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Sensors and Actuators B 148 (2010) 41–48

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

oly(o-anisidine)–tin oxide nanocomposite: Synthesis, characterization andpplication to humidity sensing

ewyani Patil a, Pradip Patil a,∗, You-Kyong Seob, Young Kyu Hwangb

Department of Physics, North Maharashtra University, Jalgaon 425 001, IndiaKorea Research Institute of Chemical Technology, Jang-dong 100, Yuseong-Gu, 305-600 Daejon, South Korea

r t i c l e i n f o

rticle history:eceived 29 September 2009eceived in revised form 12 April 2010ccepted 28 April 2010

a b s t r a c t

Poly(o-anisidine)–tin oxide (POA–SnO2) nanocomposites have been synthesized for the first time throughan in situ chemical polymerization of o-anisidine in the presence of SnO2 nanoparticles of 25–40 nm indiameters. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and field emission

vailable online 6 May 2010

eywords:onducting polymersoly(o-anisidine)

scanning electron microscopy (FESEM) were used to characterize the POA–SnO2 nanocomposites. Thecharacterization results confirmed the polymerization of o-anisidine and the strong interaction betweenPOA and SnO2 nanoparticles. Humidity sensing characteristics such as relative humidity (RH)-impedanceproperty, humidity hysteresis, response and recovery times and reproducibility of POA–SnO2 have beeninvestigated. The nanocomposite with 50 wt% SnO2 exhibited better humidity sensing properties than

nsitiv

oly(o-anisidine)–SnO2 nanocompositesumidity sensing characteristics

pure POA, such as good seexcellent repeatability.

. Introduction

Conducting polymer–metal oxide nanocomposites formed byetal oxide nanoparticles dispersed in electrically conducting

olymers, such as polyaniline (PANI) or polyprrole (PPY), haveeceived much attention in the past few years [1]. These nanocom-osites are expected to display new properties over their singleomponents, making them potential candidates for applicationn several fields such as corrosion protection, catalysis, electrode

aterials for batteries, biosensors, memory devices, sensors.The importance of humidity sensing has been well understood

nd considerable attention has been focused on the development ofew and competent humidity sensitive materials for the fabricationf humidity sensors [2,3]. Various composites of conducting poly-ers such as PANI and PPY with inorganic oxide particles have been

ynthesized in recent years and based on these composites humid-ty sensors have also been developed by several research groups4–16]. However, the conducting polymers (PANI or PPY) and theiromposites with metal oxides exhibited unsatisfactory humidityensing characteristics such as low humidity response, very highmpedance (at middle and lower RH values) and relatively higher

esponse and recovery times. Therefore, the humidity sensing char-cteristics of the humidity sensors based on conducting polymersnd their composites need to be really improved.

∗ Corresponding author. Tel.: +91 2572257474; fax: +91 257 2258403.E-mail address: pnmu@yahoo.co.in (P. Patil).

925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2010.04.046

ity, fast response (∼87 s), rapid recovery (∼13 s), hysteresis within 4% and

© 2010 Elsevier B.V. All rights reserved.

Patil et al. [5,6] synthesized conducting polymer–WO3 com-posites and evaluated their performances as sensing materials inthe fabrication of humidity sensors. It was demonstrated that asimple mechanical mixing of the WO3 with conducting polymerspoly(o-anisidine) (POA) and poly(2,5-dimethoxyaniline) (PDMA)significantly improved their humidity sensing characteristics. ThePDMA–WO3 composite (with 30 wt% of WO3) exhibited betterhumidity sensing properties than POA–WO3 composite such ashigher humidity response ∼651% at 87% RH, response time (∼27 s),narrow hysteresis (∼5%), excellent repeatability and long-term sta-bility (for more than 30 days). However, it showed longer recoverytime ∼136 s as compared to that of the POA–WO3 composite. Veryrecently, an interesting work on the humidity sensors based on thePANI–BaTiO3 composites was carried out by Patil et al. [4]. Theyobserved the maximum humidity response (∼270% at 85% RH),quick response ∼8 and 31 s at humidification and desiccation steps,respectively and excellent reproducibility for the composite (with50 wt% of BaTiO3) synthesized at pH 1.2.

