Synthesis of Numerous Edge Sites in MoS2 via SiO2 Nanorods Platform for Highly Sensitive Gas Sensor

Recent Advances in Engineering of 2D Layered Metal Chalcogenides for Resistive-Type Gas Sensor

2D nanomaterials have triggered widespread attention in sensing applications. Especially for 2D layered metal chalcogenides (LMCs), the unique semiconducting properties and high surface area endow them with great potential for gas sensors. The assembly of 2D LMCs with guest species is an effective functionalization method to produce the synergistic effects of hybridization for greatly enhancing the gas-sensing properties. This review starts with the synthetic techniques, sensing properties, and principles, and then comprehensively compiles the advanced achievements of the pristine 2D LMCs gas sensors. Key advances in the development of the functionalization of 2D LMCs for enhancing gas-sensing properties are categorized according to the spatial architectures. It is systematically discussed in three aspects: surface, lattice, and interlayer, to comprehend the benefits of the functionalized 2D LMCs from surface chemical effect, electronic properties, and structure features. The challenges and outlooks for developing high-performance 2D LMCs-based gas sensors are also proposed.

Recent Advances in Chemoresistive Gas Sensors Using Two-Dimensional Materials

Two-dimensional (2D) materials have emerged as a promising candidate in the chemoresistive gas sensor field to overcome the disadvantages of conventional metal-oxide semiconductors owing to their strong surface activities and high surface-to-volume ratio. This review summarizes the various approaches to enhance the 2D-material-based gas sensors and provides an overview of their progress. The distinctive attributes of semiconductor gas sensors employing 2D materials will be highlighted with their inherent advantages and associated challenges. The general operating principles of semiconductor gas sensors and the unique characteristics of 2D materials in gas-sensing mechanisms will be explored. The pros and cons of 2D materials in gas-sensing channels are discussed, and a route to overcome the current challenges will be delivered. Finally, the recent advancements to enhance the performance of 2D-material-based gas sensors including photo-activation, heteroatom doping, defect engineering, heterostructures, and nanostructures will be discussed. This review should offer a broad range of readers a new perspective toward the future development of 2D-material-based gas sensors.

Vertical heterostructure of graphite-MoS2 for gas sensing

2D materials, given their form-factor, high surface-to-volume ratio, and chemical functionality have immense use in sensor design. Engineering 2D heterostructures can result in robust combinations of desirable properties but sensor design methodologies require careful considerations about material properties and orientation to maximize sensor response. This study introduces a sensor approach that combines the excellent electrical transport and transduction properties of graphite film with chemical reactivity derived from the edge sites of semiconducting molybdenum disulfide (MoS2) through a two-step chemical vapour deposition method. The resulting vertical heterostructure shows potential for high-performance hybrid chemiresistors for gas sensing. This architecture offers active sensing edge sites across the MoS2 flakes. We detail the growth of vertically oriented MoS2 over a nanoscale graphite film (NGF) cross-section, enhancing the adsorption of analytes such as NO2, NH3, and water vapor. Raman spectroscopy, density functional theory calculations and scanning probe methods elucidate the influence of chemical doping by distinguishing the role of MoS2 edge sites relative to the basal plane. High-resolution imaging techniques confirm the controlled growth of highly crystalline hybrid structures. The MoS2/NGF hybrid structure exhibits exceptional chemiresistive responses at both room and elevated temperatures compared to bare graphitic layers. Quantitative analysis reveals that the sensitivity of this hybrid sensor surpasses other 2D material hybrids, particularly in parts per billion concentrations.

