Materials Sciences and Applicatio ns, 2010, 1, 39-45
doi:10.4236/msa.2010.12008 Published Online June 2010 (http://www.SciRP.org/journal/msa)
Copyright © 2010 SciRes. MSA
Synthesis and Characterization of LaNixCo1-xO3
Perovskites via Complex Precursor Methods
Grace Rafaela Oliveira Silva1,2, José Carlos Santos1, Danielle M. H. Martinelli3, Anne Michelle Garrido
Pedrosa1*, Marcelo José Barros de Souza2, Dulce Maria Araujo Melo3
1Departamento de Química, Universidade Federal de Sergipe, São Cristóvão, Brazil; 2Departamento de Engenharia Química,
Universidade Federal de Sergipe, São Cristóvão, Brazil; 3Universidade Federal do Rio Grande do Norte, Departamento de Química,
Natal, Brazil.
Email: annemgp@yahoo.com
Received February 7th, 2010; revised April 20th, 2010; accepted April 22nd, 2010.
ABSTRACT
This work presents a study on the synthesis of LaNixCo1-xO3 perovskites via complex precursor methods. Perovskite
oxides with the composition LaNixCo1-xO3 were synthesized by chelating precursor and polymeric precursor methods
using nickel and/or cobalt nitrates, lanthanum nitrate, ethylene glycol, citric acid, and EDTA as starting source. The
obtained perovskite were characterized by thermogravimetric analysis, infrared spectroscopy, X-ray diffraction and the
morphology of the samples were investigated by N2 adsorption experiments and average medium particle size. TG curves
and FTIR spectra were particularly useful in establishing of the optimal calcination temperature of the precursor
powders. X-ray diffraction patterns revealed the formation of the perovskite structure in all samples prepared by both
synthesis method and after calcinations at 700. The results showed that the preparation method resulted in oxides with
the intended structure. The specific surface area values were influenced by preparation method.
Keywords: LaNixCo1-xO3 Perovskites, EDTA, Complex Precursor Methods
1. Introduction
The perovskite-type oxides have received much attention
in the last years because of their potential application as
electrode materials in solid oxide fuel cells, as gas sensors,
in various interesting reactions, such as in the steam re-
forming and in the dried reforming of hydrocarbons, in the
catalytic combustion and as oxygen-permeable mem-
branes [1-4]. Such materials have various advantages as:
wide variety of composition and constituent elements
keeping essentially the basic structure unchanged, bulk
structure can be characterized well, their surface can be
fairly well estimated taking advantage of this well-defined
bulk structure, their valency, stoichiometry and vacancy
can be varied widely and huge information on physical
and solid-state chemical properties has been accumulated
[5-7].
A perovskite-type mixed oxide can be described by the
general stoichiometric formula ABO3, where A represents
a lanthanide or alkaline earth ion and B a transition metal
ion trivalent, generally. The perovskite lattice can ac-
commodate multiple cationic substitutions with only
small changes since the value of the structure factor (t) is
between 0.75 and 1 [8]. In this structure, the properties are
mainly determined by the occupancy of B sites, which
usually are partially substituted [8-9].
The properties studied for the different perovskite ox-
ides are potentially influenced by the synthesis method,
calcination conditions (temperature, time and atmosphere)
and substitutions of A and/or B sites [5-7,10]. The effects
of these variables have been studied in order to optimize
the performance of the material. The properties of
perovskite can be enhanced by the substitution of lan-
thanum by strontium [11-12] or even cerium [13-14].
However, the catalytic activity of perovskites is mainly
determined by the type of metal that occupies the B sites
and the partial substitution of those sites [8-9,15-17].
Substitutions in the A and/or B sites can be cause the
formation of defects that modify the catalytic properties
[15,18].
Several methods have been used so far to prepare these
materials, such as freeze-drying, spray-drying, co-precipi-
tation, hydrothermal synthesis and combustion of metal-
organic precursors [19-21]. Chelating ligands which con-
tain carboxylate groups or carboxylate and aliphatic
amine groups are essential in the water-soluble complex
precursor synthesis route. Citric acid (containing car-
boxylate groups) or ethylenediaminetetraacetic acid (ED-
40 Synthesis and Characterization of LaNiCo O Perovskites via Complex Precursor Methods
x1-x 3
TA) (containing carboxylate and aliphatic amine groups)
were often used before in the synthesis of perovskite oxide.
