Polypyrrole / Metal Sulphide Hybrid Nanocomposites : Synthesis , Characterization and Room Temperature Gas Sensing Properties

The present research work explores the room temperature gas sensing ability of hybrid nanocomposites of metal sulphide nanoparticles with polypyrrole (PPy) matrix. Sonochemically synthesized both silver sulphide (Ag2S) and cadmium sulphide (CdS) nanoparticles were mixed in varying ratios with PPy matrix by in-situ oxidative polymerization route. The resulting PPy/(Ag2S:CdS) hybrid nanocomposites were analyzed by characterizing them using FTIR and UV-Visible spectroscopy, XRD, SEM, fourprobe conductivity method and gas sensing ability by two-probe gas sensing setup. The current-voltage (I-V) characteristics of the PPy hybrid nanocomposites revealed their semiconducting nature with an electrical conductivity in the range of 10−4 to 10−3 S/cm. As compared to pure PPy, their hybrid PPy/ (Ag2S:CdS) nanocomposites showed an enhancement in sensitivity towards ammonia gas detection. The results clearly indicated a synergistic effect between the Ag2S:CdS nanoparticles and the PPy matrix.


Introduction
][3][4][5][6][7] Over the decade, there has been a steady progress in the applications of conjugated conducting polymers (CPs) such as in chemical sensors, electrochemical supercapacitors, electro chromic and photovoltaic devices, light-emitting diodes and batteries.][7][8][9][10][11][12] The physical and structural attributes of CPs are generally affected by crucial parameters such as polymerization method, type of the oxidant, pH and temperature conditions during polymerization, etc.In recent times, however, tuning of CPs' properties has also been attempted by use of nanomaterials.3][14] Using nanotechnology, various macromolecules and nanomaterials have been used for gas sensing purposes 15,16 and amongst them metal oxides 17,18 and CPs 19 are most widely used.A silver nanocrystal has been reported as a promoter for carbon nanotube-based gas sensors at room-temperature. 20Silver sulphide (Ag 2 S) is semiconducting filler, which can be used in preparation of photo electrochemical devices and solar cells.Polymers encapsulated with metal nanocrystal can be used as a promoter for room-temperature gas sensors. 1121][22][23] Bulk production of PPy is also possible by oxidative polymerization of pyrrole in aqueous or non-aqueous media.2][23] Various metals, metal oxides and metal sulphide nanoparticles have been incorporated in PPy matrix and the resulting CP nanocomposites showed an improvement in their properties.][26] Amongst the various toxic gases, ammonia (NH 3 ) gas is particularly lethal for humans as well as land and aquatic animals over 500 ppm concentration.NH 3 is commonly present in fertilizers, household cleaners and in compressors of air conditioner.It is also an important component of industrial production of synthetic polymers and explosives.It is highly toxic with a permissible exposure limit of 25 ppm for human beings and forms an explosive mixture with air at ~16-25%.These reasons make necessary the detection of NH 3 gas at room temperature.Researchers have also developed sensors based on various polymers, metal sulphides 7 , metal oxides [26][27][28] and their composites 29 , which can be used to detect NH 3 gas.But, it is more important to develop sensors that should be cheap and small in size and can be operated at room temperature.
In this study, we investigate the possibility of PPy/silver sulphide: cadmium sulphide nanoparticles (Ag 2 S:CdS) based hybrid nanocomposites as gas sensors for detection of ammonia (NH 3 ) gas.The PPy/(Ag 2 S:CdS) hybrid nanocomposites were characterized using FTIR and UV-Visible spectroscopy, XRD and SEM.We also report the effect of composition (varying

Materials
Analytical grade pyrrole and anhydrous ferric chloride (FeCl 3 ) were purchased from Merck (Mumbai, India).Sodium lauryl sulphate (SLS) and silver nitrate (AgNO 3 ) were received from S.D. Fine Chem.Ltd. (Mumbai, India).Cadmium nitrate (CdNO 3 ) was received from Loba Chemie Ltd. (Mumbai, India), while sodium sulphide (Na 2 S) was received from Himedia Laboratories Pvt. Ltd. (Mumbai, India).Ultra-pure water was prepared by Smart 2 Pure system (Thermo Electron LED GmbH, Germany) and then used for preparing solutions, cleaning as well as washing purposes.Acetone and methanol were received from S.D. Fine Chem.Ltd. (Mumbai, India) and were used for washing purposes.Ammonia and LPG (99.9% purity) gases were purchased from Master Specialty Gases Pvt. Ltd. (Mumbai, India).Microscope glass slides were purchased from the Polar India Corporation, (Mumbai, India) and used for deposition of films of PPy and its hybrid nanocomposites.

