Comparison of the sensing performance of the sensors based on the MoS2 bulk and the exfoliated MoS2 upon an exposure to 0.5 – 5 ppm NO2 at RT in terms of a gas response and b response a
LITERATURE REVIEW
Liquid-phase exfoliation (LPE) for MoS 2 NSs fabrication
1.1.1 Background on liquid-phase exfoliation
An ultrasonic electronic generator converts mains AC power into a 20 kHz electrical signal that drives a piezoelectric transducer, which then converts electrical energy into mechanical vibration along the probe; the tip’s longitudinal motion expands and contracts, and increasing the amplitude raises the sonication intensity applied to the sample In liquids, the rapid tip vibration induces cavitation—the formation and violent collapse of microscopic bubbles—whose collapse releases energy that powers fluid processing through erosion and shock effects The probe tip diameter governs the processed sample volume: microtip probes deliver high-intensity sonication in a small, concentrated area, while larger tips handle larger volumes with lower intensity The choice of generator and horns/probes is matched to the target volume, viscosity, and other application parameters Sonication power is measured in watts, while amplitude measures the tip excursion; the wattage displayed represents the energy required to drive the radiating face of the probe at a given amplitude against the current load Ultrasonic processors are designed to deliver constant amplitude to liquid samples, though the load varies with viscosity, concentration, and temperature, and amplitude control enables setting the desired level of ultrasonic vibration at the probe tip.
Figure 1.1 Schematic of sonicator ultrasonic processor system
Liquid exfoliation is a common top-down method that uses ultrasonication in a suitable solvent to realize the exfoliation of bulk crystals, breaking the weak van der Waals interlayer interactions while leaving in-plane covalent bonds intact Achieving successful exfoliation hinges on optimal ultrasonic intensity and time, and on solvent molecules that stabilize the exfoliated nanosheets and prevent reassembly; hence, the matching degree of surface free energy between solvent and nanosheets is crucial to improve exfoliation efficiency, with solvents like DMF and NMP widely used To date, ultrathin 2D TMDs such as MoS2, WS2, NbSe2, etc., have been synthesized by liquid exfoliation Liquid exfoliation complements mechanical cleavage by enabling large-scale preparation with good optoelectronic properties, but the organic solvents used are undesirable for many applications and single-layer production remains difficult, so refining experimental conditions and pursuing non-toxic solvent systems are necessary for scalable monolayer synthesis of 2D TMDs.
In this thesis, MoS2 nanosheets were fabricated by liquid phase exfoliation using the sonicator ultrasonic processor system
1.1.2 Processing-structure relationship of LPE for NSs
MoS2 offers an intrinsic band gap and high mobility, enabling it to rival graphene in certain applications, yet practical preparation and assembly challenges hinder its study Various routes exist to obtain few-layer MoS2, including mechanical exfoliation from bulk, chemical synthesis, and liquid exfoliation Coleman et al demonstrated a surfactant-free liquid exfoliation method that produces few-layer TMD nanosheets dispersed in diverse organic solvents Thermodynamically, the high surface energy of TMDs like MoS2 drives the use of high-boiling-point solvents; however, solvent removal is difficult and slow evaporation promotes aggregation, limiting device integration Although studies on liquid exfoliation of layered MoS2 in volatile solvents are still limited, this work presents a versatile, scalable mixed-solvent strategy for exfoliating MoS2 nanosheets into volatile solvents The approach yields highly stable MoS2 suspensions in low-boiling solvent mixtures by carefully selecting solvent composition, enabling easy use in gas-sensitive materials.
Hansen's solubility parameter (HSP) theory provides a semi-empirical correlation to explain dissolution behavior by linking the HSPs of solvents and solutes Three HSP parameters are used to characterize a solvent or material: dD, dP and dH, which are the dispersion, polarity and hydrogen bonding solubility parameters, respectively The dissolution process is an adaptation between the HSP parameters of solvents and solutes, and Ra is used to evaluate the level of this adaptation and is expressed in Eq 1.1, where a smaller Ra indicates greater compatibility and a higher likelihood of dissolution.
2 2 2 0.5 a D,solv D,solu P,solv P,solu H,solv H,solu
Smaller Ra values indicate higher expected solubility, and once the nanomaterial’s Hansen Solubility Parameters (HSP) are known, Ra can identify the most efficient solvent for its dispersion HSP theory also applies to solvent mixtures, enabling predictions of MoS2 nanomaterial solubility in different solvent blends.
“poor” solvents, ethanol and water have been shown to be designed to give high solubility to MoS2 nanomaterials First, 90 mg of bulk MoS2 powder was added to
Thirty milliliters of ethanol/water dispersion solvent with ethanol (EtOH) volume fractions ranging from 0 to 100% was prepared for MoS2 dispersion The mixed suspension was subjected to probe-ultrasonic treatment at 300 W for 8 hours to promote exfoliation, and then centrifuged at 2000 rpm for 20 minutes to remove unexfoliated MoS2 The collected supernatant was analyzed by UV–Vis spectroscopy to evaluate the dispersion concentration.
Figure 1.2(a) depicts photographs of MoS2 nanomaterial suspensions in different ethanol/water mixtures; at an appropriate ethanol/water ratio, a dark blue dispersion forms, demonstrating that MoS2 can be successfully dispersed at high concentrations in mixtures of two poor solvents These suspensions are stable and precipitation-free after one week of storage under ambient conditions Pure ethanol and pure water poorly disperse MoS2, whereas MoS2 exhibits markedly different dispersion behavior across ethanol/water mixtures with varying compositions The absorbance of the MoS2 suspensions in these mixtures is shown as dots, with the calculated Ra values presented as solid lines Figure 1.2(b) clearly shows that the absorbance depends on the ethanol/water composition.
Figure 1.2 presents MoS2 dispersions in ethanol–water mixtures and their optical stability under ambient storage In panel (a), photographs show how MoS2 dispersions appear after one week in different ethanol–water ratios, illustrating dispersion quality and visual stability across solvent compositions In panel (b), the absorbance of the MoS2 suspensions is plotted for varying solvent mixtures as data points, with the corresponding Ra values shown as a solid line to reveal how solvent composition influences the optical response and dispersion characteristics [18].
MoS2 concentration in dispersion is highly dependent on the ethanol–water volume fraction, with a strong correlation between the calculated Ra and the dispersion concentration Among tested solvents, the 45% ethanol–water mixture yields the smallest Ra, corresponding to the highest experimental dispersion concentration in this mixed solvent This composition-dependent dispersion behavior can be understood through Hansen Solubility Parameter (HSP) theory, since different ethanol–water ratios have distinct Ra values These Ra values can be calculated using Eq (1.1).
Based on the results of the above study [18], this thesis presents an ideal solvent system for dispersing MoS2 by using a binary ethanol–water mixture The optimal composition is a 45% ethanol/water blend, identified as the most effective solvent system for MoS2 dispersion.
Hydrothermal for ZTO fabrication
Zn2SnO4 nanomaterials can be fabricated by various methods, including chemical vapor deposition (CVD), electrospinning, thermal evaporation, and co-precipitation Each synthesis route presents its own advantages and disadvantages and enables control over nanostructure features, producing Zn2SnO4 with different sizes, shapes, properties, and potential applications By selecting an appropriate method, researchers can tailor the synthesis of nanostructured Zn2SnO4 to meet specific performance targets and application requirements.
Zn2SnO4 materials: we found that chemical synthesis, particularly the hydrothermal method, is the most suitable approach The hydrothermal method is simple and cost-effective, requiring only modest equipment Through hydrothermal synthesis, researchers have produced a wide range of semiconductor metal oxide nanomaterials with diverse shapes and sizes—including nanofibers, nanorods, nanoplates, nanoflowers, and nanoparticles—many of which exhibit porous structures.
The hydrothermal method is a synthesis approach that uses a solvent (water or no solvent) to drive heterogeneous chemical reactions at high temperature and pressures above 1 atm within a closed system, typically an autoclave It offers advantages such as easy control of reactants, large sample volumes produced in a single fabrication, crystalline products with uniform particle size, a simple process, low cost, and time efficiency By heating the chemical solution in a sealed vessel (autoclave), the pressure rises above atmospheric as the solvent’s boiling point is exceeded, which enhances solubility and accelerates the reactions of the precursors used to synthesize materials The hydrothermal crystallization process involves physicochemical and hydrodynamic principles, including solutions, solubility, phase equilibrium, thermodynamics, and kinetics, and may include the simulation of hydrothermal reactions, with the properties of water changing as temperature and pressure vary.
In hydrothermal synthesis, the dominant mechanism for crystal nucleation and oxide material growth is breakdown and recrystallization, which involves phase analysis and diffusion that drive crystallinity and the formation of the desired compound The nucleation process and the subsequent growth of crystal sprouts describe how the material initializes and develops into a well-ordered oxide phase, enabling controlled crystallization and the production of high-purity oxide materials.
