A Room-Temperature CNT/Fe₃O₄ Based Passive Wireless Gas Sensor

Recent trends in gas sensing via carbon nanomaterials: outlook and challenges

The presence of harmful and poisonous gases in the environment can have dangerous effects on human health, and therefore portable, flexible, and highly sensitive gas sensors are in high demand for environmental monitoring, pollution control, and medical diagnosis. Currently, the commercialized sensors are based on metal oxides, which generally operate at high temperatures. Additionally, the desorption of chemisorbed gas molecules is also challenging. Hence, due to the large surface area, high flexibility, and good electrical properties of carbon nanomaterials (CNMs) such as carbon nanotubes, graphene and their derivatives (graphene oxide, reduced graphene oxide, and graphene quantum dots), they are considered to be the most promising chemiresistive sensing materials, where their electrical resistance is affected by their interaction with the analyte. Further, to increase their selectivity, nanocomposites of CNMs with metal oxides, metallic nanoparticles, chalcogenides, and polymers have been studied, which exhibit better sensing capabilities even at room temperature. This review summarizes the state-of-the-art progress in research related to CNMs-based sensors. Moreover, to better understand the analyte adsorption on the surface of CNMs, various sensing mechanisms and dependent sensing parameters are discussed. Further, several existing challenges related to CNMs-based gas sensors are elucidated herein, which can pave the way for future research in this area.

Voltage standing wave ratio reading circuit design for inductance capacitance wireless passive ammonia sensors

It is not easy to conduct wired tests on sensors in harsh environments, and network analyzers are large, heavy, and inconvenient to carry. At the same time, the price of network analyzers is usually very high, which greatly limits their application. In this study, a voltage standing wave ratio reading circuit is designed to test inductively coupled (LC) wireless passive sensors. By introducing the theory of the standing wave ratio, the design concept and function of each module are analyzed from each specific module. The resonant frequency reading circuit of the sensor is designed and fabricated, and its sweep frequency range covers the frequency range of the commonly used LC wireless sensor, which widens the bandwidth measurement range. The main control chip adopts STM32 series, which makes the circuit and sensor module simple in structure and low in cost. This circuit can accurately obtain the resonant frequency of the sensor through the standing wave ratio and can measure the dynamic change in the sensor standing wave ratio. The output frequency range and output precision of the linear sweep source of the signal reading circuit were tested, and the dynamic testing ability of the circuit to the changing frequency was verified and improved the measurement accuracy.

Improving Gas-Sensing Performance Based on MOS Nanomaterials: A Review

In order to solve issues of air pollution, to monitor human health, and to promote agricultural production, gas sensors have been used widely. Metal oxide semiconductor (MOS) gas sensors have become an important area of research in the field of gas sensing due to their high sensitivity, quick response time, and short recovery time for NO₂, CO₂, acetone, etc. In our article, we mainly focus on the gas-sensing properties of MOS gas sensors and summarize the methods that are based on the interface effect of MOS materials and micro–nanostructures to improve their performance. These methods include noble metal modification, doping, and core-shell (C-S) nanostructure. Moreover, we also describe the mechanism of these methods to analyze the advantages and disadvantages of energy barrier modulation and electron transfer for gas adsorption. Finally, we put forward a variety of research ideas based on the above methods to improve the gas-sensing properties. Some perspectives for the development of MOS gas sensors are also discussed.

Green light-driven enhanced ammonia sensing at room temperature based on seed-mediated growth of gold-ferrosoferric oxide dumbbell-like heteronanostructures

Since there is excellent synergy between heterostructures and noble metals due to their unique electro-optical and catalytic properties, the introduction of noble metals into metal oxide semiconductors has substantially improved the performance of gas sensors. However, most of the reported noble metal-metal oxide composites are generally prepared as simple hybrids; hence, there is lack of control over their structure, morphology and dimension. Herein, we report a seed-mediated growth of dumbbell-like Au-Fe₃O₄ heteronanostructured gas sensors for ammonia detection under green light illumination, in which the particle sizes of Au and Fe₃O₄ were readily tuned in a wide range. The ammonia gas-sensing performances of Au-Fe₃O₄ heteronanostructures were greatly improved at room temperature by regulating their dimensions. In particular, the sensitivity improved by 30% while the response and recovery time shortened by 20 s and 50 s for the 7.5 nm Au-loaded Fe₃O₄-based sensor toward 5 ppm ammonia under 520 nm green light illumination as compared to that in the absence of light. This can be ascribed to the localized surface plasmon effect of Au and the Schottky junction formed at the interface between Au and Fe₃O₄. Interestingly, the Au-Fe₃O₄ heteronanostructure exhibits a unique p-type to n-type reversible transition for ammonia detection due to the nature of Fe₃O₄ NPs related to the trade-off between oxygen vacancies and electron transfer caused by ammonia adsorption. In addition, the calculation based on first-principle theory reveals enhanced adsorption capacities of Fe₃O₄ for ammonia after Au-doping.

Special Issue "Advanced Nanomaterials Based Gas Sensors"

During the last several years, according to the works published in research journals, many nanostructured materials have been tested as sensing materials for gas-sensing applications. This trend has been observed for both metal oxides as well as carbon-based nanomaterials. More recently, it has also been extended to other materials based on chalcogenides. The field of applications for these sensors is very wide, including air quality, industrial safety and medical diagnosis, using different transducing mechanisms. Therefore, in this Special Issue, we have put together recent advances in this area.