Morphology Controllable Synthesis of Nanostructured Boehmite and γ

Article pubs.acs.org/crystal

Morphology Controllable Synthesis of Nanostructured Boehmite and γ‑Alumina by Facile Dry Gel Conversion Shoucang Shen,*,† Wai Kiong Ng,† Leonard Sze Onn Chia,† YuanCai Dong,† and Reginald Beng Hee Tan*,†,‡ †

Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833, Republic of Singapore Department of Chemical and Biomolecular Engineering, The National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Republic of Singapore

‡

ABSTRACT: A facile dry-gel conversion process was developed for crystallization of trihydrate-alumina nanoparticles and boehmite AlOOH nanofibers with controllable morphologies from solid powder of amorphous aluminum hydroxide precipitate. By steam treatment at 200 °C, the amorphous dry gel of aluminum hydroxide precipitate was crystallized, and the morphology of resulting nanomaterials was found to be dependent on the ratio of water-to-gel applied. When the water-to-gel ratio was 1:1, nanoparticles of hydrated aluminum oxide with particle size of 80 ± 10 nm were obtained. Increase of the water-to-gel ratio to 2:1 resulted in the formation of nanoribbons of AlOOH with lengths of 1−2 μm and widths of 100 nm, and further increase of the water-to-gel ratio led to the formation of nanorods of boehmite AlOOH with diameters of 20−30 nm and lengths of 200−500 nm. After thermal treatment at 600 °C, the nanostructured hydrated aluminum oxide and boehmite was transformed to γ-Al2O3, and the morphologies were well preserved. The morphology of the nanostructures was analyzed with transmission electron microscopy (TEM) and field emission scanning electronic microscopy (FESEM). The phase transformations were characterized by X-ray diffraction (XRD), differential scanning calorimetry and thermo-gravimetric analysis (DSC-TGA), Fourier transform infrared (FTIR), Raman, and magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy.

1. INTRODUCTION Alumina (Al2O3) is most commonly used for adsorbents, catalysis, and catalyst supports due to its thermal, chemical, and mechanical stability.1−4 In addition, it has been well-known to have broad technological applications, such as advanced ceramics, catalysts, and catalyst supports, as well as dielectric microelectronics,5,6 biomedical materials,7,8 and optical devices.9 Recently, enthusiastic efforts have been devoted to the synthesis of nanostructured alumina materials with higher surface areas, more uniform pore structure and increased variety of morphologies.10−13 The fabricated fine particles with size at nanoscale could yield materials structurally different from the bulk materials.14−16 Nanoalumina based advanced ceramic showed high structural integrity, excellent significant mechanical strength (∼230 MPa) and excellent thermal shock resistance.17 The addition nano-Al2O3 has been reported to yield superplastic with a high-strain rate,18,19 bending strength, and fracture toughness of nanostructured ceramics.20,21 In addition, nanofibers of AlOOH and Al2O3, have been of great interest over the past decade because they can potentially be applied in some advanced applications involving dimensionality, size-confined quantum phenomena, electronic dielectric logic gates, and advanced ceramics. Moreover, alumina nanofibers arranged in a hierarchical structure as the separation layer in ceramic membranes have been shown to greatly improve the separation efficiency compared with conventionally fabricated ceramic membranes and remove the problems of cracks, pinholes, and serious sintering.22 An enormous effort © 2012 American Chemical Society

has been made to obtain nanorods, nanowires, and other nanofibers.23−32 Various nanofibers of Al2O3 have been produced using vapor−liquid−solid (VLS) deposition to burn aluminum with silica nanoparticles at a high temperature above 1000 °C,33,35 pyrolysis of aluminiumoxydride-HAlO at 1100 °C36 and by using other one-dimensional (1-D) materials as hard template.37,38 Many researchers have reported the synthesis of nanostructured Al2O3 precursor via liquid phase crystal growth.39 γ-AlOOH nanotubes and nanorods were reported to be synthesized by a solution-phase reaction under hydrothermal conditions.40 Lath-like mesostructured alumina was also obtained by admixing anionic and cationic surfactants in aqueous media.41 Alumina nanorods were also fabricated by supercritical technology.42 The precursors for boehmite and γAl2O3, aluminum acetate hydroxide [(CH3COO)2Al(OH)] with various morphologies, have been synthesized by an ionic liquid-assisted hydrothermal synthetic method using 1-butyl2,3-dimethyl imidazollium chloride ([bdmim][Cl]) as a template.43 Nanostructured AlOOH and γ-Al2O3 were reported to be synthesized by a hydrothermal process in the absence of template and surfactant.44 However, the high cost, complicated procedures, and low yield of these methods limit the preparation of nanostructured alumina at a large scale and for practical application. Hence, an effective synthesis method is Received: July 5, 2012 Revised: August 24, 2012 Published: August 27, 2012 4987

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3. RESULTS AND DISCUSSION Figure 1 shows the TEM images of nanostructured materials obtained by the dry gel conversion with different water-to-gel

desired to manufacture highly pure nanostructured Al2O3 with controllable morphology in a large scale with affordable cost, which is essential for transferring novel nanotechnology to practical industrial applications. In this work, a simple steam-assisted dry gel conversion route was applied to fabricate nanoparticles of hydrated alumina and nanofibers of boehmite with different morphologies under controlled steaming conditions. The process is distinguished by the simplicity of the apparatus for preparation and the high efficiency of crystal growth in large scale without the presence of solvent, surfactant, and crystal seeds. This solid form crystal transformation breaks the limitation of reactor volume for liquid reactions.45 In addition to producing high yield and purity of nanoparticles and nanofibers of boehmite, the morphology of the formed nanostructured boehmite was controllable by varying the ratio of water-to-gel. After thermal treatment, the resulting γ-Al2O3 preserved the nanostructures of boehmite well.

