Surface Composition of Layered Ruddlesden-Popper Lan+1NinO3n+1 (n = 1, 2 and 3)
Epitaxial Films
K.-T. Wua,b*, H. Téllezc, J. Drucec, M. Burriela,d, T. Ishiharab,c, J. Kilnera,c, and S.
Skinnera*
a
Department of Materials, Imperial College London, London, UK
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Japan
c
International Institute for Carbon-Neutral Energy Research, Kyushu University, Japan
d
Laboratoire des Matériaux et du Génie Physique (LMGP), Grenoble, France
b
Layered Ruddlesden-Popper (RP) type oxides, formulated
Lan+1NinO3n+1 (n = 1, 2 and 3), have recently been suggested as
candidates for IT-SOFC cathodes. The oxygen surface exchange
activity is strongly influenced by the surface composition and
atomic surface structure where the adsorption and dissociation of
molecular oxygen take place. It is therefore necessary to
understand the surface and near-surface chemical compositions of
the cathode materials. In this work, the surface and near-surface
composition of epitaxial films in the RP series was systematically
investigated using low-energy ion scattering (LEIS).
Introduction
Compared to traditional cathodes (e.g. La1-xSrxMnO3-δ), the mixed ionic-electronic
conductors (MIECs), layered RP-phase Lan+1NinO3n+1 (n = 1, 2 and 3), are expected to
possess high oxygen surface exchange kinetics. For MIEC, the oxygen reduction reaction
is not limited to the triple phase boundary (TPB), but occurs in the entire surface exposed
to the gas phase. The oxygen surface exchange activity is strongly influenced by the
surface composition and atomic surface structure where the adsorption and dissociation
of molecular oxygen take place. These factors may influence the rate-determining step of
the oxygen exchange process (1) and, consequently, the ultimate efficiency of the solid
oxide fuel cell (SOFC) device. Compared to bulk samples, the oxygen transport kinetics
in thin-film samples is significantly dominated by the surface process (2). It is therefore
of great importance to understand the surface and near-surface chemical compositions
and the structure (or termination) of these films.
In this work, a series of Lan+1NinO3n+1 (n = 1, 2 and 3) epitaxial films have been
deposited using pulsed laser deposition (PLD) (3) to investigate their surface composition
and its influence on the surface exchange properties using high-sensitivity low-energy ion
scattering (HS-LEIS). These investigations will aid in the understanding of the
electrochemical properties of lanthanum nickelate cathode materials in surface regions
(4).
The compositional analysis for the outermost and near-surface of these obtained films
was also studied. In previous studies performed on La2NiO4 polycrystalline materials (5),
we reported that the surface chemistry of the RP phase is significantly different from the
bulk due to cation segregation and subsequent near surface rearrangements at high
temperatures relevant for materials processing (e.g. 1000°C). This study reveals that the
epitaxial thin films undergo surface reconstruction even during the deposition process
due to the PLD deposition temperature used (6). This surface and near-surface
rearrangement during the deposition process of the thin-film materials may decrease the
electrochemically active surface area, having an impact on the surface exchange kinetics
and mechanism.
Experimental
Epitaxial films of RP-type Lan+1NinO3n+1 (n = 1, 2 and 3) were grown on NdGaO3 (110)
single crystal substrates by using a Neocera PLD system utilizing KrF excimer laser. The
related deposition conditions and characterization of the thin films have been reported
and discussed in literature (3). The deposition temperature was kept at 700-780°C. The
oxygen pressure in the vacuum chamber was maintained at 250 mTorr. The deposited
films were cooled in oxygen pressure of 600 Torr with a cooling rate of 10°C/min.
The chemical composition of the outer surfaces of the as-deposited films was
investigated by Low Energy Ion Scattering (LEIS) spectroscopy, which is an ideal tool
for probing the elemental composition at the near-surface and, particularly, the outermost
monoatomic layer of materials (7). Prior to the LEIS measurements, the RP films were
oxidized by exposure to reactive atomic oxygen at room temperature for a period of ~10
min in order to remove any adventitious organic contaminants or adsorbed moisture from
the atmosphere. The cleaning procedure was repeated until the absence of carbon was
confirmed by analysis using 3 keV He+ ion beam. Once this cleaning process was
completed, a fresh area was selected for LEIS surface analysis on the outmost surface.
Additionally, depth profiling analysis was also performed to obtain information about
the near-surface region. Depth profiling analyses were performed in a dual-beam mode
using a 0.5 keV Ar+ ion beam for low-energy sputtering incident at 59o and a 5 keV Ne+
beam for the analysis of the cation composition, as Ne+ beam provides better peak
separation (i.e. mass resolution) for heavier elements than the 3 keV He+ ion beam.
Results and Discussion
The obtained films were confirmed to show the intended RP phases, with low surface
roughness and expected (averaged) chemical composition measured via high-resolution
X-ray diffraction measurements, atomic force microscopy and energy dispersive X-ray
spectroscopy, respectively. These results have been discussed in a previous publication
(3).
The surface composition of the oxidized films after the cleaning protocol can be seen
in Figure 1, confirming that all organic contamination was removed, as indicated by the
absence of a carbon peak at 846 eV. As observed, the surface of the La2NiO4+δ thin film
(Figure 1 (a)) was dominated by the La and O peaks after the PLD deposition, as
previously reported for polycrystalline La2NiO4 after annealing at 1000 ºC (5). However,
for the La3Ni2O7-δ (Figure 1 (b)) and La4Ni3O10-δ (Figure 1 (c)) films, some Ni was
detected at the outer surface of the films.
