Novel Synthesis Strategy of γ-AlOOH Nanotubes: Coupling Reaction

(2, 3) Aluminum oxide hydroxides and alumina are low cost materials, which are ... and the ionic liquid, 1-octyl-3-methylimidazolium chloride ([C8mim]...
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Novel Synthesis Strategy of γ‑AlOOH Nanotubes: Coupling Reaction via Ionic Liquid-Assisted Hydrothermal Route Qing Qin,† Tongil Kim,† Xiaochuan Duan,† Jiabiao Lian,† and Wenjun Zheng*,†,‡ †

Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (MOE), and TKL of Metal and Molecule-Based Materials Chemistry, College of Chemistry and ‡Collaborative Innovation center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, P. R. China S Supporting Information *

ABSTRACT: A novel synthetic strategy, i.e., transition metal ions used to drive a coupling reaction in terms of complex formation of the metal ions and decomposition of a precursor, combined with ionic liquid-assisted hydrothermal route, was utilized to synthesize γ-AlOOH nanotubes with high purity and uniform dimension at a mild condition. These γ-AlOOH nanotubes can be easily transformed to γ-Al2O3 nanotubes by calcining at 600 °C for 2 h, without changing the morphology. More specifically, this strategy may be helpful to develop a new opportunity for synthesis of inorganic nanomaterials with novel morphologies and improved properties. synthetic strategy under mild conditions to construct γ-AlOOH nanotubes on a large scale for industrial applications becomes urgent in material science and synthetic chemistry. Herein, we report a one-step synthesis of γ-AlOOH nanotubes with uniform diameter and dimension by means of a coupling reaction in terms of the formation of the complex [Zn(CH3COO)x(H2O)4‑x](2‑x)+ and the decomposition of the precursor (CH3COO)2Al(OH) via an ionic liquid-assisted hydrothermal method at relatively low temperature. The results show that Zn2+ ions added in the reaction system drive this coupling reaction, and the ionic liquid, 1-octyl-3-methylimidazolium chloride ([C8mim]Cl), plays a key role on the morphology of the products. The γ-Al2O3 nanotubes can be obtained by calcining the as-synthesized γ-AlOOH at 600 °C for 2 h. γ-AlOOH nanotubes were synthesized at 130 °C in a molar ratio of Al3+:Zn2+:CH3COO− = 2:1:5, and the samples were labeled as S-0, S-1, S-2, and S-3, respectively, with increasing the [C8mim]Cl contents (details are listed in Table S1 in the Supporting Information). The crystal structure of the asprepared γ-AlOOH was determined by X-ray diffraction (XRD) (Figure 1a). All of the diffraction peaks can be assigned to the standard value of boehmite γ-AlOOH (JCPDS No. 21−1307), and no characteristic peaks represent any impurities. Notably, the widening of X-ray diffraction peaks demonstrated smaller size of the products, which is only 6 nm in diameter calculated according to the Scherrer formula. Figure 1b is the EDS spectrum of the γ-AlOOH, further confirming the composition;

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ne dimensional nanostructured inorganic materials, such as nanotubes and nanowires, are vital building block materials for nanoscale science and engineering.1 Despite considerable progress in controlling synthesis of inorganic materials, the difficulty dramatically increases when one or more dimensional of the object drop to sub-10 nm length scales while keeping the uniformity of morphology and composition.2 Particularly, desirable properties would be achieved in such material, such as fast transport of charge/mass/heat, drastically tunable electronic structure, confinement effect, and very high surface areas.2,3 Aluminum oxide hydroxides and alumina are low cost materials, which are widely used in industries as catalysts,4−7 catalyst supports,8−10 adsorbents,11 ceramics,12 abrasives,13 and photovoltaic device.14 As the low-temperature metastable polymorph of transition of alumina, γ-Al2O3 with a wide range of surface hydroxyls is one of the most important oxides and also the most common form of activated alumina for adsorptive and catalytic applications.15 Deeply attracted by its dielectric passivation character, γ-Al2O3 plays a very important role in solar cell, providing excellent passivation to drastically increase the silicon solar cell efficiency.16−20 The conventional synthesis of γ-Al2O3 is to dehydrate boehmite γ-AlOOH, with the morphology preserved.21−23 Up to now, many kinds of methods have been developed to prepare boehmite nanotubes, such as template-free method,24 surfactant-assisted hydrothermal method,25 a sol−gel technique,26 utilizing onedimensional inorganic nanowires or AAO porous membrane as sacrificial templates,27,28 and two-step synthesis.29 However, to the best of our knowledge, those synthetic methods, apart from being time-consuming, complex operations, needing relatively high temperature or volatile organic solvent, have the inherent disadvantages of fabricating the target products with high purity and uniform dimension.26,30 Therefore, a novel © 2016 American Chemical Society

