Fabrication of Cactus Rod-like Mesoporous Alumina with Ionic Liquid

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Fabrication of cactus rod-like mesoporous alumina with ionic liquid-supramolecular gelator as co-template Xuanxuan Lei, Tuanchun Liu, and Shaokun Tang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00293 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 6, 2018

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Crystal Growth & Design

Fabrication of cactus rod-like mesoporous alumina with ionic liquid-supramolecular gelator as co-template Xuanxuan Leia, Tuanchun Liua, Shaokun Tanga,b * a. Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, China b. Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China ABSTRACT In this article, an amino acid ionic liquid tetramethyl ammonium glycinate ([N1111][Gly]) and an organic supramolecular gelator N-lauroyl-L-glutamic acid di-n-butylamide (GP-1) were used as structure- directing agents for the fabrication of mesoporous alumina with diverse morphologies via solvothermal synthetic method. The irregular sheets and hierarchical clusters were respectively prepared with only [N1111][Gly] or GP-1/EW as the template, whereas distinct cactus rod-like morphology with interlaced lateral plates was constructed with [N1111][Gly] and GP-1 as the co-template. The co-template synergistic effects of [N1111][Gly] and GP-1 on the morphology tailoring of alumina have been investigated in depth. A self-assembly mechanism based on the bilayer arrangement model was proposed to be responsible for the formation of final cactus rods-like alumina. The intermolecular H-bonds between GP-1 molecule and [N1111][Gly]-AlOOH hybrids was found to be the main driving force for the axial growth of initial nanocrystals and the growth of the nanocrystals in lateral orientation was inhibited due to the stereo-hindrance effect of [N1111][Gly]. KEYWORDS:

alumina

ionic liquid

supramolecular gel

co-template bilayer model

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1. INTRODUCTION Recently, inorganic functional nanomaterials with different dimension hierarchies including one-dimensional nanowires, nanorods and nanotubes, two-dimensional nanoflakes, nano-films, and three-dimensional nanoclusters has been widely studied due to their interesting physical properties and potential applications.[1-4] In particular, alumina like γ-Al2O3 have received increasing attention because of its thermal and chemical stability, high specific surface area, surface acidity and enormous potential in many applications such as adsorbents, catalyst supports, advanced ceramics and abrasives.[5,6] Actually, alumina with diverse nanostructures have been fabricated via various methods. Wang et al[7] synthesized alumina from single nanosheet to nanosheets assembled flower-like nanospheres via a facile hydrothermal synthetic method. Zheng et al[8] successfully controlled the morphologies of γ-Al2O3 with nanowires, and cross-plate-like structures by ionic liquid-assisted hydrothermal route. Xu et al[9] developed an epoxide-driven sol-gel process for the fabrication of γ-Al2O3 hollow microspheres with urchin-like shell structures. In our previous work, mesoporous γ-Al2O3 with uniform rod-like shape has been synthesized via an ionic liquid-assisted sol-gel method,[10] and alumina with hierarchical meso/macroporous structure was obtained by a co-templating approach using nonionic block copolymer EO106PO70EO106(F127)/agarose hydrogel as the co-templates.[11] To the best of our knowledge, though some other functional materials such as silicas[12] and titaniums[13] have been obtained with supramolecular gelator as the soft template, few papers were focused on their applications in the architecture of mesoporous alumina due to the relatively weak interaction between alumina precursor and supramolecular gelator. Over the past two decades, room-temperature ionic liquids (RTILs) have received significant


