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Room temperature liquefied petroleum gas (LPG) sensor
Sensors and Actuators B 147 (2010) 488–494
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Room temperature liquefied petroleum gas (LPG) sensor D.S. Dhawale, D.P. Dubal, A.M. More, T.P. Gujar, C.D. Lokhande ∗ Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416004 (M.S.), India
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Article history: Received 12 November 2009 Received in revised form 21 February 2010 Accepted 26 February 2010 Available online 10 March 2010 Keywords: Heterojunction Thin films LPG sensor Selectivity LPG sensor model Stability
a b s t r a c t The p-polyaniline/n-ZnO thin film heterojunction sensor for room temperature LPG detection was fabricated by electrodepositing polyaniline on chemical bath deposited ZnO film. The heterojunction showed rectifying behavior indicating the formation of a diode with ideality factor 1.10. The formation of diffusion free interface of heterojunction was confirmed from cross-sectional FESEM. The heterojunction sensor has quick and high response towards LPG as compared to N2 and CO2 and exhibited maximum response of 81% upon exposure of 1040 ppm of LPG. The LPG sensing mechanism for heterojunction is modeled through change in height of barrier potential. © 2010 Elsevier B.V. All rights reserved.
1. Introduction LPG is a flammable gas which presents many hazards to both the humans and an environment. Due to its highly flammable characteristics, even low level concentration (ppm) poses a serious threat. With the global population boom, more and more human lives are being endangered by the effect of LPG exposure. LPG is used as an automotive fuel or as a propellant for aerosols, in addition to other specialist applications. The widespread use of LPG for cooking and as fuel for automobile vehicles requires fast and selective detection of LPG to precisely measure the leakage of gas for preventing the occurrence of accidental explosions. In spite of considerable efforts, good sensor for LPG has not been found hitherto, the problem being of vital significance to industry as well as general public. To meet this demand, considerable research for new sensors is underway, including efforts to enhance the performance of traditional devices, such as resistive metal oxide sensors, through nano-engineering. Metal oxide LPG sensors allows for the detection of lower level LPG presence, but suffer from low selectivity to specific target gases and high operation temperature and lower sensitivity [1–3]. Thus increase power consumption, reduce sensor life, limit the portability, etc. Though metal oxides have been extensively used for gas sensor, a new approach is needed to increase this selectivity and sensitivity at room temperature (300 K). The room temperature operation is also an important criterion to achieve
∗ Corresponding author. Tel.: +91 231 2609229; fax: +91 231 2692333. E-mail addresses: l [email protected], [email protected] (C.D. Lokhande). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.02.063
intrinsically safe performance in potentially hazardous situations. It has been pointed out that such sensors exhibit a fast, reversible response at room temperature [4–6]. In recent years, heterojunction between p- and n-type semiconductors have been developed for detecting various gases [7–10]. The heterojunction type sensors work on the principle of barrier mechanism [11,12], which needs no adsorption and desorption of oxygen for the detection of gas. The ZnO found potential applications in optoelectronic devices [13,14], gas sensor [15], dye sensitized solar cells (DSSC) [16], etc. However, these are effective at only temperatures above 473 K, resulting in high power consumption and complexities in integration. To meet the requirements for analyzing LPG and other poisoning species, to improve the stability and selectivity and to lower fabrication costs, conducting polymers in the form of thin films, blends, or heterojunctions have been developed and have shown great potential to enhance its sensitive characteristics. A considerable interest in the fabrication of ZnO/organic hybrid structure has also been developed for solar cell, photodiode, photovoltaic and photoelectrochemical applications [17–20]. Polyaniline (PANI) is potential p-type semiconducting material for use in junction devices [21,22]. It exhibits high electrical conductivity on doping and can be used in rechargeable batteries, electrochromic displays, electronic switches, gas sensors and photovoltaic devices. There are reports on the luminescence from nanosized ZnO/polyaniline structures [23] and UV photodetection by ZnO/PANI films [24,25]. Salomsom fabricated HCl doped p-polyaniline/n-ZnO junction by chemical bath deposition method which sensitive to UV photons [24]. The ZnO together with PANI, an inorganic/organic hybrid structure shows a promising application in the UV photodetection. The fabrication of p-polyaniline/n-ZnO heterojunction based diode
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Fig. 1. (a) Schematic and (b) actual experimental set up for the deposition of ZnO thin films.
structure and its application for room temperature gas sensor is not reported. Thus, the goal for this research is to produce a reliable, highly selective and sensitive LPG room temperature sensor, for a hand held application. Therefore, in this work, we have fabricated ppolyaniline/n-ZnO heterojunction using an (electro) chemical route and studied its room temperature LPG sensor mechanism.
