gas sensing and power harvesting polyvinylidene fluoride nanocomposites … · 2020-02-25 · gas...

Gas Sensing and Power Harvesting Polyvinylidene FluorideNanocomposites Containing Hybrid Nanotubes

DEEPALEKSHMI PONNAMMA ,1,4 ASHOK K. SHARMA,2

PRIYA SAHARAN,2 and MARIAM AL ALI AL-MAADEED1,3

1.—Center for Advanced Materials, Qatar University, P O Box 2713, Doha, Qatar.2.—Department of Materials Science and Nanotechnology, Deenbandhu Chhotu Ram Universityof Science and Technology, Murthal, Haryana 131001, India. 3.—Materials Science andTechnology Program (MATS), College of Arts and Sciences, Qatar University, P O Box 2713,Doha, Qatar. 4.—e-mail: [email protected]

Gas sensing properties at room temperature and energy harvesting perfor-mances are realized for the polyvinylidene fluoride (PVDF) nanocompositescontaining titanium dioxide (TiO2) nanotubes grown in the presence of carbonnanotubes (CNT). While hydrothermal reaction is practiced for the develop-ment of TiO2/CNT hybrid nanotubes, spin coating is done for the nanocom-posite films to be deposited on sensing electrodes. Influence of various fillerconcentrations and the synergistic combination of fillers on the sensingcharacteristics are studied by recording the response times and the stability ofthe results. Upon exposure to liquefied petroleum gas, the PVDF/TiO2-CNT(2.5 wt.%) gas sensor shows a sensing response of 0.45 s (400 ppm LPG),approximately nine times higher than the composite containing 2.5 wt.% ofTiO2 or 2.5 wt.% CNT. The piezoelectric response of the samples is also re-corded and correlated with the synergistic influence of the filler materials. Thecurrent study can stimulate a good trend in fabricating self-powered gassensors from PVDF nanocomposites.

Key words: Filler synergy, gas sensing, nanocomposite, nanogenerator,solvent vapors

INTRODUCTION

Advancement of flexible and conductive polymernanocomposites requires the presence of a highlyconducting network arranged through electricallyconducting filler units.1 Carbon based fillers includ-ing carbon black, carbon fibers, carbon nanotubes(CNT) and graphene can improve electrical conduc-tivity of various polymer nanocomposites in additionto mechanical stability.2,3 The intrinsic conductivityof such fillers enable their nanocomposites formultifunctional applications in sensing, actuators,biomedical devices for health monitoring, etc.4,5

Sensing depends on the variation in electricalresistance when the nanocomposite is exposed to

external stimulus such as temperature,6 organicsolvent,7 pressure,8 strain9 and the solvent vapors/gas.10 In the polymer nanocomposites containingconductive filler particles, conductive pathways aregenerated by direct contact between the nanomate-rials and by tunneling/electron hopping effects. Thismainly happens when the concentration of thenanomaterials becomes well above the percolationthreshold.11

Single and multi-walled carbon nanotubes(SWCNT and MWCNT) are promising nanomateri-als for improving the electrical and mechanicalproperties of polymers, and many sensors arereported based on these materials.12 However, theoverall conductivity of the polymer/CNT nanocom-posite depends on the filler concentration, nature ofconnecting networks in terms of dispersion andorientation of nanotubes and the CNT–polymerinterfacial interactions. This is often achieved by

(Received September 29, 2019; accepted December 17, 2019;published online January 3, 2020)

Journal of ELECTRONIC MATERIALS, Vol. 49, No. 4, 2020

https://doi.org/10.1007/s11664-019-07915-y� 2020 The Author(s)

2677

http://orcid.org/0000-0002-5208-4870

http://crossmark.crossref.org/dialog/?doi=10.1007/s11664-019-07915-y&domain=pdf

surface modification or compatibilization techniquesincluding chemical functionalization of nan-otubes.13,14 Wang et al. applied the principle of 3Dself-segregated structures for fabricating poly-dimethylsiloxane/MWCNT nanocomposite withhigh electrical conductivity of 0.003 S m�1, at0.2 vol.% of MWCNT, maximizing the interfacialinteraction through the formation of chemicalbonds.15 Hybrid nanocomposites are other alterna-tives to achieve maximum properties with excellentfiller dispersion due to the synergistic influences ofthe fillers. Hosseini and Yousefi16 proposed a piezo-electric sensor from the electrospun fibers ofpolyvinylidene fluoride (PVDF) containing differentratios of MWCNT and Cloisite 30B. While the claynanolayers enhanced the beta phase content of thePVDF, and thus the piezoelectric properties, theMWCNT decreased the impedance and increasedthe dielectric constant of the composite. CNT dec-orated with gold nanoparticles with a unique 1D/0Dstructure was developed on a commercial PVDFmembrane targeting an application of a surfaceenhanced Raman spectroscopy sensor.17 Those plas-monic nanostructures maintained good uniformityin dispersion and were capable of detecting variousmolecular contaminants in food and aqueous sam-ples. This hybrid architecture for the nanoparticlescan enhance the characteristic properties because ofthe synergistic effect of material behavior and bythe uniform distribution of each of them throughnetwork formation.

