Article pubs.acs.org/Langmuir
Development of Mesoporosity in Scandia-Stabilized Zirconia: Particle Size, Solvent, and Calcination Effects James T. Cahill,† Jesse N. Ruppert,‡ Bryce Wallis,§ Yanming Liu,§ and Olivia A. Graeve*,†,‡ †
Department of Mechanical and Aerospace Engineering, University of California, San Diego, 9500 Gilman Drive − MC 0411, La Jolla, California 92093-0411, United States ‡ Department of Chemical and Materials Engineering, University of Nevada, Reno, 1663 N. Virginia Street − MS 388, Reno, Nevada 89557, United States § Advanced Materials and Devices, Inc., 4750 Longley Lane #104, Reno, Nevada 89502, United States ABSTRACT: We present the mechanisms of formation of mesoporous scandia-stabilized zirconia using a surfactant-assisted process and the effects of solvent and thermal treatments on the resulting particle size of the powders. We determined that cleaning the powders with water resulted in better formation of a mesoporous structure because higher amounts of surfactant were preserved on the powders after washing. Nonetheless, this resulted in agglomerate sizes that were larger. The water-washed powders had particle sizes of >5 μm in the as-synthesized state. Calcination at 450 and 600 °C reduced the particle size to ∼1−2 and 0.5 μm, respectively. Cleaning with ethanol resulted in a mesoporous morphology that was less welldefined compared to the water-washed powders, but the agglomerate size was smaller and had an average size of ∼250 nm that did not vary with calcination temperature. Our analysis showed that surfactant-assisted formation of mesoporous structures can be a compromise between achieving a stable mesoporous architecture and material purity. We contend that removal of the surfactant in many mesoporous materials presented in the literature is not completely achieved, and the presence of these organics has to be considered during subsequent processing of the powders and/or for their use in industrial applications. The issue of material purity in mesoporous materials is one that has not been fully explored. In addition, knowledge of the particle (agglomerate) size is essential for powder handling during a variety of manufacturing techniques. Thus, the use of dynamic light scattering or any other technique that can elucidate particle size is essential if a full characterization of the powders is needed for achieving postprocessing effectiveness.
1. INTRODUCTION Zirconia-based materials can be tailored for a wide variety of applications, including gas sensors,1−4 catalyst supports,5,6 structural ceramics,7 and solid oxide fuel cells.8−10 At low temperatures zirconia has a monoclinic crystal structure, which transforms to tetragonal and then cubic at increasing temperatures. A common practice used for obtaining the high-temperature phases of zirconia at room temperature is the addition of dopants, which allows for stabilization of the tetragonal and cubic phases. As an example, yttria-stabilized zirconia (YSZ) is a ubiquitous material because as little as 10% doping allows for the stabilization of the cubic phase, which has desirable ionic conduction capabilities over a wide range of temperatures and partial oxygen pressures for solid oxide fuel cell (SOFC) applications. For most types of stabilized zirconia compounds, conductivity reaches a maximum at around 10 mol %, with a subsequent decrease in conductivity upon further doping.11 Scandia-stabilized zirconia (Sc-ZrO2) offers the highest ionic conductivity in doped zirconia systems and has shown potential for operating at temperatures nearly 100 K lower than YSZ electrodes in SOFCs.12 The material exhibits a conductivity maximum when doped with ∼10 mol % Sc; thus, © XXXX American Chemical Society
