ag doped wo3-based powder sensor for the detection of no gas in air

Ag doped WO 3 -based powder sensor for the detection of NO gas in airLing Chen, Shik Chi Tsang* Surface and Catalysis Research Centre, University of Reading, Whiteknights, Reading RG6 6A

Ag doped WO 3 -based powder sensor for the detection of NO gas in air

Ling Chen, Shik Chi Tsang*

Surface and Catalysis Research Centre, University of Reading, Whiteknights, Reading RG6 6AD, UK Received 21 August 2002; received in revised form 3 November 2002; accepted 11 November 2002

Abstract

WO3-based materials as sensors for the monitor of environmental gases such as NOx(NO ‡ NO2) have been rapidly developed for various potential applications (stationary and mobile uses) It has been reported that these materials are highly sensitive to NOxwith the sensitivity further enhanced by adding precious group metals (PGM such as Pt, Pd, Au, etc.) However, there has been limited work in revealing the sensing mechanism for these gases over the WO3-based sensors In particular, the role of promoter is not yet clear though speculations on their catalytic, electronic and structural effects have been made in the past In parallel to these PGM promoters here we report, for the ®rst time, that

Ag promotion can also enhance WO3sensitivity signi®cantly In addition, this promotion decreases the optimum sensor temperature of 300 8C for most WO3-based sensors, to below 200 8C Characterizations (XRD, TEM, and impedance measurement) reveal that there is no signi®cant bulk structure change nor particle size alteration in the WO3phases during the NO exposure However, it is found that the Ag doping creates a high concentration of oxygen vacancies in form of coordinated crystallographic shear (CS) planes onto the underneath WO3 It is thus proposed that the Ag particle facilitates the oxidative conversion of NO to NO2followed by a subsequent NO2adsorption on the defective

WOxsites created at the Ag±WO3interface; hence, accounting for the high molecular sensitivity

# 2002 Elsevier Science B.V All rights reserved

Keywords: NO; Gas sensor; Sliver; WO 3 -based powder

1 Introduction

In recent years, the demand for gas sensors based on

safety and process control requirements has been expanding

[1] Much interest is centered on studies of tin oxide as

gas sensitive resistors since it is inexpensive, robust, highly

sensitive to ¯ammable hydrocarbon gases [2] and can

be tuned to differentiate gases with good selectivity [3]

However, this sensor material is not suitable for detecting

some of the most dangerous of air pollutants, namely nitric

oxide (NO) and nitrogen dioxide (NO2) (collectively

referred to as NOx), because of its poor sensitivity Currently,

approximately one-half of all NOxemissions into the

envir-onment are due to power plants and industrial boilers[4]

NOx gas, which is the precursor to nitric and nitrous acid,

causes acid rain and photochemical smog; and hence,

con-stitutes the critical factor for the destruction of ozone in the

troposphere In fossil fuel combustion, NOx is formed by

high temperature chemical processes from both nitrogen

present in the fuel and oxidation of nitrogen in the air

Typically, the NOxemissions consist of 90±95% NO with the remainder being N2O and NO2ranging 0±4000 ppm[4,5]

In case of environmental monitoring, according to the American Conference of Governmental Industrial Hygienist (ACGIH), the threshold limit values (TLV) for NO2and NO are 3 and 25 ppm, respectively[5] Therefore, a NOxsensor requires high sensitivity for detecting low concentration of gases Another enormous driving force for developing high sensitivity NOxsensor comes from automotive industry In order to decrease both fuel consumption and carbon dioxide production, new engines with excess of air versus stoichio-metry have been developed Unfortunately, the conventional catalytic converter of exhaust gas does not work without careful control of NOx[6]

Tungsten trioxide (WO3) is a wide band-gap n-type semiconductor that has attracted much recent interest as a promising sensor because of its excellent sensitivity and selectivity [7] WO3 thin ®lms were used initially for detecting H2S and H2 Yamazoe and Miura[8]were amongst the ®rst to report that tungsten trioxide sintered ®lms are selective sensors for low concentrations of nitrogen oxides

WO3thin ®lms activated by noble metals (Pd, Pt, Au) layers have later been found to be more sensitive, selective and shown to give a quicker response to NOx[5] The sensing

* Corresponding author Tel.: ‡44-1189-316346;

fax: ‡44-1189-316632.

