金属纳米团簇(MNCs)的性能与应用*

金属纳米团簇(metal nanoclusters,MNCs)是由几个到几百个金属原子组成的内核被单层配体保护而形成的一类新型材料。MNCs的结构可在原子级精度的水平上进行调控,因其具有超小的尺寸、独特的电子能级以及大的比表面积等性质而获得了广泛的应用。主要基于ds区金属铜、银、金的纳米团簇 (CuNCs、AgNCs、AuNCs) 的相关研究介绍MNCs的概念、结构特征、主要性能、合成策略以及它们在催化、生物传感、生物成像和肿瘤治疗中的应用。     

Recent Advances in the Synthesis and Applications of Ultrasmall Bimetallic Nanoclusters

Ultrasmall bimetallic nanoclusters (or bi‐MNCs for short) have recently emerged as a new class of multi‐functional nanoparticles due to their ultrasmall size (typically below 2 nm), unique molecular‐like properties (e.g., quantized charging and strong luminescence), controlled cluster compositions (at the atomic level), synergistic physicochemical properties, and rich surface chemistry. Such intriguing properties have motivated the cluster community to develop efficient methods for the synthesis of high‐quality bi‐MNCs, which can also be seen from the quantum increase of reported synthetic protocols for bi‐MNCs. Recent advances in the development of efficient synthesis methods for high‐quality bi‐MNCs also facilitate the application explorations of bi‐MNCs in diverse fields like catalysis, sensors, and biomedicine. This Review article first surveys current progress in the synthesis of bi‐MNCs, especially for those NCs with good control of cluster size and composition, followed by a detailed discussion on some unique physicochemical properties of bi‐MNCs. The intriguing properties of bi‐MNCs have made them ideal platforms for application explorations in catalysis, sensors, and biomedicine, which are discussed in the second section. In the last section, a brief outlook on future developments of functional bi‐MNCs is presented, with a particular focus on the controlled synthesis and practical applications of bi‐MNCs.     

Assembly of Large Purely Inorganic Ce‐Stabilized/Bridged Selenotungstates: From Nanoclusters to Layers

A versatile one‐pot strategy was used to synthesize two large, purely inorganic selenotungstates, nanocluster K₆Na₁₆[Ce₆Se₆W₆₇O₂₃₀(OH)₆(H₂O)₁₇]?47 H₂O ( 1 ) and layer K₉Na₅Ce(H₂O)₄[Ce₆Se₁₀W₅₁O₁₈₇(OH)₇(H₂O)₁₈]?45 H₂O ( 2 ), by combining cerium centers and SeO₃^2? heteroanion templates. Compound 1 displays a Ce‐stabilized hexameric nanocluster with one rhombus‐like {W₄O₁₅(OH)₃} unit in the center, whereas compound 2 is the first example of a Ce‐bridged layer selenotungstate network based on linkage of the unusual {Ce₆Se₁₀W₅₁O₁₈₇(OH)₇(H₂O)₁₈} clusters and additional Ce(H₂O)₄ fragments via Ce‐O‐Se bridges. The compounds were characterized by elemental analyses, IR spectroscopy, thermogravimetric analyses, powder and single‐crystal X‐ray diffraction, and electrospray ionization mass spectrometry. Moreover, the electrochemical property of compound 1 was also investigated.     

Oxidation of CO Catalyzed by a Cu Cluster: Influence of an Electric Field

Adsorption ability and reaction rate are two essential parameters that define the efficiency of a catalyst. Herein, we implement density functional theory (DFT) and report that CO can be oxidized by a pyramidal Cu cluster with an associated reaction barrier E_b=1.317 eV. In this case, our transition state calculations reveal that the barrier can be significantly lowered after superimposing a negative electric field. Moreover, when the field intensity corresponds to F=?0.010 au, the magnitude of E_b=0.698 eV is equivalent to—or lower than—those of typical catalysts such as Pt, Rh, and Pd. The superimposition of a positive field is found to enhance the release of the nascent CO₂ molecule. Our study demonstrates that small Cu clusters have better adsorption ability than the corresponding flat surface while the field can be used to enhance the purification of the exhaust gas.     

Silver nanocluster redox-couple-promoted nonclassical electron transfer: an efficient electrochemical Wolff rearrangement of alpha-diazoketones

In this work we report the unique electrocatalytic role of benzoic acid protected silver nanoclusters (Ag(n), mean core diameter 2.5 nm) in the Wolff rearrangement (Scheme 1) of alpha-diazoketones. More specifically, the presence of a Ag(n) (0)/Ag(n) (+) redox couple facilitates a nonclassical electron-transfer process, involving chemical reaction(s) interposed between two electron-transfer steps occurring in opposite directions. Consequently, the net electron transfer between the electron mediator (Ag(n)) and alpha-diazoketone is zero. In-situ UV-visible studies using pyridine as a nucleophilic probe indicate the participation of alpha-ketocarbene/ketene as important reaction intermediates. Controlled potential coulometry of alpha-diazoketones using Ag(n) as the anode results in the formation of Wolff rearranged carboxylic acids in excellent yield, without sacrificing the electrocatalyst.     