this amount is the one of most relevance in SOFC applications, as is also the case for YSZ. For use as an SOFC electrode, stabilized zirconia must be processed with a highly porous structure. One option is a mesoporous structure of high surface area, which can allow for improved gas transport in the fuel cell. There are several synthesis methods used to produce mesoporous ceramics, many of which utilize surfactants. For example, using the cationic surfactant cetyltrimethylammonium bromide (CTAB) as a template, Mamak et al.13−15 synthesized mesoporous YSZ samples with surface areas as high as 264 m2/g. After synthesis, the material was washed and calcined in order to remove the surfactant template and induce crystallization. Zelcer and SolerIllia16 synthesized highly ordered mesoporous ZrO2 thin films by evaporation induced self-assembly (EISA) and found that one-step preparation was possible by carefully tuning the sol composition to control the hydrolysis condensation kinetics. In a similar study, Yuan et al.17 developed a sol−gel based EISA Received: December 27, 2013 Revised: April 24, 2014
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together and stirred for 20 min until a homogeneous mixture was obtained. A third mixture was prepared by dissolving 4 g of cetyltrimethylammonium bromide (CTAB) [CH3(CH2)15NBr(CH3)3, Sigma-Aldrich] and 1.6 g of sodium hydroxide in 120 mL of distilled water. The combined scandia−zirconia gel was added to the water/ CTAB solution and stirred for 30 min until a homogeneous mixture was obtained. The water−gel solution was aged for 5 days in air on a hot plate at 80 °C. Powder samples were allowed to cool to room temperature and then subjected to two different washing techniques: cleaning 3 times in water (after every wash, the solutions were decanted) or 3 times in ethanol. For each wash, 120 mL of liquid was added to the sample and then stirred for 1−2 h with a magnetic stir bar. The contents were then divided into four centrifuge bottles and spun at 5000 rpm for 20 min. After decanting the remaining liquid, an additional 40 mL of the wash liquid was added to each of the centrifuge bottles, and the process was repeated. After washing, samples were left in the centrifuge bottles to dry in air. Powders washed in water took 10−14 days to dry, while powders washed in ethanol took 2−3 days to dry. Samples were placed in CoorsTek porcelain crucibles and subjected to heat treatments using a controlled ramp to temperature. Samples of all three wash types were treated with either a 6 h ramp to 450 °C or a 12 h ramp to 600 °C with a 3 h hold at temperature. After the heat treatment, the powders were removed from the furnace and allowed to cool in air. Crystal structure and crystallite size of the powders were analyzed by X-ray diffraction (XRD) using a Phillips 3100 X-ray diffractometer with Cu Kα radiation at 35 keV. Powder samples were placed directly in a sample holder without any further preparation. Crystallite size was determined by the Williamson−Hall technique. Average particle size was determined by dynamic light scattering (DLS) using a Microtrac Nanotrac Ultra. For these measurements, 0.05 vol % (typically 0.01 g) of the powders was dispersed in 25 mL of distilled water with 0.5 g/L of sodium pyrophosphate added as a dispersant. The solution was stirred with a magnetic stir bar for a few hours before testing. Three 30 s runs were used to obtain an average for each sample. Powder morphology was examined by scanning electron microscopy (SEM) using a Hitachi S-4700 instrument operated at 3 kV with a 5 mm working distance. SEM samples were either dispersed in ethanol and drop-coated onto aluminum foil to dry in air or placed directly on carbon tape. Dry powder samples were sputter-coated with platinum to reduce charging. Phases and pore structure were examined by transmission electron microscopy (TEM) using a JEOL 2100F fieldemission transmission electron microscope operating at 200 kV. Samples were prepared by dispersing the powders in ethanol and placing a drop onto a carbon-coated copper grid to dry in air. Powder surface area was obtained using a Micromeritics Flowsorb III 2300 system and the surface area was approximated using the BET method. Thermogravimetric analysis (TGA) was completed on a TA Q-600 instrument using 30 mg of powder and a heating rate of 10 °C/min.
