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Sensors and Actuators B 120 (2006) 25–34

Fine-tuning of ceramic-based chemical sensors vianovel microstructural modification

Part II: Low level CO sensing by molybdenum oxide, MoO3

Abdul-Majeed Azad ∗Department of Chemical and Environmental Engineering, The University of Toledo, 3052 Nitschke Hall, Toledo, OH 43606-3390, USA

Received 2 November 2005; received in revised form 27 December 2005; accepted 23 January 2006Available online 28 February 2006

bstract

Employing a buffer gas mixture of CO and CO2, the equilibrium oxygen partial pressure was varied across the Mo/MoO3 proximity line byarying the ratio of concentration of the two gases, as described in an earlier communication. This led to the reduction of MoO in one case and

3

xidation of the reduced phase (Mo or MoO2) in the other with a concomitant variation in the microstructural features of the resulting species.he formation and growth of the new oxide surface under conditions of oxygen potential modulation has been found to impart enhanced sensingharacteristics to the MoO3-based chemical sensors for low levels of carbon monoxide (14–100 ppm CO) in the ambient.

2006 Elsevier B.V. All rights reserved.

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eywords: Semiconducting oxides; MoO3; Chemical sensors; Microstructure;

. Introduction

Materials with nanofeatures are of immense relevance ishe field of solid-state ceramic-based chemical sensors. Highelectivity, enhanced sensitivity and short response time areome of the key features sought in these devices. Since theensing mechanism and catalytic activity of ceramics areargely microstructure-dominated, benign surface features suchs small grain size, large surface area, high aspect ratio and,pen/connected porosity are required to realize a successful sen-or material [1–4].

A novel technique employed to impart such attributes byodifying the microstructural artifacts of ceramic-based sen-

or materials has recently been described wherein the effect ofhe variation in the ambient oxygen partial pressure across theetal/metal oxide boundary on the microstructure and gas sens-

ng characteristics (viz., enhancement of sensitivity and shorten-

ng of response time) of WO3 was thoroughly investigated [5].he methodology used to bring about these morphological varia-

ions has been explained in detail [5] and can be briefly described

∗ Tel.: +1 419 530 8103.E-mail address: Abdul-Majeed.Azad@UToledo.Edu.

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925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2006.01.038

n monoxide

s follows. At a given temperature and standard pressure (ambi-nt; 1 atm), the oxidation of a metal to its oxide or reduction ofn oxide to metal or its suboxide occurs at a well-defined finiteartial pressure of oxygen. If the two phases (metal/metal oxider metal suboxide/metal oxide) are in equilibrium, the incum-ent oxygen partial pressure is recognized as the equilibriumartial pressure. According to the Gibbs phase rule, on eitheride of this unique oxygen pressure, at a given temperature, onef the two coexisting phases must disappear. For the Mo/MoO3oexistence in accordance with the reaction:

o(s) + 32 O2(g) → Mo3(s) (1)

he equilibrium oxygen partial pressure is given by:

O2(M/MO3) = exp

(2�G◦

f(MoO3)

3RT

)(2)

here the Gibbs’ energy of formation of MoO3 according to

eaction (1) is given by [6,7]:

G◦f (cal/mol) = −177, 559 + 59.791T (K) (3)

plot of log pO2 versus T for the Mo/MoO3 coexistence in theange 450–800 ◦C is shown in Fig. 1.

26 A.-M. Azad / Sensors and Actua

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ig. 1. Temperature dependence of the equilibrium oxygen partial pressure inhe Mo/MoO3 system (solid line). The variation in pO2 by changing the CO2/COatio between 10−2 and 105 at 450, 600 and 800 ◦C is also shown (open triangles).

