Microwave-Assisted Solvothermal Synthesis of AlOOH Hierarchically

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Microwave-Assisted Solvothermal Synthesis of AlOOH Hierarchically Nanostructured Microspheres and Their Transformation to γ-Al2O3 with Similar Morphologies Ling Zhang and Ying-Jie Zhu* State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China ReceiVed: June 30, 2008; ReVised Manuscript ReceiVed: August 20, 2008

We report a microwave-assisted solvothermal method for the preparation of AlOOH hierarchically nanostructured microspheres constructed by nanosheets. The formation mechanism of AlOOH hierarchically nanostructured microspheres is discussed. γ-Al2O3 hierarchically nanostructured microspheres are obtained by heating AlOOH hierarchically nanostructured microspheres to 500 °C in air, and the morphology is well preserved during the thermal transformation process. The products are characterized by X-ray powder diffraction, transmission electron microscopy, and scanning electron microscopy. Introduction Recently, there has been increasing interest in the morphology control of nanostructured functional materials because there is a close relationship between the morphology and the properties.1-3 Considerable effort has been made in synthesizing inorganic nanomaterials with controlled shapes such as nanotubes,4 nanowires,5,6 nanoribbons,7 and hollow structures.8,9 In particular, the organization of nanostructured building blocks (nanoparticles, nanorods, nanowires, nanosheets, etc.) into threedimensional (3-D) ordered superstructures by bottom-up approaches has been an exciting research field in recent years.10 A number of preparation methods have been introduced based on self-assembly or deposition such as vapor-induced phase separation, micelle aggregation, nanosphere lithography, and vertically aligned carbon nanotubes.11-15 Up to now, inorganic materials with hierarchical shapes, such as metal oxides, hydroxides, sulfides, metals, have been successfully prepared.16-20 AlOOH and γ-Al2O3 have interesting properties and are widely used in industry as adsorbents, abrasive, catalysts, catalyst supports, and ceramics. AlOOH is also used as a template in the preparation of nanostructured materials.21 So far, various morphologies of AlOOH have been prepared such as nanotubes,22-24 nanowires,25 nanobelts,26 nanofibers,27,28 nanorods,29 whiskers,30 flowers,31 and cantaloupelike32 structures. γ-Al2O3 was usually prepared by dehydration of AlOOH at elevated temperatures. In recent years, the microwave heating has been widely applied in chemical reactions and materials synthesis due to its advantages such as rapid volumetric heating and dramatic increase in reaction rates.33-46 Nowadays, the use of microwave energy to directly heat chemical reactions has become an increasingly popular technique in the scientific community. The realization of chemical reactions in a very short time period by direct interaction of microwave energy with the reaction system as opposed to the indirect transfer of energy by utilizing the conventional heating method can certainly be considered “green”, not only because of the reduced energy consumption but also because of the associated time saving, thereby can significantly increase efficiency. * To whom correspondence should be addressed. Phone: 0086-2152412616. Fax: 0086-21-52413122. E-mail: [email protected].

Herein, we report the synthesis of AlOOH hierarchically nanostructured microspheres constructed by nanosheets using microwave-assisted solvothermal method. AlOOH hierarchically nanostructured microspheres are used as the precursor and template for the preparation of γ-Al2O3 hierarchically nanostructured microspheres by thermal transformation of AlOOH at 500 °C in air, and the morphology is well preserved during the thermal transformation process. Experimental Section All reagents were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd., and used without further purification. In a typical procedure for the preparation of AlOOH, 1 mmol of AlCl3 · 6H2O, 0.5 g of cetyltrimethyl ammonium bromide (CTAB), and 1.2 g of NaOH were dissolved in a mixture of 20 mL of deionized water and 10 mL of methanol. Then, 2.75 mL of ethyl acetate was added to the mixture and magnetically stirred for 10 min. The resultant reaction system was loaded into a 60-mL Teflon autoclave, sealed, microwave-heated to 160 °C and maintained at this temperature for 30 min, and then cooled down naturally. The microwave oven used for sample preparation was a microwave-solvothermal synthesis system (MDS-6, Sineo, Shanghai, China). The precipitate was collected by centrifugation, washed by deionized water and ethanol three times, respectively, and dried at 60 °C in air. For the preparation of γ-Al2O3, the AlOOH powder was heated to 500 °C in a tubular furnace at a heating rate of 1 °C min-1 and then cooled naturally. X-ray powder diffraction (XRD) patterns were recorded with a Rigaku D/MAX 2550V X-ray diffractometer with Cu KR radiation (λ ) 1.54178 Å) and a graphite monochromator, operating at 40 kV and 100 mA with a scan rate of 10° min-1, and the measurement was carried out at room temperature. Transmission electron microscopy (TEM) micrographs were obtained on a JEOL JEM-2100F field-emission transmission electron microscope, operating at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) micrographs were obtained on a JEOL JSM-6700F field-emission scanning electron microscope, operating at an accelerating voltage of 10 kV. The sample was dispersed in absolute ethanol by ultrasonic and then