PVDF is an electroactive polymer with stretch-able design, good biocompatibility and low acousticimpedance so that many sensors and lightweightenergy harvesting devices are made of this polymerbased nanocomposites.16,18–22 Since the beta phaseof the PVDF contributes to its polarizability andpiezoelectricity, the alpha to beta or gamma to betaphase conversion is highly anticipated while incor-porating various fillers into it. PVDF based flexiblesensors are attaining much significance as piezo-electric nanogenerators, sound absorbers and typi-cal sensors.16 Since PVDF is thermally, chemicallyand optically stable to radiation exposures, struc-tural health monitoring applications of PVDF basedmaterials in civil infrastructures and airplanes aresignificant.11 Georgousis et al. reported the electri-cal resistance variation of PVDF composites uponthe incorporation of different concentrations ofMWCNTs.2 The authors noticed that a higherpercolation threshold of 8 wt.% is more suitable forthose applications demanding high deformation orstress whereas a lower concentration limit of1.5 wt.% is suitable for measuring lower strains.The whole study focused on the impact of CNTconcentrations on the nanocomposite sensing andthe formation and breaking of filler networks. Theability of better conducting network formation of theCNTs, when compared to reduced graphene oxide,was also reported by Hu et al. by the fabricatedpolylactic acid nanocomposite strain sensors.23 This

is because of the better rearrangement of CNTnanotubes when compared to the slippage observedin reduced graphene oxide layers. Since power-generating sensors are applicable in remote sens-ing, autonomous and wireless sensors, detailedinvestigation of their fabrication has the mostsignificance.

Our group has investigated the piezoelectric andsensing properties of the PVDF composites in thepresence of semiconducting and carbonaceous fil-lers.8,18–21 In addition, the solvent sensing charac-teristics of PVDF nanocomposites containing ahybrid combination of titanium dioxide (TiO2) nan-otubes and MWCNTs were addressed in ourstudy.24 The present paper is a subsequent studyof sensing characteristics of the same PVDF/TiO2-MWCNT composites towards the liquefied petro-leum gas (LPG). Synergistic influence of the fillercombination on the electrical and sensing propertiesof piezoelectric PVDF through better interfacialinteractions and crystallinity enhancement is themain motive behind the research. The TiO2 semi-conducting nanotubes are grown in the presence ofCNTs by the hydrothermal method, and later addedto the PVDF matrix by solution casting. The sensingand piezoelectric energy harvesting properties ofthe composites are correlated with the networkformation and percolation threshold values and theLPG sensing profiles are recorded with and withoutthermal treatment. The responses of the sensortowards different solvent vapors and its self-power-ing capacity by means of piezoelectric output volt-age are investigated. The dielectric properties arealso studied to explain the energy storage perfor-mance of the samples. A good correlation is drawnbetween the presence of piezoelectric crystallinephase formation in PVDF, the synergistic influenceof hybrid filler nanotubes, and the sensingresponses towards the various gases.

EXPERIMENTAL TECHNIQUES

Materials

PVDF pellets of molecular weight Mw

1,80,000 g/mol, TiO2 anatase powder and theMWCNT were commercially obtained from SigmaAldrich. Solvents, acetone and dimethylformamide(DMF) and sodium hydroxide (NaOH) were pur-chased from British drug house (BDH) chemicals,Doha.

Synthesis of TiO2 Nanotubes and CNT-TiO2

Hybrid Nanotubes

TiO2 nanotubes were synthesized by thehydrothermal method.25 For this specific quantity(2.4 g) of TiO2 anatase micropowder was mixed with10 N NaOH (40 ml) by continuously stirring for15 min and then transferred to an autoclave kept at130�C, and the heating was continued for 10 h. Thesame procedure was done in the presence of 0.01 g

D. Ponnamma, A.K. Sharma, P. Saharan, and M.A.A. Al-Maadeed2678

carbon nanotube (CNT) so that TiO2 nanotubes aregrown on it.24

Synthesis of PVDF Nanocomposites

Solution casting method was employed for fabri-cating the PVDF nanocomposites containing CNTand TiO2 nanotubes. For this, specific amounts (2 geach) of PVDF pellets were magnetically stirred at70�C for 2–3 h in DMF/Acetone 1:1 solvent mixture.The CNT, TiO2 nanotubes and CNT-TiO2 nanohy-brids in particular weight percentages (1 wt.%,2.5 wt.% and 5 wt.%) were separately dispersed inthe same solvent mixture by ultrasonication andadded to the PVDF dissolution by room temperaturemagnetic stirring overnight. Final composite filmswere obtained by drying the dispersions in an ovenand hot pressing at 170�C. There were six sets ofsamples for the analysis as represented below.

