Exfoliation and characterization of 2D layered nanomaterials

Exfoliation and characterization of 2D layered nanomaterials

by Rozana Bari, B.Sc.

A Thesis

In CHEMICAL ENGINEERING

Submitted to the Graduate Faculty

of Texas Tech University in Partial Fulfillment of the Requirements for

the Degree of

MASTER OF SCIENCE IN

CHEMICAL ENGINEERING

Approved

Micah J. Green Chair of Committee

Rajesh Khare

Edward L. Quitevis

Mark A. Sheridan

Dean of the Graduate School

August, 2014

Copyright 2014, Rozana Bari

Texas Tech University, Rozana Bari, August 2014

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ACKNOWLEDGMENTS

I express my deepest gratitude and profound respect to my thesis supervisor

Dr. Micah J. Green, Assistant Professor, Department of Chemical Engineering at

Texas Tech University for his valuable guidance, constant encouragement and keen

interest at every stage of this study, without which this would have been extremely

difficult to accomplish.

I am grateful to Dr. Rajesh Khare and Dr. Edward L. Quitevis for kindly

serving on my thesis graduate committee and I would like to thank them for providing

valuable input and comments on my work.

I would like to thank the faculty of the Department of Chemical Engineering at

Texas Tech University for teaching and guidance. I would like to thank Dr. Mark J.

Grimson, Dr. Callum Hetherington, Dr. Lauren Gollahon, and Mary Catherine Hastert

at the TTU Imaging Center for having provided me with the microscopy facilities and

equipment which enabled me to carry out the research work.

I would like to thank the funding agencies for providing the financial support,

facilities, and resources required during my graduate studies; the Air Force Office of

Scientific Research Young Investigator Program (AFOSR FA9550-11-1-0027), and

the U.S. National Science Foundation (NSF) under CAREER award CMMI-1253085.

I would like to thank specially my groupmates; Yueyi, Sriya, Irin, Dorsa, and

Brandon. Also special thanks to all the undergrads; Robert, Francis, Shane, Garrett,

Matt, and Chris.


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Special thanks to my friends from Bangladesh. Also I would like to thank Dr.

Shamim, Dr. Bhuiyan, Dr. Bithi, Dr. Bhattacharia, and Dr. Karim for their continuous

help and suggestions. I am grateful to Irin and I would like to thank Nazir specially for

helping me in doing microscopy. Finally, I would like to thank the whole Bangladeshi

community for the support and love during my stay in Lubbock.

Finally, I would like to thank my family members who gave me love and

support to keep me focused and motivated on my research work. I am very grateful to

my siblings, Dr. Ehsanul Bari, Ruwaida Bari, and Rummana Bari for your guidance

and encouragement during my academic years. I would like to express special thanks

to my husband, Md Nadim F. Hoque, for his constant support. Last, but definitely not

the least, I would like to express my deepest gratitude to my Dad, Professor Dr. M.

Fazlul Bari and especially to my Mom, Professor Dr. Nilufar Sultana for their

continuous support and love. Without your love, guidance, and never-ending support,

this accomplishment would not have been possible.


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TABLE OF CONTENTS

ACKNOWLEDGMENTS .................................................................................... ii

ABSTRACT .......................................................................................................... vi

LIST OF TABLES ............................................................................................. viii

LIST OF FIGURES ............................................................................................. ix

1. INTRODUCTION ............................................................................................. 1

1.1 Thesis introduction ...................................................................................... 1

1.2 Problem statement ....................................................................................... 2

1.3 Research scope and objectives .................................................................... 3

1.4 Motivations ................................................................................................. 3

1.5 Organization of the dissertation .................................................................. 4

2. INTRODUCTION TO 2D LAYERED MATERIALS .................................. 6

2.1 Introduction to different 2D layered materials ............................................ 6

2.1.1 Graphene ............................................................................................ 6 2.1.2 Boron nitride nanosheets (BNNSs), and metal dichalcogenides

(MoS2 and WS2) ................................................................................. 8

2.2 Structural analogy of 2D layered materials ................................................. 9

2.3 Properties of 2D layered materials ............................................................ 10

2.3.1 Graphene .......................................................................................... 10 2.3.2 Properties inorganic 2D layered materials ....................................... 11

2.4 Applications of 2D layered material ......................................................... 14

2.4.1 Applications of Graphene ................................................................ 14 2.4.2 Applications of inorganic 2D layered materials............................... 15

3. DIRECT EXFOLIATION OF GRAPHENE IN IONIC LIQUIDS ........... 17

3.1 Introduction ............................................................................................... 17

3.2 Materials and Methods .............................................................................. 19

3.3 Results ....................................................................................................... 22

3.3.1 Preparation of graphene dispersion .................................................. 22 3.3.2 Absorbance of graphene dispersion ................................................. 22 3.3.3 Dispersion characterization .............................................................. 23


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3.3.4 Effect of ionic liquid on dispersion .................................................. 25

4. LIQUID PHASE EXFOLIATION AND CRUMPLING OF INORGANIC LAYERED MATERIALS ..................................................... 29

4.1 Introduction ............................................................................................... 29

4.2 Experimental Methods .............................................................................. 31

4.3 Results and discussion .............................................................................. 33

4.3.1 Nanosheet dispersion ....................................................................... 33 4.3.1.1 Preparation of inorganic layered material dispersion.............. 33 4.3.1.2 Absorbance of inorganic layered material dispersion..............34 4.3.1.3 Tyndall effect......................................................................... . 35 4.3.1.4 Dispersion characterization.................................................... . 36

4.3.2 Crumpling of inorganic nanosheets ................................................. 58

5. CONCLUSION AND RECOMMENDATIONS FOR FURTHER STUDY ............................................................................................................. 62

REFERENCES .................................................................................................... 66


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ABSTRACT

The objective of this research was to exfoliate 2D layered materials in liquid

phase. The layers of the nanosheets are stacked together by van der Waals attraction

forces in the parent materials. Liquid phase exfoliation is necessary to overcome the

attraction forces in the parent materials. Various techniques are applied to

synthesize these nanomaterials such as chemical vapor deposition (CVD) on a

substrate, or liquid phase exfoliation in a solvent. However, there are

difficulties associated with the use of CVD grown graphene for composite production.

Mixing of CVD grown graphene powder with polymer melts is hard due to

aggregation of graphene. On the other hand, the nanomaterials exfoliated in the liquid

phase can be easily mixed in different polymers.

Some organic solvents and ionic liquids have both been candidates to disperse

graphene directly without dispersant. However, the yield of graphene in organic

solvent is comparatively lower than in ionic liquids. Ionic liquids, also described as

designer solvent, can be synthesized with preferred functional groups which are able

to stabilize graphene. The phenyl groups in the cation of ionic liquids non-covalently

π-π stack with the graphene surface and prevent aggregation by electrostatic repulsion.

Hence, high quality and high yield production of graphene is possible by the aid of

these designer solvents.

We have utilized a dissolved polymer to disperse inorganic 2D layered

nanomaterials such as boron nitride nanosheets, molybdenum disulfide, and tungsten

disulfide in water and a range of organic solvents, which are resistant to aggregation.


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The aqueous dispersions were characterized by microscopy and dynamic light

scattering (DLS) that helped to determine the lateral size, number of layers of

nanosheets, and structure of powdered samples. The dispersions were freeze-dried

and redispersed in water to check dispersion stability against centrifugation. Finally, a

spray drying technique was utilized to convert 2D nanosheets to 3D crumpled

nanosheets.


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LIST OF TABLES

3.1. List of ionic liquids tested for stable dispersion of graphene................................. 26

4.1. Average and maximum sheet size of bn nanoparticle with sonication time. ......... 40

4.2. Concentration of nanomaterial and extinction coefficient values in different solvents ................................................................................................... 50


ix

LIST OF FIGURES

2.1. Honeycomb lattice structure of graphene with carbon-carbon bond distance. ........ 6

2.2. Graphene is the building block for different carbon structures................................ 7

2.3. Production of graphene by micromechanical cleavage method. .............................. 7

2.4. BNNSs nanosheet and nanotube .............................................................................. 8

2.5. Metal dichalcogenides (MoS2 and WS2) nanotube. ................................................. 8

2.6. Layered structure of (a) graphite, (b) hexagonal boron nitride (hBN), (c) molybdenum disulfide/ tungsten disulfide ............................................................ 10

2.7. Thermal conductivity of different carbon allotropes ............................................. 11

2.8. Electronic band gap of bulk and monolayer MoS2 ................................................ 14

2.9. Electronic band gap of bulk and monolayer WS2 .................................................. 14

2.10. Numerous applications of graphene ..................................................................... 15

2.11. Numerous applications of BNNSs, MoS2, and WS2 ............................................ 16

3.1. Chemical structure of the IL s; (a) IL-1, (b) IL-2, (c) IL-3, and (d) IL-4. ............. 20

3.2. Absorbance spectrum of IL stabilized graphene dispersion .................................. 22

3.3. TEM images of (a) graphene flakes stabilized by ionic liquid, (b) the magnified view of the flake edges indicating 2-5 layers of sheets.. ...................... 23

3.4. Raman spectra of graphene in IL-4. In the plot the shift in G-peak confirms graphene exfoliation in IL-4. ................................................................................. 24

3.5. SEM images of graphene film on filter paper (a) prepared by vacuum filtration, and (b) magnified view of the film where graphene flakes are observed. .......................................................................................................... 25

3.6. The integration between graphene and il with binding energy: (a) [C1C1im]+ cation and graphene, (b) IL-1 [BnzC1im]+ cation and graphene, and (c) IL-4 [(Bnz)2im]+ and graphene. ..................................................................................... 28

4.1. Chemical structure of polyvinylpyrrolidone (PVP) .............................................. 30


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4.2. Images of PVP stabilized aqueous dispersion of (a) BNNSs, (b) MoS2, and (c) WS2 .................................................................................................................. 34

