Microstructural Investigations of Nanostructured La(Sr)MnO3-δ Films

Chem. Mater. 2004, 16, 3733-3739

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Microstructural Investigations of Nanostructured La(Sr)MnO3-δ Films Deposited by Electrostatic Spray Deposition A. Princivalle, D. Perednis, R. Neagu, and E. Djurado* Laboratoire d’Electrochimie et de Physico-chimie des Mate´ riaux et des Interfaces, ENSEEG-INP Grenoble, Domaine Universitaire, BP 75, 1130 rue de la Piscine, 38402 St Martin d’He` res Cedex, France Received May 27, 2004. Revised Manuscript Received July 21, 2004

The deposition of (La0.85Sr0.15)0.95MnO3-δ films on an yttria-stabilized zirconia substrate was studied by the electrostatic spray deposition (ESD) technique. Films with various morphologies were obtained. The surface morphology was strongly influenced by the precursor solution flow rate and deposition temperature. Processes involved in the porous film formation were discussed. Powder X-ray diffraction analysis showed that the LSM hexagonal phase was formed and was nanostructured, consisting of crystallites of 60-75 nm on average, after thermal treatment ranging from 600 to 900 °C.

Introduction High-temperature fuel cells using solid oxide electrolytes represent a promising way of converting the chemical energy of low-cost fossil fuels into electrical energy. For long-term reliability, the cathode material requires a high electronic conductivity and in certain cases ionic conductivity, chemical stability, and a thermal expansion coefficient compatible with yttria-stabilized zirconia (YSZ), the most widely used solid electrolyte. Among the numerous oxide candidates for this application, the alkaline-earth doped lanthanum manganite La1-xSrxMnO3-δ has been widely accepted.1 Moreover, its electrocatalytic properties with respect to oxygen reduction fulfill the requirements. At present, efforts are focused on lowering the operating temperature of solid oxide fuel cells (SOFC) from 1000 °C down to 700 °C. To reduce high interfacial polarization and ohmic losses as a consequence of the temperature reduction, two research directions are suggested in the literature: (i) more careful design of cathodes2 and (ii) replacement of pure electronic conductor cathode by mixed ionic electronic conductor.3,4 The electrochemical properties of cathodes depend not only on the microstructure but also on the chemical composition. Indeed, an increase in the electrical resistivity and in the polarization of cathode was reported during long time operation depending on the perovskite composition and temperature/time treatment.5 Especially, A-deficient perovskites or B-site hyperstoichiometric perovskites seem to be favorable candidates due to their decreased tendency to form insulating zircon* Corresponding author: Tel +33-4-7682-6684; fax +33-4-76826777; e-mail [email protected]. (1) Minh, N. Q. J. Am. Ceram. Soc. 1993, 76, 563. (2) Holtappels, P.; Bagger, C. J. Eur. Ceram. Soc. 2002, 22, 41. (3) Østergård, M. J. L.; Clausen, C.; Bagger, C.; Mogensen, M. Electrochim. Acta 1995, 40, 1971. (4) Wang, S.; Jiang, Y.; Zhang, Y.; Yan, J.; Li, W. Solid State Ionics 1998, 113-115, 291. (5) Brant, M. C.; Dessemond, L. Solid State Ionics 2000, 138, 1.

ates such as La2Zr2O7 and SrZrO3 at the YSZ/LSM interfaces. In this work, we have selected the (La0.85Sr0.15)0.95MnO3-δ composition as previously reported, which will be referred to as LSM in the following.6 Various methods have been used for the fabrication of LSM cathodes, including pyrosol method,7 metallorganic chemical vapor deposition (MOCVD),8,9 vacuum plasma spraying (VPS),10 radio frequency magnetron sputtering,11 and screen printing.12 Spray pyrolysis is an alternative to the traditional methods because of its simplicity, low cost, and minimal waste production. Over the past years the electrostatic spray deposition (ESD) technique was mainly developed to prepare components for solid-state lithium-ion batteries.13 The so-called electrostatic-assisted vapor deposition (EAVD) involves the electrostatic generation of an aerosol.14 However, the authors suggest that the film deposition involves chemical reaction in the vapor phase. The assertion of a CVD-like process is doubtful, because we have not observed any indication of the vapor phase involvement in the room temperature to 500 °C temperature domain at atmospheric pressure. Golego et al.15 proposed a complex mechanism of film (6) Roux, C.; Djurado, E.; Kleitz, M. Proceedings of the International Energy Agency Joint Topical Meeting, Solid Oxide Fuel Cells under real operating conditionssMaterials and Processes, Les Diablerets, Switzerland, 28-31 January, 1997. (7) Gharbage, B.; Henault, M.; Pagnier, T.; Hammou, A. Mater. Res. Bull. 1991, 26, 1001. (8) Meda, L.; Bacaltchuk, C.; Garmestani, H. J. Mater. Sci. 2001, 12, 143. (9) Bertrand, G. L.; Caboche, G.; Dufour, L.-C. Solid State Ionics 2000, 129, 219. (10) Barthel, K.; Rambert, S.; Siegmann, S. J. Therm. Spray Technol. 2000, 9, 343. (11) Hayashi, K.; Yamamoto, O.; Nishigaki, Y.; Minoura, H. Solid State Ionics 1997, 98, 49. (12) Basu, R. N.; Pratihar, S. K.; Saha, M.; Maiti, H. S. Mater. Lett. 1997, 32, 217. (13) Chen, C.; Kelder, E. M.; van der Put, P. J. J. M.; Schoonman, J. J. Mater. Chem. 1996, 6, 765. (14) Choy, K. L.; Bai, W.; Charojrochkul, S.; Steele, B. C. H. J. Power Sources 1998, 71, 361.