2. Experimental details 2.1. Deposition of ZnO thin film The schematic of an experimental set up employed for the deposition of ZnO thin films using chemical bath deposition (CBD) is shown in Fig. 1(a). It consists of container, usually a glass beaker fixed in a water bath. The water bath is kept on the magnetic stirrerheater set up in order to maintain the desired temperature. The substrates were fitted in the bakelite holder having the slots for substrate and the holder is fixed in beaker containing the precursor solution. The actual setup of CBD method used for the deposition of ZnO films is shown in Fig. 1(b). ZnO thin films have been deposited
by CBD method from alkaline bath of zinc salt containing the substrates immersed in it. All chemicals used were A.R. grade (supplied by S.D. Fine Chem. Ltd., Mumbai). An aqueous solution of 0.1 M Zn(NO3 )2 was prepared and in this solution aqueous NH3 solution (25%) was added under constant stirring. A white precipitate was initially observed, which subsequently dissolved back into solution upon the further addition of the NH3 solution. Pre-cleaned stainless steel substrates were immersed and placed vertically in the solution. The solution was maintained at a pH of 9 and a temperature of 333 K for 2 h, resulting in the direct growth of ZnO nanorods on the substrate. The substrate was then removed from the bath, washed with deionized water, dried and annealed in air at 673 K for 4 h.
2.2. Fabrication of p-polyaniline/n-ZnO heterojunction Fig. 2(a) shows the experimental setup for the deposition of polyaniline on to ZnO thin films. It consists of conventional threeelectrode system, i.e. working electrode (ZnO), counter electrode (cathode) and reference electrode. These three electrodes are fitted in the bakelite holder having the slots for each electrode and the holder is fixed in cylindrical cell containing electrolyte. The
Fig. 2. (a) Schematic and (b) actual electrodeposition set up for the deposition of polyaniline thin films.
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cylindrical cell used is made up of glass, which serves two purposes of chemically inertness and visibility inside the bath. All the depositions were carried out using a scanning Potentiostat EG &G Princeton Applied Research Model 263A. The actual setup and electrochemical cell used for deposition of polyaniline films is shown in Fig. 2(b). For the fabrication of p-polyaniline/n-ZnO heterojunction, polyaniline film was deposited onto a previously deposited ZnO film using galvanostatic electrodeposition method [26] with a constant current 4 mA/cm2 . For this a solution containing 0.5 M H2 SO4 + 0.45 M aniline (C6 H5 NH2 ) was used. 2.3. Characterization techniques The thicknesses of the ZnO and Polyaniline films were calculated using fully computerized AMBIOS Make XP-1 surface profiler with 1 Å vertical resolution. For that sharp step is performed by applying cello tape prior to deposition. After the junction fabrication for each thickness of ZnO as well as polyaniline, the current density–voltage (J–V) characteristics were recorded. From the J–V characteristics, the junction with low ideality factor having high rectification ratio were calculated and optimized the film thickness. The optimized thicknesses of ZnO and polyaniline films were 0.5092 and 0.9012 m, respectively. The J–V characteristics of the junction were examined by making front aluminium foil press contact and back stainless steel contact to a heterojunction sample having area 1 cm × 1 cm. The schematic diagram of the p-polyaniline/n-ZnO heterojunction is depicted in inset (a) of Fig. 3. Onto stainless steel substrate ZnO and polyaniline films were subsequently deposited using CBD and electrodeposition methods, respectively. The surface morphological study of the ZnO, polyaniline and cross-sectional interface of a p-polyaniline/n-ZnO heterojunction was studied with field-emission scanning electron microscopy (FESEM, Model: JSM6160). 2.4. LPG sensing properties p-polyaniline/n-ZnO heterojunction 2.4.1. Experimental setup for gas sensor study In order to test the gas sensing ability of heterojunction, the gas sensor unit was specially designed. The gas sensor unit consists of small dome shaped glass vacuum chamber. The special mounting for sample with precise contacts was permanently installed inside the vacuum chamber. Fig. 4(a) shows the schematic of the gas sensor unit. Through the external connections, junction current–voltage (I–V) characteristic were recorded using Potentiostat (EG&G Princeton Applied Research Model 262-A). The forward biased I–V characteristics of the junction before and after exposure
Fig. 3. The current density (J)–voltage (V) plot of p-polyaniline/n-ZnO heterojunction. Inset: schematic representation of p-polyaniline/n-ZnO heterojunction.
of gases were recorded for different concentrations. From I–V characteristics, a fixed potential at which the maximum current change occurred, was noted. The electrical currents of heterojunction in air (Ia ) and in the presence of gas (Ig ) were measured and using following relation the gas response was calculated. S(%) =
Ia − Ig I × 100 = × 100 Ia Ia
(1)
The response and recovery time periods of the junction towards gas were determined at fixed forward biased potential (+1.8 V). 2.4.2. Gas volume measurement In the present work, we have developed a simple gas volume measurement unit as shown in Fig. 4(b). It consists of glass bottle containing double distilled water which is saturated with gas to be measured, in order to avoid the possibility of dissolution of inserted gas. Above this bottle, the measuring tube (pipette) is connected by vacuum seal. The cock I is connected to gas cylinder and cock II is connected to inlet of gas chamber. When the cock I is opened, the gas from the cylinder is filled in the glass bottle and the equivalent amount of water is displaced in the measurement pipette. When the cock II is opened, the desired amount of gas volume is injected in the gas chamber which is prefilled with air. Fig. 5(a) and (b) shows the photograph of actual set up of gas sensor assembly and gas volume measurement unit.
Fig. 4. Schematic diagram of the (a) gas sensor unit and (b) the gas volume measurement unit.
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Fig. 5. (a) Actual set up of gas sensor unit [inset: photograph of sample mounting and contacts] and (b) gas volume measurement unit.
