**1.7. Green synthesis**

**1.1. Conventional ceramic method**

226 Raman Spectroscopy

**1.2. High energy ball milling**

**1.3. Microwave combustion method**

**1.4. Sol-gel method**

**1.5. Hydrothermal method**

**1.6. Co-precipitation method**

necessary equipment in academic laboratories.

process control agent, temperature of milling, etc. [9].

The conventional solid-state reaction, starting from metal oxides or carbonates, is the oldest method for the synthesis of ceramics, since it is economic, efficient, and easily scalable. In fact, despite of some disadvantages, such as particle agglomeration and growth, it is still regularly used to synthesize novel materials for the first time, also for the widespread diffusion of the

The use of mechanical milling in the preparation of inorganic materials (ceramics, metals, and alloys) has a long tradition. It is a cheap and relatively rapid method that, in some cases, guarantees the formation of the desired compounds without the need of other thermal treatment. The tuning of ferrites nanoparticles could be obtained by changing milling container, speed, time, ball-to-powder weight ratio, extent of filling the vial, milling atmosphere, presence of a

This method is actually one of the most diffused due to its numerous advantages: is a wet chemical technique, enables fast reaction rate, chemical homogeneity, and high reactivity due to the uniform heating produced by microwaves. Some drawbacks are represented by the difficulty to vary the spherical form of particles and to control the valence states of the elements [10].

The sol-gel technique is a wet low-temperature method that provides products with a perfect chemical composition and takes advantages over the others, owing to good stoichiometric control and homogeneity, short preparation time and inexpensive precursors. Furthermore, it provides nanoparticles with controlled sizes and defined morphology. Generally, for the production of multicomponent oxides, alkoxides (substituted in some cases by acetates) are put together in alcohol. In the sol-gel method water, alcohol, pH, concentration of alkoxides,

The hydrothermal route is a convenient wet way to produce well crystalline materials, with particles with suitably tuned size and shape. The main advantage of hydrothermal synthesis over conventional wet-chemistry methods is that it occurs under non-standard conditions, and that non-classical crystallization pathways can be explored. Many factors can be varied such as temperature, reagents ratio, solvents, and kind of salts to customize the ferrite nanoparticles [12].

This method is probably the most favorable to produce nanoparticles, as well as simple and very productive. A high homogeneity and uniformity can be easily obtained at moderate

and controlled temperature are required for proper hydrolysis [11].

Recently, plant-based green syntheses of nanoparticles have attracted the attention of researchers. Various green resources, like micro-organisms and plants, are available for these syntheses. The extract of plants not only acts as reducing agent toward the metal ions in a short time with respect to micro-organisms or the classical organic/inorganic substances, but also provides high-yield nanosized particles. The preparation time of nanoparticles depends upon the variety of plants and the concentration of phyto-chemicals. Plant-based green synthesis of magnetic nanoparticles is still under research [14, 15].

Among the so-called physical methods, the most used to produce zinc ferrites are the following.
