Fabrication and gas sensing property of honeycomb-like ZnO

Availableonlineatwww.sciencedirect.CHINESESciencedirectCHEMICALLETTERSELSEVIERChinese Chemical Letters 19(2008)599-603www.elseviercom/locate/ccletFabrication and gas sensing property of honeycomb-like ZnoChao Li K, Zhi Shuo Yu, Shao Ming Fang Huan Xin WangYang Hai Gui, Jia Qiang Xu, Rong Feng Chen 6Henan Province Key Laboratory of Surface Interface Science, Zhengzhou University of Light IndustryZhengzhou 450002, ChinaHenan Academy of Science, Zhengzhou 450002, ChinaReceived 20 December 2007AbstractWe report the structural characterization and proposed formation mechanism of honeycomb-like ZnO conglomerationsfabricated by direct precipitation method. X-ray diffraction(XRD), energy-disperse X-ray spectrometry(EDS), scanning electronmicroscopy(SEM)showed that the as-prepared Zno calcined at 700C were micron sphere particles with honeycomb-likestructure. In the UV-vis absorbing spectrum, it was observed that there is a new additional absorption band at 260 nm, and it waspeculated that the absorption may be caused by defects on the surface and interface of honeycomb-like ZnO. The as-productsshowed high sensitivity and short response time to sulfured hydrogen gas. These results demonstrate that honeycomb-like Znoconglomerations are very promising materials for fabricating H2s gas sensorsC 2008 Chao Li. Published by Elsevier B V. on behalf of Chinese Chemical Society. All rights reservedKeywords: ZnO; Precipitation; Honeycomb-like; Gas sensing propertyAs an important wide band-gap semiconductor, zinc oxide(Zno)has unique applications [1-l1, 15] in catalysts,sensors, piezoelectric transducers and actuators, and photovoltaic and surface acoustic wave devices. Because the sizeand morphology of Zno have great effects on its properties and applications, various size and morphology of Znocrystals such as wires [2, 3], tubes [4, 5], rods [5, 6], belts [7], prisms [8], towers [9], dandelions [10] and combs [11]innano-or microscales have been fabricated by various physical and chemical techniques. These unique morphologiesand structures may have particular applications in nano-or microscale optoelectronic devices, sensors, and networks[2-11, 15]. In this work, honeycomb-like Zno conglomerations were prepared by direct precipitation method for thefirst time and it was found that the sensors based on the samples showed high sensitivity and short response time tosulfured hydrogen H2S gasIn a typical synthesis process, 6 g zinc powder and 923 mL dilute sulfuric acid (0.05 mol)were mixed togetherunder vigorous stiring. The temperature was kept at 60C. After the dilute sulfuric acid was depleted completely, thereaction of zinc and dilute sulfuric acid stopped, and then 230 mL ammonia solution(0. 2 mol/L)was added to theresulting solution drop by drop under continuous stirring. The produced gray-white precipitates were collected andCorresponding author at: Henan Province Key Laboratory of Surface InterfaceYH中国煤化工 ght Industry, ZhengzE-mail address: zzulilc @zzuli edu cn(C. LiCNMHG1001-8417/S-see front matter C 2008 Chao Li. Published by Elsevier B V. on behalf of Chinese Chemical Society. All rights reserveddoi:10.1016 cclet.200803.032C. Li et aL Chinese Chemical Letters 19(2008)599-603washed repeatedly with ethanol and distilled water, and then the product was dried in an oven at 100"C for 10 h. Thefollowing calcination procedure was carried out at 700C for 1 h. All the chemicals were of analytical grade andwithout further purificationFor fabrication of honeycomb-like ZnO, the molar ratio of Zn and H2SO4 was 2 1, in which Zn was excessive. withthe reaction proceeding, the dilute sulfuric acid was depleted completely. In the process, the surface of the excessivezinc powder was partly dissolved and the new-formed surface had relative high activity, which was unstable and easilybonded with other atoms. What's more, the release of hydrogen left small holes on the excessive zinc powder, whichcan serve as nuclei sites and pinning centers sticking the other atoms to the zinc surface. It was noticed that thehoneycomb-like ZnO conglomerations cannot be obtained when Zn was not excessive. After the addition of ammoniasolution,Zn"in solution was transformed to Zn(OH)2 and attached easily on the active surface of excessive zinc thusformed Zn/Zn(OH)2 composite particles. During calcination, Zn(OH)2 decomposed to Zno at the temperature of125C [12], and then Zn/Zn(OH transformed to Zn/Zno composite particle. Since Zn has a low melting point of419C[12], it was believed that honeycomb-like ZnO were formed by the thermal evaporation of the Zn core from zn/Zno particle during the increase of calcination temperature. With the thermal evaporation of Zn core, somesublimation of Zn powders may randomly attach on the ZnO shell and immediately oxidized because they were veryactive, and so, the surface of honeycomb-like Zno conglomerations were be with a variety of irregular pores andsurface defectsThe as-prepared