Gas-sensing Layer Research Articles

High-Entropy Sulfide (FeCoNiCrMn)S2 for Room Temperature NO2 Sensors.

Metal sulfides have been extensively studied in the field of chemical sensors working at room temperature (RT). However, compared to metal oxides, metal sulfides often suffer from incomplete recovery and unsatisfactory selectivity. In this work, we for the first time report the high-entropy sulfide (HES) (FeCoNiCrMn)S2, prepared by the solvothermal method, is a promising candidate for utilization as the sensitive layer in gas sensors. Unlike traditional single-metal sulfides, this HES (FeCoNiCrMn)S2 exhibits reversible recovery for NO2 at RT as well as excellent selectivity. To unveil the sensing mechanism, we simulated the adsorption and charge transfer between (FeCoNiCrMn)S2 and gas molecules by the density functional theory (DFT) calculation, revealing the reason (FeCoNiCrMn)S2 is uniquely selective to NO2. This work explores the application potential of HES (FeCoNiCrMn)S2 as an RT sensor and enriches the material selection for NO2 sensors.

Plasma-modified Co3O4 nanosheets for efficient CO detection: Insight into the structure-activity relationship

Exploring the correlation between crucial reactive sites and surface electron structure in the sensitive layer of gas sensors can achieve simultaneous optimization of kinetics and thermodynamics. In this work, Ar and O2 plasma sputtering are used to study how varying levels of Co3+ and oxygen vacancy affect the gas-sensitivity of Co3O4. The gas-sensing performance of Co3O4-O2PT (O2 plasma sputtering) is better than that of Co3O4-ArPT (Ar plasma sputtering), which exhibits a high response value (100 ppm-4.16) and quick response-recovery time (1.5 s/38 s). The enhancement of gas-sensing performance benefits from a high Co3+/Co2+ ratio, which provides strong CO-selective adsorption and oxidation capabilities. Therefore, the interfacial oxidation reaction reaches equilibrium rapidly. This work provides a straightforward and controllable modification approach and enhances the understanding of how oxygen vacancies and Co3+ affect Co3O4 gas sensors, which promotes the design and development of high-performance cobalt-based gas sensors.

Fluorinated polyaniline-based sensors with enhanced NH3 sensitivity

Poly(2-fluoroaniline) films were prepared by electrochemical oxidation of 2-fluoroaniline in perchloric acid solutions. These films had a characteristic globular structure, but low thickness and conductivity. For these reasons, alternative polymer films were prepared from electrolytic solutions with different aniline:2-fluoroaniline ratios. The physicochemical properties of these polymer films strongly evolved with the ratio since they were thicker and more conductive when the proportion of aniline increased while their morphology also varied significantly. The electrodeposited polymer films were then used as sensitive layers of gas sensors for ammonia detection. The addition of 2-fluoroaniline improved the performance characteristics of gas sensors for the detection of ammonia at room temperature and the poly(aniline/2-fluoroaniline)-based sensors responded to ammonia gas with good repeatability, good sensitivity (0.44 %/ppb), a very low detection limit (4 ppb), and a low dependence on humidity variations.

Inkjet-printed Pt/WO3 thin film sensor for ppb-level ammonia detection

The development of high-performance ammonia sensors is crucial for environmental monitoring and human health protection. However, traditional conductive metal oxide gas sensors encounter significant challenges such as poor selectivity and high detection limit, mainly raising from their inherent cross-sensitivity and thick gas-sensing layer caused by uncontrollable preparation. Herein, a noble metal platinum-modified tungsten oxide (Pt/WO3) gas sensor is controllably fabricated via inkjet printing technology and magnetron sputtering. In addition to a detection limit toward ammonia as low as 40 ppb, the Pt/WO3 gas sensor demonstrates a good selectivity for ammonia compared to other interfering gases, including acetone, sulfur dioxide, formaldehyde, methane, sulfur hexafluoride, and toluene, with corresponding selectivity coefficients not less than 4.7. The high sensitivity of the gas sensor is mainly attributed to the fact that more electrons can be directly controlled by the adsorbed gas benefiting from the micro-nano thickness of the sensing layer, as well as additional adsorption sites are provided by the Pt modification. Moreover, the catalytic effect of Pt on the reaction between ammonia and oxygen ions further enhances its sensitivity while also improving its selectivity. Our work establishes an important theoretical and experimental foundation for the design of high-performance ammonia sensors.

