High-Sensitive MEMS Hydrogen Sulfide Sensor made from PdRh Bimetal Hollow Nanoframe Decorated Metal Oxides and Sensitization Mechanism Study

Hierarchically porous ZnO derived from zeolitic imidazolate frameworks for high-sensitive MEMS NO2 sensor

Three-dimensional (3D) porous metal oxide nanomaterials with controllable morphology and well-defined pore size have attracted extensive attention in the field of gas sensing. Herein, hierarchically porous ZnO-450 was obtained simply by annealing Zeolitic Imidazolate Frameworks (ZIF-90) microcrystals at an optimal temperature of 450 °C, and the effect of annealing temperature on the formation of porous nanostructure was discussed. Then the as-obtained ZnO-450 was employed as sensing materials to construct a Micro-Electro-Mechanical System (MEMS) gas sensor for detecting NO2. The MEMS sensor based on ZnO-450 displays the excellent gas-sensing performances at a lower working temperature (190 °C), such as high response value (242.18% @ 10 ppm), fast response/recovery time (9/26 s) and ultralow limit of detection (35 ppb). The ZnO-450 sensor shows better sensing performance for NO2 detection than ZnO-based composites materials or commercial ZnO nanoparticles (NPs), which are attributed to its unique hierarchically structures with high porosity and larger surface area. This ZIFs driven strategy can be expected to pave a new pathway for the design of high-performance NO2 sensors.

Bimetallic PtAu-Decorated SnO2 Nanospheres Exhibiting Enhanced Gas Sensitivity for Ppb-Level Acetone Detection

Acetone is a biomarker found in the expired air of patients suffering from diabetes. Therefore, early and accurate detection of its concentration in the breath of such patients is extremely important. We prepared Tin(IV) oxide (SnO2) nanospheres via hydrothermal treatment and then decorated them with bimetallic PtAu nanoparticles (NPs) employing the approach of in situ reduction. The topology, elemental composition, as well as crystal structure of the prepared materials were studied via field emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The findings revealed that bimetallic PtAu-decorated SnO2 nanospheres (PtAu/SnO2) were effectively synthesized as well as PtAu NPs evenly deposited onto the surface of the SnO2 nanospheres. Pure SnO2 nanospheres and PtAu/SnO2 sensors were prepared, and their acetone gas sensitivity was explored. The findings demonstrated that in comparison to pristine SnO2 nanosphere sensors, the sensors based on PtAu/SnO2 displayed superior sensitivity to acetone of 0.166-100 ppm at 300 °C, providing a low theoretical limit of detection equal to 158 ppm. Moreover, the PtAu/SnO2 sensors showed excellent gas response (Ra/Rg = 492.3 to 100 ppm), along with fast response and recovery (14 s/13 s to 10 ppm), good linearity of correlation, excellent repeatability, long-term stability, and satisfactory selectivity at 300 °C. This improved gas sensitivity was because of the electron sensitization of the Pt NPs, the chemical sensitization of the Au NPs, as well as the synergistic effects of bimetallic PtAu. The PtAu/SnO2 sensors have considerable potential for the early diagnosis and screening of diabetes.

Deep Learning-Assisted Colorimetric/Electrical Dual-Sensing System for Ultrafast Detection of Hydrogen Sulfide

This study presents a colorimetric/electrical dual-sensing system (CEDS) for low-power, high-precision, adaptable, and real-time detection of hydrogen sulfide (H2S) gas. The lead acetate/poly(vinyl alcohol) (Pb(Ac)2/PVA) nanofiber film was transferred onto a polyethylene terephthalate (PET) flexible substrate by electrospinning to obtain colorimetric/electrical sensors. The CEDS was constructed to simultaneously record both the visual and electrical response of the sensor, and the improved Manhattan segmentation algorithm and deep neural network (DNN) were used as its intelligent algorithmic aids to achieve quantitative exposure to H2S. By exploring the mechanism of color change and resistance response of the sensor, a dual-sensitivity mechanism explanation model was proposed to verify that the system, as a dual-mode parallel system, can adequately solve the sensor redundancy problem. The results show that the CEDS can achieve a wide detection range of H2S from 0.1-100 ppm and identify the H2S concentration in 4 s at the fastest. The sensor can be stabilized for 180 days with excellent selectivity and a low limit of detection (LOD) to 0.1 ppm of H2S. In addition, the feasibility of the CEDS for measuring H2S levels in underground waterways was validated. This work provides a new method for adaptable, wide range of applications and low-power, high-precision H2S gas detection.

