Materials Science and Engineering C 51 (2015) 274–278

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Nacre-like calcium carbonate controlled by ionic liquid/graphene oxide composite template Chengli Yao a,b, Anjian Xie a,⁎, Yuhua Shen a, Jinmiao Zhu b, Hongying Li b a b

School of Chemistry and Chemical Engineering, Anhui University, Hefei, Anhui 230039, PR China School of Chemistry and Chemical Engineering, Hefei Normal University, Hefei, Anhui 230601, PR China

a r t i c l e

i n f o

Article history: Received 5 September 2014 Received in revised form 1 February 2015 Accepted 9 February 2015 Available online 11 February 2015 Keywords: Calcium carbonate Ionic liquid Graphene oxide Polymorphism Morphology

a b s t r a c t Nacre-like calcium carbonate nanostructures have been mediated by an ionic liquid (IL)-graphene oxide (GO) composite template. The resultant crystals were characterized by scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, and X-ray powder diffractometry (XRD). The results showed that either 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) or graphene oxide can act as a soft template for calcium carbonate formation with unusual morphologies. Based on the time-dependent morphology changes of calcium carbonate particles, it is concluded that nacre-like calcium carbonate nanostructures can be formed gradually utilizing [BMIM]BF4/GO composite template. During the process of calcium carbonate formation, [BMIM]BF4 acted not only as solvents but also as morphology templates for the fabrication of calcium carbonate materials with nacre-like morphology. Based on the observations, the possible mechanisms were also discussed. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Ionic liquids (ILs) are organic salts with polar character and low melting point (b100 °C). They are thermally and chemically stable and have a practically non-detectable vapor pressure. Their non-volatile nature allows for easy recycling after usage and, thus, endows them with high ranking in the realm of green chemistry [1,2]. For this reason, they are considered as green solvents which can meet the demands of environmental protection [3] as opposed to traditional volatile organic compounds. ILs have been explored as environmentally benign reaction media for the production of inorganic materials. Interestingly, ILs sometimes can act not only as solvents but also as reactants and morphology templates for the fabrication of inorganic materials with new or improved properties [4–9]. At the same time, graphene oxide (GO) has attracted wide interests of researchers because of its unique physicochemical and structural properties. GO sheets have one atom thickness and twodimensional structure with negatively charged edges, high specific surface area and mechanical strength, which endow them excellent performances in mediating the mineralization of CaCO 3. Thus, GO sheets as additives can greatly reduce the interfacial Gibbs energy of CaCO3/GO composite crystal formation as compared to the application of other molecular templates according to an empirical equation proposed by Nielsen [10,11]. The functional groups of GO can also act as interfacial linkers and facilitate the stress transfer from ⁎ Corresponding author. E-mail address: [email protected] (A. Xie).

http://dx.doi.org/10.1016/j.msec.2015.02.009 0928-4931/© 2015 Elsevier B.V. All rights reserved.

the polymer matrix to GO. Some materials have been induced by GO, such as metals [12], semi-conductors [13], CaCO 3 [10,14] and hydroxyapatite (HAP) [15–17]. In particular, these GO/CaCO3 (HAP) inorganic hybrid materials have excellent biocompatibility and mechanical properties. In the meantime, six widely used methods for graphene oxide preparation have been reported, such as modified Hummer method, exfoliating carbon nanotube method, chemical vapor deposition (CVD) method [18], et al. Cytotoxicity and biocompatibility of graphene oxide have also been evaluated. Y Chang et al. found GO with no obvious cytotoxicity on A549, MCF7 or SKBR3 cells when GO was prepared with the modified Hummer's method [19–21]. In addition, GO paper [22], GO-PVA, and PMMA polymer nanocomposites [23] can been formed with nacre-like structure through hydrogen bonding. Synthesis of nanomaterials with ultra-strong mechanical property greatly broadens the space for the preparation of special materials with graphene oxide. Strong electrostatic/chemical interaction between ILs and GO improves the dispersion of GO, as well as enhanced performance of matrixes induced by the introductions of ILs into functional composites and give rise to a wider range of GO applications [24–26]. Also, the combination of IL and GO provides an effective platform for the nucleation and growth of CaCO3 micro/nanoparticles. It is well known to all that natural nacre has a unique combination of remarkable strength and toughness and is often described as “brickand-mortar” structure. The unique structure is attributed to its hierarchical nano/microscale structure and precise inorganic–organic interface [27,28]. In this article, we put forward the synthesis of the nacre-like CaCO3 nanocomposites by using gas diffusion method-

