3D Architectured Graphene/Metal Oxide Hybrids for Gas Sensors: A Review
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paper_title: Novel 3D graphene aerogel–ZnO composites as efficient detection for NO2 at room temperature

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Abstract A novel three-dimensional graphene aerogel–ZnO spheres composite has been synthesized via a simple solvothermal method. The three-dimensional graphene exhibits interconnected macroporous networks and the ZnO spheres featured a size of 0.5–1 μm are anchored homogeneously on the surface of graphene layers. When being used as gas sensors for detection of NO 2 , the porous graphene–ZnO composite achieves a higher response and fast response/recovery behavior at room temperature effectively (RT). The enhancement of the sensing response is mainly attributed to the improved conductivity, the 3D porous structure of graphene and the interaction between p-type graphene and n-type zinc oxide. The sensing mechanism of the composite is also discussed.
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paper_title: Graphene–metal oxide nanohybrids for toxic gas sensor: A review

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Abstract Sensing of gas molecules is critical to environmental monitoring, control of chemical processes, agricultural, and medical applications. In particular, the detection of industrial toxic gases such as CO, NO x , and NH 3 is very important for many industries. Metal oxides have been widely studied for the sensitivity of their properties to gases even though they do have some limitations. Recently, graphene has been considered as a promising material for gas sensing since its electronic properties are strongly affected by the adsorption of foreign molecules. Intrinsic graphene has high sensitivity at low gas concentrations; but the sensor selectivity is poor which limits its use in many practical applications. Hence, hybrid architectures formed by blending of nanoparticles of metal–oxides with graphene or its derivatives have been explored by several researchers which showed improved gas sensing ability, especially the sensitivity and selectivity at room temperature. Here we review the state of the art of gas sensors based on graphene and metal oxide hybrid nanostructures for detection of various common toxic gases.
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paper_title: Gas Sensors Based on Conducting Polymers

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The gas sensors fabricated by using conducting polymers such as polyaniline (PAni), polypyrrole (PPy) and poly (3,4-ethylenedioxythiophene) (PEDOT) as the active layers have been reviewed. This review discusses the sensing mechanism and configurations of the sensors. The factors that affect the performances of the gas sensors are also addressed. The disadvantages of the sensors and a brief prospect in this research field are discussed at the end of the review.
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paper_title: Graphene-Based Chemical Sensors

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Pioneering research in 2004 by Geim and Novoselov (2010 Nobel Prize winners in Physics) of the University of Manchester led to the isolation of a monolayer graphene sheet. Graphene is a single-atom-thick sheet of sp2 hybridized carbon atoms that are packed in a hexagonal honeycomb crystalline structure. Graphene is the fundamental building block of all sp2 carbon materials including single-walled carbon nanotubes, mutliwalled carbon nanotubes, and graphite and is therefore interesting from the fundamental standpoint as well as for practical applications. One of the most promising applications of graphene that has emerged so far is its utilization as an ultrasensitive chemical or gas sensor. In this article, we review some of the significant work performed with graphene and its derivatives for gas detection and provide a perspective on the challenges that need to be overcome to enable commercially viable graphene chemical sensor technologies.
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paper_title: Three-dimensional mesoporous graphene aerogel-supported SnO2 nanocrystals for high-performance NO2 gas sensing at low temperature.

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A facile and cost-efficient hydrothermal and lyophilization two-step strategy has been developed to prepare three-dimensional (3D) SnO2/rGO composites as NO2 gas sensor. In the present study, two different metal salt precursors (Sn(2+) and Sn(4+)) were used to prepare the 3D porous composites. It was found that the products prepared from different tin salts exhibited different sensing performance for NO2 detection. The scanning electron microscopy and transmission electron microscopy characterizations clearly show the macroporous 3D hybrids, nanoporous structure of reduce graphene oxide (rGO), and the supported SnO2 nanocrystals with an average size of 2-7 nm. The specific surface area and porosity properties of the 3D mesoporous composites were analyzed by Braunauer-Emmett-Teller method. The results showed that the SnO2/rGO composite synthesized from Sn(4+) precursor (SnO2/rGO-4) has large surface area (441.9 m(2)/g), which is beneficial for its application as a gas sensing material. The gas sensing platform fabricated from the SnO2/rGO-4 composite exhibited a good linearity for NO2 detection, and the limit of detection was calculated to be as low as about 2 ppm at low temperature. The present work demonstrates that the 3D mesoporous SnO2/rGO composites with extremely large surface area and stable nanostructure are excellent candidate materials for gas sensing.
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paper_title: Enhancing NO2 gas sensing performances at room temperature based on reduced graphene oxide-ZnO nanoparticles hybrids

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a b s t r a c t NO2 gas sensor has been constructed using reduced graphene oxide-ZnO nanoparticles (ZnO-rGO) hybrids as sensing materials. Most importantly, the sensor exhibits higher sensitivity, shorter response time and recovery time than those of the sensor based on rGO, indicating that the sensing performances for NO2 sensing operating at room temperature have been enhanced by introduction of ZnO nanoparticles into rGO matrix.
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paper_title: Solvothermal Synthesis of ZnO Nanostructures and Their Morphology-Dependent Gas-Sensing Properties

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Single-crystalline ZnO nanostructures were synthesized by solvothermal method using methanol as solvent. The effect of counterions of zinc salts (nitrate, acetate, and chloride) on the morphology of ZnO nanostructures was investigated. ZnO nanorods (NRs) were formed for all kinds of zinc salts except zinc chloride, where nanoparticles (NPs) were formed. The length and width of ZnO NRs were 100–150 nm and 20–25 nm, respectively, whereas NPs were 20–25 nm in diameter. Replacing methanol to ethanol generated only NRs for all kinds of zinc salts and they were about 10 times larger than those in methanol. The effect of morphology on sensing property was investigated by comparing their response. ZnO NRs showed very high response as compared to ZnO NPs for NO2 and vice versa for CO, although the surface area of ZnO NPs (42.83 m2/g) was much higher than those of ZnO NRs (17.6 m2/g). The response of ZnO NRs was 30 times higher than those of NPs for NO2 gas, whereas 4 times lower for CO gas. The maximum response of...
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paper_title: Metal oxides for solid-state gas sensors: What determines our choice?

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The analysis of various parameters of metal oxides and the search of criteria, which could be used during material selection for solid-state gas sensor applications, were the main objectives of this review. For these purposes the correlation between electro-physical (band gap, electroconductivity, type of conductivity, oxygen diffusion), thermodynamic, surface, electronic, structural properties, catalytic activity and gas-sensing characteristics of metal oxides designed for solid-state sensors was established. It has been discussed the role of metal oxide manufacturability, chemical activity, and parameter's stability in sensing material choice as well.
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paper_title: Fast, sensitive hydrogen gas detection using single palladium nanowires that resist fracture.

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Two types of pure palladium (Pd) nanowires, differentiated by microstructure, were electrodeposited: (1) nanocrystalline Pd nanowires (grain diameter ≈ 5 nm, henceforth nc5-Pd) and (2) nanocrystalline Pd nanowires with a grain diameter of 15 nm (nc15-Pd). These nanowires were evaluated for the detection of hydrogen gas (H2). Despite their fundamental similarities, the behavior of these nanowires upon exposure to H2 was dramatically and reproducibly different: nc5-Pd nanowires spontaneously fractured upon exposure to H2 above 1−2%. Fractured nanowires continued to function as sensors for H2 concentrations above 2%, actuated by the volume change associated with the α to β phase transition of PdHx. nc15-Pd nanowires, in contrast, withstood repeated exposures to H2 up to 10% without fracturing. nc15-Pd nanowires showed a rapid (2 s at 10%) increase in resistance in the presence of H2 and a response that scaled smoothly with [H2] spanning 5 orders of magnitude down to 2 ppm.
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paper_title: Flexible graphene-based chemical sensors on paper substrates.

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Graphene-based, flexible NO(2) sensors on paper substrates exhibited an immediate response (32-39%) once exposed to 200 ppm NO(2) gas under a strain of 0.5%. Chemical vapor deposition-grown graphene with a supporting poly(methyl methacrylate) layer was transferred onto paper substrates, followed by formation of two electrodes using silver paste. Current-voltage characteristics and dynamic sensing response were obtained under both relaxed and strained conditions. We demonstrate a facile method without complex photo-lithography and high vacuum processes for fabricating graphene-based flexible NO(2) sensors on paper substrates with high sensing response.
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paper_title: Graphene and Graphene Oxide: Synthesis, Properties, and Applications

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There is intense interest in graphene in fields such as physics, chemistry, and materials science, among others. Interest in graphene's exceptional physical properties, chemical tunability, and potential for applications has generated thousands of publications and an accelerating pace of research, making review of such research timely. Here is an overview of the synthesis, properties, and applications of graphene and related materials (primarily, graphite oxide and its colloidal suspensions and materials made from them), from a materials science perspective.
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paper_title: Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene

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We measured the elastic properties and intrinsic breaking strength of free-standing monolayer graphene membranes by nanoindentation in an atomic force microscope. The force-displacement behavior is interpreted within a framework of nonlinear elastic stress-strain response, and yields second- and third-order elastic stiffnesses of 340 newtons per meter (N m –1 ) and –690 Nm –1 , respectively. The breaking strength is 42 N m –1 and represents the intrinsic strength of a defect-free sheet. These quantities correspond to a Young9s modulus of E = 1.0 terapascals, third-order elastic stiffness of D = –2.0 terapascals, and intrinsic strength of σ int = 130 gigapascals for bulk graphite. These experiments establish graphene as the strongest material ever measured, and show that atomically perfect nanoscale materials can be mechanically tested to deformations well beyond the linear regime.
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paper_title: Graphene-based gas sensors

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Graphene materials have been widely explored for the fabrication of gas sensors because of their atom-thick two-dimensional conjugated structures, high conductivity and large specific surface areas. This feature article summarizes the recent advancements on the synthesis of graphene materials for this purpose and the techniques applied for fabricating gas sensors. The effects of the compositions, structural defects and morphologies of graphene-based sensing layers and the configurations of sensing devices on the performances of gas sensors will also be discussed.
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paper_title: 3D graphene aerogel-supported SnO2 nanoparticles for efficient detection of NO2

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Three-dimensional (3D) graphene aerogel-supported SnO2 (SGA) nanoparticles (NPs) are presented by a one-pot solvothermal treatment of graphene oxide in the presence of SnCl4 followed by freeze drying. The size of SnO2 nanoparticles on the graphene aerogel is as small as 5–10 nm with uniform distribution. The 3D SnO2–graphene aerogel exhibits interconnected macroporous networks which will be beneficial for gas detection. The SGA shows excellent response and selectivity towards NO2 when other common gases are present at room temperature (RT). The gas sensing responses of the resulting nanocomposites demonstrate that macroporosity in the SGA significantly improves the response and recovery time compared with the 2D SnO2–graphene nanocomposite. The sensing mechanism and the reasons for the better response are also discussed.
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paper_title: Metal Oxide Nanostructures and Their Gas Sensing Properties: A Review

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Metal oxide gas sensors are predominant solid-state gas detecting devices for domestic, commercial and industrial applications, which have many advantages such as low cost, easy production, and compact size. However, the performance of such sensors is significantly influenced by the morphology and structure of sensing materials, resulting in a great obstacle for gas sensors based on bulk materials or dense films to achieve highly-sensitive properties. Lots of metal oxide nanostructures have been developed to improve the gas sensing properties such as sensitivity, selectivity, response speed, and so on. Here, we provide a brief overview of metal oxide nanostructures and their gas sensing properties from the aspects of particle size, morphology and doping. When the particle size of metal oxide is close to or less than double thickness of the space-charge layer, the sensitivity of the sensor will increase remarkably, which would be called "small size effect", yet small size of metal oxide nanoparticles will be compactly sintered together during the film coating process which is disadvantage for gas diffusion in them. In view of those reasons, nanostructures with many kinds of shapes such as porous nanotubes, porous nanospheres and so on have been investigated, that not only possessed large surface area and relatively mass reactive sites, but also formed relatively loose film structures which is an advantage for gas diffusion. Besides, doping is also an effective method to decrease particle size and improve gas sensing properties. Therefore, the gas sensing properties of metal oxide nanostructures assembled by nanoparticles are reviewed in this article. The effect of doping is also summarized and finally the perspectives of metal oxide gas sensor are given.
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paper_title: Gas sensors using hierarchical and hollow oxide nanostructures: Overview

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Hierarchical and hollow oxide nanostructures are very promising gas sensor materials due to their high surface area and well-aligned nanoporous structures with a less agglomerated configurations. Various synthetic strategies to prepare such hierarchical and hollow structures for gas sensor applications are reviewed and the principle parameters and mechanisms to enhance the gas sensing characteristics are investigated. The literature data clearly show that hierarchical and hollow nanostructures increase both the gas response and response speed simultaneously and substantially. This can be explained by the rapid and effective gas diffusion toward the entire sensing surfaces via the porous structures. Finally, the impact of highly sensitive and fast responding gas sensors using hierarchical and hollow nanostructures on future research directions is discussed.
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paper_title: Toxic gas detection using porphyrin dispersed polymer composites

