Preparation and Ethanol Sensing Properties of ZnO Nanofibers

CHEM. RES CHINESE UNTVERSITIES 2011, 27(5), 720-723Preparation and Ethanol Sensing Properties ofZno NanofibersFENG Cai-hui, RUAN Sheng-ping, ZHU Ling-hui, LI Chao, LI Wei, CHEN Wei-you,CHEN Li-hua and zhang Xin-dongState Key laboratory on integrated Optoelectronics, College of Electronic Science and EngineeringJilin University, Changchun 130012, P R ChinaAbstract Zno nanofibers with an average diameter of about 90 nm were prepared by an electrospinning methodcombined with a calcination process. The as-electrospun nanofibers before and after calcination were characterizeby means of differential thermal analysis(DTA), thermal gravimetric analysis(TGA), X-ray diffraction(XRD) andscanning electron microscopy(SEM). The fibers after calcination at 600C belong to the hexagonal wurtzite structureThe sensor based on Zno nanofibers exhibited excellent ethanol sensing properties at 206C such as good linear dependence in the low concentration(1-100 HL/L), high response, and good selectivity. Fast response(less than 2 s)and recovery(about 16 s)were also observed in our investigationsKeywords Electrospinning method; ZnO; Nanofiber; Ethanol; Gas sensorArticle ID1005-9040(2011)-05-720-041 Introductionare exceptionally long in length, uniform in diameter, and di-verse in compositionSemiconductor metal oxide zinc oxide(ZnO), as an im-ZnO nanofibers were prepared by an electrospinning me-portant functional material, has been widely applied in theod. The Zno nanofibers based sensors were also fabricatedfields of gas sensor",photocatalysis, solar energy )and Uv that showed excellent ethanol-sensing characteristics, whichphotodetection!4I and so on due to its superior chemical and made our product a good candidate for fabricating ethanol sen-thermal stability, wide band-gap(3.37 ev)and large exciton sors in practicebinding energy at room temperature(60 mev)5-7. Recentlyone dimensional(I D) ZnO such as nanowires, nanobelts, nano- 2 Experimentalfibers, nanosprings and nanotubes and so on has arisen an ex-tensive focus, especially in the gas sensing field 8-10). Com- 2.1 Material Preparationpared with the bulk and zero dimensional nanostructure ZnoAll the chemicals(A. R. grade) were purchased from Bei-ID Zno is a strong candidates for gas sensing because its large jing Chemicals Co, Ltd(China)and used as received withoutsurface-to-volume ratio and the congruence of the carrier further purification. To prepare the ZnO/PVP composite nanofibers, a certain amount of Zn(CH3 COO): 2H o powder washighly sensitive and efficient transducers of surface chemical dissolved in 8. 8 g of mixed solution containing N, N-dimethyl-processes into electrical signals A large surface-to-volumeformamide (DMF)ethanol(EtoH)with the mass ratio of 1: 1 andratio means that a significant fraction of the atoms(or molecules) stirred for 2 h. Then 0.8 g of polyvinylpyrrolidone(PVP)wasin such systems is surface atoms that can participate in surfacereactions.Till now, many ID nanostructures Zno basedadded to the above solution with further stirring for 6 h. theobtained solution was then loaded into a plastic syringe andsensors have been developed and widely used in various fields connected to a high-voltage power supply. A voltage of 20 kVsuch as span environmental monitoring, industrial process con-trol, medical diagnosis, combustible and toxic chemical detec-tion[ 13)anode(syringe)at a distance of 25 cm. Finally the Zno nanofi-bers were obtained by calcining the as-electrospun compositeTo date, ID nanostructure Zno has been synthesized bynanofibers in air at 600oC for 4 h.various methods such as template-assisted!4), thermal evapora-tion>, hydrothermal synthesis, electrospinning!" and so 2.2 Characterizationforth. Among these methods, electrospinning offers a simpleand versatile technique for generating ID nanostructures thatThe as-electrospun composite nanofibers before and after*Corresponding author. E-mail: xindong@jlu.edu. cr中国煤化工Received June 22, 2011; accepted August 1, 2011Supported by the National High-Tech Research and Development ProgranCNMHO02),the NationalNaturalce Foundation of China(Nos. 60977031, 50977038)and the Doctoral Fund of Ministry of Education of ChinaNo200900610040)FENG Cai-hui et alcalcination were characterized by means of a differential ther- tions 6. according to the TG-DTa analyses above, 600C wasmal analyzer(DTA) and a thermal gravimetric analyzer(tga) chosen as the suitable sintering temperature(Melttler Toledo 825); an X-ray diffractometer(XRD)10(Shimadzu XRD-6000, Cu Ka radiation); a scanning electron100microscopy(sEM, XL30 ESEM FEG)The gas sensing properties of the sensor were measured ora CGS-8( Chemical gas sensor-8)intelligent gas sensing analy-60sis system(Beijing Elite Tech Co, Ltd, China)5.The responseis defined as Response R/Rg, where Ra and Rg are the resis-tances of the sensor in air and in the target gases, respectivelyig. I shows a schematic image of the as-fabricated sensor anda photograph of the sensor on a socket, respectively.02004006008001000ZnO nanofiberAu clectrodcig 2 TG-DTA curves of Zno/PVP composite nanofibersCeramic tubePrecursors were heated up to 900C in air at a scan rate of 10C/min.The XRD pattern of Zno nanofibers after calcination atNi-Cr hot wire600C for 4 h is shown in Fig 3. It can be seen that all the dif-fraction peaks correspond to the hexagonal wurtzite zincPt lcad wireoxide(JCPDS No 36-1451)7. No other diffraction peaks weredetected, which indicated the sample had a high purity.Fig 1 Schematic image(A)and photograph of Zno2/(°)nanofibers