PtPd NPs-functionalized metal-organic framework-derived α-Fe2O3 porous spindles for efficient low-temperature detection of triethylamine

In recent years, metal-organic framework (MOF) derivatives have gradually become ideal materials for gas sensors due to their controllable composition, diverse structures and open metal sites. In this research, a simplified hydrothermal method was applied to successfully prepare MOF-derived α-Fe2O3 spindles, and an in situ reduction method was then utilized to deposit Pt, Pd and PtPd bimetallic nanoparticles (NPs) on the α-Fe2O3 spindles. The effects of noble metals Pt, Pd and PtPd on the gas-sensing properties of Fe2O3 were systematically examined. The PtPd/α-Fe2O3 sensor has enhanced gas-sensing performance for triethylamine (TEA), especially at PtPd content of 1.5 wt% and mass ratio of Pt : Pd = 90 : 10, where the response of the sensor to 100 ppm TEA at a lower temperature of 150 °C is 442, which is 34 times higher than that of the original α-Fe2O3 (response of 13). Additionally, the sensor demonstrated improved response/recovery properties and very respectable selectivity, repeatability, long-term stability within 30 days and lower detection limit (500 ppb) at 150 °C. Combining the results of XPS and O2-TPD, the enhanced gas-sensing properties of PtPd bimetallic-modified α-Fe2O3 over monometallic (Pt or Pd) modified α-Fe2O3 were analyzed, which can be attributed to the chemical and electronic sensitization of noble metals and the synergistic effect of the PtPd bimetallic NPs, resulting in more surface defects and enhanced oxygen adsorption capacity of the sensing material. This work provided an effective gas-sensing material for the low-temperature detection and analysis of triethylamine gas.

Pt-Pd Nanoalloys Functionalized Mesoporous SnO2 Spheres: Tailored Synthesis, Sensing Mechanism, and Device Integration

Methane (CH4 ), as the vital energy resource and industrial chemicals, is highly flammable and explosive for concentrations above the explosive limit, triggering potential risks to personal and production safety. Therefore, exploiting smart gas sensors for real-time monitoring of CH4 becomes extremely important. Herein, the Pt-Pd nanoalloy functionalized mesoporous SnO2 microspheres (Pt-Pd/SnO2 ) were synthesized, which show uniform diameter (≈500 nm), high surface area (40.9-56.5 m2 g-1 ), and large mesopore size (8.8-15.8 nm). The highly dispersed Pt-Pd nanoalloys are confined in the mesopores of SnO2 , causing the generation ofoxygen defects and increasing the carrier concentration of sensitive materials. The representative Pt1 -Pd4 /SnO2 exhibits superior CH4 sensing performance with ultrahigh response (Ra /Rg = 21.33 to 3000 ppm), fast response/recovery speed (4/9 s), as well as outstanding stability. Spectroscopic analyses imply that such an excellent CH4 sensing process involves the fast conversion of CH4 into formic acid and CO intermediates, and finally into CO2 . Density functional theory (DFT) calculations reveal that the attractive covalent bonding interaction and rapid electron transfer between the Pt-Pd nanoalloys and SnO2 support, dramatically promote the orbital hybridization of Pd4 sites and adsorbed CH4 molecules, enhancing the catalytic activation of CH4 over the sensing layer.

Pt-functionalized Amorphous RuOx as Excellent Stability and High-activity Catalysts for Low Temperature MEMS Sensors

