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An electronic nose (e‐nose) is a device that can detect and recognize odors and flavors using a sensor array. It has received considerable interest in the past decade because it is required in several areas such as health care, environmental monitoring, industrial applications, automobile, food storage, and military. However, there are still obstacles in developing a portable e‐nose that can be used for a wide variety of applications. For practical applications of an e‐nose, it is necessary to collect a massive amount of data from various sensing materials that can transduce interactions with molecules reliably and analyze them via pattern recognition. In addition, the possibility of miniaturizing the e‐nose and operating it with low power consumption should be considered. Moreover, it should work efficiently over a long period of time. To satisfy these requirements, several different chemoresistive material platforms including metal oxides, organics such as polymers and carbon‐based materials, and two‐dimensional materials were investigated as sensor elements for an e‐nose. As an individual material has limited selectivity, there is a continuing effort to improve the selectivity and gas sensing properties through surface decoration and compositional and structural variations. To produce a reliable e‐nose, which can be used for practical applications, researches in various fields have to be harmonized. This paper reviews the progress of research on e‐noses based on a chemoresistive gas sensor array and discusses the inherent challenges and potential solutions.

Transition metal dichalcogenides (TMDs) have attracted enormous attention in diverse research fields. Especially, gas sensors are considered in a promising application exploiting TMDs. However, the...

The use of chemoresistive gas sensors based on metal oxides has expanded to various fields such as medical diagnosis and air quality systems as well as gas leakage detectors with the development of the Internet of Things. Accordingly, sensitivity, selectivity, power consumption, and reproducibility become important factors in the development of gas sensors. Herein, we developed a facile method to fabricate a gas sensor based on porous Au-incorporated tungsten trioxide (WO3) thin films for highly sensitive and selective NO2 sensing. The mixed solution of ammonium tetrathiotungstate [(NH4)2WS4] and gold chloride (AuCl3) was transformed to Au-incorporated WO3 thin films through the spin-coating and annealing process. The gas sensors based on the Au-incorporated WO3 thin films exhibited improved sensitivity, selectivity, and response time upon exposure to NO2 with a significantly low theoretical detection limit of 28 ppt at 150 ℃ compared to gas sensors based on the pristine WO3 thin film. The high sensing properties are attributed to the porous structure and catalytic effects of Au nanoparticles. In addition to these remarkable NO2 sensing properties, the facile and cost-effective fabrication process enlarges the potential of the Au-incorporated WO3 thin films for practical and commercial gas sensing applications.

One of the well‐known strategies for achieving high‐performance light‐activated gas sensors is to design a nanostructure for effective surface responses with its geometric advances. However, no study has gone beyond the benefits of the large surface area and provided fundamental strategies to offer a rational structure for increasing their optical and chemical performances. Here, a new class of UV‐activated sensing nanoarchitecture made of highly periodic 3D TiO2, which facilitates 55 times enhanced light absorption by confining the incident light in the nanostructure, is prepared as an active gas channel. The key parameters, such as the total 3D TiO2 film and thin‐shell thicknesses, are precisely optimized by finite element analysis. Collectively, this fundamental design leads to ultrahigh chemoresistive response to NO2 with a theoretical detection limit of ≈200 ppt. The demonstration of high responses with visible light illumination proposes a future perspective for light‐activated gas sensors based on semiconducting oxides., The high‐performance light‐activated NO2 gas sensors using 3D TiO2 nanostructures are achieved by the unique light harvesting properties of the nanoporous structures. The light scattering effects from optical maze‐like nanostructures and numerous intermediate energy states from atomic layer deposition of the film electronically activate 3D TiO2 and enhance its gas sensing performance under UV illumination.

Recently, two-dimensional (2D) materials have attracted attention for gas sensor fields due to their unique properties such as high surface to volume ratio and numerous active sites. In particular, reduced graphene oxide (rGO) and MoS2 are one of the most promising materials for humidity sensor due to the oxygen containing functional groups on the surface of rGO and dangling bonds at the edge site of MoS2. Herein, we present 2D rGO/2D MoS2 hybrid composites (MS-GOs) synthesized by hydrothermal method. rGO and MoS2 are mixed with different molar ratios and drop-casted on SiO2 substrate with Pt interdigitated electrode. At an optimized molar ratio of rGO/MoS2, the sensor device exhibits high sensitivity, good selectivity, rapid response and recovery, and good linearity for humidity sensing. The enhanced sensing properties are attributed to the p-n junction between rGO and MoS2. Our work provides an efficient way for realizing high-performance and fast responding humidity sensors utilizing 2D-2D heterojunction materials based chemoresistive humidity sensors for use in diverse applications.

Reduced graphene oxide (rGO) and MoS2, the most representative two-dimensional (2D) materials, are receiving significant attention for the fabrication of sensing devices owing to their high surface area, many abundant sites, and excellent mechanical flexibility. Herein, we demonstrated chemoresistive humidity sensors with fast response, excellent selectivity, and ultrahigh sensitivity based on 2D rGO and 2D MoS2 hybrid composites (RGMSs). The RGMSs were fabricated by simple ultrasonication without the addition of additives and additional heating. Compared to pristine rGO, the RGMS exhibited a 200 times higher response to humidity at room temperature. The significant enhancement in the sensing performance of the composite was attributed to electronic sensitization due to p–n heterojunction formation and porous structures between rGO and MoS2. The synergistic combination of rGO and MoS2 could be applied to construct a flexible humidity sensor. The sensing performance of an RGMS flexible device remains the same before and after bending; this indicates that the use of rGO and MoS2 is a viable new and simple strategy to realize a humidity sensor for use in wearable electronics.

