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Self-rising approach to synthesize hierarchically porous metal oxides
Materials Research Bulletin 44 (2009) 2056–2061
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Self-rising approach to synthesize hierarchically porous metal oxides Qiang Liu, Fanglin Chen * Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 May 2009 Received in revised form 7 July 2009 Accepted 20 July 2009 Available online 25 July 2009
Hierarchically porous metal oxides are prepared using a novel self-rising approach. The method is ‘hardtemplate’ free and relatively easy to perform, utilizes inexpensive precursors and processing conditions, and is versatile, thus offering tremendous opportunities with a wide variety of conditions to explore to synthesize single or mixed metal oxides with hierarchically porous structures. Fe2O3, Sm0.2Ce0.8O1.9, LaFeO3, LaCoO3, and La0.5Sr0.5Co0.5Fe0.5O3 have been successfully synthesized while Fe2O3 and La0.5Sr0.5Co0.5Fe0.5O3 as examples for single and mixed metal oxides have been studied in detail. Sm0.2Ce0.8O1.9 has been applied as electrolyte for solid oxide fuel cells, showing good sinterability and high conductivity. A tentative scheme is provided to illustrate the pore formation mechanism using the self-rising approach. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Oxides B. Chemical synthesis C. Electron microscopy D. Microstructure
1. Introduction Hierarchically porous (mesoporous/macroporous) materials, displaying multiple length scales in pore size and combining the advantages of high surface areas from mesopores and the increased mass transport associated with macropores [1–5], exhibit many novel properties and have been widely used in the industrial fields of catalysis [6–8], bio-technology [9,10], adsorptions [11] and separations [12]. In the natural world, such highly efficient structure can also be easily found for optimal transport of fluids and gases, such as hierarchical lung and diatom [3,13]. Consequently, many efforts have been devoted to the synthesis of hierarchically porous materials, usually by employing dual supramolecular templates with appropriate reaction solutions [8,14,15] or combination of micro-molding, latex sphere templating, and cooperative assembly of a block copolymer [9,11,13,16–20]. However, most of the studies that deal with materials of hierarchical porosity are focused on silica, titania and their-based compounds for which the chemistry is relatively simple [11– 13,16–30]. Much less work has been devoted to other transition metal oxide-based porous materials [6], in which the coupling between sol–gel chemistry and self-assembly process is more difficult to control. Therefore, developing a generalized approach to synthesize hierarchically porous materials is highly appealing and of great significance.
* Corresponding author. Tel.: +1 803 777 4875; fax: +1 803 777 0106. E-mail address: [email protected] (F. Chen). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.07.013
We report here a hard-template free, low cost, very simple but versatile self-rising approach to obtain hierarchically porous metal oxides. Using Fe2O3 and La0.5Sr0.5Co0.5Fe0.5O3 (LSCF) as model materials for single and mixed metal oxide respectively, we illustrate how the self-rising approach can be used to fabricate a broad spectrum of hierarchically porous materials. 2. Experimental 2.1. Preparation of hierarchically porous metal oxides The self-rising method is essentially a modified EISA method, which has been well-characterized and confirmed to be efficient and easy to conduct [31–33]. For the synthesis of single metal oxide, Fe2O3, 0.6 g Pluronic P123 (Mav = 5800, EO20PO70EO20, Aldrich) was dissolved in 13 mL ethanol and 2 mL water inside a 50 mL beaker. 1 g urea and 2 mmol Fe(NO3)39H2O were then added. To prepare La0.5Sr0.5Co0.5Fe0.5O3 (LSCF), 0.5 mmol Co(NO3)26H2O, La(NO3)26H2O, Sr(NO3)2 and Fe(NO3)39H2O (Alfa Aesar) were added as metal precursor without changing other parameters. To prepare samaria doped ceria, Sm0.2Ce0.8O1.9 (SDC), 0.4 mmol Sm(NO3)36H2O and 1.6 mmol Ce(NO3)36H2O were added as metal precursor. For comparison, SDC samples were also synthesized by a conventional polyvinyl alcohol (PVA) combustion method. After ultrasonicating at room temperature (RT) for 15 min to dissolve the chemicals, the homogeneous sol was dried naturally at RT for solvent evaporation. After two days aging, calcination was carried out in air by slowly increasing temperature from RT to
Q. Liu, F. Chen / Materials Research Bulletin 44 (2009) 2056–2061
120 8C (1 8C min 1 ramping rate). After holding at 120 8C for 3 h, the sample was then heated to 450 8C and then held at 450 8C for 5 h. To obtain LSCF single crystalline phase, the 450 8C calcined LSCF sample was further calcined at 900 8C for 2 h. 2.2. Characterization and analyses The powder X-ray diffraction (XRD) pattern was recorded on a D/MAX-3C X-ray diffractometer with graphite-monochromatized CuKa radiation (l = 1.5418 A˚), employing a scanning rate of 58/ min in the 2u range of 20–808. The structure and morphology of the synthesized products were characterized by scanning electron microscopy (SEM, FEI Quanta and XL 30) equipped with an energy dispersive X-ray (EDX) analyzer and transmission electron microscopy (TEM, Hitachi H-800, 200 kV). The samples were also characterized by nitrogen adsorption/desorption at 196 8C by using a NOVA 2000 series volumetric adsorption system. Simultaneous thermal analysis (thermogravimetry–differential scanning calorimetry, TG–DSC) was performed on a NETZSCH STA 409. AC impedance spectra of the sintered SDC pellets were conducted on a potentiostat/galvanostat with built-in impedance analyzer (Versa STAT 3-400, Princeton Applied Research) in the frequency range of 0.05 Hz to 100 kHz at 800 8C. Both sides of the sintered SDC pellets were coated with silver paste and heat-treated at 800 8C for 30 min before testing. In all the measurements, Ag lead wires were used and the lead resistance was subtracted by measuring the impedance of a blank cell. 3. Results and discussion 3.1. Self-rising approach The self-rising approach was initially invented in food industry by Henry Jones, a baker from England, in 1845, to produce selfrising flour [34]. In fact, the self-rising flour is the mixture of allpurpose flour and a small quantity of leavening agent, usually baking powders. During baking process, the baking powders can react or decompose with temperature increasing, emit CO2 gas, introduce pores into the dough to make it in situ leaven. Inspired by this strategy that can easily introduce pores/ bubbles to bulk materials, we explore to synthesize porous metal oxides using the self-rising approach. The key influencing factor of the self-rising approach is the judicious choice of the ‘leavening agent’. The composition of the baking powder used in the food industry is mainly baking soda (NaHCO3), which, however, may not be the first choice here as the leavening agent for the synthesis of porous metal oxides because the baking soda will produce sodium oxide which might introduce impurity to the final product. After an extensive search, urea has been chosen as the leavening agent because urea will decompose to only gases (NH3 and CO2), consequently avoiding impurities and contamination to the final product. Urea has been used in the past for the preparation of catalysts to achieve high surface area and controlled porosity by precipitating metal ions in an aqueous solution [35]. However, its
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function is totally different in the self-rising approach: urea is used as macropore generator through decomposition to release gases at elevated temperatures. The self-rising method can only produce macropores/bubbles without generating mesopores inside the shells or walls. Therefore, to obtain a hierarchically macro-/meso-porous structure, other technique for the synthesis of mesoporous materials must be taken to assist the self-rising approach. The preparation of mesoporous solids with ordered or disordered structure has been wellestablished using the similar templating method, such as ‘softtemplate’ technique and ‘hard-template’ pathways and some recent reviews are recommended for further reading [36–38]. Among them, the evaporation induced self-assembly (EISA) method has been selected here to combine with the self-rising approach to synthesize hierarchically porous materials [31]. EISA is extremely useful for both controlling macroscopic form (thin films and monoliths) and enabling the synthesis of mesostructures. In the EISA method, during solvent evaporation, the self-assembly process is triggered when the concentration of the surfactant in the solution begins to exceed the critical micelle concentration. Upon high temperature calcination, the template is removed and the ordered mesoporous materials can be obtained. In addition, the precursor after solvent evaporation is viscoelastic and sticky due to the existence of a large amount of surfactant. Such physical properties of the as-prepared dough are very important for the subsequent heating process, because the dough can be easily changed to any form and shape when the leavening agent inside the dough decomposes to release gases. If the metal precursors are too rigid before calcination, the macropores would easily crack or fracture and the macroporous structure will be destroyed. The formation process of the self-rising approach to synthesize hierarchically porous metal oxides can be illustrated in Scheme 1. With the evaporation of the solvent, the concentration of nonvolatile species progressively increases and surfactant micelles form and self assemble into ordered structure while the hydrophilic head groups of the surface interact with the inorganic precursor and urea (Scheme 1a). Thus, elastic ‘dough’ with welldistributed urea, the leavening agent, is ready for ‘baking’. During the following heating process, urea gradually decomposes and releases plenty of gases at low temperature (70 8C). Consequently, abundant macropores can be successfully introduced into the sticky complex. Under the ideal condition, if urea within the complex is uniformly distributed, the pores left after urea decomposition should also be uniform (Scheme 1b). At even higher temperature treatment (450 8C), the surfactant would be burned off to leave worm-like disordered mesopores inside the macropore walls (Scheme 1c). 3.2. Hierarchically porous iron oxide The XRD pattern of the as-synthesized iron oxide is shown in Fig. 1a. The strong and sharp diffraction peaks in the XRD pattern indicate that the obtained product is well crystallized. All the diffraction peaks can be perfectly indexed as a-Fe2O3. Shown in Fig. 1b is the SEM image of Fe2O3, revealing a typical three-
Scheme 1. Schematic illustration of the self-rising approach to synthesize HPMOs.
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Fig. 2. Nitrogen adsorption/desorption isotherm of calcined iron oxide.
