Accepted Manuscript

Title: Polyaniline assisted by TiO2:SnO2 nanoparticles as ahydrogen gas sensor at environmental conditions

Author: Shahruz Nasirian Hossain Milani Moghaddam

PII: S0169-4332(14)02739-1DOI: http://dx.doi.org/doi:10.1016/j.apsusc.2014.12.051Reference: APSUSC 29286

To appear in: APSUSC

Received date: 25-9-2014Revised date: 3-12-2014Accepted date: 7-12-2014

Please cite this article as: S. Nasirian, H.M. Moghaddam, Polyaniline assisted byTiO2:SnO2 nanoparticles as a hydrogen gas sensor at environmental conditions, AppliedSurface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.12.051

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

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Highlights

►Polyaniline/TiO2:SnO2 nanocomposite (PTS) sensor was fabricated for hydrogen gas sensing.

►A sensor with 30 wt% of anatase (containing TiO2:SnO2 wt%: 2:1) has the best response

(6.18).

►A PTS sensor with 20 (30) wt% of anatase- (rutile-) TiO2 has the best response (recovery)

time.

►A PTS sensor; containing 20 wt% anatase phase of TiO2 with 10 wt% SnO2 nanoparticles; has

an excellent set of H2 gas sensing features among PTS sensors.

► The presence of SnO2 nanoparticles in PTS sensors is caused the recovery time reduced.

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Polyaniline assisted by TiO2:SnO2 nanoparticles as a hydrogen gas sensor at

environmental conditions

Shahruz Nasiriana and Hossain Milani Moghaddama,

a Solid State Physics Department, University of Mazandaran, Babolsar, Iran

Abstract

In the present research, Polyaniline assisted by TiO2:SnO2 nanoparticles was synthesized and

deposited onto an epoxy glass substrate with Cu-interdigited electrodes for gas sensing

application. To examine the efficiency of the Polyaniline/TiO2:SnO2 nanocomposite (PTS) as a

hydrogen (H2) gas sensor, its nature, stability, response, recovery/response time has been studied

with a special focus on its ability to work at environmental conditions. H2 gas sensing results

demonstrated that a PTS sensor with 20 and 10 wt% of anatase-TiO2 and SnO2 nanoparticles,

respectively, has the best response time (75 sec) with a recovery time of 117 sec at

environmental conditions. The highest (lowest) response (recovery time) was 6.18 (46 sec) in

PTS sensor with 30 and 15 wt% of anatase- (rutile-) TiO2 and SnO2 nanoparticles, respectively,

at 0.8 vol% H2 gas. Further, the H2 gas sensing mechanism of PTS sensor has also been studied.

Keywords:

Corresponding author:

Address: Solid State Physics Department, University of Mazandaran, Babolsar, Iran

E-mail addresses: milani@umz.ac.ir, hossainmilani@yahoo.com

Telefax: +98-1135302480

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Polyaniline/TiO2:SnO2 nanocomposite; Emeraldine; Metal oxide nanoparticles; Hydrogen gas

sensor; Environmental conditions

1. Introduction

In the last three decades, the intrinsically conducting polymers have emerged as a new class of

materials for the development of a new generation of important practical applications, advanced

technologies and electronic devices due to their excellent electrical properties [1-8]. Polyaniline

(PANI) is an unique among the family of conducting polymers for several important reasons: (a)

the monomer is inexpensive, (b) the polymerization reaction is straightforward, (c) the deposition

is easy, (d) the tunable connectivity is inherent and (e) PANI has a rich chemistry for structural

modification [5-11]. The half-oxidized PANI with amine (-NH-) and imine (=N-) sites in equal

proportions is one of oxidized states of PANI which is called PANI-emeraldine (PE). PE can be

reversibly switched between electrically insulating emeraldine base state (EB) and conducting

emeraldine salt state (ES) through a non-redox acid doping/base process [10,11]. These unique

properties of PANI are caused that it could be sensitive to a multitude of factors in environmental

conditions as the presence of various gases [8-10].

Among different gases, Hydrogen (H2) gas as a reducing gas with low minimum ignition energy

(0.017 mJ) and a wide flammable range (4.6-75%) has attracted much attention as a green, clean,

efficient and sustainable energy resource [12]. For the reason that H2 gas has the small molecules

that can leak easily, its monitoring at room temperature (27 oC) and air pressure (1 atm) (R.T.P)

is necessary to avoid accidents and the development of a safe hydrogen economy. Accordingly,

the use of smart sensors with high sensitivity, fast regeneration, low cost, Long-term response

and working ability at R.T.P is necessary.

