ethanol and ozone sensing characteristics of wo3 based sensors activated by au and pd

Aguira,∗ aL2MP CNRS UMR 6137, Service 152, FST St J´erˆome, Universit´e Paul CEZANNE Aix-Marseille III, 13397 Marseille Cedex 20, France bUnit´e de Recherche de Physique des Semiconducte

sensors activated by Au and Pd

A Labidia,b, E Gilleta, R Delamarea, M Maarefb, K Aguira,∗

aL2MP (CNRS UMR 6137), Service 152, FST St J´erˆome, Universit´e Paul CEZANNE Aix-Marseille III, 13397 Marseille Cedex 20, France

bUnit´e de Recherche de Physique des Semiconducteurs et Capteurs, IPEST, BP 51 La Marsa 2070, Tunis, Tunisia

Received 29 November 2005; received in revised form 7 February 2006; accepted 7 February 2006

Available online 29 March 2006

Abstract

The sensitivity towards ethanol (C2H6O) and ozone (O3) of WO3thin films based conductometric sensors was investigated The performances

of three sensing layers were compared: bare WO3, palladium (Pd) and gold (Au) activated surface WO3 The WO3 thin films were deposited

by thermal evaporation of oxide powders onto SiO2/Si transducers with platinum interdigited electrodes All the tests were performed at the

same working temperature Twork= 300◦C and under fixed gas concentrations: 2% ethanol and 0.8 ppm ozone using dry air as carrier gas The morphology of the sensor surfaces were analyzed before and after working runs by atomic force microscopy (AFM) and scanning electron microscopy (SEM) in order to control the stability of the metal deposits DC and AC electrical responses under 50 l h−1gas flows are presented and discussed

© 2006 Elsevier B.V All rights reserved

Keywords: DC/AC measurements; WO3 sensor; Pd, Au activators; Ozone; Ethanol

Contents

1 Introduction 338

2 Experimental 339

2.1 Sensing device preparation 339

2.2 Sensing tests procedure 340

3 Results and discussion 340

3.1 DC measurements 340

3.2 AC measurements 341

4 Conclusion 343

References 344

Biographies 344

1 Introduction

Numerous metal oxide semiconductor materials were

reported to be usable in conductometric gas sensors, such as

ZnO, SnO2, WO3, TiO2,␣-Fe2O3and so on These candidates

have non-stoichiometric structures, so free electrons originating

∗Corresponding author Tel.: +33 4 91 28 89 73; fax: +33 4 91 28 89 70.

E-mail address:Khalifa.aguir@l2mp.fr (K Aguir).

from oxygen vacancies contribute to electronic conductivity when the composition of the surrounding atmosphere is altered

[1–6]

application, and during the last years many works have been performed on the structural, electrical properties and sensing

different authors that WO3-based thin and thick films were both sensitive to a broad range of oxidizing or reducing gases such

0925-4005/$ – see front matter © 2006 Elsevier B.V All rights reserved.

doi: 10.1016/j.snb.2006.02.015

chosen Pd and Au, and their effect on the sensor sensitivity

towards ozone and ethanol was studied The DC transient and

AC responses were analyzed in order to discriminate the sensor

parts which are crucial for the sensing mechanisms (grains–grain

boundaries–metal/oxide interfaces ) In order to avoid bulk

parameters variation the tests were carried out at the same

measured could be attributed to surface or near surface

phe-nomena

2 Experimental

2.1 Sensing device preparation

deposi-tion at room temperature on SiO2/Si transducer with platinum

interdigitated electrodes This structure underwent a subsequent

stud-ies we have shown that such a fabrication process allows to

obtain continuous, well crystallized films with a stoichiometry

con-ditions[24,25] The thicknesses of the WO3layer and of the

electrodes were 40 nm and 50␮m, respectively, the area of the

Fig 1 AFM (3 ␮m × 3 ␮m) micrograph of the bare WO 3 surface annealed in dry air (1 h at 450 ◦C).

active part of the sensor being 4 mm× 4 mm On the top of the

WO3, 0.5 nm (mean equivalent mass thickness) of the activator

metal (Au or Pd) was vapor deposited in UHV at Tdep= 350◦C.

The morphology of each sensor was controlled by AFM (Nanoscope III-Digital Instruments) and SEM (Phillips XL30 S5).Fig 1is an image of the bare WO3layer, which is formed from small grains with a mean diameter of 40 nm The mean roughness calculated in a 1.5␮m × 1.5 ␮m area is 0.598 nm, it remains the same after some hours of working time It was dif-ficult to distinguish by AFM the metals particles from the oxide grains, so we have analyzed the activated layers by SEM.Fig 2a

respectively The mean diameter of gold particles is 5 nm, the

density The size of palladium particles is larger (9 nm) with

observed after the sensing tests, no changes were visible in the

metal layers as long as the working temperature (Twork) did not

exceed the deposition temperature (Tdep)

Fig 2 SEM images of the sensors surfaces after deposition of metal layer (5 min at 350 ◦C) (a) Au/WO and (b) Pd/WO.

