crystalline structure, defects and gas sensor response

Interesting differences were found on the sensor response between sensors based on 400 and 700 8C-annealed WO3, what motivated a structural study of these materials.. Crystalline shear p

Crystalline structure, defects and gas sensor response to

I Jime´nez*, J Arbiol, G Dezanneau, A Cornet, J.R Morante

Departament d’Electro`nica, Enginyeria i Materials Electro`nics, Universitat de Barcelona, Barcelona 08028, Spain

Abstract

Structural and NO2and H2S gas-sensing properties of nanocrystalline WO3powders are analysed in this work Sensor response of thick-film gas sensors was studied in dry and humid air Interesting differences were found on the sensor response between sensors based on 400 and

700 8C-annealed WO3, what motivated a structural study of these materials Crystalline structure and defects were characterised by X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM) Experimental results showed that both triclinic and monoclinic structures are present in the analysed materials, although their amount depends on the annealing treatment Crystalline shear planes, which are defects associated to oxygen deficient tungsten trioxide, were found in 400 8C-annealed WO3and their influence on XRD spectra was analysed by XRD simulations Moreover, XRD and Raman spectra were also acquired at normal metal oxide-based gas sensor working temperatures in order to relate both crystalline structure and sensor response

# 2003 Elsevier Science B.V All rights reserved

Keywords: WO 3 ; Gas sensor; Structural characterisation; NO 2 ; H 2 S

1 Introduction

Tungsten oxide is nowadays considered as one of the most

interesting materials in the field of gas sensors based on

metal oxides, as it is shown by the increasing number of

publications appeared in recent years Very good results in

the detection of NO2 and H2S by sensors based on this

material have been reported Most of them concern WO3

thin films obtained by physical routes such as sputtering

[1,2] or thermal evaporation [3,4] Besides, thick-films

technologies based on the use of nanopowders have been

also presented [5,6] Powder is mixed with an organic

vehicle to form a paste, which is usually deposited on a

substrate as a thick sensitive film, although compatibility

with micromachined gas sensors is also possible [7–10]

Since gas sensors based on metal oxides must usually work

at temperatures ranging from 200 to 400 8C, the sensing

material has to be previously stabilised at higher

tempera-tures This annealing step will strongly affect the structural

properties of the nanocrystalline material and thus its

gas-sensing properties too

Regarding WO3, the pyrolysis of ammonium

paratung-state has been one of the most used routes to obtain this

material as powder with nanometric grain size, being a

well-known technique in the field of gas sensors[11,12] In the same way, dehydration of tungstic acid has revealed as an interesting route to obtain WO3 with low impurities con-centration[13,14], although it is not so usual in the field of gas sensors Gas sensors based on WO3 obtained by this route showed a better sensor response to NO2than that of pyrolytic WO3-based gas sensors under the same test con-ditions, combined with a low response to CO and CH4[15] The aim of this work is to study the evolution of structural properties of WO3nanocrystalline powders as a function of annealing temperature in order to understand the sensor response to NO2and H2S of gas sensors based on differently annealed WO3 Thick-film gas sensors based on differently annealed WO3 nanopowders (400 and 700 8C) were pre-pared and their gas-sensing properties towards NO2and H2S

in dry and humid air were compared The differences found

in gas-sensing motivated a study of the crystalline properties and their evolution with annealing temperature The com-bination of characterisation techniques such as X-ray dif-fraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM) allows to precise the structure

of the synthesised compounds and the nature of possible defects Furthermore, XRD and Raman spectra were also acquired at normal working sensor temperatures for the detection of our target gases (between room temperature and 300 8C) in order to better relate structural and gas-sensing properties

Sensors and Actuators B 93 (2003) 475–485

*

Corresponding author Tel.: þ34-934-021-146; fax: þ34-934-021-148.

E-mail address: ijimenez@el.ub.es (I Jime´nez).

