PURE AND DOPED LANTHANUM COBALTITE WITH LARGE SURFACE AREA

1
PURE AND DOPED LANTHANUM COBALTITEWITH LARGE
SURFACE AREA

• Daniela Berger1 , Victor Fruth2 , Maria
  Zaharescu1
• 1Politehnica University Bucharest, Department
  of Inorganic Chemistry, 1 Polizu street,
  78126-Bucharest, Romania
• 2Institute of Physical Chemistry, 202 Splaiul
  Independentei, 77208 Bucharest, Romania

2
Introduction

• Perovskite-type oxides La(A)Co(Fe)O3 (ASr, Ca)
  having high ionic and predominate electronic
  conductivity represent one of the most promising
  groups of mixed conductors 1,2.
• Compositions with general formula
  La1-xSrxCo1-yFeyO3 have desirable properties for
  intermediate temperature solid oxide fuel cells
  (SOFCs) 3.
• Pure and doped lanthanum cobaltites have
  potential application as a catalyst for light
  hydrocarbon oxidation 4, for the control of
  automobile emissions 5, as a cathode material
  for solid oxide fuel cells (SOFCs) 3, as well
  as gas detection sensor 6.

3
Objectives

• The present study is devoted to the synthesis and
  characterization of La1-xSrxCo1-yFeyO3 (x0-0.2
  y0-0.2) nanopowders with large surface area
  values.
• To this purpose, a wet chemical route is adopted
  to obtained pure and doped lanthanum cobaltites.

4
Experimental

• La1-xSrxCo1-yFeyO3 (x0-0.3 y0-0.2) were
  obtained by thermal decomposition of the complex
  precursors prepared in the following system,
  La(NO3)3 - Sr(NO3)2 - Co(NO3)2 Fe(NO3)3
  maleic acid NH3 at molar ratio, La Sr Co
  Fe maleic acid, 1-x x 1-y y 8.6, at
  pH 7.
• La1-xSrxCo1-yFeyO3 (x0-0.2 y0-0.2) powders
  were prepared by calcining the isolated complex
  precursors at 800oC, 3h for LaCoO3, 1000oC, 4-6h
  for La1-xSrxCo1-yFeyO3
  (x0.1-0.2 y0-0.2).
• The La1-xSrxCo1-yFeyO3 samples were characterised
  by IR, XRD, SEM-EDX, specific surface area and dc
  electric conductivity measurements.

5
Results and discussion

• Pure and doped lanthanum cobaltite samples were
  characterized by XRD, IR, SEM-EDX, specific
  surface area and dc electrical conductivity
  measurements.
• Lattice parameters were calculated using the
  software Powder X. Particles sizes (D) were
  calculated by means of the Scherrer equation from
  the full width at half-maximum, FWHM, of the
  X-ray reflection.
• The dc electrical conductivity of the
  La1-xSrxCo1-yFeyO3 pellets was measured with the
  standard four probe technique in the
  temperature range 295 - 1073 K with air as
  ambient.

6
XRD data

• The XRD data for La1-xSrxCo1-yFeyO3 show that all
  the samples are single phases with hexagonal
  perovskite structure.
• The substitution of La3 ions with Sr2 ions
  causes a significant increasing of c parameter
  and a small decreasing of a and b (ab)
  parameters.
• The decreasing of a and b parameters could be
  explained by the presence of a part of cobalt
  ions in tetravalent state with smaller ionic
  radius as a result of electric charge
  compensation.
• The presence of iron ions determines an
  increasing of the parameters cell as result of
  Co3 substitution by Fe3 with larger ionic
  radius.

7
XRD data

• Fig. 1. XRD data of La1-xSrxCoO3

Fig. 2. XRD data of La0.9Sr0.1Co1-yFeyO3
8
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9
IR spectra

• Fig.3. IR spectra of La1-xSrxCoO3

• Fig.4. IR spectra of La0.9Sr0.1Co1-yFey O3

10

• The IR spectrum of LaCoO3 presents an intense
  band around 605 cm-1 that could be assigned to
  vibration of Co-O bond in octahedral coordination
  and a band from 420 cm-1 assigned to vibration
  of La-O bond in dodecahedral coordination.
• In the IR spectra of Sr- doped samples appear a
  band around 670 cm-1 maybe due to Co4 ions
  formation. The appearance of this band at higher
  frequencies could be explained by the increasing
  of constant force of Co-O as result of increasing
  of cobalt ions electrical charge. It can notice
  also the splitting of characteristic vibration of
  ?La-O that might be assigned to the presence of
  Sr2 ions.
• In the IR spectra of iron doped samples, the
  intense band around 600 cm-1 is shifted toward
  higher frequencies due to increasing of constant
  force of Fe-O in comparison of Co-O.

11
SEM -EDX investigation

• Fig.2. SEM picture of
• La0.9Sr0.1Co0.9Fe0.1O3

Fig.1. SEM picture of La0.9Sr0.1CoO3
The elemental analysis results by EDX for
lanthanum cobaltite samples show a homogenous
distribution of metallic cations
12
Electrical conductivity measurements
Fig. 4. Temperature dependence of the electrical
conductivity of La0.9Sr0.1Co1-yFeyO3
in air

• Fig. 3. Temperature dependence of the electrical
  conductivity of La1-xSrxCoO3 in air

13

• The electrical conductivity of lanthanum
  cobaltites is enhanced by increasing the Sr2
  content and decreases with Fe3 content.
• The total electroneutrality condition is
• At low temperature, the electronic conduction
  occurs through the migration of electron holes
  associated with Co4 formed by trapping a hole (
  ). The decrease in conductivity at high
  temperatures occurs in the same temperature range
  in which the loss of lattice oxygen occurs.
• The decreasing conductivity of La0.9Sr0.1Co1-yFeyO
  3 is ascribed to hole trapping, i.e.

14
Conclusions

• La1-xSrxCo1-yFeyO3 samples prepared from
  maleate-based precursors are single phases with
  hexagonal perovskite structure.
• SEM examination of of La1-xSrxCo1-yFeyO3 samples
  showed that all the samples have primary
  nanoparticles with a small tendency of
  agglomerates formation.
• La1-xSrxCo1-yFeyO3 solid solutions have the
  crystallite sizes in 37-46 nm range and large
  specific surface area values.
• Specific surface areas of 35-38 m2/g obtained for
  La1-xSrxCo1-yFeyO3 (x0-0.2 y0-0.2) represent
  rather high values for perovskite-type oxides.
• This synthetic strategy represent a viable
  alternative to the conventional ceramic routes
  due to it allows rapid synthesis of homogeneous
  powders.

15
Acknowledgements

• The authors are indebted to the Delft University
  of Technology for experimental facilities. The
  authors wish to thank Dr. Cheng Dong from
  Institute of Physics, Chinese Academy of
  Sciences, for the permission of using the
  software Powder X.

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