f-SWCNT polymer nanocomposite thin films prepared by electrochemical polymerization

Response time versus operating temperature for the PANI/f-SWNT nanocomposite thin ﬁlm based sensor. Fig[r]

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Original Article

a Department of Physics, College of Science, University of Baghdad, Baghdad, Iraq

b Department of Physics, College of Science, University of Sulaimani, Sulaymaniyah, Iraq

c Komar Research Center, Komar University of Science and Technology, Sulaymaniyah, Iraq

d Department of Physics, College of Science, University of Wassit, Wassit, Iraq

a r t i c l e i n f o

Article history:

Received 30 September 2018

Received in revised form

23 November 2018

Accepted 25 November 2018

Available online 29 November 2018

Keywords:

Conductive polymer

PANI

Nanocomposites

f-SWCNT

H 2 S gas sensor

a b s t r a c t

Hydrogen sulfide (H2S) gas sensors in the form of thinfilms based on polyaniline (PAN) incorporated with various concentrations of functionalized single wall carbon nanotubes (f-SWCNT) were prepared by electrochemical polymerization of Aniline monomer with sulfuric acid in an aqueous solution Surface morphology of the thin film nanocomposites was investigated by Field Emission Scanning Electron Microscopy (FE-SEM) and revealed that the f-SWCNTs were almost uniformly distributed on the surface

of the host PANI matrix The X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, and Hall effect measurements were used to characterize the synthesized PANI/f-SWCNT nanocomposites The Hall measurements reveal the p-type conductivity The grown FTIR band at 1145 cm1with the increase

of the f-SWCNT content evidence a formation of charge transfers due to a remarkable interaction be-tween PANI and f-SWCNTs The response of this nanocompositeﬁlm towards the H2S gas was investi-gated by monitoring the change in the electrical resistance with the time in the presence of 30% H2S at different operating temperatures The sensing analysis showed that the sensitivity increased with f-SWCNT content in the PANI matrix The rapid response/recovery times toward the H2S gas, at 50C, was achieved for a PANI/0.01% f-SWCNT nanocomposite sample

© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Hydrogen sulﬁde (H2S) is widely used in various chemical

in-dustries and research laboratories, and it is a very poisonous,

ﬂammable, and explosive gas Exposure to low concentrations of

H2S can cause various respiratory symptoms [1] However, high

exposure level can cause very serious health effects and even death

Accordingly, the fast and accurate detection of this harmful gas at

low concentrations is very important to protect human health

Conventional chemical gas sensors often rely on thin/thickﬁlms of

various sensing materials[2] In terms of their application as H2S

gas sensors, thinﬁlm semiconducting metal oxides, such as SnO2,

WO3, BaTiO3, and Fe2O3, have been extensively studied[3e5]

The metal oxides based sensors inherently suffer from some

problems, such as low selectivity, short lifetime and relatively high

operating temperature leading to high power consumption which limit their versatility[6,7] Thus, the conducting polymers, such as polyaniline (PANI), polypyrrole (PPy), and polythiophene (PTh), have been used as sensing active layers in chemical sensors due to their high sensitivity, and ability to reverse changes in their optical and electrical properties when exposed to certain liquids or gases

[8,9] The greatest advantage of conductive polymers is their ﬂex-ibility and low-cost processability, which allows a facile-fabrication

of the active layer of gas sensors As a result, more and more attention has been paid to the gas sensors, which are based on conducting polymers[10,11] However, these sensors often have a low sensitivity, and relatively high operating temperature range To improve the sensing performance, conducting polymer hybrid nanostructured materials have been employed to overcome the fundamental limitations of ﬁlm-based sensors Hybrid polymer-nanocomposite materials represent interesting strategies devel-oped to circumvent limitations of the individual components and to improve the responsibility of mechanical actuators [8] The high surface-to-volume ratio and unique size-dependent properties of

* Corresponding author Department of Physics, College of Science, University of

Sulaimani, Sulaymaniyah, Iraq.

