A high-response formaldehyde sensor based on fibrous Ag-ZnO/In2O3 with multi-level heterojunctions

Highly Sensitive Ethylene Glycol Gas Sensor Based on MIL-68(In)@ZIF-8 Derivative

Ethylene glycol, as a colorless and tasteless organic compound, is an important industrial raw material but can be hazardous to the environment and human health. Thus, the development of high-performance sensing materials is required for the monitoring of ethylene glycol. In this paper, a method to synthesize In2O3@ZnO using MIL-68(In)@ZIF-8 to serve as a sacrificial template is proposed for testing ethylene glycol sensing capabilities. For verifying an effective improvement in gas-sensitive performance by bimetallic organic skeleton (MOF) synthesized heterojunctions, we performed gas-sensitive tests on In2O3, ZnO, and In2O3@ZnO. In2O3@ZnO has the best sensitivity to ethylene glycol, including ultrahigh response value (20 ppm-200.12), moderate response/recovery time (53/50 s), and excellent selectivity. The construction of heterojunction is the main reason for enhancing the ethylene glycol response of the sensor. On this basis, the gas-sensitive enhancement mechanism of composites is analyzed. The results show that the design method of synthesizing heterojunctions using bis-MOFs proposes a new approach that enhances the properties of ethylene glycol.

VOC Gas Sensors Based on Zinc Stannate Nanoparticles Decorated with Silver

Today, air pollution is a global environmental problem. A huge amount of explosive and combustible gas emissions that negatively affect nature and human health. Gas sensors are one of the ways to prevent this impact. Several types of sensors have been developed, but the main problem with them is the high operating temperature. This leads to a decrease in reproducibility and stability over time. The aim of the work is to synthesize zinc stannate nanoparticles, study their phase composition, and modify the structure with silver nanoparticles to improve gas-sensing characteristics. This paper presents the synthesis of zinc stannate nanoparticles by the coprecipitation method and annealing at varying temperatures. A method of decorating zinc stannate with silver nanoparticles was proposed. Using XRD, it was found that a change in the annealing temperature leads to a change in the initial phase composition of the sample. Decoration with silver nanoparticles allows for increasing the sensor response of zinc stannate layers to isopropyl alcohol by 20 times. The corresponding increase in response to ethanol is 16 times.

Room-Temperature, Flexible Formaldehyde Gas Sensors Using Titanium-Incorporated 1T/2H MoS2

The development of room-temperature (RT) formaldehyde sensors is significant for indoor air quality monitoring. In this contribution, titanium (Ti)-incorporated 1T/2H molybdenum disulfide (MoS2) phase heterostructures were prepared via a facile hydrothermal method. Compared with 1T/2H MoS2, the as-prepared 1T/2H Mo1-xTixS2 nanosheets showed a boosted RT formaldehyde sensing performance, which was attributed to the enhanced gas adsorption and charge transfer processes induced by Ti incorporation and could be regulated by tuning the Mo/Ti ratio in Mo1-xTixS2. In addition, the Ti-incorporated phase heterostructures with an optimized 1T concentration showed further improved gas-sensing properties compared to the highly metallic or semiconducting nanosheets with the same chemical composition, which exhibited an obvious response of 0.035% to 1 ppm formaldehyde with an excellent limit of detection as low as 39 ppb, good repeatability, and short response and recovery times. Moreover, flexible gas sensors based on the optimized nanosheets showed well-maintained responses under moderately bent conditions. Our findings demonstrate that the creation of phase heterostructures with tunable compositions and phases may provide more opportunities to tailor their sensing properties.

Magnetic Field Assisted Enhanced Sensitivity of Nonferromagnetic Materials Boosting the Carrier Transfer: Mechanistic Studies

The performance of semiconductor sensors is determined by reaction kinetics, conductivity, and electron mobility, which are undoubtedly closely related to the electron motion behavior. Therefore, the effective regulation of electronic states is crucial for improving gas sensing properties. Previous methods of enhancing the gas-sensing performance have induced complex material modifications, and the extent of performance improvement is usually very limited. Further optimization of the gas sensing performance requires continuous efforts to advance new technologies. Toward this issue, a novel magnetic field-induced strategy is adopted to boost the carrier transfer efficiency of nonferromagnetic semiconductors. The gas sensing investigation results manifest that the applied magnetic field can effectively enhance the sensitivity and reduce the baseline resistance. The In2O3 NC-2 (In2O3 nanocubes) with an applied magnetic field have a greatly enhanced response of 161.4 toward 100 ppm formaldehyde, which is 2.5 times higher than that without magnetic field. The enhanced gas sensing properties can be mainly attributed to magnetization of reactive materials, which makes the orientation of electronic magnetic moments consistent, thus greatly contributing to reactivity. This work introduces a practical approach to effectively improve gas sensing performance without further morphology optimization, noble metal catalysis, structural modification, and material cladding. The results of this study provide new insights for designing novel gas sensors to improve the gas sensing performance.

Recent Advances in Structure Design and Application of Metal Halide Perovskite-Based Gas Sensor

Metal halide perovskites (MHPs) are emerging gas-sensing materials and have attracted considerable attention in gas sensors due to their unique bandgap structure and tunable optoelectronic properties. The past decade has witnessed significant developments in the gas-sensing field; however, their intrinsic structural instability and ambiguous gas-sensing mechanisms hamper their practical applications. Herein, we summarize the recent advances in MHP-based gas sensors. The physicochemical properties of MHPs are discussed at first. The structure design, including dimension design and engineering design, is overviewed as well as their fabrication methods, and we put forward our insights into the gas-sensing mechanism of MHPs. It is believed that enhanced understanding of gas-sensing mechanisms of MHPs are helpful for their application as gas-sensing materials, and structure design can enhance their stability, sensing sensitivity, and selectivity to target gases as gas sensors. Subsequently, the latest developments in MHP-based gas sensors are summarized according to their different application scenarios. Finally, we conclude with the current status and challenges in this field and propose future perspectives.

