Ammonia sensing and DC electrical conductivity studies of p-toluenesulfonic acid doped cetyltrimethylammonium bromide assistedV2O5@polyaniline composite nanofibers

Mudassir Hasan, Mohd Omaish Ansari, Moo Hwan Cho *, Moonyong Lee *

School of Chemical Engineering, Yeungnam University, Gyeongsan-si, Gyeongbuk 712-749, South Korea

Introduction

Organic-inorganic conducting composites have attracted con-siderable attention worldwide because of their increasing numberof applications in a range of electronic devices, such as chemicalsensors [1], light emitting diodes (LED) [2], electrochromic displays(ECD) [3,4], EMI shielding [5]. Among the conducting polymers,polyaniline (Pani) has been studied extensively in recent yearsbecause of its ease of synthesis, excellent stability, relativelyinexpensive, and a redox state that can be controlled easily bysimple acid base chemistry [6]. Recently, composites of Pani withmetal oxides have attracted considerable attention due to theenhanced properties obtained, i.e. sensing, adsorption [1,7],because of the synergism between the constituents. Among thedifferent metal oxides used, V2O5 has recently attracted theattention of researchers worldwide owing to its outstandingphysical and chemical properties and ability to alter its electricalresistance when exposed to variety of gases [8,9]. This makes it apotential candidate for gas sensing applications.

Pani composites have been readily exploited for the sensing ofvolatile organic compounds. Among these, ammonia is a hazardouschemical substance because of its toxicity. The leakage of ammoniainto the atmosphere poses serious environmental problems.Therefore, the reliable detection of NH3 gas is needed. Recently,Pani has been studied by several workers for ammonia sensingpurposes because of its high surface area exposed to the incominggas and the low penetration depth for gas molecules. Furtherstudies are expected to lower the detection limit, reduce theresponse time, and enhance the recovery of NH3 gas sensors. V2O5

nanorods [8], V2O5 nanoparticles [9], V2O5 nanowires, andnanobelts [8,10] as NH3 gas sensors have also been reported.Accordingly, it is expected that composites of Pani with V2O5 willinduce an enhanced ammonia sensing response due to the synergybetween the constituents.

Many studies have reported inorganic acid-doped Pani sensors[11,12] for NH3 gas sensing, but their main limitation is theinsufficient reversibility for practical use [13]. The lack ofreversibility can be attributed to the difficulty of desorption ofthe adsorbed NH3 due to the formation of salts, i.e., stable NH4Cl inthe case of HCl, leading to the de-doping or neutralization of thePani backbone [14]. Therefore, the aim of this study was to preparepTSA-doped V2O5@Pani composites for NH3 gas sensing. A

Journal of Industrial and Engineering Chemistry 22 (2015) 147–152

A R T I C L E I N F O

Article history:

Received 27 May 2014

Received in revised form 22 June 2014

Accepted 2 July 2014

Available online 9 July 2014

Keywords:

V2O5

Polyaniline

Conductivity

Ammonia sensing

Thermal stability

UV absorbance

A B S T R A C T

This paper reports the synthesis of V2O5@polyaniline (Pani) composite nanofibers by the in situ oxidative

polymerization of aniline in the presence of V2O5 and cetyltrimethylammonium bromide, as a surfactant.

The prepared composite nanofibers were characterized by scanning electron microscopy, transmission

electron microscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, UV–

visible diffused reflectance spectroscopy, thermogravimetric analysis, and differential scanning

calorimetry. The V2O5@Pani composite nanofibers showed higher DC electrical conductivity than Pani,

which might be due to the increased mobility of the charge carriers after the incorporation of V2O5.

V2O5@Pani also showed a better ammonia sensing and recovery response than Pani due to the

synergistic effect of V2O5 and Pani.

� 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

* Corresponding authors. Tel.: +82 10 352 78419; fax: +82 53 810 2512.

E-mail addresses: mhcho@ynu.ac.kr (M.H. Cho), mynlee@ynu.ac.kr (M. Lee).

