Sub-nanometer Particles Research Articles

The Atomic Metron – a Basis for Synthesis of Metal Sub-Nanometer Particles with a Discrete Number of Atoms

ABSTRACTFew techniques are available to systematically synthesize and characterize metal particles below 1nm in size. We build nanoparticles in an atomically defined manner through the use of a high-fidelity molecular container we call an atomic metron, which is used to select and count the metallic ions that will make up the resultant nanoparticle. After a defined number of ions are selected, the metron may be spatially isolated and the metallic ions reduced to an isolated nanoparticle. Each step in the process is characterized via analytical methods. AFM is used to demonstrate the formation of sub-nanometer particles. The counting of atoms, isolation, and formation of nanoparticles, shows high potential for easy synthesis of sub-nanometer particles with fine control over the number of atoms in each particle.

Negatively Charged Gold Atoms in Subnanometric Particles: Experimental Evidence from an X-Ray Photoelectron Spectroscopy Study

The results of an X-ray Photoelectron Spectroscopy study conducted on a series of gold nanoparticles recently reported by us, stabilized by monodentate, bidentate, tridentate and tetradentate thiolate calix[n]arene ligands, are presented here. By virtue of the different denticity of the ligands, the nuclearity of the resulting particles can be tuned down to the subnanometric range. From the present XPS results, a clear correlation among the experimental binding energy of single Au 4f peak components and the specific Au state of charge is proposed, where the smaller (i.e., nanometer) fraction of the series selectively shows negatively charged Au atoms. Our findings are relevant for the open discussion of a specific role played by negatively charged Au atoms in catalytic reactions, especially at low temperatures.

Control of Single Molecular Nanodot Patterns of Phenyl Azomethine Dendrimers by Statistical Simulation

Precisely synthesized subnanometer particles of metals or metal oxides can be prepared using dendritic polyphenyl azomethines as the template. With a goal of their arrays to a surface using a simple and quick process, such as spin-casting, statistical analyses were applied to a nanodot array of the dendrimers to obtain the relationship between the experimental condition and the results such as size, spacing, or its standard deviations. The dot patterns of a single molecular dendrimer on a substrate were able to be predicted with numerical values of the experimental parameters associated with the spin coat (concentration of the dendrimer, physical properties of solvent, the spin coating recipe, temperature of the solution, relative humidity (RH)) as the inputs for the statistical analysis.

Luminescent sub-nanometer clusters for metal ion sensing: A new direction in nanosensors

We describe the application of a recently discovered family of materials called quantum clusters, which are sub-nanometer particles composed of a few atoms with well-defined molecular formulae, exhibiting intense absorption and emission in the visible region in metal ion sensing, taking Ag25 as an example. The changes in the optical properties of the cluster, in both absorption and emission upon exposure to various metal ions in aqueous medium are explored. The cluster can detect Hg2+ down to ppb levels. It can also detect 5d block ions (Pt2+, Au3+ and Hg2+) down to ppm limits. Hg2+ interacts with the metal core as well as the functional groups of the capping agents and the interaction is concentration-dependent. To understand the mechanism behind this type of specific interaction, we have used spectroscopic and microscopic techniques such as UV–vis spectroscopy, luminescence spectroscopy, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD). Specific reasons responsible for the interaction of Hg2+ have been proposed.

CO oxidation with Au/TiO 2 aggregates encapsulated in the mesopores of MCM-48: Model studies on activation, deactivation and metal–support interaction

The activation of Au/TiO 2 clusters encapsulated in MCM-48 and related poisoning phenomena were studied. With these catalysts, which contain extremely disperse Au particles (average size below 1 nm still after exposure to 473 K according to EXAFS), light-off temperatures of 250–280 K were obtained upon activation by precursor reduction in a net oxidizing CO/O 2 feed, which is well comparable with state of the art Au/TiO 2 catalysts. This activation was, however, found to be superimposed by parallel poisoning. In an operando XAFS study with catalyst batches containing Au(III) precursor species of different reducibility for unknown reasons, it was observed that the final activity was strongly influenced by the precursor reduction. Apparently, high activities were achieved by Au particles formed at low temperatures making contact with the clean support surface. Delayed Au(III) reduction produced particles of similar size but much lower activity, probably due to predominant contact with poisoned support species. The catalysts were most active right after an initial incomplete reduction of the Au(III) precursor and deactivated at higher temperature despite further Au(0) formation. However, as complete reduction of Au ions did not cause breakdown of CO oxidation activity, Au ions do not seem to be a part of the active site. The poisoning could be effectively removed by an inert gas treatment at temperatures up to 673 K, which resulted in light-off temperatures down to 225 K. Turnover frequencies derived for this state agree with data published recently for sub-nanometer bilayered Au particles, which supports the importance of sub-nanometer particles for CO oxidation over Au catalysts. From the absence of significant contributions from support oxygen in the Au L III EXAFS spectra and of Au-derived signals in Ti K EXAFS spectra of reduced catalysts, it was concluded that there was no ordered relation between metal clusters and support surface, which appears therefore to be irrelevant for CO oxidation.

