Atomically Precise Metal Nanoclusters: Synthesis, Properties, Assembly and Catalytic Application

Nanoscience has advanced significantly over the past two decades owing to the great successes in solution phase syntheses. In early research, the polydispersity of nanoparticles (NPs) was a major issue, which motivated nanochemists to develop new synthetic methods to obtain

monodisperse NPs (e.g., size distribution <10%), and nowadays many successful methods are available that allow excellent control over size and shape. Unfortunately, no two NPs from conventional synthesis are the same at the atomic level. In the long pursuit of well-defined nanomaterials, a recent success is the synthesis of atomically precise metal nanoclusters (NCs) protected by ligands in the size range from tens to hundreds of metal atoms (equivalently 1−3 nm in core diameter). More importantly, with the help of single crystal X-ray diffraction, the total structures of these ultrasmall NPs can now be achieved. The atomic resolution from the inner kernel structure to the metal-ligand interface, then to the outer ligand shell provides the researchers unprecedented opportunities to reveal every detail of

the particle as well as the inter-particle interactions. These findings are fundamentally important to elucidate the observations in conventional NPs which cannot be fully understood. For example, the size effect is crucial in nanocatalysis but no deep mechanism could be obtained with conventional NPs. When the size goes down, it is now possible that for the same size, different atomic packing structures and even the NC isomers can be achieved, which is of major importance to catalysis at the atomic level. The variety in superlattices formed by NPs has long been a hot topic in nanoscience, however, fundamental questions such as what causes the formation of the patterns in NP self-assembly cannot be revealed, even with the most sophisticated electron microscope; thus, it calls for the creation of atomically precise NCs as a model for in-depth studies. Although a lot of breakthroughs have been done in nanochemistry over the past years, there are still many barriers to overcome. The most significant advantage of atomically precise NCs is that their outstanding properties can be directly correlated with their atomic structures. However, conclusions cannot be drawn until comparative experiments are done, and at the atomic level, there are many variables and correlated series of NCs are still rare.