Polypyrrole–Tantalum disulfide composite: An efficient material for fabrication of room temperature operable humidity sensor

Sensors and Actuators A 298 (2019) 111593

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Polypyrrole–Tantalum disulfide composite: An efficient material for fabrication of room temperature operable humidity sensor A. Sunilkumar a,1 , S. Manjunatha a,b,1 , B. Chethan c , Y.T. Ravikiran d,∗ , T. Machappa a,∗ a

VTURC, Department of Physics, Ballari Institute of Technology and Management, Karnataka, 583104, India Cambridge Institute of Technology, Bengaluru, Karnataka, 560036, India VTURC, Dept. of Physics, JNN College of Engineering, Shivamogga, Karnataka, 577201, India d Department of PG studies in Physics, Government Science College, Chitradurga, Karnataka, 577501, India b c

a r t i c l e

i n f o

Article history: Received 3 June 2019 Received in revised form 24 August 2019 Accepted 4 September 2019 Available online 5 September 2019 Keywords: Polypyrrole Tantalum disulfide Humidity sensing Response-recovery times Humidity hysteresis

a b s t r a c t Polypyrrole-tantalum disulfide (PPy/TaS2 ) composites were synthesized with varying wt% of TaS2 (10, 30 and 50) by in situ chemical polymerization. XRD spectra of the composites unraveled their crystalline nature. SEM images of the PPy/TaS2 -50% composite inferred the uniform distribution of TaS2 in the matrix of PPy with reduced grain size. TaS2 sheets embedded in the PPy matrix and crystalline nature of the composite confirmed from TEM studies. FTIR analysis revealed strong interaction between PPy and TaS2 . Humidity sensing performances of the composites were studied in the range of 10–97% RH. Among the composites synthesized, PPy/TaS2 -50% composite exhibited good linearity, sensitivity, limit of detection along with a sharp response and recovery times of 10 and 20 s respectively. The sensing mechanism has been discussed on the basis of Grotthuss reaction. © 2019 Elsevier B.V. All rights reserved.

1. Introduction The exploration as well as evolution of conducting polymers and their composites started up a new era of great opportunities in the field of materials science. It’s phenomenal growth in many aspects led to design of competent materials witnessing their applications in sensors, super capacitors, molecular electronics, actuators, electromagnetic interference (EMI) shielding, microwave absorbers and light emitting diodes [1–3]. Recently, researchers have studied the humidity sensing behavior of conducting polymers and their composites in order to overcome the drawbacks of conventional metal/ceramic based humidity sensors like high operating temperature, high cost and cumbersome fabrication [4]. So, conducting polymer composites with various inorganic fillers, especially those of polypyrrole (PPy) and polyaniline (PANI) viz. PANI/MgCrO4 [5], PANI/Y2 O3 , PANI/TiO2 [6], PANI/NiO [7], PANI/Ho2 O3 [8] PANI/graphene oxide [4], PPy/Fe2 O3 [9] all have shown their implicit humidity sensing characteristics which have overcome the limits of metal/ceramic based sensors. PPy composites are preferred because of the low oxidation potential of PPy

∗ Corresponding authors. E-mail addresses: [email protected] (Y.T. Ravikiran), [email protected] (T. Machappa). 1 Authors of equal contribution. https://doi.org/10.1016/j.sna.2019.111593 0924-4247/© 2019 Elsevier B.V. All rights reserved.

(0.8 V), lower weight, good stability and easy synthesis along with alteration in its surface charge characteristics during its synthesis. Also, NH group of PPy readily interacts with the filler material without any constraints for achieving best synergy to improve the humidity sensing performance. Further, to enhance the scope of such composites in humidity sensing, Aihua Sun et al. have showed that the quaternized/PPy composite film with a response-recovery times of 41 and 120 s respectively, with a impedance change of nearly 3 orders in 11–95% RH [10]. Pi-Guey Su et al. have studied PPy with TiO2 nanoparticles and they found best linearity and small hysteresis along with response-recovery times of 40 and 20 s respectively [11]. Jinchun Tu et al. have studied Fe+2 doped PPy for humidity sensing and obtained nearly four orders of impedance change in the range 11–95% RH with response-recovery times of 20 and 150 s respectively [12]. Higher sensitivity (s = 138), best hysteresis (0.16%) with response-recovery times of 15 and 20 s respectively were reported by Wang-De Lin et al. for PPy/ graphene composite [13]. S.K. Shukla et al. have reported good sensitivity (0.31/RH) as well as response-recovery times of 12 and 8 s respectively for PPy/ZnO composite synthesized by in situ polymerization [14]. In all these composites, reported response and recovery times as well as sensing response needs to be improved by a suitable filler. On the other hand, the current research trends with the transition metal dichalcogenides, like MoS2 , WS2 , TaS2 and phosphorene [15,16] an another novel 2D-material have gath-

