Synthesis of Layered Double Hydroxide Single-Layer Nanosheets in

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Synthesis of Layered Double Hydroxide Single-Layer Nanosheets in Formamide Jingfang Yu,† Jingjing Liu,‡ Abraham Clearfield,§ Johnathan E. Sims,∥ Michael T. Speiegle,∥ Steven L. Suib,‡,⊥ and Luyi Sun*,†,‡ †

Department of Chemical & Biomolecular Engineering, ‡Institute of Materials Science, and ⊥Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269, United States § Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States ∥ Department of Chemistry and Biochemistry, Texas State University, San Marcos, Texas 78666, United States S Supporting Information *

ABSTRACT: Layered double hydroxide (LDH) single-layer nanosheets were synthesized through a single-step process in the presence of formamide. This one-step process is simple, fast, and efficient and thus is potentially viable for large-scale production. Two key factors for the growth of LDH single-layer nanosheets, formamide concentration and LDH layer charge, were investigated thoroughly. A higher formamide concentration and a higher LDH layer charge are favorable for the growth of LDH single-layer nanosheets. The LDH single-layer nanosheets obtained at the premium formamide concentration and LDH layer charge were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and atomic force micrscopy (AFM). Poly(vinyl alcohol) (PVA)/LDH nanocomposite coatings were also prepared. The coated polyethylene terephthalate (PET) and poly(lactic acid) (PLA) films exhibited significantly improved oxygen gas barrier properties thanks to the well-dispersed and -aligned LDH single-layer nanosheets in the coating.

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INTRODUCTION Layered double hydroxides (LDHs, [M2+1−xM3+x(OH)2]x+[Ap‑x/p]·mH2O) with positively charged layers compensated by interlayer anions belong to the family of anionic clays with a wide selection of metal cations (M2+ and M3+) and interlayer anions (Ap−).1 Recently, exfoliation of LDHs into atomic-thick single-layer nanosheets has attracted much attention mainly because of their applications as building blocks to fabricate a wide range of nanostructured materials, including polymer nanocomposites,2 free-standing thin films,3 and hydrogels.4 Because of their tunable chemical compositions and two-dimensional (2D) morphology with a molecular thickness, LDH single-layer nanosheets are considered as one of the most ideal systems that can serve as a model for the physical and chemical studies of nanostructured materials. There are two categories of methods to prepare LDH singlelayer nanosheets: (1) a top-down approach to exfoliate the preformed LDH layered compounds5 and (2) a bottom-up synthesis of LDH single-layer nanosheets.5,6 LDH can be exfoliated in organic solvents including butanol,7 formamide,8 and toluene.9 Existing methods to delaminate LDH were pioneered by Adachi-Pagano et al.10 and Hibino et al.,8a who developed an intercalation-induced exfoliation method. Before exfoliation, LDH interlayer space is typically modified with © XXXX American Chemical Society

amino acids and surfactants, for example, to uptake a large amount of solvent molecules by a synergistic attractive force formed between them.5,8b Later, many researchers expanded this exfoliation method to apply different intercalation agents and dispersants to achieve a desirable interlayer environment for delamination.5 The exfoliation of ion-exchanged LDHs in formamide developed by Li et al.11 was a breakthrough. Nitrateexchanged magnesium aluminum LDH (MgAl-LDH) was exfoliated through vigorous mechanical agitation for 2 days. This mechanical force-initiated exfoliation does not require preintercalation of larger anions to expand the interlayer distance and has been widely adopted and applied in obtaining various LDH single-layer nanosheets.12 However, overall the aforementioned top-down exfoliation processes are complicated, time consuming,8b,13 and labor intensive. The recent trend to directly synthesize LDH single-layer nanosheets is attracting rising interests due to its simplicity. This bottom-up approach opened up a new route where precursors (metal salts and hydroxides) are converted to LDH single-layer nanosheets directly controlled by chemical reactions and/or physical processes in one or two steps. Received: September 14, 2016

