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Sensors and Actuators B 162 (2012) 259– 268

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

journa l h o mepage: www.elsev ier .com/ locate /snb

cetone detection properties of single crystalline tungsten oxide platesynthesized by hydrothermal method using cetyltrimethyl ammonium bromideupermolecular template

huyang Liu, Fei Zhang, He Li, Ting Chen, Yude Wang ∗

epartment of Materials Science and Engineering, Yunnan University 650091, Kunming, People’s Republic of China

r t i c l e i n f o

rticle history:eceived 15 September 2011eceived in revised form9 December 2011ccepted 22 December 2011vailable online 29 December 2011

a b s t r a c t

Single crystalline tungsten oxide plates were prepared by hydrothermal method using cetyltrimethylammonium bromide (CTAB) supermolecular template. The phase and the morphology of the resultingmaterial were characterized by X-ray diffraction (XRD), Fourier transformed infrared (FTIR) spectrum,scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM(HRTEM), and Raman spectroscopy, respectively. Indirect-heating sensors using single crystalline WO3

plates as sensitive materials were fabricated on an alumina tube with Au electrodes and platinum wires.

eywords:ydrothermal synthesisurfactant CTABO3

cetoneas-sensing properties

The as-fabricated sensor based on WO3 plates showed high response, fast response and recovery towardacetone gas. Compared with the sensor fabricated with WO3 particles prepared by a surfactant-mediatedmethod, the results show that WO3 plates sensor has about 4 times increase in response and good dynamicresponse.

© 2012 Elsevier B.V. All rights reserved.

. Introduction

Tungsten oxide (WO3) is an important wide band gap n-typeemiconductor and has many outstanding properties, such as elec-rochromic, optochromic, and gaschromic properties [1]. It haseen extensively studied as a promising material for a multi-ude of potential applications including semiconductor gas sensors,lectrode materials for secondary batteries, solar energy devices,hotocatalysts, and field-emission devices [2–10]. WO3 gas sensoras first reported for detection of H2 by Shaver [11], who showed

hat the conductivity of WO3 films changed greatly upon the expo-ure to the H2 ambient. Following this pioneering work, severalorks reported on the gas properties of WO3 to NOx, H2S, H2,H3COCH3, and so forth [2,12–16]. Most of the studies focused onO3 solid polycrystalline films which were made from large parti-

les [13–16] or WO3 nanorod films [17–19]. However, research haseldom been focused on the acetone gas sensing properties basedn the different morphologies WO3 powders.

Acetone gas, which is widely used in many industrial processes,

s harmful when it emitted into the environment. In some stud-es, concentrations of approximately 300–500 ppm were reportedo cause slight irritation of the noses, throats, lungs, and eyes.

∗ Corresponding author.E-mail address: ydwang@ynu.edu.cn (Y. Wang).

925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2011.12.076

Exposure to 250 ppm for 4 h has caused mild effects on performancein some behavioral tests (auditory tone discrimination and a moodtest). As concentrations approach 1000 ppm or higher, noticeableirritation has occurred and some people have also complained ofheadache, light-headedness, dizziness, unsteadiness, and confu-sion. Inhalation of concentrations higher than 2000 ppm can causedizziness, a feeling of drunkenness, drowsiness, nausea and vom-iting. Even higher concentrations can cause collapse, coma anddeath. With the improvement of living standards, the demands ofaccurate and dedicated sensors to provide precise process controland automation in manufacturing processes, and also to monitorand control environmental pollution for good environment qual-ity, have accelerated the development of the sensing materialsand sensor technology in the last decade. Therefore, the effectivemethods to monitor acetone have been demanded for atmosphericenvironmental measurements and control. The fabrication gas sen-sor is a desirable means for monitoring acetone and the acetonegas sensors play an important role in the fields of occupationalsafety and human body health. The focus is on the developmentof the sensing materials with new structures or morphologies toimprove sensitivity, selectivity and stability of sensors and alsoon the development of new and better fabrication techniques to

ensure reliability, safety, reproducibility, and cost reduction.