Received: May 8, 2016 Revised: September 9, 2016 Published: September 20, 2016 6139

DOI: 10.1021/acs.cgd.6b00703 Cryst. Growth Des. 2016, 16, 6139−6143

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hydroxide can be thermally transformed to boehmite at 300 °C. 3 4,3 5 This results suggest that decomposing of (CH3COO)2Al(OH) to form γ-AlOOH in the present conditions is nonspontaneous from the thermodynamics point of view. The addition of Zn(NO3)2 is, therefore, a crucial factor for the formation of γ-AlOOH in the present conditions. The phase evolution of the as-synthesized products with different reaction time was characterized by powder XRD measurements, as shown in Figure 2. When the reaction time

Figure 2. XRD patterns of the samples obtained with 8 mmol [C8mim]Cl at 130 °C for (a) 2.5, (b) 3.5, (c) 6, and (d) 8 h, demonstrating the phase evolution from (CH3COO)2Al(OH) to γAlOOH.

was 2.5 h, only pure (CH3COO)2Al(OH) was obtained (Figure 2a), and relatively pure γ-AlOOH crystals were formed after reaction time was extended to 8 h (Figure 2d). On the basis of the chemical properties of the coexistent ions in the present reaction system, we consider that the formation process of γAlOOH may be attributed to the coupling reaction in terms of complex formation of Zn2+ ions and decomposition of aluminum acetate hydroxide. It is well-known that the precipitation pH value of Al(OH)3 and Zn(OH)2 are 4.0 and 6.4, respectively.36 According to our experimental results, the present reaction system shows the original pH value of ca. 5.7 and the final one of ca. 3.4; therefore, it is difficult to form Zn(OH)2 precipitation. As shown in Figures 1b and 3b, the EDS spectra of the as-obtained and calcined samples reveal the absence of zinc in these products, which also confirm the above considerations. Moreover, in principle, Al(OH)3 can only exist as a highly active colloidal state in this pH range. Consequently, in the initial stage, aluminum acetate hydroxide precipitate can be rapidly formed rather than Al(OH)3 or AlOOH precipitates. On the other hand, Zn2+ ion as a transition metal ion shows a relatively high complexability. Despite the indifferent capability of coordination with acetate anion, the formation constant is only lgβ1 = 1.5 for the complex [Zn(CH3COO)x(H2O)4‑x](2‑x)+ (where x is 1 or 2).37 According to this formation constant, the concentration of the complex is only ca. 5.6 × 10−4 mol/dm3, accounting for about 1% of the total of Zn2+ ions. Because of the complex formation ability and positive charge, free Zn2+ ions in solution can be adsorbed on the surface of (CH3COO)2Al(OH) to construct [Zn(CH3COO)x(H2O)4‑x](2‑x)+ complex, and these complex ions thereafter can be dissolved from the surface into solution. Meanwhile, because there is appropriate concentration of OH− ions formed by the hydrolysis of potassium acetate, γ-AlOOH can be formed. After that, the complex will be dissolved into free ions under the equilibrium driving force. As previously stated, the coupling reaction is a dynamic process going forward

Figure 1. (a) XRD pattern, (b) EDS spectrum, (c,d) TEM images, (e,f) magnified TEM images (hollow structures were apparent demonstrated by yellow arrows), (g) SEM image of the γ-AlOOH nanotubes synthesized with 8 mmol [C8mim]Cl under 130 °C, (h,i) TEM image and HRTEM image of a single nanotube, showing the smaller size in diameter and hollow nature.

