xv ABBREVIATIONS 1T-MoS2 Trigonal MoS2 2H-MoS2 Hexagonal MoS2 a-MSC amorphous MoS2/CNTs nanocomposite c-MSC crystalline MoS2/CNTs nanocomposite D2PD Direct two-pot dispersion DSC Di
INTRODUCTION
Motivation
Addressing energy and environmental concerns is essential for sustainable economic and social development The current energy economy heavily relies on non-renewable sources such as coal, oil, and natural gas, which are harmful to the environment In response, there is a global movement towards adopting renewable energy alternatives like solar, wind, tidal, and biomass Despite their potential, the intermittent availability of these renewable sources poses challenges influenced by regional and seasonal variations.
Efficient energy conversion and storage technologies, along with large-scale renewable energy exploration, are essential for meeting current energy demands This necessity has driven innovations in various energy systems, particularly in recent decades Notable advancements include hydrogen production through water electrolysis, electricity generation from hydrogen using fuel cells, and energy storage solutions utilizing lithium-ion batteries.
Electrolytic water hydrogen production is a key method for generating high-purity, sustainable hydrogen energy To enhance the efficiency of hydrogen production, it is essential to identify highly efficient catalysts that exhibit high conductivity and good hydrophilicity High conductivity facilitates electron transfer during the electrode reaction, lowering the reaction's starting potential, while good hydrophilicity reduces surface tension at the electrode-electrolyte interface, allowing hydrogen ions to effectively reach active catalytic sites Although precious metals like platinum, ruthenium, and iridium are recognized as the most effective catalysts for hydrogen evolution reactions (HER), their high cost and limited availability hinder widespread use Consequently, there is a growing interest in developing new, cost-effective, highly active, scalable, and stable catalysts Catalysis, particularly electrocatalysis, plays a crucial role in advancing sustainable energy solutions.
This research investigates the two energy barriers associated with electrochemical reactions involving water, oxygen, and hydrogen in water-splitting cells and fuel cells It emphasizes the critical role of catalysis in enhancing the efficiency of electrolysis in water-splitting applications.
Lithium-ion batteries (LIBs) are the preferred power source for portable electronics and electric vehicles, addressing the urgent needs of the global energy storage market With the rising demand for long-cycle life and high-performance LIBs, significant research is focused on various components, including cathode and anode materials, binders, electrolytes, and manufacturing techniques The active materials in the electrodes play a crucial role in enhancing the energy and power densities of lithium-ion batteries.
Nanostructured materials, known for their high surface-to-volume ratios and unique transport properties, have garnered significant attention for energy-related applications Molybdenum disulfide (MoS2), a member of the layered transition-metal dichalcogenide family, has emerged as a promising low-cost alternative to platinum in electrochemical hydrogen evolution reactions The technique of phase engineering is being explored to enhance the activation of MoS2, which exists in two crystalline forms: the hexagonal 2H phase and the octahedral 1T phase Notably, 1T-MoS2 exhibits superior chemical and physical properties compared to 2H-MoS2, yet its metastability presents challenges in synthesis While hybrid 1T/2H-MoS2 has been produced, achieving a controllable ratio of these phases remains an unresolved issue This study focuses on synthesizing hybrid phase 1T/2H-MoS2 with a high concentration of the 1T phase.
MoS2 has emerged as a promising anode material for lithium-ion batteries (LIBs) due to its impressive theoretical capacity of 670 mAh g\(^{-1}\), extensive specific surface area, and short diffusion length in the thickness direction.
MoS2 exhibits abundant voids and defects that could enhance lithiation and de-lithiation; however, its practical application is hindered by poor cyclability and rate capability due to low electrical conductivity and significant volume expansion during cycling To tackle these challenges, researchers have focused on synthesizing MoS2 with varied molecular structures and developing MoS2-carbon composites Among these, carbon nanotubes (CNTs) stand out as a promising additive due to their exceptional conductivity, which enhances the electrochemical performance of the anode material Additionally, the high aspect ratio of CNTs facilitates the bridging of defects and prevents MoS2 aggregation, effectively addressing the substantial volume changes that occur during charge and discharge cycles.
Microwave-assisted (MW) methods represent a promising green strategy for synthesizing nanomaterials and nanocomposites, aligning with current green chemistry approaches These methods ensure homogeneous heating of the reaction mixture, minimizing thermal gradients and fostering a stable environment for nucleation and growth, which leads to nanomaterials with uniform size distribution The quality of microwave-generated materials is influenced by factors such as reactant choice, applied power, reaction time, and temperature Understanding the fundamentals of microwave chemistry is essential for grasping the mechanisms and phenomena that occur during microwave heating Recent advancements in MW-assisted strategies highlight their potential to create environmentally friendly nanostructured materials with diverse applications.
Objectives and scopes
The primary objective of this doctoral research is to create an efficient and straightforward process for the production of MoS2 and nano MoS2/CNTs nanomaterials This includes the development of nanostructured 1T/2H-MoS2 hybrid phases, as well as amorphous and crystalline MoS2/CNTs nanocomposites, all while prioritizing energy efficiency.
• 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
This article evaluates the structure and properties of various synthetic materials intended for use as anode electrode materials in lithium-ion batteries (LIBs) and as electrochemical catalysts for the hydrogen evolution reaction (HER).
Microwave-assisted chemical processes are essential for scaling up the industrial production of nanomaterials, particularly MoS2 and MoS2/CNTs nanocomposites This method enhances wet chemical procedures by enabling clean reactions that yield diverse morphologies and sizes while significantly reducing reaction times.
The new ideas of the research (Novelty)
A new scalable one-step microwave heating method has been introduced to achieve a high concentration of metallic 1T-MoS2 within the 1T/2H hybrid phase of MoS2, significantly improving the catalytic efficiency for the hydrogen evolution reaction (HER).
This study explores the novel application of microwave heating to manipulate the 1T/2H ratio in polyol solvents, such as ethylene glycol (EG), a mixture of ethylene glycol and glycerol, glycerol alone, and ethylene glycol with a small amount of water The effects of these various polyol solvents on the synthesis of MoS2 hybrid phases under microwave heating have not been previously reported.
Pure 1T-MoS2 exists in a metastable state, which restricts its applications However, the successful synthesis of hybrid phase 1T/2H-MoS2 stabilizes the metastable 1T phase through interaction with the 2H phase, significantly enhancing its catalytic activity This advancement lays a scientific foundation for the utilization of hybrid MoS2 in various applications.
5 phase MoS2 to replace Platinum (Pt) in the electrochemical hydrogen evolution reaction (HER)
The synthesis of crystalline and amorphous MoS2/CNT nanocomposites in polyol solvents via microwave heating remains underexplored Crystalline MoS2/CNTs show enhanced electrochemical performance for lithium-ion battery (LIB) applications, while amorphous MoS2/CNT materials are effective catalysts for the hydrogen evolution reaction (HER).
