Synthesis and Characterization of Gibbsite Nanostructures - The

Single-crystal gibbsite nanorods with a high aspect ratio were synthesized by a facile hydrothermal method. Characterizations by HRTEM, SAED, and XRD ...
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J. Phys. Chem. C 2008, 112, 4124-4128

Synthesis and Characterization of Gibbsite Nanostructures Ye Liu,† Ding Ma,*,† Ross A. Blackley,‡ Wuzong Zhou,*,‡ Xiuwen Han,† and Xinhe Bao† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China, and School of Chemistry, UniVersity of St. Andrews, St. Andrews, KY16 9ST, United Kingdom ReceiVed: October 19, 2007; In Final Form: December 14, 2007

Single-crystal gibbsite nanorods with a high aspect ratio were synthesized by a facile hydrothermal method. Characterizations by HRTEM, SAED, and XRD revealed that the nanorods prefer to grow along the c-axis. The nanorods can be easily transformed to χ-alumina by calcination at 250 °C, while the nanorod morphology is well-maintained. HRTEM indicates that the χ-alumina nanorod possesses a single-crystal-like structure formed by orientated connection of crystalline nanodomains. Altering the synthetic procedure will lead to alumina nanostructures with a different phase structure and morphology, such as alumina nanotubes.

Introduction One of the challenges in the syntheses of advanced materials is the development of synthetic methodologies that would enable the control of inorganic nanocrystal morphologies. In the past decade, one-dimensional (1-D) nanomaterials, such as nanorods, nanowires, nanobelts, nanotubes, and nanocables, have attracted extraordinary attention due to their importance in fundamental scientific research and potential technology applications.1 On the basis of their nanoscale size and 1-D anisotropy, this family of materials possesses many unique and fascinating properties, such as superior mechanical toughness,2 higher luminescence efficiency,3 enhancement of thermoelectric figure of merit,4 and a lowered lasing threshold.5 Numerous 1-D nanomaterials with different morphologies and compositions have been synthesized by two main strategies: gas-phase processes and solution-based processes.6 It is only very recently that 1-D oxide nanostructures have begun to emerge as very promising nanoscale building blocks because of their interesting properties, diverse functionalities, surface cleanness, and chemical/thermal stability.7 Gibbsite is an important semi-manufactured product of the Bayer process in aluminum production.8 It has been used as a polishing agent in toothpaste,9,10 fire retardant,9 coating and filler in paper manufacturing,10 and rheology enhancer in drilling muds.11 The size distributions of gibbsite particles obtained from Bayer liquors range from a few to several hundred micrometers. Many efforts have been made to synthesize nanosized gibbsite. Usually, a long aging time ranging from several days to weeks was used in the synthesis, and hexagonal plate-like particles with a dimension up to several hundred nanometers were obtained.12 Although rectangular gibbsite particles were observed, the aspect ratio of them was very low.13 Until now, anisotropic 1-D gibbsite nanomaterials have not been reported. As an oxide with unassailable importance, alumina has been proven to be a valuable material in nanoscience and nanotechnology because of its many excellent properties such as high strength and toughness, high elastic modulus, thermal and chemical stability, high thermal conductivity, very low perme* To whom correspondence should be addressed. E-mail: (D.M.) [email protected] or (W.Z.) [email protected]. † Chinese Academy of Sciences. ‡ University of St. Andrews.

ability, and good electrical insulation.14 Alumina whiskers and fibers are widely used in metal matrix composites,15 as catalyst supports or absorbents for scavenging precious or heavy metals,16 as radar transparent structures and antenna windows,17 and as capacitor dielectrics and gate oxides in memory devices.18 Because of their poor crystallinity, the structures of transition aluminas are less known. Although enormous effort has been extended to prepare alumina nanorods and nanowires, the structural studies of these materials with a well-defined morphology and high crystallinity synthesized by solution-based methods are very limited. In the present work, novel gibbsite nanorods were prepared for the first time, to our knowledge, by a simple hydrothermal method with a relatively short aging time. The nanorods possess a single-crystalline nature and have an aspect ratio higher than 12. They can easily transform to χ-alumina under calcination at 250 °C. Experimental Procedures The typical synthesis process of the alumina nanorods was as follows: 1.207 g of AlCl3‚6H2O and 0.80 g of NaOH were dissolved in 18 mL of deionized water to form a transparent NaAlO2 solution. A total of 1.565 g of cetyl trimethylammonium bromide (CTAB) was dissolved in 18 mL of EtOH. The CTAB solution was added to the NaAlO2 solution under stirring. After further stirring for 1 h, the mixture was transferred into a Teflonlined stainless autoclave and underwent thermal treatment at 120 °C for 12 h. After being cooled to room temperature, the final product was collected by centrifugation, washed with ethanol, and then vacuum-dried at room temperature for 12 h. The as-obtained product was further calcined at different temperatures varying from 200 to 600 °C for 4 h for the phase evolution study. X-ray powder diffraction (XRD) patterns were recorded using a Rigaku D/max-2500/PC diffractometer with Cu KR radition (λ ) 0.15418 nm). Phase identification was performed with the search/match method using a PDF2 database, and the profile fitting was performed by TOPAS using the Pawley method (TOPAS V3: general profile and structure analysis software for powder diffraction data, Bruker AXS). Scanning electron microscopy (SEM) images were obtained on a JSM-6360lv

