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Microwave hydrothermal synthesis of core-shell structured boehmite
Materials Letters 91 (2013) 249–251
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Microwave hydrothermal synthesis of core-shell structured boehmite Xiuyong Wu a,b,n, Baoquan Zhang b,nn, Zhengshui Hu b a b
Institute of Chemical Engineering, Taishan Medical College, Tai’an 271016, PR China College of Materials Science and Engineering, Qingdao University of Science & Technology, Qingdao 266042, PR China
a r t i c l e i n f o
abstract
Article history: Received 25 August 2012 Accepted 4 October 2012 Available online 13 October 2012
In this study, we successfully established an additive-free microwave hydrothermal (M-H) route by only using Al2(SO4)3 aqueous solution and urea as raw materials. Core-shell structured boehmite was synthesized at 180 1C for the first time via a M-H route. The final product was characterized by techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscope (SEM). On account of the fact that less reaction time usually means less energy consumption or more eco-friendly design, the M-H reaction time was successfully reduced to only 40 min by utilizing full microwave heating power and appropriate dosage of urea. To investigate the possible mechanism and influencing factors associated with the morphology and crystal form evolution process, samples subjected to different reaction durations were prepared and characterized. & 2012 Elsevier B.V. All rights reserved.
Keywords: Microwave Powder technology Microstructure Electron microscopy
1. Introduction In recent years, the design and synthesis of functional materials with controlled size and desired morphology have stimulated great research interest [1–4]. The simpler method and more effective control was always the immutable target of morphology-control synthesis domain. Therefore, as an important target materials, core-shell structured ultra-fine boehmite powders with nanometer to micrometer dimensions generated great interest due to their special physical and chemical properties which were different from solid particles, owing to their low density, large surface area, special core-shell structure and nanostructured wall [5–9]. Although hydrothermal method is an efficient way to synthesize core-shell structured boehmite, some approaches tend to be rather complicated and environmentally incompatible, with the obvious drawbacks that these processes required the amphiphilic copolymer [8,9] or inorganic salts(e.g. sodium tartrate [5], trisodium citrate dehydrate [6,7]) as additive, and the relevant additives removal process might compromise the structural integrity of the final products or result in environmental pollution which set great limitations on their practical application. Obviously, to prevent waste was better than to treat it or clean it up after it has formed [10]. Thus, additive-free route is a preferable choice. The other main drawback of hydrothermal method is the slow kinetics at any given temperature. On account of the fact that less reaction time usually means less
n Corresponding author at: Institute of Chemical Engineering, Taishan Medical College, Tai’an 271016, PR China. Tel.: þ 86 538 6238955; fax: þ 86 538 8426116. nn Corresponding author. Tel.: þ86 538 6238955; fax: þ 86 538 8426116. E-mail addresses: [email protected] (X. Wu), [email protected] (B. Zhang).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.017
energy consumption or more eco-friendly design, it is still an unremitting pursuit to develop time-saving boehmite synthesis routes, which will greatly facilitate their future industrial applications. As is known to all, microwave-assisted heating is generally faster, eco-friendly and most energy efficient. Such a combination is termed as the microwave hydrothermal (M-H) method [11–14]. We successfully synthesized core-shell structured amorphous aluminum hydroxide [12] via 2 h M-H process at 150 1C. However, to transform into alumina from boehmite is more energy efficient than from amorphous aluminum hydroxide, so it still remains a great challenge to directly synthesize core-shell structured boehmite via a microwave hydrothermal route. Following this development trend, in this study, we firstly report the timesaving and additive-free microwave hydrothermal syntheses of core-shell structured boehmite.
