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A new template for the synthesis of porous inorganic oxide monoliths
Journal of Non-Crystalline Solids 352 (2006) 4003–4007 www.elsevier.com/locate/jnoncrysol
A new template for the synthesis of porous inorganic oxide monoliths Zhi-Guo Shi *, Lan-Ying Xu, Yu-Qi Feng Department of Chemistry, Wuhan University, Wuhan, Hubei 430072, PR China Received 5 February 2006; received in revised form 15 July 2006 Available online 18 September 2006
Abstract A hydrophilic monolith, urea-formaldehyde (UF) resin, was for the first time proposed as a hard template for the synthesis of inorganic monoliths. Three kinds of inorganics, silica, zirconia and titania, were successfully prepared by using the new template. Scanning electron microscopy (SEM), nitrogen sorption measurements indicate that the inorganics are the replicas of their templates. Due to the characteristics of low cost, facile control of structure and ease of removal, the UF resin facilitates mass production of inorganic monoliths, which are attractive to industry. In addition, the hydrophilic characteristic of the UF resin makes it possible to use organic free precursors for preparing inorganic monoliths, which should be benign to the environment. The successful synthesis of the zirconia monolith using non-organic precursors confirms this. Ó 2006 Elsevier B.V. All rights reserved. PACS: 81.70. q; 81.20.Fw Keywords: Nanocrystals; Porosity; Silica; Processing; Sol–Gels (xerogels)
1. Introduction Because of high stability, large surface area, controllable pore structure as well as operational convenience, porous materials in the monolithic configuration are important for both academic study and technological application in catalysis, energy storage, opto-electronics, purification, separation, and adsorption [1–3]. Until now monolithic organic polymers have been extensively synthesized and their applications in bio-separation and bio-reaction have been demonstrated to be very successful [4,5]. However, due to the limited scopes of application arising from the low mechanical strength and low thermal resistance typical of polymer materials, a great deal of interest has been drawn to the development of synthesis and processing procedures for inorganic monoliths. Generally three methods have been proposed, gel casting *
Corresponding author. Fax: +86 27 68754067. E-mail address: [email protected] (Z.-G. Shi).
0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.08.008
[6–9], sol–gel technology [10–24] and nano-casting technology [25,26]. Among them, the nano-casting technology was the most widely used. Generally a deposable hard template was adopted to confine the reaction of precursors in its pores. Afterwards, the hard template was eliminated by combustion or eroding. Finally a new material with the replica structure of its template was acquired. In order to obtain monolithic replicas, monolithic templates with interconnecting pores should be utilized. To date, several kinds of such templates have been proposed for nano-casting. For example, silica monoliths have been used as hard templates for casting carbon monoliths and cobalt monoliths [27–30], monolithic polyurethane for silica and titanium silicate [31,32], polystyrene and starch gel for titania [1,33,34], carbon aerogel for magnesium oxide and ZSM-5 monolith [35,36], and native rattan stem for SiSiC/zeolite monoliths [37]. Though so many monolithic templates have been applied in nano-casting, there still remain challenges in
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their use arising from some inherent disadvantages. For instance, monolithic silica templates were commonly produced from costly precursors like tetramethoxysilane or tetraethoxysilane [27–30]. Monolithic carbons are usually prepared in severe conditions such as high temperature and inert environmental, which also increases the operational costs. Polyurethane foam and polystyrene are hydrophobic polymers, which can only serve as templates for replica materials prepared from high organic precursors [1,31,32]. Though starch gel is an economical template, the gel formation involves time-consuming procedures of freezing, thawing, and low-temperature evaporation. Furthermore, the structure as well as the strength of the starch gel cannot be guaranteed [33]. A cheap and structurally controllable monolithic template would be a solution to these problems, and it would be essential for mass production and the application of inorganic monoliths. Here we propose a new and promising template, hydrophilic urea-formaldehyde (UF) resin monolith (foam), for nano-casting. As a widely used industrial adhesive, the UF resin possesses several advantages over other templates: low cost, ease of preparation, ready elimination by combustion and facile control of structure. Furthermore, because of the hydrophilic characteristic of the UF resin, organic solvent free precursors can be also utilized for nano-casting, which should be benign to the environment. By adopting such new templates, several kinds of porous inorganic oxide monoliths (zirconia, silica, titania) have been conveniently fabricated, demonstrating the suitability of UF resin as a hard template in nano-casting. 2. Experimental 2.1. Materials Urea, formaldehyde, zirconium oxychloride (ZrOCl2 Æ 8H2O), oxalic acid, tetrabutyl titanate, acetylacetone, ethanol, poly (ethylene glycol) (PEG, Mw = 6000), hydrochloric acid, nitric acid were purchased from Shanghai General Chemical Reagent Factory (Shanghai, China). Tetraethoxysilane (TEOS) was obtained from the Chemical Factory of Wuhan University. Distilled water was from a quartz apparatus. All chemicals were used as received. 2.2. Preparation of UF resin monolith After dissolving 36 g of urea in 83.3 g formaldehyde solution (36%, w/w), the pH of this mixture was adjusted to 7–8 by 10% (w/w) sodium hydroxide solution. And then it was refluxed for 3 h to obtain milk like prepolymer of UF resin. To this UF prepolymer, 6 g of PEG and 10 ml of hydrochloric acid (pH 1.0) were added under stirring. Thereafter, the mixture was poured into a cylindrical vessel for gelation at 298 K. After 24 h, the rod like UF resin was formed and washed with a large amount of water. Then it was dried for further usage.