only the Al and O peaks are observed and the atom ratio is 33.84% to 66.16%, which is very close to the stoichiometric ratio of γ-AlOOH. Typical panoramic TEM images in Figure 1c and d reveal that the products are composed by a number of uniform monodispersed rod-like nanostructures with the length of 100−200 nm. Magnified TEM images (Figure 1e and f) are corresponding to the areas in Figure 1d marked by red rectangles. As highlighted by yellow arrows, the rod-like products own the hollow interior. High-magnification SEM image in Figure 1g indicated the rod-like morphology and high homogeneity of the products. Figure 1h and i are the TEM images of a typical single nanotube, showing the hollow nature and the smaller size in diameter (∼6 nm) and wall thickness (∼1−2 nm). The FTIR (Figure S1a in the Supporting Information) and TG-DTA (Figure S2) results are precisely consistent with the results in previous literature,16,31−33 effectively confirming the as-obtained products are a purephase γ-AlOOH. To obtain a better understanding of the effect of Zn2+ ions on the γ-AlOOH formation, a contrast experiment was carried out in the absence of Zn(NO3)2, keeping other conditions constant. The XRD patterns of the product can be perfectly assigned to the standard value of aluminum acetate hydroxide (CH3COO)2Al(OH) (JCPDS No. 54−0321/0) (Figure S3). Moreover, as shown in our previous work, aluminum acetate 6140

DOI: 10.1021/acs.cgd.6b00703 Cryst. Growth Des. 2016, 16, 6139−6143

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Scheme 1. Illustration of the Aluminum−Oxygen Core Structuresa

(a) γ-AlOOH, (b) (CH 3COO) 2Al(OH), (c) the formation mechanism of γ-AlOOH. White ball: Al. Red ball: O. Blue ball: OH. Gray ball: C. Small white ball: H. These indicated that a similar coordinate environment of aluminum exists in γ-AlOOH and aluminum acetate hydroxide in Al3+ solution. a

transition metal clusters, which have boehmite-like metal− oxygen cores surrounded by a supporting organic framework.39,42−44 Overall, based on the previous analysis, our proposed formation mechanism of γ-AlOOH nanostructures can be illustrated as in Scheme 1c. Due to the fact that a similar coordinate environment of aluminum consists in γ-AlOOH and aluminum acetate hydroxide, acetate ions are plucked up by the formation of [Zn(CH3COO)x(H2O)4‑x](2‑x)+ complex from the (CH3COO)2Al(OH) precipitates, which can be easily transformed to γ-AlOOH in coupling reaction process. Ionic liquid 1-octyl-3-methylimidazolium chloride ([C8mim]Cl) plays a crucial role in forming the γ-AlOOH nanotubes. As shown in Figure S7a,b, there was a leaf-like morphology obtained in the absence of [C8mim]Cl. Increasing the amount of [C8mim]Cl leads to production of nanotubes (Figure S7c−f and Figure S8). All the results mentioned above effectively proved that [C8mim]Cl surely had a deep effect on the structure and morphology of γ-AlOOH. As our previous work reported,34 ionic liquid can reduce the surface energy of the (CH3COO)2Al(OH) precursor and stabilize the nanostructured precursors, inducing well-defined nanostructures, due to a hydrogen bonding-co-π−π stacking mechanism.45 The possible influence of [C8mim]Cl on the morphology of products may be illustrated as Scheme S1 (see the Supporting Information). The well-defined γ-AlOOH nanotubes showed better structural stability and enough robustness to withstand the thermal annealing process at 600 °C for 2 h in the air. The annealed nanotubes were obtained without obvious morphology changes, which can be evidenced by SEM and TEM images (Figure 3c−g). All the products preserved the nanotube morphology only with a slight shrinking in length caused by dehydration. The diameter of the nanotube is about 6 nm and the length is in the range of 90−190 nm. The XRD pattern of the calcined product was shown in Figure 3a. As expected, all the diffraction peaks can be indexed to the face-centered cubic γ-Al2O3 (JCPDS No. 50−0741), without any impurity peaks. Moreover, EDS spectrum (Figure 3b) and FTIR spectrum (Figure S1b) further confirmed that the calcined products were γ-Al2O3. The specific surface area and porous nature of the γ-Al2O3 nanotubes were investigated by nitrogen adsorption−desorption isotherm measurements (Figure S11). Obviously, the

