Nanostructured materials based on molybdenum disulfide (mos2) and carbon nanotubes (cnts) for

VIETNAM NATIONAL UNIVERSITY – HO CHI MINH CITY

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

NGUYEN THI MINH NGUYET

NANOSTRUCTURED MATERIALS BASED ON

MOLYBDENUM DISULFIDE (MoS2) AND CARBON

NANOTUBES (CNTs) FOR LITHIUM-ION BATTERIES AND

HYDROGEN EVOLUTION ELECTROCATALYSTS

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF

PHILOSOPHY

HO CHI MINH CITY - 2022

VIETNAM NATIONAL UNIVERSITY – HO CHI MINH CITY

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

NANOSTRUCTURED MATERIALS BASED ON

MOLYBDENUM DISULFIDE (MoS2) AND CARBON

NANOTUBES (CNTs) FOR LITHIUM-ION BATTERIES AND

HYDROGEN EVOLUTION ELECTROCATALYSTS

Major: Materials Technology

Major code: 62520309

Independent reviewer: Assoc. Prof. Dr. Nguyen Dinh Thanh

Independent reviewer: Assoc. Prof. Dr. Hoang Thi Kim Dung

Reviewer: Prof. Dr. Nguyen Cuu Khoa

Reviewer: Assoc. Prof. Dr. Huynh Ky Phuong Ha

Reviewer: Assoc. Prof. Dr. Le Vu Tuan Hung

SUPERVISORS

1. Assoc. Prof. Dr. Le Van Thang

2. Dr. Nguyen Huu Huy Phuc

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COMMITMENT

The author hereby declare that this is the original research work. The research findings

and conclusions in this thesis are honest, and have not been copied from any source and

in any form. Sources (if any) have been properly cited and referenced.

The author

Signature

NGUYEN THI MINH NGUYET

ii

ABSTRACT

This doctoral work studies MoS2 and MoS2/CNT nanostructures and also discusses how

structural design can successfully address their challenges in lithium-ion batteries and

electrocatalysts. Recent advances in nanoparticle synthesis, nanostructure design, and

composite fabrication are summarized and discussed, as well as their impact on

electrochemical performance. Additionally, the remaining challenges and opportunities

for further improvement are discussed.

This study reviews the development of the microwave method, introduces the reaction

mechanism, and focuses on the practical application of this method. The microwave￾assisted synthesis of inorganic nanostructures in MoS2 in polyols is also discussed.

MoS2 nanoscale prepared in the presence of a strong microwave absorber is rapidly

formed in minutes, yielding clean reactions with different morphologies and sizes. In

this work, a microwave-assisted technique was successfully used to produce the hybrid

material 1T/2H-MoS2. The highest 1T concentration reached was 84.5% compared to

the 2H phase. It is believed that the hybrid nanostructures display superior

electrochemical performance due to the metallic 1T phase's enhanced electrical

conductivity.

Apart from that, the research provides a general picture of the employable materials

synthesis processes. Many review articles have been published on the microwave￾assisted synthesis of nanostructured materials. Microwave heating can affect the

reaction rate by shortening the reaction time. The rapid heating rate and/or

"superheating" may change the reaction mechanism. The scaling up of microwave￾assisted chemical reactions is very important for industrial scale production and

applications of nanostructured materials. In this study, microwave heating was also

demonstrated to be efficient in wet chemical reactions for the synthesis of MoS2 and

MoS2/CNT nanocomposites.

The formation of the MoS2/CNTs nanocomposite in the forms of crystalline and

amorphous structures was achieved using the two dispersion processes for

iii

functionalized MWNTs, indirect two–pot dispersion (I2PD) and direct two–pot

dispersion (D2PD), respectively.

The conditions for optimizing synthesis of crystalline MoS2/CNTs are as follows:

• f-CNTs (10 g/L) amount: 4mL

• Time of reaction: 60 mins.

• Solvent amount: 240 mL

• Microwave power: 240 W

• Temperature of ultrasonication: 60 oC

The conditions for optimizing synthesis of amorphous MoS2/CNTs are as follows:

• Microwave power: 240 W

• Time of reaction: 45 mins.

• Temperature of ultrasonication: 80 °C

• f-CNTs amount:40 mg.