0925-4005/03/$ – see front matter # 2003 Elsevier Science B.V All rights reserved.

doi:10.1016/S0925-4005(03)00198-9

2 Experimental

WO3 nanocrystalline powders were obtained by a soft

chemistry route based on tungstic acid Tungstic acid was

dissolved in a 50:50 volumic mixture of methanol and water

with a tungsten over water molar ratio of 25 This solution

was heated at 80 8C for 24 h under stirring in air and dried by

further heating at 110 8C in air, leading to tungsten oxide

hydrate This material was annealed in a furnace between

400 and 700 8C for 5 h under a flow of synthetic air to obtain

nanocrystalline WO3

Gas sensors were obtained by screen-printing of a paste

based on WO3over alumina substrates, which had already

printed platinum electrodes on the front side and a platinum

heater on the backside to control the operating temperature

These gas sensor devices were placed in a stainless steel test

chamber (200 ml) where a controlled atmosphere was

pro-vided by means of mass flow controllers connected to a

computer The sensor response was acquired for different

concentrations of H2S and NO2in synthetic dry and humid

air at a flow of 200 ml min1 Humidity was controlled by

mixing the appropriate quantities of dry air with

water-bubbling air, monitoring the relative humidity with a

com-mercial capacitive humidity sensor Gas sensor response was

calculated as the resistance ratio Rgas/Rair for both gases

Operating temperature of the sensor devices was varied

between 200 and 300 8C Sensor response at lower

tempera-tures was not studied in order to avoid too high sensor

resistance, which may lead to not reliable sensor response

measurements

XRD patterns of the nanopowders were obtained with a

Siemens D-500 X-ray diffractometer using Cu Ka radiation,

with operating voltage of 40 kV and current of 30 mA

Raman scattering measurements were obtained in

back-scattering geometry with a Jobin-Yvon T64000

spectro-meter coupled to an Olympus metallographic microscope

Excitation was provided by an argon-ion laser operating at a wavelength of 488.0 nm with a low incident power to avoid thermal effects Raman shifts were corrected by using silicon reference spectra after each measurement Trans-mission electron microscopy was carried out on a Phillips

300 keV with 0.19 nm point resolution For TEM observa-tions, WO3 nanopowders were ultrasonically dispersed

in ethanol and deposited on amorphous holey carbon membranes

3 Results and discussion 3.1 Gas sensor response

Fig 1a shows sensor response to 1 ppm of NO2 and

20 ppm of H2S in synthetic dry air of gas sensors based

on 400 and 700 8C-annealed WO3as a function of sensor working temperature In the range of working temperatures studied, from 200 to 300 8C, sensor response increased when temperature decreased for both gases A similar behaviour has already been described for NO2and H2S detection by

WO3[3,6], although their sensing mechanisms are comple-tely different The detection of NO2is usually based on the formation of absorbed surface-trap states NO2 ads, whereas

H2S molecules react with surface oxygen In our case, it was found that WO3 annealed at 700 8C had a higher sensor response to both gases than 400 8C-annealed WO3in this operation temperature range

Sensor response to NO2and H2S in wet air was analysed

at fixed operation temperatures (200 8C for H2S and 225 8C for NO2detection) These temperatures were selected so as

to achieve a compromise between sensor response and recovery time, which decrease when operating temperature increases in the case of NO2detection Fig 1bshows the

Fig 1 (a) Sensor response to H 2 S and NO 2 of gas sensors based on 400 and 700 8C-annealed WO 3 as functions of working temperature, (b) Sensor response

to H S (200 8C) and NO (225 8C) of gas sensors based on 400 and 700 8C-annealed WO as functions of relative humidity.

dependence of the sensor response to NO2 and H2S on

humidity for 400 and 700 8C-annealed WO3, whereas

Fig 2a and b show a comparison of the dynamic sensor

responses to these gases in dry and humidified air (50% of

relative humidity) This sensor response was evaluated

taking sensor resistance in dry or humidified synthetic air

as the reference Response and recovery times, for both

gases, have a low dependence on humidity, as sensor

response to NO2 On the other hand, sensor response to

H2S is highly dependent on humidity, especially for 700

8C-annealed WO3 In fact, only sensor resistance in H2S

atmo-sphere is highly dependent on humidity, as it will be shown

later According to sensor response and humidity

depen-dency, 400 8C-annealed WO3 was chosen for further H2S

detection studies, whereas 700 8C-annealed WO3was cho-sen for NO2 detection Sensor responses of this materials

to different concentrations of H2S (1–10 ppm) and NO2 (0.2–2 ppm) under different humidified ambiences are shown inFig 3, which still clearly shows a greater influence

of humidity on H2S detection than on NO2detection Finally, in order to study this great influence of humidity

on H2S sensor response, pulses of humidity (1 h) were introduced in atmospheres of synthetic air and H2S (2 ppm) in synthetic air, keeping the concentration of this gas constant.Fig 4shows the results for the gas sensor based

on 400 8C-annealed WO3 This experimental procedure, the study of the dynamical response of metal oxide gas sensors

to pulses of humidity in atmospheres containing a target gas,

Fig 2 Dynamic sensor response of WO 3 -based gas sensors to (a) 1ppm of NO 2 in dry and 50% relative humidity air, (b) 5 ppm of H 2 S in dry and 50% relative humidity air.

