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X-ray photoelectron spectroscopy investigation ofmagnetron sputtered MgeTieH thin films

I.J.T. Jensen a,*, A. Thøgersen b, O.M. Løvvik b, H. Schreuders d, B. Damd,S. Diplas b,c

aDepartment of Physics, University of Oslo, P/O Box 1048 Blindern, 0316 Oslo, Norwayb SINTEF Materials and Chemistry, P/O Box 124 Blindern, 0314 Oslo, NorwaycCenter for Materials Science and Nanotechnology, P/O Box 1126 Blindern, 0318 Oslo, NorwaydMaterials for Energy Conversion or Storage (MECS), DelftChemTech, Faculty of Applied Science,

Technical University Delft, P/O Box 5045, NL-2600 GA Delft, The Netherlands

a r t i c l e i n f o

Article history:

Received 22 January 2013

Received in revised form

22 May 2013

Accepted 26 May 2013

Available online 8 July 2013

Keywords:

Thin films

Magnesium

Titanium

Immiscible elements

Interface

XPS

* Corresponding author. Present address: SINE-mail address: IngvildThue.Jensen@sinte

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.05.1

a b s t r a c t

Thin film samples of Mg80Ti20 (MgeTi) and Mg, both with and without H, were investigated

in a series of X-ray photoelectron spectroscopy (XPS) measurements. The samples were

covered with a thin protective layer of Pd, which was removed by Arþ sputtering prior to

data acquisition. This sputtering was found to reduce both oxides and hydrides. A distinct,

previously unknown peak was revealed in the Mg KLL spectrum of the MgeTieH samples,

located between the metallic and the MgO component. This peak was attributed to trap-

ping of H in very stable interstitial sites at the interface between Ti nano-clusters and the

Mg matrix, based on earlier density functional theory calculations and supported by

so-called Bader analysis. The latter was performed in order to study the theoretical charge

distribution between Mg, Ti and H, establishing a link between the position of the previ-

ously unknown peak and the effect of H on the valence state of Mg. The composition of the

samples was studied both by energy dispersive spectroscopy using transmission electron

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

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 tunable electrical and optical properties [2e6]. MgeTi thin

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

TEF Materials and Chemif.no (I.J.T. Jensen).2013, Hydrogen Energy P42

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

stry, P/O Box 124 Blindern, 0314 Oslo, Norway. Tel.: þ47 98230513.

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

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 en e r g y 3 8 ( 2 0 1 3 ) 1 0 7 0 4e1 0 7 1 5 10705

[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 pureMg 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 diffi-

cult 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 ob-

tained from simulation of optical isotherms from hydro-

genography 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 signif-

icantly when the Ti atoms were arranged in small nano-

clusters 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 nano-

cluster 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 meta-

stable 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 Ver-

meulen 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 Mg100�yTiy thin films form a MgH2-

like 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 reasonswhy 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 fluo-

rite MgH2 appear to be similar to those of the conventional

rutile MgH2.

Limited published work was found on the MgeTieH sys-

tem 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 Mg100�yTiy 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 fromMg 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 distri-

bution between Mg and Ti in the theoretical model using so-

called 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 10�6 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) sub-

strates. 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 sput-

tering. The typical deposition rate was 2.2 �A/s at 150 W RF for

Mg and 0.42 �A/s at 131W 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 hy-

drogenated at room temperature in situ directly after sput-

teringwith 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 radi-

ation (hv ¼ 1486.6 eV) operated at 15 kV and 15 mA. Typical

vacuum level duringmeasurementwas in the order of 10�7 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. Tominimize 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.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 7 0 4e1 0 7 1 510706

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 oxida-

tion, comprising data fromMg,MgwithH andMgeTiwithH. 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 theMg KLLAuger 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 back-

ground [25] and calibrated setting the Mg 2pmetal component

to 49.4 � 0.05 eV [26], due to the absence of adventitious car-

bon. 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 bymeasurementnumbers (M no.). The experiments are categorized asinitial, depth profile (DP) or over-night exposure (ON). Thecompositions refer to Mg (Mg) and Mg80Ti20 (MgeTi) withand without H. Information about sample batch numberandArD sputtering parameters are also given. See text fordetails.

M no. Experiment Composition Batch no. Arþ sputtering

Volt.[kV]

Time[min]

M1 Initial Mg 1 2 3.5

M2 Initial MgeTi 1 2 3.5

M3 DP MgeTi 2 0.5 5

M4 DP MgeTi 2 0.5 10

M5 DP MgeTi 2 0.5 15

M6 DP MgeTi 2 0.5 20

M7 DP MgeTi 2 0.5 25

M8 DP MgeTi 2 0.5 30

M9 DP MgeTieH 2 0.5 5

M10 DP MgeTieH 2 0.5 10

M11 DP MgeTieH 2 0.5 15

M12 DP MgeTieH 2 0.5 20

M13 DP MgeTieH 2 0.5 25

M14 DP MgeTieH 2 0.5 30

M15 ON MgeTieH 1 2 10

M16 ON MgeTieH 1 2 a

M17 ON MgeTieH 1 2 b

M18 ON MgeH 1 2 5

M19 ON MgeH 1 2 b

M20 ON Mg 2 2 40

M21 ON Mg 2 2 b

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 grad-

ually. 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

layermay therefore be overestimated. The electron diffraction

pattern presented in Fig. 1 shows theMg [100] and Si [110] zone

axis. The Mg grains grow perpendicular to the substrate sur-

face, in the [002] direction. The lattice parameters measured

on the diffraction pattern of Mg (P63/mmc) was a ¼ b ¼ 3.11 �A

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.

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.