• ∑m (AMH+TU) : VEG = 0.06 g/mL

The Taguchi experimental method also determined the effect of factors on the efficiency

of MoS2/CNTs material synthesis. The results reveal that microwave power has the

most impact on crystallinity ‘higher-is-better’ and reaction time has the greatest impact

on the Tafel slope “lower-is-better”.

Using linear sweep voltammetry (LSV) at a scan rate of 1 mVs-1

, the catalytic potential

of MoS2/CNTs nanocomposites for the HER reaction was investigated (Tafel plot).

Amorphous MoS2/MWNTs exhibit catalytic capability and stability in the -220 ÷ -230

mV (vs. NHE) range, with a current density of -8.94 mA/cm2

(V = -350 mV vs. NHE)

and a Tafel slope of 102 mV/dec.

A microwave-assisted technique was used to successfully produce a nanocomposite of

crystalline MoS2/CNTs for use as an anode material in lithium-ion batteries. The

discovery that the crystalline MoS2/CNTs (LA-MSC-opt) electrodes retain their

performance after 54 cycles at a scan rate of 100 mV/s demonstrates that the material is

capable of stabilizing the charge-discharge capacity over an extended period of time

without structural deterioration. The crystaalline MoS2/MWNTs anode has an initial

capacity of 1.200 mAh/g, which decreases to 762 mAh/g after 60 discharge-charge

cycles. The lithiation and delithiation of Li+

and the reversible nature of the anode were

clearly demonstrated by cyclic voltammograms.

iv

TÓM TẮT LUẬN ÁN

Luận án này nghiên cứu về cấu trúc và tính chất của hai loại vật liệu nano MoS2 và

MoS2/CNTs đồng thời phân tích về mối tương quan giữa cấu trúc và hình thái vật liệu

khác nhau đến sự thay đổi tính chất điện hóa của chúng, góp phần nêu bật được những

tiềm năng cũng như thách thức của loại vật liệu này trong ứng dụng làm vật liệu điện

cực pin lithium và xúc tác điện hóa cho phản ứng điện phân nước tạo H2.

Nghiên cứu đã chứng minh được hiệu quả của phương pháp sử dụng năng lượng vi sóng

trong việc tổng hợp thành công vật liệu nano có cấu trúc lai hợp 1T/2H-MoS2 và

MoS2/CNTs nanocomposite thông qua các phương pháp phân tích tiên tiến như XPS,

Raman, XRD, SEM, TEM. Điều này có ý nghĩa khoa học quan trọng trong việc lựa

chọn phương pháp vi sóng để tạo ra vật liệu nano MoS2, MoS2/CNTs nói riêng và nhiều

loại vật liệu có cấu trúc nano khác nói chung với khả năng điều khiển đa dạng hình thái

cấu trúc và kích thước hạt vật liệu.

Với phương pháp tổng hợp vật liệu bằng vi sóng được đề xuất và xây dựng, vật liệu

MoS2 tổng hợp được có cấu trúc lai hợp 1T/2H, trong đó tỉ lệ pha 1T/2H có thể thay đổi

khi thay đổi dung môi phản ứng. Đáng chú ý rằng, phương pháp tổng hợp này có khả

năng tạo ra hàm lượng pha kim loại 1T rất cao (đạt 84.5 % pha 1T so với pha 2H khi

tổng hợp trong dung môi ethylene glycol). Pha 1T hay còn gọi là pha kim loại, có tính

chất dẫn điện tốt hơn hẳn so với pha 2H. Do đó, sự hình thành pha 1T trong vật liệu

tổng hợp có ý nghĩa rất lớn trong việc ứng dụng loại vật liệu lai hợp này vào lĩnh vực

lưu trữ và chuyển hóa năng lượng, đặc biệt là vật liệu xúc tác cho phản ứng điện phân

nước tạo H2 (HER) và ứng dụng làm vật liệu điện cực anode cho pin lithium.

