Article pubs.acs.org/Langmuir
Important Property of Polymer Spheres for the Preparation of ThreeDimensionally Ordered Macroporous (3DOM) Metal Oxides by the Ethylene Glycol Method: The Glass-Transition Temperature Masahiro Sadakane,*,† Keisuke Sasaki,‡ Hiroki Nakamura,§ Takashi Yamamoto,§ Wataru Ninomiya,‡,§ and Wataru Ueda*,‡ †
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashi-Hiroshima 739-8527, Japan ‡ Catalysis Research Center, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan § Corporate Research Laboratories, Mitsubishi Rayon Co., Ltd., Otake, Hiroshima 739-0693, Japan S Supporting Information *
ABSTRACT: We demonstrate that the glass-transition temperature (Tg) of a polymer sphere template is a crucial factor in the production of threedimensionally ordered macroporous (3DOM) materials. Metal nitrate dissolved in ethylene glycol−methanol was infiltrated into the void of a face-centered, close-packed colloidal crystal of poly(methyl methacrylate) (PMMA)-based spheres. The metal nitrate reacts with EG to form a metal oxalate (or metal glycoxylate) solid (nitrate oxidation) in the void of the template when the metal nitrate−EG−PMMA composite is heated. Further heating converts metal oxalate to metal oxide and removes PMMA to form 3DOM materials. We investigated the effect of Tg of PMMA templates and obtained clear evidence that the solidification temperature of the metal precursor solution (i.e., nitration oxidation temperature) should be lower than the Tg of the polymer spheres to obtain a well-ordered 3DOM structure.
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INTRODUCTION Recently, much interest has been shown in three-dimensionally ordered macroporous (3DOM) materials with pores sizes in the submicrometer range.1−3 3DOM materials are produced by the following procedure: (i) a colloidal crystal template is prepared by ordering monodisperse spheres (e.g., polystyrene (PS), poly(methyl methacrylate) (PMMA), or silica) into a face-centered, close-packed array (opal structure), (ii) interstices in the colloidal crystal are then filled with liquid metal precursors, either neat or in solution, that solidify in voids of the sphere templates, and (iii) an ordered form is produced after removing the template by calcination or extraction. The ordered (“inverse opals”) structure synthesized by using this method consists of a skeleton surrounding uniform closepacked macropores. The macropores are interconnected through windows that form as a result of contact between the template spheres prior to the infiltration of the precursor solution. Furthermore, the 3DOM materials have high porosity, theoretically ca. 74%. Therefore, the 3DOM materials are attractive for catalysts,4−19 electrode materials,20−24 and photonic materials.25 Properties of polymer templates should be taken into account to achieve a well-ordered 3DOM structure. Two important features have been reported. (1) A larger polymer is favored. Larger spheres have larger void spaces, and a large amount of metal precursors can be placed in the void, which is favored to maintain the 3DOM structure.26−28 (2) Surface © XXXX American Chemical Society
modification with a functional group such as carboxylic acid favors the formation of a 3DOM structure.3,29,30 Metal precursors can react with a functional group and be preferably solidified under such conditions. For these reasons, the choice of a polymer sphere is very important for the synthesis of wellordered 3DOM materials. Here we report an additional important property of a polymer template for the production of 3DOM materials. In the course of our research to prepare a 3DOM mixed iron oxide, La1 − xSrxFeO3 − δ (x = 0−0.4) and MFe2O4 (M = Zn, Ni, or Co),10,27,31 we have reported a facile heating-induced procedure (Scheme 1). Our strategy was to use an ethylene glycol−methanol mixed solution of metal nitrate salts, which could be converted to a mixed metal glyoxylate or metal oxalate derivatives by in situ nitrate oxidation (eq 1) at low temperature before the template was removed. Further calcination removed the polymer template and converted the glyoxylate salt to mixed metal oxides, resulting in well-ordered 3DOM mixed iron oxide materials in high yield. The importance of the metal precursor concentration and the role of methanol were reported.10 The advantage of our method is that mixed metal oxides can be produced with a controlled metal ratio, which is not achievable using other methods. Our Received: May 17, 2012 Revised: November 19, 2012
