Breakthroughs in the Design of Novel Carbon-Based Metal Oxides Nanocomposites for VOCs Gas Sensing

Nowadays, the detection of volatile organic compounds (VOCs) at trace levels (down to ppb) is feasible by exploiting ultra-sensitive and highly selective chemoresistors, especially in the field of medical diagnosis. By coupling metal oxide semiconductors (MOS e.g., SnO2, ZnO, WO3, CuO, TiO2 and Fe2O3) with innovative carbon-based materials (graphene, graphene oxide, reduced graphene oxide, single-wall and multi-wall carbon nanotubes), outstanding performances in terms of sensitivity, selectivity, limits of detection, response and recovery times towards specific gaseous targets (such as ethanol, acetone, formaldehyde and aromatic compounds) can be easily achieved. Notably, carbonaceous species, highly interconnected to MOS nanoparticles, enhance the sensor responses by (i) increasing the surface area and the pore content, (ii) favoring the electron migration, the transfer efficiency (spillover effect) and gas diffusion rate, (iii) promoting the active sites concomitantly limiting the nanopowders agglomeration; and (iv) forming nano-heterojunctions. Herein, the aim of the present review is to highlight the above-mentioned hybrid features in order to engineer novel flexible, miniaturized and low working temperature sensors, able to detect specific VOC biomarkers of a human's disease

Recent Advancements in TiO2 Nanostructures: Sustainable Synthesis and Gas Sensing

The search for sustainable technology-driven advancements in material synthesis is a new norm, which ensures a low impact on the environment, production cost, and workers' health. In this context, non-toxic, non-hazardous, and low-cost materials and their synthesis methods are integrated to compete with existing physical and chemical methods. From this perspective, titanium oxide (TiO2) is one of the fascinating materials because of its non-toxicity, biocompatibility, and potential of growing by sustainable methods. Accordingly, TiO2 is extensively used in gas-sensing devices. Yet, many TiO2 nanostructures are still synthesized with a lack of mindfulness of environmental impact and sustainable methods, which results in a serious burden on practical commercialization. This review provides a general outline of the advantages and disadvantages of conventional and sustainable methods of TiO2 preparation. Additionally, a detailed discussion on sustainable growth methods for green synthesis is included. Furthermore, gas-sensing applications and approaches to improve the key functionality of sensors, including response time, recovery time, repeatability, and stability, are discussed in detail in the latter parts of the review. At the end, a concluding discussion is included to provide guidelines for the selection of sustainable synthesis methods and techniques to improve the gas-sensing properties of TiO2

Wearable Nano-Based Gas Sensors for Environmental Monitoring and Encountered Challenges in Optimization

With a rising emphasis on public safety and quality of life, there is an urgent need to ensure optimal air quality, both indoors and outdoors. Detecting toxic gaseous compounds plays a pivotal role in shaping our sustainable future. This review aims to elucidate the advancements in smart wearable (nano)sensors for monitoring harmful gaseous pollutants, such as ammonia (NH3), nitric oxide (NO), nitrous oxide (N2O), nitrogen dioxide (NO2), carbon monoxide (CO), carbon dioxide (CO2), hydrogen sulfide (H2S), sulfur dioxide (SO2), ozone (O3), hydrocarbons (CxHy), and hydrogen fluoride (HF). Differentiating this review from its predecessors, we shed light on the challenges faced in enhancing sensor performance and offer a deep dive into the evolution of sensing materials, wearable substrates, electrodes, and types of sensors. Noteworthy materials for robust detection systems encompass 2D nanostructures, carbon nanomaterials, conducting polymers, nanohybrids, and metal oxide semiconductors. A dedicated section dissects the significance of circuit integration, miniaturization, real-time sensing, repeatability, reusability, power efficiency, gas-sensitive material deposition, selectivity, sensitivity, stability, and response/recovery time, pinpointing gaps in the current knowledge and offering avenues for further research. To conclude, we provide insights and suggestions for the prospective trajectory of smart wearable nanosensors in addressing the extant challenges