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Inorganic semiconductor metal oxides (SMOs) such as TiO2 and SnO2 have been well studied to

detect of H2 gas due to their suitable physical–chemical properties, good response, low cost and

simple sensing methods [4,12]. However, there is some problems such as high operating

temperature (200–500 °C) and high energy consuming with them [4,12]. Therefore, much effort

has been focused on the development of H2 gas sensors with low power consuming at low

operating temperatures by new materials. PANI has received considerable interest as an effective

conducting polymer with significant properties for H2 gas sensing at R.T.P [4,8,13-15]. PANI

also has its own shortcomings such as low thermal stability and inferior H2 gas sensing at R.T.P

which can hinder its potential applications in future [4,10,15]. The combination of PANI matrix

with nano-scale SMOs, such as TiO2 and SnO2, and create n-p heterojunction between p-type ES

and n-type SMOs seems to be a suitable strategy for the improvement of H2 gas sensor efficiency

at room working temperature [14-17]. Moreover, TiO2 and/or SnO2 nanoparticles could not only

create better directly oxidation of PANI during polymerization, but also work, presumably as a

switch to control the electric current flow and resistance in PANI/SMO nanocomposite in

practical applications such as H2 gas sensor [10,14,17-19].

Srivastava et al. [10,20] have developed the thin films of PANI/TiO2 composites for H2 gas

resistance sensors in a filled environment with H2 gas and the pressure above 2 atm at room

temperature. Their responses (response times) were approximately 1.65 and 1.42 (230 and 210

sec) for PANI-rutile TiO2 composite and PANI-tantalum composite, respectively. Sadek et al.

[13,15] have reported an H2 gas sensor based on thin films of pure PANI nanofibers which had

low responses of 1.07 and 1.11 upon exposure to 1% H2 gas at R.T.P. In all of the above reports,

H2 gas sensing based on PANI compounds have low response and/or operate at high pressure. In

order to produce a sensor based on PANI compounds with better response and response/recovery

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time at R.T.P, we expanded H2 gas sensor based on PANI/TiO2 (anatase) nanocomposite which a

sensor with 25 wt% of anatase phase had a response value of 1.63, the response/recovery time of

83/130 sec in 0.8 vol% H2 gas at R.T.P [14]. Moreover, we explained that wt% of anatase phase

of TiO2 is a significant factor for the improvement of response value and response/recovery time

of PANI /anatase-TiO2 nanocomposite sensor. In other literature, we have reported fabrication of

a thin film of PANI/TiO2 (rutile) nanocomposite as H2 gas sensor [17]. According to the results

of this work, a PANI/TiO2 (rutile) nanocomposite sensor, with 40 wt% rutile phase, has 1.54

response value, 152 sec response time and 183 recovery time in 0.8 vol% H2 gas at R.T.P and

50% relative humidity. Moreover, our results have demonstrated that all of PANI/TiO2 (rutile)

nanocomposite sensors (with 15, 25 and 40 wt% rutile phase) could be used for H2 gas sensing at

R.T.P and different relative humidity.

In spite the fact that the response and the response time are important parameters for a sensor,

however, the recovery time (the time of the regeneration) and Long-term response of the sensor

are also other significant parameters. Consequently, the improvement of the response,

response/recovery time and Long-term response of H2 gas sensor based on PANI composite still

need to more work.

Because, the work function (i.e. the minimum energy required to transfer an electron to the

vacuum level), the electronic conductivity, the octahedral arrangement, morphology and

structure of the particle surface, density packing, thermodynamic stability and charge transport of

TiO2 and SnO2 nanoparticles are various [7,16,21,22], it seems a combination of three

ingredients, including PANI, different phases of TiO2 nanoparticle (rutile and anatase phases)

and SnO2 nanoparticle, could presumably improve better H2 gas sensing feature at R.T.P

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Accordingly, in the present work, Polyaniline/TiO2:SnO2 nanocomposites (PTSs) were

synthesized by in-situ chemical oxidative polymerization of aniline. The effect of a different

wt%, type and phases of SMO nanoparticles in H2 gas sensing properties of PTS nanocomposite

thin films was investigated.

2. Experimental methods

2.1. Materials

Titanium tetrachloride (99.5%), ethanol (99.8%) aniline (99%), hydrochloric acid (HCl) (36%

concentrated), Ethylene glycol (99.5%), Stannous chloride (SnCl2·2H2O), sodium hydroxide

(98%) and 10-Camphor sulfonic acid (CSA) were purchased from Merck Co. Ammonium

peroxide sulfate (APS) (99%), Chloroform (99.9%) and ammonia solution (25% concentrated)

were purchased from Sigma-Aldrich Co.

