XPS spectrum of MnO$_2$ before (top row; red line) and after plasma reaction at room temperature (middle row; blue line) and 160 $^\circ$C (433 K) (bottom row; black line). In column (a) Mn 2p$_{3/2}$, the blue, purple, and orange fitted components represent Mn$^{2+}$, Mn$^{3+}$, and Mn$^{4+}$ species, respectively. 
    In column (b) for the O 1s peak, the green, purple and brown shaded areas correspond to lattice oxygen (O$_\text{lattice}$), surface oxygen (O$_\text{surface}$) and oxygen from water (O$_\text{water}$), respectively.
    Column (c) Mn 3s spectra showing the binding energy splitting $\Delta E$ between the two peaks.