En

Ya

b

a

ARRAA

KFTTG

1

pr[aBfip

cdecwrouripbtg

0d

Sensors and Actuators B 140 (2009) 73–78

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

nhanced toluene sensing characteristics of TiO2-doped flowerlike ZnOanostructures

i Zenga, Tong Zhanga,b,∗, Lijie Wanga, Minghui Kanga, Huitao Fana, Rui Wanga, Yuan Hea

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR ChinaKey Laboratory of Low Dimensional Materials and Application Technology of Ministry of Educations, Xiangtan University, Xiangtan 411105, PR China

r t i c l e i n f o

rticle history:eceived 7 October 2008eceived in revised form 12 February 2009

a b s t r a c t

The large-scale flowerlike ZnO nanostructures, consisting of many aggregative nanorods with the diam-eter of about 60 nm, were prepared by a simple and facile hydrothermal route. The toluene sensingproperties of the pure ZnO and TiO2-doped ZnO nanostructures were tested at different operating tem-

ccepted 31 March 2009vailable online 8 April 2009

eywords:lowerlike ZnOiO2-doped

peratures from 160 to 390 ◦C and toluene concentrations ranging from 1 to 3000 ppm. The results indicatethat the response of the ZnO nanostructures towards toluene was enhanced significantly by TiO2 doping,which was deposited onto the ZnO products by evaporation. It is found that the TiO2-doped ZnO sensorexhibits remarkably enhanced response of 17.1 at the optimal operating temperature of 290 ◦C to 100 ppmtoluene, which has been improved up from 7.4 at 390 ◦C from pure ZnO nanostructures.

olueneas sensor

. Introduction

ZnO is a chemically and thermally stable n-type II–VI com-ound semiconductor with a direct bandgap energy (3.37 eV atoom temperature) and a strong exciton binding energy (60 meV)1]. It has been extensively studied for various applications, suchs UV absorbers, optoelectronics, and field-emission devices [2–4].esides the capabilities of ZnO for the above potential applicationelds, its competence as chemical sensor has undoubtedly beenroved for detection of various oxidizing and reducing gases [5–9].

It is well known that the sensing performance of the gas sensorsan be enhanced by adjustment of the microstructure, doping ofopant or using a small amount of noble catalyst, etc. [10–16]. Gen-rally, the sputtering or thermal evaporation are used to deposit aatalyst layer on surface of the sensing materials, and the dopantith a certain concentration can be achieved using wet chemical

oute [5,15]. Although it is proved that the nanocrystalline ZnO isne of the most promising metal oxides for gas sensors due to thenique conductance characteristics and large surface to volumeatio, their sensing performances can also be improved dramat-cally by the synergistic effects of the catalyst or dopant on the

ure nanocrystalline ZnO. Recent demonstrations of gas sensorsased on nanocrystalline ZnO have stimulated substantial effortso load the metal-based catalysts, such as Au, Pt, and Pd, for highas sensitivity [17–19]. Among semiconductor oxides, TiO2, a native

∗ Corresponding author. Tel.: +86 431 85168385; fax: +86 431 85168417.E-mail address: zhangtong@jlu.edu.cn (T. Zhang).

925-4005/$ – see front matter © 2009 Published by Elsevier B.V.oi:10.1016/j.snb.2009.03.071

© 2009 Published by Elsevier B.V.

oxygen-deficient metal oxide, with a bandgap of 3.2 eV has beenundoubtedly proved to be an economically moderate dopant andpotential sensing material [20]. However, most of ZnO gas sen-sors focus on ethanol, CO, H2, and O2, rare studies concern thetoluene sensing characteristics, which are usually produced in theprocesses of making gasoline. Although most of the toluene inhala-tion is detoxified by conjugation to glutathione, the remaindermay severely damage health [21]. The toluene sensing performanceof the copolymers and multi-walled carbon nanotubes has beenreported, but the problems are that the sensor devices have notgood selectivity and sensitivity [22,23]. Meanwhile, to the best ofour knowledge, the enhanced sensing performance of the TiO2-doped ZnO nanostructure as toluene sensor is not reported.

