ammonia gas sensor based on electrosynthesized polypyrrole films

bat Eg ee = ELSEVIE Contents lists available at ScienceDirect Talanta Ammonia gas sensor based on electrosynthesized polypyrrole films Stéphanie Carquigny*”, Jean-Baptiste Sanch

bat Eg ee =

ELSEVIE

Contents lists available at ScienceDirect

Talanta

Ammonia gas sensor based on electrosynthesized polypyrrole films

Stéphanie Carquigny*”, Jean-Baptiste Sanchez?, Franck Berger?,

Boris Lakard>*, Fabrice Lallemand °

4 LCPR-AC, UMR CEA F4, Université de Franche-Comté, Batiment Propédeutique, 16 route de Gray, 25030 Besancon Cedex, France

5 Institut UTINAM, UMR CNRS 6213, Université de Franche-Comté, Batiment Propédeutique, 16 route de Gray, 25030 Besancon Cedex, France

Article history:

Received 28 July 2008

Received in revised form 24 October 2008

Accepted 31 October 2008

Available online 11 November 2008

In this work, design and fabrication of micro-gas-sensors, polymerization and deposition of poly(pyrrole) thin films as sensitive layer for the micro-gas-sensors by electrochemical processing, and characterization

of the polymer films by FTIR, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), are reported The change in conductance of thin polymer layers is used as a sensor signal The behaviours, including sensitivity, reproducibility and reversibility, to various ammonia gas concentrations

Keywords:

Gas sensor

Ammonia

Polypyrrole

Electrochemistry

ranging from 8 ppm to 1000 ppm are investigated The influence of the temperature on the electrical response of the sensors is also studied The experimental results show that these ammonia gas sensors are efficient since they are sensitive to ammonia, reversible and reproducible at room temperature

© 2008 Elsevier B.V All rights reserved

1 Introduction

Polypyrrole (PPy) has attracted considerable attention because

of the possibility to use redox reactions for transforming it into

states of strongly differing electrical conductivity [1] and because

PPy has a good stability in air and aqueous media PPy and other

conductive polymers have, therefore, also been classified as organic

metals There exists a wide range of applications to use organic

metals, such as: cell culture substrates [2,3], field effect transistors

[4-6], light-emitting diodes [7], solar cells [8-10], electrochromic

devices [11,12], electronic circuits [13,14], elastic textile composites

[15], supercapacitors for energy storage and secondary batteries

[16], protection of metals [17,18], ion exchange membranes that

respond to external stimulations [19,20], sensors and biosensors

[21-27] More, in recent years, attention has also been given to the

use of conducting polymers as active layers in chemical gas sen-

sors, and it has been proved that adsorbed gas molecules (ammonia,

NOz, COz) and organic vapors (alcohols, ethers, halocarbons) cause

a change of electrical conductivity in the polymer matrix of organic

metals [28-33] In comparison with most of the commercially avail-

able sensors, based usually on metal oxides and operating at high

temperatures, the sensors made of conducting polymers have many

improved characteristics They have high sensitivities and short

response time; especially, these feathers are ensured at room tem-

perature

* Corresponding author Tel.: +33 3 63 08 25 78

E-mail address: boris.lakard@univ-fcomte.fr (B Lakard)

0039-9140/$ - see front matter © 2008 Elsevier B.V All rights reserved

doi:10.1016/j.talanta.2008.10.056

Thus, in this paper, an original ammonia gas sensor based on micropatterned microelectrodes functionalized by electropolymer- ization of polypyrrole films is studied Electrochemical deposition has been chosen since it is the most convenient method to deposit conducting polymer films [34-36] Indeed, the thickness of the film can be controlled by the total charge passed through the elec- trochemical cell during the film growing process More, such a deposition also allows the preparation of films at a well-defined redox potential in the presence of a given counter-ion, which then also defines the level and characteristics of the doping reaction [37] Thus, electropolymerization is used in this study to fabricate

