APPLIED PHYSICS LETTERS 90, 151917 共2007兲
Short-range order of low-coverage Ti/ Al„111…: Implications for hydrogen
storage in complex metal hydrides
E. Muller, E. Sutter, and P. Zahl
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973
C. V. Ciobanu
Division of Engineering, Colorado School of Mines, Golden, Colorado 80401
P. Suttera兲
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973
共Received 25 January 2007; accepted 12 March 2007; published online 12 April 2007兲
Using scanning tunneling microscopy and density functional theory, we characterize the population
of low-coverage Ti atoms on Al共111兲 as a model surface system for transition metal doped alanate
hydrogen storage compounds, such as NaAlH4. When deposited at room temperature, Ti is
kinetically trapped in first-layer substitutional sites, avoids nearest-neighbor locations, and
preferentially forms next-nearest-neighbor pairs, similar to a structure that has been predicted to
dissociate H2 with no energy barrier. The results on this well-defined system suggest the presence
of a large population of Ti-pair complexes that may catalyze the dissociative chemisorption of
hydrogen in Ti-doped alanate storage materials. © 2007 American Institute of Physics.
关DOI: 10.1063/1.2722197兴
The storage of hydrogen in a lightweight, high-capacity
medium with fast charge/discharge kinetics has been recognized as one of the primary challenges in achieving the transition to a hydrogen economy.1 Solid-state storage in
hydrogen-rich compounds, e.g., complex hydrides, offers potentially high storage capacities, but the most promising
compounds, such as LiBH4 共18.5 wt. % H content兲, are typically not reversible, i.e., rehydrogenation of the depleted
products cannot be achieved by controlling temperature and
H2 pressure alone. The discovery that the decomposition of
sodium alanate 共NaAlH4兲 can be made reversible by doping
with transition metals 共Ti, Zr兲 共Refs. 2 and 3兲 points to a
promising strategy of using catalysts to achieve both reversible hydrogen storage and fast reaction kinetics.
A fundamental understanding of the effects of transition
metal catalysts in doped NaAlH4 could serve as a basis for a
rational search for novel reversible hydride materials with
both high storage capacity and optimized reaction kinetics.
For NaAlH4, as for other metal hydrides, a key step in the
hydrogenation reaction is the dissociative chemisorption of
H2 on the depleted materials. Large activation barriers on
metallic Al, a decomposition product of NaAlH4, suggest
that catalyzing H2 dissociation could be a major role of Ti in
enabling the reversible hydrogenation of alanates. Indeed,
barrierless H2 dissociation has been predicted for a specific
rhombus-shaped Ti–Al surface complex 关shown in Fig. 1共a兲兴
on Al共001兲.4 The characteristic of this complex responsible
for facile H2 dissociation is the nodal symmetry of the surface highest occupied molecular orbital, allowing the transfer
of electron density from Ti 3d orbitals to the antibonding ␴*
orbital of the incoming H2 molecule.4
While forming the Ti–Al complex on Al共001兲 requires a
reconstruction that would only be stabilized by an ordered
array of substitutional surface Ti at high coverage, we will
show that individual rhombus complexes with the same
a兲
Author to whom correspondence should be addressed; electronic mail:
psutter@bnl.gov
nodal symmetry and similar Ti–Ti spacing 共11% larger兲 can
be generated on Al共111兲 by substitution of second-nearestneighbor surface Al atoms 关Fig. 1共b兲兴. The corresponding
short-range order of Ti on Al共111兲 corresponds to that of
共111兲 lattice planes of a stable bulk Al3Ti intermetallic compound shown in Fig. 1共c兲.5 X-ray absorption spectroscopy on
Ti-doped bulk NaAlH4 decomposition products indeed
shows Ti with reduced coordination 共i.e., at or near surfaces兲
in an Al3Ti-like bonding environment,6 suggesting that complexes such as that shown in Fig. 1 are quite prevalent in the
bulk storage materials.
Here we use high-resolution scanning tunneling microscopy 共STM兲 in ultrahigh vacuum 共UHV兲, combined with
density functional theory 共DFT兲 calculations, to study low
coverages 共␪ ⬍ 0.1 ML兲 of Ti on Al共111兲, a model system
intended to reproduce the situation of decomposed Ti-doped
NaAlH4. We show that Ti deposited on Al共111兲 at room temperature is kinetically stabilized in the surface layer. The
resulting disordered Ti–Al surface alloy shows a particular
short-range order of substitutional Ti: nearest-neighbor pairs
of Ti atoms are avoided and as a result the population of
second-nearest-neighbor Ti pairs, i.e., of the predicted catalytic complex 关Fig. 1共b兲兴, is increased relative to that of a
random Ti–Al surface alloy. Our findings show how kinetics
共stabilizing near-surface Ti兲 and energetics 共favoring secondnearest-neighbor Ti pairs兲 in this system lead to a large population of Ti-pair complexes that may enable facile H2 dissociation.
