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Gas and humidity sensors based on iron oxide–polypyrrole nanocomposites
Sensors and Actuators B 81 (2002) 277±282
Gas and humidity sensors based on iron oxide±polypyrrole nanocomposites Komilla Surib, S. Annapoornib, A.K. Sarkara, R.P. Tandonb,* a
Chemistry Division, National Physical Laboratory, New Delhi 110012, India Department of Physics & Astrophysics, Delhi University, New Delhi 110007, India
b
Received 23 May 2001; received in revised form 9 September 2001; accepted 25 September 2001
Abstract Nanocomposites of iron oxide and polypyrrole were prepared by simultaneous gelation and polymerization process. This resulted in the formation of mixed iron oxide phase for lower polypyrrole concentration, stabilizing to a single cubic iron oxide phase at higher polypyrrole concentration. The composites in the pellet form were used for humidity and gas sensing investigations. Their sensitivity to humidity was found to increase with increasing concentration of polypyrrole. Gas sensing was performed for CO2, N2 and CH4 gases at varying pressures. The sensors showed a linear relationship between sensitivity and pressures for all the gases studied. The sensors showed highest sensitivity to CO2 gas. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Polypyrrole; Iron oxide; Sol±gel; Nanocomposites; Humidity sensors; Gas sensors
1. Introduction The increased concern about environmental protection has led to continuous expansion in sensor development. Humidity sensors have attracted lot of attention in medical and industrial ®elds. The measurement and control of humidity is important in many areas including industry (paper, electronic), domestic environment (air conditioner), medicine (respiratory equipment), etc. Also, gas sensors having good sensitivity to gases such as methane, hydrogen, carbon dioxide, carbon monoxide, etc. are utmost in demand. Different criteria are used for measuring sensitivity to humidity and gases, like changes in mechanical, optical and electrical properties [1,2]. Electrical detection is the most commonly used and is based on the change in resistance or capacitance of the sensor on exposure to water vapor and gases. In recent years, inorganic semiconducting oxides like zinc oxide (ZnO), aluminium oxide (Al2O3), titanium oxide (TiO2), tin oxide (SnO2), iron oxide (Fe2O3), etc. have been studied extensively and have emerged as economical sensors for monitoring toxic gases and humidity [3±6]. The sensitivity of these sensors to gas and humidity depends on the microstructure. This can be achieved by adopting special techniques of preparation or by doping impurities. It has * Corresponding author. Fax: 91-5736290. E-mail address: [email protected] (R.P. Tandon).
been found that doping of SO4 2 , Ti, Sn, Zn, Si, etc. in aFe2O3 has been found to improve the sensing capabilities [7±10]. The sol±gel process is an excellent method of producing highly porous and nanosized ceramics and oxides [11,12] and these have been extensively investigated as sensor materials for gas and humidity sensing [13±15]. Several organic semiconducting materials, viz. conducting polymers such as polypyrrole, polyaniline, polythiophene, polyacetylene, etc. ®nd a variety of applications as electronic and optoelectronic devices [16,17]. Many conducting polymers have shown changes in resistivity on exposure to different gases and humidity [18±20]. The demand of conducting polymers in sensor application has been tremendous due to its ease of synthesis. These materials exhibit interesting electrical properties such as their ability to oxidize and reduce at speci®c electrochemical potential. Conducting polymers like polypyrrole and polyaniline have shown capability in sensing technology and are used as sensors for air borne volatile organic compounds referred to as electric nose, especially for detection of alcohols, NO2, etc. Polypyrrole has shown interesting ammonia, NH3, and NO2 gas sensing properties at room temperature [18,19]. Though polypyrrole is highly sensitive to gases, yet it shows saturation effect at higher concentration of gases [18]. The inorganic sensors based on oxides have been found to be less sensitive to gases and humidity as compared to the
0925-4005/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 9 6 6 - 2
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conducting polymers, which show high sensing behavior due to their porous nature. However, the instability of some of the conducting polymers in air has limited their commercialization as sensors [16]. In the present paper, the inorganic±organic hybrid nanocomposite containing polypyrrole as the organic part and iron oxide as the inorganic part have been used for studying humidity and gas sensing properties. Such types of nanocomposite have shown to possess small grain size and high stability in air [21,22]. To the best of our knowledge, this is the ®rst ever attempt made to study these composites as humidity and gas sensors. In order to determine the grain size and structural properties of the nanocomposites, several structural investigations using X-ray, IR, scanning electron microscope (SEM) and transmission electron microscope (TEM) techniques have been carried out for varying polypyrrole concentration and the results are being presented here. The sensing properties of these nanocomposites were studied at different values of relative humidity (RH) and gas pressures for CO2, N2 and CH4. 