- Email: [email protected]
Enhanced sensor functionality of in situ synthesized polyaniline–SnO2 hybrids toward benzene and toluene vapors
Sensors and Actuators B 205 (2014) 74–81
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Enhanced sensor functionality of in situ synthesized polyaniline–SnO2 hybrids toward benzene and toluene vapors C. Murugan a , E. Subramanian a,∗ , D. Pathinettam Padiyan b a b
Department of Chemistry, Manonmaniam Sundaranar University, Tirunelveli 627012, Tamil Nadu, India Department of Physics, Manonmaniam Sundaranar University, Tirunelveli 627012, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 22 November 2013 Received in revised form 6 August 2014 Accepted 8 August 2014 Available online 17 August 2014 Keywords: Polyaniline–SnO2 hybrids In situ synthesis Sensor Benzene Toluene Analyte
a b s t r a c t Polyaniline–SnO2 (PANI–SnO2 ) hybrid materials with varied SnO2 content were prepared by in situ chemical oxidative polymerization method. The prepared materials were characterized by FTIR, XRD, TGA, DSC and SEM analyses. The dielectric constant of the hybrid materials was calculated via Nyquist plot of the AC impedance analysis. Sensor functionality of the hybrids was studied toward benzene and toluene vapors in dry N2 atmosphere at room temperature. The response of the materials was presented in terms of their normalized conductivity change in per cent. The PANI–SnO2 hybrid with 40 wt% SnO2 exhibited the maximal response toward both benzene (16.9%) and toluene (19.5%) vapors. In situ synthesis technique enhanced the conductivity and response of the PANI–SnO2 materials in comparison to the physical mixture. This is attributed to the formation of greater number of p–n heterojunctions through large surface interaction of the analyte species. The hybrids showed slightly higher response toward toluene than benzene. © 2014 Published by Elsevier B.V.
1. Introduction Benzene and the analogous aromatic hydrocarbons are one of the classes of volatile organic compounds (VOCs). They are hazardous and are known to cause several kinds of diseases such as allergies, asthma, cancer and emphysema [1]. Studies on metal oxide based sensors functioning at higher temperature for the detection of such benzene/benzene like hydrocarbons are highly attractive [2–4] owing to their potential applications and also from the environmental point of view. Few strategic modifications of such sensors are reported for the detection of these hydrocarbons even at room temperature [5,6]. Polyaniline (PANI) has been the focal point of study among the conducting polymers for the last three decades owing to its ease of synthesis, excellent environmental stability, high conductivity, low cost and acid-doping/base-dedoping chemistry [7,8]. It has been studied as a chemo-sensor for a limited range of VOCs [9–11]. In our previous works, the dopant-induced sensor specificity of PANI toward some VOCs [12] and the ability of PANI to detect molecular pair formation between CH2 Cl2 and CHCl3 [13]were reported. But,
∗ Corresponding author. Tel.: +91 9965178458; fax: +91 462 2334363. E-mail addresses: [email protected], [email protected] (E. Subramanian). http://dx.doi.org/10.1016/j.snb.2014.08.027 0925-4005/© 2014 Published by Elsevier B.V.
aromatics, benzene/toluene being weakly interactive are not much studied with PANI sensor. In this context organic/inorganic hybrid (OIH) materials are much promising due to their advanced properties emerging from both organic and inorganic components [11]. Many studies have illustrated this point and some are highlighted herein. PANI–SnO2 composite has been studied with optical, electrochemical [14,15], gas sensing [16,17] and super-capacitor [18] applications. The sensor studies of PANI/SnO2 materials toward polar solvents with varying mass % of SnO2 [19] and ammonia [20–22] were reported. The dielectric values of PANI–SnO2 composites strongly depend on the wt% of SnO2 present in the materials and the particle dimension of SnO2 has greater influence on the dielectric values of the composite [23]. Yadav et al. [24] proposed three controlling factors for the principal operation of the semiconductor sensor which include the grain size, grain boundaries and accessible grain surface of the semiconductor material to the target analyte molecules [25,26]. Among the various methods of synthesis of OIH materials, the in situ polymerization has the advantage where the organic monomers adsorb on the surface of the inorganic material and get polymerized producing the hybrid composite [27]. The intimate contact of the two phases is, thus, ensured. However, in physical/mechanical grinding method, the organic matrix in its polymeric form makes only peripheral contact with inorganic phase. Therefore, the in situ polymerization is the best technique to
C. Murugan et al. / Sensors and Actuators B 205 (2014) 74–81
75
achieve the maximum required properties of OIH materials from the organic and inorganic components as compared to the physical/mechanical grinding technique. The report of only fewer sensor studies for carcinogenic benzene and the related hydrocarbon toluene is almost entirely due to the weak/lack of interaction of these hydrocarbons with the sensor materials. We recently reported the sensor functionality of PANI–SnO2 physically ground mixture toward benzene and toluene vapors in dry nitrogen atmosphere [28]. In normal air atmosphere polyaniline is known to have remarkable affinity toward moisture and therefore the material as such [29–31] or its composite [32,33] is used as humidity sensor in many studies. Because of the moisture absorbing tendency of polyaniline, our materials (PANI–SnO2 hybrids) could not be studied as benzene/toluene sensor in wet air environment or normal air atmosphere. Nevertheless, in dry nitrogen atmosphere they worked, sensed the analyte and produced the response. The materials showed only moderate efficiencies (∼10%). Hence, we intend to improve the sensor efficiency of PANI–SnO2 OIH materials toward benzene and toluene in dry nitrogen atmosphere by adopting an in situ synthesis method which by making an intimate contact between the two components would increase the number of p–n heterojunctions in the materials. Also the in situ formation and coating of PANI over SnO2 particles could enhance the physical and chemical properties of the materials. Interesting results are obtained which explain their mutual interaction and enhancement in sensor functionality of in situ synthesized hybrid compared to the PANI–SnO2 physically ground mixture. Hence, in the present work, we report the enhanced sensor functionality of PANI–SnO2 hybrid materials toward benzene and toluene vapors in dry nitrogen atmosphere in comparison to the physical mixture.
flow of gases. Nitrogen was used as a carrier gas for analyte (benzene and toluene). The flow of N2 was adjusted to get the required concentration of analyte. The sensor response was measured after passing particular concentration of analyte vapors to the sensor chamber for 15 min. The current–voltage measurements of the sensor pellets both in N2 and N2 -analyte atmospheres were taken at room temperature (28–29 ◦ C) and I–V plots were made. Resistance R was computed from the slope of I–V plots and conductivity was calculated from the formula given in Eq. (1).
2. Experimental
PANI and its SnO2 hybrid materials were characterized using Fourier transform infrared spectroscopy, X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. IR spectra of the samples were obtained with KBr pellet in JASCO FTIR-410 spectrophotometer provided with computer software. XRD patterns of the samples (2 = 2–80◦ ) were obtained with Rigaku Miniflex II ˚ desktop X-ray diffractometer using CuK␣ radiation ( = 1.5418 A). Scanning electron microscope images of the gold coated samples were observed with JEOL (model 6390) microscope operating at 20 kV. Thermogravimetric analysis was carried out with Mettler Toledo instrument by heating the sample upto 800 ◦ C at the rate of 10 ◦ C/min in N2 atmosphere. Differential scanning calorimetric analysis was performed with Mettler Toledo instrument heating the sample upto 600 ◦ C at the rate of 10 ◦ C/min in N2 atmosphere.
2.1. Materials Aniline (Merck) was purified by distillation over zinc dust. Ammonium peroxodisulfate (APS; Merck), SnO2 (∼325 mesh; Sigma–Aldrich) and other chemicals were used without further purification. Water used in the preparation and washings was doubly distilled unless otherwise mentioned. 2.2. Synthesis of PANI and PANI–SnO2 composites PANI was prepared by chemical oxidative polymerization method [34]. In a typical procedure, 20 mmol aniline and calculated quantity of SnO2 were added to 50 ml of 0.2 M aqueous H2 SO4 . The solution was stirred at 0–4 ◦ C for 30 min for the formation of anilinium ion on the surface of SnO2 particles. 20 mmol APS was added to the above solution and the stirring was continued for further 1 h. The resultant mixture was kept in a refrigerator overnight for the completion of polymerization. The obtained hybrid sample was filtered, washed with 200 ml double distilled water and 50 ml of acetone–methanol (1:1) mixture. 100 ml of 0.01 M H2 SO4 solution was used for doping and then the material was dried in an air oven at 120 ◦ C for 4 h. The dried sample was stored in air-tight polythene cover. The ground materials were pelletized (13 × 2 mm size) and dried at 120 ◦ C for 30 min prior to the conductivity measurement. PANI–SnO2 hybrids with different SnO2 content (20–80 wt%) were prepared by varying the amount of SnO2 .
=
1 2RS
(1)
where S is the inter-probe distance (2 mm). The normalized conductivity change (NCC) in % was calculated from the conductivity value of each pellet in N2 ( Nitrogen ) and in analyte ( Analyte at ∼1650 ppm) atmospheres using Eq. (2). NCC (%) =
Nitrogen − Analytc Nitrogen
× 100
(2)
Dynamic sensor response-recovery study was performed on PANI–40% SnO2 measuring the change in voltage against constant current (i.e., change in conductivity) and constant benzene concentration. 30 min was given for a complete cycle of adsorption and desorption of benzene (1650 ppm). N2 was used as a carrier gas for desorption study.