10.1021/jp7101572 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/23/2008

Synthesis/Characterization of Gibbsite Nanostructure

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Figure 1. Comparison of observed XRD pattern of as-synthesized gibbsite nanorods to the Pawley analysis of monoclinic gibbsite.

Figure 2. (A) Representative SEM image of single-crystal gibbsite nanorods. (B) TEM image of one single-crystal gibbsite nanorod; and (C) HRTEM and SAED pattern of the sample in panel B.

electron microscope. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM-2011 electron microscope. TEM images were recorded by using a Gatan CCD camera at a magnification range from 50 000× to 800 000×. Thermal analyses (thermogravimetric (TG) and differential thermal analysis (DTA)) were carried out on a PE Diamond TG/DTA instrument. Samples (ca. 20 mg) were heated from ambient temperature to 700 °C at a rate of 10 °C/min in flowing air. The samples were dried at 100 °C prior to use. Results and Discussion Normally, the products obtained from caustic aluminate solution are mixed phases of monoclinic bayerite and gibbsite Al(OH)3. Bayerite may transform into a thermodynamically more stable gibbsite phase upon in situ aging for weeks.19 In the present work, the XRD pattern of the as-synthesized sample can be indexed to the monoclinic gibbsite phase (JCPDS PDF no. 33-0018). Using monoclinic gibbsite as the starting structure (in space group P21/n, No. 14), profile fitting was performed, as shown in Figure 1. A good fit between the observed and the calculated profiles was obtained with a ) 8.67881(51) Å, b ) 5.07436(27) Å, c ) 9.73081(64) Å, and β ) 94.5490(51)° (see Supporting Information), indicating a monophasic property. Notwithstanding that the reaction times were shortened remarkably, the product is the pure gibbsite phase. The preferential orientation of the nanorod growth along the c-axis has been confirmed by the SAED investigation on individual nanorods.

SEM (Figure 2A) and TEM (Figure 2B) images indicate that the as-synthesized gibbsite possesses a well-defined nanorod morphology. In addition, some particles with irregular shapes were also observed. The proportion of the nanorods is about 80%. The average diameter of the nanorods is around 120 nm. These nanorods possess a high aspect ratio (>12). Some of them have an aspect ratio of up to 40. HRTEM images and SAED patterns confirm the singlecrystalline structure of the nanorods. Figure 2C shows a typical HRTEM image viewed down the [11h0] zone axis of the monoclinic structure of gibbsite with the corresponding SAED pattern. The real space d-spacing of the horizontal diffraction spots is about 0.433 nm, corresponding to the (110) planes. The vertical diffraction spots, which are along the long axis of the nanorod, give a d-value of 0.483 nm, corresponding to the (002) diffraction index of the (001) plane. The latter group of diffraction spots has relatively higher intensities, in good agreement with the XRD pattern in Figure 1. The observed interplane angle between (110) and (002) is about 87.5°, which is also very close to the reported value of 87.7° derived from monoclinic gibbsite. Figure 3 shows the thermogravimetric analysis results of the gibbsite nanorods. Only one weight loss at around 220370 °C was detected, which corresponds to an endothermic process. The overall mass loss is around 30%, in good agreement with the water loss from gibbsite [Al(OH)3] to χ-alumina (Al2O3). The weight loss corresponding to the decomposition of surfactants was not detected, indicating that the surfactants

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Figure 3. TG/DTA plots of as-synthesized gibbsite nanorods.

