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polyaniline composite based chemiresistor type sensor for hydrogen gas sensing application
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 8 2 5 e3 8 3 2
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Preparation and characterization of tantalum/polyaniline composite based chemiresistor type sensor for hydrogen gas sensing application Subodh Srivastava*, Sumit Kumar, Y.K. Vijay Thin Film and Membrane Science Lab, University of Rajasthan, Jaipur-302055, India
article info
abstract
Article history:
In the present work we have reported the effect of Shift heavy ion (SHI) irradiation on the
Received 19 February 2011
gas sensing properties of tantalum (Ta)/Polyaniline (PANI) composite thin film based
Received in revised form
chemiresistor type gas sensor for hydrogen gas sensing application. PANI was synthesized
11 April 2011
chemically by in situ oxidative polymerization method. The thin sensing films of PANI
Accepted 12 April 2011
were deposited onto finger type Cu-interdigited electrodes using spin cast technique and
Available online 14 June 2011
a thin Ta layer was deposited on to PANI thin film to prepare Ta/PANI composite chemiresistor sensor. These chemiresistor sensing films were irradiated with energetic Auþ12
Keywords:
ions (150 MeV) at the different fluencies ranging from 1 109 to 1 1011 ions/cm2. The
Polyaniline (PANI)
structural and morphological properties of these composite thin films were characterized
Ta/PANI composite
by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements before and
Chemiresistor sensor
after SHI irradiation. The electrical properties of these composite thin films were charac-
H2 gas sensing
terized by IeV characteristic measurements. The changes in resistance of the composite
Atomic force microscopy (AFM)
thin film sensor were utilized for detection of hydrogen gas. It was observed that after SHI
X-ray diffraction (XRD)
irradiation Ta/PANI composite sensor shows a high response value and sensitivity with good repeatability in comparison to the pristine sample. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In recent years numerous sensors with polymeric materials have been developed for detecting many analytes, ionic species, organic vapours and gases [1,2]. The incorporation of metal nanoparticles into conducting polymers has been reported by a number of groups [3e6]. Generally, these efforts were directed at the development of gas sensitive materials in order to improve the sensitivity, response time and stability of gas sensors. It has been reported earlier that metal species in the conducting polymer ensures high surface area, improved conductivity and possible enhancement of the unique characteristics of the composite [7,8].
Generally, polymers are deposited as sensitive layer of sensor and then metal thin film are deposited on the surface of the sensitive polymer layer as active material to increase the area/volume ratio and favour the adsorption of gases. The deposition of active metal precursor can be made by thermal evaporation or sputtering techniques. In case of hydrogen gas platinum, palladium, silver and titanium have been widely reported as active element therefore their composite with polymer may have potential applications in gas sensors and electrocatalysis [9e15]. Among the conducting polymer, Polyaniline (PANI) has been preferred as a sensitive media for hydrogen sensing, due to its environmental stability, selectivity and sensitivity towards hydrogen gas at room
* Corresponding author. Tel.: þ91 (0) 141 2702457; fax: þ91 (0) 141 2701149. E-mail address: [email protected] (S. Srivastava). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.155
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 8 2 5 e3 8 3 2
temperature. The further incorporation of sensitive nanoparticles makes the PANI composite more sensitive towards hydrogen gas. Therefore PANI has also been used as a host matrix for the active filers like palladium, platinum, gold, silver and titanium [16e22].
1.1.
