Reversible and fast responding ppb level Cl2 sensor based on noncovalent modified carbon nanotubes with Hexadecafluorinated copper phthalocyanine

Accepted Manuscript Title: Reversible and fast responding ppb level Cl2 sensor based on noncovalent modified carbon nanotubes with Hexadecafluorinated copper phthalocyanine Authors: Anshul Kumar Sharma, Aman Mahajan, Rajan Saini, R.K. Bedi, Subodh Kumar, A.K. Debnath, D.K. Aswal PII: DOI: Reference:

S0925-4005(17)31435-1 http://dx.doi.org/doi:10.1016/j.snb.2017.08.013 SNB 22880

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

4-3-2017 28-7-2017 1-8-2017

Please cite this article as: Anshul Kumar Sharma, Aman Mahajan, Rajan Saini, R.K.Bedi, Subodh Kumar, A.K.Debnath, D.K.Aswal, Reversible and fast responding ppb level Cl2 sensor based on noncovalent modified carbon nanotubes with Hexadecafluorinated copper phthalocyanine, Sensors and Actuators B: Chemicalhttp://dx.doi.org/10.1016/j.snb.2017.08.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Reversible and fast responding ppb level Cl2 sensor based on noncovalent modified carbon nanotubes with Hexadecafluorinated copper phthalocyanine Anshul Kumar Sharmaa, Aman Mahajana*, Rajan Sainia, R.K. Bedib, Subodh Kumarc, A.K. Debnathd and D.K. Aswale a

Material Science Laboratory, Department of Physics, Guru Nanak Dev University, Amritsar 143005, India b S.I.E.T. Ram Tirath Road, Amritsar-143107, India c

Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, India

d

Technical Physics Division, Bhabha Atomic Research Centre, Mumbai, 400085, India e

CSIR-National Physical Laboratory, New Delhi, 110012, India.

*Corresponding Author: Aman Mahajan Tel.: 91-183-2258802, Ext.: 3470, Fax: 91-183-2258820 Email: [email protected]

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Graphical abstract

Highlights

1. Fabrication of highly selective ppb level Cl2 sensors using F16CuPc/CNTs hybrids. 2. F16CuPc molecules are exo-hedrally attached onto CNTs through π–π stacking. 3. The detection limit of fabricated Cl2 sensor is up to 0.27 ppb. 4. Sensors exhibit excellent base line recovery and reversibility.

Abstract Hybrids of hexadecafluorinated copper phthalocyanine (F16CuPc) with carboxylic functionalized single-walled carbon nanotubes (SWCNTs-COOH) and multi-walled carbon

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nanotubes (MWCNTs-COOH) have been synthesized using a solution assembly method. The resulting hybrid materials have been characterized by Transmission electron microscopy, Raman, UV-Visible, X-Ray photoelectron, Fourier-transform infrared spectroscopic techniques and finally studied for gas sensing application. Cl2 selective chemiresistive gas sensors have been fabricated using these hybrids with detection limit up to 0.27 ppb. The main characteristics of these sensors are their excellent baseline recovery and reversibility upon repeated exposure to Cl2. F16CuPc/SWCNTs-COOH based sensors showed a gas response as large as 35.82% with a fast response time of 9 seconds towards 2 ppm of Cl2. A plausible gas sensing mechanism for charge transfer in hybrids on interacting with Cl2 has been proposed on basis of X-ray photoelectron and impedance spectroscopic studies. These outcomes clearly indicate the great potential of the low cost solution assembly approach for sensor device. Keywords: Hexadecafluorinated copper phthalocyanine, Carbon nanotubes, Chlorine sensor, Gas sensing mechanism, X-ray photoelectron and Impedance Spectroscopy.