4.3. Absorbance spectra of PVP stabilized (a) BNNSs, (c) MoS2, and (e) WS2 dispersion and the linearity plot of PVP stabilized (b) BNNSs, (d) MoS2, and (f) WS2 dispersion to determine extinction coefficient ................................... 35

4.4. Images of water (left) and the colloids of PVP stabilized nanomaterial aqueous dispersion (right) with a laser pointer beam passing from the left side through (a) BNNSs, (b) MoS2, and (c) WS2 (tyndall effect). ........................ 36

4.5. SEM images of parent material (a) BN, (b) MoS2, and (c) WS2.. ......................... 37

4.6. HRTEM images of nanosheets. (a, b) BNNSs; (c, d) MoS2 nanosheets; (e, f) WS2 nanosheets. (a, c, e) shows folded BNNS, MoS2, and WS2 sheets. In (b, d, f) few-layer nanosheets are observed. The sheet edges indicate that the nanosheets are 2-5 layers thick. ....................................................................... 39

4.7. Particle size distribution of hBN-I after (a) 1 hour sonication, (b) 1.5 hour sonication, and (c) 2 hour sonication. .................................................................... 42

4.8. Particle size distribution of hBN-II after (a) 1 hour sonication, (b) 1.5 hour sonication, and (c) 2 hour sonication ..................................................................... 42

4.9. The final concentration of dispersed, stable nanosheets increase with the initial concentration of the parent material in the sonication vial (PVP concentration is 10 mg mL-1) (a) BNNSs, (b) MoS2, and (c) WS2........................ 45

4.10. The effect of increasing PVP concentration on the final nanomaterial concentration in the dispersion is demonstrated (20 mg mL-1 parent material in sonication vial) (a) BNNSs, (b) MoS2, and (c) WS2. ....................................... 46

4.11. Freeze-dried, PVP-stabilized dispersion in water (a) BNNSs, (b) MoS2, (c) WS2.; freeze dried powder (d) BNNSs, (e) MoS2, (f) WS2;. and redispersed in water (g) BNNSs, (h) MoS2, (i) WS2.. ........................................... 47

4.12. UV-vis absorbance spectra before and after freeze-drying-redispersion (a) BNNSs, (b) MoS2, and (c) WS2.. ..................................................................... 48

4.13. Digital photographs of PVP stabilized nanomaterial dispersion in methanol. (a) BNNSs, (b) MoS2, and (c) WS2; the absorbance spectra of (d) BNNSs, (e) MoS2, and (f) WS2. and linearity plot of PVP stabilized nanomaterial dispersion to determine the extinction coefficient (g) BNNSs, (h) MoS2, and (i) WS2 ................................................................................................................... 51


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4.14. Digital photographs of PVP stabilized nanomaterial dispersion in ethanol. ((a) BNNSs, (b) MoS2, and (c) WS2; the absorbance spectra of (d) BNNSs, (e) MoS2, and (f) WS2. and linearity plot of PVP stabilized nanomaterial dispersion to determine the extinction coefficient (g) BNNSs, (h) MoS2, and (i) WS2. ........................................................................................................... 52

4.15. Digital photographs of PVP stabilized nanomaterial dispersion in isopropanol. ((a) BNNSs, (b) MoS2, and (c) WS2; the absorbance spectra of (d) BNNSs, (e) MoS2, and (f) WS2. and linearity plot of PVP stabilized nanomaterial dispersion to determine the extinction coefficient (g) BNNSs, (h) MoS2, and (i) WS2.......... 53

4.16. Digital photographs of PVP stabilized nanomaterial dispersion in chloroform. (a) BNNSs, (b) MoS2, and (c) WS2; the absorbance spectra of (d) BNNSs, (e) MoS2, and (f) WS2. and linearity plot of PVP stabilized nanomaterial dispersion to determine the extinction coefficient (g) BNNSs, (h) MoS2, and (i) WS2.......... 54

4.17. Digital photographs of PVP stabilized nanomaterial dispersion in DMF. (a) BNNSs, (b) MoS2, and (c) WS2; the absorbance spectra of (d) BNNSs, (e) MoS2, and (f) WS2. and linearity plot of PVP stabilized nanomaterial dispersion to determine the extinction coefficient (g) BNNSs, (h) MoS2, and (i) WS2. ............. 55

4.18. Digital photographs of PVP stabilized nanomaterial dispersion in DMSO. (a) BNNSs, (b) MoS2, and (c) WS2; the absorbance spectra of (d) BNNSs, (e) MoS2, and (f) WS2. and linearity plot of PVP stabilized nanomaterial dispersion to determine the extinction coefficient (g) BNNSs, (h) MoS2, and (i) WS2.. ............ 56

4.19. Digital photographs of PVP stabilized nanomaterial dispersion in NMP. (a) BNNSs, (b) MoS2, and (c) WS2; the absorbance spectra of (d) BNNSs, (e) MoS2, and (f) WS2. and linearity plot of PVP stabilized nanomaterial dispersion to determine the extinction coefficient (g) BNNSs, (h) MoS2, and (i) WS2.......... 57

4.20. SEM images of crumpled nanosheets collected from the spray dryer; (a) BNNSs, (b) MoS2, and (c) WS2. ...................................................................... 59

4.21. Schematic diagram of the crumpling process of nanomaterial; (a) water droplet with nanosheets from atomizer, (b) shell of nanosheets formation by clustering of nanosheets, (c) coalescence and aggregation of nanosheets, (d) evaporation of water from shell and crumpling of nanosheets with wrinkles, and (e) crumpled particles with wrinkles and ridges. ................................................... 60


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CHAPTER 1

INTRODUCTION

1.1 Thesis introduction

Over the past two decades nanomaterials have attracted major attention due to

their fascinating properties and wide range of potential applications. Nanomaterials are

typically defined as materials that have at least one dimension in the range of 1-100 nm.

There are two categories of nanomaterials: organic (mostly carbon allotropes) and

inorganic nanomaterials (iron, silver, gold, boron nitride nanosheets, molybdenum

disulfide, and tungsten disulfide). The nanomaterials have completely different properties

than the bulk parent materials; these properties include high surface area, conductivity,

mechanical strength, and transparency.

Nanotechnology is expanding in academic research and also moving into industry in

recent years. The development in this field is supported by technological advancement, such

as microscopy and spectroscopy. The microscopic images help to understand the morphology

such as lateral size, thickness, and number of layers of the nanomaterials with resolution of a

few nanometers. Raman spectroscopy, X-ray photoelectron spectroscopy, and UV-vis

spectroscopy, helps to characterize nanomaterials. About 850,000 nanotechnology articles

have been published in different languages worldwide.a

a http://www.nanowerk.com/nanotechnology-news/newsid=35778.php

A significant portion of these works

focus on nanomaterials such as graphene, carbon nanotubes, fullerenes, boron nitride

nanosheets, molybdenum disulfide, and tungsten disulfide.


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1.2 Problem statement

Exfoliation of 2D layered materials from the bulk parent materials is difficult

because of the van der Waals attraction forces which hold the sheets together. However,

the use of dispersants may aid in overcoming the van der Waals attraction and disperse

them as individual sheets in liquids. While exfoliating graphene, the challenges are to

stabilize graphene sheets and prevent reaggregation by the inter-sheet van der Waals

forces. There are several methods of producing graphene. Micromechanical cleavage is

one of them. This method is also known as the scotch tape method where continuous

peeling off of layers from graphite is repeatedly done till a single layer of graphene is

isolated. This technique is not suitable to produce large scale graphene although small

quantities of high-quality graphene is produced. Another technique is epitaxial growth or

chemical vapor deposition (CVD). In both of these cases a suitable substrate (silicon

carbide) is heated at high temperature and low pressure which leads to the formation of

defect free graphene. The difficulty of these methods are to control the thickness of the

graphene and a bulk amount of graphene cannot be produced.

In the solvent route (liquid phase exfoliation), various organic solvents have the

ability to disperse graphene directly such as N-methyl-2-pyrrolidone (NMP),

dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). This technique yields

dispersions of graphene with low concentration; however, they are not stable against

prolonged centrifugation. Moreover, it is very difficult to boil off these organic solvents

due to their high boiling point.


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1.3 Research scope and objectives

We used ionic liquids (ILs) to exfoliate graphene directly with higher graphene

yield compared to other organic solvents. The cation and anion of ILs can be tuned to get

desired ILs, which can exfoliate graphene successfully. The interaction between graphene

and the ILs can be predicted by using density functional theory and then the designing of

ILs can be performed according to the simulations results. Our collaborators, Dr.

Quitevis's group from the Chemistry Department at TTU, performed these simulations to

predict the interactions between graphene and IL and designed the IL accordingly.

Moreover, we focused on the exfoliation of inorganic 2D layered materials in the

presence of dispersant in water and a wide range of organic solvents. The dispersant

makes the nanosheets aggregation resistant and the stability of the aqueous dispersions

were checked by a freeze-drying technique, and crumpled nanosheets powder were

produced by a simple spray drying technique.

1.4 Motivations

The unique properties of the 2D layered materials as mentioned above are absent

in bulk materials. This motivates us towards producing defect-free, high surface area,

single-to-few layer nanosheets dispersed in a liquid phase. The ILs are favorable solvents

for exfoliation and dispersion of graphene with a high yield. The ILs non-covalently

interact (π-π stacking) with the isolated graphene sheets. Moreover, the inorganic layered

2D materials can be exfoliated in organic solvents, although the yield is very low and the

high boiling point of the solvents are hard to boil off. Hence, we exfoliated high quality


4

inorganic layered 2D materials in water and in organic solvents in the presence of

dispersant.

1.5 Organization of the dissertation

This thesis is divided into five chapters. An introductory discussion of 2D layered

materials is provided in chapter 2. Chapters 3 and 4 are the main experimental studies

performed in past two years. Chapter 5 contains a brief summary of this research along

with suggested future works.

In chapter 2, the introduction and discovery of 2D layered materials are briefly

discussed. Significant interests have been grown up in manipulating of the properties of

inorganic layered materials since the discovery of graphene. The properties and

application of these layered materials are also highlighted in this chapter.

