Investigation of Microstructure Effect on NO2 Sensors Based on SnO2 Nanoparticles/Reduced Graphene Oxide Hybrids

Highly selective ethanol gas sensor based on CdS/Ti3C2T x MXene composites

Sensing of hazardous gases has an important role in ensuring safety in a variety of industries as well as environments. Mainly produced by the combustion of fossil fuels and other organic matter, ethanol is a dangerous gas that endangers human health and the environment. Stability and sensing sensitivity are major considerations when designing gas sensors. Here, a superior ethanol sensor with a high response and fast recovery was synthesized by "wrapping" CdS nanoparticles on metallic Ti3C2T x MXene using a simple method. CdS nanoparticles were uniformly covered on the Ti3C2T x MXene surface, forming a "rice crust"-like heterostructure. The sensor displayed good detection of ethanol gas at room temperature. Response signals up to 31% were obtained for ethanol molecules (20 ppm) with quick recovery (41 s). The performance of the ethanol sensor was evaluated across a range of concentrations (5-100 ppm) and relative humidity (60% and 90% RH) at room temperature. Our method could open up a new strategy for the development of ethanol sensors.

A Facile Method to Incorporate Di-Dopant Elements (F and Sb) into Crystalline Mesoporous Tin Dioxide Nano Powder at Ambient Temperature and Pressure

A simple two step synthetic method for di-doped crystalline mesoporous tin dioxide powder containing antimony and fluoride at ambient pressure and temperature has been developed. This approach produced materials with high surface areas and improved electrical and optoelectrical conductance. The two dopant elements; antimony and fluoride were introduced to tin dioxide by two approaches. Both approaches produced mesoporous tin dioxide with antimony and fluoride that are integrated in the framework. The structures of these materials are analyzed by powder X-ray diffraction, N2 sorption analysis, transmission electron microscopy, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The conductance of the materials improved by factor of 13-34 compared to undoped mesoporous tin dioxide. The effect of the di-doped elements on structure, conductance and optoelectronic properties of these materials are discussed in this paper.

Bismuth Ferrite-Silver Nanowire Flexible Nanocomposites for Room-Temperature Nitrogen Dioxide Sensing

Nitrogen dioxide (NO2) is a major pollutant, causing acid rain, photochemical smog, and respiratory damage. The annual safe limit is 50 parts per billion (ppb), while concentrations exceeding 1 part per million (ppm) can result in respiratory ailments. Conventionally, n-type metal oxide semiconductors operating at elevated temperatures have been utilized for NO2 detection. Recently, p-type semiconductors with their hole accumulation layer, rapid recovery post-gas exposure, and good humidity tolerance are being investigated as potential NO2 sensors, once again working at elevated temperatures. In this work, a room-temperature (27 ± 2 °C) NO2 sensor is demonstrated by using a nanocomposite based on p-type bismuth ferrite (BFO) nanoparticles and silver nanowires (Ag NWs). This nanocomposite is capable of sensing a NO2 gas concentration of up to 0.2 ppm. The BFO nanoparticles are synthesized via a sol-gel route followed by sintering at 500 °C to form the crystalline phase. Nanocomposites are obtained by formulating a dispersion of the BFO nanoparticles and Ag NWs, followed by direct writing on both flexible and rigid substrates. The Ag NWs act as the conducting pathway, reducing the overall electrical resistance and thus enabling room-temperature operation. X-ray diffraction, scanning electron microscopy, and surface area studies provide phase information and surface morphology, and the porous nature of the film helps in room-temperature gas adsorption. The current-voltage and gas-sensing behavior are studied to obtain the optimized molar ratio (4:1 BFO/Ag NWs) for the sensor. The sensor deposited on poly(ethylene terephthalate) (PET) also works under a bent condition, indicating good flexibility. Rapid NO2 sensing was achieved in a BFO-Ag/PET device with response/recovery times of 7/8.5 s and 12/15 s in straight and bent geometries, respectively. Additionally, a good sensitivity of 30 to 60% was achieved for the BFO-Ag/PET device across 100 to 1000 ppb of NO2. The development of a nanocomposite combining an active sensing element (BFO) and a charge-transport element (Ag NWs) opens up a multitude of other application areas.

Green synthesis of tellurium-doped SnO2 nanoparticles with sulfurized g-C3 N4 : Insights into methylene blue photodegradation and antibacterial capability

The construction of SnO2 nanoparticles (NPs), specifically Te-doped SnO2 NPs, using a simple and economical co-precipitation technique has been thoroughly described in this work. NH3 served as the reducing agent in this procedure, whilst polyethylene glycol served as the capping agent. The primary goals of our work were to investigate the physicochemical properties of the synthesized SnO2 NPs and assess their potential use as antibacterial agents and photocatalysts. Scanning electron microscopy-energy dispersive X-ray, ultraviolet light, Fourier transform infrared spectroscopy, X-ray diffraction (XRD), and other analytical techniques were used to thoroughly analyze the NPs. Based on the full width at half maximum of the most noticeable peaks in the XRD spectrum, the Debye-Scherrer equation was used to calculate the crystallite sizes, which indicated the presence of a single tetragonal SnO2 phase. Particularly noteworthy was the exceptional photocatalytic activity of graphene-assisted Te-doped SnO2 NPs, achieving an impressive decomposition efficiency of up to 98% in the photo-oxidation of methylene blue. Furthermore, our investigation delved into the antibacterial attributes of the synthesized SnO2 NPs against Escherichia coli and Staphylococcus aureus, demonstrating inhibitory effects on both bacteria strains. This suggests potential applications for these NPs in various environmental and medical contexts.