2. Experimental procedure In a typical M-H synthesis process, 10 mL of 0.1 mol L 1 Al2(SO4)3 aqueous solution and 30 mL distilled water were added to a double-walled Teflon-lined digestion vessel of 50 mL capacity. After 0.182 g urea (accounting for 100% theoretical dosage) was added, the vessel was sealed and placed on a turntable for uniform heating using a MDS-6 microwave hydrothermal system (Sineo, China) [11–13]. Temperature-controlled mode M-H treatments were conducted at 180 1C by non-pulsed heating type for 10–40 min, using 2.45 GHz microwave radiation under power range of 0–1000 W(full power). Afterwards, the vessel was immersed in water to cool to room temperature rapidly. The samples were collected by centrifugation and washed
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X. Wu et al. / Materials Letters 91 (2013) 249–251
with distilled water and absolute ethanol respectively for several times. The phase purity and phase structure of the samples were characterized by X-ray powder diffraction (XRD, Rigaku D/max r-A). Scanning electron microscope (SEM) images were performed on JEOL JSM-6610LV microscope. Transmission electron microscopy (TEM) images were obtained on a JEOL JSM-2100 microscope.
3. Results and discussion As shown in Fig. 1(d), diffraction peaks corresponding to boehmite (PDF no 21-1307) have been found for the final product which was prepared at 180 1C with 40 min reaction time under full microwave power range of 0–1000 W. No obvious XRD peaks arising from other phases of alumina are found indicating pure g-AlOOH phase of the microwave hydrothermal product. Fig. 2(a) shows the typical TEM image of the core-shell structured g-AlOOH. The distinct dark and light contradistinctions indicate the sample was composed of high-density spherical kernel part and incompact shell part. We can further distinguish the gaps between the core and shell part. The microscope analysis indicates that the core is around 500 nm and the shell is about 1–2 mm in diameter. Fig. 2(b) is the amplified image of Fig. 3(d). As can be seen from the SEM image of broken up core-shell structured boehmite ultra-fine particles, we can recognize that the core and shell parts were composed of granular and laminar morphology boehmite, respectively. Fig. 2(b) also reveals the laminar boehmite cumulated on the surface of spherical particles and thus formed the shell part. To investigate the possible mechanism and influencing factors associated with the core-shell structured morphology and boehmite crystal form evolution process, samples subjected to different reaction durations were studied by SEM and XRD. Fig. 3 shows SEM images of samples conducted at 180 1C for 10–40 min under full microwave power range of 0–1000 W. Compared with previous works which were accomplished under
Fig. 1. XRD patterns of M-H products with different reaction time: (a) 10 min, (b) 20 min, (c) 30 min and (d) 40 min.
the power range of 0–800 W [12,13], the utilizing of full microwave heating power obviously accelerated the core-shell morphology evolution process. As can be seen from Fig. 3(a,b), only after 10–20 min, besides some uncoated smooth spherical particles, we could find a lot of primordially formed core-shell structured particles. With the reaction time further extended to 30 min, accompanied by the disappearance of uncoated smooth spherical particles, vast majority of core-shell structured particles appeared. The distinct gaps between the core and shell could be observed clearly from the SEM images of Fig. 3(c). Even after 40 min, the homogeneous core-shell structured morphology was still maintained very well (see Fig. 3(d)). XRD patterns of above-mentioned samples were recorded to confirm the crystal form evolution process. As shown in Fig. 1(a), only after 10 min, we could indistinctly recognize the diffraction peaks of (200) and (031) lattice plane corresponding to boehmite appeared on the basis of amorphous phase. When the reaction time was extended to 20 min, the further development of the sample’s boehmite crystal form could be observed clearly. We could distinguish the diffraction peaks of (120) lattice plane corresponding to boehmite appears (see Fig. 1(b)). With the reaction time extended to 30 min, as shown in Fig. 1(c), the main diffraction peaks corresponding to boehmite have been found completely, thus indicating the basic formation of the sample’s boehmite crystal form. However, the typical diffraction peak of (020) lattice plane was still underdeveloped, and those small diffraction peaks corresponding to fine crystalline boehmite were still indistinguishable. Compared to the fully developed XRD pattern of the boehmite sample which was obtained with 40 min reaction time(see Fig. 1(d)), we preferred to select 40 min reaction time as optimal parameter. Microwaves were nonionizing electromagnetic radiation, and they produced dipole reorientation or ionic conduction when they interacted with materials that were dielectric materials or in which there were ions [15]. Because of the high penetration depth of microwave heating, the heating was rapid, which caused an increase in reaction kinetics. Microwave appeared to be particularly effective as a means of inducing nucleation and might affect the crystallization [11]. Literature manifested that the enhancement of crystallinity performed in short periods via microwave treatment could be attributed to fast heating of the precursor due to avoidance thermal gradients [15]. In order to express the ultimate capacity of experimental equipment, full microwave power range of 0–1000 W was utilized to further reduce the reaction time in this study. Since boehmite could be considered as partly dehydrated aluminum hydroxides, higher temperature was to the benefit of such crystal form transformation. In order to gain expected coreshell structured crystalline boehmite and simultaneously reduce the reaction time, after weighing gains and losses, we had a preference for 180 1C as the optimal reaction temperature in the whole M-H attempts.