1. Introduction Chlorine (Cl2) is extensively used in various industries related to plastics, textiles, agrochemicals, pharmaceuticals, water purification and household cleaning products etc. Since 3

it is highly toxic gas with an occupational exposure limit (OEL) of 500 ppb for eight hours time weighted average, hence its leakage can cause severe damage to living organisms and their surroundings [1,2]. Thus, monitoring of Cl2 requires the development of gas sensors with detection limit up to ppb level or potentially below. So far, numerous methods have been developed to detect Cl2 including solid state potentiometric [3-8], iodine stoichiometry titration [9], optical [10], opto-chemical [11], thermionic ionisation [12], colorimetric [13,14] and electrochemical methods [15-17]. The complexity and non-portability of these methods restricts the commercialization of these sensors [15,18]. In comparison to these sensors, chemiresistive sensors are comparatively simple, economical, compact in design and consume lesser power [19,20]. Different chemiresistive Cl2 sensors based on semiconductors and their oxides [21], conducting polymers [22] and organic molecules [23-25] have been fabricated in bulk, thin film and nano structured form. Recently, carbon nanotubes (CNTs) have attracted extensive attention in gas sensing applications due to their unique one-dimensional carbon nanostructure, large surface area, high electrical conductivity and excellent thermal stability [26]. Here, the charge transfer between adsorbed gas molecules and CNTs leads to variation in the resistance of CNTs that contribute to their gas sensing mechanism [27-29]. However, CNT-based chemiresistor showed several issues like, low sensitivity, slow response/recovery time due to minimum charge transfer between the pristine CNTs and gas molecules. These limitations can be further removed by functionalization of CNTs sidewalls with specific materials, such as metal oxides, noble metal nanoparticles and organic semiconductors [30-32]. Such functionalization of CNTs, not only improves the gas sensing parameters but also enhances their selectivity towards particular type of test gases [33]. Among organic semiconductors, metallo-phthalocyanines (MPcs) have been emerged as excellent sensing materials since MPcs possess high sensitivity and selectivity, fast response kinetics and high thermal and chemical stability [27]. Recently, we have fabricated highly sensitive and selective

room temperature gas sensors based upon substituted MPcs

nanostructures (nanowire, nanoflowers and nanobelts) with minimum detection limit as low as 5 ppb [19,23,24]. Further, hybrids of MPcs with CNTs have attracted extensive attention in gas sensing applications due to synergic effects on the electrical, optical and mechanical properties of two or more components [34]. A variety of CNTs based hybrid materials have been used as gas 4

sensors for the detection of various toxic gases. Wang et al. [35] fabricated lead phthalocyanine modified CNTs with enhanced NH3 sensing performance as compared to pristine CNTs. Li et al. [36] have demonstrated Cl2 selective chemiresistive sensors based on coating of SWCNTs with chlorosulfonated polyethylene and hydroxypropyl cellulose polymers. However, slow response and recovery time were the primary issues with these sensors. Gohier et al. [1] have developed Cl2 sensitive platform based upon MWCNTs with a detection limit down to 27 ppb at room temperature, but the baseline resistance recovery could only be achieved by heating the device up to 75°C desorption temperature for 60 minutes. In spite of many efforts, the properties like gas response, selectivity and stability of CNTs based sensors are still not ideal. These challenging issues have motivated us to fabricate gas sensor based upon hybrids of Copper(II)1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexa-decafluoro-29H,31Hphthalocyanine (F16CuPc) with carboxylic modified single-walled carbon nanotubes (SWCNTs-COOH) and multi-walled carbon nanotubes (MWCNTs-COOH) because of exceptional physical and chemical properties of F16CuPc molecule like its solubility, outstanding thermal stability and tuneable electronic properties due to shorter π-π stacking inter-molecular distance (~3.25 Å) of F16CuPc, which improves π-π interaction between F16CuPc and CNTs [37,38]. Also, due to molecular fluorination, the molecular orbitals are closer to the Fermi level, which leads to an increase in its ionization potential and electron affinity which presents a preferred acceptor behaviour and has shown significant performance in devices [39]. So, it will be favourable to functionalize CNTs in such a manner that the resultant hybrids can be used to detect ppb level of Cl2. Keeping these properties into consideration, we report herein the formation of hybrids of SWCNTs-COOH and MWCNTsCOOH with F16CuPc as Cl2 sensors using a low cost solution assembly method [27].