a gas sensor consisting in PPy films deposited on microstructured electrode arrays and also across the insulating gap separating the microstructured electrodes of the sensor Indeed, if the insulat- ing gap between the neighboring electrodes is close enough (a few micrometers), the growing film can cover the insulated gap and connect electrodes [38,39] This is important in fabricating chemiresistors for gas sensing A microstructured interdigitated electrode array was chosen since it represents the most suit- able geometry to serve as a transducer in chemical gas sensors, based on conductivity changes Following their deposition, the elec- tropolymerized polypyrrole layers are characterized by infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) Then, polymer films are tested

as ammonia gas sensors In particular, their response, in terms of conductance changes when exposed to different ammonia concen- tration, was studied The reproducibility and the reversibility of the signal exhibited by the PPy films to ammonia exposure but also the influence of temperature on this response are also studied

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Gas mixture a controler

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conductive

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PPy-based gas sensor

non-

- conductive Electrical substrate

bridge

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I

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Fig 1 (a) Experimental set-up used for the analysis of NH3 vapors and (b) schematic drawing of the ammonia gas sensor

2 Experimental

2.1 Fabrication of gas sensors

The gas sensors were fabricated using microsystem technolo-

gies, and in particular using lift-off process that consists in a

photolithography followed by a sputtering of platinum on a SiOz

wafer The first step of the photolithography process consisted in

drawing the required pattern (see Fig 1) with commercial mask

design software Cadence Then a Cr/Glass mask, on which the shape

of the pattern has been drawn, was made with an electromask

optical pattern generator The process started with a 100-oriented

standard 3” silicon wafer, which was thermally wet-oxidized, at

1200 °C in water vapor, in order to produce a 1.3 Lm thickness SiO»

layer Next, a 1.4-j4m thickness layer of negative photoresist (AZ

5214, from Clariant), suitable for lift-off, was deposited by spin

coating Then, the wafer was first exposed with the mask to a 36-

mJ/cm? UV radiation flux delivered by an EVG 620 apparatus, and

then without any mask to a 210-mJ/cm? UV radiation flux Thus,

the pattern was transferred to the resist, which was then devel-

oped, using AZ 726 developer, to dissolve the resist where the metal

was deposited Then, a magnetron sputtering (Alcatel SCM 441

apparatus) was used to coat microsystems with titanium (30nm,

used to improve platinum layer), then platinum (150 nm) The fab-

rication parameters for Pt and Ti films were the following ones:

base pressure: 4.6 x 10~’ mbar, pressure (Ar) during sputtering:

5 x 10-3 mbar, power: 150W, target material purity: 99.99%, film

thickness: 150 nm for Pt films and 30 nm for Ti films The remaining

resist layer was then dissolved using acetone After the gas sensors

have been fabricated, the pattern and the dimensions are controlled

using an optical microscope More details about the microsystem

fabrication can be found in a previous paper [27] A microstruc-

tured interdigitated electrode array was chosen since it represents

the most suitable geometry to serve as a transducer in chemical gas

sensors, based on conductivity changes The width and length of the

100 bands (50 bands on each microelectrode array) were 100 7m

and 9996 1m, respectively (Fig 1) The width of the gap between

the two microelectrode array was 4 4m to allow the coating of the

gap by the polypyrrole film

2.2 Electrochemistry

Pyrrole (Py) and LiClO, were obtained from Sigma-Aldrich (ana-

lytical grade) Pyrrole was used at the concentration of 0.05 M in

an aqueous solution of 0.1 M LiClO4 The electrochemical appara- tus was a classical three-electrode set-up using a Tacussel PGZ301, from Radiometer, potentiostat-galvanostat The microsystem was used as working electrode The reference electrode was a saturated calomel electrode (SCE) and the counter-electrode was a platinum wire All electrochemical experiments were carried out at room temperature (293 K) Cyclic voltammetry experiments were car- ried out with a sweep rate of 100 mVs~! between —0.3 V/SCE and +1.5V/SCE Each solution was purged by ultrahigh purity argon Chronoamperometry experiments were carried out at a potential

