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n-TiO2 heterojunction
Sensors and Actuators B 134 (2008) 988–992
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Room temperature liquefied petroleum gas (LPG) sensor based on p-polyaniline/n-TiO2 heterojunction D.S. Dhawale, R.R. Salunkhe, U.M. Patil, K.V. Gurav, A.M. More, C.D. Lokhande ∗ Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416004 (M.S.), India
a r t i c l e
i n f o
Article history: Received 31 March 2008 Received in revised form 4 July 2008 Accepted 7 July 2008 Available online 16 July 2008 Keywords: Thin films TiO2 Polyaniline Heterojunction LPG sensor
a b s t r a c t In the present work, we report on the performance of a room temperature (300 K) liquefied petroleum gas (LPG) sensor based on a p-polyaniline/n-TiO2 heterojunction. The heterojunction was fabricated using electrochemically deposited polyaniline on chemically deposited TiO2 on a stainless steel substrate. Both the methods (chemical bath deposition and electrodeposition) are simple, inexpensive and suitable for large-scale production. TiO2 and polyaniline films were characterized for their structural as well as surface morphologies and LPG response was studied. The XRD analysis showed formation of polycrystalline TiO2 while polyaniline exhibited amorphous nature. Morphological analysis using scanning electron microscopy (SEM) of the junction cross-section revealed formation of a diffusion free interface. The heterojunction showed the maximum response of 63% upon exposure to 0.1 vol% LPG at room temperature. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Many studies on various materials as gas sensors have been reported in recent years. Gas sensing materials can be classified mainly into two types, namely, organic and inorganic materials. Semiconductor inorganic gas sensors like doped or undoped SnO2, ZnO or Fe2 O3 have been well studied to detect most of reducing gases and they are considered interesting for their low cost and simple sensing methods [1–7]. Nevertheless, there still exist some problems with them, for example high working temperature of 423–623 K for SnO2 and 673–723 K for ZnO [8,9]. Heterojunction sensors are mostly based on the interface between p-type (p) and n-type (n) semiconducting ceramics [10–13]. Hazardous gases, specifically liquefied petroleum gas (LPG), have been widely used for several industrial and domestic applications. But, at certain low concentration of the gases, these metal oxide sensors show poor performance with respect to the sensitivity, long term stability, selectivity, etc. Recently, conducting polymers have been widely investigated as effective materials for room temperature chemical sensors. Polyaniline is one of the most attractive materials among the variety of conducting polymers due to its unique electrical properties, environmental stability and easy fabrication process. Due to its interesting properties, polyaniline has been a potential candidate in sensor applications [14,15], light emitting
∗ Corresponding author. Tel.: +91 231 2609229; fax: +91 231 260933. E-mail address: l [email protected] (C.D. Lokhande). 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.07.003
diodes [16], and rechargeable batteries [17]. However, the problems with these conducting polymers are their low processing ability, poor chemical stability and mechanical strength [18]. As an option, there is a room to fabricate heterojunctions between organic and inorganic materials with enhancement of the sensor characteristics and mechanical strength. By using electrochemical polymerization, polyaniline and its nanocomposite have been fabricated in a bulk form. Pd-polyaniline nanocomposite was prepared for a methanol gas sensor [19]. Tai et al. [20] fabricated a polyaniline–titanium dioxide nanocomposite for NH3 and CO sensors and reported that the resistance of the composite increased with increasing concentration of the gases. Nicho et al. [21] developed a polyaniline composite sensor for low concentration of NH3 gas. A ZnO/polyaniline layer-by-layer assembly and heterostructured polyaniline/Bi2 Te3 nanowires were fabricated by Paul et al. [22] and Xu et al. [23], respectively. Recently, Joshi et al. developed n-CdSe/p-polyaniline and n-CdTe/p-polyaniline heterojunctions for a room temperature LPG sensor [24,25]. Among the inorganic materials, nanocrystalline TiO2 is one of the most attractive and extensively used materials for detection of H2 , NH3 , NO2 and LPG gases [26,27]. However, due to the longterm instability at elevated temperature, it is desirable to develop sensors that operate at room temperature. In the present work, for the first time, we report fabrication of a p-polyaniline/n-TiO2 heterojunction with a good rectifying ratio by adopting a simple and inexpensive chemical route. Specifically, a nanocrystalline TiO2 thin film was deposited on a stainless steel substrate by chemical bath deposition (CBD), followed by
D.S. Dhawale et al. / Sensors and Actuators B 134 (2008) 988–992
989
CuK␣ radiation ( = 1.5406 Å) in a 2 range from 10◦ to 100◦ . The surface morphological study of the TiO2 , polyaniline and crosssectional interface of a p-polyaniline/n-TiO2 heterojunction was carried out using scanning electron microscopy (JEOL-6360). For this, the films were coated with a 10 nm platinum layer using a polaron scanning electron microscopy (SEM) sputter coating unit E-2500 before taking the image. 2.3. LPG sensing properties p-polyaniline/n-TiO2 heterojunction Fig. 1. A schematic representation of a p-polyaniline/n-TiO2 heterojunction.
