INTRODUCTION
Motivation
Energy and the environment are two critical societal issues that must be addressed to remain economically and socially sustainable Most of the energy economy is based on non-renewable and environmentally destructive energy sources like coal, petroleum oil, and natural gas A global push for renewable and clean energy alternatives to fossil fuels has been launched to address those issues Nature provides a diverse range of renewable energy sources, including solar, wind, tidal, and biomass However, the intermittent nature of such energy sources presents a challenge due to regional or seasonal factors
As a result, efficient energy conversion and storage technologies and large-scale renewable energy exploration are required This requirement is the impetus for numerous energy conversion and storage systems innovations Indeed, in recent decades, systems such as hydrogen production via water electrolysis, hydrogen conversion to electricity via fuel cells, and energy storage via lithium-ion batteries have received significant attention
Electrolytic water hydrogen production is a primary method for obtaining high purity, sustainable clean energy hydrogen A meaningful way to speed up the reaction process of electrolytic water hydrogen electrodes to improve hydrogen production efficiency is to find highly efficient catalysts with high conductivity and good hydrophilicity The high conductivity catalyst is beneficial to the electron transmission in the electrode reaction process, reduces the starting potential of the reaction, and good hydrophilicity can reduce the surface tension of the electrode material-electrolyte contact interface, making it easier for the hydrogen ions in the electrolyte to come into contact with the active catalytic site on the electrode surface Precious metals such as platinum, ruthenium, or iridium are best–known catalysts for HER, but the high cost and scarcity prevent them from large-scale application Therefore, the development of new catalysts which are inexpensive, high active, scalable, and stable remain of interest Among numerous factors contributing to the advancement of the future sustainable energy package, catalysis, or electrocatalysis, has been critical in overcoming the kinetic
2 energy barriers for electrochemical reactions involving water, oxygen, and hydrogen in water splitting cells and fuel cells This research is focused on the role of catalysis in electrolysis water-splitting cells
Lithium-ion batteries (LIBs) have been considered the most attractive power source for portable electronics and future electric vehicles that satisfy the urgent demands of the global energy storage market Due to the growing demand for long-cycle life and high- performance lithium-ion batteries (LIBs), extensive research is being conducted on lithium-ion batteries, including the cathode and anode materials, the binder, the electrolytes, and the battery manufacturing technique The active materials in the electrode are critical for increasing the energy and power densities of lithium-ion batteries
Nanostructured materials have several advantages, including high surface-to-volume ratios, favorable transport properties, altered physical properties, and confinement effects due to their nanoscale dimensions, and have been extensively studied for energy- related applications In recent years, molybdenum disulfide (MoS2), which belongs to the layered transition – metal dichalcogenide (LTMDs) family, with particular chemical and physical properties, has been studied extensively intensively The use of MoS2 as a low-cost alternative to platinum in the electrochemical hydrogen evolution reaction has been investigated Phase engineering is a promising technique for increasing the amount of MoS2 that can be activated In general, MoS2 crystallizes in two different crystalline phases: the hexagonal 2H phase and the octahedral 1T phase The chemical and physical properties of 1T-MoS2 are significantly superior to those of the natural semiconductor 2H-MoS2 However, 1T-MoS2 is metastable, and its synthesis continues to be a difficult task Although the hybrid 1T/2H-MoS2 has been synthesized, controlling the 1T/2H ratio is still a challenge that has not been thoroughly discussed An investigation into the synthesis methods of hybrid phase 1T/2H-MoS2 with controllable high 1T concentration is carried out in this study
In the context of lithium-ion batteries (LIBs), MoS2 has received significant attention as a prospective anode material due to its higher theoretical capacity (670 mAh g −1 ), large specific surface area, short diffusion length in the thickness direction, as well as
3 voids abundant and defects that may benefit lithiation/de-lithiation [1] However, poor cyclability and rate capability due to low electrical conductivity, and massive volume expansion during cycling, have prevented MoS2 from being used in practical applications Efforts have been made to address these issues: the synthesis of MoS2 with different molecular structures from precursors and the development of various MoS2- carbon composites Carbon nanotubes (CNTs) are among the most promising materials for using a carbonaceous matrix material as an additive to increase the conductivity of the anode material, owing to their extremely high conductivity, which ultimately contributes to improving electrochemical performance Furthermore, the high aspect ratio of CNTs allows for the connection or bridge of any defects and the prevention of MoS2 aggregation to overcome significant volume changes during discharge/charge cycling
With the continuous development and progress of material science, the research and development of new processes for material synthesis have always been an important part In this context, microwave-assisted (MW) methods can be considered a promising green strategy for synthesizing nanomaterials and nanocomposites, which can be used to confirm the green chemistry approaches currently in use Furthermore, MW-assisted strategies provide homogeneous heating to the reaction mixture, resulting in a reduction in the thermal gradients in the solution This phenomenon is responsible for the stable nucleation and growth environment, resulting in nanomaterials with uniform size distribution and uniform size distribution The final quality of microwave-generated materials depends on the reactant choice, the applied power, the reaction time, and the temperature The research deals with the fundamentals of microwave chemistry, which are critical for comprehending the mechanism and solution phase phenomena that occur during microwave heating The latest advancements and MW-assisted strategies and their claims towards creating nanostructured materials, which make chemistry more environmentally friendly and have a wide range of potential applications, are also discussed
Objectives and scopes
• The fundamental purpose of this doctoral work is to develop a simple, productive, and energy-efficient process for manufacturing MoS2 and nano MoS2/CNTs nanomaterials including nanostructured 1T/2H-MoS2 hybrid phase, amorphous MoS2/CNTs, and crystalline MoS2/CNTs nanocomposites
• Optimize the synthesis process parameters using the Taguchi experimental planning method so that the process (or product) is stable at the maximum quality level within the survey's scope
• Evaluation of the structure and properties of each type of synthetic material for use as anode electrode materials in lithium batteries (LIBs) and electrochemical catalysts for hydrogen evolution reaction (HER)
• Microwave-assisted chemical processes must be scaled up for industrial nanomaterial manufacturing and applications because microwave heating is effective in wet chemical procedures for producing MoS2 and MoS2/CNTs nanocomposites, resulting in clean reactions with varied morphologies and sizes and speed up the reaction time.
The new ideas of the research (Novelty)
• A novel scalable one-step microwave heating approach is developed to obtain a high rate of metallic 1T-MoS2 in the 1T/2H hybrid phase of MoS2 to enhance the catalytic hydrogen evolution reaction (HER)
• Using microwave heating to control the 1T/2H ratio in polyol solvents, including ethylene glycol (EG), mixed ethylene glycol and glycerol, glycerol, and ethylene glycol with a small amount of water has not been reported before This work discusses the effects of various polyol solvents on the synthesis of MoS2 hybrid phase under microwave heating
• It is worth noting that pure 1T-MoS2 has a metastable state, which severely limits its application The successful synthesis of hybrid phase 1T/2H-MoS2, in which the metastable 1T phase can be stabilized by interaction with the 2H phase, significantly increasing its catalytic activity, provides a scientific foundation for using hybrid
5 phase MoS2 to replace Platinum (Pt) in the electrochemical hydrogen evolution reaction (HER)
• Additionally, the synthesis of crystalline and amorphous MoS2/CNT nanocomposites in polyol solvents using microwave heating has not been extensively studied MoS2/CNTs, with their crystalline structure, exhibit superior electrochemical performance in lithium-ion battery (LIB) applications, whereas amorphous MoS2/CNT materials demonstrate catalytic activity in the hydrogen evolution reaction (HER).
Major contributions of the thesis
• This thesis establishes a critical foundation and initiates a new research direction in catalysis, energy storage, and conversion based on MoS2 nanostructured materials and carbon nanotubes in Vietnam in general, and VNU-HCM in particular, in order to keep pace with the global trend of advanced materials research
• The ability to control the structural morphology and particle size of hybrid structures 1T/2H-MoS2 and MoS2/CNTs nanocomposite using microwave energy have crucial scientific significance in choosing this method to create MoS2, MoS2/CNTs nanomaterials in particular and many other nanostructured materials in general With the outstanding advantages of microwaves, such as time and energy savings, the reaction at air pressure can produce a large number of synthetic products (gram scale), which is much higher than other hydro/solvothermal methods, which is vital in speeding up laboratory research
• Experiment data have a high reference value, contribute to the scientific database in the field of one-dimensional and two-dimensional nanomaterials, and open up valuable alternative research methods in material technology materials for energy storage and conversion applications, such as those found in HER and LIBs.
Research content
In order to meet the objectives, the thesis focuses on implementing the main contents listed below
• Develop the microwave-based synthesis procedure for nano molybdenum disulfide (MoS2) 1T/2H hybrid phase
• Synthesis of crystalline MoS2/CNTs nanocomposites by indirect CNTs dispersion process (I2PD), determining the most suitable synthesis conditions within the scope of investigation by Taguchi design of the experiment method with the response value is crystallinity
• Synthesis of amorphous MoS2/CNTs nanocomposites by direct CNTs dispersion process (D2PD), determining the most suitable synthesis conditions within the scope of investigation by Taguchi design of the experiment method with the response value is Tafel slope
• Evaluate the applicability of amorphous MoS2/CNTs materials as an electrochemical catalyst hydrogen evolution reaction (HER)
• Evaluate of the electrochemical performances of crystalline MoS2/CNTs as anode electrode materials in lithium - ion batteries (LIBs).
