Ni-Coated Polyaniline Nanowire As Chemical Sensing Material for

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Ni-Coated Polyaniline Nanowire As Chemical Sensing Material for Cigarette Smoke Devasish Chowdhury* Physical Sciences Division, Polymer Unit, Institute of Advanced Study in Science and Technology, Paschim Boragaon, Garchuk, Guwahati 781 035, Assam, India ABSTRACT: PANI-CSA-Ni composite nanowire was successfully synthesized using camphor sulfonic acid (CSA) as dopant as well as surfactant. In this process, CSA-ANI micelles act as soft template facilatating the formation of PANI nanowire. PANICSA-Ni nanowire composite was characterized by transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The use of PANI-CSA-Ni composite nanowire as sensing material was also investigated. It was observed that PANI-CSA-Ni nanowire composite shows nearly four order decrease in ac impedance in the presence of cigarette smoke. Therefore, such material has potential for use in cigarette smoke detector. Impedance response in the presence of controls like carbon dioxide and carbon monoxide was also measured. It was observed that whereas carbon dioxide gas has a negligible role in the change in ac impedance, there was a half-order change in impedance response in the presence of carbon monoxide (CO), hinting the possibility of CO as one of the contributing constituents of observed impedance response in cigarette smoke. However, no signature for the formation of Ni(CO)4 was identified from FTIR spectrum. We believe that derivatives of polyaromatic hydrocarbon (PAH) and alkaloid like nicotine present in cigarette smoke form a charge transfer complex with Ni as the binding site on the polymer nanowire as a result, the mobile charge carrier increases in the system thus decreasing the ac impedance.

’ INTRODUCTION Conducting polymers, since its discovery by Nobel Prize awardees Shirakawa, MacDiarmid, and Heeger,1 has come up as a new class of useful polymeric materials and has attracted huge interest and now finds its use in commercial applications in optical and microelectronic devices,2 chemical sensors,3 catalyzers,4,5 drug delivery,6,7 and energy storage systems.8,9 With the rapid development of nanoscience and nanotechnology in recent years, synthesizing nanostructures of this unique conducting polymer, especially making nanowires/nanotubes,10 has attracted growing interest. The research on nanomaterial of conducting polymer is motivated with the hope that nanostructured polymer will offer better performance or will show new properties compared with its conventional bulk counterpart. Chemical gas sensors based on nanowires can find a wide range of applications in clinical assaying, environmental emission control, explosive detection, agricultural storage and shipping, and workplace hazard monitoring. Sensors in the forms of nanowires are expected to have significantly enhanced performance due to high surface-volume ratio and quasi-1-D confinement. Conducting polymer and its nanocomposites have been demonstrated as sensing material for organic vapors,11,12 for example, methanol, ethanol, chloroform, dichloromethane, and hexane. Other analytes include amino acids, polyhydric compounds,13 water vapor,14 hydrogen sulfide,15 and hydrochloric acid and ammonia.16 Although many such examples have been demonstrated of chemical sensors based on 1-D nanostructured r 2011 American Chemical Society

materials, it still remains a challenge to discover efficient, scalable, and specific sensor systems. In this Article, a report of use of Ni-coated 1-D polyaniline (PANI-CSA-Ni) nanowire as chemical sensor for cigarette smoke is presented. The cigarette smoke causes health hazards, and it is a wellestablished fact that smoking causes lung diseases, heart diseases, hypertension, oral cancer, and host of other related problems. Cigarette smoke contains tar (includes a majority of mutagenic and carcinogenic agents), gases like carbon dioxide and carbon monoxide, nitrosamines, polynuclear aromatic hydrocarbons (PAHs), chlorinated dioxins, and furans. In this regard, Ni-coated PANI nanowire has potential use in cigarette smoke detector devices. An array of nickel nanowires enveloped in polyaniline nanotubules and its magnetic behavior17 has been previously studied, but Ni-coated PANI as sensing material has not yet been investigated.

’ EXPERIMENTAL SECTION Materials. Aniline (Merck) was distilled under reduced pressure before use. Ammonium peroxodisulfate (Merck), camphor Received: September 23, 2010 Revised: June 14, 2011 Published: June 14, 2011 13554

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Figure 1. (A) Photograph of the assembled device. (B) Experimental set up used to study the sensing property of the PANI-CSA and PANI-CSA-Ni nanowires.