Hence, we present a simple and economically attractive routefor the synthesis of the aggregative flowerlike ZnO nanorods viaa facile hydrothermal method on a large scale. Thus, to improvethe toluene sensing performance, the evaporation method is per-formed for Ti doped on the ZnO nanorods. Improved gas sensitivityand selectivity of the doped flowerlike ZnO nanorods towardstoluene are achieved. The effects of TiO2 doping on the responseand response-recovery time at different operating temperature andgas concentration towards toluene are also investigated.

2. Experimental

2.1. Preparation and characterization of sensing materials

All the chemicals, purchased from Beijing Chemicals Co. Ltd.,were analytic grade reagents and used without further purifica-

74 Y. Zeng et al. / Sensors and Actuators B 140 (2009) 73–78

tiskwvalwua6e

pR�Smreb

2

wbfiitTiFr

sdttwr

3

3

oftc

Fig. 1. The schematic structure of the gas sensor.

ion. Deionized water with a resistivity of 18.0 M� cm was usedn all experiments. In a typical synthesis process, 40 mL of aqueousolution of zinc acetate dihydrate (Zn(CH3COO)2·2H2O) (0.1 M) wasept with mild magnetic stirring for 10 min. Then, 1 M ammoniaas slowly added dropwise into the above solution under stirring

igorously until resulting in a white solution and pH 9.5. The finalqueous solution was transferred into a 50 mL Teflon-lined stain-ess steel autoclave and sealed tightly. The hydrothermal processas carried out at 95 ◦C for 2 h and subsequently cooled down nat-rally. The precipitates were centrifuged and then washed withbsolute ethyl alcohol and deionized water prior to dry in air at0 ◦C. The products were coated with Ti deposited by electron beamvaporation method and subsequently annealed at 500 ◦C for 1 h.

The crystal structure and morphology of the obtained sam-les were characterized by X-ray powder diffraction (XRD,igaku D/max-Ra) with graphite monochromatized and Cu K˛,= 0.15418 nm, field emission scanning electron microscopy (FE-EM, JEOL JSM-6700F, operated at 5 kV), transmission electronicroscopy (TEM, HITACHI H-8100, operated at 200 kV), and high-

esolution TEM (HR-TEM, JEOL JEM-3010, operated at 200 kV). Thenergy dispersive X-ray spectrometry (EDX) result was measuredy the FE-SEM attachment.

.2. Fabrication and measurement of sensor

The present products were mixed with deionized water in aeight ratio of 100:25 to form a paste. The sensors were obtained

y spin-coating the paste onto the ceramic tube to form a sensinglm with a thickness of about 10 �m. A pair of gold electrodes was

nstalled at each end of the ceramic tube before it was coated withhe paste, and each electrode was connected with two Pt wires.hen a Ni–Cr heating wire was inserted into the tube to form anndirect-heated gas sensor. The structure of the sensor is shown inig. 1. The details of the sensor fabrication were similar to thoseeported in other literatures [24].

The electrical properties of the sensor were measured by a RQ-eries intelligent test meter (China). The response of the sensor wasefined as the ratio of the sensor resistance in dry air (Ra) to that inarget gas (Rg), which is measured between 160 and 390 ◦C. The timeaken by the sensor to achieve 90% of the total resistance changeas defined as the response time in the case of adsorption or the

ecovery time in the case of desorption.

. Results and discussion

.1. Structural and morphological characteristics

Fig. 2 shows the XRD pattern of the as-prepared productsbtained after the evaporation process. All of the diffraction peaksrom the sample agree well with the wurtzite structure of ZnO withhe lattice constant of a = 3.249 Å, c = 5.206 Å (JCPDS No. 36-1451). Itan be seen that the diffraction peaks of ZnO sample exhibit a rather

Fig. 2. XRD pattern of the as-prepared products.

stronger (0 0 2) peak compared with the standard card, revealing astrong orientation along c-axis. No diffraction peaks from any otherimpurities are detected.