Origin of Intense Chiroptical Effects in Undecagold Subnanometer Particles

Time-dependent density functional theory (TDDFT) calculations are employed to examine the optical absorption and circular dichroism (CD) spectra of undecagold Au(11)L(4)X(2)(+) (X = Cl, Br) complexes and their Au(2)X(2)L precursors, where L is either 2,2'-bis(diphenylphosphino)-1,1'-binaphtyl (BINAP) or 1,4-diphosphino-1,3-butadiene (dpb). These systems exhibit intense and mirror-image Cotton effects in their CD spectra. Experimental peak positions are well reproduced in the calculations. The low energy peaks of Au(11)L(4)X(2)(+) arise primarily from transitions between delocalized metal superatom orbitals. Bidentate phosphine ligands have both a structural and electronic impact on the system. The lowest energy structure of Au(11)L(4)X(2)(+) has a chiral C(2) geometry, whereas monodentate phosphine ligands lead to a C(1) structure. In addition, the chiral core structure of Au(11)L(4)X(2)(+) is not sufficient to explain the strong Cotton effects, and the intensity of the CD spectrum is increased by the presence of the bidentate phosphine ligands.

The role of support oxygen in the epoxidation of propene over gold–titania catalysts investigated by isotopic transient kinetics

Transient kinetic experiments were performed on gold–titania catalysts for the epoxidation of propene using hydrogen and oxygen to investigate the reaction mechanism. A ‘classical’ 1 wt% gold on titania catalyst was studied, as well as a 1 wt% gold on Ti-SBA-15 catalyst. Steady-State Isotopic Transient Kinetic experiments using oxygen-18 were performed, which provide information on the types and quantities of species present on the catalyst during reaction, as well as on possible reaction pathways. The isotopic transients of the reaction products take place on a time scale of minutes, while the isotopic switch of the unreacted feed gases proceeds in a matter of seconds. This indicates that the oxygen pool on the catalysts, either in the form of adsorbed products or in the form of oxygen available for the reaction, is large in comparison to the product formation rates. The results indicate that support oxygen is playing a role in the reaction mechanism, but this could not be determined conclusively. For the water produced as a side product, it could be determined that support oxygen is ending up in the water; however, this can be explained by an exchange after its formation. The size of the oxygen pool in the neighborhood of gold, calculated based on the size of the TEM visible gold particles, is smaller than the amount of oxygen that ends up in the carbon containing oxidation products, which indicates that most likely part of the gold is present in sub-nanometer particles, not visible by TEM.

Achieving sub-nanometre particle mapping with energy-filtered TEM

A combination of state-of-the-art instrumentation and optimized data processing has enabled for the first time the chemical mapping of sub-nanometre particles using energy-filtered transmission electron microscopy (EFTEM). Multivariate statistical analysis (MSA) generated reconstructed datasets where the signal from particles smaller than 1 nm in diameter was successfully isolated from the original noisy background. The technique has been applied to the characterization of oxide dispersion strengthened (ODS) reduced activation FeCr alloys, due to their relevance as structural materials for future fusion reactors. Results revealed that most nanometer-sized particles had a core–shell structure, with an Yttrium–Chromium–Oxygen-rich core and a nano-scaled Chromium–Oxygen-rich shell. This segregation to the nanoparticles caused a decrease of the Chromium dissolved in the matrix, compromising the corrosion resistance of the alloy.

Structural Properties of the Bimetallic Cluster Au12Ag6 on the NOx (x=1,2,3) Adsorption Processes

AbstractIt has been shown that Au and Ag nanoparticles present catalytic properties of pollutant gases, product of the hydrocarbons combustion processes, whose efficiency is strongly dependant on their size, morphology and supporting material. The activity of Ag subnanometric particles supported on alumina in the NOx reduction mechanism, for the reaction HC-SCR (Hydrocarbon Selective Catalytic Reaction) is a good example of these catalytic properties. It is also well established that both Au and Ag nanoparticles are oxygen poisoning resistant and that in certain reactions the bimetallic combination may be more efficient than their isolated parts. In this work we present a DFT calculation of the NOx (x =1, 2, 3) adsorption energies on the Au12Ag6 cluster surface. It is also discussed the possible synergetic effect of the different configurations, forming small Ag islands composed of one to five atoms, on the surface of the bimetallic cluster. The analysis was done using DFT in the ZORA approximation.