The gas sensing process of metal oxide sensors generallyinvolves a catalytic reaction between the gas to be monitoredand the adsorbed oxygen on the surface of the sensor. In view of

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60 S. Liu et al. / Sensors and A

he sensing mechanism, the particle size, defects, surface, inter-ace properties, and stoichiometry directly affect the state andhe amount of oxygen species on the surface of the sensors, andonsequently the performance of metal oxide-based sensors. Its believed that sensor response can be improved both by dop-ng and the increase of sensitive surface areas of the materials. Inase of redox sensors, the materials will provide more surface sitesvailable for oxygen to be adsorbed or to make contact with theurrounding gases [20,21]. The preparation method for the sensingaterial therefore plays an important role in tailoring the mor-

hological characteristics of the sensor. In this paper, we reportmproving the acetone sensing properties of single crystalline

O3 plates obtained by hydrothermal method using cetyltrimethylmmonium bromide supermolecular template. The WO3 platesere then used as a sensing material of the sensors of indirecteating structure and tested for their ability to detect acetone.

. Experimental

.1. Synthesis of WO3 materials

All the chemical reagents used in the experiments were obtainedrom Fluka as guaranteed-grade reagents and used without furtherurification. The purity of CTAB is 99.9% and of the inorganic pre-ursors are not less than 99%, respectively. In a typical synthesisf single crystalline WO3 plates, CTAB was mixed with the dis-illed deionized water with stirring until a homogenous solutionas obtained. 10 mL NH4OH (25 wt% solution) was mixed with theistilled deionized water and then added into the CTAB solutionith stirring. When the mixing solution became homogenous, a

olution of WCl6 diluted with distilled deionized water was intro-uced and then a slurry was achieved. The synthesis mixture wastirred for 4 h before being transferred into an autoclave (Parr acidigestion bombs, Teflon cups with 50 mL inner volume). The auto-lave was taken out of the glovebox and heated at 200 ◦C for about

days. The resulting cloudy suspension was centrifuged, and therecipitate was thoroughly washed by ethanol and dried at roomemperature.

In order to compare with the sensing properties of sin-le crystalline WO3 plates, WO3 particles were prepared by aurfactant-mediated method [22]. The synthetic procedures wereased on the use of the cationic surfactant (CTAB) and the simplehemical materials (tungsten chloride and NH4OH) as inorganicrecursors. The reaction was performed at room temperature.TAB were mixed with distilled deionized water with stirring until

homogenous solution (0.08 M) was obtained. The solution ofiluted NH4OH (25 wt% solution, 10 mL) was then added into theTAB solution with stirring. When the mixing solution becameomogenous, a solution of WCl6 (0.40 M) was added, under vig-rous stirring. After stirring 4 h, the product was aged at ambientemperature for 96 h. The resulting products were filtered, washedith distilled water to remove surfactant, and then dried at ambi-

nt temperature. Complete evolution of the surfactant from thes-synthesized products to yield the oxide particles was achievedhrough thermal treatment: 2 h at 500 ◦C under flowing air atmo-phere.

.2. Characterization of WO3 materials

X-ray diffraction (XRD, Rigaku D/MAX-3B powder diffractome-er) with copper target and K�1 radiation (� = 1.54056 A) was used

or the phase identification, where the diffracted X-ray intensi-ies were recorded as a function of 2�. The sample was scannedrom 20◦ to 60◦ (2�) in steps of 0.02◦. Fourier transform infraredFTIR) investigations were performed on a Perkin-Elmer 2000 FTIR

rs B 162 (2012) 259– 268

spectrometer. Scanning electron microscopy (SEM) photographswere obtained on XL30ESEM-TMP microscope. Transmission elec-tron microscopy (TEM) measurement was performed on a ZeissEM 912 � instrument at an acceleration voltage of 120 kV, whilehigh-resolution transmission electron microscopy (HRTEM) char-acterization was done using JEOL JEM-2010 Electron Microscope(with an acceleration voltage of 200 kV). The samples for TEM wereprepared by dispersing the final samples in distilled water, and thisdispersing was then dropped on carbon–copper grids covered byan amorphous carbon film. To prevent agglomeration of nanorodsthe copper grid was placed on a filter paper at the bottom of a Petridish. X-ray photoelectron spectroscopy (XPS) was carried out atroom temperature in ESCALAB 250 system. During XPS analysis, anAl K� X-ray beam was adopted as the excitation source and the vac-uum pressure of the instrument chamber was 1 × 10−7 Pa as readon the panel. Measured spectra were decomposed into Gaussiancomponents by a least-square fitting method. Bonding energy wascalibrated with reference to C1s peak (285.0 eV). The Raman spectrawere recorded with a Renishaw in Via Raman microscope, equippedwith a CCD (charge coupled device) with the detector cooled toabout 153 K using liquid N2. The laser power was set at 30 mW.The spectral resolution was 1 cm−1. N2 adsorption–desorptionisotherms at 77 K were recorded on a Micromeritics ASAP 2010automated sorption analyzer. The samples were outgassed 20 h at150 ◦C before the analysis.

