gas sensor response of pure and activated wo3 nanoparticle

Sensitivity, response time, and recovery time of the sensors were systematically investigated as a function of annealing and operating temperature, using H2S, CO, and NO2as test gases..

Gas sensor response of pure and activated WO 3 nanoparticle

films made by advanced reactive gas deposition L.F Reyesa,∗,1, A Hoela, S Saukkob, P Heszlera,2, V Lanttob, C.G Granqvista

aDepartment of Engineering Sciences, The ˚ Angstr¨om Laboratory, Uppsala University, P.O Box 534, SE-75121 Uppsala, Sweden

bMicroelectronics and Materials Physics Laboratories, University of Oulu, Linnanmaa, FIN-90570 Oulu, Finland

Received 6 July 2005; received in revised form 2 November 2005; accepted 7 November 2005

Available online 7 December 2005

Abstract

Pure and activated (doped) nanocrystalline WO3films, produced by advanced reactive gas deposition, were investigated for gas sensing appli-cations Activation took place by co-evaporation of Al or Au with tungsten oxide as the particles were produced Structural characterization of the films was performed by electron microscopy and X-ray diffractometry Sensitivity, response time, and recovery time of the sensors were systematically investigated as a function of annealing and operating temperature, using H2S, CO, and NO2as test gases The sensitivity was found

to lie below and around the ppm level for H2S and NO2, respectively

© 2005 Elsevier B.V All rights reserved

Keywords: Microstructure; Electrical properties; WO3 ; Au; Sensor

1 Introduction

Nanocrystalline materials are in the focus of contemporary

materials research as a consequence of the superior properties

that are achievable when materials are built up from structural

units with sizes on the nanometer scale [1] Materials of this

type can be produced by a variety of techniques including gas

deposition[2], sputtering[3], sol–gel technology[4],

microbi-ological preparation[5], etc Gas-phase synthesis stands out as

being of particular interest[6]

Materials consisting of crystalline nanoparticles with a

porous structure can have extremely large surface areas and be

well suited for applications in semiconductor-based gas sensors

For example, it has been recently shown that nanocrystalline

WO3 films exhibit very interesting sensor properties and are

candidates for detecting toxic gases such as H2S[7], NO2[8],

and ozone[9] The sensor properties can be boosted by

activa-tion with metals such as Au, Pd, and Pt[10] The purpose of

∗Corresponding author Tel.: +51 1 2922528.

E-mail address: lfreyesh@yahoo.com (L.F Reyes).

1 Permanent address: Facultad de Ciencias, Universidad Nacional de

Inge-nier´ıa, Av T´upac Amaro 210, CP 31-139 Lima, Peru.

2 Also at Research Group on Laser Physics of Hungarian Academy of

Sci-ences, P.O Box 406, H-6701 Szeged, Hungary.

activation is to improve the sensitivity as well as the selectiv-ity of the sensor films for the gas to be detected We note that nanocrystalline WO3is well known also as an electrochromic material capable of sustaining reversible and persistent changes

of its optical properties[11,12] Analogously with the case for sensors, the electrochromic properties can be radically changed

by inclusion of Au nanoparticles[13] Below we present results from a study on gas sensing using nanocrystalline films of WO3in pure (intrinsic) form and after activation with Au or Al The films were prepared by reactive gas deposition Materials characterization was accomplished by X-ray diffraction (XRD) and scanning electron microscopy (SEM) The performance of sensor devices, based on the WO3films, was investigated by a two-point measuring system capable of in situ conductivity determinations as a function of sensor temperature and gas concentration Test gases of H2S, NO2, and CO were used at different concentrations on the ppm level

2 Experimental details

A reactive gas deposition unit (Ultra Fine Particle equip-ment, ULVAC Ltd., Japan) was used for producing nanocrys-talline WO3-based films A detailed description of this unit was given elsewhere[14] Briefly, the system consists of an evap-oration chamber, a deposition chamber, a transfer pipe, and an

0925-4005/$ – see front matter © 2005 Elsevier B.V All rights reserved.

doi:10.1016/j.snb.2005.11.008

evacuation pipe The chambers were evacuated to a base pressure

of∼3 × 10−2mbar prior to the experiments For producing WO

3

nanoparticles, a tungsten pellet was placed inside an induction

coil in the evaporation chamber and was heated up to∼1200◦C.

