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Preparation of montmorillonite nanosheets through freezing/thawing and ultrasonic exfoliation Tianxing Chen, Yuan Yuan, Yunliang Zhao, Feng Rao, and Shaoxian Song Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04171 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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Langmuir

Preparation of montmorillonite nanosheets through freezing/thawing and ultrasonic exfoliation

Tianxing Chen a, Yuan Yuan a, Yunliang Zhao a,b,*, Feng Rao d, Shaoxian Songb,c,* a School

of Resources and Environmental Engineering, Wuhan University of

Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China b Hubei

Key Laboratory of Mineral Resources Processing and Environment, Wuhan

University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China c Hubei

Provincial Collaborative Innovation Center for High Efficient Utilization of

Vanadium Resources, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China d Instituto

de Investigacion en Metalurgia y Materiales, Universidad Michoacana de

San Nicolas de Hidalgo, Ciudad Universitaria, Morelia, Michoacan, 58030, Mexico

* Corresponding authors. E-mail: [email protected], [email protected]

ACS Paragon Plus Environment

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Abstract Exfoliation of layered montmorillonite (MMT) into mono- or few-layer sheets is of significance for both fundamental studies and potential applications. In this report, exfoliated MMT nanosheets with different aspect ratio have been prepared via a new freezing/thawing-ultrasonic exfoliation method. Freezing/thawing processing can exfoliate MMT tactoids with low efficiency while virtually retain the original lateral size. Ultrasonic method has better exfoliation efficiency but tends to damage the nanosheets. By combining them and reasonably controlling the cycle index of freezing/thawing and ultrasonic power, the MMT nanosheets with different aspect ratio has been prepared efficiently. Such a unique exfoliation method has broad applicability for layered materials to produce monolayer nanosheets on a largescale.

Keywords: Montmorillonite, exfoliation, ultrasonic, freezing/thawing, aspect ratio, 2D nanosheets.


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Introduction Mono- to few-layer layered materials have received increasing attention owing to their exceptional role and extraordinary promise in preparation of inorganic layered compound/polymer nanocomposites in the past decade 1–6. Although a broad variety of layered materials have been studied such as graphite, molybdenite, layered manganese dioxide, etc., clay has been the focus of much research due to its low cost, chemical and thermal stability, and good mechanical properties. Montmorillonite (MMT), a major constituent of bentonite, consists of multilayer nanosheet with 10 Å in thickness. It is known that a quite thin MMT sheet consists of two kinds of sheets: two tetrahedral sheets of silica sandwiching one octahedral sheet of alumina7. Isomorphous substitutions of Mg2+ for Al3+ in the octahedral sheet of alumina and Al3+ for Si4+ in the tetrahedral sheets result in negative layer charges which is balanced by cations (e.g., Na+, Ca2+) within MMT inter-layer space. The existence of interlayer cations will cause the hydration swelling of MMT through the hydration capacity. In addition, the interaction force between the MMT sheets is weak electrostatic forces and van der Waals forces. Owing to these characteristics, MMT has the ability to exfoliate freestanding nanometer-thin sheets8. With the low cost, nontoxicity, high specific surface area and electrical anisotropy, MMT is suitable for use as raw materials of new nanocomposites in food, medicine, cosmetic, and healthcare recipients8–16. The physical and mechanical properties of polymer/MMT nanocomposites will be influenced by the property of MMT nanosheets, especially aspect ratio. MMT nanosheets with different aspect ratio have different


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applications. For example, significant research studies have shown that high aspect ratio plate-like nanosheets will significantly enhance barrier properties and mechanical properties of polymer nanocomposites17–20. However, Rawat et al. found that low aspect ratio laponite exhibited maximum efficacy as an antimicrobial agent against E. coli in comparison to high aspect ratio MMT21. This observation might be attributed to relatively wider or larger zone of inhibition caused by comparatively smaller lateral size laponite preferentially enter the cells. Therefore, it is imperative to prepare fully exfoliated and dispersed nanosheets with different aspect ratio. In MMT suspensions, water molecules penetrate into the inter-layer space of MMT and lead to increasing separation between two successive sheets. Under external force (shear stress, sonication, etc.), no further interaction occurs between the two delaminated sheets which become independently mobile in the liquid phase, this phenomenon is called exfoliation22,23. The methods used for the exfoliation of MMT divided into three main groups according to the mechanisms: physical methods, chemical method and combined physical and chemical method. Mechanical methods, especially ultrasonic exfoliation, are attributed to enough strong external energy to disrupt attractive force between MMT sheets. However, sonication tends to damage the structure of nanosheets, which trend to produce the low aspect ratio nanosheets and destruct inherent properties11,24,25. Chemical method usually exfoliates MMT by adsorbing guest molecules into the spacing between sheets via ion exchange. In this process, guest molecules will increase the layer spacing, weaken the interlayer adhesion and reduce the energy barrier to exfoliation. When the distance is large enough,


