Au-Loaded Hierarchical MoO3 Hollow Spheres with Enhanced Gas-Sensing Performance for the Detection of BTX (Benzene, Toluene, And Xylene) And the Sensing Mechanism

A Brief Review of MoO3 and MoO3-Based Materials and Recent Technological Applications in Gas Sensors, Lithium-Ion Batteries, Adsorption, and Photocatalysis

Molybdenum trioxide is an abundant natural, low-cost, and environmentally friendly material that has gained considerable attention from many researchers in a variety of high-impact applications. It is an attractive inorganic oxide that has been widely studied because of its layered structure, which results in intercalation ability through tetrahedral/octahedral holes and extension channels and leads to superior charge transfer. Shape-related properties such as high specific capacities, the presence of exposed active sites on the oxygen-rich structure, and its natural tendency to oxygen vacancy that leads to a high ionic conductivity are also attractive to technological applications. Due to its chemistry with multiple valence states, high thermal and chemical stability, high reduction potential, and electrochemical activity, many studies have focused on the development of molybdenum oxide-based systems in the last few years. Thus, this article aims to briefly review the latest advances in technological applications of MoO3 and MoO3-based materials in gas sensors, lithium-ion batteries, and water pollution treatment using adsorption and photocatalysis techniques, presenting the most relevant and new information on heterostructures, metal doping, and non-stoichiometric MoO3-x.

Conductometric Gas Sensor Based on MoO3 Nanoribbon Modified with rGO Nanosheets for Ethylenediamine Detection at Room Temperature

An ethylenediamine (EDA) gas sensor based on a composite of MoO3 nanoribbon and reduced graphene oxide (rGO) was fabricated in this work. MoO3 nanoribbon/rGO composites were synthesized using a hydrothermal process. The crystal structure, morphology, and elemental composition of MoO3/rGO were analyzed via XRD, FT-IR, Raman, TEM, SEM, XPS, and EPR characterization. The response value of MoO3/rGO to 100 ppm ethylenediamine was 843.7 at room temperature, 1.9 times higher than that of MoO3 nanoribbons. The MoO3/rGO sensor has a low detection limit (LOD) of 0.235 ppm, short response time (8 s), good selectivity, and long-term stability. The improved gas-sensitive performance of MoO3/rGO composites is mainly due to the excellent electron transport properties of graphene, the generation of heterojunctions, the higher content of oxygen vacancies, and the large specific surface area in the composites. This study presents a new approach to efficiently and selectively detect ethylenediamine vapor with low power.

An Au/SnO-SnO2 nanosheet based composite used for rapid detection of hydrogen sulphide

In this work, a new type of H₂S sensor was fabricated by means of drop-coating of an Au/SnO-SnO₂ nanosheet material, which was prepared by a one-pot hydrothermal reaction, onto a gold electrode in an alumina ceramic tube with the formation of a thin nanocomposite film. The microstructure and morphology of the nanosheet composites were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A gas-sensitivity study presented good H₂S-sensing performance of such Au/SnO-SnO₂ nanosheet composites. At an optimal operating temperature of 240 °C and ambient temperature of 25 °C, the resulting sensor showed a good linear response to H₂S in a range of 1.0 to 100 ppm with a low detection limit of 0.7 ppm, and a very fast response-recovery time of 22 s for response and 63 s for recovery, respectively. The sensor was also unaffected by ambient humidity and had good reproducibility and selectivity. When being applied to the monitoring of H₂S in an atmospheric environment in a pig farm, the response signal to H₂S was only attenuated by 4.69% within 90 days, proving that the sensor had a long and stable service lifetime for continuous running and showing its important practical application prospects.

Exclusive detection of volatile aromatic hydrocarbons using bilayer oxide chemiresistors with catalytic overlayers

The accurate detection and identification of volatile aromatic hydrocarbons, which are highly toxic pollutants, are essential for assessing indoor and outdoor air qualities and protecting humans from their sources. However, real-time and on-site monitoring of aromatic hydrocarbons has been limited by insufficient sensor selectivity. Addressing the issue, bilayer oxide chemiresistors are developed using Rh-SnO₂ gas-sensing films and catalytic CeO₂ overlayers for rapidly and cost-effectively detecting traces of aromatic hydrocarbons in a highly discriminative and quantitative manner, even in gas mixtures. The sensing mechanism underlying the exceptional performance of bilayer sensor is systematically elucidated in relation to oxidative filtering of interferants by the CeO₂ overlayer. Moreover, CeO₂-induced selective detection is validated using SnO₂, Pt-SnO₂, Au-SnO₂, In₂O₃, Rh-In₂O₃, Au-In₂O₃, WO₃, and ZnO sensors. Furthermore, sensor arrays are employed to enable pattern recognition capable of discriminating between aromatic gases and non-aromatic interferants and quantifying volatile aromatic hydrocarbon classifications.

