ethanol interference in light alkane sensing by metal-oxide solid solutions

light alkanes has been undertaken under wet condition and in presence of ethanol.. We demonstrated that the films are capable of detecting 100 ppm of light alkanes or 500 ppm of methane i

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Sensors and Actuators B: Chemical

j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / s n b

Ethanol interference in light alkane sensing by metal-oxide solid solutions 夽

M.C Carottaa, A Cervia,b, A Gibertia, V Guidia,b,∗, C Malagùa, G Martinellia,b, D Puzzovioa,b

aDepartment of Physics, University of Ferrara, Via Saragat 1/C, I-44100 Ferrara, Italy

bINFN Section of Ferrara, Via Saragat 1/C, I-44100 Ferrara, Italy

a r t i c l e i n f o

Article history:

Received 29 July 2008

Received in revised form 6 November 2008

Accepted 15 December 2008

Available online xxx

Keywords:

Metal-oxide chemoresistive sensors

Light alkane detection

Ethanol interference

Active carbon filtre

a b s t r a c t

A study of the sensing properties of chemoresistive metal-oxides vs light alkanes has been undertaken under wet condition and in presence of ethanol Screen-printed films of a solid solution of mixed Sn and

Ti oxides have been selected for the purpose We demonstrated that the films are capable of detecting

100 ppm of light alkanes or 500 ppm of methane in wet condition, two concentration levels by far lower than their alarm thresholds, respectively Ethanol is known to be a harmful interfering gas, though its concentration can be reduced to values lower than 10 ppm by proper filtering We show that, even in presence of 10 ppm of ethanol, the films steadily responded to alkanes

© 2008 Elsevier B.V All rights reserved

1 Introduction

Many different fields are interested by light-alkane detection

Methane and propane have long been recognized for their use in

combustion, while propane can also be employed to produce

hydro-gen by steam-reforming plants Detection of charged particles in

nuclear physics employs i-butane in gas-flow chambers Therefore,

many studies regarding a selective sensor system have been carried

out in order to detect the presence and, ultimately, the leakages of

these gases

Chemoresistive metal oxides are useful materials for alkane

detection due to their established advantages such as low cost,

compactness and ease of implementation with integrated-circuit

technology Some results in alkane detection via chemoresistive

materials have been published[1–4]and systematic studies have

been carried out under dry and wet conditions[5,6] However,

oper-ation of a sensor in real situoper-ations does demand the analysis of

the response in the presence of interfering gases, such as ethanol

Indeed, the interference of ethanol is a well-known disturbance

in gas detection especially in domestic applications In a previous

work we demonstrated that, under wet condition, the OH-group

of water competes in interaction with the active centres of alkane

interaction (i.e the surface sites where atmospheric oxygen species

are adsorbed)[5] Since the OH-group of ethanol is by far more

reac-夽 Paper presented at the International Meeting of Chemical Sensors 2008

(IMCS-12), July 13–16, 2008, Columbus, OH, USA.

∗ Corresponding author at: Department of Physics, University of Ferrara, Via

Sara-gat 1/C, I-44100 Ferrara, Italy Tel.: +39 0532974284; fax: +39 0532974210.

E-mail address:guidi@fe.infn.it (V Guidi).

tive than that of water, we expect an even stronger interference A possible solution to circumvent the presence of ethanol is the use of

an active carbon filtre, installed upstream the test chamber The fil-tre is capable of reducing the concentration of ethanol[7]to values lower than 10 ppm as experimentally observed

In this paper, we analyse the responses to light alkanes under wet condition with and without 10 ppm of ethanol and critically investigate the interference of the latter in the responses to alkanes

2 Experimental

2.1 Sensing element preparation

We performed a series of measurements with most traditional materials, such as SnO2and TiO2, together with their mixed oxides Such mixed oxides are as stable as SnO2, though they often exhibit superior performance as shown in Ref.[5]

The sol–gel technique was used for preparation of the SnO2 powder A given amount of deionized water was added drop-wise

to n-butanol solution 0.7 M of tin(II)2-ethylexanoate, stirring it at

room temperature for 3 h The molar ratio of water to Sn was 4 and the pH of the solution was set at the unity with HNO3 The resulting gel was dried at 95◦C for 12 h in order to obtain a yellow powder, which was subsequently calcined at 550◦C for 2 h[8]