Fig 3 Sensor response to different concentrations of NO (400 8C-annealed WO ) and H S (700 8C-annealed).

I Jime´nez et al / Sensors and Actuators B 93 (2003) 475–485 477

has revealed as a very useful method to investigate the

interaction of not only water, but also that of the target

gas with the sensing material[16] When humidified air was

introduced in a background of synthetic air, resistance

decreased very fast and afterwards increased slowly,

reach-ing a final value very close to the resistance value in dry

synthetic air Similar dynamic responses have been already

described for SnO2, In2O3and ZnO[17], although the reason

is not completely understood yet The decrease in sensor

resistance was attributed to the dissociatively reaction of

water with lattice oxygen, which leads to the formation of

oxygen vacancies and so to a resistance decrease The

following slow resistance increase could be due to the

recombination of the OH ions with the lattice oxygen

vacancies previously formed As it is shown in our case,

humidity has a low effect on base resistance in synthetic air

in the case of these WO3-based gas sensors However, it is

interesting to notice that this behaviour is completely

dif-ferent when the background is H2S in air In this case, there

is no fast decay of resistance but an important increase when

humidity is introduced, which shows that water is probably

competing with H2S molecules and preventing some oxygen

ions from reacting with H2S When dry air is reintroduced

again, previous sensor resistance value is recovered,

although response time is slower, probably due to a slow

desorption of adsorbed water Similar dynamic results were

found for 700 8C-annealed WO3, although a greater

influ-ence of humidity on resistance in the presinflu-ence of H2S was

found Therefore, this may show not only that water is

competing with H2S, but also that there are more than

one reactive oxygen species participating on the detection

of this gas, since 400 and 700 8C-annealed WO3are

differ-ently affected Therefore, the reactive site blocked by water

would be much more abundant in 700 8C-annealed than in

400 8C-annealed WO3 3.2 Structural characterisation Annealed powder was identified by XRD as nanocrystal-line WO3 Crystalline WO3presents a pseudo-cubic struc-ture with a slight distortion of the cubic ReO3-type lattice, where W atoms occupy the centre of oxygen octahedra linked by their corners At room temperature, monoclinic and triclinic are the most common structures [18] Mono-clinic is described in the P21/n space with cell parameters

a¼ 0:7301 nm, b ¼ 0:7539 nm, c ¼ 0:7689 nm and b ¼ 90:89 The triclinic structure is described in the P-1 space group with cell parameters a¼ 0:7310 nm, b ¼ 0:7524 nm,

c¼ 0:7686 nm, a ¼ 88:85, b¼ 90:91, g¼ 90:935.Fig 5

presents XRD patterns from 2y¼ 20 to 408 of WO3samples annealed at different temperatures (400, 500, 600 and

700 8C) This figure also includes theoretical diffraction diagrams of the triclinic and monoclinic compounds These diagrams have been calculated using the program FULL-PROF[19], taking the cell parameters and atomic positions

of[18]and an artificial crystallite size of 50 nm Due to the slight distortion of the lattice, the main reflection (1 0 0) of the ideal cubic cell splits in three in the range 20–308[20]; (1 0 0), (0 1 0) and (0 0 1) pseudo-cubic reflections Although these reflections are referred as (2 0 0), (0 2 0) and (0 0 2) if the monoclinic or triclinic unit cells are con-sidered, indexations will be referred herein to the pseudo-cubic representation with cell parameter a 0:38 nm It is clear fromFig 5these three main reflections can not be used

to determine if the crystalline structure is triclinic or mono-clinic, since their position and relative intensity is very Fig 4 Sensor resistance variation (400 8C-annealed WO 3 ) to pulses of humidity (30, 50 and 80% relative humidity) in a background of synthetic air and H 2 S (2 ppm) in synthetic air.

similar for both structures when dealing with nanometric

grain sizes However, it is interesting to notice the evolution

of the full width at half maximum (FWHM) and maximum

intensity with annealing temperature of these three main

peaks, shown inFig 6 Whereas relative intensities of the

three main reflections should be almost identical,

experi-mental spectra show that diffraction peak corresponding to

(0 0 1) reflection (at 23.128) is broader and not so well

defined as the other two for low annealing temperatures

This is reflected by the evolution of FWHM of (0 0 1)

reflection in Fig 6, as it only approaches the values of

the other two peaks after a 700 8C annealing A similar

behaviour is presented by the maximum peak intensity as a function of annealing temperature Since these character-istics should be similar for these three reflections, either in the case of monoclinic or triclinic hypothesis, the nature of this difference should be attributed to the presence of some bulk defects that would mainly affect the (0 0 1) reflection peak This fact will be further discussed according to TEM, selected area electron diffraction (SAED) observations and XRD simulations