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry

journal homepage: www.e lsev ier .com/ locate / j iec

http://dx.doi.org/10.1016/j.jiec.2014.07.002

1226-086X/� 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

composite of pTSA-doped Pani with V2O5 will produce a synergizedenhance response for NH3 gas sensing. Therefore, in this study,pTSA-doped V2O5@Pani composite nanofibers were prepared inthe presence of a surfactant, cetyltrimethylammonium bromide(CTAB), using a simple facile in situ oxidative polymerizationtechnique. The structure, morphology, thermal stability, electricalconductivity, and optical properties were also examined.

Experimental

Materials

Aniline and para toluene sulfonic acid (pTSA) was purchasedfrom Sigma Aldrich. Vanadium oxide (V2O5), potassium persul-phate (PPS), cetyltrimethylammonium bromide (CTAB), HCl, andmethanol were purchased from Duksan pure chemicals, Korea, andused as received. The water used in these experiments was de-ionized water from a PURE ROUP 30 water purification system.

Preparation of pTSA doped V2O5@Pani composite nanofibers

The V2O5@Pani composite nanofibers were prepared in thepresence of the surfactant, CTAB, by the in situ oxidativepolymerization of aniline with V2O5 using PPS as the oxidizingagent. The Pani/CTAB/oxidant molar ratio in all experiments was 1/0.25/0.5. V2O5 (5% of aniline monomer wt. by wt.) was dispersed ina CTAB in 1 M HCl solution and ultrasonicated for 15 min. Anilinewas then added to the above dispersion, and stirred vigorously toallow proper adsorption on V2O5, which was later polymerized bythe addition of an oxidant prepared in a 1 M HCl solution. Thereaction mixture was stirred constantly for 24 h. The resultingmixture transformed slowly into greenish black slurry, which wasthen filtered and washed thoroughly with double distilled wateruntil the filtrate was colorless to remove the excess acid and PPS.The composite nanofibers after washing were dedoped in a 1 Mammonia solution, followed by washing with methanol to removethe Pani oligomers. Subsequently, the prepared emeraldine basefrom of the V2O5@Pani composite nanofibers were doped with500 mL of 1 M pTSA by stirring the composite nanofibers in the wetstage for 2 h. The pTSA-doped V2O5@Pani composite nanofiberswere filtered and dried at 60 8C for 24 h in an air oven, converted toa fine powder, and stored in a desiccator for further experiments.

Characterization and studies

The crystal structure of the samples was analyzed by X-raydiffraction (XRD, PAN analytical, X’pert PRO-MPD) in the range,108–908 2u, using Cu Ka radiation (l = 0.15405 nm). Ramanspectroscopy (InVia Reflex UV Raman microscope, Renishaw,UK) was used to examine the type of interaction between Pani andV2O5 in the composite fibers. X-ray photoelectron spectroscopy(XPS, ESCALAB 250) was performed using a monochromatized AlKa X-ray source (hn = 1486.6 eV) with a 500 mm spot size. Thesurface morphology was examined by scanning electron micros-copy (SEM, Hitachi-4200). The optical properties were determinedby ultraviolet–visible–near infrared (UV–VIS–NIR, VARIAN, Cary5000, USA) spectrophotometry. The microstructural propertieswere analyzed by field emission transmission electron microscopy(FE-TEM, TecnaiG2 F20, FEI, USA) operating at an acceleratingvoltage of 200 KV. Thermogravimetric analysis (TGA, SDT Q600,USA) was performed by heating the samples from 25 8C to 800 8C ata rate of 10 8C min�1 in a nitrogen atmosphere. Differentialscanning calorimetry (DSC, Q 200, USA) was performed by heatingthe samples from 25 8C to 250 8C. All DC electrical conductivity (s)measurements were performed using a 4-in-line probe electricalconductivity measuring instrument with a PID controlled oven

(Scientific Equipment, Roorkee, India). The DC electrical conduc-tivity was calculated using the following equation [15].

s ¼ ln2ð2d=tÞ2pdðV=IÞ (1)

where s, I, V, d, and t are the DC electrical conductivity (S cm�1),current (A), voltage (V), probe spacing (cm), and thickness of thepellet (cm), respectively.