In Chapter 3, graphene exfoliation in liquid phase is carried out by utilizing

different ILs. We highlight the relationship of graphene yield with the anion-cation of the

ILs which is supported by the binding energy between the ions utilizing density

functional theory (DFT-D3).

Chapter 4 deals with exfoliation and stabilization of inorganic layered materials in

different solvents in presence of polymer. Prior work in our group successfully exfoliated

graphene in polymer which motivated us to exfoliate these nanosheets in presence of

polymer. It is shown that the nanomaterial is stable against centrifugation and

lyophilization. We utilized spray drying technique to process the stabilized nanomaterials

dispersions into crumpled nanosheets.


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At the end of this thesis there is a summary of the experiments and results

discussed in the other chapters. The results indicate that high yield of graphene can be

achieved by designing the ILs appropriately. Moreover, the inorganic layered materials

can be successfully dispersed in polymer solutions. A direction of future research was

also suggested in this section.


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CHAPTER 2

INTRODUCTION TO 2D LAYERED MATERIALS

2.1 Introduction to different 2D layered materials

2.1.1 Graphene

Graphene is a hexagonal structure made of a single layer of sp2 hybridized carbon

atoms.1 Graphene is defined as 'a sheet of carbon atoms bound together with double

electron bonds (called a sp2 bond) in a thin film which is only one atom thick. Atoms in

graphene are arranged in a honeycomb-style lattice pattern'.b The structure of graphite

with carbon-carbon bond distances of 0.14 nm is shown in Figure 2.1.c

This graphene is

the basis of other graphitic forms such as graphite (3D), carbon nanotubes (1D), and

buckyballs (0D) (Figure 2.2).1

Figure 2.1. Honeycomb lattice structure of graphene with carbon-carbon bond distance.

b http://physics.about.com/od/glossary/g/graphene.htm c http://www.thenanoage.com/carbon.htm


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Figure 2.2. Graphene is the building block for different carbon structures (reprinted with permission from Geim et al., Nature Materials 2007).1

Simple 'Scotch-Tape method' successfully produced the first-ever free standing

graphene flakes. In 2004, Andre Geim and Konstantin Novoselov of the University of

Manchester 'mechanical exfoliation' or 'repeated peeling off' to produce free standing

graphene from graphite. They used a scotch-tape to separate graphite layers to get few

atomic layers thick graphene (Figure 2.3). Geim and Novoselov received the Nobel Prize

in physics in 2010 'for groundbreaking experiments regarding the two-dimensional

material graphene'.d

Figure 2.3. Production of graphene by micromechanical cleavage method.e

d http://www.nobelprize.org/nobel_prizes/physics/laureates/2010/

e https://www.whiteclouds.com/article-category/commercial-3d-printers


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2.1.2 Boron nitride nanosheets (BNNSs), and metal dichalcogenides (MoS2 and WS2

In 1992 Tenne and coworkers proposed the existence of inorganic materials that

are structural analogs of carbon allotropes such as graphite, fullerene, graphene, and

CNT.2-6 They reported that boron nitride (BN), molybdenum disulfide (MoS2), and

tungsten disulfide (WS2) can form fullerene like structures. Also Tehrani et al. proposed

different WS2 structures such as spherical, sheets, and tubes.7

BNNSs are formed by alternating B and N atoms substitute C atoms and form

honeycomb-like networks, similar to graphene.8,9 Layered MoS2 and WS2 consist of a

metallic layer in between two sulfur layers with hexagonal structure. The metal (Mo or

W) is covalently bonded with sulfur.10 Tubular structures of BNNSs and metal

dichalcogenides (MoS2 and WS2 ) are shown in Figure 2.411 and Figure 2.5,12,13

respectively.

Figure 2.4. BNNSs nanosheet and nanotube (adapted from Golberg et al., ACS Nano 2010.1111


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2.2 Structural analogy of 2D layered materials

BNNSs, MoS2, and WS2 are formed of hexagonal structure analogous to graphene

(Figure 2.6). The bond between C-C (in graphene) and Mo/W-S (in MoS2 and WS2) is

covalent whereas B-N is ionic (in BN). The layers of BNNSs, MoS2, and WS2 are stacked

together by van der Waals forces similar to graphene.15 In the case of MoS2 and WS2,

weak van der Waals forces are holding adjacent sulfur sheets together.2 Three

dimensional (3D) crystals are formed by stacking up 2D sheets by van der Waals

forces.16 Carbon allotropes such as carbon nanotubes (CNTs), fullerenes (C60), and

graphene have delocalized π electrons.1,17 The key difference between the organic

(graphene) and inorganic nanosheets is the presence of the delocalized π electron

distribution in graphene.

Figure 2.5. Metal dichalcogenides (MoS2 and WS2) nanotube, (adapted from Ghorbani-Asl et al., Scientific Reports 2013).14


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Figure 2.6. Layered structure of (a) graphite, (b) hexagonal boron nitride (hBN), (c) molybdenum disulfide/ tungsten disulfide.

2.3 Properties of 2D layered materials

2.3.1 Graphene

Mechanical properties

Single layer of graphene is one of the most rigid materials. Young’s modulus of

graphene is a remarkably high ~1 TPa with an ultimate tensile strength of 130 GPa.18

Graphene is harder than diamond and stronger (~300 times) than steel. Although

graphene is robust, this nanosheet can be stretched up to 20% of its initial length.f

Thermal properties

Having

these mechanical properties, mechanically strong composite material can be produced

with graphene.

Graphene has very high thermal conductivity. The thermal conductivity of

graphene is 5000 W m−1 K−1 whereas, the thermal conductivity of single walled nanotube

(SWNT) is 3500 W m−1 K−1.19 Ho et al. reported the experimental thermal conductivity

f http://www.graphene.manchester.ac.uk/story/properties/


11

of bulk graphite as 2000 W m−1 K−1.20 Nika et al. compared thermal conductivity of bulk

graphite and graphene and they mentioned (theoretically) that thermal conductivity of

single layer graphene depends on the flake size and ranges from 3000-5000 W m−1 K−1. 21

The thermal conductivity of graphene and graphite are compared in Figure 2.7.21-23

Figure 2.7. Thermal conductivity of different carbon allotropes (reprinted with permission from Balandin et al., Nature Materials 2011).22

Electrical properties

In graphene each C atom is connected to three other C atoms and leaves one free

π electron. The electrical conductivity of graphene is 6000 S cm-1.24 Moreover, the

resistance of graphene is 10-6 Ωcm at low temperature. Graphene is a zero-band gap

semiconductor with an electronic mobility of 150,000 cm2 V-1 s-1 at room temperature.25

2.3.2 Properties inorganic 2D layered materials

Electrical insulation

Due to the ionic bonding between B and N, BNNSs are electrically insulating,

with a large electronic band gap (~4-6 eV).26,27 The electrical resistivity of BNNSs are


12

1017 and 1014 Ω cm at room temperature and at 200°C, respectively.28 BNNSs composites

can be used as electrical insulator in electronics.

Thermal conductivity

The thermal conductivity of BNNSs is 2000 W m-1 K-1.29 CNTs and graphene are

thermally and electrically conductive, therefore cannot be used in thermal management of

electronic materials. Since BNNSs are electrically insulating and thermally conductive,

BNNSs composite can be used in thermal management of high power electronics.30 In

presence of BNNSs the thermal conductivity of nanofibrillated cellulose (NFC) increases

from 0.035 W m-1 K-1 to 26.2 W m-1 K-1 while loading by 5% of nanofiller.30

For applications such as high-speed electronics and dielectronics, a material with

property of quick heat releasing as well as electrical insulation is necessary. BNNSs can

be used as nanofiller in polymer materials to improve the thermal stability, thermal

conductivity, and dielectric properties in polymeric composites. For an example, the

dielectric constant increases with BNNSs loading in polymer composite of poly(methyl

methacrylate) (PMMA).31

Chemical inertness and thermal stability

BNNSs are oxidation-resistant and chemically inert. Oxidation resistance of

metals can be improved at high temperatures by applying BNNSs coatings on metal

surface. A very recent study by Li et al. has shown that BNNSs can sustain up to

850°C.32 Another study performed by Liu et al. indicated that very thin BNNSs coating

(~5 nm) on top of nickel prevents oxidation up to 1100°C.33 Hence BNNSs are more

http://en.wikipedia.org/wiki/Poly(methyl_methacrylate)

http://en.wikipedia.org/wiki/Poly(methyl_methacrylate)


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suitable for high-temperature applications since it is more resistant to oxidation than

graphene.32

Optoelectronic properties

WS2 and MoS2 nanosheets possess different optical and electrical properties than

from bulk form. For an example, MoS2 nanosheets are semiconducting, with a direct

electronic band gap of ~1.9 eV,50,51 while the bulk material exhibits indirect band gap of

1.2 eV (Figure 2.10).34,35 Similarly, monolayer WS2 has a direct band gap of ~2.1 eV that

is different from bulk WS2 (1.3 eV) (Figure 2.11).34,35 For both of these materials, the

bulk material shows indirect band gap since the bottom of the conduction band and the

top of the valence band is situated at different reciprocal lattice points (K, M, and Γ) .