Swift heavy ions (SHI)
It is worth noting that the sensitivity of gas sensors is strongly affected by the structural and morphological parameter of sensing materials [23]. The ion irradiation has been established as a potential tool for material modification [24]. As an ion penetrates a solid, it loses energy by two distinct interactions: (1) by elastic nuclear collisions with the target atoms (nuclear stopping Sn) and (2) by excitation or ionization of atoms by inelastic collisions, (known as Electronic Stopping, and the energy spent in this process is called electronic energy loss (Se). When the energy of incoming ion is very high, the electronic stopping is dominant, where the displacement of atoms due to elastic collisions is insignificant. Such heavy ions, with energies so high that the electronic loss process dominates, are referred to as Swift heavy ions (SHI) [25]. The direct interaction between energetic ion and target atoms can lead to structural changes such as generation of point defects, amorphization of crystalline materials or phase transformations in amorphous atomic networks, surface modification, mixing of materials and formation of new compounds [26e31]. The SHI induces the mixing at the interface in order to produce novel composite materials and phases. The mixing takes place due to transfer of energy, by the energetic ions in the electronic subsystem, subsequently transfer to atomic subsystem via electron-phonon coupling which results in a rise in the lattice temperature up to 104 K. Therefore the material within few nanometers from the ion path melts for the duration of 1012 e1011 s and then quenches at very fast rate, forming the latent tracks and this induces inter-mixing in bilayer systems [32,33]. On irradiation with SHI, a dramatic change in the structural and electrical properties of polyaniline composites was also observed [34e37]. The SHI irradiated polyvinylchloride polyethylene terephthalate (PVCPET) composites have been tested for hydrogen gas and ammonia gas sensitivity [38,39]. The use of tantalum, tantalum alloys and tantalum oxide has already been suggested for sensor purposes [40,41]. Tantalum (Ta) is an active element for hydrogen gas and has been studied to investigate diffusion of hydrogen in it [42e45]. There is always a finite probability for hydrogen or deuterium atom to occupy an interstitial site in its metal lattice. Two phases, designed a and b are known to exist in the tantalum-hydrogen system and their properties determine, to a large extent, the kinetics of the diffusion process [46e48].. The presence of hydrogen, or media containing hydrogen compounds (which can liberate free hydrogen) can lead to change in mechanical and structural properties of Ta surface [49]. It has been reported earlier that Ta and Niobium react with carbon, nitrogen, oxygen, and hydrogen at room temperature [50,51]. It has also been reported that Ta, with a properly activated surface, may be a suitable element for hydrogen separation membrane [52e54]. Therefore it is impotent to use Ta as
an active material with PANI for hydrogen gas sensing. Separately PANI has been widely tested for hydrogen gas sensing application [55e58]. However, to the best of our knowledge Ta/PANI composite films have not yet been used as a sensitive layer in chemiresistor type sensor for hydrogen gas sensing. In the present work PANI was synthesized chemically by in situ oxidative polymerization of aniline using ammonium persulfate in acidic medium at low temperature. The thin sensing films of PANI were deposited onto finger type cuinterdigited electrodes using spin cast technique and a thin Ta layer was deposited on to PANI thin film to prepare Ta/PANI composite chemiresistor sensor. These chemiresistor sensing films were irradiated with Auþ12 ions at the different fluencies and the effect of SHI irradiation on the gas sensing properties of Ta/PANI composite films was studied for hydrogen gas.
2.
Experimental
2.1.
Materials
PANI was synthesized by in-situ chemical oxidative polymerization method as described elsewhere [59,60]. Tantalum sheet (99.9% pure, 1 mm thick and 2 inch diameter) was used as sputtering target for thin film deposition as shown in Fig. 1.
2.2.
Sensor preparation
CSA-PANI mixture was dissolved in 30 ml chloroform solution using magnetic stirrer. Thin films of this solution were deposited on cleaned Cu-IDE epoxy substrates using the spin coating technique and a thin tantalum layer was deposited onto spin coated PANI thin film using DC magnetron sputtering system under high vacuum of the order of 105 torr. In the sputtering process the Ta target is normally fixed at 10e12 cm apart from the substrate holder and positioned in front of the sample surface. The substrate was rotated during deposition yielding a uniform thickness throughout the sample. Argon gas was inserted as the sputtering gas through the needle valve at a constant pressure of 1 101e2 101 torr. The schematic diagram of Ta coated PANI Chemiresistor sensor is shown in Fig. 2.
2.3.
Swift heavy ions (SHI) irradiation
In the present work, a 15 UD Pelletron Accelerator facility located at IUAC New Delhi was used for SHI irradiation [61e63]. It is basically a Van de Graff type tandem electrostatic accelerator in vertical configuration, having maximum terminal voltage up to 16 MV. The prepared samples were mounted on target assembly in material science chamber under high vacuum (106 Torr) The SHI irradiation was performed at room temperature using Auþ12 ions having energy of 150 MeV at different fluencies ranging from 1 109 to 1 1011 ions/cm2 depending on time of bombardment. The beam current was kept 1 pnA and monitored intermittently with a Faraday cup. The ion beam was defocused using magnetic scanning system, so that an area of 1 1 cm2 was uniformly irradiated. The irradiated samples were stored at room temperature in air.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 8 2 5 e3 8 3 2
Fig. 1 e Ta sputtering target mounted in target holder.
3.
Result and discussion
3.1.
X-ray fluorescence (XRF)
The XRF has been used to analyze the elemental composition of composite materials. Fig. 3 shows the XRF spectra of Ta/PANI composite, which exhibits two characteristic X-ray energy peaks at Ka ¼ 8.1 keV and Kb ¼ 9.2 keV corresponding to tantalum (see inset in Fig. 3) and confirms the presence of Ta in PANI matrix. It has also been observed that after irradiation, the intensity of energy peak has been decreased with increasing ion fluence. The decreased in peak intensity is due to two reasons: there may be small variation in thickness of Ta layer during sputtering process due to the difference in target to source distance (10e12 cm) which influence the number of Ta atoms deposited at the surface of PANI and hence the intensity of corresponding x-ray energy peak of Ta decreased. Secondly there may be mixing of tantalum within PANI matrix during irradiation process therefore the relative XRF counts coming from the surface are decreased.