Fig 3 Experimental set-up used for C 2 H 6 O and O 3 sensing tests.

2.2 Sensing tests procedure

Each sensor was tested separately using the same protocol

in a small chamber where the gases are injected at a flow rate

of 50 l h−1 Dry air was used both as a reference (baseline) and

as carrier gas to obtain the desired concentration of the detected

gas.Fig 3represents schematically the experimental set-up The

dilution of C2H6O vapor in dry air was achieved using a two-arm

gas-flow device Two mass flow controllers allowed the flow rate

of the dry air that act as the carrier gas to be controlled from 0 to

94 l h−1in one arm (d1) in which the carrier gas passed through

a balloon flask containing the vapor equilibrated with 200 cm3

of the liquid, and from 0 to 94 l h−1in the other arm (d2) The

balloon flask is put into a furnace and maintained at the fixed

temperature Tvap= 30◦C, in order to fix the partial pressure of

the C2H6O vapor

In these conditions, a range of concentrations of the C2H6O

in air can be calculated by applying the following equation[26]:

where x is the molar fraction of the vapor in the balloon flask at

Tvap, given by:

x = Pvap

with Pvapthe partial pressure of the vapor at a given temperature

Tvap , and Patmthe atmospheric pressure By varying d1and d2

(d1+ d2was kept constant at 50 l h−1), different concentration

values for C2H6O in dry air can be obtained

For the test under O3the two mass flow controllers used for

C2H6O vapor will be turned off Once this condition is satisfied,

dry air controlled from 0 to 50 l h−1in arm d3and fixed also at

50 l h−1, this flow was exposed to a pen-ray UV lamp, calibrated

to give an O3concentration range between 0.03 and 0.8 ppm The

total flow charged by ethanol vapor or ozone was blowing on the

sensor placed in the test chamber on a heating holder

The working temperature (Twork) of the sensor was controlled

by a regulated power supply connected to the heating platform The basic principles of the conductometric sensors is the varia-tion of free carriers density of the active layer exposed to a gas concentration which can be correlated to a change in

conduc-tance G of the oxide G was measured by recording the current variation at the applied constant DC potential V = 50 mV, with

an HP4140B Source/Pico-ammeter In AC regime data were acquired using a Solartron 1250 frequency response analyser

in the 0.2 Hz–65 kHz frequency range Measurements were

C2H6O and 0.8 ppm of O3

3 Results and discussion

3.1 DC measurements

InTable 1are reported the conductances G0in dry air and

Ggasunder the target gases, for the sensors WO3bare, Au/WO3

and Pd/WO3at Twork= 300◦C, and resulting sensing response

“Sgas” of the sensors calculated by using the relations:

Sgas=Ggas − G0

Sgas=G0 − Ggas

larger than that for the bare WO3, when it is smaller for Pd/WO3

The relative values of G0being as Pd/WO3< Au/WO3with G0

(WO3) = 5G0(Pd) and G0(Au) = 100G0(Pd) This indicates that the activation mode of Au and Pd are different

The transient responses of the three sensors towards pulses of 2% C2H6O and 0.8 ppm O3at 300◦C are compared inFig 4a and

b, respectively The exposure time was kept constant at 15 min for each test and the time between successive pulses was also

15 min As expected under C2H6O (reducing gas) the

the pulse and a rapid increase followed by a slow decrease during

conduc-tance at the injection is followed by a slow decrease during the

pulse, and a fast increase followed by a decrease characterizes

the recovery period Such a kinetics results of the superposition

of two phenomena, one occurring on the bare oxide surface the

other on the metallic clusters Further experiments at different

temperatures and concentrations are actually in progress in order

Fig 4 Comparison of typical transient DC responses to gas pulse at 300 ◦C (a)

2% of C H O and (b) 0.8 ppm of O

dure that was established in our previous study[27] The WO3

modified sensitive layers were modeled by a serial association of three parallel RC circuits, attributed to grains (b), grains bound-aries (gb) and grains–electrodes interfaces (el) Each RC circuit

rises to a semicircle in the complex plan plot of Z(ω) versus

Z(ω) (Nyquist diagram) Under small amplitudes of sinusoidal

signal, the total impedance of sensor is given by:

ZTotal(jω) =

i

where i = b, gb and el; j is the complex number j=√−1; Z

i (ω),

Zi (ω) are the real and imaginary parts of impedance Z i (jω),

respectively

InFig 5a and b, we report the Nyquist response of the three

sensors with modeling at Twork= 300◦C, under 2% of C2H6O

The results of modeling for C2H6O and O3 were reported in

Tables 2 and 3, respectively; they confirm the DC analyses, the best response was obtained by the sensor doped by gold

