Polyaniline thin films for gas sensing - ScienceDirect

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Sensors and Actuators B 28 (1995) 173-179

Polyaniline thin films for gas sensing N.E. Agbor a*b,M.C. Petty a, A.P. Monkman b ’ School of Engineering and Centre for Molecular ElectroGcs, Universityof Durham, Lkham DHl 3LE, UK bPhpics Department and Centre for Molecular Elecbunics, Universl?, of Dwfwn, Durham DHl 3LE, UK Received 13 May 1994; in revised form 22 December 1994, accepted 9 January

1995

Abstract Thin films of polyaniline have been deposited by spinning, evaporation and by the Langmuir-Blodgett technique. The Nms are shown to possess slightly different in-plane electrical mnductivities, reflecting differences in their chemical structure and layer morphology. The conductivity is found to depend on the gas ambient. All types of polyaniline films are sensitive to H2S and NO, at concentrations down to 4 ppm. However, only spun and evaporated films are responsive to SOz. Key~or& Gas sensors; Polyaniline; Thin films

1. Introduction The importance of environmental gas monitoring is well understood and much research has focused on the development of suitable gas-sensitive materials. Recently, there has been considerable interest in exploiting organic substances such as porphyrins [l], phthalocyanines [2,3] and doped conductive polymers [4]. For maximum gas sensitivity, these compounds are usually studied as thin fihns. Among the doped conductive polymers that have been investigated are polypyrrole [5] and polythiophene [6]. Unfortunately, these materials are not readily processible. In contrast, polyaniline (PANi) is soluble in organic solvents [7] from which free-standing films can be cast [8]. In this work, polyaniline was processed into thin-film form using three different methods: spinning, vacuum ‘sublimation and the Langmuir-Blodgett (LB) technique. The gas sensitivities of the different films are compared.

2. Experimental 2.1. Substrate Fig. 1 shows a schematic diagram of the interdigitated electrode structure used in this work. It consists of gold electrodes patterned onto the surface of a quartz substrate; the overlap electrode length was 15 mm and the electrode gap was 0.38 mm. Chemiresistors were 0925-4005/95/$09.50 B 1995 Elsevier Science S.A. All rights reserved SSDI 0925-4005(95)01725-B

Fig. 1. An interdigitated electrode structure on a quartz substrate: l-15

mm, d=O.38 mm and h=75 mm.

fabricated by coating these electrodes with the polyaniline films. 2.2. spun films

Polyaniline powder (synthes&d in-house) [8] in the emeraldine base form was dissolved in N-methylpyrrolidinone (NMP), in a polymer:solvent weight ratio of l:lOO,and sonicated for 30 min. The starting material had a purity of 99.8%, as determined by NMR spectroscopy [9]. The resulting solution appeared blue in reflected light. This was spun onto the interdigitated electrode structure shown in Fig. 1. Spinning was undertaken using a Dynapert PRS 14E model spinner, at a fixed speed of 3000 rpm for 30 s. The spun !ihns were transferred to a vacuum oven and heated to a temperature of 120 “C, at lo-” mbar for 10 min. A typical film-thickness value, obtained from an Alpha Tenco surface profiling Talystep, was 2.0f 0.1 pm. Full

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details of the sample preparation elsewhere [lo].

