thermal treatment effects on the material and gas-sensing properties of room-temperaturetungstenoxidenanorodsensors

In this work, we prepared tungsten oxide films by drop-coating single-crystalline, size-controlled WO2.72 nanorod solution, and then investigated the dependence of their material and gas-

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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

Thermal treatment effects on the material and gas-sensing properties of

room-temperature tungsten oxide nanorod sensors

Yong Shin Kim∗

Department of Applied Chemistry, Hanyang University, Ansan 426-791, Republic of Korea

a r t i c l e i n f o

Article history:

Received 5 September 2008

Received in revised form 7 November 2008

Accepted 27 November 2008

Available online 9 December 2008

Keywords:

Gas sensor

Tungsten oxide nanorod

Metal oxide semiconductor

Thermal treatment

a b s t r a c t

Room-temperature tungsten oxide sensors were prepared by using a solution containing single-crystalline and monodispersed WO2.72 nanorods with an average 75 nm length and 4 nm diameter Thermal treatment-dependent gas-sensing characteristics of the sensors were examined for achieving a sensor with good performance They were explained and discussed with their material properties probed

by SEM, XRD, XPS and Raman spectroscopy Optimized thermal treatment was found to be an annealing process at around 400◦C under the flow condition of inert N2or Ar gas This treatment leads to the partial oxidation of nonstoichiometric W5+states into the fully oxidative W6+without any noticeable change in morphology or crystalline structure These changes in material properties result in a great improvement

in detection and recovery times with only a slight sacrifice of detection response

© 2008 Elsevier B.V All rights reserved

1 Introduction

Metal oxide semiconductors (MOS) have been utilized as

gas-sensing active materials for half a century[1,2] One of the most

promising solid-state MOS chemosensors is a tungsten oxide-based

gas sensor Several studies have proved that this sensor could be

used for the detection of nitrogen oxide (NO and NO2), ammonia

vapors, hydrogen sulfide, and hydrocarbons[3–7] In the last few

years, the nanostructures of tungsten oxides have been found to

be more effective sensing materials due to their high

surface-to-volume ratio and small grain size[8–12] They have demonstrated

novel sensing properties such as high sensitivity, fast response time,

and low operation temperature These properties are unattainable

by using classical MOS materials consisting of submicrometer-sized

polycrystalline particles

MOS sensors usually operate in the temperature range of

200–500◦C This operation temperature results in high electrical

power consumption, which limits the use of MOS sensor as a

sens-ing element in battery-powered portable devices To overcome

such a drawback, there have been many investigations of novel

one-dimensional (1-D) MOS nanostructure sensors able to

oper-ate at room temperature These include carbon nanotubes, SnO2

nanowires or nanobelts, In2O3 nanowires, and WO2.72 nanorods

[11–16] Among these 1-D MOS, tungsten oxide are regarded as

an encouraging material for achieving low operation temperature

∗ Tel.: +82 31 400 5507; fax: +82 31 400 3949.

E-mail address:yongshin@hanyang.ac.kr

since tungsten oxide has lower intergrain energy barrier than SnO2

or TiO2[17,18]

In this work, we prepared tungsten oxide films by drop-coating single-crystalline, size-controlled WO2.72 nanorod solution, and then investigated the dependence of their material and gas-sensing properties on thermal treatment conditions The thermal treat-ments were performed with the variation of annealing temperature

in the range of 200–700◦C under the ambient conditions of nitro-gen, Ar, oxygen or dry air Our previous works reported that WO2.72 nanorod sensors had highly sensitive detection capability even at room temperature for various reducing and oxidizing compounds [11,12] The purpose of this study is to find thermal treatment con-ditions that can optimize gas-sensing performance in a tungsten oxide nanorod system In addition, thermal treatment-dependent gas-sensing characteristics are explained and discussed in terms of observed material properties

2 Experimental

Tungsten oxide nanorods with an average 4 nm diameter and

75 nm length were synthesized in massive quantity by the colloid-based synthetic approach[19] Diluted HCl solution was added into the colloid solution to precipitate WO2.72 nanorods stabilized by amine-based surfactants The resultant precipitates were separated

by a centrifuge, and further purified by dissolving in toluene and consecutive centrifuging three times Tungsten oxide films were deposited by casting isopropyl alcohol solution of WO2.72nanorods The isopropyl alcohol solution was severely agitated to disperse the nanorods homogenously just before the deposition process As-0925-4005/$ – see front matter © 2008 Elsevier B.V All rights reserved.

