The nature of a built-in electric field within supported metal catalysts plays a crucial role in regulating gas adsorption and electron transfer during the gas-sensing process. Herein, we found an electron-supply redeployment phenomenon involving the reversal of direction of a built-in electric field between the metal palladium species and the outer S atoms in ZnS, resulting in a marked hydrogen sensing difference. It was found that Pd nanoparticles embedded into Pd NP-ZnS can induce spontaneous electron transfer from S atoms to Pd species to generate an electron-deficient sulfur (S(2-delta)-) surface. Conversely, atomically dispersed Pd species (Pd1-ZnS) prefer to generate electron-rich sulfur (S(2+delta)-) sites and thus reverse the built-in electric field. Theoretical calculations demonstrate that the electron-rich S (S(2+delta)-) surface can reduce the occupancy of antibonding orbitals in the S-Hads bond and enhance the bond energy of S-Hads, thus increasing the adsorption of hydrogen. Additionally, in situ Raman, ex situ X-ray photoelectron spectroscopy and DFT analysis demonstrate that S(2+delta)- sites in Pd1-ZnS samples can undergo strong electron transfer with hydrogen during the sensing process. Ultimately, Pd1-ZnS sensors exhibit extremely high response values (9.66/20 ppm) and fast response recovery times (5.1 s/1.8 s to 400 ppm) for hydrogen gas at a working temperature of 170 degrees C. The nature of a built-in electric field within supported metal catalysts plays a crucial role in regulating gas adsorption and electron transfer during the gas-sensing process.

Oxide semiconductor-supported metal nanoparticles often suffer from a high-temperature gas sensing process, resulting in agglomeration and coalescence, which significantly decrease their surface activity and stability. Here, we develop an in situ pyrolysis strategy to redisperse commercial Ir particles (similar to 15.6 nm) into monodisperse Ir species (similar to 5.4 nm) on ZnO supports, exhibiting excellent sintering-resistant properties and H-2 sensing. We find that large-size Ir nanoparticles can undergo an unexpected splitting decomposition process and spontaneously migrate along the encapsulated carbon layer surface during high-temperature pyrolysis of ZIF-8. This resultant monodisperse status can be integrally reserved, accompanying further oxidation sintering. The final Ir-red/ZnO-450-based sensor exhibits outstanding stability, H-2 response (10-2000 ppm), fast response/recovery capability (7/9.7 s@100 ppm), and good moisture resistance. In situ Raman and ex situ XPS further experimentally verify that highly dispersive Ir species can promote the electron transfer process during the gas sensing process. Our strategy thus provides important insights into the design of agglomeration-resistant gas sensing materials for highly effective H-2 detection.

Rational structure design of sensing materials is the most effective way to obtain a hydrogen (H2) sensor with optimal performance. Herein, a porous heterostructure WO3-C/In2O3 was designed and prepared by in-situ coupling carbon layer and WO3 into MOF-derived (metal organic framework) In2O3. In combination with Micro-Electro-Mechanical System (MEMS), a miniature H2 sensor using WO3-C/In2O3 as the active sensing material was fabricated. Compared with the bare In2O3 sensor, the sensor based on WCI-9 (the mass ratio of WO3 to In2O3 in WO3-C/In2O3 is 9 wt%) shows higher response value and lower operating temperature (Ra/Rg = 10.11@ 1000 ppm; 250 degrees C). Moreover, the WCI-9 sensor possesses a fast response-recovery speed (1.9/9.2 s@200 ppm) and a low limit of detection (LOD) (5 ppm) for H2. The good performance of the WCI-9 sensor is attributed to the hierarchical porous structure and multicomponent heterojunctions present in the material. This work not only provides an effective method for H2 detection, but also a strategy for constructing sensing materials with multicomponent heterojunctions.

Reasonably engineering the adsorption and excitation sites of oxygen and target gas molecules for expected sensing properties of semiconductor metal oxides (SMO) is still challenging. Here, we proposed a gas-segregated strategy involving spatially-separated Ir and W5+-Vo sites to respectively provide specific adsorption and excitation zones of oxygen and target gas molecules for sensitive xylene detection. We find that the introduced Ir sites can induce O2 adsorption and enhance the reactive oxygen species formation through spillover effects, while W5+-Vo sites tend to drive the target xylene molecules aggregation along the supported WO3 surface, and thus synergistically promote the sensing property. This process can be experimentally confirmed by NAP-XPS, ex situ XPS, in situ Raman and theoretical calculations results. The final Ir-c-WO3 sensor exhibits high response ability towards xylene (1-100 ppm) at 180 °C operating temperature, fast response/recovery speed (2.9s/18s @ 15 ppm) and excellent selectivity. Our study thus provides a valid method for xylene detection and opens up a new perspective for the construction of macromolecular active gas sensing materials.

