Morphology Controllable Synthesis of γ-Alumina Nanostructures via

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DOI: 10.1021/cg901422v

Morphology Controllable Synthesis of γ-Alumina Nanostructures via an Ionic Liquid-Assisted Hydrothermal Route

2010, Vol. 10 2928–2933

Tongil Kim, Jiabiao Lian, Jianmin Ma, Xiaochuan Duan, and Wenjun Zheng* Department of Materials Chemistry, College of Chemistry, Nankai University, Tianjin 300071, P. R. China Received November 14, 2009; Revised Manuscript Received March 30, 2010

ABSTRACT: The precursors for boehmite and γ-Al2O3, aluminum acetate hydroxide [(CH3COO)2Al(OH)] with various morphologies including nanoleave, nanofibers, and hierarchically nanostructured microflowers, have been successfully synthesized by an ionic liquid-assisted hydrothermal synthetic method using 1-butyl-2, 3-dimethyl imidazollium chloride ([bdmim][Cl]) as a template. The effects of the ionic liquid [bdmim][Cl] on the morphology of the precursors aluminum acetate hydroxide have been investigated systematically. The boehmite and γ-Al2O3 nanostructures were obtained by calcining the assynthesized precursors at 300 and 600 °C for 2 h, respectively, preserving the same morphology. The proposed formation mechanism of the precursors has also been investigated. The obtained products were characterized by several techniques, such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N2 adsorption-desorption technique.

1. Introduction Recently, the morphology control of nanostructured functional materials has attracted considerable interest because there is a close relationship between the morphology and properties.1-3 Many efforts have been made in synthesizing inorganic nanomaterials with controlled shapes such as nanotubes,4 nanowires,5,6 nanoribbons,7 and hollow structures.8,9 Aluminum oxide hydroxide and alumina are low cost materials widely used in industries as catalyst supports, adsorbents, ceramics, and abrasives due to their unique properties. AlOOH not only is a precursor of Al2O3 but also is used as a building unit and directing template in the preparation of core/shell materials.10 The conventional chemical method for the synthesis of Al2O3 is to dehydrate AlOOH, with the morphology remaining.11 Hence, controlling the size and shape of boehmite nanostructures is a key parameter in improving the properties and potentialities of the final alumina material. Numerous methods for the synthesis of γ-AlOOH nanostructures have been reported in the literature, such as sol-gel synthesis,12 spray pyrolysis,13 and hydrothermal treatment,14-17 and various synthetic methods of hierarchical architectures have been developed, such as vapor-phase processes and solution-based self-assembly routes.18,19 However, searching for a cost-effective and easy to scale up process still remains to be the main focus.20,21 Up to now, various morphologies of boehmite have been synthesized, such as nanospheres,22 nanofibers,23,24 nanorods,25 nanotubes,26,27 nanobelts,28 bunches of aligned boehmite nanowires,29 and flowerlike 3D nanoarchitectures.30 Recent works report the hydrothermal synthesis of γ-AlOOH nanoplatelets31 and hierarchical cantaloupe-like superstructures,32,30 and solvothermal synthesis of γ-AlOOH hierarchical nanostructures.33 However, to the best of our knowledge, there are very few reports on the morphology controllable synthesis of γ-AlOOH and γ-Al2O3 in one reaction system. *Corresponding author. Department of Materials Chemistry, College of Chemistry, Nankai University, 94 Weijin Road, Tianjin, 300071, P. R. China. Tel: þ86-022-23507951. Fax: þ86-022-23502458. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 05/06/2010

