Carbon
Carbon (C) is a chemical element, which is capable of forming many allotropes due to the various hybridized state The well-known carbon allotropes include diamond (sp 3 ) and graphite (sp 2 ) In recent decades, new carbon allotropes have been discovered, such as fullerene (0D) [78], [79], [80], carbon nanotube (1D) [81], and graphene (2D) [7], which exhibit excellent electrical, optical and mechanical properties that open the door to carbon nanomaterials Schematic illustration of carbon allotropes is shown in Fig 2.1
(a) Allotropes of carbon [7] From left to right: wrapped into buckyballs, rolled into nanotubes and stacked into graphite, (b) HRTEM images of graphene [82].
Graphene
Graphene [7], [83], an atomic layer of sp 2 bonded carbon atoms in a hexagonal lattice, is one of the most studied 2D materials Many fascinating properties of graphene such as high electron mobility (~200000 cm 2 V −1 s −1 ) [8], massless Dirac Fermions [84], quantum Hall effect [85], [86], [87], superconductivity [88] with Moiré pattern, large specific surface area (~2600 m 2 g −1 ) and excellent thermal conductivity (~5000 W −1 K −1 ) [89], all of which make it a promising candidate for various applications including gas sensors, batteries, supercapacitors, fuel cells, photovoltaic devices, solar cells and biosensors Crystalline structure and characteristic of graphene are shown in Fig 2.2
(a) Graphite crystal, (b) atomic structure of 3D graphene [90]; (c, d) TEM and HRTEM images of graphene [91]; (e, f) Atomic model and energy band of graphene [92]
For graphene, the atomic structure of the real space and the first Brillouin zone of the reciprocal space are shown in Fig 2.3 [93] According to the tight binding calculations [93], the real space lattice vectors a 1, a 2 and the reciprocal lattice vectors b 1, b 2 of the graphene can be written as Eq (2.1) and (2.2), where a = 1.42 Å is the graphene lattice constant [94]
The two points K and K′ at the corners of the graphene Brillouin zone are named Dirac points [95] which are referred to as graphene’s two valleys Their positions in momentum space are given by Eq (2.3):
The band structure of graphene depends on the number of layers and stacking orders
The full band structure of graphene is shown in Fig 2.3(c), as well as the zoom-in of the band structure close to one of the Dirac points (K or K’ point in the Brillouin zone) The conduction band and valence band of graphene meet at the Dirac points and the energy and momentum of monolayer graphene have a linear dispersion relationship near the Fermi level Electrons propagating through graphene’s honeycomb lattice effectively lose their mass, which can be regarded as quasi-particles Thus, graphene exhibits remarkable carrier mobility, with reported values of 2×10 5 cm 2 V −1 s −1
Figure 2.3 The electronic band structure of monolayer graphene
(a) The real-space atomic structure of graphene [93], (b) The first Brillouin zone of the reciprocal space for graphene [93], (c) Electronic dispersion (Dirac cone) in the honeycomb lattice [93].
Graphene oxide
Graphene oxide (GO) is obtained by treating graphite with strong oxidizers The bulk
GO disperses in basic solutions to yield monolayer sheets, known as GO analogy [96] The GO layer consists of tetrahedrally bonded sp 2 and sp 3 carbon atoms in nano clusters, which results in a random distribution of oxygen-containing functional groups along the graphene sheet [11] The functional groups on GO, such as hydroxyl (‒OH), carboxyl (‒COOH) and carbonyl (C=O) groups, expand the interlayer distance and contributes to
11 a hydrophilic surface [97] Graphene oxide layers (Fig 2.4) [98] can be partially reduced (rGO) and functionalized to desired characteristics by removing oxygen-containing groups, while preserving the structure
Figure 2.4 Lerf-Klinowski model of graphene oxide
Because of the abundant oxygenic groups, rGO has a good dispersion in solution Moreover, rGO is suitable for large-scale production and being used in transparent conductive film [99], energy storage [13], [100] and biosensors [101] The shortcomings of rGO are small-size, uneven thickness and poor conductivity.
Molydenum disulfide (MoS 2 )
Belongs to transition metal dichalcogenides (TMDs) groups [1], MoS2 has lamellar structure similar to that of graphene An individual layer of MoS2 is compacted with Mo and S atoms forming 2D S–Mo–S covalently bonded tri-layers with the adjacent layer spacing of ~0.615 nm [102] Interestingly, different coordinating of Mo and S atoms with stacking order in each layer leading to the existence of distinct polymorphs of MoS2 such as one-layer-stacked trigonal 1T-MoS2 [103] and two-layer-stacked hexagonal polymorph 2H-MoS2 [69] In 2H structure, the unit cell parameters are a = b = 3.15 Å, c = 12.30 Å, with stacking sequence (S–Mo–S’) is ABA or BAB while the unit cell parameters of 1T structure are a = b = 5.60 Å, c = 6.30 Å, the stacking sequence (S–Mo–S’) is ABC Crystalline structure of 2D MoS2 is presented in Fig 2.5 Unlike 2H phase which is semiconductor with band gap in range of ~1.2–1.9 eV [26] (as seen in Fig 2.7), the 1T phase is exhibits metallic characteristic (Fig 2.6) [43] These two phases can easily convert one to the other via intra-layer atomic plane gliding [104], which involves a transversal displacement of one of the S or Mo planes
(a) MoS 2 crystal; (b) Atomic model of MoS 2 layers [4]; (c, d) TEM and HRTEM [2] images
MoS 2 ; (e, f) Atoimic model and energy band diagrams of MoS 2 [92]
Figure 2.6 Polymorphs and atomic model structure of MoS2
(a) Three types of polymorphs of MoS 2 1T–Trigonal; 2H–Hexagonal; 3R–Rhombohedral [105]; (b) Atomic model structure of 2H- and 1T-MoS 2 [104]
Metastable nature of 1T phase leading to its easily conversion to stable 2H phase under suitable heat treatment, typically under ~300 °C [42] Metallic 1T phase of MoS2 is attracting much attention due to its high electrical conductance [43], [106], [107] and potential applications in supercapacitors [43], thermoelectric energy harvesting [108], and memristors [109] MoS2 shows high mechanical property with Young's modulus of
~270 GPa [27], and thermal stability up to 1100 ºC [4] However, low mobility of MoS2, about ~0.5–3.0 cm 2 V –1 s –1 [1] currently limits it from many applications
Figure 2.7 Thickness dependent band gap of 2D MoS2
Electronic band structures of MoS 2 calculated by DFT method [26].
Optical property of 2D MoS 2 and graphene
Among many interesting properties of monolayer MoS2, graphene and MoS2/C NC, the ability to continuously modulate the electronic structure and optical response is the most fascinating, regarding its atomic thickness and high surface-to-volume ratio, which make thier properties highly sensitive to external perturbations MoS2 exhibits a thickness-dependent band gap (Fig 2.7) [26] that changes from indirect band gap of
~1.2 eV for bulk MoS2 to direct band gap of ~1.9 eV in monolayer form, such indirect- to-direct gap transition due to quantum confinement results in giant enhancement in PL quantum yield [23], high Seebeck coefficient [25], large exciton effect [110] An exciton [111], which is also called free electron-hole pair or photoexcited state, is a bound quasi- particle state between one electron and one hole Excitons in monolayer MoS2 are formed by strong Coulomb interactions between electrons and holes which are excited across the band gap of MoS2 Furthermore, owing to the absence of inversion symmetry,
14 strong spin–orbit interactions split the valence band states at the K and K’ points by about ~0.16 eV In addition, this splitting is in the opposite direction in the two valleys, leading to opposite spin polarizations in the K and K’ valleys that allowing optical control (by circularly polarized light) of the charge carriers population separately in each of the two valleys Very recently, the optical properties and electronic structure of excitons in monolayer MoS2 have been investigated theoretically [21], [34], [112], and experimentally [23], [26] in details
Graphene has attracted much interest and attention in optoelectronic applications due to its high carrier mobility, broad absorption spectrum, and photocurrent generation with fast response time The excellent photocurrent conversion of graphene with a quantum efficiency approach to 100% has been previously reported [113] However, graphene’s low light absorbs capability of ~2.3% [114] in the visible spectra that limit it from optical applications Although various approaches such as combine graphene with hBN in moiré superlattices plasmons [115], microcavities [116], and metallic plasmons [117] have been employed to enhance light absorption in graphene, the photogain of graphene- based photodetectors still has more room to improve
The optical or photoresponse mechanisms in graphene-based devices have been identified, including the photovoltaic effect [18], [19], thermoelectric Seebeck effect [118] and the bolometric effect [119] The high carrier mobility in graphene offers a great possibility for fabricate high photoresponsivity devices Therefore, assembling graphene with various 2D layers into heterostructures or composite in order to tailor new properties has been proposed and observed in tunneling field-effect transistors [70], [120] The photoresponse efficiency of graphene devices can be greatly enhanced by exploiting a vertical geometry [121], for instance, graphene/2D semiconductor heterostructural stacking, where the whole graphene platform can be used as a junction.
Optical property of MoS 2 /C NC
In the MoS2/C NC, the MoS2 nanocrystalline phases play an important role in absorbing electromagnetic radiation and producing charge-carriers or photo-excited electron-hole pairs These electron-hole pairs are separated at the MoS2–graphene interfaces, where the electrons or holes move to graphene due to the presence of applied electrostatic field,
15 charged impurities or adsorbates Its optical properties could be engineered by controlling the layer thickness [70], [122], strain and temperature-induced lattice variations [123]; defect and doping engineering [76], [124]; substrate effect [5], and semiconductor-to-metal transitons [32], [75], [104]
Temperature effect on optical spectra of MoS2/C NC: Temperature can be used to engineer the band gap of atomic-layered MoS2 by incorporating thermal expansion of the lattice [125] Considering the quasiparticles play a key role at low temperature, the switching between quasiparticles is also an important influence on optical properties of atomic-layered MoS2 Generally, the temperature effect may be distinguished into three aspects: PL intensity, peak shift and broadening
Tuning optical properties of monolayer MoS2 via doping: Doping is known to be a convenient route to modify the carrier density in traditional semiconductors as well as 2D materials Promising approaches have been employed to investigate the effects of doping on monolayer MoS2 that can be distinguished into categories: (i) solution-based chemical doping [62], [73], [76], (ii) gas physisorption on the 2D surface [12], (iii) defect formations within the S-Mo-S layer by thermal annealing and/or plasma treatments [117], (iv) electrical doping by FET devices [5], [105], (v) the intercalation in the space between the atomic MoS2 layer and substrate [30], [120], and (vi) the intercalation with another 2D material (vdW heterostructure) [126], [127], [128]
Tuning optical properties of monolayer MoS2 via coupling with other vdW to form heterostructures or composite structures: VdW heterostructures can also be used to tune the electronic and optical properties of layered MoS2, resulting from the charge transfer and interlayer coupling between two atomic layers [129] Among them, MoS2/graphene vdW heterostructure [130] is remarkable due to the direct band gap of monolayer MoS2 and the high carrier mobility of graphene A strong PL quenching (> 85%) for monolayer MoS2 [23], [34], was observed in the experiments for the epitaxial MoS2/graphene heterostructure, which was attributed to the interfacial charge transfer from MoS2 to graphene The mechanism of the PL in the MoS2/graphene heterostructure is much more intriguing, because of the emerging of interlayer exciton transition
Synthetic approaches for MoS 2 /C NC
Synthesis of graphene and 2D MoS 2 nanocrystals
(1) Mechanical exfoliation: The exfoliation method for preparing graphene includes mechanical exfoliation [55], [131] and chemical exfoliation The mechanical exfoliation method is known as the so-called “scotch tape method” [34], [47], [83] The number of graphene layers can be controlled to a limited degree via the number of repeated peeling steps and then the graphene layers can be transferred to the targeted surfaces for the further study This method is ideally suited for the investigation of the fundamental properties of graphene because it can generate monolayer graphene sheets of high quality easily and low-cost However, the poor reproducibility, low-yield and the labour- intensive processes required make it difficult to mass production and thus lead to be used predominantly only for fundamental studies
(2) Chemical exfoliation: The chemical exfoliation method [132], [133] is using molecules or atoms to intercalate into the graphite layer, thus weakening the interlayer vdW interactions and resulting in the larger distance between graphite layers Further ultrasound process generates graphene solution [134] Compared with the mechanical exfoliation method, the graphene prepared by chemical exfoliation is low-cost, high- yield and can be applied in coating, printing, transparent conductive film and energy storage The disadvantage of this method is that prepared graphene is small-size, uneven thickness and poor conductivity
(3) Chemical vapor deposition (CVD): CVD is a chemical process used to produce high quality materials [135], [136] In typical CVD process, the substrate is exposed to one or more precursors, which react and/or decompose on the substrate surface to produce the desired materials Since the R S Ruoff group first synthesized a uniform graphene film of centimeter size on the surface of Cu foil using CVD [137], it opened a new era of graphene preparation.
Synthesis of MoS 2 /C NC
There are several approaches for synthesizing MoS2/C NC include ex-situ and in-situ strategies [52] In the ex-situ synthetic strategy, each component (MoS2, graphene or
GO) are prepared separately in advance, then the composites are fabricated by layer-by- layer assembly [53], epitaxial growth [70], liquid phase exfoliation [55], [138] and chemical exfoliation methods [132] Research team of Fu et al [139] using microwave technique to synthesize MoS2/NC for supercapacitor application Chang et al [126] used
L-cysteine asisted liquid phase synthesis of MoS2/graphene composite While David et al [140] synthesizes MoS2/rGO composite by liquid dispersion of nano plates MoS2 and rGO in distilled water is filtered in vacuum conditions forming thin layered-structure membrane Despite many advantages include low cost and scalable production, the ex- situ synthetic strategy requires multiple complex and time-consuming steps to prepare raw component materials and difficult to control the formation of the composites
For the in-situ strategy, the synthetic process involves ionic reactions such as sol–gel
[133], hydrothermal [61], [64], [63], or solvothermal methods [60], [133], which hold capability of synthesizing nanoscale materials with uniform dispersion and architectures compared to the ex-situ strategies Therefore, the in-situ synthetic strategy is receiving large amount of attention, such as the preparation of GO and g-C3N4, to synthesize
MoS2/GO and MoS2/g-C3N4 composite materials [62], [141] Wang et al [71] used the one-step liquid phase process to synthesize MoS2/graphene nano-sheets composite based on GO and Na2MoO4.2H2O in NH2CSNH2 aqueous solution Zheng et al [142] synthesized MoS2 on the GO sheets from (NH4)2MoS4 in DMF solution by using solvothermal method Although hydrothermal strategies are widely used for the preparation of MoS2/graphene composite, these methods often require long reaction time from several hours to several days as shown in Table 2.1 The effect of hydrothermal reaction temperatures on the crystallization and morphology of pristine
MoS2 has been reported with flower-like MoS2 nanoflakes (~160–240 °C) [66], MoS2 particles (~160–180 °C) [143], MoS2 nanopetal (230 °C) [144], [145], thin flakes in
(~200–220 °C) [146], and large particles at higher temperatures of 200–350 °C [65] In a low temperature range of 120–150 °C, only amorphous MoS2 nanospheres were observed However, the hydrothermal crystallization condition of MoS2 phases in the presence of graphene matrix is unexplored Therefore, further understanding of phase transition driven by temperature is essential to optimize the material properties
Table 2.1 Reported methods for synthesis of MoS2/C NC
Material Method Precursor Experiment parameter
GO suspension in acetonitrile, Mo(dedtc) 4
MoS 2 , 4–8 layers, lateral size of several micrometers
GO, Na 2 MoO 4 2H 2 , CS(NH 2 ) 2
200 °C, 24 h MoS 2 layers, thickness ~3 nm, size ~200 nm
200 °C, 24 h MoS 2 13–20 layers, lateral size of micrometers
MoS 2 /C Hydrothermal and carbonization method
PANI, MoO 3 , KSCN 210 °C, 24 h, annealed 4 h, in N 2 at 500 °C,
Graphite electrode, (NH 2 ) 2 MoS 2 , KOH (5%, wt%)
In addition, phase transition provides an important technological value for modifying materials properties without adding any other species In solid matter, phase transitions involve collective atomic displacements Previously reports [46], [47], [144] present interesting memristive and electrical properties of low-dimension MoS2/C NC However, it is intriguing to continuously modify the crystalline structure, morphology and electronic structure of MoS2 crystalline phases as a component of MoS2/C NC for electronic devices, supercapacitor or battery applications Furthermore, there is still remaining a huge gap for developing simple, reliable and economical approaches for such combination of MoS2 and graphene
Process engineering for synthesis of MoS 2 /C NC
Hydrothermal method
Hydrothermal synthetic techniques involve the synthesis of materials under heated precursor solutions with high vapour pressure and higher temperatures than the boiling point of water This condition allows an enormous degree of freedom in terms of the available chemistry The selection of solvent, pH and reagents invites the design of elegant reaction systems The moderate reaction temperatures permit the synthesis of metastable phases that may not be accessible by other approaches In laboratories, the hydrothermal reactions are usually carried out in a Teflon container enclosed in a stainless autoclave These conditions help controlling the reactions parameters that capable of synthesizing the desired material at a shorter time and at lower temperatures when compare to the solid-state reaction technique In addition, the desired nanostructure materials can also be obtained with definite particle size and morphology by controlling the synthetic conditions This technique is also useful for growing single crystals of various organic, inorganic and hybrid materials While these conditions are relatively mild when compared with ceramic methods, it is often the case that the products obtained are highly nanocrystalline in nature, without further processing steps.
Properties of supercritical water and reactor design
At temperatures above 100 °C, particularly on approaching the critical point of 374 °C and 22.4 MPa, water exhibits unusual properties At high temperatures, the density and polarity of water are vastly decreased, and this is more pronounced upon reaching the
20 critical point Such high temperatures permit reactions that would not otherwise be possible Furthermore, the dissociative behaviour changes drastically with temperature.
Nucleation and growth in hydrothermal condition
There are many factors to be considered in the hydrothermal synthesis of nanomaterials The ideal for most purposes is to obtain nanoparticles with controlled sizes and a narrow size distribution The LaMer model in Fig 2.8 [154] for the crystallization and growth of monodisperse colloids offers a simple conceptual framework for considering the formation of nanoparticles
Figure 2.8 The LaMer model of nucleation and growth
Upon mixing, or otherwise initiating a reaction, there is a build-up of precursors (Regime I) creating a degree of supersaturation When the level of supersaturation surpasses the critical nucleation threshold, nucleation occurs (Regime II), reducing the degree of supersaturation If the rate of nucleation outstrips the rate of precursor formation, the precursor concentration will drop back below the nucleation threshold Nanoparticle growth may then proceed by a number of pathways: monomer addition, in which additional precursor units deposit onto the preformed nuclei from solution; Ostwald ripening, whereby energetically disfavoured small nuclei redissolve and deposit onto more thermodynamically favourable larger nuclei; and coalescence, in which multiple nanoparticles come together and fuse (Regime III) This framework allows us to consider methods for control the size and size distribution
X-ray diffraction (XRD)
X-Ray diffraction (XRD) is a rapid analytical technique primarily used for phase identification and quantification of crystalline solids and is a non-destructive method It can be used for determination of lattice parameters, space group, crystallite size, degree of crystallinity, residual stress, etc The analyzed material is ground into a fine powder, homogenized, and the XRD patterns are recorded which determines the average bulk composition of the sample The XRD technique relies on the wave-particle duality of X-rays as described in Fig 2.9(a)
Figure 2.9 Diffraction of X-rays from crystal lattice and XRD diffractometer
(a) XRD technique principle, (b) Bruker D8 Advance XRD diffractometer is in operation at Biomass Lab., HCMUT, VNU-HCM
The wavelength of X-rays is similar to the inter-layer spacing d (hkl) of the atomic layers in crystalline solids which enables the diffraction of X-rays by crystal lattices of the material These diffracted X-rays provide information about the arrangement of atoms in the crystal lattice Interaction of monochromatic X-rays with a target material results in scattering of these X-rays which may undergo constructive and destructive interference The diffraction of X-rays is determined by the Bragg’s law [155]:
2 sin n= d (2.4) Where d (hkl) is the interplanar (hkl) spacing, nm; θ is the angle between incident X-ray beam and the atomic plane, º (degree);
22 λ is the wavelength of X-ray radiation (Cu Kα =1.5406 nm); n is an integer known as order of reflectance
The inter-planar spacing, d (hkl) can be calculated from the Bragg equation The International Centre for Diffraction Data (ICDD) offers the database of PXRD patterns of a large number of substances like inorganic, organic, polymer and minerals in the form of Joint Committee on Powder Diffraction Standards (JCPDS) files In addition, the crystallite size along a specific [hkl] direction of crytalline phase can be calculated using Scherrer’s equation with accepted shape factor of K = 0.94–1.18 for cubic or spherical shape The Scherrer’s equation is given as follows:
= (2.5) where β is the full width at half maximum (FWHM), rad;
D (hkl) is the average crystallite size, nm.