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DOI: 10.1021/acs.inorgchem.6b02203 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Various methods including reverse microemulsion,14 laser ablation,15 and one-step direct synthesis6 have been explored. Hu et al.14 reported a one-step synthesis of LDH single-layer nanosheets using a reverse microemulsion system. The traditional coprecipitation method was introduced into an oil phase of isooctane with sodium dodecyl sulfate as surfactant and 1-butanol as cosurfactant to prepare LDH single-layer nanosheets. However, the complex purification process to remove anionic surfactants limits its potential mass production. Recently, our group explored the direct synthesis of MgAl-LDH single-layer nanosheets by using formamide molecules as inhibitors to suppress the growth of LDH in the third dimension (Z direction).6 Formamide molecules were adsorbed on the surface of LDH layers through a strong interaction formed by the carbonyl functionality of formamide.16 MgAlLDH single-layer nanosheets were successfully synthesized through this facile method. In this work, we further explored this method. In particular, we aimed to understand the correlation between the degree of inhibition and the formation of LDH single-layer nanosheets. Both formamide concentration and LDH layer charge directly affect the degree of inhibition and thus were investigated in detail. The formamide molecules adsorbed on LDH layer surfaces are expected to act as a shield to weaken the electrostatic interactions between the positively charged LDH layers and the surrounding anions. Thus, a higher concentration of formamide is hypothesized to be more favorable for the growth of LDH single-layer nanosheets. Meanwhile, a stronger interaction induced by a higher LDH layer charge can probably similarly promote the formation of LDH single-layer nanosheets. Potential application of the LDH single-layer nanosheets synthesized through this direct growth method was also briefly explored.

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uniform dispersion formed after 1 h of magnetic stirring and 1 h of ultrasonication treatment (Branson 8510R-MT, 250 W, 44 kHz). The film substrates (ca. 9 cm × 13 cm) were dip coated and subsequently heated in an oven at 60 °C for 30 min to dry. Characterization. The gel-like fresh samples recovered from centrifugation were covered by a Mylar film6 for X-ray diffraction (XRD) characterization on a Bruker D8 diffractometer with Cu Kα radiation (λ = 1.5406 Å, 40 kV, 40 mA). After further treatment with water and centrifugation twice, the LDH nanosheets were well dispersed in water. The dispersion was cast onto a silicon wafer and dried at 80 °C, during which the LDH nanosheets were restacked for XRD characterization. The same LDH nanosheet dispersion was then diluted into ca. 0.01 mM and cast and dried on a clean silicon wafer for atomic force microscopy (AFM) characterization on an Asylum Research MFP-3d AFM. Tapping mode with a silicon tip coated with chromium/gold with a force constant of 40 N/m was adopted for AFM imaging. The same water dispersion sample was also cast on Cu grids for TEM imaging on a JEOL 4000EX microscope at 200 kV. A PerkinElmer Optima 7300DV inductively coupled plasma optical emission spectrometer (ICP/OES) was used to determine the Mg and Al content in the synthesized LDH. The morphology of the LDH nanosheets in the PVA/LDH nanocomposite coating was characterized on a FEI Tecnai T12 TEM with an accelerating voltage of 120 kV. The coated PET films were first embedded into epoxy, and slices of ca. 80−100 nm thickness were cut on a Reichert-Jung Ultracut E ultramicrotome from the embedded epoxy sample. The slices were deposited on 400-mesh copper grids for imaging. Oxygen transmission rates (OTRs) were tested on a MOCON (Minneapolis, MN) OX-TRAN 1/50 OTR tester (ASTM D3985) at 23 °C and 0% RH. Water vapor transmission rates (WVTRs) were tested on a MOCON PERMATRAN-W 1/50 WVTR tester (ASTM E398) at 23 °C and 50% RH.