2.3. Preparation of inorganic oxide monoliths 6 ml of TEOS and 10 ml of hydrochloric acid (pH 2.0) were mixed at 333 K to get silica sol (Si-sol). After immersing into the Si-sol for 30 min, the UF resin was taken out and put into an oven of 313 K to react for 4 h. The same procedures were repeated for three times. Thereafter, the UF resin and silica composite was put into a furnace to burn off the UF resin and strengthen the silica. A programmed temperature cycle was applied to the furnace. The temperature was first ramped from 303 K to 1173 K at 1 K/min, and then kept at 1173 K for 2 h, to ensure complete removing the UF resin. For zirconia monolith, the sol was prepared as following. Zirconium oxychloride was dissolved in water first. To this solution, oxalic acid was added drop wise, white precipitate emerged simultaneously. After being aged in situ for 10 h, the precipitate was collected and dried at 303 K for 5 days. Thereafter, the dried precipitate was re-dissolved into deionized water to obtain zirconium sol. The following procedure is similar to that of the silica monolith. For preparation of titania monolith, the sol was obtained by mixed tetrabutyl titanate, acetylacetone, ethanol, water and nitric acid. Other procedures are similar to that of silica monolith and zirconia one. The as-synthesized UF resin foam and the resulting inorganic oxides were characterized by Elemental Analysis (EA), X-ray powder diffraction (XRD) (if necessary), scanning electron microscopy (SEM), and nitrogen adsorption/ desorption measurements. SEM images were obtained by using a Hitachi (Tokyo, Japan) Model X-650 Microscope. Samples were coated with a gold layer approximately 20 nm thick by sputtering. Powder XRD patterns were acquired on a Lab-X 3000 diffractometer using CuKa radiation. After the samples were vacuum dried at 423 K for 10 h, nitrogen adsorption/desorption isotherms were measured at 76 K using a Beckman Coulter SA3100 system. Specific surface areas were measured using Brunauer–Emmett–Teller (BET) method, and pore size distributions were obtained from the desorption branches of the isotherms using BarrettJoyner-Halenda (BJH) model. 3. Results Fig. 1 shows the photographs of the UF resins, the composites and the inorganic oxides (from the bottom to the top), (a) silica, (b) zirconia, (c) titania. Apparently, monolithic inorganic oxides were successfully prepared by the new template through the nano-casting method. It can be seen from Fig. 1 that, from the UF template to the silica, the morphology as well as the dimensional size remained unchanged, but from the resin to zirconia and titania, the volume of material was sharply decreased. Though dramatic shrinkage was observed for zirconia and titania, the integrity of the monolith was retained. In addition to cylindrical inorganic oxide monoliths, materials with various morphologies can also be easily
Z.-G. Shi et al. / Journal of Non-Crystalline Solids 352 (2006) 4003–4007
Fig. 1. Photograph of UF resin, composite and inorganic oxides (a) silica; (b) zirconia and (c) titania (from the bottom to the top, scale bar in cm).
obtained just by selecting the suitable vessel when preparing the UF resin. Elemental analysis on the three inorganic oxides demonstrates the carbon content was no more than 0.1% (w/w) in the final monoliths, revealing the UF resin has been almost totally removed. XRD patterns of the three oxides disclosed that the silica was amorphous, while the zirconia was tetragonal and the titania was in its rutile form, as shown in Fig. 2.