An oxygen partial pressure in the vicinity of Mo/MoO3 prox-mity line (solid curve in Fig. 1) could be generated by manipu-ating the ratio of two gaseous species in a buffer mixture, suchs CO2/CO by virtue of the following equilibrium:

O + 12 O2 = CO2 (4a)

or which:

G◦ = −RT ln

(pCO2

(pCOpO2 )1/2

)(4b)

his gives:

O2 =(

pCO2

pCO

)2 1

e−2�G◦/RT(4c)

here

G◦ (J) = −282, 400 + 86.81T (4d)

herefore, by controlling the ratio of the concentration of CO2nd CO, it is possible to control the partial pressure of oxygen.ixing CO2 and CO in the ratio that ranges from 10−5 to 105

rovides good buffered systems. In this range, the theoreticalO2 varies between 10−35 and 10−15 atm at 600 ◦C and, between0−29 and 10−9 atm at 800 ◦C. The variation in pO2 by chang-ng the CO2/CO ratio between 0.01 and 100,000 at 450, 600 and00 ◦C is shown as open triangles in Fig. 1. In this paper, theicrostructural modification brought out in MoO3 films by pro-

essing them in an ambient that is slightly reducing or slightlyxidizing is reported. The effect of these morphological changesn the sensing behavior towards low levels of CO in the ambients also discussed.

. Experimental procedure

The materials investigated in this work included thin foils of

o and MoO3 powder procured from Alfa-Aesar (99.8% or bet-

er). The vendor-specified average particle size of the oxides wasetween 20 and 45 �m. The stoichiometry of MoO3 is knowno be significantly dependent upon the method of preparation

btes

tors B 120 (2006) 25–34

mployed [8]. To discern the effect of the synthesis methodol-gy on the microstructural evolution, molybdenum trioxide wasynthesized by two additional methods in addition to using thes-received powder from Alfa-Aesar. In the first case, commer-ial MoO3 powder was used without further processing, excepthat it was thoroughly ground, ball-milled and sieved through25-mesh screen prior to printing thick films. In the secondase, the MoO3 powder was obtained by oxidizing Mo metaloil in air at 600 ◦C for 2 h. In the third instance, MoO3 was pre-ared by a soft solution chemistry route using sodium molybdateihydrate (Na2MoO4·2H2O, AR grade from Alfa-Aesar) as therecursor; 10.5 g of sodium molybdate crystals were dissolved in50 ml of de-ionized water with constant stirring and the solutionas cooled in an ice-bath to 5 ◦C. 24 ml of 1 M HCl acid were

dded to the cooled aqueous solution, stirred and the mixture waslaced in the freezer overnight so as to maintain the temperatureround 5 ◦C. The frozen solution was thawed and heated slowlyo 80 ◦C with continuous stirring for 24 h; a pale bluish pre-ipitate of molybdic acid (H2MoO4) started to form after heat-ng. The precipitate was washed several times with de-ionizedater and centrifuged. In order to remove all the sodium ions

he conductivity of the filtrate was monitored after each washntil the conductivity was <0.05 mho. The solid thus obtainedas dried, ground well and calcined for 1 h in air at 300 ◦C.he formation of MoO3 from the precipitated molybdic acid

H2MoO4) was confirmed by following the systematic phasevolution via X-ray diffraction (XRD) and scanning electronicroscopy SEM.The metal foils were also used to demonstrate the authenticity

f the proposed concept while the oxide powders were used toabricate the sensor films whose sensing behavior towards differ-nt levels of carbon monoxide in a 10% O2–bal. N2 backgroundas monitored before and after these films were subjected to

he reduction–oxidation processes described above. Structuralnd microstructural examination of the as-received foils, rawowders and the products after each of the redox reactions waslso conducted by XRD and SEM to corroborate the observednhancement in sensing characteristics of the thick film sensors.