Edge sulfur vacancies riched MoS2 nanosheets assist PEDOT:PSS flexible film ammonia sensing enhancement for wireless greenhouse vegetables monitoring

Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is a promising NH3 sensing material owing to its super high electrical conductivity, excellent environmental stability, and reversible doping/dedoping nature. However, the low sensitivity and sluggish recovery rate limit its further application in gas sensors. Herein, exfoliated layered MoS2 nanosheets with large-specific surface area and abundant edge sulfur (S) vacancies are utilized to assist PEDOT:PSS and achieve ideal improvement in NH3 sensing performance at room temperature (RT), including high response values, fast response/recovery ability, and excellent sensing stability in complex environment. MoS2 nanosheets are combined with PEDOT:PSS to construct p-n heterojunction, the S vacancies can improve carrier transfer rate and serve as conductive bridge, effective active sites for NH3 adsorption, this series of performance improvement strategies is the significance of this work. Meanwhile, the density-functional theory (DFT), current-voltage (I-V), and in-situ FITR are firstly employed to discuss the sensing mechanisms in detail. Furthermore, integrating MoS2/PEDOT:PSS flexible sensor into a designed printed circuit board to intelligent, visual, and wireless real-time monitoring the NH3 resistance information in a simulated greenhouse vegetables equipment through the smartphone APP has also been successfully implemented.

Chemiresistive gas sensor based on Mo0.5W0.5S2 alloy nanoparticles with good selectivity and ppb-level limit of detection to ammonia

Transition metal dichalcogenides (TMDs) are promising materials for chemiresistive gas sensor, while TMD alloys (two chalcogenide or/and metal elements) with tunable electronic structures have drawn little attention in gas sensing. Herein, Mo_0.5W_0.5S₂ alloy nanoparticles (NPs) were prepared by a facile sonication exfoliation method and then tested for ammonia sensing. The crystal structure, geometric morphology, and elemental composition of Mo_0.5W_0.5S₂ NPs were investigated. The gas sensing measurements demonstrated Mo_0.5W_0.5S₂ NPs with good response to ammonia at 80 °C with a limit of detection down to 500 part per billion (ppb). The sensor also displayed good stability as well as superb selectivity to ammonia in the presence of interferences, such as methanol, acetone, benzene, and cyclohexane. The theoretical calculations revealed Mo and W atoms at edges (such as Mo_0.5W_0.5S₂ (010)) of sheet-like NPs as the active sites for ammonia adsorption. Electrons donated by the adsorbed ammonia were combined with holes in p-type Mo_0.5W_0.5S₂ NPs, and the concentration of the main charge carrier was reduced, resulting in resistance enhancement.

Patching sulfur vacancies: A versatile approach for achieving ultrasensitive gas sensors based on transition metal dichalcogenides

Transition metal dichalcogenides (TMDCs) garner significant attention for their potential to create high-performance gas sensors. Despite their favorable properties such as tunable bandgap, high carrier mobility, and large surface-to-volume ratio, the performance of TMDCs devices is compromised by sulfur vacancies, which reduce carrier mobility. To mitigate this issue, we propose a simple and universal approach for patching sulfur vacancies, wherein thiol groups are inserted to repair sulfur vacancies. The sulfur vacancy patching (SVP) approach is applied to fabricate a MoS₂-based gas sensor using mechanical exfoliation and all-dry transfer methods, and the resulting 4-nitrothiophenol (4NTP) repaired molybdenum disulfide (4NTP-MoS₂) is prepared via a sample solution process. Our results show that 4NTP-MoS₂ exhibits higher response (increased by 200 %) to ppb-level NO₂ with shorter response/recovery times (61/82 s) and better selectivity at 25 °C compared to pristine MoS₂. Notably, the limit of detection (LOD) toward NO₂ of 4NTP-MoS₂ is 10 ppb. Kelvin probe force microscopy (KPFM) and density functional theory (DFT) reveal that the improved gas sensing performance is mainly attributed to the 4NTP-induced n-doping effect on MoS₂ and the corresponding increment of surface absorption energy to NO₂. Additionally, our 4NTP-induced SVP approach is universal for enhancing gas sensing properties of other TMDCs, such as MoSe₂, WS₂, and WSe₂.

