Improved Methanol Detection Using Carbon Nanotube-Coated

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Improved Methanol Detection Using Carbon Nanotube-Coated Carbon Fibers Integrated with a Split-Ring Resonator-Based Microwave Sensor Sandeep Kumar Singh, Prakrati Azad, M. J. Akhtar, and Kamal K Kar ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00965 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 12, 2018

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ACS Applied Nano Materials

Improved Methanol Detection Using Carbon Nanotube-Coated Carbon Fibers Integrated with a Split-Ring Resonator-Based Microwave Sensor Sandeep Kumar Singh†, §, *, Prakrati Azad†, §, M.J. Akhtar †, ‡, Kamal K. Kar †, ζ †

Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur-208016, Uttar Pradesh, INDIA ‡

Microwave Imaging and Material Testing Laboratory, Department of Electrical Engineering, Indian Institute of Technology Kanpur, Kanpur-208016, Uttar Pradesh, INDIA ζ

Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur-208016, Uttar Pradesh, INDIA §: Both authors have contributed equally to this work.

KEYWORDS: Split ring resonator (SRR), Gas sensing, Carbon nanotube coated carbon fiber (CNTCF), Resonant, methanol.

Abstract A novel microwave sensor based on the electrically small split ring resonator (SRR) integrated with the hierarchal carbon nanotube coated carbon fiber (CNTCF) for the gas sensing applications is proposed. The CNTCF is synthesized via catalytic chemical vapor deposition (CVD) process, and investigated by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for their structural information. The CNTCF integrated SRR is excited by the electromagnetic field through high impendence microstrip line. The use of highly conductive CNTCF as sensing material facilitated extremely large surface area causing a substantial change in the inherent resonant frequency as a sensor response at room temperature (25 °C) for methanol, nitrogen dioxide (NO2), ethanol and chloroform gases. This change in resonant frequency facilitates the detection and sensing of the analyte gases under controlled environment and constant humidity of 32% RH. The CNTCF-SRR sensor exhibited highly

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selectivity and sensitivity towards the methanol gas with large resonance frequency shift of 400 MHz and gas sensitivity of 4.2±0.03 % at 300 ppm. Both response and recovery times are on the order of seconds for the gas concentration ranging from 100-300 ppm. In addition, sensor exhibited good reproducibility and stability with variations 0.03% and 2%, respectively. The plausible gas sensing mechanism of CNTCF-SRR sensor has been discussed. Therefore, in view of the abovementioned sensor responses it is speculated that the proposed CNTCF integrated planar microwave sensor can effectively be used for selective sensing of methanol gas in harsh condition.

Introduction Gas sensing and its associated technology has become a very important facet of the present research, especially in environmental, biomedical and industrial applied science, where the primary purpose is to find the minimal presence of analyte gas in a special controlled environment or to monitor the presence of pollutant gases in the ambient1-2. Methanol is considered one of the toxic gas and its severe presence can result in instant bronchial constriction and narrowing of the airways3-5. Over-exposures to the investigated animals have caused burning into eyes and disturbed the metabolism4. The major sources of methanol in environment are fossil fuels, agricultural waste, municipal garbage, wood and varied biomass

6-7

.Therefore, the

development of highly sensitive, selective and durable methanol gas sensor has become hotspot topic for the scientific community. Practically, a gas sensor should be in compact size at low price, suitable for the high selectivity, high sensitivity, fast responses, and useful for the room temperature (RT) working1, 8. Room temperature operation is one of the most crucial aspect for sensing devices as it reduces the operating cost and makes the assembly simpler and more portable.8-10 Recently, the carbon based nanomaterials such as carbon nanowire, carbon