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ered considerable attention of researchers in the field of sensors [17,18]. So, we thought of choosing such a novel 2D-material which would be interesting on par with the ongoing research. Among all the 2D-materials known, TaS2 in the recent days has proved to be a very efficient material in humidity sensing [19], because higher electropositivity of Ta in TaS2 promotes greater surface energy to catch water vapor. Also, the van der Waals gaps between the layers of TaS2 facilitates higher surface charge, all together makes TaS2 a good candidate for humidity sensing. Thus, the higher electropositivity with van der Waals gaps between the layers of 2D material, when incorporated in PPy matrix, helps in achieving best synergy which would further help in enhancing humidity sensing performance of any PPy-TaS2 composites [20–22]. In summary, the excellent electrical conductivity, higher surface energy and the van der Waals gaps due to 2D-nature [19], all these features caught our attention and arose curiosity of its effect on PPy. TaS2 a layered material with narrow energy gap of 0.3 eV, wherein a layer of tantalum (Ta) atoms lie in between the two layers of sulfur (S) atoms, has got an outspread demand in various fields; particularly in mid-far IR applications [23], H2 evolution [24], energy storage devices [25], field effect transistors [26], superconductors [27]. Rarely, there are reports of this material being used in sensors [28]. In this perspective, to manifest best synergy, we have considered TaS2 as a filler material with PPy to prepare PPy/TaS2 composites using in situ polymerization technique, which may enhance the humidity sensing performance of the composites. Prepared samples were characterized using various analytical techniques before humidity sensing measurements. The sensing mechanism has been brought out by discussing the various processes viz chemisorption, physisorption, capillary condensation followed by Grotthuss reaction. 2. Experimental 2.1. Materials Pyrrole [C4 H4 NH (98% purity)], ammonium persulfate [(NH4 )2 S2 O8 ], purchased from SD fine chemicals, Mumbai, India. Metal basis Tantalum disulfide -TaS2 (99.8% purity) is from Alfa Aesar. Pyrrole was refined twice, prior to use to remove the impurities if any. 2.2. Synthesis of polypyrrole/TaS2 composites To synthesize the composites, for 0.03 M pyrrole solution, grinded TaS2 powder was added (10, 30 and 50 wt%) and mixed rigorously using magnetic stirrer. Solution of ammonium persulfate of 0.06 M was added continuously to the above solution drop-wise to get PPy/TaS2 composites. The samples were refined by washing frequently with distilled water, acetone and then it is dehydrated by placing it in oven at 100 ◦ C till a constant weight is reached. Pure PPy was obtained without adding TaS2 to the above reaction. 2.3. Characterization Scanning electron microscopy (SEM) images were captured by Quanta 3D FEG instrument by mounting TaS2 , PPy and PPy/TaS2 50% samples on copper tape. Using EDAX genesis instrument, a fitted accessory to SEM, energy dispersive X-ray spectroscopy (EDX) data was obtained. Transmission electron microscopy (TEM) images were obtained through JEOL-3010 instrument attached with a CCD camera (Gatan). Sample for TEM characterization was prepared by dissolving 1 mg of PPy/TaS2 -50% composite in 1 ml ethanol high purity, sonicated for few minutes, and few more drops of ethanol were added, so as to get transparent dispersion of the

Fig. 1. Schematic diagram of the humidity sensor set up.

