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

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Composite Prepared by Electrolysis Plasma-Induced Processfor Hydrogen Evolution Reaction

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Nitrogen Doped MoS2 Nanosheets and Graphene/MoS2 CompositePrepared by Electrolysis Plasma-Induced Process for HydrogenEvolution Reaction

國國國國國國國國國國國國國國國 國國

Student: Nguyen Van TruongAdvisor: Dr Kung-Hwa Wei國國國國國國

國國國國國國國國國國國國國國國國國國國國國國國國國國國HER國國國國國國國國國 國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國 國國國國國/ 國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國 國國國國國國國 80°C國國國國國國國國國國國國國國國國國國國國國國國國國國國國 國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國 2H-國國國國 國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國 國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國 5.2 at國國國 國國國國國國國國國國國國國國 10 mA cm–2

國國國國國國國國國國國國 164mV國Tafel 國國國國國國 71 mV dec-1

–國國國國國國國國國國國國國國207 mV國82 mV dec-1

國國國 國國國國國國602 mV國198 mV dec-1

國國國國國 0.5 M 國國國國國 25 國國國國國國國國 國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國 國國國國國國國國國國國國國國國國國國國OGNs @ 國國國國國國國國國國國國國國國 國國國國國國國國GNs @ 國國國國國國國國國國國國國國國國國國國國國國 HER 國國 國國國國國國國國國國國國國國國國國國國-國國國國國國國國國國國國國國國國國國 國國國國國國國國國國國國國國國國國國 HER 國國國國國國國國國國國國國國國 OGNs@ 國國國國國國國國國國國國國國 HER 國國國國 10 mA cm-2

國國國國國國國國 118 mV 國國國國國國Tafel 國國國 dec-1 國 Tafel 國國國國國國國國國國國國國國國國國國國國 國國國國國國國國國國國國國國國國國 GNs @ 國國國國國182 mV國82 mV dec-1

國國國國 國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國國TMDCs國國國國國國國國國國國國國 TMDCs 國國國國國國國國國國國國國國國國國 國國國國

Nitrogen Doped MoS2 Nanosheets and Graphene/MoS2 Composite Prepared byElectrolysis Plasma-Induced Process for Hydrogen Evolution Reaction

Student: Nguyen Van Truong Advisor: Prof Kung-Hwa WeiDepartment of Materials Science and Engineering

National Chiao Tung University

With the goal of obtaining sustainable earth-abundant electrocatalyst materials displaying highperformance in the hydrogen evolution reaction (HER), here we propose a facile one-potplasma-induced electrochemical process for the fabrication of both nitrogen-doped MoS2

nanosheets and graphene/MoS2 composite An efficient one-step approach that involvessimultaneous plasma-induced doping and exfoliating of MoS2 nanosheets within a short timeand at a low temperature (ca 80 °C) has been developed Particularly, an active plasma zonecan be generated at the submerged cathode tip to achieve doping of nitrogen atoms into thesemiconducting 2H-MoS2 structure The electronic and transport properties were modulatedunder the synergy of the nitrogen doping and exfoliation in the MoS2 structure to enhancetheir 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 of164 mV at a current density of 10 mA cm–2 and a small Tafel slope of 71 mV dec–1—muchlower than those of exfoliated MoS2 nanosheets (207 mV, 82 mV dec–1) and bulk MoS2 (602mV, 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

a key aspect of the enhanced HER ability Therefore, we conclude that electronic coupling atthe graphene–

MoS2 nanosheet interfaces also played an important role in enhancing the HER activity Our

G Ns@MoS 2 composites exhibited high HER performance, characterized by a lowoverpotential 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 thesheet- like graphene-wrapped MoS2 composite G N s@MoS 2 (182 mV, 82 mV dec–1) Thisapproach appears to be an effective and simple strategy for tuning not only nitrogen-dopedtransition metal dichalcogenide (TMDCs) materials but also the morphologies of compositesof 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.

This dissertation presents a summary of my research work which has done in the Departmentof Materials Science and Engineering (MSE), National Chiao Tung University (NCTU) It is apleasure to express my sincere gratitude to all the people who helped and supported me duringmy Ph.D study.

