Nghiên cứu vật liệu nano siêu mỏng mos2 pha tạp nitơ và vật liệu tổng hợp graphenemos2 chế tạo bằng phương pháp điện hóa plasma ứng dụng cho phản ứng sản sinh hydrô

Nitrogen Doped MoS 2 Nanosheets and Graphene/MoS 2 Composite Prepared by Electrolysis Plasma-Induced Process for Hydrogen Evolution Reaction Student: Nguyen Van Truong Advisor: Prof.. K

國立交通大學 材料科學與工程學系

研究生: 阮文長

材料及其產氫應用

Prepared by Electrolysis Plasma-Induced Process for Hydrogen Evolution Reaction

College of Engineering National Chiao Tung University

in partial Fulfillment of the Requirements

for the Degree of Doctor of Philosophy

in Materials Science and Engineering

March 2020 Hsinchu, Taiwan, Republic of China

中華民國一零九年四月

國立交通大學 材料科學與工程學系

博士論文

摘要

為了獲得可持續的富含地球的電催化劑材料，在氫析出反應（HER）中顯示出高性能，在這裡我們提出了一種簡便的一鍋電漿電化學工藝，用於製造氮摻雜的二硫化鉬奈米片和石墨烯/ 二硫化鉬複合材料。已經開發出一種有效的一步法，該方法涉及在短時間內且在低溫（約 80°C）下同時進行二硫化鉬奈米片的電漿摻雜和剝落。特別地，可以在浸沒的陰極尖端處產生有效的電漿區，以實現將氮原子摻雜到半導體性 2H-二硫化鉬結構中。在二硫化鉬結構中氮摻雜和剝落的協同作用下調節電子和輸運性質，以增強其催化活化作用。研究發現，在摻氮的二硫化鉬奈米片上，氮原子濃度為 5.2 at％時，

的電流密度下具有 118 mV的低過電位，Tafel 斜率在 dec-1 的 Tafel 斜率，以及長期的穩定性而不會降解。該性能

）要好得多。這種方法似乎是一種有效且簡單的策略，不僅可以調節摻氮過渡金屬硫化物

Nitrogen Doped MoS 2 Nanosheets and Graphene/MoS 2 Composite Prepared by

Electrolysis Plasma-Induced Process for Hydrogen Evolution Reaction

Student: Nguyen Van Truong Advisor: Prof Kung-Hwa Wei

Department of Materials Science and Engineering

National Chiao Tung University

Abstract

With the goal of obtaining sustainable earth-abundant electrocatalyst materials displaying high performance in the hydrogen evolution reaction (HER), here we propose a facile one-pot plasma-induced electrochemical process for the fabrication of both nitrogen-doped MoS2 nanosheets and graphene/MoS2 composite An efficient one-step approach that involves simultaneous plasma-induced doping and exfoliating of MoS2 nanosheets within a short time and at a low temperature (ca 80 °C) has been developed Particularly, an active plasma zone can be generated at the submerged cathode tip to achieve doping of nitrogen atoms into the semiconducting 2H-MoS2 structure The electronic and transport properties were modulated under the synergy of the nitrogen doping and exfoliation in the MoS2 structure to enhance their catalytic activation It is found that the N concentration of 5.2 at % at N-doped MoS2 nanosheets have excellent catalytic hydrogen evolution reaction where a low over-potential of 164 mV at

a current density of 10 mA cm–2 and a small Tafel slope of 71 mV dec–1—much lower than those of exfoliated MoS2 nanosheets (207 mV, 82 mV dec–1) and bulk MoS2 (602 mV, 198 mV dec–1)—as well as an extraordinary long-term stability of >25 h in 0.5 M H2SO4 can be achieved Interestingly, through a simple selection of cathode materials in one-batch process, two different morphologies of graphene sheets were obtained, resulting in both onion-like covered MoS2 nanosheets (OGNs@MoS2) and sheets-like graphene wrapped MoS2 composites (GNs@MoS2) We found that the presence of the graphene sheets appeared to be a key aspect

of the enhanced HER ability Therefore, we conclude that electronic coupling at the graphene–

MoS2 nanosheet interfaces also played an important role in enhancing the HER activity Our OGNs@MoS2 composites exhibited high HER performance, characterized by a low overpotential of 118 mV at a current density of 10 mA cm–2, a Tafel slope of 73 mV dec–1, and long-time stability without degradation; this performance is much better than that of the sheet-like graphene-wrapped MoS2 composite GNs@MoS2 (182 mV, 82 mV dec–1) This approach appears to be an effective and simple strategy for tuning not only nitrogen-doped transition metal dichalcogenide (TMDCs) materials but also the morphologies of composites of graphene and TMDCs materials for a broad range of energy applications

KEYWORDS: MoS2, Nitrogen doped MoS2, Onion-like graphene, Graphene/MoS2 composite, One-pot Plasma-Induced exfoliation, Hydrogen evolution reaction, electrocatalyst

ACKNOWLEDGMENTS

This dissertation presents a summary of my research work which has done in the Department

of Materials Science and Engineering (MSE), National Chiao Tung University (NCTU) It is a pleasure to express my sincere gratitude to all the people who helped and supported me during

my Ph.D study

From bottom of my heart I express my deep sense of gratitude and profound respect to my supervisor Prof Kung-Hwa Wei He continually and convincingly conveyed a spirit of adventure in regard to research and scholarship, and an excitement in regard to teaching Without his generous encouragement and brief advice for those years, this dissertation would not have been completed My sincere thanks Prof Yu-Lun Chueh for his kind guidance and persistent help

I would like to thank Dr Yen Po-Jen, Dr Cheng Hao-Wen, Dr Chen Hsiu-Cheng, Dr Qui Le, Mr Phuoc Anh Le, Mr Chung-Hao Chen, Mr Tzu-Yi Yang, Mr Yung-Chi Hsu, Mr Bo-Hsien Lin for their kind supporting in my research Many thanks to all participants in Prof Kung-Hwa Wei’s lab who took part in the study and enabled this dissertation to be possible In addition, I would like to thank all members of Vietnamese Student Association-NCTU who made my life in Taiwan really pleasurable and joyful

Van-Finally, special thanks for my parent, my wife and my two angels who always standing by my side Thank you for always encouraging me to pursue my dreams I love you all so much, thanks for loving me too!

