Chloroform vapour sensor based on copper/polyaniline nanocomposite

Sensors and Actuators B 85 (2002) 131±136 Chloroform vapour sensor based on copper/polyaniline nanocomposite Satish Sharma a, Chetan Nirkhe a, Sushama Pethkar b, Anjali A. Athawale a,* a Department of Chemistry, University of Pune, Ganeshkhind Road, Pune 411 007, India b Materials and Physical Chemistry Division, National Chemical Laboratory, Pune 411 008, India Received 29 November 2001; accepted 28 January 2002 Abstract Chemically synthesised copper/polyaniline (PANi) nanocomposite has been utilised as a chloroform sensor for ppm level vapour concentration. The response in terms of increase in dc electric resistance on exposure to chloroform vapours has been observed. The FT±IR spectra of nanocomposite on exposure to chloroform show remarkable modi cations in the far IRregion indicating the interaction of chloroform with metal cluster. It is believed that the sensing mechanism mainly involves adsorption±desorption of chloroform at metal cluster surfaces. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Copper clusters; Metal/polyaniline nanocomposite; Chloroform; Vapour sensor; Chloro hydrocarbon sensor 1. Introduction Conducting polymers such as polypyrrole, polyaniline (PANi), polythiophene and their derivatives are being explored as promising materials for micro sensors, because of their ability to form good basis for chemical sensors either as a sensing element or as matrices to immobilise speci c reagents [1]. The principle sensing mechanism in conducting polymer is generally based on the modi cation of doping level due to redox interaction of analyte molecules and thereby resulting in change in conductivity. Among conducting polymers, PANi have successfully been demonstrated as an ef cient gas sensor for monitoring air borne organic and inorganic components especially alcohols, ethers, ammonia, nitrogen, (NO) x,h 2 S, SO 2,CO 2, etc. [2±12], whereas, the use of PANi as a sensing element for chlorinated hydrocarbons such as CCl 4, CHCl 3, CH 2 Cl 2, etc. is less investigated, probably due to their weak interactions with PANi. Several analytical techniques for sum parameters exist for the determination of chlorinated hydrocarbons [13], and chromatographic methods, viz., GC and HPLC also allow speci c analysis of the species. Recent developments in bre optic chemical sensors (FOCS) offer real time monitoring of volatile chlorinated hydrocarbons in water, where the contact of chlorinated hydrocarbons with uorescent indicator immobilised in polymer lm causes change in uorescence * Corresponding author. Fax: 91-020-589-3044. E-mail addresses: asha@cata.ncl.res.in, agbed@chem.unipune.ernet.in (A.A. Athawale). intensity [14]. Variety of FOCS for hydrocarbons in water has been described in the literature [14,15]. The physical properties employed by some of the optical bre sensing measurements include refractive index [16], the degree of light absorption [17] or surface plasmon resonance of metals [18] for the remote detection of chlorinated hydrocarbons. Recently, chlorinated hydrocarbons vapour sensor based on polymer-grafted carbon black conducting components [19], as well as tin oxide [20] have also been reported. Although, polymers are generally being used for the immobilisation of chloro hydrocarbon-sensing element in optical bre sensors, quartz resonators [21] and sensors etc. use of conducting polymer, as a matrix is rare. In the present work, we demonstrate the applicability of Cu/polyaniline (Cu/PANi) nanocomposite (hereafter termed as nanocomposite) as a chloroform sensor. Metal clusters are being extensively studied due to their unique surface activities imparted by huge surface areas [22,23], which can make them ideal sensing elements as microsensors. Adsorption of various gases on metal surface is well known and metal nanostructure as well as metal cluster functionalised PANi based chemical sensors have also been reported [24,25]. 2. Experimental 2.1. Materials For experimental purpose A.R. grade chemicals were used. The monomer aniline was doubly distilled prior to 0925-4005/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0925-4005(02)00064-3

