Metal-Organic-Framework Derived Co-Mo Multimetal Oxide Semiconductors: Selective Trace-Level Hydrogen Sulfide Detection

The development of a highly selective and trace-level gas sensing platform for detecting hydrogen sulfide (H2S) remains a formidable challenge. To solve this problem, Co-Mo multimetal oxide semiconductors are rationally tailored by employing metal organic frameworks (MOFs) as self-sacrificial templates. The MOF-derived Co3O4/β-CoMoO4 based gas sensors displays high sensitivity (Rg/Ra = 22) to 10 ppm of H2S and ultralow limit of detection (10 ppb H2S). The formation of p-p heterojunction and multivalence states of Mo play a crucial role in electron transfer and oxygen adsorption. A sensor array constructed from four Co3O4/β-CoMoO4 materials with different Co/Mo ratios demonstrates a superior selective discrimination of H2S from other VOCs and malodorous gases by principal component analysis (PCA). Besides, a H2S gas sensing and alarming platform was designed for monitoring the environment contaminated with H2S. This finding provides a feasible approach for the discovery of highly efficient gas sensors to monitor environmental H2S concentration.

Insights into Selective Mechanism of NiO-TiO2 Heterojunction to H2 and CO

The construction of p-n heterojunctions has become a widely adopted strategy for achieving the selective detection of reducing gases, including H2 and CO. Nevertheless, the elucidation of the gas selectivity mechanism at the nanoscale remains elusive. First-principle calculations provide an attractive avenue for comprehending the influence of coordination structures on gas-sensitive selectivity, thereby unveiling the structure-activity relationship of p-n heterojunction sites. In this study, we investigate the selective adsorption behavior of H2 and CO on a NiO-TiO2 heterojunction using density functional theory. The results of d-band center analysis confirm that the NiO-TiO2 heterojunction with adsorbed oxygen significantly enhances the adsorption stability of reducing gases. Intriguingly, our calculations reveal that H2 has a higher affinity for adsorbed oxygen on the heterojunction surface compared to that of CO, corresponding to a lower H2 adsorption energy. Density of states (DOS) results indicate that the NiO-TiO2 heterojunction, with preadsorbed oxygen, exhibits ultrahigh selectivity with an n-type gas-sensitive response to H2, effectively eliminating the cross-sensitivity observed with CO, as confirmed by gas-sensitive characterization research. The sensing mechanism of the NiO-TiO2 heterojunction's selective detection of H2 without interference from CO can be visually explained by electron transfer and potential barrier changes, paving the way for future developments in novel, selective gas-sensitive materials.

Ag2SO4-Ag2S transformation in SnO2-based nanofibers for high selectivity and response H2S sensors

Tin-based gas sensors have been developed for many years owing to their advantages of low price, high response and stability. However, selectivity remains a significant issue. Herein, Ag-SnO₂nanofibers are synthesized using AgCl as the doping reagent. The 3%Ag-SnO₂nanofibers sensors show a high response of 68 toward 1 ppm H₂S at 90 °C. Besides, the sensor with 3% AgCl possesses the shortest response time about 136 s at 150 °C which is only 30% value of the sensor without AgCl doping. It is also demonstrated that the nanofibers show a high selectivity towards H₂S. According to theex situx-ray photoelectron spectrum and x-ray diffraction results, AgCl was transferred to Ag₂S after Ag-SnO₂was exposed to H₂S, and reversible transformation between Ag₂SO₄and Ag₂S was the main mechanism for H₂S detection. Compared with pure SnO₂nanofiber sensors, the presence of Ag₂S with high conductivity greatly affects the resistance to H₂S, resulting in high selectivity and response. This mechanism differs from that of the transformation between Ag₂O and Ag₂SO₄. This study may provide a new strategy for the design and investigation of sensors with high selectivity.

Impact of a Diverse Combination of Metal Oxide Gas Sensors on Machine Learning-Based Gas Recognition in Mixed Gases

A challenge for chemiresistive-type gas sensors distinguishing mixture gases is that for highly accurate recognition, massive data processing acquired from various types of sensor configurations must be considered. The impact of data processing is indeed ineffective and time-consuming. Herein, we systemically investigate the effect of the selectivity for a target gas on the prediction accuracy of gas concentration via machine learning based on a support vector machine model. The selectivity factor S(X) of a gas sensor for a target gas "X" is introduced to reveal the correlation between the prediction accuracy and selectivity of gas sensors. The presented work suggests that (i) the strong correlation between the selectivity factor and prediction accuracy has a proportional relationship, (ii) the enhancement of the prediction accuracy of an elemental sensor with a low sensitivity factor can be attained by a complementary combination of the other sensor with a high selectivity factor, and (iii) it can also be boosted by combining the sensor having even a low selectivity factor.

Heterostructural (Sr0.6Bi0.305)2Bi2O7/ZnO for novel high-performance H2S sensor operating at low temperature

Exploring novel sensing materials enabling selective discrimination of trace ambient H₂S at lower temperature is of utmost importance for diverse practical applications. Herein, heterostructural (Sr_0.6Bi_0.305)₂Bi₂O₇/ZnO (SBO/ZnO) nanomaterials were proposed. Synergetic effect of promoting analyte adsorption (via multiplying oxygen vacancy defects) and reversible sulfuration-desulfuration reaction induced unique band alignment among SBO/ZnO/ZnS, contributes to the sensitive and selective response toward H₂S molecules. Novel SBO/ZnO (10%) sensor possesses excellent sensing H₂S performances, including a high response (107.6 for 10 ppm), low limit of detection of 20 ppb, good selectivity and long-term stability. Together with the merits of low operation temperature of 75 °C and weak humidity dependence (endowed by the hydrophobic SBO), present heterostructural SBO/ZnO sensor paves the way for the practical monitoring of trace H₂S pollutants in diverse workplaces including petroleum and natural gas industries.