2.3. Fabrication and measurement of sensors

The fabrication of an indirect-heating structure sensor wasdescribed in the literature [23]. WO3 oxides were mixed withdeionized water to form paste, and then coated onto the outside ofan alumina tube which two Au electrodes and platinum wires hadbeen installed at each end. The tube was 4 mm in length, 1.2 mmin external diameter, and 0.8 mm in internal diameter. The thick-ness of the sensitive body, which was dried and sintered in air at500 ◦C for 2 h, was about 0.5 mm. A Ni–Cr alloy wire crossing thealumina tube was used as a resistor to ensure both substrate heat-ing and temperature control. In order to improve their stability andrepeatability, the gas sensors were aged at operating temperature300 ◦C for 150 h in air. The sensor’s resistance was measured byusing a conventional circuit in which the element was connectedwith an external resistor in series at a circuit voltage of 10 V. Thegas-sensing properties were measured with static state distribu-tion in a chamber. After acetone, alcohol or ammonia liquid wasinjected into the heating apparatus in the back of the chamber by amicroinjector, the liquid would gasify quickly in the chamber. Theelectrical response of the sensors was measured with an automatictest system, controlled by a personal computer. The gas response

was defined as the ratio of the electrical resistance in air (Ro) tothat in gas (Rg), namely = Ro/Rg.

3. Results and discussion

The powder X-ray diffraction patterns revealed the formationof single-phase WO3 synthesized by hydrothermal method andsurfactant-mediated method, as shown in Fig. 1. The XRD pattern(Fig. 1(A)) of the powders synthesized with hydrothermal methodrevealed well-developed reflections of WO3 (ICDD PDF No. 83-0950), space group P21/n (14), with the average cell parametersa = 7.323 A, b = 7.513 A, and c = 7.669 A (the average cell param-eters were calculated from (0 0 2), (0 2 0), (2 0 0), (1 2 0), (1 1 2)

and (2 0 2) reflections). The intensity at (0 0 2) peak has been dra-matically improved in comparison with the standard peaks. Thisindicates that the exposed planes of plates are (0 0 2) orientationplanes, which were the preferential growth plane. No crystalline

S. Liu et al. / Sensors and Actuators B 162 (2012) 259– 268 261

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ig. 1. XRD patterns of WO3 synthesized by hydrothermal method (A) andurfactant-mediated method (B).

y-products such as CTAB, WCl6 or other tungsten oxides wereound in the pattern, indicating that the as-synthesized sampleas pure WO3 with monoclinic structure. The XRD pattern of the

ample synthesized by surfactant-mediated method is shown inig. 1(B). All reflection peaks in Fig. 1(B) can be easily indexed to theonoclinic WO3 (space group: P21/n (14)). The results indicate that

he both products synthesized via surfactant-mediated method andydrothermal method have the same crystalline structure.

In this research, the sample synthesized by hydrothermalethod was further characterized by FTIR spectroscopy in the

ange of 400–4000 cm−1 (Fig. 2). The asymmetric (2918.7 cm−1)nd symmetric (2846.4 cm−1) stretching vibrations of C–CH2 and–CH3 asymmetric stretching and N–CH3 symmetric stretchingibrations (3011.6 cm−1) are assigned to solid surfactant CTAB [24].ome bands attributed to CTAB surfactant are not observed in theegion 2800–3020 cm−1 from Fig. 2. It indicates that the surfac-ant is not present in the as-synthesized sample. The vibrationsn the infrared (IR) range at 600–900 cm−1 correspond to O–W–Otretching [25]. The bands at about 3456 cm−1 and 1636 cm−1 cane attributed to the O–H vibration in absorbed water on the sampleurface [26].