Regarding crystalline structure identification by XRD, the distribution of diffraction peak intensities in the range 32–

358 has been used in literature to distinguish between Fig 5 XRD spectra of obtained WO 3 (annealed at indicated temperature) Triclinic and monoclinic simulated spectra are also shown.

Fig 6 Evolution of FWHM and intensity of the three main XRD reflections with annealing temperature.

I Jime´nez et al / Sensors and Actuators B 93 (2003) 475–485 479

triclinic and monoclinic structures [21], as simulations of

triclinic and monoclinic structures suggest inFig 5 In our

case, intensities agree better with the hypothesis of a

mono-clinic structure Nevertheless, due to the small mean

crystal-lite size, between 30 and 70 nm[15], reflections are badly

resolved and the possibility of a mixture of both monoclinic

and triclinic phases should be considered, as Raman

inves-tigations will show As regards TEM invesinves-tigations, diffuse

ring patterns obtained by selected area electron diffraction

(SAED) (not shown) were not sufficient to determine the

amount of triclinic/monoclinic crystalline structure of the

samples either

On the other hand, a detailed analysis performed by

high-resolution electron microscopy (HRTEM) showed some

wide fringes next to the borders of some of the 400

8C-annealed WO3 nanoparticles, as it is marked with black

arrows in Fig 7 Detailed SAED pattern showed large

reflections corresponding to typical WO3and short

reflec-tions that have been assigned to Magneli phases (SAED

pattern inset inFig 7) These phases correspond to oxygen

deficient tungsten trioxide with formulas WnO3n2 [22]

Since these wide fringes were superposed to the WO3atomic

planes, a detailed digital image analysis was carried out in

order to separate these phases and study them Firstly, a

representative squared area fromFig 7was selected (Fig 8a)

and a FFT image of this squared region was obtained

(Fig 8b) Afterwards, spots corresponding to the WO3

planes and to the Magneli Phase on the FFT were selected

and filtered by using a mask filter, in order to obtain their

representation in separate images, as shown inFig 8c and d

From these images, it can be concluded that the wide planes

observed correspond to shear planes, in good agreement

with those {1 0 3} crystallographic shear (CS) defects

observed by Sloan et al [23], where {1 0 3}R refer to the family of equivalent directions expressed in the ideal ReO3 cubic cell Visible between the CS planes, 0.38 nm lattice fringes corresponding to the (0 1 0) WO3planes were found

In tungsten trioxide, it is possible to shear the structure in such a way that oxygen vacancies are eliminated and some tungsten atoms remain more closely spaced, so pairs of W5þ atoms are found in order to compensate the charge left by the oxygen deficiency These phases, called Magneli phases, correspond to oxygen deficient tungsten trioxide All of them present a crystalline structure based on WO3 zones (corner sharing WO6 octahedra) linked by units of edge sharing octahedra in the CS phases[23] These bulk defects influence the electrical transport properties: carrier concen-tration increases, as each missing oxygen atom contributes two carriers, and carrier mobility decreases[24] This kind

of defect was not found by HRTEM in samples annealed at temperatures over 400 8C

Although these substoichiometric regions seemed to be very localised, as diffraction patterns generally observed were those of typical stoichiometric WO3, it is reasonable to think that some shear planes could be also present inside the bulk material These planes may affect the intensities of XRD reflections and this effect has been studied by XRD simulations with the software Diffax [25] Two kinds of layers were defined: the first one corresponds to an unfaulted

WO3derived structure (cell no 1), whereas the other one (cell no 2) corresponds to the planar defect Both cells are represented inFig 9a, where vectors defining the unit cell 1,