Neat PVDF (Neat), PVDF with 1 wt.% CNT-TiO2

hybrid nanomaterial (PCT1), PVDF with 2.5 wt.%CNT-TiO2 hybrid nanomaterial (PCT2.5), PVDFwith 5 wt.% CNT-TiO2 hybrid nanomaterial(PCT5), PVDF with 2.5 wt.% CNT (PC), PVDF with2.5 wt.% TiO2 (PT).

Characterization Methods

Sensing experiments were done using speciallydesigned chambers.26,27 Prior to the experiments,samples were coated on pre-patterned interdigi-tated electrodes (Fig. 1) by drop drying process.These coated electrodes were dried at 40�C for 1 h inan oven. For sensing, an in-house gas sensorchamber was made with specific inlet and outletvents for the test gases to enter and exit. Theresistance changes when analyte gases contact thesamples, and these variations were monitored usingthe Keithley multimeter (Model 2750), connected to

a computer. The exit vent was connected to asuction pump to remove the gases and the responses(sensitivity) were calculated by the following Eq. 1.

S ¼ Ra �Rg

Ra; ð1Þ

where Ra and Rg represent the resistance values inair and gas, respectively.

Piezoelectric characteristics of the samples wererecorded using the laboratory set up consisting of afrequency generator, amplifier, vibrating shakerand data acquisition unit.28 While the shakervibrates according to the amplified generated fre-quency (15–50 Hz), the specific weights (2.5 N) kepton the sample surface imparted compressive forceon the sample. These vibrational forces createddipole moment variations within the samples andgenerated piezoelectric voltage signals which wereobtained from a data acquisition software. For thepiezoelectric energy harvesting experiments, thesamples were made by electrically poling the solventcasted film of 0.2 mm thickness at 8 kV for 7 s.18,22

Sample morphology was analyzed by scanningelectron microscopy (SEM) and transmission elec-tron microscopy (TEM). The SEM studies wereconducted by (SEM, XL-30E Philips Co., Holland),TEM by a Transmission Electron Microscope FEITECNAI G2. FTIR spectra of the samples were witha PerkinElmer Spectrum 400 spectrophotometer inthe range 400–4000 cm�1 with a resolution of2 cm�1. X-ray diffraction studies were performedby XRD diffractometer (Mini Flex 2, Rigaku). Nickelfiltered CuKa radiation (k = 0.1564 nm) operated at30 V and 15 mA served as the source. The patternswere recorded in the 2h range of 5�–80� at ascanning speed of 1.8�/min. Broadband dielectric/impedance spectroscopy-Novocontrol was used for

Fig. 1. Schematic representation of the sample preparation.

Gas Sensing and Power Harvesting Polyvinylidene Fluoride Nanocomposites ContainingHybrid Nanotubes

2679

measuring the dielectric properties of the samples.The dielectric constant, dielectric loss, tan d andconductivity values of the samples were checkedduring the frequency range of 10�2 Hz to 106 Hz.

RESULTS AND DISCUSSION

Figure 1 outlines the preparation processesinvolved in the PVDF nanocomposite film develop-ment. The TiO2 powders are spherical shaped andthe hydrothermal reaction ruptures the sphericalmorphology transforming them to layers and there-after tubes. The TiO2 nanotubes developed in thepresence of MWCNTs form a hybrid structuralpattern, where the nanotubes are segregated andwell distributed.24 After purifying the hybrid pow-der, it was dispersed in DMF: Acetone solventmixture and finally mixed with PVDF. Later itwas magnetically stirred and developed to flexiblenanosheets as indicated in Fig. 1. The coated sam-ples on the sensing electrodes are also shown in thefigure. Such electrodes with coated samples wereused for the LPG and solvent vapor sensingexperiments.

The sensing characteristics of the samplestowards LPG from 100 ppm to 700 ppm concentra-tion are plotted in Fig. 2. All polymer nanocompos-ite samples are not analyzed because of the non-conducting (very high resistance) behavior of cer-tain samples. This includes the neat polymer with-out any conductive filler particles embedded in itand the PCT1 sample containing conductive fillerparticles below the percolation threshold.29 Allother samples—PCT2.5, PCT5, PT and PC—showedincreasing sensor responses with respect to LPGconcentration (Fig. 2a). However the response isquite high for the sample PCT2.5 containing thehybrid CNT-TiO2 nanotubes in 2.5 wt.%. This isbecause, at the 2.5 wt.% filler amount, the concen-tration is near to/above the percolation thresholdvalue, and this initiated good network formationamong the nanotubes.30 However the PC and PTsamples, which contain CNT and TiO2 nanotubes in2.5 wt.%, showed lower sensing responses. Thisresult shows that the hybrid nanostructureenhanced the individual nanotube dispersabilitywithin the PVDF chains.31