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RESULTS AND DISCUSSION LDH layer charge is originated from the substitution of M2+ by M3+ in brucite structure.1,17 Formamide molecules form a strong interaction with the charged LDH sheets through electrostatic interactions. Thus, both of the factors, the LDH layer charge and the formamide concentration, are critical for the formed electrostatic interactions between formamide and LDH layers. To study the formamide concentration and layer charge effect on the synthesis of LDH single-layer nanosheets, an initial trial was performed where samples of three Mg/Al ratios, 2/1, 3/1, and 4/1, were prepared at 2.0 vol % formamide. The samples were first prepared and ultrasonicated for 3 min, and then they were kept motionless for 1 h (Figure S1). The result showed a clear trend. The stability of the samples decreased with an increasing Mg/Al molar ratio. The Mg/Al = 4/1 ratio sample started to precipitate almost immediately. The Mg/Al = 3/1 ratio sample precipitated slower than the Mg/Al = 4/1 ratio sample. Also, the Mg/Al = 2/1 ratio sample did not show any sign of precipitation until 2 h later. This initial observation indicates that a lower Mg/Al ratio, which corresponds to a higher layer charge x+ (x+ = M3+/(M2+ + M3+)),1,18 can better facilitate the formation of LDH singlelayer nanosheets instead of a layered structure. At a higher layer charge, the interactions between the formamide molecules and the LDH layers should be stronger, which in return can promote the adsorption of a higher density of formamide molecules on the layer surface. Thus, the inhibition effect of formamide should be stronger, favoring the formation of LDH single-layer nanosheets.

EXPERIMENTAL SECTION

Chemicals. Mg(NO3)2·6H2O (98%, Alfa Aesar), formamide (99%, Alfa Aesar), Al(NO3)3·9H2O (99%, Acros Organics), sodium hydroxide (98%, Macron), sodium nitrate (>98%, Alfa Aesar), and poly(vinyl alcohol) (PVA) (Mowiol 8-88, Mw 67 000, 86.7−88.7 mol % hydrolysis, Kuraray, Japan) were used as received without further purification. Polylactic acid (PLA) films (20 μm in thickness, BI-AX International Inc.) and polyethylene terephthalate (PET) films (ca. 24 μm in thickness, Toray Plastics (America), Inc.) were used as the coating substrates. Synthesis Method. Three sets of samples were prepared with Mg/Al formulation molar ratios at 4/1, 3/1, and 2/1. The total metal salt concentration was set at 0.05 M for all three sets of experiments for comparison. Three samples of 20.0 mL of metal salt solution [Mg(NO3)2 (0.0400 M) and Al(NO3)3 (0.0100 M) for Mg/Al = 4/1, Mg(NO3)2 (0.0375 M) and Al(NO3)3 (0.0125 M) for Mg/Al = 3/1, and Mg(NO3)2 (0.0333 M) and Al(NO3)3 (0.0167 M) for Mg/Al = 2/1] were titrated with sodium hydroxide solution in the presence of 2.0, 5.0, 15.0, or 30.0 vol % formamide. After reaction, all samples were centrifuged and washed with the same volumetric percentage of formamide solution (i.e., 2.0, 5.0, 15.0, or 30.0 vol % formamide solution accordingly) three times. After centrifugation, a gel-like sample was obtained and ready to be dispersed in water or formamide solution for characterization. Restacked samples were obtained after drying water-dispersed samples on a silicon wafer. In this study, LDH single-layer nanosheet aqueous dispersions at a concentration of ca. 0.025 g/mL were prepared as a stock solution. PVA stock solution (10.0 wt %) was prepared by dissolving PVA resin in water. The PVA/LDH nanocoating dispersion was prepared by mixing PVA solution and LDH single-layer aqueous dispersion. A B

DOI: 10.1021/acs.inorgchem.6b02203 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. XRD patterns of MgAl-LDH prepared and washed in (A) 2.0, (B) 5.0, (C) 15.0, and (D) 30.0 vol % formamide (inset in A, B, and C denotes the zoomed patterns in the 2θ range of 8−13°).

