Sub-nanometre sized metal clusters, with dimensions between metal atoms and nanoparticles, have attracted more and more attention due to their unique electronic structures and the subsequent unusual physical and chemical properties. However, the tiny size of the metal clusters brings the difficulty of their synthesis compared to the easier preparation of large nanoparticles. Up to now various synthetic techniques and routes have been successfully applied to the preparation of sub-nanometre clusters. Among the metals, gold clusters, especially the alkanethiolate monolayer protected clusters (MPCs), have been extensively investigated during the past decades. In recent years, silver and copper nanoclusters have also attracted enormous interest mainly due to their excellent photoluminescent properties. Meanwhile, more structural characteristics, particular optical, catalytic, electronic and magnetic properties and the related technical applications of the metal nanoclusters have been discovered in recent years. In this critical review, recent advances in sub-nanometre sized metal clusters (Au, Ag, Cu, etc.) including the synthetic techniques, structural characterizations, novel physical, chemical and optical properties and their potential applications are discussed in detail. We finally give a brief outlook on the future development of metal nanoclusters from the viewpoint of controlled synthesis and their potential applications.

Direct electrochemical production of hydrogen peroxide (H 2 O 2 ) through two-electron oxygen electrochemistry, the oxygen reduction in fuel cells or water oxidation in water electrolyzers, could provide an attractive alternative to locally produce such chemical on demand. The efficiency of these processes depends greatly on the availability of cost-effective catalysts with high selectivity, activity and stability. In recent years, various novel nanostructured materials have been reported to selectively produce H 2 O 2 . Through combined experimental and theoretical approaches, underlying mechanisms in the electrochemical synthesis of H 2 O 2 via oxygen electrochemistry have been unveiled. Considering the remarkable progress in this area, we summarize recent development regarding the direct production of H 2 O 2 through two-Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff))

Subnanometer-sized copper nanoclusters were prepared by a one-pot procedure based on wet chemical reduction. The structural characteristics of the 2-mercapto-5-n-propylpyrimidine-protected nanoclusters, Cu(n) (n ≤ 8), were determined by mass spectrometry. The Cu nanoclusters displayed apparent luminescence, with dual emissions at 425 and 593 nm, with quantum yields of 3.5 and 0.9%, respectively, and high electrocatalytic activity in the electoreduction of oxygen.