Filler synergy enhanced sensing capacityobserved in this report is comparable with theLPG sensing activity for the ZnO-MWCNTnanocomposite.32 Maximum average sensitivity of41.95 for 1500 ppm LPG gas with a sensingresponse of 61.57 s was observed in their report,and our results show better response time (0.45 s) atlower LPG concentration (400 ppm). In addition,the widely reported polyaniline composites also givecomparatively lower sensing response, at higherfiller concentrations. For instance the Fe2O3/polyaniline composites report response time of 60 sfor 200 ppm LPG at 3 wt.% composition 33 and theTiO2/polyaniline composite reports 90% sensing

response at 30 wt.% composition (400 ppm LPG).34

These results clearly show the significance of flex-ible sensing devices at lower filler concentration andwith better response time.

Sensing responses by different PVDF compositesdepend on various factors. During LPG exposureelectron transfer reactions occur within the sam-ples, these redox reactions have positive influence incausing resistance variations. In addition, the sam-ple surface acts as adsorption sites for gas moleculesby replacing the atmospheric oxygen. When thesample is exposed to LPG, the oxygen present in thesurface sites react with the gas molecules throughvarious intermediate reaction steps.10,26

CnH2nþ2 þ 2O� ! CnH2n : O þ e� þ H2O

CnH2n : O þ O� ! CO2 þ H2O þ e�;

where the CnH2n+2 represents the alkanes such asC4H10, C3H8 and CH4 and CnH2n:O represents thepartially oxidized reaction intermediate on thesample surface. It is clear from Fig. 2b that concen-tration of LPG enhances the sensing responses ofthe composite samples. At a particular temperatureof 50�C, all samples showed pronounced influenceon sensing the LPG analyte molecules. However,the PCT2.5 sample showed the consistent result.Figure 2c represents the variation in sensing char-acteristics with temperature at a particular concen-tration of LPG (400 ppm).27 For PT, PC and PCT5samples, the increase in sensing response is noticedwith temperature that can be due to the reductionin adsorbed water molecules causing increasedresistance.26 Good temperature sensitivity isobserved for the PCT2.5 sample, in which theCNT-TiO2 hybrid nanotubes are present at2.5 wt.%.

Resistance variations for PCT2.5 to different LPGconcentrations at room temperature is depicted inFig. 3a. A gradual increase in resistance wasobserved for the nanocomposite when the LPGconcentration was changed from 100 ppm to700 ppm. The recovery of this particular nanocom-posite was found to be good as it returns to itsoriginal stage quickly on exposure to air.30 Theresistance variation of PCT2.5 according to thetemperature differences is represented in Fig. 3b.Though large variations in resistance was seen atlower temperatures, higher temperatures above50�C imbalanced the resistance variations. Thismight be due to the higher activation energy of gasmolecules and the higher rate of electro transferreactions.35 It can be correlated with the synergisticfiller effect and the better network formation of bothfillers.

The sensing responses of the sample towardsdifferent solvent vapors were also studied to test theselective performance.19 Figure 4a shows the rela-tive resistance variations of the PVDF-2.5CTNTsample towards the vapors of benzene, acetone,


chloroform, ethanol and SO2. In the case of hybridcomposite, the interaction between the analytemolecules and the composite surface was highercompared to the only TiO2 filled or CNT filledPVDF. This causes the sensing responses to belower than that of the hybrid CTNT filled PVDF.The selective LPG sensing is attributed to the gooddiffusivity of the analyte, better nanocompositeanalyte interaction and the charge transfer reactionbetween the target molecules36,37 (all due to betterfiller dispersibility and network formation). Thereusability and long term stability of the samplewere also investigated by recording the sensingresponses as relative resistance variations over aperiod of 15 days especially for the LPG gas at400 ppm concentration. This is shown in Fig. 4b.The uniform response of the PVDF/CTNT2.5 duringthe analysis of about 30 days confirms the ability of

using this tailored nanocomposite in long-term gassensing applications.

Figure 5a illustrates the piezoelectric voltagevariations observed in the case of PVDF and itscomposite samples. The corona poled films of thesamples exhibited comparatively lower piezoelectricoutput voltage, attributed to the presence of con-ducting fillers, and thus the electron transfer reac-tions taking place within the sample.38 Though theneat PVDF and its composites containing TiO2

nanotubes at 2.5 wt.%, CNT/TiO2 nanotubes hybridat 1 wt.% and 2.5 wt.% showed output voltagevariations when a compressive force of 2.5 N isimparted on them. The samples with higher contentof hybrid nanoparticles and the one with CNT didnot show any voltage output. This can be due to thehighly conducting nature of the nanocomposites.39

Among neat PVDF, PT, PCT1 and PCT2.5, the

Fig. 2. Sensing response towards LPG at various concentrations (a) at room temperature, (b) at 50�C temperature and (c) sensing responses for400 ppm LPG with temperature variations for all composite samples.