of +1.3 V/SCE

2.3 Characterization of the polymer films 2.3.1 XPS

The polymer surface was characterized by X-ray photoelectron spectroscopy (XPS, SSX-100 spectrometer) XPS was used to control the elemental composition and to determine the oxidation state of elements All recorded spectra were recorded at a 35° take-off angle relative to the substrate with a spectrometer using the monochro- matized Al Ka radiation (1486.6 eV) The binding energies of the core-levels were calibrated against the C;, binding energy set at 285.0 eV, an energy characteristic of alkyl moieties The peaks were analyzed using mixed Gaussian—Lorentzian curves (80% of Gaussian character)

2.3.2 SEM Examinations of polymer morphologies were performed using

a high-resolution scanning electron microscope Once synthesized and dried, polymer samples were examined in a LEO microscope (SEM LEO stereoscan 440, manufactured by Zeiss—Leica, K6ln, Ger- many) with an electron beam energy of 15 keV

2.3.3, IRTF-ATR All spectra were recorded using a Shimadzu spectrometer (IR-Prestige 21) in ATR reflexion mode The specific accessory used for these analyses is the ATR Miracle Diamond/KRS5 which allowed us to record spectra between 4000cm7—! and 700cm7! Resolution was fixed at 4cm7! and 6Oscans were realized to acquire each spectrum All samples were constituted with PPy powder

S Carquigny et al / Talanta 78 (2009) 199-206

(a)

20x1033

1,5x10°4

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5.0x103

0,0 +

-04 -02 00 02 04 06 08 10 12 14 16

E/SCE (V)

(b)

201

T -

30 Time (s)

Fig 2 Electrochemical synthesis of polypyrrole films by oxidation of an aqueous solution of pyrrole and LiClO, by cyclic voltammetry (a) or chronoamperometry (b)

2.4, Gas measurements

An initial ammonia concentration equal to 1000 ppm in nitro-

gen was used for the experiment All the studies were carried

out in nitrogen atmosphere The sensor’s electrical responses were

obtained by monitoring the variations in the sensor’s instantaneous

conductance versus acquisition time, for a constant temperature of

the sensitive layer The conductance measurements lasted about

3h for each acquisition The protocol used for the conductance

experiments was the same for each sensor Each sensor’s electri-

cal response was obtained under a constant gas flow (N2 or NH3

diluted in Nz) rate of 50 mLmin7! Specially designed equipment

was developed for this study Mass flowmeters were used to obtain

different NH3 concentrations This experimental set-up allowed

PPy-based gas sensors to be exposed to the different ammonia

concentrations

The effect of gases on the sensor’s electrical properties was

recorded using a basic divisor voltage bridge (Fig 1) With these

experimental conditions, the relationship between the variation

of the sensor’s conductance and the variation of the voltage UR is

defined as:

1

ỨC = R((EJUg) — 1)

Any decrease (or increase) of the sensor’s conductance was

recorded as a decrease (or increase) of the electrical signal

Each new sensor was exposed to a constant nitrogen flow for

12h before conducing each experiment This process allowed for

the desorption of pollutant chemical compounds adsorbed onto

the sensitive layer during the storage

3 Results and discussion

3.1 Electrochemical synthesis of polymer films

Electrochemical synthesis of polypyrrole was performed by

cyclic voltammetry, from an aqueous solution containing 0.1 M pyr-

role and 0.1 M LIC]Oa, on the platinum microelectrodes of the sensor

using a potential sweep rate of 0.1 Vs~! between —0.3 V/SCE and

+1.5 V/SCE (Fig 2a) For this aqueous solution of pyrrole, the first

scan showed the oxidation of pyrrole at +1.25V/SCE Following

scans showed the oxidation peak of polypyrrole at +0.5 V/SCE and

the reduction peak of polypyrrole at about +0.2 V/SCE Moreover,

the polymer film is a conductive one since the current remains

constant during all the potential scans

PPy films can also be formed at a constant potential As in the case of polymer formation by potential scans, the films are homo- geneous and very adherent to the substrate From Fig 2a, we chose