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2. Experiment 2.1. Preparation of graphene oxide

Scheme 1. Chemical structure of the IL employed.

assisted [BMIM]BF4/GO composite template under room temperature. To precipitate CaCO3, CaCl2 and (NH4)2CO3 are selected as the sources of calcium and CO2, respectively. The use of (NH4)2CO3 instead of CO2 allows the reaction medium to maintain at a neutral or weakly basic pH caused by the dissolution of ammonia. In comparison to previous studies, this article develops a facile strategy to fabricate the nacre-like calcium carbonate materials in green solvent. Furthermore, it develops a new opportunity for the formation of calcium carbonate with superior function and morphology.

Graphene oxide was synthesized from graphite by a modified Hummers method [29,30]. Briefly, natural graphite (1.0 g), NaNO3 (0.75 g) and concentrated H2SO4 (75.0 mL) were added into KMnO4 (3.0 g) gradually with stirring and cooled in order to keep the temperature below 5 °C for 1 h. Then, the ice bath was removed and the solution was kept stirring at room temperature. Five days later, distilled water (140.0 mL) was slowly added. After 1 h, this reaction was transferred to a 98 °C water bath and stirred for 2 h. Then, 30% H2O2 solution (10.0 mL) was added and kept stirring for 2 h. The mixture was centrifuged and washed successively with 1.0 mol/L HCl aqueous solution and Milli-Q water several times until the pH of the supernatant was ~7. The bright yellow graphite oxide powder was dried in a vacuum oven less than 40 °C for 24 h.

2.2. Synthesis of CaCO3 in ionic liquid/graphene oxide solutions Anhydrous calcium chloride (CaCl2) and ammonium carbonate ((NH4)2CO3) were obtained commercially and were of pure analytical grade. All reagents above were used without further purification. Double-distilled water was employed in all experiments. The ionic

Scheme 2. Illustration of the formation of the nacre-like CaCO3. The steps describe (a) mineralization of CaCO3 in the presence of [BMIM]BF4/GO composite template and CaCl2 by CO2 gas diffusion method, (b) calcium carbonate particles grown on graphene oxide template, and (c) their conversion to nacre-like CaCO3 materials with time prolonged. Images in the right column show (d) TEM of GO sheets used for this study (inset: XRD patterns analysis) and AFM image of GO sheets (inset: height line),(e) SEM images of GO-wrapped CaCO3 microspheres, and (f) SEM images of CaCO3 microspheres induced by [BMIM]BF4/GO composite template.

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Fig. 1. SEM images of calcium carbonate induced by: (a) GO; (b) [BMIM]BF4.

2.3. Characterization The morphologies of particles were characterized by scanning electron microscopy (SEM, S-4800) after being sputter-coated with a thin layer of gold nanoparticles. X-ray powder diffractions (XRD) were obtained at a MAP18XAHF X-ray diffractometer with CuKα radiation. Infrared spectra were recorded with a Nicolet 6700 Fourier transform infrared (FT-IR) spectrometer. The atom force microscopy (AFM) images were obtained using a Veeco Multimode 8 scanning probe microscope in the tapping mode. Transmission electron microscopy measurements were examined using a JEM model 100SX electron microscopes (Japan Electron Co.) operated at an accelerating voltage at 80 kV. 3. Results and discussion GO sheets were prepared from pristine graphite with the modified Hummers method. The characteristics of GO sheets were further confirmed by X-ray diffraction (XRD) measurements. The strong and sharp peak at 2θ = 11.7° corresponds to an interlayer distance of 7.6 Å (d002). The thickness of GO nanosheets was also characterized by both AFM and TEM. As was shown in Scheme 1d, the GO sheets were multilayers with sizes of a few micrometers and heights of ~ 1.0 nm, which are characteristic parameters of the single layer GO nanosheets. After decomposition of (NH4)2CO3, CO2 and NH3 gas were

