Preparation of mesoporous La0.85Sr0.15MnO3 using a surfactant-templated sol-gel process

Abstract

Since the discovery of mesoporous silica at Mobil [1], various mesoporous materials based on silica or using silica as a template have been prepared and used for a variety of applications [1–9]. The general procedure for the preparation of these mesoporous materials involves hydroxylation of an alkoxy silane procursor by water and subsequent polymerization through water condensation to form alkanols, in the presence of a structuredirecting agent (surfactant micelles), resulting in the precipitation of surfactant-templated mesoporous solids. The surfactant micelles are then removed, by extraction or pyrolysis, leaving only a porous network of amorphous solid. The pore structure is defined by the once present micelles. This formation mechanism of the mesoporous structures was called “liquid-crystal templating” (LCT) mechanism by Beck et al. [1, 2]. It is quite a challenge, however, to apply the synthesis schemes for mesoporous materials based on silica or using silica as a template to the synthesis of mesoporous complex transition metal oxides such as ABO3 and ABO2. One difficulty is to obtain a mesophase with desired stoichiometry from a solution containing more than one types of metal ions, each having very different properties. Another difficulty is to retain the mesoporous structure of the inorganic framework during surfactant removal. Often, rearrangements of inorganic framework, even collapse of mesostructure, occur during calcination of as-synthesized mesoporous complex transition metal oxides [10–13]. Mixed ionic-electronic conductors (MIECs) with a general formula of LnMO3 (Ln: rare earth metal, M: transition metal) are attractive materials due to their wide range of electrical and catalytic properties [14, 15]. Among them, La1−x Srx MnO3 (x = 0 to 1.0) has been widely used as cathode material for solid oxide fuel cells (SOFCs) because of its high catalytic activity for oxygen reduction and its chemical and thermal compatibility with yttria-stabilized-zirconia (YSZ) electrolyte [16, 17]. Here, we reported the synthesis of mesoporous La0.9Sr0.1MnO3 (LSM) using an ionic surfactant in a sol-gel process. In our experiment, La(NO3)3·6H2O, Sr(NO3)2, and Mn(NO3)2·4H2O were used as the precursors and ionic surfactant, dodecylbenzensulfonic acid (DBSA), was used as the structure-directing agent for the preparation of mesoporous LSM. All chemicals were obtained from Aldrich or Alfa. In a typical preparation, La(NO3)3·6H2O (3.694 g), Sr(NO3)2 (0.212 g), and Mn(NO3)2·H2O (5.741 g) were dissolved in 15 g distilled water. In another beaker, DBSA (8 g) was dissolved in 12 g distilled water, followed by addition of 12.5 ml 4M NH4OH. After a clear base surfactant solution was obtained, it was slowly added to precursor solution under stirring. The obtained mixture with pH of 3.5 was stirred at room temperature for 1 to 3 h, and then aged in an oven kept at 70 to 80 ◦C for one week. The aged solution was dried at 35 to 45 ◦C for 1 to 3 days to obtain a gel, which was then dried at 80 to 90 ◦C for another 1 to 3 days and at 120 ◦C for 1 to 2 days in a vacuum oven. The completely dried samples were then ground into powder and fired at 500 ◦C in air for 1 h to remove the organics. The obtained mesoporous materials were characterized using small-angle X-ray diffraction (Scintag, XGEN4000 advanced diffraction system, Cu kα radiation) to determine the organized meso-structure and using transmission electron microscopy (Hitachi, HF2000) to reveal the detailed microscopic features. The surface area and absorption properties were studied using a surface analyzer (Coulter SA 3100). Shown in Fig. 1 are the small angle X-ray diffraction patterns of as-synthesized mesoporous LSM powders