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ters = 308 ± 10 nm, Tg = 418 K) were obtained from Mitsubishi Rayon Co., Ltd. Sphere diameters were measured by SEM images (Figure S1), and the glass-transition temperature (Tg) was determined with reference to the method of JIS K 7121 published by the Japanese Industrial Standards Committee by differential scanning calorimetry (DSC 6220C, Seiko Instruments) (Figure S2). The spheres were packed into colloidal crystals by centrifugation. The obtained template was crushed with an agate mortar, and the obtained particles were adjusted to 0.425−2.000 mm in size using testing sieves (Tokyo Screen, Co. Ltd.).27,32 Quartz sand (10−15 mesh) was purchased from Kokusan Chemical Works (Tokyo). Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a diffractometer (Rigaku, RINT UltimaIII) equipped with a graphite monochromator using Cu Kα radiation (tube voltage = 40 kV, tube current = 20 mA). Scanning electron microscopy (SEM) images were obtained with a JSM-7400F (JEOL). Synthesis of 3DOM Metal Oxides. Metal nitrate hydrates (metal concentration = 2 M) were dissolved with ca. 5 mL of ethylene glycol (EG) by slow stirring in a 100 mL beaker at room temperature or 313 K in the case of aluminum nitrate until all of the nitrate salt had dissolved, and the produced EG solution was poured into a 25 mL volumetric flask. Methanol (10 mL) and EG were added in amounts necessary to achieve the desired concentration. (The final concentration of methanol was 40% by volume.) Then the PMMA colloidal crystals were soaked in the solution for 3 h. Excess solution was removed from the impregnated PMMA colloidal crystals by vacuum filtration. The obtained sample was allowed to dry in air at room temperature overnight. A 0.5 g amount of sample was mixed with 2.5 g of quartz sand (10−15 mesh) and calcined in a tubular furnace (inner diameter of ca. 12 mm) in air flow (50 mL·min−1). The temperature was raised at a rate of 1 K·min−1 to 723 K and held for 5 h. Quartz sand (10−15 mesh) was separated from the powder sample using testing sieves.
Scheme 1. Synthesis of 3DOM Metal Oxide Using an Ethylene Glycol−Methanol Solution of Metal Nitrate as the Precursor Solution
ethylene glycol method has been applied to the production of several 3DOM materials,4,5,8−11,17,23,32,33 and improved material properties of the prepared 3DOM materials have been reported. M(NO3)3 + 3HOCH 2CH 2OH → M 2(C2O4 )3 + 3H 2O + 3NOx
(1)
However, our method has a limitation in its application; we could produce 3DOM iron, aluminum, manganese, and chromium oxides and their mixed metal oxides, but it has been difficult with other metal oxides. In comparing nitrate oxidation temperatures, the glass-transition temperature (Tg) of the PMMA template, and the formation of a 3DOM structure, we proposed that nitrate oxidation should be lower than the Tg of PMMA (ca. 378 K) to maintain the 3DOM structure.28 Tg is the critical temperature at which a polymer changes its behavior from being “glassy” (hard and brittle) to being “rubbery” (elastic and flexible). It has been reported that by heating polymers above Tg for a short time the polymers slightly melt and the size of necks between polymer spheres increases.3,27,34 Sphere shapes are completely lost if polymers are heated above Tg for a long time. The solution in the void of the colloidal crystal template should be solidified at a temperature lower than Tg. Otherwise, the solution is squeezed out from the void when the polymer is heated at a temperature higher than its Tg. In this article, we describe the preparation of 3DOM materials using PMMA-based polymer spheres, which are copolymers consisting of MMA and other methacrylates with different glass-transition temperatures. The purposes of this study are (1) proving our proposal about the relationship between the nitrate oxidation temperature and the glasstransition temperature of a polymer sphere and then (2) extending our method to produce 3DOM metal oxides with other metals that could not be achieved before.
