Coassembly of Inorganic Macromolecule of Exfoliated LDH

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J. Phys. Chem. C 2009, 113, 9157–9163

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Coassembly of Inorganic Macromolecule of Exfoliated LDH Nanosheets with Cellulose Hongliang Kang,† Gailing Huang,† Shulan Ma,† Yongxiang Bai,‡ Hui Ma,§ Yongliang Li,§ and Xiaojing Yang*,† College of Chemistry, Beijing Normal UniVersity, Beijing 100875, China, Jinghua (Guiyang) Electronic Materials Co., Ltd., Guiyang, Guizhou 550008, China, Analysis and Test Center, Beijing Normal UniVersity, Beijing 100875, China ReceiVed: January 29, 2009; ReVised Manuscript ReceiVed: April 9, 2009

Nanosheets exfoliated from layered inorganic crystals can be regarded as inorganic macromolecules. Herein, coassembly of layered double hydroxide (LDH) nanosheets with carboxymethyl cellulose (CMC) was presented. CHN analysis, XRD, FTIR, TG-DSC and SEM were employed to characterize the coassembly process. The results showed that the colloidal suspension of the exfoliated MgAl-LDH nanosheets in formamide were restacked when in contact with water. Nevertheless, CMC can prevent the colloidal state from flocculation even after all included formamide molecules were removed by water, that is the interaction of CMC molecules and the nanosheets stabilized the dispersion in aqueous media. Drying at 40 °C led the nanosheets and CMC to restack to a layered nanocomposite with a basal spacing of 1.75 nm, indicating a bilayer arrangement of CMC in the interlayer. The thermal degradation temperature of CMC in the composite was raised by about 160 °C. Introduction Many inorganic layered compounds can be exfoliated into unilamellar nanosheets.1-6 For instance, birnessite-type layered manganese oxides4 were exfoliated into brucite-layer nanosheets consisting of [MnO6] octahedrons with shared edges (Scheme 1). These nanosheets are 2D crystals with thickness on a nanoscale but width on a microscale, depending on the starting bulk crystal and exfoliation process. The exfoliated nanosheet can be regarded as an inorganic macromolecule with charge. Nanosheets commonly exist in their delaminating liquid as a colloidal suspension and can reassemble when the colloidal system is destroyed. Utilizing the exfoliation/reassembly process, (poly)ions could be introduced into the reassembled layered compounds to fabricate novel materials, which are difficult to obtain via conventional methods. Wang et al.7 reported a layered Li-Mn-oxide with disordered and porous property by restacking of exfoliated MnO2 nanosheets with Li ions. The electrochemical data identified its smooth charge/discharge feature as a cathode material. Suzuki and Miyayama8 synthesized the Liintercalated octatitanate, which exhibited excellent electrode properties. Nanocomposites of polyions and exfoliated nanosheets could be prepared by the layer-by-layer technique, through which, for example, poly(diallydimethylammonium) was restacked with MnO2 nanosheets to prepare electrode materials.9-11 However, there has been no report focusing on the coassembly of exfoliated inorganic macromolecules with a polymer. The nanosheets are mostly charged with negative electricity. In recent years, extensive efforts have been made to delaminate layered double hydroxides (LDHs), which are widely used as catalyst supports, in anion exchange, as fire retardant materials, and so forth.12-15 LDHs comprise positively charged metal hydroxide layers and counteranions as well as water in the * To whom correspondence should be addressed. Tel: +86-10-58802960. Fax: +86-10-5880-2075. E-mail: [email protected]. † College of Chemistry, Beijing Normal University. ‡ Jinghua (Guiyang) Electronic Materials Co., Ltd. § Analysis and Test Center, Beijing Normal University.

SCHEME 1: Example of 2D Crystal of Nanosheet Derived from Exfoliation of Inorganic Layered Crystals; [MnO2]x+ Obtained from the Exfoliation of Birnessite-Type Manganese Oxide