3. Results and discussion 3.1. Surface morphological studies The typical J–V characteristic of p-polyaniline/n-ZnO heterojunction for optimized film thicknesses is shown in Fig. 3. The
J–V characteristic shows rectifying behavior having rectification ratio 8.8 × 103 with ideality factor 1.10 indicating the formation of good quality diode. Fig. 6(a) and (b) shows the typical FESEM images of ZnO and polyaniline films at ×100K magnifications, respectively. It is clearly seen that ZnO films consist of nanorods which are well-defined and grown almost perpendicular
Fig. 6. The FESEM images of (a) annealed ZnO at 673 K, (b) polyaniline and (c) cross-sectional FESEM of p-polyaniline/n-ZnO heterojunction.
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Fig. 7. Forward biased I–V characteristics of p-polyaniline/n-ZnO heterojunction in the voltage range of 0 to +1.8 V and at a concentration of 520 ppm for various gases (a) without, (b) N2 , (c) CO2 , and (d) LPG. (Inset: selectivity bar diagram for different gases at 520 ppm gas concentration.)
and parallel to the substrate. The FESEM image of the polyaniline film (Fig. 6(b)) exhibits a fibrous structure with many pores and gaps among the fibers. Fig. 6(c) shows the cross-sectional FESEM image of p-polyaniline/n-ZnO heterojunction interface. The interface cross-sectional FESEM image shows the formation of diffusion free junction. The FESEM also revealed that the polyaniline is highly porous with interconnectivity of fibers and grains and the rough layer of ZnO. Due to the rough layer, a non-uniform interface is formed. 3.2. Gas sensing studies 3.2.1. Selectivity studies Fig. 7 shows the forward biased I–V characteristics of ppolyaniline/n-ZnO heterojunction for different gases at room temperature in voltage range of 0 to +1.8 V. Fig. 7(a) is the I–V characteristic before exposure to the gas and those from (b), (c) and (d) are the plots when exposed N2 , CO2 and LPG at 520 ppm concentration. From the Fig. 7 it is seen that, the maximum change in current is observed for LPG as compared to N2 and CO2 gases. Inset of Fig. 7 shows the heterojunction gas response towards N2 (1.74%), CO2 (15.46%) and LPG (28.48%). The heterojunction showed more selectivity for LPG over N2 compared to CO2 (SLPG /SN2 = 16.36 and SLPG /SCO2 = 1.86). It revealed that LPG is most selective against N2 and poor selective against CO2 . 3.2.2. LPG sensing studies Fig. 8 represents the typical forward biased I–V characteristic plots of p-polyaniline/n-ZnO heterojunction in the absence and presence of different concentrations of LPG at room temperature. Fig. 8(a) shows the I–V characteristics without LPG and those from (b) to (f) are with LPG for the concentration ranging from 260 to 1300 ppm. A shift is occurred in the I–V characteristics after exposure of LPG. As the heterojunction is exposed to LPG, the forward current drastically decreased with increase in concentration of LPG up to 1040 ppm. The decrease in forward current with increased concentration of LPG has been attributed to the decrease in charge carrier concentration as LPG molecules reach at the interface of the heterojunction through polyaniline. Such decrease in current has been attributed due to the change in work function of the polyaniline and resistance of the polyaniline increase. Thus the carrier concentration of the heterojunction decreases and current across the junction decreases and consequently barrier height of the heterojunction increases when
exposed to LPG, contrast to hydrogen gas sensors based on Pd/TiO2 [27]. The change in current may be due to the chemical reaction between interface of heterojunction and adsorbed gas molecules. Tucci et al. [28] have reported that the change in the conductivity is due to the interaction of adsorbed gas molecules onto interface of heterojunction. The LPG response of the heterojunction at an applied potential +1.8 V is depicted as inset of Fig. 8. The sensitivity increased from 15.46% to 81% with increase in concentration of LPG from 260 to 1040 ppm. The maximum LPG sensitivity of 81% was recorded under the exposure of 1040 ppm, i.e. 4% of lower explosive level (LEL) of LPG. At 1300 ppm LPG, sensitivity was decreased to 69.76%. At lower gas concentrations (<1040 ppm LPG), the unimolecular layer of gas molecules would be expected to form on the interface, which would interact with the interface more actively giving larger response. There would be multilayers of LPG molecules on the interface of junction at the higher gas concentrations (>1040 ppm LPG) resulting into decrease in the sensitivity.
3.2.3. LPG sensing model The most commonly accepted model for the operation of heterojunction gas sensors is the potential barrier dependent. Fig. 9(a) and (b) shows the schematic representation of the model for p-polyaniline/n-ZnO heterojunction for LPG detection at room temperature (300 K) with corresponding its physical model in Fig. 9(a’) and (b’), respectively before and after the exposure of LPG. Heterojunction diode is a rather simple device and can be employed to detect gas analytes. Instead of using the mechanism of adsorption–desorption of oxygen for the detection of LPG, the principle of formation of heterojunction barrier in air ambient and their disruption on exposure to LPG is employed. The effective barrier height, ‘B ’ can be modulated by analyte, through changing doping level of conducting polymer. The molecules of reducing gases (LPG), changes the potential barrier height and consequently the current across the junction. From Fig. 9(b’), it is seen that, when LPG molecules entered through the polyaniline, reach at the interface of p-polyaniline/n-ZnO heterojunction, increases the barrier height of heterojunction (Fig. 9(b’)). This leads to the interaction of LPG molecules at the interface of p-polyaniline/n-ZnO heterojunction (Fig. 9(b’)) resulting the decreases in the carrier concentration near the interface of heterojunction. This decrease in carrier concentration increases the potential barrier of heterojunction interface (Fig. 9(b’)).