Role of graphene quantum dots with discrete band gaps on SnO2 nanodomes for NO2 gas sensors with an ultralow detection limit

NO₂ is a major air pollutant that should be monitored due to its harmful effects on the environment and human health. Semiconducting metal oxide-based gas sensors have been widely explored owing to their superior sensitivity towards NO₂, but their high operating temperature (>200 °C) and low selectivity still limit their practical use in sensor devices. In this study, we decorated graphene quantum dots (GQDs) with discrete band gaps onto tin oxide nanodomes (GQD@SnO₂ nanodomes), enabling room temperature (RT) sensing towards 5 ppm NO₂ gas with a noticeable response ((R_a/R_g) - 1 = 4.8), which cannot be matched using pristine SnO₂ nanodomes. In addition, the GQD@SnO₂ nanodome based gas sensor shows an extremely low detection limit of 1.1 ppb and high selectivity compared to other pollutant gases (H₂S, CO, C₇H₈, NH₃, and CH₃COCH₃). The oxygen functional groups in GQDs specifically enhance NO₂ accessibility by increasing the adsorption energy. Strong electron transfer from SnO₂ to GQDs widens the electron depletion layer at SnO₂, thereby improving the gas response over a broad temperature range (RT-150 °C). This result provides a basic perspective for utilizing zero-dimensional GQDs in high-performance gas sensors operating over a wide range of temperatures.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Drastic Gas Sensing Selectivity in 2-Dimensional MoS2 Nanoflakes by Noble Metal Decoration

Noble metal nanoparticle decoration is a representative strategy to enhance selectivity for fabricating chemical sensor arrays based on the 2-dimensional (2D) semiconductor material, represented by molybdenum disulfide (MoS₂). However, the mechanism of selectivity tuning by noble metal decoration on 2D materials has not been fully elucidated. Here, we successfully decorated noble metal nanoparticles on MoS₂ flakes by the solution process without using reducing agents. The MoS₂ flakes showed drastic selectivity changes after surface decoration and distinguished ammonia, hydrogen, and ethanol gases clearly, which were not observed in general 3D metal oxide nanostructures. The role of noble metal nanoparticle decoration on the selectivity change is investigated by first-principles density functional theory (DFT) calculations. While the H₂ sensitivity shows a similar tendency with the calculated binding energy, that of NH₃ is strongly related to the binding site deactivation due to preferred noble metal particle decoration at the MoS₂ edge. This finding is a specific phenomenon which originates from the distinguished structure of the 2D material, with highly active edge sites. We believe that our study will provide the fundamental comprehension for the strategy to devise the highly efficient sensor array based on 2D materials.

Effect of Measurement System Configuration and Operating Conditions on 2D Material-Based Gas Sensor Sensitivity

Gas sensors applied in real-time detection of toxic gas leakage, air pollution, and respiration patterns require a reliable test platform to evaluate their characteristics, such as sensitivity and detection limits. However, securing reliable characteristics of a gas sensor is difficult, owing to the structural difference between the gas sensor measurement platform and the difference in measurement methods. This study investigates the effect of measurement conditions and system configurations on the sensitivity of two-dimensional (2D) material-based gas sensors. Herein, we developed a testbed to evaluate the response characteristics of MoS₂-based gas sensors under a NO₂ gas flow, which allows variations in their system configurations. Additionally, we demonstrated that the distance between the gas inlet and the sensor and gas inlet orientation influences the sensor performance. As the distance to the 2D gas sensor surface decreased from 4 to 2 mm, the sensitivity of the sensor improved to 9.20%. Furthermore, when the gas inlet orientation was perpendicular to the gas sensor surface, the sensitivity of the sensor was the maximum (4.29%). To attain the optimum operating conditions of the MoS₂-based gas sensor, the effects of measurement conditions, such as gas concentration and temperature, on the sensitivity of the gas sensor were investigated.

