Two-dimensional black phosphorus/tin oxide heterojunctions for high-performance chemiresistive H(2)S sensing

Bismuth Ferrite-Silver Nanowire Flexible Nanocomposites for Room-Temperature Nitrogen Dioxide Sensing

Nitrogen dioxide (NO2) is a major pollutant, causing acid rain, photochemical smog, and respiratory damage. The annual safe limit is 50 parts per billion (ppb), while concentrations exceeding 1 part per million (ppm) can result in respiratory ailments. Conventionally, n-type metal oxide semiconductors operating at elevated temperatures have been utilized for NO2 detection. Recently, p-type semiconductors with their hole accumulation layer, rapid recovery post-gas exposure, and good humidity tolerance are being investigated as potential NO2 sensors, once again working at elevated temperatures. In this work, a room-temperature (27 ± 2 °C) NO2 sensor is demonstrated by using a nanocomposite based on p-type bismuth ferrite (BFO) nanoparticles and silver nanowires (Ag NWs). This nanocomposite is capable of sensing a NO2 gas concentration of up to 0.2 ppm. The BFO nanoparticles are synthesized via a sol-gel route followed by sintering at 500 °C to form the crystalline phase. Nanocomposites are obtained by formulating a dispersion of the BFO nanoparticles and Ag NWs, followed by direct writing on both flexible and rigid substrates. The Ag NWs act as the conducting pathway, reducing the overall electrical resistance and thus enabling room-temperature operation. X-ray diffraction, scanning electron microscopy, and surface area studies provide phase information and surface morphology, and the porous nature of the film helps in room-temperature gas adsorption. The current-voltage and gas-sensing behavior are studied to obtain the optimized molar ratio (4:1 BFO/Ag NWs) for the sensor. The sensor deposited on poly(ethylene terephthalate) (PET) also works under a bent condition, indicating good flexibility. Rapid NO2 sensing was achieved in a BFO-Ag/PET device with response/recovery times of 7/8.5 s and 12/15 s in straight and bent geometries, respectively. Additionally, a good sensitivity of 30 to 60% was achieved for the BFO-Ag/PET device across 100 to 1000 ppb of NO2. The development of a nanocomposite combining an active sensing element (BFO) and a charge-transport element (Ag NWs) opens up a multitude of other application areas.

Bioinspired multi-scale interface design for wet gas sensing based on rational water management

Natural organisms have evolved multi-scale wet gas sensing interfaces with optimized mass transport pathways in biological fluid environments, which sheds light on developing artificial counterparts with improved wet gas sensing abilities and practical applications. Herein, we highlighted current advances in wet gas sensing taking advantage of optimized mass transport pathways endowed by multi-scale interface design. Common moisture resistance (e.g., employing moisture resistant sensing materials, post-modifying moisture resistant coatings, physical heating for moisture resistance, and self-removing hydroxyl groups) and moisture absorption (e.g., employing moisture absorption sensing materials and post-modifying moisture absorption coatings) strategies for wet gas sensing were discussed. Then, the design principles of bioinspired multi-scale wet gas sensing interfaces were provided, including macro-level condensation mediation, micro/nano-level transport pathway adjustment and molecular level moisture-proof design. Finally, perspectives on constructing bioinspired multi-scale wet gas sensing interfaces were presented, which will not only deepen our understanding of the underlying principles, but also promote practical applications.

Construction of a CoNiHHTP MOF/PHI Z-Scheme Heterojunction for ppb Level NO2 Photoelectric Sensing with 405 nm Irradiation at RT

Ultrasensitive photoelectric detection of nitrogen dioxide (NO2) with PHI under visible light irradiation at room temperature (RT) remains an ongoing challenge due to the low charge separation and scarce adsorption sites. In this work, a dimensionally matched ultrathin CoNiHHTP MOF/PHI Z-scheme heterojunction is successfully constructed by taking advantage of the π-π interactions existing between the CoNiHHTP MOF and PHI. The amount-optimized heterojunction possesses a record detection limit of 1 ppb (response = 15.6%) for NO2 under 405 nm irradiation at RT, with reduced responsive (3.6 min) and recovery (2.7 min) times, good selectivity and reversibility, and long-time stability (150 days) compared with PHI, even superior to others reported at RT. Based on the time-resolved photoluminescence spectra, in situ X-ray photoelectron spectra, and diffuse reflectance infrared Fourier transform spectroscopy results, the resulting sensing performance is attributed to the favorable Z-scheme charge transfer and separation. Moreover, the Ni nodes favorably present in adjacent metal sites between the lamellae contribute to charge transfer and redistribution, whereas Co nodes could act as selective centers for promoted adsorption of NO2. Interestingly, it is confirmed that the CoNiHHTP MOF/PHI heterojunction could effectively reduce the influence of O2 in the gas-sensitive reaction due to their unique bimetallic (Co and Ni) nodes, which is also favorable for the improved sensing performances for NO2. This work provides a feasible strategy to develop promising PHI-based optoelectronic gas sensors at RT.