Easy synthesis of PPy/TiO2/ZnO composites with superior photocatalytic performance, efficient supercapacitors and nitrite sensor

The synthesis of Polypyrrole (PPy)/TiO2/ZnO composites involved a chemical oxidative polymerization process, wherein the addition of TiO2/ZnO was varied from 1 to 10 wt%. The composites' photocatalytic capabilities, supercapacitor performance, and potential use as a nitrite sensor were thoroughly assessed, alongside investigations into their photoluminescence (PL) and morphological characteristics. The strong interaction between TiO2/ZnO and PPy was confirmed using FTIR, UV-Vis, and PL spectroscopy techniques. The composites demonstrated aggregated and spherical-shaped morphological features investigated by FESEM. Such morphological structures of the composites were distinct from the TiO2/ZnO (rod-like) and similar to PPy structure (spherical). However, such composites showed dominating spherical-shaped morphology ensuring a diameter in the range of 50-200 nm. The PPy/TiO2/ZnO composites exhibited significantly enhanced photocatalytic efficiency in methylene blue (MB) removal, achieving a range of 88-93% compared to PPy alone, which only achieved 77.2% MB removal. The Cyclic Voltammetry (CV) data exhibited a promising hybrid supercapacitor performance of the composites with a high capacitance value, good energy density, as well as an excellent power density. The fabricated supercapacitor was capable of lightened up a single red 5 mm LED for a few minutes, indicating the commendable energy storage capacity. A newly developed PPy/TiO2/ZnO composite is potentially used to develop as a sensor probe for the detection of nitrite chemicals using the linear sweep voltammetry (LSV) technique in three electrodes system in room conditions. It is found an excellent sensor results in terms of sensitivity as well as detection limit and satisfactory results when validated with the real samples. These results offer novel insights into the fabrication of PPy/TiO2/ZnO photocatalysts for addressing organic waste treatment, while also presenting promising prospects for potential applications in supercapacitors and sensors.

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.

Synergy of Two Intermolecular Hydrogen Bonds Promotes Highly Sensitive and Selective Room-Temperature Dimethyl Methylphosphonate Sensing: A Case of rGO-Based Gas Sensors

The development of room-temperature chemiresistive gas sensors with low limit of detection, high sensitivity, and selectivity for dimethyl methylphosphonate (DMMP) detection remains a challenge. Herein, a synergy of the two intermolecular hydrogen bond-promoted approach was proposed to fabricate a room-temperature DMMP sensor with enhanced performances. As a proof of concept, ternary p-hexafluoroisopropanol phenyl (HFIP) functionalized polypyrrole-reduced graphene oxide hybrids (HFIP-PPy-rGO) were rationally designed. During the sensing process, rGO serves as a conductive carrier, ensuring that the sensors operate at room temperature, and both HFIP and PPy act as adsorption sites for DMMP through hydrogen bonding interactions. As expected, the HFIP-PPy-rGO sensor exhibits high selectivity and sensitivity to DMMP. Besides, the HFIP-PPy-rGO sensor also possesses excellent linear response to DMMP and long-term stability. Experimental results and quartz crystal microbalance measurements prove that the specific recognition of DMMP is realized by forming two intermolecular hydrogen bonds between HFIP and DMMP, as well as PPy and DMMP. Additionally, the introduction of HFIP groups also contributes to adjusting device conductivity, enhancing signal conversion function. To put the DMMP sensor into potential practical application, the obvious sensing response to different DMMP concentrations in soil was confirmed, and a wireless detection system was built to realize real-time monitoring of DMMP concentrations in the surroundings. Overall, this study provides a facile and practical solution for improving the sensing performance of room-temperature sensors based on the hydrogen bond theory.

Preparation of MOF-derived ZnO/Co3O4 nanocages and their sensing performance toward H2S

We report a type of micro-electro-mechanical system (MEMS) H₂S gas sensors with excellent sensing performance at the ppb level (lowest detection limit is 5 ppb). The sensors were fabricated with ZnO/Co₃O₄ sensing materials derived from Zn/Co-MOFs by annealing at a suitable temperature of 500 °C. ZnO/Co₃O₄-500 exhibits the highest response when exposed to 10 ppb H₂S gas at 120 °C, and the response/recovery times are 10 s/21 s. Moreover, it exhibits outstanding selectivity, long-term stability (retained 95% response after 45 days), and moisture resistance (only a minor fluctuation of 2% even at 90% RH). This can be ascribed to the fact that ZnO/Co₃O₄-500 has regular morphology, abundant oxygen vacancies (52.8%) and high specific surface area (96.5 m² g^-1). This work provides not only a high performance H₂S MEMS gas sensor but also a systematic study of the effect of the annealing temperature on the sensing performance of ZnO/Co₃O₄ sensing materials derived from bimetal organic frameworks.