Nanocomposite Based on HA/PVTMS/Cl2FeH8O4 as a Gas and Temperature Sensor

In this paper, a novel nanocrystalline composite material of hydroxyapatite (HA)/polyvinyltrimethoxysilane (PVTMS)/iron(II)chloride tetrahydrate (Cl₂FeH₈-O₄) with hexagonal structure is proposed for the fabrication of a gas/temperature sensor. Taking into account the sensitivity of HA to high temperatures, to prevent the collapse and breakdown of bonds and the leakage of volatiles without damaging the composite structure, a freeze-drying machine is designed and fabricated. X-ray diffraction, FTIR, SEM, EDAX, TEM, absorption and photoluminescence analyses of composite are studied. XRD is used to confirm the material structure and the crystallite size of the composite is calculated by the Monshi-Scherrer method, and a value of 81.60 ± 0.06 nm is obtained. The influence of the oxygen environment on the absorption and photoluminescence measurements of the composite and the influence of vaporized ethanol, N₂ and CO on the SiO₂/composite/Ag sensor device are investigated. The sensor with a 30 nm-thick layer of composite shows the highest response to vaporized ethanol, N₂ and ambient CO. Overall, the composite and sensor exhibit a good selectivity to oxygen, vaporized ethanol, N₂ and CO environments.

Mesoporous WS2/MoO3 Hybrids for High-Performance Trace Ammonia Detection

Mesoporous WS₂/MoO₃ hybrids were synthesized by a facile two-step and additive-free hydrothermal approach and employed for high-performance trace ammonia gas (NH₃) detection. Compared with single WS₂ and MoO₃ counterparts, WS₂/MoO₃ sensors exhibited an improvement in NH₃-sensing performance at room temperature (22 ± 3 °C). Typically, the optimal WS₂/MoO₃ sensor showed a higher and quicker response of 31.58% within 57 s toward 3 ppm of NH₃, which was 17.7- and 57.4-fold larger than that of pure MoO₃ (1.78% within 251 s) and WS₂ (0.55% within 153 s) ones. Meanwhile, good reversibility, sensitivity, and selectivity, reliable long-term stability, and the lowest detection limit of 9.0 ppb were achieved. These superior properties were probably ascribed to numerous heterojunctions favorable for additional carrier-concentration modulation via the synergetic effect between WS₂ and MoO₃ components and the large specific surface area beneficial for richer sorption sites and faster molecular transfer at room temperature. Such achievements also imply that the designed WS₂/MoO₃ heterostructure nanomaterials have the potential in achieving trace NH₃ recognition catering for the requirements of high sensitivity and low power consumption in future gas sensors.

Cu2O induced Au nanochains for highly sensitive dual-mode detection of hydrogen sulfide

Colorimetric and chemoresistive gas sensing methods have aroused great interest in H₂S monitoring due to their unique merits of naked-eye readout, and highly sensitive and rapid detection. However, combining these two methods for gas detection, especially utilizing one material as their common sensing material is a grand challenge because they are inconsistent in sensing mechanism. Taking advantage of the strong chemical affinity of Cu₂O for H₂S and the excellent performance of localized surface plasmon resonance (LSPR) of Au nanoparticles (NPs) in the visible regions and its ability as a noble metal to enhance gas sensing property, the Cu₂O-Au nanochains (NCs) were prepared for dual-mode detection of H₂S gas. The Cu₂O-Au chemoresistive gas sensor shows a 5-fold higher response than Cu₂O sensor at room temperature with a low detection limit of 10 ppb. Such good performance is attributed to the spillover effect and catalytic activity of Au NPs, and the enhanced H₂S adsorption after Au loading as revealed by density functional theory calculation. Test strips containing Cu₂O-Au produced for gaseous H₂S detection show superior color gradient changes (blue, yellow, and brown). Finally, the practicability of the method was validated by real-time monitoring H₂S released from cell culture.

MoO3 Nanorods Decorated by PbMoO4 Nanoparticles for Enhanced Trimethylamine Sensing Performances at Low Working Temperature

The gas sensing performance of metal oxides is limited by the lack of conductivity and sensing activity. Inducing the release of more electrons and activating more chemisorbed oxygen ions to participate in the gas sensing reaction can effectively overcome this limitation. The development of a PbMoO₄/MoO₃ heterostructure prepared by the addition of Pb²⁺ ions with MoO₃ nanorods is reported for highly sensitive and selective trimethylamine (TMA) detection. The response of the PbMoO₄/MoO₃ sensor (33.2) to 10 ppm TMA is improved 3-fold compared to the MoO₃ sensor (10.7), and the working temperature is reduced from 170 to 133 °C. The enhanced gas sensing performance and mechanism of PbMoO₄/MoO₃ were demonstrated using the energy band diagram and X-ray photoelectron spectroscopy (XPS) analysis. It is mainly attributed to the following promotion: (1) the induction of Pb²⁺ ions increases the electron density around the Mo element, enabling the decorated MoO₃ to release electrons easily; (2) the formed PbMoO₄/MoO₃ heterojunction endows a high degree of electron transfer at the interface; (3) the formation of the potential barrier causes the device resistance to decrease significantly upon TMA exposure. Finally, the practicability of the sensor was verified by detecting TMA released from Carassius auratus and shrimp to reflect their freshness.

