Dear Seona, try to measure XANES - each Co oxidation state has a quite well defined edge energy, and thus the different Co oxidation states are well distinguishable. See e.g.the following papers:

doi:10.1016/j.jinorgbio.2006.02.015

doi:10.1016/j.apcata.2007.07.027

doi:10.1016/j.apcata.2006.06.009

doi:10.1016/j.apcata.2003.12.049

Please come back if you have any additional questions! Kind regards, Dirk

Can you trie a test hipotesis to compare the mean of each oxidation state

Resolving the oxidation states of transition metals from the 2p spectra can be pretty challenging. Some researchers have developed "recipes" for fitting oxidation states using several peaks. If you are stuck with just XPS data, then Table 5 in the paper linked here might be a good place to start for distinguishing contributions from Co2+ or Co3+. Good luck!

Article Resolving surface chemical states in XPS analysis of first r...

I would like to ask all of you, which oxidation state peak appear at lower B.E? Co3+ or Co2+? (Attached)

Some paper fit 2p3/2 and 2p1/2 peaks to two peaks, one at lower B.E is correlated to Co3+ and the other one at higher B.E is for Co2+. but some papers did the reverse.

Could any one guide me for the correct one? and why?

but in Case of Fe, Cu or Ni the higher valence state peak is at higher BE.

for example https://www.researchgate.net/publication/279862419_Hierarchical_Cu2OCuOCo3O4_core-shell_nanowires_Synthesis_and_electrochemical_properties/figures?lo=1

https://www.researchgate.net/publication/273958386_In_situ_growth_of_hierarchical_mesoporous_NiCo2S4MnO2_arrays_on_nickel_foam_for_high-performance_supercapacitors/figures?lo=1

Article Hierarchical Cu2O/CuO/Co3O4 core-shell nanowires: Synthesis ...

Article In situ growth of hierarchical mesoporous NiCo2S4@MnO2 array...