Carbon Nanotube Gas Sensor Research Articles

Dynamic Response and Swift Recovery of Filament Heater‐Integrated Low‐Power Transparent CNT Gas Sensor

AbstractDesigning a transparent carbon nanotube (CNT) gas sensor for nitrogen dioxide (NO2) detection at room temperature (RT), which is unaffected by humidity, is a critical challenge in various technologies. To solve this issue, a filament‐based memristor heater (MH) embedded low‐power CNT gas sensor is proposed to address humidity‐related issues, and dynamic response with a low power consumption of 6.15 µW and swift recovery of <1 ms/30.8 µW is successfully demonstrated, without any serious effects on humidity. This study reveals that the MH can effectively heat the surface of the active region due to the joule heating effect, which improves the absorption and desorption of the target gases and mitigates the influence on the humidity, which is in a response that is below 7.5%. As a result, it is believed that the proposed MH‐embedded transparent CNT gas sensors can address the humidity issues and slow response/recovery with ultralow‐power consumption and high‐accuracy gas sensing characteristics.

A study of strain effect on stretchable carbon nanotube gas sensors

A study of strain effect on stretchable carbon nanotube gas sensors

Effective detection of early Citrus Huanglongbing by polyethyleneimine modified multi-walled carbon nanotubes gas sensor

Citrus Huanglongbing (HLB) is incurable, so it is necessary to develop an accurate and timely detection method for citrus HLB. Volatile organic compounds (VOCs) of plants can be used as an indicator of their health status. Based on the difference of VOCs between healthy and HLB citrus leaves, we designed the polyethyleneimine (PEI)-modified carboxylic multi-walled carbon nanotubes (CNT-COOH) gas sensor and used it in the diagnosis of citrus HLB. Our PEI/CNT-COOH sensor showed significant impedance responses to four citrus HLB biomarkers VOCs (ethylhexanol, phenylacetaldehyde, linalool and methyl salicylate) with sensitivity up to single-digit ppm. We then used the sensor to detect the VOCs in citrus leaves, and found that the diseased citrus can be distinguished from the healthy citrus according to the different response values of the sensor, successfully identified HLB-infected citrus. Compared with the conventional qPCR method, PEI/CNT-COOH sensor not only has higher accuracy, but also can be used in real time field detection of citrus leaves due to its less stringent operating condition and portability.

Rate kinetics and isotherm study of step functionalised carbon nanotubes gas sensor for NO2 adsorption

The rate kinetics of adsorption of environmentally hazardous gas NO2 on pristine and SnO2 functionalized carbon nanotubes was studied using pseudo first and second order kinetic models, Elovich model and intraparticle diffusion model. Pristine carbon nanotubes were functionalized with 0.1 mg, 0.2 mg and 1 mg SnO2. The experimental data obtained for the exposure of NO2 (concentrations varying from 2 ppm to 20 ppm) to pristine and SnO2 functionalized nanotubes had been correlated with rate laws to understand the adsorption process in the duration of the reaction. Of the various kinetic models tested, the experimental data was seen to fit best with the intraparticle diffusion model. The straight line obtained for intraparticle diffusion model suggested that adsorption process in the present study might be mainly governed by intraparticle diffusion. Henry, Langmuir and Freundlich isotherm models also presented a good data fit for the studied NO2 adsorption with Freundlich model giving the highest regression coefficient. The obtained constants clearly indicate the improved adsorption process in terms of the rate of the reaction and adsorption capacity for functionalized carbon nanotubes as compared to pristine with an optimum response obtained at 0.1 mg SnO2. This may be attributed to increase in active nano sites and heterogeneity in functionalized carbon nanotubes.