In this work, LaNixCo1-xO3 perovskites were synthesized
by polymeric precursor method (PP) and chelating pre-
cursor method (PQ). The effects of chelating agents on the
synthesis procedure and the evolution of crystalline phase
have been investigated. These methods normally involve
three sequential steps: preparing an aqueous solution of
chelated complexes, concentrating the solution to form a
gel, and pyrolyzing the gel to produce an amorphous
oxide powder.
2. Experimental
Synthesis of the perovskites by chelating precursors (PQ)
method:
LaMO3 (M = Ni or Co) perovskites were synthesized by
chelating precursors method using lanthanum, nickel and
cobalt nitrates, citric acid and using ethylenediamine-
tetraacetic acid (EDTA) as chelating agent. Citrate
aqueous solution were prepared from stoichiometric
amounts of La(NO3)3·6H2O, Ni(NO3)2·6H2O and Co(NO3)2
·6H2O and citric acid in 1:2 molar ratio (metal:citric acid).
EDTA aqueous solution was prepared separately by con-
tinuous stirring until complete dissolution. This solution
was added slowly to citrate solutions under constant
mixing for 15 minutes with pH 9. With addition of
EDTA solution the resultant color was changed. Such
solutions were added to obtain a precursor solution in
which M-EDTA molar ratio was 1:1 (M = metal = La and
Ni or Co). After achieving complete dissolution, the re-
sultant solution was dried in a stove at 160 for 3 h. After
the precursor solution was maintained under vacuum
producing a viscous mass. The resultant material was
heated treated at 220 for 30 minutes at a heating rate of
10 /min. The sponge-like green resin was formed. This
solid resin precursor was calcined at 700 for 8 hours at a
heating rate of 10/min.
Synthesis of the perovskites by polymeric precursors
(PP) method:
Perovskite oxides with the composition LaNixCo1-xO3
(0.4 x 0.6; or x = 1) were synthesized using the pre-
cursor polymeric method [22]. Solutions of cobalt citrate
and nickel citrate were prepared from Co(NO3)2·6H2O,
Ni(NO3)2·6H2O and citric acid-AC (molar ratio of 2.5)
under constant stirring at 70 for 30 min. Stoichiometric
contents of La(NO3)3·6H2O solid were mixed with nickel
citrate solutions at 70 for 20 min. The cobalt citrate
solution was then added under similar conditions. After
complete mixing, the temperature was slowly increased to
90, at which point ethylene glycol was added in the
ratio of 60:40 (citric acid: ethylene glycol). The resulting
solution was maintained at that temperature for 2 h under
constant magnetic stirring. A gel was then formed and
subsequently heat-treated at 300 for 2 h resulting in the
precursor powders. These materials were calcined at
700 for 4 h at a heating rate of 10/min.
Characterizations of the perovskites
Thermogravimetric analyses of the precursor powders
were carried out on a Perkin Elmer TGA-7 instrument at a
heating rate of 5/min under air flowing at a rate of 50
cm3/min. The eventual presence of organic material after
calcination was determined by infrared spectroscopy
within the interval from 4000 to 500 cm-1 in an ABB
BOMER instrument model MB104 using KBr pellets.
X-ray diffraction patterns were obtained from a Shimadzu
XRD-6000 diffractometer by scanning the angular range
10 2 90 using CuK radiation ( = 1.5418 Å). The
specific surface area of the powders was measured by
nitrogen adsorption on a NOVA 2000 system. The spe-
cific surface area was estimated by Brunauer, Emmet and
Teller (BET) method. The microstructure of the powders
was revealed observing Au-coated samples under a Phil-
ips ESEM-XL30 scanning electron microscopy set at the
high-vacuum mode.
3. Results and Discussion
Thermal analysis has been found to yield information on
the decomposition temperatures of oxides as also about its
thermal stability [23]. Thermogravimetric curves of LaMO3
(Ni or Co) heated treated at 220 and prepared by PQ
method are shown in Figure 1(a). TG curves showed an
initial mass loss of about 21% in the temperature range of
30-315 that is attributed to water loss, EDTA initial
decomposition and citrate/nitrate decompositions. A
second loss weight of 39% can be observed in the range
315-540 which is due EDTA decomposition given
perovskite oxide, sodium carbonate and sodium oxide.
