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Available online at www.sciencedirect.com
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X-ray photoelectron spectroscopy investigation of
magnetron sputtered MgeTieH thin films
I.J.T. Jensen a,*, A. Thøgersen b, O.M. Løvvik b, H. Schreuders d, B. Dam d,
S. Diplas b,c
a
Department of Physics, University of Oslo, P/O Box 1048 Blindern, 0316 Oslo, Norway
SINTEF Materials and Chemistry, P/O Box 124 Blindern, 0314 Oslo, Norway
c
Center for Materials Science and Nanotechnology, P/O Box 1126 Blindern, 0318 Oslo, Norway
d
Materials for Energy Conversion or Storage (MECS), DelftChemTech, Faculty of Applied Science,
Technical University Delft, P/O Box 5045, NL-2600 GA Delft, The Netherlands
b
article info
abstract
Article history:
Thin film samples of Mg80Ti20 (MgeTi) and Mg, both with and without H, were investigated
Received 22 January 2013
in a series of X-ray photoelectron spectroscopy (XPS) measurements. The samples were
Received in revised form
covered with a thin protective layer of Pd, which was removed by Arþ sputtering prior to
22 May 2013
data acquisition. This sputtering was found to reduce both oxides and hydrides. A distinct,
Accepted 26 May 2013
previously unknown peak was revealed in the Mg KLL spectrum of the MgeTieH samples,
Available online 8 July 2013
located between the metallic and the MgO component. This peak was attributed to trapping of H in very stable interstitial sites at the interface between Ti nano-clusters and the
Keywords:
Mg matrix, based on earlier density functional theory calculations and supported by
Thin films
so-called Bader analysis. The latter was performed in order to study the theoretical charge
Magnesium
distribution between Mg, Ti and H, establishing a link between the position of the previ-
Titanium
ously unknown peak and the effect of H on the valence state of Mg. The composition of the
Immiscible elements
samples was studied both by energy dispersive spectroscopy using transmission electron
Interface
microscopy and by quantitative XPS analysis. Final state Auger parameters (AP) were
obtained for metallic Mg, MgO and MgH2, as well as Mg affected by trapped H. No difference
XPS
between the AP values from the metallic components was found between the Mg and the
MgeTi samples. The AP values for MgO and MgH2 were consistent with previous reports in
literature; several eV lower than the metallic value. Mg in the vicinity of trapped hydrogen,
on the other hand, showed a more metallic character, with its corresponding AP value less
than 1 eV below the AP for pure Mg.
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1.
Introduction
The discovery of so-called switchable mirrors of Y and La
hydrides [1] in 1996 sparked an interest in thin film hydrides,
leading to the discoveries of several hydride systems with
tunable electrical and optical properties [2e6]. MgeTi thin
films stand out among them, as they appear black rather than
transparent in the fully hydrogenated state [6]. Possible
applications for MgeTieH and other switchable hydride films
range from coatings on solar collectors [6] and smart windows
* Corresponding author. Present address: SINTEF Materials and Chemistry, P/O Box 124 Blindern, 0314 Oslo, Norway. Tel.: þ47 98230513.
E-mail address: IngvildThue.Jensen@sintef.no (I.J.T. Jensen).
0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ijhydene.2013.05.142
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[7] to optical hydrogen sensors [8] and semiconductor devices
[9]. In addition to their exotic optical behavior MgeTi films are
interesting from a scientific point of view for a number of
reasons: Mg and Ti are immiscible, which means that the
structure of samples produced by non-equilibrium techniques
such as magnetron sputtering [6] is difficult to predict. They
show excellent hydrogen storage capacity and kinetics, even
at room temperature [10], and what is more: they are stable
over repeated cycles of hydrogen loading and unloading [11].
The latter is really quite remarkable considering the inherent
meta-stability of MgeTi alloys. Rather, one would expect
segregation into pure Mg and Ti upon cycling, as is the case for
several other Mg containing alloys [5,12e14].
As already mentioned the limited solubility of Ti in Mg
makes the crystal structure of MgeTi thin film samples difficult to predict, either with or without hydrogen. No phase
separation has been found with X-ray diffraction (XRD) or
transmission electron microscopy (TEM); instead the samples
were found to be single phase crystalline all the way from
before hydrogen absorption to after hydrogen desorption [15].
However, evidence for chemical short range order was obtained from simulation of optical isotherms from hydrogenography by Gremaud et al. [16] in 2008 and further verified
by a combination of X-ray diffraction and extended X-ray
absorbtion fine structure (EXAFS) spectroscopy [17]. Density
functional theory (DFT) calculations later showed that the
formation enthalpy of the system was in fact lowered significantly when the Ti atoms were arranged in small nanoclusters of sizes well below the X-ray detectable range [18].
In this configuration the electron interaction between Mg and
Ti was reduced and the Ti atoms in the center of the clusters
could obtain a state close to elemental Ti. In a further DFT
investigation hydrogen was introduced to the MgeTi nanocluster model, revealing very stable H sites at the interface
between Mg and Ti, with hydrogenation energies down to
115 kJ/(mol H2). It was found that with respect to the mixing
enthalpy, which is the cohesive energy relative to standard
state elements, the MgeTieH system changes from metastable to stable for hydrogen contents above 0.07 H per
metal [19]. Further it was found that the DFT results were in
good correspondence with the experimental work of Vermeulen et al. [20], who found increased solid solubility of H in
MgeTi. In their work it was also observed that the amount of H
being retrieved from a MgeTi sample was less than the
amount initially absorbed, pointing to the presence of sites
where H was very strongly bound.