2.4. Characterization of the materials
2.5. Dielectric constant measurement of the PANI–SnO2 composites AC impedance measurement was made using Lock-in-Amplifier (Stanford Research System, Model SR830 DSP) in the frequency range 0.1–100 kHz with an amplitude of 100 mV to find the dielectric constant (εb ) of the material. Nyquist plot was constructed with real (Z ) and imaginary (Z ) components of the impedance to find frequency max [35]. The dielectric constant of each pellet was calculated at room temperature (28–29 ◦ C) using Eq. (3) to authenticate the formation of p–n heterojunction in the hybrid materials. Cb εO · A/t
2.3. Conductivity measurement
εb =
Pellets of PANI and its SnO2 composites were subjected to DC electrical conductivity measurement in a collinear four probe setup [12,13]. The pellet fitted in a collinear four probe set-up was kept in a sensor chamber provided with inlet and outlet for the
where Cb is the bulk capacitance, εo = 8.854 × 10−12 (vacuum permittivity in Fm−1 ), A is the area of cross section of the pellet (in m2 ) and t is the thickness of the pellet (in m).
(3)
76
C. Murugan et al. / Sensors and Actuators B 205 (2014) 74–81
Table 1 FT-IR spectral data (in cm−1 ) of PANI and its SnO2 composites. Sample
Quinoid C C
Benzenoid C C
Imine C N
Benzenoid amine C N
Protonated imine
PANI PANI–20% SnO2 PANI–30% SnO2 PANI–40% SnO2 a PANI–40% SnO2 b PANI–40% SnO2 c PANI–40% SnO2 PANI–60% SnO2 PANI–80% SnO2
1579 1556 1561 1562 1584 1562 1565 1561 1570
1465 1483 1482 1482 1494 1484 1481 1483 1484
1297 1301 1301 1303 1294 1303 1303 1304 1303
1234 1226 1226 1225 1242 1224 1224 1226 1227
1121 1139 1139 1141 1119 1141 1141 1143 1143
a b c
Physically ground material. Toluene adsorbed material. Benzene adsorbed material.
3. Results and discussion 3.1. Characterization of the materials Fig. 1 depicts the FTIR spectra of prepared samples and Table 1 presents the data of peak assignment. PANI shows all the characteristic peaks which reveal emeraldine salt form of PANI [36,37]. The observed conductivity value of PANI (2.489 Scm−1 ; Table 5) is in consistent with this IR observation. The quinoid C C stretching band and benzenoid secondary amine band are shifted to lower wavenumber side in the hybrid materials. However, benzenoid C C, imine C N stretching and protonated imine vibrational bands are slightly shifted to higher wavenumber in the hybrid materials. The characteristic Sn O stretching of SnO2 appears as shoulder in the hybrids with the lower wt% of SnO2 ; however that is distinctly visible (at 623 cm−1 ) in the hybrids with higher wt% of SnO2 . The intensity of Sn O band is found to increase with increase in SnO2 loading in the hybrids. These IR spectral features demonstrate firmly the mutual interaction of PANI and SnO2 in their OIH materials. Further, these features substantiate the conductivity trend. The conductivity value of PANI is increased in the hybrids at lower SnO2 content (20 and 30 wt%) and decreased gradually beyond 30 wt% (Fig. 2 and Table 5). SnO2 favors hopping of charge carriers due to the enhanced orientation and chain conformation of PANI in the hybrids at lower SnO2 content as also observed in PbO-PANI [38]. The decrease in conductivity at higher SnO2 content is due to the trapping of charge carrier by the confined SnO2 particles present in the three dimensional organization of the conducting chain. Overall an enhancement in the conductivity of in situ synthesized
materials is found as compared to the conductivity of physically ground materials (Table 5) [28]. Powder diffraction patterns of the hybrid materials (Fig. 3) clearly demonstrate the presence of SnO2 with 2 values at 26.6, 33.9, 38.0 and 51.8◦ corresponding respectively to the planes of (1 1 0), (1 0 1), (2 0 0) and (2 1 1). This pattern certainly reveals the existence of tetragonal SnO2 (JCPDS file No. 01-088-0287) [39] in the hybrid materials. The peak position of SnO2 (2 = 26.59◦ ) gets
Fig. 1. FT-IR spectra of (a) PANI, (b) PANI–20% SnO2 , (c) PANI–40% SnO2 , (d) PANI–60% SnO2 , (e) PANI–80% SnO2 and (f) SnO2.
Fig. 3. XRD spectra of (a) PANI, (b) PANI–20% SnO2 , (c) PANI–40% SnO2 , (d) PANI–60% SnO2 , and (e) PANI–80% SnO2.
Fig. 2. Plot of conductivity of materials versus wt% of SnO2 .
C. Murugan et al. / Sensors and Actuators B 205 (2014) 74–81 Table 2 XRD data of PANI, SnO2 and their hybrids.
77
Table 4 Differential scanning calorimetric analysis data of PANI–SnO2 hybrids.
Material
2 (◦ )
FWHM (◦ )
d-spacing (Å)
Crystallite size (nm)
Hybrid material
T1 (endothermic) (◦ C)
T2 (endothermic) (◦ C)
T3 (exothermic) (◦ C)
PANI SnO2 PANI–20% SnO2 PANI–40% SnO2 a PANI–40% SnO2 PANI–60% SnO2 PANI–80% SnO2
24.88 26.59 26.67 26.73 26.55 26.78 26.72
4.500 0.241 0.163 0.163 0.225 0.143 0.163
3.58 3.35 3.39 3.33 3.36 3.33 3.33
1.9 35.4 52.2 52.2 37.9 58.1 52.2
PANI–20% SnO2 PANI–40% SnO2 PANI–40% SnO2 -PM PANI–60% SnO2
82.87 77.08 94.09
279.7 273.9 258.0, 303.6
527 517 460
85.77
277.2
463
a
PM stands for physically ground material.
Physically ground material
slightly shifted to higher values with maximum for 60% loaded hybrid (Table 2). The crystallite size of SnO2 particle (35.4 nm) is enhanced in the hybrid materials with maximum value at 60% loaded hybrid (58.1 nm). This enhancement clearly shows the formation of polyaniline coating over SnO2 particles in a core-shell arrangement. In comparison to the physically ground material, the in situ synthesized materials are found to have higher crystallite sizes. That means there is a thick coating of PANI matrix around the SnO2 phase. Fig. 4 displays the SEM images of PANI, SnO2 and their composites. PANI exhibits a uniform mixture of spherical and worm-like agglomerated particles in the size range of 0.05–0.5 m whereas SnO2 has uniform spherical particles in the range of 100 nm. The particle size of the hybrid decreases leading to the low degree of agglomeration of the PANI chains with increase in SnO2 content. The worm-like particles are missing in the hybrid of higher SnO2 content. The thermogravimetric characterization data (Fig. 5a and Table 3) of the PANI–SnO2 hybrids in N2 atmosphere exhibit a fourstep weight loss from the materials up to the temperature of 800 ◦ C. The first weight loss in the temperature range of 40–140 ◦ C is due to the removal of physisorbed water. 5.83% weight loss from the physically ground material which is higher than those of in situ prepared materials reveals the presence of greater level of moisture absorption in physically ground mixture. The second weight loss in the temperature region 140–400 ◦ C is attributed to the elimination of dopant (sulphate etc.) species from the materials. The change in these values are in accordance with the polymer content (decrease with decrease in polymer content) and thus to the dopants. The hybrid with 80 wt% PANI content exhibits the maximal weight loss (25.29%) and hybrid with 40 wt% PANI exhibits the minimal weight loss (12.58%). Third step weight loss is the decomposition of organic oligomeric content and the forth step weight loss is the removal of decomposable matrix of PANI present in OIH materials. The increase in weight loss with increase in initial wt% of SnO2 in the forth step demonstrates that the OIH materials are more stable with higher PANI content and less stable with higher SnO2 content. The final residue indicates the presence of SnO2 along with carbon from thermally decomposed PANI content. The equal value of final residue in both in situ synthesized and physically ground PANI–40% SnO2 materials substantiates the presence of equal amount of SnO2 .
The differential scanning calorimetric data (Fig. 5b and Table 4) with two endothermic and one exothermic peaks are in resemblance to the DSC profile of polyaniline/polystyrene [40]. The first endothermic peak (T1 ) due to the loss of water molecule appears at 77–85 ◦ C in all in situ synthesized hybrids. However, this endothermic peak of physically ground composite is slightly shifted to higher temperature side (94 ◦ C). Another endothermic peak appearing at 273–279 ◦ C in the in situ synthesized hybrids is assigned to the decomposition and elimination of dopant. This high temperature endothermic peak (T2 ) gets split into two in physically ground composite substantiating the heterogeneity nature of the material. The onset of exothermic transition (T3 ) above 350 ◦ C is attributable to the degradation of polymeric backbone. The maximal temperature of the exothermic peak is shifted considerably to lower temperature upon increasing the SnO2 content to 60% in the hybrid material. This means lower SnO2 content hybrid has higher thermal stability which is in consistent with TGA inference mentioned above. Further, PANI–40%SnO2 , in situ synthesized material has the maximum of the exothermic degradation temperature (T3 ) at higher side by 57 ◦ C in comparison to the physically ground material (Table 4). This is attributed to the increased surface coverage by PANI over SnO2 particle followed by the increase in average crystallite size (Table 2). 3.2. Dielectric constant values of the composite materials The two components PANI and SnO2 are p- and n-type conductors respectively. When they couple together and form hybrid composite, as per the well-known concept of p–n junction formation [22,38,41,42], electron flow occurs from n-SnO2 to p-PANI and occupies the vacant energy levels in PANI until the Fermi level becomes equal in the entire composite materials. This, in turn, produces positive poles in SnO2 and negative poles in PANI and thereby the p–n heterojunctions are formed in the hybrid composite materials. When there is charge transfer, the dielectric characteristic of the material will also change proportionately. Hence, dielectric constant could be considered as a measure of formation of p–n heterojunctions. The dielectric constant values are given in Table 5. In the beginning, the dielectric constant (εb ) value gradually increases with increase in wt% of SnO2 in the composite materials. Then it reaches the maximum at PANI–40% SnO2 suggesting the formation of maximum number of dielectrics in the hybrid material. Further increase in SnO2 content in the composite results in lowering of εb value.