Fast-response MEMS xylene gas sensor based on CuO/WO3 hierarchical structure

CuO/WO₃ hierarchical hollow microspheres, assembled from irregular two dimensional (2D) nanosheets, were prepared by ultrasonic-wet chemical etching and pyrolysis in this study. The sensing performance of Micro-Electro-Mechanical System (MEMS) xylene gas sensor based on CuO/WO₃ hierarchical structure were evaluated. It was found that the CuO/WO₃ MEMS sensors showed an enhanced gas sensing performance compared with pristine WO₃ sensor. The CuO/WO₃-3 (the mass ratio of CuO to WO₃ is 3%) sensor exhibited faster response-recover speed and the highest response value to xylene. Moreover, the CuO/WO₃-3 sensor possessed higher selectivity and long-term stability. The good sensing properties can be attributed to the unique three dimensional (3D) hierarchical structure and p-n heterojunction of CuO-WO₃. Considering the above advantages, the CuO/WO₃-3 sensor has a great potential for the rapid detection and monitoring of xylene.

Rapid detection of H2S gas driven by the catalysis of flower-like α-Bi2Mo3O12 and its visual performance: A combined experimental and theoretical study

Metal oxide semiconductor (MOSs) are attractive materials for the development of H₂S gas sensors. However, detecting H₂S with short response and recovery times while also lowering the limit of detection to sub-ppb levels remains a significant challenge. We therefore developed flower-like α-Bi₂Mo₃O₁₂ microspheres for H₂S gas detection that provide fast response and recovery times (3 and 22 s, respectively, for 100 ppm H₂S), while also reducing the limit of detection to 1 ppb. The sensor performs well in terms of sensitivity, reproducibility, long-term stability, including humidity stability. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations revealed that H₂S dissociates readily on the reduced α-Bi₂Mo₃O₁₂ surface and that Mo plays a catalytic role, accelerating the rate of H₂S decomposition and enabling a fast response. Moreover, test strips containing α-Bi₂Mo₃O₁₂ were also prepared, which enabled the naked eye detection of ppm-level H₂S gas at room temperature; a light-yellow to orange to brown color change occurs when exposed to H₂S, due to its conversion into stable sulfides. This work expands the application of α-Bi₂Mo₃O₁₂ to H₂S gas sensing, and provides a strategy for the use of MOSs as sensor materials for the detection of other gases.

Electrochemical sensor based on a chitosan-molybdenum vanadate nanocomposite for detection of hydroxychloroquine in biological samples

In this study, we firstly introduce an ultra-high sensitive V_3.6Mo_2.4O₁₆-chitosan (MV-CHT) nanocomposite for electrochemical hydroxychloroquine sulfate (HCQ) monitoring toward paracetamol (PCM) and pantoprazole (PPZ) in environmental and clinical diagnostics. The single-phase MV nanostructures are prepared via the sol-gel pechini route, followed by engineering maleic acid as a structure-directing agent. The stabilization of the MV electro-catalysts is adopted by varying critical factors such as calcination temperature, different chelating ligands, chelating molality and cross-linker concentration. The structural and morphological characterizations, namely, ordered active sites, structural integrity, porous network and dispersibility on the cationic polymer are confirmed by physicochemical analyses. Also, analytical nature of the MV-CHT modified carbon paste electrode (MV-CHT/CPE) is constructed via electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and differential pulse voltammetry (DPV) techniques. As a result, the nano-MV-CHT/CPE platforms with 10% of polymeric matrixes delivered the boosted analytical performance in terms of linear ranges (0.0019-194.0 µM), lower detection limit (LOD = 0.224 nM), together with excellent sensitivity and selectivity. The novel combination of MV nanoparticles and CHT provide the fluent channels for rapid charge transport and effective surface area. Such results illustrate the synergistic and interaction capability of MV-CHT-based sensing catalysts with bioactive molecules, which make them as superior drug monitoring devices.

Controllable synthesis and enhanced gas sensing performances of AuNPs modified ZnSnO3 hollow nanocubes toward highly sensitive toluene detection

Morphology control and noble metal modification have become effective ways to improve the gas sensing performance of mixed-metal oxides sensor. In this study, ZnSnO3 nanocube structure with different morphologies were...

Facile construction of bowknot-like CuO architectures for improved xylene gas sensing properties

The accurate and rapid monitoring of xylene gas is highly desired for human health and environmental protection. Herein, the bowknot-like CuO architectures have been synthesized through a facile room temperature...