The fabrication technique of thick film sensors in chemiresis-or mode has been described in detail elsewhere [5] and is brieflyutlined here. The MoO3 (as-received, from Mo foil oxidationr obtained via precipitation from sodium molybdate aqueousolution) powders were ball-milled using 10 mm spherical zir-onia milling media (Tosoh, NJ) in 2-propanol for 8 h, dried andieved through a 325-mesh stainless steel screen. The powderas mixed with V-006 (an organic-based resinous vehicle withispersant, from Heraeus, PA), �-terpineol and tetraethoxysi-ane, TEOS (both from Alfa-Aesar) in an appropriate weightatio (solid loading ∼ 70%) and stirred thoroughly to form aomogeneous printable slurry. The sensor films were screen-rinted on high density �-alumina substrates (14 mm × 14 mm)ith gold interdigitated electrodes (12 mm × 12 mm) and con-

act pads. The films were dried in an air oven at 150 ◦C followed

y firing at 500–600 ◦C for 1–4 h in air, giving due cognizance tohe high volatility of MoO3 [9]. Gold lead wires (0.25 mm diam-ter, Alfa-Aesar) were attached to the contact pads by means ofilver paste that served as the lead wires for measuring the film

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A.-M. Azad / Sensors and

esistance. The sensor was placed on a flat platform in an all-uartz set-up and was located in the uniform temperature zonef a compact horizontal Lindberg furnace (MiniMite). A type-Khermocouple was also placed just above the sensor to monitorhe temperature and its variation (if any) during the test. Thends of the gold wires were connected to a high impedance Agi-ent 34220 A digital multimeter, which in turn was connected to aesktop PC via HPIB interface card. Sensor resistance data werecquired and displayed in real-time with the help of IntuiLinkoftware.

A gas stream consisting of 10% O2–90% N2 (v/v) mixtureas obtained by blending dry compressed air with high purityitrogen to obtain the background (reference) gas. The sensoras first heated to a selected temperature in the background

mbient, allowed to equilibrate at that temperature till a steadyaseline resistance (Ro) was established. Given amount of COrom a CO/N2 tank was then bled in and allowed to blend. Sen-itivity of a given film was measured by recording change inlm resistance with respect to Ro upon introduction of a givenmount of CO in the stream. The sensor behavior was monitoredoth with increasing and decreasing levels of CO in the ambiento confirm the reversibility attribute of the sensor. The responseime (t90) was calculated by discerning from the recorded data,he time it took for the signal to attain 90% of the differenceetween the two steady states, viz., in the background (Ro) andhat after CO was introduced (Rg).

The schematic set-up for the sensor development and testings shown in Fig. 2.

The thick film sensor made by a procedure described above,as subjected to reduction and oxidation at pO2 below and above

he theoretical line of coexistence of the Mo/MoO3. Such oxy-en partial pressures were created by mixing CO and CO2 inppropriate concentration from compressed gas cylinders as perq. (4a) so that in one case it causes reduction of MoO3 too metal and in the other case re-oxidation of metallic Mo tooO3 again. As explained in Section 3, for example, at 600 ◦C

he oxygen partial pressure for the Mo/MoO3 coexistence cane computed to be 1.2 × 10−21 atm (using reliable thermody-amic data employed in the construction of Fig. 1). However,

oO2 is also a stable phase in the Mo–O system under different

xygen partial pressure conditions (1.2 × 10−26 atm at 600 ◦C).hus, a pO2 nearly 5 orders of magnitude lower (∼10−30 atm)

han the theoretical value for the Mo/MoO2 was considered

totc

Fig. 2. Schematic of the planar resistive sensor pl

tors B 120 (2006) 25–34 27

dequate to effectively and completely reduce MoO3 to ele-ental Mo. This was generated by mixing CO and CO2 in the

atio CO2:CO = 0.00231 (pO2 = 1 × 10−30 atm). This compo-ition was used as the reducing mixture. Similarly, a pO2 nearlyorders of magnitude higher than the theoretical was gener-

ted by mixing CO and CO2 in the ratio CO2:CO ≈ 23,000pO2 = 1 × 10−16 atm). The actual mixing ratios used in thisork turned out to be 0.2% CO2 + 99.8% CO in the first case and,3 ppm CO + bal. CO2 in the second case. In order to generatehe later mixture, a gas cylinder containing 100 ppm CO + bal.O2 was used as the CO source.

