hydrogen cluster materialsP. J. Roach*, A. C. Reber†, W. H. Woodward*, S. N. Khanna†, and A. W. Castleman, Jr.*‡
*Departments of Chemistry and Physics, Pennsylvania State University, University Park, PA 16802; and †Department of Physics,Virginia Commonwealth University, Richmond, VA 23284
Contributed by A. W. Castleman, Jr., July 16, 2007 (sent for review June 5, 2007)
The formation and oxygen etching of AlnHm� clusters are charac-
terized in a flow reactor experiment with first-principles theoret-ical investigations to demonstrate the exceptional stability ofAl4H7
�. The origin of the preponderance of Al4H7� in the mass
spectra of hydrogenated aluminum anions and its resistance to O2
etching are discussed. Al4H7� is shown to have the ability to bond
with ionic partners to form stable hydrides through addition of analkali atom [XAl4H7 (X � Li-Cs)]. An intuitive model that can predictthe existence of stable hydrogenated cluster species is proposed.The potential synthetic utility of the superatom assemblies built onthese units is addressed.
Small metal clusters have great synthetic potential in novelmaterial applications because of the atom-by-atom tunability
of their electronic structure and chemical activity (1–8). Clustertunability is currently being studied rigorously to discern suitableapplications in which novel cluster assemblies may be realizedand exploited. One such application is hydrogen storage (9–10).The recent discovery of a diverse series of aluminum–hydrideclusters, or alanes, by Li and colleagues (11, 12) has given furthercredence to the existing idea that aluminum clusters might serveas hydrogen storage media with an outstanding capacity (13).Alanes have also been considered for use in the high-energy-density application of solid rocket fuels (14), wherein bothaluminum and hydrogen are burned as fuel. Although thepotential of small aluminum clusters in future fuel applicationsis apparent, questions about their properties and behaviorremain.
Through a synergistic effort that combines gas phase reactivityexperiments and first-principles electronic structure studies, wehave investigated the formation and oxidation of the Al4Hn
�
series to determine how and why they are formed, if they areamenable to storage without degradation due to oxidation, andhow they may oxidize during combustion. Al4H7
�, in particular,is shown to be an extremely robust building block that is readilyformed and is resistant to reaction with oxygen, making it a goodcandidate for further assembly. Guided by electronic structurestudies we propose chemical principles that supplement theWade–Mingos rules generally used to predict geometries inboron hydrogen systems (11, 12, 15, 16). These principlesdescribe the reactivity of the Al4Hn
� series and further enable theidentification of species that are stable in an oxygen-rich envi-ronment. The present developments share similarities withGrimm’s hydrogen-displacement theorem (17) for Hass’s con-cept of pseudoelements (18).
The identification of stable aluminum–hydride clusters beganwith flow reactor experiments wherein cluster anions wereformed by the laser-induced plasma technique (19) in a fast-f lowreactor as described in Experimental Methods. (19, 20) Theabundances of specific species produced by the laser-inducedplasma technique are governed partially by kinetic restrictions.However, distinct differences in the abundances of composition-ally similar species are representative of thermodynamic differ-ences in formation processes. Furthermore, thermalized clusters
react with other molecular species present in the fast-f lowreactor. Thus, relative thermodynamic information is obtainedby examining the differences and changes in abundance, result-ant from the chemical interactions of the clusters with oxygen orother molecular species. Theoretical investigations were carriedout to explain the observed chemical behavior. The investiga-tions spanned over neutral and anionic AlnHm clusters contain-ing one to six aluminum atoms and up to 3n � 1 hydrogen atoms.In each case, numerous geometrical structures were investigatedto find the ground state. Here we present results for the Al4Hn
�
series.
