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Polymer 55 (2014) 1866e1874

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Polymer

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Effect of different titania phases on the hydrogen gas sensing featuresof polyaniline/TiO2 nanocomposite

Shahruz Nasirian a,b, Hossain Milani Moghaddama,*

a Solid State Physics Department, University of Mazandaran, Babolsar, IranbBasic Sciences Department, Mazandaran University of Science and Technology, Babol, Iran

a r t i c l e i n f o

Article history:Received 4 December 2013Received in revised form20 January 2014Accepted 7 February 2014Available online 15 February 2014

Keywords:Polyaniline/titania nanocompositeHydrogen gasGas sensing

* Corresponding author. Tel./fax: þ98 112534 2480E-mail addresses: milani@umz.ac.ir, hossainmi

Moghaddam).

http://dx.doi.org/10.1016/j.polymer.2014.02.0300032-3861/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

This paper presents a resistance-based hydrogen gas sensor using polyaniline (emeraldine)/TiO2 nano-composite (PTeNC) thin film. It is demonstrated that different gas sensing features can arise whenvarious TiO2 phases (anatase and rutile) are applied. The different wt% of TiO2 phases were dispersed intoan acidic solution of aniline monomers and PTeNCs were synthesized by an in-situ self-assemblychemical oxidative polymerization method of aniline. PTeNCs deposited on an epoxy glass substratehaving Cu-interdigited electrodes for hydrogen gas sensing at air pressure and room temperature. Ourresults show that the better sensitivity of the sensor strongly depends on the sensor surface morphologyand its components. Furthermore, hydrogen gas sensing mechanism of the sensor based contact areasbetween Pani chains and TiO2 grains was studied.

� 2014 Elsevier Ltd. All rights reserved.

�

1. Introduction

Recently, the demand for hydrogen (H2) gas as a new cleanenergy resource is gaining momentum. However, H2 is a colorless,odorless and flammable gas (ranging from 4.65 to 93.9 vol % in air)with the smallest molecules that can leak easily duringmanufacturing, storage and consuming processes [1,2]. For thispurpose, there is an increasing interest for the environmentalmonitoring of H2 gas by smart sensors with high sensitivity, fastregeneration, low cost and faster response time as efforts to thedevelopment of a safe hydrogen economy.

The various kinds of materials are used for the detection ofreducingH2 gas. Among them, semiconductingmetal oxides such astitania (TiO2) [3,4], tin dioxide [5], tungsten trioxide [6] and zincoxide [7] have been studied extensively. TiO2 as an n-type semi-conducting metal oxide with two distinct phases of anatase andrutile is one of the most important metal oxides for a broad range ofH2 gas sensing applications [3,4]. Any phase can show varioussensing characteristics for the reason that the octahedrals ar-rangements, structure of the grains surface, thermodynamic sta-bility, charge transport, resistivity and electrical properties aredifferent in both phases [1,3,4,8,9]. Jun et al. [3] have reported amaximum sensitivity of 1.2 � 106 to one vol% H2 gas at working

.lani@yahoo.com (H. Milani

temperatureof 300 C for rutile phase TiO2 thinfilmasH2gas sensor.Shimizu et al. [4] have developed H2 gas sensor based on TiO2 thinfilms of single rutile phase, single anatase phase andmixture of bothphases at one vol% H2 gas and working temperature from 250 �C to500 �C. The best response of their sensors was about 103 at workingtemperature of 250 �C for anatase phase TiO2 thin film as H2 gassensor. However, most thin-film sensors based on TiO2 (or othermetal oxides thin film) still need to work at elevated temperatures(100/500 �C), which the result is poor long-term stability, highpower consumption with danger of explosion [1e7,10,11].

The combinations of TiO2 with polyaniline (Pani) and the for-mation of a metal oxide/organic nanocomposite seem to be a keysolution to access a suitable sensitivity, selectivity and stability inreducing gases (such as H2 gas) sensing at normal environmentalconditions [12e23]. Pani has been investigated as an intrinsicallyconducting polymer for H2 gas sensors [12,16] due to its environ-mental stability, easy synthesis, low cost monomer, tunable prop-erties, controllable electrical conductivity in the productionprocess, interesting redox properties achieved at chemical oxida-tive polymerization and doping process [12,24e28]. Pani has threemain oxidation states that are fully reduced leucoemeraldine, half-oxidized emeraldine, and fully oxidized pernigraniline[14,24,25,29,30]. Among these oxidation states, the half-oxidizedemeraldine with amine (eNHe) and imine (]Ne) sites in equalproportions can be reversibly switched between electrically insu-lating emeraldine base (EB) and conducting emeraldine salt (ES) bythe change of doping level. Doping level of Pani for obtaining a

S. Nasirian, H. Milani Moghaddam / Polymer 55 (2014) 1866e1874 1867

p-type ES can be controlled not only through a non-redox aciddoping/base process but also by the pH of a suitable protonic acidduring synthesize [24e28]. Meanwhile, the presence of nanosizedTiO2 in Pani-(emeraldine)/TiO2 nanocomposite (PTeNC) could notonly oxidize Pani directly due to its strong oxidative activity butalso improve the properties of conducting of Pani or TiO2 in H2 gassensing features [12,13,19,22,31].

