OVERVIEW OF 2D SnS2 LAYERED MATERIALS AND THEIR HETEROJUNCTION WITH SnO2 FOR GAS-SENSING APPLICATION AND THE FUNDAMENTAL OF THE FLEXIBLE GAS SENSOR TECHNOLOGY.. NO2 gas-sensing character
OVERVIEW OF 2D SnS 2 LAYERED MATERIALS AND THEIR
Overview of the SnS 2 materials
SnS2, a notable metal dichalcogenide, showcases remarkable properties when reduced to two-dimensional materials In low-dimensional settings, it demonstrates outstanding thermoelectric performance due to its high carrier mobility and low thermal conductivity This section will cover essential fundamentals related to its structural, optical, and electronic properties.
Figure 1.7 Structure schematic of SnS 2 nanosheets (A) side view, (B) Top view of multilayers [17]
The priority structure is 2H-SnS2 (Figure 1.7), where the Tin atoms are sandwiched between two layer of hexagonal close-packed Sulfur atoms to form triple layers
[18] In this configuration, a metal atom is octahedrally coordinated by sulfur
The structure of bulk SnS2 consists of 13 atoms and features a trigonal prismatic coordination, distinct from the 2H-SnS2 polytype To obtain the 2H-polytype, monolayers of 2H-SnS2 are stacked, with weak van der Waals interactions allowing for mechanical separation along the c-axis This separation can be achieved through methods like mechanical or liquid phase exfoliation The bulk SnS2 exhibits a symmetry group of 𝐷 3𝑑 3 (P3̅𝑚1), confirming its trigonal structure.
The 2H-SnS2 structure exhibits a semiconductor’s nature with an indirect bandgap for all thickness and a bandgap range of 2.18–2.41 eV, which is determined by the number of layers
The Raman spectra of few-layer 2H-SnS₂ reveal important insights into its vibrational modes Panel (a) displays both low and high-frequency modes of 5L 2H-SnS₂ measured across various laser wavelengths Panel (b) illustrates the excitation energy of the A₁g modes as the number of layers increases from 1L to 14L In panel (c), high-frequency modes of few-layer 2H-SnS₂ are examined using 523 nm lasers, while panel (d) focuses on the Eₑ mode recorded with 441.6 nm lasers Finally, panel (e) presents the Raman intensity and peak positions of the A₁g mode as a function of the number of layers, highlighting the material's layered characteristics.
Tharith Sriv et al conducted a study to examine the interactions between incident photons and lattice vibrations in five-layer 2H-SnS2 by measuring its low- and high-frequency Raman spectra across four different excitation energies Their findings, illustrated in Figure 1.8, revealed a notably intense Raman signal at a specific wavelength.
The excitation laser used at 532 nm, corresponding to a bandgap energy of 2.33 eV, highlights the dominant out-of-plane A1g mode at approximately 314 cm −1, while the Eg mode at 206 cm −1 exhibits weak intensity and is primarily observed under 2.81 eV excitation (441.6 nm) The interlayer vibrational modes, both in-plane and out-of-plane, are detailed in the low-frequency region Notably, the A1g mode displays peak intensity with the 532 nm excitation, suggesting that the bandgap of few-layer 2H-SnS2 is smaller than the theoretical prediction of 2.41 eV Additionally, the Raman spectra reveal a correlation between the number of layers and the presence of weak signals from the A1u and A1g-LA modes at 353 cm −1 for bulk and thick samples.
The Raman spectrum exhibits variations in intensity and peak position with changes in layer number, particularly showing a slight blue shift in the A1g mode from one layer (1L) to three layers (3L) This observation aligns with theoretical predictions and findings from other studies.
The thickness of SnS2 nanosheets can contribute to the electronic band structure, which is closely related to the Seebeck coefficient and electrical conductivity
Figure 1.9 Electronic band structure of SnS 2 nanosheets in various layers (a-d), the electron density distribution of conduction band minimum for SnS 2 nanoflakes (e) [22]
Figure 1.9(a-d) illustrates the splitting of the conduction band edge of SnS2 into subsets based on the number of layers For nearly all layers of SnS2 nanosheets, the conduction band minimum (CBM) is found at the M point, while the valence band maximum (VBM) is situated between the M and Γ points, confirming the indirect band structure of SnS2 despite reduced dimensions Additionally, the similar band structure characteristics near the CBM suggest a comparable effective mass The charge density for the CBM across various layers of SnS2 nanosheets is depicted in Figure 1.9(e).
10 layers, the electrons of CBM are allocated to 6 layers among them, and most of the layers are the presence of electron distribution When decreasing the thickness,
Thinner nanosheets exhibit denser electron density, with 4 layers in the 5-layer sample and all layers in the 3-layer sample showing electron occupation The conduction band primarily arises from the 5s and 5p orbitals of the Sn atom, along with the 3p orbitals of the S atom.
SnS 2 materials for gas-sensing application
In recent decades, air pollution has significantly increased, posing serious threats to human health A primary contributor to this decline in air quality is nitrogen dioxide (NO2), which, due to its colorless nature, can easily enter the circulatory and respiratory systems Long-term exposure to NO2 can lead to severe damage to vital organs such as the heart, brain, and lungs The American Conference of Governmental Industrial Hygienists (ACGIH) has established a threshold limit value (TLV) for NO2 to help mitigate these health risks.
NO2 is up to 3 ppm for 8 h time-weighted average and 5 ppm for 15-min period
Recent advancements in ultra-low NO2 gas sensors have highlighted the importance of developing new detection methods to ensure a safe environment for humans Among these innovations, layered materials like SnS2 have emerged as leading candidates for gas-sensing applications due to their high electronegativity, narrow bandgap, and responsiveness to light, which can enhance gas adsorption The electronic band structure of SnS2 is significantly temperature-dependent, allowing for optimized sensing responses at moderate temperatures Additionally, the morphology and thickness of SnS2 can be tailored to create effective gas-sensing platforms Consequently, extensive research has focused on optimizing the synthesis processes of SnS2 to meet the specific requirements of various applications.
There are various effective methods for growing 2D materials, including vapor transport, chemical vapor deposition (CVD), wet chemical processes, and atomic layer deposition (ALD), each offering optimal conditions for material formation.
Recent advancements in SnS2-based gas sensors have demonstrated significant potential for detecting NO2 gas Vertically aligned 2D SnS2 flakes, synthesized using three-zone chemical vapor deposition, exhibit the highest response to 50 ppm NO2 at 120 °C, achieving a sensitivity increase of approximately 164 times In contrast, sensors utilizing SnS2 nanoparticles fabricated through a one-step hydrothermal method can detect NO2 at room temperature, showcasing rapid response and recovery due to their favorable electronic properties The performance of these sensors is notably influenced by the thickness of the sensing layer, with thinner layers (~4 μm) providing enhanced sensitivity Additionally, SnS2 nanosheets benefit from light-induced charge transfer, which enhances gas detection capabilities, particularly under green lighting, where they show excellent selectivity for NO2 over other gases like ammonia and methanol However, challenges remain regarding the selectivity and reversibility of current gas-sensing technologies Recent studies have introduced economical sensing platforms that leverage charge transfer between physisorbed NO2 molecules and 2D SnS2 flakes, optimizing sensor performance at lower operating temperatures due to the flakes' large active surface area and low interlayer binding energy.
