PURE AND DOPED LANTHANUM COBALTITE WITH LARGE SURFACE AREA

1.  PURE AND DOPED LANTHANUM COBALTITEWITH LARGE SURFACE AREA Daniela Berger1 , Victor Fruth2 , Maria Zaharescu1 1�Politehnica� 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 (A=Sr, 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 (x=0-0.2; y=0-0.2) 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 (x=0-0.3; y=0-0.2) 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 (x=0-0.2; y=0-0.2) powders were prepared by calcining the isolated complex precursors at 800oC, 3h for LaCoO3, 1000oC, 4-6h for La1-xSrxCo1-yFeyO3 (x=0.1-0.2; y=0-0.2). 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 (a=b) 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

9. IR spectra Fig.3. IR spectra of La1-xSrxCoO3

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

12. Electrical conductivity measurements 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-yFeyO3 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 (x=0-0.2; y=0-0.2) 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.