Tripodal Ligand-Stabilized Layered Double Hydroxide Nanoparticles

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Tripodal Ligand-Stabilized Layered Double Hydroxide Nanoparticles with Highly Exchangeable CO 32–

Yoshiyuki Kuroda, Yumi Miyamoto, Mitsuhiro Hibino, Kazuya Yamaguchi, and Noritaka Mizuno Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm400846k • Publication Date (Web): 08 May 2013 Downloaded from http://pubs.acs.org on May 11, 2013

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Chemistry of Materials

Tripodal Ligand-Stabilized Layered Double Hydroxide Nanoparticles with Highly Exchangeable CO32– Yoshiyuki Kuroda, Yumi Miyamoto, Mitsuhiro Hibino, Kazuya Yamaguchi, Noritaka Mizuno* Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan *Corresponding author Fax: (+)81-3-5841-7220 E-mail: [email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

Abstract: Layered double hydroxide nanoparticles (LDHNPs) with exceptionally small particle sizes are synthesized using a tripodal ligand of tris(hydroxymethyl)aminomethane (THAM). For example, a LDHNP with the average size of 9.7 nm (denoted as LDH(10 nm), containing CO32– in the interlayer), can be synthesized using a 2.0 M THAM solution. The 13C CP/MAS NMR and FTIR analyses show that THAM is ligated to the layer as an alkoxide species. The average particle size of LDH synthesized using L-lysine (buffering base) instead of THAM is larger (47.9 nm) than that of LDH(10 nm). Therefore, the size reduction is possibly explained by the specific interaction of THAM with the layer via its multiple coordination. In addition, it is confirmed by the

13

C CP/MAS NMR analysis that

LDH(10 nm) possesses CO32– species weakly interacting with the layers. LDHNPs, in particular assynthesized LDH(10 nm) (denoted as LDH(10 nm)-as, containing CO32– and Cl– in the interlayer), 1

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possesses the extremely high anion exchange abilities, and almost all anions in LDH(10 nm)-as are potentially exchangeable with NO3– even under ambient (CO2-existing) conditions. Furthermore, LDH(10 nm)-as can act as an efficient reusable scavenger for harmful oxyanions and remove arsenic, selenium, and boron from their dilute aqueous solutions under ambient conditions.

KEYWORDS: layered double hydroxide nanoparticles, tripodal ligand, anion exchange, removal of harmful oxyanions

INTRODUCTION Layered double hydroxides (LDHs) are an important class of inorganic materials and mainly composed of positively charged brucite-like layers and interlayer anions.1 Because of their unique properties such as basicity, anion exchange ability, and low toxicity, they have been utilized as anion exchange materials, CO2 adsorbents, catalysts, catalyst supports, drug carriers, etc.2–6 LDHs with the high anion exchange abilities are promising materials for removal of harmful metals from water because they usually exist as anionic species in aqueous solutions.2b However, there is a serious problem; anion exchange reactions of LDHs should be carried out under entirely CO2-free conditions7,8 due to the exceptionally high affinity toward CO32–.9 To date there have been several reports on the anion exchange of CO32–-type LDHs using mineral acids (e.g., HCl).7 The anion exchange has been achieved by protonation of interlayer CO32– into readily exchangeable HCO3– and/or CO2. In these systems, the use of buffer solutions, ammonium salts, and/or alcohol solvents is inevitable in order to prevent dissolution of LDHs. In addition, the anion exchange reactions are typically carried out with a flow of N2. Therefore, the development of efficient anion exchange systems under more practical conditions (without acids and additives, ambient conditions, aqueous solvents, etc.) is highly desirable. Interlayer anions located on edges of LDHs are expected to be more exchangeable due to their weaker interactions with layers (i.e., smaller coordination numbers) than those in the inner part. Therefore, anion exchange abilities of LDHs would be improved by the particle size reduction. Several methods 2