Figure 4. XRD pattern of the nanorod sample after calcination at 250 °C. The diffraction peaks are indexed to the unit cell of χ-alumina.

added during the synthesis have been previously removed by washing the products with ethanol. It is interesting to note, given a slower heating rate (1 °C/ min), that the as-synthesized nanorods can be transformed to χ-alumina completely at 250 °C. The XRD pattern of calcined nanorods shown in Figure 4 is in accordance with that of hexagonal χ-alumina (JCPDS PDF no. 13--0373). Pawley profile fitting of the XRD data was performed. As there are no crystal structure parameters availaible for χ-alumina, we assume that the hexagonal χ-alumina takes the space group P6, the simplest one in a hexagonal system. The fit between the observed and the calculated profiles seems reasonable (a ) 5.674 Å and c ) 8.612 Å, space group P6, see Supporting Information). However, the line broadening of the reflections indicates a remarkably reduced particle size of the crystals. Using the crystallite size as a parameter in the profile fitting process, a mean crystallite size of about 4 nm is obtained. This demonstrates that, after calcination, the single-crystalline structure of the gibbsite nanorods probably has broken down into small crystallites. On the other hand, the TEM image shows that the morphology of the gibbsite nanorods is maintained very well after calcination (Figure 5A). However, the image contrast shows that the original crystal structure has been destroyed and that the material seems to be porous or polycrystalline. To our surprise, the SAED pattern from these nanorods still gave singlecrystal-like patterns. The inset of Figure 5A is a typical pattern recorded from the nanorod presented, when viewed down the

Liu et al. [001] zone axis of the hexagonal unit cell of χ-alumina. The diffraction spots with the smallest distance from the center correspond to a d-spacing of 0.278 nm, which can be indexed to the {11-20} atomic planes. The detailed structure of the calcined sample was revealed by HRTEM investigation. As seen in Figure 5B, the nanorods are still crystalline with many nanodomains. The line width broadening in the XRD spectrum is believed to result from the presence of those very small nanodomains. The sizes of these nanodomains measured from the HRTEM image are around 3-5 nm, which matches the value calculated from XRD very well. In addition, these nanodomains are perfectly orientated and connected with each other, giving single-crystal-like SAED patterns. A previous report on the oriented aggregation of nanocrystallites also confirms our observation.20 Such a process can be regarded as a bottom-up manner, while the alumina nanorods prepared in the present work have a similar structure but form with a top-down route (i.e., from a real piece of single crystal of gibbsite to three dimensionally connected nanocrystallites of χ-alumina). This is to some extent similar to the aggregate-like morphology observed on zeolite L, which was formed during the gel to zeolite transformation.20e The microstructure shown in Figure 5B is also similar to the porous single crystals of metal oxides developed in recent years.21 The differences between these two materials are that the crystallites in χ-alumina nanorods are much smaller, but not in an ordered arrangement, and that the pores between the crystalline domains are unlikely to be connected into a threedimensional network. The reason for the formation of the pores during the phase transformation from Al(OH)3 to Al2O3 is proposed to be due to the higher density of Al2O3 (3.97 g/cm3) in comparison to that of Al(OH)3 (2.49 g/cm3). It has been reported that structures of products by dehydrating gibbsite are different types of transition aluminas depending on the dehydration atmosphere and grain size of the gibbsite used. Air atmosphere and a fine grain size normally lead to the formation of χ-alumina. On the other hand, coarse-grained gibbsite, especially in the presence of water vapor, usually transforms to boehmite and γ-alumina after dehydration.22 The present results are in accordance with previous reports.22 In addition, as the gibbsite nanorods are small in size, the phase transformation temperature of the nanorods (250 °C) is lower than that reported (300 °C). The detailed mechanism of this complicated phase transformation process has yet to be understood, as even the crystal structure of χ-alumina has been uncertain up to now. Kogure studied the dehydration of gibbsite under in situ electron beam irradiation and suggested that χ-alumina was formed as the gibbsite layers shifted laterally, moving slightly closer to each other to form a random close-packed arrangement of anions.23 The crystallization of aluminum hydroxide, Al(OH)3, from a caustic aluminate solution is the key step in the Bayer cycle, which is widely used for alumina production.24 Because of the complexity of this process, the mechanism of Al(OH)3 crystallization is still not completely understood.25 The gibbsite crystallization process can be expressed as follows:

Na+Al(OH)4-(aq) f Al(OH)3(s) + NaOH(aq)

(1)

Usually, gibbsite with a high crystallinity is synthesized by aging at moderate temperatures. It takes several days or even weeks because the growth rate of gibbsite is very low. The grain size of the products can be up to tens of micrometers.19 To prepare gibbsite with a nanoscale size and anisotropic morphol-

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Figure 5. (A) TEM of single-crystal-like χ-alumina nanorods. The inset is a corresponding SAED pattern. (B) HRTEM image of a χ-alumina nanorod.