2. EXPERIMENTAL SECTION Typically, 30 g of Al(NO3)3·9H2O was first dissolved in 100 mL of deionized water under stirring at room temperature. Precipitation was performed by adjusting the pH of medium to 5.0 with 10%(wt) ammonia aqueous solution. The white solid precipitates were collected by filtration and dried at 55 °C for 48 h in an oven with enforced airflow. One gram of the dried gel obtained in this way was ground into a fine powder and transferred into a glass beaker (25 mL) in a Teflon vessel (200 mL), where a certain amount of water was poured at the bottom of the vessel and physically separated from the dry gel sample. The ratios of water-to-gel were controlled to (a) 1:1, (b) 2:1, (c) 4:1, and (d) 6:1, by weight, respectively. After the Teflon vessel was sealed and heated at 200 °C for 48 h, the obtained material was washed with deionized water and collected by centrifuging, then dried at 55 °C for 24 h. The dried solid material was heated from room temperature to 600 °C at a heating rate of 5 °C/min and followed by calcination in air for 2 h. X-ray diffraction measurements were performed on a D8ADVANCE (BRUKER) X-ray diffractometer in steps of 0.02° using Cu Kα radiation as X-ray source. Scanning electron microscopic (SEM) images were taken with a field emission scanning electron microscope (FESEM, JEOL JSM-6700F). The morphology and aspect ratios of nanorods of AlOOH and Al2O3 were examined by a transmission electron microscope (TEM) with selected area electron diffraction (SAED), and the measurement was performed on a TECNAI F20 (G2) (FEI) electron microscope at 125 kV. The specimens for high-resolution TEM (HRTEM)/TEM/SAED studies were prepared by suspending solid samples in acetone with ultrasonic dispersion in a water bath. The framework vibration FTIR spectra were recorded on a TFS3000MX (BIO-RAD) infrared spectrophotometer at a resolution of 2 cm−1. The samples were thoroughly ground with KBr pellets before being pressed at 4 ton to form a thin wafer. The MAS 27Al NMR spectra were obtained with a Bruker FT NMR DRX-400 MHz instrument operating at 104.3 MHz and 5 s of relax delays. Raman spectra were collected using an Invia Refex Renishaw Raman microscope with near-infrared 785 nm as power source. The thermogravimetric (TGA) and differential scanning calorimetry (DSC) were performed concurrently using a SDT 2960 simultaneous DSC-TGA thermogravimetric analyzer (TA Instrument Co). Twenty milligrams of sample was used in each experiment. The sample was heated in air with a heating rate of 10 °C/min. X-ray photoelectron spectroscopy (XPS) was measured by VG ESCALAB-250 (Thermo Electron Corp.) instrument. Aluminum Kα X-ray with an analyzer pass energy of 20 eV was operated at 15 kV. The pressure in the XPS analysis chamber was less than 10−9 Torr.

Figure 1. TEM images of nanomaterials obtained by dry gel conversion with water-to-gel ratios of (A) 1:1, (B) 2:1, (C) 4:1, and (D) 6:1.

ratios. By steaming the dried precipitates at 200 °C for 48 h, nanostructured materials were obtained. When the dried gel was steamed with a water-to-gel ratio of 1:1, nanoparticles with a size of 80 ± 10 nm were formed. With the increase of waterto-gel ratio to 2:1, fibers in lengths of 1−2 μm and width about 100 nm were harvested. Sharp tips were observed at the end of the fibers. As a comparison, nanorod-like materials were obtained when the ratio of water-to-gel was increased to 4:1 or above. Straight nanorods of boehmite AlOOH with diameters of 20−30 nm and lengths of 200−500 nm were observed (Figure 1C,D). The aspect ratios of these nanorods were up to 20. The morphologies of nanomaterials were also characterized by FESEM as shown in Figure 2. The nanoparticles were agglomerated after drying, and particle size was not uniform. The nanofibers formed with water-to-gel ratio of 2:1 was ribbon-like, and nanorods obtained at water-to-gel ratios of 4:1 and 6:1 were very similar in length as well as diameter. Figure 3 displays the XRD patterns of dry gel compared with that of the resulting nanoparticles and nanofibers obtained by steaming the dry gel with different water-to-gel ratios and the corresponding materials after calcination at 600 °C. Prior to dry gel steaming conversion, the solid dried precipitates were amorphous aluminum hydroxides mixed with an impurity of NH4NO3 crystal, as shown in Figure 3. After a steam-assisted dry gel conversion with water-to-gel ratio of 1:1 under the condition of 200 °C for 48 h, the amorphous precipitate of aluminum hydroxides was found to be transformed to crystallized trihydrate-alumina, (Al2O3·3H2O) (International Center for Diffraction Data (ICDD) 01-0263). A strong X-ray diffraction peak at 2-theta of 8.6° was observed (Figure 3A-a). However, when the ratios of water-to-gel were increased to 2:1 or higher, the dry gel was converted to boehmite (AlOOH). 4988

