Role of graphene quantum dots with discrete band gaps on SnO2 nanodomes for NO2 gas sensors with an ultralow detection limit

NO₂ is a major air pollutant that should be monitored due to its harmful effects on the environment and human health. Semiconducting metal oxide-based gas sensors have been widely explored owing to their superior sensitivity towards NO₂, but their high operating temperature (>200 °C) and low selectivity still limit their practical use in sensor devices. In this study, we decorated graphene quantum dots (GQDs) with discrete band gaps onto tin oxide nanodomes (GQD@SnO₂ nanodomes), enabling room temperature (RT) sensing towards 5 ppm NO₂ gas with a noticeable response ((R_a/R_g) - 1 = 4.8), which cannot be matched using pristine SnO₂ nanodomes. In addition, the GQD@SnO₂ nanodome based gas sensor shows an extremely low detection limit of 1.1 ppb and high selectivity compared to other pollutant gases (H₂S, CO, C₇H₈, NH₃, and CH₃COCH₃). The oxygen functional groups in GQDs specifically enhance NO₂ accessibility by increasing the adsorption energy. Strong electron transfer from SnO₂ to GQDs widens the electron depletion layer at SnO₂, thereby improving the gas response over a broad temperature range (RT-150 °C). This result provides a basic perspective for utilizing zero-dimensional GQDs in high-performance gas sensors operating over a wide range of temperatures.

Flame Spray Pyrolysis Synthesis of WO3 Sensing Materials: Effects of Flame Parameters on Particle Size Distribution and NO2 Sensing Performance

In this study, the flame spray pyrolysis (FSP) technique was employed to produce WO₃ nanoparticles, which were subsequently used as sensing materials for NO₂ sensors. To enhance the sensing performance, the effects of flame parameters on the particle properties and sensing performances for 150-1200 ppb NO₂ at 125 °C were investigated. The results indicate that WO₃ particles with an average crystal size of about 10-20 nm and a standard deviation of about 3-7.5 nm were generated by controlling the precursor and dispersion oxygen flow rate of FSP. Based on the evaluation of NO₂ sensing performance, WO₃ sensing materials synthesized under the 3/5 flame condition exhibited better sensitivity than sensors made under other flame conditions. In summary, the FSP method and the optimization of flame synthesis parameters could be an effective strategy to prepare the sensing materials with high sensing performance.

A ZnO/ZnFe2O4 n–n heterojunction and Au loading synergistically improve the sensing performance of acetone

Response of an AAZF sensor to real breath samples from healthy volunteers and diabetic patients.

On-Chip Chemiresistive Sensor Array for On-Road NO x Monitoring with Quantification

The adverse effects of air pollution on respiratory health make air quality monitoring with high spatial and temporal resolutions essential especially in cities. Despite considerable interest and efforts, the application of various types of sensors is considered immature owing to insufficient sensitivity and cross-interference under ambient conditions. Here, a fully integrated chemiresistive sensor array (CSA) with parts-per-trillion sensitivity is demonstrated with its application for on-road NO x monitoring. An analytical model is suggested to describe the kinetics of the sensor responses and quantify molecular binding affinities. Finally, the full characterization of the system is connected to implement on-road measurements on NO x vapor with quantification as its ultimate field application. The obtained results suggest that the CSA shows potential as an essential unit to realize an air-quality monitoring network with high spatial and temporal resolutions.

Enhanced NO2-Sensing Properties of Au-Loaded Porous In2O3 Gas Sensors at Low Operating Temperatures

NO2-sensing properties of semiconductor gas sensors using porous In2O3 powders loaded with and without 0.5 wt% Au (Au/In2O3 and In2O3 sensors, respectively) were examined in wet air (70% relative humidity at 25 °C). In addition, the effects of Au loading on the increased NO2 response were discussed on the basis of NO2 adsorption/desorption properties on the oxide surface. The NO2 response of the Au/In2O3 sensor monotonically increased with a decrease in the operating temperature, and the Au/In2O3 sensor showed higher NO2 responses than those of the In2O3 sensor at a temperature of 100 °C or lower. In addition, the response time of the Au/In2O3 sensor was much shorter than that of the In2O3 sensor at 30 °C. The analysis based on the Freundlich adsorption mechanism suggested that the Au loading increased the adsorption strength of NO2 on the In2O3 surface. Moreover, the Au loading was also quite effective in decreasing the baseline resistance of the In2O3 sensor in wet air (i.e., increasing the number of free electrons in the In2O3), which resulted in an increase in the number of negatively charged NO2 species on the In2O3 surface. The Au/In2O3 sensor showed high response to the low concentration of NO2 (ratio of resistance in target gas to that in air: ca. 133 to 0.1 ppm) and excellent NO2 selectivity against CO and ethanol, especially at 100 °C.