sample. X-ray diffraction (XRD) data of TaS2 , PPy and PPy/TaS2 50% were obtained in the 2␪ range 10◦ –80◦ , scanned at the rate of 1.12◦ min−1 using Bruker D8 Advance powder X-ray diffractometer, with Cu K␣ source of ␭ = 1.541 Å. Fourier transform infrared spectroscopy (FTIR) data of TaS2 , PPy and PPy/TaS2 -50% composite all were obtained in the wavenumber range 400-4000 cm-1 using a FTIR Frontier PerkinElmer instrument. 2.4. Humidity measurements Humidity sensing measurements were carried out for the composite pellets. To prepare the pellets, the powder forms of the composites were pressed separately in a hydraulic press to get the pellets of fixed diameter of 10 mm with thickness of 1.12 mm (PPy/TaS2 -10 wt%), 1.23 mm (PPy/TaS2 -30 wt%) and 1.20 mm (PPy/TaS2 -50 wt%) respectively. To get the better electrical contacts, the surface of each pellets was silver pasted. Experimental arrangement for humidity sensing measurement is shown in Fig. 1. For the measurements, the above prepared pellets were exposed to the different RH environments maintained in the simple glass chambers containing various saturated salt solutions and monitored by Mextech-DT-615 humidity meter. The top of each glass chamber was contoured with a rubber cork so as to hold the pellet in a silver electrode and to drive inside the glass chamber. One of the electrodes was connected to a programmable digital LCR meter (Hioki, Japan) to record the absolute value of impedance of the sensing material using AC voltage at 100 Hz to avoid polarization effects [29] before and after exposing to specific relative humidity. The various humidity sensing parameters for the device fabrication were calculated and analyzed. 3. Results and discussion 3.1. SEM and EDX studies Surface morphologies of PPy, TaS2 and PPy/TaS2 -50% composite are shown in Fig. 2. SEM image of PPy (Fig. 2a) shows grains of uniform size. SEM image of pristine TaS2 (Fig. 2b) shows an outspread clump of sheets of few micrometers. Whereas, SEM image of the composite (Fig. 2c) shows uniform distribution of TaS2 sheets in the matrix of PPy with reduced grains. Such morphology provides active sites for water adsorption, has been reported in the recent studies [6]. EDX of the composite (Fig. 2d) shows the peaks of carbon and nitrogen belonging to PPy, tantalum and sulfur belonging to TaS2 , confirms the formation of homogeneous composite. The average grain sizes of PPy and the composite were calculated using image-J software and histograms showing number frequency against the grain size are shown in Fig. 3. The average grain size of PPy and the composite were found to be 0.53 ␮m and

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Fig. 2. Scanning electron micrograph of (a) PPy, (b) TaS2 , (c) PPy/TaS2 -50% composite and (d) EDX of the composite.

0.41 ␮m respectively. The decrease in the grain size in the composite compared to PPy, enhanced the grain boundaries facilitating the formation of micropores. These pores provides active sites for adsorption of moisture on the surface [6], as well as favoring capillary condensation in the interiors of the composite [30].

JCPDS file No. 02-0137 confirming, hexagonal crystal system with 2H-phase of trigonal prismatic coordination [18]. Fig. 5c shows the XRD spectrum of PPy/TaS2 -50% composite exhibiting the peaks of PPy and TaS2 confirming the uniform distribution of TaS2 in PPy matrix.

3.2. TEM analysis

3.4. FTIR studies

TEM image of PPy/TaS2 -50% composite is displayed in Fig. 4a. The dark region shows TaS2 and light region indicates outspread of PPy chains. Image of the composite revealed that TaS2 has been confined in the matrix of PPy and it appears to be embedded in the polymer matrix. Such type of encapsulated structures has been reported recently [31,32]. Also, the HRTEM image of the composite as depicted in Fig. 4b, shows that TaS2 has been wrapped within the chains of PPy. SAED pattern of the composite, with assigned crystal planes is shown in Fig. 4c. From the figure, it is clear that the diffraction pattern has manifested the crystallinity of the composite which is on par with the XRD data.