route for producing mesoporous ceria-, yttria-, and scandiastabilized zirconia materials without the addition of an acid or base. The EISA method has also been used for the synthesis of mesoporous alumina and magnetic CoFe2O4-containing silica.18,19 Ma et al.20 developed a microwave-assisted, surfactant-free synthesis route for the production of Cu− Ce0.8Sm0.2O2−δ composite anodes. This method produces spherical mesoporous CuO-SDC particles by homogeneous precipitation of a microspherical precursor in aqueous solution followed by calcination. Kelly and Graeve21 illustrate similar spherical particles by using a reverse micelle process22−29 to produce mesoporous YSZ powders. In general, surfactantdirected morphology evolution is a promising strategy in the formation of a variety of nanomaterials.30 In the case of mesoporous ceramics, thorough characterization is important to accurately determine the properties of the sample. Methods such as X-ray diffraction (XRD), dynamic light scattering (DLS), and surface area analysis are useful for analyzing the “bulk” of a sample, while scanning electron microscopy (SEM) and transmission electron microscopy (TEM) can elucidate morphology at smaller scales. Employing a complete set of characterization techniques enables one to study the homogeneity of the sample and determine the effects that agglomerates and small particles have on the behavior of the material. In this contribution, we present an analysis of particle size, morphology, and porosity of Sc-ZrO2 powders produced using a self-assembly technique and how the washing procedures implemented before calcination, necessary for the removal of excess surfactant that can lead to residual impurities, results in a collapse of the pore structure upon calcination. By comparing these results with similar studies reported in recent literature, we demonstrate the importance of thoroughly characterizing porous ceramic materials with respect to impurities and agglomeration. Specifically, we show that at a calcination temperature of 600 °C the porosity in mesoporous Sc-ZrO2 (9 mol % Sc2O3) powders is almost completely eliminated. It has been previously proposed that dopants have a stabilizing effect on the mesoporous structure of zirconia,15 although this was only shown for very high dopant amounts.15 Thus, collapse of the pores in our materials is not unexpected at calcination temperatures of 600 °C, since our powders have a much lower quantity of dopant (one more commonly found in SOFCs), compared to the results presented by others.15 As it turns out, the dopant amount that results in the highest conductivity in Sc-ZrO2 is not the one most propitious for stabilizing a mesoporous structure. Thus, in spite of early reports, the use of this material as a mesoporous SOFC anode is not promising unless a completely different method for stabilization of the mesopores is identifiedone that does not rely on dopant stabilization.
3. RESULTS AND DISCUSSION All samples in the as-synthesized state were amorphous, and sample crystallization was induced by calcination, apparent in the XRD patterns of Figure 1. Both the water-washed and ethanol-washed powders had the same crystallization behavior. The powders indicate a cubic phase with reference to powder diffraction file 27-0997. Although only cubic peaks are observed in the XRD patterns of Figure 1b,c, it is difficult to resolve whether or not there is a small amount of tetragonal zirconia. Butz et al.31 showed that the determination of fully stabilized zirconia on 8 mol % YSZ cannot be justified by XRD alone due to the overlap of peaks of the cubic and tetragonal phases; thus, crystallite size values determined from X-ray line broadening may contain a systematic error associated with the presence of nanoscaled precipitates of the tetragonal phase. Keeping this in mind, the crystallite sizes of the powders calcined at 450 and 600 °C (assuming only cubic phase) were 6 and 11 nm, respectively. The particle (agglomerate) sizes were markedly
2. EXPERIMENTAL PROCEDURE A series of scandia-stabilized zirconia powders with 9 mol % Sc2O3 (16 at%) content were prepared using the sol−gel method. Zirconium gel solutions were formulated by mixing 5.00 g of zirconium ethoxide [Zr(OC2H5)4, 99.99%, Alfa Aesar] and 1.66 g of sodium hydroxide [NaOH, 99.99%, Alfa Aesar] with 50 mL of ethylene glycol [HO− CH2CH2−OH, reagent plus >99%, Sigma-Aldrich] and stirring the contents overnight under a flow of nitrogen, as described by Mamak et al.13−15 Separately, scandium gel solutions were prepared by mixing 0.8522 g of scandium acetate [(CH3CO2)3Sc·H2O, 99.99%, SigmaAldrich] with 30 mL of ethylene glycol and then stirring the contents for 2 h under a flow of nitrogen. Afterward, the two gels were mixed B
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Figure 1. X-ray diffraction patterns of (a) as-synthesized powders, (b) calcined powders at 450 °C for 6 h, and (c) calcined powders at 600 °C for 12 h.