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deposition without a CVD regime involving an equilibrium of decomposition, evaporation, and drying of the precursor solution. Taniguchi et al.16,17 used the ESD method to deposit La1-xSrxCo1-yFeyO3 on stainless steel and gadoliniadoped ceria substrates. It was observed that the cathode film consisted of a very porous superficial layer next to a relatively dense layer extending to substrate. Choy et al.18 used the so-called flame-assisted vapor deposition (FAVD) technique, where an atomized solution was sprayed through a flame. Charpentier et al.19,20 deposited porous LSM films by a modified spray pyrolysis method. In their work, thin LSM layers were prepared by spraying La0.7Sr0.3MnO3-δ powder and lanthanum nitrate, strontium acetate, and manganese nitrate solution. The aim of the present study is focused on the preparation of LSM films with a varied microstructure and compositional homogeneity as functions of processing parameters by the ESD technique. The influence of the deposition temperature and the solution flow rate on the film morphology will be systematically studied. Experimental Section Deposition of LSM Films by ESD. The LSM cathodes were prepared in a vertical ESD setup, similar to that described in the literature.21,22 LSM films were deposited on a heated polycrystalline YSZ substrate by spraying precursor solutions. Disks 20 mm in diameter and 1 mm in thickness of homemade YSZ were used as substrates. A mixture (33:67 vol %) of ethanol (C2H5OH, 99.9%; Prolabo) and diethylene glycol monobutyl ether [CH3(CH2)3OCH2CH2OCH2CH2OH, 99+%; Acros Organics), was used as solvent for lanthanum nitrate [La(NO3)3‚6H2O, 99%; Fluka], strontium chloride (SrCl2‚6H2O, 99%; Aldrich), strontium nitrate [Sr(NO3)2, 97%; Prolabo], or manganese nitrate [Mn(NO3)2‚4H2O, 98%; Aldrich]. The precursor solution was prepared according to the stoichiometry of the required film La0.8075Sr0.1425MnO3-δ. The total concentration of the salts in the solution was 0.02 mol/L. Deposition time was 1 h. Flow rate of precursor solution was varied from 0.34 to 1.17 mL/h by use of a Sage M361 syringe pump. The precursor solution was atomized with a positive high voltage from 5 to 8 kV. The deposition temperature was in the range of 225-500 °C. The nozzle-to-substrate distance was 27 mm. Characterizations. Surface morphologies and their composition were analyzed by scanning electron microscopy (SEM) (LEO 400) equipped with an energy-dispersive X-ray analyzer (EDAX). All samples were postannealed at a heating rate of 2 °C/ min in air for 2 h at 400, 500, 600, 700, 800, and 900 °C in order to investigate the influence of thermal treatment on the morphology and on the appearance of the LSM phase. (15) Golego, N.; Studenikin, S. A.; Cocivera, M. J. Mater. Res. 1999, 14, 698. (16) Taniguchi, I.; van Landschoot, R. C.; Huang, H.; Schoonman, J. Proceedings of Fifth European Solid Oxide Fuel Cell Forum, Luzern, Switzerland, 1-5 July, 2002. (17) Taniguchi, I.; van Landschoot, R. C.; Schoonman, J. Solid State Ionics 2003, 156, 1. (18) Choy, K. L.; Charojrochkul, S.; Steele, B. C. H. Solid State Ionics 1997, 96, 49. (19) Charpentier, P.; Fragnaud, P.; Schleich, D.; Lunot, C.; Gehain, E. Ionics 1997, 3, 155. (20) Charpentier, P.; Fragnaud, P.; Schleich, D.; Gehain, E. Solid State Ionics 2000, 135, 373. (21) Chen, C.; Kelder, E. M.; Jak, M. J. G.; Schoonman, J. Solid State Ionics 1996, 86-88, 1301. (22) Chen, C.; Kelder, E. M.; Schoonman, J. J. Mater. Sci. 1996, 31, 5437.