The detection limits for NO2 are below 30 ppb at 120 °C, with a remarkable response factor of about 36 when exposed to 10 ppm of NO2 Additionally, the theoretical model indicates that the NO2 adsorption and desorption processes occur without the involvement of oxygen, allowing for the potential detection of NO2 gas even in ambient conditions.
Recent publications on 2D SnS2-based gas sensors, as summarized in Table 1.1, highlight that various sensor platforms utilizing SnS2 and doped SnS2 nanomaterials exhibit exceptional sensitivity to NO2, even at low temperatures This remarkable sensitivity is characteristic of layered materials, where the sensing mechanism primarily involves charge transfer between gas molecules and the sensing surface, rather than the conventional reaction involving oxygen species.
Table 1.1 Reported publication regarding the 2D SnS 2 nanostructures in recent years
Application: Used for detecting NO2 gas at room temperature with promising response of
303 % (1 ppm) and rapid response/ recovery speed (28/2 s)
Application: Utilizing defect and interlayer engineering in the fabrication of SnS2 to achieve high response at RT (410 % to 2 ppm)
Morphology: Platelet- like/ flower-like structure
Application: SnS2 flower-like structure provides more exposed active sites with response of 32.63 % toward 50 ppm NO2 at
Application: Optical sensor demonstrates optical power variation of up to 7 W upon exposure 50 ppb and an extremely low LOD of
Method: High-energy ball miling (HEBM)
SnS2-2D into micro- heater for detecting
NO2 The sensor exhibited a response >
2000 % and response/recovery times of 6/40 s at an
Method: Hydrothermal method and chemical precipitation
Application: To boost the response and recovery properties to
Heterojunction gas sensors
1.2.1 Overview of gas sensor based on heterojunction
Recent studies indicate that 2D-material-based gas sensors offer advantages such as low energy consumption, effective room temperature operation, and enhanced target gas separation in complex environments However, the formation of a conductive network can lead to incomplete contact between the sensor flakes and gas molecules, resulting in reduced sensitivity and slower response/recovery rates at lower temperatures To address these challenges, various strategies have been explored, including the doping of metal nanoparticles to improve catalytic activity and the integration of n-type or p-type materials to create heterojunctions that leverage the strengths of each component Notably, the combination of metal oxides with 2D layered materials has shown significant potential in optimizing gas sensor performance For instance, Yuan et al successfully synthesized octahedral SnO2 nanocrystals on MoS2 microspheres, forming a Z-scheme heterostructure that enhances NO2 gas-sensing capabilities, as illustrated in their SEM images.
The Z-scheme structure exhibits exceptional physical properties, showcasing remarkable sensitivity to NO2 detection, achieving a response of approximately 264.2 at concentrations as low as 10 ppm under low-power LED illumination Additionally, the Z-scheme junction is believed to enhance the separation of photogenerated carriers, which may play a crucial role in the NO2 sensing reaction.
Figure 1.10 SEM images of MoS 2 /SnO 2 heterojunction with different magnifications (a- i) [35]
WS2 nanosheets hybridized with SnO2 quantum dots have emerged as an effective method to enhance gas sensor responses, making it a promising candidate for room temperature gas sensing applications The n-n heterostructure of MoO3/MoSe2 was synthesized using a two-pot hydrothermal process, resulting in a unique porous flower-shaped structure of MoSe2 combined with the elongated rod configuration of MoO3 This innovative structure enables the detection of a wide range of gases, from 20 ppb to 1000 ppb, within seconds, demonstrating a correlation between gas concentration and sensor response, as illustrated by a Freundlich experimental fitting curve at room temperature.
1.2.2 Overview of gas sensor based on heterojunction of SnS 2 and semiconductor oxide (SnO 2 )
Three key factors enhance gas sensing performance: geometrical effects, electronic effects, and chemical effects Recent studies have shown notable improvements in sensing performance through the formation of heterojunctions Additionally, the design of hierarchical structures plays a crucial role in optimizing gas sensors.
22 nanostructure consisting of 2D nanosheets could retain the intrinsic nature of 2D materials as well as exhibit ultra-large surface area, facilitating gas adsorption and diffusion
Figure 1.11 SEM images of MoSe 2 and MoO 3 /MoSe 2 nanocomposite sensors and the sensor response properties at room temperature [36]
The fabrication of heterojunctions is a dynamic method to enhance electronic effects through improved charge transfer Research highlights the significance of the zero-dimensional n-type semiconductor SnO2 in gas sensing applications due to its exceptional chemical and physical properties Consequently, innovative gas sensor designs utilizing SnS2 and SnO2 have been developed for detecting low concentrations of NO2 without the need for external heating For example, Wang.Y demonstrated the partial oxidation of SnS2 nanoflowers, created via a straightforward microwave method, leading to the formation of hierarchical SnS2/SnO2 nanocomposites The resulting SnS2/SnO2 sensor displayed an impressive response (Rg/Ra ~ 51.1) to 1 ppm NO2 at 100 °C, surpassing the performance of pristine SnS2 nanoflowers by over ten times Additionally, Wang.J proposed a simple in-situ high-temperature oxidation method for pure SnS2.
23 his co-author [41] The SnO2 nanoparticles decorated on SnS2 nanosheets are beneficial to sensing capability, indicating a much higher response than the pure
At lower temperatures, specifically 80 °C, SnS2 exhibits improved performance when exposed to light activation, such as UV or visible light A recent study by Zhang et al introduced a hollow SnO2@SnS2 nanostructure created through a one-step hydrothermal method This innovative sensor demonstrates a remarkable response to blue light irradiation at room temperature (25 °C), significantly enhancing conductivity and accelerating the adsorption and desorption kinetics of NO2 gas.
Gas sensor based on nano-ink
1.3.1 Overview of the low-power consumption gas sensor
Advancements in technology have led to the widespread use of gas-sensing devices across various fields, including environmental monitoring and hazardous gas detection While conventional metal oxide-based resistive gas sensors offer several advantages, their high operating temperatures pose challenges for practical applications, particularly in smart and portable devices Recent research has focused on strategies to reduce power consumption in gas sensors, such as self-heating techniques, MEMS technology, and room-temperature operation methods These low-power sensors generally consume over 1 mW, but MEMS-based sensors can potentially lower power consumption to just a few milliwatts by minimizing the size of the heater and device.
[46] developed NO2 sensing devices based on Au/SnO2: NiO thin film which were fabricated via self-assembly and magnetron sputtering technique The ordered 2D
The Au NP array demonstrates significant activation, achieving a high response and a low detection limit of 0.05 ppm for NO2 at an optimal temperature of 200 °C Chen and colleagues developed a 3D heterogeneous device that integrates a ZnO nanowire MEMS gas sensor with a blue LED, resulting in an impressive response value of 86.17% for 500 ppb of NO gas Furthermore, advancements in self-heated gas sensors have been made to reduce power consumption by eliminating the need for a microheater; instead, heat is generated through the Joule effect from applied bias voltage Son et al designed a self-heated In2O3 nanowire-based gas sensor using an on-chip growth technique, capable of detecting ethanol gas as low as 20 ppm while consuming only 1.06 mW of energy.