sulfonic acid (CSA, Merck), nickel chloride (Merck), and sodium borohydride (Merck) were used as received. Synthesis of PANI Nanowire (PANI-CSA). Polymerization of PANI was carried out by a soft template method already reported in literature.18 Aniline (ANI) monomer was distilled before use; ammonium peroxidisulfate (APS) and CSA were used as received. Polymerization of aniline was carried out by chemical oxidation of aniline by APS used as oxidant in the presence of CSA as dopant and surfactant. In this process, CSA-ANI micelles act as templates in the formation of PANI nanowire. Unlike the template synthesis, these micelles do not need to be removed after polymerization because they act as dopant of the resulting PANI nanowire. Typically 186 μL of distilled aniline was mixed with 2.5 mL of 0.1 M Camphor-10-sulfonic acid (CSA) under stirring for 30 min to obtain a uniform emulsion. Then, APS was added dropwise and the mixture was allowed to react at 0 2 °C for 10 h. The reaction was carried out without stirring and at low temperature (0 2 °C) to help in suppressing secondary growth and facilitating formation of PANI nanowires. The precipitate was washed with distilled water and methanol before being dried under vacuum for 24 h to get a dark powder of PANI-CSA. Transmission electron microscopy (JEOL JEM 2100) was done by ultrasonically dispersing the dark-colored powder in ethanol. Synthesis of Ni-Coated PANI Nanowire (PANI-CSA-Ni). Ni nanoparticles were prepared separately by reducing aqueous solution of 0.1 M NiCl2 3 6H2O with 0.05 M sodium borohydride at 60 °C with constant stirring. The black precipitates of Ni nanoparticles (NPs) thus were washed with excess of distilled water to remove the excess sodium borohydride. To obtain the Ni-coated PANI nanowire, the Ni NPs solution was mixed with PANI-CSA at room temperature with constant stirring for 1 h. Device Fabrication and Sensing Study. The ac impedance response of PANI-CSA and PANI-CSA-Ni nanowire to various analytes was studied. At first, PANI-CSA and PANI-CSA-Ni were dispersed in hexane and spin-coated on copper plates of dimension 1 cm  1 cm. The other copper plate having small perforation was placed over it. The two coppers were then connected indiviually by thin wire forming the two terminals. A photograph of the device is shown in Figure 1 A. The device was then placed in a closed chamber, as shown schematically in Figure 1B. The chamber also has an inlet for introducing analyte gases. The ac impedance response of PANI-CSA and PANICSA-Ni nanowire in the presence and absence of CHCl3, CO2, CO, and cigarette smoke was measured. All ac impedance measurements

Figure 2. (Af B) Schematic illustration of the method by which Ni NPs can be deposited on the surface of the PANI-CSA nanowire. TEM images of (A1,A2) PANI-CSA nanowire and (B1,B2) Ni NPs deposited on PANI-CSA nanowire.

were performed at room temperature under ambient condition using an impedance analyzer (Hioki 3532-50).

’ RESULTS AND DISCUSSION Figure 2Af B shows the schematic diagram of the process of deposition of Ni on PANI-CSA nanowire surface. Figure 2A1,A2 depicts the representative transmission electron microscope (TEM, JEOL JEM 2100) images of PANI-CSA nanowire synthesized. The nanowires obtained have diameters between 50 and 120 nm with length varying from 500 nm to several micrometers. Ni NPs were synthesized separately, and when mixed with PANI nanowire result in deposition of Ni NPs on the nanowire surface. Figure 2B1,B2 shows in two different magnifications the representative TEM images of Ni NP-coated PANI-CSA nanowire. 13555

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The Journal of Physical Chemistry C Ni selectively deposits on the surface of the nanowire. The adhesion of Ni NPs on the surface of nanowire is quite strong and is evident from the fact that the even after sonication of the polymer in ethanol while preparing for TEM samples, the Ni NPs remain on the surface of the nanowire. Here it should be mentioned that if required Ni deposited on the surface of PANI-CSA can be removed by treatment with 60/40% H2SO4/H2O (not shown) without affecting the PANI nanowire. Therefore, Ni deposition on the surface of PANI nanowire is reversible. Thermogravimetric analysis (TGA) measurement of PANI-CSA and PANI-CSA-Ni nanocomposite nanowire is shown in Figure 3. The thermogram clearly shows that the degradation of the polymer occurs in three steps. Major weight loss occurs after 270 300 °C. Whereas the major % weight loss starts at 270 °C for PANI-CSA, for PANI-CSA-Ni nanocomposite nanowire, it starts at ∼300 °C. However, PANI-CSA-Ni nanocomposite shows initial weight loss of 5% at 100 °C. The thermogram also shows better thermal stability for PANI-CSA-Ni nanowire composite in comparison with PANI-CSA with nanowire composite showing 10% less percentage weight loss. The use of PANI-CSA nanowire and Ni-coated PANI-CSA nanowire as ac impedance sensor for detection of cigarette smoke is investigated. The device fabricated (detail given in the Experimental Section) was placed in a closed chamber with an inlet for introduction of gases. Figure 4A red line shows the ac impedance response on a logarithmic scale measured at 42 Hz of PANI-CSA-Ni nanowire. After cigarette smoke was introduced at 420 s (the exact point noted in the graph), it took ∼2 min to sense the response, and there was around a four-fold decrease in the impedance value. This delay in

Figure 3. TGA analysis of the PANI-CSA and PANI-CSA-Ni nanowire composite.