There is, to best our knowledge, a little study based on thin filmsof pure Pani nanofiber and/or Pani nanofiber compounds for H2 gassensing. Srivastava et al. [12,32e34] have developed the compositethin films of Pani fiber/rutile TiO2 nanoparticles, Pani fiber/multi-walled carbon nanotube, Pani fiber/single-walled carbon nano-tube and Pani fiber/tantalum based on H2 gas resistance sensors atroom temperature, H2 gas high pressure from 1.3 atm to 4 atm andH2 gas-filled environment. Their responses (response times) wereapproximately 1.65, 1.42, 1.25 and 2.3 (230, 210, 40 and 80 s) for thecomposite thin films of Pani fiber/rutile TiO2 nanoparticles, Panifiber/tantalum, Pani fiber/multi-walled carbon nanotube and Panifiber/single-walled carbon nanotube, respectively. Sadek et al. [31]have reported the making of H2 gas sensor based on thin films ofpure Pani nanofibers which had low responses of 1.07 and 1.11 uponexposure to 1% H2 gas at room temperature and air pressure. In allof above reports, H2 gas sensing based on pure Pani fibers and/orPani compounds have low response and/or operate at high pres-sure. It is widely accepted that the change in resistance of thin filmsensor in H2 gas atmosphere is highly dependent on the gas vol-ume, gas pressure, working temperature, the surface morphologyof the film and the components of the film [4,12,16,32e36]. Then,H2 gas sensor based on Pani-semiconductor metal oxide nano-composite such as PTeNC sensors at normal environmental con-ditions still need more development.

In the present work, TiO2 nanoparticles were synthesized inanatase and rutile phases via a simple solegel process. PTeNCswere synthesized by in-situ chemical oxidative polymerizationwith different wt% of anatase or rutile TiO2 nanoparticles for H2 gassensing characteristics. The effect of various phases of TiO2 nano-particles with different wt% in PTeNC thin film sensors assessed forH2 gas sensing properties at normal environmental conditions.

2. Experimental

2.1. Materials

The titanium tetrachloride (99.5%), ethanol (99.8%) aniline (99%),hydrochloric acid (HCl) (36% concentrated) and 10-camphor sul-fonic acid (CSA) were purchased from Merck Co. Ammoniumperoxide sulfate (APS) (99%), chloroform (99.9%) and ammonia so-lution (25% concentrated) were purchased from SigmaeAldrich Co.

2.2. Synthesis method

In a typical synthesis of TiO2 nanoparticles, 4 ml of titaniumtetrachloride was slowly added dropwise into 40 ml of ethanolunder stirring at a room temperature. A transparent yellowish so-lution with pH of 1.0e1.5 was formed after adding all titaniumtetrachloride. After stirring of the solution for 5 days, under airatmosphere with 80% humidity, a colorless sol was formed and itaged for 3 h. We then prepared the gel using ultrasonic waves for30 min (exposed with ultrasonic waves at a frequency of 40 kHzand a 60 W power) [37,38]. The gel was heated at the 120 �C until adry-gel was obtained. The dry-gel precursor was calcined for 1 h at500 �C (900 �C), at a ramping rate of 5 �C/min, for the formation ofanatase (rutile) phase TiO2 nanopowders.

Pure Pani or PTeNCs were prepared by an in-situ chemicaloxidation polymerization of aniline (C6H5NH2) using APS

