Metal oxide nanoparticles (MONPs) have enormous applications such as in optical devices, purification systems, biomedical systems, photocatalysis, photovoltaics etc. In this review, we have explored a stable and efficient synthesis protocol of particularly four MONPs: titanium dioxide (TiO2), tin oxide (SnO2), tungsten oxide (WO3) and zinc oxide (ZnO) for getting desired chemical composition, nanostructure, and surface properties. The selection of an efficient synthesis process is a key factor that significantly influences the efficacy of the MONPs. The chemical synthesis of nanoparticles (NPs) via sol–gel route is an effective method to produce high-quality MONPs in comparison to other physical and chemical methods. Sol–gel synthesis is one of the simple, fastest and economically less expensive method, and has its own advantages like low processing temperature, homogeneity of the produced material and formation of the complex structures or composite materials. We believe that this detailed review will provide an insight into sol–gel synthesis of MONPs along with their characterization and diverse applications.

Metal oxides nanoparticles via sol–gel method: a review on synthesis, characterization and applications

Currently, thousands of tons of metallic nanoparticles (MNPs) are produced and utilized in nano-enabled devises, personal care, medicinal, food and agricultural products. It is generally accepted that the reaction compounds and the techniques used in industrial production of MNPs are not environmentally friendly. The green synthesis has been proposed as an alternative to reduce the use of hazardous compounds and harsh reaction conditions in the production of MNPs. In this endeavor, investigators have used organic compounds, microbes, plants and plant-derived materials as reducing agents. Research papers are published every year, and each one of them stresses the benefits of the green approach and the advantages over the traditional syntheses. However, after almost two decades since the explosion of the reports about the new approach, the commercial production of green-synthesized nanoparticles does not seem to find a way to scale up commercial production. This review includes descriptions of the traditional and green synthesis and applications of MNPs and highlights the factors limiting the use of plant-based synthesis as a real alternative to the traditional synthesis of MNPs.

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Plant-based green synthesis of metallic nanoparticles: scientific curiosity or a realistic alternative to chemical synthesis?

Synthesis of g-C 3 N 4 /CeO 2 nanocomposites with improved catalytic activity on the thermal decomposition of ammonium perchlorate

Nanoceria has recently received much attention, because of its widespread biomedical applications, including antibacterial, antioxidant and anticancer activity, drug/gene delivery systems, anti-diabetic property, and tissue engineering. Nanoceria exhibits excellent antibacterial activity against both Gram-positive and Gram-negative bacteria via the generation of reactive oxygen species (ROS). In healthy cells, it acts as an antioxidant by scavenging ROS (at physiological pH). Thus, it protects them, while in cancer cells (under low pH environment) it acts as pro-oxidant by generating ROS and kills them. Nanoceria has also been effectively used as a carrier for targeted drug and gene delivery in vitro and in vivo models. Besides, nanoceria can also act as an antidiabetic agent and confer protection towards diabetes-associated organ pathophysiology via decreasing the ROS level in diabetic subjects. Nanoceria also possesses excellent potential in the field of tissue engineering. In this review, firstly, we have discussed the different methods used for the synthesis of nanoceria as these are very important to control the size, shape and Ce3+/Ce4+ ratio of the particles upon which the physical, chemical, and biological properties depend. Secondly, we have extensively reviewed the different biomedical applications of nanoceria with probable mechanisms based on the literature reports. The outcome of this review will improve the understanding about the different synthetic procedures and biomedical applications of nanoceria, which should, in turn, lead to the design of novel clinical interventions associated with various health disorders.

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Synthesis and biomedical applications of nanoceria, a redox active nanoparticle

Controllable synthesis and optical properties of nano-CeO2 via a facile hydrothermal route

Nanocrystalline cubic fluorite/bixbyite CeO2 or α–Mn2O3 has been successfully synthesized by using methanol as a solvent via sol–gel method calcined at 400 °C. The obtained products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), UV–vis absorption and Photoluminescence (PL) spectroscopy. TEM reveals that the as-synthesized ultra-fine samples consist of elliptical/spherical and sheet-like morphology of crystalline particles of 8/30 nm, which are weakly aggregated. Optical absorbance spectra reveal that the absorption of ceria in the UV region originates from the charge- transfer transition between the O2− (2p) and Ce4+ (4f) orbit in CeO2. However, α–Mn2O3 nanostructures with nearly pure band gap emission should be of importance for their applications as UV emitters.

Synthesis of CeO2 or α–Mn2O3 nanoparticles via sol–gel process and their optical properties

Tin oxide (SnO2) nanoparticles were synthesized by the reaction of SnCl4·5H2O in methanol, ethanol and water via sol–gel method. The samples were characterized by X-ray diffraction (XRD), Fourier transform infrared, Scanning electron microscopy and Transmission electron microscopy. The optical properties of the as-prepared samples were investigated. The XRD analysis showed well crystallized tetragonal SnO2 can be obtained and the crystal sizes were 3.9, 4.5 and 5 nm for the sample calcined at 400 °C for 2 h. It was found that solvents played important roles in the particle size effect of nanocrystalline SnO2.

Synthesis of tin oxide nanoparticles by sol–gel process: effect of solvents on the optical properties

Facile sol-gel preparation of Cd-doped cerium oxide (CeO 2 ) nanoparticles and their photocatalytic activities

Nanocrystalline cadmium doped tin oxide (SnO2) powders of about 2.5–4.5 nm in size have been synthesized by using different solvents via sol–gel method. The obtained samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), Energy dispersive X-ray analysis (EDX), Transmission electron spectroscopy (TEM), UV-Vis absorption and Photoluminescence (PL) spectroscopy. The PL emission spectra revealed that the band centered at 452 nm might be related with oxygen vacancies. A spherical, small rod and slice like morphologies of the prepared Cd-SnO2 nanoparticles were observed in the SEM and TEM studies. The presence of Cd modifies the structural, morphological and optical properties of the tin oxide nanoparticles.

S. Gnanam

Papers

Synthesis of CeO2 or α–Mn2O3 nanoparticles via sol–gel process and their optical properties

Synthesis of tin oxide nanoparticles by sol–gel process: effect of solvents on the optical properties

Facile sol-gel preparation of Cd-doped cerium oxide (CeO 2 ) nanoparticles and their photocatalytic activities

Preparation of Cd-doped SnO2 nanoparticles by sol–gel route and their optical properties

Facile hydrothermal synthesis of alpha manganese sesquioxide (α-Mn2O3) nanodumb-bells: Structural, magnetic, optical and photocatalytic properties