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nanofibers, nanotube and nano particles have become the potential candidate for sensing devices due to their excellent sensing properties at low temperature11-13. Among them, carbon nanotubes (CNTs) have gained huge attention as they possess extremely high surface to volume ratio and hollow morphology that resulting high sensitivity, fast response, stable behavior, compact size, good thermal stability, strong reproducibility and ease of operation at room temperature 11-13. In a pioneer work by Zhang group, they reported CNTs based electrode gas sensor with excellent selectivity and high sensitivity even for 1 ppm of low methanol concentration.14 Li et al fabricated CNT-PMMA composite based methanol gas sensor and reported significant sensitivity and fast responses while working at room temperature15. Although, these sensors reported high sensitivity, but they used large CNT amount for sensor fabrication, which is very costly material and thus, increase the overall cost of the device. Besides, the agglomeration of CNTs in the form of bundles has been a major drawback, which resulted reduced apparent specific surface area and consequent decay in sensing performances16-17. Recently, Jayesh et al have used CVD method to synthesize CNTs directly on carbon fiber (CF), which help to avoid the agglomeration problem17. The modern gas sensing technologies primarily depend on the electrical and ionization response of target material-gas interaction18-19. In the case of ionization gas sensors, the sensing procedure mainly occurs due to ionization of gases and thus, this technology may easily detect even low absorption energy and inert gases. However, the bulky size, the high power consumption, and higher breakdown voltage are some major drawbacks that limit the practical uses of the ionization sensors18-19. The sensors based on change in electrical properties are simple to build and usually exhibit high sensitivity. Their sensing mechanism relies on charge transfer between the target gases and sensing materials that leads to change the resistance of the device18-19. Recently, Rong et al fabricated electrical properties based molecular imprinted



silver doped LaFeO3 core shell sensor, which exhibited excellent sensing responses for 5 ppm methanol at 195 °C5. Li et al reported SnO2-Pd-Pt-In2O3 composite structure with 0.1 ppm of low detection limit for methanol gas detection at 160 °C20. Ghosal et al prepared rGO/TiO2 nanotube base gas sensor with good response of 10-700 ppm methanol gas at 50 °C21. Zhang et al reported excellent sensitivity achieved by Co-Fe2O3/SmFeO3 for methanol detection with 5 ppm of detection limit at 155 °C22. Yang research group fabricated 3D flower and 2D sheet-like CuO nanostructures with excellent sensing performances for ethanol and ethyl-acetate at 260 °C23. In light of these recent reports, it is clear that the stated sensors have achieved high sensitivity while working on high temperatures, which requires high cost and hence, making them less popular in market. Now a days, microwave and radio frequency (RF) techniques have become hotspot in many industrial and bio-medical applications due to their simpler assembly, ease to operate at room temperature, and non-invasive and non-destructive nature24-26. These properties have already attracted researchers to use microwave techniques for different sensing applications owing to their good sensitivity and accuracy. However, for gas sensing applications, the microwave technique is not fully explored due to the fact that the planar microwave-based sensors are usually fabricated on non-reactive materials. In other words, the substrate material and the conductive coating, which together constitute any modern planar microwave sensors, do not react with gases. Thus, conventional microwave sensors cannot be employed for gas sensing. Due to above mentioned reasons, some gas sensing materials should be incorporated into the microwave sensor so that these specialized materials interact with analyte gases and cause significant change in the microwave parameters of the designed sensor. The measured parameters in the RF and microwave frequency range are usually the scattering coefficients and the resonant frequency of the device. Hence, a small change in these parameters due to the


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interaction of gases with sensing material is observed which can be detected using the RF instrument1. The microwave planar sensors have been used in the recent past to test a number of solids and liquid samples for various applications24, 27-29. Interestingly, the microwave technique has a number of advantages over other conventional gas sensing techniques such as chemical methods30. For example, the designed RF sensor can be used to detect multiple gases that requires only small sensing area and minuscule quantity of the sensing material (particularly in the high frequency regime). This is especially advantageous under the situation when the high cost material required for sensing of the gases, which ultimately helps in reducing the cost of sensor operation. In addition, the RF technique considered a robust system involving no chemical reactions, and also the overall procedure is non-destructive. Recently, Chopra et al. have analyzed the microwave circular disk resonator sensor with single and multi-walled nanotubes for sensing the ammonia with minimum detection level of ~650 ppm30. Similarly, Kim et al. have designed a DSRR based RF sensor with conducting polymer (PEDOT: PSS) coated inside the ring of resonator with the detection level of the order to 100 ppm for ethanol gas at RT31. Further, Lee et al. have employed the reflection type variable attenuator coated with the conducting polymer for sensing of ethanol gas at 28 °C32. Rossignol et al. have studied the coplanar waveguide-based sensor with phthalocyanine film coated on its surface in order to detect different levels of toluene and ammonia gases33. Whereas, Bailly et al. have observed that incorporating TiO2 nanoparticles with the microstrip inter-digital capacitor provides significant sensing behavior for NH3 at the microwave frequency34. From above discussion, it is observed that most of the earlier designed conductive/nano-materials based microwave sensors are not versatile enough to detect multiple gases with a reasonable amount of