interlayer. However, a high charge density of the layers associated with a high content of anions results in strong interlayer electrostatic interactions, which makes it difficult to exfoliate the LDHs. Investigations have found that intercalating organophilic16-18 or monovalent anions19 into the interlayer can facilitate the exfoliation and that formamide is an excellent delaminating reagent.16 Using formamide, Sasaki group20,21 successfully delaminated many highly crystallized LDHs such as MgAl-, CoAl-, NiAl-, and ZnAl-LDHs and investigated systematically the exfoliation process. Cellulose (part a of Scheme 2), a complicated carbohydrate, (C6H10O5)n, is the main constituent of cell walls in most cases and the most abundant nature organic in the world. Carboxymethyl cellulose (CMC), a water-soluble ether derivative (part b of Scheme 2) of cellulose, is biodegradable and environmental friendly material that can be widely applied in various fields.22-25 CMC may have different degrees of substitution (Ds) of carboxymethyl group from 0.8 to 2.2. In this work, positively charged LDH nanosheets and negatively charged CMC were coassembled. It was found that the colloidal suspension of the nanosheets can be stable in water after introducing CMC. This behavior assures the uniform dispersion of CMC and nanosheets, for which we proposed a model to describe the coassembling process. In addition, the thermostability of CMC in the nanocomposite was investigated.

10.1021/jp900861k CCC: $40.75  2009 American Chemical Society Published on Web 04/29/2009

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SCHEME 2: Structure of (a) Cellulose and (b) Carboxymethyl Cellulose (CMC)

Experimental Section Starting Materials. Carboxymethyl cellulose (CMC) sodium salt (Acros, Mw ) 90 000, Ds ) 0.7) was obtained from Aldrich. MgAl-LDH was synthesized using the homogeneous precipitation method.26 A mixed solution of Mg(NO3)2 (0.1 M), Al(NO3)3 (0.05 M), and urea (0.5 M) was hydrothermally treated at 100 °C for 34 h in a Teflon-autoclave. The precipitate was filtered, washed, and then dried at 30 °C in vacuum. The obtained CO32--LDH was converted to NO3- form by ionexchange using a HNO3-NaNO3 solution.19 The delamination was performed as that: a 0.375 g of NO3--LDH was mixed with 250 mL formamide in a flask sealed after purging with N2 gas, and then shaken at 160 rpm for 1 day, giving a colloidal suspension. The nanosheet slurry was obtained by centrifugation of the colloid at 9600 rpm. Coassembly of LDH Nanosheets and CMC. CMC sodium salt was dissolved in 200 mL of formamide (10 g/L) with heating at 80 °C and then cooled down to room temperature. The solution was poured into 200 mL colloidal suspension of the LDH nanosheets. After stirring for 3 h accompanied by purging with N2, the mixture was divided to four portions followed by centrifugation, one of which was directly characterized and three were done after water-washed 1, 2, and 3 times (40 mL each time), respectively. For comparison, 200 mL colloidal suspension of the nanosheets without addition of CMC was also washed with water. After being water-washed 3 times, the obtained samples were dried at 40 °C in vacuum. Characterizations. The chemical compositions of the samples were identified by the CHN analysis and inductively

coupled plasma (ICP) atomic emission spectroscopy (JarrelASH, ICAP-9000). For the ICP measurement, the samples were dissolved in a dilute acid solution and filtered with semipermeable membrane to exclude CMC. The Fourier transform infrared (FTIR) spectra were recorded by Nicolet 380 FTIR spectrometer at room temperature. The powder X-ray diffraction (XRD) patterns were collected using a Philips X’Pert diffractometer with Cu KR radiation. SEM observation was carried out on an S-4800 microscope (Hitachi, Ltd.) operating at 5.0 kV. Thermal gravimetric (TG) and differential scanning calorimeters (DSC) data were collected using a NETZSCH STA 409 PC/PG thermal analyzer. The sample was put into alumina crucible and the heating rate was 10 °C/min. Results and Discussion Preparation of LDH Nanosheets. The obtained CO32-LDH crystals are hexagonal platelets with a size of 1.5 µm across the hexagonal surface and a thickness of 0.3 µm (part a of Figure 1), and the XRD pattern (part a of Figure 2) shows that they have a hexagonal symmetry, the same as reported in the literature.21 After the ion exchange, the NO3--LDH product retains the morphology (part b of Figure 1) and the crystal symmetry (part b of Figure 2). The lattice parameters are calculated as a ) 0.30362(5) and c ) 2.6714(4) nm. The FTIR adsorption bands at 1354 cm-1 (part a of Figure 3) and 1385 cm-1 (part b of Figure 3) prove the presence of CO32- and NO3-, respectively. After the NO3--LDH is treated with formamide and centrifuged, the obtained slurry shows an XRD pattern

Figure 1. SEM images of (a) CO32--LDH, (b) NO3--LDH, (c) restacked LDH, and (d) CMC-LDH nanocomposite.

