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Applied Surface Science 269 (2013) 180– 183

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

jou rn al h om epa g e: www.elsev ier .com/ locate /apsusc

lectrochemical gas sensors based on polypyrrole-porous silicon

atma-Zohra Tebizi-Tighilt a,b,∗, Fawzi Zaneb, Naima Belhaneche-Bensemrab, Samia Belhoussea,abrina Sama, Nour-Eddine Gabouzea

UDTS, 2, Bd Frantz-Fanon, B.P. 140 Alger-7 merveilles, 16200 Algiers, AlgeriaENP, 10, Avenue Hassen Badi, B.P. 182-16200, El Harrach, Algiers, Algeria

r t i c l e i n f o

rticle history:eceived 13 July 2012eceived in revised form 13 October 2012ccepted 14 October 2012

a b s t r a c t

This work consists in elaborating a gas sensor based on porous silicon and a polypyrrole obtained bycovalent grafting and studying its answer in different environments. At first, we were interested in theformation of the nanoporous layers by cyclovoltammetric (CV) in a hydrofluoric acid solution followedby an electrochemical grafting of the polypyrrole (PPy) on the porous silicon and oxide porous silicon

vailable online 23 October 2012

eywords:lectrochemical synthesisorous siliconolypyrrole

surfaces.The various interfaces were characterized using different techniques such as Fourier transform infrared

spectroscopy (FT-IR), energy dispersive X-ray spectroscopy (EDX) and scanning electron microscopy(SEM). Finally, a series of electric characterizations to study the answer of the structures in the contactof the carbon dioxide was achieved.

as sensors

. Introduction

Nowadays, there is great interest in using sensing devices tomprove the environmental and safety control/monitoring of CO2as. Conducting polymers such as polypyrrole, polythiophene andolyaniline, due to their imprinting and templating ability, haveeen widely used for the development of highly selective sensors1]. Many conducting polymers have shown changes in resisti-ity on expose to different gases and humidity. Polypyrrole (PPy)s one of the most extensively used conducting polymers in theesign of chemical sensors. This polymer has good stability, facileynthesis, higher conductivity and versatility compared to manyther conducting polymers [2]. Further, PPy can be easily coated ashin adherent films on various metal or semiconductors substratesy electropolymerization from aqueous or organic solvents. Thesetructures are highly sensitive to gases but they show a saturationffect at higher concentration of gases [3]. Suri et al. [4] reportedhat iron oxide–polypyrrole nanocomposite sensors showed the

aximum response to CO2 gas as compared to N2 and CH4 gases.aghuley et al. [5] reported the increased sensitivity of a PPy com-

osite sensor in the presence of CO2 gas.A comparative study between two structures Si/PS/PPy and

i/oxide PS/PPy, was carried out, in order to determine the mostensitive structure for CO2 sensing.

∗ Corresponding author at: UDTS, 2, Bd Frantz-Fanon, B.P. 140 Alger-7 merveilles,6200 Algiers, Algeria. Tel.: +213 771426393; fax: +213 21432630.

E-mail address: mli zola@yahoo.fr (F.-Z. Tebizi-Tighilt).

169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2012.10.080

© 2012 Elsevier B.V. All rights reserved.

2. Experimental

2.1. Materials

All chemicals used were purchased from Merck Company. Pyr-role was stored below 5 ◦C in dark. Nanoporous silicon (PS) wasobtained by electrochemical etching of (boron) doped p-type Si(1 0 0) wafers (� = 1 � cm resistivity) in hydrofluodric acid solu-tion (HF-distilled water–ethanol) for 5 min at a current densityof 10 mA/cm2. After etching, the samples were rinsed with pureethanol and dried under a stream of dry nitrogen prior to use.Porous silicon was subsequently stabilized by ozone treatment for10 min.

2.2. Electrochemical polymerization

Electropolymerization was performed in three electrodes sys-tem contained in a single compartment cell. A porous silicon (PS)and an oxide porous silicon were used as working electrode. Thecounter electrode was comprised of a platinum wire. An Ag/AgClelectrode was used as reference electrode. Polymerization was car-ried out at ambient temperature (25 ± 2 ◦C). Modification of theporous silicon by polypyrrole may be used to increase the sensi-tivity to gas.

2.3. Characterization

Voltametry was performed on EG&G Princeton Applied ResearchPotentiostat model 362 used as the source of power supply. The

F.-Z. Tebizi-Tighilt et al. / Applied Surface Science 269 (2013) 180– 183 181

-0,5 0,0 0,5 1,0 1,5

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2

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0,05 M

0,01 M

0,1 M

0.8V

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[9] appeared whereas those at 2200–2400 cm corresponding to�Si H diminished. This feature is a signature of silicon oxidation.