Fig. 2. TEM (a) and SEM (b) images of the M-H products.
X. Wu et al. / Materials Letters 91 (2013) 249–251
251
Fig. 3. SEM images of M-H products with different reaction time: (a) 10 min, (b) 20 min, (c) 30 min and (d) 40 min.
¨ In addition, urea is widely used as a Bronsted base for homogeneous precipitation of metal ions and synthesis of oxides or hydroxide since urea hydrolyzes steadily and continuously into carbon dioxide and ammonia [5,8,9,12–14]. In this study, the dosage of urea was also increased from previous 0.164 g [12] to 0.182 g (accounting for 100% theoretical dosage), so as to yield more carbonate and hydroxide in water over equal periods of time, and finally speed up the M-H reaction process. As the integration of the above conditions, by using the full microwave power range of 0–1000 W and 100% theoretical dosage of urea, homogeneous core-shell structured boehmite with sufficiently developed crystal form was synthesized at 180 1C for only 40 min via microwave hydrothermal route for the first time.
4. Conclusions To sum up, we conclude that it was the introduction of full microwave heating power and 180 1C reaction temperature that has enabled us to accomplish the morphology evolution of core-shell structured boehmite in only 30–40 min. In the meantime, the final product could also complete its crystal form transformation process from amorphous Al(OH)3 to boehmite completely. In addition, the appropriate dosage of urea we used also helped us to control the core-shell morphology transformation process precisely.
Acknowledgments This work has been supported by the National Natural Science Foundation of China(Grant no. 21136008) and the Taishan Scholars Program of Shandong Province, China (ts20081119).
References [1] Shen QM, Min QH, Shi JJ, Jiang LP, Zhang JR, Hou WH, et al. J Phys Chem C 2009;113:1267–73. [2] Hu P, Yu LJ, Zuo AH, Guo CY, Yuan FL. J Phys Chem C 2009;113:900–6. [3] Liu B, Zeng HC. Small 2005;5:566–71. [4] Li ZM, Lai XY, Wang H, Mao D, Xing CJ, Wang D. J Phys Chem C 2009;113: 2792–7. [5] Cai WQ, Yu JG, Cheng B, Su BL, Jaroniec M. J Phys Chem C 2009;113: 14739–46. [6] Lu¨ Y, Lu WC, Zhang LM, Yue BH, Shang XF, Ni JP. Acta Phys-Chim Sin 2009;25: 1391–6. [7] Zhang LM, Lu WC, Cui RR, Shen SS. Mater Res Bull 2010;45:429–36. [8] Wu XY, Hu ZS. Chin J Inorg Chem 2008;24:760–4. [9] Wu XY, Wang DB, Hu ZS, Gu GH. Mater Chem Phys 2008;109:560–4. [10] Hu XL, Yu JC, Li Q. J Mater Chem 2006;16:748–51. [11] Huang JF, Xia CK, Cao LY, Zeng XR. Mater Sci Eng B 2008;150:187–93. [12] Wu XY, Meng XF, Ding J, Kong Q, Wang DB, Hu ZS. Mater Lett 2011;65: 2133–5. [13] Wu XY, Zhang BQ, Hu ZS. Mater Lett 2012;73:169–71. [14] Sreeja V, Joy PA. Mater Res Bull 2007;42:1570–6. [15] Xu LP, Ding YS, Chen CH, Zhao LL, Rimkus C, Joesten R, et al. Chem Mater 2008;20:308–16.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Microwave hydrothermal synthesis of core-shell structured boehmite Xiuyong Wu a,b,n, Baoquan Zhang b,nn, Zhengshui Hu b a b
Institute of Chemical Engineering, Taishan Medical College, Tai’an 271016, PR China College of Materials Science and Engineering, Qingdao University of Science & Technology, Qingdao 266042, PR China
a r t i c l e i n f o
abstract
Article history: Received 25 August 2012 Accepted 4 October 2012 Available online 13 October 2012
In this study, we successfully established an additive-free microwave hydrothermal (M-H) route by only using Al2(SO4)3 aqueous solution and urea as raw materials. Core-shell structured boehmite was synthesized at 180 1C for the first time via a M-H route. The final product was characterized by techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscope (SEM). On account of the fact that less reaction time usually means less energy consumption or more eco-friendly design, the M-H reaction time was successfully reduced to only 40 min by utilizing full microwave heating power and appropriate dosage of urea. To investigate the possible mechanism and influencing factors associated with the morphology and crystal form evolution process, samples subjected to different reaction durations were prepared and characterized. & 2012 Elsevier B.V. All rights reserved.