Fig. 2 e Schematic diagram of Ta coated PANI thin film chemiresistor sensor and (b) prepared sensor.
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Fig. 3 e XRF spectra of Ta/PANI composite before and after SHI irradiation.
3.2.
X-ray diffraction (XRD)
The XRD pattern of Ta/PANI composite films before and after the SHI irradiation is shown in Fig. 4. The XRD pattern of PANI (ES) exhibits two broad amorphous peaks, observed at 2q z 20.1 and 2q z 25.2 (Fig. 4a), which may be attributed to periodicity parallel and perpendicular to PANI conjugation chains, respectively [64e66]. It has been observed that all Ta/ PANI composite films exhibit a broad peak appeared at 2q z 25.2 corresponding to the amorphous nature of PANI and two sharp crystalline peaks, centered at 2q z 38 and 2q z 70.1 corresponding to the crystalline nature of Ta. The Peak observed at 2q z 38 can be ascribed to either (110) of bccTa or (200) of b-Ta, while the peak at 2q z 70 corresponds to the (400) of b-Ta [67e69]. It was observed that after irradiation the peaks become more sharpen as the ion fluence increases. This indicates that after SHI irradiation the crystallinity of Ta in PANI matrix has been increases. No new structural order has found to be generated within composites after the irradiation as shown in Fig. 4ced. It has been earlier reported that
Fig. 4 e X-ray diffraction patterns of pure PANI and Ta/PANI composite films before and after SHI irradiation.
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Fig. 5 e AFM images of Ta/PANI composite film (a) before irradiation, (b) after irradiated at 1 3 109 ion/cm2 and (c) after irradiated at 1 3 1011 ion/cm2.
the density of the polymer increases during the SHI irradiation making the polymer more compact which results in more crystalline regions in polymer films resulting in an increase in the degree of crystallinity [30,43] of composite.
3.3.
Atomic force microscopy (AFM)
Surface morphology of pristine and irradiated Ta/PANI composite films has been examined by AFM measurements.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 8 2 5 e3 8 3 2
The Fig. 5 shows AFM images of Ta/PANI composite films before and after SHI irradiation. The pristine Ta/PANI composite film shows a highly dense random shaped uneven granular structure on the surface (Fig. 5a). The composite film irradiated at low fluencies (1 109d1 1010 ions/cm2) shows a typical polycrystalline clustered structure of irregular grains aligned in small crystallized domains (Fig. 5b). The film irradiated at 1 1011 ions/cm2 shows more compact, closely packed interconnected rope like structure at the surface which results in an increased crystallinity of the composite film and thus show good agreement with XRD measurement results after irradiation. The AFM images revealed a continuous increase in the granular cluster in composite film with the increase of ion fluence. The cluster formation may be attributed to large amount of electronic energy loss induced collision cascades therefore particles agglomerated due to the partially melted Ta layer. This took place near the surface and is responsible for the displaced atoms forming clusters.
3.4.
Fig. 6 e Current-Voltage (IeV) characteristics curve of unirradiated and Irradiated Ta/PANI composite films at room temperature.
IeV characteristics
Fig. 6 shows the IeV characteristics of Ta/PANI composite film before and after SHI irradiation at room temperature. From the IeV characteristics curve it has been observed that at low voltages, the current is proportional to the applied voltage corresponding to an ohmic regime, which extends almost up to 0.3 V. With increasing bias voltage beyond 0.3 V, an increasing trend in the current was observed showing the non linear region. IeV curve of pristine Ta/PANI thin film shows almost similar characteristics as that of pure PANI thin film. This suggests that Ta thin film onto PANI surface does not affect the density as well as transportation of charge carrier within PANI matrix. In case of irradiated Ta/PANI composite thin films, it was observed that from the ohmic regime to the non linear region, the current increases slightly with increasing fluence. This implies that the background free charge carrier density increases slightly with increasing ion fluence [70]. Also, due to irradiation, Ta melts and diffuses into PANI matrix, which provides more conducting path for easy charge transport between consecutive PANI chains and hence current increase with increasing ion fluence. B. Scrosati and Hussein et. al. have reported that the increase in the crystallinity of the composite films upon SHI irradiation may also contribute to the increase in conductivity of the films [30,71], which is in good agreement with our XRD and AFM measurements. Fig. 7 shows the IeV characteristics of unirradiated and irradiated Ta/PANI composites thin film with increasing temperature. The increasing trend in current was observed for both unirradiated and irradiated composite samples with increasing temperature. It may be attributed to the increase in the number of thermally activated charge carriers with temperature, which indicates that composite thin films are semiconductor in nature.
3.5.