Raman spectroscopy
Raman Spectroscopy technique is based on Raman scattering, in which light is inelastically scattered by an atom or molecule It probes the vibrational and rotational modes of a molecule An incident photon is absorbed and is reemitted by the atom or molecule at a shifted frequency The energy difference between the emitted photon and the incoming photon is called the Raman shift and it is normally presented in reciprocal centimeters (cm −1 ) The reemitted photon may have a higher (anti-Stokes scattering) or lower (Stokes scattering) frequency than the incident photon (Fig 2.10) We usually measure the Stokes peaks as they are typically more intense Since each molecule has its unique vibrational modes, this technique is widely used as a way to nondestructively characterize a material It gives information about the atomic structure of the material, bond strength, and bond environment This leads to that Raman spectroscopy is a widely used method for characterization, especially of graphene
Figure 2.10 The basic principle of Raman spectroscopy
(a) Three types of scattering processes that can occur when light interacts with a molecule, (b) Perrin-Jablonski diagram of molecular energy level.
High resolution Transmission electron microscopy (TEM, HRTEM)
Transmission electron microscopy (TEM) is a microscopic technique in which a beam of high energy electrons (200–300 kV) is transmitted through an ultra-thin specimen which interacts with the specimen as it passes through This interaction produces an image which is enlarged and focused onto an imaging device such as a fluorescent screen or a photographic film High-resolution TEM (HRTEM) produces a high-resolution image from the interaction between the specimen and energetic electrons in a vacuum chamber The electrons then pass through a series of electromagnetic lenses down the column and make contact with the screen where the electrons are converted to light and form an image of the specimen By varying the strength of these lenses, magnification of the image can be adjusted These images provide information on the structure, texture, shape and size of the sample.
Field emission Scanning electron microscopy (FESEM)
Scanning electron microscopy (SEM) is a microscopic technique that uses electron beams for imaging an object Upon the impingement of electron beam on the specimen, it interacts with atoms on the surface which gives rise to different types of radiation When a part of the primary electrons striking the specimen, surface is deflected through large angles and re-emitted without energy loss (elastic scattering) from the surface, they
24 are called as backscattered electrons Since they arise from atomic nucleus interactions, the intensity of the backscattered image depends upon the atomic number of the elements present in specimen Hence heavy elements are able to backscatter the electrons more strongly and appear brighter in the image while the lighter elements look less bright Secondary electrons are low energy electrons (< 50 eV) arising from electron-electron interactions (inelastic events) and are emitted from the top 5–10 nm zone of excitation area These secondary electrons are collected by a secondary electron detector (SED) and constitute the basis for the three-dimensional imaging of a specimen surface with a scanning electron microscope The secondary electrons are most valuable as they provide information about the morphology and topography of samples while the backscattered electrons used for illustrating contrasts especially in multiphase samples for phase discrimination.
Energy Dispersive X-ray Spectroscopy (EDX)
While there are several other types of signals that are generated by the primary electrons, X-ray signal is typically the only other signal widely used in SEM This technique called as energy dispersive X-ray spectroscopy (EDS or EDX) is used for the elemental analysis of a sample X-rays are generated when the primary electrons undergo inelastic collisions with the sample resulting in the excitation of electrons in discrete orbitals As these excited electrons return to lower energy states, they yield X-rays which exhibit characteristic wavelength and energy patterns of the elements present which leads to its elemental composition.
X-ray Photoelectron Spectroscopy (XPS)
X-ray photoelectron spectroscopy (XPS) is a surface analytical method based on the photoelectric effect where X-ray photons (Al Kα (~1486 eV) or Mg Kα (~1253 eV)) interacts with core level electrons of an atom and a photoelectron is emitted (as seen in Fig 2.11) The photoelectron has a kinetic energy (E BE) corresponding to the difference of the X-ray photon energy (h) and the binding energy of the electron to the atomic nucleus (E BE) with a correction for the work function (ϕ) which is the energy difference between the Fermi level and the vacuum level The analysis is performed by measuring
25 the electrons kinetic energy and subsequently calculating the electrons binding energy by using Eq 2.6 [156]
Figure 2.11 Principle of X-ray Photoelectron Spectroscopy
(a) Schematic illustration of the X-Ray photoelectron generation, (b) Typical XPS spectrum
(ref from Thermal Fisher Scientific Inc.)
Since all electrons has a specfic binding energy, which also differs from the chemical environment, a spectrum of E BE and the intensity count provides information on the elemental composition of the sample as well as the oxidation state and bonding type of the materials
The Current-Voltage characteristic (I – V) curves of an electrical device or component, are a set of graphical curves which are used to define its operation within an electrical circuit As its name suggests, I – V characteristic curves show the relationship between the current I flowing through an electronic device and the applied voltage V across its terminals The I – V characteristic curves are generally used as a tool to determine and understand the basic parameters of a component or device and which can also be used to mathematically model its behaviour within an electronic circuit
The 4-point probe is employed to measure the resistivity of any semiconductor material Basically, the 4-point probe setup consists of four equally spaced tungsten metal tips with finite radius Each tip is supported by springs on the other end to minimize sample damage during probing The four metal tips are part of an automechanical stage which
26 travels up and down during measurements A high impedance current source is used to supply current through the outer two probes; a voltmeter measures the voltage across the inner two probes to determine the sample resistivity Typical probe spacing s ~ 1 mm The volume resistivity (ρ) and sheet resistance (σ) are calculated as follows: ln 2
= = (2.8) where: ρ is volume resistivity, Ω cm;
I is the source current, A; t is the sample thickness, cm; k: a correction factor based on the ratio of the probe to wafer diameter and on the ratio of wafer thicknessto probe separation
Figure 2.12 Schematic illustration of the Volt-Ampere measurement
For some materials such as thin films and coatings, the sheet resistance, or surface resistivity, is determined instead, which does not take the thickness (t) into account The
27 sheet resistance (σ, Ω m −2 or Ω) is calculated by Eq (2.8) In this thesis, the I – V characteristic curves are use for investigating the electrical property of the MoS2/C NC
Electrochemical Impedance Spectroscopy (EIS) is an electroanalytical tool used for the evaluation of mechanistic and kinetic information of a wide range of materials such as supercapacitors, batteries, fuel cells and corrosion inhibitors In EIS studies of supercapacitor systems, the cell is held at equilibrium at a constant voltage and a small amplitude (δv) AC-signal v(ω) = V S + δv×sin(ωt) is applied The response of the system to this perturbation from equilibrium is measured in terms of the amplitude and phase of the resultant (δi) current i(ω) = I S + δi×sin(ωt + φ) This technique provides information about the overall impedance of the cell The frequency (f = ω/2π) of the
AC-signal is varied and the impedance (Z) of the cell is recorded as a function of frequency The impedance is represented as a complex quantity Z comprising of in- phase (Z Im) and out-of-phase (Z Re) impedances The plots of imaginary versus the real impedance at different frequencies are called Nyquist plots (Fig 2.13(a)) or Bode plot (Fig 2.13(b))
Figure 2.13 Impedance plot of electrochemical cells
EIS provides a means to understand the kinetic and mechanistic mechanism to monitor the battery or supercapacitor properties under different electrochemical conditions Impedance is expressed as a complex function through real impedance Z Re (Ω cm –2 ) and virtual impedance Z Im (Ω cm –2 ), with j = − 1:
The Impedance modulus and phase angle φ are determined as:
An electrochemical cell with impedance Z WE can be considered as the circuit includes the following components: the double-layer capacitance C dl (F cm –2 ), the faradic impedance Z F (Ω cm –2 ) which includes charge-transfer resistance R CT (Ω cm –2 ), diffused resistance or Warburg resistance W (S s –1/2 cm –2 ), and resistance at the electrode/electrolyte interface R S (Ω cm –2 ) The total impedance of a cell is the combination of different processes occurring during cycling, namely, diffusion, electron transfer kinetics, charge transfer impedance, bulk impedance, passivating layers, Warburg impedance and intercalation capacitance The relative contributions of these different processes depend on frequency The electron transfer kinetics dominates at high (HF) to intermediate frequency range (1.0 MHz to 1.0 kHz) The diffusion process dominates in the low frequency (LF) range (0.01 Hz to 3.0 MHz) Therefore, the EIS measurements were carried over wide frequency range of 100 kHz to 0.001 Hz Total capacitance C’(ω) of cell is included:
Where the imaginary part of capacitance C”(ω) represents for the energy dissipation of the cell The electrochemical surface area (S E, m 2 g –1 ) of electrode materials can be calculated by Eq (2.14): dl E d
29 where C d = 20 μF cm –2 [157], [158] is the capacitance of an atomic unit cell surface of metallic single crystal Electrical double layer capacitance (C dl) refers to the polarization of ionic charge at the surface of EIS system electrodes The amount of charge build up is directly related to the electrode surface area (S E) and ion size Therefore, the larger the electrode surface area the higher the double layer capacitance The C dl is calculated by Eq (2.15), where f ~ 0.01 Hz and m (g) are the mass of electrode materials: dl
The relaxation time τ o (τ o = 1/(2πf o)) is definded as minimum time for completely discharge cell energy with efficiency greater than 50% where f o is the frequency at maxima (“knee” frequency) of −Z Im(ω) in HF region
Cyclic Voltametry (CV) measuring technique based on the comparison of the voltage variation versus responsed current when applying linear variable potential to electrodes over time from V 1 to V 2 By measuring the response current corresponding to the result is the graph i = f(V) shows the relationship voltage electricity lines Scanning rate may vary depending on the object of experiments The voltage applied to the electrodes as in
Eq (2.16) and (2.17), where as, the current I (A) is described as in Eq (2.18), where v (V s –1 ) is the scan rate; V 1, V 2 (V) are potential window
Figure 2.14 Cyclic voltammograms (CV) and measuring system
(a) CV curves of typical supercapacitor; (b) schematic illustration of a 3-electrode, (c) photograph of PGSTAT302N instrument
For the EDLCs electrode materials, CV curve have rectangular shape as shown in Fig 2.14(a) Equation (2.21) is often used to determine the capacitance and potential window of supercapacitor electrodes For two or three-electrodes cells, the electrode's capacitance is determined through the variable scan rate (v s, V s −1 ) (by Eq (2.19), with current density at the mid-point potential value according to the Eq (2.20) and (2.21)
Photoluminescence Spectroscopy (PL)
Photoluminescence spectroscopy is a contactless, nondestructive method of probing the band gap structure of materials The emission of visible light, occurs when a photon (hν) (energy source) excites an electron out of its stable ground state (valence band, VB) and elevates it to an excited state (conduct band, CB) As the electrons rapidly returns to its normal ground state, energy is released, much of it in the form of visible light In almost all cases, the emitted light is of a lower energy in the electromagnetic spectrum than the original light from the energy source Since wavelength and energy are inversely proportional, a lower energy always translates to a higher wavelength Therefore, PL spectra always encompass a wavelength range that is higher than the wavelength of the excitation source The relationship between energy (E g, eV), wavelength (λ, nm), and wavenumber (ν, cm –1 ) is given by Eq (2.22):
= , eV (2.22) where E g is energy band gap, eV;
31 λ is the incident wavelength, nm; h is the Planck’s constant (6.6261 × 10 –34 J s); c is the speed of light (2.9979 × 10 8 m s –1 )
Optical absorbance (UV-Vis)
Ultraviolet-visible (UV-vis) spectroscopy is used to determine the optical band gap (OBG) of the MoS2/C NC To calculate the OBG, the absorption spectrum is converted to Tauc’s plot method [159] The basic procedure for a Tauc analysis is to acquire optical absorbance data for the sample in question that spans a range of energies from below the band-gap transition to above it The optical absorption intensity depends on the difference between the photon energy (hν) and the band gap (E g, eV) as follows:
= − (2.23) where α is the absorption coefficient, ν is the photon’s frequency, cm −1
The absorption coefficient α is given by A/t, where A is the optical absorbance and t is the thickness of the MoS2/C NC film The value of the exponent denotes the nature of the electronic transition, namely are, direct allowed (s = 1/2), indirect allowed (s = 2) or direct forbidden (s = 3/2) and indirect forbidden (s = 3) Typically, the allowed transitions dominate the basic absorption processes, giving either s = 1/2 or s = 2, for direct and indirect transitions, respectively The characteristic features of Tauc plots are evident: at low photon energies the absorption approaches zero, the material is transparent; near the band gap value the absorption gets stronger and shows a region of linearity in this squared-exponent plot This linear region has been used to extrapolate to the x-axis intercept to find the band gap value At even higher energies, the absorption processes saturate and the curve again deviates from linear
In this chapter, a series of experimental work in the hydrothermal synthesis of MoS2/C
NC have been reported An aqueous dispersion containing suspended GO, ammonium molybdate and thioacetamide was heated in a high-pressure stainless-steel autoclave reactor to grow 2D MoS2 crystals on GO templates
All majority chemicals used in synthetic process were purchased from Sigma Aldrich included amonium heptamolybdate tetrahydrate ((NH4)6Mo7O24.4H2O), thioacetamide (CH3CSNH2, 98%), H2SO4 (98%), H3PO4 (85%), KMnO4 (98%), H2O2 (30 wt %), ethanol and deionized (DI) water The dispersion of GO nanosheets were prepared by Hummer’s modified method [160]
Figure 3.1 High pressure autocalve reactor
(a) Photograph and (b) Schematic view of the high-pressure stainless-steel autoclave
In this dissertation, the MoS2/C NC were synthezied utilizing a high-pressure autoclave reactor (Parr Instruments Co.) with volume of 50 mL as shown in Fig 3.1 The design of the autoclave reactor is ideal for hydrothermal synthesis, include:
(1) Instant and uniform mixing of the reagents to obtain a high number of nuclei; (2) Minimize reaction time to control the size distributions of products;
(3) Minimal heating of the precursor solution prior to avoids premature nucleation and unwanted reactions;
(4) The autoclave reactor with a specific sampling valve which enables to withdraw samples at a specific reaction time for instant analysis during the growth of MoS2/C NC This sampling valve provides the access to the information of the in-situ crystallization process that benefit for determination of suitable reaction time, instant monitoring of the crystallization, microstructure, morphology of MoS2 crystalline phase in MoS2/C NC
Hydrothermal synthesis of MoS 2 /C NC
In the experiments, MoS2/C NC were synthesized by two-steps process hydrothermal route, include following:
(1) Prepare GO by Hummer’s method [160] with some modification The prepared GO is employed as a substrate for growing 2D MoS2 crystals in the hydrothermal condition
(2) In-situ grow 2D MoS2 nanocrystals on GO to form MoS2/C NC The 2D MoS2 nanostructures were grown from the precursor contain a dispersion of GO 1.0 g L −1 in
DI water, Mo 4+ and S 2− sources in the controlled hydrothermal condition.
Preparation of GO
To prepare GO dispersion, ~2.0 g graphite flakes (~1–5 μm, 99.8%, Sigma Aldrich) was added to ~120 mL solution of H2SO4 (~92 mL, 98%) and H3PO4 (~24 mL, 85%) which was maintained at ~0–5 °C Then, ~24.0 g of KMnO4 (98%) was added to this mixture and stirred for ~3 h After that, ~180 mL of DI water was added and stirred at ~36 ± 3 °C for 8 h Next, ~800 mL of DI water was added, followed by adding ~90 mL of H2O2
(30%) for terminating the reaction The obtained mixture was purified by centrifuging at ~12,000 rpm for 30 minutes and washing for several times by adding HCl (5%) and deionized (DI) until the pH of its supernatant reached ~7.0 After the final centrifugation and removal of supernatant, a relatively high concentration of GO pastes (~3.5 wt %) was obtained The as-prepared GO dispersion was employed as a substrate for growing 2D MoS2 crystals and forming MoS2/C NC The synthetic procedure of GO is shown in Fig 3.2
Figure 3.2 The synthetic procedure of graphene oxide
Synthesis of MoS 2 /C NC
The MoS2/C NC was synthesized by hydrothermal route as seen in Fig 3.3 In order to prepare a homogenous suspension of GO, the GO paste (in section 3.2.1) was diluted several times with DI water and ultrasonicated for 4 h to obtain concentration of 1.0 g
L –1 The ultrasonication was conducted by using an Elma P120H (Germany) Sonicator Ultrasonic Bath (~80 kHz, 1.33 kW) In a typical synthetic procedure, ~30.0 mL of GO (1.0 g L –1 ) dispersion in DI was prepared Subsequently, ~0.1506 g ammonium molybdate tetrahydrate and ~0.3060 g of thioacetamide were added to this solution (pH was kept at ~7.0–8.0) and stirred at ~30 ± 3 °C for 15 minutes The resulting mixture was transferred to a high-pressure Teflon-lined stainless-steel autoclave reactor (~75% volume filled) Nitrogen gas (N2, 99.9%) was introduced into the autoclave through inject valve to purge and control the pressure inside the system at ~50 bar The mixture was heated at 230 °C for 2 h in stirring condition After cooling down to room temperature, a precipitate was colected and washed with DI water and ethanol several times, then centrifuged and dried at 60 ± 5 °C for 24 h in a vacuum furnace The resulting MoS2/C NC black powder was collected for further characterization
Figure 3.3 The synthetic procedure of nanocomposite MoS2/C NC
Synthesis of bare 2D MoS 2
For comparison, the bare 2D MoS2 was synthesized with the same conditions as described in Section 3.2.2, except for the addition of GO to the initial precursor.