Fig. 1. (a) XRD, (b) SEM, (c) low magnification TEM and (d) high magnification TEM of as-synthesized iron oxide.
dimensionally macroporous morphology with an average macrospore size around 2 mm. The ‘honeycomb-like’ macrostructure can be observed over a range of hundreds of micrometers. A low magnitude TEM image is shown in Fig. 1c, which confirms that the macropores in these materials are highly interconnected. High magnitude TEM study on the macropores’ walls (Fig. 1d) clearly shows disordered mesoporous structure, these irregular mesopores are formed by Fe2O3 nanoparticles with diameter around 10 nm, which has been further confirmed by the followed nitrogen adsorption/desorption analysis. Fig. 2 shows the nitrogen adsorption and desorption isotherms of the calcined Fe2O3 with honeycomb structure, displaying a type IV isotherm with type H1 hysteresis loops in the relative pressure
range of 0.75–0.98. The high relative pressure indicates large pore size in these samples [6]. The pore size distribution is determined by the Barrett–Joyner–Halenda (BJH) method and shows a broad distribution in the range of 2–45 nm (inset of Fig. 2). The Brunauer– Emmett–Teller (BET) surface area and pore volume are 48 m2/g and 0.12 cm3/g, respectively. Such hierarchically porous structures are expected to have broad applications in catalysis and adsorption. 3.3. Honeycomb LSCF mixed metal oxide The interest in LSCF mixed metal oxides (MMOs) stems from its possible applications as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs), because LSCF possesses mixed ionic and electronic conduction with adequate oxide ion and electronic conductivities [39–41]. Cathodes within the IT-SOFCs have stringent porous requirements to fulfill the function of facile mass transport and effective charge-transfer [40].
Fig. 3. SEM and macropore size distribution of as-synthesized samples. (a–c) LSCF450 and (d–f) LSCF900.
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Fig. 4. XRD patterns of the differently heat-treated samples: (a) 450 8C, (b) 600 8C, (c) 750 8C, and (d) 900 8C; (e) TG–DSC of LSCF450.
Several researchers have attempted to tailor the microstructure to improve its porosity [40–42]. However, due to the difficulty and high cost of obtaining metal alkoxide precursors and the high calcination temperature needed to form a single phase LSCF, there has been no report on the synthesis of macroporous LSCF. The reports on successful synthesis of other mixed metal oxides are also very limited [43,44]. Recently, Ueda et al. reported a modified method to synthesize MMOs using metal nitrates with an ethylene glycol–methanol mixed solvent for precursors. The amorphous products obtained after template removal, but these converted to skeleton structures upon crystallization of the products [45,46]. Shown in Fig. 3a and b are the low and high magnitude SEM graphs of the as-synthesized LSCF calcined at 450 8C (LSCF450), revealing a three-dimensionally honeycomb-like morphology, which is very similar to that of Fe2O3. Large fractions (over 90%) of the sample (shown in supporting information) have highly ordered porous structure in three-dimensions over a range of hundreds of micrometers. However, XRD results indicate that the LSCF450 is amorphous (Fig. 4a). In order to get well-crystallized
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LSCF, further higher temperature calcination is required. TG–DSC (Fig. 4e) was performed on the LSCF450 sample in order to obtain insight into the reaction process to form the perovskite phase. No further reaction occurred above 850 8C, consistent with the XRD analysis (Fig. 4b–d). After calcination at 900 8C for 2 h, high crystalline perovskite LSCF900 is obtained (Fig. 4d). One significant feature of this self-rising approach is that the high temperature calcination does not destroy the original macroporous structure, as shown in Fig. 4d. The low magnitude SEM of LSCF900 exhibits the same ordered macroporous structure as that of LSCF450. This result is quite different from the previous reports in which, even after calcinations at 500 8C, the macroporous structure of MMOs started to collapse when poly(methyl methacrylate) was used as hardtemplate [46]. The larger and thicker channel walls in LSCF450 most probably lead to enhanced thermal stability of the 3D macrostructure in this work [47]. High magnitude SEM (Fig. 3e and figure S2 in supporting information) and TEM (Fig. 5b) of LSCF900 shows randomly distributed pores with pore diameter around 150 nm in the macropore walls. Such porous structure is expected to provide facile gas transport pathways in the LSCF cathodes for IT-SOFCs. Another difference between LSCF450 and LSCF900 is the average size of the macropores. After measuring the adjacent 60 pores from the SEM images, apparent shrinkage of the LSCF macroporous structure can be seen after high temperature treatment. The average pore diameter of LSCF450 (Fig. 3c) is about 5 mm, which is twice as large as that of LSCF900 (Fig. 3f). This may be due to LSCF crystallite growth and the mesopores inside the macropore walls of LSCF450 sintering at high temperature, resulting in the macroscopic volume decreasing. This speculation can be further confirmed by N2 adsorption and desorption as well as TEM study. Shown in Fig. 6a is the nitrogen adsorption–desorption isotherm of LSCF450, which can be ascribed to type IV with H1shaped hysteresis loops with the P/P0 position of the inflection point corresponding to a diameter in the mesoporous range [35]. The BET surface area and pore volume of LSCF450 are 65 m2/g and 0.11 cm3/g, respectively. BJH calculations reveal that the pore size distribution is centered around 6.7 nm (Fig. 6b), consistent well with the TEM results of the sample (Fig. 5a). However, after the
Fig. 5. (a) TEM graphs of LSCF450 and (b) LSCF900.