Table 3 Thermogravimetric analysis data of PANI–SnO2 hybrids. Samples
PANI–20% SnO2 PANI–40% SnO2 PANI–40% SnO2 -PM PANI–60% SnO2
Weight loss in the temperature range (%) 40–140 ◦ C
140–400 ◦ C
400–620 ◦ C
620–800 ◦ C
<800 ◦ C
2.40 1.82 5.83 2.52
25.29 18.88 12.58 12.58
9.52 6.66 9.85 3.97
10.38 13.65 13.53 16.51
52.41 58.99 58.66 64.42
PM stands for physically ground material.
78
C. Murugan et al. / Sensors and Actuators B 205 (2014) 74–81
Fig. 4. SEM images of (A) PANI, (B) SnO2 , (C) PANI–20% SnO2 , (D) PANI–40% SnO2 and (E) PANI–60% SnO2.
Fig. 5. Thermogravimetric (a) and differential scanning calorimetric (b) profile of PANI–SnO2 hybrids.
C. Murugan et al. / Sensors and Actuators B 205 (2014) 74–81
79
Table 5 Physical and sensor properties of PANI and its SnO2 hybrids. Sample
SnO2 loading (wt%)
Dielectric constant εb (Fm−1 )
Conductivity (Scm−1 )
PANI PANI–20% SnO2 PANI–30% SnO2 PANI–40% SnO2 a PANI–40% SnO2 PANI–50% SnO2 PANI–60% SnO2 PANI–70% SnO2 PANI–80% SnO2 SnO2
– 20 30 40 40 50 60 70 80 100
7.65 × 103 8.01 × 105 6.58 × 106 2.89 × 107 5.71 × 104 9.47 × 106 4.28 × 106 NA 8.99 × 105 3.82 × 10−2
2.489 3.247 2.711 1.942 0.682 1.461 1.141 0.941 0.737 –
a
NCC (%) Benzene
Toluene
1.49 5.39 13.23 16.93 9.82 13.27 12.23 7.72 3.46 –
1.57 6.84 15.49 19.53 10.41 14.79 13.15 8.7 3.55 –
Physically ground material. [Analyte] = ∼1650 ppm. NCC represents normalized conductivity change.
This is quite logical because of lower PANI (p-type) content relative to n-SnO2 in the composite and the incomplete saturation of p- and n-regions. The physically ground material exhibits only lower dielectric constant (∼500 times) than the in situ synthesized material and this observation explains the formation of greater number of p–n heterojunctions via in situ synthesis method. This also explains higher conductivity of in situ synthesized material than that of physically ground mixture (Table 5). 3.3. Sensor behavior and mechanism The sensor function of the composite materials (in pellet form) in normal air atmosphere was insignificant. In normal air atmosphere PANI has the tendency to adsorb moisture and functions as humidity sensor [29–33]. Therefore, in the present work when the pellets are exposed to benzene/toluene analyte in normal air atmosphere, moisture preferentially adsorbs in competition to analyte and causes an increase in conductivity. This sensor response due to moisture is, in fact, opposite to the sensor response of the materials toward benzene/toluene analyte in dry N2 atmosphere (conductivity decrease, Table 5). Hence, the sensor response of PANI–SnO2 hybrid materials could be studied only in dry N2 atmosphere and therefore the sensor is operable only in dry N2 atmosphere. Table 5 displays the conductivity data of PANI and its SnO2 composites in N2 atmosphere. The presence of analyte in N2 atmosphere definitely influences the conductivity of PANI and its SnO2 composites by reducing the value. This sensor response is given in terms of NCC%. PANI alone shows only ∼1.5 NCC% upon exposure to the analyte vapors while pure SnO2 shows no change in conductivity. That means the individual component produces very bleak/nil sensor response. However, the NCC values of PANI–SnO2 composites are remarkably higher; they initially increase, reach the maximum at PANI–40% SnO2 and then decrease with increase in SnO2 content. The hybrid with 40 wt% SnO2 exhibits the maximum sensor efficiency which coincides with the maximum dielectric constant value for this material (Table 5). Further, good correlation exists between sensor efficiency and p–n heterojunctions (i.e., dielectric constant values) of the materials (Fig. 6). That means sensor efficiency of the hybrid materials (Table 5) is supported and facilitated by the number of p–n heterojunctions present in them. The sensor activity is rapid, reversible and regenerative for all the materials, because when the analyte flow was replaced by pure N2 flow, the conductivities of the materials returned nearly to their original values (approximately 96% regeneration). To confirm and to make this observation explicit, dynamic sensor study was made representatively for PANI–40% SnO2 composite. Fig. 7 illustrates the dynamic response-recovery profile toward benzene. It seems within 15 min of analyte vapor exposure (benzene), the material gets saturated and reaches its maximum level of response (decrease in conductivity); on benzene vapor off, the material returns to its
Fig. 6. Dependence of sensor efficiencies on dielectric constant of the materials at [Analyte] = 1650 ppm.
near original conductivity value in 15 min. Retain of roughly 4% of the conductivity value during regeneration suggests some sort of diffusion of benzene vapor into the pores of the materials in addition to surface adsorption. This profile in first cycle is repeated in 2nd and 3rd cycles also, demonstrating reversibility and regeneration. A plausible sensor mechanism is as follows. The benzene/toluene analyte molecules adsorb on the surface of the pellet (physisorption), tend to attract electrons from the hybrid material,
Fig. 7. Dynamic sensor profile of PANI–40% SnO2 toward benzene vapor.
80
C. Murugan et al. / Sensors and Actuators B 205 (2014) 74–81
sensor function toward benzene/toluene vapor is insignificant. However, in dry nitrogen atmosphere the hybrid materials function very well and produce significant sensor response. The individual component SnO2 does not show sensor response while pristine PANI produces very feeble value even in dry nitrogen atmosphere. However, the hybrid materials exhibit enhanced responses and the sensor efficiency becomes maximal at 40 wt% loading of SnO2 . The in situ synthesized materials have nearly two-times higher response than those of physical mixtures. The intimate contact and strong interaction between PANI and SnO2 in former as revealed in characterization techniques lead to the enhanced sensor response. Although the present sensor materials are operable only in dry nitrogen atmosphere, they do have some positive aspects as listed hereunder: (1) PANI sensor for weakly interactive benzene/toluene analyte, (2) sensor function at ambient temperature compared to higher operable temperature of metal oxide sensors and (3) considerably higher sensor efficiency for the weakly interacting analytes. Fig. 8. FT-IR spectra of PANI–40% SnO2 (line) and its benzene adsorbed form (dotted line).
particularly from p-doped PANI in the heterojunction region (electron rich by charge transfer from SnO2 ), disturb/perturb electron density and thereby they provide friction to the movement of electrons in the conjugate system of PANI, reducing the conductivity of the material. Toluene shows slightly higher influence on conductivity change than does benzene. The methyl group in toluene shifts the electron density from the aromatic ring and makes the molecule little polar. This polarity provides some extra friction for free-moving polarons and thus decreases the conductivity little higher than that by benzene [43]. This type of physisorption and reduction in conductivity has been reported as the sensor mechanism in TiO2 -dispersed-poly(vinylidenfluoride) polymer film [6] and also in polyaniline-lead oxide composites [38]. Upon analyte off, the carrier gas nitrogen lifts off the adsorbed analyte, decreases its concentration on the surface of the sensor material and increases the conductivity by facilitating the flow of charge carriers. In situ synthesized hybrid materials exhibit good physical strength with a little lesser brittle nature in comparison to the physically ground composite. These materials exhibit nearly two times higher response than the physically ground PANI–SnO2 composites [28]. Certainly, this is due to the formation of larger number of p–n heterojunctions (2–3 orders) in former. In an attempt to identify the chemical group(s) of PANI involved in analyte association, the IR spectrum of benzene vapor adsorbed material was recorded (Fig. 8). The intensity of benzenoid (C C) stretching is increased with a decrease of the intensity of quinoid (C C) stretching. However, the intensity of the protonated imine band (also known as conductivity band) for the existence of positive charge distribution in the dihedral angle between benzenoid +
+