Tungsten oxide films prepared on Si were utilized to

character-ize their material properties through various methods of analysis

Thickness was measured by using a mechanical profiler (Alpha-Step

500, KLA-Tencor), and surface microstructures were measured by

scanning electron microscopy (SEM; Sirion, FEI) X-ray diffraction

(XRD; D/MaxRC, Rigaku) having a Cu K␣ anode and Raman spectra

excited at 514 nm were utilized for probing a crystalline structure

In addition, atomic constituents and chemical bonding states were

evaluated through X-ray photoelectron spectroscopy (XPS; SCALAB

200R, VG Scientific) Curve fittings of XPS spectra were performed

with the freeware program of XPSPEAK 4.1 using the sum of

Gaus-sian and Lorentzian functions as the model functions During fitting,

the spin-orbit splitting energy between W 4f7/2and W 4f5/2was

fixed at 2.1 eV The fitting was started with the initial W 4f7/2

bind-ing energies of 35.5 and 34.3 eV for the W6+ and W5+oxidation

states, respectively[20]

Tungsten oxide sensors were fabricated by using Au-patterned

glass substrates The sensor substrate was prepared by the

consec-utive e-beam depositions of 5 nm Cr and 100 nm Au layers through

a shadow metal mask It possesses two interdigitated Au

elec-trodes with a sensing area of 3 mm× 10 mm and an electrode gap of

300␮m Sensing characteristics were measured by using the flow

injection system as previously described[21] They were carried

out by placing a gas sensor in a detection chamber and blowing

analyte vapors over it with a flow rate of 1000 ml/min Analyte

con-centrations were regulated with the relative flow ratio between

the carrier dry air and diluted analyte vapor The sensing

mea-surements were performed upon exposure to the four different

(NI) environments with a typical sampling rate of 5 Hz

3 Results and discussion

3.1 Thermal treatment effects on the material properties

As-deposited tungsten oxide films were measured to have a thickness of 3–7␮m by the mechanical profiler Their surface mor-phology observed by SEM displayed a porous appearance resulting from randomly arranged agglomerates, favorably formed by par-allel alignment of high anisotropic WO2.72 nanorods [11] This appearance is almost identical to the SEM image of a N2-annealed sample at 500◦C (seeFig 1A), suggesting no noticeable change

in surface morphology In fact, we were not able to observe any microstructural change in SEM images of samples annealed at below 500◦C in ambient N2 conditions The surface appearance however begins to change significantly for N2-annealed samples

at above 550◦C (seeFig 1B–D) In spite of the similarity in over-all size, the surface appearance of a sample annealed at 550◦C exhibits that tungsten oxide nanorod agglomerates are no longer basic constitutional components Instead, new larger, collapsed crystalline particles have appeared They seem to form through the recrystallization of nanorod aggregates As the anneal temperature increases, the surface morphology reveals more dramatic changes Above 600◦C, the newly born crystals must undergo severe arrange-ment, resulting in a favorable columnar crystal growth along the vertical direction Moreover crystal and void sizes become larger when the anneal temperature increases from 600 to 700◦C The average surface crystal size of N2-annealed sample at 700◦C finds

Fig 1 Typical surface SEM images of tungsten oxide nanorod films annealed under the ambient nitrogen conditions at the four different temperatures of (A) 500◦ C, (B)

Fig 2 Typical surface SEM images of tungsten oxide nanorod films annealed under

the ambient oxygen conditions at the two different temperatures of (A) 400 ◦ C and

(B) 500◦C.

