The term graphene-based gas sensors may be too broad, as there are many physicochemical differences within the graphene-based materials (GBM) used for chemiresistive gas sensors. These differences condition the sensitivity, selectivity, recovery, and ultimately the sensing performance of these devices towards air pollutants. Continuous ultraviolet irradiation aids in the desorption of gas molecules and enhances sensor performance. Under these conditions, the devices from this work can reliably monitor NO₂ and CO at room temperature, below the human-recommended exposure limits, presenting NO₂ LoD down to ∼20 ppb. By selecting GBMs with different levels of defectivity, which influence gas adsorption dynamics, and through comprehensive characterization, including D, D′, D″, 2D, and G Raman bands, graphene-based gas sensors can be tailored to meet specific sensing requirements. This study examines five different non-oxidized GBM to develop tools and gain a deeper understanding of the relationships between GBM properties and their sensing performance. This research introduces a new standard for defect assessment, moving beyond graphene's D and G Raman band intensity ratio, to facilitate the successful integration of graphene-based gas sensors into everyday applications, such as environmental monitoring and industrial safety, and potentially impacting other 2D materials, thereby reducing health risks associated with air pollution.

This work reports, for the first time, the use of spark ablation with impaction printing to selectively deposit silver (Ag) and gold (Au) nanoparticle (NP) functionalization on single-layer graphene (SLG) based gas sensors. This method avoids lithography and chemical processes, maintaining the device's quality while potentially lowering the fabrication costs. Ag-decorated sensors reveal a three-fold improvement in nitrogen dioxide (NO₂) gas response over pristine-SLG sensors. We demonstrate detection capabilities down to 50 ppb at room temperature, negating the requirement for external thermal or photoactivation. In contrast to pristine or Au-decorated SLG sensors, Ag-decorated devices exhibit 96% recovery at room temperature (RT). These results highlight the potential of using spark ablation with impaction printing for functionalizing graphene-based sensors.

In this work, we demonstrate the use of a micro-hotplate (MHP) with graphene integrated without transfer step for NO₂ sensing. The MHP can rapidly recover the device to its initial conditions by applying a brief heat pulse. Moreover, by employing a process without graphene transfer, we prevent random polymer contamination from the transfer process. The (up to 8) graphene sensors on a single MHP show a very similar response. Finally, we demonstrate that we can extract the relative humidity from the device's response immediately after the MHP is turned off in a humid environment.

Depending on the applications based on graphene, single-layer or few-layer graphene would be more beneficial. Ideally, graphene could be nucleated directly with the required thickness. However, some aspects related to graphene thickness and uniformity control still need to be solved. This work aims to better understand graphene formation using Mo thin films as a catalyst. The grown graphene films were characterized using SEM, TEM, XPS, AFM, standard Raman spectroscopy and 3D Raman surface imaging. A correlation between the catalyst thickness and the number of layers is established. All the characterization techniques show that the number of graphene layers inversely scales with the Mo catalyst thickness used for the graphene synthesis. Then, by simply adjusting the catalyst thickness, the number of graphene layers can be engineered from few-layer graphene (FLG) up to multi-layer graphene (MLG). A pinhole distribution of 1 % was detected on the films synthesized on 50 nm and 100 nm Mo thicknesses after the catalyst was etched. On the synthesized FLG (500 nm Mo), no holes were observed on the surface film after the etching process and even after a transfer onto another substrate. These results can enable the formation of FLG with a controlled thickness and good uniformity.

Nitrogen dioxide (NO₂) is a potential hazard to human health at low concentrations, below one part per million (ppm). NO₂ can be monitored using gas sensors based on multi-layered graphene operating at ambient temperature. However, reliable detection of concentrations on the order of parts per million and lower is hindered by partial recovery and lack of reproducibility of the sensors after exposure. We show how to overcome these longstanding problems using ultraviolet (UV) light. When exposed to NO₂, the sensor response is enhanced by 290 % − 550 % under a 275 nm wavelength light emitting diode irradiation. Furthermore, the sensor's initial state is completely restored after exposure to the target gas. UV irradiation at 68 W/m2 reduces the NO₂ detection limit to 30 parts per billion (ppb) at room temperature. We investigated sensor performance optimization for UV irradiation with different power densities and target gases, such as carbon oxide and ammonia. Improved sensitivity, recovery, and reproducibility of UV-assisted graphene-based gas sensors make them suitable for widespread environmental applications.

Ionic polymer metal composites (IPMCs) are a class of materials with a rising appeal in biological micro-electromechanical systems (bio-MEMS) due to their unique properties (low voltage output, bio-compatibility, affinity with ionic medium). While tailoring and improving actuation capabilities of IPMCs is a key motivator in almost all IPMC manufacturing reports, very little efforts have been dedicated to sensing using IPMC thinner than 100 µm. Most reports on IPMC manufacturing and utilization rely on 180 µm-thick Nafion with platinum electrodes, too stiff for bio-MEMS applications. The same fabrication process on thinner membranes does yield in very poor electrodes and performance, and needs to be studied to increase flexibility and sensitivity in the microscale range. This study demonstrates an electroless Pt deposition method for fabricating bio-MEMS-suitable 50 µm-thick IPMC samples. First, we perform a comparative study on the platinum distribution within the Nafion backbone as well as on the surface for the standard electroless deposition recipe for thin (50 µm) and thick (180 µm) Nafion. We report strong differences in platinum distribution for thick and thin IPMC that experienced the same manufacturing process. By varying chemical concentrations from the standard recipe we obtain convenient platinum distribution on thin Nafion, with platinum mainly localized in proximity of surface, as well as electrodes with lower sheet resistance. We could measure the flexural rigidity as 3.43 × 10 − 8 N·m², 46 times lower than standard 180 µm-thick IPMC. The calculated sensitivity is 0.476 ± 0.02 mV mm⁻¹ and the limit of detection for our sensor is 500 ± 20 µm. This procedure sets a milestone for manufacturing 50 µm-thick IPMC for transducers and sensors in bio-MEMS applications.