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attention due to their distinct advantages such as negligible vapor pressure, good thermal stability, good dissolving ability, high ionic conductivity, low melting temperature and designable structures, and have widespread applications in organic catalysis, electrochemical as well as inorganic synthesis.[14] Notably, as a green substitute for conventional solvents，ILs can interact with reactants via hydrogen-bond and electrostatic force in liquid system. Therefore, ILs have been utilized as the structure-directing agent for the formation of well-defined and extended ordering nanostructures, especially in inorganic materials field including metal elements (silver nanowires[15]), non-metal elements (tellurium rods[16]), metal oxides (CuO nanoplates,[17] Fe2O3 spheres,[18] Al2O3 particles[19]) and zeolites.[20] In general, imidazole-based ILs are the most popular ones in controllable synthesis of nanostructures due to their multi-type and special imidazole structures, moreover, the imidazolium cations can interact with crystal surface through strong binding affinities to stabilize nanoparticles.[21,22] However, up to now, few researches have focused on the applications of other types of ILs in nanostructured architecture. Supramolecular gels can self-assemble in specific solvents through weak intermolecular interactions including hydrogen bonding, π-π stacking, and van der Waals forces, leading to the formation of three-dimensional fiber network micro/nanostructure. As so far, they have been extended to variable application fields such as drug delivery, scaffolds for tissue engineering as well as the design of highly self-assembly nanostructures.[23-25] Sanchez et al[26] have presented a general overview about the templated synthesis of one-dimensional inorganic and organic–inorganic hybrid fibrous nanomaterials by the aid of self-assembled organogelator-based supramolecule. Noticeably, most oxide materials transcribed through organogel via three template strategies, including in situ coassembly with organogel, in situ coassembly with precipitate formation, and organogel self-assembly followed by



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post-transcription. Nevertheless, all these synthetic routes typically suffer an aging process of about several days which is very time-consuming. Thus a creative method is in need for the utilization of supermolecular gels as template to direct the growth of nanomaterials. Herein, an innovative templated solvothermal method has been developed to synthesize mesoporous alumina with a distinct cactus rod-like shape. N-lauroyl-L-glutamic acid di-n-butyl-amide organogelator with ethanol-water system (GP-1/EW) is introduced into the solvothermal synthesis process for the supramolecular assembly. Besides, an environmentally friendly amino acids-based IL ([N1111][Gly]) is first employed as the co-template in the synthesis of alumina, which can induce the oriented arrangement and growth of nanocrystals and thus the final formation of cactus rod-like morphology. The synergy mechanism on the controllable synthesis of alumina with [N1111][Gly]/GP-1 co-template will be discussed in depth.

2. EXPERIMENTAL SECTION Materials Aluminium nitrate and urea were acquired from Guangfu Fine Chemical Industry Research Institute. N-lauroyl-L-glutamic acid di-n-butylamide (GP-1) was obtained from Kishi-moto Sangyo Asia. Tetramethyl ammonium glycinate ([N1111][Gly]) was acquired from Sigma-Aldrich. All chemicals were analytical grade and used without any pretreatment. The molecular structures of GP-1 and [N1111][Gly] are shown below in Scheme 1.


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Scheme 1. Molecular structures of (a) N-lauroyl-L-glutamic acid di-n-butylamide and (b) Tetramethyl ammonium glycinate

Synthesis of morphology-controlled mesoporous γ-Al2O3 In a typical synthesis, 0.01 mol aluminium nitrate, 0.0025 mol [N1111][Gly] and 0.1 mol urea were dissolved in 30 mL mixed solvent of ethanol-water (2:1 v/v) under magnetic stirring. 0.3 g GP-1 was then dissolved in the above solution under vigorous stirring at 70 °C. The obtained homogeneous solution was transferred into a 50 mL stainless-steel autoclave, sealed and heated at 120 °C for 12 h. After reaction, the autoclave was cooled to room temperature naturally. Washed with hot absolute ethanol several times, the white product was obtained and dried at 60 °C for 12 h, followed by calcination in air from room temperature to 500 ºC with a heating rate of 1 ºC/min and keeping at 500 ºC for 5 h to obtain the γ-Al2O3. For comparison, reference samples were prepared using the same method with single [N1111][Gly] or GP-1 as the template agent. Here the final samples are abbreviated as MA-x-y-T, where x and y represent the molar ratio of [N1111][Gly]/aluminium nitrate and the mass percent of GP-1 in ethanol-water respectivly, T represents the calcination temperature. For example, the typical product described above is labeled as MA-0.25-1.2-500. Characterizations



