Polyaniline Thin Films for Gas Sensing2

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ELSEVIER 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, University f Durham, Lk ham DHl 3LE, UK

bPhpics Department and Centre for MolecularElecbunics, 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 iswell understood and much research has focused on thedevelopment of suitable gas-sensitive materials. Re-cently, there has been considerable interest in exploiting

organic substances such as porphyrins [l], phthalocy-anines [2,3] and doped conductive polymers [4]. Formaximum gas sensitivity, these compounds are usuallystudied as thin fihns.

Among the doped conductive polymers that havebeen investigated are polypyrrole [5] and polythiophene[6]. Unfortunately, these materials are not readily pro-cessible. In contrast, polyaniline (PANi) is soluble inorganic solvents [7] from which free-standing films canbe cast [8]. In this work, polyaniline was processedinto thin-film form using three different methods: spin-

ning, vacuum ‘sublimation and the Langmuir-Blodgett(LB) technique. The gas sensitivities of the differentfilms are compared.

2. Experimental

2.1. Substrate

Fig. 1 shows a schematic diagram of the interdigitatedelectrode structure used in this work. It consists ofgold electrodes patterned onto the surface of a quartzsubstrate; the overlap electrode length was 15 mm andthe 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 poly-aniline films.

2.2. spun films

Polyaniline powder (synthes&d in-house) [8] in theemeraldine base form was dissolved in N-methylpyr-rolidinone (NMP), in a polymer:solvent weight ratioof l:lOO,and sonicated for 30 min. The starting materialhad a purity of 99.8%, as determined by NMR spec-troscopy [9]. The resulting solution appeared blue inreflected light. This was spun onto the interdigitatedelectrode structure shown in Fig. 1. Spinning was un-dertaken using a Dynapert PRS 14E model spinner,at a fixed speed of 3000 rpm for 30 s. The spun !ihnswere transferred to a vacuum oven and heated to atemperature of 120 “C, at lo-” mbar for 10 min. Atypical film-thickness value, obtained from an AlphaTenco surface profiling Talystep, was 2.0 f 0.1 pm. Full



174 N.E. A&w e: a!. I Sensors and Actuators B 28 (1995) 173-179

details of the sample preparation have been published

elsewhere [lo].

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 awater-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= 0e3

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 tem-

perature ( =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-Blodgett ilms

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 micro-

electronics clean room. Full details have been describedpreviously 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 char-

acteristics of the uncoated and coated interdigitated

electrode structures were measured using a Time Elec-

tronics 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 con-

centration was varied using a Signal Instrument Series850 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 ob-

tained. 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

steady reading had been obtained, the next highest

concentration of the active gas was admitted to the

chamber and the entire measurement procedure re-

peated. 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 st nrcture

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 ringfor 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 characterisaion

Both the spun and LB polyaniline films on glass

appeared blue. This colour is indicative of the emer-

aldine 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.



N.E. A&r et al. I Sensors and Actuators B 28 (1995) 173-179 175

The current versus voltage characteristics of spin-coated emeraldine base polyaniline are shown in Fig.3. The measurements were undertaken in an atmosphereof nitrogen, at room temperature and after the currenthad stabilized (see next section). Data for the uncoatedelectrode (under nitrogen a current of 1.2 f 0.2x lo-l2A was measured with 10 V applied) confirmed that

the current was flowing through the polyaniline filmrather than through the substrate. The change in re-sistance for different film thicknesses (1.0, 2.0, 4.0 pm)indicates that Ohmic contacts have been establishedbetween the gold electrodes and the polymer. Usingthe thickness values from the surface profiler, theaverage room-temperature in-plane d.c. conductivitywas 4.4~hO.9XlO-‘~ S cm-‘, which is comparable tothe literature value of 1.0X 10-l’ S cm-’ for the baseform of emeraldine 171.The current versus voltagecharacteristics for both the evaporated and LB films

of polyaniline were qualitatively similar to those ofshown in Fig. 3 (including the linearity with film thick-ness).

The average room-temperature d.c. conductivity offreshly evaporated polyaniline film, in nitrogen, was1.0*0.2x lo-” S cm-‘. This compares with a valueof 2.0~10~~ S cm-’ reported in the literature forsimilar material [15]. The conductivity is slightly higherthan that of our spin-coated films (4.4&0.9x lo-” Scm-‘). This can be explained by the absence of quinoidrings to disrupt rr-r mixing between adjacent benzoidrings in the polymer chain [16].