Crystal seeds are the foundation for explaining how nanomaterials evolve into three-dimensional, anisotropic crystalline architectures To achieve unidirectional growth through hydrothermal synthesis, researchers commonly add appropriate organic additives or surfactants that orient the crystals and control abnormal crystal growth.
Hydrothermal synthesis of three-dimensional Zn2SnO4 nanostructures typically begins with the formation of ZnSn(OH)6 intermediates (or other precursor phases) from zinc and tin salts in the presence of surfactants under alkaline conditions The solution is then heated in an autoclave at elevated temperature for a defined period, enabling the decomposition of ZnSn(OH)6 and other intermediates to yield Zn2SnO4 as the final product By varying the solvent system, surfactant type and concentration, and the pH, researchers can tailor Zn2SnO4 nanostructures into a variety of shapes Systematic studies show that reaction temperature, reaction time, surfactants, and precursor pH collectively influence the morphology, size, and properties of the resulting Zn2SnO4 nanostructures.
Hydrothermal synthesis of Zn2SnO4 using diverse precursors and surfactants has yielded a variety of nanostructures Lili Wang et al fabricated Zn2SnO4 sheets, spheres, and cubes via hydrothermal reactions assisted by CTAB, SDS, and hexamethylenetetramine (HMT) Xueli Yang et al reported the synthesis of Zn2SnO4 nano-octahedra by hydrothermal processes with the aid of CTAB Fengjun Liu et al prepared Zn2SnO4 nanorods through a hydrothermal method starting from commercial oxide powder precursors Chao Chen et al achieved 3D flower-like Zn2SnO4 structures using a CTAB-assisted hydrothermal route Tingting Zhou et al studied Zn2SnO4 nanostructures including nanocubes, nanorods, and nano-octahedra from precursors Zn(CH3COO)2·2H2O and ZnSO4·7H2O.
Na2SnO3.3H2O, SnCl4.5H2O, cetyl trimethyl ammonium bromide (CTAB),
Using hydrothermal synthesis, we fabricate nanostructured Zn2SnO4 materials for gas sensors, leveraging the method’s advantages to enable high-performance VOC detection The novelty of this work lies in surfactant-free synthesis of Zn2SnO4 from precursors Zn(CH3COO)2·2H2O, SnCl4·5H2O, and NaOH without any additional surfactants We identify optimal conditions for precursor ratio, hydrothermal temperature, and pH to produce Zn2SnO4 nanomaterials with a porous octahedral structure, a large surface area, and uniform particle size This porous octahedral Zn2SnO4 is expected to exhibit heightened sensitivity to volatile organic compounds such as TEA, acetone, and ethanol, offering practical applications in breath analysis and food quality assessment.
MoS 2 NSs for gas-sensing application
Following graphene's huge research momentum, MoS2 has emerged as the next prominent two-dimensional material for device applications Its direct bandgap provides advantages over graphene for optical sensors and field-effect transistors MoS2 belongs to the transition metal dichalcogenides (TMDs) family, whose members follow the MX2 formula, with M representing a transition metal (groups 4–12) and X a chalcogen (group 16) Specifically, MoS2 is molybdenum disulfide.
MoS2 exhibits a hexagonal layered crystal structure in which a sheet of sulfur atoms sits on both sides of a molybdenum sheet, forming S–Mo–S trilayers Strong covalent bonds bind atoms within each trilayer, while much weaker van der Waals forces hold adjacent trilayers together to create a stacked crystal This layered architecture enables easy exfoliation into thin sheets and underpins MoS2’s distinctive lubricating and electronic properties.
Mo and S atoms, but weak van der Waals forcing holding layers together as shown in Fig 1.3 This allows them to be mechanically separated to form 2-dimensional sheets of MoS2
MoS2 in its monolayer form has a thickness of about 6.5 Å The basic structural unit of MoS2 organizes into a hexagonal pattern, reflecting its hexagonal lattice structure The Mo–S bond length is 2.42 Å, and the MoS2 monolayer lattice constant is 3.18 Å.
Figure 1.3 provides a three-dimensional representation of the MoS2 structure and presents the optimized MoS2 monolayer with four adsorption sites: (1) hollow site, (2) top site of the sulfur (S) atom, (3) Mo–S bridge site, and (4) top site of the molybdenum (Mo) atom, as cited in reference [28].
MoS2 exhibits three common crystal structures: 1T, 2H, and 3R The digits indicate the number of S–Mo–S layers in a unit cell, while the letters H, R, and T denote the arrangement of Mo atoms—hexagonal, rhombohedral, and trigonal, respectively These polytypes differ in how the sulfur atoms are arranged around the Mo atoms and in the stacking order of each layer Of these, only the 2H and 3R phases are thermodynamically stable and found in nature; both feature triangular prismatic coordination of sulfur around Mo but differ in Mo–S–Mo layer stacking, which leads to distinct space groups In the 1T phase, six sulfur atoms are arranged octahedrally around the Mo atom to form the base cell, whereas in the 2H phase, each Mo atom occupies a trigonal-prismatic site surrounded by six sulfur atoms.
S ligands are arranged in a triangular prism geometry with two S–Mo–S units per base cell in the 2H phase In the 3R phase, a similar triangular prismatic arrangement exists, but each base cell along the c-axis contains three S–Mo–S units Both the 2H and 3R phases exhibit similar crystal sizes, reflecting comparable structural dimensions.
Mo atom to the nearest S atom about 2.41 A o
Figure 1.4 (a) Top view of 2H/1T MoS 2 monolayer (b) Polymorphic structures of
Bulk MoS2 is a semiconductor with an indirect bandgap of about 1.2 eV, which limits its appeal for optoelectronic applications By contrast, a MoS2 monolayer—where interlayer interactions are removed and electrons are confined to a two-dimensional plane—exhibits a direct bandgap of roughly 1.89 eV, corresponding to visible red light This dimensional crossover from bulk to single-layer MoS2 highlights the dramatic change in electronic properties and the enhanced optoelectronic potential of 2D MoS2.
Figure 1.5 Band structure diagram of (left) bulk and (right) monolayer MoS 2 showing the crossover from indirect to direct bandgap accompanied by a widening of the bandgap [28]
The more obvious difference in band gap structure between bulk and layered materials is shown in Fig 1.5 The oblique band gap structure gradually changes
As the material transitions from bulk to layered and ultimately to a monolayer, its bandgap structure evolves toward a direct bandgap The valence-band top and the conduction-band bottom shift progressively during this transition, culminating in a direct bandgap for the monolayer.
Bulk MoS2 has its valence-band maximum at the Γ point and its conduction-band minimum near the K point, with the Mo d orbitals and the antibonding S p orbitals shaping the valence-band top and strongly influencing the conduction-band edge through orbital interactions These orbitals help drive the conduction-band minimum toward the K point within the S–Mo–S trilayers, and as the number of layers decreases, the K state remains nearly constant while other states shift downward In the monolayer, the valence-band maximum moves to K and becomes the top of the valence band, while the conduction-band minimum also occurs at K, giving MoS2 a direct band gap at the K point Therefore, the MoS2 monolayer is a semiconductor with a direct (straight) band gap at the K point.
1.3.2 MoS 2 NSs in NO 2 gas-sensing application
The escalating emissions of hazardous gases from chemical plants, industrial facilities, and fossil-fuel vehicles are a growing threat to the environment and public health Exposure to these toxic gases, even at very low concentrations, can cause serious health damage, underscoring the urgency of stronger air-quality standards and emission reductions Among these pollutants, nitrogen dioxide (NO2) is both deadly and highly common, making it a key focus for environmental protection and health-safety initiatives.
Exposure to extremely low levels of nitrogen dioxide (NO2) can cause serious respiratory damage, contributing to pediatric asthma and even lung cancer NO2 is a major toxic byproduct released during the burning of fossil fuels in factories and vehicles Even concentrations below 5 ppm of NO2 can trigger severe respiratory disorders and may lead to life-threatening diseases The Occupational Safety and Health Administration (OSHA) sets a short-term exposure limit of 1 ppm for NO2 over a 15-minute period, underscoring the need for continuous air monitoring To mitigate these risks, there is a growing emphasis on using sensitive and selective gas sensors to monitor air quality in both workplace and residential environments.
1.3.2.2 MoS2 NSs in NO2 gas-sensing application
Transition metal dichalcogenides (TMDs) have become highly promising materials for gas sensing due to their layered structure, large surface area, and abundant active sites for redox reactions, enabling sensitive detection at room temperature without the need for external heating and with a direct electrical readout Their unique properties include a band gap that depends on the number of layers, a high surface-to-volume ratio, and a strong adsorption coefficient, all of which enhance sensor performance Among TMDs, MoS2 and WS2 offer additional advantages of mechanical flexibility and robustness, making them well suited for flexible gas sensors; for example, ultrathin MoS2 exhibits a high Young's modulus of about 0.33 ± 0.07 TPa, surpassing stainless steel and graphene oxide, which underscores its suitability for durable, wearable sensing devices.