2. Experimental F16CuPc, SWCNTs and MWCNTs have been purchased from Sigma-Aldrich. The CNTs have been first acidified bearing acidic groups (CNTs-COOH) according to the procedures developed by Smalley and co-workers as pristine CNTs are insoluble or have poor dispersibility in common solvents due to aggregation and entanglement of CNTs. Nevertheless, introduction of the carboxyl group by acid treatment, imparts negative charges and create the long-term electrostatic stability required for dispersion of CNTs [40,41]. In a typical experimental procedure, 15 mg of F16CuPc has been stirred in 5 ml of dimethylformamide (DMF) to form F16CuPc/DMF solution. Subsequently, we have made suspensions of 5

SWCNTs-COOH and MWCNTs-COOH (30 mg each) in DMF (10 ml) by stirring for 1 hour. Further, F16CuPc/DMF solution has been added drop wise to these suspensions of SWCNTsCOOH and MWCNTs-COOH in DMF. The resulting mixtures have been sonicated at room temperature (25°C) for 3 hours and subsequently stirred in dark for 6 hours at 100°C. After centrifugation of the mixtures, the supernatants have been collected and filtered through PTEF filter (0.22 µm, Milipore). These filtered products have been washed with DMF to remove excess of F16CuPc derivative, followed by rinsing with ethanol for several times and finally dried to obtain desired F16CuPc/SWCNTs-COOH (H1) and F16CuPc/MWCNTs-COOH (H2) hybrids. Fig. 1 shows the schematic of the formation of H1 and H2 hybrids. The surface morphologies of H1 and H2 hybrids have been studied by transmission electron microscopy (Jeol, TEM-2100). Raman spectroscopic measurements have been performed using Renishaw invia micro-Raman spectrometer. Fourier transform infrared (FTIR) and Ultraviolet-Visible (UV-Visible) spectra of H1 and H2 hybrids have been recorded on Perkin

Elmer

Frontier

FT-IR

spectrometer

and

UV-1601PC (Shimadzu,

Japan)

spectrophotometer, respectively. X-ray photoelectron spectroscopy (XPS) measurements have been carried out using Mg Kα X-ray beams as the excitation source (1253.6 eV) and a DESA150 electron analyzer (Staib Instruments, Germany). To fabricate the gas sensors of H1 and H2 hybrids, a typical protocol has been adopted: as prepared hybrids have been dispersed in DMF 2 mg/ml by stirring for two hours at room temperature (25°C and 50% relative humidity). The multiple sensors with an effective area of 3 mm × 1 mm have been fabricated by drop casting 50 µL of hybrid solution onto glass substrate with two precoated gold electrodes (3 mm × 3 mm at a spacing of 1 mm). The silver paste has been used for making contacts of silver wires to the gold electrodes. The gas sensing studies have been performed in a home-built stainless steel test chamber (1000 ml) containing sample holder geometry, using electrometer (Keithley 6517A) as shown in Fig. 2. The sensor resistance has been measured as a function of time by applying a constant bias of 3 V. Cl2, NO2, NO and NH3 dry gases have been commercially procured from M/s Chemtron Science Pvt. Ltd., India, in the gas filled canister (gas pressure 20 kg/cm2) of volume 0.5 litres with a concentration of 1080 ppm (rest content is N2) [25]. The ambient room air was used as carrying gas. A desired concentration of a gas in the test chamber has been achieved by injecting a known quantity of gas using a micro-syringe. After gas exposure, once a steady state was achieved, the sensor resistance has been recovered by opening the lid of test chamber.