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response can be attributed to slow penetration of gases into the conducting polymer backbone. The impedance response of exposure of cigarette smoke to bare PANI-CSA nanowire was also measured and presented in Figure 4A (black line). The impedance response of bare PANI-CSA after cigarette smoke exposure is less drastic. Although, in this case, too, there is observed decrease in the ac impedance value, but the impedance response is not stable, and the ac impedance increases with time. It is attributed to the fact that unlike in PANI-CSA-Ni, where the Ni NPs on the surface of PANI-CSA-Ni nanowire act as effective site for the formation of Ni complex, rendering more stability to the system for better stable response, in the case of bare PANI-CSA nanowire, the analyte (cigarette smoke) absorbs on the polymer backbone and results in a change in the charge carriers in the system, but the analyte desorbs with time, making the response unstable.The ac impedance response of PANI-CSA-Ni in the presence and absence of cigarette smoke in different frequency from 42 Hz to 1 MHz was recorded, and the plot is shown in Figure 4 B. PANI-CSA-Ni in both the presence and the absence of cigarette smoke show similar change in impedance with frequency, and thus the ac impedance response can be recorded at any frequency. The impedance response in different frequency was studied in different device, and it is different set of experiement. In fact, a different set of experiments was done on the new device each time because the device cannot be reused once exposed to analytes. Cigarette smoke contains ∼4000 chemicals, but main constituents are tar, gases like carbon dioxide and carbon monoxide, nitrosamines, PAHs, chlorinated dioxins, and furans. What constituent in cigarette smoke is responsible for observed impedance response is not easy to answer, but impedance response of the device in the presence of carbon monoxide (CO) and carbon dioxide gas (CO2) was carried out to know the impedance response in the presence of these gases. Figure 5 shows the ac impedance response with time of PANI-CSANi nanowire composite in the presence of 250 ppm of carbon dioxide gas. The plot shows (Figure 5) that after the introduction of CO2 gas, there was only slight a increase in impedance value. This result shows that the carbon dioxide gas present in cigarette smoke has a negligible role in the change of impedance response. Figure 6 shows the impedance response with time of PANICSA-Ni composite in the presence of carbon monoxide. The carbon monoxide gas was prepared in laboratory from heating a mixture of calcium carbonate and zinc powder. After the introduction of CO gas, a half order decrease in impedance response was observed. Because cigarette smoke contains CO, it may be one of the contributing constitutes responsible for sensing cigarette smoke,

Figure 4. (A) Log Impedance (Z) versus time plot of the PANI-CSA-Ni nanowire (red line) after exposure of cigarette smoke and Log impedance response with time of PANI-CSA (black line) in the presence of cigarette smoke. The impedance was measured at 42 Hz and at 30 °C. (B) Log impedance (Z) versus frequency plot of PANI-CSA-Ni nanowire and PANI-CSA nanowire. 13556

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The Journal of Physical Chemistry C although the formation of nickel carboyl was not detected in FTIR measurement. It is already reported in literature that in the presence of CO gas, resistance of PANI is decreased as a consequence of reduction of surface potential and barrier height.19 We believe in our case mainly the derivatives of PAH, alkaloids like nicotine absorb onto PANI-CSA-Ni nanowire to form a weakly associated charge transfer complex with Ni as the binding site on the polymer nanowire. This results in an increase in the mobile charge carrier in the system, thus decreasing the ac impedance. The resultant

Figure 5. Impedence response of PANI-CSA-Ni in the presence (∼ 250 ppm) CO2. The impedance was measured at 42 Hz and at 30 °C.

Figure 6. Impedence response of PANI-CSA-Ni in the presence CO. The impedance was measured at 100 Hz and at 30 °C.