((NH4)2S2O8) as an oxidant in the absence or presence of well-dispersed TiO2 nanoparticles at 5e7 �C, respectively. In a typicalprocedure of Pani synthesize, 0.1 M APS solution slowly dropwiseadding to 0.1 M double distillated aniline under a continuousstirring at 5e7 �C. After 3 h, a dark green color was recovered andthen the solution was aged for 12 h. The solution was filtered,washed repeatedly with 1 M HCl and dried under relative vacuumat 55 �C for 24e36 h. This product was grounded using mortar andpestle. The obtained uniform powder was conductive Pani emer-aldine salt (Pani-ES) with a green color. Pani-ES was stirred for12 h in a 0.1 M aqueous solution of ammonia, then washed withdeionized water several times and finally dried under relativevacuum at 55 �C for 24e36 h to obtain Pani emeraldine base (Pani-EB). For preparation of PTeNCs, TiO2 nanoparticles with 15, 25,40 wt% of anatase or rutile phase were suspended in 0.1 M anilineand then sonicated and mixed for 1 h at room temperature toreduce aggregation of TiO2 nanoparticles. APS solution with anequal molar ratio to aniline was slowly added in dropwise to an-ilineeTiO2 suspension with a continuous stirring at 5e7 �C. Afteraging for half day, the solution filtered, washed repeatedly with1 M HCl and dried under relative vacuum at 55 �C. This productwas grounded using mortar and pestle and the uniform PTeNCsalt powders obtained. PTeNC salt powders kept in 0.1 Mammonia solution and stirred for 12 h. The product was filteredand washed with deionized water and then dried in a relativevacuum at 55 �C to obtain PTeNC base. 0.2 g of Pani-EB or PTeNCbase powder was separately mixed with CSA by grinding in asmooth agate mortar. The pure Pani/CSA and (PTeNC)/CSA mix-tures were separately added in 25 ml chloroform to prepare theconducting solution. The solution requires 5 days of continuousstirring to make the homogenous solution.

For sensor preparation, a finger type Cu-interdigited electrodespatterned onto 2 � 2 cm2 area of epoxy glass substrate. The widthof overlap electrode and the gap between two successive electrodeswas 0.4 mm. Before use, the epoxy glass substrates were cleaned byultrasonic treatment with hot soap-water and acetone solutions,then rinsed with deionized water and dried under relative vacuum.The prepared homogenous solutions were deposited on an epoxyglass substrate having cleaned Cu-interdigited electrodes using thespin coating technique at a speed of 2500 rpm. These sensor filmswere dried at 55 �C in a relative vacuum for a day. The obtained thinfilm sensors from doped CSA/PTeNC with 15, 25 and 40 wt% ofanatase or rutile phases were denoted as PA15, PA25, PA40, PR15,PR25 and PR40, respectively. Hereinafter, doped CSA/PTeNCswith anatase or rutile phase were denoted PAeNC or PReNC,respectively.

2.3. Characterization techniques

Characterization was accomplished using X-ray powderdiffraction (XRD). XRDwas carried out using a Bruker-D8 ADVANCEX-ray diffractometer with copper radiation (Cu-Ka, l ¼ 1.54 �A)through a graphite monochromator and step-scanning measure-ments in a range from 10� to 60� 2q, with a step of 0.04 2q and acounting time of 3 s per step with a working voltage of 30 kV.Absorption and transition spectra was taken by UVevis spec-trometer with type PG Instrument-T80þ. Spectroscopic analysis ofthe samples was performed using a Fourier transform infraredspectroscopy (FT-IR) with type BrukereTensor 27 IR. The samplingmethod was a KBr pellet method. The instrument’s spectral reso-lution in units of wave numbers (cm�1) and our setting was 2 cm�1

at 14 s, and 10 kHz scan speed. Field emission scanning electronmicroscopy (FE-SEM) with type Hitachi-S4160 at 26 kV and trans-mission electronmicroscopy (TEM)with type Zeiss-EM10C at 80 kVused to study of morphology of the samples.

S. Nasirian, H. Milani Moghaddam / Polymer 55 (2014) 1866e18741868

3. Results

3.1. Characterization

Fig. 1aeb illustrates FE-SEM images taken from pure TiO2nanoparticles at anatase and rutile phases, respectively. These im-ages clearly show although two samples have almost good homo-geneity and spherical morphology, the rutile phase has a greatersize than anatase phase. TEM images for pure Pani-EB and PA25 areshown in Fig. 1ced. As seen in Fig. 1c, Pani-EB has fibrous networkmorphology with intertwined multiple branches. Fig. 1d reveals adisordered cylindrical morphology of Pani fiber by TiO2 grains. TheTiO2 rich regions in composite seem to be composed of many smallTiO2 nanoparticles of about 15e25 nm in size. TiO2 grains arecoated by Pani matrix. Meanwhile, Fig. 1d confirms that there is thecontact between TiO2 grains with neighboring grains and betweenPani fiber and TiO2 grain in PTeNC.

Fig. 2aec shows FE-SEM images of Pani, PA25 and PR25,respectively. Fig. 2a reveals that the Pani-ES nanofiber layerdeposited on the substrate consists of a large quantity of grill-likenanostructures. Fig. 2b shows the surface of PA25 with a cylindri-cal morphology which is porous with the interestingly inter-connected nanofibrous architecture when deposited as a thin filmon the substrate. Fig. 2c exposed the formation of nanofibernetwork of PR25 which has a cylindrical morphology. According toFig. 2bec, it is evident that the diameter and length of the nano-composite fibers change with the variation of TiO2 phase. Thepresence of anatase nanoparticles with lower size in the nano-composite fibers caused the diameter of PAeNC fibers is shorterthan PReNC fibers. In addition, the PTeNC fibers can be approxi-mated as cylindrical objects that present a porous three-

Fig. 1. FE-SEM images of (a) anatase and (b) rutile TiO2 nano

dimensional surface to hydrogen gas sensing. It is well-knownthat gas diffusion is much more rapid into cylinders than intotwo-dimensional slabs of comparable dimensions [19].