Titanium butoxide (TB) was used as a source of titanium to syn-thesize the TiO2 TB dissolved in the absolute ethanol (0.23 M) was added drop by drop to a solution of ethanol/water 1:1 vol under mild stirring This step was followed by 20 min of vigorous stirring The obtained suspension was treated by means of the sol–gel pro-cess After stirring, 16 h resting followed the suspension was filtered 0925-4005/$ – see front matter © 2008 Elsevier B.V All rights reserved.

doi: 10.1016/j.snb.2008.12.052

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Table 1

Lower and upper explosive limits for each alkane.

to obtain a white precipitate, which was dried in air (100◦C) for 16 h

Finally, the powders were calcined at 400◦C in air for 2 h[9]

The solid solutions of Sn and Ti mixed oxide were produced

via symplectic gel coprecipitation (SGC) of stoichiometric Sn(4+)

and Ti(4+) hydroalcoholic solutions and further calcination of the

resulting xerogels Calcination was performed at 550◦C for 2 h

under air flow condition The obtained powders resulted in a

particle-size distribution averaging about 20 nm, as determined by

SEM[10] The solid solution of TixSn1−xO2 with two values of x

(x = 0.3 and 0.5) will be hereinafter labeled as ST30 and ST50 Nb

was added to the pure solution of ST30 by coprecipitation, in the

proportion of Sn:Ti:Nb = 100:42:5 in order to enhance the

conduc-tivity

The synthesized powders were mixed together with

␣-terpineol, ethyl-cellulose and 2(2-butoxyethoxy) ethyl acetate in

order to obtain the serigraphyc paste Then, this paste was

screen-printed onto miniaturized laser pre-cut alumina equipped with

a heater on the backside, a Pt-100 resistor controlling sensor

operating temperature and a golden front interdigitated contact

Successively, firing was performed for 1 h at 650◦C under air flow

condition, resulting in 20–30␮m film thickness[8]

2.2 Measurement of gas response

Film conductance was measured by the gas-flow technique in

a sealed test chamber The sensor is inserted in an electric circuit,

equipped with an inverting operational amplifier, thus its transfer

function is

VS= −RC

where V B is the bias tension (set at−5 V), R Cthe feedback

resis-tance and R S is the resistance of the sensor Voltage V Sis measured

through the four-point method Thus, the expression for the

con-ductance, G, holds

G =R1

S = −VVS

Finally, the response of the sensor is the ratio of the film

conduc-tance in gas and that in air (Ggas/Gair)

Relative humidity (RH) of the gas flow was monitored through

a HIH-3610-Series Honeywell humidity sensor We operated with

wet (40% < RH < 50% at 25◦C) carrier In this condition, a typical

resistance value for the sensing films is 1011 at room

tempera-ture and 106 at about 500◦C Alkanes and ethanol were supplied

by certified bottles and fed into the test chamber at 500 cm3/min

total flow In particular we chose: 100 ppm of C2–C4 alkanes and

500 ppm of methane because such values are by far lower than the

alarm levels (Table 1); 10 ppm of ethanol, because higher

concen-trations are prevented by the use of a filtre (NORIT RB1-activated

carbons), installed upstream the test chamber Thus, we chose the

size of the filtre and the total flow in order to obtain a 99.9% filtering

efficiency and to obtain an estimated maximum of 5 ppm of ethanol

to be fed into the test chamber, when 5000 ppm were supplied

Electrical measurements were performed at several operating

tem-peratures: within 300◦C and 650◦C The ethanol filtre was tested

vs 2500 ppm or 1670 ppm of methane and 5000 ppm of ethanol

Fig 1 Responses to 2500 ppm of CH4 with and without filtre at 550 ◦ C under wet condition.

3 Results and discussion

We investigated first the behaviour of two films, SnO2and STN,

in the presence of an active carbon filtre at an intermediate temper-ature (550◦C) under wet condition The insertion of the filtre makes the relative humidity decrease from about 45% down to about 25%

at 25◦C InFig 1, we show the response of the films vs 2500 ppm of methane with and without the filtre The response was found to be higher with the filtre and the STN film turned out to be the best per-forming sensor According to the specifications of the manufacturer

of the filtre, absorption of methane by the filtre is negligible The response increases with the filtre on, because it reduces the inter-ference of water in the reaction between methane and the film[5] Fig 2shows the responses vs 5000 ppm of ethanol and 1670 ppm

of methane with the filtre on, separately (a) and simultaneously (b) In the latter, normalization is made relative to the conductance achieved at a 5000 ppm of ethanol (with filtre on)

