Direct Correlation between Nanostructure and Particle Morphology

Communication pubs.acs.org/crystal

Direct Correlation between Nanostructure and Particle Morphology during Intercalation M. Ogawa*,†,‡ and M. Hiramine‡ †

Department of Earth Sciences, Waseda University Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan Graduate School of Creative Science and Engineering Waseda University Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan

‡

ABSTRACT: Ion exchange of the interlayer carbonate anion of Co(II)/Al(III) layered double hydroxide with dodecylsulfate was conducted by the reaction in aqueous media under hydrothermal conditions. While the hexagonal plate particle shape was retained, the thickness of the plate increased. The ratio of the average particle thickness (from 440 to 1380 nm, 3.1 times) before and after the exchange with dodecylsulfate was consistent with the ratio of the basal spacing (from 0.76 to 2.6 nm, 3.4 times) before and after ion exchange. This correlation between the basal spacing and the particle size is the first example to confirm the topochemical reaction of the intercalation.

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INTRODUCTION Intercalation is a reaction of inserting guest species topochemically into the solid. Intercalation compounds of various layered solids have been synthesized and their properties have been examined extensively for the advanced materials applications.1 Due to the wide range of possible applications of layered materials and their intercalates, the precise design of the nanostructure and the particle morphology is worth investigating. The visualization of intercalation processes has been seen using large crystals of such layered materials as a layered double hydroxide2 and a layered titanate3 and niobate4 by microscopy. Here, we report a very convincing example of the hierarchical design of nanostructure and particle morphology using monodispersed platy particle of a layered double hydroxide. Layered double hydroxides (abbreviated as LDHs) are a class of layered materials consisting of a positively charged brucitelike layer, where some M2+ cations are substituted with M3+ cations to give positive charge, and the charge compensating interlayer exchangeable anions.5 Besides the mineralogical and structural interests, studies on the possible applications6 of LDHs in such fields as pharmaceutical and biochemical uses,7 catalyst,8 and adsorbent9 filler for the polymer10 have been conducted. For the applications, the morphology of LDHs is a key issue to control the performance. Homogeneous precipitation method utilizing urea hydrolysis is a promising way to prepare wellcrystallized and large particles of various oxides and hydroxide particles because pH rises homogeneously in the solution. LDHs with particle sizes from 2−5 μm have been synthesized by the urea method.11 We have used hydrothermal conditions in the urea method to successfully prepare large platy hydrotalcite particles12−15 with the size as large as 25 μm and associated to a relatively narrow particle size distribution. Using the well-defined LDHs, we followed the intercalation reaction © 2014 American Chemical Society

by scanning electron microscopy (SEM) in addition to the Xray diffraction. Carbonate form of CoAl-LDH (abbreviated as CO32−-CoAlLDH) was synthesized by the hydrothermal method based on the previous reports.11 An aqueous stock solution of 0.01 M CoCl2·6H2O, 0.01 M AlCl3·6H2O, and 0.1 M (NH2)2CO were mixed at the molar Co:Al:(NH2)2CO ratio of 2:1:10. Glycerol was added to the aqueous mixture in order to control the morphology of LDH. Though the role of glycerol is not clear at present, well-defined particles of LDHs have been obtained from the aqueous glycerol solution.13,14 The amount of glycerol was 1.0 g to 40 mL of the aqueous solution of CoCl2, AlCl3, and (NH2)2CO. The aqueous mixtures were allowed to react in a Teflon-lined autoclave of 300 mL capacity, sealed in a stainless steel bottle (Taiatsu Glass Ind. Company) at 80 °C for 48 h with rotating at 15 rpm. After cooling to room temperature, the solid products were collected by centrifugation (3500 rpm for 5 min), washed with deionized water, and dried under reduced pressure. The ion exchange of CO32−-CoAlLDH with dodecylsulfate (DS−) was conducted in aqueous solution of sodium dodecylsulfate (SDS) (10 and 50 times of the anion exchange capacity of the LDH) under hydrothermal condition (120 °C for 24 h). After cooling to room temperature, the solid products were collected by centrifugation (3500 rpm for 5 min), washed with deionized water three times, and dried under a reduced pressure. As reported previously,14,15 CO32−-CoAl-LDH were successfully synthesized by the present hydrothermal synthesis. Figure 1 shows the XRD patterns of CO32−-CoAl-LDH prepared in Received: November 11, 2013 Revised: February 13, 2014 Published: March 4, 2014 1516

dx.doi.org/10.1021/cg401684u | Cryst. Growth Des. 2014, 14, 1516−1519

Crystal Growth & Design

Communication

Figure 1. XRD patterns of (a) CO32−-CoAl-LDH, (b) DS−-CoAl-LDH, and (c) DS−/CO32−-CoAl-LDH.