sensitivity at RT. In addition, the conductive/nano-materials used by most of the research groups in the past are not very easy to fabricate in the lab environment and the overall synthesis procedure of these nano-materials is quite complicated, expensive and lacks in efficiency. In this work, the RF planar gas sensors have been designed for the purpose of qualitative and quantitative sensing of methanol gas. The RF/microwave sensor in the presented work is designed using the double split ring resonators (SRR), which are basically small electrical resonators and have become quite popular in recent years due to their compact structure and high sensitivity for sensing dielectric materials. The SRR based microwave sensors can be used for dielectric solids and solvents, but it is quite difficult to use them for gases as they are conventionally designed using simple dielectric substrates and metal coating, which are usually non-reactive with gases1, 24, 27-29. Hence, for gas sensing, conventional microwave sensors should be loaded with specialized materials, which can react with gases. One of the main advantages of the present work is that it includes a very small amount (micrograms) of gas sensing nanostructured material inside the inner ring of the double SRR structure in order to obtain significant sensing property. Importantly, the most common gases like C2H2, N2 and H2 are used for the synthesis of CNTs on CF. In the composite, the amount of CNT is also very little and major component is CF, which is cheap material. Therefore, from the overall cost to performance ratio the material is cost effective and desirable for the device. Another important advantage of this work is that it uses very simple and effective technique for integration of the sensing material with the SRR based microwave sensor requiring very less time for coating. These features make overall CNTCF integrated SRR microwave sensor cost effective.

Experimental section


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Synthesis of hierarchical CNT coated carbon fiber (CNTCF): The complete synthesis process of CNTCF is already reported in our previous work35. In detail, first, nickel particles (30-100 nm) were coated on heat treated CF surfaces using electroless coating method, and kept in a horizontal quartz tube reactor17, 35. Then, the mixture of N2, H2 and C2H2 gases were flowed in the CVD chamber with 40/100 sccm of flow rate for 20 minutes at 700 °C of temperature under normal pressure condition (1 Atmospheric pressure). The chamber was then cooled down to room temperature under nitrogen environment. After completion of growth processes, the as synthesized CNTCFs were taken out from the CVD reactor for the further characterizations. A schematic is portrayed in Figure S2, which exhibits the complete processes involved in the synthesis of the CNTCF material. Characterizations. Scanning electron microscope (SEM; Zeiss EVO MA-15) was carried out on CNTCF and CF to scrutinize their morphologies. In addition, to investigate the structural information of CNTCF sample, the transmission electron microscopes (TEM, HR-TEM), FEI Titan G2 60 -300) characterization was performed. Fourier transform infrared spectroscopy (FTIR) measurement was carried out via infrared spectrometer (Perkin Elmer Spectrum 1) in the range 4000-500 cm−1 using transmission mode spectrum. The X-ray photoelectron spectroscope (XPS; PHI 5000 Versa Probe II, FEI Inc.) was utilized to analyze the presence of chemical bonding states on the surface of CNT coated CF. S-parameter of the SRR sensing device was measured through Agilent vector network analyzer (VNA) E8364B in the frequency range of 8.2-12.4 GHz. Measurement Set-up: The CNTCFs integrated microwave SRR sensor (design and fabrication details are provided in supplementary information) is kept inside the gas-controlled chamber. The complete setup



consists of the microwave gas sensor inside the gas chamber with two cables coming out of the closed gas chamber and connected to the VNA for measurement. The experimental set-up containing the RF sensor is then exposed to analyte gases one at a time, and changes in resonant frequency and the transmission coefficient are recorded as a sensor response.