to be around 100 nm, which is two times larger than that of 600◦C

Similar changes in surface morphology were also examined under

the thermal treatments with different ambient conditions of dry

air, O2or Ar An identical result was observed in the Ar-annealing

condition: the onset of recrystallization is around 550◦C and the

columnar crystals grow larger when an anneal temperature reaches

above 600◦C On the other hand, dry air and O2anneal treatments

result in a different onset temperature.Fig 2A and B shows surface

SEM images of O2-annealed WO2.72films at 400 and 500◦C,

respec-tively They distinctly demonstrate that crystal growth takes place

at a temperature between 400 and 500◦C Since the average crystal

size of a 500◦C O2-annealing sample is observed to be larger than

that of 600◦C N2-annealed, oxygen environments must

encour-age the coalescence processes among the aggregated nanorods An

identical enhancement in crystal growth was also observed for

air-annealed samples The dependence of an onset temperature on the

ambient environments suggests that oxygen molecules can act as

a crystallization enhancer Due to the nonstoichiometric nature of

WO2.72nanorods, oxygen infiltration might easily initiate the

oxida-tive recrystallization into the most stable WO3state Even under

the flow condition of inert N2or Ar, the oxidation process seems to

proceed at higher temperature due to a small number of residual

oxygen-containing compounds within a furnace

Fig 3 shows the XRD patterns of the N2-annealed samples

shown in Fig 1 The deflection pattern of 500◦C sample shown

inFig 3A displays no discernable change from the as-deposited

films, which reported broad background peaks due to the small

nano-sized dimension of nanorods and the very weak (0 1 0)

mon-oclinic peak at the diffraction angle of 23.3◦assigned to the growth

direction[12] The asterisk-designated peak at around 33◦ comes

from a used Si substrate However, there are many new peaks

for the samples annealed at over 550◦C, indicating the

forma-Fig 3 X-ray deflection spectra of the same tungsten oxide samples displayed in

Fig 1 They were annealed under the ambient nitrogen condition at the four different temperatures of (A) 500 ◦ C, (B) 550 ◦ C, (C) 600 ◦ C and (D) 700 ◦ C The Miller indices are determined from the monoclinic structure (JCPDS no 05-0363), and the peak indicated with an asterisk results from a used Si substrate.

tion of new crystalline phases These peaks have higher intensity and narrower FWHM as temperature increases from 550 to 700◦C

It is well coincident with previous observations in SEM that the higher temperature-annealed sample shows a larger crystal struc-ture Stoichiometric WO3crystals have known to have monoclinic

or triclinic structures at room temperature[22] The most inten-sive three diffraction peaks at the angle of 22–25◦correspond to the pseudo-cubic reflections originating from the slight distortion

of the ideal cubic{1 0 0} lattice planes These three reflections

can-not be used solely to determine whether the crystalline structure

is triclinic or monoclinic, since their position and relative inten-sity is very similar On the other hand, the inteninten-sity distribution

of the diffraction peaks in the range of 32–35◦can provide a clue

to distinguish between triclinic and monoclinic structures[23] Our intensity distributions were found to agree better with that of mon-oclinic WO3, so that the newborn peaks were assigned based on monoclinic structure (JCPDS no 05-0363)

The temperature-dependent crystalline changes were also investigated for the samples annealed under different ambient con-ditions All observed XRD peaks were well interpreted with the monoclinic WO3structure even though there is a little difference in preferential crystal structure They exhibited an identical tendency

as observed in SEM: N2and Ar anneal treatments displayed the same temperature dependence while the dry air and O2 environ-ments induced the recrystallization at the lower onset temperature

of less than 500◦C

Raman shift spectra are shown inFig 4A and B for the films annealed under ambient N2 and O2 environments, respectively Each consists of two spectra obtained at two different anneal temperatures, which correspond to before and after the onset of

an extensive crystal growth The two spectra obtained at 400◦C observed to be similar to that of as-deposited sample[12] All the spectra display four distinct bands at 270, 327, 712, and 807 cm−1 except for the lattice mode band below 200 cm−1 and a small instrumental artifact around 520 cm−1 The four bands fall very

Fig 4 Raman shift spectra of tungsten oxide films annealed at two different

tem-peratures under the ambient conditions of (A) nitrogen and (B) oxygen They were

observed with an Ar-ion laser excitation at 514 nm.

