Formation Of Small Metal Clusters Research Articles

Irradiation-driven molecular dynamics simulation of the FEBID process for Pt(PF3)4.

This paper presents a detailed computational protocol for the atomistic simulation of formation and growth of metal-containing nanostructures during focused electron beam-induced deposition (FEBID). The protocol is based upon irradiation-driven molecular dynamics (IDMD), a novel and general methodology for computer simulations of irradiation-driven transformations of complex molecular systems by means of the advanced software packages MBN Explorer and MBN Studio. Atomistic simulations performed following the formulated protocol provide valuable insights into the fundamental mechanisms of electron-induced precursor fragmentation and the related mechanism of nanostructure formation and growth using FEBID, which are essential for the further advancement of FEBID-based nanofabrication. The developed computational methodology is general and applicable to different precursor molecules, substrate types, and irradiation regimes. The methodology can also be adjusted to simulate the nanostructure formation by other nanofabrication techniques using electron beams, such as direct electron beam lithography. In the present study, the methodology is applied to the IDMD simulation of the FEBID of Pt(PF3)4, a widely studied precursor molecule, on a SiO2 surface. The simulations reveal the processes driving the initial phase of nanostructure formation during FEBID, including the nucleation of Pt atoms and the formation of small metal clusters on the surface, followed by their aggregation and the formation of dendritic platinum nanostructures. The analysis of the simulation results provides spatially resolved relative metal content, height, and growth rate of the deposits, which represents valuable reference data for the experimental characterization of the nanostructures grown by FEBID.

Degradation mechanism of silicone glues under UV irradiation and options for designing materials with increased stability

The degradation of silicone glues used, for example, in the assembly of solar modules for use in space, has been investigated and possible mechanisms which lead to colouration and possible embrittlement are analysed. Both effects are connected to the generation of radicals upon exposure to UV irradiation. As a follow up-reaction in vacuum, cross-linking and subsequently embrittlement and cracking due to shrinkage can occur. The degradation of the polymers via a radical mechanism can also lead to the formation of small metal clusters containing residues of the Karstedt catalyst and also silicon. Both types of nanoparticles contribute to colouration of the silicone glues. Possible ways to avoid this is the use of ultra-pure starting substances, a minimum amount of catalyst and a higher cross-link density of the silicone, minimizing the mobility of dangling chains formed upon UV-exposure and their tendency to form metal clusters.

Au/CuPc interface: A valence band photoemission investigation

The formation of the metal-organic interface depositing gold, by thermal evaporation in ultrahigh vacuum conditions, onto a copper phthalocyanine film has been studied by means of ultraviolet photoemission spectroscopy. The evolution of the valence band spectra and in particular of the highest occupied molecular orbitals (HOMO) and of the Au upper states has been followed as a function of gold film thickness. The HOMO peak position is strongly influenced by the Au deposition, suggesting an n-doping process of the organic film. The gold 6s state behavior indicates the formation of small metal clusters whose size increases as a function of the gold deposited. The doping of the organic film determines a strong reduction of the film ionization energy.

Magnetic and X-ray absorption investigations of Co-doped ZnO films

We present an investigation of magnetic and structural properties of Co-doped ZnO (ZCO) film grown by pulsed laser deposition at different dopant concentrations (cCo). X-ray diffraction patterns show that the films are single phase and exhibit ferromagnetism (FM) above room temperature (RT) with coercive fields up to 700 Oe. X-ray absorption fine-structure spectroscopy (XAFS) at the Co edge suggests that in films grown below 600°C dopant clustering involve less than 10% of the Co atoms in the alloy. In samples grown at higher temperature a larger fraction of Co atoms is involved in the formation of small metallic clusters. The experimental work has been accompanied by preliminary first-principles Density Functional Theory calculations.

Restructuring of hex-Pt(100) under CO Gas Environments: Formation of 2-D Nanoclusters

The atomic-scale restructuring of hex-Pt(100) induced by carbon monoxide with a wide pressure range was studied with a newly designed chamber-in-chamber high-pressure STM and theoretical calculations. Both experimental and DFT calculation results show that CO molecules are bound to Pt nanoclusters through a tilted on-top configuration with a separation of approximately 3.7-4.1 A. The phenomenon of restructuring of metal catalyst surfaces induced by adsorption and, in particular, the formation of small metallic clusters suggests the importance of studying structures of catalyst surfaces under high-pressure conditions for understanding catalytic mechanisms.