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The nitrogen adsorption/desorption isotherms were performed on an ASAP analyzer (Tristar3000, Micromeritics, USA). The morphologies were observed by a Nanosem 430 field emission scanning electronic microscopy. Transmission electronic microscopy was taken on a JEM-2100F transmission electron microscope under a working voltage of 200KV. TGA experiments (Du Pont Instruments 951 thermogravimetric analyzer) were conducted on a 10 mg sample from room temperature to 800℃ in flowing air at a heating rate of 10 °C/min. FTIR spectra (KBr pellets) were measured by a VECTOR-22 (Bruker, Germany) spectrometer within the range of 400–4000 cm-1.

3. RESULTS AND DISCUSSION TG analysis The TG-DTG measurement was performed to investigate the conversion process from the as-prepared precursor to Al2O3, and the result is shown in Figure 1. From the TG curve, the weight loss of 8 % at 100-200 °C should be attributed to the removal of structural water. And a significant weight loss of 39.8 % with a sharp endothermic peak at around 200-250 °C in DTG curve, should be owing to the decomposition of residual organic compounds including GP-1 and [N1111][Gly]. With the temperature increasing, a slight mass loss of about 8.2 % appears at 250-500 °C, which could be associated with the decomposition of precursor into Al2O3. And no distinct weight loss can be observed within the calcination temperature from 500 °C to 1000 °C, demonstrating the high thermal stability of final products.


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Figure 1. TGA (a) and DTG (b) curves of the precursor of MA-0.25-1.2-500 synthesized within [N1111][Gly]/GP-1 co-template

Morphology of alumina product Typical SEM and TEM images were recorded in Figure 2 to detect the morphology of the product (MA-0.25-1.2-500) synthesized within [N1111][Gly]/GP-1 co-template. Figure 2a shows the typical low magnification SEM image, which clearly illustrates that the sample is composed of uniform cactus rod-like particles with the average diameter of about 630 nm. The magnified SEM image in Figure 2b reveals the detailed morphology of cactus rod with several lateral plates and sharp head. Figure 2c, 2d and 2e display the TEM images with different magnifications, the lateral plates are relatively thin with the thickness of about 85 nm and have some cracks. Especially, the lateral plates branched from a truncated rod stem and the wormlike mesopores can be clearly observed in Figure 2e. From the corresponding HRTEM image, clear lattice f ringes are observed with the interplanar distance of 0.14 nm and 0.20 nm, which correspond to (440) and (400) planes for γ-Al2O3 respectively. It suggests the preferred growth direction of lateral plate along [100] and slow growth along [110].



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Figure 2. Typical morphologies and structures of as-prapared product (MA-0.25-1.2-500): (a,b) SEM images, (c,d,e) TEM images and (f) corresponding HRTEM image

Crystalline structures of various samples The crystalline phases of as-prepared aluminum oxide hydroxide precursors and aluminum oxide with [N1111][Gly]/GP-1 co-template were characterized by wide-angle XRD patterns. As shown in Figure 3a, all of the diffraction peaks of the product without any thermal treatment can be perfectly indexed to the orthorhombic structure γ- AlOOH (JCPDS Card no. 21-1307). And when the precursors was calcined in air at 500 °C for 5h (MA-0.25-1.2-500), even though the peak intensity is relatively weak, the XRD pattern presented with six diffraction peaks in Fig. 3b could be readily indexed to the reflections of cubic γ-Al2O3 with lattice constants a=b=c=7.924 Ǻ (JCPDS Card no. 29-0063). From Figure 3b to 3d, the diffraction peaks intensity intensified gradually with the increase of the calcination


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temperature from 500 to 900 °C and no peaks from other phase are observed, indicating the enhanced crystallizability and high purity of the products. Moreover, the γ-phase alumina that obtained under relatively low temperature can be maintained even under the calcination temperature of 900 °C, demonstrating the excellent thermal stability of the products which is in accordance with the result of TG-DTG measurement.

