*2.3.2 Plasma assisted synthesis*

The plasma state is obtained from the ionization of noble gases, which increases the reactivity of the medium rich in electrons and chemical species upon reaching a very high temperature, up to 10000K [29]. Non-thermal plasma is an advantageous alternative that can be operated at atmospheric pressure [30]. Among the different types of systems used for plasma production, gliding arc discharge is reported to attractive for MO-NPs synthesis.

Under humid air as feeding gas, glidarc discharge produces acidic and very oxidizing environment in which many transitions metal ions precipitate as hydroxides or oxides [31–33]. However, one of the major disadvantages associated to this technique is the poor control on the size range and morphology of the particles.

## *2.3.3 Extraction from ores*

Mineral ores are naturally occurring rock, from which the metal content can be extracted via hydrometallurgy [34]. During MO extraction, the ore is successively crushed, roasted, leached, and precipitated. The leaching step consists in the solubilization of the metal species under its ionic forms by the action of a strong acid or a strong base. Usually, impurities remain insoluble and are removed from the mixture.

The metal ions remaining in solution are then concentrated by solvent (water) evaporation before further treatment for the precipitation. When the mineral contains one major species, single MO particles are produced, but for more complex ore, composite materials can be obtained [35, 36].