The unsaturated coordination and abundant active sites endow amorphous metals with tremendous potential in improving metal oxide semiconductors' gas-sensing properties. However, the amorphous materials maintain the metastable status and easily transfer into the lower-active crystals during the gas-sensing process at high working temperatures, significantly limiting their further applications. Here, a bimetal amorphous PtRu catalyst is developed by accurately regulating the introduction of Pt species into amorphous RuOx supports to realize the highly active and stable H2 S gas-sensing detection. It is found that incorporation of low-concentration Pt species can effectively maintain the amorphous state of initial RuOx and delay the crystallization temperature as high as 100 °C. Further, ex situ XPS and in situ Raman spectroscopy analysis confirm that active Pt species can facilitate H2 S adsorption by strong Pt-S coordination and dissociate the sulfur species to the surrounding support, which contribute to the chemisorption and sensitization of H2 S. Meanwhile, electron transport at the interface between Pt, RuOx and ZnO further activates the reaction process at the surface of the gas-sensitive material. The final PtRu-modified ZnO (PtRu/ZnO) sensor enables the detection of H2 S in the ultra-low concentration range of 15-2000 ppb with remarkable stability.

Preparation of MOF-derived ZnO/Co3O4 nanocages and their sensing performance toward H2S

We report a type of micro-electro-mechanical system (MEMS) H₂S gas sensors with excellent sensing performance at the ppb level (lowest detection limit is 5 ppb). The sensors were fabricated with ZnO/Co₃O₄ sensing materials derived from Zn/Co-MOFs by annealing at a suitable temperature of 500 °C. ZnO/Co₃O₄-500 exhibits the highest response when exposed to 10 ppb H₂S gas at 120 °C, and the response/recovery times are 10 s/21 s. Moreover, it exhibits outstanding selectivity, long-term stability (retained 95% response after 45 days), and moisture resistance (only a minor fluctuation of 2% even at 90% RH). This can be ascribed to the fact that ZnO/Co₃O₄-500 has regular morphology, abundant oxygen vacancies (52.8%) and high specific surface area (96.5 m² g^-1). This work provides not only a high performance H₂S MEMS gas sensor but also a systematic study of the effect of the annealing temperature on the sensing performance of ZnO/Co₃O₄ sensing materials derived from bimetal organic frameworks.

Advances in Noble Metal-Decorated Metal Oxide Nanomaterials for Chemiresistive Gas Sensors: Overview

Highly sensitive gas sensors with remarkably low detection limits are attractive for diverse practical application fields including real-time environmental monitoring, exhaled breath diagnosis, and food freshness analysis. Among various chemiresistive sensing materials, noble metal-decorated semiconducting metal oxides (SMOs) have currently aroused extensive attention by virtue of the unique electronic and catalytic properties of noble metals. This review highlights the research progress on the designs and applications of different noble metal-decorated SMOs with diverse nanostructures (e.g., nanoparticles, nanowires, nanorods, nanosheets, nanoflowers, and microspheres) for high-performance gas sensors with higher response, faster response/recovery speed, lower operating temperature, and ultra-low detection limits. The key topics include Pt, Pd, Au, other noble metals (e.g., Ag, Ru, and Rh.), and bimetals-decorated SMOs containing ZnO, SnO2, WO3, other SMOs (e.g., In2O3, Fe2O3, and CuO), and heterostructured SMOs. In addition to conventional devices, the innovative applications like photo-assisted room temperature gas sensors and mechanically flexible smart wearable devices are also discussed. Moreover, the relevant mechanisms for the sensing performance improvement caused by noble metal decoration, including the electronic sensitization effect and the chemical sensitization effect, have also been summarized in detail. Finally, major challenges and future perspectives towards noble metal-decorated SMOs-based chemiresistive gas sensors are proposed.

Three-Dimensional van der Waals Heterostructure-Based Nanocages as Supersensitive 3-Hydroxy-2-butanone Gas Sensors at Room Temperature

3-Hydroxy-2-butanone is one of the biomarkers of Listeria monocytogenes, which is quite important for the intelligent detection of 3H-2B. However, it is still a challenge to fabricate sensing materials obtaining excellent sensitivity and selectivity under the ppb-level detection limit. Herein, a plasma-assisted synthetic approach was proposed for the construction of hierarchical nanostructures and the simultaneous loading of TAPP-COFs, which could reduce interlayer interaction and convert the metallized sites on the surface of predesigned porphyrin rings into quantum nanoparticles. These multichannel pathways of Co-TAPP-COFs@SnO₂@MWCNTs nanocages contributed to the gas adsorption and diffusion, thus enhancing the sensing behavior. The nanocages exhibited a highly specific sensing performance toward 3H-2B with the highest sensitivity (R_a/R_g = 100.9 to 0.5 ppm) in all reported sensing materials. The 3H-2B sensor presented outstanding long-term stability, and the detection limit was 100 ppb at room temperature. Furthermore, the synthesized materials were integrated into the sensing module connecting to an Internet of Things platform, providing rapid and real-time detection of 3H-2B. We also applied machine learning methods to analyze the nanocage-based sensors and found that the combined effects of modified sites on the heterointerfaces contributed to the improvement of the sensing performance.