Upon hydrogenation Mg100yTiy thin films form a MgH2like rutile structure (body centered tetragonal) or a TiH2-like
fluorite structure (face centered cubic) depending on the
amount of Ti [21]. The turning point between formation of a
MgH2-like and a TiH2-like hydride phase was reported to be at
a composition 10 < y < 13 [15]. However, the reasons why such
relatively small amounts of Ti would cause Mg to form a cubic
hydride remains a riddle. What is known, from extensive
optical and electrical investigations [15], is that regardless of
the Ti content, there must be a phase with a similar band gap
as the conventional MgH2 rutile structure present in the
hydrogenated samples. Thus the electrical properties of fluorite MgH2 appear to be similar to those of the conventional
rutile MgH2.
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Limited published work was found on the MgeTieH system using spectroscopic techniques like X-ray photoelectron
spectroscopy (XPS). The close overlap of the hydride, oxide,
and hydroxide components of the Mg peaks makes data
interpretation a challenge. Friederichs et al. [22] studied
different powders of Mg and Mg hydride under controlled
exposure to oxygen and air. In their analysis, the MgH2, MgO
and Mg(OH)2 components were not separated in the Mg 2p
peak, while in the Mg KLL peak the Mg(OH)2 component was
shifted to slightly lower kinetic energy EK compared to MgO.
Nevertheless they did detect distinct Mg 2p-KLL Auger
parameter (AP) values for MgH2 and MgO, 1231.8 eV and
1231.2 eV, respectively. Vapor deposited Mg100yTiy samples
with y up to 25.3 were investigated by high energy XPS by
Mitchell et al. [23] From AP and charge-transfer calculations,
small indications of charge-transfer from Mg to Ti were found.
In this work thin films of Mg80Ti20 with and without H were
studied by XPS and compared to their Mg counterparts. The
results were interpreted in view of recent DFT calculations,
which were extended to include a study of the charge distribution between Mg and Ti in the theoretical model using socalled Bader analysis.
2.
Experimental
Mg80Ti20 (MgeTi) and Mg films covered with a thin protective
layer of Pd were deposited in a UHV system (base
pressure ¼ 106 Pa) by DC/RF magnetron sputtering. The
former was made by co-sputtering of Mg (99.95%) and Ti
(99.999%) targets in 0.3 Pa of Ar, on single-crystal Si(100) substrates. The substrate was kept at room temperature. In order
to increase the homogeneity and ensure an even thickness of
the film the substrate was continuously rotated during sputtering. The typical deposition rate was 2.2 A/s at 150 W RF for
Mg and 0.42 A/s at 131 W DC for Ti. A Pd (99.98%) caplayer was
sputtered with a rate of 0.58 A/s at 25 W DC to prevent
oxidation. The Mg films were deposited at similar conditions.
The investigated samples were produced in two batches. In
batch 1 the investigated film was 100 nm thick with a 5 nm Pd
layer on top and in batch 2 the film was 200 nm thick with a
1.5 nm Pd layer. The first batch was hydrogenated ex situ in
10 bar H2 at 80 C for about a day. The second batch was hydrogenated at room temperature in situ directly after sputtering with 100% hydrogen at 1 bar for about a day. The thin Pd
layer of 1.5 nm is just enough to load the sample with H, but
not enough to stop oxidation completely. Formation of oxide
is believed to prevent unloading of H.
The samples were investigated by XPS using a KRATOS
AXIS ULTRADLD instrument with monochromatic Al Ka radiation (hv ¼ 1486.6 eV) operated at 15 kV and 15 mA. Typical
vacuum level during measurement was in the order of 107 Pa.
The Pd layer was removed by sputtering an area typically
about 2 mm 2 mm with an Arþ ion beam of 0.5 or 2 kV
delivering 100 mA of current. To minimize cratering effects and
avoid detection of Pd the measurements were done using
a “small spot” aperture with a diameter of 110 mm. High
resolution spectra were acquired with pass energy 20 eV and
step size 0.1 eV/s. Two different series of experiments were
performed.
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In the over-night exposure (ON) experiments the measured
sample was left in position in the XPS analysis chamber for
about 24 h before the same measurement was repeated.
Magnesium is highly reactive to oxygen, which means that it
will oxidize even in UHV. Thus the results from the ON
experiment show the changes in spectra upon gentle oxidation, comprising data from Mg, Mg with H and MgeTi with H. A
comparative depth profile (DP) experiment was performed for
MgeTi with and without H. For each sample measurements
where done at intervals of 5 min of Arþ sputtering at 0.5 kV,
starting at 5 min and ending at 30 min total sputtering time.
The sputtering time was not converted to sample depth as the
craters were too shallow to be measured by available
methods, e.g., stylus profilometry or atomic force microscopy.