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Abstract The optochemical sensing performances of the porphyrin dyes dispersed in ethylcellulose (EC) matrix to toxic gases such as HCl, NO 2 and SO 2 were examined using a photo-reflection spectrum technique. The intensity of the characteristic Soret- and Q-bands of the porphyrins sensitively changed with the concentration of the gasses. Introducing electron-donating OCH 3 and OH groups to the phenyl-group in the tetraphenylporphyrin (TPPH 2 ) molecule enhanced the sensitivity and that was in the order of TP(4-OH) 4 PH 2 > TP(3,5-OCH 3 ) 4 PH 2 > TPPH 2 to HCl. In contrast, electron-withdrawing COOH group decreased in the sensitivity. It was interpreted that the electron density at the pyrrole nitrogen increased with the electron-donationing groups. Doping of dioctylphthalate (DOP) as a plasticizer to the porphyrin–EC composites remarkably modified their sensitivity and response behavior. The sensitivity to NO 2 was extremely high and sub-ppm level of detection was possible for all the composites. In the case of SO 2 , the sensitivity was lower than NO 2 , but the composite of TP(4-OH) 4 PH 2 exhibited enough sensitivity to detect the concentration of 0.5 ppm. The sensing performance was strongly dependent on the humidity.
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paper_title: Self-assembly fabrication of 3D flower-like ZnO hierarchical nanostructures and their gas sensing properties

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In this paper, three-dimensional flower-like ZnO hierarchical nanostructures were fabricated from the thermal-decomposition of 3D zinc hydroxide carbonate precursor, which was synthesized by a urea hydrothermal method with block copolymer F127 (EO106-PO70-EO106) as the morphology director. XRD, IR, UV-vis, SEM, TEM, TG and N2 adsorption–desorption isotherms have been employed to characterize the products. The influences of synthesis parameters such as reaction time, the type of zinc sources, and species concentration on the morphologies of the products were systematically studied. It was found that the reaction time played a key role in determining the final morphology of porous ZnO. On the basis of experimental results, a possible formation mechanism of the 3D flower-like ZnO hierarchical nanostructures was discussed. More importantly, the gas sensing tests indicated that the sensor made from porous ZnO hierarchical nanostructures exhibited better gas sensing properties to n-butanol compared with the sensor based on the commercial ZnO nanoparticles. The enhancement in gas sensing properties was attributed to their unique 3D hierarchical nanostructures, high surface areas, and greater number of surface active sites.
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paper_title: Micromachined polymer-based chemical gas sensor array

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We have developed a miniature polymer-based chemical gas sensor array on silicon using micromachining technology. The sensors are polymer‐carbon black composite films, which swell reversibly and cause a resistance change upon exposure to a wide variety of gases. We have fabricated two types of devices which can measure this resistance change using a ‘‘well’’ design. These ‘‘wells’’ contain the polymer‐ carbon black‐solvent liquid volume present during deposition and allow the sensor film to be placed reproducibly in a specific and wellconstrained area. After deposition, the solvents evaporate and leave behind a polymer‐carbon black residue crust between metal leads on each side of the ‘‘well’’. Two types of devices, a bulk micromachined sensor and a patterned thick-film sensor, have been fabricated, ranging in size from 500mm 600m mt o 100mm 100 mm. Since the composite film sensors are not specific to any one gas, an array of these sensors, each with a different sensing film, is used to identify gases and gas mixtures through the pattern response of the array. Six polymer‐carbon black composite films were deposited into the sensor array and exposed to three chemical gases at five different concentration levels. The sensors were able to uniquely detect these gas vapors and demonstrated a linear response to concentration levels between 2000 and 10,000 ppm. Results also indicate that a reduction in sensor area by an order of magnitude (from 4.32 to 0.30 mm 2 ) does not reduce sensor response. This design allows the integration of circuits to process the changes in resistance which will permit the realization of a completely integrated miniature gas sensor. # 2001 Published by Elsevier Science B.V.
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paper_title: Chemical methods for the production of graphenes

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Interest in graphene centres on its excellent mechanical, electrical, thermal and optical properties, its very high specific surface area, and our ability to influence these properties through chemical functionalization. There are a number of methods for generating graphene and chemically modified graphene from graphite and derivatives of graphite, each with different advantages and disadvantages. Here we review the use of colloidal suspensions to produce new materials composed of graphene and chemically modified graphene. This approach is both versatile and scalable, and is adaptable to a wide variety of applications.
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paper_title: The rise of graphene

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Graphene is a rapidly rising star on the horizon of materials science and condensed-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discussed here. Whereas one can be certain of the realness of applications only when commercial products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condensed-matter physics, where quantum relativistic phenomena, some of which are unobservable in high-energy physics, can now be mimicked and tested in table-top experiments. More generally, graphene represents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications.
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paper_title: Graphene-based hybrids for chemiresistive gas sensors

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Gas sensors can detect combustible, explosive and toxic gases, and have been widely used in safety monitoring and process control in residential buildings, industries and mines. Recently, graphene-based hybrids were widely investigated as chemiresistive gas sensors with high sensitivity and selectivity. This systematic review is therefore timely and necessary to evaluate the success of graphene-based hybrids in gas detection and to identify their challenges. We review the sensing principles and the synthesis process of the graphene-based hybrids with noble metals, metal oxides and conducting polymers to achieve better understanding and design of novel gas sensors. Our review will assist researchers to understand the evolution and the challenges of graphene-based hybrids, and create interest in development of gas-sensing techniques.
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paper_title: Fabrication of Palladium Nanotubes and Their Application in Hydrogen Sensing

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A new electroless plating method was used to deposit palladium nanotubes within the pores of track-etched polycarbonate membranes. The morphology of the deposited palladium nanostructures were characterized with the use of field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectrometry (EDX), and transmission electron microscopy (TEM). The largely expanded surface area and granular nature of the nanotubes make them ideal for applications that require high surface area. One of the applications investigated here is hydrogen sensing. Palladium nanotubes excel in high sensitivity, low detection limit, and fast response times in hydrogen sensing compared to sputtered Pd thin film on glass. The response time ranges from a few seconds to tens of seconds depending on the concentration of hydrogen. The hydrogen sensing behavior can be understood with the Langmuir adsorption isotherm theory at very low hydrogen concentration. The high sensitivity and fast response time in the nanotube imbe...
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paper_title: Detection of individual gas molecules adsorbed on graphene

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The ultimate aim of any detection method is to achieve such a level of sensitivity that individual quanta of a measured entity can be resolved. In the case of chemical sensors, the quantum is one atom or molecule. Such resolution has so far been beyond the reach of any detection technique, including solid-state gas sensors hailed for their exceptional sensitivity1, 2, 3, 4. The fundamental reason limiting the resolution of such sensors is fluctuations due to thermal motion of charges and defects5, which lead to intrinsic noise exceeding the sought-after signal from individual molecules, usually by many orders of magnitude. Here, we show that micrometre-size sensors made from graphene are capable of detecting individual events when a gas molecule attaches to or detaches from graphene's surface. The adsorbed molecules change the local carrier concentration in graphene one by one electron, which leads to step-like changes in resistance. The achieved sensitivity is due to the fact that graphene is an exceptionally low-noise material electronically, which makes it a promising candidate not only for chemical detectors but also for other applications where local probes sensitive to external charge, magnetic field or mechanical strain are required.
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paper_title: From conception to realization: an historial account of graphene and some perspectives for its future.

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There has been an intense surge in interest in graphene during recent years. However, graphene-like materials derived from graphite oxide were reported in 1962, and related chemical modifications of graphite were described as early as 1840. In this detailed account of the fascinating development of the synthesis and characterization of graphene, we hope to demonstrate that the rich history of graphene chemistry laid the foundation for the exciting research that continues to this day. Important challenges remain, however; many with great technological relevance.
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paper_title: Porous ZnO Polygonal Nanoflakes: Synthesis, Use in High-Sensitivity NO2Gas Sensor, and Proposed Mechanism of Gas Sensing

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Unique porous ZnO polygonal nanoflakes were synthesized by the microwave hydrothermal method. The structural properties of the products were investigated by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), and high-resolution TEM techniques. In situ diffuse reflectance infrared Fourier transform spectroscopy technique was employed to investigate the mechanism of NO2 sensing. Free nitrate ions, nitrate ions, and nitrite anions were the main adsorbed species. N2O was formed via NO– and N2O2– that were stemmed from NO. Comparative tests for gas sensing between gas sensors based on the as-prepared porous ZnO nanoflakes and purchased ZnO nanoparticles clearly showed that the former exhibited more excellent NO2 sensing performances. Photoluminescence and X-ray photoelectron spectroscopy spectra further proved that the intensities of donors (oxygen vacancy (VO) and/or zinc interstitial (Zni)) and surface oxygen species (O2– and O2), which were involved in the mechani...
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paper_title: Electric Field Effect in Atomically Thin Carbon Films

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tection efficiency for the read beam is $ ; 0.04, so we infer the efficiency of quantum state transfer from the atoms onto the photon, O ; 0.03. We have realized a quantum node by combining the entanglement of atomic and photonic qubits with the atom-photon quantum state transfer. By implementing the second node at a different location and performing a joint detection of the signal photons from the two nodes, the quantum repeater protocol (11), as well as distant teleportation of an atomic qubit, may be realized. Based on this work, we estimate the rate for these protocols to be R 2 ; ($O"n s ) 2 R ; 3 � 10 j7 s j1 . However, improvements in O that are based on increasing the optical thickness of atomic samples (16), as well as elimination of transmission losses, could provide several orders of magnitude increase in R 2 . Our results also demonstrate the possibility of realizing quantum nodes consisting of multiple atomic qubits by using multiple beams of light. This approach shows promise for implementation of distributed quantum computation (20, 21).
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paper_title: Nanostructured Materials for Room-Temperature Gas Sensors

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Sensor technology has an important effect on many aspects in our society, and has gained much progress, propelled by the development of nanoscience and nanotechnology. Current research efforts are directed toward developing high-performance gas sensors with low operating temperature at low fabrication costs. A gas sensor working at room temperature is very appealing as it provides very low power consumption and does not require a heater for high-temperature operation, and hence simplifies the fabrication of sensor devices and reduces the operating cost. Nanostructured materials are at the core of the development of any room-temperature sensing platform. The most important advances with regard to fundamental research, sensing mechanisms, and application of nanostructured materials for room-temperature conductometric sensor devices are reviewed here. Particular emphasis is given to the relation between the nanostructure and sensor properties in an attempt to address structure–property correlations. Finally, some future research perspectives and new challenges that the field of room-temperature sensors will have to address are also discussed.
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paper_title: Preparation of porous flower-like CuO/ZnO nanostructures and analysis of their gas-sensing property

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Abstract Porous flower-like CuO/ZnO nanostructures were obtained using a facile chemical solution method combined with subsequent calcination. The calcination of the precursors produced flower-like CuO/ZnO nanostructures, which is comprised of interconnected highly porous CuO/ZnO nanosheets that resulted from the thermal decomposition of the as-prepared precursors, i.e., flower-like zinc copper hydroxide carbonate. Moreover, the nanostructures were characterized through X-ray diffraction, field emission scanning electron microscopy, thermogravimetric–differential thermal analysis, and Brunauer–Emmett–Teller N 2 adsorption–desorption analyses, among others. The BET surface area of the hierarchically porous CuO/ZnO nanostructures was calculated at 17.1 m 2 g −1 . Furthermore, the gas sensing properties of the as-prepared porous flower-like CuO/ZnO nanostructures were investigated using volatile organic compounds. Compared with the porous flower-like ZnO nanostructures, the porous flower-like CuO/ZnO nanostructures exhibited a higher response and lower working temperature with certain organic vapors, such as ethanol, acetone, and formaldehyde. The responses to 100 ppm ethanol and formaldehyde were 25.5 and 28.9, respectively, at a working temperature of 220 °C. These results showed that the porous flower-like CuO/ZnO nanostructures are highly promising candidates gas sensing applications.
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paper_title: Composite nanofibers of conducting polymers and hydrophobic insulating polymers: Preparation and sensing applications

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Abstract Various conducting polymer/hydrophobic insulating polymer (CP/HIP) composite nanofibers have been prepared by electrospinning and vapor deposition polymerization (VDP) with benzoyl peroxide (BPO) as oxidant. BPO is soluble in N , N -dimethylformamide (DMF) and can form homogenous solutions with hydrophobic polymers such as poly(methyl methacrylate) (PMMA) and polystyrene (PS). High-quality nanofibers of PMMA or PS containing a certain amount of BPO were produced by electrospinning and used as the templates for VDP of pyrrole, 3,4-ethylenedioxythiophene (EDOT), and aniline. The non-woven mats of the resulting CP/HIP composite fibers can be used as the high-sensitive sensing elements of gas sensors. A gas senor based on polypyrrole (PPy)/PMMA composite fibers was fabricated for sensing ammonia or chloroform vapor, and exhibited greatly improved performances comparing with those of the device based on a PPy flat film.
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paper_title: Flexible Palladium-Based H2 Sensor with Fast Response and Low Leakage Detection by Nanoimprint Lithography