SnO2 is a celebrated n-type semiconductor oxide sensing mate-rial, which exhibits exceptional gas sensing property [17] as wellas humidity sensing behavior [18]. However, the resistance of theSnO2 is very high (∼1010 �) at lower RH values, which brings aboutgreat problems for measurement [19]. One of the solutions to thisproblem is to prepare the composite of SnO2 with the conducting

polymers such as PANI, so as to decrease the resistance at lower RHvalues. There are only three reports dealing with the compositesof PANI and PPY with SnO2. Geng et al. [7] synthesized PANI–SnO2hybrid material by a hydrothermal method and examined ethanoland acetone gas sensing properties. The synthesis of PPY–SnO2

4 Actuators B 148 (2010) 41–48

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2 D. Patil et al. / Sensors and

omposite by using chemical polymerization route for lithium-ionattery application was reported by Yuan et al. [8]. Dutta and De [9]repared PPY–SnO2 nanocomposite by the soft chemical route and

nvestigated the optical properties and current–voltage character-stics. However, hardly any attempt has been made to investigatehe humidity sensing characteristics of conducting polymer–SnO2omposites.

In the work reported in this paper, we have synthesized theanocomposite of SnO2 nanoparticles and POA by using an initu chemical polymerization route and used it as new humid-ty sensitive material. The o-anisidine is a substituted derivativef PANI with methoxy (–OCH3) group substituted at ortho posi-ions. The POA was chosen as an organic counterpart for this studyo explore the possibility of utilizing it as an alternative to PANIor humidity sensing application. To the best of our knowledge,o reports have been found in THE literature on the synthesis ofOA–SnO2 nanocomposites and the investigation of their humid-ty sensing characteristics. The humidity sensing characteristics ofhe nanocomposite were measured at room temperature and com-ared with those of pure POA. The improved humidity response ofhe nanocomposite was explained by considering the contributionsf both POA and the SnO2 nanoparticles.

. Experimental

.1. Materials

All reagents tin oxide (SnO2), ammonium persulphate(NH4)2S2O8) and hydrochloric acid (HCl) were purchasedrom Sigma, except for the monomer o-anisidine, which wasrocured from Fluka. All chemicals were of analytical grade andsed as-received except for o-anisidine which was doubly distilledrior to use.

.2. Synthesis of POA/SnO2 nanocomposite

The POA–SnO2 nanocomposites were synthesized by using ann situ chemical polymerization of o-anisidine with (NH4)2S2O8 asn oxidant in the presence of SnO2 nanoparticles of 25–40 nm iniameters. Initially, SnO2 nanoparticles were added to 1 M HCl andonicated for 2 h in order to keep them well dispersed and sus-ended in the solution. After sonication, o-anisidine was addedo this dispersion of SnO2. The 0.1 M (NH4)2S2O8 was then slowlydded dropwise to well-dispersed reaction mixture with continu-us stirring at 0–5 ◦C and the reaction was continued for another4 h. The good degree of polymerization is achieved after 24 h andhe suspension was dark green in color. The product was filterednd washed with deionized water and methanol, then dried underacuum at 50 ◦C for 24 h. Three kinds of POA–SnO2 nanocompos-tes were synthesized using 10, 30 and 50 wt% of SnO2 with respecto o-anisidine, which are referred as POA–SnO2-10, POA–SnO2-0 and POA–SnO2-50, respectively. For comparison, pure POA wasynthesized following the same procedure without SnO2 nanopar-icles.

.3. Characterization

The POA–SnO2 nanocomposites were characterized by using-ray diffraction (XRD), Fourier transform infrared (FTIR) spec-

roscopy and scanning electron microscopy (SEM). The XRD

easurements were performed with a Rigaku diffractometer

Miniflex Model, Rigaku, Japan) using Cu K� (� = 0.1542 nm) radia-ion over the 2� range from 1.5◦ to 80◦ at a scan rate of 2◦/min. TheTIR transmission spectra were recorded with a Nicolet FTIR spec-rometer (IMPACT 420 DSP) in the spectral range 4000–400 cm−1.

