p–n Heterojunction formation in polyaniline–SnO2 organic–inorganic hybrid composite materials leading to enhancement in sensor functionality toward benzene and toluene vapors at room temperature

Synthetic Metals 192 (2014) 106–112

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p–n Heterojunction formation in polyaniline–SnO2 organic–inorganic hybrid composite materials leading to enhancement in sensor functionality toward benzene and toluene vapors at room temperature 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 11 November 2013 Received in revised form 20 February 2014 Accepted 18 March 2014 Keywords: Polyaniline–SnO2 composite Sensor Benzene toluene analyte

a b s t r a c t Organic–inorganic hybrid composite (OIHC) materials were synthesized through physical grinding of conducting p-polyaniline and n-SnO2 with different weight %. Characterization by FT-IR, XRD, SEM and AC impedance studies revealed a mutual interaction and formation of p–n heterojunctions. In N2 atmosphere OIHC materials had enhancement in sensor functionality toward benzene and toluene vapors at room temperature. Good correlation existed between number of p–n heterojunctions and sensor efficiencies. An optimized material with 40 wt% SnO2 showed 10% sensor efficiency for benzene and slightly higher value for toluene. Considering weak/nil interaction of analyte, the present work produced fairly efficient hybrid sensor materials from almost insensitive organic and inorganic components. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Volatile organic compounds (VOC) such as hydrocarbon and halogen-substituted paraffin solvents easily evaporate and deteriorate the indoor air quality at industrial sites and households as well. Most of the VOCs are hazardous and are known to cause several kinds of diseases such as allergies, asthma, cancer and emphysema [1]. For instance, benzene is widely used in industrial processes, despite being a known carcinogen. For precautionary measures, identification and an estimation of such hazardous vapors are essential. In this context, the development of sensor materials for the detection of benzene and related hydrocarbon like toluene has attracted researchers [2–6]. In respect of sensor functionality and other salient features like ease of synthesis, environmental stability and good conductivity, the conducting polyaniline (PANI) has been receiving much attention worldwide [7–9]. Sengupta et al. [10] have shown the gas sensing property of polyaniline toward ammonia. Roh et al. [11] have investigated the possible utilization of bare PANI toward

∗ Corresponding author. Tel.:+91 996 517 8458(mob.); fax: +91 462 233 4363/+91 462 232 2973. E-mail addresses: [email protected], [email protected] (E. Subramanian). http://dx.doi.org/10.1016/j.synthmet.2014.03.017 0379-6779/© 2014 Elsevier B.V. All rights reserved.

benzene, toluene and chloroform. We have also investigated the dopant-induced sensor specificity of PANI toward chlorinated hydrocarbon vapors [12] and the ability of doped PANI to recognize and expose the molecular pair formation between CH2 Cl2 and CHCl3 [13]. Metal oxides are also efficient sensors. Benzene sensing properties of TiO2 [2], Y2 O3 [3], WO3 [4] and SnO2 [14] have been reported. In a recent report, SnO2 is found to enhance the gas sensing behavior of TiO2 nanobelt [15]. Though many studies are found, the metal oxide sensors are effective only at higher temperature [4,5,16]. Organic–inorganic hybrid composite (OIHC) materials with synergic/complement property could overcome the shortcomings of metal oxide sensors [17–20]. PANI is a p-type organic semiconductor with a linear conjugate ␲-electronic system while SnO2 is a n-type semiconductor. Their composite may have p–n heterojunction formation and could thus behave as a better sensor than pure SnO2 . Therefore PANI–SnO2 OIHC material has become the most attractive subject of research, particularly for supercapacitor [21], optical and non-linear electrical properties [22] and ammonia gas-sensing [20,23]. As far as the sensor studies for carcinogenic benzene and related hydrocarbon toluene are concerned, besides the above, no further study was made. This is almost entirely due to the weak/lack of interaction of these hydrocarbons with the sensor materials. Hence in the present work our focus was to develop sensor material for

