Carbon Nanotube Based Gas Sensors Research Articles

Engineering hierarchical heterostructure material based on multiwalled carbon nanotubes/SnO2 nanorod-flowers for high-efficient ammonia detection

The rational manufacturing of multi-component materials with rich heterostructure is emerging as a possible strategy to develop improved carbon nanotube-based gas sensors. Herein, hierarchical heterostructure of multiwalled carbon nanotubes/SnO2 nanorod-flowers (HMCNS) was synthesized by a facile water-bathing combined hydrothermal approach for highly sensitive ammonia detection. Exploration in the microstructure reveals that hierarchical flowers-like SnO2 consisted of nanorods demonstrates a hollow geometry for wrapping multiwalled carbon nanotubes, featuring a unique three-dimensional (3D) hierarchical architecture. Gas sensor based on SnO2@4.5%CNTs hierarchical heterostructure exhibits highly enhanced ammonia-sensing performances, including ultra-high response of 1650 toward 20 ppm ammonia, good selectivity, 250 ppb level detection, and superior long-term stability. The improvement in ammonia sensing capability could be attributed to the peculiar hierarchical architecture and the creation of Mott-Schottky heterostructure which has been strongly validated by electrochemical research. This work will provide a concise and effective avenue to construct hierarchical composite with Mott-Schottky heterostructure for high-performance ammonia gas sensor.

The Influence of Surfactants on the Deposition and Performance of Single-Walled Carbon Nanotube-Based Gas Sensors for NO2 and NH3 Detection

Solid-state chemiresistive gas sensors have attracted a lot of researchers’ attention during the last half-century thanks to their ability to detect different gases with high sensitivity, low power consumption, low cost, and high portability. Among the most promising sensitive materials, carbon nanotubes (CNTs) have attracted a lot of interest due to their large active surface area (in the range of 50–1400 m2/g, depending on their composition) and the fact that they can operate at room temperature. In this study, single-walled carbon nanotube (SWCNT)-based sensing films were prepared and deposited by spray deposition for the fabrication of gas sensors. For the deposition, various SWCNTs were prepared in deionized water with the addition of specific surfactants, i.e., carboxymethyl cellulose (CMC) and sodium dodecyl sulfate (SDS), which act as dispersing agents to create a suitable ink for deposition. This study aims to elucidate the possible differences in the sensing performance of the fabricated devices due to the use of the two different surfactants. To achieve this goal, all the devices were tested versus ethanol (C2H5OH), carbon monoxide (CO), nitrogen dioxide (NO2), and ammonia (NH3). The produced devices demonstrated high selectivity towards NH3 and NO2. The different sensors, prepared with different deposition thicknesses (from 0.51 nm to 18.41 nm), were tested in dry and wet conditions (40% humidity), highlighting an enhanced response as a function of relative humidity. In addition, sensor performance was evaluated at different working temperatures, showing the best performance when heated up to 150 °C. The best sensing conditions we found were against NO2, sensors with 10 layers of deposition and an operating temperature of 150 °C; in this condition, sensors showed high responses compared those found in the literature (62.5%—SDS-based and 78.6%—CMC-based). Finally, cross-sensitivity measurements showed how the produced sensors are good candidates for the practical and selective detection of NO2, even in the presence of the most important interfering gases identified, i.e., NH3.

Internal doping of metallic carbon nanotubes for chemiresistive sensing of explosive molecules

Carbon nanotubes (CNTs) have been the focus of much chemical sensing research, typically employing a chemiresistive sensing mechanism. Their effective application in hazardous gas sensing has been hindered by a number of drawbacks, including: poor selectivity, irreversibility, temperature instability, and relatively high sensitivity to oxygen. All of these issues may be addressed by deploying metallic CNTs in a new configuration: nanotube sensor arrays internally doped by encapsulated species (halogens, alkali metals, interhalogens, and alkali metal halides) which p-dope or n-dope the pristine material. Such a sensor design offers: selectivity via the dissimilar response of the diversely doped tubes to various analytes, reversibility by establishing the fully cleaned CNT surface as a sensor reference state, temperature stability by encapsulation of the dopants, and doping-induced sensitivity to a range of analytes, including ammonia, nitrogen dioxide, and both nitramine and nitroaromatic explosives. Ab initio analysis suggests that internally doped metallic CNT sensor arrays can dramatically improve the performance of CNT based gas sensors, capitalizing on both the high mass specific surface area and the stable encapsulation features of one dimensional materials.