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have previously been reported for preparation of LDH nanoparticles (LDHNPs),10–16 though control of crystal growth is usually very complex and difficult because the crystal growth is highly dependent on various synthetic parameters such as concentrations of metal solutions, pH, reaction times, temperatures, etc. Ligand-stabilization is useful for controlling crystal growth of inorganic nanoparticles17 and nanosheets18 by a simple parameter (i.e., concentration of ligands). Although modification of the layers by using alcohols,19 carboxylates,20 and phosphonates21 has been reported, the effective control of crystal growth has never been achieved with these ligands likely due to their weak interactions with the layers.22 We focused on a tripodal ligand of tris(hydroxymethyl)aminomethane (THAM) which is known for functionalization of Anderson-type polyoxometalates (POMs).23 Inspired by the similarity of the local structure of LDHs to that of Anderson-type POMs, we expected specific and strong interactions of THAM with the layer via its multiple coordination (Figures 1a–c). To the best of our knowledge, there have been no reports on synthesis of LDHs using THAM.24

Figure 1. Structural models of (a) Anderson-type POM, (b) POM–THAM hybrid, and (c) LDH (M2+/M3+ = 2). Dark blue and light blue octahedra represent the sites of different metals. Orange and yellow octahedra represent the sites of M2+ and M3+, respectively. THAM is indicated by a ball-andstick model (gray: CH2 or C, red: O or OH, and blue: NH2). The local structure of LDH similar to that of Anderson-type POM is highlighted with a light blue line in (c). (d) Schematic illustration of the synthesis of ligand-stabilized LDHNPs.

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In this paper, we demonstrate for the first time synthesis of well-defined LDHNPs in the presence of THAM (Figure 1d). LDHNPs with the average size of 9.7 nm could successfully be synthesized using a 2.0 M THAM solution. Surprisingly, the LDHNPs possessed the extremely high anion exchange abilities, and CO32– in the interlayer could be exchanged with NO3– even under ambient (CO2-existing) conditions. In addition, they could act as efficient reusable scavengers for harmful oxyanions of arsenic, selenium, and boron.

EXPERIMENTAL SECTION Materials. Tris(hydroxymethyl)aminomethane (THAM), MgCl2·6H2O, AlCl3·6H2O, NaOH, ethanol, acetone, NaNO3, Na2HAsO4·7H2O, Na2SeO4, and a standard solution of boron for atomic absorption analysis (1000 ppm) were purchased from Kanto Chemical. Na2CO3 was purchased from Nacalai Tesque. L-Lysine was purchased from Aldrich. 2-Amino-1,3-propanediol was purchased from TCI. All reagents were used as received. Deionized water was used without a degassing treatment. Synthesis of LDHNPs. LDHNPs prepared by using 2.0 M and 0.5 M THAM solutions are denoted as LDH(10 nm) and LDH(26 nm), respectively. A procedure for synthesis of LDH(10 nm) is as follows. A 2.0 M THAM solution (100 mL) and a mixed salt solution (100 mL) containing MgCl2 (13.1 mM) and AlCl3 (6.9 mM) were mixed in a single step, followed by aging at 80 °C for 12 h in a closed polypropylene bottle. The gel consisting of LDH(10 nm) was separated by vacuum filtration using a membrane filter (average pore size: 0.2 µm). The obtained gel was redispersed in 100 mL of a 0.5 M Na2CO3 solution and stirred for 1 h, followed by washing twice with deionized water and drying in an oven at 120 °C, giving the aggregated LDH sample. LDH(26 nm) was synthesized via the same procedure except that the concentrations of THAM, MgCl2, and AlCl3 were 0.5 M, 15 mM, and 5.0 mM, respectively. In the present procedure, the pH adjustment was not necessary because of the buffering action of THAM. The pH values of the mixtures using 2.0 and 0.5 M THAM solutions were 9.8 and 9.4, respectively. For comparison, LDHs were synthesized via the above procedure using L-lysine (1.0 M) and 2-amino-1,3-propanediol (1.0 M) instead of THAM. 4