132 S. Sharma et al. / Sensors and Actuators B 85 (2002) 131±136 use. Ammonium Persulphate ((NH 4 ) 2 S 2 O 8 ), Sodium Borohydride (NaBH 4 ), Cupric Nitrate (Cu (NO 3 ) 2 ) and Hydrochloric acid (HCl) were used as received. Hexane and Chloroform (A.R. grade) were distilled before use. For synthesis double distilled water was used. 2.2. Synthesis of Cu/ polyaniline nanocomposite A volume of 100 ml copper sol in aqueous medium was prepared by the controlled reduction of 10 3 M cupric nitrate using desired quantity of sodium borohydride. To this solution, 0.93 ml of aniline and 2 ml HCl was added under constant stirring. Polymerisation was effected by the addition of 2.28 g of ammonium persulphate dissolved separately in 5 ml water. The oxidising agent was added dropwise using a soxhlet funnel at 5 8C under constant stirring and the polymerisation was allowed to proceed for 2 h. The precipitated nanocomposite powder recovered was vacuum ltered and washed with deionised water. The washing procedure was repeated till the ph of the ltrate became neutral. Finally, the nanocomposite was dried in oven for 24 h to achieve a constant weight. Blank PANi was synthesised using similar procedure as described above in the absence of copper sol. 2.3. Characterisation The surface morphology and the size of the incorporated copper clusters were determined by TEM. By dispersing nanocmposite powder on a carbon substrate supported 400- mesh copper grid TEM imaging of the nanocomposite was enabled. Transmission electron micrograph was obtained on a JEOL JEM 1200 EX transmission electron microscope operating at 100 kv. The synthesised nanocomposite was made in the pellet form (diameter: 12 mm, thickness: 3 mm) by applying pressure of 7 t using Pye±Unicam system and used as a sensor. The sensing performance of the nanocomposite was tested by subjecting the nanocomposite pellets to chloroform vapours at room temperature. Hexane was used as a diluent, for preparing various analyte concentrations. The dc electric resistance of the nanocomposite exposed to analyte vapour was measured by hanging the pellet in a closed glass tube container as shown in Fig. 1. The distance between the sensing element and the solvent surface was 3 cm. Resistance measurements were performed in a simple two probe con- guration. The contact leads were xed 1 cm apart on one of the surfaces of the pellet using conductive silver paste. The resistance was measured as a function of time on a digital multimeter. The responses to chloroform were recorded by injecting the desired volume of chloroform in the solvent. The sensing measurements were carried out for different chloroform concentrations such as 10, 50 and 100 ppm for three different sets of samples. The resistance data was collected for 5 min while the pellet was exposed to chloroform vapours at ambient conditions and recovered in dry air for 5 min. Fig. 1. Apparatus used to study sensing performance. The infrared spectra of the materials were collected on a PERKIN-ELMER 1700 spectrophotometer in the range of 400±4000 cm 1. The sample was prepared in the pellet form by mixing 1 mg of material in 150 mg of KBr. The FT±IR spectra were collected after exposing pellet to various chloroform concentrations in the sample port. 3. Results and discussion Fig. 2 shows typical transmission electron micrograph of the nanocomposite. Copper nanoclusters appearing as black spots associated with polymer bre structures are clearly seen in the bright eld image. The average particle size of 50 nm is estimated from the image, which was found to be Fig. 2. Typical transmission electron image of Cu/ PANi nanocomposite.