Major contributions of the thesis
This thesis lays a crucial groundwork and opens a new research avenue in catalysis, energy storage, and conversion utilizing MoS2 nanostructured materials and carbon nanotubes, specifically in Vietnam and VNU-HCM, to align with the global advancements in materials research.
The control of structural morphology and particle size in hybrid structures like 1T/2H-MoS2 and MoS2/CNTs nanocomposites using microwave energy is scientifically significant for the synthesis of MoS2 and other nanostructured materials Microwaves offer notable advantages, including time and energy efficiency, enabling reactions at air pressure to yield a substantial amount of synthetic products on a gram scale, surpassing traditional hydro/solvothermal methods This capability is essential for accelerating laboratory research.
Experimental data play a crucial role in enhancing the scientific database related to one-dimensional and two-dimensional nanomaterials This research not only provides valuable insights but also introduces alternative methods in material technology for energy storage and conversion applications, particularly in hydrogen evolution reactions (HER) and lithium-ion batteries (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
The synthesis of crystalline MoS2/CNTs nanocomposites was achieved through an indirect CNTs dispersion process (I2PD) This study focused on identifying the optimal synthesis conditions using the Taguchi design of experiments method, with crystallinity as the key response variable.
The synthesis of amorphous MoS2/CNTs nanocomposites was achieved through a direct CNTs dispersion process (D2PD) The study focused on identifying optimal synthesis conditions using the Taguchi design of experiments method, with the Tafel slope serving as the response variable.
• 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
The article discusses the synthesis of 1T/2H-MoS2 and MoS2/CNTs using the microwave energy method, highlighting its advantages and efficiency It also explores innovative strategies to enhance the electrocatalytic water splitting performance of MoS2 and MoS2/CNTs, along with their electrochemical performance in lithium batteries.
Chapter 3 outlines the comprehensive methodology for synthesizing nanostructured materials, specifically 1T/2H-MoS2 and MoS2/CNTs nanocomposites, utilizing microwave synthesis techniques It details the Taguchi experimental method for optimizing synthesis conditions, along with various characterization methods for physical and structural analysis, catalyst preparation, electrode preparation, and electrochemical characterization Key techniques employed include X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) to assess the structure and morphology of the synthesized materials, alongside galvanostatic charge-discharge and cyclic voltammetry for electrochemical evaluation.
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, amorphous and crystalline MoS2/CNTs nanocomposites are fabricated using the microwave heating method The synthesis conditions are optimized through the Taguchi experimental design method Additionally, the catalytic activity of the nanostructured amorphous MoS2/CNTs is examined.
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 represent significant advancements in science and technology, encompassing various new allotropic forms of carbon, including single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs), graphene, graphene oxide (GO), fullerene, carbon nanohorns, carbon nanocones, and carbon nano-onions Notably, SWNTs, MWNTs, graphene, and graphene oxide are carbon nanomaterials that exhibit exceptional physical, chemical, and mechanical properties, making them suitable for a diverse array of applications.
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]
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
MoS2 exhibits a layered structure held together by van der Waals interactions, with each layer measuring approximately 6.2 Å in thickness This structure consists of a hexagonally packed layer of metal atoms situated between two layers of chalcogen atoms, where the lone-pair electrons of the chalcogen atoms stabilize the surfaces by preventing reactions with environmental species Typically, MoS2 exists in a thermodynamically stable trigonal 2H phase, characterized by semiconducting properties and low electrical conductivity To enhance electrical conductivity and achieve moderate electrochemical performance in batteries, hybrid electrodes combining MoS2 with high-conductivity materials such as graphene, carbon nanotubes, carbon nanofibers, and polyanilines are utilized.
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
The MoS2 material exhibits various phases, including tetragonal (1T), hexagonal (1H and 2H), and rhombohedral (3R), each characterized by different layer arrangements and repeat unit symmetries The 2H phase is the most stable and has hexagonal symmetry, while the 3R phase displays rhombohedral symmetry Notably, intercalation of alkali metals into 2H-MoS2 can induce a phase transformation to the distorted metallic 1T-MoS2, which is gaining attention for its superior properties in energy storage applications The unique structural features of 1T-MoS2 enhance electron transport and ion diffusion, making it advantageous for electrochemical technologies Unlike the semiconducting 2H-MoS2, which has limited active sites, 1T-MoS2 offers a greater number of electrochemically active sites, improved electronic conductivity, and chemical activity, albeit with stability challenges The presence of a mixed-phase heterostructure of 1T/2H-MoS2 reduces kinetic barriers and promotes electron transfer, thereby increasing active site availability and enhancing catalytic activity.
Hybrid nanostructures of 1T and 2H phases demonstrate exceptional electrochemical properties, characterized by high specific capacitance, remarkable rate capability, and outstanding long-cycle durability This performance is attributed to the intrinsic higher activity and superior electrical conductivity of the 1T phase, along with the enhanced ability of the swollen lamellar structures to accommodate electrolyte ions, positioning them as an impressive electrode material for supercapacitors.
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 shows significant potential as a cost-effective substitute for platinum-based catalysts in the hydrogen evolution reaction (HER) Its advantages include a large surface area, near-zero adsorption free energy (\(\Delta G_{\text{ads}}\)), and various opportunities for structural engineering.
Hydrogen is poised to be a key energy carrier for future carbon-neutral energy production, with water being its most abundant and carbon-free source The process of hydrogen production from water involves two half-cell reactions, generating both hydrogen and oxygen The hydrogen evolution reaction (HER), represented by the equation \(2H^+ + 2e^- \rightarrow H_2\), necessitates catalysts to reduce the overvoltage and enhance energetic efficiency While precious metals like platinum, ruthenium, and iridium are effective catalysts for HER, their high cost and scarcity limit large-scale applications Consequently, there is ongoing interest in developing new catalysts that are inexpensive, highly active, scalable, and stable, such as a class of MoS2 nanostructures with edge-terminated and interlayer-expanded features.
Figure 2.3 illustrates the structural polytypes of bulk MoS2 crystals, highlighting the unit cells marked by dashed lines The inset provides a summary of the space group and Mo-S coordination for various polytypes, including the 2H, 1T, and 3R phases.
Recent studies highlight that molybdenum disulfide (MoS2) synthesized via microwave heating exhibits exceptional catalytic performance The edge-terminated structure enhances active edge sites, while an increased interlayer distance optimizes the electronic properties, leading to improved efficiency Additionally, the metallic 1T polymorph of MoS2 has been identified as catalytically active In comparison, amorphous MoSx demonstrates superior hydrogen evolution activity over crystalline MoS2, likely due to a higher number of unsaturated sites compared to the active edge sites found in single crystal forms, which contain coordinately unsaturated molybdenum and sulfur atoms.