The experiments were performed in an UHV system
共base pressure of 10−10 torr兲 with in situ growth/preparation,
Auger electron spectroscopy 共AES兲 and STM. Thick
共⬎500 Å兲 epitaxial Al共111兲 films, grown by dc-magnetron
sputtering on clean Si共111兲-共7 ⫻ 7兲,7 were used as substrates.
Ti was deposited by dc-magnetron sputtering from a Ti target
共purity of 99.95%兲 to 0.01⬍ ␪ ⬍ 0.1 ML, using a calibrated
growth rate that, for the same deposition conditions, is reproducible for several months. Research Grade Ar, purified by a
heated getter purifier, ensured background impurities below
0003-6951/2007/90共15兲/151917/3/$23.00
90, 151917-1
© 2007 American Institute of Physics
Downloaded 14 May 2007 to 138.67.20.124. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
151917-2
Muller et al.
Appl. Phys. Lett. 90, 151917 共2007兲
FIG. 1. 共Color兲 共a兲 Structure and Ti–Ti spacing of the Ti:Al共001兲 reconstruction predicted in Ref. 4, with the periodic cell 共solid rectangle兲 and the
catalytic rhombus complex indicated. Black dots: hollow sites where H atoms reside after H2 chemisorption. 共b兲 Individual complex with the same
nodal symmetry and similar Ti–Ti spacing, forming upon deposition of Ti on
Al共111兲 at 300 K. 共c兲 Unit cell of a bulk D022 – Al3Ti intermetallic compound 共Ref. 5兲.
5 ⫻ 10−10 torr during growth. Two types of samples were prepared with Ti deposited at 300 and at 520 K, respectively.
STM was performed at 300 K using W tips that, when carefully dipped into the Al surface, reproducibly enabled chemical contrast of individual Ti atoms.8 All STM images shown
were obtained in constant-current mode at a sample bias 共VS兲
of +1.8 V and tunneling current 共I兲 of 0.2 nA.
DFT calculations were carried out in the generalized gradient approximation with the VASP package,9 using ultrasoft
pseudopotentials and the Ceperley-Alder functional form for
the exchange-correlation energy.10 The 96 atom supercell 共97
for adatom and interstitial atom simulations兲 had dimensions
of 9.88⫻ 11.40⫻ 24 Å3 along the 关101̄兴, 关12̄1兴, and 关111兴
directions, respectively. The Brillouin zone was sampled on a
⌫-centered 4 ⫻ 4 ⫻ 1 grid, which yielded eight irreducible k
points. A vacuum thickness of 12 Å and six 共111兲 layers of
aluminum were used, with the bottom two layers kept fixed
to simulate the underlying bulk. The energy cutoff for the
plane waves was set to 181.35 eV 共13.33 Ry兲. The atoms in
the first four layers were relaxed via a conjugate-gradient
algorithm until the total energy converged to less than
0.001 eV. We performed DFT calculations for systems with
one Ti atom 共first- and second-layer substitutional, adatom,
interstitial兲, and for pairs of substitutional atoms in the surface layer. After each electronic structure calculation,
Tersoff-Hamann11 empty-state STM simulations were performed for +1.8 V sample bias.
Ti deposition at 300 K generates a disordered array of
shallow protrusions covering the flat Al terraces. Their apparent height varies between 0.2 and 0.4 Å, depending on
the tip condition but is uniform for any given STM image. In
addition, the Ti growth induces the nucleation of 1 ML tall
islands on wider terraces 关Fig. 2共a兲兴, on which the same protrusions are observed. Annealing such samples to 520 K 共or,
alternatively, depositing Ti at 520 K兲 leads to wide terraces
without additional islands. In the 520 K samples an inversion
of the empty-state STM contrast is observed 关Fig. 2共b兲兴: the
Al共111兲 terraces now carry primarily shallow depressions
with about the same density. The area density of protrusions
and depressions is proportional to the nominal Ti coverage
关Fig. 2共c兲兴. Hence, we conclude that these features are due to
individual Ti atoms. AES shows only Ti and no measurable
oxygen or other contaminants. An observed linear increase of
the integrated Ti AES intensity with Ti deposition up to ␪
= 0.6 ML supports our identification of the STM features as
Ti atoms. STM image simulations performed for several configurations of one Ti atom in the supercell show that the
FIG. 2. 共Color兲 共a兲 Empty-state STM image of 0.04 ML Ti on Al共111兲,
deposited at 300 K 共VS = + 1.8 V, I = 0.2 nA, 320⫻ 120 Å2兲. 共b兲 STM image
of the same Ti coverage, deposited at T = 520 K. 共c兲 Density of protrusions
共300 K兲 and depressions 共520 K兲 determined from the STM images as a
function of Ti deposition time. 共d兲 Empty-state STM image simulation 共VS
= + 1.8 V兲 of individual Ti atoms substituting for Al in the surface layer. The
height profile shows that the substitutional Ti gives rise to a small
protrusion.
adatom and the first-layer substitutional Ti generate protrusions in empty-state STM, but only the latter has the small
height consistent with the experiments 关Fig. 2共d兲兴. Hence, the
deposition of Ti at room temperature gives rise to a disordered Ti–Al substitutional alloy in the surface layer.