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RESULTS AND DISCUSSION Selection of Polymer Spheres. We selected three PMMA-based polymer spheres: PMMA (Tg = 382 K, diameters = 365 ± 8 nm), P(MMA-IBXMA) (Tg = 389 K, diameters = 324 ± 25 nm), and P(MMA-BA- IBXMA) (Tg = 418 K, diameters = 308 ± 10 nm). The glass-transition temperature (Tg) was increased by the copolymerization of isobornyl metacylate (IBXMA) and butyl acrylate (BA). IBXMA and BA are acrylate derivatives without functional groups such as carboxylic acid, which is known to aid the formation of 3DOM structures. It is known that a larger sphere is preferred for the production of a 3DOM structure. Therefore, we selected polymer spheres with smaller diameters when the glasstransition temperature was increased in order to observe the effect of the glass-transition temperature more carefully. Formation of 3DOM Materials. An ethylene glycol− methanol mixed solution of metal nitrate was infiltrated into the void of a colloidal crystal template of polymer spheres, and the metal nitrate−ethylene glycol−polymer composite was calcined at 723 K to remove polymers. We have reported that a temperature of 673−700 K is enough to remove PMMA templates under air flow,6,27,28 although a small amount of carbon remained in the sample. The calcination temperature of 723 K was set to be slightly higher than the combustion temperature of polymers to observe the influence of the glasstransition temperature on the formation of the 3DOM structure. It has been reported that further heating decomposes the 3DOM structure by crystal formation in some cases.28 The formation of a 3DOM structure of the obtained solid was observed by SEM images (Figures 1 and S3−S5) and classified into four categories. More than 20 particles were randomly chosen by SEM, and images with magnifications of ×5000 to
EXPERIMENTAL SECTION
Materials. All chemicals used were reagent grade and used as supplied. A suspension of monodisperse poly(methyl methacrylate) (PMMA) spheres (diameters = 365 ± 8 nm, Tg = 382 K) were synthesized by literature techniques.27,32 Suspensions of monodisperse poly(methyl methacrylate-isobornyl methacrylate) (P(MMA-IBXMA) (diameters = 324 ± 25 nm, Tg = 389 K) and poly(methacrylate-butyl acrylic acid-isobornyl methacrylate) (P(MMA-BA-IBXMA)) (diameB
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but the 3DOM structure was not well-ordered and the samples had only partial 3DOM structures (category C). 3DOM materials of other metal oxides (Cr, Cu, and Zn) could not be obtained (category D). When Tg was further increased to 418 K, well-ordered 3DOM materials of Fe, Al, Ce, Co, Zn, Ni, La, and Mg oxides were obtained (category A). These results clearly indicate that Tg should be higher than the nitrate oxidation temperature, which is the solidification temperature of metals. Even when we used the template with Tg = 418 K, the obtained Mn, Cr, and Cu oxide materials did not have wellordered 3DOM structure. In the case of Mn oxide, the fraction of the 3DOM structure was not large (category C). Moreover, 3DOM materials of other metal oxides (Cr and Cu) could not be obtained at all (category D). It has been reported that 3DOM materials of Cr2O3 need polymer spheres larger than 400 nm because of the crystal growth of Cr2O3.28 Therefore, larger polymer spheres are necessary for the production of 3DOM Cr2O3. Unfortunately, we do not have an explanation for the unsuccessful formation of 3DOM Mn and Cu oxides. 3DOM Cu2O has been prepared only by electrochemical deposition methods.35,36 Recently, 3DOM La2CuO4 was also prepared using an ethylene glycol method together with citric acid.33 However, the obtained 3DOM structure was not wellordered. Because Mn- and Cu-based oxides are important materials for catalysis, further investigations to produce wellordered 3DOM Mn- and Cu-based oxides are currently underway in our group. We could extend the scope of metals with our ethylene glycol method by increasing the Tg of polymers. It became clear that our method using an EG solution of metal nitrate can produce 3DOM metal oxides with Fe, Cr, Mn, Al, Ce, Co, Zn, Ni, La, and Mg. 3DOM metal oxides with these metals have been prepared using two-step precipitation methods and the organic acid chelating method. In the two-step precipitation methods, metal precursors such as nitrates or acetates were infiltrated into the voids of the templates. Then these metal salts were reacted with oxalic acid,37 ethylenediamine tetraacetate (EDTA),38 or ammonia26,39 to form a solid in the void. It has been reported that 3DOM Fe,40 Mg,41−43 Ce,44,45 La,44 Sm,46 and Eu46 oxides can be produced if metal salts were
Figure 1. SEM images of 3DOM: (a) La oxide and (b) MgO prepared using a colloidal crystal of P(MMA-BA-IBMXM) (Tg = 418 K, diameters = 308 ± 10 nm) as a template.
