Sensors and Actuators B 134 (2008) 133–139

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Sensors and Actuators B: Chemical

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Au-doped WO3-based sensor for NO2 detection at lowoperating temperature

Huijuan Xia, Yan Wang, Fanhong Kong, Shurong Wang, Baolin Zhu, Xianzhi Guo,Jun Zhang, Yanmei Wang, Shihua Wu ∗

Department of Chemistry, Nankai University, Tanjin 300071, China

a r t i c l e i n f o

Article history:Received 8 January 2008Received in revised form 16 April 2008Accepted 17 April 2008Available online 22 April 2008

Keywords:Au-doped WO3

a b s t r a c t

Pure and Au-doped WO3 powders for NO2 gas detection were prepared by a colloidal chemical method,and characterized via X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and X-rayphotoelectron spectroscopy (XPS). The NO2 sensing properties of the sensors based on pure and Au-dopedWO3 powders were investigated by HW-30A gas sensing measurement. The results showed that the gassensing properties of the doped WO3 sensors were superior to those of the undoped one. Especially, the1.0 wt% Au-doped WO3 sensor possessed larger response, better selectivity, faster response/recovery andbetter longer term stability to NO2 than the others at relatively low operating temperature (150 ◦C).

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. Introduction

In recent years, air pollution has been becoming more and moreerious with the development of industry and the increase of peopleife. Especially, nitrogen oxide NOx (NO2 or NO) is toxic itself and,urthermore, is a main source of acid rain and photochemical smog1]. In order to detect such hazardous NOx gas, there have beenots of efforts in developing a variety of NOx gas sensors such aslectrochemical sensors [2], SAW sensors [3], and polymer sensors4]. As we all know, NO gas is very easily oxidized into NO2 gas inir. As a result, it is an urgent assignment to develop a small NO2as sensor with high sensitivity and excellent selectivity to NO2 gasf low concentrations.

Recently, many semiconductive oxides have been widely inves-igated as sensing materials for NO2 detection by either thin orhick film fabrication technique [5–7]. Among the metallic oxides,ungsten trioxide (WO3), which is a wide band-gap n-type semi-onductor, has been considered as a promising sensing material of

olid-state semiconductor gas sensors for NO2 monitoring becausef its excellent sensitivity and selectivity. Many attempts have beenade to enhance the gas sensitivity of semiconductor gas sensors,

ne of which involved the doping of impurities in the films. Penza

∗ Corresponding author. Tel.: +86 22 2350 5896; fax: +86 22 2350 2458.E-mail address: wushh@nankai.edu.cn (S. Wu).

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925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2008.04.018

t al. [7,8] have shown that a high sensitivity was achieved whenoble metals such as Pt, Au and Pd were deposited as activator lay-rs on WO3 films. However, there are few reports on the study ofO2 gas sensing properties of Au-doped WO3 sensors, especially at

elatively low operating temperature.In this paper, thick film sensors based on pure and Au-doped

O3 powders were prepared by a simple method. The structuralroperties of the prepared Au-doped WO3 were analyzed by XRD,EM and XPS. The gas sensing properties of pure and Au-dopedO3 sensors were investigated in detail by an HW-30A gas sens-

ng measurement system. The sensing mechanism of the Au-dopedO3 sensors to NO2 was also discussed.

. Experimental

.1. Preparation of sensor materials

Au-doped WO3 nanocrystalline powders were synthesized by aolloidal chemical method based on sodium tungstate dihydrateNa2WO4·2H2O, analytical grade). Na2WO4·2H2O was dissolvednto a certain amount of deionized water. Then a certain con-

entration of aqueous solution of HCl was dropwise added to theodium tungstate solution under stirring at room temperature tillo white precipitate was further formed. Then an aqueous solutionf HAuCl4·4H2O (0.25, 0.5, 1.0 and 1.5 wt%) was immediately addedo the above solution. The pH of the solution was adjusted with an

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dathe JCPDS file no. 20-1324 for orthorhombic WO3. The sharp peakssuggest that the crystal of WO3 is perfect.