High-Performance Ppb Level NO2 Gas Sensor Based on Colloidal SnO2 Quantum Wires/Ti3C2Tx MXene Composite

Nitrogen dioxide is one origin of air pollution from fossil fuels with the potential to cause great harm to human health in low concentrations. Therefore, low-cost, low-power-consumption sensors for low-concentration NO₂ detection are essential. Herein, heterojunction by SnO₂ quantum wires, a traditional metal oxide NO₂ sensing material, and Ti₃C₂T_x MXene, a novel type of 2D layered material, was synthesized using a simple solvothermal method for enhancing gas-sensing performance and reducing operating temperature. The operating temperature was reduced to 80 °C, with a best performance of 27.8 and a fast response and recovery time (11 s and 23 s, respectively). The SnO₂ and Ti₃C₂T_x MXene composite exhibits high speed and low detection limit due to the construction of the heterojunction with high conductive Ti₃C₂T_x MXene. The selectivity and stability of gas sensors are carried out. This could enable the realization of fast response, high-sensitivity, and selective NO₂ sensing under low operating temperatures.

Highly Sensitive Sub-ppm CH3COOH Detection by Improved Assembly of Sn3O4-RGO Nanocomposite

Detection of sub-ppm acetic acid (CH₃COOH) is in demand for environmental gas monitoring. In this article, we propose a CH₃COOH gas sensor based on Sn₃O₄ and reduced graphene oxide (RGO), where the assembly of Sn₃O₄-RGO nanocomposites is dependent on the synthesis method. Three nanocomposites prepared by three different synthesis methods are investigated. The optimum assembly is by hydrothermal reactions of Sn⁴⁺ salts and pre-reduced RGO (designated as RS nanocomposite). Raman spectra verified the fingerprint of RGO in the synthesized RS nanocomposite. The Sn₃O₄ planes of (111), (210), (130), (13¯2) are observed from the X-ray diffractogram, and its average crystallite size is 3.94 nm. X-ray photoelectron spectroscopy on Sn3d and O1s spectra confirm the stoichiometry of Sn₃O₄ with Sn:O ratio = 0.76. Sn₃O₄-RGO-RS exhibits the highest response of 74% and 4% at 2 and 0.3 ppm, respectively. The sensitivity within sub-ppm CH₃COOH is 64%/ppm. Its superior sensing performance is owing to the embedded and uniformly wrapped Sn₃O₄ nanoparticles on RGO sheets. This allows a massive relative change in electron concentration at the Sn₃O₄-RGO heterojunction during the on/off exposure of CH₃COOH. Additionally, the operation is performed at room temperature, possesses good repeatability, and consumes only ~4 µW, and is a step closer to the development of a commercial CH₃COOH sensor.

Gas-Sensitive Characteristics of Graphene Composite Tungsten Disulfide to Ammonia

Two-dimensional materials have outstanding application prospects in gas sensing. By constructing composite structures of various gas-sensitive materials, more-efficient and sensitive gas sensors can be further developed. After graphene is compounded with WS₂, the composite material can improve the gas detection performance. In this work, the adsorption energy and the electronic properties of a graphene/WS₂ structure were calculated by first-principles before and after adsorption of NH₃. The calculation results indicate that the bandgap of the material was appreciably reduced after NH₃ was adsorbed. In addition, a graphene/WS₂ gas sensor was prepared. The response of the sensor to NH₃ at a concentration of 100 ppm was 2.42% and 1.73% at 30 °C and 60 °C, respectively. Combining simulation with experiment, it is feasible to use graphene composite WS₂ to detect NH₃, which provides a new idea for applications of graphene and other composite materials in gas sensing.

Edge-enriched MoS2 nanosheets modified porous nanosheet-assembled hierarchical In2O3 microflowers for room temperature detection of NO2 with ultrahigh sensitivity and selectivity

Nitrogen dioxide (NO₂) is one of the most hazardous toxic pollutants to human health and the environment. However, deficiencies of low sensitivity and poor selectivity at room temperature (RT) restrain the application of NO₂ sensors. Herein, the edge-enriched MoS₂ nanosheets modified porous nanosheets-assembled three-dimensional (3D) In₂O₃ microflowers have been synthesized to improve the sensitivity and selectivity of NO₂ detection at RT. The results show that the In₂O₃/MoS₂ composite sensor exhibits a response as high as 343.09-5 ppm NO₂, which is 309 and 72.5 times higher than the sensors based on the pristine MoS₂ and In₂O₃. The composite sensor also shows short recovery time (37 s), excellent repeatability and long-term stability. Furthermore, the response of the In₂O₃/MoS₂ sensor to NO₂ is at least 30 times higher than that of other gases, proving the ultrahigh selectivity of the sensor. The outstanding sensing performance of the In₂O₃/MoS₂ sensor can be attributed to the synergistic effect and abundant active sites originating from the p-n heterojunction, exposed edge structures and the designed 2D/3D hybrid structure. The strategy proposed herein is expected to provide a useful reference for the development of high-performance RT NO₂ sensors.

