Recent progress in applications of graphene oxide for gas sensing: A review

This paper is a review of the recent progress on gas sensors using graphene oxide (GO). GO is not a new material but its unique features have recently been of interest for gas sensing applications, and not just as an intermediate for reduced graphene oxide (RGO). Graphene and RGO have been well known gas-sensing materials, but GO is also an attractive sensing material that has been well studied these last few years. The functional groups on GO nanosheets play important roles in adsorbing gas molecules, and the electric or optical properties of GO materials change with exposure to certain gases. Addition of metal nanoparticles and metal oxide nanocomposites is an effective way to make GO materials selective and sensitive to analyte gases. In this paper, several applications of GO based sensors are summarized for detection of water vapor, NO₂, H₂, NH₃, H₂S, and organic vapors. Also binding energies of gas molecules onto graphene and the oxygenous functional groups are summarized, and problems and possible solutions are discussed for the GO-based gas sensors.     

Graphdiyne:a Highly Sensitive Material for ppb-Level NO2 Gas Sensing at Room Temperature

Detection of a trace amount of NO2 at room temperature has very important applications in air quality monitoring,protection of human health and medical diagnose.However,the existing NO2 sensors often suffer from low sensitivity when the concentration at the ppb-level.Here,we report a new kind of materials based on graphdiyne(GDY)for highly sensitive detection of ppb-level(ppb:part per billion)NO2 at room temperature.After thermal treatment of the as-prepared GDY at 600℃under argon atmosphere for 2 h(the obtained sample denoted as GDY-600),the prepared sensor with GDY-600 displays excellent sensitivity with a response value of 6.2％towards 250 ppb NO2 at room temperature,which is better than most of reported sensing materials.In addition,the sensor exhibits significantly high selectivity to NO2 against typical interfering gases including CO,CO2,NH3,H2,H2S and toluene.Moreover,the sensor shows remarkable stability after repetitive measurements.The superior sensing performance of GDY-600 can be ascribed to the highly π-conjugated structure with special acetylenic bonds and abundant oxygen-containing functional groups,which are all beneficial for the gas adsorption and redox reaction on the surface.     

Graphene based chemiresistor sensors for the detection of toxic air pollutants: A theoretical overview

The advancements in present world like industrial revolution, transport system, agriculture, food industry, pharmaceutical industry and electricity generation have made our life easy and comfortable in many aspects but they also have deleterious effects on our life. The pollution from industries, household waste, combustion of fossils etc. has introduced many harmful chemicals in air and water which have badly affected the life on our planet. The presence of undesirable gases in air has serious adverse effects on health and quality of life. Therefore, monitoring of these substances becomes important.Graphene based chemiresistor sensor are proved to be promising materials for the detection of toxic air pollutants. This article summarizes the recent progress of Density functional theory (DFT) based studies in the field of graphene based gas sensors. This article discusses the working mechanism of graphene based chemiresistor and also provides the information that how the sensing ability can be enhanced. The information given in the article will help the young researchers in the selection of suitable dopant and nano-clusters for graphene based surfaces to make them more selective and sensitive towards the analytes.     

An Analysis of Exhaust Emission of the Internal Combustion Engine Treated by the Non-Thermal Plasma

Industries’ air pollution causes serious challenges to modern society, among them exhaust gases from internal combustion engines, which are currently one of the main sources. This study proposes a non-thermal plasma (NTP) system for placement in the exhaust system of internal combustion engines to reduce the toxic contaminants (HC, CO, and NO_x) of exhaust gases. This NTP system generates a high-voltage discharge that not only responds to the ion chemical reaction to eliminate NO_x and CO, but that also generates a combustion reaction at the local high temperature of plasma to reduce HC. The NTP system was designed on both the front and rear of the exhaust pipe to analyze the difference of different exhaust flow rates under the specified frequency. The results indicate that the NTP system can greatly reduce toxic contaminants. The NTP reactor placed in the front of exhaust pipe gave HC and CO removal efficiency of about 34.5% and 16.0%, respectively, while the NTP reactor placed in the rear of exhaust pipe gave NO_x removal efficiency of about 41.3%. In addition, the voltage and material directly affect the exhaust gases obviously. In conclusion, the proposed NTP system installed in the exhaust system can significantly reduce air pollutants. These results suggest that applying NTP to the combustion engine should be a useful tool to simultaneously reduce both emissions of NO_x and CO.     