2.2. Synthesis of TiO2 and SnO2 nanoparticles and PANI/TiO2:SnO2 nanocomposites

For synthesis of TiO2 nanoparticles, 3 ml of titanium tetrachloride was slowly added dropwise

into 30 ml of ethanol under stirring. The pH of the transparent yellowish solution was nearly 1.0-

1.5 after adding all titanium tetrachloride. After stirring of the solution for about 5 days, the

solution aged for 3 h. The prepared gel sonicated for 30 minutes with ultrasonic waves at a

frequency of 40 kHz and a 60 Watt power. The gel was dried at 120 oC until a dry-gel was

obtained. This dry-gel was calcined for 1 h at 500 oC (900 oC) with ramping rate of 5 oC/min and

anatase- (rutile-) TiO2 nanopowder formed.

In a typical procedure, 1 g of SnCl2.2H2O was added to 60 ml of Ethylene glycol that was hosted

in a round-bottom flask, following the solution stirred for one hour at R.T.P. If needed, 1 M

sodium hydroxide was added to adapt the pH value of the solution toward 6 [23]. After the

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solution was refluxed at 190 °C for a certain time with constant stirring, the clear solution was

turned into a slight yellow colloid. Then yellow precipitate was filtrated, washed with deionized

water. The result dried for 24 h at 70 ºC and calcined for 2 h at 500 oC with ramping rate of 5

oC/min and SnO2 nanopowder formed.

For the synthesis of PTSs, physical mixtures of SMO nanoparticles (containing TiO2:SnO2 wt%

= 2:1) were dispersed and suspended in 0.1 M aniline solution. After mixing and sonicating for 1

h, APS solution with an equal molar ratio to aniline, added dropwise to this suspension at 5-7 oC.

A good degree of polymerization was achieved with dark green color after 3 h. The solution was

aged for 12 h and then filtered, washed repeatedly with 1 M HCl and dried under a relative

vacuum (10 torr) for 24-36 h. To improve the doping level of PTS (or a better conductivity of

PTS), it is doped with 0.1 M ammonia solution for 12 h. Then the solution washed with the

dionized water several times and finally dried under the relative vacuum for 24-36 h at 55 oC. 0.3

g of the product was separately mixed with CSA by grinding in a smooth agate mortar. The

mixture was then added in 30 ml chloroform to prepare the conducting solution [14]. The

solution stirred about 5 days to make a homogenous solution. The prepared homogenous

solutions were deposited on a substrate using the spin-coating technique for 60 sec with a speed

of 2500 RPM. This sensor film was dried under the relative vacuum (about 10 torr) for 24-36 h

at 60 oC.

CSA-doped PTS with the anatase or rutile phase of TiO2, containing SnO2, was denoted as PAS

or PRS, respectively. Moreover, the obtained CSA-doped PTS with 10, 20 and 30 wt% of

anatase (PAS) or rutile (PRS) phase (containing TiO2:SnO2 wt%: 2:1) were denoted as PAS1

(containing 10 wt% anatase phase of TiO2 with 5 wt% SnO2 nanoparticles), PAS2 (containing 20

wt% anatase phase of TiO2 with 10 wt% SnO2 nanoparticles), PAS3 (containing 30 wt% anatase

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phase of TiO2 with 15 wt% SnO2 nanoparticles) or PRS1 (containing 10 wt% rutile phase of

TiO2 with 5 wt% SnO2 nanoparticles), PRS2 (containing 20 wt% rutile phase of TiO2 with 10

wt% SnO2 nanoparticles), PRS3 (containing 30 wt% rutile phase of TiO2 with 15 wt% SnO2

nanoparticles), respectively.

2.3. Characterization

Structural characterization of the thin layers was accomplished through X-ray powder diffraction

(XRD) using a Bruker-D8 ADVANCE X-ray diffractometer with copper radiation (Cu-Kα,

λ1=1.5418

A ) through a graphite monochromator and step-scanning measurements in a range

from 10o to 60o 2θ with a working voltage of 30 kV. Absorption spectra was taken by UV-vis

spectrometer with type PG Instrument-T80+. Spectroscopic analysis of the samples was

performed using a Fourier transform infrared (FT-IR) spectroscopy with type Bruker-Tensor 27

IR. Field emission scanning electron microscopy (FE-SEM) with type Hitachi-S4160 at 26 kV

equipped with energy dispersive X-ray (EDX) spectroscopy and transmission electron

microscopy (TEM) with type Zeiss-EM10C at 80 kV used to study of the morphology of the

samples.

3. Results and Discussion

3.1. FE-SEM and TEM images

Figure 1a-c illustrates FE-SEM images taken from anatase-TiO2, rutile-TiO2 and SnO2

nanoparticles, respectively. As seen in Figures, the samples have an almost spherical

morphology consists of small grains distributed randomly in a range of under 50 nm.