Princivalle et al.

Figure 1. TG and DTA of strontium nitrate (heating rate 10 °C min-1). X-ray powder diffraction was carried out on a Siemens D500 θ/2θ diffractometer in the Bragg Brentano geometry from 25° to 95° in 2θ (0.04° in 2θ step, 8 s as a counting time) with Fe KR radiation (λ ) 0.1936 nm). High-purity silicon was used as the standard in order to precisely measure the instrumental resolution. The average crystallite size was calculated by applying the Scherrer law, corrected with silicon on the (024) XRD peak of LSM:

D)

0.9λ β cos θ

where D is the crystallite size (in nanometers), λ is the wavelength (in nanometers), β is the full width at halfmaximum (fwhm) corrected from high-purity silicon (in radians), and θ is the diffraction angle. Phases were identified by use of DIFFRAC-AT software systems (Socabim, Paris). The cell parameters (a and c) of the hexagonal form were refined by the Rietveld23 structural refinement approach as implemented in the FULLPROF package24 for the sample annealed at 900 °C. The reflection positions were determined by fitting peaks with pseudo-Voigt functions. Thermal decomposition of salts was performed in air at a 10 °C/min heating rate on a Netzsch simultaneous thermal analyzer STA 409 instrument.

Results and Discussion Decomposition of Precursors. Thermogravimetric (TG) and differential thermal analysis (DTA) traces of the thermal decomposition of strontium nitrate, strontium chloride, manganese nitrate, and lanthanum nitrate are shown in Figures 1-4, respectively. The full decomposition of manganese nitrate and strontium chloride was accomplished at 220 °C, whereas strontium nitrate decomposition was found in the 600-700 °C temperature range. Lanthanum nitrate was decomposed differently compared to the other salts in three steps up to 700 °C. It is worth noting that all precursors do not decompose simultaneously; namely, lanthanum and strontium nitrates decompose at a higher temperature than manganese nitrate. Influence of the Salt in the Precursor Solution. The nature of salts plays a significant role in the film morphology. In a previous study, Taniguchi et al.16,17 used strontium chloride and lanthanum and manganese nitrates as precursors. A comparative study is necessary to select the most suitable salt and to understand its (23) Rietveld, H. M. Acta Crystallogr. 1967, 22, 151. (24) Rodriguez-Carvajal, J. In Abstracts of the Satellite Meeting on Powder Diffraction of the XV Congress of the IUCR, Toulouse, France, 1990.

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Figure 2. TG and DTA of strontium chloride (heating rate 10 °C min-1).

Figure 5. SEM micrographs of La0.8075Sr0.1425MnO3 films deposited on YSZ for 1 h by spraying a solution containing (a) SrCl2 or (b) Sr(NO3)2. Deposition temperature was 300 °C; flow rate was 0.67 mL/h. Figure 3. TG and DTA of manganese nitrate (heating rate 10 °C min-1).

Figure 6. SEM observation of the cross-section of a La0.8075Sr0.1425MnO3 layer deposited on YSZ for 1 h at 300 °C (solution containing SrCl2). Precursor solution flow rate was 0.67 mL/ h. Figure 4. TG and DTA of lanthanum nitrate (heating rate 10 °C min-1).

role in the film formation. Figure 5 shows the microstructure of the films deposited with strontium chloride and strontium nitrate. The film is very porous in the case of strontium chloride (Figure 5a), whereas the coating prepared with strontium nitrate (Figure 5b) is cracked with reticulated surface morphology. Cracks in the coating are probably the consequence of different drying processes of droplets starting from strontium nitrate and strontium chloride. At 300 °C, as expected from TG/DTA results, strontium chloride is fully decomposed into strontium oxide, whereas this is not the case for strontium nitrate. Therefore, most of the droplets based on the nitrate will be still wet. So, in this case, the drying step will be more heterogeneous due to the presence of strontium nitrate decomposition residues. Consequently, mechanical stresses have led to crack formation. Figure 6 shows the cross-section of an LSM thin film deposited on a YSZ substrate starting

from a strontium chloride precursor. The thickness of the layer was approximately 3 µm. Henceforth, strontium chloride was always used instead of strontium nitrate in order to avoid crack formation. Influence of Deposition Temperature. Figure 7 shows different surface morphologies of LSM thin films deposited at different temperatures ranging from 225 to 500 °C and starting from strontium chloride. As shown in Figure 7a, the coating is cracked at 225 °C. This can be attributed to the stresses that have been developed during the drying process of a large quantity of liquid on the substrate. At higher temperature the arriving droplets contain less solvent. Therefore, the drying step is more homogeneous and cracks disappeared, leading to porous LSM films. The deposition temperature for porous morphology ranges from 250 to 300 °C. This reticulated microstructure can be the consequence of simultaneous boiling and drying of precursor solution, which is possible when substrate surface temperature is close to the boiling point of the solvent as in our case of butylcarbitol (230 °C). From