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2.3. Thermal evaporation An Edwards 6E4 vacuum-deposition system was used to evaporate polyaniline. The equipment possessed an evaporation chamber of 30 cm diameter and used a water-cooled diffusion pump. The base vacuum level was 10e3 mbar. The source temperature was maintained by means of a Radio Spares temperature controller. 40 mg of emeraldine base polyaniline was weighted into a source boat, placed into the system and pumped down. When the pressure in the chamber was E=10e3 mbar and stable, the temperature of the boat was increased to 400 “C. Once equilibrium had been achieved, the shutter was opened and film deposition carried out for a tixed length of time. After evaporation, the system was allowed to cool down to room temperature ( =5 h). It was then opened to air and the substrates were removed. A typical film thickness from the Talystep was 210& 10 nm for a 60 min evaporation. 2.4. Langmuir-Blodgettfilms A floating layer on a water subphase was formed by spreading a solution made from polyaniline mixed with acetic acid and dissolved in a chloroform/NMP mixture. The deposition of LB films was undertaken using troughs designed and built in Durham and housed in a microelectronics clean room. Full details have been described previously Ill]. The film thickness was measured to be approximately 6.0*0.1 nm per layer. 2.5. Gas measurements The room-temperature current versus voltage characteristics of the uncoated and coated interdigitated electrode structures were measured using a Time Electronics d.c. voltage calibrator and a Keithley 410A picoammeter. The samples were placed in a chamber through which a gas could be passed. The gas concentration was varied using a Signal Instrument Series 850 gas blender. The gases used (NO,, HzS, SO,, CO and CH,) were all diluted with nitrogen. These were obtained from Air Products Limited and had purity levels of 99.99%. The procedure for measuring the electrical conductivity in the presence of a gas was as follows. With a fixed voltage applied to the thin-film structure, pure nitrogen was passed through the sample chamber until a steady current reading had been obtained. The active gas was then admitted in its lowest concentration and the current recorded after a fixed period. This isochronal approach was used because current saturation was not obtained in some of the samples studied. The active gas was then turned off and the sample left to recover in nitrogen. When a

B 28 (1995) 173-179

steady reading had been obtained, the next highest concentration of the active gas was admitted to the chamber and the entire measurement procedure repeated. Using this procedure, the gas responses reported in #is paper were repeatable to 10% with the same sample. The uncoated electrode (i.e., no polymer) showed no response to any of the gases at the maximum concentrations used.

3. Results and discussion 3.1. Chemical stnrcture Polyaniline is known to possess a number of reversible oxidation states, each with a distinct backbone structure composed of different ratios of quinoid to benzoid rings. These are shown in Fig. 2. For example, emeraldine base polyaniline (Fig. 2(a)) possesses one quinoid ring for every three benzoid rings. Other states include leucoemeraldine base (Fig. 2(b)), in which there are no quinoid units, and pernigraniline (Fig. 2(c)), in which there are equal numbers of quinoid and benzoid rings. 3.2. Film characterisation Both the spun and LB polyaniline films on glass appeared blue. This colour is indicative of the emeraldine base form of the polymer [12]. In contrast, the evaporated films initially appeared colourless on glass microscopy slides, suggesting that the film was in a state close to leucoemeraldine base [13,14]. However, upon prolonged exposure to air/moisture (at least two weeks), the colour of the evaporated film changed to purple and eventually to blue, similar to that of the spun and LB layers. This effect was almost certainly due to the oxidation of the film in the atmosphere, as reported elsewhere [14].

Fig. 2. The chemical structures of polyaniline: (a) emerald&

base; (b) leucnemeraldine base; (c) pemigraniline base; (d) a generic formula for polyaniline where n is a positive integer, x= l/2 for emeraldine base,1 = 0 for Ieucoemeraldine and x = 1 for pemigraniline.

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The current versus voltage characteristics of spincoated emeraldine base polyaniline are shown in Fig. 3. The measurements were undertaken in an atmosphere of nitrogen, at room temperature and after the current had stabilized (see next section). Data for the uncoated electrode (under nitrogen a current of 1.2f 0.2x lo-l2 A was measured with 10 V applied) confirmed that the current was flowing through the polyaniline film rather than through the substrate. The change in resistance for different film thicknesses (1.0, 2.0, 4.0 pm) indicates that Ohmic contacts have been established between the gold electrodes and the polymer. Using the thickness values from the surface profiler, the average room-temperature in-plane d.c. conductivity was 4.4~hO.9XlO-‘~S cm-‘, which is comparable to the literature value of 1.0X 10-l’ S cm-’ for the base form of emeraldine 171.The current versus voltage characteristics for both the evaporated and LB films of polyaniline were qualitatively similar to those of shown in Fig. 3 (including the linearity with film thickness). The average room-temperature d.c. conductivity of freshly evaporated polyaniline film, in nitrogen, was 1.0*0.2x lo-” S cm-‘. This compares with a value of 2.0~10~~ S cm-’ reported in the literature for similar material [15].The conductivity is slightly higher than that of our spin-coated films (4.4&0.9x lo-” S cm-‘). This can be explained by the absence of quinoid rings to disrupt rr-r mixing between adjacent benzoid rings in the polymer chain [16]. Electrical measurements on polyaniline LB filmshave been reported previously [ll]. The film has a roomtemperature conductivity in nitrogen of 10W8S cm-‘. This is significantly higher than that of the emeraldine base form of polyaniline, suggesting that a degree of protonation, possibly by the acetic acid, had occurred. In general, the agreement between the conductivity values reported here and those in the literature is not unreasonable considering that (a) polyaniline exists in