Trong nghiên cứu này, vật liệu nanocomposite MoS2/CNTs cũng đã được tổng hợp

thành công bằng phản ứng hóa học pha lỏng với sự hỗ trợ của vi sóng. Thông qua hai

quy trình phân tán vật liệu f-CNTs tiền phản ứng: phân tán gián tiếp qua 2 bình phản

ứng (I2PD) và phân tán trực tiếp qua 2 bình phản ứng (D2PD), hai loại cấu trúc vật liệu

nanocomposite MoS2/CNTs tinh thể (crystalline-MoS2/CNTs) và vô định hình

(amorphous-MoS2/CNTs) đã được hình thành. Các điều kiện tổng hợp tối ưu để tạo ra

v

hai cấu trúc vật liệu naocompostite MoS2/CNTs tinh thể và vô định hình đã được xác

định bằng phương pháp quy hoạch thực nghiệm Taguchi. Cụ thể như sau:

Điều kiện tổng hợp vật liệu crystalline-MoS2/CNTs:

• Lượng f-CNTs (10g/L): 4 mL

• Thời gian phản ứng: 45 phút

• Thể tích dung môi: 240 mL

• Công suất vi sóng: 240 W

• Nhiệt độ siêu âm (giai đoạn tiền phản ứng): 60 oC

Điều kiện tổng hợp vật liệu amorphous-MoS2/CNTs:

• Công suất vi sóng: 240 W

• Thời gian phản ứng: 45 phút

• Nhiệt độ siêu âm (giai đoạn tiền phản ứng): 80 °C

• Lượng f-CNTs: 40 mg

• Tỉ lệ tác chất và dung môi ∑m (AMH+TU):VEG = 0.06 g/mL

Phương pháp quy hoạch thực nghiệm Taguchi cũng xác định được mức độ ảnh hưởng

của các thông số đến hiệu quả tổng hợp vật liệu MoS2/CNTs. Kết quả tính toán cho thấy

công suất vi sóng có mức độ ảnh hưởng mạnh nhất đến độ tinh thể hóa của vật liệu

MoS2/CNTs tổng hợp bằng quy trình I2PD và thời gian phản ứng có mức độ ảnh hưởng

mạnh nhất đến hệ số Tafel của vật liệu vô định hình MoS2/CNTs tổng hợp bằng quy

trình D2PD.

MoS2/CNTs cấu trúc tinh thể được chứng minh là vật liệu phù hợp làm điện cực anode

cho LIB. Phương pháp quét thế vòng tuần hoàn của vật liệu này cho thấy khả năng đan

cài và giải phóng ion Li+ xảy ra tốt và vật liệu có độ bền sạc/xả cao. Đồ thị phóng nạp

của vật liệu anode cũng cho thấy vật liệu có độ bền phóng nạp cao với dung lượng ổn

định trong khoảng 1000 ÷ 1100 mAh/g sau 50 chu kỳ phóng–nạp và giảm còn 762

mAh/g sau 60 chu kỳ phóng-nạp.

Nghiên cứu đã đánh giá được khả năng ứng dụng làm xúc tác cho phản ứng HER của

vật liệu nanocomposite MoS2/CNTs bằng phương pháp quét thế tuyến tính và Tafel

plot. Kết quả cho thấy vật liệu MoS2/CNTs cấu trúc vô định hình thể hiện khả năng xúc

tác tốt và ổn định trong khoảng điện thế -220 mV đến -230 mV (so với NHE), mật độ

dòng điện -8,94 mA/cm2

(V = –350 mV so với NHE), độ dốc Tafel là 102 mV/dec.

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ACKNOWLEDGEMENTS

First of all, I would like to express my heartfelt appreciation to my supervisors, Assoc.

Prof. Dr. Le Van Thang and Dr. Nguyen Huu Huy Phuc, for guiding, supporting, and

imparting valuable knowledge to me throughout the process of implementing this thesis.

My sincere appreciation goes to Vietnam National University, Ho Chi Minh City, Ho

Chi Minh City University of Technology (HCMUT), and Project 911 for their financial

assistance in allowing me to complete my dissertation.

Finally, I would also like to give thanks to the Faculty of Materials Technology -

HCMUT, as well as VNU-HCM Key Laboratory for Material Technologies - HCMUT

for assisting me and establishing the best conditions for completing this PhD thesis.

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TABLE OF CONTENTS

ABSTRACT ............................................................................................................ ii

TÓM TẮT LUẬN ÁN...................................................................................................iv

ACKNOWLEDGEMENTS ..........................................................................................vi

TABLE OF CONTENTS ............................................................................................ vii

LIST OF FIGURES........................................................................................................x

LIST OF TABLES...................................................................................................... xiii

ABBREVIATIONS......................................................................................................xv

CHAPTER 1 INTRODUCTION...............................................................................1

1.1Motivation.................................................................................................................1

1.2Objectives and scopes...............................................................................................4

1.3The new ideas of the research (Novelty) ..................................................................4

1.4Major contributions of the thesis ..............................................................................5

1.5Research content .......................................................................................................5

1.6Research outline........................................................................................................6

CHAPTER 2 LITERATURE REVIEW....................................................................8

2.1Structure and properties of carbon nanotubes (CNTs) .............................................8

2.2Structures and properties molybdenum disulfide (MoS2).........................................9

2.3MoS2 and their composite with carbon nanomaterials as electrocatalyst for HER ....