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Mesoporous properties of various samples The porous structures of Al2O3 with cactus rod-like structures were further investigated as shown in Figure 4. Figure 4a presents the corresponding N2 adsorption and desorption isotherms of the products (MA-0.25-1.2) that calcined under different temperatures. It can be observed that all the samples display the typical type-IV curves, suggesting the obtained cactus rod-like alumina possesses



mesoporous characteristics. The sample calcined under 500 ºC exhibited H2 hysteresis loops, indicating the presence of ink bottle-like mesoporous structure, while the samples calcined under 700 and 900 ºC presented the H3 hysteresis loops attribute to the slit pore channels. The pore-size distributions of all alumina products calculated by BIH method using desorption data are shown in Figure 4b. The sample calcined at 500 ºC displays a narrow pore distribution centering at 4 nm with the average pore size of 4.76 nm, whereas the samples obtained at 700 ºC and 900 ºC show broad pore distributions from 4-30 nm with the average pore size of 8.42 nm and 13.44 nm respectively. The result of the enlarged pore diameter and widen pore-size distribution should be assigned to the partial collapse of pore structure at higher calcination temperature. Furthermore, the specific surface area gradually decreases from 141 m2g-1 to 58 m2g-1 as the calcination temperature rising from 500 to 900 ºC due to the pore channels collapse at the higher temperature. 300

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Formation mechanism of Cactus rod-like alumina

Figure 5. SEM images of as-prepared products (MA-0.25-1.2-500) at different reaction time: (a) 6h, (b) 8h, (c) 10h, (d) 12h

A series of time-dependent experiments were carried out to demonstrate the crystal growth process of cactus rod-like alumina. Figure 5 shows the representative SEM images of the synthesized products. Noticeablely, a mass of irregular nanoparticles with slight cactus rods coexisted in the product after reaction time of 6 h. As time goes on, an increasing number of cactus rods were formed while the number of nanoparticles gradually droped off, and the nanoparticles almostly disappeared when the reaction time prolonged to 12 h. Generally, the process of nanoparticle formation should be divided into nucleation and growth, and the initial crystals typically suffer dissolving-recrystallization to grow up in high temperature. In this synthetic system, the crystals are in the nucleation stage during the initial reaction period, and small nanocrystals are obtained in this process. Then they can gradually dissolve, recrystallize and grow into cactus rod-like structure under the co-template synergistic effects.



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Figure 6. SEM and corresponding TEM images of products synthesized with only GP-1 (a,b) or [N1111][Gly] (c,d) as the template

Figure 6 illustrates the SEM and TEM images of products synthesized with single GP-1 supramolecular gelator or [N1111][Gly] as the template. There only irregular sheets were obtained in pure GP-1/EW system, indicating the weak interaction between the nanocrystals and GP-1 gelator. Whereas the hierarchical clusters were formed with ionic liquid [N1111][Gly] as the template as shown in Figure 6c. The corresponding TEM image displays the amplifying morphology of the assembled leaf-like sheets, which suggest that [N1111][Gly] can form strong interactions with the intial nanocrystals, and possesses a structure-directing effect in the growth of precursor. Besides, the comparative experiments were performed to exemplify the synergistic effects of the [N1111][Gly]/GP-1 co-template on the formation of cactus rod-like Al2O3. Figure S1 shows the microstructures of products synthesized at various concentrations of [N1111][Gly]. Interestingly, with the addition of [N1111][Gly] at low concentrations, most irregular sheets were modified into rod-like structures. And the rods became thicker and more uniform at a relatively higher [N1111][Gly] concentration. The average diameter of the