Enhanced NO2 Gas Sensing Properties Based on Rb-Doped ZnO/In2O3 Heterojunctions at Room Temperature: A Combined DFT and Experimental Study

In this work, alkali metal Rb-loaded ZnO/In2O3 heterojunctions were synthesized using a combination of hydrothermal and impregnation methods. The morphology and structure of the synthesized samples were characterized by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. The enhancement mechanism of the nitrogen dioxide gas sensing performance of the Rb-loaded ZnO/In2O3 heterojunctions was systematically investigated at room temperature using density-functional theory calculations and experimental validation. The experimental tests showed that the Rb-loaded ZnO/In2O3 sensor achieved an excellent response value of 24.2 for 1 ppm NO2, with response and recovery times of 55 and 21 s, respectively. This result is 20 times higher than that of pure ZnO sensors and two times higher than that of ZnO/In2O3 sensors, indicating that the Rb-loaded ZnO/In2O3 sensor has a more pronounced enhancement in performance for NO2. This study not only revealed the mechanism by which Rb loading affects the electronic structure and gas molecule adsorption behavior on the surface of ZnO/In2O3 heterojunctions but also provides theoretical guidance and technical support for the development of high-performance room-temperature NO2 sensors.

Sensitive detection of H2S based on Ce doped ZnCo2O4 hollow microspheres at low working temperature

In order to develop a highly efficient H2S gas sensor at low working temperature, in this work, a kind of novel Ce-doped ZnCo2O4 hollow microspheres (Ce/ZnCo2O4 HMSs) were successfully synthesized using a template-free one-pot method, showing a sensitive response toward H2S. The microstructure and morphology of the material were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The gas-sensing performance of the composite was investigated, showing that the ZnCo2O4 doped with 6 mol% Ce had the highest response to 20 ppm H2S at a low operating temperature of 160 °C with a response value of 67.42, which was about 2 times higher than that of original ZnCo2O4. The prepared Ce/ZnCo2O4 HMS sensor in response to H2S exhibited a linear range of 0.1-200 ppm with a low detection limit of 0.1 ppm under the conditions of ambient humidity of 45% and ambient temperature of 20 °C. Meanwhile, it also possessed good selectivity, repeatability and reproducibility. The response value of the sensor decreased by 5.32% after 7 months of continuous monitoring of H2S in an atmospheric environment of a pig farm, indicating that the sensor had a long-term stability and continuous service life with important application prospects.

Preparation of Pt and bamboo charcoal co-modified TiO2 for formaldehyde sensing at room temperature

Anatase TiO2 has evolved into one of the most attractive materials for gas sensing owing to its strong oxidation activity and excellent sensing properties. In this study, we prepared Pt and bamboo charcoal co-modified nano-TiO2 using a one-pot hydrothermal process and applied it to detect formaldehyde. The successful incorporation of the precious metal Pt and bamboo charcoal onto TiO2 was confirmed by scanning electron microscope, transmission electron microscopy, energy dispersive spectrometer, X-ray diffraction and X-ray photoelectron spectroscopy. Detailed analysis revealed a homogeneous distribution of Pt nanoparticles and bamboo charcoal on the TiO2 surface, which significantly improved the surface area and facilitated gas adsorption. These modifiers significantly enhanced the response of TiO2 to formaldehyde, for instance, the response signal increased fourfold, while the response time decreased from 91 to 68 s. The sample with 0.5@Pt and 0.5@C bamboo charcoal performed the best, showcasing the synergistic effect of metal nanoparticles and carbonaceous materials on gas-sensing properties. Our work highlighted the potential of using biomass-derived carbon to enhance the detection of formaldehyde and demonstrated the importance of material characteristics in designing effective gas sensors.

Enhanced removal of Pb(II) from acid mine drainage using green reduced graphene oxide/silver nanoparticles

Mining activities can potentially release high levels of Pb(II) in acid mine drainage (AMD), which thereafter poses a significant threat to ecological security. In this study, green reduced graphene oxide/silver nanoparticles (rGO/Ag NPs) were successfully synthesized via a one-step approach using a green tea extract and subsequently used as a cost-effective absorbent to remove Pb(II) from AMD. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy indicated that organic functional groups in the green tea extracts, such as C=O-C, CO, and CC, acted both as reductants and stabilizers in the synthesis of rGO/Ag NPs. In addition, the removal efficiency of Pb(II) by rGO/Ag NPs (84.2 %) was much better than either rGO (75.4 %) or Ag NPs (12.3 %) alone. Also, in real AMD, the distribution coefficient (Kd) of Pb(II) (4528 mL/g), was much higher than other heavy metal indicating the adsorbent had a high selective affinity for Pb(II). Interestingly, after five cycles of use, the removal efficiency of Pb(II) by rGO/Ag NPs from AMD actually increased from 46.4 to 65.2 % due to iron oxides (i.e., Fe2O3 and Fe3O4) being generated when rGO/Ag NPs was exposed to AMD. The removal of Pb(II) via adsorption on the rGO/Ag NPs surface involved formation of hexagonal rod-like precipitates. This work demonstrated the potential of rGO/Ag NPs to be continuously used for the removal of Pb(II) from AMD.

