Efficient growth of ordered thin oxide films on Ni (1 1 1) by NO 2 oxidation

Article abstract of DOI:10.1016/j.susc.2004.03.052

The efficient growth of ordered thin oxide films on Ni(1 1 1) using low NO₂ gas exposures under ultrahigh vacuum conditions has been achieved. The reaction mechanism of NO₂ on Ni(1 1 1) at 500-600 K changes from total decomposition to partial dissociation into NO and O after a critical surface adatom coverage has been attained. Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) reveal the creation of an intermediate NiO(1 0 0) phase, followed by the eventual formation of clean NiO(1 1 1) overlayers. Reflection absorption infrared spectroscopy (RAIRS) using CO molecules as probes of the surface adsorption sites present confirms the conversion of Ni⁰ to Ni²⁺ during the oxidation process.

Full text of DOI:10.1016/j.susc.2004.03.052

Surface Science 557 (2004) 201–207
www.elsevier.com/locate/susc
Efficient growth of ordered thin oxide films on Nið111Þ
by NO2 oxidation
*
B.S. Yeo, Z.H. Chen, W.S. Sim
Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 119260, Singapore
Received 17 January 2004; accepted for publication 24 March 2004
Abstract
The efficient growth of ordered thin oxide films on Nið111Þ using low NO2 gas exposures under ultrahigh vacuum
conditions has been achieved. The reaction mechanism of NO2 on Nið111Þ at 500–600 K changes from total
decomposition to partial dissociation into NO and O after a critical surface adatom coverage has been attained. Auger
electron spectroscopy (AES) and low energy electron diffraction (LEED) reveal the creation of an intermediate
NiOð100Þ phase, followed by the eventual formation of clean NiOð111Þ overlayers. Reflection absorption infrared
spectroscopy (RAIRS) using CO molecules as probes of the surface adsorption sites present confirms the conversion of
Ni0 to Ni2þ during the oxidation process.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Nickel oxides; Nitrogen oxides; Auger electron spectroscopy; Low energy electron diffraction (LEED); Infrared absorption
spectroscopy; Low index single crystal surfaces; Growth
1. Introduction
mechanism for this process involves dissociative
adsorption, oxide island formation and thick
Understanding the properties and reactivity of
NiO surfaces is essential from a technological and
commercial standpoint because of its utility as a
key material in many catalytic processes [1]. Due
to the insulating nature of oxide materials that
results in sample charging problems when probed
with electron-based spectroscopies, surface sci-
ence studies are normally conducted on NiO thin
films prepared by the direct oxidation of Ni single
crystal surfaces with molecular O2. The general
oxide growth. On Nið111Þ, the formation of
pffiffiffi pffiffiffi
ordered pð2 ꢀ 2Þ and pð 3 ꢀ 3ÞR30ꢁ O over-
layers has been observed during the initial
chemisorption stages. NiO islands then grow and
amalgamate until the entire surface is oxidized
[2].
This method of synthesizing metal oxide thin
films requires relatively large gas exposures as the
probability of dissociative O2 chemisorption tends
to decrease rapidly with increasing surface O cov-
erages, especially for Ni surfaces [3]. NO2 is a much
more efficient oxidant as it is able to maintain a
high dissociative sticking probability to yield NO
and atomic O even in the presence of large con-
centrations of surface O [4,5]. This property makes
* Corresponding author. Tel.: +65-6874-2680; fax: +65-6779-
1691.
E-mail address: chmsimws@nus.edu.sg (W.S. Sim).
0039-6028/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2004.03.052