The catalytic performance of catalysts for hydrogen evolution reaction (HER) can be assessed using standardized indices such as overpotential (η), Tafel slope (b), exchange current density (j0), turnover frequency (TOF), stability, Gibbs free energy, faradic efficiency (FE), and electrochemically active surface area (ECSA) Among these, overpotential, Tafel slope, and durability are the most commonly utilized metrics for evaluating HER electrocatalyst performance.
The catalytic performance of MoS2 is significantly affected by its crystal properties, including crystallinity, size, and morphology An effective electrocatalyst should possess a high surface area, a dense concentration of active sites, excellent electrical conductivity, robust chemical stability, superior adsorption characteristics, and a straightforward, scalable synthesis method Smaller MoS2 particles offer a greater number of surface and edge sites, leading to enhanced catalytic activity compared to larger MoS2 particles.
The HER mechanism has been extensively studied, revealing two widely accepted mechanisms that depend on the electrode material and its hydrogen atom adsorption strength These mechanisms lead to three detailed reaction steps The first step, known as the Volmer reaction, involves the initial electron transfer where protons gain electrons to form adsorbed hydrogen atoms (Hads) on the catalyst's surface The second step is the Tafel reaction, where Hads combine.
The electrochemical absorption process involves the transfer of electrons through electrolyte protons, culminating in the Heyrovsky reaction, which produces H2 molecules from two Hads atoms The reaction equations for these fundamental processes vary between acidic and alkaline media, as detailed 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 -
The rate of the hydrogen evolution reaction (HER) is primarily determined by one of three steps, with hydrogen adsorption often being the rate-limiting factor Electrode materials featuring more edges and cavities enhance adsorption sites, thereby improving electron transfer Additionally, desorption and diffusion of molecular hydrogen can also serve as rate-limiting steps Rough or perforated electrode surfaces increase the reaction area and prevent bubble formation, facilitating better electron transfer The rate-determining step varies with potential; at low potentials, hydrogen adsorption is critical, while at high potentials, desorption becomes the limiting factor To enhance the catalytic performance of MoS2, research predominantly targets increasing edge sites and sulfur vacancies, with significant improvements achievable by effectively utilizing inert MoS2 basal planes.
The schematic illustration of the hydrogen evolution reaction (HER) mechanism for the MoS2 catalyst involves two key steps: Step I is the adsorption of H\(^+\) on the catalytic site, known as the Volmer reaction, while Step II is the release of H\(_2\) from active sites, referred to as the Heyrovsky reaction Notably, the distance between two adjacent sulfur (S) atoms is measured at 3.18 Å for the 2H-MoS2 phase and 3.22 Å for the 1T phase.
The rational design of defects on the basal plane of MoS2 can enhance its hydrogen evolution reaction (HER) performance, as the basal plane is inactive while the edge sites are significantly more active Active centers in MoS2 are primarily found at edge sites and sulfur vacancies, with the basal plane considered chemically inert Research by Hinnemann et al in 2005 confirmed the activity of MoS2 edges through DFT calculations Additionally, studies indicate that mixed-phase 1T/2H MoS2 functions as both a photocatalytic and electrocatalytic hydrogen evolution catalyst The octahedral metallic 1T phase of MoS2 is 10^7 times more conductive and offers a greater number of active sites for hydrogen formation compared to the semiconducting 2H phase.
The conductivity of MoS2 significantly influences its hydrogen evolution reaction (HER) efficiency, primarily due to its chemical nature across various polymorphs, particularly the 2H and 1T phases Additionally, effective electrical transport between the active site and the electrode is crucial Research by Lukowski et al highlighted the benefits of converting semiconducting 2H-phase MoS2 nanostructures that are directly grown on graphite.
16 to the metallic 1T phase via exfoliation with lithium intercalation improved HER performance [39]
The advancement of high-performance molybdenum sulfide hydrogen evolution reaction (HER) catalysts is hindered by a limited understanding of the catalytic mechanisms involved Key parameters influencing catalytic performance include exchange current density, Tafel slope, and stability, which require synergistic optimization for an ideal catalyst Despite previous findings that highlight the catalytic activity of edge sites in molybdenum disulfide (MoS2) and their linear relationship with exchange current density, recent studies indicate that the materials' electrical conductivity also significantly affects this parameter.
The electrical catalytic performance of MoS2 and its composites is hindered by factors such as active site density, reactivity, and inefficient electrical transport and contact To enhance electrical contact with MoS2's active sites, high electrical conductivity supports have been explored Carbon materials, including carbon nanotubes, macroporous–mesoporous carbon, graphene, and reduced graphene oxide, are commonly utilized as electrocatalyst supports in hydrogen evolution reactions (HER) and other electrochemical applications Among these, multiwall carbon nanotubes (MWNTs) stand out due to their excellent chemical stability, strong electrical conductivity, and large surface area, making them a cost-effective option—20 times cheaper than graphene—and a promising support for nanoscale catalysts.
The metastable 1T form of metallic MoS2 has garnered significant interest due to its outstanding properties for energy storage applications Its unique structural characteristics enhance electron transport and ion diffusion, making it ideal for electrochemical energy storage technologies The distorted octahedral coordination and improved hydrophilicity of 1T-MoS2 contribute to superior electron transport capabilities.
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
The diffusion of Li\(^+\) ions is facilitated by MoS\(_2\) without significant volumetric changes during lithiation and delithiation However, its poor electrical conductivity, low cycling stability, and high risk of agglomeration are major drawbacks To address these issues, one-dimensional carbon nanotubes (CNTs) have been introduced, offering superior electrical conductivity, excellent mechanical properties, and high structural stability.
Carbon nanotubes (CNTs) serve as ideal nanometer-sized templates that can significantly reduce the aggregation rates of MoS2 The MoS2/CNT hybrid effectively merges the excellent electrical conductivity of CNTs with the superior electrochemical performance of individual MoS2 layers during cycling This structure features 1D carbon nanotubes as the backbone, with 2D MoS2 layers growing on their surface, providing a large surface area for accommodating Li\(^+\) ions Additionally, the increased layer distance of S-Mo-S leads to reduced strain and a smaller intercalation barrier for Li ions.
MoS2 is widely recognized for its low electronic conductivity during electrochemical reactions Furthermore, the intercalation of lithium into the MoS2 layer leads to significant volume expansion Many studies have focused on enhancing the electrochemical performance of MoS2 anodes.
Shuhua Li et al developed nitrogen-doped MoS2 nano-flowers for use as an anode material in lithium-ion batteries The N-doped MoS2 anodes demonstrated a specific capacity of 786 mAh g\(^{-1}\) after 100 cycles at a high current density of 0.5 C, showcasing remarkable cycle stability compared to pristine MoS2 anodes.