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Fig. 3 presents the SEM images of the fractured surface of the resulting UF resin, silica, zirconia and titania. Obviously, all the samples show an interconnected morphology. The skeletons and the structure pores (macropores) were woven together, leading to the formation of an interpenetrating network. Comparing the SEM image of the UF resin with those of the silica, zirconia and titania, it can be seen that the skeletons of the UF resin was composed of small spherical aggregates, while the skeletons of the zirconia and titania were sheet-like conglomerates. Additionally, the macropores of the silica are larger than those of the zirconia and titania, which is ascribed to the shrinkage of the latter two materials. Fig. 4 shows the nitrogen adsorption–desorption isotherms and pore size distributions of the UF resin and a representative inorganic oxide (silica). Both the materials show similar isotherms and pore size distributions, indicating their similar mesoporous structures. Fig. 4(a) also reveals that the pore volume of the silica was larger than that of the UF resin. The BET surface area, pore volume (mesopore volume) and mean pore size of the UF resin were found to be 9.5 m2/g, 0.02 cm3/g and ca. 3 nm, while those for the silica were 26 m2/g, 0.05 cm3/g and ca. 3 nm, respectively. The pore parameters of the other two inorganics are listed in Table 1. 4. Discussion
Fig. 2. X-ray diffraction pattern of (a) zirconia and (b) titania calcinated at 1173 K.
In the UF resin synthesis, the proton in the acidic solution catalyzed the co-polymerization of urea and formaldehyde. As the reaction progressed, the degree of polymerization increased. The increase induced phase separation with the help of PEG, leading to the formation of UF resin rich phase and solvent rich phase. Through some necessary post synthesis treatment, the UF skeletons remained while the solvent was removed, leaving macropores in the material. Therefore, the UF resin exhibited an interpenetrating network structure. The high porosity and the connection of these pores are expected to facilitate other species to penetrate into the network of the UF resin, rendering itself an ideal template in nano-casting. Furthermore, because of the hydrophilic characteristic of the UF resin, organic solvent free precursors can be utilized, which should be environmental friendly. For example, in our experiment, zirconyl oxalate aqueous solution was adopted for the nano-casting of zirconia monolith, free of organic additive. By repeated penetration of inorganic sols or precursors, the pores in the UF resin were gradually filled with inorganic species. The subsequent temperature treatment strengthened the inorganics, leading to the formation of inorganic network in the UF material. The inorganic network and the UF resin were interpenetrated with each other. The calcination process burned off the UF resin while the inorganic network was remained. Apparently, the UF resin acted as a sacrifice template for the synthesis of inorganic monoliths.
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Fig. 3. Scanning electron micrograph of (A) UF resin; (B) silica; (C) zirconia and (D) titania.
60
0.010
UFresin silica Pore Volume (cc/g*nm)
Volume Adsorbed (cc/g)
50 40 30 20 10
UFresin silica
0.008 0.006 0.004 0.002 0.000
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (Ps/Po)
0
5
10
15
20
25
30
35
40
Pore Size (nm)
Fig. 4. (a) Nitrogen adsorption/desorption isotherms of the UF resin and the silica monolith; (b) pore size distribution for them.