Tar formation resulting during lignocellulosic biomass gasification is a major impediment to utilizing biomass energy sources, in that it blocks and fouls the processing equipment; as such, any tar present in the produced syngas much be effectively removed. This study analyzes the ability of commercially available Ni and Ru based CH4 reforming catalysts to effect tar removal and compares deactivation characteristics. Toluene was used as the model biomass tar at concentrations of 30 and 100 g/Nm3. Several additional parameters were also tested, including reaction temperatures (400–800 °C), space velocities (5000–30,000 h−1), and the steam/toluene ratios (2–20). The variation of toluene conversion and product gas composition with reaction conditions was analyzed. Overall, H2 and CO production were favored by the Ru catalyst and generally increased with temperature. Conversion also increased with temperature, with conversions higher than 90% obtained at 800 °C.

Transition metal dichalcogenides (TMDs) have attracted enormous attention in diverse research fields. Especially, gas sensors are considered in a promising application exploiting TMDs. However, the studies are confined to only major TMDs such as MoS

To take advantage of nanostructures beyond their large surface area for light‐activated gas sensing, in article number 2001883, Seokwoo Jeon, Young‐Seok Shim, Ho Won Jang, and co‐workers introduce a new class of UV‐activated sensing of rationally designed highly periodic 3D TiO(2). The 3D TiO(2) facilitates 55 times enhanced light absorption and ultra‐sensitive NO(2) sensing properties with a detection limit of ∼200 ppt at room temperature. [Image: see text]

Chemoresistive gas sensors based on two-dimensional (2D) materials including graphene-based materials have attracted significant research interest owing to their potential use in next-generation technologies including the Internet of Things (IoT). The functionalization of 2D materials is considered as a key strategy to achieve superior gas sensing properties such as high selectivity, high sensitivity, and reversible response and recovery, because it can modulate the chemical and electrical properties of 2D materials for more efficient gas sensing. Herein, we present a facile solution process and the room temperature gas sensing properties of chemically fluorinated graphene oxide (CFGO). The CFGO sensors exhibit improved sensitivity, selectivity, and reversibility upon exposure to NH3 with a significantly low theoretical detection limit of ∼6 ppb at room temperature in comparison to NO2 sensing properties. The effect of fluorine doping on the sensing mechanism is examined by first-principles calculations based on density functional theory. The calculations reveal that the fluorine dopant changes the charge distribution on the oxygen containing functional groups in graphene oxide, resulting in the preferred selective adsorption and desorption of NH3 molecules. We believe that the remarkable NH3 sensing properties of CFGO and investigation by first-principles calculations would enlarge the possibility of functionalized 2D materials for practical gas sensing applications such as the IoT.

Graphene is one of the most promising materials for high-performance gas sensors due to its unique properties such as high sensitivity at room temperature, transparency, and flexibility. However, the low selectivity and irreversible behavior of graphene-based gas sensors are major problems. Here, we present unprecedented room temperature hydrogen detection by Au nanoclusters supported on self-activated graphene. Compared to pristine graphene sensors, the Au-decorated graphene sensors exhibit highly improved gas-sensing properties upon exposure to various gases. In particular, an unexpected substantial enhancement in H2 detection is found, which has never been reported for Au decoration on any type of chemoresistive material. Density functional theory calculations reveal that Au nanoclusters on graphene contribute to the adsorption of H atoms, whereas the surfaces of Au and graphene do not bind with H atoms individually. The discovery of such a new functionality in the existing material platform holds the key to diverse research areas based on metal nanocluster/graphene heterostructures.

Transition metal disulfides have been attracting significant attentions in recent years. There are extensive applications of transition metal disulfides, especially on gas sensing applications, due to their large specific surface-to-volume ratios, high sensitivity to adsorption of gas molecules and tunable surface functionality depending on the decoration species or functional groups. However, there are several drawbacks such as poor gas selectivity, sluggish recovery characteristics and difficulty in the fabrication of large-scale devices. Here, we provide a review of recent progress on the chemoresistive gas sensing properties of two-dimensional transition metal disulfides. This review also provides various methods to enhance the gas sensing performance of two-dimensional disulfides, such as surface functionalization, decoration receptor functions and developing nanostructures.

The decoration of p-type nickel oxide (NiO) with n-type hematite (α-Fe2O3) to achieve vertically ordered 1D nanostructures is an attractive strategy to enhance gas sensing properties. Herein, the authors report a facile method for α-Fe2O3 decoration of the whole surface of vertical NiO nanorods. An NiO/Fe heterostructure is deposited in multiple steps using a glancing angle deposition method, which is followed by the oxidation of Fe into α-Fe2O3. Thermally agglomerated α-Fe2O3 nanoparticles are uniformly distributed on the whole surface of the NiO nanorods. Due to the α-Fe2O3 decoration, the NiO nanorods exhibit a coral-like rough surface and, more interestingly, their preferential crystallographic orientation changed from (111) to (200). Compared to bare NiO nanorods, the α-Fe2O3-decorated NiO nanocorals exhibit a 45.4 times higher response to 50 ppm toluene (C7H8) at 350 °C. Their theoretical detection limit for C7H8 is calculated to be ≈22 ppb. The observed unprecedented synergetic effects of α-Fe2O3-decorated NiO nanocorals for the extremely selective C7H8 sensing, as well as their facile synthetic route, establish a new perspective on heterostructured metal oxide 1D nanostructures for selective gas sensing.