X-Ray diffraction patterns of the pure Pani, PAeNCs, PReNCs,pristine anatase and rutile phases of TiO2 nanoparticles are shownin Fig. 3a. According to Fig. 3a1, a maximum peak is around2q ¼ 25.3� for Pani with two broad amorphous peaks centered at2q ¼ 15.1� and 2q ¼ 20.5�, which may be assigned to the scatteringfrom Pani conjugation chains at interplanar spacing. Wide-angle X-ray diffraction pattern of Fig. 3a1 at 2q ¼ 25.3� reveals that Pani hasalso some degree of crystallinity [12,14,17,22,39,40]. Fig. 3a8 showsXRD pattern of the anatase TiO2 powder with a sharp peak at2q¼ 25.3� that indicates the formation of pure anatase phase (101).

Fig. 3a9 shows the X-ray diffraction pattern of rutile TiO2 powderwith a sharp peak at 2q ¼ 27.3� that indicates the formation of purerutile phase (110). No peaks of rutile phase (anatase phase) weredetected in Fig. 3a8 (Fig. 3a9). Adopting the Scherrer formula, thecalculated size of TiO2 nanocrystallites is 25 nm and 50 nm foranatase and rutile phase, respectively [37,38]. PAeNC patterns(Fig. 3a2ea4) show that there is no significant difference betweenthe position of the anatase peak (101) and Pani centered at2q ¼ 25.3�. XRD patterns of PAeNCs and pristine anatase TiO2nanoparticles (Fig. 3a8) also indicates deposition of Pani on thesurface of TiO2 nanoparticles has a little effect on the crystallinity ofTiO2 nanoparticles and the polymorph of TiO2 in the PAeNC is stillanatase. In spite of the fact that Pani peak intensity at 2q¼ 15.1� and2q ¼ 20.5� decreased, the peak intensity at 2q ¼ 25.3� increasedwith an increase of wt% of anatase phase from 15 to 40 in the PAeNC that the result is the hampered crystallization of Pani in PAeNCs. Fig. 3a5ea7 shows PReNC patterns. The diffraction patterns of2q ¼ 15.1�, 20.5� and 27.3� are evident in PR15. With an increase of

particle. TEM images for (c) pure Pani-EB and (d) PA25.

Fig. 2. FE-SEM images of (a) Pani-ES, (b) Pani/TiO2 (anatase) and (c) Pani/TiO2 (rutile).

S. Nasirian, H. Milani Moghaddam / Polymer 55 (2014) 1866e1874 1869

wt% of rutile nanoparticles in PReNCs (from 15% to 40%), intensityof two weak peaks centered at 2q ¼ 15.1� and 20.5� decreased.However, an increase of wt% of rutile nanoparticles in PReNC up to40% caused the intensity of peak centered at 2q ¼ 27.3� increased.The results of Fig. 3a show that when Pani chains are adsorbed onthe surface of TiO2 nanoparticles, the interaction of Pani andnanoparticles restricts the growth of Pani chains around nano-particles [12,40e42]. It is presumably one of a reason for the changeof the surface morphology from pure Pani to PTeNC.

FT-IR spectroscopy was used to characterize the prepared Pani,PA25, PR25, PA40, PR40 and TiO2 nanoparticles. The band of673 cm�1 in TiO2 (Fig. 3b1) indicates TieOeTi band [43,44], while

Fig. 3. (a) The XRD pattern for Pani, PTeNCs, anatase and rutile TiO2. FT-IR spectra of (b1)spectra between PAeNCs and PReNCs thin films.

the band centered at 1625 cm�1 is associated with TieO stretchingmode [45]. Fig. 3b2 shows the main characteristic peaks of Panidoped with CSA. According to Fig. 3b2, the band at 3440 cm�1 isattributable to NeH stretching mode. The transmission peaks ofPani at 2923 cm�1 and 2852 cm�1 correspond to eNH2

þ and NeHbond present in aromatic amines, respectively, while the bands at1740 cm�1 and 1654 cm�1 correspond to the CeN imine stretchingvibration [20,41]. The peaks assigned at 1487 cm�1 and 1282 cm�1

correspond to the stretching modes of the C]C and CeN bonds ofthe benzenoid rings, respectively, while the band centered at792 cm�1 is associated with the CeC and CeH bands of benzenoidring [16,40,42,46]. The C]N stretching mode for the quinoid ring

TiO2 nanoparticle, (b2) Pani, (b3) PA25 and (b4) PR25. (c)e(d) Comparison of UVevis

Fig. 4. The schematic block diagram of our handmade hydrogen gas sensing setup.

