**1. Introduction**

Although heavily exploited in recent decades, the domain of oxide nanostructures remains of interest to researchers throughout the world. This is because that the shapes and sizes of oxide nanomaterials greatly influence their properties, which is reflected in their use in the most diverse fields [1, 2]. Oxide nanostructures have applications in catalysis, energy storage, environmental decontamination, microelectronics, medical technology, ceramics, cosmetics, and so on [3–5].

Among the most studied branches of nanostructures are metal oxides, with representatives such as TiO2, ZnO, CuO, Fe3O4, WO3, Cr2O3, Co3O4 [6].

The structure, morphology, and properties of the oxide nanostructures depend significantly on the obtaining method. A large number of available synthesis methods underlies the continuous interest in obtaining oxide nanostructures that can be used successfully in specific areas [1, 7]. However, most of these methods are limited due to the use of toxic reagents, high processing temperatures, high vacuum, expensive equipment, or long reaction times [8, 9].

Although physical methods have the advantage of high reproducibility, chemical methods in the liquid phase are more often used to obtain oxide nanostructures due to their advantages, such as low production temperature, homogeneous mixing of precursors at the molecular scale, design and control of the physico-chemical properties of final products, depending on the precursors, and the experimental conditions used [10, 11].

Among the various chemical procedures, the sol–gel method gained increasing importance in the field of materials science because it is cheap, simple, allows the introduction of dopants in large quantities, ensures high purity, and homogeneity, allows control of size, shape, and size distribution of the obtained nanomaterials [12–14].

Lately, for the preparation of functional nanomaterials, more and more attention is being paid to the use of microwave as the energy source for carrying out a chemical reaction [1, 15]. The microwave (MW) assisted sol–gel method is reported to be a simple, cheap, faster, more energy-saving, and efficient process as compared to conventional heating methods [16–18]. The use of microwaves has received increased attention in the technological field because, among other things, it reduces the reaction time from days to minutes or hours, improves the properties of synthesized nanostructures, and allows obtaining oxide nanocrystalline films on various substrates [8, 19, 20].

The improved properties of the oxide nanostructures obtained by microwaves assisted sol–gel method could be correlated to the influence of the microwaves on the chemical reactions that take place during the sol–gel synthesis, leading to the formation of different molecular species. Results on the influence of the microwaves on the chemical reactions during the sol–gel synthesis will be discussed in the present chapter.