to carry out the potentiostatic depositions at +1.3 V/SCE Fig 2b shows the /-t curves obtained for a fixed potential of +1.3 V/SCE and for a deposition time of 60s This chronoamperometric curve shows that, after an increase corresponding to the formation of pyrrole cation radicals, the current decreases following a linear rela- tionship with t-!/2 This behaviour indicates a diffusion-controlled process This current response is due to the nucleation and growth

of the polymer At longer times, after the nucleation transient, the chronoamperometry shows a constant /-t response for PPy

3.2 XPS characterization of the polypyrrole films Fig 3a shows the XPS of the films obtained from the oxidation of

an aqueous solution containing pyrrole and LiClO, since this tech- nique is widely used to control the elemental composition of a solid film The XPS analyses confirm the presence of PPy, incorporating ClO,4~ doping agents, on the platinum surfaces Indeed, XPS spec- tra of polymer samples reveal the presence of C, N, O, Cl, Pt for all polymers Thus, Cy; signal (Fig 3b) can be fitted by five differ- ent carbon species at 284.0, 284.8, 286.1, 287.8 and 289.8 eV The two components at the lowest binding energy relevant to B and

a carbon atoms, respectively, revealed the first interesting finding

In fact, the comparison of these two carbon atoms areas showed that, following overoxidation, the B carbons in the film were less abundant than the a ones This indicates, that the B positions were the ones involved in the polymer functionalization The third peak

at 286.1 eV is attributed to carbons of the polymer C=N or C—N*; the fourth one at 287.8eV to C=N* carbons and the peak much weaker at 289.8 eV to carbonyl C=O groups The appearance of a C=O component may be associated with the overoxidation of PPy

at the B carbon site in the pyrrole rings The Nj, spectra (Fig 3c) indicate the presence of four peaks in the case of PPy It contains a main signal at 399.6 eV which is characteristic of pyrrolylium nitro- gens (—NH-structure) and a high BE tail (BE=400.4 and 402.0 eV) attributable to the positively charged nitrogen (—NH* (polaron) and

=NH' (bipolaron) The spectra also show a small contribution at 397.0 eV that we associate with =N-structure Fig 3d represents the Cly, core-level XPS spectrum at 207.5 eV binding energy due

to the perchlorate anions present in the film as a doping agent Consequently, these XPS spectra confirm that polypyrrole films incorporating ClO4~ doping agents are obtained from the oxidation

of pyrrole in various solvents

202

Binding Energy (eV)

1000 800

Binding Energy (eV)

406 404

S Carquigny et al / Talanta 78 (2009) 199-206

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Fig 3 Survey-scan XPS of polypyrrole films electrosynthesized by oxidation of an aqueous solution of pyrrole and LiClO, (a) (b) C 1s (c) N 1s (d) Cl 2p XPS spectra of the same polypyrrole film

3.3 Morphological characterization

Scanning electron microscopy was used to determine the sur-

face morphology of the polypyrrole films on the microstructured

electrode arrays but also to check that PPy films were deposited

across the insulating gap on the microstructured electrode arrays

Thus, Fig 4 shows that the whole surface of the platinum micro-

electrodes is coated by a homogeneous and very compact film of

polypyrrole composed of many nodules (1-2 zm long) The mean

a |

= 22 nm

Fig 4 SEM image of PPy film grown on the sensor surface

film thickness of this polypyrrole film (x) was estimated to 2.25 4m from the electrical charge (q), associated with pyrrole oxidation by application of Faraday’s law and assuming 100% current efficiency for polypyrrole formation: x =qM/pAzF, where M is the molar mass

of the polymer, F is the Faraday constant, ¢ is the density of the polymer and z is the number of electrons involved The nominal density of the polypyrrole films (p) was taken as 1.5 gcm~ and an electron loss z of 2.25 was considered