Nguyen Van Truong Hsinchu, Taiwan April 2020

Table of Content

摘要 i

Abstract ii

Acknowledgment iv

Table of Content v

Figures list vii

Tables list xi

Chapter 1 Introduction 1

1.1 Introduction of Transition metal dichalcogenides 1

1.2 Production of Transition Metal Dichalcogenides materials 3

1.3 Introduction of cathodic plasma exfoliation method 6

1.4 Introduction of Electrocatalytic Hydrogen Evolution Reaction 9

1.5 Introduction of nitrogen doped MoS2 12

1.6 Introduction of graphene/MoS2 composite 14

1.7 Strategies to enhancing MoS2 catalytic activity 16

1.8 Thesis outline 20

Chapter 2 Production Nitrogen-Doped Molybdenum Disulfide nanosheets through Plasma-Induced process and their electrocatalyst performance 21

2.1 Introduction 21

Chapter 3 Production Graphene/MoS 2 composite through One-Pot Plasma-Induced

process and their Electrocatalyst performance 51

3.1 Introduction 51

3.2 Experimental section 54

3.3 Results and discussion 58

3.4 Conclusions 83

Chapter 4 Conclusions 84

References 87

Vita 99

Publications list 101

Figures list

Figure 1.1 The periodic table with highlighted transition metal and chalcogenide elements that

form layered TMDCs materials 1

Figure 1.2 The crystal struture of TMDCs with Octahedral (1T), Trigonal prismatic (2H) and (3R) coordination 2

Figure 1.3 Six main production methods of TMDCs and their content 3

Figure 1.4 Several TMDCs nanosheets production methods 5

Figure 1.5 Typical of plasma electrolysis and its applications 6

Figure 1.6 Experimental setup and mechanism of cathodic plasma exfoliation 7

Figure 1.7 Schematic representation of the proposed mechanism of plasma exfoliation and nitrogen-doping 8

Figure 1.8 I-V curve of overall water splitting 10

Figure 1.9 Schematic of the covalent nitrogen doping in MoS2 upon N2 plasma surface treatment 13

Figure 1.10 (a) Schematic illustration of the electrochemical deposition set-up; (b) Comparison of MoS2-3D graphene hybrid in solution and solid state supercapacitor 15

Figure 1.11 Synthesis procedure and structural model for mesoporous MoS2 with a double-gyroid morphology 17

Figure 1.12 the schematic preparation process of MoS2/N-RGO nanocomposite 19

Figure 2.1 (a) Experimental setup for plasma-induced exfoliation and (b) proposed mechanism of exfoliation and nitrogen-doping process 27

Figure 2.3 SEM images of exfoliated (a) MoS2, (b) MoSe2, (c) WS2 and (d) WSe2 nanosheets AFM images of exfoliated (e) MoS2, (f) MoSe2 and (h) WSe2 nanosheets Raman spectra of exfoliated (i) MoS2, (j) MoSe2, (k) WS2 and (l) WSe2 nanosheets 31

Figure 2.4 UV–Vis spectra of (a) MoS2, (b) MoSe2, (c) WS2 and (d) WSe2 nanosheets 32

Figure 2.5 Low-magnification TEM images of (a) MoS2, (b) MoSe2, (c) WSe2 and (d) WS2 nanosheets Insets show the corresponding SAED patterns HRTEM images recorded along the [001] zone axis Insets: their filtered of (e) MoS2 (f) MoSe2, (g) WSe2, and (h) WS2 STEM bright-field images of (i) MoS2, (j) MoSe2, (k)WS2 and (l) WSe2 nanosheets, and their element mapping images, respectively 33

Figure 2.6 (a) Difference in frequency between E1 2g and A1g in Raman spectra and (b) the lateral size of exfoliated MoS2 using different applied biases 35

Figure 2.7 (a) Mechanism of the N-doped MoS2 nanosheets (b-f) Dark-field STEM images of undoped MoS2 and N-doped MoS2 nanosheets and the corresponding EELS elemental mapping images of Mo, S and N with different electrolytes and/or plasma-induced time, respectively 36

Figure 2.8 The statistical distribution of the lateral size of (a) undoped MoS2, (b) N-doped MoS2 and (c) the thickness of MoS2 nanosheets 38

Figure 2.0.21 XPS spectra (a) survey, (b) S 2p and (c) Mo 3d of Undoped MoS2 and N-doped MoS2 nanosheets, respectively 39

Figure 2.9 SEM images of N-doped MoS2 after the plasma-induced exfoliation at (a) 200 oC, (b) 300 oC (c) 500 oC and their BF-STEM images(d-f), respectively, correspond with EDS mapping of Mo, S and N elements 41

Figure 2.10 Raman spectra of N-doped MoS2 nanosheets after the thermal annealing at (a) 200, (b)300, and (c)500 oC, respectively 42

Figure 2.11 (a) LSV curves (recorded on a glassy-carbon electrode) of bulk MoS2, undoped MoS2 and N-doped MoS2 (b) Corresponding Tafel plots derived from (a) (c) Nyquist plots acquired at –200 mV vs RHE of the bulk MoS2, undoped MoS2 and N-doped MoS2 (d) Durability test of the N-doped MoS2 catalyst, performed at an overpotential of 165mV vs RHE 45