Optimizing electrode structure of carbon nanotube gas sensors for sensitivity improvement based on electric field enhancement effect of fractal geometry

With the rapid development of carbon nanotubes gas sensor, the sensitivity of the sensing response is becoming more and more demanding. Different from the traditional studies on gas-sensitive materials, this paper combines the microscopic dimensional effects and physical properties of fractal geometry theory from the structure and morphology of sensor devices. The electrode structures of carbon nanotubes gas sensor is designed and optimized by Hilbert–Piano curve. Simulation experiments demonstrate that the electric field intensity and hot spot distribution of the fractal electrode are superior to those of the traditional interdigital electrode. Moreover, a novel chemiresistive gas sensor is fabricated combining the characteristics of carbon nanotubes and fractal geometry, and a test with exposure to nitric oxide showed that the sensors with fractal electrode structures improved the gas sensing sensitivity over sensors with traditional geometrical structures. It provides a new idea for the exploration of gas sensing technology.

Black phosphorus–carbon nanotube gas sensor for detection of dimethyl methylphosphonate

As a nerve agent simulant, the effective detection of dimethyl methylphosphonate (DMMP) is essential for the development of toxic gas sensors. Herein, a resistive gas sensor was developed for the DMMP. The gas-sensitive material in the sensor includes black phosphorus (BP), carbon nanotubes (CNT), and black phosphorus–carbon nanotube (BP–CNT) composite materials. The sensor was prepared by making a solid raw material into slurry and coating it on a microelectromechanical systems (MEMS) sensor substrate. The size of the MEMS sensor was 1 × 1 × 0.3 mm. The sensitivity ranges of the CNT, BP and BP–CNT sensors are 0.3%–1.6%, 4.2–13.5% and 1.8–4.8% respectively in the concentration range of 1.5–30 mg/m3.

Understanding and improving carbon nanotube-electrode contact in bottom-contacted nanotube gas sensors

• Carbon nanotube-metal contact is improved by electrode surface etching with Ar-ion. • Self-heating operation of suspended carbon nanotube gas sensors is facilitated. • Surface analysis identifies main causes of poor nanotube-metal contact. Suspended carbon nanotube field-effect transistors fabricated with a dry transfer technique demonstrate strong promise as ultra-low-power, hysteresis-free gas sensors. However, the difficulty of establishing a good electrical contact between a nanotube and the electrode surface often limits the yield of low-resistance devices that can operate as low-power gas sensors. In this work, the contact resistance at the nanotube-metal interface and the distribution thereof are reduced significantly by removing the top layer of electrode surface with Ar-ion etching directly before nanotube placement. Combined with post-transfer annealing, this pre-transfer electrode surface cleaning reduces the median ON-resistance of transistors by an order of magnitude--from 1.56 MOhm to 143 kOhm--and the interquartile range by more than two orders of magnitude--from 9.38 MOhm to 59 kOhm. The ability to consistently improve nanotube-metal contact demonstrated in this work is a significant advance in the fabrication of ultraclean nanotube transistors and carbon nanotube gas sensors.

Enhancement of functionalized carbon nanotubes gas sensor by adding metal oxide nanoparticles

Functionalized-multi wall carbon nanotubes (F-MWCNTs) and functionalized-single wall carbon nanotubes (F-SWCNTs) were well enhanced using CoO Nanoparticles. The sensor device consisted of a film of sensitive material (F-MWCNTs/CoONPs) and (F-SWCNTs/CoO NPs) deposited by drop- casting on an n-type porous silicon substrate. The two sensors perform high sensitivity to NO2 gas at room temperatures. The analysis indicated that the (F-MWCNTs/CoONPs) have a better performance than (F-SWCNTs/CoONPs). The F-SWCNTs/CoONPs gas sensor shows high sensitivity (19.1 %) at RT with response time 17 sec, while F-MWCNTs/CoONPs gas sensor show better sensitivity (39 %) at RT with response time 13 sec. The device shows a very reproducible sensor performance, with high repeatability, complete recovery, and adequate response. A demonstration of the improvement in sensing of NO2 gas using CoO-functionalized nanotubes is provided.