The influence of the synthesis conditions of graphite/tin nanoparticle materials on their electrode electrochemical performance in Li-ion battery anodes

Electrochemical lithium insertion was carried out in tin–graphite composites obtained by two different preparation processes. In the first graphite was mixed with the products obtained by reduction of SnCl 4 with Na tert-Butanoate ( t-BuONa)-activated NaH (two-step synthesis). The second used materials synthesized by reducing SnCl 4 with a graphite and ( t-BuONa)-activated NaH suspension in THF (one-pot synthesis). Both composites were characterized by X-ray diffraction and transmission electron microscopy . It appeared that the tin particle size was controlled by the reduction time of SnCl 4. The stability of the electrochemical capacity of composites prepared by the two-step synthesis is dependent on the tin particle size: a stable capacity upon cycling was shown with subnanometer particles while a capacity fade was observed with larger nanoparticles. In materials prepared by the one pot synthesis, tin was present either as nanopartcles supported on graphite or as free aggregates. An initial reversible capacity of 630 mA hg −1 decayed to a constant value of 415 mA hg −1 after 12 charge/discharge cycles. It was hypothesized that the fraction of tin bound to graphite contributed to the stable reversible capacity while free tin aggregates were responsible for its decay.

Thiol-Functionalized Undecagold Clusters by Ligand Exchange: Synthesis, Mechanism, and Properties

Ligand exchange of phosphine-stabilized undecagold precursor particles, Au11(PPh3)8Cl3, with omega-functionalized thiols provides a convenient and general approach for the rapid preparation of large families of thiol-stabilized, subnanometer (dCORE approximately 0.8 nm) particles. The approach permits rapid incorporation of specific functionality into the stabilizing ligand shell, is tolerant of a wide range of functional groups, and provides convenient access to new materials inaccessible by other methods. Mechanistic studies and trapping experiments give insight into the progression of the ligand exchange, providing evidence that the core size of the phosphine-stabilized undecagold precursor particles is preserved during ligand exchange. The optical properties of the thiol-stabilized nanoparticles depend strongly on the composition of the ligand shell, and a series of studies suggests that this dependence is a result of the ligand shell's influence on the electronic structure of the particle core, as opposed to a structural change within the nanoparticle core.

Comparison of Au Catalysts Supported on Mesoporous Titania and Silica: Investigation of Au Particle Size Effects and Metal-Support Interactions

Au catalysts supported on mesoporous silica and titania supports were synthesized and tested for the oxidation of CO. Two approaches were used to prepare the silica-supported catalysts utilizing complexing triamine ligands which resulted in mesoporous silica with wormhole and hexagonal structures. The use of triamine ligands is the key for the formation of uniformly sized 2–3 nm Au nanoparticles in the silica pores. On mesoporous titania, high gold dispersions were obtained without the need of a functional ligand. Au supported on titania exhibited a much higher activity for CO oxidation, even though the Au particle sizes were essentially identical on the titania and the wormhole silica supports. The results suggest that the presence of 2–3 nm particle size alone is not sufficient to achieve high activity in CO oxidation. Instead, the support may influence the activity through other possible ways including stabilization of active sub-nanometer particles, formation of active oxygen-containing reactant intermediates (such as hydroxyls or O2−), or stabilization of optimal Au structures.

Ligand Exchange Reactions Yield Subnanometer, Thiol-Stabilized Gold Particles with Defined Optical Transitions

A convenient preparation is reported for a series of functionalized, thiol-stabilized gold particles with subnanometer core diameters (dCORE = 0.8 ± 0.2 nm). The preparation produces previously inaccessible materials through ligand exchange reactions of a phosphine-stabilized precursor, Au11(PPh3)8Cl3, with ω-functionalized alkanethiols. Convenient access to these new materials is a prerequisite to the detailed study of the electronic and optical properties of subnanometer particles and the investigation of the utility of these building blocks in nanoscale devices. Preliminary investigations of the optical properties of these new materials by UV−vis spectroscopy revealed that the particles have defined optical transitions. These results confirm the presence of discrete energy levels in the electronic structure of these materials that might be expected due to quantum size effects. Initial studies also suggest that the optical properties depend on the nature of the stabilizing ligand shell. The ligand exchange method described is applicable for a diverse family of alkanethiols to produce both organic- and water-soluble particles that show increased stability over the phosphine-stabilized precursor. Extensive characterization indicates that the thiol-stabilized exchange products have well-defined core sizes and dispersities.