Results and discussion

Structural studies

Fig. 1 shows XRD patterns of Pani and V2O5@Pani compositenanofibers. In the case of Pani, the peak at�208 2u clearly indicatedan amorphous structure, which can be attributed to periodicityperpendicular to the polymer chain [16,17]. The XRD pattern of theV2O5@Pani composite nanofibers showed all the characteristicpeaks of V2O5 and Pani. The peaks at 2u �158, 208, 218, 268, 318confirmed the existence of V2O5 in composite system, and can beattributed to the reflections from (2 0 0), (0 0 1), (1 0 1), (1 1 0),(4 0 0) planes, respectively (JCPDS file No. 09-0387). However, incase of V2O5@Pani composite nanofibers, the peak of both Pani andV2O5 at�208 2u superimposed, resulting into increase in sharpnessof the peak compared to the much broader peak obtained in the caseof Pani. The reduced intensity of all the peaks of V2O5@Panicomposite nanofibers can be explained on the basis that Pani beingamorphous has significant effect on the crystallinity of V2O5 in termsof beige formation, resulting into reduction of peak intensity [18,19].

Morphological studies

Fig. 2a and b shows SEM images of Pani and the V2O5@Panicomposite nanofibers, respectively. In the case of Pani, a uniforminterconnected fibrous network-like structure was observed(Fig. 2a). The fibrous morphology can be attributed to thepolymerization of aniline inside the micelle structure of CTAB[20,21]. SEM of the V2O5@Pani composite revealed similar featuresbut with slightly more agglomeration compared to Pani, whichmight be due to the nucleation effect of V2O5. A similar nucleationeffect has also been reported for TiO2 in the case of a pTSA-dopedTiO2@Pani nanocomposite [1]. The non-visibility of V2O5 can beexplained by aniline monomers being first absorbed on V2O5 andlater its polymerization results in the encapsulation of V2O5.

Fig. 2c and d shows TEM images of the V2O5@Pani compositenanofibers. Somewhat tubular with some flaky structures can alsobe seen. This tubular morphology can be explained on the basis[(Fig._1)TD$FIG]

Fig. 1. XRD patterns of Pani and V2O5@Pani composite nanofibers.

M. Hasan et al. / Journal of Industrial and Engineering Chemistry 22 (2015) 147–152148

that aniline is polymerized inside the template channel, i.e. tubularmicelle provided by CTAB. Hence, fibers or all the flakes are alignedin a specific direction to give a tubular morphology. The HR-TEMimages show V2O5 particles distributed throughout the Pani matrixwith high degree of structural uniformity.

Raman spectroscopy

Fig. 3 shows Raman spectra of V2O5@Pani composite nanofi-bers. Raman spectroscopy was used to examine the structural

defects in Pani, if any in the composite system, owing to itsresonant effect [22]. The Raman spectra were found to beconsistent with the previous reported result for Pani spectra[22]. The bands at 1170, 1331, and 1590 cm�1 can be attributed tobenzenoid rings, polaronic structure, and quinoid segments,respectively. The high intensity band at 1331 cm�1 confirmedemeraldine salt form of Pani and can be attributed to C–N8+

stretching. The spectra also shows two other bands at 600 and1600 cm�1, which can be ascribed to the cross-linked segmentsresulting into formation of other structures, such as ‘‘phenazine’’segments [23].

[(Fig._2)TD$FIG]

Fig. 2. SEM images of (a) Pani, (b) V2O5@Pani composite nanofibers, TEM images of V2O5@Pani composite nanofibers (c and d).

[(Fig._3)TD$FIG]

Fig. 3. Raman spectroscopy of V2O5@Pani composite nanofibers.

[(Fig._4)TD$FIG]

Fig. 4. TGA–DSC of the V2O5@Pani composite nanofibers.