Keywords: Microwave Powder technology Microstructure Electron microscopy
1. Introduction In recent years, the design and synthesis of functional materials with controlled size and desired morphology have stimulated great research interest [1–4]. The simpler method and more effective control was always the immutable target of morphology-control synthesis domain. Therefore, as an important target materials, core-shell structured ultra-fine boehmite powders with nanometer to micrometer dimensions generated great interest due to their special physical and chemical properties which were different from solid particles, owing to their low density, large surface area, special core-shell structure and nanostructured wall [5–9]. Although hydrothermal method is an efficient way to synthesize core-shell structured boehmite, some approaches tend to be rather complicated and environmentally incompatible, with the obvious drawbacks that these processes required the amphiphilic copolymer [8,9] or inorganic salts(e.g. sodium tartrate [5], trisodium citrate dehydrate [6,7]) as additive, and the relevant additives removal process might compromise the structural integrity of the final products or result in environmental pollution which set great limitations on their practical application. Obviously, to prevent waste was better than to treat it or clean it up after it has formed [10]. Thus, additive-free route is a preferable choice. The other main drawback of hydrothermal method is the slow kinetics at any given temperature. On account of the fact that less reaction time usually means less
n Corresponding author at: Institute of Chemical Engineering, Taishan Medical College, Tai’an 271016, PR China. Tel.: þ 86 538 6238955; fax: þ 86 538 8426116. nn Corresponding author. Tel.: þ86 538 6238955; fax: þ 86 538 8426116. E-mail addresses: [email protected] (X. Wu), [email protected] (B. Zhang).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.017
energy consumption or more eco-friendly design, it is still an unremitting pursuit to develop time-saving boehmite synthesis routes, which will greatly facilitate their future industrial applications. As is known to all, microwave-assisted heating is generally faster, eco-friendly and most energy efficient. Such a combination is termed as the microwave hydrothermal (M-H) method [11–14]. We successfully synthesized core-shell structured amorphous aluminum hydroxide [12] via 2 h M-H process at 150 1C. However, to transform into alumina from boehmite is more energy efficient than from amorphous aluminum hydroxide, so it still remains a great challenge to directly synthesize core-shell structured boehmite via a microwave hydrothermal route. Following this development trend, in this study, we firstly report the timesaving and additive-free microwave hydrothermal syntheses of core-shell structured boehmite.