Wireless Gas Sensor Based on the Mesoporous ZnO-SnO2 Heterostructure Enables Ultrasensitive and Rapid Detection of 3-Methylbutyraldehyde

Achieving ultrasensitive and rapid detection of 3-methylbutyraldehyde is crucial for monitoring chemical intermediate leakage in pharmaceutical and chemical industries as well as diagnosing ventilator-associated pneumonia by monitoring exhaled gas. However, developing a sensitive and rapid method for detecting 3-methylbutyraldehyde poses challenges. Herein, a wireless chemiresistive gas sensor based on a mesoporous ZnO-SnO2 heterostructure is fabricated to enable the ultrasensitive and rapid detection of 3-methylbutyraldehyde for the first time. The mesoporous ZnO-SnO2 heterostructure exhibits a uniform spherical shape (∼79 nm in diameter), a high specific surface area (54.8 m2 g-1), a small crystal size (∼4 nm), and a large pore size (6.7 nm). The gas sensor demonstrates high response (18.98@20 ppm), short response/recovery times (13/13 s), and a low detection limit (0.48 ppm) toward 3-methylbutyraldehyde. Furthermore, a real-time monitoring system is developed utilizing microelectromechanical systems gas sensors. The modification of amorphous ZnO on the mesoporous SnO2 pore wall can effectively increase the chemisorbed oxygen content and the thickness of the electron depletion layer at the gas-solid interface, which facilitates the interface redox reaction and enhances the sensing performance. This work presents an initial example of semiconductor metal oxide gas sensors for efficient detection of 3-methylbutyraldehyde that holds great potential for ensuring safety during chemical production and disease diagnosis.

General Model for Predicting Response of Gas-Sensitive Materials to Target Gas Based on Machine Learning

Gas sensors play a crucial role in various industries and applications. In recent years, there has been an increasing demand for gas sensors in society. However, the current method for screening gas-sensitive materials is time-, energy-, and cost-consuming. Consequently, an imperative exists to enhance the screening efficiency. In this study, we proposed a collaborative screening strategy through integration of density functional theory and machine learning. Taking zinc oxide (ZnO) as an example, the responsiveness of ZnO to the target gas was determined quickly on the basis of the changes in the electronic state and structure before and after gas adsorption. In this work, the adsorption energy and electronic and structural characteristics of ZnO after adsorbing 24 kinds of gases were calculated. These computed features served as the basis for training a machine learning model. Subsequently, various machine learning and evaluation algorithms were utilized to train the fast screening model. The importance of feature values was evaluated by the AdaBoost, Random Forest, and Extra Trees models. Specifically, charge transfer was assigned importance values of 0.160, 0.127, and 0.122, respectively, ranking as the highest among the 11 features. Following closely was the d-band center, which was presumed to exert influence on electrical conductivity and, consequently, adsorption properties. With 5-fold cross-validation using the Extra Tree accuracy, the 24-sample data set achieved an accuracy of 88%. The 72-sample data set achieved an accuracy of 78% using multilayer perceptron after 5-fold cross-validation, with both data sets exhibiting low standard deviations. This verified the accuracy and reliability of the strategy, showcasing its potential for rapidly screening a material's responsiveness to the target gas.

Thermodynamically and Kinetically Enhanced Benzene Vapor Sensor Based on the Cu-TCPP-Cu MOF with Extremely Low Limit of Detection

As a carcinogenic and highly neurotoxic hazardous gas, benzene vapor is particularly difficult to be distinguished in BTEX (benzene, toluene, ethylbenzene, xylene) atmosphere and be detected in low concentrations due to its chemical inertness. Herein, we develop a depth-related pore structure in Cu-TCPP-Cu to thermodynamically and kinetically enhance the adsorption of benzene vapor and realize the detection of ultralow-temperature benzene gas. We find that the in-plane π electronic nature and proper pore sizes in Cu-TCPP-Cu can selectively induce the adsorption and diffusion of BTEX. Interestingly, the theoretical calculations (including density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations) exhibit that benzene molecules are preferred to adsorb and array as a consecutive arrangement mode in the Cu-TCPP-Cu pore, while the TEX (toluene, ethylbenzene, xylene) dominate the jumping arrangement model. The differences in distribution behaviors can allow adsorption and diffusion of more benzene molecules within limited room. Furthermore, the optimal pore-depth range (60-65 nm) of Cu-TCPP-Cu allows more exposure of active sites and hinders the gas-blocking process. The optimized sensor exhibits ultrahigh sensitivity to benzene vapor (155 Hz/μg@1 ppm), fast response time (less than 10 s), extremely low limit of detection (65 ppb), and excellent selectivity (83%). Our research thus provides a fundamental understanding to design and optimize two-dimensional metal-organic framework (MOF)-based gas sensors.