(B) and quinoid (Q) rings (Q NH B or B NH B) [37] remains unchanged. An additional peak emerged at 673 cm−1 in the benzene adsorbed material corresponds to the breathing ring vibration of benzene [37]. All these IR observations clearly demonstrate that (i) Q and B rings of PANI undergo electronic perturbation, (ii) the conductivity decrease of the sensor material by analyte association is not permanent but is transient and (iii) benzene ring of analyte has overlapping interaction with B unit of PANI. 4. Conclusions Polyaniline–SnO2 hybrid materials with various wt% of SnO2 were prepared by in situ method and investigated for benzene/toluene analyte vapor sensor. In normal air atmosphere moisture preferentially adsorbs in competition to analyte and the
Acknowledgement Authors are thankful to the Department of Science and Technology, Govt. of India for the financial support under the project SR/S2/CMP-22/2009. References [1] T. Godish, Air Quality, 2nd ed., Lewis Publishers, Chelsea, MI, 1991. [2] Z. Rao, L. Liu, J. Xie, Y. Zeng, Development of a benzene vapour sensor utilizing chemiluminescence on Y2 O3 , Luminescence 23 (2008) 163–168. [3] M. Ke, M. Lee, C. Lee, L. Fu, A MEMS-based benzene gas sensor with a self-heating WO3 sensing layer, Sensors 9 (2009) 2895–2906. [4] B. Ghaddab, F. Berger, J.B. Sanchez, P. Menini, C. Mavon, P. Yoboue, V. Potin, Benzene monitoring by micro-machined sensors with SnO2 layer obtained by using micro-droplet deposition technique, Sens. Actuators B 152 (2011) 68–72. [5] R. Leghrib, A. Felten, F. Demoisson, F. Reniers, J. Pireaux, E. Llobet, Room-temperature, selective detection of benzene at trace levels using plasmatreated metal-decorated multiwalled carbon nanotubes, Carbon 48 (2010) 3477–3484. [6] M. Mabrook, P. Hawkins, A rapidly-responding sensor for benzene, methanol and ethanol vapors based on films of titanium dioxide dispersed in a polymer operating at room temperature, Sens. Actuators B 75 (2001) 197–202. [7] J.-C. Chiang, A.G. MacDiarmid, ‘Polyaniline’: protonic acid doping of the emeraldine form to the metallic regime, Synth. Met. 13 (1986) 193–205. ´ Marjanovic, ´ Recent advances in polyaniline research: polymerization [8] G.C. mechanisms, structural aspects, properties and applications, Synth. Met. 177 (2013) 1–47. [9] P.P. Sengupta, S. Barik, B. Adhikari, Polyaniline as a gas-sensor material, Manufact. Proc. 21 (2006) 263–270. [10] U. Lange, N.V. Roznyatovskaya, V.M. Mirsky, Conducting polymers in chemical sensors and arrays, Anal. Chim. Acta 614 (2008) 1–26. [11] S. Wang, Y. Kang, L. Wang, H. Zhang, Y. Wang, Y. Wang, Organic/inorganic hybrid sensors: a review, Sens. Actuators B 122 (2013) 467–481. [12] G. Anitha, E. Subramanian, Dopant induced specificity in sensor behavior of conducting polyaniline materials with organic solvents, Sens. Actuators B 92 (2003) 49–59. [13] G. Anitha, E. Subramanian, Recognition and exposition of intermolecular interaction between CH2 Cl2 and CHCl3 by conducting polyaniline materials, Sens. Actuators B 107 (2005) 605–615. [14] K. Dutta, S.K. De, Optical and nonlinear electrical properties of SnO2 -polyaniline nanocomposites, Mater. Lett. 61 (2007) 4967–4971. [15] P. Manivel, S. Ramakrishnan, N.K. Kothurkar, A. Balamurugan, N. Ponpandian, D. Mangalaraj, C. Viswanathan, Optical and electrochemical studies of polyaniline/SnO2 fibrous nanocomposites, Mater. Res. Bull. 48 (2) (2013) 640–645. [16] L. Geng, Y. Zhao, X. Huang, S. Wang, S. Zhang, S. Wu, Characterization and gas sensitivity of polyaniline/SnO2 hybrid material prepared by hydrothermal route, Sens. Actuators B 120 (2007) 568–572. [17] H. Tai, Y. Jiang, G. Xie, J. Yu, Preparation, characterization and comparative NH3 -sensing characteristic studies of PANI/inorganic oxides nanocomposite thin films, J. Mater. Sci. Technol. 26 (2010) 605–613. [18] Z. Hu, Y. Xie, Y. Wang, L. Mo, Y. Yang, Z. Zhang, Polyaniline/SnO2 nanocomposite for supercapacitor applications, Mater. Chem. Phys. 114 (2009) 990–995. [19] L. Geng, Gas sensitivity of polyaniline/SnO2 hybrids to volatile organic compounds, Trans. Nonferrous Met. Soc. China 19 (2009) s678–s683. [20] N.G. Deshpande, Y.G. Gudage, R. Sharma, J.C. Vyas, J.B. Kim, Y.P. Lee, Studies on tin oxide-intercalated polyaniline nanocomposite for ammonia gas sensing applications, Sens. Actuators B: Chem. 138 (2009) 76–84.
C. Murugan et al. / Sensors and Actuators B 205 (2014) 74–81 [21] G.D. Khuspe, S.T. Navale, M.A. Chougule, V.B. Patil, Ammonia gas sensing properties of CSA doped PANI–SnO2 nanohybrid thin films, Synth. Met. 185–186 (2013) 1–8. [22] G.D. Khuspe, S.T. Navale, D.K. Bandgar, R.D. Sakhare, M.A. Chougule, V.B. Patil, SnO2 -nanoparticles modified polyaniline films as highly selective, sensitive, reproducible and stable ammonia sensors, Electron, Mater. Lett. 10 (1) (2014) 191–197. [23] S. Manjunath, A.K. Koppalkar, M.V.N.A. Prasad, Dielectric spectroscopy of polyaniline/stanic oxide (PANI/SnO2 ) composites, Ferroelectrics 366 (2008) 22–28. [24] B.C. Yadav, A. Yadav, V. Shukla, S. Sing, Solid-state titania-based gas sensor for liquefied petroleum gas detection at room temperature, Bull. Mater. Sci. 34 (2011) 1639–1644. [25] A.M. Ruiz, A. Cornet, K. Shimanoe, J.R. Morante, N. Yamazoe, Effects of various metal additives on the gas sensing performances of TiO2 nanocrystals obtained from hydrothermal treatments, Sens. Actuators B 108 (2005) 34–40. [26] N. Barsan, D. Koziej, U. Weimar, Metal oxide-based gas sensor research: How to? Sens. Actuators B 121 (2007) 18–35. [27] R. Hossein, H. Vahid, S. Mehdi, In situ controlled radical polymerization: a review on synthesis of well-defined nanocomposites, Polym. Rev. 52 (2012) 142–188. [28] C. Murugan, E. Subramanian, D.P. Padiyan, p–n heterojunction formation in polyaniline–SnO2 organic–inorganic hybrid composite materials leading to enhancement in sensor functionality towards benzene and toluene vapors at room temperature, Synth. Met. 192 (2014) 106–112. [29] M.T.S. Chani, K.S. Karimov, F.A. Khalid, S.A. Moiz, Polyaniline based impedance humidity sensors, Solid State Sci. 18 (2013) 78–82. [30] F.W. Zeng, X.X. Liu, D. Diamond, K.T. Lau, Humidity sensors based on polyaniline nanofibres, Sens. Actuators B 143 (2010) 530–534. [31] M. Angelopoulos, A. Ray, A.G. MacDiarmid, A.J. Epstein, Polyaniline: processability from aqueous solutions and effect of water vapor on conductivity, Synth. Met. 21 (1987) 21–30. [32] A.T. Ramaprasad, V. Rao, Chitin–polyaniline blend as humidity sensor, Sens. Actuators B 148 (2010) 117–125. [33] M.T.S. Chani, K.S. Karimov, F.A. Khalid, S.Z. Abbas, M.B. Bhatty, Orange-dyepolyaniline composite based impedance humidity sensors, Chin. Phys. B 22 (2013), 010701 1–6. [34] A.G. MacDiarmid, A.J. Epstein, Polyaniline A novel class of conducting polymers, Faraday Discuss. Chem. Soc. 88 (1989) 317–332. [35] M. Palencia, B.L. Rivas, E. Pereira, A. Arrieta, Polyelectrolyte adsorption study on polyethersulfone membrane during polymer-enhanced ultrafiltration by electrochemical impedance spectroscopy, Polym. Bull. 65 (2010) 145–156.
81
[36] T. Abdiryim, Z.X. Gang, R. Jamal, Comparative studies of solid-state synthesized polyaniline doped with inorganic acids, Mater. Chem. Phys. 90 (2005) 367–372. [37] S. Stafström, B. Sjögren, O. Wennerström, T. Hjertberg, Geometry of polyaniline, Synth. Met. 16 (1986) 31–39. [38] A. Parveen, R. Dashpande, S. Ahmed, A.S. Roy, Synthesis, characterization, and DC conductivity of polyaniline-lead oxide composites, Chem. Pap. 67 (3) (2013) 350–356. [39] R.K. Selvan, I. Perelshtein, N. Perkas, A. Gedanken, Synthesis of hexagonalshaped SnO2 nanocrystals and SnO2 @C nanocomposites for electrochemical redox supercapacitors, J. Phys. Chem. C 112 (2008) 1825–1830. [40] N. Gupta, D. Kumar, S.K. Tomar, Thermal behavior of chemically synthesized polyanilines/polystyrene sulphonic acid composites, Int. J. Mater. Chem. 2 (2) (2012) 79–85. [41] J. Zhang, S. Wang, M. Xu, Y. Wang, H. Xia, S. Zhang, X. Guo, S. Wu, Polypyrrolecoated SnO2 hollow spheres and their application for ammonia sensor, J. Phys. Chem. C 113 (2009) 1662–1665. [42] X. Liu, J. Zhang, X. Guo, S. Wu, S. Wang, Enhanced sensor response of Ni-doped SnO2 hollow spheres, Sens. Actuators B: Chem. 152 (2011) 162–167. [43] M.D. Imisides, R. Jhon, G.G. Wallace, Microsensors based on conducting polymers, Chemtech 26 (1996) 19–25.