Fig. 8. Forward biased I–V characteristics of p-polyaniline/n-ZnO heterojunction at various concentrations of LPG in (a) without, (b) 260 ppm, (c) 520 ppm, (d) 780 ppm (e) 1040 ppm, and (f) 1300 ppm. (Inset: the gas response (%) vs. LPG concentration.)
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Fig. 9. The LPG sensing model of p-polyaniline/n-ZnO heterojunction before (a) and after (b) exposure of LPG and their corresponding physical models (a’) and (b’).
3.2.4. Dynamic response and stability studies The dynamic variation of gas response with time at the fix concentration of 1040 ppm LPG is shown in Fig. 10. For the measurement of response and recovery time periods, we have taken I–V characteristics in forward biased region for different LPG concentrations and we found as the LPG concentration increases from 260 to 1040 ppm, response time decreases from 200 to 100 s. Also, the corresponding recovery time periods increase from 110 to 150 s. The heterojunction attained the maximum sensitivity in short time upon exposure of 1040 ppm LPG and dropped rapidly when the gas was removed from testing atmosphere indicating that the sensor has the good response (100 s) and recovery time (150 s). For the stability studies of the p-polyaniline/n-ZnO heterojunction sensor element, the forward biased I–V characteristics were performed at room temperature upon exposure of fixed concentra-
tion of 1040 ppm LPG for 30 days with 5 days interval after the first measurement. The gas response with time is illustrated in inset of Fig. 10. The LPG performance remained almost the same after initial decrease (10%) from which it is concluded that p-polyaniline/nZnO heterojunction can stand as a reliable sensor element for room temperature LPG sensor application. 4. Conclusions In summary, the present work has been addressed LPG sensing performance of p-polyaniline/n-ZnO heterojunction, which appeared as a promising material for such an application due to its room temperature operation. The results highlighted that this material can be applied in gas sensing field to develop LPG sensors with performances suitable for practical application. The present heterojunction is stable, robust, compact, easy to fabricate and diffusion free and compatible with integrated circuit fabrication technology. Acknowledgement Authors are grateful to the Department of Science and Technology, New Delhi for financial support through the scheme no. SR/S2/CMP-82/2006. References
Fig. 10. The gas response (%) vs. time(s) plot of the p-polyaniline/n-ZnO heterojunction at voltage of +1.8 V and at a concentration of 1040 ppm LPG. (Inset: stability studies.)
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Biographies D.S. Dhawale received his B.Sc. degree (2005) in general physics, M.Sc. degree (2007) in materials science and Ph.D. (2009) in liquefied petroleum gas sensor performance of polyaniline based heterojunctions from the Shivaji University, Kolhapur, India (M.S.). His present research interest includes synthesis of polyaniline based heterojunctions, metal oxides for solar cells, dye sensitized solar cells, room temperature (300 K) gas sensors and supercapacitors. D.P. Dubal received his B.Sc. degree (2006) in general physics, M.Sc. degree (2008) in solid state physics and presently doing Ph.D. in preparation and characterization of electrodeposited Fe doped MnO2 thin films for supercapacitor application, from the Shivaji University, Kolhapur, India (M.S.). His present research interest includes synthesis of oxide thin films and their applications. A.M. More received his B.Sc. (2003) in general physics from Shivaji University, Kolhapur (India), M.Sc. (2005) in general physics from Pune University, Pune, and Ph.D. (2009) in synthesis of nanocrystalline TiO2 thin films by chemical methods and their applications in dye sensitized solar cells. He is presently working as a post doc. fellow at Hanyang University, South Korea. His research interests are in the field of synthesis of thin films of metal oxide by chemical, electrochemical methods and their applications in solar cells, dye sensitized solar cells, gas sensors and supercapacitors. T.P. Gujar received his B.Sc. (2001) in general physics, M.Sc. (2003) in materials science and Ph.D. (2006) in oxide film preparation and application in supercapacitors, from Shivaji University, Kolhapur (India). He is presently working as a post doc. fellow at KIST, Republic of Korea. His research interests are in the field of synthesis of thin films of metal oxide by vacuum, chemical, electrochemical methods, and their applications in supercapacitors and gas sensors. C.D. Lokhande received his Ph.D. in 1984. He was a Humboldtian (Hahn-Meitner Institute, Berlin, Germany). He is fellow of Institute of Physics. He is currently a professor in the Department of Physics, Shivaji University, Kolhapur, India (M.S.). He has been continuously engaged in the research field more than last 30 years. His research interest includes the synthesis of thin films of metal chalcogenides, metal oxides, conducting polymers and ferrites by chemical, electrochemical methods and their applications in dye sensitized solar cells, gas sensors, energy storage devices, etc.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Room temperature liquefied petroleum gas (LPG) sensor D.S. Dhawale, D.P. Dubal, A.M. More, T.P. Gujar, C.D. Lokhande ∗ Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416004 (M.S.), India
a r t i c l e
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Article history: Received 12 November 2009 Received in revised form 21 February 2010 Accepted 26 February 2010 Available online 10 March 2010 Keywords: Heterojunction Thin films LPG sensor Selectivity LPG sensor model Stability
a b s t r a c t The p-polyaniline/n-ZnO thin film heterojunction sensor for room temperature LPG detection was fabricated by electrodepositing polyaniline on chemical bath deposited ZnO film. The heterojunction showed rectifying behavior indicating the formation of a diode with ideality factor 1.10. The formation of diffusion free interface of heterojunction was confirmed from cross-sectional FESEM. The heterojunction sensor has quick and high response towards LPG as compared to N2 and CO2 and exhibited maximum response of 81% upon exposure of 1040 ppm of LPG. The LPG sensing mechanism for heterojunction is modeled through change in height of barrier potential. © 2010 Elsevier B.V. All rights reserved.