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benzene and toluene with room temperature functionality. In this respect both PANI and SnO2 as the individual component were not helpful because of their insensitivity or non-functionality in air atmosphere; in N2 atmosphere only PANI not SnO2 had very marginal sensitivity. Hence our investigation was on OIHC material of PANI and nano SnO2 components. Simple method i.e., physical grinding was adopted to prepare OIHC materials of PANI with different wt% of SnO2 . Interesting results are obtained concerning the wt% dependent-interaction of SnO2 with PANI and its impact on p–n heterojunction formation and sensor functionality of resultant OIHC materials. 2. Materials and methods Aniline (Merck) was purified by distillation over zinc dust. Ammonium peroxodisulfate (APS; Merck), SnO2 (∼325 mesh; Sigma-Aldrich) and other chemicals (SD fine chemicals) were used without further purification. Water used in the preparation and washings was doubly distilled unless otherwise mentioned. 2.1. Synthesis of PANI and PANI–SnO2 composites

2.3. Characterization of the samples

PANI was prepared by chemical oxidative polymerization method [7–9]. In a typical procedure, 0.2 M aniline in 0.2 M H2 SO4 (100 ml) was stirred at 0–4 ◦ C for 30 min. 0.2 M APS (100 ml) in ice cold solution was added dropwise into the aniline solution with constant stirring and the agitation was continued for 30 min. The resultant mixture was kept in a refrigerator overnight for the completion of polymerization. The obtained polymer sample was filtered, washed several times with double distilled water, 100 ml of acetone-methanol (1:1) mixture and finally with 0.1 M H2 SO4 and dried in an air oven at 120 ◦ C for 4 h. The dried sample was ground into fine powder, stored in air-tight polythene cover and used for characterization and sensor studies. PANI–SnO2 nanocomposites of varied SnO2 (10–80 wt%) composition were prepared by mechanically grinding the physical mixture of appropriate amount of PANI and SnO2 in an agate mortar. The ground materials were pelletized and dried at 120 ◦ C for 30 min prior to the conductivity measurement.

PANI and its SnO2 composite pellets were subjected to dc electrical conductivity measurement in a four probe setup [12]. The pellet mounted on a four probe was kept in a sensor chamber (900 ml) provided with inlet and outlet for the flow of gases. Nitrogen bubbling through benzene/toluene solvent was used as the carrier gas for analyte vapors. By maintaining different flow rates of N2 carrier gas, the analyte concentration in the sensor test chamber was varied. The sensor response was measured after passing analyte vapor of particular concentration into the sensor chamber for 15 min. The current–voltage measurements of the pellets both in pure N2 and N2 -analyte atmospheres were made. The conductivity of each pellet was calculated from the slope of I–V plot. The normalized conductivity change (NCC) in % was calculated from the conductivity value of each pellet in N2 (N2 ) and in N2 -analyte ( Analyte ) atmospheres using Eq. (1). Nitrogen − Analyte Nitrogen

× 100

PANI and its SnO2 composites were characterized using Fourier transform infrared spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) techniques. IR spectra of the samples were obtained with KBr pellet in JASCO FTIR-410 spectrophotometer provided with computer software for fresh and benzene vapor adsorbed samples. XRD patterns of the samples (2 = 2–80o ) were obtained with Rigaku ˚ Miniflex II X-ray diffractometer using CuK␣ radiation ( = 1.5418 A). The average crystallite size of each material was calculated from 2 and FWHM values of maximum intensity peak using Scherrer formula. 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 up to 800 ◦ C at the rate 10 ◦ C/min in N2 atmosphere. 2.4. Dielectric constant determination of the PANI–SnO2 composites

2.2. Conductivity and sensor experiments

NCC (%) =

Fig. 1. FT-IR spectra of PANI and its SnO2 composites.

AC impedance measurement was carried out using Lock-inAmplifier (Stanford Research System, Model SR830 DSP) in the frequency range 0.1 to 100 kHz with an amplitude of 100 mV to find the dielectric constant (εb ) of the material. Bode plot was constructed with the logarithmic of frequency (log ω) on x-axis and the absolute value of impedance (|Z|) on y-axis [24]. The dielectric constant of each pellet was calculated at room temperature using Eq. (2) εb =

Cb ε0 × A × t

(2)

where Cb is the bulk capacitance of the material, ε0 = 8.854 × 10−12 (vacuum permittivity in F m−1 ), A is the area of cross section of the pellet (in m2 ) and t is the thickness of the pellet (in m). 3. Results and discussion

(1)

Dynamic sensor study was also carried out with a representative material of PANI–40% SnO2 composite measuring the change in voltage against constant current and constant benzene concentration (∼1650 ppm). Thirty minutes was given for a complete cycle of adsorption and desorption of benzene. N2 used as the carrier gas was also used for desorption of benzene vapor.