........................................................................................................................12

2.4MoS2 and their composite with carbon nanomaterials for lithium-ion batteries........

........................................................................................................................19

2.5Microwave synthesis of nanomolybdenum disulfide (MoS2) and MoS2/CNTs

nanocomposites ............................................................................................................22

2.6Conclusion ..............................................................................................................26

CHAPTER 3 METHODOLOGY............................................................................28

3.1Overall research procedure .....................................................................................28

3.2Chemicals/Materials................................................................................................28

3.3Synthesis of 1T/2H-MoS2 hybrid phase .................................................................30

3.4Synthesis of MoS2/CNTs nanocomposite...............................................................32

viii

3.4.1 Taguchi experimental method for investigating the factors affecting the

synthesis of MoS2/CNTs.........................................................................................34

3.4.2 Synthesis of crystalline MoS2/CNTs from the I2PD procedure ...................40

3.4.3 Synthesis of amorphous MoS2/CNTs from the D2PD procedure.................41

3.5Structural and Physical Characterization Method...................................................42

3.5.1 X-ray Diffraction (XRD)...............................................................................42

3.5.2 Raman spectroscopy......................................................................................43

3.5.3 Scanning electron microscopy (SEM) ..........................................................44

3.5.4 Transmission electron microscopy (TEM)....................................................45

3.5.5 X-ray photoelectron spectroscopy (XPS)......................................................45

3.6Electrochemical measurements for catalysts..........................................................48

3.6.1 Preparation of working electrode ..................................................................49

3.6.2 Tafel plot .......................................................................................................50

3.7Electrochemical characterizations for lithium-ion batteries (LIBs) .......................51

3.7.1 Anode preparation .........................................................................................51

3.7.2 Cyclic Voltammetry (CV).............................................................................55

3.7.3 Cell assembly for cyclic voltammetry (CV) testing......................................55

3.7.4 Galvanostatic charge-discharge testing.........................................................57

3.7.5 Electrochemical impedance spectroscopy (EIS)...........................................57

CHAPTER 4 MICROWAVE-ASSISTED SYNTHESIS OF NANO

MOLYBDENUM DISULFIDE (MoS2) 1T/2H HYBRID PHASE AND

STRUCTURAL CHARACTERIZATION ..................................................................59

4.1Explain the reason for the choosing of microwave synthesis process parameters .59

4.2X-ray diffraction (XRD) patterns of 1T/2H-MoS2 .................................................61

4.3Raman spectra of 1T/2H- MoS2..............................................................................63

4.4SEM – TEM images of 1T/2H-MoS2 .....................................................................66

4.5X-ray photoelectron spectroscopy (XPS) of 1T/2H-MoS2 .....................................71

4.6Explain the mechanism for synthesis reaction and propose the structure of as￾prepared material ..........................................................................................................76

4.7Explain the phase transitions between 1T and 2H under microwaves irradiation..79

4.8Predict the effect of water content in the ethylene glycol (EG) solvent on the size of

MoS2 nanoparticles synthesized using microwave assisted synthesis .........................81

ix

4.9Conclusion ..............................................................................................................84

CHAPTER 5 MICROWAVE-ASSISTED SYNTHESIS OF MOLYBDENUM

DISULFIDE/CARBON NANOTUBES (MoS2/CNTs) NANOCOMPOSITE AND

THEIR CATALYTIC ACTIVITY FOR HYDROGEN EVOLUTION REACTION

(HER) ...........................................................................................................85

5.1Synthesis of crystalline MoS2/CNTs from the I2PD procedure .............................85

5.1.1 Taguchi experimental design for synthesis of crystalline MoS2/CNTs via

I2PD process...........................................................................................................85

5.1.2 TEM images of crystalline MoS2/CNTs (MSC-I2PD-opt)...........................91

5.1.3 XRD pattern of crystalline MoS2/CNTs (MSC-I2PD-opt)...........................93

5.1.4 Raman spectra of crystalline MoS2/CNTs (MSC-I2PD-otp)........................94

5.2Synthesis of amorphous MoS2/CNTs from the D2PD procedure...........................95

