Effect of oxidation state of manganese in manganese oxide thin films on their capacitance performances

Mn oxides, which are relatively low cost with low toxicity and environmental friendly, are regarded as one of the most promising candidate materials for pseudo-capacitors. The oxidation state of Mn ion in the oxides (e.g.: +2, +3, +4, +6 and +7) is a very critical factor affecting the specific capacitance. Three Mn oxide films (crystalline Mn2O3, amorphous Mn2Ox, x similar to 3.74 and crystalline Mn3O4) with different oxidation states of Mn were prepared using a pulsed laser deposition (PLD) method, which show different capacitance performances (crystalline Mn2O3 film > amorphous MnOx film > crystalline Mn3O4 film). X-ray absorption near edge structures (XANES) is applied to investigate the electronic structures of these films. Mn K-edge and L-3,L-2-dege XANES results confirm that the bulk of the amorphous MnOx film is composed of MnO2 and Mn2O3 with Mn4+/Mn3+ ratio of about 3:1. Total electron yield (TEY) XANES at the Mn L-3,L-2-edge which is surface-sensitive, suggests that the amorphous film also contains some Mn3O4 on the surface, which is proposed to be the reason why the amorphous MnOx film shows moderate pseudo-capacitance behavior in between the pure Mn2O3 and Mn3O4 films. (C) 2017 Elsevier B.V. All rights reserved.