EXAFS, TDPAC and TPR characterization of PtInFerrierite: The role of surface species in the SCR of NOx with CH4

Ferrierite exchanged with In and with Pt, In (InFerrierite and PtInFerrierite) is characterized by extended X-ray absorption fine structure (EXAFS), time differential perturbed angular correlation (TDPAC) and temperature-programmed reduction (TPR). The presence of different In and Pt species is correlated with activity and selectivity during the NO selective reduction (SCR) with CH4 in the presence of excess oxygen. Both catalysts are active and selective for the said reaction. When a dry feed is used, both monometallic and bimetallic catalysts behave similarly. However, when water is present in the feed, while InFerrierite loses activity in the whole temperature range, PtInFerrierite increases its activity at temperatures above 450°C. EXAFS characterization indicates that in PtInFerrierite only a fraction of Pt is at exchange positions, the remaining fractions adopting the form of PtO2 crystals located in the outer-surface of the zeolitic structure. When this bimetallic catalyst is treated under wet reaction conditions, the formation of small metal clusters due to the reduction of PtO2 crystals occurs. Thus, the increase in activity could be due to a catalytic effect of Pt metal clusters on the NO oxidation reaction (NO+1/2O2→NO2).TDPAC characterization indicates the presence of two In species both in the monometallic and in the bimetallic catalysts: In2O3 and (InO)+Z− (In at exchange positions), the latter being the active In species for the reaction under study. TPR results also suggest the presence of highly dispersed non-crystalline In oxide species not bonded to the zeolite matrix. Neither EXAFS results nor TDPAC characterization show the presence of intermetallic Pt–In species. The results of this work prove that EXAFS and TDPAC techniques, combined with TPR and activity measurements, are powerful tools for the characterization of bimetallic catalysts.

Characterization of metallic species on Ni- and Co-containing ZSM-5 catalysts—reduction behavior and catalytic properties

A series of Co- and Ni-containing ZSM-5 zeolites with varying metal loading was prepared by solid-state ion exchange and by wet impregnation. Solid-state ion exchange led to highly dispersed metal ions inside the channels of ZSM-5. For the impregnated samples metal oxide clusters, mainly located on the exterior of the crystallites, were observed. Reduction at 573 K of NiZSM-5 prepared by solid-state ion exchange led to the formation of small metallic clusters inside the pores, whereas reduction at 773 K led to the formation of large Ni clusters outside the channels. CoZSM-5 could be only reduced at 773 K, where larger particles were rapidly formed. After low-temperature reduction, NiHZSM-5 showed in the hydroconversion of n-heptane an enhanced selectivity for isomerization, and after high-temperature reduction, for hydrocracking reactions. On CoHZSM-5 increased metal loading led to an extended hydrogenolysis activity in the hydroconversion of n-heptane.

Characterization and catalysis studies of small metal particles on planar model oxide supports

Planar model oxide-supported metal catalysts have been prepared by evaporating metals of catalytic interest onto various oxide thin films supported on refractory metal single crystals, followed by annealing to promote the formation of small metal clusters. The employment of these relatively simple supported catalysts prepared in ultrahigh vacuum (UHV) represent a promising method of simulating heterogeneous catalytic processes over high surface area supported catalysts in an experimentally tractable manner. While providing many of the same practical advantages associated with single crystals relative to high surface area supported catalysts, the experiments using planar model catalysts address important issues such as intrinsic particle size effects and metal support interactions that are beyond the scope of single crystal studies. Because these samples are suitable for study using modern UHV based surface analysis techniques, including scanning tunneling and atomic force microscopy, they can be characterized on the atomic level to a degree unlikely to be achieved for the analogous powder catalysts. Kinetics studies for several reactions performed in parallel over single crystals, planar model supported catalysts and conventional high surface area supported powder catalysts allow the catalytic behavior exhibited for a particular reaction to be correlated with specific structural features. The experiments described here establish the viability of these relatively simple systems as representative model catalysts and demonstrate their utility in elucidating heterogeneous catalytic systems, particularly in the context of combined studies involving single crystals and powder catalysts.

Pt Species in Zeolite X: Catalytic Activity in18O Exchange of O2with Zeolitic Oxygen,18O2–16O2Equilibration, H2–D2Equilibration, and the CO–NO Reaction

Pt0, Pt2+and Ptoxwere prepared by different treatments of platinum tetrammine complexes in alkali faujasites. The observed behavior of Pt, especially in TPR experiments, provided evidence that the decomposition of ammine complexes in an oxygen stream yields predominantly Pt cations, which following reduction by hydrogen leads to the formation of small metallic clusters, whose reoxidation gives Ptoxin zeolite cavities. The activity of both latter species was markedly higher than that of Pt2+in the reaction studied, i.e., in the isotopic exchange of gaseous oxygen with the oxygens of the zeolite framework, in the equilibration of hydrogen and oxygen molecules (D2+H2and18O2+16O2reaction, respectively), and in the NO reduction by CO. The catalytic reactions were studied over NaX and KX zeolites. Isotopic exchange of18O2with zeolitic oxygen occurred even below 400°C, oxygen activated on Pt centers (3Pt per unit cell) being spilled over to the oxygen of the zeolite skeleton. The equilibration of the gaseous oxygen molecules began above 300°C, while that of hydrogen molecules was easily measured at −78°C.

Formation of small metal clusters by ion bombardment of single crystal surfaces

The mechanism for the formation of small metal clusters ejected from an ion bombarded metal surface is examined in detail. The analysis is performed by classical trajectory methods which determine the positions and momenta of all particles in a model microcrystallite as a function of time. The calculation utilizes pair potentials for Cu derived from elastic constants of the solid and is performed for 600 eV Ar+ ion at normal incidence to the crystal. The results show that cluster species do not leave the surface as intact parts of the solid but form in a region above the surface. A trajectory for Cu5 formation is traced in detail showing a typical mechanism which is valid for Cun formation where n?7.