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attraction between the sheets disappears and the lamellar structure breaks down. The main drawback of this method is very difficult to obtain 100% purity and controlled aspect ratio nanosheets11,26,27. Therefore, a novel and efficient exfoliation method is required to prepare controlled aspect ratio MMT nanosheets. It is well known that freezing-thawing cycles are widely utilized to test mechanical and thermal properties of building material. The volume of water gradually increases in the freezing process due to the formation of ordered structure and shrink in the thawing process. Multiple freezing-thawing cycles would result in volume expansion and crack occurrence of material28–31. Based on this phenomenon, we hypothesized that freezing-thawing is an effective assistant proceeding for exfoliation of MMT with massive interlayer water. In this work, we investigate the effect of freezing/thawing on the exfoliation of MMT tactoids. The aim is to report a novel and efficient freezing/thawing-ultrasonic exfoliation method to prepare controlled aspect ratio 2D MMT nanosheets.

Experimental Materials. The original MMT used in the present study was collected from Sanding Technology Co., Ltd, Zhejiang province, China. A common method for obtaining purified colloidal MMT is fractionation by sedimentation after removal of carbonates, oxides, and organic materials. The original MMT is carefully dispersed in 1 M NaCl solution by shaking for about 12 h, and separated by centrifugation. This procedure is repeated several times to ensure the interlayer cations of MMT is Na+. The


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MMT suspensions was dried at 100 ℃ and ground to minus 74 μm 32. The powder XRD pattern and FESEM image of the original MMT sample are shown in Fig. 1. It shows that the pristine MMT particles were very high grade and contained negligible impurities with lamellar structure. Millipore Milli-Q Direct 8/16 ultrapure water (18.2 MΩ) was used in this study.

(a)

(b) M

M-Montmorillonite B-Beidellite

Intensity (a.u.)

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B 0

10

M

 m

M 20

30

40

50

60

70

80

2 Theta (degree)

Figure 1. (a) XRD pattern and (b) FESEM image of the original MMT sample.

Exfoliation

of

MMT

by

the

freezing/thawing-ultrasonic

exfoliation method. MMT suspensions (0.1%, w/v) was prepared by disperse MMT powder in distilled water under stirring for 12 h at room temperature. The MMT suspensions was poured onto plastic bottle and then frozen at -20 °C for 24 h. Then, the frozen MMT suspensions was subsequently thawed and conditioned at 20 °C for 24 h. This freezing/thawing cycle was repeated 1-2 times. After shaking, the MMT suspension was retained for analysis. The MMT sample after different cycles of the freezing/thawing process was exfoliated further by a ultrasonic exfoliation method in common with that studied by


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Song et al.33 The MMT sample was exfoliated by a ultrasonic dispersion instrument (Vernon Hills, Illinois, CP505, 40 kHz) with different ultrasonic power (150, 300 and 450W) for 4min. The samples were named X-Y W (X represent the number of freezing/thawing cycles, Y represent the ultrasonic power). Then the exfoliated MMT sample was dispersed with a high speed mixer (Fluko, FA25, 10000 r/min) for 1 min and retained for analysis.

Characterization. X-ray powder diffraction (XRD) analysis was carried out using a Bruker D8 Advance X-ray diffractometer with CuKα radiation. The diffraction patterns in the 2θ range from 10° to 90° were collected with a step-scanning speed of 10°/min. Field emission scanning electron microscopy (FESEM, Ultra Plus, Zeiss, Germany) was used to characterize the surface morphologies of the MMT samples at an accelerating voltage of 5 kV. The MMT samples were characterized using an atomic force microscope (Bruker MultiMode 8, USA) with ScanAsyst-Air Si3N4 probe tip (radius = 2 nm) in peak force tapping mode. The height resolution and lateral resolution of the AFM scanner were of 1 Å and 1 nm, respectively. For the analysis of MMT sheets using AFM, the MMT suspension was first diluted to 10-4 g/L and completely dispersed through adjusting pH value to 9. Then a drop of MMT suspension (10μL) was dropped onto a freshly cleaved mica sheet (1 cm2) and dried at room temperature to ensure the MMT particles dispersed without coagulation and overlapping. In order to accurately get the thickness and lateral size distribution of MMT samples, the AFM scanner was routinely calibrated using standard calibration gratings with 104 nm deep trenches and 5×5 μm2 square


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pits for vertical and horizontal directions. Then 150 sheets of every sample were got for the statistics in the AFM measurement used with NanoScope Analysis 1.5 software.