Selective monitoring of natural gas sulphur-based odorant mixture of t-butyl mercaptan and methyl ethyl sulphide using an array of microfluidic gas sensors

A natural gas (NG) odorization system requires continuous monitoring as well as an optimal injection to satisfy the odorization guidelines, minimize over-odorization, and prevent hazardous gas leaks. NG consists of hydrocarbons such as methane, odorants such as tert-butyl mercaptan, and other sulphur-based VOCs such as hydrogen sulphide; therefore, selectivity is paramount for the reliable and accurate monitoring of odorants. In this study, we developed a portable device integrated with an array of five different sensors to detect a mixture of tert-butyl mercaptan and methyl ethyl sulphide for a concentration range of 1 ppm to 10 ppm. A machine learning model was developed to predict the presence and concentration of NG odorants from the sensor data. The best-performing sensors in the array achieved high sensitivity and selectivity indicators (measured using the Davies-Bouldin index) of 0.3667 (1⁄ppm) and 0.125, respectively. The sensor system achieved a classification accuracy of 98.75% between NG odorants and hydrogen sulphide, with an overall Mean Squared Error (MSE) and R² error (for the regression model) of 0.50 and 95.16%. These results indicate that the developed portable device and the machine learning model have promising applications for the selective monitoring of NG odorants.

Bidimensional Engineered Amorphous a-SnO2 Interfaces: Synthesis and Gas Sensing Response to H2S and Humidity

Two-dimensional (2D) transition metal dichalcogenides (TMDs) and metal chalcogenides (MCs), despite their excellent gas sensing properties, are subjected to spontaneous oxidation in ambient air, negatively affecting the sensor's signal reproducibility in the long run. Taking advantage of spontaneous oxidation, we synthesized fully amorphous a-SnO₂ 2D flakes (≈30 nm thick) by annealing in air 2D SnSe₂ for two weeks at temperatures below the crystallization temperature of SnO₂ (T < 280 °C). These engineered a-SnO₂ interfaces, preserving all the precursor's 2D surface-to-volume features, are stable in dry/wet air up to 250 °C, with excellent baseline and sensor's signal reproducibility to H₂S (400 ppb to 1.5 ppm) and humidity (10-80% relative humidity (RH)) at 100 °C for one year. Specifically, by combined density functional theory and ab initio molecular dynamics, we demonstrated that H₂S and H₂O compete by dissociative chemisorption over the same a-SnO₂ adsorption sites, disclosing the humidity cross-response to H₂S sensing. Tests confirmed that humidity decreases the baseline resistance, hampers the H₂S sensor's signal (i.e., relative response (RR) = R_a/R_g), and increases the limit of detection (LOD). At 1 ppm, the H₂S sensor's signal decreases from an RR of 2.4 ± 0.1 at 0% RH to 1.9 ± 0.1 at 80% RH, while the LOD increases from 210 to 380 ppb. Utilizing a suitable thermal treatment, here, we report an amorphization procedure that can be easily extended to a large variety of TMDs and MCs, opening extraordinary applications for 2D layered amorphous metal oxide gas sensors.

Multishell SnO2 Hollow Microspheres Loaded with Bimetal PdPt Nanoparticles for Ultrasensitive and Rapid Formaldehyde MEMS Sensors

