Oxidation-resistant gold-55 clusters

Gold nanoparticles ranging in diameter from 1 to 8 nanometers were prepared on top of silicon wafers in order to study the size dependence of their oxidation behavior when exposed to atomic oxygen. X-ray photoelectron spectroscopy showed a maximum oxidation resistance for "magic-number" clusters containing 55 gold atoms. This inertness is not related to electron confinement leading to a size-induced metal-to-insulator transition, but rather seems to be linked to the closed-shell structure of such magic clusters. The result additionally suggests that gold-55 clusters may act as especially effective oxidation catalysts, such as for oxidizing carbon monoxide.     

Identification of active gold nanoclusters on iron oxide supports for CO oxidation

Gold nanocrystals absorbed on metal oxides have exceptional properties in oxidation catalysis, including the oxidation of carbon monoxide at ambient temperatures, but the identification of the active catalytic gold species among the many present on real catalysts is challenging. We have used aberration-corrected scanning transmission electron microscopy to analyze several iron oxide-supported catalyst samples, ranging from those with little or no activity to others with high activities. High catalytic activity for carbon monoxide oxidation is correlated with the presence of bilayer clusters that are approximately 0.5 nanometer in diameter and contain only approximately 10 gold atoms. The activity of these bilayer clusters is consistent with that demonstrated previously with the use of model catalyst systems.     

The structure of catalytically active gold on titania

The high catalytic activity of gold clusters on oxides has been attributed to structural effects (including particle thickness and shape and metal oxidation state), as well as to support effects. We have created well-ordered gold mono-layers and bilayers that completely wet (cover) the oxide support, thus eliminating particle shape and direct support effects. High-resolution electron energy loss spectroscopy and carbon monoxide adsorption confirm that the gold atoms are bonded to titanium atoms. Kinetic measurements for the catalytic oxidation of carbon monoxide show that the gold bilayer structure is significantly more active (by more than an order of magnitude) than the monolayer.     

Covalent Group IV Atomic Clusters

Atomic clusters containing from two to several hundred atoms offer the possibility of studying the transition from molecules to crystalline solids. The covalent group IV elements carbon, silicon, and germanium are now being examined with this long-range objective. These elements are particularly interesting because of the very different character of their crystalline solids and because they are intermediate between metals and insulators in the nature of their bonding. Small mass-selected atom cluster ions are formed by pulsed laser techniques and identified by time-of-flight methods. Laser photoexcitation is used to study the relative stability of these clusters and their modes of fragmentation. These modes for C(n)(+) clusters, which tend to fragment with a characteristic loss of a neutral C(3), are found to be different from the modes for Si(n)(+) and Ge(n)(+) clusters, which tend to fragment to "magic" clusters such as Si(4)(+), Si(6)(+) and Si(10)(+). These experimental results can be accounted for by recent theoretical calculations of the ground-state structure and stability of small silicon and carbon clusters. Several theoretical approaches give consistent results, showing that small silicon clusters are compact and different from small fragments of the bulk crystal. Calculations show that carbon clusters change from linear structures toward cyclic structures as the cluster size increases, but with significant odd-even differences.     

Is soot composed predominantly of carbon clusters?

Soot generated from diesel fuel in a combustion tube is characterized by microanalysis, x-ray diffraction, chemical reactivity, and nuclear magnetic resonance to address the recent proposal of the significance of carbon clusters in soot. The data support a traditional model of soot as polynuclear aromatic compounds rather than as clusters of carbon atoms with minimal edge site density. The amounts of noncarbon atoms in the soot (hydrogen, oxygen, nitrogen, and sulfur) are commensurate with the edge density of the crystallites (2 by 2 nanometers) inferred from diffraction. The chemistry of soot, in being reduced by potassium metal and alkylated by alkyl iodides, is that known for aromatic compounds and not that anticipated for materials such as graphite, with a small fraction of carbon atoms on edges.     

Porous semiconducting gels and aerogels from chalcogenide clusters

Inorganic porous materials are being developed for use as molecular sieves, ion exchangers, and catalysts, but most are oxides. We show that various sulfide and selenide clusters, when bound to metal ions, yield gels having porous frameworks. These gels are transformed to aerogels after supercritical drying with carbon dioxide. The aerogels have high internal surface area (up to 327 square meters per gram) and broad pore size distribution, depending on the precursors used. The pores of these sulfide and selenide materials preferentially absorb heavy metals. These materials have narrow energy gaps (between 0.2 and 2.0 electron volts) and low densities, and they may be useful in optoelectronics, as photocatalysts, or in the removal of heavy metals from water.     