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electrostatic attraction provides a stabilizing force for the charge transfer complex. It is already known that an additive that can form a charge transfer complex can enhance the conductivity of the polymer.20 It is also known that Ni can form a complex with nicotinic acid21 and furan.22 Al Ni alloy23 powder also helps in reducing nitrosamines; all three of them are constituents in cigarette smoke. So, Ni NPs on the surface of the PANI-CSA nanowire react with these chemicals present in cigarette smoke and form Ni complex on the surface of the nanowire. The performance of the device in the presence of other analytes, e.g. chloroform, toulene, and methanol were checked. The sensing response in the presence of chloroform is only discussed here. In the presence of 20 ppm of chloroform vapor, PANICSA-Ni nanowire composite shows an increase in impedance response by nearly one order. The plot of Log Z versus time is shown in Figure 7. The impedance was also measured with different frequency from 42 Hz to 1 MHz. It was observed that a change in impedance with different frequency has the same trend in the presence and in the absence of chloroform vapor (Figure 7B). The FTIR studies were also carried out on the PANI-CSA and PANI-CSA-Ni nanowire before and after exposure to cigarette smoke to determine the chemical changes taking place in the nanowire and shown in Figure 8. The FTIR study of the PANICSA shows the vibrational band at 1633, 1579, and 1502 cm 1. The band in the region of 1502 cm 1 was attributed mainly to the benzenoid (B) ring stretching vibration in the polymer,24 and a band near 1579 cm 1 is related to the quinoid (Q) structure of the polymer chain.25 The relative absorbance intensity of quinoid ring versus the benzenoid ring tells about the pernigraniline form and the emeraldine form present in PANI.26 The high absorption intensity of 1579 cm 1 indicates the existence of a higher amount of benzenoid structure unit in the polymer chain. A 1300 cm 1 peak is assigned to C Nstr. The 1124 cm 1 band can be assigned to a vibrational mode of a B NHdQ structure, which is formed during the protonation process and indicates the existence of positive charges on the chain. The appearance of C H out-ofplane bending vibrational band of the 1,4 ring at 842 cm 1 suggests that head-to-tail configuration taken by PANI is the present synthesis. Other peaks at 995 and 823 cm 1 are attributed to C Hinplane and C Houtofplane bending vibration modes of the aromatic ring. After deposition of Ni NPs on PANI-CSA, referred to as PANI-CSA-Ni, there is observed splitting of 1579 cm 1, which is related to the quinoid (Q) structure of

Figure 7. (A) Log impedance (Z) versus time plot of the PANI-CSA-Ni nanowire after exposure of 100 ppm of CHCl3. The impedance was measured at 42 Hz and at 30 °C. (B) Log impedance (Z) versus frequency plot of PANI-CSA-Ni nanowire before and after exposure to CHCl3. 13557

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +91 361 2912073. Fax: +91 361 2279909.

’ ACKNOWLEDGMENT I thank Mr. Indrajit Talukdar for TEM measurements. Special thank to Dr. Neelotpal Sen Sarma and Mr. Samiul Hoque for helping with impedance measurements and device fabrication. I thank Dr. Gitanjali Majumdar for helpful discussion. I also thank Department of Science & Technology, Govt. of India for project under SERC Fast Track Scheme (SR/FTP/CS-45/2007). Thanks are also due to reviewers for their critical comments, which have helped to improve the manuscript. ’ REFERENCES

Figure 8. FTIR of (a) PANI-CSA, (b) PANI-CSA-Ni, and (c) PANICSA-Ni after cigarette exposure.

the polymer chain into two small peaks at 1579 and 1588 cm 1, indicating a change with the degree of doping of the polymer backbone. There is also changes in the vibrational mode of a B NHdQ structure with splitting of 1124 peak to 1100 cm 1 and emergence of an additional peak at 1152 cm 1. The shifting of some peaks can be an indication of interaction between the nickel NPs and the polymer. On exposure to cigarette smoke, the changes observed are primarily in peak because of the vibrational mode of B NHdQ, where the 1124 cm 1 peaks splits into a number of peaks at 1157, 1136, 1124, and 1105 cm 1, which shows more doping of the polymer backbone. CO peak due to absorption of CO27 on Ni2+ at 2201 and 2184 2181 cm 1 in the cigarette-smokeexposed PANI-CSA-Ni nanowires was not detected, which confirms that the Ni does not form complex with CO. There is also apparent change in the absorption intensity of 1579 and 1502 cm 1 peaks, indicating some changes in the benzenoid quinoid ring structure. In summary, Ni-coated PANI-CSA nanowire was synthesized and found to be effective sensing material for cigarette smoke in comparison with bare PANI-CSA. A four-order change was observed in the impedance response of Ni-coated PANI-CSA on exposure to cigarette smoke, and the response was stable with time, unlike bare PANI-CSA, whose response was not stable with time. The response time of the device was ∼2 min because of slow penetration of gases into the conducting polymer backbone. Therefore, PANI-CSA-Ni has potential for use in cigarette smoke detector devices.

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