Fig. 3

Three-dimensional distribution of Ni valence states over the polycrystalline ensembles after the heating sequence illustrated in Fig. 1a. a The 3D distribution of nickel valence states represented by Ni K-edge energy. The single-voxel thick slices through different depths of a particle (30 nm thick slices at 50 voxels apart through the largest particle in a are shown in b-e. The scale bar is 8 μm