Discriminating BTX Molecules by the Nonselective Metal Oxide Sensor-Based Smart Sensing System

Discriminating structurally similar volatile organic compounds (VOCs) molecules, such as benzene, toluene, and three xylene isomers (BTX), remains a significant challenge, especially, for metal oxide semiconductor (MOS) sensors, in which selectivity is a long-standing challenge. Recent progress indicates that temperature modulation of a single MOS sensor offers a powerful route in extracting the features of adsorbed gas analytes than conventional isothermal operation. Herein, a rectangular heating waveform is applied on NiO-, WO₃-, and SnO₂-based sensors to gradually activate the specific gas/oxide interfacial redox reaction and generate rich (electrical) features of adsorbed BTX molecules. Upon several signal preprocessing steps, the intrinsic feature of BTX molecules can be extracted by the linear discrimination analysis (LDA) or convolutional neural network (CNN) analysis. The combination of three distinct MOS sensors noticeably benefits the recognition accuracy (with a reduced number of training iterations). Finally, a prototype of a smart BTX recognition system (including sensing electronics, sensors, Wi-Fi module, UI, PC, etc.) based on temperature modulation has been explored, which enables a prompt, accurate, and stable identification of xylene isomers in the ambient air background and raises the hope of innovating the future advanced machine olfactory system.

Advanced Strategies to Improve Performances of Molybdenum-Based Gas Sensors

Molybdenum-based materials have been intensively investigated for high-performance gas sensor applications. Particularly, molybdenum oxides and dichalcogenides nanostructures have been widely examined due to their tunable structural and physicochemical properties that meet sensor requirements. These materials have good durability, are naturally abundant, low cost, and have facile preparation, allowing scalable fabrication to fulfill the growing demand of susceptible sensor devices. Significant advances have been made in recent decades to design and fabricate various molybdenum oxides- and dichalcogenides-based sensing materials, though it is still challenging to achieve high performances. Therefore, many experimental and theoretical investigations have been devoted to exploring suitable approaches which can significantly enhance their gas sensing properties. This review comprehensively examines recent advanced strategies to improve the nanostructured molybdenum-based material performance for detecting harmful pollutants, dangerous gases, or even exhaled breath monitoring. The summary and future challenges to advance their gas sensing performances will also be presented.

Room-Temperature Benzene Sensing with Au-Doped ZnO Nanorods/Exfoliated WSe2 Nanosheets and Density Functional Theory Simulations

A gold-doped zinc oxide (Au-ZnO)/exfoliated tungsten diselenide (exfoliated WSe₂) nanocomposite-based gas sensor toward benzene with high sensing properties was demonstrated. Epoxy resin was used as the matrix of the Au-ZnO/exfoliated WSe₂ nanocomposite sensor. The straw-shaped Au-ZnO was synthesized by the hydrothermal method, and WSe₂ nanosheets (NSs) were prepared via hydrothermal and liquid-phase exfoliation methods. The properties of Au-ZnO/exfoliated WSe₂ nanoheterostructures constructed by self-assembly technology have been confirmed via a series of characterization methods. The benzene-sensing performances of sensors were tested at 25 °C. Compared with Au-ZnO, WSe_2, and their composites, the Au-ZnO/exfoliated WSe₂ sensor has a significant performance improvement, including a higher response and linear fit degree, better selectivity and repeatability, and faster detection rate. The significantly enhanced sensing properties of the Au-ZnO/exfoliated WSe₂ sensor can be ascribed to the doping of Au nanoparticles, the increase in the specific surface area and adsorption sites of NSs after exfoliation, and the cooperative interface combination of the ZnO/WSe₂ heterojunction. Furthermore, the sensitivity mechanism of the composite sensor to benzene was explored by density functional theory simulations.

Ex-situ XPS analysis of yolk-shell Sb2O3/WO3 for ultra-fast acetone resistive sensor

The preparation of fast, highly responsive and reliable gas sensing devices for the detection of acetone gas is considered to be a key challenge for the development of accurate disease diagnosis systems through exhaled respiratory gases. In the paper, yolk shell Sb₂O₃/WO₃ is synthesized and its gas sensing performance was studied by static test system. Special, the maximum response value of 1:1 Sb₂O₃/WO₃ yolk-shell (WO₃-1 YSL) sensor to 100 ppm acetone can reach as high as 50.0 at 200 ℃. And it also exhibits excellent response/recover time (4 s/5 s), low detection limit (2 ppm) and superior selectivity towards acetone. More importantly, in mixed selective gas test, the sensor shows high selectivity towards acetone. And the mechanism is analyzed by ex-situ XPS. The excellent gas-sensing performance can be attributed to unique yolk-shell structure, which facilitates the rapid transport of charge carriers from the surface to the bulk and provides more active sites for gas adsorption and desorption; the heterojunction between of Sb₂O₃ and WO₃, which promotes oxygen pre-adsorption on the surface and increasing the interfacial potential; the increased oxygen vacancies which allowing more chemisorbed oxygen to form.