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different oxidation states and (b) external influences (impurities) may result in doping of the material. 3.3. Gas sensitivity 3.3-l. Nitrogen Fig. 4(a) shows the effect of dry N2 on the d.c. conductivity of a spun polyaniline chemiresistor. The conductivity decreased very rapidly upon the introduction of N2 and became stable after approximately 60 min. This can be associated with the removal of surface/bulk trapped water molecules. A similar response was also obtained for polyaniline in LB film form. Fig. 4(b) shows the effect of dry nitrogen on the d.c. conductivity of an evaporated polyaniline layer. In this case, the shorter time to achieve a stable conductivity value can be attributed to the lower level of water and the evaporated film. The effects of water on the conductivity of polyaniline are well documented [17,18]. No evidence for oxidation (see previous section) was noted for the evaporated film in the nitrogen environment. N2m 1.0 09 0.8 0.7 0.6 g

05 0.4 03 0.2

01

0 4

12 20 28 30 44 52 60

Zdnys 3days &days

Tinehitlsl

0.8 I

-500

I

500

I 1500

Supply voltage [mVl Fig. 3. The room-temperature current vs. voltage characteristics for spun emeraldine base polyaniline on gold-plated interdigitated copper electrodes for different film thicknesses: (a) 1.0 pm; (b) 2.0 pm: (c) 4.0 p.m.

0 0

’ ’ 12 26

’ 36

’ 48

’ 60

’ 72

’ ’ ’ ’ SL 96 106 120

Tine Imins~ Fig. 4. ‘l%e effect of dry nitrogen on the d.c. conductivity of: (a) 1.0 Frn thick spun PANi; (b) 210 MI thick evaporated PANi films at room temperature.

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3.3.2. Nitrogen dioxide Fig. 5 shows the effect of 10 ppm NO, on the spun polyaniline. It can be seen that exposure to the gas produced an increase in conductivity, which continued to rise until the gas was turned off. The original conductivity was restored approximately 90 min after the NO, had been turned off, in an atmosphere of nitrogen. Fig. 6(a) shows how the conductivity change, after a fixed exposure time, varies with NO, concentration. The threshold (limited by the measuring equipment) concentration level is about 4 ppm. The interaction between NO, and the spun polyaniline can be explained as follows. NO, is a well-known oxidizing gas which, on contact with the T-electron network of polyaniline (or any other system with electron lone pairs), is likely to result in the transfer of an electron from the polymer to the gas. When this occurs, the polymer becomes positively charged. The charge carriers thus created give rise to the increased conductivity of the film. This is analogous to the well-known increase in conductivity upon protonation for emeraldine. The effect of NO, on a 210 nm thick freshly evaporated polyaniline chemiresistor in a nitrogen atmosphere at room temperature was similar to that observed for spincoated polyaniline, i.e., an increase in conductivity with increasing gas concentration. However, at high gas concentration (> 40 ppm), the response was found to be only partially reversible. The response to different NO, gas concentrations is shown in Fig. 6(b). NO, also produced an increase in conductivity for 18 LB layers of polyaniline (approximately 110 nm in thickness) at room temperature. However, the effect was not reversible when .the gas was turned off. Furthermore, the calibration graph (Fig. 6(c)) exhibits a detection threshold of approximately 30 ppm, compared to 4 ppm for both the evaporated and spin-coated material. The lack of reversibility and reduced sensitivity

rw

IlnSl

Fig. 5. The effect of 10 ppm NOz on a spun polyaniline chemiresistor at room temperature (2 V supply, film thickness 1.0 pm and temperature 20f2 “C).