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Synthesis of a Conventional LDH. A conventional LDH (denoted as LDH(107 nm)-c) was prepared by the coprecipitation method.25 A mixed salt solution (30 mL) containing MgCl2 (333 mM) and AlCl3 (167 mM) was added dropwise to a 0.1 M Na2CO3 solution (50 mL) at room temperature. During the addition, the pH value of the solution was kept at 10 using a 2.0 M aqueous NaOH solution. Then, the slurry was aged at 80 °C for 12 h. The precipitates formed were separated by filtration, washed with water, and dried at 80 °C. Anion Exchange with NaNO3. The anion exchange experiments were performed under ambient (CO2-existing) conditions. A typical procedure is as follows. LDH(10 nm) (2 mg mL–1) was dispersed in an aqueous NaNO3 solution (1.0, 2.5, 5.0, or 10 M) and stirred at 32 °C. After 2 h, LDH(10 nm) was separated by filtration, washed twice with water and once with acetone, and dried at room temperature. Removal of Harmful Oxyanions. The solutions of arsenic and selenium (ca. 2 ppm) were prepared by dissolving Na2HAsO4·7H2O and Na2SeO4, respectively. The solution of boron (ca. 100 ppm) was prepared by diluting a standard solution of boron for atomic absorption analysis. The pH of the solutions of HAsO42– and SeO42– was 8. The pH value of the solution of boron was adjusted to 11 with a 1.0 M NaOH solution. In this condition, boron is present mainly as B(OH)4–. A typical procedure is as follows. LDH(10 nm) was dispersed in model polluted water and stirred for 2 h at room temperature in a closed PTFE bottle. After the removal experiment, the LDH(10 nm) was separated by filtration, and then the anion concentration was determined by ICP-AES analysis. LDH(10 nm) used for the removal of harmful oxyanions could easily be recovered by simple filtration. For example, the LDH(10 nm) could be separated within 1 min (>95% recovery) by vacuum filtration using a membrane filter (pore size: 0.2 µm, diameter: 47 mm) from the 20 mL solutions containing 20 mg of LDH(10 nm). The recovered LDH(10 nm) was reused for the next removal experiment without any special pretreatments. Characterization. XRD patterns were recorded on Rigaku Smartlab using CuKα radiation (45 kV, 200 mA). SEM images were obtained by Hitachi S-4700. LDH samples were dispersed in ethanol, sonicated for 5 min, and dropped on a carbon-coated Cu grid, and SEM images were observed without metal coating. Highly dispersed primary particles of LDH(26 nm) and LDH(10 nm) could not be 5



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observed by TEM because of their too low contrast. On the other hand, clear TEM images of aggregates could be observed (Figure S1). The size distributions of LDHNPs obtained by TEM were consistent with those obtained by SEM. We used the SEM images to evaluate the particle size distributions because the primary particles could be observed by SEM with high contrast. The BET surface areas were measured by N2 adsorption at –196 °C using Micromeritics ASAP 2000. Elemental analyses (for Mg, Al, As, Se, and B) were performed by ICP-AES using Shimadzu ICPS-8100. Solid-state

13

C

CP/MAS NMR spectra (MAS rate = 5 kHz) were recorded on Chemagnetics CMX-300 Infinity operating at 75.6 MHz with a contact time of 5.0 ms, and the repetition time was 5 s. FTIR spectra were recorded on Jasco FT/IR-4100 using KBr discs.

RESULTS AND DISCUSSION Structural Characterization. The XRD patterns of the samples showed that all diffraction lines were assignable to those of typical MgAl-type LDHs (Figure 2).1 The colloidal dispersions of LDHNPs were transparent and showed the Tyndall effect, indicating the presence of highly dispersed colloidal LDHNPs (Figures 3a and b). The SEM images of LDH(26 nm) and LDH(10 nm) showed formation of uniform nanoparticles (Figures 3c–e). LDHNPs with a rectangular shape (average size: 2.9 nm × 10.3 nm) were observed by TEM (Figure S1c, Supporting Information). Moreover, TEM images of LDHNPs viewed along the c axis (stacking direction) showed uniform contrast. Therefore, LDHNPs possess platy shapes. The average sizes of LDH(107 nm)-c, LDH(26 nm), and LDH(10 nm) were 106.9 nm (standard deviation (σ) = 29.8%), 25.6 nm (22.2%), and 9.7 nm (17.6%), respectively (Figures 3f–h). These results show that the sizes of LDHNPs can effectively be reduced by addition of THAM. The BET surface areas of LDH(107 nm)-c, LDH(26 nm), and LDH(10 nm) were 74, 93, and 65 m2 g–1, respectively.26

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Figure 2. XRD patterns of (a) LDH(107 nm)-c, (b) LDH(26 nm), and (c) LDH(10 nm).