1. Introduction LPG is a flammable gas which presents many hazards to both the humans and an environment. Due to its highly flammable characteristics, even low level concentration (ppm) poses a serious threat. With the global population boom, more and more human lives are being endangered by the effect of LPG exposure. LPG is used as an automotive fuel or as a propellant for aerosols, in addition to other specialist applications. The widespread use of LPG for cooking and as fuel for automobile vehicles requires fast and selective detection of LPG to precisely measure the leakage of gas for preventing the occurrence of accidental explosions. In spite of considerable efforts, good sensor for LPG has not been found hitherto, the problem being of vital significance to industry as well as general public. To meet this demand, considerable research for new sensors is underway, including efforts to enhance the performance of traditional devices, such as resistive metal oxide sensors, through nano-engineering. Metal oxide LPG sensors allows for the detection of lower level LPG presence, but suffer from low selectivity to specific target gases and high operation temperature and lower sensitivity [1–3]. Thus increase power consumption, reduce sensor life, limit the portability, etc. Though metal oxides have been extensively used for gas sensor, a new approach is needed to increase this selectivity and sensitivity at room temperature (300 K). The room temperature operation is also an important criterion to achieve
∗ Corresponding author. Tel.: +91 231 2609229; fax: +91 231 2692333. E-mail addresses: l [email protected], [email protected] (C.D. Lokhande). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.02.063
intrinsically safe performance in potentially hazardous situations. It has been pointed out that such sensors exhibit a fast, reversible response at room temperature [4–6]. In recent years, heterojunction between p- and n-type semiconductors have been developed for detecting various gases [7–10]. The heterojunction type sensors work on the principle of barrier mechanism [11,12], which needs no adsorption and desorption of oxygen for the detection of gas. The ZnO found potential applications in optoelectronic devices [13,14], gas sensor [15], dye sensitized solar cells (DSSC) [16], etc. However, these are effective at only temperatures above 473 K, resulting in high power consumption and complexities in integration. To meet the requirements for analyzing LPG and other poisoning species, to improve the stability and selectivity and to lower fabrication costs, conducting polymers in the form of thin films, blends, or heterojunctions have been developed and have shown great potential to enhance its sensitive characteristics. A considerable interest in the fabrication of ZnO/organic hybrid structure has also been developed for solar cell, photodiode, photovoltaic and photoelectrochemical applications [17–20]. Polyaniline (PANI) is potential p-type semiconducting material for use in junction devices [21,22]. It exhibits high electrical conductivity on doping and can be used in rechargeable batteries, electrochromic displays, electronic switches, gas sensors and photovoltaic devices. There are reports on the luminescence from nanosized ZnO/polyaniline structures [23] and UV photodetection by ZnO/PANI films [24,25]. Salomsom fabricated HCl doped p-polyaniline/n-ZnO junction by chemical bath deposition method which sensitive to UV photons [24]. The ZnO together with PANI, an inorganic/organic hybrid structure shows a promising application in the UV photodetection. The fabrication of p-polyaniline/n-ZnO heterojunction based diode
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Fig. 1. (a) Schematic and (b) actual experimental set up for the deposition of ZnO thin films.
structure and its application for room temperature gas sensor is not reported. Thus, the goal for this research is to produce a reliable, highly selective and sensitive LPG room temperature sensor, for a hand held application. Therefore, in this work, we have fabricated ppolyaniline/n-ZnO heterojunction using an (electro) chemical route and studied its room temperature LPG sensor mechanism.
