Reduction reactions play significant roles in industrial manufacture, and supported metal nanoparticles (NPs) show great prospect for mass production. The gold NPs have achieved expanding progress in selective reduction in recent years and will lead to a hot issue in the future. However, the designing of efficient catalysts is currently quite challenging because of the lack of systematic understandings on the fundamentals of the catalytic efficiency. Fortunately, tremendous advances can be expected with the aid of quasi-homogeneous NPs. This review highlights the unsupported gold NPs or nanoclusters (NCs) catalyzed reduction reactions, which include single-electron-transfer catalysis, selective hydrogenation of carbonyl and nitro groups. The tentative catalytic mechanism and the size-dependence of gold nanocatalyst have also been discussed in this review.

Herein, we report the synthesis and atomic structure of the cluster-assembled [Au₆₀Se₂(Ph₃P)₁₀(SeR)₁₅]⁺ material. Five icosahedral Au₁₃ building blocks from a closed gold ring with Au–Se–Au linkages. Interestingly, two Se atoms (without the phenyl tail) locate in the center of the cluster, stabilized by the Se-(Au)₅ interactions. The ring-like nanocluster is unprecedented in previous experimental and theoretical studies of gold nanocluster structures. In addition, our optical and electrochemical studies show that the electronic properties of the icosahedral Au₁₃ units still remain unchanged in the penta-twinned Au₆₀ nanocluster, and this new material might be a promising in optical limiting material. This work offers a basis for deep understanding on controlling the cluster-assembled materials for tailoring their functionalities.

Herein, we report the synthesis and atomic structure of the cluster-assembled [Au₆₀Se₂(Ph₃P)₁₀(SeR)₁₅]⁺ material. Five icosahedral Au₁₃ building blocks from a closed gold ring with Au–Se–Au linkages. Interestingly, two Se atoms (without the phenyl tail) locate in the center of the cluster, stabilized by the Se-(Au)₅ interactions. The ring-like nanocluster is unprecedented in previous experimental and theoretical studies of gold nanocluster structures. In addition, our optical and electrochemical studies show that the electronic properties of the icosahedral Au₁₃ units still remain unchanged in the penta-twinned Au₆₀ nanocluster, and this new material might be a promising in optical limiting material. This work offers a basis for deep understanding on controlling the cluster-assembled materials for tailoring their functionalities.

The rod-shaped Au₂₅ nanocluster possesses a low photoluminescence quantum yield (QY=0.1 %) and hence is not of practical use in bioimaging and related applications. Herein, we show that substituting silver atoms for gold in the 25-atom matrix can drastically enhance the photoluminescence. The obtained Ag_xAu_25−x (x=1–13) nanoclusters exhibit high quantum yield (QY=40.1 %), which is in striking contrast with the normally weakly luminescent Ag_xAu_25−x species (x=1–12, QY=0.21 %). X-ray crystallography further determines the substitution sites of Ag atoms in the Ag_xAu_25−x cluster through partial occupancy analysis, which provides further insight into the mechanism of photoluminescence enhancement.

The rod-shaped Au₂₅ nanocluster possesses a low photoluminescence quantum yield (QY=0.1 %) and hence is not of practical use in bioimaging and related applications. Herein, we show that substituting silver atoms for gold in the 25-atom matrix can drastically enhance the photoluminescence. The obtained Ag_xAu_25−x (x=1–13) nanoclusters exhibit high quantum yield (QY=40.1 %), which is in striking contrast with the normally weakly luminescent Ag_xAu_25−x species (x=1–12, QY=0.21 %). X-ray crystallography further determines the substitution sites of Ag atoms in the Ag_xAu_25−x cluster through partial occupancy analysis, which provides further insight into the mechanism of photoluminescence enhancement.

We report two synthetic routes for concurrent formation of phenylmethanethiolate (-SCH₂Ph)-protected Au₂₀(SR)₁₆ and Au₂₄(SR)₂₄ nanoclusters in one-pot by kinetic control. Unlike the previously reported methods for thiolate-protected gold nanoclusters, which typically involve rapid reduction of the gold precursor by excess NaBH₄ and subsequent size focusing into atomically monodisperse clusters of a specific size, the present work reveals some insight into the kinetic control in gold–thiolate cluster synthesis. We demonstrate that the synthesis of -SCH₂Ph-protected Au₂₀ and Au₂₄ nanoclusters can be obtained through two different, kinetically controlled methods. Specifically, route 1 employs slow addition of a relatively large amount of NaBH₄ under slow stirring of the reaction mixture, while route 2 employs rapid addition of a small amount of NaBH₄ under rapid stirring of the reaction mixture. At first glance, these two methods apparently possess quite different reaction kinetics, but interestingly they give rise to exactly the same product (i.e., the coproduction of Au₂₀(SCH₂Ph)₁₆ and Au₂₄(SCH₂Ph)₂₀ clusters). Our results explicitly demonstrate the complex interplay between the kinetic factors that include the addition speed and amount of NaBH₄ solution as well as the stirring speed of the reaction mixture. Such insight is important for devising synthetic routes for different sized nanoclusters. We also compared the photoluminescence and electrochemical properties of PhCH₂S-protected Au₂₀ and Au₂₄ nanoclusters with the PhC₂H₄S-protected counterparts. A surprising 2.5 times photoluminescence enhancement was observed for the PhCH₂S-capped nanoclusters when compared to the PhC₂H₄S-capped analogues, thereby indicating a drastic effect of the ligand that is merely one carbon shorter.