Oxygen Plasma-Assisted Defect Engineering of Graphene Nanocomposites with Ultrasmall Co3O4 Nanocrystals for Monitoring Toxic Nitrogen Dioxide at Room Temperature

Functional adjustment of graphene with metal oxide can in fact progress the affectability of graphene-based gas sensors. However, it could be a huge challenge to upgrade the detecting execution of nitrogen dioxide (NO₂) sensors at room temperature. The ultrasmall size of nanocrystals (NCs) and copious defects are two key variables for moving forward gas detecting execution. Herein, we provide an effective strategy that the hydrothermal reaction is combined with room-temperature oxygen plasma treatment to prepare Co₃O₄ NCs and reduced graphene oxide (RGO) nanohybrids (Co₃O₄-RGO). Among all of Co₃O₄-RGO nanohybrids, Co₃O₄-RGO-60 W exhibits the most superior NO₂ sensing properties and achieves the low-concentration detection of NO₂. The sensitivity of Co₃O₄-RGO-60 W to 20 ppm NO₂ at room temperature is the highest (72.36%). The excellent sensing properties can mainly depend on the change in the microstructure of Co₃O₄-RGO. Compared with Co₃O₄-RGO, Co₃O₄-RGO-60 W with oxygen plasma treatment shows more favorable properties for NO₂ adsorption, including the smaller size of Co₃O₄ NCs, larger specific surface area, pore size, and more oxygen vacancies (OVs). Especially, OVs make the surface of NCs have a unique chemical state, which can increase active sites and improve the adsorption property of NO₂. Besides, the agreeable impact of the p-p heterojunction (Co₃O₄ and RGO) and the doping of N molecule contribute to the improved NO₂ detecting properties. It is demonstrated that the Co₃O₄-RGO-60 W sensor is expected to monitor NO₂ at room temperature sensitively.

The Combination of Two-Dimensional Nanomaterials with Metal Oxide Nanoparticles for Gas Sensors: A Review

Metal oxide nanoparticles have been widely utilized for the fabrication of functional gas sensors to determine various flammable, explosive, toxic, and harmful gases due to their advantages of low cost, fast response, and high sensitivity. However, metal oxide-based gas sensors reveal the shortcomings of high operating temperature, high power requirement, and low selectivity, which limited their rapid development in the fabrication of high-performance gas sensors. The combination of metal oxides with two-dimensional (2D) nanomaterials to construct a heterostructure can hybridize the advantages of each other and overcome their respective shortcomings, thereby improving the sensing performance of the fabricated gas sensors. In this review, we present recent advances in the fabrication of metal oxide-, 2D nanomaterials-, as well as 2D material/metal oxide composite-based gas sensors with highly sensitive and selective functions. To achieve this aim, we firstly introduce the working principles of various gas sensors, and then discuss the factors that could affect the sensitivity of gas sensors. After that, a lot of cases on the fabrication of gas sensors by using metal oxides, 2D materials, and 2D material/metal oxide composites are demonstrated. Finally, we summarize the current development and discuss potential research directions in this promising topic. We believe in this work is helpful for the readers in multidiscipline research fields like materials science, nanotechnology, chemical engineering, environmental science, and other related aspects.

Core-shell Fe3O4@SnO2 nanochains toward the application of radar-infrared-visible compatible stealth

Multiband-compatible stealth materials play an increasingly crucial role in the field of modern military defence because they can enable the targeted objects to dodge advance detection technologies. In this study, chain-like Fe₃O₄@poly(ethyleneglycol dimethacrylate-co-methacrylic acid) nanocomposites were constructed as precursors through the magnetic field-induced distillation precipitation polymerisation. Then, the liquid-phase seed-mediated growth method, together with subsequent calcination, was applied to introduce SnO₂ shells and remove poly(ethyleneglycol dimethacrylate-co-methacrylic acid) shells, which led to the successful preparation of innovative core-shell Fe₃O₄@SnO₂ nanochains. The unique microstructure and appropriate components endowed nanochains with multiple functional applications. The minimum reflection loss value was approximately -39.4 dB (5.67 GHz), exhibiting excellent microwave absorption performance. The possible microwave absorption mechanisms involve interfacial polarisation, space charge polarisation, natural resonance, and multiple reflections and scatterings. The optimal infrared reflectivity reached 0.64, 0.51, and 0.37 in three atmospheric windows, indicating outstanding infrared stealth performance, which was attributed to the intense infrared reflection of SnO₂ shells. Furthermore, three nanochains showed different colours (dark green, brick red, and bright orange), revealing selection absorption for visible light. This can be attributed to the combined effect of visible responses of SnO₂ shells along with Bragg diffraction from the periodic arrangement of Fe₃O₄ particles in a single nanochain. Thus, core-shell Fe₃O₄@SnO₂ nanochains can be considered as promising radar-infrared-visible compatible stealth materials. This discovery opens a new means to exploit multiband-compatible stealth materials.

Flexible, non-contact and multifunctional humidity sensors based on two-dimensional phytic acid doped co-metal organic frameworks nanosheets

The development of high-performance humidity sensors is of great significance to explore their practical applications in the fields of environment, energy saving and safety monitoring. Herein, a flexible, non-contact and multifunctional humidity sensor based on two-dimensional Co-metal organic frameworks (Co-MOF) nanosheets is proposed, which is fabricated by simple bottom-up synthesis method. Furthermore, environmentally friendly, renewable and abundant biomass phytic acid (PA) is modified on the surface of Co-MOF nanosheets, which releases free protons being capable of etching the framework of MOF to improve the hydrophilicity and conductivity of MOF. Compared with Co-MOF-based sensor, the Co-MOF@PA-based sensor exhibits significantly enhanced sensitivity and broadened response range within 23-95% relative humidity (RH). The humidity sensor has an excellent humidity sensing response over 2 × 10³. The Co-MOF@PA-based sensor shows good flexibility and humidity sensing properties, endowing it with multifunctional applications in real-time facial respiration monitoring, skin humidity perception, cosmetic moisturizing evaluation and fruit freshness testing. Four respiration patterns, including slow breath, deep breath, normal breath and fast breath are wirelessly monitored in real time by Co-MOF@PA-based sensor and recorded by mobile phone software. The research work presents potential applications in human-machine interactions (HMI) devices in future.