Sensitive Cross-Linked SnO2:NiO Networks for MEMS Compatible Ethanol Gas Sensors

Nowadays, it is still technologically challenging to prepare highly sensitive sensing films using microelectrical mechanical system (MEMS) compatible methods for miniaturized sensors with low power consumption and high yield. Here, sensitive cross-linked SnO₂:NiO networks were successfully fabricated by sputtering SnO₂:NiO target onto the etched self-assembled triangle polystyrene (PS) microsphere arrays and then ultrasonically removing the PS microsphere templates in acetone. The optimum line width (~ 600 nm) and film thickness (~ 50 nm) of SnO₂:NiO networks were obtained by varying the plasma etching time and the sputtering time. Then, thermal annealing at 500 °C in H₂ was implemented to activate and reorganize the as-deposited amorphous SnO₂:NiO thin films. Compared with continuous SnO₂:NiO thin film counterparts, these cross-linked films show the highest response of ~ 9 to 50 ppm ethanol, low detection limits (< 5 ppm) at 300 °C, and also high selectivity against NO₂, SO₂, NH₃, C₇H₈, and acetone. The gas-sensing enhancement could be mainly attributed to the creating of more active adsorption sites by increased stepped surface in cross-linked SnO₂:NiO network. Furthermore, this method is MEMS compatible and of generality to effectively fabricate other cross-linked sensing films, showing the promising potency in the production of low energy consumption and wafer-scale MEMS gas sensors.

Electrospinning Nanoparticles-Based Materials Interfaces for Sensor Applications

Electrospinning is a facile technique to fabricate nanofibrous materials with adjustable structure, property, and functions. Electrospun materials have exhibited wide applications in the fields of materials science, biomedicine, tissue engineering, energy storage, environmental science, sensing, and others. In this review, we present recent advance in the fabrication of nanoparticles (NPs)-based materials interfaces through electrospinning technique and their applications for high-performance sensors. To achieve this aim, first the strategies for fabricating various materials interfaces through electrospinning NPs, such as metallic, oxide, alloy/metal oxide, and carbon NPs, are demonstrated and discussed, and then the sensor applications of the fabricated NPs-based materials interfaces in electrochemical, electric, fluorescent, colorimetric, surface-enhanced Raman scattering, photoelectric, and chemoresistance-based sensing and detection are presented and discussed in detail. We believe that this study will be helpful for readers to understand the fabrication of functional materials interfaces by electrospinning, and at the same time will promote the design and fabrication of electrospun nano/micro-devices for wider applications in bioanalysis and label-free sensors.

Gas sensors using ordered macroporous oxide nanostructures

Detection and monitoring of harmful and toxic gases have gained increased interest in relation to worldwide environmental issues. Semiconducting metal oxide gas sensors have been considered promising for the facile remote detection of gases and vapors over the past decades. However, their sensing performance is still a challenge to meet the demands for practical applications where excellent sensitivity, selectivity, stability, and response/recovery rate are imperative. Therefore, sensing materials with novel architectures and fabrication processes have been pursued with a flurry of research activity. In particular, the preparation of ordered macroporous metal oxide nanostructures is regarded as an intriguing candidate wherein ordered aperture sizes in the range from 50 nm to 1.5 μm can increase the chemical diffusion rate and considerably strengthen the performance stability and repeatability. This review highlights the recent advances in the fabrication of ordered macroporous nanostructures with different dimensions and compositions, discusses the sensing behavior evolution governed by structural layouts, hierarchy, doping, and heterojunctions, as well as considering their general principles and future prospects. This would provide a clear scale for others to tune the sensing performance of porous materials in terms of specific components and structural designs.