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Flexible palladium-based H2 sensors have a great potential in advanced sensing applications, as they offer advantages such as light weight, space conservation, and mechanical durability. Despite these advantages, the paucity of such sensors is due to the fact that they are difficult to fabricate while maintaining excellent sensing performance. Here, we demonstrate, using direct nanoimprint lithography of palladium, the fabrication of a flexible, durable, and fast responsive H2 sensor that is capable of detecting H2 gas concentration as low as 50 ppm. High resolution and high throughput patterning of palladium gratings over a 2 cm × 1 cm area on a rigid substrate was achieved by heat-treating nanoimprinted palladium benzyl mercaptide at 250 °C for 1 h. The flexible and robust H2 sensing device was fabricated by subsequent transfer nanoimprinting of these gratings into a polycarbonate film at its glass transition temperature. This technique produces flexible H2 sensors with improved durability, sensitivity,...
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paper_title: Three-dimensional graphene architectures

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Graphene has been widely explored for applications in electronics, sensors, actuators, catalysis and bio- or energy related systems. For these purposes, graphene materials usually have to be fabricated or assembled into desired micro-/nano-architectures for tuning and/or controlling their electrical, optical, mechanical, chemical or electrochemical properties. In this feature article, we review recent achievements in the studies of three-dimensional (3D) graphene architectures. The methodology for preparing these 3D graphene micro-/nano-architectures and their potential applications have been summarized.
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paper_title: Nanoseed-assisted rapid formation of ultrathin ZnO nanorods for efficient room temperature NO 2 detection

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Abstract The influence of ZnO nanoseeds on the formation of ZnO nanorods from e-Zn(OH)2 in NaOH solution at 80 °C was investigated, using ZnO nanoparticles with a diameter of 4–10 nm as the seeds. The experimental results indicated that the presence of ZnO nanoseeds promoted the rapid heterogeneous formation of ultrathin ZnO nanorods. Compared with the ZnO submicron rods with a diameter of 0.5–1.0 µm, the ultrathin ZnO nanorods with a diameter of 10–15 nm were found to be more sensitive for detecting NO2 at room temperature owing to their higher variation of channel conduction to the diameter.
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paper_title: Mesoporous materials as gas sensors.

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Ordered mesoporous materials have great potential in the field of gas sensing. Today various template-assisted synthesis methods facilitate the preparation of silica (SiO2) as well as numerous metal oxides with well-defined, uniform and regular pore systems. The unique nanostructural properties of such materials are particularly useful for their application as active layers in gas sensors based on various operating principles, such as capacitive, resistive, or optical sensing. This review summarizes the basic aspects of materials synthesis, discusses some structural properties relevant in gas sensing, and gives an overview of the literature on ordered mesoporous gas sensors.
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paper_title: Graphene for electrochemical sensing and biosensing

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Graphene has proved to be an excellent nanomaterial for applications in electrochemistry. We review progress in constructing high-performance electrochemical sensors and biosensors. We also discuss: different routes for graphene fabrication; graphene-modified electrodes and graphene-composite electrodes for sensing, including those based on ionic liquids; incorporation of biorecognition elements into graphene-based electrodes; and, graphene-supported electrocatalytic nanoparticle-based electrochemical sensors and biosensors.
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paper_title: Graphene Oxide, Highly Reduced Graphene Oxide, and Graphene: Versatile Building Blocks for Carbon-Based Materials†

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Isolated graphene, a nanometer-thick two-dimensional analog of fullerenes and carbon nanotubes, has recently sparked great excitement in the scientific community given its excellent mechanical and electronic properties. Particularly attractive is the availability of bulk quantities of graphene as both colloidal dispersions and powders, which enables the facile fabrication of many carbon-based materials. The fact that such large amounts of graphene are most easily produced via the reduction of graphene oxide—oxygenated graphene sheets covered with epoxy, hydroxyl, and carboxyl groups—offers tremendous opportunities for access to functionalized graphene-based materials. Both graphene oxide and graphene can be processed into a wide variety of novel materials with distinctly different morphological features, where the carbonaceous nanosheets can serve as either the sole component, as in papers and thin films, or as fillers in polymer and/or inorganic nanocomposites. This Review summarizes techniques for preparing such advanced materials via stable graphene oxide, highly reduced graphene oxide, and graphene dispersions in aqueous and organic media. The excellent mechanical and electronic properties of the resulting materials are highlighted with a forward outlook on their applications.
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paper_title: Highly sensitive NO2 gas sensor based on ozone treated graphene

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Abstract In the present study, we report a simple and reproducible method to improve the sensing performance of a graphene gas sensor using ozone treatment and demonstrate it with nitrogen dioxide (NO 2 ) gas. The ozone-treated graphene (OTG) sensor demonstrated remarkable enhancement of the sensing performances such as percentage response, detection limit and response time. The percentage response of the OTG sensor was twofold higher than that of a pristine graphene sensor when it was exposed to 200 ppm concentration of NO 2 at room temperature. It is noteworthy that significant improvement was achieved in the response time by a factor of 8. Extremely low parts-per-billion (ppb) concentrations were clearly detectable, while the pristine graphene sensor could not detect NO 2 molecules below 10 ppm concentration. The detection limit of the OTG sensor was estimated to be 1.3 ppb based on the signal to noise ratio, which is the cutting-edge resolution. The present ozone treatment may provide an effective way to improve the performance of the graphene-based sensor, given its simple process, practical usability and cost effectiveness.
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paper_title: Synthesis and Characterization of Chromium‐Doped Mesoporous Tungsten Oxide for Gas Sensing Applications

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SBA-15 (2D hexagonal structure) and KIT-6 (3D cubic structure) silica materials are used as templates for the synthesis of two different crystalline mesoporous WO 3 replicas usable as NO 2 gas sensors. High-resolution transmission electron microscopy (HRTEM) studies reveal that single-crystal hexagonal rings set up the atomic morphology of the WO 3 KIT-6 replica, whereas the SBA-15 replica is composed of randomly oriented nanoparticles. A model capable of explaining the KIT-6 replica mesostructure is described. A small amount of chromium is added to the WO 3 matrix in order to enhance sensor response. It is demonstrated that chromium does not form clusters, but well-distributed centers. Pure WO 3 KIT-6 replica displays a higher response rate as well as a lower response time to NO 2 gas than the SBA-15 replica. This behavior is explained by taking into account that the KIT-6 replica has a higher surface area as demonstrated by Brunauer-Emmett-Teller analyses and its mesostructure is fully maintained after the screen-printing step involved in sensors preparation. The presence of chromium in the material results in a shorter response time and improved sensor response to the lowest NO 2 concentrations tested. Electrical differences related to mesostructure are reduced as a result of additive introduction.
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paper_title: Preparation of a leaf-like CdS micro-/nanostructure and its enhanced gas-sensing properties for detecting volatile organic compounds

paper_content:

A novel leaf-like CdS micro-/nanostructure was prepared via a hydrothermal method using dimethyl sulfoxide as the growth template. The as-synthesized hierarchical micro-/nanostructures were characterized by field emission scanning electronic microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectrometry (XPS). The elemental mapping and the line scans were also performed in the high-angle annular dark-field (HAADF) mode on the same TEM. The growth mechanism was demonstrated from the aspects of time-dependent growth processes and different influencing factors including temperatures and templates. In gas-sensing measurements, typical volatile organic compounds at a series of concentrations were employed as the target analytes, including ether, methanol, acetone and isopropanol. We found that the gas sensor based on the special leaf-like CdS micro-/nanostructures exhibited a fascinating performance including high response, short response/recovery times and good recognition ability towards different analytes. The detecting limits towards ether, methanol, acetone and isopropanol could be lower than 25, 50, 50, and 10 ppb, respectively. By comparing with a conventional spherical structure, the leaf-like micro-/nanostructure was revealed to possess an improved ability for diffusion and adsorption/desorption. The structure could be generally significant for developing some other novel materials which possess a morphology inspired from nature for specific applications, such as sensors, energy transfer and storage devices, and catalysts.
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paper_title: Fabrication of ultrafine metal-oxide-decorated carbon nanofibers for DMMP sensor application.

paper_content:

Ultrafine metal-oxide-decorated hybrid carbon nanofibers (CNFs) were fabricated by a single-nozzle co-electrospinning process using a phase-separated mixed polymer composite solution and heat treatment. To decorate metal oxides on the CNF surface, core (PAN) and shell (PVP) structured nanofibers (NFs) were fabricated as starting materials. The core-shell NF structure was prepared by single-nozzle co-electrospinning because of the incompatibility of the two polymers. Ultrafine hybrid CNFs were then formed by decomposing the PVP phase, converting the metal precursors to metal oxide nanonodules, and transforming the PAN to CNFs of ca. 40 nm diameter during heat treatment. The decoration morphology of the metal oxide nanonodules could be controlled by precursor concentration in the PVP solution. These ultrafine hybrid CNFs were applied to a dimethyl methylphosphonate (DMMP) chemical sensor at room temperature with excellent sensitivity. The minimum detectable level (MDL) of hybrid CNFs was as low as 0.1 ppb, which is 10-100 times higher than for a chemical sensor based on carbon nanotubes. This is because the metal oxide nanonodules of hybrid CNFs increase the surface area and affinity to DMMP vapor. Our new synthetic methodology promises to be an effective approach to fabricating hybrid CNF/inorganic nanostructures for future sensing technologies.
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paper_title: Pyrenyl Excimers Induced by the Crystallization of POSS Moieties: Spectroscopic Studies and Sensing Applications

paper_content:

An ester (PBPOSS) of 1-pyrenebutyric acid (PBA) and 1-(2,3-propanediol)propoxy-3,5,7,9, 11,13, 15-isobutylpentacyclo-[9.5.1.1-(3,9).1(5,15).1(7,13)]octasiloxane (a polyhedral oligomeric silsesquioxane, POSS) is synthesized by simple one-step esterification. Thin films of PBPOSS fabricated by the spin-coating technique exhibit strong excimer emissions of visible light. Upon exposure to the vapours of nitroaromatic compounds, including trinitrotoluene and dinitrotoluene, the films show fast fluorescence quenching. The high performance of these gas sensors is due to the high excimer contents and good vapour permeability of the PBPOSS films. Spectroscopic studies indicate that the crystallization of the POSS moieties in PBPOSS films induce the formation of pyrenyl excimers.
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paper_title: Porous Metal Oxides as Gas Sensors

paper_content:

Semiconducting metal oxides are frequently used as gas-sensing materials. Apart from large surface-to-volume ratios, well-defined and uniform pore structures are particularly desired for improved sensing performance. This article addresses the role of some key structural aspects in porous gas sensors, such as grain size and agglomeration, pore size or crack-free film morphology. New synthesis concepts, for example, the utilisation of rigid matrices for structure replication, allow to control these parameters independently, providing the opportunity to create self-diagnostic sensors with enhanced sensitivity and reproducible selectivity.
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paper_title: Transfer of CVD-grown graphene for room temperature gas sensors

paper_content:

An easy transfer procedure to obtain graphene-based gas sensing devices operating at room temperature (RT) is presented. Starting from chemical vapor deposition-grown graphene on copper foil, we obtained single layer graphene which could be transferred onto arbitrary substrates. In particular, we placed single layer graphene on top of a SiO2/Si substrate with pre-patterned Pt electrodes to realize a chemiresistor gas sensor able to operate at RT. The responses to ammonia (10, 20, 30 ppm) and nitrogen dioxide (1, 2, 3 ppm) are shown at different values of relative humidity, in dark and under 254 nm UV light. In order to check the sensor selectivity, gas response has also been tested towards hydrogen, ethanol, acetone and carbon oxide. Finally, a model based on linear dispersion relation characteristic of graphene, which take into account humidity and UV light effects, has been proposed.
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paper_title: Nanocrystalline Metal Oxides from the Injection of Metal Oxide Sols in Coordinating Solutions: Synthesis, Characterization, Thermal Stabilization, Device Processing, and Gas-Sensing Properties

paper_content:

Metal oxide (SnO 2 , TiO 2 , In 2 O 3 , ZnO) sols are prepared by various sol-gel processes in such a way as to hinder the condensation reactions. The obtained sols are injected at 160 °C into a solution of tetradecene and dodecylamine, and kept under heating for different periods of time. Depending on the starting sol, variously crystallized oxide nanoparticles are obtained, whose phase compositions and chemical structure have been studied by X-ray diffraction (XRD) and Fourier transform IR spectroscopy. The elimination of the organic residuals has been carried out by thermal treatment, and the thermal evolution of the nanoparticles has been studied by thermal analyses and Raman spectroscopy. High-resolution transmission electron microscopy studies coupled with XRD measurements show that the thermal treatment does not markedly affect the particle size, which remains in the nanometer-sized regime (from 3.5 to 8.5 nm, depending on the system), except in the case of ZnO. The thermally purified and stabilized powders, drop-coated onto alumina substrates with pre-deposited electrical contacts, have been tested as gas-sensing devices, displaying outstanding sensing properties even at room temperature.
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paper_title: Nanostructured Materials for Room-Temperature Gas Sensors

paper_content:

Sensor technology has an important effect on many aspects in our society, and has gained much progress, propelled by the development of nanoscience and nanotechnology. Current research efforts are directed toward developing high-performance gas sensors with low operating temperature at low fabrication costs. A gas sensor working at room temperature is very appealing as it provides very low power consumption and does not require a heater for high-temperature operation, and hence simplifies the fabrication of sensor devices and reduces the operating cost. Nanostructured materials are at the core of the development of any room-temperature sensing platform. The most important advances with regard to fundamental research, sensing mechanisms, and application of nanostructured materials for room-temperature conductometric sensor devices are reviewed here. Particular emphasis is given to the relation between the nanostructure and sensor properties in an attempt to address structure–property correlations. Finally, some future research perspectives and new challenges that the field of room-temperature sensors will have to address are also discussed.
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paper_title: Fast responding exhaled-breath sensors using WO3 hemitubes functionalized by graphene-based electronic sensitizers for diagnosis of diseases.

paper_content:

Diagnostic sensing device using exhaled breath of human have critical advantages due to the noninvasive diagnosis and high potential for portable device with simple analysis process. Here, we report ultrafast as well as highly sensitive bumpy WO3 hemitube nanostructure assisted by O2 plasma surface modification with functionalization of graphene-based material for the detection of acetone (CH3COCH3) and hydrogen sulfide (H2S) which are biomarkers for the diagnosis of diabetes and halitosis, respectively. 0.1 wt % graphene oxide (GO)- and 0.1 wt % thin layered graphite (GR)- WO3 hemitube composites showed response times of 11.5 ± 2.5 s and 13.5 ± 3.4 s to 1 ppm acetone as well as 12.5 ± 1.9 s and 10.0 ± 1.6 s to 1 ppm of H2S, respectively. In addition, low limits of detection (LOD) of 100 ppb (Rair/Rgas = 1.7 for acetone and Rair/Rgas = 3.3 for H2S at 300 °C) were achieved. The superior sensing properties were ascribed to the electronic sensitization of graphene based materials by modulating space charged layers at the interfaces between n-type WO3 hemitubes and p-type graphene based materials, as identified by Kelvin Probe Force Microscopy (KPFM). Rapid response and superior sensitivity of the proposed sensing materials following cyclic thermal aging demonstrates good potential for real-time exhaled breath diagnosis of diseases.
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paper_title: Templated synthesis of nanostructured materials

paper_content:

Templating is one of the most important techniques for the controlled synthesis of nanostructured materials. This powerful tool uses a pre-existing guide with desired nanoscale features to direct the formation of nanomaterials into forms that are otherwise difficult to obtain. As a result, templated synthesis is capable of producing nanostructures with unique structures, morphologies and properties. In this review, we summarize the general principles of templated synthesis and cover recent developments in this area. As a wide variety of synthesis techniques are utilized to produce nanomaterials using template-based methods, the discussion is organized around the various types of common templates. We examine the use of both physical and chemical hard colloidal templates, soft templates, and other non-colloidal templates, followed by our perspective on the state of the field and potential future directions.
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paper_title: Reduced graphene oxide/α-Fe2O3 composite nanofibers for application in gas sensors

paper_content:

Pure α-Fe2O3 nanobelts (NBs) and 0.5, 1.0, 3.0 wt% reduced graphene oxide (rGO)/α-Fe2O3 composite nanofibers (NFs) have been successfully synthesized by a homotaxial electrospinning method. Various techniques were employed for the characterization of the nanostructures and morphologies of the as-obtained nanofibers. The results indicate that pure α-Fe2O3 NBs have uniform widths more than 200 nm and series of rGO/α-Fe2O3 NFs have diameters approximately 100 nm. In addition, the gas sensing properties of the obtained nanofibers were investigated. It is found that 1 wt% rGO/α-Fe2O3 shows high response, excellent repeatability and long-term stability to 100 ppm acetone at the operating temperature of 375 °C. The response of 1 wt% rGO/α-Fe2O3 nanofibers to 100 ppm acetone is about 8.9, which is about 4.5 times higher than pure α-Fe2O3 NBs. ::: ::: Furthermore, the combination of rGO with α-Fe2O3 can also improve the response/recovery properties. The response and recovery times of the sensor were about 3 and 9s, respectively, which were superior to pure α-Fe2O3 NBs. Finally, we have proposed that the creation of ohmic contact, defects on the rGO nanosheets surface play significant roles in raising the sensitivity of rGO/α-Fe2O3 NFs.
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paper_title: Confined Formation of Ultrathin ZnO Nanorods/Reduced Graphene Oxide Mesoporous Nanocomposites for High-Performance Room-Temperature NO2 Sensors

paper_content:

Here we demonstrate high-performance room-temperature NO2 sensors based on ultrathin ZnO nanorods/reduced graphene oxide (rGO) mesoporous nanocomposites. Ultrathin ZnO nanorods were loaded on rGO nanosheets by a facile two-step additive-free solution synthesis involving anchored seeding followed by oriented growth. The ZnO nanorod diameters were simply controlled by the seed diameters associated with the spatial confinement effects of graphene oxide (GO) nanosheets. Compared to the solely ZnO nanorods and rGO-based sensors, the optimal sensor based on ultrathin ZnO nanorods/rGO nanocomposites exhibited higher sensitivity and quicker p-type response to parts per million level of NO2 at room temperature, and the sensitivity to 1 ppm of NO2 was 119% with the response and recovery time being 75 and 132 s. Moreover, the sensor exhibited full reversibility, excellent selectivity, and a low detection limit (50 ppb) to NO2 at room temperature. In addition to the high transport capability of rGO as well as excelle...
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paper_title: Two-step self-assembly of hierarchically-ordered nanostructures

paper_content:

Due to their unique size- and shape-dependent physical and chemical properties, highly hierarchically-ordered nanostructures have attracted great attention with a view to application in emerging technologies, such as novel energy generation, harvesting, and storage devices. The question of how to get controllable ensembles of nanostructures, however, still remains a challenge. This concept paper first summarizes and clarifies the concept of the two-step self-assembly approach for the synthesis of hierarchically-ordered nanostructures with complex morphology. Based on the preparation processes, two-step self-assembly can be classified into two typical types, namely, two-step self-assembly with two discontinuous processes and two-step self-assembly completed in one-pot solutions with two continuous processes. Compared to the conventional one-step self-assembly, the two-step self-assembly approach allows the combination of multiple synthetic techniques and the realization of complex nanostructures with hierarchically-ordered multiscale structures. Moreover, this approach also allows the self-assembly of heterostructures or hybrid nanomaterials in a cost-effective way. It is expected that widespread application of two-step self-assembly will give us a new way to fabricate multifunctional nanostructures with deliberately designed architectures. The concept of two-step self-assembly can also be extended to syntheses including more than two chemical/physical reaction steps (multiple-step self-assembly).
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paper_title: Heterogeneous photocatalyst materials for water splitting.

paper_content:

This critical review shows the basis of photocatalytic water splitting and experimental points, and surveys heterogeneous photocatalyst materials for water splitting into H2 and O2, and H2 or O2 evolution from an aqueous solution containing a sacrificial reagent. Many oxides consisting of metal cations with d0 and d10 configurations, metal (oxy)sulfide and metal (oxy)nitride photocatalysts have been reported, especially during the latest decade. The fruitful photocatalyst library gives important information on factors affecting photocatalytic performances and design of new materials. Photocatalytic water splitting and H2 evolution using abundant compounds as electron donors are expected to contribute to construction of a clean and simple system for solar hydrogen production, and a solution of global energy and environmental issues in the future (361 references).
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paper_title: Extraordinary Improvement of Gas-Sensing Performances in SnO2 Nanofibers Due to Creation of Local p–n Heterojunctions by Loading Reduced Graphene Oxide Nanosheets

paper_content:

We propose a novel approach to improve the gas-sensing properties of n-type nanofibers (NFs) that involves creation of local p–n heterojunctions with p-type reduced graphene oxide (RGO) nanosheets (NSs). This work investigates the sensing behaviors of n-SnO2 NFs loaded with p-RGO NSs as a model system. n-SnO2 NFs demonstrated greatly improved gas-sensing performances when loaded with an optimized amount of p-RGO NSs. Loading an optimized amount of RGOs resulted in a 20-fold higher sensor response than that of pristine SnO2 NFs. The sensing mechanism of monolithic SnO2 NFs is based on the joint effects of modulation of the potential barrier at nanograin boundaries and radial modulation of the electron-depletion layer. In addition to the sensing mechanisms described above, enhanced sensing was obtained for p-RGO NS-loaded SnO2 NFs due to creation of local p–n heterojunctions, which not only provided a potential barrier, but also functioned as a local electron absorption reservoir. These mechanisms markedly ...
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paper_title: Excellent gas detection of ZnO nanofibers by loading with reduced graphene oxide nanosheets

paper_content:

Abstract We have fabricated sensors with reduced graphene oxide (RGO) nanosheets (NSs)-loaded ZnO nanofibers (NFs) via an electrospinning method. The RGO NSs-loaded ZnO NFs were comprised of nanograins with an average diameter of 20 nm. Transmission electron microscopy and X-ray diffraction both revealed the presence of RGO NSs in the ZnO NFs. The sensing properties of RGO NSs-loaded ZnO NFs were examined after exposure to various gases, including O 2 , SO 2 , NO 2 , CO, C 6 H 6 , and C 2 H 5 OH. The sensor responses showed a bell-shaped behavior with respect to the weight ratio of RGO NSs. It is remarkable that our sensors exhibited significantly higher responses than pure ZnO NFs. We propose a novel hybrid sensing mechanism for the drastic improvement in the sensing behavior that is caused by loading RGO NSs into ZnO NFs. This hybrid sensing mechanism combines the resistance modulation of ZnO/ZnO homointerfaces and RGO-NSs/ZnO heterointerfaces in addition to the radial modulation of the surface depletion layer of ZnO NFs. In the heterointerfaces, the creation of local heterojunctions plays a significant role in raising the sensitivity of RGO-loaded ZnO NFs.
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paper_title: Novel porous single-crystalline ZnO nanosheets fabricated by annealing ZnS(en)0.5 (en = ethylenediamine) precursor. Application in a gas sensor for indoor air contaminant detection

paper_content:

Novel single-crystalline ZnO nanosheets with porous structure have been fabricated by annealing ZnS(en)0.5 (en = ethylenediamine) complex precursor. The morphology and structure observations performed by field emission scanning electronic microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) indicate that numerous mesopores with a diameter of about 26.1 nm distribute all through each nanosheet with a high density. The transformation of structure and composition of samples obtained during thermal treatment processes were investigated by x-ray diffraction (XRD), x-ray photoelectron spectrometry (XPS), thermogravimetric analysis (TGA), and Fourier transform infrared (FTIR) absorption spectroscopy. The formation mechanism of the porous structure is proposed. For indoor air contaminant detection in which formaldehyde and ammonia are employed as target gases, the as-prepared ZnO nanosheets were applied for the fabrication of gas sensors. It was found that the as-fabricated sensors not only exhibit highly sensitive performance, e.g., high gas-sensing responses, short response and recovery time, but also possess significant long-term stability. It is indicated that these ZnO nanostructures could promisingly be applied in electronic devices for environmental evaluation.
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paper_title: Synthesis of single-crystalline potassium-doped tungsten oxide nanosheets as high-sensitive gas sensors

paper_content:

Abstract The single-crystalline potassium-doped tungsten oxide nanosheets were prepared by simply heating a treated foil of tungsten. The as-prepared products were characterized using X-ray powder diffraction (XRD), scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and selected-area electron diffraction (SAED). XRD and SAED results revealed that the products are pure, single-crystalline. The nanosheets width reaches to 30 μm and thickness is about 80 nm. The nanosheets have high sensitivity, fast response time and good stability to H 2 S, acetone and other gases.
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paper_title: Fabrication of a novel 2D-graphene/2D-NiO nanosheet-based hybrid nanostructure and its use in highly sensitive NO2 sensors

paper_content:

Abstract A highly sensitive gas sensor based on novel hybrid structures composed of 2D graphene and 2D NiO nanosheets (NSs) is fabricated using a low-cost, low temperature and large area scalable solution-based process. The highly developed hierarchically porous structures of 2D NiO sheets are grown on reduced graphene oxide (RGO) surfaces. Sensors fabricated with hybrid structures showed a responsivity and sensitivity two orders higher than that of a NiO NS alone toward NO 2 even at 1 ppm level. This is attributed to the effective carrier transfer from NiO NS to graphene and to the well-developed 2D NiO structure. The sensing results are similar when reducing gases such as H 2 , NH 3 and H 2 S are tested, but the responsivity toward NO 2 was highest among all the gases tested.
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paper_title: ZnO nanosheets/graphene oxide nanocomposites for highly effective acetone vapor detection

paper_content:

Abstract ZnO nanosheets/graphene oxide (GO) nanocomposites were synthesized for highly selective, quick response acetone detection. Field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the morphologies, microstructures and compositions of the nanocomposites. The effect of GO on the sensing performances of the nanocomposites was investigated. It was found that 10 wt% GO was the optimized loading and the response value reached 35.8 under the exposure of 100 ppm acetone. The unique sensing properties were attributed to the synergistic effects of rigid ZnO nanosheets and flexible GO, including large specific surface area, rich functional groups of GO etc. This work will contribute to the development of new acetone sensors and broaden the application of GO composite materials.
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paper_title: Graphene-based hybrids for chemiresistive gas sensors

paper_content:

