Humidity-Tolerant Chemiresistive Gas Sensors Based on Hydrophobic CeO2/SnO2 Heterostructure Films

Amorphous Carbon-Loaded WO3 Nanosheet Arrays for ppb-Level NO2 Sensing with Improved Anti-Humidity Property

Metal-oxide-based gas sensors have attracted considerable interest due to their low cost, high sensitivity, quick response, and ease of miniaturization and integration. Unfortunately, metal oxides are susceptible to the interference of moisture, which brings about hydroxyl poisoning of the sensing layer and decrement in baseline resistance and sensor performance. In this study, WO3 films were modified with a porous amorphous carbon layer. The as-loaded amorphous carbon layer improves simultaneously the adsorption capacity of WO3 and hydrophobicity of the surface, which endows the composite films with not only surpassing NO2 sensitivity at the parts per billion level but also enhanced immunity to humidity. The incorporation of amorphous carbon with metal oxides is expected to be a general route for the fabrication of high performance, antihumidity gas sensors.

Construction of trace nitric oxide sensors at low temperature based on bulk embedded BiVO4 in SnO2 nanofibers with nano-heterointerfaces

Constructing heterostructures is an effective way to improve the carrier mobility for metal oxide sensing material, since heterojunctions are usually built only on the surface of the material, the carrier transport efficiency inside the material still needs to be improved. In this paper, BiVO4 nanocrystals (BVO NCs) with an average size of 1 nm generated by pulsed laser irradiation were embedded in situ at the particle boundaries (PBs) of SnO2 nanofibers to form an effective n-n heterojunctions inside the material. After embedding the BVO NCs in the SnO2 samples, the response value for 10 ppm NO was improved to 48.91, which was 2.5 times higher than that of pure SnO2 at near room temperature (50 °C). Meanwhile, the detection limit was lowered to 50 ppb with excellent long term stability. Detailed analysis and theoretical calculations demonstrated that the formation of abundant n-n heterojunctions not only promotes the electron-hole separation and the carrier mobility, but also reduces the conductivity and adsorption energy of the material, which significantly improves its sensing performance. This work demonstrates a new approach to modulate the gas-sensing performance of metal oxide semiconductors by generating heterostructure inside the bulk of the material.

Vitalizing Perovskite Oxide-Based Acetone Sensors with Metal-Organic Framework-Derived Heterogeneous Oxide Catalysts

Perovskite oxides are promising candidates for chemiresistive-type gas sensors owing to their exceptional thermal and chemical stability during solid-gas reactions. However, perovskites suffer from critical issues such as low surface area and poor surface activity, which negatively influence the sensing characteristics. While metal nanoparticles can be incorporated in perovskites to improve their reactivity, the fundamental incompatibility between catalytic metals and perovskite oxides often leads to substantial structural degradation as well as phase instability. Herein, we overcome this challenge through the introduction of an intermediary phase that forms coherent interfaces with both the perovskite phase and catalyst metals. Specifically, we present the case study of p-type La0.8Ca0.2Fe0.98Pt0.02O3 perovskite, whose hole accumulation layer was modulated by the incorporation of metal-organic framework (MOF)-derived n-type α-Fe2O3 nanoparticles decorated with highly dispersed Pt catalysts. The resulting composite exhibited significantly improved surface activity over the nonmodified La0.8Ca0.2FeO3 perovskite, leading to exceptional chemiresistive sensing performance toward acetone gas (Rg/Ra = 39.8 toward 10 ppm of acetone at 250 °C) with high cross-sensitivity against interfering gases. Importantly, our findings reaffirm the critical influence of interfacial engineering in facilitating surface chemical reactions on perovskite oxides and, by doing so, effectively provide a general synthetic guideline to the design of perovskite-based chemiresistors.

Ultrahigh humidity-resistance ppb-level formaldehyde sensing at room temperature induced by fluorinated dipole based "umbrella" and "bridge"

Formaldehyde (HCHO) is a major indoor pollutant that is extremely harmful to human health even at ppb-level. Meanwhile, ppb-level HCHO is also a potential disease marker in the exhalation of patients with respiratory diseases. Higher humidity resistance and lower practical limit of detection (pLOD) both have to be pursued for practical HCHO sensors. In this work, by assembling indium oxide (In2O3) and fluorinated dipole modified reduced graphene oxide (rGO), we prepared a high-performance room temperature HCHO sensor (In2O3 @ATQ-rGO). Excellent sensing properties toward HCHO under visible illumination have been achieved, including ultra-low pLOD of 3 ppb and high humidity-resistance. By control experiments and density functional theory calculation, it is indicated that the introduced fluorinated dipoles act as not only an "umbrella" to improve the humidity resistance of the composite, but also a "bridge" to accelerate the electron transport, improving the sensitivity of the material. The significant practicality and reliability of the obtained sensors were verified by in-situ simulation experiments using a 3 m3 test chamber with a humidity control system and by detection of the simulated lung disease patient's exhalation. This work provides an effective strategy of simultaneously achieving high humidity-resistance and low pLOD of room temperature formaldehyde sensing materials.