Reactive evaporation was performed using synthetic air (80% N2

and 20% O2) introduced with a flow of 10 l/min at the bottom

of the induction coil so that an operating pressure of∼20 mbar

was maintained The surface of the tungsten pellet was oxidized,

and the chosen temperature allowed vaporization of the

tung-sten oxide layer The gas flow moved the tungtung-sten oxide vapor

upward, leading to a cooling of the vapor as it was removed

from the heating zone and thereby yielding first molecular oxide

clusters and then WO3nanoparticles One part of the gas flow

in the nanoparticle formation zone was diverted by the transfer

pipe to the deposition chamber This selection, and the fact that

the gas flow is highly laminar, results in a narrow size

distribu-tion of the particles[15,16] For producing the activated WO3

nanoparticle films, additional Au or Al pellets were placed in the

heating zone, thus resulting in co-evaporation or sublimation of

the metallic dopant Doping was manifest as a color induced in

the film, which made the visual impression different from that of

the unactivated WO3 nanoparticle films Both unactivated and

activated WO3films were deposited on alumina substrates with

previously prepared gold electrodes, separated by 0.3 mm, on

the front side and a platinum wire heater on the back side

Sam-ples were heat treated at temperatures between 100 and 600◦C

for 1 h to examine the annealing effect on the sensitivity of the

WO3nanoparticle films

Thickness measurements were made by a Tencor Alpha-Step

200 mechanical stylus instrument with a maximal resolution of

0.5 nm An average thickness of 11␮m was found for the

as-prepared films The crystal structure of the deposited materials

was examined by X-ray diffraction, using a Siemens D5000

diffractrometer (40 kV, 40 mA) with monochromatized Cu K␣

radiation having a wavelength of 1.5406 ˚A The mean grain size

was determined using Scherrer’s equation[17] Microstructures

of the nanocrystalline WO3films were investigated by scanning

electron microscopy using a LEO 1550 Gemini instrument with

an in-lens detector

The conductance of the samples was measured by a

two-point set-up in an evacuated system comprising a gas source, a

gas blender (Signal series 850), and a test chamber Synthetic

air was used as a carrier gas at a constant flow rate 1 l/min

Diverse parameters – such as sensor temperature, gas

concen-tration, exposition time to different gases, etc – were computer

controlled and monitored in real time

3 Results and discussion

3.1 XRD analysis

Fig 1shows X-ray diffraction patterns of as-deposited and

annealed (sintered) nanocrystalline WO3films Reflection peaks

due to tetragonal (t) and monoclinic (m) phases of WO3and of

the substrate (Al2O3) can be seen Clearly the WO3films

con-sist of a mixture of tetragonal and monoclinic phases It should

be noted, though, that films produced at low heating powers

Fig 1 X-ray diffractograms of nanocrystalline WO 3 films on alumina

sub-strates in as-deposited state and after sintering at a temperature T between 100

and 600 ◦C Peaks denoted by m, t, and (*) Al2O3are characteristic for the

mon-oclinic and tetragonal phases of WO 3 and for the alumina substrate, respectively.

only gave evidence for the tetragonal phase[7,18] The tetrag-onal structure corresponds to a high-temperature phase of WO3

that is stable above 740◦C, while a monoclinic phase is stable

at lower temperatures It is evident that the high temperature associated with the W oxide evaporation – being ∼1200◦C

during film fabrication – can lead to a tetragonal phase which remains metastable during cooling in the gas stream No sig-nificant changes in the diffractograms could be observed for sintering temperatures up to 300◦C However, the tetragonal

phase started to transform to monoclinic above 300◦C, and only

the monoclinic phase appeared to be present at a sintering tem-perature of 600◦C The crystallite grain size was∼10 nm for the as-deposited films, and no grain growth could be seen up

to 300◦C However, grain growth could be observed above this

temperature, and the crystallite size was∼40 nm at 600◦C.

3.2 SEM analysis

Scanning electron microscopy data on as-deposited nanocrystalline WO3 films are shown in Fig 2 A minimum feature size of ∼12 nm can be seen, which is in agreement with XRD measurements analyzed by using Scherrer’s equa-tion Aggregates in a porous network-like structure could be observed; this is an important characteristic for gas sensors[19]

Fig 2 Scanning electron micrograph (inset with higher magnification) of an as-deposited nanocrystalline WO film.

Transmission electron microscopy data on samples produced at

slightly lower temperatures were reported elsewhere[18]

4 Sensitivity measurements

The sensitivity of the active layer upon gas exposure was

defined as the ratio Ggas/Gair, where Ggas and Gair denote the

conductance of the sensor exposed to a test gas (H2S, NO2,

or CO) and in pure synthetic air, respectively First the pure

nanocrystalline WO3 films, sintered at temperatures between

100 and 600◦C, were tested upon exposure to 10 ppm of H

2S

at room temperature.Fig 3shows that the sample annealed at

300◦C displayed the highest sensitivity.