For comparison, TG/DTG curves for EDTA pure are
shown in the Figure 1(b). The curves shown typically
four regions of mass loss in the temperature range studied.
EDTA thermo decomposition starting at 30and fol-
lowing until 750. In the temperature range of 750-850
there is formation of a stable product that was attrib-
uted to sodium carbonate and/or sodium oxide. The resi-
due of pure EDTA decomposition at 773 for 2 hours
was characterized by XRD (Figure not shown) and was
observed the formation of predominantly sodium car-
bonate. It can be seen from Figure 1a that the decompo-
sition of EDTA molecules in the LaMO3 (Ni or Co) heated
treated at 220 and prepared by PQ method starts at
different temperature than in EDTA pure. TG/DTG curves
of the EDTA residue obtained by calcinations at 773 for
2 hours (Figure not shown) showed an initial mass loss of
about 9% in the temperature range of 80-108 that was
attributed to water loss. A second loss weight of about
90% in the temperature range of 860-1200 was attrib-
uted to decomposition of sodium carbonate, revealing the
predominant presence of this material in the temperature
range of 750 at 860. For the samples heated treated at
220, the complexation of nickel or cobalt and lantha-
Copyright © 2010 SciRes. MSA
Synthesis and Characterization of LaNiCoO Perovskites via Complex Precursor Methods 41
x1-x 3
num ions with EDTA ligand influence on the EDTA in-
teractions and consequently in their decomposition tem-
perature. Therefore, in these samples the temperature
required for EDTA decomposition is at 540 (Figure
1(a)) and the residue is composed perovskite oxide, so-
dium carbonate and sodium oxide. A loss mass observe at
temperature up to 850 is due of sodium carbonate de-
composition.
TG curves of LaNixCo1-xO3 (0.4 x 0.6; x = 1) heated
treated at 300 and prepared by PP method are shown in
Figure 1c. The first step took place at about 30 with the
onset of decomposition of the residual material synthesis
and following until 350. Full decomposition of residual
material occurred between 350 and 650 ºC, followed by
perovskite oxide formation.
For the samples LaNixCo1-xO3 (0.4 x 0.6; x = 1)
calcined at 700 and prepared by PP method, TG curves
not shown loss mass in the range of 30-900.
For the other hand, for the samples LaMO3 (Ni or Co),
calcined at 700 and prepared by PQ method (Figure not
shown), TG curves showed that the thermal decomposi-
tion took place in two steps stabilizing at temperatures
high than 1100. The mass loss observed between
30-140 was attributed to several simultaneous decom-
position reactions of the residual material of the synthesis
process. The samples still showed another decomposition
step above 850 which can be attributed to decomposi-
tion of carbonates compounds formed.
Figure 2(a) shown FTIR spectra for the samples
LaMO3 (Ni or Co), heated treated at 220 and prepared
by PQ method. Several bands were observed in such
spectra with central point at 1654 cm-1, 1473 cm-1, 866
cm-1 and 600 cm-1 which can be attributed to the N-H, C =
O, C-N, N-O and metal-O bonds [24,25]. The bands due
EDTA vibrations still were observed in the calcined
samples at 700(Figure not shown). Such results indi-
cated that the temperature and time of calcinations were
not sufficient for complete decomposition of residual
material of the synthesis process.
FTIR spectra of LaNixCo1-xO3 (0.4 x 0.6; x =1)
heated treated at 300 and prepared by PP method
showed bands between 1643-1538 and at 1410, 1249,
1067 and 642 cm-1 (Figure 2(b)). The bands observed at
1643-1538 and 1410 cm-1 were assigned to the asymmet-
ric and symmetric carbonyl group stretching vibrations of
ionized carboxylate. The bands at 1249 and 1067 cm-1
could be attributed to nitrate ions. These bands were no
longer observed as the materials were calcined at 700,
suggesting the decomposition of the residual material of
the synthesis process.
X-ray diffraction patterns of the precursor powders
prepared by polymeric precursor and chelating precursor
methods revealed the presence of amorphous structures.