Fig. 1. XRD patterns of (a) POA, (b) SnO2 nanoparticles and (c) POA–SnO2-30nanocomposite.

The field emission scanning electron microscopy (FESEM) analysiswas carried out with a Hitachi (S-4800, Hitachi, Japan) system.

2.4. Humidity sensing study

The POA–SnO2 nanocomposite was pressed into pellet form ofdiameter ∼1 cm and thickness ∼0.1 cm for the humidity sensingstudy. The electrical contact leads were fixed 0.7 cm apart with thehelp of silver paste on the surface of the pellet. The impedance ofthe pellet was measured as a function of RH at an applied voltageof 1 V and frequency 100 Hz by using a simple two-probe configu-ration with a Impedance/Gain phase analyser (SI 1260, Solartron,UK) controlled by Solartron Materials Research and Test (SMaRT)software supplied by Solartron Mobrey Ltd., UK.

The continuous variation in humidity was achieved in a sim-ple experimental set-up fabricated in our laboratory in order toinvestigate the humidity sensing properties. The two-temperaturemethod is used to measure the relative humidity. The experimentalset-up mainly consisted of a closed flask (1000 ml) with two necksfor inserting thermometers and the sensing element (i.e. the pelletof the composite). The flask was partially filled with water and keptin a glass container. The sensing element along with the thermome-ter was mounted on the sensor holder. The sensor holder was keptinside the flask at a height of 6 cm from the surface of the water.The water from the cryostat was circulated continuously throughthe external glass container to vary the temperature of the waterinside the flask. As a result, the temperature of the water (Tw) insidethe flask changes and consequently, the change in the temperatureof the sensing element (Ts) is observed. The system was allowed toequilibrate at each temperature change for 5 min before noting theimpedance of the pellet. The relative humidity (RH) in percentageinside the flask was calculated by using the relation [2]:

%RH = Pw(Tw)Pw(Ts)

(1)

where Pw(Tw) and Pw(Ts) denote the saturated water vapor pres-sures at the temperatures of the water and the sensing element,respectively.

3. Results and discussion

3.1. XRD results

The XRD patterns of pure POA, SnO2 nanoparticles andPOA–SnO2-30 nanocomposite are shown in Fig. 1. The XRD pattern

Actuators B 148 (2010) 41–48 43

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D. Patil et al. / Sensors and

f pure POA [Fig. 1(a)] indicates the broad diffraction peaks at 2�alues of 7.96◦ and 24.98◦, which are attributed to amorphousature of POA. The XRD pattern of SnO2 nanoparticles [Fig. 1(b)]

ndicates the prominent diffraction peaks at 2� values of 25.72◦,5.30◦, 37.94◦, 43.50◦, 52.70◦, 57.66◦, 61.44◦, 66.66◦, 68.32◦ and7.0◦, which are attributed to the typical tetragonal type of SnO2JCPDS data card 71-0652). The crystallite size was estimated bysing the Scherrer formula:

= k�

B cos �(2)

here t is the average size of the crystallite, assuming that therains are spherical, k is 0.9, � is the wavelength of X-ray radia-ion, B is the peak full width at half maximum (FWHM) and � is thengle of diffraction. The crystalline size of pure SnO2 nanoparticless found to be ∼26.60 nm.

As expected, the XRD pattern of the POA–SnO2 nanocompos-te [Fig. 1(c)] exhibits the characteristic diffraction peaks of thenO2 and the broad diffraction peaks at 2� values of 7.96◦ and4.68◦ due to POA. It is important to note that the XRD pat-ern of the POA–SnO2 nanocomposite retains the characteristiciffraction peaks due to SnO2. This indicates that the crystallinetructure of SnO2 is preserved in the nanocomposite even thought was dispersed in reaction mixture during polymerization process.

oreover, the intensities of the diffraction peaks for the nanocom-osite are weaker than those for the pure SnO2 nanoparticles. Thisuggests that there is a kind of interaction between POA and SnO2anoparticles, which resulted in the reduced intensities of diffrac-ion peaks of SnO2 nanoparticles in the nanocomposite.

