Al Nanocluster Research Articles

Enhanced Hydrogen Generation through Low-Temperature Plasma Treatment of Waste Aluminum for Hydrolysis Reaction.

This study investigates the low-temperature hydrogen plasma treatment approach for the improvement of hydrogen generation through waste aluminum (Al) reactions with water and electricity generation via proton-exchange membrane fuel cell (PEM FC). Waste Al scraps were subjected to ball milling and treated using two different low-temperature plasma regimes: Diode and magnetron-initiated plasma treatment. Hydrolysis experiments were conducted using powders with different treatments, varying molarities, and reaction temperatures to assess hydrogen generation, reaction kinetics, and activation energy. The results indicate that magnetron-initiated plasma treatment significantly enhances the hydrolysis reaction kinetics compared to untreated powders or those treated with diode-generated plasma. Analysis of chemical bonds revealed that magnetron-initiated hydrogen plasma treatment takes advantage by promoting a dual procedure: Surface cleaning and Al nanocluster deposition on top of Al powders. Moreover, it was modeled that such H2 plasma could penetrate up to 150 Å depth. Meanwhile, electricity generation tests demonstrate that only 0.2 g of treated Al powder can generate approximately 1 V for over 300 s under a constant 2.5 Ω load and 1.5 V for 2700 s with a spinning fan.

Oxygen Activation on Four-Atom Metal Clusters and Alloys

The dissociation of oxygen molecules and generation of two separated oxygen atoms is an important step in fuel cells. The activation of the bond linking two oxygen atoms in the oxygen molecule usually needs large activation barriers on metals utilized as catalysts. In the present study, the characterization and catalytic performance of four-atom Al nanocluster (Al4) as well as XAl3 (X = Ti and Sc) nanoalloys in activation of the oxygen molecule are investigated using density functional theory. Our results show that the tetrahedral isomer of the Al4 nanocluster lies lower in energy (Ecoh = −33.35 kcal/mol) than does the corresponding rhombus isomer (Ecoh = −31.05 kcal/mol). Also, transition metal (Ti and Sc) doping reduces the reactivity of the Al4 nanocluster. In consistent with the suggestion of reactivity descriptors used in the present study, our results reveal that the maximum catalytic performance in activation of the oxygen-oxygen bond is related to the pristine Al4 clusters. The energy barrier of O2 activation on the Al4 system (21.26 kcal/mol) exhibits that the activation of the bond linking of two oxygen atoms on Al4 is kinetically preferable. In this respect, our study shows that the Al4 nanocluster can be considered as an excellent catalyst for activation the oxygen molecule.

Oxygen activation on four-atom metal clusters and alloys

The dissociation of oxygen molecules and generation of two separated oxygen atoms is an important step in fuel cells. The activation of the bond linking two oxygen atoms in the oxygen molecule usually needs large activation barriers on metals utilized as catalysts. In the present study, the characterization and catalytic performance of four-atom Al nanocluster (Al4) as well as XAl3 (X = Ti and Sc) nanoalloys in activation of the oxygen molecule are investigated using density functional theory. Our results show that the tetrahedral isomer of the Al4 nanocluster lies lower in energy (Ecoh = –33.35 kcal/mol) than does the corresponding rhombus isomer (Ecoh = –31.05 kcal/mol). Also, transition metal (Ti and Sc) doping reduces the reacti­vity of the Al4 nanocluster. In consistent with the suggestion of reactivity descriptors used in the present study, our results reveal that the maximum catalytic performance in activation of the oxygen-oxygen bond is related to the pristine Al4 clusters. The energy barrier of O2 activation on the Al4 system (21.26 kcal/mol) exhibits that the activation of the bond linking of two oxygen atoms on Al4 is kinetically preferable. In this respect, our study shows that the Al4 nanocluster can be considered as an excellent catalyst for activation the oxygen molecule.

