High-temperature resistive gas sensors based on ZnO/SiC nanocomposites

Progress of One-Dimensional SiC Nanomaterials: Design, Fabrication and Sensing Applications

One-dimensional silicon carbide (SiC) nanomaterials hold great promise for a series of applications, such as nanoelectronic devices, sensors, supercapacitors, and catalyst carriers, attributed to their unique electrical, mechanical, and physicochemical properties. Recent progress in their design and fabrication has led to a deep understanding of the structural evolution and structure-property correlation. Several unique attributes, such as high electron mobility, offer SiC nanomaterials an opportunity in the design of SiC-based sensors with high sensitivity. In this review, a brief introduction to the structure and properties of SiC is first presented, and the latest progress in design and fabrication of one-dimensional SiC nanomaterials is summarized. Then, the sensing applications of one-dimensional SiC nanomaterials are reviewed. Finally, our perspectives on the important research direction and future opportunities of one-dimensional SiC nanomaterial for sensors are proposed.

Tailoring Structural, Chemical, and Photocatalytic Properties of ZnO@β-SiC Composites: The Effect of Annealing Temperature and Environment

For achieving unified functionalities of rare-earth free materials, the development of innovative zinc oxide and β-silicon carbide (ZnO@β-SiC) composites by a solid-state reaction method is presented. The evolution of zinc silicate (Zn₂SiO₄) is evidenced by X-ray diffraction when annealed in air beyond 700 °C. Detailed X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses reveal the involvement of silicon dioxide in forming Zn₂SiO₄. Transmission electron microscopy and the associated energy-dispersive X-ray spectroscopy elucidate the evolution of the zinc silicate phase at the ZnO/β-SiC interface, though it can be averted by vacuum annealing. These results manifest the importance of air in oxidizing SiC before a chemical reaction with ZnO from 700 °C. Finally, ZnO@β-SiC composites are found to be promising for methylene blue dye degradation under ultraviolet radiation, but the annealing above 700 °C is detrimental due to the evolution of a potential barrier in the presence of Zn₂SiO₄ at the ZnO/β-SiC interface.

Novel preparation of functional β-SiC fiber based In2O3 nanocomposite and controlling of influence factors for the chemical gas sensing

The gas sensing ability of a pure β-SiC fiber is limited due to its low-sensitivity and selectivity with poor recovery time during a gas sensing test. The combination of functional β-SiC fibers with metal-oxide (MO) can lead to excellent electronic conductivity, boosted chemical activity, and high reaction activity with the target gas and β-SiC–In₂O₃ sensor material. Influence factors such as amounts of MO, current collectors, and gas species (CO₂, O₂ and without gas) for the gas sensing ability of β-SiC–In₂O₃ nanocomposite were determined at standard room temperature (25 °C) and high temperature (350 °C) conditions. The gas sensing ability of the functional β-SiC fiber was significantly enhanced by the loading of In₂O₃ metal-oxide. In addition, the MO junction on the β-SiC fiber was mainly subjected to the Si–C–O–In bond sensor layer with an effective electron-transfer ability. The gas sensing mechanism was based on the transfer of charges, in which the sensing material acted as an absorber or a donor of charges. The sensor material could use different current- collectors to support the electron transfer and gas sensing ability of the material. A 1:0.5M SiC–In₂O₃ coated Ni-foil current collector sensor showed better sensing ability for CO₂ and O₂ gases than other gas sensors at room temperature and high temperature conditions. The sensing result of the electrode was obtained with different current density values without or with gas purging conditions because CO₂ and O₂ gases had electron acceptor properties. During the gas sensing test, the sensor material donated electrons to target gases. The current value on the CV graph then significantly changed. Our obtained sample analysis data and the gas sensing test adequately demonstrated that MO junctions on functional β-SiC fibers could improve the sensitivity of a sensor material and particularly upgrade the sensor material for gas sensing.