The shallow depressions observed after 520 K annealing
indicate that the Ti atoms diffuse into new near-surface sites
at higher T. STM image simulations show depressions for
both interstitial and second-layer substitutional Ti, i.e., Ti
tends to occupy subsurface sites once kinetic limitations are
lifted. The two subsurface Ti sites show small differences in
STM contrast that have not been distinguishable in the experiment. DFT calculations of the stability of Ti/ Al共111兲
show a general trend to lower energy when Ti is incorporated
into subsurface sites. The energy of second-layer substitutional Ti is lower than that of first-layer substitutional Ti by
0.43 eV, consistent with the Al surface termination expected
based on the surface energies of Ti共111兲 and Al共111兲.12
Our STM results show a complex energy landscape with
local minima for near-surface Ti. Under strongly kinetically
limited conditions 共300 K兲, virtually all Ti is incorporated
into the surface layer. Even after deposition or annealing at
520 K, higher than typical cycling temperatures of complex
metal hydride storage materials,2 the amount of Ti in nearsurface sites remains unchanged 关Fig. 2共c兲兴, consistent with
Rutherford backscattering spectrometry, showing that Ti diffusion into deeper subsurface sites is activated only above
700 K.13 At 300 K, we find Ti trapped in metastable surface
sites, where a maximal catalytic effect for dissociative H2
chemisorption can be expected under practical conditions of
hydrogen storage in doped alanates. The surface Ti population may be enhanced further if additional Ti is promoted to
Downloaded 14 May 2007 to 138.67.20.124. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
151917-3
Appl. Phys. Lett. 90, 151917 共2007兲
Muller et al.
FIG. 3. 共Color online兲 共a兲 STM image of 0.04 ML Ti on Al共111兲, deposited
at T = 300 K 共190⫻ 120 Å2兲. The ellipses mark Ti pairs with Ti–Ti distance
of ⬃5 Å, whose orientation differs by 60°. 共b兲 Simulated STM image of a
second-nearest neighbor Ti pair. 关共c兲—共e兲兴 Ti-pair correlation function g共r兲
obtained from the experimental data, from a simulation of a random surface
alloy with forbidden nearest-neighbor pairs, and from a simulation of a
random alloy where such pairs are allowed.
the surface under high pressure H2. Atomistic simulations
indeed show that hydrogen termination can cause surface
segregation of Ti.14
The STM results in Fig. 2 show a disordered Ti–Al surface alloy with substitutional Ti in the surface layer, consistent with previous studies showing a disordered mixed Ti–Al
surface phase at higher Ti coverages 共⬎1 ML兲.15 We used
chemically specific STM images, showing the location of
individual Ti atoms, to determine the short-range order of the
alloy at low Ti coverages. The Ti-pair rhombus complex
关Fig. 1共b兲兴 forms spontaneously, and we have quantified its
abundance after Ti deposition on Al共111兲. Figure 3共a兲 shows
one of several STM images used to analyze the two-particle
correlation of Ti atoms embedded in Al共111兲. Individual Ti
atoms are clearly resolved, and Ti pairs with equal Ti–Ti
separation align in directions that differ by multiples of 60°,
as expected from the threefold symmetry of the surface. The
in-plane coordinates of all Ti atoms were determined by twodimensional Gaussian fits of the local maxima in the STM
images. Given the broadening of features in the STM images, this procedure would resolve Ti pairs with spacing below 3 Å, i.e., Ti atoms in nearest-neighbor sites. However,
no separations smaller than 3.5 Å are found, which means
that nearest-neighbor Ti pairs do not form on the surface.
This result is supported by DFT calculations, which indicate
that two Ti atoms repel when placed in nearest-neighbor
sites. DFT calculations for supercells with one Ti pair show
that the energy of the nearest-neighbor pair exceeds that of
the second-nearest-neighbor configuration by 0.21 eV. The
latter gives a simulated STM image 关Fig. 3共b兲兴 with a height
profile and distance between height maxima agreeing well
with those of the most frequently observed Ti atom pairs.