Electrical measurements on polyaniline LB films havebeen reported previously [ll]. The film has a room-temperature conductivity in nitrogen of 10W8 cm-‘.This is significantly higher than that of the emeraldinebase 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

I I I-500 500 1500

Supply voltage [mVl

Fig. 3. The room-temperature current vs. voltage characteristics forspun 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.

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. Theconductivity decreased very rapidly upon the intro-duction of N2 and became stable after approximately60 min. This can be associated with the removal ofsurface/bulk trapped water molecules. A similar re-sponse was also obtained for polyaniline in LB filmform. Fig. 4(b) shows the effect of dry nitrogen on thed.c. conductivity of an evaporated polyaniline layer. Inthis case, the shorter time to achieve a stable conductivityvalue can be attributed to the lower level of water and

the evaporated film. The effects of water on the con-ductivity of polyaniline are well documented [17,18].

No evidence for oxidation (see previous section) was

noted for the evaporated film in the nitrogen envi-

ronment.

N2m

1.0

09

0.8

0.7

0.6

g 05

0.4

03

0.2

010 4 12 20 28 30 44 52 60 Zdnys 3days &days

Tinehitlsl

0.8 -

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.



176 N.E. Agbor et al. / Sensors and Achtators B 28 (1995) 173479

3.3.2. N it rogen diox ide

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 createdgive 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 spin-

coated 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. Fur-

thermore, 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

rwIlnSl

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 tem-

perature 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. Hydrogen sulfide

H,S was found to produce an increase in the con-

ductivity 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 B 28 (1995) 173-179 I77

of the same thickness. Complete recovery for 10 ppmof the gas was achieved after a period of about 60

min. A similar effect was observed with an 18 LB layerpolyaniline chemiresistor. The change in conductivity,after a fixed exposure time, for both spun and LB

polyaniline chemiresistors is shown in Fig. 7. The thresh-

old 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 polyaniline chemiresistors. The observed increase in con-ductivity indicates that either more than one type ofreaction site is available or that a number of differentreactions are possible. At room temperature and pres-sure, 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]+ (0

where the equ~jb~um is shifted to the right duringexposure and to the left after exposure. The protonationagain produces charge carriers (semiquinone radicals)resulting in an increase in the d.c. conductivity. Thisreaction is likely to involve different sites in the polymerthan for the NO, response. As a result, the sensit~itiesof 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. Thisreveals an irreversible decrease in conductivity at room

oL-----J0 4 8 12

H2S cone. [vpml

vE$ 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).

aqueousphaseH++HS-

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

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).

temperature. The device response to different H,S gasconcentrations is shown in Fig. 9(b). A threshold de-tection value of 10 ppm is evident. Note that spun andLB films are likely to possess more H,O than theevaporated material (Fig. 4), thus increasing the like-lihood of the reaction given by Eq. (1).

3.3.4. suZfir dioxide

SO 2 produced an increase in conductivity of spunpolyaniline as well as complete reversibility at roomtemperature. The effect of different SO, gas concen-

trations is shown in Fig. 10(a), revealing that the deviceis capable of measuring changes down to 2 ppm.Fig. 10(b) shows the response of an evaporated

polyaniline chemiresistor to different concentrations ofSOz. Here, the detection threshold is about 10 ppm,



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

60 80 100

1” so cont. [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 evap-

orated polyaniline films showed a much higher sensi-

tivity, down to 0.5 ppm. This increased to 10 ppm as

the film was cycled continuously between air and ni-

trogen. This result suggests that the conversion of phenylrings 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 tech-

nique. The as-deposited films on interdigitated elec-

trodes have been shown to possess different electrical

conductivities in different gaseous environments. It may

well be possible to exploit the different sensor sensi-

tivities 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

Dr Alex Milton and Dr Phil Adams for the synthesis

of the polyaniline.

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[21

131

141

I51

1‘31

I71

I81

191

PI

IllI

1121

1131

u41

1151

WI

1171

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Biographies

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

of Manchester in 1990. He subsequently received hisPh.D. from the University of Durham for work on gassensing using organic films.

Andy Monkman obtained his BSc. and Ph.D. degreesfrom Queen Mary College, University of London. Cur-rently he heads the Organic Electroactive Materials

Group in the Department of Physics, University ofDurham. His research activities include the charac-terization and applications of conductive polymers, es-

pecially polyaniline, and laser spectroscopy, includingfemtosecond 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 Mo-lecular Electronics. He gained his B.Sc. and D.Sc. fromthe University of Sussex and his Ph.D. from ImperialCollege, London. His research interests include thedevelopment of organic materials, particularly Lang-muir-Blodgett films, and their incorporation in novelelectronic and optoelectronic devices.

Polyaniline Thin Films for Gas Sensing2

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