2. Experimental details 2.1. Deposition of ZnO thin film The schematic of an experimental set up employed for the deposition of ZnO thin films using chemical bath deposition (CBD) is shown in Fig. 1(a). It consists of container, usually a glass beaker fixed in a water bath. The water bath is kept on the magnetic stirrerheater set up in order to maintain the desired temperature. The substrates were fitted in the bakelite holder having the slots for substrate and the holder is fixed in beaker containing the precursor solution. The actual setup of CBD method used for the deposition of ZnO films is shown in Fig. 1(b). ZnO thin films have been deposited
by CBD method from alkaline bath of zinc salt containing the substrates immersed in it. All chemicals used were A.R. grade (supplied by S.D. Fine Chem. Ltd., Mumbai). An aqueous solution of 0.1 M Zn(NO3 )2 was prepared and in this solution aqueous NH3 solution (25%) was added under constant stirring. A white precipitate was initially observed, which subsequently dissolved back into solution upon the further addition of the NH3 solution. Pre-cleaned stainless steel substrates were immersed and placed vertically in the solution. The solution was maintained at a pH of 9 and a temperature of 333 K for 2 h, resulting in the direct growth of ZnO nanorods on the substrate. The substrate was then removed from the bath, washed with deionized water, dried and annealed in air at 673 K for 4 h.
2.2. Fabrication of p-polyaniline/n-ZnO heterojunction Fig. 2(a) shows the experimental setup for the deposition of polyaniline on to ZnO thin films. It consists of conventional threeelectrode system, i.e. working electrode (ZnO), counter electrode (cathode) and reference electrode. These three electrodes are fitted in the bakelite holder having the slots for each electrode and the holder is fixed in cylindrical cell containing electrolyte. The
Fig. 2. (a) Schematic and (b) actual electrodeposition set up for the deposition of polyaniline thin films.
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cylindrical cell used is made up of glass, which serves two purposes of chemically inertness and visibility inside the bath. All the depositions were carried out using a scanning Potentiostat EG &G Princeton Applied Research Model 263A. The actual setup and electrochemical cell used for deposition of polyaniline films is shown in Fig. 2(b). For the fabrication of p-polyaniline/n-ZnO heterojunction, polyaniline film was deposited onto a previously deposited ZnO film using galvanostatic electrodeposition method [26] with a constant current 4 mA/cm2 . For this a solution containing 0.5 M H2 SO4 + 0.45 M aniline (C6 H5 NH2 ) was used. 2.3. Characterization techniques The thicknesses of the ZnO and Polyaniline films were calculated using fully computerized AMBIOS Make XP-1 surface profiler with 1 Å vertical resolution. For that sharp step is performed by applying cello tape prior to deposition. After the junction fabrication for each thickness of ZnO as well as polyaniline, the current density–voltage (J–V) characteristics were recorded. From the J–V characteristics, the junction with low ideality factor having high rectification ratio were calculated and optimized the film thickness. The optimized thicknesses of ZnO and polyaniline films were 0.5092 and 0.9012 m, respectively. The J–V characteristics of the junction were examined by making front aluminium foil press contact and back stainless steel contact to a heterojunction sample having area 1 cm × 1 cm. The schematic diagram of the p-polyaniline/n-ZnO heterojunction is depicted in inset (a) of Fig. 3. Onto stainless steel substrate ZnO and polyaniline films were subsequently deposited using CBD and electrodeposition methods, respectively. The surface morphological study of the ZnO, polyaniline and cross-sectional interface of a p-polyaniline/n-ZnO heterojunction was studied with field-emission scanning electron microscopy (FESEM, Model: JSM6160). 2.4. LPG sensing properties p-polyaniline/n-ZnO heterojunction 2.4.1. Experimental setup for gas sensor study In order to test the gas sensing ability of heterojunction, the gas sensor unit was specially designed. The gas sensor unit consists of small dome shaped glass vacuum chamber. The special mounting for sample with precise contacts was permanently installed inside the vacuum chamber. Fig. 4(a) shows the schematic of the gas sensor unit. Through the external connections, junction current–voltage (I–V) characteristic were recorded using Potentiostat (EG&G Princeton Applied Research Model 262-A). The forward biased I–V characteristics of the junction before and after exposure
Fig. 3. The current density (J)–voltage (V) plot of p-polyaniline/n-ZnO heterojunction. Inset: schematic representation of p-polyaniline/n-ZnO heterojunction.
of gases were recorded for different concentrations. From I–V characteristics, a fixed potential at which the maximum current change occurred, was noted. The electrical currents of heterojunction in air (Ia ) and in the presence of gas (Ig ) were measured and using following relation the gas response was calculated. S(%) =
Ia − Ig I × 100 = × 100 Ia Ia
(1)
The response and recovery time periods of the junction towards gas were determined at fixed forward biased potential (+1.8 V). 2.4.2. Gas volume measurement In the present work, we have developed a simple gas volume measurement unit as shown in Fig. 4(b). It consists of glass bottle containing double distilled water which is saturated with gas to be measured, in order to avoid the possibility of dissolution of inserted gas. Above this bottle, the measuring tube (pipette) is connected by vacuum seal. The cock I is connected to gas cylinder and cock II is connected to inlet of gas chamber. When the cock I is opened, the gas from the cylinder is filled in the glass bottle and the equivalent amount of water is displaced in the measurement pipette. When the cock II is opened, the desired amount of gas volume is injected in the gas chamber which is prefilled with air. Fig. 5(a) and (b) shows the photograph of actual set up of gas sensor assembly and gas volume measurement unit.
Fig. 4. Schematic diagram of the (a) gas sensor unit and (b) the gas volume measurement unit.
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Fig. 5. (a) Actual set up of gas sensor unit [inset: photograph of sample mounting and contacts] and (b) gas volume measurement unit.