Figure 1. Gas sensing measurement setup. In detail, the gas chamber is made up of 3 mm thick Perspex® sheet using sophisticated laser cutting process. For the measurement, the chamber was made in rectangular shape with dimension 15 X 10 X 10 cm3, which has two openings through which the vector network analyzer (VNA) probes are connected to the sensors. Three more openings are made in the chamber for the introduction of analyte gas, outlet for gas to exit and to create vacuum for cleaning purpose as shown in Figure 1. To probe the gas responses at room temperature (25±2°C), the shift in resonance frequency of the proposed SRR sensing device was measured with gas and without gas conditions at humidity of 32% RH. The experiment carried out over the


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frequency range 8-11 GHz using VNA E8364B, which was calibrated with 1600 points. The desired concentrations of the analyte gases like methanol, ethanol, NO2 and chloroform in the air tight gas chamber was achieved by injecting a known volume of required gas (mixed with dry air) with help of a syringe. When sensor reached to its steady state, the recovery attained by introducing air to the sensor that accomplished with help of opening lid. In the presence of these analyte gases, the resonance frequency was found to shift towards lower frequency regime within the tested domain, which was expected due to p-type behavior of CNTCF. Further, methanol gas with sequence of 0, 300, 0, 300, 0 and 300 ppm was introduced in the chamber for 10 min each concentration step at room temperature to measure the reproducibility of the device. For stability performance test, we exposed CNTCF-SRR sensor to 300 ppm concentration of methanol for one month of period at an interval of five days under the same conditions i. e. room temperature and humidity 32% RH. The dynamic responses of the sensor was carried out by abruptly changing the gas concentration in the chamber at the end of 10 minutes of step. The response to different gasses i.e. gas sensitivity (Sg) of the proposed CNTCF integrated SRR sensor can be calculated as follow1,  fg - fa  Sg(%) =   ×100  fa 

(1)

where, fg and fa represents the resonance frequency of the designed sensor with and without the analyte gas condition, respectively. The selectivity coefficient (K) is another important parameter, which confirms the high selectivity of sensor towards the particular gas and typically expressed as36; K = Sg (methanol)/ Sg (X)


(2)


where, Sg (methanol) and Sg (X) represents the gas sensitivities of sensor in presence of methanol and other gasses, respectively.

Results and discussions Figure 2a-e shows the SEM image of CF and as synthesized CNTCF with different magnifications. The results exhibited that hierarchal CNTCF has diameter more than twice the order of uncoated CF. This signifies that grown CNTs is actually very thick and CNTCFs fabrication productivity is very high. This also explains that weight share of CNT in CNTCF are significant. It is also clear that CF holds CNTs firmly and hence, acting as the preventer to stop any kind of CNT bundle type agglomeration, which consequent weak sensing responses. Therefore, CNTCF would be very beneficial in developing the CNT based gas sensor with high sensing capability. In addition, it is important to observe that the CF surface is completely covered by CNTs. Significantly, the well dispersed CNTs on CF exhibit extremely large surface area35, which provides favorable conditions for gas molecules to interact over the large area of CNTCF thereby improving the sensitivity of the designed microwave sensor. The surface area of the CNTCFs calculated using Brunauer–Emmett–Teller (BET) method through N2 sorption is 553.8 m2g−1. The high value of surface area shows that the CNTCFs formed do not have an entangled structure. The above statement can be justified by observing SEM images of CNTCFs (Figure 2b-e). The uncovered openings are utilized for the purpose of N2 sorption process by gas molecules17. This large number of uncovered openings on the surface morphology gives a high surface area for gas molecules to interact. This property makes CNTCFs very effective candidate for gas sensing applications.


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Figure 2. SEM images (a) CFs, (b)-(e) CNTCF with different magnifications, and TEMHRTEM images (f)-(h) of CNTCF. The TEM and high resolution-TEM characterizations were additionally performed to reveal the structure and morphology of the grown CNTs as shown in Figure 2f-h. Figure 2f (TEM imaging) disclosed that CNT type formation has been successfully achieved by the CVD process. At high magnification, from Figure 2g-h, it is observed that CNT exhibit multiwalled structure with 0.34 nm of wall separation, which corresponds to be (002) plane of multi-walled CNT (MWCNT)35, 37 .