Palladium (Pd)-modified metal oxide semiconductors (MOSs) gas sensors often exhibit unexpected hydrogen (H2 ) sensing activity through a spillover effect. However, sluggish kinetics over a limited Pd-MOS surface seriously restrict the sensing process. Here, a hollow Pd-NiO/SnO2 buffered nanocavity is engineered to kinetically drive the H2 spillover over dual yolk-shell surface for the ultrasensitive H2 sensing. This unique nanocavity is found and can induce more H2 absorption and markedly improve kinetical H2 ab/desorption rates. Meanwhile, the limited buffer-room allows the H2 molecules to adequately spillover in the inside-layer surface and thus realize dual H2 spillover effect. Ex situ XPS, in situ Raman, and density functional theory (DFT) analysis further confirm that the Pd species can effectively combine H2 to form Pd-H bonds and then dissociate the hydrogen species to NiO/SnO2 surface. The final Pd-NiO/SnO2 sensors exhibit an ultrasensitive response (0.1-1000 ppm H2 ) and low actual detection limit (100 ppb) at the operating temperature of 230 °C, which surpass that of most reported H2 sensors.

Amorphous/crystalline heterophase engineering is emerging as an attractive strategy to adjust the properties and functions of nanomaterials. Here, we reveal a heterophase interface role by precisely tailoring the crystalline Pt coverage density on an amorphous Ru surface (cPt/aRu) for ultrasensitive H2S detection. We found that when the atomic ratio of Pt/Ru increased from 10 to 50%, the loading modes of Pt changed from island coverage (1cPt/aRu) to cross-linkable coverage (3cPt/aRu) and further to dense coverage (5cPt/aRu). The differences in coverage models further regulate the chemical adsorption of H2S on Pt and the electronic transformation process on Ru, which can be proved by ex situ X-ray photoelectron spectroscopy experiments. Notably, a special cross-linkable coverage 3cPt/aRu on ZnO shows the best gas-sensitive performance, in which the operating temperature reduces from 240 to 160 °C compared with pristine ZnO and the selectivity coefficient for H2S gas improves from ∼1.2 to ∼4.6. This is mainly benefit from the maximized exposure of the amorphous/crystalline heterophase interface. Our work thus provides a new platform for future applications of amorphous/crystalline heterogeneous nanostructures in gas sensors and catalysis.

Controllable and efficient construction of oxygen vacancies on the surface of metal oxide semiconductors (MOSs) is essential for their application in the gas sensor. Herein, a general H2 reduction method is developed to synthesize SnO2 with oxygen vacancies defect (SnO2-D) by annealing the SnO2 in a H2 atmosphere at different temperature (300 degrees C, 400 degrees C and 500 degrees C), and then named SnO2-D3, SnO2-D4 and SnO2-D5, respectively. It was found that although the determined specific surface areas for pristine SnO2, SnO2-D3, SnO2-D4 and SnO2-D5 are 103.749 m2g 1, 63.316 m2g 1, 47.652 m2g 1 and 15.541 m2g1, respectively, the gas sensitivity test results indicate that the SnO2-D4 Micro-Electro-Mechanical System (MEMS) sensor shows improved response and excellent low-concentration detection capability (down to 0.1 ppm) to H2 compared with that fabricated with pristine SnO2, SnO2-D3 and SnO2-D5. The abnormal relationship between specific surface area and gas sensing performance is attributed to the more oxygen vacancies of SnO2-D4 surface. In addition, ZnO (ZnO-D) and In2O3 (In2O3-D) with oxygen vacancy defects based on the H2 reduction method show better gas sensitivity than ZnO and In2O3 sensors, which further proves that oxygen vacancy defects can effectively improve the gas-sensing performance of MOSs.

Constructing porous structure has proved to be an effective strategy to improve the gas sensing properties of metal oxide semiconducting materials. In this work, high-performance hydrogen sulfide (H2S) gas sensing material with porous structure were synthesized by growing nickel oxide (NiO) on vanadium trioxide (V2O3) seeds. Morphology and structure characterizations reveal that the novel three-dimensional (3D) nanoflowers are formed by self-assembling two-dimensional (2D) porous nanosheets with high surface area and abundant active sites. Compared with pristine NiO, the sensing performance of V2O3-NiO (VN8, VN7, VN6) with different ratios of V3+: Ni2+ is enhanced due to their porous structure. Among them, VN7 sensor shows excellent sensing properties at 200 °C. The response to 500 ppb H2S can reach 65, which increases as high as 2.2 times compared with the pristine NiO sensor. The response time to 500 ppb H2S is further decreased from 13 s of the pristine NiO sensors to 8 s of VN7 sensor. The VN7 sensor also shows a wide linear range from 20 ppb to 500 ppb, high selectivity, good repeatability, long-term stability, moisture resistance and low detection limit (20 ppb), indicating its potential candidate for ppb-level H2S detection in complex environment of industrial mine.