Effect of Humidity on Gas Sensing Performance of Carbon Nanotube Gas Sensors Operated at Room Temperature

Carbon nanotubes (CNTs) have shown promising results for gas sensing due to their high surface area. Since humidity has a great impact on the electrical conductivity of resistive CNT gas sensors, we have investigated the change of humidity on their sensing properties. In this study, we fabricated vertically aligned CNT-based gas sensors for the detection of volatile organic compounds. The morphologies and phase structures of the fabricated samples were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR), confirming the presence of CNT with some surface impurities. It was found that a relative humidity increase from 10% to 80% can reduce the electrical conductivity of the sensor by around 4%. On the other hand, for a humidity above 80% the conductivity increased slightly. The fabricated device has been used as a gas sensor for volatile organic compounds, and the cross-sensitivity to humidity was investigated. It was found that in the fabricated sensors, a change in humidity up to 80% results in a 40% decrease in the response for the studied organic compounds.

Response of Multi-Walled Carbon Nanotube Gas Sensor Based on Langmuir Isotherm Model

Response of Multi-Walled Carbon Nanotube Gas Sensor Based on Langmuir Isotherm Model

The Functionalized Single-Walled Carbon Nanotubes Gas Sensor With Pd Nanoparticles for Hydrogen Detection in the High-Voltage Transformers.

Single-walled carbon nanotubes (SWCNTs) have been widely discussed and applied as novel gas sensing nanomaterials. Hydrogen is one of the remarkable fault characteristic gases in high-voltage oil-paper insulated transformers. In this paper, 3.07 wt% Pd nanoparticles (NPs) were used to decorate SWCNTs. The unloaded, the carboxylated, and the Pd-doped SWCNTs were fabricated into three planar gas sensors, and their gas sensing properties to hydrogen were studied. Gas sensing mechanism was analyzed. Results show that the optimal operating temperature of a Pd-doped SWCNTs-based gas sensor is 125°C lower than that of the unloaded SWCNTs-based gas sensor, and it shows the highest gas sensing response value. This is attributed to the decreasing work function of Pd, which reduces the hole carries in the nanotubes.

Transient and steady-state readout of nanowire gas sensors in the presence of low-frequency noise

Nanowire sensors show great promise in a variety of sensing applications due to their potential for high sensitivities. Practical nanowire sensor systems, however, are often limited by low-frequency, 1/f noise. This work presents theoretical and experimental results comparing the performance metrics of sensing schemes using transient and steady-state parameters in the presence of 1/f noise. Criteria are derived for when the considered transient or steady-state sensing schemes will have a better signal-to-noise ratio (SNR). The theoretical results for the SNR of these sensing schemes are applied to experimental data from carbon nanotube NO2 sensors. These data and theoretical results demonstrate that due to the Langmuir binding behavior of the sensor-analyte system, sensing using the considered transient parameters increases linearity and decreases response time relative to steady-state sensing. Noise analysis further shows that with current devices, transient sensing has a lower SNR relative to steady-state sensing, however this may change if functionalization is considered. The use of transient parameters also has the potential to reduce sensor drift due to 1/f noise, improving system stability. In addition to providing useful considerations towards the design of carbon nanotube gas sensors, these results are relevant towards understanding the SNR of other chemical and biological sensors limited by 1/f noise.

Time stability of carbon nanotube gas sensors

In this communication, we report on the long-term stability of a carbon nanotube (CNT) thin-film based NH3 gas sensor. These sensors were fabricated with a reproducible, large-scale, low-cost spray deposition process and are examined in terms of NH3 sensitivity and long-term stability. The devices are measured directly after fabrication; 1-year and 4-years post fabrication while being stored in ambient conditions without further encapsulation. The retainment of up to 96% of response demonstrates the stability of such CNT-based devices. Hence, it solves a critical issue for stepping beyond the proof of concept stage. Not only can CNT-based gas sensors be utilized for years, but such sensors also do not require any special recalibration over the years.

Ni-CNT Chemical Sensor for SF₆ Decomposition Components Detection: A Combined Experimental and Theoretical Study.