1.3.2 Overview of ink-jet printing technique and fabrication nano-ink 1.3.2.1 Overview of ink-jet printing
Traditional methods for manufacturing microelectronic components, such as photolithography and thin-film deposition techniques (sputtering, CVD, thermal evaporation), face significant limitations including bulkiness, high costs, complex operational requirements, and substantial environmental waste As the demand for wearable and portable electronic devices grows, there is an increasing need for low-cost, user-friendly, and efficient manufacturing equipment This shift has led to a remarkable surge in innovative technological trends within the electronics industry.
Figure 1.12 Printed, flexible, and organic electronics evolution in 2020 – 2030 [50]
Printing electronics represents a significant advancement in manufacturing techniques, offering benefits such as sustainability and the ability to pattern on various flexible substrates, including paper, fabrics, glass, metal, and plastics This innovative printing method is increasingly utilized in electrochemical devices, including sensors for monitoring, memory storage, portable displays, and self-powered devices The process of printed electronics consists of three key stages: material selection, printing, and post-printing.
Figure 1.13 The typical stages in electronic printing process [52]
Layer deposition processes are essential for creating stacked structures in electronic components, including conductive layers, insulating layers, and electrodes Various printing techniques are employed in the fabrication of these electronic components, ensuring precision and efficiency.
There are 25 applied printing techniques, categorized by their physical contact with the substrate Contact printing methods involve the direct transfer of ink onto the target substrate Notable contact printing techniques include screen printing, gravure printing, offset printing, and flexographic printing.
[56] Conversely, the non-contact technologies expel the materials using an actuator assisted-nozzle without any physical contact with the substrate, for instance, inkjet printing [57] and spray printing [58] as Figure 1.14
Figure 1.14 The ink-based printing technologies are applied in flexible electronic devices [59] a Ink-jet print-head
Figure 1.15 The schematic of continuous inkjet printer (a) and on-demand inkjet printer [60]
Ink-jet printing is a leading technology in the production of flexible and wearable sensors due to its mask-free approach and versatility with various flexible substrates This technique utilizes two primary operation modes: continuous inkjet (CIJ) and drop-on-demand (DOD), which are controlled by programming the motion of the printing nozzle DOD inkjet printing commonly employs thermal and piezoelectric print heads The basic print head features a piezoelectric diaphragm that, when activated by a pulse voltage, generates pressure to eject ink through the nozzle, creating a liquid jet that forms droplets Continuous inkjet printing offers a rapid ejection rate of 0.5 µL droplets at frequencies of 80-100 kHz, while drop-on-demand printing produces smaller droplets (2-500 pL) at a slower rate of up to 30 kHz, without the need for a recirculation system.
Figure 1.16 (a-d) illustrates various piezo-driven print-head designs tailored by manufacturers to fulfill specific needs Some of these print-heads incorporate a heater to manage thermal phase-change materials that exhibit high viscosities at room temperature The inclusion of a nozzle heater enhances the stability of ink properties under varying ambient conditions and effectively lowers viscosity.
Figure 1.16 Variety configuration of piezo-driven head printing (a) squeeze, (b) shear,
Table 1.2 outlines the specifications and practical performance of commercial inkjet printheads, highlighting that droplet volumes can be precisely controlled between 1 to 80 pL These printheads achieve high resolutions of 1200 dots per inch (dpi) at frequencies exceeding 40 kHz Furthermore, certain models incorporate heaters designed for thermal phase-change materials, enhancing their functionality.
The ink supply system for inkjet print-heads is enhanced by the use of a nozzle heater, which stabilizes ink properties and significantly reduces viscosity, particularly at room temperature under varying ambient conditions.
In inkjet printing systems, supplying ink to the print-head is crucial, typically utilizing bottles or syringe barrels to maintain the necessary pressure A standard ink supply setup is depicted in Figure 1.17 During manufacturing, ink containing nanoparticles is continuously fed to the print-head, but the accumulation of these particles can lead to nozzle blockages To mitigate this issue, ink recirculation systems are frequently employed in large-scale production.
Table 1.2 The commercial inkjet print-heads
100 116 3 - 21 1220 5680 c Driver and driving waveform for inkjet print-head
Inkjet print-head nozzles require ink supply and pulse voltage to the piezo actuators, creating pressure waves for jetting To effectively manage this bias voltage, electronic drivers are essential for regulating jetting by delivering the appropriate voltage to each nozzle There are two main types of inkjet drivers: drive per nozzle (DPN), which allows independent voltage application to each nozzle, and shared drivers, which control groups of nozzles, typically around 128 or 256.
Figure 1.17 The graph of ink supply getting the ink to print-head [62]
Nano-ink formulation is essential in ink-based printing, involving the mixing of primary materials, solvents, and functional additives to create effective printing inks Key factors influencing this process include ink rheology, substrate conditions, and post-treatment parameters, all of which affect the designed patterned structure Structural qualification relies on various parameters such as processing time, temperature, shear rate, and content Inks can be categorized into non-Newtonian and Newtonian types, with Newtonian inks maintaining constant viscosity regardless of shear rate changes These inks, often low in viscosity, are suitable for inkjet printing and spray coating when dispersing feedstocks like nanoparticles or graphene in solvents such as ethanol or deionized water The characteristics of ink, including viscosity, surface tension, particle size, and solid content, are crucial, as indicated in Table 1.3, which outlines compatible viscosities for different printing technologies Regulating ink parameters poses challenges, as stable electrical properties are vital for electronic devices, even with modifications to ink specifications Solvents and surfactants can adjust ink viscosity, with low-viscosity inks being preferred for inkjet printing to meet specific utilization requirements, necessitating modifications in precursor content to achieve desired conductive and semiconducting properties.
Fernandes I.J et al conducted a study on various conductive ink formulations utilizing silver nanoparticles synthesized via the chemical reduction of silver nitrate Their findings indicated that the resistivity of the inks varied from 3.3 to 5.6 × 10^-6 Ω·m, while the viscosity ranged from 3.7 to 7.4 mPa·s, making these formulations suitable for inkjet printing applications.
A study reported the fabrication of CuO nano ink using a microwave-assisted solvothermal sealed vessel method To meet the parameters required for ink-jet printing, the ink composition was enhanced with a polymeric steric surfactant and dispersant.
Table 1.3 Ink characteristics for different printing techniques [68]
Gravure Flexography Offset Screen Inkjet Aerosol
Maximum preferred particle size (nm)
1.3.3 Overview of the flexible gas sensor based on inkjet printing
EXPERIMENTAL SECTION
Preparation of SnS 2 -based nano-ink
Ethanol (EtOH, >99%) was utilized as a solvent, and polyvinylpyrrolidone (PVP, >98%) served as a stabilizer in the preparation of SnS2/SnO2 nanopowders The nano-ink was created through probe sonication, effectively breaking down the initial particles into smaller sizes to prevent nozzle clogging This SnS2/SnO2-based composite is designed for the detection of hazardous gases.
In a study, 0.5 g of SnS2/SnO2 was combined with 10 mL of ethanol solution containing 0.1 g of PVP, achieving a PVP/ethanol ratio of 10 mg/ml Sonication was performed using an ultrasonic probe at a power output of 50% (300 W) for 30 minutes, with an on/off pulse cycle of 5 seconds each To maintain a stable temperature during the process, a cooled-water system was utilized, ensuring that the temperature remained below 25°C The pH of the solution was measured at approximately 7, confirming the stability of the ink and its compatibility with the printhead.