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Fig. 6. (a) Nitrogen adsorption and desorption isotherms, (b) BJH methods calculated pore size distribution of LSCF450.
high temperature treatment, the BET surface area of LSCF900 drops sharply to only about 0.25 m2/g (although some pores with diameter around 150 nm can be found, these pores are too big to contribute to the BET surface area). Further, no change of N2 volume adsorbed can be observed with the increase/decrease in pressure during the nitrogen adsorption–desorption process, indicating that sintering of the nanocrystals occurred by adsorbing surrounding primary particles via Oswald ripening [46], thus resulting in the disappearance of the mesopores inside the macropore walls. This postulation can be confirmed by the TEM analysis, as shown in Fig. 5b, after the high temperature (900 8C) calcinations, the mesopores have disappeared, only some bigger pores are left which are formed by sintered large particles. However, as mentioned above, the thick macropore walls give strong support to the whole structure and the macroporous morphology maintains well even after such fast crystallite growth. 3.4. Application in the field of SOFCs Due to the remarkable volume expansion during the self-rising process, the obtained hierarchically porous metal oxides show extremely low filled density, making it possible to prepare thin membranes by a uniaxial dry pressing method [48]. Consequently, we have explored the application of hierarchically porous SDC
Fig. 8. Conductivity of SDC_SR and SDC_CB at 800 8C versus sintering temperature.
powders to make thin and dense electrolyte SDC films for IT-SOFCs. Using the self-rising approach, hierarchically porous SDC powders (SDC_SR), i.e. foam like macroporous structure shown under SEM (Fig. 7a) and randomly distributed mesopores shown under TEM (Fig. 7b), have been successfully synthesized with a high BET surface area of 54.4 m2/g. The theoretical density of bulk SDC is about 7.15 g cm 3, while the filled density of the SDC_SR powder is measured to be 0.0438 g cm 3, only 0.61% of the bulk value, implying that the apparent volume of this hierarchically porous powder is about 165 times that of the same amount of bulk material. Consequently, using this extremely loose powders, anode supported thin membranes with thickness of only 20 mm can be obtained (Fig. 7c) by dry pressing method. Such thin electrolyte films will dramatically reduce the ohmic resistance and improve their electrical performance accordingly. In addition, SDC powder from the self-rising also displays better sinterability and higher conductivity than the SDC powder obtained by a conventional combustion method (SDC_CB) using PVA as fuel. For SDC_SR, fully densified pellets can be obtained after sintering at 1400 8C while a sintering temperature of 1500 8C is needed to obtain fully densified pellets using SDC powders from the combustion method. Fig. 8 shows the conductivity measured at 800 8C of the SDC pellets sintered at different temperatures and the SDC conductivity from the self-rising powders is higher than that
Fig. 7. (a) SEM and (b) TEM of SDC powders after 500 8C calcinations for 5 h; (c) cross-section SEM of a fuel cell with thin SDC electrolyte obtained by dry pressing method.
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from the combustion powders. The SDC_SR pellet sintered at 1500 8C has a conductivity value of 0.081 S cm 1 at 800 8C, which is in the upper range of the reported data [49]. 4. Conclusion In conclusion, self-rising method has been successfully developed to synthesize HPMOs. Compared with the previously reported methods to prepare hierarchically porous microstructures, the self-rising approach demonstrates the following remarkable advantages: (1) cost effective. Metal nitrites together with urea are used as the precursors, eliminating the need of expensive hard-template typically employed from the other approaches; (2) versatile. Not only single metal oxide but also mixed metal oxide such as SDC, LaFeO3, LaCoO3, and even LSCF hierarchically porous structures can be successfully obtained. (3) simple and reproducible. Preliminary work has shown that the self-rising approach is an efficient way to produce hierarchically porous metal oxides with extremely low filled density and high BET surface area, which have been successfully applied to obtain thin and dense SDC electrolyte films for SOFCs, showing good sinterability and high conductivity. Acknowledgment The financial support of the Department of Energy (contract no. DE-FG36-08GO88116) is acknowledged gratefully. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.materresbull.2009.07.013. References [1] Y.N. Xia, B. Gates, Y.D. Yin, Y.F. Lu, Adv. Mater. 12 (2000) 693. [2] Y. Sakatani, C. Boissie`re, D. Grosso, L. Nicole, G.J. Soler-Illia, C. Sanchez, Chem. Mater. 20 (2008) 1049. [3] E.S. Toberer, R. Seshadri, Adv. Mater. 17 (2005) 2244. [4] E.S. Toberer, R. Seshadri, Chem. Commun. (2006) 3159. [5] S.H. Im, U.Y. Jeong, Y.N. Xia, Nat. Mater. 4 (2005) 671. [6] F. Xu, P. Zhang, A. Navrotsky, Z.Y. Yuan, T.Z. Ren, M. Halasa, B.L. Su, Chem. Mater. 19 (2007) 5680. [7] J.G. Yu, L.J. Zhang, B. Cheng, Y.R. Su, J. Phys. Chem. C 111 (2007) 10582. [8] X.C. Wang, J.C. Yu, C. Ho, Y.D. Hou, X.Z. Fu, Langmuir 21 (2005) 2552. [9] H.S. Yun, S.E. Kim, Y.T. Hyeon, Chem. Commun. (2007) 2139.