After testing the filtre, we carried out a set of systematic mea-surements of the conductance of the sensors Since a maximum concentration of 5 ppm of ethanol can be released by the filtre,

we decided to perform all electrical measurements with 10 ppm

of ethanol and under wet condition, in order to overestimate any possible interfering condition and to take into account of the active carbon aging

Ethanol was measured at temperatures within 300◦C and

650◦C, in order to estimate the extent of the response The signal of ethanol (seeFig 3) initially increases with temperature then starts decreasing at higher temperatures.Fig 3shows that the response

Fig 2 Responses to 1670 ppm of CH4 and 5000 ppm of C 2 H 6 O with filtre at 550 ◦ C under wet condition (a); responses to 1670 ppm of CH 4 in presence of 5000 ppm of

C 2 H 6 O (b).

Fig 3 Responses to 10 ppm of C2 H 6 O at temperatures within 300 ◦ C and 650 ◦ C

under wet condition.

peaks at 400◦C for the SnO2 layer and at 550◦C for the TiO2 The

other layers peak at intermediate temperatures The behaviour of

ethanol is due to its favoured interaction with the sensor surface in

comparison to that of the alkanes, as it occurs with water[5] The

oxidation of ethanol takes place on the surface of the sensor via the

dehydrogenation to acetaldehyde, involving the adsorbed oxygens

and the OH−group[11]:

First we focus on the electrical measurements achieved for alkanes,

then for alkanes in presence of ethanol On the basis of the

previ-ous studies[5,6], the response vs light alkanes is significantly high,

when operating above 450◦C.Figs 4–6show the responses vs

alka-nes compared with that achieved for ethanol at temperature within

450◦C and 650◦C under wet condition The STx and STN films

turned out to be the best performing sensors, while TiO2was too

resistive to appreciate the variations induced by gases In general,

the response of the sensors vs alkanes increases with the

num-ber of carbon atoms, and with temperature[5,6] This behaviour is

explained through alkane oxidation via heterogeneous catalysis of

metal-oxide materials[12–14] Very high- (over 1000◦C) and

low-temperature (400–700◦C) mechanisms are possible, though our

results are well explained by the low-temperature model[6] The

first step consists of homolytic C–H bond breaking, which occurs

at the most reactive carbon atom of the chain As a consequence,

Fig 4 Response to 500 ppm of methane, 100 ppm of C2–C4 alkanes and 10 ppm of

C 2 H 6 O at 450 ◦ C under wet conditions.

Fig 5 Response to 500 ppm of methane, 100 ppm of C2–C4 alkanes and 10 ppm of

C 2 H 6 O at 550 ◦ C under wet conditions.

O−species at surface (oxygen adsorbed on the sensor surface) trap hydrogen and an alkyl radical is being created In the second step, the radical reacts to give a second homolytic C–H bond dissociation and to form an alkene and a second OH−bond on the surface In the end, the whole process yields an alkene, which then proceeds

to oxidation of carbon to CO, CO2and cracking sub-products Finally, we investigated the interference of ethanol by measur-ing the response of the sensors vs alkanes under wet condition within 450◦C and 650◦C On the basis of the previous observations,

we expect that the SnO2 layer sensitively responds to alkanes in presence of ethanol at 650◦C, while the TiO2layer should be insen-sitive and the other layers should respond similarly to the SnO2 film Indeed, we observed a negligible response below 550◦C and significant sensing performance within 550◦C and 650◦C for all the sensors (seeFigs 7 and 8) Ethanol lowered the response to alka-nes, even if the sensors still sensitively detected such gases at high temperatures STN and SnO2turned out to be the best performing films, especially at 650◦C

All the electrical measurements are characterized by sensor response and recovery times of about 10 min and 20 min, respec-tively

The present study demonstrates that it is possible to sensi-tively detect alkanes in presence of a maximum concentration

Fig 6 Response to 500 ppm of methane, 100 ppm of C2–C4 alkanes and 10 ppm of

C 2 H 6 O at 650 ◦ C under wet conditions.

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Fig 7 Response to 500 ppm of methane, 100 ppm of C2–C4 alkanes in presence of

10 ppm of C 2 H 6 O at 550 ◦ C in wet conditions.