Figure 3共c兲 shows the experimentally determined histogram of the Ti–Ti spacings, with the vertical axis representing the pair correlation function g共r兲 of the substitutional Ti
atoms. While a large number of Ti–Ti configurations produce
a quasicontinuous distribution for separations greater than
8 Å, Ti pairs with smaller separations give rise to distinct
peaks in the histogram. The dominant feature is a peak centered at about 4.7 Å due to Ti atoms in second-nearestneighbor sites. The suppression of nearest-neighbor Ti pairs
has important consequences on the overall short-range order
of Ti on Al共111兲, as can be illustrated by comparing the
lattice occupation statistics of random alloys with forbidden
nearest neighbors 关Fig. 3共d兲兴 with that of unconstrained surface alloys 关Fig. 3共e兲兴. As seen in Fig. 3共e兲, Ti substituting
randomly for surface Al would create a sizable number of
nearest-neighbor pairs. A similar number of second- and
third-nearest-neighbor Ti pairs would be expected. If Ti atoms avoid the nearest-neighbor configuration, the correlation
function g共r兲 corresponding to second- and third-nearestneighbor sites increases by about 50% and 20%, respectively.
Therefore, a substantial increase in the actual number of predicted catalytic sites for H2 dissociation over the number
expected for a random Ti distribution must be expected.
In conclusion, combined STM experiments and DFT calculations have been used to establish the population of Ti
near the Al共111兲 surface, a system used to model hydrogendepleted transition metal doped alanate hydrogen storage
materials. Although calculations predict that Ti would occupy subsurface sites in equilibrium, experiments show severe kinetic limitations that stabilize Ti in the topmost two
atomic layers up to temperatures of at least 520 K. Due to
strong repulsion, substitutional Ti atoms in the surface layer
avoid the occupation of nearest-neighbor sites. As a result, a
rhombus-shaped complex consisting of two Ti atoms in
second-nearest-neighbor sites, predicted to catalyze the dissociative chemisorption of H2, is significantly more likely to
form than in a random distribution of surface Ti. These results suggest the formation of active sites for H2 dissociation,
a key step in the solid-state reaction to hydrogen-rich alanate
phases, as a likely primary effect of transition metal dopants
in NaAlH4.
This work was supported by the Office of Basic Energy
Sciences, U.S. Department of Energy 共Hydrogen Fuel Initiative兲. Work performed under the auspices of the U.S. Department of Energy under Contract No. DE-AC02-98CH1-886.
1
U.S. Department of Energy, Office of Basic Energy Sciences, Workshop
Report on Basic Research Needs for the Hydrogen Economy, 共2003兲
http://www.sc.doe.gov/bes/reports/NHE_rpt.pdf.
2
B. Bogdanovic and M. Schwickardi, J. Alloys Compd. 253–254, 1 共1997兲.
3
R. A. Zidan, S. Takara, A. G. Hee, and C. M. Jensen, J. Alloys Compd.
285, 119 共1999兲.
4
S. Chaudhuri and J. T. Muckerman, J. Phys. Chem. B 109, 6952 共2005兲.
5
T. Hong, T. J. Watson-Yang, X. Guo, A. J. Freeman, T. Oguchi, and J. H.
Xu, Phys. Rev. B 41, 12462 共1990兲.
6
J. Graetz, A. Y. Ignatov, T. A. Tyson, J. J. Reilly, and J. Johnson, Mater.
Res. Soc. Symp. Proc. 83, N.2.4.1 共2005兲.
7
See EPAPS Document No. E-APPLAB-90-110714 for details on the
Al/ Si共111兲 growth and STM of the Al共111兲 prior to Ti deposition. This
document can be reached via a direct link in the online article’s HTML
reference section or via EPAPS home page 共http://www.aip.org/pubserves/
epaps.html兲.
8
M. Schmid, H. Stadler, and P. Varga, Phys. Rev. Lett. 70, 1441 共1993兲.
9
G. Kresse and J. Furthmuller, Phys. Rev. B 54, 11169 共1996兲; G. Kresse,
Comput. Mater. Sci. 6, 15 共1996兲.
10
D. M. Ceperley and B. J. Alder, Phys. Rev. Lett. 45, 566 共1980兲; J. P.
Perdew and A. Zunger, Phys. Rev. B 23, 5048 共1981兲.
11
J. Tersoff and D. R. Hamann, Phys. Rev. B 31, 805 共1985兲.
12
L. Z. Mezey and J. Giber, Jpn. J. Appl. Phys., Part 1 21, 1569 共1982兲.
13
C. V. Ramana, B. S. Choi, R. J. Smith, R. Hutchinson, S. P. Stuk, B. S.
Park, A. A. Saleh, and D. R. Jeon, J. Vac. Sci. Technol. A 21, 1326
共2003兲.
14
R. Stumpf 共private communication兲.
15
Y. W. Kim, G. A. White, N. R. Shivaparan, M. A. Teter, and R. J. Smith,
Surf. Rev. Lett. 6, 775 共1999兲.
Downloaded 14 May 2007 to 138.67.20.124. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp