Nanosegregated composites of an imidazolium salt and a layered inorganic compound: Organization of both anions and cations in interlayer space

Abstract

The organization of an imidazolium salt in the interlayer space of a layered inorganic compound leads to the formation of nanosegregated composites.

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Development of organic–inorganic nanocomposites has attracted much attention because of their great potential as innovative materials exhibiting versatile functions.^1,2 It is important to achieve controlled complexation of both organic and inorganic components on the nanoscale although it is not easy. If functional organic molecules and inorganic compounds form ordered structures at nanometre length scales in the composites, a new type of material can be obtained. Herein, we report on nanosegregated composites consisting of an imidazolium salt and a layered inorganic compound.

A wide variety of composite materials has been prepared using layered inorganic compounds such as clay minerals.² The layered structures are suitable for the formation of composites on the nanoscale. Generally, the guest functional organic molecules can be intercalated in the interlayer space of the host inorganic compounds through ion-exchange reactions. The intercalation of functional organic molecules leads to photochemical,³ electronic,⁴ ionic,⁵ and magnetic⁶ properties. These properties are different from those of their bulk or solution states. Chemical reactions of the intercalated molecules also proceed in the interlayer spaces.⁷ The intercalation and exfoliation of layered compounds based on transition metal oxides have been studied for the application of their electrochemical and magnetic properties.⁸

Intensive studies have focused on imidazolium and ammonium salts as ionic liquids, which are room-temperature molten salts.⁹ They can be used as materials in a variety of fields such as electrochemistry and organic syntheses. Self-organization of the derivatives of these ionic molecules has been achieved using nanosegregated liquid crystalline (LC) structures.^10–12 As these low-dimensional organic nanomaterials are useful, our intention here is to prepare nanosegregated organic–inorganic composites of imidazolium salts and layered inorganic compounds. Recently, ionic liquids were introduced into sol–gel derived inorganic materials.¹³ Nanomaterials of inorganic compounds have been synthesized using ionic liquids as solvents and templates.¹⁴ However, the organization of imidazolium salts in the inorganic materials is not easy. If imidazolium or ammonium salts can be organized in the interlayer spaces of transition metal oxides, a novel type of functional composites can be obtained. Since transition metal related compounds and organic salts are used as materials for electrochemical devices,^9e,f,15 their combinations may lead to unprecedented properties. However, a key problem is the charge balance between the organic salts and layered inorganic compounds. The intercalation of both anions and cations in the charged interlayer spaces cannot proceed through typical ion-exchange reactions under mild conditions.¹⁶ A new rational approach is required for the organization of organic salts in layered compounds. In the present work, an imidazolium salt is organized in the interlayer spaces of layered α-cobalt hydroxide [α-Co(OH)₂] through a two-step approach (Fig. 1).

Fig. 1 Schematic illustrations of the preparation of nanosegregated composites consisting of the imidazolium salt and a layered compound.

In the first step, the self-organized composites of imidazoleacetic acid and α-Co(OH)₂ are directly prepared through a one-pot synthesis. Recently, we have reported the selective synthesis of α-Co(OH)₂ under mild conditions.¹⁷ Since α-Co(OH)₂ forms a layered structure of positively charged two-dimensional monolayers consisting of edge-sharing {CoO₆} octahedra, anionic molecules can be spontaneously introduced in the interlayer spaces.^6a,d,18 In the second step, the quaternization of the imidazole rings result in the formation of imidazolium salts in the interlayer spaces of the layered Co(OH)₂. This two-step approach is key to preparing nanosegregated composites of the imidazolium salts and the layered compounds (Fig. 1). It was difficult to intercalate the imidazolium salts into the interlayer spaces of Co(OH)₂ through ion-exchange reactions under mild conditions.

