Graphene, a single, one-atom-thick sheet of carbon atoms arranged in a honeycomb lattice and the two-dimensional building block for carbon materials, has attracted great interest for a wide range of applications. Due to its superior properties such as thermo-electric conduction, surface area and mechanical strength, graphene materials have inspired huge interest in sensing of various chemical species. In this timely review, we discuss the recent advancement in the field of graphene based gas sensors with emphasis on the use of modified graphene materials. Further, insights of theoretical and experimental aspects associated with such systems are also discussed with significance on the sensitivity and selectivity of graphene towards various gas molecules. The first section introduces graphene, its synthesis methods and its physico-chemical properties. The second part focuses on the theoretical approaches that discuss the structural improvisations of graphene for its effective use as gas sensing materials. The third section discusses the applications of pristine and modified graphene materials in gas sensing applications. Various graphene modification methods are discussed including using dopants and defects, decoration with metal/metal oxide nanoparticles, and functionalization with polymers. Finally, a discussion on the future challenges and perspectives of this enticing field of graphene sensors for gas detection is provided.

Recent advances in graphene based gas sensors

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

During the last decade there has been a rapidly growing interest in integrated optical (IO) sensors, expecially because many of them principally allow for sensitive, real time, label-free-on-site measurements of the concentration of (bio-)chemical species. This review aims at giving an overview of the most relevant developments in this area. After a general introduction into the field of IO sensors for the chemical domain, relevant aspects of integrated optics and chemical sensing are presented in short. A large variety of IO sensing platforms are introduced and discussed: interferometers, resonators, coupling-based devices such as grating couplers and surface plasmon resonance based sensors and finally a new class of sensors based on chemically induced field profile changes. Strong and weak points of principle and of configuration based on these principles are indicated and the main performance data of the IO sensing platforms, especially the obtained resolution, are indicated. Best resolutions of the chemically induced refractive indices on the order of magnitude $10^{-6}-10^{-8}RIU$ can be obtained, corresponding to a resolution of $10^{-3}-10^{-5}$nm in the chemically induced growth of layer thickness of chemo-optical transducer materials. Depending on the anlalyte and the type of transduction layer chemical concentrations down to some ppb or some pg $ml^{-1}$can be determined. Several IO sensing systems are commercially available. Extension of individual sensors to sensor arrays is treated and finally an outlook for the future is given.

Integrated Optical sensors for the chemical domain

Two-dimensional (2D) nanomaterials have attracted a large amount of attention regarding gas sensing applications, because of their high surface-to-volume ratio and unique chemical or physical gas adsorption capabilities. As an important research method, theoretical calculations have been massively applied in predicting the potentially excellent gas sensing properties of these 2D nanomaterials. In this review, we discuss the contributions of theoretical calculations in the study of the gas sensing properties of 2D nanomaterials. Firstly, we elaborate on the gas sensing mechanisms of 2D layered nanomaterials, such as the traditional charge transfer mechanism, and a standard for distinguishing between physical and chemical adsorption, from the perspective of theoretical calculations. Then, we describe how to conduct a theoretical analysis to explain or predict the gas sensing properties of 2D nanomaterials. Thirdly, we discuss three important methods that have been applied in order to improve the gas sensing properties, that is, defect functionalization (vacancy, edge, grain boundary, and doping), heterojunctions, and electric fields. Among these strategies, theoretical calculations play a very important role in explaining the mechanisms underlying the enhanced gas sensing properties. Finally, we summarize both the advantages and limitations of the theoretical calculations, and present perspectives for further research on the 2D nanomaterials-based gas sensors.

https://www.mdpi.com/2079-4991/8/10/851/pdf

Two-Dimensional Nanomaterials for Gas Sensing Applications: The Role of Theoretical Calculations

The paper deals with investigations concerning the construction of sensors based on a quartz crystal microbalance (QCM) containing a TiO2 nanostructures sensor layer. A chemical method of synthesizing these nanostructures is presented. The prepared prototype of the QCM sensing system, as well as the results of tests for detecting low NO2 concentrations in an atmosphere of synthetic air have been described. The constructed NO2 sensors operate at room temperature, which is a great advantage, because resistance sensors based on wide gap semiconductors often require much higher operation temperatures, sometimes as high as 500 °C. The sensors constructed by the authors can be used, among other applications, in medical and chemical diagnostics, and also for the purpose of detecting explosive vapours. Reactions of the sensor to nitroglycerine vapours are presented as an example of its application. The influence of humidity on the operation of the sensor was studied.