Gas-solid interfacial charge transfer in volatile organic compound detection by CuCrO2nanoparticles

Nanostructured metal oxide semiconductors have received great attention used as the chemiresistive layer of gas sensor to detect the volatile organic compound recently. As indispensable complementary parts for dominative n-type semiconductors, the p-type metal oxides based gas sensors fail to be studied sufficiently, which hampers their practical applications. In this work, the p-type delafossite CuCrO₂nanoparticles were synthesized, characterized, and tested for gas sensing, followed by the first principles calculations to simulate the generation of chemiresistive signal. The hydrothermal synthesis time of CuCrO₂nanoparticles is optimized as 24 h with a higher proportion of oxygen vacancies but a smaller size, which is confirmed by the microscopy and spectrum characterization and allows for a prevailing gas sensitivity. Meanwhile, this CuCrO₂gas sensor is proven to perform a higher selectivity to n-propanol and a low detection limit of 1 ppm. The adsorption sites and charge variations of dehydrogenation at the gas-solid interface predicted by the theoretical analysis are claimed to be crucial to such selectivity. It is an innovative approach to understand the chemiresistive gas sensing by evaluating the preference of charge transfer between the sensor and target gaseous molecule, which provides a new route to precisely design and develop the advanced sensing devices for the diverse applications.

Highly Selective Detection of Benzene and Discrimination of Volatile Aromatic Compounds Using Oxide Chemiresistors with Tunable Rh-TiO2 Catalytic Overlayers

Volatile aromatic compounds are major air pollutants, and their health impacts should be assessed accurately based on the concentration and composition of gas mixtures. Herein, novel bilayer sensors consisting of a SnO2 sensing layer and three different xRh-TiO2 catalytic overlayers (x = 0.5, 1, and 2 wt%) are designed for the new functionalities such as the selective detection, discrimination, and analysis of benzene, toluene, and p-xylene. The 2Rh-TiO2/SnO2 bilayer sensor shows a high selectivity and response toward ppm- and sub-ppm-levels of benzene over a wide range of sensing temperatures (325-425 °C). An array of 0.5Rh-, 1Rh-, and 2Rh-TiO2/SnO2 sensors exhibits discrimination and composition analyses of aromatic compounds. The conversion of gases into more active species at moderate catalytic activation and the complete oxidation of gases into non-reactive forms by excessive catalytic promotion are proposed as the reasons behind the enhancement and suppression of analyte gases, respectively. Analysis using proton transfer reaction-quadrupole mass spectrometer (PTR-QMS) is performed to verify the above proposals. Although the sensing characteristics exhibit mild moisture interference, bilayer sensors with systematic and tailored control of gas selectivity and response provide new pathways for monitoring aromatic air pollutants and evaluating their health impacts.

Inorganic-Diverse Nanostructured Materials for Volatile Organic Compound Sensing

Environmental pollution related to volatile organic compounds (VOCs) has become a global issue which attracts intensive work towards their controlling and monitoring. To this direction various regulations and research towards VOCs detection have been laid down and conducted by many countries. Distinct devices are proposed to monitor the VOCs pollution. Among them, chemiresistor devices comprised of inorganic-semiconducting materials with diverse nanostructures are most attractive because they are cost-effective and eco-friendly. These diverse nanostructured materials-based devices are usually made up of nanoparticles, nanowires/rods, nanocrystals, nanotubes, nanocages, nanocubes, nanocomposites, etc. They can be employed in monitoring the VOCs present in the reliable sources. This review outlines the device-based VOC detection using diverse semiconducting-nanostructured materials and covers more than 340 references that have been published since 2016.

Enhanced room-temperature ethanol sensing performance of porous MoO3/V0.13Mo0.87O2.935 heterostructures self-assembled with 2D nanosheets

Porous MoO₃/V_0.13Mo_0.87O_2.935 heterostructures self-assembled with 2D nanosheets have been primarily prepared by a facile method for effectively detecting ethanol at room temperature.

A density functional theory study on the interaction of toluene with transition metal decorated carbon nanotubes: a promising platform for early detection of lung cancer from human breath

In this study, single-wall carbon nanotubes (SWCNTs) decorated by platinum-group transition metals (Pt, Pd, Rh, or Ru) were introduced as promising nanosensors for the detection of toluene, an important biomarker in the exhaled breath of the lung cancer patients. First-principle calculations based on density functional theory (DFT) was employed to scrutinize the impact of an individual toluene gas molecule on the structural, electronic, and magnetic properties of pristine and metal decorated SWCNTs. It was discovered that toluene is physisorbed on the pristine SWCNT through the interaction of the π orbitals of the carbon atoms in the toluene and the nanotube. Decoration of the SWCNT with metal atoms enhanced the adsorption energies significantly by means of strong overlapping between d orbital of the metal atoms and p orbital of C atoms in the benzene ring of toluene. Investigations showed that toluene is strongly chemisorbed on Rh- and Ru-SWCNT systems via strong covalent bonds with the superior response (-96.98% and -99.98%, respectively), and moderately chemisorbed on Pt-SWCNTs (-27.3%) and Pd-SWCNTs (61.60%). Our findings propose metal decorated SWCNT molecular sensors are attractive candidates for the detection of toluene and other lung cancer biomarkers in the exhaled breath of the lung cancer patients.

