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
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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-TiOz 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
Article history:
Received 31 March 2008
Received in revised form 4 July 2008
Accepted 7 July 2008
Available online 16 July 2008
In the present work, we report on the performance of a room temperature (300K) liquefied petroleum gas (LPG) sensor based on a p-polyaniline/n-TiOz 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 suit- able for large-scale production TiO and polyaniline films were characterized for their structural as well
as surface morphologies and LPG response was studied The XRD analysis showed formation of poly- eas ores: crystalline TiO02 while polyaniline exhibited amorphous nature Morphological analysis using scanning TiO; electron microscopy (SEM) of the junction cross-section revealed formation of a diffusion free inter-
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 Fe203 have been well studied to detect most of reduc-
ing 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 SnOz and 673-723 K for ZnO [8,9] Heterojunc-
tion 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 cer-
tain low concentration of the gases, these metal oxide sensors show
poor performance with respect to the sensitivity, long term sta-
bility, selectivity, etc Recently, conducting polymers have been
widely investigated as effective materials for room temperature
chemical sensors Polyaniline is one of the most attractive mate-
rials 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: |_chandrakant@yahoo.com (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 prob- lems 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 electrochemi- cal polymerization, polyaniline and its nanocomposite have been fabricated in a bulk form Pd-polyaniline nanocomposite was pre- pared for a methanol gas sensor [19] Tai et al [20] fabricated a polyaniline-titanium dioxide nanocomposite for NH3 and CO sen- sors and reported that the resistance of the composite increased with increasing concentration of the gases Nicho et al [21] devel- oped a polyaniline composite sensor for low concentration of NH3 gas A ZnO/polyaniline layer-by-layer assembly and heterostruc- tured polyaniline/BizTe3; 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 hetero- junctions for a room temperature LPG sensor [24,25]
Among the inorganic materials, nanocrystalline TiOz is one of the most attractive and extensively used materials for detection
of Hz, NH3, NO2 and LPG gases [26,27] However, due to the long-
term 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-TiO, heterojunction with a good rectifying ratio
by adopting a simple and inexpensive chemical route Specifically,
a nanocrystalline TiO, thin film was deposited on a stainless steel substrate by chemical bath deposition (CBD), followed by
Trang 2Front aluminium
Back Contact
Fig 1 A schematic representation of a p-polyaniline/n-TiOz 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 (J-V)
characteristics under the forward bias condition
2 Experimental details
2.1 Fabrication of p-polyaniline/n-TiOz heterojunction
Preparation of a TiO thin film by the CBD method is based on
the heating of an acidic solution of titanium (III) chloride contain-
ing 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 (NH2CONH3) while constantly stir-
ring at room temperature for 30 min A stainless steel substrate was
immersed vertically in the above bath and the bath was heated At
353K, the precipitation was started in the bath During the pre-
cipitation, heterogeneous reaction occurred and deposition of TiO
took place on the substrates The substrate coated with TiO, thin
film were removed after 2h, 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 TiO film by an electrodeposition (ED) method using a
galvanostatic mode by applying a constant current of 4mAjcm2
The electrodeposition (ED) cell employed a standard three elec-
trode configuration comprising a TiO, 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 H2SO4 + 0.45 M ani-
line (CgH5NH2) was used The thickness of the film was calculated
by a weight-difference method, employing a sensitive microbal-
ance The optimized thicknesses of TiO2 and polyaniline films
were 0.55 and 0.9 1m, respectively, and roughness of the sur-
face was 0.869 jum 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 heterojunc-
tion sample of area 1cmx1cm The schematic diagram of the
p-polyaniline/n-TiO2 heterojunction is depicted in Fig 1 It consists
of a stainless steel substrate, onto which TiO, and polyaniline films
were subsequently deposited by the chemical bath deposition and
electrodeposition methods
2.2 Characterization techniques
The structural characterization of the TiOz and polyaniline films
was catried out using a Philips (PW 3710) X-ray diffractometer with
CuK, radiation (A = 1.5406 A) in a 20 range from 10° to 100° The surface morphological study of the TiOz, polyaniline and cross- sectional interface of a p-polyaniline/n-TiOz heterojunction was carried out using scanning electron microscopy (JEOL-6360) For this, the films were coated with a 10nm 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-TiOz heterojunction The LPG sensing properties of the p-polyaniline/n-TiO, het- erojunction were studied by using a home-made gas sensor unit, described elsewhere [24] Through the external connections, junc- tion 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 electri- cal currents of a p-polyaniline/n-TiO, heterojunction in air ([,) and
in the presence of LPG ([g) were measured and using the following relation the gas response was calculated
da da
The response and recovery times of the junction to various concen- trations 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 TiO, film The planes corresponding to(110),(101),(111)and (2 10) are in good agreement with the Joint Committee on Powder Diffraction Standard (JCPDS) (no 21-1276), confirming the forma- tion of nanocrystalline TiO2 The same kind of result was reported
A - Stainless steel A A
26 ( degree) Fig 2 X-Ray diffraction patterns of (a) TiOz annealed at 673 K and (b) polyaniline thin film.
