polyaniline assisted by tio2:sno2 nanoparticles as a hydrogen gas sensor at environmental conditions
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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.
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