I. JELOVICA BADOVINAC et al.: OXIDATION OF MOLYBDENUM BY LOW-ENERGY OXYGEN-ION BOMBARDMENT
617–621
OXIDATION OF MOLYBDENUM BY LOW-ENERGY
OXYGEN-ION BOMBARDMENT
OKSIDACIJA MOLIBDENA Z NIZKOENERGETSKIM KISIKOVIM
IONSKIM OBSTRELJEVANJEM
Ivana Jelovica Badovinac, Ivna Kavre Piltaver, Iva [ari}, Robert Peter,
Mladen Petravi}
University of Rijeka, Department of Physics and Center for Micro- and Nanosciences and Technologies, 51000 Rijeka, Croatia
ijelov@phy.uniri.hr
Prejem rokopisa – received: 2016-07-15; sprejem za objavo – accepted for publication: 2016-11-17
doi:10.17222/mit.2016.199
We have studied the oxidation of pure molybdenum metal at room temperature (RT) by low-energy oxygen-ion bombardment
within an analytical, ultrahigh vacuum chamber, using x-ray photoemission spectroscopy (XPS) around the Mo 3d core level.
We provide a detailed characterization of the oxidation states of pure Mo and a comparison with the Mo oxidation in the
CoCrMo alloy. After the thermal oxidation of pure metal at RT up to 5000 L of oxygen, the Mo 3d photoelectron remained
almost unchanged. On the other hand, Mo oxidizes quite easily within the CoCrMo alloy by thermal oxidation at RT. The XPS
spectra recorded after the ion bombardment showed a complex structure consisting of Mo4+, Mo5+ and Mo6+ oxidation states.
Our study shows that pure Mo metal oxidizes more efficiently under oxygen-ion bombardment or when incorporated into the
CoCrMo alloy. We explain our results in terms of the implantation dynamics of oxygen ions and the enhanced mobility and
reactivity of oxygen atoms in the alloy, compared to the pure Mo metal.
Keywords: molybdenum, CoCrMo alloy, ion-beam oxidation, thermal oxidation, x-ray photoelectron spectroscopy
Z rentgensko fotoemisijsko spektroskopijo (angl. XPS) smo preu~evali oksidacijo ~iste kovine molibdena (Mo) pri sobni tempe-
raturi (angl. RT) z obstreljevanjem z nizkoenergetskim ionskim `arkom kisika znotraj komore z ultravisokim vakuumom. V
{tudiji smo predstavili podrobno karakterizacijo oksidacije ~iste kovine Mo in jo primerjali z oksidacijo Mo v CoCrMo zlitini.
Po termi~ni oksidaciji Mo v ~isti kovini pri sobni temperaturi z odmerki kisika do 5000 L, fotoemisijski spekter Mo 3d lupin-
skega nivoja ostane skoraj nespremenjen. Nasprotno temu, Mo oksidira z lahkoto v CoCrMo zlitini s postopkom termi~ne
oksidacije pri sobni temperaturi. Po drugi strani pa so XPS-spektri, posneti po ionskem obstreljevanju, pokazali kompleksno
strukturo, ki jo sestavljajo oksidacijska stanja Mo4+, Mo5+ ter Mo6+. Na{a {tudija ka`e, da ~isti Mo oksidira bolj u~inkovito, ~e je
obstreljevan z ioni kisika ali ~e je vklju~en v CoCrMo zlitino. Na{e rezultate lahko interpretiramo v smislu implementacijske
dinamike kisikovih ionov ter ve~je mobilnosti in reaktivnosti kisikovih atomov v zlitini, v primerjavi s ~isto kovino Mo.
Klju~ne besede: molibden, CoCrMo zlitina, oksidacija z ionskim `arkom, termi~na oksidacija, rentgenska fotoemisijska spek-
troskopija
1 INTRODUCTION
The CoCrMo alloy is currently one of the most
important materials for biomedical applications due to its
superior wear resistance, hardness and high corrosion
resistance. It is frequently used for metal-on-metal hip
resurfacing joints.1 The possible release of metal ions
due to corrosion is thought to have adverse effects on the
surrounding body tissue and ultimately leads to the
failure of the implant. The release of the metallic ions
into the body tissue can be prevented by the formation of
a thin oxide layer on the surface of the implant.2 Thus, it
is important to fully understand the oxidation mechanism
of the CoCrMo alloy, which seems to be quite complex3
and requires a knowledge of the oxidation mechanisms
of each metallic component. In the present paper we
focus on the oxidation behaviour of molybdenum.