Gas sensors can detect combustible, explosive and toxic gases, and have been widely used in safety monitoring and process control in residential buildings, industries and mines. Recently, graphene-based hybrids were widely investigated as chemiresistive gas sensors with high sensitivity and selectivity. This systematic review is therefore timely and necessary to evaluate the success of graphene-based hybrids in gas detection and to identify their challenges. We review the sensing principles and the synthesis process of the graphene-based hybrids with noble metals, metal oxides and conducting polymers to achieve better understanding and design of novel gas sensors. Our review will assist researchers to understand the evolution and the challenges of graphene-based hybrids, and create interest in development of gas-sensing techniques.
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paper_title: Multilayered ZnO Nanosheets with 3D Porous Architectures: Synthesis and Gas Sensing Application

paper_content:

Multilayered ZnO nanosheets with three-dimensional (3D) porous architectures were synthesized by calcining a layered precursor of zinc hydroxide carbonate. The structural properties were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and selected-area electron diffraction techniques. It was observed that the concentration of the urea was the key parameter determining the final morphology. Room-temperature photoluminescence data showed interesting optical properties of the ZnO architectures. Gas sensing tests showed that these 3D porous ZnO architectures were highly promising for gas sensor applications, as the gas diffusion and mass transportation in sensing materials were significantly enhanced by their unique structures. Our results indicate that the 3D porous ZnO nanosheets have potential applications in fabricating optoelectrical devices and gas sensors.
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paper_title: Self-assembly fabrication of 3D flower-like ZnO hierarchical nanostructures and their gas sensing properties

paper_content:

In this paper, three-dimensional flower-like ZnO hierarchical nanostructures were fabricated from the thermal-decomposition of 3D zinc hydroxide carbonate precursor, which was synthesized by a urea hydrothermal method with block copolymer F127 (EO106-PO70-EO106) as the morphology director. XRD, IR, UV-vis, SEM, TEM, TG and N2 adsorption–desorption isotherms have been employed to characterize the products. The influences of synthesis parameters such as reaction time, the type of zinc sources, and species concentration on the morphologies of the products were systematically studied. It was found that the reaction time played a key role in determining the final morphology of porous ZnO. On the basis of experimental results, a possible formation mechanism of the 3D flower-like ZnO hierarchical nanostructures was discussed. More importantly, the gas sensing tests indicated that the sensor made from porous ZnO hierarchical nanostructures exhibited better gas sensing properties to n-butanol compared with the sensor based on the commercial ZnO nanoparticles. The enhancement in gas sensing properties was attributed to their unique 3D hierarchical nanostructures, high surface areas, and greater number of surface active sites.
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paper_title: Preparation of porous flower-like CuO/ZnO nanostructures and analysis of their gas-sensing property

paper_content:

Abstract Porous flower-like CuO/ZnO nanostructures were obtained using a facile chemical solution method combined with subsequent calcination. The calcination of the precursors produced flower-like CuO/ZnO nanostructures, which is comprised of interconnected highly porous CuO/ZnO nanosheets that resulted from the thermal decomposition of the as-prepared precursors, i.e., flower-like zinc copper hydroxide carbonate. Moreover, the nanostructures were characterized through X-ray diffraction, field emission scanning electron microscopy, thermogravimetric–differential thermal analysis, and Brunauer–Emmett–Teller N 2 adsorption–desorption analyses, among others. The BET surface area of the hierarchically porous CuO/ZnO nanostructures was calculated at 17.1 m 2 g −1 . Furthermore, the gas sensing properties of the as-prepared porous flower-like CuO/ZnO nanostructures were investigated using volatile organic compounds. Compared with the porous flower-like ZnO nanostructures, the porous flower-like CuO/ZnO nanostructures exhibited a higher response and lower working temperature with certain organic vapors, such as ethanol, acetone, and formaldehyde. The responses to 100 ppm ethanol and formaldehyde were 25.5 and 28.9, respectively, at a working temperature of 220 °C. These results showed that the porous flower-like CuO/ZnO nanostructures are highly promising candidates gas sensing applications.
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paper_title: Preparation of a leaf-like CdS micro-/nanostructure and its enhanced gas-sensing properties for detecting volatile organic compounds

paper_content:

A novel leaf-like CdS micro-/nanostructure was prepared via a hydrothermal method using dimethyl sulfoxide as the growth template. The as-synthesized hierarchical micro-/nanostructures were characterized by field emission scanning electronic microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectrometry (XPS). The elemental mapping and the line scans were also performed in the high-angle annular dark-field (HAADF) mode on the same TEM. The growth mechanism was demonstrated from the aspects of time-dependent growth processes and different influencing factors including temperatures and templates. In gas-sensing measurements, typical volatile organic compounds at a series of concentrations were employed as the target analytes, including ether, methanol, acetone and isopropanol. We found that the gas sensor based on the special leaf-like CdS micro-/nanostructures exhibited a fascinating performance including high response, short response/recovery times and good recognition ability towards different analytes. The detecting limits towards ether, methanol, acetone and isopropanol could be lower than 25, 50, 50, and 10 ppb, respectively. By comparing with a conventional spherical structure, the leaf-like micro-/nanostructure was revealed to possess an improved ability for diffusion and adsorption/desorption. The structure could be generally significant for developing some other novel materials which possess a morphology inspired from nature for specific applications, such as sensors, energy transfer and storage devices, and catalysts.
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paper_title: Improvement of NO2 gas sensing performance based on discoid tin oxide modified by reduced graphene oxide

paper_content:

Abstract A facile one-step hydrothermal method for a novel discoid crystal of rutile SnO 2 modified by reduced graphene oxide (rGO) is reported in this work. X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were performed to characterize the structure and morphology of the SnO 2 /rGO composites. Uniform discoid rutile SnO 2 monocrystal with a diameter of approximately 100 nm and a center thickness of 40 nm was anchored on both sides of rGO nanosheets. The SnO 2 /rGO composite exhibited preferential detection toward NO 2 with high response, good selectivity and reproducibility. The response of the sensor to 1 ppm NO 2 at 75 °C was nearly one order of magnitude higher than that of SnO 2 , and the detection limit was improved to 50 ppb. The improved response was discussed and the gas sensing mechanism was established.
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paper_title: C-doped and N-doped reduced graphene oxide/TiO2 composites with exposed (0 0 1) and (1 0 1) facets controllably synthesized by a hydrothermal route and their gas sensing characteristics

paper_content:

Abstract Element doping and controllably facet exposing are efficient solutions for enhancing gas sensing performances of TiO 2 nanomaterials. In this study, C-doped and N-doped reduced graphene oxide/TiO 2 composites with special exposed facets C-RGO/TiO 2 (with HF) and N-RGO/TiO 2 (with HF) were controllably synthesized via a hydrothermal method using HF as the morphology-controlling agent at 180 °C for 12 h. The as-prepared composites were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and other measurements. Their gas sensing results demonstrate that the gas sensing performance of N-RGO/TiO 2 (with HF) is much better than that of C-RGO/TiO 2 (with HF), such as higher sensitivity, and shorter response and recovery time. The sensor based on N-RGO/TiO 2 (with HF) exhibits the highest gas response toward isopropanol, ethanol, and acetone at a working temperature of 210, 240, and 270 °C, respectively. The lowest detection of these gases was 1 ppm. The gas sensing mechanism was also carefully analyzed. The TiO 2 particles of composite with exposed facets generate electron–hole pairs efficiently. The N element dopant plays the roles of narrowing the band gap of TiO 2 based composite, and strengthening the chemical binding between N-RGO and TiO 2 , which is of benefit to charge separation and electron mobility.
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paper_title: Impact of reduced graphene oxide on the ethanol sensing performance of hollow SnO2 nanoparticles under humid atmosphere

paper_content:

Abstract The interference of humidity is a key factor to be considered in metal oxide semiconductors gas sensing performance. However, an efficient gas detection under humid conditions is a challenge. Herein, we report the effect of reduced graphene oxide (RGO) on volatile organic compounds (VOCs) sensing performance of hollow SnO 2 nanoparticles (NPs) under wet atmosphere. For this purpose, RGO-SnO 2 nanocomposite was obtained by a one-pot microwave-assisted solvothermal synthesis. The sensing tests for VOCs were conducted under dry air and at a relative humidity (RH) between 24 and 98%. The samples exhibited better response toward ethanol than to other VOCs such as acetone, benzene, methanol, m-xylene, and toluene, at the optimum operating temperature of 300 °C. Furthermore, RGO-SnO 2 nanocomposite showed an enhanced ethanol response in comparison with pure hollow SnO 2 NPs. Even under 98% of RH, the RGO-SnO 2 nanocomposite showed a response of 43.0 toward 100 ppm of ethanol with a response time of 8 s. The excellent sensor performance is related to the hollow structure of SnO 2 NPs, and the heterojunction between RGO and SnO 2 . Therefore, the RGO content can be a promising approach to minimize the humidity effect on SnO 2 ethanol sensing performance.
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paper_title: Hierarchical NiO Cube/Nitrogen-Doped Reduced Graphene Oxide Composite with Enhanced H2S Sensing Properties at Low Temperature.

paper_content:

A novel hierarchical NiO cube (hc-NiO)/nitrogen-doped reduced graphene oxide (N-rGO) composite is synthesized via a facile hydrothermal method and a postcalcination treatment without any templates and surfactants added. The NiO cubes assembled by abundant nanoparticles in situ grow on the surface of N-rGO layers. The combination of hc-NiO and N-rGO results in enhanced sensing properties with the contributions of the N-rGO providing high specific surface area and more efficient active sites for the adsorption of H2S molecules and the hierarchically structured NiO cubes providing high sensitivity and distinctive selectivity to H2S gas. At the optimal operating temperature of 92 °C, the hc-NiO/N-rGO composite based sensor shows not only high response to H2S in a range of 0.1-100 ppm but also excellent selectivity for H2S against the other seven gases. The gaseous product, produced from the contact of H2S with the hc-NiO/N-rGO composite at 92 °C, is measured by GC-MS technique. The change of the surface composition and the chemical state of the hc-NiO/N-rGO composite before and after exposure to H2S are investigated by XPS. The possible sensing mechanism of the hc-NiO/N-rGO composite is similar to that of semiconductor oxides. The H2S molecules that absorbed on the sensor surface transform to SO2 by reacting with the adsorbed oxygen anions. Meanwhile, the electrons restricted by the surface-adsorbed oxygen return to the bulk and neutralize the holes, producing a change in resistance.
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paper_title: Reduced graphene oxide (rGO) decorated TiO2 microspheres for selective room-temperature gas sensors

paper_content:

Reduced graphene oxide (rGO) decorated TiO2 microspheres are prepared by using the hydrothermal method. The as-synthesized sample shows that TiO2 particles are connected by the rGO nanosheets while some TiO2 particles are found to be partly covered by rGO sheets. The rGO-decorated TiO2 based chemoresistive-type sensor shows good sensitivity and excellent selectivity to different concentrations of ammonia from 5 ppm to 50 ppm at room temperature. However, the response and recovery speeds of the sensor to ammonia are sluggish and need further optimization.
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paper_title: Facile synthesis of novel 3D nanoflower-like CuxO/multilayer graphene composites for room temperature NOx gas sensor application

paper_content:

3D nanoflower-like CuxO/multilayer graphene composites (CuMGCs) have been successfully synthesized as a new type of room temperature NOx gas sensor. Firstly, the expanded graphite (EG) was activated by KOH and many moderate functional groups were generated; secondly, Cu(CH3COO)2 and CTAB underwent full infusion into the interlayers of activated EG (aEG) by means of a vacuum-assisted technique and then reacted with the functional groups of aEG accompanied by the exfoliation of aEG via reflux. Eventually, the 3D nanoflower consisting of 5–9 nm CuxO nanoparticles homogeneously grow in situ on aEG. The KOH activation of EG plays a key role in the uniform formation of CuMGCs. When being used as gas sensors for detection of NOx, the CuMGCs achieved a higher response at room temperature than that of the corresponding CuxO. In detail, the CuMGCs show a higher NOx gas sensing performance with low detection limit of 97 ppb, high gas response of 95.1% and short response time of 9.6 s to 97.0 ppm NOx at room temperature. Meanwhile, the CuMGC sensor presents a favorable linearity, good selectivity and stability. The enhancement of the sensing response is mainly attributed to the improved conductivity of the CuMGCs. A series of Mott–Schottky and EIS measurements demonstrated that the CuMGCs have much higher donor densities than CuxO and can easily capture and migrate electrons from the conduction band, resulting in the enhancement of electrical conductivity.
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paper_title: Flower-like In 2 O 3 modified by reduced graphene oxide sheets serving as a highly sensitive gas sensor for trace NO 2 detection

paper_content:

Abstract In this work, we described gas sensors based on the materials composed of hierarchical flower-likeIn 2 O 3 and reduced graphene oxide (rGO), which were fabricated by a facile one-step hydrothermal method. The rGO-In 2 O 3 composites exhibited enhanced sensing performance towards NO 2 through comparison with the pure In 2 O 3 sample. The operating temperature can be tuned by the percentage of rGO in the composites. The sensor based on 5 wt% rGO-In 2 O 3 could work at room temperature with a high response value to 1 ppm NO 2 . 3 wt% rGO-In 2 O 3 composite was adopted for the ultra-sensitivity gas sensor owing to its extremely low limit of detection of 10 ppb with rapid response time to NO 2 . The sensor also exhibited excellent selectivity and stability. The ultra-sensitivity of rGO-In 2 O 3 should be related to synergistic effect of the hierarchical structure of In 2 O 3 and the presence of rGO in the composites, which provided enhanced surface area and local p-n heterojunctions in rGO/In 2 O 3 composites.
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paper_title: Ultrasensitive and low detection limit of nitrogen dioxide gas sensor based on flower-like ZnO hierarchical nanostructure modified by reduced graphene oxide

paper_content:

Abstract Hierarchical rGO/ZnO hybrids with a flower-like morphology of ZnO and flexible rGO sheets were synthesized by a facile solution-processed method. The structures and morphologies of the hybrids were investigated by different kinds of techniques, including X-ray diffraction, field-emission electron scanning microscopy, transmission electron microscopy, and energy dispersive spectroscopy. The gas sensing properties of hierarchical rGO/ZnO hybrids toward nitrogen dioxide were studied via a static system. The response of rGO/ZnO hybrids to 50 ppb NO 2 was 12, which was seven times higher than that of pristine ZnO at 100 °C. The limit of detection could be achieved as low as 5 ppb. The enhanced sensor response was attributed to the presence of local p - n heterojunctions between rGO sheets and hierarchical structure of ZnO.
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paper_title: Influence of rGO Cladding in Improving the Sensitivity and Selectivity of ZnO Nanoflowers-Based Alcohol Sensor

paper_content:

Development of ultra-sensitive and selective 2-propanol sensor employing hybrid structure of reduced graphene oxide (rGO) and 3-D ZnO nanoflowers (NFs) is reported in this paper. The 3-D ZnO NFs were synthesized on n-Si wafer, using liquid phase deposition technique, while a dip coating technique was adopted for the deposition of rGO, in the form of discrete claddings, on the ZnO NFs layer. The detailed morphological (field emission scanning electron microscopy) and structural (Raman and XPS) characterizations were carried out to compare the structural and morphological properties of pristine ZnO NFs and rGO-ZnO NFs hybrid structure. The sensing properties of ZnO NFs and rGO-ZnO NFs devices were investigated toward different alcohol vapors (2-propanol, ethanol, and methanol) in the concentration range of 0.5–700 ppm at 100 °C–275 °C. The rGO-ZnO NFs 3-D hybrid device offered ultra-high sensitivity (645% response magnitude at 700 ppm) toward 2-propanol at ~160 °C. In addition, the same sensor also offered excellent selectivity against interfering neighbors (i.e., ethanol, methanol, acetone, and benzene). The underlying mechanism for such experimental findings has been discussed with the properties of rGO–ZnO hybrid junction and respective band bending in air and target species.
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paper_title: Low-temperature NO2 gas sensor fabricated with NiO and reduced graphene oxide hybrid structure

paper_content:

Abstract The fabrication of highly sensitive gas sensors that can monitor environmental toxic gases, such as nitrogen dioxide (NO 2 ), which is one the most harmful gases to human health, attracts considerable attention. In this study, a hybrid structure composed of reduced graphene oxide (rGO) and a p-type NiO hierarchical structure (NiO-rGO) is fabricated using a facile solution process. The morphology and surface properties are analyzed by Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The NO 2 sensitivity is enhanced by the high surface to volume ratio of the NiO hierarchical structures as well as the enhanced charge transfer of NiO to rGO. The fabricated sensor with a hierarchical NiO and rGO hybrid structure exhibiting high sensitivity and selectivity at a low temperature (100 °C) with a rapid response time.
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paper_title: Enhanced NO2 detection using hierarchical porous ZnO nanoflowers modified with graphene

paper_content:

Abstract Because of their potential applications in gas sensing and catalysis, reduced graphene oxide (RGO) and ZnO have been the focus of much recent attention. However, few reported materials have been produced via the combination of hierarchical ZnO structures with RGO to achieve high sensing performances. In this paper, a hydrothermal method was used to synthesize hierarchical porous ZnO nanoflowers, which were then combined with graphene to enhance their sensing performances. The rapid detection of 1 ppm NO 2 was achieved at 174 °C. The morphologies and structures of these materials were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. Photoluminescence measurements and X-ray photoelectron spectroscopy were also used to investigate the mechanism of gas sensing by these materials.
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paper_title: Humidity-Sensing Properties of Urchinlike CuO Nanostructures Modified by Reduced Graphene Oxide

paper_content:

Urchinlike CuO modified by reduced graphene oxide (rGO) was synthesized by a one-pot microwave-assisted hydrothermal method. The as-prepared composites were characterized using various characterization methods. A humidity sensor based on the CuO/rGO composites was fabricated and tested. The results revealed that the sensor based on the composites showed much higher impedance than pure CuO. Compared with the sensors based on pristine rGO and CuO, the sensor fabricated with the composites exhibited relatively good humidity-sensing performance in terms of response time and response value. The humidity-sensing mechanism was also briefly introduced. The enlargement of the impedance and improvement of the humidity-sensing properties are briefly explained by the Schottky junction theory.
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paper_title: Faster response of NO 2 sensing in graphene-WO 3 nanocomposites

paper_content:

Graphene-based nanocomposites have proven to be very promising materials for gas sensing applications. In this paper, we present a general approach for the preparation of graphene‐WO3 nanocomposites. Graphene‐WO3 nanocomposite thin-layer sensors were prepared by drop coating the dispersed solution onto the alumina substrate. These nanocomposites were used for the detection of NO2 for the first time. TEM micrographs revealed that WO3 nanoparticles were well distributed on graphene nanosheets. Three different compositions (0.2, 0.5 and 0.1 wt%) of graphene with WO3 were used for the gas sensing measurements. It was observed that the sensor response to NO2 increased nearly three times in the case of graphene‐WO3 nanocomposite layer as compared to a pure WO3 layer at room temperature. The best response of the graphene‐WO3 nanocomposite was obtained at 250 C. (Some figures may appear in colour only in the online journal)
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paper_title: Enhancing the NO2 gas sensing properties of rGO/SnO2 nanocomposite films by using microporous substrates

paper_content:

Abstract Sensitive detection for low-concentration NO 2 gas is imperative in the fields of environmental monitoring and human healthcare. In this work, reduced graphene oxide (rGO)/tin oxide (SnO 2 ) nanocomposites were prepared by a simple blending and deposited onto different microporous substrates to explore their sensing performance to NO 2 gas, combining with multiple characterization techniques such as SEM, TEM, XRD and XPS. The experimental results revealed an increased sensing response with rising SnO 2 concentration ranging from 0 to 10 mg/ml. Especially, the sensing response at 10 mg/ml attained nearly three times larger than that of pure rGO. Investigation on long-term stability manifested a smaller response attenuation of rGO/SnO 2 sensors toward NO 2 gas of high concentration regime than that of low one. Besides, the largest sensing response of rGO film to 1 ppm NO 2 was achieved under 200-μm microporous substrate, which was 350% enhancement of that under flat one. To analyze the dynamic equilibrium of adsorption/desorption process, two kinds of measurement methods were contrastively studied. Additionally, interfering factors including operation temperature and relative humidity were probed as well. The prepared rGO/SnO 2 sensors exhibited an excellent selectivity toward NO 2 gas as well as a nice detection limit as low as 15.7 ppb.
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paper_title: Acetylene gas sensing properties of an Ag-loaded hierarchical ZnO nanostructure-decorated reduced graphene oxide hybrid

paper_content:

Abstract Acetylene (C 2 H 2 ) gas sensors were developed by synthesizing an Ag-loaded hierarchical (Hrc) ZnO nanostructure-reduced graphene oxide (Ag/ZnO Hrc-RGO) hybrid using a facile and rapid photochemical method. Morphological characterizations showed the formation of well-dispersed Ag nanoparticles-loaded hierarchical ZnO nanostructures onto the 3- to 5-layer thick RGO sheets. The hierarchical ZnO nanostructures were composed of numerous randomly oriented porous ZnO nanosheets with an average sheet thickness of 6 nm. It was also observed that Ag nanoparticles with an average diameter of 40 nm were closely affixed onto the ZnO nanosheets. Structural analysis revealed good agreement with the standard crystalline phases of ZnO, Ag and RGO. Gas-sensing characteristics of the synthesized materials were carried out in a temperature range of 25–300 °C at different C 2 H 2 gas concentrations. At 200 °C, 5 wt% Ag/ZnO Hrc-RGO hybrid showed a preferable detection of C 2 H 2 with high sensor response (12.3 toward 100 ppm), fast response time (57 s), a limit of detection (LOD) of 3 ppm, good linearity, with excellent reproducibility and selectivity. The fabricated sensor also showed less humidity effect in an open air environment.
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paper_title: Pd–WO3/reduced graphene oxide hierarchical nanostructures as efficient hydrogen gas sensors

paper_content:

Abstract Pd–WO 3 nanostructures were incorporated on graphene oxide (GO) and partially reduced graphene oxide (PRGO) sheets using a controlled hydrothermal process to fabricate effective hydrogen gas sensors. Pd–WO 3 nanostructures showed ribbon-like morphologies and Pd–WO 3 /GO presented an irregular nanostructured form, while Pd–WO 3 /PRGO exhibited a hierarchical nanostructure with a high surface area. Gas sensing properties of thin films of these materials were studied for different hydrogen concentrations (from 20 to 10,000 ppm) at various temperatures (from room temperature to 250 °C). Although adding GO in the Pd–WO 3 , after hydrothermal process could increase the film conductivity, gas sensitivity was reduced to half, due to lower surface area of the irregular morphology in comparison with the ribbon-like morphology. The Pd–WO 3 /PRGO films showed an optimum sensitivity (∼10 folds better than the sensitivity of Pd–WO 3 /GO), and a fast response and recovery time ( 3 /PRGO-based gas sensor was sensitive to 20 ppm concentration of hydrogen gas at room temperature. The results confirmed the effect of residual oxygen-containing functional groups of PRGO on the growth and morphology of Pd–WO 3 as well as gas sensing properties of metal oxide/graphene based hybrid nanostructures.
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paper_title: Novel 3D graphene aerogel–ZnO composites as efficient detection for NO2 at room temperature

paper_content:

Abstract A novel three-dimensional graphene aerogel–ZnO spheres composite has been synthesized via a simple solvothermal method. The three-dimensional graphene exhibits interconnected macroporous networks and the ZnO spheres featured a size of 0.5–1 μm are anchored homogeneously on the surface of graphene layers. When being used as gas sensors for detection of NO 2 , the porous graphene–ZnO composite achieves a higher response and fast response/recovery behavior at room temperature effectively (RT). The enhancement of the sensing response is mainly attributed to the improved conductivity, the 3D porous structure of graphene and the interaction between p-type graphene and n-type zinc oxide. The sensing mechanism of the composite is also discussed.
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paper_title: PVP assisted in situ synthesis of functionalized graphene/ZnO (FGZnO) nanohybrids with enhanced gas-sensing property

paper_content:

Functionalized graphene/ZnO (FGZnO) nanohybrids with strong chemical coupling and electronic interaction were synthesized through a modified in situ synthesis method by combining two procedures: functionalization of graphene oxides and solution-based synthesis of FGZnO. ZnO species with morphologies of nanorods or nanoplates were densely attached on the graphene sheets. The effectiveness of this method for loading ZnO onto graphene with good interface contact was confirmed by using scanning electron microscopy, transmission electron microscopy, XPS measurements, and UV–Vis spectroscopy. Gas sensors based on the nanohybrids have faster response, higher sensitivity, and better selectivity to ethanol compared with pure ZnO nanorods. The mechanisms to interpret the formation of nanohybrids and their enhanced gas-sensing performance were put forward.
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paper_title: Normal-pressure microwave rapid synthesis of hierarchical SnO2@rGO nanostructures with superhigh surface areas as high-quality gas-sensing and electrochemical active materials

paper_content:

Hierarchical SnO2@rGO nanostructures with superhigh surface areas are synthesized via a simple redox reaction between Sn2+ ions and graphene oxide (GO) nanosheets under microwave irradiation. XRD, SEM, TEM, XPS, TG-DTA and N2 adsorption–desorption are used to characterize the compositions and microstructures of the SnO2@rGO samples obtained. The SnO2@rGO nanostructures are used as gas-sensing and electroactive materials to evaluate their property–microstructure relationship. The results show that SnO2 nanoparticles (NPs) with particle sizes of 3–5 nm are uniformly anchored on the surfaces of reduced graphene oxide (rGO) nanosheets through a heteronucleation and growth process. The as-obtained SnO2@rGO sample with a hierarchically sesame cake-like microstructure and a superhigh specific surface area of 2110.9 m2 g−1 consists of 92 mass% SnO2 NPs and ∼8 mass% rGO nanosheets. The sensitivity of the SnO2@rGO sensor upon exposure to 10 ppm H2S is up to 78 at the optimal operating temperature of 100 °C, and its response time is as short as 7 s. Compared with SnO2 nanocrystals (5–10 nm), the hierarchical SnO2@rGO nanostructures have enhanced gas-sensing behaviors (i.e., high sensitivity, rapid response and good selectivity). The SnO2@rGO nanostructures also show excellent electroactivity in detecting sunset yellow (SY) in 0.1 M phosphate buffer solution (pH = 2.0). The enhancement in gas-sensing and electroactive performance is mainly attributed to the unique hierarchical microstructure, high surface areas and the synergistic effect of SnO2 NPs and rGO nanosheets.
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paper_title: Facile Synthesis of ZnO–Reduced Graphene Oxide Nanocomposites for NO2 Gas Sensing Applications

paper_content:

Graphene and its exotic forms have been widely recognized as exceptional materials for gas-sensing applications because of their extraordinary electrical conductivity and large surface area to volume ratios. Herein, chemically reduced graphene oxide (rGO) and zinc oxide-reduced graphene oxide (ZrGO) nanocomposite powders have been successfully synthesized through a simple hydrolysis method followed by annealing in ambient N-2 gas. The reduction of graphene oxide by hydrazine hydrate and the decoration of the graphene surface by ZnO nanoparticles have occurred during the synthesis process. The prepared samples were characterized by various microscopic techniques to explore the surface morphology and uniformity. Spectroscopic techniques were used to investigate the quality of the as-synthesized powder samples as well as the extent of graphitization of the samples. Coil sensors with two Pt terminals and a heating element have been designed to extensively monitor the effect of temperature on the electrical and gas-sensing properties of the rGO and ZrGO nanocomposite samples. The ZrGO nanocomposites possess better electrical and NO2 gas sensing properties than the pristine rGO. The ZrGO nanocomposite sensor exhibits a high response (ca. 32%) for 50 ppm NO2 at relatively low temperature (50 degrees C). Our results suggest that the ZrGO nanocomposite material could be used to fabricate a new generation of low-power portable NO2 sensors.
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paper_title: Three-dimensional mesoporous graphene aerogel-supported SnO2 nanocrystals for high-performance NO2 gas sensing at low temperature.

paper_content:

A facile and cost-efficient hydrothermal and lyophilization two-step strategy has been developed to prepare three-dimensional (3D) SnO2/rGO composites as NO2 gas sensor. In the present study, two different metal salt precursors (Sn(2+) and Sn(4+)) were used to prepare the 3D porous composites. It was found that the products prepared from different tin salts exhibited different sensing performance for NO2 detection. The scanning electron microscopy and transmission electron microscopy characterizations clearly show the macroporous 3D hybrids, nanoporous structure of reduce graphene oxide (rGO), and the supported SnO2 nanocrystals with an average size of 2-7 nm. The specific surface area and porosity properties of the 3D mesoporous composites were analyzed by Braunauer-Emmett-Teller method. The results showed that the SnO2/rGO composite synthesized from Sn(4+) precursor (SnO2/rGO-4) has large surface area (441.9 m(2)/g), which is beneficial for its application as a gas sensing material. The gas sensing platform fabricated from the SnO2/rGO-4 composite exhibited a good linearity for NO2 detection, and the limit of detection was calculated to be as low as about 2 ppm at low temperature. The present work demonstrates that the 3D mesoporous SnO2/rGO composites with extremely large surface area and stable nanostructure are excellent candidate materials for gas sensing.
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paper_title: Rapid ethanol sensor based on electrolytically-exfoliated graphene-loaded flame-made In-doped SnO2 composite film

paper_content:

Abstract In this research, flame-made SnO 2 nanoparticles doped with 0.2–1 wt% indium and loaded with 0.1–5 wt% electrolytically-exfoliated graphene are systematically investigated for ethanol sensing applications. The sensing films (∼10–50 μm in thickness) were prepared by spin coating technique on Au/Al 2 O 3 substrates and evaluated for ethanol sensing performances at operating temperatures ranging from 150 to 350 °C in dry air. Characterizations by XRD, XPS, SEM, TEM and Raman spectroscopy demonstrated that In-doped SnO 2 nanostructures had spheriodal morphology with polycrystalline tetragonal SnO 2 phase and indium was confirmed to form solid solution with SnO 2 lattice while graphene in the sensing film after annealing and testing still retained high-quality multilayer structure with low oxygen content. Gas-sensing measurement showed that SnO 2 sensing film with 0.5 wt% In-doping concentration exhibited an optimal response of 110 and short response time of 2 s towards 1000 ppm C 2 H 5 OH at an optimal operating temperature of 300 °C. The additional loading of graphene at 5 wt% into 0.5 wt% In-doped SnO 2 led to a drastic response enhancement to 965 with very short response time of 1.8 s and fast recovery stabilization at optimal operating temperature of 350 °C. The superior gas sensing performances of In-doped SnO 2 nanoparticles loaded with graphene may be attributed to large specific surface area of the composite, high density of reactive sites of highly porous non-agglomerated graphene-SnO 2 nanoparticle structure and high electronic conductivity of graphene, which allowed fast gas response and recovery. Therefore, the graphene loaded In-doped SnO 2 sensor is a promising candidate for fast, sensitive and selective detection of ethanol.
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paper_title: Novel 3D graphene aerogel–ZnO composites as efficient detection for NO2 at room temperature

paper_content:

Abstract A novel three-dimensional graphene aerogel–ZnO spheres composite has been synthesized via a simple solvothermal method. The three-dimensional graphene exhibits interconnected macroporous networks and the ZnO spheres featured a size of 0.5–1 μm are anchored homogeneously on the surface of graphene layers. When being used as gas sensors for detection of NO 2 , the porous graphene–ZnO composite achieves a higher response and fast response/recovery behavior at room temperature effectively (RT). The enhancement of the sensing response is mainly attributed to the improved conductivity, the 3D porous structure of graphene and the interaction between p-type graphene and n-type zinc oxide. The sensing mechanism of the composite is also discussed.
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paper_title: Tuning gas-sensing properties of reduced graphene oxide using tin oxide nanocrystals

paper_content:

We report a novel and selective gas-sensing platform with reduced graphene oxide (RGO) decorated with tin oxide (SnO2) nanocrystals (NCs). This hybrid SnO2 NC–RGO platform showed enhanced NO2 but weakened NH3 sensing compared with bare RGO, showing promise in tuning the sensitivity and selectivity of RGO-based gas sensors.
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paper_title: Reduced graphene oxide/hierarchical flower-like zinc oxide hybrid films for room temperature formaldehyde detection

paper_content:

Abstract We report a potential method to improve the room temperature formaldehyde (HCHO) gas-sensing properties of the layer-by-layer self-assembled reduced graphene oxide (rGO) film by introducing a special hierarchical flower-like zinc oxide (ZnO) structure. Hierarchical flower-like ZnO structures are developed by one step hydrothermal synthesis methods. Both rGO and rGO/ZnO sensitive films respond at once when exposed to HCHO at room temperature. The gas-sensing responses to 2–10 ppm formaldehyde are enhanced in rGO/ZnO hybrid films compared with pure rGO films. The enhanced formaldehyde gas-sensing responses are attributed to the more adsorption sites on the surface of the rGO/ZnO hybrid films and the reduced barrier height for the electron transfer during the gas sensing process.
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paper_title: Confined Formation of Ultrathin ZnO Nanorods/Reduced Graphene Oxide Mesoporous Nanocomposites for High-Performance Room-Temperature NO2 Sensors

paper_content:

Here we demonstrate high-performance room-temperature NO2 sensors based on ultrathin ZnO nanorods/reduced graphene oxide (rGO) mesoporous nanocomposites. Ultrathin ZnO nanorods were loaded on rGO nanosheets by a facile two-step additive-free solution synthesis involving anchored seeding followed by oriented growth. The ZnO nanorod diameters were simply controlled by the seed diameters associated with the spatial confinement effects of graphene oxide (GO) nanosheets. Compared to the solely ZnO nanorods and rGO-based sensors, the optimal sensor based on ultrathin ZnO nanorods/rGO nanocomposites exhibited higher sensitivity and quicker p-type response to parts per million level of NO2 at room temperature, and the sensitivity to 1 ppm of NO2 was 119% with the response and recovery time being 75 and 132 s. Moreover, the sensor exhibited full reversibility, excellent selectivity, and a low detection limit (50 ppb) to NO2 at room temperature. In addition to the high transport capability of rGO as well as excelle...
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paper_title: Three-dimensional mesoporous graphene aerogel-supported SnO2 nanocrystals for high-performance NO2 gas sensing at low temperature.

paper_content:

A facile and cost-efficient hydrothermal and lyophilization two-step strategy has been developed to prepare three-dimensional (3D) SnO2/rGO composites as NO2 gas sensor. In the present study, two different metal salt precursors (Sn(2+) and Sn(4+)) were used to prepare the 3D porous composites. It was found that the products prepared from different tin salts exhibited different sensing performance for NO2 detection. The scanning electron microscopy and transmission electron microscopy characterizations clearly show the macroporous 3D hybrids, nanoporous structure of reduce graphene oxide (rGO), and the supported SnO2 nanocrystals with an average size of 2-7 nm. The specific surface area and porosity properties of the 3D mesoporous composites were analyzed by Braunauer-Emmett-Teller method. The results showed that the SnO2/rGO composite synthesized from Sn(4+) precursor (SnO2/rGO-4) has large surface area (441.9 m(2)/g), which is beneficial for its application as a gas sensing material. The gas sensing platform fabricated from the SnO2/rGO-4 composite exhibited a good linearity for NO2 detection, and the limit of detection was calculated to be as low as about 2 ppm at low temperature. The present work demonstrates that the 3D mesoporous SnO2/rGO composites with extremely large surface area and stable nanostructure are excellent candidate materials for gas sensing.
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paper_title: Synthesis of water soluble graphene.

paper_content:

A facile and scalable preparation of aqueous solutions of isolated, sparingly sulfonated graphene is reported. 13C NMR and FTIR spectra indicate that the bulk of the oxygen-containing functional groups was removed from graphene oxide. The electrical conductivity of thin evaporated films of graphene (1250 S/m) relative to similarly prepared graphite (6120 S/m) implies that an extended conjugated sp2 network is restored in the water soluble graphene.
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paper_title: Facile synthesis of novel 3D nanoflower-like CuxO/multilayer graphene composites for room temperature NOx gas sensor application

paper_content:

3D nanoflower-like CuxO/multilayer graphene composites (CuMGCs) have been successfully synthesized as a new type of room temperature NOx gas sensor. Firstly, the expanded graphite (EG) was activated by KOH and many moderate functional groups were generated; secondly, Cu(CH3COO)2 and CTAB underwent full infusion into the interlayers of activated EG (aEG) by means of a vacuum-assisted technique and then reacted with the functional groups of aEG accompanied by the exfoliation of aEG via reflux. Eventually, the 3D nanoflower consisting of 5–9 nm CuxO nanoparticles homogeneously grow in situ on aEG. The KOH activation of EG plays a key role in the uniform formation of CuMGCs. When being used as gas sensors for detection of NOx, the CuMGCs achieved a higher response at room temperature than that of the corresponding CuxO. In detail, the CuMGCs show a higher NOx gas sensing performance with low detection limit of 97 ppb, high gas response of 95.1% and short response time of 9.6 s to 97.0 ppm NOx at room temperature. Meanwhile, the CuMGC sensor presents a favorable linearity, good selectivity and stability. The enhancement of the sensing response is mainly attributed to the improved conductivity of the CuMGCs. A series of Mott–Schottky and EIS measurements demonstrated that the CuMGCs have much higher donor densities than CuxO and can easily capture and migrate electrons from the conduction band, resulting in the enhancement of electrical conductivity.
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paper_title: Flower-like In 2 O 3 modified by reduced graphene oxide sheets serving as a highly sensitive gas sensor for trace NO 2 detection

paper_content:

Abstract In this work, we described gas sensors based on the materials composed of hierarchical flower-likeIn 2 O 3 and reduced graphene oxide (rGO), which were fabricated by a facile one-step hydrothermal method. The rGO-In 2 O 3 composites exhibited enhanced sensing performance towards NO 2 through comparison with the pure In 2 O 3 sample. The operating temperature can be tuned by the percentage of rGO in the composites. The sensor based on 5 wt% rGO-In 2 O 3 could work at room temperature with a high response value to 1 ppm NO 2 . 3 wt% rGO-In 2 O 3 composite was adopted for the ultra-sensitivity gas sensor owing to its extremely low limit of detection of 10 ppb with rapid response time to NO 2 . The sensor also exhibited excellent selectivity and stability. The ultra-sensitivity of rGO-In 2 O 3 should be related to synergistic effect of the hierarchical structure of In 2 O 3 and the presence of rGO in the composites, which provided enhanced surface area and local p-n heterojunctions in rGO/In 2 O 3 composites.
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paper_title: Room-Temperature Hydrogen Sensing with Heteronanostructures Based on Reduced Graphene Oxide and Tin Oxide

paper_content:

Hydrogen is used in many industrial processes and is regarded as one of the most promising clean-energy carriers for future applications in transportation and power generation. However, it is also a highly flammable gas and explosive when mixed with air at volume concentrations higher than about 4%; consequently, hydrogen manipulation requires constant monitoring for leaks. There are several types of H2 sensors commercially available or in development, but the prospect of the widespread use of hydrogen as a fuel in the near future would require sensors that are highly sensitive, accurate, durable, small in size, and easy to fabricate, with short response and recovery times and low power consumption. None of the sensors presently available can fulfill all these requirements simultaneously. Metal-oxide semiconductors, such as SnO2, are widely used as sensing layers for the detection of gases, including hydrogen, owing to their high sensitivity, fast response and recovery, stability, small size, simplicity, low cost, and simple interface electronics. However, one significant disadvantage of sensors of this type is the high temperatures necessary for their operation (usually well above 200 8C), which increase the power consumption of the device. Hence, if the operating temperature of sensors based on semiconductor metal oxides could be decreased considerably without compromising other important aspects of their performance, such as sensitivity or response and recovery times, the potential of these sensors for widespread application would be improved significantly. Different strategies have been employed to enhance the performance of semiconductor metal oxides in H2 sensing, including the fabrication of 1D nanostructures (i.e. nanotubes, nanowires, and nanorods) and the combination of the metal oxides with organic or carbon nanomaterials. In particular, the incorporation of carbon nanostructures, such as carbon nanotubes (CNTs) or graphene, in H2-sensing devices seems to be a promising approach, owing to the unique electronic and structural properties of these materials. Several different types of H2 sensors with enhanced performance at low temperature have been reported previously, including optical sensors and field-effect-transistor devices, such as those developed by Janata, Doll, Eisele, and coworkers. However, in this study, we focused essentially on resistive H2 sensors based on carbon nanostructures. Tin oxide/carbon nanotube nanocomposites have performed well in the past in terms of their operating temperature, response times, or sensitivity. The properties of graphene have also been explored for the detection of hydrogen. In addition to its high electronic conductivity, graphene has a very large theoretical surface area (twice as high as that of multiwalled carbon nanotubes) and shows structural flexibility and low electrical noise as a result of its crystal lattice and 2D nature: properties that are extremely desirable for gas-sensing applications. However, pure graphene is not sensitive to hydrogen. Therefore, most of the resistive hydrogen sensors based on graphene that have been reported so far consist of graphene layers functionalized with Pd or Pt nanoparticles and films, as it is well-known that these metals interact selectively with hydrogen molecules and catalyze their dissociation. The use of nanocomposites based on metal oxides and graphene as hydrogen-sensing layers is, on the other hand, relatively unexplored. Herein, we report on the hydrogen-sensing performance of nanostructures fabricated from reduced graphene oxide (RGO), tin oxide, and platinum by simple, fast, and readily scaled up microwave-assisted methods. The SnO2/RGO and Pt–SnO2/RGO nanostructures showed enhanced sensing performance relative to that of the corresponding pure and binary systems as a consequence of synergistic effects between the different components in the composites. In particular, the Pt–SnO2/RGO sensor can detect H2 even at [*] Dr. P. A. Russo, Dr. D. E. Conte, Prof. N. Pinna Department of Chemistry, CICECO, University of Aveiro 3810-193 Aveiro (Portugal)
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paper_title: Highly sensitive and rapidly responding room-temperature NO2 gas sensors based on WO3 nanorods/sulfonated graphene nanocomposites

paper_content:

Metal oxide/graphene nanocomposites are emerging as promising materials for developing room-temperature gas sensors. However, the unsatisfactory performances owing to the relatively low sensitivity, slow response, and recovery kinetics limit their applications. Herein, a highly sensitive and rapidly responding room-temperature NO2 gas sensor based on WO3 nanorods/sulfonated reduced graphene oxide (S-rGO) was prepared via a simple and cost-effective hydrothermal method. The optimal sensor response of the WO3/S-rGO sensor toward 20 ppm NO2 is 149% in 6 s, which is 4.7 times higher and 100 times faster than that of the corresponding WO3/rGO sensors. In addition, the sensor exhibits excellent reproducibility, selectivity, and extremely fast recovery kinetics. The mechanism of the WO3/S-rGO nanocomposite gas sensor is investigated in detail. In addition to the high transport capability of S-rGO as well as its excellent NO2 adsorption ability, the superior sensing performance of the S-rGO/WO3 sensor can be attributed to the favorable charge transfer occurring at the S-rGO/WO3 interfaces. We believe that the strategy of compositing a metal oxide with functionalized graphene provides a new insight for the future development of room-temperature gas sensors.
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paper_title: Room temperature formaldehyde sensor with enhanced performance based on reduced graphene oxide/titanium dioxide

paper_content:

Abstract Reduced graphene oxide/titanium dioxide (rGO/TiO 2 ) layered film deposition on the interdigital electrode substrate has been developed through simple thermal treatment for trace detection of formaldehyde (HCHO) at room temperature. The layered film was characterized by UV–vis spectroscopy, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy, and the formaldehyde sensing performances of rGO/TiO 2 layered film were investigated over low detection concentrations from 0.1 ppmv to 0.5 ppmv. An interesting abruption behavior of rGO sheets during the heating process was observed and discussed. However, the dissociation of rGO honeycomb network was prevented for rGO/TiO 2 layered film upon elevated temperature, which might be induced by the synergetic interaction between TiO 2 nanoparticles and graphene sheets effectively. The sensing-test results revealed that the rGO/TiO 2 sensor exhibited rapid response, excellent selectivity, good reproducibility and remarkable sensitivity of 0.8 ppmv −1 , which was higher than that of rGO-based one (0.5 ppmv −1 ). Furthermore, the formaldehyde-sensing mechanism was analyzed based on the combination of catalytic reactions and supporting function of TiO 2 nanoparticles.
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paper_title: High-Performance Schottky Diode Gas Sensor Based on the Heterojunction of Three-Dimensional Nanohybrids of Reduced Graphene Oxide–Vertical ZnO Nanorods on an AlGaN/GaN Layer

paper_content:

A Schottky diode based on a heterojunction of three-dimensional (3D) nanohybrid materials, formed by hybridizing reduced graphene oxide (RGO) with epitaxial vertical zinc oxide nanorods (ZnO NRs) and Al0.27GaN0.73(∼25 nm)/GaN is presented as a new class of high-performance chemical sensors. The RGO nanosheet layer coated on the ZnO NRs enables the formation of a direct Schottky contact with the AlGaN layer. The sensing results of the Schottky diode with respect to NO2, SO2, and HCHO gases exhibit high sensitivity (0.88–1.88 ppm–1), fast response (∼2 min), and good reproducibility down to 120 ppb concentration levels at room temperature. The sensing mechanism of the Schottky diode can be explained by the effective modulation of the reverse saturation current due to the change in thermionic emission carrier transport caused by ultrasensitive changes in the Schottky barrier of a van der Waals heterostructure between RGO and AlGaN layers upon interaction with gas molecules. Advances in the design of a Schottk...
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paper_title: Flexible Graphene-Based Wearable Gas and Chemical Sensors

paper_content:

Wearable electronics is expected to be one of the most active research areas in the next decade; therefore, nanomaterials possessing high carrier mobility, optical transparency, mechanical robustness and flexibility, lightweight, and environmental stability will be in immense demand. Graphene is one of the nanomaterials that fulfill all these requirements, along with other inherently unique properties and convenience to fabricate into different morphological nanostructures, from atomically thin single layers to nanoribbons. Graphene-based materials have also been investigated in sensor technologies, from chemical sensing to detection of cancer biomarkers. The progress of graphene-based flexible gas and chemical sensors in terms of material preparation, sensor fabrication, and their performance are reviewed here. The article provides a brief introduction to graphene-based materials and their potential applications in flexible and stretchable wearable electronic devices. The role of graphene in fabricating ...
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paper_title: Fully Printed, Rapid-Response Sensors Based on Chemically Modified Graphene for Detecting NO2at Room Temperature

paper_content:

Reduced graphene oxide (RGO) has proven to be effective in trace gas detection at room temperature ambient conditions. However, the slow response–recovery characteristic is a major hurdle for the RGO-based gas sensors. Herein, we report a gravure-printed chemoresistor-type NO2 sensor based on sulfonated RGO (S-RGO) decorated with Ag nanoparticles (Ag–S-RGO). Large amounts of silver nanoparticles with an average particle size of 10–20 nm were uniformly assembled on flat S-RGO surfaces. The printed Ag–S-RGO sensor possesses a high sensitivity and fast response–recovery characteristic over NO2 concentrations ranging from 0.5 to 50 ppm. Upon exposure to 50 ppm NO2 at room temperature, the Ag–S-RGO sensor shows a sensitivity of 74.6%, a response time of 12 s and a recovery time of 20 s. In addition, the Ag–S-RGO sensors exhibit satisfactory flexibility with an almost constant resistance after 1000 bending cycles. The printed and high-performance Ag–S-RGO sensors described here will be a good prospect in enviro...
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paper_title: Reduced graphene oxide (rGO) decorated TiO2 microspheres for selective room-temperature gas sensors

paper_content:

Reduced graphene oxide (rGO) decorated TiO2 microspheres are prepared by using the hydrothermal method. The as-synthesized sample shows that TiO2 particles are connected by the rGO nanosheets while some TiO2 particles are found to be partly covered by rGO sheets. The rGO-decorated TiO2 based chemoresistive-type sensor shows good sensitivity and excellent selectivity to different concentrations of ammonia from 5 ppm to 50 ppm at room temperature. However, the response and recovery speeds of the sensor to ammonia are sluggish and need further optimization.
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paper_title: Enhancing the NO2 gas sensing properties of rGO/SnO2 nanocomposite films by using microporous substrates

paper_content:

Abstract Sensitive detection for low-concentration NO 2 gas is imperative in the fields of environmental monitoring and human healthcare. In this work, reduced graphene oxide (rGO)/tin oxide (SnO 2 ) nanocomposites were prepared by a simple blending and deposited onto different microporous substrates to explore their sensing performance to NO 2 gas, combining with multiple characterization techniques such as SEM, TEM, XRD and XPS. The experimental results revealed an increased sensing response with rising SnO 2 concentration ranging from 0 to 10 mg/ml. Especially, the sensing response at 10 mg/ml attained nearly three times larger than that of pure rGO. Investigation on long-term stability manifested a smaller response attenuation of rGO/SnO 2 sensors toward NO 2 gas of high concentration regime than that of low one. Besides, the largest sensing response of rGO film to 1 ppm NO 2 was achieved under 200-μm microporous substrate, which was 350% enhancement of that under flat one. To analyze the dynamic equilibrium of adsorption/desorption process, two kinds of measurement methods were contrastively studied. Additionally, interfering factors including operation temperature and relative humidity were probed as well. The prepared rGO/SnO 2 sensors exhibited an excellent selectivity toward NO 2 gas as well as a nice detection limit as low as 15.7 ppb.
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Title: 3D Architectured Graphene/Metal Oxide Hybrids for Gas Sensors: A Review
Section 1: Introduction
Description 1: This section introduces the importance of gas sensors in modern industry, the materials used for their fabrication, and specifically focuses on the potential of graphene/metal oxide (MOx) hybrids for gas sensing applications.

Section 2: 2D Graphene/1D MOx Based Architectures
Description 2: This section covers the advantages of one-dimensional metal oxides in gas sensing and their integration with graphene to form 2D/1D hybrid architectures.

Section 3: Template-Assisted Synthesis
Description 3: This section discusses the methods and benefits of using templates to create 3D graphene/metal oxide architectures.

Section 4: Multistep Sequential Growth
Description 4: This section explains the development of 3D hybrid structures through multistep sequential growth techniques, highlighting specific examples and processes.

Section 5: 2D Graphene/2D MOx Based Architectures
Description 5: This section explores the formation and challenges of combining two-dimensional metal oxide nanosheets with graphene to form 2D/2D hybrid architectures.

Section 6: 2D Graphene/3D MOx Architectured Hybrids
Description 6: This section focuses on the construction and benefits of 3D metal oxide nanostructures combined with 2D graphene for improved gas sensing performance.

Section 7: Graphene/Regular 3D MOx Nanostructures
Description 7: This section reviews the combination of graphene with regularly shaped 3D metal oxide nanostructures such as spheres and cubes.

Section 8: Graphene/3D MOx Hierarchical Assemblies
Description 8: This section discusses the use of hierarchical 3D metal oxide assemblies with graphene for enhanced gas sensing through improved surface area and porosity.

Section 9: Graphene-MOx Based Ternary 3D Hybrids
Description 9: This section covers the decoration of graphene/MOx composites with additional elements like noble metals to form ternary hybrids for better gas sensor performance.

Section 10: Graphene-MOx Hybrids Assembled within 3D Graphene-Multilayer Network
Description 10: This section discusses the construction of multilayer graphene networks serving as a framework for MOx growth to create advanced 3D hybrid systems.

Section 11: Applications of 3D Graphene/MOx Hybrids for Room Temperature Gas Sensing
Description 11: This section details the gas sensing applications of 3D graphene/MOx hybrids, particularly focusing on room temperature detection of various hazardous gases.

Section 12: Conclusions and Perspectives
Description 12: This section summarizes the advancements in 3D graphene/metal oxide hybrid-based gas sensors, discusses current challenges, and suggests future research directions.