Low temperature NO2 gas sensing with ZnO nanostructured by laser interference lithography

ZnO conductometric gas sensors have been widely studied due to their good sensitivity, cost-efficiency, long stability and simple fabrication. This work is focused on NO2 sensing, which is a toxic and irritating gas. The developed sensor consists of interdigitated electrodes covered by a ZnO sensing layer. ZnO has been grown by means of the aerosol assisted chemical vapor deposition technique and then nanostructured by laser interference lithography with a UV laser. The SEM and XRD results show vertically oriented growth of ZnO grains and a 2D periodic nanopatterning of the material with a period of 800 nm. Nanostructuring lowers the base resistance of the developed sensors and modifies the sensor response to NO2. Maximum sensitivity is obtained at 175 °C achieving a change of 600% in sensor resistance for 4 ppm NO2 versus a 400% change for the non-nanostructured material. However, the most relevant results have been obtained at temperatures below 125 °C. While the non-nanostructured material does not respond to NO2 at such low temperatures, nanostructured ZnO allows NO2 sensing even at room temperature. The room temperature sensing capability possibly derives from the increase of both the surface defects and the surface-to-volume ratio. The long stability and the gas sensing under humid conditions have also been tested, showing improvements of sensitivity for the nanostructured sensors.

Fast detection of NO2 by porous SnO2 nanotoast sensor at low temperature

In order to challenge high working temperature, low response and low selectivity of present NO₂ sensor, porous SnO₂ nanotoasts with a large surface area (79.94 m²/g) were synthesized. Thick film sensors fabricated by the SnO₂ nanotoasts exhibited a high response to NO₂ gas operating at room temperature. Excellent performance for NO₂ sensing gas at 50 °C, included the high response of 105.2 (10 ppm), low detection limitation of 0.1 ppm, fast response within 10 s, and wide range of 0.1-10 ppm (R² = 0.9931). These sensors also demonstrated perfect selectivity, moisture resistance and 90 days of long-term stability. SnO₂ nanotoasts sensor has excellent detection ability in actual detection. The superior response of porous SnO₂ nanotoasts towards NO₂ was attributed to the special porous structure with large specific surface area and oxygen vacancies in sensing material, which helped adsorption of the target gas molecules onto the sensing surfaces and transfer of the charge.

High-performance and wearable hazardous gases sensor based on n-n heterojunction film of NGO and tetrakis(1-pyrenyl)porphyrin

The popularity of "Internet of Things" and portable electronic devices creates unprecedented demands for wearable gas sensors with excellent performance. In this study, the flexible n-n heterojunction film is firstly produced from metalloporphyrin complex 5,10,15,20-tetrakis(1-pyrenyl) porphyrin cobalt (II) (CoTPyrP) and nitrogen-doped graphene oxide (NGO) film, using solution-processing quasi-Langmuir-Shäfer (QLS) method and employed as the electrochemical identification layer for the wearable sensor. Thanks to the attractive electron-transfer properties from porphyrin to NGO, and the local regulation of electron transport by N and C atoms with different electronegativity on NGO, the resulting sensor shows good responses to NO₂, SO₂, NH₃, H₂S gases with the low detection limit (LOD) of 6, 74, 113 and 178 ppb, respectively. The uniform and compact structure of the heterojunction films provide excellent mechanical flexibility and suppress the penetration of gases into the film to obtain fast recovery speed. In addition, a sensor array consisting of NGO/CoTPyrP heterojunction and CoTPyrP film sensor is established, achieving selective identification of four hazardous gases. The present work provides potential application for hazardous gases identification in actual systems, and proposes an effective method to develop new flexible n-n heterojunctions for wearable gas sensors.

Humidity-Insensitive NO2 Sensors Based on SnO2/rGO Composites

This study reported a novel humidity-insensitive nitrogen dioxide (NO₂) gas sensor based on tin dioxide (SnO₂)/reduced graphene oxide (rGO) composites through the sol-gel method. The sensor demonstrated ppb-level NO₂ detection in p-type sensing behaviors (13.6% response to 750 ppb). Because of the synergistic effect on SnO₂/rGO p-n heterojunction, the sensing performance was greatly enhanced compared to that of bare rGO. The limit of detection of sensors was as low as 6.7 ppb under dry air. Moreover, benefited from the formed superhydrophobic structure of the SnO₂/rGO composites (contact angle: 149.0°), the humidity showed a negligible influence on the dynamic response (S_g) of the sensor to different concentration of NO₂ when increasing the relative humidity (RH) from 0 to 70% at 116°C. The relative conductivity of the sensor to 83% relative humidity was 0.11%. In addition, the response ratio (S_g/S_RH) between 750 ppb NO₂ and 83% RH was 649.0, indicating the negligible impaction of high-level ambient humidity on the sensor. The as-fabricated humidity-insensitive gas sensor can promise NO₂ detection in real-world applications such as safety alarm, chemical engineering, and so on.

