Low Temperature Formation of Ruddlesden–Popper-Type Layered La2CoO4±δ Perovskite Monitored via In Situ X-ray Powder Diffraction

Article abstract of DOI:10.1002/ejic.201801162

In this contribution low temperature formation of Ruddlesden–Popper (RP)-type layered La₂CoO_4±δ perovskite was optimized via in situ X-ray powder diffraction (XRPD). Starting from LaCoO₃ a stoichiometric transformation to La₂CoO_4±δ and CoO can be achieved by controlled reduction with H₂. The challenge of this reaction is the use of appropriate amounts of H₂ in a defined temperature region. If the amount of H₂ is too high, complete reduction of the perovskite occurs. If temperatures are not appropriate, intermediate phases seem to hinder the transformation to La₂CoO_4±δ or lead to a complete decomposition to simple oxides. Based on in situ XRPD experiments, the temperature window and required amount of H₂ for the transformation of LaCoO₃ to La₂CoO_4±δ were determined. Systematic experiments reveal that 650 °C is the optimal temperature for the complete transformation of LaCoO₃ into La₂CoO_4±δ and CoO/Co⁰. The information was then transferred to realize bulk synthesis of La₂CoO_4±δ at 650 °C in a tube furnace without extended heat treatments at elevated temperatures.

Full text of DOI:10.1002/ejic.201801162

A Journal of
Accepted Article
Title: Low Temperature Formation of Ruddlesden-Popper-Type
Layered La₁CoO4±δ Perovskite Monitored via in situ X-ray
Powder Diffraction
Authors: Seyma Ortatatli, Jan Ternieden, and Claudia Weidenthaler
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201801162
Link to VoR: http://dx.doi.org/10.1002/ejic.201801162

European Journal of Inorganic Chemistry
10.1002/ejic.201801162
FULL PAPER
employed as electrode materials in solid oxide fuel cells
[
8,9,11-15]
[16,17]
(
SOFCs),
rechargeable metal-air batteries,
as well as
Low Temperature Formation of
Ruddlesden-Popper-Type
[
18]
[19]
solar cell materials and oxygen separation membranes. The
physicochemical properties of pure or cation-substituted
2
La MO4+δ (M = Ni, Cu) compounds have been comprehensively
studied. Due to their high oxygen ion conductivity, they are
interesting cathode materials particularly for intermediate
Layered La CoO Perovskite
2
4±δ
[
5,9,11,20-24]
temperature (IT: 600<T<800 °C) SOFC applications.
Monitored via in situ X-ray
Cationic substitution as well as the oxygen content of RP-type
perovskites change not only their structural but also their
electronical properties significantly which is favorable for IT-
Powder Diffraction
[
10, 20, 25, 26]
SOFCs.
Beside energy-oriented applications, such as
[a]
[a]
Şeyma Ortatatlı, Jan Ternieden, and Claudia
in SOFCs, RP-type perovskites are used as catalysts in
numerous heterogeneously catalyzed reactions. Among others,
[a]
Weidenthaler*
[
27]
they are used as catalysts for oxidation of CO, decomposition
[
28]
[29]
[30]
Abstract: In this contribution low temperature formation of
Ruddlesden-Popper (RP)-type layered La CoO4±δ perovskite was
optimized via in situ X-ray powder diffraction (XRPD). Starting from
LaCoO a stoichiometric transformation to La CoO4±δ and CoO can
be achieved by controlled reduction with H . The challenge of this
reaction is the use of appropriate amounts of H in a defined
temperature region. If the amount of H is too high, complete
2
of NO, combustion of methane, decomposition of N O, or
[
31-34]
oxygen evolution reaction (OER).
2
3
2
2
2
2
reduction of the perovskite occurs. If temperatures are not
appropriate, intermediate phases seem to hinder the transformation
to La
Based on in situ XRPD experiments, temperature window and
required amount of H for the transformation of LaCoO to La
were determined. Systematic experiments reveal that 650 °C is the
optimal temperature for the complete transformation of LaCoO into
CoO4±δ and CoO/Co . The information was then transferred to
realize bulk synthesis of La CoO4±δ at 650 °C in a tube furnace
2
CoO4±δ or lead to a complete decomposition to simple oxides.
2
3
2
CoO4±δ
3
0
La
2
2
without extended heat treatments at elevated temperatures.