An additional motivation to study the oxidation of
molybdenum is related to the variety of applications of
molybdenum oxide films, such as heterogeneous che-
mistry and catalysis, microelectronics and nanotechnol-
ogy.4,5 Molybdenum oxides are important catalysts or
catalyst precursors exhibiting catalytic properties in both
selective oxidation and selective hydrogenation reac-
tions.6 Also, both pure molybdenum oxides or mixtures
with tungsten oxide are promising display materials due
to changes of their colour reversibly upon light
irradiation.7 Although molybdenum oxides are used in a
variety of applications, the nature and catalytic
properties of these materials are not yet fully understood.
Since molybdenum has several oxidation states, ranging
from +2 to +6, a variety of its oxide compounds may
co-exist on the same surface. Therefore, any application
of molybdenum oxides requires a detailed knowledge of
the molybdenum oxidation states within the sample.
Molybdenum oxide surfaces of varying chemical
composition, complexity, crystallinity and intercalation
have been extensively analyzed, especially with XPS.8–12
However, the interpretation of the results has not been
consistent in the literature due to the complexity of the
XPS spectra, particularly when samples include different
oxidation states.13
Materiali in tehnologije / Materials and technology 51 (2017) 4, 617–621 617
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 669.017.1:67.017:543.428.3 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(4)617(2017)
It has been shown previously that the Mo surface is
quite resistant to the thermal oxidation at temperatures
below 300 °C.8 On the other hand, the ion-bombardment
represents an efficient method for the oxidation of
different metallic and semiconductor surfaces with
controlled amount of oxidation and oxide thickness, even
at room temperature.7–10 In our previous studies, we have
shown that oxygen bombardment is more efficient in
creating the oxide films on Ni and Co surfaces than
oxidation by electrochemical methods or thermal oxi-
dation.14,15
In this paper we provide a detailed characterization of
the RT oxidation of pure Mo metal by low-energy oxy-
gen-ion bombardment or thermal oxidation in an oxygen
atmosphere using X-ray photoelectron spectroscopy
(XPS) and compare these results with the oxidation of
Mo in the CoCrMo alloy.
2 EXPERIMENTAL PART
The Mo metal foil (M. Woite GmbH, 99.95 % of
mass fractions of Mo) and CoCrMo alloy (58 % Co,
32 % Cr, 6 % Mo, 1.3 % Si) samples were first polished
with SiC papers of 800–2500 grit and then washed with
ethanol and redistilled water. Before any oxidation step,
the Mo and CoCrMo surfaces were additionally cleaned
within the analytical ultrahigh vacuum (UHV) chamber
by cycles of low energy Ar+ bombardment at room tem-
perature (these samples are referred to as cleaned
samples). All the oxidation was conducted in situ in the
main chamber of the XPS instrument with a broad beam
of 500 eV O2+ ions of the typical current density of
2 μA/cm2. The thermal oxidation was performed by
supplying the pure oxygen gas into the UHV chamber to
a pressure of 6.5 × 10–4 Pa. In this case, the oxidation
dose is expressed in units of Langmuir, defined as 1 L =
1.33 × 10–4 Pa.