×10 000 were taken. The fraction of 3DOM was calculated from the number of particles containing a 3DOM structure/ total number of particles, and 3DOM materials were classified into four categories: (1) a well-ordered 3DOM structure observed by SEM with the fraction of 3DOM being more than 95% (category A), (2) more than 95% of the samples containing a 3DOM structure but not well-ordered (category B), (3) 80−40% of the samples containing a 3DOM structure (category C), and (4) only a very small fraction of the samples having a 3DOM structure (fraction of 3DOM < 5%) or porous structure hardly being observed (category D). The results are summarized together with the nitrate oxidation temperatures in Table 1. The order of the nitrate oxidation temperature of the metals used is Fe < Cr < Al < Cu < Mn < Ce < Co < Zn < Ni < La < Mg, ranging from 328 to 413 K.28 When PMMA (Tg = 382 K, diameter = 365 nm) was used, well-ordered 3DOM materials of Fe and Al oxides were obtained in high fraction yields (category A). Though 3DOM CeO2 was obtained, the 3DOM structure was not well-ordered (category B). In the case of Mn oxide, the fraction of the 3DOM structure was not large (category C). 3DOM materials of other metal oxides (Cr, Cu, Co, Zn, Ni, La, and Mg) could not be obtained (category D). When Tg was increased to 388 K (P(MMA-IBXMA)), wellordered 3DOM materials of Fe, Al, and Ce oxides were obtained (category A). 3DOM Co3O4 oxide was obtained, but the 3DOM structure was not well-ordered (category B). 3DOM structures of Mn, Ni, La, and Mg oxides were observed,
Table 1. Formation of 3DOM Metal Oxide Materials Using PMMA-Based Polymers with Different Glass-Transition Temperatures metal
Fe
Cr
Al
Cu
Mn
Ce
Co
Zn
Ni
La
Mg
ox. temp. (K)a crystalb
328 Fe2O3
335 Cr2O3
342 Amor
375 CuO
377 Mn2O3, Mn3O4
383 CeO2
386 Co3O4
388 ZnO
392 NiO
403 La5O7NO3, La2O2CO3
413 MgO
C
B
D
D
D
D
D
polymer: P(MMA-IBXMA) (diameter =324 ± 25 nm, Tg = 389 K) 3DOMc A D A D C
A
B
D
C
C
C
polymer: P(MMA-BA-IBXMA) (diameter = 308 ± 10 nm, Tg = 418 K) 3DOMc A D A D C
A
A
A
A
A
A
polymer: PMMA (diameter = 365 ± 8 nm, Tg = 382 K) 3DOMc A D A D
a
Obtained from TG-DTA.28 bObtained from XRD. c(A) A well-ordered 3DOM structure was observed by SEM, and the fraction of 3DOM was more than 95% (more than 20 particles were randomly chosen by SEM, and images with magnifications of ×5000 to ×10 000 were taken. The fraction of 3DOM was calculated as follows: number of particles containing a 3DOM structure/total number of particles). (B) More than 95% of the samples contained a 3DOM structure that was not well-ordered. (C) 80−40% of the samples contained a 3DOM structure. (D) Only a very small fraction of the samples had a 3DOM structure (fraction of 3DOM being less than 5%) or a porous structure was hardly observed by SEM. C