Exfoliated LDH Nanosheets with Cellulose

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Figure 2. XRD patterns of (a) CO32--LDH, (b) NO3--LDH, and (c) the slurry from colloidal suspension of LDH nanosheets in formamide; d-values in nanometers.

Figure 3. FTIR spectra of (a) CO32--LDH and (b) NO3--LDH.

(part c of Figure 2) with a halo at 2θ region of ∼20-30°, produced by formamide,20,21 and a faint reflection at 2θ ) 62° (marked with *), indicating that the 2D crystalline of the LDH layer is preserved. The above results identify that the wellcrystallized NO3--LDH crystals are exfoliated to nanosheets. Influence of Water on the Restacking of LDH Nanosheets. The samples become opaque after water-washing. The sample washed once with 40 mL water (part b of Figure 4) depicts the XRD reflections at 2θ ) 24, 36, and 41°, suggesting the nanosheets are immediately restacked. In the FTIR spectrum (part b of Figure 5), the bands at 1690 and 1312 cm-1, assigned to stretching vibrations of CdO and C-N respectively in

Figure 4. XRD patterns of (a) colloidal suspension of LDH in formamide, and the samples (b-d) water-washed 1, 2, and 3 times respectively, and (e) dried; d-values in nanometers.

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Figure 5. FTIR spectra of (a) colloidal suspension of LDH in formamide, and the samples (b-d) water-washed 1, 2, and 3 times, respectively.

formamide, are weakened compared with that of the unwashed sample (part a of Figure 5), suggesting that the formamide is partially removed by water. Further washing removes more formamide (part c of Figure 5), and 3 times washing (120 mL water) can remove formamide completely (part d of Figure 5). The three water-washed samples show a halo at 2θ region of 25-45° in the XRD patterns, different from that (20-30°) in the unwashed sample (part a of Figure 4), indicating the removal of formamide. These results indicate that the colloid of the exfoliated LDH nanosheets is not stable in water, and the partial removal of formamide can result in an immediate restacking of the nanosheets. Hibino27 has suggested that formamide has a very high ability to form hydrogen bonding with LDHs. In the exfoliated suspension, the carbonyl group of formamide has a strong interaction with the LDH hydroxyl slabs, whereas the other end (-NH2) bonds weakly with NO3-.28 The -NH2 can bond strongly to water molecules and the interaction between formamide and LDH hydroxyl slabs can be weakened. Thus, formamide tends to be dissolved in the added water, and the removal of formamide leads to the restacking of LDH layers. Because of the electrostatic interaction, NO3- anions exist around LDH nanosheets but not being washed out, as testified by the IR adsorption band at 1384 cm-1 (Figure 5). The XRD pattern of the dried sample (part e of Figure 4) shows that the restacked one has a layered structure with a basal spacing of 0.87 nm, which corresponds to the interlayer distance of NO3--LDH; but the broad peaks suggest that the interlayer galleries are not uniform. The SEM image (part c of Figure 1) reveals that the particles become irregular, although the plateshape morphology is still visible. In other words, the restacked NO3--LDH recovers hardly to the morphology of the original crystals. Coassembly of LDH Nanosheets and CMC. The Tyndall light scattering (Figure 6) indicates the addition of CMC into the nanosheet suspension does not destroy the colloidal state. After being centrifuged, the obtained transparent colloidal slurry has the same XRD pattern (part a of Figure 7) as that of the LDH colloid (part a of Figure 4). The water-washed samples have almost the same XRD patterns (parts b-d of Figure 7), with only a halo and the small peak (d ) 0.15 nm) of the (110) plane of LDH layer, suggesting that the nanosheets are not restacked. The halo is shifted to higher 2θ than that of the unwashed sample, showing the removal of formamide. The FTIR spectra (Figure 8) prove the gradual removal of formamide, characteristic by the 1690 cm-1 band of CdO vibration, and formamide is entirely removed after washed 3 times (part d of Figure 8). At the same time, the bands at 1069 cm-1, ascribed to C-O-C in CMC, and 1384 cm-1 indicate the

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Figure 6. Tyndall light scattering of the suspension after CMC was added into colloid of LDH nanosheets in formamide.