Room-temperature ionic liquids (RTILs) have received much attention as functional materials offering a wide range of possibilities for applications in catalysis,34,35 electrochemistry,36 separation,37 etc.,38 due to their favorable properties, such as extremely low volatility, good thermal stability, good dissolving ability, wide liquid temperature range, designable structures, high ionic conductivity, and wide electrochemical windows. In particular, RTILs have received considerable attention as a templating agent in the synthesis of functional nanomaterials. For instance, Park et al. have synthesized a large mesoporous γ-Al2O3 by using the ionic liquid [C16mim][Cl] as a template.39 More recently, our research group has also successfully synthesized various functional nanomaterials by using ionic liquids as functional materials in terms of solvent and template.40-42 Herein, we have reported the synthesis of AlOOH and γ-Al2O3 with various morphologies including nanoleaves, nanofibers, and hierarchically nanostructured microflowers by a two-step ionic liquid-assisted hydrothermal method using aluminum acetate hydroxide as a precursor. The ionic liquid [bdmim][Cl] plays a key role in the formation of the precursors with various morphologies as a soft template or a capping agent. The proposed formation mechanism has been given on the basis of a series of FE-SEM studies of the products obtained at different durations. 2. Experimental Section 2.1. Synthesis of the Aluminum Acetate Hydroxide Precursors. All of the reagents were analytical grade and used without further purification. The ionic liquid 1-butyl-2,3-dimethylimidazolium chloride ([bdmim][Cl]) was prepared according to the literature,43 and its general structural feature is shown bellow:

In the typical synthesis procedure, 2.1 mmol of AlCl3 3 6H2O and 0, 2.1, 5.3, and 10.6 mmol of ionic lipuid [bdmim][Cl] were placed into 5 mL of deionized water under stirring to form a homogeneous solution, and the samples were labeled as S-0, S-1, r 2010 American Chemical Society

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Figure 1. TG-DTA curves of the as-prepared aluminum acetate hydroxide precursor. S-2, and S-3, respectively. Subsequently, 10 mL of 0.52 M CH3COOK aqueous solution was added to the above homogeneous solution under continuous stirring. Then, the total solution was transferred into a stainless-steel autoclave with a capacity of 20 mL, sealed, and heated at 120 °C for 12 h. When the reaction was completed, the autoclave was cooled to room temperature naturally. The resultant product was collected and washed several times with deionized water, and anhydrous ethanol until the solution was neutral. The final white product was dried in a vacuum at 80 °C for 3 h. The synthetic conditions for preparing some typical samples are summarized in Table S1 (see Supporting Information). 2.2. Synthesis of Boehmite and γ-Al2O3 Nanostructures. The AlOOH and γ-Al2O3 nanostructures have been obtained by calcining the above products in air at 300 and 600 °C (heating rate is 10 °C/ min) for 2 h, respectively, preserving the same morphology. 2.3. Characterization. The product was characterized by XRD, FTIR, TGA, SEM, and TEM. XRD measurements were performed on a Rigaku D/max 2500 diffractometer with Cu KR radiation (λ= 0.154056 nm) at V=40 kV and I=150 mA, and the scanning speed was 8°/min. The FTIR spectroscopy of the sample was conducted at room temperature with a KBr pellet on a VECTOR-22 (Bruker) spectrometer ranging from 400 to 4000 cm-1. TGA experiments (Du Pont Instruments 951 thermogravimetric analyzer) were performed on a 100 mg sample from room temperature to 700 °C in flowing nitrogen gas at a heating rate of 5 °C/min. Morphology observations were performed on a JEOLJSM-6700F field emission scanning electron microscope (FE-SEM). TEM images were recorded with a Tecnai G2 20S-Twin transmission electron microscope operating at an accelerating voltage of 120 kV. N2 adsorptiondesorption isotherms were collected at liquid nitrogen temperature using a Quantachrome Nova 2000e sorption analyzer. The pore diameter and the pore size distributions were determined by the Barret-Joyner-Halenda (BJH) method. The specific surface areas (SBET) of the samples were calculated following the multipoint Brunauer-Emmett-Teller (BET) procedure.