NO, mnc. [vpml T

NOx cone. [vpml

?

Fig. 6. The response of PANi films to different NO, concentrations: (a) 1.0 w spun film: (b) 210 nm evaporated film; (c) 100 nm LB film (2 V supply and temperature 203~2 “C).

of the LB film could be due to the fact that acetic acid molecules have occupied and chemically blocked sites responsive to NO,. It is difficult to make direct comparisons between the results presented in Fig. 6 for the three different films, as these are in different chemical and physical forms. For example, the higher sensitivity and faster response time for the (thinner) evaporated film (Fig. 6(b)) over the (thicker) spun layer (Fig. 6(a)) may be the result of the gas reaction taking place throughout the bulk of the polymer layer. The data may also be an indication of a reaction occurring at the polymer/ substrate interface. However, these effects may also result from differences in the sensing materials: the spun layers are in the emeraldine base state while the evaporated film is in a form close to leucoemeraldine. 3.3.3. Hydrogensulfide H,S was found to produce an increase in the conductivity of the spun polyaniline chemiresistor. No significant difference in response was observed between a film previously exposed to NO, and a fresh sample

N.E. Agbor et ai. f Sen.wn and Acmatm

of the same thickness. Complete recovery for 10 ppm of the gas was achieved after a period of about 60 min. A similar effect was observed with an 18 LB layer polyaniline chemiresistor. The change in conductivity, after a fixed exposure time, for both spun and LB polyaniline chemiresistors is shown in Fig. 7. The threshold for detection is about 4 ppm H,S for both films, H2S is a known reducing gas. Thus, we would expect to observe a decrease in the conductivity of the poly aniline chemiresistors. The observed increase in conductivity indicates that either more than one type of reaction site is available or that a number of different reactions are possible. At room temperature and pressure, H,S dissociates in water into H+ and HS- [19-221 as illustrated in Fig. 8. The H’ ion may subsequently protonate the polymer, i.e., FANi]+[H]*

c== [PANiH]+

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I77

H++HS-

aqueousphase

I

Fig. 8. An illustration of the state of HsS in different environments [21]. In this work, the vapour phase is equivalent to H&surface bound water molecules and the aqueous phase is equivalent to HsS/ water molecules trapped in the bulk of the film.

nine[ruins] 0

4

8

12

16

20

24

28

32

36

40

44

48

(0

where the equ~jb~um is shifted to the right during exposure and to the left after exposure. The protonation again produces charge carriers (semiquinone radicals) resulting in an increase in the d.c. conductivity. This reaction is likely to involve different sites in the polymer than for the NO, response. As a result, the sensit~ities of the LB and spun films are similar (compare the poor sensitivity of the LB film to NO, in Fig. 6). Fig. 9(a) shows the effect of 10 ppm H2S on a 210 nm freshly evaporated polyaniline chemiresistor. This reveals an irreversible decrease in conductivity at room

Gas off

Fig. 9. (a) The effect of 10 ppm HaS on an evaporated polyaniline chemiresistor at room temperature. (b)The msponse of an evaporated polyaniline chemiresistor to different concentrations of H&3 at room temperature (2 V supply, film thickness 210 nm and temperature 205~2 “C in both cases).

oL-----J 0 4

8

12

H2S cone. [vpml

v

E$ cone. @ml

Fig. 7. The response of PANi films to different HsS concentrations: (a) 1.0 pm spun film; (b) 110 nm LB film (2 V supply and temperature 20f2 “c).