Figure 7. XRD patterns of (a) colloid suspension of CMC and LDH nanosheets in formamide, and the samples (b-d) water-washed 1, 2, and 3 times, respectively, and (e and e′) dried (CMC-LDH nanocomposite); d-values in nanometers.

presence of CMC and NO3-. The water-soluble CMC is not removed by water-washing, indicating a very strong interaction of CMC and the nanosheets. This interaction ensures the nanosheets in the exfoliated state in water. For the sample water-washed 3 times, drying causes it to restack to a layered structure; and from the first rational orders corresponding to the 00l reflections, the basal spacing is obtained to be 1.75 nm (parts e and e′ of Figure 7). Correspondingly, a plate-shaped morphology can be observed in the SEM image (part d of Figure 1). Taking the thickness (0.48 nm29-32) of the LDH layer into account, the interlayer distance (∆d) of the obtained composite is estimated to be about 1.27 nm. The interlayer gallery could be considered as a multilayer arrangement of CMC because the size of a glucose ring in cellulose crystal is ca. 0.40 nm.33 The relatively strong and sharp peak at d ) 0.15 nm in part e of Figure 7 shows that the intralayer structure is regular, whereas the other broad reflection peaks imply that the interlayer distances are not uniform. The FTIR

Exfoliated LDH Nanosheets with Cellulose

Figure 8. FTIR spectra of (a) colloidal suspension of CMC and LDH nanosheets in formamide, the samples (b-f) water-washed 1, 2, and 3 times, respectively, and (e) dried, and (f) CMC.

Figure 9. TG and DSC curves of (a) CMC, (b) NO3--LDH, and (c) CMC-LDH nanocomposite.

spectrum (part e of Figure 8) testifies to the formation of the CMC-LDH composite. The bands at 682 and 449 cm-1 are assigned to metal-oxygen bonds of the LDH layer. The CdO vibration appears at a lower wavenumber of 1616 cm-1 than

J. Phys. Chem. C, Vol. 113, No. 21, 2009 9161 that (1637 cm-1) in CMC (part f of Figure 8), indicating an interaction between carboxyl groups of CMC and the LDH layers. The structure of LDH is considered to be derived from the layered lattice of brucite, Mg(OH)2.34 The Mg and Al atoms are octahedrally coordinated by six oxygen atoms belonging to six OH groups. Each OH group is, in turn, shared by three cations and points to the interlayer space. The valence difference between Al3+ and Mg2+ causes the layers with positive charge. The LDH nanosheet can be characterized as a 2D crystal whose structure retains the intralayer structure. Consequently, on the basis of the crystal parameter a and the composition of the NO3--LDH sample, the average area per unit charge (Scharge)35 of the nanosheet is calculated as (1/0.5) · a2 sin 60° ) 0.16 nm2/ charge, where 0.5 is the layer charge in unit cell, calculated as 0.62 (Mg content) × 2 + 0.41 (Al content) × 3 - 2 (OH content) (Table 1). Scharge of CMC (DS ) 0.7) can be estimated to be 0.31 nm2/charge, according to Meyer-Mish model33 of cellulose I crystal (a ) 0.835, b ) 1.03, c ) 0.79 nm, β ) 84°). The Scharge value of CMC is larger than that of the nanosheet, so a bilayer arrangement should be formed.35,36 A few NO3- anions remain in the interlayer to compensate the surplus positive charge. The composition analysis (Table 1) shows that the composite contains 49 wt % (32 mol %) of CMC and 2.6 wt % of NO3-. The XRD pattern has a peak at d ) 0.88 nm (Figure 7e), the same as the basal spacing of NO3-–LDH (0.89 or 0.87 nm in part b of Figure 2 or part e of Figure 4). However, the patterns of the wet samples (parts b-d of Figure 7) negate the presence of a separate NO3--LDH phase because, if there is a separate NO3--LDH phase, it should give reflection peaks after water-washing, as in the system without CMC (parts b-d of Figure 4). Thus, the NO3- anions exist only among the CMC macromolecules. The CMC-LDH composite has a smaller Mg/Al molar ratio (1.13) than that (1.52) of the NO3--LDH precursor, whereas the latter is comparable to that (1.43) of the starting CO32--LDH sample (Table 1). Sasaki et al. also found that the Mg/Al ratio of the exfoliated nanosheets became smaller than that of the starting precursor,21 and they considered this change as a result of the gradual dissolution of the nanosheets in formamide.28 Accordingly, the dissolution of nanosheets in formamide might be responsible for the decrease of the Mg/Al ratio in the present case. Also, the small difference of the Mg/ Al ratio between the CO32-- and NO3--LDH samples probably shows the influence of the acidic aqueous solution on the LDH layer during the ion-exchange process. Further study will be needed on the changes of Mg/Al ratio. Scheme 3 is a schematic model proposed for the coassembly process according to the above results. The delamination of NO3--LDH in formamide follows a two-step process,28 (1) a certain volume of formamide replace the water molecules and produce a highly swollen phase, and (2) the mechanical shaking or ultrasonic treatment leads to exfoliation. The colloid of the exfoliated nanosheets is not stable in water. The substitution of water for formamide leads the nanosheets to restack to a layered structure of NO3--LDH. When CMC is introduced into the colloid, the nanosheet with a smaller Scharge could attach CMC on the both surfaces. CMC does not destroy the colloidal state. A stronger electrostatic interaction of the nanosheet and CMC may take place. The nanosheets encapsulated by CMC chains are in a stable exfoliated state in water. In other words, the interaction of CMC molecules and the nanosheets stabilizes the dispersion in aqueous media. Drying results in the flocculation