In this paper, tin oxidation (SnO x )/tin-sulfide (SnS) heterostructures are synthesized by the post-oxidation of liquid-phase exfoliated SnS nanosheets in air. We comparatively analyzed the NO2 gas response of samples with different oxidation levels to study the gas sensing mechanisms. The results show that the samples oxidized at 325 °C are the most sensitive to NO2 gas molecules, followed by the samples oxidated at 350 °C, 400 °C and 450 °C. The repeatabilities of 350 °C samples are better than that of 325 °C, and there is almost no shift in the baseline. Thus this work systematically analyzed the gas sensing performance of SnO x/SnS-based sensor oxidized at 350 °C. It exhibits a high response of 171% towards 1 ppb NO2, a wide detecting range (from 1 ppb to 1 ppm), and an ultra-low theoretical detection limit of 5 ppt, and excellent repeatability at room temperature. The sensor also shows superior gas selectivity to NO2 in comparison to several other gas molecules, such as NO, H2, SO2, CO, NH3, and H2O. After X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscope, and electron paramagnetic resonance characterizations combining first principle analysis, it is found that the outstanding NO2 sensing behavior may be attributed to three factors: The Schottky contact between electrodes and SnO x/SnS; active charge transfer in the surface and the interface layer of SnO x/SnS heterostructures; and numerous oxygen vacancies generated during the post-oxidation process, which provides more adsorption sites and superior bandgap modulation. Such a heterostructure-based room-temperature sensor can be fabricated in miniaturized size with low cost, making it possible for large-scale applications.

2D and nanostructured metal sulfide materials are promising in the advancement of several gas sensing applications due to the abundant choice of materials with easily tunable electronic, optical, physical, and chemical properties. These applications are particularly attractive for gas sensing in environmental monitoring and breath analysis. This review gives a systematic description of various gas sensors based on 2D and nanostructured metal sulfide materials. Firstly, the crystal structures of metal sulfides are introduced. Secondly, the gas sensing mechanisms of different metal sulfides based on density functional theory analysis are summarised. Various gas-sensing concepts of metal sulfide-based devices, including chemiresistors, functionalized metal sulfides, Schottky junctions, heterojunctions, field-effect transistors, and optical and surface acoustic wave sensors, are compared and presented. It then discusses the extensive applications of metal sulfide-based sensors for different gas molecules, including volatile organic compounds (i.e., acetone, benzene, methane, formaldehyde, ethanol, and liquefied petroleum gas) and inorganic gas (i.e., CO2, O2, NH3, H2S, SO2, NOx, CH4, H2, and humidity). Finally, a strengths-weaknesses-opportunities-threats (SWOT) analysis is proposed for future development and commercialization in this field. This journal is

In this work, a thin-film transistor gas sensor based on the p-N heterojunction is fabricated by stacking chemical vapor deposition-grown tungsten disulfide (WS₂) with a sputtered indium-gallium-zinc-oxide (IGZO) film. To the best of our knowledge, the present device has the best NO₂ gas sensor response compared to all the gas sensors based on transition-metal dichalcogenide materials. The gas-sensing response is investigated under different NO₂ concentrations, adopting heterojunction device mode and transistor mode. High sensing response is obtained of p-N diode in the range of 1-300 ppm with values of 230% for 5 ppm and 18 170% for 300 ppm. On the transistor mode, the gas-sensing response can be modulated by the gate bias, and the transistor shows an ultrahigh response after exposure to NO₂, with sensitivity values of 6820% for 5 ppm and 499 400% for 300 ppm. Interestingly, the transistor has a typical ambipolar behavior under dry air, while the transistor becomes p-type as the amount of NO₂ increases. The assembly of these results demonstrates that the WS₂/IGZO device is a promising platform for the NO₂-gas detection, and its gas-modulated transistor properties show a potential application in tunable engineering for two-dimensional material heterojunction-based transistor device.

The present work aims to become a standard step for the fabrication of the next generation porous anodic alumina (PAA) templates based devices and to provide new insights on the mechanism involved in the porous anodic alumina formation. The oxide barrier layer at the bottom of the pores has been successfully thinned by applying an exponential voltage decrease process followed by a wet chemical etching. The impact of the potential drop on the PAA structure has been deeply investigated, as well as the electrolyte temperature, the number of potential steps and the exponential decay rate. The presented results underline that a smart adjustment between the anodization conditions and exponential voltage decay parameters can simultaneously give PAA structures with straight pores and remove the dielectric layer in spite of applying the exponential voltage decay step. Additionally, the PAA structure has been tuned to fabricate hierarchically nanoporous templates with secondary pores ranging from 2 up to 10 branches.