this material [19]. Synthesis of cathodes of this battery is also reported by this method [20].

Figure 4.

Synthesis process of aluminium (Al)-doped lithium lanthanum zirconium oxide (LLZO) by taking carbonates and nitrates as starting materials. A schematic showing the formation of nanoparticles is drawn, based on the description given in the report [19].

Sol-gel processing is also able to produce natural minerals such as calcium carbonate in the laboratory [21, 22]. Sensitivity of annealing temperature on the phase of calcium carbonate can be seen in Figure 5.

Figure 5.

Synthesis amorphous and crystalline carbonate. Reprinted with permission from [22].

This methodology is also able to produce nanostructure of different shapes, such as nanocubes [23], nanotubes [24], nanoplates [25], etc. Sol-gel processing also plays a vital role in thin film growth technology. By taking the help of dip/spin coater, thin films of appropriate material can be grown effectively [26]. Sol-gel is a low-temperature method of fabricating glass in shapes that can range from simple to very intricate. High-quality optical elements are possible, as are quality optical element-doped materials (even containing organic dyes) suitable for laser-gain media [27]. Thus, easy processing and its suitability to grow nanostructures of different materials make it favourable for commercial purposes too [28].

Acknowledgments

JPS is thankful to the Science and Engineering Board, Department of Science and Technology, New Delhi, for providing the Ramanujan Fellowship via Grant Number RJF/2021/000115.

Conflict of interest

The authors declare no conflict of interest.