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The gas response of sensor has been calculated by using the equation (1): S (%) = |(

−

)⁄

| × 100

(1)

Here, Rg and Ra represents the sensor resistance in gas and air environment, respectively. The response time was taken as the time required for sensor resistance to reach 90% of its equilibrium value after the gas was introduced into the test chamber and recovery time was the time required for the sensor resistance to regain 90% of its baseline value after the removal of gas from test chamber [35]. The saturated aqueous solutions of LiCl, MgCl2, K2CO3, NaBr, KI, NaCl and KCl at an ambient temperature of 25°C have been used for maintaining homogeneous and stable environment with relative humidity of nearly 11.2%, 32.9%, 43.2%, 57.4%, 68.7%, 75.2% and 84.7% respectively [42]. The relative humidity (RH) levels has been independently monitored by using a hygrometer (Keithley 6517 A). Impedance spectroscopy study of H1 and H2 hybrids have been carried out using a frequency response analyser (FRA) attached with a potentiostat (Autolab) in the frequency range of 10 Hz-1 MHz.

3. Result and discussion 3.1 Material characterization The surface morphologies of SWCNTs-COOH, MWCNTs-COOH, H1 and H2 hybrids have been studied by transmission electron microscopic (TEM) investigations as shown in Fig. 3. TEM images (Fig. 3(a and b)) have shown that SWCNTs-COOH and MWCNTs-COOH were long tube like structures having a mean diameter of around 10 and 14 nm, respectively. TEM analysis (Fig. 3(c and d)) of the H1 and H2 hybrids highlights the exohedral attachment of F16CuPc molecules onto the surface of CNTs. F16CuPc anchored CNTs have discontinuous and irregular layer on outer surface of CNTs. In comparison to pristine CNTs, H1 and H2 hybrids were comparatively rough and thicker with a mean diameter of around 19 and 23 nm, respectively. In order to investigate the interaction between the F16CuPc molecules and CNTs, Raman spectra of all the samples have been studied (Fig. 4). The characteristic peaks up to 1000 cm-1 in the Raman spectrum of F16CuPc were due to vibrations of isoindole moieties [43]. The peaks between 1200 and 1600 cm-1 correspond to pyrrole groups and the peak around 1619 cm-1 has been assigned to C-H bending, respectively. Further, the peak at 1522 cm-1 was due

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to displacement of C-N-C bridge bond, closely linked to central metal ion of phthalocyanine molecule [44-46]. Raman spectra of MWCNTs-COOH and SWCNTs-COOH exhibited the characteristic G-band around 1593 cm-1 related to C-C vibration of the carbon material with a sp2 orbital structure and D band around 1360 cm-1 associated with sp2 C with defects [47,48]. Also, the radial breathing mode (RBM) of SWCNTs-COOH exhibited a characteristic peak at 164 cm-1 due to the distribution of diameters in SWCNTs-COOH sample [49]. Raman spectra of H1 and H2 hybrids showed a combination of the characteristic Raman peaks of both F16CuPc and CNTs, but with different intensities. The relative intensity ratio (ID/IG) known as a ratio of sp3hybridized carbon atom relative to sp2 -bonded carbon atom was estimated to be 0.3 and 0.12 for SWCNTs-COOH and H1 samples, respectively, whereas for MWCNTs-COOH and H2 samples it was 1.16 and 1.02 respectively [47,49]. The observed small variation in ID/IG suggested that F16CuPc molecules were non-covalently attached on the surface of MWCNTsCOOH and SWCNTs-COOH respectively [47]. The non-covalent attachment of F16CuPc with CNTs was further confirmed by FTIR spectroscopy (Fig. 5). FTIR spectrum of F16CuPc exhibited characteristic peaks at 603, 765, 965, 1152 cm-1 due to hexa-decafluoro substituents and the peaks at 1330, 1499, 1537, 1623, 1728 and 2920 cm-1 were due to aliphatic C-H vibrations [27]. FTIR spectra of SWCNTsCOOH and MWCNTs-COOH possessed C-O stretching vibration peaks at 1021 and 1037 cm1