Results and DiscussionFormation kinetics within the laser vaporization (lava) sourcewere investigated by monitoring the relative abundances ofindividual species in the Al4Hn
� series as the concentration ofhydrogen was varied, as is shown in Fig. 1. The clusters areproduced in a continuous source, and the distribution is rela-tively insensitive to laser intensity. The hydrogen atoms addsequentially without an even/odd alteration, suggesting fulldissociation of H2 within the source. As the hydrogen concen-tration is increased, the series fills in starting at species with lesshydrogen. However, by Fig. 1C a buildup at Al4H7
� is readilydiscernable. As the concentration is further increased in Fig. 1Dthe abundance of Al4H7
� is further enhanced and can rightly becalled magic. The damming of growth at Al4H7
� suggests thatthere is an underlying phenomenon that stops hydrogen uptakeat this number.
To study the oxidation of the Al4Hn� series a representative
population was formed (shown in Fig. 2A). Upon addition ofoxygen (Fig. 2B), the clusters containing even numbers ofhydrogen atoms readily react away whereas species containingan odd number of hydrogen atoms are resistant to oxygenetching.
It is important to use caution in peak assignment, especially in thecase of Al4H9 and Al4H11, when oxygen is present. This is becauseof the existence of a mass degeneracy between Al3O2Hx
� andAl4H(x�5)
� . We have assigned all species as the pure aluminum–hydrides after careful consideration. Experiments in the absence ofoxygen (Fig. 1) show that the Al4H7
� population increases contin-uously as the partial pressure of hydrogen is increased and disap-pears when the source of hydrogen is removed. Additionally,molecular oxygen was intentionally leaked into the source withresults very different from those reported, further supporting thesemass assignments. Furthermore, it is suggested that the appearanceof Al4H9
� and Al4H11� is resultant from the etching removal of
aluminum atoms from larger species as has been observed inprevious investigations (3).
Author contributions: P.J.R., A.C.R., W.H.W., S.N.K., and A.W.C. designed research, per-formed research, analyzed data, and wrote the paper.
The observed chemical behavior yields three distinct phenom-ena that require explanation: (i) the electronic features of Al4H7
�
that make it a unique species and the damming of cluster growthat this size, (ii) the selective reactivity with oxygen of even overodd hydrogen-containing species, and (iii) the exceptional re-sistance of the species Al4Hn
� (n � 1, 3, 5, and 7) against oxygenetching. Phenomena ii and iii refer to the general features ofAl4Hn
� species and will be addressed first.To determine the energetic differences between species, the
ground-state geometries of the Al4Hn� series were calculated as
shown in Fig. 3. In these structures hydrogen atoms occupy eitheron-top, bridge, or hollow positions, and their addition frequentlyresults in considerable changes in the geometry of the aluminumcore. Starting from planar Al4
�, the initial effect of hydrogen isto drive the four Al atoms into a square aromatic molecule (21)and then into a compact tetrahedral core. Further addition ofhydrogen eventually breaks the AlOAl bonds in favor of AlOH
bonds, driving the system to a lower energy structure where Alatoms are connected only by sharing the H atoms.
Explanation of these phenomena requires a complete inves-tigation of the ground-state energies of species involved so thechange in properties may be understood. The hydrogen removalenergy (HRE) represents the energy required to remove a singleH atom from a given cluster, or, equivalently, the energy gainedby adding a single H to Al4Hn�1
� . Thus, HRE functions as anindicator of the stability of Al4Hn
� in the formation process. Alarge highest occupied molecular orbital (HOMO)–lowest un-occupied molecular orbital (LUMO) gap is indicative of elec-tronic stability; the cluster prefers to neither donate nor receivecharge. The electron affinity (EA) represents the adiabaticenergy required to remove an electron from the anion species.This quantity represents the energy difference between theground states of the anion and the corresponding neutral species.
Fig. 1. Mass spectra of aluminum–hydride formation with carrier gas hy-drogen concentration increasing from A to D.
Fig. 2. Oxygen etching of Al4Hx�. (A) Mass spectrum after aluminum plasma
is exposed to H2 gas. (B) The clusters are subsequently exposed to molecularoxygen.
