Synergistic Effects of a Combination of Cr2O3-Functionalization and UV-Irradiation Techniques on the Ethanol Gas Sensing Performance of ZnO Nanorod Gas Sensors

The Effect of Rare Earths on the Response of Photo UV-Activate ZnO Gas Sensors

In this work, ZnO nanoparticle resistive sensors decorated with rare earths (REs; including Er, Tb, Eu and Dy) were used at room temperature to detect atmospheric pollutant gases (NO₂, CO and CH₄). Sensitive films were prepared by drop casting from aqueous solutions of ZnO nanoparticles (NPs) and trivalent RE ions. The sensors were continuously illuminated by ultraviolet light during the detection processes. The effect of photoactivation of the sensitive films was studied, as well as the influence of humidity on the response of the sensors to polluting gases. Comparative studies on the detection properties of the sensors showed how the presence of REs increased the response to the gases detected. Low concentrations of pollutant gases (50 ppb NO₂, 1 ppm CO and 3 ppm CH₄) were detected at room temperature. The detection mechanisms were then discussed in terms of the possible oxidation-reduction (redox) reaction in both dry and humid air atmospheres.

Proof of Concept for Operando Infrared Spectroscopy Investigation of Light-Excited Metal Oxide-Based Gas Sensors

Light-excitation of semiconducting metal-oxide (SMOX)-based gas sensors is a promising means to lower their operation temperature, thereby reducing power consumption, which would allow for their broader application. Despite increased research interest in light-excited gas sensors, progress has been slow because of a lack of mechanistic understanding. Notably, significant differences between light-excitation and, the better understood, thermal-excitation mechanisms have been identified. Insights from operando spectroscopic studies have been key to understanding the surface chemistry that determines the performance of thermally activated SMOX, but they have not yet been performed on illuminated sensors. Here, for the first time, we demonstrate that it is possible to perform operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy measurements on sensors under illumination. We demonstrate the benefits of the approach and show that under light illumination the splitting of water on the WO₃ surface is responsible for the increased resistance of the sensor during exposure to high humidity.

Oxygen sensing with individual ZnO:Sb micro-wires: effects of temperature and light exposure on the sensitivity and stability

Nanostructured ZnO has been widely investigated as a gas sensing material. Antimony is an important dopant for ZnO that catalyses its surface reactivity and thus strengthens its gas sensing capability. However, there are not enough studies on the gas sensing of antimony-doped ZnO single wires. We fabricated and characterized ZnO/ZnO:Sb core-shell micro-wires and demonstrated that individual wires are sensitive to oxygen gas flow. Temperature and light illumination strongly affect the oxygen gas sensitivity and stability of these individual wires. It was found that these micro- and nano-wire oxygen sensors at 200°C give the highest response to oxygen, yet a vanishingly small effect of light and temperature variations. The underlying physics and the interplay between these effects are discussed in terms of surface-adsorbed oxygen, oxygen vacancies and hydrogen doping.

Size-Controlled Au Nanoparticles Incorporating Mesoporous ZnO for Sensitive Ethanol Sensing

Zinc oxide (ZnO) as a commonly used semiconductor material has aroused extensive research attention in various fields, such as field-effect transistors, solar cells, luminescent devices, and sensors, because of its excellent light-electrical features and large exciton bonding energy. Herein, ultrasmall Au nanoparticles with tunable size decorated mesoporous ZnO nanospheres were synthesized via facile formaldehyde-assisted metal-ligand cross-linking strategy, where these active Au species could be transferred into Au nanoparticles in the frameworks by various reduction strategies. Typically, mesoporous ZnO-Au with a photoreduction technique showed superior ethanol sensing performance (ca. 159 for 50 ppm at 200 °C) because of its high surface area, dual-mesoporous structure, and interface effect (electron effect, surface catalytic/adsorption). Moreover, the mesoporous ZnO-Au composites by photoreduction show much better performance than those via H2 reduction and NaBH4 reduction, which is ascribed to the providential size of Au nanoparticles (ca. 6.6 nm) and abundant oxygen defects in the composites. In particular, the selectivity and sensitivity of mesoporous ZnO-Au far exceeds those of materials loaded with other noble metals (Pt, Pd, and Ag). The sensing mechanism of mesoporous ZnO-Au for ethanol is attributed to classical surface adsorption/catalytic reaction, where strong sensitization effect (electron and chemical) and the spillover effect of Au nanoparticles in the catalytic reaction cause superior ethanol sensing performances. In situ FTIR and GC-MS measurement revealed that the catalytic oxidation of ethanol follows the process of dehydrogenation and deep oxidation, that is, dehydrogenation to acetaldehyde, and then further oxidation to carbon dioxide and water.