https://www.mdpi.com/1424-8220/15/4/9563/pdf

A Study of a QCM Sensor Based on TiO2 Nanostructures for the Detection of NO2 and Explosives Vapours in Air

The paper presents a resistance structures with sensor layers based on nanostructures elaborated on the base of TiO2 and ZnO. The structures were tested concerning their sensitivities to the effects of nitrogen dioxide in the atmosphere of synthetic air. The TiO2 and ZnO nanostructures played the role of sensor layers. Investigations have proved that the elaborated resistance structures with TiO2 and ZnO layers are sensitive to the presence of NO2 in the atmosphere of synthetic air. The resistance of the structure amounted to about 20Ω in the case of ZnO structures and to about 200Ω in the case of TiO2 structures. The investigations confirmed that resistance structures with ZnO and TiO2, exposed to the effect of nitrogen dioxide in the atmosphere of synthetic air changes their resistances relatively fast. This indicates that such structures might be practically applied in sensors of nitrogen dioxide ensuring a short time of response.

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Gas sensors based on nanostructures of semiconductors ZnO and TiO2

Layered nanostructures of tungsten trioxide WO3-x about 62 nm thick, with a very thin film of palladium (about 3.3 nm) on the top, have been studied for gas-sensing application at temperatures 50◦C and 120–130◦C and low NO2 and NH3 concentrations in 6%, 30% or 45% relative humidity in the air. Thin film WO3-x nanostructures were obtained by vacuum deposition on a common Si-SiO2 substrate at room temperature and 120◦C. The palladium was coated by vacuum evaporation at room temperature and 4 · 10 mbar on WO3-x layers obtained at two different substrate temperatures. The average rate of growth of the films, controlled by a QCM, was 0.1–0.2 nm/s. A multi-channel (four-channel interdigital gold electrodes) planar resistance gas sensor structure was used in the experiments. The surface of the nanostructures was characterized by means of the AFM method. Good sensor results have been observed at these layered nanostructures with an increasing resistance for NO2 molecules and decreasing resistance for NH3 molecules in a humid air atmosphere. The interaction and recovery speed were higher in the case of the nanostructure obtained at room temperature.

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Layered thin film nanostructures of Pd/WO3-x as resistance gas sensors

The paper presents the results of investigations concerning the measurement of the refractive index and the thickness of planar waveguide structures, obtained by photo polymerization of the polymer SU8. In the paper the mode sensitivity has been calculated as a function of the thickness in a single-mode structure. The thickness of the layer has been determined in the case when the interferometer is most sensitive to changes of the refractive index.

Differential interference in a polymer waveguide

The paper presents the results of investigations on the resistive structure with a graphene oxide (GO) sensing layer. The effects of dangerous gases (hydrogen and nitrogen dioxide) on the structure were studied; the resistance changes were examined during the flow of the selected gas in the atmosphere of synthetic air. Measurements were performed with a special emphasis on the detection of low concentrations of the analyzed gases. The reactions of the sensing structure to the effect of nitrogen and synthetic air at different humidity were also tested. Much attention was also paid to the fast response of the sensor to the changes in the gas atmosphere. The thin palladium layer (�2 nm) has been applied in order to improve the sensing properties of the structure. The investigations were performed in the temperature range from RT to 120 ◦ C and the analyzed gases in synthetic air were batched alternately with pure synthetic air.

/pdf/the-sensitivity-of-sensor-structures-with-oxide-graphene-xxy0fdylr8.pdf

The sensitivity of sensor structures with oxide graphene exposed to selected gaseous atmospheres

/pdf/the-sensibility-of-resistance-sensor-structures-with-3yqxfoqxt6.pdf

K. Gut

Papers

Gas sensors based on nanostructures of semiconductors ZnO and TiO2

Layered thin film nanostructures of Pd/WO3-x as resistance gas sensors

Differential interference in a polymer waveguide

The sensitivity of sensor structures with oxide graphene exposed to selected gaseous atmospheres

The sensibility of resistance sensor structures with graphene to the action of selected gaseous media