Different morphologies of aluminum nanoclusters: Effect of pressure on solid-liquid phase transition of the nanoclusters using molecular dynamics simulations

Abstract In this work, we have simulated the melting points of Al 55 nanoclusters in the icosahedral and cuboctahedral and for the Al 54 in the decahedral morphologies at different exerted pressures of 0.0001, 0.1, 0.5, 1, 2, and 3 GPa. We have investigated the effect of the exerted pressure on some of the thermodynamics and structural properties of the Al nanoclusters with the three different morphologies. Our results indicated that the thermal stability of the Al nanocluster in the icosahedral morphology is greater than other structures from low pressures to 2 GPa. After that, the Al nanocluster in the cuboctahedral morphology is more stable than the others. Our results also showed that the molar enthalpy and molar volume changes of melting are almost constant for all of the Al nanoclusters as the external pressure increase. Our structural investigations indicated that the Al nanocluster in the icosahedral morphology shows a solid-solid transformation to somewhat HCP symmetry near the melting point. The Al nanocluster in the cuboctahedral morphology shows a transformation to icosahedral morphology with increasing the temperature, then it changes to somewhat HCP structure near the melting point. The Al nanocluster in the decahedral morphology shows also a transformation to icosahedral morphology with increasing the temperature, then it changes to an amphorous-like structure near the melting point. These transformations exist for the Al nanoclusters at the different pressures.

Investigation of Size Effects on the Al Nanoclusters Physical Properties via Molecular Dynamics Simulations

Molecular dynamics (MD) simulation has been carried out for study of size effects on physical properties of Al nanoclusters with different sizes (N = 256, 500, 864, 1372, 2048 and 4000) in the temperature range 300 K ≤ T ≤ 1200 K for both free and periodic boundary conditions. Energy per site for periodic and free boundary conditions was calculated for the mentioned sizes and temperature. By increasing size of Al nanocluster, the energy per atom was convergence to that of the bulk. By use of molecular dynamics some physical properties such as melting point, surface energy, sublimation energy, thermodynamic limit and pair distribution function have been obtained. Melting point of bulk state is 875 K which proportional the jump of energy and sharp peak in heat capacity. Our calculated sublimation energy (ΔHs-calculated = 330 kJ/mol) was in good agreement with experimental result (ΔHs-experimental = 318 kJ/mol).

Sputtering of Al nanoclusters by 1–13 keV monatomic or polyatomic ions studied by Molecular Dynamics simulations

Molecular Dynamics (MD) simulations were employed to study sputtering of freestanding and supported spherical Al nanoclusters with 2–10nm diameters under bombardment by Al1 and Al13 projectiles with energies of 1–13keV at normal and oblique incidence. Both monatomic and clustered yields of secondary emission are found to be larger than those for (111) flat surface of the bulk Al (at equal irradiation conditions). In some events, target nanocluster receives a backward momentum and therefore its major part (more than 1/2 of the mass) is ejected due to produced secondary emission mainly towards the substrate direction. This “recoil effect” is found more pronounced under the impact of cluster projectiles and its probability decreases with increase of the target cluster size. A restricted number of MD simulations were performed to verify whether this “recoil effect” is strong enough to desorb a 4nm Al nanocluster off an Al (111) substrate. Desorption was observed under oblique Al13 impact within the impact parameter range of 0.6–0.9.

Initial Adsorption of C60 Molecules on Si(111)-7*7 Surface with Al Nanocluster Array

The initial adsorption of C60 molecules on an Al nanocluster array template, an Si(111)-7× 7 surface with an Al nanocluster array, was observed at various temperatures using a variable temperature scanning tunneling microscope (STM). STM images showed that C60 molecules stay above the dimer lines near the corner hole, with one of the 6—6 bonds facing toward the adsorbing surface and forming chemical bonds with two neighboring corner Si adatoms. The strong interaction immobilized C60 molecules on the adsorbing template. [DOI: 10.1380/ejssnt.2010.354]

Reversible atomic modification of nanostructures on surfaces using direction-depended tip-surface interaction with a trimer-apex tip

Using first-principles simulations, we propose a simple mechanism and an easy-controlled method for reversible modification of supported nanoclusters on surfaces with atomic precision. As illustrated, individual atoms at edges of a Al nanocluster on a Al(111) surface can be extracted vertically and repositioned with a Al trimer-apex tip, which allows to rearrange a ten-atom hexagonal nanocluster to a triangular one in a reversible way. The governing mechanism is the atomic tip-surface interaction whose distinct dependence on directions plays a key role in manipulations.