Adsorption of Hydrogen Sulfide on Reduced Graphene Oxide-Wrapped Titanium Dioxide Nanofibers

This work presents a fabrication of room-temperature gas sensor for hydrogen sulfide (H2S) adsorption. Pristine titanium dioxide (TiO2) nanofibers, reduced graphene oxide (rGO) sheets, and reduced graphene oxide-wrapped titanium dioxide (rGO-wrapped TiO2) nanofibers were presented in the form of integrated suspension used for a gas-sensing layer. The TiO2 nanofibers were firstly synthesized by using an electrospinning method with a polyvinylpyrrolidone (PVP) polymer. The rGO sheets were then wrapped around TiO2 nanofibers by a hydrothermal method. Scanning electron microscope, transmission electron microscope, X-ray diffractometer, and Raman spectrometer confirmed the presence of rGO sheets onto the surface of TiO2 nanofibers. Ultraviolet-visible spectrophotometer was also considered and displayed to calculate the band gap of TiO2 and rGO-wrapped TiO2 nanofibers. After preparing the gas-sensing suspensions, they were dropped onto the polyethylene terephthalate substrates with silver-interdigitated electrodes. The gas-sensing properties of sensors were evaluated for H2S adsorption at room temperature. Based on the results, the rGO-wrapped TiO2 nanofiber gas sensor exhibited higher H2S sensitivity and selectivity than pristine TiO2 nanofiber and pure rGO gas sensors. The H2S-sensing mechanism of rGO-wrapped TiO2 nanofiber gas sensor was discussed based on a formation of p-n heterojunctions between p-type rGO sheets and n-type TiO2 nanofibers. Furthermore, a direct charge-transfer process by physisorption was also highlighted as a second H2S-sensing mechanism.

An NO2 sensor based on WO3 thin films for automotive applications in the microwave frequency range

A microwave system dedicated to the detection of nitrogen dioxide in the harsh environment of the Norway highways is proposed. An optimized transmission line type of sensor coated with a tungsten trioxide thin film that changes its electrical properties under NO2 gas exposure is developed. The sensors' response (S) is given in °/GHz and it is calculated based on wideband measurements. The advantage of wideband measurements in comparison to a single value is based on multiple measurements taken at different frequencies, which greatly suppresses noise and enables measuring low target-gas concentrations within environments of high interfering compounds. Herein, the developed system works in 1.5 GHz – 4.5 GHz, and NO2 varies in the 0–20 ppm range. The optimal thickness of the gas-sensing layer is estimated to be around 410 nm taking the advantage of the magnetron sputtering technology with the glancing angle deposition technique. The advantage of the developed sensor is the possibility to work at ambient temperature without the need to heat up the sensors. The disadvantage of the developed sensors is longer response and recovery times; however, this issue will be a subject of research in the future.

Radiation hardness of solution-processed amorphous ZnSnO gas sensors against gamma rays

Metal oxide semiconductors, such as ZnO and SnO2, are widely used for civil gas sensing applications. However, the performance of these gas sensors degrades significantly if they are irradiated with high levels of gamma rays so they are not suited to use in harsh environments. This study demonstrates a gamma ray tolerance of 150 kilo-Gray (kGy) for solution-processed amorphous ZnSnO (a-ZTO) thin film due to its radiation hardness. The gas sensing response measurement shows that the gas response of gas sensor is increased after gamma irradiation because oxygen vacancies are induced by gamma radiation. The increased gas response is maintained up to 150 kGy. This study demonstrates that a-ZTO thin film acts as a radiation-resistant sensing layer for gas sensors in a space radiation environment.

Study on the gas sensor at room temperature based on polypyrrole materials

This paper presents the results of synthesising polypyrrole (PPy) modified Dodecylbenzene sulfonic acid (DBSA) with nanostructures oriented to applications for NH3 gas sensors operating at room temperature. The outcomes of surface morphology and chemical composition structure analysis of PPy film were studied by field-emission scanning electron microscopy (FE-SEM), fourier-transform infrared spectroscopy (FT-IR), and ultraviolet-visible spectroscopy (UV-Vis). The products were obtained by electrochemical method with the nanostructured PPy film, the conductivity of the PPy film depends on the concentration of DBSA denaturant. The experimental results showed that these PPy films have a great potential application as a new sensitive layer for gas sensors.