different depending on the cleaning procedure. Kaliszewski and Heuer32 have provided a generally accepted and straightforward explanation for this phenomenon. The interaction of ethanol with the as-synthesized amorphous Zr(OH)4 during washing results in the replacement of hydroxyl groups with ethoxy groups, which are terminal rather than bridging, preventing the formation of hard agglomerates during calcination. Water, on the other hand, results in the formation of bridging hydrogen bonds that eventually result in necks between precipitates, thus forming hard agglomerates, especially as calcination temperatures are increased and/or calcination times are extended. Our powders before calcination had particle sizes of ∼250 nm and >5 μm, with the much larger particles a product of water washing (Figure 2a). This is not unexpected considering the bridging effects associated with water, as described earlier. Calcination of the powders cleaned only with water exhibited dramatic differences in particle size (Figure 2c). At a calcination temperature of 450 °C, the particle size reduced from >5 μm before calcination to a bimodal distribution centered at ∼1 and 2 μm. Calcination at 600 °C further reduced the particle size to around 0.5 μm. We attribute this to the presence of significant surfactant in these materials, even after thrice washing, which behaves as a binder between powder particles. Figure 3 confirms this to be the case from TGA analysis on the samples. The weight loss exhibited by the water-washed powders is significantly higher compared to the ethanol-washed powders. This weight loss is not associated with removal of excess water, as both samples are dry when placed in the TGA instrument, but with the removal of surfactant as well as the conversion from Zr(OH)4 to ZrO2. Since both sets of powders were analyzed using the same amount of material (i.e., 30 mg), the loss of water in converting from Zr(OH)4 to ZrO2 is the same for both the water-washed and ethanol-washed powders; thus, the significant difference in weight loss in the water-washed powders must be attributed to surfactant volatilization, thus corroborating that water is ineffective in the removal of surfactant compared to ethanol. As the surfactant is volatilized during calcination, the agglomerates disperse to submicrometer sizes. If the calcination
Figure 2. Particle size distributions of powders (a) cleaned in water or ethanol, (b) cleaned in ethanol and calcined at 450 and 600 °C, and (c) cleaned in water and calcined at 450 and 600 °C.
process had been performed at higher temperatures and/or for longer times, the powders would then start sintering and forming larger hard agglomerates, as we have previously seen in a variety of other materials,22,33,34 but this does not happen in the powders in this study because the temperature of calcination is mild and the time of calcination is short. On the other hand, calcination of the powders cleaned with ethanol (Figure 2b) does not modify the particle size. The level of agglomeration in these powders is fixed at ∼250 nm even before crystallization. This occurs because the surfactant is removed effectively by the ethanol and the powders have no polymeric species that can glue them together; thus, a very small particle size forms even before calcination. Since ethanol does not form bridging bonds between the as-synthesized precipitates, the calcination process does not lead to hard agglomeration, and the powders maintain their 250 nm particle size at the mild calcination conditions in this study. The morphology of the powders is illustrated in Figure 4. As a relevant comparison, Pattanayak et al.35 and Grosso et al.36 C
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attain an adequate mesostructure. In our case, the powders washed in water took 10−14 days to dry, while the powders washed in ethanol took 2−3 days to dry; thus, one would expect the ethanol powders to exhibit a higher mesoporous order. As they do not, differences in surfactant amount must explain the enhanced morphology of the water-washed powders. Removal of surfactant using ethanol is more efficient compared to water; thus, after completion of the wash cycles, the powders cleaned with water contain higher amounts of surfactant. This, then, results in the formation of a more ordered structure, even though evaporation of the ethanol is faster. A proposed mechanism is illustrated in Figure 5. As a
Figure 3. Thermogravimetric analysis of water- and ethanol-washed powders.
Figure 5. Scheme for the formation of the Sc-ZrO2 mesoporous powders: (a) as a first step a glycolate gel containing ordered micelles is formed, (b) as the powders are washed with either water or ethanol some surfactant is removed, and (c) the subsequent calcination process then results in the elimination of surface surfactant by volatilization, crystallization, and coarsening of the pores.
Figure 4. Scanning electron micrographs of (a) as-synthesized powders cleaned with water, (b, c) water-washed powders calcined at 450 °C, (d) as-synthesized powders cleaned with ethanol, and (e, f) ethanol-washed powders calcined at 450 °C.