Ab

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PS

PS/PPy

ig. 1. Cyclic voltammogram of porous silicon coated with PPy films, in 0.05 Mcetonitrile solution. Scan rate = 100 mV s−1.

tructure of the PPy film was analyzed by FT-IR spectroscopyPerkin-Elmer) in the range of 450–4000 cm−1. Surface morphol-gy of the Polypyrrole was investigated by scanning microscopyEM and EDX.

A series of electric characterizations to study the answer of thetructures in the contact of the carbon dioxide was carried out.

. Results and discussion

.1. Study of electropolymerization of pyrrole by cyclicoltametry

Polymerization was carried out on a porous silicon and on anxide porous silicon surfaces from an acetonitrile solution con-aining 0.05 M pyrrole monomer and 0.1 M (C16H36BF4N) by cyclicoltametry. Many cycles were completed from −0.5 V to 2.5 Vg/AgCl at 100 mV s−1 scan rate.

Fig. 1 shows the cyclic voltammogram of porous siliconoated with PPy films, in 0.05 M acetonitrile solution. Scanate = 100 mV s−1. During the first scan, the sudden increase in cur-ent density corresponds to the creation of first active centers athe electrode surface. Afterwards, the PPy film starts growing onhe porous silicon surface. The oxidation potential was found to bequal to 1.3 V (vs. Ag/AgCl).

The nucleation starts at relatively low potentials and the growthn PS occurs inside the deep pores and after pore filling this cov-rs the surface completely and uniformly. Indeed, the structureecomes thicker and rougher after each cycle and a black deposit ofPy is formed on the porous silicon surface. The same technique wastilized to deposit polypyrrole on an oxide porous silicon surface.he oxidation potential was equal to 1.2 V (vs. Ag/AgCl).

Fig. 2 shows the cyclic voltammetry of pyrrole at different con-entrations in acetonitrile containing 1% (v/v) of water and 0.1 MC16H36BF4N) on porous silicon. The peak current decrease linearlyith the increase of the pyrrole concentration [6]. This apparentnlimited polymer film growth was observed in this solution butot in water-free acetonitrile showing the importance of smallmounts of water on the PPy oxidation process.

.2. FT-IR spectroscopy analysis

Fig. 3 shows the infrared spectrum of porous silicon before andfter deposition of PPy, which displays absorptions characteristic ofurface SiH, SiH2 and SiH3 stretching vibrations at 2090, 2110 and150 cm−1, respectively, and SiH2 bending vibrations at 910 cm−1.

Fig. 2. Cyclic voltammogram of PPy electrodeposition on porous silicon at a poten-tial sweep rate of 100 mV s−1 at different concentration of pyrrole.

The bands at 630 and 680 cm−1 are assigned to both Si–Si latticeand Si–Hx, respectively [7,8].

The spectra in the range 450–400 cm−1 show the presence ofmajor expected peaks of the PPy. The three bands located at 794,939 and 965 cm−1 are attributed to the C H out-of-plane vibra-tion. The sharp band at 1035 cm−1 corresponds to the N H in-planedeformation. The weak bands located at 1310 and 1223 cm−1 arerelated to the stretching vibration of the Py ring. The bands corre-sponding to the C H in-plane deformation appear at 1220 cm−1.We see three bands situated at 1636, 1583 and 1490 cm−1 whichare attributed to the C C stretching mode. The sharp band locatedat 1705 cm−1 can be attributed to the carboxylic acid groups.Some authors assigned this band to carbonyl C O groups fixed in�-position of some Py rings and which result from a light overoxi-dation of the polymer occurring during its electrosynthesis.

After deposition of polypyrrole on oxide porous silicon, theinfrared spectra confirm a significant oxidation of the surface, asit is indicated by the decrease of �Si H and increase of �Si Ovalence bends. A band centered at 906 cm−1 due to Si O Si links

−1

4000 3500 3000 2500 2000 1500 1000 500

Longueur d'onde (cm-1 )

Fig. 3. FT-IR spectra of pure PS and PS/PPy structure in the region 3550–2000 cm−1.

182 F.-Z. Tebizi-Tighilt et al. / Applied Surface Science 269 (2013) 180– 183

opolym

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Fig. 4. SEM micrographs of PPy electr

.3. SEM analysis

In order to obtain information about the morphology of the PPyoating, SEM measurements were performed on PPy films synthe-ized electrochemically and deposited on porous silicon obtainedefore and after oxidation.

Fig. 4 shows SEM micrographs of PPy films electrochemicallyeposited on porous silicon surface by cyclic voltammetry from0.5 V to 2.5 V Ag/AgCl at 100 mV s−1 scan rate. The surface of thebtained polymer was characterized by palmer-warm-like withluster. An apparently dendrite deposit of bulk polymer can be seenfter complete pore filling with polymer [10]. It should be notedhat the polymer coatings are poorly adherent and can be easilyeeled-off from the porous silicon electrode.