Investigating the effect of experimental parameters
In order to investigate the effect of experimental parameters, MoS2/C NC were synthesized with systematically variable parameters such as reaction temperature, reaction time, pH value and molar ratio of reactants in the precursor Details of the controlled hydrothermal parameters are listed in Table 3.1
(1) Investigate the effect of reaction temperature: To investigate the effect of hydrothermal reaction temperature, the hydrothermal reaction temperature program was set as following profile with four stages: heated to 190 ± 3 °C (the heating rate of ~20 °C/min), then increase reaction temperature to 210, 230 and 250 °C, while the reaction time was kept at 2 h for every stage under stirring condition At every period of reaction temperature of 190, 210, 230 and 250 °C, a volume of ~3.0 mL sample was taken out of
36 the reactor through a sampling valve for characterization These corresponding samples were denoted as MoS2/C(190), MoS2/C(210), MoS2/C(230) and MoS2/C(250) Other reaction parameters were maintained
Table 3.1 Strategies to control experimental parameters
3 Effect of Mo 4+ : C molar ratio
(2) Investigate the effect of reaction time: In a series of typical experiment, the MoS2/C
NC was synthesized at 230 °C, the (Mo 4+ : C) molar ratio and pH of the precursor are fit at (1.5:1) and ~7.0–8.0, respectively In order to investigate the formation and growth of MoS2 on graphene surface versus reaction time, a volume of ~3.0 mL sample was taken out of the reactor at reaction time of 15, 30, 45, 60, 120, 240 and 360 minutes for XRD, FESEM, HRTEM, HAADF-STEM/EDS, HXPES, and Raman analysis The samples were withdrawn to 25 mL glass vials which have already contained ~10 mL DI water for rapid cooling the samples to room temperature and stopped the crystallization of the materials
(3) Investigate the effect of precursor’s molar ratio (Mo 4+ : C): In the experiments, the MoS2/C NC were synthesized at 230 °C, 2 h and pH ~7.0–8.0 To investigate the effect of (Mo 4+ : C) molar ratio, five groups of MoS2/C NC samples with different (Mo 4+ : C)
37 molar ratios were prepared, denoted as MoS2/C(0.5:1), MoS2/C(1:1), MoS2/C(1.5:1), MoS2/C(2.0:1) and MoS2/C(2.5:1)
(4) Investigate the effect of pH of the precursor: In order to investigate the effect of pH to the formation and crystalline structure of MoS2/C NC, a series of precursor with different pH values were prepared The initial pH of the precursors was adjusted to pH in range of ~3.0–4.0, ~5.0–6.0, ~7.0–8.0 and ~9.0–10.0 by dropwise adding 0.1 M of
CH3COOH and 1.6 M of NH4OH Other reaction conditions were maintained The pH value of all dispersions was measured by HI2020 edge® Multiparameter pH Meter, Hanna Instruments, with measuring range of −2.00–16.00 ± 0.01 pH
In order to characterize the crystalline structure, elemental composition, morphologies and growth mechanisms of the MoS2/C NC samples, it is essential to adopt a variety of characterization methods and a combination of characterization information is required Such characterization methods are briefly described as following:
Powder X-ray diffraction (XRD): The X-ray diffraction (XRD) patterns were recorded using a XRD Bruker D8 Venture diffractometer equipped with a Cu-Kα radiation (λ=1.5406 Å) with a p-type Silicon zero-background cell holder, over an angle 2θ range of 5–80º, with a step size of 0.0194º, step time of 0.25 s and operated at 40 kV and 200 mA Peak search, peaks fit and indexing were performed using DIFFRAC.SUITE EVA- XRD Software, Bruker
Raman spectroscopy: MicroRaman measurements were carried out using Horiba
XploRA ONE spectrometer equipped with an Olympus BX50 microscope The BX50 microscope is attached to focus the laser beam on a 180×120 μm 2 selected area of the sample, a green argon laser (λ = 532 nm, 15 mW) (~2.33 eV) was used as an excitation source with exposition time of 15 s along with a 900 lines per mm grating monochromator with liquid nitrogencooled CCD
High resolution Transmission Electron Microscopy (TEM, HRTEM): the microstructure of the samples was observed by HRTEM (JEM-2100F, JEOL) at 200 kV in the bright field TEM samples were prepared by drop casting the dispersions of MoS2/C NC
38 sample on a carbon-coated copper grid, followed by drying under vacuum for 24 h at 50 °C The High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM) images were obtained with a JEM-ARM200F (JEOL) at an acceleration voltage of 200 kV
Field emission scanning electron microscopy (FESEM): the morphology of the samples was observed by FESEM (S-4800, Hitachi), with an acceleration voltage of 10 kV Energy dispersive X-ray spectrum and mappings of the MoS2/C NC samples were obtained using an EDX spectrometer (EDX, Oxford Inca X-max 80) by STEM with an acceleration voltage of 200 kV
X-ray Photoelectron Spectroscopy (XPS): The Hard X-ray Photoelectron Spectroscopy
(HXPES) measurements were performed at the synchrotron radiation facility BL15XU in 15 SPring-8 by using a VG Scienta R4000 analyzer The energy of the irradiated X- ray beam was 5.95 keV The binding energy were corrected by referred to the Fermi level of a thin Au film as an internal standard The detailed machine setup and experimental conditions of the HXPES measurements were reported in a previous paper
In order to investigate the (I – V) characteristic of the as-synthesized MoS2/C NC and their transport property, eight-Platinum electrodes (30 nm thick) devices with 400 nm gap size on 100 nm n-type SiO2/Si substrate was fabricated by standard photolithography method (Fig 3.4(a)) Devices were fabricated at Semiconductor Center, Kitakyushu Science and Research Park, Graduate School of Life Science and Systems Engineering, KYUTECH (Wakamatsu Campus), Japan For I – V measurement, a single thin flake of MoS2/C NC was attached on the center of these electrodes by dielectrophoresis (DEP) method [161], [162] as presented in Fig 3.4(b))
For this fabrication process, the dispersion of MoS2/C(1.5:1) NC in ethanol were prepared Briefly, ~3.3 mg of MoS2/C NC powder was dispersed in ~10.0 mL ethanol (98%), ultrasonicated for 3 minutes and ready for usage To attach a MoS2/C NC flake on the device (Fig 3.4(c)), dropwise an aqueous dispersion of the prepared solution on the very center of electrodes on the device, and a sinusoidal potential difference is
Structural characteristic of GO
As the prepared GO was employed as a substrate for growing 2D MoS2 crystals in the hydrothermal condition to synthesized MoS2/C NC materials, the insight analysis of GO microstructure and chemical composition is important Therefore, the crystalline structure and characteristic of the prepared GO materials were first examined by XRD as seen in Fig 4.1(a) The XRD pattern of GO shows a strong (001) peak at 2θ ~ 10.8º and a lower (002) peak at ~23.5º that matched well with standard diffraction pattern of graphene oxide (JCPDS #00-065-1528) The strong (001) peak centered at 2θ ~ 10.8º, corresponding to interlayer distance d (001) of ~8.18 Å (calculated by Bragg equation) Such d-spacing is considerably larger than that of graphite (3.35 Å) [163], indicating that GO contains a large number of oxygenated functional groups on both sides and surfaces of the basal sheets which was described in [98], [164] These functional groups endow GO with a good hydrophilicity and favorable water solubility, which is beneficial to an effective dispersion of GO as precursor in aqueous solution [165] Additionally, a broad (002) peak near ~23.5 to 26.3º may be due to incomplete oxidation of graphite or partially reduced to rGO in synthetic procedure as described in [63], [64], [97]
Raman spectra shown in Fig 4.1(b) are used to analyze the structures of the as-prepared
GO Raman spectrum of the prepared GO exhibit two dominant peaks at ~1352 and
~1583 cm –1 , that match well with the D and G bands of graphene oxide and graphitic materials [166], [167], respectively The ratio of D and G band intensity (I D/I G) is employed to quantify the structural defects of the investigating graphene and graphene
42 oxide [168] The calculated I D/I G value for the prepared GO is about ~1.14, indicating the rich-defects structure and the sp 3 conjugation are dominant in the prepared GO and well agreed with that of GO in previously reported [64], [100], [169]
Figure 4.1 XRD patterns and Raman spectra of GO
(a) XRD patterns of GO; (b) Raman spectra of GO
The chemical composition of the GO was analyzed by employing XPS, STEM-EDX and elemental mapping as seen in Fig 4.2 EDX spectrum in Fig 4.2(d) shows two strong characterized peaks located at energy of ~0.277 and ∼0.525 keV The appeared peaks are originated from the X-ray energy of C Kα (∼0.225 eV) and O Kα (∼0.525 keV) which indicating that the main chemical composition of GO is carbon (C) and oxygen (O) In addition, the high-resolution core-level spectra of C 1s in Fig 4.3(h) and
O 1s in Fig 4.2(i) show strong peaks centered at binding energy of ~284.4 eV and
~531.6 eV, respectively, which characterized for the sp 2 bonding state of graphene domain and oxygenated groups in the prepared GO
Table 4.1 The EDX chemical composition of the synthesized GO
From Table 4.1, the atomic percentage of C and O is ~86.64% and 13.36%, respectively The calculated C : O atomic ratio is ∼5.549 which indicates the low oxygen in the as-
43 synthesized GO The prepared GO may be considered as reduced GO as suggested by Drey et al [96], and Bagri et al [97] The oxygen mainly originates from oxygenated groups on rGO crystalline structure Such functional groups as ‒OH, ‒COOH and C=O expand the interlayer distance and contributes to a hydrophilic surface, also play an important role for functionalization and fabricating graphene based composite materials
Figure 4.2 Characteristics of graphite and the as-prepared GO
(a) FESEM image of graphite flakes, (b, c) FESEM and HRTEM images of the prepared GO, (d, e, f) STEM-EDX spectrum and elemental mapping of C and O in the GO, (g) Full-scan XPS spectrum (0–1000 eV) of GO; (h, i) high-resolution core-level spectra of C 1s and O 1s
Structural characteristic of bare 2D MoS 2
For the bare 2D MoS2 sample synthesized at hydrothermal reaction temperatures ~230 °C, in ~2 h, and pH of precursor is ~7.6, the XRD pattern exhibits high crystallinity, as seen in Fig 4.3(a), and the diffraction peaks observed at ~14.4º, 32.5º, 39.6º, 49.3º and 58.9º correspond to (002), (100), (103), (105) and (110) crystal planes of 2D MoS2 crystalline phase respectively, which is in good agreement with the standard diffraction pattern of MoS2 (2H-MoS2, JCPDS 00-037-1492) The strong (002) peak at ~14.3º corresponds to the d (002) of ~0.615 nm, indicating that individual layer of 2D MoS2 grows well along the [001] direction during the hydrothermal crystallization process as mentioned in previously reports [59], [126], [133]
Figure 4.3 XRD and Raman characteristics of the bare 2D MoS2
(a) XRD patterns, and (b) Raman spectrum of the bare 2D MoS 2
As shown in Fig 4.3(b), Lorentz fitting results of Raman spectra show three distinct vibrational modes located at ~379.8, 404.1 and 459.1 cm −1 which correspond to the in- plane E 1 2g , out-of-plane A 1g and the second-order longitudinal acoustic phonon modes (2LA(M)) [170], [171], [172] of 2H-MoS2 crystalline phase, respectively These Raman vibration modes further confirming the successfully synthesis of 2H-MoS2 layered materials that have also been previously reported in literatures [30], [173] Furthermore, the difference in Raman shift of E 1 2g and A 1g modes can be used as a reliable identification for the number of layers MoS2 [172] By measuring the wavenumber difference between the two modes, one can determine the layer-thickness of MoS2 based
45 materials [172] In the present study, the frequency difference (Δῡ, cm –1 ) measured between two modes is ~21.3 cm –1 , implying that these 2D MoS2 nanosheets are compacted in several S–Mo–S layers (~3–6 layers) Microstructural and morphological characteristics of the synthesized 2D MoS2 are shown in Fig 4.5 For comparision and indentification of 2D MoS2 and GO phases, the lattice parameters and refinement details of the GO and bare 2D MoS2 and are provided in Table 4.2
Table 4.2 Lattice parameters of GO and 2H-MoS2
Material name Graphene oxide, GO, nano Molydenum disulfide,
Table 4.3 The chemical composition of the bare 2D MoS2
The elemental composition of the bare 2D MoS2 samples were analysized by STEM-EDX as shown in Fig 4.4 The recorded EDX energy spectrum of the synthesized 2D
MoS2 nanosheets in Fig 4.4(a) exhibits two highest peaks which correspond to the X- ray energy of Mo Kα (~2.293 keV) and S Kα (~2.307 keV)) elements that reflecting the main chemical composition of the samples
Figure 4.4 Chemical composition of the synthesized 2D MoS2
(a) STEM-EDX spectrum and (b, c) elemental mapping of molybdenum (Mo) and oxygen (O) in the synthesized 2D MoS 2 nanocrystalline materials Scale bar in the inset of (a) is 500 nm
Figure 4.5 Microstructure and morphology of the synthesized 2D MoS2
(a–c) FESEM, TEM and HRTEM images of the bare 2D MoS 2 ; (d) Schematic illustration of 2D layered MoS 2 atomic structure
Furthermore, the quantification of elemental composition in Table 4.3 indicates that the atomic ratio between Mo and S is about (1 : 2.06), which is close to the stoichiometric of MoS2 (Mo : S = 1 : 2) This results confirm the low impurities of the sythesized 2D MoS2 nanosheets and also the success of our hydrothermal synthetic approach The morphologies of the 2D MoS2 nanostructures were evaluated by FESEM and HRTEM analysis as seen in Fig 4.5 The MoS2 occupied space with nanopetal-like morphology with average lateral size of ~300–400 nm and thickness of ~0.63–3.69 nm It is easy to realise that the prepared MoS2 samples compacted of ~1–6 layers (Fig 4.5(c)) with the schematic view of model structure in Fig 4.5(d) As reported by X Zhou et al [147], the MoS2 flowers was obtain by the same hydrothermal method with thickness ~3 nm, lateral size ~200 nm, however, the reaction time was about 24 h that was rather long when compare to that of ~2 h, as obtaining in this work These results indicate that the proposed hydrothermal approach for synthesizing 2D MoS2 nanomaterials in this dissertation is more effective in term of reducing reation time while still maintaining the controllable crystalline size and morphology of the as-synthesized 2D MoS2 material
Structural characteristic of MoS 2 /C NC
Microstructure and morphology of MoS 2 /C NC
The formation and crystalline structures of the MoS2/C NC can be well characterized by XRD analysis At reaction temperatures around 230 °C, 2D MoS2 crystalline phase was obtained in ~2 h when the pH of precursor is ~7.2–8.8 and (Mo 4+ : C) molar ratio of
~(1.5 : 1) As seen in Fig 4.6(a), the XRD pattern of the MoS2/C NC shows diffraction peaks at 2θ ~ 14.4º, 32.5º, 39.6º and 58.9º correspond to (002), (100), (103) and (110) crystal planes of 2D MoS2, respectively, which is in good agreement with 2H-MoS2
(JCPDS #00-037-1492) without other peaks of impurities phases The strong (002) peak centered at ~14.4º corresponds to the d (002) of ~0.63 nm, indicating that 2D layered MoS2 grows well along the c-axis during the synthetic process that well agreed with previous reports of Y Hou et al [74], and Z H Deng et al [174] In addition, the broad and low intensity of diffraction peaks compared to that of bare 2D MoS2 sample indicating that the 2D MoS2 phase in MoS2/C NC is in a short-range order crystalline state
Raman spectra shown in Fig 4.6(b) are used to analyze the structures of MoS2/C NC For sample MoS2/C(1.5:1), two distinct peaks at ~380.7 and ~406.5 cm –1 represent for the in-plane E 1 2g and out-of-plane A 1 g vibrational modes of 2H-MoS2 which can be clearly identified in both spectra of MoS2/C NC and pristine MoS2 as reported elsewhere [36], [135] The MoS2/C NC exhibit two dominant Raman peaks at ~1352 and ~1583 cm –1 , which match well with the D and G bands of graphene, respectively, and well agreed with that of GO The I D/I G calculated value for the MoS2/C NC (I D/I G = 0.10) is much smaller than that of GO (I D/I G = 1.14), indicating that the sp 2 conjugation is restored during the hybridization process which were mentioned in [168], [175], [176] The decreased frequency difference [172] of the MoS2/C NC compared to that of bare 2D MoS2 confirms the ultrathin layers of MoS2 crystals in accordance with HRTEM images in Fig 4.7(c) and Fig 4.7(k)
Figure 4.6 XRD patterns and Raman spectra of GO, bare MoS2 and MoS2/C NC
(a) XRD patterns, (b) Full Raman shift (0–3000 cm −1 ) with applied Lorentz fitting of E 1 2g and
A g peaks 2D MoS 2 ((350–450 cm −1 ) and D, G and 2D band of graphene domain
The formation of 2D MoS2 crystalline phase in the graphene matrix can be explained by the reduction of MoO4 2– to form MoS2, while the GO is also in-situ reduced to graphene by H2S during the hydrothermal process In this case, graphene provides a platform for the nucleation and growth of 2D MoS2 nanostructures However, it is found that almost all of the diffraction peaks are broad and low intensity compared to that of bare MoS2 sample, indicating that the sample is in a poorly crystalline state with short-range order crystallization In addition, the broadening of XRD peaks may result from small grain sizes Overall, all of the diffraction peaks in this XRD pattern can be well indexed to the hexagonal phase of MoS2 without diffration peaks of other impurities Additionally, there is no obvious difference in the XRD pattern of sample MoS2/C(1.5:1) compared to that of bare MoS2, implying that the introduction of GO in the synthetic process does not change the phase of 2D MoS2
The microstructure and morphology of MoS2/C NC were examined by FESEM, TEM and HRTEM with results are shown in Fig 4.7, the typical morphologies of MoS2/C NC are shown in TEM image in Fig 4.7(e), where the well distributed MoS2 nanostructures can be clearly observed on graphene surfaces and edges It is obvious that the lateral size of the individual of graphene sheets is quite large up to several micrometers, while petal-like MoS2 nanosheets have diameters in range of ~200–300 nm (Fig 4.7(b, c)) with average thickness of ~0.63–6.60 nm (1–10 layers) As reported by W Zhou et al [150], who also used the same hydrothermal method to synthesize MoS2/graphene composite, the MoS2 flowers was obtained with 13–20 layers, in micrometer lateral size However, the reaction time was rather long (~24 h) when compare to that of ~2 h, as obtaining in this dissertation These results show that the proposed hydrothermal approach for synthesizing MoS2/C NC in this work is more effective in term of reducing reation time while still maintaining the controllable crystalline size and morphology of the as-synthesized MoS2 /C NC materials Digital photograph in Fig 4.7(d) shows a 3 × 3 cm black paper-like film of MoS2/C(1.5:1) sample collected after drying, while the dispersion in DI water is in brown color (on top-right) The three-dimensional (3D) architecture of the MoS2/C NC are shown in Fig 4.8 The 3D images were reconstructed form HRTEM image in Fig 4.7(f)
Figure 4.7 Microstructure and morphology of MoS2/C NC
(a–c) FESEM images of GO, 2D MoS 2 and MoS 2 /C NC; (d) digital photo of a paper-like film of MoS 2 /C NC, the inset (top-right) shows suspension of MoS 2 /C NC in deionized water; (e) typical TEM image of MoS 2 /C NC morphology, (f) HRTEM image of the MoS 2 /C(1.5:1) sample and high magnification of the white rectangles in (f) showing MoS 2 phase (g) and graphene (h); (i) schematic view of MoS 2 /C NC atomic structures with MoS 2 several layers vertical stacked on graphene; (k) high magnification of region in HRTEM image (f) elucidating the MoS 2 /C NC structures with 2D MoS 2 several layers vertical stacked on graphene
To better elucidate the crystalline structure and morphological of MoS2/C NC, HRTEM analysis was carried out as recorded in Fig 4.7(f–h) Ultrathin 2D MoS2 layers are grown directly on both sides of the graphene surface which forming vertically stacked architectures The 2D MoS2 nanocrystals occupied space with distinctive lamellar structure (Fig 4.7(k)) that can be easily distinguished from the surrounding area where hexagonal lattice of carbon in graphene clearly observed From HRTEM images, the spacing of ~0.63–0.64 nm can be identified and assigned to the distance between (002) planes of 2H-MoS2, as well as distance of ~0.25 nm which can be attributed to that of zig-zag chain gap in a unit cell of graphene lattice (0.246 nm) [67] as shown in Fig
4.7(h) The projection in Fig 4.7(i) illustrates the vertical stacked along [001] direction of ultrathin 2D MoS2 layers on graphene sheet These insight observations help confirming the directly growth of 2D MoS2 nanostructures on graphene
Figure 4.8 Vertical stacked architecture of MoS2/C NC
Figure 4.9 Chemical composition of the MoS2/C NC
(a) HAADF-STEM image and (b, c, d) EDX elemental mapping of C, Mo, S; (e) EDX spectrum and chemical composition of MoS 2 /C NC synthesized at 230 °C, 2 h
To confirm the existence of graphene and 2D MoS2 in MoS2/C NC, the HAADF-STEM and EDX spectroscopy mapping analysis were employed with the results are shown in Fig 4.9 The selected EDX mapping region (500 nm 2 ) is observed in Fig 4.9(a) with the elemental mapping of C, Mo, and S (Fig 4.9(b–d)), respectively, indicating the co- existence of C, Mo and S elements These analyses suggest that 2D MoS2 nanosheets are well attached on graphene with no serious aggregation The chemical composition of the MoS2/C NC is investigated by STEM-EDX spectrum as recorded in Fig 4.9(e) The measured (Mo : S) atomic ratio is around ~0.54, which is consistent with the stoichiometric ratio of MoS2 (1 : 2) However, the measured (Mo : C) ratio is around
~(1.3 : 1) which slightly differs from their corresponding precursor ratio of (1.5 : 1) This phenomenon could be attributed to the partially oxidized MoS2 to MoO3 that consistent to later XPS investigation as elucidated in Fig 4.10
Figure 4.10 Chemical states and bonding structures of MoS2/C NC
(a) Full-scan XPS spectrum (0–1000 eV) spectra of MoS 2 /C NC synthesized at 230 °C, 2 h; (b) high-resolution core-level spectra of (b) C 1s, (c) Mo 3d, (d) S 2p
In order to elucidate the chemical composition and chemical states of elements in MoS2/C NC, the XPS measurements were performed We applied Lorentz function for curves fitting procedure to analyze the XPS spectra, in which the experimental data points are displayed as dots and the solid line is the cumulative of fitted components As shown in Fig 4.10(a), the survey wide range spectrum of sample MoS2/C(230) recorded from 0 to 1000 eV reveals the presence of Mo, S and C elements in the sample Fig 4.10(b) shows the high-resolution scan XPS with Lorentz peak-fitting result of C 1s region One can see three resolved peaks of sp 2 -hydridized C−C/C=C at ~284.2 eV, sp 3 (amorphous carbon) C−C at ~285.9 eV and oxygenated functional groups (C−O) at
~288.3 eV The sp 2 -hydridized C−C/C=C is well known signature of graphene domain material [99], [127], [129] The percentage of sp 2 domain was calculated from the sum of the total area of each peak (sp 2 , sp 3 andoxygenated groups) As seen in Fig 4.10(b), the sample MoS2/C(230) is dominant by ~83.3% sp 2 domain The absence of the C(O)−O and C=O peaks indicates that the GO sheets have been almost reduced to graphene Details of XPS analysis are listed in Table 4.4
Table 4.4 Composition of graphene (sp 2 ) domain in MoS2/C NC
Percentage, (%) sp 2 sp 3 Oxides sp 2 sp 3 Oxides
Table 4.5 Composition of 1T, 2H-MoS2 domains in MoS2/C NC
From the high resolution scan spectrum of the Mo 3d region in Fig 4.10(c), two major peaks at ~229.2 and ~232.9 eV are observed which is assigned to the Mo +4 3d 5/2 and
Mo +4 3d 3/2 in 2H-MoS2, confirming the dominance of Mo(IV) in sample MoS2/C(230) [38] Besides the Mo(IV) 3d 5/2 signal, a peak appears at ~226.1 eV which could be from
54 the S 2s orbital Also, two shoulders located at ~228.9 and 232.2 eV are attributed to
