X-ray photoelectron spectroscopy (XPS) measurements were done to analyze the contribution of surface oxygen sites for charge accumulation in the layer. Exfoliation of Mn3O4 membranes in the solvent resulted in Mn with both +2 and +3 oxidation states (Figure ). The surface of the exfoliated Mn3O4 membranes has more Mn (+3) at 64.87 at%, while the bulk has more of Mn (+2) at 69.13 at%. The three main deconvoluted peaks in the O 1s XPS spectra correspond to metal-oxygen bonds at 529 eV, loosely adsorbed oxygen species, which come from the creation of oxygen vacancies, at 531 eV, and surface adsorbed hydroxide intermediates at 532.7