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Figure 7. Surface morphologies of La0.8075Sr0.1425MnO3 films deposited for 1 h at 0.5 mL/h flow rate from precursor solution containing SrCl2 at different temperatures: (a) 225, (b) 250, (c) 275, (d) 300, (e) 325, (f) 350, (g) 400, and (h) 500 °C.

these data, we have shown that the simultaneous boiling and drying steps are strongly related to the porous microstructure. Moreover, three different views were previously reported in the literature to explain this surface morphology: (i) Golego et al.15 showed that the formation of porous films was mainly due to the evolution of gaseous salt decomposition products. However, the authors did not consider the boiling of solvent in their interpretation. (ii) Chen et al.13 suggested that simultaneous spreading of liquid droplets and precipitation of salts lead to a porous microstructure. (iii) The model proposed by Choy et al.18 involves formation of porous sintered film from small particles, which were formed in the gaseous phase. This mechanism is doubtful, because the strontium nitrate, which was used in their study, does not fully decompose in the 500-700 °C temperature range as shown in Figure 1.

The morphology of films becomes dense and smooth at 325 and 350 °C (see Figure 7e,f), and some small additional particles on the surface are observed at 400 °C (Figure 7g). In this temperature domain, some droplets are completely dried before reaching the substrate. At the highest deposition temperature, most dry droplets arrived on the surface due to faster evaporation of solvent at 500 °C (Figure 7h), leading to granular coating. Influence of the Solution Flow Rate. Figure 8 shows a systematic study of the influence of the solution flow rate on the surface morphologies of LSM films deposited at 275, 300, and 325 °C for 1 h. The flow rate was varied from 0.34 to 1.17 mL/h. The film is dense at the lowest flow rate of 0.34 mL/h for all deposition temperatures (see Figure 8a-c). A small quantity of liquid sprayed droplets arrives on the

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Figure 8. Influence of the deposition temperature and precursor solution flow rate on the surface morphology of La0.8075Sr0.1425MnO3 films.

substrate and consequently a wet film is not formed, because the droplets spread and dry rapidly. At higher flow rates, a larger number of liquid droplets are now in contact with the substrate. The

simultaneous boiling and drying of a wetter film on the substrate surface occurs. As shown in Figure 8d,e, porous coatings were deposited at the flow rate of 0.5 mL/h. At 325 °C and for 0.5 mL/h, dense film was

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Table 1. Composition of La0.8075Sr0.1425MnO3 Film Deposited by ESD element

precursors solution (at. %)

observed (at. %)

La Mn Sr

41.4 51.3 7.3

39.8 50.0 10.2

deposited (Figure 8f). A similar morphology was obtained at 300 °C for 0.34 mL/h (Figure 8b). It is interesting to notice the combined effect of temperature and flow rate parameters: when the flow rate is decreased, a smaller quantity of liquid droplets arrives on the substrate. Consequently, they dried at lower temperature. When the flow rate is further increased, the volume change during the drying step is too large due to the presence of a larger amount of solution on the substrate surface. Consequently, stresses and cracking of the coating appear. Therefore, at the lowest deposition temperature (275 °C), all coatings were cracked when the flow rate was larger than or equal to 0.67 mL/h (see Figure 8g,j,m). In contrast, the coatings deposited at 300 and 325 °C do not crack for a flow rate of 0.67 mL/h (see Figure 8n,o,k,l,h,i). In these cases the substrate temperature was sufficiently high to avoid the drying cracks. As shown in Figure 8e,h, the porosity increases with increasing flow rate from 0.5 to 0.67 mL/h at 300 °C. Taniguchi et al.17 also observed this behavior when the flow rate was increased from low to high values. The morphology of the deposited films changed from microscopic to a mesoscopic net. The authors did not report the formation of dense films as in our work. This difference could probably be explained by the use of a shorter nozzle to substrate distance and a higher flow rate compared to our study. A larger quantity of liquid impacts the substrate at the next two higher flow rates (0.86 and 1.17 mL/h), leading to an unequilibrated simultaneous drying and boiling on the substrate surface. The liquid in excess penetrates into formed pores and consequently densifies the porous structure. Therefore, dense films with netlike structure on the surface are formed at 300 and 325 °C as shown in Figure 8n,o,k,l. If one looks carefully at all these microstructures, we can extract a general trend for the formation of a selected morphology. From Figure 8 panels d and h to l, if one follows the flow rate/temperature diagonal, one can retrieve the same porous reticular microstructure. It is also valid for dense microstructure when one goes from panel b to f. As a conclusion, as far as the flow rate is increased, the substrate temperature is also increased in order to keep a selected morphology, dense or porous. Physicochemical Characterizations of LSM Films. The composition of as-deposited LSM films was measured by EDX analysis. The film stoichiometry was found to be close to that of the precursor solution within experimental error (Table 1). It shows that the ESD technique allows easy control of multicomponent film stoichiometry. Figure 9 shows a quite similar porous morphology of the films before and after thermal treatment at 800 °C for 2 h. A denser reticular net consisting of LSM was observed after the treatment at 800 °C.