In this work, the nanocluster Co 12 was successfully prepared by reflux reaction and decorated by varied ZnO contents (1-7 wt%). The gas sensing results show that both Co 12 and ZnO/Co 12 Micro-Electro-Mechanical System (MEMS) sensors exhibit superior selectivity to H 2 S. Among these clusters and composites, the ZnO/Co 12 -5 composite exhibited the largest response value of 116.85 to 500 ppb H 2 S at lower working temperature of 115 °C with an ultralow limit of detection of 10 ppb, and the response/recovery speed was also the fastest, reaching 19/20 s. The optimized performance of ZnO/Co 12 -5 to H 2 S attributed to formation of p-n heterojunction at the ZnO/Co 12 interface. Here, we developed the application of cobalt clusters and their composites in MEMS sensors, and the detection of H 2 S can reach ppb level, which opens up a direction for the application of cobalt clusters in gas sensing.

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.

Sensors based on RuOx/ZnO possess better gas sensing performance than ZnO and RuO2/ZnO based sensors and amorphous RuOx can work stably as a sensitizer at high temperature (210 °C).

A p-n heterostructure has been proved as a good strategy to enhance the gas sensing performance of metal oxide semiconductor (MOS). Here, we reported a xylene gas sensor based on CuO/WO3 p-n hierarchical structure. CuO/WO3 were prepared by a two-step route. First, WO3 was synthesized by a reaction between CaWO4 and HNO3 under reaction condition was at 25˚C for 18 h, and then CuO/WO3 were prepared by immersion pyrolysis method. The prepared hierarchical CuO/WO3 hollow microspheres features an ordered assembly of two dimensional (2D) nanosheets with the thickness about 20 nm. Gas sensing tests displayed CuO/WO3 sensor had a good linearity property between the sensing responses and xylene concentrations range from 0.3 ppm to 50 ppm, and CuO/WO3 -3 (the mass ratio of CuO to WO3 is 3) sensor exhibited the highest response of 304.8 toward 20 ppm xylene gas at the optimal working temperature of 260˚C. Furthermore, it is obvious that CuO/WO3 -3 not only possessed superior response and recovery properties of 3 s and 6 s under 20 ppm xylene gas, respectively, but also showed outstanding selectivity and stability. The advantages of high sensitivity, simple manufacturing and outstanding selectivity indicate the promising application of CuO/WO3 xylene sensors in the field of air-quality and environmental monitoring, and p-n heterostructure can significantly enhance the gas-sensing performance of metal oxide semiconductor. Figure 1

In this paper, tungsten oxide nanorods were successfully synthesized by using ordered mesoporous silica of SBA-15 as a template and 12-phosphotungstic acid as a precursor of tungsten oxide under a hydrogen (H2) atmosphere. XRD and HRTEM characterization show that the composition of tungsten oxide synthesized is WO2.83. The gas sensing test results show that WO2.83 can detect 5 ppb H2S. Moreover, the response of the sensor fabricated with WO2.83 to 200 ppb H2S at the optimal operating temperature of 280 °C is as high as 1.8, which is increased by nearly 50% than that of the common WO3 without redundant oxygen vacancies. The improved gas sensing properties can be attributed to rich oxygen vacancies in the WO2.83. Our present results also demonstrate that WO2.83 with a lot of oxygen vacancies can significantly enhance the gas response to ultra-low concentration H2S, and this method of manufacturing defects has great potential in improving the gas-sensing performance of metal oxide semiconductor. Figure 1

Here we report the fabrication of a high performance metal oxide semiconductor (MOS) sensor for the detection of hydrogen sulfide (H2S) using PdRh bimetal hollow nanocube (HC) with Rh-rich hollow frame and Pd-rich core frame as sensitizing materials. PdRh bimetal HC with the edge-length about 10 nm was prepared by chemical etching PdRh bimetal solid nanocube (SC) in HNO3 aqueous solution. The results of gas-sensing tests indicate that the response value order of the MEMS gas sensors based on MOSs (including ZnO, MoO3 and SnO2) is as follows: RPdRh HC/MOS > RPdRh SC/MOS > RMOS. First, in the system of ZnO, gas sensor modified by PdRh (PdRh SC/ZnO and PdRh HC/ZnO) possess enhanced H2S sensing performance with a better response and excellent low-concentration detection capability (down to 15 ppb) comparing to pure ZnO. The improved H2S sensing performance could be attributed to the good conductivity of Rh-rich frame, the high catalytic activity of PdRh bimetal and formation of Schottky barrier-type junctions and defect. Second, PdRh HC/ZnO sensor shows better response (185-1 ppm of H2S) compared to PdRh SC/ZnO sensor (108-1 ppm of H2S), which is due to the higher specific surface area of PdRh HC/ZnO and good gas diffusion of the hollow structure. This work indicate that the sensitization characteristics of PdRh bimetal HC will provide new paradigms for the future development of the high performance sensor.