Sonochemical synthesis of metal sulphide nanoparticles
Sonochemical synthesis of both Ag 2 S and CdS nanoparticles was carried out as per our previous work. 7In brief, aqueous solution of Na 2 S was added drop-wise to an aqueous solution of AgNO 3 or Cd(NO 3 ) 2 under ultrasound (BO3 Ultrasonic Processor UP1200, Cromtech, India) at room temperature (30 ± 2 °C).Both metal sulphide nanoparticles were obtained in the precipitate form at the end of the reaction, which were centrifuged at 8000 rpm, washed with acetone and then finally with water.These metal sulphide nanoparticles were then allowed to dry in air at temperature of 60 °C.During the reactions involved in the sonochemical synthesis, the metal nitrates get converted into metal sulphides.

Synthesis of PPy and their nanocomposites
Films of PPy were prepared by oxidative polymerization (as per our previous work 7 ) using FeCl 3 as the oxidant.A solution of 0.2M of pyrrole and 0.466 M of FeCl 3 was prepared in 100 mL distilled water by keeping monomer to oxidant (FeCl 3 to pyrrole) molar ratio to be (1:2.33).A solution of 0.2M pyrrole was stirred continuously on magnetic stirrer at 400 rpm and simultaneously 0.466M FeCl 3 was added drop wise by keeping rate of one drop per second, simultaneously induction time was noted.The reaction was allowed to proceed for 2 h and then left to stand overnight.Films of PPy were deposited on microscope glass slides during the polymerization of pyrrole.At the end of polymerization reaction, the glass slides were taken out of the reaction mixture, sequentially washed with water and ethanol and then dried under vacuum.The black precipitate of PPy was repeatedly washed with water and acetone and then dried at room temperature.
For the preparation of PPy hybrid nanocomposites, predetermined weight ratios of Ag 2 S and CdS nanoparticles were dispersed into the pyrrole solution by ultrasound treatment and then pyrrole was polymerized as per the procedure mentioned above.Different compositions of PPy/ (Ag 2 S:CdS) nanocomposites were prepared and deposited on glass slides, wherein the Ag 2 S:CdS nanoparticles were taken in different ratios, viz., 1:1, 2:1 and 1:2, and are henceforth labelled as AC1, AC2 and AC3, respectively.The glass slides for thin film deposition of PPy and its hybrid nanocomposites were cleaned.These slides were cleaned by completely immersing them in 0.1M HCl solution for 1 h and then successively sonicated in water bath and methanol for 15 min each followed by vacuum drying.

Preparation of pellets
To measure electrical conductivity and gas sensing performance, the pellets (diameter of 10 mm and 2 mm thick) of PPy and its hybrid nanocomposites were prepared in a Hydraulic Press Machine, Kimaya Engineering (Mumbai, India).A 0.2 mg of sample was taken between two die plates and pressurized by the hydraulic press to obtain pellets having a diameter of 15 mm.

