Organic Electronics 15 (2014) 71–81

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Organic Electronics

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H2S sensing using in situ photo-polymerized polyaniline–silvernanocomposite films on flexible substrates

1566-1199/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.orgel.2013.10.012

⇑ Corresponding authors. Address: Technical Physics Division, BhabhaAtomic Research Centre (BARC), Mumbai 400085, India. Tel.: +91 2225591664; fax: +91 22 25505296 (A. Singh).

E-mail addresses: asb_barc@yahoo.com (A. Singh), chehimi@univ-paris-diderot.fr (M.M. Chehimi), dkaswal@yahoo.com (D.K. Aswal).

1 These authors contributed equally to this work.

Ahmed Mekki a,c, Nirav Joshi b, Ajay Singh a,b,⇑,1, Zakaria Salmi a, Purushottam Jha b,Philippe Decorse a, Stéphanie Lau-Truong a, Rachid Mahmoud c, Mohamed M. Chehimi a,⇑,1,Dinesh K. Aswal b,⇑,1, Shiv K. Gupta b

a Univ Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR CNRS 7086, 15 rue J-A de Baïf, 75013 Paris, Franceb Technical Physics Division, Bhabha Atomic Research Centre (BARC), Mumbai 400085, Indiac Ecole Militaire Polytechnique, BP 17, Bordj El Bahri 16111, Alger, Algeria

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 July 2013Received in revised form 12 October 2013Accepted 13 October 2013Available online 5 November 2013

Keywords:Polymer–metal nanocomposites filmsPhoto-polymerizationChemi-resistive gas sensorXPS

We demonstrate the preparation of flexible polyaniline–silver (PANI–Ag) nanocompositefilms via an in situ facile UV induced polymerization of aniline in presence of AgNO3.The flexible substrates used were (3-aminopropyl)trimethoxysilane (APTMS) modifiedbiaxially oriented polyethylene terephthalate (BOPET) substrates. The APTMS modificationof BOPET surface has two advantages: (i) improved adhesion of the films, and (ii) direc-tional growth of polymer perpendicular to the substrate plane, leading to nanobrush-likemorphology. The PANI–Ag films have been characterized by various techniques, such as,UV/Vis, FTIR, Raman, SEM and XPS. These films were found to be highly selective and sen-sitive to the H2S i.e. chemiresistive response of �100% at 10 ppm with a reasonably fastresponse time of 6 min. PANI–Ag films prepared on pristine BOPET exhibits chemiresistiveresponse of �67% at 10 ppm of H2S exposure. In contrast pure PANI films did not exhibitany response on exposure to H2S. The plausible mechanism(s) of H2S sensing have beendiscussed. This study highlights the importance of surface modification and the role ofAg in PANI matrix for H2S sensing.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Conducting polymer–metal nanocomposites are a newclass of materials, which provide device design possibil-ity with new functionality, where best of both organicand inorganic can be utilized [1]. Different approacheshave been used to prepare conducting polymer-metalnanocomposites in the form of metal nanostructures cov-ered with a polymer shell [2] or vice versa [3], or nano-

structures embedded in the polymer matrix [4]. Amongvarious conducting polymers, polyaniline (PANI) has at-tracted much attention due to ease of synthesis, excel-lent stability under environmental conditions, tunableelectrical conductivity, and biocompatibility [5]. Embed-ding metal nanoparticles (MNp) in PANI is one of theattractive strategies to harness the sensing [6], electro-chemical [7] and catalytic properties [8]. In the contextof chemiresistive gas sensor based on PANI, the incorpo-ration of suitable metal in PANI matrix not only im-proves the sensitivity but also the selectivity towards aparticular gas [9]. In literature, nanocomposites of PANIwith different metals, such as, PANI–Ag [7–11], PANI–Au [12], PANI–Pt [13] and PANI–Cu [14] etc. have beenreported. The synthesis of PANI–metal nanocompositescan be broadly classified into two categories: (i) post

72 A. Mekki et al. / Organic Electronics 15 (2014) 71–81

addition of MNp in the preformed polymer solution [7],however the associated problem in this case is anagglomeration of MNp in the polymer matrix; and (ii)in situ addition of MNp via reduction of metal saltduring the polymerization process, and this approachusually yields a homogenous distribution of MNp [11].Among various nanocomposites, PANI–Ag is gaining con-siderable attention because it can be easily prepared byin situ approach i.e. using direct oxidation of aniline byAg+ in the presence of UV irradiation [7], c irradiation[15], ultrasonication in aqueous media [16] or chemicalpolymerisation in ionic liquid media [17]. Most of the re-ported studies on the PANI–Ag nanocomposites aremainly focussed on the preparation of this material inthe form of powder [11] or drop casted films [7]. How-ever, for many emerging flexible devices, such as, dis-plays, electronics, gas sensors and, it is essential thatthe PANI–Ag films are directly deposited onto the flexiblesubstrates. In fact, conducting polymer (CP) thin films onflexible substrates have been deposited by chemical,electrochemical or photo-polymerization. Electrochemicalmethod has an inherent disadvantage as it requireselectrically conducting substrates. On the other hand,chemical or photo-polymerization process allows in situpolymerization providing thin CP layers on a wide rangeof insulating substrates [18–20]. Unfortunately, thoughthese methods are simple but have two majorlimitations: (i) poor adhesion of CP layer to thesubstrate; and (ii) non-uniform film thickness and mor-phology over a large surface area. The commonly usedstrategy to overcome these problems is the surfacemodification of substrate using suitable coupling agents[20].