polyaniline film by an electrodeposition (ED) method. These films were characterized using XRD and SEM techniques. The sensing performance at different concentrations of LPG (0.04–0.12 vol%) was studied at room temperature (300 K) by current–voltage (I–V) characteristics under the forward bias condition. 2. Experimental details 2.1. Fabrication of p-polyaniline/n-TiO2 heterojunction Preparation of a TiO2 thin film by the CBD method is based on the heating of an acidic solution of titanium (III) chloride containing a substrate immersed in it. The titanium (III) chloride solution was mixed with double distilled water in appropriate quantities. Specifically, 2.5 ml of TiCl3 (30 wt% in HCl, Loba Chemie, India) was added to 50 ml of double distilled water. The pH of the solution was adjusted to ∼1 using urea (NH2 CONH2 ) while constantly stirring at room temperature for 30 min. A stainless steel substrate was immersed vertically in the above bath and the bath was heated. At 353 K, the precipitation was started in the bath. During the precipitation, heterogeneous reaction occurred and deposition of TiO2 took place on the substrates. The substrate coated with TiO2 thin film were removed after 2 h, washed with double distilled water, and dried in air. Further the film was annealed at 673 K for 2 h. The deposited film was specularly reflecting, uniform and well adherent to the substrate. For fabrication of a p-polyaniline/n-TiO2 heterojunction, a polyaniline film was deposited onto a previously chemically deposited TiO2 film by an electrodeposition (ED) method using a galvanostatic mode by applying a constant current of 4 mA/cm2 . The electrodeposition (ED) cell employed a standard three electrode configuration comprising a TiO2 thin film based stainless steel substrate, a graphite rod and a saturated calomel electrode as working, counter and reference electrodes, respectively. To deposit a polyaniline film, a solution containing 0.5 M H2 SO4 + 0.45 M aniline (C6 H5 NH2 ) was used. The thickness of the film was calculated by a weight-difference method, employing a sensitive microbalance. The optimized thicknesses of TiO2 and polyaniline films were 0.55 and 0.9 m, respectively, and roughness of the surface was 0.869 m. The forward biased junction current–voltage (I–V) characteristic was examined by making front aluminium foil press contact and back stainless steel contact to a heterojunction sample of area 1 cm × 1 cm. The schematic diagram of the p-polyaniline/n-TiO2 heterojunction is depicted in Fig. 1. It consists of a stainless steel substrate, onto which TiO2 and polyaniline films were subsequently deposited by the chemical bath deposition and electrodeposition methods.
The LPG sensing properties of the p-polyaniline/n-TiO2 heterojunction were studied by using a home-made gas sensor unit, described elsewhere [24]. Through the external connections, junction I–V characteristics were recorded using a potentiostat (EG& G Princeton Applied Research Model 262-A). The forward biased I–V characteristics of the junction before and after exposure to LPG were recorded at different concentrations in the range of 0.04–0.12 vol% in a voltage range of 0–2 V. From the plot, maximum current change was recorded at a fixed voltage (+2 V). The electrical currents of a p-polyaniline/n-TiO2 heterojunction in air (Ia ) and in the presence of LPG (Ig ) were measured and using the following relation the gas response was calculated. S (%) =
Ia − Ig I × 100 = × 100 Ia Ia
(1)
The response and recovery times of the junction to various concentrations of LPG were determined by holding the junction to a fixed potential (+2 V) and the junction current change was recorded with time. 3. Results and discussion 3.1. Crystal structural studies Figs. 2(a) and (b) shows the X-ray diffraction patterns of TiO2 and polyaniline films, respectively. From Fig. 2(a), the presence of broad, small and well distinct peaks indicates the nanocrystalline nature of the TiO2 film. The planes corresponding to (1 1 0), (1 0 1), (1 1 1) and (2 1 0) are in good agreement with the Joint Committee on Powder Diffraction Standard (JCPDS) (no. 21-1276), confirming the formation of nanocrystalline TiO2 . The same kind of result was reported
2.2. Characterization techniques The structural characterization of the TiO2 and polyaniline films was carried out using a Philips (PW 3710) X-ray diffractometer with
Fig. 2. X-Ray diffraction patterns of (a) TiO2 annealed at 673 K and (b) polyaniline thin film.