Recent Advances of SnO2-Based Sensors for Detecting Volatile Organic Compounds

SnO₂ based sensors has received extensive attention in the field of toxic gas detection due to their excellent performances with high sensitivity, fast response, long-term stability. Volatile organic compounds (VOCs), originate from industrial production, fuel burning, detergent, adhesives, and painting, are poisonous gases with significant effects on air quality and human health. This mini-review focuses on significant improvement of SnO₂ based sensors in VOCs detection in recent years. In this review, the sensing mechanism of SnO₂-based sensors detecting VOCs are discussed. Furthermore, the improvement strategies of the SnO₂ sensor from the perspective of nanomaterials are presented. Finally, this paper summarizes the sensing performances of these SnO₂ nanomaterial sensors in VOCs detection, and the future development prospect and challenges is proposed.

Au-Decorated ZnFe2O4 Yolk-Shell Spheres for Trace Sensing of Chlorobenzene

Noble metals supported on metal oxides are promising materials for widely applying on gas sensors because of their enviable physical and chemical properties in enhancing the sensitivity and selectivity. Herein, pristine ZFO yolk-shell spheres composed of ultrathin nanosheets and ultrasmall nanoparticles decorated with nanosized Au particles with a diameter of 1-2 nm are fabricated using the method of solution-phase deposition-precipitation. As a result, the Au@ZFO yolk-shell sphere based sensor exhibits significantly sensing performances for chlorobenzene (CB). In comparison with pristine ZFO, the response (R_air/R_gas= 90.9) of a Au@ZFO based sensor with a low detection limit of 100 ppb increases 4-fold when exposed to 10 ppm chlorobezene at 150 °C. Excitingly, the sensing response for chlorobenzene is the highest among metal oxides semiconductor based sensors. Moreover, the sensors can be further applied in the field of chlorobenzene monitoring, owing to its outstanding selectivity. The results elaborated that the enhanced sensing mechanism is mainly attributed to the effects of electronic sensitization and chemical sensitization, which are induced by the Au nanoparticles on the surface of ZFO yolk-shell spheres. Density functional theory (DFT) calculations further illustrated that the existence of Au nanoparticles exhibits higher adsorption energy and net charge transfer for CB. In addition, the relationship between the sensing performances of pristine ZFO and Au@ZFO yolk-shell spheres for chlorobenzene and the factors of Au loading amount, operating temperature, and humidity was also fully investigated in this work.

Highly Sensitive Detection of Benzene, Toluene, and Xylene Based on CoPP-Functionalized TiO2 Nanoparticles with Low Power Consumption

Among various metal oxides, titanium dioxide (TiO₂) has received considerable interest as a gas-sensing material owing to its high reliability at high operating temperatures. Nonetheless, TiO₂ generally has low sensitivity to target gases. In particular, TiO₂-based sensors have difficulty in sensitively detecting benzene, toluene, and xylene (referred to as BTX). Moreover, the reported TiO₂-based sensors have not simultaneously satisfied the demand for tens of ppb BTX detection and operation with low power consumption. This work proposes a BTX sensor using cobalt porphyrin (CoPP)-functionalized TiO₂ nanoparticles as a sensing material on a suspended microheater fabricated by bulk micromachining for low power consumption. TiO₂ nanoparticles show an enhanced sensitivity (245%) to 10 ppm toluene with CoPP functionalization. The proposed sensor exhibits high sensitivity to BTX at concentrations ranging from 10 ppm down to several ppb. The high reliability of the sensor is also explored through the long-time operation with repeated exposure to 10 ppm toluene for 14 h.

In situ deposited hierarchical CuO/NiO nanowall arrays film sensor with enhanced gas sensing performance to H2S

Hierarchical and heterogeneous CuO/NiO nanowall arrays were in situ grown on ceramic tubes via a facile template-free hydrothermal route, and then were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen adsorption-desorption techniques. The resultant composites exhibit network-like CuO/NiO array structures constructed by interconnected porous nanosheets, in which the decoration of CuO nanoparticles in NiO nanowall arrays was confirmed by XRD, XPS and TEM analyses. The 2.84 at % CuO decorated NiO sensor exhibits excellent sensing properties at 133 °C. The response to 5 ppm H₂S attains 36.9, which increases as high as 5.6 times compared to the NiO one. The detection limit to H₂S is further decreased from 1 ppb for the pure NiO sensor to 0.5 ppb. The CuO/NiO sensor shows a wide linear range from 50 to 1000 ppb, good repeatability, selectivity and long-term stability, which is expected to be a candidate for ppb-level H₂S detection in real and complex environment of industrial production. Furthermore, the dominant H₂S sensing mechanism is discussed from the view of the homo- and hierarchical architecture of the CuO/NiO arrays as well as the chemical and electronic sensitization effects of CuO decoration.

The effects of Zr-doping on improving the sensitivity and selectivity of a one-dimensional α-MoO3-based xylene gas sensor

One dimensional Zr-doped α-MoO₃ nanobelts were synthesized, and the influence of Zr doping on xylene sensing properties was studied.