In this paper, we report preparation of PANI–Ag nano-composites films on (3-aminopropyl)trimethoxysilane(APTMS) modified bi-axially oriented polyethylene tera-phthalate (BOPET) substrates by in situ photo-polymeriza-tion. We demonstrate that APTMS modified BOPET surfacenot only improves the adhesion of the deposited PANI–Agfilms but also provides a unique brush-like morphology.These PANI–Ag nanocomposites films exhibit superior re-sponse to H2S gas (at parts per million (ppm) level) as com-pared those prepared onto the unmodified BOPET. Theroom temperature detection of H2S using PANI–Ag filmsis very important from application point of view. H2S is atoxic, corrosive, and inflammable gas produced in sewage,coal mines, oil, and natural gas industries and utilized inmany chemical industries. It is important to detect H2S be-cause it is colorless, flammable, heavier than air, and itstoxic limit is 10 ppm [21]. H2S can cause blood poisoningand at high enough concentrations higher than 250 ppmmay lead to death. Therefore, H2S sensor that is sensitiveand rapid in its response at ppm level of H2S exposure isneeded. In this context, thick or thin film sensors basedon semiconductor oxides such as tin oxide, tungsten oxide,and have been widely reported [21a]. A major disadvan-tage of these sensors is their high operating temperaturesthat make them highly power inefficient and reduce theirlong term stability. More important, our PANI–Ag filmsexhibit reversible gas sensing characteristics and are

adherent, flexible in nature, therefore may have longeroperating life.

2. Experimental

2.1. Materials

Aniline (Sigma–Aldrich, purity 98%) was refrigerated inthe dark prior to synthesis. Before using, it was passedthrough a basic alumina powder (MERCK, size �63 lm)filled column to remove the impurities. Silver nitrate (Sig-ma–Aldrich, purity �99.9%) was of analytical grade andwas used as received. The organic solvents used were ofanalytical grade and deionized (DI) water was used forwashing and solution preparation. BOPET sheets of thick-ness �100 lm were procured from DuPont. These sheetswere cut into size of 25 mm � 10 mm using steel scissorfor the experiments. (3-Aminopropyl)trimethoxysilane(APTMS) (Sigma–Aldrich, purity �97%) used for surfacemodification of BOPET. BOPET substrates were ultrasoni-cally washed with chloroform, and ethanol for 30 min,after that dried for 4 h at 70 �C in an oven.

2.2. Surface modification of BOPET substrate by grafting ofAPTMS

The surface modification of BOPET sheets were carriedout by the APTMS i.e. (3-aminopropyl)trimethoxysilanelayer. To graft APTMS on BOPET, first the surface of BOPETwas hydroxylated by dipping it in the potassium hydroxide(KOH) containing dimethyl sulphoxide (DMSO) solution(4 mg of KOH dissolved in 30 ml of DMSO). The sheetswere left to react with KOH for 30 min, then thoroughlywashed in distilled water and dried. The hydroxylatedsheets were then dipped in an APTMS solution (115 mgin 25 ml of ethanol and 1 ml of acetic acid) and left to reactfor 72 h. After this the substrates were thoroughly cleanedusing ethanol and dried for 2 h at 70 �C in an oven.

2.3. Preparation of PANI–Ag nanocomposite films on APTMSmodified BOPET

The photo-polymerization of aniline on APTMS modi-fied BOPET sheets was done in a glass bottle containingfixed amount of distilled aniline 931.3 mg (1 M) along with2 M HNO3 in 10 ml of DI water. HNO3 is added to aniline10 min prior to the photo-polymerization for the proton-ation of monomer. After protonation step of aniline,849.4 mg (0.5 M) AgNO3 is added and finally after a uni-form mixing, solutions were placed under a UV lamp(Spectrolinker, XL-1500UV cross linker) set at wavelength�365 nm, UV source to sample distance �13 cm, intensity�5 mW/cm2 for 6 h. After UV exposure the samples werethoroughly cleaned with deionized water and ethanol toremove the unreacted species. This washing treatmentmay partially de-dope the PANI–Ag films and thereforethe films were subsequently exposed to the HCl vaporsfor about 2 min. Finally the samples were dried for 4 h at70 �C in an oven. In order to investigate the effect of Ag

A. Mekki et al. / Organic Electronics 15 (2014) 71–81 73

on gas sensing characteristics of PANI–Ag films, sampleswith fixed concentration of aniline (1 M) and varying con-centration of AgNO3 (0.5–3 M) were also prepared underidentical experimental condition. Pure PANI films wereprepared with chemical oxidation method using ammo-nium persulphate ((NH4)2S2O8) as oxidant.

