Formation and Bonding of Alane Clusters on Al(111) Surfaces Studied by Infrared Absorption Spectroscopy and Theoretical Modeling

Abstract

Alanes are believed to be the mass transport intermediate in many hydrogen storage reactions and thus important for understanding rehydrogenation kinetics for alanates and AlH3. Combining density functional theory (DFT) and surface infrared (IR) spectroscopy, we provide atomistic details about the formation of alanes on the Al(111) surface, a model environment for the rehydrogenation reactions. At low coverage, DFT predicts a 2-fold bridge site adsorption for atomic hydrogen at 1150 cm(-1), which is too weak to be detected by IR but was previously observed in electron energy loss spectroscopy. At higher coverage, steps are the most favorable adsorption sites for atomic H adsorption, and it is likely that the AlH3 molecules form (initially strongly bound to steps) at saturation. With increasing exposures AlH3 is extracted from the step edge and becomes highly mobile on the terraces in a weakly bound state, accounting for step etching observed in previous STM studies. The mobility of these weakly bound AlH3 molecules is the key factor leading to the growth of larger alanes through AlH3 oligomerization. The subsequent decomposition and desorption of alanes is also investigated and compared to previous temperature programmed desorption studies.

Nanostructures from hydrogen implantation of metals.

“…This finding is consistent with the theory AlH 3 in a physisorbed state, as proposed in the mechanism by Chaudhuri et al [12]. Assuming this binding energy, we find desorption and step-etching of the Al (111) surface at 340 K, as shown in Figure 5.6, in agreement with experiments.…”

“…While there is little in the literature regarding the formation, DFT calculations have shown Al 2 H 6 and Al 3 H 9 to be stable surface species [12]. In the current model, migration from a step and transitioning from a chemisorbed state to a physisorbed state are not understood or accounted for.…”

“…The surface alanes are more weakly bound to Al adatoms [12] and etch the smooth Al surface. However, the mechanistic details behind both the surface reactions and mass transfer are uncertain and only inferred from experimental observations.…”

“…However, the mechanistic details behind both the surface reactions and mass transfer are uncertain and only inferred from experimental observations. While the details are unknown, density functional theory (DFT) calculations show that AlH 2 and AlH 3 are more weakly bound than Al and AlH adatoms [12], which helps explain the surface morphology changes seen by Go et al…”

“…However, we are interested in the inverse process of CVD, so this uncertainty is immaterial. In another study, Chaudhuri et al invested the formation and oligomerization of alane clusters from adsorbed hydrogen on the Al (111) surface using both experimental (surface IR) and theoretical techniques (DFT) [12]. They found that at high H coverages, step edges become favorable sites for adsorption of atomic hydrogen, and AlH 3 likely forms here at saturation.…”