Introduction
The interest in Ruddlesden-Popper (RP)-type layered
perovskites began after the studies on the structure of
potassium nickel fluoride (K
Plieth in 1955. Later on, Ruddlesden and Popper introduced
2
NiF
4
) single crystals by Balz and
[
1]
[2]
Sr
2
TiO
4
, Ca
2
4
MnO , and SrLaAlO
4
to be isostructural to K
2
4
NiF .
Today, materials possessing K
2
NiF
4
structure are referred to as
2 4
Figure 1. Crystal structure of an RP-type (A BO ) tetragonal perovskite for
n = 1. Color codes of the atoms: (gray) A, (blue) B, (red) O.
“
RP-type”. The general formula of an RP-type perovskite can be
given as An+1BO3n+1 or AO(A BO3n) perovskite layer sandwiched
n
between the rock-salt type layers (Fig. 1).
RP-type layered perovskites attract great attention due to
their mixed ionic-electronic conductivity (MIEC) and excellent
In general, conventional solid-state reactions performed above
[4,13,35-
1
000 °C are used for the synthesis of RP-type perovskites.
[
3-10]
4
1]
oxygen-ion diffusion as well as proton conductivity.
They are
However, also other methods are used including sol-gel
[42]
[41,43]
[15]
synthesis,
co-precipitation,
spray
pyrolysis,
[44]
[24,45]
hydrothermal flow synthesis,
or combustion methods.
[
a]
Dr. S. Ortatatli, J. Ternieden, Dr. C. Weidenthaler
These methods are usually implemented in combination with
high temperature treatments (≥ 1000 °C). On the other hand, Du
et al. reported the low temperature synthesis of several RP-type
Max-Planck-Institut für Kohlenforschung, Department of
Heterogeneous Catalysis, Kaiser- Wilhelm-Platz 1, D-45470
Mülheim an der Ruhr
E-mail:weidenthaler@mpi-muelheim.mpg.de
Homepage:http://www.kofo.mpg.de
x 2 7
La Ca3-xMn O compounds, starting from mixing metal nitrate
solutions with citric acid, subsequent calcination at 700 °C
[
29]
Supporting information for this article is given via a link at the end of
the document.
followed by hydrothermal treatment.
Diffraction patterns
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European Journal of Inorganic Chemistry
10.1002/ejic.201801162
FULL PAPER
showed the presence of impurities such as La
Weng et al. achieved the formation of La NiO
2 4
2
O
3
. In addition,
via a continuous
hydrothermal flow synthesis (CHFS) after heat treatment at
(Fig. 2, inset). Rietveld refinements performed prior to the in situ
experiments are summarized in the Supporting Information (Fig.
S1).
7
00 °C for 3h which was claimed to be the lowest temperature
[
46]
used for the synthesis of La
2
NiO
4
.
In fact, the synthesis of an
RP-type perovskite can be rather challenging, depending on the
desired chemical composition. In this regard, RP-type La
perovskite has been less in the focus of scientific reports
2
CoO4+δ
[
38]
compared to its Ni or Cu analogues,
significant amounts of oxygen up to
spontaneous topotactic oxidation at room temperature in air.
This makes La CoO4+δ a very good candidate as a cathode
material for IT-SOFCs. The main drawback in the synthesis of
La CoO4±δ is mainly caused by the extremely easy oxidation of
although it can tolerate
a
δ
of 0.16 upon
[35]
2
[9]
2
2
+
3+
Co to Co resulting in the formation of undesired perovskite
3
+ [38, 47]
compounds containing Co .
oxygen partial pressures (p(O
Therefore, relatively low
2
)) or reductive atmosphere is
[
35]
required for a successful synthesis.
of La CoO4±δ as well as of the A- or B-site substituted analogues
require synthesis temperatures higher than 850 °C in order to
Moreover, the formation
2
[
3, 35, 36, 38, 39, 42, 48-53]
acquire almost phase pure products.
For
Figure 2. Qualitative phase analysis of the in situ XRPD data of the as-
instance, Guo et al. reported the sol-gel synthesis of RP-type
[
42]
synthesized LaCoO
marks represent the reference reflection positions of LaCoO
the Inorganic Crystal Structure Database (ICSD): ICSD-236314,
crystal structure of LaCoO
(space group: R 3� c). The symbol (*) represents the
3
collected in pure N
2
gas at room temperature (RT). Tick
La
temperature of 850 °C did not lead to formation of pure La
but to a mixture of La CoO and LaCoO . The formation of
La CoO was completed only at higher temperature
1100 °C). Similarly, Le Dréau et al., as well as Babkevich
2
XO
4
(X = Ni, Co) perovskites. They show that the synthesis
3
as obtained from
2
4
CoO ,
[55]
(inset)
2
4
3
3
perovskite reflections coming from secondary W radiation. Color codes of the
atoms: (gray) La, (blue) Co, and (red) O.