Biographies C. Murugan is currently pursuing his Ph.D. under the guidance of Prof. E. Subramanian in the Department of Chemistry, Manonmaniam Sundaranar University, Tirunelveli, India. He has done his B.Sc. (2000) and M.Sc. (2006) in Chemistry from the affiliated colleges of this University. His main research interest includes organic–inorganic hybrid materials for sensor and photocatalytic applications. E. Subramanian is currently working as Professor and Head in Department of Chemistry, Manonmanian Sundaranar University, Tirunelveli, India. He obtained his Ph.D. degree from University of Madras, in 1989. He has about 23 years of teaching and research experience. His research interests include chemical sensors, photocatalysis and environmental water pollution studies. D. Pathinettam Padiyan is currently working as Professor in Department of Physics, Manonmaniam Sundaranar University, Tirunelveli, India. He was also the Dean of Science in this University. He received his Ph.D. in Physics from Madurai Kamaraj University, India, in 1988. He is actively engaged in thin films, hydrogen production, nano-materials and sensor materials.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Enhanced sensor functionality of in situ synthesized polyaniline–SnO2 hybrids toward benzene and toluene vapors C. Murugan a , E. Subramanian a,∗ , D. Pathinettam Padiyan b a b
Department of Chemistry, Manonmaniam Sundaranar University, Tirunelveli 627012, Tamil Nadu, India Department of Physics, Manonmaniam Sundaranar University, Tirunelveli 627012, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 22 November 2013 Received in revised form 6 August 2014 Accepted 8 August 2014 Available online 17 August 2014 Keywords: Polyaniline–SnO2 hybrids In situ synthesis Sensor Benzene Toluene Analyte
a b s t r a c t Polyaniline–SnO2 (PANI–SnO2 ) hybrid materials with varied SnO2 content were prepared by in situ chemical oxidative polymerization method. The prepared materials were characterized by FTIR, XRD, TGA, DSC and SEM analyses. The dielectric constant of the hybrid materials was calculated via Nyquist plot of the AC impedance analysis. Sensor functionality of the hybrids was studied toward benzene and toluene vapors in dry N2 atmosphere at room temperature. The response of the materials was presented in terms of their normalized conductivity change in per cent. The PANI–SnO2 hybrid with 40 wt% SnO2 exhibited the maximal response toward both benzene (16.9%) and toluene (19.5%) vapors. In situ synthesis technique enhanced the conductivity and response of the PANI–SnO2 materials in comparison to the physical mixture. This is attributed to the formation of greater number of p–n heterojunctions through large surface interaction of the analyte species. The hybrids showed slightly higher response toward toluene than benzene. © 2014 Published by Elsevier B.V.
1. Introduction Benzene and the analogous aromatic hydrocarbons are one of the classes of volatile organic compounds (VOCs). They are hazardous and are known to cause several kinds of diseases such as allergies, asthma, cancer and emphysema [1]. Studies on metal oxide based sensors functioning at higher temperature for the detection of such benzene/benzene like hydrocarbons are highly attractive [2–4] owing to their potential applications and also from the environmental point of view. Few strategic modifications of such sensors are reported for the detection of these hydrocarbons even at room temperature [5,6]. Polyaniline (PANI) has been the focal point of study among the conducting polymers for the last three decades owing to its ease of synthesis, excellent environmental stability, high conductivity, low cost and acid-doping/base-dedoping chemistry [7,8]. It has been studied as a chemo-sensor for a limited range of VOCs [9–11]. In our previous works, the dopant-induced sensor specificity of PANI toward some VOCs [12] and the ability of PANI to detect molecular pair formation between CH2 Cl2 and CHCl3 [13]were reported. But,
∗ Corresponding author. Tel.: +91 9965178458; fax: +91 462 2334363. E-mail addresses: [email protected], [email protected] (E. Subramanian). http://dx.doi.org/10.1016/j.snb.2014.08.027 0925-4005/© 2014 Published by Elsevier B.V.
aromatics, benzene/toluene being weakly interactive are not much studied with PANI sensor. In this context organic/inorganic hybrid (OIH) materials are much promising due to their advanced properties emerging from both organic and inorganic components [11]. Many studies have illustrated this point and some are highlighted herein. PANI–SnO2 composite has been studied with optical, electrochemical [14,15], gas sensing [16,17] and super-capacitor [18] applications. The sensor studies of PANI/SnO2 materials toward polar solvents with varying mass % of SnO2 [19] and ammonia [20–22] were reported. The dielectric values of PANI–SnO2 composites strongly depend on the wt% of SnO2 present in the materials and the particle dimension of SnO2 has greater influence on the dielectric values of the composite [23]. Yadav et al. [24] proposed three controlling factors for the principal operation of the semiconductor sensor which include the grain size, grain boundaries and accessible grain surface of the semiconductor material to the target analyte molecules [25,26]. Among the various methods of synthesis of OIH materials, the in situ polymerization has the advantage where the organic monomers adsorb on the surface of the inorganic material and get polymerized producing the hybrid composite [27]. The intimate contact of the two phases is, thus, ensured. However, in physical/mechanical grinding method, the organic matrix in its polymeric form makes only peripheral contact with inorganic phase. Therefore, the in situ polymerization is the best technique to
C. Murugan et al. / Sensors and Actuators B 205 (2014) 74–81
75
achieve the maximum required properties of OIH materials from the organic and inorganic components as compared to the physical/mechanical grinding technique. The report of only fewer sensor studies for carcinogenic benzene and the related hydrocarbon toluene is almost entirely due to the weak/lack of interaction of these hydrocarbons with the sensor materials. We recently reported the sensor functionality of PANI–SnO2 physically ground mixture toward benzene and toluene vapors in dry nitrogen atmosphere [28]. In normal air atmosphere polyaniline is known to have remarkable affinity toward moisture and therefore the material as such [29–31] or its composite [32,33] is used as humidity sensor in many studies. Because of the moisture absorbing tendency of polyaniline, our materials (PANI–SnO2 hybrids) could not be studied as benzene/toluene sensor in wet air environment or normal air atmosphere. Nevertheless, in dry nitrogen atmosphere they worked, sensed the analyte and produced the response. The materials showed only moderate efficiencies (∼10%). Hence, we intend to improve the sensor efficiency of PANI–SnO2 OIH materials toward benzene and toluene in dry nitrogen atmosphere by adopting an in situ synthesis method which by making an intimate contact between the two components would increase the number of p–n heterojunctions in the materials. Also the in situ formation and coating of PANI over SnO2 particles could enhance the physical and chemical properties of the materials. Interesting results are obtained which explain their mutual interaction and enhancement in sensor functionality of in situ synthesized hybrid compared to the PANI–SnO2 physically ground mixture. Hence, in the present work, we report the enhanced sensor functionality of PANI–SnO2 hybrid materials toward benzene and toluene vapors in dry nitrogen atmosphere in comparison to the physical mixture.
flow of gases. Nitrogen was used as a carrier gas for analyte (benzene and toluene). The flow of N2 was adjusted to get the required concentration of analyte. The sensor response was measured after passing particular concentration of analyte vapors to the sensor chamber for 15 min. The current–voltage measurements of the sensor pellets both in N2 and N2 -analyte atmospheres were taken at room temperature (28–29 ◦ C) and I–V plots were made. Resistance R was computed from the slope of I–V plots and conductivity was calculated from the formula given in Eq. (1).
2. Experimental
PANI and its SnO2 hybrid materials were characterized using Fourier transform infrared spectroscopy, X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. IR spectra of the samples were obtained with KBr pellet in JASCO FTIR-410 spectrophotometer provided with computer software. XRD patterns of the samples (2 = 2–80◦ ) were obtained with Rigaku Miniflex II ˚ desktop X-ray diffractometer using CuK␣ radiation ( = 1.5418 A). Scanning electron microscope images of the gold coated samples were observed with JEOL (model 6390) microscope operating at 20 kV. Thermogravimetric analysis was carried out with Mettler Toledo instrument by heating the sample upto 800 ◦ C at the rate of 10 ◦ C/min in N2 atmosphere. Differential scanning calorimetric analysis was performed with Mettler Toledo instrument heating the sample upto 600 ◦ C at the rate of 10 ◦ C/min in N2 atmosphere.
2.1. Materials Aniline (Merck) was purified by distillation over zinc dust. Ammonium peroxodisulfate (APS; Merck), SnO2 (∼325 mesh; Sigma–Aldrich) and other chemicals were used without further purification. Water used in the preparation and washings was doubly distilled unless otherwise mentioned. 2.2. Synthesis of PANI and PANI–SnO2 composites PANI was prepared by chemical oxidative polymerization method [34]. In a typical procedure, 20 mmol aniline and calculated quantity of SnO2 were added to 50 ml of 0.2 M aqueous H2 SO4 . The solution was stirred at 0–4 ◦ C for 30 min for the formation of anilinium ion on the surface of SnO2 particles. 20 mmol APS was added to the above solution and the stirring was continued for further 1 h. The resultant mixture was kept in a refrigerator overnight for the completion of polymerization. The obtained hybrid sample was filtered, washed with 200 ml double distilled water and 50 ml of acetone–methanol (1:1) mixture. 100 ml of 0.01 M H2 SO4 solution was used for doping and then the material was dried in an air oven at 120 ◦ C for 4 h. The dried sample was stored in air-tight polythene cover. The ground materials were pelletized (13 × 2 mm size) and dried at 120 ◦ C for 30 min prior to the conductivity measurement. PANI–SnO2 hybrids with different SnO2 content (20–80 wt%) were prepared by varying the amount of SnO2 .
=
1 2RS
(1)
where S is the inter-probe distance (2 mm). The normalized conductivity change (NCC) in % was calculated from the conductivity value of each pellet in N2 ( Nitrogen ) and in analyte ( Analyte at ∼1650 ppm) atmospheres using Eq. (2). NCC (%) =
Nitrogen − Analytc Nitrogen
× 100
(2)
Dynamic sensor response-recovery study was performed on PANI–40% SnO2 measuring the change in voltage against constant current (i.e., change in conductivity) and constant benzene concentration. 30 min was given for a complete cycle of adsorption and desorption of benzene (1650 ppm). N2 was used as a carrier gas for desorption study.
