Nghiên cứu quá trình làm sạch và phân tán ống nano cacbon đa thành (mwnts) trong nước bằng chất hoạt động bề mặt

NGUYỄN THỊ MINH NGUYỆT NGHIÊN CỨU QUÁ TRÌNH LÀM SẠCH VÀ PHÂN TÁN ỐNG NANO CACBON ĐA THÀNH MWNTs TRONG NƯỚC BẰNG CHẤT HOẠT ĐỘNG BỀ MẶT Mã ngành: 605294 LUẬN VĂN THẠC SỸ TPHCM, 2012...

NGUYEN THI MINH NGUYET

“STUDY THE PURIFICATION AND

SURFACTANT ASSISTED DISPERSION OF MULTI-WALLED CARBON NANOTUBES (MWNTs) IN AQUEOUS SOLUTION”

Major: Technology of High Molecular and Composite Materials

605294

THESIS

HCMC, 2012

NGUYỄN THỊ MINH NGUYỆT

NGHIÊN CỨU QUÁ TRÌNH LÀM SẠCH VÀ PHÂN TÁN ỐNG NANO CACBON ĐA THÀNH (MWNTs) TRONG NƯỚC BẰNG CHẤT HOẠT ĐỘNG BỀ MẶT

Mã ngành: 605294

LUẬN VĂN THẠC SỸ

TPHCM, 2012

NHIỆM VỤ LUẬN VĂN THẠC SĨ

Họ tên học viên: NGUYỄN THỊ MINH NGUYỆT MSHV: 09030932

Ngày, tháng, năm sinh: 24/07/1986 Nơi sinh: Lâm Đồng

Chuyên ngành: Công nghệ Vật liệu cao phân tử và tổ hợp

Tên đề tài:

NGHIÊN CỨU QUÁ TRÌNH LÀM SẠCH VÀ PHÂN TÁN ỐNG NANO CARBON ĐA

THÀNH (MWNTs) TRONG NƯỚC BẰNG CHẤT HOẠT ĐỘNG BỀ MẶT

NHIỆM VỤ VÀ NỘI DUNG :

• Thực hiện và đánh giá hiệu quả quy trình làm sạch ống nano cacbon đa thành

• Đánh giá vai trò của quá trình làm sạch đến khả năng phân tán của ống nano cacbon

đa thành

dụng hai loại chất hoạt động bề mặt khác nhau: Sodium Dodecyl Sulfate (SDS) and

Triton X – 100 Xác đinh hệ số hấp thụ (ε) – một thông số quan trọng trong việc

đánh giá định lượng sự phân tán của ống nano cacbon trong nước

• So sánh hiệu quả quả trợ phân tán của hai chất hoạt động bề mặt: SDS and Triton X

– 100 Xác định tỉ lệ tối ưu giữa ống nano cacbon đa thành/chất hoạt động bề mặt

thành

TP HCM ngày tháng năm 201

CÁN BỘ HƯỚNG DẪN CHỦ NHIỆM BỘ MÔN ĐÀO TẠO TRƯỞNG KHOA

Cán bộ hướng dẫn khoa học : TS LÊ VĂN THĂNG

Thành phần Hội đồng đánh giá luận văn thạc sĩ gồm:

1 GS NGUYỄN HỮU NIẾU

2 PGS TS HÀ THÚC HUY

3 PGS TS NGUYỄN ĐẮC THÀNH

4 TS LA THỊ THÁI HÀ

5 TS LÊ VĂN THĂNG

Xác nhận của Chủ tịch Hội đồng đánh giá LV và Trưởng Khoa quản lý chuyên ngành sau khi luận văn đã được sửa chữa (nếu có)

1

ACKNOWLEDGEMENT

It is impossible to complete this thesis without the help of many people

First of all, I wish to express my sincere appreciation to my research supervisor, Dr Van Thang Le, for his technical guidance and support during the course of this research work His assistance and suggestions were crucial in the realization of this work

I would like to thank my colleagues and friends in Department of Materials Science Fundamentals as well as the Key Laboratory of Materials Technology for their help related to carbon nanotubes, ideas for my research and most importantly,

an enjoyable working atmosphere

In addition, I would like to acknowledge and thank the Faculty of Materials Technology at Ho Chi Minh University of Technology for providing the opportunity and the financial wherewithal to accomplish my goals at HCMUT I am also thankful to National Key Lab of Polymer and Composite for transmission electron microscope facility

Finally, I would like to thank my parents for their love and support

Ho Chi Minh City, 12/2011

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ABSTRACT

The discovery of carbon nanotubes (CNTs) has attracted tremendous attention of many researchers due to their exceptional electronic, optical, mechanical and chemical properties With these particular characteristics, they

nanoelectronics, probe tips, optical filters and various biomedical applications However, in order to utilize CNTs in a wide range of applications, it is necessary to disperse them both at micro-scale and nano-scale