2. Experimental details The composites of iron oxide and polypyrrole were prepared by adding different amounts of pyrrole monomer to a mixture of ferric nitrate and methoxy ethanol in a certain speci®c ratio. This mixture was heated at 150 8C to evaporate the solvent, resulting in composite powders. These powders were then annealed at 300 8C. The powders were then compressed into pellets of diameter 1.2 cm and thickness 0.1 cm for further studies. The crystal structure of the powders was examined by using a Rigaku Rota¯ex diffractometer using a Cu Ka Ê ) at 40 keV. The surface morphology was (l 1:5418 A analyzed with the help of JEOL (JSM 840) SEM and JEOL JEM 2000 EX TEM. The Fourier transform infrared (FTIR) spectra was recorded using the Nicolet 510 P FTIR spectrophotometer. Powders were used in pellet form to study humidity and gas sensing properties. The pellets were placed in the sample holder having two pressure contacts. The contacts were made using silver paint. This was enclosed in a metallic chamber provided with two holes, one for attaching the hygrometer to measure the RH and the other for the contacts from the sample holder to determine the resistance. The testo 601 capacitative hygrometer was used to measure the humidity and Keithley electrometer 610 was used to measure the resistance. The humidity inside the chamber was generated using two pressure method. By this method, the RH could be controlled upto 1% approximately. The resistance variations were studied as a function of RH values at room temperature. Gas sensing properties of the pellets were also studied for CO2, N2 and CH4 gases. Same chamber which was used for
humidity (described in previous paragraphs) was used for gas studies, with hygrometer replaced by a pressure gauge. The gas pressure was controlled using a regulator. The resistance variations were studied as a function of gas pressures. 3. Results and discussions Fig. 1a and b shows the X-ray diffraction pattern for the nanocomposites. The X-ray diffraction pattern for powder having lower concentration of polypyrrole (5%) has shown presence of both hexagonal and cubic phases of iron oxide (Fig. 1a). Addition of 15% polypyrrole results in a single Ê. cubic phase (Fig. 1b) with lattice constant a 8:54 A The FTIR transmission spectra of the powders using KBr pellets having different concentration of polypyrrole was recorded in the range 400±4000 cm 1 to con®rm polymerization of pyrrole. The spectra for all the samples showed peaks at 780, 1090 and 3400 cm 1 indicating the presence of polypyrrole. The intensity of these peaks was found to increase with increase in the concentration of polypyrrole. Fig. 2 shows a typical FTIR spectra of powder with maximum polypyrrole. The SEM studies performed on all the samples indicated a transformation from a cluster pattern to highly branched chain structure (or a ®brillar morphology) with increase in polypyrrole concentration. The SEM picture indicating a chain pattern for 15% polypyrrole is shown in Fig. 3. The TEM picture shown in Fig. 4, shows the presence of nanosized particles for the same concentration of polypyrrole. 3.1. Humidity sensors: RH versus sensitivity for different concentrations For humidity sensing, sensitivity is de®ned as (Rd/Rh), where Rd is the resistance under dry conditions (in this case, with RH 20%) and Rh is the resistance at speci®ed humidity. The sensitivity was determined for varying RH for different nanocomposites.
Fig. 1. X-ray diffraction pattern of the nanocomposites having (a) 5% and (b) 15% polypyrrole.
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Fig. 2. FTIR transmission spectra of KBr pellet of nanocomposite with 15% polypyrrole.
Fig. 3. SEM picture of the nanocomposite with 15% polypyrrole.
Fig. 4. TEM picture of the nanocomposite.
Fig. 5a±d shows the sensitivity as a function of humidity for nanocomposites having different concentrations of polypyrrole. It is seen from Fig. 5a that the sensor with very low concentration (1%) of polypyrrole has a very less response to humidity in the range 20±80% and a very steep increase in sensitivity only beyond 80% RH. As the concentration of polypyrrole is increased to 5%, the sensitivity is increased to a maximum value of 8 (Fig. 5b) and the value of RH at which a large increase in sensitivity occurs is lowered to RH 70%. When the sensor having still higher concentration of polymer (10%) is exposed to different RH values, the steep rise in sensitivity occurs at RH 60% and sensitivity is increased further reaching a value of 20. The maximum response was obtained for the sensor having (15%) higher concentration of polypyrrole. It is seen from Fig. 5d that, not
Fig. 5. Variation of sensitivity with humidity for sensors having (a) 1%, (b) 5%, (c) 10% and (d) 15% polypyrrole.
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sites for reaction to occur and, hence, no change in resistance beyond a certain humidity was observed in earlier case [14].
Fig. 6. Variation of sensitivity with concentration of polypyrrole at (a) 40%, (b) 60% and (c) 80% RHs, respectively.