first step in the process (Figure 5a), a glycolate gel with ordered micelles forms. As the powders are washed with either water or ethanol, some surfactant is removed (Figure 5b). The calcination process then results in the elimination of further surfactant by volatilization (Figure 5c), although complete elimination is not likely, since some of the surfactant is probably trapped in the closed pores of the structure deep in the powders. The calcination process also results in crystallization of the powders (Figure 1) and coarsening of the pores. The surface area is low (the values we obtained were 18.6 ± 0.1 and 24.8 ± 0.3 m2/g for the ethanol and waterwashed powders, respectively, calcined at 450 °C); thus, the porosity is only present on the surfaces, and there is no continuous porosity that runs deep into the powder particles. At a calcination temperature of 600 °C, the porosity in both the water-washed and ethanol-washed powders is almost
obtained similar morphologies using templating processes. Both the water-washed and ethanol-washed powders form a macroporous structure when calcining at 450 °C (Figures 4c and 4f, respectively) with pore sizes around 100 nm. The waterwashed powders exhibit a more ordered porosity, whereas the ethanol-washed powders have some areas of collapsed pores and a less homogeneous pore size distribution. Soler-Illia et al.37 have determined that the formation of organized mesostructures does not take place when evaporating the solvent (ethanol in their study) at room temperature, since this results in longer evaporation times that can inhibit the desired segregation of surfactant species at the mesoscale. That is, they conclude that solvent evaporation must be quick in order to D
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likely as a carbonaceous residue. This effect may be very prominent in powder samples, but less apparent in films with arrays of pore “channels” that allow for a more accessible route for the surfactant to escape. Thus, surfactant-assisted formation of mesoporous structures can be a compromise between achieving a stable mesoporous architecture and material purity. The issue of material purity in mesoporous materials is one that has not been fully explored. We propose that removal of the surfactant in many mesoporous materials presented in the literature is not completely achieved, and the presence of these organics has to be considered during subsequent processing of the powders and/or for their use in industrial applications. In addition, knowledge of the particle (agglomerate) size is essential for powder handling during a variety of manufacturing techniques (i.e., slip casting, tape casting, injection molding) that can result in useful components and/or devices. Thus, the use of dynamic light scattering or any other technique that can elucidate particle size is essential if a full characterization of the powders is needed for achieving postprocessing effectiveness.
completely eliminated. This is in contrast to the results described by Mamak et al.14,15 on mesoporous YSZ powders washed with water. These powders must contain organics in the as-synthesized state, even after water washing, and seem not to loss these organics until a calcination temperature above 300 °C is reached. Unfortunately, the number of wash cycles performed on the powders was not specified; thus, it is impossible to even roughly estimate the level of purity of the powders before calcination. What is shown is that when calcining these powders at 600 °C, there appears to be negligible loss of the low-angle XRD peak (located below 5° 2θ), presumably demonstrating the stability of these materials. Nonetheless, the surface area, when comparing powders calcined at 450 and 600 °C decreases from 264 to 116 m2/g, respectively, demonstrating that pore collapse is occurring to a significant extent. Thus, we postulate that these powders are not able to survive the removal of surfactant or the required operating temperatures of SOFCs, which typically operate at temperatures greater than 600 °C. Our own synthesis and calcination treatments are nearly equivalent to the ones followed by Mamak et al.,13−15 with the exception of the washing procedures, for which we did not have specifics that could be replicated. As