Laser-generated BiVO4 colloidal particles with tailoring size and native oxygen defect for highly efficient gas sensing

To alleviate the poor sensing performance of BiVO₄, developing new strategies for the fabrication of unique device with improved sensing properties is very necessary and has great practical significance. In this work, size-tailored and uniform black BiVO₄ colloids with abundant oxygen vacancy were synthesized by a unique method of pulsed laser irradiation of colloidal nanoparticles (PLICN). The corresponding laser irradiation effects on the sensing properties are comparatively investigated. The results indicate that the BiVO₄ nanospheres with average size of 50 nm shows best sensing properties with high sensitivity, superior selectivity, low detection limit (44 ppb) to H₂S at low working temperature (75 °C). Its sensing response is over 4 times higher when comparing with that of the raw material. Further investigation manifests that laser irradiation could induce quantity of the oxygen vacancy and decrease the resistance of the sensing device, which is mainly responsible for the enhanced sensing performance. Moreover, the density functional theories (DFT) calculations suggest that the oxygen vacancies can greatly decrease the surface absorption energy with enhanced H₂S absorption capability on BiVO₄ surface and lower the bader charger transfer from the absorbed H₂S molecules to the BiVO₄, thus enabling the implementation for the enhanced gas-sensing properties.

An in situ electrospinning route to fabricate NiO-SnO2 based detectors for fast H2S sensing

Hydrogen sulfide (H₂S) is a toxic and flammable chemical, even in low concentration. In this study, an in situ electrospinning strategy was developed to directly deposit the sensitive materials of nickel oxide (NiO)-doped SnO₂ nanofiber on alumina substrates, resulting in the fast H₂S detection. The electrospun fiber could be deposited on to the alumina tube directly, and remain there during calcination. Using this method, the NiO-doped SnO₂ nanofibers fabricated and manifested a fast response, fast recovery, and high selectivity at a low temperature (150 °C). A 15% atom NiO-doped SnO₂ nanofiber-containing H₂S detector presented a high response (1352), low response time (23 s), and low recovery time (38 s) while detecting a concentration of 50 ppm H₂S at 150 °C. Compared to conventional methods, the H₂S detector based on the in situ electrospinning method showed a higher sensitivity, faster response, and faster recovery. Furthermore, the superior performance of the detector can be ascribed to the thinner film and non-interrupted fiber structure. Additionally, the transformation of NiO to Ni₃S₂, confirmed by the x-ray photoelectron spectroscopy under a H₂S atmosphere, suggested the main reason for the detector's high performance. The high performance of the NiO-doped SnO₂ suggests a strategy for gas detectors, biodetectors, and semiconductor devices.

A silver nanoparticle-anchored UiO-66(Zr) metal–organic framework (MOF)-based capacitive H2S gas sensor

Metal–organic frameworks anchored with metal oxide nanoparticles for the detection of H₂S gas with enhanced sensitivity.

ZnO-carbon nanofibers for stable, high response, and selective H2S sensors

Hydrogen sulfide (H₂S), as a typical atmospheric pollutant, is neurotoxic and flammable even at a very low concentration. In this study, we design stable H₂S sensors based on ZnO-carbon nanofibers. Nanofibers with 30.34 wt% carbon are prepared by a facial electrospinning route followed by an annealing treatment. The resulting H₂S sensors show excellent selectivity and response compared to the pure ZnO nanofiber H₂S sensors, particularly the response in the range of 102-50 ppm of H₂S. Besides, they exhibited a nearly constant response of approximately 40-20 ppm of H₂S over 60 days. The superior performance of these H₂S sensors can be attributed to the protection of carbon, which ensures the high stability of ZnO, and oxygen vacancies that improve the response and selectivity of H₂S. The good performance of ZnO-carbon H₂S sensors suggests that composites with oxygen vacancies prepared by a facial electrospinning route may provide a new research strategy in the field of gas sensors, photocatalysts, and semiconductor devices.

Metal Oxide Nanostructures in Food Applications: Quality Control and Packaging

Metal oxide materials have been applied in different fields due to their excellent functional properties. Metal oxides nanostructuration, preparation with the various morphologies, and their coupling with other structures enhance the unique properties of the materials and open new perspectives for their application in the food industry. Chemical gas sensors that are based on semiconducting metal oxide materials can detect the presence of toxins and volatile organic compounds that are produced in food products due to their spoilage and hazardous processes that may take place during the food aging and transportation. Metal oxide nanomaterials can be used in food processing, packaging, and the preservation industry as well. Moreover, the metal oxide-based nanocomposite structures can provide many advantageous features to the final food packaging material, such as antimicrobial activity, enzyme immobilization, oxygen scavenging, mechanical strength, increasing the stability and the shelf life of food, and securing the food against humidity, temperature, and other physiological factors. In this paper, we review the most recent achievements on the synthesis of metal oxide-based nanostructures and their applications in food quality monitoring and active and intelligent packaging.