Research outline
An outline of the thesis is briefly presented as follows:
Chapter 1 provides an overview of the thesis's research motivation, objectives, and novelty
Chapter 2 reviews a comprehensive structure and various approaches for fabricating
1T/2H-MoS2 and MoS2/CNTs and the advantages and efficiency of the microwave energy method This session also covers novel approaches for improving the electrocatalytic water splitting performance of MoS2 and MoS2/CNTs and their electrochemical performance in lithium batteries
The overall methodology is described in Chapter 3, which includes microwave synthesis techniques for nanostructured materials including 1T/2H-MoS2 and MoS2/CNTs nanocomposite, Taguchi experimental method for optimizing the synthesis conditions The physical and structural characterization methods, catalyst preparation and characterization methods, electrode preparation, and electrochemical characterization methods are also described X-ray diffraction, Raman spectroscopy, X- ray photoelectron spectroscopy, SEM, and TEM are used to identify the structure and morphologies of as-prepared materials Galvanostatic charge-discharge, cyclic
7 voltammetry (CV), and electrochemical impedance spectra (EIS) are used to assess the electrochemical performance of LIBs
Chapter 4 describes a rapid microwave-assisted route for achieving the 1T/2H hybrid phase of MoS2 in a short period This study investigated the influence of different polyol solvents on the microwave synthesis of molybdenum disulfide (MoS2)
In Chapter 5, the microwave heating method is used to fabricate amorphous and crystalline MoS2/CNTs nanocomposites The Taguchi experimental design method is used to optimize the synthesis conditions The catalytic activity of nanostructured amorphous MoS2/CNTs is also discussed
Chapter 6 examines the effect of crystalline MoS2/CNTs nanocomposites as active anode materials on the electrochemical performance of lithium-ion batteries
The conclusion discusses the research work summary, prospects and implications
LITERATURE REVIEW
Structure and properties of carbon nanotubes (CNTs)
Carbon nanotubes are the result of recent scientific and technological advancements The materials in this group are all new allotropic forms of carbon, such as single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs), graphene, graphene oxide (GO), fullerene, carbon nanohorn, carbon nanocones, carbon nano- onions, (Figure 2.1) SWNTs, MWNTs, graphene, and graphene oxide, in particular, are carbon nanomaterials with a wide range of applications due to their superior physical, chemical, and mechanical properties (Table 2.1)
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) [2]
9 Table 2.1 Fundamental properties of carbon nanomaterials – Graphene, SWNTs, and
Properties Graphene SWNTs MWNTs References
Young modulus ∽ 1 TPa ~ 1 TPa ~ 0.3 – 1 TPa [5, 6]
Tensile strength ∽ 1100 GPa 50 – 500 GPa 10 – 60 GPa [5]
Electrical resistivity 5 – 50 μΩ cm 5 – 50 μΩcm 5 – 50 μΩcm [9]
Structures and properties molybdenum disulfide (MoS 2 )
Molybdenum disulfide (MoS2) has a crystal structure composed of weakly coupled layers of S–Mo–S atoms, with a Mo atom layer sandwiched between two S atom layers
It has a large direct band gap, which has been determined experimentally to be around 1.8 eV MoS2 crystals are composed of layers that are stacked vertically and are held
10 together by van der Waals interactions Layered structure of MoS2 is shown in figure 2.2, each layer typically has thickness of 6.2 Å, which consists of a hexagonally packed layer of metal atoms sandwiched between two layers of chalcogen atoms The lone-pair electrons of the chalcogen atoms terminate the surfaces of the layers, and the absence of dangling bonds renders those layers stable against reactions with environmental species MoS2 usually exists in a thermodynamically stable, trigonal 2H phase with semiconducting and poorly conductive characteristics Thus, hybrid electrodes of semiconductor MoS2 and other high electrical conductivity materials such as graphene, carbon nanotubes, carbon nanofibers and polyanilines [1, 10-15] are used to improve the electrical conductivity and show moderate electrochemical performance in battery
Figure 2.2 Structure of hexagonal molybdenum disulfide (2H-MoS2)
Depending on the electron filling in the valence d-orbitals with different coordinates of
Mo and S atoms, MoS2 can be classified into four main phases, namely, the distorted
11 tetragonal phase (1T), hexagonal phases (1H and 2H), and rhombohedral (3R) (as shown in figure 2.3 [16]) For the 1T and 1H phases, there is one layer per repeat unit; for the 2H phase, there are two layers per repeat unit; and for the 3R phase, there are three layers per repeat unit For the 1H, 2H, and 3R phases, the arrangement of atomic layers (S-Mo-S) is the same (trigonal prismatic coordination), but the constitution of the repeat unit is different However, the arrangement of atomic layers (S-Mo-S) in 1T- phase MoS2 is much different from that in the 1H, 2H, and 3R phases In the 2H-phase MoS2, the repeat unit has hexagonal symmetry, whereas the repeat unit in the 3R phase has rhombohedral symmetry The 2H-MoS2 phase is the most stable among the three established polytypes of MoS2 and it is also well known that the intercalation of alkali metals such as Li, Na, or K into 2H-MoS2 cause a phase transformation to the distorted 1T-MoS2 which exhibits a metallic nature [17] The metastable form of metallic 1T has also recently received attention because of its excellent properties in energy storage applications [18-29] 1T-MoS2 has different structural features that facilitate both electron transport and ion diffusion for electrochemical energy storage technologies The distorted octahedral coordination and better hydrophilicity of 1T-MoS2 are beneficial to obtain higher electron transport efficiency and better ion adsorption kinetic for electrochemical energy storage technologies [23, 29-31] In comparison to semiconducting 2H-MoS2, which has active sites only on the edges, metallic 1T MoS2 has a greater number of electrochemically active sites (both on the edges and the basal surfaces), a larger interlayer distance, a high electronic conductivity, and chemical activity, but has stability problems When partial transformation from the 2H to the 1T phase occurs in the 1T/2H-MoS2 mixed-phase heterostructure, the kinetic barrier decreases, and more electron transfer occurs, resulting in an increased number of active sites The metastable 1T phase is stabilized in this case by the interaction with the 2H phase, significantly increasing its catalytic activity
The hybrid nanostructures of 1T and 2H phases exhibit fantastic electrochemical properties, including large specific capacitance, super high-rate capability, and excellent long-cycle durability, due to the intrinsic higher activity and better electrical conductivity of the 1T phase and greater ability of the swollen lamellar structures to insert electrolyte ions, making it an impressive electrode material for supercapacitor
12 devices [24, 32] As previously reported, the presence of 1T phase plays a crucial rule for the catalytic activity [30, 33].
MoS 2 and their composite with carbon nanomaterials as electrocatalyst for HER
Layered molybdenum disulfide has also demonstrated great promise as a low-cost alternative to platinum-based catalysts for hydrogen evolution reaction (HER) due to its large surface area, near-zero ΔGads, and numerous structure engineering possibilities [34] Hydrogen is considered as a major energy carrier for the future carbon-neutral energy production [35, 36] The most abundant source of hydrogen is water as it is available everywhere and carbon-free Hydrogen is produced from water (water splitting) via two half-cell reactions which results in both hydrogen and oxygen The HER (2H + + 2e → H2) requires catalysts to minimize the overvoltage necessary to drive it and to achieve high energetic efficiency Precious metals such as platinum, ruthenium or iridium are best – known catalysts for HER but the high cost and scarcity prevent them from large scale application Therefore, development of new catalysts which are inexpensive, high active, scalable and stable is still remained of interests A class of MoS2 nanostructure with edge-terminated and interlayer-expanded features that was 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 [8]
13 synthesized through a microwave heating strategy performing among the best of current molybdenum disulfide catalyst [37, 38] Beside the edge-terminated structure that allows more active edge sites, the expanded interlayer distance can further optimize its electronic structure and therefore permits the performance gains [37] It was also suggested that metallic 1T polymorph of MoS2 materials is catalytically active [30, 39] Amorphous MoSx are highly active hydrogen evolution catalyst when compared to crystalline forms of MoS2 Their amorphous nature may give them such more unsaturated sites than the active sites on the edges of single crystals which have coordinately unsaturated Mo and/or S atoms [40-43]
Generally, the catalytic performance of the catalysts towards HER activity based on these three principal steps can be evaluated by some standardized indices, which include overpotential (η), Tafel slope (b) and exchange current density (j0), turnover frequency (TOF), stability, the Gibbs free energy, faradic efficiency (FE) and electrochemically active surface area (ECSA) [44-46] Among all these indices, overpotential, Tafel slope and durability are the most widely used indices for evaluating the performance of HER electrocatalysts [46, 47]
The catalytic activities of MoS2 are highly influenced by the impact of crystal properties such as crystallinity, crystal size, and morphology [48] The ideal electrocatalyst exhibits attributes such as a high surface area, a high density of active sites, high electrical conductivity, high chemical stability, excellent adsorption properties, and a simple (scalable) method of synthesis MoS2 of small size has more surface and edge sites compared to MoS2 of large size, expecting higher catalytic activity than MoS2 of large size [49]
The HER mechanism (Figure 2.5) has been extensively studied, and two widely accepted mechanisms have been identified [50-53] Which mechanism is dominant or controlling depends on the electrode material, particularly its hydrogen atom adsorption strength And, based on these two mechanisms, the following three reaction steps can be discussed in detail The first step is the Volmer reaction, which is the initial electron transfer process in which protons receive electrons to produce adsorbed hydrogen atoms (Hads) on the catalyst's surface The second is the Tafel reaction, in which Hads combine
14 with electrolyte protons to obtain electrons while completing the electrochemical absorption process The Heyrovsky reaction, a chemical desorption process that results in the formation of H2 molecules from the combination of two Hads atoms, is the final step The reaction equations for these three basic reactions differ in acidic and alkaline media, as shown in Table 2.2
Table 2.2 Fundamental principles of HER electrocatalysts in both acidic and alkaline media [54]
Steps Acidic media Alkaline media
Volmer reaction H + (aq) + e - → Hads H2O + e - → Hads + OH-
Tafel reaction Hads + Hads → H 2 (g) Hads + Hads → H 2 (g)
Heyrovsky reaction H + (aq) + e - + Hads → H2 H2O + e - + Hads → H2 + OH -
In general, one of the three steps outlined above determines the rate of HER [55] If hydrogen adsorption is the rate limiting step in HER, an electrode material with more edges and cavities in its surface structure provides more adsorption sites and thus improves electron transfer In some cases, the desorption and diffusion of molecular hydrogen may also be the rate limiting step Rough or perforated surface structures of electrode materials either increase electron transfer by increasing reaction area or prevent bubbles from growing [56] The rate-determining step of HER changes in different potential ranges At low potentials, hydrogen adsorption will determine the rate, but electron transfer will be slower than desorption When the potential is high, the hydrogen adsorption rate can be higher than the desorption rate, making the desorption reaction the rate-determining step In order to improve the catalytic performance of MoS2, the majority of research focuses on increasing the edge sites and S vacancies In fact, the catalytic performance will be greatly boosted if a large number of inert MoS2 basal planes are exploited properly [57]
15 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 [58]
Theoretically, the rational design of defects on the basal plane of MoS2 can improve the HER performance of MoS2 because the basal plane of MoS2 is inactive, whereas the edge sites are significantly more active [59] The active centers of MoS2 are primarily located at edge sites and S vacancies, and its basal plane is believed to be chemically inert In 2005, Hinnemann et al demonstrated that the edges of MoS2 are active sites using DFT calculations [60] On account of several literature studies, it has been deduced that similar to 2H and 1T MoS2, the mixed-phase 1T/2H MoS2 can also be regarded as both a photocatalytic and electrocatalytic hydrogen evolution catalyst In 1T/2H-MoS2, the octahedral metallic 1T phase MoS2 is 10 7 times more conductive and provides a greater number of rich active sites for H2 formation compared to that of the semiconducting 2H-phase
The conductivity of the MoS2 is also an important factor in their HER efficiency This is due to the chemical nature of the MoS2 when it exists in different polymorphs [61], with 2H and 1T phases being the most commonly encountered, as well as electrical transport between the active site and the electrode Lukowski et al demonstrated that converting semiconducting 2H-phase MoS2 nanostructures grown directly on graphite
16 to the metallic 1T phase via exfoliation with lithium intercalation improved HER performance [39]
The development of a high-performance molybdenum sulfide HER catalyst is currently limited by a lack of understanding of the catalytic reaction Three main material parameters indicate catalytic performance: exchange current density (turnover frequency), Tafel slope, and stability Synergistic optimization is required to achieve a high exchange current density, a low Tafel slope, and high stability in an ideal catalyst However, understanding the relationship between each of these parameters and the physical properties of molybdenum sulfide materials remains elusive For example, it has previously been demonstrated that the edge sites of molybdenum disulfide (MoS2) are catalytically active and that the exchange current density is linearly proportional to the number of edge sites [62, 63], but recent research has suggested that the electrical conductivity of the materials also plays a role in the exchange current density [18, 64, 65]
Electrical catalytic performance of MoS2 and their composites is currently limited by active site density and reactivity, inefficient electrical transport, and inefficient electrical contact with the catalyst Some supports with high electrical conductivity have been investigated to improve electrical contact with the active sites of MoS2 Carbon materials, in general, are frequently used as electrocatalyst support in HER and other electrochemical applications These materials include carbon nanotubes, macroporous– mesoporous carbon materials, graphene, and reduced graphene oxide sheets Multiwall carbon nanotubes (MWNTs) are one-dimensional materials with great chemical stability, strong electrical conductivity, and a huge surface area (they are also 20 times cheaper than graphene) Due to these benefits, MWNTs are among the most promising supports for nanoscale catalysts [66, 67]