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Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Self-rising approach to synthesize hierarchically porous metal oxides Qiang Liu, Fanglin Chen * Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 May 2009 Received in revised form 7 July 2009 Accepted 20 July 2009 Available online 25 July 2009
Hierarchically porous metal oxides are prepared using a novel self-rising approach. The method is ‘hardtemplate’ free and relatively easy to perform, utilizes inexpensive precursors and processing conditions, and is versatile, thus offering tremendous opportunities with a wide variety of conditions to explore to synthesize single or mixed metal oxides with hierarchically porous structures. Fe2O3, Sm0.2Ce0.8O1.9, LaFeO3, LaCoO3, and La0.5Sr0.5Co0.5Fe0.5O3 have been successfully synthesized while Fe2O3 and La0.5Sr0.5Co0.5Fe0.5O3 as examples for single and mixed metal oxides have been studied in detail. Sm0.2Ce0.8O1.9 has been applied as electrolyte for solid oxide fuel cells, showing good sinterability and high conductivity. A tentative scheme is provided to illustrate the pore formation mechanism using the self-rising approach. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Oxides B. Chemical synthesis C. Electron microscopy D. Microstructure
1. Introduction Hierarchically porous (mesoporous/macroporous) materials, displaying multiple length scales in pore size and combining the advantages of high surface areas from mesopores and the increased mass transport associated with macropores [1–5], exhibit many novel properties and have been widely used in the industrial fields of catalysis [6–8], bio-technology [9,10], adsorptions [11] and separations [12]. In the natural world, such highly efficient structure can also be easily found for optimal transport of fluids and gases, such as hierarchical lung and diatom [3,13]. Consequently, many efforts have been devoted to the synthesis of hierarchically porous materials, usually by employing dual supramolecular templates with appropriate reaction solutions [8,14,15] or combination of micro-molding, latex sphere templating, and cooperative assembly of a block copolymer [9,11,13,16–20]. However, most of the studies that deal with materials of hierarchical porosity are focused on silica, titania and their-based compounds for which the chemistry is relatively simple [11– 13,16–30]. Much less work has been devoted to other transition metal oxide-based porous materials [6], in which the coupling between sol–gel chemistry and self-assembly process is more difficult to control. Therefore, developing a generalized approach to synthesize hierarchically porous materials is highly appealing and of great significance.
* Corresponding author. Tel.: +1 803 777 4875; fax: +1 803 777 0106. E-mail address: [email protected] (F. Chen). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.07.013
We report here a hard-template free, low cost, very simple but versatile self-rising approach to obtain hierarchically porous metal oxides. Using Fe2O3 and La0.5Sr0.5Co0.5Fe0.5O3 (LSCF) as model materials for single and mixed metal oxide respectively, we illustrate how the self-rising approach can be used to fabricate a broad spectrum of hierarchically porous materials. 2. Experimental 2.1. Preparation of hierarchically porous metal oxides The self-rising method is essentially a modified EISA method, which has been well-characterized and confirmed to be efficient and easy to conduct [31–33]. For the synthesis of single metal oxide, Fe2O3, 0.6 g Pluronic P123 (Mav = 5800, EO20PO70EO20, Aldrich) was dissolved in 13 mL ethanol and 2 mL water inside a 50 mL beaker. 1 g urea and 2 mmol Fe(NO3)39H2O were then added. To prepare La0.5Sr0.5Co0.5Fe0.5O3 (LSCF), 0.5 mmol Co(NO3)26H2O, La(NO3)26H2O, Sr(NO3)2 and Fe(NO3)39H2O (Alfa Aesar) were added as metal precursor without changing other parameters. To prepare samaria doped ceria, Sm0.2Ce0.8O1.9 (SDC), 0.4 mmol Sm(NO3)36H2O and 1.6 mmol Ce(NO3)36H2O were added as metal precursor. For comparison, SDC samples were also synthesized by a conventional polyvinyl alcohol (PVA) combustion method. After ultrasonicating at room temperature (RT) for 15 min to dissolve the chemicals, the homogeneous sol was dried naturally at RT for solvent evaporation. After two days aging, calcination was carried out in air by slowly increasing temperature from RT to
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120 8C (1 8C min 1 ramping rate). After holding at 120 8C for 3 h, the sample was then heated to 450 8C and then held at 450 8C for 5 h. To obtain LSCF single crystalline phase, the 450 8C calcined LSCF sample was further calcined at 900 8C for 2 h. 2.2. Characterization and analyses The powder X-ray diffraction (XRD) pattern was recorded on a D/MAX-3C X-ray diffractometer with graphite-monochromatized CuKa radiation (l = 1.5418 A˚), employing a scanning rate of 58/ min in the 2u range of 20–808. The structure and morphology of the synthesized products were characterized by scanning electron microscopy (SEM, FEI Quanta and XL 30) equipped with an energy dispersive X-ray (EDX) analyzer and transmission electron microscopy (TEM, Hitachi H-800, 200 kV). The samples were also characterized by nitrogen adsorption/desorption at 196 8C by using a NOVA 2000 series volumetric adsorption system. Simultaneous thermal analysis (thermogravimetry–differential scanning calorimetry, TG–DSC) was performed on