However, for the monolayer materials, the positions of bottom of the conduction band

and the top of the valence band are at same K point of the x (wave vector) axis. In the

figures, 1 Hartree (Eh) is equivalent to 27.21 eV. Since these materials exhibit direct band

gap behavior, they have strong photoluminescence36,37 and spin polarization (degree of

alignment of electron's spin).38 These materials are also suitable for photovoltaics in a

sense that they can be used as absorber materials in thin film solar cell.39 So, these

transition metal dichalcogenides show various potential applications in nano- and

optoelectronics.13


14

Figure 2.8. Electronic band gap of bulk and monolayer MoS2 (reprinted with permission from Wang et al., 2012).34

Figure 2.9. Electronic band gap of bulk and monolayer WS2 (reprinted with permission from Wang et al., Nature Nanotechnology 2012).34

2.4 Applications of 2D layered material

2.4.1 Applications of Graphene

Graphene is extensively used in fabricating sensors, solar cells, flexible

electronics, electrically conductive composites, and also as thermal management material

in electronic circuits.23 Moreover, the high surface area of graphene is also relevant for

various applications such as catalysis, oil absorption, electrodes for super-capacitors, and

batteries, energy storage applications. Graphene is also used as nanofiller in polymer to

make polymer nanocomposite. The incorporation of graphene (with a very low filler

content) makes the insulating polymer to electrically conductive nanocomposite and these


15

composite are widely used in electrostatic dissipation devices and electromagnetic

interference shielding which require electrical conductivity at the range of about 10-6 S m-

1.40 The cost effectiveness and lightweight of these composites are attractive features.41-54

Figure 2.10 shows the diversity of applications of graphene.

Figure 2.10. Numerous applications of graphene.

2.4.2 Applications of inorganic 2D layered materials

BNNSs adsorbs organic contaminations and thermally stable.32 Also BNNSs are

thermally conductive and mechanically strong; these collective properties are used to

fabricate cellulose wires with BNNSs.11,30 The combination of thermal conductivity and

electrical insulating property of BNNSs has potential applications in fabricating die

attachments, encapsulation of electric wire, and electronic packaging materials.31,55 The

direct band gap of MoS2 nanosheets can be potentially used in transistors, light-emitting

diodes, solar cells,56 and field-effect transistors (FET),57,58 ultra-high strength

nanocomposites. WS2 is a potential semiconducting material for solar energy

conversion59-63 and also extensively used as electrode in lithium batteries,4,64,65 catalyst,66


16

shock absorbers,6,67 and hydrogen storage.68 These different types of applications are

shown in Figure 2.11.

Figure 2.11. Numerous applications of BNNSs, MoS2, and WS2.

In the next two chapters, nanomaterial dispersion technique for graphene in

different ionic liquids and inorganic layered materials in different solvents in presence of

polymer will be discussed.


17

CHAPTER 3

DIRECT EXFOLIATION OF GRAPHENE IN IONIC LIQUIDS

3.1 Introduction

There are two common methods to exfoliate graphene from graphite:

micromechanical cleavage of graphite and liquid phase exfoliation.

Micromechanical cleavage of graphite

Initially the micromechanical cleavage technique was applied to isolate graphene

from graphite which is also known as the ‘scotch tape method’.1,17,69 In this method,

highly ordered pyrolytic graphite (HOPG) is etched with oxygen initially. Single-to-few

layer graphene sheets are separated from the HOPG by using a scotch tape. High-quality

graphene sheets are prepared by this method and the graphene is used for electrical or

mechanical characterization. This process is unsuitable for isolating graphene at a large

scale.

Liquid phase processing

In order to scale up the production of graphene, liquid-phase processing is a

better approach. Graphene layers are held together by van der Waals forces in graphite;

this creates problems both in exfoliating graphene from graphite as well as keeping

graphene dispersed in a liquid medium rather than aggregating. The liquid phase

processing has 2 main routes the graphene oxide route and the ‘pristine’ graphene route.

The most common approach to exfoliate graphene from graphite involves

oxidizing the graphite to produce graphene oxide by Hummer’s method. Graphene oxide

(GO) may subsequently be chemically or thermally treated to remove the covalent


18

functional groups and produce reduced graphene oxide (rGO).70-75 However some of the

functional groups remain in the rGO sheets since the reduction from GO to rGO is

incomplete.71 The properties of rGO sheets are not comparable to those of the pristine

graphene such as the electrical conductivity of rGO is lower than pristine graphene.71,76

Another way of exfoliation of graphene is to exfoliate (through sonication or high

shear) graphite powder in a solution of stabilizer molecules such as micelle-forming

surfactants, polymers, and aromatic hydrocarbons.77-79 However, there are some solvents

(organic and ILs) those can exfoliate graphene directly.80 Among the organic solvents

such as N-methyl pyrrolidone (NMP), and dimethylformamide (DMF) can successfully

disperse graphene sheets with low graphene yield.81 Moreover this requires extensive

sonication and graphene is unstable against centrifugation.80 Ionic liquids (ILs) can also

exfoliate graphene directly with high yield.82

ILs are molten salts, which are composed of ions and they are liquid below 100

ºC. ILs are chemically and thermally stable.83,84 The other properties of ILs include low

vapor pressure,85 low dielectric constant, and high electrical conductivity.84 Moreover,

ILs are non-flammable, recyclable,86 and capable of dissolving a range of solutes. The

properties of ILs such as miscibility and viscosity can be tuned via chemical changes to

the cation or anion.87 ILs are broadly used in the fields of analytical chemistry,88

electrochemical devices,89-91 fuel cells,84 separation process,89,92 bioscience,93,94 and in

various engineering fields.95,96

Wang et al. exfoliated graphene from graphite flakes by tip sonicating in 1-butyl-

3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide.97 Nuvoli et al. dispersed


19

graphene by sonicating graphite in l-hexyl-3-methylimidazolium hexafluorophosphate

and depending on different sonication time they reported yield of graphene ranging from

2.3 to 5.3 mg mL-1.98 Moreover, IL (1-butyl-3-methylimidazolium hexafluorophosphate)

can be used as solvent to exfoliate graphene and subsequently stabilize the graphene by

polymer IL (poly(1-vinyl-3-butylimidazolium chloride) with a concentration of graphene

of 0.05 mg mL-1.82

Prior reports have indicated that ILs can disperse graphene directly by sonicating

graphite in ILs without dispersant.97,98 However, the mechanisms behind IL-graphene

interactions are not well understood, and the existing literature gives little insight into

effective IL design for graphene dispersion. In this chapter, we demonstrate that ILs can

be used to directly disperse unfunctionalized or pristine graphene at high concentrations.

3.2 Materials and Methods

Materials

Expanded graphite (EG) was purchased from Asbury Carbons (CAS# 7782-42-5,

Grade 3772). Four different types of ILs were investigated : 1-benzyl-3-

methylimidazolium bis(trifluoromethylsulfonyl)amide ([BnzC1im][NTf2], IL-1), 1-butyl-

3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([bmim][NTf2], IL-2), 1-benzyl-

3-methylimidazolium bromide ([BnzC1im][Br], IL-3), and 1,3-

bis(phenylmethyl)imidazolium bis(trifluoromethylsulfonyl)imide ([(Bnz)2im][NTf2], IL-

4). Their chemical structures are presented in Figure 3.1. The ILs were synthesized and


20

provided by Dr. Tamas from Dr Quitevis group in Chemistry Dept, TTU. The synthetic

procedures of the ILs have been published elsewhere.99-101

Figure 3.1. Chemical structure of the ILs; (a) IL-1, (b) IL-2, (c) IL-3, and (d) IL-4. There are two phenyl groups in IL-4 and one in IL-1.

Preparation of graphene dispersions

Expanded graphite (10 mg mL-1) was added to the IL and then tip sonicated at

output wattage of 10W for one hour in an ice-water bath. After sonication, the dispersion

was centrifuged (Centrific Centrifuge 225, Fischer Scientific) at ~5000 rpm for 6 hours to

remove large aggregates. Further characterizations were performed on the collected

supernatant. Four different ILs varied in chemical composition (anion and cation) were

investigated. The concentration of graphene was measured by vacuum filtrating samples

through Teflon filter paper (Millipore, 0.2 µm). Dichloromethane was used to wash the

excess IL from the filter paper, and the weight difference yields a graphene concentration

estimate of 5.8 mg mL-1 in IL-4. The absorbance spectrum of the dispersion was

measured by Shimadzu UV-Vis spectrophotometer 2550 at wavelengths of 200-800 nm.

The Lambert-Beer law, A = αLC, where the absorbance A is proportional to the product

of concentration C and path length L was utilized to measure the extinction coefficient


21

(α) of the IL stabilized graphene dispersion. The pure IL was used as a blank to eliminate

the background effect.

Characterization of graphene/ionic liquid dispersions

Raman spectroscopy

In order to measure the degree of exfoliation, Raman spectroscopy was used. A

Thermo Scientific™ DXR Raman microscope was utilized. The filtered sample was used

to measure Raman spectrum using a 532 nm laser.

Microscopy

To measure the number of layers of graphene in a typical dispersed flake,

transmission electron microscopy (TEM) was performed. Hitachi H8100 was utilized to

image the sample and the accelerating voltage was 75 kV. The graphene dispersion was

drop cast on a lacy carbon coated 200-mesh copper grid. The grid was washed with

ethanol to remove the IL. Moreover, the thickness of the graphene film on filter paper

was measured by scanning electron microscope (SEM). It was observed in a Hitachi S-

4300. Also the morphology of graphene film was observed by SEM. The filtered sample

was mounted on double sided carbon tape and the accelerating voltage was 2 kV.

Electrical conductivity

Electrical resistivity of the vacuum filtered film was measured by standard two-

point probe method. The vacuum filtered sample was used to measure the resistivity by a

high resistance meter (Model-HR2, AlphaLab, Inc.). A number of measurements were

measured on different position of the film and the statistical average of the conductivity

(as calculated from resistance) was reported.


22

3.3 Results

3.3.1 Preparation of graphene dispersion

Graphene dispersions were obtained by tip sonicating and centrifuging expanded

graphite directly in IL. Four different ILs varied in chemical composition (anion and

cation) were investigated and the final graphene yield is summarized in Table 3.1. The

IL-4 was most effective in terms of exfoliating graphene. The sonicated sample in IL-4

was centrifuged to remove large aggregates. The supernatant containing graphene was

collected as shown in inset of Figure 3.2. The dispersion is stable after centrifugation.