Imidazoleacetic acid-intercalated Co(OH)₂ was directly obtained by precipitation from an aqueous solution containing imidazoleacetic acid (20 mM) and cobalt chloride hexahydrate (10 mM). The precursor solutions were maintained in a closed vessel with ammonia solution (5 wt%) for 12 h at 25 °C. Ammonia vapor was diffused in the precursor solutions and then precipitates with blue-green color were formed by increasing the pH. The precipitates were washed and collected by centrifugation with purified water. Before drying, the precipitates were heated with an excess amount of bromoethane for 12 h at 80 °C. After evaporation of the remaining bromoethane, the precipitates were dried under vacuum conditions at room temperature.

The intercalated structures were studied by the powder X-ray diffraction (XRD) analysis (Fig. 2). The basal spacing of the pure layered α-Co(OH)₂ is about 0.80 nm. In contrast, the spacing of the each layer increases up to 1.2 nm with intercalation of imidazoleacetic acid in the interlayer spaces. The formation of the intercalated structures is facilitated by the electrostatic interactions between the positively charged monolayers of Co(OH)₂ and the carboxylate groups of imidazoleacetic acid. Although the detailed structures of α-Co(OH)₂ have not been clarified, anionic organic acids were intercalated with tilted bilayer structures in previous reports.^6d,e The size and the optimized conformation of imidazoleacetic acid in the vacuum state were calculated by density functional theory (See Fig. S1 of the ESI†). Based on these results, the intercalated molecules formed tilted bilayer structures in the interlayer spaces.

Fig. 2 XRD patterns of the resultant layered compounds. (a) α-Co(OH)₂ without intercalated compounds, (b) the imidazoleacetic acid-intercalated Co(OH)₂ synthesized from the precursor solution, (c) the imidazolium salt intercalated phase after quaternization in the interlayer spaces.

After the quaternization, 1-carboxymethyl-3-ethyl-imidazolium bromide was formed in the interlayer space as expected (Fig. 1). The ethylene groups and bromine anions are introduced into the imidazoleacetic acid organized in the interlayer space. The peak positions are not shifted in the XRD patterns even after the quaternization (Fig. 2). But, the peak broadening after the quaternization implies low crytallinity with structural modifications including the intercalation of the anions. Five characteristic changes are observed in the Fourier-transform infrared (FT-IR) spectra after the quaternization (Fig. 3a). C–H stretching and bending vibrations appear around 2800 cm⁻¹ and 1450 cm⁻¹, respectively. The disappearance of C=C and C=N stretching vibrations resulting from heteroaromatic amines around 1500 cm⁻¹ indicates the quaternization of the imidazole rings. The C–N stretching vibration of the tertiary amine around 1300 cm⁻¹ also disappears. Vibration of the carboxylic groups is seen around 1720 cm⁻¹. These characteristic changes in the FT-IR spectra are the same as those of the model compounds not intercalated in the layered compound (Fig. S2 of the ESI†).

Fig. 3 FT-IR spectra (a) and TG curves (b) of the resultant layered compounds. (i) α-Co(OH)₂ without intercalated compounds, (ii) the imidazoleacetic acid-intercalated Co(OH)₂ synthesized from the precursor solution, (iii) the imidazolium salt intercalated phase after quaternization in the interlayer space. The arrows in the panel (a) indicate the characteristic spectral changes with the quaternization.

In the thermogravimetry (TG) analysis under nitrogen flow conditions, the weight-loss curves are characteristic of each intercalated compound (Fig. 3b). The weight losses around 150 °C can be attributed to the dehydration of adsorbed and intercalated water molecules. The dehydroxylation of –OH species and the decomposition of the intercalated compounds cause the weight losses from 150 °C to 400 °C. Cobalt oxide (Co₃O₄) is eventually transformed to CoO with release of oxygen around 800 °C.¹⁹ The amount of the cobalt species and the intercalated imidazole can be estimated from the differences in the TG curves. The molar ratio of the cobalt to the imidazolium salt can be calculated to be approximately 80 : 20. Energy-dispersive X-ray (EDX) analysis suggests that the atomic ratio of cobalt to bromine is 76 : 24 in the composite (Fig. S3 of the ESI†), which is consistent with TG analysis of the molar ratio of the cobalt and imidazolim ions. In contrast, the presence of bromine anions is not detected in the EDX spectrum of the imidazoleacetic acid-intercalated Co(OH)₂. Therefore, the imidazolium salts are organized in the interlayer space with the bilayer structures through a two-step process including the intercalation and subsequent quaternization (Fig. 1).