Fabrication of the gas sensor based on SnS 2 material
In this section, the different approaches of gas sensor fabrication will be described and compared deeply Two techniques applied are conventional and nano-ink printing technology
Conventional gas sensors have been developed using microelectronic techniques such as photolithography and sputtering, with the sensing layer applied through a drop casting method These sensors are designed to fulfill practical requirements for operation in real-world environments, emphasizing features like robust structure, high reliability, long-term stability, user-friendliness, and scalability This thesis focuses on creating a gas sensor capable of functioning at room temperature without the need for additional heating.
Flexible gas sensors were fabricated on a PET substrate using an ink-jet printing process A nano-ink solution was applied to an existing electrode, creating a conductive channel essential for gas detection.
35 signal The sensor resistance was tested by hand-made gas sensing measurement and basic circuit using an Arduino microcontroller
2.3.1 Conventional technique for fabrication of gas sensors on SiO 2 /Si substrate
Figure 2.3 The Inter Digitated Electrodes (IDE) fabrication via microelectronic technology
A gas sensor consists of three primary components: a non-conductive substrate made of SiO2/Si, electrodes of Pt/Au, and a sensing layer Unlike typical MEMS structures, this resistive gas sensor does not incorporate an integrated microheater, making external heating necessary for its operation.
Figure 2.3 illustrates the process of IDE electrode fabrication, which begins with the thermal oxidation of n-Si water to create a thin, non-conductive SiO2 layer Subsequently, a primer substance, Hexamethyldisiloxane, is applied to enhance the electrode's performance.
To enhance the surface adhesivity of the SiO2/n-Si wafer, a coating of C6H19NSi2 is applied, enabling a uniform deposition of photoresist Following this, a soft baking process at 95 °C for 90 seconds is conducted Photolithography is then utilized to pattern electrodes using a UV light source, with the Cr mask playing a crucial role in translating designs created with software like Coreldraw and Clewin The wafer is subsequently immersed in a developer for 60 seconds, revealing the electrode patterns; in this process, the positive photoresist allows for the removal of exposed areas while preserving the unexposed sections Finally, a hard baking step at 125 °C is performed to prepare the Si wafer for the deposition of a Pt/Au thin layer, which is later lifted off.
36 acetone solvent in an ultrasonic bath to eliminate all the Pt/Au film on the photoresist
Figure 2.4 The drop-casting process of the SnS 2 gas sensor
The SnS2 gas sensor is fabricated using a drop-casting process, where SnS2 nanopowder is dispersed in NVP solvent to create a suspension A specific volume of this solution is applied to the electrode and then subjected to hard baking at 150 °C for 15 minutes to eliminate any residual solvent The sensor's performance is influenced by various synthesis parameters outlined in Table 2.1 To ensure proper contact between the electrode and the sensing layer, the sensor undergoes a heating treatment in a vacuum at 300 °C for 2 hours, with a heating rate of 2 °C/min A photo of the completed sensor from the experiment is also provided.
Table 2.1 The parameter of the SnS 2 -based gas sensor
2.3.2 Ink-jet printing on a PET substrate
Before the printing process, the viscosity and surface tension of the SnS2/SnO2 nano-ink solution were evaluated using a tensiometer and a viscometer to ensure the ink met the necessary parameters for ink-jet printing, as illustrated in Figure 2.5(A-B).
In this study, a commercial lab-scale Dimatix ink-jet printer (DMP-2800 model, FujiFilm) is used and equipped with a 10 pL cartridge The full image of the DMP Time reaction (h)
The inkjet printing system depicted in Figure 2.6(A) operates with an ink solution that has a surface tension of 30 dynes, viscosity ranging from 2 to 30 cPs, and a pH level between 4 and 9 To ensure optimal performance, filtration is employed to eliminate larger particles from the nano-ink solution Before printing, a detailed electrode structure was designed using EAGLE software, as shown in Figure 2.6(B), featuring a comb size of 300 x 770 μm and a gap of approximately 300 μm between the combs The drawing file of this structural design was then uploaded to the printer for execution.
Figure 2.5 Testing nano-ink parameters using KiBron Aquapi tensionmeter (A) and
The fabrication schematic for the SnS2-based flexible gas sensor is illustrated in Figure 2.6(C) Prior to processing, the non-conductive PET substrate underwent a thorough cleaning with acetone, isopropanol, and deionized water, followed by drying on a hotplate.
Figure 2.6 The DMP-2800 printer for lab-scale printing (A), the IDE electrode designing (B), and gas sensor fabrication using SnS 2 /SnO 2 composite nano-ink printing
Commercial Ag nano-ink was patterned on a PET surface using 10 pairs of combs, with a spacing of 300 µm between each IDE pair The electrode was hydrated at room temperature, leading to the formation of a complete electrode structure Subsequently, SnS2/SnO2 ink, prepared through a sonicating process, was deposited onto the Ag electrode The sensing layer covered an area of 0.6 cm² and exhibited varying thicknesses, which were measured using the DektakXT® stylus profilometer (Bruker) at a scanning distance of 2 µm and a sweeping rate of 0.025 µm/s.
Characterization Techniques and gas sensing measurement
The synthesized materials' morphological structures and elemental composition were analyzed using various advanced techniques, including field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD) with a Cu-Kα source over a 2θ range of 10° to 80° Additionally, energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, and thermogravimetric analysis (TGA) were employed The binding states of the elements in the prepared samples were further investigated using X-ray photoelectron spectroscopy (XPS).
This study investigates gas-sensing properties using a custom-built setup that features a high-speed gas flow system transitioning between air and standard gases The system comprises five key components: (i) standard gas cylinders and an air compressor, (ii) a mass flow controller (MFC) system for adjusting gas concentration ratios, (iii) gas valves managed by two normal-ON and two normal-OFF valves to ensure continuous gas flow, (iv) a testing chamber equipped with a supporting pump for rapid gas evacuation, and (v) a data recording system utilizing a Keithley source meter and MFC controller interfaced with a computer through USB, operated via the LABVIEW graphical programming environment The sensor is positioned on a precisely regulated plate within the testing chamber, which includes gas inlet and outlet features Four mass flow controllers (MSK Instrument, USA) are employed to control the concentrations of various analytes, such as NO2, SO2, NH3, H2S, and C2H4, by mixing these target gases with dried air.
During the testing cycle, dried air was injected into the chamber to stabilize and eliminate turbulence before introducing the testing gas The threshold concentrations for the mass flow controller (MFC) component were adjusted based on the standard concentration of the gas, ranging from 10 to varying limits.
500 sccm The concentration of a kind of gas was calculated via the equation
Figure 2.7 The schematic diagram of the gas-sensing measuring system
Qgas and Qair represent the flow rates of standard gas and carrier gas, respectively, while C0 denotes the concentration of the standard gas cylinder, and MF is the calibration constant In essence, the carrier and mixing gas consist of dried and refreshed air.
The relative humidity (RH) in the chamber was continuously monitored using a DHT22 commercial humidity sensor A bias voltage was applied to the sensor, and real-time electrical signals were recorded at various temperatures with a Keithley instrument.