2 and cubic unit cell are also shown In this representation,

W atoms occupy the centre of oxygen octahedra For the sake of simplicity, the atomic positions have been first described in a cubic representation, the unit cells being Fig 7 HRTEM micrograph from a 400 8C-annealed WO 3 nanoparticle The inset corresponds to the SAED pattern of this nanoparticle Wide fringes have been marked with black arrows The squared region has been digitally analyzed in Fig 4

artificially distorted at the end (orthorhombic unit cells) to

reflect the distortion of WO3structure The cells parameters

used for the pseudo-cubic unit cell were a0¼ 0:365 nm,

b0¼ 0:377 nm, c0¼ 0:384 nm and the shear plane direction

was [0 1 3], according to HRTEM investigations

Simula-tions have been performed supposing a random stacking of

layer 1 and layer 2 with a given probability associated to

each layer OnFig 9b, simulations for different probabilities

of shear planes are shown It can be seen that for a zero

percent probability for layer 2, i.e a stacking of unfaulted

WO3layers, the distribution of intensities for the three main diffraction peaks remain similar to that observed for mono-clinic or trimono-clinic structures, confirming the validity of the approximation done on atomic positions When the prob-ability of layer 2 presence increase, the intensities of calcu-lated (0 0 1) reflection diminishes while its width increases

A similar tendency is observed for the (0 1 0) reflection but

to a lower extent These results agree with the evolution of

Fig 8 (a) Magnified detail of the squared region in Fig 7 , (b) FFT of image 4.a, (c) 0.38 nm WO 3 lattice fringes corresponding to the {1 0 0} R planes obtained after selecting the large reflections in 4.b, (d) {1 0 3} R crystallographic shear (CS) defects from Magneli phases obtained after selecting the short reflections in 4.b.

Fig 9 (a) Representation of the layers used for XRD simulation: the first one corresponds to an unfaulted WO 3 derived structure (cell no 1), whereas the other one (cell no 2) corresponds to the planar defect, (b) XRD simulations considering different probabilities of shear planes.

I Jime´nez et al / Sensors and Actuators B 93 (2003) 475–485 481

experimental XRD diffraction patterns of samples annealed

between 400 and 700 8C (Fig 5), confirming that the TEM

observed CS planes could be responsible for the anomalous

XRD pattern of the (0 0 1) reflection after a 400

8C-anneal-ing Therefore, this anomalous reflection behaviour can be

used as an indirect proof of the presence of bulk oxygen

deficiencies Higher annealing temperatures would reduce

the presence of these defects, according to experimental a

simulated XRD patterns and TEM observations

Regarding Raman spectroscopy, it is able to give a clearer

evidence of the monoclinic or triclinic nature of the WO3

phase, since the lowest frequency phonon modes are

dif-ferent for both structures Cazzanelli et al.[21]reported that

monoclinic structure presents a peak at 34 cm1, while

triclinic presents a peak at 43 cm1.Fig 10shows that only

the peak corresponding to triclinic phase was present in

powders annealed at 4008 When annealing temperature is

increased, the peak at 34 cm1appears and both peaks seem

to coexist in the range of annealing temperatures studied

Similar results concerning the evidence of

triclinic-mono-clinic coexistence in WO3 by Raman have also been

reported by Souza-Filho et al [26] Therefore, powders

annealed at 400 8C have mainly a triclinic phase and both

monoclinic and triclinic phases coexist at higher annealing

temperatures

Finally, XRD and Raman spectra of 400 and 700

8C-annealed powders were also acquired under controlled

temperature (from room temperature to 300 8C for both

techniques) and ambient conditions (synthetic dry air only

for Raman spectra) The aim of this study was to determine if

the previously shown structural differences could also be

present at normal sensor operating temperatures

Concern-ing XRD (not shown), displacements lower than 0.18 respect

room temperature were found and they were attributed to thermal effects However, apart from these displacements,

no appreciable changes on spectra were found Therefore, taking into account the previous results, this would mean that CS defects are also present at normal sensor working temperatures in 400 8C-annealed WO3 Regarding Raman spectra, no trace of the 34 cm1 monoclinic vibration was found on the 400 8C-annealed sample in the range of temperatures studied (Fig 11a) This fact is remarkable, since transition temperature between triclinic and monocli-nic temperature is set around 20 8C [18] In the case of

700 8C-annealed WO3 (Fig 11b), 34 cm1 monoclinic vibration became broader and collapsed over 100 8C with the peak at 43 cm1, typical of triclinic structure Therefore, this would lead to think that 400 8C-annealed WO3is able to maintain its triclinic structure at least up to 100 8C Over this temperature, unfortunately, it is not possible to determine the predominant crystalline structure since 700 8C-annealed