To systematically investigate the observed phenomenon as well as to study the formamide concentration effect, three additional sets of samples were prepared where three Mg/Al ratios, 2/1, 3/1, and 4/1, were investigated under 5.0, 15.0, and 30.0 vol % formamide. All prepared samples were characterized by XRD (Figure 1). The Mg/Al = 4/1 ratio sample showed a relatively strong LDH characteristic peak at ca. 11.57° (corresponding to LDH layered structure)19 at 2.0 vol % formamide (Figure 1A). The Mylar film exhibits a strong diffraction peak at ca. 25.8°, ideally serving as an internal reference for our investigation. The ratio of the LDH characteristic peak to the PET peak decreased with a decreasing Mg/Al molar ratio, which corresponds well with the aforementioned observation that a lower Mg/Al ratio (a higher LDH layer charge) favors the formation of LDH single-layer nanosheets. When the three samples were prepared in 5.0 and 15.0 vol % formamide, the same trend was observed (shown in Figure 1B and 1C). At a formamide concentration of 30.0 vol %, none of the three samples exhibited the LDH characteristic peak (Figure 1D), indicating the lack of a long-range ordering. Thus, LDH layered structure did not exist or existed at a very low concentration, below the detection limit of XRD. Effect of Formamide Concentration. In theory, at any given LDH layer charge, a higher formamide concentration will lead to stronger interactions with the charged LDH layers and thus a more significant layer growth inhibition effect. The diffraction peak from Mylar film was used as an internal reference to estimate the concentration change of the layered LDH in the samples (data in Figure 1). As shown in Figure 2, the Y axis represents the ratio of LDH characteristic peak (i.e., LDH layered structure) area to the Mylar film internal reference peak area. The randomly distributed LDH singlelayer nanosheets could not induce X-ray diffraction. Thus, the Y axis roughly represents the concentration scale of layered LDH in the samples.

Figure 2. Summary of LDH characteristic peak to internal reference peak area ratios.

When the formamide concentration was low (2.0 vol %, black square shown in Figure 2), the concentration of layered LDH was the highest for all three Mg/Al ratios. With an increasing formamide concentration (5.0 vol %, red sphere; 15.0 vol %, blue triangle in Figure 2), the concentration of layered LDH decreased for all three Mg/Al ratios. On further increasing the formamide concentration to 30.0 vol % (pink obtriangular in Figure 2), no layered LDH was detected by the X-ray diffraction for all three samples, indicating that the layered LDH was not formed or below the detection limit. Thus, at 30.0 vol % formamide, LDH single-layer nanosheets can be prepared with virtually no layered LDH regardless of Mg/Al ratio. Thus, it is clear that a high formamide concentration strongly favors the formation of LDH singlelayer nanosheets. Effect of LDH Layer Charge. At the same formamide concentration (2.0, 5.0, and 15.0 vol % as shown in Figure 2), the concentration of layered LDH decreased when the Mg/Al C

DOI: 10.1021/acs.inorgchem.6b02203 Inorg. Chem. XXXX, XXX, XXX−XXX

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above results, a formulation with a molar ratio of Mg/Al = 2/1 in combination with 30.0 vol % of formamide are most favorable for growth of LDH single-layer nanosheets. As such, the LDH single-layer nanosheets synthesized from such a condition (Mg2Al-LDH) were selected for further characterization. Figure 4A shows the XRD pattern of the restacked Mg2AlLDH nanosheets on a silicon wafer. The d spacing of 7.8 Å agreed very well with the reported interlayer distance of Mg2AlLDH with CO32− residing in the interlayer region.21 The Mg2Al-LDH nanosheets prepared in 30.0 vol % formamide clearly exhibited a sheet-like structure as shown in Figure 4B. AFM imaging was performed to measure the thickness of the samples. Figure 5A and 5B shows that the diameter of the LDH

ratio was lowered, which suggests that a higher LDH layer charge promotes the formation of LDH single-layer nanosheets instead of layered LDH. While the layer charge of the synthesized LDH is usually well correlated with the Mg/Al formulation ratio in the starting materials,1 the exact layer charge was characterized based on elemental analysis. ICP analysis was performed to determine the exact Mg and Al content in each synthesized LDH sample, as the Mg/Al ratio in the LDH product usually slightly varies from the initial formulation ratio.20 The test results as well as the calculated Mg/Al atomic ratio and layer charge (x) are listed in Table S1. The calculated Mg/Al molar ratios and layer charges were ploted as a function of initial Mg/Al molar ratio and are shown in Figure 3A and 3B. Overall, the measured Mg/

Figure 3. Summary of (A) Mg/Al measured molar ratios and (B) MgAl-LDH layer charge at different Mg/Al formulation molar ratios and formamide concentrations.