introduced into a solution containing exfoliated GO, [BMIM]BF4 and CaCl2 (Scheme 2a). With the equilibrium of crystallization and dissolution, a GO–CaCO3 hybrid film that consisted of numerous CaCO3 microspheres interconnected by a GO network was synthesized, followed by filtering and drying of the resultant suspension (Scheme 2b). The diameters of the CaCO3 microspheres were approximately 150 nm (Scheme 2d). Each microsphere apparently “grows in” well-stretched

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liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4), used in the present work was purchased from Aladdin Reagent (Shanghai) Co. Ltd., whose chemical structure was schematically shown in Scheme 1. In a typical synthesis procedure, 20.0 mL of [BMIM]BF4 was dropped into the solution of GO (8.0 mg/mL, 40.0 mL) with ultrasonic vibration. Then, a solution of CaCl2 (0.2 mol/L, 60.0 mL) was injected into the mixed solution under continued ultrasonic oscillation. The mixture was stirred for 30 min at room temperature to form the reaction mother liquor and was later divided into two parts with the same volume (named A and B, respectively). The two parts were then transferred to two 100 mL beakers, respectively. Next, a small (50 mL) beaker containing 10.0 g of crushed ammonium carbonate solid was also covered with PVC film, punched with some holes, and placed at the bottom of a desiccator. This desiccator was placed at room temperature (20–23 °C) for 4 days. In addition, when the reaction time reaches 24, 36 and 48 h, precipitates of beaker B were taken out for kinetics measurement (the precipitates were characterized beforehand). The precipitates of beaker A were continued to the end of the reaction without any disturbance. The precipitates were separated by centrifugation (4000 r/min), washed three times with double distilled water and ethanol, respectively, and then vacuum dried for further determination. For comparison, the above [BMIM]BF4 and GO solution were used as the template to induce calcium carbonate under the same condition, respectively.

873 746 713

e 713 873

f 873 2500

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1500 1000 Wavenumbers(cm-1)

713 500

Fig. 2. XRD patterns (A) and FTIR spectra (B) of calcium carbonate mediated by: (a,d) IL; (b,e) GO; (c,f) GO/IL.

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GO sheets, as shown in the scanning electron microscopy (SEM) image (Scheme 2e). With the extension of time, GO–CaCO3 hybrid films became nacre-like structure which was constructed by the layer stacking under the synergistic effect of ionic liquid after 72 h (Scheme 2c,f). As we know, ILs exhibit long-range order and liquid crystallinity which endow them not only solvent characters but molecular precursors with a well-defined composition, structure, and reactivity. In these cases, ILs can be viewed as true solvent-template-reactant systems [31,32]. Second, ILs are good solvents which will help graphene oxide to dissolve. The curling and folding of graphene oxide could be avoided. Under the combined action of GO and [BMIM]BF4, nacre-like CaCO3 can be easily induced. In order to evaluate the effect of graphene oxide and ionic liquids as templates on the crystallization of calcium carbonate, GO and [BMIM] BF4 were selected to induce the nucleation of calcium carbonate, respectively. Fig. 1 represented the SEM images of calcium carbonate after induction. Graphene oxide tended to induce rhombic shaped calcium carbonate (Fig. 1a). However, the [BMIM]BF4 as a template induced calcium carbonate with a fractal-like structure with spindle and long range order (Fig. 1b). There were significant differences between their morphologies. From Scheme 2e and Fig. 1, it can be seen that GO and [BMIM]BF4 composite templates play an important role in the formation of nacrelike calcium carbonate. According to literature, the orientation of GO nanosheets was the main reason for nacre-like structure formation [33]. It is assumed that the functional groups (carboxyl groups, et al.) on GO can coordinate with Ca2 + ion to provide calcium carbonate nucleation sites. Then there is a lateral growth rate supported by GO nanosheets. Under the assistance of [BMIM]BF4, the calcium carbonate/GO hybrid particles carry out vertical growth method. Accompanied by precipitation and dissolution equilibrium for a certain time, nacrelike calcium carbonate structures are formed finally. Calcium carbonate samples formed in the GO and [BMIM]BF4 medium, were used to investigate crystal polymorph by means of powder XRD patterns analysis and FTIR spectral analysis. Fig. 2A depicted typical XRD patterns of calcium carbonate crystals and it can be observed that obtained CaCO3 crystals have a similar polymorph. The diffraction peaks of 2θ at 29.41°, 35.91° and 39.51° correspond to (104), (110)