Table 1 The surface area, mean pore size and pore volume (mesopore volume) of the zirconia and the titania
Zirconia Titania
Surface area (m2/g)
Mean pore size
Pore volume (cm3/g)
26 (±1) 4 (±1)
4 (±0.5) 3 (±0.5)
0.07 (±0.01) 0.01 (±0.01)
Theoretically the magnitude of the inorganic monolith should be predetermined by the dimension size of the UF
template. However, due to the inherent characteristic of the different inorganic materials, shrinkage or distortion may be observed in the template synthesis. It can be found from Fig. 1 that, the zirconia and titania demonstrated dramatic shrinkage in comparison with the UF template, while the silica showed no this phenomenon. It is probably ascribed to the crystalline phase transformation of the zirconia and titania. As can be seen in Fig. 2, the zirconia was tetragonal and the titania was in rutile. The zirconia and
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titania prepared at low temperatures by sol–gel process were generally amorphous. But at high temperature treatment, the atoms of zirconia and titania rearranged into a crystalline structure, which commonly causes shrinkage of the materials. The silica is very stable in a large temperature range. It underwent no crystalline phase transformation in the experiment conditions. Therefore, no detectable shrinkage was found for the silica replica. 5. Conclusion In summary, UF resin was for the first time proposed as a hard template for preparing series of inorganic oxide monoliths including silica, zirconia and titania by nanocasting technology. The UF resin was proved to be a promising hard template, due to its ideal properties, low cost, controllable structure, and the hydrophilic characteristics as well as its ease of removal, These properties make it attractive for industrial applications. Acknowledgements The authors gratefully acknowledge the Nature Science Foundation of Hubei Province and the National Nature Science Foundation of China (Grant No: 20475040). References [1] H. Maekawa, J. Esquena, S. Bishop, C. Solans, B.F. Chmelka, Adv. Mater. 15 (2003) 591. [2] K.K. Mistry, D. Saha, K. Sengupta, Sens. Actuator B-Chem. B 106 (2005) 258. [3] K.K. Unger, D. Kumar, M. Grun, G. Buchel, S. Ludtke, T.H. Adam, K. Schumacher, S. Renker, J. Chromatogr. A 892 (2000) 47. [4] S. Hjerten, J.L. Liao, R. Zhang, J. Chromatogr. 473 (1989) 273. [5] S.F. Xie, F. Svec, J.M.J. Frechet, J. Chromatogr. A 775 (1997) 65. [6] H. Wang, L. Huang, Z. Wang, A. Mitra, Y. Yan, Chem. Commun. (2001) 1364. [7] R. Asiaie, X. Huang, D. Farnan, C. Horvath, J. Chromatogr. A 806 (1998) 251. [8] D. Wistuba, V. Schurig, Electrophoresis 21 (2000) 3152. [9] C.D. Liang, S. Dai, G. Guiochon, Chem. Commun. (2002) 2680. [10] T. Amatani, K. Nakanishi, K. Hirao, T. Kodaira, Chem. Mater. 17 (2005) 2114.
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[11] M. Brandhorst, J. Zajac, D.J. Jones, J. Roziere, M. Womes, A. Jimenez-Lopez, E. Rodriguez-Castellon, Appl. Catal. B-Environ. 55 (2005) 267. [12] Y.H. Zhang, N. Guo, C. Wang, Y.B. Bai, Y. Wei, Polymer Preprints (Am. Chem. Soc., Div. Polymer Chem.) 42 (2001) 481. [13] R.L. Brutchey, T.D. Tilley, Abstracts of Papers, 224th ACS National Meeting, Boston, MA, United States, (2002) INOR-493. [14] R. Takahashi, S. Sato, T. Sodesawa, Y. Tomita, J. Ceram. Soc. Jpn. 113 (2005) 92. [15] K. Nakanishi, N. Soga, J. Non-Cryst. Solids 139 (1992) 1. [16] J. El Haskouri, D.O. de Zarate, C. Guillem, J. Latorre, M. Caldes, A. Beltran, D. Beltran, A.B. Descalzo, G. Rodriguez-Lopez, R. Martinez-Manez, M.D. Marcos, P. Amoros, Chem. Commun. (2002) 330. [17] S.A. El-Safty, T. Hanaoka, F. Mizukami, Chem. Mater. 