Al ratios of all samples agreed well with the initial Mg/Al formulation ratios. The calculation results showed that at Mg/ Al formulation molar ratios of 2/1 and 3/1, the respective actual Mg/Al ratio in the samples decreased (Figure 3A), where the layer charge x+ (Figure 3B) increased as the formamide concentration was increased. For a Mg/Al formulation molar ratio at 4/1, the trend was not so obvious as the results were close to each other where the difference may just come from testing error. Overall, the results agreed well with XRD characterization where a higher formamide concentration and a higher LDH layer charge favored the formation of LDH singlelayer nanosheets. It indicated that at a high LDH layer charge more formamide molecules can be absorbed on the layer surface to enhance the blocking effect during layer growth. Characterization of Mg2Al-LDH Nanosheets Synthesized from 30.0 Vol % Formamide. On the basis of the

Figure 5. Representative AFM images of Mg2Al-LDH prepared in 30.0 vol % formamide.

nanosheets ranged from ca. 40−60 to ca.100−150 nm. Regardless of the diameter of the LDH nanosheets, the average thickness was ca. 0.8 nm, which corresponds to the thickness of a single-layer LDH sheet with surface-absorbed anions and formamide as reported.11,16 Figure 5B (cross-section line 1) shows a configuration composed of a double-layered structure where a thickness transition from ca. 1.5 to 0.8 nm was observed. The mismatch of the sheet area suggests that the double layer was from the stacking of two individual single-layer nanosheets during AFM sample preparation instead of a

Figure 4. XRD pattern of Mg2Al-LDH nanosheets (A) from 30.0 vol % formamide restacked on silicon wafer and the corresponding TEM image (B). D

DOI: 10.1021/acs.inorgchem.6b02203 Inorg. Chem. XXXX, XXX, XXX−XXX

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aligned on the substrate. Overall, the TEM observation was consistent with the XRD pattern, both of which suggested a high level of alignment of LDH single-layer nanosheets along the polymer film surface. The coated PET films were tested at 1 atm for OTR and WVTR. The results showed that the gas barrier properties of the coated films were significantly improved. The testing results are summarized in Table 1. The OTR of neat PET film was

synthesized double-layered LDH. Overall, XRD and AFM characterization confirmed that LDH single-layer nanosheets were synthesized. Preparation of PVA/LDH Nanocoatings on PET and PLA. Inorganic nanosheets can effectively block the transport of small molecules; thus, they have been widely used to mix with polymers to prepare composite barrier materials for various applications such as food packaging.22 In general, polymers are permeable to gas molecules, but inorganic nanosheets are assumed to be impermeable; thus, gas molecules are forced to circle around inorganic nanosheets to diffuse through the film, which results in a very tortuous and thus longer pathway (Figure 6).23 On the basis of both simulation

Table 1. Barrier Properties of the Coated PET and PLA Films samples PET PET coated with layered LDH (control) PET coated with LDH singlelayer nanosheets PLA PLA coated with layered LDH (control) PLA coated with LDH singlelayer nanosheets

Figure 6. Comparison of gas molecule passing through a neat polymer film (left) and a polymer film containing a high concentration of wellaligned impermeable inorganic nanosheets.

OTR [mL/(m2·day)]

WVTR [g/(m2·day)]

64.0 8.8

4.1 3.9

5.3

2.6

1205.0 13.7

98.2 38.0

3.3

22.1

64.0 mL/(m2·day), which dropped significantly to 5.3 mL/(m2· day) after the PET film was coated with a very thin layer of nanocoating. At the same time, the WVTR of the coated PET was decreased to 2.6 g/(m2·day) from 4.1 g/(m2·day) of the uncoated PET film. In addition to PET, we also coated PLA films for the same barrier performance evaluation. Similarly, the OTR of the coated PLA film was 3.3 mL/(m2·day), which decreased from 1205.0 mL/(m2·day) of the uncoated PLA film. The WVTR of the coated PLA was decreased to 22.1 g/(m2·day) from 98.2 g/ (m2·day) of the uncoated PLA film. PVA is very hydrophilic, which might be one of the key reasons that the water vapor barrier performance improvement was not as impressive as the oxygen barrier. Compared to the films coated with conventional layered LDH synthesized in the absence of formamide (Table 1), the films coated with LDH single-layer nanosheets exhibited much better barrier properties owing to the much higher aspect ratio of the LDH single-layer nanosheets.