Ultrafast NH3 gas sensor based on phthalocyanine-optimized non-covalent hybrid of carbon nanotubes with pyrrole

The development of innovative methods for optimizing carbon nanotube-based gas sensors for rapid and sensitive detection of ammonia remains challenging. Here, MCNT@PPy/TfmpoPcCo hybrid with a uniform 3-D network were prepared by tightly combining tetra-β-trifluoromethylphenoxyphthalocyanine cobalt (TfmpoPcCo) on the surface of MCNT@PPy through π-π interactions and used to construct NH3 sensor, while the MCNT@PPy were obtained by in-situ polymerization of PPy onto the side wall of MCNT. TfmpoPcCo not only adsorbs on the MCNT@PPy side wall due to its structure advantages, but also acts as a sensor accelerator. MCNT@PPy/TfmpoPcCo sensor show higher NH3 sensing performance than the MCNT, PPy and TfmpoPcCo under same operating conditions, especially in ultrafast response/recovery time (11.7 s/91.8 s to 50 ppm NH3) with low limit-of-detection of 11 ppb at room temperature (20 °C) and excellent gas selectivity. In addition, the MCNT@PPy/TfmpoPcCo sensor preserve superior gas response (26.2% to 50 ppm NH3) as well as outstanding stability over 60 days and humidity resistance. The excellent NH3 sensing properties are attributed to the excellent gas selectivity of TfmpoPcCo, the unique redox characteristics of PPy to ammonia (NH3), and the good electron transport ability and stability of MCNT. The synergistic optimization strategy of ternary materials is of great significance towards the exploration and construction of ideal materials for gas sensor in the future.

Structure Optimization of Carbon Nanotubes Based on Swarm Intelligence Algorithm and Evolutionary Computation

Swarm intelligence algorithm is an emerging evolutionary computing technology, which has become the focus of more and more researchers. It has a very special connection with artificial life, especially evolutionary strategies and genetic algorithms. The swarm intelligence algorithms you see include genetic algorithm, particle swarm optimization algorithm, and ant colony algorithm. This part of the content has been supplemented in the article. Evolutionary computing is a group‐oriented random search technology and method produced by simulating the evolutionary process of organisms in nature. Evolutionary computing is based on natural selection strategy: survival of the fittest, elimination of the unfit, and individuals with large fitness values have a higher survival probability than individuals with small fitness values. The purpose of this paper is to study the structure optimization of carbon nanotubes based on swarm intelligence algorithm and evolutionary computation. It is expected to optimize the structure of carbon nanotube materials with the help of intelligent evolution algorithm, so that it can be used in more fields. In this paper, the preparation process and principle of carbon nanotube‐based gas sensors are studied, and the preparation process of the side‐heated gas sensor is selected. This paper focuses on the strain sensing performance of carbon nanotubes, analyzes various parameters that characterize the sensing performance, and proposes feasible technical routes for improvement, optimization and improvement. The experimental results in this paper show that when different proportions of oxides are added, the tensile strength of carbon nanotube materials is increased by about 8%, and the elastic modulus is increased by up to 40%. After adding CNFs, the tensile strength increased by up to 18%, and the elastic modulus increased by up to 50%.

A Review of Inkjet Printed Graphene and Carbon Nanotubes Based Gas Sensors.

Graphene and carbon nanotube (CNT)-based gas/vapor sensors have gained much traction for numerous applications over the last decade due to their excellent sensing performance at ambient conditions. Inkjet printing various forms of graphene (reduced graphene oxide or modified graphene) and CNT (single-wall nanotubes (SWNTs) or multiwall nanotubes (MWNTs)) nanomaterials allows fabrication onto flexible substrates which enable gas sensing applications in flexible electronics. This review focuses on their recent developments and provides an overview of the state-of-the-art in inkjet printing of graphene and CNT based sensors targeting gases, such as NO2, Cl2, CO2, NH3, and organic vapors. Moreover, this review presents the current enhancements and challenges of printing CNT and graphene-based gas/vapor sensors, the role of defects, and advanced printing techniques using these nanomaterials, while highlighting challenges in reliability and reproducibility. The future potential and outlook of this rapidly growing research are analyzed as well.

