2018, Vol.4 No.1 The cover picture shows an ingenious “top-down” method to prepare various transition metal compounds by etching A ions out from the precursor multi metal oxides (AxMyOz). The obtained products have diverse and fascinating structures, e.g., porous bulk MOn/2, morphology-retained bulk MOn/2, monodis-persed MOn/2 nanoparticles, assembled MOn/2 nanostructure, hollow MOn/2, bulk HMO(n+1)/2, and core-shell structure by controlling the reaction conditions. These compounds may possess great potential applications in catalysis and energy storage. More details are discussed in the article by Dong et al. on page 170022.