Water Ice

Freezing MMT Thawing

Sonication

Water

Scheme 1. Exfoliation of MMT via (1) freezing of hydrated MMT tactoids in suspension, (2) subsequent thawing of the resultant solid and (3) sonication exfoliation of the treated MMT sample

Results and Discussions Exfoliation of MMT by the Freezing/thawing Method. To study the effect of freezing/thawing method on the exfoliation of MMT, the MMT sample after different cycles of the freezing/thawing process was dispersed and dried on a mica substrate and characterized by AFM measurement. Fig. 2 shows a representative AFM image and distributions of thickness and lateral size of MMT sheets obtained after different cycles of the freezing/thawing process. The images show that MMT sheets are not at all regularly shaped and have different thickness from the cross-sectional analysis.


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The thickness distribution of the original MMT was mainly distributed in the range of >10 nm with 58.67% percentage. With the freezing/thawing process, the thickness distribution of MMT in the range of 1–2 nm ranges from 2% to 15.77% and the range of >10 nm ranges from 58.67% to 22.46%. It shows that the thickness of MMT decreased monotonously as the increase of the cycles of the freezing/thawing process. In other words, the degree of exfoliation of MMT increased as the increase of the cycles of the freezing/thawing process. Howerer, the exfoliation degree of this method is low efficient than the ultrasonic exfoliation method23,34. Unlike the strong ultrasonic force, the freezing/thawing processing exfoliate the MMT tactoids through the volume expansion of inter-layer water. This processing is mild so that high degree of exfoliation needs multiple-times freezing/thawing process. However, the lateral sizes of the MMT sheets after the freezing/thawing process appear to be similar to the lateral sizes distribution of the original MMT sheets. The data indicate that these MMT sheets had simply been peeled off from the MMT particles with minimal fragmentation. Therefore, multiple-times freezing/thawing process may be a promising exfoliation method for MMT tactoids which require the integrity of sheets.


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(a)

(b) Thickness (nm)

2.0

1.5

1.0

0.5

0.0 0

(c)

60

58.67

Number of cycles: 0

0

(d)

0 60 40 20 0 60

2

3.33 5.33 7.33

2

6

Number of cycles: 1 27.7 7.43

5.4

10.81 9.46 6.76 10.81 10.14

5.4

6.08

Number of cycles: 2

0

14.77 14.09 13.11 8.72 8.05 7.38 1-2

2-3

3-4

4-5

200

300

5-6

6-7

22.46 4.7

3.36 3.36

7-8

8-9

9-10

60

49.61

500

600

Number of cycles: 0 22.58 11.96

8.86

5.03

1.31

60 0

20

21.44 6.71

0.65

Number of cycles: 1

37.52

40 10.84

12.75

5.37

5.37

60 0

Number of cycles: 2

36.13 17.33

20 0

>10

400

Length (nm)

40

40 20

100

20

5.33 6.67 3.33

Percentage (%)

20

0

40

40

Percentage (%)

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25.33 9.33

3.33

2

80-120 120-160 160-200 200-240 240-280 280-320

Thickness (nm)

5.33 >320

Lateral Size (nm)

Figure 2. Typical AFM morphology images of MMT sheets (a) and topographic profile along the red line in the corresponding image (b). Distribution of mean thickness (c) and lateral size (d) of MMT sheets after different cycles of the freezing/thawing process. The error bars correspond to the standard deviation obtained from at least three samples.

To obtain direct proof of MMT exfoliation after different freezing/thawing process, X-ray diffraction pattern of MMT sample after vacuum freeze-drying was determined over the scanning range 3°10

0

1-2

2-3

Thickness (nm)

8.92

3-4

1.91

2.55

0.64

1.91

4-5

5-6

6-7

7-8

1.91

8-9

9-10

>10

Thickness (nm)

Figure 4. Thickness distribution of the MMT sheets after different cycle index of freezing/thawing and ultrasonic power. The error bars correspond to the standard deviation obtained from at least three samples.

Preparation of montmorillonite nanosheets with different aspect ratios. The aspect ratio generally defined as the length of the long axis to the length of the short axis irrespective of shape, is a key factor in determining nanocomposite physical and mechanical properties. For MMT sheets, this aspect ratio is often expressed as the ratio of the lateral size to the thickness. For each sheet, we accurately know its mean thickness as well as its lateral size used by AFM, which makes it possible to produce the true distribution of the sheet aspect ratio from all of the sheets in several images.