Characterizations
Powder form of pristine PPy and PPy/(Ag 2 S:CdS) hybrid nanocomposites were prepared by mixing with potassium bromide (KBr) and their FTIR spectra was recorded on Shimadzu FTIR-8400 spectrophotometer (Tokyo, Japan) within the wavenumber range of 400-4000 cm −1 .UV-visible absorption spectra of pristine PPy and PPy/(Ag 2 S:CdS) nanocomposites taken in dimethyl formamide (DMF) on a Hitachi U-2900 spectrophotometer (Tokyo, Japan) in the range of 200-800 nm.Blank DMF (without reactants) was used as reference in UV spectrophotometer.The excitation wavelength was 800 nm and slit width was taken as 10 nm for excitation and emission.UV spectra were obtained from the quartz cuvettes filled solution of different PPy hybrid nanocomposites and DMF.X-ray diffraction (XRD) analysis of pristine PPy and PPy/(Ag 2 S:CdS) nanocomposites (AC1, AC2 and AC3) were carried out by Advance X-ray diffractometer (Brukers D8, Germany) with CuKα1 radiation (λ = 1.5404Å).The surface morphology of PPy and PPy/ (Ag 2 S:CdS) nanocomposites films was observed in a field emission scanning electron microscope (Hitachi S-4800, Tokyo, Japan) by mounting samples on a specimen stub.The electrical conductivity of various composites was measured by four-point probe conductivity method, using a current source (CCS-01) and a micro voltmeter (DMV-001, SES Instruments, Scientific Equipments, Roorkee, India).The resistance of the nanocomposite sensors was measured with a programmable 4.5 digital multimeter (SM 5015, Scientific MES-Technik Private Limited, Indore, India) connected to a computer (QT035AV, Hewlett Packard, Banglore, India) using a RS232C interface that was fully automated and logged by a program.Current-Voltage (I-V) characteristics of the all pellets were recorded in the bias range of 0.15 to −0.15 V and the electrical conductivity of the films was deduced from the plot of Current-Voltage (I-V) graphs.Pellets of PPy and PPy/(Ag 2 S:CdS) nanocomposites (AC1, AC2 and AC3) were loaded into a sensing chamber (Figure 1) and two conducting probes were placed in contact with the pellets.The sensor response was measured from current-time characteristics of each sample recorded first in ambient air and then in different concentration (100, 250 and 500 ppm) of NH 3 and LPG gases.All electrical data were recorded at room temperature (30 ± 2 o C).The response factor of the sensor was measured from the change in electrical current observed in presence and absence of testing gas using the following Eq.( 1), in which, I a is current measured in the presence of air and I g is current measured in the presence of testing gas.cation.This is followed by dimerization of the radical cations with expulsion of two protons.The chain propagation occurs through the addition of a new radical cation.The preparation of nanocomposites, as shown in Figure 2, involves the attachment of the hydrophobic oligomeric units of pyrrole on the nanoparticle surface.These nanoparticles serve as a nucleation site for the polymer.As polymerization proceeds, the both nanoparticles get embedded into the polymer matrix resulting in the formation of hybrid nanocomposites.The use of ultrasound facilitates uniform dispersion of the nanoparticles in the monomer solution during polymerization, which is crucial for the better performance of a nanocomposite.

Oxidative polymerization mechanism
PPy and PPy/(Ag 2 S:CdS) nanocomposites were prepared (as per our previous work 7 ) by an oxidative polymerization method at room temperature.The first step in the polymerization reaction is the oxidation of Fe 3+ ion to give pyrrole radical

FTIR spectra
Figure 3 shows the FTIR spectra of PPy and PPy/ (Ag 2 S:CdS) nanocomposites.A peak near around 467 cm -1 in the spectra of the all PPy nanocomposites is associated with Ag 2 S nanoparticles.The spectra of PPy hybrid nanocomposites (AC1, AC2 and AC3) show slightly shifted peaks: band at 1043 cm -1 shifted to 1045 cm -1 due to the N-H in-plane deformation 10,12 caused by interaction between Ag 2 S:CdS nanoparticles and PPy matrix.The peak at 1375 cm -1 is related to the asymmetric C-H stretching in the PPy ring, while 778 cm -1 is due to the plane deformation of C-N in PPy 9 .The characteristic peaks for pristine PPy can be observed at 1542 and 1466 cm -1 that are related to the fundamental vibrations of pyrrole ring. 7,12The peak at 2928 cm -1 associated with C-H vibrations.The broad region at about 3400 cm -1 refers to vibrations of N-H stretching. 10,12These results indicated that the extent of oxidation of PPy increased by increasing the molar ratio of the oxidant and the monomer.

UV spectra
The UV-visible spectra of PPy and PPy/(Ag 2 S:CdS) nanocomposites is shown in  observed at ~ 273-275 nm in AC1 and AC3 nanocomposites.
The shift in peaks in the spectrum indicates interaction of Ag 2 S and CdS nanoparticles with PPy chains in the AC3 hybrid nanocomposites.The peak at 650 cm -1 shows the slight stretching of CdS in AC3 hybrid nanocomposites, while for Ag 2 S metal stretching band appears at 467-487 cm -1 in AC1 hybrid nanocomposites. 7No obvious absorbance for Ag 2 S and CdS nanoparticles was observed in the spectra of AC1 hybrid nanocomposites due to the agglomeration of the both nanoparticles, which might have affected the UV visible absorption spectrum.Since the interaction of PPy with solvent molecules may be very low, hence its corresponding absorption could be absent in the spectra.The peak may shift towards higher energy that accounts to reduce the PPy after addition of NPs 12 .The absorption peak at λ > 300 nm is possibly due to interaction of the polymer with the solvent.