the present study. The XRD pattern exhibits the characteristic reflections corresponding to CO32−-CoAl-LDH. The lattice parameters are a[2 × d(110)] = 0.306 nm, c[3 × d(003)] = 2.25 nm, which are in good agreement with those of CoAĺ et al.16 It is known that the LDHs reported by Pérez-Ramirez ion exchange of carbonate with other anions is difficult probably due to the small size and bidentate nature of carbonate. There are several methods to replace interlayer carbonate anions with other anions in such ways as controlled pH conditions as well as reconstruction by calcined LDHs.17,18 In the present study, the ion exchange carbonate with dodecylsulfate was done using aqueous solution of sodium dodecyl sulfate (SDS). Since the anion exchange at room temperature was not successful, the anion exchange was conducted in the presence of large excess of SDS under hydrothermal conditions. By the hydrothermal reactions at 120 °C for 24 h, DS was successfully exchanged with the carbonate anion of CO32−-CoAl-LDH as evidenced by the expansion of the interlayer space from 0.76 to 2.6 nm, which is consistent with the reported dodecylsulfate intercalated LDHs.19 The infrared spectrum and the TG-DTA curves (data not shown) of the product revealed the presence of dodecylsulfate with the amount of the anion exchange capacity of the present LDH. Figure 2a shows scanning electron micrographs of the present CO32−-CoAl-LDH. The particle size distributions derived from the SEM images are shown in Figure 3a. The particle size distributions were obtained by the scanning electron micrographs for no less than 100 particles. Hexagonal plates with the average diameters of 9400 nm and the average thickness of 440 nm were observed for the present CO32−CoAl-LDH. As reported previously,15 the size distribution of the particle radius of the present CO32−-CoAl-LDH is narrow with the coefficient of variation (CV) of 0.12. In addition to the radius, the size distribution of the particle thickness was also discussed in the present study to estimate the relatively narrow thickness distribution with the coefficient of variation (CV) of 0.25. The scanning electron micrographs of the DS−-CoAl-LDH particle are shown in Figure 2b. The particle size distribution (both radius and thickness) derived from the SEM images is

Figure 2. SEM images of (a) CO32−-CoAl-LDH, (b) DS−-CoAl-LDH, and (c) DS−/CO32−-CoAl-LDH.

shown in Figure 3b, where the average radius (9300 nm; CV = 0.11) was almost the same as that (9400 nm) of CO32−-CoAlLDH and the average thickness (1380 nm, CV = 0.23) was much larger than that (440 nm) of CO32−-CoAl-LDH. The coefficient of variation of the size distribution did not change by the intercalation of DS−. The expansion of the particle thickness reflects the expansion of the interlayer space, and the degree was consistent with the expansion of the basal spacing. The ratio of the expansion of the particle thickness (440 to 1380 nm, 3.1 times) is close to that of the basal spacing (from 0.76 to 2.6 nm, 3.4 times), confirming the ion exchange occurred topochemically. To the best of our knowledge, there are no reports on the precise 1517



Communication

that the segregation in the present case occurred at the interparticle level. Particle morphology of layered solids has been a topic of interest because it limits their applications. Accordingly, LDHs with a wide range of sizes from nanometers to micrometers have been synthesized using various synthetic means.11−15,21−24 The morphological change of LDHs with varied sized during ion exchange is worth investigating further to understand the reaction mechanism and to apply LDHs. In summary, we found that the urea hydrolysis under hydrothermal conditions (80 °C for 24 h) led to well-defined hexagonal platy particles of carbonate CoAl-LDH, not only with narrow radius distribution but also with narrow thickness distribution. It was possible to replace carbonate anions with dodecylsulfate by an anion exchange reaction under a hydrothermal condition (120 °C for 24 h). While the hexagonal platy particle shape was retained, the thickness of the plate increased by the anion exchange. The ratio of the average particle thickness (from 440 to 1380 nm, 3.1 times) before and after the exchange with dodecylsulfate was consistent with the ratio of the basal spacing (from 0.76 to 2.6 nm, 3.4 times) before and after the ion exchange. This correlation between the basal spacing and the particle size is the first example to confirm the topochemical reaction of the intercalation.

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Figure 3. Particle size distribution (left, radius; right, thickness) of (a) CO32−-CoAl-LDH, (b) DS−-CoAl-LDH, and (c) DS−/CO32−-CoAlLDH.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +81-3-3207-4950. hierarchical correlation between the nanostructure and the particle size, though so many intercalation compounds have been synthesized and the formation mechanism was thought to be topochemical intercalation.1 From the X-ray diffraction patterns shown in Figure 1, crystallite size (Lc) was derived by the Scherrer equation as 32.1 and 35.4 nm for CO32−-CoAlLDH and DS−-CoAl-LDH, respectively. The crystallite size was not directly correlated with the particles size observed by the SEM images (Figure 2), suggesting there are some crystalline domains in the platy particles seen in Figure 2. When the anion exchange with dodecylsulfate was conducted using smaller amount of SDS (10 times of LDH’s ion exchange capacity), a product with mixed phase (named as DS−/CO32−CoAl-LDH), where the 003 reflection in the XRD pattern was split into two (0.76 and 2.6 nm, as shown in Figure 1c), was obtained. The SEM image of DS−/CO32−-CoAl-LDH was also examined. (The SEM image is shown in Figure 2c, and the particle size distribution derived from the SEM images is shown in Figure 3c. The particle radius was almost the same (9700 nm, CV = 0.13) as those of CO32−-CoAl-LDH and DS−-CoAlLDH, while the thickness was intermediate of those two. The thickness distribution became broad (average thickness of 1000 nm, CV = 0.43), indicating a kind of segregation had occurred during the ion exchange. “Segregation” has been reported so far, especially for the intercalation into naturally occurring smectites;20 it has not been possible to image the particle level segregation due to the difficulty of evaluating morphological changes of smectites, which are finite particles with broad particle size distribution. In the present study, thanks to the well-defined particles of LDH (both radius and thickness), it became possible to directly correlate the change in the nanostructure and particle morphology during intercalation. From the SEM image (Figure 3c), some particles did not react with SDS, while some reacted with SDS. It can be concluded

Funding

This work was supported financially by Waseda University Grant for Special Research Projects (Grants 2013B-054 and 2013B-055). Notes

The authors declare no competing financial interest.

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REFERENCES

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Direct Correlation between Nanostructure and Particle Morphology

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