A CONDENSATION NUCLEUS COUNTER (CNC) SENSITIVE TO SINGLY CHARGED SUB-NANOMETER PARTICLES

The mixing-type particle size magnifier (PSM) of Okuyama et al. (1984, Aerosol. Sci. Technol. 3, 353–366) has recently shown an ability to detect positively charged particles with mobility diameters as small as 1.5 nm (Seto et al., 1997, J. Chem. Phys. 107, 1576–1585), well below the sensitivity limit of 2.3 nm available in commercial CNCs. Gamero-Castaño (1999, Ph.D. thesis, Yale University) has shown that the activation of even smaller particles is limited in this PSM by homogeneous nucleation due to cooling of the saturated dibutyl phthalate vapor stream before it mixes with the aerosol. This problem is overcome here by means of a reheater, whereby ions with mobilities (in air) higher than 2 cm 2 V −1 s −1 can be activated at supersaturations well below the threshold for homogeneous nucleation. The critical supersaturation required to activate ions whose mobility exceeds 0.78 cm 2 V −1 s −1 (mobility diameter smaller than 1.6 nm ) is seen to be independent of size, indicating that this instrument can detect arbitrarily small (charged) particles. The reheater makes it possible also to run the PSM at fairly small vapor flow rates with essentially no vapor loss. Varying the vapor flow rate hence provides a rapid means to scan over the supersaturation in the mixing region, and hence infer the particle diameter (or its charge) with the PSM alone. Coupled to an optical detector this PSM constitutes a condensation nucleus counter (CNC) suitable for the analysis of charged sub-nanometer particles, ions, as well as charged biomolecules and their fragments, with effectively no lower size limit.

Growth Kinetics of Cu Clusters on a α-Al2O3Surface Studied by SEXAFS

We show that SEXAFS can be used to determine the growth law of subnanometric particles. In the case of copper deposited on alumina the dynamic scaling of the mean cluster size distribution, in the radius range 2-10 A, is demonstrated. This scaling is explained by the coalescence of copper clusters, as in breath figures due to the condensation of liquid droplets. This growth law can be fitted by a power law with an exponent 1.2 and is fairly well reproduced by a purely stochastic coalescence model.

Study on Growth Processes of Subnanometer Particles in Early Phase of Silane RF Discharge

Growth processes of particles formed in an early phase of silane RF plasmas are investigated using the modified Langmuir probe, spectroscopic and photodetachment methods. From the probe measurements, the particles are estimated to be on the order of subnanometer size, and they are found to nucleate and grow principally around the plasma/sheath boundary near the RF electrode, where optical emission intensity of SiH radicals is high. These spatial profiles are different from those of positive and negative ion densities during discharging periods. These results suggest that many short-lifetime neutral radicals are necessary for the nucleation and subsequent growth of a particle. The photodetachment experiments and scanning electron microscope (SEM) observation also show that the particles are in the size ranges below about 200 atoms and about 1 nm. Spatial profile measurements of probe currents and negatively charged particle density obtained by the photodetachment method confirm that the mass of the particles around the plasma/sheath boundary near the RF electrode is large.

HREM image simulations for supported metal particle catalysts

The chemical properties of catalysts often depend on the size, shape and structure of the supported metal particles. To characterize these morphological features and relate them to catalysis is one of the main objectives for HREM study of catalysts. However, in plan view imaging, details of the shape and structure of ultra-fine supported particles (<2nm) are often obscured by the overlapping contrast from the support, and supported sub-nanometer particles are sometimes even invisible. Image simulations may help in the interpretation at HREM images of supported particles in particular to extract useful information about the size, shape and structure of the particles. It should also be a useful tool for evaluating the imaging conditions in terms of visibility of supported particles. P. L. Gai et al have studied contrast from metal particles supported on amorphous material using multislice simulations. In order to better understand the influence of a crystalline support on the visibility and apparent morphological features of supported fine particles, we have calculated images of Pt and Re particles supported on TiO2(rutile) in both plan view and profile view.

Imaging and analysis of surfaces with high spatial resolution

New electron microscopes designed to operate with specimens prepared, treated, characterized, and observed in an ultrahigh vacuum environment, provide several techniques for the study of surfaces with atomic or near‐atomic resolution. The method of profile imaging allows the best attainable resolution (1.6 Å) to be applied to determine surface atom positions and atomic motions may be recorded at television rates. Reflection electron microscopy is sensitive to fractions of a monolayer of foreign atoms on a surface and shows atom‐high surface steps with good contrast. Reflection electron energy loss spectroscopy, under surfaces resonance diffraction conditions is sensitive to the composition of surface monolayers. Microdiffraction allows structure analysis of subnanometer particles or crystal defect regions. Spectroscopic analysis or imaging with secondary or Auger electrons provides nanometer resolution on variations of surface composition.