Fabrication of a Highly NO2-Sensitive Gas Sensor Based on a Defective ZnO Nanofilm and Using Electron Beam Lithography

Hazardous substances produced by anthropic activities threaten human health and the green environment. Gas sensors, especially those based on metal oxides, are widely used to monitor toxic gases with low cost and efficient performance. In this study, electron beam lithography with two-step exposure was used to minimize the geometries of the gas sensor hotplate to a submicron size in order to reduce the power consumption, reaching 100 °C with 0.09 W. The sensing capabilities of the ZnO nanofilm against NO2 were optimized by introducing an enrichment of oxygen vacancies through N2 calcination at 650 °C. The presence of oxygen vacancies was proven using EDX and XPS. It was found that oxygen vacancies did not significantly change the crystallographic structure of ZnO, but they significantly improved the electrical conductivity and sensing behaviors of ZnO film toward 5 ppm of dry air

Research status and prospect of graphene materials in aviation

Among various 2D materials, graphene has received extensive research attention in the past 2-30 years due to its fascinating properties. The discovery of graphene has provided a huge boost and a new dimension to materials research and nanotechnology. Many lightweight materials with good performance have been widely used in the aviation field, which has greatly promoted the development of military and civilian industries and promoted technological innovation. Based on the introduction of the structure and properties of graphene, this paper summarizes the application value of graphene in the aerospace field in three aspects: energy equipment, sensors, and composite materials used outside aircraft.Comment: (22 pages, 23 figures

Zinc oxide nanostructures with carbon nanotube and gold additives for co gas sensing application

Abstract: Zinc oxide (ZnO) nanostructures were synthesised for gas sensing application. In an attempt to improve the surface area and the electrical conductivity of the ZnO, nanomaterials such as the carbon nanotubes (CNTs) and gold nanoparticles (AuNPs) were used separately to produce CNTs/ZnO and Au/ZnO nanocomposites, respectively. The addition of these nanomaterials onto the ZnO nanostructures significantly improved the gas sensing properties such as the sensitivity and response time. Synthesis of gold nanoparticles was successfully achieved via gold salt (HAuCl4.3H2O) reduction using the Turkevich method. Citrate molecules were used as the stabiliser and to systematically control the sizes of the AuNPs. The sizes of AuNPs were found to increase from 14 nm to 40 nm when the concentration of citrate ions was reduced from 1 mM to 0.3 mM. The size distribution of AuNPs was relatively wider as the particle size increased. The synthesized AuNPs were stable for over a period of 4 weeks. Carbon nanotubes synthesis was achieved using chemical vapour deposition (CVD) method using acetylene gas as the carbon source and ferrocene as the catalyst. An increase in the flowrate of the precursor gas (acetylene) yielded an increase in amorphous carbon, which was attached to the walls of the carbon nanotubes. The optimum flowrate of acetylene was found to be 150 m3/min that yielded CNTs with an average diameter of 95 nm and a relatively narrow size distribution. The hydrothermal chemical precipitation method was used to synthesise ZnO nanostructures. Zinc sulphate (ZnSO4) and sodium hydroxide (NaOH) were used as a metal precursor and reducing agent, respectively. The NaOH concentration of 0.3 M yielded ZnO nanosheets with relatively the highest surface area of 102 m2/g. Gas sensing analysis was conducted using carbon monoxide (CO) gas at 250°C. The sensitivity and response time were calculated to be 9.8% and 114 seconds, respectively, at a CO concentration of 200 ppm. The composites CNTs/ZnO and Au/ZnO were prepared, separately. The average surface area of the Au/ZnO composite was 131 m2/g and that of CNTs/ZnO composite was 153 m2/g. The CNTs/ZnO composite showed an optimum sensitivity of 9.9% and the response time of 49 seconds when exposed to 200 ppm of CO gas at 250°C.M.Tech. (Chemical Engineering