Recent Progress in Graphene Derivatives/Metal Oxides Binary Nanocomposites Based Chemi-resistive Sensors for Disease Diagnosis by Breath Analysis

Background::

The scientific and clinical interest of breath analysis for non-invasive disease diagnosis has been focused by the scientific community over the past decade. This was due to the exhalation of prominent volatile organic compounds (VOCs) corresponding to the metabolic activities in the body and their concentration variation. To identify these biomarkers, various analytical techniques have been used in the past and the threshold concentration was established between a healthy and diseased state. Subsequently, various nanomaterials-based gas sensors were explored for their demand in quantifying these biomarkers for real-time, low cost and portable breathalyzers along with the essential sensor performances.

Methods::

We focus on the classification of graphene derivatives and their composites’ gas sensing efficiency for the application in the development of breathalyzers. The review begins with the feasibility of the application of nanomaterial gas sensors for healthcare applications. Then, we systematically report the gas sensing performance of various graphene derivatives/semiconductor metal oxides (SMO) binary nanocomposites and their optimizing strategies in selective detection of biomarkers specific to diseases. Finally, we provide insights on the challenges, opportunity and future research directions for the development of breathalyzers using other graphene derivatives/SMO binary nanocomposites.

Results::

On the basis of these analyses, graphene and its derivatives/metal oxides based binary nanocomposites have been a choice for gas sensing material owing to their high electrical conductivity and extraordinary thickness-dependent physicochemical properties. Moreover, the presence of oxygen vacancies in SMO does not only alter the conductivity but also accelerates the carrier transport rate and influence the adsorption behavior of target analyte on the sensing materials. Hence researchers are exploring the search of ultrathin graphene and metal oxide counterpart for high sensing performances.

Conclusion::

Their impressive properties compared to their bulk counterpart have been uncovered towards sensitive and selective detection of biomarkers for its use in portable breathalyzers.

Hierarchically Porous SnO2 Coupled Organic Carbon for CO2 Electroreduction

Converting CO₂ into value-added chemicals or fuels by electrochemical CO₂ reduction reaction (CO₂ RR) has aroused great interest, whereas designing highly active and selective electrocatalysts is still a challenge. Herein, a novel kind of electrochemical catalyst composed with SnO₂ and organic carbon (OC), named as SnO₂ /OC, was facilely constructed for CO₂ RR. The obtained SnO₂ /OC exhibits both high faradaic efficiency for formate (∼75 %) and carbon products (∼95 %) as well as excellent stability. High surface area with hierarchically porous structure and the homogeneous formation of Sn-O-C linkages in SnO₂ /OC jointly promote the adsorption and activation of CO₂ , as well as fast transport of reactants and products.

Synthesis, Antimicrobial Activity, and Photocatalytic Performance of Ce Doped SnO2 Nanoparticles

This work represented the synthesis of Ce doped SnO2 nanoparticles by a wet chemical method and was characterized by various characterization techniques. PXRD confirmed the presence of the rutile phase for Ce doped SnO2 nanoparticles. SEM image and elemental mapping showed agglomerated irregular shaped particles and uniform distribution of 5% Ce ions within the SnO2 lattice, respectively. Ce doped SnO2 nanoparticles showed antimicrobial activity against E. coli and prevented the growth of bacteria. The nanoparticles were found photocatalytic active and photocatalytic behavior was elucidated by the degradation of Malachite Green dye under UV light irradiation.

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.

Room-Temperature, Highly Durable Ti3C2Tx MXene/Graphene Hybrid Fibers for NH3 Gas Sensing

Graphene-based fibers (GFs) have aroused enormous interest in portable, wearable electronics because of their excellent mechanical flexibility, electrical conductivity, and weavability, which make them advantageous for wearable electronic devices. Herein, we report the development of metal binder-free Ti₃C₂T_x MXene/graphene hybrid fibers by a scalable wet-spinning process. These hybrid fibers exhibit excellent mechanical and electrical properties for applications in flexible wearable gas sensors. The synergistic effects of electronic properties and gas-adsorption capabilities of MXene/graphene allow the created fibers to show high NH₃ gas sensitivity at room temperature. The hybrid fibers exhibited significantly improved NH₃ sensing response (ΔR/R₀ = 6.77%) compared with individual MXene and graphene. The hybrid fibers also showed excellent mechanical flexibility with a minimal fluctuation of resistance of ±0.2% and low noise resistance even after bending over 2000 cycles, enabling gas sensing during deformation. Furthermore, flexible MXene/graphene hybrid fibers were woven into a lab coat, demonstrating their high potential for wearable devices. We envisage that these exciting features of 2D hybrid materials will provide a novel pathway for designing next-generation portable wearable gas sensors.

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.