SnS2 Nanograins on Porous SiO2 Nanorods Template for Highly Sensitive NO2 Sensor at Room Temperature with Excellent Recovery

In order to develop high performance chemoresistive gas sensors for Internet of Everything applications, low power consumption should be achieved due to the limited battery capacity of portable devices. One of the most efficient ways to reduce power consumption is to lower the operating temperature to room temperature. Herein, we report superior gas sensing properties of SnS₂ nanograins on SiO₂ nanorods toward NO₂ at room temperature. The gas response is as high as 701% for 10 ppm of NO₂ with excellent recovery characteristics and the theoretical detection limit is evaluated to be 408.9 ppb at room temperature, which has not been reported for SnS₂-based gas sensors to the best of our knowledge. The SnS₂ nanograins on the template used in this study have excessive sulfur component (Sn:S = 1:2.33) and exhibit p-type conduction behavior. These results will provide a new perspective of nanostructured two-dimensional materials for gas sensor applications on demand.

p-p Heterojunction of Nickel Oxide-Decorated Cobalt Oxide Nanorods for Enhanced Sensitivity and Selectivity toward Volatile Organic Compounds

The utilization of p-p isotype heterojunctions is an effective strategy to enhance the gas sensing properties of metal-oxide semiconductors, but most previous studies focused on p-n heterojunctions owing to their simple mechanism of formation of depletion layers. However, a proper choice of isotype semiconductors with appropriate energy bands can also contribute to the enhancement of the gas sensing performance. Herein, we report nickel oxide (NiO)-decorated cobalt oxide (Co₃O₄) nanorods (NRs) fabricated using the multiple-step glancing angle deposition method. The effective decoration of NiO on the entire surface of Co₃O₄ NRs enabled the formation of numerous p-p heterojunctions, and they exhibited a 16.78 times higher gas response to 50 ppm of C₆H₆ at 350 °C compared to that of bare Co₃O₄ NRs with the calculated detection limit of approximately 13.91 ppb. Apart from the p-p heterojunctions, increased active sites owing to the changes in the orientation of the exposed lattice surface and the catalytic effects of NiO also contributed to the enhanced gas sensing properties. The advantages of p-p heterojunctions for gas sensing applications demonstrated in this work will provide a new perspective of heterostructured metal-oxide nanostructures for sensitive and selective gas sensing.

ppb level detection of NO2 using a WO3 thin film-based sensor: material optimization, device fabrication and packaging

In this study, we have investigated the thickness-dependent nitrogen dioxide (NO₂) sensing characteristics of a reactive-ion magnetron sputtered tungsten trioxide (WO₃) film, followed by morphological and electrical characterizations.

Highly dispersed Au nanoparticles decorated WO3 nanoplatelets: Laser-assisted synthesis and superior performance for detecting ethanol vapor

Loading of noble metal nanoparticles (NPs) on the surfaces of semiconductor oxides to form a hybrid nanostructure is an effective strategy to improve gas-sensing performance. In this study, WO₃ nanoplatelets decorated with Au NPs were prepared by laser ablation in liquids (LAL) with subsequent aging and annealing treatments. Results indicated that Au NPs with an average size of 7.8 ± 2.5 nm were highly dispersed on the surface of WO₃ nanoplatelets. As gas-sensing materials, the obtained Au-decorated WO₃ nanoplatelets showed lower operating temperature of 320 °C and higher response value of 3.5-fold in detecting ethanol molecules compared with pure WO₃ nanoplatelets. Moreover, Au-decorated WO₃ nanoplatelets displayed good selectivity toward ethanol compared with other tested vapors and excellent stability within several cycled measurements. These results can be ascribed to the supported Au NPs, which promote the adsorption and dissociation of oxygen species, eventually resulting in accelerated electron depletion on the surface of Au-WO₃ hybrids.

Chemiresistive Electronic Nose toward Detection of Biomarkers in Exhaled Breath

Detection of gas-phase chemicals finds a wide variety of applications, including food and beverages, fragrances, environmental monitoring, chemical and biochemical processing, medical diagnostics, and transportation. One approach for these tasks is to use arrays of highly sensitive and selective sensors as an electronic nose. Here, we present a high performance chemiresistive electronic nose (CEN) based on an array of metal oxide thin films, metal-catalyzed thin films, and nanostructured thin films. The gas sensing properties of the CEN show enhanced sensitive detection of H2S, NH3, and NO in an 80% relative humidity (RH) atmosphere similar to the composition of exhaled breath. The detection limits of the sensor elements we fabricated are in the following ranges: 534 ppt to 2.87 ppb for H2S, 4.45 to 42.29 ppb for NH3, and 206 ppt to 2.06 ppb for NO. The enhanced sensitivity is attributed to the spillover effect by Au nanoparticles and the high porosity of villi-like nanostructures, providing a large surface-to-volume ratio. The remarkable selectivity based on the collection of sensor responses manifests itself in the principal component analysis (PCA). The excellent sensing performance indicates that the CEN can detect the biomarkers of H2S, NH3, and NO in exhaled breath and even distinguish them clearly in the PCA. Our results show high potential of the CEN as an inexpensive and noninvasive diagnostic tool for halitosis, kidney disorder, and asthma.