2
4
[
42]
[38]
(
[39]
et al. performed solid-state reactions with extended heating at
very high temperatures (>1000 °C) for 24 or 48 h to produce
Figure 3 shows the bright field scanning transmission electron
microscopy (BF-STEM) image and energy dispersive X-ray
spectroscopy (EDX) analyses of the as-synthesized LaCoO .
3
The average particle size is in the range of 30-35 nm (Fig. 3a).
The nanoparticles are agglomerated due to sintering at elevated
temperature (700 °C). EDX analyses in Fig. 3b and c indicate a
homogeneous distribution of lanthanum and cobalt.
2 2 4
La CoO4+δ single crystal or La CoO polycrystalline material,
respectively.
The most convenient strategy to investigate structural
changes of a material or formation of new phases under non-
ambient conditions is the use of in situ structure characterization
techniques. In this respect, in situ diffraction is an ideal tool.
Several papers reported in situ diffraction studies on structural
[
38]
phase transitions of La
2
CoO4+δ single crystals
or of
[
36]
polycrystalline La
transitions of La
calcination were also reported. However, the formation of non-
substituted polycrystalline La CoO4±δ at temperatures as low as
50 °C has not been reported so far.
The main goal of this work was to establish a strategy
for a low temperature synthesis of an RP-type La
controlled reduction of LaCoO . In order to mimic the reaction
2
CoO4+δ.
In addition, the structural phase
[
50]
[54]
2
CoO4-δ
or Ni-substituted La
2
CoO4±δ
upon
2
6
Figure 3. Investigation on the morphology and the elemental analysis of the
3
as-synthesized LaCoO : (a) BF-STEM image, (b, c) EDX mapping analyses:
2
CoO4±δ by
(b) La (6.3 at%, green), and (c) Co (4.2 at%, red).
3
conditions with respect to temperature and gas flow, we
performed in situ X-ray powder diffraction (XRPD) experiments
under controlled reductive atmosphere at different temperatures.
The N
LaCoO
2
gas adsorption-desorption isotherms of as-synthesized
3
are shown in the Supplementary Information (Fig. S2).
2
-1
A specific surface area of 5 m g was calculated with the BET
[
56]
method.
sintering followed by severe agglomeration of particles is the
reason for the low specific surface area of LaCoO
In order to mimic the reaction parameters for the formation of
the as-synthesized LaCoO was heated under
protective atmosphere to different temperatures (550, 650, and
50 °C) and then controlled reduction was performed. First of all,
the sample was heated in pure N gas from room temperature
Since the synthesis temperature was 700 °C,
Results and Discussion
3
.
In Figure 2, the XRPD pattern of the phase-pure LaCoO
synthesized at 700 °C via hard templating method is shown. The
crystal structure of LaCoO exhibits a rhombohedral symmetry
with the space group R3c and consists of eight corner-sharing
CoO octahedral units with one La ion positioned in the center
3
La
2
CoO4±δ
,
3
3
�
7
6
2
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European Journal of Inorganic Chemistry
10.1002/ejic.201801162
FULL PAPER
(
2
r.t.) to the according temperatures with a heating rate of
After exposure to the H
550 °C, formation of some intermediate phase(s) was observed
(pattern 4). These are most probably oxygen deficient, so-called
2
/N
2
gas stream for about 15 min at
-1
0 °Cmin . After reaching the desired temperatures, the gas
-1
atmosphere was changed to a continuous flow (10 mLn min ) of
a 10% H /90% N gas mixture. In the next step, diffraction
2
2
brownmillerite-type, La
La Co Co
Co-O compounds propose the formation of La
LaCoO under controlled p(O ) at temperatures ≥1100 °C.
La Co 10±δ belongs to the family of RP-type perovskites (with n
= 3) and was investigated previously for its electronic and
2
Co
perovskites. The phase diagrams for La-
Co
2
O
5
or La
3
Co
3
O
8
and/or RP-type
patterns were collected under isothermal conditions. The
reduction is a very fast process which is not easy to monitor with
in-house diffractometers. Therefore, for monitoring the reduction,
data were collected in a small 2θ range between 28 and 40°.