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converted to γ-Al2O3 through dehydration of internal water, as shown in Figure 3B. All X-ray diffraction peaks of the calcined sample were well indexed to γ-Al2O3. The low diffraction intensity of the calcined nanoparticles implies the small primary particle size of the resultant γ-Al2O3. The results indicated that the water-to-gel ratio substantially affected the morphology and crystal structures of resulting nanomaterials by dry gel conversion of amorphous aluminum hydroxides. In this synthesis system, the structural transformation and material morphologies follows these reaction series: Al3 + + 3NO−3 + 3NH4(OH) → Al(OH)3 (amorphous) + 3NH4NO3

Figure 2. SEM images of nanomaterials obtained by dry gel conversion with water-to-gel ratios of (A) 1:1, (B) 2:1, (C) 4:1, and (D) 6:1.

(1)

Al 2O3 ·3H 2O → γ‐Al 2O3 + 3H 2O

(3)

2AlOOH → γ‐Al 2O3 + H 2O

(4)

The precipitation reaction 1 was conducted by mixing Al(NO3)3 aqueous solution with NH4(OH) solution. The dry gel of aluminum hydroxides obtained by precipitation followed by drying at 55 °C was in amorphous form or the primary nuclei without crystal growth were smaller than the limitation detectable by XRD. To simplify the synthesis procedure, the precipitate was collected by filtration without washing with deionized water, thus small amount of NH4NO3 was still retained in the solid of amorphous aluminum hydroxides as indicated in Figure 3. Nevertheless, the existence of NH4NO3 was not found to hinder the crystal growth of trihydratealumina and boehmite under the steaming conditions. The ratios of water-to-gel used for steam treatment substantially control the crystal growth behavior. When water-to-gel ratio was 1:1, aluminum hydroxide nuclei were self-assembled to nanoparticles of trihydrate-alumina nanoparticles. The amount of steam is not enough to create a local hydrothermal condition to drive the crystal transformation and growth. The amorphous aluminum hydroxide small particles were assembled by weak hydrogen bonds to form trihydrate-alumina (Al2O3·3H2O or Al(OH)3) nanosized particles. With the increase of water-to-gel ratio to 2:1 or above, the amorphous aluminum hydroxides were crystallized to AlOOH nanofibers by directed one-dimensional growth. It has been extensively reported that one-dimensional nanostructure of boehmite can be grown under solution-based hydrothermal conditions.25,39,46−48 In this study, when the water-to-gel ratio was above 2:1, steam could assist to form a local hydrothermal condition for the small particles of Al(OH)3 to initialize a crystal growth of AlOOH. While converting aluminum hydroxide to AlOOH, the extra water molecules formed during the crystallization of AlOOH maintained the suitable local hydrothermal condition for the nanofibers to continuously grow although the crystallization was not performed in a solution. When steam was enough to maintain the local

Figure 3. XRD patterns of (A) solid powder obtained by dry gel conversion with water-to-gel ratios of (a) 1:1, (b) 2:1, (c) 4:1, and (d) 6:1; (B) corresponding materials after thermal treatment at 600 °C for 2 h.

The obtained nanoribbons or nanorods exhibited a well-defined XRD pattern and all diffraction peaks were perfectly indexed to the boehmite AlOOH as indicated in Figure 3A-b−d. The cell parameters of AlOOH were calculated to be a = 3.7093 Å, b = 12.2365 Å, and c = 2.8685 Å, which are in good agreement with the value of bulk AlOOH (a = 3.6936 Å, b = 12.2141 Å, and c = 2.8679 Å, ICDD 83-2384). No diffraction peaks representing other phases were detected, indicating the high purity of the resultant crystalline AlOOH after the steaming treatment of dry gel. After being calcined at 600 °C for 5 h, all the trihydratealumina nanoparticles and AlOOH 1-D nanofibers were 4989

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hydrothermal condition, the growth of AlOOH is believed to follow the mechanism under solution-based hydrothermal conditions. Popa and co-workers49 proposed that the Al atoms in the boehmite lattice form a deformed octahedron with four oxygen and two hydroxide neighbors. Such octahedra joined by edges result in AlO(OH) polymeric layers. These layers are held together by hydrogen bonds between the hydroxyl groups of each octahedron. The crystallization of AlOOH has a preferential growth direction to form 1-D nanofibers due to the presence of the hydrogen bonds although the growth is in the absence of templates. Figure 4 displays the HRTEM images of nanoribbons and nanorods of boehmite AlOOH formed by dry gel conversion at

Figure 5. FTIR spectra of (a) nanoparticles of trihydrate-alumina, (b) nanoribbons of boehmite AlOOH, and (c,d) nanorods of boehmite AlOOH obtained with water-to-gel ratios of 4:1 and 6:1, respectively.