Three-Dimensional Graphene Hydrogel Decorated with SnO2 for High-Performance NO2 Sensing with Enhanced Immunity to Humidity

A facile, one-step hydrothermal route was exploited to prepare SnO₂-decorated reduced graphene oxide hydrogel (SnO₂/RGOH) with three-dimensional (3D) porous structures for NO₂ gas detection. Various material characterizations demonstrate the effective deoxygenation of graphene oxide and in situ growth of rutile SnO₂ nanoparticles (NPs) on 3D RGOH. Compared with the pristine RGOH, the SnO₂/RGOH displayed much lower limit of detection (LOD) and an order of magnitude higher sensitivity, revealing the distinct impact of SnO₂ NPs in improving the NO₂-sensing properties. An exceptional low theoretical LOD of 2.8 ppb was obtained at room temperature. The p-n heterojunction formed at the interface between RGOH and SnO₂ facilitates the charge transfer, improving both the sensitivity in NO₂ detection and the conductivity of hybrid material. Considering that existing SnO₂/RGO-based NO₂ sensors suffer from great vulnerability to humidity, here we employed integrated microheaters to effectively suppress the response to humidity, with nearly unimpaired response to NO₂, which boosted the selectivity. Notably, a flexible NO₂ sensor was constructed on a liquid crystal polymer substrate with endurance to mechanical deformation. This work indicates the feasibility of optimizing the gas-sensing performance of sensors by combining rational material hybridization, 3D structural engineering with temperature modulation.

Construction of ZnO/SnO2 Heterostructure on Reduced Graphene Oxide for Enhanced Nitrogen Dioxide Sensitive Performances at Room Temperature

The employment of n-n homotypic heterogeneous junctions is an efficient method to improve sensitive performance of metal oxide-based gas sensors owing to the generation of charge accumulation regions. Herein, in order to further enhance nitrogen dioxide (NO₂) sensing properties of the sensors based on reduced graphene oxide (RGO) at room temperature (RT), n-type ZnO nanoparticles (NPs) decorated n-type SnO₂ NPs heterojunctions were successfully constructed on RGO nanosheets (NSs) by combination of the hydrothermal method and the wet-chemical deposition method. The formation of heterostructures between ZnO NPs and SnO₂ NPs was confirmed by the nonlinear behavior of current versus voltage (I-V) curve of ZnO/SnO₂-RGO. ZnO/SnO₂-RGO based sensor displayed remarkably enhanced response (141.0%) for detecting 5 ppm of NO₂ at RT, which is almost 4 and 3 times higher than that of SnO₂-RGO (34.8%) and ZnO-RGO (43.3%), respectively. Moreover, as far as the ZnO/SnO₂-RGO-based sensor is concerned, its response and recovery time (33 and 92 s) are also significantly decreased, compared to SnO₂-RGO-based sensor (70 and 39 s) and ZnO-RGO-based sensor (272 and 1297 s). In this work, the improved NO₂ sensing properties of the sensors based on RGO not only benefit from the effects of the heterostructures between SnO₂ and ZnO, but also derive from the superior electrical characteristics of RGO. In particular, the n-n heterojunctions could offer facile access to effective electronic interaction and improve transfer efficiency of the charges at the interface to adsorbed oxygen. Meanwhile, the n-n heterojunctions can also provide additional reaction center for adsorbing gas.

SnS2/SnS p-n heterojunctions with an accumulation layer for ultrasensitive room-temperature NO2 detection

The unique features of SnS₂ make it a sensitive material ideal for preparing high-performance nitrogen dioxide (NO₂) gas sensors. However, sensors based on pristine tin disulfide (SnS₂) fail to work at room temperature (RT) owing to their poor intrinsic conductivity and weak adsorptivity toward the target gas, thereby impeding their wide application. Herein, an ultrasensitive and fully recoverable room-temperature NO₂ gas sensor based on SnS₂/SnS p-n heterojunctions with an accumulation layer was fabricated. The amounts of SnS₂/SnS heterojunctions can be effectively controlled by tuning the ratios of tin and sulfur precursors in the easy one-step solvothermal synthesis. Compared with pristine SnS₂, the conductivity of SnS₂/SnS heterostructures improved considerably. Such improvement was caused by the electron transfer from p-type SnS to n-type SnS₂ because the Fermi level of SnS was higher than that of SnS₂. The sensing response of optimized SnS₂/SnS toward 4 ppm NO₂ was 660% at room temperature, which was higher than most reported sensitivity values of other two-dimensional (2D) materials at room temperature. The superior sensing response of SnS₂/SnS heterostructures was attributed to the enhanced electron transport and the increased adsorption sites caused by the SnS₂/SnS p-n heterojunctions. Moreover, the SnS₂/SnS sensor showed good selectivity and long-term stability. These achievements of SnS₂/SnS heterostructured sensors make them highly desirable for practical applications.

A nanocomposite consisting of ZnO decorated graphene oxide nanoribbons for resistive sensing of NO2 gas at room temperature

A composite prepared from zinc oxide and graphene oxide nanoribbons (ZnO/GONR) is demonstrated to enable improved room temperature (RT) detection of nitrogen dioxide (NO₂). Low-cost hydrothermal synthesis is used to construct the composite. The properties of the resistive sensor, including the sensitivity, response and recovery times, repeatability and selectivity, were investigated in the NO₂ concentration range from 1 to 50 ppm at RT. The sensor, typically operated at a voltage of 5 V, exhibits a low detection limit of 1 ppm, a fast response-recovery time, and excellent repeatability which outperforms that of pure ZnO sensors. The sensing mechanism is explained in terms of a redox reaction between NO₂ and oxygen anions on the surface of the ZnO/GONR composite. Graphical abstract Schematic representation of the NO₂ sensing mechanisms on the surface of the ZnO/GONR composite and overall improved NO₂ gas-sensing performance.