Improving sensing properties of entangled carbon nanotube-based gas sensors by atmospheric plasma surface treatment

Entangled multi-walled carbon nanotubes (MWCNTs) on polyurethane (PUR) after Ar plasma-treatment and He plasma-treatment have been tested as gas sensors for ethanol sensing. It was found that plasma-treated sensors exhibit higher sensitivity compared to the non-treated samples along with different ethanol concentration. Non-treated sensors exhibit similar sensor response with the increase in ethanol concentration, while Ar plasma-treated sensors displays ~5 times improvement and He plasma-treated sensors show ~3 times improvement with an increase in ethanol concentration. The sensitivity of the plasma-treated sensors is also stable for following two-weeks after the preparation compared to the non-treated sensor. Entangled nanotube network exhibits a significant shift in the baseline resistance after both plasma-treatments. The response time of the sensor was increased after the plasma-treatment, while the recovery was rather quick. Surface analyses revealed that plasma-treatment did not make any significant morphological changes. Thus, the improvements in stability and sensitivity after plasma-treatment are attributed to the plasma-enhanced surface modification and formation of functional bonds on the surface of nanotubes, which are sensitive to the ethanol vapour.

Carbon Nanotube Based Gas Sensors toward Breath Analysis.

Breath analysis is a promising method for rapid, inexpensive, noninvasive disease diagnosis and health monitoring owing to the correlative relationship between breath biomarker concentrations and abnormal health conditions. However, current methods to identify and quantify breath components rely on large, bench-top analytical instruments. Carbon nanotube (CNT)-based gas sensors are desirable candidates to replace benchtop instruments because of their sensitive chemical-to-electrical transducer capability, high degree of chemical functionality options, and potential for miniaturization. This review seeks to give an overview of the synthetic methods used to functionalize CNT-based gas sensors, specifically those sensors that target biologically relevant breath markers. Specific examples are provided to highlight the sensing mechanisms behind different classes of CNT hybrid composites. Finally, the current challenges and prospective solutions of applying CNT-based sensors to breath analysis are discussed.

Controlling the Cross-Sensitivity of Carbon Nanotube-Based Gas Sensors to Water Using Zeolites.

Carbon nanotube-based gas sensors can be used to detect harmful environmental pollutants such as NO2 at room temperature. Although they show promise as low-powered, sensitive, and affordable monitoring devices, cross-sensitivity of functionalized carbon nanotubes to water vapor often obscures the detection of target molecules. This is a barrier to adoption for monitoring of airborne pollutants because of the varying humidity levels found in real world environments. Zeolites, also known as molecular sieves because of their selective adsorption properties, are used in this work to control the cross-sensitivity of single-walled carbon nanotube (SWCNT)-based sensors to water vapor. Zeolites incorporated into the sensing layer are found to reduce interference effects that would otherwise obscure the identification of NO2 gas, permitting repeatable detection over a range of relative humidities. This significant improvement is found to depend on the arrangement of the SWCNT-zeolite layers in the sensing device, as well as the hydrophilicity of the chosen zeolite.

Powerful greenhouse gas nitrous oxide adsorption onto intrinsic and Pd doped Single walled carbon nanotube

Density functional studies on the adsorption behavior of nitrous oxide (N2O) onto intrinsic carbon nanotube (CNT) and Pd-doped (5,5) single-walled carbon nanotube (Pd-CNT) have been reported. Introduction of Pd dopant facilitates in adsorption of N2O on the otherwise inert nanotube as observed from the adsorption energies and global reactivity descriptor values. Among three adsorption features of N2O onto CNT, the horizontal adsorption with Eads=−0.16eV exhibits higher adsorption energy. On the other hand the Pd-CNT exhibit strong affinity toward gas molecule and would cause a huge increase in N2O adsorption energies. Chemical and electronic properties of CNT and Pd-CNT in the absence and presence of N2O were investigated. Adsorption of N2O gas molecule would affect the electronic conductance of Pd-CNT that can serve as a signal of gas sensors and the increased energy gaps demonstrate the formation of more stable systems. The atoms in molecules (AIM) theory and the natural bond orbital (NBO) calculations were performed to get more details about the nature and charge transfers in intermolecular interactions within adsorption process. As a final point, the density of states (DOSs) calculations was achieved to confirm previous results. According to our results, intrinsic CNT cannot act as a suitable adsorbent while Pd-CNT can be introduced as novel detectable complex for designing high sensitive, fast response and high efficient carbon nanotube based gas sensor to detect N2O gas as an air pollutant. Our results could provide helpful information for the design and fabrication of the N2O sensors.