while the O-H stretching vibration peak was observed at 3440 cm-1 in both samples due to

carboxylic group [41]. Both SWCNTs-COOH and MWCNTs-COOH exhibited a characteristic peak at 1637 cm-1 due to C=C stretching which confirmed the graphitic structure of CNTs [41]. Moreover, the peaks at 2855 and 2921 cm-1 in the CNTs samples were due to asymmetric and symmetric CH2 stretching [50].These results suggested that carboxylic group was successfully attached onto the surfaces of SWCNTs and MWCNTs by acid treatment. After hybrid formation, the characteristic peaks of F16CuPc and SWCNTs-COOH were observed in H1 hybrid at 668, 765 , 965, 1152, 1320, 1496, 1526, 1616, 1728 and 2921 cm-1 respectively [51]. Similarly the characteristic peaks of F16CuPc and MWCNTs-COOH were observed in H2 hybrid at 684, 1086, 1220, 1384, 1496, 1637, 1728 and 2921 cm-1 respectively [27]. The characteristic peaks of F16CuPc in H1 and H2 hybrids were slightly shifted due to electron delocalization by the non-covalent attachment of F16CuPc onto CNTs which is in consonance with Raman spectroscopic studies of these samples [52].

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Fig. 6(a and b) depicts the UV-Visible absorption spectra of F16CuPc, SWCNTs-COOH, MWCNTs-COOH, H1 and H2 hybrids. The UV-Visible spectrum of F16CuPc exhibited two strong absorption bands, one broad B band in the wavelength range 300-450 nm due to the electronic transitions from HOMO a2u to LUMO eg level and the other Q band doublet (640 and 677 nm) arising from the electronic transitions from HOMO a1u level to LUMO eg level [51]. The UV-Visible absorption spectra of SWCNTs-COOH and MWCNTs-COOH were found to be featureless [53]. However, in case of H1 and H2 hybrids, the Q-band was found to be comparatively broadened and red shifted by 16 and 70 nm, respectively as compared to that of F16CuPc. This red shift confirmed the π-π interaction and the charge transfer between the F16CuPc macrocycle and CNTs [49,54]. 3.2 Gas-sensing characteristics To study the gas sensing characteristics, H1 and H2 hybrids based sensors have been exposed to 1 ppm of Cl2, NO2, NO and NH3 gases at room temperature (25°C). The response of H1 and H2 sensors towards different test gases at room temperature was very sluggish except for Cl2, indicating the high selectivity of sensors towards Cl2 among all the tested gases. The response of H1 and H2 sensors for 1 ppm of Cl2 was 24.5% and 17.7% and respective response time were 4 and 5 minutes respectively. Further, to compare the Cl2 sensing capability of SWCNTs-COOH, MWCNTs-COOH, H1 and H2 based sensors, the resistance-time variation of these sensors have been investigated for different doses of Cl2 (Fig. 7). Sensors exhibited an irreversible behaviour at room temperature as sensors did not recover to baseline resistance even after a long interval of time. Nevertheless, on heating the recovery characteristics of sensors were greatly improved. Further in order to make sensors reversible, the operating temperature of sensors have been optimized. Here, both sensors H1 and H2 have been exposed to 1 ppm of Cl2 at different operating temperatures ranging from 25°C to 175°C. A plot of sensor response for 1 ppm of Cl2 as a function of temperature is shown in Fig. 8(a). These results clearly indicated that both sensors became fully reversible at 150°C. Both sensors showed a maximum response towards Cl2 at an operating temperature of 150°C while beyond 150°C, sensor response decreases due to desorption of Cl2 from the sensor surface. Fig. 8(b) shows the selectivity histogram of H1 and H2 sensors at room temperature (25°C) and 150°C for 1 ppm of different test gases. H1 sensor exhibited a comparatively higher response of 27.4% for Cl2 with a response time of 9 seconds in comparison to 4 minutes at 9