Outstanding Room-Temperature Hydrogen Gas Detection by Plasma-Assisted and Graphene-Functionalized Core-Shell Assembly of SnO2 Nanoburflower

Here, we have reported the synthesis of three-dimensional, mesoporous, nano-SnO₂ cores encapsulated in nonstoichiometric SnO₂ shells grown by chemical as well as physical synthesis procedures such as plasma-enhanced chemical vapor deposition, followed by functionalization with reduced graphene oxide (rGO) on the surface. The main motif to fabricate such morphology, i.e., core-shell assembly of burflower-like SnO₂ nanobid is to distinguish gases quantitatively at reduced operating temperatures. Electrochemical results reveal that rGO anchored on SnO₂ surface offers excellent gas detection performances at room temperature. It exhibits outstanding H₂ selectivity through a wide range, from ∼10 ppm to 1 vol %, with very little cross-sensitivity against other similar types of reducing gases. Good recovery as well as prompt responses also added flair in its quality due to the highly mesoporous architecture. Without using any expensive dopant/catalyst/filler or any special class of surfactants, these unique SnO₂ mesoporous nanostructures have exhibited exceptional gas sensing performances at room temperature and are thus helpful to fabricate sensing devices in most cost-effective and eco-friendly manner.

Mesoporous Ultrathin SnO2 Nanosheets in Situ Modified by Graphene Oxide for Extraordinary Formaldehyde Detection at Low Temperatures

In this article, we demonstrated an extremely high-sensitivity formaldehyde (HCHO) gas sensor, where the graphene oxide (GO) in situ modified two-dimensional (2D) SnO₂ nanosheets with in-plane mesopores were utilized as the sensing materials. The sensor response ( R_a/ R_g) was larger than 2000 toward 100 ppm HCHO at 60 °C. In addition, the selectivity for detecting HCHO was excellent against other interferences including ethanol, acetone, methanol, toluene, ammonia, water, etc. The outstanding sensing performance of 2D mesoporous GO/SnO₂ nanosheets was attributed to the synergism of the sensitizer effect of GO, large surface areas of 2D nanostructure, suitable particle size, and abundant in-plane mesopores. The high sensitivity, high selectivity, and low working temperature of the sensor reported here endowed it a great potential in selective detection of HCHO. Meanwhile, the design and synthesis of GO/SnO₂ nanocomposites will provide new paradigms in the future development of HCHO-sensitive materials.

Octahedral SnO2/Graphene Composites with Enhanced Gas-Sensing Performance at Room Temperature

Although high-energy facets on metal oxides are usually active and preferred for gas sensing, it is difficult to expose them according to thermodynamics. In this work, nanocomposites of SnO₂ and graphene are prepared by a hydrothermal method. The SnO₂ nanoparticles change from a lance shape to an octahedral shape as the concentration of HCl in the solution is increased gradually from 6.5 to 10 vol %. However, the SnO₂ nanoparticles have an elongated octahedral shape if the concentration of HCl is increased further. The octahedral SnO₂ nanoparticles are mainly surrounded by high-surface-energy {221} facets, thus facilitating gas sensing. First-principles calculation shows that the surface energy and adsorption energy of the {221} facets are larger than those of the stable {110} facets, and so, the gas adsorption capacity on the {221} facets is better. Furthermore, because the Fermi level of the SnO₂{221} facet is higher than that of graphene, the electrons are transferred from SnO₂ nanoparticles to graphene sheets, enabling effective electron exchange between the composite and external NO₂ gas. The excellent gas-sensing properties of the octahedral SnO₂/graphene composites are ascribed to the high-surface-energy {221} facets exposed.

Toward a Fast and Highly Responsive SnSe2-Based Photodiode by Exploiting the Mobility of the Counter Semiconductor

In photodetection, the response time is mainly controlled by the device architecture and electron/hole mobility, while the absorption coefficient and the effective separation of the electrons/holes are the key parameters for high responsivity. Here, we report an approach toward the fast and highly responsive infrared photodetection using an n-type SnSe₂ thin film on a p-Si(100) substrate keeping the overall performance of the device. The I- V characteristics of the device show a rectification ratio of ∼147 at ±5 V and enhanced optoelectronic properties under 1064 nm radiation. The responsivity is 0.12 A/W at 5 V, and the response/recovery time constants were estimated as ∼57 ± 25/34 ± 15 μs, respectively. Overall, the response times are shown to be controlled by the mobility of the constituent semiconductors of a photodiode. Further, our findings suggest that n-SnSe₂ can be integrated with well-established Si technology with enhanced optoelectronic properties and also pave the way in the design of fast response photodetectors for other wavelengths as well.

Assembly with copper(ii) ions and D–π–A molecules on a graphene surface for ultra-fast acetic acid sensing at room temperature

The as-prepared 4HQ-rGO/Cu²⁺ sensor possessed a high response, outstanding selectivity and fast response-recovery characteristic, which was mainly attributed to the supramolecularly assemble of 4-hydroxyquinoline, and Cu²⁺ with graphene nanosheets.