Magnetic gas sensing: working principles and recent developments

Gas sensors work on the principle of transforming the gas adsorption effects on the surface of the active material into a detectable signal in terms of its changed electrical, optical, thermal, mechanical, magnetic (magnetization and spin), and piezoelectric properties. In magnetic gas sensors, the change in the magnetic properties of the active materials is measured by one of the approaches such as Hall effect, magnetization, spin orientation, ferromagnetic resonance, magneto-optical Kerr effect, and magneto-static wave oscillation effect. The disadvantages of different types of gas sensors include their chemical selectivity and sensitivity to humidity and high-temperature operation. For example, in the case of chemiresistive-type gas sensors, the change in the sensor resistance can drastically vary in the real environment due to the presence of other gas species and the overall electrical effect is quite complex due to simultaneous surface reactions. Further, it is not easy to make stable contacts for powdered samples for the conventional electrical property-based gas sensors. Fire hazard is another issue for the electrical property-based hydrogen gas sensors due to their flammable nature at higher operating temperature. In this regard, to solve these issues, magnetic gas sensor concepts have emerged, in which the magnetic properties of the materials get modified when exposed to gas molecules. In this review article, the working principles, fundamentals, recent developments, and future perspectives in magnetic gas sensors are reviewed. Finally, the prospects and opportunities in these exciting fields are also commented upon based on their current progress.

Water Electrolysis Using Thin Pt and RuO x Catalysts Deposited by a Flame-Annealing Method on Pencil-Lead Graphite-Rod Electrodes

An inexpensive, simple, and high-activity catalyst preparation method has been introduced in this work. Pt and RuO _x catalysts were fabricated by soaking inexpensive graphite electrodes (pencil-lead graphite rod: PGR) in catalyst precursor solutions and using a simple flame-annealing method, which results in lower amount of Pt and RuO _x catalyst layers. From X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure analysis, it has been found that platinum and ruthenium were deposited as zero-valence metal (Pt) and oxide (RuO _x ), respectively. Catalytic activities of Pt/PGR and RuO _x /PGR for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were evaluated using neutral 1 M Na₂SO₄ aqueous electrolyte, respectively. Although HER and OER currents using PGR without catalysts were -16 mA cm^-2 (at -1.5 V vs Ag/AgCl) and +20 mA cm^-2 (at +2.0 V vs Ag/AgCl), they were improved to -110 and +80 mA cm^-2 with catalysts (Pt and RuO _x ), respectively. Such an inexpensive and rapid catalyst electrode preparation method on PGR using flame-annealing is a very significant method in the initial catalyst activity evaluation requiring a large amount of trial and error.

CrI3-WTe2: A Novel Two-Dimensional Heterostructure as Multisensor for BrF3 and COCL2 Toxic Gases

A new multisensor (i.e. resistive and magnetic) CrI₃-WTe₂ heterostructure (HS) to detect the toxic gases BrF₃ and COCl₂ (Phosgene) has been theoretically studied in our present investigation. The HS has demonstrated sensitivity towards both the gases by varying its electronic and magnetic properties when gas molecule interacts with the HS. Fast recovery time (<0.14 fs) under UV radiation has been observed. We have considered two configurations of BrF₃ adsorbed HS; (1) when F ion interacts with HS (C1) and (2) when Br ion interacts with HS (C2). In C1 case the adsorption energy E_ad is observed to be −0.66 eV while in C2 it is −0.95 eV. On the other hand in case of COCl₂E_ad is found to be −0.42 eV. Magnetic moments of atoms are also found to vary upon gas adsorption indicates the suitability of the HS as a magnetic gas sensor. Our observations suggest the suitability of CrI₃-WTe₂ HS to respond detection of the toxic gases like BrF₃ and COCl₂.

Giant magneto-electric coupling in 100 nm thick Co capped by ZnO nanorods

Here we report a giant, completely reversible magneto-electric coupling of 100 nm polycrystalline Co layer in contact with ZnO nanorods. When the sample is under an applied bias of ±2 V, the Co magnetic coercivity is reduced by a factor 5 from the un-poled case, with additionally a reduction of total magnetic moment in Co. Taking into account the chemical properties of ZnO nanorods measured by X-rays absorption near edge spectroscopy under bias, we conclude that these macroscopic effects on the magnetic response of the Co layer are due to the microstructure and the strong strain-driven magneto-electric coupling induced by the ZnO nanorods, whose nanostructuration maximizes the piezoelectric response under bias.