Details of the measurements are listed in Table 1. The Mg 2p,
Ti 2p and O 1s photoelectron peaks and the Mg KLL Auger peak
were acquired. In some of the measurements the Mg 1s peak
was also obtained. Data processing was done using CasaXPS
[24]. All spectra were fitted with a standard Shirley background [25] and calibrated setting the Mg 2p metal component
to 49.4 0.05 eV [26], due to the absence of adventitious carbon. In quantifications Scofield cross-sections were used for
the relative sensitivity factors [27].
Cross-sectional transmission electron microscopy (TEM)
samples were prepared by ion-milling using a Gatan precision
Table 1 e List of measurements, labeled by measurement
numbers (M no.). The experiments are categorized as
initial, depth profile (DP) or over-night exposure (ON). The
compositions refer to Mg (Mg) and Mg80Ti20 (MgeTi) with
and without H. Information about sample batch number
and ArD sputtering parameters are also given. See text for
details.
M no. Experiment Composition Batch no. Arþ sputtering
Volt.
[kV]
Time
[min]
M1
M2
Initial
Initial
Mg
MgeTi
1
1
2
2
3.5
3.5
M3
M4
M5
M6
M7
M8
DP
DP
DP
DP
DP
DP
MgeTi
MgeTi
MgeTi
MgeTi
MgeTi
MgeTi
2
2
2
2
2
2
0.5
0.5
0.5
0.5
0.5
0.5
5
10
15
20
25
30
M9
M10
M11
M12
M13
M14
DP
DP
DP
DP
DP
DP
MgeTieH
MgeTieH
MgeTieH
MgeTieH
MgeTieH
MgeTieH
2
2
2
2
2
2
0.5
0.5
0.5
0.5
0.5
0.5
5
10
15
20
25
30
M15
M16
M17
M18
M19
ON
ON
ON
ON
ON
MgeTieH
MgeTieH
MgeTieH
MgeH
MgeH
1
1
1
1
1
2
2
2
2
2
10
M20
M21
ON
ON
Mg
Mg
2
2
2
2
a Same as above, measured w2 h later.
b Same as above, next day measurement.
ion polishing system with 5 kV gun voltage. The sample was
analyzed in a 200 keV JEOL 2010F microscope with a Gatan
imaging filter and detector, and a NORAN Vantage DIþ energy
dispersive spectroscopy (EDS) system. A low intensity beam
and minimal exposure were used in order to reduce the
knock-on damage by the electron beam.
DFT calculations at the PBE-GGA level [28] were performed
for a Mg81.25Ti18.75 composition using the Vienna ab-initio
simulation package (VASP) [29,30]. A 4 4 2 Mg unit cell
(64 atoms) was used as a starting point, with Ti arranged in
nano-clusters. Hydrogen was introduced to the system gradually. In this structure the sites available for hydrogen will
have the amount of Ti in the first coordination sphere (nTi)
varying from 6 to 0. The sites where filled according to their
stability, in the following order: nTi ¼ 3, 6, 2, 1 and 0. A detailed
description of the calculations can be found in Ref. [20]. In the
present work Bader analysis [31] was employed to investigate
the charge transfer upon hydrogenation. This is a method to
determine which electrons belong to which atoms in the DFT
unit cell, where a surface is drawn around each atom
perpendicular to minima in the charge density.
3.
Results and discussion
3.1.
Microstructure and composition
The TEM image in Fig. 1 shows a 200 nm thick MgeTieH film
from sample batch 2, on a Si substrate. A 3 nm thick SiOx film
is visible at the MgeTieH/Si interface, and the Pd capping
layer at the surface is estimated to be about 10 nm thick.
However, the surface is rough and the thickness of the Pd
layer may therefore be overestimated. The electron diffraction
pattern presented in Fig. 1 shows the Mg [100] and Si [110] zone
axis. The Mg grains grow perpendicular to the substrate surface, in the [002] direction. The lattice parameters measured
on the diffraction pattern of Mg (P63/mmc) was a ¼ b ¼ 3.11 A
a
b
5
b
40
b
Fig. 1 e Cross-sectional TEM image of the film, with an
electron diffraction pattern as an inset for the Mg [100] and
the Si [110] zone axis.
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and c ¼ 5.10 A. The composition of bulk film was measured by
EDS to be 84 at.% Mg and 16 at.% Ti within an uncertainty of
1 at.%. No Ti clusters could be observed with TEM.
Fig. 2 shows compositional quantification obtained by XPS,
based on the Mg 2p, Ti 2p and O 1s spectra. The amount of Ti
detected is generally higher than the expected 20 at.%, especially for the unhydrogenated samples. However, the accuracy
of such quantifications rely on many factors, especially for
inhomogeneous samples, and can typically be anywhere from
1 to 10 at.%. Nevertheless, there appears to be a relative difference in Ti content between the DP measurements of MgeTi
with and without H (M9-14 and M3-8, respectively). An option
to consider is whether this is related to differences in volume
per metal atom before and after hydrogenation. The apparent
drop in Ti content upon attempted hydrogenation (M9-14) is
accompanied by a slight increase in oxygen content. As will be
shown later, the H to metal ratio is also found to be higher in
the Ti rich regions of the samples compared to the Mg rich
regions. For the limited analysis depth of the XPS technique
such reductions of Ti atoms per volume in the hydrogenated
sample could possibly be sufficient to cause the apparent drop
in the Ti content of the sample. Fig. 2 also shows that the
oxygen content was somewhat increased in the first DP
measurements (M3 and M9) closest to the surface and the
initial ON measurements (M15 and M18) that were hydrogenated ex situ. The final ON measurements (M16, M17 and M19),
which were left in UHV in the XPS after removal of the protective Pd layer, show a substantial increase of oxygen.