E-mail address: s.c.e.tsang@reading.ac.uk (S.C Tsang).

0925-4005/02/$ ± see front matter # 2002 Elsevier Science B.V All rights reserved.

PII: S 0 9 25 - 4 0 0 5 ( 0 2 ) 0 0 4 3 0 - 6

materials of WO3 promoted with 1 wt.% metal or metal

oxides for NO detection were then systematically screened

It was found that some of these promoters were critical for

improving the sensitivity[9]

The sensing mechanism of semiconductor oxides lie, in

general, on the changes in the resistance resulting from

physisorption, chemisorption, catalytic reactions and

spe-cies migration (spillover) on the promoter±sensor particles

upon analytical gas exposure[10] It has been acknowledged

that the thin ®lm microstructure plays a major role in this

behavior [11,12] This is mainly because grains and grain

boundary regions are expected to have signi®cantly different

electronic properties and, accordingly, to show a different

electronic response after the interaction with gases [11]

However, there is limited work reported in the literature on

addressing the precise sensing mechanism for the NOxgases

detection over WO3-based sensors and there is no work to

reveal the roles of the promoter It is known that sensitivity

of NO2is many times higher than NO over the WO3-based

sensors and most of the best-reported promoters are also

known to be good oxidation catalysts As a result, one of the

roles of promoter may be to provide a surface for the

catalytic conversion of NO to NO2 that are responsible

for the high sensitivity associated with the promotion

However, whether a speci®c interaction(s) between the

promoter and the WO3exists is not yet clear In addition,

WO3phases generally regarded as distortions of the cubic

ReO3structure[13]are known to easily form

non-stoichio-metric oxides in which the oxidation state of the W is less

than 6[14] Their stability, relationship with promoter and to

the overall NOxsensing are still unclear

In this paper, we report the screening of WO3 powder

doped with different promoters as sensor candidates

Amongst all the promoters tested, Ag/WO3shows the best

response to 50 ppm NO in air at the temperature range of

150±350 8C It is found that the intrinsic high sensitivity of

Ag/WO3could not be mimicked via passing the same gas

mixture over a separate Ag/Al2O3bed (a good catalyst for

NO to NO2) prior to a pure WO3sensor arranged in series

under identical conditions This suggests that Ag particles

supported on WO3not only provide NO2but also give some

kinds of bene®cial interfacial effects with the underneath

WO3 It is evident from the XRD and the TEM that the

Ag doping creates oxygen-de®cient sites that are believed to

be crucial for the enhanced sensitivity obtained over this

material

2 Experimental

2.1 Sensor material preparation

Analytical grade WO3 powder (Aldrich, 99.9%) was

purchased and used without further puri®cation Doping

of WO3was achieved by mixing 1 wt.% precursors (soluble

salts) dissolved in a minimum amount of deionized water

(DI) with a dried WO3powder The resulting mixture was kept stirring at 80 8C until it was completely dried All solids were calcined at 600 8C for 2h in air The calcined powder, 0.6 g, was cold pressed at 10 tonnes into a pellet (area ˆ

1 cm2) using an infra-red press

2.2 Testing conditions The sensor pellet was then sandwiched between two platinum electrodes (Fig 1), which were in turn connected

to an impedance analyzer (Solartron SI 1260) via two jacketed co-axial cables The electrical responses of the sensor materials were measured in a ¯owing stream of pure air or in 40 ppm NO in air (60:40 mixture from an air cylinder with another cylinder (BOC special gases) contain-ing ca 100 ppm NO balanced with air) by the ac impedance analyzer (Fig 1) The ¯ow rate of the gases was kept at

100 ml/min throughout the measurement using mass ¯ow controllers The schematic diagram is presented below

