Interaction of electrochemically deposited aluminium nanoparticles with reactive gases

Article abstract of DOI:10.1016/j.susc.2007.04.013

Metastable induced electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) were used to study the interaction of nanocrystalline aluminium with oxygen and carbon monoxide, respectively. High resolution scanning electron microscopy (HRSEM) was used to investigate the morphology of the nanocrystalline aluminium films. These films were prepared by electrodeposition from the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide containing 1.6 Mol per litre AlCl₃ in an argon filled glove box. Only a slight oxidation under exposure to oxygen and carbon monoxide was observed. After carbon monoxide dosage, no significant amount of carbon contamination was detected on the sample. These results indicate that the nanocrystalline aluminium is rather inert.

Full text of DOI:10.1016/j.susc.2007.04.013

Surface Science 601 (2007) 3769–3773
www.elsevier.com/locate/susc
Interaction of electrochemically deposited aluminium nanoparticles
with reactive gases
a
b
b,
c
F. Bebensee , L. Klarho¨fer , W. Maus-Friedrichs *, F. Endres
a
Lehrstuhl fu¨r Physikalische Chemie II, Universita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3, 91058 Erlangen, Germany
Institut fu¨r Physik und Physikalische Technologien, Technische Universita¨t Clausthal, Leibnizstrasse 4, 38678 Clausthal-Zellerfeld, Germany
b
c
Fakulta¨t fu¨r Naturwissenschaften, Technische Universita¨t Clausthal, Robert-Koch-Strasse 42, 38678 Clausthal-Zellerfeld, Germany
Available online 7 April 2007
Abstract
Metastable induced electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectros-
copy (XPS) were used to study the interaction of nanocrystalline aluminium with oxygen and carbon monoxide, respectively. High res-
olution scanning electron microscopy (HRSEM) was used to investigate the morphology of the nanocrystalline aluminium films. These
films were prepared by electrodeposition from the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide contain-
ing 1.6 Mol per litre AlCl3 in an argon filled glove box.
Only a slight oxidation under exposure to oxygen and carbon monoxide was observed. After carbon monoxide dosage, no significant
amount of carbon contamination was detected on the sample. These results indicate that the nanocrystalline aluminium is rather inert.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: MIES; UPS; XPS; SEM; Aluminium
1. Introduction
products. A good example for the use of aluminium is
the remarkable increase in wear resistance of stainless steel
after dip coating with sols prepared from dispersed boehm-
ite nano-powders observed by Hubert and co-workers [3].
The corrosion behaviour of aluminium compounds with
surface science methods (MIES, UPS, XPS) has been inves-
tigated by Frerichs et al. [4], the interaction of aluminium
films prepared by physical vapour deposition with reactive
gases under UHV conditions has been investigated inten-
sively by Frerichs et al. [5]. In this paper, we present the
first MIES/UPS-data on the interaction between nanocrys-
talline aluminium and O2 and CO, respectively.
Nowadays, a considerable amount of procedures for the
generation of nanoparticles of various kinds exist: thermal
spraying, sputter deposition, laser ablation, electrodeposit-
ion from ionic liquids and many more. The advantages of
electrodeposition from ionic liquids of the second genera-
tion, such as 1-butyl-1-methylpyrrolidinium bis(trifluoro-
methylsolfonly) imide ([BMP]Tf2N), include the feasibility
of deposition at room temperature and the possibility to
influence the mean grain size by controllable parameters
The unique properties (optical, electrical, magnetic,
mechanical and chemical) of nanomaterials, which are
mostly a function of the grain size, have attracted a lot
of interest. The increasing number of atoms located at
grain boundaries with decreasing grain size leads to dra-
matic effects e.g., in the mechanical properties of the mate-
rial. The oxidation behaviour of various nanoscale metals
has been found to be strongly dependent on the particle
size [1] and according to Natter et al., the microhardness
of nanocrystalline aluminium showed grain size dependent
values between 1.44 GPa (100 nm average grain size) and
3.40 GPa (14 nm average grain size) [2]. This gives support
to a potential use of nanocrystalline aluminium in corro-
sion resistance applications, which is interesting consider-
ing that aluminium is already widely used in technical
*
Corresponding author. Tel.: +49 5353 722310; fax: +49 5323 723600.
E-mail address: w.maus-friedrichs@pe.tu-clausthal.de (W. Maus-Frie-
drichs).
0039-6028/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2007.04.013