From bottom of my heart I express my deep sense of gratitude and profound respect to mysupervisor Prof Kung-Hwa Wei He continually and convincingly conveyed a spirit ofadventure 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 wouldnot have been completed My sincere thanks Prof Yu-Lun Chueh for his kind guidance andpersistent 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 whomade 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 myside Thank you for always encouraging me to pursue my dreams I love you all so much,thanks for loving me too!

Nguyen Van TruongHsinchu, TaiwanApril 2020

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 throughPlasma-Induced process and their electrocatalyst performance .

2.4 Conclusions 50

Chapter 3 Production Graphene/MoS2 composite through One-Pot Plasma-Induced

process and their Electrocatalyst performance

513.1 Introduction 51

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

F ig u r e 1.6 E x p e rim e ntal s e tup and m ec h a nism of ca thod i c plasma e x foli a t i on 7

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

Figure 1.8 I-V curve of overall water splitting 10

F ig u r e 1.9 S c h e m a ti c o f the c ov a lent ni t r o g e n doping in MoS 2 upon N 2 plasma surf a c e

tr ea t m e nt 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 gyroid morphology 17

double-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) proposedmechanism of exfoliation and nitrogen-doping process 27

Figure 2.2 FE-SEM images of bulk commercial samples of (a) MoS2, (b) MoSe2, (c) WS2 and(d) WSe2, respectively .29

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 ofexfoliated (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

2g and A1g in Raman spectra and (b) thelateral size of exfoliated MoS2 using different applied biases 35

undoped MoS2 and N-doped MoS2 nanosheets and the corresponding EELS elementalmapping images of Mo, S and N with different electrolytes and/or plasma-induced time,respectively 36

(b) 300 oC (c) 500 oC and their BF-STEM images(d-f), respectively, correspond with EDSmapping of Mo, S and N elements .41

Figure 2.11 (a) LSV curves (recorded on a glassy-carbon electrode) of bulk MoS2, undopedMoS2 and N-doped MoS2 (b) Corresponding Tafel plots derived from (a) (c) Nyquist plotsacquired 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

ig u r e 3.1 ( a) P ro c e dure a nd s e tup for the p r e p a r a t i on of MoS 2 n anosh eets cov e r e d b y onion -

l i ke g r a ph e n e shee ts ( O G N s@MoS 2 ) a nd MoS 2 n a noshee ts d e c or a t e d on sh eet - l i ke g r a ph e ne

( G Ns@MoS 2 ); ( b) Sche matic re p r e s e ntation of t he prop os e d me c h a nism of O G N s@MoS 2 and

G N s@MoS 2 58

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

Figure 3.3 a–c) SEM and (d–f) TEM images of (a, d) MoS2 nanosheets, (b, e) O G Ns@MoS 2 ,

and (c, f) G Ns@MoS 2 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, GN s@MoS 2 and OG Ns@MoS 2 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 O G Ns @MoS 2 sample; (b)expanded view; and (c) HR-TEM image of the same sample recorded from another position.(d) STEM bright-field image of the O G Ns@ MoS 2 sample and corresponding elementalmapping of C, Mo, and S atoms .65

Figure 3.9 (a) HR-TEM image of the G Ns@MoS 2 sample and expanded views of its (b)MoS2 and (c) GNs region; insets: corresponding SAED patterns (d) STEM dark-field imageof the G N s@MoS 2 structure and corresponding elemental mapping of C, Mo, and Satoms 67

Figure 3.12 High resolution XPS spectra of O1s of (a) O G Ns@Mo S 2 and (b) MoS2

71

Figure 3.13 (a) LSV curves of the OGNs, MoS2, G N s@MoS 2 , and O GN s @MoS 2 samples andthe Pt electrode (b) Tafel plots obtained from LSV curves .72

current density of 10 mA cm-2 of MoS2, GN s@M o S 2 and OG Ns@MoS 2 73

Figure 3.15 (a,b) SEM, (c,d) TEM and (e,f) images of MoS2 nanosheets prepared in 2MH2SO4

and 2M NaOH 78

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

Figure 3.17 (a-c) SEM images and (d-f) corresponding of EDS spectra of O G Ns@MoS 2