2.4. Characterization of the materials
2.5. Dielectric constant measurement of the PANI–SnO2 composites AC impedance measurement was made using Lock-in-Amplifier (Stanford Research System, Model SR830 DSP) in the frequency range 0.1–100 kHz with an amplitude of 100 mV to find the dielectric constant (εb ) of the material. Nyquist plot was constructed with real (Z ) and imaginary (Z ) components of the impedance to find frequency max [35]. The dielectric constant of each pellet was calculated at room temperature (28–29 ◦ C) using Eq. (3) to authenticate the formation of p–n heterojunction in the hybrid materials. Cb εO · A/t
2.3. Conductivity measurement
εb =
Pellets of PANI and its SnO2 composites were subjected to DC electrical conductivity measurement in a collinear four probe setup [12,13]. The pellet fitted in a collinear four probe set-up was kept in a sensor chamber provided with inlet and outlet for the
where Cb is the bulk capacitance, εo = 8.854 × 10−12 (vacuum permittivity in Fm−1 ), A is the area of cross section of the pellet (in m2 ) and t is the thickness of the pellet (in m).
(3)
76
C. Murugan et al. / Sensors and Actuators B 205 (2014) 74–81
Table 1 FT-IR spectral data (in cm−1 ) of PANI and its SnO2 composites. Sample
Quinoid C C
Benzenoid C C
Imine C N
Benzenoid amine C N
Protonated imine
PANI PANI–20% SnO2 PANI–30% SnO2 PANI–40% SnO2 a PANI–40% SnO2 b PANI–40% SnO2 c PANI–40% SnO2 PANI–60% SnO2 PANI–80% SnO2
1579 1556 1561 1562 1584 1562 1565 1561 1570
1465 1483 1482 1482 1494 1484 1481 1483 1484
1297 1301 1301 1303 1294 1303 1303 1304 1303
1234 1226 1226 1225 1242 1224 1224 1226 1227
1121 1139 1139 1141 1119 1141 1141 1143 1143
a b c
Physically ground material. Toluene adsorbed material. Benzene adsorbed material.
3. Results and discussion 3.1. Characterization of the materials Fig. 1 depicts the FTIR spectra of prepared samples and Table 1 presents the data of peak assignment. PANI shows all the characteristic peaks which reveal emeraldine salt form of PANI [36,37]. The observed conductivity value of PANI (2.489 Scm−1 ; Table 5) is in consistent with this IR observation. The quinoid C C stretching band and benzenoid secondary amine band are shifted to lower wavenumber side in the hybrid materials. However, benzenoid C C, imine C N stretching and protonated imine vibrational bands are slightly shifted to higher wavenumber in the hybrid materials. The characteristic Sn O stretching of SnO2 appears as shoulder in the hybrids with the lower wt% of SnO2 ; however that is distinctly visible (at 623 cm−1 ) in the hybrids with higher wt% of SnO2 . The intensity of Sn O band is found to increase with increase in SnO2 loading in the hybrids. These IR spectral features demonstrate firmly the mutual interaction of PANI and SnO2 in their OIH materials. Further, these features substantiate the conductivity trend. The conductivity value of PANI is increased in the hybrids at lower SnO2 content (20 and 30 wt%) and decreased gradually beyond 30 wt% (Fig. 2 and Table 5). SnO2 favors hopping of charge carriers due to the enhanced orientation and chain conformation of PANI in the hybrids at lower SnO2 content as also observed in PbO-PANI [38]. The decrease in conductivity at higher SnO2 content is due to the trapping of charge carrier by the confined SnO2 particles present in the three dimensional organization of the conducting chain. Overall an enhancement in the conductivity of in situ synthesized
materials is found as compared to the conductivity of physically ground materials (Table 5) [28]. Powder diffraction patterns of the hybrid materials (Fig. 3) clearly demonstrate the presence of SnO2 with 2 values at 26.6, 33.9, 38.0 and 51.8◦ corresponding respectively to the planes of (1 1 0), (1 0 1), (2 0 0) and (2 1 1). This pattern certainly reveals the existence of tetragonal SnO2 (JCPDS file No. 01-088-0287) [39] in the hybrid materials. The peak position of SnO2 (2 = 26.59◦ ) gets
Fig. 1. FT-IR spectra of (a) PANI, (b) PANI–20% SnO2 , (c) PANI–40% SnO2 , (d) PANI–60% SnO2 , (e) PANI–80% SnO2 and (f) SnO2.
Fig. 3. XRD spectra of (a) PANI, (b) PANI–20% SnO2 , (c) PANI–40% SnO2 , (d) PANI–60% SnO2 , and (e) PANI–80% SnO2.
Fig. 2. Plot of conductivity of materials versus wt% of SnO2 .
C. Murugan et al. / Sensors and Actuators B 205 (2014) 74–81 Table 2 XRD data of PANI, SnO2 and their hybrids.
77
Table 4 Differential scanning calorimetric analysis data of PANI–SnO2 hybrids.
Material
2 (◦ )
FWHM (◦ )
d-spacing (Å)
Crystallite size (nm)
Hybrid material
T1 (endothermic) (◦ C)
T2 (endothermic) (◦ C)
T3 (exothermic) (◦ C)
PANI SnO2 PANI–20% SnO2 PANI–40% SnO2 a PANI–40% SnO2 PANI–60% SnO2 PANI–80% SnO2
24.88 26.59 26.67 26.73 26.55 26.78 26.72
4.500 0.241 0.163 0.163 0.225 0.143 0.163
3.58 3.35 3.39 3.33 3.36 3.33 3.33
1.9 35.4 52.2 52.2 37.9 58.1 52.2
PANI–20% SnO2 PANI–40% SnO2 PANI–40% SnO2 -PM PANI–60% SnO2
82.87 77.08 94.09
279.7 273.9 258.0, 303.6
527 517 460
85.77
277.2
463
a
PM stands for physically ground material.
Physically ground material
slightly shifted to higher values with maximum for 60% loaded hybrid (Table 2). The crystallite size of SnO2 particle (35.4 nm) is enhanced in the hybrid materials with maximum value at 60% loaded hybrid (58.1 nm). This enhancement clearly shows the formation of polyaniline coating over SnO2 particles in a core-shell arrangement. In comparison to the physically ground material, the in situ synthesized materials are found to have higher crystallite sizes. That means there is a thick coating of PANI matrix around the SnO2 phase. Fig. 4 displays the SEM images of PANI, SnO2 and their composites. PANI exhibits a uniform mixture of spherical and worm-like agglomerated particles in the size range of 0.05–0.5 m whereas SnO2 has uniform spherical particles in the range of 100 nm. The particle size of the hybrid decreases leading to the low degree of agglomeration of the PANI chains with increase in SnO2 content. The worm-like particles are missing in the hybrid of higher SnO2 content. The thermogravimetric characterization data (Fig. 5a and Table 3) of the PANI–SnO2 hybrids in N2 atmosphere exhibit a fourstep weight loss from the materials up to the temperature of 800 ◦ C. The first weight loss in the temperature range of 40–140 ◦ C is due to the removal of physisorbed water. 5.83% weight loss from the physically ground material which is higher than those of in situ prepared materials reveals the presence of greater level of moisture absorption in physically ground mixture. The second weight loss in the temperature region 140–400 ◦ C is attributed to the elimination of dopant (sulphate etc.) species from the materials. The change in these values are in accordance with the polymer content (decrease with decrease in polymer content) and thus to the dopants. The hybrid with 80 wt% PANI content exhibits the maximal weight loss (25.29%) and hybrid with 40 wt% PANI exhibits the minimal weight loss (12.58%). Third step weight loss is the decomposition of organic oligomeric content and the forth step weight loss is the removal of decomposable matrix of PANI present in OIH materials. The increase in weight loss with increase in initial wt% of SnO2 in the forth step demonstrates that the OIH materials are more stable with higher PANI content and less stable with higher SnO2 content. The final residue indicates the presence of SnO2 along with carbon from thermally decomposed PANI content. The equal value of final residue in both in situ synthesized and physically ground PANI–40% SnO2 materials substantiates the presence of equal amount of SnO2 .
The differential scanning calorimetric data (Fig. 5b and Table 4) with two endothermic and one exothermic peaks are in resemblance to the DSC profile of polyaniline/polystyrene [40]. The first endothermic peak (T1 ) due to the loss of water molecule appears at 77–85 ◦ C in all in situ synthesized hybrids. However, this endothermic peak of physically ground composite is slightly shifted to higher temperature side (94 ◦ C). Another endothermic peak appearing at 273–279 ◦ C in the in situ synthesized hybrids is assigned to the decomposition and elimination of dopant. This high temperature endothermic peak (T2 ) gets split into two in physically ground composite substantiating the heterogeneity nature of the material. The onset of exothermic transition (T3 ) above 350 ◦ C is attributable to the degradation of polymeric backbone. The maximal temperature of the exothermic peak is shifted considerably to lower temperature upon increasing the SnO2 content to 60% in the hybrid material. This means lower SnO2 content hybrid has higher thermal stability which is in consistent with TGA inference mentioned above. Further, PANI–40%SnO2 , in situ synthesized material has the maximum of the exothermic degradation temperature (T3 ) at higher side by 57 ◦ C in comparison to the physically ground material (Table 4). This is attributed to the increased surface coverage by PANI over SnO2 particle followed by the increase in average crystallite size (Table 2). 3.2. Dielectric constant values of the composite materials The two components PANI and SnO2 are p- and n-type conductors respectively. When they couple together and form hybrid composite, as per the well-known concept of p–n junction formation [22,38,41,42], electron flow occurs from n-SnO2 to p-PANI and occupies the vacant energy levels in PANI until the Fermi level becomes equal in the entire composite materials. This, in turn, produces positive poles in SnO2 and negative poles in PANI and thereby the p–n heterojunctions are formed in the hybrid composite materials. When there is charge transfer, the dielectric characteristic of the material will also change proportionately. Hence, dielectric constant could be considered as a measure of formation of p–n heterojunctions. The dielectric constant values are given in Table 5. In the beginning, the dielectric constant (εb ) value gradually increases with increase in wt% of SnO2 in the composite materials. Then it reaches the maximum at PANI–40% SnO2 suggesting the formation of maximum number of dielectrics in the hybrid material. Further increase in SnO2 content in the composite results in lowering of εb value.