The two different approaches are currently being used to disperse CNTs, i.e., mechanical (or physical) methods and chemical methods Mechanical approaches, based primarily on ultrasonication, are time – consuming and less efficient Chemical methods use surfactants or chemical moieties to change the surface energy of the nanotubes Covalent functionalization involves the attachment of various chemical functional groups on the sidewalls of carbon nanotubes However, the aggressive chemical functionalization causes an increase in the defects on the sidewalls This can alter the electrical and mechanical properties of CNTs To diminish these defects and get highly dispersing, we integrate an effective but non-destructive purification and non-covalent modification to ensure that their structure

is not significantly disturbed

In this study, we first purified MWNTs with the facile process involved oxidation in the air and hydrochloric acid treatment, then dispersing them in aqueous solution with different surfactants: Sodium Dodecyl Sulfate (SDS) and Triton X-100 The final products got higher purity than 95%wt and the role of purification was strongly expressed in highly dispersing of MWNTs at nano-scale

By using UV-Vis spectroscopy, the extinction coefficient (ε) of MWNTs was determined Through this value, a comparative analysis on dispersion of MWNTs with two surfactants – Triton X-100 and SDS was reported Triton-X100 was

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observed to be higher dispersion than SDS An optimum CNT-to-surfactant ratio has been determined for each surfactant This parameter was shown to affect the nanotubes dispersion significantly Surfactant concentration above or below this ratio was shown to deteriorate the quality of nanotubes dispersion

The pH dependence of these surfactant assisted MWNT dispersions was also examined Deviations from neutral pH demonstrated negligible influence on non-ionic surfactant adsorption (Triton X-100) In contrast, anionic surfactant (SDS) was found to be poor dispersing aids for highly acidic and basic solutions and showed the maximum solubility near neutral pH conditions

Keywords: Multiwall carbon nanotubes (MWNTs), Dispersion, Surfactant, Purification, Extinction coefficient

vi rộng như thế, việc phân tán ống nano cacbon ở cả kích thước micro và nano là vấn đề vô cùng cần thiết và đáng được quan tâm

Hiện nay có hai cách tiếp cận khác nhau đang được sử dụng để phân tán CNTs là phương pháp cơ học (vật lý) và phương pháp hóa học Phương pháp vật lý chủ yếu dựa trên việc sử dụng sóng siêu âm (ultrasonication) nên tốn nhiều thời gian và hiệu quả không cao Phương pháp hóa học sử dụng các chất hoạt động bề mặt hoặc gắn các nhóm chức để thay đổi năng lượng bề mặt của các ống nano cacbon Tuy nhiên việc gắn các nhóm chức khác nhau sẽ làm gia tăng khuyết tật trên bề mặt ống nano cacbon Điều này có thể làm thay đổi tính chất điện và tính chất cơ của CNTs Để tránh được tình trạng này và cải thiện đáng kể độ phân tán của CNTs, chúng tôi đã đưa ra một phương pháp kết hợp đơn giản mà hiệu quả giữa quá trình làm sạch và biến tính bề mặt CNTs bằng cách sử dụng chất hoạt động bề mặt để đảm bảo rằng cấu trúc của chúng không bị ảnh hưởng đáng kể

Trong phạm vi luận văn này, đầu tiên chúng tôi sẽ làm sạch MWNTs thông qua một quy trình đơn giản bao gồm hai giai đoạn: oxi hóa trong không khí và xử lý bằng axit clohydric (HCl), sau đó phân tán chúng trong môi trường nước bằng cách

sử dụng các loại chất hoạt động bề mặt khác nhau: Sodium Dodecyl Sulfate (SDS)

và Triton X-100 Kết quả nghiên cứu cho thấy rằng sản phẩm cuối cùng có độ tinh khiết cao (95%) và vai trò của quá trình làm sạch cũng được thể hiện rất rõ nét trong việc phân tán MWNTs ở cấp độ nano

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Bằng cách sử dụng quang phổ UV-Vis, hệ số hấp thụ (ε) của MWNTs đã được xác định Thông qua giá trị này, phép phân tích so sánh sự phân tán của MWNTs khi sử dụng hai chất hoạt động bề mặt – Triton X-100 và SDS đã được thực hiện Kết quả cho thấy rằng Triton X-100 mang lại hiệu quả phân tán cao hơn SDS Tỷ lệ tối ưu giữa CNT với từng chất hoạt động bề mặt cũng được xác định Thông số này cho thấy có ảnh hưởng đáng kể đến sự phân tán ống nano cacbon Khi nồng độ chất hoạt động bề mặt sử dụng cao hơn hay thấp hơn tỉ lệ này đều làm giảm mức độ phân tán của MWNTs

Sự phụ thuộc pH của quá trình phân tán bằng chất hoạt động bề mặt cũng được khảo sát Kết quả cho thấy, quá trình phân tán MWNTs bằng Triton X-100 không bị ảnh hưởng đáng kể bởi pH của dung dịch Trái lại, quá trình phân tán sử dụng SDS lại giảm sút đáng kể trong môi trường acid hoặc bazơ và độ phân tán cao nhất có thể đạt được tại điểm có pH lân cận miền trung tính

Từ khóa: Ống nano cacbon đa thành (MWNTs), sự phân tán, chất hoạt động

bề mặt, quá trình làm sạch, hệ số hấp thụ

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LIST OF PUBLICATIONS

1 Cao Duy Vinh, Le Van Thang, Nguyen Thi Minh Nguyet “The Quantitative

Characterization of The Dispersion State of Multi-walled Carbon Nanotubes (MWNTs)” Journal of Chemistry, Vol.49 (3A), P.279-284, 2011, ISSN 0866-7144