3.2.
Mg KLL spectra
Fig. 3 shows high resolution Mg KLL spectra from MgeTi and
Mg before and after hydrogenation. In the unhydrogenated
state (Fig. 3a and b) the spectra are dominated by the metallic
peak at about 1186 eV [32]. Also prominent is the peak at about
1180.6 eV, which is due to MgO [33]. Traces of Mg(OH)2 is
hidden in the shoulder on the low EK side of MgO [22]. The bulk
plasmon loss peak is clearly seen at 1175.2 eV for Mg [34] and is
less intense for MgeTi at 1174.2 eV (Fig. 3a and b, respectively.). Between the bulk plasmon and the oxygen related
Auger peak the surface plasmon loss peak can be vaguely
Fig. 2 e Quantification data obtained by XPS. The
measurement numbers refer to Table 1.
Fig. 3 e Mg KLL spectra from Mg (M1), MgeTi (M2), MgeH
(M18) and MgeTieH (M13) (bottom upwards). The
components are labeled as follows: M [ metallic, O [ MgO,
OH [ Mg(OH)2, H [ MgH2, PL [ bulk plasmon. For roman
numbered components see text.
discerned [34]. One or two weak peaks (I and II) are found
between the metallic and the oxygen related Auger peaks.
After hydrogenation the Mg sample (Fig. 3c) shows a drastic
increase in intensity in the region of MgO and Mg(OH)2. This
corresponds well with previous work [22] where MgH2 was
found to overlap with MgO. The metallic peak is still clearly
visible, meaning that either the sample was not fully loaded
from the start, or the sample has lost H during the sputtering
and/or measurement. The hydrogenated MgeTi spectrum on
the other hand (Fig. 3d), is more surprising. Rather than an
increase in intensity of the region of MgO consistent with
MgH2, hydrogenated MgeTi shows a strong peak much closer
to the metallic one, at about 1183.9 eV (peak IH). Along with
this comes a broader, less distinct feature just above 1182 eV
(peak IIH). The spectrum for the hydrogenated Mg sample also
exhibits increased intensity in the region between metallic Mg
and MgH2, which can be fitted with one or more components.
For simplicity the region was fitted with just one component
(IIIH). Peaks I, II and IIIH can be attributed to understoichiometric oxide, hydroxide and hydride, respectively, as
will be discussed below. The extra peaks IH and IIH in the hydrogenated MgeTi sample, however, appear to be previously
unknown. No contaminating species other than traces of Pd
have been found in the survey spectra of these samples.
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The positions of peaks I and IH appear to be quite close to
each other. One option to consider is whether these peaks
have similar origins, for instance from interactions between
Mg and the Pd caplayer. The formation of MgePd alloys
(Mg6Pd, Mg5Pd2) is often observed in hydrogen cycling of Pdcapped Mg and Mg2Ni thin films [35,36]. Mg KLL spectra from
such MgePd alloys are not readily accessible in literature, so a
direct comparison has not been possible. As shown in Fig. 4,
comparisons of the DP spectra for MgeTi with and without H
do indeed show increased levels of Pd consistent with MgePd
alloy formation upon hydrogenation, as described in literature
[35,36]. Fig. 5 shows the corresponding Mg KLL spectra from
the unhydrogenated sample. From this it is evident that while
the Pd 3d peak vanishes with sample depth, the I peak in the
Mg KLL spectrum only decreases slightly. Electron attenuation
lengths l were calculated according to Tanuma et al. [37]
assuming a Mg matrix, in order to compare the maximum
sample depth d ¼ 3l where the signal arise from for Mg KLL
and Pd 3d. The values were l ¼ 2.96 nm and 2.90 nm, respectively. Thus the sample volume analyzed in Figs. 4 and 5
should be practically the same. In general the relative Pd
levels in the samples after Arþ sputtering do not correlate with
the relative intensities of the unknown peaks. For instance,
the Pd levels in the deepest MgeTieH DP measurements are
similar to that of the Mg sample, see Fig. 6. From Fig. 3 on the
other hand, it is obvious that the corresponding intensities of
the I and IH peaks are very different. Furthermore, if the IH
peak was caused by enhanced alloying of Mg and Pd and
subsequent MgePd hydride formation it should also be present in similar amounts in the MgeH spectra, which is not the
case.
More likely the I peak can be understood after a close look
at the corresponding O 1s spectra. These spectra, both in
MgeTi and Mg, exhibit a fairly strong shoulder at the higher
binding energy (EB) side of the MgO component. Fig. 7 compares the O 1s spectrum of unhydrogenated MgeTi fitted with
one and two oxide peaks. According to Göpel et al. [38] the
Fig. 4 e Pd 3d spectra from the DP measurements of MgeTi
(left) and MgeTieH (right). The Pd content at the first depth
is about 40% higher in the hydrogenated sample. Sample
depth increases from top downwards.