Fluorophenol-Containing Hydrogen-Bond Acidic Polysiloxane for Gas Sensing-Synthesis and Characterization.

In this work, the synthesis of a new polysiloxane, poly {dimethylsiloxane-co-[4-(2,3-difluoro-4-hydroxyphenoxy) butyl] methylsiloxane} (dubbed PMFOS), is presented. This polymer exhibits high hydrogen bond acidity and was designed to be used as a sensor layer in gas sensors. The description of the synthetic route of the PMFOS has been divided into two main stages: the synthesis of the functional substituent 4-(but-3-en-1-yloxy)-2,3-difluorophenol, and the post-polymerization functionalization of the polysiloxane chain (methylhydrosiloxane-dimethylsiloxane copolymer) via hydrosilylation. The synthesized material was subjected to instrumental analysis, which confirmed its structure. The performed thermal analysis made it possible to determine some properties important for the sensor application, such as glass transition temperature and decomposition temperature. The results showed that PMFOS meets the requirements for materials intended for use in gas sensors based on acoustoelectric transducers.

A review on chemi-resistive human exhaled breath biosensors for early diagnosis of disease

Human breath analysis provides a non-invasive and fast method for identifying numerous volatile organic compounds that are disease markers. Breath analysis on a small device that is connected to the Internet of Things and uses chemi-resistive semiconductor with extremely low energy usage will build new roads for detection of diseases and monitoring patients. Nano-structures of various forms, synthesized by diverse nano-meter-scale synthesis methods, can be utilized to achieve this aim. Metal oxide nano-structures allow a well-built gas-sensing layer interaction and, as a result, show a higher sensitivity than traditional materials due to features such as high surface-to-volume ratios and, consequentlya large number of surface sites exposed to gas. This paper provides an overview of advances in chemi-resistive nano-structures as gas sensing materials for exhaled breath detection, with the goal of assisting patients with various conditions in their disease screening. The many types of chemi-resistive materials utilized in breath sensors have been discussed, as well as their limits and future prospects also presented.

Air Stable Nickel-Decorated Black Phosphorus and Its Room-Temperature Chemiresistive Gas Sensor Capabilities.

In the rapidly emerging field of layered two-dimensional functional materials, black phosphorus, the P-counterpart of graphene, is a potential candidate for various applications, e.g., nanoscale optoelectronics, rechargeable ion batteries, electrocatalysts, thermoelectrics, solar cells, and sensors. Black phosphorus has shown superior chemical sensing performance; in particular, it is selective for the detection of NO2, an environmental toxic gas, for which black phosphorus has highlighted high sensitivity at a ppb level. In this work, by applying a multiscale characterization approach, we demonstrated a stability and functionality improvement of nickel-decorated black phosphorus films for gas sensing prepared by a simple, reproducible, and affordable deposition technique. Furthermore, we studied the electrical behavior of these films once implemented as functional layers in gas sensors by exposing them to different gaseous compounds and under different relative humidity conditions. Finally, the influence on sensing performance of nickel nanoparticle dimensions and concentration correlated to the decoration technique and film thickness was investigated.

Fluoro-Substituted Metal Phthalocyanines for Active Layers of Chemical Sensors

Metal phthalocyanines bearing electron-withdrawing fluorine substituents were synthesized a long time ago, but interest in the study of their films has emerged in recent decades. This is due to the fact that, unlike unsubstituted phthalocyanines, films of some fluorinated phthalocyanines exhibit the properties of n-type semiconductors, which makes them promising candidates for application in ambipolar transistors. Apart from this, it was shown that the introduction of fluorine substituents led to an increase in the sensitivity of phthalocyanine films to reducing gases. This review analyzes the state of research over the last fifteen years in the field of applications of fluoro-substituted metal phthalocyanines as active layers of gas sensors, with a primary focus on chemiresistive ones. The active layers on the basis of phthalocyanines with fluorine and fluorine-containing substituents of optical and quartz crystal microbalance sensors are also considered. Attention is paid to the analysis of the effect of molecular structure (central metal, number and type of fluorine substituent etc.) on sensor properties of fluorinated phthalocyanine films.