Oxygen Vacancy-Rich Bimetallic Au@Pt Core-Shell Nanosphere-Functionalized Electrospun ZnFe2O4 Nanofibers for Chemiresistive Breath Acetone Detection

Sensitive and selective acetone detection is of great significance in the fields of environmental protection, industrial production, and individual health monitoring from exhaled breath. To achieve this goal, bimetallic Au@Pt core-shell nanospheres (BNSs) functionalized-electrospun ZnFe2O4 nanofibers (ZFO NFs) are prepared in this work. Compared to pure NFs-650 analogue, the ZFO NFs/BNSs-2 sensor exhibits a stronger mean response (3.32 vs 1.84), quicker response/recovery speeds (33 s/28 s vs 54 s/42 s), and lower operating temperature (188 vs 273 °C) toward 0.5 ppm acetone. Note that an experimental detection limit of 30 ppb is achieved, which ranks among the best cases reported thus far. Besides the demonstrated excellent repeatability, humidity-enhanced response, and long-term stability, the selectivity toward acetone is remarkably improved after BNSs functionalization. Through material characterizations and DFT calculations, all these improvements could be attributed to the boosted oxygen vacancies and abundant Schottky junctions between ZFO NFs and BNSs, and the synergistic catalytic effect of BNSs. This work offers an alternative strategy to realize selective subppm acetone under high-humidity conditions catering for the future requirements of noninvasive breath diabetes diagnosis in the field of individual healthcare.

Black Phosphorus-Tungsten Oxide Sandwich-like Nanostructures for Highly Selective NO2 Detection

Modern chemical production processes often emit complex mixtures of gases, including hazardous pollutants such as NO2. Although widely used, gas sensors based on metal oxide semiconductors such as WO3 respond to a wide range of interfering gases other than NO2. Consequently, developing WO3 gas sensors with high NO2 selectivity is challenging. In this study, a simple one-step hydrothermal method was used to prepare WO3 nanorods modified with black phosphorus (BP) flakes as sensitive materials for NO2 sensing, and BP-WO3-based micro-electromechanical system gas sensors were fabricated. The characterization of the as-prepared BP-WO3 composite through X-ray diffraction scanning electron microscopy and X-ray photoelectron spectroscopy confirmed the successful formation of the sandwich-like nanostructures. The result of gas-sensing tests with 2-14 ppm NO2 indicated that the sensor response was 1.25-2.21 with response-recovery times of 36 and 36 s, respectively, at 190 °C. In contrast to pure WO3, which exhibited a response of 1.07-2.2 to 0.3-5 ppm H2S at 160 °C, BP-WO3 showed almost no response to H2S. Thus, compared with pure WO3, BP-WO3 exhibited significantly improved NO2 selectivity. Overall, the BP-WO3 composite with sandwich-like nanostructures is a promising material for developing highly selective NO2 sensors for practical applications.

Highly Sensitive H2S Gas Sensor Based on a Lead-Free CsCu2I3 Perovskite Film at Room Temperature

Recently, there have been reports of lead halide perovskite-based sensors demonstrating their potential for gas sensing applications. However, the toxicity of lead and the instability of lead-based perovskites have limited their applications. This study addressed this issue by developing a H2S gas sensor based on a lead-free CsCu2I3 film prepared using a one-step CVD method. The sensor demonstrated excellent sensing properties, including a high response and selectivity toward H2S, even at low concentrations (0.2 ppm) at room temperature. Furthermore, a reasonable sensing mechanism was proposed. It is suggested that the sensing mechanism sheds light on the role of defects in perovskite materials, the impact of H2S as an electron donor, and the occurrence of reversible chemical reactions. These findings suggest that lead-free CsCu2I3 has great potential in the field of H2S gas sensing.