3770
F. Bebensee et al. / Surface Science 601 (2007) 3769–3773
such as deposition temperature and choice of the specific
ionic liquid. The electrodeposition of aluminium was the
subject of detailed studies in the past [6–8]. The air–mois-
ture stability of newly developed ionic liquids render this
technique interesting even for industrial applications
(coatings).
level obtained from the high energy cutoff observed for
metallic samples in UPS.
The fitting of XPS spectra was performed using Origin-
Pro7 (OriginLab Corporation) including the PFM add-on.
The full width at half maximum (FWHM) and the position
for the metallic Al2p peak were obtained in previous mea-
surements (not shown here) and used as fitting parameters
in order to get more reliable results.
2. Experimental
Several different mechanisms for the interaction of meta-
stable He* atoms with surfaces are known. For the nano-
crystalline aluminium samples studied here, only the
Auger deexcitation (AD) occurs. During AD, an electron
from the sample surface fills the 1s orbital of the impinging
He*. Simultaneously, the He* 2s carrying the excess energy
is emitted. Detailed descriptions of the different interaction
processes between He* and surfaces may be found in recent
reviews [9,10].
A commercial X-ray source is used for XPS. The pho-
tons hit the surface under an angle of 80ꢁ with respect to
the surface normal. Electrons emitted from the surface
are analyzed under 10ꢁ to the surface normal using a hemi-
spherical analyzer. Survey spectra are recorded with an en-
ergy resolution of 2.2 eV, detail spectra with 1.1 eV,
respectively.
The ionic liquid [BMP]Tf2N of the highest available
quality was purchased. It was subsequently dried at
100 ꢁC under vacuum conditions to water-content below
3 ppm and stored in an argon filled glove box (water-
and oxygen-content below 2 ppm). Anhydrous AlCl3
(Fluka, 99%) was used as a source of aluminium. A concen-
tration of 1.6 Mol per litre AlCl3 was used in this study.
The electrochemical cell with a geometric surface area of
0.79 cm2 was made of polytetraflouroethylene and clamped
over a teflon covered viton o-ring onto the hydrogen termi-
nated Si(100) substrate. The hydrogen termination was
achieved by immersion in HF and NH4F after extensive
cleaning procedures. The deposition was carried out inside
the glove box using a Parstat 2263 Potentiostat/Galvano-
stat (Princeton Applied Research) controlled by Power
CV and PowerStep software at a voltage of À1.4 V (vs.
Al/AlCl3 quasi reference electrode) for a duration of 2 h.
Upon deposition, the ionic liquid is removed by washing
the sample with acetonitrile inside the glove box.
3. Results
Fig. 1 shows an HRSEM micrograph of a nanocrystal-
line aluminium film deposited onto a Si(100) substrate in
the manner described above. Manually size evaluation of
about 100 particles gives a value of 16 3 nm for the mean
grain size. This size could also be corroborated with X-ray
diffraction measurements (not shown here) giving a similar
mean grain size. The particles constitute a film covering the
substrate completely. In the micrograph, residues of neither
the ionic liquid nor the solvent (acetonitrile) are visible.
Fig. 2 shows a XPS survey spectrum (0–1100 eV) of a
nanocrystalline aluminium film after sputtering. All peaks
are labelled with the corresponding element. Striking is
The surface morphology of the film was investigated
with a high resolution scanning electron microscope
(HRSEM) (Carl Zeiss DSM 982 Gemini). X-ray photo-
electron spectroscopy (XPS), metastable impact electron
spectroscopy (MIES) and ultraviolet photoelectron spec-
troscopy (UPS) were performed at the ‘‘Institut fur Physik
¨
und Physikalische Technologien’’. For that purpose, the
samples were transferred to a vacuum-tight transportation
chamber inside the glove box. The transportation chamber
could be docked to the load lock chamber of the spectros-
copy apparatus, so that the samples were not exposed to
the ambient atmosphere.
The spectroscopy apparatus with a base pressure of
2 · 10À11 mbar is equipped with a combined MIES-
UPS(HeI)-source, X-ray source (combined Al and Mg
cathode, Specs RQ-20/38C) and a hemispherical electron
energy analyser (VSW HA100). Details on this apparatus
can be found elsewhere [5].
A cold-cathode discharge adapted via two pumping
stages to the chamber is utilized to perform MIES and
UPS. An integrated time-of-flight technique is used to sep-
arate signals arising by electron emission from He* (MIES)
and HeI (UPS) interaction with the surface. Both MIES
and UPS spectra, are recorded under normal emission
within 140 s with an energy resolution of 220 meV. The
mixed H*–HeI beam exhibits an angle of incidence of
45ꢁ. All MIES and UPS spectra are displayed as a function
of the electron binding energy with respect to the Fermi
Fig. 1. HRSEM micrograph of a nanocrystalline aluminium film.