To investigate the chemical bonding states of the fabricated CNTCF, the XPS measurement was carried out as shown in Figure 3a-c. Figure 3a represents the survey scan, which confirms the presence of oxygen and carbon elements. Figure 3b-c illustrate the high resolution XPS spectra corresponding to C 1s and O 1s peaks of CNTCF, respectively. The deconvoluted C 1s peaks are positioned at 284.03, 285.6, 286.8 and 290.5 eV, which are assigned to be the sp2 carbons (C-C; graphitic carbon), sp3 carbons (defective carbon), carbon bonded to oxygen (O-C=O) and π–π* respectively38. The O 1s peak is deconvoluted in three parts as shown in Figure 3c. The peaks centered at 530.3, 531.2 and 532.1 eV corresponds to C=O, C-O-C and C-O bonds, respectively38-39. In view of this results, it is clear that the fabricated CNTCFs are more graphitic and hence, the XPS findings matches well with the earlier reported Raman results of CNTCF35. The FTIR study of CNTCF has been performed in the wave number range 500-4000 cm-1 to identify the presence of functional groups. Figure 3d represents the typical FT-IR spectra of CNTCF. In Figure 3d, the broad and intense band located at 3428 cm-1 corresponds to the O-H stretching of hydroxyl groups38. The peak related to 1631 cm−1 assigned to the skeletal vibration of sp2 C=C bonds38-39. The peaks positioned at 1349, 1248 and 1170 cm−1 corresponds to the CH, C-OH and C-O-C bonds, respectively17, 39. In view of these information, it is obvious that primarily several functional groups are present on the surface of CNTCFs.


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Figure 3. XPS (a) survey scan, (b) C 1s, (c) O 1s spectra and (d) FT-IR of CNTCF. The typical Raman spectra of CNTCF has been reported in our previous work35. In which, the Raman spectra exhibited two peaks situated at ~1330 and 1600 cm−1 that confirming the D and G bands of graphitic carbon, respectively35. The intense G band was observed in Raman spectra of CNTCF, which represents the high graphitic nature of grown CNTs. In addition, low ID/IG ratio (0.86) indicates that fabricated CNTs are indeed highly conductive. Gas sensing properties:



As discussed in the previous sections, the integration of the CNTCF with the SRR sensor affects the microwave parameters and the inherent S-parameters of the sensor resulting into a shift at the designated frequency. Figure S4 shows the change in the transmission coefficient (S21) of the SRR based microwave sensor after integration of CNTCFs on the inner ring of the SRR. The fabricated SRR based microwave sensor resonates at resonant frequency (fr) of 9.57 GHz and transmission coefficient of -10.77 dB, which after integration of CNTCFs comes down to 9.37 GHz and transmission coefficient of -8.13 dB. This change in resonant frequency and the transmission coefficient primarily occurs due to the change in the effective electrical properties of the inner ring of SRR because of the presence of CNTCF thus providing perturbation of the original fields40. The new S-parameters recorded after integration of CNTCFs on the inner ring of the SRR sensor are taken as reference for the gas sensing application. In the next step, change in resonant frequency in the presence of various gas species were recorded as a sensor responses at room temperature. Figure 4a-d exhibits the CNTCF-SRR sensor performances for 200 ppm concentration of different gases such as methanol, nitrogen dioxide, ethanol and chloroform. For the reference, the resonance phenomenon of the sensor is recorded without and with gas condition in the chamber. The downward shift in resonance frequencies from the initial 9.37 GHz position where ~ 9.12, 9.19, 9.24 and 9.26 GHz for methanol, NO2, ethanol and chloroform, respectively. Hence, it is obvious from the results that sensor exhibits significant change in the resonance frequency for methanol, NO2, ethanol and chloroform gases and among these, the best performance is obtained for the methanol gas. Further, the gas sensitivity of the CNTCF-SRR sensor with 200 ppm of various gasses is shown in Figure 5a. It is obvious from the results that the sensor exhibits different sensitivity for different gas species. Practically, gas sensor should show high sensitivity towards the particular


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gas than the others in the same familiar ambient41. This selective behavior appears because different gases have different electronegativity and energies for interaction36. Figure 5a exhibits highest ~ 2.68% of sensitivity for methanol gas than the other investigated gas working at room temperature. The calculated selectivity coefficient (K) values are ~1.19, 1.94 and 2.24 for NO2, ethanol and chloroform, respectively. The high selectivity coefficient factor means the sensor has greater ability to discriminate methanol gas from the mixture of the various gases. Hence, it is justifiable to say that CNTCF-SRR sensor exhibits excellent performances for methanol gas.

