Differential-Difference Equations. By Richard Bellman and Kenneth L. Cooke. Pp. xvi, 465. 1963. (Academic Press)

In recent years, gas sensors have been increasingly used in industrial production and daily life. Metal oxide semiconductor gas sensing materials are favoured for their outstanding physical and chemical properties, low cost and simple preparation methods. However, the gas sensing mechanisms of metal oxide semiconductors have not been considered by researchers, resulting in omissions and errors in the interpretation of gas sensing mechanisms in many articles. This review organizes and introduces several common gas sensing mechanisms of metal oxide semiconductors in detail and classifies them into two categories. The scope and relationship of these mechanisms are clarified. In addition, this review selects four strategies for enhancing the gas sensing properties of metal oxide semiconductors and analyses the gas sensing mechanisms to highlight the importance of the gas sensing mechanism. Finally, some perspectives for future investigations on the gas sensing mechanisms of metal oxide semiconductors are discussed as well.

Gas sensing mechanisms of metal oxide semiconductors: a focus review

A biosensor is an integrated receptor-transducer device, which can convert a biological response into an electrical signal The design and development of biosensors have taken a center stage for researchers or scientists in the recent decade owing to the wide range of biosensor applications, such as health care and disease diagnosis, environmental monitoring, water and food quality monitoring, and drug delivery The main challenges involved in the biosensor progress are (i) the efficient capturing of biorecognition signals and the transformation of these signals into electrochemical, electrical, optical, gravimetric, or acoustic signals (transduction process), (ii) enhancing transducer performance ie, increasing sensitivity, shorter response time, reproducibility, and low detection limits even to detect individual molecules, and (iii) miniaturization of the biosensing devices using micro-and nano-fabrication technologies Those challenges can be met through the integration of sensing technology with nanomaterials, which range from zero- to three-dimensional, possessing a high surface-to-volume ratio, good conductivities, shock-bearing abilities, and color tunability Nanomaterials (NMs) employed in the fabrication and nanobiosensors include nanoparticles (NPs) (high stability and high carrier capacity), nanowires (NWs) and nanorods (NRs) (capable of high detection sensitivity), carbon nanotubes (CNTs) (large surface area, high electrical and thermal conductivity), and quantum dots (QDs) (color tunability) Furthermore, these nanomaterials can themselves act as transduction elements This review summarizes the evolution of biosensors, the types of biosensors based on their receptors, transducers, and modern approaches employed in biosensors using nanomaterials such as NPs (eg, noble metal NPs and metal oxide NPs), NWs, NRs, CNTs, QDs, and dendrimers and their recent advancement in biosensing technology with the expansion of nanotechnology

A Review on Biosensors and Recent Development of Nanostructured Materials-Enabled Biosensors.

Metal oxide semiconductor (MOS) gas sensors possess extensive applications due to their high sensitivity, low cost, and simplicity To boost their excellent sensing performance and meet the growing demand for applications, a series of strategies have been developed, such as the surface morphology engineering and function manipulation Recently, the controlled morphology with exposed high-energy facets and the facet-dependent sensing properties have attracted much attention Because of its abundant unsaturated active sites, the crystal planes with high surface energy usually serve as promising platform for gas sensing After a lot of survey of literature, the authors provide a review of recent efforts on engineering crystal structures with exposed high-energy facets of MOS nanomaterials and their improved gas-sensitive performance, emphasis on six kinds of common gas-sensitive MOS including ZnO, SnO2, TiO2, α-Fe2O3, NiO and Cu2O Also, the relationship between dangling bonds density and gas-sensing properties has been systematically discussed and used as one significant factor to evaluate superior sensing surface of MOS According to the research and calculation, surface engineering by selectively exposing high-energy facets provides an effective way to obtain MOS gas-sensitive materials with superior performance The understanding of the facet-dependent properties of MOS will assist in and guide the fabrication of more excellent gas sensors in the future

An overview: Facet-dependent metal oxide semiconductor gas sensors

A review on recent progress of p-type nickel oxide based gas sensors: Future perspectives