More, Fig 4 shows that nodules of PPy are also present in the insulating gap between the microelectrodes indicating that the growing film covers the insulated gap and connect microelectrodes This point is important since the PPy layer must connect each pair

of interdigitated electrodes in order to obtain the sensitive layer of the gas sensor

3.4 Evaluation of the sensor’s electrical signal under NH3 flow Firstly, the sensor was exposed to a NH3 flow at a concentration equal to 500 ppm with a temperature of the sensitive layer near to room temperature The signal’s electrical variation was recorded versus time Fig 5a represents the evolution of the PPy’s conduc- tance in presence of ammonia and nitrogen The curve shows the evolution of the electrical signal when the sensor is first stabilised under N2 flow (until 300 s.), second exposed to NH3 flow (from 300 s

to 6000s) and then to nitrogen flow (from 600s to 14,000s) This acquisition protocol was used for all the experiments described in this paper

In presence of pollutant in the gas chamber (NH3), we clearly notice a decrease in sensor’s sensitive layer conductance Ammo- nia reacts with the PPy and induces a modification of the sensor’s sensitive layer electrical properties After ammonia exposition, the

1

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(a)

Slope

160}:

wh XS _—

100'

sey

605 2000 4000 6000 8000 10000 12000 14000

Acquistion time (seconds)

(b) 200

180

160 44oll

120

100

80

Acquistion time (seconds)

15000

Fig 5 Sensor’s electrical response under NH3 (500 ppm) and Nz flow

sensor is submitted to a nitrogen flow (after 6000s) Fig 5a indi-

cates an increase of the sensor’s conductance This modification of

conductance can be attributed to the desorption of ammonia from

sensitive layer Among to this electrical variation under NH3 flow,

it is possible to obtain one supplementary information Looking at

the beginning of the exposition to pollutant flow, the conductance

variation is linear with time In this way, the calculation of the slope

value gives us information about the sensitivity of the gas sensor

For a concentration of ammonia equal to 500 ppm the value of the

slope equals to 63.20nSs7!

In order to evaluate a possible reproductibility of the gas sensor

under ammonia flow at room temperature, we studied two succes-

sive electrical responses of the same gas sensor under pollutant

The purpose was to compare the sensor’s conductance between two

successive acquisitions In Fig 5b is represented the first electrical

response obtained under a constant NH3 flow and nitrogen flow

The second curve shows the successive response under NH3 flow

and N> flow after a nitrogen flow exposition of the sensitive layer

during 12h at room temperature As shown in Fig 5b, we notice

a superposition of the signal under ammonia flow during the first

minutes of acquisition If we consider that the sensor’s electrical

response is measured by referring to experimental point obtained

at the beginning of the exposition of the sensor, one can say that the

electrical signal is reproductible with the same sensor at room tem-

perature The values of the slope are nearly the same (63.20 nS s~!

and 65.42 nS s~1),

By comparing the two acquisitions (Fig 5b curves 1 and 2),

when the sensor is rinsed with nitrogen flow, the second electrical

response is slightly shifted This phenomenon is probably due to a

chemical modification of the sensitive surface after a first detection This point will be confirmed with infrared analysis

Various ammonia concentrations from 8 ppm to 1000 ppm were tested using the same sensor Fig 6a shows some of the electri- cal responses obtained for various ammonia concentrations Before each pollutant expositions, the gas sensor is stabilised by nitrogen flow during 12h at room temperature