Selective detection of complex gas mixtures using point contacts: concept, method and tools

Of all modern nanosensors using the principle of measuring variations in electric conductance, point-contact sensors stand out in having a number of original sensor properties not manifested by their analogues. The nontrivial nature of point-contact sensors is based on the unique properties of Yanson point contacts used as the sensing elements. The quantum properties of Yanson point contacts enable the solution of some of the problems that could not be solved using conventional sensors measuring conductance. In the present paper, we demonstrate this by showing the potential of quantum point-contact sensors to selectively detect components of a gas mixture in real time. To demonstrate the high efficiency of the proposed approach, we analyze the human breath, which is the most complex of the currently known natural gas mixtures with extremely low concentrations of its components. Point-contact sensors allow us to obtain a spectroscopic profile of the mixture. This profile contains information about the complete set of energy interactions occurring in the point contact/breath system when the breath constituents adsorb to and desorb from the surface of the point-contact conduction channel. With this information we can unambiguously characterize the analyzed system, since knowing the energy parameters is key to successfully identifying and modeling the physicochemical properties of various quantum objects. Using the point-contact spectroscopic profile of a complex gas mixture it is possible to get a functional dependence of the concentration of particular breath components on the amplitude of the sensor output signal. To demonstrate the feasibility of the proposed approach, we analyze the point-contact profiles from the breath of several patients and compare them with the concentrations of serotonin and cortisol in the body of each patient. The obtained results demonstrate that the proposed methodology allows one to get an effective calibration function for a non-invasive analysis of the level of serotonin and cortisol in the human body using the point-contact breath test. The present study indicates some necessary prerequisites for the design of fast detection methods using differential sensor analysis in real time, which can be implemented in various areas of science and technology, among which medicine is one of the most important.

Nanostructured ZnO/Ag Film Prepared by Magnetron Sputtering Method for Fast Response of Ammonia Gas Detection

Possessing a large surface-to-volume ratio is significant to the sensitive gas detection of semiconductor nanostructures. Here, we propose a fast-response ammonia gas sensor based on porous nanostructured zinc oxide (ZnO) film, which is fabricated through physical vapor deposition and subsequent thermal annealing. In general, an extremely thin silver (Ag) layer (1, 3, 5 nm) and a 100 nm ZnO film are sequentially deposited on the SiO₂/Si substrate by a magnetron sputtering method. The porous nanostructure of ZnO film is formed after thermal annealing contributed by the diffusion of Ag among ZnO crystal grains and the expansion of the ZnO film. Different thicknesses of the Ag layer help the formation of different sizes and quantities of hollows uniformly distributed in the ZnO film, which is demonstrated to hold superior gas sensing abilities than the compact ZnO film. The responses of the different porous ZnO films were also investigated in the ammonia concentration range of 10 to 300 ppm. Experimental results demonstrate that the ZnO/Ag(3 nm) sensor possesses a good electrical resistance variation of 85.74% after exposing the sample to 300 ppm ammonia gas for 310 s. Interestingly, a fast response of 61.18% in 60 s for 300 ppm ammonia gas has been achieved from the ZnO/Ag(5 nm) sensor, which costs only 6 s for the response increase to 10%. Therefore, this controllable, porous, nanostructured ZnO film maintaining a sensitive gas response, fabricated by the physical deposition approach, will be of great interest to the gas-sensing community.

Langford S, Mohd Said S, R. Sarker M, Julai S, Hamid Md Ali S. Amino Anthraquinone: Synthesis, Characterization, and Its Application as an Active Material in Environmental Sensors

This work reports synthesis, thin film characterizations, and study of an organic semiconductor 2-aminoanthraquinone (AAq) for humidity and temperature sensing applications. The morphological and phase studies of AAq thin films are carried out by scanning electron microscope (SEM), atomic force microscope (AFM), and X-ray diffraction (XRD) analysis. To study the sensing properties of AAq, a surface type Au/AAq/Au sensor is fabricated by thermally depositing a 60 nm layer of AAq at a pressure of ~10⁻⁵ mbar on a pre-patterned gold (Au) electrodes with inter-electrode gap of 45 µm. To measure sensing capability of the Au/AAq/Au device, the variations in its capacitance and resistance are studied as a function of humidity and temperature. The Au/AAq/Au device measures and exhibits a linear change in capacitance and resistance when relative humidity (%RH) and temperature are varied. The AAq is a hydrophobic material which makes it one of the best candidates to be used as an active material in humidity sensors; on the other hand, its high melting point (575 K) is another appealing property that enables it for its potential applications in temperature sensors.