5.2.1 XRD patterns of 16 samples MSC-D2PD-n .................................................96

5.2.2 Raman spetra of MSC-D2PD-n.....................................................................99

5.2.3 SEM-TEM images of MSC-D2PD-n ..........................................................101

5.2.4 Taguchi experimental design for synthesis of amorphous MoS2/CNTs via

D2PD process........................................................................................................103

5.2.5 Discuss the catalytic activity of nanostructured amorphous MoS2/CNTs........

................................................................................................................108

5.3Conclusion ............................................................................................................112

CHAPTER 6 ELECTROCHEMICAL PERFORMANCE OF CRYSTALLINE

MoS2/CNTs FOR LITHIUM-ION BATTERIES (LIBs) APPLICATION................114

6.1Cyclic voltammetry (CV) of crystalline MoS2/CNTs...........................................114

6.2Galvanostatic charge-discharge (GCD) of crystalline MoS2/CNTs.....................119

6.3Electrochemical Impedance Spectroscopy (EIS) of crystalline MoS2/CNTs.............

......................................................................................................................121

6.4EIS of 16 samples of crystalline MoS2/CNTs (MSC-I2PD-n) .............................124

6.5Conclusion ............................................................................................................129

CONCLUSION .........................................................................................................130

LIST OF PUBLICATIONS........................................................................................133

REFERENCES .........................................................................................................135

APPENDIX .........................................................................................................150

x

LIST OF FIGURES

Figure 2.1 Structures of carbon nanomaterials: SWNTs (a), MWNTs (b), graphene (c),

graphene oxide (d), fullerene C60 (e), carbon nanohorn (f), carbon nanocones (g),

carbon nano–onions (h) .................................................................................................8

Figure 2.2 Structure of hexagonal molybdenum disulfide (2H-MoS2)........................10

Figure 2.3 Structural polytypes of bulk MoS2 crystals based on known structure. The

unit cells are enclosed by dashed lines. Inset summarizes the space group and Mo-S

coordination of different polytypes, including 2H, 1T and 3R phase ..........................12

Figure 2.4 Schematic illustration of HER mechanism of the prepared MoS2 catalyst.

Step I - The adsorption of H+ on the catalytic site (Volmer reaction). Step II - The release

of H2 from active sites (Heyrovsky reaction). The distance between two adjacent S

atoms is 3.18 Å for 2H-MoS2 and 3.22 A for 1T phase. ..............................................15

Figure 2.5 Stability of MoS2. (A) SCAN-calculated Mo-S Pourbaix diagram generated

with aqueous ion concentrations 10-6 M at 25 °C. (B) Calculated Pourbaix

decomposition free energy (ΔGpbx) of MoS2 from the potential -0.6 to 0.5 V vs RHE

at pH = 1. ......................................................................................................................19

Figure 3.1 Framework of the overall procedures of the research.................................28

Figure 3.2 The flowchart for synthesis of MoS2 nano powders under microwave heating

......................................................................................................................................31

Figure 3.3 Flowchart for the synthesis of amorphous and crystalline MoS2/CNTs

nanocomposites via Indirect 2-pot dispersion (I2PD) and Direct 2-pot dispersion

(D2PD) procedures.......................................................................................................34

Figure 3.4 Taguchi design of experiment – modeling the influence of control factors on

performance output.......................................................................................................35

Figure 3.5 Principle of Raman scattering .....................................................................44

Figure 3.6 Principle of XPS..........................................................................................46

Figure 3.7 Three-electrode configuration for electrochemical tests.............................50

Figure 3.8 Anode preparation for electrochemical performance testing......................54

Figure 3.9 PARSTAT 2273 (AMETEK) instrument ...................................................56

Figure 3.10 Swagelok type cell ....................................................................................56

Figure 3.11 Electrode arrangement for electrochemical measurements ......................57

Figure 4.1 XRD patterns of S1-EG, S2-EG + H2O, S3-EG+G, and S4-G...................62

Figure 4.2 (A) Raman spectra of S1-EG, S2-EG + H2O, S3-EG+G, and S4-G; (B) the

magnified Raman signals of S2-EG + H2O; (C) the maginified Raman signals of green

area from figure (A) and (D) symmetric displacement of Mo and S atoms in E1g, E

1

2g

and A1g vibrational modes............................................................................................65