3. Results and discussion 3.1. Surface morphological studies The typical J–V characteristic of p-polyaniline/n-ZnO heterojunction for optimized film thicknesses is shown in Fig. 3. The
J–V characteristic shows rectifying behavior having rectification ratio 8.8 × 103 with ideality factor 1.10 indicating the formation of good quality diode. Fig. 6(a) and (b) shows the typical FESEM images of ZnO and polyaniline films at ×100K magnifications, respectively. It is clearly seen that ZnO films consist of nanorods which are well-defined and grown almost perpendicular
Fig. 6. The FESEM images of (a) annealed ZnO at 673 K, (b) polyaniline and (c) cross-sectional FESEM of p-polyaniline/n-ZnO heterojunction.
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Fig. 7. Forward biased I–V characteristics of p-polyaniline/n-ZnO heterojunction in the voltage range of 0 to +1.8 V and at a concentration of 520 ppm for various gases (a) without, (b) N2 , (c) CO2 , and (d) LPG. (Inset: selectivity bar diagram for different gases at 520 ppm gas concentration.)
and parallel to the substrate. The FESEM image of the polyaniline film (Fig. 6(b)) exhibits a fibrous structure with many pores and gaps among the fibers. Fig. 6(c) shows the cross-sectional FESEM image of p-polyaniline/n-ZnO heterojunction interface. The interface cross-sectional FESEM image shows the formation of diffusion free junction. The FESEM also revealed that the polyaniline is highly porous with interconnectivity of fibers and grains and the rough layer of ZnO. Due to the rough layer, a non-uniform interface is formed. 3.2. Gas sensing studies 3.2.1. Selectivity studies Fig. 7 shows the forward biased I–V characteristics of ppolyaniline/n-ZnO heterojunction for different gases at room temperature in voltage range of 0 to +1.8 V. Fig. 7(a) is the I–V characteristic before exposure to the gas and those from (b), (c) and (d) are the plots when exposed N2 , CO2 and LPG at 520 ppm concentration. From the Fig. 7 it is seen that, the maximum change in current is observed for LPG as compared to N2 and CO2 gases. Inset of Fig. 7 shows the heterojunction gas response towards N2 (1.74%), CO2 (15.46%) and LPG (28.48%). The heterojunction showed more selectivity for LPG over N2 compared to CO2 (SLPG /SN2 = 16.36 and SLPG /SCO2 = 1.86). It revealed that LPG is most selective against N2 and poor selective against CO2 . 3.2.2. LPG sensing studies Fig. 8 represents the typical forward biased I–V characteristic plots of p-polyaniline/n-ZnO heterojunction in the absence and presence of different concentrations of LPG at room temperature. Fig. 8(a) shows the I–V characteristics without LPG and those from (b) to (f) are with LPG for the concentration ranging from 260 to 1300 ppm. A shift is occurred in the I–V characteristics after exposure of LPG. As the heterojunction is exposed to LPG, the forward current drastically decreased with increase in concentration of LPG up to 1040 ppm. The decrease in forward current with increased concentration of LPG has been attributed to the decrease in charge carrier concentration as LPG molecules reach at the interface of the heterojunction through polyaniline. Such decrease in current has been attributed due to the change in work function of the polyaniline and resistance of the polyaniline increase. Thus the carrier concentration of the heterojunction decreases and current across the junction decreases and consequently barrier height of the heterojunction increases when
exposed to LPG, contrast to hydrogen gas sensors based on Pd/TiO2 [27]. The change in current may be due to the chemical reaction between interface of heterojunction and adsorbed gas molecules. Tucci et al. [28] have reported that the change in the conductivity is due to the interaction of adsorbed gas molecules onto interface of heterojunction. The LPG response of the heterojunction at an applied potential +1.8 V is depicted as inset of Fig. 8. The sensitivity increased from 15.46% to 81% with increase in concentration of LPG from 260 to 1040 ppm. The maximum LPG sensitivity of 81% was recorded under the exposure of 1040 ppm, i.e. 4% of lower explosive level (LEL) of LPG. At 1300 ppm LPG, sensitivity was decreased to 69.76%. At lower gas concentrations (<1040 ppm LPG), the unimolecular layer of gas molecules would be expected to form on the interface, which would interact with the interface more actively giving larger response. There would be multilayers of LPG molecules on the interface of junction at the higher gas concentrations (>1040 ppm LPG) resulting into decrease in the sensitivity.
3.2.3. LPG sensing model The most commonly accepted model for the operation of heterojunction gas sensors is the potential barrier dependent. Fig. 9(a) and (b) shows the schematic representation of the model for p-polyaniline/n-ZnO heterojunction for LPG detection at room temperature (300 K) with corresponding its physical model in Fig. 9(a’) and (b’), respectively before and after the exposure of LPG. Heterojunction diode is a rather simple device and can be employed to detect gas analytes. Instead of using the mechanism of adsorption–desorption of oxygen for the detection of LPG, the principle of formation of heterojunction barrier in air ambient and their disruption on exposure to LPG is employed. The effective barrier height, ‘B ’ can be modulated by analyte, through changing doping level of conducting polymer. The molecules of reducing gases (LPG), changes the potential barrier height and consequently the current across the junction. From Fig. 9(b’), it is seen that, when LPG molecules entered through the polyaniline, reach at the interface of p-polyaniline/n-ZnO heterojunction, increases the barrier height of heterojunction (Fig. 9(b’)). This leads to the interaction of LPG molecules at the interface of p-polyaniline/n-ZnO heterojunction (Fig. 9(b’)) resulting the decreases in the carrier concentration near the interface of heterojunction. This decrease in carrier concentration increases the potential barrier of heterojunction interface (Fig. 9(b’)).