3 Results and discussion 3.1 Pure WO3

Before conducting the screening 1% doped WO3sensors, pure tungsten trioxide sensor pellet was initially tested for the detection of NO in air in order to optimize acquisition parameters It was found that the resistance (collected from the impedance at 0.5 Hz) of the material decreased at increasing temperature, thus the observation is consistent with the semiconducting behavior of the material The resistance of our WO3 increased when the sensor was exposed to NO gas in a similar way to WO3sensors reported

in[5,9,15] WO3is an n-type semiconductor, which means that NO forms an anionic adsorbate on the surface of WO3

particles under the present conditions Although the material responded almost instantaneously to the change of air to NO gas, it required 20±30 min for the resistance to settle (response time to reach steady state) This presumably is the minimum time for the gas molecules to diffuse through the compressed WO3powder with reference to this parti-cular sensor con®guration (the gaseous molecules likely

Fig 1 The apparatus for the sensing measurements.

diffuse into the compressed powder through the edges of the

pellets) As a result, all data presented in this paper were then

collected at least 30 min after the gas exposure It is

inter-esting to note from Table 1 that the sensitivity, S

(S ˆ resistance in NO/resistance in air) peaked at around

300 8C This is in a good agreement with the optimum

temperature of around 300 8C reported in the literature using

WO3-based sensors [5,15] It is accepted that lower

sensi-tivity is obtained at higher temperatures since the thermal

energy of the carrier (electrons) can overcome the charge

depletion layer in WO3 On the other hand, the poor

sensi-tivity at low temperature is related to the high activation

barrier for NO to form adsorbate on the oxide surface As

typical of compressed powder, there are some variations in

the absolute response to the same gas mixture over different

sensors, but comparisons with different temperature, gas

composition, ¯ow, etc can be reliably made by using the

same sensor [16] As seen from Table 2, the optimum

sensitivity for 40 ppm NO in air using our pure WO3 is

about S ˆ 9  2collected at 300 8C This value is slightly

higher than but comparable with the results presented by

Yamazoe and co-workers[15], who obtained S ˆ 4:6 using a

milled WO3powder sensor It is noted that Penza et al.[5]

achieved S ˆ 4 using a WO3 thin ®lm sensor and

Tom-chenko et al.[17] achieved S ˆ 1:5 using a WO3(Bi2O3)

thick ®lm sensor at 300 8C at atmosphere of 40 ppm NO in

air

3.2 Screening 1% doped WO3sensors

After the ®rst report of using WO3-based sensor for NO

analysis, Yamazoe and co-workers[9]carried out extensive

screening of the WO3sensors doped with each of the 48

elements at 1% level Elements such as Pb and Re were

shown to be exceptionally effective in enhancing the sensi-tivity for NO gas Penza et al [5] reported the bene®cial effect of doping (Pd, Pt, Au) It is noted that these dopants are also known to be effective as NO oxidation catalysts In light of these works, here we investigated only doping with a few elements to see the promotion effect (Pb, Ag, Sr, La) Particular emphasis was on the effect of Ag since Ag/Al2O3

is a well-accepted catalyst in the catalysis community for

NO conversion to NO2in air As seen from Table 3, the sensitivity is indeed enormously improved by doping the

WO3with silver giving S ˆ 21:5 The order of effectiveness

of dopants is Ag @ Sr > Pb > La

3.3 Ag doped WO3

With regards to the signi®cant improvement in sensitivity

at 300 8C as the WO3is promoted with silver, it is interesting

to investigate whether this doped sensor gives the same temperature response as the pure WO3(Table 1) Table 4

shows that the sensitivity of this sensor increases with decreasing temperature (250 8C versus 300 and 350 8C) This is interesting and may be important since this new sensor offers lower operation temperature than most of the reported WO3-based sensors A sensitivity as high as 38.3 can be obtained at 250 8C over this Ag/WO3 sensor as compared to 2.8 without the Ag promotion (13.7 times higher in sensitivity) There was no apparent change in the time (20±30 min) required to reach steady conditions

Table 1

Temperature effect for NO sensitivity

Temperature (8C) Sensitivity (5 min) Sensitivity (30 min)

Typical conditions: 0.6 g WO 3 compressed pellet in a flowing stream of

40 ppm NO in air at 100 ml/min.

Table 2

Variations over different WO 3 sensors using same experimental conditions

Temperature (8C) Sensitivity

Typical conditions: 0.6 g WO 3 compressed pellet in a flowing stream of

40 ppm NO in air at 100 ml/min.