Mo +4 3d 5/2 and Mo +4 3d 3/2 in 1T-MoS2 phase [75], [77] Another peak at higher binding energy of ~235.2 eV relates to the Mo ions in the +6-oxidation state, which may be due to the inadequate reduction of MoO4 2− species during the hydrothermal synthesis From the higher resolution XPS spectrum of the S 2s in Fig 4.10(d), the main doublet located at binding energies of ~162.0 and ~163.2 eV corresponds to the S 2p 1/2 and S 2p 3/2 of pristine MoS2, respectively, which is consistent with previous reports [108], [149] Furthermore, deconvoluted XPS spectra of the Mo 3d peak in Fig 4.10(c) and S 2p in Fig 4.10(d) allow the percentage of characteristic binding energy in 1T- and 2H-MoS2 phase of the MoS2/C(230) sample to be quantified [177], [178] The percentage of each binding energy was calculated from the sum of area under each peak (Mo +4 3d 3/2 and
Mo +4 3d 5/2) As seen in Fig 4.10(c, d) and Table 4.5, the MoS2/C(230) sample composed of ~47.53% 1T and ~36.57% 2H domains Our experimental results are similar to previously reports for 1T–2H phase conversion ratio of bare MoS2 materials [178], [179] Based on the XPS investigation, the formation of MoS2/C NC and the extent of reduction of GO to graphene can be successfully explained
Related hydrothermal approaches have been reported by Y Xu et al [57] for synthesizing graphene hydrogel, or published by D Zhang et al [58] (MoS2/GO) and
Electrical property of MoS 2 /C NC
It is well known that non-volatile resistance switching [180] recently has been observed in various solution-processed multi-layer 2D material morphologies including pristine MoS2 [181], reduced and functionalized GO mixtures [182], partially degraded black phosphorus quantum dots [183], functionalized MoS2 and their composites [48], [184], and TMDs based hybrids [46], [121], [185], [186], where the resistance can be modulated between a high-resistance state (HRS) and a low-resistance state (LRS) and subsequently retained absent any electric field supply In this section, we also briefly reported the resistance switching behavior of the MoS2/C(1.5:1) NC in which 2D MoS2 nanopetal-like shape randomly distributed on graphene surfaces The MoS2/C(1.5:1) sample was deposited on a nano-gap electronic device with Ohmic electrodes on SiO2/n-
Si substrate, as shown in Fig 4.12 and Fig 4.13 The I − V characteristics are measured using a semiconductor parameter analyzer and shielded probe station with a power supply under dark conditions
Firstly, the I − V measurement was carried out for the devices fabricated from GO and 2D MoS2 with the recorded results as shown in Fig 4.11 The I − V curve in Fig 4.11(a) of 2D MoS2 device presents an ohmic-like behavior while GO device show a non-ohmic relation with the semi-log plot and dI/dV – V curves as shown in Fig 4.11(b, c) As seen in Fig 4.11(c, d) the prepared GO has low conductance (G = dI/dV) with value of several nS in the low resistance states (LRS) (–10.5 V to 2.5 V) and ((2.4 V to 10.0 V) The
56 semiconductive property of the device was also confirmed by a Plateau width obtained in a dI/dV – V curve In the high resistance state (HRS) (–2.5 V to 2.6 V) the dI/dV – V plot show Plateau with of ~4.8 V indicating the semiconductive of the prepared GO
Figure 4.11 Volt-Ampere characteristic of GO and 2D MoS2
(a) I – V curves (b) semilog plots of I – V curves versus V; (c) Plots of conductance G ≡ dI/dV – V, (d) Resistance R = 1/G of GO and MoS 2 samples
In case of 2D MoS2 device, the I – V curve presents an ohmic-like behavior with lower conductance G ~ 0.36 nS and rather high resistance (R ~ 2.7×10 3 MΩ) The electrical transport property of the synthesized GO and 2D MoS2 are important characteristic and basic for comparision with the synthesized MoS2/C NC This observation help determine the MoS2/C NC system with high conductance for electronic applications For the MoS2/C NC system, the I − V measurement was first probed for the graphene region located inside the pairs of electrodes ( 2–3 ) as shown in Fig 4.14(a, d) One can see that the connection between two electrodes is the graphene flake due to the lack of 2D MoS2 petals as observed in FESEM image (Fig 4.13(d)) The I − V curve in Fig 4.14(a) presents a high conductive ohmic-like (linear) behavior and high conductance (G ~ 24 μS) with the semi-log plot and dI/dV – V curve as shown in Fig 4.14(b, c)
Figure 4.12 Device fabrication and I – V measurement set up
(a) Schematic illustration of a device model and I – V measurement set up; (b) Photograph image of a SiO 2 /n-type Si devices fabricated with deposited MoS 2 /C NC membrane; (c) I – V vacuum 4-probes chamber
Figure 4.13 Nano-gap devices construction
(a, b) SEM images of a device before attaching of MoS 2 /C NC sheet; (c) optical microscope image of a measuring device in a I – V 4-probes system, (d) higher magnification SEM image of device #1 shows a single MoS 2 /C NC sheet trapped on electrodes, (e) HRTEM image of a MoS 2 /C NC sheet
Then, the I − V curve of the MoS2/C NC region located inside the pairs of electrodes ( 1 –
8 ), as shown in Fig 4.13(a, d), was examined in which there are some of 2D MoS2 nanopetals distributed on graphene surface and only graphene contacts to the electrodes
Figure 4.14 I – V curves of device fabricated from MoS2/C NC
(a) I – V curves probed along the (2-3) pair of current collectors (b) semilog plots of I – V curves versus V in (a); (c) plots of conductance G = dI/dV – V in (a), (d) I – V curves probed along the
(1-8) pair of current collectors; semilog plots of I – V curves versus V in (d) and (f) plots of conductance G = dI/dV – V in (d)
The I – V characteristic (Fig 4.14(d, e)) has non-ohmic behavior and lower conductive in the sweeping bias range of –10 to +10 V In addition, the existence of Plateau width (PW ~ 4.1 V) defined as the zero-current region in the dI/dV – V curve (Fig 4.14(f)) also confirm the semiconducting property of the synthesized MoS2/C NC It is noted that the semi-log plot of I – V in this case (Fig 4.114(e)) depicts the large hysteresis loop in both forward and reverse sweepings and also the pinched hysteresis which are a characteristic for memristive or charge storage (capacitance) property [48], [187]: the initial LRS changed to a HRS in the 0 to –10 V voltage sweeping The device remained in the HRS and progressively changed to the LRS only in the +10 V to 0 voltage sweeping This remarkable hysteresis is strongly related to underlying 2D MoS2 nano structures, as there is no appreciable hysteresis in the transfer curve of region with the pairs of electrodes ( 2–3 ), a graphene device on the same sample but without underlying MoS2 nano structures It has been reported that memristive property can be obtained from MoS2 materials [32], [109] The fact that these atomically thin sheets allow great thickness scalability along with high strength makes them viable for conventional
59 semiconductor and flexible electronics devices These results demonstrate that the hysteresis of the MoS2/C(1.5:1) device is due to charge trapped in graphene and 2D MoS2 layers as well as at interfaces between the layers A number of reports have utilized graphene or GO in non-volatile memory devices, such as an FET channel [182], a charge-trapping layer [186] or an electrode [47] It was shown that the large hysteresis in the gate characterization curves of graphene FETs (GFETs) [188] can be applied for memory devices operation [187] It was also reported that this hysteresis arises due to trapped charge in the oxide dielectric layer [185] However, the relatively slow dynamics and poor controllability of the trap density in these graphene-based memory devices require further improvement for realistic applications In these cases, the 2D MoS2, can be great candidates thanks to their superior electrical properties
Throughout the details about structural characterization and electrical tranport property investigation, one can sumerize that the MoS2/C NC system with low dimensions, unique and robust structures, high electrical conductance (G ~24 μS) and memristive property has been succesfully synthesized In this context, gaphene and 2D MoS2 phase were employed as both channel and charge-trapping layers In these heterostructured memory devices, the atomically ultrathin 2D MoS2 or graphene-trapping layer stores charge tunnelled through vdW gap (described in section 4.5.3) These devices show high mobility, large memory window and stable retention, providing a promising route towards flexible and transparent memory devices utilizing atomically thin 2D materials
Effect of hydrothermal reaction time
The morphology evolution of MoS 2 /C NC versus reaction time
In hydrothermal approach, the reaction time is determined as one of the most important experimental parameters that directly affects the microstructure and morphology of synthesized materials To investigate the effect of hydrothermal reaction time and access to the in-situ information on the crystallization process, the sample extraction technique was employed In the experiments, the reaction was carried out at different reaction time of 15–360 minutes to obtain the MoS2/C NC samples The reaction temperature, the (Mo 4+ : C) molar ratio and pH value were controlled at ~230 °C, ~(1.5 : 1), and in range
60 of ~7.2–8.8, respectively The crystalline structure and phase components of the MoS2/C
NC under different reaction time were examined by XRD, as seen in Fig 4.15(a)
Figure 4.15 XRD patterns of MoS2/C NC versus reaction time
(a) XRD patterns of GO, pristine 2H-MoS 2 and MoS 2 /C NC versus reaction time; (b) Deconvolution of (002) characteristic peak of MoS 2 phases, (c) Evolution of 2θ and d (002) versus reaction time, (d) Plot of FWHM versus reaction time
The MoS2/C NC have the same crystalline structure as that of the bare 2D MoS2 synthesized at 120 minutes, denoted at MoS2(120), but with lower crystallinity and can be observed for samples with the growth time less than 60 minutes In fact, there is no
61 clear (002) characteristic peaks of the MoS2 phase in MoS2/C NC were observed in the XRD patterns of the samples with growth time from 15 to 45 mininutes (Fig 4.15(b)), indicating that there are no significant crystalline phase of MoS2 in these MoS2/C NC samples The lack of MoS2 crystalline phase may be attributed to the incorporation of the graphene in the initial formation stage of MoS2, resulting in the inhibition of the growth of the layered MoS2 crystal during the hydrothermal process When reaction time increases to 60 minutes, the growth of the MoS2 crystalline phase starts to occur, as indicated by the appearance of three diffraction peaks centered at around 2θ ~14.1º, 32.5º and 58.9º, corresponding respectively to (002), (100), and (110) planes of the hexagonal MoS2 phase (JCPDS #00-037-1492) [71], [72] When reaction time reaches to 120 minutes, the crystalline structure of sample MoS2/C(120) exhibits significantly changes The three characteristic peaks of MoS2 phase appear more clearly and the peak shape is narrower, accompanied by the appearance of a new peak centered at 2θ ~ 40.1º which originates from the diffraction of (103) plane of MoS2 The emergence of the new peak might be due to the fact that MoS2 crystals are growing bigger and stacking thicker in dimensionality
With the further increasing of the reaction time to 240–360 minutes, the four main diffraction peaks exhibit more intensive while the peak positions shift closer to that of bare MoS2 as seen in Fig 4.15(c) In addition, longer reaction time show strong (002) peaks with narrow FWHM compare to samples synthesized at shorter reaction time (Fig 4.15(d)) This phenomenon can be explained by considering the phase transition and structural refinement of MoS2 when increasing reaction time All diffraction peaks in this XRD patterns can be well indexed to the 2H- phase of MoS2, without other impurities [61], [71], [189] In addition, there was no obvious difference in the XRD pattern of this MoS2/C NC compared with that of bare 2D MoS2, implying that the inclusion of rGO matrix in the synthesis did not affect the phase structure of 2D MoS2
As seen in Fig 4.16(a), Raman spectra of all MoS2/C NC exhibit two typical peaks in range of ~1344–1352 cm −1 and ~1590–1583 cm −1 These peaks can be attributed to D and G characteristic bands of graphene Raman spectra of the as-prepared MoS2/C
NC has two distinct vibrational modes located at ~378.2 and 404.1 cm −1 which
62 correspond to the in-plane (E 1 2g) and out-of plane (A 1g) of 2H-MoS2 crystalline phase, respectively
Figure 4.16 Characteristic Raman modes of graphene domain in MoS2/C NC
(a) Raman shift (0–3000 cm −1 ) of the MoS 2 /C NC versus reaction time, (b) Characteristic vibration modes D, G bands of graphene (1100–1700 cm −1 ) The characteristic Raman mode in (b) were fitted by applying Lorenzt function
In addition, by using the I D/I G peak intensity ratio, the degree of defects and disorder in graphene can be well characterized The structural disorder in graphene increases, I D/I G will increase as higher defect density produces more elastic scattering and vice versa [168], [190] The calculated (I D/I G) of sample MoS2/C(15) is ~1.15 indicating that the
GO sheets mainly dominance by disorder structure Intensity ratios (I D/I G) of sample MoS2/C(60) (reation time ~60 minutes) is around ~0.97 which is obviously smaller than that of GO (I D/I G ~1.13) When reation time increase to 120–360 minutes, the I D/I G decrease significantly from ~0.62 to 0.18 (D-band is almost absent) while the 2D band raising at ~ 2730 cm −1 as seen in Table 4.6
Table 4.6 Raman modes of graphene domain in MoS2/C NC
Samples Reaction time D band G band 2D band I D/I G ratio min Raman shift, cm −1
Interestingly, the calculated value of I D/I G for the MoS2/C NC decrease while the 2D band raising up in samples with increasing of reaction time, indicating that the GO sheets have been almost reduced to graphene [168], [191] Such a dramatic change may be caused by the anchoring of 2D MoS2 species onto the GO sheets, benefiting the restoration of the aromatic structures (sp 2 hybridization) by repairing defects This observation suggests an effective method for reducing the structural defects on GO to obtain rGO or chemically reduced graphene In a general way, the longer the reaction time, the thicker the nanopetal-like MoS2 will be For this reason, the total hydrothermal reaction time should be less than 120 minutes to get the desire crystalline MoS2/C NC
X-ray photoelectron spectroscopy (XPS) is a powerful tool to elucidate the chemicals presenting in the materials because the recorded spectrum is perfectly reflecting the chemical bonding states of elements through their binding energy signatures Therefore, XPS analysis is employed to investigated the formation and chemical states bonding structure of the as-synthesized MoS2/C NC as shown in Fig 3.17(a) As observed in Fig 4.17(a), wide scan (0–1000 eV) XPS spectra of the sample MoS2/C(1.5:1) synthesized at 230 ºC, pH ~7.2–8.8, with different reaction time reveal the presence of carbon (C), molybdenum (Mo) and sulfur (S) elements We applied Voigt function for fitting the high-resolution san XPS spectra, in which the experimental data points are displayed as dots and the solid line is the cumulative of fitted components
Figure 4.17 Chemical states and bonding structures of MoS2/C NC
(a) Wide scan (0–1000 eV) XPS spectra of the MoS 2 /C(1.5:1) NC samples synthesized at 230 ºC, with different reaction time showing carbon (C), molybdenum (Mo) and sulfur (S) elements; high-resolution core-level binding energy regions for (b) Mo 3d, (c) S 2p, respectively
It can be seen that it is not until the reaction time reaches 30 to 45 minutes, the presence of MoS2 phases started to appear The high resolution XPS spectrum of Mo 3d in Fig 4.17(b) clearly shows prominent peaks at ~232.3 and ~229.1 eV that are assigned to the
Mo +4 3d 3/2 and Mo +4 3d 5/2 orbitals in 2H-MoS2, respectively Whereas, XPS spectrum
65 of the S 2s in Fig 4.17(c) shows a main doublet located at binding energies of ~163.1 and ~161.9 eV, corresponding to the S 2p 1/2 and S 2p 3/2 orbitals of 2H-MoS2 In addition, the increasing of total Mo 3d binding energy versus the reaction time as calculated in Table 4.7 indicating the increasing of MoS2 domain in the as-synthesized MoS2/C NC
Table 4.7 Percentage of Mo 3d domain in MoS2/C NC
Sample (s) Mo 3d 3/2 Mo 3d 5/2 Total Mo 3d,
Area under fitted peak (eV × counts)
Area under fitted peak (eV × counts) MoS2/C(360) 232.72 136837.16 229.32 9531.89 91.20 MoS2/C(240) 232.44 112937.31 229.26 84883.06 88.62 MoS2/C(120) 232.64 99279.51 229.48 78276.34 81.44 MoS2/C(60) 232.55 26470.77 229.32 12652.50 79.34 MoS2/C(45) 232.02 32824.06 229.72 42112.83 74.06 MoS2/C(30) 232.67 25037.25 229.17 17430.04 70.97 MoS2/C(15) 232.26 128827.34 229.14 40745.38 66.54
From the high resolution scan spectrum of the Mo 3d region in Fig 4.17(b), two major peaks at ~229.2 and ~232.9 eV are observed which is assigned to the Mo +4 3d 5/2 and
Mo +4 3d 3/2 in 2H-MoS2, confirming the dominance of Mo(IV) in MoS2/C NC sample Besides the Mo(IV) 3d 5/2 signal, a peak appears at ~226.1 eV which could be from the
S 2s orbital Another peak at higher binding energy of ~235.2 eV relates to the Mo ions in the +6-oxidation state, which may be due to the inadequate reduction of MoO4 2− species during the hydrothermal synthesis From the higher resolution XPS spectrum of the S 2s in Fig 4.17(c), the main doublet located at binding energies of ~162.0 and
~163.2 eV corresponds to the S 2p 1/2 and S 2p 3/2 of pristine MoS2, respectively, which is consistent with previous reports [38], [108], [149] Figure 4.18 shows the high- resolution scan XPS result of C 1s region in Fig 4.17(a) One can see three resolved peaks of sp 2 -hydridized C−C/C=C at ~284.4 eV, sp 3 C−C at ~285.9 eV [192] When reaction time under 45 minustes, the C 1s region is dominanced by oxygenated functional groups (C−O, C=O, COO−) at binding energy of ~288–290 eV The large
66 quantity of these oxygenated groups is mainly originating from the precursors (CH3COO − and GO) that had not yet to converted
Figure 4.18 High-resolution XPS core-level binding energy regions for C 1s
Growth mechanism of 2D MoS 2 crystals on graphene
The microstructure, morphorlogy and hydrothermal growth mechanism of 2D nanopetal-like MoS2 on graphene are well characterized by FESEM and HRTEM As seen in FESEM images in Fig 4.19(a–g), and corresponding HRTEM in Fig 4.19(h– n), it is not until the reaction time reaches 45 min, the presence of MoS2 phases started to appear in which can be clearly distinguished on the carbon surface Although MoS2 morphology looks like particles distributed on carbon surface at around 30 min with an average size of ~0.7 nm in diameter as in Fig 4.19(b), but it rapidly gathered to form the petal-like shaped with the increasing of reaction time The thickness (w) of MoS2 nano petals is ~0.63–12.3 nm (Fig 4.19(c–f)) even as the wall itself continues to extend laterally over lengths (L) of order from ~50–360 nm or larger The resulting low-aspect ratio structure w/L ~ 0.0126–0.0342 may thus be well approximated as a surface that is left behind by a space curve, which represents the growth front and evolves over time
We interpret the crystallization of 2D MoS2 on graphene as a growth instability characteristic of a diffusion-limited process [193] which taken place under four regimes:
In the first regime: When dispersed in water by ultrasonication, rGO forming a negatively charged colloids due to the existence of defects and the ionization of the oxygen functional groups such as (epoxy (–O–), hydroxyl (–OH), and edges can comprise carboxyls (–C(O)O–) and ketones (–C=O), as illustrated in Fig 4.20(a)
Figure 4.19 The morphology evolution of MoS2 nanosheets on graphene surfaces
(a–g) FESEM and (h–n) HRTEM images indicating the growth process of MoS 2 nanostructures on graphene versus the reaction time
Under hydrothermal conditions, the tetrathiomolybdate MoO4 2– anions released from (NH4)6Mo7O24.4H2O and randomly adsorbed on the edges and surfaces of GO At the same time, the hydrosulfide/sulfide radicals (HS – •, S 2– •) were also forming from thioacetamide (CH3CSNH2) as strong reducing agents In this condition, the MoS4 2− anions were formed as illustrated in Fig 4.20(b), while the GO/rGO matrix was dopped by HS – • or S 2– • radicals forming SH-dopped or S-dopped rGO domains The formation of SH-dopped or S-dopped rGO domains in the synthesized MoS2/C NC is evidenced and supported by XPS data in Fig 4.17(b) in which the R-SH chemical states could be observed (R refers to the carbon region where the SH- attachs to) Interestingly, the R-HS peaks disappeared when reaction time approaching to 2 h The absence of the R-HS could be explained by the in-situ chemical reaction that occurring at the formed MoS4 2− radicals at the active sites to form amorphous MoSx clusters on rGO matrix The insight analysis indicates that the GO or rGO domains were almost reduced to graphene
Ultrasound employed in the pre-treatment provides acoustic cavitation near the graphene surface, which can weaken the vdW interactions among graphene sheets, rip the platelets apart, and restrict the aggregation of rGO sheets to thoroughly disperse in water This feature provides higher active surface for chemical interaction (require less energy, reduce reaction time) Since GO is not thermally stable material above 200 °C, the local high temperature (~5000 K) and high pressure (~1000 bar) with a heating and
69 cooling rate over ~10 10 K s –1 produced during the ultrasound irradiation itself able to reduce GO into rGO or graphene The radicals (–O•, –OH•, –C(O)O•, –C=O•) resulting from ultrasound irradiating oxygen functional groups on the basal plane of GO provide the highly active platform for inducing the chemical reaction as illustrated on Fig 4.21 The high local temperatures and pressures lead to the formation of MoO4 2–• radicals with uniform distribution on rGO, that facilitated or pre-conditioned the chemical reduction reactions with speeding up the reaction rate significantly
Figure 4.20 Illustration of reaction mechanism between Mo, S radicals with GO
(a) Atomic model of GO functionalized groups, (b) reaction mechanism between Mo-S radicals and active sites on GO to form MoS 2 /C NC
Figure 4.21 Schematic view of ultrasonic-assisted hydrothermal synthetic strategy
In the second regime: When thioacetamide is soluble in water, it releases H2S gas as a sulfide source (HS – , S 2– ), also a reducing agent (process (2)) while the buffer solution form of acetic acid help maintaining pH (~6–7) of the reaction system during the hydrothermal process These (MoO4 2–•) radicals were captured by oxygen functional on the GO through coordination leading to the restoration of sp 2 domains The acidic
70 conditions promotes the reduction of Mo(VI) to Mo(IV) In this regime, MoS2 first formation (process (3)) under the disordered in thin layers or islands forms
Figure 4.22 Illustration of the growth mechanism of 2D MoS2 crystals on graphene
In the third regime: The reducing agent (S 2– •) forms and interacts with (MoO4 2–•) radicals to form MoS2 clusters in-situ on graphene These MoS2 clusters immediately undergo a phase transition into 2H-MoS2 vertically along (002) planes on reduced graphene oxide surfaces and shaped in to thin 2D crystals with one to several layers of 2H-MoS2 This shaping process leaves behind an ultrathin wall of 2D MoS2 crystals after restacking of graphene and forming a layer-by-layer composite structure, as illustrated in Fig 4.22 The formation of 2D MoS2 nano petals in graphene matrix can be explained by chemical reaction as described in Eq (4.1) to (4.5):
CH3CSNH2 (s) + 2H2O → CH3COO − + 2H + + NH4 + + HS − (4.2)
MoO4 2– + 4HS – → MoS4 2– + 4OH – (4.3) MoS4 2– + H + → MoS3 + HS– (4.4)
In the fourth regime: Due to the continuous supplying of MoO4 2– building blocks to the reaction zones, the formation of 2H-MoS2 is also continuing to grow leading to the formation of petal-like geomertry of MoS2 nanocrystals In fact, the concentration of
Mo cations in the precursor is limited, the growing front of MoS2 petals will stop at a specific size And, the increasing of the reaction time may lead MoS2 petals to restack and form large spherical shapes which construct from several dozens of Mo-S-Mo single layer After front splitting, a diffusion-limited growth front lays down another thin- walled in the MoS2/C NC structure These petals continue growing and branching until the supplying of MoO4 2– and is disconnected
By the insight analysis the formation and crystallization of 2D MoS2 phase in MoS2/C
NC versus reaction time during the hydrothermal synthesis, we can determine the suitable reaction time ~2 h for successfully synthesizing MoS2/C NC system with low- dimension and robust structures.