Layered Double Oxides for Sensing Reducing Volatile Organic Compounds: The Effect of Local Charge Region Modulation

Multi-metal oxides with uniform distribution of various metal elements have potential for an enhanced gas-sensing response due to the strong heterogeneous and synergistic effects involved. In this study, three layered double oxides, labeled as CuCr-, ZnCr-, and ZnTi-LDOs, respectively, were prepared with corresponding LDHs (layered double hydroxides) as precursors and self-sacrificial templates. The elemental mapping confirms the uniform distribution of hetero-metal elements in whole LDOs. The CuCr-LDOs exhibits a much larger sensing response towards reducing VOCs at room temperature, which is 3.5 or 13.3 times that of ZnCr- or ZnTi-LDOs, respectively. The response differences are analyzed in terms of the local charge region modulation associated with heterojunction formation, and it is further demonstrated based on first-principles calculations and valence electron theory. The present work suggests a possible strategy for developing highly sensitive oxide-based gas sensors for VOCs detection.

Rational Design of Semiconductor-Based Chemiresistors and their Libraries for Next-Generation Artificial Olfaction

Artificial olfaction based on gas sensor arrays aims to substitute for, support, and surpass human olfaction. Like mammalian olfaction, a larger number of sensors and more signal processing are crucial for strengthening artificial olfaction. Due to rapid progress in computing capabilities and machine-learning algorithms, on-demand high-performance artificial olfaction that can eclipse human olfaction becomes inevitable once diverse and versatile gas sensing materials are provided. Here, rational strategies to design a myriad of different semiconductor-based chemiresistors and to grow gas sensing libraries enough to identify a wide range of odors and gases are reviewed, discussed, and suggested. Key approaches include the use of p-type oxide semiconductors, multinary perovskite and spinel oxides, carbon-based materials, metal chalcogenides, their heterostructures, as well as heterocomposites as distinctive sensing materials, the utilization of bilayer sensor design, the design of robust sensing materials, and the high-throughput screening of sensing materials. In addition, the state-of-the-art and key issues in the implementation of electronic noses are discussed. Finally, a perspective on chemiresistive sensing materials for next-generation artificial olfaction is provided.

Growth and NO2-Sensing Properties of Biaxial p-SnO/n-ZnO Heterostructured Nanowires

Biaxial p-SnO/n-ZnO heterostructured nanowires (average length of 10 μm) were grown onto a glass substrate by thermal evaporation in vacuum. These nanowires had spherical ball tips, and the size of the SnO part increased gradually from the top to the bottom of the nanowire, but the corresponding size of ZnO varied slightly. The Sn-Zn alloy formed in the tips resulted in determined as the catalyst of the growth of the ZnO nanowires. The growth process of the p-SnO/n-ZnO biaxial nanowires is discussed based on vapor-liquid-solid (VLS) based on the subsequent growth process: the VLS catalytic growth of the ZnO nanowire and subsequent epitaxial SnO growth on the sidewall of the pregrown ZnO nanowire. An epitaxial relationship, (001)_SnO//(110)_ZnO and [110]_SnO//[002]_ZnO, was observed in the biaxial p-SnO/n-ZnO heterostructured nanowires. The gas-sensing properties of the as-synthesized p-SnO/n-ZnO nanowires were investigated. The results show that the device exhibit a good performance to the ppb-level NO₂ at room temperature (25 °C) without light illumination. The detection limit of the p-SnO/n-ZnO sensor to NO₂ is 50 ppb. Moreover, the NO₂-sensing properties of the p-SnO/n-ZnO device were investigated under various relative humidity. Finally, the NO₂-sensing mechanism of the p-SnO/n-ZnO nanowires was proposed and discussed.