F. Bebensee et al. / Surface Science 601 (2007) 3769–3773
3771
O2p
XPS
Al Kα
O 1s
UPS
O(KVV)
O2 exposure
in langmuir
600
100
50
0
200
400
600
800
1000
binding energy / eV
5
Fig. 2. XPS survey spectrum (0–1100 eV) of the nanocrystalline alumin-
ium film.
20
15
10
5
-5
0
binding energy / eV
Fig. 3b. UPS spectra of the nanocrystalline aluminium film recorded
during oxygen exposure.
the low contamination level – only small contributions
from chlorine, carbon and fluor are visible – indicating a
clean aluminium film. The small molybdenum signal is
caused by the sample holder and does not affect our further
discussion.
Fig. 3 displays the MIES (a) and UPS (b) spectra re-
corded during oxygen exposure. The spectra are displayed
in a waterfall manner, with the amount of oxygen indicated
by the arrow on the right hand side. Two relevant features
are visible in the MIES spectra: one clearly visible peak
with a binding energy of 10.5 eV and a shoulder at a bind-
ing energy of 7.5 eV. In the UPS spectra, only the feature
with a binding energy of 7.5 eV is visible. The only signifi-
cant change in the spectra with higher oxygen dosage is a
rise in the feature at 7.5 eV binding energy, but only in
the UPS spectra.
Fig. 4 shows detail XPS spectra of the Al2p peak before
(a) and after (b) oxygen dosage. In both spectra, clear con-
tributions arising from metallic (binding energy 74.4 eV)
and oxidized (binding energy 77.0 eV) aluminium are visi-
ble. The relative areas and the individually fitted peaks
change only slightly: the ratio of metallic to oxidized alu-
minium decreases from 0.30 to 0.28 after exposure to
oxygen.
Fig. 5 displays the MIES (a) and UPS (b) spectra re-
corded during carbon monoxide exposure. Again, the ar-
row on the right-hand side indicates the amount of CO
dosed. The MIES spectra display, similar to those shown
in Fig. 4, two features: a peak at a binding energy of
11.5 eV and a shoulder at a binding energy of 7.5 eV.
The UPS spectra display a single feature at a binding en-
ergy of 7.4 eV; again this feature is slightly increasing with
dosage.
organic residues
MIES
O2p
O2 exposure
in langmuir
4. Discussion
600
The XPS survey spectrum depicted in Fig. 2 documents
clearly that by the method described above, relatively
clean, nanocrystalline aluminium films can be obtained.
Only hints of contaminations from carbon, chlorine and
fluorine are visible. Although the O1s peak appears to be
rather large, the oxide layer thickness was estimated to be
2.1 nm from an XPS detail spectrum of the Al2p peak
(not shown here).
100
50
5
20
15
10
5
0
-5
binding energy / eV
The peak in the MIES spectra displayed in Fig. 3a are
due to organic contaminations. The detection of this
Fig. 3a. MIES spectra of the nanocrystalline aluminium film recorded
during oxygen exposure.

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F. Bebensee et al. / Surface Science 601 (2007) 3769–3773
organic
residues
XPS
Al Kα
MIES
O2p
AlOX
CO exposure
in languir
160
Almet
60
50
10
5
1
20
15
10
5
0
-5
binding energy / eV
Fig. 5a. MIES spectra of the nanocrystalline aluminium film recorded
during carbon monoxide exposure.
72
74
76
78
80
82
binding energy / eV
XPS
Al Kα
UPS
O2p
AlOX
CO exposure
in languir
160
Almet
60
50
10
5
1
20
15
10
5
0
-5
binding energy / eV
72
74
76
78
80
82
Fig. 5b. UPS spectra of the nanocrystalline aluminium film recorded
during carbon monoxide exposure.
binding energy / eV
Fig. 4. XPS spectrum of the Al2p peak of the nanocrystalline aluminium
film: (a) before oxygen exposure and (b) after oxygen exposure.
peak seems to enlarge with prolonged oxygen dosage. This
suggests the sub-surface incorporation of oxygen.
contamination underlines the extremely high surface sensi-
tivity of the MIES technique, because they are neither vis-
ible in the HRSEM micrograph (Fig. 1) nor in the XPS or
UPS spectra. The second feature in the MIES spectra with
a binding energy of 7.5 eV is attributed to electron emission
from the O2p orbital.
The UPS spectra in Fig. 3b only show one peak at a
binding energy of 7.5 eV, which is also assigned to emission
from the O2p orbital. In contrast to the evolution of the
MIES spectra, where basically no change is observable, this
The perception of a small amount of oxygen being
incorporated into the bulk is corroborated by the XPS
spectra in Fig. 4. The ratio of metallic to aluminium to oxi-
dized aluminium is decreased slightly from 0.3 to 0.28.
The interpretation of the MIES and UPS spectra during
CO exposure shown in Fig. 5 is similar: the peak with a
binding energy of 11.5 eV is again attributed to organic
contaminations. The feature at a binding energy of 7.5 eV
attributed to the emission from the O2p orbital is more

F. Bebensee et al. / Surface Science 601 (2007) 3769–3773
3773
pronounced in this case. This is probably caused by a
Acknowledgements
The authors are thankful for the technical assistance of
Mrs. Munster (HRSEM images) and D. Schwendt (spec-
¨
troscopy lab). Furthermore, the authors thank Dr. S. Zein
el Abedin for help with the ionic liquid.
slightly higher degree of pre-oxidation in the glove box.
The UPS spectra, again show only the peak attributed to
oxygen, which is slightly increasing with dosage of carbon
monoxide. XPS spectra not shown here indicate a slight
oxidation of the aluminium, while not a considerable
amount of carbon accumulates on the surface.
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