a NETZSCH STA 409. AC impedance spectra of the sintered SDC pellets were conducted on a potentiostat/galvanostat with built-in impedance analyzer (Versa STAT 3-400, Princeton Applied Research) in the frequency range of 0.05 Hz to 100 kHz at 800 8C. Both sides of the sintered SDC pellets were coated with silver paste and heat-treated at 800 8C for 30 min before testing. In all the measurements, Ag lead wires were used and the lead resistance was subtracted by measuring the impedance of a blank cell. 3. Results and discussion 3.1. Self-rising approach The self-rising approach was initially invented in food industry by Henry Jones, a baker from England, in 1845, to produce selfrising flour [34]. In fact, the self-rising flour is the mixture of allpurpose flour and a small quantity of leavening agent, usually baking powders. During baking process, the baking powders can react or decompose with temperature increasing, emit CO2 gas, introduce pores into the dough to make it in situ leaven. Inspired by this strategy that can easily introduce pores/ bubbles to bulk materials, we explore to synthesize porous metal oxides using the self-rising approach. The key influencing factor of the self-rising approach is the judicious choice of the ‘leavening agent’. The composition of the baking powder used in the food industry is mainly baking soda (NaHCO3), which, however, may not be the first choice here as the leavening agent for the synthesis of porous metal oxides because the baking soda will produce sodium oxide which might introduce impurity to the final product. After an extensive search, urea has been chosen as the leavening agent because urea will decompose to only gases (NH3 and CO2), consequently avoiding impurities and contamination to the final product. Urea has been used in the past for the preparation of catalysts to achieve high surface area and controlled porosity by precipitating metal ions in an aqueous solution [35]. However, its
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function is totally different in the self-rising approach: urea is used as macropore generator through decomposition to release gases at elevated temperatures. The self-rising method can only produce macropores/bubbles without generating mesopores inside the shells or walls. Therefore, to obtain a hierarchically macro-/meso-porous structure, other technique for the synthesis of mesoporous materials must be taken to assist the self-rising approach. The preparation of mesoporous solids with ordered or disordered structure has been wellestablished using the similar templating method, such as ‘softtemplate’ technique and ‘hard-template’ pathways and some recent reviews are recommended for further reading [36–38]. Among them, the evaporation induced self-assembly (EISA) method has been selected here to combine with the self-rising approach to synthesize hierarchically porous materials [31]. EISA is extremely useful for both controlling macroscopic form (thin films and monoliths) and enabling the synthesis of mesostructures. In the EISA method, during solvent evaporation, the self-assembly process is triggered when the concentration of the surfactant in the solution begins to exceed the critical micelle concentration. Upon high temperature calcination, the template is removed and the ordered mesoporous materials can be obtained. In addition, the precursor after solvent evaporation is viscoelastic and sticky due to the existence of a large amount of surfactant. Such physical properties of the as-prepared dough are very important for the subsequent heating process, because the dough can be easily changed to any form and shape when the leavening agent inside the dough decomposes to release gases. If the metal precursors are too rigid before calcination, the macropores would easily crack or fracture and the macroporous structure will be destroyed. The formation process of the self-rising approach to synthesize hierarchically porous metal oxides can be illustrated in Scheme 1. With the evaporation of the solvent, the concentration of nonvolatile species progressively increases and surfactant micelles form and self assemble into ordered structure while the hydrophilic head groups of the surface interact with the inorganic precursor and urea (Scheme 1a). Thus, elastic ‘dough’ with welldistributed urea, the leavening agent, is ready for ‘baking’. During the following heating process, urea gradually decomposes and releases plenty of gases at low temperature (70 8C). Consequently, abundant macropores can be successfully introduced into the sticky complex. Under the ideal condition, if urea within the complex is uniformly distributed, the pores left after urea decomposition should also be uniform (Scheme 1b). At even higher temperature treatment (450 8C), the surfactant would be burned off to leave worm-like disordered mesopores inside the macropore walls (Scheme 1c). 3.2. Hierarchically porous iron oxide The XRD pattern of the as-synthesized iron oxide is shown in Fig. 1a. The strong and sharp diffraction peaks in the XRD pattern indicate that the obtained product is well crystallized. All the diffraction peaks can be perfectly indexed as a-Fe2O3. Shown in Fig. 1b is the SEM image of Fe2O3, revealing a typical three-
Scheme 1. Schematic illustration of the self-rising approach to synthesize HPMOs.
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Fig. 2. Nitrogen adsorption/desorption isotherm of calcined iron oxide.