The formation of crystalline phases can be seen from the
patterns of powders calcined at 700 (Figure 3). For the
(a)
(b)
(c)
Figure 1. TG curves for (a) LaMO3 (M = Ni or Co); (b)
EDTA and (c) LaNixCo1-xO3
samples LaNixCo1-xO3 (0.4 x 0.6; x = 1) calcined at
700 and prepared by PP method (Figure 3(b)), all the
Copyright © 2010 SciRes. MSA
42 Synthesis and Characterization of LaNiCo O Perovskites via Complex Precursor Methods
x1-x 3
(a)
(b)
Figure 2. FTIR sp ectra fo r (a) LaMO3 ( M = Ni or Co) and (b)
LaNixCo1-xO3
samples consisted of single-phase materials exhibiting the
perov- skite structure, except for a minor phase observed
on the LaNi0.5Co0.5O3 sample. The perovskite structure
was char- acterized by rather intense peaks at 2 = 33.1º,
47.4º and 59.5º. All perovskite peaks were attributed to
rhombohedral unit cells, space group R3c (JCPDS
32-0296). According to XRD data, no significant changes
occurred in the lattice parameters of the perovskite
structure due to partial substitution of Ni for Co in its B
sites. Such result was expected since the ionic radii of Ni3+
and Co3+ are rather similar. XRD also revealed that the
structure of the resulting perovskite is similar to that of
non-substituted lanthanum nickelate or cobalt perovskites
[26-27]. Moreover, powders prepared by the polymeric
precursor method depicted similar structure compared to
the same powders prepared by other methods.
Well-resolved X-ray patterns were obtained for the
samples LaMO3 (Ni or Co), calcined at 700 and pre-
pared by PQ method (Figure 3(a)) clearly suggesting the
formation of a highly crystallized powder. For these
samples, X-ray diffraction patterns showed peaks at 2 =
(a)
(b)
Figure 3. XRD patterns for (a) LaMO3 (M = Ni or Co) and (b)
LaNixCo1-xO3
32.4º, 46.7º and 58.1º which were attributed to perovskite
structure [13]. All perovskite structure peaks identificated
were indexed as rhombohedral system. Peaks referent to
carbonate sodium and sodium oxide also were observed
suggesting that the crystallinity is not a function of the
monophase structure. Carbonate phase also was con-
firmed by FTIR and TG data. Therefore, the perovskite
structure crystallizes from amorphous precursors and
simultaneity occurs the formation of intermediate phase
which can be decomposed at high temperatures. the
LaNi0.5Co0.5O3 sample. The perovskite structure was
characterized by rather intense peaks at 2 = 33.1º, 47.4º
and 59.5º. All perovskite peaks were attributed to rhom-
bohedral unit cells, space group R3c (JCPDS 32-0296).
According to XRD data, no significant changes occurred
in the lattice parameters of the perovskite structure due to
partial substitution of Ni for Co in its B sites. Such result
was expected since the ionic radii of Ni3+ and Co3+ are
rather similar. XRD also revealed that the structure of the
Copyright © 2010 SciRes. MSA
Synthesis and Characterization of LaNiCoO Perovskites via Complex Precursor Methods 43
x1-x 3
resulting perovskite is similar to that of non-substituted
lanthanum nickelate or cobalt perovskites [26,27]. More-
over, powders prepared by the polymeric precursor
method depicted similar structure compared to the same
powders prepared by other methods.
Well-resolved X-ray patterns were obtained for the
samples LaMO3 (Ni or Co), calcined at 700 and pre-
pared by PQ method (Figure 3(a)) clearly suggesting the
formation of a highly crystallized powder. For these
samples, X-ray diffraction patterns showed peaks at 2 =
32.4º, 46.7º and 58.1º which were attributed to perovskite
structure [13]. All perovskite structure peaks identificated
were indexed as rhombohedral system. Peaks referent to
carbonate sodium and sodium oxide also were observed
suggesting that the crystallinity is not a function of the
monophase structure. Carbonate phase also was con-
firmed by FTIR and TG data. Therefore, the perovskite
structure crystallizes from amorphous precursors and
simultaneity occurs the formation of intermediate phase
which can be decomposed at high temperatures.