Worldwide, transition metal dichalcogenides (TMDs) are a family of two-dimensional (2D) materials that have attracted significant attention from gas-sensing researchers, ranking just behind graphene because of their unique structural, chemical, and electronic properties The general MX2 formula describes TMDs, where M is a transition metal and X is a chalcogen; among the most studied combinations are MoS2, WS2, MoSe2, and WSe2, with molybdenum (Mo) and tungsten (W) as the metallic components and sulfur (S) or selenium (Se) as the chalcogen partners.
Transition metal dichalcogenides (TMDs), composed of metals such as molybdenum (Mo) or tungsten (W) and chalcogen elements like sulfur (S) or selenium (Se) (X), feature a layered structure where a sheet of transition metal atoms is sandwiched between two sheets of chalcogen atoms Within each layer, atoms form strong covalent bonds, while the layers are held together by weak van der Waals forces (Fig 1.6) This weak interlayer interaction enables liquid phase exfoliation (LPE) to produce monolayer or few-layer 2D nanosheets with thicknesses under 10 nm and lateral sizes ranging from 50 nm to 1 μm.
Figure 1.6 Schematic diagram of the layered structure of MoS 2 [33]
Among the 2D materials studied for gas sensing, transition metal dichalcogenides (TMDs) offer chemical versatility and tunable band gaps, making them attractive for practical sensor design Layer-number dependent properties and multiple activation sites have been explored to improve the sensing performance of TMDs Unlike graphene, a common 2D material with a near-zero band gap, some TMDs such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) possess band gaps around 1–2 eV due to their intrinsic carrier concentration Delaminated MoS2 nanosheets exhibit markedly different physico-chemical properties compared with bulk multilayer MoS2 For example, bulk MoS2 is a semiconductor with a band gap of about 1.29 eV, while layered MoS2 nanosheets can show a larger effective band gap (~1.9 eV) as thickness decreases This band-gap tuning can be leveraged to enhance the gas-sensing properties of MoS2 nanosheets In addition, their high specific surface areas, high surface-to-volume ratios, and abundant activation sites contribute to improved sensitivity and selectivity in gas detection.
MoS 2 combine with SMO (ZTO) for gas-sensing application
1.4.1 Overview on ZTO and its application in gas-sensing field
Among the semiconductor metal oxide materials being studied, Zn2SnO4
(ZTO) materials have attracted great interest, with interesting properties such as high strength, stability and high electron mobility ZTO can be applied in many
15 fields such as being used as an electrode material for Li-ion batteries, solar cells, photocatalysts to decompose organic pollutants, etc
Zinc stannate (ZTO) adopts a cube-based spinel structure in which the oxygen ions form the framework and the crystal symmetry is Fd3m (JCPDS PDF 24-1470), placing it in the AIIBIVO4 family where A is the first metal ion that can undergo chemical transformation and B is the second metal ion with its valence This structure can be regarded as a mixed tin–zinc salt arrangement Like other spinel oxides, a unit cell of zinc stannate contains eight formula units, i.e., 56 ions in total, comprising 32 oxygen ions and 24 metal ions (see Fig 1.7) In the inverse spinel arrangement of ZTO, a Zn atom sits at the center of a ZnO4 tetrahedron surrounded by four nearest oxygen atoms, while an equal number of Zn and Sn atoms occupy the centers of the octahedral ZnO6 or SnO6 units, each octahedron coordinated by six oxygen atoms.
Figure 1.7 The lattice structure of the Zn 2 SnO 4 nanomaterial [59]
Zn2SnO4 (ZTO) is a wide-band-gap n-type semiconductor with Eg ≈ 3.6 eV The n-type behavior arises from oxygen vacancies formed during fabrication, which introduce free-electron states in the lattice Each oxygen vacancy donates a pair of free electrons, making electrons the primary charge carriers in Zn2SnO4 Consequently, the metal oxide semiconductor ZTO exhibits n-type conductivity driven by oxygen-vacancy–induced free electrons.
More oxygen vacancies per unit volume lead to higher electron concentration, which increases the material's conductivity and lowers its resistance The amount of oxygen deficiency in the ZTO lattice can be controlled by heat treatments at various temperatures or in different environments, enabling tunable electrical properties.
1.4.1.2 ZTO in gas-sensing application
ZTO materials offer high electronic mobility, good thermal stability, and high chemical sensitivity, making them suitable for gas-sensing applications However, the reported results show that working temperature, response, gas selectivity, and long-term stability remain limited and require improvement For example, ZTO nanowires fabricated by thermal evaporation exhibit a response of 21.6 to 50 ppm ethanol at 500 °C Additionally, ZTO hollow cubic structures have been fabricated by hydrothermal methods for gas sensing.
16 method for acetone gas sensing application showed a response values were 47.80 for 125 ppm of acetone at 450 ºC [8]
Gas sensor performance of semiconductor metal oxides is highly influenced by morphology, crystal size, porosity, defect density, and other structural features For food-quality analysis through VOC detection, sensors must achieve detection limits at the ppb level This drives the development of novel ZTO nanostructures to enhance working temperature, response speed, selectivity, and long-term stability of gas sensors Compared with granular materials, porous ZTO nanostructures offer greater surface activity, higher surface-to-volume ratios, and faster diffusion, thereby boosting sensor efficiency.
1.4.2 MoS 2 combine with SMO (ZTO) for Triethylamine (TEA) gas- sensing application
1.4.2.1 MoS 2 combine SMO for gas sensor
Transition metal dichalcogenide (TMD) gas sensors can detect a broad range of gases at room temperature, making them a promising platform for the next generation of gas-sensing devices However, their practical application is hindered by low gas response, poor stability, and insufficient reversibility A comparative summary of the advantages and disadvantages of TMDs versus SMOs in gas sensing is presented in Table 1.1, outlining the current trade-offs and guiding future optimization for room-temperature gas sensing.
Recent studies have demonstrated that combining transition metal dichalcogenides (TMDs) with metal oxide semiconductors (SMOs) can significantly enhance gas-sensing performance while lowering the operating temperature A common method to synthesize TMD/SMO nanocomposite gas sensors is to use chemical routes in which one component forms in the presence of the other during the reaction, fostering intimate interfacial contact This approach yields homogeneous, well-dispersed interfaces that promote synergistic interactions, improving adsorption, charge transfer, and response speed, and ultimately delivering higher sensitivity and selectivity at reduced temperatures.
Table 1.1 Summary of advantages and disadvantages of TMDs and SMOs in terms of gas sensor application [55]
High electron mobility at low temperature
Low energy consumption Good compatibility Mechanical flexibility
Short response time Low cost Long-term stability Scalable fabrication High response
Low gas response Long response/recovery time Relatively high cost Lack of long-term stability Lack of scalable fabrication
Low electron mobility at low temperatures, combined with high operating temperatures and resulting high power consumption, presents a key challenge in electronic materials The as-obtained composites show better contact between the components and can be constructed into interesting structures This tendency is well recognized in the literature, where optimized composite architectures are noted for enhanced interfacial bonding and potential performance gains under varying temperature conditions.
MoS2 and SnO2 are among the most studied materials in the TMDs and SMOs families, respectively Bai et al synthesized a MoS2/SnO2 nanocomposite by growing thin-layer MoS2 nanoflakes on SnO2 nanotubes via a hydrothermal process, forming a gas sensor with a high NO2 response at room temperature (RT) of 34.67 The device also demonstrates fast response and recovery dynamics along with excellent selectivity, underscoring the potential of MoS2/SnO2 nanocomposites for sensitive NO2 monitoring.
Figure 1.8 presents SEM images of pure SnO2 and the optimized MoS2@SnO2, the dynamic response–recovery curve and response time of the MoS2@SnO2 gas sensor to NO2 at room temperature across 0.01–100 ppm, the response of MoS2@SnO2 with different MoS2 loadings and pure SnO2 sensors to various NO2 concentrations, the overall response and response time for 0.01–100 ppm NO2, and the selectivity of the optimal MoS2@SnO2 sensor toward NO2, H2S, NH3, and CO [47].
Wang et al fabricated 3D multilayer MoS2 nanosheets and subsequently modified them with SnO2 nanoparticles via hydrothermal processing The SnO2 decoration significantly enhances NH3 adsorption on the MoS2 nanosheets, resulting in a gas sensor with a remarkable 27.5 times higher gas response, fast response and recovery times, and outstanding selectivity toward NH3 at room temperature compared with the MoS2 sensor.