The metastable form of metallic 1T has also recently received attention because of its excellent properties in energy storage applications [18-30] 1T-MoS2 has different structural features that facilitate both electron transport and ion diffusion for electrochemical energy storage technologies The distorted octahedral coordination and better hydrophilicity of 1T-MoS2 are beneficial to obtain higher electron transport
17 efficiency and better ion adsorption kinetic for electrochemical energy storage technologies [23, 29-31] In comparison to semiconducting 2H-MoS2, which has active sites only on the edges, metallic 1T-MoS2 has a greater number of electrochemically active sites (both on the edges and the basal surfaces), a larger interlayer distance, a high electronic conductivity, and chemical activity, but has stability problems When partial transformation from the 2H to the 1T phase occurs in the 1T/2H-MoS2 mixed- phase heterostructure, the kinetic barrier decreases, and more electron transfer occurs, resulting in an increased number of active sites The metastable 1T phase is stabilized in this case by the interaction with the 2H phase, significantly increasing its catalytic activity [18] The HER performance of mixed-phase 1T/2H-MoS2 and other types of MoS2 materials, as well as their composites with carbon nanomaterials, is summarized in Table 2.2
Table 2.3 Comprehensive table of electrochemistry of MoS2 and their composite with carbon nanomaterials including materials, properties, synthetic method, and key electrochemical data
Porous MoS2 one-pot and versatile calcination 30 mV b ; 130 mV c [68] exposed edge sites of MoS2
Microwave assisted hydrothermal 104 mV c ; 0.2 mA/cm 2 d [62] Edge-terminated
49mV/dec a ;-103mV b ; 9.62x10 -3 mA.cm -2 d [37]
1T/2H-MoS2 nanosheet one-step hydrothermal reaction
1T/2H MoS2 hydrothermal 61 mV/dec a ; 220 mV c [64] 1T/2H MoS2 hydrothermal 88 mV/dec a , 180 mV c [71] 1T-MoS2 phase liquid-phase sonication exfoliation of bulk MoS2 solvothermally synthesized 1T-MoS2
18 onto 2H-phase liquid- exfoliated MoS2
1T-MoS2 hydrothermal 41 mV/dec a ; -175 mV c [30] 1T-MoS2 nanosheet solvent free intercalation method 40 mV/dec a [20]
MoSx/P-CNTs urea-assisted synthesis via a facile hydrothermal process
MoS2/CNFs hydrothermal 60 mV/dec a [69]
MoS2/MWCNTs hydrothermal 43 mV/dec a ; 50 mV b [67] MoS2/N-doped
MoS2/SWCNT composites Liquid-phase synthesis 40.82 mV/decade a [74] a HER Tafel slope (mV/dec) b HER onset potential at -0.1 mA/cm 2 ~ Vonset c HER overpotential at -10 mA/cm 2 (mV) d Exchange current density n e number of transferred electrons
MoS 2 and their composite with carbon nanomaterials for lithium-ion batteries
MoS2 layered structure has emerged as promising electrode material in lithium-ion batteries (LIBs) [81-84] The weak van der Waals interaction between MoS2 layers
20 allows the diffusion of Li + ion without the significant volumetric changes in the lithiation and delithiation processes However, the poor electrical conductivity, low cycling stability and high agglomerate risk remain the major drawbacks of MoS2 To overcome these problems, 1D carbon nanotubes (CNTs) were introduced due to their superior electrical conductivity, good mechanical property and high structural stability [85, 86] In addition, CNTs in the role of ideal nanometer sized templates could reduce the rates of MoS2 aggregation The hybrid of MoS2/CNTs could effectively combine the merits of the good electrical conductivity of CNTs and excellent electrochemical performance of individual MoS2 layer throughout cycling [87-89] The structure where 1D carbon nanotubes as the back bones, while the 2D MoS2 layers grown on the surface of CNTs, which provide a large surface area of the active material to accommodate Li + Furthermore, an increased layer distance of S-Mo-S can be expected, which results in less strain and smaller intercalation barrier of Li ions [1, 86, 88, 90, 91]
As is commonly known, MoS2 has a low electronic conductivity during the electrochemical reaction [92, 93] Additionally, volume expansion occurs when lithium is intercalated into the MoS2 layer Numerous publications have been published on the subject of modifying the electrochemical performance of the MoS2 anode [86, 94, 95]
Shuhua Li et al prepared N-doped MoS2 nano-flowers as a lithium-ion battery anode material [92] As a result, comparing with the pristine MoS2 anode electrodes, the as- prepared N-doped MoS2 anode electrodes exhibit specific capacity value of 786 mAh g -1 after 100 cycles at the high current density of 0.5 C, demonstrating excellent cycle stability
The conductivity of 1T-MoS2 was found to be 10 7 times that of 2H-MoS2 due to the highly exposed basal and edge planes, which provide more active sites, but the difficult synthetic procedure and instability limited its wide range of applications, including the performance of electrochemical storage devices [96] In this regard, the highly promising hybrid 1T/2H mixed phase MoS2 prepared via phase engineering is regarded as a promising energy storage device, as both the semiconducting and metallic phases contribute to the device's electrochemical properties for energy storage MoS2 1T/2H is
21 a versatile material that can be used in both supercapacitors and batteries, depending on the reaction conditions [56]
Wang and co-workers [24] developed swollen ammoniated MoS2 with 1T/2H mixed- phase MoS2, in which the NH 4+ ions increased the lamellar structure from 0.615 to 0.99 nm, thereby increasing the concentration of the 1T phase The as-fabricated ammoniated 1T/2H-MoS2 electrode material exhibits superior charge storage performance when used in conjunction with Li-ion batteries due to the presence of a distinct surface redox reaction
Nguyen Quoc Hai et al have demonstrated that the mixing of CNTs with MoS2 through the high – energy mechanical milling (HEMM) process not only enhances the electric conductivity, but also prevents the volume expansion based on various characterizations and electrochemical performances.In addition, upon studying the effects of the weight ratio between MoS2 and the CNTs at (1:2), (1:1), and (2:1), MoS2/CNT-(1:2) was found to be the highest specific capacity (∼765 mAh g -1 after 70 cycles) and the best rate capability due to increased conductivity [85] In 2015, S K Srivastava and B Kartick have been prepared and characterized MoS2–MWCNT hybrids with MoS2 : MWCNT (1 : 1) in weight ratio The hybrid in Li-ion batteries exhibits exceptional reversible capacity and cycling stability (1214 and 1030 mAhg -1 after the 1st and 60th cycles) and enhanced rate performance over pristine MoS2, MWCNTs and other hybrids Such enhanced performance has been attributed to the synergistic effects of MoS2 and MWCNTs [87] Similarly, Yumeng Shi et al., have also demonstrated thatthe unique hierarchicalnanostructures with MWNTs backbone and nanosheets of MoSx have significantly promoted the electrodemperformance in LIBs Every single MoSx nanosheet interconnect to MWNTs centers with maximized exposed electrochemical active sites, which significantly enhance ion diffusion efficiency and accommodate volume expansion during the electrochemical reaction A remarkably high specific capacity (i.e >1000 mAh/g) was achieved at the current density of 50 mAg -1 , which is much higher than theoretical numbers for either MWNTs or MoS2 along (~372 and
22 Recently, JinglongWang et al., have prepared N-doped carbon-carbon nanotube as the basic structure for MoS2 growth, and a 1T/2H–MoS2 heterostructure is fabricated on this structure The double-phase 1T/2H–MoS2 with high content (60%) of the metallic 1T-MoS2 gives a larger layer spacing, higher internal electron conductivity, and more active lithiation and de-lithiation reactions To serve as anode for lithium-ion batteries, the synthesized 1T/2H–MoS2@NC–CNT has a reversible capacity of 764.99 mAh g −1 at 200 mA g −1 and a high-rate capacity of 547.7 mAh g −1 at 1000 mA g −1 even after
Microwave synthesis of nanomolybdenum disulfide (MoS 2 ) and MoS 2 /CNTs
Microwave heating can affect 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 and this topic will be an important future research trend This review focuses on the microwave synthesis of nano MoS2 and MoS2/CNTs composites
Table 2.4 A summary comparison of the syntheis method for nano MoS2 and
Reaction period Solvent Size distribution
Simple, high pressure 220 Hours, ca Days Water-ethanol Relatively narrow
180 minutes Organic/polyol Relatively narrow
23 Table 2.4 compares the benefits and drawbacks of several nano MoS2 and nano MoS2/CNTs manufacturing techniques Our interest has been on materials prepared by chemical routes other than the vapor transport, using microwave heating other than conventional heating These alternative methods include the thermal decomposition of the corresponding salts and the low temperature precipitation from solutions Materials obtained by these routes have an amorphous or poorly crystallized structure, depending on the temperature of preparation Interestingly, these highly disordered materials are found to have unique properties not present in their corresponding crystalline phases [107]
Microwaves have been shown to efficiently heat chemical reactions Utilizing microwave reactors has two major effects: The first is the thermal effect, which is associated with dielectric heating and results from molecular dipoles trying to adjust to the changing electric field of microwave radiation; the second is the nonthermal microwave effect, which is associated with the dipole-dipole-like interaction between the charges of the electric field and molecules with the dipole moment
The crucial law to remember here is the definition of isobaric heat capacity, 𝑑𝑄 𝑚 𝐶𝑝 𝑑𝑇, which states that there is no difference between conventional and MW heating because the heat transferred depends solely on the sample's mass, Cp, and the temperature differential
This technique is commonly applied to the targeted heating of active regions in supported metal catalysts [108] It also enables the effective synthesis of metal chalcogenide complexes, which are notoriously difficult to synthesize due to the volatility of chalcogen starting materials Using MWs, the metal components of the reaction mixture are heated preferentially and react rapidly with the chalcogen before the chalcogen has a time to volatilize [109] Rapidly increasing the temperature with microwaves minimized atomic diffusion and aggregation to ensure the uniform distribution of individual metal atoms throughout the crystal lattice [110]
Solvents play important roles in microwave-assisted synthesis in liquid phase Therefore, the solvent is a crucial factor for the microwave-assisted formation inorganic
24 nanostructures One of the most important properties of a solvent is its polarity The more polar a solvent is, the higher its ability to couple with the microwave energy, leading to a rapid increase in temperature and fast reaction rate The heating rate and efficiency of the microwaves strongly depend on the properties of the reaction system The use of excellent microwave absorbing solvents results in high heating rates The solvents used in microwave heating can be classified on the basis of their loss tangent (tan δ): high (tan δ > 0.5), medium (tan δ ≈ 0.1 – 0.5), and low (tan δ < 0.1) [111]
Table 2.5 Loss tangent (tan δ) values at 2.45 GHz and 20 o C and boiling points of different solvents [111, 112]
1,2-dichloroethane 84 0.127 water 100 0.123 chlorobenzene 131 0.101 acetone 56-57 0.054 tetrahydrofuran 66 0.047 dichloromethane 39.8 0.042 toluene 111 0.040 hexane 68-69 0.020
Table 2.5 shows loss tangent (tan δ) values at 2.45 GHz and 20 o C and boiling points of some typical solvents Among the solvents commonly used for the microwave-assisted preparation of inorganic nanostructures, water (tan δ = 0.123) and alcohols are good solvents for microwave heating Ethylene glycol (tan δ = 1.350) has high boiling point
25 (~ 198 o C) and reductive ability, allowing relatively high temperatures for the preparation of inorganic nanostructures in an open reaction system
Beside the single-solvent microwave-assisted synthesis, mixed solvents are also frequently used in the microwave-assisted formation of nanostructures Different kinds of solvents can be selected and volume ratios of solvents can be varied in the mixed solvent reaction systems; that is, more experimental parameters can be adjusted to control the chemical composition, structure, size, and morphology of the product Nanostructured chalcogenides have also been prepared by the microwave-assisted method in mixed solvents of water and polyols [113]
Liu et al synthesized 1T@2H-MoS2 nanospheres using a microwave-assisted hydrothermal approach using ammonium molybdate, thiourea, and deionized water as the solvent; they also utilized a simple hydrothermal method with a certain Mo and S precursor proportions Both hydrothermal reactions occur at 220 °C, although with distinct reaction durations The microwave-assisted procedure took 10 minutes for the first step and 4 minutes for the second, compared to 24 and 10 hours for the hydrothermal process [114]
Utilizing microwaves to conduct inorganic synthesis enables the direct transfer of electromagnetic energy within the reaction mixture, regardless of its temperature The conversion of microwave (MW) radiation into heat is useful for overcoming activation energy limits associated with chemical processes, but the usage of microwaves can be expanded to higher temperatures, thereby generating very high-energy environments [115] The particle size of MoS2 materials prepared under microwave heating is usually smaller than those prepared under conventional heating, which suggests that the nucleation is fast and homogeneous under microwave heating conditions Kim et al [44] reported that the additive of ethylene glycol (EG) could improve the crystallinity and morphology of MoS2 due to the higher dissipation factor of EG than that of water leading to fast energy transformation from microwave energy to thermal energy
Danyun Xu et al developed a simple, fast, and efficient microwave strategy for converting MoS2 from 1T to 2H phase and successfully preparing processable 2H-MoS2 nanosheets Traditionally, the phase change was accomplished by annealing the
26 deposited MoS2 nanosheets or heating the corresponding solution mixture at high temperatures for hours They discovered, however, that microwave irradiation could achieve the phase change in minutes Because they are well dispersed in the solution, the obtained 2H-MoS2 nanosheets are easily processed [105] The microwave strategy described in this study should be easily adaptable to achieve the phase change between 1T and 2H in the synthesis of hybrid material 1T/2H – MoS2
MoS2/MoO2@CNT nanocomposites was also fabricated by an ultrafast microwave approach was reported by Yunrui Tian et al It was found that MoS2/MoO2 nanoparticles could grow uniformly on the surface of CNTs, and the CNTs can provide a transport path for ions in electrolytes and reduce energy loss, thus leading to enhanced electrochemical performance of the resultant nanocomposites [116].