Fig. 1. (a) XRD, (b) SEM, (c) low magnification TEM and (d) high magnification TEM of as-synthesized iron oxide.
dimensionally macroporous morphology with an average macrospore size around 2 mm. The ‘honeycomb-like’ macrostructure can be observed over a range of hundreds of micrometers. A low magnitude TEM image is shown in Fig. 1c, which confirms that the macropores in these materials are highly interconnected. High magnitude TEM study on the macropores’ walls (Fig. 1d) clearly shows disordered mesoporous structure, these irregular mesopores are formed by Fe2O3 nanoparticles with diameter around 10 nm, which has been further confirmed by the followed nitrogen adsorption/desorption analysis. Fig. 2 shows the nitrogen adsorption and desorption isotherms of the calcined Fe2O3 with honeycomb structure, displaying a type IV isotherm with type H1 hysteresis loops in the relative pressure
range of 0.75–0.98. The high relative pressure indicates large pore size in these samples [6]. The pore size distribution is determined by the Barrett–Joyner–Halenda (BJH) method and shows a broad distribution in the range of 2–45 nm (inset of Fig. 2). The Brunauer– Emmett–Teller (BET) surface area and pore volume are 48 m2/g and 0.12 cm3/g, respectively. Such hierarchically porous structures are expected to have broad applications in catalysis and adsorption. 3.3. Honeycomb LSCF mixed metal oxide The interest in LSCF mixed metal oxides (MMOs) stems from its possible applications as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs), because LSCF possesses mixed ionic and electronic conduction with adequate oxide ion and electronic conductivities [39–41]. Cathodes within the IT-SOFCs have stringent porous requirements to fulfill the function of facile mass transport and effective charge-transfer [40].
Fig. 3. SEM and macropore size distribution of as-synthesized samples. (a–c) LSCF450 and (d–f) LSCF900.
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Fig. 4. XRD patterns of the differently heat-treated samples: (a) 450 8C, (b) 600 8C, (c) 750 8C, and (d) 900 8C; (e) TG–DSC of LSCF450.
Several researchers have attempted to tailor the microstructure to improve its porosity [40–42]. However, due to the difficulty and high cost of obtaining metal alkoxide precursors and the high calcination temperature needed to form a single phase LSCF, there has been no report on the synthesis of macroporous LSCF. The reports on successful synthesis of other mixed metal oxides are also very limited [43,44]. Recently, Ueda et al. reported a modified method to synthesize MMOs using metal nitrates with an ethylene glycol–methanol mixed solvent for precursors. The amorphous products obtained after template removal, but these converted to skeleton structures upon crystallization of the products [45,46]. Shown in Fig. 3a and b are the low and high magnitude SEM graphs of the as-synthesized LSCF calcined at 450 8C (LSCF450), revealing a three-dimensionally honeycomb-like morphology, which is very similar to that of Fe2O3. Large fractions (over 90%) of the sample (shown in supporting information) have highly ordered porous structure in three-dimensions over a range of hundreds of micrometers. However, XRD results indicate that the LSCF450 is amorphous (Fig. 4a). In order to get well-crystallized
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LSCF, further higher temperature calcination is required. TG–DSC (Fig. 4e) was performed on the LSCF450 sample in order to obtain insight into the reaction process to form the perovskite phase. No further reaction occurred above 850 8C, consistent with the XRD analysis (Fig. 4b–d). After calcination at 900 8C for 2 h, high crystalline perovskite LSCF900 is obtained (Fig. 4d). One significant feature of this self-rising approach is that the high temperature calcination does not destroy the original macroporous structure, as shown in Fig. 4d. The low magnitude SEM of LSCF900 exhibits the same ordered macroporous structure as that of LSCF450. This result is quite different from the previous reports in which, even after calcinations at 500 8C, the macroporous structure of MMOs started to collapse when poly(methyl methacrylate) was used as hardtemplate [46]. The larger and thicker channel walls in LSCF450 most probably lead to enhanced thermal stability of the 3D macrostructure in this work [47]. High magnitude SEM (Fig. 3e and figure S2 in supporting information) and TEM (Fig. 5b) of LSCF900 shows randomly distributed pores with pore diameter around 150 nm in the macropore walls. Such porous structure is expected to provide facile gas transport pathways in the LSCF cathodes for IT-SOFCs. Another difference between LSCF450 and LSCF900 is the average size of the macropores. After measuring the adjacent 60 pores from the SEM images, apparent shrinkage of the LSCF macroporous structure can be seen after high temperature treatment. The average pore diameter of LSCF450 (Fig. 3c) is about 5 mm, which is twice as large as that of LSCF900 (Fig. 3f). This may be due to LSCF crystallite growth and the mesopores inside the macropore walls of LSCF450 sintering at high temperature, resulting in the macroscopic volume decreasing. This speculation can be further confirmed by N2 adsorption and desorption as well as TEM study. Shown in Fig. 6a is the nitrogen adsorption–desorption isotherm of LSCF450, which can be ascribed to type IV with H1shaped hysteresis loops with the P/P0 position of the inflection point corresponding to a diameter in the mesoporous range [35]. The BET surface area and pore volume of LSCF450 are 65 m2/g and 0.11 cm3/g, respectively. BJH calculations reveal that the pore size distribution is centered around 6.7 nm (Fig. 6b), consistent well with the TEM results of the sample (Fig. 5a). However, after the
Fig. 5. (a) TEM graphs of LSCF450 and (b) LSCF900.