A conductance gas sensor operates on the principle of detecting changes in resistance when exposed to target gases Key performance indicators for gas sensing include sensitivity, selectivity, response and recovery time, stability, and low detection limits Furthermore, external environmental factors such as temperature and humidity can influence the recorded signals These parameters play a crucial role in determining the effectiveness of gas sensors.
The sensor response is defined as the ratio between the sensor resistance in air and tested gas Conventionally, the sensor response () is determined by the equation
Ra/Rg represents the ratio for tested reducing gas, while Rg/Ra indicates the ratio for oxidizing gas In this context, Ra and Rg refer to the sensor's resistance in dried air and the tested gas, respectively.
Sensitivity refers to the variation in the electrical signal of a sensing material relative to the concentration of an analyte, often represented by the slope of the calibration curve The sensitivity of a sensor is typically estimated using a specific calculation method.
The sensor's response, denoted as S(c), varies with concentration, demonstrating increased sensitivity at higher levels The calibration curve indicates a constant sensitivity within the dynamic range, although precision may vary across different conditions Consequently, the sensor's quantity is mathematically expressed as a derivative at a specific point.
According to this equation, the sensitivity modulates versus the measured concentration, indicating the capability to discriminate various concentrations, for instance, there is a saturation phenomenon at high concentration
Selectivity in gas sensors is the ability to accurately detect a specific gas or a group of analytes while distinguishing them from interfering gases This selectivity is quantified by comparing the sensor's response to one gas in relation to another.
Where ST is the sensor respone to the target gas, and SI denotes the response to an interfering gas
(iii) Response and recovery time
Response time and recovery time refer to the duration required for a sensor's electrical signal to achieve 90% of its equilibrium value after the target gas and air have been introduced into the sensing chamber.
Stability is defined as the ability to maintain a relatively stable and repeatable signal over a long-term period in terms of sensitivity, selectivity, response, and recovery time
The International Union of Pure and Applied Chemistry (IUPAC) defines the detection limit (DL) as the minimum quantity or concentration of an analyte that can be distinctly identified in comparison to an appropriate blank.
Determining the detection limit can be achieved through various methods, one of which involves the signal-to-noise ratio approach This method compares the signal from a sample containing low concentrations of the analyte to that of a blank sample A signal-to-noise ratio of 3.3 is deemed acceptable for estimating the detection limit The methodology for calculating the limit of detection is illustrated in Figure 2.8.
Figure 2.8 The visualization of detection limit determination
In general, the detection limit is expressed as:
The standard deviation of the signal at low concentrations () and the slope of the calibration curve (S) are critical parameters in gas-sensing performance Additionally, key gas-sensing properties such as dynamic range, reversibility, working temperature, and humidity are evaluated during testing to guarantee the sensor's accuracy, reliability, and stability in various real-world environments.
This chapter details the preparation of two-dimensional SnS2 material and its heterojunction with the semiconductor metal oxide SnO2 using a hydrothermal method, followed by an annealing process It also covers the synthesis of nano-ink and the characterization techniques employed for testing the ink.
42 performed and analyzed Finally, the analysis method of morphological and microstructural properties as well as the gas-sensing measuring system was also discussed deeply
SnS 2 NANOFLAKES AND THEIR DECORATING WITH SnO 2
NO 2 gas sensor based on SnS 2 nanoflakes
3.1.1 Morphology and structure of SnS 2 nanoflakes
The morphology and microstructure of SnS2 nanoflakes are greatly influenced by the precursor content As shown in Figure 3.1 (A-C), the FESEM images illustrate the morphological characteristics of SnS2, highlighting changes that occur with varying amounts of SC(NH2)2, all captured at low magnification.
Figure 3.1 FESEM images of 2D SnS 2 nanoflakes prepared with various precursor molar ratios (A) SS-4, (B) SS-6, and (C) SS-8
The SnS2 materials typically exhibit a hexagonal-like shape of nanoflakes with random orientation, while the SS-4 sample transitions to a nanoplate-like morphology, albeit with some irregularities in the hexagonal fringes These nanosheets tend to stack, forming thick layers rather than separating completely Increasing the SC(NH2)2/SnCl4 molar ratio to 6:1 results in more regular hexagon-shaped nanoflakes, and further increasing the ratio to 8:1 leads to thinner, well-separated nanoflakes with an ordered structure, averaging around 500 nm in diameter Cross-sectional SEM images reveal that the thickness of the SnS2 nanoflakes ranges from 25 to 50 nm across different samples The large surface area and thin thickness enhance the sensing properties of the pristine SnS2 nanoflakes, promoting high gas diffusion Variations in grain sizes observed in SEM images and calculated using the Scherrer formula are attributed to the shape and aggregation of the SnS2 nanoparticles, which decrease in size with increased thiourea amounts due to the nucleation and crystal growth mechanisms under hydrothermal conditions.
The X-ray diffraction patterns of calcined SS-x (x = 4, 6, and 8) were depicted in Figure 3.2 All as-synthesized products were observed obvious and intensive peaks at the diffraction angle (2) of 14.2 , 27.4 , 31.5 , 41.2 , 45.5 , 49.1 , 51.76 , 54.2 , 60.0 , 62.71 , 66.57 , and 69.81 which were ascribed with reflection planes (001),
The peaks observed in the diffraction patterns are consistent with the hexagonal structure of SnS2 (JCPDS Card no 23-0677) As the molar ratio increases from 4:1 to 8:1, sharper and more intense diffraction peaks indicate enhanced crystallinity and larger nanograin sizes The crystallite size of SnS2 nanoflakes increases from 18.83 nm to 24.40 nm with the higher molar ratio This degree of crystallinity is closely linked to the density of charge carriers, which significantly affects the adsorption and desorption of gaseous species Improved crystallinity leads to a reduced number of carriers and a notable change in resistance upon exposure to gas.
Figure 3.2 XRD patterns of the SS-x product synthesized hydrothermally with different
SC(NH 2 ) 2 /SnCl 4 molar ratios at 200 C for 48 h after annealing
SEM images of SnS2 nanosheets synthesized at temperatures ranging from 160 °C to 240 °C reveal a transformation in morphology from irregular nanoparticles to well-defined nanoplates as the reaction temperature increases Furthermore, higher temperatures contribute to a more uniform orientation of the nanosheets, enhancing their structural consistency.
At low temperatures, 45 nanosheets tend to aggregate, resulting in the formation of several nanoflowers However, when the temperature is increased to 200 °C, the nanosheets distinctly separate but stack randomly As the reaction temperature rises, the nanosheets arrange orientally, achieving a dense thickness.
Figure 3.3 SEM images of the prepared SnS 2 at temperature of (A-B) 160 C, (C-D)
200 C, and (E-F) 240 C in 24 hours Note that the left-side has a low-magnification image and right-side exhibit high-magnification images
The morphological properties of synthesized SnS2 products, produced at varying temperatures and annealed at 300 °C in vacuum for 48 hours, are illustrated in Figure 3.4(A-F) In comparison to samples obtained after 24 hours, the SnS2 nanosheets exhibit significant accumulation, resulting in the formation of a thicker layer Analyzing the hydrothermal conditions, such as precursor concentrations, reaction temperature, and reaction time, provides insights into the growth principles of SnS2 nucleation.