17 (2005) 3137. [18] D. Brandhuber, N. Huesing, C.K. Raab, V. Torma, H. Peterlik, J. Mater. Chem. 15 (2005) 1801. [19] S.A. El-Safty, F. Mizukami, T. Hanaoka, J. Phys. Chem. B 109 (2005) 9255. [20] X.F. Zhou, C.Z. Yu, J.W. Tang, X.X. Yan, D.Y. Zhao, Microp. Mesop. Mat. 79 (2005) 283. [21] H. Yokoyama, L. Li, T. Nemoto, K. Sugiyama, Adv. Mater. 16 (2004) 1542. [22] A.S. Zalusky, R. Olayo-Valles, C.J. Taylor, M.A. Hillmyer, J. Am. Chem. Soc. 123 (2001) 1519. [23] K.T. Lee, J.C. Lytle, N.S. Ergang, S.M. Oh, A. Stein, Adv. Funct. Mater. 15 (2005) 547. [24] P.Y. Feng, X. Bu, G.D. Stucky, D.J. Pine, J. Am. Chem. Soc. 122 (2000) 994. [25] H.F. Yang, D.Y. Zhao, J. Mater. Chem. 15 (2005) 1217. [26] F. Schu¨th, Angew. Chem. Int. Ed. 42 (2003) 3604. [27] Z.G. Shi, Y.Q. Feng, L. Xu, S.L. Da, M. Zhang, Carbon 42 (2004) 1677. [28] Z.G. Shi, Y.Q. Feng, L. Xu, S.L. Da, M. Zhang, Carbon 41 (2003) 2677. [29] A.H. Lu, J.H. Smatt, M. Linden, Adv. Funct. Mater. 15 (2005) 865. [30] J.H. Smatt, B. Spliethoff, J.B. Rosenholm, M. Linden, Chem. Commun. (2004) 2188. [31] L. Huerta, C. Guillem, J. Latorre, A. Beltran, D. Beltran, P. Amoros, Solid State Sci. 7 (2005) 405. [32] W.J. Kim, T.J. Kim, W.S. Ahn, Y.J. Lee, K.B. Yoon, Catal. Lett. 91 (2003) 123. [33] M. Iwasaki, S.A. Davis, S. Mann, J. Sol–Gel Sci. Technol. 32 (2004) 99. [34] J. Ren, Z.J. Du, H.Q. Li, PMSE Preprints 92 (2005) 494. [35] W.C. Li, A.H. Lu, C. Weidenthaler, F. Schuth, Chem. Mater. 16 (2004) 5676. [36] Y.S. Tao, H. Kanoh, K. Kaneko, J. Am. Chem. Soc. 125 (2003) 6044. [37] A. Zampieri, H. Sieber, T. Selvam, G.T.P. Mabande, W. Schwieger, F. Scheffler, M. Scheffler, P. Greil, Adv. Mater. 17 (2005) 344.
A new template for the synthesis of porous inorganic oxide monoliths Zhi-Guo Shi *, Lan-Ying Xu, Yu-Qi Feng Department of Chemistry, Wuhan University, Wuhan, Hubei 430072, PR China Received 5 February 2006; received in revised form 15 July 2006 Available online 18 September 2006
Abstract A hydrophilic monolith, urea-formaldehyde (UF) resin, was for the first time proposed as a hard template for the synthesis of inorganic monoliths. Three kinds of inorganics, silica, zirconia and titania, were successfully prepared by using the new template. Scanning electron microscopy (SEM), nitrogen sorption measurements indicate that the inorganics are the replicas of their templates. Due to the characteristics of low cost, facile control of structure and ease of removal, the UF resin facilitates mass production of inorganic monoliths, which are attractive to industry. In addition, the hydrophilic characteristic of the UF resin makes it possible to use organic free precursors for preparing inorganic monoliths, which should be benign to the environment. The successful synthesis of the zirconia monolith using non-organic precursors confirms this. Ó 2006 Elsevier B.V. All rights reserved. PACS: 81.70. q; 81.20.Fw Keywords: Nanocrystals; Porosity; Silica; Processing; Sol–Gels (xerogels)
1. Introduction Because of high stability, large surface area, controllable pore structure as well as operational convenience, porous materials in the monolithic configuration are important for both academic study and technological application in catalysis, energy storage, opto-electronics, purification, separation, and adsorption [1–3]. Until now monolithic organic polymers have been extensively synthesized and their applications in bio-separation and bio-reaction have been demonstrated to be very successful [4,5]. However, due to the limited scopes of application arising from the low mechanical strength and low thermal resistance typical of polymer materials, a great deal of interest has been drawn to the development of synthesis and processing procedures for inorganic monoliths. Generally three methods have been proposed, gel casting *
Corresponding author. Fax: +86 27 68754067. E-mail address: [email protected] (Z.-G. Shi).