Wearable Inorganic Oxide Chemiresistor Based on Flexible Al2O3-Stabilized ZrO2 Ceramic Sponge Substrate for NO2 Sensing

Wearable gas sensors, possessing the advantages of high sensitivity, excellent flexibility, high permeability, low weight, and workability at ambient conditions, hold great promise for real-time health monitoring and early warnings of poisonous gases. However, obtaining high-performance wearable gas sensors utilizing the current well-developed inorganic semiconductor oxide sensing materials is still very limited due to their fragile and rigid nature. Herein, a newly designed wearable gas sensor based on an all-inorganic ASZ (Al2O3-stabilized ZrO2)/ZnO/SnO2 nanofibers is introduced for the first time. The flexible ASZ ceramic sponge substrate (with a Young's modulus of 4.15 MPa) and ultrathin ZnO/SnO2 sensing layer endow the wearable gas sensor with promising properties such as super flexibility (with a bending radius of 5 mm), high gas permeability, and low weight. Furthermore, driven by UV light irradiation, this all-inorganic wearable sensor also demonstrates a stable NO2 sensing response under different bending states at room temperature, which enables the gas sensor to be more compatible with wearable sensing applications. This work offers a general method to achieve a high-performance wearable gas sensor based on inorganic materials and provides new insights into their potential in wearable gas-sensing applications.

Phase-Engineered MoSe2/CeO2 Composites for Room-Temperature Gas Sensing with a Drastic Discrimination of NH3 and TEA Gases

Detecting and distinguishing between hazardous gases with similar odors by using conventional sensor technology for safeguarding human health and ensuring food safety are significant challenges. Bulky, costly, and power-hungry devices, such as that used for gas chromatography-mass spectrometry (GC-MS), are widely employed for gas sensing. Using a single chemiresistive semiconductor or electric nose (e-nose) gas sensor to achieve this objective is difficult, mainly because of its selectivity issue. Thus, there is a need to develop new materials with tunable and versatile sensing characteristics. Phase engineering of two-dimensional materials to better utilize their physiochemical properties has attracted considerable attention. Here, we show that MoSe2 phase-transition/CeO2 composites can be effectively used to distinguish ammonia (NH3) and triethylamine (TEA) at room temperature. The phase transition of nanocomposite samples from semimetallic (1T) to semiconducting (2H) prepared at different synthesis temperatures is confirmed via X-ray photoelectron spectroscopy (XPS). A composite sensor in which the 2H phase of MoSe2 is predominant lacks discrimination capability and is less responsive to NH3 and TEA. An MoSe2/CeO2 composite sensor with a higher 1T phase content exhibits high selectivity for NH3, whereas one with a higher 2H phase content (2H > 1T) shows more selective behavior toward TEA. For example, for 50% relative humidity, the MoSe2/CeO2 sensor's signal changes from the baseline by 45% and 58% for 1 ppm of NH3 and TEA, respectively, indicating a low limit of detection (LOD) of 70 and 160 ppb, respectively. The composites' superior sensing characteristics are mainly attributed to their large specific surface area, their numerous active sites, presence of defects, and the n-n type heterojunction between MoSe2 and CeO2. The sensing mechanism is elucidated using Raman spectroscopy, XPS, and GC-MS results. Their phase-transition characteristics render MoSe2/CeO2 sensors promising for use in distributed, low-cost, and room-temperature sensor networks, and they offer new opportunities for the development of integrated advanced smart sensing technologies for environmental and healthcare.

Enhanced Interface Charge Carrier Transport of SnO2/CeO2 via Oxygen Vacancy Synergized Heterojunction for Triethylamine Sensing Property

Efficient charge carrier transport characteristics are critical to achieving the excellent performance of metal-oxide semiconductor gas sensors. Herein, SnO2/CeO2 heterojunction layered nanosheets with abundant oxygen vacancies were successfully synthesized through a simple solvothermal assisted high-temperature calcination method. The synergistic effect of oxygen vacancies and heterojunctions promoting the charge carrier transport properties at the SnO2/CeO2 interface for the enhanced sensing properties of triethylamine (TEA) was highlighted. As a result, the optimized SnO2/CeO2 exhibits improved gas sensing performance at 173 °C to 50 ppm of TEA. These include high response (205), excellent selectivity, low detection limit, and good long-term stability. This enhanced gas sensing property of SnO2/CeO2 is mainly attributed to the fact that the heterojunction and oxygen vacancies act as dual active sites synergistically inducing electron transfer, thereby effectively modulating the transport properties of the interfacial charge carriers, and thus facilitate the surface reactions efficiently. In this work, the dual-engineering strategy of synergistic interaction of heterojunction and oxygen vacancies can provide new perspectives for the design of advanced gas sensing materials.