Fig. 3. The optimized structures of the Al4Hx� clusters where x � 0–13. Bronze
indicates Al atoms, and white indicates H atoms. The bond lengths are markedin angstroms.
14566 � www.pnas.org�cgi�doi�10.1073�pnas.0706613104 Roach et al.
Fig. 4A shows the HRE, HOMO–LUMO gap, and EA of theAl4Hn
� cluster series. At low hydrogenation levels there areminor oscillations in the HRE, but with the addition of the eighthhydrogen atom it decreases significantly. Hydrogen is boundstrongly or weakly in the Al4Hn
� species when n is odd or even,respectively. The HOMO–LUMO gap also shows strong oscil-lations with clusters having more than six hydrogen atoms.Because clusters with a large HOMO–LUMO gap do not preferto donate or receive charge, the gap is indicative of a possiblebarrier and resistance to further growth. Although Al4H7
� ex-hibits the first major jump in the HOMO–LUMO gap of the oddH series, the gap continues to increase at larger odd hydrogen-containing clusters, culminating in Al4H13
� , which has a vast gapof 5.17 eV. Furthermore, the stability of alanates is usuallyconnected to stability of AlH4
� motifs (22). Our studies indicatethat AlH4
� motifs have a HOMO–LUMO gap of 5.31 eV, whichis comparable to the HOMO–LUMO gap of Al4H13
� . These showthat hydrogenated aluminum clusters have gaps that are com-parable with known materials.
Variations in the adiabatic EA, which vary concomitantly withthe HOMO–LUMO gap, also show a substantial increase at Al4H7
�
compared with previous sizes. The calculated vertical detachment
energy of 3.05 eV is consistent with the recent measurement of 3.0eV (11). Anions with high EA are more likely to acquire andmaintain an extra electron. Furthermore, the high EAs of thespecies Al4Hn
� (n � 9 and 11) support our previous assumption thatthey may be etching down from clusters with greater numbers ofaluminum atoms because their EAs are higher than those of thecommon etching products AlO2, Al2O2, or HO2.
To understand the etching behavior more quantitatively, weexamined the energetics of the process where an O2 molecule isattached to the cluster and the gain in energy is sufficient to releasean AlO2
� molecule (or Al� anion) from the cluster (23). To this end,we calculated the gain in energy starting from molecular oxygenapproaching the cluster from a significant separation. Fig. 4B showsthe binding energy of the O2 plotted as a function of the number ofhydrogen atoms. (The energy to remove an Al� from pure AlnHm
�
is also shown to help gauge the thinking.) Note that the clusters withodd numbers of electrons (even numbers of hydrogen atoms) showbinding energies in excess of 8 eV, whereas clusters with evennumbers of electrons have significantly reduced binding. We alsofound that although the OOO bond is broken in clusters with evennumbers of hydrogens, it remains intact for clusters with oddnumbers of hydrogens. This is consistent with the observation thatclusters with odd numbers of electrons are etched, whereas thosewith even numbers of electrons are resistant. The origin of lowbinding in these even electron systems can be linked to the tripletmultiplicity of molecular oxygen (24). Because the ground state ineven electron systems is a singlet, the binding of O2 requires a spinexcitation of the cluster to a higher spin state. This interplaybetween spin and binding will be discussed, in detail, in a futurepublication.