Fig 4(a) depicts room-temperature sensitivity upon 10 min

exposures to several concentrations of H2S gas of a sample

sin-tered at 300◦C This exposure produced a drastic increase of

the sensitivity; specifically an increment by more than three

orders of magnitude can be observed at 35 ppm of H2S One can

also note that the increase in sensitivity does not reach

satura-tion for the applied concentrasatura-tion range, between 5 and 35 ppm,

but the sensitivity increases linearly with the H2S concentration

as apparent fromFig 4b The slope of the sensitivity curve is

somewhat below 100 per ppm, indicating sensitivity below the

ppm level for H2S at room temperature However, the change

of the sensitivity as a function of time was very slow at room

temperature; the response time was several minutes, and the

recovery time amounted to some hours after evacuation of the

gas However, high-temperature operation at 600 K significantly

improved the response and recovery times of the sensors (see

Fig 5a), but there is a concomitant decrease of the sensitivity

by more than two orders of magnitude compared to the case of

room-temperature operation (seeFig 5b) The sensitivity always

reached its initial value after the H2S gas had been evacuated,

which shows that the adsorption and desorption processes were

reversible under cycles of gas exposure and ensuing evacuation

A slightly non-linear behavior of the sensitivity versus H2S

con-centration was apparent at 600 K (seeFig 5b)

Fig 3 Sensitivity as a function of annealing temperature for a pure

nanocrys-talline WO 3 sensor exposed to 10 ppm of H 2 S at room-temperature operation.

The line is a guide for the eye.

Fig 4 (a) Sensitivity as a function of time for a pure nanocrystalline WO 3

sensor (sintered at 300 ◦C) exposed to several H2S concentrations at

room-temperature operation H 2 S injection and evacuation took place after 5 and

15 min, respectively (b) Sensitivity S (maximum values) as a function of H2 S

concentration C for the experiment described in (a) The line corresponds to the

stated equation.

A comparison of the room-temperature sensitivity for as-deposited unactivated and activated films is presented inFig 6 Clearly the WO3 nanoparticle film activated by Al exhibited

a sensitivity increase by more than one order of magnitude, while activation with Au increased the sensitivity approximately

by a factor of two Neither the concentration nor phase of the dopant was investigated in this study, but it is likely that the

Al co-evaporation resulted in an inclusion of aluminum oxide Sensitivity measurements are in progress to determine optimal concentrations of the activation material

Slow response and long recovery time prevailed for room-temperature operation of activated films (seeFig 6), just as for unactivated films (seeFig 4a) However, these characteristics could be significantly improved by having the sensor oper-ate at higher temperature (see Fig 7a and b) Fig 7a shows that the processes of adsorption and desorption were reversible for several concentrations of H2S gas at 600 K We note that these processes were irreversible for continued H2S exposures in

10 min-intervals for the 300–450 K temperature range It is also observed that Au is a better dopant for high-temperature oper-ation than Al, contrary to the situoper-ation for room-temperature sensing (seeFig 6) A slight non-linearity can be seen in the

sensitivity versus concentration curves (seeFig 7b)

Fig 5 (a) Sensitivity as a function of time for a pure nanocrystalline WO 3 film

(sintered at 300 ◦C) exposed to several H2S concentrations at 600 K operation

temperature The solid lines represent the H 2 S concentration (right-hand scale),

while the dotted curves denote measured data (left-hand scale) (b) Sensitivity

S as a function of H2S concentration C for the experiment described in part (a).

The curve corresponds to the stated equation.

Fig 8depicts the conductance response of an Au-activated

sensor film under exposure to CO and NO2 gases at an

operating temperature of 700 K For a reducing gas such

as CO, the conductance was increased, while a conductance

Fig 6 Sensitivity as a function of time for pure and activated as-deposited

nanocrystalline WO 3 sensors upon exposure to 10 ppm H 2 S at room-temperature

operation H 2 S injection and evacuation took place after 5 and 15 min,

respec-tively.

Fig 7 (a) Sensitivity as a function of time for pure and activated as-deposited nanocrystalline WO 3 sensors exposed to several H 2 S concentrations at 600 K operation temperature Solid bars represent H 2 S concentrations (right-hand scale), while curves represent measured data (left-hand scale) (b) Sensitivity

as a function of H 2 S concentration for the experiments described in (a).

decrease could be observed for an oxidizing gas such as

NO2

In order to determine the optimal operating temperature, the sensitivity of an Al-activated sensor film was tested at different operation temperatures in H2S, CO, and NO2 Results are shown

in Fig 9for gas concentrations being 10 ppm H2S, 100 ppm

CO, and 5 ppm NO2 The sensor exhibited different optimum operating temperatures, specifically being 400, 525, and 700 K for H2S, NO2, and CO, respectively Importantly, the sensor did

Fig 8 Conductance response of an as-deposited Au-activated WO 3 sensor upon

CO (50 and 100 ppm) and NO 2 (5 and 10 ppm) exposures at 700 K operating temperature.