3.1. FTIR characterization of the samples 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 [25]. The observed conductivity value of PANI (2.489 S cm−1 ; Table 2) is in consistent with this IR observation. On compositing with SnO2 with

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Table 1 FTIR spectral data of PANI and its SnO2 composites. Sample

Quinoid C C

Benzenoid C C

Imine C N

Benzenoid C N

Protonated imine

PANI PANI–10% SnO2 PANI–20% SnO2 PANI–40% SnO2 * PANI–40% SnO2 PANI–60% SnO2 PANI–80% SnO2

1579 1565 1579 1584 1580 1581 1582

1465 1467 1478 1494 1489 1493 1488

1297 1296 1290 1294 1295 1297 1299

1234 1237 1236 1242 1244 1244 1245

1121 1131 1121 1119 1117 1117 1119

*

Stands for benzene adsorbed material.

Table 2 Change in conductivity of PANI and its SnO2 composites on analyte exposure. Sample

PANI PANI–10% SnO2 PANI–20% SnO2 PANI–30% SnO2 PANI–40% SnO2 PANI–50% SnO2 PANI–60% SnO2 PANI–80% SnO2 SnO2

SnO2 loading (wt%)

Dielectric constant εb (F m−1 )

– 10 20 30 40 50 60 80 100

9.96 × 10 1.17 × 1010 3.33 × 1010 9.91 × 1010 1.07 × 1011 9.20 × 109 1.84 × 106 1.89 × 106 1.06 × 106 9

Conductivity with (S cm−1 )

NCC (%)

N2

Benzene

Toluene

Benzene

Toluene

2.489 2.703 1.754 0.969 0.682 0.561 0.333 0.048 –

2.452 2.655 1.709 0.909 0.615 0.536 0.322 0.048 –

2.450 2.652 1.705 0.908 0.611 0.532 0.318 0.048 –

1.49 1.78 2.57 6.19 9.82 4.46 3.30 0 –

1.57 1.89 2.79 6.30 10.41 5.17 4.50 0 –

[Analyte] = ∼1650 ppm. NCC represents normalized conductivity change. – stands for unmeasurable data due to the brittle nature of the pellet.

gradual increase in wt%, the impact on IR spectra of resulting OIHC materials is quite interesting. Except with 10 wt% of SnO2 where the trend is quite amazing, in all other cases, as we see in Table 1, the C C and C N stretching vibrations of benzenoid with amine group undergo a blue shift, i.e., higher wavenumber-side shift by almost 20–30 cm−1 and 10 cm−1 , respectively while the protonated imine band (conductivity band) a small red-shift (by ∼4 cm−1 ) when compared to pristine PANI. With 10 wt% SnO2 the C C stretching of quinoid ring has a red-shift (by 14 cm−1 ) and the conductivity band a blue-shift (by 10 cm−1 ). With respect of SnO2 , its O Sn O antisymmetric stretching is not observable in 10 and 20 wt% SnO2 OIHC materials because of its overlapping with PANI peak at 616 cm−1 . With 40% SnO2 however the Sn O band has considerable intensity equal to the intensity of quinoid/benzenoid and with 60 and 80 wt% SnO2 the intensity increases proportionally. Consequently in PANI–80% SnO2 OIHC material the Sn O band is very broad similar to the one in pure SnO2 . All these changes have their impact on conductivity of OIHC materials. Such type of IR spectral shifts has been noticed in earlier works also [17,18,20]. The different trends in 10 wt% and 20–80 wt% of SnO2 with PANI in IR spectral shifts reflect in the conductivity values of OIHC materials (Table 2). The conductivity of pristine PANI is reduced continuously but not linearly as shown in Fig. 2 with increase in SnO2 content in 20–80 wt% range, but it takes up an opposite trend, (i.e., an increase) with 10 wt% SnO2 . As we thought that this should not be an experimental error, we measured the conductivity at several times with repeatedly synthesized samples. The same trend was observed every time not only with fresh samples but also with benzene/toluene adsorbed samples (Table 2). The decrease in conductivity of PANI by SnO2 is quite understandable and logically acceptable by the very known fact that SnO2 at room temperature is an insulator and its addition to PANI conductor will definitely have a negative effect. In the light of this discussion, the nearly 9% increase in conductivity of PANI with 10 wt% SnO2 could have different origin only. As is evident in Fig. 1, the unequal or less than one quinoid to benzenoid peak intensity ratio in pristine PANI is made almost equal in PANI–10 wt% SnO2 and the sharp peak of N H vibration in benzenoid appearing at 1401 cm−1 in PANI disappears in composite. That means the number of quinoid (Q) group