Interface Defects and Carrier Regulation in MOF-Derived Co3O4/In2O3 Composite Materials for Enhanced Selective Detection of HCHO

Gas sensors for real-time monitoring of low HCHO concentrations have promising applications in the field of health protection and air treatment, and this work reports a novel resistive gas sensor with high sensitivity and selectivity to HCHO. The MOF-derived hollow In2O3 was mixed with ZIF-67(Co) and calcined twice to obtain a hollow Co3O4/In2O3 (hereafter collectively termed MZO-6) composite enriched with oxygen vacancies, and tests such as XPS and EPR proved that the strong interfacial electronic coupling increased the oxygen vacancies. The gas-sensitive test results show that the hollow composite MZO-6 with abundant oxygen vacancies has a higher response value (11,003) to 10 ppm of HCHO and achieves a fast response/recovery time (11/181 s) for HCHO at a lower operating temperature (140 °C). The MZO-6 material significantly enhances the selectivity to HCHO and reduces the interference of common pollutant gases such as ethanol, acetone, and xylene. There is no significant fluctuation of resistance and response values in the 30-day long-term stability test, and the material has good stability. The synergistic effect of the heterostructure and oxygen vacancies altered the formaldehyde adsorption intermediate pathway and reduced the reaction activation energy, enhancing the HCHO responsiveness and selectivity of the MZO-6 material.

Proposals for gas-detection improvement of the FeMPc monolayer towards ethylene and formaldehyde by using bimetallic synergy

Development and fabrication of a novel gas sensor with superb performance are crucial for enabling real-time monitoring of ethylene (C2H4) and formaldehyde (H2CO) emissions from industrial manufacture. Herein, first-principles calculations and AIMD simulations were carried out to investigate the effect of the Fe-M dimer on the adsorption of C2H4 and H2CO on metal dimer phthalocyanine (FeMPc, M = Ti-Zn) monolayers, and the electronic structures and sensing properties of the above adsorption systems were systematically discussed. The results show that the FeMPc (M = Ti, V, Cr, Mn) monolayers interact with C2H4 and H2CO by chemisorption except for the FeMnPc/H2CO system, while the other adsorption systems are all characterized by physisorption. Interestingly, the adsorption strength of C2H4 and H2CO can be effectively regulated by the bimetallic synergy of the Fe-M dimer. Moreover, the FeCrPc and FeMnPc monolayers exhibit excellent sensitivity towards C2H4 and H2CO, and have short recovery time (4.69 ms-2.31 s) for these gases at room temperature due to the effective surface diffusion at 300 K. Consequently, the FeCrPc and FeMnPc materials can be utilized as high-performance, reusable gas sensors for detecting C2H4 and H2CO, and have promising applications in monitoring the release of ethylene and formaldehyde from industrial processes.

Recent Advances in Electrochemical Sensors for Formaldehyde

Formaldehyde, a ubiquitous indoor air pollutant, plays a significant role in various biological processes, posing both environmental and health challenges. This comprehensive review delves into the latest advancements in electrochemical methods for detecting formaldehyde, a compound of growing concern due to its widespread use and potential health hazards. This review underscores the inherent advantages of electrochemical techniques, such as high sensitivity, selectivity, and capability for real-time analysis, making them highly effective for formaldehyde monitoring. We explore the fundamental principles, mechanisms, and diverse methodologies employed in electrochemical formaldehyde detection, highlighting the role of innovative sensing materials and electrodes. Special attention is given to recent developments in nanotechnology and sensor design, which significantly enhance the sensitivity and selectivity of these detection systems. Moreover, this review identifies current challenges and discusses future research directions. Our aim is to encourage ongoing research and innovation in this field, ultimately leading to the development of advanced, practical solutions for formaldehyde detection in various environmental and biological contexts.

Nonstoichiometric Doping of La0.9FexSn1-xO3 Hollow Microspheres for an Ultrasensitive Formaldehyde Sensor

Oxygen vacancies play an essential role in gas-sensitive materials, but the intrinsic oxides are poorly controlled and contain low oxygen vacancy concentrations. In this work, we prepared La0.9Fe1-xSnxO3 microspheres with high sensitivity and controllability by a simple hydrothermal method, and then, we demonstrated that it has many oxygen ion defects by X-ray photoelectron spectroscopy and electron paramagnetic resonance characterization. The gas sensor exhibited ultrahigh response, specific recognition of formaldehyde gas, and excellent moisture resistance. By comparing the composites with different doping ratios, it was found that the highest catalytic activity was reached when x = 0.75, and the response value of La0.9Fe0.75Sn0.25O3 hollow microspheres at 200 °C reached 73-10 ppm of formaldehyde, which is 188% higher than that of intrinsic LaFeO3 hollow microspheres. On the one hand, due to the absence of A-site La3+ and the replacement of B-site Fe3+ by Sn4+, a large number of oxygen vacancies are induced on the surface and in the interior of the materials; on the other hand, it is also related to the large specific surface area and gas channels caused by the particular structure.

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.

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.

Highly Dispersed Pt-Incorporated Mesoporous Fe2O3 for Low-Level Sensing of Formaldehyde Gas

Highly dispersed Pt-incorporated mesoporous Fe₂O₃ (Pt/m-Fe₂O₃) of 4 μm size is prepared through a simple hydrothermal reaction and thermal decomposition procedures. Furthermore, the formaldehyde gas-sensing properties of Pt/m-Fe₂O₃ are investigated. Compared with our previous mesoporous Fe₂O₃-based gas sensors, a gas sensor based on 0.2% Pt/m-Fe₂O₃ shows improved gas response by over 90% in detecting low-level formaldehyde gas at 50 ppb concentration, an enhanced selectivity of formaldehyde gas, and a lower degradation of sensing performance in high-humidity environments. Additionally, the gas sensor exhibits similar properties as the previous sensor, such as operating temperature (275 °C) and long-term stability. The enhancement in formaldehyde gas-sensing performance is attributed to the attractive catalytic chemical sensitization of highly dispersed Pt nanoparticles in the mesoporous Fe₂O₃ microcube architecture.