SF6 decomposition components detection is a key technology to evaluate and diagnose the insulation status of SF6-insulated equipment online, especially when insulation defects-induced discharge occurs in equipment. In order to detect the type and concentration of SF6 decomposition components, a Ni-modified carbon nanotube (Ni-CNT) gas sensor has been prepared to analyze its gas sensitivity and selectivity to SF6 decomposition components based on an experimental and density functional theory (DFT) theoretical study. Experimental results show that a Ni-CNT gas sensor presents an outstanding gas sensing property according to the significant change of conductivity during the gas molecule adsorption. The conductivity increases in the following order: H2S > SOF2 > SO2 > SO2F2. The limit of detection of the Ni-CNT gas sensor reaches 1 ppm. In addition, the excellent recovery property of the Ni-CNT gas sensor makes it easy to be widely used. A DFT theoretical study was applied to analyze the influence mechanism of Ni modification on SF6 decomposition components detection. In summary, the Ni-CNT gas sensor prepared in this study can be an effective way to evaluate and diagnose the insulation status of SF6-insulated equipment online.

Carbon nanotube thin-film-transistors for gas identification

Single-walled carbon nanotube (CNT) based gas sensors have enormous potential in pollution monitoring in low concentration level because of its high sensitivity, fast response, and physical/chemical stability. However, the lack of selectivity has been a major drawback for its wide range employment. In this work, we fabricate thin film transistors (TFTs) using randomly distributed CNTs and investigate them for ammonia and nitrogen dioxide detection in air at low ppm concentrations. A sensing mechanism is proposed based on the interaction between gas molecules and different types of dwelling spots inside the channel area of a TFT. We present double exponential-convolution model to decipher sensor response as well as to explore its application in gas identification. In this context, the consistency in time constants is recognized, which is independent of gas concentration. More importantly, the time constants vary with respect to different gas types and TFTs. The uniqueness of time constants can work as identity verification for different sensing gases, which demonstrates that the sensor response is a distinctive behavior determined by the unique channel structure of each TFT. This work provides us a general strategy for gas identification in ppm level and a practical path to realize the advantages of CNT gas sensors in air quality detection as well as the industrial emission control.

The porcine odorant-binding protein as molecular probe for benzene detection.

In recent years, air pollution has been a subject of great scientific and public interests for the strong impact on human health. Air pollution is due to the presence in the atmosphere of polluting substances, such as carbon monoxide, sulfur and nitrogen oxides, particulates and volatile organic compounds (VOCs), derived predominantly from various combustion processes. Benzene is a VOC belonging to group-I carcinogens with a toxicity widely demonstrated. The emission limit values and the daily exposure time to benzene (TLV-TWA) are 5μg/m3 (0.00157 ppm) and 1.6mg/m3 (0.5 ppm), respectively. Currently, expensive and time-consuming analytical methods are used for detection of benzene. These methods require to perform a few preliminary steps such as sampling, and matrices pre-treatments. In addition, it is also needed the support of specialized personnel. Recently, single-walled carbon nanotube (SWNTs) gas sensors with a limit detection (LOD) of 20 ppm were developed for benzene detection. Other innovative bioassay, called bio-report systems, were proposed. They use a whole cell (Pseudomona putida or Escherichia coli) as molecular recognition element and exhibit a LOD of about 10 μM. Here, we report on the design of a highly sensitive fluorescence assay for monitoring atmospheric level of benzene. For this purpose, we used as molecular recognition element the porcine odorant-binding protein (pOBP). 1-Aminoanthracene was selected as extrinsic fluorescence probe for designing a competitive fluorescence resonance energy transfer (FRET) assay for benzene detection. The detection limit of our assay was 3.9μg/m3, a value lower than the actual emission limit value of benzene as regulated by European law.