Ag Nanoparticle-Sensitized WO3 Hollow Nanosphere for Localized Surface Plasmon Enhanced Gas Sensors

Ag nanoparticle (NP)-sensitized WO3 hollow nanospheres (Ag-WO3-HNSs) are fabricated via a simple sonochemical synthesis route. It is found that the Ag-WO3-HNS shows remarkable performance in gas sensors. Field-emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM) images reveal that the Agx-WO3 adopts the HNS structure in which WO3 forms the outer shell framework and the Ag NPs are grown on the inner wall of the WO3 hollow sphere. The size of the Ag NPs can be controlled by adjusting the addition amount of WCl6 during the reaction. The sensor Agx-WO3 exhibits extremely high sensitivity and selectivity toward alcohol vapor. In particular, the Ag(15nm)-WO3 sensor shows significantly lower operating temperature (230 °C), superior detection limits as low as 0.09 ppb, and faster response (7 s). Light illumination was found to boost the sensor performance effectively, especially at 405 and 900 nm, where the light wavelength resonates with the absorption of Ag NPs and the surface oxygen vacancies of WO3, respectively. The improved sensor performance is attributed to the localized surface plasmon resonance (LSPR) effect.

Structural and electronic engineering of 3DOM WO3 by alkali metal doping for improved NO2 sensing performance

Novel alkali metal doped 3DOM WO3 materials were prepared using a simple colloidal crystal template method. Raman, XRD, SEM, TEM, XPS, PL, Hall and UV-Vis techniques were used to characterize the structural and electronic properties of all the products, while the corresponding sensing performances targeting ppb level NO2 were determined at different working temperatures. For the overall goal of structural and electronic engineering, the co-effect of structural and electronic properties on the improved NO2 sensing performance of alkali metal doped 3DOM WO3 was studied. The test results showed that the gas sensing properties of 3DOM WO3/Li improved the most, with the fast response-recovery time and excellent selectivity. More importantly, the response of 3DOM WO3/Li to 500 ppb NO2 was up to 55 at room temperature (25 °C). The especially high response to ppb level NO2 at room temperature (25 °C) in this work has a very important practical significance. The best sensing performance of 3DOM WO3/Li could be ascribed to the most structure defects and the highest carrier mobility. And the possible gas sensing mechanism based on the model of the depletion layer was proposed to demonstrate that both structural and electronic properties are responsible for the NO2 sensing behavior.

Room-temperature volatile organic compounds sensing based on WO3·0.33H2O, hexagonal-WO3, and their reduced graphene oxide composites

The sensors based on WO₃·0.33H₂O, RGO-WO₃·0.33H₂O, h-WO₃, and RGO-h-WO₃ showed great VOCs sensing properties at room temperature and 55% relative humidity. The materials exhibited a p-type behavior. RGO improved the acetone sensing response.

Synthesis and highly enhanced acetylene sensing properties of Au nanoparticle-decorated hexagonal ZnO nanorings

Au nanoparticles with a size of 3–10 nm were decorated discretely on the surface of ZnO nanorings with the porous Au–ZnO nanorings showing highly enhanced acetylene-sensing properties.

Bio-templated fabrication of hierarchically porous WO3 microspheres from lotus pollens for NO gas sensing at low temperatures

Lotus pollen was used as a template to prepare WO₃ microspheres. The porous structure of the microspheres is ideal for gas sensing. The microsphere-based sensor has high sensitivity (S = 46.2) to 100 ppm NO gas with fast response and recovery speed 62 s/223 s) at 200 °C.

Bio-green synthesis of Ni-doped tin oxide nanoparticles and its influence on gas sensing properties

Using a novel, cost-effective and environmentally friendly biosynthesis method, Ni-doped SnO₂ nanoparticles have been synthesized. Gas sensing results suggest that the Ni-dopant is a promising additive to fabricate low cost SnO₂ based sensors.

Plate-like p-n heterogeneous NiO/WO₃ nanocomposites for high performance room temperature NO₂ sensors

Plate-like heterogeneous NiO/WO₃ nanocomposites have been successfully prepared by annealing Ni(OH)₂ and H₂WO₄ in air. These NiO/WO₃ nanocomposites have shown excellent sensitivity towards NO₂ and ultrafast response at room temperature due to their p-n heterogeneous characteristics.