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at room temperature. This Figure clearly reveals that resistance of all irradiated composite films decreases very rapidly by introduction of hydrogen gas and become stable after few seconds. This may be attributed to the reducing nature of
Gas sensing measurements
Fig. 8 shows the variation in the resistance of pristine and irradiated Ta/PANI composite sensors towards hydrogen gas
Fig. 7 e Current-Voltage (IeV) characteristics curve of (a) unirradiated and (b) Irradiated Ta/PANI composite films with temperature.
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Fig. 8 e Change in resistance of unirradiated and irradiated Ta/PANI composite sensors with time after exposed to hydrogen gas at room temperature.
Fig. 10 e Variation in %sensitivity of Ta/PANI composite sensors irradiated at different ion fluencies.
hydrogen gas. Introduction of hydrogen gas in the composite thin films injected electrons to the film, and thus significantly increase the number of charge carrier in the film. As a result, more electrons flowed in the film and at the same time reduced the resistance of the film. An exception was observed in the pristine Ta/PANI composite film, where the resistance change is very small or almost negligible. Fig. 9 shows the response of pristine and irradiated Ta/PANI composite sensors towards the hydrogen gas at the room temperature. It was observed that unirradiated Ta/PANI composite sensor shows almost negligible response in comparison to irradiated sensors. It may be due to the Ta layer coated over the PANI surface, which does not react with hydrogen at room temperature and inhibited the hydrogen to diffuse in to the PANI matrix. Therefore at room temperature pristine Ta/PANI
sensor dose not shows any response for hydrogen. While upon irradiation, it was observed that Ta/PANI composite sensor show a higher response and the response increases slightly with increasing ion fluence. The response value has been found z1.1 (i.e. % Sensitivity z 9.2%) for Ta/PANI composite sensor irradiated at fluence 1 109 ion/cm2, which was increased up to 1.42 (i.e. % Sensitivity z 30%) for composite sensor irradiated at fluence 1 x 1011 ion/cm2. The % Sensitivity of unirradiated and irradiated Ta/PANI composite sensors is shown in Fig. 10. In case of irradiated Ta/PANI composites sensor the interaction of hydrogen with PANI is predominantly responsible for higher response of sensor towards hydrogen gas. It may suggest that due to the SHI irradiation Ta melts and diffuses into PANI matrix, which provides comparatively
Fig. 9 e Response versus time plot for unirradiated and Irradiated Ta/PANI composite sensors after hydrogen exposure at room temperature.
Fig. 11 e Reproducibility of Ta/PANI composite sensor exposed to hydrogen gas at room temperature.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 8 2 5 e3 8 3 2
rough, and higher surface area for hydrogen adsorption and rapid diffusion, therefore more interaction sites are available for hydrogen sensing and hence the sensing response is increased. It has been reported that rough and fiber like structure of PANI shows a faster and higher response for hydrogen than conventional PANI film, because the three dimensional porous structure of a PANI nanofibers allows for easy and rapid diffusion of hydrogen gas into PANI [57,58]. Although, a few work has been published on effects of hydrogen on the mechanical properties of tantalum material [50,51,72,73], but the exact mechanism of direct interaction between hydrogen and tantalum is not yet fully understood. It may also suggest that hydrogen molecules could be absorbed between the interstitial sites within the tantalum lattice and then dissociate into hydrogen atoms. The following formation of new NeH bonds between the hydrogen atoms and nitrogen atoms of PANI can reduce the resistance of Composite sensor [74]. Fig. 11 shows the response e recovery property of Ta/PANI composite sensor upon SHI irradiation at 1 1011 ion/cm2. Over long periods of hydrogen exposure it was observed that composite film sensors exhibited a good stability and repeatability as gas sensors. It was also observed that after first cycle the sensor takes longer time to reach at the stable value of response magnitude. It may be due to the slow diffusion rate of hydrogen gas with time.
4.
Conclusion
The Ta/PANI composite based chemiresistor type gas sensors were fabricated on interdigitated electrodes and irradiated with 150 MeV Auþ12 ions at different fluence ranging from 1 109 to 1 1011 ions/cm2. XRD measurements revealed that Ta/PANI composite film exhibit both amorphous and crystalline nature due to presence of PANI and Ta respectively. Upon irradiation, the crystalline nature of Ta/PANI composite films increased with increasing ion fluence due to the mixing of tantalum atoms in PANI matrix and hence the resistance of composite film decreased. AFM study shows that the ion beam irradiation leads to formation of clusters and craters in Ta/PANI composite films. The response behavior was monitored in terms of resistance change of unirradiated and irradiated Ta/PANI composite sensors towards H2 gas in air at room temperature. The irradiated Ta/PANI composite sensors showed high response value and sensitivity with good repeatability than pristine one. The role of tantalum as per its contribution in higher response of irradiated TA/PANI composite sensor towards H2 is not clear at present and some more work is required to explore the use of tantalum as sensitive material in hydrogen gas sensing application.