Figure 3.1 (a) Procedure and setup for the preparation of MoS2 nanosheets covered by like graphene sheets (OGNs@MoS2) and MoS2 nanosheets decorated on sheet-like graphene (GNs@MoS2); (b) Schematic representation of the proposed mechanism of OGNs@MoS2 and GNs@MoS2 58

onion-Figure 3.2 Digital images of plasma-induced experiments of fabricating MoS2 nanosheets wrapped with graphene nanosheets: (a) step 1: making MoS2 nanosheets, (b) step 2: making graphene nanosheets on MoS2 nanosheets 59

Figure 3.3 a–c) SEM and (d–f) TEM images of (a, d) MoS2 nanosheets, (b, e) OGNs@MoS2, and (c, f) GNs@MoS2 sample prepared through plasma-induced exfoliation 60

Figure 3.4 (a) SEM and (b) low magnification TEM image of OGNs 60 Figure 3.5 EDS spectra of MoS2, GNs@MoS2 and OGNs@MoS2 62

Figure 3.6 High resolution TEM image and insets SEAD of (a) MoS2 nanosheets and (b) OGNs 63

Figure 3.7 AFM images of MoS2 nanosheets and corresponding height profile 63

Figure 3.8 (a) HR-TEM image and SAED pattern (inset) of the OGNs@MoS2 sample; (b) expanded view; and (c) HR-TEM image of the same sample recorded from another position (d) STEM bright-field image of the OGNs@MoS2 sample and corresponding elemental mapping of C, Mo, and S atoms 65

Figure 3.9 (a) HR-TEM image of the GNs@MoS2 sample and expanded views of its (b) MoS2 and (c) GNs region; insets: corresponding SAED patterns (d) STEM dark-field image of the GNs@MoS2 structure and corresponding elemental mapping of C, Mo, and S atoms 67

Figure 3.10 (a, b) Raman spectra and (c) XRD patterns of the OGN, MoS2, and OGNs@MoS2 samples 68

Figure 3.11 (a) XPS survey spectra of the OGN and OGNs@MoS2 samples (b–d)

High-Figure 3.13 (a) LSV curves of the OGNs, MoS2, GNs@MoS2, and OGNs@MoS2 samples and the Pt electrode (b) Tafel plots obtained from LSV curves 72

Figure 3.14 Relationship between total amount of the MoS2 1T phase and Overpotential at current density of 10 mA cm-2 of MoS2, GNs@MoS2 and OGNs@MoS2 73

Figure 3.15 (a,b) SEM, (c,d) TEM and (e,f) images of MoS2 nanosheets prepared in 2M H2SO4 and 2M NaOH 78

Figure 3.16 LSV curves of MoS2 nanosheets prepared in acid (black) and base (red) electrolytes 79

Figure 3.17 (a-c) SEM images and (d-f) corresponding of EDS spectra of OGNs@MoS2 prepared at different plasma electrolysis time conditions 80

Figure 3.18 (a) Nyquist plots of the MoS2, GNs@MoS2, and OGNs@MoS2 samples; inset: expanded view of the boxed area (b) I–t test of the OGNs@MoS2 sample, recorded over 105 s

at an overpotential of 120 mV 82

Tables list Table 2.1 Elements concentrations after the plasma-induced exfoliation at various

Table 3.1 Component ratio of MoS2, GNs@MoS2 and OGNs@MoS2 62

Table 3.2 Synthetic methods and HER performances of recently reported MoS2-based materials 77

Table 3.3 Component ratio of OGNs@MoS2 prepared at different electrolysis plasma time conditions 80

Table 3.4 HER performance of OGNs@MoS2 samples prepared at various of plasma time and pulse supply power conditions 81

Chapter 1 Introduction

1.1 Introduction of Transition metal dichalcogenides

Owing to numerous of fascinating properties, the most famous of two-dimentional materials (2D), graphene has been very populated for a worldwide range of possible applications Other members in family of layered inorganic material, transition metal dichalcogenides (TMDCs) which are semiconducting materials with a typical MX2, where M

is a transition metal such as Mo, W and X is a chalcogenide, such as S, Se exhibited many intriguing scientifically and technologically properties Discovered their structure in 1923 by Linus Pauling1, figure 1.1 shows more than 40 type of TMDCs which were known as layer structure by late 1960s To 1986, the first research on monolayer MoS2 was reported2 In parallel with the exploding of research on graphene when discovered by A Geim and K Novoselov in

20043, which was opened the new studying way for TMDCs As a typical of layered material, TMDCs is the lamellar hexagonal structure with each layer were formed by X-M-X layer without dangling bonds between layers which related to other by the weak van der Waals forces

Figure 1.1 The periodic table with highlighted transition metal and chalcogenide elements that

form layered TMDCs materials.4

Figure 1.2 The crystal struture of TMDCs with Octahedral (1T), Trigonal prismatic (2H) and

(3R) coordination

Figure 1.2 shows the three possible phase of MX2 materials by the crystallography5, which include 1T-, 2H and 3R- phase In the 2H- and 3R- phase, the Mo atoms locate at the center of triangular prisms while the Mo atoms are at center of the octadedral in the 1T- phase case Furthermore, the staking method of “A-b-A” is the feature of 2H- and 3R- phase and “A-b-C” for 1T- phase In addition, the 3R- phase is less stable than that of the 2H phase, therefore, 3R- phase can easily convert to 2H- phase The 1T- phase is the octahedral metallic phase which can be converted from semiconducting 2H phase by several methods such as liquid exfoliation6,7 or microwave assisted8,9 Specially, the appearance of 1T- phase can be enhanced the electrocatalytic activity of TMDCs

Due to the unconventional electronic properties of TMDCs, the stable 2H phase is the

simultaneously with the thickness decreasing, the indirect bandgap semiconductor in bulk counterpart change to direct bandgap in monolayer For example, the experiment value for bandgap of bulk and monolayer semiconducting 2H-MoS2 are around 1.3 eV and 2.1 eV, respectively This changing from indirect to direct bandgap of bulk to monolayer material appears from quantum confinement effects These properties of TMDCs can open up promising for valleytronics and/or electrochemical energy storage applications