Photothermally Crumpled MoS2 Film as an Omnidirectionally Stretchable Platform

Molybdenum disulfide (MoS₂ ) is considered a fascinating material for next-generation semiconducting applications due to its outstanding mechanical stability and direct transition characteristics comparable to silicon. However, its application to stretchable platforms still is a challenging issue in wearable logic devices and sensors with noble form-factors required for future industry. Here, an omnidirectionally stretchable MoS₂ platform with laser-induced strained structures is demonstrated. The laser patterning induces the pyrolysis of MoS₂ precursors as well as the weak adhesion between Si and SiO₂ layers. The photothermal expansion of the Si layer results in the crumpling of SiO₂ and MoS₂ layers and the field-effect transistors with the crumpled MoS₂ are found to be suitable for strain sensor applications. The electrical performance of the crumpled MoS₂ depends on the degree of stretching, showing the stable omnidirectional stretchability up to 8% with approximately four times higher saturation current than its initial state. This platform is expected to be applied to future electronic devices, sensors, and so on.

Gate-Tuned Gas Molecule Sensitivity of a Two-Dimensional Semiconductor

In this work, we develop a gate-tunable gas sensor based on a MoS2/hBN heterostructure field effect transistor. Through experimental measurements and numerical simulations, we systematically reveal a principle that relates the concentration of the target gas and sensing signals (ΔI/I0) as a function of gate bias. Because a linear relationship between ΔI/I0 and the gas concentration guarantees reliable sensor operation, the optimal gate bias condition for linearity was investigated. Taking NO2 and NH3 as target molecules, it is clarified that the bias condition greatly depends on the electron accepting/donating nature of the gas. The effects of the bandgap and polarity of the transition metal dichalcogenides (TMDC) channel are also discussed. In order to achieve linearly increasing signals that are stable with respect to the gas concentration, a sufficiently large VBG within VBG > 0 is required. We expect this work will shed light on a way to precisely design reliable semiconducting gas sensors based on the characteristics of TMDC and target gas molecules.

Flexible and high-sensitivity sensor based on Ti3C2-MoS2 MXene composite for the detection of toxic gases

It is vital to have high sensitivity in gas sensors to allow the exact detection of dangerous gases in the air and at room temperature. In this study, we used 2D MXenes and MoS₂ materials to create a Ti₃C₂-MoS₂ composite with high metallic conductivity and a wholly functionalized surface for a significant signal. At room temperature, the Ti₃C₂-MoS₂ composite demonstrated clear signals, cyclic response curves to NO₂ gas, and gas concentration-dependent. The sensitivities of the standard Ti₃C₂-MoS₂ (TM_2) composite (20 wt% MoS₂) rose dramatically to 35.8%, 63.4%, and 72.5% when increasing NO₂ concentrations to 10 ppm, 50 ppm, and 100 ppm, respectively. In addition, the composite showed reaction signals to additional hazardous gases, such as ammonia and methane. Our findings suggest that highly functionalized metallic sensing channels could be used to construct multigas-detecting sensors that are very sensitive in air and at room temperature.

Rationally Designed TiO2 Nanostructures of Continuous Pore Network for Fast-Responding and Highly Sensitive Acetone Sensor

For the last several years, indoor air quality monitoring has been a significant issue due to the increasing time portion of indoor human activities. Especially, the early detection of volatile organic compounds potentially harmful to the human body by the prolonged exposure is the primary concern for public human health, and such technology is imperatively desired. In this study, highly porous and periodic 3D TiO₂ nanostructures are designed and studied for this concern. Specifically, extremely high gas molecule accessibility throughout the whole nanostructures and precisely controlled internecks of 3D TiO₂ nanostructures can achieve an unprecedented gas response of 299 to 50 ppm CH₃ COCH₃ with an extremely fast response time of less than 1s. The systematic approach to utilize the whole inner and outer surfaces of the gas sensing materials and periodically formed internecks to localize the current paths in this study can provide highly promising perspectives to advance the development of chemoresistive gas sensors using metal oxide nanostructures for the Internet of Everything application.