temperature. The device response to different H,S gas concentrations is shown in Fig. 9(b). A threshold detection value of 10 ppm is evident. Note that spun and LB films are likely to possess more H,O than the evaporated material (Fig. 4), thus increasing the likelihood of the reaction given by Eq. (1). 3.3.4. suZfir dioxide SO2 produced an increase in conductivity of spun polyaniline as well as complete reversibility at room temperature. The effect of different SO, gas concentrations is shown in Fig. 10(a), revealing that the device is capable of measuring changes down to 2 ppm. Fig. 10(b) shows the response of an evaporated polyaniline chemiresistor to different concentrations of SOz. Here, the detection threshold is about 10 ppm,

N.E. Agbor et al. I Sensors and Achutors B 28 (1995) 173-l 79

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Dr Alex Milton and Dr Phil Adams for the synthesis of the polyaniline.

References 111 D.G. Zhu, D.-F. Cui, M. Harris and M.C. Petty, An optical

60

1 ”

so

2

cont.

80

100

[vpm]

Fig. 10. The response of (a) a 1.0 pm spun film; (b) a 210 nm evaporated film to different concentrations of SO? (2 V supply and temperature 20*2 “C).

compared to 2 ppm for NO,. However, freshly evaporated polyaniline films showed a much higher sensitivity, down to 0.5 ppm. This increased to 10 ppm as the film was cycled continuously between air and nitrogen. This result suggests that the conversion of phenyl rings into the quinoid structures on exposure to air (film oxidation) is equivalent to blocking the SO, reactive sites. SO, had no effect on LB polyaniline films. Again a possible explanation is that the acetic acid, used for the LB film formation, could have blocked the chemically sensitive sites. No effect was observed using CO and CH, gases on spun, evaporated or LB polyaniline films.

4. Conclusions Thin films of polyaniline have been deposited by evaporation, spinning and the Langmuir-Blodgett technique. The as-deposited films on interdigitated electrodes have been shown to possess different electrical conductivities in different gaseous environments. It may well be possible to exploit the different sensor sensitivities to make a highly specific gas-sensor array [23].

Acknowledgements This work was sponsored by British Gas plc and the Cameroon government. We would also like to thank

sensor for nitrogen dioxide based on a copper phthalocyanine Langmuir-Blodgett film, Sensors and Actuators B, 12 (1993) 111-114. [21 J.D. Wright, P. Roisin, G.P. Rigby, R.J.M. Nolte, M. Cook and S.C. Thorpe, Crowned and liquid-c~talline phthalocyanine as gas-sensor materials, Sewers and Actuators B, 13-14 (1993) 276-280. 131 P.S. Vukusic and J.R. Sambles, Cobalt phthalocyanine as a basis for the optical sensing of nitrogen dioxide using surface plasmon resonance, Thin Solid Films, 221 (1992) 311-317. 141 P.N. Barlett and S.K. Ling-Chung, Conducting polymer gas sensors, Part III: results for four different polymers and fwe different vapours, Senson and Actuorm, 20 (1989) 2X7-292. I51 G. Gustafsson, Spectroscopic and electrical studies of some conjugated polyheterocycles, Ph.D. Thesk, Linkiiping University, Sweden, 1990. 1‘31 T. Hanawa, S. Kuwabata, H. Hashimoto and H. Yoneyama, Gas sensitivities of electropolymerized polythiophene films, Synth. Met., 30 (1989) 173-181. I71 A.G. MacDiarmid. J.C. Chiang, A.F. Richter and A.J. Epstein, Polyaniline: a new concept in conducting polymers, Synth. Met, 18 (1987) 285-290. I81 A.P. Monkman and P. Adams, Optical and electronic properties of stretch-oriented solution-cast polyaniline films, Synth. Met., 40 (1991) 87-96. 191 A.M. Kenwright, W.J. Feast, P. Adams, A.J. Milton, AP. Monkman and B.J. Say, Solution state NMR studies of polyaniline structure, Synrh. Mef., 55-57 (1993) 666-671. M.T. Scully, M.C. Petty, A.P. Monkman and M. Harris, Optical PI properties of polyaniline thin films, Synth. Met., 55-57 (1993) 183-187. IllI N.E. Agbor, M.C. Petty, A.P. Monkman and M. Harris, Langmuir-Blodgett films of polyaniline, Synth. Met., X5-57 (1993) 3789-3794. 1121 A.P. Monkman, Characterization of the electroactive polymer polyaniline, Ph.D. Thea%, London University, 1989. 1131 J.W. Chevalier, J. Bergeron and L.H. Dao, Poly(N-benzylaniline): a soluble electrochromic conducting polymer, Polymer Commun., 30 (1989) 31X-310. u41 P. Dannetun, K. Uvdal, W.R. Salaneck, S. StafstrGm, L. Bertilsson. A.G. MacDiarmid, A. Ray, E.M. Scherr, M. Laglund, T. Hjertberg and A.J. Epstein, Vapor deposited poiyaniline, Synth. Mel., 29 (1989) 451-162. 1151 M. Aktar, H.A. Weakliem, R.M. Paiste and K. Gaugban, Polyaniiine thin film electrwhromic devices, Synth. Met., 26 (1988) 203-208. WI (a) S. Stafstrdm and J.L. Brhdas, Evolution of structure and electronic properties in oxidized polyaniline as a function of the torsion angle between adjacent rings, Synth. Met., 14 (1986) 297-308; (b) S. Stafstrijm, Defect states in polyaniline, Synrh Met., 18 (1987) 387-392. 1171 M. Nechtschein, C. Santier, J.P. Travers, J. Chroboczek, A. AIix and M. Ripert, Water effects in pdyaniline: NMR and transport properties, Symh. Met., 18 (1987) 311-316. WI H.H.S. Javadi, M. Angelopoulos, A.G. MacDiarmid and A.J. Epstein, Conduction mechanism in polyaniline: effect of moisture, Synfh. Mrt., 26 (1988) l-8.