Low-Voltage-Driven Sensors Based on ZnO Nanowires for Room-Temperature Detection of NO2 and CO Gases

Herein, we report the synthesis of pristine and Au-functionalized ZnO nanowires (NWs) for low power consumption (self-heated) gas sensors at room temperature. The ZnO NWs were produced via a vapor-liquid-solid growth technique, and Au layers with different thicknesses were sputter-deposited on the ZnO NWs, followed by subsequent annealing. Microscopic characterization methods demonstrated that ZnO NWs were successfully formed. Pristine ZnO NW gas sensors showed the best sensitivity toward either CO or NO₂ gases at 300 and 350 °C, respectively. Also, the sensitivities of pristine ZnO NW gas sensors were tested toward NO₂ gas under different applied voltages; the sensors revealed a good response and selectivity under an applied voltage of 7 V. Au-functionalized ZnO NW gas sensors exhibited the best response for CO gas at an applied voltage of 7 V and showed a much higher response relative to the pristine ZnO NWs. The sensing mechanisms for pristine and functionalized gas sensors are comprehensively discussed.

A highly responsive NH3 sensor based on Pd-loaded ZnO nanoparticles prepared via a chemical precipitation approach

The gas-detecting ability of nanostructured ZnO has led to significant attention being paid to the development of a unique and effective approach to its synthesis. However, its poor sensitivity, cross-sensitivity to humidity, long response/recovery times and poor selectivity hinder its practical use in environmental and health monitoring. In this context, the addition of noble metals, as dopants or catalysts to modify the ZnO surface has been examined to enhance its sensing performance. Herein, we report preparation of Pd-loaded ZnO nanoparticles via a chemical precipitation approach. Various Pd loadings were employed to produce surface-modified ZnO nanostructure sensors, and their resulting NH₃ sensing capabilities both in dry and humid environments were investigated. Through a comparative gas sensing study between the pure and Pd-loaded ZnO sensors upon exposure to NH₃ at an optimal operating temperature of 350 °C, the Pd-loaded ZnO sensors were found to exhibit enhanced sensor responses and fast response/recovery times. The influence of Pd loading and its successful incorporation into ZnO nanostructure was examined by X-ray diffraction, high resolution-transmission electron microscopy, and X-ray photoelectron spectroscopy. XPS studies demonstrated that in all samples, Pd existed in two chemical states, namely Pd° and Pd²⁺. The possible sensing mechanism related to NH₃ gas is also discussed in detail.

Polyvinyl Acetate Film-Based Quartz Crystal Microbalance for the Detection of Benzene, Toluene, and Xylene Vapors in Air

Vapors of volatile organic compounds such as benzene, toluene, and xylene (BTX) may cause health concerns. The sensitive detection of these compounds in air remains challenging. In this study, we reported on modification of the Quartz Crystal Microbalance (QCM) sensing chip using polyvinyl acetate (PVAc) film as active coating for the analysis of BTX vapors. The PVAc film was deposited on the QCM sensing chip surface by a spin coating technique. The morphology of the PVAc films was confirmed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The sensitivities of PVAc based QCM system for benzene, toluene, and xylene analyses were 0.018, 0.041, and 0.081 Hz/ppm, respectively. The high sensitivity of the proposed QCM system for analysis of BTX vapors is believed to be due to the effective interaction between the PVAc film and BTX molecules. The analyte vapor pressure appears to also affect the sensitivity. These data show that the prepared QCM sensor has a low time constant, good reproducibility, and excellent stability. It offers an alternative to the developed methods for detection of BTX and possibly other aromatic hydrocarbons in the air.

Semiconductor Metal Oxides as Chemoresistive Sensors for Detecting Volatile Organic Compounds

Volatile organic compounds (VOCs), which originate from painting, oil refining and vehicle exhaust emissions, are hazardous gases that have significant effects on air quality and human health. The detection of VOCs is of special importance to environmental safety. Among the various detection methods, chemoresistive semiconductor metal oxide gas sensors are considered to be the most promising technique due to their easy production, low cost and good portability. Sensitivity is an important parameter of gas sensors and is greatly affected by the microstructure, defects, catalyst, heterojunction and humidity. By adjusting the aforementioned factors, the sensitivity of gas sensors can be improved further. In this review, attention will be focused on how to improve the sensitivity of chemoresistive gas sensors towards certain common VOCs with respect to the five factors mentioned above.

P–N heterointerface-determined acetone sensing characteristics of α-MoO3@NiO core@shell nanobelts

α-MoO₃@NiO nanocomposite with well-defined core@shell P–N heterojunction nanobelts was prepared which exhibited heterointerface-determined acetone sensing characteristics.