Conclusion
This review outlines the studies in the same field of MoS2 and MoS2/CNTs nanostructures and also discusses how nanostructure 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
Many review articles have been published on the microwave-assisted synthesis of nanostructured materials The scaling up of microwave-assisted chemical reactions is very important for industrial scale production and applications of nanostructured materials In this study, microwave-assisted (MW) methods can be considered a promising green strategy for synthesizing nanostructured 1T/2H-MoS2 hybrid phases, amorphous MoS2/CNTs, and crystalline MoS2/CNTs nanocomposite, which can be used to confirm the green chemistry approaches currently in use The final quality of microwave-generated materials depends on the reactant choice, the applied power, the reaction time, and the temperature The research deals with the fundamentals of microwave chemistry, which are critical for comprehending the mechanisms and solution-phase phenomena that occur during microwave heating
It is still unknown how solvent properties affect the microwave synthesis of nano MoS2 and nano MoS2/CNTs This work will focus on a fundamental understanding of solvent influence, specifically polyol solvents like ethylene glycol and glycerol
Phase engineering is a promising technique for increasing the amount of MoS2 that can be activated In general, MoS2 crystallizes in two different crystalline phases: the hexagonal 2H phase and the octahedral 1T phase The chemical and physical properties of 1T-MoS2 are significantly superior to those of the natural semiconductor 2H-MoS2 However, 1T-MoS2 is metastable, and its synthesis continues to be a difficult task Although the hybrid 1T/2H-MoS2 has been synthesized, controlling the 1T/2H ratio is still a challenge that has not been thoroughly discussed An investigation into the synthesis methods of hybrid phase 1T/2H-MoS2 with controllable high 1T concentration is carried out in this study
Additionally, this study examines how structural design can solve these issues successfully in lithium-ion batteries and electrocatalysts
As a result of the abovementioned literature review, this study chooses the following research topics: “Nanostructured materials based on molybdenum disulfide (MoS2) and carbon nanotubes (CNTs) for lithium-ion batteries and hydrogen evolution electrocatalysts.”
METHODOLOGY
Overall research procedure
The overall experimental procedures for designing nanostructure materials are shown in Figure 3.1 This research work mainly consists of three steps: (1) the preparation of nanostructure materials; (2) the characterization of as-prepared materials; (3) testing of the electrochemical performances of the as-prepared nanostructure materials for lithium-ion batteries and catalytic activity for hydrogen evolution reaction
Figure 3.1 Framework of the overall procedures of the research
Chemicals/Materials
All the chemicals used to synthesize MoS2 and MoS2/CNTs nanomaterials were reagent grade, as shown in table 3.1
29 Table 3.1 Chemicals used in the synthesis of 1T/2H MoS2 and MoS2/CNTs nanocomposites Chemical Chemical formula Abbreviations Origin Purity/Concentration
(NH4)6Mo7O24.4H2O AHM VWR, Japan ≥ 98%
Thiourea (NH2)2CS TU VWR, Europe 99,8%
Ethylene glycol C2H4(OH)2 EG Xilong, China 99%
VNU-HCM Key Laboratory for Material Technologies (MTLab)
Ammonium heptamolybdate is an odorless crystalline compound ranging in color from white to yellow-green Usually, tetrahydrate, whose formula is (NH4)6Mo7O24ã4H2O It is known as ammonium paramolybdate or simply as ammonium molybdate, although ammonium molybdate can also refer to ammonium orthomolybdate (NH4)2MoO4
Multi-wall carbon nanotubes (MWCNTs) was synthesized via T-CVD method by MTLab [117] Purification of the raw materials is accomplished in two steps: heat treatment at 460 o C with air supply to remove carbonaceous impurities and expose metal or metal oxide catalysts; acid treatment in HNO3 and HCl at 60 o C to remove metallic catalysts and oxidize remaining carbonaceous impurities after heat treatment Finally, the purified MWCNTs are functionalized in an HNO3/H2SO4 solution to generate
30 carboxyl groups (–COOH) on the MWCNT surface (referred to as the f-CNTs) [118]
In Chapter 5, f-CNTs are used as a starting material to synthesize a MoS2/CNTs nanocomposite
The dosmetic microwave oven utilized for this thesis is a SHARP R-201VN-W model that operates at 2.45 GHz.
Synthesis of 1T/2H-MoS 2 hybrid phase
In this work, the ammonium heptamolybdate tetrahydrate (NH4)6Mo7O24.4H2O (AHM) and thiourea CSN2H4 (TU) were used as the starting materials under microwave heating to obtain the MoS2 mixing phases Four solvents were investigated in this study, including ethylene glycol (EG), ethylene glycol mixing with 4 mL of water, glycerol (G), ethylene glycol (EG) mixing with glycerol (G) in a volume ratio of 1:1 Figure 3.2 depicts the steps in the the self-construction microwave synthesis process The main reasons for choosing the parameters in this process will be explained in Chapter 4 (Section 4.1)
In a typical reaction, 1.24 g (0.001 mol) ammonium heptamolybdate tetrahydrate (NH4)6Mo7O24.4H2O and 2.28 g (0.03 mol) thiourea (CSN2H4) were immersed in 60 mL of various solvents and kept at 60 o C for 30 minutes on a hot plate This study investigated four solvents: ethylene glycol (EG), ethylene glycol mixed with 4 mL of water, glycerol (G), and ethylene glycol mixed with glycerol in a volume ratio of 1:1 Following that, the homogeneous solution was placed in a microwave oven The reaction mixture was rapidly heated to the boiling point of the solvent/mixing solvents for four solvents, and the microwave irradiation power was maintained at 240 W for 15 minutes After a significant amount of black precipitate formed, the reaction mixture was cooled to room temperature and diluted with 200 mL ethanol Centrifugation was used to collect the black precipitates, which were then filtered and washed several times with ethanol Finally, the powder was dried for 5 hours under vacuum at 80 o C The final samples of MoS2 nanostructures were labeled S1 – EG, S2 – (EG + H2O), S3 – (EG + G), and S4 – (G) and are summarized in Table 3.2 In Chapter 4, the synthesis
31 efficiency of 1T/2H-MoS2 hybrid materials, the structure, and properties, and the reaction mechanism will be thoroughly examined
Dissolving in various solvents (60mL)
Microwave heating at 240W for 15 mins
As-prepared MoS2 nano powders (NH4)6Mo7O24.