Figure 4.3 (A) the symmetry of the sulfur in the 1T and 2H phase of MoS2 structures;

(B) the basal plane in MoS2 structure ...........................................................................66

xi

Figure 4.4 TEM images of 1T/2H-MoS2 in S1-EG, S2-EG+H2O, S3-EG+G and S4-G

......................................................................................................................................68

Figure 4.5 SEM and TEM images of prepared samples with different morphologies:

nanoflakes (A, C); nanoparticles (B, D); the particle in image (E) is enlarged to

demonstrate the hexagonal structure of MoS2..............................................................69

Figure 4.6 (A) HRTEM images of S1-EG; (B) Image of the region enclosed by the red

rectangle of (A) and schematic structure of the unit cells of the 1T phase; (C) Image of

the region enclosed by the red rectangle in (A) and schematic structure of the unit cells

of the 2H phase; (D) measurement of interlayer distances by ImageJ to calculate the d

spacing between the (100) planes of 1T phase MoS2...................................................70

Figure 4.7 (B) and (D) are histograms of the particle size distribution of MoS2

nanoparticles calculated by ImageJ from (A) and (C) .................................................71

Figure 4.8 Mo 3d XPS spectra of 1T/2H-MoS2 in S1-EG, S2-EG+H2O, S3-EG+G and

S4-G..............................................................................................................................72

Figure 4.9 S 2p XPS spectra of 1T/2H-MoS2 in S1-EG, S2-EG+H2O, S3-EG+G and

S4-G..............................................................................................................................73

Figure 4.10 N 1s XPS spectra of 1T/2H-MoS2 in S1-EG, S2-EG+H2O, S3-EG+G and

S4-G..............................................................................................................................74

Figure 4.11 A possible mechanism for the formation of MoS2 nanoparticles in various

solutions via microwave synthesis, crystal growth occurred concurrently with the 1T

↔2H phase transition when NH4

+

ions intercalated to the MoS2 layers......................78

Figure 4.12 Mechanism of substitution of the O atom in the [Mo7O24]

6- of AHM by the

S atom in thiourea.........................................................................................................79

Figure 4.13 (a) The 2H-MoS2 crystal structure and the ligand splitting for 4d orbitals of

Mo atoms with trigonal-prismatic coordination, (b) The 1T-MoS2 crystal structure and

the ligand splitting for 4d orbitals of Mo atoms with octahedral coordination............80

Figure 4.14 An illustration of the phase distribution in a 1T/2H-MoS2 material in which

the 1T is the dominant phase ........................................................................................81

Figure 4.15 TEM images of nano MoS2 material synthesized in EG + 4 mL H2O .....82

Figure 4.16 Predict the shape development of MoS2 nanoparticles for (a) 4 mL H2O, (b)

5 mL H2O, and (c) 6 mL H2O in ethylene glycol (EG) based on the similar growth of

ZnO nanoparticles in the reference ..............................................................................83

Figure 5.1 (a) XRD patterns of 16 samples MSC-I2PD-n (n = 1, 2, 3..., n) from Taguchi

Table 3.7; (b) Gaussian fitting to define the crystalline area and total area for calculating

crystallinity in equation (5.1) .......................................................................................87

Figure 5.2 S/N (Ra) analysis exported from Minitab ...................................................88

Figure 5.3 HRTEM images of crystalline MoS2/CNTs (MSC-I2PD-opt)...................92

Figure 5.4 XRD pattern of crystalline MoS2/CNTs (MSC-I2PD-opt).........................94

Figure 5.5 (A) Raman spectra of (a) crystalline MoS2/CNTs (MSC-I2PD-otp) and (b)

bare CNTs; (B) comparing the magnified Raman signature of MoS2 in prepared

xii

MoS2/CNTs and bare CNTs; Symmetric displacement of Mo and S atoms in A1g and

E

1

2g vibrational modes (inset figure (B))......................................................................95

Figure 5.6 XRD patterns of 16 samples investigated from Taguchi method (Table 3.8)

......................................................................................................................................97

Figure 5.7 The relative position of the XRD spectrum center from MSC-D2DP-n (n =

1, 2,...16) in comparison to the standard spectrum.......................................................98

Figure 5.8 Raman spectra of 16 samples investigated from Taguchi method (Table 3.8)

......................................................................................................................................99