has increased at the cost of benzenoid (B) in PANI–10 wt% SnO2 composite. The observed red-shift of quinoid ring stretching and the blue-shift of conductivity band with 10 wt% SnO2 clearly suggest bond weakening/electron displacement from quinoid and N H +

bond strengthening (by electron addition) in Q = NH − B group in PANI by SnO2 . This could happen only when SnO2 functions as a dopant with its oxygen negative pole or hydroxide group. In all other PANI–SnO2 composites, this dopant activity of SnO2 might have outweighed by n-type functionality in p–n junction formation with p-type PANI. 3.2. Structural characterization of the samples XRD patterns of PANI, SnO2 and PANI–SnO2 composites are shown in Fig. 3 and the data are given in Table 3. The reflections at 2 values 26.6, 33.9, 38.0 and 51.8o correspond, respectively to the planes (1 1 0), (1 0 1), (2 0 0) and (2 1 1) and reveal the presence of tetragonal SnO2 (JCPDS file no. 01-088-0287) [26]. There was no

Fig. 2. Plot of conductivity of materials versus wt% of SnO2 .

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Table 3 XRD data of PANI, SnO2 and PANI–40% SnO2 .

Fig. 3. XRD spectra of PANI, SnO2 and PANI–40% SnO2 .

shift of peak positions of SnO2 upon mixing with PANI confirming retain of crystal structure of SnO2 in the composite. However, the presence of PANI in the composite slightly (2.5 nm) enhanced the

Material

2-Theta (◦ )

FWHM (◦ )

d-Spacing (Å)

Crystallite size (nm)

PANI SnO2 PANI–40% SnO2

24.88 26.59 26.55

4.500 0.241 0.225

3.58 3.35 3.36

1.9 35.4 37.9

crystallite size of SnO2 . This might be due to the formation of PANI layer over SnO2 upon grinding. Fig. 4 presents the SEM images of PANI, SnO2 and their composite materials. PANI exhibits more or less uniform spherically shaped agglomerated secondary particles with nanobristles projection of primary particles on their surfaces. Fresh SnO2 has tiny spherical grains of submicron size/nanosize (∼100 nm) with a few agglomerations (Fig. 4B). PANI–20% SnO2 composite has a distinct morphology of flake-like structure (Fig. 4C). A few SnO2 grains can be seen as white dots embedded in the flakes. A strong interaction cementing PANI and SnO2 together has resulted in flakes morphology. With increase in loading of SnO2 to 40 and 60 wt% (Fig. 4D and E), the extent of flakes structure is reduced and more number of discrete clumps with many white dots/patches is formed. Thus with incorporation of more amount of SnO2 , the morphology of OIHC

Fig. 4. SEM images of (A) PANI, (B) SnO2 , (C) PANI–20% SnO2 , (D) PANI–40% SnO2 and (E) PANI–60% SnO2 .

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Table 4 Thermogravimetric analysis data of PANI–SnO2 compositesa . % Weight loss in the temperature range (◦ C)

Sample

PANI PANI–20% SnO2 PANI–40% SnO2 PANI–60% SnO2 SnO2 a

Residue at 800 ◦ C

<140

140–400

400–620

620–800

7.13 6.80 5.35 3.75 0.15

19.13 14.82 12.59 8.43 0.50

14.31 9.84 9.81 5.84 0.40

4.67 12.15 13.52 17.98 0.29

54.76 56.39 58.73 64.00 98.66

Analysis conditions: 30–800 ◦ C heating at the rate of 10 ◦ C/min; N2 atmosphere.