3. Results and Discussions 3.1. Structure Characterization. The TG-DTA measurement was performed to study the conversion process of the as-prepared aluminum acetate hydroxide precursor during calcination, and the result is shown in Figure 1. From the TG curve of Figure 1, it can be seen that the decomposition of aluminum acetate hydroxide appears to occur in three steps with a total weight loss of approximately 72.8% up to 700 °C (theoretical value is 68.5%). The first step can be attributed to the desorption of physically adsorbed water (weight loss of about 5.1%) and the second and third steps to the conversion of aluminum acetate hydroxide into γ-AlOOH (about 60.8%) and γ-AlOOH into γ-Al2O3 (about 6.9%), respectively. Correspondingly, there are two endothermic peaks

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Figure 2. XRD patterns of the as-prepared precursor (a), γ-AlOOH obtained by calcining the precursor at 300 °C (b), and γ-Al2O3 obtained by calcining the precursor at 600 °C (c).

and an exothermic peak on the DTA curve which may be ascribed to the removal of the structural water molecules and the crystallization process of γ-AlOOH and γ-Al2O3, respectively. Above 500 °C, the weight of the precursor no longer changes, which indicates that the stable residue can reasonably be ascribed to the pure γ-Al2O3 phase. This result can also be confirmed by the following XRD analysis results. As expected, boehmite and γ-Al2O3 were obtained by calcining the as-synthesized precursors at 300 and 600 °C for 2 h, respectively. Figure 2 shows the XRD patterns of the samples. The deflection peaks of the as-prepared precursor (Figure 2a) can be perfectly assigned to the standard value of aluminum acetate hydroxide (CH3COO)2Al(OH) (JCPDS card number 54-0321). When the precursor was calcined in air at 300 °C (Figure 2b) and 600 °C (Figure 2c) for 2 h, respectively, all of the deflection peaks of the product were in agreement with the standard data of orthorhombic γ-AlOOH (JCPDS card number 21-1307) and face-centered cubic γ-Al2O3 (JCPDS card number 50-0741), respectively. No characteristic peaks are observed for impurities in all patterns, indicating the high purity of the products. Further evidence for the formation of the boehmite and γ-Al2O3 by calcining the as-synthesized precursor can be obtained from the FTIR spectra of the samples, as shown in Figure 3. In Figure 3a, the intensive bands at 3301 and 3095 cm-1 belong to the υas (Al)O-H and υs (Al)O-H stretching vibrations. The two weak bands at 2092 and 1971 cm-1 are the combination bands. The band at 1065 cm-1 and the shoulder at 1156 cm-1 are assigned to the δs Al-O-H and δas Al-O-H modes, respectively. The three bands at 747, 631, and 480 cm-1 represent the vibration mode of AlO6. The shoulder at 1638 cm-1 is the feature of the bending mode of absorbed water. In Figure 3b, the intensive band at 3460 cm-1 and weak band at 1640 cm-1 are attributed to the stretching vibrations of OH groups in the hydroxide structure as well as in physically adsorbed water, and the intensive bands at 781 and 567 cm-1 represent the vibration mode of AlO6 octahedra. These absorption bands are precisely consistent with the results of the previous literature.32,44 3.2. Morphology Characterization. The FE-SEM image of S-0 obtained without [bdmim][Cl] is shown in Figure S1 (see Supporting Information). It can be seen that there were only irregular particles in the product. Detailed morphology studies of the samples obtained with [bdmim][Cl] have been carried out in Figure 4. Figure 4a and b show the low-magnification and

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high-magnification FE-SEM images of S-1, respectively. Figure 4d shows the low-magnification FE-SEM image of S-2, which is composed of highly dispersed nanofibers. The higher magnification FE-SEM image shown in Figure 4e indicates that the average length and diameter of the products are approximately 1.5 μm and 90 nm, respectively. As shown in Figure 4g and h, the flowerlike nanoarchitectures are, in fact, composed of smaller two-dimensional nanoleaves. Figure 4c, f, and i are TEM or FE-SEM images of γ-Al2O3 obtained by calcining S-1, S-2, and S-3 at 600 °C, respectively, which have the same morphology as their corresponding precursors, except that their sizes were considerably decreased due to the decomporsition of acetic acid groups.