Synthesis of a CeO2/Co3O4 catalyst with a remarkable performance for the soot oxidation reaction

A highly active catalyst based on CeO₂/Co₃O₄ for soot oxidation at low temperatures with complete selectivity toward CO₂.

Ultrasensitive resistivity-based ethanol sensor based on the use of CeO2-Fe2O3 core-shell microclusters

This paper presents a method for synthesis of CeO₂-Fe₂O₃ core-shell nanoparticles (CSNPs). These are shown to display enhanced ethanol sensing properties. Synthesis was done via a two-step process, starting with co-precipitation and followed by applying a sol-gel method. High resolution electron microscopy results revealed the core-shell nature of the particles. Surface morphological studies of the CSNPs showed a microcluster-like structure which is assumed to be responsible for the enhanced sensing response. X-ray photoelectron spectroscopy revealed valence states of Fe(III) and Ce(IV). The material was used in a resisitive sensor for ethanol vapor at room temperature (RT), at a typically applied voltage of 5 V. The response of the sensor is higher than that of pristine CeO₂ or Fe₂O₃ sensors towards 100 ppm of ethanol at RT. The lower detection limit is 1 ppm (with a signal change of 23). The response and recovery times are as short as 3 and 7 s, respectively. The sensing mechanism is discussed in detail with respect to n-n heterojunctions formed between n-CeO₂ and n-Fe₂O₃, high catalytic activity of the Fe₂O₃, and microcluster-like structures of the particles. Graphical abstract Schematic representation of gas sensing mechanism of CeO₂-Fe₂O₃ core-shell nanoparticles (c) along with their morphological images (a&b).

On-chip grown ZnO nanosheet-array with interconnected nanojunction interfaces for enhanced optoelectronic NO2 gas sensing at room temperature

Herein, we demonstrated the on-chip growth of nanostructured ZnO films with abundant nanojunctions for the fabrication of high-performance optoelectronic NO₂ sensors. A fast solution approach allowed the controllable growth of ZnO nanorod- and nanosheet-arrays directly on flexible substrates, which were endowed with abundant nanojunctions. Electron microscopy observations revealed the existence of two types of the nanojunction interfaces, i.e., the attached and interconnected interfaces within the nanostructure networks. Compared with the attached nanorods, the optoelectronic NO₂ sensors based on interconnected ZnO nanosheets showed higher responses and faster response/recovery rates under UV illumination at room temperature. The responses of the nanosheet-based sensor ranged from 28% to 610% toward NO₂ concentrations of 10 ppb to 1000 ppb. Moreover, the optoelectronic sensors exhibited excellent reversibility, and mechanical and long-term stabilities along with low detection limits. The enhanced optoelectronic NO₂ sensing properties of the interconnected ZnO nanosheets could be attributed to different types of nanojunction interfaces, which played a key role in modulating the interfacial potential barrier heights of the nanojunctions according to the surface depletion model. The presently developed strategy of nanojunction interface engineering is expected to have wide interest for semiconductor-based sensor applications.

Gold and ZnO-Based Metal-Semiconductor Network for Highly Sensitive Room-Temperature Gas Sensing

Metal-semiconductor junctions and interfaces have been studied for many years due to their importance in applications such as semiconductor electronics and solar cells. However, semiconductor-metal networks are less studied because there is a lack of effective methods to fabricate such structures. Here, we report a novel Au-ZnO-based metal-semiconductor (M-S)_n network in which ZnO nanowires were grown horizontally on gold particles and extended to reach the neighboring particles, forming an (M-S)_n network. The (M-S)_n network was further used as a gas sensor for sensing ethanol and acetone gases. The results show that the (M-S)_n network is sensitive to ethanol (28.1 ppm) and acetone (22.3 ppm) gases and has the capacity to recognize the two gases based on differences in the saturation time. This study provides a method for producing a new type of metal-semiconductor network structure and demonstrates its application in gas sensing.