Charging effect induced by electron beam irradiation: a review

Charging effect frequently occurs when characterizing nonconductive materials using electrons as probes and/or signals and can impede the acquisition of useful information about the material under investigation. It is not adequate to investigate it merely by experiments, but theoretical investigations, for which the Monte Carlo method is a suitable tool, are also necessary. In this paper we review Monte Carlo simulations and selected experiments, intending to provide general insight into the charging effects induced by electron beam irradiation. We will introduce categories of the charging effect, the theoretical framework that is adopted in Monte Carlo modeling of the charging effect and present some typical simulation results. At last, with the knowledge on charging effect imparted by the above contents, we will discuss the measures that can be used for minimizing it.

Edge-enriched WS2 nanosheets on carbon nanofibers boosts NO2 detection at room temperature

Two-dimensional (2D) transition metal dichalcogenides (TMDs) hold great promise for room temperature (RT) NO₂ sensors. However, the exposure of the edges of TMDs with high adsorption capability and electronic activity remains a great obstacle to achieve high sensor sensitivity. Herein, we demonstrate a high-performance RT NO₂ gas sensor based on WS₂ nanosheets/carbon nanofibers (CNFs) composite with abundant intentionally exposed WS₂ edges. Few-layer WS₂ nanosheets are anchored on CNFs through a hydrothermal process. The approach permits to achieve a coating presenting an optimized active surface area and accessibility of the sensing layers. The exposure of WS₂ edges remarkably improves the sensing properties. Consequently, the WS₂@CNFs composite exhibits excellent selectivity to NO₂ at RT with improved response and much lower detection limit in comparison to the WS₂ and CNFs counterparts. Density functional theory (DFT) calculations verify a surprisingly strong NO₂ adsorption on WS₂ edge sites (adsorption energy 3.40 eV) with a partial charge transfer of 0.394e, while a week adsorption on the basal surface of WS₂ (adsorption energy 0.25 eV) with a partial charge transfer of 0.171e. The strategy proposed herein will be instructive to the design of efficient material structures for low-power NO₂ sensors with optimized performances.

Recent Development of Gas Sensing Platforms Based on 2D Atomic Crystals

Sensors, capable of detecting trace amounts of gas molecules or volatile organic compounds (VOCs), are in great demand for environmental monitoring, food safety, health diagnostics, and national defense. In the era of the Internet of Things (IoT) and big data, the requirements on gas sensors, in addition to sensitivity and selectivity, have been increasingly placed on sensor simplicity, room temperature operation, ease for integration, and flexibility. The key to meet these requirements is the development of high-performance gas sensing materials. Two-dimensional (2D) atomic crystals, emerged after graphene, have demonstrated a number of attractive properties that are beneficial to gas sensing, such as the versatile and tunable electronic/optoelectronic properties of metal chalcogenides (MCs), the rich surface chemistry and good conductivity of MXenes, and the anisotropic structural and electronic properties of black phosphorus (BP). While most gas sensors based on 2D atomic crystals have been incorporated in the setup of a chemiresistor, field-effect transistor (FET), quartz crystal microbalance (QCM), or optical fiber, their working principles that involve gas adsorption, charge transfer, surface reaction, mass loading, and/or change of the refractive index vary from material to material. Understanding the gas-solid interaction and the subsequent signal transduction pathways is essential not only for improving the performance of existing sensing materials but also for searching new and advanced ones. In this review, we aim to provide an overview of the recent development of gas sensors based on various 2D atomic crystals from both the experimental and theoretical investigations. We will particularly focus on the sensing mechanisms and working principles of the related sensors, as well as approaches to enhance their sensing performances. Finally, we summarize the whole article and provide future perspectives for the development of gas sensors with 2D materials.

Optically Activated 3D Thin-Shell TiO2 for Super-Sensitive Chemoresistive Responses: Toward Visible Light Activation

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.