1.2 Production of Transition Metal Dichalcogenides materials

TMDCs have many of morphology with numerous of shapes, sizes or phases, such as nanosheets, nanoparticle, nanostructure, nanofibers Among them, the ultrathin TMDCs materials have been exhibited the different chemical, physical and electronic properties with their bulk counterpart which give great promising for a board of applications Up to date, there are two main approaches to prepare mono or few layers TMDCs nanosheets are top-down and bottom up routes Basically, top-down methods isolated TMDCs nanosheets from bulk materials such as mechanical, liquid exfoliation, or electrochemical exfoliation Bottom up methods include chemical vapour deposition (CVD), physical vapour deposition (PVD) and/or wet solution, where the layered 2D materials are mostly formed by self-assembly which can

deposit large-area of monolayer or few layers TMDCs with high-quality and uniform thickness However, cost effect, scalable, high modern technique, high temperature and vacuum requirements are their disadvantages The mechanical cleavage method which use the adhesive tape to produce high-quality TMDCs nanosheets however the long-time process and slow rate are unchanged This method can only suitable for fabrication of individual devices or fundamental characterization Recently, the laser spot is more attractive in producing monolayer TMDCs, although the scalable and laser rating is the barrier for scale-up producing The most commonly and promising method is liquid phase exfoliation With high quantity of sub-micrometer to sub-nanometer size of TMDCs nanosheets production, the liquid phase exfoliation method can be allowed for industrially scalable Particularly, the exfoliated TMDCs nanosheets electronics structures are changed from semiconducting (2H) phase to metallic phase (1T) phase The intercalation and exfoliation in the liquid phase method produce TMDCs nanosheets into different sizes with structure distortions although they show high rate production The lithium ions intercalation was discovered from 1975 by Martin B Dines11 Bulk TMDCs was immersed into n-butyl lithium one day for the intercalation of lithium ions The TMDCs nanosheets were exfoliated by a sonication step This method was widened by using many kind of intercalation ions Furthermore, an electrochemical system using discharge mode to control Li ion from lithium foil anode intercalate into TMDCs layers, subsequently TMDCs nanosheets was exfoliated by sonication step12 Although lithium-intercalated method still exists several disadvantages such as long durations, contaminators, this method is one of most efficient method to procedure mass production of TMDCs nanosheets In addition, with the expanding of the scope of TMDCs application, synthesis methods to produce TMDCs nanosheets achieve structure defects, heterostructure, and/or doping Essentially, the plasma

Figure 1.4 Several TMDCs nanosheets production methods

1.3 Introduction of cathodic plasma exfoliation method

The plasma electrolysis is a coherency between conventional electrolysis and atmospheric plasma process15

The typical plasma electrolysis includes anodic and cathodic plasma base on the apply voltage separation Figure 1.5 shows the typical of plasma electrolysis and its applications They were used to produce nanoparticles, coating, cleaning or heat treatment For setup, a traditional electrochemical system which includes two electrodes set into an electrolyte is used The active electrode is smaller than another one is At high voltage, the rapid exploding of gas surround active electrode form to the spark plasma when the gas pressure exceeded the threshold pressure The plasma envelope can reach to high temperature and they can be dislocated the space around the active electrode

Owing to the features of electrochemical cathodic plasma process, this method was firstly used

the anode is stainless steel and the mixing of (NH4)2SO4 and KOH is electrolyte Figure 1.6 shows the experimental setup and the mechanism of the cathodic plasma exfoliation process17

When using cathodic plasma method the high temperature and atmosphere located at the cathode tip is the main reason for graphite exfoliation in to graphite oxide The plasma zone was established by the bombardment of the bubble gas which formed at the cathodic by the strong electrical field in the electrolyte Furthermore, the oxygen-containing radicals and exfoliation graphene was simultaneously produced In addition, the graphene sheets were also produced by this method when the high-purity graphite rod was selected

More interesting, the different morphology of graphene nanosheets were prepared in the

different electrolyte media Wei and Yen et al was reported a facile method to tune graphene

stack structure from sheet-like to onion-like by using different ions in the plasma process18

Figure 1.6 Experimental setup and mechanism of cathodic plasma exfoliation

Graphene nanosheets was produced by cathodic plasma method when using NaOH as an electrolyte On the other hand, the Onion-like graphene was obtained when using H2SO4 They assigned that the different ions size of H+ and Na+ in the plasma process was facilitated the bond breaking and dissociation of radical to form different morphology of exfoliated graphene sheets from high-purity graphite rod

Furthermore, a report for the nitrogen doping was also obtained when using cathodic plasma

process by Yen et al19 They demonstrated a one-step, simple and green method to produce the nitrogen-doped graphene sheets The doping mechanism was proposed: under the strong electric field with the electrolysis plasma phenomenon was formed at cathode in electrolyte The large temperature gradient could be the main driving force for the graphite layer surrounding cathode expansion Resulting in partially radicalized graphene formed during exfoliation process as depicted in figure 1.7 Simultaneously, the various of radicals from (NH)4OH such as H*, NH3 , NH2 , NH* generated in the plasma zone which provide N source

to form nitrogen-doped graphene19

1.4 Introduction of Electrocatalytic Hydrogen Evolution Reaction

Up to date, the top challenges human facing on the world are environmental pollution, global warming, and energy crisis Therefore, development sustainable, renewable and clean energy sources for traditional fossil fuels replacement is vital significant There are many kinds

of renewable energy sources have been widely explored such as solar, wind or geothermal Nevertheless, the limitation of spatial and temporal application for spreading of these energy sources is still a prime challenge Hydrogen energy, an earth-abundance and green energy with extremely high energy carrier, is one of the best promising candidates for future solution Hitherto, the hydrogen gas has been mostly produced by steam-reforming and/or gasification system where metal and coal have been the foremost sources However, these massive energy consuming methods involve the copious pollutants emission with low quality hydrogen purity