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[19] JA. Barbero, KG. McCurdy and P.R. Tremaine, Apparent molar heat capacities and volumes of aqueous hydrogen sulphide and sodium hydrogen sulphide near 25 “C: the temperature dependencies of H2S ionization, Can. J. Chem, 60 (1982) 1873-1880. [ZO] J.J. Carroll and A.E. Mather, The solubility of hydrogen sulphide in water from 0 to 90 “C and pressure to 1 MPa, Gwchim Cosmochim., 53 (1989) 1163-1170. [21] E.C.W. Clarke and D.N. Glew, Aqueous nonelectrolyte solutions. Part VIII. Deuterium and hydrogen sulphide solubilities in deuterium oxide and water, Can. J. Chem., 49 (1971) 691-698. [22] W. Geaard, Solubility of hydrogen sulphide, dimethyl ether, methyl chloride and sulphur dioxide in liquids. The prediction of solubility of all gases, J. Appl. Chem. Biokzhnol., 22 (1972) 623450. (231 P.S. Barker, J.R. Chen, N.E. Agbor, A.P. Monkman, P. Mars and M.C. Petty, Vapour recognition using organic films and artificial neural networks, Sensors and Actudors B, I7 (1994) 143-147.

Biographies

NE. Agbor was awarded a B.Sc. from Keele University in 1988 and obtained an M.Sc. from the University

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of Manchester in 1990. He subsequently received his Ph.D. from the University of Durham for work on gas sensing using organic films. Andy Monkman obtained his BSc. and Ph.D. degrees from Queen Mary College, University of London. Currently he heads the Organic Electroactive Materials Group in the Department of Physics, University of Durham. His research activities include the characterization and applications of conductive polymers, especially polyaniline, and laser spectroscopy, including femtosecond time-resolved measurements. Michael Petty is a professor of electronics in the School of Engineering at the University of Durham. He is also co-director of the Durham Centre for Molecular Electronics. He gained his B.Sc. and D.Sc. from the University of Sussex and his Ph.D. from Imperial College, London. His research interests include the development of organic materials, particularly Langmuir-Blodgett films, and their incorporation in novel electronic and optoelectronic devices.