E-mail address: omed.abdullah@univsul.edu.iq (O.Gh Abdullah).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2018.11.006

Journal of Science: Advanced Materials and Devices 4 (2019) 143e149

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improvements in the sensing performance [2] The gas-sensing

mechanism of thin-ﬁlm gas sensors is essentially based on the

change in the electrical resistance of the sensing element, when

speciﬁc gases interact with its surface[12e14]

In recent years, a great deal of research effort has been directed

to develop a sensor based on conducting polymers incorporated

with carbon nanotubes (CNTs), due to their high stiffness, and good

electrical conductivity at relatively low concentrations of CNTs[15]

CNTs have shown as a new class of one-dimensional crystal

struc-ture having extraordinary mechanical, thermal and electrical

properties[16,17]

Among all, PANI is considered to be one of the most

techno-logically promising conducting polymers because of its easy

prep-aration, low cost, environmental stability, and controllable

electrical conductivity[18] Moreover, PANI has also been used in

various applications, such as electrodes for batteries, sensors,

photovoltaic cells and electrochemical displays[19]

It has previously been established that incorporating

nano-particles in polar-polymers often modiﬁes the electron structures of

the compound which resulted in changes to both the bulk and

surface properties[12] Consequently, the resulting polymer

nano-composite can achieve a sensitivity and selectivity for gas detection

far exceeds those achievable performance with the individual

con-stituent of the composite Until recently, many researchers have

demonstrated that the PANI based nanocomposites can be widely

used as sensors to detect various gases[20] Srivastava et al.[21]

reported multiwall carbon nanotube (MWCNT) doped polyaniline

(PANI) composite thinﬁlms for hydrogen gas sensing applications

Their results reveal that the MWCNT/PANI compositeﬁlm shows a

higher sensitivity in comparison to pure PANI and it decreases with

increasing hydrogen gas pressure Zhang et al [22] developed a

PANI-SWCNT thinﬁlm nanocomposite based sensor for ammonia

(NH3) gas sensing, and they concluded that the electrochemical

functionalization of SWCNTs provides a promising new method

with improved sensitivity, response time, and reproducibility

The inﬂuence of the morphology on the gas sensing

perfor-mance is another important factor, and therefore, should be

considered The literature survey of polymer nanocomposites

re-veals that the sensors composition is a key factor that affects the

surface morphology of sensing materials which depend primarily

on the nature of the components and the processing conditions

[23] In the present work, structural, morphology, and H2S sensing

properties of polyaniline functionalized single-wall carbon

polymerization were systematically investigated The effect of

f-SWCNT concentration as well as operating temperature on sensing

parameters was also studied

2 Experimental

2.1 Preparation of thinﬁlm sensors

The electrochemical method was used to polymerize

PANI/f-SWCNTs thinﬁlm nanocomposites using the aniline monomer in

the aqueous acid medium at room temperature A titanium plate

was used as the working electrode and indium tin oxide (ITO) as a

reference electrode ITO substrates were ultrasonically cleaned by

typical methods The nanocomposite solution was prepared by

dissolving 0.3 M aniline monomer in 0.1 M sulfuric acid (H2SO4)

and mixed with the f-SWCNTs at different ratios (0.005 and 0.01%)

in the 150 ml of distilled water The synthesized electrodes were

carefully washed with distilled water thoroughly to avoid the

possible presence of electrolyte species on the surface of the

polymerﬁlm PANI/CNT ﬁlms were deposited at voltages of 2.4 and

2.2 V with two different ratios of f-SWCNT in 3 min The prepared

adherent to the ITO substrate The thickness of the samples was

~100 nm measured by the optical interferometer technique using the HeeNe laser (632 nm) A mask was used to deposit the 100 nm thin aluminum layer on theﬁlms surface by thermal evaporation 2.2 Thinﬁlms characterization

The crystallinity and phase of the PANI/f-SWCNT nanocomposites were characterized using an X-ray diffractometer (XRD, Shimadzu-6000) with a 2q scan from 10 to 80, and Cu Ka radiation (l¼ 1.5414 Å) was used as the X-ray source The surface micro-structures were analyzed by using the ﬁeld emission scanning electron microscope (FE-SEM) Hitachi model S-4160 operating at