We now discuss the special stability and electronic propertiesthat define Al4H7
� as a potentially unique species. We firstconsider the energetics that favor the formation of Al4H7
�. Thegrowth of the clusters occurs by the addition of hydrogen atomsas well as poaching reactions where the hydrogen atoms aretransferred between Al4Hm
� species (shown in the followingequation): Al4Hn
� � Al4Hm�3 Al4Hn�k
� � Al4Hm�k� . Analysis of
the energetics of the poaching process shows that Al4H8� will
react with all of the Al4Hm� clusters except Al4H7
� to form Al4H7�
species, indicating that it is energetically unstable. Similarly,Al4H6
� combines with Al4Hm� clusters with fewer than four H
atoms to generate seven atom species. Hence, the formation ofAl4H7
� is energetically favorable from either the growth or thebreaking of the larger species. A similar poaching reactionbetween the anions and neutral species also favors Al4H7
�
because it has the largest EA of all of the observed species. Inaddition to being stable, as shown in Fig. 4A, the EA and theHOMO–LUMO gap show a jump at Al4H7
�. All of these featuresaccount for the unusual stability of Al4H7
�. The large EA alsomakes it ideal for developing cluster materials.
Important questions are (i) what ultimately governs the elec-tronic structure of metal hydrogen systems and leads to theobserved trends in electronic properties? and (ii) are there simplerules that can describe stability? Previous researchers (11, 12) haveargued that the Wade–Mingos rules (15, 16) are the key tounderstanding the stability of metal hydrogen systems. However,both Al4H5
� and Al4H7� are consistent with these rules, yet Al4H7
�
is the largest even electron peak in the spectrum whereas Al4H5� is
the smallest. Although the Wade–Mingos rules are useful forpredicting structure, a clear understanding of reactivity requiresfurther analysis.
To provide insight that may facilitate future predictions of theexistence of stable species such as Al4H7
�, we have carried out anin-depth investigation of the electronic levels of the Al4Hn
� seriesto show how the molecular orbitals are affected by the additionof hydrogen atoms. This analysis provides an intuitive under-standing of how the local electronic structure of a cluster isperturbed by the addition of hydrogen.
Fig. 4. Energetic comparisons of the Al4Hx� series. (A) The HOMO–LUMO gap,
EA, and HRE for Al4Hx� clusters. (B) The binding energy of molecular oxygen
with the Al4Hx� clusters and Al� removal energy from Al4Hx
� clusters. (C) Theone-electron states in Al4Hx
� clusters. The states in red have appreciabledensity at the H sites.
Roach et al. PNAS � September 11, 2007 � vol. 104 � no. 37 � 14567
CHEM
ISTR
Y
Fig. 4C shows the occupied and unoccupied one-electronlevels in odd Al4Hn
� clusters containing up to 13 hydrogen atoms.For each cluster, the electronic charge densities in individualelectronic levels were analyzed to differentiate AlOAl andAlOH bonding states from unfilled levels.
An H atom has a deep 1s filled level and an unfilled 1s state.When the H atom is added to an Al4Hn�1
� cluster, the spin-up andspin-down 1s levels interact with the cluster levels close to theHOMO to form a deep pair of states. Two such states appear foreach spin. Because the H atom has only one electron in the 1s state,the other state must be filled by an electron transferred from theHOMO state of the preceding cluster. Fig. 4C shows the transfor-mation of AlOAl and AlOH states upon successive hydrogenation.The depopulation of HOMOs that are marked by AlOAl bonds todeeper AlOH levels affects the geometry of the clusters. As Fig. 3shows, the addition of hydrogen leads to structures wherein theAlOAl bonds are broken in favor of the formation of the AlOHbonds. In fact, for Al4H13
� , the aluminum atoms are linked only viahydrogen atoms, producing a HOMO–LUMO gap comparable toAlH4
�, which is a known building block (XAlH4, X � Li-Cs).The depopulation of states close to the HOMO results in
jumps of EA when the electron level moves from one group ofstates to another. This simple picture is only a rough correlation,however, because the added hydrogen atom also modifies theoverall electronic spectrum through structural rearrangement.Nevertheless, the electronic structure of the pure cluster maypotentially yield information concerning the jumps in theHOMO–LUMO gap and EA of the species. As an example, thecalculated electronic levels in Al4
� have groups of five, two, two,two, and two states that are expected to show HOMO–LUMOjumps at five, seven, nine, and 11 and a larger jump at 13 Hatoms. This roughly correlates with Fig. 4A.