The surface FE-SEM images of PAS and PRS sensors are shown in Fig. 1d-f and Fig. 1g-i,

respectively. These images clearly reveal that the surface of sensors is not smooth and have some

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degree of rod morphology with uneven lumps and holes, which are suitable for gas penetration

and adsorption [7,10,14,24]. According to Fig. 1, there is extra rod morphology with the

interestingly interconnected nanofibrous architecture in PAS2 with the respect to other sensors.

Figure 1d-i exposed that the formation and surface morphology of PTS nanofiber networks

change with the variation of wt% components. Figure 1f and I illustrate that there is almost the

same porous morphology in samples.

Figure 2b shows the cross-sectional FE-SEM image of PAS3 sensor surface which has about 170

nm thickness. Figure 2a shows TEM image of PAS2 with the fibrous network morphology of

PANI which is hampered by TiO2 and/or SnO2 nanoparticles. The TiO2 and/or SnO2

nanoparticles have a size of about 25-30 nm in PANI matrix.

3.2. XRD characterization

X-ray diffraction patterns of PAS2 thin layer, PRS2 thin layer, SnO2 nanoparticles powder,

anatase and rutile phases of TiO2 nanoparticle powders are shown in Fig. 3a. Figure 3a3 shows

XRD pattern of the anatase-TiO2 sample with a sharp peak at 2= 25.3 o which indicates the

formation of pure anatase phase (101) [10]. Figure 3a4 shows the X-ray diffraction pattern of

rutile-TiO2 sample with a sharp peak at 2= 27.3 o that signifies the formation of pure rutile

phase (110) [21]. No peaks of rutile phase (anatase phase) were detected in Fig. 3a3 (Fig. 3a4).

Figure 3a5 shows a highly crystallinity in pure sample of SnO2 nanoparticles with a sharp peak at

2= 26.7 o [22,25]. Adopting the Scherrer formula, the calculated size of anatase- (rutile-) TiO2

nanocrystallites is 25 nm (50 nm) and the size of SnO2 nanocrystallites is 25 nm. XRD patterns

of PAS2 and PRS2 in Fig. 3a1 and a2, respectively, reveals that both anatase/ rutile phase of TiO2

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and SnO2 peaks are existent, but their intensity changed due to the interaction between SMOs

nanoparticles and PANI chains.

3.3. FT-IR spectroscopy

Figure 3b1 and b2 shows the FT-IR spectroscopy of PAS2 and PRS2, respectively. According to

FT-IR spectrum of PAS2 in Fig. 3b1, the band of 673 cm-1 indicates Ti-O-Ti band [14,26,27],

while the band of 588 cm-1 shows a strong band associated with anti-symmetric Sn-O-Sn

stretching mode [28]. The band due to water contents was observed at 3426 cm-1 and assigned to

O–H vibrational bond [26,27]. The FT-IR studies reveal the formation of polyaniline with a

transmission band at 2933 cm-1 assigned to –NH2+. The transmission peak of PANI at 2871 cm-1

corresponds to N–H characteristic bond present in aromatic amines. The band at 1734 cm-1

corresponds to the C-N imine stretching vibration [6,26]. The peaks assigned at 1461 cm-1 and

1280 cm-1 correspond to the stretching modes of the C=C and C-N bonds of benzenoid rings,

respectively. The band centered at 785 cm-1 is associated with the C-C and C-H bands of the

benzenoid ring [26-28]. The C=N stretching mode for the quinoid ring occurs at 1596 cm-1

[6,26,27]. The band at 1042 cm-1 is due to a quinoid unit of CSA-doped PANI, while the band at

1170 cm-1 is assigned to an in-plan bending vibration of C-H, which is formed during

protonation [27,28]. These characteristic bands confirm that the PANI matrix contained the

conducting emeraldine salt phase. The band at 1130 cm-1 and 1131 cm-1 are assigned to Ti–O–C

and/or Sn–O–C stretching mode in PAS2 and PRS2, respectively [26, 18]. It has been

demonstrated that not only the particle packing density, the lattice structure and the octahedral

connect in the rutile and anatase-TiO2 layers are different, but also the anatase phase has more

vacancies in lattice structure than rutile phase [14,17,21,22]. Then, according to Fig. 3b1 and b2,

all bands that are present in PAS2 shifted slightly in PRS2 which it is presumably due to

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different of the particle packing density, the lattice structure, the lattice vacancies in the anatase

and rutile-TiO2 and the number of contacts between SMOs nanoparticles and PANI chains. The

results of FT-IR spectroscopy clearly illustrate that PANI (emeraldine)/TiO2:SnO2

nanocomposite formed.

