Properties Of Cu Clusters Research Articles

Influence of the zeolite support on the catalytic properties of confined metal clusters: a periodic DFT study of O2 dissociation on Cun clusters in CHA.

The catalytic properties of sub-nanometer Cun clusters are modified by interactions with inorganic supports used for their stabilization. In this work, the reactivity towards O2 dissociation of Cu5 and Cu7 clusters confined within the cavities of the CHA zeolite is theoretically investigated by means of periodic DFT calculations. Increasing the Al content in the zeolite framework not only modifies the cluster morphology, but also leads to a decrease in the electronic density available on the supported Cun clusters, which in turn leads to higher activation energies for O2 dissociation. Together with the cluster size and shape, the Si/Al ratio in the zeolite support appears as a potential parameter to finely tune the stability and oxidation properties of Cu-based catalysts.

Dendrimer-Encapsulated Copper Cluster as a Chemoselective and Regenerable Hydrogenation Catalyst

A copper cluster encapsulated within a poly(amidoamine) dendrimer with hydroxyl surface groups (PAMAM–OH) acted as a chemoselective catalyst for hydrogenation of carbonyl and olefin groups in water. The activity was dependent on the molar ratio of Cu2+ and PAMAM–OH used in the synthesis, which was ascribed to the size-specific chemical properties of Cu clusters. The dendrimer-encapsulated Cu cluster was oxidized into Cu2+ ions under aerobic conditions, but could be regenerated by reduction with NaBH4 for catalytic application.

Optical response of Cu clusters in zeolite template

Optical properties of Cu clusters embedded in mordenite are studied experimentally and theoretically. In this work we discuss spectral features of the system at various reduction steps and compare then with the results of spectra obtained within a theoretical model. The model employed consists of Cu clusters embedded in a homogeneous matrix. A second model employed introduced further variation considering a three component system where air or water can be present. The macroscopic dielectric response of the system is obtained within the Maxwell Garnett approximation. In this approach the complex non-local in homogeneous dielectric response of the zeolite+copper system is replaced by an effective homogeneous dielectric function. Metallic clusters can occupy specific available cavities in the zeolite framework. The presence of clusters that are smaller than the cavities in which they reside can lead to an air–Cu or water–Cu interface which allows shifts in surface plasmon resonance energies. As observed experimentally the energy of the main resonance is seen to be insensitive to the filling fraction ratios and highly susceptible to the embedding matrix properties. Reflectance spectra have been obtained which can be explained within this model.

Structural properties of Cu clusters on Si(1 1 1):Cu 2Si magic family

Basing on the results of the scanning tunneling microscopy (STM) observations and density functional theory (DFT) calculations, the structural model for the Cu magic clusters formed on Si(1 1 1)7 × 7 surface has been proposed. Using STM, composition of the Cu magic clusters has been evaluated from the quantitative analysis of the Cu and Si mass transport occurring during magic cluster converting into the Si(1 1 1)‘5.5 × 5.5’-Cu reconstruction upon annealing. Evaluation yields that Cu magic cluster accommodates ∼20 Cu atoms with ∼20 Si atoms being expelled from the corresponding 7 × 7 half unit cell (HUC). In order to fit these values, it has been suggested that the Cu magic clusters resemble fragments of the Cu 2Si-silicide monolayer incorporated into the rest-atom layer of the Si(1 1 1)7 × 7 HUCs. Using DFT calculations, stability of the nineteen models has been tested of which five models appeared to have formation energies lower than that of the original Si(1 1 1)7 × 7 surface. The three of five models having the lowest formation energies have been concluded to be the most plausible ones. They resemble well the evaluated composition and their counterparts are found in the experimental STM images.

Growth of Copper on Single Crystalline ZnO: Surface Study of a Model Catalyst

Copper, vapor-deposited on the polar, Zn-terminated ZnO(0001) surface is investigated in view of its suitability as model system for the technologically important Cu/ZnO catalyst. The structure and electronic properties of Cu clusters on ZnO(0001)–Zn have been studied with scanning tunneling microscopy (STM), low energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS), and low-energy He+ ion scattering (LEIS). At room temperature copper grows as two-dimensional (2D) clusters only at very low coverages of 0.001–0.05 equivalent monolayers (ML). At coverages greater than 0.01 ML, 3D clusters start to develop. This is contrasted to Cu growth on the oxygen-terminated ZnO(0001bar) surface, where a strong adhesion between Cu and the ZnO substrate results in an initial wetting of the surface by Cu. On ZnO(0001)–Zn, surface roughness and sputter damage change the growth mode to more 2D-like. Annealing in UHV results in well-separated, hexagonal clusters rotationally aligned with the substrate. Annealing of 2–5 ML Cu deposits on the ZnO(0001)–Zn surface in 10−6 mbar O2 results in the formation of a (√3 × √3)R30° superstructure with respect to the ZnO lattice. This superstructure likely contains Cu+ sites. The suitability of the different surface morphologies to probe specific sites that are thought to be active for catalytic processes is discussed.