close to the wavenumbers of the strongest modes of monoclinic

tungsten oxide The two bands at 270 and 327 cm−1 had been

assigned to O W O bending modes of the bridging oxide, while the

712 and 807 cm−1bands are the corresponding stretching modes

[24,25] Moreover, the band intensities of the samples annealed

at higher temperature increase by more than one order in

mag-nitude compared with those at lower temperature Consequently,

these results give further evidence that the monoclinic crystals

are grown and become larger as the anneal temperature goes

higher

Main constituent elements of as-deposited films were observed

to be tungsten and oxygen atoms from wide-scan XPS

measure-ments, except for additional minor peaks resulting from carbon and

Si elements The appearance of Si 2s and 2p peaks can be explained

with photoelectrons ejected from a Si substrate due to the highly

porous nature of nanorod films Small amount of carbon

impuri-ties were confirmed to exist within the film probably due to the

incorporation of carbon atoms originated from carbon-containing

chemicals used in the synthesis processes, together with a

consider-able amount of surface-adsorbed carbon moieties However, there

was no other discernable impurity except for the carbon atoms

Narrow-scan XPS measurements were performed to quantify

the amount of carbon impurities in the binding energy range of

240–295 eV.Fig 5shows spectra obtained from the samples with

different thermal treatments: (A) no annealing, (B) N2 annealing

at 400◦C, (C) N2 annealing at 600◦C, (D) O2 annealing at 300◦C

and (E) O2 annealing at 500◦C In the case of N2annealing

treat-ment, the intensity of C 1s peak begins to decrease considerably at

600◦C under the condition of maintaining the W 4d peak

inten-sities (seeFig 5A–C) It implies that carbon contaminants are not

removed noticeably for the N2400◦C annealing while they decrease

by about one third of as-deposited films for the 600◦C annealing

A similar decrease in carbon content was also observed in the O2

annealing treatments: the C 1s intensity decreases with

ascend-ing O2annealing temperature (seeFig 5D and E) Judging from the

slight decrease in C 1s intensity even for the sample annealed at

Fig 5 XPS spectra of tungsten oxide samples in the binding energy range of

240–295 eV They have undergone different thermal treatments: (A) no annealing, (B) N 2 annealing at 400 ◦ C, (C) N 2 annealing at 600 ◦ C, (D) O 2 annealing at 300 ◦ C and (E) O 2 annealing at 500 ◦ C.

300◦C, carbon-containing impurities could be favorably eliminated

in ambient O2 conditions, probably through oxidative reaction pathways

The W 4f XPS spectrum of an as-deposited blue WO2.72film was previously reported to decompose into two components resulting from W5+and W6+ oxidation states[12] Since the nanorod has crystallographic shear planes relative to a ReO3-type structure, the cations will have the W5+formal oxidation state at shear plane boundaries in which a WO6 octahedron has three corner-sharing and two edge-sharing octahedral neighbors Each component con-sists of W 4f7/2 and W 4f5/2 doublet peaks with the spin-orbit splitting energy of 2.1 eV.Fig 6A and B shows W 4f XPS spectra

of N2annealed samples at the two temperatures of 400 and 600◦C, respectively The two dashed lines correspond to best-fitted curves for the W6+and W5+oxidation states while the solid line displays a sum spectrum of the two components The sum spectrum displays fairly good agreement with the raw data displayed by open circles

In addition, the W 4f XPS spectra obtained from 300 and 500◦C O2 annealed samples are shown inFig 7A and B, respectively These spectra are also well interpreted with the two components corre-sponding to W6+and W5+oxidation states.Table 1summarizes the

parameters used in the deconvolution of W 4f XPS peaks, i.e the

binding energy of the W 4f7/2, FWHM and relative contribution of the two oxidation states All the samples exhibit identical binding energies of around 35.7 and 34.2 eV for the W6+and W5+ oxida-tion states, respectively, which are well consistent with the values

in the literatures[20,26,27] This observation suggests that chemi-cal bonding characteristics of the two oxidation states do not alter noticeably according to the thermal treatments However, the rela-tive W5+population steadily decreases as N2annealing temperature increases: 19% for the as-deposited sample, 12% at 400◦C, and 6% at

600◦C It must be attributed to the oxidation of partially-reduced

W5+states into fully oxidized stable W6+states This interpreta-tion is consistent with the SEM and XRD results indicating the extensive monoclinic WO crystal growth at above 600◦C It is

Table 1

Parameters used in the deconvolution of W 4f XPS peaks into the two possible oxidation state components.

a BE and FWHM are abbreviations for binding energy and full width at half maximum of the main W 4f 7/2 peak, respectively.

b These results were obtained from our previous work [12]

worth mentioning that the oxidation significantly occurs for the

400◦C N2-annealing sample with the same surface morphology

of as-deposited films It suggests that the nanorod elements first

undergo partial oxidation at relatively low temperature and then

agglomerate to form a larger crystal above the onset temperature

Furthermore, an identical trend is observed for the O2-annealing

samples: the relative W5+populations are 15% at 300◦C and 6% at

500◦C Judging from the relative W5+populations, the oxidation

process can be activated more easily in the O2-containing ambient

conditions

Fig 6 Narrow-scan W 4f XPS spectra of tungsten oxide samples N2 -annealed at the

two different temperatures of (A) 400◦C and (B) 600◦C The open circle symbols

show raw data, and the solid line corresponds to a sum spectrum of the two dashed