Fig. 3 shows the FE-SEM and TEM images of the undoped ZnOproducts, which exhibits a rod-assembled structure with flower-like shape. The SAED pattern and HRTEM image taken from a ZnOnanorod are also recorded in the insets of Fig. 3(b). It suggeststhat the growth of ZnO nanorods is single crystalline and pref-erentially along the c-axis corresponding to the [0 0 1] direction.The morphology and structure of the TiO2-doped ZnO products arealso characterized by FE-SEM and TEM, as shown in Fig. 4(a–e),which indicates that there are no obvious morphological distinct-ness between the undoped ZnO and doped ZnO products. As shownin Fig. 4(a), it can be seen clearly that the ZnO samples dispersein the space without any aggregation and exhibit approximatelyuniform morphologies in a large-scale area. The magnified FE-SEMimage shown in Fig. 4(b) indicates that the flowerlike ZnO are com-posed of some taper-like branches consisting of plenty of tightlyaggregative nanorods, which have homogeneous size distribution.At a higher magnification, an individual branch of a single ZnO prod-uct exhibits a typical rod-assembled structure with a rough surface(Fig. 4(c)). The nanorods serving as building blocks are in the flowershape and their diameter are in the range of 40–80 nm. The struc-ture of the ZnO products is further investigated by TEM. Fig. 4(d)shows a typical TEM image of one ZnO microstructure, which is ingood agreement with the FE-SEM results. The enlarged TEM imageshown in Fig. 4(e) indicates that the surface of TiO2-doped ZnOnanorod is very coarse. The surface composition of the product isfurther identified by EDX measurement. The EDX result shown inFig. 4(f) demonstrates that the peaks of Zn, O and Ti can be clearlyseen in the survey spectrum.

3.2. Toluene sensing properties

Fig. 5 shows the response of undoped and TiO2-doped ZnO as afunction of the operating temperature upon exposure to 100 ppmtoluene and ethanol, respectively. It can be seen from the figure that,for undoped ZnO to toluene, the response increases slowly withincreasing the operating temperature from 170 to 370 ◦C. Above370 ◦C, the response increases rapidly, which can be attributed

to the facts that the thermal energy obtained is high enough toovercome the activation energy barrier to the reaction and a sig-nificant increase in the electron concentration results in the highresponse [25]. The maximum value of response is 7.4 at 390 ◦C nearthe detection limit. On the other hand, for TiO2-doped ZnO, the

Y. Zeng et al. / Sensors and Actuators B 140 (2009) 73–78 75

Fp

Fe

ig. 3. Morphological and structural characterizations of the undoped ZnO products: (a) Fattern and HRTEM image of a single nanorod, respectively.

ig. 4. (a–c) Different magnification FE-SEM images, (d and e) different magnification Tnlarged TEM image of one part of an individual ZnO product.

E-SEM and (b) TEM images. The upper right and lower left insets of (b) are the SAED

EM images, and (f) EDX result of TiO2-doped ZnO nanorods. The inset of (d) is an

76 Y. Zeng et al. / Sensors and Actuators B 140 (2009) 73–78

Fp

riortutslt1murt

ZvCo

Fa

suggests that the TiO2-doped ZnO nanorods are more favourable to

ig. 5. Response of TiO2-doped and undoped ZnO products versus operating tem-erature to 100 ppm toluene and ethanol.

esponse does not exhibit significant difference below the operat-ng temperature of 230 ◦C. Then a rapid increase in the response isbserved as the operating temperature increases to 290 ◦C, and theesponse reaches the maximum value is 17.1, which is almost eightimes higher than that of the obtained response (about 2.0) of thendoped ZnO achieved at the same temperature. The response forhe doped ZnO above the operating temperature of 290 ◦C reducesignificantly when compared with that of the undoped ZnO. Simi-ar enhancing effect is found for ethanol. At 250 ◦C, the response ofhe doped ZnO to ethanol is found to reach the maximum value of9.2, which shows an enhanced response and a shift of the responseaximum towards lower temperature. This quality is potentially

seful for an improvement of gas sensor selectivity. The possibleeason is attributed to the TiO2 doping, which efficiently activateshe dissociation of molecular oxygen at lower temperature.