S. Nasirian, H. Milani Moghaddam / Polymer 55 (2014) 1866e18741870

occur at 1560 cm�1 [26,40]. The band at 1043 cm�1 is due to aquinoid unit of CSA-doped Pani, while the band at 1124 cm�1 isassigned to an in-plan bending vibration of CeH, which is formedduring protonation [41,42]. These characteristic bonds confirm thatPani contains the conducting emeraldine salt phase [40,42,46].Fig. 3b3eb6 indicates that the main peaks of doped Pani with CSAhave appeared in PA25, PR25, PA40 and PR40 respectively. Fouradditional bands at 1111 cm�1, 1107 cm�1, 1130 cm�1 and 1131 cm�1

are assigned to TieOeC stretching mode in PA25, PR25, PA40 andPR40, respectively, furthermore the bands at 667 cm�1, 663 cm�1,657 cm�1 and 667 cm�1 correspond to the TieOeTi band [45].

It can be concluded that Ti-organic compound with an align-ment structure of TiO2 particles is formed. According to Fig. 3b, allbands of Pani in PA25, PR25, PA40 and PR40 shifted slightly and alsothe intensity ratio of bands have changed. These obvious shifts ofbands reveal that an interaction presumably exists at the interfaceof Pani macromolecules and TiO2 nanoparticles [26,40,42,45,46].This interaction may be associated with the contact of titanics and/or O� ions (chemisorbed on the surface of the grains) and nitrogenatoms in Pani macromolecules. Moreover, the interaction betweentitanics or O� ions on the grains surface and nitrogen atoms in Panimacromolecules was caused not only the trapped electrons byoxygen ions return to the surface of the grains but also the electrondrawing nature of TiO2 may be redistributed or localized some ofthe electrons around contact area between the nitrogen atoms ofPani chains and the surface of TiO2 grains.

Fig. 3ced shows UVevis absorption spectra of pure Pani, PAeNCsand PReNCs thinfilms dopedwith CSAon a clean glass substrate. It isobvious that the absorption intensity increases as the concentrationof TiO2 increases in PAeNC and PReNC. According to Fig. 3ced, purePani-ES thin film shows three absorption bands at around 366, 445and810nmthat are associated topep* transition in thebenzoid ring,polaron e p* and p e polaron transition, respectively. The chains ofPani have an extended conformation, in which the interaction be-tween the adjacent polarons is stronger, and the polaron band be-comes more dispersed in energy (that is, more delocalized). As aresult, the absorption peak at around 400 nm associated with theprotonation of Pani (polaron and bipolaron in semiconductivestate) and intra-band transitions within the half-filled polaronband [12,14,19,39]. Moreover, the peak around 400 nm is moreevident inPani, PA15andPR25 thanothercurveswhich indicatemoreintra-band transitions of the Pani backbone in these samples [19,39].

Fig. 3c1 and d1 exhibits an absorption edge of around 385 nmand405 nm for anatase and rutile TiO2, respectively, that associatedwith the electronic transition between the valence band and theconduction band of TiO2 [8,9]. Fig. 3c2ec5 shows 35 nm blue shiftfrom Pani to PA40 (from 775 to 810 nm) and a 10 nm blue shift fromPani to PA40 (from 435 to 445 nm). The blue shift indicates aredistribution of polaron density in the band gap of Pani-ES due tothe impact of TiO2 (anatase) nanoparticles [13]. Fig. 3d2ed5 shows a30 nm red shift fromPani to PR40 (from 810 to 840 nm) and a 10 nmred shift from Pani to PR40 (from 445 to 455 nm). The red shiftindicates the formation of an interface with localized states thatmay be related to the trapped charge at Pani/TiO2 (rutile) interface[39]. These results indicate that the carrier transport may presum-ably be better in PAeNC than PReNC. Fig. 4 shows the schematicdiagram of our handmade gas sensor setup. The typical structure ofour resistance-based hydrogen sensor consists of a layer of Pani,PAeNC or PReNC on a finger type Cu-interdigited electrodespatterned area of an epoxy glass substrate and two electrodes.

3.2. H2 gas sensing behavior of thin film sensors

The resistance change of the thin film sensor due to phys-isorption and chemisorption can be directly correlated to the

concentration of the gas. The gas sensing behavior of resistancetype sensors was studied by calculating a change in the resistanceof sensing film with time when exposed to H2 gas at air pressureand room temperature. For a reducing gas such as H2, the sensorresponse is defined as [3] “Response ¼ Rair/Rgas” where Rgas is theresistance in the presence of the gas and Rair is the resistance inpure air. The response/recovery time is an important parameter forcharacterizing a sensor. It is defined as the time required to reach90% of the final change in resistance, when the gas is turned on andoff, respectively. The difference in the resistance of Pani, PAeNC andPReNC sensors is shown in Fig. 5aebwhen they exposed to 0.8 vol%H2 gas at room temperature, air pressure and a relative humidity of45%e55% RH.