2681

PCT1 composite showed the highest piezoelectricoutput voltage of 1.3 V. This is attributed to theuniform distribution of hybrid40 nanotubes withinthe PVDF medium and the ability of the nanotubesto impart good b-phase crystallinity to the basepolymer.41 Influence of nanoparticles on the crys-tallinity of polymers and their piezoelectric voltagegeneration was well investigated by our group. Thecorona poled samples of PVDF copolymer (PVDF-HFP) containing barium titanate/hexagonal boronnitride (3 wt.%/1 wt.%)18 and graphene oxide-tita-nia/strontium titanate (1:2)22 respectively showedmaximum output voltages of 2.4 V and 2.0 V, bothbeing higher than the current performance. This isdue to the piezoelectric nature of the filler particlesused in the reported works and the larger fillerconcentration.

Though the best output voltage was observed forthe PCT1 sample, best LPG sensing was observed

for the PCT2.5 and so this particular sample isselected to test the variations in maximum outputvoltage during exposure to the gas molecules. Theresults are shown in Fig. 5b. The PCT2.5 showed amaximum piezoelectric output voltage of 0.8 Vwhich is observed to decrease during the exposureto LPG. The exposure to LPG was done at regularintervals on the same sample between which N2 gaswas passed through the sample to achieve neutralnature. During the LPG exposure at 400 ppm thevoltage changes to 0.3 V. This voltage variation ispronounced when compared to the similar outputpiezo voltage (0.533 V in air shifts to 0.089 V LPG)variations observed for ZnSnO3/ZnO nanowirestowards 8000 ppm LPG.42 As illustrated in thesensing characteristics, the sample exposure toLPG generates electrons within and such electronsmove towards the conduction band. This cases thecarrier density of the nanotubes to be enhanced and

Fig. 3. Dynamic sensor response of the PCT2.5 sample (a) at room temperature for different LPG concentrations and (b) at varioustemperatures towards the 400 ppm LPG.

Fig. 4. Solvent vapor sensing performance by the (a) samples and (b) long term stability results for PCT2.5 towards 400 ppm LPG.


decreases the surface depletion layer width.43 Thecompressive force applied during the piezoelectricexperiment enhances the charge carrier density andcauses piezoelectric screening effect. This finallyresults in the decreased output voltage.43–45

Morphology and structural investigations of thesynthesized hybrid nanotubes are well demon-strated in our previously published research.24

The hydrothermal synthesis of TiO2 nanotubesand the CNT-TiO2 nano hybrid material exhibitedtubular morphology. The dense, interconnected andcoiled network of tubes was seen in the hybrid CNT-TiO2. The tube-in-tube and tube-on-tube structureas TiO2 tubes grow on the walls and inner surfacesof the CNTs were observed. No rupture or formationof chemical bonds was seen in the case of CNT-TiO2

hybrid nanomaterial, indicating only physical/elec-tronic interactions between both tubes.24 Morphol-ogy images for the cryocut surfaces of PVDFnanocomposites are provided in Fig. 6.

The planar morphology of the PVDF neat polymerwas transformed when specific nanomaterials areincorporated in it. The projections observed inFig. 6a, b, c, and d indicates the tubular CNT andTiO2 nanomaterials. Better uniformity in dispersionof nanotubes was observed in Fig. 6b and c, respec-tively, where CNT and CNT-TiO2 were present at2.5 wt.%. This indicates the ability of CNTs inuniformly distributing the TiO2 tubes, as they weregrown within the CNT structures.24 This resultconfirms the hybrid networking effect of nanomate-rials in uniformly distributing themselves withinthe polymer matrix. However, at 5 wt.% (Fig. 6d),the higher concentrations result in uniformity losein dispersion and thus agglomerated structuraldeformations are observed. These results are in

good agreement with the sensing responsesobtained for all samples, and the piezoelectricproperty enhancement.

Structural investigations on the PVDF nanocom-posites were done using the FTIR and XRD studies.As shown in Fig. 7a, the FTIR spectra for allsamples are similar as the pronounced peaks aredue to the C-C stretching, C-H stretching andbending vibrations of the PVDF skeleton.20,28 Nosignificant variation in the peak positions, or peakintensities confirm the absence of any kind ofchemical interactions that the nanomaterialsimpart on the polymer chains. The FTIR spectrashown in Fig. 7a does not provide any informationfor chemical grafting between the polymer and thenanofillers. This also indicates the weak physicalinteractions between the filler and PVDF.18 XRDpatterns of the samples are shown in Fig. 7b withpeaks oriented at 17.5�, 18.5�, 20� and 26.5�attributed to the crystalline phases of PVDF. It isknown that the peaks at 18.5�, 20� and 26.5�correspond to the crystalline alpha phase, withsimilar peak around 20� corresponding to the betaphase. The large intensity of the peak around 20� isattributed to the combined influence of the alphaand beta crystalline phases. There is a slightenhancement in the intensity of XRD peaks indi-cating the effect of tubular filler materials on thecrystallinity values of PVDF.8,18,20–22 This enhance-ment in crystallinity is the main factor behind thehigher piezoelectric output voltage obtained for thesample containing CNT and TiO2 nanotubes.