Acknowledgements Authors are grateful to the UGC, New Delhi, for the financial support in the form of a research project.
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references
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Preparation and characterization of tantalum/polyaniline composite based chemiresistor type sensor for hydrogen gas sensing application Subodh Srivastava*, Sumit Kumar, Y.K. Vijay Thin Film and Membrane Science Lab, University of Rajasthan, Jaipur-302055, India
article info
abstract
Article history:
In the present work we have reported the effect of Shift heavy ion (SHI) irradiation on the
Received 19 February 2011
gas sensing properties of tantalum (Ta)/Polyaniline (PANI) composite thin film based
Received in revised form
chemiresistor type gas sensor for hydrogen gas sensing application. PANI was synthesized
11 April 2011
chemically by in situ oxidative polymerization method. The thin sensing films of PANI
Accepted 12 April 2011
were deposited onto finger type Cu-interdigited electrodes using spin cast technique and
Available online 14 June 2011
a thin Ta layer was deposited on to PANI thin film to prepare Ta/PANI composite chemiresistor sensor. These chemiresistor sensing films were irradiated with energetic Auþ12
Keywords:
ions (150 MeV) at the different fluencies ranging from 1 109 to 1 1011 ions/cm2. The
Polyaniline (PANI)
structural and morphological properties of these composite thin films were characterized
Ta/PANI composite
by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements before and
Chemiresistor sensor
after SHI irradiation. The electrical properties of these composite thin films were charac-
H2 gas sensing
terized by IeV characteristic measurements. The changes in resistance of the composite
Atomic force microscopy (AFM)
thin film sensor were utilized for detection of hydrogen gas. It was observed that after SHI
X-ray diffraction (XRD)
irradiation Ta/PANI composite sensor shows a high response value and sensitivity with good repeatability in comparison to the pristine sample. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In recent years numerous sensors with polymeric materials have been developed for detecting many analytes, ionic species, organic vapours and gases [1,2]. The incorporation of metal nanoparticles into conducting polymers has been reported by a number of groups [3e6]. Generally, these efforts were directed at the development of gas sensitive materials in order to improve the sensitivity, response time and stability of gas sensors. It has been reported earlier that metal species in the conducting polymer ensures high surface area, improved conductivity and possible enhancement of the unique characteristics of the composite [7,8].
Generally, polymers are deposited as sensitive layer of sensor and then metal thin film are deposited on the surface of the sensitive polymer layer as active material to increase the area/volume ratio and favour the adsorption of gases. The deposition of active metal precursor can be made by thermal evaporation or sputtering techniques. In case of hydrogen gas platinum, palladium, silver and titanium have been widely reported as active element therefore their composite with polymer may have potential applications in gas sensors and electrocatalysis [9e15]. Among the conducting polymer, Polyaniline (PANI) has been preferred as a sensitive media for hydrogen sensing, due to its environmental stability, selectivity and sensitivity towards hydrogen gas at room
* Corresponding author. Tel.: þ91 (0) 141 2702457; fax: þ91 (0) 141 2701149. E-mail address: [email protected] (S. Srivastava). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.155
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temperature. The further incorporation of sensitive nanoparticles makes the PANI composite more sensitive towards hydrogen gas. Therefore PANI has also been used as a host matrix for the active filers like palladium, platinum, gold, silver and titanium [16e22].
1.1.
Swift heavy ions (SHI)
It is worth noting that the sensitivity of gas sensors is strongly affected by the structural and morphological parameter of sensing materials [23]. The ion irradiation has been established as a potential tool for material modification [24]. As an ion penetrates a solid, it loses energy by two distinct interactions: (1) by elastic nuclear collisions with the target atoms (nuclear stopping Sn) and (2) by excitation or ionization of atoms by inelastic collisions, (known as Electronic Stopping, and the energy spent in this process is called electronic energy loss (Se). When the energy of incoming ion is very high, the electronic stopping is dominant, where the displacement of atoms due to elastic collisions is insignificant. Such heavy ions, with energies so high that the electronic loss process dominates, are referred to as Swift heavy ions (SHI) [25]. The direct interaction between energetic ion and target atoms can lead to structural changes such as generation of point defects, amorphization of crystalline materials or phase transformations in amorphous atomic networks, surface modification, mixing of materials and formation of new compounds [26e31]. The SHI induces the mixing at the interface in order to produce novel composite materials and phases. The mixing takes place due to transfer of energy, by the energetic ions in the electronic subsystem, subsequently transfer to atomic subsystem via electron-phonon coupling which results in a rise in the