Prior to mixing, the mass flow meters were thoroughly cali-rated over the entire range (0–100%) for each of the two gasesnd effect of temperature variation in the laboratory was incor-orated to offset the errors. In order to ensure that a uniformuffer mixture containing the two gases in a pre-determinedoncentration ratio was admitted into the reactor for reductionr oxidation, the chamber was thoroughly evacuated and flushedith high purity nitrogen (twice in that order) to drive off any

emnant of air and hence oxygen in the vicinity of the samplender investigation. The two gases were made to flow throughwo separate lines and come into a large volume bottle for effec-ive mixing and homogenization prior to allowing the mixtureo enter the reactor via a two-way stainless steel non-greasedalve. The pO2 in the buffer mixture so created was measuredy a zirconia-based oxygen sensor at the exit port.

. Results and discussion

MoO3 possesses good catalytic response since it haseen used in the field of catalysis for oxidation reactions ofydrocarbons [9]. A number of studies have shown MoO3 toe a potential gas sensor material even though the meltingoint of MoO3 is only 795 ◦C—relatively low compared toiO2, SnO2 and ZnO. MoO3 has a band gap of 3.2 eV andigh electrical resistance near room temperature (∼1010 �).lthough the system Mo–O is known sufficiently well regardingarious closely-related coexisting oxide phases [10], the phaseelationship is somewhat vague in terms of the mechanism of

ransformation during reduction of a given oxide or re-oxidationf a suboxide. Furthermore, depending upon the extent of reac-ion and conditions of reduction, several oxide phases mightoexist, showing a multilayer structure. Ferroni et al. [11] have

atform (a) and the sensor testing set-up (b).

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8 A.-M. Azad / Sensors and

tudied the behavior of MoO3 as a sensor for NO2 and CO inhe temperature range of 200–400 ◦C. They also outlined howignificant improvements in sensing behavior can be made byodifying the microstructure of a metal oxide [12]. This helps

n optimization of experimental parameters and establishes aorrelation between structural properties and the resulting elec-rical response. It was shown that the response time of MoO3or CO and NO2 is much smaller than that of SnO2 and ZnO.

Reduction of MoO3 has been investigated using traditionalpproach, i.e. by H2. The reduction scheme studied by Schul-eyer and Ortner [13] has been shown to follow the path:

oO3 → Mo4O11 → MoO2 → Mo.

owever, as stated above, the published literature offers littlenformation about the actual mechanisms during the reductionf molybdenum oxides.

Calculations based on precise thermodynamic data in theo/MoO2, Mo/MoO3 and MoO2/MoO3 coexisting phases

eveal that the equilibrium oxygen partial pressures over theo/MoO2 and Mo/MoO3 systems differ only by a few orders ofagnitudes (5.3 × 10−37 atm versus 1.9 × 10−30 atm at 400 ◦C

nd 1.2 × 10−26 atm versus 1.2 × 10−21 atm at 600 ◦C) [6,7].hus, by exposing molybdenum trioxide at say 600 ◦C to a pO2

f the order of ∼10−30 atm its reduction to elemental Mo with-ut going through the intermediate phases (such as Mo4O11 oroO2) is ensured. Similarly, by selecting an incumbent pO2 of

he order of ∼10−16 atm in the vicinity of a Mo surface (eitherresh foil or that obtained after reduction of a MoO3 film) is capa-le of oxidizing it to the ultimate phase, viz., MoO3 at 600 ◦C.imilar considerations are applicable at other temperatures asell. It should, however, be pointed out that the temperatures

or bringing about microstructural changes in MoO3-based filmshould be carefully selected by giving due cognizance to its pref-rential volatility even at moderate temperatures [14].

.1. Microstructural evolution in MoO3 derived from Mooil oxidation

In order to first verify that modulation of oxygen potentialn the vicinity of a metal or its oxide affects the morphologicalrtifacts, small (25 mm × 12.5 mm) coupons cut from a Mo foil

j

tt

Fig. 3. (a and b) Microstructural features of a pure Mo fo

tors B 120 (2006) 25–34

ere subjected to several redox schemes and were characterizedt the end of each treatment by XRD and SEM.