samples were characterized by XRD (D/MAX-BA, Rigaku, using Cu Ka,A=0.15418 nm), SEM(JEM-5600 electron microscopes with EDS), UV-vis absorption spectrum(T6 spectrophotometer, Beijing PurkinGeneral Instrument Co, Ltd ) The typical SEM images of as-synthesized ZnO are given in Fig. 1. By abovesynthesization procedure, honeycomb-like ZnO samples were obtained, Fig. 1(a) shows the typical honeycomb-likeZno conglomeration is a porous micron sphere with a variety of irregular pores and surface defects, which isanalogous to real honeycomb. The enlarged images of the surface of the honeycomb-like sphere are shown in Fig. 1(b)and(c). Careful observation can reveal that the bore diameters rang from hundreds of nanometers to several tensnanometersThe XRD pattern of the ZnO sample is shown in Fig. 2. The diffraction peak positions in the XRD spectrum of theproducts can be indexed to a pure hexagonal Zno wurtzite structure with lattice constants a=3. 24982(9)A,c=5.20661(15)A(JCPDS No 36-1451). No peak due to other phases is detected, indicating Zn(OH)2 has completelythermal decomposited. Similar result can also be obtained by EDS patternFig 3 shows the UV-vis absorption spectrum of obtained ZnO product. The measured onset of the absorption curvegives a value at about 373 nm(3. 26 V), which is close to the band-gap of Zno 1s-1s electron transition(3.37 ev)Meanwhile, it was observed that there is also a weak absorption band at 260 nm in the spectrum. The appearance ofunusual additional absorption bands of Zno has not been reported to our knowledge. Many authors [13, 14] havesystematically investigated additional optical absorption in single crystals of a-Al2O3. According to their results, it isreasonable to assume, that the new additional absorption band at 260 nm in the spectrum may be caused by the colorcenter with high concentration converted by numerous defects, such as vacancy sites, cavities and impurities in thelarge surface of the as-products(a)(c中国煤化工CNMHFig. l. The typical SEM images of the as-synthesized znoC Li et aL /Chinese Chemical Letters 19(2008)599-6030ofFig 3. The UV-vis spectrum of the as-synthesized ZnOWe selected four kinds of reducing gases as a detecting gas to characterize the gas sensing properties of samplesThe four reducing gases include H2S, CH4, CO and H2, whose concentration are all 50 ppm. The test was operated inameasuring system of HW-30A( Hanwei Electronics Co Ltd, Henan, PR China). The gas sensitivity was measured instatic state. The basic fabrication process of gas sensors based on as-prepared samples is as literature [1]. The circuitvoltage was 10 V, and outputs Vout were the terminal voltage of the load resistor. The working temperature of a sensorwas adjusted through varying the heating voltage. The resistance of a sensor in air or test gas was measured bymonitoring Vout. The gas sensitivity(response magnitude)in this paper was defined as S= Ra/rg, where Ra and rgwere the resistances of a sensor in air and in a test gas, respectively. The response time was defined as the time requiredfor the variation in conductance to reach 90% of the equilibriumColumn chart in Fig 4 shows the comparison of sensitivity of the sensor to four reducing gases at 300C. We cansee that the resistance response is 26 to H2S gas, much higher than other three gases. Hence it can be determined thatthe material would have a good interface resistance property as a Hrs detection. It is well known that the sensingmechanism of ZnO belongs to the surface-controlled type [1]. When Zno was contacted with H2s gas, the strongreducing gas may react with O2-easily and put back the electrons中国煤化工 reby the resistance ofZno would decrease. Its gas sensitivity is relative to grain size,CNMHGantity, active energyof oxygen adsorption and lattice defects. Compared to the sensu vastu ou uva puils Luo prepared by directprecipitation, whose sensitivity values were usually less than 15, the honeycomb-like ZnO sensors exhibited a highersensitivity. It is confirmed that the surface activity of the semiconductor material was enhanced as the surface area isC Li et al. /Chinese Chemical Letters 19(2008)599-603Fig. 4. Sensitivity of the as-synthesized Zno curve to different gases of sensor.050100150200250Time(s)Fig. 5. Typical response and reversion of ZnO sensor to H2sincreased [1, 15]. Fig. 5 shows the sensing and recovery test of Zno for Hs carried out under the conditions:Vn=5.5V, R=4.7 kn, relative humidity 48% at room temperature 18C. From this curve, we can see thatresponse time is 12 s, quick enough to satisfy users'requirementAcknowledgmentsThe financial support from the National Natural Science Foundation of China (No. 20771095)and He'nanOutstanding Youth Science Fund(No. 0612002700)is gratefully acknowledgedReferences[1]JQ. Xu, Y P. Chen, D Y. Chen, et al. Sens. Actuators B 113(2006)526[2] M H. Huang, S. Mao, H. Feick, et al. Science 292(2001)(189[3]Q. Xiang, Q.Y. Pan, J.Q. Xu, et al. Chin J. Inorg. 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