2.4. Characterization

The prepared PANI–Ag films were characterized by Fou-rier-transform infrared spectroscopy (Bruker 80 V), Ramanspectroscopy (Horiba Jobin Yvon). Surface morphology ofthe films was imaged using scanning electron microscopy(SEM) (ZEISS SUPRA). X-ray diffraction analysis was carriedout on films using X’Pert PRO (PANalytical) diffractometerusing Co Ka (1.789 Å) radiation. In order to estimate thecrystallite size using Scherrer formula, the instrument con-tribution for broadening of the XRD peak was estimated byusing a polycrystalline Si (with large grains and no stress)as a standard. A pure lorentzian fit is done estimate the fullwidth at half maximum (FWHM) of each peak of the stan-dard, and a caglioti function is obtained (FWHM2 =f(tanh)). In this way we can estimate instruments contri-bution in FWHM at any angle, and this value is subtractedto estimate correct value of the crystallite size.

X-ray photoelectron spectra (XPS) were obtained usinga Thermo VG ESCALAB 250 instrument fitted with a mono-chromated Al K X-ray source. An electron flood gun wasused for charge compensation. The analyzer was operatedat 40 and 100 eV pass energy for the narrow regions andsurvey spectra, respectively. Elemental atomic concentra-tions were calculated from the XPS peak areas and the cor-responding Scofield sensitivity factors corrected for theanalyzer transmission work function. Electrical conductiv-ity of the samples was measured using conventional fourprobe technique (with typical probe separation of 1 mm)and Keithley 2400 source meter. For measurement 10 lAcurrent was passed through the outer probe in the sampleand voltage was measured at inner probe.

In order to perform the gas sensing measurement, fewpairs of Au electrodes (size: 3 mm � 2 mm, thickness�1 lm) separated by 12 lm gap was first thermally depos-ited onto the films (shown in Fig. 6(b)). Gold was chosen asthe electrode materials due to following reasons: (i) it has avery good work function (�5.2 eV) matching with thePANI, hence the films electrode contact is ohmic and (ii)being a noble metal gold does not react with most of thetest gases. The sensor films with Au electrodes weremounted in a leak tight stainless steel chamber having vol-ume of 1000 cm3 and electrical wires were connected be-tween gold electrodes and pins at electrical feed throughof the chamber. The pins at electrical feed through is al-ready connected to and electrical property measurementsetup (Keithley 6487 voltage source/picoammeter). The re-sponse curves towards various test gases were measuredby applying a fix bias of 1 V across the gold electrode andthe time dependence of the current passing between theelectrodes was recorded using computer based data acqui-sition system using Labview software. Gas sensing mea-surements were performed in a static environmentmethod. Required concentration of a test gas in the

chamber was attained by introducing a measured quantityof desired gas using a syringe. All the test gases were com-mercially procured from M/s Chemtron Science Pvt. Ltd.Mumbai, India. For these measurement Keithley 6487 volt-age source/picoammeter was used. Once a steady state wasachieved, recovery of sensors was recorded by exposingthe sensors to air, which is achieved by opening the lid ofthe chamber. The sensitivity (S) of the sensors was calcu-lated from the response curves using the relation:

Sð%Þ ¼ jIg � IajIa

� 100% ð1Þ

where Ig and Ia are the current values of the sensor films intest gas and fresh air, respectively. Response and recoverytimes were defined as the times needed for 90% of totalchange in conductance upon exposure to test gas and freshair, respectively.

Impedance spectroscopy measurement of the sampleswas carried out using Wayne Kerr precision impedanceanalyzer (model: 6500 B) in the frequency range20 Hz–10 MHz.

3. Results and discussion

3.1. Grafting of (3-aminopropyl)trimethoxysilane on BOPETsubstrate

The surface modification of BOPET was carried out bythe APTMS (i.e. (3-aminopropyl)trimethoxysilane) layer.The scheme for grafting of APTMS layer at hydroxylatedBOPET surface is shown in Fig. 1. In brief, the silanizationprocess in general takes place in following steps as [22]:(i) hydrolyse of silane head-groups arriving close to thesubstrate due to adsorbed water layer (inherently present)on the surface, (ii) covalent bonds formation betweenSi(OH)3 groups with the hydroxyl groups on BOPET.

The pristine, hydroxylated and APTMS modified BO-PET substrates were further characterized by the XPS.The XPS results shown in Fig. 2 suggest that pristineand hydroxylated BOPET substrate shows the presenceof only C1s and O1s i.e. whereas the survey scan of theAPTMS modified BOPET shows the two additional peak,i.e. N1s peak at 399.6 eV and Si2p peak at 102.1 eV;which confirms the presence of APTMS layer at the sur-face of BOPET [23]. The elemental composition (in at%)obtained from the high resolution XPS spectra for pure,hydroxylated and APTMS treated BOPET are reported inTable 1.