The conductivity of 1T-MoS2 is significantly higher, being 10^7 times that of 2H-MoS2, due to its highly exposed basal and edge planes that offer more active sites However, its complex synthesis and instability hinder its widespread application, particularly in electrochemical storage devices In contrast, the hybrid 1T/2H mixed phase MoS2, developed through phase engineering, shows great promise as an energy storage device, as it leverages the electrochemical properties of both the semiconducting and metallic phases.
21 a versatile material that can be used in both supercapacitors and batteries, depending on the reaction conditions [56]
Wang and colleagues developed swollen ammoniated MoS2 featuring a mixed-phase of 1T/2H MoS2, where NH4+ ions enhanced the lamellar structure from 0.615 nm to 0.99 nm, thereby increasing the concentration of the 1T phase The resulting ammoniated 1T/2H-MoS2 electrode material demonstrates exceptional charge storage performance in Li-ion batteries, attributed to a unique surface redox reaction.
Nguyen Quoc Hai et al demonstrated that mixing carbon nanotubes (CNTs) with molybdenum disulfide (MoS2) using the high-energy mechanical milling (HEMM) process enhances electrical conductivity and mitigates volume expansion Their study revealed that the MoS2/CNT ratio of (1:2) achieved the highest specific capacity of approximately 765 mAh g\(^{-1}\) after 70 cycles, along with superior rate capability due to improved conductivity Additionally, in 2015, S K Srivastava and B Kartick prepared and characterized MoS2–multi-walled carbon nanotube (MWCNT) hybrids.
The hybrid structure in Li-ion batteries, composed of MoS2 and MWCNTs in a 1:1 weight ratio, demonstrates outstanding reversible capacity and cycling stability, achieving 1214 mAh/g after the first cycle and 1030 mAh/g after the sixtieth cycle This enhanced performance is attributed to the synergistic effects of MoS2 and MWCNTs Research by Yumeng Shi et al highlights that the unique hierarchical nanostructures, featuring a MWNTs backbone and MoSx nanosheets, significantly improve electrode performance in lithium-ion batteries Each MoSx nanosheet connects to MWNTs, maximizing exposed electrochemical active sites, which enhances ion diffusion efficiency and accommodates volume expansion during electrochemical reactions A remarkable specific capacity exceeding 1000 mAh/g is achieved at a current density of 50 mA/g, surpassing the theoretical capacities of both MWNTs and MoS2, which are approximately 372 mAh/g.
Jinglong Wang et al have developed N-doped carbon-carbon nanotubes as a foundational structure for the growth of a 1T/2H–MoS2 heterostructure This double-phase 1T/2H–MoS2, featuring a significant 60% metallic 1T-MoS2 content, exhibits increased layer spacing, enhanced internal electron conductivity, and improved lithiation and de-lithiation reactions As an anode material for lithium-ion batteries, the synthesized 1T/2H–MoS2@NC–CNT demonstrates a remarkable reversible capacity of 764.99 mAh g⁻¹ at 200 mA g⁻¹ and maintains a high-rate capacity of 547.7 mAh g⁻¹ at 1000 mA g⁻¹.
Microwave synthesis of nanomolybdenum disulfide (MoS 2 ) and MoS 2 /CNTs
Microwave heating significantly accelerates reaction rates by reducing reaction times, and the rapid heating or "superheating" can alter reaction mechanisms The scalability of microwave-assisted chemical reactions is crucial for industrial production and the application of nanostructured materials, making it a key area for future research This review specifically examines 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
Table 2.4 highlights the advantages and disadvantages of various manufacturing techniques for nano MoS2 and nano MoS2/CNTs Our focus is on chemical methods, specifically those utilizing microwave heating instead of traditional heating These methods encompass thermal decomposition of salts and low-temperature precipitation from solutions The resulting materials exhibit an amorphous or poorly crystallized structure, influenced by the preparation temperature Notably, these highly disordered materials possess unique properties that are absent in their crystalline counterparts.
Microwave reactors effectively enhance chemical reactions through two primary mechanisms The first mechanism is the thermal effect, which arises from dielectric heating as molecular dipoles attempt to align with the fluctuating electric field of microwave radiation The second mechanism is the nonthermal microwave effect, characterized by dipole-dipole-like interactions between the electric field's charges and molecules possessing a dipole moment.
The key principle to understand is the definition of isobaric heat capacity, expressed as \$dQ = m \cdot C_p \cdot dT\$ This indicates that there is no distinction between conventional and microwave heating, as the heat transferred is determined solely by the mass of the sample, its heat capacity (\$C_p\$), and the temperature change.
Microwave (MW) heating is an effective technique for targeting active regions in supported metal catalysts, facilitating the synthesis of challenging metal chalcogenide complexes This method preferentially heats the metal components in the reaction mixture, allowing them to react swiftly with chalcogen materials before they can volatilize By rapidly increasing the temperature, MW heating minimizes atomic diffusion and aggregation, ensuring a uniform distribution of individual metal atoms within the crystal lattice.
Solvents play important roles in microwave-assisted synthesis in liquid phase Therefore, the solvent is a crucial factor for the microwave-assisted formation inorganic
The polarity of a solvent is crucial for its ability to couple with microwave energy, resulting in rapid temperature increases and fast reaction rates The efficiency of microwave heating is significantly influenced by the properties of the reaction system, with excellent microwave-absorbing solvents leading to higher heating rates Solvents for microwave heating can be categorized based on their loss tangent (tan δ): high (tan δ > 0.5), medium (tan δ ≈ 0.1 – 0.5), and low (tan δ < 0.1).
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 presents the loss tangent (tan δ) values at 2.45 GHz and 20 °C, along with the boiling points of various typical solvents Water (tan δ = 0.123) and alcohols are effective solvents for microwave-assisted preparation of inorganic nanostructures Notably, ethylene glycol, with a tan δ of 1.350, has a high boiling point, making it a suitable choice for such applications.
(~ 198 o C) and reductive ability, allowing relatively high temperatures for the preparation of inorganic nanostructures in an open reaction system
In addition to single-solvent microwave-assisted synthesis, mixed solvents are commonly employed in the microwave-assisted formation of nanostructures The selection of various solvents and the adjustment of their volume ratios in mixed solvent systems allow for greater control over the chemical composition, structure, size, and morphology of the resulting products Notably, nanostructured chalcogenides have been successfully synthesized using the microwave-assisted method in mixed solvents, such as water and polyols.
Liu et al developed 1T@2H-MoS2 nanospheres through a microwave-assisted hydrothermal method, utilizing ammonium molybdate, thiourea, and deionized water as solvents They also employed a straightforward hydrothermal technique with specific ratios of Mo and S precursors Both methods were conducted at 220 °C, but the reaction times varied significantly; the microwave-assisted approach required only 10 minutes for the first step and 4 minutes for the second, while the traditional hydrothermal process took 24 hours and 10 hours, respectively.