Applications of cerium-based materials in food monitoring

With the rapid development of society, food safety to public health has been a topic that cannot be ignored. In recent years, lanthanide-based materials are studied to be potential candidates in the detection of food samples. Cerium (Ce)-based materials (such as Ce ions, CeO2, Ce-metal organic framework (Ce-MOF), etc.) have also attracted more attention in food detection by virtue of colorimetric, fluorescence, sensing, and other methods. This is because the mixed valence of Ce (Ce3+ and Ce4+), the formation of oxygen vacancies, and their optical and electrochemical properties. In this review, Ce-based materials will be introduced and discussed in the field of food detection, including biogenesis, construction, catalytic mechanisms, combination, and applications. In addition, the current challenges and future development trend of these Ce-based materials in food safety detection are also proposed and discussed. Therefore, it is meaningful to explore the Ce-based materials for detection of biomarkers in food samples.

Ultrathin Serpentine Insulation Layer Architecture for Ultralow Power Gas Sensor

Toxic gases have surreptitiously influenced the health and environment of contemporary society with their odorless/colorless characteristics. As a result, a pressing need for reliable and portable gas-sensing devices has continuously increased. However, with their negligence to efficiently microstructure their bulky supportive layer on which the sensing and heating materials are located, previous semiconductor metal-oxide gas sensors have been unable to fully enhance their power efficiency, a critical factor in power-stringent portable devices. Herein, an ultrathin insulation layer with a unique serpentine architecture is proposed for the development of a power-efficient gas sensor, consuming only 2.3 mW with an operating temperature of 300 °C (≈6% of the leading commercial product). Utilizing a mechanically robust serpentine design, this work presents a fully suspended standalone device with a supportive layer thickness of only ≈50 nm. The developed gas sensor shows excellent mechanical durability, operating over 10 000 on/off cycles and ≈2 years of life expectancy under continuous operation. The gas sensor detected carbon monoxide concentrations from 30 to 1 ppm with an average response time of ≈15 s and distinguishable sensitivity to 1 ppm (ΔR/R0 = 5%). The mass-producible fabrication and heating efficiency presented here provide an exemplary platform for diverse power-efficient-related devices.

Room-temperature response of MOF-derived Pd@PdO core shell/γ-Fe2O3 microcubes decorated graphitic carbon based ultrasensitive and highly selective H2 gas sensor

With the current upsurge in hydrogen economies all over the world, an increased demand for improved chemiresistive H2 sensors that are highly responsive and fast acting when exposed to gases is expected. Owing to safety concerns about explosive and highly flammable H2 gas, it is important to develop resistive sensors that can detect the leakage of H2 gas swiftly and selectively. Currently, interest in metal-organic frameworks (MOFs) for gas-sensor applications is increasing due to their open-metal sites, large surface area, and unique surface morphologies. In this research, a highly selective and sensitive H2-sensor was established based on graphitic carbon (GC) anchored spherical Pd@PdO core-shells over γ-Fe2O3 microcube (Pd@PdO/γ-Fe2O3@GC which is termed as S3) heterostructure materials. The combined solvothermal followed by controlled calcination-assisted S3 exhibited a specific morphology with the highest surface area of 79.12 m2 g-1, resulting in fast response and recovery times (21 and 29 s, respectively), and excellent sensing performance (ΔR/R0∼ 96.2 ± 1.5), outstanding long-term stability, and a 100 ppb detection limit when detecting H2-gas at room temperature (mainly in very humid surroundings). This result proves that adsorption sites provided by S3 can promote surface reactions (adsorption and desorption) for ultrasensitive and selective H2gas sensors.

2D/2D Dy2O3 Nanosheet/MoO3 Nanoflake Heterostructures for Humidity-Independent and Sensitive Ammonia Detection