3.3.2 Absorbance of graphene dispersion

The absorbance spectrum of graphene dispersion is shown in Figure 3.2. The

extinction coefficient was computed to be 3.2 x 103 mL mg-1 m-1 for IL-4 using Lambert-

Beer law. The value of the extinction coefficient is comparable (same order of

magnitude) with another system (l-hexyl-3-methylimidazolium hexafluorophosphate and

graphene) reported in literature as 1172 mL mg-1 m-1.98

Figure 3.2. Absorbance spectrum of IL stabilized graphene dispersion. The extinction co-efficient of the IL-4 stabilized graphene dispersion is computed as 3.2 x 103 mL mg-1 m-1 at 660 nm.


23

3.3.3 Dispersion characterization

Transmission electron microscopy (TEM)

We performed TEM analysis on the graphene dispersion in IL-4 to investigate the

number of layers in graphene sheets. The graphene dispersion was drop cast on a lacy

carbon coated 200-mesh copper grid. The grid was washed with ethanol to remove the IL.

The TEM images are depicted in Figure 3.3. Folded graphene flakes are observed in

Figure 3.3a. The magnified view of the flake (Figure 3.3b) indicates that there are 2-5

layers of graphene sheets present. The TEM shows that the flake size of graphene varies

from ~300 nm to 1200 nm.

Figure 3.3. TEM images of (a) graphene flakes stabilized by ionic liquid, (b) the magnified view of the flake edges indicating 2-5 layers of sheets. The inset of (a) shows an image of ionic liquid stabilized graphene dispersion which is stable after centrifugation.

Raman spectroscopy

To characterize the graphene dispersion in IL-4, Raman spectroscopy on a

vacuum-filtered film was utilized. In the spectrum (Figure 3.4), we observe a G peak


24

(associated with sp2 hybridization, common to graphene) at ~1600 cm-1 and a 2D peak at

~2700 cm-1. The shape of the 2D peak is characteristic of graphene rather than graphite,

confirming the presence of stable, exfoliated graphene in the dispersion.77,102 Near 1330

cm-1 an additional peak (D peak) is obseved which is associated with graphene edges in

sonicated pristine graphene dispersions.78

Figure 3.4. Raman spectrum of graphene in IL-4. In the plot the shift in G-peak confirms graphene exfoliation in IL-4.

Electrical conductivity

The electrical conductivity of the same vacuum-filtered graphene film was

measured to be 1.07 × 10-2 S/m. The SEM images of the top surface of the film are

depicted in Figure 3.5 and the inset of Figure 3.5a shows the vacuum filtered film on a

filter paper. The thickness of the film was measured from the SEM (Figure 3.5a) as ~6

µm. Individual graphene flakes are visible in the magnified view of the film (Figure

3.5b).


25

Figure 3.5. SEM images of graphene film on filter paper (a) prepared by vacuum filtration, and (b) magnified view of the film where graphene flakes are observed. The thickness of the film is ~6 µm and the electrical conductivity of the film is measured to be 1.07 x 10-2 S/m.

3.3.4 Effect of ionic liquid on dispersion

We compared the ability of the various ILs to disperse graphene as a function of

their chemical composition. Our results are summarized in Table 3.1. IL-2 and IL-3 did

not disperse graphene at measurable levels. However, both IL-1 and IL-4 did successfully

disperse graphene, and notably, IL-4 yielded an unusually high graphene concentration of

5.8 mg mL-1. These results indicate a much stronger interaction of graphene with IL-4

than with IL-1. This is correlated with the presence of two phenyl groups in IL-4 and one

phenyl group in IL-1. The two phenyl groups in IL-4 increases the interaction between

graphene and IL-4. Our prior work indicates that aromatic hydrocarbons undergo non-

covalent π-π stacking interactions with graphene.79


26

Table 3.1. List of ionic liquids tested for stable dispersion of graphene

Graphene was not dispersed when IL-2 was used. This may be associated with the

lack of aromatic groups in this IL. The cation in IL-2 contains a butyl group in contrast

with the phenyl group in IL-1. One prior study reported graphene dispersion up to 0.95

mg mL-1 using IL-2.97 The difference between their results and ours may be attributed to

several factors. First, the high sonication power (750 W) in the prior study used may have

caused some reductions in average sheet lateral size, which is correlated with higher

Type of Ionic Liquid Chemical Name Chemical formula

Stability of the dispersion

after centrifugation

Concentration (mg mL-1)

IL-1 [BnzC1im][NTf

2]

1-benzyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

Stable dispersion 0.081

IL-2 [bmim][NTf2]

1-butyl-3- methylimidazoliumbis (trifluoromethylsulfonyl)imide

Unstable dispersion -

IL-3 [BnzC1im][Br]

1-benzyl-3-metylimidazolium bromide

Unstable dispersion -

IL-4 [(Bnz)2im][NTf

2]

1,3-bis(phenylmethyl)imidazolium bis(trifluoromethylsulfonyl)imide

Stable dispersion 5.8


27

concentrations.103 A more likely cause is that insufficient centrifugation (only 20 minutes

in their case) and experimental error in measuring the sediment rather than the

supernatant introduced error into their estimates. It should be noted that a 47.5% yield as

they report is unprecedented and quite unlikely for sonication-based exfoliation. Another

paper reported high concentrations (5.33 mg mL-1) for 1-hexyl-3-methylimidazolium

hexafluorophosphate similar to ours (5.8 mg mL-1 in IL-4), but for extremely high (24 h)

sonication time.98 Again, high sonication times are known to decrease graphene sheet

lateral size, resulting in higher yields at the cost of smaller sheets.104

The difference between IL-1 and IL-3 lies in the anion. The bromide anion in IL-3

causes a sharp increase in viscosity such that exfoliation via sonication becomes

ineffective; this is a practical concern that prevents us from examining the actual cation-

graphene interactions in the case of IL-3.

The binding energy between graphene, the cations and the anion were calculated

utilizing density functional theory (DFT-D3) by Dr. Tamas and Dr. Quitevis (our

collaborators from Department of Chemistry, TTU).105,106 These calculations support the

experimental observations. The optimized structures and the intermolecular distances are

shown in Figure 3.6. The calculated binding energy between the dimethyl imidazolium

ring and graphene was -27.4 kcal mol-1 (Figure 3.6a).39 A more favorable interaction is

observed between the 1-benzyl-3-methylimidazolium cation and graphene with binding

energy of -28.7 kcal mol-1 (Figure 3.6b).39 The interaction between 1,3-

bis(phenylmethyl)imidazolium cation and graphene is the strongest with a binding

energy of -30.6 kcal mol-1 (Figure 3.6c). 39 The distance between graphene and cation in


28

Figure 3.6c is shorter than Figure 3.6b indicating presence of increased number of π-π

interactions; this pattern is also supported by the slight curvature of the graphene toward

the cation in case of IL4 (Figure 3.6c).

Figure 3.6. The integration between Graphene and IL with binding energy: (a) [C1C1im]+ cation and graphene, (b) IL-1 [BnzC1im]+ cation and graphene, and (c) IL-4 [(Bnz)2im]+ and graphene. The graphene sheet is slightly bent toward the cation as a result of additional phenyl groups (c). (the calculations were carried out by Dr. Tamas from Chemistry Dept. TTU

The computational calculations predict that π-π interactions between cation and

graphene are favorable for stabilizing graphene-ILs complexes. Hence, ILs can be

synthesized accordingly to successfully exfoliate and stabilize graphene.

a) ΔE = -27.4 kcal/mol

b) ΔE = -28.7 kcal/mol

c) ΔE = -30.6 kcal/mol


29

CHAPTER 4

LIQUID PHASE EXFOLIATION AND CRUMPLING OF INORGANIC LAYERED MATERIALS

4.1 Introduction

The emergence of graphene has prompted novel investigations of a wide array of

inorganic 2D nanosheets such as boron nitride nanosheets (BNNSs), molybdenum

disulfide (MoS2), and tungsten disulfide (WS2), whose structure is analogous to graphene.

The processing of these inorganic 2D nanosheets are carried out in liquid phase

exfoliation.16,107,108 These nanosheets need to be dispersed in liquid phase for applications

such as nanocomposites, dry lubricant, electronics, electronic insulator etc. Inorganic

nanosheet exfoliation and stabilization has been accomplished in organic solvents without

stabilizer but it requires extensive sonication and yields a comparatively low

concentration of nanosheets.108-110 Coleman and coworkers reported exfoliation of

BNNSs in isopropanol (IPA) as well as MoS2 and WS2 in N-methyl-2-pyrrolidone

(NMP).108 Additionally, they exfoliated BNNSs and MoS2 in water using sodium cholate

as a surfactant.16 A very recent study by Guardia et al. reported dispersing these inorganic

nanosheets in water using various non-ionic and anionic surfactants and at the same time

we were investigating the use of dispersant to stabilize inorganic layered materials in

water and some organic solvents.107 They used polyvinylpyrrolidone as well as other

dispersants to stabilize nanomaterials by using ultrasound bath cleaner. They reported

higher nanomaterial (metal dichalogenides) to surfactant ratio as 2.4-3.5. The sonication

time is a major factor of higher concentration (0.110 mg ml-1 BNNSs at 1 mg ml-1 PVP


30

concentration, 0.22 mg ml-1 MoS2 at 2 mg ml-1 PVP concentration, and 0.6 mg ml-1 WS2

at 10 mg ml-1 PVP concentration) of the nanomaterials. However, insufficient

centrifugation (at 1500 rpm for only 20 minutes in their case) may have contributed to the

higher concentrations, as lower centrifugation is unable to remove all unexfoliated

materials.

Polyvinylpyrrolidone (PVP) stabilizes graphene in water and wide range of

organic solvent as reported in our prior work.78 The structure of PVP is shown in Figure

4.1. We have generalized the dispersion technique to exfoliate inorganic 2D layered

materials (BNNSs, MoS2, and WS2) in water and other organic solvents using PVP as

stabilizer. Prior work on dispersion of inorganic nanosheets used organic solvents (NMP,

DMF, IPA etc.) or surfactants for aqueous dispersion.16,108 Our major goal was to develop

a facile technique to produce high quality and stable inorganic nanosheets (BNNSs,

MoS2, and WS2) dispersions in a range of solvents.