This reduced data collection time per dataset to 5 min. After
finishing the reduction, diffraction data for Rietveld refinements
were collected over a larger 2θ range.
4
3
O
10 or La
4
3 9
O
4
3
O10 from
[
57-59]
3
2
4
3
O
[57, 60-63]
magnetic properties as well as its kinetic stability.
triple layered (n = 3) perovskite that can form during a
transformation from LaCoO to La CoO4±δ under reductive
atmosphere, as proposed by reaction 1:
It is a
3
2
[
57]
4
LaCoO
According to in situ XRPD studies reported by Radovic et al.,
commercial LaCoO was heated up to 900 °C in a gas mixture of
Their study showed the formation of some
brownmillerite-type perovskites such as La Co and La Co
in a temperature range between 375-425 °C. The formation of
La Co 10 was observed at 425 °C, while the sample was
completely decomposed to La and reduced to metallic Co
and Co were stable upon heating to
It is important to note that La Co 10±δ compounds
3
→ La
4
Co
3
O
10±δ + CoO + (1-(±δ)/2) O
2
(reaction 1)
3
[
66]
2 2
4% H /96% N .
3
3
O
8
2
2 5
O
4
3
O
2
O
3
0
0
(
2 3
Co ) at 600 °C. La O
[
66]
900 °C.
4
3
O
are known to be thermodynamically unstable below ca. 1400 °C
[
57]
in air.
dependent on the temperature and p(O
phase diagram reported by Adachi et al., a precisely controlled
4 3
The oxygen stoichiometry of La Co O10±δ is highly
[60]
2
).
According to the
[57]
p(O
2
) is required to stabilize La
Not only the structures of intermediate RP-phases, such as
Co 10, but also the structure of La CoO depend strongly on
the p(O as oxygen stoichiometry changes with p(O ). The
crystal structure of La CoO changes as function of p(O ) and
In this regard, mainly four different phases are
known to exist for La CoO . These are high temperature
4 3 10
Co O .
La
4
3
O
2
4
[63]
2
),
2
2
4
2
[
38,54]
temperature.
2
4
tetragonal (HTT), low temperature orthorhombic (LTO), low
temperature tetragonal (LTT) and high temperature
[
36]
orthorhombic (HTO) phases.
In addition, Le Dréau et al.
reported another phase transition from LTO to high temperature
less orthorhombic (HTLO) phase in the temperature range
Figure 4. Temperature-dependent in situ XRPD patterns of the as-synthesized
LaCoO heated to 550 °C. After reaching the desired temperature, data were
3
collected isothermally. Data collection time per patterns was 5 min. Each
pattern is numbered for the sake of simplicity in the discussion. The reference
[
38]
between 140-160 °C.
In Figure 5 the crystal structures of
LaCoO and the RP-type oxygen stoichiometric HTT- and LTO-
3
reflection positions were obtained from ICSD as follows: LaCoO
3
: ICSD-
La
2
CoO
4
are shown. The crystal structure of LaCoO
3
is
[
55]
[54]
236314,
HTT-La
2
CoO4±δ
:
ICSD-245657;
LTO-La CoO4±δ: ICSD-
2
�
[
64]
[65]
rhombohedral with space group R3ꢀ (Fig. 5a), whereas La
2
CoO
4
2 3
173514; La O : ICSD-56771 . Patterns 1 and 22 were recorded in pure N
2
at r.t., while patterns 2 to 15 were recorded under reductive atmosphere at
50 °C and patterns 16 to 21 were recorded under pure N at 550 °C.
can crystallize in various symmetries depending on temperature
treatments. Therefore, determination of the space group is not
5
2
2 4
straightforward. It is known that the HTT-phase of La CoO
crystallizes in tetragonal symmetry with space groups I4/mmm or
In Figure 4, temperature-dependent in situ XRPD patterns
are shown which display significant structural changes of the as-
synthesized LaCoO under reductive atmosphere at 550 °C.
P4 /nmc (Fig. 5b), while the LTO-phase crystallizes in
2
orthorhombic symmetry with the space groups Bmab or Pmcb
[
50]
3
(Fig. 5c).