Sodium/lithium 3d transition metalates for chemisorption of gaseous pollutants: a review

Sodium/lithium transition metalates have found tremendous potential as cathode materials for Na/Li batteries. These metalates have acid-base and redox characteristics, which could be utilized for the chemisorption of gaseous pollutants. Their use was focused mainly on CO₂ gas chemisorption at elevated temperatures. In recent years, these materials have found interesting applications in the chemisorption of CO, NO, SO₂, and H₂S gases. Some of these alkali ceramics have shown tremendous potential for the wet-oxidative removal of acidic gases, even in ambient conditions. The review presents an up-to-date account of alkali ceramics of 3d transition metals for the chemisorption of toxic gases, including CO₂, CO, NO, SO₂, and H₂S. To the best of our knowledge, Na/Li 3d transition metalates have never been reviewed in the context of air decontamination, which needs to be presented to the readers for air purification applications.     

Graphene Based Electrochemical Sensors and Biosensors: A Review

Graphene, emerging as a true 2‐dimensional material, has received increasing attention due to its unique physicochemical properties (high surface area, excellent conductivity, high mechanical strength, and ease of functionalization and mass production). This article selectively reviews recent advances in graphene‐based electrochemical sensors and biosensors. In particular, graphene for direct electrochemistry of enzyme, its electrocatalytic activity toward small biomolecules (hydrogen peroxide, NADH, dopamine, etc.), and graphene‐based enzyme biosensors have been summarized in more detail; Graphene‐based DNA sensing and environmental analysis have been discussed. Future perspectives in this rapidly developing field are also discussed.     

Synthesis of LaFeO<Subscript>3</Subscript> catalytic materials and their sensing properties

LaFeO₃ is a p-type semiconductor catalytic material of perovskite structure (ABO₃). Its magnetic and photocatalytic properties have been widely investigated, but the gas sensing properties are seldom reported, especially for toxic and noxious gases of NO₂ and CO. The nanocomposites of LaFeO₃ and LaFe_1−x Mg _x O₃ (x = 0.02, 0.04, 0.06) were prepared by various methods of the wet chemical process and their exact composition, crystal structures, grain sizes, specific surfaces, morphology and the electronic interaction between components were characterized by EDX, XRD, BET, SEM and XPS analysis. The sensors based on these nanocomposites have been fabricated to examine the sensing responses to gases, and the results show that these sensors exhibited high response to both oxidizing gas (NO₂) and reducing gas (CO), and the response was greatly enhanced by the surface modification of MgO. The additive method, amount of additives, and their effects on the LaFeO₃ structure and gas response have been analyzed and discussed by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopic (XPS) analysis.     

Electrochemically exfoliating graphite into N-doped graphene and its use as a high efficient electrocatalyst for oxygen reduction reaction

N-doped graphene has been extensively explored because of their intriguing properties. However, most of the conventional heat-processed N-doped graphene (HNG) suffer from the poor hydrophilic property and low electric conductivity when using electrode materials. Herein, we present a facile solution-processed strategy to fabricate N-doped graphene through electrochemical exfoliation of graphite in inorganic electrolyte solution. The resulting electrochemically exfoliated N-doped graphene (ENG) has high level of nitrogen (7.9 at.%) and oxygen (16.5 at.%), moreover, excellent electric conductivity (19 s cm^?1). As a binder-free electrode material for oxygen reduction reaction (ORR), ENG exhibits much better electroactivity than HNG and electrochemically exfoliated graphene (EG), moreover, much better methanol tolerance and long-term durability than that commercial Pt/C catalyst. The results provide new sights into scalable production of noble metal-free catalyst towards ORR.     

Fast response speed of mechanically exfoliated MoS2 modified by PbS in detecting NO2

MoS₂, acting as a promising gas sensing material, has shown huge potential in monitoring of toxic and harmful gases at room temperature. However, MoS₂-based gas sensors still suffer from poor gas sensing performance such as poor sensitivity, long response time. Constructing the heterostructure is an effective approach to improve gas-sensing performance of MoS₂. Herein, PbS@MoS₂ composites synthesized by mechanical exfoliation combining with wet-chemical precipitation are used to investigate its performance in detecting NO₂ at room temperature. The response value of PbS@MoS₂ gas sensor against NO₂ is significantly improved compared with the pure MoS₂ gas sensor. At the same time, the modification with PbS also accelerates the response speed of MoS₂, and the response time is almost reduced by two orders of magnitude, from hundreds of seconds to less than ten seconds. The enhanced response value and fast response time are mainly benefited from the modulation effect of NO₂ to PbS@MoS₂ heterostructure and the mechanically exfoliated MoS₂ surface with few defects. This work can be expected to provide useful guidance for designing composite materials with excellent gas sensing properties.     