Fig Ga shows a decrease of the instantaneous conductance for each NH3 concentrations This decrease depends on the concen- trations of the pollutant in the gas chamber In particular, if we determine the value of the slope of the electrical responses obtained for each ammonia concentrations, we can plot the variation of the slope versus ammonia concentrations Fig 6b represents the evolution of the slope versus the ammonia concentration Above concentrations of 500 ppm, we noticed a smooth plate which was due to the saturation of the sensitive layer For lower concentra- tions, there is a linear relationship between the value of the slope and the ammonia’s concentration

3.5 Influence of the sensitive layer’s temperature on the sensor’s electrical signal under NH3 flow

Chemiresistors, based on metallic oxides, generally works at high temperatures (about 450°C) in order to optimize the elec- trical response In order to evaluate the impact of temperatures on the electrical signal of the PPy-based gas sensor, the sensing layer was heated at temperatures ranging from 25°C to 100°C (Table 1)

A concentration of ammonia equals to 500 ppm was used for this experiment Curves show the evolution of the electrical signal when

Œ) N2 NH3 60 : in

5 170 F 2 40 L rã _ i

{ 4

Fig 6 Sensor’s electrical responses to various ammonia concentrations (a) Slope of the gas sensor’s electrical response vs NH3 concentrations (b).

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Fig 7 Infrared spectra of polypyrrole powder before and after an exposition to ammonia flow for 1 h

Table 1

Values of the slope under ammonia flow at different temperatures

the sensor is first stabilised under N> flow (until 400s), second

exposed to NH3 flow (from 400s to 1200s) and then to nitrogen

flow (from 1200s to 3500s)

Looking at Table 1 which represents the values of the slope for

each temperature, we understand that an increase of the PPy’s sen-

sitive layer temperature decreases the sensitivity of the gas sensor

The best sensitivity was obtained at room temperature Conse-

quently, in term of power consumption, the PPy-based gas sensor

show very interesting detection properties compared to resistive

sensors which have higher working temperatures (300-500 °C)

3.6 Interaction mechanism

In order to understand the interaction mechanism between

ammonia and the gas sensor’s sensitive surface we proceed to

an infrared characterization of PPy powder before and after being

exposed to NH3 flow

First we characterized the PPy powder unexposed to NH3 vapors The spectrum represented in Fig 7a shows the presence

of broad absorption bands characteristics of polypyrrole material These absorption bands corresponds to: N—H stretching in sec- ondary amine (3600 cm~!), N—H deformation in secondary amine

(1600 cm—!), C=C aromatic stretching (1400 cm~!) et C—N stretch- ing (1050 cm~!)

Then, in order to understand the mechanism of ammonia adsorption onto PPy surfaces, polypyrrole samples were exposed during 1h to NH3 1000 ppm flow The spectra obtained for this sample, compared to PPy under Np, is represented in Fig 7b Com- pared to the PPy powder spectrum (Fig 7a), two broad absorption bands appeared when PPy was exposed to NH3 flow The first one at 3260cm~! may be attributed to the stretching vibration

of N—H binding in NH3°° radical group The second one seems

to be superposed to the C=C aromatic stretching band centered

at 1400cm_~! This latter band may be attributed to N—H bend- ing vibration These results confirmed that the interaction between NH3 and PPy-induced chemical modifications of the sensitive layer These infrared analyses explained the shift observed between two successive NH3 detections using PPy-based gas sensors

According to the infrared results, we propose an interaction mechanism for the adsorption of ammonia onto polypyrrole thin films The different stages of ammonia adsorption onto the PPy layer which is considered as a p-type semi-conducting material (positive hole conduction) are the following ones:

Conductance decrease

The first step of this mechanism is the lost of an electron by the

doublet of nitrogen of some nitrogens of the polymer backbone

This electron transfer between ammonia molecule and the poly-

mer’s positive hole induces a diminution of the sensitive positive

charge density which leads to a decrease in the conductance layer

After adsorption of NH3, the polymer becomes less conducting In

this mechanism, it is proposed that ammonia is adsorbed onto PPy

surface forming NH3* radical groups according to infra-red spec-

tra This mechanism is completing the various works realised on the

ammonia detection studies using PPy-based gas sensors [40,41 ]