Understanding Heterostructure Chemiresistive Gas Sensing at Room Temperature

Chemiresistive sensors are the most widely investigated gas sensors due to their ease in fabrication, cost-effectiveness, simplicity of operation, and offer advances in miniaturization. Up to date, typical and well-researched resistive-type sensing materials include semiconductor metal oxides, noble metals, carbon-based nanomaterials (e.g., graphene and carbon nanotubes), and conducting polymers. Gas sensors based on a single material were found difficult to meet the practical requirements for multi-sensing properties, including sensitivity, selectivity, speed of response/recovery, stability, limit of detection, and room temperature operation. Rational design through a combination of chemically or electronically dissimilar nanomaterials is an effective route to enhancing gas sensing performance. Because the chemical composition varies with position, especially at the interface between two dissimilar materials, the newly hybridized structure is defined as a heterostructure. During the past decades, there has been significant research effort in exploring the nanocomposite heterostructures for chemiresistive room-temperature gas sensors. However, sensing mechanisms for such heterostructures are still elusive without solid analysis or direct characterization results. The objective of this dissertation study is to understand the sensing mechanisms of heterostructure-based chemiresistive gas sensors through in situ investigation and analysis under real operating conditions. Various novel heterostructures have been developed for specific types of gas sensing, with a variety of in situ/operando techniques applied to investigate the sensing mechanisms toward different gases. Firstly, nickel oxide-tungsten oxide (NiO-WO3) nanowire-based heterostructures with various component ratios were fabricated via a facile, sonication-based solution mixing method. The exhibited heterojunction effect is maximally observed for W3N1 (75 mol% WO3-25 mol% NiO) and confirmed by observation of the increase in resistance due to the formation of a diode-like p-n junction at the NiO-WO3 interface. The excellent hydrogen sulfide (H2S) sensing performance for W3N1 is attributed to the p-n junction effect, sulfurization by H2S (formation of tungsten sulfides (WS2-x), and nickel sulfides (NiS1-x)), and the ideal ratio of the NiO component in the composite. The formation of reactive semi-metallic products due to sulfurization on the sensor surface was confirmed by in situ X-ray diffraction (XRD) analyses. Operando impedance measurements and resistor-capacitor (RC) equivalent circuit analyses during gas sensing experiments were performed to evaluate the effect of grain-grain boundary or the p-n junction on the sensing performance. It was found that for pure WO3 and W3N1 samples, these contributing effects are in the same direction, resulting in a cooperative and highly sensitive performance, whereas, for other compositions, the samples exhibited competing influences, resulting in low sensitivity. Secondly, the gold doped tin oxide/reduced graphene oxide (Au-SnO2/rGO) ternary nanohybrid heterostructure was designed with improved room temperature hydrogen (H2) sensing performance. The sputtered Au nanoparticles enhanced both sensitivity and recovery of the SnO2-rGO platform. Such an enhancement was attributed to the increased surface area and the oxygen ions spillover effect of loaded Au nanoparticles. The catalytic effect of Au nanoparticles for hydrogen adsorption and desorption was then revealed through the temperature-dependent gas sensing test and the Arrhenius analysis. A better balance between sensitivity and recovery can be further achieved in the future by tuning the deposition conditions of Au nanoparticles. A prototype handheld device based on the Au-SnO2/rGO composites was finally developed for hydrogen detection. The prototype device demonstrates the potential for real-time hydrogen monitoring. The availability of such sensors will contribute to promoting a sustainable hydrogen economy, protecting public safety, and enhancing lead-acid battery safety in a wide range of applications. Thirdly, the nickel-doped tin oxide-reduced graphene oxide (Ni/SnO2-rGO) ternary nanohybrid heterostructure was prepared with enhanced room temperature sulfur dioxide (SO2) sensing performance. The Ni additives significantly improved the lower detection limit (ppb level) of the SnO2-rGO platform. The SO2 concentration calibration curve is well fitted by the Langmuir isotherm. The humidity effect on the sensing performance was also investigated. The results suggested that current nanohybrid materials still suffer from the humidity effect. Metal oxide nanocomposite doping enhanced the SO2 sensing and activated the adsorption of water molecules, which diminished the sensor response to sulfur dioxide gas. Finally, the Poly[3-(3carboxypropyl)thiophene-2,5-diyl]regioregular (PT-COOH)-GO binary nanocomposite heterostructure was prepared. The gas sensing properties were investigated toward NO2, NH3, SO2, and CO. The PT-COOH based sensors exhibited tunable sensing performance through the drain voltage modulation. PT-COOH-GO sensors indicated enhanced NO2 sensing performance with good sensitivity, recovery, and stable responses. The statistical signal analysis was conducted to obtain proof-of-concept results for gas discrimination through signal processing. This study reveals the electronic conduction gas sensing model of multi-metal oxide -nanowires-based chemiresistive gas sensors through the combination of direct current (DC) and alternating current (AC) impedance measurements. The research also suggests that two-dimensional (2D) rGO with proper modifications can be efficient gas sensing materials toward various gaseous analytes. Combining in situ characterization and critical sensing factor analyses, results from the study will offer valuable and comprehensive insights for the rational design of superior heterostructure-based chemiresistive gas sensors