prepared at different plasma electrolysis time conditions .80

Table 3.1 Component ratio of MoS2, GN s@MoS 2 and O GNs@MoS 2 62

Chapter 1 Introduction1.1 Introduction of Transition metal dichalcogenides

Owing to numerous of fascinating properties, the most famous of two-dimentionalmaterials (2D), graphene has been very populated for a worldwide range of possibleapplications Other members in family of layered inorganic material, transition metaldichalcogenides (TMDCs) which are semiconducting materials with a typical MX2, where Mis a transition metal such as Mo, W and X is a chalcogenide, such as S, Se exhibited manyintriguing scientifically and technologically properties Discovered their structure in 1923 byLinus Pauling1, figure 1.1 shows more than 40 type of TMDCs which were known as layerstructure by late 1960s To 1986, the first research on monolayer MoS2 was reported2 Inparallel 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 layerwithout dangling bonds between layers which related to other by the weak van der Waalsforces.

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 thecenter of triangular prisms while the Mo atoms are at center of the octadedral in the 1T- phasecase 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 metallicphase which can be converted from semiconducting 2H phase by several methods such asliquid exfoliation6,7 or microwave assisted8,9 Specially, the appearance of 1T- phase can beenhanced the electrocatalytic activity of TMDCs.

simultaneously with the thickness decreasing, the indirect bandgap semiconductor in bulkcounterpart change to direct bandgap in monolayer For example, the experiment value forbandgap 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 materialappears from quantum confinement effects These properties of TMDCs can open uppromising for valleytronics and/or electrochemical energy storage applications.

1.2 Production of Transition Metal Dichalcogenides materials.

Figure 1.3 Six main production methods of TMDCs and their content10

TMDCs have many of morphology with numerous of shapes, sizes or phases, such asnanosheets, nanoparticle, nanostructure, nanofibers Among them, the ultrathin TMDCsmaterials have been exhibited the different chemical, physical and electronic properties withtheir bulk counterpart which give great promising for a board of applications Up to date, thereare two main approaches to prepare mono or few layers TMDCs nanosheets are top-down andbottom up routes Basically, top-down methods isolated TMDCs nanosheets from bulkmaterials such as mechanical, liquid exfoliation, or electrochemical exfoliation Bottom upmethods include chemical vapour deposition (CVD), physical vapour deposition (PVD) and/orwet 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 uniformthickness However, cost effect, scalable, high modern technique, high temperature andvacuum requirements are their disadvantages The mechanical cleavage method which use theadhesive tape to produce high-quality TMDCs nanosheets however the long-time process andslow rate are unchanged This method can only suitable for fabrication of individual devicesor fundamental characterization Recently, the laser spot is more attractive in producingmonolayer TMDCs, although the scalable and laser rating is the barrier for scale-upproducing The most commonly and promising method is liquid phase exfoliation With highquantity of sub-micrometer to sub-nanometer size of TMDCs nanosheets production, theliquid phase exfoliation method can be allowed for industrially scalable Particularly, theexfoliated TMDCs nanosheets electronics structures are changed from semiconducting (2H)phase to metallic phase (1T) phase The intercalation and exfoliation in the liquid phasemethod produce TMDCs nanosheets into different sizes with structure distortions althoughthey show high rate production The lithium ions intercalation was discovered from 1975 byMartin B Dines11 Bulk TMDCs was immersed into n-butyl lithium one day for theintercalation of lithium ions The TMDCs nanosheets were exfoliated by a sonication step.This method was widened by using many kind of intercalation ions Furthermore, anelectrochemical system using discharge mode to control Li ion from lithium foil anodeintercalate into TMDCs layers, subsequently TMDCs nanosheets was exfoliated by sonicationstep12 Although lithium-intercalated method still exists several disadvantages such as longdurations, contaminators, this method is one of most efficient method to procedure massproduction of TMDCs nanosheets In addition, with the expanding of the scope of TMDCs

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 atmosphericplasma process15.

