Each Mn 2p peak could be deconvoluted into two peaks, as shown in Fig. 4b and c. In the 7.9% sample, 60W(250), the two deconvoluted peaks of Mn 2p3/2 at 641.6 eV and 643.8 eV are attributed to the contribution of Mn3+ and Mn4+[48], with Mn3+ as the major component of oxidation states, owing to its higher intensity.