Phase Transfer Induced Enhanced Stability of Monolayer Protected Silver Quantum Clusters

Mercaptosuccinic acid (MSA) protected silver clusters (Ag@MSA) were synthesized and their stability was monitored both in an aqueous and in a toluene phase. In the former, the pH of the cluster solution was tuned between 3 and 10 using ammonium acetate buffer, and the cluster stability was tested. The clusters were unstable at all measured pH values, whereas in the toluene phase, by using sterically hindered counter ion (tertraoctylammonium, TOA), the stability of the clusters was improved significantly. The phase transferred clusters in the toluene phase showed very high stability over months at room temperature. Transmission electron microscopy images show an average cluster size of ~ 2.05 nm. Size exclusion chromatography and Electrospray Ionisation Mass Spectrometry (ESI–MS) measurements were employed to analyse the phase transferred clusters. Importantly, Ag₄MSA₄ ^? type stable fragments were observed in ESI–MS measurements. The [Ag₈(MSA)₈(TOA)₅]^2? was the largest cluster identified by ESI–MS.     

In situ observation of spontaneous alloying of antimony into nm-sized indium clusters by optical absorption spectroscopy

The alloying behavior of antimony atoms into nm-sized indium clusters has been studied in situ by uvvisible absorption spectroscopy. Using a vacuum evaporator installed in the specimen chamber of a spectrophotometer, antimony atoms are vapor-deposited onto nm-sized indium clusters kept at ambient temperature. Spectra from clusters after antimony deposition onto pure indium clusters can be characterized by a doublet structure with an energy separation corresponding to the spin-orbit splitting of the L ₃ valence band state of InSb. It has been confirmed that when antimony is vapor-deposited onto indium clusters, rapid dissolution of antimony into indium clusters takes place to form InSb clusters in the absence of any electron irradiation.     

Size effects in the solid phase transition of SF6 clusters

The structure of SF₆ clusters produced in a free jet expansion is studied by electron diffraction methods. A solid phase transition is known to occur when clusters are warmed up by changing several experimental conditions in the expansion of a Ne + SF₆ mixture. In the present study, the total stagnation pressure and the SF₆ mole fraction are varied in order to understand how these parameters influence the structural state of the clusters and further to observe the phase transition for different cluster sizes. When the stagnation pressurep ₀ is larger than about 10 bar, a given mole fraction results in clusters with identical structure and probably identical temperature. Whenp ₀ is decreased below 10 bar, identical structures are found for lower and lower mole fractions. This structural behaviour suggests that for small clusters, containing less than about 500 molecules, the transition steps occur at temperatures lower than those observed for larger clusters. The possibility of detecting a temperature variation in the diffraction patterns of small cubic clusters is discussed.     

Spontaneous alloying of gold clusters into nanometer-sized antimony clusters

Alloying behavior of gold into nm-sized amorphous antimony (a-Sb) clusters has been studied by transmission electron microscopy (TEM), employing gold clusters in contact with a-Sb clusters. In order to produce gold clusters on individual a-Sb clusters, a-Sb clusters on an amorphous carbon film were cooled down to 96 K, and gold was then condenced on the film. When gold clusters in contact with a-Sb clusters are gradually heated from 96 to 290 K, dissolution of gold into a-Sb clusters sets in around 200K and clusters of a-(Sb-Au) alloys are produced. With increasing annealing temperture, more gold is absorbed into individual a-Sb clusters, and when the gold concentration in a-(Sb-Au) clusters reaches to the stoichiometric composition of AuSb₂, these amorphous clusters crystallize into AuSb2 clusters. The crystallization temperature decreases with decreasing size of initial a-Sb clusters.     

Reversible Interconversion between Luminescent Isomeric Metal–Organic Frameworks of [Cu4I4(DABCO)2] (DABCO=1,4‐Diazabicyclo[2.2.2]octane)

The metal–organic frameworks (MOF) of cluster [Cu₄I₄(DABCO)₂] (DABCO=1,4‐diazabicyclo[2.2.2]octane) have been prepared and characterized as two different crystalline forms, I and II . Form I is obtained by reaction of DABCO and CuI in aqueous solution or by solvothermal reaction, while form II is obtained by reacting DABCO and CuI in acetonitrile. Their luminescence properties in the solid state have been analyzed at room temperature and at 77 K. MOF II has bright emission with a maximum at 556 nm that shifts bathochromically at low temperature in conjunction with a marked change in the colour of the emission. The emission of MOF I has a maximum at 580 nm and a less pronounced temperature dependence. The peculiar luminescence properties of the two isomers have been interpreted by utilising current knowledge on the excited states properties of Cu^I cubane clusters. The two isomers exhibit a high degree of porosity and can release the disordered solvent molecules trapped in the channels, whilst preserving the crystal structure. Isomer I can be converted into II on exposure to acetonitrile or methanol vapour, whereas II reverts to I when heated in a closed pan at 250 °C.     