This simple picture also provides a more formal basis for thewell known Grimm’s hydrogen displacement theorem (17),which states that the addition of a hydrogen atom to elementscan change their effective valence by one. Furthermore, thisanalysis is supported by the existence of known stable motifsAlH3, NaAlH4, Li3AlH6 (9), and BaAlH5 (10), which obey thisanalysis in that the number of H atoms to form the stable speciesequals the number of valence electrons. The model agrees withmany naturally occurring species through hydrogenation. H2O,NH3, CH4, and SiH4 are all commonly known to form molecularsolids, and each of these molecules is stabilized by the numberof hydrogen atoms equal to the chemical valence.
Although the identification of Al4H7� as a stable species with a
large EA is an interesting finding, the next issue is to find structuralmotifs in which the cluster can be used as a building block andstabilized in a new material. To this end, we have explored thepossibility where the extra charge could be donated by an alkaliatom and investigated the stability of LiAl4Hn and KAl4Hn species.Here we report only our findings on the lithium species. Fig. 5Ashows the geometries of the species, and Fig. 5B shows the energyto remove a Li or H atom as well as the HOMO–LUMO gap. Notethe jump at LiAl4H7 indicating it to be the stable species ideal forforming cluster materials in this series.
ConclusionsIt is shown that aluminum clusters coated with selected numbersof hydrogen atoms can exhibit unusual stability and resilience tooxidation. In particular, Al4H7
� is an outstanding example of astable cluster because it is marked by a large HOMO–LUMOgap and increased EA. By using an alkali atom to donate charge,the stable neutral complex LiAl4H7 can be formed. This complexhas potential to form cluster-assembled hydrides.
Experimental MethodsBriefly, in the restricted volume of a laser vaporization (lava)source a Nd:YAG laser (532 nm) impinges on a rotating/
translating aluminum rod, thus ejecting aluminum vapor intohelium and hydrogen carrier gases, which are concomitantlyf lowing through the source. The plasma created in the vapor-ization event frees atoms (i.e., hydrogen/protons) and electronsto combine with the aluminum vapor and form mixed clusters.The separated atoms and charges recombine through collisionsand electron capture events, respectively, as the plasma cools.Formation processes are arrested as the gas escapes the sourcewaiting room, entering expansively into a lower-pressure laminarflow tube. Carrier gas is leaked into the source at 8,000 standardcubic centimeters per minute while an �0.3-Torr pressure ismaintained in the flow tube by a high-volume roots blower. Theclusters are cooled to room temperature through transfer ofexcess thermal energy to the chamber walls in collisions withcarrier gas within the laminar flow region.
Theoretical MethodsThe studies used a first-principles molecular orbital approachwhere the wave function of the cluster is expressed as a linearcombination of atomic orbitals centered at the atomic sites in thecluster. The actual studies were carried out by using the NRL-MOL set of codes developed by Pederson and coworkers (25–27). The basis set was comprised of 6s, 5p, and 3d functions forAl and 4s, 3p, and 1d Gaussians for H and were supplementedwith additional Gaussians. The method incorporated the gen-eralized gradient approximation of Perdew et al. (28) for ex-change and correlation contributions.
We gratefully acknowledge funding by the U.S. Air Force Office ofScientific Research (Grants FA9550-04-1-0066 and FA9550-05-1-0186),the U.S. Department of the Army (Multidisciplinary University Re-search Initiative Grant W911NF-06-1-0280), and the U.S. Department ofEnergy (Grant DE-FG02-02ER46009) for the systematic studies onLiAl4Hn and KAl4Hn species.
Fig. 5. Charge-passivated Al4Hx� clusters. (A) The structure of the odd LiAl4Hx
clusters. (B) The Li removal energy, HOMO–LUMO gap, and HRE for the LiAl4Hx
clusters.
14568 � www.pnas.org�cgi�doi�10.1073�pnas.0706613104 Roach et al.
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