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and (113) crystallographic planes of calcite, respectively, while the peak at 27.18° corresponds to (112) crystallographic plane of vaterite [34]. As indicated by Fig. 2, nacre-like CaCO3 which mediated by GO/ [BMIM]BF4 composite template was made of calcite (Fig. 2A-a) just like CaCO3 partials induced by only GO template (Fig. 2A-b). However, there was a certain amount of vaterite in CaCO3 partials which formed in [BMIM]BF4 medium (Fig. 2A-c). The corresponding FT-IR spectra of precipitates were plotted in Fig. 2B. The spectra showed that particles were mainly composed of calcite (the characteristic vibrational bands: 713 cm− 1 and 876 cm− 1) [35]. The characteristic bands (713 cm− 1 and 876 cm−1) become weaker as can be referred to Fig. 2B-d. At the same time, a new characteristic band (746 cm−1) indicated a certain amount of vaterite. Both XRD and FTIR results indicated that the synthesized CaCO3 crystals were mainly calcite, while a small amount of vaterite crystals were mediated by [BMIM]BF4. In order to prove our speculation and gain a deeper understanding of the crystallization processes of calcium carbonate induced by [BMIM] BF4/GO composite template, the samples were collected at different stages and observed by SEM. Representative SEM snapshots of the products obtained at different time intervals were presented in Fig. 3. After 12 h, calcium carbonate particles can be described as “brick-and-mortar” structure. Calcium carbonate layers of ~0.5 μm in thickness were orderly stacked together and formed a three-dimensional structure (Fig. 3a). After 48 h of reaction, the above 3D structure turned into the vertical direction as indicated by SEM (Fig. 3b). A perfectly nacre-like calcium carbonate structure can be observed with much more thinner layers and more orderly packing in the following 24 h (Fig. 3c), which show perfect match with natural nacre (Fig. 3d). According to previous studies, artificial nacre-like composites offer many advantages: they are stiffer, harder, can be used at higher temperatures and display superior wear properties [37]. Artificial magnetic graphene based nacre-like films can act as shielding to mitigate electromagnetic pollution [38]. Nacre-like heparin/layered double hydroxide films have potential applications in the areas of optical applications, transportation and construction [39]. Here, the formation of nacre-like CaCO3/GO composites is reported. Nacre-like calcium carbonate as coating is strong and light. Nacre-like CaCO3/GO composites can be used as hard templates to induce carbonate hydroxyapatite/GO composites

Fig. 3. Scanning electron microscope (SEM) images of: (a–c) fabricated composite and (d) natural nacre from Haliotis laevigata. Reproduced with permission [36].