Trang 3| 1Ù rf t A l5 5 u K = P H hp
Z8kU XI8; 883
f
co, a nh A L
L ee Ns XS” eee
rd r ø
-
p-polyaniline § N v
substrate
12:50:32 PM | 10.00 kV| 4.5 mm
z8kV <18; 888 lim 8É SUK-FPHY
40 000 x | TLD} High vacuum
Fig 3 Scanning electron micrographs of (a) TiOz annealed at 673 K, (b) polyaniline, and (c) an interface cross-section of a p-polyaniline/n-TiOz heterojunction
elsewhere [28] for TiOz 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, sim-
ilar 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
TiO and polyaniline films at x 10,000 magnification, respectively
It is seen that the TiOz (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-TiOz heterojunc-
tion at high magnification of x40,000, 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-TiO2z heterojunction
Fig 4represents the typical forward biased I-V characteristics of the p-polyaniline/n-TiOz heterojunction in the absence and pres- ence 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/TiO, [30] The LPG response of the p-polyaniline/n-TiOz heterojunction at
an applied potential of +2 V is depicted in Fig 5 From the figure, it
Trang 4
2.0 +-
1.5 +
1.0 +
0.0 T : : : : T : : * : T l : : ` T : : h :
Voltage ( V) Fig 4 Forward biased ï—-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
is concluded that the gas response is a function of LPG concentra-
tion 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
140s 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 140s when LPG con-
centration 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 prob-
ably be due to the heavier nature of LPG and the reaction products
are not leaving from the interface immediately after the reaction
60 T
50 T
+> oO 1
30 T
LPG concentration (vol%) Fig 5 Gas response (%) vs LPG concentration of a p-polyaniline/n-TiOz heterojunc-
tion
70 +
50 +
40 +
30 +
20 +
10 +
Gas on
Time (S) Fig 6 Gas response (3) vs time (s) of a p-polyaniline/n-TiO; heteroJunction at a fixed voltage of +2 V and at a concentration of 0.1 vol% LPG
200 + —®— Response time (s) |
190 +
©
®©
140 +
+ 100
130 LU 7 [ on 1 — Lư pc : l = I :
0.04 0.05 0.06 0.07 0.08 0.09 0.10
LPG concentration (vol %) 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-TiOz heterojunction for a room temperature (300 K) liquefied petroleum gas (LPG) sensor Morpho- logical analysis using SEM of the junction cross-section revealed the formation of a diffusion free interface The gas sensing prop- erties of heterojunction to LPG indicated that the thin film of p-polyaniline/n-TiOz heterojunction is a candidate for LPG detec- tion The maximum gas response of 63% was achieved upon exposure to 0.1 vol% LPG
Acknowledgement Authors are grateful to the Department of Science and Tech- nology, New Delhi for financial support through the scheme no SR/S2/CMP-82/2006
Trang 5References
[1] N Yamazoe, New approaches for improving semiconductor gas sensors, Sens
Actuators B 5 (1991) 7-19
[2] W Gopel, K.D Schierbaum, SnOz sensors: current status and future prospects,
Sens Actuators B 26 (1995) 1-12
[3] G Behr, W Fliegel, Electrical properties and improvement of the gas sensitivity
in multiple-doped SnOzg, Sens Actuators B 26 (1995) 33-37
[4] C.H Kwon, H.K Hong, D.H Yun, K Lee, S.T Kim, Y.H Roh, B.H Lee, Thick-
film zinc-oxide gas sensor for the control of lean air-to-fuel ratio in domestic
combustion systems, Sens Actuators B 24 (1995) 610-613
[5] G Mangamma, V Jayaraman, T Gnanasekaran, G Periaswami, Effects of silica
additions on H2S sensing properties of CuO-SnOz sensors, Sens Actuators B 53
(1998) 133-139
[6] Y Yamada, Y Seno, Y Masuoka, K Yamashita, Nitrogen oxides sensing charac-
teristics of Zn2SnO, thin film, Sens Actuators B 49 (1998) 248-252
[7] Z Jiao, S Wang, L Bian, J Liu, Stability of SnO2/Fe203 multilayer thin film gas
sensor, Mater Res Bull 35 (2000) 741-745
[8] R Ionescu, A Vancu, F Buta, Diode-like SnO2 gas detection devices, Sens Actu-
ators B 43 (1997) 126-131
[9] T Seiyama, H Futata, F Era, N Yamazoe, Detection of gases by an activated
semiconductive sensor, Mater Sci 8 (1972) 63-65