only the sensitivity reaches a higher value of 130, but also the value of RH, at which sensitivity increases very rapidly, decreases to 50%. Thus, in the present study, we observe a continuous increase in the sensitivity even at high RH values. This behavior is different from a-Fe2O3 humidity sensor which shows a distinct saturation at higher humidity values, shown as dotted line in Fig. 5 (obtained from [14]). Fig. 6a±c shows a plot of sensitivity versus concentration for three different values of humidity, viz. 40, 60 and 80%, which clearly indicate an increase in sensitivity with concentration of polypyrrole for a ®xed value of humidity, thus, suggesting that the addition of polypyrrole plays an important role in improving the humidity sensing property. The sensor response to humidity with different concentration of polypyrrole could be explained as combination of the following two processes:
In this present study, a continuous increase in sensitivity with increase in RH values is observed. This could be probably due to the presence of polypyrrole which also reacts with water vapor, causing a counter balance or canceling of the saturation characteristic of a-Fe2O3 humidity sensor [16]. Moreover, this polymer seems to have hydrophilic ions for charge compensation that are electrostatically bonded to the matrix and adsorption of water increases mobility of ions and consequently resistance decreases. The combined mechanisms hence result in an increased response of the sensor at high relative humidities. 3.2. Gas sensors: sensor response to CO2, N2 and CH4 gases In the case of gas sensing, sensitivity is de®ned as (Rgas/ Rair), where Rgas is the resistance in the presence of gas and Rair is the resistance in clean air at atmospheric pressure. The response of the sensors to different gases, viz. CO2, N2 and CH4 at various pressures have been investigated. All the measurements were performed in the pressure range 5± 40 psi. Figs. 7±9 show the response of the sensors with varying polypyrrole concentration to various gases, viz. CO2, N2 and CH4 gases, respectively. In each of these cases, the
(a) The absorption of water on iron oxide surface has been shown to be a dissociative mechanism to form hydroxyl groups which are bound to lattice iron as discussed by earlier workers [23]. The water vapor is adsorbed on the grain surface and in nanopores and react reversibly with lattice iron as H2 O OO 2FeFe ?2 OH Fe VO00 2e ; where OO is lattice oxygen at the oxygen site and VO00 is vacancy created at oxygen site according to the reaction OO ?O2 VO00 : (b) The doubly ionized oxygen displaced from lattice reacts with H coming from dissociation of water vapor to form hydroxyl groups as given as follows: H O2 ?OH : Because of the free electrons given by this reaction, the resistance decreases with increase in humidity. But, the sensors based on iron oxide attain a saturation at high RH suggesting the absence of any further active iron
Fig. 7. Variation of sensitivity with pressures of CO2 gas (a) and with polypyrrole concentration at three different CO2 gas pressures (b).
K. Suri et al. / Sensors and Actuators B 81 (2002) 277±282
sensitivity has been determined for different applied gas pressures. Fig. 7a shows very high sensitivity to CO2 gas with the value of sensitivity approaching 123 at the highest pressure applied (40 psi). A linear behavior was observed as a function of pressure. A plot of sensitivity as a function of polypyrrole concentration at three different gas pressures is shown in Fig. 7b. It is observed that the sensitivity increases linearly with the concentration of polypyrrole for all pressures. Fig. 8a shows the response of all sensors to different pressures of N2 gas. The sensors show linear response with sensitivity reaching a value of 70 for maximum polypyrrole concentration and maximum pressure of 40 psi. Thus, a comparatively lower response as compared to CO2 gas was observed. A plot of concentration versus sensitivity at three different pressures is shown in Fig. 8b. It is observed that with increased pressure, the sensitivity of the sensor varies linearly with increase in concentration of polypyrrole. When the sensors were exposed to CH4 gas, the sensors sensitivity decreased to a value as low as 40 for the maximum pressure applied (Fig. 9a). Thus, indicating the lowest sensitivity to this gas. The sensitivity as a function of concentration at different pressures is shown in Fig. 9b, exhibit similar behavior as that of other gases. It is seen that sensor with 15% polypyrrole gave maximum response for all gases. This is in accordance with SEM studies, which show the presence of ®brillar morphology with microscopic voids for this sensor (Fig. 3). This kind of structure is expected to be highly permeable to all the gases. The permeability depends on the kinetic diameter
281
Fig. 9. Variation of sensitivity with pressures of CH4 gas (a) and with polypyrrole concentration at three different CH4 gas pressures (b).
of the gas molecule [24] and also on the size of the pores in the sensor. The response of all the sensors was maximum to CO2 gas as compared to other gases which could be due to the fact that larger the molecule, lesser is the permeability of the gas. Ê ) is lesser As the kinetic diameter of CO2 molecule (3.3 A Ê Ê than that of N2 (3.64 A) and CH4 (3.8 A), therefore, permeability of CH4 is minimum and CO2 is maximum, resulting in highest response and sensitivity of all sensors to CO2 and least to CH4 gas. The variation in resistance was very instantaneous for all the gases studied. 4. Conclusion
Fig. 8. Variation of sensitivity with pressures of N2 gas (a) and with polypyrrole concentration at three different N2 gas pressures (b).