Method for restoring sp 2 domain on rich-defect GO crystal
Through the investigation the nucleus formation, crystallization and growth mechanism of 2D MoS2 nanocrystals on the rGO sheets versus the reaction time during the hydrothermal synthesis of MoS2/C NC as previously analysis by XRD (Fig 4.15), XPS (Fig 4.17 and Fig 4.18), FESEM and HRTEM (Fig 4.19), and specifically Raman results (Fig 4.16), one can observe the significant decreasing in the I D/I G peak intensity ratio from ~1.13 (for GO) to ~ 0.18 (for the MoS2/C NC) after 360 minutes of hydrothermal growth as calculated in Table 4.6 The decreasing in the I D/I G ratio of the MoS2/C NC systems indicate that the degree of structural defects and disorder in graphene (sp 2 domain) versus the reaction time were significant reduced (D-band is almost absent) as reported in [168], [191] In addition, the characteristic 2D band of graphene domain located at ~2730 cm −1 re-appeared which implying that a large number of sp 2 domains in the chemically synthesized graphene oxide have been restored
Interestingly, the I – V characteristic of the prepared GO, bare 2D MoS2 and MoS2/C(1.5:1) nanocompsite systems synthesized at 230 °C, for 2 h was measured as seen in Fig 4 23 It can be clearly seen that the conductance G of the MoS2/C(1.5:1) is about ~ 63.9 μS at bias around zero, which is significant high when compared to that of the prepared GO (G ~ 0.026 nS) and 2D MoS2 (G ~ 0.35 nS) In addition, the non-linear
I – V curve of the prepared GO (in Fig 4.23(a)) and Plateau width of ~4.8 V (in Fig
4.23(b) transform to a linear curve (Fig 4.23(d)) and the closing of Plateau width as
72 observed in Fig 4.23(e) This indicate that the semiconductive property of the prepared
GO has been changed to high electrical conductivity Such a dramatic change in the electrical transport property and increasing in conductance may be caused by the anchoring of 2D MoS2 species onto the GO sheets, benefiting the restoring aromatic structures (sp 2 hybridization) and defects
Figure 4.23 Volt-Ampere characteristic of GO, 2D MoS2 and MoS2/C(1.5:1) NC
(a, d) I – V curves, (b, e) Plots of conductance G ≡ dI/dV – V, (c, f) Resistance R = 1/G of the prepared GO, bare 2D MoS 2 and MoS 2 /C(1.5:1) NC samples
Figure 4.24 Schematic illustration of method for restoring sp 2 hybridization in GO This observation suggests an effective method for restoration the sp 2 hybridization domain of chemically synthesized graphene from rich-defect structure of GO sheets, and
73 improving its high electrical conductance by in-situ anchoring 2D MoS2 crystals via the donor-acceptor structure at the GO defects sites as illustrated in Fig 4.24
Effect of hydrothermal reaction temperature
Temperature inducing the 1T to 2H-MoS 2 phase transition
In order to investigate the effect of the hydrothermal reaction to the formation of MoS2/C
NC, a series of experiments were carried out at different temperatures of 190, 210, 230 and 250 °C to obtain the MoS2/C NC samples (denoted as MoS2/C(190), MoS2/C(210), MoS2/C(230) and MoS2/C(250)) The hydrothermal reation time was controlled at ~2h The (Mo 4+ : C) molar ratio were fixed at ~(1.5 : 1) and pH value of the precursors were adjusted in range of ~7.2–8.8 The formation and crystalline structures of the MoS2/C
NC at different reaction temperature can be characterized by XRD analysis
Figure 4.25 XRD patterns of MoS2/C NC versus reaction temperature
(a) XRD patterns of GO, pristine 2H-MoS 2 and MoS 2 /C NC versus reaction temperatures; (b) Deconvolution of (002) characteristic peak of MoS 2 phases revealing the dominance of semiconductive (2H) and metallic (1T) phases with shifting tendency versus reaction temperature
As seen in Fig 4.25(a), the XRD patterns of the all MoS2/C NC show diffraction peaks at 2θ ~ 13.8º, 33.1º, 40.1º 49.3º and 59.1º correspond to (002), (101), (103), (105) and (110) crystal planes of MoS2, respectively, which is in good agreement with 2H-MoS2
(JCPDS 00-037-1492) without other peaks of impurities phases Theses observed peaks indicating the existence of MoS2 crystalline phases in four all samples and well agreed with previous reports [60], [72], [73] Samples MoS2/C(230) and MoS2/C(250) with higher reaction temperature show strong (002) peaks with narrow FWHM compare to samples MoS2/C(190) and MoS2/C(210) synthesized at lower reaction temperature as observed in Fig 4.25(b) These observations indicating that high crystalline phase MoS2 form at higher reaction temperature while lower reaction temperature result in low crystalline phase as evidenced in HRTEM images Fig 4.26 Interestingly, carefully investigate the strong (002) characteristic peak centered at around 2θ ~14.3º of MoS2 phase with applying the Gaussian peak fitting technique (Fig 4.25(b)) revealing the co- existance of both metallic (1T) MoS2 and semiconductive (2H) MoS2 and the transition 1T to 2H phase is induced when varying the reaction temperature from 190 °C to 250°C
The 1T to 2H phase transition in pristine MoS2 has been previously documented [32], [75], [76] However, in the MoS2/C NC this phenomenon has not been reported so far
In the present study, the strong (002) characteristic peak centered at ~14.3º of MoS2 crystalline phase can be used as an indicator to elucidate the 1T to 2H MoS2 phase transformation [43], [108], [178] The metallic phase 1T-MoS2 has been documented in superstructures with lattice constants such as 2a × 2a, 3a × √3a and 2a × a rather than ideal elementary cell of a × a unit cell by density functional theory (DFT) calculations [77], [178], electron diffraction [194] and scanning tunneling microscopy experimental studies [103], [104], [178] Therefore, a shifting tendency of (002) peaks versus diffraction angle (2θ) should be observed in XRD patterns of MoS2 phase according to Bragg’s equation due to the expanding of lattice constants Carefully examine the (002) peaks of all samples with 2θ in range of ~11–17º as plotted in Fig 4.25(b) showing the shifting tendency of (002) peaks to larger diffraction angles when the reaction temperatures increasing from 190 to 250 °C Upon increasing temperature, the structure of MoS2 phase is dramatically modified leading to the broaden of diffraction peaks in XRD patterns, indicating a short structural coherence length
For sample MoS2/C(250), the peaks centered at 2θ ~ 13.8º corresponds to (002) signature peak of 2H-MoS2, confirming the dominance of semiconductive phase Interestingly, deconvolution of (002) peak (located at 2θ ~ 14.3º) of sample MoS2/C(230) reveals the two peaks centered at 2θ ~ 13.8 and 14.4º The peaks centered at 2θ ~ 13.8º can be attributed to the (002) peaks of 2H-MoS2 while the peak at 2θ ~ 14.4º with d (002) ~0.63 nm matched with (002) peak of the 1T-MoS2 revealing the existence of a crystalline 1T-MoS2 phase in this sample The same result was also reported by M Acerce et al [43] for metallic 1T-MoS2 obtained by chemical exfoliation method, and Y Yu et al [107] for unstable 1T’-MoS2 phases prepared by thermal annealing combine with laser irradiation Similar phenomena can also be observed in sample MoS2/C(210) in which, Gaussian fitting technique help separated (002) peak at 2θ ~ 14.4º into two peaks centered at 2θ ~ 14.0 and 14.6º These first peak can be indexed to the (002) peak of 2H- and 1T-MoS2 phases existing in this sample For sample MoS2/C(190), there is only one diffraction peak of (002) planes located at 2θ ~14.6º that can only be addressed to the (002) peak of 1T MoS2 phase with reference to the prepared 1T-MoS2
The 1T to 2H phase transition versus the increase of reaction temperature is evidenced by the absence of the 2H phase in sample MoS2/C(190) It worth noticed that the shifting tendency of (002) peaks of 1T phases in samples MoS2/C(230), MoS2/C(210) and MoS2/C(190) is clearly observed that consistent with previously reports of Y Yu et al [107] and X Fan et al [179] All the aforementioned results confirm that the 1T-MoS2 phase in sample MoS2/C(190) is almost reversed to 2H phase in sample MoS2/C(250) This observation provides strong evidence support for the semiconductive to metallic phase transformation in MoS2/C composite have been successfully induced by controlling the reaction temperature Previously reports confirmed that monolayer 1T- and 2H-MoS2 can be directly distinguished by the intensity of sulfur atom columns in the STEM-ADF images that reported in [103], [125], [178], [195]
Figure 4.26 Microstructure of MoS2/C NC versus reaction temperature
(a–d) HRTEM images of the MoS 2 /C NC synthesized at different reaction temperature, (e–h) enlargement of dash square regions in (a–d)
Figure 4.27 STEM images of MoS2/C NC versus reaction temperature
Color enhanced of STEM images of single layered MoS 2 in (a) sample MoS 2 /C(190); (b) MoS 2 /C(210); (c) (MoS 2 /C(230) and (d) MoS 2 /C(250) The projection of the 1T and 2H phases are indicated by white triangle and yellow hexagon The white dash lines show the boundaries between these phases; (e–h) extracted surface profile along the dash lines in (a–d) showing the arrangement of Mo and S atoms in 1T and 2H symmetry
Therefore, to analyze the nanostructure and 1T to 2H-MoS2 phase transition the in MoS2/C NC, HRTEM and high-magnification STEM characterizations are conducted as shown in Fig 4.26 and Fig 4.27 The difference in symmetry between the octahedral coordinated 1T-MoS2 and the trigonal prismatic coordinated 2H-MoS2 crystal structures can be directly identified For sample MoS2/C(250), the MoS2 phase (Fig 4.27(d)) exhibits the lattice with a hexagonal shape projection confirming the 2H phase While two samples MoS2/C(210) and MoS2/C(230) (Fig 4.27(b, c)) exhibit both clearly observable triangular and hexagonal lattice regions indicating the existence of both 1T- and 2H- phases that well agreed with previous reports [103], [179], [195] As seen in Fig 4.27(a), sample MoS2/C(190) demonstrates the triangular lattice projection that representing for the dominance of 1T-MoS2 phase The white dashed lines in the high- magnification STEM images (Fig 4.27(b, c)) help separating boundaries between 2H and 1T phase regions The implanted 2H phase in the 1T-MoS2 matrix to form mixture of 2H- and 1T-MoS2/C composite, implying that the increasing of reaction temperature has induced the 1T to 2H phase transformation
The 1T to 2H phase transition creates interleaved nano sized domains with different orientation variants; the nano crystallinity engendered with the phase change allows a more homogeneous phase conversion and uniform charge distribution compared to the as-prepared 2H-MoS2 Interestingly, 2H phase is expanding with increasing of the reaction temperature from 210 °C to 230 °C and increasing to larger area when temperature up to 250 °C The underlying mechanism of the 1T–2H phase transition is attributed to shear-deformation of electronic band structure due to the hot electron- phonon interaction [196] between graphene surfaces and S-planes of MoS2 phase in MoS2/C NC as demonstrated in Fig 4.28(b) In 1T phase, the local strain in the MoS2 lattice remain occurring due to the local compression along these Mo–Mo zigzag chains
It is an intermediate state, but forms a stable (2H) structure at high temperatures The local strains, therefore, must be released by changing the Mo–S bond angles in such a S–S or Mo–Mo plane gliding fashion As a result, when temperature rapidly increased, the phase transformation from 1T to 2H was induced to excess strain in the MoS2 lattice lowering the system energy That increasing the reaction temperature rapidly allows a greater observable number of 2H-MoS2 regions
Figure 4.28 Mechanism of the 1T to 2H-MoS2 phase transition in MoS2/C NC
(a) Schematic illustration of 1T and 2H-MoS 2 atomic structures, (b) Explanation of 1T to 2H- MoS 2 phase transition mechanism induced by the generation of coherent acoustic phonons on graphene platform, (c) Schematic view of 1T to 2H-MoS 2 /C NC band alignment
The co-existence of both 2H and 1T phases is responsible for the deconvoluted of (002) diffraction peaks of samples MoS2/C(230) and MoS2/C(210) into two peaks (attributed to (002) peak of 2H and 1T-MoS2 phases), as well as their shifting tendency versus reaction temperature The insightful analysis by STEM as presented help confirming that the hydrothermal reaction temperature link to the structural transition from metallic (1T)to semiconductive (2H) MoS2 phase
In addition, the 1T to 2H phase transition of the MoS2/C NC samples could be well characterized by Raman spectra In Fig 4.29, we show Raman spectra of the synthesized
GO, MoS2/C NC and 1T phases of MoS2 compare with the spectra of the 2H phases, respectively A crucial difference between 1T-MoS2 and 2H-MoS2 lies within the symmetry of the S atoms in their structures Changes in the symmetry of S atoms in these structures lead to significant differences in their characteristic Raman vibration
Figure 4.29 Raman spectra of MoS2/C NC versus reaction temperature
(a) Raman shift (0–3000 cm −1 ) of the MoS 2 /C NC versus reaction temperature with characteristic vibration modes of (b) E 1 2g và A 1 g of MoS 2 (60–600 cm −1 ) and (c) D, G of graphene (1100–1700 cm −1 ) The characteristic Raman mode in (b) and (c) were fitted by applying Lorenzt function
Effect of reaction temperature on the electrical property of MoS 2 /C NC 84
To analyze the transport property of the MoS2/C NC with different reaction temperature, the I – V measurement of the corresponding devices were conducted The synthesis and characterization of MoS2/C NC samples at different reaction temperature were described in section 3.2.4 The typical (I − V) characteristic was carried out for the fabricated devices that made from samples MoS2/C(250) (device #1), MoS2/C(230) (device #2), MoS2/C(210) (device #3) and MoS2/C(230) (device #4) Representative source-drain
85 transport characteristics of the fabricated devices with the I – V curves, semi-log plot of
I – V curves, differential resistance, and dI/dV – V curves are shown in Fig 4.31 As previously examined by STEM and HRTEM in Fig 4.26(a–d), the heterostructure that formed by the growing of 2D MoS2 phase on graphene in the MoS2/C NC were clearly observed Therefore, the Schottky barrier [128] formed at the surface of the graphene and MoS2 phases may responsible for the asymmetric I – V curves The forward and reverse current are driven by the negative and positive bias voltage, respectively The forward (I F) and reverse (I R) current can be expressed as following equations [198]: exp( )
= − (4.7) where n is the ideality factor, k B is the Boltzmann constant, T is the Kelvin temperature,
A* is the effective Richardson constant, A is the contact area, and ϕ B is the Schottky barrier height
The Schottky barrier suppresses the reverse current, considering ϕ B is bias voltage- independent For samples MoS2/C(250) (device #1) and MoS2/C(230) (device #2), the I – V curves in Fig 4.31(a, b) has non-ohmic behavior and lower conductive in the sweeping bias (−10 to +10 V) The hysteretic I – V can be explained by the dynamic and nonlinear relation between drift (driven by electric field) and diffusion (driven by the concentration gradient) of charged carriers to form depletion regions on either side of the MoS2–graphene interfaces To investigate the current transport mechanism, the I –
V curves in forward bias sweep were fitted to the HRS and LRS conduction mechanisms