Yan and co-workers demonstrated that SnO2 can act as the primary sensing material, with MoS2 nanoflowers reducing the size and activation energy of SnO2 nanoparticles to yield a MoS2/SnO2 gas sensor that exhibits enhanced ethanol sensing, lower operating temperature, and excellent selectivity Moreover, MoS2/SnO2 sensors have been reported to deliver higher gas responses at room temperature to NO2, NH3, and CH4, or operate at lower temperatures compared with pure MoS2 sensors.
Figure 1.9 illustrates a two-step hydrothermal route for synthesizing the SnO2/MoS2 nanocomposite, accompanied by SEM images that reveal the morphology of MoS2 nanosheets and the MoS2/SnO2 hybrid The figure also presents sensor responses and corresponding function-fitting curves for MoS2, SnO2, and the SnO2/MoS2 nanocomposite across different NH3 concentrations, highlighting the enhanced sensing performance of the composite Additionally, a selectivity study at 50 ppm shows the SnO2/MoS2 nanocomposites’ responses to various gases, underscoring their potential for selective ammonia detection.
ZnO is another well-studied SMO material for gas sensors besides SnO2 In one study, Yan et al used a two-step hydrothermal synthesis to fabricate a ZnO nanoparticle-coated MoS2 nanosheet gas sensor, which showed a greatly higher gas response to ethanol, a reduced optimal working temperature, and better selectivity than the ZnO nanoparticle–based sensor In another study, Zhang and co-authors fabricated MoS2 and ZnO separately by two hydrothermal processes, and prepared the MoS2/ZnO nanocomposite film by alternate immersion into each material’s solution for several cycles This self-assembled multilayer film possessed a much greater specific surface area and strong MoS2–ZnO bonding, resulting in an enhanced gas response toward ammonia at room temperature.
Nanocomposites formed by transition metal dichalcogenides (TMDs) and semiconducting metal oxides (SMOs) can be synthesized using physical methods, with liquid-phase exfoliation being particularly effective In a study by Jha et al [63], a WS2/WO3 composite was fabricated through sequential grinding and exfoliation processes The grinding step was found to homogenize the size distribution of the micro-sized bulk powders prior to the co-exfoliation step The as-obtained WS2/WO3 hybrid sensor.
19 when being tested with ammonia as the analyte, showed an optimal working temperature (250 ºC) in the range between that of WS2 sensor (80 ºC) and WO3
(350 ºC), with a higher response (12.6 to 2000 ppm ammonia) than that of WS2 sensor at optimal temperature (0.09 to 2000 ppm ammonia) and that of WO3 (6 to
Gas-sensing mechanism
1.5.1 General sensing mechanisms of gas sensors
Gas sensors rely on an active sensing material that detects analyte gas molecules through changes in charge carrier concentration, making sensor performance inherently linked to the underlying sensing mechanisms In 2D materials and metal oxides, these mechanisms are broadly classified into two categories: the surface-adsorbed oxygen ions mechanism and the charge transfer mechanism The widely studied surface-adsorbed oxygen ions mechanism involves surface reactions between target gas molecules and pre-adsorbed oxygen ions (such as O2− and O−) on the sensor surface, which modulates the charge carrier density and, consequently, the sensor resistance Depending on the target gas, the material, and the operating conditions, these interactions lead to detectable changes in electrical signals, enabling effective gas detection and performance optimization.
22 working temperature of the sensor device, a particular type of oxygen ion species is considered to explain the sensing mechanism [76]
These oxygen ion species form a depletion layer on the surface of the 2D materials When the oxygen molecules are adsorbed on the surface of the active materials, they extract electrons from the conduction band and trap the electrons at the surface in the form of ions This process leads to the band bending and formation of electron depletion or space charge layer During the exposure of gas molecules (electron-donating or withdrawing), change in the thickness of the depletion layer occurs due to the reaction of the gas molecules with the adsorbed oxygen species and transfer of electrons The conductivity of the active materials either decreases or increases depending on the p-type and n-type nature of the active sensor material and the type of analyte species, respectively (Fig 1.11)
Figure 1.11 Schematic Illustration of gas sensing mechanisms in semiconductors for reducing and oxidizing gases: Surface adsorbed oxygen ions mechanism [88,89]
In the case of the charge transfer mechanism, the charge (electron or hole) transfer occurs between the target analyte gas molecules and the active sensing material (Fig 1.12 (a,b)) Similarly, the direction of charge transfer depends on the oxidizing or reducing nature of the analyte gas The weak or strong interactions depending on the reactivity of the adsorbates and adsorbents result in the
The modification of the electronic properties of the sensing material and the emergence of new transport channels are critical factors in gas-sensing performance For example, Yu et al verified the charge-transfer mechanism of MoS2-based sensors during the adsorption of various gas molecules (H2, O2, H2O, NH3, NO, NO2, and NO) using first-principles calculations (Fig 1.12).
Figure 1.12 Schematic Illustration of gas sensing mechanisms in semiconductors for reducing and oxidizing gases: (a) Schematic diagram of charge transfer process [81];
(b) Charge density difference plots of MoS 2 based sensor [80]
1.5.2 Sensing mechanisms of p-n heterojunction material-based gas sensors
Gas sensor devices based on p–n heterojunctions leverage two-dimensional materials that act as p-type and/or n-type semiconductors to form the junction, with the interface region and the interaction of charge carriers across it playing a key role in sensor performance The sensing mechanism arises from the involvement of two semiconducting components and their interface, which drives a sequence of steps in operation In the initial stage, the spillover mechanism prevails: atmospheric oxygen molecules adsorb on the surface and capture the conduction band electrons of one of the semiconductors in the p–n junction, forming surface oxygen ions (O2−, O−, O2−) that modulate the sensor's electrical response.
Adsorbed oxygen species establish a space-charge layer at the surface, interface, and junctions of p-n heterojunctions formed from 2D materials When the p-type and n-type layers come into electrical contact, electrons flow from high-energy states to unoccupied low-energy states (and holes move in the opposite direction) until the Fermi levels equilibrate After equilibration, carrier movement ceases and a depletion region forms at the interface, accompanied by band bending and the development of a potential energy barrier Depending on the analyte gas (oxidizing or reducing), gas molecules donate or withdraw electrons from the p-n junction, altering the carrier concentration and thereby the width of the space-charge and depletion regions These changes critically enhance the sensing performance of 2D-material p-n heterojunctions Figure 1.13 presents a representative schematic of energy-band diagrams and band bending for p-MoS2/n-SnO2 and p-MoS2/n-SnS2, illustrating sensing mechanisms for reducing gases (TEA) and oxidizing gases (NO2).
Figure 1.13 schematically explains the gas sensing mechanisms of p-n heterojunctions based on two-dimensional (2D) materials Panel (a) shows the energy band diagrams of the MoS2/SnO2 p-n heterojunction in air and under exposure to the reducing gas vapor TEA, highlighting how gas interaction modulates band alignment and carrier dynamics at the junction [73] Panel (b) depicts the corresponding energy band diagrams for the other configuration, illustrating the role of interfacial band structure in determining the sensor response.
MoS 2 / SnS 2 p-n heterojunction in air and in oxidizing gas (NO 2 ) [84]
As shown in Fig 1.13(a), the higher resistance of MoS2/SnO2 heterojunctions arises from the formation of the depletion layer and space-charge region caused by adsorbed oxygen species near the surface and interface When the device is exposed to TEA gas molecules, the adsorbed oxygen species (O2) participate in surface reactions that modulate this depletion region This surface chemical interaction alters the electrical conductivity, linking the adsorption dynamics of oxygen on MoS2/SnO2 to the gas-sensing response.
During these processes, excess electrons are released back to SnO2, resulting in a decrease in resistance Simultaneously, electrons donated by TEA molecules are injected into the p-MoS2, leading to a decrease in the hole concentration Together, these electron transfer events modulate the material’s electronic behavior by lowering resistance and reducing hole density in p-MoS2.
Based on the semiconductor mass-action law (n0 × p0 = ni), a reduction in hole concentration in MoS2 increases electron concentration and flattens the concentration gradient in p-n heterojunctions, leading to a smaller depletion layer width and lower barrier height; a similar mechanism operates for other p-n heterojunctions built from 2D materials, facilitating the detection of reducing gases.
In NO2 sensors based on a p-MoS2/n-SnS2 heterojunction, NO2 gas captures free electrons from the sensor’s acceptor level, triggering the formation of NO2− and NO3− ion species; this mechanism is shown in Fig 1.13(b).
NO2 + e ‒ → NO2 ‒ + e ‒ (1.3) 2NO2 + O2 ‒ + 2e ‒ → 2NO3 ‒ (1.4) Accumulation of the NO2 ‒/NO3 ‒ at the interface, surface, and near depletion layer of the p-n heterojunction affects the charge transfer in the sensing process
Enhanced sensing performance is achieved in sensors based on 2D material p-n heterojunctions, where barrier height is tuned by variations in the total built-in potential and in the ratio of majority carrier concentrations in the p- and n-type regions Similar mechanisms govern the detection of oxidizing gases using 2D material p-n heterojunction sensors.