Table 3 Thermogravimetric analysis data of PANI–SnO2 hybrids. Samples
PANI–20% SnO2 PANI–40% SnO2 PANI–40% SnO2 -PM PANI–60% SnO2
Weight loss in the temperature range (%) 40–140 ◦ C
140–400 ◦ C
400–620 ◦ C
620–800 ◦ C
<800 ◦ C
2.40 1.82 5.83 2.52
25.29 18.88 12.58 12.58
9.52 6.66 9.85 3.97
10.38 13.65 13.53 16.51
52.41 58.99 58.66 64.42
PM stands for physically ground material.
78
C. Murugan et al. / Sensors and Actuators B 205 (2014) 74–81
Fig. 4. SEM images of (A) PANI, (B) SnO2 , (C) PANI–20% SnO2 , (D) PANI–40% SnO2 and (E) PANI–60% SnO2.
Fig. 5. Thermogravimetric (a) and differential scanning calorimetric (b) profile of PANI–SnO2 hybrids.
C. Murugan et al. / Sensors and Actuators B 205 (2014) 74–81
79
Table 5 Physical and sensor properties of PANI and its SnO2 hybrids. Sample
SnO2 loading (wt%)
Dielectric constant εb (Fm−1 )
Conductivity (Scm−1 )
PANI PANI–20% SnO2 PANI–30% SnO2 PANI–40% SnO2 a PANI–40% SnO2 PANI–50% SnO2 PANI–60% SnO2 PANI–70% SnO2 PANI–80% SnO2 SnO2
– 20 30 40 40 50 60 70 80 100
7.65 × 103 8.01 × 105 6.58 × 106 2.89 × 107 5.71 × 104 9.47 × 106 4.28 × 106 NA 8.99 × 105 3.82 × 10−2
2.489 3.247 2.711 1.942 0.682 1.461 1.141 0.941 0.737 –
a
NCC (%) Benzene
Toluene
1.49 5.39 13.23 16.93 9.82 13.27 12.23 7.72 3.46 –
1.57 6.84 15.49 19.53 10.41 14.79 13.15 8.7 3.55 –
Physically ground material. [Analyte] = ∼1650 ppm. NCC represents normalized conductivity change.
This is quite logical because of lower PANI (p-type) content relative to n-SnO2 in the composite and the incomplete saturation of p- and n-regions. The physically ground material exhibits only lower dielectric constant (∼500 times) than the in situ synthesized material and this observation explains the formation of greater number of p–n heterojunctions via in situ synthesis method. This also explains higher conductivity of in situ synthesized material than that of physically ground mixture (Table 5). 3.3. Sensor behavior and mechanism The sensor function of the composite materials (in pellet form) in normal air atmosphere was insignificant. In normal air atmosphere PANI has the tendency to adsorb moisture and functions as humidity sensor [29–33]. Therefore, in the present work when the pellets are exposed to benzene/toluene analyte in normal air atmosphere, moisture preferentially adsorbs in competition to analyte and causes an increase in conductivity. This sensor response due to moisture is, in fact, opposite to the sensor response of the materials toward benzene/toluene analyte in dry N2 atmosphere (conductivity decrease, Table 5). Hence, the sensor response of PANI–SnO2 hybrid materials could be studied only in dry N2 atmosphere and therefore the sensor is operable only in dry N2 atmosphere. Table 5 displays the conductivity data of PANI and its SnO2 composites in N2 atmosphere. The presence of analyte in N2 atmosphere definitely influences the conductivity of PANI and its SnO2 composites by reducing the value. This sensor response is given in terms of NCC%. PANI alone shows only ∼1.5 NCC% upon exposure to the analyte vapors while pure SnO2 shows no change in conductivity. That means the individual component produces very bleak/nil sensor response. However, the NCC values of PANI–SnO2 composites are remarkably higher; they initially increase, reach the maximum at PANI–40% SnO2 and then decrease with increase in SnO2 content. The hybrid with 40 wt% SnO2 exhibits the maximum sensor efficiency which coincides with the maximum dielectric constant value for this material (Table 5). Further, good correlation exists between sensor efficiency and p–n heterojunctions (i.e., dielectric constant values) of the materials (Fig. 6). That means sensor efficiency of the hybrid materials (Table 5) is supported and facilitated by the number of p–n heterojunctions present in them. The sensor activity is rapid, reversible and regenerative for all the materials, because when the analyte flow was replaced by pure N2 flow, the conductivities of the materials returned nearly to their original values (approximately 96% regeneration). To confirm and to make this observation explicit, dynamic sensor study was made representatively for PANI–40% SnO2 composite. Fig. 7 illustrates the dynamic response-recovery profile toward benzene. It seems within 15 min of analyte vapor exposure (benzene), the material gets saturated and reaches its maximum level of response (decrease in conductivity); on benzene vapor off, the material returns to its
Fig. 6. Dependence of sensor efficiencies on dielectric constant of the materials at [Analyte] = 1650 ppm.
near original conductivity value in 15 min. Retain of roughly 4% of the conductivity value during regeneration suggests some sort of diffusion of benzene vapor into the pores of the materials in addition to surface adsorption. This profile in first cycle is repeated in 2nd and 3rd cycles also, demonstrating reversibility and regeneration. A plausible sensor mechanism is as follows. The benzene/toluene analyte molecules adsorb on the surface of the pellet (physisorption), tend to attract electrons from the hybrid material,
Fig. 7. Dynamic sensor profile of PANI–40% SnO2 toward benzene vapor.
80
C. Murugan et al. / Sensors and Actuators B 205 (2014) 74–81
sensor function toward benzene/toluene vapor is insignificant. However, in dry nitrogen atmosphere the hybrid materials function very well and produce significant sensor response. The individual component SnO2 does not show sensor response while pristine PANI produces very feeble value even in dry nitrogen atmosphere. However, the hybrid materials exhibit enhanced responses and the sensor efficiency becomes maximal at 40 wt% loading of SnO2 . The in situ synthesized materials have nearly two-times higher response than those of physical mixtures. The intimate contact and strong interaction between PANI and SnO2 in former as revealed in characterization techniques lead to the enhanced sensor response. Although the present sensor materials are operable only in dry nitrogen atmosphere, they do have some positive aspects as listed hereunder: (1) PANI sensor for weakly interactive benzene/toluene analyte, (2) sensor function at ambient temperature compared to higher operable temperature of metal oxide sensors and (3) considerably higher sensor efficiency for the weakly interacting analytes. Fig. 8. FT-IR spectra of PANI–40% SnO2 (line) and its benzene adsorbed form (dotted line).
particularly from p-doped PANI in the heterojunction region (electron rich by charge transfer from SnO2 ), disturb/perturb electron density and thereby they provide friction to the movement of electrons in the conjugate system of PANI, reducing the conductivity of the material. Toluene shows slightly higher influence on conductivity change than does benzene. The methyl group in toluene shifts the electron density from the aromatic ring and makes the molecule little polar. This polarity provides some extra friction for free-moving polarons and thus decreases the conductivity little higher than that by benzene [43]. This type of physisorption and reduction in conductivity has been reported as the sensor mechanism in TiO2 -dispersed-poly(vinylidenfluoride) polymer film [6] and also in polyaniline-lead oxide composites [38]. Upon analyte off, the carrier gas nitrogen lifts off the adsorbed analyte, decreases its concentration on the surface of the sensor material and increases the conductivity by facilitating the flow of charge carriers. In situ synthesized hybrid materials exhibit good physical strength with a little lesser brittle nature in comparison to the physically ground composite. These materials exhibit nearly two times higher response than the physically ground PANI–SnO2 composites [28]. Certainly, this is due to the formation of larger number of p–n heterojunctions (2–3 orders) in former. In an attempt to identify the chemical group(s) of PANI involved in analyte association, the IR spectrum of benzene vapor adsorbed material was recorded (Fig. 8). The intensity of benzenoid (C C) stretching is increased with a decrease of the intensity of quinoid (C C) stretching. However, the intensity of the protonated imine band (also known as conductivity band) for the existence of positive charge distribution in the dihedral angle between benzenoid +
+
(B) and quinoid (Q) rings (Q NH B or B NH B) [37] remains unchanged. An additional peak emerged at 673 cm−1 in the benzene adsorbed material corresponds to the breathing ring vibration of benzene [37]. All these IR observations clearly demonstrate that (i) Q and B rings of PANI undergo electronic perturbation, (ii) the conductivity decrease of the sensor material by analyte association is not permanent but is transient and (iii) benzene ring of analyte has overlapping interaction with B unit of PANI. 4. Conclusions Polyaniline–SnO2 hybrid materials with various wt% of SnO2 were prepared by in situ method and investigated for benzene/toluene analyte vapor sensor. In normal air atmosphere moisture preferentially adsorbs in competition to analyte and the