Synthesis of Mesoporous Ag2O/SnO2 Nanospheres for Selective Sensing of Formaldehyde at a Low Working Temperature

Formaldehyde (HCHO) is a prevalent indoor gas pollutant that has been seriously endangering human health. Developing semiconductor metal oxide (SMO) gas sensors for selective measurement of formaldehyde at low working temperatures remains a great challenge. In this work, silver/tin-polyphenol hybrid spheres are applied as a sacrificial template for the fabrication of spherical mesoporous Ag₂O/SnO₂ sensing materials. The obtained mesoporous Ag₂O/SnO₂ spheres have a uniform particle size (∼80 nm), large pore size (5.8 nm), and high specific surface area (71.3 m² g^-1). The response is 140 toward formaldehyde (10 ppm) at a low working temperature (75 °C). The detection limit reaches a low level of 23.6 ppb. Most importantly, it has excellent selectivity toward interfering gases. When the concentration of the interfering gas (e.g., ethanol) is 5 times as high as that of formaldehyde, the response is little affected. Theoretical calculations suggest that the addition of Ag₂O can significantly enhance the adsorption energy toward formaldehyde, thus improving formaldehyde sensing performance. This work demonstrates an efficient self-template synthesis strategy for noble metal catalyst-decorated mesoporous metal oxide spheres, which could boost gas sensing performance at a lower working temperature.

Spray-Deposited Rare Earth Metal Ion (Sm3+, Ce3+, Pr3+, La3+)-Doped CdO Thin Films for Enhanced Formaldehyde Gas Sensing Characteristics

This study shows the electrical conductivity-dependent gas sensing characteristics of spray-deposited rare earth (RE) metal ion (Sm³⁺, Ce³⁺, Pr³⁺, La³⁺)-doped cadmium oxide (CdO) thin films on soda-lime microscope glass substrates at 300 °C. We examined the deposited films' structural, surface microstructural, DC electrical, and gas sensing features. The X-ray diffraction study indicates that all samples were polycrystalline, with the favored growth direction shifting from the (111) plane to the (200) plane. The highest root-mean-square values were obtained for the Pr-doped CdO thin film (5.86 nm). The surface microstructure of CdO thin films was significantly influenced by the RE metal ion dopant, with typical grain size values ranging from 64 nm to 134 nm depending on the dopant. The carrier concentration and resistivity of CdO films vary based on the RE metal ions used as dopants. Low resistivity (3.01 × 10^-4 Ω.cm) was achieved for the CdO thin film doped with La. High gas sensitivity (71.42%) was achieved for CdO thin films doped with La. The donor dopant regulated the electrical conductivity and gas sensing capabilities of CdO thin films.

Development of a Conductometric Sensor Based on Al,Ca-Doped ZnO for the Detection of Formaldehyde

In the present study, the development of a conductometric gas sensor based on Al,Ca-doped zinc oxide composite which is finalized to the detection of formaldehyde (HCHO) at a low concentration in air is investigated. The electrical and sensing properties of the composite based on ZnO doped with different loadings of Al and/or Ca (from 0 up to 5 at%) were evaluated. The gas-sensing mechanism of Al,Ca-doped zinc oxide nanocomposite-based sensors was also discussed. The optimized 3%Al,3%Ca-ZnO sensor displayed a formaldehyde response of 3.5 (@ 4 ppm HCHO/air) and an experimental low detection limit of 125 ppb HCHO/air, at the operating temperature of 400 °C. The sensor was also shown to be selective to HCHO with respect to many interferent indoor gases, but NO₂ changed the baseline resistance in an opposite way compared to the target gas. The developed device for monitoring HCHO in indoor and workplace environments has the advantage of a simple planar structure and can be easily fabricated for mass production by using low-cost materials and easy fabrication methods.

Thermal-Driven Formation of Silver Clusters Inside Na/Li FAUY Zeolites for Formaldehyde Detection

In this research, the LiY zeolite was firstly synthesized by using NaY as the parent zeolite; thereafter, the LiYAg and NaYAg zeolites created for formaldehyde gas detection were prepared with further Ag⁺-Li⁺/Na⁺ exchange and a mild thermal treatment at 300 °C to promote the formation of luminescent Ag CLs. The spectra experimental results indicated that Ag CLs showed stronger and blue-shifted emissions in LiYAg compared with in NaYAg, and the emission intensity of Ag CLs in both zeolites monotonously decreased when exposed to increasing formaldehyde gas content. Moreover, the linear dependence of the Ag CLs' emission intensity variation on formaldehyde content indicated a reliable method for fast and sensitive formaldehyde detection. According to the XPS, UV-vis absorption, and N₂ adsorption-desorption isotherm studies, the formaldehyde-gas-induced luminescence quenching of Ag CLs is due to the formation of Ag₂O and Ag NPs, in which the higher content of Ag⁺/Ag⁰ redox couples in LiYAg and larger surface area of NaYAg benefit the precise detection of formaldehyde gas in low- and high-content ranges, respectively. Furthermore, the blue-shifted peak position and widened FWHM of Ag CLs can also be used for the indication of formaldehyde gas and the detection limit of NaYAg and LiYAg, which both meet with the standards of the WHO and OSHA.