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B.S. Yeo et al. / Surface Science 557 (2004) 201–207
it particularly useful for oxidation reactions under
ultrahigh vacuum (UHV) conditions, where prac-
tical considerations require the minimization of gas
loading in the system. NO2 oxidation has been used
to successfully fabricate metal oxide thin films on
Moð110Þ [6], Ruð001Þ [7], Pdð111Þ [5], Cuð100Þ
[8] and Agð111Þ [9,10] under much milder condi-
tions than that needed for O2 oxidation.
In this work, Auger electron spectroscopy
(AES), low energy electron diffraction (LEED)
and reflection absorption infrared spectroscopy
(RAIRS) have been utilized in tandem to investi-
gate the oxidation of Nið111Þ by NO2. It will be
shown here that NO2 decomposes readily on
Nið111Þ to give atomic O that undergoes facile
reaction with the metal substrate to produce clean
and ordered NiOð111Þ thin films.
2. Experimental
All experiments were performed in a stainless
steel UHV chamber with a base pressure <2 ·
10ꢁ10 Torr that has been described in detail else-
where [11]. The Nið111Þ sample was cleaned by
repeated cycles of Ar ion sputtering and annealing,
with its cleanliness and surface structure verified
by the absence of any detectable contaminant AES
signals and the observation of a sharp hexagonal
LEED pattern with little background intensity.
The oxidation process was carried out by exposing
the clean Nið111Þ crystal held at a temperature of
500 K to NO2 (Soxal, 99.5%) dosed through a
variable leak valve, followed by annealing to 600
K to desorb any molecular species present on the
surface. All LEED images and AES spectra were
collected at 300 K while RAIRS was performed at
120 K under an ambient CO background pressure
of 5 · 10ꢁ8 Torr in order to maintain a saturation
population of CO on Ni adsorption sites that may
be energetically unfavourable towards Ni–CO
bonding.
Fig. 1. Plots of AES peak intensities of the O5 1 0 (KLL), Ni8 4 8
(LMM) and N3 5 2 (KLL) signals as a function NO2 exposure.
Nið111Þ sample using the peak-to-peak heights
of the O5 1 0 (KLL), Ni8 4 8 (LMM) and N3 5 2
(KLL) signals (dN=dE). In Fig. 1, the intensities
of the O, Ni and N signals measured during the
oxidation process are plotted as a function of
NO2 exposure, with the common feature of all
three plots being the concerted increase (O) or
decrease (Ni and N) followed by plateauing of
the AES signals at >80 L NO2 exposure. Previous
reports have indicated that the growth of NiO
thin films on Nið111Þ can be self-limiting [3], and
similar mechanisms may be operating in the
present case.
Assuming that n layers of NiO grow uniformly
in a ð111Þ-orientation on the Nið111Þ substrate
such that there is an exponential attenuation of
the AES intensities with film thickness, the ratio
of the O to Ni AES signals may be expressed
as:
3. Results and discussion
AES was employed to quantify the elemental
composition of the oxide film grown on the

B.S. Yeo et al. / Surface Science 557 (2004) 201–207
203
P
n
IO
SO
i¼1 CO;oxide exp ð ꢁ ði ꢁ 1Þxoxide cos h=kO;oxide
Þ
ꢀ
ꢁ
¼
ꢀ
P
C
Ni;oxide exp ð ꢁ ði ꢁ 1Þxoxide cos h=kNi;oxide
Þ
INi SNi
n
i¼1
P1
þ exp ð ꢁ nxoxide cos h=kNi;oxide
Þ
j¼1 CNi;metal exp ð ꢁ ðj ꢁ 1Þxmetal cos h=kNi;metal
Þ
For the NiOð111Þ layers, the concentrations of
Upon 0.3 L NO2 exposure, a complex LEED
pattern reminiscent of that obtained when NO
dissociates on Nið111Þ into an overlayer of coad-
sorbed O and N is observed (Fig. 2(b)) [14]. At 18
L NO2 exposure, the LEED image consists of the
six Nið111Þ substrate spots, a smaller hexagonal
ring of six spots oriented parallel to the original
hexagonal substrate array, and two dodecagonal
rings of 24 spots (Figs. 2(c) and 3). The attenua-
tion of the complex LEED pattern suggests dis-
ordering of the O and N overlayer. Above 80 L
NO2 exposure, the substrate spots have disap-
peared, the two rings of 24 spots have also faded,
while the new hexagonal ring of six spots has be-
come noticeably brighter (Fig. 2(d)).
ꢁ2
ꢀ
O and Ni are CO;oxide ¼ CNi;oxide ¼ 0:133 ions A
,
ꢀ
the interlayer distance xoxide ¼ 2:41 A, and the
electron mean free paths for the O and Ni electrons
ꢀ
ꢀ
are kO;oxide ¼ 9:5 A and kNi;oxide ¼ 10:8 A respec-
tively [12]. The corresponding values for the
ꢁ2
ꢀ
Nið111Þ substrate are CNi;metal ¼ 0:186 atoms A
,
ꢀ
ꢀ
xmetal ¼ 2:03 A, and kNi;metal ¼ 10.2 A [12]. Using a
collection angle h ¼ 90ꢁ and a relative sensitivity
factor SO=SNi ¼ 4 calibrated for our retarding field
analyzer, the O to Ni AES intensity ratio IO=
INi ¼ 3:05 at saturation (>80 L) NO2 exposure
yields a NiO film thickness of 9–10 layers. This
result agrees well with the reported thicknesses of
NiO films grown using O2 gas as the oxidant, which
range from 3–4 layers to 8–21 layers when oxida-
tion was carried out at 300 K [3,12] and 570 K [13]
respectively.
The ꢂ15% reduction in the reciprocal surface
lattice parameter of the structure giving rise to
these six spots with respect to that of the Nið111Þ
substrate is in excellent agreement with the differ-
The N AES signal is observed to have the
highest intensity at the lowest NO2 exposure, and
this can be attributed to the presence of atomic N
formed by the complete decomposition of NO2 on
Nið111Þ at the earliest stages of the oxidation
process. This probably occurs via an NO inter-
mediate species, which is itself known to dissociate
on clean Nið111Þ but remains intact in the pres-
ence of preadsorbed O [14]. As the oxidation
proceeds, the buildup of O and N adatoms in the
overlayer eventually inhibits the NO dissociation
[15] and results in a mechanism switch from
complete to partial NO2 decomposition, with the
NO formed desorbing under the experimental
conditions (>500 K) [16]. The rapid decay of the N
AES signal indicates that the atomic N formed is
only embedded in the initial chemisorbed over-
layer and absent in the growing NiO film.
ꢀ
ence in lattice constants of NiO (4.17 A) and Ni
ꢀ
(3.52 A). In addition, the hexagonal LEED pattern
coupled with the absence of any half-order spots
implies that ordered layers with a NiOð111Þ-
pð1 ꢀ 1Þ structure have been formed. The posi-
tions of the observed LEED spots are also
consistent with the preferred growth behaviour of
NiOð111Þ on Nið111Þ, where the LEED patterns
of both the oxide overlayers and the substrate are
known to be aligned [17].
The intensity attenuation observed for the two
rings of 24 spots as the oxidation proceeds indicates
that they originate from an intermediate type
structure, possibly formed at the interfacial region
between the NiOð111Þ overlayers and Nið111Þ sub-
strate. The positions of these spots are consistent
with the presence of three 120ꢁ rotational domains
of NiOð100Þ, which give rise to a reciprocal surface
lattice parameter contraction of ꢂ28% with respect
to the Nið111Þ substrate, as shown schematically in
Fig. 3. These domain structures have also been
observed by scanning tunneling microscopy (STM)
LEED images at 84 eV of the various ordered
surface structures present during the NO2 oxida-
tion process are shown in Fig. 2. The clean
Nið111Þ surface is characterized by a regular
hexagonal array of six sharp spots (Fig. 2(a)).