Chemiresistive ammonia gas (NH3) sensors have been playing a significant role in the fields of environmental protection, food safety monitoring, and air quality evaluation. Nevertheless, balancing the high sensitivity and humidity tolerance remains challenging. Herein, the two-dimensional (2D) heterostructures of molybdenum trioxide (MoO3) nanoflakes decorated with dysprosium oxide (Dy2O3) nanosheets (termed Dy2O3/MoO3) were synthesized via a facile probe-sonication method. With respect to pristine MoO3 counterparts, the optimal Dy2O3/MoO3 sensors possessed a 4.49-fold larger response at a lower temperature (30.52@328.2 °C vs 6.8@369.7 °C toward 10 ppm of NH3), shorter response/recovery times (11.6/2.9 s vs 26.9/43.4 s), 52.6-fold higher sensitivity (17.35/ppm vs 0.33/ppm), and a lower theoretical detection limit (1.02 vs 32.82 ppb). Besides the nice reversibility, wide detection range (0.45-100 ppm) and robust long-term stability, inspiringly, the Dy2O3/MoO3 sensors showed a nearly humidity-independent response. These impressive improvements in the NH3-sensing performance were attributed to numerous heterojunctions to strengthen the carrier concentration modulation and the compensation/protection effect of Dy2O3 to mitigate the humidity effect. Moreover, the Dy2O3/MoO3 sensors showed preliminary application potential in monitoring pork freshness. This work provides a universal methodology for constructing NH3 gas sensors with high sensitivity and good humidity resistance and probably extends the application scenarios of MoO3-based sensors in the Internet of Things in the future.

Construction of Flower-like PtOx@ZnO/In2O3 Hollow Microspheres for Ultrasensitive and Rapid Trace Detection of Isopropanol

The design of a highly effective isopropanol gas sensor with high response and trace detection capability is extremely important for environmental surveillance and human health. Here, novel flower-like PtO_x@ZnO/In₂O₃ hollow microspheres were prepared by a three-step approach. The hollow structure was composed of an In₂O₃ shell inside and layered ZnO/In₂O₃ nanosheets outside with PtO_x nanoparticles (NPs) on the surface. Meanwhile, the gas sensing performances of the ZnO/In₂O₃ composite with different Zn/In ratios and PtO_x@ZnO/In₂O₃ composites were evaluated and compared systematically. The measurement results indicated that the ratio of Zn/In affected the sensing performance and the ZnIn₂ sensor presented a higher response, which was then modified with PtO_x NPs to further enhance its sensing property. The Pt@ZnIn₂ sensor exhibited outstanding isopropanol detection performance with ultrahigh response values under 22 and 95% relative humidity (RH). In addition, it also showed a rapid response/recovery speed, good linearity, and low theoretical limit of detection (LOD) regardless of being under a relatively dry or ultrahumid atmosphere. The enhancement of isopropanol sensing properties might be ascribed to the unique structure of PtO_x@ZnO/In₂O₃, heterojunctions between the components, and catalytic effect of Pt NPs.

Metal-oxide-semiconductor resistive gas sensors for fish freshness detection

Fish are prone to spoilage and deterioration during processing, storage, or transportation. Therefore, there is a need for rapid and efficient techniques to detect and evaluate fish freshness during different periods or conditions. Gas sensors are increasingly important in the qualitative and quantitative evaluation of high-protein foods, including fish. Among them, metal-oxide-semiconductor resistive (MOSR) sensors with advantages such as low cost, small size, easy integration, and high sensitivity have been extensively studied in the past few years, which gradually show promising practical application prospects. Herein, we take the detection, classification, and assessment of fish freshness as the actual demand, and summarize the physical and chemical changes of fish during the spoilage process, the volatile marker gases released, and their production mechanisms. Then, we introduce the advantages, performance parameters, and working principles of gas sensors, and summarize the MOSR gas sensors aimed at detecting different kinds of volatile marker gases of fish spoiling in the last 5 years. After that, this paper reviews the research and application progress of MOSR gas sensor arrays and electronic nose technology for various odor indicators and fish freshness detection. Finally, this review points out the multifaceted challenges (sampling system, sensing module, and pattern recognition technology) faced by the rapid detection technology of fish freshness based on metal oxide gas sensors, and the potential solutions and development directions are proposed from the view of multidisciplinary intersection.

Recent Progress on Anti-Humidity Strategies of Chemiresistive Gas Sensors

In recent decades, chemiresistive gas sensors (CGS) have been widely studied due to their unique advantages of expedient miniaturization, simple fabrication, easy operation, and low cost. As one ubiquitous interference factor, humidity dramatically affects the performance of CGS, which has been neglected for a long time. With the rapid development of technologies based on gas sensors, including the internet of things (IoT), healthcare, environment monitoring, and food quality assessing, the humidity interference on gas sensors has been attracting increasing attention. Inspiringly, various anti-humidity strategies have been proposed to alleviate the humidity interference in this field; however, comprehensive summaries of these strategies are rarely reported. Therefore, this review aims to summarize the latest research advances on humidity-independent CGS. First, we discussed the humidity interference mechanism on gas sensors. Then, the anti-humidity strategies mainly including surface engineering, physical isolation, working parameters modulation, humidity compensation, and developing novel gas-sensing materials were successively introduced in detail. Finally, challenges and perspectives of improving the humidity tolerance of gas sensors were proposed for future research.