2 Cao Duy Vinh, Le Van Thang, Nguyen Thi Minh Nguyet “Controlling the

Dispersion of Multi-wall Carbon Nanotubes in Deionized Water and Evaluating the Effects of Modified MWNTs on Electrical Conductivity of Polyvinylalcohol

International Workshop on Nanotechnology and Application IWNA 2011 - Vung Tau City, Vietnam, Nov 12-

14, 2011

3 Nguyen Thi Minh Nguyet, Le Van Thang, Nguyen Van Dong, Nguyen Thi

Hang, Luu Tuan Anh, Cao Duy Vinh “A Facile and Effective Purification Method

Conference of Materials Science and Technology, 2011, ISBN 978-604-73-0611-4

4 Cao Duy Vinh, Le Van Thang, Nguyen Thi Minh Nguyet, Luu Tuan Anh, Tran

Khac Bien Cuong “The Facile Process to Disperse and Separate MWNTs in Deionized Water”, Proceedings of The 12th

Conference of Materials Science and Technology, 2011, ISBN 978-604-73-0611-4

5 Cao Duy Vinh, Le Van Thang, Nguyen Thi Minh Nguyet, Luu Tuan Anh, Tran

on Modified Multi-Walled Carbon Nanotubes (MWNTs) Dispersion by Uv-Vis Spectroscopy”, Proceedings of The 12th

Conference of Materials Science and Technology, 2011, ISBN 978-604-73-0611-4

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TABLE OF CONTENTS

ACKNOWLEDGEMENT 1

ABSTRACT 2

TÓM TẮT LUẬN VĂN 4

LIST OF PUBLICATIONS 6

LIST OF SYMBOLS AND ABBREVIATIONS 11

LIST OF FIGURES 12

LIST OF TABLES 16

CHAPTER 1 INTRODUCTION 17

CHAPTER 2 OVERVIEW 19

2.1 CARBON NANOTUBES 19

2.1.1 Carbon allotropes 19

2.1.2 Bonding of carbon atoms 22

2.1.3 Structure of carbon nanotubes 25

2.1.4 Properties of carbon nanotubes 28

2.2 COMMON SYNTHESIS TECHNIQUES 30

2.2.1 Arc Discharge and Laser Ablation [24] 31

2.2.2 Chemical Vapor Deposition [26] 33

2.3 APPLICATIONS 33

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2.3.1 Nanotube as SPM tips 33

2.3.2 Nanotube transistors 34

2.3.3 Nanotube sensors 35

2.3.4 Solar cells 36

2.3.5 Nanocomposite 36

2.4 UNDERSTANDING SURFACTANTS IN DISPERSING CARBON NANOTUBES 37

2.4.1 General structural features and behavior of surfactants 38

2.4.2 Classification of surfactants 40

2.4.3 Micelle formation by surfactants 42

2.4.4 Mechanism of surfactant adsorption 45

2.4.5 Aggregation and current approaches for dispersing carbon nanotubes 46

2.4.6 The role of ultrasonication in surfactant adsorption for dispersing CNTs.48 2.5 CHARACTERIZATION METHODS 49

2.5.1 Scanning Electron Microscopy (SEM) 50

2.5.2 Transmission Electron Microscopy (TEM) 50

2.5.3 Raman Spectroscopy 51

2.5.4Thermal gravimetric analysis (TGA) 51

2.5.5 Infrared radiation (IR) spectroscopy 52

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2.5.6 Ultraviolet Visible (UV-Vis) spectroscopy 52

CHAPTER 3 METHODOLOGY AND EXPERIMENTAL 54

3.1 MATERIALS 54

3.2 APPARATUSES 55

3.2.1 Sonication bath 55

3.2.2 Ultrasonic processor 55

3.2.3 Magnetic and hotplate stirrer 56

3.2.4 Furnace 56

3.2.5 Centrifugal machine 57

3.2.6 Universal oven 57

3.2.7 pH meter 58

3.3 EXPERIMENTAL 59

3.3.1 Purification process 59

3.3.2 Determine the extinction coefficient of MWNTs using UV-Vis spectroscopy 60

3.3.3 Comparing the dispersing power of the two surfactants: SDS and Triton-X 63

3.3.4 Determination of optimum CNT to surfactant ratio using UV–Vis spectroscopy 64

3.3.4 Comparing the stability of MWNTs before and after purification 64

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3.3.5 Preparation of dispersion of MWNTs in various pH values 64

CHAPTER 4 RESULTS AND DISCUSSIONS 66

4.1 Properties of MWNTs source 66

4.2 Purification process 70

4.2.1 SEM and TEM 70

4.2.2 TGA 73

4.2.3 FTIR 75

4.2.4 Raman spectroscopy 76

4.3 The role of purification in the dispersion of MWNTs 77

4.4 Determining the extinction coefficient of MWNTs 80

4.5 Comparing the dispersing power of two surfactants 84

4.6 Determination of optimum CNT to surfactant ratio using UV–Vis spectroscopy 88

4.7 Effects of pH values on the stability of MWNTs’ suspension 90

CHAPTER 5 CONCLUSIONS AND PERSPECTIVES 95

REFERENCES 97

APPENDIX 109

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LIST OF SYMBOLS AND ABBREVIATIONS