2. Experimental procedure In a typical M-H synthesis process, 10 mL of 0.1 mol L 1 Al2(SO4)3 aqueous solution and 30 mL distilled water were added to a double-walled Teflon-lined digestion vessel of 50 mL capacity. After 0.182 g urea (accounting for 100% theoretical dosage) was added, the vessel was sealed and placed on a turntable for uniform heating using a MDS-6 microwave hydrothermal system (Sineo, China) [11–13]. Temperature-controlled mode M-H treatments were conducted at 180 1C by non-pulsed heating type for 10–40 min, using 2.45 GHz microwave radiation under power range of 0–1000 W(full power). Afterwards, the vessel was immersed in water to cool to room temperature rapidly. The samples were collected by centrifugation and washed
250
X. Wu et al. / Materials Letters 91 (2013) 249–251
with distilled water and absolute ethanol respectively for several times. The phase purity and phase structure of the samples were characterized by X-ray powder diffraction (XRD, Rigaku D/max r-A). Scanning electron microscope (SEM) images were performed on JEOL JSM-6610LV microscope. Transmission electron microscopy (TEM) images were obtained on a JEOL JSM-2100 microscope.
3. Results and discussion As shown in Fig. 1(d), diffraction peaks corresponding to boehmite (PDF no 21-1307) have been found for the final product which was prepared at 180 1C with 40 min reaction time under full microwave power range of 0–1000 W. No obvious XRD peaks arising from other phases of alumina are found indicating pure g-AlOOH phase of the microwave hydrothermal product. Fig. 2(a) shows the typical TEM image of the core-shell structured g-AlOOH. The distinct dark and light contradistinctions indicate the sample was composed of high-density spherical kernel part and incompact shell part. We can further distinguish the gaps between the core and shell part. The microscope analysis indicates that the core is around 500 nm and the shell is about 1–2 mm in diameter. Fig. 2(b) is the amplified image of Fig. 3(d). As can be seen from the SEM image of broken up core-shell structured boehmite ultra-fine particles, we can recognize that the core and shell parts were composed of granular and laminar morphology boehmite, respectively. Fig. 2(b) also reveals the laminar boehmite cumulated on the surface of spherical particles and thus formed the shell part. To investigate the possible mechanism and influencing factors associated with the core-shell structured morphology and boehmite crystal form evolution process, samples subjected to different reaction durations were studied by SEM and XRD. Fig. 3 shows SEM images of samples conducted at 180 1C for 10–40 min under full microwave power range of 0–1000 W. Compared with previous works which were accomplished under
Fig. 1. XRD patterns of M-H products with different reaction time: (a) 10 min, (b) 20 min, (c) 30 min and (d) 40 min.
the power range of 0–800 W [12,13], the utilizing of full microwave heating power obviously accelerated the core-shell morphology evolution process. As can be seen from Fig. 3(a,b), only after 10–20 min, besides some uncoated smooth spherical particles, we could find a lot of primordially formed core-shell structured particles. With the reaction time further extended to 30 min, accompanied by the disappearance of uncoated smooth spherical particles, vast majority of core-shell structured particles appeared. The distinct gaps between the core and shell could be observed clearly from the SEM images of Fig. 3(c). Even after 40 min, the homogeneous core-shell structured morphology was still maintained very well (see Fig. 3(d)). XRD patterns of above-mentioned samples were recorded to confirm the crystal form evolution process. As shown in Fig. 1(a), only after 10 min, we could indistinctly recognize the diffraction peaks of (200) and (031) lattice plane corresponding to boehmite appeared on the basis of amorphous phase. When the reaction time was extended to 20 min, the further development of the sample’s boehmite crystal form could be observed clearly. We could distinguish the diffraction peaks of (120) lattice plane corresponding to boehmite appears (see Fig. 1(b)). With the reaction time extended to 30 min, as shown in Fig. 1(c), the main diffraction peaks corresponding to boehmite have been found completely, thus indicating the basic formation of the sample’s boehmite crystal form. However, the typical diffraction peak of (020) lattice plane was still underdeveloped, and those small diffraction peaks corresponding to fine crystalline boehmite were still indistinguishable. Compared to the fully developed XRD pattern of the boehmite sample which was obtained with 40 min reaction time(see Fig. 1(d)), we preferred to select 40 min reaction time as optimal parameter. Microwaves were nonionizing electromagnetic radiation, and they produced dipole reorientation or ionic conduction when they interacted with materials that were dielectric materials or in which there were ions [15]. Because of the high penetration depth of microwave heating, the heating was rapid, which caused an increase in reaction kinetics. Microwave appeared to be particularly effective as a means of inducing nucleation and might affect the crystallization [11]. Literature manifested that the enhancement of crystallinity performed in short periods via microwave treatment could be attributed to fast heating of the precursor due to avoidance thermal gradients [15]. In order to express the ultimate capacity of experimental equipment, full microwave power range of 0–1000 W was utilized to further reduce the reaction time in this study. Since boehmite could be considered as partly dehydrated aluminum hydroxides, higher temperature was to the benefit of such crystal form transformation. In order to gain expected coreshell structured crystalline boehmite and simultaneously reduce the reaction time, after weighing gains and losses, we had a preference for 180 1C as the optimal reaction temperature in the whole M-H attempts.