3.2 Thermal treatment effects on the gas-sensing properties

Gas-sensing measurements were performed for WO2.72nanorod sensors annealed at different temperatures under ambient N2 con-ditions Fig 8 shows annealing temperature-dependent sensor response profiles at room temperature as a function of a detec-tion time The sensors were exposed to air-diluted ethanol vapors

of 1000 ppm with the concentration variation of step function The lowest plot inFig 8displays the expected time profile of ethanol concentration for the sequence of 5 min injection and 20 min recov-ery period In our detection chamber, the elapsed times of filling and removing ethanol vapors were observed to be less than 10 s from

Fig 7 Narrow-scan W 4f XPS spectra of tungsten oxide samples O2 -annealed at the two different temperatures of (A) 300◦C and (B) 500◦C The open circle symbols show raw data, and the solid line corresponds to a sum spectrum of the two dashed

Fig 8 Time-dependent response profiles of WO2.72 nanorod sensors having the

dif-ferent N 2 anneal temperatures in the range of 300–700 ◦ C No annealed sample is

also given for comparison All sensors were measured under the condition of

flow-injecting air-diluted ethanol vapors of 1000 ppm as presented in the lowest curve.

The vertical arrow corresponds to the magnitude of 10 in sensor response.

separated measurements using a fast response sensor The length

of the vertical arrow corresponds to the magnitude of 10 in

sen-sor response, which was defined by a relative percentage change

of a sensor resistance R with respect to a stabilized initial value

R0, i.e response = 100 × (R–R0)/R0= 100× R/R0 The response

pro-files exhibit an increase change in resistance upon exposure to

the reducing ethanol vapors, corresponding to p-type response

This phenomenon was previously explained with the

competi-tion adsorpcompeti-tion between ambient molecular oxygen and analyte

vapors on the surface of WO2.72nanorods instead of the

conven-tional mechanism observed at above 200◦C in MOS sensor, i.e.

the reaction between ionosorbed oxygen moieties and analytes

[11] The adsorbed analyte molecules may act as active

scatter-ing centers, thus suppressscatter-ing the electrical conductance of free

electron carriers in n-type tungsten oxide system This scattering

results in the resistance increase for both oxidizing and reducing

analytes

The response of a no-annealed sample steadily increases for

the ethanol injection period and eventually reaches to a maximum

response of 9.1 It slowly decreases into the initial position for a

fol-lowing recovery time of 20 min The recovery time of no-annealed

sensor was observed to be around 30 min which is too long to use

as a practical MOS gas sensor Such a long recovery time was

previ-ously observed in other MOS sensor systems operated at ambient

temperature [13–15] It might be attributed to the low

opera-tion temperature leading to slow desorpopera-tion rate of pre-adsorbed

moieties As an annealing temperature increases until 500◦C, the

response profile becomes closer to a square shape, indicating that

the sensor has fast response and short recovery time In the case

of a 400◦C annealed sensor, the response abruptly increases to the

90% level of a maximum value within a time of less than 1 min, and

the recovery time becomes two times shorter compared with the

no-annealed sample However, there is a slight decrease in

maxi-mum response: a maximaxi-mum response of the 400◦C annealed sensor

istics as stated below The material properties of N2-annealed films

at below 500◦C have been confirmed to be identical to those of as-deposited samples except for the population difference in W6+

and W5+oxidation states Therefore, the temperature-dependent response variation might result from the difference in sensing abil-ity of the two oxidation states Fully oxidized WO3 is the most stable tungsten oxide compound so that the W6+state may be more inert to the sensing-related surface reactions than the W5+

state The slight decrease in response with the increase in the anneal temperature can be interpreted as the local oxidation of more reactive W5+states to W6+as probed by XPS Furthermore, the W5+states have a stronger interaction with analyte molecules

so that the long recovery time of as-deposited sensor can be also understood with the high W5+population Consequently, N2 ther-mal treatment of WO2.72 nanorod films below 500◦C results in the partial oxidation of nonstoichiometric W5+states without any noticeable change in morphology or crystalline structure, which gives a chance to modulate systematically gas-sensing characteris-tics such as response magnitude and detection time by means of careful control of the relative population ratio between W5+and