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Fig. 3. Scanning electron micrographs of (a) TiO2 annealed at 673 K, (b) polyaniline, and (c) an interface cross-section of a p-polyaniline/n-TiO2 heterojunction.
elsewhere [28] for TiO2 thin films deposited by a hydrothermal route. Fig. 2(b) declares the absence of any sharp diffraction lines, indicating that the deposited polyaniline film is amorphous, similar to the results reported by Joshi and Lokhande [29]. The peaks marked by triangles are due to the contribution from the stainless steel substrate. 3.2. Surface morphological studies Figs. 3(a) and (b) shows the scanning electron micrographs of TiO2 and polyaniline films at × 10,000 magnification, respectively. It is seen that the TiO2 (Fig. 3(a)) film is more compact and has strong adhesion with the stainless steel substrate. The SEM image of the polyaniline film (Fig. 3(b)) exhibits a fibrous structure with many pores and gaps among the fibers. Fig. 3(c) shows interface cross-sectional SEM image of a p-polyaniline/n-TiO2 heterojunction at high magnification of ×40,000, which clearly indicates the formation of a diffusion free interface. It is evident that there are many pores on the polyaniline surface, which seem to contribute to the short response and recovery times. Due to the porous struc-
ture, LPG diffusion as well as reaction between gas molecules and the interface occurs more easily. 3.3. LPG sensing properties of p-polyaniline/n-TiO2 heterojunction Fig. 4 represents the typical forward biased I–V characteristics of the p-polyaniline/n-TiO2 heterojunction in the absence and presence of LPG at room temperature (300 K). Curve (a) in Fig. 4 shows the I–V characteristic in the absence of LPG and curves (b–e) are in the presence of LPG for the concentrations ranging from 0.04 to 0.12 vol%. As the heterojunction was exposed to LPG, the forward current drastically decreased with an increase in concentration of LPG up to 0.1 vol%. A similar type of result is also observed by Tai et al. [20] for NH3 and CO gases. The decrease in current has been attributed due to an increase in resistance of polyaniline or an increase in potential barrier height at the interface when exposed to LPG, in contrast to hydrogen gas sensors based on Pd/TiO2 [30]. The LPG response of the p-polyaniline/n-TiO2 heterojunction at an applied potential of +2 V is depicted in Fig. 5. From the figure, it
D.S. Dhawale et al. / Sensors and Actuators B 134 (2008) 988–992
Fig. 4. Forward biased I–V characteristics of a p-polyaniline/n-TiO2 heterojunction at various concentrations of LPG (a) in air, (b) 0.04 vol%, (c) 0.06 vol%, (d) 0.1 vol% and (e) 0.12 vol% LPG.
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Fig. 6. Gas response (%) vs. time (s) of a p-polyaniline/n-TiO2 heterojunction at a fixed voltage of +2 V and at a concentration of 0.1 vol% LPG.
is concluded that the gas response is a function of LPG concentration. The gas response increased from 15 to 63% with an increase in concentration of LPG from 0.04 to 0.1 vol%. The maximum gas response of 63% was observed at 0.1 vol%. At 0.12 vol% of LPG, the response decreased to 25%. The response/recovery time is an important parameter used for characterizing a sensor. It is defined as the time required to reach 90% of the final change in current, when the gas is turned on and off, respectively. The device response vs. time is shown in Fig. 6 for 0.1 vol% of LPG. From the plot, it is seen that the response time is 140 s and the recovery time is 180 s. Fig. 7 shows the heterojunction response and recovery times for different vol% of LPG. It is revealed that the response time decreased from 200 to 140 s when LPG concentration increased from 0.02 to 0.1 vol%. This may be due to the presence of sufficient gas molecules at the interface of the junction for reaction to occur. From the same graph, it is found that for higher concentrations of LPG, the recovery time was long. This may probably be due to the heavier nature of LPG and the reaction products are not leaving from the interface immediately after the reaction. Fig. 7. Variation of response and recovery time of the heterojunction sensor with LPG concentration.