Figure 4.1. Chemical structure of polyvinylpyrrolidone (PVP).

Furthermore, we demonstrated that these 2D nanosheets can be converted to 3D

nanostructures by utilizing a spray drying technique. This work follows on prior

demonstrations that graphene and graphene oxide can be crumpled through an


31

aerosolization process. We demonstrated that this crumpling process can be

accomplished using simple industrial spray dryer. The nanosheets show a crumpled

morphology, and they are resistant to aggregation. This development is critical for the

deployment of inorganic nanosheets in high-performance materials.

4.2 Experimental Methods

Materials

Hexagonal boron nitride (hBN), molybdenum disulfide (MoS2) and

polyvinylpyrrolidone (PVP) (MW 10,000 g mol-1) were purchased from Sigma Aldrich.

Tungsten disulfide (WS2) was brought from Santa Cruz Biotechnology. Another type of

hBN was purchased from Industrial Supply Inc. The hBN form Sigma Aldrich (hBN-I)

has smaller flake size than the other one (hBN-II). The average size of small flake hBN is

~1 µm and large flake is ~3 µm. The organic solvents methanol, ethanol, isopropanol,

choloroform, dimethylformamide, dimethyl sulfoxide, and N-methyl-2-pyrrolidone were

purchased from Sigma Aldrich.

Preparation of layered material dispersions

Stable aqueous dispersions of nanomaterials were prepared using PVP (polymer)

as a dispersant. Using magnetic stirring, PVP (10 mg mL-1) was dissolved in water and

20 mg mL-1 parent material was added to this solution. The solution was then tip

sonicated in a water bath at output wattage of 10 W for an hour using a Misonix sonicator

(XL 2000). To remove large aggregates, the dispersion was centrifuged (Centrific


32

Centrifuge 225, Fischer Scientific) for 4 hours at ~5000 rpm. The same procedure was

followed to make dispersion of nanosheets in organic solvents.

The absorbance on the collected supernatant was measured by Shimadzu UV-vis

spectrophotometer 2550 at wavelengths of 200 nm to 800 nm. The Lambert-Beer law was

utilized to determine the concentration of nanomaterials using UV-vis spectroscopy. To

find the extinction coefficients, the nanomaterial concentrations were measured by

vacuum filtration by using Teflon filter paper (Millipore, 0.2 µm). The weight of the filter

paper before and after filtration were recorded. From the difference between these two

values the concentration of the nanomaterial was determined.

Characterization

The dispersion quality of PVP-stabilized nanomaterials in water was accessed by

high resolution transmission electron microscopy (TEM) in a Hitachi H8100 electron

microscope. The sample preparation for TEM imaging was performed by dropping 20 µL

of dispersion on to holey carbon grids and drying in air for a minute. The acceleration

voltage was 75 kV to image the samples.

To prepare freeze-dried powders of PVP-stabilized nanomaterial dispersions, the

dispersions were frozen in a freezer at -20°C. Then the frozen dispersions were freeze-

dried in a freeze-dryer (VitrisBenchtop Freeze Dryer) overnight. This resulted dry

powdered samples of nanomaterials. The redispersion of the freeze-dried powders were

performed by adding water without any need of sonication. These samples were

centrifuged to confirm the stability after redispersion.


33

The morphology of the powdered nanomaterial were observed in a HITACHI S-

4300 electron microscope. The accelerating voltage was 10 kV and the working distance

was 8 mm. Scanning electron microscope (SEM) samples were sputter coated with Au/Pd

in Hummer V Technics sputter coater at 10 mA current and 10 kV voltage for a minute at

a rate of 10 nm min-1.

Particle size distribution of the nanomaterial was determined at room temperature

using the principles of dynamic light scattering by a Zetatrac analyzer from Microtrac

Inc. In this measurement setup, two laser beams at 780 nm were irradiated to the aqueous

dispersion and the velocity distribution of particles suspended in the dispersing medium

gave the particle size distribution for the sample.

PVP-stabilized nanomaterial dispersions were processed in a spray dryer (Buchi

290 mini spray dryer) to yield crumpled nanosheets. The operating temperature and

pressure was 220°C and 60 psi respectively.

4.3 Results and discussion

4.3.1 Nanosheet dispersion

4.3.1.1 Preparation of inorganic layered material dispersion

The parent materials was tip sonicated in PVP solution and centrifuged to remove

large aggregates. The supernatant was collected and used for further characterization.

Figure 4.2a, Figure 4.2b, and Figure 4.2c show PVP-stabilized aqueous dispersions of

BNNSs, MoS2, and WS2, respectively.


34

Figure 4.2. Images of PVP stabilized aqueous dispersion of (a) BNNSs, (b) MoS2, and (c) WS2.

4.3.1.2 Absorbance of inorganic layered material dispersion

The absorbance was measured by a UV-vis spectrophotometer. The concentration

was then calculated from the Lambert-Beer law (A = αlC; where l is cell length, α is the

extinction coefficient). These results are somewhat surprising; these stable dispersions

demonstrate that PVP acts as a physisorbing dispersant in water for not only graphene but

disperse inorganic nanosheets as well.78 The absorbance spectra of PVP stabilized

nanomaterial dispersion and the linear plots are shown in Figure 4.3. The absorbance

spectra for BNNSs, MoS2, and WS2 are plotted in Figure 4.3a, Figure 4.3c, and Figure

4.3e, respectively. According to the Lambert-Beer, the absorbance (A) varies linearly

with concentration (C). This linear relationship between the absorbance and

concentration was utilized to calculate the value of α. The value of α were determined to

be 1.1 x 103 mL mg-1 m-1 for BNNSs at 350 nm (Figure 4.3b), 5.2 x 102 mL mg-1 m-1 for


35

MoS2 at 674 nm (Figure 4.3d), and 4.2 x 102 mL mg-1 m-1 for WS2 at 630 nm (Figure

4.3f) respectively.

Figure 4.3. Absorbance spectra of PVP stabilized (a) BNNSs, (c) MoS2, (e) WS2 dispersion and the linearity plot of PVP stabilized (b) BNNSs, (d) MoS2, and (f) WS2 dispersion to determine extinction coefficient.

4.3.1.3 Tyndall effect

To confirm the colloidal suspension of the nanomaterials, we used a 635 nm laser

light to confirm the Tyndall effect. (The nanomaterials present in the dispersion scatters

the laser light and the path of the laser light becomes visible in the dispersion.111) Such an

effect does not occur if the particle sizes are small (~1 nm). The Tyndall effect is visible

when the particle size (~40-900 nm) is below or near the wavelength of light, and here

the wavelength of red laser light is 635 nm. Figure 4.4 shows Tyndall effect on BNNSs

(Figure 4.4a), MoS2 (Figure 4.4b), and WS2 (Figure 4.4c).


36

Figure 4.4. Images of water (left) and the colloids of PVP stabilized nanomaterial aqueous dispersion (right) with a laser pointer beam passing from the left side through (a) BNNSs, (b) MoS2, and (c) WS2 (Tyndall effect).

4.3.1.4 Dispersion characterization

Scanning electron microscopy (SEM)

All the parent materials are initially compact and flat as shown in the SEM images

(Figure 4.5). The usual flake size of parent hBN powder (Figure 4.5a) is the smallest (~1

µm) compared to MoS2 and WS2. The apparent lateral size of parent MoS2 is ~2 µm

(Figure 4.5b) and WS2 is ~3 µm (Figure 4.5c), respectively. The magnified view of parent

BN flakes (inset of Figure 4.5a) shows an oval-shaped morphology whereas MoS2 is

irregularly shaped with rough edges (inset of Figure 4.5b), and WS2 flakes are also

irregularly shaped with straight, smooth edges (inset of Figure 4.5c).


37

Figure 4.5. SEM images of parent material (a) BN, (b) MoS2, and (c) WS2.

Transmission electron microscopy (TEM)

TEM was performed on the nanosheet dispersions to determine lateral size,

morphology, and the number of nanosheets layers. The TEM images of these dispersions

are presented in Figure 4.6. Most of the BNNSs exhibit smooth, curved edges with an

overall oval shape; the typical size of the nanosheets is ~700nm (Figure 4.6a). The shape

of the BNNSs matches with the TEM images of the exfoliated BNNSs.112,113 This shape

of the dispersed nanomaterial is quite unusual compared with the irregular complex

shapes seen in dispersed graphene; this regular shape of the dispersed nanosheets matches


38

the parent material depicted in Figure 4.5a. The similarity in shape between the parent

material and the dispersed nanosheet indicates that sonication does not radically alter or

break the nanosheet structure, in contrast to graphene nanosheets; these images are

similar to others depicted in recent studies on BN exfoliation.112,113 In the case of MoS2

nanosheets, irregular edges are observed and the typical sheet size is ~500 nm (Figure

4.6c). The nanosheets of WS2 exhibit straight edges with a “folded paper” appearance,

and typical sheet size is ~400 nm (Figure 4.6e). The number of layers in each dispersed

nanosheet were determined by the magnified view of the sheet edges (Figure 4.6b, Figure

4.6d, and Figure 4.6f) for BNNSs, MoS2, and WS2, respectively. The number of visible,

distinct layers at the edge confirm that the nanosheets are 2-5 layers thick, which is

consistent with prior work in stable nanosheet dispersions. Unusually, the BNNSs and

WS2 nanosheets show distinct, paper-like folds. In contrast, TEM images of graphene in

prior studies show that graphene has more tendencies to form irregular wrinkles and

folds.114,115 Some of these features (the regular oval shape) are indicative of differences in

parent material morphology; however, the straight edges and folds also indicate

differences in bending moduli and in sonication response.


39

Figure 4.6. TEM images of nanosheets. (a, b) BNNSs; (c, d) MoS2 nanosheets; (e, f) WS2 nanosheets. (a, c, e) shows folded BNNS, MoS2, and WS2 sheets. In (b, d, f) few-layer nanosheets are observed. The sheet edges indicate that the nanosheets are 2-5 layers thick.