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(6) Sadakane, M.; Sasaki, K.; Kunioku, H.; Ohtani, B.; Abe, R.; Ueda, W. Preparation of 3-D Ordered Macroporous Tungsten Oxides and Nano Particle Tungsten Oxides Using a Colloidal Crystal Template Method, and Their Structural Characterization and Application as Photocatalysts under Visible Light Irradiation. J. Mater. Chem. 2010, 20, 1811−1818. (7) Sadakane, M.; Sasaki, K.; Kunioku, H.; Ohtani, B.; Ueda, W.; Abe, R. Preparation of Nano-Structured Crystalline WO3 and Improved Photocatalytic Activity for Decomposition of Organic Compounds under Visible Light Irradiation. Chem. Commun. 2008, 6552−6554. (8) Xu, J.; Liu, J.; Zhao, Z.; Xu, C.; Zheng, J.; Duan, A.; Jiang, G. Easy Synthesis of Three-Dimensionally Ordered Macroporous La1‑xKxCoO3 Catalysts and Their High Activities for the Catalytic Combustion of soot. J. Catal. 2011, 282, 1−12. (9) Zhang, X.; Hirota, R.; Kubota, T.; Yoneyama, Y.; Tsubaki, N. Preparation of Hierarchically Meso-Macroporous Hematite Fe2O3 Using PMMA as Imprint Template and Its Reaction Performance for Fischer−Tropsch Synthesis. Catal. Commun. 2011, 13, 44−48. (10) Sadakane, M.; Asanuma, T.; Kubo, J.; Ueda, W. Facile Procedure to Prepare Three-Dimensionally Ordered Macroporous (3DOM) Perovskite-Type Mixed Metal Oxides by Collidal Crystal Templating Method. Chem. Mater. 2005, 17, 3546−3551. (11) Wei, Y.; Liu, J.; Zhao, Z.; Chen, Y.; Xu, C.; Duan, A.; Jiang, G.; He, H. Highly Active Catalysts of Gold Nanoparticles Supported on Three-Dimensionally Ordered Macroporous LaFeO3 for Soot Oxidation. Angew. Chem., Int. Ed. 2011, 50, 2326−2320. (12) Han, D.; Li, X.; Zhang, L.; Wang, Y.; Yan, Z.; Liu, S. Hierarchically Ordered Meso/Macroporous Gamma-Alumina for Enhanced Hydrodesulfurization Performance. Microporous Mesoporous Mater. 2012, 158, 1−6. (13) Zhao, Z.; Dai, H.; Deng, J.; Du, Y.; Liu, Y.; Zhang, L. ThreeDimensionally Ordered Macroporous La0.6Sr0.4FeO3‑d: High-Efficiency Catalysts for the Oxidative Removal of Toluene. Microporous Mesoporous Mater. 2012, 163, 131−139. (14) Liu, Y.; Dai, H.; Du, Y.; Deng, J.; Zhang, L.; Zhao, Z. LysineAided PMMA-Templating Preparation and High Performance of Three-Dimensionally Ordered Macroporous LaMnO3 with Mesoporous Walls for the Catalytic Combustion of Toluene. Appl. Catal. B 2012, 119−120, 20−31. (15) Ji, K.; Dai, H.; Deng, J.; Zhang, L.; Wang, F.; Jiang, H.; Au, C. T. Three-Dimensionally Ordered Macroporous SrFeO3‑δ with High Surface Area: Active Catalysts for the Complete Oxidation of Toluene. Appl. Catal. A 2012, 425−426, 153−160. (16) Liu, Y.; Dai, H.; Du, Y.; Deng, J.; Zhang, L.; Zhao, Z.; Au, C. T. Controlled Preparation and High Catalytic Performance of ThreeDimensionally Ordered Macroporous LaMnO3 with Nanovoid Skeletons for the Combustion of Toluene. J. Catal. 