Fig. 5 e Mg KLL spectra from the DP measurements of
MgeTi without H. Sample depth increases from top
downwards. The components are labeled as follows:
M [ metallic, O [ MgO, OH [ Mg(OH)2, I: see text.
presence of under-stoichiometric oxide should give an O 1s
peak shifted to higher EB accompanied by a shift to lower EB in
the corresponding cation peak. In an investigation of the
stoichiometry and morphology of MgO films Wollenschläger
et al. [39] assigned a peak at 1183.5 eV in the Mg KLL spectrum
Fig. 6 e Pd 3d spectra from the Mg measurement M1 (a) and
the MgeTieH measurements M13 (b) and M14 (c) showing
Pd levels of about 1 at.%.
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and IIH peaks in the Mg KLL spectrum from MgeTieH will be
discussed further in separate sections.
3.3.
Mg 1s, Mg 2p and Ti 2p spectra
Figs. 8 and 9 show the Ti 2p and Mg 2p spectra corresponding
to Fig. 3. In the Ti 2p spectra the key challenge is to be able to
distinguish correctly between the metallic, hydride and oxide
components. For instance, the intensity of the TiO component
will depend strongly on the peak shape of the metallic
component. For the unhydrogenated samples we have chosen
to follow the fitting procedure proposed by Biesinger et al. [43].
For the metallic peak an asymmetric peak-shape La(1.1,5,7)
was used, where the first two parameters define the spread on
either side of a Lorentzian component. The Lorentzian curve is
convoluted by a Gaussian with the width specified by the last
parameter. For more details about the LA peak shape see
Ref. [43]. The oxide peaks were fitted with a mixed GaussianeLorentzian component. This resulted in a fit consisting
almost entirely of metallic Ti with only about 5% of the intensity attributed to TiO and TiO2. For comparison purposes
and lack of better suggestions from literature the hydride
component was fitted with the same peak shape as the
metallic one. Unlike MgH2, TiH2 is conducting, thus the use of
the metallic peak shape seems reasonable. This choice of
fitting parameters resulted in increased intensity in the regions of the TiO, Ti2O3 and TiO2 components in the hydrogenated sample compared to in the unhydrogenated state. The
Fig. 7 e O 1s spectra from MgeTi (M2), fitted with one
(bottom) or two (top) Mg oxide peaks.
to under-stoichiometric magnesium oxide. Arþ sputter
reduction of oxides is a well known phenomenon [40]. In the
work of Kurth et al. [41] the initial stages of oxidation of Mg
was studied at oxygen partial pressures between 1.3 108
and 1.3 105 Pa, revealing a clear oxygen deficiency with
respect to MgO for the early oxide. Thus we find the presence
of under-stoichiometric magnesium oxide to be a plausible
explanation for peak I in the Mg KLL spectrum of unhydrogenated samples. Along the same lines peak II can be thought
of as under-stoichiometric Mg hydroxide. This is supported by
a tendency of peak II to disappear with increasing sample
depth (not shown). Compositional changes due to Arþ ion
sputtering have also been reported for Ti hydrides by Lamartine et al. [42] Similar to the oxide case we believe that the
broad feature in the MgeH spectrum (peak IIIH) is due to
under-stoichiometric Mg hydride, MgHx<2, caused by the Arþ
sputtering. This would mean that in reality the IIIH peak is
made up of several components arising from a gradual
decrease of hydrogen content. H is generally thought to be
more easily desorbed from MgeTieH than from MgH2 [20].
Thus the MgeTieH samples should also be affected by Arþ
sputtering, yet the region between the metal and the MgH2
peak positions in the Mg KLL spectrum is not smooth as in the
spectrum from MgeH. On the contrary, the IH peak is a very
distinct feature, likely to have an equally distinct origin. The IH
Fig. 8 e Ti 2p spectra from MgeTi (M2, bottom) and
MgeTieH (M13, top).
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Fig. 9 e Mg 2p spectra from Mg (M1), MgeTi (M2), MgeH
(M18) and MgeTieH (M13) (bottom upwards). The spectra
are normalized using the metallic peak, but in reality this
peak will have lost intensity in the spectra from
hydrogenated samples compared to the unhydrogenated
ones. The components are labeled as follows:
M [ metallic, O [ MgO, OH [ Mg(OH)2, H [ MgH2.
hydride component was shifted about 0.3 eV to higher EB as
compared to the metallic component. The position of the titanium hydride peak at 454.3 eV corresponds fairly well with
literature: 454.4 eV was reported (without Ti metal reference)
by Lesiak et al. [44], 454.6 eV, and a 0.6 eV shift compared to Ti
metal was reported by Lisowski et al. [45] and by Kang et al.
[46]. However, the shift observed in Fig. 8 is smaller than what
would be expected from pure TiH2. Lamartine et al. [42] found
a linear relationship between the H:Ti ratio and the shift in Ti
2p, and according to this the Ti in our samples appears not to
be fully hydrogenated. Rather it seems that the H:Ti ratio was
around 1, half of what is the case in fully loaded TiH2.