mentioned earlier, the BET surface area for the ethanol-washed powders subjected to a 6 h ramp to 450 °C was found to be 18.6 ± 0.1 m2/g, a value that is quite low compared to Mamak et al.’s results. Mamak et al.15 found that samples with 12.5 at% yttrium did not maintain a mesoporous structure during calcination, whereas samples with dopant amounts greater than 20 at. % were improved in this regard. The conjecture is that yttrium has a stabilizing effect on the mesoporous structure. Since our powders have a lower quantity of dopant, one more commonly found in SOFCs, collapse of the pores is not unexpected at calcination temperatures of 600 °C. Clearly, the dopant amount that results in the highest conductivity in Sc-ZrO212 is not the one that stabilizes a mesoporous structure. Thus, the use of this material as a mesoporous SOFC anode is not promising, although dense Sc-ZrO2 might still have potential as an SOFC electrolyte. The reader is referred to several sources38−44 for a general review of the operation of an SOFC and its various components. It has been repeatedly demonstrated that in order to maintain a porous structure, a rigid 3D inorganic framework based on metal−oxygen−metal bonds needs to be formed before the removal of the surfactant template. However, there is a fine balance in the process of removing the surfactant. Temperatures should be high enough to remove surfactant, yet a temperature that is too high will lead to pore collapse. This has been shown in a variety of materials, including most recently on mesoporous MgO.45 The desired surfactant concentration can be obtained by washing away excess surfactant after the desired structure has been achieved, but before the calcination process. While this method is effective at producing mesoporosity, there is no guarantee that all of the surfactant will be removed by the heat treatment, which may result in an impure final product in which the surfactant is present as an organic or one in which the surfactant has converted to a carbonaceous residue. The surfactant that remains after washing, ideally only present as a templating agent, is presumably removed during the calcination process. However, depending on the length and temperature of the heat treatment, surfactant present deep inside the powders may be unable to fully volatilize and will remain in the sample, most
4. CONCLUSIONS We present an analysis of agglomeration behavior in mesoporous 9 mol % scandia-stabilized zirconia materials. Materials preparation was achieved through the use of a sol−gel process with a glycolate gel as intermediate. After synthesis, the powders were washed with either ethanol or water and subsequently calcined at 450 and 600 °C. This approach permitted the careful characterization of particle/agglomerate size that can be correlated to cleaning effectiveness and thermal processing. We determined that cleaning the powders with water resulted in better formation of a mesoporous structure because higher amounts of surfactant are preserved on the powders after washing. Nonetheless, this results in agglomerate sizes that are larger. The water-washed powders have particle sizes of >5 μm in the as-synthesized state. Calcination at 450 and 600 °C reduces the particle size to ∼1−2 and 0.5 μm, respectively. Cleaning with ethanol results in a mesoporous morphology that is less well-defined compared to the waterwashed powders, but the agglomerate size is smaller and has an average size of ∼250 nm that does not vary with calcination temperature. We attribute this to the effective removal of surfactant by ethanol, which does not form bridging bonds between the as-synthesized precipitates. Our analysis shows that surfactant-assisted formation of mesoporous structures can be a compromise between achieving a stable mesoporous architecture and material purity. We contend that removal of the surfactant in many mesoporous materials presented in the literature is not completely achieved and the presence of these organics has to be considered during subsequent processing of the powders and/or for their use in industrial applications. In addition, knowledge of the particle (agglomerate) size is essential for powder handling during a variety of manufacturing techniques (i.e., slip casting, tape casting, injection molding) that can result in useful components and/or devices. Thus, the use of dynamic light scattering or any other technique that can elucidate particle size is essential if a full characterization of the powders is needed for achieving postprocessing effectiveness.