2012, 287, 149− 160. (17) Zheng, J.; Liu, J.; Zhao, Z.; Xu, J.; Duan, A.; Jiang, G. The Synthesis and Catalytic Performances of Three-Dimensionally Ordered Macroporous Perovskite-Type LaMn1‑xFexO3 Complex Oxide Catalysts with Different Pore Diameters for Diesel Soot Combustion. Catal. Today 2012, 191, 146−153. (18) Xu, J.; Liu, J.; Zhao, Z.; Zheng, J.; Zhang, G.; Duan, A.; Jiang, G. Three-Dimensionally Ordered Macroporous LaCoxFe1‑xO3 PerovskiteType Complex Oxide Catalytsts for Diesel Soot Combustion. Catal. Today 2010, 153, 136−142. (19) Liu, Y.; Dai, H.; Deng, J.; Zhang, L.; Au, C. T. ThreeDimensional Ordered Macroporous Bismuth Vanadates: PMMATemplating Fabrication and Excellent Visible Light-Driven Photocatalytic Performance for Phenol Degradation. Nanoscale 2012, 4, 2317−2325. (20) Bosco, J. P.; Sasaki, K.; Sadakane, M.; Ueda, W.; Chen, J. G. Synthesis and Characterization of Three-Dimensionally Ordered Macroporous (3DOM) Tungsten Carbide: Application to Direct Methanol Fuel Cells. Chem. Mater. 2010, 22, 966−973. (21) Chai, G. S.; Yoon, S. B.; Yu, Y.-S.; Choi, J.-H.; Sung, Y.-E. Ordered Porous Carbons with Tunable Pore Sizes as Catalyst
infiltrated into the void of the template together with organic acids such as citric acid as chelating reagents (organic acid chelating method). These methods are not suitable for the preparation of 3DOM materials of mixed-metal oxides. Each metal has a different reactivity with an organic acid or base, and the produced salts have different degrees of solubility in the reacting media, resulting in mixed-metal oxides with undesired metal ratios.4,47−49 On the other hand, our ethylene glycol method has the advantage that 3DOM mixed metal oxides can be obtained with a controlled metal ratio, and 3DOM Fe 2 O 3 , 9 , 4 0 La 1 − x Sr x FeO 3 , 10,11 La 1 − x K x CoO 3 , 8 LaMn 1 − x Fe x O 3 , 17 CoFe 2 O 4 , 2 3 Ce x Zr 1 − x O 2 , 4 Ce−Pr−Zr oxides, 5 and La2CuO433 materials prepared with ethylene glycol showed enhanced catalytic properties for several reactions. As shown here, we can extend the scope of metals, which aids the development of 3DOM metal oxide catalysts, including 3DOM mixed-metal oxide catalysts, in the future.
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CONCLUSIONS We clearly demonstrated that metal precursors in the voids of colloidal crystal templates of polymer spheres should be solidified before the glass-transition temperature (Tg) of the polymer, where the polymer starts to change its shape, for the successful production of three-dimensionally ordered macroporous (3DOM) materials. According to our procedure using ethylene glycol, well-ordered 3DOM materials of Fe-, Al-, Ce-, Co-, Zn-, Ni-, La-, and Mg-based oxides were successfully synthesized in combination with PMMA-based templates with high Tg.
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ASSOCIATED CONTENT
S Supporting Information *
SEM images and TG-DSC curves. SEM images of metal oxides prepared by using colloidal crystal templates of polymer spheres of polymer templates. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*(M.S.) Tel: 81-82-424-4456. Fax: 81-82-424-5494. E-mail:
[email protected]. (W.U.) Tel: 81-11-706-9166. Fax: 81-11-706-9163. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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dx.doi.org/10.1021/la303921u | Langmuir XXXX, XXX, XXX−XXX