0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.08.008
[6–9], sol–gel technology [10–24] and nano-casting technology [25,26]. Among them, the nano-casting technology was the most widely used. Generally a deposable hard template was adopted to confine the reaction of precursors in its pores. Afterwards, the hard template was eliminated by combustion or eroding. Finally a new material with the replica structure of its template was acquired. In order to obtain monolithic replicas, monolithic templates with interconnecting pores should be utilized. To date, several kinds of such templates have been proposed for nano-casting. For example, silica monoliths have been used as hard templates for casting carbon monoliths and cobalt monoliths [27–30], monolithic polyurethane for silica and titanium silicate [31,32], polystyrene and starch gel for titania [1,33,34], carbon aerogel for magnesium oxide and ZSM-5 monolith [35,36], and native rattan stem for SiSiC/zeolite monoliths [37]. Though so many monolithic templates have been applied in nano-casting, there still remain challenges in
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their use arising from some inherent disadvantages. For instance, monolithic silica templates were commonly produced from costly precursors like tetramethoxysilane or tetraethoxysilane [27–30]. Monolithic carbons are usually prepared in severe conditions such as high temperature and inert environmental, which also increases the operational costs. Polyurethane foam and polystyrene are hydrophobic polymers, which can only serve as templates for replica materials prepared from high organic precursors [1,31,32]. Though starch gel is an economical template, the gel formation involves time-consuming procedures of freezing, thawing, and low-temperature evaporation. Furthermore, the structure as well as the strength of the starch gel cannot be guaranteed [33]. A cheap and structurally controllable monolithic template would be a solution to these problems, and it would be essential for mass production and the application of inorganic monoliths. Here we propose a new and promising template, hydrophilic urea-formaldehyde (UF) resin monolith (foam), for nano-casting. As a widely used industrial adhesive, the UF resin possesses several advantages over other templates: low cost, ease of preparation, ready elimination by combustion and facile control of structure. Furthermore, because of the hydrophilic characteristic of the UF resin, organic solvent free precursors can be also utilized for nano-casting, which should be benign to the environment. By adopting such new templates, several kinds of porous inorganic oxide monoliths (zirconia, silica, titania) have been conveniently fabricated, demonstrating the suitability of UF resin as a hard template in nano-casting. 2. Experimental 2.1. Materials Urea, formaldehyde, zirconium oxychloride (ZrOCl2 Æ 8H2O), oxalic acid, tetrabutyl titanate, acetylacetone, ethanol, poly (ethylene glycol) (PEG, Mw = 6000), hydrochloric acid, nitric acid were purchased from Shanghai General Chemical Reagent Factory (Shanghai, China). Tetraethoxysilane (TEOS) was obtained from the Chemical Factory of Wuhan University. Distilled water was from a quartz apparatus. All chemicals were used as received. 2.2. Preparation of UF resin monolith After dissolving 36 g of urea in 83.3 g formaldehyde solution (36%, w/w), the pH of this mixture was adjusted to 7–8 by 10% (w/w) sodium hydroxide solution. And then it was refluxed for 3 h to obtain milk like prepolymer of UF resin. To this UF prepolymer, 6 g of PEG and 10 ml of hydrochloric acid (pH 1.0) were added under stirring. Thereafter, the mixture was poured into a cylindrical vessel for gelation at 298 K. After 24 h, the rod like UF resin was formed and washed with a large amount of water. Then it was dried for further usage.