room temperature while the response for NO2, NO and NH3 gases remained lower as 4.9%, 2.5% and1.1%, respectively. Similarly, the response of H2 sensor for Cl2 was 20.6% with a response time of 20 seconds while for NO2, NO and NH3 response was 4.3%, 2.3% and 1.0% respectively. These results indicated that both sensor were highly selective towards Cl2 even at elevated operating temperature. Further, the typical resistance-time characteristics of H1 and H2 sensors for different doses of Cl2 (0.1-2 ppm) at 150°C were studied and the obtained results are shown in Fig. 8(c). The response of H1 and H2 sensors was found to lie in the range 8.435.8% and 3.3-23% respectively for 0.1-2 ppm of Cl2 (Fig. 8(d)). Fig. 9(a and b) represents the response curves of H1 and H2 sensors for successive exposures of Cl2. The nearly same value of sensor response without any drift in the baseline resistance reflected the highly reproducible response characteristic of the sensors. The faster response time, reversible and reproducible Cl2 sensing characteristic of these hybrid sensors in comparison to other CNTs based Cl2 sensors reported in literature [1,36] make them as promising candidates for ppb level Cl2 detection. This increase in the sensor response with increase in Cl2 concentration can be explained on the basis of surface area and number of active sites available on the sensor surface [55]. At lower concentration, Cl2 covers lesser surface area and hence interacts with lesser number of active sites available on sensor surface, thus leading to lower sensor response. While at higher concentration Cl2 covers comparatively larger surface area and interacts with larger number of active sites leading to higher sensor response. Fig. 10 represents the variation in Cl2 response of H1 and H2 sensors with relative humidity (10-85%) for 2000 ppb of Cl2 at room temperature. It has been observed that both sensors showed only small variation (32.6-29.3% for H1 and 23.2-20.7% for H2 sensor) in their Cl2 response as humidity level was varied from 10% to 85% indicating that humidity has negligible effect on the Cl2 response of these sensors. Further, the variation of response with the gas concentration has been studied using the equation (2) [56,57]: = α [

]

(2)

10

where α and β are coefficients that depends upon the operating temperature and testing material. The constant α is adsorption capacity, whereas β is the strength of adsorption [56] and have been calculated by curve fitting of response curve. The smaller the value of β, greater is the expected heterogeneity and its value lies between 0 and 1 for normal adsorption. The value of α and β are found to be 1.279 and 0.440 for H1 and 0.818 and 0.452 for H2 hybrids, respectively. Here the value of β is found to be less than one, which reveals heterogeneous surface of sensors and normal adsorption. Further, detection limit (DL) [58,59] of the sensors H1 and H2 has been calculated using equation (3): =

×





(3)

The signal to noise ratio is defined as the ratio of maximum resistance change to root mean square noise ( points,



) of the baseline resistance of sensor. For N number of data ∑( − )

, with

as the measured data points and Y as the

corresponding value calculated from the curve fitting equation. A set of 20 data points from Fig. 8(c), corresponding to baseline resistance before exposure to gas has been selected and is found to be best fitted by third-order polynomial. The corresponding to fitted curve is found to be 0.199 for H1 and 0.100 for H2 sensor with corresponding detection limit of 0.27 ppb and 0.85 ppb respectively.

3.3 Gas Sensing Mechanism To investigate the gas sensing mechanism of hybrids, X-ray photoelectron spectroscopy (XPS) analysis of fresh, Cl2 exposed and fully recovered H1 and H2 sensors samples has been carried out (Fig. 11 and 12). XPS spectrum of fresh H1 hybrid exhibited the characteristic peaks at 284.8, 531.2, 533.4, 399.0, 687.0, 933.9, 953.7 eV corresponding to C1s, O1s, N1s, F1s, Cu-2p3/2 and Cu-2p1/2 levels. Whereas, fresh H2 hybrid (Fig. 12) showed the characteristic XPS peaks at 284.8, 531.9, 533.9, 399.9, 687.3, 933.7 and 954.1 eV corresponding to C1s, O1s, N1s, F1s, Cu-2p3/2 and Cu-2p1/2 levels respectively [23,49]. After exposure to 25 ppm of Cl2, H1 hybrid (Fig. 11) exhibited an additional peak at 199.9 eV due to formation of metal chloride along with a peak shift of 0.1 eV in spectrum of core level C1s, O1s, F1s, a shift of 11