Figure 3. Photographs of the dispersions of (a) LDH(26 nm) and (b) LDH(10 nm). Red laser from the left shows the Tyndall effects. SEM images of (c) LDH(107 nm)-c, (d) LDH(26 nm), and (e) LDH(10 nm). Particle size distributions of (f) LDH(107 nm)-c, (g) LDH(26 nm), and (h) LDH(10 nm). See Figure S2 for the images in the wider areas. The average size of LDH(107 nm)-c, LDH(26 nm), LHD(10 nm) were 106.9 nm (σ = 29.8%), 25.6 nm(22.2%), and 9.7 nm (17.6%), respectively.

The elemental analyses showed that Mg/Al ratios of all LDH samples were in the range of 2.01–2.22 (Tables S1 and S2, Supporting Information). These values were close to the highest limit of the 7



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incorporation of Al3+ in brucite layers.1 The amounts of CO32– in LDH(26 nm) and LDH(107 nm)-c were almost the same as those estimated by the amounts of Al3+. In the case of LDH(10 nm), the amount of CO32– was larger than the estimated one. The excess amount of CO32– in LDH(10 nm) is possibly explained by the generation of additional anion exchange sites due to the amino group of THAM and/or broken bonds on the edge. Since the pH control is possibly an important factor for size reduction of LDHs,15,27 we used L-lysine (1.0 M solution) as a buffering base28 instead of THAM. In this case, the average size of the product was 47.9 nm (σ = 29.2%), and the XRD pattern showed sharper diffraction lines than those of the LDHNPs (Figure S3, Supporting Information), while the pH value of the solution was 9.6 and similar to those with THAM (9.8 and 9.4, see the Experimental Section). Therefore, it is suggested that the size reduction does not result simply from the pH control. It was confirmed by the elemental analyses of LDHNPs that nitrogen was still present even after washing (Tables S1 and S2, Supporting Information). The solid-state

13

C CP/MAS NMR spectrum of

LDH(10 nm) showed the signals at 50.5, 58.8, and 70.6 ppm (Figure 4b). The signals at 58.8 and 70.6 ppm are assignable to the methylene carbons (–CH2O–) of free and ligated THAM, respectively. Such a downfield shift of the methylene signal is usually observed upon formation of metal alkoxide species.29 The signal at 50.5 ppm is also assignable to the quaternary carbon (–CNH2) of THAM ligated to the layer as an alkoxide species.30 The FTIR spectra of LDHNPs showed bands at 1087 and 1135 cm– 1

assignable to the M–O–C and C–C–O stretching vibrations, respectively (Figure S4, Supporting

Information). Such bands have been observed for layered clay minerals, e.g., ZnAl-LDH, brucite, and kaolinite possessing Al or Mg–alkoxide species.29a,31 These NMR and IR results clearly indicate the ligation of THAM to the layer as an alkoxide species. According to the above-mentioned results, the size reduction of LDHs is possibly explained by the specific and strong interaction of THAM with the layer via its multiple coordination22 rather than the buffering action of THAM.