30 kV The Fourier transform infrared (FTIR) spectrum of the pre-pared samples was recorded using the Shimadzu IR Affinity-1 system, in the range of 400e4000 cm1 Room temperature Hall Effect measurements were carried out using the Van der Pauw method Hall measurements were used to quantify important elec-trical parameters, such as Hall coefficient, Hall carrier concentration, and Hall mobility The gas sensing properties, such as the sensitivity, the response and recovery time were subsequently measured and evaluated, at exposuring the nanocomposite thinfilms to a 30% H2S gas -air mixture at different operating temperatures of 20, 50, 100,

150, 200C for different f-SWCNT concentrations

3 Results and discussion 3.1 XRD analysis

The XRD patterns of pure and f-SWCNT doped PANI nano-compositeﬁlms are shown inFig 1 The diffraction patterns of the samples exhibit two crystalline peaks at around 2q¼ 25and 50

which referred, respectively, to (200) and (210) plane directions representing the characteristic peaks of PANI[24] It can be clearly seen that the intensity of the observed peaks for the crystalline nanocompositeﬁlms are higher and the lines are sharper as for the pure PANIﬁlm

The average grain size of theﬁlms were estimated by using the DebyeeScherrer formula[25] The results obtained are shown in

Table 1 As seen fromTable 1, the average grain size increases with the increasing f-SWCNT concentration

3.2 FTIR analysis FTIR spectroscopy was conducted on the pure and f-SWCNT doped PANI nanocomposite thinﬁlm samples, and the results are shown inFig 2 The main characteristic band of PANI observed at

3466 cm1is assigned to the asymmetric NeH2stretching vibration

[26] The two bonds situated at 1461 cm1and 1554 cm1 corre-spond to the C]C stretching modes for the benzenoid and quinoid rings, respectively[27,28] The prominent band at 775 cm1may be attributed to the CeH out of plane deformation

It is observed that all the characteristic bands in theﬁngerprint region of PANI appear in the FTIR spectra of the PANI/f-SWCNT nanocomposite samples, indicating that the main constituents of PANI and its nanocomposites with f-SWCNTs have the same chemical structure However, the incorporation of f-SWCNT results into a slight shift in the peak's position to lower or higher wave-numbers from its original position A noticeable shift in the char-acteristic peak positions depicted inTable 2reveals the presence of the interaction between PANI and f-SWCNT during the electro-chemical polymerization Such an interaction was also reported by Patil et al.[29]between PANI and ZnO nanoparticle thinﬁlms M.H Suhail et al / Journal of Science: Advanced Materials and Devices 4 (2019) 143e149

144

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The polymer shows an interaction promoting and stabilizing the

quinoid ring structure in the polymer nanocomposite This

inter-action between PANI and f-SWCNTs may result in a charge transfer

between them[30e32] Thep-bonded surface of f-SWCNT might

interact strongly with the conjugated structure of PANI, especially

through the quinoid ring The strong band at 1145 cm1 is

considered to be a measure of the degree of the delocalization of

electrons, and it is, thus, the characteristic peak of the PANI's

con-ductivity[33] It appears that the interaction between PANI and

f-SWCNTs increases the effective degree of the electron

delocaliza-tion, and thus enhances the conductivity of the polymer composite

ﬁlms[34]