All the Mg 2p spectra show a clear metallic peak, which
was set to 49.4 eV in order to calibrate the energy scale [26]. On
the high EB side of the metallic peak Mg oxide, hydroxide and
hydride components partly overlap to form a broader
continuous peak [22]. For the unhydrogenated samples this
region is weak in intensity, reflecting the low level of oxygen
contamination in the samples. For the hydrogenated samples,
on the other hand, the intensity in this region is strongly
increased due to hydride overlapping with the existing oxide.
A certain shift in the peak position is observed between
MgeTieH and MgeH, consistent with the differences seen
also in the Mg KLL spectra.
Fig. 10 shows the Mg 1s peak for Mg (M1), MgeTi (M2),
MgeH (M18) and MgeTieH (M15). In the first two the metallic
component at 1302.9 eV is clearly seen, while in the hydrogenated samples it is present as a shoulder on the low EB side.
In the MgeTi spectrum the oxide peak is increased quite a lot
compared to in the Mg spectrum. The quantification in Fig. 12
also shows increased oxygen levels in the MgeTi measurement compared to Mg. This difference will be enhanced
further in the Mg 1s peak, which is very surface sensitive. Due
to low EK it is collected from shallow sample depths only. For
the MgeH sample the hydride/oxide component is dominant,
Fig. 10 e Mg 1s spectra from Mg (M1), MgeTi (M2), MgeH
(M18) and MgeTieH (M15) (bottom upwards). The
components are labeled as follows: M [ metallic, O [ MgO,
OH [ Mg(OH)2, H [ MgH2.
with the MgeTieH peak shifted to slightly lower EB
(1304.35 eV) compared to MgeH (1304.71 eV), as seen also in
the case of Mg 2p.
3.4.
Auger parameters
Final state Auger parameters (AP) obtained in this work are
listed in Table 2. The Mg 1s-KLL final state AP for the metallic
components was 2488.8 0.1 eV for both Mg (M1) and MgeTi
(M2), while the Mg 2p-KLL final state AP for the metallic
components was 1235.3 0.1 eV for all samples. No significant
differences between Mg and MgeTi were found in either of the
cases and the AP values were in correspondence with literature values for pure Mg [47]. The final state AP is defined as:
a ¼ EB ðphotoelectronÞ þ EK ðAuger electronÞ;
(1)
where EB and EK are the binding energy and kinetic energy,
respectively. The final state Auger parameter a is essentially
the energy distance between a photoelectron peak and a
related Auger-peak in the spectra, making it unaffected by
energy referencing and charging problems. Differences in the
Auger parameter reflect differences in the system’s ability to
screen the core hole created upon photoemission. It can be
divided into the relaxation of the photoionized atom itself,
e.g., shrinkage of the valence band, and the response of the
surroundings through either charge transfer or polarization.
Table 2 e Final state Auger parameters for Mg.
Composition
Mg
MgeTi
MgeH
MgeTieH
MgeTieH
Phase
1s-KLL AP [eV]
2p-KLL AP [eV]
Metallic
Metallic
MgH2
IH
MgO
2488.8
2488.8
2485.7
2488.4
e
1235.3
1235.3
1232.1
1234.6
1231.2
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The fact that there was no difference between the AP values of
Mg and MgeTi means that no strong effect of interactions
between Mg and Ti could be detected.
Fig. 11 shows Mg 2p-Mg KLL final state Auger parameters
for the metallic and the dominant non-metallic phases in the
hydrogenated samples with and without Ti. The AP is unaffected by energy referencing problems provided that the
photoelectron and Auger peaks being compared originate
from the same phase. In the Mg 2p spectra the oxide, hydroxide and hydride phase overlap to form a broad peak (the
non-metallic peak), which represent a problem in this context.
Rather than trying to deconvolute the individual components
this peak was fitted with just one component, based on the
assumption that the dominant peak in the Mg KLL spectrum
would also dominate the position of the peak maximum in the
Mg 2p spectrum. For the MgeTieH samples two APs were
measured, combining the non-metallic Mg 2p peak with the
Mg KLL MgO and IH component, respectively. MgO is not a
dominant phase in this case, which means that the previous
argument regarding the fitting of the Mg 2p component is not
valid. It is however a well known phase with AP values for
comparison readily available from literature. The AP for MgO
shows good consistency across the data set for all but two
measurements (M9 and M17). M9 is the first of the DP measurements, suggesting that the analyzed area was still
affected by the surface of the film at this sputtering depth, e.g.,
possible alloying with Pd. M17 is an ON next day measurement, which showed extensive formation of hydroxide
possibly affecting the position of the non-metallic Mg 2p peak.
Disregarding these two anomalies, the AP was found to be
1231.25 eV with a standard deviation of 0.02 eV. This is
consistent with AP ¼ 1231.2 eV found for MgO by Diplas et al.
[48] and Friederichs et al. [22] The AP for the IH peak was found
to be 1234.60 eV with a standard deviation of 0.07 eV, still
disregarding M9 and M17. For the MgeH samples (M18 and
M19) only one AP was calculated, relating the non-metallic Mg
2p peak to the Mg KLL peak with expected overlapping
contribution from MgH2 and MgO. This AP shows a significant
deviation from the AP of MgO found above. The AP for M18
was 1232.1 eV, while the AP for the next day measurement
(M19) with hydroxide formation had decreased slightly to
Fig. 11 e Mg 2p-KLL Auger parameter values (AP) of the
hydrogenated samples with standard deviations. The
measurement numbers refer to Table 1.
