Berichte der Bunsengesellschaft für physikalische Chemie最新文献

Computer simulation of possible distributions of H_ad and D_ad atoms developed on nickel surface as a result of partial reactions of anodic hypophosphite oxidation and cathodic proton (deuteron) discharge from water respectively was used for modeling of the catalyst surface state according to on-line electrochemical mass spectrometry data. The simpliest lattice-gas model gives a probable qualitative description of the catalyst surface state and allows the genesis of electrocatalytic properties of nickel with the electrode potential to be followed. Location of anodic and cathodic half-reactions at special types of sites was evidenced, leading to formation of non-equilibrium H₂, HD and D₂ mixtures with the lower HD content than that predicted theoretically.

In this paper we show that the use of colloidal assemblies as templates favors the control of the size and shape of nanoparticles. As expected theoretically, the change in size and shape of copper metal nanosized particles induces changes in their optical properties. Cylindrical copper metal particles having the same size and shape can be obtained in various regions of the phase diagram when the template is made of interconnected cylinders.

Self-assembly of silver and silver sulfide nanoparticles is reported. Monolayers of particles organized in a hexagonal network are formed over very large domains. Small or large aggregates can also be produced and, in these aggregates, the particles are highly organized and form pseudo-crystals with a face-centered cubic structure for various particle sizes.

Metal clusters exhibit unique optical properties due to the excitation of Mie plasmon resonances. It is well known since decades that measured resonances of clusters, surrounded by some adsorbate, or some solid or liquid embedding material (as e.g., in colloidal systems), are often not described quantitatively by Mie's theory. Only recently, these discrepancies were traced back to complex physical and chemical influences of the cluster-matrix interlayer onto the optical response. They prove often to be more important than cluster size effects.

These findings opened a new field of surface/interface research where deviations of measured Mie resonances from the predictions of Mie's theory are used as sensitive sensors for physical and chemical interface properties and processes in cluster-matter.

By combining optical spectroscopy experiments on free clusters in UHV and on the same clusters after embedding, this method was calibrated to separate, quantitatively, the cluster-matrix interface effects from other cluster effects like shape and structure effects, nonlocal dielectric effects and cluster size effects.

Among all metals, silver exhibits the most pronounced Mie resonances, so silver clusters were used as model systems and were embedded in a broad variety of solid and liquid embedding media, in course of the investigations reported in the present Progress Report.

A theoretical description of the obtained data, based upon static and dynamic charge transfer processes of the cluster electrons into/out of adsorbate states is, however, only at its beginning. It allows to ascribe the extremely short decay times of the resonances of the order of 1 to 10 fs to phase relaxation processes; the decay times are in good correspondence with results of direct femtosecond-experiments.

The electronics of colloidal nano-particles of semiconductors and metals has been one of the fastest growing research fields in physical chemistry for the past 10 years. The historical development is briefly reviewed and a number of fundamental effects and processes are described. They include the transition from the compact material to the atom, heterogeneous photo-catalysis, and various aspects of nano-electrochemistry, such as the storage of excess electrons and metal deposition on nano-particles. Preparative aspects, such as the synthesis of composite metal and semiconductor particles and of nano-crystals of semiconductors are also discussed.

Temperature-dependent EXAFS (Extended X-ray Absorption Fine Structure) measurements between 5 and 290 K show that structural and dynamic properties of CdS nanoparticles with 13-120 Å diameter depend on their particle size and the way of surface stabilization. The mean Cd-S distance of thiolcapped CdS nanoparticles (13-40 Å) increases with decreasing particle size with respect to CdS bulk. In contrast, mean interatomic distances in polyphosphate-stabilized particles (30-120 Å) are slightly contracted. This demonstrates that the covalently bound thiol ligands induce the nanocluster lattice expansion. From the temperature dependence of the mean-square relative displacement (MSRD), a slight stiffening of the Cd-S bond can be observed with decreasing particle size. The static part of the MSRD, reflecting static disorder of the Cd-S bond, exhibits a maximum at cluster sizes of about 30 Å. For particles larger than 30 Å diameter, the increase of the static disorder follows the change of the surface to volume ratio. This ratio also determines the size dependence of the asymmetry of the interatomic Cd-S pair potential. Furthermore, in CdS bulk and CdS nanoparticles with the thermodynamically favored wurtzite structure this asymmetry is significantly enlarged in comparison to nanoparticles with zincblende structure.

Precipitation of lead ions by hydrogen sulfide in the presence of amphiphilic block copoloymer micelles composed of polystyrene-b-poly(4)vinylpridine yields PbS nanoparticles. By adjusting the reaction conditions with respect to the lead-to-polymer ratio, temperature or proton concentration particles of different size (2 to 20 nm) and morphology (spheres, cubes or needles) have been prepared. These particles have been characterized by powder X-ray diffraction and High Resolution Transmission Electron Microscopy (HRTEM).

At a low lead content of the polymer micelles and relevated temperatures the formation of nanocrystalline PbS needles is preferred. These exhibit a structured absorption spectrum with two peaks at 375 and 580 nm. In polymer micelles with high lead content, PbS spheres (1 to 2 nm) are formed. Growth of these particles can be induced by increasing the proton concentration, resulting in particles with diameters up to 20 nm.

The chemistry of tungsten chlorides in the LiCl/KCl eutectic has been investigated by Raman spectroscopy between 400 and 500°. It has been found that W(VI) is reduced to W(V) directly by the free Cl⁻ ion in the melt and the octahedral complex ion WCL⁻₆ is formed. Addition of metals (Al, Zn, Pb, W) causes further reduction of W(V). Depending on the metal added WCl^2-₆ or W metal can be obtained.

Phase equilibria in ambient temperature molten salt systems prepared from aluminium bromide and dimethylethylphenylammonium bromide or trimethylphenylammonium iodide are described. The experimental densities of some solutions of the mentioned mixtures in aromatic hydrocarbon and the compositions of these layering solutions are given depending on temperature.

The enthalpies of formation of amorphous Zr-Cu, Zr-Ni, Zr-Cu-Al, Zr-Ni-Al and Zr-Ni-Ti alloys from pure crystalline elements at 298 K were measured in an isoperibolic Calvet-type solution calorimeter which is operating in the temperature range 298–1300 K. The alloys of the following compositions were investigated:

Zr_1-xCu_x (x = 0.30, 0.40, 0.50, 0.60, 0.65, 0.70)

Zr_1-xNi_x (x = 0.33, 0.40, 0.67)

(Zr_0.65Cu_0.35)_1-x (Zr_0.65Cu_0.35)_x (x = 0, 0.10, 0.20, 0.40, 0.50, 0.60, 0.70, 0.80)

(Zr_0.60Ni_0.40)_1-x (Zr_0.60Ni_0.40)_x (x = 0, 0.125, 0.250, 0.374, 0.500)

(Zr_0.33Ni_0.67)_1-x (Ti_0.33Ni_0.67)_x (x = 0, 0.15, 0.30, 0.45)

The results are discussed in comparison with thermodynamic data about enthalpies of formation of liquid alloys and intermetallic compounds available in the literature. In all investigated systems the enthalpies of formation show great negative values and a strong chemical interaction between the atoms.