Fig 8 Response to 500 ppm of methane, 100 ppm of C2–C4 alkanes in presence of

10 ppm of C 2 H 6 O at 650 ◦ C in wet conditions.

of 10 ppm of ethanol under wet condition In particular, the STN

and the SnO2 films are suitable for this purpose at 650◦C If the

concentration of ethanol were constant, it would be possible to

determine the concentration of alkanes through the knowledge of

the calibration curve However, unexpected changes in the

envi-ronment or worsening of the filtre condition would change the

concentration of ethanol In this case, an additional sensor

capa-ble of detecting specifically the concentration of ethanol is needed

This device could either be the STN or the SnO2 layer

operat-ing at maximum 400◦C In fact, at this temperature, the films

proved to be insensitive to alkanes When two interfering gases

are present, a two-dimension calibration surface is needed This

surface is determined through the interpolation of several

experi-mental calibration curves of one gas Each curve is built, fixing the

concentration of the second gas, according to the method in Ref

[15] In this case, the knowledge of the calibration curve of the film

sensitive to only ethanol and the calibration surface of the film

sen-sitive to alkanes and ethanol will lead to the concentration of the

alkane under consideration

4 Conclusions

A study of the sensing properties of SnO2and TiO2-based oxides

vs light alkanes has been undertaken under wet condition and in

presence of ethanol We were able to considerably hinder the

inter-ference of ethanol and partly of humidity with an active carbon filtre

in a preliminary test Positive indication regarding the development

of a device capable to detect light alkanes in presence of a max-imum concentration of 10 ppm of ethanol have been determined and a strategy for precise measurement of alkane concentration has been highlighted

Acknowledgments

This work has been partly financed by project DeGIMon by INFN and by MIST-ER project by Regione Emilia Romagna

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Biographies

Maria Cristina Carotta became Doctor in physics at the University of Ferrara in 1973.

Since 1981 she has been researcher at the Department of Physics of the same Uni-versity; she is also researcher within CNR-INFM (Department for the Physics of the Matter within Research National Council) Since 1983 she has focused her research activity on semiconductor physics, mainly on electrical, optical and transport prop-erties of silicon and of semiconductor oxides for gas sensors She is currently involved

in research projects concerning the development and characterization of nanostruc-tured thick-film gas sensors for industrial and environmental applications.

Alan Cervi obtained master degree with honours in Nuclear and Subnuclear Physics

at the University of Ferrara in 2005 He is a Ph.D student in Physics of Micro and Nanotechnologies at the Physics Department of the University of Ferrara since 2006 His research activity is mainly addressed to the characterization of nanostructured metal oxides for gas sensing applications.

Alessio Giberti obtained bachelor in theoretical physics at the University of Ferrara

in 2000, and Ph.D in physics of matter in 2004 at the Physics Department of the University of Ferrara His work since Ph.D is focused on the field of semiconductor

gas sensors based on nanostructured metal oxides, with particular interest toward

the electrical, transport and selectivity properties.

Vincenzo Guidi obtained bachelor in physics at the University of Ferrara in 1990,

and was a fellow at “Budker Institute for Nuclear Physics” of Novosibirsk (Russia)

in 1991 Thesis of doctorate was in experimental physics at Legnaro National

Labo-ratories in 1994 He is associate professor in experimental physics at University of

Ferrara Research activity, carried out at the Sensors and Semiconductors Laboratory

of the University of Ferrara, has consisted of investigations on basic phenomena in

semiconductors and to practical implementations of sensing devices.

Cesare Malagù got bachelor in physics at the University of Ferrara in 1997 and his

Ph.D in 2001 in experimental physics He was Post doc with the National Institute of

Physics of Matter in 2001 His current research activity regards analytical modeling

of polycrystalline semiconductors He is a lecturer at the University of Ferrara and teaches general physics at the Department of Chemistry.

Giuliano Martinelli received his doctorate degree in physics at the University of

Ferrara (Italy) in 1968 Based at the Physics Department of the University of Ferrara

as an associate professor since 1980 and then as a full professor His research interests include silicon crystals growth, photovoltaic technology and thick-film gas sensors.

He coordinated several European projects both in P.V and gas-sensor fields.

Delia Puzzovio received master degree in material science at the University of

Padova in 2005 She is a Ph.D student in physics of matter at the Department

of Physics of the University of Ferrara since 2006 Her research activity is mainly addressed to the characterization of nanostructured metal oxides for gas sensing applications.