The comparative morphological features of the as-receivedo foil and that subjected to oxidation in air at 600 ◦C for 2 h are

hown in Fig. 3. The mechanical stress causing the disintegra-ion of the foil upon oxidation is due to the phase and structuralhange from cubic Mo metal (ICDD 42-1120, unit cell volume1.17 A3) to orthorhombic MoO3 (ICDD 05-0508, unit cell vol-me 202.99 A3) which is attended by ∼85% volume change.

On the other hand, totally different microstructure resultshen the Mo foil is oxidized and reduced at 600 ◦C for 24 h

n manipulative pO2 regimes (∼5 orders of magnitudes acrosshe Mo/MoO3 line in Fig. 1) generated by different ratios ofO/CO2. This is shown in Fig. 4a and b, respectively. Theanoscale features of the oxides subsequent to both oxidationnd reduction treatment in CO/CO2 mixture can be clearly seen.he lack of open porosity and presence of well-defined grainoundaries in both the cases is probably due to sintering viaapor phase transport (VPT) as a result of long soak duration24 h at 600 ◦C).

Interestingly, when the foil oxidized and reduced in CO/CO2uffer mixture (Fig. 4) is heated again in air at 600 ◦C for 4 h,hin oriented platelets of MoO3 of large aspect ratios and surfacerea result. The resulting morphology is quite similar to that seenn the commercial MoO3 powders, albeit with much smaller and

ore uniform grain size and narrower particle size distribution.his is shown in Fig. 5.

The microstructural features developed in films fabricatedrom the powder obtained after Mo foil oxidation in air at 600 ◦Cor 2 h foil are compared with those in the films subjected toedox treatments in appropriate pO2 regimes at 600 ◦C in Fig. 6.

.2. Microstructural evolution in MoO3 thick filmsabricated from the commercial powder

The morphological features in the MoO3 thick films that wereade by using the commercial oxide from Alfa-Aesar, and sub-

ected to different redox treatments at 600 ◦C are shown in Fig. 7.It is apparent that while the grains maintain the unique platelet

exture, so characteristics of MoO3, the microstructure lackshe uniformity that was obtained as a consequence of identical

il (left) and that heated in air at 600 ◦C/2 h (right).

A.-M. Azad / Sensors and Actuators B 120 (2006) 25–34 29

Fig. 4. Scanning electron micrographs of Mo foil after: (a) oxidation and (b) reduction by CO/CO2 buffer mixtures at 600 ◦C/24 h.

Fig. 5. SEM pictures of: (a) Mo foil after redox treatment in CO/CO2 buffer for 24 h followed by air oxidation for 4 h and (b) commercial MoO3 powder.

Fig. 6. Microstructural evolution in the MoO3 films fabricated from Mo foil oxidation: (a) film calcined for 1 h; (b) a reduced by CO/CO2 for 12 h; (c) b oxidizedby CO/CO2 for 12 h; (d) c oxidized by air for 1 h. Size bar = 5 �m.

30 A.-M. Azad / Sensors and Actuators B 120 (2006) 25–34

Fig. 7. Microstructural evolution in the MoO thick films fabricated from MoO3 commercial powder: (a) film calcined for 1 h; (b) a reduced by CO/CO for 12 h;(

ttd

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Ft

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pdat

3

c) b oxidized by CO/CO2 for 12 h; (d) c oxidized by air for 1 h.

reatment given to the MoO3 powder derived from foil oxida-ion (nearly monosized small platelets with narrow plate sizeistribution; see Fig. 6).

The nearly identical morphological features after subjectingolybdenum oxide films to different redox treatments under var-

ous oxygen partial pressure regimes suggest that the ultimatehemical state of the oxide were also restored. The XRD pat-erns collected at the end of each of such treatment were identical

orthorhombic MoO3 [15]) except for some preferential textur-ng which corroborate and validate the stated assumption. Oneuch pattern is shown in Fig. 8.

ig. 8. A typical XRD pattern of the MoO3 thick films after various redoxreatments with preferential texturing in 〈0 6 0〉 direction (2 − θ = 38.976◦) [15].