sensor ( B)Fig 3 XRD pattern of ZnO nanofibersThe SEM images of the as-electrospun composite nanofi2.3 Sensor Fabricationers before and after calcination are shown in Fig 4. As shownin Fig 4(A), the PVP/Zno composite nanofibers, which wereThe as-synthesized sample was mixed with deionized wa-ter in a mass ratio of 100: 25 to form a paste. The paste was thencoated on a ceramic tube to form a sensing film (with a thick-ness of about 300 um) on which a pair of Au electrodes werepreviously printed. Pt lead wires attaching to these electrodeswere used as electrical contacts. After the ceramic tube wascalcined at 300C for 2 h, an Ni-Cr heating wire was insertedin the tube as a heater for controlling the operating temperature3 Results and Discussion3.1 Material CharacterizationTG-DTA curves of ZnO/PVP composite nanofibers areshown in Fig. 2. It is clear from the TGa curve that all the vola-tiles(H,0, ethanol), organic components(PVP, DMF)andCH3Coo groups were removed completely below 556C,which resulted in a metal oxide phase. The first minor massloss below 286C corresponded to the removal of the free sol-vent in the precursors. The significant mass loss of approxi100mately 79.4%(dominant exothermic DTA peak at 458 an中国煤化工520C)between 286 and 556C was attributed to the completeFig 4 SElCNH Gosite sanofi-decomposition of Zn(CH3 COO) and the degradation of PVP,bers lenore(A auu aer(D) calcination in airwhich involved both intra- and inter-molecular transfer reacat600°cfor4h722CHEM. RES CHINESE UNTVERSITIESVol 27collected as randomly oriented structures in the form of non- the sensors reached saturation at above 9000 HL/L. In addition,woven fabrics, had smooth and uniform surfaces. Their diame- thegood linece as shown in the insters were ranging from 150 nm to 350 nm(with an average cated that the Zno nanofibers can be used as promising matediameter about 250 nm) and their lengths can reach several rials for ethanol sensorsmicrons. As shown in Fig 4(B), after calcination at 600C the&&s. The responses of the Zno nanofibers to 100 HL/L differentnanofibers shrank and became bending and rough but main- gases( CrHsOH, C2H2, H2, HCHO, CO and CH4)at 206C aretained the continuous structures and the average diameter of shown in Fig. 7. The results indicate that the ZnO nanofibersthese fibers, which were composed of nanoparticles of 21.3 nm based sensor exhibited a little response to C2H2 and H2, andin diameter(estimated from the XRd peaks using the Scherrer was almost insensitive to other gases. The highest response offormula), was reduced to about 90 nm, indicating the complete the sensor is about 21.2 to ethanol, while the responses to otherremoval of organic molecules and the formation of Zno crygases are not greater than 5. It is obvious that the selectivity oftallinethe sensor to ethanol with respect to those of other gases wasincreased almost by 4 times. This was possiblely because the3.2 Gas Sensitive Properties of Samplesoptimum working temperature of the sensor will shift when thesensor was towards to different gases!]. Thus, we can conThe response of the ZnO nanofiber sensor at different clude that the Zno nanofibers may be promising sensitive ma-working temperatures was investigated at an ethanol concentra-terials for ethanol detectiontion of 100 HL/L. As shown in Fig. 5, the response increasedrapidly as the temperature was raised from 160C to 210CThis is mainly because of the enhanced reaction between theethanol and the absorbed oxygen at an elevated temperatures).At the near-optimal operating temperature of 206'C, the sensorexhibited relatively the highest response 21.2 to 100 HL/Lethanol. So 206C was approximately chosen as the near-optimal operating temperature and used in the following expe-rimentC?H, OH C2H, H2 HCHO CO CHFg.7 Responses of Zno nanofibers to100μ儿Ldifferent gases atThe response and recovery characteristics of the Zno na10nofiber sensor were also investigated. The sensor exhibited ashort response time of less than 2 s and relatively a longer re-covery time of 16The sensing mechanism of Zno nanofiber based gas sen-20160200240sors was based on the conductance changes which is caused bythe adsorption and desorption of the molecules on the surfaceFig 5 Response of Zno nanofibers vs.of the Zno nanofiber. when the Zno nanofiber is surroundedworking temperatureby air, oxygen molecules will be adsorbed on the nanofiberThe responses of Zno nanofibers to different concentra- surface to generate chemisorbed oxygen species(02, 02, andtions of ethanol at 206C are shown in Fig. 6. It can be easily 0)by capturing electrons from the conductance band and thusfound that the response increased rapidly with increasing etha- lowers the conductance of ZnO. When ethanol gas was intro-nol concentration(1-5000 uL/L), and then the increase of the duced,by reacting with the oxygen species on the nanofiberresponse gradually slowed down(5000--9000 HL/L). Finally surface, the reducing gas reduced the concentration of oxygenductance of zno nanofibers/]4 ConclusionsIn summary, Zno nanofibers with an average diameter ofround 90 nm were fabricated and their ethanol sensing proper-ties were also intEthanol(uL. L')2060150030004500600075009000sensitivity, quick response and recovery speed and good selectivity to ethanoldidates in fab中国煤化工fibers good can-Fig. 6 Response of Zno nanofibers to differentconcentrations of ethanol at 206'CCNMHGReferencesconcentration range of 1-1001] Wei S H, Yu Y, Zhou M. H, Mater. Lef, 2010, 64(21), 2284FENG Cai-hui et al723[2]Lin DD, Wu H, Zhang R. 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