The morphologies of the products were characterized by scan-ing electron microscopy (SEM). SEM images for the samples under

nvestigation are shown in Fig. 3. Fig. 3(a) is the low-magnificationmage, showing that more than 90% of the samples are plates with

Fig. 2. FTIR spectrum of WO3 synthesized by hydrothermal method.

lengths up to several micrometers. The enlarged SEM pattern ofWO3 synthesized with surfactant-mediated method is shown inFig. 3(b). All of the plates have uniform morphology, while thesample synthesized with surfactant-mediated method is generallyirregular morphology, and the particles are agglomerated (Fig. 3(c)and (d)). The particle size is in the range of 200–400 nm. Whileon the surface of the sample, there are many spherically shapedparticles which are uniform in size and well dispersed.

TEM investigations give further insight into the structural fea-tures of WO3 materials. As shown in Fig. 4(a), the sample iscomposed of a large number of plates with lengths up to sev-eral micrometers. A typical HRTEM image of WO3 plates is shownin Fig. 4(b). The clear two-dimensional ordered lattice structureindicates that the obtained WO3 plates are single-crystalline. Theselected area electron diffraction (SAED) pattern (Fig. 4(c)) iscomposed of systematic bright spots and specified as single crys-talline WO3. The lattice distance (0.383 nm) measured from theHRTEM image of the WO3 palate corresponds well with the d-value(0.384 nm) of (0 0 2) plane of monoclinic WO3 according to JCPDSNo. 83-0950. The average particle size of WO3 particles as shownin Fig. 4(d) is found to be less than 200 nm. The lattice distance(0.378 nm) measured from the HRTEM image (Fig. 4(e)) of the WO3particles corresponds well with the d-value (0.377 nm) of (0 2 0)plane of monoclinic WO3 according to JCPDS No. 83-0950. The SAEDpattern of WO3 particles (Fig. 4(f)) shows also the characteristicsingle crystalline WO3.

The Raman spectrum of as-synthesized single crystalline WO3plates is shown in Fig. 5. In the spectrum, the two bands at 272and 324 cm−1 can be attributed to O–W–O bending modes of thebridging oxide, while the 715 and 805 cm−1 bands are assigned tothe stretching modes [27,28]. The peaks correspond to monoclinicphase [27] (space group: P21/n) which is the stable form of WO3 atroom temperature.

Brunauer–Emmett–Teller (BET) nitrogenadsorption–desorption isotherms and surface area measure-ments were performed. The adsorption–desorption isotherms areshown in Fig. 6. The BET specific surface area of WO3 particles ismeasured to be 8.83 m2 g−1, while that of single crystalline WO3plates was measured to be 33.41 m2 g−1, and it showed higher

values of about 4 times comparing to that of WO3 particles.

The surface/near surface chemical compositions of the WO3materials were further analyzed by XPS, as shown in Fig. 7. Apart

262 S. Liu et al. / Sensors and Actuators B 162 (2012) 259– 268

Fig. 3. SEM image of the as-synthesized samples show the surface structure and morphology. WO3 plates at low magnification (a) and at high magnification (b), and WO3

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articles at low magnification (c) and at high magnification (d).

rom the C1s peak positioned at 286.0 eV, which originated frompurious amounts of surface carbon of the decomposed CTAB tem-late adsorbed onto the inorganic framework of WO3, XPS surveypectrum confirmed the high chemical purity of the WO3 materials,onsisting solely of W and O. The O1s components also show theirarticular binding energies in the spectra (Fig. 7(a) and (d)). Espe-ially, the state of O1s indicated that there are two sorts oxygen inhe surface, the lattice oxygen (Olattice) and the adsorbed oxygenOads). The lattice oxygen could not be interacted with the reduc-ng gas, and unable to affect the formation of main charge-carrieroles in n-type semiconductor. However, the adsorbed oxygen iseactable with the gas and then enhances the holes concentration29]. Therefore, increasing the adsorbed oxygen contributes to theas response. Judging from Fig. 7(a) and (d), the adsorbed oxygenn single crystalline WO3 plates is more than in WO3 particles. Forhe single crystalline WO3 plates, calculations of the areas of theads and Olattice emission lines resulted in the Oads to Olattice ratiof 0.91, which is generally bigger as compared to of the WO3 par-icles (0.32). It can be attributed to the bigger specific surface areaf single crystalline WO3 plates than of WO3 particles. Spectra ofndividual line of W4d measured at high resolution show (Fig. 7(b)nd (e)) narrow range scans for single crystalline WO3 plates andO3 particles. The XPS spectra show two peaks of 4d5/2 and 4d3/2

