Hydrogen sulfide (H2S) gas sensors in the form of thin films 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.
Journal of Science: Advanced Materials and Devices (2019) 143e149 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Hydrogen sulfide sensors based on PANI/f-SWCNT polymer nanocomposite thin films prepared by electrochemical polymerization Mahdi Hasan Suhail a, Omed Gh Abdullah b, c, *, Ghada Ayad Kadhim d a Department of Physics, College of Science, University of Baghdad, Baghdad, Iraq 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 b a r t i c l e i n f o a b s t r a c t Article history: Received 30 September 2018 Received in revised form 23 November 2018 Accepted 25 November 2018 Available online 29 November 2018 Hydrogen sulfide (H2S) gas sensors in the form of thin films 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 cmÀ1 with the increase of the f-SWCNT content evidence a formation of charge transfers due to a remarkable interaction between PANI and f-SWCNTs The response of this nanocomposite film towards the H2S gas was investigated 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 fSWCNT content in the PANI matrix The rapid response/recovery times toward the H2S gas, at 50 C, 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/) Keywords: Conductive polymer PANI Nanocomposites f-SWCNT H2S gas sensor Introduction Hydrogen sulfide (H2S) is widely used in various chemical industries 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/thick films of various sensing materials [2] In terms of their application as H2S gas sensors, thin film 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 * 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 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 flexibility 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 polymernanocomposite materials represent interesting strategies developed 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 nanomaterials have resulted in very promising further https://doi.org/10.1016/j.jsamd.2018.11.006 2468-2179/© 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/) 144 M.H Suhail et al / Journal of Science: Advanced Materials and Devices (2019) 143e149 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 structure having extraordinary mechanical, thermal and electrical properties [16,17] Among all, PANI is considered to be one of the most technologically promising conducting polymers because of its easy preparation, 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 nanoparticles 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 nanocomposite can achieve a sensitivity and selectivity for gas detection far exceeds those achievable performance with the individual constituent 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 thin films for hydrogen gas sensing applications Their results reveal that the MWCNT/PANI composite film 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 thin film 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 performance is another important factor, and therefore, should be considered The literature survey of polymer nanocomposites reveals 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 nanotubes (PANI/SWCNTs) thin films prepared by electrochemical polymerization were systematically investigated The effect of fSWCNT concentration as well as operating temperature on sensing parameters was also studied Experimental 2.1 Preparation of thin film sensors The electrochemical method was used to polymerize PANI/fSWCNTs thin film 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 polymer film PANI/CNT films were deposited at voltages of 2.4 and 2.2 V with two different ratios of f-SWCNT in The prepared nanocomposite thin films were green, uniform, and strongly 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 the films surface by thermal evaporation 2.2 Thin films characterization The crystallinity and phase of the PANI/f-SWCNT nanocomposites were characterized using an X-ray diffractometer (XRD, Shimadzu6000) with a 2q scan from 10 to 80 , and Cu Ka radiation (l ¼ 1.5414 Å) was used as the X-ray source The surface microstructures 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 prepared samples was recorded using the Shimadzu IR Affinity-1 system, in the range of 400e4000 cmÀ1 Room temperature Hall Effect measurements were carried out using the Van der Pauw method Hall measurements were used to quantify important electrical 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 thin films to a 30% H2S gas -air mixture at different operating temperatures of 20, 50, 100, 150, 200 C for different f-SWCNT concentrations Results and discussion 3.1 XRD analysis The XRD patterns of pure and f-SWCNT doped PANI nanocomposite films are shown in Fig The diffraction patterns of the samples exhibit two crystalline peaks at around 2q ¼ 25 and 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 nanocomposite films are higher and the lines are sharper as for the pure PANI film The average grain size of the films were estimated by using the DebyeeScherrer formula [25] The results obtained are shown in Table As seen from Table 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 thin film samples, and the results are shown in Fig The main characteristic band of PANI observed at 3466 cmÀ1 is assigned to the asymmetric NeH2 stretching vibration [26] The two bonds situated at 1461 cmÀ1 and 1554 cmÀ1 correspond to the C]C stretching modes for the benzenoid and quinoid rings, respectively [27,28] The prominent band at 775 cmÀ1 may be attributed to the CeH out of plane deformation It is observed that all the characteristic bands in the fingerprint 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 wavenumbers from its original position A noticeable shift in the characteristic peak positions depicted in Table reveals the presence of the interaction between PANI and f-SWCNT during the electrochemical polymerization Such an interaction was also reported by Patil et al [29] between PANI and ZnO nanoparticle thin films M.H Suhail et al / Journal of Science: Advanced Materials and Devices (2019) 143e149 145 Fig The XRD pattern of pure and f-SWCNT doped PANI nanocomposite thin films The polymer shows an interaction promoting and stabilizing the quinoid ring structure in the polymer nanocomposite This interaction between PANI and f-SWCNTs may result in a charge transfer between them [30e32] The p-bonded surface of f-SWCNT might interact strongly with the conjugated structure of PANI, especially through the quinoid ring The strong band at 1145 cmÀ1 