for the Al2O3·3H2O nanoparticles is obviously different from the boehmite nanofibers. The broad vibrational bands at 3454 and 1647 cm−1 are assigned to the asymmetric and symmetric vibrations of the water molecule, respectively.50 The acute peak in 1384 cm−1 corresponds to the coexistence of the nitrate anion with Al 2O 3 ·3H2 O nanoparticles, which was not thoroughly removed by filtration. In addition, Al2O3·3H2O nanoparticles exhibited three well resolved bands at 963, 735, and 538 cm−1, respectively. For AlOOH nanofibers formed under steaming condition at water-to-gel ratio of 2:1 or above, the FTIR spectra are similar regardless of the difference in morphologies. Both nanoribbons and nanorods showed two broad and resolved bands at 3105 and 3305 cm−1, which is attributed to symmetric and asymmetric stretching vibrations in OH groups. As boehmite AlOOH is a nonlinear molecule with 4 atoms, there are 6 fundamental modes (FM) of vibrations. Among all IR vibrations, assuming (AlO) is a single-point mass, OH groups have three kinds of important vibrations: OH group stretching, (AlO)−O−H angle bending and (AlO)−O−H deformation. In addition, supposing (OH) is a single-point mass, the other three vibration modes are described as (OH)−AlO symmetric bond stretching (ν1), (OH)−AlO angle bending (ν2), and (OH)−AlO asymmetric bond stretching (ν3).51,52 In total, six FM vibrations provided the theoretical explanation for the IR observation of the boehmite AlOOH resulting from the dry gel steam-assisted crystallization. At the region of 400− 1200 cm−1, well resolved sharper absorbance bands were observed. As shown in Figure 5, for the nanofibers of boehmite AlOOH, five strong absorbance bands (476, 673, 783, 1068, and 1179 cm−1) were observed. The strong and sharp band at 1068 cm−1 is assigned to the (OH)−AlO asymmetric stretching vibration. The small shoulder at 1179 cm−1 is assigned to the O−H bending. The (AlO)−O−H angle bending results in the absorbance band at 783 cm−1 and the absorbance band at 673 cm−1 is attributed to the (OH)−AlO angle bending. The sharp band at 476 cm−1 is assigned to the angle deformation (wagging) of OA−(OH). The absorbance bands in the region of 1500−2500 cm−1 are relatively weaker as compared with the other two regions. The broad weak band at 1647 cm−1 in this group is the well-known H−O−H angle

Figure 4. HRTEM images and SAED patterns of nanoribbons and nanorods obtained by dry gel conversion at water-to-gel ratios of (A) 2:1 and (B) 4:1, respectively.

water-to-gel ratios of 2:1 and 4:1, respectively. The layered crystal lattice of boehmite AlOOH is shown for both nanoribbons and nanorods. For nanorods formed at a waterto-gel ratio of 4:1 or above, the d-space of this layered stacking was 0.19 ± 0.01 nm, and the lattice stacking was along the axial direction, suggesting that nanorods grew along the (200) direction. The water-to-gel ratio was also found to affect the crystal growth direction during formation of the nanofibers. For the nanoribbons formed at a water-to-gel ratio of 2:1, the (200) lattice stacking with d-space with 0.19 nm was almost parallel to the axial direction of nanoribbons. The SAED pattern taken at lower magnification of both samples of the nanoribbons and nanorods presented in Figure 4 indicated that the crystal structure was of boehmite phase with orthorhombic crystal structure, which matched well with the data in Figure 3 and the JCPDS powder diffraction file 83-2384. The SAED pattern appeared in rings, indicating that dry gel conversion with waterto-gel ratio above 2:1 yielded polycrystalline nanofibers of boehmite AlOOH with controllable morphology. Figure 5 exhibits the FTIR spectrum of the nanoparticles and nanofibers of boehmite AlOOH obtained by steaming the dry gel at 200 °C for 48 h. It is observed that the FTIR spectrum 4990

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bands for vibration of atoms were not observed for the sample of Al2O3·3H2O nanoparticles obtained by steaming treatment with water-to-gel of 1:1. Figure 7 shows the O(1s) and Al(2p) XPS spectra of dry gel and nanostructured solid materials resulted from steaming the dry gel with different water-to-gel ratios. The binding energy of O(1s) of the amorphous dry gel was observed at 533.1 eV, which can be assigned to adsorbed water mixed with hydroxyl groups and oxygen impurity of NO3−.56 After being steamed with water-to-gel at 1:1, the binding energy of O(1s) was not obviously shifted, implying that the oxygen chemical environment was not changed after transformation to Al2O3·3H2O nanoparticles. It has been reported that oxygen in the crystal structure of oxides was characterized by a binding energy of about 530.6 eV in all minerals. Hydroxyl groups, present either in the crystal structure or on the surface, exhibited binding energies around 531.9 eV, while water on the surface showed binding energies around 533.0 eV. 57 Water molecules associated with the Al2O3·3H2O nanoparticles are the major source of oxygen, thus binding energy of O(1s) is observed at 533.1 eV. However, after being steamed with a water-to-gel ratio of 2:1 or above, the amorphous dry gel was transformed to nanostructured boehmite AlOOH, and the binding energy of O(1s) was shifted to 531.5 eV, attributing to the OH group in the crystal structure and oxygen anion in the crystal structure of Al−O.58 The binding energy of O(1s) was not dependent on the morphology of boehmite AlOOH nanofibers obtained under different conditions. The binding energy of Al(2p) in the dry gel was observed at 74.8 eV, which is assigned to the aluminum hydroxides in amorphous state. After being crystallized to Al2O3·3H2O nanoparticles, the binding energy of Al(2p) was not obviously shifted, implying that aluminum atoms were not reorganized with new chemical bonds. By comparison, for boehmite nanoribbons and nanorods, the binding energy of Al(2p) was shifted to 73.5 eV, indicating that the chemical environment of Al atoms was changed by the steaming treatment with water-to-gel ratios of 2:1 or above. Figure 8 shows differential scanning calorimetry (DSC)-TGA curves of Al2O3·3H2O nanoparticles and nanofibers of AlOOH with different morphologies. In the DSC signal profile, all