As seen from Table 1, for pristine BOPET the C/O ratiowas found to be very high (�6.6), which does not matchwith the chemical formula (C10H8O4)n for BOPET [24]. Onthe hydroxylation of BOPET the C/O ratio was found to be�2.7, indicating that BOPET surface is regaining its intrin-sic composition on hydroxylation. For APTMS modified BO-PET the ratio of N/Si was found to be 1.8, which closelymatches with the reported value for other silanes graftedon ITO surface [23,25]. The presence of Si 2p peak at102.1 eV suggests the presence of SiO kind of layer asshown in Fig. 1. It is interesting to note that for the APTMStreated BOPET, the N1s spectra shows the presence of two

BOPET BOPET

OH OH OH

KOH+ DMSO

CH3OH+ HOAc(72 h)

OH

30 min BOPET

(CH2)3

Si

H3CO OCH3OCH3

NH2

(CH2)3

SiO O

O

NH2

(CH2)3

SiO O

δ

δ

OH

NH2

Fig. 1. Scheme showing the surface modification of BOPET substrate by APTMS.

0 200 400 600 800

O1s

N2p

C1s

APTMS modified BOPET

Hydroxylated BOPETI (

arb.

uni

ts)

Binding energy (eV)

BOPET

Si2p

(a)

I (a

rb. u

nits

)

Binding energy (eV)

N1s(c)

396 398 400 402

100 102 104 106

I (a

rb. u

nits

)Binding energy (eV)

Si2p(b)

Fig. 2. (a) XPS survey spectrums of the pristine, hydroxylated and APTMS modified BOPET substrates. (b) Si2p and (c) N1s spectra for APTMS modifiedBOPET.

Table 1Chemical composition of the pristine, hydroxylated, and APTMS modifiedBOPET surface determined from the analysis of the XPS data.

Sample C1s N1s Si2p O1s Comment

BOPET 86.9 – – 13.1 C/O � 6.7Hydroxylated BOPET 73.1 – – 27.1 C/O � 2.7APTMS modified BOPET 70.6 2.4 1.3 25.7 N/Si � 1.8

74 A. Mekki et al. / Organic Electronics 15 (2014) 71–81

peaks at binding energy values of 398.7 and 400.2 eV. Thelow binding energy peak is associated with the nitrogen infree NH2 group while the higher binding energy peak400.2 eV is attributed to the slightly positively chargednitrogen of the NH2. The presence of positively chargednitrogen suggests the possibility of APTMS multilayers for-mation. Process of multilayers formation can be describedby the interaction of head group of the hydrolyzed APTMSmolecules (i.e. Si(OH)3 which is weak acid) in the presenceof adsorbed water layer with the surface NH2 groups (i.e.basic in nature) of the monolayer through the dipole–di-pole interaction as shown in Fig. 1 [23].

3.2. Synthesis of PANI–Ag composite films

In the present work, PANI–Ag nanocomposite filmswere prepared on the APTMS modified and pristine BOPETsubstrates by the photo-polymerization approach. Brieflythe process of photo-polymerization of aniline is given inFig. 3(a). On exposure to UV light, the preformed proton-ated aniline monomer gets excited and then electrons getstransferred from the excited monomer to the Ag+ ions leav-ing aniline radical cation. This radical cation further reactswith another radical cation and resulting in the polymeri-zation of aniline. In this process, Ag+ ion after receive elec-trons from protonated aniline monomer reduced to themetallic state [11].

When APTMS modified BOPET is placed in the mixedsolution of protonated aniline and AgNO3, the surfaceNH2 group of APTMS layer are converted into radicals(NHþ2 ) on exposure to UV [20]. These amino radicals initi-ate the growth of PANI by photo-polymerization processas shown in Fig. 3(b).

Protonatedmonomer

Excitedmonomer

Radicalcation Polymerisedaniline

NH3 Aghv

NH3 Ag365 nm

NH2 Ag0

N

H

N

n

nAg02

BOPET BOPETAgNO3+UV(365 nm)

+HNO3

NH2

(CH2)3

SiO O

O

NH2

(CH2)3

SiO O

δ

δ

OH

NH2

(CH2)3

SiO O

O

NH2

(CH2)3

SiO O

NH

NH

δ

δ

OH

NH

Gro

wth

dir

ecti

on

(a)

(b)

Fig. 3. (a) Mechanism of photo-polymerization of aniline in presence of AgNO3 photo-initiators and UV light (365 nm). (b) Scheme showing the synthesis ofPANI–Ag nanocomposite films on the APTMS modified BOPET substrate using photo-polymerization approach. In this scheme the N atoms denoted by redcolor corresponds to the aniline while the one denoted by black correspond to the APTMS layer. (For interpretation of the references to colour in this figurelegend, the reader is referred to the web version of this article.)

A. Mekki et al. / Organic Electronics 15 (2014) 71–81 75

3.3. Morphology and structural characterization

Fig. 4(a) shows the low magnification SEM image of thesynthesized PANI–Ag films (prepared with AgNO3

� 0.5 M), which reveals the growth of nano-brush kind ofmorphology. A more clear view of nano-brush type of mor-phology is clearly seen in Fig. 4(b), which reveals the nano-fibers with a typical average diameter <20 nm. It may benoted that PANI–Ag films prepared under identical condi-tion on pristine BOPET substrate shows a smooth morphol-ogy (shown in inset of Fig. 4(a)). These results suggests, thepresence of APTMS layers at the BOPET surface controls thephoto-polymerization of aniline in the direction normal ofthe substrate, as schematically shown in Fig. 3(b). Fig. 4(c)shows the high magnification SEM image of the PANI–Agfilms prepared with higher concentration of AgNO3

(�3 M), it reveals the presence of Ag clusters in the PANImatrix. The presence of these Ag clusters in the PANImatrix could be due to the limited solubility of Ag in thePANI matrix.