Fig 9 Sensitivity as a function of operating temperature for as-deposited

Al-activated WO 3 films exposed to 10 ppm H 2 S (right-hand scale), 5 ppm NO 2

(left-hand scale), and 100 ppm CO (left-hand scale) The lines are guide for the

eye.

not show any overlap in its maximum gas-specific sensitivities,

which implies that chemical selectivity can be obtained when the

sensor is operated at 400, 525, and 700 K At optimal operating

temperatures, the sensor is∼20 and ∼2000 times more sensitive

to H2S than to NO2 and CO, respectively, normalized to the

concentration

5 Discussion

Semiconductor gas sensors are mostly used at in air at

atmo-spheric pressure, so that the surface of the sensor is continuously

exposed to oxygen with a partial pressure of∼0.2 atm

Stud-ies on the operating mechanism of semiconductor gas sensors

indicated that most target gases are detected via the influence

they exert on the adsorbed oxygen [20] In particular, several

investigations showed that the key reaction for detecting

reduc-ing gases involves oxygen ions on the surface of the sensor

[19]

We first consider processes that are – or at least may be –

responsible for conductivity changes in sensor films At low

temperatures, it has been suggested[20]that resistance changes

due to H2S adsorption occur as a consequence of the reactions

O2(g)+ e−↔ O2 −(ads) (1)

2H2S(g)+ 3O2 −(ads)↔ 2H2O+ 2SO2+ 3e− (2)

where (g) and (ads) denote gas phase and adsorbed species,

respectively Reaction(1)takes place prior to sensing and creates

a thin electron-depleted layer at the surfaces of the WO3grains

As H2S is adsorbed, electrons are released into the conduction

band according to reaction (2), resulting in conductivity and

thus sensitivity increase For high-temperature operation, the

reactions

O2 −(ads)+ e−↔ 2O−(ads) (3)

H2S(ads)+ O−(ads)↔ H2O+ S + e− (4)

are relevant [20] Reaction(3) assumes that O2 − has already

been formed on the surface by reaction(1)

Another mechanism, that can play a role in the gas sensing,

is the formation of additional surface oxygen vacancies, created

by the interaction of H2S with lattice oxygen according to[21]

3WO3+ 7H2S→ 3WS2+ SO2+ 7H2O (5) This reaction takes place on the surface and involves a reduction

of W6+ to W4+ Oxygen leaves the surface thereby releasing electrons into the grains so that the conductivity of the film is increased However, re-oxidation of the vacancies by O2results

in a competition with the formation of the oxygen vacancies by

H2S

Samples sintered at 300◦C exhibited the largest sensitivity.

This may be associated with structural modifications, shown by the XRD analysis which indicated that a transformation from

a tetragonal to a monoclinic phase commenced at∼300◦C It

therefore appears that a certain ratio of tetragonal to monoclinic phase is required for optimal sensitivity

The high sensitivity at room temperature is remarkable and in accordance with earlier results of ours[22] It may be explained

as a consequence of several different effects such as (i) the occur-rence of a tetragonal phase of WO3in the film[22]; (ii) a high surface area of the nanocrystalline films; and (iii) a porous struc-ture with percolating networks that can be extremely sensitive

to minor changes in the conductivity of the individual nanopar-ticles ensuing from H2S adsorption

The room-temperature sensitivity could be increased by active materials, especially by Al as found fromFig 6 Although

we could not identify the pertinent phase of Al, it is very likely that it was in oxide form in the films since the Al evaporation took place in synthetic air It is known that Al2O3can be active for oxygen adsorption in an air ambient[23], and it follows that it can exert a catalytic role for reaction(1)in the WO3layers This leads

to a lower intrinsic conductivity for the Al-doped films than for the non-activated ones According to reaction(2), H2S exposure increases the conductivity of the sensor In addition, active Al-oxide centers can be neutralized by water produced by reaction

(2), thus resulting in further conductivity increase Considera-tions of this kind may explain the enhanced sensing capability for the Al-doped films at room temperature For high-temperature sensing, the significant water desorption makes the neutraliza-tion of the active Al oxide centers less effective, thereby yielding less sensitivity than at low-temperature operation