Figure 4. Shift in resonant frequency with 200 ppm of (a) methanol (b) NO2 (c) ethanol (d) chloroform gas concentrations.



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We now describe the possible phenomena behind the shift in resonance frequency (fr) upon exposure of the designed SRR sensor to various gases. It is well reported in literature that the electrical conductivity of the carbon nanostructured based sensing material changes significantly, when it is exposed to the gaseous environment. However, the change in the conductivity of such type of material under influence of various gases is quite complicated phenomenon which is difficult to be measured directly. It is mainly due to this reason that the change in conductivity of the CNTCF sensing material in the present situation is observed indirectly by noticing the fact that any change in the electrical conductivity of the sensing material loaded on the CSRR sensor would result into variation of the overall capacitance of the resonator structure thereby causing a shift in the resonant frequency (fr ∝ √1/LC) of the device1, 42-44. It is also observed that the shift in the measured resonant frequency is different when the CNTCF loaded CSRR sensor is exposed to various type of gases thus signifying the fact the sensitivity of the proposed sensor is different for different gases. On the micro level, when the analyte gas molecules interact with the plentiful sp2 bonding cites containing specific surfaces of CNTCF, the transfer of electrons from CNTCF to analyte gas occurrs due to the electron accepting nature of these gas species as shown in Figure 5b1. In detail, the interaction mechanism of CNTCF in air can be described by the following equations45; (3)

O 2(gas) = O 2(ads) O 2(ads) + e - → O 2

_

(ads)

(4)

Therefore, it is obvious that the oxygen molecules are mainly adsorbed on the CNTCFs surface and are transformed into O2− ion by capturing electrons from CNTCFs45. When the SRRCNTCF sensor is exposed to methanol, NO2, ethanol and chloroform gas molecules they react with O2− ion and re-collect electrons from the CNTCF, resulting in more deficiency of


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electrons in the CNTCF. This causes shift in the resonance frequency of SRR-CNTCF sensor towards the lower side, which indirectly suggests that the conductivity of the CNTCF increases on exposure to gas species. In view of this, it is reasonable to propose that CNTCF behaves as ptype semiconductor. Hence, because of these electron acceptor gases, the Fermi levels moves towards the conduction band, creating more holes and thus increasing the overall electrical conductivity45-46. Further, the overall procedure is found to be sensitive to presence of various types of gases such as ethanol, nitrogen dioxide, chloroform, methanol etc. In summary, it may be postulated that the sensitivity of the proposed sensor is maximum for methanol as it possesses high ability to extract electrons from the CNTCF surfaces among the tested gas species47.

Figure 5. (a) Gas sensitivity of sensor towards methanol, NO2, ethanol and chloroform with 200 ppm of concentration and (b) Sensing mechanism of CNTCF-SRR sensor. In view of the high selectivity towards methanol, the further investigation of the proposed sensor against various methanol gas concentration was conducted at room temperature. Gas sensing performance of CNTCF-SRR sensor towards methanol gas: The gas sensitivity towards various concentrations of methanol gas (100-300 ppm) is presented in Figure 6a. From Figure 6a, it is observed that sensitivity of the sensor raised monotonically in the presence of the gas and reached to the constant value then drops quickly to its initial position



after being exposed to air. Specially, the sensor responses at higher elevated gas concentration consequent almost saturation state because most of the reactive sites in CNTCF may be covered with the gaseous species. At 100 ppm, CNTCF-SRR sensor exhibited 32 seconds of response time and 42 seconds of recovery time. However, 51 seconds of response and 60 seconds of recovery times were noticed at 300 pm of gas. The variation in the response time and recovery time of the sensor for different gas levels in the chamber depends on the available reactive surfaces and diffusion of the analyte gas molecules1, 36. In detail, at lower methanol level, the low populated gas molecules spreads faster on the large available active surfaces and consequent rapid saturation that resulting faster response and recovery. However, at higher methanol level, the population of gas molecules increases then the increased gas molecules required larger time to cover the active surfaces and reach the saturation state, which led to enhance the response and recovery times. From the aforementioned properties of the CNTCF-SRR sensor it is anticipated that sensor has great potential to detect methanol gas in this range. In addition, the comparison of response and recover time of the proposed gas sensor with other well-established sensors are tabulated in Table 1. The room temperature operation with fast recovery and response is the most vital aspect for the practical utility of gas sensing devices. From Table 1, it can be seen that the proposed CNTCF-SRR based microwave sensor exhibits fast response-recovery times at room temperature towards methanol gas as compared to the other available sensors in the literature4849

. Soon afterwards, the detection limit of the device was investigated by exposing 0-100 ppm of

methanol gas as shown in Figure S5. From Figure S5, it is obvious that sensor is capable to detect even 10 ppm of low methanol traces at room temperature, which established its substantial potential towards methanol detection.