NO x gas detection characteristics in FET-type multi-walled carbon nanotube-based gas sensors for various electrode spacings

In this study, we fabricated a p-channel FET-type NOx gas sensor by using multi-walled carbon nanotubes (MWCNTs). Carbon nanotubes (CNTs) have good electronic, chemical-stability, and sensitivity characteristics. In particular, gas sensors require characteristics such as high speed, selectivity, and sensitivity. The fabricated sensor was used to detect NOx gas for different values of the gate-source voltage (Vgs) and the electrode spacings (30, 60, 90, and 120 μm). The gas sensor that absorbed NOx gas molecules showed a decrease in resistance. The sensitivity of the gas sensor was increased by increasing the electrode spacing. Additionally, while changing the Vgs and the temperature inside the chamber for the MWCNT gas sensor, we obtained the sensitivity and the normalized response for detecting NOx gas. We also obtained the adsorption energy (Ua) by using Arrhenius plots based on the reduction of resistance due to voltage variations. The adsorption energy was found to increase with increasing applied voltage.

Highly sensitive and selective carbon nanotube-based gas sensor arrays functionalized with different metallic nanoparticles

We present the fabrication and characterization of selective CNT-based gas sensor, which is capable of discriminating between four test gases (NH3, ethanol, CO and CO2). Different metals are utilized in order to alter the response of each sensing element towards the different test gases. We discuss as well the different possibilities for metal combinations in the sensor arrays. The optimum combination between the metallic nanoparticles is obtained through an efficient and quick framework. We study the effect of the thickness of the CNT thin-film on the reproducibility and the response of the sensor array. Furthermore, we investigate the response of the fabricated CNT sensor array towards a mixture of two gases. Such kind of characterization was performed by exposing the CNT sensor array to a mixture of (NH3, ethanol) and (NH3, CO) at different ratios for each mixture. We show that the response towards a mixture of two gases lies in the plane between the data points of each gas separately.

Highly efficient Ceramic-SWCNT based free standing films for gas sensing application

This paper presents a method of preparation of ceramic-carbon nanotube nanocomposite films using a gel-cast technique to improve the sensitivity of the carbon nanotubes based gas sensor for toxic gases (NH3, NO2) to a sub-ppm levels. A detailed study is presented on nanocompostite synthesis, sensing mechanism and performance of free standing nanocomposites film. The method is simple, low cost, environment friendly and allows batch fabrication process. The film was prepared by a motor driven machine fitted with doctor׳s blade where single wall carbon nanotube(SWCNT) powder dispersed in sol–gel based alumina solution was poured and rolled out on Mylar tape at specific speed to have control on film thickness. Electron microscopy, Energy-dispersive X-ray and Raman Spectroscopy were used to characterize the morphology and composition of prepared material. The proposed free standing nanocomposite film shows excellent and stable sensitivity to NH3 and NO2 molecules are 15% and 32% for 1ppm respectively

Enhanced hydrogen gas response of Pd nanoparticles-decorated single walled carbon nanotube film/SiO2/Si heterostructure

A new type carbon nanotube-based gas sensor: palladium nanoparticles decorated single walled carbon nanotube film/SiO2/Si (Pd-SWCNT film/SiO2/Si) heterostructures were fabricated by a simple and practical filtration method. When used for hydrogen (H2) sensing, the Pd-SWCNT film/SiO2/p-Si heterostructure shows very high H2 response, which is tens of times higher than that of Pd-SWCNT film resistance-type H2 sensor in this paper and is superior to those of carbon nanotube-based resistance-type H2 sensors reported previously. The mechanism of the enhanced H2 response can be explained by thermionic emission theory and interfacial effect.

Analytical modeling and simulation of I–V characteristics in carbon nanotube based gas sensors using ANN and SVR methods

As one of the most interesting advancements in the field of nanotechnology, carbon nanotubes (CNTs) have been given special attention because of their remarkable mechanical and electrical properties and are being used in many scientific and engineering research projects. One such application facilitated by the fact that CNTs experience changes in electrical conductivity when exposed to different gases is the use of these materials as part of gas detection sensors. These are typically constructed on a field effect transistor (FET) based structure in which the CNT is employed as the channel between the source and the drain. In this study, an analytical model has been proposed and developed with the initial assumption that the gate voltage is directly proportional to the gas concentration as well as its temperature. Using the corresponding formulae for CNT conductance, the proposed mathematical model is derived. artificial neural network (ANN) and support vector regression (SVR) algorithms have also been incorporated to obtain other models for the current–voltage (I–V) characteristic in which the experimental data extracted from a recent work by N. Peng et al. has been used as the training data set. The comparative study of the results from ANN, SVR, and the analytical models with the experimental data in hand shows a satisfactory agreement which validates the proposed models. However, SVR outperforms the ANN approach and gives more accurate results.