Surface structure and solidification morphology of aluminum nanoclusters

Classical molecular dynamics simulation with embedded atom method potential had been performed to investigate the surface structure and solidification morphology of aluminum nanoclusters Aln (n=256, 604, 1220 and 2048). It is found that Al cluster surfaces are comprised of (1 1 1) and (0 0 1) crystal planes. (1 1 0) crystal plane is not found on Al cluster surfaces in our simulation. On the surfaces of smaller Al clusters (n=256 and 604), (1 1 1) crystal planes are dominant. On larger Al clusters (n=1220 and 2048), (1 1 1) planes are still dominant but (0 0 1) planes cannot be neglected. Atomic density on cluster (1 1 1)/(0 0 1) surface is smaller/larger than the corresponding value on bulk surface. Computational analysis on total surface area and surface energies indicates that the total surface energy of an ideal Al nanocluster has the minimum value when (0 0 1) planes occupy 25% of the total surface area. We predict that a melted Al cluster will be a truncated octahedron after equilibrium solidification.

Amplification properties of Tb (III) green emission in polymeric waveguide doped with Tb–Al nanocluster

This paper describes optical amplification properties in a polymeric waveguide doped with Tb–Al nanoclusters. The Tb–Al nanocluster is a promising fluorescent material for polymeric waveguides, which can be uniformly dispersed in polymer matrices while restraining the concentration quenching of Tb 3+. Under the continuous optical pumping by 488 nm laser light, optical amplification for the green emission of Tb 3+ was achieved. The optical gain coefficients were estimated to be as high as 0.25 and 0.56 mm −1 at the Tb–Al nanocluster concentrations of 4.5 and 5.0 wt%, respectively. Taking into account our previous works for the polymeric waveguide doped with Eu–Al nanocluster, the rare-earth-metal nanocluster is believed to be a promising candidate for various photonic applications such as multicolor polymer lasers and waveguide-type optical amplifiers.

Scanning tunneling spectroscopy ofSmFeAsO0.85: Possible evidence ford-wave order-parameter symmetry

Scanning tunneling microscopy and scanning tunneling spectroscopy (STS) have been examined for an Al nanocluster periodic array grown on p-type silicon substrate. Local density of states at Al and Si sites within the nanocluster has been extracted from site-resolved STS spectra by taking into account tip-induced band bending (TIBB) effect. Besides, it has been clarified that the surface-potential- energy shift caused by TIBB effect directly influences the tunneling current spectra. Consequently, a good correspondence has been found between the experimental spectra and the theoretical local density of states except for the energy gap. The energy gap has been experimentally determined as 1.7 eV, which is much larger than the predicted value. This discrepancy could be ascribed to the local-density approximation rather than the TIBB effect.

Tip-induced band bending effect and local electronic structure of Al nanoclusters on Si(111)

Scanning tunneling microscopy and scanning tunneling spectroscopy (STS) have been examined for an Al nanocluster periodic array grown on $p$-type silicon substrate. Local density of states at Al and Si sites within the nanocluster has been extracted from site-resolved STS spectra by taking into account tip-induced band bending (TIBB) effect. Besides, it has been clarified that the surface-potential-energy shift caused by TIBB effect directly influences the tunneling current spectra. Consequently, a good correspondence has been found between the experimental spectra and the theoretical local density of states except for the energy gap. The energy gap has been experimentally determined as 1.7 eV, which is much larger than the predicted value. This discrepancy could be ascribed to the local-density approximation rather than the TIBB effect.

Intermediate surface structure of Al nanoclusters restricted to Si(111) half-unit cells observed via scanning tunneling microscopy

We have observed an intermediate surface structure between two different surface phases of $\mathrm{Al}∕\mathrm{Si}$ (111) using the scanning tunneling microscope. The intermediate structure between the Al nanocluster and the $\ensuremath{\gamma}$ phase shows a large cluster with the size of a half-unit cell. The structural model of the $\ensuremath{\gamma}$-phase $\mathrm{Al}∕\mathrm{Si}$ (111) has been reasonably proposed through the intermediate Al cluster.

Continuous-wave stimulated emission and optical amplification in europium (III)-aluminum nanocluster-doped polymeric waveguide

The authors have demonstrated stimulated emission from polymer-based planar waveguide doped with Eu–Al nanoclusters under continuous-wave optical pumping. For the waveguide sample with a nanocluster concentration of 5.0wt%, amplified spontaneous emission was observed in the spectra of the light decoupled from the waveguide edge under a pumping density of 0.16–1.55W∕cm2. The optical gain coefficient was 0.92mm−1 when pumped at 1.55W∕cm2. The well-suppressed concentration quenching and multiphonon quenching may cause the high-gain and low-threshold optical amplification. These results show that the Eu–Al nanocluster is a promising material for polymer-based solid-state lasers and waveguide-type optical amplifiers.