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Figure 6. (a) Dynamic responses of sensor for various concentration of methanol gas, (b) reproducibility and (c) stability of CNTCF-SRR gas sensor with 300 ppm of methanol gas. Reproducibility and stability analysis of CNTCF-SRR sensor: We performed the reproducibility study of our proposed gas sensor. An example is illustrated in Figure 6b in which the sensing responses were repeated three times using 300 ppm of methanol gas concentration at room temperature. All responses are consistent with sensitivity 4.23±0.03 that exhibit excellent reproducibility and reversibility of the CNTCF-SRR sensor. Stability of the sensing device is one of the most crucial parameter that needs to be studied extensively before exploring it to the real-world application4. In view of this, the stability analysis of the CNTCF-SRR sensor was investigated at room temperature by re-testing them with 300 ppm of fixed methanol gas concentration for one month at an interval of five days. The gas responses collected from the first test and till the seventh test are exclusively represented in



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Figure 6c. It can be seen that the CNTCF-SRR sensor response is quite constant and the variation of the gas sensitivity factor was ±2% relative to the first test over the thirty days investigation period. This measurement clearly establishes the durability of the proposed CNTCF-SRR gas sensors. The small variation in the sensitivity of the sensor may appeared due to humidity or may be affected by the multiple cycles of testing that can decay the sensing performances36, 49. Table 1: Comparison of response and recovery time of CNTCF-SRR sensor with other popular sensors in the literature.

Sensor

Gas

G-ALFO composite TiO2 NT-RGO GaN at Silicon Ag-LaFeO3 fiber SnO2-ZnO Sr added NiAl2O4 Silver-doped LaFeO CNF VOX/CNF Graphitic CNF CNTCF-SRR

Methanol Methanol Methanol Methanol Methanol Methanol Methanol Ammonia NO2 CO Methanol

Working Temp. (°C) 5 102 10-800 RT 500 350 5 125 50 250 1000 159 5 215 30 RT -150-300 2500 RT 100 RT

Level (ppm)

Response time (Sec.)

Recovery time (Sec.)

References

30 18 8 40 18 240 45 336 -144 32

28 61 7 60 25 210 50 1440 -53 42

50 51 3 4 52 53 5 54 55 56

In this work

Conclusion A microwave gas sensor based on the CNTCF has been designed, fabricated and tested. The analysis of microwave-based gas sensor is performed by observing shift in the resonant frequency of the sensor after it is exposed to the gaseous environment containing various analyte gases. The gas sensing material CNTCF was synthesized using catalytic CVD method and investigated by the SEM and TEM characterizations. The shift in the resonance frequency of the


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designed sensor was observed because of the large specific surface area of CNTCF which can effectively interact with gas molecules in quick time. The gas sensing behavior of the sensor was tested at room temperature (25°C) that showed high selectivity and sensitivity for methanol gas with 100-300 ppm of concentration. Moreover, the proposed sensor exhibited excellent stability and reproducibility with 0.03 and 2 % of sensitivity variations, respectively. The response and recovery times were found of the order of seconds for the gas concentration ranging from 100300 ppm. All the aforementioned sensing performances demonstrate that, the proposed CNTCF integrated SRR microwave sensor has great potential for highly sensitive and selective methanol gas sensing applications at room temperature. AUTHOR INFORMATION Corresponding Author [email protected] [email protected] Present Addresses Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur-208016, Uttar Pradesh, INDIA. Notes The authors declare no competing financial interests.

Supporting Information



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BRIEFS In the present work, the high sensitive and selective methanol gas at room temperature is proposed using hierarchal MWCNTs coated carbon fiber incorporated split ring resonator based microwave gas sensor.



“TOC”


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Improved Methanol Detection Using Carbon Nanotube-Coated

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