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tecbwor(amu

2

.3. Microstructural evolution in MoO3 films fabricatedrom sodium molybdate precursor

The morphological features of the molybdic acid (H2MoO4)recipitated from ammonium molybdate and of the MoO3 pow-er obtained after the calcination of molybdic acid at 300 ◦C/1 hre shown in Fig. 9a and b, respectively. The triclinic habits ofhe molybdic acid crystals (ICDD 26-1449) are easily discern-ble as are the plate-like structure characteristic of MoO3 in thered sample.

The phase evolution in MoO3 upon the molybdic acid dehy-ration appears to be textured along 〈0 2 0〉 direction rather than0 2 1〉 plane as is the case with the bulk oxide [15]. This is shownn Fig. 10.

The microstructural evolution in films made from this powdernd subjected to different redox treatment is shown in Fig. 11.

It is worth-mentioning that while the films undergo dras-ic morphological variations as result of exposure to the gasnvironment of varying oxygen potential, the XRD signaturesollected on these samples reveal that the phase structures of a,and d belong to those of MoO3 (akin to that shown in Fig. 8ith texturing along the directions of preferred growth) and thatf c belongs to MoO2 [16]. It is also interesting to note thateduction–oxidation by the conventional H2–air combination

Fig. 11b) leads to a rather compact and dense microstructurelbeit with large aspect ratio; in a clear contrast to this, theicrostructure is rather open in Fig. 11d when CO/CO2 buffer is

sed as the redox medium. This is suggestive of layer-by-layer

A.-M. Azad / Sensors and Actuators B 120 (2006) 25–34 31

Fig. 9. Morphology of: (a) H2MoO4 precipitate and (b) H2MoO4 precipitatefired at 300 ◦C/h.

Fig. 10. XRD pattern of the oxide obtained from molybdic acid fired at 300 ◦Cf〈

biapWteao

3p

r

Fig. 11. Morphology in: (a) film fabricated from H2MoO4-derived MoO3 and calcinin static air; (c) a reduced by CO/CO2; (d) c oxidized by CO/CO2 (both at 600 ◦C for

or 1 h. The pattern conforms to that of MoO3 with preferred texturing along0 2 0〉 plane.

uildup of the parent oxide from the reduced intermediate phasen the case of redox treatment under conditions of restrictedvailability of oxygen. Similar observations have been madereviously in the case of regeneration of the oxide phase inO3 system [5]. This vividly illustrates that the oxygen poten-

ial prevalent in the vicinity of an oxide phase has profoundffect on its morphological features, which could be tailored toccentuate the sensing behavior of a potential semiconductingxide.

.4. Preparatory technique–microstructure–gas sensing

roperty correlation in MoO3 films

The response characteristics (such as the variation in filmesistance in the presence of different levels of CO in the

ed at 500 ◦C/2 h; (b) a heated in 10% H2–N2 mixture at 500 ◦C/h and oxidized12 h). Note the length of the scale bars.

32 A.-M. Azad / Sensors and Actuators B 120 (2006) 25–34

F s andfi d (iii)

aftiCdt(aptptd

rtpat

rwCq

cMntptrioCs

ig. 12. Variation in resistance of MoO3 films prepared by different techniquelms in (i and ii) were subjected to redox in CO/CO2 mixtures (scheme ‘b’) an

mbient, sensitivity and response time) of thick film sensorsabricated from MoO3 derived from three different prepara-ory techniques to CO gas (14–100 ppm) at 450 ◦C are shownn Figs. 12–15 (a: as-prepared, b: a reduced and oxidized inO/CO2 mixtures, and c: a reduced in CO/CO2 mixture and oxi-ized in air). The response of the sensor toward CO is defined ashe normalized variation of resistance of the n-type film, namelyRo − Rg)/Ro, where Ro is the steady-state resistance (identifieds a time-independent plateau) in the background gas (in theresent case, 10% O2–bal. N2) and Rg is the steady-state resis-ance of the sensor film (identified again as a time-independentlateau) when exposed to the gas of interest (CO). The responseime (t90) was calculated as the time taken to reach 90% of theifference (Ro − Rg).