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European Journal of Inorganic Chemistry
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FULL PAPER
In the next experiment, LaCoO
under N
3
was heated from r.t. to 650 °C
atmosphere (Fig. 6, patterns 1 and 2). The significant
2
peak shift to lower 2θ values in pattern 2 compared to pattern 1
is due to thermal expansion at elevated temperatures. After data
collection at 650°C under pure N
2
, the atmosphere was switched
to H /N (patterns 3 to 11). During exposure to reductive
2
2
atmosphere for about 10 min, formation of some intermediate
phase(s) was observed. However, those phases disappeared
upon further reduction at 650 °C. Pattern 5 in Fig. 6 shows the
rapid formation of a new phase, as two new reflections emerged
at 30.9 and 32.2° (2θ). The positions of these two new
Figure 5. Crystal structures: (a) LaCoO
I4/mmm), and (c) LTO-La CoO (space group: Bmab). Color codes of the
atoms: (gray) La, (dark blue) Co, and (red) O.
3 2 4
, (b) HTT-La CoO (space group:
reflections indicated the formation of HTT-La
proposed to occur according to reaction 2. The formation of
HTT-La CoO4±δ was completed at 650 °C after about 15-20 min
2
CoO4±δ which is
2
4
2
of reduction.
2 4 3
The crystal structure of La CoO consists of one LaCoO
perovskite layer positioned in the center of the unit cell along the
crystallographic c-axis which is squeezed between intercalating
LaO layers (Fig. 5b, c). In our study, the crystal structure of
2
La CoO4±δ formed at 650 °C is in accordance with the HTT-
phase (space group I4/mmm), while the structure obtained after
cooling to r.t. is likely the LTO-phase (space group Bmab). Exact
identification of the intermediate phase(s) is not straightforward
in the current study due to the complicated crystal structures of
the RP-phases caused by non-stoichiometric oxygen contents.
Additionally, the positions of the reflections of the intermediate
phases are located relatively close to each other and reflection
broadening complicates the situation as well. Only neutron
diffraction experiments (not subject of the present study) would
allow assigning the intermediate phases more accurately with
2
respect to oxygen content of La CoO4±δ.
Starting the reduction at 550 °C, transformation from
unassigned intermediate phase(s) to high temperature
tetragonal (HTT) La
min of exposure to reductive atmosphere (Fig. 4, pattern 6). The
formation of La CoO4±δ is proposed to occur according to
2
CoO4±δ is observed after approximately 20
2
[
57]
reaction 2:
La Co O10±δ → 2La CoO4±δ + CoO + (1-(±δ)/2) O (reaction 2)
4
3
2
2
The reflection intensities of HTT-La
while those of the intermediate phases decreased accordingly
patterns 6 to 15). On the other hand, La was detected
starting from pattern 13. This corresponds to 60 min treatment
under reductive atmosphere and is evidence for the
decomposition of La CoO4±δ. Although HTT-La CoO4±δ was
2 2
obtained already at 550 °C under controlled gas atmosphere,
the presence of intermediate phases seem to hinder the
2
CoO4±δ increased with time,
Figure 6. Temperature dependent in situ XRPD patterns of the as-synthesized
LaCoO performed during heating to 650 °C. After reaching the temperature,
3
data were collected isothermally. Data collection time per patterns was 5 min.
Each pattern is numbered for the sake of simplicity in the discussion. Patterns
(
2 3
O
1 and 2 were recorded in pure N
to 11 were recorded under reductive atmosphere at 650 °C, while pattern 12
was recorded under pure N at r.t..
2
at r.t. and at 650 °C, respectively. Patterns 3
2
complete formation of HTT-La
La
was changed back to pure N
be stopped and in the end, a mixture of La O , intermediate RP-
2 3
phases and low temperature orthorhombic (LTO) La
2
CoO4±δ. Instead, decomposition to
O is observed. After 70 min (pattern 21), the atmosphere
3
Rietveld refinements of a dataset collected in situ at 650 °C
provide information about the phase composition of the sample
2
2
but the decomposition could not
(
Fig. 7). From the refinement a phase composition of 88 wt%
0
2
La CoO4±δ and 12 wt% Co was obtained. Formation of metallic
2
CoO4±δ was
Co is an indication that CoO, which forms during the reduction,
is further reduced (reactions 3). The values obtained correspond
obtained (pattern 22).
exactly to the theoretical composition expected for
a
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European Journal of Inorganic Chemistry
10.1002/ejic.201801162
FULL PAPER
stoichiometric reduction of LaCoO
3
to the RP phase and metallic
transferred to the ex situ synthesis of La
tube furnace. In order to probe the time required for H
100 mg of pure LaCoO (Fig. 8a, Fig. S6) were heated in a
continuous N flow to 650 °C. Then the gas changed to a
2 2
mixture of 10% H /90% N (with a flow rate of 10 mLn min ). Fig.
