Precision synthesis of subnanoparticles using dendrimers as a superatom synthesizer

Classical metal-based nanomaterials come in two prominent types: a mononuclear or multinuclear complex chemically stabilized by organic ligands or a nanoparticle (also called a nanorod, nanosheet, or nanocrystal) physically stabilized by inorganic or polymer supports. Over the last decade, a class of superatoms that lies between these categories of materials has attracted attention because their properties are dramatically different from those typically ascribed to their component elements. Typically the superatoms include a specific, low number of metallic atoms. Because a one-atom difference can alter the properties of these superatoms, their synthesis must be ultraprecise, requiring one-atom resolution. To date, researchers have only been able to prepare monodisperse superatoms using gas-phase synthesis followed by purification through a flight tube. Though this technique provides monodisperse superatoms, it does not allow researchers to produce them in large quantites. Other researchers have proposed ligand-assisted liquid-phase synthesis as an alternative, but this technique is only useful for a few stable "magic number" clusters. Recently researchers have developed a new approach for the synthesis of superatoms that employs a novel class of molecular templates, which can define the number of metal ions or salts precisely. As a result, researchers can now synthesize nanoparticles or even subnanoparticles successfully. A dendrimer-type template has proven to be especially useful for ultraprecise control of the atomicity of the product, but it works with a full range of metal elements. In this Account, we highlight recent advances in the precise preparation of metal-assembling complexes using the dendrimer as a template. Next we discuss the selective assembly of subnanoparticles that utilize the dendrimer as a superatom synthesizer. The resulting subnanoparticles are almost monodisperse, and as a result, some of them exhibited distinctive characteristics based on their atomicity. For example, because of the quantum-size effect, the reduction in particle size of TiO2 and other metal-oxide subnanoparticles led to a significant shift in the band-gap energy. In addition, a miniaturized platinum particle less than 1 nm in diameter showed unexpectedly high catalytic activity for the oxygen reduction reaction (ORR) and other related reactions. Of particular note, in all these examples, this substantial change in their properties arose out of a single-atom difference in the atomicity. These results suggest that next-generation subnanoparticle design could play an important role in new materials and offer an additional palette of physical and chemical properties for new applications.