2.3. Preparation of inorganic oxide monoliths 6 ml of TEOS and 10 ml of hydrochloric acid (pH 2.0) were mixed at 333 K to get silica sol (Si-sol). After immersing into the Si-sol for 30 min, the UF resin was taken out and put into an oven of 313 K to react for 4 h. The same procedures were repeated for three times. Thereafter, the UF resin and silica composite was put into a furnace to burn off the UF resin and strengthen the silica. A programmed temperature cycle was applied to the furnace. The temperature was first ramped from 303 K to 1173 K at 1 K/min, and then kept at 1173 K for 2 h, to ensure complete removing the UF resin. For zirconia monolith, the sol was prepared as following. Zirconium oxychloride was dissolved in water first. To this solution, oxalic acid was added drop wise, white precipitate emerged simultaneously. After being aged in situ for 10 h, the precipitate was collected and dried at 303 K for 5 days. Thereafter, the dried precipitate was re-dissolved into deionized water to obtain zirconium sol. The following procedure is similar to that of the silica monolith. For preparation of titania monolith, the sol was obtained by mixed tetrabutyl titanate, acetylacetone, ethanol, water and nitric acid. Other procedures are similar to that of silica monolith and zirconia one. The as-synthesized UF resin foam and the resulting inorganic oxides were characterized by Elemental Analysis (EA), X-ray powder diffraction (XRD) (if necessary), scanning electron microscopy (SEM), and nitrogen adsorption/ desorption measurements. SEM images were obtained by using a Hitachi (Tokyo, Japan) Model X-650 Microscope. Samples were coated with a gold layer approximately 20 nm thick by sputtering. Powder XRD patterns were acquired on a Lab-X 3000 diffractometer using CuKa radiation. After the samples were vacuum dried at 423 K for 10 h, nitrogen adsorption/desorption isotherms were measured at 76 K using a Beckman Coulter SA3100 system. Specific surface areas were measured using Brunauer–Emmett–Teller (BET) method, and pore size distributions were obtained from the desorption branches of the isotherms using BarrettJoyner-Halenda (BJH) model. 3. Results Fig. 1 shows the photographs of the UF resins, the composites and the inorganic oxides (from the bottom to the top), (a) silica, (b) zirconia, (c) titania. Apparently, monolithic inorganic oxides were successfully prepared by the new template through the nano-casting method. It can be seen from Fig. 1 that, from the UF template to the silica, the morphology as well as the dimensional size remained unchanged, but from the resin to zirconia and titania, the volume of material was sharply decreased. Though dramatic shrinkage was observed for zirconia and titania, the integrity of the monolith was retained. In addition to cylindrical inorganic oxide monoliths, materials with various morphologies can also be easily
Z.-G. Shi et al. / Journal of Non-Crystalline Solids 352 (2006) 4003–4007
Fig. 1. Photograph of UF resin, composite and inorganic oxides (a) silica; (b) zirconia and (c) titania (from the bottom to the top, scale bar in cm).
obtained just by selecting the suitable vessel when preparing the UF resin. Elemental analysis on the three inorganic oxides demonstrates the carbon content was no more than 0.1% (w/w) in the final monoliths, revealing the UF resin has been almost totally removed. XRD patterns of the three oxides disclosed that the silica was amorphous, while the zirconia was tetragonal and the titania was in its rutile form, as shown in Fig. 2.
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Fig. 3 presents the SEM images of the fractured surface of the resulting UF resin, silica, zirconia and titania. Obviously, all the samples show an interconnected morphology. The skeletons and the structure pores (macropores) were woven together, leading to the formation of an interpenetrating network. Comparing the SEM image of the UF resin with those of the silica, zirconia and titania, it can be seen that the skeletons of the UF resin was composed of small spherical aggregates, while the skeletons of the zirconia and titania were sheet-like conglomerates. Additionally, the macropores of the silica are larger than those of the zirconia and titania, which is ascribed to the shrinkage of the latter two materials. Fig. 4 shows the nitrogen adsorption–desorption isotherms and pore size distributions of the UF resin and a representative inorganic oxide (silica). Both the materials show similar isotherms and pore size distributions, indicating their similar mesoporous structures. Fig. 4(a) also reveals that the pore volume of the silica was larger than that of the UF resin. The BET surface area, pore volume (mesopore volume) and mean pore size of the UF resin were found to be 9.5 m2/g, 0.02 cm3/g and ca. 3 nm, while those for the silica were 26 m2/g, 0.05 cm3/g and ca. 3 nm, respectively. The pore parameters of the other two inorganics are listed in Table 1. 4. Discussion
Fig. 2. X-ray diffraction pattern of (a) zirconia and (b) titania calcinated at 1173 K.