Surface morphology and elemental analysis
PPy has a globular 7,12 and porous 10 morphology, which can be confirmed from Figure 6 (a-b).Uniform, nanospheres of Ag 2 S particles have diameter of ~ 40-65 nm (Figure 6 (c)), while CdS nanoparticles possess diameter of ~ 60-100 nm can be seen from Figure 6 (d).This indicates the effectiveness of ultrasound in producing non-aggregated nanoparticles even in the absence of an encapsulating agent.The morphology of PPy/(Ag 2 S:CdS) hybrid nanocomposites (AC2) can be observed to be spherical as shown in Figure 6 (e).It is evident from the FE-SEM micrographs that ultrasound assisted in-situ polymerization can produce non-aggregated structures of PPy matrix.
Figure 7 (a-b) shows the energy dispersive X-ray (EDX) spectra of AC1 and AC2 nanocomposites.The EDX result confirmed that the PPy hybrid nanocomposites were properly embedded metal sulphide nanoparticles.It can be also observed from FE-SEM micrographs in Figure 6 (c-d) that two metal sulphide nanoparticles are uniformly dispersed in PPy matrix.Elements like carbon (C) and nitrogen (N) are observed due to the presence of pyrrole, while other elements such as silver (Ag), cadmium (Cd) and sulphur (S) are due to the presence of metal sulphide nanoparticles and their proper dispersion in PPy matrix.The EDX spectrum (Figure 7    The formation of AC3 nanocomposites can be confirmed from Figure 7 (b) showing peaks of Ag, Cd and S as well as C and N.However, it can be seen from the spectrum, peaks of Ag (2.39 wt%), Cd (0.34 wt%) and S (0.55 wt%) were high, which indicates that the composition of PPy nanocomposites was more dominated by Ag 2 S nanoparticles than CdS nanoparticles.

Electrical behavior
The electrical conductivity of PPy was recorded as 1.9 × 10 -3 S/cm with ohmic behavior 7,10 (Figure 8).The conductivity of the PPy hybrid nanocomposites was observed to be varied with the addition of different ratios of (Ag 2 S-CdS) nanoparticles.The current-voltage (I-V) characteristics of the PPy/(Ag 2 S:CdS) hybrid nanocomposites showed a semiconducting behavior.The electrical  conductivity of the PPy hybrid nanocomposites was observed to be decreasing with addition of Ag 2 S:CdS nanoparticles owing to charge compensation by these nanoparticles.Interestingly, the nanocomposites exhibited a lower conductivity when the Ag 2 S and CdS content were equal; but still CdS nanoparticles content increased the conductivity.The order of electrical conductivities was observed to be PPy > AC3 > AC2 > AC1.The result reveals that lower content of metal sulphide nanoparticles decreases the electrical conductivity of the PPy based hybrid nanocomposites, while higher content possibly increases the conductivity by forming a conducting channel within the hybrid nanocomposites.However, the drop in conductivity may also due to localization of charge in p-type PPy by n-type Ag 2 S nanoparticles 7 .