In addition, these methods require complex system using at high temperature and pressure Hydrogen production by water splitting is an alternative strategy for solar to hydrogen conversion, where photoelectrochemical and electrochemical are two possible routes Electrocatalytic water splitting for production of H2 is an alternative approach for the conversion of solar energy into chemical fuels The strongly absorbed of hydrogen atoms on the catalyst surface is one of most important factor for hydrogen evolution reaction (HER) To date, platinum is the most efficient HER electrocatalyst which exhibit the lowest overpotential among the present electrocatalysts However, the high cost and scarce is the challenge for their widely application Hence, looking for an abundance, low cost and high efficiency electrocatalytic activity material is crucial By calculation of adsorption energies, a predictive model of HER was introduced by Norskøv and his group20 Accordingly, MoS2 system appeared as the most suitable candidate Their Gibbs free energy for atomic adsorption (ΔGH) shows closely with Pt on the top of the volcano curve21

Figure 1.5 shows the I-V curve for overall water splitting Basically, there are two half reactions generate at both anode and cathode in an overall water splitting process:

H2O → H2 + 1/2O2 – 1.23 V (1)

An oxygen evolution reaction (OER) locates at the anode via 2H2O ↔ O2 + 4H+ + 4e– (2)

where Ea = 1.23 V – 0.059*pH (3) (V versus normal hydrogen electrode (NHE)) Another part

is the hydrogen evolution reaction (HER) at the cathode via 4H+ + 4e– = 2H2 (4) Generally, the overpotential at a current density of 10 mA cm-2 is the value to comparison of the HER activity corresponding to the 10% of solar-to-devices efficiency22

Besides, an important kinetic parameter for ranking of HER catalytic comparison is Tafel slope valuation The Tafel plot can be estimated from the Tafel equation:

η = b log(j) + a (5)

Where j is the current density, η is the overpotential at j While a is a constant, b is the slope The rate-determining of an electrocatalyst material uses the Tafel plot value to indentify where the smaller of slope revealing the better electrocatalytic activity The Tafel slope can derive

from the linear fit of I-V curve More specifically, three equations for multiple steps of HER occurs on the surface of material in electrocatalyst process which are: (6) H3O+ + e− → Hads + H2O; (7) 2Hads → H2; (8) Hads + e− + H+ → H2 First, the Volmer reaction which is a process

allow the adsorption of a hydrogen atom on the active sites as equation (6) Second, in the Tafel (eq 7) and Heyrovsky (eq 8), two nearby H atoms absorbance or second proton and an absorbed

H atom have been combined Finally, the H2 is generated on the material surface4 Another important parameter for the electrocatalyst is the stability The durability of the catalysts can assign to the decreasing of current density at the fixed potential The better stability shows the smaller decreasing of current density

1.5 Introduction of nitrogen doped MoS 2

The most common conventional strategy to improve the mass transfer, tune electronic structure

or the synergetic effect is doping The nitrogen doped MoS2 is one of the best approaches to increase active sites, optimize the electronic structure also transfer the phase structure Up to date, there are various method to generate nitrogen doped MoS2 such as hydrothermal/solvothermal, plasma treatment and/or thermal treatment These method, however, require multi-steps and a high-temperature annealing treatment For example, Si Qin

et al reported the tunable N atom-doped MoS2 nanosheets with the range from ca 5.8 to 7.6 at% through the sol−gel process and subsequent annealing treatment (350 – 1150 oC, 3hr)24 They can tune the nitrogen containing by controlling the precursor source for nitrogen ions (Thiourea) Besides, the hydrothermal method is very common method to produce the nitrogen doped MoS2 Wang and his collaborators reported a Fluorine and nitrogen co-doped MoS2 active basal plane toward hydrogen evolution reaction25 They described the synergistic effects

of codoped fluorine and nitrogen can active the inert basal planes Subsequence, the codoped MoS2 enhanced HER activity They used the simulation to demonstrate that the nitrogen and fluorine were doped into the basal plane Recently, the plasma treatment emerged as the promising candidate for synthesis TMDCs doping Among them, using nitrogen plasma is very popular for nitrogen doped into MoS2 Azcatl and Wallace et al13 described the covalent nitrogen doping and compressive strain in MoS2 by remote nitrogen plasma exposure The controllable of nitrogen containing in the MoS2 by plasma time was obtained They also demonstrated that the nitrogen can substitute the sulfur in the nitrogen doped MoS2 structure

Figure 1.9 shows the schematic of the covalent nitrogen doping in MoS2 upon N2 plasma surface treatment

Figure 1.9 Schematic of the covalent nitrogen doping in MoS 2 upon N 2 plasma surface treatment

1.6 Introduction of graphene/MoS 2 composite

Recently, the heterostructures of graphene-base materials and ultrathin TMDCs have emerged for opening up extraordinary in board of application because of their unique optical and electrical properties26–28 Among them, graphene-base/MoS2 heterostructures/composite have been attracted various researchers because their widely applications Therefore, numerous of approaches in the synthesis graphene-base/MoS2 materials have been reported, including

hydrothermal, solvothermal, electrochemical, CVD Li et al synthesized MoS2 grown on

reduced graphene oxide (RGO) by solvothermal method using for HER in 201129 They claim that the MoS2 particles stacked on RGO nanosheets were exposed their edge sites The high loaded of MoS2 particles on RGO nanosheets based on the strong chemical and electronic coupling between MoS2 and the RGO sheets Consequently, the electrocatalytical activity of MoS2/RGO were improved In 2017 year, Wan and his group described an electrochemical deposition to produce large area MoS2 directly on CVD graphene sheets for photodetectors as shown in figure 1.10(a)30 The vertical MoS2/graphene heterostructures were synthesized by electrochemical deposition in water, followed an annealing step The controllable thickness of vertical MoS2/graphene has been confirmed by AFM, XPS and SEM The vertical MoS2/graphene exhibited high photoelectric performance at 1.7x107 W A-1 Figure 1.10(b) shows the comparison of MoS2-3D graphene hybrid in solution a three-dimensional graphene and MoS2 hybrid for supercapacitor The MoS2-3D graphene hybrid was characterized by XRD, SEM, TEM and XPS analysis They claimed that the 3D graphene can support to the electrolytic ions transfer which increase the charge storage capacity