Chapter overview: This section provides an overview of transition metal dichalcogenide (TMD) nanosheets, notably MoS2, and MoS2-based semiconductor metal oxide (SMO) sensors, fabricated by liquid phase exfoliation (LPE) and by LPE combined with hydrothermal synthesis The results show that MoS2 nanosheets and MoS2 NSs integrated with SMO sensors exhibit rapid response, low operating temperatures, and high sensitivity due to their large specific surface area, high porosity, and the formation of potential barriers at homojunctions or heterojunctions In addition, the chapter discusses the gas-sensing mechanisms of MoS2 NSs and their interactions with SMO materials.
EXPERIMENTAL
Synthesis
Figure 2.1 General procedure for fabrication of nanosheet TMDs by probe ultrasonic- assisted liquid-phase exfoliation
MoS2 nanosheets were prepared by liquid-phase exfoliation as shown in Fig 2.1, using environmentally friendly solvents (ethanol and water) instead of conventional organic solvents such as N-methyl-2-pyrrolidone, formamide, acetone, isopropanol, or dimethylformamide In a representative procedure, 90 mg of bulk MoS2 powder (NAYATE, 99.99% purity) was dispersed in 30 mL of a 45% ethanol/water mixture and subjected to probe-ultrasonication for 8 h at 180–420 W with a 5 s on/5 s off regime The mixture was then centrifuged at 2000 rpm for 20 min to remove unexfoliated MoS2; the sediment was discarded, and the supernatant was centrifuged again at 4000 rpm and 6000 rpm for 30 min The resulting sediment was collected for further use The detailed procedure is illustrated in Fig 2.1.
Figure 2.2 Schematic of liquid phase exfoliation of MoS 2 nanosheets [87]
Zn2SnO4 materials are synthesized by a hydrothermal method using a defined set of precursors and solvents: Zn(CH3COO)2·2H2O, SnCl4·5H2O, NaOH, ethanol (C2H5OH), and deionized water No other surface-active agents or surfactants are used in the process All chemicals employed are analytical chemicals, ensuring high purity for reproducible hydrothermal synthesis and high-quality Zn2SnO4 materials.
Equipment and tools: Electronic balance, magnetic stirrer, pH meter, hydrothermal vessel, annealing furnace, centrifuge, ultrasonic vibrator All equipment for material synthesis is available at the Laboratory (Fig 2.3)
Figure 2.3 shows photos of the main equipment used to synthesize Zn2SnO4 nanomaterials by the hydrothermal method, including an electronic balance, a magnetic stirrer, a pH meter, a stainless-steel autoclave, an annealing furnace, a centrifugal rotary machine, and an ultrasonic vibrator.
To synthesize ZTO porous octahedra, Zn(CH3COO)2.2H2O (0.264 g) and SnCl4.5H2O (0.21 g) were dissolved in 30 mL of DI deionized water After stirring
Starting with precursors having a Zn:Sn:OH molar ratio of 2:1:10, a white suspension was prepared Then, 30 mL of NaOH solution (0.24 g) was slowly added dropwise while stirring for 15 minutes to ensure uniform alkalinity The resulting white suspension was transferred to a Teflon-lined high-pressure autoclave for the next step.
A 100 mL reaction system was used for hydrothermal synthesis and the process was maintained at 200 °C for 30 hours in an annealing furnace After 30 hours, the oven was turned off and the mixture was allowed to cool naturally to room temperature The precipitated product was washed and centrifuged several times with deionized water, with the final two washes using an ethanol solution, and collected by centrifugation at 4000 rpm The resulting white product was then dried in air in an oven at 80 °C for 24 hours.
2.1.3 Preparation MoS 2 NSs combine with ZTO porous octahedra
Few-layer MoS2 nanosheets were prepared following the previously reported method Briefly, 90 mg of bulk MoS2 powder was dispersed in 30 mL of a 45% ethanol–water mixture and subjected to probe-ultrasonic treatment at 420 W for 8 hours in a 5 s on/5 s off regime The dispersion was then centrifuged at 2000 rpm for 10 minutes to remove unexfoliated MoS2, the sediments were discarded, and the supernatant was collected for further use.
Figure 2.4 Schematic diagram of the preparation process for MoS 2 /Zn 2 SnO 4 nanocomposite on the electrode for gas sensors
Similar to the ZTO porous octahedra synthesis process, this method emphasizes maintaining the solution’s pH, preventing any drift during reaction To achieve pH stability and reproducible results, the volume of the solution in the Teflon vessel was kept at a constant level This controlled volume ensures consistent reaction conditions, enabling uniform growth of ZTO porous octahedra By preserving both pH and volume, the synthesis yields reliable nanostructures with well-defined porosity.
Sixty milliliters of the white suspension served as the base, and different volume fractions of MoS2 nanosheets (NSs) were gradually added with continuous stirring for 15 minutes to achieve a homogeneous mixture and form the ZTO–MoS2 heterostructure The MoS2 NSs were added in volumes of 1, 3, 5 and 10 mL, producing samples labeled as ZM1, ZM3, ZM5 and ZM10, respectively The subsequent hydrothermal process, followed by precipitation, washing, and drying steps, was performed using the same procedures as in the previous ZTO synthesis.
Five samples of MoS2/Zn2SnO4 nanocomposites—ZM0, ZM1, ZM3, ZM5, and ZM10—were prepared, and the names of these samples reflect their distinct synthesis conditions Table 2.1 presents the synthesis conditions for each MoS2/Zn2SnO4 nanocomposite sample.
No Sample Volume of MoS 2 (ml) Volume of solution
Characterization Techniques
Raman spectroscopy is an inelastic-scattering technique that uses lasers in the visible, near-infrared, or near-ultraviolet range to probe vibrational, rotational, and other low-frequency modes in materials, with the scattered photons shifting in frequency relative to the incident light It is widely used in condensed matter physics and chemistry to extract information about vibrational energy levels of atoms, molecules, or crystal lattices from the Raman spectrum The Raman effect arises from the interaction between light and matter, producing Stokes and anti-Stokes lines that reveal structural and dynamical properties For materials like MoS2, the spectrum typically shows two characteristic peaks, the E2g and A1g modes, whose positions and separation provide insights into layer thickness, strain, and material quality.
2.5) The E 1 2g spectral peak is the oscillation where the Mo atoms vibrate
Figure 2.5 presents the two characteristic Raman peaks of MoS2, corresponding to distinct vibrational modes: the in-plane E2g^1 mode and the out-of-plane A1g mode The A1g spectral peak arises from sulfur atoms oscillating in opposite directions perpendicular to the plane, while the E2g^1 mode involves in-plane vibrations of the Mo and S atoms Together, these peaks provide a signature of MoS2's crystal structure and are essential for characterizing layer thickness and material quality in Raman spectroscopy.
In this thesis, the as-synthesized MoS2 and ZTO was examined by Raman spectra using a Raman spectrometer (Renishaw, InVia confocal micro-Raman) with a laser wavelength of 633 nm
The X-ray diffraction method is based on the interaction between the X-ray beam and the crystal lattice structure When the X-ray beam reaches the crystal surface and enters the crystal lattice, the lattice acts as a special diffraction grating
In a crystal lattice, atoms or ions can be distributed in planes parallel to each other When excited by an X-ray beam, they become centers of reflected rays
The basic principle of the Ronghen diffraction method to study the crystal lattice structure is based on the Vulf-Bragg equation:
2d sin θ = n λ (2.1) Where n: order of diffraction (n = 1, 2, 3, etc.) λ: wavelength of X-ray (nm) d: distance between crystal planes θ: reflection angle
X-ray diffraction (XRD) analysis begins by locating the diffraction peak and determining the angle 2θ, from which the interplanar spacing d is calculated using Bragg's law Each material has a characteristic set of d-values, so comparing the sample’s d-spacing with standard reference data identifies its crystal lattice structure and material characteristics This nondestructive method requires minimal sample preparation and underpins the wide use of XRD in characterizing materials, addressing crystal structure, lattice constants and geometry, identification of unknown materials, orientation of single crystals, and preferred orientation of polycrystals, as well as defects and residual stresses in solids In addition, the XRD spectrum enables estimation of nanoparticle size via the Scherrer formula.
Crystallite size D (in nanometers) can be estimated from X-ray diffraction data using the Debye–Scherrer relation D = 0.9 λ / (β cos θ) In this equation, λ is the X-ray wavelength, θ is the diffraction angle of the observed planes in the nanofibers (NFs), and β is the full width at half maximum (FWHM) of the corresponding diffraction peak.
In this thesis, crystal structure of the specimens was tested with a D8 Advance,
X-ray diffraction measurements were performed with a Bruker instrument (Germany) using Cu Kα radiation (λ = 0.154056 nm) Diffractograms were recorded over a 2θ range of 10° to 80° with a step size of 0.03° and a dwell time of 1.0 s Phase identification was carried out by comparing the measured diffractograms with the registered patterns from the Joint Committee on Powder Diffraction Standards (JCPDS).