Strategy and Future Prospects to Develop Room-Temperature-Recoverable NO2 Gas Sensor Based on Two-Dimensional Molybdenum Disulfide

Nitrogen dioxide (NO2), a hazardous gas with acidic nature, is continuously being liberated in the atmosphere due to human activity. The NO2 sensors based on traditional materials have limitations of high-temperature requirements, slow recovery, and performance degradation under harsh environmental conditions. These limitations of traditional materials are forcing the scientific community to discover future alternative NO2 sensitive materials. Molybdenum disulfide (MoS2) has emerged as a potential candidate for developing next-generation NO2 gas sensors. MoS2 has a large surface area for NO2 molecules adsorption with controllable morphologies, facile integration with other materials and compatibility with internet of things (IoT) devices. The aim of this review is to provide a detailed overview of the fabrication of MoS2 chemiresistance sensors in terms of devices (resistor and transistor), layer thickness, morphology control, defect tailoring, heterostructure, metal nanoparticle doping, and through light illumination. Moreover, the experimental and theoretical aspects used in designing MoS2-based NO2 sensors are also discussed extensively. Finally, the review concludes the challenges and future perspectives to further enhance the gas-sensing performance of MoS2. Understanding and addressing these issues are expected to yield the development of highly reliable and industry standard chemiresistance NO2 gas sensors for environmental monitoring.

Tellurene Nanoflake-Based NO2 Sensors with Superior Sensitivity and a Sub-Parts-per-Billion Detection Limit

Industrial production, environmental monitoring, and clinical medicine put forward urgent demands for high-performance gas sensors. Two-dimensional (2D) materials are regarded as promising gas-sensing materials owing to their large surface-to-volume ratio, high surface activity, and abundant surface-active sites. However, it is still challenging to achieve facilely prepared materials with high sensitivity, fast response, full recovery, and robustness in harsh environments for gas sensing. Here, a combination of experiments and density functional theory (DFT) calculations is performed to explore the application of tellurene in gas sensors. The prepared tellurene nanoflakes via facile liquid-phase exfoliation show an excellent response to NO₂ (25 ppb, 201.8% and 150 ppb, 264.3%) and an ultralow theory detection limit (DL) of 0.214 ppb at room temperature, which is excellent compared to that of most reported 2D materials. Furthermore, tellurene sensors present a fast response (25 ppb, 83 s and 100 ppb, 26 s) and recovery (25 ppb, 458 s and 100 ppb, 290 s). The DFT calculations further clarify the reasons for enhanced electrical conductivity after NO₂ adsorption because of the interfacial electron transfer from tellurene to NO₂, revealing an underlying explanation for tellurene-based gas sensors. These results indicate that tellurene is eminently promising for detecting NO₂ with superior sensitivity, favorable selectivity, an ultralow DL, fast response-recovery, and high stability.

Phase-dependent gas sensitivity of MoS2 chemical sensors investigated with phase-locked MoS2

In the present study, phase-dependent gas sensitivities of MoS₂ chemical sensors were examined. While 1T-phase MoS₂ (1T-MoS₂) has shown better chemical sensitivity than has 2H-phase MoS₂ (2H-MoS₂), the instability of the 1T phase has been hindering applications of 1T-MoS₂ as chemical sensors. Here, the chemical sensitivity of MoS₂ locked in its 1T phase by using a ZnO phase lock was investigated. To develop MoS₂ chemical sensors locked in the 1T phase, we synthesized a multi-dimensional nanomaterial by growing ZnO nanorods onto MoS₂ nanosheets (ZnO@1T-MoS₂). Raman spectroscopy and x-ray photoelectron spectroscopy analyses of such phase-locked 1T-MoS₂ subjected to flash light irradiation 100 times confirmed its robustness. ZnO nanomaterials hybridized on MoS₂ nanosheets not only froze the MoS₂ at its 1T phase, but also increased the active surface area for chemical sensing. The resulting hybridized material showed better response, namely better sensitivity, to NO₂ gas exposure at room temperature than did 1T-MoS₂ and 2H-MoS₂. This result indicated that increased surface area and heterojunction formation between MoS₂ and ZnO constitute a more promising route for improving sensitivity than using the 1T phase itself.