The nanocomposites of iron oxide and polypyrrole prepared by the sol±gel process were used to investigate the humidity and gas sensing properties. These nanocomposites showed better humidity sensing as compared to a-Fe2O3. The sensitivity was found to be high for the sensor with higher polypyrrole concentration. The possible mechanism for such an increase has also been suggested. A sharp increase in sensitivity was observed at higher values of RH, depending on the polypyrrole concentration. Higher the concentration of polypyrrole, lower was the RH at which the sensor showed a steep variation. The gas sensing properties of all the nanocomposites were also studied for CO2, N2 and CH4 gases. The order of sensitivity was CO2 > N2 > CH4 and this was due to the varying kinetic diameter of the gas molecules having the following order CO2 < N2 < CH4 . The response of the sensor to both
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humidity and gases was very instantaneous which is an important feature for its use as commercial sensors. Acknowledgements The authors wish to thank Dr. Harkishan and Mr. Bheekam Singh, National Physical Laboratory, New Delhi for extending help in carrying out humidity studies. We would also like to thank Dr. N.C. Mehra, USIC, New Delhi for carrying out SEM and TEM studies. One of the authors (K.S.) wishes to thank CSIR, New Delhi, for providing fellowship and all other ®nancial helps. References [1] N.M. White, J.D. Turner, Thick film sensors: past, present and future, Measur. Sci. Technol. 8 (1997)1±20. [2] W. Qu, J.U. Meyer, A novel thick film ceramic humidity sensor, Sens. Actuators B 40 (1997)175±182. [3] C. Cantalini, M. Pelino, Microstructure and humidity sensitive characteristics of a-Fe2O3 ceramic sensor, J. Am Ceram. Soc. 75 (1992)546±551. [4] M. Li, Y. Chen, An investigation of response time of TiO2 thin film oxygen sensors, Sens. Actuators B 32 (1996)83±85. [5] H. Yagi, M. Nakata, Humidity sensor using Al2O3, TiO2 and SnO2 prepared by sol±gel method, J. Ceram. Soc. Jpn. 100 (1992)152±156. [6] W.Y. Chung, D.D. Lee, Characteristics of a-Fe2O3 thick film gas sensors, Thin Solid Films 200 (1991)329±339. [7] C. Cantalini, M. Faccio, G. Ferri, M. Pelino, Microstructure and electrical properties of Si doped a-Fe2O3 humidity sensor, Sens. Actuators B 15/16 (1993)298. [8] Y. Nakatani, M. Sakai, M. Matsuoka, Enhancement of gas sensitivity by controlling microstructure of a-Fe2O3 ceramics, Jpn. J. Appl. Phys. 22 (1983)912±916. [9] F.J. He, T. Yao, B.D. Qu, J.S. Han, A.B. Yu, Gas sensitivity of Zn doped a-Fe2O3 (SO4 , Sn, Zn) to carbon monoxide, Sens. Actuator B 40 (1997)183±186. [10] Y. Nakatani, M. Matsuoka, Effects of sulphate ions on gas sensitive properties of a-Fe2O3 ceramics, Jpn. J. Appl. Phys. 21 (1982)L758± L760. [11] Y. Yamanolie, K. Yagamuchi, K. Matsunoto, T. Fuji, Iron oxide powders by sol±gel method, Jpn. J. Appl. Phys. 36 (3) (1991) 478±L483. [12] P. Chauhan, S. Annapoorni, S.K. Trikha, Preparation, characterization and optical properties of a-Fe2O3 films by sol spinning process, Bull. Mater. Sci. 21 (1998)381±385. [13] G. Montesperelli, A. Pumo, E. Traversa, G. Gusmano, A. Bearzotti, A. Montenero, G. Gnappi, Sol±gel processed TiO2-based thin film as innovative humidity sensors, Sens. Actuators B 25 (1995) 705±709.
[14] P. Chauhan, S. Annapoorni, S.K. Trikha, Humidity sensing properties of nanocrystalline haematite thin films prepared by sol±gel processing, Thin Solid Films 346 (1999)266±268. [15] E. Traversa, G. Gnappi, A. Montenero, G. Gusmano, Ceramic thin films by sol±gel processing as novel materials for integrated humidity sensors, Sens. Actuators B 31 (1996)59±70. [16] T.J. Skotheim (Ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986. [17] A.G. MacDiarmid, J.C. Chiang, A.F. Richter, N.L.D. Samasiri, A.J. Epstein, in: L. Alacer (Ed.), Conducting Polymers, Reidel, Dordrecht, The Netherlands, 1987, pp. 105±120. [18] M. Penza, E. Milella, V.I. Anisimkin, Monitoring of NH3 gas by LB polypyrrole-based SAW sensor, Sens. Actuators B 47 (1998)218± 224. [19] J.J. Miasik, A. Hooper, B.C. Tofield, Conducting polymer gas sensors, J. Chem. Soc., Faraday Trans. 82 (1986)1117±1126. [20] G. Bidan, Electroconducting conjugated polymers: new sensitive matrices to build up chemical or eletrochemical sensors: a review, Sens. Actuators B 6 (1992)45±56. [21] S. Armes, S.P. Maeda, Preparation and characterization of polypyrrole±tin(IV) oxide nanocomposite colloids, Chem. Mater. 7 (1995)171±179. [22] R. Partch, S.G. Gangolli, E. Matijevic, W. Cal, S. Arajs, Surface induced polymerization of pyrrole on iron(III) and cerium(IV) oxide particles, J. Colloid Interf. Sci. 144 (1) (1991)27±34. [23] V.E. Henrich, P.A. Cox, The Surface Science of Metal Oxides, Cambridge University Press, New York, USA, 1994. [24] D.W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1974, pp. 593±724.