According to Fig. 5aeb, there is a lower resistance in PTeNC thinfilms than pure Pani before H2 gas injection. In pure air, thedecrease of the resistance in PTeNC thin films with respect to purePani is presumably due to the presence of TiO2 grains in PTeNC andwell-matching between Pani chains and TiO2 nanoparticles whichcauses the PTeNC conductivity become better. Moreover, in thepresence of H2 gas and due to the difference in the particle packingdensity of anatase and rutile phases and faster electron transport ofanatase phase with respect to rutile phase [8,9], the change ofresistance in PAeNC thin films was more than PReNC thin films. Inaddition, the resistance of the PTeNC thin film sensors graduallyincreases until about the baseline resistance when H2 gas wasremoved.

Responses of Pani and PTeNC thin films vs. time are plotted inFig. 5c. The response time (tresponse), recovery time (trecovery), theresponse value (Rgas/Rair) and a resistance shift for Pani and PTeNCthin film sensors are noted in Table 1. Fig. 5 and Table 1 clearlyindicate that almost all of the PTeNC thin film sensors show ahigher and faster response value, a better shift in resistance and afaster response/recovery time due to pure Pani thin film sensor,except the recovery time of PR15 and the shift in resistance of PR25.The most change in resistance with the best response/recoverytime in PReNCs family belong to PR40 with a shift in resistance of183 U, the response time of 153 s and the recovery time of 170 s.

Moreover, the maximum resistance change with the bestresponse/recovery time in PAeNCs family belongs to PA25 with ashift in resistance of 223 U, the response time and recovery time of83 s and 130 s, respectively. As a result, the PA25 thin film resis-tance sensor is a good candidate at PTeNCs family for H2 gas sensor.

Fig. 6 shows the resistance shift and the dynamic responseerecovery property of PA25 and PR40 thin film sensors exposed to

Fig. 6. Dynamic responseserecovery of the PTeNC thin films exposed to different vol%H2 gas at room temperature, air pressure and a humidity of 45%e55% RH.

Fig. 5. The shift of the resistance in (a) PAeNC and (b) PReNC thin film sensors, and (c)response vs. time plot for PTeNC thin film sensors after the exposure of 0.8 vol% H2 gasat room temperature, ambient pressure and a humidity of 45%e55% RH.

280

1.2

1.4

1.6

1.8

2.0

2.20.4 0.5 0.6 0.7 0.8 0.9 1.0

(a)Triangle point : tresponse for PA25Circle point : trecovery for PA25

(b)

Res

pons

e

Star point : Response of PR40Circle point : Response of PA25

S. Nasirian, H. Milani Moghaddam / Polymer 55 (2014) 1866e1874 1871

different concentrations of H2 gas ranging from 0.4 to 1 vol%. Fig. 7shows the response/recovery time and response value of PA25 andPR40 for different vol% of H2 gas. Fig. 7 reveals that although theresponse time in PA25 (PR40) decreases from 130 s (198 s) to 73 s(126 s), the recovery time increases from86 s (126 s) to 162 s (225 s)and also the response value enhances from 1.19 (1.22) to 1.93 (2),respectively when H2 concentration increases from 0.4 to 1 vol%.The decrease of the response time and the increase of response toan increase of H2 gas concentration may be due to the presence andpenetration of very sufficient gas molecules on the surface of thethin film.

Table 1The response time, recovery time, response value (Rair/Rgas) and a shift in resistancefor different thin film sensors.

Type of thinfilm sensor

Shift inresistance (U)

Responsevalue (Rgas/Rair)

tresponse (s) trecovery (s)

Pani 73 1.13 264 218PA15 169 1.58 144 161PA25 223 1.63 83 130PA40 106 1.22 216 175PR15 100 1.57 252 252PR25 51 1.18 154 183PR40 183 1.54 153 170

Moreover, an increase of the recovery time in 1 vol% H2 gascould be created due to more reaction between gas molecules andthe surface of the sensor which causes the reaction products do notleave immediately the sample surface after the reaction. Further-more, there is a non-linear relationship between response,response/recovery time and H2 gas vol% in PA25 and PR40 sensors.As a result, in spite the fact that with an enhancement of H2 gasconcentration, the response in PA25 and PR40 sensors does nothave a significant change, the change of the response/recovery timeis completely dissimilar in two sensors.