Microwave-assisted inorganic synthesis allows for the direct transfer of electromagnetic energy to the reaction mixture, independent of temperature This method effectively converts microwave radiation into heat, helping to surpass activation energy barriers in chemical reactions Additionally, the application of microwaves can be extended to higher temperatures, creating extremely high-energy environments conducive to various chemical processes.
Microwave heating typically results in smaller particle sizes of MoS2 materials compared to conventional heating, indicating faster and more homogeneous nucleation Additionally, the use of ethylene glycol (EG) as an additive enhances the crystallinity and morphology of MoS2, attributed to EG's higher dissipation factor than water, which facilitates rapid energy conversion from microwave to thermal energy.
Danyun Xu and colleagues have introduced a rapid and efficient microwave method to convert MoS2 from the 1T phase to the 2H phase, successfully producing processable 2H-MoS2 nanosheets This innovative approach offers a simpler alternative to the traditional phase change method, which typically relies on annealing.
Microwave irradiation has been found to facilitate the phase change of MoS2 nanosheets from 1T to 2H in just minutes, significantly faster than traditional methods that require high temperatures for hours The well-dispersed 2H-MoS2 nanosheets produced through this method are easily processed, making the microwave strategy a promising approach for synthesizing hybrid materials like 1T/2H-MoS2.
Yunrui Tian et al reported the fabrication of MoS2/MoO2@CNT nanocomposites using an ultrafast microwave approach The study revealed that MoS2/MoO2 nanoparticles grow uniformly on the surface of carbon nanotubes (CNTs), which serve as a transport path for ions in electrolytes This structure reduces energy loss and significantly enhances the electrochemical performance of the resulting nanocomposites.
Conclusion
This review examines the research on MoS2 and MoS2/CNTs nanostructures, highlighting their potential to overcome challenges in lithium-ion batteries and electrocatalysts It summarizes recent advancements in nanoparticle synthesis, nanostructure design, and composite fabrication, emphasizing their effects on electrochemical performance Furthermore, it addresses the ongoing challenges and opportunities for future enhancements in this field.
Microwave-assisted synthesis of nanostructured materials has gained significant attention, particularly for its potential in industrial-scale production This study highlights microwave methods as an effective green strategy for synthesizing nanostructured 1T/2H-MoS2 hybrid phases, amorphous MoS2/CNTs, and crystalline MoS2/CNTs nanocomposites, aligning with current green chemistry practices The quality of materials produced through microwave techniques is influenced by factors such as reactant selection, power, reaction time, and temperature Understanding the fundamentals of microwave chemistry is essential for grasping the mechanisms and phenomena that occur during microwave heating.
The impact of solvent properties on the microwave synthesis of nano MoS2 and nano MoS2/CNTs remains unclear This study aims to enhance the fundamental understanding of how solvents, particularly polyol solvents such as ethylene glycol and glycerol, influence the synthesis process.
Phase engineering is an innovative approach aimed at enhancing the activation of MoS2, which exists in two crystalline forms: the hexagonal 2H phase and the octahedral 1T phase The 1T-MoS2 phase exhibits superior chemical and physical properties compared to the natural semiconductor 2H-MoS2 However, the metastable nature of 1T-MoS2 presents significant challenges in its synthesis While hybrid 1T/2H-MoS2 has been produced, achieving precise control over the 1T/2H ratio remains an unresolved issue This study focuses on exploring synthesis methods for hybrid phase 1T/2H-MoS2 that allow for a controllable high concentration of the 1T phase.
Additionally, this study examines how structural design can solve these issues successfully in lithium-ion batteries and electrocatalysts
This study focuses on the research of nanostructured materials, specifically molybdenum disulfide (MoS2) and carbon nanotubes (CNTs), for their applications in lithium-ion batteries and hydrogen evolution electrocatalysts.
METHODOLOGY
Overall research procedure
The experimental procedures for designing nanostructure materials are illustrated in Figure 3.1 and involve three key steps: first, the preparation of nanostructure materials; second, the characterization of these materials; and third, the evaluation of their electrochemical performance in lithium-ion batteries and catalytic activity for the 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
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 a colorless to yellow-green crystalline compound, typically found in its tetrahydrate form with the chemical formula (NH4)6Mo7O24·4H2O Commonly referred to as ammonium paramolybdate or ammonium molybdate, it is important to note that the term ammonium molybdate can also denote ammonium orthomolybdate, represented by the formula (NH4)2MoO4.
Multi-wall carbon nanotubes (MWCNTs) were synthesized using the T-CVD method by MTLab The purification process involved two key steps: first, a heat treatment at 460 °C with air to eliminate carbonaceous impurities and expose metal or metal oxide catalysts; second, an acid treatment with HNO3 and HCl at 60 °C to remove metallic catalysts and oxidize any remaining carbonaceous impurities The final step involved functionalizing the purified MWCNTs in an HNO3/H2SO4 solution.
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
This study utilized ammonium heptamolybdate tetrahydrate (NH4)6Mo7O24.4H2O (AHM) and thiourea (CSN2H4) as starting materials, employing microwave heating to synthesize MoS2 mixing phases Four solvents were examined: ethylene glycol (EG), a mixture of ethylene glycol with 4 mL of water, glycerol (G), and a 1:1 volume ratio of ethylene glycol and glycerol The synthesis process is illustrated in Figure 3.2, with detailed explanations of the chosen parameters provided in Chapter 4, Section 4.1.
In this study, 1.24 g of ammonium heptamolybdate tetrahydrate and 2.28 g of thiourea were dissolved in 60 mL of various solvents and heated at 60 °C for 30 minutes The solvents investigated included ethylene glycol, a mixture of ethylene glycol and water, glycerol, and a 1:1 mixture of ethylene glycol and glycerol The resulting homogeneous solution was then subjected to microwave irradiation, rapidly heating the mixture to the boiling point for 15 minutes at 240 W After the formation of a significant amount of black precipitate, the mixture was cooled, diluted with ethanol, and the precipitates were collected through centrifugation, followed by filtration and washing with ethanol The final MoS2 nanostructure samples were labeled S1 – EG, S2 – (EG + H2O), S3 – (EG + G), and S4 – (G), as summarized in Table 3.2.