W6+states

The sensing characteristics become suddenly worse as the N2 annealing temperature increases beyond 600◦C at which point the nanorods begin to coalesce and develop large, columnar WO3

crystals Considering that porous film made of small crystalline nanorods with a high aspect ratio are favorable for achieving a high sensitive MOS sensor, the deterioration in sensing capability can be explained by the formation of larger crystals with a columnar crack Response profiles were also evaluated for the sensors annealed

in ambient O2 conditions at a temperature range of 300–600◦C

On the whole, O2-annealed sensors had exhibited bad sensing properties The most serious problem was their weak operation durability: the O2-annealed sensors easily broke down in spite of material properties similar to N2-annealed analogues This propen-sity was also observed for the sensors annealed in ambient air conditions The thermal treatment under the O2-containing ambi-ence therefore gives adverse effects on gas-sensing characteristics Even though there has been no direct clue to explain this effect,

it seems to be ascribed to delicate differences in microstructures induced by O2thermal treatment

Additional measurements were performed at room tempera-ture upon exposure to other volatile vapors for evaluating the sensing characteristics of our nanorod sensors in terms of sensi-tivity and selecsensi-tivity.Fig 9shows time-dependent sensor response profiles of 400 and 600◦C N2-annealing samples upon exposure

to the three different analytes of 1000 ppm hexane, 1000 ppm benzene, and 10 ppm NH3 The bottom square pulses display the time-dependent profiles of supplying air-diluted analyte vapors The middle profiles were obtained from the sensor annealed at

400◦C while the uppers correspond to the sample annealed at

600◦C The upper curves were shifted upward with respect to the middle ones in order to distinguish easily between them The scale in sensor response is, however, identical for the two cases and the vertical arrow size matches up to the magnitude of 2 The relative detection magnitudes in sensor response exhibit great differences for the three different analytes: the 400◦C sensor is pos-sible to achieve feapos-sible detection of hexane and benzene while the 600◦C sensor can detect NH3vapors more sensitively In fact, the detection responses of the 400◦C sensor are fond to be 1.2 for hexane and 2.1 for benzene These values are much larger com-pared with the 600◦C response of <0.1 On the other hand the

NH detection response of 600◦C sensor is about six times larger

Fig 9 Time-dependent response profiles of WO2.72 nanorod sensors for the three

different analyte exposures of 1000 ppm hexane, 1000 ppm benzene, and 10 ppm

NH 3 The top profiles are sensor responses of the 600 ◦ C N 2 -annealing sample while

the middles correspond to those of the 400 ◦ C N 2 -annealing The bottom square

pulses exhibit the time profiles of supplying three different analytes The vertical

arrow corresponds to the magnitude of 2 in sensor response.

than that of 400◦C As a result these response variation according

to anneal temperatures implies that carefully controlled thermal

treatments could provide a chance to improve detection selectivity

which is regarded as obstacles in a tungsten oxide nanorod

sys-tem for achieving a good performance sensor operated at room

temperature

4 Conclusion

We have investigated the dependence of gas-sensing

character-istics on thermal treatment conditions in a tungsten oxide nanorod

system that demonstrated the facile detection of various analytes at

ambient temperature As the annealing temperature increases, the

surface morphology of as-deposited films consisting of randomly

arranged WO2.72nanorod agglomerates were observed to develop

larger WO3particles with a columnar monoclinic crystal structures,

probably through a recrystallization process of nanorod aggregates

The onset temperatures for the recrystallization were found to

be around 550◦C for the inert N2 or Ar annealing conditions and

450◦C for the O2-containing ambient, which suggests that oxygen

molecules act as a crystallization enhancer Since such a

crystal-lization process leads to large crystals and many cracks between

grain boundaries, the thermal treatments above the onset

temper-ature were found to deteriorate the detection capability of WO2.72

sensors In addition, thermal treatments under the O2-containing

active environments were found to result in bad reproducibility in

sensor response compared with the inert N2conditions As a result,

the recommendable thermal treatment conditions for WO2.72

sen-sors were an annealing temperature of 300–500◦C under inert N2

or Ar ambient The annealing temperature within the range could

be utilized as a parameter to regulate a relative population ratio

between W5+and W6+states without any noticeable change in

mor-phology or crystalline structure We can have an opportunity to

optimize gas-sensing properties such as detection selectivity and

a recovery time through the careful control of relative populations

between the two oxidation states

Acknowledgement

This work was supported by the research funds of KOSEF (R01-2008-000-20460-0) and SMBA (S708002511 and S6070381211)

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