Scanning electron microscopy (SEM) uses a focused electron beam to scan a research sample and generate a high-resolution image The electron beam is emitted from a cathode, passes through two condenser lenses to control and focus its convergence, and is then directed onto the sample where electron–sample interactions produce signals that are captured and displayed as a magnified image on a phosphor (fluorescent) screen or detectors By adjusting lens currents and scan parameters, the SEM achieves the desired magnification and resolution, revealing detailed surface morphology of the specimen.
During SEM operation, the electron beam strikes the sample and causes secondary electrons to be emitted from its surface These emitted electrons are accelerated into the detector, where they are converted into light signals, amplified, and sent to the control electronics to generate brightness on the display that maps the surface topography of the sample Scanning electron microscopy (SEM) is capable of producing high-resolution images of a sample surface.
Energy-dispersive X-ray spectroscopy (EDX), also called energy-dispersive spectroscopy (EDS), is a technique for analyzing the chemical composition of a solid by recording the X-ray spectrum emitted when a high-energy electron beam interacts with the sample in an electron microscope The emitted X-ray frequencies are characteristic of the atoms present, enabling identification of which elements are in the material and their relative proportions Although EDS can be performed with various instruments, it has been primarily developed for use in electron microscopes, where the high-energy electron beam and electromagnetic lens systems enable detailed analysis of a solid’s microstructure.
This study investigates the morphology and microstructure of nanomaterials using a field emission scanning electron microscope (FESEM), specifically the JEOL-JSM7600F model The instrument is integrated with an energy-dispersive X-ray spectrometer (EDX), enabling simultaneous imaging and elemental analysis, and serving as a key tool for identifying the chemical composition of the specimens.
Transmission electron microscopy (TEM) uses an electron beam generated at the cathode and focused by condenser lenses to illuminate a thin specimen As electrons pass through, some are transmitted while others are scattered, and the transmitted electrons are accelerated, collected by the detector, converted into a light signal, amplified, and displayed as brightness on a screen or camera HR-TEM, the high-resolution imaging mode of TEM, achieves sub-nanometer detail with pixel resolutions around 0.17 nm and magnifications up to about 1,000,000x TEM is used to investigate the shape, size, grain boundaries, and microstructure of materials, including visualizing atomic-layer contrast in crystalline solids, and like SEM it can be used to characterize geometric features of specimens.
32 properties such as diameter, diameter distribution, orientation and morphology HR-TEM is one of the tools to observe microstructure to the atomic level
Selected Area Electron Diffraction (SAED) illustrates the diffraction pattern from a selected area of the specimen The process begins by examining the specimen in image mode to identify a feature of interest An intermediate aperture is then inserted and precisely positioned around this feature, after which the microscope is switched to diffraction mode SAED can analyze regions as small as 10^-4 cm in diameter.
In this study, all samples were prepared by dispersing the nanomaterials in ethanol with a sonicator The resulting dispersion was deposited onto copper grids and air-dried, after which morphology and structure were examined by TEM, HRTEM, and SAED on a Tecnai G2 20 S-TWIN/FEI microscope.
RESULTS AND DISCUSSIONS
Introduction
Recently, a wide range of two-dimensional (2D) nanomaterials—including graphene and its derivatives, transition-metal dichalcogenides (TMDs), black phosphorus, and transition-metal carbides/nitrides (MXenes)—have been actively used to fabricate room-temperature (RT) gas sensors Owing to their exceptional surface area, high charge-carrier mobility, and tunable surface chemistry, these 2D materials enable highly sensitive and selective gas detection at ambient conditions, driving significant advances in RT gas sensor performance.
Inspired by graphene for gas sensing, extensive research has focused on other layered materials, notably transition metal dichalcogenides (TMDs) The reduced dimensionality of 2D layered materials yields unique properties and bandgap-dependent applications Unlike graphene, which has zero bandgap, MoS2 exhibits a thickness-dependent bandgap and greater potential for device applications Recent theoretical and experimental studies have explored MoS2 as a gas sensor, with the direct bandgap of monolayer MoS2 and its large surface-area-to-volume ratio making it highly attractive for gas sensing applications This is supported by various studies, such as Yue et al.'s first-principles work showing the effect of an electric field on the adsorption of gas molecules H2, O2, and H2O.
MoS2-based gas sensors detect NH3, NO, NO2, and CO, with the electrical response of few-layer MoS2 to NO2 and other gases demonstrated in experiments by Donarelli et al (94) Li et al showed that multilayer MoS2 transistors are sensitive to NO, while He et al reported NH3 detection down to 1 ppb and proposed single-molecule sensitivity for NO2 detection (95) Further experimental studies have investigated the effectiveness of multi-layer and few-layer MoS2 structures for gas-sensing performance.
Figure 3.1 An overview of resistive-based gas sensors based on their energy consumption point of view [3]
2D MoS2 for successful gas sensing applications Multilayer MoS2 films based on transistor sensors have been experimentally demonstrated to show stable sensitivity towards NO gas molecules by Late et al [96]
Although several studies have investigated low-temperature gas sensors based on MoS2 nanomaterials prepared by different methods and have examined their morphology, structure, and gas-sensing properties, the gas response of these sensors remains relatively low, at only a small percentage Moreover, there is currently no research on applying liquid-phase exfoliation of MoS2 nanosheets from bulk materials for gas-sensing applications.
Transition-metal dichalcogenides (TMDs), including molybdenum disulfide (MoS2), have attracted tremendous attention due to their high stability, numerous surface active sites for functionalization and gas adsorption, and good conductivity at room temperature.
Pristine MoS2 gas sensors exhibit low sensitivity and slow dynamics at room temperature (RT) [99], so their RT sensing performance needs enhancement in sensitivity, selectivity, and response/recovery Integrating these 2D materials with a MOS structure significantly improves sensing properties, and the formation of p-n heterojunctions further enhances gas sensing characteristics, which is why such composites are commonly used For example, Han et al fabricated MoS2/ZnO heterostructures via a wet chemical route [100], achieving a ΔI/Ia of 30.50 toward 5 ppm NO2 with an ultrafast response time of 40 s, explained by the synergistic effects of the unique 2D/0D morphology and the ZnO–MoS2 p-n junctions [101] In another study, Chen et al demonstrated surface functionalization of MoS2 by Au NPs for selective detection of acetone at RT.
Although a number of studies have explored TMDs and metal oxide hybrid materials for gas sensing, there are no clear studies demonstrating how MoS2 nanosheets can enhance the gas sensing performance of MoS2/Zn2SnO4 nanomaterials.
In this study, MoS2 sensors and their integration with ZTO were prepared by liquid-phase exfoliation followed by hydrothermal treatment We investigated the effects of ultrasonic vibration power and ultrasonic vibration time on the morphology, structure, and NO2 gas-sensing performance of MoS2 nanosheet sensors We also examined how the concentration of MoS2 NSs influences the TEA gas-sensing properties of the MoS2/ZTO sensors.
NO 2 gas sensors based on MoS 2 NSs
3.2.1 Morphologies and structure of MoS 2 NSs
3.2.1.1 Effects of ultrasonic vibration power
To study how ultrasonic vibration power affects the size and morphology of MoS2 materials, we conducted ultrasonic processing at three power levels—180 W, 300 W, and 420 W This investigation examines the influence of ultrasonic power on the size distribution and morphological features of the fabricated MoS2 samples, highlighting the relationship between processing power and material structure.
SEM characterization of MoS2 materials is shown in Figure 3.2 The results indicate that the size and thickness of MoS2 bulk material decrease significantly as ultrasonic vibration power is gradually increased from 180 W to 420 W, i.e., from 30% to 70% of the maximum power Following ultrasonic-assisted exfoliation, the MoS2 sheets attain lateral sizes of several hundred nanometers, which are considerably smaller than the bulk MoS2 dimensions of several micrometers This demonstrates that ultrasonic vibration effectively exfoliates bulk MoS2 into nanoscale sheets.
At the highest realized ultrasonic vibration power of 420 W, ultra-thin, small nanosheets are formed In general, increasing ultrasonic vibration power leads to smaller nanosheet sizes, and this reduction in size is expected to enhance NO2 gas sensing sensitivity at room temperature.
Figure 3.2 SEM images of (a) the bulk MoS 2 powder; the exfoliated MoS 2 nanosheets at different ultrasonic vibration power rates (b) 180 W; (c) 300 W; (d) 420 W
Following the evaluation of ultrasonic vibration power on size and morphology, we studied the effect of centrifugal rotation speed on the size and morphology of MoS2 nanosheets fabricated at 420 W Centrifugation speeds of 2000 rpm, 4000 rpm, and 6000 rpm were applied to the MoS2-containing solution after ultrasonic vibration at 420 W At 2000 rpm, relatively large bulk materials that had not delaminated remained in the liquid phase (Fig 3.3(b)) Two-thirds of the solution after centrifugation at 2000 rpm was collected and re-centrifuged at 4000 rpm; a similar procedure was used for 6000 rpm to collect MoS2 nanosheets The nanosheets obtained after centrifugation at 4000 rpm and 6000 rpm showed a significant reduction in size, indicating that higher centrifugal speeds promote delamination and size decrease.