S. Sharma et al. / Sensors and Actuators B 85 (2002) 131±136 133 Fig. 3. The response of PANi on exposure to 50 ppm chloroform in hexane. consistent with that deduced from the XRD pattern. Anchoring of metal clusters on polymer bres implies that the cluster surface can be ef ciently accessed by the analyte species. The responses of nanocomposite to various chloroform vapour concentrations are presented in Fig. 4. The response of pure PANi to 50 ppm chloroform in hexane (Fig. 3) and also the response of nanocomposite to neat hexane (Fig. 4a) have also been presented for the comparison. The initial resistance of pure PANi and that of nanocomposite before exposure to chloroform vapours were 251 and 135 O, respectively. The responses are presented in terms of DR/R, where DR is the change in the resistance on exposure to chloroform and R is the initial resistance of the sensing pellet. Pure PANi shows substantial increase in DR/R upon rst exposure to chloroform but fails to return to the baseline value (initial resistance) after being transferred to dry air as clearly seen in Fig. 3. On the other hand, the nanocomposite indicates stable baseline and improved responses for all chloroform concentrations, viz., 10, 50, 100 ppm (Fig. 4(b±d)). This remarkable difference in responses can occur mainly due to the change in the sensing mechanism governing the response of PANi and that after incorporation of Cu clusters. Chloroform exhibits weak acidic characteristics and is also known to act like a reducing gas. The increased resistance of pure PANi on exposure to chloroform is certainly indicative of interaction of chloroform molecules at dopant sites, viz., N ±H sites created by Cl. There is high probability that chloroform molecules may attack dopant sites causing neutralization of loosely bound couple formed with Cl leading to appreciable increase in the Fig. 4. The responses of the nanocomposite on exposure to various chloroform concentrations: (a) 0 ppm (neat hexane), (b) 10 ppm; (c) 50 ppm, (d) 100 ppm. The sample was exposed to chloroform vapours for 5 min and the to dry air for 5 min alternately.

134 S. Sharma et al. / Sensors and Actuators B 85 (2002) 131±136 resistance (and apparently DR/R value) of pure PANi on rst exposure. So, on successive exposures considerable shift in the baseline is observed with complete deterioration of overall response. In contrast, the responses of nanocomposite are reproducible and show increase in resistance of nanocomposite on exposure to chloroform vapours, which is fairly reversible in dry air. This implies that the mechanism governing the sensing in nanocomposite is different and incorporated Cu clusters may play dominant role due to their unique surface activities being con ned. In addition, adsorption±desorption of analyte vapours on Cu clusters is quite possible owing to their large active surface areas. The preliminary information of such phenomena at metal cluster surfaces can be accessed from simple FT±IRstudies as vibrational spectroscopy reveals the identi cation of certain functionalised groups within limitations imposed by surface dipole selection rules. The adsorption of chloroform species on metal cluster surface has been identi ed from the investigations of molecular vibrations. To visualise the induced modi cations clearly, FT±IRspectra only in the region of 2000± 400 cm 1 are presented. Fig. 5 shows FT±IRspectrum of nanocomposite (Fig. 5a) and that of nanocomposite on exposure to different analyte concentrations (Fig. 5b±d). It was noted that the FT±IRspectrum of PANi and that of the nanocomposite appeared indifferent before exposure to analyte. Secondly, the spectrum of PANi did not show additional vibrational bands as seen for nanocomposite (Fig. 5b±d) on exposure to chloroform. This is indicative of role of embedded Cu clusters in determining sensing behaviour. The major absorption bands observed before and on exposure to chloroform having usual signi cance are quoted in Table 1. The broad band ca. 3100±3700 cm 1 (not shown in the gure) and 1110 cm 1 represents partial doping and protonation of amine and imine nitrogen [26]. The presence of peaks ca. 2917 and 2854 cm 1 (not shown in the gure) can be attributed to the C±H stretching vibrations of PANi whereas, vibration at ca. 1296 cm 1 represents C±N aromatic stretching. The absorption peaks at ca. 1542 and 1461 cm 1 are assigned to the quinoid (Q) and Fig. 5. FT±IRspectra of nanocomposite (a) before exposure to chloroform and on exposure various chloroform concentrations: (b) 10 ppm, (c) 50 ppm, (d) 100 ppm. the benzenoid (B) structures of emaraldiene form of the PANi, respectively. On the other hand, Fig. 5(b±d) depict FT±IRspectra of nanocomposite samples exposed to various concentrations of chloroform. A comparison of these spectra with the spectrum of the unexposed nanocomposite