Figure 3.2 The flowchart for synthesis of MoS2 nano powders under microwave heating
32 Table 3.2 Samples mark with various solvents and the corresponding boiling points
The boiling point of solvent/mixing solvents (corresponding to the reaction temperature)
S2 - (EG + H 2 O) Ethylene glycol and 4ml
Synthesis of MoS 2 /CNTs nanocomposite
In this study, MoS2/CNTs nanocomposites were successfully synthesized from two different reaction mixtures using a microwave-assisted liquid phase chemical reaction Each reaction mixture consists of a dispersion of functionalized carbon nanotubes (f- CNTs) in an ethylene glycol solution containing the precursor ammonium molybdate tetrahydrate, thiourea (referred to as the "precursor solution") Two distinct methods are used to disperse functionalized carbon nanotubes in precursor solution: indirect two-pot dispersion (I2PD) and direct two-pot dispersion MoS2/CNTs with crystalline and amorphous MoS2/CNTs were successfully synthesized using I2PD and D2PD reaction mixtures The Taguchi experimental design was used to thoroughly investigate the conditions for MoS2/CNTs synthesis to develop a structurally controlled synthesis process for MoS2/CNTs materials
As is well known, the homogeneity of the reaction mixture is critical to the success of the MoS2/CNT synthesis The AHM and TU precursors were utterly soluble in EG, whereas f–CNTs were only dispersed in EG The medium and dispersion conditions
33 affect the dispersion efficiency of f–CNTs The f–CNTs in this content are dispersed in the two following ways (Figure 3.3):
• The f–CNTs powder was dispersed in distilled water to make the f–CNTs/H2O dispersion, which was then combined with the EG solution containing dissolved AHM and TU The processes are carried out in two pots, and the f–CNTs are dispersed indirectly through water before being added to the EG solvent, which is referred to as “indirectly dispersed f–CNTs through two pots” (”indirect two–pot dispersion (I2PD) of f–CNTs”) This process accurately weighed 2.47 g (2 mmol) and 4.56 g (60 mmol) of AHM and TU precursors The precursors were added directly to 120 mL of EG solvent and stirred continuously at 50°C for 1h until wholly dissolved to form a precursor solution (POT 1) In a separate pot (POT 2), 40 mg of f–CNTs were mixed with 4 mL of distilled water and sonicated for 30 minutes at 50°C to produce a f–CNTs/H2O solution (concentration 10 g/L) Then, 4 mL of f– CNTs/H2O solution was added to the solution in pot 1 and sonicated for 30 minutes at 50°C before being transferred to the microwave system for reaction
• The f–CNTs powder was dispersed in EG to form the f–CNTs/EG dispersion solution, which was then added to the EG solution that dissolved the AHM and TU, a process known as "direct two–pot dispersion (D2PD) of f–CNTs" (Figure 3.3) In the D2PD procedure, 40 mg of f–CNTs were mixed with 4 mL of EG and sonicated for 30 minutes at 50°C to create a f–CNTs/EG solution (concentration 10 g/L) Then
4 mL of f–CNTs/EG solution was added to the precursor solution until a total of 120 mL EG was reached The D2PD reaction mixture was sonicated for 30 minutes at 50°C before being transferred to the microwave system for reaction
34 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
3.4.1 Taguchi experimental method for investigating the factors affecting the synthesis of MoS 2 /CNTs
Conventional optimization studies that involve changing one parameter while keeping the others constant are frequently regarded as time-consuming and expensive The Taguchi method, however, is a simpler and equally effective method for optimizing multiple operational variables in statistical design of experimental methods Taguchi's optimization technique is a unique and efficient optimization method that permits optimization with a minimum number of experiments Taguchi experimental design reduces costs, improves quality, and provides trustworthy design solutions With the Taguchi method, multiple factors can be optimized simultaneously, and more quantitative data can be extracted from fewer experimental trials than with other methods The selection of the control factors is the most crucial stage in designing an experiment As a result, a number of factors are included in order to identify non- significant variables as quickly as possible Taguchi's experiment design is an effective tool for modeling and analyzing the impact of control factors on performance output This method has been applied to nanomaterials for a limited number of syntheses,
35 including carbon nanotubes [120], graphene/cotton nanocomposite [121] or other nanoparticles such as TiO2 [122]
The five general steps of Taguchi method are shown in Figure 3.4 and each step is detailed as follow:
Step 1 - Define the number of factors to be studied and the number of levels for each factor
In the present study, the numbers of factor to be studied are 5
Figure 3.4 Taguchi design of experiment – modeling the influence of control factors on performance output
For optimizing the synthesis of crystalline MoS2/CNTs via the I2PD process, the five factors are as follows:
For optimizing the synthesis of amorphous MoS2/CNTs via the D2PD process, the five factors are as follows:
• ∑m (AMH+TU):VEG ratio (g/mL) – P5
The number of levels considered for each factor is 4 The factors and levels are detailed in table 3.3 for I2PD process and in table 3.4 for D2PD process
Step 2 - Define the response value of experiment
In this work, the response values considered are the degree of crystallinity for crystalline MoS2/CNTs synthesis (from XRD results) and the Tafel slope value (from LSV results) for amorphous MoS2/CNTs synthesis The target value of this process is to maximize the output crystallinity and to minimize Tafel slope The deviation in the performance characteristic from the target value is used to define the loss function for the process Table 3.3 Five factors and 4 levels for each factor to be investigated in I2PD process
Table 3.4 Five factors and 4 levels for each factor to be investigated in D2PD process
Ultrasonication temperature (°C) f-CNTs amount (mg)
Step 3 – Select an appropriate orthogonal array (OA)
Create orthogonal arrays for the parameter design based on the number of factors and the number of levels for each factor Because this study has five factors with four levels each to consider, we determine that the appropriate orthogonal array is L'16 [123] A total 16 experiments were designed where different factors and their levels used in each experiment are shown in table 3.5
Table 3.5 The appropriate orthogonal array L’16
Conduct the experiments following the designed matrix experiment L16, and the experimental conditions are presented in Table 3.7 and Table 3.8 below After setting the experimental parameters for each experiment, the responses are characterized as crystalline and Tafel slope The responses obtained according to each experiment conducted are shown in Table 5.1 and Table 5.5 in Chapter 5
Step 5 – Analyze the data to identify the optimal level
The "Signal" of the desired effect is a quality characteristic of a product under investigation in response to a factor introduced in the experimental design The effect
38 of uncontrollable external factors on the outcome of the quality characteristic under test is referred to as "noise." The Signal-to-Noise ratio (S/N ratio) measures the sensitivity of the Quality characteristic under investigation to external influencing factors (Noise factors) that are not under control The S/N ratio is the loss function's transformed figure of merit In a single metric, the S/N ratio combines both parameters - the mean level of quality and the variation around this mean The goal of any experiment is to find the highest possible S/N ratio for the result (crystallinity and Tafel slope); a high S/N ratio indicates that the signal is much stronger than the random effects of noise factors
Normalization of response values are divided into three types according to expect nature of the response values
• The first normalization is the “Lower-the-better” values where the lowest values of the objective function are expected In the synthesis of amorphous MoS2/CNTs, the values for Tafel slope should decrease for the better catalytic activity The formula for “lower-the-better” normalization criteria considered is as follow
• Second is “nominal the better” where the objective function has average values
• The third one is the “Higher-the-better” where the highest values of the results are expected The crystallinity values in the synthesis of crystalline MoS2/CNTs should be increased for improved electrochemical performance in lithium batteries The formula for “higher-the-better” normalization criteria considered is as follow
) where 𝑦 𝑖 is the signal measured in each experiment (mean value)
The effect of a parameter level on the S/N ratio, i.e., the deviation it causes from the overall mean of signal, is obtained by analysis of mean (ANOM) The relative effect of process parameter can be obtained from analysis of variance (ANOVA) of S/N ratios Computation of ANOM and ANOVA are done by using following relations
Where 𝑚 𝑖 represents the contribution of each parameter level to S/N ration and 𝑁 𝑙 represents the number of times the experiment is conducted with the same factor level in the entire experimental region
For example, after calculating the SN ratio for each experiment in table 3.5, the average
SN value is calculated for each factor and level Once these SN ratio values are calculated for each factor and level, they are tabulated as shown below in Table 3.6 and the range Δ (Δ = high SN - low SN) of the SN for each parameter is calculated and entered into the table The larger the Δ value for a parameter, the more significant the effect the variable has on the process because the exact change in signal causes a larger effect on the output variable being measured
4 Other parameters 𝑆𝑁 𝑃 𝑖,𝑗 (𝑖 = 1, 4; 𝑗 = 1, 4) in table 3.6 are calculated in the same way as above
Table 3.6 The average SN value for each factor and level
1 SNP1,1 SNP2,1 SNP3,1 SNP4,1 SNP5,1
2 SNP1,2 SNP2,2 SNP3,2 SNP4,2 SNP5,2
3 SNP1,3 SNP2,3 SNP3,3 SNP4,3 SNP5,3
4 SNP1,4 SNP2,4 SNP3,4 SNP4,4 SNP5,4 Δ RP1 RP2 RP3 RP4 RP5
3.4.2 Synthesis of crystalline MoS 2 /CNTs from the I2PD procedure
Structural and Physical Characterization Method
At the nanoscale, materials exhibit different properties than at the macro level, often quite dramatically different The characterization of nanomaterials naturally requires imaging techniques with resolution at the same scale or better, so local property variations can be discerned, and defects properly detected; only with this understanding can the material properties be engineered to meet the performance requirements of next- generation devices
Characterization methods are essential for analyzing nanomaterials' various properties/aspects in order to investigate their novel physical properties, structure, and morphology This project will present some of the most advanced characterization techniques for MoS2 and MoS2/CNTs nanomaterials, including XRD, SEM, TEM, Raman, and XPS
Characterizations of nanomaterials are performed at different levels Some characterization methods are used to study nanostructures' sizes, shapes, and morphology, whereas others are used to obtain detailed structural information
In this doctoral work, X-ray diffraction (XRD) measurements are performed on a Bruker D8 ADVANCE diffractometer with Cu Kα radiation (𝜆 = 1.5406 Å) (Institute of Applied Materials Science – VAST) The diffraction was performed from 5 o to 80 o at a scan rate of 1° min -1 and step size of 0.0194 ° The sample for XRD measurements is a fine powder, forming a flat surface with a thickness of about 100 Å, and measured
43 at room temperature The method's mean error is 2-5%, which can be reduced to less than ±0.5% under ideal conditions The sample is rotated at a constant speed, and the probe rotates twice as fast to ensure that when the sample rotates by an angle θ, the probe rotates by an angle of 2θ
As shown in table 3.9, the characteristic peaks of 2H-MoS2 can be observed at the (002) (2θ = 14.4°), (100) (2θ = 32.7°), (101) (2θ = 33.5°), (103) (2θ = 39.6°), and (110) (2θ 58.3°) planes, respectively The absence of the (002) and (103) planes, together with a newly-emerged (004) plane at 17.8° refers to the 1T-MoS2 pattern