Cleaned and oxidised Mo surfaces were characterized
by XPS in a SPECS XPS spectrometer equipped with a
Phoibos MCD 100 electron analyser and a monochro-
matized source of Al-K X-rays of 1486.74 eV. The
typical pressure in the UHV chamber during the XPS
analysis was in the 10–7 Pa range. For the electron pass
energy of the hemispherical electron energy analyser of
10 eV used in the present study, the overall energy
resolution was around 0.8 eV. The photoemission spectra
were simulated with several sets of mixed Gaussian-
Lorentzian functions with Shirley background subtrac-
tion using Unifit software.16
3 RESULTS AND DISCUSSION
The formation of different Mo-O bonds, related to
the different oxidation states of the Mo, can be extracted
from the chemical shifts in the photoemission spectra
around the Mo 3d core-levels. We start with the thermal
oxidation of Mo metal. In Figure 1 we show several
characteristic Mo 3d photoemission spectra taken on the
cleaned surface and surfaces oxidized with two different
oxygen doses at RT. The cleaned sample shows a
characteristic spin-orbit splitting of 3.1 eV of the Mo 3d
level to Mo 3d3/2 and Mo 3d5/2, at binding energies (BEs)
of 231.2 and 228.1 eV, respectively. This doublet is
assigned to the metallic state of Mo, Mo0, in good
agreement with the data from the literature.8,9,11 After
thermal oxidation at RT, shown in Figure 1, the Mo 3d
photoemission remains almost unchanged up to the
highest oxygen dose of 5000 L used in this study. This is
in agreement with the previous studies of Mo oxidation,
in which the thermal oxidation of Mo requires
temperatures above 300 °C.8
On the other hand, bombardment with O2+ ions leads
to significant changes in the photoemission spectra, with
the development of several new and well-defined peaks
around the Mo 3d level. As an example, we show in
Figure 2 several Mo 3d spectra taken from a cleaned Mo
surface and surfaces bombarded with 500 eV O2+ ions
for two different bombardment times of 5 min and
20 min, corresponding to the oxygen doses of 4.75 × 1015
O atoms/cm2 and 1.5 × 1016 O atoms/cm2, respectively.
Obviously, the oxygen bombardment is more efficient
for the formation of some new Mo bonds on the metal
surface than the thermal oxidation.
The Mo 3d spectra of ion-bombarded surfaces show a
quite complex structure. To identify the different com-
ponents within that structure we fitted the experimental
curves from Figure 2 with the set of Voigt functions, as
shown in Figure 3. We first note that the deconvolution
I. JELOVICA BADOVINAC et al.: OXIDATION OF MOLYBDENUM BY LOW-ENERGY OXYGEN-ION BOMBARDMENT
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: Mo 3d core-level photoemission spectra from a cleaned
molybdenum surface and surfaces oxidized at room temperature (RT)
with oxygen doses of 500 L and 5000 L
of the Mo 3d spectrum from the cleaned sample is
dominated by the Mo 3d5/2–Mo 3d3/2 doublet assigned to
the Mo0 emission. However, a good fit of the experi-
mental curve is obtained only by introducing two small
additional doublets, with the main Mo 3d5/2 components
at 229.4 eV and 230.7 eV, respectively. We assign these
components to the intrinsic oxygen, most likely in the
form of MoO2 or Mo2O5, present in the Mo foil that has
not been removed with our cleaning procedure.
Deconvolution of the spectrum from a sample bom-
barded to a high ion fluence of 1.5 × 1016 O atoms/cm2
(i.e., for the 20-minute bombardment time, shown in
Figure 3), exhibits three well-resolved doublets in
addition to the Mo0 doublet, with the dominant Mo 3d5/2
peaks at BEs of 229.4, 230.8 and 232.2 eV, respectively.
We assign these additional components to the emission
from MoO2, Mo2O5 and MoO3, respectively, i.e., from
the Mo4+, Mo5+ and Mo6+ oxidation states of Mo,
respectively, in good agreement with the data from the
literature.8,9,11
Turning now to the oxidation of the CoCrMo alloy
shown in Figure 4, we first note that the Mo 3d XPS
spectrum of the cleaned CoCrMo sample is almost
identical to the spectrum from the cleaned Mo metal foil
from Figure 1. Furthermore, the thermal oxidation of
Mo at RT in the alloy is more efficient than the thermal
oxidation of the pure metal at RT to the same dose of
5000 L, clearly showing the formation of MoO2 (Mo4+
oxidation state), Mo2O5 (Mo5+ oxidation state) and MoO3
(Mo6+ oxidation state). These oxidation states are
represented in Figure 4 by three fitting doublets, in
addition to the Mo0 doublet. Finally, the ion-bombard-
ment of CoCrMo shows the different oxide structure of
Mo, compared to the metallic molybdenum. Now the
contribution from the Mo6+ states dominates the spect-
rum, as indicated in Figure 4 for the sample bombarded
with 500 eV O2+ ions for 20 min. At the same time, the
I. JELOVICA BADOVINAC et al.: OXIDATION OF MOLYBDENUM BY LOW-ENERGY OXYGEN-ION BOMBARDMENT
Materiali in tehnologije / Materials and technology 51 (2017) 4, 617–621 619
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 4: Comparison of Mo 3d core-level XPS spectra from a
cleaned CoCrMo surface and surfaces oxidized at RT with 5000 L of
oxygen or bombarded with 500 eV O2+ ions for 20 min (closed
circles); the three fitting doublets of each spectrum (solid lines,
representing product of Gaussians and Lorentzians) correspond to the
emission from MoO2, Mo2O5 and MoO3
Figure 2: Mo 3d core-level photoemission spectra from a cleaned
molybdenum surface and surfaces bombarded with 500 eV O2+ ions
for 5 min and 20 min at room temperature (RT)
Figure 3: The deconvoluted Mo 3d spectra from a cleaned molyb-
denum sample and a sample exposed to 500 eV O2+ ions at RT for 20
min; solid lines represent product of Gaussians and Lorentzians, while
closed circles represent experimental XPS spectra
Mo0 component in the spectrum is greatly reduced. We
stress here that the Mo0 peak is present in all ion-bom-
barded or thermally oxidised samples, indicating the
formation of very thin oxide films on the surface. In such
case, the XPS signal includes a contribution from the
underlying metallic molybdenum.