10711
1231.9 eV. Variations in O 1s positions in the data set (e.g., Figs.
7 and 14) indicate a certain effect of sample charging, making
it difficult to reliably determine exact positions for the individual oxide and hydride peaks. The AP values however, are
not affected by charging and thus the present result indicates
that it should be possible to separate the oxide and hydride
phase of Mg in a dedicated XPS experiment. The unhydrogenated samples have AP values for MgO similar to the
MgeTieH samples, but the low oxygen content makes it hard
to determine the exact position of the non-metallic Mg 2p
peak in each spectrum. The Mg 1s-KLL AP values corresponding to the dominant peak in the Mg 1s spectra were
found to be 2488.4 eV and 2485.7 eV for MgeTieH (M15) and
MgeH (M18), respectively. The overall trend in shifts between
MgO, MgH2 and the phase responsible for the IH peak corresponds very well with the Mg 2p-KLL AP results.
3.5.
The origin of the IH peak
If Mg and Ti do not interact to any large extent one would
expect MgH2 and TiH2 to be formed more or less independently, but given the difference between the spectrum c and
d in Fig. 3 this is clearly not what we observe in our sample. As
mentioned in Section 1, MgH2 forms a TiH2-like structure (face
centered cubic rather than body centered tetragonal) for the
composition of our sample. Could the difference in crystal
structure account for the difference observed in the Mg KLL
spectra? Probably not, as it is known from optical and electrical investigations that the band gap must be similar to that
of conventional rutile MgH2 [15]. Thus one would not expect a
shift of several eV in the Auger spectrum and it seems that
fluorite MgH2 can be ruled out as a cause of the IH peak.
Another option to consider is that the hydrogenated MgeTi
samples undergo H desorption in the XPS instrument, for
instance due to the initial Arþ sputtering, as mentioned also
for MgeH in Section 3.2. From the Ti 2p spectrum a H:Ti ratio
of about 1 was estimated, half of what is expected for fully
loaded TiH2. Whether there is any hydrogen connected to Mg
is more of an open question, since TiH2 is significantly more
stable than MgH2 in bulk form [15]. As the Mg KLL peaks of MgO
Fig. 12 e Ratio of the peak with contribution from MgO and/
or MgH2 relative to total amount of Mg in the Mg KLL
spectrum. The measurement numbers refer to Table 1.
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Fig. 13 e Mg KLL spectra from MgeTieH taken immediately
after Pd removal (M15, bottom)and after 24 h in UHV (M17,
top). The components are labeled as follows: M [ metallic,
O [ MgO, OH [ Mg(OH)2, IH and IIH [ see text.
and MgH2 are not easily separated, there is no direct way to
determine whether the latter is present in the hydrogenated
MgeTi samples. Instead we look at the ratio of the peak with
contribution from MgO and/or MgH2 relative to total amount
of Mg in the Mg KLL spectrum, Fig. 12. We see that for the two
MgeH measurements (M18 and M19) the ratio is dramatically
increased, giving clear indications of the presence of conventional MgH2. In the MgeTieH case, however, nothing like
that happens. In fact, if one compares the depth profile data
with and without H (M3-M8 and M9-M14), which should be
directly comparable, the MgO level is nearly the same. Thus,
the content of conventional MgH2 in the MgeTieH samples
seems to be very low, if any at all. We will return to this issue
in Section 3.6.
Fig. 13 shows what happens when a hydrogenated MgeTi
sample is left without the Pd protection in UHV for 24 h (the
ON experiments). The corresponding fitting parameters are
given in Table 3. The metallic peak suffers a reduction of more
than 50% in intensity while the Mg(OH)2 peak intensity increases drastically. The latter is supported also by a corresponding change in the O 1s spectrum, see Fig. 14. The
decrease of the metallic and increase of the hydroxide peak is
similar for the hydrogenated Mg sample (not shown). For
comparison Fig. 14 also shows oxidation of unhydrogenated
Mg after 24 h in UHV, where both the oxide and hydroxide
components in the O 1s peak increase evenly. Thus the rapid
growth of Mg(OH)2 in M17 gives strong indications of the
presence of H in both the MgeH and the MgeTieH samples.
Fig. 13 shows that the relative area of the IH peak decreases
while IIH increases in vacuum. This leads us to conclude that
IH must be related primarily to hydrogen while IIH is somehow
related to the oxidation process.
A possible explanation of the IH peak can be found in our
previously published DFT work, where we investigated the
introduction of H into a model with Ti nano-clusters in an Mg
matrix [19]. It was found that a certain amount of H was likely to
remain trapped in especially stable sites on the interface between Mg and the Ti clusters, even after dehydrogenation had
been attempted. As already mentioned it could be that the
hydrogenated MgeTi sample was actually desorbing H due to
the Arþ sputtering and what was measured was the metallic
rather than the hydride phase. In this case the unknown peak
IH in Figs. 3 and 13 could be due to H trapped on the very stable
interstitial sites on the interface between Mg and the Ti nanocluster. To match the experimental deviation from maximum
M:H ratio, it was proposed from our calculation that sites with
6, 3 and 2 nearest neighbor Ti atoms would take part in the
trapping, i.e., the sites within the Ti nano-clusters as well as the
ones on the interface. Such a picture would fit well with our
current experimental findings: In the metallic hcp structure H
prefers to occupy octahedral voids, leading to a H:M ratio of 1.