When the dopant content is 0.25 wt%, there are no Au peaksin the pattern, which may be due to the low content of Au. But

34 H. Xia et al. / Sensors and A

queous solution of HCl in the reaction process. Then the solutionas aged for 24 h, after which a quantity of cetyltrimethyl ammo-ium bromide (CTAB) solution (15 ml, 0.15 mol/l) was immediatelydded to the solution and abundant white flocculent precipi-ate was formed. The white flocculent precipitate was treated byltrasonication for 40 min. Then the precipitate was filtrated andentrifuged with deionized water to remove Cl−, Br− and any otherossible remnants, dried at 80 ◦C and calcined at 600 ◦C for 2 h. Aeries of 0.25, 0.5, 1.0 and 1.5 wt% Au-doped WO3 powders werebtained.

.2. Characterization of materials

The X-ray diffraction (XRD) analyses of the as-prepared pow-ers were carried out on a D/MAX-RAX diffractometer operatingt 40 kV and 100 mA, using Cu K� radiation (� = 1.5418 A, scanningange 2�: 10–80◦). Diffraction peaks of the crystalline phase wereompared with those of the standard compounds reported in theCPDS Data Files. The mean particle sizes were estimated by usingeby–Scherrer equation.

TEM experiments were carried out using a Phillips-T20ST elec-ron microscope, operating at 200 kV.

The electronic structure of the surface of WO3 was performed bymethod of X-ray photoelectron spectroscopy (XPS) with a Kratosxis Ultra DLD spectrometer employing a monochromated Al K� X-ay source (h� = 1.486.6 eV), hybrid (magnetic/electrostatic) opticsnd a multi-channel plate and delay line detector (DLD). All XPSpectra were recorded using an aperture slot of 300 �m × 700 �m;urvey spectra were recorded with a pass energy of 150 eV, andigh-resolution spectra with a pass energy of 40 eV. All bindingnergies were referred to the adventitious C 1s line at 284.6 eV.

.3. Gas sensing properties test

The gas sensing properties were detected by the gas sensingeasurement system of HW-30A (a computer-controlled static gas

ensing characterization system) from Henan Hanwei Electronicalechnology Co. Ltd. Gas sensors were screen printed on an aluminaubstrate (4 mm in length). Electrical contacts were made with twolatinum wires attached to the electrodes. A small Ni–Cr alloy coilas placed through the tube to control the operating temperature

rom 100 to 500 ◦C. The schematic diagram of a typical gas sensors shown in Fig. 1(a). The Au-doped WO3 powders were mixed witheionized water to form pastes. Then the pastes were coated ontohe alumina tubes. Then the gas sensors were sintered at 300 ◦Cor 10 days in air in order to improve their stability and repeatabil-ty. These gas sensor devices were placed in a glass chamber (15 l).as response of the side-heated gas sensors was measured understeady-state condition. The measuring electric circuit is shown inig. 1(b). The operating voltage (Vh) was supplied to either of theoils for heating the sensors and the circuit voltage (Vc) was sup-lied across. A load resistor RL was connected in series with theensor element. By monitoring Vout, the resistance of the sensorn air or a test gas can be measured. In the gas response measure-

ent, a given amount of sample gases were injected into a closedhamber by a microinjector and mixed by a fan for several secondsliquids were firstly evaporated and then mixed by a fan). Afterach measurement, the sensor was exposed to the atmosphericir by opening the chamber. The gas response was defined as theatio (S = Rg/Ra) of the electrical resistance in a target gas (Rg) to

hat in air (Ra). The target gas was produced by chemical reac-ion (3NaNO2(s) + 3HCl(aq) = 2NO(g) + H2O(l) + 3NaCl(l) + HNO3(l);NO(g) + O2(g) = 2NO2(g)). Operating temperature of the sensorevices was varied between 100 and 300 ◦C. The operating tem-erature was controlled through adjusting the heating power.

ig. 1. (a) Schematic diagram of the Au-doped WO3 thick film sensor: (1) Pt wire;2) Ni–Cr-heated wire; (3) Al2O3 tube; (4) Au/WO3 thick film; (5) Au electrode andb) graphic of testing principle.