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Fig. 6. (a) Nitrogen adsorption and desorption isotherms, (b) BJH methods calculated pore size distribution of LSCF450.
high temperature treatment, the BET surface area of LSCF900 drops sharply to only about 0.25 m2/g (although some pores with diameter around 150 nm can be found, these pores are too big to contribute to the BET surface area). Further, no change of N2 volume adsorbed can be observed with the increase/decrease in pressure during the nitrogen adsorption–desorption process, indicating that sintering of the nanocrystals occurred by adsorbing surrounding primary particles via Oswald ripening [46], thus resulting in the disappearance of the mesopores inside the macropore walls. This postulation can be confirmed by the TEM analysis, as shown in Fig. 5b, after the high temperature (900 8C) calcinations, the mesopores have disappeared, only some bigger pores are left which are formed by sintered large particles. However, as mentioned above, the thick macropore walls give strong support to the whole structure and the macroporous morphology maintains well even after such fast crystallite growth. 3.4. Application in the field of SOFCs Due to the remarkable volume expansion during the self-rising process, the obtained hierarchically porous metal oxides show extremely low filled density, making it possible to prepare thin membranes by a uniaxial dry pressing method [48]. Consequently, we have explored the application of hierarchically porous SDC
Fig. 8. Conductivity of SDC_SR and SDC_CB at 800 8C versus sintering temperature.
powders to make thin and dense electrolyte SDC films for IT-SOFCs. Using the self-rising approach, hierarchically porous SDC powders (SDC_SR), i.e. foam like macroporous structure shown under SEM (Fig. 7a) and randomly distributed mesopores shown under TEM (Fig. 7b), have been successfully synthesized with a high BET surface area of 54.4 m2/g. The theoretical density of bulk SDC is about 7.15 g cm 3, while the filled density of the SDC_SR powder is measured to be 0.0438 g cm 3, only 0.61% of the bulk value, implying that the apparent volume of this hierarchically porous powder is about 165 times that of the same amount of bulk material. Consequently, using this extremely loose powders, anode supported thin membranes with thickness of only 20 mm can be obtained (Fig. 7c) by dry pressing method. Such thin electrolyte films will dramatically reduce the ohmic resistance and improve their electrical performance accordingly. In addition, SDC powder from the self-rising also displays better sinterability and higher conductivity than the SDC powder obtained by a conventional combustion method (SDC_CB) using PVA as fuel. For SDC_SR, fully densified pellets can be obtained after sintering at 1400 8C while a sintering temperature of 1500 8C is needed to obtain fully densified pellets using SDC powders from the combustion method. Fig. 8 shows the conductivity measured at 800 8C of the SDC pellets sintered at different temperatures and the SDC conductivity from the self-rising powders is higher than that
Fig. 7. (a) SEM and (b) TEM of SDC powders after 500 8C calcinations for 5 h; (c) cross-section SEM of a fuel cell with thin SDC electrolyte obtained by dry pressing method.
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from the combustion powders. The SDC_SR pellet sintered at 1500 8C has a conductivity value of 0.081 S cm 1 at 800 8C, which is in the upper range of the reported data [49]. 4. Conclusion In conclusion, self-rising method has been successfully developed to synthesize HPMOs. Compared with the previously reported methods to prepare hierarchically porous microstructures, the self-rising approach demonstrates the following remarkable advantages: (1) cost effective. Metal nitrites together with urea are used as the precursors, eliminating the need of expensive hard-template typically employed from the other approaches; (2) versatile. Not only single metal oxide but also mixed metal oxide such as SDC, LaFeO3, LaCoO3, and even LSCF hierarchically porous structures can be successfully obtained. (3) simple and reproducible. Preliminary work has shown that the self-rising approach is an efficient way to produce hierarchically porous metal oxides with extremely low filled density and high BET surface area, which have been successfully applied to obtain thin and dense SDC electrolyte films for SOFCs, showing good sinterability and high conductivity. Acknowledgment The financial support of the Department of Energy (contract no. DE-FG36-08GO88116) is acknowledged gratefully. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.materresbull.2009.07.013. References [1] Y.N. Xia, B. Gates, Y.D. Yin, Y.F. Lu, Adv. Mater. 12 (2000) 693. [2] Y. Sakatani, C. Boissie`re, D. Grosso, L. Nicole, G.J. Soler-Illia, C. Sanchez, Chem. Mater. 20 (2008) 1049. [3] E.S. Toberer, R. Seshadri, Adv. Mater. 17 (2005) 2244. [4] E.S. Toberer, R. Seshadri, Chem. Commun. (2006) 3159. [5] S.H. Im, U.Y. Jeong, Y.N. Xia, Nat. Mater. 4 (2005) 671. [6] F. Xu, P. Zhang, A. Navrotsky, Z.Y. Yuan, T.Z. Ren, M. Halasa, B.L. Su, Chem. Mater. 19 (2007) 5680. [7] J.G. Yu, L.J. Zhang, B. Cheng, Y.R. Su, J. Phys. Chem. C 111 (2007) 10582. [8] X.C. Wang, J.C. Yu, C. Ho, Y.D. Hou, X.Z. Fu, Langmuir 21 (2005) 2552. [9] H.S. Yun, S.E. Kim, Y.T. Hyeon, Chem. Commun. (2007) 2139.
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