1, 2

surfactant

12

LIST OF FIGURES

Figure 2.1 Carbon phase diagram 19

Figure 2.2 Model of carbon allotropies 21

Figure 2.3 Bonding structures of diamond, graphite, nanotubes, and fullerenes 24

Figure 2.4 Computer-generated images of carbon nanotubes [17] 25

Figure 2.5 Chiral vector and unit cell of CNT 26

Figure 2.6 Rolling the graphite sheet on different directions 27

Figure 2.7 Schematic of an arc-discharge apparatus, along with electron microscopy pictures of the products with doped and pure anodes 31

Figure 2.8 Schematics of a laser ablation set-up [25] 32

Figure 2.9 Schematic of CVD deposition oven [26] 33

Figure 2.10 SPM tips [28] 34

Figure 2.11 Diagram of nanotube transitor [29] 35

Figure 2.12 Electrical response of a semiconducting SWNT to NO2 gas molecules The evolution of the conductance with time depends clearly on the gas flow [30] 35 Figure 2.13 In a carbon nanotube-based photodiode, electrons (blue) and holes (red) - the positively charged areas where electrons used to be before becoming excited - release their excess energy to efficiently create more electron-hole pairs when light is shined on the device [31] 36

Figure 2.14 General structure of surfactant 39

Figure 2.15 Some common types of surfactants 42

Figure 2.16 Micellization [39] 43

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Figure 2.17 Schematic illustration of basic surfactant assembly structures [41] 45

Figure 2.18 Covalent addition reactions on the sidewall of carbon nanotubes [57] 47 Figure 2.19 Concept of particles separation by surfactants [65] 48

Figure 2.20 Mechanism of nanotube isolation from bundle obtained by ultrasonication and surfactant stabilization [68] 49

Figure 3.1 Elma (T460H) sonication bath 55

Figure 3.2 Ultrasonic processor Sonic Vibracell VC505 56

Figure 3.3 Magnetic and hotplate stirrer 56

Figure 3.4 Nabertherm furnace 57

Figure 3.5 EBA 21 centrifugal machine 57

Figure 3.6 Memert (UNB 400) Universal oven-Germany 58

Figure 3.7 Melter Toledo pH meter 58

Figure 3.8 Schematic flowchart of MWNTs purification 59

Figure 3.9 Schematic of multi-step treatment to make the stable suspension for estimating the extinction coefficient 62

Figure 4.1 XRF spectra of M-raw 66

Figure 4.2 a) SEM image and b) TEM image of raw MWNTs 67

Figure 4.3 TGA of M-raw 67

Figure 4.4 Raman spectra of M-raw 69

Figure 4.5 SEM images of a) Mraw and b) M-P 71

Figure 4.6 TEM images of a) Mraw, b) M460 and c) M-P 72

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Figure 4.7 TGA of MWNTs sample after each purification step 75

Figure 4.8 FTIR spectroscopy of a) M-raw and b) M-P 76

Figure 4.9 Raman spectra of M460 and M-P 76

Figure 4.10 Schematic of state of suspended MWNTs at nanoscale in water after

removing catalyst 78

Figure 4.11 Absorbance of 1.5mg/l suspension (diluting with a factor of 50) of raw-

MWNTs and purified-MWNTs after centrifuging at 3000rpm for 20 minutes (using SDS as dispersant) 79

Figure 4.12 Images of suspensions of M-P at concentration of 1.15 mg/ml during

various times (using SDS as dispersant) 80

Figure 4.13 UV-Vis spectrum of SDS/M-Pf (40:1) solution a) after 45 mins

sonicating b) after 55 mins sonicating and c) after centrifuging 82

Figure 4.14 a) UV-Vis absorbance of M-Pf dispersed in the presence of SDS for different concentrations and b) Beer – Lambert curve 82

Figure 4.15 a) UV-Vis absorbance of M-Pf dispersed in the presence of Triton

X-100 for different concentrations and b) Beer – Lambert curve 84

Figure 4.16 (a) UV-Vis spectra of carbon nanotubes in SDS solution (diluted with

a factor of 50) and (b) Beer – Lambert curve 85

Figure 4.17 (a) UV-Vis spectra of carbon nanotubes in Triton X-100 solution

(diluted with a factor of 50) and (b) Beer – Lambert curve 86

Figure 4.18 Percentage extractability vs concentration trend of carbon nanotubes

for (a) SDS and (b) Triton X-100 87

Figure 4.19 Chemical structures of (a) SDS and (b) Triton X-100 88

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Figure 4.20 Variation of percentage extractability with variation of concentration of

surfactant for (a) SDS and (b) Triton X-100 89

Figure 4.21 Mechanism of flocculation of CNTs via surfactant molecules [62] 90

Figure 4.22 Suspendability of MWNTs in (a) SDS and (b) Triton X-100 solution at different pH 91

