Study of the growth of NiO on highly oriented pyrolytic graphite by X-ray absorption spectroscopy

Article abstract of DOI:10.1016/j.elspec.2006.11.030

In this work, we present a X-ray absorption spectroscopy (XAS) study of the growth of NiO on highly oriented pyrolytic graphite (HOPG). NiO has been grown by reactive evaporation of metallic Ni in an oxygen atmosphere (2 × 10^-5 Torr) at room temperature. We paid special attention to the study of the early stages of growth. Both, Ni 2p and O 1s core-level XAS spectra were measured. For large NiO coverages, the spectra resemble that of a NiO single crystal, thus indicating the formation of a stoichiometric NiO thin film on the HOPG substrate. The Ni 2p XAS spectra remain similar during the whole growth process, indicating that Ni atoms are present in the high spin Ni²⁺ form, as supported by multiplet calculations. In contrast, for low coverages the line-shape of the O 1s XAS spectra differ strongly from that of bulk NiO. Cluster calculations of the spectra in octahedral and pyramidal symmetries support the formation of nanometric planar NiO islands at the graphite steps as previously observed by atomic force microscopy (AFM) in this system.

Full text of DOI:10.1016/j.elspec.2006.11.030

Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 111–114
Study of the growth of NiO on highly oriented pyrolytic graphite
by X-ray absorption spectroscopy
a
b
a
a
c
a,∗
I. Preda , M. Abbate , A. Guti e´ rrez , S. Palac ´ı n , A. Vollmer , L. Soriano
a
Departamento de F ´ı sica Aplicada, Universidad Aut o´ noma de Madrid, Cantoblanco, 28049 Madrid, Spain
Departamento de F ´ı sica, Universidade Federal do Paran a´ , Caixa Postal 19091, 81531-990 Curitiba PR, Brazil
b
c
BESSY, Albert Einstein Strasse15, D-12489 Berlin, Germany
Available online 28 November 2006
Abstract
In this work, we present a X-ray absorption spectroscopy (XAS) study of the growth of NiO on highly oriented pyrolytic graphite (HOPG). NiO
−
5
has been grown by reactive evaporation of metallic Ni in an oxygen atmosphere (2 × 10 Torr) at room temperature. We paid special attention to
the study of the early stages of growth. Both, Ni 2p and O 1s core-level XAS spectra were measured. For large NiO coverages, the spectra resemble
that of a NiO single crystal, thus indicating the formation of a stoichiometric NiO thin film on the HOPG substrate. The Ni 2p XAS spectra remain
2
+
similar during the whole growth process, indicating that Ni atoms are present in the high spin Ni form, as supported by multiplet calculations.
In contrast, for low coverages the line-shape of the O 1s XAS spectra differ strongly from that of bulk NiO. Cluster calculations of the spectra
in octahedral and pyramidal symmetries support the formation of nanometric planar NiO islands at the graphite steps as previously observed by
atomic force microscopy (AFM) in this system.
©
2006 Elsevier B.V. All rights reserved.
Keywords: Nickel oxide; NiO nanostructures; X-ray absorption spectroscopy
1
. Introduction
X-ray absorption spectroscopy is a well established technique
which, in a first approximation, probes unoccupied electronic
states. However, the Ni 2p XAS spectra are dominated by the
Ni 2p spin-orbit splitting, as well as the Ni 2p–3d Coulomb
and exchange interactions. These interactions give rise to strong
multiplet effects in the resulting spectra, hiding the informa-
tion on the density of states [2]. These effects are, in turn,
very sensitive to the local symmetry of the initial state, thus
being a suitable technique to determine the chemical state of the
metal [3].
The main purpose of this paper is the study of the electronic
structure of the early and final stages of growth of a NiO thin
film on highly oriented pyrolytic graphite (HOPG). This work is
motivated by the increasing importance of nanostructured oxide
coatings in technological applications. In fact, NiO is an oxide
material with applications in catalysis and as magnetic mate-
rials. Another important motivation is that a previous atomic
force microscopy (AFM) study on this system shows that for
the early stages of growth, NiO grows by forming planar islands
The O 1s XAS spectra correspond to O s → p transitions
where, according to the dipole selection rule (ꢀꢁ = ± 1) in XAS,
only unoccupied O states of p-character can be reached. The
spectra are thus, directly related to unoccupied O p states mixed
with metal-3d and metal-4sp states via hybridization. The near-
edge region of the spectra is dominated by hybridization with
metal-3d states, thus mapping the metallic 3d density of states.
This property of XAS allowed us to observe the splitting of the
Ni 3d states located at the NiO surface in nanostructured NiO
(
100–200 nm in size and 3–10 nm in height), mainly located at
the HOPG steps. For further NiO deposition, the NiO islands
spread throughout the terraces, reaching coalescence for higher
NiO coverages [1]. Therefore, the study of the electronic struc-
ture of this nanostructured system seems to be well justified.
We show below that the X-ray absorption spectroscopy (XAS)
spectra reflect surface effects due to the large surface to volume
ratio typical of nanometric structures.
8
systems [4]. It is worthy to note here that NiO is a 3d charge
8
9
transfer oxide with the ground state as a mixture of 3d + 3d
L + 3d10 L states [5–7] so that only e states remain unoccupied.
2
∗
Corresponding author. Tel.: +34 914974192; fax: +34 914973969.
g
E-mail address: l.soriano@uam.es (L. Soriano).
We show below that surface effects are clearly observed in the O
0
368-2048/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.elspec.2006.11.030

