FIB plan view lift-out sample preparation for TEM characterization of periodic nanostructures obtained by spinodal decomposition in Co_1.7Fe_1.3O₄ thin films

Abstract

There is a miscibility gap in the CoFe2O4–Co3O4 phase diagram in which the oxides can be subjected to a spinodal transformation.

3D atomic-scale imaging of mixed Co-Fe spinel oxide nanoparticles during oxygen evolution reaction

“…Comprehensive information regarding the surface state changes is obtained by the scale-bridging method, including oxidation state measurements of bulk volume and top surface layer (5–10 nm) of nanoparticles by XAS and XPS, respectively, along with nanoscale and atomic-scale elemental and structural characterisation of individual nanoparticles by APT and HRTEM. Our study reveals the presence of Co-rich and Fe-rich nanodomains, created by spinodal decomposition, in pristine Co 2 FeO 4 and most likely in most mixed Co x Fe (3- x ) O 4 spinel oxides when x is in the range of 1.1–2.7 due to the miscibility gap 21 , 22 . Interestingly, hydroxyl groups were trapped at the interface between the nanodomains, possibly yielding a significantly enhanced OER activity of pristine Co 2 FeO 4 compared to CoFe 2 O 4 .…”

“…Similarly, the stoichiometry of the non-segregated nanoparticles is Co 1.7 Fe 1.3 O 3.3 (based on values given in Tables 1 and 2 ). The formation of Fe-rich and Co-rich nanodomains in the pristine Co 2 FeO 4 nanoparticles is most likely the result of spinodal decomposition that is driven by the miscibility gap in the composition range 0.37 < Co/(Co+Fe) < 0.9 at temperatures below 700 °C 21 , 22 . According to the CoFe 2 O 4 -Co 3 O 4 phase diagram 21 , 22 , Co 2 FeO 4 is expected to decompose to Co 1.4 Fe 1.6 O 4 (Co/Fe = 0.875) and Co 2.4 Fe 0.6 O 4 (Co/Fe = 4).…”

“…The formation of Fe-rich and Co-rich nanodomains in the pristine Co 2 FeO 4 nanoparticles is most likely the result of spinodal decomposition that is driven by the miscibility gap in the composition range 0.37 < Co/(Co+Fe) < 0.9 at temperatures below 700 °C 21 , 22 . According to the CoFe 2 O 4 -Co 3 O 4 phase diagram 21 , 22 , Co 2 FeO 4 is expected to decompose to Co 1.4 Fe 1.6 O 4 (Co/Fe = 0.875) and Co 2.4 Fe 0.6 O 4 (Co/Fe = 4). The discrepancy of compositions between our study and previous work possibly arises from the fact that these nanoparticles do not reach thermodynamic equilibrium after synthesis, and we expect the phase stability of nanoparticles to deviate from that of bulk materials in previous studies 21 , 22 .…”

The Roles of Composition and Mesostructure of Cobalt‐Based Spinel Catalysts in Oxygen Evolution Reactions

“…[22] Co 2 AlO 4 and CoFe 2 O 4 do not show this additional thermal reduction step as the spinel structure with these compositions is stable up to higher temperatures investigated here. [23] Thermogravimetric analysis suggests a sufficient calcination temperature of 400 °C for all samples except CoFe 2 O 4. For this composition, a small amount of a hematite by-phase was observed upon thermal treatment at 400 °C but vanished at 600 °C.…”

Evaluation of CoxFe3-xO4 cobaltites as ethanol sensing material

“…As expected, the spectra obtained by XRF are in good agreement with the desired composition. The Figure 3 shows the vibrational spectra of the samples, it is not easy to compare these results with the literature because of those particular compositions, however works undergone on Co3O4 [7] confirm the found vibrational mode and results on Co2FeO4 [8] and Co2.7Fe0.3O4 [9] enable to situate the obtained spectrum in the correct zone.…”