Advances in functional guest materials for resistive gas sensors

Resistive gas sensors are considered promising candidates for gas detection, benefiting from their small size, ease of fabrication and operation convenience. The development history, performance index, device type and common host materials (metal oxide semiconductors, conductive polymers, carbon-based materials and transition metal dichalcogenides) of resistive gas sensors are firstly reviewed. This review systematically summarizes the functions, functional mechanisms, features and applications of seven kinds of guest materials (noble metals, metal heteroatoms, metal oxides, metal-organic frameworks, transition metal dichalcogenides, polymers, and multiple guest materials) used for the modification and optimization of the host materials. The introduction of guest materials enables synergistic effects and complementary advantages, introduces catalytic sites, constructs heterojunctions, promotes charge transfer, improves carrier transport, or introduces protective/sieving/enrichment layers, thereby effectively improving the sensitivity, selectivity and stability of the gas sensors. The perspectives and challenges regarding the host-guest hybrid materials-based gas sensors are also discussed.

Edge-enriched MoS2 nanosheets modified porous nanosheet-assembled hierarchical In2O3 microflowers for room temperature detection of NO2 with ultrahigh sensitivity and selectivity

Nitrogen dioxide (NO₂) is one of the most hazardous toxic pollutants to human health and the environment. However, deficiencies of low sensitivity and poor selectivity at room temperature (RT) restrain the application of NO₂ sensors. Herein, the edge-enriched MoS₂ nanosheets modified porous nanosheets-assembled three-dimensional (3D) In₂O₃ microflowers have been synthesized to improve the sensitivity and selectivity of NO₂ detection at RT. The results show that the In₂O₃/MoS₂ composite sensor exhibits a response as high as 343.09-5 ppm NO₂, which is 309 and 72.5 times higher than the sensors based on the pristine MoS₂ and In₂O₃. The composite sensor also shows short recovery time (37 s), excellent repeatability and long-term stability. Furthermore, the response of the In₂O₃/MoS₂ sensor to NO₂ is at least 30 times higher than that of other gases, proving the ultrahigh selectivity of the sensor. The outstanding sensing performance of the In₂O₃/MoS₂ sensor can be attributed to the synergistic effect and abundant active sites originating from the p-n heterojunction, exposed edge structures and the designed 2D/3D hybrid structure. The strategy proposed herein is expected to provide a useful reference for the development of high-performance RT NO₂ sensors.

Constructing a Channel to Regulate the Electron-Transfer Behavior of CO Adsorption and Light-Driven CO Reduction by H2 over CuO-ZnO

Photocatalytic conversions of C1 molecules under mild conditions have been widely researched in many fields. Adsorption of reactants at a catalyst surface is an indispensable process for C1 conversion and thus it might play a key role in reaction behavior. Herein, for a ZnO sample without photocatalytic activity for CO + H₂ reduction, CuO is introduced into ZnO to regulate the adsorption behavior of CO on the CuO-ZnO surface and then to drive the reduction of CO by H₂ under UV irradiation. The results of gas sensitivity tests and various in situ characterization methods are as expected. Specifically, surface zinc vacancies and Cu²⁺ sites at the interface of ZnO and CuO cooperate to construct a special electron-transfer channel (Zn-O-Cu-O) for CO adsorption [CO (ads)]. A new linear adsorption mode of CO at Cu²⁺ sites occurs, and this successfully changes the electron-transfer behavior of CO (ads) from donating electrons (to ZnO) to accepting electrons (from CuO-ZnO) via electron-transfer channels and d-electrons of Cu²⁺ matching. Then, CO molecules are reduced by H₂ under UV irradiation. The strategy here provides an insight into the design of highly effective catalysts as well as an in-depth understanding of the mechanism of C1 photocatalytic conversion.

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.

Boosting room-temperature ppb-level NO2 sensing over reduced graphene oxide by co-decoration of α-Fe2O3 and SnO2 nanocrystals

As promising sensing materials, reduced graphene oxide (RGO)-based nanomaterials have drawn considerable attention in the fields of gas monitoring owing to their low operating temperature. However, constructing RGO-based room-temperature gas sensors possessing ppb-level limit of detection with high sensitivity remains challenging. In this work, a series of highly sensitive NO₂ sensors were fabricated using α-Fe₂O₃ and SnO₂ co-decorated RGO hybrids (designated as α-Fe₂O₃/SnO₂-RGO) as sensing materials. They were rationally synthesized by a one-pot hydrothermal method. Compared to SnO₂ modified RGO hybrids (SnO₂-RGO with bandgap of 3.88 eV), the bandgap energy of α-Fe₂O₃/SnO₂-RGO hybrids (3.53 eV) was reduced by adding α-Fe₂O_3; the narrower bandgap facilitated the sensing materials to release more electrons and form more oxygen ions at room temperature. Besides, the high carrier migration of RGO, which served as continuous phase, identical structure with ultrasmall particle size of α-Fe₂O₃ and SnO₂ (about 3-6 nm), and abundant chemisorbed oxygen species on the surface (20.8%) of the sensing materials, as well as their suitable bandgap (3.53 eV) in the sensing materials, significantly improved NO₂ response at room temperature. Among the sensors fabricated, α-Fe₂O₃/SnO₂-RGO-15-based NO₂ sensor had the highest response of 7.4 with a short response time of 59 s towards 1 ppm NO₂; it could even reach a response of 2.6 towards 100 ppb NO₂. Notably, α-Fe₂O₃/SnO₂-RGO-15 sample has excellent capability to recognize NO₂, where the response value (7.4) towards 1 ppm NO₂ is about 7 times higher than that of 100 ppm ammonia and common volatile organic compounds (formaldehyde, toluene, ethanol and acetone). Such NO₂ sensor has superior repeatability with negligible response deviation towards 1 ppm NO₂ for four reversible cycles. This makes it to have a great potential application in the field of NO₂ detection.