3.3 FE-SEM analysis

The microstructure and the surface morphology of the pure and

f-SWCNT doped PANI nanocompositeﬁlms were studied by FE-SEM

analysis The images were taken at 60,000 times magniﬁcation

Fig 3shows the FE-SEM images and the interactive 3D surface plot

(the insets) of the pure and the f-SWCNT doped PANI thinﬁlms The

image of the pure PANI sample shows the formation of

structured conducting PANI, in which the spherical PANI

nano-particles are distributed almost uniformly and the average grain

size of them was estimated to be 36.62 nm As it is seen, the grain

size in the sample increased to 49.84 and 84.86 nm upon

incor-porating 0.005% and 0.010% of f-SWCNT, respectively The

micro-graph also shows some clusters made up from aggregates of many

PANI nanoparticles

The micrographs of the prepared nanocomposite thin films show the effect of the f-SWCNT dopant on the morphology of these films The increase in the f-SWCNT concentration causes an in-crease in the average surface grain size and the surface roughness of the nanocompositefilms, which is well confirmed by the obtained results from XRD As a result of the strong interaction between the f-SWCNT and the polar groups of PANI (which was confirmed by FTIR analysis as previously described), a homogeneous interaction

is typically obtained on the polymerization of aniline in the pres-ence of f-SWCNT

A higher degree of roughness is observed for the f-SWCNT doped PANI based nanocomposite surface when compared with that of the pure PANI These features are shown in the inset ofFig 2 The high degree of roughness, as identiﬁed in the 3D surface im-ages, is associated with an increase in the exposed surface area It is well reported in the literature that, the high exposed surface area of

a sensing element usually has positive effects on the gas-sensing performance by providing more active sites for the adsorption of gas molecules[35,36]

3.4 Hall measurements The Hall-effect measurements are a useful mean for the characterization of materials, which provides the basic elec-trical parameters to identify the suitable material for partic-ular applications [37] The values of the Hall coefﬁcient (RH), the carrier concentration (nH), the Hall mobility (mH), and the type of charge carrier conductivity have been estimated from Hall measurements on the pure and the f-SWCNT doped PANI

following equations:

where;

Fig 1 The XRD pattern of pure and f-SWCNT doped PANI nanocomposite thin ﬁlms.

Table 1

Structural parameters of XRD pattern for pure and f-SWCNT doped PANI

nano-composite thin ﬁlms.

Samples 2q(Deg.) FWHM d hkl (Å) G.S (nm) (hkl)

50.080 0.116 1.8200 86.9 (210) PANI/0.005% f-SWCNT 25.600 0.156 3.4769 57.4 (200)

50.090 0.104 1.8196 94.3 (210) PANI/0.01% f-SWCNT 25.599 0.141 3.4770 60.4 (200)

50.100 0.087 1.8193 112.5 (210)

M.H Suhail et al / Journal of Science: Advanced Materials and Devices 4 (2019) 143e149 145

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I

d

here, VHis Hall voltage, I is constant current,sis conductivity of the

material, e is the electron charge, and H is the applied magnetic

ﬁeld

The positive sign of the Hall coefﬁcients (RH) for all

composi-tions of PANI/f-SWCNT conﬁrmed the p-type nature of the

con-ductivity of this system It can also be noted that the magnitude of

RH decreases with the increasing f-SWCNT concentration During

the electropolymerization, emeraldine salt is formed onto the

surface of carbon nanotubes, making the polyaniline a p-type

semiconductor.Table 3presents the electrical parameters for the

prepared PANI/f-SWCNT nanocomposite thinﬁlms It is seen that

the carrier concentration (n ) and the Hall mobility (m ) increase

with the increasing f-SWCNT concentration, which indicates the reduction of theﬁlm resistivity

3.5 Sensor measurements The gas sensing tests aim to find the optimal conditions of operation of the sensor elements for detecting any specific gas It is well accepted that the sensing performance of polymer-based gas sensors can be improved upon doping with suitable dopants[38] The sensing properties of the PANI/f-SWCNT nanocomposite thin films with respect to the H2S gas were measured by detecting the changes in the electrical resistance with the time over the two sensing electrodes under the H2S gas.Fig 4shows the variation of the normalized resistance (DR=Ro) as a function of time of the on/ off gas valve of the pure PANI and the PANI/f-SWCNT films at different operating temperatures

From the Hall measurements the present polymer nano-composite thinﬁlms are identiﬁed as p-type semiconductors with holes as the major charge carriers The H2S gas is believed to partly dissociate into Hþ and HSas it is a weak acid, resulting in the partial protonation of PANI This causes a band bending and space-charge layer near the surface of each grain boundary [39] By introducing HS gas in a gas sensing process, the electrical

Fig 2 FTIR spectra of the pure and f-SWCNT doped PANI nanocomposite ﬁlms.