The XRD pattern recorded for PANI–Ag films shown inFig. 4(d), which reveals the presence of Ag (111) diffrac-tion peak at 44.6 degree along with the BOPET diffractionpeaks at 30 and 65 degrees due to the low thickness ofthe films [26]. From the full width half maximum (FWHM)of the highest intensity (111) peak of Ag, the size of the Agcrystallites was estimated by Scherrer formula as �70 nm.

3.4. Spectroscopic characterization of PANI–Ag films

PANI–Ag films were characterized by UV–visible, FTIR,Raman and XPS. The details such as assignment of thepeak and fitting results are given in Ref. [37]. The UV–visible and FTIR spectra for PANI–Ag films are shownin Fig. 5(a) and (b) respectively. In the UV–visible spectrathree absorption peaks are observed at 340 nm, 692 nmand 430 nm and which are respectively assigned top–p� transition, polaron band to p� band and p bandto polaron band [11]. To clearly distinguish the IR peakof PANI–Ag films from the BOPET substrate, we have alsoplotted the IR data for BOPET substrate in Fig. 5(b). Theappearances of the bands at 1568 cm�1 and 1490 cm�1

benzenoid suggest that both quinoid and benzenoid ringstructure are present in PANI–Ag films. The ratio of peakarea gives the relative concentration of quinoid to thebenzenoid as 43% [27]. The band at 1235 cm�1 is charac-teristics of the electrically conducting form of the PANI[28].

A comparison of the Raman spectra of PANI–Ag film andBOPET (shown in Fig. 5(c)) suggest that BOPET has no con-tribution in the observed pattern for PANI–Ag films. Thedetailed peak assignment given in Ref. [37], suggest thatsample exhibit the characteristic of the protonated stateof PANI and correspond to the formation of delocalizedpolaronic structures in the films [7,28].

0 20 40 60 80

PANI-Ag

I (a

rb. u

nits

)

2θ (degree)

BOPET

(111

)

1 μm

1 μm

(a) (b)

(d)

100 nm

(b)

100 nm

(c) Ag clusters

Fig. 4. (a) SEM image PANI–Ag films (prepared with AgNO3 � 0.5 M) grown on APTMS modified BOPET substrate at low magnification. Inset shows thePANI–Ag films grown on pristine BOPET (b) SEM image taken at 25�tilt, (c) SEM image of the PANI–Ag films (prepared with AgNO3 � 3 M) grown on APTMSmodified BOPET. (d) XRD data for pristine BOPET and PANI–Ag coated APTMS modified BOPET.

692

430

Abs

orba

nce

Wavelength (nm)

340

1490

1178

1568

1444

1290

PANI-Ag

Abs

orba

nce

Wavenumber (cm-1)

Wavenumber (cm-1)

BOPET

3100 3200 3300

3231

(a) (b)

1164

1185

1310

1500

1613

1463

1414

1220

1290

PANI-Ag

I (a

rb. u

nits

)

BOPET

300 450 600 750 900 1200 1400 1600 1800

1200 1300 1400 1500 1600 1700 0 200 400 600 800

PANI-Ag

Cl2

p

O1s

N1s

Ag3

d

I (a

rb. u

nits

)

Binding energy (eV)

C1s

(c) (d)

Fig. 5. Characterization of PANI–Ag nanocomposite films: (a) UV–vis spectra. (b) FTIR spectra. Inset shows FTIR spectra in higher wave number region. (c)Raman spectrum. (d) XPS survey scan.

76 A. Mekki et al. / Organic Electronics 15 (2014) 71–81

A. Mekki et al. / Organic Electronics 15 (2014) 71–81 77

XPS spectra were recorded to obtain the informationabout the various intrinsic/redox states of the PANI–Agfilms. Survey XPS spectrum for PANI–Ag films is shownin Fig. 5(d); it shows the presence of nitrogen(N1s � 400 eV), carbon (C1s � 285 eV), chlorine(Cl2p � 199 eV) and Ag 3d doublet centered at 368.3–374.4 eV [29]. In order to obtain the information aboutthe doping level, high resolution C1s and N1s XPS spectrawere recorded and the quantitative information obtainedfrom the fitting is given in Table 2 (details of the fittingis given in Ref. [37]). The ratio of positively charge nitrogen(N+) with the neutral nitrogen corresponding to amine orthe ratio of counter ions (Cl� and NO�3 ) to the total nitrogengive a doping level of 36%.

In summary, the spectroscopic characterization of thesample indicates that synthesized PANI–Ag films are inthe doped emeraldine form.

3.5. Flexibility and adhesion

In order to demonstrate the mechanical flexibility of thePANI–Ag nanocomposites films synthesized on APTMSmodified BOPET sheet, films were bent to various radius

Table 2Quantitative analysis (in at.%) of N1s and C1s XPS spectra.