10.1021/jp805751t CCC: $40.75  2008 American Chemical Society Published on Web 10/08/2008

AlOOH Hierarchically Nanostructured Microspheres

Figure 1. XRD patterns of the products prepared by microwaveassisted solvothermal method at 120 and 160 °C for 30 min.

the suspension was dropped on the copper grid or copper platform for characterization. Results and Discussion The XRD patterns of the products prepared by microwaveassisted solvothermal method at 120 and 160 °C for 30 min are shown in Figure 1. All the diffraction peaks can be indexed to the single-phase AlOOH with an orthorhombic structure (JCPDS No. 21-1307). No impurities such as Al(OH)3 and Al2O3 were detected by XRD. SEM and TEM micrographs of the as-prepared AlOOH are shown in Figure 2, from which one can see hierarchically nanostructured microspheres with chrysanthemum-like morphology. The average diameter of the hierarchically nanostructured microspheres was about 600 nm. The high-magnification TEM and SEM micrographs (parts b and c of Figure 2) reveal that hierarchically nanostructured microspheres were built up by nanosheets with average thickness of about 20 nm. Figure 2d shows the morphology of the sample prepared in the absence of methanol, indicating that hierarchically nanostructured microspheres were less densely assembled by nanosheets and the sizes were bigger compared with those obtained in the presence of methanol. In our preparation method, we employed a double-hydrolysis homogeneous precipitation process. Before the microwave irradiation, the reactants of AlCl3 and NaOH were dissolved in

J. Phys. Chem. C, Vol. 112, No. 43, 2008 16765 a mixture of deionized water and methanol. Al3+ existed in the form of AlO2- in the solution because of the excess of NaOH. The reaction proceeded as eq 1. Then, ethyl acetate was added to form a transparent mixture. When the microwave irradiation was introduced and the system temperature increased, the double-hydrolysis precipitation process occurred. Ethyl acetate and AlO2- hydrolyzed at the same time, and AlOOH formed, as shown in eqs 2 and 3. With the help of the catalysis by the excessive sodium hydroxide, ethyl acetate hydrolyzed to form ethanol and acetic acid. As a weak acid, acetic acid consumed OH- ions. On the other hand, AlO2- hydrolyzed to form AlOOH and OH-. The two processes accelerated each other, leading to the formation of AlOOH. The overall reaction is summarized in eq 4. The whole process is illustrated as follows

Al3+ + 4OH- f AlO2- + 2H2O

(1)

CH3COOC2H5 + H2O f CH3COOH + C2H5OH

(2)

AlO2- + H2O f AlOOH + OH-

(3)

CH3COOC2H5 + AlO2- + H2O f AlOOH + CH3COO- + C2H5OH (4) The crystal growth is usually sensitive to the initial nucleation process. In the conventional solvothermal synthesis, crystals tend to nucleate on container walls or impurity particles, leading to a slow growth rate due to few nuclei. We employed the microwave heating instead of conventional heating, leading to rapid nucleation rate and the formation of massive nuclei throughout the bulk solution. To form a homogeneous reaction system, we used mixed solvents of water and methanol, in which the reagents were dissolved completely. In the absence of methanol, there was an interface between water and ethyl acetate, and the reaction occurred at the interface. CTAB acted as a surfactant and a soft template for the formation of AlOOH hierarchically nanostructured microspheres. CTAB molecules may form the micelles in the solution, and they act as a soft template, leading to the formation of hierarchically nanostructured microspheres. On the other hand, CTAB molecules may

Figure 2. (a and b) TEM and (c) SEM micrographs of AlOOH hierarchically nanostructured microspheres prepared by microwave-assisted solvothermal method at 160 °C for 30 min. (d) SEM micrograph of AlOOH hierarchically nanostructured microspheres prepared in the absence of methanol.

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Figure 3. SEM micrographs of AlOOH hierarchically nanostructured microspheres prepared by microwave-assisted solvothermal method at 160 °C for (a) 0, (b) 10, and (c) 60 min.

Figure 4. SEM micrographs of AlOOH hierarchically nanostructured microspheres prepared by microwave-assisted solvothermal method for 30 min at (a) 120, (b) 140, and (c) 180 °C.