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.

Alkalized Cellulose Nanofiber-Interweaved PEDOT:PSS Thin-Film Sensors via Layer-by-Layer Spraying Assembly for Ultrafast Molecular Ammonia Detection

As a typical representative of conductive polymers (CPs), poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS) is intensively employed for chemiresistive ammonia (NH3) sensing on account of its favorable aqueous solubility, benign environmental stability, and outstanding room-temperature conductivity; however, it is severely plagued by low sensitivity and sluggish reaction kinetics. To circumvent these limitations, the guest-alkalized cellulose nanofibers (AC) were introduced into the host PEDOT:PSS matrix by the layer-by-layer spraying assembly method (LBLSA) in this work. The componential proportion-optimized PEDOT:PSS/AC/PEDOT:PSS (P/AC/P) sensor delivered a large sensitivity of 20.2%/ppm within 0.1-3 ppm of NH3 at 21 °C@26% RH, an experimental limit of detection (LoD) as low as 30 ppb, a high response of 18.1%, and a short response/recovery times (4.8/4.0 s) toward 1 ppm of NH3, which ranked among the best cases thus far. Also, excellent repeatability and long-term stability and selectivity were demonstrated. Meanwhile, the flexible P/AC/P sensors worked well under various bending angles and bending times. This work combines a green material system and a facile film deposition method to overcome the liquid dispersion incompatibility when preparing a multicomponent mixture for swift trace NH3 detection. The universality and extensibility of this methodology endow a broad prospect in the field of future wearable optoelectronic systems.

Amplification of ammonia sensing performance through gate induced carrier modulation in Cur-rGO Silk-FET

Uncontrolled human and industrial activities lead to the increase in demand for selective gas sensors for detection of poisonous gases in our environment. Conventional resistive gas sensors suffer from predetermined sensitivity and poor selectivity among gases. This paper demonstrates curcumin reduced graphene oxide-silk field effect transistor for selective and sensitive detection of ammonia in air. The sensing layer was characterized by X-ray diffraction, FESEM and HRTEM to confirm its structural and morphological features. Raman spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy was carried out to analyze the functional moieties present in the sensing layer. Curcumin reduced graphene oxide introduces sufficient hydroxyl groups in the sensing layer to provide high degree of selectivity towards ammonia vapors. The performance of the sensor device was evaluated at positive, negative and zero gate voltage. Carrier modulation in the channel through gate electrostatics revealed that the minority carriers (electrons) in p-type reduced graphene oxide plays a pivotal role in enhancement of sensitivity of the sensor device. The sensor response was enhanced to 634% for 50 ppm ammonia at 0.6 V gate voltage compared to 23.2% and 39.3% at 0 V and - 3 V respectively. The sensor exhibited faster response and recovery at 0.6 V owing to higher mobility of electrons and quick charge transfer mechanism. The sensor exhibited satisfactory humidity resistant characteristics and high stability. Hence, curcumin reduced graphene oxide-silk field effect transistor device with proper gate bias elucidates excellent ammonia detection and may be a potential candidate for future room temperature, low power, portable gas detection system.

Highly Sensitive Chemiresistive H2S Detection at Subzero Temperature over the Sb-Doped SnO2@g-C3N4 Heterojunctions under UV Illumination

NASA has detected H₂S in the persistently shadowed region of the lunar South Pole through NIR and UV/vis spectroscopy remotely, but in situ detection is generally considered to be more accurate and convincing. However, subzero temperatures in space drastically reduce chemisorbed oxygen ions for gas sensing reactions, making gas sensing at subzero temperature something that has rarely been attempted. Herein, we report an in situ semiconductor H₂S gas sensor assisted by UV illumination at subzero temperature. We constructed a g-C₃N₄ network to wrap the porous Sb doped SnO₂ microspheres to form type II heterojunctions, which facilitate the separation and transport of photoinduced charge carriers under UV irradiation. This UV-driven technique affords the gas sensor a fast response time of 14 s and a response value of 20.1 toward 2 ppm H₂S at -20 °C, realizing the sensitive response of the semiconductor gas sensor at subzero temperature for the first time. Both the experimental observations and theoretical calculation results provide evidence that UV irradiation and the formation of type II heterojunctions together promote the performance at subzero temperature. This work fills the gap of semiconductor gas sensors working at subzero temperature and suggests a feasible method for deep space gas detection.