4.1. Hydrothermal/Solvothermal Method

Hydrothermal and solvothermal techniques are the most important methods for the synthesis of various kinds of monodispersed and highly homogeneous nanomaterials. Using these methods, nanomaterials are fabricated using a typical wet-chemical approach, with high pressure and temperature, in aqueous solvents that dissolve and recover the materials. All the reactants are dissolved in an autoclave with a suitable solvent under low or high pressure and temperature conditions depending on the desired composition, crystal structure, size, and shape of the nanomaterials. The main advantage of this approach is that, with high vapor pressures and minimal loss of nanomaterials, the procedure is well controlled through liquid-phase or multiphase chemical reactions. Its disadvantages include the use of expensive equipment and high temperatures. Many bimetallic NPs, such as Ni–Fe, co-doped Zn1−xCoxMn₂O, and NiFe₂O₄ NPs, have been designed using the hydrothermal method [98,99,100]. Commonly, the hydrothermal process involves the use of solvent and surfactant; however, the surfactant-free synthesis of multimetallic NPs on an electrode surface was recently reported successfully [101].