Atomically precise pieces of matter of nanometer dimensions composed of noble metals are new categories of materials with many unusual properties. Over 100 molecules of this kind with formulas such as Au25(SR)18, Au38(SR)24, and Au102(SR)44 as well as Ag25(SR)18, Ag29(S2R)12, and Ag44(SR)30 (often with a few counterions to compensate charges) are known now. They can be made reproducibly with robust synthetic protocols, resulting in colored solutions, yielding powders or diffractable crystals. They are distinctly different from nanoparticles in their spectroscopic properties such as optical absorption and emission, showing well-defined features, just like molecules. They show isotopically resolved molecular ion peaks in mass spectra and provide diverse information when examined through multiple instrumental methods. Most important of these properties is luminescence, often in the visible–near-infrared window, useful in biological applications. Luminescence in the visible region, especially by clusters prot...

Atomically Precise Clusters of Noble Metals: Emerging Link between Atoms and Nanoparticles

Rational design and development of new-generation photocatalysts with high hydrogen evolution activity is recognized as an effective strategy to settle energy crisis. To this regard, hybrid photocatalysts of Au quantum dots embedded in rimous cadmium sulfide nanospheres are synthesized by using a simple hydrothermal process followed by photoreduction. The rimous cadmium sulfide nanospheres with rough surface and irregular fissures greatly strengthen their adhesion and interaction with Au quantum dots, which effectively facilitates the separation, restrains the recombination, and accelerates the consumption of photoinduced electron-hole pairs. Impressively, the highest photocatalytic activity for hydrogen generation (601.2 μmol h-1 g-1 ) and organic pollutant degradation (100% degradation in 80 min) is obtained by adjusting the Au mass loading to achieve uniform distribution. This work paves new way to the exploitation of highly efficient metal/semiconductor hybrid photocatalysts for clean energy generation and environment restoration.

Embedding Au Quantum Dots in Rimous Cadmium Sulfide Nanospheres for Enhanced Photocatalytic Hydrogen Evolution.

This Review article provides a report on progress in the synthesis, properties and catalytic applications of noble metal based composite nanomaterials. We begin with a brief discussion on the categories of various composite materials. We then present some important colloidal synthetic approaches to the composite nanostructures; here, major attention has been paid to bimetallic nanoparticles. We also introduce some important physiochemical properties that are beneficial from composite nanomaterials. Finally, we highlight the catalytic applications of such composite nanoparticles and conclude with remarks on prospective future directions.

Recent advances in noble metal based composite nanocatalysts: colloidal synthesis, properties, and catalytic applications

Plasmonic photocatalysts composed of Au and bimetallic Au–Pd alloy nanoparticles (NPs) on one-dimensional TiO2 nanobelts (TiO2-NBs) were used for the aerobic oxidation of benzyl alcohol under visible light irradiation. Remarkable light-promoted activity was observed for the as-synthesized M/TiO2-NB (M = Au, Au–Pd) nanostructures based on the TiO2(B)/anatase heterostructured nanobelt. The difference in band structure and the well matched interface between the TiO2(B) and anatase phases, coupled with the one-dimensional nanostructure, enable an enhanced charge transfer within the heterostructured nanobelt. This inter-phase charge transfer greatly facilitates the flow of hot electrons from the metal NPs to TiO2 and promotes benzyl alcohol oxidation. This efficient electron transfer was identified by the much higher photocurrent response measured for the Au/TiO2-NB nanostructure with the TiO2(B)/anatase heterojunction than those with either of the single phases under visible light irradiation. Alloying Au with Pd in Au–Pd/TiO2-NB results in a significant improvement in the visible light-promoted activity compared to the monometallic Au/TiO2-NB sample. It is supposed that the plasmon-mediated charge distribution within the alloy NPs is mainly responsible for the enhanced photocatalytic activity of the bimetallic nanostructures.

Gold and gold–palladium alloy nanoparticles on heterostructured TiO2 nanobelts as plasmonic photocatalysts for benzyl alcohol oxidation

Industrially, oxidation of alcohols is very important as they are starting materials for a variety of ketones, aldehydes, acids, etc. required to produce plastics, detergents, paints, cosmetics, food additives and drug intermediates. The potential of gold nanoparticles (AuNPs) in oxidation, in general, is immense owing to its tendency to dissociate dioxygen. Supports play a vital role not only in the synthesis and stabilization of nanoparticles, but can also look after the issues of sustainability by facilitating separation, recycling and recovery of catalyst. In the oxidation of alcohols, supports also control the outcome of the reaction. This review aims to systematically discuss the recent developments in the selective oxidation of alcohols catalyzed by supported gold nanoparticles. We draw attention to the impressive progress that has taken place in the oxidation of alcohols in the last thirteen years, leading to several green, sustainable and highly efficient protocols. The effect of a support on the remarkable properties of gold nanoparticles in terms of its catalytic activity, selectivity, recyclability, and stability is discussed at length. Mechanistic aspects of these conversions are discussed in brief.

Selective oxidation of alcohols by supported gold nanoparticles: recent advances

Au–Cu bimetallic nanoparticles supported on a TiO2-nanobelt (TiO2-NB) have been designed and synthesized by a one-pot photodeposition-galvanic replacement method. TEM observation revealed that small-sized metal nanoparticles (less than 2 nm) were uniformly and finely dispersed on the TiO2 nanobelt. Characterization by XRD coupled with XPS demonstrated that the Au–Cu bimetallic nanoparticles are composed of an Au-rich core/CuOx shell structure. The as-synthesized one-dimensional Au–Cu/TiO2-NB nanostructure can be easily assembled into a paper-like porous monolithic catalyst and applied in heterogeneous catalysis. The formed bimetallic nanopaper catalysts presented synergistically enhanced activity and improved stability for catalyzing the aerobic oxidation of benzyl alcohol compared to their monometallic counterparts. It is likely that the Au–CuOx heterostructure is responsible for the superior catalytic properties of the bimetallic Au–Cu/TiO2-NB catalysts, and the catalytic activity can be significantly affected by the Au/Cu ratio. The uniform and high dispersion of metal nanoparticles on TiO2 nanobelts is also believed to contribute to the stability of Au–Cu/TiO2-NB catalysts, suggesting that the one-dimensional TiO2 nanobelts are a desirable support for the preparation of nanoscale metal catalysts.

Nan Bao

Papers

Synergistic catalysis of Au–Cu/TiO2-NB nanopaper in aerobic oxidation of benzyl alcohol