adsorb on the surface of the newly formed AlOOH, reduce the surface energy and stabilize the nanostructured microspheres. From the thermodynamics point of view, the surface energy of an individual nanosheet with two main exposed planes is quite high, and thus nanosheets tend to aggregate perpendicularly to the surface planes in order to decrease the surface energy by reducing exposed areas.47,48 To further investigate the formation mechanism, time-dependent experiments were conducted to reveal the crystal growth process. We set the holding time of the reaction system to be 0, 10, 30, and 60 min, respectively, when the reaction temperature reached 160 °C and then cooled down by cold water. SEM and TEM were used to observe the morphology evolution as the reaction time increased, as shown in Figure 3. When the reaction time increased from 0 to 60 min, the morphologies of the as-prepared samples were similar. When the temperature reached 160 °C, microspheres constructed

by nanosheets formed (Figure 3a), the diameters of microspheres were about 500 nm, and the interstices between the nanosheets were narrow. When the holding time at 160 °C increased to 10 min, one can see from Figure 3b that the microspheres still consisted of nanosheets and that the sizes of nanosheets became larger and the interstices between them increased. When the holding time at 160 °C elongated to 30 min, the density of nanosheets in the microsphere further decreased (Figure 2c). The morphology of hierarchical nanostructures prepared for 60 min at 160 °C was irregular (not spherical), and the sizes were about 500 nm, the interstices between the nanosheets were enlarged to be micropores. Hierarchically nanostructured microspheres are constructed by assembly of low dimensional nanostructures such as nanoparticles, nanowires and nanosheets by bottom-up approaches. We did not observe the assembly process of nanosheets to form microspheres in the time-dependent experiments because the

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Figure 5. SEM micrographs of samples obtained by conventional solvothermal process: (a and b) 3 h; (c and d) 18 h.

Figure 6. (a and b) TEM micrographs and (c) XRD pattern of γ-Al2O3 hierarchically nanostructured microspheres.

formation process was too rapid. We propose the formation mechanism of AlOOH hierarchically nanostructured microspheres as follows: First, by microwave irradiation, the temperature rose rapidly, leading to rapid reaction and nucleation. Newly formed nuclei grew to form small nanosheets rapidly. The nanosheets assembled to microspheres in order to reduce the surface energy. By increasing the holding time at 160 °C,

nanosheets, which were at the periphery of the microsphere, grew outward by consuming the newly formed nuclei and stopped when the reactants were exhausted. This process was a typical Ostwald ripening process. When the holding time at 160 °C was long enough, the dissolution process of nanosheets in the microsphere occurred, leading to smaller size and irregular shape of the microspheres.

16768 J. Phys. Chem. C, Vol. 112, No. 43, 2008 We investigated the formation of hierarchically nanostructured microspheres at different temperatures. The temperature did not have significant effect on the morphology of the product. Figure 4 shows the SEM micrographs of the samples obtained at 120, 140, and 180 °C, respectively. The sizes of microspheres prepared at a lower temperature were not uniform, while the sizes of microspheres became uniform when the temperature increased. As shown in Figure 1, we obtained well crystallized single-phase AlOOH even at a temperature as low as 120 °C. To investigate the effect of heating method on the product, the conventional solvothermal method instead of microwavesolvothermal method was employed. The SEM micrographs of samples prepared by conventional solvothermal method are shown in Figure 5. Because of the slow heat transfer rate, there was no precipitate obtained when the heating time was 1 h. When the heating time increased to 3 h, we obtained microspheres with diameters of about 1 µm; however, the microspheres were obviously aggregated (parts a and b of Figure 5). By increasing the heating time to 18 h, the diameters of the microspheres increased (parts c and d of Figure 5). AlOOH hierarchically nanostructured microspheres were used as the precursor and template for the preparation of γ-Al2O3 hierarchically nanostructured microspheres by thermal transformation of AlOOH at 500 °C in air. The XRD pattern and TEM micrographs are shown in Figure 6. The XRD pattern indicates that AlOOH was transformed to γ-Al2O3 by thermal transformation of AlOOH at 500 °C in air. TEM micrographs show that the morphology of AlOOH was well preserved during the thermal transformation process. High-magnification TEM micrograph (Figure 6b) shows that there were nanometer-size holes in the nanosheets, which were derived from the losing of H2O during the thermal transformation process. Conclusion Herein, we report rapid microwave-assisted solvothermal synthesis of hierarchically nanostructured microsphere of AlOOH constructed by nanosheets. Time and temperature dependent experiments are performed to explore the formation mechanism of the hierarchical nanostructures. AlOOH hierarchically nanostructured microspheres are used as the precursor and template for the preparation of γ-Al2O3 hierarchically nanostructured microspheres by thermal transformation of AlOOH at 500 °C in air, and the morphology is well preserved during the thermal transformation process. Acknowledgment. Financial support from the National Natural Science Foundation of China (50772124, the Fund for Innovative Research Groups), the Program of Shanghai Subject Chief Scientist (07XD14031), CAS International Partnership Program for Innovative Research Team, and the fund of State Key Laboratory of High Performance Ceramics and Superfine Microstructure is gratefully acknowledged. References and Notes (1) Wang, Z. L.; Song, J. H. Science 2006, 312, 242. (2) Huang, J. X.; Tao, A. R.; Connor, S.; He, R. R.; Yang, P. D. Nano Lett. 2006, 6, 524. (3) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (4) Tong, H.; Zhu, Y. J.; Yang, L. X.; Li, L.; Zhang, L. Angew. Chem., Int. Ed. 2006, 45, 7739.

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