Figure 1.5 Typical of plasma electrolysis and its applications16

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

The recycle graphite from wasted battery was using as a cathode in a two electrodes systemwhere

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

Figure 1.6 Experimental setup and mechanism of cathodic plasma exfoliation

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

Graphene nanosheets was produced by cathodic plasma method when using NaOH as anelectrolyte On the other hand, the Onion-like graphene was obtained when using H2SO4 Theyassigned that the different ions size of H+ and Na+ in the plasma process was facilitated thebond breaking and dissociation of radical to form different morphology of exfoliated graphenesheets 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 producethe nitrogen-doped graphene sheets The doping mechanism was proposed: under thestrong electric field with the electrolysis plasma phenomenon was formed at cathode inelectrolyte The large temperature gradient could be the main driving force for the graphitelayer surrounding cathode expansion Resulting in partially radicalized graphene formedduring exfoliation process as depicted in figure 1.7 Simultaneously, the various of radicalsfrom (NH)4OH such as H*, NH3 , NH2 , NH* generated in the plasma zone which provide Nsource to form nitrogen-doped graphene19.

Figure 1.7 Schematic representation of the proposed mechanism of plasma exfoliation and

nitrogen-doping19

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 cleanenergy sources for traditional fossil fuels replacement is vital significant There are manykinds of renewable energy sources have been widely explored such as solar, wind orgeothermal Nevertheless, the limitation of spatial and temporal application for spreading ofthese energy sources is still a prime challenge Hydrogen energy, an earth-abundance andgreen energy with extremely high energy carrier, is one of the best promising candidates forfuture solution Hitherto, the hydrogen gas has been mostly produced by steam-reformingand/or gasification system where metal and coal have been the foremost sources However,these massive energy consuming methods involve the copious pollutants emission with lowquality hydrogen purity In addition, these methods require complex system using at hightemperature and pressure.

Hydrogen production by water splitting is an alternative strategy for solar to hydrogenconversion, where photoelectrochemical and electrochemical are two possible routes.Electrocatalytic water splitting for production of H2 is an alternative approach for theconversion of solar energy into chemical fuels The strongly absorbed of hydrogen atoms onthe catalyst surface is one of most important factor for hydrogen evolution reaction (HER) Todate, platinum is the most efficient HER electrocatalyst which exhibit the lowest overpotentialamong the present electrocatalysts However, the high cost and scarce is the challenge for theirwidely application Hence, looking for an abundance, low cost and high efficiency

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 partis 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.

Figure 1.8 I-V curve of overall water splitting23

Besides, an important kinetic parameter for ranking of HER catalytic comparison is Tafelslope 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 wherethe 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 HERoccurs 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 theTafel (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 thecatalysts can assign to the decreasing of current density at the fixed potential The betterstability shows the smaller decreasing of current density.

1.5 Introduction of nitrogen doped MoS2

The most common conventional strategy to improve the mass transfer, tune electronicstructure or the synergetic effect is doping The nitrogen doped MoS2 is one of the bestapproaches to increase active sites, optimize the electronic structure also transfer the phasestructure Up to date, there are various method to generate nitrogen doped MoS2 such ashydrothermal/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 7.6at% 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 thenitrogen doped MoS2 Wang and his collaborators reported a Fluorine and nitrogen co-dopedMoS2 active basal plane toward hydrogen evolution reaction25 They described the synergisticeffects of codoped fluorine and nitrogen can active the inert basal planes Subsequence, thecodoped MoS2 enhanced HER activity They used the simulation to demonstrate that thenitrogen and fluorine were doped into the basal plane Recently, the plasma treatmentemerged as the promising candidate for synthesis TMDCs doping Among them, usingnitrogen 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 nitrogenplasma exposure The controllable of nitrogen containing in the MoS2 by plasma time wasobtained They also demonstrated that the nitrogen can substitute the sulfur in the nitrogendoped MoS2 structure.