(Fig 4.31(f)) The initial LRS (from −10 to −2.5 V) changed to a HRS (from −2.5 to +2.5 V) and return to the LRS (from +2.5 V to −10 V) in both forward and reverse bias sweeping The nonlinear I – V behaviors of the device #1 and device #2 in both HRS and LRS can be explained by Schottky emission mechanism [198], which the current voltage characteristics are expressed as ln(I) ∝ V 1/2 The current fitting results in Fig 4.32(e, f) show that HRS of samples MoS2/C (250) and MoS2/C (230) are dominated by the Schottky emission mechanism (ln (I) ∝ V 1/2 )
Figure 4.31 Characteristic (I – V) curves of the as-fabricated devices
(a) Device #1 (sample MoS 2 /C(250)), (b) device #2 (sample MoS 2 /C(230), (c) device #3 (sample MoS 2 /C(210)) and (d) device #4 (sample MoS 2 /C(190)) The lower inset (a–d) shown the SEM images of the above corresponding devices Comparison of I –V characteristic (e), semi-log plot (f) and dI/dV–V curves (g) of all the fabricated devices
In case of sample MoS2/C(210) (device #3) (Fig 4.31(c)) and MoS2/C(190) (device #4) (Fig 4.31(d)), the I – V curves present a high conductive ohmic-like behavior The current is linear with source-drain biases, indicating that the contact between graphene and MoS2 phase is ohmic The current fitting results in Fig 4.32(g, h) of samples MoS2/C(210) and MoS2/C(190) further verify the Ohmic behavior (I ∝ V) in both LRS and HRS
Figure 4.32 Transport properties of the MoS2/C NC
(a) Semi-log plot and (b) dI/dV – V curves of the as-fabricated devices in forward bias I – V characteristic of n-type (c) and p-type semiconductors (d); (e, f) The current conduction mechanism of device #1 and #2 fitted to Schottky emission (ln (I) ∝ V 1/2 ); (g, h) The current conduction mechanism of device #3 and #4 fitted to Ohmic (I ∝ V) in forward bias (0–10 V)
In addition, the symmetry of the current with respect to the positive and negative biases and the sharper slope of the I – V curves further verifies the ohmic conductivity nature
88 of the samples with higher electronic mobility Therefore, the steady increase in conductance of samples MoS2/C(210) and MoS2/C(190) are observed in Fig 4.31(h) and Fig 4.32(b) when compare to that of samples MoS2/C(250) and MoS2/C(230) The semi-log plot of I – V curves of device #1 and device #2 (in Fig 4.31(g) and Fig 4.32(a)) have V-shaped with several escalated steps of HRS (or low conductance) while samples MoS2/C(210) and MoS2/C(190) show smooth curves without other shoulders Semi-log plot of I – V curves of device #3 and device #4 are smooth V-shaped due to their linear energy-dependent density of state (DoS) [199] The dI/dV – V plots or differential conductance (G ≡ ∂I/∂V or dI/dV) further highlights the different electronic properties of the MoS2/C NC It is known that the Plateau width was defined as the zero-current region (G ≈ 0) in the dI/dV plot The existence of Plateau width of ~4.9 V in bias range of (−2.9 to +2.0 V) for sample MoS2/C(250) and ~1.8 V in bias (−1.0 to +0.8 V) for sample MoS2/C(230) also confirm the semiconductive property of these devices The Plateau width (~4.9 V) of sample MoS2/C(250) become narrow compare to sample MoS2/C(230) (~1.8 V) When the reaction temperature vary from 210 °C to 230 °C, we observed electrical gaps opening at Femi level E f in samples MoS2/C(230) compare to MoS2/C(210) as seen in Fig 4.31(h) and Fig 4.32(b) The gapless dI/dV – V spectra in samples MoS2/C(210) and MoS2/C(190) indicate the forming of conduction paths that lead to the increase of conductivity of these samples The lower charge neutrality points (G = 0 at zero bias) of samples MoS2/C(250) and MoS2/C(230) shift to higher point for samples MoS2/C(210) and MoS2/C(190) Correspondingly, the conductance of samples MoS2/C(210) and MoS2/C(190) is ~0.033 and ~0.180 μS, respectively, indicating their metallic property with significantly increasing in conductance (by ~5.45 fold) Therefore, to obtain high electrical conductance the MoS2/C NC should be synthesized at the reaction temperature ~190–210 °C
The increasing of electrical conductivity of the fabricated devices can be explained by the metallic boundary underlying phase transition (metallic (1T) to semiconductive (2H)) occurred when increasing reaction temperature from 190 to 250 °C More specifically, the dominance of 2H phase of MoS2 in samples MoS2/C(250) and MoS2/C(230) as well as the presence of defects, grain boundary in the MoS2/C NC that develop mid-gap states (or trap states) [200] These localized mid-gap states that
89 appeared mainly in the projected density of states (PDOS) [29], of the atoms in the boundary region led to the existence of band gap of these samples Therefore, the corresponding dI/dV – V curves show HRS around their charge neutrality point as seen in Fig 4.31(h) and Fig 4.32(b) The implanting and growing of metallic (1T) phase resulting the co-existence of 2H- and 1T- phases in two samples MoS2/C(230) and MoS2/C(210) that injects charged carriers or n-doped to 2H-MoS2 domains in the MoS2/C NC For the phase transition from the 1T- to 2H-MoS2, the charge doping of either n- or p-type lowers the transition barrier and induces the phase transition as illustrated in Fig 4.33
Figure 4.33 Transport property mechanism of the MoS2/C NC
(a) Schematic illustration of the MoS 2 /C NC structure, (b) top-view and (c) side view of the contact layer between MoS 2 and graphene crystalline phases, (d–f) band alignment of MoS 2 and graphene energy diagrams explain the conductivity mechanism under applied electric field
Therefore, the semi-log plots in Fig 4.30(a) of device #1 shows p-type behaviors of charged carriers while device #2 indicates n-type characteristic with reference from Fig 4.32(c, d) Carefully examine the dI/dV – V curve of sample MoS2/C(250) revealing the suddenly change in the dynamic positive differential conductance (G) to negative differential conductance [185] at bias centered around −5.2 V (−0.046 μS) and +4.0 V (−0.014 μS), as clearly observed in Fig 4.31(h) and Fig 4.32(b) Whereas, sample
MoS2/C(230) shows negative conductance at bias centered at −8.8 V (−0.176 μS), +4.8
V (−0.069 μS), +8.0 V (−0.159 μS) and +8.6 V (−0.026 μS) The conductance drops due to the access high resistive states
In the MoS2/C NC system, considering MoS2 phases and graphene sheets are closely stacked, defects like charge impurities or structural defects, which acts as the long-range Coulomb scattering centers [201] The formation and rupture of conducting paths made up of many traps result in from the charge-trapping characteristics of graphene and MoS2
[186] The mid-gap states serve as charge scattering centers in the MoS2/C NC samples that lead to the comprehensive decrease of mobility Carriers are trapped and stored as quantum capacitance [185] leading to the negative electrical conductance as seen in Fig 4.32(a, b) The expanding of metallic (1T) phase of MoS2 the samples MoS2/C(210) and MoS2/C(190) is mainly responsible for the increasing of conductivity of these samples The semi-log (Fig 4.32(a)) and dI/dV – V plots (Fig 4.32(b)) of devices MoS2/C(250) further show sudden increase in current (ΔI ≈ 100 μA at ΔV = 0.6 V) within several bias step of 0.2 V This current spike is followed by a negative dynamic differential resistance (Fig 4.31(f)) that is commonly observed in memristive systems [46], [185] and capacitors This observed characteristic fascinating the MoS2/C NC for various potential applications such as multifunctional memory devices or supercapacitor.
Effect of reaction temperature on the PL property of MoS 2 /C NC
In this section, we further investigate the photoluminescence property of the as- synthesized MoS2/C NC by engineering their optical band gap The combination of 2D MoS2 and graphene in the composite architecture guarantees enhanced light-matter interactions, leading to enhanced photon absorption and electron-hole formation This allows development of extremely efficient flexible photovoltaic devices with high photoresponsivity For the sake of developing high performance photonic devices, it is undoubtedly important to have a comprehensive understanding on the fundamental optical properties of the working nanocomposite materials
As previously described in section 4.4.2, temperature strongly affect to the formation and microstructure of the synthesized MoS2/C NC In brief, lower hydrothermal reaction temperatures (~190–210 °C) leading to the formation of 2D MoS2 nanocrystalline
91 layered structure with short-range ordered distribution on the graphene sheets And higher reaction temperatures (~230–250 °C) will result in a highly crystalliny 2D MoS2 nanolayers with long-range ordered as seen in Fig 4.34 In addition, the co-existence of metallic (1T) and semiconductive (2H) phase of MoS2 component in the MoS2/C NC and their phase transition (1T to 2H) leading to the modification of energy band structure that induced significant variation in their corresponding optical response and photoluminescence emission properties In this part of the thesis, the author reports the investigation of optical response and photoluminescence emission of MoS2/C NC with different growing temperature related to their specific microstructure, providing a straight forward clarification for the above-mentioned arguments
Figure 4.34 HRTEM images of MoS2/C NC versus reaction temperature
It is well establishes that the reduced graphene oxide (rGO) behaves like semi-metal or semiconductor, therefore, the band gap in GO is ~2.7 eV while band gap rGO can vary from ~1.00 to 1.69 eV depending on the degree of reduction [202] In addition, previously studies reported that PL spectra of monolayer MoS2 at room temperature have two peaks, located at ~1.8 and ~1.92 eV which are assigned to the luminescence from exciton (A), exciton (B) [25], [131], respectively In this dissertation, the PL spectra of the MoS2/C NC systems synthesized at different hydrothermal reaction temperatures (190–250 °C) are shown in Fig 4.35(a), in which two distinct peaks at
~1.68 eV (A) and ~1.81 eV (B) and another peak at ~1.31 eV, are clearly observed
Figure 4.35 PL spectra of MoS2/C NC synthesized at different temperatures
(a) The PL emission of the the MoS 2 /C NC with various hydrothermal temperature growth, (b) Deconvolution of PL spectra in (a) reveal the mid-gap band luminescence, (c) Schematic illustration of the photoluminescence generation in MoS 2 /C NC, (d) photoelectron transfer dynamics in MoS 2 /C NC, (e) the compositions of exciton and trion in monolayer MoS 2 and their optical transitions
The PL spectra in Fig 4.35(a) exhibit a strong peak centered at photon energy (hv) ~1.68 eV (from exciton (A)), and other two detectable peaks at ~1.64 and ~1.84 eV which could possibly be attributed to the luminescence electron-bound exciton (or trion (A – )) and exciton (B) of ultrathin 2D MoS2 crystalline phase in the MoS2/C NC The formation mechanism of exciton A, B and trion A – under photon excitation are illutrated in Fig 4.35(e) In addition, the lower peak at ~1.33 eV could be the result of the rupture of conducting paths made up of traps states due to the charge-trapping characteristics at the contact phase of graphene and MoS2 These PL emission spectra are consistent with those reported in previous studies for few-layer MoS2 [23], [203]
In general, the PL intensity of 2D MoS2 based composite materials could depend on many parameters such as chemical doping, height, strain, thickness, the number of layers and defects of nanosheets It has been reported that the indirect gap of bulk MoS2 and MoS2 nanosheets with thickness greater than 5 nm do not exhibit photoluminescence [25], [122] As the layer number of MoS2 decreases, the indirect band gap changes into direct band gap with enhanced photoluminescence, which is due to confinement effect Interestingly, the as-synthesized MoS2/C NC still exhibits PL signal though thickness of 2D MoS2 nanosheets is about ~3.6–5.7 nm from the HRTTEM observation (Fig 4.34) The enhanced PL signal for few-layered MoS2 nanosheets anchored on graphene may be due to the structural discontinuity at the nanosheet edges, which induces variation of the electronic structure of MoS2 nanosheets Therefore, to optain the MoS2/C NC with large optical band gap, the reaction temperature should be controlled at ~230 °C
Carefully examine the PL emission spectra of the MoS2/C NC by employing Lorentz fitting technique as demostrated in Fig 4.35(b) PL spectrum of samples MoS2/C(190), MoS2/C(210), and MoS2/C(230) exhibit wide range emission at ~1.4–2.0 eV which can be resolved to five sharp peaks centered at ~1.51, 1.62, 1.68, 174 and 1.82 eV The splitting of these emission peaks can be an evidence for the mid-gap states (or mid-gap band) [200], [127] that originated from zero or one dimension defects in MoS2/C NC as illustrated in Fig 4.35(d) In addition, the PL intensity increase reaction temperature from 190 to 230 °C implying that the hydrothermal reaction strongly affect the PL emission of the MoS2/C NC samples as seen in Fig 4.35(a) The presence of these
94 luminescence peaks in PL spectra can be explained when considering the existence of structural defects in the MoS2/C NC Due to the lattice mismatch between 2D MoS2 and graphene, when MoS2 phase grow on graphene surfaces and edges, the interfaces between vertical hetero-layers can also be considered as 2D defects In this case, vdW force couples the MoS2 layers and graphene sheets Calculated band gap values of the MoS2/C NC for different reaction temperatures are listed in Table 4.11
The vdW interfaces has strong impact on the photoluminescence of few-layered MoS2/C
NC and could introduce a larger band gap that resulting two emission peaks at ~1.55 and 1.74 eV However, with increasing temperature to 250 °C, the A and B excitons are broadened and shift to lower energies PL intensity increase significantly as well as the absence of two resolved peaks at ~1.59 and 1.82 eV as observed in sample MoS2/C(250) Previously chapter (section 3.6, Chapter 3) show that the MoS2/C NC is dominant by metallic (1T-MoS2) and semiconductive (2H-MoS2) phases in the composite The metallic 1T-MoS2 phase is known to be more conductive due to the formation of conducting paths the charge-trapping characteristics inter-layer and intralayer at the contact phase of graphene and MoS2 When reaction temperature changes, the 1T to 2H phase transition occurs that injecting charged carriers or n-doped to 2H-MoS2 domains in the MoS2/C NC, leading to the the rupture of conducting paths made up of traps states and therefore expanding the band gap of this sample
Table 4.11 Band gap values of the MoS2/C NC for different reaction temperatures
Samples MoS2/C(190) MoS2/C(210) MoS2/C(230) MoS2/C(250)
In summary for this section, we obtained a series of MoS2/C NC systems with different hydrothermal reaction temperatures The PL intensity increase gradually with the
95 increasing of temperature The dependence of PL intensity and emission band are attributed to the formation and rupture of conducting paths made up of many mid-gap states which acts as the long-range Coulomb scattering centers that lead to the comprehensive decrease of mobility Defects like charge impurities or structural defects in the MoS2/C NC samples leading to the formation of mid-gap band PL emission and they could be tuned by controlling temperature This result provides a feasible approach to obtain vdWs heterostructures with controlled optical band and open up a possible route towards engineered band gap in the recently emerging vdW heterostructures.