Ag2S-Sensitized NiO-ZnO Heterostructures with Enhanced Visible Light Photocatalytic Activity and Acetone Sensing Property

Visible light-driven Ag2S-grafted NiO-ZnO ternary nanocomposites are synthesized using a facile and cost-effective homogeneous precipitation method. The structural, morphological, and optical properties were extensively studied, confirming the formation of ternary nanocomposites. The surface area of the synthesized nanocomposites was calculated by electrochemical double-layer capacitance (C dl). Ternary Ag2S/NiO-ZnO nanocomposites showed excellent visible light photocatalytic property which increases further with the concentration of Ag2S. The maximum photocatalytic activity was shown by 8% Ag2S/NiO-ZnO with a RhB degradation efficiency of 95%. Hydroxyl and superoxide radicals were found to be dominant species for photodegradation of RhB, confirmed by scavenging experiments. It is noteworthy that the recycling experiments demonstrated high stability and recyclable nature of the photocatalyst. Moreover, the electrochemical results indicated that the prepared nanocomposite exhibits remarkable activity toward detection of acetone. The fabricated nanocomposite sensor showed high sensitivity (4.0764 μA mmol L-1 cm-2) and a lower detection limit (0.06 mmol L-1) for the detection of acetone. The enhanced photocatalytic and the sensing property of Ag2S/NiO-ZnO can be attributed to the synergistic effects of strong visible light absorption, excellent charge separation, and remarkable surface properties.

Thermo-Electro-Mechanical Simulation of Semiconductor Metal Oxide Gas Sensors

There is a growing demand in the semiconductor industry to integrate many functionalities on a single portable device. The integration of sensor fabrication with the mature CMOS technology has made this level of integration a reality. However, sensors still require calibration and optimization before full integration. For this, modeling and simulation is essential, since attempting new, innovative designs in a laboratory requires a long time and expensive tests. In this manuscript we address aspects for the modeling and simulation of semiconductor metal oxide gas sensors, devices which have the highest potential for integration because of their CMOS-friendly fabrication capability and low operating power. We analyze recent advancements using FEM models to simulate the thermo-electro-mechanical behavior of the sensors. These simulations are essentials to calibrate the design choices and ensure low operating power and improve reliability. The primary consumer of power is a microheater which is essential to heat the sensing film to appropriately high temperatures in order to initiate the sensing mechanism. Electro-thermal models to simulate its operation are presented here, using FEM and the Cauer network model. We show that the simpler Cauer model, which uses an electrical circuit to model the thermo-electrical behavior, can efficiently reproduce experimental observations.

Improvement on Fluorine Migration from SF6 to SiF4 by an Efficient Mediator of Fe2O3/Cr2O3 Composites

An economic and facile method was urgently required for the degradation of SF₆ to replace the high-energy excitation treatment. Both theoretical calculations and experimental observations were conducted to reveal the synergy of Cr/Fe/Si composites on a new technique of SF₆ degradation through reacting silicon dioxide. Density functional theory (DFT) calculations show that strong adsorption of SF₆ on Cr₂O₃, and then the fast F/O exchange between CrF₃ and Fe₂O₃ (energy barrier was 1.45 eV) as well as FeF₃ and SiO₂ (energy barrier was 1.69 eV) enhanced mediated efficiency from SF₆ to SiF₄. The fluorine (F) migration between solid interfaces in Cr₂O₃&Fe₂O₃@SBA15 was responsible for efficient SF₆ removal. The F migration route was composed of SF₆ to CrF₃, CrF₃ to FeF₃, and FeF₃ to SiF₄ with the lowest thermodynamic driving. Enhanced specific accumulative converted amount (SACA) of SF₆ on Cr₂O₃&Fe₂O₃@SBA15 was achieved and the highest SACA was 13.98 mmol/g within 7 h, significantly higher than that on Fe₂O₃@SBA15 (5.74 mmol/g) and Cr₂O₃@SBA15 (2.71 mmol/g). Moreover, X-ray diffractometry and X-ray photoelectron spectroscopy were performed to support DFT calculations, including ion intensities detected using mass spectroscopy and composition analysis of the mediator during the reaction. Therefore, our work put forward a novel approach for economic and efficient SF₆ decomposition through reacting with silicon dioxide under the mediation of Cr₂O₃&Fe₂O₃. This method was also potentially used in effective degradation of refractory non-metal halides.