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As described before, the exfoliation degree of MMT at 300W after once freezing/thawing process is almost same with the MMT at 450W without freezing/thawing process. However, freezing/thawing processing can virtually retain the original lateral size of MMT sheets. As a result, the lateral size of the two kinds of exfoliated MMT sheets is quite different. As can be seen from Fig. 5, MMT sheets with different aspect ratio have been successfully prepared through the above two approaches. The aspect ratio distribution of the former MMT nanosheets was mainly distributed in the range of 1-20 with 45% percentage. And the aspect ratio distribution of the later MMT nanosheets was mainly distributed in the range of 40-60 with 42% percentage. The variance ranges and magnitudes of aspect ratio are equivalent to others research that investigate the role of the aspect ratio on the properties of layered silicate nanocomposites36,37. By preparation of MMT nanosheets with different aspect ratio, fundamental knowledge on exactly how the aspect ratio influence the physical and mechanical properties of nanocomposites can be clearly addressed.


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60

(a)

(b)

55

0-450W

60

53.91

40

30

20

16.33

40

30

20

10 10

0

(c)

1-300W

50

Percentage (%)

Percentage (%)

50

3.33 4.67

0 1-2

2-3

3-4

4-5

5-6

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2

1.33

0 7-8

8-9

46.67

50

10.16 9.38 8.59

10 2.67

9-10

4.69

3.33

0

0

>10

Thickness (nm)

60

(d)

0-450W

2.34 2.34

0 1-2

2-3

3-4

4-5

5-6

6-7

7-8

0.78 8-9

3.12 9-10

4.69 >10

Thickness (nm)

60

1-300W

48

50

Percentage (%)

40

30

20

9.33

10

0

(e)

40-80

50

Lateral Size (nm)

(f)

28 20

17

18 9

10

4 0

1-20

7.33 0.67 1-40

40-80

3

1-300W 42

40

27

30

20

16

15

10

13

12 8 3

2

20-40 40-60 60-80 80-100 100-120120-140140-160160-180 >180

0.67

Lateral Size (nm)

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80-120 120-160 160-200 200-240 240-280 >280

50

40

20

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60

0-450W

45

30

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0

80-120 120-160 160-200 200-240 240-280 >280

60

30

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0.67 1-40

40

10

Percentage (%)

Percentage (%)

42

Percentage (%)

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0

1-20

4

20-40 40-60 60-80 80-100 100-120120-140140-160160-180 >180

Aspect Ratio

Aspect Ratio

Figure 5. Distributions of thickness, lateral size, and aspect ratio of two kinds of MMT nanosheets (0450W and 1-300W). The error bars correspond to the standard deviation obtained from at least three samples.

To further support the AFM results, the XRD patterns and FESEM images of different MMT nanosheets (0-450W and 1-300W) were investigated. As explained before, order degree of lamellar structure is characterized by XRD patterns. As shown in Fig. 6 a, the basal plane (001) reflection of different MMT nanosheets (0-450W and 1-300W) are almost disappeared, indicating that the MMT nanosheets prepared by the two exfoliation methods are consist of a single layer of MMT nanosheet. The FESEM


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data (Fig.6 b,c) show two kinds of MMT nanosheets have completely different lateral sizes. The MMT nanosheet (1-300W) is found to be larger than the other nanosheet (0450W) due to freezing/thawing processing is help to retain the original lateral size. In summary, the new exfoliation approach by reasonably controlling the cycle index of freezing/thawing and ultrasonic power can efficiently prepared the MMT nanosheets with different aspect ratio.

(a)

0-450W

Intensity (a.u.)

Un-exfoliated (b) MMT

1-300W Intensity (a.u.)

200 nm

(c)

1-300W

0-450W Intensity (a.u.)

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200 nm 3

4

5

6

7

8

9

10

11

12

13

14

15

2 Theta (degree)

Figure 6. XRD patterns and FESEM images of two kinds of MMT nanosheets (0-450W and 1-300W).


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Conclusions MMT nanosheets with different aspect ratios can be efficiently prepared in water by a new freezing/thawing-ultrasonic exfoliation method. This freezing/thawing pretreatment involves multiple cycles of freezing of an MMT suspensions and subsequent thawing of the resultant solid lead to increasing separation between two successive MMT sheets. After freezing/thawing pretreatment, the low ultrasonic power also can yield MMT nanosheets efficiently and virtually avoid the fragmentation of original nanosheet caused by high ultrasonic power. By reasonably controlling the cycle index of freezing/thawing and ultrasonic power, the MMT nanosheets with different aspect ratio has been prepared efficiently. The simple, effective, and scalable exfoliation method has the potential to be applied to the large-scale production of MMT nanosheets. The synthesis strategy for a new material through similar exfoliation and reassembly using the phase change of water may be general and applicable to other types of layered materials, which exhibit suitable interfacial interactions with water.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China under the project Nos. 51474167 and 51674183 and the Fundamental Research Funds for the Central Universities under the project No. 2017-YB-025.


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Exfoliation of MMT via (1) freezing of hydrated MMT tactoids in suspension, (2) subsequent thawing of the resultant solid and (3) sonication exfoliation of the treated MMT sample 275x190mm (96 x 96 DPI)


thawing

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