Table 1 shows the results of the crystallite size of the
perovskite phase. The crystallite size was calculated by
the Scherrer laws. The crystallites are smaller when the
perovskites are synthesized by PQ method when com-
pared with the perovskite synthesized by PP method. In
the Table 1 also is shown the specific surface area values
and average medium particle size for all the perovskites.
For the samples LaMO3 (Ni or Co), calcined at 700 and
prepared by PQ method, the surface area were smaller
than 137 m2/g. These values not are characteristics for the
perovskite type oxides prepared by conventional synthesis
method. This result is a consequence of the presence of
carbonate phase showed in the XRD analysis. From the
results showed in the Table 1 also is possible to see that
with the substitution of nickel by cobalt practically not
provokes greats changes in the surface area. Such result
was expected since the ionic radii of Ni3+ and Co3+ are
rather similar. The surface area for the samples LaNix
Co1-xO3 (0.4 x 0.6; x = 1) calcined at 700 and pre-
pared by PP method were lower than 15 m2/g. These
values are characteristics for the perovskite type oxides
prepared by polymeric precursor method and from the
calcinations conditions used.
Figure 4 shown SEM micrographs for all perovskites
samples calcined at 700 and prepared by PQ and PP
Table 1. Specific surface area (SSA), average medium particle
(Dm) and crystallite size (d) for the perovskites synthesized
Samples/
Method Dm
(m) SSA
(m2/g) d
(nm)
LaNiO3, PQ 2.6 143.7 10.8
LaCoO3, PQ 2.7 132.1 9.5
LaNiO3, PP 21.4 28.7 12.9
LaNi0,6Co0,4O3, PP 39.6 12.2 16.2
LaNi0,5Co0,5O3, PP 38.4 12.7 14.7
LaNi0,4Co0,6O3, PP 41.3 11.5 14.1
(a) (b)
(c) (d)
(e) (f)
Figure 4. SEM for (a) LaNiO3, PQ; (b) LaCoO3, PQ; (c)
LaNi0.6Co0.4O3,PP; (d) LaNi0.5Co0.5O3, PP; (e) LaNi0.4Co0.6
O3, PP; and (a) LaNiO3, PP
methods. SEM micrographs showed non-uniform parti-
cles with sizes ranging from 2 to 20 m. Such distribution
in particle size may be attributed to the preparation meth-
od and calcination temperature range used. The perovs-
kites prepared by PQ and PP methods showed similar
non-uniform particles when were compared to the same
powders prepared by other methods.
4. Conclusions
This work was carried out to investigate the effect of the
preparation methods in the perovskite structure formation.
The effect of partial substitution of Ni for Co in the B sites
of LaNiO3 perovskites on the structural, morphological
and surface properties also was studied. In accordance
with TG curves and FTIR spectra, all organic residual
material of the synthesis process decomposed up to 650
for the precursor powders synthesized by polymeric pre-
cursor method. For the other hand, for the precursor
powders prepared by chelating precursor method, all
organic residual material of the synthesis process also
decomposed up to 650, but in this case the decomposi-
tions lead to the formation of secondary phase in this
temperature.
X-ray diffraction patterns revealed the formation of the
rhombohedral perovskite structure in all samples prepared
by both synthesis method and after calcinations at 700.
The results showed that the preparation method resulted in
oxides with the intended structure. Minor phases attrib-
Copyright © 2010 SciRes. MSA
44 Synthesis and Characterization of LaNiCo O Perovskites via Complex Precursor Methods
x1-x 3
uted to impurities were observed in the LaNi0.5Co0.5O3
prepared by polymeric precursor method and in all the
samples prepared by chelating precursor method, which
was attributed to carbonate phase.
In the perovskites prepared by PP method, the replac-
ement of Ni by Co did not cause significant changes in the
lattice parameters. The perovskites prepared by PP met-
hod are characterized by relatively wide particle size
distributions with particles ranging from 2 to 20 m and
specific surface area about 15 m2/g. The perovskites pre-
pared by PQ method showed similar structure compared
to the same powders prepared by other methods, having
advantage strength how a little particle size and surface
area higher 100 m2/g.
5. Acknowledgements
The authors would like to thank CNPq/Projeto nº550244/
2007-7 and UFS/Edital PAIRD 2008 for the financial
support of this study.
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