Fig. 8. Forward biased I–V characteristics of p-polyaniline/n-ZnO heterojunction at various concentrations of LPG in (a) without, (b) 260 ppm, (c) 520 ppm, (d) 780 ppm (e) 1040 ppm, and (f) 1300 ppm. (Inset: the gas response (%) vs. LPG concentration.)
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Fig. 9. The LPG sensing model of p-polyaniline/n-ZnO heterojunction before (a) and after (b) exposure of LPG and their corresponding physical models (a’) and (b’).
3.2.4. Dynamic response and stability studies The dynamic variation of gas response with time at the fix concentration of 1040 ppm LPG is shown in Fig. 10. For the measurement of response and recovery time periods, we have taken I–V characteristics in forward biased region for different LPG concentrations and we found as the LPG concentration increases from 260 to 1040 ppm, response time decreases from 200 to 100 s. Also, the corresponding recovery time periods increase from 110 to 150 s. The heterojunction attained the maximum sensitivity in short time upon exposure of 1040 ppm LPG and dropped rapidly when the gas was removed from testing atmosphere indicating that the sensor has the good response (100 s) and recovery time (150 s). For the stability studies of the p-polyaniline/n-ZnO heterojunction sensor element, the forward biased I–V characteristics were performed at room temperature upon exposure of fixed concentra-
tion of 1040 ppm LPG for 30 days with 5 days interval after the first measurement. The gas response with time is illustrated in inset of Fig. 10. The LPG performance remained almost the same after initial decrease (10%) from which it is concluded that p-polyaniline/nZnO heterojunction can stand as a reliable sensor element for room temperature LPG sensor application. 4. Conclusions In summary, the present work has been addressed LPG sensing performance of p-polyaniline/n-ZnO heterojunction, which appeared as a promising material for such an application due to its room temperature operation. The results highlighted that this material can be applied in gas sensing field to develop LPG sensors with performances suitable for practical application. The present heterojunction is stable, robust, compact, easy to fabricate and diffusion free and compatible with integrated circuit fabrication technology. Acknowledgement Authors are grateful to the Department of Science and Technology, New Delhi for financial support through the scheme no. SR/S2/CMP-82/2006. References
Fig. 10. The gas response (%) vs. time(s) plot of the p-polyaniline/n-ZnO heterojunction at voltage of +1.8 V and at a concentration of 1040 ppm LPG. (Inset: stability studies.)
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Biographies D.S. Dhawale received his B.Sc. degree (2005) in general physics, M.Sc. degree (2007) in materials science and Ph.D. (2009) in liquefied petroleum gas sensor performance of polyaniline based heterojunctions from the Shivaji University, Kolhapur, India (M.S.). His present research interest includes synthesis of polyaniline based heterojunctions, metal oxides for solar cells, dye sensitized solar cells, room temperature (300 K) gas sensors and supercapacitors. D.P. Dubal received his B.Sc. degree (2006) in general physics, M.Sc. degree (2008) in solid state physics and presently doing Ph.D. in preparation and characterization of electrodeposited Fe doped MnO2 thin films for supercapacitor application, from the Shivaji University, Kolhapur, India (M.S.). His present research interest includes synthesis of oxide thin films and their applications. A.M. More received his B.Sc. (2003) in general physics from Shivaji University, Kolhapur (India), M.Sc. (2005) in general physics from Pune University, Pune, and Ph.D. (2009) in synthesis of nanocrystalline TiO2 thin films by chemical methods and their applications in dye sensitized solar cells. He is presently working as a post doc. fellow at Hanyang University, South Korea. His research interests are in the field of synthesis of thin films of metal oxide by chemical, electrochemical methods and their applications in solar cells, dye sensitized solar cells, gas sensors and supercapacitors. T.P. Gujar received his B.Sc. (2001) in general physics, M.Sc. (2003) in materials science and Ph.D. (2006) in oxide film preparation and application in supercapacitors, from Shivaji University, Kolhapur (India). He is presently working as a post doc. fellow at KIST, Republic of Korea. His research interests are in the field of synthesis of thin films of metal oxide by vacuum, chemical, electrochemical methods, and their applications in supercapacitors and gas sensors. C.D. Lokhande received his Ph.D. in 1984. He was a Humboldtian (Hahn-Meitner Institute, Berlin, Germany). He is fellow of Institute of Physics. He is currently a professor in the Department of Physics, Shivaji University, Kolhapur, India (M.S.). He has been continuously engaged in the research field more than last 30 years. His research interest includes the synthesis of thin films of metal chalcogenides, metal oxides, conducting polymers and ferrites by chemical, electrochemical methods and their applications in dye sensitized solar cells, gas sensors, energy storage devices, etc.