M. Hasan et al. / Journal of Industrial and Engineering Chemistry 22 (2015) 147–152 149

TGA–DSC

Fig. 4 shows the results of thermogravimetric and differentialthermal analysis of V2O5@Pani composite nanofibers. Thermaldecomposition of V2O5@Pani composite nanofibers occurredthrough three different weight loss steps. The initial weight lossbelow 150 8C was attributed to the evaporation of moistureadsorbed and the other volatile matter retained in the composite

system. The second major weight loss was observed at �245 8C,which might be due to the removal of higher oligomers of Pani andCTAB [24]. The third major weight loss at approximately 481 8Cwas attributed to the thermo-oxidative decomposition of Pani,resulting in various degradation products, such as ammonia,aniline, methane, acetylene, etc. [25] The peaks in the DSCthermogram shows the heat variations associated with anexothermic and endothermic reaction. An endothermic peakwas observed at approximately 265 8C, which was attributed tothe thermal decomposition of the oligomers and CTAB. Noexothermic or endothermic peaks were observed due to thepresence of V2O5 in a composite structure, suggesting that only thePani backbone experienced the changes.

Optical studies

Fig. 5 shows the UV–vis diffuse absorbance spectra of Pani andthe V2O5@Pani composite nanofibers. In both cases, a p–p*absorption peak at approximately 283 nm and two polaronicbands at around 420 and 798 nm were observed, confirming thesynthesis of pTSA-doped Pani [26]. V2O5 had only slight effect onthe absorption spectra as V2O5@Pani composite nanofibers showedstronger absorbance in the region lower than 463 nm than Pani. Incase of V2O5, an absorption band was observed in the wavelengthrange, 210–605 nm, which was assigned to the electronictransition of the O 2p to V 3d [27].

[(Fig._5)TD$FIG]

Fig. 5. UV–visible diffuse absorbance spectra of Pani, V2O5 and the V2O5@Pani

composite nanofibers.

[(Fig._6)TD$FIG]

Fig. 6. XP spectra of the V2O5@Pani composite nanofibers for (a) C 1s peak, (b) N 1s peak, (c) O 1s peak and (d) V 2p peaks.

M. Hasan et al. / Journal of Industrial and Engineering Chemistry 22 (2015) 147–152150

XPS analysis

The surface composition and chemical state of the V2O5@Panicomposite nanofibers was carried out by XPS analysis. The XPSsurvey scan of the V2O5@Pani composite nanofibers (Fig. 6)revealed the chemical state of V, C, N and O present in thecomposite structure. The XP spectra of carbon (1s level) showed apeak centered at approximately 285 eV, which was assigned to thecarbon backbone of Pani. The XP spectra of N (1s level) revealed apeak at approximately 400 eV, which can be attributed to the Panistructure [28]. The XP spectra of O (1s level) showed a broad peakat approximately 530 eV due to the adsorbed oxygen and latticeoxygen [29]. The XP spectra of V (2p3/2, 2p1/2 level) in theV2O5@Pani composite nanofibers showed broad and diffused peaksat approximately 518 and 524 eV, respectively, which wereassigned to the oxide form of V5+ [30] and also indicated thepresence of V2O5 in the composite nanofibers. The diffused peaks ofV2O5 in the V2O5@Pani composite nanofibers were assigned to theencapsulation of V2O5 by Pani. Similar diffused peaks of Ag/TiO2

due to encapsulation by Pani have also been reported for Ag/TiO2@Pani nanocomposite system [31].