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Figure 4. Solid-state 13C CP/MAS NMR spectra of (a) LDH(107 nm)-c and (b) LDH(10 nm). Light blue, dark blue, green, pink, orange, and red curves show the deconvolution of the signals. The 13C CP/MAS NMR spectrum of LDH(107 nm)-c showed a signal at 170.8 ppm and a shoulder at 164.8 ppm with the respective integrated ratio of 1 : 0.16 (Figure 4a). These two signals are assignable to interlayer CO32– species. It has been reported that signals of CO32– species at upperfield appear upon weak interaction with cations or polar molecules.32–34 For example, CO32– species in monovalent (with weaker interaction) and trivalent anionic (with stronger interaction) sites of apatites show signals at 166.6 and 170.0 ppm, respectively.32 Therefore, the upperfield signal in Figure 4a is assignable to CO32– species interacting weakly with the layers. The 13C CP/MAS NMR spectrum of LDH(10 nm) showed a broad signal, and the signal could be deconvoluted into two components at 169.1 and 166.5 ppm with the respective integrated ratio of 1 : 0.40 (Figure 4b). Therefore, LDH(10 nm) possesses a larger amount of CO32– interacting weakly with the layers, which is expected to be readily exchangeable even with monovalent anions. Exchange Ability of CO32–. It is well known that anion exchange of CO32– in aqueous media hardly proceeds even in the presence of small amounts of CO2 in the case of conventional LDHs.7 Using deionized water without degassing treatment, we assessed the properties of LDHNPs by the anion exchange with NO3– under ambient (CO2-existing) conditions. Figure 5A shows the equilibrium amounts of NO3– exchanged into LDHs (Q) as a function of its concentrations (C). The results could be 9



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adequately reproduced by the Langmuir adsorption model. From the linear fits of Figure S5 (C/Q vs. C plots), percentages of exchangeable anions in the LDHs (Qmax) and adsorption equilibrium constants of NO3– (K) were calculated (Table 1). The Qmax values and basal spacings significantly increased with a decrease in the sizes of LDHs (see also Figure 5B). In the case of LDH(10 nm), 52% of CO32– were potentially exchangeable with NO3– even under the present conditions. The K value for LDH(10 nm) was smaller than those of LDH(26 nm) and LDH(107 nm)-c. This is likely due to the weak interaction among the layers and anions located on edges (readily exchangeable anions). The size reduction causes an increase in the amounts of CO32– interacting weakly with the layers, resulting in facile use of LDHNPs for anion exchange reaction even under ambient conditions without careful exclusion of CO2. In contrast, the anion exchange with NO3– hardly proceeded in the case of LDH(107 nm)-c under the same conditions. It has also been suggested that nanoparticles of layered materials other than LDHs (e.g., sodium titanate, cesium tungstate, and sodium manganate) can smoothly exchange their interlayer cations with bulky alkylammonium cations, resulting in exfoliation without fracture of nanosheets.18b LDH(10 nm) used after the NO3–-exchange reaction was treated with an aqueous Na2CO3 solution to regenerate the original CO32– form, and then the CO32– in the sample was again exchanged using an aqueous NaNO3 solution (10 M). In this case, the Q value (0.159 mmol g–1) and the basal spacing (0.861 nm) were almost the same as those of the first exchange reaction (Q: 0.161 mmol g–1, basal spacing 0.872 nm), indicating that the anion exchange reaction is reversible. The driving force of anion exchange was considered to be a concentration gradient. The exchange of CO32– in LDH(107 nm)-c was likely suppressed due to the strong interaction with the layers even when a large concentration gradient was given by using 10 M NaNO3 solution. In contrast, in the case of LDHNPs, the CO32– species could be exchanged under the same conditions because of their weak interactions with the layers. Notably, in the case of as-synthesized LDH(10 nm) containing both CO32– and readily exchangeable Cl– (denoted as LDH(10 nm)-as), almost all anions were assumed to be potentially exchangeable (Qmax = 98%, Table 1). 10


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Figure 5. (A) Equilibrium amounts of NO3– exchanged into LDHs (Q) as a function of NO3– concentrations (C); (a) LDH(107 nm)-c, (b) LDH(26 nm), (c) LDH(10 nm), and (d) LDH(10 nm)-as. (B) The basal spacings after the anion exchange; (a) LDH(107 nm)-c, (b) LDH(26 nm), (c) LDH(10 nm), and (d) LDH(10 nm)-as. The basal spacing of a conventional LDH (Mg/Al = 2) completely exchanged with NO3– is 0.89 nm.35

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Table 1. Anion Exchange of LDHs with NO3– sample

exchange levela (%)

basal spacingb (nm)

Qmaxc (%)

Kc (L mol–1)

LDH(107 nm)-c

2.1

0.759

3.8

0.80

LDH(26 nm)

36

0.813

40

0.80

LDH(10 nm)

48

0.847

52

0.45

LDH(10 nm)-as

73

0.877

98

0.28

a

Exchange level [%] = amount of exchanged NO3– at 10 M NaNO3 / amount of anion exchange sites calculated based on the Al3+ content × 100. b Basal spacings of LDHs after the anion exchange with 10 M NaNO3. c Qmax and K values were calculated based on the Langmuir adsorption model (see Figure S5, Supporting Information). Qmax: Percentages of exchangeable anions in the LDHs, K: Adsorption equilibrium constants of NO3–.

