# Spinel Iron Oxide by the Co-Precipitation Method: Effect of the Reaction Atmosphere
## 3 Results and Discussions
### Morpho-Structural Properties
SAED patterns are composed of diffraction rings having the same diameters and intensities for both samples (see supporting information for further details). The interplanar distances associated with the diffraction rings reveal that they are both composed of polycrystalline iron oxide with a spinel structure, confirming XRD results. High\(-\)resolution TEM (HR\(-\)TEM) observations allowed us to better investigate the particles morphology. Figure 3c,d show HR\(-\)TEM images of FO\(-\)CA and FO\(-\)NA particles, respectively. Looking at the atomic planes, it is evident that FO\(-\)CA particles have a quasi-spherical shape while the FO\(-\)NA ones are generally faceted. Generally speaking, in colloidal synthesis, the formation of crystals is discussed in terms of nucleation and growth theory where chemical species are supposed to assemble directly from solution [38]. Accordingly, the best way to control the particle size and shape, is to separate these two steps (i.e., the nucleation step should be avoided during the growth of the nuclei [39]). Unfortunately in the co-precipitation method, it is not possible to separate the nucleation and the growth steps [40; 41]. In typical co-precipitation \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{**Sites**} & \(\delta\) & \(2\epsilon\) & \(B_{\textit{eff}}\) & \(B_{\textit{Hup}}\) & \(\theta\) & \(\psi\) \\  & & (mm.S\({}^{-1}\)) & (mm.S\({}^{-1}\)) & (T) & (T) & (T) & (\({}^{\circ}\)) & \% \\ \hline \multirow{2}{*}{FO\(-\)CA} & A & 0.36 & \(-\)0.01 & 60.5 & 52.8 & 16 & 38 \\ \cline{2-7}  & B & 0.50 & \(-\)0.00 & 46.2 & 53.8 & 20 & 62 \\ \hline \multirow{2}{*}{FO\(-\)NA} & A & 0.35 & \(-\)0.03 & 60.4 & 52.8 & 16 & 39 \\ \cline{2-7}  & B & 0.51 & \(-\)0.03 & 46.1 & 53.9 & 15 & 61 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of obtained values. of hyperfine parameters (Isomer Shift, Quadrupolar Shift, Effective and Hyperfine fields, Angle \(\theta\), and weight) obtained at 12 K under external field of 8T. Figure 2: \({}^{\mathrm{SF}}\)Fe Mössbauer spectra measured at 12 K under high magnetic field (8 T) and fitted with 2 sextets corresponding to iron in A\(-\)site, and B\(-\)site (fitted using a Lorentzian lines) for (**a**) FO\(-\)CA and (**b**) FO\(-\)NA. synthesis, the pH of ferrous and ferric solution is increased via the addition of the base. Depending on the method of base addition [38] (e.g., continuous or abrupt addition) different formation pathways of magnetite nanoparticles can be observed [42; 43]. In the investigated synthesis procedure, the base was added in abrupt way (i.e., pH quickly increases from 2 to 9.2), resulting in significant pH gradient leading to the formation of lepidocrocite(\(\gamma\)-FeOOH) at pH \(7-8\)[44; 45; 46; 47],then transformed into spinel iron oxide at higher pH. Moreover, the presence of oxygen in a reaction medium is a key factor in promoting the oxidation of Fe\({}^{2+}\) to Fe\({}^{3+}\)[48; 49]. In our case, the produced nanoparticles under different synthesis conditions(O\({}_{2}\)/N\({}_{2}\)) show different particles shape. The surface structure passes from quasi-spherical in FO\(-\)CA sample to faceted for FO\(-\)NA sample. For the latter, the excess of O\({}_{2}\) (air) creates higher Fe\({}^{2+}\)/Fe\({}^{3+}\) oxidation rate, results in transforming most of the Fe(OH)\({}_{2}\) to \(\gamma\)-FeOOH then through a topotactic process transforms into magnetite [47; 50]. This formation path way leads to irregularly shaped particles as observed in TEM image. In case of FO\(-\)CA sample, the synthesis under inert atmosphere lead to slow oxidation rate until the complete mixing of the solution (i.e., homogeneity in the pH of the solution) then direct transformation to magnetite involving the remaining Fe\({}^{2+}\) in solution resulting in more uniformed and spherical particles shape [51].