Electrical conductivity studies

Fig. 7 shows the DC electrical conductivity of the V2O5@Panicomposite nanofibers. The measured room temperature DCconductivity of pTSA-doped Pani was 1.33 S cm�1, which wasconsiderably higher than that of HCl-doped Pani reportedelsewhere [16]. Much higher conductivity obtained in our casefor pTSA-doped Pani in comparison to HCl-doped Pani is that pTSAresults in a decrease in the hopping distance and consequentlyenhances the conduction by hopping mechanism [1]. After theincorporation of V2O5 into Pani, the conductivity further increasedas is evident from Fig. 6. As doping can facilitate charge transfer,associate the insertion of a counter ion, and simultaneously controlthe Fermi level, hence doping by V2O5 can be helpful in controllingthe electrical properties of Pani. The increased conductivity can beattributed to the successful incorporation of V2O5 particles intoPani, which might increase the mobility of charge carriers alongthe p–electronic system of Pani. As charge transport is believed tooccur mainly along the conjugated chains of Pani with interchainpolaron hopping as the charge transfer mechanism, thus there isalso the possibility of reduced hopping distance as V2O5 mayfacilitate charge transfer by acting as bridge, leading to an increasein the mobility of the charge carriers [32].

Ammonia vapor sensing studies

Fig. 8 shows the sensing response to a low concentration(0.01 M) NH3 vapor by the pTSA-doped Pani and V2O5@Panicomposite nanofiber sensors under ambient conditions. The

V2O5@pani composite nanofiber sensor showed a much greaterresponse than the pTSA-doped Pani nanofiber sensor prepared inthe presence of the surfactant CTAB. Since both V2O5 [8–10] andPani [1] independently show ammonia vapor sensing properties,the enhanced response obtained in our case can be attributed tothe synergistic/additional effect of both V2O5 and Pani. Apart fromthis, polymerization of aniline on V2O5 may increase the surfacearea of Pani, which also leads to an increase in the sensingresponse. A similar result has also been reported for studies of theNH3 gas sensing of V2O5/PVAC fibers [33]. Furthermore, comparedto the pTSA-doped Pani nanofiber sensor, the V2O5@panicomposite nanofiber sensor showed excellent recovery, as wellas high sensitivity toward NH3 vapor due to the obvious reasonsmentioned above.

Conclusions

pTSA-doped, CTAB-assisted V2O5@Pani composite nanofiberswere synthesized by in situ oxidative polymerization, and thesurface, structure, thermal properties, DC electrical conductivity,and ammonia sensing behaviors of the composite nanofibers wereinvestigated using range of different techniques. pTSA-doped Panishowed greater electrical conductivity than HCl-doped Pani.Further V2O5@Pani composite nanofibers compared to pTSA-doped Pani showed enhanced DC electrical conductivity, whichmight be due to the increased mobility of the charge carriers afterthe incorporation of V2O5. Moreover, V2O5@Pani compositenanofibers showed stronger response and better recovery towardammonia sensing than pTSA-doped Pani, which might be due tosynergism between Pani and V2O5.

Acknowledgement

This study was supported by a Yeungnam University ResearchGrant (2013).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in

the online version, at doi:10.1016/j.jiec.2014.07.002.

References

[1] M.O. Ansari, F. Mohammad, Sens. Actuators B 157 (2011) 122.[2] J. Zhang, D. Shan, S.L. Mu, J. Power Sour. 161 (2006) 685.[3] D.Y. Kim, J. Kim, J. Kim, A.-Y. Kim, G. Lee, M. Kang, J. Ind. Eng. Chem. 18 (2012) 1.[4] J. Bae, J. Jang, J. Ind. Eng. Chem. 18 (6) (2012) 1921.

[(Fig._7)TD$FIG]

Fig. 7. DC electrical conductivities of pTSA-doped Pani and V2O5@Pani composite

nanofibers.

[(Fig._8)TD$FIG]

Fig. 8. Variation in the resistivity of pTSA-doped Pani and V2O5@Pani composite

nanofibers upon intermittent exposure to 0.1 M ammonia.

M. Hasan et al. / Journal of Industrial and Engineering Chemistry 22 (2015) 147–152 151

[5] J.A. Pomposo, J. Rodrıguez, H. Grande, Synth. Met. 104 (1999) 107.[6] R.B. Kaner, A.G. Macdiarmid, Sci. Am. 106–111 (1988) 258.[7] M. Bhaumika, A. Maityb, V.V. Srinivasuc, M.S. Onyangoa, J. Hazard. Mater. 190

(2011) 381.[8] A. Dhayal Raj, T. Pazhanivel, P. Suresh Kumar, D. Mangalaraj, D. Nataraj, N.

