Supplemental Information:
Activation of Erbium Films for Hydrogen Storage
Michael T. Brumbach1*, James A. Ohlhausen1, Kevin R. Zavadil2, Clark S. Snow3, Joseph C. Woicik4
1
Materials Characterization Department, Sandia National Laboratories
Albuquerque, NM 87185, U.S.A.
2
Materials Reliability Department, Sandia National Laboratories
Albuquerque, NM 87185, U.S.A.
3
Applied Science and Technology Maturation Department, Sandia National Laboratories
Albuquerque, NM 87185, U.S.A.
4
National Institute of Standards and Technology
Gaithersburg, MD 20899, U.S.A.
*mtbrumb@sandia.gov
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Erbium films were sputter cleaned to determine the spectra for the clean metal surface. The valence
band spectra are shown in Figure SI.1.
C.
B.
A.
Figure SI.1. The valence band spectra are shown for erbium films analyzed via XPS with
hυ=1486.6 eV. (A) As-received erbium film, (B) sputtered with 4 keV Ar+ for 20 seconds, and
(C) sputtered with 4 keV Ar+ for 60 seconds.
For thermally treated films, the oxide component decreases in FWHM and shifts slightly to higher BE
for all analyses after the first heating cycle; from 530.1 to 530.9 eV (BE) for 3.0 keV, from 530.7 to
530.8 eV (BE) for 2.2 keV, and from 530.8 to 531.3 eV (BE) for 1486.6 eV, Figure SI.2. The
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differences in BE separations between the three XPS analyses may be due to the presence of additional
adventitious species which are not accounted for in the fitting, however, it was the intent to keep the
fitting as simple as possible and two peaks can be used to adequately fit the spectra. The high BE
shoulder is actually eliminated after initial stages of annealing (observed using 2.2 keV after 10 minutes
and 3.0 keV after 5 minutes) followed by a re-emergence of intensity on the high BE side of the O(1s)
with continued annealing.
hν = 1486.6 eV
52min
1.8
hν = 2200 eV
50min
hν = 3000 eV
50min
2.0
3.5min
5min
1.9
5min
(1.4)
2.1
(1.7)
0min
2.2
0min
0min
2.3
2.9
Figure SI.2. The O(1s) spectra from three different XPS excitation energies show that the oxide
component shifts to higher BE and narrows with initial heating. For the O(1s) measured using
2.2 and 3.0 keV, the high BE component is virtually eliminated after 5-10 minutes of annealing,
but with continued annealing the high BE component reemerges. The separation between high
and low BE components decreases for all three spectra after 50 minutes of annealing.
Figure SI.3 shows the O(1s) spectra for fully activated erbium films analyzed with all three excitation
energies. Identical peak fitting conditions have been applied to all three spectra.
Oxygen vacancies are observed at the oxide/metal interface as the erbium films are heated in to Stage II
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activation. Upon exposure to air, as with the LEIS and ToF-SIMS profiles, the oxygen vacancies are
apparently partially filled by ambient conditions leading to an apparent thickening of the oxide.
However, in an in vacuo condition where hydriding would occur immediately after activation, the
highly defective oxide would be an optimal surface for adsorption and absorption of hydrogen.
The space group, Ia3, is the same for In2O3 and Er2O3 with comparable lattice constants, 1.0117 and
1.0536 nm, respectively. The band gap of Er2O3 is given as 7.6 eV compared to 3.6 for In2O3.1-2
The results of Müller et al. suggest that the sticking of H2 to erbium metal is significantly greater than
for erbium oxide.3 The decrease in oxygen content at the surface upon activation might suggest that the
H2 sticking probability should increase, but Fromm and Uchida have shown that even low coverages of
oxygen on tantalum can significantly reduce the sticking probability.4 The sticking probabilities for the
untreated and activated films, which still both show substantial amounts of oxygen, are assumed to be
similar.
4
hν = 1486.6 eV
hν = 2200 eV
hν = 3000 eV
Figure SI.3. The O(1s) spectra for fully activated erbium films (annealed 50 minutes) can be effectively fit with three identical
components with fixed energy separations. All components are 1.3 eV FWHM and the energy separation between components is 1.1 eV.
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4.
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J. Muller, N. A. Surplice and B. Singh, Journal of Physics D-Applied Physics 5 (6), 1177& (1972).
E. Fromm and H. Uchida, Journal of the Less-Common Metals 66 (1), 77-88 (1979).
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