Reconstruction and disordering on (110) fcc metal surfaces

Due to the large number of possible defects available to disorder the fcc metal (110) 2x1 missing row reconstruction, there is a particularly rich variety of possible disordered phases. The five distinct disordering scenarios that have been predicted for this reconstruction depend on the relative energy cost to generate the different defects that can form on this surface. Recent high resolution LEED measurements on the clean and alkali covered Pd(110) surfaces have explored a few of these phases. For clean Pd(110), a surface consisting of semi-ordered islands has been demonstrated to exist up to 950 K. The periodic island structure can be predicted by a simple model that includes step-step interactions and strain relaxation in the islands. Subsequent alkali adsorption destroys the island structure and forms the 2x1 missing row reconstruction. Diffraction results will be presented as a function of alkali coverage that show that the Pd(110)+K 2x1→1x1 transition is an example of deconstruction to the “even” flat phase. The existence of this intermediate phase has important implications to the growth of the missing row reconstruction and the range of the alkali-substrate interaction. In addition to these observations, higher potassium coverage data demonstrates that the Pd(110)+K work function minimum is correlated with a surface roughening transition.     

Superconductivity of some transition metal compounds

Single crystals of niobium carbonitride were made by zone melting growth methods and single crystals of γ-NbN and δ-NbN by zone annealing crystal growth. The crystals are nonstoichiometric in contrast to the niobium carbonitride or niobium nitride prepared in reaction with nitrogen gas and niobium-niobium carbide mixtures and niobium metal, respectively. The transition temperature for superconductivity (T_c) decreases with increasing deviation from stoichiometry, and a determination of T_c is a nondestructive determination of this deviation. An instrument using the Wheatstone bridge principle is described and T_c values are listed for some nonstoichiometric single crystals of niobium carbonitride and niobium nitride.     

Effect of soluble calcium on the determination of the labile metal content of sediments with ion-exchangers

Equilibration of sediments with cation-exchangers results in a transfer of loosely bound labile metal species to the exchanger phase. Dissolution of the matrix is also promoted and selectivity rules suggest that some of the cations released (particularly Ca) could effectively compete with metal ions for exchange sites. This potential source of error has been evaluated by studying synthetic mixtures of Ca²⁺ and other metal ions (Cu²⁺, Pb²⁺, Cd²⁺, Zn²⁺) and by analysis of two calcium-rich wastes (a calcine and a jarosite). The ion uptake most influenced by calcium competition was that of zinc; uptake of lead was least affected. For minimum error, i.e., optimum transfer of “available” or “labile” metal ion, the level of free Ca²⁺ introduced into the solution should not exceed 300 mg/l., and the amount of exchanger added must provide an excess of exchange sites relative to the amount of cations released from the sample. By use of exchangers of different types it is possible to attempt some classification of the labile metal content, e.g., acid-displaced, exchangeable, salts of weak acids.     

Formation and relaxation of 2D island arrays in metal(100) homoepitaxy

We present a comprehensive analysis of both the formation of near-square islands during deposition in submonolayer metal (100) homoepitaxy, as well as the subsequent post-deposition relaxation of these island arrays. We highlight recent fundamental advances in our understanding of the nucleation and growth of islands, as well as of the kinetic pathways controlling the relaxation of island arrays (including a study of the “collision” and coalescence of diffusing islands). Extensive Scanning Tunneling Microscopy results are presented for the Ag/Ag(100) system at 295K, and these are analyzed utilizing kinetic Monte Carlo simulations of appropriate lattice-gas models.     

C-H activation of ethane by group 4 metal atoms: observation and characterization of the MH-CH2CH3, MH2-(CH2)2, and MH3-CH=CH2 complexes

Group 4 metal atoms excited in the laser ablation process activate ethane to form the C-H insertion product, the metallacyclopropane dihydride, and vinyl metal trihydride complexes as major products. These three new metal hydrides are characterized by their strong M-H stretching absorptions and other weaker modes as predicted by density functional theory vibrational frequency calculations.     

Some general aspects of hydrogen chemisorption on metal surfaces

Chemisorption of hydrogen on metal surfaces requires the dissociation of the H₂ molecule in the first place; this process has been experimentally investigated and theoretically described in terms of multi-dimensional potential energy diagrams. The adsorption of atomic hydrogen is frequently accompanied by displacements of the metal surface atoms leading to phenomena such as layer relaxation or surface reconstruction. Especially surface reconstruction may be regarded as a precursor stage for a progressive chemical attack of the hydrogen atoms also on the bulk metal, leading to the occupation of so-called “subsurface” sites, to bulk diffusion and, finally, to hydride compound formation. All these processes depend sensitively on the crystallographic structure of the surface, and some examples for H on Rh, Co and Pd surfaces will demonstrate the general correlation between the hydrogen surface concentration and the metal's cohesive energy, surface crystallography, and its tendency to reconstruct.     

The structures of transition metal-transition metal alloys

A structure map using the average electron count and d orbital energy difference as indices is used to sort transition metal alloys of stoichiometry AB. The gross features of the map are mimicked by tight-binding calculations. The inclusion of s orbitals on the metal atoms appear to be important in the determination of alloy structure in some parts of the calculated map. The correct coloring of the elemental lattice as a function of electron count is reproduced by calculation (i.e., AuCd vs WC and CsCl vs CuTi). Two new stability fields for the WC and CuTi structures are predicted. The calculations fail to really distinguish bcc, fcc, and hcp derivative structures in the region of 6–8 d + s valence electrons per atom. In this part of the structure map the calculations appear to be sensitive to small geometrical changes.     

Development of capillary electrophoresis for the determination of metal ions using mixed partial and complete complexation techniques

A new capillary electrophoretic (CE) method was developed for the selective and sensitive determination of common metal ions. The proposed method is based on conventional CE separation of metal cations followed by complete complexation of separated analytes with 1,10-phenanthroline using the zone-passing technique. This approach combines both partial and complete complexation modes and, thus, enables rapid, selective, efficient separation together with sensitive direct UV detection of metal species. The optimal conditions for the separation and derivatization reaction were established by varying type of electrolyte, electrolyte pH, introduction time and concentration of 1,10-phenanthroline. The optimized separations were carried out in 50 mmol l(-1) glycolic acid electrolyte (pH 6.0 with imidazole) using direct UV detection at 254 nm. Five common metal cations (Fe2+, Co2+, Ni2+, Cu2+ and Zn2+) were separated in less than 4 min. The proposed system was applied to the determination of Fe(II) and Zn(II) in snow samples. The recovery tests established for snow samples were within the range 100+/-12%.     

Potentiometric and spectrophotometric flow-injection determinations of metal ions with use of metal ion buffers

Potentiometric and spectrophotometric flow-injection determinations of metal ions, based on metal ion buffers, are described. A copper(II) ion-selective electrode and copper(II) ion buffers containing nitrilotriacetic acid (NTA) or ethylenebis(oxyethylenedinitrilo)tetraacetic acid (EGTA) are used for determination of ca. 10^?3 M transition metal ions or of calcium in the presence of magnesium. Spectrophotometric determination of transition metal ions is achieved by using a zinc ion buffer solution containing NTA and xylenol orange as indicator. Zinc concentrations up to 2 M can be determined by using large dispersion in the manifold. The factors influencing the sensitivity of the proposed methods are discussed.     

Application of carbon sorbents for the concentration and separation of metal ions.

Solid-phase extraction is an attractive approach in the preparation of many kinds of samples prior to analysis; highly selective sorbents are desirable for this purpose. The objective of this review is to provide updated information about carbon-based sorbents, their interaction modes and potential application for the concentration and separation of metal ions from environmental samples prior to their determination. New selective phases such as fullerenes and carbon nanotubes are described. Selected examples illustrate the potential of these sorbents.     

Ion chromatographic separation of alkali metal and ammonium cations on a C18 reversed-phase column

The separation of alkali metal (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) and ammonium cations on a C₁₈ reversed-phase column using three anionic surfactants [sodium 1-eicosyl sulphate, sodium dodecyl benzenesulphonate and sodium dodecyl sulphate (SDS)] is described. Two methods were examined: (a) “permanent” coating, with the use of a C₁₈ reversed-phase column previously coated with the surfactants; and (b) dynamic coating, with addition of the surfactants to the mobile phase. With method (a) the separation of the six cations was achieved with SDS. However, the retention times gradually decreased owing to dissolution of the SDS coating. Good separation was obtained with method (b), where 10 mM HNO₃ containing 0.1 mM SDS was used as the mobile phase with conductivity detection, and it was applied satisfactorily to real samples. The effect of system peaks on determination is also discussed.     

Towards accurate ab initio calculations on the vibrational modes of the alkaline earth metal hydrides

The fundamental modes of the alkaline earth metal hydrides (BeH2, MgH2, CaH2) and their dimers, HX(H)2XH, have been studied by vibrational configuration-interaction calculations based on very accurate potential energy surfaces. Comparison with experimental data obtained from matrix isolation and gas phase measurements is provided and the agreement was found to be excellent for the monomers but poor for the dimers. In addition, many fundamental bands are predicted which have not yet been detected experimentally.     

Fluorescence quenching near small metal nanoparticles

We develop a microscopic model for fluorescence of a molecule (or semiconductor quantum dot) near a small metal nanoparticle. When a molecule is situated close to metal surface, its fluorescence is quenched due to energy transfer to the metal. We perform quantum-mechanical calculations of energy transfer rates for nanometer-sized Au nanoparticles and find that nonlocal and quantum-size effects significantly enhance dissipation in metal as compared to those predicted by semiclassical electromagnetic models. However, the dependence of transfer rates on molecule's distance to metal nanoparticle surface, d, is significantly weaker than the d(-4) behavior for flat metal surface with a sharp boundary predicted by previous calculations within random phase approximation.     

New chelating sorbents for noble metals

The properties of the new ehelate-forming “POLYORGS” sorbents for concentration of noble metals are discussed. POLYORGS are made from different polymeric matrices (polystyrene, copolymers of styréne with divinylbenzene, fibrous materials). They contain heterocyclic amine and amidoxime groups, and are selective for noble metals. Some methods of noble metal determination after preliminary concentration of POLYORGS sorbents are given.     

Vibrational modes of metal nanoshells and bimetallic core-shell nanoparticles

We theoretically study the spectrum of radial vibrational modes in composite metal nanostructures such as bimetallic core-shell particles and metal nanoshells with dielectric core in an environment. We calculate frequencies and damping rates of fundamental (breathing) modes for these nanostructures along with those of two higher-order modes. For metal nanoshells, we find that the breathing mode frequency is always lower than the one for solid particles of the same size, while the damping is higher and increases with a reduction in the shell thickness. We identify two regimes that can be characterized as weakly damped and overdamped vibrations in the presence of external medium. For bimetallic particles, we find periodic dependence of frequency and damping rate on the shell thickness with period being determined by the mode number. For both types of nanostructures, the frequency of higher modes is nearly independent of the environment, while the damping rate shows a strong sensitivity to the outside medium.     

Determination of copper, lead and cadmium in whole blood by stripping voltammetry with the use of graphite electrodes

The problem with toxic metal ion determination in blood is the adsorption of organic compounds on the electrode surface and the formation of complexes between metal ions and organic constituents of blood. This is the reason why usually preliminary acid digestion or other sample pretreatment is used. Two kinds of electrodes have been used: “Ultra-Trace Electrode”, made from impregnated graphite (I), and thick film graphite disposable electrodes (II). The analysis of whole blood with different sample preparation methods shows, that chemical digestion is not necessary for the analysis. Electrochemical two-stage sample preparation provides the possibility for analysing whole blood with the mentioned electrodes. Thick film disposable electrodes are less sensitive to the interference of organic constituents of blood. These electrodes give the possibility to determine total cadmium, lead and copper concentration in whole blood without special sample pretreatment. The application of “Ultra-Trace Electrode” for blood analysis is possible only after preliminary pretreatment of blood by chemical digestion or electrochemical sample preparation.     

Core distortions in metal atoms and the use of effective core potentials

The recent experimental determination of the geometry of Ti(CH₃)₂Cl₂ shows it to be inconsistent with the VSEPR model, a result not uncommon for molecules containing transition metal atoms. The valence shell charge concentrations (CCs) that appear as maxima in L(r)=−²ρ(r), provide a physical basis for the VSEPR model of molecular geometry for main group molecules. The same model accounts for the geometry of transition metal molecules with the proviso that the CCs are formed within the outer shell of the core of the metal atom, as defined by the shell structure of L(r). This observation appears to be in conflict with calculations for Ti(CH₃)₂Cl₂ showing that its geometry can be predicted using an effective core potential for the metal atom, a procedure that would appear to preclude the presence of core distortions. The apparent contradiction is resolved by distinguishing between the definition of the core using L(r) and one based on the orbital model.     

Artificial atom solids based on metal nanocrystals: Formation and electrical properties

Metal nanocrystals can behave as “artificial atoms” due to their diameter-dependent single electron charging energy. Organically passivated nanocrystals with narrow size distributions can self-assemble into ordered arrays, offering the possibility of artificial atom solids with unique collective electronic properties, derived from both the size-dependent electronic properties of the individual nanocrystal cores and the inter-nanocrystal electronic coupling mechanisms. We review our recent progress on probing the electronic properties of artificial atom solids via variable temperature charge transport measurements on laterally contacted arrays of metal nanocrystals, together with development of combined synthesis and processing routes to manipulate these properties.     

Catalytic activity of noble metal ions for the degradation of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphine in the presence of oxidizing agent, and its application to the determination of ultra trace amounts of ruthenium

Trace quantities of ruthenium(II) ion catalyze the oxidation of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphine (TPPS) by potassium bromate. This reaction can be used for the determination of ultra trace amounts of ruthenium using the water-soluble porphyrin with its high molar absorbance. The scope of the reaction was investigated in terms of the reaction conditions and selectivity with respect to other noble metal ions. The effect of pH, concentration of bromate, the reaction time, and the type of metalloporphyrin were studied so as to optimize the method for the determination of trace amounts of Ru(III). The apparent reaction rate constant for the disappearance of TPPS (or metal-TPPS) is proportional to the root of the concentration of bromate, and directly related to that of Ru(III). The limit of detection is 0.11?nM (equal to 10.7?pg?mL^?1) at pH?4.25, where the turnover number is 201. The reproducibility for five measurements at 2.7?nM of Ru(III) was 2.9%.

Resonance energy transfer from a fluorescent dye to a metal nanoparticle

A quantum mechanical theory of the rate of excitation energy transfer from a fluorescent dye molecule to the surface plasmonic modes of a spherical metal nanoparticle is presented. The theory predicts the distance dependence of the transfer rate to vary as 1/d(sigma), with sigma=3-4 at intermediate distances, in partial agreement with the recent experimental results. F?rster's 1/d(6) dependence is recovered at large separations. The predicted rate exhibits nontrivial nanoparticle size dependence, ultimately going over to an asymptotic, a(3) size dependence. Unlike in conventional fluorescence resonance energy transfer, the orientational factor is found to vary between 1 and 4.