Sizes of MoS2 nanosheets obtained at different centrifugation times are relatively similar, indicating that changing spin durations does not significantly affect lateral dimensions Consequently, centrifugation at different speeds is expected to produce MoS2 nanosheets that are uniform in thickness with few layers, enabling consistent material properties and scalable synthesis.
Figure 3.3 SEM images of (a) the bulk MoS 2 powder; the exfoliated MoS 2 nanosheets at centrifugal rotational speeds (b) 2000 rpm; (c) 4000 rpm; (d) 6000 rpm
Figure 3.4 Raman spectra of the bulk MoS 2 (a) and the exfoliated MoS 2 nanosheets at centrifugal rotational speeds (b) 2000 rpm; (c) 4000 rpm; (d) 6000 rpm
Fig 3.4 shows the Raman scattering spectrum of the samples fabricated with different centrifugation rotational speeds The first two peaks indexed as E 1 2g and
A1g arose from the in-plane and out-of-plane vibrational modes of the hexagonal
MoS2 consists of layers with a molybdenum atom sandwiched between sulfur layers, and Raman-active modes E2g, A1g, and 2LA(M) appear for samples prepared under various conditions All samples show peaks at the E2g, A1g, and 2LA(M) modes, but the A1g–E2g frequency separation differs: about 27.1 cm^-1 for bulk MoS2, as shown in Fig 3.4(a) For the MoS2-2000, MoS2-4000, and MoS2-6000 samples, the A1g–E2g spacings are 24, 23, and 22 cm^-1, respectively, indicating nanosheet thickness decreases from approximately four layers to two layers with increasing centrifugation rate These thickness and layer-number changes in MoS2 nanosheets are expected to directly affect NO2 gas sensitivity.
By studying how centrifugal rotation speed affects the morphology of MoS2 nanosheets, we also evaluated their NO2 gas sensing performance to identify the optimal fabrication conditions Our results indicate that 420 W and a centrifugal rotation speed of 4000 rpm are the optimal parameters for producing MoS2 nanosheets with favorable morphology for gas sensing Consequently, we conducted a detailed examination of the morphology and NO2 sensing properties of the optimally fabricated MoS2 nanosheet sample.
Field emission scanning electron microscopy (FESEM) was used to characterize the morphologies and microstructures of bulk MoS2 powder and exfoliated MoS2 nanosheets under optimal conditions The low- and high-magnification FESEM images show that the layer-stacked bulk MoS2 powder consists of flat, thick layers with lateral dimensions ranging from a few micrometers to tens of micrometers In contrast, the exfoliated MoS2 nanosheets imaged at comparable magnifications reveal significantly reduced lateral sizes, and following ultrasonic processing, their thicknesses are greatly diminished compared with the bulk powder, indicating effective exfoliation.
Figure 3.5 Low- and high- resolution SEM images of (a) and (b) the bulk MoS 2 powder; (b) and (d) the exfoliated MoS 2 nanosheets [87]
Figure 3.6 presents a low-magnification TEM image of MoS2 nanosheets with average sizes ranging from 100 to 200 nm High-resolution TEM reveals clear parallel lattice fringes, signifying a well-crystallized MoS2 structure The interplanar spacings of about 0.62 nm and 0.27 nm match the (002) and (100) planes of the MoS2 hexagonal lattice The selected-area electron diffraction pattern shows sharp spots consistent with hexagonal symmetry, confirming the high crystallinity and single-crystal nature of the MoS2 nanosheets (JCPDS 65-7025) and aligning with the XRD results.
Figure 3.6 (a,b) Transmission electron microscopy (TEM) images of MoS 2 NSs; (c)
HR-TEM image and (d) SAED pattern of MoS 2 NSs
Fig 3.7 (a) and (b) illustrate the XRD patterns and Raman spectra of the bulk
Figure 3.7(a) compares the XRD patterns of bulk MoS2 and MoS2 nanosheets, with both forms displaying very sharp, well-defined peaks at 2θ = 14.40°, 32.69°, 35.84°, 39.57°, 44.18°, 49.82°, 58.35°, 70.20°, 72.78°, 76.06°, and 78.12°, corresponding to the (002), (100), and other crystal planes This observation indicates preserved crystallinity in both bulk MoS2 and MoS2 nanosheets.
(103), (102), (006), (105), (110) and (108), (203), (116), and (109) planes are shown, indicating the appearance of highly crystalline 2H-MoS2 phase (JCPDS
No abnormal diffraction peaks could be seen in the XRD pattern of the MoS2 nanosheets, suggesting that the exfoliation process successfully created intact nanosheets from the bulk material without the introduction of impurities The most noticeable change in the diffraction patterns was the reduction in the intensity of the (002) peak, which is indexed to the chemically inert basal plane of 2D MoS2 nanosheets [104] This reduction indicates changes in the stacking layers of the MoS2 nanosheets as a result of exfoliation.
MoS2 along the c-axis decreased, which in turn created abundant active edge sites that contributed to the enhanced gas-sensing performance
Figure 3.7 (a) XRD patterns and (b) Raman spectra of the bulk MoS 2 and the exfoliated MoS 2 nanosheets [87]
Fig 3.7(b) compares the Raman spectra of bulk MoS2 and MoS2 nanosheets The in-plane vibrational mode A1g shifts from 402.1 cm−1 in bulk MoS2 to 405.3 cm−1 in the nanosheets, while the out-of-plane vibrational mode E1 2g near 379 cm−1 shows almost no change The A1g–E1 2g frequency difference is about 27.1 cm−1 for the bulk MoS2 spectrum (Fig 3.8(b) bottom), confirming the bulk material, whereas the exfoliated MoS2 nanosheets exhibit a gap of 23.1 cm−1, indicating a thin thickness of about three layers [103].
3.2.2 NO 2 gas-sensing properties of MoS 2 NSs sensors
3.2.2.3 Effects of ultrasonic vibration power
Figure 3.8 demonstrates the effect of ultrasonic vibration power on the gas-detection performance of MoS2 nanosheet sensors fabricated at different vibration powers and tested with 5 ppm NO2 at room temperature Figure 3.8(a) illustrates the dynamic response of the NSs sensors under three vibration-power conditions, where the sensor resistance drops rapidly upon exposure to the oxidizing NO2 gas and gradually returns to its baseline after the chamber is flushed with dry air This response reflects the p-type semiconductor nature of the synthesized MoS2 NSs.
Figure 3.8(b) shows the sensor response to higher ultrasonic vibration power For 5 ppm NO2, the response reaches 68% at a fabrication power of 180 W and increases to 76% at 420 W.
42 response was calculated using the formula (S = (R a – R g )/R a ) %) The sensor's resistance also decreased as the fabricated power increased from 180 W to 420 W (Fig 3.8 (c))
Figure 3.8 (a) Transient resistances of the sensor based on synthesized MoS 2 nanosheets at different ultrasonic vibration power to 5 ppm NO 2 ; (b) Response and (c) resistance of different samples
Figure 3.9 shows (a–c) the transient resistance of a sensor based on synthesized MoS2 nanosheets at different centrifugation speeds during exposure to NO2 in the 0.25–5 ppm range, while panel (d) summarizes the gas response as a function of NO2 concentration for the different samples The transient resistance curves reveal that centrifugation speed influences the dynamic resistance changes, indicating processing-dependent sensing performance The concentration-dependent plot in panel (d) demonstrates how NO2 response scales with concentration across samples, illustrating the sensor’s sensitivity to NO2 Overall, the figure highlights measurable, concentration-dependent resistance changes in NO2 sensing and underscores the impact of synthesis/centrifugation conditions on the transient response and detection efficiency.
Triethylamine (TEA) gas sensors based on MoS 2 NSs combine with
This section presented the influence of MoS2 NSs concentration on morphology, structure and TEA gas-sensing properties of MoS2/ZTO nanocomposite
3.3.1 Morphologies and structure of MoS 2 NSs combine with ZTO porous octahedra
During the initial stage, under relatively low temperature and short reaction time, metastable ZnSnO3 nanocubes and ZnO nanoparticles form When the hydrothermal temperature increases to 200°C, a thermodynamically driven phase transformation occurs, causing the cubic Zn2SnO4 to dissolve and recrystallize into a smooth-surfaced octahedron As hydrothermal time extends, secondary nucleation begins, and Zn2SnO4 hexagonal nanoflakes start to grow on the octahedron surface via internal dissolution, contributing to the porous octahedral Zn2SnO4 structure.