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cactus rods increased to 1.2 µm when the molar ratio of [N1111][Gly]/aluminium nitrate was 0.5 (MA-0.5-1.2-500). Similarly, the effect of GP-1 concentration on the morphology of alumina product

was also investigated as shown in Figure S2. When 0.6 wt% GP-1 was added in the system, the cactus rod-like shape could be observed, although there still remained some sheets. And these sheets completely disappeared with the GP-1 concentration increased to 1.2 wt%. However, when the GP-1 concentration was too high, some shorter rods without clear lateral plates emerged in the final product. As a result, well-defined cactus rod-like morphology can be constructed only by the [N1111][Gly]/GP-1 co-template with the appropriate concentrations. The gelling tests were conducted as shown in Figure S3 to study how IL ([N1111][Gly]) affects the gelation process of GP-1. It is obviously that [N1111][Gly] can significantly expedite the GP-1 gelling rate. The corresponding SEM and TEM images of GP-1 organogels without or with IL are shown in Figure S4. Compared with the fiber network formed in only GP-1/EW system, the interconnected fibers are more compact and the crosslinking degree of the fiber network becomes higher with the addition of [N1111][Gly]. It demonstrates the existence of [N1111][Gly] molecules display more than a kinetic effect on the gelation of GP-1. We believe that [N1111][Gly] can form definite interactions with GP-1 to promote the self-assembly of GP-1 and construct denser supramolecular network. Figure 7 shows the FTIR spectra of the pure [N1111][Gly], 1.2 wt% GP-1/EW xerogel formed with or without [N1111][Gly] and the as-prepared AlOOH precursorof MA-0.25-1.2-12. The absorption bands at aroud 1642cm-1, 1590 cm-1 in Fig 7a as well as 1639 cm-1, 1548 cm-1 in Figure 7b are assigned to the C=O stretching vibration and N-H bending vibration of the pure [N1111][Gly] and GP-1/EW xeroge without [N1111][Gly] additive, respectively. However, After the addition of [N1111][Gly] to GP-1/EW



organogel system, the peaks of C=O stretching vibration and N-H bending vibration in Fig 7c shift to lower wave numbers (σC=O:1632 cm-1, δN-H:1535 cm-1), which should be attributed to the formation of intermolecular hydrogen bonds between [N1111][Gly] and GP-1 molecules. Figure 7d illustrates the FTIR of AlOOH precursor of MA-0.25-1.2-12. It is clearly that the peaks ascribed to N-H bending vibration appeared at 1542cm-1, which is lower than the 1590 cm-1 in pure [N1111][Gly]. The result confirms the strong interactions between AlOOH and [N1111][Gly] through H-bonds. b Transmission(a.u.)

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Figure 7. FTIR spectra of (a) [N1111][Gly], (b) GP-1/EW xerogel without [N1111][Gly], (c) GP-1/EW xerogel with [N1111][Gly] and (d) MA-0.25-1.2-12 before calcination

From the above, it is reasonable to propose that the [N1111][Gly]/GP-1 co-templates are effective in the controllable synthesis of cactus rod-like alumina. The possible formation mechanism of such a distinct architecture is illustrated in Scheme 2. At the beginning, GP-1 molecules can self-assemble into a interdigitated lamellar structure through the hydrophobic interactions of GP-1 alkyl chains, and the