.2. FTIR spectroscopy results

The FTIR spectra of SnO2 nanoparticles, pure POA andOA–SnO2-30 nanocomposite are shown in Fig. 2. The FTIR spec-rum of the SnO2 nanoparticles [Fig. 2(a)] exhibits a broad bandentered at 600 cm−1 a characteristic of Sn–O vibration and thether at ∼450 cm−1 due to Sn–OH vibration [20]. In addition,broad and weak band at ∼3460 cm−1 is observed which is

ttributed to the O–H stretching vibrations. The FTIR spectrumf POA [Fig. 2(b)] exhibits the following spectral features [5]: (i)broad and weak band at ∼3436 cm−1 due to the characteristic–H stretching vibration suggests the presence of –NH– groups

n o-anisidine units; (ii) the band at ∼1568 cm−1 is an indica-ive of stretching vibrations in quinoid (Q) rings; (iii) the band1490 cm−1 represents the stretching vibrations of the benzoid

B) rings; (iv) the bands at ∼1254 and 1014 cm−1 are assignedo the presence of an o-methoxy group in the POA; (v) a bandt ∼1123 cm−1 is attributed to a plane bending vibration of C–H,hich is formed during protonation; (vi) a band at ∼1207 cm−1 is

onsidered as a measure of the degree of delocalization of elec-rons on POA and it is referred to as the electronic like band. It is aharacteristic peak of emeraldine salt phase of POA and (vii) a bandt ∼805 cm−1 reveals the presence of ortho-substituted benzeneing. As expected, the spectrum of the POA–SnO2-30 nanocom-osite [Fig. 2(c)] clearly exhibits bands attributable to both POA3434, 1580, 1489, 1286, 1207, 1123 and 1014 cm−1) and SnO2anoparticles (590 and 450 cm−1). In FTIR spectra of POA andOA–SnO2-30 nanocomposite similar bands are observed between00 and 4000 cm−1 indicating that the main constituents of poly-er and its nanocomposite have the same chemical structures.owever, the incorporation of SnO2 nanoparticles results into the

hift of FTIR bands of POA to higher wavenumbers. This indicateshat there is some interaction between POA and SnO2 nanoparti-les. Thus, the FTIR spectroscopy result confirms that the POA–SnO2anocomposite has been formed during the chemical polymeriza-ion of o-anisidine in the presence of SnO2 nanoparticles.

Fig. 2. FTIR spectra of (a) SnO2 nanoparticles, (b) POA and (c) POA–SnO2-30nanocomposite.

3.3. Surface morphology

The FESEM images of the SnO2 nanoparticles and POA–SnO2-30 nanocomposite are shown in Fig. 3(a) and (b), respectively.The SnO2 nanoparticles exhibit a granular structure, while themorphology of the POA–SnO2-30 nanocomposite is characterizedby the presence of large globules. The granular POA deposits areclearly visible at the surface of SnO2, probably due to the secondarynucleation of the POA growth on the already existing POA. The pre-cipitation of the POA in the composite has not been observed whichindicates that the growth of POA at the surface of SnO2 nanopar-ticles is favoured rather than the polymerization process in thereaction mixture. Thus, the results of the XRD, FTIR and FESEMstudies have provided clear evidence that the polymerization ofo-anisidine has been successfully achieved on the surface of theSnO2 nanoparticles.

3.4. Polymerization mechanism

When the SnO2 nanoparticles are introduced into the reactionmixture, the surfaces of individual nanoparticles act as template

areas for the growth of POA. Due to the polymerization in theacidic medium, the surface of the SnO2 is positively charged andtherefore, adsorption of anions such as Cl− may compensate thepositive charges on the SnO2 surface. It is assumed that o-anisidineoligomers are produced at first and because of their reduced

44 D. Patil et al. / Sensors and Actuators B 148 (2010) 41–48

and (b

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Fig. 3. FESEM images of (a) SnO2

olubility in water, they adsorb at the available SnO2 surfaces. Asconsequence, the interactions between adsorbed anions and o-

nisidine oligomers initiate the growth of POA chains on the SnO2urface. It is known that the polymerization of aniline and its substi-uted derivatives is auto-accelerated process [21]. That is, the POAormation is preferred in the region where some POA has alreadyeen produced. This leads to a formation of the POA layer on theurface of the SnO2. The schematic diagram for the formation of theOA–SnO2 nanocomposite is shown in Fig. 4.