Figure 5.9 SEM images of 16 samples investigated from Taguchi table 3.8.............102

Figure 5.10 TEM images of MSC-D2PD-n samples .................................................103

Figure 5.11 (a) Tafel plots of EC-D2DP-n (n = 1, 2, 3… 16); (b) LSV curves of EC￾D2PD-n (n=1, 2, 3,…16) electrodes in 0.5 M H2SO4 ................................................104

Figure 5.12 S/N (Ra) analysis exported from Minitab ...............................................106

Figure 5.13 The effect of microwave power and reaction energy on the Tafel slope

....................................................................................................................................111

Figure 6.1 Cyclic voltammogram of the 2nd cycle at the scan rate of 50 mV/s of LA￾MW; LA-MS; LA-a-MSC and LA-c-MSC samples..................................................115

Figure 6.2 Cyclic voltammogram of LA-c-MSC at different scan rate .....................116

Figure 6.3 Cyclic voltammogram of LA-c-MSC at scan rate of 100 mV/s...............118

Figure 6.4 Charge - discharge profile of crystalline MoS2/CNTs (LA-c-MSC)........120

Figure 6.5 Bode plots (a) and EIS Nyquist plots (b) of MWNTs (LA-MW), 1T/2H￾MoS2 (LA-MS), amorphous MoS2/CNTs (LA-a-MSC) and crystalline MoS2/CNTs

(LA-c-MSC) ...............................................................................................................122

Figure 6.6 Equivalent curcuit model for fitting the experimental EIS data ...............122

Figure 6.7 (a) Nyquist plots and (b) Bode plots representing the same EIS data

simulated by the circuit shown in Figure (b)..............................................................127

Figure 6.8 Equivalent curcuit for EIS measurement ..................................................127

xiii

LIST OF TABLES

Table 2.1 Fundamental properties of carbon nanomaterials – Graphene, SWNTs, and

MWNTs..........................................................................................................................9

Table 2.2 Fundamental principles of HER electrocatalysts in both acidic and alkaline

media ...........................................................................................................................14

Table 2.3 Comprehensive table of electrochemistry of MoS2 and their composite with

carbon nanomaterials including materials, properties, synthetic method, and key

electrochemical data .....................................................................................................17

Table 2.4 A summary comparison of the syntheis method for nano MoS2 and

MoS2/CNTs ..................................................................................................................22

Table 2.5 Loss tangent (tan δ) values at 2.45 GHz and 20 oC and boiling points of

different solvents .........................................................................................................24

Table 3.1 Chemicals used in the synthesis of 1T/2H MoS2 and MoS2/CNTs

nanocomposites ............................................................................................................29

Table 3.2 Samples mark with various solvents and the corresponding boiling points 32

Table 3.3 Five factors and 4 levels for each factor to be investigated in I2PD process

......................................................................................................................................36

Table 3.4 Five factors and 4 levels for each factor to be investigated in D2PD process

......................................................................................................................................36

Table 3.5 The appropriate orthogonal array L’16 ........................................................37

Table 3.6 The average SN value for each factor and level...........................................39

Table 3.7 Investigate reaction conditions with an I2PD reaction mixture - Experimental

plan for the first round of optimizations (following Taguchi method) ........................40

Table 3.8 Investigate reaction conditions with an D2PD reaction mixture -Experimental

plan for the first round of optimizations (following Taguchi method) ........................41

Table 3.9 XRD diffraction peaks of 1T and 2H phase of MoS2 (from literature review)

......................................................................................................................................43

Table 3.10 Raman modes of 1T and 2H forms of MoS2 ..............................................44

Table 3.11 XPS binding energies (eV) of the different phases and bonding in

molybdenum compounds..............................................................................................47

Table 3.12 Catalytic electrode sample markers............................................................49

Table 3.13 Anode paste and anode electrode samples identification symbols ............53

Table 3.14 Circuit elements used in the equivalent circuit mode ................................58

Table 4.1 Summarize the vibration modes (position peaks) in S1, S2, S3, S4 (cm-1

) .64

Table 4.2 Binding energy of the doublet 3d5/2 and 3d3/2 in Mo+4 of 1T and 2H phase the

1T/2H -MoS2 concentration of 4 samples in different solvents...................................76

Table 5.1 The crystallinity of the I2PD samples investigated by Taguchi method

(calculated from XRD results)......................................................................................87

Table 5.2 Response Table for Means exported from Minitab......................................89