Metal carbonyl clusters as precursors of heterogeneous catalysts: Hydrogenation and disproportionation of monoenes, dienes, and arenes in the presence of H2Ru4(CO)13, H2FeRu3(CO)13, and HRuCo3(CO)12 supported and activated on chromosorb. Characterization of the metal particles derived from H2Ru4(CO)13 by means of IR spectroscopy and TEM microscopy

The tetrahedral hydridic clusters H₂Ru₄(CO)₁₃ (1), H₂FeRu₃(CO)₁₃ (2), and HRuCo₃(CO)₁₂ (3) were supported on Chromosorb P and activated under dihydrogen flow. The resulting metal particles are active in the hydrogenation of pentenes, cyclic monoenenes and dienes, benzene, and toluene; these catalysts are effective under mild conditions and with a low metal loading. Experiments under dinitrogen showed that complex hydrogenation-dehydrogenation processes occur, as already observed for the same clusters during the homogeneous hydrogenation of cyclohexadienes. After the gas-chromatographic catalytic runs with cluster 1 as precursor, TEM microscopy showed the presence of very small supported metal particles (mean size 7.5 nm). The decomposition of cluster 1 to metal particles upon thermal treatment on Aerosil under vacuum or under dihydrogen was followed by means of IR spectroscopy; this catalyst hydrogenates benzene at room temperature with 100% conversion in a very short time (calculated activity was about 3200 TOFs).     

单分子磁体

单分子磁体是涉及合成化学、材料科学和凝聚态物理等边缘学科的一个新颖课题。本文对单分子磁体的主要性质、功能、研究方法和最新进展做了评述。重点介绍了含Mn和Fe这两类重要的单分子磁体。     

Extra Silver Atom Triggers Room‐Temperature Photoluminescence in Atomically Precise Radarlike Silver Clusters

Luminescent metal clusters show promise for applications in imaging and sensing. However, promoting emission from metal clusters at room temperature is a challenging task owing to the lack of an efficient approach to suppress the nonradiative decay process in metal cores. We report herein that the addition of a silver atom into a metal interstice of the radarlike thiolated silver cluster [Ag₂₇(S^tBu)₁₄(S)₂(CF₃COO)₉(DMAc)₄]?DMAc ( NC1 , DMAc=dimethylacetamide), which is non‐emissive under ambient conditions, produced another silver cluster [Ag₂₈(AdmS)₁₄(S)₂(CF₃COO)₁₀(H₂O)₄] ( NC2 ) that displayed bright green room‐temperature photoluminescence aided by the new ligand 1‐adamantanethiol (AdmSH). The 28th Ag atom, which hardly affects the geometrical and electronic structures of the Ag–S skeleton, triggered the emission of green light as a result of the rigidity of the cluster structure.     

Frequency and field dependent conductivity in Pd-based metal cluster compounds

Ligand stabilized metal cluster compounds and colloids consist of nanometer-size metal cores surrounded by ligand shells. We study the dielectric properties of randomly packed aggregates of these compounds as a function of temperature T, electric field E and frequency ω. Cores were formed by Pd-atoms and stabilized by a selection of ligands. The core size varied between 2.4 and 3.6 nm (monodisperse) and 18 nm (5–10% dispersion). The dielectric properties of all compounds show scaling: the same functional dependence of the normalized conductivity on normalized frequency irrespective of temperature. The results imply that at room temperature at all frequencies below 1 THz exclusively hopping between the particles is seen. More details of the hopping processes are obtained by varying E. The observed dependences are consistent with stochastic models that use random packing of clusters inside the sample.     