Fig. 6 exhibits the histogram showing the response of the doped

nO and pure ZnO sensor at 290 ◦C to 500 ppm of various gasapours, including toluene (C7H8), C2H5OH, NO, CH4, H2S, H2, andO. The results imply that the TiO2-doped ZnO sensor exhibits obvi-us response for toluene, and less effect for C2H5OH, NO, CH4, H2S,

ig. 6. Response of doped and undoped ZnO versus 500 ppm of various gas vapourst 290 ◦C.

Fig. 7. Response of TiO2-doped ZnO at 290 ◦C and undoped ZnO at different temper-ature (290 and 390 ◦C) versus toluene concentrations.

H2, and CO. Therefore, it is concluded that the effect of TiO2 dopingcan improve the gas response and selectivity to toluene of the ZnOsensor.

Fig. 7 shows the response of TiO2-doped ZnO nanorods at 290 ◦Cand pure ZnO nanorods at different temperature (290 and 390 ◦C)versus the toluene concentration, respectively. Similarly to pureZnO nanorods, the response of doped ZnO nanorods increases withincreasing the toluene concentration. The response does not exhibitsignificant difference for the two ZnO products when the tolueneconcentration is low. As the toluene concentration increases, theincrease of the response of doped ZnO nanorods is faster. Above3000 ppm, for doped ZnO, the response increases slowly withincreasing the gas concentration, which indicates that the sensorbecomes saturated gradually. Finally, the response reaches satura-tion at about 4000 ppm. The TiO2-doped ZnO sensor shows quiteenhanced response to toluene with 23.3 times higher than that ofthe pure ZnO at 290 ◦C. The inset of Fig. 7 shows that the increasein the response depends near linearly on the gas concentration inthe range from 1 to 200 ppm for the TiO2-doped ZnO products. This

detect toluene with low concentration.The dynamic response of the doped ZnO and undoped ZnO

nanorods at the operating temperature of 290 and 390 ◦C to 50 ppmtoluene are shown in Fig. 8, respectively. When the toluene is intro-

Fig. 8. Response of the TiO2-doped ZnO and undoped ZnO to 50 ppm toluene at theoperating temperature of 290 and 390 ◦C, respectively.

Y. Zeng et al. / Sensors and Actua

F2

ddaati

aod1Wwtrtraffa

do

ig. 9. Response of the doped ZnO to 100 ppm toluene at operating temperature of70, 290, and 310 ◦C, respectively.

uced, the response increases with the time, especially for theoped ZnO at 290 ◦C with the response of 10.9. The response timend recovery time are found to decrease with increasing the oper-ting temperature for all the ZnO products. It should also be notedhat the sensor resistivity returns to its initial value after the sensors exposed to the atmospheric air.

Fig. 9 shows the dynamic variation of the response of doped ZnOt different various temperatures to 100 ppm toluene for a periodf time. In these measurements, the response curves are obviouslyifferent at different operating temperature. The responses are 10.3,7.1 and 7.6 at 270, 290 and 310 ◦C to 100 ppm toluene, respectively.

hen the toluene is introduced, the response increases rapidlyith the increase of the operation time, especially at the operating

emperature of 290 ◦C. When the toluene is turned-off, the longestecovery time of about 25 s is obtained at the operating tempera-ure of 270 ◦C. The recovery of the resistance when the toluene isemoved is determined by both the oxygen re-adsorption from thembient to the surface and re-oxidation of the oxide [26]. There-ore, 290 ◦C is believed to be the optimum operating temperatureor high response and rapid sensing behavior, which is applied to

ll the investigations hereinafter.