As can be seen from Fig. 13, neither the sensitivity nor theesponse time changed significantly as a result of various redox

reatment given to the films made from the commercial MoO3owder. The lack of improvement in either of the two char-cteristics is believed be due to almost negligible variation inhe microstructural features of the films (see Fig. 7). The film

adTd

subjected to different redox treatment, upon exposure to various levels of CO;to scheme ‘c’.

educed and re-oxidized by CO/CO2 (scheme b) showed some-hat marginally better sensitivity, while the film reduced byO/CO2 buffer and re-oxidized by air (scheme c) showed theuickest response (∼27 s).

There was no improvement in the sensing behavior in thease of films made from the powders obtained after oxidation ofo foil (Fig. 14) either. This is understandable in the light of the

egligible change in the microstructural features as a result of thehree redox treatments (Fig. 5). On the other hand, the film pre-ared from MoO3 powders derived via precipitation technique,uned out to be the most promising in terms of sensitivity andesponse time. As can be seen from Figs. 12(iii) and 15, signif-cant improvement in the sensitivity and the response time wasbserved in this case. The film subjected to redox treatment byO/CO2 showed improved sensitivity compared to the untreated

ample. However, the film reduced by CO/CO2 followed by

ir oxidation showed an increase of ∼40% in sensitivity andecrease of ∼300% in the response time at 100 ppm CO level.his is believed to be due to the unique microstructural artifactseveloped in these films (Fig. 11).

A.-M. Azad / Sensors and Actuators B 120 (2006) 25–34 33

Fig. 13. Effect of surface treatment on: (a) the gas sensitivity and (b) responsetime of thick films made from the commercial MoO3 powder.

Fig. 14. Effect of surface treatment on: (a) the gas sensitivity and (b) responsetime of thick films made from the MoO3 powder obtained via Mo oxidation.

Fig. 15. Effect of surface treatment on: (a) the gas sensitivity and (b) responsetime of MoO3 thick films made from sodium molybdate precursor.

Fig. 16. Comparison of the sensitivity (i) and the response time (ii) of the filmsderived from three methods: (a) MoO3 from vendor; (b) MoO3 from Mo oxida-tion; (c) MoO3 from sodium molybdate and subjected to reduction by CO/CO2

buffer followed by air oxidation. The sensor measurements were carried out at450 ◦C.

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4 A.-M. Azad / Sensors and

These results are summarized in Fig. 16, where the sensitivitynd response time for samples from each of the three methods areompared. Clearly, the third method resulted in sensor films withhe most improved sensitivity to all levels of CO and particularlyhe lowest response time for 100 ppm CO.

. Conclusions

High selectivity, enhanced sensitivity, short response timend long shelf-life are some of the key features sought in theolid-state ceramic-based chemical sensors. Since the sensingechanism and catalytic activity of ceramics are predominantly

urface-dominated, benign surface features in terms of smallrain size, large surface area and, open and connected poros-ty, are sought to realize a successful material. We have shownhat these features could be incorporated in a given semicon-ucting oxide, such as MoO3, via a novel gas phase redoxcheme, thereby resulting in better sensor performance. Theffect of the variation in the ambient oxygen partial pressurecross the metal/metal oxide boundary on the microstructurend gas sensing characteristics (viz., enhancement of sensitivitynd shortening of response time) of MoO3 was studied. It washown that the regenerated oxide phase with unusual microstruc-ure shows better gas sensing behavior.

cknowledgment

The help provided by Mr. M. Hammoud in the data collections gratefully acknowledged.

eferences

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[

[[

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[4] A.-M. Azad, E.D. Wachsman, Solid state chemical sensors for CO gas,in: E.D. Wachsman, W. Weppner, E. Traversa, M. Liu, P. Vanysek, N.Yamazoe (Eds.), Proceedings of the 198th Meeting of the Electrochemi-cal Society—Solid State Ionic Devices II, Ceramic Sensors, vol. 2000-32,Phoenix, AZ, October 23–27, 2000, pp. 455–466.

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15] ICDD Reference Card #05-0408.16] ICDD Reference Card #32-0671.