Table 3.9 XRD diffraction peaks of 1T and 2H phase of MoS2 (from literature review) Planes (002) (004) (100) (101) (103) (110) Ref
Shift to lower angle 17.8 Broaden Broaden -
Raman spectroscopy is a spectroscopic technique used to investigate a system's vibrational, rotational, and other low-frequency modes It is based on the inelastic scattering of monochromatic light, such as a laser or Raman scattering When the laser light interacts with molecular vibrations, phonons, or other system excitations, the energy of the laser photons is shifted up or down The energy shift provides information about the system's vibrational modes The simplified energy diagram depicted in Figure 3.5 demonstrates the concepts of a simplified energy diagram
The structure information for the as-prepared materials was obtained in this doctoral work using a Labram HR VIS Mico Raman spectrometer at room temperature with a He-Ne excitation wavelength of 632.8 nm on a 300 lines/mm grating The laser power was kept at 1 mW to avoid the oxidation effect on the MoS2 samples Raman
44 spectroscopy can quickly identify the generation of the 1T-phase and the 2H-MoS2 in mixed-phase MoS2
Figure 3.5 Principle of Raman scattering
The Raman spectrum signals shown in Table 3.10 are employed to distinguish between the two phases of MoS2 nanomaterials, 1T and 2H
Table 3.10 Raman modes of 1T and 2H forms of MoS2
Raman modes 2H – MoS2 (cm -1 ) 1T-MoS2 (cm -1 ) References
• J1 is attributed to Mo-Mo stretching vibration in 1T-MoS2
• E 1 2g is the in-plane vibrational mode
• A1g is the out-of-plane vibrational mode
Electron microscopy has been an essential enabling technology for advancing our understanding of the structure and behavior of materials: atomic-scale imaging of atoms and defects has led to substantial breakthroughs and is a mainstay of modern materials science With the ongoing development of new microscopy hardware and novel
45 imaging and analysis techniques, electron microscopy will continue to aid in pushing the boundaries of our knowledge of every type of material The scanning electron microscope (SEM) is an electron microscope that scans a sample with a high-energy electron beam in a raster scan pattern to image it Electrons interact with the atoms in the sample, producing signals that contain information about the surface topography, composition, and other properties such as electrical conductivity
The morphological features of as-prepared materials in this research were examined using a field emission scanning electron microscope (FE-SEM, JEOL-JSM-7401F) operated at an accelerating voltage of 15 kV
Transmission electron microscopy (TEM) is a type of microscopy that can be used to examine the morphology, crystal structure, and electronic structure of a variety of materials Because electrons have a small de Broglie wavelength, transmission electron microscopes can produce images with significantly higher resolution than light microscopes A beam of electrons is passed through an ultra-thin specimen, interacting with it as it passes The interaction of electrons transmitted through the specimen can produce an image, which is magnified and focused onto an imaging device (e.g., a fluorescent screen, layer of photographic film), or detected by a sensor such as a CCD camera TEM samples were prepared by drop casting nano MoS2 or nano MoS2/CNTs dispersions onto a carbon-coated copper grid, followed by drying for 24 hours at 50 o C under vacuum The morphology and structure of the prepared samples were analyzed by TEM (JEOL-TEM-1400) at the National Key laboratory of polymer and composite materials (PCKLABS) - Ho Chi Minh City University of Technology The equipment operated at an acceleration voltage of 200 kV in the bright field image mode
XPS is a surface analysis that involves irradiating a solid in a vacuum with monoenergetic soft x-rays and analyzing the emitted electrons by energy If the surface chemistry or thickness is critical to its function and safety, XPS analysis is a proven choice It provides a detailed breakdown of the elemental composition (at the parts per
46 thousand range), empirical formula, chemical state, and electronic state of the elements found on the material's surface It is considered a non-destructive method because it produces soft x-rays to induce photoelectron emission from the sample surface The spectrum is obtained to plot the number of detected electrons per energy interval versus their kinetic energy Each element has a unique spectrum The spectrum from a mixture of elements is approximately the sum of the peaks of the individual constituents Because the mean free path of electrons in solid is very small, the detected electrons originate from only the top few atomic layers, making XPS a unique surface-sensitive technique for chemical analysis Mg Kα (1253.6 eV) or Al Kα (1486.6 eV) sources of X-rays are usually used These photons have limited penetrating power in a solid on 1-
10 nano meters They interact with atoms in the surface region, causing electrons to be emitted by the photoelectric effect
The emitted electrons have measured kinetic energies given by:
𝐾𝐸 = ℎ𝑣 − 𝐵𝐸 − ∅ 𝑠 (3.1) Where 𝐾𝐸 is the kinetic energy (measured in the XPS spectrometer)
ℎ𝑣 is the energy of the photon from the X-ray source (controlled)
47 ∅ 𝑠 is the spectrometer work function It is a few eV, and it gets more complicated because materials in the instrument will affect it This parameter can be found by calibration
𝐵𝐸 is the binding energy of the atomic orbital from which the electron originates
The above equation (1) will calculate the energy needed to get an electron out from the surface of the solid The BE can be calculated if the 𝐾𝐸, ℎ𝑣 and ∅ 𝑠 are known
Important information on the surface electronic state and the composition of the final product can be provided by XPS The synthesized MoS2 nanopowders were analyzed through an XPS ESCALAB250 system with Al (Kα) radiation as the probe under vacuum A survey scan was performed over a wide binding energy range of 0-1000 eV for a detailed analysis of Mo and S elements, focusing on the appropriate and specific energy windows The energies of C 1s peak at 284.6 eV to calibrate the samples' binding energies Meanwhile, the values of full width at half maximum (FWHM) were kept equal between the spin-orbit splitting doublet
The oxidation state of a specific metal in any material surface can be determined using X-ray photoelectron spectroscopy (XPS) In this case, each oxidation state of molybdenum sulfide nanomaterials gives rise to a distinct Mo 3d and S 2p spectral component Generally, the electrons of the 3d orbitals possess stronger peak intensities than electrons of other orbitals for Mo species So, the high-resolution spectra of the selected energy range appropriate for Mo 3d orbitals, before and after curve peak fitting in all XPS results reviewed in table 3.6 below
Table 3.11 XPS binding energies (eV) of the different phases and bonding in molybdenum compounds
Many studies have shown that the high-resolution XPS Mo 3d spectrum of Mo +4 in MoS2 after fitting exhibits two main peaks at about 229.1 and 232.3 eV [142-144], which correspond to the Mo 3d5/2 and Mo 3d3/2 orbitals Another peak at about 226.1 eV often appeared in the Mo 3d spectrum can be attributed to the S 2s, indicating the presence of Mo-S bond in MoS2 The binding energies at 162.0 and 163.3 eV indicate the S 2p3/2 and 2p1/2 orbits, respectively [15, 144] The binding energies summarized in table 3.1 also suggest that the emergence of 1T phase MoS2 is often accompanied by debasing binding energy compared to 2H phase MoS2 [18, 135, 136, 139]Peaks of S 2p3/2 and S 2p1/2 around 162.2 and 163.4 eV in 1T phase moved to a lower energy direction than 2H-MoS2 [11, 12, 17, 18] Similarly, Mo 3d peaks of 1T-MoS2 at around 231.1 and 228.1 eV [20, 25, 135-137] corresponding to Mo 3d3/2 and 3d5/2 shifted to lower energy than those of 2H-MoS2 [18, 24, 139, 140].
Electrochemical measurements for catalysts
Electrochemical analysis using Tafel equations is widely used to evaluate and characterize electrocatalysts in the hydrogen evolution reaction The Tafel slope and exchange current density are the only two parameters that are invariably determined and discussed in the literature concerning the Tafel equation The Tafel constant is proposed to be regarded as the HER onset potential (Vonset) When other parameters like Tafel slope or exchange current density (j) become equal, the Tafel constant becomes
49 the defining parameter between two electrocatalysts Tafel constant becomes complementary to Tafel slope from an electrochemical standpoint
For preparation of working electrode, 10 mg of as–prepared materials (catalysts) from Table 3.12 was combined with 2.5 mg polyvinylidene fluoride (PVDF) and ultrasonically dispersed for 60 min at 50 o C in the solutions containing 1 àL polyvinyl alcohol (PVA) 0.1 wt% and 1 mL EG Subsequently, 50 μL of the homogeneous suspension were transferred into the 3 mm-diameter glassy carbon electrode GCE)
Table 3.12 Catalytic electrode sample markers
Catalysts/As-prepared materials Binder Electrode area
Crystalline MoS2/CNTs synthesized by optimized conditions
16 samples of MoS2/CNTs synthesized from Taguchi table
The following electrolyte solution was prepared for the Tafel plot analysis:
• Dilute H2SO4 98% solution with distilled water to a concentration of 0.5 mol/L
• Add 0.2 g sodium dodecyl sulfate (SDS) to 100 mL of 0.5 mol/L H2SO4 solution and stir thoroughly before inserting the working electrode
• Throughout the analysis, the solution temperature was maintained at 25 ± 1°C
A three-electrode system was adopted to perform electrochemical tests in 0.5 mol/L
H2SO4 (pH = 0.3) solutions at room temperature on the PARSTAT 2273 (AMETEK) electrochemical instrument (Figure 3.7) A glassy carbon electrode (GCE) loaded
50 catalysts (3 mm in diameter), Pt (99.99%) (10 x 10 mm), and a saturated Cu/Cu 2+ in CuSO4 were used respectively as the working, counter, and reference electrodes The value saturated Cu/Cu 2+ in CuSO4 is 0.34 V vs NHE All final potentials were calibrated to NHE according to the Nernst equation ENHE = ECu/CuSO4 + 0.34 V + 0.059 × pH The linear sweep voltammetry (LSV) was measured from 0 to 1 V (vs NHE) at a sweep rate of 1 mVãs -1
Figure 3.7 Three-electrode configuration for electrochemical tests
Several voltammetric cycles were recorded with a scan rate of 1 mV s -1 before recording the polarization curve All the recorded current densities were normalized by dividing the current response by the geometric area of the electrode To minimize the electrical double layer charging current, linear sweep voltammetry (LSV) scans were recorded at a scan rate of 1 mVs -1 The uncompensated resistance was eliminated by iR correction Tafel slope values were obtained from the Tafel equation below and is related to the increase in the HER rate of MoS2 with enhanced overpotential
51 where η is the overpotential, 𝑗 is the current density, and 𝑏 is the Tafel slope For a hydrogen evolution reaction, the overpotential (η) = 0 - ENHE
Tafel plots illustrate the logarithmic relationship between electrochemical current density (j) and various overpotentials, as indicated by the Tafel equation (η = a + b log j) This equation can be used to derive two significant parameters, including the Tafel slope (b) and the exchange current density (j0) at a zero value of η The Tafel slope represents the intrinsic catalytic activity of the calatyst used An efficient catalytic material must generally have a high j0 and a small Tafel slope
Exchange current density is the rate of reaction at the reversible potential At the reversible potential, the reaction is in equilibrium, meaning that the forward and reverse reactions progress at the same rates We have used the linear part of the polarization curve at small over-potentials in H2-saturated 0.5 M H2SO4 solution to obtain j0 values The exchange current density can be calculated by equation below
Here 𝑛 represents the number of electrons exchanged, F (96485 C mol -1 ) is the Faraday constant, and R (8.314 J mol -1 K -1 ) is the gas constant The exchange current densities of were obtained from the polarization curves.