Fig. 10 shows the response of doped ZnO gas sensor exposed toifferent concentrations of toluene at the operating temperaturef 290 ◦C. It is clear that the response and recovery characteristics

Fig. 10. Response of doped ZnO versus toluene concentrations at 290 ◦C.

tors B 140 (2009) 73–78 77

are almost reproducible as well as the quick response and recoverytime. The response time and recovery time are 8 and 20 s, respec-tively. The response is about 1.9, 2.7, 4.3, 5.8, 10.9, and 17.1 to 1,5, 10, 20, 50 and 100 ppm toluene, respectively. It is worth notingthat the doped ZnO shows an obvious response even to tolueneconcentration as low as 1 ppm.

Semiconductor gas sensors are based on the conductivitychanges of the semiconducting materials upon interaction with tar-get gas molecules [27]. It is well known that when ZnO nanorodsare exposed to air, oxygen molecules can be adsorbed on the sur-face of the ZnO sample and form O2

−, O−, O2− ions by capturingelectrons from the conduction band, which in turn produces anelectron-depleted space-charge layer in the surface region of thenanorod and results in a higher resistance [28]. The interactionbetween these adsorbed oxygen species and the target gas reducesthe concentration of oxygen species on the nanorods surface andincreases the electron concentration. The interactions result in theresistance variety and response, which could be related to the targetgas species, the amount of adsorbed oxygen, the chemical compo-sition of the semiconductor, and the operating temperature [29].As the toluene is introduced, the ZnO nanorods is exposed to thetraces of reductive gas. The toluene molecules will react with theadsorbed oxygen species on the ZnO surface and release the trappedelectrons back to the conduction band, which is depicted in Eq. (1).

C7H8 + 2O− ↔ H2O + C7H6–O + 2e− (1)

For doped ZnO, the increase in response at the reduced oper-ating temperature of 290 ◦C can be attributed to the increase ofthe oxygen ions quantity and the decrease of the activation energybarrier to allow more gas molecules to react with oxygen species,resulting in the improved increase of electron concentration. WhenTiO2 is present on the surface of ZnO nanostructures, an electronsaccumulation is formed around this region, resulting in the forma-tion of a deeper electron-depleted space-charge layer to increasethe adsorption of oxygen species. The response of the TiO2-dopedZnO nanorods is distinctly higher than that of the undoped ZnO,while the operating temperature sharply decreases by about 100 ◦C.Thus, the TiO2-doped flowerlike ZnO nanorods are very promisingfor fabricating low power consumption sensors, and TiO2 dop-ing is a very efficient method for exploring synthesized route andimproving sensing characteristics of other semiconducting oxidegas sensors.

4. Conclusions

This paper describes a simple wet chemical synthesis route andsubsequent evaporation process for fabricating flowerlike shapeof TiO2-doped ZnO nanostructures. The effects of TiO2 dopingon the toluene sensing properties of flowerlike ZnO productshave been investigated. The results reveal that the sensor exhibitsmuch higher response, faster response and recovery than that ofundoped ZnO products, as well as the low operating temperature.A response of 17.1–100 ppm toluene is obtained with the responsetime shorter than 10 s at 290 ◦C. Our results indicate that TiO2doping can significantly improve the toluene sensing propertiesof flowerlike ZnO, which has excellent potential applications asgas sensors.

Acknowledgements

This research was financially supported by Science and Tech-nology Office, Jilin Province, China (grant number 2006528) andthe Open Project of Key Laboratory of Low Dimensional Materials& Application Technology (Xiangtan University), Ministry of Edu-cation, China (grant number KF0706).

7 Actua

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

8 Y. Zeng et al. / Sensors and

eferences

[1] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D.Yang, Room-temperature ultraviolet nanowire nanolasers, Science 292 (2001)1897–1899.

[2] P.X. Gao, Z.L. Wang, Mesoporous polyhedral cages and shells formed by texturedself-assembly of ZnO nanocrystals, J. Am. Chem. Soc. 125 (2003) 11299–11305.

[3] X.D. Wang, C.J. Summers, Z.L. Wang, Large-scale hexagonal-patterned growthof aligned ZnO nanorods for nano-optoelectronics and nanosensor arrays, NanoLett. 4 (2004) 423–426.