models and experimental results, a high concentration of wellaligned LDH nanosheets with a high aspect ratio is the key to achieve the highest possible barrier property. The single-layer Mg2Al-LDH nanosheets with a high aspect ratio synthesized in 30 vol % formamide appear to be an ideal candidate to form barrier coatings. Thus, they were adopted for the preparation of a PVA/LDH nanocomposite barrier coating on PET films as a brief demonstration. The PET films remained highly transparent after being coated, and the coating layer is invisible under naked eyes. The XRD pattern of the coated PET (Figure 7) showed a strong

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CONCLUSIONS Two critical preparation conditions concerning the LDH layer charge and formamide concentration were explored to prepare LDH single-layer nanosheets. The results shown above indicate that a high concentration of formamide and a high LDH layer charge are beneficial for the formation of single-layer LDH nanosheets, where more formamide can be absorbed on the layer surface to weaken the electrostatic attractions between the anions and the positively charged LDH layers. A relatively low concentration of formamide (such as 2.0, 5.0, and 15.0 vol %) may not be sufficient to completely block the growth of LDH in the Z direction regardless of Mg/Al molar ratio, resulting in the formation of layered LDH as evidenced by the XRD characterization. On the other hand, LDH layer growth in the Z direction was effectively inhibited at a higher formamide concentration (30.0 vol %), where no layered LDH was detected by XRD. Elemental analysis was adopted to measure the Mg and Al content in each sample, and the result agrees well with XRD results that at a higher LDH layer charge LDH single-layer nanosheets can be better synthesized with less layered LDH present. AFM characterization confirmed the

Figure 7. XRD pattern of the coated PET film (inset: TEM image of the cross-section of the PVA/LDH nanocomposite coating).

diffraction peak at 2.85° corresponding to an interlayer distance of 31.1 Å. Such an intensive and narrow first-order diffraction peak indicates that the LDH single-layer nanosheets were well aligned on the substrates. In addition, the very low interlayer distance of 31.1 Å suggested that the nanosheets were closely packed. Both the high level of orientation and the dense packing are very critical to achieve excellent barrier properties. The strong diffraction peak at 26.00° is from the semicrystalline structure of PET. TEM images were obtained to further assess the alignment of the LDH single-layer nanosheets in the nanocoatings. As shown in Figure 7, inset, LDH single-layer nanosheets were well E