bending vibration band of H2O, and this absorbance in the spectra of AlOOH nanofibers is very weak, indicating a very small amount of physically adsorbed water molecules. As a comparison, the intensity of this band is stronger for Al2O3·3H2O nanoparticles as more H2O molecules are associated in the crystal. Figure 6 shows the Raman spectra of Al2O3·3H2O nanoparticles and nanofibers of AlOOH with different morpholo-

Figure 6. Raman spectra of (a) nanoparticles of trihydrate-alumina, (b) nanoribbons of boehmite AlOOH, and (c,d) nanorods of boehmite AlOOH obtained with water-to-gel ratios of 4:1 and 6:1, respectively.

gies. For the fully crystallized boehmite nanofibers obtained by steaming treatment with water-to-gel at 2:1 or above, three characteristic strong bands at 362, 495, and 675 cm −1 representing AlOOH nanorods at the region of 300−1000 cm−1 were observed. The strongest Raman band for AlOOH nanorods was observed at 362 cm−1, which was assigned to the vibration of the fully symmetric Ag mode in which all aluminum and oxygen atoms move parallel to the b-axis.53 The bands at 495 and 675 cm−1 originated from a doubly degenerate mode of the AlO6 octahedron.54 In addition, three associated weak bands at 338, 448, and 728 cm−1, attributing to OH− defect mode, were observed.55 As a comparison, these characteristic

Figure 7. XPS O(1s) (left) and Al(2p) (right) spectra of (a) nanoparticles of trihydrate-alumina, (b) nanoribbons of boehmite AlOOH, and (c,d) nanorods of boehmite AlOOH obtained with water-to-gel ratios of 4:1 and 6:1, respectively. 4991

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Figure 9 displays the TEM images of γ-Al2O3 resulting from the thermal treatment of Al2O3·3H2O nanoparticles and

Figure 8. DSC (top) and TGA (bottom) curves of (a) nanoparticles of trihydrate-alumina, (b) nanoribbons of boehmite AlOOH, and (c,d) nanorods of boehmite AlOOH obtained with water-to-gel ratios of 4:1 and 6:1, respectively.

Figure 9. TEM images of nanostructured γ-Al2O3 obtained by thermal treatment of (a) nanoparticles of trihydrate-alumina, (b) nanoribbons of boehmite AlOOH, and (c,d) nanorods of boehmite AlOOH obtained with water-to-gel ratios of 4:1 and 6:1, respectively.

samples exhibited an endothermic peak below 100 °C, which was caused by the desorption of physically adsorbed water. The Al2O3·3H2O nanoparticles exhibited a broad endothermic DSC signal at temperatures of 300−450 °C (peak at 400 °C), corresponding to 30% of the total weight loss in the TGA curve, caused by removing most of the crystal water molecules. For up to the temperature of 800 °C, the total weight of Al2O3·3H2O nanoparticles is 34.5%, which is very close to theoretical weight loss of 34.6% after being transformed to γAl2O3. As a comparison, the temperature for removing water from AlOOH nanofibers is higher than that of Al2O3·3H2O nanoparticles. The fully crystallized nanofibers of boehmite AlOOH exhibited a strong endothermic peak in DSC curves at a temperature range of 400−500 °C. This strong endothermic peak is attributed to the dehydroxylation of boehmite, and it corresponds to 12−14 wt % weight loss in its TGA curve, corresponding to 15 wt % of theoretical total weight loss. The crystal transformation of boehmite to alumina started with the diffusion of protons and reaction with hydroxyl ions to form water, followed by desorption from the internal surface. This process removed the hydrogen bond as the binding forces between layers in boehmite and caused a change in crystal structure as well as a change of chemical composition. Because of the thin structure of nanoribbons, the temperature for the removal of water from nanoribbons was shifted to a lower temperature as water molecules were easy to diffuse and desorb from nanoribbons. A DSC endothermic peak at 460 °C was observed for nanoribbons; instead, it was 480 °C for the nanorods of AlOOH obtained by steam treatment with waterto-gel at 4:1 or above. Nevertheless, as a comparison, the dehydration temperatures for the nanostructured AlOOH materials are much lower than that of bulk AlOOH microparticles, which exhibited a DSC peak for dehydration at 540 °C.59

