Large-scale aqueous synthesis of layered double hydroxide single-layer nanosheets

Colloids and Surfaces A: Physicochem. Eng. Aspects 501 (2016) 49–54

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Large-scale aqueous synthesis of layered double hydroxide single-layer nanosheets Yaping Zhang a , Haiping Li b , Na Du a , Renjie Zhang a , Wanguo Hou a,b,∗ a b

Key Laboratory of Colloid and Interface Chemistry (Ministry of Education), Shandong University, Jinan 250100, PR China National Engineering Technology Research Center for Colloidal Materials, Shandong University, Jinan 250100, PR China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Layered double hydroxide single• • • •

layer nanosheets (SLNSs) were synthesized. The synthesis includes three steps: coprecipitation, washing, and redispersion. No organic modifiers or organic solvents were used in the synthesis. The SLNSs in dispersion remains stable for at least 2 d at room temperature. The SLNSs can be directly used as building blocks for functional materials.

a r t i c l e

i n f o

Article history: Received 24 February 2016 Received in revised form 15 April 2016 Accepted 19 April 2016 Available online 23 April 2016 Keywords: Layered double hydroxide Coprecipitation Nanosheet Basic building block

a b s t r a c t Inorganic layered double hydroxide (LDH) single-layer nanosheets (SLNSs) are traditionally synthesized with the assistance of organic chemicals or in organic solvents, resulting in organic-coated SLNSs or organic dispersions of SLNSs, which may limit their potential applications. Herein, we report, for the first time, a simple aqueous synthetic route to naked (uncoated) LDH SLNSs; this route, which is termed the PWD route for simplicity, includes three steps: aqueous coprecipitation, water-washing, and redispersion in water. The obtained LDH SLNSs were characterized using TEM, AFM, XRD, and dynamic light scattering techniques, and the stability of the LDH SLNS dispersions was determined. Moreover, the co-assemblies of the LDH SLNSs with negatively charged guests, sodium cholate and graphene oxide, were investigated. Results showed that the SLNSs can remain stable (i.e., without layer-by-layer stacking) for a period of time, depending on temperature, and can be directly used as building blocks for functional materials. This new route has many advantages including its simple operation, environmental friendliness, low cost, and ease of large-scale application. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The synthesis of ultimate two-dimensional (2D) nanosheets (NSs) of layered solids, such as metal chalcogenides, metal phos-

∗ Corresponding author at: Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Shandong University, Jinan 250100, PR China. E-mail address: [email protected] (W. Hou). http://dx.doi.org/10.1016/j.colsurfa.2016.04.046 0927-7757/© 2016 Elsevier B.V. All rights reserved.

phates or phosphonates, layered metal oxides, and layered double hydroxides (LDHs), has drawn considerable attention [1–6], this is because the anisotropy of NSs, with thicknesses of around one nanometer and lateral sizes ranging from submicrometer to several tens of micrometers, allows them to serve either as an ideal quantum system for fundamental studies or as a basic building block for functional materials. Among these solids, LDHs are one of the few layered materials with structural positively charged host layers [1].

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LDHs can be represented by the following general formula [1,2]: [MII 1−x MIII x (OH)2 ]x+ [An− x /n ]x− ·mH2 O. They consist of positively charged Brucite-type metal hydroxide layers and anionic counterions located between the layers. The synthesis of LDH NSs has attracted intense interest [1–4,7–12], owing to that the positively charged NSs have great potential as basic building blocks for constructing functional materials, such as catalysts [13], drug carriers [14], polymer–LDH nanocomposites [15], hydrogels [16], and electromagnetic materials [17], via charge-directed co-assembly with negatively charged basic building blocks. However, it was found that the synthesis of well-dispersed LDH NSs is difficult, due to their high structural charge density and high guest anion content, which leads to strong stacking of the LDH layers [2,4,18,19]. Therefore, inorganic LDH NSs are traditionally synthesized with the assistance of organic chemicals or in organic solvents, such as using delamination of organic intercalated LDHs in appropriate solvents [18–22] direct delamination of LDHs with NO3 − or ClO4 − as counter anions in formamide [4,23–27] and microemulsion method [28–31]. As a result, the LDH NSs obtained using these methods are either coated by organic chemicals or dispersed in organic solvents (such as toxic formamide [19]), which may limit their potential applications. From an application, environmental, and cost perspective, there remains an urgent need to develop aqueous direct synthetic methods for naked (i.e., uncoated) LDH SLNSs without using organic chemicals or solvents. Herein, we report a new route for synthesizing naked Mg2 AlNO3 LDH SLNSs. This route, which is called the PWD route for simplicity, comprises the following three steps: Aqueous co-precipitation, water-washing, and redispersion in water. The obtained LDH SLNSs can be directly used as basic building blocks to construct function materials via co-assembly with molecular or layered guests. To the best of our knowledge, there have been no reports on the synthesis of naked LDH SLNSs without using organic solvents or modifiers, although co-precipitation is a general route to get inorganic NSs. Compared with previously reported synthetic methods [7–12,18–31], this new route has many advantages including its simple operation, environmental friendliness, low cost, and ease of large-scale application. 2. Experimental