Electrochemical characterizations for lithium-ion batteries (LIBs)
The following materials were chosen to make anode paste in order to evaluate and compare the electrochemical properties of various synthetic materials as anode electrode materials in lithium batteries
• Crystalline MoS2/CNTs synthesized under optimal conditions using the I2PD process (MSC–I2PD–n) in section 3.3.2
• Amorphous MoS2/CNTs synthesized under optimal conditions using the D2PD process (MSC–D2PD–n) in section 3.3.3
• 1T/2H-MoS2 nano powder synthesized in an EG+H2O mixture solvent (S2- EG+H2O) with a high concentration of 2H phase (section 3.2)
• MWNTs powder is synthesized by MTLab with a carbon content of 95% and a fiber length of 10-20 nm
The completed anodes for lithium-ion battery testing were established in three stages below (Figure 3.8)
(1) Preparation of electrode materials (anode paste)
Mixing 100 mg the as-prepared materials with 10 mg of MWNTs and 100 mL of 10% PVA (mPVA ≈ 10 mg) in 100 mL EG The mixture was sonicated for 40 mins to form a uniformly dispersed solution The solution was dried at 80 °C until it reached a mass of about 1 g and formed a homogeneous slurry (anode paste) In the next stage, the slurry was uniformly pasted onto Cu foil (anode current collector) by a doctor blade in the next stages and dried in an oven at 80 - 100 °C overnight under vacuum The anode paste samples were completely dried before the morphology and structure were evaluated by XRD and SEM
(2) Anode current collector treatment (Cu foil)
Copper foil was cut into a circular sheet with a 10 mm diameter The copper foil surface was softly polished with 3 àm grain sandpaper to remove the copper oxide layer and washed with absolute alcohol to eliminate rust and oil The anode current collector was then dried under vacuum
(3) Finishing anode electrode for testing
Weigh the current collector before coating the anode paste Weigh the sample after coating it with copper foil The electrode was hot rolled by a two-axis rolling mill (the distance between the two axes is fixed at 200 àm) to create a uniform coating layer thickness (the anode material layer thickness is about 100 àm), with the temperature maintained at 120°C during rolling The anode sample was rolled until the mass was constant, the flash (residual anode paste) was removed, and the mass of the anode
53 attached to the electrode was determined XRD, SEM, and TEM are used to evaluate the anode paste on the electrode surface
The electrode is a complex composite material that is adhered to the current collector (copper foil for the anode) It is made up of active materials, conductors, and a binder Since the electronic conductivity of most active materials is relatively low, adding materials conductive (e.g MWNTs) increases the electrode's electronic conductivity Typically, the binder is a long-chain polymer Polyvinyl alcohol (PVA) capable of holding all of the electrode materials on the current collector together Table 3.13 denoted the electrode samples whose electrochemical properties will be investigated in Chapter 6
Table 3.13 Anode paste and anode electrode samples identification symbols
Mass of anode (mg) Anode Active anode material
LA–c-MSC MSC–I2PD-opt 37.1
LA–a-MSC MSC–D2PD-opt 28.2
16 samples of MoS2/CNTs synthesized from Taguchi table 3.7 MSC–I2PD–n (n = 1; 2;… 16)
35.6 ± 3.8 (The values are the average of 16 samples)
54 Figure 3.8 Anode preparation for electrochemical performance testing
Completed anode Flash removing, weigh again
55 The electrochemical measurements for evaluating the electrochemical performance of lithium-ion batteries included galvanostatic charge-discharge testing, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS)
CV is a potentiodynamic electrochemical measurement technique that has been extensively used to characterize the electrochemical performance of lithium-ion batteries The working electrode potential is ramped linearly versus time in a cyclic voltammetry experiment This ramping is referred to as the scan rate of the experiment (V/s) The potential is applied between a reference electrode and a working electrode in three-electrode systems, and the current is measured between a working electrode and a counter electrode CV measurements were made in this dissertation using two- electrode systems, with a lithium anode serving as both the counter and reference electrode This collected data was plotted as current 𝑖 versus potential 𝐸 The current peaks appear as the potential reaches the reduction or oxidation potential of the analyte
As a result, the electrode materials' redox potential and electrochemical reaction rates can be determined
3.7.3 Cell assembly for cyclic voltammetry (CV) testing
The electrochemical measurements were carried out on a Swagelok cell using the PARSTAT 2273 (AMETEK) instrument (Figure 3.9) Swagelok cell made of Teflon (polytetrafluoroethylene – PTFE) with an outer diameter of 25.4 mm (1 inch) and an inner diameter of 10 mm with stainless steel SS316 caps (electrodes) A schematic diagram of the configuration of the Swagelok type cell is shown in Figure 3.10 The cells contained the as-prepared electrode as the working electrode, lithium foil as the counter and Li/Li + as the reference electrode, cellulose acetate (OE67 Whatman) membrane with the pore size 0.45 àm as the separator, and 1M LiPF6 in dimethyl carbonate (DMC) liquid as the electrolyte The following steps were taken to prepare the working electrode:
• Cellulose acetate membrane was soaked with a few drops of 1 M LiPF6 in EC solution, then sandwiched between the composite working electrode and counter electrode
• The entire assembly was secured in a cylinder of Swagelok cell
All electrode preparation and cyclic voltammetry were performed in an argon-filled glovebox with the relative humidity RH < 1% Cyclic voltammograms were acquired over a voltage range of 0 to 3.5 V (versus Li/Li + ) at a scanning rate of 1 mV/s The electrodes are arranged as shown in Figure 3.11
57 Figure 3.11 Electrode arrangement for electrochemical measurements
Generally, the capacity of the electrode material was calculated by galvanostatic charge and discharge testing The measurement was conducted under a constant current density The charge/discharge capacities (Q) can be calculated using the following formula:
Q = I × t (3.4) where I is the current density and t is the charge/discharge time Lithium-ion batteries' galvanostatic testing voltage cut-offs were 2.0-4.3 V for cathode materials and 0.01-3.0
V for anode materials In lithium-ion battery testing, the C-rate performance was used to evaluate the capacity of the electrode at different charge/discharge current densities Charge/discharge the cell at C/n rate means completely charge/discharge the cell within n hour
Electrochemical impedance spectroscopy (EIS) has been widely used to examine a complex sequence of coupled electrochemical processes, such as electron transfer and mass transfer A very small amplitude signal (5 mV – 10 mV) is applied to the testing system over frequencies from 100 mHz to 2 MHz The variation of resistance with frequency can be examined by monitoring the current response Charge-transfer
58 resistance (𝑅ct), which can qualitatively characterize the electrode reaction speed, can be calculated through EIS measurements A typical impedance Nyquist curve of a lithium-ion battery system consists of a compressed semicircle in the medium- frequency region assigned to the charge-transfer resistance Rct and an inclined line in the low-frequency range assigned to be Warburg impedance The Warburg impedance in a lithium-ion battery has the following relationship W=σω −1/2 − jσω −1/2 (σ : Warburg coefficient), and its details are shown in Table 3.14
Table 3.14 Circuit elements used in the equivalent circuit mode
QCPE (Constant Phase Element) 1/Q(jω) α (α = 1 for ideal capacitor)
The impedance depends on the frequency of the potential perturbation At high frequencies, the Warburg impedance is small since diffusing reactants don't have to move very far At low frequencies, the reactants have to diffuse farther, increasing the Warburg-impedance On a Nyquist Plot the Warburg impedance appears as a diagonal line with a slope of 45° On a Bode Plot, the Warburg impedance exhibits a phase shift of 45°.This form of the Warburg impedance is only valid if the diffusion layer has an infinite thickness If the diffusion layer is bounded, the impedance at lower frequencies no longer obeys the equation above
MICROWAVE-ASSISTED SYNTHESIS OF NANO
Explain the reason for the choosing of microwave synthesis process parameters 59
In the self-construction microwave synthesis described in figure 3.2, ammonium molybdate [(NH4)6Mo7O24.4H2O] is used as the Mo source rather than other precursors because it produces the NH 4+ ion during microwave decomposition, resulting in high- quality mixed-phase MoS2 with a high concentration of the 1T-MoS2 phase [149] Thiourea (NH2CSNH2) was used as a sulfur source and reducing agent Through its interaction with molybdenum, thiourea can stabilize the MoS2 surface, restrict growth, and helps in the formation of nanosized MoS2 domains along the basal planes AHM and TU can easily react with each other to form MoS2, producing high-quality mixed- phase MoS2 with a high content of the 1T phase in the shortest time [114, 150-152]
Ethylene glycol (EG) and glycerol (G) have a high boiling point (>180 o C), are microwave-compatible, have an appropriate viscosity, and promote nanoparticle nucleation and development in high boiling polyols The polyols' high boiling points enable synthesis at temperatures ranging from 200 to 320 °C without high pressure or
60 autoclaves [126, 153, 154] The nucleation and growth of nanoparticles in high boiling polyols such as ethylene glycol (EG) and glycerol are the primary reasons for their use in nano MoS2 production The polyol acts as both a solvent and a stabilizing agent, in this case, restricting particle growth while limiting particle agglomeration and aggregation Additionally, the synthesis is simple and does not require complex experimental conditions or professional reactors [155]
Furthermore, high boiling point polyols such as ethylene glycol (boiling point ~198 o C) and glycerol (boiling point ~ 290 o C) are common solvents for microwave-assisted preparation in open reaction systems Due to its higher dissipation factor than water, ethylene glycol (EG) could improve the crystallinity and morphology of nanomaterials, resulting in a faster energy transformation from microwave energy to thermal energy Glycerol has the potential to be an excellent renewable solvent in modern chemical processes when irradiated with microwaves and/or ultrasounds Because glycerol is nontoxic and biodegradable, it will provide significant environmental benefits to new platform products It has a high boiling point and a low vapour pressure Many organic and inorganic compounds can be dissolved in glycerol [156]
Concerning the reason for choosing the VEG:VG = 1:1 ratio, mixing these two solvents together is a method of changing the reaction's boiling point, i.e., to investigate the change in reaction temperature because the chemical reaction with microwave heating occurred at the boiling point of the solvent Table 3.2 of the thesis shows the increasing boiling points of different solvents: EG, EG + G, and G
The following are the reasons for using a specific and precise amount of 4 mL H2O without calculating the volume ratio to EG:
• Since the purpose of this research was to use a small amount of water added to the primary solvent (< 10 mL H2O injected into a 60 - 240 mL volume of polyol), it was desirable to produce a significant change in synthesis efficiency or change the morphology and structure of the synthesized materials without significantly changing the boiling point of the solvent The addition of only a small amount of
H2O to the reaction allows the role of H2O to be evaluated independently without
61 being influenced by changes in the reaction temperature parameter (ie, the boiling point of the solvent)
• The specific amount of water of 4 mL chosen for the initial test in this new process is also based on references from a study of the effect of a small amount of water in a certain organic solvent on the synthesis efficiency of nanomaterials [113, 157]
Since high initial power results in superheating (10 - 20 o C), 240 W of microwave power is chosen (see Appendix – Table A1), when the microwave power increases, the longer the duration of the irradiation causes superheating and degrades product quality [156]
Due to the rapid heating nature of microwave radiation, most chemical reactions involving microwave radiation have reaction times ranging from 2 to 15 minutes The reaction time was set at 15 minutes to ensure a complete reaction [105, 150, 158, 159].
X-ray diffraction (XRD) patterns of 1T/2H-MoS 2
XRD provides information about the crystal structure and size of the crystal, as well as the crystalline phase and purity of the crystal Intercalation and heat treatment during the microwave irradiation process significantly affects the crystal structure of the host materials It results in the phase transition from semiconducting (2H) to metallic (1T), as illustrated by the XRD pattern in figure 4.1 Only by comparing the XRD spectrum of mixed-phase 1T/2H-MoS2 to bulk 2H-MoS2 can the XRD spectrum of mixed-phase 1T/2H-MoS2 be determined [18, 124]
62 Figure 4.1 XRD patterns of S1-EG, S2-EG + H2O, S3-EG+G, and S4-G
As shown in Figure 4.1, all samples had XRD peaks at 31.9 o , 43.3 o , and 56.8 o , which correspond to the (100), (103), and (110) planes of MoS2, respectively For the (002) plane of 2H-MoS2, the characteristic peak was not observed at ~ 14.41 o but exhibited peak splitting lower and higher 2θ angles (red dashed rectangle in figure 4.1) This disordered structure matches the previously reported ammoniated 1T-MoS2 [24], and indicates the formation of a 1T/2H-MoS2 hybrid phase The largest interlayer spacing of the as-prepared MoS2 is similar to those of S1-EG (2θ ~ 10.6 o ), S3-EG+G (2θ ~ 10.9 o ), S4-G (2θ ~ 10.9 o ), which is due to the high content of 1T-MoS2 in the samples, and decreased in S2 (2θ ~ 13.5 o ), indicating the presence of both 1T and 2H phases, with the predominant of 2H phase
The S2-EG+H2O powder exhibits peaks at 2θ and 13.5 o that is almost identical to the (002) planes of hexagonal-phase MoS2, showing that it contains the 2H phase's primary phase component A small amount of H2O (4 mL) in the EG solvent is expected to cause a phase transition toward forming a 2H phase structure The interlayer distance of S1-
EG, as calculated by Bragg's equation, is approximately 8.3 Å, which is larger than the interlayer distance of bulk MoS2 (6.15 Å) (JCPDS 00-037-1492), which is mainly
63 attributed to the introduction of guest ions or molecules [29] When annealed at 800 o C for 2 hours, the expansion and NH4 + ion can be eliminated, resulting in a decrease in the interlayer distance (Figure A1.1 in Appendix) Additionally, the weaker and broader diffraction peaks observed in all samples compared to bulk 2H-MoS2 indicate that basal plane growth was restricted, meaning that MoS2 contains more interface defects and nanocrystals of different sizes [37, 58, 149, 150] In summary, the XRD data indicate that the microwave irradiation approach successfully prepared the hybrid system of MoS2.
