3.4 The characteristics of oxygen vacancies
The surface Mn state and oxygen species were investigated by XPS. The magnitude of Mn3s multiplet splitting could be used to calculate the average oxidation state(AOS) of Mn, according to the following relationship [32]:
AOS=8.956-1.126ΔE S (7)
Where ∆E s is the binding energy difference between the doublet Mn3s peaks shown in Figure 6a.The results were summarized in Table 1. The calculated AOSs of MnO2, Mn2O3 and Mn3O4 were lower than their theoretical values owing of surface oxygen defects. Moreover, the AOS of MnO2-H-200 decreased to 3.22 from 3.78 of MnO2 suggesting that the density of oxygen vacancy was enhanced via hydrogen reduction.
For the above MnOx, the O1s spectrum shown in Figure 6b could be deconvoluted into two peaks to gain information on the nature of oxygen species. The one with lower binding energy at 529.5-530.2eV was assigned as the lattice oxygen (denoted as Olatt), and the other with higher binding energy at 531.3-531.4eV was ascribed as the surface-adsorbed oxygen (denoted as Oads). Generally oxygen molecules were adsorbed at the oxygen vacancies of an oxide material [33]. And the ratio of Oads/Olatt was an efficient parameter to characterize the relative abundance of oxygen vacancy in MnOx. The more oxygen species adsorbed on the MnOx surface, the higher oxygen vacancy density was. As was shown in Table 1, there were no remarkable differences on the ratios of Oads/Olatt for MnO2, Mn2O3 and Mn3O4. While MnO2-H-200 exhibited the higher value of Oads/Olattratio than those of MnO2, Mn2O3 and Mn3O4, indicating that MnO2-H-200 owned the most abundant surface adsorbed oxygen species, which was consistent with the results revealed by O2-TPD.