Structural properties of Cu clusters on the O-terminated ZnO [formula omitted] surface

We have studied the growth of Cu deposited by molecular beam epitaxy at room temperature on the O-terminated ZnO (0 0 0 1 ̄ ) surface. The structural properties of Cu on ZnO were investigated by X-ray scattering, atomic force microscopy (AFM) and scanning electron microscopy (SEM). Chemical characterisation was performed by X-ray photoemission spectroscopy (XPS). X-ray diffraction studies reveal a predominant (1 1 1) out-of plane texture and the [1 1 ̄ 0] Cu | |[1 1 2 ̄ 0] ZnO and [1 1 2 ̄ ] Cu | |[1 0 1 ̄ 0] ZnO azimuthal orientation of the Cu. The XPS spectra show that the ZnO (0 0 0 1 ̄ ) surface is completely covered after a Cu deposition with a nominal thickness of 50 Å. AFM and SEM images clearly indicate that at later deposition states Cu clusters increase laterally and later coalesce.

Electronic properties of Cu clusters and islands and their reaction with O2 on SnO2(110) surfaces

The deposition of Cu on SnO2(110) surfaces, and its oxidation to CuxO, have been studied by low-energy electron diffraction (LEED) and angle-integrated photoemission using synchrotron radiation photoemission spectroscopy (SRPES). With the growth of copper on SnO2(110), which was found to follow the Volmer-Weber (“islanding”) growth mode, a small amount of metal-phase Sn segregates to the surface, and even when the copper thickness reaches several tens of A, Sn metal still is seen at the surface. But when this surface is annealed at 800 K in 5 × 10−6 mbar O2 for 20 min, the Sn atoms are totally converted to SnO2. Simultaneously, the deposited Cu atoms become oxidized. The surface charges up both during LEED and SRPES data acquisition. The clean SnO2(110) surface shows a 1 × 1 structure. With Cu deposition, the substrate LEED pattern gradually becomes weaker. With even more copper deposited, a Cu(111)-1 × 1-oriented particle structure appears, indicating coalescence of the Cu islands to 3-dimensional Cu(111) epitaxy. After subsequent heating to 500 K, the substrate signal appears again, and we see the SnO2 1 × 1 pattern. In conclusion, Cu atoms quite easily form clusters on the SnO2(110) surface already after a slight heat treatment. The results show that this system is quite active towards O2 gas exposure, and that the surface conductivity changes during O2 exposure.

Formation of titled clusters in the electrochemical deposition of copper on n-GaAs(001)

Using in-situ synchrotron X-ray diffraction, we have studied the epitaxial properties of Cu clusters electrochemically deposited on n-GaAs(001) substrates. The Cu clusters have (001) base planes and their 〈100〉 directions are aligned with the 〈110〉 directions of the GaAs(001) surface unit cell, but with a large angular spread of 4.5°. Moreover, the Cu(001) planes are tilted with respect to the GaAs(001) substrate. The tilt of about 6° did not show a preference for any substrate azimuth, giving rise to a ring-shaped (111) Cu reflection. This epitaxial tilt seems to be a novel mechanism of strain relief in an epitaxial system with a large mismatch of lattice constants and a large surface roughness.

The coupling between atomic and electronic structure in small Cu clusters

Thermodynamic and ground-state properties of Cu clusters have been studied with the effective-medium theory including a tight-binding description of the one-electron spectrum. Simulated-annealing Monte Carlo calculations have been performed for cluster sizes between 3 and 29 to determine ground-state energies and structures. Finite-temperature ensembles have been generated for a range of temperatures. The magic numbers 8, 18 and 20 are reproduced and remain stable at temperatures close to 1000 K, where the clusters may be regarded as liquid. The coupling between the atomic and electronic degrees of freedom through a Jahn-Teller-like effect is shown to play a key role in understanding the stability of the magic clusters at high temperatures. For the non-magic clusters this Jahn-Teller effect gives rise to large Fermi gaps even at high temperatures and a pronounced stability of the even-sized clusters relative to those with odd sizes. Finite-temperature electronic spectra are calculated. The fluctuations in the atomic positions give rise to a large broadening of the electronic levels, in agreement with experimental observations. Cu13 exhibits a rather sharp melting transition, whereas clusters of other sizes show more complex behaviour.

First-principles study of the structural and electronic properties of Cu clusters.

The structural and electronic properties of small (n\ensuremath{\le}5) Cu clusters are determined from first-principles calculations based on the local-spin-density approximation, using an all-electron, Gaussian-orbital formalism. The computational method includes use of a systematically refined numerical integration mesh, providing extremely accurate total energies and atomic forces, and a variational technique for treating accidental degeneracies at the Fermi level. Equilibrium geometries have been determined for the neutral clusters for n\ensuremath{\le}5, as well as for the ${\mathrm{Cu}}_{2}$ and ${\mathrm{Cu}}_{3}$ anions. The calculated properties of the dimer and trimer structures are examined in detail and compared to existing experimental measurements. The influence of the Cu 3d states on cluster properties is analyzed. It is shown that there is significant hybridization between the 3d and 4s in the cluster bonding states. One effect of this hybridization is a strong bond-length preference in the clusters.