. Results and discussion

.1. X-ray diffraction (XRD)

The diffraction spectra of pure WO3 and Au-doped WO3 withifferent Au contents are presented in Fig. 2. These diffraction peaksnd their relative intensities match very well with those given by

Fig. 2. XRD patterns of pure and Au-doped WO3 samples.

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pAu-doped WO3 sensor calcined in air at 600 C for 2 h. Fig. 4 showsW 4f, O 1s and Au 4f of 1.0 wt% Au-doped WO3 thick film. In Fig. 4a,the binding energy of W 4f7/2 and W 4f5/2 were determined tobe 35.7 and 37.9 eV, respectively, which well agree with literaturevalues for W6+ [9].

H. Xia et al. / Sensors and A

ith an increase of Au concent, the Au peaks can be observedbviously at the angle of around 38.1 and 64.5◦ of 2�. The meanize of Au-doped WO3 particles was close to that of pure WO3,round 28 nm, calculated by the Deby–Scherrer equation, whichndicated that the size of WO3 particle was not affected by Auoping.

.2. Transmission electron microscopy (TEM)

The TEM micrograph pattern of 1.0 wt% Au-doped WO3 is shownn Fig. 3. It can be seen that the particles are anomalistic andhe mean size of Au-doped WO3 nanoparticles is larger than thatalculated from XRD data, which can be attributed to the parti-

les conglomeration because of the high-temperature treatment.ts corresponding selected-area electron diffraction (SAED) patternFig. 3b) shows that the Au-doped WO3 powder was single crys-alline in structure.

ig. 3. (a) A TEM micrograph and (b) the corresponding SAED pattern of 1.0 wt%u-doped WO3.

ors B 134 (2008) 133–139 135

.3. X-ray photoelectron spectroscopy (XPS)

The XPS were performed in order to illuminate the surface com-osition and the chemical state of the elements existed in the

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Fig. 4. XPS of 1.0 wt% Au-doped WO3: (a) W 4f; (b) O 1s; (c) Au 4f.

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O 1s XPS peaks are shown in Fig. 4b. An intense peak is observedt 530.5 eV, and a second component is at 531.5 eV. The intenseeak can be assigned to the presence of “O2−” ions. And the secondeak can be described as the existence of “O−” ions. The two opin-

ons can be supported by the previous paper [9]. The analyses onransition metal oxides have shown the following peaks: (i) in the29.5–530.5 eV range, O 1s peaks characteristic of the “O2−” ionsf the crystalline network; and (ii) in the 531–532 eV range, O 1sateral structures of variable intensity. Although a precise assign-

ent is not possible it seems reasonable to consider ionizations ofeakly adsorbed species and also of oxygen ions with particular

oordination more specifically integrated in the subsurface (bulktructure near the surface). This suggests the existence in the sub-urface of oxygen ions which bear lower electron density than theO2−” ions; formally these oxide ions could be described as “O−”pecies. They can be associated with sites where the coordinationumber of oxygen ions is smaller than in a regular site, with a higherovalence of the M O bonds. A reasonable hypothesis is to considerhe existence, in variable proportions for the different transition

etal oxides of defects in the subsurface.As shown in Fig. 4c, the gold chemical state has two separate

eaks located at 84.0 and 87.7 eV which are due to Au 4f7/2 and Auf5/2 transitions, respectively. These values are in good agreementith the previous report for Au [10]. The peaks located at 84.0 and

7.7 eV were assigned to the spin–orbit splitting component of Au4 f7/2) level in metallic Au [10,11].