Improvement of Sensing Properties for Copper Phthalocyanine Sensors Based on Polymer Nanofibers Scaffolds

An effectual and understandable route for the fabrication techniques of stereoscopic NO₂ sensor is provided in this work. As the gas-sensing layer of the sensor, copper phthalocyanine (CuPc) grew on the top of poly(vinyl alcohol) (PVA) nanofibers (NFs). The sensitivity of the CuPc/PVA NFs stereoscopic sensors to NO₂ was over 829%/ppm, while the sensitivity of the continuous CuPc films sensors was 2 orders of magnitude lower than that of the stereoscopic ones. To the responsivities at 25 ppm of NO₂, the CuPc/PVA NFs stereoscopic sensors were about four times stronger than that of the continuous CuPc films sensors. For the recovery time, the CuPc/PVA NFs stereoscopic sensors were over eight times faster than the continuous CuPc films sensors. This general tactic can be used to prepare various toxic gas sensors to improve the overall performance of the devices.

3D synergistical rGO/Eu(TPyP)(Pc) hybrid aerogel for high-performance NO2 gas sensor with enhanced immunity to humidity

The prevalence of Internet of Things and portable electronics create an unprecedented demand for the high performance gas sensors. To pursuit such sensor, sandwich-type (phthalocyaninato)(porphyrinato) europium double-decker complex Eu(TPyP)(Pc) [TPyP = meso-tetra(4-pyridyl)porphyrin; Pc = phthalocyanine] was in situ self-assembled on the surface of reduced graphene oxide (rGO) driven by the π-π interaction, forming a 3D synergistical rGO/Eu(TPyP)(Pc) hybrid aerogel. The resulting aerogel not only effectively integrates the gas sensing of Eu(TPyP)(Pc) and good conductivity of rGO, but also exhibited a prominent synergy effect. Ascribed to the attractive properties, the fabricated NO₂ gas sensor exhibits superior sensitivity and selectivity in the range of 0.5 to 100 ppm with an extremely low theoretical limit level of detection (80 ppb) at ambient temperature. The response and recovery time of rGO/Eu(TPyP)(Pc) hybrid aerogel based sensor to20 ppm NO₂ were 172 and 828 s, respectively. Remarkably, the hydrophobic porous structure of rGO/Eu(TPyP)(Pc) hybrid aerogel endows the prepared sensor with excellent immunity to high relative humidity, which conquered the key technical issue of real application. The present sensor, simultaneously featured with high performance, low-power consumption, and good tolerance to environmental variations, is anticipated to offer the "on-site" and "on-line" measurement tool in real samples.

Two-Dimensional NbS2 Gas Sensors for Selective and Reversible NO2 Detection at Room Temperature

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 MoS₂ and WS₂. Particularly, the chemoresistive sensing properties of two-dimensional (2D) NbS₂ have never been explored. For the first time, we report room temperature NO₂ sensing characteristics of 2D NbS₂ nanosheets and the sensing mechanisms using first-principles calculations based on density functional theory. The results demonstrate that the NbS₂ edges possessing different configurations depending on synthetic conditions differ in the sensing ability of the TMD nanosheets. This study not only broadens the potential of 2D NbS₂ for gas sensing applications, but also presents the important role of edge configuration of TMDs depending on synthetic conditions for further studies.