4. Conclusions In the present work, for the first time, we have succeeded in fabrication of a p-polyaniline/n-TiO2 heterojunction for a room temperature (300 K) liquefied petroleum gas (LPG) sensor. Morphological analysis using SEM of the junction cross-section revealed the formation of a diffusion free interface. The gas sensing properties of heterojunction to LPG indicated that the thin film of p-polyaniline/n-TiO2 heterojunction is a candidate for LPG detection. The maximum gas response of 63% was achieved upon exposure to 0.1 vol% LPG. Acknowledgement
Fig. 5. Gas response (%) vs. LPG concentration of a p-polyaniline/n-TiO2 heterojunction.
Authors are grateful to the Department of Science and Technology, New Delhi for financial support through the scheme no. SR/S2/CMP-82/2006.
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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 presently doing Ph.D. 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 and their application in gas sensor at room temperature (300 K). R.R. Salunkhe received his B.Sc. (2003) in general physics, M.Sc. (2005) in solid-state physics and presently he is doing his Ph.D. (2007) in chemical preparation of CdO thin films and application in gas sensors, from Shivaji University, Kolhapur, India. His present research interests include mainly the synthesis of nanocrystalline metal oxide thin films and their applications in gas sensor. U.M. Patil received his B.Sc. degree (2004) in general physics, M.Sc. degree (2006) in Solid state Physics and presently doing Ph.D. in supercapacitive behavior of synthesized RuO2 –TiO2 thin films from the Shivaji University, Kolhapur, India (M.S.). His present research interest includes synthesis of TiO2 and RuO2 thin films by chemical methods and their application in supercapacitor. K.V. Gurav received his B.Sc. degree (2004) in general physics, M.Sc. degree (2006) in Solid state Physics and presently doing Ph.D. in nanostructured ZnO: synthesis and application in LPG sensors from the Shivaji University, Kolhapur, India (M.S.). His present research interest includes synthesis of ZnO thin films by chemical methods and their application in sensor. 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, India (M.S.). Presently, he is working as a Ph.D. scholar in Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur. His research interests include mainly the synthesis of nanocrystalline TiO2 thin films by chemical methods and their applications in dye sensitized solar cells and gas sensor. 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 reader 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.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Room temperature liquefied petroleum gas (LPG) sensor based on p-polyaniline/n-TiO2 heterojunction D.S. Dhawale, R.R. Salunkhe, U.M. Patil, K.V. Gurav, A.M. More, C.D. Lokhande ∗ Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416004 (M.S.), India
a r t i c l e
i n f o
Article history: Received 31 March 2008 Received in revised form 4 July 2008 Accepted 7 July 2008 Available online 16 July 2008 Keywords: Thin films TiO2 Polyaniline Heterojunction LPG sensor
a b s t r a c t In the present work, we report on the performance of a room temperature (300 K) liquefied petroleum gas (LPG) sensor based on a p-polyaniline/n-TiO2 heterojunction. The heterojunction was fabricated using electrochemically deposited polyaniline on chemically deposited TiO2 on a stainless steel substrate. Both the methods (chemical bath deposition and electrodeposition) are simple, inexpensive and suitable for large-scale production. TiO2 and polyaniline films were characterized for their structural as well as surface morphologies and LPG response was studied. The XRD analysis showed formation of polycrystalline TiO2 while polyaniline exhibited amorphous nature. Morphological analysis using scanning electron microscopy (SEM) of the junction cross-section revealed formation of a diffusion free interface. The heterojunction showed the maximum response of 63% upon exposure to 0.1 vol% LPG at room temperature. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Many studies on various materials as gas sensors have been reported in recent years. Gas sensing materials can be classified mainly into two types, namely, organic and inorganic materials. Semiconductor inorganic gas sensors like doped or undoped SnO2, ZnO or Fe2 O3 have been well studied to detect most of reducing gases and they are considered interesting for their low cost and simple sensing methods [1–7]. Nevertheless, there still exist some problems with them, for example high working temperature of 423–623 K for SnO2 and 673–723 K for ZnO [8,9]. Heterojunction sensors are mostly based on the interface between p-type (p) and n-type (n) semiconducting ceramics [10–13]. Hazardous gases, specifically liquefied petroleum gas (LPG), have been widely used for several industrial and domestic applications. But, at certain low concentration of the gases, these metal oxide sensors show poor performance with respect to the sensitivity, long term stability, selectivity, etc. Recently, conducting polymers have been widely investigated as effective materials for room temperature chemical sensors. Polyaniline is one of the most attractive materials among the variety of conducting polymers due to its unique electrical properties, environmental stability and easy fabrication process. Due to its interesting properties, polyaniline has been a potential candidate in sensor applications [14,15], light emitting