Size characterization by Dynamic Light Scattering (DLS)

The effect of sonication time and source material (parent material with different

flake size) on the nanosheets size (hydrodynamic radius) of BNNSs in the dispersion was

investigated by dynamic light scattering (DLS) measurements. We utilized two different

source materials and investigated the size characterization. The different source materials

are described in the experimental section; the parent hBN-I (~1 µm) has a smaller flake

size than parent hBN-II (~3 µm). Size characterization was also performed at the

dispersion with different sonication time (1, 1.5, and 2 hours). Sonication is used to

exfoliate the nanosheets and reduce the number of layers; however, prior studies have


40

shown that sonication also simultaneously reduces the lateral area of dispersed

graphene.116 For the inorganic nanosheets in this study, Figure 4.7 and Figure 4.8

confirms that a similar process does occur. The observed final flake size depends on the

initial flake size, which indicates that the dispersed nanosheets have not reached a

“terminal flake size” analogous to those observed for sonicated carbon nanotubes.117 The

average particle size for hBN-I is 198, 225, and 253.6 nm after 1 hour, 1.5 hour, and 2

hour of sonication time, respectively (Table 4.1). Moreover, the average particle size for

hBN-II is 204, 174, and 206 nm after 1 hour, 1.5 hour, and 2 hour of sonication time,

respectively (Table 4.1). The effect of longer sonication time results in size reduction of

large nanosheets. Hence, with an increase in sonication time, the population of the

maximum sheet size decreases but the average sheet size increases. Nazarov et al.

reported average hydrodynamic radius of functionalized BNNSs in DMF as 334 nm.118

This phenomenon is plotted in Figure 4.7a, Figure 4.7b, Figure 4.7c (hBN-I) after 1 hour,

1.5 hour, and 2 hour of sonication time, respectively and Figure 4.8a, Figure 4.8b, Figure

4.8c (hBN-II) after 1 hour, 1.5 hour, and 2 hour of sonication time, respectively.

Table 4.1. Average and maximum sheet size of BN nanoparticle with sonication time.

Sample Sonication time (hour)

Average size (nm) hBN-I hBN-II

BNNSs/PVP/water

1 198 204

1.5 225 174

2 253 206


41

Figure 4.7. Particle size distribution of hBN-I after (a) 1 hour sonication, (b) 1.5 hour sonication, and (c) 2 hour sonication.


42

Figure 4.8. size distribution of hBN-II after (a) 1 hour sonication, (b) 1.5 hour sonication, and (c) 2 hour sonication.


43

Effect of parent material and PVP concentration on nanomaterial concentration

The effect of initial parent material concentration on the final concentration of

inorganic nanosheets was investigated. In all these experiments, 10 mg mL-1 PVP

concentration was used. The final dispersed nanomaterial concentration increases with

the initial parent material concentration as shown in Figure 4.9. This is in agreement

with our prior work on pristine graphene produced from graphite.78,79,115 The

proportionality of nanomaterial concentration to parent material concentration has

implications for the scalability of sonication-based methods; at low initial material

concentrations, there is a linear relationship describing the yield of nanosheets from

the raw material. At higher initial material concentrations, this trend plateaus due to

limitations in sonication effectiveness caused by the additional material and increased

viscosity. The effect of PVP concentration on the supernatant concentration was also

investigated. The concentration of parent material was taken as 20 mg mL-1. Figure

4.10 shows the nanomaterial concentration as a function of PVP concentration. Again,

at higher PVP concentrations, there is a plateau in obtained nanosheet concentration;

this is likely due to increase in viscosity and a resulting decrease in sonication

efficiency. A similar effect was seen for PVP-stabilized graphene.119

Error Estimations

To determine the variation in the concentration values for the inorganic

nanomaterials from each other, a fractional error was calculated for each of them using

the UV-vis spectrophotometer. Four samples of each nanomaterials were sonicated by

taking an identical concentration of ~20 mg mL-1 of parent materials and ~10 mg mL-1


44

of PVP stabilizer to determine the variation. At first, the average nanomaterials

concentration and standard deviation were calculated for each of the nanomaterials set.

Then, standard deviation was divided by the average concentration value to calculate

the fractional error for each case, which values were applied to all other data points

later. Since, the main source of error for this experiment was non-uniformity in

sonication efficiency, the fractional value of error was used to conduct the error

analysis of this experiment. From our prior measurement, we have seen that the error

in absorbance measurement is comparatively less than variation in dispersion. Thus,

the main reason behind the variable absorbance and concentration is the variation in

dispersion itself.


45

Figure 4.9. The final concentration of dispersed, stable nanosheets increase with the initial concentration of the parent material in the sonication vial (PVP concentration is 10 mg mL-1) (a) BNNSs, (b) MoS2, and (c) WS2. As in the case of graphene, the concentration of dispersed nanosheets increases with increasing initial material but plateaus at high initial concentration, possibly due to increased viscosity and decreased sonication efficiency.


46

Figure 4.10. The effect of increasing PVP concentration on the final nanomaterial concentration in the dispersion is demonstrated (20 mg mL-1 parent material in sonication vial) (a) BNNSs, (b) MoS2, and (c) WS2. Again, there is a plateau at higher PVP concentrations; this may be due to increasing viscosity and decreasing sonication efficiency.


47

Freeze-drying and redispersion

The PVP stabilized nanomaterial dispersions were freeze dried and redispersed

in water. All three freeze dried powders were redispersible in water without

sonication, as illustrated in Figure 4.11. The redispersed samples were centrifuged,

and no sedimentation of nanomaterial was observed. The absorption spectra of the

PVP stabilized nanomaterial dispersions before and after freeze-drying are plotted in

Figure 4.12. From the absorbance spectra we observe that the value of absorbance

changes to a significant amount before and after freeze-drying and we are currently

revisiting this factor.

Figure 4.11. Freeze-dried, PVP-stabilized dispersion in water (a) BNNSs, (b) MoS2, (c) WS2; freeze dried powder (d) BNNSs, (e) MoS2, (f) WS2; and redispersed in water (g) BNNSs, (h) MoS2, (i) WS2.


48

Figure 4.12. UV-Vis absorbance spectra before and after freeze-drying-redispersion (a) BNNSs, (b) MoS2, and (c) WS2.

Nanomaterial dispersion in organic solvents

Apart from making aqueous dispersion of inorganic nanomaterials (BNNSs,

MoS2, and WS2) stabilized by PVP, we also investigated the applicability of PVP as a

stabilizer in organic solvents such as methanol, ethanol, isopropanol, choloroform,

dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and N-methyl-2-pyrrolidone

(NMP). PVP successfully stabilizes BNNSs, MoS2, and WS2 methanol (Figure 4.13),

http://en.wikipedia.org/wiki/Dimethylformamide


49

ethanol (Figure 4.14), isopropanol (Figure 4.15), choloroform (Figure 4.16), DMF

(Figure 4.17), DMSO (Figure 4.18), and NMP (Figure 4.19) at 10 mg mL-1 PVP

concentration and 20 mg mL-1 parent material concentration. The nanomaterial

concentrations were determined by vacuum filtrating samples through Teflon filter

paper (Millipore, 0.2 µm) and dried overnight. The concentrations of the nanomaterial

were calculated from the weight difference of the filter paper before and after

filtration. The nanomaterial concentration depends on the sonication efficiency. The

absorbance spectra of the PVP stabilized nanomaterial dispersions are plotted. The

linear plots of absorbance as a function of concentration were utilized to calculate the

extinction coefficient . The supernatant concentration and the extinction coefficient

(BNNSs at 350 nm, MoS2 at 674 nm and WS2 630 nm) in the organic solvents are

reported in Table 4.2.


50

Table 4.2. Concentration of nanomaterial and extinction coefficient values in different solvents

Nano- materials Parameters Methanol Ethanol Isopropanol Chloroform DMF DMSO NMP

BNNSs

Conc (mg mL-1) 0.23 0.24 0.25 0.17 0.15 0.030 1.1

alpha (mL mg-1 m-1) 1.2 x 103 4.2 x 102 2.9 x 103 2.5 x 102 1.5

x103 2.0 x 103 1.1x 103

MoS2

Conc (mg mL-1) 0.080 0.26 0.030 0.10 0.10 0.10 0.68

alpha (mL mg-1 m-1) 6.9 x 102 8.5 x 102 1.3 x 103 5.9 x 101 4.2 x

103 2.3 x 103 1.9 x 102

WS2

Conc (mg mL-1) 0.13 0.33 0.070 0.28 0.30 0.050 0.13

alpha (mL mg-1 m-1) 8.3 x 102 3.4 x 101 2.2 x 102 3.2 x 101 5.5 x

102 1.2 x 103 5.0 x 102


51

Figure 4.13. Digital photographs of PVP stabilized nanomaterial dispersion in methanol. (a) BNNSs, (b) MoS2, and (c) WS2; the absorbance spectra of (d) BNNSs, (e) MoS2, (f) WS2; and linearity plot of PVP stabilized nanomaterial dispersion to determine the extinction coefficient (g) BNNSs, (h) MoS2, (i) WS2.


52

Figure 4.14. Digital photographs of PVP stabilized nanomaterial dispersion in ethanol. (a) BNNSs, (b) MoS2, and (c) WS2;the absorbance spectra of (d) BNNSs, (e) MoS2, (f) WS2; and linearity plot of PVP stabilized nanomaterial dispersion to determine the extinction coefficient (g) BNNSs, (h) MoS2, (i) WS2.


53

Figure 4.15. Digital photographs of PVP stabilized nanomaterial dispersion in isopropanol. (a) BNNSs, (b) MoS2, and (c) WS2; the absorbance spectra of (d) BNNSs, (e) MoS2, (f) WS2; and linearity plot of PVP stabilized nanomaterial dispersion to determine the extinction coefficient (g) BNNSs, (h) MoS2, (i) WS2.