Thermal stability of Al nanocluster arrays onSi (111)-7 × 7 surfaces

By means of in situ high-temperature scanning tunnelling microscopy(STM), the thermal stability of highly ordered artificial Al nanoclusterarrays, so-called two-dimensional nanocluster crystals, on vicinalSi (111)-7 × 7 surfaces has been investigated. It was found that Al nanocluster crystals are stable below500 °C.Above 500 °C, the Al nanocluster crystal transforms into a surface phase with singular triangle shapes on down-terraces (the down-step side of aterrace). However, on up-terraces (the up-step side of a terrace), Al nanoclusters can move onSi (111)-7 × 7 surfaces over a large distance and the Al nanocluster crystal retracts toward the phase until the bare Si (111)-7 × 7 surface is completely recovered on up-terraces. The thermallyactivated motion of Al nanoclusters was not observed apparently below400 °C, though Al nanocluster crystals can be formed even at200 °C. This indicates that it is atom diffusion, not cluster diffusion, whichis responsible for the formation process of metal cluster crystals onSi (111)-7 × 7 surfaces.

Local Electronic Structure of Al Nanocluster Array Fabricated on Si(111)7 ×7 Surface

We have applied scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) as well as current imaging tunneling spectroscopy (CITS) [1] to an Al nanocluster periodic array of a submonolayer-Al/Si(111) system. These clusters become visible with an Al deposition thickness of 0.24–0.5 ML at a substrate temperature of 550 °C. The site-resolved STS spectra of the Al nanocluster at 78 K show an insulating energy gap of 3.4 eV. In the CITS images, we have found different features of space-resolved tunneling current for the filled and empty states, which markedly depend on atomic sites and also on inequivalent half unit cells.

Site-resolved electronic structure of Al nanocluster fabricated on Si(111)7 × 7 surface

Site-resolved electronic structure of Al nanocluster fabricated on Si(111)7 × 7 surface

Si(111)7×7再構成表面上Alナノクラスターの局在電子構造

Si(111)7×7再構成表面上Alナノクラスターの局在電子構造

Quantum Growth of Magnetic Nanoplatelets of Co on Si with High Blocking Temperature

Self-organized Co nanoplatelets with a singular height, quantized lateral sizes, and unique shape and orientation have been fabricated on a template consisting of ordered Al nanocluster arrays on Si(111)-7 x 7 surfaces. Despite their small volume (a few nm(3)), these nanomagnets exhibit an unusually high blocking temperature (>100 K). The perpendicular direction for easy magnetization, the high blocking temperature, the size tunability, and the epitaxial growth on Si substrates make these nanomagnets important for applications in information technology.

Quantitative characterization of the Al nanoclustering induced by H interaction with Si(1 0 0)c(4 × 12)-Al surface phase

Abstract Self-organized formation of Al nanoclusters (i) induced by interaction of atomic hydrogen with the Si(1 0 0) c (4 × 12)-Al surface and (ii) in the course of Al deposition onto the Si(1 0 0) c (4 × 12)-Al surface has been quantitatively characterized using scanning tunneling microscopy measurements of the Al nanocluster number density. The obtained data have been evaluated within the framework of nucleation rate theory. In the case of Al nanoclustering upon Al deposition at 350–600 K, the changeover of the critical cluster size has been revealed at about 500 K. In the case of H-induced nanoclustering at 400–650 K, the changeover of the rate-limiting process has been detected at about 550 K, namely, at lower temperatures the rate-limiting process is the H-induced detachment of Al atoms from the Si(1 0 0) c (4 × 12)-Al phase, while at higher temperatures this is a competition between H adsorption and desorption at the surface. Activation energy of surface diffusion of Al atoms on the Si(1 0 0) c (4 × 12)-Al surface has been found to be 0.99 eV. Activation barrier for Al detachment from the Si(1 0 0) c (4 × 12)-Al has been estimated to be 1.53 eV.