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

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

1.6 Introduction of graphene/MoS2 composite

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

hydrothermal, solvothermal, electrochemical, CVD Li et al synthesized MoS2 grown onreduced graphene oxide (RGO) by solvothermal method using for HER in 201129 They claimthat the MoS2 particles stacked on RGO nanosheets were exposed their edge sites The highloaded of MoS2 particles on RGO nanosheets based on the strong chemical and electroniccoupling between MoS2 and the RGO sheets Consequently, the electrocatalytical activity ofMoS2/RGO were improved In 2017 year, Wan and his group described an electrochemicaldeposition to produce large area MoS2 directly on CVD graphene sheets for photodetectors asshown in figure 1.10(a)30 The vertical MoS2/graphene heterostructures were synthesized byelectrochemical deposition in water, followed an annealing step The controllable thickness ofvertical MoS2/graphene has been confirmed by AFM, XPS and SEM The verticalMoS2/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 grapheneand MoS2 hybrid for supercapacitor The MoS2-3D graphene hybrid was characterized byXRD, SEM, TEM and XPS analysis They claimed that the 3D graphene can support to theelectrolytic ions transfer which increase the charge storage capacity.

Figure 1.10 (a) Schematic illustration of the electrochemical deposition set-up30, (b)Comparison of MoS2-3D graphene hybrid in solution and solid state supercapacitor31

1.7 Strategies to enhancing MoS2 catalytic activity

Both theoretical calculation an experiment demonstrated that MoS2 is one of the bestelectrocatalyst candidate to select Pt However, it still exists various of challenge such as thelow conductivity, limited number of active sites and deactivate on the basal plane32,33.Therefore, numerous of strategies have been developed to improve the intrinsicelectrocatalytic activity of MoS2 making it a highly competitive HER catalyst such asincreasing the number of active sites, doping, coupling with the conductive material, tuningthe phase or the electronic properties26,32,34,35 Theoretical has been demonstrated that the edgesites of MoS2 have the free energy of the adsorption of hydrogen close to that of Pt21, henceexposing more edge sites of MoS2 has been recognized as a valid strategy toward increasingthe HER catalytic activity of MoS2 To maximize the exposed active sites, there are manyform of MoS2 nanostructure have been developed such as nanoparticles, vertical nanoflakes,nanowires, and mesoporous structures By the oxidation-sulfidation approach, Hu and Zhu etal reported a vertical MoS2 nanofilms on Mo foils as efficient HER catalysts36 Anothersingle-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 theHER activity was enhanced In 2012 year, Kibsgaard and Jaramillo et al described anengineering surface structure of MoS2 to produce double-gyroid morphology mesoporousMoS2 which expose the active edge sites as shown in figure 1.11 38.

Figure 1.11 Synthesis procedure and structural model for mesoporous MoS2 with a double-gyroidmorphology38

As discussed, three phases of MoS2 structure are 3R, 2H, and 1T, and the transforming the 2Hto 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 semiconducting2H-MoS2 nanostructures grown directly on graphite can be enhanced HER activity wasconducted by Lukowski et al 40 The 1T phase of MoS2 nanosheets exhibited not only facileelectrode kinetics but also low-loss electrical transport resulted in the enhancement of catalyticactivity Interestingly, a suggestion for the no longer limited to the edges of the 1T metallicMoS2 as in the case of semiconducting 2H-MoS2 was demonstrated by Voiry and hiscollaborators41 So, the catalytic active sites can be also located on the basal plane By usinghydrogen plasma treatment activated the basal-plane of MoS to enhance the catalytic activity

hydrogen evolution Subsequently, the nonmetal such as B, N, O were further used to tunethe reaction.

Therefore, nonmetal doping is an efficient strategy for modulating electronic structure, andoptimizing 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 MoO2nanobelt s@nit rogen self-doped MoS2 nanosheets as effective electrocatalysts for hydrogenevolution reaction44 They claimed that the stable electrocatalytic activity in hydrogenevolution reaction (HER) and electronic conductivity was optimized by the nitrogen dopingand exposed active edges Another research describing the effect of nitrogen doping inMoS2 for HER application was reported by Li et al.45

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

Figure 1.12 the schematic preparation process of MoS2/N-RGO nanocomposite28

As discussed above, the coupling MoS2 with graphene-base and the nitrogen doping intoMoS2 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 dopedMoS2 and the graphene/MoS2 composite We suspect that this simple approach will openup new possibilities for preparing both nitrogen doped MoS2 and the graphene/MoS2

composite for board of applications.

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

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

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

4 Chapter 4 shows the summary of the research.