materials tend to become the morphology of pure SnO2 shown in Fig. 4B. 3.3. Thermal characterization of the samples TGA curves are depicted in Fig. 5 and the data are entered in Table 4. Pristine PANI exhibits a three-step weight loss, pure SnO2 almost no weight loss while PANI–SnO2 composites show four-step weight loss profiles. The first weight loss occurring below 140 ◦ C for all the materials is resulted from the elimination of absorbed moisture. The second weight loss occurring in the temperature range 140–400 ◦ C is due to the thermal elimination of dopant species like SO4 2− ion and low molecular weight polymer fragments. The third weight loss at 400–620 ◦ C is attributable to the thermal degradation of polyaniline backbone and the elimination of decomposed molecular fragments [27,28]. In all these three weight-loss steps, pristine PANI exhibits the maximum per cent of weight loss and the PANI–SnO2 composites show a gradual decrease with increase in wt% of SnO2 or correspondingly the decrease in polyaniline content. This decrement in weight-loss of PANI–SnO2 composites at each step of thermal decomposition is not in proportion to the polyaniline content; indeed, as is evident from the third step weight loss values, polyaniline undergoes thermal degradation only to a lesser level in its SnO2 composites than in its pristine form. This is because of the stabilization of PANI with SnO2 that originates from their mutual interaction [27]. Also even in pristine PANI, the thermal degradation at 400–620 ◦ C is not complete and a considerable portion (54.76%) remains as charcoal [28] at 800 ◦ C. These observations clearly suggest that PANI is stabilized by SnO2 in its composites up to 620 ◦ C owing to their mutual interaction. But the trend is reversed above 620 ◦ C at the fourth step. The percentage of weight loss at 620–800 ◦ C increases steadily with increase in SnO2 content. As there could be only weak/nil interaction between residual charcoal and SnO2 , charcoal elimination is favored to a

larger degree with SnO2 . In the case of PANI–60% SnO2 , the residue at 800 ◦ C is 64% which is close to the 60 wt% of SnO2 initially taken in the composite preparation. That means 36% of PANI has degraded up to 800 ◦ C and only 4% of PANI is still remaining as carbon residue. Such type of quantitative estimation of PANI content in other PANI–SnO2 composites is not possible because the remaining residue per cent at 800 ◦ C is far higher. The TGA studies thus illustrate that there is mutual interaction between PANI and SnO2 in their composites and consequently PANI is thermally stabilized at least up to 620 ◦ C. 3.4. Sensor behaviors of PANI, SnO2 and their composites The sensor functionality of the materials toward benzene and toluene analyte vapors was initially studied in normal air atmosphere, but it was only miniscule or insignificant. Perhaps the materials being polar/hydrophilic while the analyte being nonpolar vapors, moisture in air preferentially adsorbs on the surface of pellet materials leaving no room to the analyte vapors. Hence the sensor activities were investigated only in N2 atmosphere. Table 2 displays all the relevant data. PANI and its SnO2 composites upto 60 wt% SnO2 show sensor activities by reduction in conductivity upon interaction with analyte vapors of benzene and toluene (∼1650 ppm). PANI–80% SnO2 and pure SnO2 do not show sensor functionality, perhaps due to their very low/nil intrinsic conductivity. Pristine PANI also exhibits only very meager sensitivities toward benzene and toluene vapors. But its SnO2 composites show enhancement in sensor efficiencies. NCC values for both benzene and toluene vapors steadily increase with increase in wt% of SnO2 , reach the maximum at 40 wt% SnO2 and then decline. Also the materials show varying sensor response according to the analyte concentration. Representatively the data are shown for PANI–40% SnO2 in Table 5. The sensor activity studied in static method is rapid, reversible and regenerative for all the materials, because when the analyte vapor is replaced with pure N2 , the conductivities of the materials return to their original values. To confirm and to make this observation explicit, dynamic sensor study was made representatively for PANI–40% SnO2 composite. Fig. 6 illustrates the dynamic response–recovery curve toward benzene. It seems within 5 min of analyte vapor exposure (benzene), the material gets saturated and reaches its maximum level of sensing (increase in volt or decrease in conductivity); on benzene vapor off, the material returns to its original value within 10 min. The profile in cycle 1 is repeated in cycles 2 and 3 also, demonstrating reversibility, recovery and regeneration of sensor material. The sensitivity data of PANI–40% SnO2 at different benzene concentrations (Table 5) along with Fig. 6 clearly demonstrate Table 5 Sensitivity of PANI–40% SnO2 at different benzene concentrations with response time of 15 min.