Biographies Komilla Suri received her MSc from Department of Physics, Delhi University and is currently working as senior research fellow in the same department. As a PhD student, her research interests include conducting polymers for various applications. S. Annapoorni is a PhD in Physics from IIT, Chennai (1990). At present she is reader in Delhi University. Her research interests include hydrogen absorption in metals, magnetic properties of rare earth transition metals, metal oxides and conducting polymers. The recent interest is in magnetic and sensing properties of nanocomposites. A.K. Sarkar is a PhD in Analytical Chemistry from Burdwan University, India. He is working as senior scientist and head of chemistry section at the National Physical Laboratory, New Delhi. He has 35 years research experience in modern instrumental methods of analysis and has published over 90 research papers in different fields of chemistry. He holds five Patents including one each from USA and South Africa. R.P. Tandon is a PhD from the Department of Physics, Delhi University. He is a visiting scientist at MIT (USA), Queens University (Canada) and is currently a professor of physics in the Delhi University. Earlier, he was senior scientist at the National Physical Laboratory, New Delhi. His research interests include ceramics, polymers and glasses and has published over 100 research papers in these areas.
Gas and humidity sensors based on iron oxide±polypyrrole nanocomposites Komilla Surib, S. Annapoornib, A.K. Sarkara, R.P. Tandonb,* a
Chemistry Division, National Physical Laboratory, New Delhi 110012, India Department of Physics & Astrophysics, Delhi University, New Delhi 110007, India
b
Received 23 May 2001; received in revised form 9 September 2001; accepted 25 September 2001
Abstract Nanocomposites of iron oxide and polypyrrole were prepared by simultaneous gelation and polymerization process. This resulted in the formation of mixed iron oxide phase for lower polypyrrole concentration, stabilizing to a single cubic iron oxide phase at higher polypyrrole concentration. The composites in the pellet form were used for humidity and gas sensing investigations. Their sensitivity to humidity was found to increase with increasing concentration of polypyrrole. Gas sensing was performed for CO2, N2 and CH4 gases at varying pressures. The sensors showed a linear relationship between sensitivity and pressures for all the gases studied. The sensors showed highest sensitivity to CO2 gas. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Polypyrrole; Iron oxide; Sol±gel; Nanocomposites; Humidity sensors; Gas sensors
1. Introduction The increased concern about environmental protection has led to continuous expansion in sensor development. Humidity sensors have attracted lot of attention in medical and industrial ®elds. The measurement and control of humidity is important in many areas including industry (paper, electronic), domestic environment (air conditioner), medicine (respiratory equipment), etc. Also, gas sensors having good sensitivity to gases such as methane, hydrogen, carbon dioxide, carbon monoxide, etc. are utmost in demand. Different criteria are used for measuring sensitivity to humidity and gases, like changes in mechanical, optical and electrical properties [1,2]. Electrical detection is the most commonly used and is based on the change in resistance or capacitance of the sensor on exposure to water vapor and gases. In recent years, inorganic semiconducting oxides like zinc oxide (ZnO), aluminium oxide (Al2O3), titanium oxide (TiO2), tin oxide (SnO2), iron oxide (Fe2O3), etc. have been studied extensively and have emerged as economical sensors for monitoring toxic gases and humidity [3±6]. The sensitivity of these sensors to gas and humidity depends on the microstructure. This can be achieved by adopting special techniques of preparation or by doping impurities. It has * Corresponding author. Fax: 91-5736290. E-mail address: [email protected] (R.P. Tandon).
been found that doping of SO4 2 , Ti, Sn, Zn, Si, etc. in aFe2O3 has been found to improve the sensing capabilities [7±10]. The sol±gel process is an excellent method of producing highly porous and nanosized ceramics and oxides [11,12] and these have been extensively investigated as sensor materials for gas and humidity sensing [13±15]. Several organic semiconducting materials, viz. conducting polymers such as polypyrrole, polyaniline, polythiophene, polyacetylene, etc. ®nd a variety of applications as electronic and optoelectronic devices [16,17]. Many conducting polymers have shown changes in resistivity on exposure to different gases and humidity [18±20]. The demand of conducting polymers in sensor application has been tremendous due to its ease of synthesis. These materials exhibit interesting electrical properties such as their ability to oxidize and reduce at speci®c electrochemical potential. Conducting polymers like polypyrrole and polyaniline have shown capability in sensing technology and are used as sensors for air borne volatile organic compounds referred to as electric nose, especially for detection of alcohols, NO2, etc. Polypyrrole has shown interesting ammonia, NH3, and NO2 gas sensing properties at room temperature [18,19]. Though polypyrrole is highly sensitive to gases, yet it shows saturation effect at higher concentration of gases [18]. The inorganic sensors based on oxides have been found to be less sensitive to gases and humidity as compared to the
0925-4005/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 9 6 6 - 2