t 247.5 or 247.6 eV and 260.3 or 260.4 eV with a better symmetry,espectively, and they are assigned to the lattice tungsten in tung-

ten oxide. They have a peak separation of 12.8 eV between thesewo peaks. The values correspond to a binding energy of W(VI) ionindexed Standard ESCA Spectra of the Elements and Line Energynformation, Co., USA). The binding energy of Wf7/2 is determined to

be 35.6 ± 0.1 eV for single crystalline WO3 plates and 35.8 ± 0.1 eVfor WO3 particles, respectively. Each material consists of W 4f7/2and W 4f5/2 doublet peaks with the spin-orbit splitting energy of2.1 eV (Fig. 7(c) and (f)). These values agree well with literature val-ues for W6+ [30,31]. The W4f XPS spectrum of tungsten oxide waspreviously reported to decompose into two components resultingfrom W5+ (34.2 eV) and W6+ (35.7 eV) oxidation states [17,30,32].However, the single crystalline WO3 plates and WO3 particles onlyexhibit identical binding energies of around 35.7 eV for the W6+

oxidation states.As known from the literature, the electrical conductivity of

sensor depends not only on the gas atmosphere, but also onthe operating temperature of the sensing material exposed tothe test gas [33]. The operating temperature has a great influ-ence on the resistance of the sensors, as shown in Fig. 8. Theproperties of the resistance-operating temperature show the char-acteristic of a typical surface-controlled model [34–36]. Electronconcentration of WO3 materials is determined mainly by the con-centration of stoichiometric defects such as oxygen vacancy likeother metal oxide semiconductors. From 180 ◦C to 360 ◦C, the shapeof their resistance-operating temperature curves is attributed tothe change in charge state of the chemisorbed oxygen-relatedspecies, such as O2ads

−, Oads−, OHads

−, and Oads2−. The resistance

decreases with increase of temperature in the range of 180–300 ◦C.The probable factor is that the intrinsic defects become responsi-

ble for the conductance of the sensor at higher temperature, andthe thermal energy causes the electrons to emit from low energylevels (such as donor levels or valence band) to conduction band[37] and the increased mobility of the charge carriers. For the

S. Liu et al. / Sensors and Actuators B 162 (2012) 259– 268 263

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ig. 4. (a) TEM image of WO3 plates, (b) HRTEM image of WO3 plates, (c) SAED pattf WO3 particles, (f) SAED pattern of WO3 particles.

ensor based on single crystalline WO3 plates, the resistance isearly independent of the operating temperature above 300 ◦C,ecause the electrons of donor level are ionized completely andhe electronic concentration of intrinsic exciting is less than the

lectronic concentration of donors in this temperature region [37].he experiment was carried out at a relative humidity (RH) of 60%.dditional tests were performed at RHs of 30, 80, and 90% with

he result that the resistances were almost constant for all RHs,

m the individual palate sample; (d) TEM image of WO3 particles, (e) HRTEM image

suggesting that this sensor can be applied over a wide range ofhumidity.

The gas sensing properties of the WO3 materials toward ace-tone gas were analyzed between 180 and 360 ◦C in dry air. Fig. 9

depicts the relations between the response and the operating tem-perature for the sensors in the different acetone gas concentrations.The operating temperature has a great influence on the response. Asexpected, the response first gradually increases and then decreases

264 S. Liu et al. / Sensors and Actuato

Fig. 5. Roman spectrum of single crystalline WO3 plates.

Fig. 6. BET nitrogen adsorption–desorption of WO3 plates (a) and WO3 particles.

rs B 162 (2012) 259– 268

with increasing the operating temperature. As shown in Fig. 9(a),the response of the single crystalline WO3 plates based sensor oper-ated at 307 ◦C shows the best response and good dependence on thegas concentration. The straight line is the calibration curve and theexperimental data were fitted as:

ˇ = m lg Cacetone − 191.82 (1)

where m1 is the sensitivity coefficient and Cacetone is the ace-tone concentration. From the straight line obtained, we calculatedm = 99.1 ± 8.2.