reveals a signi cant modi cation as well as additional bands in the far IRregion, viz., <800 cm 1. The appearance of the vibration at 665 cm 1 is indicative of the presence of chloroform in the nanocomposite. This is also consistent Table 1 FTIRspectra of Cu/polyaniline nanocomposite before and after exposure to various concentrations of chloroform Cu/polyaniline nanocomposite Chloroform concentration 10 ppm Chloroform concentration 50 ppm Chloroform concentration 100 ppm Peak assignment 484 478 473 Cu±C stretching 604 614 639 594 Aromatic deformation 651 665 669 Chloroform vibrations (669 cm 1 ) 751 755 731 798 Out of plane ±C±H bending vibration And chloroform (758 cm 1 ) 1110 1102 1104 1104 B±N H±B stretching vibration 1296 1290 1285 1294 Aromatic (C±N) stretching band 1461 1466 1455 1482 Benzenoid ring stretching 1542 1538 1556 1549 Quinoid ring stretching 2917 2917 2919 2915 ±C±H stretching band 3100±700 3100±700 3100±700 3100±700 ±N±H stretching band

S. Sharma et al. / Sensors and Actuators B 85 (2002) 131±136 135 with the strengthening of the shoulder peak 751 cm 1 (Fig. 5a) on exposure to chloroform. This vibration is usually assigned to the C±H out of plane bending in 1±2 ring in intrinsic PANi. The neat chloroform is known to exhibit vibrations at 668 and at 758 cm 1. The development of peak at 758 cm 1 as well as substantial shift to higher frequency on exposure to higher chloroform concentration supports the adsorption of chloroform molecules. The appearance of new band in the range 490±470 cm 1 and it is predominant increase in sharpness with chloroform concentration is also suggestive of interaction of chloroform with the metal cluster surfaces. Such band is generally observed for metal±carbon stretching vibrations for chemisorbed organic species [27]. The presence of this band can be attributed to Cu±C stretching frequency arising due to probable electrostatic interaction of chloroform molecules with Cu. It is reasonable to believe that chloroform molecules, which possesses slight polar character due to high electron af nity of chlorine atoms leaves poor electron density at carbon. This in turn may lead to weak electrostatic interaction between carbon and metal conduction electrons. This band has been found to shift to lower frequency on exposure to higher concentration of chloroform due to increased surface concentration of the analyte on metal. This agrees well with the coverage induced frequency shifts generally attributed to the dipole coupled interactions for chemisorbed gases on metals [27]. These observed modi cations in FT±IRspectra of nanocomposite on exposure to chloroform fairly supports suggested adsorption±desorption phenomenon at metal cluster surfaces as a possible sensing mechanism in the nanocomposite, which needs further studies. The sensitivity values (DR/R) obtained for various chloroform concentrations was found to fall in the range of 1.5±3.5. However, at higher concentrations, the observed DR/R seems to drop remarkably, which may be due to low concentration of accessible metal clusters and resulting in diffusion of chloroform molecules in the matrix. This may result in the development of sharp absorptions at 794 and 668 cm 1 in the FT±IRspectrum of nanocomposite on exposure to 100 ppm of chloroform (Fig. 5d). However, for low concentrations, typically 10 ppm, DR/R reduces appreciably on successive exposures to chloroform suggestive of competent interaction of analyte at dopant sites of the host polymer. 4. Conclusions In this paper, we have presented a chloroform sensor based on Cu/PANi nanocomposite. In comparison to pure PANi, the nanocomposite exhibited reversible and fairly good response for comparatively low concentrations of chloroform, viz., <100 ppm indicating different sensing mechanisms and probable interaction of chloroform with metal cluster surfaces in nanocomposite. The preliminary results as obtained from vibrational spectroscopy supports adsorption±desorption of chloroform on metal cluster surfaces as one of the competent sensing mechanisms, which needs further studies. The sensitivity of nanocomposite can further be improved by increasing incorporated metal cluster concentration. Metal cluster incorporated conducting polymers can selectively and ef ciently be used as chemical sensors. Acknowledgements Dr. Anjali A. Athawale acknowledges UGC, India for the nancial assistance. The authors would like to acknowledge the division of EM, NCL, Pune, for technical assistance. One of the authors (Sushama Pethkar) would like to thank CSIR, India for the nancial support. References [1] G. Bidan, Electroconducting conjugated polymers: new sensitive matrices to build up chemical or electrochemical sensors. A review, Sens. Actuators B 6 (1992) 45±56. [2] P.N. Bartlett, P.B.M. Archer, S.K. 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