This is the ratio of H:Ti estimated from the Ti 2p spectrum.
In the present work the theoretical charge distribution
between Mg, Ti and H was investigated by performing Bader
Table 3 e Fitting parameters for the spectra in Fig. 13.
Peak
Mg met.
IH
IIH
MgOc
Mg(OH)2
Directly after Pd removal (M15)
a
After 24 h in UHV (M17)
Position EK [eV]
FWHM
Rel. area [%]
Position EK [eV]
FWHMa
Rel. area [%]
1185.9
1184.0
1182.1
1180.6
1179.3
0.8
2.1
2.0
1.5
2.0
31.7
36.9
16.3
10.3
4.8
1186.0
1183.9
1181.8
1180.7
1179.5
0.8
2.3
2.2
1.6b
2.4b
11.1
18.1
24.5
8.6
37.8
a The metallic peak was fitted with peak shape GL(99)T(1.2) and the non-metallic peaks with GL(50). Please refer to CasaXPS [24] for definition of
peak shapes.
b Upwards restricted.
c Position restricted to 5.3 eV below metallic peak, based on data from non-hydrogenated samples.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 0 7 0 4 e1 0 7 1 5
10713
the Mg core hole screening efficiency caused by hydrogen
trapped on especially stable interstitial sites at the interface
between Mg and the Ti nano-clusters.
3.6.
Fig. 14 e O 1s spectra from MgeTieH (left, M15 and M17)
and unhydrogenated Mg (right, M20 and M21) taken
immediately after Pd removal (bottom) and after 24 h in
UHV (top).
analysis [31] on the DFT results obtained in our previous work
[19]. Fig. 15 shows the calculated average number of valence
electrons per atom as function of hydrogen content x, as obtained by Bader analysis. Here it is clearly seen how Mg loose
electrons compared to the elemental state, regardless of the H
content. The average number of remaining valence electrons
per Mg is found to be w1.5, which is less than the elemental 2
electrons, but more than the ionic MgO state without
remaining valence electrons on the Mg sites. Thus the Bader
analysis supports that introduction of H to MgeTi would result
in a peak situated between the metal and the oxide component
in the Mg KLL spectrum. It is also consistent with the AP values
presented in Section 3.4. The AP of metallic Mg and MgO are
separated by several eV, reflecting the substantially reduced
core hole screening on Mg sites in the ionic oxide. For MgH2
the Mg AP is close to that of MgO, while the AP calculated for
the IH phase is much closer to the metallic value. In view of our
theoretical findings we think this is due to a small reduction of
The origin of the IIH peak
The IIH peak remains a topic for discussion. The region between IH and MgO is very broad, without distinct features, not
unlike the IIIH peak. For simplicity it has been fitted with just
one component (IIH), but there might be more than one, with
under-stoichiometric MgHx<2 as a likely candidate. From the
ON experiment, however, we found IIH to increase upon oxygen exposure, thus pointing to the involvement of oxygen.
One possibility is that oxygen connects to understoichiometric MgHx<2 to produce under-stoichiometric
Mg(OH)2. Additionally, the strong affinity of H for the sites
on the interface between the Mg matrix and the Ti nanoclusters could perhaps be extended to O as well. In the hydrogenation process O could move to the interface between
Mg and Ti and interact with H to result in a hydroxide-like
MgeTieOeH constellation. This would also explain the
increased oxygen levels seen in the Ti 2p spectra of the MgeTi
samples after hydrogenation, giving different peak positions
depending on the site occupied by O. However, such claims
would have to be investigated further both experimentally
and by DFT calculations.
4.
Conclusions
In this work thin films of Mg80Ti20 were studied by XPS before
and after hydrogen loading. No significant trace of conventional MgH2 was found. Instead new, previously unknown
peaks were revealed between the position of the metal and the
oxide in the Mg KLL spectrum. From the shift in the Ti 2p
spectrum the H:Ti ratio appeared to be 1 rather than 2, which
would have been the fully loaded state. It was concluded that
most probably the sample had undergone hydrogen desorption due to the Arþ sputtering prior to XPS measurement.
However, the presence of remaining H was evident from over
night exposure-experiments in the analysis chamber, where
oxygen increase was seen primarily in the form of Mg(OH)2.
The most prominent new peak in the Mg KLL spectrum (IH)
was proposed to be due to H trapped in especially stable
interstitial sites at the interface between Ti nano-clusters and
the Mg matrix. Such stable H sites have previously been predicted both experimentally [20] and theoretically [19]. An
additional peak (IIH) in the Mg KLL spectrum was found to
increase upon oxygen exposure. This peak was suggested to
be due to MgeTi hydroxide formation.
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Average number of valence electrons per atom as function
of hydrogen content x. Horizontal lines give values for
elemental Ti, Mg and H; 4, 2 end 1 valence electrons
respectively.
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