The surface characteristics of the nanocompositesare explored by means of contact angle measure-ments as provided in Table I. The contact anglevalues show good variation between various

Fig. 5. (a) Piezoelectric output voltage obtained for the samples, neat polymer, PT, PC, PCT1 and PCT2.5 composites and (b) maximum outputvoltage variations when the PCT2.5 sample is exposed to and recovered from 400 ppm LPG.


2683

samples, according to the nature of the nanotubes.It is clear that both nanotubes enhance thehydrophobicity of PVDF, however, at 2.5 wt.%, allnanocomposites show similar contact angle values.This can be due to the better dispersion of nan-otubes in correlation with the SEM results.

The melting and crystallization curves for thePVDF nanocomposites are represented in Fig. 8.The crystallization peaks (Fig. 8a) are sharp indi-cating the uniform crystalline structure of thecomposites. The crystallization kinetics indicatesthe formation of crystallites at higher temperature.When the values are compared there is a slight

Fig. 6. Scanning electron microscopy images for (a) PT, (b) PC, (c) PCT2.5 and (d) PCT5 nanocomposites.

Fig. 7. (a) FTIR spectra and (b) x-ray diffraction patterns for the PVDF/CTNT composites.


change in the crystallization temperature indicatingthe stability of the nanocomposites.22 This is a goodsign as far as the piezoelectric property enhance-ment is considered. The fillers have the ability toenhance the crystallinity of the PVDF, and this inturn influence the piezoelectric output voltage.

The increase in concentration of the filler parti-cles and the mode of dispersion and interfacialstability achieved by the presence of hybrid nan-otubes have pronounced effect on the variation inthese characteristic parameters. In Fig. 8b also, themelting curves show slight change with the additionof filler particles. This indicates the formation ofuniform filler networks within the polymer and theenhanced thermal stability and glass transition forthe PVDF nanocomposites containing CNT-TiO2

hybrid nanotubes.24

The ability to store electric charge in a material isindicated by its dielectric properties, and it ismeasured in terms of dielectric constant, dielectricloss and conductivity values as shown in Fig. 9. Thedielectric constant values enhanced with the nan-otubes addition as clear from Fig. 9a. This dielectricpermittivity enhancement with the presence of fillerparticles is generally explained in terms of Max-well–Wagnar–Sillars effect,10,13 which explains the

significance of heterogeneous interfaces (filler–poly-mer) in regulating electron transfer mechanism.Lower amounts of the particles and the less disper-sion rates do not cause much filler–filler interac-tions and network formation within the polymernanocomposites, causing many defects and voids inthe interfacial junctions. However, increased fillerpercentages connects throughout and make a con-ductive network, this again supports the goodsensing response observed for the PCT2.5 sample.The conductivity values showed in Fig. 9b explainsthe significance of CNT networks in impartingbetter electron transfer and movement as the CNTcontaining samples shows relatively higher conduc-tivity. The TiO2 nanotubes grown on CNT alsoimproves the conductivity through better dispersionand network formation. Thus, the dielectric resultsare also in correlation with the sensing, piezoelec-tric and morphology data.

CONCLUSIONS

This paper describes the fabrication of PVDF-based composite films containing CNT-TiO2 hybridnanotube architecture for their effective applicationin LPG sensing. The hybrid architecture for thenanotubes was obtained by the hydrothermalmethod and solution casted sample films were usedfor conventional characteristic property assessment.The sensor responses depend on the rupture andformation of the conducting network within thesamples, and this is ensured by the well dispersedhybrid CNT-TiO2 network. The piezoelectric outputvoltage varies from 0.8 V to 0.3 V with the exposureto LPG. The dielectric permittivity and tan deltavalues for the composites also followed the similarexplanations of electron transfer and conductivefiller particles-based mechanism. Variation in piezo-electric output voltage of the PCT2.5 sample showsthe applicability of this nanocomposite in fabricat-ing self-powering gas sensors.

Table I. Contact angle measurements for the PVDFcomposites

Sample Contact angle

PVDF 90.83 ± 1.87PT 91.34 ± 3.34PC 92.40 ± 1.92PCT1 90.28 ± 3.54PCT2.5 91.78 ± 3.02PCT5 93.07 ± 1.52

Fig. 8. (a) Melting and (b) crystallization curves of PVDF nanocomposites containing CNT, TiO2 and CNT-TiO2 hybrid nanotubes.