The sensing characteristics of a Pt/NiO thin film-based resistor-type ammonia gas sensor are comprehensively studied and demonstrated. Experimentally, the studied Pt/NiO ammonia gas sensor exhibits improved performance, including a higher sensing response of ratio of 1278%, an extremely low detection limit of 10 ppb NH3/air, and fast speeds, at an optimal operating temperature of 300 °C. Based on the advantages indicated above and the benefits of its simple structure, relatively easy fabrication, and inherent p-type semiconductor properties, the studied device is promising for high-performance ammonia gas sensing and complementary metal oxide sensor (CMOS) array applications.

Characteristics of a Pt/NiO thin film-based ammonia gas sensor

An interesting ammonia gas sensor based on a p-type NiO thin film, prepared by a radio frequency sputtering process, is studied and demonstrated. As compared with conventional n-type metal-oxide sensors, the studied device shows comparable and good sensing performance, including a high-sensing response ratio of 289%, a lower response time of 31 s, and a lower recovery time of 78 s, under an introduced 1000 ppm NH3/air gas at 250 °C and 350 °C, respectively. In addition, the studied sensor device exhibits a lower detection limit (<5 ppm NH3/air) at 250 °C. Consequently, based on these advantages and inherent benefits of low cost, chemical stability, and easy fabrication, etc., the studied NiO thin-film sensor shows the promise for high-performance ammonia gas sensing applications.

On the Ammonia Gas Sensing Performance of a RF Sputtered NiO Thin-Film Sensor

Platinum nanoparticles (Pt NPs) are decorated on a sputtered tungsten tri-oxide (WO3) thin film to fabricate a high-performance ammonia gas sensor. These Pt NPs are synthesized by drop coating combined with UV irradiation approaches. Compared to a sensor comprising pristine WO3 thin film, the studied Pt NP/WO3 device shows a significant improvement in ammonia sensing response. The experimental results indicate that a high sensing response of 26.9 is measured under 1000 ppm NH3/air gas at 250 °C. Furthermore, even at a low concentration of 1 ppm NH3/air, a good sensing response of 3.73 is acquired from this device. The superior performance is attributed mainly to the catalytic activity and increased surface-to-volume ratio of Pt NPs. The relevant ammonia sensing properties and thermodynamic characteristics are also investigated. In addition, the Pt NP/WO3 device demonstrated in this study has advantages including a simple structure, low cost, a relatively easy fabrication process, and a wide concentration detection range.

Ammonia sensing performance of a platinum nanoparticle-decorated tungsten trioxide gas sensor

Hydrogen sensing performance of a nickel oxide (NiO) thin film-based device

An ammonia sensor based on a Pt/AlGaN/GaN Schottky diode, fabricated by the electroless plating (EP) technique, has been studied in this work. The studied sensor device shows a significant sensing response under an extremely low ammonia concentration of 10 ppb NH 3 /air at 115 °C. As exposed to a 1000 ppm NH 3 /air gas, a high sensing response of 16.22 with a response (recovery) time of 8.73 (2.04) min is obtained. Even at room temperature (25 °C), the studied sensor exhibits good ammonia sensing performance with a sensing response of 2.68 at 1000 ppm NH 3 /air and a low detection limit of 1 ppm NH 3 /air. Based on the excellent sensing performance and inherent advantages of low-power consumption and low-temperature operation, the studied sensor device provides the promise for high-performance ammonia sensing applications.

I-Ping Liu

Papers

Characteristics of a Pt/NiO thin film-based ammonia gas sensor

On the Ammonia Gas Sensing Performance of a RF Sputtered NiO Thin-Film Sensor

Ammonia sensing performance of a platinum nanoparticle-decorated tungsten trioxide gas sensor

Hydrogen sensing performance of a nickel oxide (NiO) thin film-based device

Study of an electroless plating (EP)-based Pt/AlGaN/GaN Schottky diode-type ammonia sensor