ANH (399.1 eV) AN@ (397.2 eV) AN+ (4

60.99 2.41 36.59CAC or CAH (284 eV) CAN/C@N (284.9 eV) CAN+/37.58 23.42 24.96

6 4 20.25

0.30

0.35

Res

ista

nce

(MΩ

)

Bend radius (mm)

FlatFlat

(a)

(c) (d)

(b)

Fig. 6. (a) Digital photograph of PANI–Ag films deposited on silanised APTMS marrangement along with resistance measurement circuit. Right side shows the pPPy-Ag films. (c) Variation in resistance of PANI–Ag films for different bend radBOPET (1) APTMS modified BOPET substrate (2). The films are glued on adhesivewhich shows the material detached from the films by the adhesive tape after 9

of curvatures (r) at room temperature. A photograph show-ing the flexible nature of PANI–Ag nanocomposite films isshown in Fig. 6(a).

In order to measure the electrical resistance (R) of thesamples, few pairs of gold electrodes were deposited onthe PANI–Ag films (photograph shown in right side ofFig. 6(b)). Left side of Fig. 6(b) shows the schematic ofour bending set up along with resistance measurement cir-cuit. The variation of room temperature resistance of theflexible PANI–Ag films as a function of r is plotted inFig. 6(c). It is seen that under flat condition (without bend-ing i.e. r ? /) the films exhibit a resistance of 0.3 MX. Onincreasing the film bending, i.e. a decrease in r, results in avery minor variation in the resistance. The stability ofresistance on bending is only possible if the films areadherent to the substrate and does not create any defectstate or cracks on bending. Thus, a highly stable resistancewith bending implies that these PANI–Ag films are suitablefor the flexible organic devices.

Qualitative adhesion test (90� peel test) was carried outon PANI–Ag films prepared under identical experimentalcondition on pristine and APTMS modified BOPET. The re-sults of adhesion test are shown in Fig. 6(d). The left side

00.4; 402.6 eV) Doping level (N+/N; {Cl-+NO3-}/N)

36.6, 36.04C@N+/CACl (285.5 eV) C@O/CAO (286.7 eV)

14.04

PANI-Ag films on (1) BOPET (2) APTMS modified BOPET

PANI-Agfilm

Au electrodes (separated by 12 µm)

A

90˚ peel test

odified BOPET substrate. (b) Left side shows the schematic of the bendinghotograph of few pairs of gold electrodes (gap � 12 lm) deposited on theius. (d) Left side photograph showing PANI–Ag film prepared on pristinetape under similar load. Right hand side photograph of the adhesive tape,

0� peel test.

78 A. Mekki et al. / Organic Electronics 15 (2014) 71–81

of Fig. 6(d) shows the photograph of the PANI–Ag films (1correspond to films on pristine BOPET and 2 correspond tofilms on APTMS modified BOPET) glued with adhesive tapafter application of uniform weight (�3 kg) for 10 min.Right side photograph of Fig. 6(d) shows the material de-tached from the films by the adhesive tape after 90� peeltest. It can be seen that adhesive tape removes moreamount of material from the films grown on pristine BO-PET surface, while for the films prepared on APTMS modi-fied BOPET negligible material gets detached from thesurface. These results qualitatively suggest that strongeradhesion of PANI–Ag film on APTMS modified BOPETsubstrates.

3.6. Electrical and chemiresistive gas sensing characteristics

Typical room temperature current–voltage (I–V) char-acteristics of the PANI–Ag films (with AgNO3 � 0.5 M) inthe bias range of ±1 V are shown in Fig. 7(a). I–V curvesfor these films are linear indicating an ohmic contact be-tween Au electrode and PANI–Ag films. Similar linear I–Vcharacteristics were observed for all other PANI–Ag filmsprepared with different AgNO3 concentration and the esti-mated conductivity (r) of the films is plotted in the inset ofFig. 7(a). It can be seen that r of the films systematicallyincreases up to AgNO3 concentration �2 M and after thatit decreases at high concentrations �3 M. The decrease inr at higher Ag concentration is quite surprising because

-24

-18

-12

-6

0

6

12

18

Cur

rent

(μA

)

Bias (V)

PANI-Ag (0.5)(a)

0

150

300

450

600

PANI-Ag (0.5M)

Res

pons

e (%

)

Time (min)

PANI-Ag (2M)

(c)

0

20

40

60

80

100

CO

C2H

5OH

Cl 2

NO

NO

2

NH

3Res

pons

e (%

)

Test gases

H2S

10 ppm

-1.0 -0.5 0.0 0.5 1.0

0 20 40 60 80 100 120 140

0

20

40

σ (x

10-3) Ω

-1cm

-1

AgNO3 conc.(M)

0 1 2 3

Fig. 7. (a) Current–voltage characteristics of PANI–Ag (0.5 M) films. Inset showsResponse curve for PANI–Ag(0.5 M) films at various H2S concentration. Comparfilms prepared on APTMS modified BOPET and pristine BOPET. (c) Comparativeselectivity histogram of PANI–Ag(0.5 M) at 10 ppm concentration of differenconcentration. Inset shows the response (%) of PANI–Ag films prepared with dif

by increasing the Ag content in the PANI matrix should in-crease the r. As discussed earlier, PANI–Ag films preparedwith AgNO3 � 3 M exhibit the presence of large number ofAg clusters embedded in the PANI matrix. These big size Agclusters can form schottky barriers at the PANI/Ag inter-faces impeding the easy flow of the charge carriers, andtherefore lowers r. Earlier we have observed similarbehavior for the polypyrrole-silver (PPy-Ag) nanocompos-ite films, where r is low for the samples prepared at veryhigh AgNO3 concentration [26].