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(9) (a) Jobbagy, M. I.̀ a.; Regazzoni, A. E. Delamination and restacking of hybrid layered double hydroxides assessed by in situ XRD. J. Colloid Interface Sci. 2004, 275, 345−348. (b) Naik, V. V.; Ramesh, T. N.; Vasudevan, S. Neutral Nanosheets that Gel: Exfoliated Layered Double Hydroxides in Toluene. J. Phys. Chem. Lett. 2011, 2, 1193−1198. (10) Adachi-Pagano, M.; Forano, C.; Besse, J.-P. Delamination of layered double hydroxides by use of surfactants. Chem. Commun. 2000, 91−92. (11) Li, L.; Ma, R.; Ebina, Y.; Iyi, N.; Sasaki, T. Positively Charged Nanosheets Derived via Total Delamination of Layered Double Hydroxides. Chem. Mater. 2005, 17, 4386−4391. (12) (a) Liu, Z.; Ma, R.; Ebina, Y.; Iyi, N.; Takada, K.; Sasaki, T. General Synthesis and Delamination of Highly Crystalline TransitionMetal-Bearing Layered Double Hydroxides. Langmuir 2007, 23, 861− 867. (b) Liu, Z.; Ma, R.; Osada, M.; Iyi, N.; Ebina, Y.; Takada, K.; Sasaki, T. Synthesis, Anion Exchange, and Delamination of Co-Al Layered Double Hydroxide. Assembly of the Exfoliated Nanosheet/ Polyanion Composite Films and Magneto-Optical Studies. J. Am. Chem. Soc. 2006, 128, 4872−4880. (c) Ma, R.; Liu, Z.; Takada, K.; Iyi, N.; Bando, Y.; Sasaki, T. Synthesis and Exfoliation of Co2+−Fe3+ Layered Double Hydroxides: An Innovative Topochemical Approach. J. Am. Chem. Soc. 2007, 129, 5257−5263. (13) Liu, Z.; Ma, R.; Osada, M.; Iyi, N.; Ebina, Y.; Takada, K.; Sasaki, T. Synthesis, Anion Exchange, and Delamination of Co−Al Layered Double Hydroxide: Assembly of the Exfoliated Nanosheet/Polyanion Composite Films and Magneto-Optical Studies. J. Am. Chem. Soc. 2006, 128, 4872−4880. (14) Hu, G.; Wang, N.; O’Hare, D.; Davis, J. One-step synthesis and AFM imaging of hydrophobic LDH monolayers. Chem. Commun. 2006, 287−289. (15) Hur, T.-B.; Phuoc, T. X.; Chyu, M. K. New approach to the synthesis of layered double hydroxides and associated ultrathin nanosheets in de-ionized water by laser ablation. J. Appl. Phys. 2010, 108, 114312. (16) Ma, R.; Liu, Z.; Li, L.; Iyi, N.; Sasaki, T. Exfoliating layered double hydroxides in formamide: a method to obtain positively charged nanosheets. J. Mater. Chem. 2006, 16, 3809−3813. (17) Leroux, F.; Moujahid, E. M.; Taviot-Guého, C.; Besse, J.-P. Effect of layer charge modification for Co−Al layered double hydroxides: study by X-ray absorption spectroscopy. Solid State Sci. 2001, 3, 81−92. (18) Marappa, S.; Radha, S.; Kamath, P. V. Nitrate-Intercalated Layered Double Hydroxides − Structure Model, Order, and Disorder. Eur. J. Inorg. Chem. 2013, 2013, 2122−2128. (19) Yang, W.; Kim, Y.; Liu, P. K.; Sahimi, M.; Tsotsis, T. T. A study by in situ techniques of the thermal evolution of the structure of a Mg−Al−CO 3 layered double hydroxide. Chem. Eng. Sci. 2002, 57, 2945−2953. (20) (a) Kaneyoshi, M.; Jones, W. Formation of Mg-Al layered double hydroxides intercalated with nitrilotriacetate anions. J. Mater. Chem. 1999, 9, 805−811. (b) Hibino, T.; Tsunashima, A. Synthesis of Paramolybdate Intercalates of Hydrotalcite-like Compounds by Ion Exchange in Ethanol/Water Solution. Chem. Mater. 1997, 9, 2082− 2089. (21) Olanrewaju, J.; Newalkar, B. L.; Mancino, C.; Komarneni, S. Simplified synthesis of nitrate form of layered double hydroxide. Mater. Lett. 2000, 45, 307−310. (22) (a) Compton, O. C.; Kim, S.; Pierre, C.; Torkelson, J. M.; Nguyen, S. T. Crumpled Graphene Nanosheets as Highly Effective Barrier Property Enhancers. Adv. Mater. 2010, 22, 4759−4763. (b) Xu, H.; Xie, L.; Wu, D.; Hakkarainen, M. Immobilized Graphene Oxide Nanosheets as Thin but Strong Nanointerfaces in Biocomposites. ACS Sustainable Chem. Eng. 2016, 4, 2211−2222. (23) Sun, L.; Boo, W.-J.; Clearfield, A.; Sue, H.-J.; Pham, H. Barrier properties of model epoxy nanocomposites. J. Membr. Sci. 2008, 318, 129−136.

formation of Mg2Al-LDH single-layer nanosheets. This detailed exploration is expected to facilitate the large-scale production of LDH single-layer nanosheets. The synthesized LDH single-layer nanosheets were successfully applied to prepare nanocoatings to improve polymer film barrier properties. The LDH single-layer nanosheets prepared in 30.0 vol % formamide with a Mg to Al initial molar ratio of 2 were selected to prepare PVA/LDH nanocoatings on PET and PLA. The oxygen and water vapor barrier properties can be greatly improved upon coating. Such high-quality LDH singlelayer nanosheets through a facile and low-cost top-down approach thus should be promising for many other applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02203. Elemental analysis data and additional data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: (860) 486-6895. Fax: (860) 486-4745. E-mail: luyi. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially support by the National Science Foundation (CMMI-1562907) and the Air Force Office of Scientific Research (FA9550-12-1-0159). A.C. thanks the Robert A. Welch Foundation (Grant No. A-0673) for support.

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DOI: 10.1021/acs.inorgchem.6b02203 Inorg. Chem. XXXX, XXX, XXX−XXX