.4. NO2 sensing properties

The gas sensing properties of pure and Au-doped WO3 wereested. It is well known that the gas sensitivity as well as theate of gas response to NO2 is much dependent on the operatingemperatures and the amounts of additives. Such tendencies arellustrated in Fig. 5. It can be seen that the response of these sensorso 10 ppm NO2 varies with not only the operating temperature butlso the concentration of Au. In the range of the operating temper-tures studied, from 100 to 300 ◦C, the response values increasedharply at first and decreased rapidly for all the sensors excepthe 1.5 wt% Au-doped WO3 sensor with an increase in operating

emperature, which is similar to the behavior of WO3 nanowireensors to NO2 [12]. Each curve presents a maximum at an opti-al operating temperature. The pure and 1.5 wt% Au-doped WO3

ensors have the maximum gas response at 200 ◦C, while the 0.25,.5 and 1.0 wt% Au-doped WO3 sensors have the maximum gas

Fig. 5. Responses of pure and Au-doped WO3 sensors to 10 ppm NO2.

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ors B 134 (2008) 133–139

esponse at 150 ◦C. The operating temperature of Au-doped WO3ensors is lower, compared with previous reports about NO2 sen-ors [13–15]. Furthermore, the doped sensors exhibit much higheresponse than the undoped one. Especially, the 1.0 wt% Au-doped

O3 sensor presents the largest response to NO2 at 150 ◦C, whicheads us to believe that the Au-doped WO3 sensor is superior tothers in monitoring NO2 gas.

As has been reported, WO3 is a typical n-type semiconductor,nd its gas-sensing mechanism belongs to the surface-controlledype, and the change of resistance is dependent on the speciesnd the amount of chemisorbed oxygen on the surface. In the casef n-type semiconductor oxides [16,17], the intrinsic conductancencreases with increasing the temperature, whereas the adsorbedxygen molecules transform into oxygen ions (O−, O2− and O2

−) byapturing free electrons from the oxide, which causes a decreasen conductance of the oxide with increasing the temperature. Therocess can be expressed in the following reactions [18]:

2 (gas) → O2 (ads) (1)

2 (ads) + e− → O2− (ads) (2)

2− (ads) + e− → 2O− (ads) (3)

− (ads) + e− → O2− (ads) (4)

Adsorption of NO2 on WO3 nanoparticles results in a decreasen conductivity, which may be explained by the following reactions19]:

O2 (g) + e− ↔ NO2− (ads) (5)

O2 (g) + e− ↔ NO (g) + O− (ads) (6)

Both of these reactions require electrons from the conductionand of WO3, which then leads to a decrease in conductivity. WhenO2 gas was introduced into the reaction chamber, it would be

ubjected to two stages: the chemisorption and the removal ofhemisorbed oxygen. They can be described as in the literature20] ((Z)s and (ZO)s present the chemisorption sites and the surfacepecies, respectively).

O2 (g) + (Z)s ↔ (ZO)s + NO (g) (7)

(ZO)s ↔ 2 (Z)s + O2 (g) (8)

Reaction (8) is limited in the decomposition of NO2 on thexides, which is testified by the long recovery time after treatmentith NO2 and it is dependent on the operating temperature.

Noble metals (Pt, Pd, Au and Ag) are the most common metalatalysts that are used to improve the gas sensing properties ofemiconductor gas sensors. Catalytic reactions on the semiconduc-or surfaces form the receptor function of the semiconductor gasensors and catalytically active metal surfaces are used as additiveso strengthen these reactions in order to increase the selectivity andensitivity of the sensors, and also to reduce the response and recov-ry times. Correlative works have been widely studied [7,21–23].t present, as for the effects of the noble metals on the improve-ent of sensor response, two types of mechanisms are discussed

ommonly to explain the role of metallic dopants. The two mecha-isms are the chemical and electronic sensitization. The observed

ncrease in sensor response with Au is predominantly due to thehange which is termed as the electronic sensitization [24]. In thelectronic mechanism, the reaction with target gas molecules takeslace on the surface of the additives and not in the substrates. These

dditives change their charge state, which results in a variation ofhe surface barrier height and as a result leads to a conductancehange on the metallic oxide. Besides, the Au additive as an activeatalyst creates more active sites that are believed to be crucial forhe enhanced sensitivity obtained over this material.