NiO decorated CeO2 nanostructures as room temperature isopropanol gas sensors

Heterostructures developed using CeO2 show promising peculiarities in the field of metal oxide gas sensors due to the great variations in the resistance during the adsorption and desorption processes. NiO decorated CeO2 nanostructures (NiO/CeO2) were synthesized via a facile two-step process. High resolution transmission electron microscopy (HRTEM) results revealed the perfect decoration of NiO on the CeO2 surface. The porous nature of the NiO/CeO2 sensor surface was confirmed from scanning electron microscopy (SEM) analysis. Gas sensing studies of pristine CeO2 and NiO/CeO2 sensors were performed under room conditions and enhanced gas sensing properties for the NiO/CeO2 sensor towards isopropanol were observed. Decoration of NiO on the CeO2 surface develops a built-in potential at the interface of NiO and CeO2 which played a vital role in the superior sensing performance of the NiO/CeO2 sensor. Sharp response and recovery times (15 s/19 s) were observed for the NiO/CeO2 sensor towards 100 ppm isopropanol at room temperature. Long-term stability of the NiO/CeO2 sensor was also studied and discussed. From all the results it is concluded that the decoration of NiO on the CeO2 surface could significantly enhance the sensing performance and it has great advantages in designing best performing isopropanol gas sensors.

Large-Scale Synthesis of Hierarchically Porous ZnO Hollow Tubule for Fast Response to ppb-Level H2S Gas

Response and recovery time to toxic and inflammable hydrogen sulfide (H₂S) gas are important indexes for metal oxide sensors in real-time environmental monitoring. However, large-scale production of ZnO-based sensing materials for fast response to ppb-level H₂S has been rarely reported. In this work, hierarchically porous hexagonal ZnO hollow tubule was simply fabricated by zinc salt impregnation and subsequently calcination using absorbent cotton as the template. The influence of calcination temperature on the corresponding morphology and sensing properties is also explored. The hollow tubules calcined at 600 °C are constructed from abundant cross-linked nanoparticles (∼20 nm). Its Brunauer-Emmett-Teller surface area is 31 m²·g^-1 and the meso- and macroporous sizes are centered at 35 and 115 nm, respectively. The sensor with a lower detection limit of 10 ppb exhibits a fast response speed of 29 s toward the 50 ppb H₂S rather than those of the reported intrinsic and doped ZnO-based sensing materials. Furthermore, the sensor shows a wide linear range (10-1000 ppb), good reproducibility, and stability. Such excellent trace ppb-level H₂S performances are mainly related to the inherent characteristics of hierarchically porous hollow tubular structure and the surface-adsorbed oxygen control type mechanism.

One-Dimensional Zinc Oxide Decorated Cobalt Oxide Nanospheres for Enhanced Gas-Sensing Properties

In this study, one-dimensional (1D) zinc oxide was loaded on the surface of cobalt oxide microspheres, which were assembled by single-crystalline porous nanosheets, via a simple heteroepitaxial growth process. This elaborate structure possessed an excellent transducer function from the single-crystalline feature of Co₃O₄ nanosheets and the receptor function from the zinc oxide nanorods. The structure of the as-prepared hybrid was confirmed via a Scanning Electron Microscope (SEM), X-ray diffraction (XRD), and a Transmission Electron Microscope (TEM). Gas-sensing tests showed that the gas-sensing properties of the as-designed hybrid were largely improved. The response was about 161 (R_a/R_g) to 100 ppm ethanol, which is 110 and 10 times higher than that of Co₃O₄ (R_g/R_a = 1.47) and ZnO (R_a/R_g = 15), respectively. And the as-designed ZnO/Co₃O₄ hybrid also showed a high selectivity to ethanol. The superior gas-sensing properties were mainly attributed to the as-designed nanostructures that contained a super transducer function and a super receptor function. The design strategy for gas-sensing materials in this work shed a new light on the exploration of high-performance gas sensors.