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TABLE 1: Results of Chemical Analysis of LDH and CMC-LDH Nanocomposite Content, Found (Calcd)/wt %a Sample CO32--LDH NO3--LDH CMC-LDH nanocomposite a

Mg

Al

C

H

N

Formula

16.46 (16.68) 16.10 (15.65) 7.89 (8.08)

12.77 (12.94) 11.73 (11.40) 7.73 (7.92)

2.86 (2.90) 0.88 (0.85) 20.13 (20.61)

4.07 (4.13) 3.50 (3.38) 4.67 (4.75)

5.07 (4.93) 0.58 (0.59)

Mg0.59Al0.41(OH)2(CO3)0.21 · 0.76H2O Mg0.62Al0.41(OH)2 (NO3)0.34(CO3)0.07 · 0.64H2O Mg0.46Al0.40 (OH)2(NO3)0.06 [C6H10O5 + 0.7(C2HO2)]0.320.24- · 0.6H2O

No Na content was detected by ICP analysis.

SCHEME 3: Sketch Presentation of the Formation Process of CMC-LDH Nanocomposite

mide by water can result in the restacking of the nanosheets, forming NO3--LDH. CMC does not destroy the colloidal state of the nanosheets in formamide, and the interaction of CMC and the nanosheets stabilizes their dispersion state in aqueous media. During drying, the nanosheets and CMC molecules coassemble to a layered nanocomposite with a basal spacing of 1.75 nm, showing a bilayer arrangement of CMC molecules in the interlayer. The thermal degradation temperature of CMC in the composite is raised by 160 °C. This provides a new approach for coassembly of polymer and inorganic macromolecules. Acknowledgment. This work is supported by the National Science Foundations of China 20671012, 50872012, and 20871018. References and Notes

of the colloidal suspension to construct the layered nanocomposite with a bilayer arrangement of CMC in the interlayer. Thermal Analysis. TG-DSC curves of pure CMC, NO3--LDH, and the CMC-LDH composite are shown in Figure 9. For CMC (part a of Figure 9), the weight loss below 200 °C, with an endothermic peak at around 70 °C, is attributed to the evaporation of physically adsorbed water. The weight losses from 200 to 500 °C, accompanying two exothermic peaks at 302 and 399 °C, are attributed to a total result of the thermal degradation and combustion, and the weight loss from 650 °C, with an exothermic peak at 692 °C, may be attributed to the decomposition of, mainly, resultant sodium salts. For the NO3--LDH (part b of Figure 9), the endothermic peak at 142 °C indicates the loss of water (12% between 20 and 200 °C). The weight losses (about 40%) between 200 and 550 °C, with two endothermic peaks at 270 and 369 °C, are produced by the removal of the interlayer NO3- anions and the dehydroxylation of the layers.37 Part c of Figure 9 is the curves of the CMC-LDH nanocomposite. A weight loss of about 16% between 20 and 160 °C with an endothermic peak is related to the loss of water. In the temperature region of 160-400 °C, the weight loss (25%) is attributed to the dehydroxylation of the LDH layers and the partial degradation of CMC. The weight loss (30%) from 400 to 800 °C corresponding to an exothermic peak at 561 °C is related to the further degradation and combustion of CMC. From the starting degradation temperature of CMC (260 °C) and the composite (420 °C), the thermal stability of CMC in the composite is raised by about 160 °C. Conclusions LDH nanosheets are obtained by exfoliation of the MgAl-NO3--LDH in formamide. The removal of forma-

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