nanofibers of boehmite AlOOH at 600 °C. It was observed that the basic morphology of nanoparticles and nanofibers could be well preserved. The transformation from boehmite to γ-Al2O3 has been theoretically investigated, and Krokidis and co-workers60 proposed the mechanism for structure transformation during thermal treatment. The transformation usually takes place by removal of a water molecule from two AlO(OH) octaedra with the help of internal condensation of a proton and a hydroxyl ion. The rest of the oxygen ions serve as an anion link between two Al atoms. The skeleton of γ-Al2O3 inherited from the original AlOOH network and the skeleton could be regarded as a primary matrix providing the basic structural properties of γ-Al2O3. Thus, the nanostructured γAl2O3 obtained in this work retained the overall morphologies of nanoribbons and nanorods after the thermal treatment. This kind of crystal structural transformation to γ-Al2O3 without a substantial change in morphology of nanostructure was also observed by other researchers.61,62 Similar to AlOOH, Al2O3·3H2O nanoparticles also preserved the morphology after removing the crystal water molecules and transforming to γ-Al2O3 nanoparticles. Porous structures with bright dots (10−15 nm) were observed on nanoparticles and nanorods.63 In comparison, the nanoribbons did not show the porous structures. The porous structures may contribute to the larger pore volume as indicated in Table 1. The γ-Al2O3 nanoparticles exhibited the largest pore volumes of 0.53 cm3/g, whereas the nanoribbons of Al2O3 showed the lowest pore volumes of 0.3 cm3/g. The Al2O3 nanoparticles had the highest specific area of 279.2 m2/g, and all nanofibers of Al2O3 showed similar values of surface areas of about 140−160 m2/g. Figure 10 shows the MAS 27Al NMR spectra of Al2O3·3H2O nanoparticles and nanofibers of boehmite AlOOH obtained by 4992

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crystal water molecules were removed from the Al2O3·3H2O nanoparticles, the 27Al NMR spectrum was substantially changed. During the thermal treatment, the Al and O atoms were rearranged to form the γ-Al2O3 with defective spinel (AB2O4) crystal structure, Φ0.33Al2.66O4 or (Al0.66Al2O4), where Φ is a cation vacancy. As γ-Al2O3 has a distorted spinel crystalline structure, more aluminum atoms are located at 6coordinated than 4-coordinated sites in the resultant γ-Al2O3 nanoparticles and nanofibers regardless of their different morphologies.68,69 It is noted that these types of 4-coordinated Al species were not observed for trihydrate-alumina nanoparticles, indicating the complete crystal structure transformation during the thermal treatment.

Table 1. Surface Area and Pore Volume of Nanostructured Al2O3 samples

specific area (m2/g)

pore volume (cm3/g)

a b c d

279.2 160.6 153.2 145.5

0.53 0.30 0.43 0.42

dry gel conversion at different water-to-gel ratio as well as the corresponding γ-Al2O3 materials resulting from calcination. The dry gel showed a broad peak at 4.8 ppm, which is assigned to 6coordianted Al species.64 In addition, two very weak signals with chemical shifts about 33.0 ppm and 63.0 ppm are attributed to the existence of a small portion of 5-coordinated and 4-coordinated Al species.65,66 The result indicates that the dry gel of aluminum hydroxide precipitates comprises various species of Al in heterogeneous chemical environments.52 After dry gel conversion with water-to-gel ratio of 1:1 at 200 °C for 48 h, the aluminum species were reorganized to the crystal structure of Al2O3·3H2O. The width of the 27Al NMR peak for 6-coordianted Al species became narrow as compared with the dry gel, and the signal for 5-coordinated and 4-coordinated Al species was not observable, implying uniform chemical environment in the crystallized Al2O3·3H2O nanoparticles. When the dry gel was steamed at a water-to-gel ratio of 2:1 or above, the obtained nanofibers of AlOOH exhibited a sharp 27 Al NMR peak at 7.8 ppm, indicating that Al atoms in well crystallized AlOOH had a very uniform chemical environment. Thus, the resultant nanofibers showed only one narrow 27Al NMR peak corresponding to 6-coordinated Al species. After thermal treatment at 600 °C for 5 h, all samples of Al2O3·3H2O nanoparticles and AlOOH nanofibers with different morphologies were converted to γ-Al2O3. A major peak at 6.8 ppm for nanoparticles, nanoribbons, and nanorods of γ-Al2O3 is attributed to 6-coordinated aluminum and a smaller peak around 63.6 ppm, which is assigned to 4coordinated aluminum species at tetrahedral sites.65,67 After

4. CONCLUSIONS Hydrated alumina nanoparticles and boehmite nanofibers with morphologies of nanoribbons and nanorods were fabricated via a dry gel conversion process, where solid form crystallization was performed by steam treatment of amorphous aluminum hydroxide dry powder. The crystal growth behavior and morphology of the obtained nanostructured materials were found to be dependent on the water-to-gel ratio for steam treatment. After being calcined at 600 °C for 5 h, nanostructured γ-Al2O3 was obtained, and the morphology was well preserved corresponding to their precursors. Among these nanostructured γ-Al2O3, the nanoparticles of γ-Al2O3 possessed the highest specific surface areas of 270 m2/g, and other nanofibers exhibited surface areas of 140−160 m2/g. This solid form steam crystallization technique overcomes the limitation of a reactor’s volume for solution based hydrothermal synthesis and can potentially be further developed for fabrication of nanostructured boehmite and γ-Al2O3 with desired morphologies at large scale with low cost.

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*E-mail: [email protected] (S.S.); reginald_ [email protected] (R.B.H.T.).