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conducting polymers, which show high sensing behavior due to their porous nature. However, the instability of some of the conducting polymers in air has limited their commercialization as sensors [16]. In the present paper, the inorganic±organic hybrid nanocomposite containing polypyrrole as the organic part and iron oxide as the inorganic part have been used for studying humidity and gas sensing properties. Such types of nanocomposite have shown to possess small grain size and high stability in air [21,22]. To the best of our knowledge, this is the ®rst ever attempt made to study these composites as humidity and gas sensors. In order to determine the grain size and structural properties of the nanocomposites, several structural investigations using X-ray, IR, scanning electron microscope (SEM) and transmission electron microscope (TEM) techniques have been carried out for varying polypyrrole concentration and the results are being presented here. The sensing properties of these nanocomposites were studied at different values of relative humidity (RH) and gas pressures for CO2, N2 and CH4. 2. Experimental details The composites of iron oxide and polypyrrole were prepared by adding different amounts of pyrrole monomer to a mixture of ferric nitrate and methoxy ethanol in a certain speci®c ratio. This mixture was heated at 150 8C to evaporate the solvent, resulting in composite powders. These powders were then annealed at 300 8C. The powders were then compressed into pellets of diameter 1.2 cm and thickness 0.1 cm for further studies. The crystal structure of the powders was examined by using a Rigaku Rota¯ex diffractometer using a Cu Ka Ê ) at 40 keV. The surface morphology was (l 1:5418 A analyzed with the help of JEOL (JSM 840) SEM and JEOL JEM 2000 EX TEM. The Fourier transform infrared (FTIR) spectra was recorded using the Nicolet 510 P FTIR spectrophotometer. Powders were used in pellet form to study humidity and gas sensing properties. The pellets were placed in the sample holder having two pressure contacts. The contacts were made using silver paint. This was enclosed in a metallic chamber provided with two holes, one for attaching the hygrometer to measure the RH and the other for the contacts from the sample holder to determine the resistance. The testo 601 capacitative hygrometer was used to measure the humidity and Keithley electrometer 610 was used to measure the resistance. The humidity inside the chamber was generated using two pressure method. By this method, the RH could be controlled upto 1% approximately. The resistance variations were studied as a function of RH values at room temperature. Gas sensing properties of the pellets were also studied for CO2, N2 and CH4 gases. Same chamber which was used for
humidity (described in previous paragraphs) was used for gas studies, with hygrometer replaced by a pressure gauge. The gas pressure was controlled using a regulator. The resistance variations were studied as a function of gas pressures. 3. Results and discussions Fig. 1a and b shows the X-ray diffraction pattern for the nanocomposites. The X-ray diffraction pattern for powder having lower concentration of polypyrrole (5%) has shown presence of both hexagonal and cubic phases of iron oxide (Fig. 1a). Addition of 15% polypyrrole results in a single Ê. cubic phase (Fig. 1b) with lattice constant a 8:54 A The FTIR transmission spectra of the powders using KBr pellets having different concentration of polypyrrole was recorded in the range 400±4000 cm 1 to con®rm polymerization of pyrrole. The spectra for all the samples showed peaks at 780, 1090 and 3400 cm 1 indicating the presence of polypyrrole. The intensity of these peaks was found to increase with increase in the concentration of polypyrrole. Fig. 2 shows a typical FTIR spectra of powder with maximum polypyrrole. The SEM studies performed on all the samples indicated a transformation from a cluster pattern to highly branched chain structure (or a ®brillar morphology) with increase in polypyrrole concentration. The SEM picture indicating a chain pattern for 15% polypyrrole is shown in Fig. 3. The TEM picture shown in Fig. 4, shows the presence of nanosized particles for the same concentration of polypyrrole. 3.1. Humidity sensors: RH versus sensitivity for different concentrations For humidity sensing, sensitivity is de®ned as (Rd/Rh), where Rd is the resistance under dry conditions (in this case, with RH 20%) and Rh is the resistance at speci®ed humidity. The sensitivity was determined for varying RH for different nanocomposites.
Fig. 1. X-ray diffraction pattern of the nanocomposites having (a) 5% and (b) 15% polypyrrole.
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Fig. 2. FTIR transmission spectra of KBr pellet of nanocomposite with 15% polypyrrole.
Fig. 3. SEM picture of the nanocomposite with 15% polypyrrole.
Fig. 4. TEM picture of the nanocomposite.
Fig. 5a±d shows the sensitivity as a function of humidity for nanocomposites having different concentrations of polypyrrole. It is seen from Fig. 5a that the sensor with very low concentration (1%) of polypyrrole has a very less response to humidity in the range 20±80% and a very steep increase in sensitivity only beyond 80% RH. As the concentration of polypyrrole is increased to 5%, the sensitivity is increased to a maximum value of 8 (Fig. 5b) and the value of RH at which a large increase in sensitivity occurs is lowered to RH 70%. When the sensor having still higher concentration of polymer (10%) is exposed to different RH values, the steep rise in sensitivity occurs at RH 60% and sensitivity is increased further reaching a value of 20. The maximum response was obtained for the sensor having (15%) higher concentration of polypyrrole. It is seen from Fig. 5d that, not
Fig. 5. Variation of sensitivity with humidity for sensors having (a) 1%, (b) 5%, (c) 10% and (d) 15% polypyrrole.
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sites for reaction to occur and, hence, no change in resistance beyond a certain humidity was observed in earlier case [14].