Table 2

The value of FTIR bonds for PANI/f-SWCNT thin ﬁlms.

PANI/0.005 f-SWCNT 3469 1557, 1463 1137 775

PANI/0.01 f-SWCNT 3470 1560, 1464 1141 780

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conductivity of theﬁlm changes due to the interactions between

the surface grains and the gas molecules, causing the removal of

electrons from the aromatic rings of PANI The electron transfer can

cause the changes in the work function and hence the resistance of

the sensing element When this occurs for the p-type conductive

polymer, the electrical conductivity of the conductive polymer is

enhanced[10]

The variation of the H2S gas sensitivity versus operating

tem-perature for the PANI/f-SWCNT polymer nanocomposite thinﬁlms

is shown inFig 5 The dopedﬁlms exhibited an improvement in the sensitivity in comparison with the pure PANIﬁlm This is attributed

to an increase in the rate of the surface reaction of the target gas, which is conﬁrmed by SEM analysis The highest sensitivity is found

at 50 C, and the value decreases with a further increase in the operating temperature The reason might be due to a reduction of the intensity of the reaction between theﬁlm surface and the gas stream at a higher temperature[40]

The response time of the sensor is dependent on how rapidly gas molecules can diffuse and reacts with the sensitive active layer

Figs 6 and 7, respectively, exhibit the variation of the response time and the recovery time versus the operating temperature for the pure and the f-SWCNT doped PANIﬁlms The results reveal that the f-SWCNT doped PANIﬁlm has faster response/recovery times in

Fig 3 FE-SEM images of (a) pure PANI, (b) PANI/0.005% SWNT, and (c) PANI/0.01%

f-SWNT thin ﬁlms (scale bar 500 nm).

Table 3

Effect of f-SWCNT concentration on the Hall measurements results of PANI/f-SWCNT nanocomposite thin ﬁlms.

f-SWCNT Content % sRT 10 2 (U1 cm1) R H (cm 3 /C) n H 10 19 (cm3) mH (cm 2 /V.sec)

Fig 4 The variation of normalized resistance with time for: (a) pure PANI, (b) PANI/ 0.005% f-SWNT, and (c) PANI/0.01% f-SWNT.

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comparison to the pure one This may be attributed to the

forma-tion of conducting paths and to the electron hopping through the

conducting channels of the carbon nanotubes The presence of

f-SWCNT in PANI may promote the H2S absorption due to their

centrally hollow core structure Further, their large surface area

provides more interaction sites within the PANIﬁlm On the other

hand, both the response and recovery times of the sensor were

found to decrease with the increasing operating temperature This

can be explained as the following: The gas sensing process involves

the adsorption and the diffusion of the gas molecules on the sensor

active layer and their reaction with the sensing ﬁlm Since the

adsorption takes place at low temperature and decreases with the

increasing temperature [41], consequently, the gas sensing

response will decrease with the increasing temperature

4 Conclusion The electrochemical polymerization technique was used to prepare pure and f-SWCNT doped PANI nanocomposite thinfilms XRD and FTIR spectrum revealed the incorporation of f-SWCNT into the conducting PANI matrix FE-SEM images confirmed that the f-SWCNTs were uniformly dispersed on the surface of the nano-compositefilm The Hall effect measurements confirmed that the

conductors The most sensitive nanocomposite thinﬁlm to H2S gas was obtained by incorporating 0.01% f-SWCNT into the PANI ma-trix The sensing analysis showed an excellent sensitivity, with rapid response and recovery times toward H2S gas, at a low oper-ating temperature of 50C

Acknowledgements The authors would like to express their sincere appreciation to the Department of Physics, College of Science, at the University of Baghdad, for the facility in their laboratories during this research References

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