Two-Dimensional Tetragonal GaN as Potential Molecule Sensors for NO and NO2 Detection: A First-Principle Study

Properties of gas molecules (NO, NH₃, and NO₂) adsorbed on two-dimensional GaN with a tetragonal structure (T-GaN) are studied using first-principles methods. Adsorption energy, adsorption distance, Hirshfeld charge, electronic properties, electric conductivity, and recovery time are calculated. It is found that these three molecules are all chemisorbed on the T-GaN with reasonable adsorption energies and apparent charge transfer. The electronic properties of the T-GaN present dramatic changes after the adsorption of NO₂ and NO molecules, especially its electric conductivity, but NH₃ molecule hardly changes the electronic properties of the T-GaN. Furthermore, the recovery time of the T-GaN sensor at T = 300 K is estimated to be quite short for NO₂ and NO but very long for NH₃. Moreover, the magnetic properties of the T-GaN are changed obviously due to the adsorption of NO (or NO₂) molecule. Therefore, we suggest that the T-GaN can be a prominent candidate for application as NO₂ and NO molecule sensors.

The Zn12O12 cluster-assembled nanowires as a highly sensitive and selective gas sensor for NO and NO2

Motivated by the recent realization of cluster-assembled nanomaterials as gas sensors, first-principles calculations are carried out to explore the stability and electronic properties of Zn₁₂O₁₂ cluster-assembled nanowires and the adsorption behaviors of environmental gases on the Zn₁₂O₁₂-based nanowires, including CO, NO, NO₂, SO₂, NH₃, CH₄, CO₂, O₂ and H₂. Our results indicate that the ultrathin Zn₁₂O₁₂ cluster-assembled nanowires are particularly thermodynamic stable at room temperature. The CO, NO, NO₂, SO₂, and NH₃ molecules are all chemisorbed on the Zn₁₂O₁₂-based nanowires with reasonable adsorption energies, but CH₄, CO₂, O₂ and H₂ molecules are only physically adsorbed on the nanowire. The electronic properties of the Zn₁₂O₁₂-based nanowire present dramatic changes after the adsorption of the NO and NO₂ molecules, especially their electric conductivity and magnetic properties, however, the other molecules adsorption hardly change the electric conductivity of the nanowire. Meanwhile, the recovery time of the nanowire sensor at T = 300 K is estimated at 1.5 μs and 16.7 μs for NO and NO₂ molecules, respectively. Furthermore, the sensitivities of NO and NO₂ are much larger than that of the other molecules. Our results thus conclude that the Zn₁₂O₁₂-based nanowire is a potential candidate for gas sensors with highly sensitivity for NO and NO₂.

Metal Oxide Gas Sensors, a Survey of Selectivity Issues Addressed at the SENSOR Lab, Brescia (Italy)

This work reports the recent results achieved at the SENSOR Lab, Brescia (Italy) to address the selectivity of metal oxide based gas sensors. In particular, two main strategies are being developed for this purpose: (i) investigating different sensing mechanisms featuring different response spectra that may be potentially integrated in a single device; (ii) exploiting the electronic nose (EN) approach. The former has been addressed only recently and activities are mainly focused on determining the most suitable configuration and measurements to exploit the novel mechanism. Devices suitable to exploit optical (photoluminescence), magnetic (magneto-optical Kerr effect) and surface ionization in addition to the traditional chemiresistor device are here discussed together with the sensing performance measured so far. The electronic nose is a much more consolidated technology, and results are shown concerning its suitability to respond to industrial and societal needs in the fields of food quality control and detection of microbial activity in human sweat.

Adsorption of gas molecules on a graphitic GaN sheet and its implications for molecule sensors

Motivated by the recent realization of two-dimensional nanomaterials as gas sensors, we have investigated the adsorption of gas molecules (SO₂, NO₂, HCN, NH₃, H₂S, CO, NO, O₂, H₂, CO₂, and H₂O) on the graphitic GaN sheet (PL-GaN) using density functional theory calculations.