Analysis on Mars Atmospheric Composition Probing Technology in the Future

Mars atmospheric composition probing is one of critical tasks of mars exploration. The mars atmosphere may have the existence of water, methane, organic matter, conducting mars atmospheric composition probing is very critical to studying whether the mars has, or once had the existence of living matter, analyzing the forming, evolution of mars atmosphere as well as its change with season. Restricted by launching technology, platform technology, mars exploration task has very strict requirement to weight, volume and power consumption of probing load. At present, the mars atmospheric composition in-situ probing load in abroad usually adopts mass spectrometry with load weight from several kilogram to tens kilogram, and power consumption from several watts to tens watts. Carbon Nanotubes (CNTs) has large surface area with sensitivity to gas absorption reaction, and can be used to prepare the high performance gas sensor with high sensitivity, quick response, light weight, small volume, and low power consumption, and it is possible to realize atmospheric composition probing load with 100g weight and milliwatt power consumption. CNTs gas sensor technology shall be the critical development trend of mars atmospheric exploration technology in the future.

A High-Sensitivity Hydrogen Gas Sensor Based on Carbon Nanotubes Fabricated on Glass Substrate

A hydrogen (H2) gas sensor had been fabricated using a carbon nanotube (CNT) which was produced via a conventional microwave oven. It was deposited onto a glass substrate between two Palladium (Pd) electrodes using the dielectrophoretic deposition method. The hydrogen sensing capability of the multiwall carbon nanotube (MWCNT) gas sensor was determined at different operating temperatures and gas concentrations, ranging from 20 ppm to 1000 ppm. The results indicate that the gas sensor exhibits high sensitivity, up to 240% upon exposure to 1000 ppm of H2 at room temperature (RT). Additionally, at room temperature, the sensor shows an excellent sensitivity of 31% when exposed to a very low H2 gas concentration of 20 ppm. The fabricated MWCNT sensors displayed excellent reproducible and reversible responses towards hydrogen at room temperature. Furthermore, MWCNTs were fabricated onto a glass substrate for the detection of H2 gas without the supplementary assistance of metal decoration, vacuum system, or additional film coatings. The high response, low cost, fast recovery, and low power consumption of the fabricated MWCNTs gas sensor validates its superior potential for H2 gas sensing ability at low temperatures.

Ag-Decorated Multi-Wall Carbon Nanotubes for Acetone Gas Sensing

In this study, room-temperature Acetone gas platform with enhanced sensitivity, superfast response and recovery, and good stability, using Ag nanocrystal-functionalized multiwalled carbon nanotubes. High-density carbon nanotubes (CNTs) are grown on an oxidized Si substrate by thermal chemical vapor deposition (CVD). The average length and diameter of the CNTs were about ~4.52μm and ~45nm, respectively. Ag nanoparticles were coated onto nanotube surfaces to a thickness of approximately 5nm and the CNTs thus formed could be utilized to sense acetone vapor. Functionalized CNT gas sensors exhibited better performances compared to unmodified CNTs, making them highly promising candidates for environmental air monitoring applications.

Transfer-Free In Situ CCVD Grown Nanocrystalline Graphene for Sub-PPMV Ammonia Detection

Reasonably-priced devices for the detection of toxic species in the atmosphere are critical for reasons of health. Previous research work shows the promising detection capabilities of graphene. Thus, we demonstrate the gaseous response of our nanocrystalline graphene field-effect transistors that can be fabricated hundredfold on a two inch substrate by our transfer-free in situ catalytic chemical vapor deposition process. By means of Raman spectroscopy and near edge X-ray absorption fine structure nanocrystallinity of the CCVD grown graphene films within the devices can be confirmed. Using a self-constructed vacuum probing station the sensitivity of the fabricated devices is extracted from dynamic electrical sampling measurements. With respect to ammonia it is found that the sensitivity is being higher than previously reported from other groups. Moreover, a comparable responsivity is achieved. A deeper understanding of the origin of the high sensitivity, which we attribute to the nanocrystallinity, is given by backgate input characteristics under varying ammonia concentration as well as from comparison with literature results on carbon nanotube gas sensors. Furthermore, the origin and influence of ammonia on the hysteresis of our nanocrystalline graphene field-effect transistors is discussed.