The response of the WO3 particles based sensor operated at307 ◦C (Fig. 9(b)) shows the following linear relation:

= 16.9 lg Cacetone − 29.88 (2)

The WO3 plates show about 4 times increase in response to ace-tone gas compared with the WO3 particles. Also, a good responseand quick response/recovery time were observed with single crys-talline WO3 plates sensor at the optimal operating temperature of307 ◦C. Response and recovery times calculated from the case of2000 ppm are 10 and 26 s, respectively. The sensing performancehas been improved in the gas responses and the dynamic responseto different acetone concentrations, comparing with other acetonesensors fabricated by WO3 hollow-sphere [1], WO3 thick film [38],WO3 nanoplates [39], ZnO thin film [40], fine grained nickel ferrite[41], porous �-Fe2O3 hollow microspheres [42], nano particulateSnO2 [43], and Co doped ZnO nanofibers [44].

The principle of gas detection of the resistance-type sensors isbased on the conductance variation of the sensing element, whichdepends on the gas atmosphere and on the operating temperatureof the sensing material exposed to the test gas, thus resulting inthe space-charge layer changes and band modulation [45]. It typi-cally involves physisorption, chemisorption, and electron transferprocesses. For n-type conductor, oxygen species were adsorbed onthe surface of particles in the air, and then were ionized into Oads

−

or O2ads− by capturing free electrons from the particles, thus lead-

ing to the formation of thick space charge layer and increasing ofpotential barrier. Here, the resistance of the sensor was high. WO3is a kind of the acidic oxide and can react with the alkali. Besidesthe state of oxygen adsorbed on the surface of WO3, there is OH−

that comes from water. The existing form of chemisorbed water onWO3 is more complicated. The reaction can be summarized as:

Wlat6+ + H2O ↔ (Wlat

6+–OH−) + Hads+ (3)

where Wlat6+ is Lewis acid site, which can form covalent bond with

OH−, Hads+ is the adsorbed hydrogen ion, which is Bronsted acid site

that is can be removed easily in catalytic reaction. OH−, O2ads− and

Oads− are dominating oxygen-related species on materials surface.

The reducing gas reacted with oxygen adsorbed on the surface ofthe sensor and the reaction between reducing gas and lattice oxy-gen was neglected. Acetone is adsorbed and reacts with Lewis acidsite of surface (that is W ion) and adsorbed oxygen. To maintainneutrality, the electrons release back WO3 material, resulting inthe increase of the electron concentration and the decrease of theresistance. This change of the electrical resistance determined sen-sitivity of the WO3-based sensor to reducing gases. The possibleprocess of the reaction with acetone can be explained as follows[46,47]:

CH3COCH3 + Wlat6+ → (CH3)2C O–Wlat

6+ (4)

(CH3)2C O–Wlat6+ ↔ CH2 C(CH3)OH–Wlat

6+ (5)

(CH ) C O–W 6+ + O − → (CH ) C–O–W–O–W + e− (6)

3 2 lat ads 3 2

CH2 C(CH3)OH–Wlat6+ + Oads

− → CH2

C(CH3)OW + HO–W + e− (7)

S. Liu et al. / Sensors and Actuators B 162 (2012) 259– 268 265

F es, an

C

2

ig. 7. The high-resolution XPS spectra of O1s (a), W4d (b), and W4f (c) for WO3 plat

H2 C(CH3)OH–Wlat6+ + OH− → Wlat

6+–O−

+H2O + CH2 C(CH3) + e− (8)

Hads+ + Oads

− → H2O (9)

d the high-resolution XPS spectra of O1s (d), W4d (e), and W4f (f) for WO3 particles.

CH3COCH3 + 8Oads− → 3CO2 + 3H2O + 8e− (10)

Furthermore, the selectivity of the WO3 sensors was also investi-

gated. As shown in Fig. 10, we tried to detect alcohol and ammoniagases under the same conditions at an operating temperature of307 ◦C. From the curve of the sensor response vs gas concentration(Fig. 10(a)), the response of the sensor based on single crystalline

266 S. Liu et al. / Sensors and Actuators B 162 (2012) 259– 268

Fig. 8. Relations between the resistance and the operating temperature for gassensors based on single crystalline WO3 plates and WO3 particles.