Revealing the Relationship between Energy Level and Gas Sensing Performance in Heteroatom-Doped Semiconducting Nanostructures

The cation substitutional doping of metal oxide semiconductors plays pivotal roles in improving the gas sensing performances, but the doping effect on surface sensing reaction is still not well understood. In this study, indium oxides doped with various heteroatoms are investigated to obtain in-depth understanding of how doping (or the resulting change in the electronic structure) alters the surface-absorbed oxygen chemistry and subsequent sensing process. The experimental results reveal that energy level of In₂O₃ can be modulated by introduction of these dopants, some of which (e.g., Al, Ga, and Zr) lead to the elevation of Fermi level, whereas others (e.g., Ti, V, Cr, Mo, W, and Sn) bring about relative drop in Fermi level. However, only the former can improve the response to formaldehyde, indicating a strong link between Fermi level and sensing properties. Mechanistic study suggests that the elevation of Fermi level increases energy level difference between oxide semiconductor and oxygen molecules and facilitates the surface absorption of oxygen species, resulting in superior formaldehyde sensing activity. Especially, Al-doped In₂O₃ exhibits remarkably enhanced sensing performances toward formaldehyde at low working temperature (150 °C) with high response, good selectivity, ultralow limit of detection (60 ppb), and short response time (2-23 s). Our findings not only promote the understanding of sensing reaction process and its correlation with the semiconductor electronic structure but also offer a general guideline for large-scale screening of promising oxide semiconductor-based sensing materials for gas detection.

Pt-decorated zinc oxide nanorod arrays with graphitic carbon nitride nanosheets for highly efficient dual-functional gas sensing

In this work, well-aligned ZnO nanorods were grown on the substrate of exfoliated g-C₃N₄ nanosheets via a microwave-assisted hydrothermal synthesis, and then Pt/ZnO/g-C₃N₄ nanostructures were obtained after the deposition of Pt nanoparticles. The growth of vertically ordered ZnO nanorods was occurred on g-C₃N₄ nanosheets through the bonding interaction between Zn and N atoms, which was confirmed by XPS, FT-IR data and molecular orbital theory. The Pt/ZnO/g-C₃N₄ nanostructures sensor exhibited the remarkable sensitivity, selectivity, and fast response/recovery time for air pollutants of ethanol and NO₂. The application of Pt/ZnO/g-C₃N₄ nanostructures could be used as a dual-functional gas sensor through the controlled working temperature. Besides, the Pt/ZnO/g-C₃N₄ nanostructures sensor could be applied to the repeating detection of ethanol and NO₂ in the natural environment. The synergistic effect and improved the separation of electron-hole pairs in Pt/ZnO/g-C₃N₄ nanostructures had been verified for the gas sensing mechanism. Additionally, Pt/ZnO/g-C₃N₄ nanostructures revealed the excellent charge carriers transport properties in electrochemical impedance spectroscopy (EIS), such as the longer electron lifetime (τ_n), higher electron diffusion coefficient (D_n) and bigger effective diffusion length (L_n), which also played an important role for Pt/ZnO/g-C₃N₄ nanostructures with striking gas sensing activities.

Octahedral-Like CuO/In2O3 Mesocages with Double-Shell Architectures: Rational Preparation and Application in Hydrogen Sulfide Detection

This contribution describes a facile strategy for constructing octahedral-like CuO/In₂O₃ mesocages with double-shell architectures. The synthetic method included first preparation of unifrom Cu₂O as an ideal self-sacrificial template and then decoration by a In₂O₃ outer layer through room-temperature Cu₂O-engaged redox etching reaction combined with subsequent annealing process. Various characterization techniques manifested that In₂O₃ nanoparticles were uniformly grown on the surface of CuO mesocages, resulting in a well-defined double-shelled heterostructure. When evaluated as a novel sensing material for hydrogen sulfide (H₂S) detection, the resultant octahedral-like CuO/In₂O₃ heterostructures exhibited obviously enhanced sensing response, lower operating temperature as well as faster response/recover speed during the dynamic measurement compared to the pristine CuO particles, which is likely related to the high-level of adsorbed oxygen concentration, resistance modulation effect, and unique microstructure of as-prepared CuO/In₂O₃ heterostructure.