Chemists cluster in chicago to confer on cagey compounds

Shrink a clump of matter to a few hundred atoms, and it enters a curious netherworld. Neither ordinary solids nor conventional small molecules, such atomic clusters have unique chemical, optical, and electronic properties. Since the mid-1970s, when advances in synthesizing and analyzing clusters first enabled researchers to explore this netherworld in earnest, clusters have nucleated a sizable field. Many of its participants gathered in Chicago on 15 to 22 September for the Sixth International Symposium on Small Particles and Inorganic Clusters, the first of the series to be held in the United States. At the top of the agenda for the 300 attendees were the field's old stalwarts, various kinds of metal clusters; its current darlings, the carbon clusters known as fullerenes; and a possible future superstar, silicon clusters.     

镍小团簇中金属键的E L F和L O L函数研究

用非限制性密度泛函方法UBP86泛函结合def2-TZVP基组获得了Ni2～6团簇的几何结构并用ELF(ElectronLocalized Function)和LOL(Localized Orbital Locator)函数对小镍团簇的电子结构进行拓扑分析.研究结果表明,除Ni2团簇外,团簇中Ni—Ni键的成键临界点不在键轴上,为弯曲键.镍团簇中Ni—Ni间的金属键为部分共价键,其价层Basin的布局数均小于1e,并且团簇中存在多个多中心价层Basin.团簇中各个化学键均强烈离域化.团簇中的单电子主要位于内层轨道,并没有参与成键.     

Magnetic clusters on single-walled carbon nanotubes: the Kondo effect in a one-dimensional host

Single-walled carbon nanotubes are ideal systems for investigating fundamental properties and applications of one-dimensional electronic systems. The interaction of magnetic impurities with electrons confined in one dimension has been studied by spatially resolving the local electronic density of states of small cobalt clusters on metallic single-walled nanotubes with a low-temperature scanning tunneling microscope. Spectroscopic measurements performed on and near these clusters exhibit a narrow peak near the Fermi level that has been identified as a Kondo resonance. Using the scanning tunneling microscope to fabricate ultrasmall magnetic nanostructures consisting of small cobalt clusters on short nanotube pieces, spectroscopic studies of this quantum box structure exhibited features characteristic of the bulk Kondo resonance, but also new features due to finite size.     

Electron solvation in finite systems: femtosecond dynamics of iodide. (Water)n anion clusters

Electron solvation dynamics in photoexcited anion clusters of I-(D2O)n=4-6 and I-(H2O)4-6 were probed by using femtosecond photoelectron spectroscopy (FPES). An ultrafast pump pulse excited the anion to the cluster analog of the charge-transfer-to-solvent state seen for I- in aqueous solution. Evolution of this state was monitored by time-resolved photoelectron spectroscopy using an ultrafast probe pulse. The excited n = 4 clusters showed simple population decay, but in the n = 5 and 6 clusters the solvent molecules rearranged to stabilize and localize the excess electron, showing characteristics associated with electron solvation dynamics in bulk water. Comparison of the FPES of I-(D2O)n with I-(H2O)n indicates more rapid solvation in the H2O clusters.     

An inverse compton process for the excess diffuse EUV emission from the virgo and coma galaxy clusters

The excess extreme-ultraviolet (EUV) emission detected in the Virgo and Coma clusters is explained by inverse Compton scattering of cosmic microwave background photons, which are scattered by the relativistic electrons that account for the extended radio synchrotron emission of these clusters. The lower limits of the average magnetic fields of these clusters estimated from the EUV excess are close to the equipartition magnetic fields derived from radio observations, indicating that the electron energies and magnetic field energies might be close to equipartition. The excess emission suggests energy reservoirs of approximately 10(61) and approximately 10(60) ergs for the Coma and Virgo clusters, respectively.     

Observation of large water-cluster anions with surface-bound excess electrons

Anionic water clusters have long been studied to infer properties of the bulk hydrated electron. We used photoelectron imaging to characterize a class of (H2O)n- and (D2O)n- cluster anions (n 相似文献   

Conversion of supramolecular clusters to macromolecular objects

In a reaction proceeding within a nanoscopic volume, supramolecular clusters were transformed to polymer objects while retaining their shape and size. Spatial isolation of the cross-linkable blocks of oligobutadiene that were involved in this stitching reaction was achieved by self-assembly of the molecules that made up the clusters. Thermal activation of cross-linking yielded macromolecules (molecular weight of 70,000) with a narrow size distribution that was similar to that of the supramolecular clusters. The macromolecules obtained have an anisotropic shape (2 nanometers by 8 nanometers), as determined by electron microscopy and small-angle x-ray scattering, and form materials that exhibit a liquid crystalline state.     