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 conductivity [33] It appears that the interaction between PANI and fSWCNTs increases the effective degree of the electron delocalization, 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 nanocomposite films were studied by FE-SEM analysis The images were taken at 60,000 times magnification Fig shows the FE-SEM images and the interactive 3D surface plot (the insets) of the pure and the f-SWCNT doped PANI thin films The image of the pure PANI sample shows the formation of nanostructured conducting PANI, in which the spherical PANI nanoparticles 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 incorporating 0.005% and 0.010% of f-SWCNT, respectively The micrograph also shows some clusters made up from aggregates of many PANI nanoparticles Table Structural parameters of XRD pattern for pure and f-SWCNT doped PANI nanocomposite thin films Samples 2q (Deg.) FWHM dhkl (Å) G.S (nm) (hkl) Pure PANI 25.630 50.080 25.600 50.090 25.599 50.100 0.172 0.116 0.156 0.104 0.141 0.087 3.4729 1.8200 3.4769 1.8196 3.4770 1.8193 49.7 86.9 57.4 94.3 60.4 112.5 (200) (210) (200) (210) (200) (210) PANI/0.005% f-SWCNT PANI/0.01% f-SWCNT 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 increase in the average surface grain size and the surface roughness of the nanocomposite films, 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 presence 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 of Fig The high degree of roughness, as identified in the 3D surface images, 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 electrical parameters to identify the suitable material for particular 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 nanocomposite thin films at room temperature, using the following equations: nH ¼ RH e mH ¼ sjRH j where; (1) (2) 146 M.H Suhail et al / Journal of Science: Advanced Materials and Devices (2019) 143e149 Fig FTIR spectra of the pure and f-SWCNT doped PANI nanocomposite films with the increasing f-SWCNT concentration, which indicates the reduction of the film resistivity Table The value of FTIR bonds for PANI/f-SWCNT thin films Samples NeH C¼C S¼O CeH Pure PANI PANI/0.005 f-SWCNT PANI/0.01 f-SWCNT 3466 3469 3470 1554, 1461 1557, 1463 1560, 1464 1145 1137 1141 775 775 780 RH ¼ VH d I H (3) here, VH is Hall voltage, I is constant current, s is 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 compositions of PANI/f-SWCNT confirmed the p-type nature of the conductivity 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 presents the electrical parameters for the prepared PANI/f-SWCNT nanocomposite thin films It is seen that the carrier concentration (nH ) and the Hall mobility (mH ) increase 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 shows 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 nanocomposite thin films are identified as p-type semiconductors with holes as the major charge carriers The H2S gas is believed to partly dissociate into Hỵ and HS as it is a weak acid, resulting in the partial protonation of PANI This causes a band bending and spacecharge layer near the surface of each grain boundary [39] By introducing H2S gas in a gas sensing process, the electrical M.H Suhail et al / Journal of Science: Advanced Materials and Devices (2019) 143e149 147 Fig The variation of normalized resistance with time for: (a) pure PANI, (b) PANI/ 0.005% f-SWNT, and (c) PANI/0.01% f-SWNT Fig FE-SEM images of (a) pure PANI, (b) PANI/0.005% f-SWNT, and (c) PANI/0.01% fSWNT thin films (scale bar 500 nm) conductivity of the film 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 temperature for the PANI/f-SWCNT polymer nanocomposite thin films is shown in Fig The doped films exhibited an improvement in the sensitivity in comparison with the pure PANI film 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 the film 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 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 PANI films The results reveal that the f-SWCNT doped PANI film has faster response/recovery times in Table Effect of f-SWCNT concentration on the Hall measurements results of PANI/f-SWCNT nanocomposite thin films f-SWCNT Content % sRT Â102 (UÀ1 cmÀ1) RH (cm3/C) nH Â1019 (cmÀ3) mH (cm2/V.sec) 0.000 0.005 0.010 2.21 4.51 6.22 0.08987 0.06657 0.05567 69.5 93.8 112.2 19.89 30.04 34.62 148 M.H Suhail et al / Journal of Science: Advanced Materials and Devices (2019) 143e149 Conclusion Fig Sensitivity versus operating temperature for PANI/f-SWNT thin films nanocomposite based sensor The electrochemical polymerization technique was used to prepare pure and f-SWCNT doped PANI nanocomposite thin films XRD and FTIR spectrum revealed the incorporation of f-SWCNT into the conducting PANI matrix FE-SEM images confirmed that the fSWCNTs were uniformly dispersed on the surface of the nanocomposite film The Hall effect measurements confirmed that the PANI/f-SWCNT nanocomposite films behave as p-type electric conductors The most sensitive nanocomposite thin film to H2S gas was obtained by incorporating 0.01% f-SWCNT into the PANI matrix The sensing analysis showed an excellent sensitivity, with rapid response and recovery times toward H2S gas, at a low operating temperature of 50 C 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 Fig Response time versus operating temperature for the PANI/f-SWNT nanocomposite thin film based sensor Fig Recovery time versus operating temperature for the PANI/f-SWNT nanocomposite thin film based sensor comparison to the pure one This may be attributed to the formation of conducting paths and to the electron hopping through the conducting channels of the carbon nanotubes The presence of fSWCNT in PANI may promote the H2S absorption due to their centrally hollow core structure Further, their large surface area 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polypyrrole/ SWCNTs nanocomposites-based NH3 sensor operated at room temperature, Sensor Actuator B Chem 140 (2009) 500e507 ... and Devices (2019) 143e149 Conclusion Fig Sensitivity versus operating temperature for PANI/f-SWNT thin films nanocomposite based sensor The electrochemical polymerization technique was used to prepare... Park, H Yoon, Chemo-electrical gas sensors based on conducting polymer hybrids, Polymers (2017) 155 [12] A.F Abdulameer, M.H Suhail, O.G Abdullah, I.M Al-Essa, Fabrication and characterization of... processing conditions [23] In the present work, structural, morphology, and H2S sensing properties of polyaniline functionalized single-wall carbon nanotubes (PANI/SWCNTs) thin films prepared by electrochemical