3.4. UV-vis spectra

Figures 3c and d show UV-vis absorption spectra of PASs and PRSs on a clean glass substrate.

According to Fig. 3c and d, PTS thin film shows three absorption bands at 370-400, 410-440 and

790-830 nm that are associated to π-π*, polaron- π* and π- polaron transition of PANI chains,

respectively [10, 24].

Figure 3c1-c3 shows 20 nm blue shift from PAS1 to PAS3 (from 830 to 810 nm) and a 20 nm

blue shift from PAS1 to PAS3 (from 440 to 420 nm). Moreover, Fig. 3d1-d3 shows 35 nm blue

shift from PRS1 to PRS3 (from 825 to 790 nm) and a 20 nm blue shift from PRS1 to PRS3 (from

440 to 420 nm). The blue shift indicates a redistribution of polaron density in the band gap of ES

due to the impact of SMO nanoparticles (the anatase- or rutile-TiO2 with SnO2) [14, 24,27].

3.5. EDX pattern of PAS2

FE-SEM image of a selected region of PAS2 sensor surface is shown in Fig. 4a. According to

Fig. 4a (the approved by Fig. 2a), TiO2 or SnO2 nanoparticles distributed in PANI network and

there are the uneven lumps under PANI shell layers. It can be reasonably inferred that these

‘‘buried’’ nanoparticles are TiO2 and/or SnO2 nanoparticles, because XRD and FT-IR analysis

revealed that TiO2 and/or SnO2 nanoparticles existed with remarkable peaks in PTSs. The

existence of TiO2 and/or SnO2 nanoparticles in PAS2 thin film was analyzed by energy

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dispersive X-ray spectrometry (EDX). The EDX patterns (punctual mode) of PAS2 in Fig. 4b

indicate that the as-prepared nanocomposite is composed of Ti, Sn, O, S, N and C elements

[29,30]. The H element was too light to be detected in the EDX analysis. As can be seen from

Fig. 4b, the spectrum shows two and four characteristic peaks of Ti and Sn elements,

respectively, which are presumably due to the x-ray aroused various energy levels in Ti and Sn

atoms. According to Fig. 4b, Ti peak is almost two times longer than Sn peak. Moreover, EDX

analysis was done to prove the incorporation of the doping acids as counter ions in the PANI

structure. As can be seen, the presence of a band attributed to sulfur (sKa 2.21 keV) in the EDX

spectrum indicates the incorporation of –SO3– as counter ion in the structure of PANI nanofibers

of PAS2 [30,31].

3.6. H2 gas sensing behavior of sensors

H2 gas sensing properties of PTS thin film sensors with a substrate temperature of 27 oC are

characterized in a two liter chamber which is filled with pure air at R.T.P and 45-50% relative

humidity (R.T.P.H). Figure 5 shows the schematic diagram of our handmade gas sensor setup. The

typical structure of our resistance-based hydrogen sensor consists of a layer of PTS on a finger type Cu-

interdigited electrodes patterned area of an epoxy glass substrate and two electrodes. The sensor

response is defined as “Response = Rair/ Rgas” where Ra is the resistance in pure air and Rg is the

resistance under a reducing gas such as H2. The response time (tresponse) is defined as the time

required to reach 90% of the steady response signal. The recovery time (trecovery) denotes the time

needed to recover 90% of the original baseline resistance [14,17].

Fig. 6 shows the resistance shift of PTS sensors toward 0.8 vol% H2 gas at R.T.P.H. According

to the results of Fig. 6, the baseline resistance of sensors decreases when wt% of SMOs

nanoparticles increases in PTS sensors. Moreover, the highest baseline resistance in PAS sensors

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is about the lowest baseline resistance in PRS sensors. This variation is presumably due to the

presence of different phases of TiO2 in PTS. After H2 gas injection and due to directly

chemisorption of gas molecules on the sensor surface, the sensors with various components have

different gas sensing feature. This variation is presumably due to different in wt% of sensor

components, the surface morphology of sensors, well-matching between PANI chains and SMO

nanoparticles, the particle packing density and electron transport of various SMOs in sensors.

The resistance of PTS sensors gradually increases until about the baseline resistance when H2 gas

is removed. The response/recovery time, the response and a resistance shift (Ω) for sensors are

noted in Table 1. Table 1 clearly illustrates that the recovery time has been significantly reduced

up to 57 sec and 46 sec in PAS3 and PRS3 sensors, respectively. A comparison between these

results and the results of References [14, 17] demonstrates that the presence of SnO2

nanoparticles in PTS sensors caused the recovery time had a significant decrease.

