Supporting information
The particle size distribution of bare iron oxide nanoparticles and their related composites was
determined from transmission electron microscopy images (Fig. SI.1), using a digital camera and the
SAISAM software (Microvision Instruments), calculating the surface-average particle diameter dP as
follows:
d P   ni d i2 /  ni d i
i
(SI.1)
i
where ni is the number of particles with di diameter. The statistical result of the particle size was
obtained by counting about three hundred particles considering a spherical particle shape (dP is given
in nanometer). A lognormal analysis of the collected data permits the calculus of the average diameter
and its standard deviation for each series of particles. The obtained values are 10.4 ± 1.9 and 12.0 ±
2.4 nm, respectively, suggesting that iron oxide seeds were successfully coated by an almost
continuous and uniform CoO shell of about 1.0-1.5 nm of thickness.
(a)
50 nm
(b)
Fig. SI.1 Micrographs of an assembly of bare iron oxide NPs (a) and their related nanocomposites (b)
Number (%)
30.0
(a)
20.0
10.0
0.0
0.0
10.0
20.0
di (nm)
Number (%)
20.0
(b)
10.0
0.0
0.0
10.0
20.0
di (nm)
Fig. SI.2 Size distribution of bare iron oxide NPs (a) and their related nanocomposites (b)
To determine the CoO thickness using XPS data it is important to calculate the R parameter, defined
as follows:
R
I(Co) (Fe) Ts (Fe) [Fe(Fe3O 4 )] C(Fe)




I(Fe) (Co) Ts (Co) [Co(CoO)] C(Co)
(SI.2)
where I(Co) and I(Fe) are the intensity of the 2p3/2 Co and Fe XPS peak, respectively; σ(Fe) and σ(Co)
are the photo-ionization cross section for Fe 2p3/2 and Co 2p3/2 signals, respectively; Ts(Fe) and Ts(Co)
are the analyzer transmission functions of the spectrometer for each element; λ[Fe(Fe 3O4)],
λ[Fe(CoO)] and λ[Co(CoO)] are the free path of the Fe 2p 3/2 or Co(2p3/2) photoelectrons in the Fe3O4
and CoO oxide matrices, respectively; C(Fe) and C(Co) are the number of Fe and Co atoms per unit
oxide volume. One can also express the relation between R and d(CoO), the thickness of the CoO
shell, as follows:
  d(CoO) 

1  exp 
[Co(CoO)] 

R
  d(CoO) 

exp 
 [Fe(CoO)] 
(SI.3)
The quantitative analysis of the high resolution Fe 2p and Co 2p XPS spectra permits to calculate the λ
values (in nm) using the Dench and Seah equation specifically established for oxides (Beamson 1992):
(k)  0.55E 0k.5  a1k.5
(SI.4)
where Ek and ak are the kinetic energy (eV) of the ejected core-hole electron and the k atom size,
respectively. Ek values are tabulated while ak ones are calculated using the following equation:
ak 
A  1024
  n  Na
(SI.5)
where A is the molecular weight (g mol-1) of the k based oxide phase, ρ its density (kg m-3) and n the
number of atoms per molecular formula unit. Na is the Avogadro number. So, using Equations (SI.4)
and (SI.5), we obtained exactly [Fe(CoO)] = 1.51 nm, [Co(CoO)] = 1.44 nm and λ[Fe(Fe3O4)] =
1.58 nm, which introduced in Equation SI.3 gives directly d(CoO) value.