Fig. 9. Effect of the operating temperature on the gas response of the gas sensorsat different concentrations of acetone gas: (a) gas sensor based on single crystallineWO3 plates and (b) gas sensor based on WO3 particles.

Fig. 10. Gas response of the sensors to different gases at the different gas concen-

trations under an operating temperature of 307 ◦C.

WO3 plates to acetone is the highest, and is roughly 10 times higherthan that to other gases at the concentration of 1000 ppm, indi-cating that the single crystalline WO3 plates sensor has a highselectivity to acetone among the examined gases. From Fig. 10(b),one can see that the selectivity of the single crystalline WO3 platessensor is higher than of the WO3 particles sensor at the same exper-imental conditions. It is difficult to explain the selectivity to acetonefrom the reaction processes (4)–(10). The evidences studied fromthe ferroelectric materials show that the dipole moment of a polarmolecule may interact with the electric polarization of some ferro-electric domains on the surface [47–50]. As a type of ferroelectricmaterial, the monoclinic WO3 has a spontaneous electric dipolemoment, which plays an important role on the selective detectionof acetone. As a consequence, the interaction between the WO3surface dipole and acetone molecules could be much stronger thanany other gas, leading to the observed selectivity to acetone detec-tion [46]. The single crystalline WO3 plates have higher surface areathan WO3 particles, which can lead to more surface dipole on thesurface of WO3 plates. It is one of the factors to influence on sensorselectivity for acetone. On the other hand, some factors can influ-

ence on sensor selectivity, such as the different morphology of WO3,which can lead to different crystalline faces exposed to gas phasewhich could have different affinity to acetone. The difference in

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orphology can influence on the state of intergrain contacts whichan play an important role in sensor response.

. Conclusion

The successful synthesis of single crystalline tungsten oxidelates by using hydrothermal method is presented. The XRD, FT-IR,EM, Raman, XPS, and BET nitrogen adsorption–desorption resultshowed that the single crystalline WO3 plates with high specificurface area and high concentration of adsorbed oxygen werebtained. The single crystalline WO3 plates can be used directlyo prepare gas sensor devices by fabrication of the WO3 plates onhe alumina tubes with Au electrodes and Pt wires. The fabricatedensor showed good response, high selectivity, quick responses,nd good recovery to acetone gas at an operating temperature of07 ◦C. Compared with the sensor fabricated with WO3 particlesrepared by a surfactant-mediated method, the WO3 plates sensoras about 4 times increase in response and good dynamic responseo acetone.

cknowledgements

This work was supported by the Department of Science andechnology of Yunnan Province via the Key Project for the Sci-nce and Technology (Grant No. 2011FA001), the Key Project ofhinese Ministry of Education (Grant No. 210206), and Nationalatural Science Foundation of China (Grant No. 50662006).

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68 S. Liu et al. / Sensors and A

iographies

huyang Liu received her B.S. degree in Department of Materials Science and Engi-eering from Qinghai University in 2010. She is currently a graduate student inunnan University and devotes to nanostructured functional materials and theirpplication in gas sensors.

ei Zhang received her B.S. degree in Chemical Science from Nanchang Hangkongniversity in 2010. She is currently a graduate student in Yunnan University and

evotes to nanostructured functional materials and their applications.

e Li received her B.S. degree from The PLA Information Engineering Universityn 2010. She is currently a graduate student in Yunnan University and devotes toanostructured functional materials and their applications.

rs B 162 (2012) 259– 268

Ting Chen received her B.S. and M.S. degrees in Department of Materials Scienceand Engineering from Yunnan University in 2005 and 2008, respectively. She iscurrently a Ph.D. student in University of Science and Technology Beijing and devotesto inorganic functional materials.

Yude Wang obtained his M.S. degree in Physics Condensed State from YunnanUniversity in 1997 and Ph.D. in Materials Physics and Chemistry from TsinghuaUniversity in 2003. From 2005 to 2007, he was a guest scientist in Max-Planck-

of Colloids and Interfaces, Germany. Currently, he is a professor at the Collegeof Physics Science and Technology, Yunnan University. His work is devoted tochemical and biochemical sensors, nanostructured functional materials and theirapplications.