3.7 Comparison with other works

Before this study, other authors used conducting polymer

films to develop gas sensors These polymer films were obtained

using different techniques The most often used technique was

the chemical deposition by dip-coating [42-45], and the oth-

ers were: spin-coating from soluble conducting polymers [46,47],

thermal evaporation by heating and deposition of the conduct-

ing polymer on a substrate [48], vapor deposition polymerization

{49], drop-coating of a dried polymer solution [50,51], UV-

photopolymerization [52], deposition of Langmuir—Blodgett film

{53} and electrochemical deposition [54,55] We decide to use this

latter technique since the thickness of the film can be controlled

by the total charge passed through the electrochemical cell dur-

ing film growing process Moreover, the film can be deposited

on patterned microelectrode arrays {38| However, if the insulat-

ing gap between the neighboring electrodes is close enough, the

growing film can cover the insulated gap and connect electrodes

{39}

Amongst the various polymer films, polypyrrole is one of the

most studied and interesting in particular thanks to its high conduc-

tivity Consequently, many papers have already used this polymer

as active layer of gas sensors Thus, PPy obtained by chemical oxi-

dation was used for the detection of CO [56], CO, [57], xylene

{58}, alcohols [43,59,60] or acetone [61] PPy obtained by vapor

deposition polymerization was also used for the detection of

methanol, ethanol, CCl, and benzene [62] PPy obtained by UV-

photopolymerization was used for the detection of sevoflurane

{52] It can also be noticed that the surfaces coated by PPy in all

these studies were either gold microelectrode arrays deposited on

aluminia substrates {43,52,56,58,60] or ITO substrates {57,59,62]

Polypyrrole deposited by electrochemical way was also incorpo-

rated in gas sensors for the detection of ethanol gas [54], benzene,

xylene and toluene [55]

Other studies focused on ammonia gas sensors using polypyr-

role as sensitive layer Thus, an ammonia gas sensor based on

Langmuir-Blodgett PPy film was developed but its lower detectable

limit was of 100 ppm of NH3 in N2 [53] Bai et al have electrochemi-

cally co-polymerized polypyrrole and sulfonated polyaniline on an

ITO substrate to obtain an ammonia sensor but it was efficient only

for ammonia concentration higher than 20 ppm [63] Another study

from Brie et al presents an ammonia sensor using electrosynthe-

sized PPy film, with various doping agents, but this study is limited

to concentrations higher than 10 ppm [32] Thus, only a study by

Guernion et al presented an ammonia sensor giving a response

below 10 ppm but in this study PPy is chemically oxidized on a

poly(etheretherketone) surface [64] Concerning ammonia sensors

using polymer films deposited on microelectrode arrays, an inter-

esting work was carried out by Lin et al [65] In this work an

electrosynthesized copolymer PPy—poly(vinyl alcohol) was used

and was efficient for ammonia gas concentrations ranging from

50 ppm to 150 ppm Consequently, the results obtained in our study

are competitive with all these results since the ammonia gas sen-

sors developed in this paper showed a detectable limit of 8 ppm

of NH3 in Nz More, the best responses were obtained at room temperature and were reproductible

4 Conclusion The aim of this work was to validate the use of polypyrrole-based gas sensor for the detection of ammonia at concentrations lower than 10 ppm From this study we first electrosynthesized PPy films doped with small anions ClO~,4 on metallic electrodes to develop

a chemical resistor gas sensor A homogeneous polymer deposited film with a thickness close to the micrometer was obtained The various tests conducted under ammonia flow showed an interest- ing sensitivity (lower than 10 ppm) and a good reproductibility By comparison with most of chemiresistors gas sensors, our PPy-based sensor presents best sensitivity at room temperature

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