Figure 4.23 The influence of pH on zeta potentials of CNTs The standard deviations were calculated with three replications [97] 92

Figure 4.24 Schematic representation of surfactant assisted adsorption of nanotubes using the ionic surfactants SDS with deference to pH effects on the nanotube surface 94

Figure 4.25 Flocculation occurs via surfactant tails at low pH (acidic conditions) 94 Figure A.1 TGA curve of SDS surfactant 112

Figure A.2 TGA curve of M-P and M-Pf after removing SDS 113

Figure A.3 FTIR spectra of a Mraw , b M-Pf and c M-P with Triton X-100 113

Figure A.4 FTIR spectra of a Mraw, b M-Pf and c M-P with SDS 113

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LIST OF TABLES

Table 2.1 Mechanical properties of CNTs – A comparison [22] 29

Table 2.2 Thermal conductivity of CNTs compare to other materials 30

Table 2.3 CMC values (g/l) of anionic, cationic and nonionic surfactants [40] 44

Table 3.1: Materials used in experiments 54

Table 4.1 The MWNTs’ outer diameter of Mraw and M460 73

Table 4.2 The remained weight of the CNTs sample after each purification step 75

Table 4.3 Comparison ID/IG ratio of Mraw, M460 and M-P 77

Table 4.4 Remained concentration of suspension raw MWNTs and purified-MWNTs samples after centrifuging at 3000rpm for 20 minutes 79

Table 4.5 The extinction coefficient value of M-Pf (using SDS as dispersant) 83

Table 4.6 The extinction coefficient value of M-Pf (using Triton X-100 as dispersant) 83

Table A.1 The outer diameter of Mraw, M460 and M-P 109

Table A.2 % extractability calculated for various concentrations of M-P dispersed in 1% SDS solution 109

Table A.3 % extractability calculated for various concentrations of M-P dispersed in 1% Triton X-100 solution 110

Table A.4 % extractability calculated for concentration of 1.6 mg/ml M-P dispersed in various concentrations of SDS solution 110

Table A.5 % extractability calculated for concentration of 2 mg/ml M-P dispersed in various concentrations of Triton X-100 solution 111

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CHAPTER 1 INTRODUCTION

Since their discovery by Iijima [1], carbon nanotubes (CNTs) have attracted considerable attention due to their exceptional mechanical, thermal, and electrical properties These unique properties have facilitated interest in CNTs for a wide array of applications including biomaterials [2], multi-functional composites [3,4], and electronic components [5] However, their high aspect ratio and propensity to aggregate into bundles makes disentanglement and dispersion non-trivial processes limiting commercial applicability [6] Dispersing nanotubes in solvents typically involves chemical treatment to enable debundling while simultaneously driving favorable interactions between the nanotube surface and supporting solvent Covalent methodologies rely on directly binding organic moieties to nanotube sidewalls or defect sites [7] Unfortunately, such bonding disrupts the intrinsic sp2hybridized network that gives rise to the nanotubes’ exceptional properties [8] In contrast, non-covalent approaches focus on spurring non-disruptive interactions such as π–π stacking, adsorption, or Coulomb interactions through insertion of a chemical bridging agent These approaches preserve the delocalized π-electron network of the nanotube sidewall ensuring minimal perturbation of the defect sensitive electrical and thermal properties [9] Surfactants and polymers are generally selected as the chemical bridging agents of choice [6]

In Vietnam, numerous studies have been carried out on nano materials, especially carbon nanotubes The Institute of Materials Science is one of the earliest institute produced CNTs successfully in 2002 After a few years, many different institutes such as The International Training Institute for Materials Science (ITIMS) and Institute of Engineering Physics - Hanoi University of Technology, R&D center

of Saigon High Tech Park have also shown their interest in this new material by producing MWNTs in large scale but the quality of products was restricted Furthermore, the National Key Laboratory of Materials and Electronic Device- Materials Science Institute - Vietnam Science - Technical Institute has also given an

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economical large – scale in production of MWNTs In 2009, Prof Phan Hong Khoi and Phan Ngoc Minh [10] applied successfully MWNTs in tungsten tips for field emission devices as well as Ni-MWNTs, Cr-MWNTs composite plating film In the same year, Bui Van Ga et al published his investigation about the super-hydrophobicity of PS/MWNTs composite which could be applied in the biogas storage [11] Recent years, some groups in Ho Chi Minh University of Technology have studied CNTs synthesis and application processes and obtained acceptable results Cao Duy Vinh et al have succeeded in investigating the MWNTs' functionalization process by mixture of sulfuric acid and nitric acid [12] Furthermore, by blending modified MWNTs with PVA, he has shown the potential results in improving the electricity conductivity of this composite [13] Prof Nguyen Huu Nieu et al [14] have also used modified MWNTs as the supporter for fabricating magnetic iron (III) oxide

Until now, there is no report in studying the dispersion of MWNTs using surfactants Knowing the very important role of dispersing process, this thesis will

be focusing on the following issues:

 Purifying and evaluating the effectiveness of purification process

 The role of purification in the dispersion of carbon nanotubes

 Providing new method for fast dispersing carbon nanotubes in aqueous solution using different surfactants (Sodium Dodecyl Sulfate (SDS) and Triton X – 100)