H. Xia et al. / Sensors and Actuators B 134 (2008) 133–139 137

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ig. 6. Responses of pure and Au-doped WO3 sensors to different concentration NO2

t 150 ◦C.

An interesting phenomenon is noticed in Fig. 5; the response of.5 wt% Au-doped WO3 sensor is the lowest compared with that ofhe others. From the XRD patterns, it can be seen that the size ofu particles of the 1.5 wt% Au-doped WO3 is larger than those ofhe other samples. The result indicates that the large Au particle isdverse to the gas sensitivity of sensor. The fact that Au particlesith diameters of about 5 nm or less have unique catalytic proper-

ies, which has been widely reported in previous papers [25–28],n which these small Au particles were calcined below 400 ◦C orputtered on the base semiconductors. However, in our study, theamples were calcined at 600 ◦C, the size of Au particles calculatedy the Deby–Scherrer equation from the data of XRD patterns areround 20 nm, and it can be observed from the TEM micrograph thathe particles are agglomerated because of the high-temperaturealcination. The particle size of gold cannot be gotten from the TEMecause of the poor contrast between gold and WO3. Though theu particles are relatively large, the gas response of Au-doped WO3ensors is higher than that of the undoped one. The gas responseay be further improved if the Au particles are smaller; the further

tudies about it are being carried out.The responses of pure and Au-doped WO3 sensors to differ-

nt concentrations of NO2 (5–20 ppm) were measured at the sameperating temperature (150 ◦C). The results are shown in Fig. 6. Itan be seen that the gas response of the doped sensors is higher

ig. 7. Responses of 1.0 wt% Au-doped WO3 to various gases at different operatingemperatures.

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ig. 8. Comparison of responses of pure and 1.0 wt% Au-doped WO3 sensors to var-ous gases at their optimal operating temperatures (150 ◦C for NO2, 100 ◦C for H2S,00 ◦C for C2H5OH, 250 ◦C for NH3 and acetone, respectively).

han that of the undoped one. And the response values of the sen-ors increased with an increase of the NO2 gas concentration inhe range of 5–20 ppm except the 1.5 wt% Au-doped WO3 sensor.urthermore, the increasing rate of the response of the sensor tobove 10 ppm is smaller than that in the range of 5–10 ppm, whichuggests that the sensors could meet the application demand toonitor NO2 gas, especially of low concentrations.Selectivity is an important factor in gas sensing, and the sen-

or has to present rather high selectivity for its application. Ase all know, a WO3 sensor also responds to other gases such as2S, NH3 and C2H5OH. Therefore, we investigated the responsesf the 1.0 wt% Au-doped WO3 sensor to other five gases at differ-nt operating temperatures from 100 to 300 ◦C. From Fig. 7 it cane observed that the sensor exhibits the largest response to NO2,mong all the tested gases. Furthermore, the optimal operatingemperature is 250 ◦C to NH3 and acetone and 200 ◦C to C2H5OH,oth being higher than that to NO2. The selectivity to NO2 is higherhen the operating temperature is 150 ◦C. As a result, it is possible

ures.The gas responses of pure and 1.0 wt% Au-doped WO3 sensors to

arious gases at their optimal operating temperature are comparednd the results are shown in Fig. 8. It can be seen that the responses

ig. 9. Response–recovery characteristics of a 1.0 wt% Au-doped WO3 sensor to NO2

f different concentrations at 150 ◦C.

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ig. 10. Response–recovery characteristics of a 1.0 wt% Au-doped WO3 sensor to0 ppm NO2 at different operating temperatures.

f the two sensors to NO2 are much higher than those to other fiveases. Furthermore, the responses of 1.0 wt% Au-doped WO3 sen-or to all of these gases are higher than those of the undoped one,specially to NO2. As a result, the Au-doped WO3 sensor is a veryromising semiconductor to monitor NO2 at relatively low temper-ture; both sensitivity and selectivity are taken into consideration.