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
Figure 3.2 The flowchart for synthesis of MoS2 nano powders under microwave heating
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
This study successfully synthesized MoS2/CNTs nanocomposites through microwave-assisted liquid phase chemical reactions using two different reaction mixtures Each mixture involved a dispersion of functionalized carbon nanotubes (f-CNTs) in an ethylene glycol solution with ammonium molybdate tetrahydrate and thiourea as precursors Two methods, indirect two-pot dispersion (I2PD) and direct two-pot dispersion (D2PD), were employed to disperse the f-CNTs in the precursor solution The synthesis resulted in both crystalline and amorphous MoS2/CNTs Additionally, the Taguchi experimental design was utilized to optimize the synthesis conditions, aiming to establish a structurally controlled process for MoS2/CNTs materials.
The success of MoS2/CNT synthesis heavily relies on the homogeneity of the reaction mixture While AHM and TU precursors are completely soluble in EG, f–CNTs are merely dispersed in the same medium The conditions of the medium and dispersion play a crucial role in this process.
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 create a f–CNTs/H2O dispersion, which was then combined with an ethylene glycol (EG) solution containing dissolved AHM and TU This method, known as “indirectly dispersed f–CNTs through two pots” (I2PD), involves dispersing f–CNTs in water before adding them to the EG solvent Specifically, 2.47 g (2 mmol) of AHM and 4.56 g (60 mmol) of TU precursors were accurately weighed and added directly to 120 mL of EG solvent, stirring continuously at 50°C for 1 hour 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 with a concentration of 10 g/L Subsequently, 4 mL of the f–CNTs/H2O solution was added to the precursor solution in pot 1 and sonicated for an additional 30 minutes at 50°C before being transferred to the microwave system for the reaction.
The f–CNTs powder was dispersed in ethylene glycol (EG) to create a f–CNTs/EG dispersion solution, which was subsequently combined with an EG solution containing AHM and TU, a method referred to as "direct two–pot dispersion (D2PD) of f–CNTs." In this D2PD process, 40 mg of f–CNTs were sonicated with 4 mL of EG for 30 minutes at 50°C, resulting in a f–CNTs/EG solution with a concentration of 10 g/L.
A total of 120 mL of ethylene glycol (EG) was achieved by adding 4 mL of f–CNTs/EG solution to the precursor solution The D2PD reaction mixture was then sonicated for 30 minutes at 50°C before being placed in the microwave system for the reaction.
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
The Taguchi method offers a simpler and more efficient approach to optimizing multiple operational variables in experimental design, reducing the time and costs associated with conventional optimization studies This technique allows for the simultaneous optimization of various factors while requiring fewer experiments, leading to improved quality and reliable design solutions A critical aspect of Taguchi's experimental design is the careful selection of control factors, enabling the quick identification of non-significant variables This method has also been effectively applied to the synthesis of nanomaterials, demonstrating its versatility in modeling and analyzing the impact of control factors on performance output.
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
This study focuses on optimizing the synthesis of MoS2/CNTs by analyzing the degree of crystallinity from XRD results and the Tafel slope from LSV results The primary objective is to enhance crystallinity while reducing the Tafel slope The difference between the actual performance and the target values is utilized to establish the loss function for the synthesis process Additionally, Table 3.3 outlines five factors, each with four levels, to be examined in the 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)
To create orthogonal arrays for parameter design, we consider five factors, each with four levels Consequently, the suitable orthogonal array identified for this study is L'16 [123] This results in a total of 16 experiments, with the specific factors and their levels utilized in each experiment detailed in Table 3.5.
Table 3.5 The appropriate orthogonal array L’16
The experiments were conducted according to the L16 matrix design, with the experimental conditions detailed in Tables 3.7 and 3.8 Each experiment's parameters were set, and the responses were characterized by crystalline structure and Tafel slope The results from these experiments are presented in Tables 5.1 and 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
The term "noise" refers to uncontrollable external factors that affect the outcome of the quality characteristic being tested The Signal-to-Noise ratio (S/N ratio) quantifies the sensitivity of this quality characteristic to these external noise factors Serving as a transformed figure of merit for the loss function, the S/N ratio encapsulates both the mean quality level and the variation around it The primary objective of any experiment is to achieve the highest possible S/N ratio, particularly for results such as crystallinity and Tafel slope, as a high S/N ratio signifies that the signal significantly outweighs the random effects of noise factors.
Normalization of response values are divided into three types according to expect nature of the response values
The "Lower-the-better" normalization criterion emphasizes that lower values of the objective function are preferred In the synthesis of amorphous MoS2/CNTs, a decrease in the Tafel slope values is indicative of improved catalytic activity The formula used for this normalization approach is as follows.
• Second is “nominal the better” where the objective function has average values
The "Higher-the-better" criterion emphasizes the importance of maximizing results, particularly in the synthesis of crystalline MoS2/CNTs, where increased crystallinity is essential for enhancing electrochemical performance in lithium batteries The normalization formula for this criterion is as follows.
) where 𝑦 𝑖 is the signal measured in each experiment (mean value)
The impact of a parameter level on the signal-to-noise (S/N) ratio, specifically the deviation from the overall mean signal, is determined through analysis of means (ANOM) Additionally, the relative influence of process parameters is assessed using analysis of variance (ANOVA) on the S/N ratios The calculations for both ANOM and ANOVA are performed using specific mathematical relations.
In the context of the experiment, the contribution of each parameter level to the signal-to-noise (S/N) ratio is denoted by \( m_i \), while \( N_l \) represents the frequency of conducting the experiment at the same factor level across the entire experimental region.
For example, after calculating the SN ratio for each experiment in table 3.5, the average
The SN value is determined for each factor and level, and these values are organized in Table 3.6 The range Δ, defined as the difference between the high and low SN values, is calculated for each parameter and included in the table A larger Δ indicates a more significant impact of the variable on the process, as it reflects that a precise change in the signal results in a greater effect on the measured output variable.
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 display significantly different properties compared to their macro counterparts To effectively characterize nanomaterials, imaging techniques with high resolution are essential to identify local property variations and detect defects This detailed understanding is crucial for engineering material properties to fulfill the performance demands of next-generation devices.
Characterization methods play a crucial role in analyzing the diverse properties of nanomaterials, enabling the exploration of their unique physical characteristics, structure, and morphology This project will showcase advanced techniques for characterizing MoS2 and MoS2/CNTs nanomaterials, including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (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
This doctoral research utilizes X-ray diffraction (XRD) measurements conducted on a Bruker D8 ADVANCE diffractometer, employing Cu Kα radiation (𝜆 = 1.5406 Å) at the Institute of Applied Materials Science – VAST The diffraction analysis spans an angle range of 5° to 80° with a scan rate of 1° min⁻¹ and a step size of 0.0194° The XRD samples consist of fine powder, creating a flat surface approximately 100 Å thick for accurate measurement.
At room temperature, the method exhibits a mean error of 2-5%, which can be minimized to less than ±0.5% under optimal conditions The sample is rotated at a constant speed, while the probe rotates at double that speed, ensuring that for every angle θ the sample rotates, the probe rotates by an angle of 2θ.