Potentiometric NOx sensor based on stabilized zirconia and NiCr2O4 sensing electrode operating at high temperatures

The two types of electrochemical sensors using stabilized zirconia and the oxide sensing electrode (SE) were developed for NO_x detection at high temperatures. For the mixed-potential-type sensor, NiCr₂O₄ was found to give fairly excellent NO_x sensing characteristics in air among several spinel-type oxides tested. This NO_x sensor provided a linear correlation between EMF and the logarithm of NO or NO₂ concentration in the range 25–436 ppm and in the temperature range 550–650°C. With fixed bias voltage being applied between the SE (oxide) and the counter (Pt) electrode (CE), the EMF between SE and the reference (Pt) electrode (RE) was measured as a sensing signal. The NiCr₂O₄-attached tubular device was found to provide selective response to NO over NO₂ if SE was polarized at +175 mV versus RE. It was also found that this device gave selective response to NO₂ over NO, if SE was polarized at −250 mV versus CE. The new design of the planar device was proposed to avoid the cross-sensitivities to the others gases usually coexisting in car exhausts.     

HFIP-Functionalized Co3O4 Micro-Nano-Octahedra/rGO as a Double-Layer Sensing Material for Chemical Warfare Agents

Semiconductor metal oxides (SMO)-based gas-sensing materials suffer from insufficient detection of a specific target gas. Reliable selectivity, high sensitivity, and rapid response–recovery times under various working conditions are the main requirements for optimal gas sensors. Chemical warfare agents (CWA) such as sarin are fatal inhibitors of acetylcholinesterase in the nerve system. So, sensing materials with high sensitivity and selectivity toward CWA are urgently needed. Herein, micro-nano octahedral Co₃O₄ functionalized with hexafluoroisopropanol (HFIP) were deposited on a layer of reduced graphene oxide (rGO) as a double-layer sensing materials. The Co₃O₄ micro-nano octahedra were synthesized by direct growth from electrospun fiber templates calcined in ambient air. The double-layer rGO/Co₃O₄-HFIP sensing materials presented high selectivity toward DMMP (sarin agent simulant, dimethyl methyl phosphonate) versus rGO/Co₃O₄ and Co₃O₄ sensors after the exposure to various gases owing to hydrogen bonding between the DMMP molecules and Co₃O₄-HFIP. The rGO/Co₃O₄-HFIP sensors showed high stability with a response signal around 11.8 toward 0.5 ppm DMMP at 125 °C, and more than 75 % of the initial response was maintained under a saturated humid environment (85 % relative humidity). These results prove that these double-layer inorganic–organic composite sensing materials are excellent candidates to serve as optimal gas-sensing materials.     

Hydrothermal synthesis of one-dimensional α-MoO3 nanomaterials and its unique sensing mechanism for ethanol

In gas sensor applications, the availability of highly sensitive and rapid response/recovery detector for ethanol gas is sparse. One-dimensional orthogonal crystalline molybdenum trioxide nanomaterials were synthesized by an economical and environmentally friendly hydrothermal method. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy spectroscopy (EDS) were used to investigate the structure and morphology of the nanometer materials. The relevant characterization shows that nanobelts are highly crystalline layered structures with a width of about 200 nm and a length of a few micrometers. The synthesized ethanol gas sensors based on α-MoO₃ semiconductor material show the highest response at 350 °C. Gas sensitivity tests indicated that α-MoO₃ nanobelts respond well to 50 ～ 600 ppm ethanol at optimal operating temperatures. The selectivity test among various reducing gases shows that the sensor responds better to ethanol compared to other gases such as xylene, NO₂, CO, and H₂ gases. This excellent sensing performance is attributed to the unique sensing mechanism formed in the layered MoO₃ nanobelts through the catalytic reaction between ethanol and MoO₃ lattice oxygen and adsorbed oxygen. The sensing mechanism of the co-catalytic effect of lattice oxygen and adsorbed oxygen on ethanol is also discussed in depth.     

Electrochemical sensors based on graphene materials

Single?Clayered graphene, emerging as a true two?Cdimensional nanomaterial, has tremendous potential for electrochemical catalysis and biosensing as a novel electrode material. Considering the excellent properties of graphene, such as large surface?Cto?Cvolume ratio, high conductivity and electron mobility at room temperature, low energy dynamics of electrons with atomic thickness, robust mechanical and flexibility, we give a general view of recent advances in electrochemical sensors based on graphene. We are highlighting here important applications of graphene and graphene nanocomposites, and the assay strategies in electrochemical sensors for DNA, proteins, neurotransmitters, phytohormones, pollutants, metal ions, gases, hydrogen peroxide, and in medical, enzymatic and immunosensors.