2685

ACKNOWLEDGMENTS

Open Access funding provided by the QatarNational Library. This publication was made pos-sible by NPRP Grant 6-282-2-119 from the QatarNational Research Fund (a member of QatarFoundation). The statements made herein are solelythe responsibility of the authors.

CONFLICT OF INTEREST

The authors declare that they have no conflict ofinterest.

OPEN ACCESS

This article is licensed under a Creative CommonsAttribution 4.0 International License, which per-mits use, sharing, adaptation, distribution and re-production in any medium or format, as long as yougive appropriate credit to the original author(s) andthe source, provide a link to the Creative Commonslicence, and indicate if changes were made. Theimages or other third party material in this articleare included in the article’s Creative Commons li-

cence, unless indicated otherwise in a credit line tothe material. If material is not included in the ar-ticle’s Creative Commons licence and your intendeduse is not permitted by statutory regulation or ex-ceeds the permitted use, you will need to obtainpermission directly from the copyright holder. Toview a copy of this licence, visit http://creative-commons.org/licenses/by/4.0/.

REFERENCES

1. D. Ponnamma, K.K. Sadasivuni, C. Wan, S. Thomas, andM.A. AlMa’adeed, eds., Flexible and Stretchable ElectronicComposites (Berlin: Springer, 2015).

2. G. Georgousis, C. Pandis, A. Kalamiotis, P. Georgiopoulos,A. Kyritsis, E. Kontou, P. Pissis, M. Micusik, K. Czanikova,J. Kulicek, and M. Omastova, Compos. B Eng. 68, 162(2015).

3. K.K. Sadasivuni, D. Ponnamma, J. Kim, and S. Thomas,eds., Graphene-Based Polymer Nanocomposites in Elec-tronics (Cham: Springer, 2015).

4. V. Eswaraiah, K. Balasubramaniam, and S. Ramaprabhu, J.Mater. Chem. 21, 12626 (2011).

5. D. Ponnamma, Q. Guo, I. Krupa, M.A. Al-Maadeed, K.T.Varughese, S. Thomas, and K.K. Sadasivuni, Phys. Chem.Chem. Phys. 17, 3954 (2015).

Fig. 9. Dielectric data for PVDF/CTNT composites versus frequency (a) dielectric strength and dielectric loss, (b) conductivity and (c) tan d.


6. D. Ponnamma, A. Saiter, J.M. Saiter, S. Thomas, Y. Gro-hens, M.A. AlMaadeed, and K.K. Sadasivuni, J. Polym. Res.23, 125 (2016).

7. D. Ponnamma, K.T. Varughese, M.A. Al-Maadeed, and S.Thomas, Polym. Int. 66, 931 (2017).

8. A. Al-Saygh, D. Ponnamma, M.A. AlMaadeed, P. Vijayan, A.Karim, and M. Hassan, Polymers 9, 33 (2017).

9. A. Ferrreira, J.G. Rocha, A. Anson-Casaos, M.T. Martınez,F. Vaz, and S. Lanceros-Mendez, Sens. Actuators A Phys.178, 10 (2012).

10. J.G. Thangamani, K. Deshmukh, K.K. Sadasivuni, D. Pon-namma, S. Goutham, K.V. Rao, K. Chidambaram, M.B.Ahamed, A.N. Grace, M. Faisal, and S.K. Pasha, Microchim.Acta 184, 3977 (2017).

11. K. Ke, P. Potschke, N. Wiegand, B. Krause, B. Voit, andA.C.S. Appl, Mater. Interfaces 8, 14190 (2016).

12. Z. Zhu, L. Garcia-Gancedo, A.J. Flewitt, H. Xie, F. Moussy,and W.I. Milne, Sensors 12, 5996 (2012).

13. N. Raphael, K. Namratha, B.N. Chandrashekar, K.K.Sadasivuni, D. Ponnamma, A.S. Smitha, S. Krishnaveni, C.Cheng, and K. Byrappa, Prog. Cryst. Growth Charact. Ma-ter. 64, 75 (2018).

14. D. Ponnamma, S.H. Sung, J.S. Hong, K.H. Ahn, K.T.Varughese, and S. Thomas, Eur. Polym. J. 53, 147 (2014).

15. M. Wang, K. Zhang, X.X. Dai, Y. Li, J. Guo, H. Liu, G.H. Li,Y.J. Tan, J.B. Zeng, and Z. Guo, Nanoscale 9, 11017 (2017).