Figure 9. SEM images of La0.8075Sr0.1425MnO3 films deposited on YSZ for 1 h at 300 °C with 0.67 mL/h flow rate from precursor solution containing SrCl2: (a) as prepared and (b) after thermal treatment at 800 °C for 2 h.

Figure 10. XRD patterns of LSM films deposited from precursor solution containing SrCl2 on YSZ for 1 h at 300 °C with 0.67 mL/h flow rate and heat-treated for 2 h in air at (a) 400, (b) 500, (c) 600, (d) 700, (e) 800, and (f) 900 °C.

All films deposited at 300 °C did not contain LSM phase whether amorphous or crystalline as shown in Figure 10. The thermal treatment above 600 °C leads to a striking appearance of crystalline LSM films (Figure 10d). Indeed, XRD patterns of the films annealed up to 600 °C consisted of a single YSZ phase, the signature of the substrate. The presence of LSM was not detected. This behavior is consistent with TG/DTA data. Indeed, the final decomposition of lanthanum nitrate is expected to be above 700 °C. The higher the annealing temperature, the larger the crystallinity of LSM films (Figure 10d-f). Grain size dependence on the temperature is shown in Figure 11. The average crystallite size increased from 60 to 75 nm after heat treatments at 700 and 900 °C, respectively. The XRD pattern of LSM film annealed at 900 °C was refined by the Rietveld method. a and c cell parameters were found to equal 5.4980 ( 0.0006 Å and 13.352 ( 0.001 Å, respectively, in the hexagonal R3 h c space group (no. 167).

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polycrystalline LSM film at 900 °C due to the different decomposition into oxides of La, Sr, and Mn precursors. Further electrical and electrocatalytic characterizations are in progress in order to understand the role of the microstructure on the reduction reaction of oxygen on LSM electrodes. Conclusions

Figure 11. Evolution of the crystallite size as a function of thermal treatment.

Figure 12. XRD patterns (0 2 4) of LSM layer deposited from precursor solution containing SrCl2 on YSZ annealed for 2 h in air at (a) 400, (b) 500, (c) 600, (d) 700, (e) 800, and (f) 900 °C.

As shown in Figure 12, a shift of the (0 2 4) line is observed toward lower 2θ as a function of annealing temperature. This behavior probably shows that the La+Sr/Mn ratio of 0.95 is only effective in the final

This paper presents an experimental study of La0.8075Sr0.1425MnO3-δ coatings prepared by the ESD process. Microstructural properties of LSM films were systematically investigated as functions of the type of salt, deposition temperature, and flow rate. In contrast to porous and crack-free film prepared from strontium chloride, the coating deposited from the precursor solution with dissolved strontium nitrate was cracked with a netlike structure on the surface. The deposition temperature for the formation of a porous morphology ranged from 250 to 300 °C, where simultaneous boiling and drying of the wet film on the substrate surface occurred. The higher the flow rate of the precursor solution is, the higher the substrate temperature will be in order to obtain a typical coating, whether dense or porous. The porous layer of the best quality was obtained at a deposition temperature of 300 °C and 0.67 mL/h flow rate of the precursor solution containing strontium chloride. The films were cracked below 250 °C and dense above 300 °C. Nanostructured and hexagonal single-phased LSM films were detected after annealing above 600 °C by XRD. All coatings were free of cracks and consisted of only 75 nm grains on average after annealing at 900 °C. Acknowledgment. We are grateful to Dr. G. Dezanneau for the Rietveld refinement and to Dr. C. Roux for thermal analyses. CM049158Z