FESEM images show that the composites of the imidazolium salt and the layered compound form flower-like aggregates of nanosheets ca. 20 nm in thickness (Fig. 4a and b). The morphologies are not changed after the conversion to the imidaozlium salt. In contrast, the hexagonal plates of α-Co(OH)₂ that are ca. 50 nm in thickness were formed in the absence of imidazoleacetic acid.¹⁷ The selected-area electron diffraction (SAED) pattern with TEM imaging shows nearly hexagonal spots corresponding to the (100) and (110) planes of the layered Co(OH)₂ (Fig. 4c).‡ The results suggest that the nanosheets exhibit the (001) faces of layered Co(OH)₂. The interaction of imidazoleacetic acid with the (001) faces of the layered Co(OH)₂ results in the formation of the nanosheets. It is estimated that each nanosheet contains about 15 monolayers separated by the intercalated imidazolium salts (Fig. 4d).

Fig. 4 Electron microscope images (a–c) and schematic illustration (d) of the layered Co(OH)₂ with intercalation of the imidazolium salt. (a and b) FESEM images of the flower-like aggregates consisting of the nanosheets. (c) TEM image and corresponding SAED pattern of the nanosheet. (d) the nanosegregated composite structures of the layered Co(OH)₂ and the imidazolium salt in the nanosheet.

In summary, we have synthesized nanosegregated composites of an imidazolium salt organized in the interlayer space of a transition metal based layered compound. In the first step, the layered compound with intercalation of the precursor for an imidazolium salt is spontaneously formed under mild conditions. In the second step, the intercalated precursors are converted to the organic salt through reaction in the interlayer space. These two-step processes are key routes to obtaining composites of organic salts and layered compounds. Since we can prepare thin films of transition metal oxides on substrates,¹⁷ these nanosegregated composites may be formed in these films using the same method. Functional ionic liquids have been developed in recent years.^9a–c The approach in the present report could be widely applied to other combinations. The nanosegregated composites of functional ionic liquids and layered compounds in the nanosheets have potential for ion conduction with anisotropy, as studied in nanosegregated LC materials.^10–12 If we can fabricate nanosegregated structures in the electrodes of batteries, the ions related to charge–discharge reactions could be efficiently transported. The synthetic processes and the resultant structures of these nanosegregate composites may be useful for the development of electrochemical devices and ion conductors.

Acknowledgements

This study was partially supported by the Grant-in-Aid for the Global COE Program for Chemistry Innovation (T. K.), Exploratory Research (20655022) (T. K.), and Grant-in-Aid for Scientific Research (A) (19205017) (T. K.) from the Japan Society for the Promotion of Science (JSPS). The authors are grateful to Ms. Junko Kagiomoto at Tokyo University of Agriculture and Technology for discussion about the properties of the imidazolium salt. We also thank the Center for Nano Lithography & Analysis of the University of Tokyo for the TEM measurements. One of the authors (Y. O.) is grateful for a JSPS research fellowship (19-8820) for young scientists.

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Footnotes

† Electronic supplementary information (ESI) available: Characterization of the resultant materials, the size and optimized conformation of imidazoleacetic acid, the FT-IR spectra of the reference compounds and the EDX spectra of the intercalated compounds. See DOI: 10.1039/c0nr00393j

‡ The stacking of the each monolayer in the c-axis direction is slightly disordered in a nanosheet containing imidazoleacetic acid or its quaternized compound. In the XRD pattern (Fig. 2), the two saw-toothed peaks of the (100) and (110) planes result from the disordered stacking of the monolayers in each nanosheet or the thin nature of the each nanosheet (ca. 20 nm in thickness).

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