Fig. 6. Variation of sensitivity with concentration of polypyrrole at (a) 40%, (b) 60% and (c) 80% RHs, respectively.
only the sensitivity reaches a higher value of 130, but also the value of RH, at which sensitivity increases very rapidly, decreases to 50%. Thus, in the present study, we observe a continuous increase in the sensitivity even at high RH values. This behavior is different from a-Fe2O3 humidity sensor which shows a distinct saturation at higher humidity values, shown as dotted line in Fig. 5 (obtained from [14]). Fig. 6a±c shows a plot of sensitivity versus concentration for three different values of humidity, viz. 40, 60 and 80%, which clearly indicate an increase in sensitivity with concentration of polypyrrole for a ®xed value of humidity, thus, suggesting that the addition of polypyrrole plays an important role in improving the humidity sensing property. The sensor response to humidity with different concentration of polypyrrole could be explained as combination of the following two processes:
In this present study, a continuous increase in sensitivity with increase in RH values is observed. This could be probably due to the presence of polypyrrole which also reacts with water vapor, causing a counter balance or canceling of the saturation characteristic of a-Fe2O3 humidity sensor [16]. Moreover, this polymer seems to have hydrophilic ions for charge compensation that are electrostatically bonded to the matrix and adsorption of water increases mobility of ions and consequently resistance decreases. The combined mechanisms hence result in an increased response of the sensor at high relative humidities. 3.2. Gas sensors: sensor response to CO2, N2 and CH4 gases In the case of gas sensing, sensitivity is de®ned as (Rgas/ Rair), where Rgas is the resistance in the presence of gas and Rair is the resistance in clean air at atmospheric pressure. The response of the sensors to different gases, viz. CO2, N2 and CH4 at various pressures have been investigated. All the measurements were performed in the pressure range 5± 40 psi. Figs. 7±9 show the response of the sensors with varying polypyrrole concentration to various gases, viz. CO2, N2 and CH4 gases, respectively. In each of these cases, the
(a) The absorption of water on iron oxide surface has been shown to be a dissociative mechanism to form hydroxyl groups which are bound to lattice iron as discussed by earlier workers [23]. The water vapor is adsorbed on the grain surface and in nanopores and react reversibly with lattice iron as H2 O OO 2FeFe ?2 OH Fe VO00 2e ; where OO is lattice oxygen at the oxygen site and VO00 is vacancy created at oxygen site according to the reaction OO ?O2 VO00 : (b) The doubly ionized oxygen displaced from lattice reacts with H coming from dissociation of water vapor to form hydroxyl groups as given as follows: H O2 ?OH : Because of the free electrons given by this reaction, the resistance decreases with increase in humidity. But, the sensors based on iron oxide attain a saturation at high RH suggesting the absence of any further active iron
Fig. 7. Variation of sensitivity with pressures of CO2 gas (a) and with polypyrrole concentration at three different CO2 gas pressures (b).
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sensitivity has been determined for different applied gas pressures. Fig. 7a shows very high sensitivity to CO2 gas with the value of sensitivity approaching 123 at the highest pressure applied (40 psi). A linear behavior was observed as a function of pressure. A plot of sensitivity as a function of polypyrrole concentration at three different gas pressures is shown in Fig. 7b. It is observed that the sensitivity increases linearly with the concentration of polypyrrole for all pressures. Fig. 8a shows the response of all sensors to different pressures of N2 gas. The sensors show linear response with sensitivity reaching a value of 70 for maximum polypyrrole concentration and maximum pressure of 40 psi. Thus, a comparatively lower response as compared to CO2 gas was observed. A plot of concentration versus sensitivity at three different pressures is shown in Fig. 8b. It is observed that with increased pressure, the sensitivity of the sensor varies linearly with increase in concentration of polypyrrole. When the sensors were exposed to CH4 gas, the sensors sensitivity decreased to a value as low as 40 for the maximum pressure applied (Fig. 9a). Thus, indicating the lowest sensitivity to this gas. The sensitivity as a function of concentration at different pressures is shown in Fig. 9b, exhibit similar behavior as that of other gases. It is seen that sensor with 15% polypyrrole gave maximum response for all gases. This is in accordance with SEM studies, which show the presence of ®brillar morphology with microscopic voids for this sensor (Fig. 3). This kind of structure is expected to be highly permeable to all the gases. The permeability depends on the kinetic diameter
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Fig. 9. Variation of sensitivity with pressures of CH4 gas (a) and with polypyrrole concentration at three different CH4 gas pressures (b).
of the gas molecule [24] and also on the size of the pores in the sensor. The response of all the sensors was maximum to CO2 gas as compared to other gases which could be due to the fact that larger the molecule, lesser is the permeability of the gas. Ê ) is lesser As the kinetic diameter of CO2 molecule (3.3 A Ê Ê than that of N2 (3.64 A) and CH4 (3.8 A), therefore, permeability of CH4 is minimum and CO2 is maximum, resulting in highest response and sensitivity of all sensors to CO2 and least to CH4 gas. The variation in resistance was very instantaneous for all the gases studied. 4. Conclusion
Fig. 8. Variation of sensitivity with pressures of N2 gas (a) and with polypyrrole concentration at three different N2 gas pressures (b).