FTIR spectrum of PPy (Fig. 6a) shows the bands at 919, 1050 and 1217 cm−1 due to C H wagging, C N stretching and C H deformation vibrations of PPy respectively. The bands at 1366, 1566, 2971 and 3896 cm−1 corresponds to stretching vibrations of C N, C C, C H and N H in the pyrrole ring respectively [34]. FTIR of TaS2 (Fig. 6b) shows absorption bands at 587 cm−1 which can be attributed to the Ta-S stretching vibrations and band at 862 cm−1 represents S S stretching vibrations as reported in the literature [35]. Fig. 6c shows strong characteristic absorption bands of PPy and TaS2 with slight shifts confirming the formation of the integrated composite.

3.3. XRD studies

3.5. Humidity sensing studies

X-ray diffraction pattern of pristine PPy, TaS2 and PPy-TaS2 50 wt% composite along with the assigned hkl values are shown in Fig. 5. The XRD pattern of pristine PPy (Fig. 5a) accompanied by its broad peak at 2␪ = 24.77◦ confirming amorphous nature. The appearance of this broad peak is due to the short range of chain ordering of PPy [33]. Fig. 5b shows XRD of TaS2 with assigned planes at 15.0◦ (001), 30.6◦ (002), 34.2◦ (101), 43.6◦ (102), 46.2◦ (003), 54.4◦ (110), 56.4◦ (103), 63.1◦ (004) matches very well with the

Change in impedance modulus of PPy and PPy/TaS2 composites with relative humidity range 11–97% RH is shown in Fig. 7. Initially, PPy and PPy/TaS2 -10%, 30% and 50% were exposed to various RH environments and the respective impedance variation with %RH were observed and related plots are shown in Fig. 7. It can be seen from the plots that the absolute value of impedance found to decrease linearly with %RH. However, among the three composites, PPy/TaS2 -50% exhibited a better linearity and larger

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Fig. 5. XRD patterns of (a) PPy, (b) TaS2 and (c) PPy/TaS2 -50% composite.

Fig. 3. Histograms showing the number frequency versus size distribution of (a) PPy and (b) PPy/TaS2 -50% composite.

variation in impedance with %RH. Thus, this composite offered itself as a potential candidate to device it as a sensor, henceforth its response-recovery times, hysteresis loss, sensing stability, linearity, LOD studies were carried out and comparatively analyzed with pristine PPy. Humidity sensing response of pristine PPy and PPy/TaS2 -50% composite was calculated using Eq. (1) and their variation with %RH is shown in Fig. 8 SH =

Zo − ZRH X 100 Zo

(1)

Where, SH - Humidity sensing response, Zo - impedance at lowest % RH, ZRH - impedance at various % RH. The maximum sensing response of PPy and PPy/TaS2 -50% composite was found to be 73% and 92% respectively. Such a high sensing response of the compos-

ite can be explained as follows: the synergetic effects which has manifested suitable variations in the morphology of the composite as convinced from SEM images. Change in morphology of the composite is due to the best synergy achieved between TaS2 and PPy matrix. This has facilitated an unprecedented upsurge in the adsorption of water molecules, along with increased conduction and a better sensing response. Humidity sensing mechanism can be explained in three steps; namely, chemisorption, physisorption and capillary condensation. At lower RH environments, chemical adsorption of water molecules occur on the active sites of the surface of sensing material, which in turn dissociates into H+ and OH− ions [36] due to self-ionization [29] as shown in Eq. (2). H2 O  H+ +OH− OH−

(2)

ions adsorb chemically on the metal cations due to These the high charge density as well as strong electric field and eventually form a chemisorbed layer. On the other side, H+ ions hop from one active site to the other, which is also associated with the hopping of electrons and thus leading to the overall increase in conduction of the composite [37]. On the other side, at higher RH environments each water molecule join with two OH− ions of the

Fig. 4. (a) TEM (b) HRTEM image of the PPy/TaS2 -50% composite, (c) SAED pattern.

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Fig. 8. Variation of sensing response of PPy and PPy/TaS2 -50% composite with relative humidity.

accounts for conduction due to the hopping of electrons associated with the proton transfer mechanism. This proton transfer takes place rapidly between the water and hydronium ion [29] as shown in Eq. (5). Fig. 6. FTIR of (a) PPy, (b) TaS2 , (c) PPy/TaS2 -50% composite.