Photoluminescence mechanism of MoS 2 /C NC
A strong PL emission intensity was observed in the experiments for the MoS2/C NC, which was attributed to the interfacial charge transfer from MoS2 nanocrystals to graphene matrix The mechanism of the PL in the MoS2/C NC is much more intriguing, because of the emerging of interlayer exciton transition The corresponding schematic illustration of the photoelectron transfer process is presented in Fig 4.36
Figure 4.36 Photoinduced electrons transfer from 2D MoS2 to graphene
The corresponding band alignment at the MoS 2 –graphene interface with (a) slightly p-doped graphene, (b) neutralized state, and (c) n-doped graphene
In the MoS2/C NC systems with the covalent donor–acceptor structure, the Fermi energy of MoS2 (E F) is higher than that of graphene Therefore, MoS2 occupies crystalline space as an electron donor through covalently bonding with graphene, thus facilitating the electron transfer from MoS2 to graphene It is well known that graphene as a perfect electron acceptor has been proven in many systems [15], [52], [173] The gapless spectra in mono- and bilayer graphene are protected by the symmetry between
96 the sublattices However, in chemical obtained graphene, such symmetry can be easily lifted by selective chemical doping of one of the layers or by applying a transverse electric field (gating) This leads to the opening of a significant gap, which can be seen in optical absorption and photoluminescence
The band gap structure of disorderd graphene is drastically different from that of the pristine monolayer The valence and conduction bands have parabolic dispersion and touch at zero energy (E F) [84] However, it also contains additional mid-gap bands that are offset from Fermi level by ~0.33 eV [95] On the other hand, the reported MoS2
Fermi level (~4.4 eV) [204] as shown in Fig 4.36(b), the estimated HUMO level of graphene is ~4.3 eV [127] which makes the photoexcited electrons can be effectively transferred from the graphene sheet to the MoS2 monolayers In addition, strong optical absorption (~10 7 m −1 ) [28] and a visible range band gap (~1.2–1.9 eV) [26] of MoS2 allow an exceptional photoresponse in graphene–MoS2 contact interface, with large quantum efficiency and photocurrent generation Upon photoexcitation, electrons can be rapidly excited from the valence band (VB) to the conduction band (CB), in both graphene with its gapless band structure and multilayer MoS2 with its indirect gap energy of below ~1.48 eV [25] The excited state electrons of MoS2 can be efficiently transferred to graphene, due to the slower excited state electron relaxation (hundred ps) of the pristine MoS2 The generated holes however, were left in the graphene sheet, which resulted a p-type doping of graphene, and suppressed the fluorescence from the garphene On the other hand, with the n-type doping process, MoS2 crystal with atomically thin thickness further push the doped electrons interacting with electron–hole pairs, and couple them into the trion [131] state, where two electrons and one hole are bound together (Fig 4.36(c)) When the charge transfer continues, the abundant doping electrons suppress the recombination of the electron–hole pair, and results in the PL red- shifting in Fig 4.35(b) This phenomenon is the signature of electron transfer from MoS2 to the graphene which hinders the recombination of electron-hole pairs created by the photoexcitation This electron transfer is not attributed to a strong coupling between MoS2 and graphene, but rather to a standard hopping from an electron in MoS2 conduction band to an unoccupied state at the same energy in graphene
In summary for this section, the investigations of optical response of 2D layered MoS2/C
NC and their tunability are the key to understand the fundamental of their band gap structures and electronic transitions Among several physical routes have been employed to modify the optical properties of graphene, 2D MoS2 and MoS2/C NC, such as layer thickness, mechanical stress and doping, our results suggest that the wide range band gap distribution of few-layers MoS2/C NC could be engineered by controlling hydrothermal reaction temperature and the pH values of the precursor’s solution in the synthetic procedure The MoS2/C NC are emerging as highly light sensitive with strong photoluminescence emission ability and absorption capacity Nevertheless, the demonstrated remarkable structural stability and optical band tunability of the MoS2/C
NC make these promising for applications such as new class of scalable optoelectronic memory devices, photodetectors, phototransitors, and photonic nanodevices
Effect of (Mo 4+ : C) molar ratio
Effect of (Mo 4+ : C) molar ratio on the architectures of MoS 2 /C NC
In order to investigate the effect of (Mo 4+ : C) molar ratio to the structural formation of MoS2/C NC, the XRD, STEM-EDX and HRTEM analyses were employed Different MoS2/C NC systems were synthesized at 230 °C, in ~2 h and the pH value of the precursors was adjusted in range of ~7.2–8.8 A serie of MoS2/C NC samples which are denoted as MoS2/C(0.5:1), MoS2/C(1.0:1), MoS2/C(1.5:1), MoS2/C(2.0:1) and MoS2/C(2.5:1) were controlled with (Mo 4+ : C) molar ratios of ~(0.5 : 1), (1.0 : 1), (1.5 : 1), (2.0 : 1) and (2.5 : 1), respectively The XRD patterns of the synthesized MoS2/C
NC samples are shown in Fig 4.37 while the EDX spectra in Fig 4.38 were used to indentifying the actual (Mo 4+ : C) molar ratio compared to that of the experimental design As seen in Fig 4.37, the XRD patterns of the all MoS2/C NC systems show diffraction peaks at 2θ ~ 13.8º, 33.1º, 40.1º 49.3º and 59.1º correspond to (002), (101), (103), (105) and (110) crystal planes of MoS2, respectively, which is in good agreement with 2H-MoS2 (JCPDS #00-037-1492) Theses observed peaks indicating the existence of MoS2 crystalline phases in four all samples In addition, for samples MoS2/C(0.5:1) and MoS2/C(1.0:1), the sharp peaks (marked by ▼) at ~8.7, 9.2º and a broad peak (marked by #) at ~18.1, 17.5º, respectively, appeared in XRD patterns along with clearly
98 observable characteristic peaks of 2H-MoS2 The higher (Mo 4+ : C) molar ratio is, the more intensity of these two peaks increases However, one can see that these peaks disappeared as the ratio of (Mo 4+ : C) molar exceeds value of (1.5 : 1) as recorded for samples MoS2/C(1.5:1), MoS2/C(2.0:1) and MoS2/C(2.5:1) These two new peaks with the interlayer spacing calculated by Bragg’s equation are ~0.96–1.10 and ~0.48–0.56 nm, are certainly not characteristic peaks of pristine MoS2 nor GO as two main components of MoS2/C NC that suggesting the underlying new crystalline structures
Figure 4.37 XRD patterns of MoS2/C NC with different (Mo 4+ : C) molar ratios
The elemental composition of the synthesized MoS2/C NC samples with diffrent (Mo 4+ : C) molar ratios were analyzed by STEM-EDX as shown in Fig 4.38 The recorded STEM-EDX energy spectrum indicates that the main chemical composition of MoS2/C samples are carbon (C), molydenum (Mo) and sulfur (S) The characterized peaks located at energy of ∼0.277 eV is originated from the X-ray energy of C Kα (∼0.225 eV) and other highest peaks at ~2.263 and 2.330 eV which correspond to the X-ray
99 energy of Mo Lα (~2.293 keV), Mo Kα (~17.453 keV) and S Kα (~2.307 keV)) elements that reflecting the main chemical composition of the samples From the quantification of elemental composition in Table 4.12, the chemical composition include (Mo : S) and (Mo 4+ : C) molar ratios can be calculated
Figure 4.38 STEM-EDX spectra of MoS2/C NC samples
Table 4.12 Chemical compositions of the MoS2/C NC
A careful inspection of HRTEM images (Fig 4.39) reveals that 2D MoS2 layers stacked with graphene sheets forming a layer-by-layer structure when (Mo 4+ : C) molar ratio is controlled around ~0.465 Details of this type of composite architecture is evidenced in HRTEM images (Fig 4.39(a, f)) and schematic illustration presented in Fig 4.39(k) When precursor with (Mo 4+ : C) molar ratio of ~1.167, the MoS2 layers stacked with several carbon sheets forming a sandwich-liked structure which is can be observed in Fig 4.39(b, g, l) And precursors with higher (Mo 4+ : C) molar ratios from ~1.464–
2.040, we found that 2D MoS2 grows on graphene surface along (002) planes forming a vertical-stacked architecture (Fig 4.39(b, g, l)) The number of individual MoS2 layer increased from 2–6 layers with thickness of ~1.3–3.6 nm The thickness of the MoS2 nanocrystals was measured from extracted depth profile along the dash lines as illustrated in Fig 4.39(c, d, h, i, m, n) When precursor with (Mo 4+ : C) molar ratio higher than ~2.0, the MoS2 nanoflakes restacked on the graphene sheets forming an anchored-like architecture Fig 4.39(e, f, n)
As described in the shaping mechanism in Section 4.3.2, the lower precursor molar ratio of ~1.0, the lesser concentration of MoO4 2– precursors in hydrothermal is, followed by less diffusion or supply of the MoO4 2– building blocks during the synthesis This results in a thin layer of MoO4 2– that electrostatic attachment on negative charged centers on
GO edges or surfaces Under a reduction of HS − in the hydrothermal condition, Mo 4+ ions were reduced and shaped into thin layers of 2H-MoS2 from one to several layers This shaping process leaves behind an ultrathin wall of MoS2 after restacking of graphene to form a layer-by-layer composite structure This structure enables a two new diffraction peaks as observed in XRD spectra as shown in Fig 4.37 The similar phenomena were attributed to the intercalation of oxidized DMF species into two S– Mo–S layers [30], oxygen incorporation into MoS2 layers [124], or caused by the inclusion of the CTA + ion into the MoS2/graphene structure [60] without necessary supported evidence For sample MoS2/C(2.5:1) with higher (Mo 4+ : C) ratio of ~2.444, the XRD data in Fig 4.37 of four samples appear similar to that of 2H-MoS2 containing a few layers as nanopetal-like shape anchored on graphene without graphitic diffraction peaks as described earlier according to our proposed mechanism
According to our experimental results, four types of MoS2/C nanocomposite structures could be observed as layer-by-layer (Fig 4.37(a, f)) for sample MoS2/C(0.5:1), sandwich-like (Fig 4.37(b, g)) for MoS2/C(1.0:1) sample, vertical stacked (Fig 4.37(c, h)) for MoS2/C (1.5:1) and Fig 4.37(d, i) for sample MoS2/C (2.0:1), and anchored (Fig 4.37(e, j)) for sample MoS2/C(2.5:1)
Figure 4.39 Microstructure of MoS2/C NC with different (Mo 4+ : C) molar ratios
(a–e) HRTEM images and (f–j) higher magnification of dash rectangles in HRTEM images (a– e), and schematic view of crystalline structure of the MoS 2 /C NC with the extracted depth profile along the dash lines
Effect of precursor molar ratio on the conductance of the MoS 2 /C NC 102
In this part, we analyze the transport property of the MoS2/C NC with different (Mo 4+ : C) According to our experimental results, when the (Mo 4+ : C) molar ratios were controlled at different values, the MoS2 nanocrystalline layers grow on graphene sheets that forming four types of MoS2/C NC structures include layer-by-layer, sandwich-like, vertical stacked, and anchored as seen in Fig 4.39 To investigate the I – V characteristic of the corresponding MoS2/C NC structures, the devices were fabricated include samples MoS2/C(0.5:1), MoS2/C(1.0:1), MoS2/C(1.5:1), MoS2/C(2.0:1) and MoS2/C(2.5:1) The representative I – V of the fabricated devices are shown in Fig 4.40
Figure 4.40 I – V curves of MoS2/C NC with different (Mo 4+ : C) molar ratios
(a) I – V curves, (b) semilog plots of I – V curves, (c) conductance G = dI/dV – V plots, (d)
Resistance R = 1/G of samples MoS 2 /C(0.5:1), MoS 2 /C(1.0:1), MoS 2 /C(1.5:1), MoS 2 /C(2.0:1) and MoS 2 /C(2.5:1)
The I − V curves of all devices in Fig 4.40(a) presents a high conductive ohmic-like behavior with the semi-log plot and dI–dV curve as shown in Fig 4.40(b) In these cases, the current is linear with source-drain biases (I ∝ V) in both LRS and HRS, indicating
103 that the contact between graphene and MoS2 phase is ohmic Semi-log plot of I – V curves of devices are smooth V-shaped due to their linear energy-dependent density of state (DoS) [184] The dI/dV – V plots or differential conductance (G ≡ ∂I/∂V or dI/dV) further highlights the different electronic properties of the MoS2/C NC systems
Table 4.13 Calculated conductance and resistance of MoS2/C NC
GO 3.0 0.193×10 −3 (0.150 ± 0.017) ×10 −3 6.483×10 6 Bare MoS2 3.0 1.055×10 −3 (0.360 ± 0.018) ×10 −3 2.819×10 6 MoS2/C(0.5:1) 3.0 278.118 98.815 ± 3.090 10.1 ± 0.4 MoS2/C(1.0:1) 3.0 255.200 89.885 ± 2.836 11.1 ± 0.4 MoS2/C(1.5:1) 3.0 197.080 68.272 ± 2.190 14.7 ± 0.5 MoS2/C(2.0:1) 3.0 66.455 23.391 ± 0.739 42.8 ± 1.5 MoS2/C(2.5:1) 3.0 35.821 14.114 ± 0.398 70.9 ± 2.8
Carefully examine the dI/dV curve of all MoS2/C NC devices showing that the conductance (G) is significant high at bias range from −10.0 to +10.0 V, and is decreasing with the increase of (Mo 4+ : C) molar ratios (Fig 4.40(a)) It also noted that the resisistance of the corresponding devices increase linearly with the increasing of (Mo 4+ : C) molar ratios, however, the resistance turns in to the HRS (non-linear) when the molar ratios increasing from (2.0 : 1) to (2.5 : 1) (Fig 4.40(d)) This phenomenon can be attributed to the underlying microstructure of samples MoS2/C(2.0:1) and MoS2/C(2.5:1) as observed in Fig 4.39(d, e) In these cases, the multilayred MoS2 crystalline phase anchored on graphene sheets to form composite structures The multi- layers 2H-MoS2 sheets with semiconductive property that led to the decreasing of electrical conductivity (G ~14.1−23.4 μS) as seen in Fig 4.40(c) and Table 4.13 However, with the (Mo 4+ : C) molar ratios lower than (1.5 : 1), the devices MoS2/C(0.5:1), MoS2/C(1.0:1) and MoS2/C(1.5:1) show higher conductance (G ~ 68.3−98.8 μS) when comparing to that of devices MoS2/C(2.0:1) and MoS2/C(2.5:1) In these devices, the MoS2/C NC are dominance with utrathin 2D MoS2 nanosheets, lower structural defects and charge impurities These kinds of microstructures characteristic could lead to the increasing of conductance of these MoS2/C NC systems
Table 4.14 Reported conductance of MoS2/C NC and MoS2 materials
MoS2 CVD 0.5 4.000 B Radisavljevic et al [4]
MoS2/graphene Exfoliated MoS2 flake stacked on CVD- grown graphene
MoS2 CVD 0.017–15.000 V K Sangwan et al [109]
Exfoliated-MoS2 flake stacked on CVD grown graphene
MoS2 CVD 0.1 0.400–0.500 H Schmidt et al [135]
There have been a number of studies reporting on the electrical conductance of GO, MoS2 and MoS2/graphene composites as summarized in Table 4.14 For instant, research group of Radisavljevic et al [4], synthesized MoS2 materials by CVD method and investigated its conductance by I – V measurement Their results showed that conductance reached about ~4.0 μS Or, as reported by Roy et al [47], the
MoS2/graphene composite was fabricated by combining exfoliated MoS2 in CVD- graphene composite system The conductance of this material is measured about ~3.0 μS When compare to the electrical conductance of the synthesized MoS2/C NC, one can clearly see the significant increase in conductance as seen in Table 4.14 The increasing in conductance can be fully explained based on the structural characteristics as previously analyzed by XRD, HRTEM, Raman and XPS results It has been elucidated that the MoS2 crystalline phase tend to grow in form of mono- or several layer structures such as “layer-by-layer” and “sandwich-like”, and dominated by metallic phase (1T) which could be responsible for the high conductance and significant increasing compared to GO, bare 2D MoS2 as observed
Electrochemical impedance spectroscopy of MoS 2 /C NC
Electrochemical impedance spectroscopy (EIS) is an important technique used to analyse electrode structure and kinetics To further examine the ion transport property and fundamental electrochemical behavior of the MoS2/C NC, the EIS analysis has been introduced to measure the impedance in the frequency range of 10 mHz–100 kHz at open circuit voltage (OCV) (~1.2 V), with an AC perturbation of 10 mV Impedance experiments were measured in 5 mM ferro/ferricyanide in 0.1 M KCl Electrochemical impedance of the electrodes fabricated from MoS2/C NC include MoS2/C(0.5:1), MoS2/C(1.0:1), MoS2/C(1.5:1), MoS2/C(2.0:1), and MoS2/C(2.5:1) are shown in Fig 4.41(a, c) For comparison, GO and bare 2D MoS2 based electrodes were also prepared The results are plotted as Nyquist plots (−Z Im vs Z Re), where Z Re (Ω cm –2 ) and Z Im (Ω cm –2 ) are the real and imaginary parts of cell impedance Z (Ω cm –2 ), respectively In addition, an impedance (Z, Ω cm –2 ) and phase angle plot (or Bode’s plot) versus frequency (f, Hz) of the MoS2/C NC electrode cell is also performed in Fig 4.41(b, d) The Nyquist plots obtained were modeled and interpreted with the help of an appropriate electric equivalent circuit (Fig 4.41(e, f))
As seen in Fig 4.41(c, d), the Nyquist and Bode plot for the fabricated electrodes made from bare 2D MoS2 show a single semi-circle region at high (HF) to medium frequency range indicating the capacitive or charge storage property of the MoS2 material This semi-circle defines the equivalent resistance includes the charge transfer resistance (R CT) and the corresponding double-layer capacitance (C dl) and Warburg diffused resistance
W (S s –1/2 cm –2 ) At OCV, the electrolyte resistance R S is found to be ~30.3 Ω, the total resistance (R CT) is about ~20.6 Ω cm –2 , and the C dl is ~0.23 mF cm –2 In contrast to the bare MoS2 electrode, the GO electrode shows a low resistance (R CT) of ~5 Ω cm –2 with the C dl of ~13 μF cm –2 For the MoS2/C NC with different (Mo 4+ : C) molar ratios, the corresponding electrodes were fabricated which are denoted as MoS2/C(0.5:1), MoS2/C(1.0:1), MoS2/C(1.5:1), MoS2/C(2.0:1), and MoS2/C(2.5:1) The Nyquist plot in Fig 4.41(a) show linear part in low-frequency (LF) region and single semi-circles region at HF region indicating the capacitive characteristic of the MoS2/C NC With the increase of (Mo 4+ : C) molar ratio from (0.5 : 1) to (1.5 :1), the equivalent resistance
(R CT) increase from ~12.0 to 12.5 Ω cm –2 , along with the significant increase of C dl from
~10.4 to 72.7 mF cm –2 indicating that the (Mo 4+ : C) molar ratio strongly affects the charge storage capacity of the MoS2/C NC
Figure 4.41 EIS characteristics of the electrodes fabricated from MoS2/C NC
(a, c) Nyquist plots and (b, d) Bode plot of the MoS 2 /C NC electrodes fabricated with different (Mo 4+ : C) molar ratios of GO and bare MoS 2 , schematic illustration of equivalent circuit (e) of MoS 2 /C NC electrode (f)
It can be observed that the integration of 2D MoS2 into graphene matrix to form MoS2/C
NC electrode decreases the charge transfer resistance compared to that of bare 2D MoS2
(~20.6 Ω cm –2 ) and GO (~5.5 Ω cm –2 ) electrodes as seen in Fig 4.41(a, c) and Table 4.15 The impedance of prepared MoS2/C NC electrodes in the current study are lesser owing to its good conductivity produced by the combined effect of graphene sheets with good conductivity and the distribution of ultrathin 2D MoS2 crystals in the composite
It also notices that the MoS2/C NC electrodes have much smaller equivalent series resistances than the bare 2D MoS2 and a nearly vertical line at the end of the semicircular region In addition, the “knee” frequency (f o, φ = 45º), which is defined as the maximum frequency at which the dominant behaviour of the typical capacitor can be observed for these electrodes The knee frequency (in Fig 4.41(b)) or the frequency at maxima of
−Z Im(ω) in HF region (f o) of the MoS2/C(1.5:1) nanocomposite electrode is ~0.33 Hz at a phase of −36.9º, which exhibits a rather pure capacitive behavior [43], [178]
Table 4.15 Capacitance and electrochemical parameters of the MoS2/C NC
The knee frequency relates to the rate or power capability of the supercapacitor; It also can be used to determine the electrochemical capacitance retention ability and indicate the limit between the resistive and the capacitive behaviours The higher the f o value is, the more rapidly (characterized by τ o (ms)) such a supercapacitor can be charged and discharged Most of its stored energy is accessible at frequencies below ~3 kHz At open circuit voltage, the double-layer capacitance C dl of MoS2/C NC electrodes are about
~10.4–72.7mF cm –2 , which are higher than the bare 2D MoS2 and GO as seen in Table 4.14 The high double-layer capacitance of the MoS2/C NC suggest it as a good candidate for supercapacitor applications However, the double-layer capacitacne C dl
108 decreases from ~72.7 to 32.5 mF cm –2 when the (Mo 4+ : C) molar ratio increase from (1.5 : 1) to (2.5 : 1) as seen for electrodes MoS2/C(2.0:1) and MoS2/C(2.5:1), respectively It may be attributed to the vertical-stack and anchored structures of thicker 2D MoS2 flakes on graphene surfaces that decreasing the electrochemical active surface These nanocomposite structures could increase the R CT ~12.5 to 47.2 Ω cm –2 and decrease the double-layer capacitance (C dl) of as-fabricated MoS2/C NC electrodes
The capacitive or energy-storage mechanism of bare 2D MoS2 in aqueous supercapacitor is well described in the literatures [43] It is the combined phenomena involving transition from electric double-layer capacitors (EDLCs) (non-Faradaic) to pseudocapacitive (Faradaic/redox) process and increased active surface area due to possible minimizing the number of layers in MoS2 crystals First, there is the accumulation of ions at the double layer interface between the MoS2 nanopetals and the electrolyte as illustrated in Eq (4.8a) and Fig 4.39(f) This is subsequently accompanied by a redox process: upon charging (reduction) the alkali metal ions in the electrolyte (K + , in the present experiments) adsorb onto the surface and intercalate between the MoS2 layers, followed by deintercalation upon discharging (oxidation), as shown in Eq (4.8b)
The presence of graphene in the nanocomposite system reduces the interlayer resistance and contact resistance with external circuit, provides a high conductive large specific area for distributing 2D MoS2 nanostructures on its surfaces and edges It also prevents the agglomeration of the 2D MoS2 nanosheets due to intercalating capability of ions through the van der Waals gap of MoS2 Therefore, the in-situ growth of 2D MoS2 nanostructures on graphene possesses more active sites towards the accessibility of electrolyte ion and thereby enhances the capacitance behaviour of the hybrid material However, the increase of MoS2 phase or (Mo 4+ : C) molar ratio, significantly decreases the capacitance of the MoS2/C NC electrodes The repeating intercalation- deintercalation process of the potasium ions over several cycles leads to partial
109 exfoliation of the 2D MoS2 layers, resulting in an increased surface area and enhanced specific capacitance.
Specific capacitance of MoS 2 /C NC
Cyclic voltammetry (CV) is generally used to characterize the capacitive behavior of an electrode material Figure 4.42(a) shows the typical CV curves of as-prepared MoS2/C(1.5:1) electrodes at different scan rates from 5 to 100 mV s −1 using 3 M KCl Ag/AgCl solution CV curve of MoS2/C(1.5:1) nanocomposite electrodes show high specific capacitance and relatively more rectangular in shape, approaching the ideal capacitive behavior This is ascribed to the well-dispersing of 2D MoS2 nanopetals on highly conductive graphene surface The specific capacitance (C sp, F g −1 ) of the electrode can be calculated according to the following Eq (4.9):
(4.9) where |Q A| (C) and |Q C| (C) are the charges stored in the anode and cathode (recorded from the PGSTAT302N instrument), respectively The mass of working electrode is m, (g) and the potential window of the CV curve is expressed as ΔV, (V) The energy density (E, Wh kg –1 ) and power density (P, W kg –1 ) were calculated from cyclic voltammograms by following the Eq (4.10) and (4.11), where ΔV is the potential window, ν (mV s −1 ) is the scan rate
The specific capacitance of GO, the bare 2D MoS2 and MoS2/C NC electrodes with different (Mo 4+ : C) molar ratios include samples MoS2/C(0.5:1), MoS2/C (1.0:1), MoS2/C(1.5:1), MoS2/C(2.0:1) and MoS2/C(2.5:1) are plotted as a function of scan rate in Fig 4.42(a) The calculated specific capacitance of all samples is presented at dots while the solid lines are curve fitting results of experimental values
Figure 4.42 Electrochemical performance of MoS2/C NC in liquid electrolytes
(a) Cyclic voltammograms of MoS 2 /C(1.5:1) electrode at different scan rate, (b) comparision of specific capacitance of MoS 2 /C NC electrodes with different (Mo 4+ : C) molar ratios The measured values are presented as dots and the curves are fitting values
As seen in Fig 4.42(b) the MoS2/C NC electrodes exhibits higher rate performance than bare 2D MoS2 and GO particualrly at higher scan rate, which is contributed from extra interface at the hybridized interlayer areas The electrochemical tests of the electrode materials were performed in a three-electrode cell using aqueous 3 M KCl vs Ag/AgCl electrolyte The MoS2/C(1.5:1) nanocomposite electrodes exhibit better electrochemical performance than pure GO and 2D MoS2 with a working potential window of −1.10 to 0.10 V (vs Ag/AgCl) The working electrode fabricated from MoS2/C(1.5:1) nanocomposite exhibits a higher specific capacitance of ~122.2 F g −1 at 5 mV s −1 , much better than GO (~1.8 F g −1 ) and 2D MoS2 (~18.2 F g −1 ) As seen in Fig 4.42(b) and Table 4.16, the increase of (Mo 4+ : C) from 0.5 to 1.5 in samples MoS2/C(0.5:1), MoS2/C(1.0:1) and MoS2/C(1.5:1), leading to the increase of specific capacitance However, when the (Mo 4+ : C) increase from (1.5 : 1) to (2.5 : 1) in samples MoS2/C(1.5:1), MoS2/C(2.0:1) and MoS2/C(2.5:1), the specific capacitance tend to decrease In addition, the increasing of scan rate may lead to the decrease of capacitive value Usually, at higher scan rate, sufficient ion diffusion cannot take place within a constant time, and therefore, reflects a reduction of capacitance values
The enhancement of capacitance and rate capability of the MoS2/C NC can be attributed to the two factors: (i) mesoporous structure of 2D MoS2 nanopetals dispersed on
111 graphene which exhibit short diffusion path lengths and favoring fast diffusion of electrolyte ions during charged/discharge process, and, (ii) graphene acts as better electron transport channel and offers better interconnectivity among 2D MoS2 nanostructures
Table 4.16 Specific capacitance of the GO, 2D MoS2 and MoS2/C NC
Table 4.17 Electrochemical properties of the MoS2/C NC
This futher provides electrochemical double layer capacitance to increase total capacitance In case of samples MoS2/C(2.0:1), MoS2/C(2.5:1) with high (Mo 4+ : C) molar ratios, the MoS2/C NC compose of multi-layers MoS2 anchored on graphene surface and restack to form bulk 2H-MoS2 crytalline phase This microstructure (Fig 4.39) result in lower conductivity as described in Section 4.5.1, that limit the diffusion
112 of electrolyte ions during charged/discharge process As a result, the specific capacitance of these samples was decreased The specific capacitance and performance of the MoS2/C NC electrodes are summarized in Table 4.17
Table 4.18 Comparison of specific capacitance with reported literatures
MoS 2 /C NC 3-Electrode 1.2 122.2 87.98 366.6 This work
As seen in Table 4.18, when comparing with the reported specific capacitance of MoS2- rGO composite (~75 F g −1 ) obtained by G Huang et at [61], or MoS2 nanospheres (~103.0 F g −1 ) synthesized by X Zhou et al [147], the specific capacitance of the as- synthesized MoS2/C NC in this work is significant high, with larger operation potential window (~1.2 V) These results suggest that the hydrothermal method employed in this work is more effective for synthesizing MoS2/C NC with short reaction time and the ability to control crystalline size and morphology of the as-synthesized MoS2/C NC as well as their high specific capacitance and excellent electrochemical properties.