0.2 eV in spectrum of N1s and a prominent shift of 0.5 eV in the core level spectrum of Cu 2p3/2 and 2p1/2 peaks toward higher binding energy (BE) value. Similarly, H2 hybrid (Fig. 12) on exposure to 25 ppm of Cl2, showed an additional metal chloride peak at 200.8 eV along with a prominent shift of 0.5 eV in Cu 2p3/2 and 2p1/2 peaks towards higher BE value and peak shift of 0.1 eV in C1s and O1s, respectively. The observed significant shift of 0.5 eV towards higher BE side in the Cu-2p core level peak in both hybrids materials revealed the decrease in electron density of hybrids due to charge transfer interaction between strong electron acceptor Cl2 and hybrid materials through central metal copper ions [24,60]. Chlorine has withdrawn electrons from F16CuPc molecules which were non-covalently attached to CNTs [32,49], thus leading to charge transfer from CNTs to F16CuPc molecules and increases the hole concentration in CNTs [32,49,60]. Thus, on Cl2 exposure, H1 and H2 sensors showed fast variation in the sensor resistance [61]. XPS spectra of H1 and H2 hybrids, recorded after recovery time (as found in sensing studies) exhibit identical peaks as noticed in fresh samples. The absence of any chlorine signal and shift observed in the exposed samples confirm that sensing process is highly reproducible. As per TEM analysis discussed earlier, the sensing layer consists of grains of F16CuPcSWCNTs/MWCNTs and respective grain boundaries. The exact contribution of these grains and grain boundaries towards sensing mechanism can be evaluated by impedance spectroscopy of fabricated sensors. Fig. 13(a and b) shows cole-cole plot [62,63] between imaginary component of impedance (-Z′′) as a function of real component (Z′) of H1 and H2 sensors in the presence of air and after exposure to 1 ppm of Cl2. These plots exhibit a circular arc before and after exposure to Cl2 with an equivalent circuit (Fig. 13(c)) consisting of RC network in series with a resistor Ro. Here, Ro is the grain resistance and can be estimated from the intercept of the arc at high frequency with the real axis. R1 and C1 are resistance and capacitance across grain boundaries, where R1 can be estimated from the diameter of the arc in Fig. 13, while C1 can be found from the relation ωmax R1C1 = 1, where ωmax is the frequency corresponding to the top of the arc [63]. The obtained values of Ro, R1 and C1 for the sensors using equivalent circuit are tabulated in Table 1. The mathematical formulation for this equivalent circuit [62] can be given as: = where

=

+[

/(1 +

+

′′

(4)

) ] and 12

=[

/(1 +

) ].

It was observed that the parameter Ro remained the same in air and presence of Cl2, whereas R1 decreased and C1 increased with exposure of Cl2. It reveals that incoming Cl2 molecules adsorbed onto the outer surfaces of grains and improved hole conductivity through charge transfer between phthalocyanines and CNTs in a similar manner as observed in XPS studies [62].

4. Conclusion We have demonstrated a convenient solution assembly method for fabricating fast responding and fully reversible chemiresistive Cl2 sensor with detection limit down to 0.27 ppb using F16CuPc-SWCNTs/MWCNTs hybrids. F16CuPc molecules are found to be exo-hedrally attached onto the surface of CNTs through π–π stacking and helps in the formation of charge transfer conjugate complex. The obtained hybrids were found to be favourable Cl2 sensing materials up to ppb level with a good sensor response, high selectivity, fast response and recovery time, good stability and repeatability. H1 sensor displays comparatively higher response and faster response/ recovery time. X-ray photoelectron and impedance spectroscopic studies permitted to describe the possible Cl2 interaction with F16CuPc-SWCNTs/MWCNTs hybrid material. These findings have important implication in designing a new low cost Cl2 sensors with superior gas sensing parameters as compared to earlier reported CNTs based Cl2 sensors.