Functionalized Carbon Nanotubes-Based Gas Sensors for Pollutants Detection: Investigation on the Use of a Double Transduction Mode

With an objective to fabricate Carbon nanotubes (CNTs) based sensors, the solution route is investigated. The dispersion routes are chosen here to avoid the CNTs to form bundles which can reduce their surface area. The results show that SWNTs-based gas sensors made by the surfactant method is possible if the annealing temperature is correctly chosen. The use of a surfactant allows preparing sensing layers which present responses to NO2 exposure in the 50-200 ppb Range. In a second procedure the CNTs are noncovalently functionalized and used as sensing material for BTX (Benzene, Toluene and Xylenes) detection. The noncovalent functionalisation occurs through p-p stacking between the SWNTs framework and the highly delocalized π-system of the macrocycle which are phthalocyanines and porphyrines derivatives. The SWNTs materials are characterized by standard techniques (UV-Vis spectroscopy, TGA, TEM, Raman analysis). For BTX detection, we used a double transduction mode: IDEs (Interdigitated electrodes) and QCM (Quartz Crystal Microbalance) in order to get insight into the sensing mechanism.

Metallic nanoparticles functionalizing carbon nanotube networks for gas sensing applications

We report the fabrication of carbon nanotube (CNT) based gas sensors functionalized with different metallic nanoparticles (NPs) (Au, Pd, Ag) with exceptionally high responses towards four test gases (NH3, CO2, CO and ethanol). The CNT networks were fabricated through a low cost spray deposition process while the NPs were deposited by a thermal evaporation process. CNT based gas sensors functionalized with Au with a nominal thickness of 1.0 nm showed superior response towards NH3, CO and ethanol. The sensors’ normalized responses reached 92%, 22% and 32% with concentrations of 100 ppm, 50 ppm and 100 ppm for NH3, CO and ethanol respectively. CNT based gas sensors functionalized with Pd with a nominal thickness of 1.5 nm showed the best performance with CO2. The normalized response reached 3%, 6%, 12% and 17% with concentrations of 500 ppm, 1000 ppm, 2500 ppm and 5000 ppm of CO2 respectively. We also investigated the morphological and optical changes that occur to the NPs upon thermal treatment. Functionalization of CNT films deposited on glass with Au and Ag showed surface plasmon resonance effects that are dependent on the nominal thickness of the functionalization layer.

Flexible Carbon Nanotube Based Gas Sensors Fabricated by Large-Scale Spray Deposition

Carbon nanotubes (CNTs) are demonstrated as one of the most interesting materials for gas sensing. The possibility to obtain CNT thin-films via solution-processing techniques has paved the way for low-cost applications of such sensors. In this paper, we demonstrate that high performance gas sensors based on CNT thin-films can be obtained by direct spray deposition onto flexible substrates. The results obtained for NH3 sensing using CNT films deposited on polyimide show exceptionally high as well as immediate response to the test gas, with performance comparable with that obtained on oxidized silicon substrates. In addition, a good repeatability of the sensor response to defined gas concentrations is demonstrated. This represents a major step toward low-cost large-scale production of this class of devices.

Role of the Material Electrodes on Resistive Behaviour of Carbon Nanotube-Based Gas Sensors for Detection

Miniaturized gas-sensing devices that use single-walled carbon nanotubes as active material have been fabricated using two different electrode materials, namely, Au/Cr and NbN. The resistive sensors have been assembled aligning by dielectrophoresis the nanotube bundles between 40 μm spaced Au/Cr or NbN multifinger electrodes. The sensing devices have been tested for detection of the H2S gas, in the concentration range 10–100 ppm, using N2as carrier gas. No resistance changes were detected using sensor fabricated with NbN electrodes, whereas the response of the sensor fabricated with Au/Cr electrodes was characterized by an increase of the resistance upon gas exposure. The main performances of this sensor are a detection limit for H2S of 10 ppm and a recovery time of few minutes. The present study suggests that the mechanism involved in H2S gas detection is not a direct charge transfer between molecules and nanotubes. The hypothesis is that detection occurs through passivation of the Au surfaces by H2S molecules and modification of the contact resistance at the Au/nanotube interface.

Functionalized Carbon Nanotubes Based Gas Sensors: Effects of Heat and Vacuum Treatment

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