Removal of Harmful Oxyanions. As mentioned in the former section, LDHNPs (in particular LDH(10 nm) and LDH(10 nm)-as) showed the high anion exchange abilities. Thus, LDHNPs are potentially useful for purification of water containing harmful oxyanions of arsenic, selenium, and boron under ambient (CO2-existing) conditions. The concentrations of these elements should be reduced (e.g., ppb levels for drinking water (arsenic and selenium) and below 10 ppm for effluent standard (boron)),36 while the adsorptive removal of those anions in dilute solutions is generally very difficult. Because LDHNPs showed the extremely high anion exchange abilities, they are expected to be useful for removal of these oxyanions from their dilute aqueous solutions. Firstly, we used ca. 2.0 ppm aqueous solutions of arsenic and selenium as model polluted water. By using LDH(10 nm)-as, concentrations of arsenic and selenium could successfully be reduced to below the WHO guideline values for drinking water (below 10 ppb,36 >99.6% removal, Table 2). For these experiments, the amounts of LDH(10 nm)-as required for the removal of arsenic and selenium were only 0.01 wt% and 0.1 wt% with respect to the solutions, respectively. After the removal of arsenic, LDHNPs could easily be recovered by simple filtration. The recovered LDHNPs (LDH(10 nm) and LDH(10 nm)-as) could be reused for removal of arsenic without loss of their high performance (Table 2).

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Secondly, we used a ca. 100 ppm aqueous solution of boron as model polluted water. It is known that the selectivity of conventional LDHs to oxyanions of boron is very low, and that boron can not be removed using conventional LDHs.16 Indeed, in our hand, the LDH(107 nm)-c hardly removed boron under the conditions described in Table 2. In contrast, in the case of LDH(10 nm)-as, the concentration of boron could be reduced to 3.2 ppm (96.8% removal, Table 2) by two cycles of removal experiments. The concentration of boron was below the national effluent standard value applied in Japan.36

Table 2. Removal of Harmful Oxyanions Using LDHsa concentration (ppm) adsorbent (loading amount (wt%))

element

initial

final

removal (%)

LDH(107 nm)-c (0.01)

As

1.97

0.811

58.8

LDH(107 nm)-c (0.10)

Se

2.08

0.827

60.2

LDH(107 nm)-c (1.0)

B

101

96.8

4.50

LDH(10 nm)-as (0.01)

As

1.97

99.6

LDH(10 nm)-as reuse-1

As

1.85

99.6

LDH(10 nm)-as (0.10)

Se

2.08

99.6

LDH(10 nm)-as (1.0)

B

101

3.29c

96.7c

LDH(10 nm) (0.01)

As

1.97

0.0104

99.5

LDH(10 nm) (0.10)

Se

2.02

0.149

92.6

LDH(10 nm) (1.0)

B

108

81.5

24.8

LDH(10 nm) (0.10)d

As

2.13

99.6

LDH(10 nm) reuse-1d

As

2.13

99.6

LDH(10 nm) reuse-2d

As

2.13

99.6

LDH(10 nm) reuse-3d

As

2.13

99.6

a

Reaction conditions: model polluted water (5 mL for As and Se, and 3 mL for B), LDHs (0.5 mg for As, 5.0 mg for Se, and 30 mg for B), at room temperature for 2 h. All experiments were carried out under ambient conditions without exclusion of CO2. b The value is below the detection limit. The detection limit was determined by 3.3σ of blank response. c The result after sequential two adsorption cycles. The concentration after the first cycle was 22.5 ppm (77.8% removal). d At 32 °C for 1 h.