Fabrication of a WS2/p-Si Heterostructure Photocathode Using Direct Hybrid Thermolysis

P-N heterostructures based on transition-metal dichelcongenides (TMDs) and a conventional semiconductor, such as p-Si, have been considered a promising structure for next-generation electronic devices and applications. However, synthesis of high-quality, wafer-scale TMDs, particularly WS₂ on p-Si, is challenging. Herein, we propose an efficient method to directly grow WS₂ crystals on p-Si via a hybrid thermolysis process. The WO₃ is deposited to prepare the p-Si surface for coating of the (NH₄)₂WS₄ precursor and converted to WS₂/p-Si during thermolysis. Moreover, the WS₂/p-Si heterojunction photocathode is fabricated and used in solar hydrogen production. The fabricated n-WS₂/p-Si heterojunction provided an onset potential of +0.022 V at 10 mA/cm² and a benchmark current density of -9.8 ± 1.2 mA/cm² at 0 V. This method reliably and efficiently produced high-quality, wafer-scale WS₂ crystals and overcame the challenges associated with previous approaches. The approach developed in this research demonstrates a magnificent progress in the fabrication of 2D material-based electronic devices.

Gas Sensors Based on Mechanically Exfoliated MoS2 Nanosheets for Room-Temperature NO2 Detection

The unique properties of MoS2 nanosheets make them a promising candidate for high-performance room temperature gas detection. Herein, few-layer MoS2 nanosheets (FLMN) prepared via mechanical exfoliation are coated on a substrate with interdigital electrodes for room-temperature NO2 detection. Interestingly, compared with other NO2 gas sensors based on MoS2, FLMN gas sensors exhibit high responsivity for room-temperature NO2 detection, and NO2 is easily desorbed from the sensor surface with an ultrafast recovery behavior, with recovery times around 2 s. The high responsivity is related to the fact that the adsorbed NO2 can affect the electron states within the entire material, which is attributed to the very small thickness of the MoS2 nanosheets. First-principles calculations were carried out based on the density functional theory (DFT) to verify that the ultrafast recovery behavior arises from the weak van der Waals binding between NO2 and the MoS2 surface. Our work suggests that FLMN prepared via mechanical exfoliation have a great potential for fabricating high-performance NO2 gas sensors.

SnS2 Nanograins on Porous SiO2 Nanorods Template for Highly Sensitive NO2 Sensor at Room Temperature with Excellent Recovery

In order to develop high performance chemoresistive gas sensors for Internet of Everything applications, low power consumption should be achieved due to the limited battery capacity of portable devices. One of the most efficient ways to reduce power consumption is to lower the operating temperature to room temperature. Herein, we report superior gas sensing properties of SnS₂ nanograins on SiO₂ nanorods toward NO₂ at room temperature. The gas response is as high as 701% for 10 ppm of NO₂ with excellent recovery characteristics and the theoretical detection limit is evaluated to be 408.9 ppb at room temperature, which has not been reported for SnS₂-based gas sensors to the best of our knowledge. The SnS₂ nanograins on the template used in this study have excessive sulfur component (Sn:S = 1:2.33) and exhibit p-type conduction behavior. These results will provide a new perspective of nanostructured two-dimensional materials for gas sensor applications on demand.

Morphological Evolution Induced through a Heterojunction of W-Decorated NiO Nanoigloos: Synergistic Effect on High-Performance Gas Sensors

Morphological evolution accompanying a surface roughening and preferred orientation is an effective way to realize a high-performance gas sensor because of its significant potential as a chemical catalyst through chemical potentials and atomic energy states. In this work, we investigated a heterojunction of double-side-W-decorated NiO nanoigloos fabricated through radio frequency sputtering and a soft-template method. Interestingly, a morphological evolution characterized by a pyramidal rough surface and the preferred orientation of the (111) plane was observed upon decorating the bare NiO nanoigloos with W. The underlying mechanism of the morphological evolution was precisely demonstrated based on the van der Drift competitive growth model originating from the oxygen transport and chemical strain in the lattice. The gas sensing properties of W-decorated NiO show an excellent NO₂ response and selectivity when compared to other gases. In addition, high response stability was evaluated under interference gas and humidity conditions. The synergistic effects on the sensing performance were interpreted on the basis of the morphological evolution of W-decorated NiO nanoigloos.