.5. Humidity sensing performance

.5.1. Impedance—RH characteristics

The dependence of impedance on RH for SnO2 nanoparti-

les measured at 1 V and the frequency 100 Hz is shown inig. 5. The impedance of SnO2 nanoparticles decreased expo-entially from 6.1 × 106 to 1.0 × 106 � for humidity change from0% to 100% RH. The dashed line shows the exponential fit

) POA–SnO2-30 nanocomposite.

(y = 7.3 × 104 + 9.5 × 106 × e(−x/42.04), R2 = 0.99, where x, y and R2

represent the %RH, resistance and correlation coefficient, respec-tively.) to the experimental data, illustrating clearly good qualityof the fit. The impedance was as high as 6.1 × 106 � at 20% RH andbecame too high to be measured at even lower humidity. Hence,SnO2 nanoparticles cannot be used to detect very low humidity.

The dependence of impedance on RH for pure POA andnanocomposites POA–SnO2-10, POA–SnO2-30 and POA–SnO2-50measured at 1 V and the frequency 100 Hz is shown in Fig. 6. Thedecrease in the impedance of both POA and its nanocompositeswith SnO2 with an increase in RH can be attributed to the absorp-tion of water. This implies that POA and its nanocomposites withSnO2 are humidity sensitive. The impedance of pure POA [Fig. 6(a)]

5 3

decreased exponentially from 1.4 × 10 to 4.1 × 10 � for humid-ity change from 20% to 100% RH. Thus, the impedance changedapproximately by two orders of magnitude over the RH range of20–100% RH, showing good sensitivity to humidity. The dashedline shows the exponential fit (y = 43.18 × 104 × e(−x/18.39), R2 = 0.99)

D. Patil et al. / Sensors and Actuators B 148 (2010) 41–48 45

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ig. 4. Schematic diagram of the formation mechanism of POA and POA–SnO2

anocomposite.

o the experimental data, illustrating clearly good quality of thet.

When the nanocomposite having 10 wt% of SnO2 (i.e.OA–SnO2-10) was exposed to different RH values, the variation ofhe humidity response [Fig. 6(b)] with RH is qualitatively similar tohat observed in the case of pure POA. The impedance of POA–SnO2-0 is about 2.0 × 105 � in 20% RH air, which is 7.7 times than that2.6 × 103 �) in 100% RH air, showing good sensitivity to humidity.

t is found that the response in the humidity range 20–100% RH cane empirically represented as y = −2.9 × 104 + 3.2 × 105 × e(−x/57.51),2 = 0.9985. The dashed line shows the exponential fit to the exper-

mental data, illustrating clearly good quality of the fit.

ig. 5. Dependence of impedance on RH for SnO2 nanoparticles measured at 1 V,00 Hz. The dashed line represents the exponential fit to the experimental data.

Fig. 6. Dependence of impedance on RH for (a) pure POA and nanocomposites, (b)POA–SnO2-10, (c) POA–SnO2-30 and (d) POA–SnO2-50 measured at 1 V, 100 Hz. Thedashed lines represent the theoretical fit to the experimental data.

When the concentration of SnO2 in the nanocomposite isincreased to 30 wt%, the impedance of the nanocomposite [Fig. 6(c)]decreased systematically and exponentially with increasing RH.The impedance is about 3.6 × 105 � in 20% RH air, which is∼5.7 times than that (6.3 × 104 �) in 100% RH air. The dashedline shows the exponential fit (y = 2.5 × 104 + 5.9 × 105 × e(−x/35.58),R2 = 0.9980) to the experimental data, illustrating clearly good qual-ity of the fit.