2
CoO4±δ in a laboratory
Co (reaction 3b). The formation of either CoO or metallic cobalt
is expected for the stoichiometric reduction reaction of
2
dosing,
3
[
67, 68]
LaCoO
3
.
If the reduction stops at CoO (reaction 3a), 15.8
CoO4±δ need to be formed. Even
2
-1
wt% of CoO and 84.2 wt% La
2
0
though the intensities of the CoO and/or Co are low and
correspond only to max 16 wt% CoO and 13 wt% Co , one
8 shows the ex situ XRPD patterns and the corresponding
qualitative phase analyses. It is evident that the formation of
0
should note that on a molar basis 1 mole of CoO is formed per
La
after 10 min, 68 wt% La
8 wt% LaCoO remained (Fig.8b, Fig. S7). Increasing dosing
time to 15 min leads to about 78 wt% La CoO4±δ, CoO (5 wt%),
and metallic Co (2 wt%), and 15 wt% remaining LaCoO (Fig. 8c,
Fig.S8). After 17 min dosing LaCoO is completely reduced and
2
CoO4±δ is influenced significantly by H
2
dosing time. Already
mole of La
2
CoO
4
.
2
CoO4±δ and 4 wt% CoO have formed.
2
3
2
2
LaCoO
LaCoO
3
→ La
→ La
2
CoO4±δ + CoO + (1-(±δ)/2) O
2
(reaction 3a)
(reaction 3b)
2
0
3
2
CoO4±δ + Co + (2-(±δ))/2) O
2
3
3
The data also show the presence of an additional reflection
which cannot be assigned to any of the known brownmillerite or
RP-phases. Since the reflection is not present after cooling
anymore, the formation of an additional, so far unknown high
temperature metastable phase can be assumed.
the phase composition has changed to 88 wt% La
wt% CoO and 5 wt% metallic Co (Fig. 8d, Fig. S9).
2
CoO4±δ, 7
Figure 7. Rietveld refinement plot of data collected at 650 °C under reductive
atmosphere. The star indicates the presence of an unidentified phase which is
0
[69]
2
only present at 650 °C. Tick marks: upper HTT- La CoO4±δ, lower Co
After finishing the reduction, the gas atmosphere was switched
back to pure N , the sample was cooled to r.t., and data were
2
collected accordingly (pattern 12, Fig. 6). Rietveld refinements of
the data indicated the transformation from HTT to LTO-
La CoO4±δ after cooling in pure N (Supporting Information
2 2
Fig.S3). Some of the HTT RP-phase remained but disappeared
after exposure of the sample to air (not shown here). In the
absence of intermediate phase(s), the sample is stable as RP-
type La
phase(s) destabilizes the structure of La
sample oxidizes back to LaCoO under atmospheric conditions
Supplementary Information, Fig. S4) or completely decomposes.
The in situ studies evidence the important influence of
2
CoO4±δ. On the other hand, the presence of intermediate
Figure 8. Ex situ XRPD patterns collected after reduction of LaCoO
3
in a tube
2
CoO4±δ so that the
furnace showing the influence of H dosing time on the formation of La CoO4±δ.
2
2
3
From bottom to top: (a) before dosing at r.t., after dosing at constant
temperature (650 °C): (b) 10 min, (c) 15 min, (d) 17 min, and (e) 20 min.
Crystal structure data for CoO were taken from ref [70].
(
reduction time on the formation of the desired RP-phase. As
expected, reduction at higher temperature (750 °C) is faster but
since low reaction temperatures are preferred, this experiment
will not be discussed here. Nonetheless, in situ XRPD data
collected at 750 °C under controlled atmosphere can be found in
the Supplementary Information (Fig. S5).