The RC modeling show the existence of two semicircles under dry air, contrary to the sensor without metals (WO3bare) that gives only one semicircle either under dry air or C2H6O The first semicircle is attributed to WO3surface and bulk phe-nomena, the second one could be attributed probably to the

under dry air this circle appears only when Au is added to the

WO3surface When ethanol is introduced the second semicircle disappears, this is could be explained by the fact that the

grain boundaries, is consumed by the C2H6O oxidation follow-ing the reaction paths(6)and(7) [29]:

C2H5OH(vap)+ O−(ads)↔ CH3CHO(ads)+ H2O(vap)+ e− (6)

CH3CHO(ads)+ O(lattice)↔ CH3COOH(vap)+ VO (7) The electrons produced by this reaction are injected into the con-duction band of WO3, which induces a decrease of the resistance

Fig 5 Impedance measurements (symbols) and modeling (lines) at 300 ◦C (a) 2% of C2H6O and (b) 0.8 ppm of O3.

Table 3

Modeled RC for WO 3 bare, Au/WO 3 and Pd/WO 3 based sensors towards 0.8 ppm of O 3 and dry air (baseline) at 300 ◦C

0.8 ppm of Ozone (O 3 )

Rs-b ( ×10 6

a Sensors.

b Gases.

an opposite result was obtained with the addition of Pd to the

WO3surface, i.e increase of resistance and decrease of

sensi-tivity This could be caused by the oxidation of palladium on the

surface or by the formation of a bimetallic component as it was

found on SnO2and CeO2[30–32]

with a more pronounced grain boundaries effect The transient

responses ofFig 4b, suggested that one of the two steps of the

reaction(8)resulting of the O3dissociation is enhanced by the

presence of the metal particles[33–35]

p

Op(ads) + qe ↔ O q−

p(ads) electron transfer step (8) The AC analysis evidenced that it is the electron transfer step

which is improved, in effect when O3is introduced the RC

mod-eling shows a decrease of the grains boundaries capacitance due

to the increases of the depletion zone and consequently increases

the resistance and the electrons capture by oxygen through the

interface Au/WO3 In this case the conductance is mainly

con-trolled by the grains boundaries phenomena, i.e by the electron

transfer step in these regions, which is confirmed by the

model with two circles, whose results are reproduced inTable 2,

represents well the experimental results at the high frequencies

(first semicircle) The more important variation observed at the

low frequencies between the experimental model and results can

be due to the existence of a third semicircle, due to the apparitions

of some diffusions phenomena in the interface grains/electrode (Pt) Unfortunately our work frequency range, does not allow us

to modeling this third semicircle, that is why we have a small deviation between AC measurements and modeling in the low frequency range, as illustrated inFig 5b, for the sensor doped

The other important remark is the non-sensitivity of the Pd-doped WO3gas sensor, which became practically insensitive to the O3gas The origin of such a behavior should be the adverse effect of the PdO formation on the free carrier density

4 Conclusion

films The sensitivities of Au/WO3sensors to ethanol and ozone are in the 2/1 ratio; therefore, at a working point of 300◦C they

can provide a stable, sensitive element for ethanol gas On the contrary Pd/WO3sensors are practically insensitive in this tem-perature range to the tested gases and in these senses could be used as selective elements against ozone The characteristics

of the dynamic responses of the activated WO3thin films sug-gest complex phenomena which depend on the strength of the metal–substrate interaction and consequently could be induced

by the formation of oxide or bimetallic species on the metal par-ticles In the actual knowledge state in the behavior of doped

oxide layers one needs an understanding of activation processes

at an atomic level if one want to progress in the design of

predictable sensing properties In this way we have initiated a

research by XPS and synchrotron radiation photoelectron

spec-troscopy (SRPES) in order to elucidate the electronic structure

of the atomic species under various working conditions

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Biographies Ahmed Labidi, was born in La Marsa, Tunis, Tunisia in 1975 He received the

DEA (post-graduate diploma) in quantum physics 2002 from the University of Tunis El Manar (Tunis, Tunisia) He is currently preparing his PhD degree in physics and Materials science in the L2MP laboratory at the Paul CEZANNE, Aix-Marseille III University (France), in cooperation with the URPSC Labora-tory at the 7 November University (Tunisia) His research interest is the electrical studies of the WO 3 gas sensors under oxidizing and reducing gases by impedance spectroscopy.

Eveline Gillet, born in 1937, graduated from the University of Poitiers (France),

Docteur `es Sciences (Universit´e de Provence-1969) She is Professor in Physics

at Paul C´ezanne University – Aix-Marseille (France) She worked in the area

of Surface Science In particular she studied chemisorption on transition metal nanoparticles (model catalysts) Actually she is involved in a research devoted to the electrical properties of nanostructured metal oxide semiconductor thin films and nanorods for applications to new sensing devices.

Romain Delamare, was born in 1973 He is professor assistant at Paul

CEZANNE, Aix Marseille III University (France) He was awarded his PhD degree in semiconductors physic from University of Orl´eans (France) in 2003 His principal research interests are now directed towards WO 3 gas sensors and selectivity enhancement strategies including noise spectroscopy and modelling

of sensor responses.