Fig. 7

Distribution of defects in the surface layer. a Light area displays the sub-surface defect layer. Gray area reflects the depth of nanoparticles, where defects are almost absent in comparison with the sub-surface layer. Proportionality between the size of sub-surface defect area and defect-free inner core is broken in this figure for more clear representation of the defects distribution in the sub-surface layer. Fe atoms are distributed uniformly in the depth of the defect layer, the oxygen vacancies V(O) are located near Fe atoms. b Zoom of the defective layer (a); d is the distance between defects