Nickel-Decorated Black Phosphorus for Room Temperature NO2 Detection

Introduction In the rapidly emerging field of layered two-dimensional (2D) functional materials, bP, the all P-counterpart of graphene, is a potential candidate for various applications e.g., nanoscale optoelectronics, rechargeable ion batteries, electrocatalysts, thermoelectrics, solar cells, and sensors [1]. bP has been reported to exhibit superior chemical sensing performance. In particular, bP is selective for the detection of paramagnetic molecules, e.g., NO2, in addition to high sensitivity at a limit of detection (LOD) of ppb levels.[2] In this work, by applying a multiscale characterization approach we demonstrated a stability and functionality improvement of Nickel-decorated bP (Ni/bP) films for gas sensing prepared by a simple, reproducible and affordable deposition technique. Furthermore, we studied pos-sible electrical activity of these films for their employment as functional layers in gas sensors by exposing them to different gaseous compounds (NO2, CO2, H2, NH3, CO, Benzene, etha-nol, ethylene, Formaldehyde) in different relative humidity (RH%) conditions. Moreover, the influence in sensing perfor-mance of nickel nanoparticle (NP) dimensions in related to the decoration technique and the film thickness were investigated. Materials and Methods Black phosphorus microcrystals were suspended in dimethylsulfoxide and kept under ultrasonication for several hours. The exfoliated bP nanosheets were decorated with nickel nanoparticles following two different procedures: 1) preformed Ni NPs were immobilized on bP, 2) Ni NPs were grown directly on bP flakes. Electrical conductance measurements have been performed on Ni/bP flakes deposited via spin coating on Al2O3 substrates with built-in interdigitated gold electrodes, in order to better control the film thickness. Electrical conductance measurements have been performed at RT (25±2°C) by exposing the sensing film to certified mixtures of NO2 diluted in dry air (20% O2 and 80% N2) with different NO2 concentration ranging from 200 ppb to 1 ppm. Results and Conclusions Exfoliated black phosphorus nanoflakes decorated with nickel nanoparticles (Ni/bP) have been prepared and characterized with particular attention to the role of Ni nanoparticle dimension in the sensing mechanism. Electrical activity towards different concentrations of NO2 at room temperature in a dry air environment highlighted fast and stable response over the time. This latter characteristic is particularly relevant since environmental instability of exfoliated BP has been an issue so far, under ambient conditions few-layer bP degrades completely in less than a week, preventing its applications in several fields. In situ grown Ni/bP (2) shows stable response during all the four weeks measurements period with no dependence of the response on the film thickness. Ni decoration effectively suppress ambient degradation, for at least 1 week in Ni/bP (1) and over 4 weeks in Ni/bP (2) in ambient conditions. This result highlighted that Ni/bP (2) sensor can be practically used under ambient conditions for a reasonable period without performance degradation of the devices.The obtained results pave the way for the use of chemically exfoliated bP flakes for gas sensing applications, avoiding the use of a time-consuming technique for the exfoliation and for the fabrication of the metallic contacts, like electron beam lithography, focused ion beam lithography, etc.

Copper Oxide Thin Films for Sub-Ppm Acetone Detection Obtained By Glancing Angle Magnetron Sputtering Deposition