16. S.M. Hosseini and A.A. Yousefi, Org. Electron. 50, 121(2017).

17. K. Zhang, J. Ji, X. Fang, L. Yan, and B. Liu, Analyst 140,134 (2015).

18. D. Ponnamma and M. Al-Maadeed, Sustain. Energy Fuel 3,774 (2019).

19. D. Ponnamma, S. Goutham, K.K. Sadasivuni, K.V. Rao, J.J.Cabibihan, and M.A. Al-Maadeed, Synth. Metals 243, 34(2018).

20. H. Parangusan, D. Ponnamma, and M.A. AlMaadeed, SoftMatter 14, 8803 (2018).

21. A. Issa, M.A. Al-Maadeed, A. Luyt, D. Ponnamma, and M.Hassan, Carbon Mater. 3, 30 (2017).

22. D. Ponnamma, A. Erturk, H. Parangusan, K. Deshmukh,M.B. Ahamed, and M.A. Al-Maadeed, Emerg. Mater. 1, 55(2018).

23. C. Hu, Z. Li, Y. Wang, J. Gao, K. Dai, G. Zheng, C. Liu, C.Shen, H. Song, and Z. Guo, J. Mater. Chem. C 5, 2318(2017).

24. M.M. Chamakh, D. Ponnamma, and M.A. Al-Maadeed, J.Mater. Sci. Mater. Electron. 29, 4402 (2018).

25. D. Ponnamma and M.A. Al-Maadeed, Mater. Des. 117, 203(2017).

26. S. Goutham, S. Bykkam, K.K. Sadasivuni, D.S. Kumar, M.Ahmadipour, Z.A. Ahmad, and K.V. Rao, Microchim. Acta185, 69 (2018).

27. J.M. Park, G.Y. Gu, Z.J. Wang, D.J. Kwon, and K.L. DeV-ries, Appl. Surf. Sci. 287, 75 (2013).

28. H. Parangusan, D. Ponnamma, and M.A. AlMaadeed, RSCAdv. 7, 50156 (2017).

29. M. Knite, V. Tupureina, A. Fuith, J. Zavickis, and V. Te-teris, Mater. Sci. Eng. C 27, 1125 (2007).

30. R. Thirupathi, G. Solleti, T. Sreekanth, K.K. Sadasivuni,and K.V. Rao, J. Electron. Mater. 47, 3468 (2018).

31. W.L. Ong, M. Gao, and G.W. Ho, Nanoscale. 5, 11283 (2013).32. R.K. Sonker, M. Singh, U. Kumar, and B.C. Yadav, J. Inorg.

Organomet. Polym. Mater. 26, 1434 (2016).33. T. Sen, N.G. Shimpi, S. Mishra, and R. Sharma, Sens.

Actuators B 190, 120 (2014).34. A. Parveen, A. Koppalkar, and A.S. Roy, Sens. Lett. 11, 242

(2013).35. A. Kaushik, R. Kumar, S.K. Arya, M. Nair, B.D. Malhotra,

and S. Bhansali, Chem. Rev. 115, 4571 (2015).36. R.K. Mishra, S.B. Upadhyay, A. Kushwaha, T.H. Kim, G.

Murali, R. Verma, M. Srivastava, J. Singh, P.P. Sahay, andS.H. Lee, Nanoscale 7, 11971 (2015).

37. A. Wei, L. Pan, and W. Huang, Mater. Sci. Eng. B 176, 1409(2011).

38. F. Yang, Y.M. Wen, P. Li, M. Zheng, and L.X. Bian, Sens.Actuators A Phys. 141, 129 (2008).

39. H. Zhao, J. Bai, and A.C.S. Appl, Mater. Interfaces 7, 9652(2015).

40. W. Brostow, S. Lohse, X. Lu, and A.T. Osmanson, Emerg.Mater. 2, 23 (2018).

41. Y. Ahn, J.Y. Lim, S.M. Hong, J. Lee, J. Ha, H.J. Choi, and Y.Seo, J. Phys. Chem. C 117, 11791 (2013).

42. Y. Fu, Y. Nie, Y. Zhao, P. Wang, L. Xing, Y. Zhang, and X.Xue, ACS Appl. Mater. Interfaces 7, 10482 (2015).

43. P. Wang, Y. Fu, B. Yu, Y. Zhao, L. Xing, and X. Xue, J.Mater. Chem. A 3, 3529 (2015).

44. Z. Qu, Y. Fu, B. Yu, P. Deng, L. Xing, and X. Xue, Sens.Actuators B Chem. 222, 78 (2016).

45. H. He, Y. Fu, W. Zang, Q. Wang, L. Xing, Y. Zhang, and X.Xue, Nano Energy 31, 37 (2017).

Publisher’s Note Springer Nature remains neutral withregard to jurisdictional claims in published maps and institu-tional affiliations.


2687

gas sensing and power harvesting polyvinylidene fluoride nanocomposites … · 2020-02-25 · gas...

Documents