Figure 3.4 SEM images of the prepared SnS 2 at temperature of (A-B) 160 o C, (C-D)
200 C, and (E-F) 240 C in 48 hours Note that left-side are low-magnification image and right-side exhibit high-magnification images
The XRD patterns of the SnS2 sample, illustrated in Figure 3.5, reveal its crystallite features, showing distinct peaks characteristic of the hexagonal phase of SnS2 without any impurity diffraction peaks The data suggests that as the reaction temperature increases and the hydrothermal process is extended to 48 hours, the intensity of these peaks also rises, indicating a size-changing trend in SnS2 nanoparticles The formation mechanism involves the use of SnCl4.5H2O, thiourea, and HCl acid as precursors during the hydrothermal process Initially, the dissociation of SnCl4.5H2O and water releases Sn4+ cations, which then combine with HCl acid to form a (Sn(HCl)4+) complex, while thiourea undergoes hydrolysis.
47 the aqueous solutions to produce H2S (E.q 3.2) which further dissociates to generate S 2− anions (E.q 3.4 and E.q 3.5)
Figure 3.5 XRD patterns of SnS 2 products after annealing at 300 C for 2 hours under different hydrothermal conditions
The dissociation as well as hydrolysis reaction of complex and thiourea occur in short-time in higher temperature
The release of Sn 4+ ions combines with S 2− to form SnS2 precipitate, as described in Equation 3.6 Initially, the reaction between Sn(HCl) 4+ and thiourea gradually produces S 2−, limiting the formation of SnS2 crystal nuclei at the early stages Consequently, a significant amount of unreacted Sn 4+ and S 2− ions remain in the solution Increasing the temperature and extending the reaction time enhance the nucleation rate of SnS2, which is directly influenced by the release rate of S 2− anions Since the energy required for nucleation in the heterogeneous phase is lower than in the homogeneous phase, subsequent nucleation of SnS2 predominantly occurs on the surfaces of pre-formed SnS2 crystal nuclei, resulting in the formation of complete SnS2 nanosheets.
Apparently, the larger in the size of SnS2 nanocrystals with increasing of the reaction temperature and reaction time is contributed by the resulting viscosity of
48 the solution which boost the diffusion of Sn 4+ and S 2− ion towards the forming SnS2 crystal, or using Ostwald ripening process [79], [83]
To assess the elemental composition of SnS2 nanoflakes, EDS analyses were performed using a 15 keV electron source The results, illustrated in Figure 3.6, reveal a chemical composition of 62.94% Sn and 37.06% S, indicating an approximate Sn to S atom ratio of 1:2, which aligns well with findings from previous studies [84], [85] The elemental mapping images from the EDS analysis demonstrate a well-defined and uniform distribution of Sn and S throughout the nanoflakes (Figure 3.6(B-D)) These findings confirm the high purity and homogeneity of the SnS2 nanoflakes produced.
Figure 3.6 (A) EDS analysis, and (C-D) elemental mapping images of the SnS 2 nanoflakes
TEM and HRTEM analyses were conducted to investigate the nanostructure of SnS2 nanoflakes The TEM micrograph revealed flake-like features and uniform thickness, indicating a hexagonal geometry with an approximate size of 500 nm Higher magnification images illustrated the layered structure of the hexagonal SnS2 nanoflakes Additionally, HRTEM confirmed that the SnS2 nanoplate is single crystalline, with an inter-planar spacing of 0.31 nm, corresponding to the lattice fringes of the (100) plane of typical hexagonal SnS2.
The selected area electron diffraction (SEAD) pattern depicted in Figure 3D indicates the presence of SnS2 crystals, characterized by luminous dots These findings align with previous reports on single crystal SnS2 nanosheets.
Figure 3.7 (A, B) TEM micrographs of the SnS 2 nanoflakes; (C ) HRTEM micrograph, and (D) corresponding SAED patterns of the SnS 2 nanoflakes
Raman spectroscopy is an effective and reliable method for measuring the thickness of SnS2 and identifying vibrational modes within the material The typical Raman spectra of SnS2 nanoflakes, shown in Figure 3.8(A), exhibit a frequency range of 150 – 500 cm−1 Notable peaks at 314 cm−1 and 206 cm−1 correspond to the out-of-plane mode (A1g) and in-plane mode (Eg) of the Sn-S bonds in the 2H polytype of SnS2 Research indicates that the Eg photon mode vanishes when the thickness is reduced to a monolayer, attributed to a decrease in scattering centers for in-plane scattering.
To evaluate the thermal stability of the synthesized SnS2, we employed TGA−DTA analysis across a temperature range of 25 °C to 1000 °C The results, illustrated in Figure 3.8 (B), reveal two distinct stages of weight loss in the TG analysis, with the initial stage showing a minor weight reduction.
The weight loss of 1% in the nanopowders is primarily due to the evaporation of water molecules trapped between the interlayers A significant weight reduction of approximately 15% occurs in the temperature range of 280–400 °C, attributed to the thermal decomposition of SnS2 powder into SnO2 The DTA curve shows a marked reduction at 378 °C, indicating the phase transformation temperature from SnS2 to SnO2 This suggests that SnS2 exhibits thermal stability at temperatures up to 378 °C before undergoing decomposition.
NO 2 gas sensors based on heterojunction of SnS 2 /SnO 2
This section explores how the functionalization of SnO2 nanoparticles influences the structure, morphology, and gas-sensing capabilities of SnS2 when combined with SnO2 It also examines the significant effects of moisture on the sensing behavior Furthermore, the synergistic impact of the n-n heterojunction on gas sensitivity performance is discussed in detail.
The 3D self-assembled SnS2 nanoflowers are formed through the arrangement of hexagonal-like SnS2 nanoflakes, utilizing mechanisms such as angle-to-angle and edge-to-edge interactions This process is significantly influenced by the hydrolysis reaction of Sn 4+ ions.
The formation of SnS2 nanoflowers occurs simultaneously with the generation of SnO2 nanoparticles, which tend to aggregate and transform into ultrathin SnS2 nanoflakes in situ As depicted in Figure 3.21, the SEM image reveals that hierarchical flower-like microspheres are approximately 1 µm in size At higher magnification, the detailed interaction between SnO2 nanoparticles and hexagonal SnS2 plates becomes evident The SnS2 nanoflowers consist of nanoplates with a thickness of around 10 nm, featuring a rough surface that facilitates the adsorption of SnO2 nanoparticles, which have an average diameter of 20−25 nm, primarily located along the edges of the nanoflakes in both single and aggregated forms.
Figure 3.21 The FESEM images of the obtained SnO 2 -doped SnS 2 nanoflower-like morphology under different magnification
The TEM analysis of the synthesized SnS2/SnO2 hierarchical nanostructure, illustrated in Figure 3.22, reveals the 2D structure of SnS2 nanosheets that self-assemble into distinct flower-like formations Under both low and high magnification, the images show two contrasting regions: the dark areas indicate nanosheets arranged perpendicular to the growth orientation, while the brighter regions represent nanosheets aligned along the assembly direction A significant number of hexagonal-shaped thin nanosheets are observed stacked together, with careful observation revealing their rough fringes The magnified insets highlight the vertically stacked sheet-like crystals, emphasizing the unique structural characteristics of the material.