According to the measurement methods introduced by the sen-or measurement section, the response–recovery characteristicsf the sensors are shown in Figs. 9 and 10. From the two figures,he voltage of sensor decreased rapidly, when NO2 was introducednto the measurement chamber. Upon exposing the sensor to airy opening the chamber, the voltage of the sensor increased to thenitial value gradually. From Fig. 9, we can see that the fast responseharacteristics were almost unchanged at all the concentrationsf NO2, but the recovery times were longer and longer with anncrease of NO2 concentration. From Fig. 10, it is clear that the initial

oltage values were higher and higher with an increase of oper-ting temperature. The response time decreased with an increasef operating temperature. On the other hand, the recovery timesere long but became shorter when the operating temperature was

ncreased.

ig. 11. The long-term response values of a 1.0-wt% Au-doped WO3 sensor to 10 ppmO2 at 150 ◦C.

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ors B 134 (2008) 133–139

The long-term stability is another important factor for gasensors. The stability of the 1.0 wt% Au-doped WO3 sensor was mea-ured at a level of 10 ppm NO2 for 3 months. The results are shownn Fig. 11. It can be observed that the sensor still showed excellentesponse performance to NO2 gas even after 3 months, which indi-ated that the sensors based on Au-doped WO3 have the enoughtability to detect NO2 gas for a relatively long period.

. Conclusions

Pure and Au-doped WO3 nanoparticles were prepared by a col-oidal chemical method. The crystalline phase of WO3 nanoparticlesalcined in air at 600 ◦C for 2 h was orthorhombic. XPS analyseshowed that W6+ and Au0 existed on the surface of the as-preparedaterials. The sensors based on these materials showed high gas

esponse, excellent selectivity and long-term stability to NO2 gas.he response of the sensors were strongly dependent on the oper-ting temperature and the amounts of additives. The optimumerformance was obtained at 150 ◦C for the WO3 sensor dopedith 1.0 wt% Au. The doped sensors also presented rapid response

haracteristics. However, the gas sensors need to be further investi-ated to shorten the recovery times. In summary, the sensor basedn WO3 with a proper Au loading is suitable for detecting NO2 atelatively low operating temperature.

cknowledgements

This work was supported by the National Nature Science Foun-ation of China (No. 20771061) and 973 program (2005CB623607).

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iographies

uijuan Xia is studying in Shihua Wu’s group for her MS degree in Department ofhemistry, Nankai University in China. Her research focuses on the synthesis andas-sensing properties of metal oxide nanomaterials.

S1Utn

ors B 134 (2008) 133–139 139

an Wang received her MS degree in chemistry from Nankai University in006. She is now studying in Shihua Wu’s group for her PhD degree inepartment of Chemistry, Nankai University in China. Her research focusesn the synthesis, characterization and gas-sensing properties of metal oxideanomaterials.

anhong Kong is student in Department of Chemistry, Nankai University now.er interest is devoted to the preparation and application of gas-sensitiveaterials.

hurong Wang is working at Nankai University. She received her PhD degree inhemistry from Nankai University in 2007. Her research covers nanomaterials, catal-sis and gas sensors.

aolin Zhu received her PhD degree in chemistry from Nankai University in 2006.ow, she is working at Nankai University. Her research is focused on the preparationf nanomaterials.

ianzhi Guo associate professor. Her research is focused on the field of macro-olecule chemistry and physics.

un Zhang received his bachelor degree in chemistry from Qufu normal University in006. He is currently a postgraduate in the Department of Chemistry in Nankai Uni-ersity. His research is focused on the development and application of gas-sensitiveaterials.

anmei Wang is currently a postgraduate in the College of Chemistry in Nankai Uni-ersity. Her research is focused on the development and application of gas-sensitiveaterials.

hihua Wu received his degree in chemistry from Nankai University in970. At present he is professor of chemistry at the Department of Nankainiversity, where he has been working for many years in the field of prepara-

ion, characterization and catalytic and gas-sensing properties of metal oxidesanomaterials.