Low-cost and real-time formaldehyde (HCHO) monitoring is of great importance due to its volatility, extreme toxicity, and ready accessibility. In this work, a low-cost and integrated microelectromechanical system (MEMS) HCHO sensor is developed based on SnO₂ multishell hollow microspheres loaded with a bimetallic PdPt (PdPt/SnO₂-M) sensitizer. The MEMS sensor exhibits a high sensitivity to HCHO ((R_a/R_g - 1) % = 83.7 @ 1 ppm), ultralow detection limit of 50 ppb, and ultrashort response/recovery time (5.0/7.0 s @ 1 ppm). These excellent HCHO sensing properties are attributed to its unique multishell hollow structure with a large and accessible surface, abundant interfaces, suitable mesoporous structure, and synergistic catalytic effects of bimetal PdPt. The well-defined multishell hollow structure also shows fascinating capacities as good hosts for noble metal loading. Therefore, PdPt bimetallic nanoparticles can be employed to construct a synergistic sensitizer with a high content and good dispersity on this multishell hollow structure, further exhibiting a reduced working temperature and ultrasensitive detection of HCHO. This PdPt/SnO₂-M-based MEMS sensor presents a unique and highly sensitive means to detect HCHO, establishing its great promise for potential application in environmental monitoring.

Porous Cr2O3 Architecture Assembled by Nano-Sized Cylinders/Ellipsoids for Enhanced Sensing to Trace H2S Gas

How to achieve high sensing of Cr₂O₃-based sensors for harmful inorganic gases is still a challenge. To this end, Cr₂O₃ nanomaterials assembled from different building blocks were simply prepared by chromium salt immersion and air calcination with waste scallion roots as the biomass template. The hierarchical architecture calcined at 600 °C is constructed from nanocylinders and nanoellipsoids (named as Cr₂O₃-600), and also possesses multistage pore distribution for target gas accessibility. Interestingly, the synergism of two shapes of nanocrystals enables the Cr₂O₃-based sensor to realize highly sensitive detection of trace H₂S gas. At 170 °C, Cr₂O₃-600 exhibits a high response of 42.8 to 100 ppm H₂S gas, which is 3.45 times larger than that of Cr₂O₃-500 assembled from nanocylinders. Meanwhile, this sensor has a low detection limit of 1.0 ppb (S = 1.4), good selectivity, stability, and moisture resistance. These results show that the combination of nanosized cylinders/ellipsoids together with exposed (104) facet can effectively improve the sensing performance of the p-type Cr₂O₃ material. In addition, the Cr₂O₃-600 sensor shows satisfactory results for actual monitoring of the corruption process of fresh chicken.

Fast-response MEMS xylene gas sensor based on CuO/WO3 hierarchical structure

CuO/WO₃ hierarchical hollow microspheres, assembled from irregular two dimensional (2D) nanosheets, were prepared by ultrasonic-wet chemical etching and pyrolysis in this study. The sensing performance of Micro-Electro-Mechanical System (MEMS) xylene gas sensor based on CuO/WO₃ hierarchical structure were evaluated. It was found that the CuO/WO₃ MEMS sensors showed an enhanced gas sensing performance compared with pristine WO₃ sensor. The CuO/WO₃-3 (the mass ratio of CuO to WO₃ is 3%) sensor exhibited faster response-recover speed and the highest response value to xylene. Moreover, the CuO/WO₃-3 sensor possessed higher selectivity and long-term stability. The good sensing properties can be attributed to the unique three dimensional (3D) hierarchical structure and p-n heterojunction of CuO-WO₃. Considering the above advantages, the CuO/WO₃-3 sensor has a great potential for the rapid detection and monitoring of xylene.

Research Progress on Coating of Sensitive Materials for Micro-Hotplate Gas Sensor

Micro-hotplate gas sensors are widely used in air quality monitoring, identification of hazardous chemicals, human health monitoring, and other fields due to their advantages of small size, low power consumption, excellent consistency, and fast response speed. The micro-hotplate gas sensor comprises a micro-hotplate and a gas-sensitive material layer. The micro-hotplate is responsible for providing temperature conditions for the sensor to work. The gas-sensitive material layer is responsible for the redox reaction with the gas molecules to be measured, causing the resistance value to change. The gas-sensitive material film with high stability, fantastic adhesion, and amazing uniformity is prepared on the surface of the micro-hotplate to realize the reliable assembly of the gas-sensitive material and the micro-hotplate, which can improve the response speed, response value, and selectivity. This paper first introduces the classification and structural characteristics of micro-hotplates. Then the assembly process and characteristics of various gas-sensing materials and micro-hotplates are summarized. Finally, the assembly method of the gas-sensing material and the micro-hotplate prospects.