Figure 3. Composition and structure characterization of γ-Al2O3 obtained by calcining S-3 at 600 °C for 2 h: (a) XRD pattern, (b) EDS spectrum, (c,d) TEM images, (e) SEM image, (f,g) magnified TEM images, demonstrating the smaller size in diameter and hollow nature.

circularly, and the formation reaction of γ-AlOOH can be achieved by the repeated circles. In order to further confirm the previous discussions, a contrast experiment was carried out in the same reaction system by using Mg(NO3)2 instead of Zn(NO3)2 (Figures S4 and S5). The results were precisely consistent with the ones by using Zn(NO3)2, which also proved our hypothesis, the formation of γ-AlOOH through the coupling reaction in terms of the formation of complex containing acetate ion and the decomposition of (CH3COO)2Al(OH) due to the adsorption of divalent metal ions on (CH3COO)2Al(OH) surface. Therefore, the coupling reaction can rationally explain the influence of Zn2+ ion on the one-step synthesis of γ-AlOOH in the present reaction system. In order to clarify the coupling reaction and the formation mechanism of γ-AlOOH, it may be helpful to compare the coordinate environment of aluminum at boehmite with aluminum acetate hydroxide. Boehmite can be depicted as a lamellar structure: the basic layer is composed of two sheets of edge sharing AlO6 octahedra, and the sheets are fused again via common edges of the octahedra (see Figure S6).38 Thus, the aluminum atom is coordinated by four O atoms, which are inside the layer and shared by adjacent AlO6 octahedra, and another two O atoms are connected to the hydrogen atoms and accommodated in between the layers (Scheme 1a).39 For aluminum acetate hydroxide, its structure may be considered to consist of a core of tricoordinate oxygen atom (OAl3), linked by carboxylate groups in bridging positions across two adjacent OAl3 cores, as shown in Scheme 1b.40 Although direct crystallographic evidence to support this structure model is unavailable, the structure model has been determined not only by 27Al MAS NMR technique,41 but also entirely feasible based on literature precedents for a wide range of main group and 6141

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nitrogen adsorption−desorption curve is assigned to type-IV, according to IUPAC classification,34 which shows a stepwise behavior and end at a significant hysteresis at relative pressures P/P0 above 0.55 (S-3), indicating an excellent pore structure.46 The BET specific surface area of the sample obtained by calcining S-3 at 600 °C for 2 h is as high as a 285.4 m2/g and the total pore volume is 0.46 cm3/g. Barrett−Joyner−Halenda (BJH) calculations for the pore-size distribution, derived from desorption data, reveal a narrow distribution with one apex centered at 4.78 nm. These mesopores undoubtedly arise from the spaces formed by the dehydration of γ-AlOOH crystals during the calcination process. The results display that the obtained γ-Al2O3 nanotubes own high surface area and mesoporous properties. All these endow the as-prepared γAl 2O 3 with particular features, combined with unique morphologies, high purity, making them a promising candidate for application in high-temperature environments, such as catalysis, gas sensors, adsorption, and photovoltaic industry as dielectric passivation material.47−49 In summary, the well-developed γ-AlOOH nanotubes have been successfully synthesized by using coupling reaction in terms of the formation of the complex [Zn(CH3COO)x(H2O)4‑x](2‑x)+ and the decomposition of the precursor (CH3COO)2Al(OH) via an ionic liquid-assisted hydrothermal method at relatively low temperature. In the present work, not only were γ-AlOOH nanotubes with high purity and uniform dimension obtained in a mild condition, but this also could be an efficient and environmentally friendly route for large-scale synthesis. We highly expect that this synthesis strategy could be developed into an efficient route for synthesis of other inorganic nanomaterials and nanostructures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00703. Experimental method; the structural view of boehmite; supplementary characterizations PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Programs of National Natural Science Foundation of China (21371101, 51672135 and 21421001) and MOE (B12015).

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DOI: 10.1021/acs.cgd.6b00703 Cryst. Growth Des. 2016, 16, 6139−6143