Catalan solids from superionic nanoparticles

The self-assembly of inorganic nanoparticles (NPs) into ordered structures (superlattices) has led to a wide range of nanomaterials with unique optical, magnetic, electronic, and catalytic properties. Various interactions have been employed to direct the crystallization of NPs, including van der Waals forces, hydrogen bonding, as well as electric and magnetic dipolar interactions. Among them, Coulombic interactions—ubiquitous in nature and the main driving force behind the formation of many minerals, such as fluorite or rock salt—have remained largely underexplored, owing to the rapid charge exchange between NPs bearing high densities of opposite charges (superionic NPs). Here, we worked with superionic NPs under conditions (room temperature, concentrated salt solutions) that preserved their native surface charge density. We demonstrate that under these conditions, the Coulombic interactions between superionic NPs are reminiscent of short-range intermolecular interactions. Our methodology was used to assemble oppositely charged NPs into high-quality superlattices exhibiting Catalan shapes. Depending on their size ratio, the NPs assembled into either rhombic dodecahedra or triakis tetrahedra with structures mimicking those of the ionic solids CsCl and Th3P4, respectively. We envision that the methodology described here can be applied to a wide range of charged NPs of various sizes, shapes, and compositions, thus facilitating the discovery of new nanomaterials.