Controllable Evolution of Dual Defect Zni and VO Associate-Rich ZnO Nanodishes with (0001) Exposed Facet and Its Multiple Sensitization Effect for Ethanol Detection

Building an effective way for finding the role of surface defects in gas sensing property remains a big challenge. In the present work, we synthesized the ZnO nanodishes (NDs) and first explored the formation process of rich electron donor surface defects by means of studying mechanism for the ZnO NDs synthesis. The test results revealed that ZnO-6, added by 6 mmol Zn powder, had the best gas-sensing properties with the excellent selectivity to ethanol than the others. Specially, the ZnO-6 sensor exhibited the best response (about 49) to 100 ppm ethanol at 230 °C among four as-synthesized samples, while noncustomized ZnO was only 28. It was mainly caused by the following two reasons: the exposure of target (0001) crystal facet and rich electron donor surface defects zinc interstitial (Zn_i) and oxygen vacancy (V_O). As a guide, the formation process of surface defects was revealed by an ideal defect model. By the small-angle XRD and TEM patterns, we could conclude that ZnO NDs, changing stoichiometric ratio, increased the content of Zn_i by adding Zn powder, while excessive Zn powder promoted the growth of c axis of ZnO NDs in the self-assembly engineering. Besides, a depletion model has been provided to explain how the surface defects work on the sensors and the complex mechanism of gas sensing performance. These findings will develop the application of ZnO-based gas sensor in health and security.

ZnO Nanowire Application in Chemoresistive Sensing: A Review

This article provides an overview of the recent development of ZnO nanowires (NWs) for chemoresistive sensing. Working mechanisms of chemoresistive sensors are unified for gas, ultraviolet (UV) and bio sensor types: single nanowire and nanowire junction sensors are described, giving the overview for a simple sensor manufacture by multiple nanowire junctions. ZnO NW surface functionalization is discussed, and how this effects the sensing is explained. Further, novel approaches for sensing, using ZnO NW functionalization with other materials such as metal nanoparticles or heterojunctions, are explained, and limiting factors and possible improvements are discussed. The review concludes with the insights and recommendations for the future improvement of the ZnO NW chemoresistive sensing.

Wafer-Scale Synthesized MoS2/Porous Silicon Nanostructures for Efficient and Selective Ethanol Sensing at Room Temperature

This paper presents the performance of a highly selective ethanol sensor based on MoS₂-functionalized porous silicon (PSi). The uniqueness of the sensor includes its method of fabrication, wafer scalability, affinity for ethanol, and high sensitivity. MoS₂ nanoflakes (NFs) were synthesized by sulfurization of oxidized radio-frequency (RF)-sputtered Mo thin films. The MoS₂ NFs synthesis technique is superior in comparison to other methods, because it is chip-scalable and low in cost. Interdigitated electrodes (IDEs) were used to record resistive measurements from MoS₂/PSi sensors in the presence of volatile organic compound (VOC) and moisture at room temperature. With the effect of MoS₂ on PSi, an enhancement in sensitivity and a selective response for ethanol were observed, with a minimum detection limit of 1 ppm. The ethanol sensitivity was found to increase by a factor of 5, in comparison to the single-layer counterpart levels. This impressive response is explained on the basis of an analytical resistive model, the band gap of MoS₂/PSi/Si, the interface formed between MoS₂ and PSi, and the chemical interaction of the vapor molecules and the surface. This two-dimensional (2D) composite material with PSi paves the way for efficient, highly responsive, and stable sensors.

Modeling the sensing characteristics of chemi-resistive thin film semi-conducting gas sensors

Modeling of sensor response with the operating temperature and thickness of the sensing film.

“Close network” effect of a ZnO micro/nanoporous array allows high UV-irradiated NO2 sensing performance

The “close network” effect of a ZnO micro/nanoporous array allows high UV-irradiated NO₂ sensing performance at room temperature.

Improved ethanol gas sensing performances of a ZnO/Co3O4 composite induced by its flytrap-like structure

Nanocomposite materials with excellent receptor and transducer functions are promising in ameliorating their gas sensing properties.