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Notes
Low-Temperature Ceramic Fuel Cells. Int. J. Hydrogen Energy 2013, 38, 597−602. (21) Kelly, J. P.; Graeve, O. A. Effect of Powder Characteristics on Nanosintering. In Sintering Mechanisms of Convention Nanodensification and Field Assisted Processes; Springer: New York, 2013, pp 77. DOI: 10.1007/978-3-642-31009-6_4. (22) Graeve, O. A.; Fathi, H.; Kelly, J. P.; Saterlie, M. S.; Sinha, K.; Rojas-George, G.; Kanakala, R.; Brown, D. R.; Lopez, E. A. Reverse Micelle Synthesis of Oxide Nanopowders: Mechanisms of Precipitate Formation and Agglomeration Effects. J. Colloid Interface Sci. 2013, 407, 302−309. (23) Fathi, H.; Kelly, J. P.; Vasquez, V. R.; Graeve, O. A. Ionic Concentration Effects on Reverse Micelle Size and Stability: Implications for the Synthesis of Nanoparticles. Langmuir 2012, 28, 9267−9274. (24) Vasquez, V. R.; Williams, B. C.; Graeve, O. A. Stability and Comparative Analysis of AOT/Water/Isooctane Reverse Micelles System Using Dynamic Light Scattering and Molecular Dynamics. J. Phys. Chem. B 2011, 115, 2979−2987. (25) Gardner, A.; Vasquez, V. R.; Clifton, A.; Graeve, O. A. Molecular Dynamics Analysis of the AOT/Water/Isooctane System: Effect of Simulation Time, Initial Configuration, and Model Salts. Fluid Phase Equilib. 2007, 262, 264−270. (26) Graeve, O. A.; Sinha, K. Dynamic Light Scattering Study of Reverse Micellar Systems for the Synthesis of Iron-Based Nanofluids. Int. J. Mod. Phys. B 2007, 21, 4774−4781. (27) Graeve, O. A.; Corral, J. O. Preparation and Characterization of Rare-Earth Doped Y2O3 Luminescent Ceramics by the Use of Reverse Micelles. Opt. Mater. 2006, 29, 24−30. (28) Graeve, O. A.; Singh, H.; Clifton, A. Synthesis and Consolidation of Zirconia Powders via a Unique Reverse Micelle Synthesis Process and Spark Plasma Sintering. Ceram. Trans. 2006, 194, 209−223. (29) Singh, H.; Graeve, O. A. Reverse Micelle Synthesis of Zirconia Powders: The Use of Hydrogen Peroxide as Washing Solvent. Mater. Res. Soc. Symp. Proc. 2005, 879E, Z10.28. (30) Wang, F.; Jakes, J. E.; Geng, D.; Wang, X. Spontaneous Phase Transformation and Exfoliation of Rectangular Single-Crystal Zinc Hydroxy Dodecylsulfate Nanomembranes. ACS Nano 2013, 7, 6007− 6016. (31) Butz, B.; Störmer, H.; Gerthsen, D.; Bockmeyer, M.; Krüger, R.; Ivers-Tiffée, E.; Luysberg, M. Microstructure of Nanocrystalline Yttriadoped Zirconia Thin Films Obtained by Sol-Gel Processing. J. Am. Ceram. Soc. 2008, 91, 2281−2289. (32) Kaliszewski, M. S.; Heuer, A. H. Alcohol Interaction with Zirconia Powders. J. Am. Ceram. Soc. 1990, 73, 1504−1509. (33) Sinha, K.; Kavlicoglu, B.; Liu, Y.; Gordaninejad, F.; Graeve, O. A. A Comparative Study of Thermal Behavior of Iron and Copper Nanofluids. J. Appl. Phys. 2009, 106, 064307. (34) Graeve, O. A.; Madadi, A.; Kanakala, R.; Sinha, K. Analysis of Particle and Crystallite Size in Tungsten Nanopowder Synthesis. Metall. Mater. Trans. A 2010, 41, 2691−2697. (35) Pattanayak, A.; Subramanian, A. Production of Meso- and GigaPorous Zirconia Particles − An Improved Multi-step Particle Aggregation Process. Powder Technol. 2009, 192, 359−366. (36) Grosso, D.; Soler Illia, G. de A. A.; Crepaldi, E. L.; Charleux, B.; Sanchez, C. Nanocrystalline Transition-Metal Oxide Spheres with Controlled Multi-scale Porosity. Adv. Funct. Mater. 2003, 13, 37−42. (37) Soler-Illia, G. J. de A. A.; Louis, A.; Sanchez, C. Synthesis and Characterization of Mesostructured Titania-Based Materials through Evaporation-Induced Self-Assembly. Chem. Mater. 2002, 14, 750−759. (38) Badwal, S. P. S.; Foger, K. Solid Oxide Electrolyte Fuel Cell Review. Ceram. Int. 1996, 22, 257−265. (39) Boudghene Stambouli, A.; Traversa, E. Solid Oxide Fuel Cells (SOFCs): A Review of an Environmentally Clean and Efficient Source of Energy. Renew. Sust. Energy Rev. 2002, 6, 433−455. (40) Haile, S. M. Fuel Cell Materials and Components. Acta Mater. 2003, 51, 5981−6000.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was funded by grant #0839146 from the National Science Foundation.