The nanostructure of PS leads to an enormous increase in surfacerea but the stress produced during the fabrication process includ-ng anodization, drying and storage leads to fracture and fragilitynhibiting its application for gas detection. To this end the PS surface

as oxidized in order to stabilize it.The surface morphology of PPy coated over oxide porous sil-

con substrates are shown in Fig. 5. The PPy films obtained areomogeneous, compact and theirs surfaces are characterized by

ig. 5. SEM micrographs of PPy electropolymerized on oxide PS by cyclic voltam-etry.

erized on PS by cyclic voltammetry.

a cauliflower-like structure with average size of 1 �m [11]. How-ever these larger grains are made up by the agglomerates of smallergrains. It can be observed that the PPy films are strongly adherentto the oxide porous silicon [12,13].

3.4. EDX analysis

The structure oxide porous silicon/PPy was analyzed by EDXmeasurement (Fig. 6) which indicates the presence of carbon andazote. These elements are the principal components of the PPywhile the presence of oxygen is characteristic of the oxide existingon the porous silicon.

3.5. Si/oxidePS/polypyrrole and Si/oxidePS/PPy structures assensors

The current–voltage (I–V) characteristics of the Si/PS/PPy andSi/oxidePS/PPy structures, were performed for a −1 V to + 2 V biasvoltage change against CO2 gas. We note from these I–V curves ofthe sensor under vacuum and in CO2 gas at 500 ppm pressure, arectifying behavior. It can be noted that introduction of CO2 gasinto the gas chamber leads to a decrease of the dc current. In addi-tion, it is observed that exposure to CO2 did not change the shapeof current–voltage dependence but only change the current mag-

nitude at a fixed voltage.

Figs. 7 and 8 show the absolute sensitivity (|�I|/I0) of the sensorsas a function of polarization for gas CO2, where I0 and I corre-spond to currents acquired before and after contact with CO2 gas.

Fig. 6. EDX spectra of polypyrrole deposit on porous silicon by cyclic voltammetry.

F.-Z. Tebizi-Tighilt et al. / Applied Surface Science 269 (2013) 180– 183 183

0,0 0,2 0,4 0,6 0,8 1,0 1,20,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

CO2

V (V)

ΔΙ/Ι 0

Fig. 7. The sensitivity of the sensors Si/oxide PS/polypyrrole.

0,0 0,5 1,0 1,5 2,00,0

0,1

0,2

0,3

0,4

ΔI/I

0

Ib

Pfoas

crsarcrasfdtn

[[

[12] A. Pascual, J.F. Fernandez, C.R. Sanchez, S. Manotas, F. Agulló-Rueda, J. PorousMater. 99 (2002) 57–66.

V (V)

Fig. 8. The sensitivity of the sensors Si/PS/polypyrrole.

t shows that the highest sensitivities are observed for low appliedias voltage.

A maximum sensitivity was found at 0.45 V for Si/oxideS/polypyrrole. This is slightly different from the results obtainedor Si/PS/polypyrrole structure where the maximum sensitivity wasbserved at 1 V. These results indicate that the sensors can operatet low bias voltage which is very advantageous for energy con-umption.

Fig. 9 depicts the dynamic response of the sensors toward a con-entration of 500 ppm of CO2 gas at 1.4 V. It shows an instantaneousesponse when Si/PS/polypyrrole and Si/oxide PS/polypyrroletructures are exposed to various gases. The response time isbout 50 s of the Si/oxide PS/polypyrrole sensor compare to theesponse time obtained with only polypyrrole [14]. We note theurrent recovers rapidly and completely to the initial value, theecovery time is found about 2 min inside Fig. 9 we show severaldsorption–desorption cycles of CO2 on an oxide PS/polypyrroleurface in order to study the reproducibility of the sensor. We per-

ormed several measurements of current vs. time by injecting andegassing the same amount of gas. We note that the intensity ofhe response decreases suggesting that the pores are saturated andeed more time to give a better answer.

[[

Fig. 9. (a) I (t) characterization of the Si/PS/PPy structure. (b) I (t) characterizationof the Si/oxide PS/PPy structure.

In the �-orbital overlap of neighboring molecules of the PPystructure, the �-electrons delocalized along the entire chain, pro-vided semiconducting and conducting properties. CO2 moleculesformed weak bonds with �-electrons of PPy.

4. Conclusion

Polymerization of pyrrole into regularly nanostructured Si/PSand Si/oxide PS from CH3CN solution was investigated. The poly-merization process showed different characteristic stages in bothpotential and current transients. The oxidation of PS has a prefer-ential nucleation of PPy at the pore bottom, and deposition of PPyinside the pores followed by polymerization at the outer surface.FT-IR and EDX results reveal that the systems Si/oxide PSi/PPy andSi/PS/PPy, are likely obtained.

The results presented in this work show that Si/oxide PSi/PPyand Si/PS/PPy structures can be used as a sensor for CO2 gas. Thesesensors are extremely sensitive, have a good response and recov-ery time, and can operate at low voltages. Therefore, the Si/oxidePS/PPy structure gives a better sensitivity compared to poroussilicon.

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