Introduction The number of gas-sensing applications increases year by year including industrial application as well as new approaches such as medical diagnostics, where gas sensors are used for measurement of volatile organic compounds present in the exhaled human breath. Over 3500 various VOCs have been already detected in the exhaled human breath, and a single breath consists of 500 various VOCs [1]. Apart from the conventional breath alcohol detectors, the novel applications appeared such as the detection of biomarkers [2]. One of the promising compounds that can be used as a biomarker is acetone, which is considered to be the biomarker of diabetes since people with diabetes tend to have higher acetone detection that healthy people [3,4]. The exhaled acetone is usually in the range of 0.2 - 0.8 ppm and over 2.5 ppm for people without and with diabetes, respectively. In order to measure such low, sub-ppm, concentrations, laboratory systems are applied including GC-MS [5], PTR-MS [6], and recently e-noses [7] as well as single sensors [8]. In this paper, the investigation results on the copper oxide (CuO) deposited by utilization of magnetron sputtering technology with glancing angle technique are presented, where CuO was deposited at various parameters such as deposition angle, argon/oxygen ratio. The responses to various sub-ppm acetone concentrations are presented.CuO depositionThe gas-sensing applications of CuO have been previously confirmed and described [9]. Here, the CuO thin films were deposited by the UHV magnetron sputtering system from Kurt J. Lesker with a glancing angle deposition technique realized by utilization of a commercial ECR manipulator, which allows us to change the deposition angle, substrate rotation, and it has integrated heater that can be heated up to 850oC. The base vacuum pressure and deposition vacuum pressure were 0.001 Pa and 0.3 Pa, respectively. The temperature was fixed at 200oC, the power at 100W, the argon/oxygen ratios were 33%, 50%, 66%, 80% and 100% (pure oxygen without argon). After deposition, the samples were annealed at 400oC for 4 hours in the air. Gas-sensing measurements The developed sensors were tested in the dedicated measurement setup based on the quartz-tube oven with gas-dosing lines, where various target gas concentrations are obtained by utilization of mass flow controllers 1179B (MKS Instruments, USA) and acetone canisters with 5 ppm of acetone (Air Products, UK). The gas sensor response (S) was defined as resistance ratio S=(Rgas-Rair)/Rair, where Rgas and Rair are electrical resistances in gas and air, respectively. Results and Conclusions The sensors have been measured under exposure to acetone in the 0.25–2.5 ppm range, which covers the exhaled acetone range. The results have confirmed the influence of the glancing angle deposition for gas detection, in this case for acetone detection. The highest responses have been obtained for samples deposited at 85o (Fig.1b), however, the SEM verification will be carried out to compare the morphology and nanorods distribution. Moreover, the deposition system allows us to deposit the copper oxides at various argon/oxygen concentrations including deposition in pure oxygen. The results presented in Fig.1a shows, that the highest responses have been obtained for samples deposited at 20%/80% Ar/O2 ratio. It has been underlined, that the samples were annealed after deposition and preheated for 4h at 300oC, 350oC, and 400oC before the measurements. Further investigations are focused on choosing the optimal deposition conditions for acetone-sensors as well as long-term stability tests are performing.Fig.1. The sensors’ responses under exposure to acetone in the 0.25–2.5 ppm range at various operating temperatures for gas-sensing layer deposited at various (a) oxygen concentrations; (b) deposition angles.

Gas-solid interfacial charge transfer in volatile organic compound detection by CuCrO2 nanoparticles

Nanostructured metal oxide semiconductors have received great attention used as the chemiresistive layer of gas sensor to detect the volatile organic compound recently. As indispensable complementary parts for dominative n-type semiconductors, the p-type metal oxides based gas sensors fail to be studied sufficiently, which hampers their practical applications. In this work, the p-type delafossite CuCrO2 nanoparticles were synthesized, characterized, and tested for gas sensing, followed by the first principles calculations to simulate the generation of chemiresistive signal. The hydrothermal synthesis time of CuCrO2 nanoparticles is optimized as 24 h with a higher proportion of oxygen vacancies but a smaller size, which is confirmed by the microscopy and spectrum characterization and allows for a prevailing gas sensitivity. Meanwhile, this CuCrO2 gas sensor is proven to perform a higher selectivity to n-propanol and a low detection limit of 1 ppm. The adsorption sites and charge variations of dehydrogenation at the gas-solid interface predicted by the theoretical analysis are claimed to be crucial to such selectivity. It is an innovative approach to understand the chemiresistive gas sensing by evaluating the preference of charge transfer between the sensor and target gaseous molecule, which provides a new route to precisely design and develop the advanced sensing devices for the diverse applications.