For room-temperature sensing, the long response times can

be explained by the slow reaction rate of H2S with adsorbed oxygen ions (reaction(2)), and the long recovery time is due

to incomplete desorption of the adsorbed reaction products and

H2S after finishing the H2S exposure By increasing the temper-ature, the reactions described by(1)and(2)as well as desorption

of the products are faster, according to an Arrhenius-type behav-ior, which explains the decrease of the response time as well

as the recovery time (see Fig 7a) However, both pure and activated WO3 nanoparticle films exhibit low sensitivity for high-temperature operation, as found fromFig 7b In addition, Au-activated films were more sensitive than Al-activated ones – in contrast to the behavior at room temperature (seeFig 6) – indicating that the catalytic effects are temperature sensitive

Several reasons can be given for the sensitivity loss at high

temperatures The rates of the reactions(3)and(4)become

sig-nificant at high temperatures and as a consequence – although

the net result of these reactions is neutral in terms of the number

of electrons – the involved electrons tend to block reaction(2)so

that fewer electrons are injected into the conduction band, thus

leading to a loss of sensitivity Formation of hydrogen tungsten

bronze upon exposure to H2S may also contribute to the

sensitiv-ity loss at high temperatures Similar effects have been reported

in electrochromic tungsten oxide[11,12] It is also worth noting

that transformation to WS2occur for WO3powder reacting with

H2S at atmospheric pressure and elevated temperature according

to reaction (5) [24] This implies that the amount of

surface-adsorbed oxygen available for reaction’s(2)and(4)decreases,

i.e., a thin layer containing W S bonds works as an inhibitor,

which would result in less sensitivity The decrease of the amount

of the tetragonal phase as the operating temperature was elevated

may also contribute to the sensitivity loss[22] Furthermore, the

desorption rate for adsorbed O2 −ions is accelerated at high

tem-peratures, thus leading to less sensitivity toward H2S according

to reaction(2)

The WO3 nanoparticle sensor films are also sensitive to

CO and NO2at 700 K, as seen from Fig 9 The conductivity

increased for CO exposure while it decreased for NO2

expo-sure Reactions similar to those described by(1)–(4)above can

be used to account for the conductivity increase due to CO As

the molecule is adsorbed, it is oxidized to CO2by the help of

an adsorbed12O2 −, thereby leaving one electron to the

conduc-tion band Adsorpconduc-tion of NO2on WO3nanoparticles results in

a decrease of the conductivity, which may be explained by the

reactions[25]

NO2(g)+ e−↔ NO2 −(ads) (6)

NO2(g)+ e−↔ NO(g) + O−(ads) (7)

Both of these reactions require electrons from the conduction

band of WO3, which then leads to a decrease of the conductivity

It is very likely that the reactions indicated for the various test

gases have different rates depending on the operating

tempera-ture This may lead to different optimal operating temperatures

and, indeed, these temperatures were found to be 400 K for H2S,

525 K for NO2, and 700 K for CO, as seen fromFig 9 It is

impor-tant to observe that this effect, with no noticeable cross-over, can

be exploited for chemical selectivity

6 Conclusions

Pure and activated nanocrystalline WO3 films were

pro-duced by advanced reactive gas deposition on alumina substrates

prepared for gas sensor application Activation took place by

co-evaporation of Al and Au with the tungsten source

As-deposited films exhibited ∼10 nm average crystal size, high

electrical resistivity, and tetragonal structure Agglomerates of

WO3nanoparticles with a porous structure – hence being

suit-able for gas sensing – could be observed by electron microscopy

The tetragonal phase changed gradually to a monoclinic phase

as the sintering temperature was elevated from 100 to 600◦C.

Importantly, the WO3-nanoparticle-based sensors were sensitive

to H2S even at room temperature However, room-temperature operation resulted in a slow response (some minutes) and a long recovery time (several hours) Both of these times could be sig-nificantly decreased by operating at elevated temperature Activation by Al or Au increased the sensitivity of the WO3

nanoparticle-based devices by about an order of magnitude It was also found that the activity of the dopants is temperature dependent Al is more effective at room temperature, while Au

is preferable at high-temperature operation The sensitivity of the sensors was measured for different test gases and at various temperatures The optimum sensing temperatures for H2S, NO2, and CO were 400, 525, and 700 K, respectively There is no significant cross-over in the sensitivity curves for the three test gases, thus pointing at chemical selectivity The sensitivity of the sensors is below and around the ppm level for H2S and NO2, respectively

Acknowledgement

One of us (L.F.R.) thanks the International Science Pro-gramme of Uppsala University for a scholarship which made

it possible to carry out Ph.D work at Uppsala University and at the University of Oulu

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