Acknowledgements The authors are thankful to Council of Scientific and Industrial Research (CSIR), New Delhi, India for providing financial assistance to accomplish this research work. References

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20

Figure Captions 1. Schematic of the formation of H1 and H2 hybrids. 2.

Gas sensing set-up used in the present study.

3. TEM images of (a) SWCNTs-COOH; (b) MWCNTs-COOH; (c) H1 and (d) H2 hybrid (Inset shows a magnified view of (a) SWCNTs-COOH; (b) MWCNTs-COOH; (c) H1 and (d) H2 hybrid). 4. Raman spectra of (a) F16CuPc, SWCNTs-COOH, H1 hybrid, MWCNTs-COOH and H2 hybrid; (b) Phthalocyanine vibrations of H1 and H2 hybrid in the range 250-1200 cm-1 (magnified view) and (c) Radial breathing mode of SWCNTs-COOH and H1 hybrid in the range of 120-200 cm-1 (magnified view). 5. FTIR spectra of (a) F16CuPc; (b) SWCNTs-COOH; (c) H1 hybrid; (d) MWCNTs-COOH and (e) H2 hybrid. 6. UV-Visible spectra of (a) F16CuPc, SWCNTs-COOH and H1 hybrid; (b) F16CuPc, MWCNTs-COOH and H2 hybrid (Inset shows an enlarged view of H2 hybrid). 7. Response curves of (a) SWCNTs-COOH; (b) H1; (c) MWCNTs-COOH and (d) H2 sensor for different doses of Cl2 at room temperature. 8.(a) Sensor response as a function of temperature for 1 ppm of Cl2 concentration for H1 and H2 sensors; (b) Response histogram of H1 and H2 sensors for 1 ppm of Cl2, NO2, NO and NH3 at room temperature (25°C) and 150°C; (c) Response curves of H1 and H2 sensors for different doses of Cl2 at 150°C and (d) Variation in the response of H1 and H2 sensors with Cl2 concentration experimental curve (dotted lines) at 150°C and fitting curve (solid lines). 9. Reproducibility of the response curves of (a) H1 and (b) H2 hybrid sensors to 0.5 ppm of Cl2 at 150°C. 10. Variation in Cl2 response of H1 and H2 sensor with humidity for 2 ppm of Cl2 at room temperature. 11. XPS spectra of fresh (black curve), Cl2 exposed (red curve) and fully recovered (blue curve) H1 sensor. 12. XPS spectra of fresh (black curve), Cl2 exposed (red curve) and fully recovered (blue curve) H2 sensor. 21

13. Impedance spectra of fresh and Cl2 exposed (a) H1 and (b) H2 sensors (Insets show the equivalent circuit used for analysis of data obtained from H1 and H2 sensors).

22

Figure 1

Figure 2

23

Figure 3

24

Figure 4

25

Figure 5

26

Figure 6

27

Figure 7

28

Figure 8

29

Figure 9 30

Figure 10

31

Figure 11

32

Figure 12

33

Figure 13 TABLE 1. Impedance parameters obtained for H1 and H2 sensors by fitting experimental data to the equivalent circuit. Sensors

H1

H2

Conditions

Parameters Ro (Ω)

R1 (Ω)

C1 (nF)

Unexposed

1047.20

1527

54

Exposed to 1 ppm Cl2

1047.20

879.2

63

Unexposed

127.42

1015.17

8.0

Exposed to 1 ppm Cl2

127.42

763.32

8.95

34