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which will be acted as bone substitute or drug carrier materials in the future. 4. Conclusion A simple and convenient gas diffusion method utilizing the interaction of graphene oxide and ionic liquid to mediate calcium carbonate mineral growth is developed. From the results, it is concluded that graphene oxide is helpful for the lateral growth of calcium carbonate while ionic liquid contributes to the vertical growth. Although the mechanism of the nacre-like calcium carbonate formation is discussed, elastic modulus, hardness values and biocompatibility have not been sufficiently investigated. We will focus them in our future work. None the less, biomineralization of nature continues and we are inspired to design and duplicate a detailed copy from nature intrigued by its simple processing routes and engineering. Acknowledgment This work is supported by the National Nature Science Foundation of China (91022032, 21171001, 21173001, 51372004, 21371003 and 21101054), the Lab for Clean Energy & Green Catalysis of Anhui University, the Important Project of Anhui Provincial Education Department (2012SQRL167ZD, 2013SQRL067ZD), the Project of Hefei Normal University (2014cxy25). References [1] Przemysław Kubisa, Ionic liquids in the synthesis and modification of polymers, J. Polym. Sci. A Polym. Chem. 43 (2005) 4675–4683. [2] Shengdong Zhu, Yuanxin Wu, Qiming Chen, Ziniu Yu, Cunwen Wang, Shiwei Jin, Yigang Ding, Gang Wu, Dissolution of cellulose with ionic liquids and its application: a mini review, Green Chem. 8 (2006) 325–327. [3] Anna Chen, Zhiping Luo, Mustafa Akbulut, Ionic liquid mediated auto-templating assembly of CaCO3–chitosan hybrid nanoboxes and nanoframes, Chem. Commun. 47 (2011) 2312–2314. [4] Mariana Fernandes, Filipe A. Almeida Paz, Verónica de Zea Bermudez, Ionic-liquidassisted morphology tuning of calcium carbonate in ethanolic solution, Eur. J. Inorg. Chem. 13 (2012) 2183–2192. [5] Sasan Rabieh, Mozhgan Bagheri, Mojgan Heydari, Elahe Badiei, Microwave assisted synthesis of ZnO nanoparticles in ionic liquid [Bmim]cl and their photocatalytic investigation, Mat. Sci. Semicon. Proc. 26 (2014) 244–250. [6] Pedro Migowski, Jaïrton Dupont, Catalytic applications of metal nanoparticles in imidazolium ionic liquids, Chem. Eur. J. 13 (2007) 32–39. [7] Markus Antonietti, Daibin Kuang, Bernd Smarsly, Yong Zhou, Ionic liquids for the convenient synthesis of functional nanoparticles and other inorganic nanostructures, Angew. Chem. Int. Ed. 43 (2004) 4988–4992. [8] Zhonghao Li, Zhimin Liu, Jianling Zhang, Buxing Han, Jimin Du, Yanan Gao, Tao Jiang, J. Phys. Chem. B109 (2005) 14445–14448. [9] Dolly Batra, Sönke Seifert, Legna M. Varela, Amelia C.Y. Liu, Millicent A. Firestone, Solvent-mediated plasmon tuning in a gold-nanoparticle–poly(ionic liquid) composite, Adv. Funct. Mater. 17 (2007) 1279–1287. [10] Xiluan Wang, Hua Bai, Yuying Jia, Linjie Zhi, Liangti Qu, Yuxi Xu, Chun Li, Gaoquan Shi, Synthesis of CaCO3/graphene composite crystals for ultra-strong structural materials, RSC Adv. 2 (2012) 2154–2160. [11] A.E. Nielsen, Theory of electrolyte crystal growth: the parabolic rate law, Pure Appl. Chem. 53 (1981) 2025–2039. [12] Xiao Huang, Xiaozhu Zhou, Wu. Shixin, Yanyan Wei, Xiaoying Qi, Juan Zhang, Freddy Boey, Hua Zhang, Reduced graphene oxide-templated photochemical synthesis and in situ assembly of Au nanodots to orderly patterned au nanodot chains, Small 6 (2010) 513–516. [13] Yong-Tae Kim, Jung Hee Han, Byung Hee Hong, Young-Uk Kwon, Electrochemical synthesis of CdSe quantum-dot arrays on a graphene basal plane using mesoporous silica thin-film templates, Adv. Mater. 22 (2010) 515–518.

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