The SnO2/MXene Composite Ethanol Sensor Based on MEMS Platform

In recent years, two-dimensional layered material MXene has attracted extensive attention in the fields of sensors due to its large specific surface area and rich active sites. So, we employed multilayer Ti3C2TX and SnO2 microspheres to prepare SnO2/MXene composites for enhancing gas-sensing properties of pristine SnO2. The composite was brushed on a microelectromechanical system (MEMS) platform to make resistance-type gas sensors with low power consumption. The gas-sensing results show that the SnO2/MXene sensor with the best composite ratio (SnO2: MXene mass ratio is 5:1, named SM-5) greatly improves gas sensitivity of SnO2 sensor, among which the sensitivity to ethanol gas is the highest. At the same time, the composite also speeds up the response recovery speed of the sensor. When the SM-5 sensor worked at its optimal temperature 230 °C, its response value to 10 ppm ethanol reaches 5.0, which is twice that of the pristine SnO2 sensor. Its response and recovery time are only 14 s and 26 s, respectively. The sensing mechanism of the composite is discussed according to the classical the space charge or depletion layer model. It is concluded that the Schottky barrier of composites and the metal properties of Ti3C2Tx are responsible for improvement of the gas-sensing properties of the composite.

Ultra-sensitive H2S sensor based on sunflower-like In-doped ZnO with enriched oxygen vacancies

In–ZnO with oxygen vacancies exhibits a higher sensing response and a shorter recovery time for H2S compared to ZnO.

Bimetallic Nanocrystals: Structure, Controllable Synthesis and Applications in Catalysis, Energy and Sensing

In recent years, bimetallic nanocrystals have attracted great interest from many researchers. Bimetallic nanocrystals are expected to exhibit improved physical and chemical properties due to the synergistic effect between the two metals, not just a combination of two monometallic properties. More importantly, the properties of bimetallic nanocrystals are significantly affected by their morphology, structure, and atomic arrangement. Reasonable regulation of these parameters of nanocrystals can effectively control their properties and enhance their practicality in a given application. This review summarizes some recent research progress in the controlled synthesis of shape, composition and structure, as well as some important applications of bimetallic nanocrystals. We first give a brief introduction to the development of bimetals, followed by the architectural diversity of bimetallic nanocrystals. The most commonly used and typical synthesis methods are also summarized, and the possible morphologies under different conditions are also discussed. Finally, we discuss the composition-dependent and shape-dependent properties of bimetals in terms of highlighting applications such as catalysis, energy conversion, gas sensing and bio-detection applications.

AuPt Bimetal-Functionalized SnSe2 Microflower-Based Sensors for Detecting Sub-ppm NO2 at Low Temperatures

A novel chemiresistive-type sensor for detecting sub-ppm NO₂ has been fabricated using AuPt bimetal-decorated SnSe₂ microflowers, which was synthesized by the hydrothermal treatment followed by in situ chemical reduction of the bimetal precursors on the surface of the petals of the microflowers. The as-prepared sensor registers a superior performance in detection of sub-ppm concentration of NO₂. Functionalized by the AuPt bimetal, the SnSe₂ microflower-based sensor shows a response of approximately 4.62 to 8 ppm NO₂ at 130 °C. It is significantly higher than those of the sensors using the pristine SnSe₂ (∼2.29) and the modified SnSe₂ samples by a single metal, either Au (∼3.03) or Pt (∼3.97). The sensor demonstrates excellent long-term stability, signal repeatability, and selectivity to some typical interfering gaseous species including ammonia, acetone, formaldehyde, ethanol, methanol, benzene, CO₂, SO₂, and CO. The remarkable improvement of the sensitive characteristics could be induced by the electronic and chemical sensitization and the synergistic effect of the AuPt bimetal. Density functional theory (DFT) is implemented to calculate the adsorption states of NO₂ on the sensing materials and thus to possibly reveal the sensing mechanism. The significantly enhanced response of the SnSe₂-based sensor decorated with AuPt bimetallic nanoparticles has been found to be possibly caused by the orbital hybridization of O, Au, and Pt atoms leading to the redistribution of electrons, which is beneficial for NO₂ molecules to obtain more electrons from the composite material.