Modeling the basal XRD reflections shown in Fig. 4(a) using the Bragg-rod approach. The volume-averaged nanoparticles are modeled with a stack of δ-MnO2 bi-layers with a layer-to-layer separation of 7.2 Å. The misfit of the 002 reflection comes from the non-uniformity of the interlayer spacing, as shown in Fig. 2.