# Spinel Magnetic Iron Oxide Nanoparticles: Properties, Synthesis and Washing Methods
## 3 Main Synthesis Methods of Magnetic and Maghemite Nanoparticles
### Synthesis by Co-Precipitation
It is therefore preferable that the nucleation step is very short so that all the germs are generated at the same time and the next stage of growth is longer. In fact, considering a closed system, the number of germs obtained during the nucleation phase will determine the number of particles which can grow. The size of particles obtained at the end of the phase III will be determined by the quantity of precursors initially introduced by the number of germs formed in phase II. Another way to better control the size of nanoparticles obtained at the end of the synthesis involves limiting the effect of phase IV, in particular by limiting the Ostwald ripening. [PERSON] et al. [52] compensated for the consumption of precursor required for the growth of nanoparticles by adding precursor continuously during the synthesis. They thus maintained the constant concentration of the precursor (located between C\({}_{\rm min}\) and C\({}_{\rm S}\)) so that neither subsequent nucleation nor Ostwald ripening occurs. The nanoparticles growth is predictable, which makes it possible to obtain nanoparticles of the desired size. Figure 6: Lamer diagrams representing the concentration of aqua-hydroxy precursors complexes as well as the number in the reaction as a function of time. We can distinguish on these diagrams the different stages of the reaction. The shape and the size of nanoparticles obtained by co-precipitation can be adjusted relatively effectively by controlling the parameters of the synthesis. [PERSON], [PERSON] et al. [21; 53], one of the pioneers of this synthesis method of magnetic iron oxide nanoparticles, have carried out in-depth studies on the influence of different parameters such as the basis used (NH\({}_{4}\)OH, CH\({}_{3}\)NH\({}_{2}\), NaOH, KOH), the pH of precipitation, the adding of cations in the solution (N(CH\({}_{3}\))\({}^{+}\), CH\({}_{3}\)NH\({}_{3}\)\({}^{+}\), Li\({}^{+}\), Na\({}^{+}\), NH\({}_{4}\)\({}^{+}\)), and the effect of Fe\({}^{2+}\)/Fe\({}^{3+}\) ratio on the size and polydispersity of synthesized nanoparticles. They have shown in particular that, by adjusting these parameters, they can obtain samples with a controlled size between 4.2 and 16.6 nm. They also highlighted that certain conditions, especially the ratio of Fe\({}^{2+}\)/Fe\({}^{3+}\) introduced and the precipitation pH of nanoparticles, can affect the formation of a second phase constituted of oxohydroxide \(\alpha\)-FeOOH, denoted goethite, non-magnetic in addition to the magnetic oxides. [PERSON], [PERSON] et al. [23; 54] also studied that the synthesis parameters can influence the size and the monodispersity of nanoparticles. They have shown in particular that by adjusting the ionic force of the solution during the synthesis of nanoparticles by co-precipitation, they can obtain nanoparticles with a controlled size between 2 and 15 nm. They were able to define the best conditions to obtain batches of particles of uniform size, i.e., a pH of precipitation between 8.5 and 12.0 and an ionic force of the solution between 0.5 and 3.0 mol\(\cdot\)L\({}^{-1}\). This team also extended the studies around the influence of the ration Fe\({}^{2+}\)/Fe\({}^{3+}\) on the yield of magnetic materials. They have shown that for a ratio of 0.3, the two phases that are the maghemite/magnetite mixture and the goethite coexisted; for a ratio higher than 3.5, the goethite is not present but the magnetite formed is not stoichiometric; for a ratio of 0.5, the magnetite approaches stoichiometry and the size and the composition of nanoparticles become more homogeneous. Parallel to this marked influence of the molar ratio Fe\({}^{2+}\)/Fe\({}^{3+}\) on the composition of nanoparticles formed, [PERSON] et al. [55] observed that the average size of nanoparticles increases with the ratio Fe\({}^{2+}\)/Fe\({}^{3+}\). They also established that the nanoparticles formed kept interesting magnetic properties only for molar ratios Fe\({}^{2+}\)/Fe\({}^{3+}\) between 0.4 and 0.6. On the other hand, [PERSON] et al.