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CONCLUSIONS In conclusion, well-defined LDHNPs were synthesized by ligand-stabilization. Their particle sizes were controlled simply by the concentration of THAM. The CO32– species in the LDHNPs was easily exchanged with NO3– even in the presence of CO2. Moreover, the LDHNPs, in particular LDH(10 nm)-as, showed significant performance for removal of harmful oxyanions of arsenic, selenium, and boron from their dilute aqueous solutions. We hope that the present findings will contribute to the development of functionalized LDH-based materials that are useful for various applications such as anion exchangers, catalysts, and catalyst supports.

ASSOCIATED CONTENT Supporting Information. Composition of the LDHs (Table S1 and S2), TEM images of the LDHNPs (Figure S1), SEM images of the LDHNPs in wide area (Figure S2), LDHNPs prepared by using L-lysine (Figure S3), FTIR spectra of the LDHs (Figure S4), Langmuir plots of LDHs exchanged with NO3– (Figure S5), and LDHNPs prepared by using 2-amino-1,3-propanediol (Figure S6) are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT This work was supported in part by the Japan Society for the Promotion of Science through its FIRST Program and Grants-in-Aid for Scientific Researches from Ministry of Education, Culture, Sports, Science and Technology.

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(14) (a) Zhang, Y.; Wang, L.; Zou, L.; Xue, D. J. Cryst. Growth 2010, 312, 3367–3372; (b) Hibino, T. Appl. Clay Sci. 2011, 54, 83–89. (15)Wang, Q.; Zhang, X.; Zhu, J.; Guo, Z.; O'Hare, D. Chem. Commun. 2012, 48, 7450–7452. (16) Poorly crystalline LDHs showed reduced selectivity to CO32– in the presence of CO2, whereas size control by rapid coprecipitation is very difficult: Hongo, T.; Takahashi, M.; Yamazaki, A. Chem. Lett. 2006, 35, 1296–1297. (17) (a) Cushing, B. L.; Kolesnichenko, V. L.; O'Connor, C. J. Chem. Rev. 2004, 104, 3893–3946; (b) Mann, S. Nat. Mater. 2009, 8, 781–792; (c) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem. Int. Ed. 2009, 48, 60–103. (18) (a) Oaki, Y.; Imai, H. Angew. Chem. Int. Ed. 2007, 46, 4951–4955; (b) Nakamura, K.; Oaki, Y.; Imai, H. J. Am. Chem. Soc. 2013, 135, 4501–4508. (19) Gardner, E.; Huntoon, K. M.; Pinnavaia, T. J. Adv. Mater. 2001, 13, 1263–1266. (20) Morioka, H.; Tagaya, H.; Karasu, M.; Kadokawa, J.; Chiba, K. J. Solid State Chem. 1995, 117, 337–342. (21) Wang, J. D.; Serrette, G.; Tian, Y.; Clearfield, A. Appl. Clay Sci. 1995, 10, 103–115. (22) When 2-amino-1,3-propanediol (1.0 M solution) was used as a dipodal ligand, relatively larger LDHNPs (average size: 30.7 nm, σ = 20.0%) were obtained (Figure S6, Supporting Information). (23) Hasenknopf, B.; Delmont, R.; Herson, P.; Gouzerh, P. Eur. J. Inorg. Chem. 2002, 1081–1087. (24) THAM has been utilized for synthesis of silica nanoparticles: Choi, K.-M.; Kuroda, K. Chem. Commun. 2011, 47, 10933–10935. (25) Hibino, T.; Tsunashima, A. Chem. Mater. 1998, 10, 4055–4061.

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(36) The WHO guideline values of arsenic, selenium, and boron in drinking water are 0.01, 0.01, and 0.5 ppm, respectively, and the national effluent standard values of arsenic, selenium, and boron (noncoastal area) applied in Japan are 0.1, 0.1, and 10 ppm, respectively; see the websites of WHO (http://www.who.int/water_sanitation_health/dwq/gdwq3rev/en/) and Ministry of the Environment, Government of Japan (http://www.env.go.jp/en/water/wq/nes.html).

SYNOPSIS TOC

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Tripodal Ligand-Stabilized Layered Double Hydroxide Nanoparticles

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