The study reveals that the structure consists of 15 single layers of SnS2, featuring an interplanar spacing of 0.28 nm, corresponding to the (011) plane High-resolution imaging of the SnS2/SnO2 heterojunction nanostructure, depicted in Figure 3.22(D), shows three distinct SnS2 flakes indicated by white arrows Additionally, SnO2 nanoparticles are observed to be distributed on the SnS2 flakes, with nanograin sizes varying within the structure.
10 – 20 nm The characteristic seven spots (Figure 3.22D inset) in the Fast Fourier Transform (FFT) image of the SnO2 nanoparticles can be ascribed to the planes
(110) and (101) of the typical tetragonal phase [102] corresponding to interplanar distances 0.34 and 0.267 nm accordingly In the planar direction, lattice fringes with d-spacing of 0.26, 0.27 and 0.31 nm attributed to the crystallographic planes
(101), (011) and (100) [40], [41], [103] of hexagonal-like structure being displayed in small inset FFT graph in the top corner
The article presents a detailed examination of SnS2/SnO2 nanoflowers through low and high magnification transmission electron microscopy (TEM) images Figure 3.22(A) and (B) illustrate the structural features of the nanoflowers, highlighting the well-defined nanoflakes at their edges The inset in (A) displays a magnified view of a specific area, while (C) offers a high-resolution image of the layered SnS2 flakes Additionally, the Fast Fourier Transform (FFT) patterns of SnO2 nanoparticles are depicted in (D), alongside a high-resolution image of the SnS2 nanoflake from the marked area The top right inset of (D) shows the FFT patterns corresponding to the hexagonal structure of SnS2, and the selected area electron diffraction (SAED) patterns of the flake’s surface are also included.
The Moire pattern indicates that the intersection angle of two overlaid grids in the plane directions (100) and (011) is 60 degrees The distinct sharp diffraction spots in the SEAD pattern, as shown in Figure 3.22 F, confirm the single crystalline nature of SnS2 nanosheets These findings affirm that the SnS2 nanosheets exhibit a single crystalline structure and achieve a 2D layered configuration Additionally, SnO2 nanoparticles are formed on the surface of the SnS2 nanosheets.
65 nanosheets is evident the successful fabrication of heterojunction structure generating between layered material and metal semiconductor
The growth mechanisms of the 3D hierarchical SnS2 structure and the formation of SnO2 on the SnS2 surface are clearly explained Initially, Sn4+ ions readily hydrolyze in water due to the low solubility-product constant (Ksp) of Sn(OH)4.
SnO2 nanoparticles are produced through the hydrolysis of Sn(OH)4, which subsequently congregates to minimize surface energy until a sulfide reaction occurs Viable SnS2 nanocrystals can be synthesized from Sn4+ or SnO2, with competitive reactions influencing the ratio of SnS2 to SnO2 The hydrolysis of Sn4+ to form SnO2 crystals competes with the sulfide reaction, while the concentration of S2− in the initial solution significantly regulates the assembly rate of SnS2 nuclei A higher sulfide content promotes rapid emergence of SnS2 nuclei, leading to 2D nanostructure growth influenced by precursor conditions In this study, a fixed Sn/S precursor ratio of 1:4 is used, resulting in flattened SnS2 nanosheets that reduce surface energy in a water medium at temperatures between 160–200 °C Throughout the synthesis, various tin oxide sources are consumed, disappearing due to the sulfide reaction, which significantly increases the number of nanosheets This initiates a self-assembly process where nanosheets interconnect to form three-dimensional hierarchical nanoflowers Following the generation of SnS2 nanoflowers, a ripening process enhances the development of thicker sheet-like nanostructures, improving crystallization and elemental composition.
The EDX analysis presented in Figure 3.23(A) illustrates the atomic percentage and atomic ratio of S/Sn for the synthesized product, revealing the presence of Sn, S, and O with atomic percentages of 28.41%, 25.95%, and 45.65%, respectively Unlike the SnS2 sample, which has a Sn to S ratio of 1:2, the observed ratio in this case approximates 1:1 due to the limited concentration of S− ions generated from the thiourea precursor reacting with Sn2+.
The synthesized SnS2 nanostructures retain a significant amount of SnO2 nanocrystallites derived from the hydrolysis of Sn(OH)4, resulting in a (S+O)/Sn ratio of approximately 2.52, which indicates the presence of oxide phases alongside SnS2 EDS mapping analysis, illustrated in Figure 3.23(C-D), reveals a uniform distribution of the three elements across the surface of the SnS2/SnO2 nanocomposite.
Figure 3.23 Energy-dispersive X-Ray elemental mapping analysis (A) EDX spectrum of SnS 2 /SnO 2 , (B-D) Elemental mapping of Sn, S, and O elements respectively
The crystallographic structures of the synthesized SnS2 and SnS2/SnO2 nanocomposite were examined using X-ray diffraction, as shown in Figure 3.24(A) The analysis revealed no impurity phase peaks for SnS and Sn2S3 in the obtained samples The XRD peaks of SnS2 correspond to a hexagonal phase, while the SnS2/SnO2 nanostructure displays additional peaks at 27°, 32°, 36°, and 52°, which are associated with the tetragonal phase of SnO2, specifically corresponding to the lattice planes of (110), (101), and (200).
Raman spectroscopy, referenced by Card JCPDS No 41-1445, is utilized to gain insights into the crystalline structure of materials In Figure 3.24(B), the Raman spectra of pristine SnS2 and the SnS2/SnO2 composite are compared, highlighting distinct peaks that reveal important information about the structural differences between the two samples.
The SnS2 exhibits a peak at approximately 312 cm −1, while a weak-intensity peak of SnO2 appears at 584 cm −1 This peak corresponds to the A2g vibrational mode of the tetragonal phase, suggesting the formation of a nanocomposite between SnS2 and SnO2.
Figure 3.24 XRD patterns of the as-prepared SnS 2 /SnO 2 nanoflower and SnS 2 nanoflakes (A) and Raman spectroscopy
Figure 3.25 The TGA and DTA curve of SnS 2 /SnO 2 composite
To effectively analyze the impact of SnS2 surface reduction, Thermogravimetric Analysis (TGA) was conducted to assess the changes in physical and chemical properties of the prepared sample within a temperature range of 25 to 900 °C, as illustrated in Figure 3.25 The graph indicates that the weight loss of the specimen occurs in three distinct stages.
SnS 2 -based nanoink in NO 2 gas sensing
3.3.1 SnS 2 /SnO 2 ink formation for inkjet printing
The SnS2/SnO2 nano-ink was created through probe sonication of synthesized powder in a mixture of ethanol and PVP This sonication process effectively reduced the initial powder into smaller nanodroplets, which were modified by the PVP surfactant due to hydrophobic and hydrophilic interactions, ultimately resulting in the formation of colloidal nanoparticles within the ethanol solvent.
Figure 3.33 The PVP stabilized SnS 2 /SnO 2 nano-ink suspended in EtOH The photo was captured after standing for 24 hours (A), 30 days storage (B), and SEM image of
SnS 2 /SnO 2 nano-ink dispersed on Silicon substrate
As shown in Figure 3.33(A), the PVP make the nano-ink was well-dispersed and stabilized in EtOH regardless of storage at room temperature within 24 hours
PVP surfactant exhibits remarkable stability over a 30-day storage period, making it essential for practical and industrial applications in printed electronics SEM images reveal that untreated thermal nano-ink maintains a uniform distribution of nanopowders in solution, preventing significant sedimentation after three days Additionally, the SEM image of SnS2/SnO2 nano-ink dispersed in ethanol shows uniformly allocated flower-like structures on a silicon substrate.