Gas sensing technologies -- status, trends, perspectives and novel applications

The strong, continuous progresses in gas sensors and electronic noses resulted in improved performance and enabled an increasing range of applications with large impact on modern societies, such as environmental monitoring, food quality control and diagnostics by breath analysis. Here we review this field with special attention to established and emerging approaches as well as the most recent breakthroughs, challenges and perspectives. In particular, we focus on (1) the transduction principles employed in different architectures of gas sensors, analysing their advantages and limitations; (2) the sensing layers including recent trends toward nanostructured, low-dimensional and composite materials; (3) advances in signal processing methodologies, including the recent advent of artificial neural networks. Finally, we conclude with a summary on the latest achievements and trends in terms of applications.Comment: arXiv admin comment: This version has been removed by arXiv administrators as the submitter did not have the rights to agree to the license at the time of submissio

Carbon-based materials for humidity sensing: a short review

Humidity sensors are widespread in many industrial applications, ranging from environmental and meteorological monitoring, soil water content determination in agriculture, air conditioning systems, food quality monitoring, and medical equipment to many other fields. Thus, an accurate and reliable measurement of water content in dierent environments and materials is of paramount importance. Due to their rich surface chemistry and structure designability, carbon materials have become interesting in humidity sensing. In addition, they can be easily miniaturized and applied in flexible electronics. Therefore, this short review aims at providing a survey of recent research dealing with carbonaceous materials used as capacitive and resistive humidity sensors. This work collects some successful examples of devices based on carbon nanotubes, graphene, carbon black, carbon fibers, carbon soot, and more recently, biochar produced from agricultural wastes. The pros and cons of the dierent sensors are also discussed in the present review

Two-Dimensional Electronics - Prospects and Challenges

During the past 10 years, two-dimensional materials have found incredible attention in the scientific community. The first two-dimensional material studied in detail was graphene, and many groups explored its potential for electronic applications. Meanwhile, researchers have extended their work to two-dimensional materials beyond graphene. At present, several hundred of these materials are known and part of them is considered to be useful for electronic applications. Rapid progress has been made in research concerning two-dimensional electronics, and a variety of transistors of different two-dimensional materials, including graphene, transition metal dichalcogenides, e.g., MoS2 and WS2, and phosphorene, have been reported. Other areas where two-dimensional materials are considered promising are sensors, transparent electrodes, or displays, to name just a few. This Special Issue of Electronics is devoted to all aspects of two-dimensional materials for electronic applications, including material preparation and analysis, device fabrication and characterization, device physics, modeling and simulation, and circuits. The devices of interest include, but are not limited to transistors (both field-effect transistors and alternative transistor concepts), sensors, optoelectronics devices, MEMS and NEMS, and displays