Formation Mechanism,Geometric Stability and Catalytic Activity of a Single Iron Atom Supported on N-Doped Graphene

Based on density functional theory (DFT) calculations, the formation geometries, stability and catalytic properties of single-atom iron anchored on xN-doped graphene (xN-graphene-Fe, x=1, 2, 3) sheet are systemically investigated. It is found that the different kinds and numbers of gas reactants can effectively regulate the electronic structure and magnetic properties of the 3 N-graphene-Fe system. For NO and CO oxidation reactions, the coadsorption configurations of NO/O₂ and CO/O₂ molecules on a reactive substrate as the initial state are comparably analyzed. The NO oxidation reactions through the Langmuir–Hinshelwood (LH) and Eley-Rideal (ER) mechanisms have relatively smaller energy barriers than those of the CO oxidation processes. In comparison, the preadsorbed 2NO reacting with 2CO molecules (2NO+2CO→2CO₂+N₂) through ER reactions (<0.4 eV) are energetically more favorable processes. These results can provide beneficial references for theoretical studies on NO and CO oxidation and designing graphene-based catalyst for toxic gas removal.     

Conducting polymer-based hybrid assemblies for electrochemical sensing: a materials science perspective

In this topical review, progress achieved in amperometric sensing of different analytes over conducting polymer-based hybrid electrocatalysts is summarized. We report a variety of synthetic methods and the resulting hybrid assemblies, with the effectiveness of such strategies, for designing conjugated polymer-based hybrids as robust sensors for amperometric detection. Beyond incorporation of metal nanoparticles, metal-oxide and non-oxide semiconductors, carbon-based nanomaterials (nanotubes, graphene, and graphene oxide), and special dopant ions are also discussed. Moreover, some particularly interesting miscellaneous approaches, for example photo-amperometric sensing or use of overoxidized polymers, are also emphasized. Determination of dissolved gases (for example O₂, NO, and NO₂), ions (sulfite, nitrite, nitrate, chlorate, bromate, and iodate) and smaller and larger molecules (for example H₂O₂, ascorbic acid (AA), dopamine (DA), urea (UA), amino acids, hydrazine, NADH, serotonin, and epinephrine) is discussed. These achievements are reviewed from the materials perspective, addressing both synthetic and electrocatalytic aspects of the polymer-based modified electrodes. Beyond simple or more sophisticated mixing, a wide range of methods of preparation is presented, including chemical (one-pot polymerization, impregnation), electrochemical (co-deposition, doping type inclusion, etc.) and combined strategies. Classification of such synthetic routes is also included. However, it is important to note that we omit studies in which conducting polymers alone were used for determination of different species. Furthermore, because excellent reviews—cited in this work also—are available on immobilization of biomolecules (for example enzymes) for biosensing purposes, this topic, also, is excluded.     

Highly selective CO2 capture on N-doped carbon produced by chemical activation of polypyrrole functionalized graphene sheets

N-doped porous carbon produced via chemical activation of polypyrrole functionalized graphene sheets shows selective adsorption of CO(2) (4.3 mmol g(-1)) over N(2) (0.27 mmol g(-1)) at 298 K. The potential for large scale production and facile regeneration makes this material useful for industrial applications.     

Laser-ablated violet phosphorus/graphene heterojunction as ultrasensitive ppb-level room-temperature NO sensor

Two dimensional(2D) materials are promising gas sensing materials, but the most of them need to be heated to show promising sensing performance. Sensing structures with high sensing performance at room-temperature are urgent. Here, another 2D material, violet phosphorus(VP) nanoflake is investigated as gas sensing material. The VP nanoflakes have been effectively ablated to have layers of 1–5 layers by laser ablation in glycol. The VP nanoflakes are combined with graphene to form VP/G heterostru...     

Cu?Ti?O catalyst for complex purification of gases

Cu–Ti–O catalysts activity in the reactions of complete oxidation of CO and C₄H₁₀, selective catalytic reduction of NO by ammonia, SO₂ oxidation to SO₃, as well as the catalyst resistance to sulfur poisoning were studied. We suggest these catalysts for the combined removal of NO, CO and toxic organics from flue gases.