[10] S.J Jung, H Ohsawa, Y Nakamura, H Yanagida, K Hasumik, O Okada, Effects of
Na203 addition on the gas sensing characteristic of CuO/ZnO heterocontact, J
Electrochem Soc 141 (1994) 53-55
[11] SJ Jung, H Yanagida, The characterisation of CuO/ZnO heterocontact type gas
sensor having selectivity for CO gas, Sens Actuators B 37 (1996) 55-60
[12] E Traversa, M Miyayama, H Yanagida, Gas sensitivity of ZnO/LazCuO, hetero-
contacts, Sens Actuators B 17 (1994) 257-261
[13] R Mochinaga, T Yamasaki, T Arakawa, The gas-sensing of SmCoO,/MO, (M = Fe,
Zn, In, Sn) having a heterojunction, Sens Actuators B 52 (1998) 96-99
[14] P.N Bartlett, K Sim, L Chung, Conducting polymer gas sensors part II: response
of polypyrrole to methanol vapour, Sens Actuators B 19 (1989) 141-150
[15] R Nohria, R.K Khillan, Y Su, R Dikshit, Y Lvov, K Varahramya, Humidity
sensor based on ultrathin polyaniline film deposited using layer-by-layer nano-
assembly, Sens Actuators B 114 (2006) 218-222
[16] S.A Chen, K.R Chuang, C.I Chao, H.T Lee, White-light emission from electrolu-
minescence diode with polyaniline as the emitting layer, Synth Met 82 (1996)
207-210
[17] A.G MacDiarmid, L.S Yang, W.S Huang, B.D Humphrey, Polyaniline: electro-
chemistry and application to rechargeable batteries, Synth Met 18 (1987)
393-398
[18] M Matsuguchi, A Okamoto, Y Sakai, Effect of humidity on NH3 gas sensitivity
of polyaniline blend films, Sens Actuators B 94 (2003) 46-52
[19] A.A Athawale, S.V Bhagwat, P.P Katre, Nanocomposite of Pd - polyaniline as a
selective methanol, Sens Actuators B 114 (2006) 263-267
[20] H Tai, Y Jiang, G Xie, J Yu, X Chen, Fabrication and gas sensitivity of
polyaniline-titanium dioxide nanocomposite thin film, Sens Actuators B 25
(2007) 644-650
[21] M.E Nicho, M Trejo, A.G Valenzaela, J.M Saniger, J Palacios, H Hu, Polyaniline
composite coatings interrogated by a nulling optical bridge for sensing low
concentration of ammonia gas, Sens Actuators B 76 (2001) 18-24
[22] G.K Paul, A Bhaumika, A.S Patra, S.K Bera, Enhanced photo-electric response
of ZnO/polyaniline layer-by-layer self-assembled films, Mater Chem Phys 106
(2007) 360-363
[23] X Xu, L Chen, C Wang, Q Yao, C Feng, Template synthesis of het-
erostructured polyaniline/BizTe; nanowires, J Solid State Chem 178 (2005)
2163-2166
[24] S.S Joshi, C.D Lokhande, S.H Han, A room temperature liquefied petroleum
gas sensor based on all-electrodeposited n-CdSe/p-polyaniline junction, Sens
Actuators B 123 (2007) 240-245
[25] S.S Joshi, T.P Gujar, V.R Shinde, C.D Lokhande, Fabrication of n-CdTe/p- polyaniline heterojunction-based room temperature LPG sensor, Sens Actuators B 132 (2008) 349-355
[26] Y Shimizu, T Okamoto, Y Takao, M Egashira, Desorption behavior of ammo- nia from TiO2-based specimens-ammonia sensing mechanism of double-layer sensors with TiOz-based catalyst layers, J Mol Catal A: Chem 155 (2000) 183-191
[27] A.M More, J.L Gunjakar, C.D Lokhande, Liquefied petroleum gas (LPG) sen- sor properties of interconnected web-like structured sprayed TiOz films, Sens Actuators B 129 (2008) 671-677
[28] L Liu, Y Zhao, H Liu, H.Z Kou, Y Wang, Directed growth of TiOz nanorods into microspheres, Nanotechnology 17 (2006) 5046-5050
[29] S.S Joshi, C.D Lokhande, Electrodeposited heterojunctions based on cadmium chalcogenide, CdX (X=S, Se, Te) and polyaniline, J Mater Sci 42 (2007) 1304-1308
[30] N Yamamoto, S Tonomura, T Matsuoka, H Tsubomura, A study of a palladium- titanium oxide schottky diode as a detector for gaseous components, Surf Sci
92 (1980) 400-406
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 per- formance 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 (300K) 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 synthe- sized RuO;-TiO; thin films from the Shivaji University, Kolhapur, India (MLS.) His present research interest includes synthesis of TiO2 and RuO; 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, Kol- hapur (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 TiOz 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.