Table 3.9 illustrates the characteristic peaks of 2H-MoS2, which are identified at the (002) plane (2θ = 14.4°), (100) plane (2θ = 32.7°), (101) plane (2θ = 33.5°), (103) plane (2θ = 39.6°), and (110) plane (2θ = 58.3°) Notably, the absence of the (002) and (103) planes, along with the appearance of a new (004) plane at 17.8°, indicates the presence of 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 technique that explores a system's vibrational, rotational, and low-frequency modes through the inelastic scattering of monochromatic light, such as lasers This method reveals energy shifts in laser photons when they interact with molecular vibrations or phonons, providing insights into the system's vibrational modes A simplified energy diagram illustrates these concepts effectively.
In this doctoral research, the structural information of the prepared materials was analyzed using a Labram HR VIS Micro Raman spectrometer at room temperature, employing a He-Ne laser with an excitation wavelength of 632.8 nm and a 300 lines/mm grating To prevent oxidation of the MoS2 samples, the laser power was maintained at 1 mW.
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 is crucial for enhancing our understanding of material structures and behaviors It provides atomic-scale imaging of atoms and defects, leading to significant breakthroughs in modern materials science The continuous advancement of microscopy hardware and innovative techniques further supports this field.
Electron microscopy, particularly through techniques like scanning electron microscopy (SEM), plays a crucial role in advancing our understanding of various materials SEM utilizes a high-energy electron beam to scan samples in a raster pattern, generating images by interacting with the sample's atoms This interaction produces signals that provide valuable insights into surface topography, composition, and 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 powerful technique for examining the morphology, crystal structure, and electronic properties of various materials, offering significantly higher resolution than light microscopy due to the small de Broglie wavelength of electrons In TEM, a beam of electrons is transmitted through an ultra-thin specimen, creating an image that is magnified and focused onto an imaging device or detected by a sensor For this study, samples were prepared by drop casting nano MoS2 or nano MoS2/CNTs dispersions onto a carbon-coated copper grid, followed by a 24-hour drying process at 50 °C under vacuum The morphology and structure of the samples were analyzed using a JEOL-TEM-1400 at the National Key Laboratory of Polymer and Composite Materials (PCKLABS) at Ho Chi Minh City University of Technology, operating at an acceleration voltage of 200 kV in bright field image mode.
X-ray Photoelectron Spectroscopy (XPS) is a surface analysis technique that uses monoenergetic soft X-rays to irradiate a solid in a vacuum, allowing for the analysis of emitted electrons based on their energy This method is particularly effective when surface chemistry or thickness is crucial for the functionality and safety of materials XPS delivers a comprehensive analysis of the elemental composition, making it a reliable choice for detailed surface characterization.
X-ray photoelectron spectroscopy (XPS) is a non-destructive analytical technique that utilizes soft X-rays to induce photoelectron emission from a material's surface, allowing for the determination of the empirical formula, chemical state, and electronic state of surface elements The resulting spectrum, which plots the number of detected electrons against their kinetic energy, is unique for each element, and for mixtures, it approximates the sum of individual peaks Due to the short mean free path of electrons in solids, XPS is particularly sensitive to the top few atomic layers, making it an effective method for surface chemical analysis Common X-ray sources used in XPS include Mg Kα (1253.6 eV) and Al Kα (1486.6 eV), which have limited penetration power in solids.
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)
∅ 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
X-ray Photoelectron Spectroscopy (XPS) provides crucial insights into the surface electronic states and composition of final products The MoS2 nanopowders were analyzed using an XPS ESCALAB250 system with Al (Kα) radiation under vacuum conditions A comprehensive survey scan was conducted across a binding energy range of 0-1000 eV to examine the Mo and S elements, with a focus on specific energy windows The C 1s peak was calibrated at 284.6 eV to ensure accurate binding energy measurements, while maintaining equal full width at half maximum (FWHM) values for the spin-orbit splitting doublet.
X-ray photoelectron spectroscopy (XPS) is an effective method for determining the oxidation state of metals on material surfaces, such as molybdenum sulfide nanomaterials Each oxidation state of molybdenum results in unique Mo 3d and S 2p spectral components, with the 3d orbital electrons typically exhibiting stronger peak intensities compared to other orbitals The high-resolution spectra for the Mo 3d orbitals, both before and after curve peak fitting, are presented in the XPS results summarized in Table 3.6.
Table 3.11 XPS binding energies (eV) of the different phases and bonding in molybdenum compounds
High-resolution XPS Mo 3d spectra of Mo +4 in MoS2 reveal two primary peaks at approximately 229.1 eV and 232.3 eV, corresponding to the Mo 3d5/2 and Mo 3d3/2 orbitals Additionally, a peak around 226.1 eV, attributed to S 2s, indicates the presence of Mo-S bonds in MoS2 The binding energies of 162.0 eV and 163.3 eV represent the S 2p3/2 and 2p1/2 orbits, respectively Notably, the emergence of the 1T phase MoS2 is associated with lower binding energies compared to the 2H phase The S 2p3/2 and S 2p1/2 peaks in the 1T phase shift to lower energy at around 162.2 eV and 163.4 eV, respectively, compared to 2H-MoS2 Similarly, the Mo 3d peaks for 1T-MoS2 at approximately 231.1 eV and 228.1 eV also shift to lower energy than those of 2H-MoS2.
Electrochemical measurements for catalysts
Electrochemical analysis utilizing Tafel equations is essential for evaluating and characterizing electrocatalysts in the hydrogen evolution reaction (HER) The literature primarily focuses on two key parameters derived from the Tafel equation: the Tafel slope and the exchange current density Additionally, the Tafel constant is suggested to represent the HER onset potential (Vonset) Notably, when the Tafel slope and exchange current density (j) are equal, the significance of the Tafel constant is highlighted.
49 the defining parameter between two electrocatalysts Tafel constant becomes complementary to Tafel slope from an electrochemical standpoint
To prepare the working electrode, 10 mg of the catalysts from Table 3.12 was mixed with 2.5 mg of polyvinylidene fluoride (PVDF) and ultrasonically dispersed for 60 minutes at 50 °C in a solution containing 1 µL of 0.1 wt% polyvinyl alcohol (PVA) and 1 mL of ethylene glycol (EG) Following this, 50 µL of the resulting homogeneous suspension was applied to a glassy carbon electrode (GCE) with a diameter of 3 mm.