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intermolecular chiral H-bonds between –NH and –CO in acylamino groups.[27,28] Meanwhile, [N1111][Gly] can be embedded into the lamellar structure via intermolecular H-bonding interactions, inducing the formation of bilayer arrangement. When the initial nanocrystals are generated with the continuous hydrolysis of aluminium nitrate, [N1111][Gly]-AlOOH hybrids can be formed via strong H-bonds and electrostatic force between AlOOH and [N1111][Gly] molecules, and align along the bilayer model (Scheme 2a). The cross section of the self-assembled structure are shown in Scheme 2b. With the reaction proceeding, AlOOH nanoparticles present oriented growth along the lengthways direction. Furthermore, the growth of the nanocrystals in lateral orientation will be inhibited due to the stereo-hindrance effect of [N1111][Gly] and the axial growth of the nanocrystals is induced to achieve the final cactus rod-like morphology (Scheme 2c). When [N1111][Gly] is at a relatively low concentration, the aggregation of GP-1 molecules as well as the amount of the combined AlOOH nanocrystals tend to decrease and the steric-hindrance effect of IL will also be weakened due to the decrease of IL adsorbed on the interdugitated lamellar structure of GP-1, resulting in the formation of some short rods (Figure S1(a)). On the contrary, with the excessive [N1111][Gly], more AlOOH nanocrystals will be adsorbed on the bilayer. Its growth in lateral direction will be promoted, which leads to the increase of diameter (Figure S1(b)). As for the addition of GP-1, a part of IL is hardly able to adsorb on the lamellar structure formed by GP-1, but a single-template effect of [N1111][Gly] is employed to control the growth of AlOOH into sheets at a low content of GP-1 (Figure S2(a)). When the GP-1 concentration is too high, it means that the relative content of [N1111][Gly] is reduced, and some rods without lateral plates appear again (Figure S2(b)).



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Scheme 2. The schematic mechanism of the formation of cactus rod-like mesoporous alumina

4. CONCLUSIONS In this work, mesoporous alumina with novel cactus rod-like morphology has been first synthesized with [N1111][Gly] and GP-1 as the co-template via a one-step solvothermal method. The cactus rod-like alumina presents a uniform morphology with interlaced lateral plates and sharp head and its average diameter is around 630 nm. The formation mechanism has also been investigated in depth. The result demonstrates that only irregular sheets or alumina clusters assembled by leaf-like sheets can be obtained with only GP-1 organogelator or [N1111][Gly] as the structure-directing agent. And the synergistic effects of [N1111][Gly] and GP-1/EW plays a crucial role in the morphology control of γ-Al2O3. The possible synergistic mechanism has been proposed that the addition of [N1111][Gly] tends


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to promote the self-assembly of GP-1 and interact with GP-1 linear structure via strong intermolecular hydrogen bonds, which result in the formation of a bilayer arrangement model. Then the AlOOH nanocrystals can be adsorbed to form [N1111][Gly]-AlOOH hybrids and be induced axial growth along the bilayer. Meanwhile their lateral growth is inhibited because of the stereo-hindrance effect of [N1111][Gly]. The strategy presented in our study may provide a new insight into the structure design of various inorganic functional materials. ASSOCIATED CONTENT Supporting Information The pictures of gelling process, SEM images, TEM images of GP-1 organogels and alumina products prepared under different conditions are listed in Supporting Information. AUTHOR INFORMATION Corresponding Author *Email: [email protected] ORCID Shaokun Tang: 000-0002-5481-1680 ACKNOWLEDGMENT This project was financially supported by the National Natural Science Foundation of China (21206118) and PetroChina Innovation Foundation (2013D-5006-0402). REFERENCES [1] Lou, Z.; Shen, G. Flexible Photodetectors Based on 1D Inorganic Nanostructures. Adv. Sci. 2016, 3, 1500287. [2] Luo, B.; Smith, J. W.; Wu, Z.; Kim, J.; Ou, Z.; Chen, Q. Polymerization-like co-assembly of silver nanoplates and patchy spheres. ACS Nano 2017, 11, 7626−7633.



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Fabrication of cactus rod-like mesoporous alumina with ionic liquid-supramolecular gelator as co-template Xuanxuan Leia, Tuanchun Liua, Shaokun Tanga,b *

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Synopsis • Mesoporous alumina with novel cactus rod-like morphology has been synthesized via solvothermal route. • IL [N1111][Gly] and GP-1 organogel have been firstly employed as the co-template for the fabrication of alumina. • The synergistic effects of the [N1111][Gly]/GP-1 co-template in inducing the growth of nanocrystals has been studied in deepth.


Fabrication of Cactus Rod-like Mesoporous Alumina with Ionic Liquid

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