∗ Corresponding author. Tel.: +91 231 2609229; fax: +91 231 260933. E-mail address: l [email protected] (C.D. Lokhande). 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.07.003
diodes [16], and rechargeable batteries [17]. However, the problems with these conducting polymers are their low processing ability, poor chemical stability and mechanical strength [18]. As an option, there is a room to fabricate heterojunctions between organic and inorganic materials with enhancement of the sensor characteristics and mechanical strength. By using electrochemical polymerization, polyaniline and its nanocomposite have been fabricated in a bulk form. Pd-polyaniline nanocomposite was prepared for a methanol gas sensor [19]. Tai et al. [20] fabricated a polyaniline–titanium dioxide nanocomposite for NH3 and CO sensors and reported that the resistance of the composite increased with increasing concentration of the gases. Nicho et al. [21] developed a polyaniline composite sensor for low concentration of NH3 gas. A ZnO/polyaniline layer-by-layer assembly and heterostructured polyaniline/Bi2 Te3 nanowires were fabricated by Paul et al. [22] and Xu et al. [23], respectively. Recently, Joshi et al. developed n-CdSe/p-polyaniline and n-CdTe/p-polyaniline heterojunctions for a room temperature LPG sensor [24,25]. Among the inorganic materials, nanocrystalline TiO2 is one of the most attractive and extensively used materials for detection of H2 , NH3 , NO2 and LPG gases [26,27]. However, due to the longterm instability at elevated temperature, it is desirable to develop sensors that operate at room temperature. In the present work, for the first time, we report fabrication of a p-polyaniline/n-TiO2 heterojunction with a good rectifying ratio by adopting a simple and inexpensive chemical route. Specifically, a nanocrystalline TiO2 thin film was deposited on a stainless steel substrate by chemical bath deposition (CBD), followed by
D.S. Dhawale et al. / Sensors and Actuators B 134 (2008) 988–992
989
CuK␣ radiation ( = 1.5406 Å) in a 2 range from 10◦ to 100◦ . The surface morphological study of the TiO2 , polyaniline and crosssectional interface of a p-polyaniline/n-TiO2 heterojunction was carried out using scanning electron microscopy (JEOL-6360). For this, the films were coated with a 10 nm platinum layer using a polaron scanning electron microscopy (SEM) sputter coating unit E-2500 before taking the image. 2.3. LPG sensing properties p-polyaniline/n-TiO2 heterojunction Fig. 1. A schematic representation of a p-polyaniline/n-TiO2 heterojunction.
polyaniline film by an electrodeposition (ED) method. These films were characterized using XRD and SEM techniques. The sensing performance at different concentrations of LPG (0.04–0.12 vol%) was studied at room temperature (300 K) by current–voltage (I–V) characteristics under the forward bias condition. 2. Experimental details 2.1. Fabrication of p-polyaniline/n-TiO2 heterojunction Preparation of a TiO2 thin film by the CBD method is based on the heating of an acidic solution of titanium (III) chloride containing a substrate immersed in it. The titanium (III) chloride solution was mixed with double distilled water in appropriate quantities. Specifically, 2.5 ml of TiCl3 (30 wt% in HCl, Loba Chemie, India) was added to 50 ml of double distilled water. The pH of the solution was adjusted to ∼1 using urea (NH2 CONH2 ) while constantly stirring at room temperature for 30 min. A stainless steel substrate was immersed vertically in the above bath and the bath was heated. At 353 K, the precipitation was started in the bath. During the precipitation, heterogeneous reaction occurred and deposition of TiO2 took place on the substrates. The substrate coated with TiO2 thin film were removed after 2 h, washed with double distilled water, and dried in air. Further the film was annealed at 673 K for 2 h. The deposited film was specularly reflecting, uniform and well adherent to the substrate. For fabrication of a p-polyaniline/n-TiO2 heterojunction, a polyaniline film was deposited onto a previously chemically deposited TiO2 film by an electrodeposition (ED) method using a galvanostatic mode by applying a constant current of 4 mA/cm2 . The electrodeposition (ED) cell employed a standard three electrode configuration comprising a TiO2 thin film based stainless steel substrate, a graphite rod and a saturated calomel electrode as working, counter and reference electrodes, respectively. To deposit a polyaniline film, a solution containing 0.5 M H2 SO4 + 0.45 M aniline (C6 H5 NH2 ) was used. The thickness of the film was calculated by a weight-difference method, employing a sensitive microbalance. The optimized thicknesses of TiO2 and polyaniline films were 0.55 and 0.9 m, respectively, and roughness of the surface was 0.869 m. The forward biased junction current–voltage (I–V) characteristic was examined by making front aluminium foil press contact and back stainless steel contact to a heterojunction sample of area 1 cm × 1 cm. The schematic diagram of the p-polyaniline/n-TiO2 heterojunction is depicted in Fig. 1. It consists of a stainless steel substrate, onto which TiO2 and polyaniline films were subsequently deposited by the chemical bath deposition and electrodeposition methods.