WO3showed only a single broad peak over 100 8C Similar situations have been already described for WO3, such as a tetragonal metastable phase at room temperature[27], while

it is considered to appear over 1000 8K

Basically, Raman spectroscopy showed that both triclinic and monoclinic structures are present in WO3 nanocrystal-line powder obtained from tungstic acid Their abundance depends on annealing temperature and seems to be stable at least up to 100 8C TEM showed the presence of CS planes, associated to oxygen deficiencies, in 400 8C-annealed WO3, whereas they were not observed in materials annealed at higher temperatures XRD simulations based on the pre-sence of this defect showed that it might be responsible for the anomalous XRD pattern of the 0 0 1 reflection, confirm-ing its disappearance with annealconfirm-ing over 400 8C However,

Fig 10 Low-frequency Raman spectra of obtained WO (annealed at indicated temperature).

no change of the 400 8C-annealed WO3XRD pattern was

observed when XRD measuring temperature was changed

between room temperature and 300 8C As a result, this

defect may be present at usual working sensor temperatures

3.3 Discussion

In addition to an obvious difference of mean grain size

and crystalline quality between materials annealed at 400

and 700 8C[15], previously reported structural differences

have to be taken into account to explain differences on

sensor response Some influences of structural properties on

sensor response to NO2and H2S have been already discussed

in literature For instance, it has been reported that high annealing temperature leads to better NO2sensor response, despite grain size increase, in screen-printed gas sensors based on SnO2nanopowders[28] This was attributed to the improvement of the crystalline quality and the faceting of nanograins, which improved NO2adsorption Additionally, sensor response of WO3 to H2S has been reported to be highly dependent on crystalline structure, with especially good results in the case of tetragonal structure[23], although there is no reference about influence of triclinic or mono-clinic structure influence, to the best of our knowledge

Fig 11 (a) evolution of the low-frequency Raman spectra of 400 8C-annealed WO 3 with measurement temperature, (b) evolution of the low-frequency Raman spectra of 700 8C-annealed WO 3 with measurement temperature.

I Jime´nez et al / Sensors and Actuators B 93 (2003) 475–485 483

Finally, influence of crystalline shear planes and bulk

oxy-gen deficiencies on electrical conduction (carrier

concentra-tion and mobility) has also been reported[23] In our case, as

these defects disappear when annealing temperature

increases over 400 8C, oxygen deficiencies may drift to

the surface of the grain and become reactive sites for the

adsorption of NO2and oxygen molecules[29,30], the latter

being consumed by H2S, and this may thus improve sensor

response It is well known that the formation of chemical

bonds between gaseous species and metal oxides depends on

the presence of unsaturated bonds on the surface of the

material, so the amount of chemisorbed species increases

with surface defect concentration[31] As revealed by the

introduction of humidity pulses in the presence of H2S, it is

clear that water is competing with H2S on the grain surface

and some sites are blocked for H2S reaction Since gas

sensors based on 700 8C-annealed is much more affected,

this may lead to think these sites are more abundant in

700 8C-annealed WO3, provided there is more than one

reactive site Nevertheless, more spectroscopic in situ

mea-surements are in progress in order to confirm these

hypoth-eses, which may explain reported differences on sensor

response

4 Conclusions

Crystalline structure, defects and gas sensor response to

NO2and H2S of nanocrystalline WO3were analysed in this

work Annealing temperature was varied between 400 and

700 8C Gas sensors based on 700 8C-annealed tungsten

trioxide showed a better response to NO2and H2S in dry

air than the ones based on 400 8C-annealed WO3 Influence

of humidity on the detection of these gases was also

ana-lysed Gas sensors based on 700 8C-annealed tungsten

tri-oxide exhibited a lower influence of humidity on NO2

response, whereas gas sensors based on 400 8C-annealed

WO3showed a lower influence of humidity on H2S

detec-tion Regarding structural characterisation, two crystalline

structures (triclinic and monoclinic) in WO3nanocrystalline

powders obtained from tungstic acid were identified by

Raman spectroscopy The abundance of these structures

was dependent on annealing temperature Crystalline shear

planes defects, related to oxygen deficiencies, were present

in 400 8C-annealed WO3, as revealed by TEM

investiga-tions Comparison between XRD simulations and

experi-mental data showed that the amount of these defects in bulk

is decreasing when annealing temperature increases from

400 to 700 8C This fact may mean that oxygen deficiencies

are displaced to the outermost part of the grain, increasing

thus reactive sites These structural characteristics were also

studied at normal sensor working temperatures by Raman

and XRD It was found that triclinic structure was stable, at

least, up to 100 in 400 8C-annealed WO3 Besides,

crystal-line shear plane defects were present at operating

tempera-tures under 300 8C, according to XRD results

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