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B.S. Yeo et al. / Surface Science 557 (2004) 201–207
Fig. 2. LEED images of Nið111Þ at 84 eV after oxidation by (a) 0 L, (b) 0.3 L, (c) 18 L and (d) >80 L NO2.
during the initial stages of NiOð111Þ growth on
Nið111Þ via O2 oxidation [18].
clean surface has two m(CO) bands at 2054 and
1917 cmꢁ1, consistent with CO adsorbed at on-top
The m(CO) frequency of CO adsorbed on the
Nið111Þ surface at different stages of the oxidation
process gives valuable information about the oxi-
dation state of the Ni atoms present as it is a sen-
sitive function of the degree of electron transfer
between the CO molecular orbitals and the metallic
adsorption sites. In general, the m(CO) frequency
increases as the metal atom becomes more posi-
tively charged [19]. This generalization is applica-
ble to the current work where the relative
invariance of the m(CO) frequencies with band size
on the oxidized surface phases indicates that the
CO exists as islands of constant local coverage with
the same degree of dipole coupling that is inde-
pendent of total surface coverage within each
phase. Fig. 4 shows a series of RAIR spectra for
saturation coverages of CO adsorbed on Nið111Þ
under 5 · 10ꢁ8 Torr ambient gas pressure at 120 K
after oxidation with various NO2 exposures. The
and two-fold bridging Ni0 sites respectively in the
pffiffiffi pffiffi
pð 7 ꢀ 7ÞR19.1ꢁ structure [20,21].
After 0.3 L NO2 exposure, two new m(CO) bands
are observed at 2126 and 2090 cmꢁ1, together with
broad and weak absorption features centered
around 2066 and 1920 cmꢁ1 that arise from CO
adsorbed on residual patches of clean Nið111Þ [22].
The AES and LEED data at this stage indicate that
O and N adatoms are both present on the surface,
probably as separate phases given that the N–N
and N–O repulsive pairwise lateral interaction
energies are twice that for O–O [15]. The com-
plex LEED pattern then presumably arises from
pffiffi
six domain orientations of a cð5 3 ꢀ 9Þrect-N
structure, in which the N atoms occupy four-fold
hollow sites in a reconstructed Nið100Þ-c(2 · 2)-
2N-like top layer [23]. The Nið100Þ-cð2 ꢀ 2Þ-2O
surface has been reported to be inert towards CO
adsorption [24], and we propose that a similar