The reproducibility dynamic response of PTS sensors versus time are plotted in Fig. 7. Figure 8

shows the difference in the response, the response/recovery time of PTS thin film sensors versus

wt% of SMO nanoparticles (containing anatase or rutile phase of TiO2 with SnO2 nanoparticles)

in 0.8 vol% H2 gas at R.T.P.H. The increase of TiO2 and consequently SnO2 nanoparticles wt%

in PTS is caused that the response of PTS sensors increases. This race is very higher in PAS than

a PRS thin film sensor., PAS2 sensor has faster response time (75 sec) and acceptable recovery

time (119 sec). Furthermore, PAS3 sensor has the highest response (6.18).

One of the important parameters which are generally ignored by researchers is Long-term

response of sensor. Figure 9 displays the resistance shift and response versus time for PAS2 thin

film sensor after 3 months of the initial experiment toward 0.8 vol% H2 gas at R.T.P.H. The

experimental results demonstrated that the response and the resistance shift of PAS2 thin film

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sensor decreased to 1.22 (about 2.5% less than the initial experiment) and 91 Ω (about 10% less

than the initial experiment), respectively. Because the response of PAS2 thin film sensor did not

exhibit a significant change, our handmade PAS2 thin film sensor has Long-term stability.

The current-voltage characteristics of PTS thin film sensors are shown in Fig. 10. According to

Fig. 10, a nonlinear current was observed with a symmetric current plot on both sides in positive

and negative voltage regions within the range of ±1 V with 0.1 V intervals. Moreover, an

increasing (a decreasing) trend of current (resistance) in each voltage range was observed in

PRS1, PRS2, PRS3, PAS1, PAS2 and PAS3 thin film sensors, respectively. PRS1 (PAS3) thin

film sensor has the lowest (highest) current on any voltage range (Fig. 10). A comparison

between these results and the results of Fig. 6 shows that there is a good correlation between the

results of the baseline resistance (the resistance before H2 gas injection).

3.7. H2 gas sensing mechanism

At PE, the imine nitrogen sites could be protonated, through doping with a suitable dopant (such

as CSA) which the result is a polaron lattice on PANI backbone. This Polaron lattice induces

charge carriers in PANI network, which can be created more conductivity than EB. Higher

conductivity of ES depends on its ability to the charge carriers transport along the inter-PANI

chain through polaron lattice and the carriers to hop between the polymer chains in PANI

network.

Moreover, it is accepted that the adsorbed oxygen molecules on the surface of different n-type

SMO grains have the form of O2-, O- and O2- ions which create a positive charge (depletion

region or energy band bending) by extracting electrons from their conduction band [21,22]. This

depletion region with the potential barrier of the grain boundaries and/or the necks creates a

height basic potential barrier which it hampers the carrier transport in the grains region

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[21,22,32]. When dispersed SMO grains (anatase- or rutile-TiO2 grains with SnO2 grains)

suspend in aniline solution along with APS, the aniline monomers transform to the anilinium

cations and an electrostatic interaction between them and adsorbed anions on the surface of

grains could be created [10,16,33]. After reaction, the trapped electrons by the adsorbed oxygen

on the grain surface release [16,33]. This liberation leads to not only a decrease in the height of

the basic potential barrier of the grain surface, but also a redistribution of polaron density in the

interface of PANI chains and SMO grains (see Fig. 3c and d). Consequently, it could create the

enhanced carrier mobility in not only PTSs region, which depends on the number of grains

contacts and their type (boundary or necks), morphology, type, size and wt% of components, but

also between SMO grains and PANI chains which depends on the style and the number of

contacts between them [1,9, 33-36].

The gas sensing mechanism of PTS sensor is governed by the reaction between the surface layers

of the sensor and H2 gas molecules. According to the results of PTS characterization and scheme

in Fig. 11a, the grains are distributed in PANI network and only PANI chains are exposed to H2

gas. MacDiarmid [37] has presented a possible mechanism for the interaction of H2 with PANI

chain(s). According to this mechanism [37], it is possible that the gas molecules react with

nitrogen sites of the charged amine of the protonated PANI-emeraldine salt which located on the

sensor surface after H2 gas injection. H2 molecules might form a bridge between nitrogen atoms

of two adjacent chains (see Fig. 11b). H2 bonds dissociation presumably follows with the

formation of new N-H bonds to the amine nitrogen of the PANI chains. Subsequently, the charge

transport within a PANI chain returns it to its polaron lattice state (a combination of the charge

and the chain deformation) with a redistribution of polaron density in the forbidden energy gap

that causing new charge transport features create [9,36].