The results for the samples reduced in a tube furnace confirm
the data obtained by in situ XRD experiments. The formation of
La
after about 15-20 min. Dosing H
decomposition of the sample that is visible by small La
reflections (Fig. 8e). From these results it can be concluded that
2
CoO4±δ was completed at 650 °C for a given flow of hydrogen
2
for 20 min caused partial
2
O
3
Based on the results obtained from the in situ studies,
information on H dosing time as well as temperature was then
2
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European Journal of Inorganic Chemistry
10.1002/ejic.201801162
FULL PAPER
-
1
dosing of 10% H
1
2
/90% N
2
(with a flow rate of 10 mLn min ) for
and/or Co does not negatively affect the properties of the
material for a specific application, the low temperature synthesis
reported in this paper is more convenient and practical
compared to what has been reported elsewhere for the
7 min is an upper limit for the successful formation of
La CoO4±δ under the experimental conditions used in this work.
2
Further dosing may cause a full decomposition and reduction of
the product.
synthesis of polycrystalline La
Nonetheless, it is important to note that the crystal structure of
CoO4±δ is governed by temperature treatment as well as
2
CoO4±δ.
La
2
oxygen stoichiometry, as discussed before. From XRPD data, it
is very difficult to assign the oxygen stoichiometry for the in situ
and the ex situ synthesized La CoO4±δ in the present work. As a
2
consequence, determination of the oxygen stoichiometry using
neutron diffraction is mandatory, although the mobility of oxygen
ions is pronounced at high temperatures which can make the
2
analysis of in situ diffraction data of La CoO4±δ rather difficult.
Figure 9. (a) STEM image of the sample obtained after reduction for 17 min
and (b) overlay of the elemental distribution of La and Co as obtained by EDX
for the same agglomerate (green La-Co phase, red Co-containing phase).
Experimental Section
Synthesis of LaCoO
3
In order to investigate the distribution of La and Co after the
reduction experiment, high resolution scanning transmission
electron microscopy (STEM) was performed. The images
demonstrate the formation of particles consisting mainly of Co
Hard templating method was used as illustrated in Fig. 10. A commercial
porous carbon (Carbotech, Rütgers 1407) with a specific surface area of
2
-1
3 -1
1200 m g and a pore volume of 0.81 cm g was used as the template.
The metals were introduced into the pores of the carbon host via hard
(
2
CoO and/or Co) on the surface of La CoO4±δ (Fig. 9). An
[
71, 72]
templating method, as described by Schwickardi and co-workers.
overlay of the element distribution determined by EDX analysis
shows areas with a more homogeneous La-Co distribution
The details regarding the amounts and the concentrations of the metal
nitrate precursors are provided in Table 1. In brief, 5 g of porous carbon
was impregnated with solutions of the mixed precursors upon continuous
stirring. The impregnated sample was first dried overnight at 50 °C and
then calcined in air in a muffle furnace according to a fixed temperature
program. First, the samples were heated to 350 °C (heating rate of
(
2 4
green) belonging to La CoO and Co-containing (red) particles.
EDX analyses of the particles shown in red reveal that they
consist mainly of Co and/ or CoO.
-1
2
°C min ) and kept at this temperature for 2 h. Then, the temperature
-1
was increased to 700 °C (heating rate of 0.5 °C min ) and the sample
was kept at this temperature for 20 min. Afterwards, the sample was left
in the furnace to cool to room temperature.
Conclusions
In this work, we present the design of
temperature synthesis method for an RP-type layered La
perovskite. Starting from LaCoO , a stoichiometric reduction
reaction to La CoO4±δ, CoO and/or metallic Co takes place. In
a practical low
2
CoO4±δ
3
2
this respect, in situ XRPD played a key role for the identification
of optimal synthesis parameters and provided a direct insight
into the formation of La
2 3
CoO4±δ. Starting from LaCoO , the
formation of La CoO4±δ was found to be completed at 650 °C in
2
less than 30 min under in situ reactions conditions while at lower
temperatures intermediates are formed as by–phases. The
presence of intermediate phases caused by too low reaction
2
temperatures was shown to destabilize La CoO4±δ. Additionally,
for a successful reduction, an appropriate amount of hydrogen
has to be calculated. Too high amounts of hydrogen cause
excessive reduction of the RP phases and La O and Co are
2 3
formed. Using hydrogen already at the start of heating will also
lead to a complete decomposition of the perovskites.
3
Figure 10. The synthesis scheme used for the production of LaCoO : hard-
templating method using porous carbon (represented by the gray balls) as the
template. Impregnated carbon was dried at 50 °C overnight in air and
subsequently calcined at 700 °C for 20 min to obtain the product.