Fig. 2. TEM (a) and SEM (b) images of the M-H products.
X. Wu et al. / Materials Letters 91 (2013) 249–251
251
Fig. 3. SEM images of M-H products with different reaction time: (a) 10 min, (b) 20 min, (c) 30 min and (d) 40 min.
¨ In addition, urea is widely used as a Bronsted base for homogeneous precipitation of metal ions and synthesis of oxides or hydroxide since urea hydrolyzes steadily and continuously into carbon dioxide and ammonia [5,8,9,12–14]. In this study, the dosage of urea was also increased from previous 0.164 g [12] to 0.182 g (accounting for 100% theoretical dosage), so as to yield more carbonate and hydroxide in water over equal periods of time, and finally speed up the M-H reaction process. As the integration of the above conditions, by using the full microwave power range of 0–1000 W and 100% theoretical dosage of urea, homogeneous core-shell structured boehmite with sufficiently developed crystal form was synthesized at 180 1C for only 40 min via microwave hydrothermal route for the first time.
4. Conclusions To sum up, we conclude that it was the introduction of full microwave heating power and 180 1C reaction temperature that has enabled us to accomplish the morphology evolution of core-shell structured boehmite in only 30–40 min. In the meantime, the final product could also complete its crystal form transformation process from amorphous Al(OH)3 to boehmite completely. In addition, the appropriate dosage of urea we used also helped us to control the core-shell morphology transformation process precisely.
Acknowledgments This work has been supported by the National Natural Science Foundation of China(Grant no. 21136008) and the Taishan Scholars Program of Shandong Province, China (ts20081119).
References [1] Shen QM, Min QH, Shi JJ, Jiang LP, Zhang JR, Hou WH, et al. J Phys Chem C 2009;113:1267–73. [2] Hu P, Yu LJ, Zuo AH, Guo CY, Yuan FL. J Phys Chem C 2009;113:900–6. [3] Liu B, Zeng HC. Small 2005;5:566–71. [4] Li ZM, Lai XY, Wang H, Mao D, Xing CJ, Wang D. J Phys Chem C 2009;113: 2792–7. [5] Cai WQ, Yu JG, Cheng B, Su BL, Jaroniec M. J Phys Chem C 2009;113: 14739–46. [6] Lu¨ Y, Lu WC, Zhang LM, Yue BH, Shang XF, Ni JP. Acta Phys-Chim Sin 2009;25: 1391–6. [7] Zhang LM, Lu WC, Cui RR, Shen SS. Mater Res Bull 2010;45:429–36. [8] Wu XY, Hu ZS. Chin J Inorg Chem 2008;24:760–4. [9] Wu XY, Wang DB, Hu ZS, Gu GH. Mater Chem Phys 2008;109:560–4. [10] Hu XL, Yu JC, Li Q. J Mater Chem 2006;16:748–51. [11] Huang JF, Xia CK, Cao LY, Zeng XR. Mater Sci Eng B 2008;150:187–93. [12] Wu XY, Meng XF, Ding J, Kong Q, Wang DB, Hu ZS. Mater Lett 2011;65: 2133–5. [13] Wu XY, Zhang BQ, Hu ZS. Mater Lett 2012;73:169–71. [14] Sreeja V, Joy PA. Mater Res Bull 2007;42:1570–6. [15] Xu LP, Ding YS, Chen CH, Zhao LL, Rimkus C, Joesten R, et al. Chem Mater 2008;20:308–16.