Raman spectra of 1T/2H- MoS 2
Additionally, Raman spectroscopy was employed to characterize the nano MoS2 structure Raman experiments were performed with an excitation wavelength of 632.8 nm The quality of signals was affected by the strains, defects, and dislocations in lamellar materials with imperfect crystalline structures Raman is also a powerful technique for identifying the 1T and 2H phases in MoS2 The most notable difference between 1T-MoS2 and 2H-MoS2 is the symmetry of the sulfur in their structures (figure 4.3A), and the considerable differences in their Raman properties are due to structural variations
The Raman spectra of S1, S2, S3, and S4 synthesized in various solvents, including EG, EG+H2O, EG+G, and glycerol (G), are shown in Figure 4.2 The vibrations detected in all four samples are summarized in Table 4.1 The strong peaks at 141, 190, 230, and
331 cm -1 originated from stretching vibration of Mo–Mo and the phonon mode of 1T- MoS2 in all samples, indicating the presence of 1T-MoS2 [18, 58, 160] The remaining three peaks at 278, 372, and 398 cm -1 (in S2-EG+H2O) were assigned to the typical E1g,
E 1 2g, and A1g, respectively, suggesting the coexistence of 2H-MoS2 The intensity ratio of the A1g to J1 peak in a spatially resolved Raman spectra of MoS2 can be used to calculate the phase content A1g/J1 is inversely proportional to the amount of 1T phase present The J1 peak is visible in S1, S3, and S4, but the A1g/J1 value is extremely low and cannot be seen when zoomed in on figure 4.2C, indicating that the 1T phase content is high in this case When the 2H phase was dominant in S2-EG+H2O, as shown in
64 figure 4.2B (magnified S2-EG+H2O) spectra from figure 4.2 A), the intensity of the J1 peak decreased compared to the S1, S3, S4, and the A1g peak became visible The value of A1g/J1 increased, indicating that the content of the 1T phase is decreasing, and a mixed phase of 1T/2H has formed [161] This also explains the absence of A1g peaks in the Raman spectra of samples S1, S3, and S4, as shown in figure 4.2C The intensity of typical E 1 2g and A1g peaks over 1T/2H MoS2 in S1, S3, S4 is greatly reduced, which could be due to the low crystallinity and lack of 2H phase in the MoS2 material [18, 30]
Table 4.1 Summarize the vibration modes (position peaks) in S1, S2, S3, S4 (cm -1 )
Figure 4.2 (B) illustrates the E1g, E 1 2g, and A1g vibration modes located at 278, 372, and
398 cm -1 , respectively, in S2-EG+H2O The presence of crystal defects in the MoS2 basal planes is indicated by the broad and weak band of E 1 2g The A1g indicates the strength of the interaction between the adjacent layers The weak A1g band indicates weakened interactions between layers, which could be attributed to chaotic alternations of intercalated molecules and MoS2 layers along the c axis in intermediate MoS2 structures Notice that the Raman peak corresponding to the out-of plane Mo-S phonon mode (A1g) is preferentially excited for the edge-terminated film due to the polarization dependence In contrast the in-plane Mo-S phonon mode (E 1 2g) is preferentially excited for a terrace-terminated film [162] In comparison to the absence of H2O in sample S1-
EG, adding a small amount of H2O (4 mL) to the EG solvent in sample S2 boosted the
65 2H phase content in the hybrids considerably A minor quantity of H2O (4 mL) in EG solvent helps in increasing the concentration of 2H phase in the 1T/2H-MoS2 material
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 no signal of
SEM – TEM images of 1T/2H-MoS 2
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) was also employed to investigate further the effect of the reaction conditions on the structures, morphologies, and the particle sizes of the MoS2 obtained in all four solvents investigated
The formation of various morphologies of MoS2 materials is depicted in Figure 4.4 In comparison to the nanoparticle morphology of the S1-EG, S2-EG+H2O, and S4-G samples, the TEM images of the S3-EG+G sample revealed numerous wrinkles of nanoflakes morphology They are all agglomerates/aggregates made of homogeneous MoS2 particles or flakes The shape of nanoparticles can vary from spherical to elliptical or hexagonal (Figure 4.5 (D) & (E)) For a small amount of water in the precursor, the dominant population is nanoparticles (S2-EG+H2O) Particle boundaries were more clearly in S2-EG+H2O and S4-G than in S1-EG and S3- EG+G, so the particle size distribution of S2- EG+H2O and S4-G can be determined in Figure 4.7 Typically, one MoS2 particle comprises several crystallites, which means particle sizes are much more significant than crystallite sizes The diameter of the MoS2 particles increases with increasing temperature in various polyols (TEG < TEG+G < TG), indicating that higher temperatures promote the growth of larger MoS2 crystals It is worth noting that the
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
67 average particle size decreases significantly with the presence of H2O in EG solvent The sample S2- EG+H2O, with the addition of 4 mL water, can form crystallites with a smaller size than the other samples This result is consistent with those of Jacek Wojnarowicz et al [113], who examined the effect of water content in an ethylene glycol solvent on the size of nanoparticles synthesized via microwave solvothermal synthesis
Particle size between 15 and 80 nm was observed from SEM and TEM images of S1, S2, and S4 Another image of nanoflake materials of S3-EG+G is given in Fig 4.5(A) and (C) The flakes were observed with corrugation, indicating the flexible and ultrathin nature of the material Therefore, thanks to the results obtained from TEM images, it can confirm that materials at the nanoscale were successfully prepared with various morphologies Moreover, one can see that the size and shape of metal sulfide nanoparticles are different by the same microwave synthesis route It also shows that microwave irradiation can selectively influence the nucleation and growth rates of different compounds More detailed studies about microwave synthesis are in progress [163]
High-resolution TEM (HRTEM) is performed to visualize the 1T/2H crystal surface structures with lattice fringes (Figure 4.6) The red rectangle in Figure 4.6 (A) is enlarged, as shown in Figures 4.6 (B) & (C), evidently displays the trigonal lattice area of the 1T phase; and displays the common honeycomb lattice area of the trigonal prismatic coordination in the 2H phase As shown in Figure 4.6 (D), the interlayer distances were measured to be 0.236 nm which is attributed to the d spacing of the (100) planes of 1T phase MoS2 [164]
Figure 4.4 TEM images of 1T/2H-MoS2 in S1-EG, S2-EG+H2O, S3-EG+G and S4-G
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
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 d (100) = 3.55/15 = 2.36 Å d (100) = 2.36 Å
Figure 4.7 (B) and (D) are histograms of the particle size distribution of MoS2 nanoparticles calculated by ImageJ from (A) and (C) [165]
X-ray photoelectron spectroscopy (XPS) of 1T/2H-MoS 2
XPS was employed to investigate the chemical composition of the products In theory, peak positions in binding energy give information about a material's chemical state This section recorded the high-resolution XPS spectra of as-prepared MoS2 synthesized in various solvents to investigate the structures and the coexistence of different phases in the samples The selection of an appropriate solvent for the reaction is a critical factor influencing the morphology, structure, and efficiency of material synthesis in a microwave oven The analysis of the binding energy from the peak of the Mo 3d and S 2p orbitals in typical XPS spectrums of Mo 3d and S 2p orbitals proves the formation of MoS2 The XPS data have shown well-defined signals for 1T-MoS2, 2H-MoS2, and a small content of MoO3 in all the solvents investigated Since the mixed-phase 1T/2H-
72 MoS2 contains both the trigonal prismatic (2H-MoS2) and octahedral (1T-MoS2) coordination structure, it possesses different atomic states for Mo and S, having different binding energies The relevant data will be explained in detail in the discussion below
Figure 4.8 Mo 3d XPS spectra of 1T/2H-MoS2 in S1-EG, S2-EG+H2O, S3-EG+G and S4-G
The hybridized 1T/2H structure could be identified in Mo 3d and S 2p XPS spectra, as shown in Figure 4.8 and Figure 4.9 It is reported that, for any oxidation state of Mo, the 3d doublet (i.e., 3d5/2 and 3d3/2) has an energy separation of ~ 3.2 eV and a characteristic 3d5/2 : 3d3/2 peak intensity ratio of 3:2 [145] Furthermore, the 3d5/2 and 3d3/2 binding energies of all oxidation states of Mo show a higher shift compared to the respective binding energy values for elemental Mo, 228 and 231.1 eV [166] The peak pair (228.394 eV and 231.668 eV), which has a shift from 3d doublet of elemental Mo,
73 lower than that of Mo (IV) in 2H-MoS2 (~1.3 eV), can be respectively assigned to 3d5/2 and 3d3/2 (i.e., the 3d doublet) binding energies of Mo (IV) species in 1T-MoS2
Figure 4.9 S 2p XPS spectra of 1T/2H-MoS2 in S1-EG, S2-EG+H2O, S3-EG+G and S4-G
The listed peak identification (identity and orbital), peak position, FWHMs, and peak areas of the S1, S2, S3, and S4 samples were shown in table A.1 (Appendix) From figure 4.9, the two characteristic peaks around 229.803 and 233.167 eV were observed in all four samples, indicating the 2H phase of MoS2 [18, 24, 139, 140, 167] The binding energy of the Mo 3d peaks in 1T-MoS2 (Figure 4.9) was about ~1.3 eV lower than that in 2H-MoS2, which is consistent with the previously reported [131, 140, 160, 168] Similarly, a downshift of bonding energies can also be observed in the S 2p peaks shown in Figure 4.9 As the surrounding electron density of Mo and S increases in 1T-MoS2, the binding energy of Mo and S shifts to the lower binding energy The lone peak 3d3/2 at about 235 eV in Mo 3d orbitals (Figure 4.9) appears due to Mo (VI) in MoO3, whereas
74 its 3d5/2 counterpart of 3d doublet, with binding energy at ~232 eV, may have merged with the broad peak at 231.611 eV This trace amount of MoO3 may be formed during the handling of the sample in the ambient atmosphere Another peak at about 225.868 eV often appeared in the Mo 3d spectrum can be attributed to the S 2s, indicating Mo-
Figure 4.10 N 1s XPS spectra of 1T/2H-MoS2 in S1-EG, S2-EG+H2O, S3-EG+G and S4-G
Peaks in S 2p spectrum in S1-EG at 161.466 and 162.840 eV attributed to S 2p3/2 and S 2p1/2 of S 2- in MoS2 [169] As shown in the high-resolution N 1s spectra (Figure 4.10), three main peaks located at around 398.047 and 400.194 eV, which can be assigned to C-N, C=N, and N-Mo bonds, respectively [40] Additionally, Mo 3p3/2 and N 1s electrons can be identified at 394.311 eV and 397.872 eV, respectively, due to the formation Mo–N bonding linkages [147] The XPS study indicated that N atoms were
75 successfully doped into MoS2 via the development of a Mo–N bond, which can also be responsible for the 1T and 2H phase transitions in the MoS2 material structure There are only a few reports for fabricating stable 1T phase MoS2 doped with nitrogen element experimentally [22]
The quantitative analysis of 1T and 2H phase of MoS 2 with X-ray photoelectron spectroscopy
The relative concentration of the 1T and 2H phases in various solvents was calculated, using XPS as the ratio between the Mo 3d peak areas Each phase's fraction was calculated following the equation (4.1) below and then summarized in table 4.2 [170] All the MoS2 products comprise both phases – 1T and 2H - but their compositions vary depending on the solvents (reaction temperatures) Notably, 1T phase proportion was decreased when increasing the reaction temperatures (TEG < T(EG+G)