Figure 3. FTIR spectra of γ-AlOOH (a) and γ-Al2O3 (b) obtained by calcining the precursor at 300 and 600 °C, respectively.

Kim et al.

3.3. Effect of [bdmim][Cl] on the Morphology of the Aluminum Acetate Hydroxide Precursor. In order to investigate the effect of the ionic liquid [bdmim][Cl] on the formation of the aluminum acetate hydroxide precursor with various morphologies, a series of comparative experiments (as shown in Table S1, Supporting Information) were carried out. As mentioned above, there were only irregular particles obtained without [bdmim][Cl]. When the amount of [bdmim][Cl] was 2.1 and 5.3 mmol, the nanoleaves (S-1) and nanofibers (S-2) were obtained, respectively. When the amount of [bdmim][Cl] was increased to 10.6 mmol, the hierarchical flower-like architectures (S-3) were observed. According to these results, it is reasonable to consider that [bdmim][Cl] is a very important factor on the growth process of the (CH3COO)2Al(OH) nanostructures. As is well known, from the thermodynamics point of view, the surface energy of individual nanostructures is quite high, and thus, nanostructures tend to aggregate together in order to decrease the surface energy by reducing exposed areas.45,46 In the RTILstemplated system, the nanostructures of inorganic oxides were induced by hydrogen bonding-co-π-π stacking mechanism as discussed in previous reports.39,41,42,47,48 In the present case, the [bdmim][Cl] molecules may be easily adsorbed on the aluminum acetate hydroxide precursor by hydrogen bonding, due to the existence of (-Al-O-H) groups and (-Al-OOC-CH3) groups on the surface of crystallites. Consequently, they can reduce the surface energy and stabilize the nanostructured precursors, inducing well grown nanoleaves and nanofibers. With increasing the [bdmim][Cl] amount in the reaction system, the number of [bdmim][Cl]

Figure 4. Typical SEM images of (CH3COO)2Al(OH) obtained at 120 °C for 12 h; a and b, nanoleaf (S-1); d and e, nanofibers (S-2); g and h, microflowers assembled by nanoleaves (S-3). The TEM (c and f) and SEM (i) images of γ-Al2O3 obtained by calcining the corresponding precursors at 600 °C for 2 h.

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Figure 5. N2 absorption-desorption isothermal and pore-size distribution curves (inset) for the γ-Al2O3 products obtained by calcining (a) leaf-like, (b) fiber-like, and (c) flower-like precursors at 600 °C for 2 h.

Scheme 1. Schematic Illustration for the Growth Process of the Precursor Aluminum Acetate Hydroxide

molecules adsorbed on the surfaces of crystals is higher and higher, which favors the self-assembly of 2D or 1D nanostructures by van der Waals forces and the hydrogen bonding into flower-like 3D architectures.49 In addition, the increasing of the [bdmim][Cl] amount increased the viscosity of the system, which also favors the formation of the 3D architectures.50 As is well known, the viscosity of ionic liquidwater mixtures can generally be described by the exponential expression47,51 η ¼ ηIL 3 exp½- xC =a where xC is the mole fraction of water, a is a characteristic constant of the mixture, and ηIL is the viscosity of the pure ionic liquid. The empirical equation above indicated that the viscosity of ionic liquid-water mixtures increased exponentially when the mole fraction of water (xC) decreased. The increasing of the [bdmim][Cl] amount increased the viscosity of the system, which hindered the diffusion of monomers. The influence of viscosity on the Ostwald ripening of aluminum acetate hydroxide precursors may be thereby significant and the growth of nanocrystals for a diffusion-limited growth model, that is, the resolved aluminum acetate hydroxide