Table 3 The sensitivity of doped materials

Typical conditions: 0.6 g WO 3 -based compressed pellet in a flowing stream of 40 ppm NO in air at 100 ml/min 8C at 300 8C The powder mixture was filtered, washed, dried and calcined at 600 8C for 2h Characteristic FTIR bands confirmed the nature of theWO 3

a Commercial WO 3

b WO 3 prepared by a spray method using 4 g Na 2 WO 4 (Aldrich, 99.8%) pre-dissolved in 100 ml DM water, followed by spraying into a

6 M HCl solution with stirring at 80 8C.

Table 4 Sensitivity change of Ag doped WO 3 at different temperature Temperature (8C) Sensitivity

Typical conditions: 0.6 g WO 3 compressed pellet in a flowing stream of

40 ppm NO in air at 100 ml/min.

(rate of response) at different temperatures, which suggests

that the gas diffusion into the porous powder is not critically

dependent on the operation temperature but perhaps on the

¯ow characteristics/porosity within the powder The

sensi-tivity enhancement with the decrease of optimum

tempera-ture for NO detection in air associated with the Ag doping

on WO3 has not been reported before this present study

However, Penza et al [5] reported that noble metals (Pd,

Pt, Au) promotions to WO3 give similar observations

Typically, WO3promoted with Pd shifts the optimum

tem-perature from 300 to 200 8C with a four-fold sensitivity

increase (from S ˆ 4 to 16 for 40 ppm NO) It is noted that

these metal promoters including Ag are also well known to

be capable of catalyzing NO to NO2and NO2is known to

display a much higher molecular sensitivity than NO at the

same concentration

Experiments were designed to investigate the role of Ag

particles on WO3 A catalyst of 2% Ag/Al2O3was

synthe-sized according to[18], which is ef®cient for the conversion

of NO to NO2in air The material was sandwiched between

two silica wool plugs in a 5 cm in length, 4 mm i.d vertical

Pyrex tube housed in a temperature controlled furnace

Thus, this bed of 5 g of 2% Ag/Al2O3catalyst was inserted

between the gas supply and the pure WO3sensor This setup

allowed the physical separation of the Ag from WO3with

different temperature controls Fig 2Ashows clearly that

when the temperature of the Ag catalyst is increased to

250 8C, the sensitivity of the WO3sensor kept at 150 8C

increased by three times Without heating the Ag catalyst or bypassing the gas stream from the Ag catalyst bed no sensitivity enhancement was encountered Similar observa-tion was obtained when the sensor was kept at 250 8C (Fig 2B) Although no attempt was carried out to monitor the concentration of NO and NO2gases because of the low concentration, one of the roles of the Ag is apparent to provide surface for the conversion of NO to NO2according

to the literature[18] It is worth noting that the sensitivity enhancement (three to four times) for separate beds (Ag/

Al2O3‡ WO3) is still far less than those observed using the

Ag doped WO3 sensor (13.7 times) despite using a long catalyst bed and/or a higher silver content This indicates that Ag doping onto the WO3 may also provide a unique interface for the interactions with NOxmolecules

3.4 Characterization of the sensor materials Impedance analysis has been increasingly used to char-acterize the inter-granular particles and interface phenom-ena It has been found that the simultaneous measurement of resistance and capacitance of a specimen over a wide range

of frequency gives much more information than dc or a single-frequency ac measurement[19,20].Fig 3shows the complex impedance spectra of 1% Ag/WO3at 350 8C in air and in 40 ppm NO in air mixture, respectively Both of the impedance values (resistance at variable frequencies) are found to depend on the input frequency, giving typical

Fig 2 Histogram of NO sensitivity vs Ag/Al 2 O 3 bed temperature An increase in temperature of the Ag/Al 2 O 3 bed (production of NO 2 ) results in the increase in NO sensitivity over the WO 3 sensor.