It is interesting to observe that the humidity response of thePOA–SnO2-50 nanocomposite [Fig. 6(d)] increases linearly overthe whole humidity range (i.e. 20–100% RH) used in the presentexperiment. This may be probably due to the presence of higheramount of SnO2 (i.e. 50 wt%) in the nanocomposite, which alsointeracts with water vapor. The impedance is about 7.6 × 105 � in20% RH air, which is ∼10.6 times than that (7.2 × 104 �) in 100%RH air, showing very good sensitivity and good linearity. The lin-ear fit (y = 9.8 × 105 − 9.1 × 103 × x, R2 = 0.9992) to the experimentaldata indicates good quality of the fit. It is important to note thatthe impedance of the POA–SnO2-50 nanocomposite is as low as7.6 × 105 � at 20% RH and thus, the problem of high impedancemeasurement can be solved. The linearity of the humidity responsein the humidity range (20–100% RH) suggests that the POA–SnO2-50 nanocomposite can be reliably used to monitor the RH over thisrange. Therefore, the POA–SnO2-50 nanocomposite for its best lin-earity and good sensitivity was chosen in further study to evaluatethe hysteresis, response and recovery times and reproducibility.

3.5.2. HysteresisHysteresis is an important parameter for evaluating the per-

formance of a humidity sensor. The hysteresis curves for POA andPOA–SnO2-50 nanocomposite were obtained by measuring theresistances as a function of RH for the high (100% RH) – low (20% RH)– high (100% RH) cycle and the corresponding hysteresis curves arepresented in Fig. 7. Pure POA exhibited a relatively wide hystere-sis loop [Fig. 7(a)], which indicates that the regeneration processis slower. It is seen that the differences in resistance values of thenanocomposite [Fig. 7(b)] for low (20% RH) − high (100% RH) (i.e.humidification process) and high (100% RH) − low (20% RH) (i.e.

desiccation process) are within 4%, which indicates a good reli-ability of POA–SnO2-50 nanocomposite. These results reveal thatthe presence of SnO2 significantly improves the humidity sensingcharacteristics of pure POA.

46 D. Patil et al. / Sensors and Actuators B 148 (2010) 41–48

Fm

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ig. 7. Humidity hysteresis for (a) pure POA and (b) POA–SnO2-50 nanocompositeeasured at 1 V, 100 Hz.

.5.3. Response and recovery timesIn order to utilize the POA–SnO2 nanocomposite as a humid-

ty sensor, it is important to know the rate of response to theariation of RH. The impedances of SnO2 nanoparticles, pure POAnd POA–SnO2-50 nanocomposite were monitored at two extremeumid atmospheres (20% and 100% RH). The sensor was trans-

erred from a chamber (20% RH) to another chamber (100% RH) andhen transferred back. The resulting response and recovery charac-eristics are shown in Fig. 8. Although, the sensing element wasxposed to the surrounding air (i.e. laboratory environment) dur-ng the transfer process, the transfer time (∼1–2 s) is much smallerhan the response and recovery times of the sensor and therefore,he correctness of this experiment is acceptable. According to litera-ure, the time taken by a sensor to achieve 90% of the total resistancehange is defined as response time in the case of adsorption or theecovery time in the case of desorption. The SnO2 nanoparticlesxhibited the response and recovery times of ∼99 and 110 s at theumidification and desiccation steps, respectively. It can be seenhat pure POA has the response and recovery times of ∼221 and65 s at the humidification and desiccation steps, respectively. By

ontrast, the POA–SnO2-50 nanocomposite exhibited fast responseor both humidification and desiccation processes than those ofure POA and SnO2 nanoparticles. The response and recovery timesor the POA–SnO2-50 nanocomposite were found to be of ∼87 and3 s at the humidification and desiccation steps, respectively.

Fig. 8. Response and recovery characteristics of (a) SnO2 nanoparticles, (b) pure POAand (c) POA–SnO2-50 nanocomposite measured at 1 V, 100 Hz.

3.5.4. Reproducibility and reversibilityThe reproducibility and reversibility are important parameters

to consider when evaluating the performance of a sensor. Theimpedance of SnO2 nanoparticles, pure POA and POA–SnO2-50nanocomposite were measured by exposing them repeatedly to

20% RH and then to 100% RH atmospheres to examine the repro-ducibility and reversibility. The measurements were repeated forfour cycles and the resulting response and recovery characteristicsare shown in Fig. 9. The asymmetric variation in impedance of pure

D. Patil et al. / Sensors and Actua

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ig. 9. Repetitive response of (a) SnO2 nanoparticles, (b) pure POA and (c)OA–SnO2-50 nanocomposite when exposed to four high (100% RH)-low (20% RH)-igh (100% RH) cycles.