The nanocomposites of iron oxide and polypyrrole prepared by the sol±gel process were used to investigate the humidity and gas sensing properties. These nanocomposites showed better humidity sensing as compared to a-Fe2O3. The sensitivity was found to be high for the sensor with higher polypyrrole concentration. The possible mechanism for such an increase has also been suggested. A sharp increase in sensitivity was observed at higher values of RH, depending on the polypyrrole concentration. Higher the concentration of polypyrrole, lower was the RH at which the sensor showed a steep variation. The gas sensing properties of all the nanocomposites were also studied for CO2, N2 and CH4 gases. The order of sensitivity was CO2 > N2 > CH4 and this was due to the varying kinetic diameter of the gas molecules having the following order CO2 < N2 < CH4 . The response of the sensor to both
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humidity and gases was very instantaneous which is an important feature for its use as commercial sensors. Acknowledgements The authors wish to thank Dr. Harkishan and Mr. Bheekam Singh, National Physical Laboratory, New Delhi for extending help in carrying out humidity studies. We would also like to thank Dr. N.C. Mehra, USIC, New Delhi for carrying out SEM and TEM studies. One of the authors (K.S.) wishes to thank CSIR, New Delhi, for providing fellowship and all other ®nancial helps. References [1] N.M. White, J.D. Turner, Thick film sensors: past, present and future, Measur. Sci. Technol. 8 (1997)1±20. [2] W. Qu, J.U. Meyer, A novel thick film ceramic humidity sensor, Sens. Actuators B 40 (1997)175±182. [3] C. Cantalini, M. Pelino, Microstructure and humidity sensitive characteristics of a-Fe2O3 ceramic sensor, J. Am Ceram. Soc. 75 (1992)546±551. [4] M. Li, Y. Chen, An investigation of response time of TiO2 thin film oxygen sensors, Sens. Actuators B 32 (1996)83±85. [5] H. Yagi, M. Nakata, Humidity sensor using Al2O3, TiO2 and SnO2 prepared by sol±gel method, J. Ceram. Soc. Jpn. 100 (1992)152±156. [6] W.Y. Chung, D.D. Lee, Characteristics of a-Fe2O3 thick film gas sensors, Thin Solid Films 200 (1991)329±339. [7] C. Cantalini, M. Faccio, G. Ferri, M. Pelino, Microstructure and electrical properties of Si doped a-Fe2O3 humidity sensor, Sens. Actuators B 15/16 (1993)298. [8] Y. Nakatani, M. Sakai, M. Matsuoka, Enhancement of gas sensitivity by controlling microstructure of a-Fe2O3 ceramics, Jpn. J. Appl. Phys. 22 (1983)912±916. [9] F.J. He, T. Yao, B.D. Qu, J.S. Han, A.B. Yu, Gas sensitivity of Zn doped a-Fe2O3 (SO4 , Sn, Zn) to carbon monoxide, Sens. Actuator B 40 (1997)183±186. [10] Y. Nakatani, M. Matsuoka, Effects of sulphate ions on gas sensitive properties of a-Fe2O3 ceramics, Jpn. J. Appl. Phys. 21 (1982)L758± L760. [11] Y. Yamanolie, K. Yagamuchi, K. Matsunoto, T. Fuji, Iron oxide powders by sol±gel method, Jpn. J. Appl. Phys. 36 (3) (1991) 478±L483. [12] P. Chauhan, S. Annapoorni, S.K. Trikha, Preparation, characterization and optical properties of a-Fe2O3 films by sol spinning process, Bull. Mater. Sci. 21 (1998)381±385. [13] G. Montesperelli, A. Pumo, E. Traversa, G. Gusmano, A. Bearzotti, A. Montenero, G. Gnappi, Sol±gel processed TiO2-based thin film as innovative humidity sensors, Sens. Actuators B 25 (1995) 705±709.
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Biographies Komilla Suri received her MSc from Department of Physics, Delhi University and is currently working as senior research fellow in the same department. As a PhD student, her research interests include conducting polymers for various applications. S. Annapoorni is a PhD in Physics from IIT, Chennai (1990). At present she is reader in Delhi University. Her research interests include hydrogen absorption in metals, magnetic properties of rare earth transition metals, metal oxides and conducting polymers. The recent interest is in magnetic and sensing properties of nanocomposites. A.K. Sarkar is a PhD in Analytical Chemistry from Burdwan University, India. He is working as senior scientist and head of chemistry section at the National Physical Laboratory, New Delhi. He has 35 years research experience in modern instrumental methods of analysis and has published over 90 research papers in different fields of chemistry. He holds five Patents including one each from USA and South Africa. R.P. Tandon is a PhD from the Department of Physics, Delhi University. He is a visiting scientist at MIT (USA), Queens University (Canada) and is currently a professor of physics in the Delhi University. Earlier, he was senior scientist at the National Physical Laboratory, New Delhi. His research interests include ceramics, polymers and glasses and has published over 100 research papers in these areas.