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

National University HoChiMinh city
University of Technology

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


Trường Đại Học Bách Khoa TPHồ Chí Minh
Khoa Cơng Nghệ Vật Liệu

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
Chuyên ngành: Vật Liệu Cao Phân Tử và Tổ Hợp
Mã ngành: 605294

LUẬN VĂN THẠC SỸ

TPHCM, 2012

ĐẠI HỌC QUỐC GIA TP.HCM
TRƯỜNG ĐẠI HỌC BÁCH KHOA

CỘNG HÒA XÃ HỘI CHỦ NGHĨA VIỆT NAM
Độc lập – Tự do – Hạnh phúc

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 trị của q trình làm sạch đến khả năng phân tán của ống nano cacbon
đa thành.
• Đưa ra phương pháp thực nghiệm phân tán ống nano cacbon đa thành trong nước sử
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.

• Nghiên cứu sự ảnh hưởng của pH đến khả năng phân tán của ống nano cacbon đa
thành.
I. NGÀY GIAO NHIỆM VỤ :

14/02/2011

II.

02/12/2011

NGÀY HOÀN THÀNH NHIỆM VỤ :

III. CÁN BỘ HƯỚNG DẪN:

TS. LÊ VĂN THĂNG

TP. HCM ngày
CÁN BỘ HƯỚNG DẪN

CHỦ NHIỆM BỘ MÔN ĐÀO TẠO

tháng

năm 201

TRƯỞNG KHOA


CƠNG TRÌNH ĐƯỢC HỒN THÀNH TẠI
TRƯỜNG ĐẠI HỌC BÁCH KHOA –ĐHQG –HCM

Cán bộ hướng dẫn khoa học : TS. LÊ VĂN THĂNG
(Ghi rõ họ, tên, học hàm, học vị và chữ ký)

Cán bộ chấm nhận xét 1 : PGS. TS HÀ THÚC HUY
(Ghi rõ họ, tên, học hàm, học vị và chữ ký)

Cán bộ chấm nhận xét 2 : PGS. TS NGUYỄN ĐẮC THÀNH
(Ghi rõ họ, tên, học hàm, học vị và chữ ký)

Luận văn thạc sĩ được bảo vệ tại Trường Đại học Bách Khoa, ĐHQG Tp.
HCM ngày

tháng

năm 201

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).

CHỦ TỊCH HỘI ĐỒNG

TRƯỞNG KHOA

Nguyen Thi Minh Nguyet

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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Nguyen Thi Minh Nguyet

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
show

potential

applications,

e.g.

composites,

field

emission

devices,

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 nondestructive 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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Nguyen Thi Minh Nguyet

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 nonionic 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.

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Nguyen Thi Minh Nguyet

TÓM TẮT LUẬN VĂN
Việc phát hiện ra ống nano cacbon (CNTs) đã thu hút sự quan tâm to lớn của
nhiều nhà nghiên cứu trên thế giới do các tính chất đặc biệt của chúng như tính chất
về điện, quang, cơ, hóa học…Với những đặc tính hiếm có này, sản phẩm ống nano
carbon được đánh giá là có rất nhiều tiềm năng như ứng dụng trong vật liệu
composite, các thiết bị phát xạ trường, các vi điện tử, đầu dò, bộ lọc quang học và
các ứng dụng sinh học khác. Tuy nhiên, để có thể ứng dụng được CNTs trong phạm
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
q 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 khơng khí và xử lý
bằng axit clohydric (HCl), sau đó phân tán chúng trong mơi 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 trị của q trình làm sạch cũng được thể hiện rất rõ nét
trong việc phân tán MWNTs ở cấp độ nano.

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Nguyen Thi Minh Nguyet

Bằng cách sử dụng quang phổ UV-Vis, hệ số hấp thụ (ε) của MWNTs đã
được xác định. Thơng 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.
Thơng 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
Composite Thin Film”. Proceedings of The 3nd International Workshop on
Nanotechnology and Application IWNA 2011 - Vung Tau City, Vietnam, Nov 1214, 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
for Multi-Walled Carbon Nanotubes (MWNTs)”, Proceedings of The 12th
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
Khac Bien Cuong. “H2SO4/HNO3—Functionalization and Quantitative Assessment
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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Nguyen Thi Minh Nguyet

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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Nguyen Thi Minh Nguyet

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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Nguyen Thi Minh Nguyet

LIST OF SYMBOLS AND ABBREVIATIONS
CNTs

Carbon nanotubes

MWNTs

Multi walled carbon nanotubes

SWNTs

Single walled carbon nanotubes

CVD

Chemical vapor deposition

DI water

Deionized water

a1 , a2

Basic vectors of graphite

θ

Chiral angle

dt

The diameter of the nanotubes

ε

The extinction coefficient

M - raw

Raw, crude MWNTs

M -Pf

Purified MWNTs after filtering to remove
surfactant.

SEM

Scanning electron microscope

TEM

Transmission electron microscope

FTIR

Transform Fourier infrared spectroscopy

XRF

X-Ray fluorescence

TGA

Thermal gravimetric analysis

UV-Vis spectroscopy

Ultraviolet Visible spectroscopy
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Nguyen Thi Minh Nguyet

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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Nguyen Thi Minh Nguyet

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 rawMWNTs 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 X100 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 purifiedMWNTs 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 sp2
hybridized 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 DeviceMaterials 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 superhydrophobicity 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.
 Determining the optimum CNT-to-surfactant ratio for each surfactant.
 Influence of pH value on the stability of MWNTs’ suspension.

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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 hexagonaltype diamond; E: upper ends of shock compression/quench cycles that convert hextype 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.
 The transition line, the boundary between the graphite and stable diamond
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).

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Figure 2.2 Model of carbon allotropies
These allotropic solids can be classified into three major categories (Figure
2.2):
• The sp2 structures include graphite, the graphitic materials, amorphous carbon,
and other carbon materials.
• The sp3 structures involve diamond and lonsdaleite (a form detected in
meteorites).

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