 Determining the extinction coefficient – a very important parameter for quantitative assessment of carbon nanotube dispersions

 Comparing the dispersing power of two surfactants: SDS and Triton X –

100

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CHAPTER 2 OVERVIEW

2.1 CARBON NANOTUBES

2.1.1 Carbon allotropes [15]

 Carbon phase diagram

The carbon phase diagram at high pressure (>1 GPa) is shown in Figure 2.1 below The phase diagram presents several main features:

Figure 2.1 Carbon phase diagram

Solid lines represent equilibrium phase boundaries A: synthesis of diamond from graphite by catalysis; B: P/T threshold of very fast solid-solid transformation of graphite to diamond; C: P/T threshold of very fast transformation of diamond to graphite; D: single crystal hexagonal graphite transforms to retrievable hexagonal- type diamond; E: upper ends of shock compression/quench cycles that convert hex- type graphite particles to hex-type diamond; F:upper ends of shock

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compression/quench cycles that convert hex-type graphite to cubic-type diamond;

B, F, G: threshold of fast P/T cycles, however generated, that convert either type of graphite or hexagonal diamond into cubic-type diamond; H, I, J: path along which

a single crystal hex-type graphite compressed in the c-direction at room

temperature

regions, runs from 1.7GPa/ 0°K to the graphite/diamond/liquid triple point I

at 12GPa/5000°K

 The graphite/liquid/vapor triple point, the graphite/vapor phase boundary and the liquid/vapor phase boundary occur at pressures too low for scale of diagram (not presented here)

 The melting line of graphite extending from the graphite/liquid/vapor triple point at 0.011 GPa/ 5000°K to the graphite/ diamond/ liquid triple point at 12 GPa/5000°K

– The dotted line (diamond GFB) represents the graphite-diamond kinetic transformation under shock compression and quenches cycles

– The diamond melting line runs at high P and T , above the triple point

 The Carbon allotropes

In the above sections, we discussed the various ways that carbon atoms bond together to form solids These solids are the allotropes (or polymorphs) of carbon They have the same building block but with different atomic hybrid configurations:

sp3 (tetragonal), sp2 (trigonal) or sp (digonal)

21

Figure 2.2 Model of carbon allotropies

These allotropic solids can be classified into three major categories (Figure 2.2):

2.1.2 Bonding of carbon atoms [16]

To understand the structure and properties of nanotubes, the bonding structure and properties of carbon atoms are discussed first A carbon atom has six electrons with two of them filling the 1s orbital The remaining four electrons fill the sp3 or sp2 as well as the sp hybrid orbital, responsible for bonding structures of diamond, graphite, nanotubes, or fullerenes, as shown in Figure 2.3

orbital and create four equivalent σ covalent bonds to connect four other carbons in the four tetrahedral directions This three-dimensional interlocking structure makes diamond the hardest known material Because the electrons in diamond form covalent σ bonds and no delocalized π bonds, diamond is electrically insulating The electrons within diamond are tightly held within the bonds among the carbon atoms These electrons absorb light in the ultraviolet region but not in the visible or infrared region, so pure diamond appears clear to human eyes Diamond also has a high index of refraction, which makes large diamond single crystals gems Diamond has unusually high thermal conductivity

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In graphite, three outer-shell electrons of each carbon atom occupy the planar

sp2 hybrid orbital to form three in-plane σ bonds with an out-of-plane π orbital (bond) This makes a planar hexagonal network Van der Waals force holds sheets

of hexagonal networks parallel with each other with a spacing of 0.34 nm The σ

kcal/mol in sp3 configuration Therefore, graphite is stronger in-plane than diamond

In addition, an out-of-plane π orbital or electron is distributed over a graphite plane and makes it more thermally and electrically conductive The interaction of the loose π electron with light causes graphite to appear black The weak Van der Waals interaction among graphite sheets makes graphite soft and hence ideal as a lubricant because the sheets are easy to glide relative to each other

A CNT can be viewed as a hollow cylinder formed by rolling graphite sheets Bonding in nanotubes is essentially sp2 However, the circular curvature will cause quantum confinement and σ – π rehybridization in which three σ bonds are slightly out of plane; for compensation, the π orbital is more delocalized outside the tube This makes nanotubes mechanically stronger, electrically and thermally more conductive, and chemically and biologically more active than graphite In addition, they allow topological defects such as pentagons and heptagons to be incorporated into the hexagonal network to form capped, bent, toroidal, and helical nanotubes whereas electrons will be localized in pentagons and heptagons because of redistribution of π electrons For convention, we call a nanotube defect free if it is

of only hexagonal network and defective if it also contains topological defects such

as pentagon and heptagon or other chemical and structural defects

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Figure 2.3 Bonding structures of diamond, graphite, nanotubes, and fullerenes:

when a graphite sheet is rolled over to form a nanotube, the sp 2 hybrid orbital is deformed for rehybridization of sp 2 toward sp 3 orbital or σ - π bond mixing This rehybridization structural feature, together with π electron confinement, gives nanotubes unique, extraordinary electronic, mechanical, chemical, thermal, magnetic, and optical properties