molecules, preferred to aggregate on the surface of the already existed crystallites, rather than forming new nuclei. Consequently, the well developed flower-like 3D architectures were obtained at the higher amount of [bdmim][Cl] (10.6 mmol). 3.4. Possible Growth Mechanism. To investigate the formation process of the fiber-like nanostructures, samples subjected to different reaction durations were studied by FE-SEM, as shown in Figure S2 (see Supporting Information). Our time-dependent experiments indeed agree with the selfassembly mechanism described above. As shown in Figure S2a (Supporting Information), irregular particles first were formed at 120 °C for 0.5 h, and then with increasing reaction time, the irregular nanoparticles become more and more long structures (Figure S2b and c, Supporting Information), and finally grow up into nanofibers (Figure S2d, Supporting Information). From the observed morphologies of the products at different evolutionary stages, a possible formation process is proposed. In first stage, the irregular particles are rapidly formed at the relatively high temperature. At the subsequent stage, the self-assembly and dissolutionrecrystallization process dominates, and [bdmim][Cl] molecules

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are adsorbed on the surface of the precursor aluminum acetate hydroxide as a soft template to control their growth direction. At the last stage, an Ostwald ripening process dominates. On the basis of the discussions above, we consider that the growth mechanism of aluminum acetate hydroxide crystals can be illustrated in Scheme 1. 3.5. Surface Properties of γ-Al2O3 Products Obtained by Calcining Precursor Crystals. To investigate the specific surface area and porous nature of the γ-Al2O3 samples obtained by calcining at 600 °C for 2 h, Brunauer-Emmett-Teller (BET) gas-sorption measurements were carried out. The recorded nitrogen adsorption/desorption isotherms for the samples showed significant hysteresis at relative pressures P/P0 above 0.60 (Figure 5). The BET specific surface areas of the samples calculated from N2 isotherms at 77 K are found to be as high as about 245.4 (leaf-like), 280.9 (fiber-like), 384.7 (flower-like) m2/g, respectively, where one can see that the BET specific surface area of the sample increases with increasing IL amounts. These results have also confirmed that as a larger amount of ionic liquid moecules was adsorbed on the surface of the crystal, the surface energy of the crystal decreased more and more, which induced the formation of thinner nanostructures or superstructures assembled by them; consequently, the superstructure of higher specific surface area could be obtained, as discussed above. BarrettJoyner-Halenda (BJH) calculations for the pore-size distribution, derived from desorption data, reveal a narrow distribution with one apex centered at 3.5 to 3.7 nm (Figure 5, inset). These mesopores presumably arise from the spaces formed by decomposition and hydration of the precursor aluminum acetate hydroxide crystals during the calcination process. The results display that the obtained γ-Al2O3 products have excellent porous properties. 4. Conclusions In summary, the γ-Al2O3 nanostructures with various morphologies, including leaf-like, fiber-like, and flower-like morphologies, have been successfully synthesized by using ionic liquid [bdmim][Cl] as a template and aluminum acetate hydroxide as a precursor. The corresponding AlOOH and γ-Al2O3 nanostructures have also been obtained by calcining precursors at 300 and 600 °C for 2 h, respectively, with the morphology well preserved during the thermal transformation process. It has been demonstrated that [bdmim][Cl] plays a crucial role in the morphology of the precursors. The growth mechanism has also been demonstrated in detail. The BET specific surface areas of the γ-Al2O3 products obtained by calcining the precursor aluminum acetate hydroxide crystals with leaf-like, fiber-like, and flower-like morphologies are as high as 240380 m2/g. It is expected to be used in some applications, such as adsorbents, catalysts, catalyst supports, and ceramics, etc. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant numbers 20571044 and 20971070) and the Project Fundamental and Applied Research of Tianjin. Supporting Information Available: Details of the synthetic conditions and characterizations. This material is available free of charge via the Internet at http://pubs.acs.org.

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