Figure 10. MAS 27Al NMR spectra of (left) (a) nanoparticles of trihydrate-alumina, (b) nanoribbons of boehmite AlOOH, and (c,d) nanorods of boehmite AlOOH obtained with water-to-gel ratios of 4:1 and 6:1, respectively; (right) nanostructured γ-Al2O3 obtained by thermal treatment of corresponding trihydrate-alumina nanoparticles and AlOOH nanofibers. 4993

dx.doi.org/10.1021/cg300915p | Cryst. Growth Des. 2012, 12, 4987−4994

Crystal Growth & Design

Article

Notes

(37) Hwang, J.; Min, B. D.; Lee, J. S.; Keem, K.; Cho, K.; Sung, M. Y.; Lee, M. S.; Kim, S. Adv. Mater. 2004, 16, 422. (38) Lee, J. S.; Min, B.; Cho, K.; Kim, S.; Park, J.; Lee, Y. T.; Kim, N. S.; Lee, M. S.; Park, S. O.; Moon, J. T. J. Cryst. Growth 2003, 254, 443. (39) Kuiry, S. C.; Megen, E.; Patil, S. D.; Deshpande, S. A.; Seal, S. J. Phys. Chem. B 2005, 109, 3868. (40) Hou, H. W.; Xie, Y.; Yang, Q.; Guo, Q. X.; Tan, C. R. Nanotechnology 2005, 16, 741. (41) Valange, S.; Guth, J. L.; Kolenda, F.; Lacombe, S.; Gabelica, Z. Microporous Mesoporous Mater. 2000, 35−36, 597. (42) Yu, J.; Wang, J.; Li, Z. S.; Li, L.; Liu, Q.; Zhang, M. L.; Liu, L. H. Cryst. Growth Des. 2012, 12, 2872. (43) Kim, T.; Lian, J. B.; Ma, J. M.; Duan, X. C.; Zheng, W. J. Cryst. Growth Des. 2010, 10, 2928. (44) Yang, Q. Bull. Mater. Sci. 2011, 34, 239. (45) Naik, S. P.; Chiang, A. S. T.; Thompson, R. W. J. Phys. Chem. B 2003, 107, 7006. (46) Musić, S.; Dragćević, O.; Popović, S. Mater. Lett. 1999, 40, 269. (47) Tang, B.; Ge, J. C.; Zhuo, L. H.; Wang, G. L.; Niu, J. Y.; Shi, Z. Q.; Dong, Y. B. Eur. J. Inorg. Chem. 2005, 4366. (48) Zhao, Y. Y.; Frost, R. L.; Martens, W. N.; Zhu, H. Y. Langmuir 2007, 23, 9850. (49) Popa, A. F.; Rossignol, S.; Kappenstein, C. J. Non-Cryst. Solids 2002, 306, 169. (50) Chaouche, S.; Ouarsal, R.; Bali, B. E.; Lachkar, M.; Bolte, M.; Dusek, M. J. Chem. Crystallogr. 2010, 40, 526. (51) Ram, S. Infrared Phys. Technol. 2001, 42, 547. (52) Rana, S.; Ram, S. J. Solid State Chem. 2001, 157, 40. (53) Doss, C. J.; Zallen, R. Phys. Rev. B 1993, 48, 15626. (54) Kiss, A. B.; Keresztury, G.; Farkas, L. Spectrochim. Acta, Part A 1980, 36A (7), 653. (55) Assih, T.; Ayral, A.; Abenoza, M.; Phalippou, J. J. Mater. Sci. 1988, 23, 3326. (56) Gredelj, S.; Gerson, A. R.; Kumar, S.; Cavallaro, G. P. Appl. Surf. Sci. 2001, 174, 240. (57) Kloprogge, J. T.; Duong, L. V.; Wood, B. J.; Frost, R. L. J. Colloid Interface Sci. 2006, 296, 572. (58) Ozao, R.; Yoshida, H.; Inada, T.; Ochiai, M. J. Therm. Anal. Calorim. 2003, 72, 113. (59) Xu, B. G.; Smith, P. Thermochim. Acta 2012, 531, 46. (60) Krokidis, X.; Raybaud, P.; Gobichon, A. E.; Rebours, B.; Euzen, P.; Toulhout, H. J. Phys. Chem. 2001, 105, 5121. (61) Trimm, D. L.; Stanislaus, A. Appl. Catal. 1986, 21, 215. (62) Wilson, S. J. J. Solid State Chem. 1979, 30, 247. (63) Zhong, Z. Y.; Ho, J.; Teo, J.; Shen, S. C.; Gedanken, A. Chem. Mater. 2007, 19, 4776. (64) Pozarnsky, G. A.; McCormick, A. V. J. Non-Cryst. Solids 1995, 190, 212. (65) Bagshaw, S. A.; Pinnavaia, T. J. Angew. Chem., Int. Ed. 1996, 35, 1102. (66) Chen, F. R.; Davis, J. G.; Fripiat, J. J. J. Catal. 1992, 133, 263. (67) MacKenziea, K. J. D.; Temuujinb, J.; Smithc, M. E.; Angererd, P.; Kameshima, Y. Thermochim. Acta 2000, 359, 87. (68) Baraton, M. I.; Quintard, P. J. Mol. Struct. 1982, 79, 337. (69) Tsyganenko, A. A.; Mardilovich, P. P. J. Chem. Soc., Faraday Trans. 1996, 92, 4843.

The authors declare no competing financial interest.