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Fullerenes (C60) are made of 20 hexagons and 12 pentagons The bonding is also sp2, although once again mixed with sp3 character because of high curvature The special bonded structures in fullerene molecules have provided several surprises such as metal–insulator transition, unusual magnetic correlations, very rich electronic and optical band structures and properties, chemical functionalizations, and molecular packing Because of these properties, fullerenes have been widely exploited for electronic, magnetic, optical, chemical, biological, and medical applications

2.1.3 Structure of carbon nanotubes

Iijima was first to recognize that nanotubes were concentrically rolled graphene sheets with a large number of potential helicities and chiralities rather than

a graphene sheet rolled up like a scroll as originally proposed by Bacon Iijima initially observed only MWNTs with between 2 and 20 layers, but in a subsequent publication in 1993, he confirmed the existence of single-walled carbon nanotubes (SWNTs) and elucidated their structure The exact properties of CNTs are extremely sensitive to their degree of graphitization, diameter (or chirality), and whether they are in single wall or multi wall form (Figure 2.4a and b, respectively) Single-walled carbon nanotubes (SWNTs), which are seamless cylinders, each made of a single graphene sheet, were first reported in 1993 Multi-walled carbon nanotubes (MWNTs), consisting of two or more seamless graphene cylinders concentrically arranged, were discovered two years previously [17]

Figure 2.4 Computer-generated images of carbon nanotubes [17]

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In order to describe fundamental characteristic of the nanotubes, two

vectors, Ch and T, are introduced in Figure 2.5 [15]

Figure 2.5 Chiral vector and unit cell of CNT

Ch is the vector that defines the circumference on the surface of the nanotube connecting two equivalent carbon atoms

The chiral angle is used to separate carbon nanotubes into three different classes by their electronic

Where: and are two basic vectors of graphite

n and m are integers

n and m are also called indexes and determine the chiral angle

Figure 2.6 Rolling the graphite sheet on different directions

In three classes of nanotubes, armchair carbon nanotubes are metallic (a degenerate semi-metallic with zero band gap), zig-zag and chiral nanotubes can be semimetals with a finite band gap if n-m/3 = i (i being an integer and n ≠m ) or semiconductors in all other cases (Figure 2.6)

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2.1.4 Properties of carbon nanotubes

Carbon nanotubes have many magnificent properties that have attracted researchers of many disciplines into intensive studies [16, 17, 18] A compilation of the mechanical, thermal and electrical properties are discussed below

2.1.4.1 Electronic properties

The most important electrical property is the conductance/resistivity

of any material Another property is the maximum current density Carbon based materials have shown interesting electrical properties Graphite is a good conductor whereas diamond is a very good insulator Carbon nanotubes’ chirality is related to its electrical behavior For a given (n, m) nanotube, armchair carbon nanotubes are metallic (a degenerate semimetal with zero band gap), zig-zag and chiral nanotubes can be semimetals with a finite band gap if n-m/3 = i (i being an integer and n ≠ m)

or semiconductors in all other cases

Several experiments have concluded that CNTs behave like quantum wires

In a study [19], multi walled nanotubes were found to exhibit ballistic conductance

In another investigation [20] the resistivity of ropes of metallic SWCNTs was found out to be 10-4 Ω.cm at 300 K In the same study the current density is reported to be greater than 107A/cm2 Another study [21] reports that the current density can be increased as high as 1013 A/cm2 The extremely small size and superior electrical properties makes carbon nanotubes very suitable for small scale electronics applications

2.1.4.2 Mechanical properties

Nanotubes are rolled-up graphene sheets, and graphene is one of the stiffest materials when subjected to deformations parallel to the sheet It comes therefore not as a surprise that nanotubes show exceptional mechanical properties, especially

a high strength-to-weight ratio

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Several theoretical, experimental and computational approaches have been used to find the mechanical properties that include the stiffness and strength Reported values of the properties vary very widely A brief comparison of the elastic properties (Young’s modulus and the tensile strength) and density of SWCNTs, SWNT bundles, MWCNTs, graphite (in-plane) and steel is shown in Table 2.1

Table 2.1 Mechanical properties of CNTs – A comparison [22]

Young’s modulus (GPa)

Tensile strength (GPa)

Density (g/cm3)

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Prior to CNTs, diamond was the best thermal conductor CNTs have now been shown to have a thermal conductivity at least twice that of diamond Indeed, recent theoretical work [23] has predicted that the room-temperature thermal conductivity of individual nanotubes is as high as 6600W/m.K Measurements show

a room-temperature thermal conductivity over 200 W/m.K for bulk samples of single-walled nanotubes, and over 3000 W/m.K for individual multi-walled nanotubes (Table 2.2) Additions of nanotubes to epoxy resin can double the thermal conductivity for a loading of only 1%, showing that nanotube composite materials may be useful for thermal management applications

Table 2.2 Thermal conductivity of CNTs compare to other materials

2.2 COMMON SYNTHESIS TECHNIQUES

There are many techniques used to produce MWNTs or SWNTs Methods such as electric arc discharge, laser ablation and chemical vapour deposition techniques are well established to produce a wide variety of CNTs These methods are described in following sections