Nitrogen doped In2O3-ZnO nanocomposite mesoporous thin film based highly sensitive and selective ethanol sensors

Nanocomposite metal oxide thin films exhibit promising qualities in the field of gas sensors due to the opportunities provided by the heterointerface formation. In this work, we present the synthesis of nitrogen doped mesoporous In₂O₃-ZnO nanocomposite thin films by a simple wet chemical method using urea as the nitrogen precursor. SEM investigation suggests the formation of mesoporous nanocomposite thin films, where the uniformity of the surface pore distribution depends on the relative proportion of In₂O₃ and ZnO in the composites. HRTEM investigation suggests the formation of sharp interfaces between N-In₂O₃ and N-ZnO grains in the nanocomposite thin films. The nanocomposite thin films have been tested for their ethanol sensing performance over an extensive range of temperatures, ethanol vapor concentrations and relative humidities. Nitrogen doped nanocomposite thin films with an equal proportion of In₂O₃ and ZnO exhibit excellent ethanol sensing performance at a reasonable operating temperature (∼94% at 200 °C for 50 ppm of ethanol), fast response time (∼two seconds), stability over time, enhanced resilience against humidity and selectivity to ethanol over various other volatile organic compounds. All the results indicated that nitrogen doped In₂O₃/ZnO nanocomposite thin films portray great possibilities in designing improved performance ethanol sensors.

Semiconductor Gas Sensor for Triethylamine Detection

With the demanding detection of unique toxic gas, semiconductor gas sensors have attracted tremendous attention due to their intriguing features, such as, high sensitivity, online detection, portability, ease of use, and low cost. Triethylamine, a typical gas of volatile organic compounds, is an important raw material for industrial development, but it is also a hazard to human health. This review presents a concise compilation of the advances in triethylamine detection based on chemiresistive sensors. Specifically, the testing system and sensing parameters are described in detail. Besides, the sensing mechanism with characterizing tactics is analyzed. The research status based on various chemiresistive sensors is also surveyed. Finally, the conclusion and challenges, as well as some perspectives toward this area, are presented.

N-doped three-dimensional needle-like CoS2 bridge connection Co3O4 core-shell structure as high-efficiency room temperature NO2 gas sensor

The N-doped three-dimensional (3D) needle bridge connection core-shell structure N-CoS₂@Co₃O₄ synthesized in this work was prepared by simple hydrothermal and high-temperature vulcanization methods. The optimized N-CoS₂@Co₃O₄^-2 composite response to NO₂ is 62.3-100 ppm, a response time of 1.3 s, the recovery time of 17.98 s, the detection limit of 5 ppb and stability of as long as 10 weeks at room temperature (RT). Its excellent NO₂ sensing performance is attributed to the unique porous and bridge connection core-shell structure of the N-CoS₂@Co₃O₄^-2 with high specific surface area, interconnected internal channels, abundant exposed S edge active sites, and high catalytic performance promoted by N-doping. This simple manufacturing method of high-performance sensing materials paves the way for the design of N-doped bridge connection core-shell structures.

Nanomaterials for IoT Sensing Platforms and Point-of-Care Applications in South Korea

Herein, state-of-the-art research advances in South Korea regarding the development of chemical sensing materials and fully integrated Internet of Things (IoT) sensing platforms were comprehensively reviewed for verifying the applicability of such sensing systems in point-of-care testing (POCT). Various organic/inorganic nanomaterials were synthesized and characterized to understand their fundamental chemical sensing mechanisms upon exposure to target analytes. Moreover, the applicability of nanomaterials integrated with IoT-based signal transducers for the real-time and on-site analysis of chemical species was verified. In this review, we focused on the development of noble nanostructures and signal transduction techniques for use in IoT sensing platforms, and based on their applications, such systems were classified into gas sensors, ion sensors, and biosensors. A future perspective for the development of chemical sensors was discussed for application to next-generation POCT systems that facilitate rapid and multiplexed screening of various analytes.

Electronic and morphological dual modulation of NiO by indium-doping for highly improved xylene sensing

Ultrathin indium-doped NiO nanosheets with simultaneously optimized nanostructures and electronic properties were developed to achieve a high response to a low concentration of xylene.

Enhanced Sensitivity of Hydrogenated Cu0.27Co2.73O4 Nanooctahedrons Having {111} Facets and the Sensing Mechanism of 3-Coordinated Co/Cu Atoms as Active Centers

Cu_0.27Co_2.73O₄ nanooctahedrons enclosed by polar {111} planes have been prepared through the selective adsorption of Cl^-. Hydrogenation has been successfully used to enhance the responses of the Cu_0.27Co_2.73O₄ nanooctahedron sensors to acetone, ethanol, and n-butylamine. The enhancement of the response results from the increase in the number of 3-coordinated Co/Cu atoms (Co_3c/Cu_3c) at the (111) plane of Cu_0.27Co_2.73O₄ through removing O-H groups and Cl^- ions at the surface by hydrogenation. The Co_3c/Cu_3c atoms on the (111) plane of Cu_0.27Co_2.73O₄ are considered to function as the gas response active centers. These Co_3c/Cu_3c active atoms have three functions: generating electrons, adsorbing oxygen from air, and catalyzing the sensing reactions. The hydrogenation polar surface approach can be applied to improve the performances of other sensing materials. Such sensing mechanisms of the Co_3c/Cu_3c unsaturated atoms as the active centers can be conducive to understanding the gas-sensing essence and the development of sensing materials with high performances.