Acknowledgement Authors are thankful to the Department of Science and Technology, Govt. of India for the financial support under the project SR/S2/CMP-22/2009. References [1] T. Godish, Air Quality, 2nd ed., Lewis Publishers, Chelsea, MI, 1991. [2] Z. Rao, L. Liu, J. Xie, Y. Zeng, Development of a benzene vapour sensor utilizing chemiluminescence on Y2 O3 , Luminescence 23 (2008) 163–168. [3] M. Ke, M. Lee, C. Lee, L. Fu, A MEMS-based benzene gas sensor with a self-heating WO3 sensing layer, Sensors 9 (2009) 2895–2906. [4] B. Ghaddab, F. Berger, J.B. Sanchez, P. Menini, C. Mavon, P. Yoboue, V. Potin, Benzene monitoring by micro-machined sensors with SnO2 layer obtained by using micro-droplet deposition technique, Sens. Actuators B 152 (2011) 68–72. [5] R. Leghrib, A. Felten, F. Demoisson, F. Reniers, J. Pireaux, E. Llobet, Room-temperature, selective detection of benzene at trace levels using plasmatreated metal-decorated multiwalled carbon nanotubes, Carbon 48 (2010) 3477–3484. [6] M. Mabrook, P. Hawkins, A rapidly-responding sensor for benzene, methanol and ethanol vapors based on films of titanium dioxide dispersed in a polymer operating at room temperature, Sens. Actuators B 75 (2001) 197–202. [7] J.-C. Chiang, A.G. MacDiarmid, ‘Polyaniline’: protonic acid doping of the emeraldine form to the metallic regime, Synth. Met. 13 (1986) 193–205. ´ Marjanovic, ´ Recent advances in polyaniline research: polymerization [8] G.C. mechanisms, structural aspects, properties and applications, Synth. Met. 177 (2013) 1–47. [9] P.P. Sengupta, S. Barik, B. Adhikari, Polyaniline as a gas-sensor material, Manufact. Proc. 21 (2006) 263–270. [10] U. Lange, N.V. Roznyatovskaya, V.M. Mirsky, Conducting polymers in chemical sensors and arrays, Anal. Chim. Acta 614 (2008) 1–26. [11] S. Wang, Y. Kang, L. Wang, H. Zhang, Y. Wang, Y. Wang, Organic/inorganic hybrid sensors: a review, Sens. Actuators B 122 (2013) 467–481. [12] G. Anitha, E. Subramanian, Dopant induced specificity in sensor behavior of conducting polyaniline materials with organic solvents, Sens. Actuators B 92 (2003) 49–59. [13] G. Anitha, E. Subramanian, Recognition and exposition of intermolecular interaction between CH2 Cl2 and CHCl3 by conducting polyaniline materials, Sens. Actuators B 107 (2005) 605–615. [14] K. Dutta, S.K. De, Optical and nonlinear electrical properties of SnO2 -polyaniline nanocomposites, Mater. Lett. 61 (2007) 4967–4971. [15] P. Manivel, S. Ramakrishnan, N.K. Kothurkar, A. Balamurugan, N. Ponpandian, D. Mangalaraj, C. Viswanathan, Optical and electrochemical studies of polyaniline/SnO2 fibrous nanocomposites, Mater. Res. Bull. 48 (2) (2013) 640–645. [16] L. Geng, Y. Zhao, X. Huang, S. Wang, S. Zhang, S. Wu, Characterization and gas sensitivity of polyaniline/SnO2 hybrid material prepared by hydrothermal route, Sens. Actuators B 120 (2007) 568–572. [17] H. Tai, Y. Jiang, G. Xie, J. Yu, Preparation, characterization and comparative NH3 -sensing characteristic studies of PANI/inorganic oxides nanocomposite thin films, J. Mater. Sci. Technol. 26 (2010) 605–613. [18] Z. Hu, Y. Xie, Y. Wang, L. Mo, Y. Yang, Z. Zhang, Polyaniline/SnO2 nanocomposite for supercapacitor applications, Mater. Chem. Phys. 114 (2009) 990–995. [19] L. Geng, Gas sensitivity of polyaniline/SnO2 hybrids to volatile organic compounds, Trans. Nonferrous Met. Soc. China 19 (2009) s678–s683. [20] N.G. Deshpande, Y.G. Gudage, R. Sharma, J.C. Vyas, J.B. Kim, Y.P. Lee, Studies on tin oxide-intercalated polyaniline nanocomposite for ammonia gas sensing applications, Sens. Actuators B: Chem. 138 (2009) 76–84.
C. Murugan et al. / Sensors and Actuators B 205 (2014) 74–81 [21] G.D. Khuspe, S.T. Navale, M.A. Chougule, V.B. Patil, Ammonia gas sensing properties of CSA doped PANI–SnO2 nanohybrid thin films, Synth. Met. 185–186 (2013) 1–8. [22] G.D. Khuspe, S.T. Navale, D.K. Bandgar, R.D. Sakhare, M.A. Chougule, V.B. Patil, SnO2 -nanoparticles modified polyaniline films as highly selective, sensitive, reproducible and stable ammonia sensors, Electron, Mater. Lett. 10 (1) (2014) 191–197. [23] S. Manjunath, A.K. Koppalkar, M.V.N.A. Prasad, Dielectric spectroscopy of polyaniline/stanic oxide (PANI/SnO2 ) composites, Ferroelectrics 366 (2008) 22–28. [24] B.C. Yadav, A. Yadav, V. Shukla, S. Sing, Solid-state titania-based gas sensor for liquefied petroleum gas detection at room temperature, Bull. Mater. Sci. 34 (2011) 1639–1644. [25] A.M. Ruiz, A. Cornet, K. Shimanoe, J.R. Morante, N. Yamazoe, Effects of various metal additives on the gas sensing performances of TiO2 nanocrystals obtained from hydrothermal treatments, Sens. Actuators B 108 (2005) 34–40. [26] N. Barsan, D. Koziej, U. Weimar, Metal oxide-based gas sensor research: How to? Sens. Actuators B 121 (2007) 18–35. [27] R. Hossein, H. Vahid, S. Mehdi, In situ controlled radical polymerization: a review on synthesis of well-defined nanocomposites, Polym. Rev. 52 (2012) 142–188. [28] C. Murugan, E. Subramanian, D.P. Padiyan, p–n heterojunction formation in polyaniline–SnO2 organic–inorganic hybrid composite materials leading to enhancement in sensor functionality towards benzene and toluene vapors at room temperature, Synth. Met. 192 (2014) 106–112. [29] M.T.S. Chani, K.S. Karimov, F.A. Khalid, S.A. Moiz, Polyaniline based impedance humidity sensors, Solid State Sci. 18 (2013) 78–82. [30] F.W. Zeng, X.X. Liu, D. Diamond, K.T. Lau, Humidity sensors based on polyaniline nanofibres, Sens. Actuators B 143 (2010) 530–534. [31] M. Angelopoulos, A. Ray, A.G. MacDiarmid, A.J. Epstein, Polyaniline: processability from aqueous solutions and effect of water vapor on conductivity, Synth. Met. 21 (1987) 21–30. [32] A.T. Ramaprasad, V. Rao, Chitin–polyaniline blend as humidity sensor, Sens. Actuators B 148 (2010) 117–125. [33] M.T.S. Chani, K.S. Karimov, F.A. Khalid, S.Z. Abbas, M.B. Bhatty, Orange-dyepolyaniline composite based impedance humidity sensors, Chin. Phys. B 22 (2013), 010701 1–6. [34] A.G. MacDiarmid, A.J. Epstein, Polyaniline A novel class of conducting polymers, Faraday Discuss. Chem. Soc. 88 (1989) 317–332. [35] M. Palencia, B.L. Rivas, E. Pereira, A. Arrieta, Polyelectrolyte adsorption study on polyethersulfone membrane during polymer-enhanced ultrafiltration by electrochemical impedance spectroscopy, Polym. Bull. 65 (2010) 145–156.
81
[36] T. Abdiryim, Z.X. Gang, R. Jamal, Comparative studies of solid-state synthesized polyaniline doped with inorganic acids, Mater. Chem. Phys. 90 (2005) 367–372. [37] S. Stafström, B. Sjögren, O. Wennerström, T. Hjertberg, Geometry of polyaniline, Synth. Met. 16 (1986) 31–39. [38] A. Parveen, R. Dashpande, S. Ahmed, A.S. Roy, Synthesis, characterization, and DC conductivity of polyaniline-lead oxide composites, Chem. Pap. 67 (3) (2013) 350–356. [39] R.K. Selvan, I. Perelshtein, N. Perkas, A. Gedanken, Synthesis of hexagonalshaped SnO2 nanocrystals and SnO2 @C nanocomposites for electrochemical redox supercapacitors, J. Phys. Chem. C 112 (2008) 1825–1830. [40] N. Gupta, D. Kumar, S.K. Tomar, Thermal behavior of chemically synthesized polyanilines/polystyrene sulphonic acid composites, Int. J. Mater. Chem. 2 (2) (2012) 79–85. [41] J. Zhang, S. Wang, M. Xu, Y. Wang, H. Xia, S. Zhang, X. Guo, S. Wu, Polypyrrolecoated SnO2 hollow spheres and their application for ammonia sensor, J. Phys. Chem. C 113 (2009) 1662–1665. [42] X. Liu, J. Zhang, X. Guo, S. Wu, S. Wang, Enhanced sensor response of Ni-doped SnO2 hollow spheres, Sens. Actuators B: Chem. 152 (2011) 162–167. [43] M.D. Imisides, R. Jhon, G.G. Wallace, Microsensors based on conducting polymers, Chemtech 26 (1996) 19–25.
Biographies C. Murugan is currently pursuing his Ph.D. under the guidance of Prof. E. Subramanian in the Department of Chemistry, Manonmaniam Sundaranar University, Tirunelveli, India. He has done his B.Sc. (2000) and M.Sc. (2006) in Chemistry from the affiliated colleges of this University. His main research interest includes organic–inorganic hybrid materials for sensor and photocatalytic applications. E. Subramanian is currently working as Professor and Head in Department of Chemistry, Manonmanian Sundaranar University, Tirunelveli, India. He obtained his Ph.D. degree from University of Madras, in 1989. He has about 23 years of teaching and research experience. His research interests include chemical sensors, photocatalysis and environmental water pollution studies. D. Pathinettam Padiyan is currently working as Professor in Department of Physics, Manonmaniam Sundaranar University, Tirunelveli, India. He was also the Dean of Science in this University. He received his Ph.D. in Physics from Madurai Kamaraj University, India, in 1988. He is actively engaged in thin films, hydrogen production, nano-materials and sensor materials.