Catalytic conversions of chloroolefines over iron oxide nanoparticles 3. Electronic and magnetic properties of γ-Fe2O3 nanoparticles immobilized on different silicas

Catalytic properties of superparamagnetic γ-ferric oxide nanoclusters, which are uniform in terms of size and magnetic properties were studied. The catalysts were supported on the activated silica gel matrix (AGM) prepared from the KSK-2 silica gel of globular structure and on the activated silica matrix (ASM) prepared from layered natural vermiculite. The clusters are active in some reactions of chloroolefin conversions: isomerization of dichlorobutenes and alkylation of benzene with allyl chloride. Their activity in these reactions is many times higher that of usual supported catalysts based on α-ferric oxide. Analysis of the Mössbauer spectra of the 2.5 wt.% Fe/AGM and 2.5 wt.%Fe/ASM samples before and after the reaction at T = 3–300 K shows that during the reaction some Fe^III ions arranged in ～2–3-nm γ-Fe₂O₃ nanoclusters magnetically ordered at 6 K are reduced to form a high-spin Fe^II complex in the paramagnetic state. According to the macroscopic magnetization data (SQUID) of the initial clusters, curves with hysteresis are observed at 2 K in the plots of forward and backward magnetization, while the 2.5 wt.%Fe/ASM catalyst after the reaction at T = 2 K demonstrates a linear field dependence of the magnetization passing through the coordinate origin. Analysis of the Mössbauer spectra and magnetic properties suggests that during the catalytic reaction the Fe^III ions in the γ-Fe₂O₃ nanoclusters interact with chloroolefin with the allylic structure to be partially reduced to the Fe^II ions that are bound in a complex containing chloride ions and O^II ion(s) of the silicate matrix as ligands. This is a reason, probably, for the high catalytic activity of γ-Fe₂O₃ nanoparticles.     

Photoluminescence as a Function of Aggregated Size from n-Butyl-Terminated Silicon Nanoclusters

Photoluminescence (PL) from alkyl-terminated silicon nanocrystallites as a function of size has been studied. Ultraviolet–blue luminescence (390–410 nm) is observed from as-prepared silicon nanoclusters with diameters from 3 to 8 nm. After 1 h of annealing at 162°C in 2-methoxyethyl ether (diglyme), the _max of PL shifts from 360 to 420 nm. High-resolution transmission electron microscopy (HRTEM) images show that individual silicon nanoparticles are fused to form pairs of nanoparticles. FTIR spectra show that the alkyl groups remain on the surface of silicon nanoparticles. As the temperature is raised to 250°C for 1 h, the PL no longer shows any peak in the visible light region. TEM images show that the silicon nanoparticles are aggregated and fused uniformly in one single dimension, to form a strip, and these strips parallel each other. When the temperature is raised to 350°C these silicon nanoparticles form a large piece of silicon textile network, showing that functionalized alkyl surface does not persist above this temperature. A strong Si–O–Si asymmetric stretching vibration appears between 1000 and 1100 cm^–1 at the expense of the C–H vibrational modes and there is no more change after 3 h of annealing at 250 or 350°C. These results provide strong evidence that the PL originates from quantum confinement.     

Template Controlled Synthesis (TCS) of size-controlled metal nanoclusters: preparation of nanostructured metals supported by inorganic supports

Strictly size-controlled metal nanoclusters of palladium(0) and gold(0) can be easily formed in the organic component of an organic-inorganic composite between a gel-type resin (DV44) and silica. To this purpose, silica is impregnated at incipient wetness with a solution of the required co-monomers (N,N-dimethylacrylamide, 4-vinylpyridine and N,N′-methylene-bis-acrylamide), which are co-polymerized to give the composite. The silica-supported polymeric framework is then loaded with palladium(II) or gold(III) precursors. Their chemical reduction with aqueous solutions of sodium boronhydride yields the metal nanoclusters. Their size is controlled by the nanomorphology of the polymeric framework, which acts as a template during the nanocluster growth (Template Controlled Synthesis). Upon thermal decomposition of the polymeric template at 600°C under nitrogen, the nanostructured metals are deposited onto the silica surface. In the case of palladium the final step occurs with retention of the original size of the nanoclusters.