GaAs Clusters in the Quantum Size Regime: Growth on High Surface Area Silica by Molecular Beam Epitaxy

Molecular beam epitaxy has been used to grow microcrystalline clusters of gallium arsenide (GaAs) in the size range from 2.5 to 60 nanometers on high-purity, amorphous silica supports. High-resolution transmission electron microscopy reveals that clusters as small as 3.5 nanometers have good crystalline order with a lattice constant equal to that of bulk GaAs. Study of the microcrystallite surfaces by x-ray photoelectron spectroscopy shows that they are covered with a shell (1.0 to 1.5 nanometers thick) of native oxides of gallium and arsenic (Ga(2)O(3) and As(2)O(3)), whose presence could explain the low luminescence efficiency of the clusters. Optical absorption spectra of the supported GaAs are consistent with the blue-shifted band edge expected for semiconductor microcrystallites in the quantum size regime.     

微生物反硝化及其电化学强化研究进展

微生物反硝化过程是在硝酸盐还原酶、亚硝酸盐还原酶、一氧化氮还原酶、一氧化二氮还原酶及其他一些脱氢酶的作用下,将电子供体的电子传递给NO3-的过程.在此综述了反硝化的各种电子供体及各种碳源.根据碳源的不同,反硝化菌可分为异养型反硝化菌和自养型反硝化菌.针对反硝化的过程,可以通过提供电子供体、加入电子载体、提高酶活性、加速辅酶再生、电刺激提高微生物活性等方法来强化反硝化过程.     

Hydrated electron dynamics: from clusters to bulk

The electronic relaxation dynamics of size-selected (H2O)n-/(D2O)n[25 eaq-(s(dagger)) internal conversion lifetime.     

How do small water clusters bind an excess electron?

The arrangement of water molecules around a hydrated electron has eluded explanation for more than 40 years. Here we report sharp vibrational bands for small gas-phase water cluster anions, (H2O)(4-6)- and (D2O)(4-6)-. Analysis of these bands reveals a detailed picture of the diffuse electron-binding site. The electron is closely associated with a single water molecule attached to the supporting network through a double H-bond acceptor motif. The local OH stretching bands of this molecule are dramatically distorted in the pentamer and smaller clusters because the excited vibrational levels are strongly coupled to the electron continuum. The vibration-to-electronic energy transfer rates, as revealed by line shape analysis, are mode-specific and remarkably fast, with the symmetric stretching mode surviving for less than 10 vibrational periods [50 fs in (H2O)4-].     

多壁碳纳米管表面邻二氯苯分子印迹聚合物材料的制备

在多壁碳纳米管表面接枝碳碳双键,以邻二氯苯为模板、苯乙烯为功能单体、乙二醇二甲基丙烯酸酯为交联剂,采用沉淀聚合技术,在碳纳米管表面制备邻二氯苯印迹材料,用红外光谱、热重分析、透射电镜对材料性能进行表征。结果表明,在碳纳米管表面接枝一层稳定、均匀、30～50 nm厚的印迹材料。用高效液相色谱研究该印迹材料的吸附动力学及吸附容量,结果表明,印迹材料的吸附平衡时间为100 min,通过Scatchard分析,该聚合物与邻二氯苯的结合位点有2个,最大吸附容量为92.732 mol/g,平衡常数为17.986 mol/L,表明能成功将该材料用于检测环境中邻二氯苯的前处理。     

Nanotube nanotweezers

Nanoscale electromechanical systems-nanotweezers-based on carbon nanotubes have been developed for manipulation and interrogation of nanostructures. Electrically conducting and mechanically robust carbon nanotubes were attached to independent electrodes fabricated on pulled glass micropipettes. Voltages applied to the electrodes closed and opened the free ends of the nanotubes, and this electromechanical response was simulated quantitatively using known nanotweezer structure and nanotube properties. The mechanical capabilities of the nanotweezers were demonstrated by grabbing and manipulating submicron clusters and nanowires. The conducting nanotube arms of the tweezers were also used for measuring the electrical properties of silicon carbide nanoclusters and gallium arsenide nanowires.