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REFERENCES
(1) Akbar, S.; Dutta, P.; Lee, C. High-Temperature Ceramic Gas Sensors: A Review. Int. J. Appl. Ceram. Technol. 2006, 3, 302−311. (2) Lee, J.-H. Review on Zirconia Air-fuel Ratio Sensors for Automotive Applications. J. Mater. Sci. 2003, 38, 4247−4257. (3) Rheaume, J. M.; Pisano, A. P. A Review of Recent Progress in Sensing of Gas Concentration by Impedance Change. Ionics 2011, 17, 99−108. (4) Benammar, M. Techniques for Measurement of Oxygen and Airto-fuel Ratio Using Zirconia Sensors. A Review. Meas. Sci. Technol. 1994, 5, 757−767. (5) Bej, S. K.; Maity, S. K.; Turaga, U. T. Search for an Efficient 4,6DMDBT Hydrodesulfurization Catalyst: A Review of Recent Studies. Energy Fuels 2004, 18, 1227−1237. (6) Sharma, S.; Pollet, B. G. Support Materials for PEMFC and DMFC Electrocatalysts − A Review. J. Power Sources 2012, 208, 96− 119. (7) Rodriguez-Suarez, R.; Bartolomé, J. F.; Moya, J. S. Mechanical and Tribological Properties of Ceramic/Metal Composites: A Review of Phenomena Spanning from the Nanometer to the Micrometer Length Scale. J. Eur. Ceram. Soc. 2012, 32, 3887−3898. (8) Gamble, S. Fabrication − Microstructure − Performance Relationships of Reversible Solid Oxide Fuel Cell Electrodes − Review. Mater. Sci. Technol. 2011, 27 (10), 1485−1497. (9) Laguna-Bercero, M. A. Recent Advances in High Temperature Electrolysis Using Solid Oxide Fuel Cells: A Review. J. Power Sources 2012, 203, 4−16. (10) Sarantaridis, D.; Atkinson, A. Redox Cycling of Ni-Based Solid Oxide Fuel Cell Anodes: A Review. Fuel Cells 2007, 7, 246−258. (11) Kharton, V. V.; Marques, F. M. B.; Atkinson, A. Transport Properties of Solid Oxide Electrolyte Ceramics: A Brief Review. Solid State Ionics 2004, 174, 135−149. (12) Haering, C.; Roosen, A.; Schichl, H.; Schnöller, M. Degradation of the Electrical Conductivity in Stabilised Zirconia System Part II: Scandia-Stabilised Zirconia. Solid State Ionics 2005, 176, 261−268. (13) Mamak, M.; Coombs, N.; Ozin, G. A. Mesoporous NickelYttria-Zirconia Fuel Cell Materials. Chem. Mater. 2001, 13, 3564− 3570. (14) Mamak, M.; Coombs, N.; Ozin, G. Self-assembling Solid Oxide Fuel Cell Material: Mesoporous Yttria-Zirconia and Metal-YttriaZirconia Solid Solutions. J. Am. Chem. Soc. 2000, 122, 8932−8939. (15) Mamak, M.; Coombs, N.; Ozin, G. Mesoporous Yttria-zirconia and Metal-Yttria-Zirconia Solid Solutions for Fuel Cells. Adv. Mater. 2000, 12, 198−202. (16) Zelcer, A.; Soler-Illia, G. J. A. A. One-Step Preparation of UV Transparent Highly Ordered Mesoporous Zirconia Thin Films. J. Mater. Chem. C 2013, 1, 1359−1367. (17) Yuan, Q.; Li, L.-L.; Lu, S.-L.; Duan, H.-H.; Li, Z.-X.; Zhu, Y.-X.; Yan, C.-H. Facile Synthesis of Zr-Based Functional Materials with Highly Ordered Mesoporous Structures. J. Phys. Chem. C 2009, 113, 4117−4124. (18) Wang, X.; Pan, D.; Guo, M.; He, M.; Niu, P.; Li, R. Facile Synthesis of Highly Ordered Mesoporous Alumina with High Thermal and Hydrothermal Stability Using Zirconia as Promoter. Mater. Lett. 2013, 97, 27−30. (19) Yuan, X.; Cheng, L.; Liu, W.; Zhang, L. Synthesis of Ordered Mesoporous CoFe2O4-Containing Silica by Self-Assembly Process. J. Magn. Magn. Mater. 2013, 331, 198−203. (20) Ma, Y.; Wang, X.; Zhu, B.; Muhammed, M. Microwave Synthesis of Mesoporous Cu-Ce0.8Sm0.2O2−δ Composite Anode for F
dx.doi.org/10.1021/la4049743 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
(41) Singhal, S. C. Advances in Solid Oxide Fuel Cell Technology. Solid State Ionics 2000, 135, 305−313. (42) Steele, B. C. H. Material Science and Engineering: The Enabling Technology for the Commercialisation of Fuel Cell Systems. J. Mater. Sci. 2001, 36, 1053−1068. (43) Tietz, F.; Buchkremer, H.-P.; Stö ver, D. Components Manufacturing for Solid Oxide Fuel Cells. Solid State Ionics 2002, 152−153, 373−381. (44) Zhu, W. Z.; Deevi, S. C. A Review on the Status of Anode Materials for Solid Oxide Fuel Cells. Mater. Sci. Eng., A 2003, 362, 228−239. (45) Kim, T. K.; Lee, K. J.; Cheon, J. Y.; Lee, J. H.; Joo, S. H.; Moon, H. R. Nanoporous Metal Oxides with Tunable and Nanocrystalline Frameworks via Conversion of Metal-Organic Frameworks. J. Am. Chem. Soc. 2013, 135, 8940−8946.
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