Figure 1. LEIS energy spectra of as-deposited (a) La2NiO4+δ, (b) La3Ni2O7-δ and (c)
La4Ni3O10-δ epitaxial films performed on the outer surface by 3 keV He+ analysis beam
after cleaning with atomic oxygen plasma.
The expected peak positions for Na and K/Ca (which would not be resolved by 3 keV
He+ scattering) are also marked in Figure 1 (a). Although these elements were detected
for the films after annealing, they are not detectable in the as-deposited samples. The
characterization of the films after annealing will be reported in a forthcoming work.
The LEIS spectra obtained by 5 keV Ne+ analysis of the outer surface of the series of
Lan+1NinO3n+1 (n = 1, 2 and 3) epitaxial films are shown in Figure 2. The 5 keV Ne+ beam
provides better peak resolution between the cation surface peaks and sensitivity for
heavier species (i.e. La and Ni), compared to the spectra obtained using the 3 keV He+
beam. The results reveal that the immediate surface of these as-deposited samples is
dominated by A-site cations, showing a predominant LaO-termination for the RP thin
films after the deposition. Nevertheless, as discussed for Figure 1, the high RP index
films (n=2 and 3), showed a mixed termination at the outermost atomic layer.
Figure 2. LEIS spectra of as-deposited (a) La2NiO4+δ, (b) La3Ni2O7-δ and (c) La4Ni3O10-δ
epitaxial thin films. Spectra were collected from the outer sample surface using a 5 keV
Ne+ analysis beam.
Figure 3 shows the LEIS depth profile analysis of the series of as-deposited film
samples. Due to the absence of matrix effects, the intensity of a peak in a LEIS spectrum
directly reflects the coverage of the corresponding cation (7). The surface coverage of La
and Ni cations was normalized to the plateau signals corresponding to the bulk
composition of the films, outlined by dashed lines in figure 3 (a-c). These profiles show
the apparent cation coverage for the La and Ni cations of each compound compared to
the bulk stoichiometry. As seen in Figure 3, there is a sub-surface Ni enrichment
compared to the bulk composition, suggesting that atomic rearrangements take place
during the film growth (6, 8). Interestingly, it appears that a larger Ni-enrichment is
found for the higher n-index RP phase. These results of LaO-termination are also in good
agreement with recent experimental studies in the compounds of Lan+1NinO3n+1 (n = 1, 2
and 3) powder (9), La2-xSrxNiO4+δ single crystal (10), and Pr2-xLaxNiO4+δ and La2NiO4+δ
polycrystalline ceramics (5, 11); the ones of Ni-enrichment are in agreement with the
studies in the La2NiO4+δ polycrystalline ceramics (5).
Figure 3. LEIS depth profiles of as-deposited (a) La2NiO4+δ, (b) La3Ni2O7-δ and (c)
La4Ni3O10-δ epitaxial films, showing the cation coverage at the surface and near-surface
region.
Due to entropy, the surface of a real material at our temperatures of interest is
thermodynamically unlikely to present a perfect structure, which implies that a certain
amount of defects (both point defects such as anion and cation vacancies, as well as
extended defects such as dislocations) must exist. These defects may facilitate the
occurrence of reconstruction during the cooling process in an atmosphere of high oxygen
partial pressure (600 Torr) after deposition. Indeed, recent research showed evidence for
dynamic layer rearrangement for the series of thin-film materials during growth the by
molecular beam expitaxy method (6). Whilst the former work was observed via in-situ
synchrotron X-ray scattering, our LEIS findings presented here add further evidence for
such layer rearrangement for the RP epitaxial films during deposition.
Conclusion
Using the technique of highly sensitive LEIS, significant deviations from the bulk
composition have been observed for the surface and near-surface region of a series of asdeposited Lan+1NinO3n+1 (n = 1, 2 and 3) epitaxial films grown by PLD. The results show
predominant LaO-termination and Ni-enrichment at the outermost atomic surface and
sub-surface region, respectively. Such predominant surface termination may have
profound implications for the surface catalytic activity of the series of RP-type materials.
From previous theoretical studies (12, 13), the transition metals (NiOx-termination in this
case) are usually assumed to actively participate in the oxygen reduction reaction with
oxygen gaseous species. However, studies by LEIS, which is uniquely capable of
determining the composition of the very outer atomic layer, consistently show that these
transition metals are hidden behind the first atomic surface layer. This would imply the
presence of point defects such as oxygen vacancies. These oxygen vacancies could
provide oxygen molecules access to the NiO-sites and facilitate the oxygen reduction
reaction. Therefore, it is also important to understand how these surfaces are modified
under the operational conditions of high temperature and pO2. Further work will be
addressed to study these PLD films after annealing in order to obtain a better
understanding of these promising materials under high-temperature operational
conditions.
Acknowledgments
The authors gratefully acknowledge Dr. Richard Chater (Imperial College London) for
useful discussions, and Dr. Peter K Petrov and Dr. Bin Zou (Imperial College London)
for assistance with PLD system. We also gratefully acknowledge the support of the
International Institute for Carbon Neutral Energy Research (wpi-I2CNER). H.T. also
acknowledges the financial support of the Japanese Society for Promotion of Science for
her JSPS postdoctoral fellowship and the Kakenhi Grant-in-Aid project (25-03770).
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