Figure 1.10 (a) Schematic illustration of the electrochemical deposition set-up30, (b) Comparison of MoS 2 -3D graphene hybrid in solution and solid state supercapacitor31

(a)

(b)

1.7 Strategies to enhancing MoS 2 catalytic activity

Both theoretical calculation an experiment demonstrated that MoS2 is one of the best electrocatalyst candidate to select Pt However, it still exists various of challenge such as the low conductivity, limited number of active sites and deactivate on the basal plane32,33 Therefore, numerous of strategies have been developed to improve the intrinsic electrocatalytic activity of MoS2 making it a highly competitive HER catalyst such as increasing the number of active sites, doping, coupling with the conductive material, tuning the phase or the electronic properties26,32,34,35 Theoretical has been demonstrated that the edge sites of MoS2 have the free energy of the adsorption of hydrogen close to that of Pt21, hence exposing more edge sites of MoS2 has been recognized as a valid strategy toward increasing the HER catalytic activity of MoS2 To maximize the exposed active sites, there are many form of MoS2 nanostructure have been developed such as nanoparticles, vertical nanoflakes, nanowires, and mesoporous structures By the oxidation-sulfidation approach, Hu and Zhu et al reported a vertical MoS2 nanofilms on Mo foils as efficient HER catalysts36 Another single-crystal atomic layered MoS2 nanobelts were synthesized by Yang and his group37 They demonstrated the highly active MoS2 surface were fully covered by edge sites, so the HER activity was enhanced In 2012 year, Kibsgaard and Jaramillo et al described an engineering surface structure of MoS2 to produce double-gyroid morphology mesoporous MoS2 which expose the active edge sites as shown in figure 1.11 38

Figure 1.11 Synthesis procedure and structural model for mesoporous MoS 2 with a double-gyroid

morphology38

As discussed, three phases of MoS2 structure are 3R, 2H, and 1T, and the transforming the 2H

to the 1T phase is an effective strategy to improve the HER performance of MoS2 catalysts39

A report for the metallic nanosheets of 1T-MoS2 chemically exfoliated from semiconducting 2H-MoS2 nanostructures grown directly on graphite can be enhanced HER activity was conducted by Lukowski et al 40 The 1T phase of MoS2 nanosheets exhibited not only facile electrode kinetics but also low-loss electrical transport resulted in the enhancement of catalytic activity Interestingly, a suggestion for the no longer limited to the edges of the 1T metallic MoS2 as in the case of semiconducting 2H-MoS2 was demonstrated by Voiry and his collaborators41 So, the catalytic active sites can be also located on the basal plane By using hydrogen plasma treatment activated the basal-plane of MoS2 to enhance the catalytic activity was performed by Cheng et al.42 Similarly, the defect on the basal plane of MoS2 was calculated

by Shi-Hsin Lin and Jer-Lai Kuo43 They demonstrated that the defected MoS2 can adsorb hydrogen atoms at defect sites which shows an appropriate adsorption energy for hydrogen evolution Subsequently, the nonmetal such as B, N, O were further used to tune the reaction

Therefore, nonmetal doping is an efficient strategy for modulating electronic structure, and optimizing active sites of MoS2 to boost the HER perfomance Nitrogen dope into MoS2 structure was used popular to enrich HER performance Zhou et al reported a MoO2 nanobelts@nitrogen self-doped MoS2 nanosheets as effective electrocatalysts for hydrogen evolution reaction44 They claimed that the stable electrocatalytic activity in hydrogen evolution reaction (HER) and electronic conductivity was optimized by the nitrogen doping and exposed active edges Another research describing the effect of nitrogen doping in MoS2 for HER application was reported by Li et al.45

Owing the poor conductivity, coupling MoS2 with graphene-base is an effective approach to improve its electrical conductivity, hence increasing the overall HER catalytic performance The poor conducting MoS2 was optimized by the internal electron-transport from the graphene-base material which further not permitted MoS2 re-stacking Tang et al reported a hydrothermal method to produce molybdenum disulfide/nitrogen-doped reduced graphene oxide nanocomposite with enlarged interlayer spacing for HER28 Figure 1.12 shows the schematic preparation process of MoS2/N-RGO nanocomposite The fast electrons transfer is a cause of the enlarged interlayer spacing of 9.5 Å of MoS2/N-RGO, resulted in highly efficient HER performance

Figure 1.12 the schematic preparation process of MoS 2 /N-RGO nanocomposite28

As discussed above, the coupling MoS2 with graphene-base and the nitrogen doping into MoS2 structure are the best appropriate strategy to improve HER performance In this thesis, we developed a facile and efficient plasma-induced method to produce both nitrogen doped MoS2 and the graphene/MoS2 composite We suspect that this simple approach will open up new possibilities for preparing both nitrogen doped MoS2 and the graphene/MoS2 composite for board of applications

1.8 Thesis outline

In this thesis, we developed a facile and efficient method to produce MoS2 nanosheets By utilizing active plasma generated the surrounding of the immersed cathode tip the bulk MoS2 dispersed in the electrolyte was exfoliated Furthermore, the nitrogen precursor was provided

by adding the NH3+ sources into electrolyte during plasma-induced process Subsequently, the nitrogen doped MoS2 was obtained simultaneously with the exfoliation process Moreover, by simple selection from hard-metal rod to high-purity rod, the graphene/MoS2 composite with different morphology was achieved Both nitrogen doped MoS2 and graphene/MoS2 composite exhibited promising properties for electrocatalyst application This thesis is divided into four main parts:

1 Chapter 1 presents the fundamental knowledge about two dimensional materials such as transition metal dichalcogenides and graphene and their unique properties and applications

2 A simultaneous exfoliation and nitrogen doping MoS2 nanosheets process was studied The nitrogen doped MoS2 with the turning nitrogen containing was acquired They exhibited excellent electrocatalytic performance (see details in chapter 2)

3 Different morphology of graphene/MoS2 composite was obtained by plasma-induced exfoliation process The onion-like graphene surrounded MoS2 exhibited great electrocatalytic performance (the details present in chapter 3)

4 Chapter 4 shows the summary of the research

Chapter 2 Production Nitrogen-Doped Molybdenum Disulfide nanosheets through

Plasma-Induced process and their electrocatalyst performance

2.1 Introduction

Two-dimensional (2D) materials have been studied extensively because of their unusual properties that are suitable for advanced technological applications For example, transition metal dichalcogenides (TMDs) such as MoS2, WS2, TiS2, TaS2, MoSe2 and WSe2 that can be prepared in the form of a single or a few layers are emerging 2D materials, exhibiting novel electronic and optical properties.46–51 They can have great potentials for applications in electronics, optical devices, actuators, sensors, solar cell and storage devices.46–51 2D TMDs displaying high catalytic activity and stability are particularly suitable for the production of hydrogen fuel, a clean and high-density energy carrier, through the electrolysis of water, and they are promising substitutes for traditional Pt-group metal alloys, which are expensive and scarce For example, MoS2 nanosheets, a well-established 2D TMD materials, displaying a unusually high electro-catalyst activity; they appear to be an ideal candidate for replacing Pt-based metals in the hydrogen evolution reaction (HER).46,52

Up to date, “bottom-up” and “top-down” are two typical ways for the production of layered MX2 materials where M is Mo or W and X is S or Se For the “bottom-up” process, physical/chemical vapor deposition (PVD/CVD) 48,49 and hydrothermal synthesis have been used where the concept of self-assembly is the main reason for the formation of layered 2D materials.50 For “top-down” methods, mechanical/thermal cleavage 51,53 and liquid exfoliation 54–56 have been demonstrated where the mechanical exfoliation of bulk 2D materials into

layered 2D materials by physical or chemical intercalation utilizing gas or metal molecules is the main concept While the “bottom-up” methods utilize specific substrates in a high temperature and vacuum with a limited mass of materials57 , the “top-down” methods often use

simple approaches to produce TMD materials in quantity.58,59 Among the top-down approaches, liquid exfoliation is one of the simplest and most convenient methods for producing large amounts of high-quality MX2 nanosheets.60 This method is, however, sensitive to environmental conditions, consumes large amounts of chemical reagents that can cause environmental issues and requires long processing time.61

Several methods—such as engineering the crystal structure and controlling the degree of heteroatom doping62–64—have been developed to improve the HER activity of layered TMDs.65Structural engineering usually involves tuning complicate growth parameters and additional template.66 Controlling the degree of heteroatom doping in layered TMDs can in turn tune the active surface area, edge density and defect levels in its basal planes 42,67 —the active sites are determined by these parameters—and thus the electro-catalytic activity since.68,69 For the doping of non-metal atoms, high temperature annealing treatments, however, are necessary steps For example, Se-doped MoS2 has shown improved electrical conductivity and a greater number of active edge sites, resulting in the enhanced HER activity.35 Si Qui et al described the tunable N atom doped MoS2 nanosheets through sol–gel process and subsequent annealing treatment24; the N-doped MoS2 exhibited good performance for the storage of lithium ions 70 Although N-doped MoS2 nanosheets appear to be one of the most highly active and durable catalysts for HER, particularly in acidic media 45,71, they are typically synthesized through multi-step processes involving high temperature annealing 24,72 or through the use of special equipment (e.g., N2 plasma systems).13 The simultaneous one-step plasma-induced exfoliation and doping of MoS2 for application in the HER process has not been described previously Herein, we demonstrate such a facile and one-step plasma-induced exfoliation approach—

microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), Raman spectra and atomic force microscopy (AFM) Different kinds of MX2 nanosheets such as MoSe2, WSe2, MoS2 and WS2 from bulk counterparts were demonstrated Furthermore, an active surface plasma can be generated at the submerged cathode tip to induce doping of nitrogen atoms into the MX2 structure, namely N-doped MX2, from electrolytes containing nitrogen atoms Here, N-doped MoS2 nanosheets were selected for the concept demonstration because of the highly catalytic hydrogen production ability In addition, selection of electrolytes and plasma-induced exfoliation times were investigated to achieve different concentrations of

N atoms in MoS2 nanosheets The maximum doping concentration can be up to 7.6 at % using the ammonia solution in the electrolyte, as confirmed by X-ray photoelectron spectroscopy and electron energy loss spectroscopy The optimal N-doped MoS2 nanosheets with the N concentration of 5.2 at % with the excellent catalytic hydrogen evolution reaction can be found where a low over-potential of 164 mV at a current density of 10 mA cm–2 and a small Tafel slope of 71 dec mV–1—much lower than those of exfoliated MoS2 nanosheets (207 mV, 82 dec

mV–1) and bulk MoS2 (602 mV, 198 dec mV–1)—as well as an extraordinary long-term stability

of >25 h in 0.5 M H2SO4 can be achieved The as-made N-doped MoS2 nanosheets displayed highly efficient HER performance, suggesting that the plasma-induced method has an excellent potential for producing N-doped TMDs used in promising HER applications

2.2 Experimental section

Plasma-Induced Exfoliation on bulk TMDs into layered TMD nanosheets:

TMD nanosheets (MoS2, MoSe2, WS2 and WSe2) were prepared by dispersing micrometer-size (>2 μm) TMD powders (500 mg) into 1.5 м aqueous NaOH (200 mL) Each dispersion was stirred and warmed on a hot plate to 80 °C and then maintained at 80 °C for 3 h For the setup

of the electrolytic cell, a W (or Mo) rod was used as the cathode and a Pt foil as the anode These electrodes were immersed at depths of 0.2 and 1 cm the electrolyte, respectively A bias was applied to the system through a DC power supply (100 V/10 A, LinVAC TE CH) The bias was increased gradually to 65 V, at which the point plasma appeared at the immersed part of the cathode This plasma became stronger as the bias was increased further The cathode and anode were then immersed in the electrolyte at various depths, from 0.2 to 7 cm and from 1 to

5 cm, respectively, while maintaining the bias at 95 V To exfoliate MX2, the system was stirred

at 350 rpm for 30 min During the plasma-induced treatment process, deionized water was added to keep the electrolyte at 200 mL in the electrolyte After the plasma treatment, the dispersion was subjected to ultrasonication in a bath for 60 min at 20 kHz under a power of 130

W After the sonication, the exfoliated MoS2 powder was collected through vacuum filtration onto an AAO membrane filter (Millipore; pore size: 200 nm) and washed repeatedly (at least three times) with distilled water and EtOH and then rinsed with dilute HCl The powder was dried in a vacuum oven at 70 °C overnight, then dispersed in Ethanol (concentration: 1 mg mL–

1) and sonicated for 15 min The dispersed MoS2 solution was centrifuged (1000 rpm, 30 min)

to remove any aggregated or un-exfoliated flakes The top one-third of the solution was

carefully removed and placed dropwise onto a cleaned Si wafer for further characterization

In-situ N doping during Plasma-Induced Exfoliation:

N-doped MoS2 nanosheets were prepared by adding 50 mL NH4(OH) 25%, 50mL NH4NO3 1

м or 50 mL (NH4)2SO4 1 м, respectively, in the solution which bulk commercial MoS2 powder (500 mg) were dispersed into an aqueous solution of 2 M NaOH (150 mL) The mixture solution was stirred on a hot plate while warming to 80 °C, maintaining the temperature at 80 °C for 3

h into a three necks flask as the electrolyte Similar as preparation of TMD nanosheets, the electrolytic cell was setup with a cathode (W rod) and an anode (Pt foil); these electrodes were immersed at depths of 0.2 and 1 cm, respectively, in the electrolyte A DC potential was applied and regularly increased to plasma-induce point appeared at the immersed part of the cathode; The voltage was increased to 95V, meanwhile the cathode and anode were immersed in the electrolyte with the depths 7 cm and 5 cm, respectively The plasma-induce was maintained at 95V under 350 rpm stirring for 30 min To offset the water was steamed, the deionized was added to keep electrolyte around 200 mL Similarly, 3 mL NH4OH, NH4NO3 or (NH4)2SO4 was slowly dropped into electrolyte twice after 10 and 20 min plasma, respectively After plasma-induced treatment at a DC voltage of 95 V, the N-doped MoS2 nanosheets were collected using the same procedure as described above for the pristine MoS2 nanosheets

excitation wavelength: 514.5 nm) UV–Vis spectra were recorded using a Hitachi U-4100 spectrophotometer X-ray photoelectron spectroscopy (XPS) was performed at beamline (BL) 09A2 U5 Spectroscopy at the National Synchrotron Radiation Research Center (NSRRC), Taiwan Atomic force microscopy (AFM) was performed using a diInnova scanning probe microscope (Bruker) operated in the tapping mode

Electrocatalytic hydrogen evolution:

For HER measurements, linear sweep voltammetry (LSV) was performed at room temperature

in 0.5 M H2SO4 at a scan rate of 5 mV s–1 using a CHI 611B electrochemical workstation, with

a Pt wire as a counter electrode and a 3 M NaCl Ag/AgCl (CHI equipment) as a reference electrode A drop (5 μL) of a solution of MoS2 ink at a concentration of 3 mg mL–1 (0.214 mg

cm–2) in DMF was placed onto glassy carbon (diameter: 0.3 cm) as the working electrode The working electrode was protected by doped 5 μL Nafion solution and then natural drying at room temperature The averages of at least five LSV curves were measured to calculate the Tafel slope The performance of the hydrogen evolution catalyst was recorded while performing LSV

from –0.5 to +0.2 V versus a reversible hydrogen electrode (RHE) The I–t stability was

measured at overpotentials of 165 mV for 25 h on N-doped MoS2 Electrochemical impedance spectroscopy (EIS) was performed from 100 kHz to 0.1 Hz (amplitude: 10 mV) under an overpotential of 200 mV (vs RHE), using a Zahner Zennium workstation The electrolyte was purged with N2 or Ar gas for 30 min prior to each measurement All potentials were calibrated with respect to the RHE

2.3 Results and discussion

Figure 2.1 (a) Experimental setup for plasma-induced exfoliation and (b) proposed

mechanism of exfoliation and nitrogen-doping process

Figure 2.1(a) provides a schematic of the experimental setup for the plasma-induced

exfoliation process Because a W (or Mo) cathode has a high Mohs hardness of 7.5 (5.5) and a high melting temperature of approximately 3400 °C, it is expected that the surface plasma (ca

2600 °C) would not damage the cathode Figure 2.1(a) inset indicated that the tip of the cathode

is always engulfed in many discrete plasma discharge bubbles, instead of a single plasma bubble

(see the movie in the supporting information) Figure 2.1(b) show the energy of the strong and

instant plasma leading to in the form of thermal and rapid gas exploding energy can simultaneously exfoliate bulk MX2 into MX2 nanosheets and dope nitrogen atoms into MX2 structure Inside the bubbles, the gas expanded and reached a pressure of several hundred megapascals.73 When the gas pressure surpassed the threshold pressure, the discrete plasma bubbles exploded The plasma energy can be released into the electrolyte via high-energy jets that attacked the edges of the bulk MX2 and disrupted the weak out-of-plane van der Waals