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The increase of the number of carriers and charge transfer rate in the presence of H2 molecules

could cause fully reversible carrier transport (carrier hopping) would be faster in inter- (intra-)

PANI chain(s) (see Fig. 11c). Furthermore, this principle with the carrier concentration in PANI

chains/SMO grains interface leads to a more decrease in the potential barrier height between

SMO grains.

According to EDX spectrum (Fig. 4b) there is presumably a heterojunction between SMO grains

and a carrier hopping from PANI chain to the SMO grain surface (or reversely) is presumably a

reasonable reason for the increase of the conductivity and the decrease of the resistance of PTS

sensors in the absence or the presence of H2 gas (see Fig. 11c). The change of the resistance and

conductivity are different in PTSs due to the change of the phase, size, morphology and wt% of

grains; the type and number of contacts; surface morphology of sensor; inter- and intra-PANI

chain separation; and PANI chains packing [14,17,21,22,35]. Then, the result is not only a lower

resistance (a better response), in the absence (presence) of H2 gas, but also a better relative

response of PAS sensors than PRS sensors.

4. Conclusion

PTS sensors were produced by the different amounts of dispersed TiO2 along with SnO2 grains

in aniline solution and oxidation polymerization of aniline. CSA-doped PTS sensors were

designed and fabricated for H2 gas sensing. Our results have demonstrated not only the

morphology of the sensor surface layers has a significant effect on the resistance shift, the

response and response/recovery time of the sensor but also the change of type, wt% and phase of

grains; and the type and number of contacts in PTS are important factors to alter them. PAS3

sensor has a better (very high) response between PTS sensors and PAS2 sensor has lower

response time in 0.8 vol% H2 gas at R.T.P.H. Moreover, an increase of SMO components wt%

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with a change in the sensor surface morphology caused lower recovery time in PAS3 (57 sec)

and PRS3 (46 sec) occur. The current-voltage characteristics of sensors showed a nonlinear

current versus voltage which increased with the increasing of SMO wt%. The comparison of our

results to the other results of PANI composite systems for H2 gas sensing [14,17] demonstrates

that the present of SnO2 nanoparticles in PTSs caused the response and recovery time of the

sensors had a significant decrease.

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Table 1. The response time, recovery time, response and resistance shift of PTS sensors.

Type of sensor Shift in resistance (Ω) Response (Rgas/Rair) tresponse (sec) trecovery (sec)

PAS1 67 1.16 114 210

PAS2 100 1.25 75 117

PAS3 382 6.18 245 57

PRS1 102 1.16 261 127

PRS2 78 1.12 247 146

PRS3 129 1.28 308 46

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Graphical abstract

10 15 20 25 30 35 40 45 500

1

2

3

4

5

6

Metal oxide nanoparticles (wt%)

Hollow star point : PRSCircle point : PASR

esponse

Graphical Abstract (for review)

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Fig. 1. FE-SEM images of (a) anatase phase of TiO2, (b) rutile phase of TiO2, (c) SnO2

nanoparticles, (d) PAS1, (e) PAS2, (f) PAS3, (g) PRS1, (h) PRS2 and (i) PRS3.

Figure 1

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Fig. 2. (a) TEM image of PAS3. (b) The cross-sectional FE-SEM image of PAS2.

Figure 2

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0.6

0.8

1.0

1.2

1.4300 400 500 600 700 800 900 300 400 500 600 700 800 900 1000

0.4

0.5

0.6

0.7

0.8

0.9

10 20 30 40 50 500 1000 1500 2000 2500 3000 3500 4000

(c)

A

bsorb

ance

Wavelength (nm)

(c3):PAS3

(c2):PAS2

(c1):PAS1

420 nm

440 nm

810 nm

830 nm

(d)

Wavelength (nm)

Absorb

ance

790 nm

410 nm

440 nm825 nm

(d3):PRS3

(d2):PRS2

(d1):PRS1

(a5): SnO

2

(a4): Rutile

(a3): Anatase

(a2): PRS2

(a1): PAS2

(a)

Sn(110)

2 theta (degree)

Counts

(a.u

.)

Sn(200)

Sn(211)Sn(101)

Sn(110)

A(101)

A(101)

R(110)

R(110)

Sn(110)

A : Anatase of TiO2

R : Rutile of TiO2

Sn: SnO2

1174

1170

2873

2871

(b)

Wavenumber (cm-1)

Tra

nsm

itta

nce

(b 2): P

RS2

(b 1): P

AS2

3433

3426

2934

2933

1736

1734

1605

1596

1462

1461

1281

1280

1131

1130

1043

1042

786

785

588

590

684

673

Fig. 3. (a) The XRD pattern for PAS2, PRS2, SnO2, anatase and rutile phases of TiO2. FT-IR

spectra of (b1) PAS2 and (b2) PRS2. Comparison of UV-vis spectra between PAS and PRS thin

films (c-d).