The chemiresistive gas sensing properties of PANI–Agfilms was investigated by the exposure of 10 ppm of eachtest gases such as NH3, H2S, Cl2, NO, NO2, CO, CH4, andC2H5OH. Among all gases PANI–Ag films showed the re-sponse for H2S only. The typical response curves recordedof PANI–Ag films (prepared with AgNO3 � 0.5 M) for differ-ent concentration of H2S exposure are shown in Fig. 7(b),which reveals that there is an increase in current on expo-sure to H2S. Considering the electron donating nature ofH2S (reducing gas); the increase of current instead of ex-pected decrease for PANI–Ag films for H2S gas is quiteinteresting.

Now we discuss the mechanism for response of PANIfilms towards H2S gas. H2S gas is a weak acid and PANIgives a robust response to strong acids since they havethe ability to fully dope PANI (resulting a very large changein conductivity). Weak acids only partially dope the PANIand hence the response of PANI towards H2S is expected

Cur

rent

(μA

)

5

10

15

20

1510

15

25 p

pm

Time (min)

(b)

0 100 200 300 400 500

0 5 10 15 20 250

100

200

300

400

500

600

700

2 M

1 M0.5 M

Res

pons

e (%

)

H2S concentration (ppm)

3 M

(d)

225

300

375

450

Res

pons

e (%

)

AgNO3 conc.(M)0 1 2 3

0 5 10 15 20 250

75

150

225

300

PANI-Ag/BOPET

Res

pons

e (%

)

H2S concentration (ppm)

PANI-Ag/A

PTMS/BOPET

the base resistance of the films as a function of AgNO3 concentration. (b)ative response (%) as a function of H2S concentration for PANI–Ag(0.5 M)response curve for PANI–Ag (0.5 M) and PANI–Ag (2 M). Inset shows thet gases. (d) Response as a function of H2S concentration and AgNO3

ferent AgNO3 concentration at 25 ppm of H2S exposure.

A. Mekki et al. / Organic Electronics 15 (2014) 71–81 79

to be minimal. Therefore in order to improve the responseof PANI for H2S, their composites have been used [12,30–33]. Although in literature various mechanism have beenproposed to explain the interaction of H2S with PANI basedcomposites but the most suitable explanation related toour work can be described on the basis of dissociationH2S on the metal surface under ambient condition becauseit is a weak acid (acid dissociation constant pKa = 7.05)[34]. The dissociation of H2S results into H+ and HS� ions.The resulting HS� anion compensates for the positive N+

charges in the PANI chains but there is also proton libera-tion in the films. Since the mobility of cation (H+) is muchlarger than the anion (HS�), therefore the overall effect isthe slight conductance rise on exposure to H2S [35]. Thismechanism is supported by the two facts: (i) with increas-ing the AgNO3 concentration the response of PANI–Agfilms enhances (shown in Fig. 7(c) and (d)) due to availabil-ity of large numbers of Ag sites. (ii) In this work, we havealso prepared pure PANI films on APTMS modified BOPETusing chemical oxidation, but these samples do not exhibitany response to H2S, which suggest the importance of Agparticles in the PANI matrix for H2S sensing [12a].

From Fig. 7(d), it can also be seen that PANI–Ag filmsprepared at very high concentration of AgNO3 � 3 M (>ani-line concentration � 1 M) exhibit less reponse due to thefact that the effective area (which govern the conductivitychange on H2S exposure) of PANI matrix is reduced due tolarger size of embedded Ag clusters.

For comparison purpose, we have also investigated theresponse of PANI–Ag films (prepared with AgNO3 � 0.5)prepared on pristine BOPET surface, these films exhibit re-sponse �67% at 10 ppm of H2S exposure (inset of Fig. 7(b)).The lower response of these films might be mostly due tothe smooth surface exhibiting smaller surface area for

0

40

80

120

160

H2S exposed (50 ppm)

PANI-Ag (2M)

Fresh

R0

R1

C1

(a)

0 50 100 150 200 250

-Z"

(kΩ

)

Z' (kΩ)

(c)

Fig. 8. Impedance spectra of the fresh and H2S exposed (a) PANI–Ag (2 M) films (resistance–capacitor (RC) network model shown in (c).

reaction. As discussed earlier in the APTMS modified BO-PET, the PANI–Ag films exhibit nano-brush type of mor-phology with large surface area, which not only offersvery large area for interaction with H2S but also providesan easy access of the Ag interaction sites for fast re-sponse/recovery. The gas sensing results (such as lowestdetection limit of 1 ppm with a high response �100% andfast response time �6 min at 10 ppm) obtained in presentwork are better than earlier reported results on PANI-inor-ganic nanocomposites films [12,30–33]. The reliable detec-tion of H2S gases in ppm level using flexible PANI–Ag filmsmakes them attractive candidates for gas sensingapplication.