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12
I. Preda et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 111–114
1
s XAS spectra of the nanostructured NiO system studied here
which are completely consistent with the AFM data.
2
. Experimental
Theexperimentwasperformedinanultrahighvacuumcham-
ber located at the PM4 beam-line in the Synchrotron BESSY
Berlin). Successive depositions were performed and analyzed
(
by XAS. NiO was deposited at room temperature on HOPG
by reactive evaporation from a pure Ni filament. The evapora-
−
5
tion was performed in an oxygen atmosphere (5 × 10 Torr)
at room temperature. The evaporation rate was constant and
maintained low enough to allow the study of the early stages
of NiO growth. After deposition for large coverages, the
◦
sample was annealed at 400 C for 30 min in ultra high vac-
uum. More details on the preparation procedure can be found
elsewhere [4].
The XAS measurements were performed at the PM4 beam-
line of the BESSY II storage ring (Berlin). The spectra were
collected in the total electron yield detection mode at graz-
ing incidence to take advantage of the linear polarization of
the synchrotron light in order to enhance transitions towards
atoms in the direction to the surface normal (surface enhance-
ment). The estimated overall resolution of the plane grating
monochromator (PM4) was better than 100 meV at the O 1s
edge (530 eV). The spectra were normalized to the I0 current
coming from a fresh gold sample in order to correct the spec-
tra from the contamination of the optical elements and beam
losses.
Fig. 1. Ni 2p XAS spectra as a function of the NiO coverage.
state of the Ni atoms remains unchanged and the same as in bulk
NiO.
3
. Results and discussion
In order to corroborate the above results, we have compared
the Ni 2p XAS spectra with theory. According to de Groot et al.
[2,3], the spectra can be reproduced by calculating the 2p → 3d
multiplet transitions as a first step. In the case of NiO, these
transitions have to be projected in an octahedral crystal field of
certain strength. We have performed 2p → 3d multiplet calcula-
The Ni 2p XAS spectra as a function of the NiO coverage
are shown in Fig. 1. We have to note here that the coverages
have been calculated from the intensities of the XAS spectra
following conventional methods and assuming a layer-by-layer
way of growth at constant evaporation rate. Since the AFM data
show that the way of growth of NiO/HOPG is dominated by the
formation of nanometric planar islands, the coverage should be
understood as an indication of the equivalent material needed to
complete such layers. At first sight, all the spectra through the
series are similar. First of all we want to note that the spectrum
2+
tion for Ni ions in different symmetries: high spin Ni (Fig. 2a),
low spin Ni 2 (Fig. 2b) and Ni (Fig. 2c). The crystal field
parameter (10Dq) was set to 1.8 eV in all calculations. From
the comparison of the calculations with the experimental Ni 2p
spectra it is inferred that, indeed, the Ni atoms involved in the
+
3+
◦
2+
of the NiO thin film annealed at 400 C matches other published
experiment remain in the high spin Ni form.
Ni 2p spectra for bulk NiO [8,9]. Also, the spectra for large cov-
erages, prior to annealing, are almost identical. This is a clear
indication that a stoichiometric NiO thin film can be grown on
the HOPG substrate. However, the most important feature is
that the Ni 2p XAS spectra show the same line-shape during the
whole growth process. Even for low coverages, the structures
observed in the spectra are the same, although less defined and
slightly broader, than those for bulk NiO. These minor discrep-
ancies can be attributed to disorder and consequently to a lower
value of the crystal field. The spectra also slightly shift towards
higher photon energies as the coverage increases. Although XAS
measurements cannot be referred to the Fermi level, this shift is
directly related to the increase of the thickness of the NiO iso-
lating thin film. These results seem to indicate that the oxidation
Fig. 3 shows the O 1s XAS spectra as a function of the
coverage. Once again, the spectrum of the annealed thin film
is identical to other spectra published elsewhere for bulk NiO
[10,11]. The first peak at 530.5 eV is assigned to the hybridiza-
tion of O p states with Ni 3d (eg) states whereas the broad
peaks located at 536.0 and 539.0 eV correspond to O p states
hybridized with Ni 4s and Ni 4p, respectively. The spectra for
high coverages prior to annealing are also similar to that of bulk
NiO. This supports again the growth of a stoichiometric NiO
thin film on the HOPG substrate.
However, for low coverages (0.5 ML in Fig. 3) the spec-
trum differs strongly from the bulk spectrum. The single peak at
threshold, assigned to eg states in bulk NiO, is now split forming
a broad band. It is clearly seen that the initial broad band for the