water via redispersion/centrifugation, yielding the LDH SLNS gel with a solid content (Cs ) of ∼8.5 wt%. A specific amount of the LDH SLNS gel (∼1.2 g) was redispersed in a predetermined amount of water (∼49 mL) by ultrasonication to produce an LDH SLNS dispersion (Cs = 2.0 g/L). For comparison, a portion of the LDH SLNS gel was peptized at 80 ◦ C for 24 h to produce a conventional LDH sol sample. 2.3. Co-assembly of LDH SLNSs and organic guests Co-assemblies of the LDH SLNSs with cholate (Ch) and graphene oxide (GO) were performed to explore the potential of the LDH SLNSs as building blocks for functional materials. Briefly, Ch aqueous solution (50 mL, 0.01 mol/L) or GO dispersion (50 mL, 0.4 g/L) was added to the LDH SLNS dispersion (50 mL, 2.0 g/L) under magnetic stirring at ambient temperature (∼25 ◦ C). The suspension was centrifuged at 12000 rpm for 10 min and then washed with water, resulting in LDH nanohybrids intercalated with organic Ch or GO guests, denoted as Ch-LDH or GO-LDH nanohybrids. 2.4. Characterization X-Ray diffraction (XRD) patterns were recorded on a D8 Advance diffractometer (Bruker, Germany) with Cu K␣ radiation ( = 0.15418 nm) at 40 kV and 40 mA. Small-angle X-ray scattering (SAXS) patterns were recorded on a SAXSess system (Anton-Paar, Austria) with Cu K␣ radiation operating at 50 kV and 40 mA. Transmission electron microscopy (TEM) images were collected on a JEM-1011 microscope (JEOL, Japan) operating at 120 kV. A Nanoscope IIIa Multimode atomic force microscope (AFM, Digital Instruments, USA) was used to examine the morphology of LDH NSs deposited on mica wafers. AFM images were acquired in tapping mode using a Si tip cantilever with a force constant of 40 N m−1 . The particle size distribution and zeta potential of the LDH dispersions were determined by dynamic light scattering (DLS) method, using a nano-ZS90 zetasizer analyzer (Malvern, UK). The test dispersions (∼0.1 wt%) were prepared by diluting the original samples using water and equilibrated for ∼1 h at ambient temperature before DLS measurements. The transmittance (Tr) of the LDH dispersions was measured on an SP-4100 UV–vis spectrometer (Shanghai Spectrum Instruments Co., Ltd., China) at  = 590 nm.

2.1. Chemicals 3. Results and discussion Analytical grade Al(NO3 )3 ·9H2 O, Mg(NO3 )2 ·6H2 O, and NH3 ·H2 O were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Sodium cholate hydrate (Ch; 99% purity) was purchased from Alfa Aesar. Graphene oxide (GO) was purchased from the Institute of Coal Chemistry, Chinese Academy of Sciences, China. All chemicals were used as received. The water used in the current study was purified using a Hitech-Kflow water-purification system (China). 2.2. Synthesis of LDH SLNSs Mg2 Al-NO3 LDH SLNSs were synthesized using the PWD route. In a typical procedure, a Mg(NO3 )2 /Al(NO3 )3 mixed solution with a total salt concentration of 0.3 mol/L and Mg/Al molar ratio of 2 was prepared by dissolving 3.75 g (0.01 mol) of Al(NO3 )3 ·9H2 O and 5.13 g (0.02 mol) of Mg(NO3 )2 ·6H2 O in 100 mL of water. An alkali solution (∼7 wt%) was prepared by diluting an NH3 ·H2 O solution (25–28 wt%) with water. The mixed salt solution (∼100 mL) and alkali solution (∼80 mL) were simultaneously added to a beaker within ∼10 min under magnetic stirring and N2 protection, during this process, the pH of the reaction system was held at ∼10 by controlling the relative addition rates of the two raw material solutions. After stirring for ∼10 min, the resultant precipitate was collected by centrifugation at 12000 rpm and washed thrice with

3.1. Synthesis of LDH SLNSs LDH SLNSs were synthesized by a PWD route that includes three steps: Aqueous coprecipitation, water-washing, and redispersion in water. The key to successfully obtaining LDH SLNSs is low (room)-temperature operation, to avoid the layer-by-layer stacking of the SLNSs formed. After co-precipitation and centrifugal water-washing, the obtained LDH SLNS gel had a low Cs of ∼8.5 wt%; even using a higher centrifugal speed, the Cs was also difficult to exceed 10 wt%, indicating that the SLNSs have a strong ability to bond (or solidify) water. Most likely, the SLNSs in the gel form a 3D ‘house-of-cards’ aggregate structure [32], the water molecules are hydrogen-bonded to hydroxyl groups of the SLNSs and, simultaneously, coordinated to anions (NO3 − ), forming a 3D hydrogen-bonding network [2,23]. Redispersion of the LDH SLNS gel (such as ∼1.2 g) in water (such as ∼49 mL) by manual shaking (or ultrasonication for 1 min) resulted in a milk-white dispersion (Cs = 2 g/L), as shown in Fig. 1A. Irregular aggregates consisting of curved ultrathin LDH sheets and with a size of ∼200–400 nm were observed using TEM in the gel dispersion (Fig. 2A, and Fig. S1A in the Supporting information, SI). After ultrasonication for ∼5 min, the milk-white dispersion

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NSs synthesized by the PWD route. Their crystalline nature was further determined by XRD and compared with that of the conventional peptized LDH platelets, as shown in Fig. 3. The peptized LDH platelets exhibit all the characteristic reflections of hydrotalcite (JCPDS card No. 51-1528), and their d003 value (d-spacing) is 0.87 nm (Fig. 3c), which is close to the reported value for Mg-AlNO3 LDHs [23,24]. The average crystallite size along the c-axis was estimated to be ∼10.3 nm from the (003) diffraction peak using the Debye-Scherrer equation. Therefore, a peptized LDH platelet is estimated to contain ∼12 Brucite-like layers. In contrast to the peptized LDH, the LDH SLNS gel showed no characteristic reflections of hydrotalcite in the low 2 range (Fig. 3a), and only a pronounced halo at a 2 of 25–45◦ from the glass support (Fig. S3 in the SI) Fig. 1. Photos of (A) SLNS gel dispersion (2 g/L), (B) LDH SLNS dispersion (2 g/L) with a clear Tyndall effect, and (C) LDH SLNS dispersion after standing for 6 d at ambient temperature.

became a transparent ‘solution’ (Fig. 1B). However, a clear Tyndall effect confirms the colloid nature of the solution [8,25]. TEM image showed well-dispersed and disk-like ultrathin LDH sheets with a faint contrast and an average lateral size of ∼80 nm (Fig. 2B). The selected area electron diffraction (SAED) pattern showed hexagonally arranged spots (Fig. 2C) corresponding to the (110) crystal plane [4], suggesting the single crystalline nature of the ultrathin LDH sheets. High resolution TEM (HR-TEM) images clearly showed the LDH sheet domain (Figs. 2 D and S1B in SI). The lattice fringe spacing of about 0.15 nm in Fig. 2D corresponds to (110) facets of the hexagonal phase LDH. Both the SAED and HR-TEM results indicate that the LDH sheets arose from the growth of the crystal nucleus along [110] direction with (001) facets exposed dominantly. These results are similar to previous reports on the delamination of conventional LDH platelets in formamide [4,14,33]. Besides, DLS analyses showed that the average hydrodynamic diameters (Dh ) of the particles in the milk-white and transparent dispersions were ∼250 and 80 nm with the polydispersity indexes (PDIs) of 0.233 and 0.205 (Fig. S2 in the SI), respectively, which are consistent with the TEM data. These results demonstrate that the milk-white dispersion is essentially a colloidal gel dispersion in which the dispersed gel particles consist of LDH SLNSs and solidified water. Under ultrasonication, the aggregated SLNSs in the gel particles disperse into discrete SLNSs, forming a transparent LDH SLNS dispersion. We noted that, after standing for ∼6 d at ambient temperature (∼25 ◦ C), the transparent dispersion again became semitransparent (Fig. 1C). TEM showed disk-like thick LDH platelets with an obviously high contrast and an average lateral size of ∼93 nm (Fig. 2E). This indicates that layer-by-layer stacking of the discrete SLNSs occurred. The close lateral sizes of the discrete SLNSs and thick platelets indicate that only stacking of SLNSs along the c-axis but no growth of LDH crystals along the a–b plane occurred at ambient temperature (∼25 ◦ C). For comparison, a conventional LDH sol sample was synthesized by peptizing the LDH SLNS gel at 80 ◦ C for 24 h. As expected, hexagonal thick LDH platelets were clearly observed in the peptized sample (Fig. 2F); the average lateral size and Dh of the peptized LDH particles, as determined from TEM and DLS (Fig. S2 in the SI), respectively, were very similar at ∼180 nm. The PDI value of the DLS measurement was 0.285. Evidently, peptizing the LDH SLNS gel at a high temperature (80 ◦ C) induces both stacking of LDH SLNSs along the c-axis and growth of LDH crystals along the a–b plane. 3.2. Characterization of LDH SLNSs The TEM images and SAED pattern shown in Fig. 2B–D preliminarily suggest the 2D single-crystalline nature of the LDH

Fig. 2. TEM images of dispersions of (A) LDH SLNS gel, (B) LDH SLNSs, (E) LDH SLNSs after stacking at ∼25 ◦ C, and (F) peptized LDH platelets; (C) HR-TEM image and (D) SAED pattern of LDH SLNSs; (G) AFM image and sectional analysis of the LDH SLNSs.

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100

b

(003)

80 Tr (%)

Intensity (a.u.)

a

d(110)=0.15 nm

(006) (009)

(110)

30

40

1.00 g/L 5.00 g/L 10.0 g/L

0

JCPDS 51-1528

20

60

20

(113)

c 10

30 ºC

(A)

0 40 2θ (°)

50

60

40

70

(B)

Fig. 3. XRD patterns of (a) LDH gel, (b) LDH gel after storage at 4 ◦ C for 20 d, and (c) peptized LDH.

3.3. Stability of LDH SLNSs Understanding the dispersion, stability, and layer-by-layer stacking behaviors of LDH SLNSs in water is important for their practical applications. To monitor these processes, we determined the change in the Tr of LDH SLNS gel dispersions with various Cs values, obtained by either manual shaking (Fig. 4A) or 30 min of ultrasonication (Fig. 4B), with respect to standing time (tst , without mechanical stirring or manual shaking) at different temperatures (Tst ). The results showed that, with increasing tst , the change of Tr for the LDH SLNS gel dispersions can be divided into three stages: initial increase, near plateau, and final decrease, as marked with the dotted lines in Fig. 4A. The initial increase and final decrease stages correspond to the dispersion of aggregated LDH SLNSs and stacking of discrete SLNSs, respectively. The driving force of the dispersion of aggregated LDH SLNSs is probably the electrostatic repulsion between the LDH SLNSs, and that of the layer-by-layer stacking of discrete SLNSs is probably the electrostatic attraction

120

160

5.00 g/L

80 Tr (%)

was observed. The absence of LDH Bragg reflections indicates the lack of a crystal-plane stacking structure [2,12]. However, the (110) and (113) reflections at a 2 of ∼61◦ are apparent and indicate the formation of 2D crystalline LDH layers [24,25], which is consistent with the SAED and HR-TEM results (Fig. 2C and D). The d110 value is 0.15 nm, which indicates that the lattice parameter a value (a = 2d110 ) [34] of the 2D crystalline LDH is 0.30 nm. These XRD results are in good agreement with those of delaminated LDH NSs [28−31], thereby demonstrating that well-crystallized LDH SLNSs formed during aqueous coprecipitation. Subsequent peptizing treatment of the LDH SLNS gel induced stacking of the LDH SLNSs along the c-axis. The single-layer characteristics of the LDH NSs were revealed using AFM. The AFM image (Fig. 2G) showed that the ultrathin LDH sheets are ∼0.8 nm thick (Fig. S1C in the SI), which agrees well with reported values for formamide-delaminated LDH NSs [12,23,26,35]. Owing to the fact that the thickness of a Brucitelike layer is ∼0.48 nm and the adsorption layer of counter-anions (NO3 − ) on the NS surface is ∼0.3 nm thick [23,26], the AFM data suggest that the ultrathin LDH sheets consist of a single Brucitelike layer [4,12]. That is, we successfully synthesized Mg2 Al-NO3 LDH SLNSs using the simple PWD route. The Mg/Al ratio of the LDH SLNSs was determined via energy dispersive X-ray spectroscopy analyses to be ∼1.89, which is close to the initial stoichiometry (2.0), the O/Mg ratio is ∼3.9, which is consistent with the expected value [23,12]. In addition, the PWD route described above was expanded to synthesize Mg2 Al-Cl, Zn2 Al-NO3 , Mg2 Fe-NO3 , and Ni2 Al-NO3 LDH SLNSs. Similar results were obtained (Figs. S4 and S5F igs. S4 and S5 in the SI), indicating that the PWD route is versatile for synthesizing LDH SLNSs.

80 tst (h)

30 ºC 40 ºC 50 ºC 60 ºC 70 ºC 80 ºC

60 40 20

1

10 tst (h)

100

Fig. 4. Change in transmittance versus standing time for LDH SLNS gel dispersions (A) with various solid contents at 30 ◦ C without ultrasonication and (B) with a solid content of 5.00 g/L at various temperatures after 30 min of ultrasonication.

between the LDH SLNSs and the counter anions (NO3 − ). In the second stage, the lack of obvious change in the Tr value reveals that the well-dispersed SLNSs are stable (i.e., do not undergo layer-by-layer stacking). The existence of a stable period for SLNSs is important or essential for their practical applications. This dynamic stability of LDH SLNS dispersions can be attributed to the electrostatic repulsion between the LDH SLNSs and strong hydration of the LDH SLNSs. The zeta potential of the LDH SLNSs was measured at a pH of 9 to be 27.8 mV. In addition, an increasing Cs induces an increase in the duration of the first stage (tD ), but appears to have no effect on the duration of the second stage (tS ). Ultrasonication and high Tst values can dramatically decrease the tD and tS values, respectively, as shown in Fig. 4B and Fig. S6 in the SI. For example, at the same Tst of 30 ◦ C, the tD values of the LDH dispersions with a Cs of 5.00 mg/L obtained by manual shaking and 30 min of ultrasonication are ∼38 and 4 h, respectively; with increasing Tst from 30 to 80 ◦ C, the tS value of the LDH dispersion with a Cs of 5.00 mg/L obtained by 30 min of ultrasonication decreased from ∼60 h (or ∼2.5 d) to less than 0.5 h. We noted that, at a Tst of 40 ◦ C, the LDH SLNSs in dispersions with Cs values in the range of 5−20 g/L can remain stable for ∼10 h. From the perspective of practical applications, a Tst greater than 40 ◦ C should be avoided so that a relatively long stable period is achieved for unhurried operations. Moreover, the above results indicate that the dispersion state and stability of LDH SLNSs can be determined by monitoring the Tr change of the dispersion [24], the highest Tr value corresponds to their fully discrete state. Notably, for the LDH SLNS gel dispersions with high Cs values (>2 g/L), a standing period after ultrasonication is commonly required to achieve fully dispersed states, and can be used to avoid an excessively long ultrasonication period. Furthermore, we found that, under storage at 4 ◦ C in a refrigerator, the LDH SLNS gel can remain stable for a long time (∼20 d). After storage at 4 ◦ C for 20 d, no obvious changes in the XRD patterns of the SLNS gels were observed (Fig. 3b); in addition, the Tr values and particle morphologies of the LDH SLNS dispersions obtained from the stored gel did not change relative to those of the freshly prepared gel. With further storage time, the Tr values of the LDH SLNS dispersions obtained from the stored gel began to

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Scheme 1. Schematic illustration of the formation mechanism of LDH SLNSs.

3.4. Co-assembly of LDH SLNSs and organic guests To explore the potential of the LDH SLNSs as building blocks for functional materials, two examples of guest incorporation, i.e., coassemblies of the LDH SLNSs with the molecular guest Ch and the nanosheet guest GO, were investigated. Mixing the LDH SLNS dispersion with Ch solution or GO dispersion immediately generated a cloudy solution, indicating the occurrence of a co-assembly process. The driving force for the co-assembly is electrostatic attraction between the positively charged LDH SLNSs and the negatively charged guests. SAXS analyses confirmed the formation of the targeted co-assemblies (Fig. 5). Bragg basal reflections appeared in the SAXS patterns of the Ch-LDH and GO-LDH nanohybrids. The dspacing values of the peptized LDH (d003 ) and GO (d001 ) were 0.86 and 0.67 nm, respectively, while those of the Ch-LDH and GO-LDH nanohybrids were 3.90 and 0.78 nm, respectively. The difference in the d-spacing between the nanohybrids and peptized LDH arises from intercalation of the guests into the LDH interlayers. These results are consistent with those in previous reports on the nanohybrids that have potential applications such as in drug carriers [14] and electrode materials [17], and thereby demonstrate that the obtained LDH SLNSs can be directly used as building blocks for functional materials. Also, these SLNSs can be functionally modified for more specific applications.

d001 = 0.78 nm

d Intensity (a.u.)

decrease and the LDH particles became more regular and thicker than fresh LDH SLNSs, suggesting that layer-by-layer stacking of the LDH SLNSs had occurred in the gel. An obvious change in the appearance of the LDH gel sample from a gel state to a slurry state was also observed when the storage time exceeded 25 d. The stable period of ∼20 d for LDH SLNSs in gel stored at 4 ◦ C is promising for their practical applications. The mechanisms for LDH formation using conventional coprecipitation have been proposed [36–38]. Boclair et al. [36,37] suggested that poorly crystallized LDH forms during coprecipitation and is then converted into well-crystallized LDH during subsequent peptizing (or aging). Eliseev et al. [38] believed that the agglomerates that form during coprecipitation are amorphous, but then convert into a layered structure during peptizing. Our observations suggest that well-crystallized LDH SLNSs are formed during co-precipitation and subsequent water-washing, which arises from the crystal habit of LDHs, and the SLNSs along with solidified water aggregate to form a gel. When the SLNS gel is redispersed in water, the aggregated SLNSs can gradually disaggregate into discrete SLNSs, as shown in Scheme 1. During the low (ambient)-temperature synthetic procedure, the electrostatic repulsion between the LDH SLNSs and the strong hydration of the LDH SLNSs can effectively delay the layer-by-layer stacking of the LDH SLNSs, thus we can obtain well-dispersed LDH SLNSs.

d001 = 0.67 nm

c

d003 = 3.90 nm

b d003 = 0.86 nm

a 0

5

10 15 -1 q (nm )

20

25

Fig. 5. SAXS patterns of the (a) peptized LDH, (b) Ch-LDH nanohybrid, (c) GO, and (d) GO-LDH nanohybrid.

4. Conclusions We report, for the first time, a simple synthesis of naked LDH SLNSs dispersed in water using the PWD route. The dispersion state and stability of LDH SLNSs can be determined by monitoring the Tr change of a dispersion; the highest Tr value corresponds to their fully discrete state. The obtained LDH SLNS dispersion can remain stable (i.e., without layer-by-layer stacking) for at least 2 d at ambient temperature. These SLNSs can be directly used as building blocks for functional materials. This new route has many advantages including its simple operation, environmental friendliness, low cost, and ease of large-scale application. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21573133, 21403128, and 21273135) and the Fundamental Research Funds of Shandong University in China (No. 12320075614004). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2016.04. 046. References [1] R. Ma, T. Sasaki, Nanosheets of oxides and hydroxides: ultimate 2D charge-bearing functional crystallites, Adv. Mater. 22 (2010) 5082–5104. [2] Q. Wang, D. O’Hare, Recent advances in the synthesis and application of layered double hydroxide (LDH) nanosheets, Chem. Rev. 112 (2012) 4124–4155. [3] V. Nicolosi, M. Chhowalla, M.G. Kanatzidis, M.S. Strano, J.N. Coleman, Liquid exfoliation of layered materials, Science 340 (2013) 1226419. [4] F. Song, X. Hu, Exfoliation of layered double hydroxides for enhanced oxygen evolution catalysis, Nat. Commun. 5 (2014) 4477.

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