54

Figure 4.16. Digital photographs of PVP stabilized nanomaterial dispersion in chloroform. (a) BNNSs, (b) MoS2, and (c) WS2; the absorbance spectra of (d) BNNSs, (e) MoS2, (f) WS2; and linearity plot of PVP stabilized nanomaterial dispersion to determine the extinction coefficient (g) BNNSs, (h) MoS2, (i) WS2.


55

Figure 4.17. Digital photographs of PVP stabilized nanomaterial dispersion in DMF. (a) BNNSs, (b) MoS2, and (c) WS2; the absorbance spectra of (d) BNNSs, (e) MoS2, (f) WS2; and linearity plot of PVP stabilized nanomaterial dispersion to determine the extinction coefficient (g) BNNSs, (h) MoS2, (i) WS2.


56

Figure 4.18. Digital photographs of PVP stabilized nanomaterial dispersion in DMSO. (a) BNNSs, (b) MoS2, and (c) WS2; the absorbance spectra of (d) BNNSs, (e) MoS2, (f) WS2; and linearity plot of PVP stabilized nanomaterial dispersion to determine the extinction coefficient (g) BNNSs, (h) MoS2, (i) WS2.


57

Figure 4.19. Digital photographs of PVP stabilized nanomaterial dispersion in NMP. (a) BNNSs, (b) MoS2, and (c) WS2;the absorbance spectra of (d) BNNSs, (e) MoS2, (f) WS2; and linearity plot of PVP stabilized nanomaterial dispersion to determine the extinction coefficient (g) BNNSs, (h) MoS2, (i) WS2.


58

4.3.2 Crumpling of inorganic nanosheets

It has been demonstrated that colloidal graphene dispersions can be

manipulated to yield a “crumpled ball” graphene morphology through aerosolization

and drying. Huang and coworkers developed a technique to crumple graphene oxide

(GO) and graphene nanosheets; they utilized a tube furnace to rapidly evaporate

aerosolized GO droplets and produced crumpled GO.120-123 The rapid drying of

aerosolized graphene oxide (GO) dispersion results in shrinkage of droplets; as these

droplets evaporate, surface tension compacts the GO to induce a highly wrinkled

structure.120-123 These crumpled graphene nanosheets are aggregation-and

compression-resistant in the solid state.120,122

Spray drying technique can be applied to crumple 2D layered nanosheets.

Spray dryer is easy to operate, can be widely used in spraying a wide range of

colloidal samples. In this process, micron sized droplets are produced using an

atomizer. The droplets evaporate in the hot air flow and produce a dry powder. The

dry powders are settled by a cyclone separator. The air is discharged from the

separator along with small entrained particles.

We applied the spray drying technique to BNNSs, MoS2, and WS2 to

investigate the universal use of this process; we demonstrated for the first time that

inorganic nanosheets can be crumpled. The dispersions were spray dried at 220°C and

60 psi. Figure 4.20a, 4.20b, and 4.20c illustrates the first-ever SEM images of

crumpled BNNSs, MoS2, and WS2, respectively. The inset shows a magnified view of

the crumpled sheets. The morphology of the crumpled particles indicates that all three


59

nanosheets deformed due to capillary forces exerted on them during droplet

evaporation. Our group is currently studying the mechanism of the nanosheets

deformation. High surface tension at the water-air interface induces buckling of the

nanosheets. Complete removal of the water results in 3D particles with several

dimples all over their surface. The typical particle size for all the samples was 1-2 µm

and display a near-monodisperse appearance from the SEM images.

Figure 4.20. SEM images of crumpled nanosheets collected from the spray dryer; (a) BNNSs, (b) MoS2, and (c) WS2. The inset shows magnified view of the crumpled nanosheets. The wrinkles and ridges indicate that they are hollow sheets with sides buckled in; this has been observed in the prior literature on elastic powder shells formed during spray drying of other particulate dispersions as well. The crumpled nanosheets are fairly monodisperse.


60

The schematic process of crumpling of nanosheets is illustrated in Figure 4.21.

In Figure 4.21a water droplet coming out of atomizer with nanosheets flown through

the spray drier by hot air. Evaporation of water concentrates the nanosheets which

forms shell of nanosheets on the surface of the droplet (Figure 4.21b). The final

crumpled morphology can be explained by the elastic shell deformation theory.124,125

As the water evaporates from the surface of the droplet, the pressure gradient at the

interface increases and leads to enhanced capillary forces. Consequently, the

nanosheets yield and buckle due to strong compression forces. Crumpled particles

comprised of many nanosheets are produced due to compression and squeezing of

nanosheets by isotropic capillary compression, and the final dry crumpled particles are

shown in Figure 4.21e.120,126 Depending on the wettability of the sheets, the

deformation may occur in the form of dimples on the surface (pristine graphene) or

wrinkles (as was observed in the case of GO due to its hydrophilic nature).120,127,128

Inorganic nanosheets have similar structure as pristine graphene sheets; all of them

lack the functional groups of graphene oxide and are less hydrophilic than GO. Hence,

deformation of inorganic nanosheets follows the same pattern as pristine graphene

Figure 4.21. Schematic diagram of the crumpling process of nanomaterial; (a) water droplet with nanosheets from atomizer, (b) shell of nanosheets formation by clustering of nanosheets, (c) coalescence and aggregation of nanosheets, (d) evaporation of water from shell and crumpling of nanosheets with wrinkles, and (e) crumpled particles with wrinkles and ridges.


61

sheets. The final morphology of crumpled nanosheets is similar to crumpled pristine

graphene sheets, both represent dimples on the surface instead of the wrinkle and

ripples that has been observed in the GO sheets.120 These results indicate that

crumpling of dispersed nanosheets through spray drying is generic to any 2D

nanosheet type.


62

CHAPTER 5

CONCLUSION AND RECOMMENDATIONS FOR FURTHER STUDY

In conclusion, we have investigated two separate novel techniques for

nanosheet dispersion. First, graphene was exfoliated in ILs without covalent

functionalization. Second, inorganic nanomaterials such as boron nitride nanosheets

(BNNSs), molybdenum disulfide (MoS2), and tungsten disulfide (WS2) were

exfoliated in presence of polymer in water and in different organic solvents. The

pristine nanosheets were characterized through TEM, SEM, and DLS.

Exfoliation and stabilization with high yield of pristine graphene nanosheets

has been obtained by using ILs without any dispersant. We performed experiments to

stabilize graphene in four different ILs with and without aromatic groups in the cation.

We found that the ILs without aromatic groups in the cation are not capable of

exfoliating graphene, whereas ILs with aromatic groups in the cation exfoliate

graphene successfully. We explored that the phenyl groups in the cation of the IL

interact non-covalently with graphene surface. The yield of graphene depends on the

number of phenyl groups in the cation. We also established that the interaction

between graphene and the cation depends on the ability of cation to π-π stack with

graphene. Among the four different ILs we explored to exfoliate graphene, IL-1 and

IL-4 were able to disperse graphene successfully. IL-4, containing two phenyl groups

in the cation, was the most effective one in stabilizing graphene with concentration of

5.8 mg mL-1. The number of layers and lateral size of graphene were observed from

the TEM images. The thickness of the graphene film was measured from SEM images.


63

The electrical conductivity of the graphene film was measured as 1.07 × 10-2 S m-1.

The most remarkable achievement was the high yield of exfoliated graphene in ILs

without covalent functionalization. The high yield of unfunctionalized graphene opens

a range of potential applications for fabrication of graphene based polymer

composites.

In the next section of the thesis, we demonstrated a simple and effective

technique to disperse inorganic layered nanomaterials in a range of solvents using a

polymer (polyvinylpyrrolidone, PVP) as dispersant. These nanosheets include boron

nitride nanosheets (BNNSs), molybdenum disulfide (MoS2), and tungsten disulfide

(WS2). The aqueous dispersions of the inorganic nanomaterials in presence of polymer

were stable after centrifugation. Moreover, the aqueous dispersions were freeze-dried

and the stability of the redispered sample in water were confirmed by centrifugation.

The value of the absorbance from the UV-Vis spectroscopy of the dispersion before

and after freeze-dry indicates that the concentration of the nanomaterials remain

unchanged. The morphology (lateral size, number of layers, thickness of film, and

hydrodynamic radius) of the nanosheets were measured by TEM, SEM, and DLS.

Furthermore, the aqueous dispersions were spray dried to form a 3D crumpled

nanosheet powder. The SEM images of the first ever crumpled nanosheets confirmed

the 3D morphology.


64

Recommendations for further study

This research deals with stabilizing nanomaterials as well as characterization

of them. The following needs have been identified for further study that would

make logical extensions of this research.

• The result from this research indicated that the yield of graphene depends

on the cation as well as the anion of the ILs. The phenyl groups in the

cation non-covalently interacts with graphene. However, the degree of

exfoliation depends on the anion as well. The anion can alter the viscosity

of the IL. For highly viscous IL, exfoliation via sonication is not effective

even in the presence of phenyl groups in the cation. The interaction

between graphene ILs can be predicted by density functional theory

calculation and the ILs with desired ions can be synthesized to exfoliate

graphene with high yield.

• The exfoliation in presence of dispersant is necessary to stabilize inorganic

layered materials. The stabilizer to nanomaterial ratio is very high which

can have adverse effects in composite application. Our group has

performed tests that support this claim in the case of graphene based

composites. In future studies, we can move forward towards removing

excess stabilizer from the dispersion to have high quality nanocomposites.

• The structure of the nanosheets such as crumpled and platelet would affect

the different properties in composites. The crumpled nanosheets are more

aggregation resistant particle with different properties compared to freeze-


65

dried powders. The two different types (2D and 3D) of nanosheets can be

used to fabricate composites and characterize different physical properties

such as the electrical insulation, electrical conductivity, thermal

conductivity, and chemical inertness.


66

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Exfoliation and characterization of 2D layered nanomaterials