lattice temperature up to 104 K. Therefore the material within few nanometers from the ion path melts for the duration of 1012 e1011 s and then quenches at very fast rate, forming the latent tracks and this induces inter-mixing in bilayer systems [32,33]. On irradiation with SHI, a dramatic change in the structural and electrical properties of polyaniline composites was also observed [34e37]. The SHI irradiated polyvinylchloride polyethylene terephthalate (PVCPET) composites have been tested for hydrogen gas and ammonia gas sensitivity [38,39]. The use of tantalum, tantalum alloys and tantalum oxide has already been suggested for sensor purposes [40,41]. Tantalum (Ta) is an active element for hydrogen gas and has been studied to investigate diffusion of hydrogen in it [42e45]. There is always a finite probability for hydrogen or deuterium atom to occupy an interstitial site in its metal lattice. Two phases, designed a and b are known to exist in the tantalum-hydrogen system and their properties determine, to a large extent, the kinetics of the diffusion process [46e48].. The presence of hydrogen, or media containing hydrogen compounds (which can liberate free hydrogen) can lead to change in mechanical and structural properties of Ta surface [49]. It has been reported earlier that Ta and Niobium react with carbon, nitrogen, oxygen, and hydrogen at room temperature [50,51]. It has also been reported that Ta, with a properly activated surface, may be a suitable element for hydrogen separation membrane [52e54]. Therefore it is impotent to use Ta as
an active material with PANI for hydrogen gas sensing. Separately PANI has been widely tested for hydrogen gas sensing application [55e58]. However, to the best of our knowledge Ta/PANI composite films have not yet been used as a sensitive layer in chemiresistor type sensor for hydrogen gas sensing. In the present work PANI was synthesized chemically by in situ oxidative polymerization of aniline using ammonium persulfate in acidic medium at low temperature. The thin sensing films of PANI were deposited onto finger type cuinterdigited electrodes using spin cast technique and a thin Ta layer was deposited on to PANI thin film to prepare Ta/PANI composite chemiresistor sensor. These chemiresistor sensing films were irradiated with Auþ12 ions at the different fluencies and the effect of SHI irradiation on the gas sensing properties of Ta/PANI composite films was studied for hydrogen gas.
2.
Experimental
2.1.
Materials
PANI was synthesized by in-situ chemical oxidative polymerization method as described elsewhere [59,60]. Tantalum sheet (99.9% pure, 1 mm thick and 2 inch diameter) was used as sputtering target for thin film deposition as shown in Fig. 1.
2.2.
Sensor preparation
CSA-PANI mixture was dissolved in 30 ml chloroform solution using magnetic stirrer. Thin films of this solution were deposited on cleaned Cu-IDE epoxy substrates using the spin coating technique and a thin tantalum layer was deposited onto spin coated PANI thin film using DC magnetron sputtering system under high vacuum of the order of 105 torr. In the sputtering process the Ta target is normally fixed at 10e12 cm apart from the substrate holder and positioned in front of the sample surface. The substrate was rotated during deposition yielding a uniform thickness throughout the sample. Argon gas was inserted as the sputtering gas through the needle valve at a constant pressure of 1 101e2 101 torr. The schematic diagram of Ta coated PANI Chemiresistor sensor is shown in Fig. 2.
2.3.
Swift heavy ions (SHI) irradiation
In the present work, a 15 UD Pelletron Accelerator facility located at IUAC New Delhi was used for SHI irradiation [61e63]. It is basically a Van de Graff type tandem electrostatic accelerator in vertical configuration, having maximum terminal voltage up to 16 MV. The prepared samples were mounted on target assembly in material science chamber under high vacuum (106 Torr) The SHI irradiation was performed at room temperature using Auþ12 ions having energy of 150 MeV at different fluencies ranging from 1 109 to 1 1011 ions/cm2 depending on time of bombardment. The beam current was kept 1 pnA and monitored intermittently with a Faraday cup. The ion beam was defocused using magnetic scanning system, so that an area of 1 1 cm2 was uniformly irradiated. The irradiated samples were stored at room temperature in air.
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Fig. 1 e Ta sputtering target mounted in target holder.
3.
Result and discussion
3.1.
X-ray fluorescence (XRF)
The XRF has been used to analyze the elemental composition of composite materials. Fig. 3 shows the XRF spectra of Ta/PANI composite, which exhibits two characteristic X-ray energy peaks at Ka ¼ 8.1 keV and Kb ¼ 9.2 keV corresponding to tantalum (see inset in Fig. 3) and confirms the presence of Ta in PANI matrix. It has also been observed that after irradiation, the intensity of energy peak has been decreased with increasing ion fluence. The decreased in peak intensity is due to two reasons: there may be small variation in thickness of Ta layer during sputtering process due to the difference in target to source distance (10e12 cm) which influence the number of Ta atoms deposited at the surface of PANI and hence the intensity of corresponding x-ray energy peak of Ta decreased. Secondly there may be mixing of tantalum within PANI matrix during irradiation process therefore the relative XRF counts coming from the surface are decreased.