Realization of graphene oxide nanosheets as a potential mass-type gas sensor for detecting NO2, SO2, CO, and NH3

Graphene oxide (GO) is an excellent suitable material as a gas-sensing layer due to its adsorption ability towards toxic gas molecules. In this work, GO nanosheets (GO-NSs) were employed in a mass-type toxic gas sensor using a quartz crystal microbalance (QCM) transducer. The sensor was prepared by spraying the GO-NS suspension on the working electrode surface of the QCM. The gas-sensing properties of the sensor were examined with various concentrations of toxic gases, including NO2, SO2, CO, and NH3. The results demonstrated that the adsorption potential of the prepared GO-NSs makes them a promising candidate for the sensing layer of mass-type gas sensors to detect toxic gases.

UV Light-Modulated Fluctuation-Enhanced Gas Sensing by Layers of Graphene Flakes/TiO2 Nanoparticles

We present experimental results of fluctuation-enhanced gas sensing by low-cost resistive sensors made of a mixture of graphene flakes and TiO2 nanoparticles. Both components are photocatalytic and activated by UV light. Two UV LEDs of different wavelengths (362 and 394 nm) were applied to modulate the gas sensing of the layers. Resistance noise was recorded at low frequencies, between 8 Hz and 10 kHz. The sensors’ response was observed in an ambient atmosphere of synthetic air and toxic NO2 at selected concentrations (5, 10, and 15 ppm). We observed that flicker noise changed its frequency dependence at different UV light wavelengths, thereby providing additional information about the ambient atmosphere. The power spectral density changed by a few times as a result of UV light irradiation. The sensors were operated at 60 and 120°C, and the effect of UV light on gas sensing was most apparent at low operating temperature. We conclude that UV light activates the gas-sensing layer and improves gas detection at low concentrations of NO2. This result is desirable for the detection of the components of gas mixtures, and the modulated sensor can replace an array of independent resistive sensors which would consume much more energy for heating. We also suggest that a more advanced technology for preparing the gas-sensing layer, by use of spin coating, will produce corresponding layers with thickness of about a few μm, which is about ten times less than that for the tested samples. The effects induced by the applied UV light, having a penetration depth of only a few μm, would then be amplified.

Nickel-Decorated Black Phosphorus for Room Temperature NO2 detection

We study the electrical characterization at room temperature (RT) of black phosphorus nanoflakes decorated with nickel nanoparticles (Ni/bP). The latter has been exposed to different concentrations of NO2 at RT in a dry air environment highlighting fast and stable response over the time. This latter characteristic is particularly relevant since environmental instability of exfoliated BP has been an issue so far, under ambient conditions few-layer BP degrades completely in less than a week, preventing its applications in several fields. Introduction In the rapidly emerging field of layered two-dimensional (2D) functional materials, bP, the all P-counterpart of graphene, is a potential candidate for various applications e.g., nanoscale optoelectronics, rechargeable ion batteries, electrocatalysts, thermoelectrics, solar cells, and sensors [1]. bP has been reported to exhibit superior chemical sensing performance. In particular, bP is selective for the detection of paramagnetic molecules, e.g., NO2, in addition to high sensitivity at a limit of detection (LOD) of ppb levels.[2] In this work, by applying a multiscale characterization approach we demonstrated a stability and functionality improvement of Nickel-decorated bP (Ni/bP) films for gas sensing prepared by a simple, reproducible and affordable deposition technique. Furthermore, we studied possible electrical activity of these films for their employment as functional layers in gas sensors by exposing them to different gaseous compounds (NO2, CO2, H2, NH3, CO, Benzene, etha-nol, ethylene, Formaldehyde) in different relative humidity (RH%) conditions. Moreover, the influence in sensing perfor-mance of nickel nanoparticle (NP) dimensions in related to the decoration technique and the film thickness were investigated. Materials and Methods Black phosphorus microcrystals were suspended in dimethylsulfoxide and kept under ultrasonication for several hours. The exfoliated bP nanosheets were decorated with nickel nanoparticles following two different procedures: 1) preformed Ni NPs were immobilized on bP, 2) Ni NPs were grown directly on bP flakes. Electrical conductance measurements have been performed on Ni/bP flakes deposited via spin coating on Al2O3 substrates with built-in interdigitated gold electrodes, in order to better control the film thickness. Electrical conductance measurements have been performed at RT (25±2°C) by exposing the sensing film to certified mixtures of NO2 diluted in dry air (20% O2 and 80% N2) with different NO2 concentration ranging from 200 ppb to 1 ppm. Results and Conclusions Exfoliated black phosphorus nanoflakes decorated with nickel nanoparticles (Ni/bP) have been prepared and characterized with particular attention to the role of Ni nanoparticle dimension in the sensing mechanism. Electrical activity towards different concentrations of NO2 at room temperature in a dry air environment highlighted fast and stable response over the time. This latter characteristic is particularly relevant since environmental instability of exfoliated BP has been an issue so far, under ambient conditions few-layer bP degrades completely in less than a week, preventing its applications in several fields. In situ grown Ni/bP (2) shows stable response during all the four weeks measurements period with no dependence of the response on the film thickness. Ni decoration effectively suppress ambient degradation, for at least 1 week in Ni/bP (1) and over 4 weeks in Ni/bP (2) in ambient conditions. This result highlighted that Ni/bP (2) sensor can be practically used under ambient conditions for a reasonable period without performance degradation of the devices.The obtained results pave the way for the use of chemically exfoliated bP flakes for gas sensing applications, avoiding the use of a time-consuming technique for the exfoliation and for the fabrication of the metallic contacts, like electron beam lithography, focused ion beam lithography, etc.

