Sol–gel synthesis method

The sol–gel method is a wet chemical method, being the most widespread method for the synthesis of photocatalytic semiconductors (Medina-Ramírez et al. 2015; Adeola et al. 2022; Thiagarajan et al. 2017). The final products can be crystalline and non-crystalline nanoparticles, ceramics, aerosols, and xerosols, depending on the final stage of thermal treatment. Precursor soil can be deposited on a substrate by coating or spin (Jamjoum et al. 2021). The method is cheap and allows control of the composition and the final product (Adeola et al. 2022). Through this method, semiconductors such as TiO₂ can be doped with boron and nitrogen in order to develop materials with advanced properties applied to water decontamination processes with various pollutants, such as methyl orange (MO) (Gombac et al. 2007). Other examples would be obtaining hybrid nanocomposite magnesium aminoclay (MgAC)-Fe₂O₃/TiO₂ used for the degradation of about 95% methylene blue (MB) and about 80% phenol from water (Bui et al. 2019).

Coupled oxide semiconductors of the p-n heterojunction-type ZnO/GO, Fe₂O₃/GO, ZnO/CuO, Nb₂O₅/TiO₂, Ta₂O₅/TiO₂, and SnO₂/TiO₂ were obtained by the sol–gel method, the metal oxide precursors being hydrolyzed under stirring, and the surface area of the metal oxide synthesized coupled leads to increased photocatalytic activity (Medina-Ramírez et al. 2015; Bayode et al. 2021a, b; Gajendiran and Rajendran 2014; Arbuj et al. 2013b, a; Nur et al. 2007).

The sol–gel technique is a versatile but complex method for preparing metal oxide nanoparticles. It involves a series of steps, including sol preparation, hydrolysis, polymerization, gel formation, solvent removal, and heat treatment, which can be time-consuming and intricate. One of the key challenges is achieving precise control over particle size and distribution, as uniformity and prevention of agglomeration can be difficult (Navas et al. 2021). Additionally, maintaining purity and minimizing contamination are crucial, as impurities from reagents or equipment can easily affect the quality of the final nanoparticles. The choice of precursor materials, cracking, shrinkage during drying, and the energy-intensive heating process further add to the complexities (Simon et al. 2009). Safety concerns and the need for specialized equipment and high-purity chemicals can also increase costs. Reproducibility can be a challenge due to the sensitivity of the process to various parameters (Modan and Schiopu 2020; Verma et al. 2021).

Despite these drawbacks, the sol–gel technique is valuable for tailoring metal oxide nanoparticles with unique properties. Researchers are actively working to refine the process and address these challenges to make it more efficient and reliable. Advancements in controlling particle size, minimizing impurities, and optimizing the process parameters are ongoing efforts to improve the utility of the sol–gel technique for nanoparticle synthesis.