In the UF resin synthesis, the proton in the acidic solution catalyzed the co-polymerization of urea and formaldehyde. As the reaction progressed, the degree of polymerization increased. The increase induced phase separation with the help of PEG, leading to the formation of UF resin rich phase and solvent rich phase. Through some necessary post synthesis treatment, the UF skeletons remained while the solvent was removed, leaving macropores in the material. Therefore, the UF resin exhibited an interpenetrating network structure. The high porosity and the connection of these pores are expected to facilitate other species to penetrate into the network of the UF resin, rendering itself an ideal template in nano-casting. Furthermore, because of the hydrophilic characteristic of the UF resin, organic solvent free precursors can be utilized, which should be environmental friendly. For example, in our experiment, zirconyl oxalate aqueous solution was adopted for the nano-casting of zirconia monolith, free of organic additive. By repeated penetration of inorganic sols or precursors, the pores in the UF resin were gradually filled with inorganic species. The subsequent temperature treatment strengthened the inorganics, leading to the formation of inorganic network in the UF material. The inorganic network and the UF resin were interpenetrated with each other. The calcination process burned off the UF resin while the inorganic network was remained. Apparently, the UF resin acted as a sacrifice template for the synthesis of inorganic monoliths.
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Fig. 3. Scanning electron micrograph of (A) UF resin; (B) silica; (C) zirconia and (D) titania.
60
0.010
UFresin silica Pore Volume (cc/g*nm)
Volume Adsorbed (cc/g)
50 40 30 20 10
UFresin silica
0.008 0.006 0.004 0.002 0.000
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (Ps/Po)
0
5
10
15
20
25
30
35
40
Pore Size (nm)
Fig. 4. (a) Nitrogen adsorption/desorption isotherms of the UF resin and the silica monolith; (b) pore size distribution for them.
Table 1 The surface area, mean pore size and pore volume (mesopore volume) of the zirconia and the titania
Zirconia Titania
Surface area (m2/g)
Mean pore size
Pore volume (cm3/g)
26 (±1) 4 (±1)
4 (±0.5) 3 (±0.5)
0.07 (±0.01) 0.01 (±0.01)
Theoretically the magnitude of the inorganic monolith should be predetermined by the dimension size of the UF
template. However, due to the inherent characteristic of the different inorganic materials, shrinkage or distortion may be observed in the template synthesis. It can be found from Fig. 1 that, the zirconia and titania demonstrated dramatic shrinkage in comparison with the UF template, while the silica showed no this phenomenon. It is probably ascribed to the crystalline phase transformation of the zirconia and titania. As can be seen in Fig. 2, the zirconia was tetragonal and the titania was in rutile. The zirconia and
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titania prepared at low temperatures by sol–gel process were generally amorphous. But at high temperature treatment, the atoms of zirconia and titania rearranged into a crystalline structure, which commonly causes shrinkage of the materials. The silica is very stable in a large temperature range. It underwent no crystalline phase transformation in the experiment conditions. Therefore, no detectable shrinkage was found for the silica replica. 5. Conclusion In summary, UF resin was for the first time proposed as a hard template for preparing series of inorganic oxide monoliths including silica, zirconia and titania by nanocasting technology. The UF resin was proved to be a promising hard template, due to its ideal properties, low cost, controllable structure, and the hydrophilic characteristics as well as its ease of removal, These properties make it attractive for industrial applications. Acknowledgements The authors gratefully acknowledge the Nature Science Foundation of Hubei Province and the National Nature Science Foundation of China (Grant No: 20475040). References [1] H. Maekawa, J. Esquena, S. Bishop, C. Solans, B.F. Chmelka, Adv. Mater. 15 (2003) 591. [2] K.K. Mistry, D. Saha, K. Sengupta, Sens. Actuator B-Chem. B 106 (2005) 258. [3] K.K. Unger, D. Kumar, M. Grun, G. Buchel, S. Ludtke, T.H. Adam, K. Schumacher, S. Renker, J. Chromatogr. A 892 (2000) 47. [4] S. Hjerten, J.L. Liao, R. Zhang, J. Chromatogr. 473 (1989) 273. [5] S.F. Xie, F. Svec, J.M.J. Frechet, J. Chromatogr. A 775 (1997) 65. [6] H. Wang, L. Huang, Z. Wang, A. Mitra, Y. Yan, Chem. Commun. (2001) 1364. [7] R. Asiaie, X. Huang, D. Farnan, C. Horvath, J. Chromatogr. A 806 (1998) 251. [8] D. Wistuba, V. Schurig, Electrophoresis 21 (2000) 3152. [9] C.D. Liang, S. Dai, G. Guiochon, Chem. Commun. (2002) 2680. [10] T. Amatani, K. Nakanishi, K. Hirao, T. Kodaira, Chem. Mater. 17 (2005) 2114.
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