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2.2.1 Arc Discharge and Laser Ablation [24]

Figure 2.7 Schematic of an arc-discharge apparatus, along with electron

microscopy pictures of the products with doped and pure anodes

The first method that was successfully used to synthesize CNTs in small quantities was the arc discharge method (Figure 2.7) Opposing anode and cathode terminals made of 6-mm and 9-mm graphite rods respectively are placed in an inert environment (He or Ar at ~500 Torr) A strong current, typically around 100 A (DC

or AC), is passed between the terminals generating arc-induced plasma that evaporates the carbon atoms in the graphite The nanotubes grow from the surface

of these terminals A catalyst can be introduced into the graphite terminal Although MWNTs can be formed without a catalyst, it has been found that SWNTs can only

be formed with the use of a metal catalyst such as iron or cobalt

A process called Laser Ablation (Figure 2.8), first developed in 1995, uses a similar principle to produce nanotubes Carbon is evaporated at high temperatures

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from a graphite target using a powerful and focused laser beam In the most basic laser ablation technique, a 1.25-cm diameter graphite target is placed in a 2.5-cm

99.99% pure argon to a pressure of 500 Torr A pulsed Nd-YAG laser beam at 250mJ (10 Hz) is focused using a circular lens and the beam is swept uniformly across the graphite target surface The nanotubes, mixed with undesired amorphous carbon, are collected on a cooled substrate at the end of the chamber

Both of these methods have limited potential for scale-up Solid graphite must be evaporated at >3000°C to source the carbon needed, the nanotubes produced are in an entangled form, and extensive purification is required to remove the amorphous carbon and fullerenes that are naturally produced in the process

Figure 2.8 Schematics of a laser ablation set-up [25]

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2.2.2 Chemical Vapor Deposition [26]

Figure 2.9 Schematic of CVD deposition oven [26]

In the CVD process, growth involves heating a catalyst material to high temperatures (160–12000C) in a tube furnace using a hydrocarbon gas through the tube reactor over a period of time The basic mechanism in this process is the dissociation of hydrocarbon molecules catalyzed by the transition metal and saturation of carbon atoms in the metal nanoparticle Precipitation of carbon from the metal particle leads to the formation of tubular carbon solids in a sp2 structure

The characteristics of the carbon nanotubes produced by CVD method depend on the working conditions such as the temperature and the pressure of operation, the volume and concentration of hydrocarbon gas such as methane, acetylene, methylene or carbon monoxide, the size and the pretreatment of metallic catalyst, and the time of reaction

2.3 APPLICATIONS

Because of their special physico-chemical properties, CNTs are expected to play a major role in numerous applications

2.3.1 Nanotube as SPM tips [27]

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The exceptional mechanical strength of nanotubes makes them also attractive

as tips for scanning probe microscopies Such tips are usually silicon cantilevers or metal wires that are etched to form a sharp point They can achieve high resolution because of small protusions, but they seldom survive a tip crash (accidental contact with the observation surface) Nanotubes are in this respect far more resistant The great advantage of nanotubes is however their slender shape and well-defined end Because of their large aspect-ratio, they are able to reach down deep trenches and to image sharp topographies with good resolution when attached to the silicon cantilevers of conventional atomic force microscopes

Figure 2.10 SPM tips [28]

2.3.2 Nanotube transistors [27]

Carbon nanotubes are ideal candidates for novel molecular devices because

of their electronic properties depend rather on their geometry than on doping by impurities, which results in high thermal stabilities There have been thus several realizations of carbon-based electronic devices, more specifically of transistors

Such a switching device that consists of one semiconducting single-wall nanotube connected to two metal electrodes (Figure 2.11) By applying a voltage to

a gate electrode, the nanotube can be switched from a conducting to an insulating state, even at room temperature

single-resistance of a semiconducting SWNT was found to dramatically increase or

decrease These nanotube sensors exhibit a fast response and a substantially higher sensitivity than that of existing solid-state sensors at room temperature An

individual nanotube can be furthermore used to detect different types of molecules The selectivity is achieved by adjusting the electrical gate to set the SWNT in an initial conducting or insulating state

Figure 2.12 Electrical response of a semiconducting SWNT to NO2 gas molecules The evolution of the conductance with time depends clearly on the gas flow [30]

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2.3.4 Solar cells

Using a carbon nanotube instead of traditional silicon, researchers have created the basic elements of a solar cell that hopefully will lead to much more efficient ways of converting light to electricity than now used in calculators and on rooftops

Figure 2.13 In a carbon nanotube-based photodiode, electrons (blue) and holes

(red) - the positively charged areas where electrons used to be before becoming excited - release their excess energy to efficiently create more electron-hole pairs

when light is shined on the device [31]

2.3.5 Nanocomposite

One of the first commercial applications of multi–walled carbon nanotubes is

in its use as electrically conducting materials in polymer composites The combination of high aspect ratio, stiffness, mechanical strength, low density, small size and high conductivity makes carbon nanotubes ideal substitutes to carbon fibers as reinforcements in high strength, low–weight and high performance polymer composites In addition, incorporation of carbon nanotubes in plastics can potentially result in remarkable increase in the modulus and strength of structural materials