3.3.2 Morphology characterization of SnS 2 /SnO 2 nano-ink
Printed sensors undergo a pre-heating process at 80 °C for 30 minutes, resulting in an electrode structure characterized by a line width of 200 µm and a spacing of 300 µm between the lines The electrode comb displays rough fringes, highlighting a significant level of resolution (Figure 3.34(A-B)).
Figure 3.34 SEM micrograph of the sensor The printed sensors without printing sensing layer at low- and high- magnification (A-B), after printing nano-ink
The SnS2/SnO2 nano-ink, characterized by its homogeneous deposition between silver electrodes and distinct grey spots, demonstrates excellent dispersion The measured viscosity and surface tension of the ink are 3.75 cp and 23.6 mN/m, respectively, indicating that this nano-ink is well-suited for ink-jet printing applications.
The thickness of the sensing layer significantly influences sensing performance In this study, we varied three different thicknesses to examine their impact on sensing response under similar conditions The SEM micrograph of the sensors, shown in Figure 3.35(A), reveals that the PET substrate beneath the electrode layer appears as a transparent white material.
The silver electrode, measuring 8-9 µm in thickness, is centrally located, while a grey SnS2/SnO2 coating layer is applied on top and in contact with the silver electrode The thickness of the sensing layer is accurately determined using Stylus profilemetry (Bruker) Figure 3.35(B) illustrates a typical tapping mode image along with the height cross-section profile of the SnS2/SnO2 sensing layer, which has three different thicknesses deposited on a PET substrate Height profile analysis estimates the thickness of the SnS2/SnO2 layers to be approximately 2, 3, and 4 µm.
The SEM micrograph in Figure 3.35 illustrates the cross-section of a flexible gas sensor, revealing its thickness profile composed of three distinct layers: PET, Ag, and SnS2/SnO2 Notably, the Ag and SnS2 materials are directly printed onto the PET substrate, enhancing the sensor's functionality and structure.
3.3.3 Gas sensing performance of SnS 2 /SnO 2 nano-ink
3.3.3.1 Evaluation of flexible gas sensor based on SnS 2 /SnO 2 nano-ink with working temperature
The SnS2/SnO2-based flexible gas sensor was evaluated under various conditions to understand the relationship between operating temperatures and sensing behavior As illustrated in Figure 3.36(A), the sensor's resistance was measured over time within a temperature range of room temperature to 150 °C The baseline resistance increased from room temperature to 50 °C, followed by a gradual decrease as the temperature continued to rise Notably, the nano-ink exhibited a tenfold increase in resistance compared to SnS2/SnO2 powder, primarily due to the presence of PVP precursor, a common polymer This polymer addition reduces the sensor's conductance by obstructing carrier flow in the conducting channel Techniques such as mechanical sintering and high-temperature thermal treatment (>300 °C) can be employed to eliminate polymer residues from the ink solution, thus enhancing the sensor’s conductance; however, this study focused on a heating treatment process for the sensor.
At 200 °C, the SnS2/SnO2 nano-ink demonstrates incomplete surfactant removal Upon exposure to NO2 gas, the sensor's resistance increases rapidly before returning to its original baseline, showcasing impressive response and recovery characteristics ideal for flexible sensors However, the presence of polymer slows down the reaction rate As illustrated in Figure 3.36 (B), the flexible sensor reaches its peak response at 100 °C, achieving a remarkable response value of 170 times when exposed to 5 ppm of NO2.
NO2 Especially, the sensor can detect at room temperature with the response of approximately 2 times toward 5 ppm gas
Figure 3.36 Dynamic resistance of SnS 2 /SnO 2 nano-ink-based gas sensor to different
NO 2 gas concentration of 0.25 – 5 ppm (A) and response curve as function of NO 2 gas concentration at different temperatures (room temperature – 150 C)
3.3.3.2 The investigation of sensing layer thickness on gas-sensing performance
Numerous studies have highlighted the significance of sensing material thickness on gas-sensing performance In this research, we explored the relationship between gas adsorption mechanisms and the thickness of SnS2/SnO2 nano-ink, which was deposited on a 0.6 x 1.0 cm² inkjet-printed rectangular silver electrode By adjusting the number of printed layers, we achieved thicknesses of 2, 3, and 4 μm for the SnS2/SnO2 layer The resistance transient of the flexible gas sensor was measured against time for 5 ppm NO2 at various reaction temperatures, revealing that sensor resistance is influenced by thickness across all tested temperatures Notably, thinner sensing layers resulted in a higher baseline resistance, with the 2 μm SnS2/SnO2 layer-based flexible sensor reaching an impressive 100 MΩ at room temperature.
The conductance of a printed layer is significantly influenced by the electron carrier transition between two silver electrodes, exhibiting a five-fold to a hundred-fold increase compared to layers of 3 and 4 µm thickness In essence, conductance is directly proportional to the layer's thickness, while conductivity remains a constant value that is independent of dimensions for uniform printed layers This relationship can be expressed mathematically.
The area for current transport is represented by A, while w, t, and l refer to the conducting channel's width, thickness, and length, respectively The equation suggests that a reduction in thickness is likely to lead to a decrease in conductance.
The dynamic resistance of gas sensors utilizing varying thicknesses of SnS2/SnO2 nano-ink (2, 3, and 4 µm) was analyzed upon exposure to 5 ppm NO2 in dried air at a relative humidity of 60% The study examined the response of these sensors across a temperature range of 25 to 150 °C, highlighting the relationship between sensing layer thickness and operating temperature.
The sensor’s response of the 2 m sensing layer exceeds thicker sensing layers of
The gas adsorption capability of the sensing film is enhanced with thicknesses of 3 and 4 µm, as gas diffusion is proportional to the sensing layer thickness The structure of the sensing material facilitates easy access for gas molecules, allowing direct reactions at the interface through charge transfer Increasing the temperature to 50 °C leads to a rise in baseline resistance due to oxygen vacancies in SnO2, while further heating to 150 °C significantly reduces resistance, attributed to the dominance of thermally generated electron carriers Upon exposure to NO2 gas, the sensor exhibits a rapid increase in response, returning to its original state once the gas injection ceases, demonstrating a remarkable reversible response across varying thicknesses Evaluating the sensing performance in dry air, the relative response (Rg/Ra) shows that the 2 µm sensor achieves the highest response across nearly all temperatures, with a notable response of 50.
200, 50 corresponding room temperature, 50 C, and 100 C The response with
80 thicknesses of 3 m experienced lower responses toward 5 ppm NO2 The response of the 4 m sensor surpassed the 2 m sensor when the temperature exceeded
100 o C This phenomenon is explained by the effect of temperature, when rising temperature, the thinner layer is easy to be devastated by thermal and the recorded signal become noise
The performance repeatability of a printed 2 µm SnS₂/SnO₂ sensor for detecting 1 ppm NO₂ at 25 °C is illustrated in Figure 3.38 This includes a graph showcasing the mechanical bending of a flexible gas sensor, along with real-time resistance curves recorded before and after bending Additionally, the figure highlights the effect of bending angle on the sensor's response to 1 ppm NO₂ at room temperature.