Flexible Gas Sensing Fibre Based on Bismuthous Sulfide Nanobelts-Sensitized Graphene Nanoplatelets (GNPs) Nanocomposites

Introduction Gas sensors with high flexibility and favorable extensibility have increasingly attractive prospects. Conventional rigid ceramics or semiconductor-based gas sensors have restricted flexibility and ductility, limiting their use in flexible devices and wearable electronics. Flexible gas sensors possess broad application prospects in the fields of wearable electronics, artificial intelligence, medical health, etc. Two-dimensional (2D) layered nanomaterials have attracted attention due to unique properties, which possess tunable band gaps and excellent mobility inherently. For example, graphene is usually utilized as gas sensing material due to high specific surface area and high carrier mobility [1]. However, the properties of pristine graphene nanoplatelets (GNPs) are influenced by significant adsorption of oxygen in an air environment, leading to instability and carrier mobility recession, limiting the practical application of GNPs gas sensors in atmosphere. The combination of quantum dots or nanobelts and 2D materials could build sensitized nanocomposites, which improves sensing performance of gas sensors. Flexible and ultrafast-response nitrogen dioxide (NO2) gas sensors based on cotton fibre that applies the mechanism of coupling and hydrogen bonding interaction were prepared by facile layered dip-coating methods. The gas sensors were fabricated utilizing cotton fibre as flexible substrate and bismuth sulfide (Bi2S3) nanobelts-sensitized GNPs nanocomposites as sensing layer. The flexible gas sensing fibre demonstrated ultrafast and recoverable response (67 s/45 s for 500 ppb gas), good reproducibility and mechanical robustness. The favorable sensing performance and high flexibility of fibre gas sensors will open potential prospects in intelligent sensing system and wearable electronics. Experimental Preparation of coupling reagent modified cotton fibre: The cleansed fibre substrate was first modified by 3-aminopropyl) triethoxysilane (APTES: coupling reagent) via dipping in APTES ethanol solution, and then rinsed with deionized water and dried in drying oven (100 ℃, 15 min). The organic functional group in coupling agent can be combined with the fiber substrate, meanwhile the silanoxy group in coupling agent is reactive with inorganic substances. When coupling agent is positioned between fiber substrate and gas sensing layer, bonding structure of cotton fibre-coupling agent-sensing layer can be formed. Synthesis of Bi2S3 nanobelts-sensitized graphene nanoplatelets nanocomposites: Solvothermal method was used for Bi2S3 nanobelts synthesis reported in literatures [2]. And GNPs/Sodium laurylsulfonate (SDL) ethanol solution (solution A) and Bi2S3/polyethylene glycol (PEG) ethanol solution (solution B) were prepared for next synthesis. Finally, solution A were injected into solution B under magnetic stirring for six hours to prepare Bi2S3/GNPs nanocomposites. Flexible gas sensor fabrication: The modified fibre was dipped in Bi2S3/GNPs solution for 30 min through dip-coating method, then treated with Pb(NO3)2 for surface ligand exchange to remove insulating organic ligands capping on Bi2S3 surface, lastly annealed in drying oven (60 ℃, 30 min). Results and Conclusions The stereoscopic illustration of electronic fibre gas sensor is shown in figure 1 (a). After two steps dip-coating processes, modified layer and outer Bi2S3/GNPs nanocomposites layer were successively coated on the surface of cotton fibre. Figure 1 (b) shows cross-section structure of fiber-based gas sensors, illustrating bonding structure of fibre-coupling agent-sensing layer. As for Bi2S3/GNPs nanocomposites layer, the intermolecular hydrogen bond [3] between sulfonic groups (-SO3H) of SDL and hydroxyl groups (-OH) of PEG could contribute to the interfacial adhesion between substrate and sensing layer (figure 1 (c)). For one thing, cotton fiber has advantages of large specific surface area and rich skeletal structure, which helps the combination of fiber skeleton and sensing materials. For another, Bi2S3-sensitized GNPs nanocomposites constructed an excellent receptor-transducer gas-sensing layer. Considering the dual-mode characteristics of fast carrier transfer on graphene and strong adsorption of gas molecules in Bi2S3 nanobelts, fiber-based gas sensors demonstrated ultrafast and recoverable response (67 s/45 s for 500 ppb gas), good reproducibility and mechanical robustness (figure 1(f-g)). The device showed excellent reversibility and negligible baseline drift following cycling responses (figure 1(g)). Compared with recently reported fibre-based gas sensors (Figure 1(h)), fibre-based gas sensors showed significantly enhanced response and recovery time. This work indicates that the flexible gas sensing fibre could be used to establish wearable electronics and real-time atmosphere information monitoring system.