Ponpandian, Curr. Appl. Phys. 10 (2010) 531.[9] G. Rizzo, A. Arena, A. Bonavita, N. Donato, G. Neri, G. Saitta, Thin Solid Films 518

(2010) 7124.[10] I. Raible, M. Burghard, U. Schlecht, A. Yasuda, T. Vossmeyer, Sens. Actuators B 106

(2005) 730.[11] J. Huang, S. Virji, B.H. Weiller, R.B. Kaner, Chem. Eur. J. 10 (2004) 1314.[12] D.S. Sutar, N. Padma, D.K. Aswal, S.K. Deshpande, S.K. Gupta, J.V. Yakhmi, Sens.

Actuators B 128 (2007) 286.[13] M. Matsuguchi, J. Io, G. Sugiyama, Y. Sakai, Synth. Met. 128 (2002) 15.[14] J. Chen, J. Yang, X. Yan, Q. Xue, Synth. Met. 160 (2010) 2452.[15] C.W. Wang, Z. Wang, M.K. Li, H.L. Li, Chem. Phys. Lett. 341 (2001) 431.[16] M.O. Ansari, S.K. Yadav, J.W. Cho, F. Mohammad, Composites B 47 (2013) 155.[17] D. Han, Y. Chu, L. Yang, Y. Liu, Z. Lv, Colloids Surfaces A: Physicochem. Eng. Aspects

259 (2005) 179.[18] M. Hasan, et al. J. Ind. Eng. Chem. (2014), http://dx.doi.org/10.1016/j.jiec.

2014.04.019.[19] M.O. Ansari, F. Mohammad, Composites B 43 (2012) 3541.

[20] M.O. Ansari, S.P. Ansari, S.K. Yadav, T. Anwer, M.H. Cho, F. Mohammad, J. Ind. Eng.Chem. 20 (2014) 2010.

[21] M.O. Ansari, M.M. Khan, S.A. Ansari, I. Amal, J. Lee, M.H. Cho, Chem. Eng. J. 242(2014) 155.

[22] J.E. Pereira da Silva, D.L.A. de Faria, S.I. Cordoba de Torresi, M.L.A. Temperini,Macromolecules 33 (2000) 3077.

[23] M. Malta, G. Louran, N. Errien, R.M. Torresi, J. Power Sour. 156 (2006) 533.[24] G. Chakraborty, S. Ghatak, A.K. Meikap, T. Woods, R. Babu, W.J. Blau, J. Polym. Sci.

B: Polym. Phys. 48 (2010) 1767.[25] F. Mohammad, Handbook of Organic Conductive Molecules and Polymers, John

Wiley & Sons, Chichester, 1997p. 834.[26] Y.B. Kim, J.K. Choi, J.A. Yu, J.W. Hong, Synth. Met. 131 (2002) 79.[27] G.Y. Liang, C.Q. Yuan, T.H. Xia, H.H. Ping, Q. Dong, Y.Y. Hui, Z.J. Liang, J. Cent. South

Univ. Technol. 16 (2009) 919.[28] N. Asim, S. Radiman, M.A. Bin yarmo, Mater. Lett. 62 (2008) 1044.[29] P.R. Somani, R. Marimuthu, A.B. Mandale, Polymer 42 (2001) 2991.[30] W.F.S. Moulder, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer

Corporation, Eden Prairie, MN, USA, 1992.[31] M.O. Ansari, et al. ACS Appl. Mater. Interfaces 6 (2014) 8124.[32] P.M. Grant, I.P. Batra, Solid State Comm. 29 (1979) 225.[33] V. Modafferi, G. Panzera, A. Donato, P.L. Antonucci, C. Cannilla, N. Donato, D.

Spadaro, G. Neri, Sens. Actuators B 163 (2012) 61.

M. Hasan et al. / Journal of Industrial and Engineering Chemistry 22 (2015) 147–152152