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
The study utilized 50 catalysts with a diameter of 3 mm, platinum electrodes (99.99%) measuring 10 x 10 mm, and a saturated Cu/Cu²⁺ reference electrode in CuSO₄ The potential of the saturated Cu/Cu²⁺ in CuSO₄ was established at 0.34 V versus the Normal Hydrogen Electrode (NHE) All final potentials were calibrated to NHE using the Nernst equation: \( E_{NHE} = E_{Cu/CuSO₄} + 0.34 \, \text{V} + 0.059 \times \text{pH} \) Linear sweep voltammetry (LSV) was conducted over a potential range from 0 to 1 V (vs NHE) at a sweep rate of 1 mV/s.
Figure 3.7 Three-electrode configuration for electrochemical tests
Voltammetric cycles were conducted at a scan rate of 1 mV s\(^{-1}\) prior to the polarization curve recording, with current densities normalized by the electrode's geometric area To reduce the electrical double layer charging current, linear sweep voltammetry (LSV) was performed at the same scan rate The uncompensated resistance was corrected using iR compensation Tafel slope values, derived from the Tafel equation, indicate the relationship between the increase in the hydrogen evolution reaction (HER) rate of MoS\(_2\) and the 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 demonstrate the logarithmic relationship between electrochemical current density (j) and overpotentials, as described by the Tafel equation (\$η = a + b \log j\$) This equation allows for the determination of key parameters such as the Tafel slope (b) and the exchange current density (j0) at zero overpotential The Tafel slope indicates the intrinsic catalytic activity of the catalyst, with an effective catalytic material typically exhibiting a high j0 and a low Tafel slope.
Exchange current density represents the reaction rate at reversible potential, where the forward and reverse reactions are in equilibrium We determined the j0 values using the linear section of the polarization curve at small over-potentials in a H2-saturated 0.5 M H2SO4 solution The exchange current density can be calculated using the equation provided.
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)
A mixture of 100 mg of prepared materials, 10 mg of multi-walled carbon nanotubes (MWNTs), and 100 mL of 10% polyvinyl alcohol (PVA) was sonicated for 40 minutes to create a uniformly dispersed solution This solution was then dried at 80 °C until it reached approximately 1 g, resulting in a homogeneous slurry suitable for anode paste The slurry was applied to copper foil using a doctor blade and subsequently dried in an oven at 80 - 100 °C overnight under vacuum The anode paste samples were fully dried before undergoing morphological and structural evaluation through X-ray diffraction (XRD) and scanning electron microscopy (SEM).
(2) Anode current collector treatment (Cu foil)
A circular sheet of copper foil with a diameter of 10 mm was prepared by cutting and then gently polishing its surface using 3 µm grain sandpaper to remove the copper oxide layer Following this, the sheet was washed with absolute alcohol to eliminate any rust and oil, and the anode current collector was subsequently dried under vacuum.
(3) Finishing anode electrode for testing
Before applying the anode paste, weigh the current collector After coating with copper foil, weigh the sample again The electrode undergoes hot rolling using a two-axis rolling mill, maintaining a fixed distance of 200 µm between the axes, to achieve a uniform coating layer thickness of approximately 100 µm The rolling process is conducted at a temperature of 120°C until the mass stabilizes, followed by the removal of any residual anode paste, ensuring accurate mass measurement 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 adhered to a copper foil current collector for the anode, consisting of active materials, conductors, and a binder To enhance the electronic conductivity, which is typically low in most active materials, conductive materials such as multi-walled carbon nanotubes (MWNTs) are added The binder used is a long-chain polymer, Polyvinyl alcohol (PVA), which effectively holds all electrode components together on the current collector The electrochemical properties of the electrode samples listed in Table 3.13 will be examined 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
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)
Cyclic voltammetry (CV) is a powerful electrochemical technique widely utilized to assess the performance of lithium-ion batteries In a CV experiment, the working electrode potential is linearly ramped over time, known as the scan rate (V/s) This technique involves applying potential between a reference electrode and a working electrode within a three-electrode system, while measuring the current between the working and counter electrodes In this dissertation, CV measurements were conducted using a two-electrode system, where a lithium anode acted as both the counter and reference electrode The resulting data was plotted as current (\$i\$) versus potential (\$E\$), with current peaks indicating the reduction or oxidation potentials 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
Electrochemical measurements were conducted using a Swagelok cell and the PARSTAT 2273 (AMETEK) instrument The Swagelok cell, constructed from Teflon (PTFE), features an outer diameter of 25.4 mm and an inner diameter of 10 mm, with stainless steel SS316 caps serving as electrodes The cell configuration is illustrated in a schematic diagram It includes the as-prepared electrode as the working electrode, lithium foil as the counter electrode, and Li/Li⁺ as the reference electrode A cellulose acetate (OE67 Whatman) membrane with a pore size of 0.45 µm acts as the separator, while 1M LiPF6 in dimethyl carbonate (DMC) serves as the electrolyte The preparation steps for the working electrode were meticulously followed.
• 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 conducted in an argon-filled glovebox with relative humidity below 1% Cyclic voltammograms were recorded over a voltage range of 0 to 3.5 V (versus Li/Li⁺) at a scanning rate of 1 mV/s, with the electrode arrangement illustrated in Figure 3.11.
Figure 3.11 Electrode arrangement for electrochemical measurements
The electrode material's capacity was determined through galvanostatic charge and discharge testing at a constant current density The charge and discharge capacities (Q) are calculated using a specific 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
In lithium-ion battery testing, the performance of anode materials is assessed using C-rate metrics, which evaluate the electrode's capacity at varying charge and discharge current densities Charging or discharging the cell at a C/n rate indicates that the cell is fully charged or discharged within n hours.
Electrochemical impedance spectroscopy (EIS) is a valuable technique for analyzing complex coupled electrochemical processes, including electron and mass transfer By applying a small amplitude signal ranging from 5 mV to 10 mV across a frequency spectrum of 100 mHz to 2 MHz, researchers can investigate how resistance varies with frequency through the monitoring of current responses.
The charge-transfer resistance (𝑅ct) of a lithium-ion battery, which indicates the speed of the electrode reaction, can be determined through electrochemical impedance spectroscopy (EIS) measurements The impedance Nyquist curve typically features a compressed semicircle in the medium-frequency region, representing the charge-transfer resistance, and an inclined line in the low-frequency range, associated with Warburg impedance The relationship for Warburg impedance in lithium-ion batteries is given by \( W = \sigma \omega^{-1/2} - j\sigma \omega^{-1/2} \), where \( \sigma \) is the Warburg coefficient, with further details provided 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)
Impedance is frequency-dependent, with Warburg impedance being small at high frequencies due to minimal diffusion distance for reactants Conversely, at low frequencies, increased diffusion distance results in higher Warburg impedance On a Nyquist Plot, this impedance manifests as a diagonal line with a 45° slope, while a Bode Plot shows a corresponding 45° phase shift However, this representation of Warburg impedance is only applicable when the diffusion layer is infinitely thick; if the layer is bounded, the impedance behavior at lower frequencies deviates from this equation.