Mesocrystals are superstructures with a crystallographically ordered alignment of nanoparticles.1 Owing to their organized structures, the mesocrystals have unique characteristics such as a high surface area, pore accessibility, and good electronic conductivity.2 However, there remain challenges that are relevant to the fabrication of the organized assembly of structure-controlled nanoparticles up to the micrometer scale or more. Such hierarchical superstructures will play crucial roles in their physicochemical properties for efficient solar energy conversion.
     So far, we have developed developed developed hierarchical mesocrystals for photocatalytic water splitting reaction and organics degradation. For example, SrTiO3 mesocrystals (SMCs) were synthesized by topotactic epitaxy from TiO2 mesocrystals (TMCs) during hydrothermal treatment.3 The optimized SMCs, which exhibited synergetic integration of an internal nanocube network and larger external nanocubes, showed superior activity in the hydrogen evolution when compared with conventional disordered nanoparticle systems in alkaline aqueous solution. It also exhibits a high quantum yield of ca. 7% at 360 nm in overall water splitting and even good durability up to 1 day. Furthermore, we applied single-particle spectroelectrochemistry to determine the active sites on the mesocrystal surface and reveal the role of structural disorders.3,4 Interestingly, the optimized SMC has a limited number of emitting sites on its surface decorated with particles several hundred nanometers in size. This finding suggests that the synergy of the efficient electron flow along the internal nanocube network and efficient collection at the larger external cubes produces remarkably long-lived charges for enhanced photocatalysis.