4. Discussion

Pani-ES is inherently conducting in nature due to the presenceof a conjugated electron system in its structure [24,28,31]. A highlevel of conductivity (near metallic) can be achieved in Panithrough oxidationereduction as well as doping with a suitable

0.4 0.5 0.6 0.7 0.8 0.9 1.0

80

120

160

200

240 Star point : tresponse for PR40Square point : trecovery for PR40

Tim

e (s

ec)

vol %

Fig. 7. (a) The response/recovery time and (b) the response value of PA25 and PR40 atdifferent vol% of H2 gas. The lines are only a guide to the eye.

S. Nasirian, H. Milani Moghaddam / Polymer 55 (2014) 1866e18741872

dopant. Sadek et al. [31] and Han et al. [47] have illustrated theelectronic conductivity of CSA-doped Pani thin film is higher thanHCl-doping. Pani or PTeNC was re-doped with CSA in chloroformsolvent which it leads to the substitution of a CSA� functional groupinstead Cl� ions in imine sites of HCl-doped Pani [23e25]. Thisprocess schematic is given in Fig. 8.

In addition, the protonated imine sites in CSA-doped Pani canachieve a polaron lattice (radical cations). Furthermore, the polaronlattice could be attained an intermediate bipolaron form (dications)by dissociation (carrier hopping). The result is a carrier transport ininter-chain of doped Pani-ES [28,29,31,48]. It is widely accepted thatthe adsorbed oxygen molecules on the surface layer of TiO2 grainshave the formofO2

�, O� andO2� ionswhich create a positive chargeon the surface of TiO2 grains by extract electrons from the conduc-tion band of TiO2 [8,9,39,46]. Therefore, when dispersed TiO2 grainssuspend in aniline solution along with APS as oxidant, the trans-formation of anilinemonomers to the anilinium cations couldmakean electrostatic interaction with adsorbed anions on the surface ofTiO2 grains. The surface of TiO2 grains is then partly covered by Paninanosheets during polymerization of aniline monomers. The sche-matic of PTeNC formation is shown in Fig. 9aeb.

According to Figs. 1ced, 2 and 9, there are three contact regionsin PTeNCs that could accomplish or disrupt carrier transport. Thesethree areas are (1) the connection of the adjacent Pani chains, (2)the contact between n-type TiO2 grainwith neighboring grains and(3) contact between p-type Pani backbone and n-type TiO2 grain.The carrier transport could be functional as follows:

(1) In Pani chain with a polaron structure, a cation radical of onenitrogen acts as the hole which these holes are charge carriers. Theelectron from the adjacent nitrogen jumps to that hole which be-comes neutral. Consequently, this activity continues along Panichain (Fig. 9). There is then an electron hopping from a chain to anadjacent chainwith a potential barrier between chains which limitsand disrupts electron transport. The electrons should overcome onthis potential barrier for intera-backbone transport. It is mentionedthat the potential barrier between chains and the connectivity ofPani could be altered with a change of molecular weight, the per-centage of crystallinity, morphology, shape, intra-chain separation,width and length of the polymer chain(s), the percentage of dopingand type of dopant [28,49,50].

Fig. 8. Scheme of the molecule obtained aft

(2) The region of TiO2 grains could be schematically representedby the presence of grain boundaries and necks between grains(Fig. 9d). According to Fig. 9d, it has been shown [4,35] that theresistance in region of TiO2 grains is dominated by the potentialbarrier of the grain boundaries and/or the necks (PB-GB/N).Moreover, the chemisorbed negative oxygen ions on the surface ofthe TiO2 grains create a developed depletion or space-charge regionon the surface of the positively TiO2 grains which trap and extractelectrons from the conduction band of the n-type TiO2 due to theirhigh electron affinity. The carriers for a transport from a grain to anadjacent grain should also overcome on this depletion region orinsulating potential barrier (IPB). Consequently, PB-GB/N and IPBmake a basic potential barrier for the electron transport betweengrains which depends to grain structure, octahedral arrangement,number of boundaries and/or necks, size, morphology and densitypacking of grain(s). During PTeNCs production, the negative anionsof the surface of the grains make an electrostatic interaction withpositive anilinium ions of Pani chains. After this reaction, thetrapped electrons by the adsorbed oxygen on the grain surfacerelease which it leads to a decrease in the basic potential barrierheight. Consequently, it gets an increase in carrier mobility on thegrain surface.

(3) There is a potential barrier in TiO2 grains-emeraldine chainsjunction as the third contact region for carrier transport[20,28,39,46]. The injected electrons from Pani chains (throughinter- and intra-charge carrier) to the surface of grains shouldovercome on this potential barrier in PTeNC. According to our re-sults in Fig. 5aeb, the initial resistance of the Pani sensor is higherthan any initial resistance of PTeNC sensors that probably is due tothe protective effect (well-matching) of TiO2 grains in (with) Pani-ES matrix before H2 gas injection. As a result, this well-matchingbetween TiO2 nanoparticles and Pani chain(s) with the reducedpotential barrier are presumably caused the easier carrier transferoccurs in PTeNC thin films with respect to pure Pani thin film.