H2 O + H3 O+  H3 O+ +H2 O

(5)

Proton transfer occurs through the water molecules by the concomitant cleavage of the hydrogen bond network causing further decrease in impedance, thus sets the humidity sensing mechanism as depicted in Fig. 9. Interestingly, the limit of detection (LOD) is a crucial parameter for devising a material as a humidity sensor. This means, higher the efficiency of the sensing material, lower is the LOD [40]. Hence, LOD of the sensing material under study is given by Eq. (6) using the standard error of estimate from linear regression analysis [41]. LOD values for PPy and PPy/TaS2 -50% composite was calculated to be 15.66 and 10.58 respectively and the corresponding plots are shown in Fig. 10. Such a low LOD of the composite material is potential for the fabrication of humidity sensors. LOD = (3.3D )/b

Fig. 7. Variation of impedance modulus of PPy and PPy/TaS2 composites with relative humidity.

chemisorbed layer through hydrogen bonding as shown in Eq. (3) and this accounts for the first immobile layer of physisorption [29]. H2 O → H3 O+ +OH−

(3)

Further at high RH environments, the water molecules of the humidity adsorb over the first physisorbed layer and these adsorbed molecules move upon the first layer, as if like a bulk water molecule. At these circumstances, water molecules singly bond to the OH− groups and thus form hydronium H3 O+ ions. As per the Grotthuss mechanism [38,39], these ions dissociate in to H2 O and H+ ions as shown in Eq. (4). H3 O+ → H2 O + H+

(4)

Increase in conduction due to the adsorption of water molecules result in donor doping, as water molecules act as electron donors [22]. Water molecules adsorbed on the surface of the composite,

(6)

where, b- slope of the impedance versus %RH curve and D - standard deviation. Further, for any sensing material, timing behavior is an important feature in the design of sensor. Hence response-recovery timing curves for PPy/TaS2 -50% sample is shown in Fig. 11. Initially the composite was exposed to 11% RH environment and instantly it was then transferred to 97% RH, thereby obtaining an appreciable response time of 10 s and then brought back quickly to 11% RH chamber and witnessed with a recovery time of 20 s. The good response and recovery times of the composite can be attributed to its lower grain size as revealed by FESEM image. This reduction in grain size causing increase in surface to volume ratio, leading to the more adsorption of water molecules through macro channels of the composite structures. The longer recovery time is because of slower rate of desorption which is endothermic, due to the greater bonding energy between the adsorbed water molecule and the sensing surface [42]. Contrary to this, response time is fast due to the comparatively faster rate of adsorption, which is spontaneous exothermic reaction [43]. The timing behavior of various PPy composites on comparison with the present composite under study are depicted in Table 1. Now, the present work has evidenced a fair response and recovery parameters of 10 and 20 s respectively and

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Fig. 9. Illustration of the humidity sensing mechanism in PPy/TaS2 composites. Table 1 Comparative study of humidity sensing parameters of various PPy based composites. Hybrid material

Sensing type

%RHrange

Response time (s)

Recoverytime (s)

References

PPy/Quaternized film PPy/TiO2 PPy/Fe2+ PPy/graphene PPy/ZnO PPy/TaS2

impedance type impedance type impedance type impedance type resistive type impedance type

11–95 30–84 11–95 12–90 5–95 11–97

41 40 20 15 12 10

120 20 150 20 8 20

[10] [11] [12] [13] [14] present work

Fig. 10. Linear fitting of the variation of impedance modulus of PPy and PPy/TaS2 50% composite with relative humidity.

proved to be an optimistic material for designing a humidity sensor device. Humidity hysteresis plot for PPy/TaS2 -50% sample is depicted in Fig. 12. During adsorption, sample was exposed to vivid RH environments, in the escalating order from 11 to 97% RH and then adsorption curve was traced. Subsequently, the sample replaced to its previous states in the diminishing order of RH environments from 97 to 11% RH, the respective absolute impedance values were noted and then desorption curve was traced. From Fig. 12, it is clear that the process of desorption occurred at a slower pace than the adsorption. Further, at 53% RH the maximum hysteresis or deviation in impedances between humidification and desiccation processes was found to be around 10% using Eq. (7) as given below [29].