The O 1s photoemission (not shown) was also re-
corded from all the oxidized surfaces. While the inten-
sity of the oxygen peaks increases with the bombardment
time or dose of oxygen, indicating the increase of
oxygen concentration within the surface layer, the shape
of O 1s peak does not show any significant changes. It is
well known from the literature that the O 1s binding
energies of many metal oxides fall within a very narrow
range and it is not possible to separate the individual
oxide contributions.17 The main O 1s peak, observed in
our spectra around 530 eV, is usually assigned to all
metal oxides, while the small shoulder, observed in our
spectra around 531.5 eV, is assigned to the surface
hydroxide groups or oxygen defects.18,19
We conclude from the comparison of our results from
Figures 1 to 4 that the ion bombardment provides a more
efficient oxidation process for molybdenum than the
thermal oxidation, while Mo oxidizes more efficiently in
the CoCrMo alloy compared to the pure Mo metal. We
attribute this behaviour to the enhanced mobility and
reactivity of oxygen atoms in the alloy, compared to the
pure Mo metal. On the other hand, in contrast to thermal
processes, the ion-induced oxidation eliminates the need
for elevated temperatures and bypasses several thermally
activated processes, such as absorption, dissociation,
diffusion or bond breaking. The atomic collisions bet-
ween the impinging oxygen and matrix atoms create
different defects within the material, which are known to
enhance the mobility and reactivity of oxygen anions and
metal cations.20–23 Our measurements of ion-bombarded
samples show, indeed, the effects expected from the
radiation-enhanced oxidation, as observed previously for
some other metals and semiconductors.15,20,21,23
4 CONCLUSION
The thermal oxidation of Mo in pure Mo metal is not
efficient at RT. However, Mo oxidises quite easily at RT
within the CoCrMo alloy, showing characteristic contri-
butions from the Mo4+, Mo5+ and Mo6+ oxidation states.
The presence of Co and Cr reduces the surface barrier
and enables a more efficient intake of oxygen within the
sample.
On the other hand, the ion-induced oxidation of Mo
is quite efficient at RT in both the Mo metal and the
CoCrMo alloy. It is explained by the radiation-enhanced
oxidation of Mo, as observed previously for some other
metals and semiconductors. Again, contributions from
the Mo4+, Mo5+ and Mo6+ oxidation states are present in
all the oxides with the dominant contribution from the
Mo6+ oxidation states for the high implantation doses of
oxygen. Finally, the RT ion-induced oxidation of Mo is
more efficient than thermal oxidation in both the
CoCrMo alloy and the pure Mo metal samples.
Acknowledgements
This work was supported by the European Fund for
Regional Development and the Ministry of Science,
Education and Sports of the Republic of Croatia under
the project Research Infrastructure for Campus-Based
Laboratories at the University of Rijeka (grant number
RC.2.2.06-0001); and the European Social Fund for
Human Resources Development under the project SIZIF
(grant number HR.3.2.01-0310).
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I. JELOVICA BADOVINAC et al.: OXIDATION OF MOLYBDENUM BY LOW-ENERGY OXYGEN-ION BOMBARDMENT
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