UV-Vis absorbance of MoS 2 /C NC
Atomically thin graphene and MoS2 materials hold great promise in electronic and photonic applications because they possess unique properties inherited from the ultrathin planar structures, which allow for the fabrication of thinner, more flexible and more efficient devices Graphene has high carrier mobility [8], broad absorption spectrum [117], and fast response time [206] It can also absorb and convert light into a
113 photocurrent However, it absorbs only ~2.3% [114] of visible electromagnetic spectrum and its fast recombination of photoexcited carriers might limit it from many optoelectronic applications On the contrary, MoS2 is an emerging 2D nanomaterial with a direct and finite band gap that led to its strong photoluminescence (PL) capability This unique property originates form the quantum confinement effect [26] associated with the transition from an indirect band gap to direct band gap when its thickness is reduced to mono or several layer For the MoS2/C NC systems, the exciton-related phenomena and the intrinsic properties of excitons in the composite architectures are of great interest but remaining unexplored Therefore, in this section, the optical response and photonic properties of the MoS2/C NC were investigated
The absorption spectroscopic characterization was carried out for the bare MoS2 and GO dispersions as seen in Fig 4.43(a) The 2D MoS2 is well doccumented for the unique band gap transition from indirect to direct when the layered structure reduces from bulk to monolayer [25], [26], [105] While the absorption spectra of GO dispersion show characteristic absorbance band around ~295–325 nm which is due to the π to π* transition in the GO electronic structure [207] The absorption spectra of bare MoS2 dispersion exhibit obvious saturable absorption response (~616 and 676 nm) from the visible (~400–760 nm) to near Infared (NIR) regions (from ~780 nm) implying a broadband non-linear optical response performance
These absorption spectra are comparable with those of previous reports [179], [208], confirming the existence of high-quality MoS2 nanosheets in the dispersions For mono or several layers MoS2 crystals, the splitting in valence band (VB) at K point in first Brillouin zone gives rise to two direct interband optical transitions, known as the A and B excitonic transitions [131] Therefore, it can be clearly observed that the characteristic A and B exciton peaks of MoS2 originating from the interband excitonic transition at the K point are located at ~676 nm (~1.83 eV) and ~616 nm (~2.01 eV), respectively, indicating a pristine 2H-MoS2 poly-type Referring to the relationship between exciton energy of the A peak and the thickness of the MoS2 crystals [30], [122] the average thickness of the nanosheets in the MoS2/C NC dispersions could to be
114 estimated ~3.6–5.8 nm, equivalent to 6–9 of Mo–S–Mo monolayers which is well agree with experiment results in Section 3.4.3
To investigate the dependence of the optical response characteristic of the MoS2/C NC with different quantity of MoS2 crystalline phase or (Mo 4+ :C) molar ratio, and thus, bandgap structure of the MoS2/C NC, we prepared another batch of MoS2/C NC dispersion, which are denoted as MoS2/C(0.5:1), MoS2/C(1.0:1), MoS2/C(1.5:1), MoS2/C(2.0:1) and MoS2/C(2.5:1) The absorption spectra of these MoS2/C NC dispersions are presented in Fig 4.43(b)
Figure 4.43 UV-Vis spectra of GO, bare 2D MoS2 and MoS2/C NC
(a) UV-Vis absorpbance spectra of GO and MoS 2 dissolved in DI water, (b) UV-Vis absorpbance spectra of MoS 2 /C NC synthesized with different (Mo 4+ : C) molar ratios, (c) Aqueous dispersion of GO, MoS 2 and MoS 2 /C(1.0:1) samples, (d) Optical micrograph of MoS 2 /C NC
In contrast with GO and bare 2D MoS2, the MoS2/C(0.5:1) dispersion exhibit obvious saturable absorption response from the visible (~616 and 668 nm) regions which are attributed to the A and B exciton peaks of MoS2 phase Whereas, the absorption band at
~325 nm certainly originates from the π to π* transition in the GO The larger absorbance
115 values (~38%) of MoS2/C(0.5:1) with the same input light intensity, indicating an enhanced light-matter interaction compared to graphene (~3.9%) or bare MoS2 (~16%) The absorption response of the MoS2/C NC dispersions are listed in Table 4.19
Table 4.19 Optical response of the MoS2/C NC
Sample(s) Absorption wavelength λ max, nm
When the (Mo 4+ : C) molar ratio increase as observed in MoS2/C(1.0:1), and MoS2/C(1.5:1), the graphene matrix significantly alters the electronic band structure and optical properties due to the change in interlayer interactions, microstructures and morphologies of MoS2 nanocrystals in the MoS2/C NC The increase of light absorption at wavelengths shorter than 550 nm is due to the intrinsic absorption from graphene (~325 nm) and synergetic effect between graphene and distribution of MoS2 nanolayers (~467 and ~531 nm) in the MoS2/C NC Due to the difference in thickness distribution for MoS2 nanosheets in the MoS2/C NC dispersions upon varying the (Mo 4+ : C) molar ratios, the light energy absorbance will change accordingly As the (Mo 4+ : C) molar ratio increase, the 2D MoS2 with few-layers verticle-stacked or anchored on graphene sheets to form MoS2/C NC leading to the strong absorption of the samples MoS2/C(2.0:1) (~41–52%) and MoS2/C(2.5:1) (~52–82%) in the visible range (400–760 nm) This result indicated that MoS2/C NC can be considered to be a broadband saturable absorber with an operation regime from the visible to the near-infrared
Effect of precursor pH value
Effect of pH on the stacking density characteristic of MoS 2 /C NC
The pH value plays an important role in controlling the growth of crystalline materials, specifically MoS2 nanostructure In this section, the effect of pH values of the precursor was systematically investigated To synthesize MoS2/C NC with different pH condition include acidic (~3.0–6.6), near-neutral (~6.0–7.0) and alkaline (~8.0–10.0) conditions The reaction temperature, reaction time and (Mo 4+ : C) molar ratio were controlled at
~230 C, ~2 h and ~ (1.5 : 1), respectively In the experiments, the pH values of the four precursors were prepared with ~3.0–4.0, ~5.0–6.0, ~7.0–8.0 and 9.0–10.0 However, the actual pH values of these samples were measured at ~3.3–4.8, ~5.4–6.6, ~7.2–8.8 and 9.3–10.8, respectively Therefore, the corresponding experimental results of the MoS2/C
NC systems were referenced with these above measured pH values
In XRD patterns of four samples (Fig 4.44(a)), the presence of all characteristic peaks centered at 2θ ~14.2; 33.4; 39.3; 44.5, 49.1 and 58.2º that matched well with diffraction peaks of (002), (100), (103), (006), (105) and (110) planes of MoS2 (2H-MoS2, JCPDS
#00-037-1492) These observed characteristic peaks indicating the existence of 2D MoS2 crystalline phase in the MoS2/C NC samples The XRD patterns of results show that, MoS2/C NC systems obtained in pH values of ~3.3–4.8 and ~5.4–6.6, have the lower (002) peaks intensity, indicating that the growth of (002) plane can be inhibited at low pH values While the precursors with pH of ~7.2–8.8 and ~9.3–10.8 can accelerate the growth rate of 2D MoS2 crystals leading to the increase of (002) peaks intensity and area under these peaks as observed in Fig 4.44(b) The large FWHM of the (002) peaks shows that a short-range MoS2 nanocrystal is dominant in the MoS2/C
NC synthesized with acidic precursors (pH ~3.3–4.8 and ~5.4–6.6) However, the intensity of (002) peaks and the area under these peaks increased significantly with increasing pH value from neutral to alkaline precursor as seen in the synthesized MoS2/C
NC systems (pH ~7.2–8.8 and ~9.3–10.8) In addition, the FWHM narrowing of these (002) peaks compared with the increasing in pH value indicates an increase in crystallinity and thickness of the 2D MoS2 nanocrystals phase along the [001] direction
Figure 4.44 XRD patterns of the MoS2/C NC with different pH values
Figure 4.45 HRTEM images of MoS2/C NC with different pH values
(a–d) HRTEM images of the MoS 2 /C NC with pH = ~3.3–4.8; pH = ~5.4–6.6 (c) pH = ~7.2–8.8, and pH = ~9.3–10.8, respectively; (e–h) higher magnification of HRTEM images in (a–d), respectively
In aqueous solution, family of polymolybdate ions such as [MoO4] 2− , [Mo7O24] 6− , and [Mo8O26] 4− are in rapid equilibria The resulting species depend on the pH and the concentration In the hydrothermal condition, the anion O2 2− in [MoO4] 2− is easy to be replaced by S 2− during the sulfidation process owing to weak Mo–O bond in [MoO4] 2− Meanwhile, it is easy to dehydrate and condense into polymolybdate group in acid solution [22] Comparing to [MoO4] 2− it is more difficult for the polymolybdate group to generate MoS2 In the alkaline solution (pH ~ 7.2–10.8), the precursors dominated by [MoO4] 2− which can be easily reduced to MoS2 under hydrothermal condition As previously reported [75], [132], the (002) crystalline plane is the thermodynamically stable plane of 2D MoS2 nanostructure which is favourable for 2D MoS2 nanolayers to grow along this direction Therefore, the 2D MoS2 nanopetals grow rapidly along (002) plane with fast stacking number of layers, as identified by the bulk morphology of MoS2 as observed in Fig 4.45(d, h)
However, in the acidic solution (pH ~ 3.3–4.8), the precursors are dominance by polymolybdate group [Mo8O26] 4− rather than [MoO4 2−], that limted its interaction with reducing agents (SH − , S 2− ) and decreased the growth rate of 2D MoS2 crystals along (002) planes The obtained 2D MoS2 nano crystals are ultrathin layers as observed in Fig 4.45(a, e) and Fig 4.45(b, f), which could be attributed to the less stacking along (002) owing to the decreasing growth rate of MoS2 For the near neutral condition (pH
~ 7.2–8.8), both [MoO4 2−] and polymolybdate group [Mo8O26] 4− coexist in the precursor, the morphology of 2D MoS2 nanocrystalline phase in MoS2/C NC is between the above cases as seen in Fig 4.45(c, g) These results suggest that a low pH condition can help to control the growth rate of (002) plane, thereby, controlling the microstructure and morphology of the MoS2/C NC systems.
Effect of pH value on the photoluminescence of MoS 2 /C NC
As detailed analysis in Section 4.6.1, the pH value plays an important role in controlling the crystalline process, microstructure and morphology of MoS2/C NC Therefore, pH value of the precursor strongly affects the electronic band structure an optical band gap of the MoS2/C NC systems In brief, in the acidic solution (pH ~ 3.3–6.8), the obtained 2D MoS2 nanocrystals are low crystallinity as seen in Fig 4.46(a, b) However, in the
119 alkaline solution (pH ~ 8.0–10.8), the 2D MoS2 phase grow rapidly to form nanosheets with high crystallinity as observed in Fig 4.46(d) And for the neutral condition (pH ~ 7.2–8.8), the 2D MoS2 phase occupied space in MoS2/C NC with ultrathin layers morphology (several monolayers) as seen in Fig 4.46(c) To investigate the effect of the precursor pH value to the optical property of MoS2/C NC, the prepared samples were characterized with photoluminescence measurement
It is notice that the PL emission signal is strongly enhanced when a direct band gap is present in the electronic band structure of material The PL spectra of the MoS2/C NC synthesized at different pH values are shown in Fig 4.47(a) One can see that the MoS2/C NC exhibit a broad range luminescence in the visible (~350–760 nm) and NIR (~800–1000 nm) electromagnetic radiation spectrum, where two distinct peaks at ~1.76 eV (A) and ~1.89 eV (B) and two other peaks at ~2.31 eV (C) and ~1.67 eV (A – ) are clearly observed In addition, deconvolution of the PL spectra by employing Voigt fitting function could extract more information about the optical band structure of the composite samples as seen in Fig 4.47 (b) The strong peak centered at photon energy of ~1.76 eV originates from exciton (A), and other two detectable peaks at ~1.67 and
~1.89 eV which could be attributed to the trion (A – ), and exciton (B) of ultrathin 2D MoS2 crystalline phase in the MoS2/C NC
Furthermore, PL peaks at ~2.23–2.34 eV, can be attributed to PL emission from the transfer process on the graphene matrix to the MoS2 interfaces that have also been reported by D Pierucci et al in previous study [127] This process is reflected on the
PL spectra of MoS2/C NC denoted as peak (C) Other two peaks located at ~1.54 and
~1.60 eV could be originated to the mid-gap band that forming from structural defects characteristic of the MoS2/C NC vdW architecture as previously described [127] The MoS2/C NC systems synthesized with low pH value (acidic precursor) show a low PL intensity when compare to that of MoS2/C NC samples prepared at high pH values (alkaline precursor) As pH increases, PL emission increase significantly (Fig 4.47(a)) These features can be explained by considering the favourable growth and refinement of crystalline structure of 2D MoS2 phase in the MoS2/C NC systems
Figure 4.46 HRTEM and photographs images of the MoS2/C NC
HRTEM images of MoS 2 /C NC synthesized at (a) pH ~ 3.3–4.8; (b) pH ~ 5.4–6.6; (c) pH ~ 7.2–8.8, (d) pH ~ 9.3–10.8, (e) Photographs of bare 2D MoS 2 , GO and MoS 2 /C NC dispersions in DI water
Figure 4.47 PL spectra of MoS2/C NC synthesized at different pH values
(a) Area stacked plot of PL emission of MoS 2 /C NC with different pH values, (b) Voigt fitting results of MoS 2 /C NC samples The dot (○) represents the experiments values while the solid lines are the cumulative fitting values
The insight HRTEM images as seen in Fig 4.46(a–d) elucidated the short-range crystalline and wide-range thickness distribution of semiconductive 2D MoS2 phase in MoS2/C NC obtained from low pH condition, while at high pH values, 2D MoS2 phase forming long-range crystalline structure with high crystallinity The wide range distribution of PL emission for MoS2/C NC may be due to the structural discontinuity at the nanosheet edges, which induces variation of the electronic structure of MoS2 nanosheets In addition, the structural formation and crystallinity refinement of 2D MoS2 nanocystals in the MoS2/C NC under pH conditions significantly alters their electronic band structure and optical properties Therefore, to obtain the MoS2/C NC with large optical band gap, the pH value of the precursor should be controlled at ~7.2–8.8 The calculated band gap values of the MoS2/C NC are listed in Table 4.20
Table 4.20 Calculated band gap values of the MoS2/C NC for different pH values
MoS2/graphene nanocomposites pH ~3.3–4.8 pH ~5.4–6.6 pH ~7.2–8.8 pH ~9.3–10.8
In summary for Chapter 4, by systematically investigating the hydrothermal systhesis of MoS2/C NC and the effect of experimental parameters, the suitable hydrothermal conditions have been determined to successfully synthesized MoS2/C NC materials with metallic (1T- MoS2) or semiconductive (2H-MoS2) phase composition dispersed on graphene matrix The MoS2/C NC systems with low dimensions, unique and robust structures, high electrical conductivity, wide band gap and “memristive” properties of 2D MoS2 phase provide great potential for applications in energy storage devices, wideband electromagnetic wave absorbers, optoelectronics and memristors
The essential conclusions can be extracted from this dissertation include:
(1) The MoS2/C NC systems have been successfully synthesized at 230 °C, in ~2 h, from graphene oxide (GO) dispersion (1.0 mg L −1 , ~84.73% C) and Mo 4+ and S 2− sources The molar ratio (Mo 4+ : C) and pH are controlled at ~(1.46 : 1) and ~7.2–8.8, respectively Two-dimensional (2D) MoS2 petal-liked crystals, thickness of ~0.63–3.69 nm was in-situ grown on graphene sheets forming “sandwich”, “layer-by-layer”,
“vertical-stacked” and “anchored” nanocomposite structures
(2) The underlying crystallized mechanism of 2D MoS2 on GO undergoes four regimes that could be addressed as, i) a diffusion-limited process and interaction of molybdate ions with precursors, ii) hydrothermal reaction between molybdates and sulfur ions at the rich-active sites GO matrix to form nucleus, iii) the growth instability characteristic of a MoSx in unsaturated hydrothermal medium, and iv) the phase transition of MoSx polymorphs to 2D MoS2 nanostructures In the context, the important role of graphene oxide is highlight as a suitable platform for growing such 2D MoS2 nanostructures
(3) The suitable hydrothermal conditions have been determined to fabricate MoS2/C NC with metallic phase 1T-MoS2 (1–6 monolayers) dispersed on graphene, that exhibit high electrical conductivity (G ~ 0.180–98.815 μS) and large specific capacitance (C sp ~ 122.2 F g −1 ) including: temperature ~ 190–210 °C, time ~2 h, pH ~ 7.2–8.8 and (Mo 4+ :
C ) molar ratio below (1.5 : 1) The synthesized MoS2/C NC systems could be promising for energy storage devices applications such as batteries or supercapacitors
(4) The suitable conditions have been determined to fabricate MoS2/C NC materials with 2H-MoS2 semiconductor phase composition with strong optical absorption (~ 82%) and luminescence properties with large band gap of ~1.31–2.34 eV, including: temperature
~230–250 °C, time 2 h, pH ~ 7.2; (Mo 4+ : C) molar ratio above (1.5 : 1) The synthesized MoS2/C NC systems could be great potential for electronics device applications such as optical sensors, wideband electromagnetic wave absorbers, optoelectronics and memristors
(5) The highly electrical conductivity sp 2 hybridization domains of chemically synthesized graphene could be restored from rich-defect structure of GO sheets by in- situ hydrothermal anchoring 2D MoS2 crystals via the donor-acceptor structure at the
GO defect sites And is, hereof, an effective method for improving the high electrical conductance of the synthesized nanocomposite materials
(6) The 1T to 2H phase transition of 2D MoS2 dispersed phase can be engineered by controlling reaction temperature through the synthesis of MoS2/C NC These results suggest a facile method to fabricate the MoS2/C NC system with the crystalline structures and property such as metallic or semiconductive for desired applications