At 30 hours, nanosheets completely cover the octahedral surface, forming a texture-like morphology The porous octahedral structure arises from dissolution–recrystallization during the hydrothermal reaction, first producing a cubic, smooth-surfaced octahedron and finally yielding a porous octahedron composed of Zn2SnO4 hexagonal nanoflakes interlaced on the surface The Zn2SnO4 formation follows the general reaction shown in equation (3.11).
2Zn 2+ + Sn 4+ + 8OH ‒ → Zn2SnO4 + 4H2O (3.11) SEM results show that MoS2 nanosheets are successfully intercalated on
Zn2SnO4 porous octahedra were synthesized by a hydrothermal method that preserves morphology and microstructure The porous octahedral Zn2SnO4 produced in this study is compared with the octahedral Zn2SnO4 prepared by hydrothermal synthesis reported by Hanh et al (2021) [125] Literature indicates a hydrothermal time of 24 hours at 200 °C, yielding a smooth surface; this observation is consistent with our results at 30 hours of fabrication, which also show a smooth surface while maintaining the same morphology and microstructure.
X-ray diffraction (XRD) analysis was conducted on the as-prepared samples to determine their composition and crystal structure, as shown in Fig 3.16 The XRD patterns of all samples show no significant differences after the addition of the additive, indicating that the overall crystalline phases remain unchanged by the modification.
MoS2 NSs, which indicates that the MoS2 NSs assembly did not affect the crystal structure of ZTO The positions of typical diffraction peaks of ZTO at 2θ (2-theta) are 17.7 o , 29.1 o , 34.3 o , 35.9 o , 41.7 o , 45.6 o , 51.6 o , 55.1 o , 60.4 o , 63.5 o , 68.5 o , 71.4 o ,
72.3 o , 76.1 o , 79.0 o , 83.5 o and 86.2 o are consistent with the cubic spinel structure of Zn2SnO4 with space group Fd3m, (red color, JCPDS No 24-1470) The crystal planes corresponding to these diffraction peaks are (111), (220), (311), (222),
(400), (331), (422), (511), (440), (531), (533), (622), (444) and (711), respectively The individual corresponding XRD patterns of ZM-0, 1, 3, 5, 10 were shown to identify the peaks clearly The results show that the composite samples with MoS2
The composition of MoS2 nanosheets increases from ZM1 to ZM10, with the MoS2 diffraction peaks appearing and intensifying, confirming the formation of ZTO–MoS2 hybrids The characteristic MoS2 peaks at 2θ values of 14.4°, 32.7°, 39.6°, 49.8°, 58.4°, and 60.8° correspond to the (002), (100), (103), (105), (110), and (112) planes (JCPDS No 65-7025) These peaks are predominantly observed in the ZM10 sample and are absent or of low intensity in ZM1 due to the smaller MoS2 nanosheets content in the composites The strong diffraction peak intensity and the absence of impurity peaks confirm the high purity and good crystallinity of the products.
Figure 3.16 XRD patterns of pure Zn 2 SnO 4 and ZM1, ZM3, ZM5 and ZM10 samples.
SEM analysis of the samples (Fig 3.17) reveals hexagonal Zn2SnO4 hollow octahedra assembled from nanoflakes with a relatively uniform size and distribution of 3–5 μm, and shows that the MoS2 nanosheet assembly does not significantly alter the Zn2SnO4 morphology The octahedra exhibit more space between adjacent nanoflakes, promoting deep diffusion of gas molecules and increasing the gas adsorption surface area, which is expected to improve sensor response and sensitivity The surface of the pure ZTO octahedron is fairly uniform, but higher MoS2 concentrations roughen the surface while preserving the overall octahedral morphology, indicating that MoS2 does not damage the ZTO octahedron structure MoS2 nanosheets, spanning several hundred nanometers, are distributed on the ZTO surface and are difficult to observe clearly in SEM images.
Figure 3.17 SEM images of (a, b) pure Zn 2 SnO 4 octahedrons, (c, d) ZM1, (e, f) ZM3,
Figure 3.18 (a) Transmission electron microscopy (TEM) images of ZM3 sample; (b-d)
HR-TEM image of ZM3 sample
Figure 3.18a shows the TEM image of the ZM3 sample, revealing an octahedral framework formed by interlaced MoS2 and ZTO nanosheets that can be visually distinguished In this composite, the MoS2 and ZTO nanosheets are in contact and interwoven, though SEM images make the exact MoS2 position difficult to determine HR-TEM images (Figure 3.18a,b) reveal parallel lattice fringes for MoS2 and ZTO, with a lattice spacing of ~0.62 nm assigned to the MoS2 (002) plane, confirming a few-layer MoS2 structure with d(002) ~ 6 Å For ZTO, the interplanar distance is ~0.306 nm corresponding to the (220) planes The SAED pattern (Figure 3.18d) shows sharp spots, indicating that the MoS2/ZTO composite is a single-crystalline material.
(111) of ZTO found in the SAED
EDS elemental analysis of pure ZTO confirms high purity with no detectable impurities in the sample The analysis identifies oxygen (O), zinc (Zn), and tin (Sn) as the constituent elements, with atomic compositions of 55.6%, 20.9%, and 23.6%, respectively, as shown in Fig 3.19.
(a)) Meanwhile, EDX analysis of sample ZM3 shows the existence of elements S,
Mo, O, Sn and Zn (Fig 3.19 (b)) The proportions of elements O, Zn, Sn, Mo and
S are 47.2, 16.9, 15, 0.5 and 0.5%, respectively This confirms the presence of MoS2 NSs in the composite structure with ZTO
Figure 3.19 EDX analysis and Raman spectrum of the (a, c) ZTO and (b, d) ZM3 sample
Raman spectroscopy is used to study the vibrational properties of materials For the Raman spectrum of pure ZTO, as shown in Fig 3.19(c), two prominent peaks occur at about 669 cm^-1 and 537 cm^-1, corresponding to the symmetric A1g and E2g lattice vibrational modes of ZTO [126] The peak near 668 cm^-1 is attributed to the symmetric elongation of Zn–O bonds in the ZnO4 tetrahedra of the ZTO inverted spinel, while the peak around 536–537 cm^-1 reflects the internal oscillations of the O tetrahedra [127].
Figure 3.19(d) shows the Raman spectrum of the ZM3 porous octahedron The in-plane and out-of-plane vibrations between Mo and S atoms are represented by two characteristic peaks, E2g^1 at 384 cm−1 and A1g at 408 cm−1, respectively.
Raman analysis of the exfoliated MoS2 nanosheets reveals a ~24 cm^-1 separation between the in-plane and out-of-plane vibrational modes, indicating that the MoS2 nanosheets, when integrated with ZTO, have a thickness of about three to four layers.
Figure 3.20 TEA sensing transients of sensors based on different concentration MoS 2
NSs combine with ZTO at different working temperatures (a) ZTO, (b) ZM1, (c) ZM3, (d) ZM5 and (e) ZM10; (f) Response to 50 ppm TEA of different sensors at various working temperatures
To evaluate the effect of working temperature on gas sensing properties, we compared the TEA gas sensing characteristics of sensors at different operating temperatures This helps to find the optimal working temperature of the sensors
Figure 3.20 presents the transient resistance versus time for the pure ZTO sensor exposed to different TEA concentrations at 150–300 °C, showing that resistance decreases with increasing temperature and TEA exposure, which confirms the n-type semiconductor behavior of ZTO While similar to the ZM1–ZM10 sensors, the base resistance increases when MoS2 nanosheets are coupled with ZTO, as shown in the data The pure ZTO sensor achieves its highest response to 50 ppm TEA at 250 °C, whereas the MoS2/ZTO composite sensors have an optimal operating temperature of 150 °C, indicating that decorating ZTO porous octahedra with MoS2 nanosheets significantly lowers the working temperature and markedly enhances the gas response compared to pure ZTO.
The responses of ZTO and MoS2/Zn2SnO4 sensors to TEA concentrations from 50 to 200 ppm were measured at different temperatures, showing that the pure ZTO sensor’s response increases as the temperature rises from 150 °C to 250 °C, with the response growing from 7.4 at 50 ppm TEA to 13.4 at 200 ppm TEA at 250 °C, while at 300 °C the TEA response declines, indicating an optimal operating temperature of 250 °C for the pure ZTO sensor; after adding MoS2 nanosheets, all sensors exhibit an optimal TEA operating temperature of 150 °C.
Figure 3.21 presents the TEA response characteristics of different MoS2/ZTO sensors, including ZTO, ZM1, ZM3, ZM5, and ZM10 Panel (f) shows the sensor responses as a function of acetone concentration and the response to 200 ppm TEA at different working temperatures, enabling a direct comparison of sensitivity, selectivity, and temperature stability across the MoS2/ZTO variants.