[4] D.C. Reynolds, D.C. Look, B. Jogai, Optically pumped ultraviolet lasing from ZnO,Solid State Commun. 99 (1996) 873–875.

[5] A.P. Chatterjee, P. Mitra, A.K. Mukhopadhyay, Chemically deposited zinc oxidethin film gas sensor, J. Mater. Sci. 34 (1999) 4225–4231.

[6] H.W. Ryu, B.S. Park, A.A. Sheikh, W.S. Lee, K.J. Hong, Y.J. Seo, D.C. Shin, J.S. Park,G.P. Choi, ZnO sol–gel derived porous film for CO gas sensing, Sens. Actuators B96 (2003) 717–722.

[7] Á. Németh, E. Horváth, Z. Lábadi, L. Fedák, I. Bársony, Single step depositionof different morphology ZnO gas sensing films, Sens. Actuators B 127 (2007)157–160.

[8] S.T. Shishiyanu, T.S. Shishiyanu, O.I. Lupan, Sensing characteristics of tin-dopedZnO thin films as NO2 gas sensor, Sens. Actuators B 107 (2005) 379–386.

[9] G. Sberveglieri, S. Groppelli, P. Nelli, F. Quaranta, A. Valentini, L. Vasanelli, Oxy-gen gas-sensing characteristics for ZnO(Li) sputtered thin films, Sens. ActuatorsB 7 (1992) 747–751.

10] X. Cao, W. Ning, L.D. Li, L. Guo, Synthesis and characterization of waxberry-likemicrostructures ZnO for biosensors, Sens. Actuators B 129 (2008) 268–273.

11] P. Feng, Q. Wan, T.H. Wang, Contact-controlled sensing properties of flowerlikeZnO nanostructures, Appl. Phys. Lett. 87 (2005) 213111.

12] H. Gong, J.Q. Hu, J.H. Wang, C.H. Ong, F.R. Zhu, Nano-crystalline Cu-doped ZnOthin film gas sensor for CO, Sens. Actuators B 115 (2006) 247–251.

13] V.R. Shinde, T.P. Gujar, C.D. Lokhande, Enhanced response of porous ZnOnanobeads towards LPG: effect of Pd sensitization, Sens. Actuators B 123 (2007)701–706.

14] C.C. Li, L.M. Li, Z.F. Du, H.C. Yu, Y.Y. Xiang, Y. Li, Y. Cai, T.H. Wang, Rapid andultrahigh ethanol sensing based on Au-coated ZnO nanorods, Nanotechnology19 (2008) 035501.

15] P. Mitra, H.S. Maiti, A wet-chemical process to form palladium oxide sensitiserlayer on thin film zinc oxide based LPG sensor, Sens. Actuators B 97 (2004)49–58.

16] P.T. Moseley, New trends and future prospects of thick- and thin-film gas sen-sors, Sens. Actuators B 3 (1991) 167–174.

17] J. Kong, M.G. Chapline, H. Dai, Functionalized carbon nanotubes for molecularhydrogen sensors, Adv. Mater. 13 (2001) 1384–1386.

18] L. Madler, A. Roessler, S.E. Pratsinis, T. Sahm, A. Gurlo, N. Barsan, U. Weimar,Direct formation of highly porous gas-sensing films by in situ thermophoreticdeposition of flame-made Pt/SnO2 nanoparticles, Sens. Actuators B 114 (2006)283–295.

19] U.S. Choi, G. Sakai, K. Shimanoe, N. Yamazoe, Sensing properties of Au-loadedSnO2–Co3O4 composites to CO and H2, Sens. Actuators B 107 (2005) 397–401.

20] C. Garzella, E. Comini, E. Bontempi, L.E. Depero, C. Frigeri, G. Sberveglieri, Sol–gelTiO2 and W/TiO2 nanostructured thin films for control drunken driving, Sens.Actuators B 83 (2002) 230–237.