Fig 3 Complex impedance plots of Ag/WO 3 (with simulated RC elements) in air and in 40 ppm NO.

semi-circle impedance spectra Their simulated values using

equivalent circuitry with RC elements (C to mimic the

surface charge depletion layers on particle and between

particles, R ˆ granular and inter-granular resistance of the

sensor material in the parallel mode) are also presented in

Fig 3

It is interesting to note that when 40 ppm NO in air

mixture was passed over the sensor, the impedance of the

material was dramatically increased In general, resistance

of WO3-based sensors reported in the literature increases

upon exposure to NO in air using dc measurement It is

ascribed to the NO or NO2(product of NO oxidation over

metal doper) adsorption on WO3surface generating strongly

anionic adsorbates which create a large charge depletion

layer (increase in capacitance) to restrict the dc current ¯ow

[15] It is noted however, our simulation results indicate that

the capacitance and the contact resistance (Ra) of the Ag/

WO3were only marginally altered to a small extent but the

main change was in its granular/inter-granular resistance

Thus, it is likely that adsorption of these gases may somehow interfere with the main pathway for a current ¯ow through the sensor particles in this particular sensor Studies of electrical conductivity of WO3have been extensively carried out in the past It is generally accepted that the electrical conductivity of WO3depends critically on its stoichiometry and particularly the presence of oxygen vacancies in WO3 Electrical conductivity increases at higher defect concentra-tion [21,22] Thus, NOx adsorption on these oxygen-de®-cient W sites blocking the current passage through the material is therefore envisaged

3.4.1 X-ray diffraction (XRD) XRD analyses of this sensor material were carried out using a Spectrolab Series 1300 CPS 120 X-ray powder diffractometer equipped with a capillary sample holder The data were collected in Debye±Scherrer geometry using

a monochromated X-ray beam of nickel ®ltered Cu Ka radiation (l ˆ 0:154 nm) This was obtained from a curved Fig 4 XRD spectra of WO 3 before and after the NO exposure.

Fig 5 XRD spectra of 1% Ag/WO 3 before and after the NO exposure.

quartz monochromatic crystal The X-ray beam was passed

through the capillary, which contained the sample and was

then diffracted onto the detector The detector collected data

for all angles in the range 4±648, 2y, simultaneously

Fig 4gives the diffraction spectra of the WO3before and

after the NO exposure, which shows a triclinic

polycrystal-line structure matching most of the major peaks collected

FromFig 4, we can see that the (0 0 1) peak has the highest

intensity The lattice constants of the WO3were computed to

be a ˆ 7:311(2) AÊ, b ˆ 7:512(3) AÊ, c ˆ 3:846(2) AÊ These

values and their relative intensities match very well with

those given by the JCPDS ®le no 20±1323 (a ˆ 7:300 AÊ,

b ˆ 7:520 AÊ, c ˆ 3:845 AÊ) for triclinic WO3 It was

observed that there is no new peak or any peak shift before

or after the NO treatment This suggests that the crystal

structures were identical throughout the sensing It is noted

that this observation is in an agreement with the previous

report [19] that revealed no bulk structural change in the

WO3after its exposure to NO gas in air

In contrast, it is interesting to ®nd that there is a very low

but clearly observable diffraction hump at the 2y angle

between 10 and 188 in the 1% Ag/WO3 sample (Fig 5)

Such low angles do not match with any possible

diffrac-tion peaks from silver or possible compounds (in fact, no

peaks can be assigned to Ag at this doping level) but merely

re¯ect some local orders with relatively large lattice

para-meters WO3 is easy to form local non-stochiometric

regions, especially oxygen de®ciency in WO3can generate

crystallographic shear (CS) structures giving a wide range of

local non-stoichiometry of WO3 x[23] Detailed XRD char-acterizations of intermediate phases of W5O14, W18O49and

W20O58derived from the parent WO3with long-range order structures have been reported[24] It is therefore speculated that silver doping might have introduced some degree of local order non-stoichiometry structure in the sensor material However, identi®cation of this phase(s) by XRD in the background of WO3was proved to be very dif®cult because

of the low concentration and the poor diffraction region It is noted however, that the well-characterized W5O14 phases

Fig 6 A low-resolution TEM micrograph showing the 1 wt.% doped

WO 3 material after the NO exposure (magnification ˆ 152,000 showing average particle size of 300 nm).