The Design of Nano/Micro Structure of the Composite Metal Oxides and the Gas Sensing Performances

The gas sensor based on metal oxide semiconductors (MOS), which is known as one of the most widely popular gas sensors, plays an important role in gas detection in all walks of life. The sensing principle of MOS-based gas sensors results from changes in resistance caused by the adsorption–desorption reaction between target gases and chemisorbed oxygen ions on the surface of the sensing layers [1-3]. To generate adjustable oxygen vacancies on the surfaces of MOS is an effective strategy to improve the gas sensing performances [4, 5]. This work reported the synthesis of spinel type cobalt-based MOS (MCo2O4, M=Zn, Ni and Mn) (Fig. 1(a)) and the performance optimization by controlling the stoichiometric proportion of sensing materials. The results show that sensing properties can be significantly enhanced by substituting Co with other transition metals in the lattices. ZnCo2O4 with typical normal spinel structure can be regarded as a representative sensing material among the studied cobaltates, which exhibits the best sensing performances to acetone. The sensitivity of the ZnCo2O4 sensor is 24 and 106 times higher than those of the NiCo2O4-sensor and MnCo2O4-sensor at 190 °C, respectively. Zn substitution in tetrahedrally coordinated positions can provide more adsorbed oxygen molecules and oxygen vacancies and enable a better gas-sensing capability (Fig. 1(b-d)). In addition, the transition metal content can also be adjusted to further increase oxygen vacancy. The Zn content was controlled in ZnxCo3-xO4 bimetallic oxides during the growth process. When tested as an acetone sensing layer for gas sensors, the prepared ZnxCo3-xO4 materials (Zn/Co=0.37) composed of nano-sized building blocks exhibited a response of 35.6 to 200 ppm acetone within 60 s (Fig. 1(e)). In addition, the sensor device also demonstrates a low detection limit of 0.5 ppm to acetone, even in a high humidity environment. The better acetone sensing performance is attributed to the adsorbed oxygen and abundant oxygen vacancies of ZnCo-0.37 samples. More adsorbed oxygen molecules that take part in the sensing reaction are beneficial for the release of a mass of electrons to increase the extent of the change in resistance. Oxygen vacancies, serving as electron donors, endow ZnCo-n nanocages with unpaired electrons to provide free electrons and active sites to enhance the gas sensing properties. This approach sheds light on the rational design of bimetallic oxides with abundant oxygen vacancy and the development of superior sensors targeted for ultratrace acetone gas.