21] R. van Doorn, C. Leijdekkers, R. Bos, R. Brouns, P. Henderson, Alcohol and sul-phate intermediates in the metabolism of toluene and xylenes to mercapturicacids, J. Appl. Toxicol. 1 (1981) 236–242.

22] M. Matsuguchi, T. Uno, T. Aoki, M. Yoshida, Chemically modified copolymercoatings for mass-sensitive toluene vapor sensors, Sens. Actuators B 131 (2008)652–659.

tors B 140 (2009) 73–78

23] M. Penza, G. Gassano, P. Aversa, F. Antolini, A. Cusano, A. Cutolo, M. Giordano,L. Nicolais, Alcohol detection using carbon nanotubes acoustic and optical sen-sors, Appl. Phys. Lett. 85 (2004) 2379–2381.

24] J.P. Ge, J. Wang, H.X. Zhang, X. Wang, Q. Peng, Y.D. Li, High ethanol sensitiveSnO2 microspheres, Sens. Actuators B 113 (2006) 937–943.

25] V.R. Shinde, T.P. Gujar, C.D. Lokhande, LPG sensing properties of ZnO films pre-pared by spray pyrolysis method: effect of molarity of precursor solution, Sens.Actuators B 120 (2007) 551–559.

26] J.F. Chang, H.H. Kuo, I.C. Leu, M.H. Hon, The effects of thickness and operationtemperature on ZnO: Al thin film CO gas sensor, Sens. Actuators B 84 (2002)258–264.

27] J. Cerda Belmonte, J. Manzano, J. Arbiol, A. Cirera, J. Puigcorbe, A. Vila, N. Sabate,I. Gracia, C. Can’e, J.R. Morante, Micromachined twin gas sensor for CO and O2

quantification based on catalytically modified nano-SnO2, Sens. Actuators B 114(2006) 881–892.

28] N.J. Dayan, S.R. Sainkar, R.N. Karekar, R.C. Aiyer, Formulation and charac-terization of ZnO:Sb thick-film gas sensors, Thin Solid Films 325 (1998)254–258.

29] K. Arshak, I. Gaidan, Development of a novel gas sensor based on oxide thickfilms, Mater. Sci. Eng. B 118 (2005) 44–49.

Biographies

Yi Zeng received his MS degree from National Laboratory of Superhard Materi-als, Jilin University, China in 2007. He entered the PhD course in 2007, majored inmicroelectronics and solid-state electronics. Now, he is engaged in the synthesis andcharacterization of the semiconducting functional materials and gas sensors.

Tong Zhang received her MS degree in major of semiconductor materials in 1992,and PhD degree in the field of microelectronics and solid-state electronics in 2001from Jilin University. She was appointed as a full-time professor in college of Elec-tronics Science and Engineering, Jilin University in 2001. Now, she is interested inthe field of sensing functional materials, gas sensors and humidity sensors.

Lijie Wang received her BS and MS degree from the College of Electronics Scienceand Engineering, Jilin University, China in 2002 and 2005, respectively. She enteredthe PhD course in 2006, majored in microelectronics and solid-state electronics, andengaged in synthesis mesoporous materials and IC design.

Minghui Kang received her BS degree from the College of Electronics Science andEngineering, Jilin University, China in 2007. He entered the MS course in 2007,majored in microelectronics and solid-state electronics, and engaged in novel sens-ing materials and gas sensors.

Huitao Fan received her BS and MS degree from the College of Electronics Scienceand Engineering, Jilin University, China in 2006 and 2008. She entered the PhD coursein 2008, majored in microelectronics and solid-state electronics, and engaged innovel sensing materials and gas sensors.

Rui Wang received her BS and MS degrees from the College of Electronics Scienceand Engineering, Jilin University, China in 2005 and 2007, respectively. She entered

the PhD course in 2007, majored in microelectronics and solid-state electronics, andengaged in novel sensing materials and humidity sensors.

Yuan He received her BS and MS degrees from the College of Electronics Science andEngineering, Jilin University, China in 2006 and 2008. Presently, she is a graduatestudent, majored in microelectronics and solid-state electronics.