微量钛掺杂的纳米二氧化锆相变过程中的光学性质

微量钛掺杂的纳米氧化锆由于纳米“小尺寸效应”的影响,在材料中产生了过量的氧空位缺陷,这些氧空位缺陷使得氧化锆在室温下呈现出高温的四方相。这种晶相的变化对氧化锆的光学性质产生了很大的影响。在氧化锆从四方相向单斜相的转变过程中,位于470 nm位置的Ti发光中心的宽带发光以及相同位置的余辉发光得到了增强。     

Computer Simulation of Water Clusterization on Chlorine Ions: 1. Thermodynamic Properties

Cl^–(H₂O) _n clusters, n = 1–60, in equilibrium with vapor were simulated using the Monte Carlo method. Free energy and the work of clusters formation at room temperature and temperature corresponding to polar stratosphere were calculated. Clusters retain their stability over the entire investigated size range even at multiple vapor supersaturation; however, when supersaturation increases further, the cluster grows in an avalanche-like manner. In clusters with n > 20, the effect of ion field on the free energy of added molecules diminishes dramatically retaining, however, its stabilizing function.     

A dielectric study of the structure of propylene glycol

The dielectric spectra of propylene glycol over the frequency and temperature ranges 10 mHz–75 GHz and 175–423 K, respectively, were analyzed using the Dissado-Hill cluster model. A correlation between relaxation processes of breaking and formation of intermolecular H-bonds in clusters was obtained. A correlation of fluctuation processes of synchronous exchange of molecules between neighboring clusters corresponded to the redistribution of H-bonds between them. The Dissado-Hill theory was used to determine the integral relaxation times, n _DH and m _DH parameters and calculate the mean dipole moments of propylene glycol clusters and the energy characteristics of processes of their rearrangement. The mean dipole moments of clusters (23617–18.65 D) were compared with those of molecules in the liquid phase (3.67–3.03 D). The apparent activation enthalpy of processes of cluster rearrangements decreased from 141.8 to 25.2 kJ/mol, the activation energy decreased from 46.03 to 18.47 kJ/mol, and the energy of orientation dipole-dipole interactions, from 3.78 to 3.45 kJ/mol as the temperature increased.     

CTAB辅助微波水热合成花簇状SnS微球

以氯化亚锡(SnCl2.2H2O)和硫代乙酰胺(TAA)为前驱物,十六烷基三甲基溴化铵(CTAB)为表面活性剂,采用微波水热法控制合成花簇状SnS微球。采用XRD和FESEM等分析手段对制备的样品进行表征。结果表明:合成的产物为正交晶系的SnS微晶,且结晶性良好;SnS微晶是由长方形纳米片自组装而成的花簇状微球。通过改变CTAB用量,可以实现花簇状SnS微晶的形貌和尺寸的调控,并初步分析了其形成过程。利用紫外-可见吸收光谱分析,产物的光学带隙约为1.51 eV;室温光致发光光谱表明,产物在832 nm处具有近红外发光特性。     

Size dependent melting mechanisms of iron nanoclusters

Molecular dynamics simulations were used to study the change in the mechanism of iron cluster melting with increasing cluster size. Melting of smaller clusters (e.g., Fe₅₅ and Fe₁₀₀) occurs over a large temperature interval where the phase of the cluster repeatedly oscillates between liquid and solid. In contrast, larger clusters (e.g., Fe₃₀₀) have sharper melting points with surface melting preceding bulk melting. The importance of the simulation time, the force field and the definition of cluster melting is also discussed.     

Ion-pair evaporation from ionic liquid clusters

A differential mobility analyzer (DMA) is used in atmospheric pressure N₂ to select a narrow range of electrical mobilities from a complex mix of cluster ions of composition (CA)_n(C⁺)_z. The clusters are introduced into the N₂ gas by electrospraying concentrated (～20 mM) acetonitrile solutions of ionic liquids (molten salts) of composition CA (C⁺ = cation, A^? = anion). Mass analysis of these mobility-selected ions reveals the occurrence of individual neutral ion-pair evaporation events from the smallest singly charged clusters: (CA)_nC⁺→(CA)_{n? 1}C⁺+CA. Although bulk ionic liquids are effectively involatile at room temperature, up to six sequential evaporation events are observed. Because this requires far more internal energy than available in the original clusters, substantial heating (～10 eV) must take place in the ion guides leading to the mass analyzer. The observed increase in IL evaporation rate with decreasing size is drastic, in qualitative agreement with the exponential vapor pressure dependence predicted by Kelvin’s formula. A single evaporation event is barely detectable at n = 13, while two or more are prominent for n ≤ 9. Magic number clusters (CA)₄C⁺ with singularly low volatilities are found in three of the four ionic liquids studied. Like their recently reported liquid phase prenucleation cluster analogs, these magic number clusters could play a key role as gas-phase nucleation seeds. All the singularly involatile clusters seen are cations, which may help understand commonly observed sign effects in ion-induced nucleation. No other charge-sign asymmetry is seen on cluster evaporation.     