Effect of Ag Addition on the Gas-Sensing Properties of Nanostructured Resistive-Based Gas Sensors: An Overview

Nanostructured semiconducting metal oxides (SMOs) are among the most popular sensing materials for integration into resistive-type gas sensors owing to their low costs and high sensing performances. SMOs can be decorated or doped with noble metals to further enhance their gas sensing properties. Ag is one of the cheapest noble metals, and it is extensively used in the decoration or doping of SMOs to boost the overall gas-sensing performances of SMOs. In this review, we discussed the impact of Ag addition on the gas-sensing properties of nanostructured resistive-based gas sensors. Ag-decorated or -doped SMOs often exhibit better responsivities/selectivities at low sensing temperatures and shorter response times than those of their pristine counterparts. Herein, the focus was on the detection mechanism of SMO-based gas sensors in the presence of Ag. This review can provide insights for research on SMO-based gas sensors.

CdO-ZnO nanorices for enhanced and selective formaldehyde gas sensing applications

This paper reports synthesis, properties and gas sensing applications of ZnO nanoflowers and CdO-ZnO nanorices prepared by hydrothermal process. The morphological characterizations confirmed the formation of well-defined nanoflowers and nanorices structures for ZnO and CdO-ZnO nanomaterials, respectively. The structural properties revealed the wurtzite hexagonal phase of the synthesized materials. The sensor devices based on ZnO nanoflowers and CdO-ZnO nanorices were fabricated and tested towards various gases including ethanol, methanol, ammonia, carbon monoxide, methane and formaldehyde. The fabricated gas sensor based on CdO-ZnO nanorices exhibited a high response (34.5) towards 300 ppm formaldehyde gas at 350 °C compared to ZnO nanoflowers (14.5) under the same experimental conditions. The response and recovery times for ZnO nanoflowers-based sensor were~9.8 s and ~6 s while for CdO-ZnO based sensor, these were ~10s and ~6s, respectively. A rapid response (34.5) for CdO-ZnO nanorices based formaldehyde gas sensor was observed as compared to other gases such as ammonia (12.3), methanol (16.5), ethanol (20), carbon monoxide (16.3) and methane (12.4), which confirm the high-selectivity towards formaldehyde gas. Finally, a plausible formaldehyde gas sensing mechanism is proposed.

Strategies for Improving the Sensing Performance of Semiconductor Gas Sensors for High-Performance Formaldehyde Detection: A Review

Formaldehyde is a poisonous and harmful gas, which is ubiquitous in our daily life. Long-term exposure to formaldehyde harms human body functions; therefore, it is urgent to fabricate sensors for the real-time monitoring of formaldehyde concentrations. Metal oxide semiconductor (MOS) gas sensors is favored by researchers as a result of their low cost, simple operation and portability. In this paper, the mechanism of formaldehyde detection by gas sensors is introduced, and then the ways of ameliorating the response of gas sensors for formaldehyde detection in recent years are summarized. These methods include the control of the microstructure and morphology of sensing materials, the doping modification of matrix materials, the development of new semiconductor sensing materials, the outfield control strategy and the construction of the filter membrane. These five methods will provide a good prerequisite for the preparation of better performing formaldehyde gas sensors.

TiO2hierarchical nano blooming-flower decorated by Pt for formaldehyde detection

To achieve an ultra-low concentration formaldehyde detection at low temperature, a platinum (Pt) assisted TiO₂hierarchical nano blooming-flower sphere material is synthesized through hydrothermal method. SEM and transmission electron microscope characterizations show that the diameter of the nano sphere was around 2μm with dissilient rods of 60 nm in diameter and 1μm in length on the surface. The response (R_a/R_g) achieved form this nanomaterial to HCHO is 1.08 (100 ppb) and 5.82 (5 ppm) at 130 °C without an involvement of any light source or solution. The relationship curve between the responses and concentrations shows regular exponential trend. The verification of sensor stability done by a 3 month reliability test shows no response-degradation. The optimal response and stability is attributed to the massive dissilient rods on the surface of TiO₂spheres and the assistance of Pt as a catalyzer disperses to intensify the formation of depletion area on the surface of TiO₂. This study provide an attractive and cost effective solution for the detection of HCHO in air at a relatively low temperature.

0D/2D CdS/ZnO composite with n-n heterojunction for efficient detection of triethylamine

It is imperative to seek for novel materials with pronounced gas sensing performance towards triethylamine for the sake of human health. Herein, we successfully fabricate an outstanding triethylamine sensor based on CdS/ZnO composite with 0D/2D structure, which are prepared by in-situ growth of CdS quantum dots on ultra-thin ZnO nanosheets. The ratios between the two ingredients are adjusted and their effect is evaluated. The optimal sample exhibits the lowest operating temperature of 200 °C, the highest response value of ~20 and the fastest response time of 2 s. Besides, it also has the virtues of durable stability, excellent selectivity and superior anti-interference ability. The mechanism behind the aforementioned intriguing performance is investigated by X-ray photoelectron spectroscopy, Kelvin probe and density function theory (DFT) simulation. All the results verify that the enhanced gas sensing properties are derived from splendid 0D/2D structure, n-n heterojunction and large specific surface area. Additionally, this study opens an avenue for designing sensors with 0D/2D structure.