Fig 7 A high-resolution TEM micrograph and a model showing that the 1 wt.% doped WO 3 material after the NO exposure generates a high concentration

of local ordered CS structures.

show a maximum peak value at 2y of 148[24] which ®ts

coincidentally into the hump region of our XRD

3.4.2 Transmission electron microscopy (TEM)

TEM experiments were carried out using a Phillips CM

20, 200 kV (0.26 nm point resolution) under bright ®eld

conditions It was shown fromFig 6that the average particle

size of the 1% Ag/WO3-based material before or after test is

about 300 nm in diameter There was no obvious change in

particle size and morphology after this material was

quenched from 40 ppm NO in air during the sensing

Fig 7shows the high-resolution TEM images of 1% Ag/

WO3 which revealed high concentration of the defective

regions (enriched with CS structure at [0 1 0] with respect to

[0 0 1] WO3) It is found that the separation of these

defective lines is not always identical re¯ecting their lack

of a long-range order (non-stoichiometry region of disorder

nature) However, some local regions show ordered patterns

(the lack of their long-range order and their heterogeneity

may explain the poor re¯ection hump obtained in XRD)

Typically, as seen in the ®gure, a region containing ca

6.93 AÊ planar separation can be visualized It is

inter-esting to note that these defective lines in WO3are much

easily found in the regions at a close proximity to the Ag

particles (at or near to surface) The reason for the creation of

CS defects associated with Ag doping is not yet known So

far, there is no evidence that Ag atoms are incorporated into

these local ordered defective structures (from XRD,

HRTEM) It is however, interesting to point out that silver

nanoparticle, upon melting, can absorb a large amount of

oxygen from environment (reaching about 10 times its

volume, or 0.3% of its weight in oxygen) On cooling to

a few degrees above solidi®cation, it abruptly releases much

of its oxygen adsorbed in a dramatic phenomenon known as

`spit' [25] There are many recent studies on catalytic

oxidation using supported silver nanoparticles based upon

these interesting properties Thus, when the Ag particles are

in a close contact with WO3, it is likely that oxygen

migration between the silver nanoparticles and the WO3

support is involved The phenomenon of metal±oxide

sup-port interaction and the implications on catalysis are

cur-rently being explored in copper, silver or gold on oxide

supports[26]

4 Conclusion

Initial development of NO sensor to monitor NO gas in air

based on WO3is presented here As far as we are aware, 1%

Ag promotion to WO3is for the ®rst time disclosed in the

open literature Thus, the Ag promotion can dramatically

enhance sensitivity and decrease the optimum sensor

tem-perature of the compressed WO3powder sensor Attempts to

understand the bene®cial effects of Ag have been carried

out It is evident that Ag surface can oxidize NO to NO2that

shows a high molecular response (sensitivity) to WO3-based

materials Thus, one of the roles of Ag promotion is to provide metal surface for the NO conversion to NO2 It is however, also found that small silver particles residented on

WO3produce a much higher sensitivity than those using two separate beds (Ag/Al2O3and WO3) This result indicates a structural synergy between silver and WO3(metal±support interaction) for optimum sensitivity Impedance measure-ment suggests that adsorption of NOx, possibly takes place

on the oxygen defective WO3 x sites; hence, interrupting this main electrical conductive pathway of the material Thus, other roles of Ag promotion may include creation

of these defective sites on the WO3at and near the interface between the small Ag particles and the bulk WO3particles as indicated by the present XRD and the TEM results A detailed study on the WO3promoted at different Ag loadings will be very useful in correlating the active sites (created at the Ag±WO3interface) with the sensing sensitivity, which is being carried out at the present laboratory

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Biographies Ling Chen received a BSc degree in chemistry at Guangxi Teachers College, China in 1995 She is currently completing her postgraduate course under the supervision of Dr Tsang in the Catalysis Research Centre

at Reading Her research was concerned with solid state sensors and catalysis.

Shik Chi Tsang obtained a first class degree in chemistry from Birbeck College, London University in 1987 He received his PhD from Reading University in 1991 After spending 4 years as a departmental research fellow in the Inorganic Chemistry Laboratory at Oxford University, he returned to Reading in 1995 Dr Tsang currently holds a readership at Reading University and a Royal Society University Fellowship for catalysis research His main research interests are catalysis, mesoporous materials, supercritical CO 2 and sensors.