Figure 6. TEM images of boehmite nanotubes.

ogy, accelerating the growth rate of gibbsite is desired. It is well-known that hydrothermal conditions are helpful for the growth of crystals.26 Using a modified hydrothermal method, crystalline gibbsite can be synthesized within 12-24 h in our system. The use of mixed solvents of water and alcohol is a key factor in the process. On the one hand, it is reported that the addition of alcohol will promote the micellar sphere-rod transition and lengthen the rod-shaped micelles.27 The existence of long rod-shaped micelles is obviously beneficial for the synthesis of gibbsite nanorods. On the other hand, the polarity of an alcohol-containing system is lower than the polarity of a water system. It is easy to understand that positively charged CTAB tends to combine with electronegative Al(III)-containing species in polarity-decreased circumstances. Our controlled experiment confirmed the key role of ethanol in the system. The product obtained from a system only using water as a solvent mainly gave spherical particles instead of well-defined nanorods (see Supporting Information).

After a series of optimization experiments, we found that it was difficult to prepare products with 100% nanorods in morphology. We propose that this resulted from the complexity of Al(III)-containing speciation in the caustic aluminate solution and the uncertain crystallization mechanism.28 It was observed that a higher precursor concentration and higher temperature will accelerate the growth of gibbsite crystals, enhancing the formation of single-crystal gibbsite nanorods. Therefore, a higher precursor concentration benefits the production of a pure gibbsite phase. At the same time, the higher precursor concentration will aggravate the agglomeration of the gibbsite crystal nuclei, leading to particles with an enlarged size and irregular morphology by secondary nucleation. As a result, under optimized conditions, the yield of single-crystal gibbsite nanorods is around 80%. It is worth noting that Kuang et al. obtained boehmite nanotubes from a system similar to that in our report.29 In our studies, we noticed that the sequence of raw material addition

4128 J. Phys. Chem. C, Vol. 112, No. 11, 2008 is pivotal for the phase and morphology control of the products. If the NaOH solution was mixed with the CTAB solution prior to the addition of the AlCl3 solution, boehmite nanotubes were obtained after hydrothermal treatment (Figure 6 and XRD patterns, see Supporting Information). Instead, if a clear sodium aluminate solution is prepared first, followed by the addition of the CTAB solution, gibbsite nanorods are obtained after the reaction. It is well-known that the nature of the Al(III)-containing particles and species in a freshly prepared sodium aluminate solution is mainly determined by the concentration of the NaOH solution used in synthesis. With a higher concentration of the NaOH solution, small Al(III)-containing species, in forms of monomers, dimers, and trimers, are the dominant species. Instead, at a lower NaOH concentration, the relatively larger Al(III)-polycondensed structures (e.g., Keggin ions) are dominant.28 In the former case, as the NaOH solution was premixed with the CTAB solution, the small Al(III)-containing species such as Al(OH)4- or AlO2- formed ion pairs with CTAB. Templated by the rod-like micelles of CTAB, these Al(III)containing species underwent condensation and mineralization under hydrothermal conditions to form boehmite nanotubes. In the latter case, it is difficult for the large Al(III)-polycondensed species to form ion pairs with CTAB. Instead, the CTAB added will act as the oriented attachment reagent that enables anisotropic growth of gibbsite crystals. Therefore, the gibbsite nanorods were obtained. Conclusion In summary, using a simple surfactant-assisted hydrothermal method, novel single-crystal gibbsite nanorods with a high aspect ratio (>12) were synthesized. It is noteworthy that in contrast to the previous methods to synthesize crystalline gibbsite, the crystallization process was shortened greatly in the present study. These single-crystal gibbsite nanorods can be easily transformed to single-crystal-like χ-alumina nanorods at a low temperature (250 °C). The syntheses of single-crystal gibbsite and χ-alumina are highly helpful in studying various properties of these anisotropic structures. Acknowledgment. Dr. Yingxia Wang (Peking University) is thanked for her valuable assistance with XRD profile fitting and for helpful discussions. Supporting Information Available: Pawley profile fitting results, XRD patterns, TEM image of the product, and experimental procedures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Heath, J. R.; Kuekes, P. J.; Synder, G.; Williams, R. S. Science (Washington, DC, U.S.) 1998, 280, 717. (b) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science (Washington, DC, U.S.) 2001, 291, 1947. (c) Yang, P. D.; Yan, H. Q.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R. R.; Choi, H. J. AdV. Funct. Mater. 2002, 12, 323. (d) Edmondson, M. J.; Zhou, W. Z.; Seiber, S.; Gameson, I.; Anderson, P. A.; Edwards, P. P. AdV. Mater. 2001, 13, 1608. (e) Zhu, K.; He, H.; Xie, S.; Zhang, X.; Zhou, W. Z.; Jin, S.; Yue, B. Chem. Phys. Lett. 2003, 377, 317. (2) Wang, E. W.; Sheehan, P. E.; Lieber, C. M. Science (Washington, DC, U.S.) 1997, 277, 1971.

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