OA [Fig. 9(a)] is observed when RH was changed between 20% and00%. This suggests that the humidity sensing process is partiallyeversible, which substantiates the hysteresis observed betweenhe humidification and desiccation for pure POA.

It was seen that the impedance values of the SnO2 nanoparti-les [Fig. 9(b)] and POA–SnO2-50 nanocomposite [Fig. 9(c)] revertlways to the original one when RH is restored to the former state,hich indicates that the humidity sensing process is extremely

eversible. The response and recovery times does not change during

tors B 148 (2010) 41–48 47

the four cycles of measurements, indicating a good reproducibil-ity of the humidity response. Thus, the SnO2 nanoparticles andPOA–SnO2-50 nanocomposite exhibited good stability as well asan excellent reproducibility of the response. This suggests that thePOA–SnO2-50 nanocomposite can be used as a reusable sensingmaterial for humidity.

3.6. Humidity sensing mechanism

The results of humidity measurement experiments demonstratethat an excellent humidity sensing characteristics can be achievedby synthesizing a nanocomposite of POA and SnO2 nanoparticles.It was observed that the presence of SnO2 in the nanocompositesignificantly improves the humidity sensing performance of purePOA and SnO2 nanoparticles as well. The whole range of humidityi.e. 20–100% RH used in the present experiment was observed tobe the operational range for the nanocomposites. When the similarexperiment was performed using only pure POA, the impedanceis about 1.4 × 105 � in 20% RH air, which is ∼34 times than that(4.1 × 103 �) in 100% RH air, indicating that pure POA was itselfcontributing to the humidity sensing in this range. The impedanceof SnO2 nanoparticles is about 6.1 × 106� in 20% RH air, which is∼6.1 times than that (1.0 × 106 �) in 100% RH air. The linearity ofthe humidity response, small hysteresis, relatively fast response(∼87 s), rapid recovery (∼13 s) and excellent repeatability suggeststhat the POA–SnO2-50 nanocomposite exhibited better humiditysensing properties than those of pure POA and SnO2 nanoparticles.

The response of the nanocomposite to RH is due to the inter-action of water molecules with both the POA as well as SnO2. Asdiscussed earlier, the layer of the POA was deposited on the surfaceof the SnO2 nanoparticles during the formation of nanocomposite.Consequently, when the nanocomposite was exposed to the differ-ent RH values, initially the water molecules are adsorbed on thesurface of the POA and due to its hydrophilic character it hold theadsorbed water in the matrix.

The interaction of POA with water occurs via the protonexchange mechanism as depicted below, which is in accordancewith that reported by Jain et al. [22] for PANI. The POA is com-paratively less compact as compared to that of nanocomposites.Consequently, when the POA is exposed to the humid atmospherethe water molecules are easily adsorbed onto the POA particles andit quickly becomes saturated with water content.

Alternatively, the interaction of water molecules at the SnO2surface takes place via the electron-donating adsorption [23]. Theadsorption of the water molecules on SnO2 surface takes place viadissociative chemisorption process which results into the forma-tion of hydroxyl groups at the surface [16]. The overall dissociativechemisorption process can be described in two step process asgiven below: (a) the water molecules absorbed on the SnO2 grainsurface reacts with tin lattice as

H2O + Oo + SnSn � 2OH − SnSn + Vo•• + 2e′

where Oo represents the lattice oxygen at the oxygen site and V••o

is the vacancy created at the oxygen site according to the reaction:

Oo � O2− + V ••o

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8 D. Patil et al. / Sensors and

(b) The doubly ionized oxygen, displaced from the lattice, reactsith the H+ coming from the dissociation of water molecules to

orm the hydroxyl group as given below:

+ + O2− � OH−

As a result of this reaction, the electrons are accumulated at thenO2 surface and consequently, the impedance of SnO2 decreasesith increase in the RH. Thus, the harmony between the electron-onating adsorption of water molecules at the SnO2 surface and

nteraction of the water molecules with POA via proton exchangeechanism results into the observed humidity sensing behavior of

he POA–SnO2 nanocomposites.At low RH range, only a few water molecules were adsorbed

ue to the compact nature of the nanocomposites and as a resulthe coverage of the water on the surface was not continuous. Dueo the discontinuous water layer, the transfer of H+ and H3O+ is soifficult that POA exhibits higher impedance at lower RH. At theame time, the interaction of water with SnO2 is also very weakue to the adsorption of a few water molecules. This results intohe higher impedance of the nanocomposite as compared to that ofure POA at low RH.

When the RH was increased to middle region, one or severalerial water layers were formed which accelerate the transfer of+ or H3O+. The quick transfer of ions on the water layer results

n a rapid decrease in the impedance of the POA. Furthermore, dueo the higher adsorption of water molecules on the SnO2 surfacet middle and higher RH, more electrons will be accumulated athe SnO2 surface. These combined reactions hence result in a rapidecrease in the impedance of the composites at middle and higherH.

. Conclusions

In this work, the POA–SnO2 nanocomposites exhibiting humid-ty sensing properties have been successfully synthesized by usingn in situ chemical polymerization route. The results of the XRD,TIR and SEM studies reveal that the polymerization of o-anisidineas been successfully achieved on the surface of the SnO2 nanopar-icles and indicates that there is some interaction between POAnd SnO2 nanoparticles. The humidity sensing characteristics of theOA–SnO2 nanocomposites are observed to be better than those ofure POA and SnO2 nanoparticles. The nanocomposite with 50 wt%f SnO2 exhibits humidity sensing properties such as good sen-itivity, fast response (∼87 s), rapid recovery (∼13 s), hysteresisithin 4% and excellent repeatability. Due to the fact that it has an

xcellent humidity sensing characteristics and can be synthesizedasily, the POA–SnO2 nanocomposite would be an ideal candidateor application in humidity sensors.

cknowledgements

The financial support from Board of Research in Nuclear Sci-nce (BRNS), Department of Atomic Energy (DAE), Mumbai, Indiahrough the major research project No.2008/37/44/BRNS is grate-ully acknowledged. We also thank Mr. T. Sai Kamaraju and Mr. B.V.holkar, Forevision Instruments (India) Pvt. Ltd. for FESEM analy-is of the samples. This work is partially supported by the Nationalesearch Foundation of Korea (NRF) funded by the Ministry of Edu-ation, Science and Technology (KN-0942).

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Biographies

Dewyani Patil is a research scholar in Department of Physics, North MaharashtraUniveristy, Jalgaon, India, doing research work in the field of conducting polymers.She has obtained master’s degree (M.Sc.) in electronics from North MaharashtraUniversity, Jalgaon, India in 2004. She is currently pursuing a Ph.D. in physics. Herresearch interests include conducting polymers and conducting polymer compositesfor chemical and biological sensors.

Pradip Patil is working as professor in physics at Department of Physics, NorthMaharashtra University, Jalgaon, India. He received his master’s degree (M.Sc.) inphysics in 1983 and Ph.D. degree in 1988 from the University of Pune, Pune, India.He joined North Maharashtra University, Jalgaon, India as the Head, Departmentof Physics since its inception. His main interests include development of conduct-ing polymers and conducting polymer nanocomposites for their applications ascorrosive protective coatings, chemical and biological sensors.

You-Kyong Seo received his B.S. and M.S. in 2004 and 2006, respectively, in chem-istry from Inje University. He is currently pursuing a Ph.D. in Chemistry at KoreaResearch Institute of Chemical Technology (KRICT), South Korea. His research workis focused on development of nanoporous materials, sensors and mesoporous mate-rials.

Young Kyu Hwang received his B.E., M.S. and Ph.D. degrees in 1995, 1997 and 2003,respectively in chemistry from Sungkyunkwan University. He has been a seniorscientist at Korea Research Institute of Chemical Technology since 2003. His currentresearch work is focused on development of new types of nanoporous materials,thin films, and inorganic functional materials as well as gas sensors.