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REFERENCES

(1) Misra, C. Industrial Alumina Catalysis, ACS Monograph Series 1986, 184, 133. (2) KnÖ zinger, H.; Ratnasarmy, P. Catal. Rev. Sci. Eng. 1978, 31, 17. (3) Cortright, R. D.; Davda, R. R.; Dumesic, J. A. Nature 2002, 418, 964. (4) Argo, A. M.; Odzak, J. F.; Lai, F. S.; Gates, B. C. Nature 2002, 415, 623. (5) Rajesh, S.; Jantunen, H.; Letz, M.; Pichler-Willhelm, S. Int. J. Appl. Ceram. Technol. 2012, 9, 52. (6) Keon-kook, H.; Soonmin, S. Jpn. J. Appl. Phys. 2011, 50, 04DK17. (7) Webster, J. T.; Hellenmeyer, E. L.; Price, R. L. Biomaterials 2005, 26, 953. (8) Fadli, A.; Sopyan, I. Mater. Res. Innovations 2009, 13, 327. (9) Santos, A.; Balderrama, V. S.; Alba, M.; Formentin, P.; FerreBorrull, J.; Pallares, J.; Marsal, L. F. Adv. Mater. 2012, 24, 1050. (10) Lepota, N.; Van Baela, M. K.; Van den Rula, H.; DHaenc, J.; Peetersb, R.; Francob, D.; Mullensa, J. Ceram. Int. 2008, 34, 1971. (11) Kanti, N. M. J. J. Am. Ceram. Soc. 2010, 93, 1260. (12) Mathieu, Y.; Lebeau, B.; Valtchev, V. Langmuir 2007, 23, 9435. (13) Zhu, Z. F.; Sun, H. J.; Liu, H.; Yang, D. J. Mater. Sci. 2010, 45, 46. (14) Gutkin, M. Y.; Mikaelyan, K. N.; Ovidko, I. A. Rev. Adv. Mater. Sci. 2010, 24, 81. (15) Srikanth, N.; Zhong, X. L.; Gupta, M. Mater. Lett. 2005, 29−30, 3851. (16) Nguyena, Q. B.; Guptaa, M.; Srivatsanb, T. S. Mater. Sci. Eng., A 2009, 500, 233. (17) Mishra, S.; Ranjana, R.; Balasubramanian, K. J. Alloys Compd. 2012, 524, 83. (18) Kim, B. N.; Hiraga, K.; Morita, K.; Sakka, Y. Nature 2001, 413, 288. (19) Thomson, K. T.; Wentzcovitch, R. M. Science 1996, 274, 1880. (20) Li, G. H.; Jiang, Z.; Jiang, A. Q.; Zhang, L. D. Nanostruct. Mater. 1997, 8, 749. (21) Bhaduri, S.; Bhaduri, S. B. Nanostruct. Mater. 1997, 8, 755. (22) Ke, X. B.; Zhu, H. Y.; Gao, X. P.; Liu, J. W.; Zheng, Z. F. Adv. Mater. 2007, 19, 785. (23) Jin, Y. Z.; Zhu, Y. Q.; Brigatti, K.; Kroto, H. W.; Walton, D. R. M. Appl. Phys. A: Mater. Sci. Process. 2003, 77, 113. (24) Zhang, Z. R.; Pinnavaia, T. J. J. Am. Chem. Soc. 2002, 124, 12294. (25) Lee, H. C.; Kim, H. J.; Chung, S. H.; Lee, K. H.; Lee, H. C.; Lee, J. S. J. Am. Chem. Soc. 2003, 125, 2882. (26) Kim, H. J.; Lee, H. C.; Rhee, C. H.; Chung, S. H.; Lee, H. C.; Lee, J. S. J. Am. Chem. Soc. 2003, 125, 13354. (27) Kung, D.; Fang, Y.; Liu, H.; Frommen, C.; Fenske, D. J. Am. Chem. Soc. 2003, 13 (4), 660. (28) Shen, S. C.; Chen, Q.; Chow, P. S.; Tan, G. H.; Zeng, X. T.; Wang, Z.; Tan, R. B. H. J. Phys. Chem. C 2007, 111, 700. (29) Shen, S. C.; Ng, W. K.; Chen, Q.; Zeng, X. T.; Chew, M. Z.; Tan, R. B. H. J. Nanosci. Nanotechnol. 2007, 7, 2726. (30) Ling, T. G.; Yong, L. K.; Mahmood, W. A. K. J. Sol-Gel Sci. Tech.nol 2007, 44, 177. (31) Yang, Q.; Deng, Y. D.; Hu, W. B. Ceram. Int. 2009, 35, 531. (32) Shen, S. C.; Ng, W. K.; Zhong, Z. Y.; Dong, Y. C.; Chia, L.; Tan, R. B. H. J. Am. Ceram. Soc. 2009, 92, 1311. (33) Valcarcel, V.; Perez, A.; Cyrklaff, M.; Guitian, F. Adv. Mater. 1998, 10, 1370. (34) Peng, X. S.; Zhang, L. D.; Meng, G. W.; Wang, X. F.; Wang, Y. W.; Wang, C. Z.; Wu, G. S. J. Phys. Chem. B 2002, 106, 11163. (35) Zhang, Y.; Li, R. Y.; Zhou, X. R.; Cai, M.; Sun, X. L. Cryst. Growth Des. 2009, 9, 4230. (36) Liu, S.; Wehmschulte, R. J.; Burba, C. M. J. Mater. Chem. 2003, 13, 3107. 4994

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