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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.%, espe-

cially 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 dif-

ference in Ti content between the DPmeasurements 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 hydroge-

nated ex situ. The final ONmeasurements (M16, M17 andM19),

which were left in UHV in the XPS after removal of the pro-

tective 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 ofMgO [22]. The bulk

plasmon loss peak is clearly seen at 1175.2 eV forMg [34] and is

less intense for MgeTi at 1174.2 eV (Fig. 3a and b, respec-

tively.). 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.

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 betweenmetallic 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 under-

stoichiometric oxide, hydroxide and hydride, respectively, as

will be discussed below. The extra peaks IH and IIH in the hy-

drogenated 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.

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.

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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 Pd-

capped 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, respec-

tively. 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 correlatewith

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 pre-

sent 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 com-

pares the O 1s spectrum of unhydrogenated MgeTi fitted with

one and two oxide peaks. According to Gopel 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.

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 Wollenschlager

et al. [39] assigned a peak at 1183.5 eV in the Mg KLL spectrum

Fig. 6 e Pd 3d spectra from the Mgmeasurement M1 (a) and

the MgeTieH measurements M13 (b) and M14 (c) showing

Pd levels of about 1 at.%.

Fig. 7 e O 1s spectra from MgeTi (M2), fitted with one

(bottom) or two (top) Mg oxide peaks.

Fig. 8 e Ti 2p spectra from MgeTi (M2, bottom) and

MgeTieH (M13, top).

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 en e r g y 3 8 ( 2 0 1 3 ) 1 0 7 0 4e1 0 7 1 5 10709

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 � 10�8

and 1.3 � 10�5 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 unhydro-

genated 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 Lamar-

tine 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

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 Gaus-

sianeLorentzian component. This resulted in a fit consisting

almost entirely of metallic Ti with only about 5% of the in-

tensity 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 re-

gions of the TiO, Ti2O3 and TiO2 components in the hydroge-

nated sample compared to in the unhydrogenated state. The

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.

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.

Table 2 e Final state Auger parameters for Mg.

Composition Phase 1s-KLL AP [eV] 2p-KLL AP [eV]

Mg Metallic 2488.8 1235.3

MgeTi Metallic 2488.8 1235.3

MgeH MgH2 2485.7 1232.1

MgeTieH IH 2488.4 1234.6

MgeTieH MgO e 1231.2

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hydride component was shifted about 0.3 eV to higher EB as

compared to the metallic component. The position of the ti-

tanium 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 hydro-

genated 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 measure-

ment 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,

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

componentswas 1235.3� 0.1 eV for all samples. No significant

differences betweenMg andMgeTi were found in either of the

cases and the AP values were in correspondence with litera-

ture values for pure Mg [47]. The final state AP is defined as:

a ¼ EBðphotoelectronÞ þ EKðAuger 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.

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 en e r g y 3 8 ( 2 0 1 3 ) 1 0 7 0 4e1 0 7 1 5 10711

The fact that therewas 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 unaf-

fected 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, hy-

droxide 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 peakmaximum 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 mea-

surements, 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 measure-

ment, 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 fromMgH2 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.

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 indi-

vidual 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 unhydro-

genated 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 corre-

sponding 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 corre-

sponds 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 indepen-

dently, 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 elec-

trical 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 thanMgH2 in bulk form [15]. As theMgKLL peaks ofMgO

Fig. 12 e Ratio of the peak with contribution fromMgO and/

or MgH2 relative to total amount of Mg in the Mg KLL

spectrum. The measurement numbers refer to Table 1.

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.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 7 0 4e1 0 7 1 510712

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 con-

ventional 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

Table 3 e Fitting parameters for the spectra in Fig. 13.

Peak Directly after Pd removal (M15)

Position EK [eV] FWHMa Rel. area [%

Mg met. 1185.9 0.8 31.7

IH 1184.0 2.1 36.9

IIH 1182.1 2.0 16.3

MgOc 1180.6 1.5 10.3

Mg(OH)2 1179.3 2.0 4.8

a Themetallic peak was fitted with peak shape GL(99)T(1.2) and the non-m

peak shapes.

b Upwards restricted.

c Position restricted to 5.3 eV below metallic peak, based on data from n

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. Themetallic peak suffers a reduction ofmore

than 50% in intensity while the Mg(OH)2 peak intensity in-

creases drastically. The latter is supported also by a corre-

sponding 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 hydrogenwhile 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]. Itwas foundthatacertainamountofHwas likely to

remain trapped in especially stable sites on the interface be-

tweenMg 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 nano-

cluster. 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., thesiteswithin theTinano-clustersaswell 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

After 24 h in UHV (M17)

] Position EK [eV] FWHMa Rel. area [%]

1186.0 0.8 11.1

1183.9 2.3 18.1

1181.8 2.2 24.5

1180.7 1.6b 8.6

1179.5 2.4b 37.8

etallic peaks with GL(50). Please refer to CasaXPS [24] for definition of

on-hydrogenated samples.

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).

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 en e r g y 3 8 ( 2 0 1 3 ) 1 0 7 0 4e1 0 7 1 5 10713

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 ob-

tained 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 toMgeTiwould result

in a peak situated between the metal and the oxide component

in theMg KLL spectrum. It is also consistentwith 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 ismuch closer to themetallic value. In view of our

theoretical findingswe think this is due to a small reduction of

Fig. 15 e Results from Bader analysis of Mg81.25Ti18.75Hx.

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.

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. The origin of the IIH peak

The IIH peak remains a topic for discussion. The region be-

tween 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 ox-

ygen exposure, thus pointing to the involvement of oxygen.

One possibility is that oxygen connects to under-

stoichiometric 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 nano-

clusters could perhaps be extended to O as well. In the hy-

drogenation 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 conven-

tional MgH2 was found. Instead new, previously unknown

peakswere revealed between the position of themetal 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 desorp-

tion 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 pre-

dicted 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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