The ex situ synthesis experiments in a tube furnace showed
that the formation of La
time at constant temperature. The formation of La
achieved at 650 °C with a defined hydrogen flow rate after
relatively short time. We found that the formation of La
2
CoO4±δ is a matter of hydrogen dosing
2
CoO4±δ was
2
CoO4±δ
was completed in less than 20 min, while further dosing caused
partial decomposition of the sample. If the presence of CoO
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European Journal of Inorganic Chemistry
10.1002/ejic.201801162
FULL PAPER
Fisher Scientific EDX system (NORAN™ System 7) with an UltraDry
EDS detector. The powder sample was embedded in Spurr-resin and
sectioned in slices of 30-50 nm thick. The sample was measured on a Cu
grid with a lacey carbon film.
Table 1. Details about the metal nitrate precursors.
Molar
concentration of
the aqueous
solution
Molecular
weight
Purity
%)
Precursor
Supplier
(
-1
(gmol )
N
2
gas adsorption-desorption isotherms of the as-synthesized LaCoO
were measured using the automated gas sorption system
Quantachrome NOVA 3200e apparatus). The sample was outgassed at
90 °C for about 24 h before the measurement. The measurement was
performed using N gas at liquid N temperature and the data were
3
-
1
(mol L )
(
La(NO
3
)
3
.6H
2
O
Fluka
Chemika
[
a]
≥99.0
433.02
291.03
2.65
2
2
analyzed according to the Brunauer-Emmett-Teller (BET) method.
Co(NO
3
)
2
.6H
2
O
Sigma-
Aldrich
[
b]
>98
3.75
Data visualization and analysis
[a] 1. 898 mL, [b] 1.342 mL: Corresponding amounts of the aqueous
solutions used for 5 g of porous carbon.
The simulation and visualization of crystal structures were performed
[
73]
using the Diamond software. The reference crystallographic data were
obtained from the Inorganic Crystal Structure Database (ICSD). The
references are provided in the figure captions. Rietveld refinements were
2
Bulk synthesis of La CoO4±δ
[
74]
performed with TOPAS5.
1
2
6
00 mg of the as-synthesized LaCoO
5 to 650 °C in a continuous N flow (heating rate of 20 °C min ). At
/90% N gas mixture was flushed into
/N gas mixture was dosed
3
was heated in a tube furnace from
-1
2
-1
50 °C, 10 mLn min of a 10% H
2
2
Acknowledgements
the furnace at constant temperature. The H
2
2
for various time periods, i.e., 10, 15, 17 and 20 min, in order to evaluate
the influence of dosing time on the products. A fresh sample was used
for each experiment. After dosing, the sample was cooled to room
The authors are grateful to Hans Josef Bongard (Max-Planck-
Institut für Kohlenforschung) and Norbert Pfänder (Max Planck
Institute for Chemical Energy Conversion) for the electron
2
temperature under pure N gas.
micrographs and EDX analysis of the as-synthesized LaCoO
This project was funded by the Max Planck Society.
3
.
Characterization
An Anton Paar XRK900 reaction chamber mounted on a PANalytical
X’Pert Pro diffractometer working with CuKα1,2 radiation (λ = 1.5406 Å)
was used for the heating experiments. The powder sample was placed
on a MACOR sample holder with 10 mm diameter. Data were collected in
Bragg-Brentano geometry. A secondary monochromator was mounted
on the diffractometer. Data of the starting compound were collected at
Keywords: Ruddlesden-Popper phases, perovskite phases,
X-ray diffraction, phase transitions
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European Journal of Inorganic Chemistry
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Table of Contents
FULL PAPER
In this work, we present the low
Şeyma Ortatatlı, Jan Ternieden,
temperature
Ruddlesden-Popper
La
in situ X-ray powder diffraction
XRPD) performed under controlled
formation
(RP)-type
CoO4±δ layered perovskite using
of
Claudia Weidenthaler*
Page No. – Page No.
2
Low Temperature Formation of
Ruddlesden-Popper-Type Layered
(
gas atmosphere. After determination
of the hydrogen dosing time and
2
La CoO4±δ Perovskite Monitored via
in situ X-ray Powder Diffraction
temperature
required
for
a
successful synthesis of the RP-type
perovskite, we could achieve the
stoichiometric reduction to the RP
0
phase and CoO and/or Co already
at 650 °C.
This article is protected by copyright. All rights reserved.