The gas sensing mechanism in Pani and PTeNCs gas sensors,after exposure of H2 gas, is governed by the reaction between thesurface layers of the sensor and the penetrating H2 gas molecules.According to Figs. 1d and 2, only Pani surface layers are exposed toH2 gas in the pure Pani and PTeNC films. After H2 gas permeation,the gas molecules react with the charged amine nitrogen sites on

er a reaction between Pani-EB and CSA.

Fig. 9. (aec) Scheme of the interaction of TiO2 nanoparticles with Pani-ES to form a PTeNC. (d) Potential barrier between grains. (eef) Three contact regions in the PTeNC fibers andhydrogen sensing mechanism of PTeNCs with the change of potential barrier between grains and also n-type TiO2 grain and p-type emeraldine after exposure to hydrogen gas.

S. Nasirian, H. Milani Moghaddam / Polymer 55 (2014) 1866e1874 1873

the Pani surface layers. According to MacDiarmid proposal [51],hydrogen molecules might form a bridge between nitrogen atomsof two adjacent chains (see Fig. 9e). H2 bond dissociation followswith the formation of new NeH bonds to the amine nitrogen of thePani-ES chain. Subsequently, the charge transfer between adjacentamine nitrogens returns the Pani chain to its polaron lattice statewith a release of hydrogen atom.

The increase of number of carriers and charge transfer rate inpresence of aggressive hydrogen molecules causes not only fullyreversible carrier transport would be faster in inter- and intra-Panichains but also a decrease of the potential barrier between adjacentchains occurs.

As a result, an increase of the penetrating H2 gas moleculescauses the degree of polaron formation increases. This promotedpolaron formation leads to an enhancement in the carrier concen-tration and mobility of the charge carriers in both intra- and inter-chain transport. Moreover, the increase of number of the carriersand their easier hopping in both intra- and inter-Pani chains with agood energy band gap matching between the conduction band ofTiO2 and the lowest unoccupiedmolecular orbital of Pani lead to notonly a more decrease in the potential barrier height between TiO2grain and Pani-ES but also a reduction in basic potential barrierheight between neighboring TiO2 grains. The result is the increase ofthe conductivity and lower resistance in PTeNC thin films than pure

S. Nasirian, H. Milani Moghaddam / Polymer 55 (2014) 1866e18741874

Pani in the presence of H2 gas molecules. The change of the resis-tance and conductivity are different in PTeNCs due to the change ofthe phase, size and wt% of TiO2 nanoparticles, the number of con-tacts, surface morphology of thin films, inter-Pani chain separationand a shell depth of Pani chains around TiO2 nanoparticles. It hasbeen demonstrated that not only electron transport is slower in therutile layers than in the anatase layers due to the difference in theparticle packing density but also the anatase phase has more va-cancies in lattice structure than rutile phase [8,9]. According toFig. 3c, the redistribution of polaron density in the band gap of PAeNCwith respect to the localized states of the trapped charges of PReNC indicates that the electronhopping in PAeNC layers is easier thanPReNC layers. Furthermore, more change in resistance of PAeNCsthan PReNCs is presumably due to better well-matching (betterpacking) of anatase TiO2 nanoparticles and Pani matrix than rutilenanoparticles and Pani matrix.

5. Conclusion

We have designed and fabricated H2 gas sensors based on CSA-doped Pani or PTeNCs on an epoxy glass substrate containing Cu-interdigited electrodes. The morphology of the films changedfrom a grill-like structure in Pani to a network porous cylindricalmorphology in PTeNCs. Meanwhile, TEM analysis reveals a fibrousmorphology of pure Pani and a coreeshell structure of PTeNC. Thesensors’ performancewas investigatedwith a capable of working atroom temperature and air pressure towards H2 gas. PTeNCs areexcellent candidates for recognition and detection of H2 gas.However, the better sensitivity of the sensors strongly depends onthe components. The results demonstrate that there is a goodrelatively response/recovery time with excellent repeatability andbase-line stability in sensors based on the Pani composition withTiO2. Moreover, the phase of TiO2, the diameter and length of fibers,porosity, orientation and surface decoration are effective parame-ters in the sensing performance of PTeNC sensors at normalenvironmental conditions. The TiO2 phases into Pani matrix may bemake a well-matching with Pani chains which could work as aswitch to control the electric current flow in PTeNCs. The result is abetter H2 gas sensitivity in PTeNCs than Pani. Furthermore, itdemonstrates that the resistance shift, the response value andresponse/recovery time were improved in PAeNCs with respect toPReNCs at similar condition of H2 gas atmosphere.

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