3.7. Impedance spectroscopy study

Detection of gases is generally carried out by measure-ment of dc resistance of films. Sample resistance measuredusing dc current has contributions from different regions ofthe sample such as intra-grain, grain boundaries and elec-trode–sample interface [36]. Impedance spectroscopy is animportant technique that has been widely used for distin-guishing between different contributions to sensor re-sponse. In this technique, system is perturbed by a timedependent potential of the form V = V0sin(xt) and the out-put signal is described by I = I0sin(xt + /), where / is thephase angle. The ratio V/I is a complex number whichdetermines the impedance (Z) at the corresponding fre-quency. The real (Z0) and imaginary part (Z0 0) of the imped-ance and phase angle depends on the particular nature ofthe dominant conductive behavior (such as resistive,capacitive, or inductive) present in the system at a givenfrequency range.

0 200 400 600 8000

100

200

300

400

500

-Z"

(kΩ

)

Z' (kΩ)

H2S exposed (50 ppm)

Fresh

PANI(b)

b) PANI films. Solid line represents the fitting of the spectra using a single

Table 3Impedance parameters obtained for PANI–Ag (2 M) and PANI films byfitting experimental data to the equivalent circuit.

Sample R0 (X) R1 (kX) C1 (nF)

Fresh PANI–Ag (2 M) 2 255 2.1H2S Exposed PANI–Ag (2 M) 2 36.5 125Fresh PANI 13 758 100H2S Exposed PANI 13 669 105

80 A. Mekki et al. / Organic Electronics 15 (2014) 71–81

We have performed the impedance measurement onthe PANI–Ag (2 M) films, which exhibits highest responsetoward H2S gas among all synthesized samples. Theimpedance measurements were carried out on fresh film(in air) and under exposure of 50 ppm of H2S gas, obtainedspectra is plotted in Fig. 8(a) in the form of Nyquist plot. Itcan be seen that impedance spectra shows a perfect singlesemicircle from low frequency to high frequency region,and it can be fitted into an equivalent circuit as shown inFig. 8(c), which consist of frequency independent resis-tance (R0) and a resistor (R1) and capacitor (C1) in parallel.The corresponding mathematical equation for this equiva-lent circuit can be described as:

Z ¼ Z0 þ jZ00 ð2Þ

where Z0 ¼ R0 þ R1

1þðxR1C1Þ2and Z00 ¼ xR2

1C1

1þðxR1C1Þ2.

The physical significance of this model can be describedas: R0 is the resistance between PANI–Ag films and Auelectrodes, while R1 and C1 is the respective interfacialresistance and capacitance between PANI nanofibers. Herex ¼ 2pf , and f is the frequency of A.C. Signal. From Fig. 8(a)it can be seen that there is very good agreement betweenthe simulated (shown by solid lines) pattern and experi-mental data and the values of fitted parameters are givenin Table 3. It is seen that on exposure with H2S gas, R1 fallswhile C1 increases. This result is supported by the fact thaton interaction of H2S with PANI–Ag, large numbers ofcharge carriers are generated at the surface of PANInano-fibers, which will lowers the inter-fiber resistanceand will also increase the inter-fiber capacitance.

The impedance spectra for pure PANI films under air andunder 50 ppm of H2S exposure are given in Fig. 8(b). A com-parison of the impedance spectra for pure PANI and PANI–Agfilms indicates that impedance spectra for pure PANI filmsexhibits a very small changes even on exposure of very highH2S concentration of 50 ppm. For pure PANI films imped-ance spectra also shows a perfect single semicircle fromlow frequency to high frequency region, which can be fittedinto an equivalent circuit as shown in Fig. 8(c). The obtainedparameters for pure PANI film are shown in Table 3, it can beseen that on exposure to H2S, parameter R1 decreases and C1

enhances marginally. This result is also supported by the factthat on exposure to very high H2S concentration of 50 ppm,pure PANI films exhibit a response �15% while PANI–Agfilms exhibit a very high response �550%.

4. Conclusions

In summary, we have modified the surface of BOPET sub-strates by APTMS for the covalent anchoring of PANI–Ag

nanocomposite films synthesized via photo-polymerizationprocess. We demonstrated that PANI–Ag films prepared onAPTMS modified BOPET substrate results in a nano-brushkind of morphology with very high surface area. The PANI–Ag films show high response with reversible conductivitychange on ppm level (1–25 ppm) exposure of H2S. Thesesensors have fast response time � few min. Interestinglyhigher response of the PANI–Ag films prepared on APTMSmodified BOPET was noted compared to the films preparedon pristine BOPET, which highlights the paramount impor-tance of surface modification of the substrate in tuning theapplication potential of the deposited materials.

Acknowledgements

The authors would like to thank the Indo-French Centrefor the Promotion of Advanced Research (IFCPAR) forfinancial support through the ‘‘Flexi-Sensors’’ Project No.4705-2. This work is also supported by ‘‘DAE-SRC Out-standing Research Investigator Award’’ (2008/21/05-BRNS)granted to D.K.A.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.orgel.2013.10.012.

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