B.S. Yeo et al. / Surface Science 557 (2004) 201–207
205
Fig. 3. Schematic diagram of the LEED spots observed in Fig.
2(c) for Nið111Þ at 84 eV after oxidation by 18 L NO2, showing
the three 120ꢁ rotational domains of NiOð100Þ.
situation occurs here. The m(CO) band at 2090
cmꢁ1 is characteristic of CO adsorbed on-top of
O-perturbed Ni0 atoms [22]. However, the absence
of any other known LEED patterns associated with
adsorbed O on Nið111Þ suggests that the O adlayer
is in a disordered state. The m(CO) band at 2126
cmꢁ1 lies approximately midway between that of
CO adsorbed on O-perturbed Ni0 and fully
oxidized Ni2þ sites. On the basis of electronic
considerations, it can be reasonably ascribed to CO
linearly bonded on-top of partially oxidized Nidþ
sites. This assignment is in agreement with infrared
spectroscopic studies of CO adsorption on NiO
supported on Nið111Þ [25] and Al2O3 [19,26],
where m(CO) bands at 2120–2124 cmꢁ1 were also
postulated to originate from CO adsorbed on such
Nidþ sites.
Fig. 4. RAIR spectra of Nið111Þ under an ambient CO gas
pressure of 5 · 10ꢁ8 Torr at 120 K after oxidation by (a) 0 L,
(b) 0.3 L, (c) 18 L and (d) >80 L NO2.
be assigned to CO adsorbed on-top of Ni2þ sites in
the NiOð111Þ thin film. Its absorption frequency is
consistent with the higher oxidation state of the
Ni2þ substrate atoms, and in excellent agreement
with previous infrared and high resolution electron
energy loss spectroscopy (HREELS) studies of CO
adsorption on NiOð111Þ thin films, which have
also yielded m(CO) frequencies at ꢂ2150 cmꢁ1 for
the Ni2þ-bound species [27,28]. The efficacy of NO2
in effecting the complete and homogeneous oxida-
tion of the Nið111Þ surface is demonstrated by the
absence of any other m(CO) absorption bands be-
tween 1800 and 2130 cmꢁ1 in the RAIR spectra at
>80 L NO2 exposure. While the possibility of the
oxidized surface being terminated with OH groups
At 18 L NO2 exposure, the bands observed
earlier are attenuated and a new m(CO) band ap-
pears at 2142 cmꢁ1, which grows to become the
main feature in the RAIR spectrum at NO2 expo-
sures >80 L. This sharp and well-defined band may

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B.S. Yeo et al. / Surface Science 557 (2004) 201–207
Acknowledgements
from dissociative background adsorption of resid-
ual H2O or H2 cannot be completely ruled out, the
presence of these coadsorbates does not appear to
affect the chemisorption behaviour of CO on the
oxidized surface [27].
We acknowledge financial support for this work
from the National University of Singapore (Grant
No. R-143-000-074-112).
The current results show that the reaction
pathway of NO2 on Nið111Þ rapidly switches from
complete decomposition on the clean surface to
partial dissociation in the presence of adsorbed O
and N. The net result is the inclusion of a small
quantity of N atoms into a bridging interfacial
layer, and the subsequent growth of N-free or-
dered NiO thin films of predominantly ð111Þ ori-
entation. The crucial factor in this behaviour is the
change from dissociative to molecular chemisorp-
tion of the NO intermediate on Ni when the dis-
sociation products reach a critical surface coverage
due to strong repulsive interactions between the O
and N adatoms [15]. Except for Moð110Þ [6],
previous studies of NO2 oxidation of transition
metal surfaces have generally not made any men-
tion of the presence of N in the oxide films grown
[5,7–10]. Assuming that similar mechanisms exist
in some of these cases involving the more reactive
transition metals whose clean surfaces are known
to dissociate NO, then any N contamination
would be expected to be localized within the
interfacial region between the substrate and the
growing oxide film and may not have been de-
tected. The amount of N incorporated depends on
the propensity of the clean metal surface towards
NO dissociation, and can probably be minimized
by covering the surface with O adatoms from an
O2 gas source beforehand.
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