Iron luminescence thermal effects in sodium β″-alumina

Powder NaAl₅O₈ doped with Fe³⁺ emits two broadbands at 705 and 758 nm. The shape of the combined emission depends on the annealing time of the samples and it is affected by the temperature of the samples, even below room temperature. It is concluded that these features are due to the occupation of at least two crystallographic inequivalent sites by the Fe ions in the sodium β″-aluminate: the BR sites (I) in the conduction region and Al(3) sites (II) of the spinel blocks near to the conduction plane. It is proposed that iron diffusion from I sites to vacant II sites should also occur in the temperature range below room temperature after the samples are submitted to long annealing times at synthesis temperature.     

Oxygen adsorption on hydrated gold cluster anions: experiment and theory

The discovery that supported gold clusters act as highly efficient catalysts for low-temperature oxidation reactions has led to a great deal of work aimed at understanding the origins of the catalytic activity. Several studies have shown that the presence of trace moisture is required for the catalysts to function. Using near-atmospheric pressure flow reactor techniques, we have studied humidity and temperature effects on the reactivity of gas-phase gold cluster anions with O2. Near room temperature, the humid source produces abundant gold-hydroxy cluster anions, Au(N)OH(-), and these have a reversed O2 adsorption activity: Nonreactive bare gold clusters become active when in the form Au(N)OH(-), while active bare clusters are inactive when -OH is bound. The binding energies for the stable structures obtained from density functional calculations confirm fully these findings. Moreover, the theory provides evidence that electron-transfer induced by the binding of a OH group enhances the reactivity toward molecular oxygen for odd anionic gold clusters and suppresses the reactivity for the even ones. The temperature dependence of O2 addition to Au(3)OH(-) and Au(4)(-) indicates deviations from equilibrium control at temperatures below room temperature. The effects of humidity on gold cluster adsorption activity support the conclusion drawn for the mechanism of O2 adsorption on "dry" gold cluster anions and provides insight into the possible role of water in the enhanced activity of supported gold cluster catalysts.     

(HBNH)n团簇的结构和稳定性

Structures and thermodynamic properties of the imidoboranes （HBNH）n （n=1-16） have been investigated theoretically at the B3LYP/6-31G^＊ level of theory. Needle-shaped oligomers that violate the isolated square rule were found to be more stable than cage isomers. The needle-shaped oligomer with n=16 was predicted to be exceptionally stable at low temperature, hexamer and octamer clusters dominated the gas phase at higher temperature. The highest oligomerization degree of the spontaneous cluster fomation has been estimated. It was concluded that generation of the gas phase （HBNH）n clusters with oligomerization degree n ≥24 was viable, making these species possible intermediates involved in the gas phase generation of BN nanoparticles.     

Many-cluster-systems: models of inhomogeneous matter

First, some general remarks concerning macroscopic “cluster matter” are given. In the second part, three recent, mainly optically and electron-microscopically performed investigations are discussed which deal with special properties of noble metal cluster systems forming the building units of this kind of matter:(1) dressed Au-55 clusters,(2) electromagnetic coupling effects among coagulated clusters,(3) the transition towards compact inhomogeneous matter caused by coalescence of clusters.     

Mechanical,thermomechanical, and thermal properties of polystyrene crosslinked with a multifunctional zirconium oxo cluster

Inorganic–organic hybrid materials were prepared by free radical polymerization of styrene in the presence of varying amounts of the cluster Zr₆O₄(OH)₄ (methacrylate)₁₂. Stepwise polymerization allowed the preparation of bubble‐ and crack‐free, transparent bulk samples on a 30 g scale with dimensions required for mechanical testing. Small‐angle X‐ray scattering investigations and transmission electron micrographs revealed that the clusters formed randomly distributed aggregates of random size. Solvent uptake in swelling experiments was related to the cluster proportion. Storage moduli in the glassy state were slightly increased when compared with neat polystyrene, but pronounced plateau moduli were observed above the glass transition temperature, which correlated to the cluster proportion. Plateau moduli were used to calculate network parameters such as network density. Onset temperatures of thermal decomposition and the glass transition temperatures of the cluster‐crosslinked polymers were higher than that of neat polystyrene. Thermal expansion coefficients were unaffected in the glassy state, but were gradually reduced above the glass transition temperature with increasing cluster proportion. Both the tensile moduli at room temperature and the yield points increased when polystyrene was doped with the cluster. The strain hardening moduli, as determined in compression tests at large deformations, increased linearly with the cluster proportion. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2215–2231, 2007