Fig. 5. Thermogravimetric analysis profiles of PANI, SnO2 and their composites.

[Benzene]

NCC (%)

∼500 ∼800 ∼1650

1.79 3.25 9.82

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Fig. 7. Dependence of sensor efficiencies on dielectric constant of the materials. Fig. 6. Dynamic profile of PANI–40% SnO2 toward benzene at ∼1650 ppm.

the analyte-concentration-dependent sensor functionality and the fast response–recovery ability of PANI–SnO2 nanocomposite sensors. Pure SnO2 has nil while pristine PANI has a meager sensor activity. However, their composites have considerably higher values, the enhancement being 6.6 times for 40 wt% SnO2 composite. The remarkable improvement in sensor functionality of PANI–SnO2 composites has arisen undoubtedly from the mutual interaction between PANI and SnO2 components, which is definitely complementary and synergic. Knowledge of this interaction is essential in order to understand and appreciate the synergic role and the trend in sensor efficiencies of the composite materials. Applying the well-known general concept of p–n junction formation [19,29,30] to the present system, it is inferable that upon compositing PANI and SnO2 , electron flow occurs from n-SnO2 to p-PANI, saturates the holes (polaron and bipolaron sites in PANI) until establishment of equilibrium, raises the Fermi level and make it equal in n and p regions. Electron flow also occurs in the reverse direction, i.e., from PANI to SnO2 , of course, in minor level. The mutual interaction thus makes charge separation and produces p–n heterojunctions in PANI–SnO2 composites. The p–n heterojunctions formed in the PANI–SnO2 system could increase the depletion barrier height [30], thus leading to an improved response of the sensor as observed by Zhang et al. with polypyrrole–SnO2 system [19]. An assessment of such p–n heterojunction formation was made through a measure of dielectric constant values of the composites in AC impedance study. As seen in Table 2, the value increases with increase in wt% of SnO2 , attains maximum at 40 wt% and then declines. The dielectric constant of PANI is increased by 10 fold while that of SnO2 by 5 orders (i.e., 100,000 fold). As shown in Fig. 7, there is good correlation between dielectric constant values and sensor efficiencies (NCC%). Therefore, the resulting conclusion is that greater the number of p–n heterojunctions, greater is the sensor efficiency of the PANI–SnO2 composites. An intuitive picture regarding the sensing of analyte by composite at the micro-environmental level would be as follows. Benzene/toluene a primary aromatic hydrocarbon is basically an electron acceptor. When this analyte molecule associates, it draws electron from the composite material particularly from p-doped PANI in the junction region and provides friction/resistance to the electron movement in ␲ conjugate system of PANI. The material with greater number of p–n heterojunctions is the larger supplier of electrons and hence maximal sufferer in conductivity. The slightly greater sensing level of toluene compared to benzene results from methyl substituent which, being a non-polar sp3 group, provides a greater resistance to ␲ electron system in PANI chain.

Fig. 8. FT-IR spectra of fresh PANI–40% SnO2 (a) and its benzene-adsorbed form (b).

To have a chemical insight into the association of analyte molecule with PANI and its composite materials, FTIR spectra of benzene-vapor adsorbed samples were recorded (Fig. 8). No much shifting in peak positions was observed. However, the intensity of benzenoid band at 1460–80 cm−1 and peak at 1401–03 cm−1 increased; in addition, the C H out-of-plane bending band at ∼800 cm−1 also has slight intensity modification. These intensity changes are maximal with PANI–40% SnO2 composite and hence representatively its FTIR spectra in the fresh and benzene-adsorbed forms are displayed in Fig. 8. These spectral observations clearly suggest that the analyte associates preferentially with benzenoid unit, probably in stacking configuration and causes its electronic effect. A weak interaction of van der Waals type may exist delivering only a smaller degree of influence on sensor’s conductivity. Pure N2 carrier gas can easily wipe off the associated analyte and bring the sensor back to its original condition. 4. Conclusions Physical grinding of conducting PANI with various wt% of nSnO2 results in the formation of OIHC materials. Characterization by FTIR, XRD, SEM and TGA techniques reveal the mutual interaction of the two components in the composites. As a result, p–n heterojunctions are developed in the materials, which enhance many folds/orders the sensor activities of the composite materials relative to the insignificant/nil sensor efficiencies of the two individual

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