I. Preda et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 111–114
113
2
+
Fig. 2. Multiplet calculations for: (a) Ni with high spin symmetry; (b) low
2
+
3+
spin Ni ; (c) Ni
.
Fig. 4. Experimental and calculated near edge region of the O 1s XAS spectra
(
dots) of: (a) large NiO coverage (bulk); (b) 0.5 ML NiO/HOPG; (c) 3 nm NiO
nanoparticles. Gaussian curves represent the results of the calculations in: octa-
hedral (short dash lines) and pyramidal symmetry (short dot lines). Solid lines
represent typical curves used to fit the background.
very low stages of growth is progressively changing in shape
throughout the series to give the final bulk line-shape.
To understand this effect we have calculated the O 1s absorp-
tion edge by means of cluster calculations in octahedral (NiO6)
and pyramidal (NiO ) symmetries. The results of the calcu-
5
lations are presented in Fig. 4. The calculations in octahedral
symmetry (short dash line in Fig. 4a) give a single peak which
corresponds to the hybridization with the unoccupied eg states
in bulk NiO, as it is seen by the comparison with the exper-
imental O 1s XAS spectra of bulk NiO (dots in Fig. 4a). In
turn, the calculations for pyramidal symmetry (short dot line
in Fig. 4c) give two peaks separated by 1.5 eV and with relative
intensity of 32% and 50% with respect to the intensity of the bulk
line.
2
These results show that unoccupied eg states split into z
2
2
and x − y states due to the lack of the apical oxygen at the
NiO surface. The calculations in pyramidal symmetry have been
compared with the experimental O 1s XAS spectra of 3 nm
NiO nanoparticles taken from Ref. [12]. This nanometric system
shows important surface effects due to the high surface to vol-
umeratio. Thecalculationsperfectlyagreewiththespectraofthe
NiO nanoparticles. However, in the case of the spectra of 0.5 ML
of NiO/HOPG, the best fitting has been obtained by introducing
Fig. 3. O 1s XAS spectra as a function of the NiO coverage.

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I. Preda et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 111–114
a small component of the bulk calculations. We have to note here
that all the XAS spectra shown here have been taken at grazing
incidence to enhance the surface of the planar NiO islands. The
quasi-bidimensional morphology of the NiO islands is reflected
in the XAS spectra. The planar islands formed at the early stages
of growth present a large surface to volume ratio, allowing the
observation of this surface effect. It can be seen that further evap-
oration produces the increase of the bulk component, indicating
an increase of the island thickness. Only when coalescence has
been reached (25 ML in Fig. 3), the thin film spectra resemble
those of bulk NiO. Another relevant aspect of the results is the
Acknowledgments
We want to thank the support of the CICYT of Spain under
contract: BFM2003-03277 and the European Union through the
R II 3.CT-2004-506008 contract. Two of the authors (A.G. and
I.P.) thank the Spanish Ministerio de Educaci o´ n y Ciencia for
financial support through the “Ram o´ n y Cajal” and FPI Pro-
grams, respectively. We also thank the BESSY staff by technical
support.
References
2
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5
(
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. Conclusions
We have studied the electronic structure of the growth of NiO
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