Coordination Polymer-Derived Multishelled Mixed Ni-Co Oxide Microspheres for Robust and Selective Detection of Xylene

Multishell, stable, porous metal-oxide microspheres (Ni-Co oxides, Co₃O₄ and NiO) have been synthesized through the amorphous coordination polymer-based self-templated method. Both oxides of Ni and Co show poor selectivity to xylene, but the composite phase has substantial selectivity (e.g., S_xylene/ S_ethanol = 2.69) and remarkable sensitivity (11.5-5 ppm xylene at 255 °C). The short response and recovery times (6 and 9 s), excellent humidity-resistance performance (with coefficient of variation = 11.4%), good cyclability, and long-term stability (sensitivity attenuation of ∼9.5% after 30 days and stable sensitivity thereafter) all show that this composite is a competitive solution to the problem of xylene sensing. The sensing performances are evidently due to the high specific surface area and the nano-heterostructure in the composite phase.

Impact of Oxygen Functional Groups on Reduced Graphene Oxide-Based Sensors for Ammonia and Toluene Detection at Room Temperature

The chemically reduced graphene oxide (rGO) was prepared by the reduction of graphene oxide by hydrazine hydrate. By varying the reduction time (10 min, 1 h, and 15 h), oxygen functional groups on rGO were tremendously controlled and they were named RG1, RG2, and RG3, respectively. Here, we investigate the impact of oxygen functional groups on the detection of ammonia and toluene at room temperature. Their effect on sensing mechanism was analyzed by first-principles calculation-based density functional theory. The sensing material was fabricated, and the effect of reduction time shown improved the recovery of ammonia and toluene sensing at room temperature. Structural, morphological, and electrical characterizations were performed on both RG1 and RG3. The sensor response toward toluene vapor of 300 ppm was found to vary 4.4, 2.5, and 3.8% for RG1, RG2, and RG3, respectively. Though RG1 shows higher sensing response with poor recovery, RG3 exhibited complete desorption of toluene after the sensing process with response and recovery times of approximately 40 and 75 s, respectively. The complete recovery of toluene molecules on RG3 is due to the generation of new sites after the reduction of oxygen functionalities on its surface. It could be suggested that these sites provided anchor to ammonia and toluene molecules and good recovery under N₂ purge. Both theoretical and experimental studies revealed that tuning the oxygen functional groups on rGO could play a vital role in the detection of volatile organic compounds (VOCs) on rGO sheets and was discussed in detail. This study could provoke knowledge about rGO-based sensor dependency with oxygen functional groups and shed light on effective monitoring of VOCs under ambient conditions for air quality monitoring applications.

The facile synthesis of MoO3microsheets and their excellent gas-sensing performance toward triethylamine: high selectivity, excellent stability and superior repeatability

In this work, MoO₃microsheets were successfully prepared by thermally oxidizing the MoO₂nanospheres synthesized by a hydrothermal method.

Improved gas sensing properties of silver-functionalized ZnSnO3 hollow nanocubes

Porous silver-functionalized ZnSnO₃ hollow nanocubes as a gas sensor with an ultra-fast response and recovery speed for acetone detection.

Synthesis of Pd-loaded mesoporous SnO2 hollow spheres for highly sensitive and stable methane gas sensors

This work reports a simple, rapid, effective and reliable CH₄ sensor based on Pd-loaded SnO₂ hollow spheres with high surface area and porosity, which is of great importance to gas sensing performance.

Ppt-level benzene detection and gas sensing mechanism using (C4H9NH3)2PbI2Br2 organic–inorganic layered perovskite

Organic–inorganic layered perovskites employed as resistive gas sensing candidates for ppt-level benzene detection at a working temperature of 160 °C.

The Morphologies of the Semiconductor Oxides and Their Gas-Sensing Properties

Semiconductor oxide chemoresistive gas sensors are widely used for detecting deleterious gases due to low cost, simple preparation, rapid response and high sensitivity. The performance of gas sensor is greatly affected by the morphology of the semiconductor oxide. There are many semiconductor oxide morphologies, including zero-dimensional, one-dimensional, two-dimensional and three-dimensional ones. The semiconductor oxides with different morphologies significantly enhance the gas-sensing performance. Among the various morphologies, hollow nanostructures and core-shell nanostructures are always the focus of research in the field of gas sensors due to their distinctive structural characteristics and superior performance. Herein the morphologies of semiconductor oxides and their gas-sensing properties are reviewed. This review also proposes a potential strategy for the enhancement of gas-sensing performance in the future.

Hierarchical and Hollow Fe2O3 Nanoboxes Derived from Metal-Organic Frameworks with Excellent Sensitivity to H2S

Hierarchical and hollow porous Fe₂O₃ nanoboxes (with an average edge length of ∼500 nm) were derived from metal-organic frameworks (MOFs) and the gas sensing characteristics were investigated. Sensors based on Fe₂O₃ nanoboxes exhibited a response (resistance ratio) of 1.23 to 0.25 ppm (ppm) hydrogen sulfide (H₂S) at 200 °C, the response/recovery speed is fast and the selectivity to H₂S is excellent. Remarkably, the sensor showed fully reversible response to 5 ppm of H₂S at 50 °C, demonstrating its promise for operating at near room temperature, which is favorable for medical diagnosis and indoor/outdoor environment monitoring. The excellent performance of the Fe₂O₃ nanoboxes can be ascribed to the unique morphology with high specific surface area (SSA) and porous nanostructure.