The LPG sensing properties of the p-polyaniline/n-TiO2 heterojunction were studied by using a home-made gas sensor unit, described elsewhere [24]. Through the external connections, junction I–V characteristics were recorded using a potentiostat (EG& G Princeton Applied Research Model 262-A). The forward biased I–V characteristics of the junction before and after exposure to LPG were recorded at different concentrations in the range of 0.04–0.12 vol% in a voltage range of 0–2 V. From the plot, maximum current change was recorded at a fixed voltage (+2 V). The electrical currents of a p-polyaniline/n-TiO2 heterojunction in air (Ia ) and in the presence of LPG (Ig ) were measured and using the following relation the gas response was calculated. S (%) =
Ia − Ig I × 100 = × 100 Ia Ia
(1)
The response and recovery times of the junction to various concentrations of LPG were determined by holding the junction to a fixed potential (+2 V) and the junction current change was recorded with time. 3. Results and discussion 3.1. Crystal structural studies Figs. 2(a) and (b) shows the X-ray diffraction patterns of TiO2 and polyaniline films, respectively. From Fig. 2(a), the presence of broad, small and well distinct peaks indicates the nanocrystalline nature of the TiO2 film. The planes corresponding to (1 1 0), (1 0 1), (1 1 1) and (2 1 0) are in good agreement with the Joint Committee on Powder Diffraction Standard (JCPDS) (no. 21-1276), confirming the formation of nanocrystalline TiO2 . The same kind of result was reported
2.2. Characterization techniques The structural characterization of the TiO2 and polyaniline films was carried out using a Philips (PW 3710) X-ray diffractometer with
Fig. 2. X-Ray diffraction patterns of (a) TiO2 annealed at 673 K and (b) polyaniline thin film.
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Fig. 3. Scanning electron micrographs of (a) TiO2 annealed at 673 K, (b) polyaniline, and (c) an interface cross-section of a p-polyaniline/n-TiO2 heterojunction.
elsewhere [28] for TiO2 thin films deposited by a hydrothermal route. Fig. 2(b) declares the absence of any sharp diffraction lines, indicating that the deposited polyaniline film is amorphous, similar to the results reported by Joshi and Lokhande [29]. The peaks marked by triangles are due to the contribution from the stainless steel substrate. 3.2. Surface morphological studies Figs. 3(a) and (b) shows the scanning electron micrographs of TiO2 and polyaniline films at × 10,000 magnification, respectively. It is seen that the TiO2 (Fig. 3(a)) film is more compact and has strong adhesion with the stainless steel substrate. The SEM image of the polyaniline film (Fig. 3(b)) exhibits a fibrous structure with many pores and gaps among the fibers. Fig. 3(c) shows interface cross-sectional SEM image of a p-polyaniline/n-TiO2 heterojunction at high magnification of ×40,000, which clearly indicates the formation of a diffusion free interface. It is evident that there are many pores on the polyaniline surface, which seem to contribute to the short response and recovery times. Due to the porous struc-
ture, LPG diffusion as well as reaction between gas molecules and the interface occurs more easily. 3.3. LPG sensing properties of p-polyaniline/n-TiO2 heterojunction Fig. 4 represents the typical forward biased I–V characteristics of the p-polyaniline/n-TiO2 heterojunction in the absence and presence of LPG at room temperature (300 K). Curve (a) in Fig. 4 shows the I–V characteristic in the absence of LPG and curves (b–e) are in the presence of LPG for the concentrations ranging from 0.04 to 0.12 vol%. As the heterojunction was exposed to LPG, the forward current drastically decreased with an increase in concentration of LPG up to 0.1 vol%. A similar type of result is also observed by Tai et al. [20] for NH3 and CO gases. The decrease in current has been attributed due to an increase in resistance of polyaniline or an increase in potential barrier height at the interface when exposed to LPG, in contrast to hydrogen gas sensors based on Pd/TiO2 [30]. The LPG response of the p-polyaniline/n-TiO2 heterojunction at an applied potential of +2 V is depicted in Fig. 5. From the figure, it
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Fig. 4. Forward biased I–V characteristics of a p-polyaniline/n-TiO2 heterojunction at various concentrations of LPG (a) in air, (b) 0.04 vol%, (c) 0.06 vol%, (d) 0.1 vol% and (e) 0.12 vol% LPG.