Figure 3

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0 1 2 3 4 50

(b)

O

SnSn

SnSN

C

O

Sn

Ti

Ti

Inte

nsity (

a. u.)

Energy (keV)

Fig. 4. FE-SEM image of PAS2 with a selected region of EDX pattern.

Figure 4

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Fig. 5. The schematic block diagram of our handmade hydrogen gas sensing setup.

Figure 5

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0 100 200 300 400 500

100

200

300

400

500

600

500

600

700

8000 100 200 300 400 500

PAS3

PAS2

PAS1

Time (sec)

gas out

gas in

Time (sec)

PRS3

PRS2

PRS1R

esis

tance

(ohm

)

Fig. 6. The resistance shift in PTS thin film sensors toward 0.8 vol% H2 gas at R.T.P.H.

Figure 6

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1.00

1.05

1.10

1.15

1.200 200 400 600 800 1000 1200 0 400 800 1200 1600

1.00

1.05

1.10

1.15

200 400 600 800

1.00

1.05

1.10

1.15

1.20

1.25

400 800 1200 1600

1.00

1.05

1.10

1.15

0 400 800 12001

2

3

4

5

6

7

0 400 800 1200

1.0

1.1

1.2

1.3

PAS1

PRS1

PAS2

Re

sp

on

se

PRS2

Re

sp

on

se

PAS3

Time (sec)

PRS3

Time (sec)

Fig. 7. The reproducibility dynamic response of PTS sensors exposed to 0.8 vol% H2 gas at

R.T.P.H.

Figure 7

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15 20 25 30 35 40 45

50

100

150

200

250

300

350

1

2

3

4

5

6

(b)

Hollow star point: trecovery

for PRS

Star point: tresponse

for PRS

Hollow circle point: trecovery

for PAS

Circle point: tresponse

for PAS

Tim

e (

se

c)

SMO nanoparticles (wt%)

(a)

Hollow star point : PRS sensors

Circle point : PAS sensors

Re

sp

on

se

Fig. 8. Variations in the response (a), the response/recovery time (b) of PTSs thin film sensors

vs. wt% of SMOs (containing anatase or rutile phase of TiO2 with SnO2) nanoparticles toward

0.8 vol% H2 gas at R.T.P.H. The lines are only a guide to the eye.

Figure 8

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0 200 400 600 800 1000390

420

450

480

510

1.0

1.1

1.2

1.30 200 400 600 800 1000

after

90 d

ays

(a)Resis

tance (

ohm

)

Time (sec)

afte

r 90 d

ays

(b)

Time (sec)

Response

Fig. 9. Reproducibility (a) resistance shift and (b) response of PAS2 thin film sensors exposed to

0.8 vol% H2 gas at R.T.P.H. The diagrams with star points show response and shift of resistance

after 3 months.

Figure 9

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-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Voltage (V)

Cu

rre

nt

(mA

)

Hollow Circle point : PRS2

Hollow star point : PAS1

Circle point : PRS1

Triangle point : PAS2

Star point : PRS3

Hollow triangle point : PAS3

Fig. 10. Current-voltage characteristics of the thin films of PTSs. The lines are only a guide to

the eye.

Figure 10

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Fig. 11. (a) Schematic representation of different contact region between SMO grains and SMO

grains-Pani chains with their potential barrier. (b-c) H2 gas sensing mechanism of PTS sensor.

Figure 11

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Biographies

Hossain Milani Moghaddam has got his B.S. degree from Ferdowsi University on Solid state

physics (1995), M.S. degree from Iran University of Science and Technology on Condensed

matter physics (1998) and PhD degree from Ferdowsi University on Condensed matter physics

(2007). Dr. Milani is currently working as an assistant professor in Solid State Physics

Department and a researcher at Nano and Biotechnology Research Group at University of

Mazandaran. His research interests include functional nanomaterials such as semiconductor

metal oxides, different gas sensor technology, nanostrucucture physics and molecular wires.

Shahruz Nasirian received his B.S. degree from Isfahan University of Technology on Solid

state physics and M.S. degree from University of Mazandaran on Nanophysics in 2010. He is a

Ph.D. candidate at University of Mazandaran on Nanophysics since 2011. His research interests

include the synthesis and characterization of inorganic semiconductor metal oxides such as TiO2,

SnO2, SiO2; nanofibers; conducting polymers; functional thin films and conducting

polymer/semiconductor metal oxide composites in order to gas sensor technology.