Fig. 2 e Schematic diagram of Ta coated PANI thin film chemiresistor sensor and (b) prepared sensor.
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Fig. 3 e XRF spectra of Ta/PANI composite before and after SHI irradiation.
3.2.
X-ray diffraction (XRD)
The XRD pattern of Ta/PANI composite films before and after the SHI irradiation is shown in Fig. 4. The XRD pattern of PANI (ES) exhibits two broad amorphous peaks, observed at 2q z 20.1 and 2q z 25.2 (Fig. 4a), which may be attributed to periodicity parallel and perpendicular to PANI conjugation chains, respectively [64e66]. It has been observed that all Ta/ PANI composite films exhibit a broad peak appeared at 2q z 25.2 corresponding to the amorphous nature of PANI and two sharp crystalline peaks, centered at 2q z 38 and 2q z 70.1 corresponding to the crystalline nature of Ta. The Peak observed at 2q z 38 can be ascribed to either (110) of bccTa or (200) of b-Ta, while the peak at 2q z 70 corresponds to the (400) of b-Ta [67e69]. It was observed that after irradiation the peaks become more sharpen as the ion fluence increases. This indicates that after SHI irradiation the crystallinity of Ta in PANI matrix has been increases. No new structural order has found to be generated within composites after the irradiation as shown in Fig. 4ced. It has been earlier reported that
Fig. 4 e X-ray diffraction patterns of pure PANI and Ta/PANI composite films before and after SHI irradiation.
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Fig. 5 e AFM images of Ta/PANI composite film (a) before irradiation, (b) after irradiated at 1 3 109 ion/cm2 and (c) after irradiated at 1 3 1011 ion/cm2.
the density of the polymer increases during the SHI irradiation making the polymer more compact which results in more crystalline regions in polymer films resulting in an increase in the degree of crystallinity [30,43] of composite.
3.3.
Atomic force microscopy (AFM)
Surface morphology of pristine and irradiated Ta/PANI composite films has been examined by AFM measurements.
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The Fig. 5 shows AFM images of Ta/PANI composite films before and after SHI irradiation. The pristine Ta/PANI composite film shows a highly dense random shaped uneven granular structure on the surface (Fig. 5a). The composite film irradiated at low fluencies (1 109d1 1010 ions/cm2) shows a typical polycrystalline clustered structure of irregular grains aligned in small crystallized domains (Fig. 5b). The film irradiated at 1 1011 ions/cm2 shows more compact, closely packed interconnected rope like structure at the surface which results in an increased crystallinity of the composite film and thus show good agreement with XRD measurement results after irradiation. The AFM images revealed a continuous increase in the granular cluster in composite film with the increase of ion fluence. The cluster formation may be attributed to large amount of electronic energy loss induced collision cascades therefore particles agglomerated due to the partially melted Ta layer. This took place near the surface and is responsible for the displaced atoms forming clusters.
3.4.
Fig. 6 e Current-Voltage (IeV) characteristics curve of unirradiated and Irradiated Ta/PANI composite films at room temperature.
IeV characteristics
Fig. 6 shows the IeV characteristics of Ta/PANI composite film before and after SHI irradiation at room temperature. From the IeV characteristics curve it has been observed that at low voltages, the current is proportional to the applied voltage corresponding to an ohmic regime, which extends almost up to 0.3 V. With increasing bias voltage beyond 0.3 V, an increasing trend in the current was observed showing the non linear region. IeV curve of pristine Ta/PANI thin film shows almost similar characteristics as that of pure PANI thin film. This suggests that Ta thin film onto PANI surface does not affect the density as well as transportation of charge carrier within PANI matrix. In case of irradiated Ta/PANI composite thin films, it was observed that from the ohmic regime to the non linear region, the current increases slightly with increasing fluence. This implies that the background free charge carrier density increases slightly with increasing ion fluence [70]. Also, due to irradiation, Ta melts and diffuses into PANI matrix, which provides more conducting path for easy charge transport between consecutive PANI chains and hence current increase with increasing ion fluence. B. Scrosati and Hussein et. al. have reported that the increase in the crystallinity of the composite films upon SHI irradiation may also contribute to the increase in conductivity of the films [30,71], which is in good agreement with our XRD and AFM measurements. Fig. 7 shows the IeV characteristics of unirradiated and irradiated Ta/PANI composites thin film with increasing temperature. The increasing trend in current was observed for both unirradiated and irradiated composite samples with increasing temperature. It may be attributed to the increase in the number of thermally activated charge carriers with temperature, which indicates that composite thin films are semiconductor in nature.
3.5.