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Fig. 6. Gas response (%) vs. time (s) of a p-polyaniline/n-TiO2 heterojunction at a fixed voltage of +2 V and at a concentration of 0.1 vol% LPG.
is concluded that the gas response is a function of LPG concentration. The gas response increased from 15 to 63% with an increase in concentration of LPG from 0.04 to 0.1 vol%. The maximum gas response of 63% was observed at 0.1 vol%. At 0.12 vol% of LPG, the response decreased to 25%. The response/recovery time is an important parameter used for characterizing a sensor. It is defined as the time required to reach 90% of the final change in current, when the gas is turned on and off, respectively. The device response vs. time is shown in Fig. 6 for 0.1 vol% of LPG. From the plot, it is seen that the response time is 140 s and the recovery time is 180 s. Fig. 7 shows the heterojunction response and recovery times for different vol% of LPG. It is revealed that the response time decreased from 200 to 140 s when LPG concentration increased from 0.02 to 0.1 vol%. This may be due to the presence of sufficient gas molecules at the interface of the junction for reaction to occur. From the same graph, it is found that for higher concentrations of LPG, the recovery time was long. This may probably be due to the heavier nature of LPG and the reaction products are not leaving from the interface immediately after the reaction. Fig. 7. Variation of response and recovery time of the heterojunction sensor with LPG concentration.
4. Conclusions In the present work, for the first time, we have succeeded in fabrication of a p-polyaniline/n-TiO2 heterojunction for a room temperature (300 K) liquefied petroleum gas (LPG) sensor. Morphological analysis using SEM of the junction cross-section revealed the formation of a diffusion free interface. The gas sensing properties of heterojunction to LPG indicated that the thin film of p-polyaniline/n-TiO2 heterojunction is a candidate for LPG detection. The maximum gas response of 63% was achieved upon exposure to 0.1 vol% LPG. Acknowledgement
Fig. 5. Gas response (%) vs. LPG concentration of a p-polyaniline/n-TiO2 heterojunction.
Authors are grateful to the Department of Science and Technology, New Delhi for financial support through the scheme no. SR/S2/CMP-82/2006.
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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 presently doing Ph.D. 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 and their application in gas sensor at room temperature (300 K). R.R. Salunkhe received his B.Sc. (2003) in general physics, M.Sc. (2005) in solid-state physics and presently he is doing his Ph.D. (2007) in chemical preparation of CdO thin films and application in gas sensors, from Shivaji University, Kolhapur, India. His present research interests include mainly the synthesis of nanocrystalline metal oxide thin films and their applications in gas sensor. U.M. Patil received his B.Sc. degree (2004) in general physics, M.Sc. degree (2006) in Solid state Physics and presently doing Ph.D. in supercapacitive behavior of synthesized RuO2 –TiO2 thin films from the Shivaji University, Kolhapur, India (M.S.). His present research interest includes synthesis of TiO2 and RuO2 thin films by chemical methods and their application in supercapacitor. K.V. Gurav received his B.Sc. degree (2004) in general physics, M.Sc. degree (2006) in Solid state Physics and presently doing Ph.D. in nanostructured ZnO: synthesis and application in LPG sensors from the Shivaji University, Kolhapur, India (M.S.). His present research interest includes synthesis of ZnO thin films by chemical methods and their application in sensor. 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, India (M.S.). Presently, he is working as a Ph.D. scholar in Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur. His research interests include mainly the synthesis of nanocrystalline TiO2 thin films by chemical methods and their applications in dye sensitized solar cells and gas sensor. 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 reader 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.