Gas sensing performance
Room temperature gas sensing studies were carried out for the detection of NH 3 and LPG gases using two pointprobe gas sensing setup.PPy and its hybrid nanocomposites (i.e.AC1, AC2 and AC3) as gas sensors were studied to determine their sensitivity and selectivity to specific gases at room temperature, and also to understand the underlying mechanism.A drop in current was observed with respect to the time when the samples were exposed to NH 3 gas (Figure 9 (a)).The corresponding response graph (Figure 9 (b)) shows an increase in gas response with respect to time when exposed to NH 3 gas.After an initial increase, the response curve shows no change which indicates saturation of the sensor surface by the analytes.As it has been discussed in our previous work 7 , when a p-type semiconductor such as PPy is exposed to a reducing gas such as NH 3 there is a drop in charge concentration resulting from the donation of lone pairs of electrons of NH 3 to the PPy leading to a sudden rise in resistance.The gas response data are presented in Table 1.The AC1 nanocomposites show the maximum gas sensing response factor of 1.0 for the detection of 500 ppm NH 3 gas.For the same concentration, the response factors of AC2 and AC3 were recorded as 0.5 and 0.6, respectively.A decrease in response factor was observed with increasing the content of metal sulphide nanoparticles in the PPy matrix.This could be a result of charge compensation by the nanoparticles when present in higher content that renders the sensor incapable of accepting electrons from NH 3 gas.Therefore, the response of the sensor decreased with increase in nanoparticles content.Moreover, the most of the PPy based hybrid nanocomposites sensors exhibited a response time of ~ 20 s.
The change in response of the sensor with respect to variation in gas concentration was also studied.Figure 10 (a) shows that increasing the NH 3 gas concentration increases the response of the sensor.This can be well explained by the fact that incorporating the nanoparticles leads to increase in the surface roughness, which in turn facilitates adsorption of gas molecules easily.Higher the number of adsorbed gas molecules, higher would be the change in sensor's resistance due to the accepting electrons from NH 3 gas.This is in contrast with the   results observed for PPy (Table 2).In case of PPy, the sensor response decreased with increase in gas concentration due to saturation of the PPy surface with NH 3 gas, which was otherwise prevented in the nanocomposites by adding the metal sulphide nanoparticles.When pure PPy doped with FeCl 3 was exposed to NH 3 gas, an increment in the current was observed.During an oxidative polymerization for the preparation of PPy, the Cl -ion is incorporated as counter ion to maintain its electro neutrality.
Usually partial oxidation state (up to + 0.33) is produced in the process.In addition, it can be said that some amount of ions may remain within the PPy in the Fe 2+ state.Thus, there is a possibility that higher oxidation state can be induced in the interaction with NH 3 gas molecules.In such a situation the higher charge on cations may lead to increase in current.The response of PPy and its all hybrid nanocomposites was also studied against LPG at room temperature.But they exhibited good sensitivity towards NH 3 gas, their response to LPG was found to be negligible for detection of 100 ppm of respective gases.Figure 10 (b) presents the gas response and selectivity of PPy and its hybrid nanocomposites towards detection of  2) that PPy and its nanocomposite based sensors are selective towards detection of ammonia as compared to LPG at higher concentration (250 and 500 ppm) also.Therefore, not only the nanoparticles aided in a better sensing behavior, but also induced selectivity into the PPy nanocomposites.

Conclusions
Both Ag 2 S and CdS nanoparticles were mixed in different ratios and incorporated into PPy matrix by ultrasound assisted in-situ polymerization for the preparation of PPy/(Ag 2 S:CdS) hybrid nanocomposites.PPy and its hybrid nanocomposites were characterized using FTIR and UV-visible spectra, XRD and FESEM.Size of both nanoparticles of Ag 2 S (40-65 nm) and CdS (60-100 nm) were observed at nanoscale, while the PPy showed globular and porous structures.The (I-V) characteristics revealed a semiconducting behavior of the resulting PPy hybrid nanocomposites.The gas sensing ability of the nanocomposite sensors at room temperature was determined using NH 3 gas.The sensing study underlines three important findings: (i) A low nanoparticle content aids in enhancing the sensitivity of the PPy/(Ag 2 S:CdS) hybrid nanocomposite based sensors.At higher nanoparticles content the sensor loses its sensitivity either through the agglomeration of nanoparticles or by charge compensation by the nanoparticles.(ii) Unlike with PPy sensor, a gas response of the PPy/(Ag 2 S:CdS) hybrid nanocomposite sensors increases with increase in gas concentration.This is possibly due to the increase in surface roughness, which facilitates adsorption of gas molecules on the sensor surface.(iii) Finally, the metal sulphide nanoparticles do not only impinge upon the sensor's sensitivity but also on its selectivity towards an analyte.The PPy/(Ag 2 S-CdS) hybrid nanocomposites were found to be more selective towards NH 3 gas than LPG.Hence, metal sulphide nanoparticles provide a possibility to improve the existing CP based sensors in terms of their gas sensing performance.

Acknowledgment
Authors are thankful to University Grants Commission, New Delhi, Government of India for providing financial assistance under One Time Grant (UGC-BSR project no.: F.4-10/2010, dated 20 Sept., 2011) to carry out this research work.

Figure 1 .
Figure 1.Photographic view of gas sensing setup.

Figure 9 .
Figure 9. Various PPy/(Ag 2 S-CdS) hybrid nanocomposites showing (a) Drop in current and (b) change in response factor with respect to time.
Department of Plastic Technology, University Institute of Chemical Technology, North MaharashtraUniversity, Jalgaon 425001, Maharashtra, India amounts of Ag 2 S:CdS nanoparticles) on the structural, morphological, electrical and sensing properties of the hybrid nanocomposites (PPy/Ag 2 S:CdS).The sensing performance of the PPy based hybrid nanocomposites towards detection of ammonia and LPG was investigated at room temperature and the results are discussed in detail. a

Table 1 .
Response factor values for sensing different concentration of NH 3 by PPy/(Ag 2 S:CdS) hybrid nanocomposites.

Table 2 .
Response factor values for sensing LPG and ammonia gas by PPy.