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at room temperature. This Figure clearly reveals that resistance of all irradiated composite films decreases very rapidly by introduction of hydrogen gas and become stable after few seconds. This may be attributed to the reducing nature of
Gas sensing measurements
Fig. 8 shows the variation in the resistance of pristine and irradiated Ta/PANI composite sensors towards hydrogen gas
Fig. 7 e Current-Voltage (IeV) characteristics curve of (a) unirradiated and (b) Irradiated Ta/PANI composite films with temperature.
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Fig. 8 e Change in resistance of unirradiated and irradiated Ta/PANI composite sensors with time after exposed to hydrogen gas at room temperature.
Fig. 10 e Variation in %sensitivity of Ta/PANI composite sensors irradiated at different ion fluencies.
hydrogen gas. Introduction of hydrogen gas in the composite thin films injected electrons to the film, and thus significantly increase the number of charge carrier in the film. As a result, more electrons flowed in the film and at the same time reduced the resistance of the film. An exception was observed in the pristine Ta/PANI composite film, where the resistance change is very small or almost negligible. Fig. 9 shows the response of pristine and irradiated Ta/PANI composite sensors towards the hydrogen gas at the room temperature. It was observed that unirradiated Ta/PANI composite sensor shows almost negligible response in comparison to irradiated sensors. It may be due to the Ta layer coated over the PANI surface, which does not react with hydrogen at room temperature and inhibited the hydrogen to diffuse in to the PANI matrix. Therefore at room temperature pristine Ta/PANI
sensor dose not shows any response for hydrogen. While upon irradiation, it was observed that Ta/PANI composite sensor show a higher response and the response increases slightly with increasing ion fluence. The response value has been found z1.1 (i.e. % Sensitivity z 9.2%) for Ta/PANI composite sensor irradiated at fluence 1 109 ion/cm2, which was increased up to 1.42 (i.e. % Sensitivity z 30%) for composite sensor irradiated at fluence 1 x 1011 ion/cm2. The % Sensitivity of unirradiated and irradiated Ta/PANI composite sensors is shown in Fig. 10. In case of irradiated Ta/PANI composites sensor the interaction of hydrogen with PANI is predominantly responsible for higher response of sensor towards hydrogen gas. It may suggest that due to the SHI irradiation Ta melts and diffuses into PANI matrix, which provides comparatively
Fig. 9 e Response versus time plot for unirradiated and Irradiated Ta/PANI composite sensors after hydrogen exposure at room temperature.
Fig. 11 e Reproducibility of Ta/PANI composite sensor exposed to hydrogen gas at room temperature.
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rough, and higher surface area for hydrogen adsorption and rapid diffusion, therefore more interaction sites are available for hydrogen sensing and hence the sensing response is increased. It has been reported that rough and fiber like structure of PANI shows a faster and higher response for hydrogen than conventional PANI film, because the three dimensional porous structure of a PANI nanofibers allows for easy and rapid diffusion of hydrogen gas into PANI [57,58]. Although, a few work has been published on effects of hydrogen on the mechanical properties of tantalum material [50,51,72,73], but the exact mechanism of direct interaction between hydrogen and tantalum is not yet fully understood. It may also suggest that hydrogen molecules could be absorbed between the interstitial sites within the tantalum lattice and then dissociate into hydrogen atoms. The following formation of new NeH bonds between the hydrogen atoms and nitrogen atoms of PANI can reduce the resistance of Composite sensor [74]. Fig. 11 shows the response e recovery property of Ta/PANI composite sensor upon SHI irradiation at 1 1011 ion/cm2. Over long periods of hydrogen exposure it was observed that composite film sensors exhibited a good stability and repeatability as gas sensors. It was also observed that after first cycle the sensor takes longer time to reach at the stable value of response magnitude. It may be due to the slow diffusion rate of hydrogen gas with time.
4.
Conclusion
The Ta/PANI composite based chemiresistor type gas sensors were fabricated on interdigitated electrodes and irradiated with 150 MeV Auþ12 ions at different fluence ranging from 1 109 to 1 1011 ions/cm2. XRD measurements revealed that Ta/PANI composite film exhibit both amorphous and crystalline nature due to presence of PANI and Ta respectively. Upon irradiation, the crystalline nature of Ta/PANI composite films increased with increasing ion fluence due to the mixing of tantalum atoms in PANI matrix and hence the resistance of composite film decreased. AFM study shows that the ion beam irradiation leads to formation of clusters and craters in Ta/PANI composite films. The response behavior was monitored in terms of resistance change of unirradiated and irradiated Ta/PANI composite sensors towards H2 gas in air at room temperature. The irradiated Ta/PANI composite sensors showed high response value and sensitivity with good repeatability than pristine one. The role of tantalum as per its contribution in higher response of irradiated TA/PANI composite sensor towards H2 is not clear at present and some more work is required to explore the use of tantalum as sensitive material in hydrogen gas sensing application.
Acknowledgements Authors are grateful to the UGC, New Delhi, for the financial support in the form of a research project.
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references
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