% Hysteresis =

Zmn − Zmp





(Zmax − Zmin )

× 100 %

(7)

Fig. 11. Response and recovery curves of the PPy/TaS2 -50% composite.

Where, Zmn is the maximum value and Zmp is the minimum value of Z at the mean of the %RH, Zmax and Zmin are the maximum and minimum values of the measured absolute impedance in the the range 11–97% RH. Humidity sensing stability of PPy/TaS2 -50% composite is shown in Fig. 13. To check the stability; at every ten days, the composite was taken for investigations at two different %RH and the absolute values of impedance were noted. Even up to sixty days, the impedance was found to be almost identical, inferring that the composite is stable in its sensing performance. This stable sensing ability is due to low oxidation potential of PPy present in the composite.

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Fig. 12. Humidity hysteresis characteristic curve of PPy/TaS2 -50% composite.

Fig. 13. Stability of the PPy/TaS2 -50% composite in humidity sensing.

4. Conclusions PPy/TaS2 composites synthesized using in situ chemical polymerization technique, were successfully studied for the humidity sensing performance. PPy/TaS2 -50% composite evidenced an excellent linear correlation between impedance and relative humidity over the entire range of 11–97% relative humidity. PPy/TaS2 50% composite exhibited superior response and recovery times of 10 and 20 s respectively with good linearity, maximum sensing response of 92%, low LOD, all these turned out to be the salient features in fabrication of a potential room temperature impedance-type humidity sensor. In conclusion, transition metal dichalcogenide TaS2 in the matrix of PPy has exhibited superior humidity sensing properties and thus offering it as a competent material to develop an efficient impedance-type humidity sensor. Acknowledgements All the authors thank Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bengaluru, in providing facilities for structural characterization of the samples. References

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Biographies Mr. A. Sunilkumar has completed his M.Sc. (Physics) from Karnatak University, Dharwad. Now, working as an Asst. Prof of Physics and simultaneously perceiving his PhD at the Dept. of Physics, VTURC, Ballari Institute of Technology and Management, Ballari, affiliated to Visvesvaraya Technological University. His areas of research include impedance, dielectric and humidity sensor studies of conducting polymer composites. Mr. S. Manjunatha has received his M.Sc. (Physics) from Gulbarga University, Kalaburagi, Post-Graduate Diploma in Materials Science from Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bengaluru. He has also worked as an R&D Asst. in Prof. C.N.R. Rao’s lab and Prof. S.M. Shivaprasad’s Lab at JNCASR in 2016-18. Now, working as an Asst. Prof of Physics at Cambridge Institute of Technology, Bengaluru and perceiving his Ph.D. at the Dept. of Physics, VTURC, Ballari Institute of Technology and Management, Ballari. His research interests are functionalization of 2D nanomaterials, electrical characterization and sensor studies of conducting polymer composites. Mr. B. Chethan received his M.Sc. degree in Physics in 2016 from Davangere University, India. He is currently working as a Ph.D student at the Research center, Jawaharlal Nehru National College of Engineering, Shimoga, India. His research interests include studies on humidity and vapour sensors by nano ferrites and nanostructured conducting polymer composites. Dr. Y.T. Ravikiran received his M.Sc. degree in Physics in 2002 and M.Phil. degree in 2003, and Ph.D degree in Physics in 2008 from Gulbarga University, India. He is currently Assistant Professor of Physics in Government Science College, Chitradurga, India. He has published 48 articles in international peer-reviewed journals. His research interests are, studies on humidity and gas sensing properties of conducting polymers and their composites, solid state battery and super capacitor technology using polymer electrolytes. Dr. T. Machappa received his M.Sc. (Physics) in 1994 with second rank and Ph.D. in 2010 from Gulbarga University, Kalaburagi. Presently, he is working as Professor and Head of the Dept. of Physics, Ballari Institute of Technology and Management, Ballari. He has put 25 years of teaching experience and 10 years of research experience, his research fields include conductivity, dielectric, humidity and gas sensor studies of conducting polymer composites.