Snapshots of four being adsorbed on Al(111) surface. Both the aluminum slab and the alanes were kept at a temperature of 300 K throughout this simulation run.
Oligomerization of smaller alanes into a large alanes at 800 K on Al(111) surface. The calculated agglomeration energy is −6.56 kcal/mol per . Notice the stringlike conformation.
Gas phase oligomerization of four alanes into a local minimum of . The system exhibits drop in energy due to the exothermic nature of the oligomerization process. The calculated agglomeration energy is ca. −23.88 kcal/mol per . This simulation run was conducted at 300 K.
The geometrical orientations of some of the possible isomers of . In (a), singly bridged, all four aluminum atoms are tetrahedrally coordinated to hydrogen atoms. In (b), doubly bridged, all four aluminum atoms are coordinated to five hydrogen atoms. In (c), mixed singles and double bridges, three aluminum atoms are tetrahedrally coordinated to hydrogen atoms, and one Al atom is coordinated to five hydrogen atoms. Isomer (c) has not been reported in the literature before. The isomer was found by ReaxFF to be a local minima during MD simulation. It was then optimized in Gaussian at the level of theory. It was found to be stable and very close in energy to the singly bridged isomer (a).
Aluminum atoms notoriously love each other as shown by the fractionation of an aluminum hydride molecule by a vacancy. (a) approaching the vacancy. The vacancy is indicated by the white circle. [(b) and (c)] flips over and is trapped at the vacancy. (d) Surface aluminum atoms at the edges of the vacancy become attached to the Al atom in . (e) The vacancy is repaired (self-healing) and (f) the hydrogen atoms diffuse away.
The mean square displacements, , of atomic hydrogen and mass-centers of and at different temperatures.
MD simulations trajectories of atomic hydrogen on Al(111) surface at 650 and 700 K. At 650 K the hydrogen atom hops between the threefold hollow sites via the bridge site. At 700 K the hydrogen atom not only hops between the threefold sites but also diffuses much faster over a wider surface area.
(a) The complex energy landscape with deep valleys and mountains spanned by x and y coordinates of the diffusing hydrogen atom at a temperature of 675 K. (b) The trajectory of atomic hydrogen at 675 K.
Schematic representation of the deposition of atomic hydrogen on Al(111) surface. The hydrogen atoms etch aluminum atoms from the surface leading to the formation of alanes.
(Top view) A snapshot from MD simulations. Oligomerization of the alanes to form an aluminum-hydride complex. At the middle of the aluminum surface we have an almost perfect Al(111) terrace instead of being corrugated or having atomic vacancies as one would expect. This surface is surrounded by an alanes complex. The formation of the flat terrace is due to reconstruction of the Al(111) surface.
Charge distributions of the aluminum atoms on the top layer of the slab plus the adsorbed hydrogen atoms, after minimization and at the end of the equilibration run (125 ps). The figure shows that at the end of the simulation there is a significant increase in the number of aluminum atom with charge of approximately 0. At the same time there is also an increase in the number of aluminum atoms with charges between and .
Surface IR spectra from Al(111) surface after sequential dosing of hydrogen on a stepped Al(111) surface at 90 K at as indicated on the right side. The vibration modes become broad at higher surface coverage. This is due to oligomerization of alane clusters and their minimal surface attachment leads to averaging of net perpendicular surface dipoles.
The surface IR spectra from Al(111) surface after hydrogen saturation dosing at 90 K (green curve) and 250 K (red curve). Top: on a stepped surface; Bottom: on a flat surface. Dots indicate the experimental data, plain lines are fit.
Schematic diagram showing (i) a hydrogen atom trapped at the bridge site due to localized minimum on the potential energy surface of this site. (ii) Potential energy diagram of the Al(111) surface, site b represents the potential energy well where the hydrogen atom is trapped in. Site d might be the bridge site with double vacancies. is the migration energy barrier and is the binding energy of the vacancy. The binding energies for site b are −5.23 and −1.01 kcal/mol using ReaxFF and DFT, respectively.
The mean square displacement of atomic hydrogen attached to a vacancy at 675 K. Atomic H diffuses away but is again attracted to the vacancy. The first portion (region A) where there is dramatic increase in MSD is due to the hydrogen atom breaking free from the bound hydrogen-vacancy complex. It then diffuses about the surface. However, after some time it is attracted back to the vacancy. Region B represents that time frame where the hydrogen atom is attracted to the vacancy and slowly loses its mobility. In region C (where the MSD has a value of ) the hydrogen atom is vibrating about its mean position (Al–H–Al bridge) at the vacancy.
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