Gas-sensing properties of nanocomplex metal oxide

PROGRESS IN NATURAL SCIENCEVo.15,No.7,Juy2005Gas-sensing properties of nanocomplex metal oxideCHEN Liangyuan, HE Zhusheng, YAN Tao, BAI ShouliCHEN Aifand liu Chung Ch1, Electronics Design Center, Case Western Reserve University, Cleveland, OH 44106, USA: 2. Department of Chemistry and Chemical Engineering, Baoji University of Arts and Science, Baoji 721007, China: 3. College of Science, Beijing University of Chemical Technology, Beijing 100029, ChinaReceived October 9, 2004Abstract The development of CuO/Ceo nanocomposites as the sensing material of semiconductor gas sensors is reporteCuO/CeO, nanocomplex oxide is prepared by modified sol-gel method that uses copper nitrate, cerium nitrate and ethylene glycol as precursors. The optimized synthesis parameters and processing condition have been established. The particle size and distribution, phase morphology, specific surface, electronic states of atoms and gas sensing properties have been systematically characterized by TransmissionElectron Microscope(TEM), Brunauer Emmett Teller(BET), X-ray diffraction(XRD),X-ray Photoelectron Spectroscopy(XPS)angas sensitivity measurement. The results show that the sensor sensitivity depends on particle size, Cu/Ce cation ratio and calcination tem-perature. The effects of calcinations temperature and Cuo loading on the gas sensitivity are also examined. The optimum calcination temperature and the Cuo content for the highest sensitivity are 600C and 12%, respectively. The combination of excellent thermal stabilityand tunable sensing properties through careful control of the processing parameters and selection of material composition gives rise to novelnanocomposites attractive to the sensitive and selective detection of a variety of toxic and combustible gasesKeywords: copper oxide, cerium oxide, nanocomposite, CO, gas sensitivitGas sensors based on semiconducting metal oxide and surface adsorption activity. Various experimentalare widely used for industrial usage and environmental techniques have been employed to characterize themonitoring because of their advantages of simple granularity, distribution, specific surface, phasestructure, higher sensitivity and low cost. The sensor structure and surface electronic states. The effects ocharacteristics are greatly affected by the grain size of the preparation technique, calcination temperaturethe metal oxide. Nanoparticles have attracted a great Cuo content and operating temperature on the surfacedeal of research attention due to their potential use as area and gas sensitivity were also studied. Thecatalysts, sensors, ceramic and biomaterial, etc. LI. catalytic and adsorption properties of semiconductorUnfortunately, sensor fabrication often requires calci- sensor surface also have a significant influence on theirnation temperature 2600. Even if nanocrystals of a sensitivity and selectivity by temperature-programmedsingle-component metal oxide are used as the sensing desorption studies and X-ray photoelectron spectro-material,they will undergo significant grain growth scopic analysisfabrication, greatly compromising thesensitivity of the resulting device. This drawback Experimentpresents challenges that would require better design of1.1 Preparation of nanocompositessensing materials through manipulating the Cu/Cecation ratio and solution pH value, nano- crystallinepreparation of composite oxide was mainlycomposites powder derived with finer grain sizes and based on precipitation method and dippingsuperior thermal stability compared to the single com- method, 33. The nanostructured CuO/CeO2 compoponent nanocrystalline oxide. We should consider not itesd b, mad fied sol-gel method 4]inmpositionure of theanocomplex materials in terms of providing superior actedGYH中国煤化工 matrix in cuo/CO2CNMH Gr the dispersion of thethermal stability, but also the electronic and chemical Cuo particles and had the predominance that thesynergism effects of the different oxide components chance of introducing impurity was few and the partiSupported by National Natural Science Foundation of China( Grant No. 20377004) and the Natural Science Foundation of Beijing( grant Nos2032014an数据关并 To whonpendenceshouldbeaddressedE-mail:chenaf@mailbuct.edu.cn670www.tandf.co.uk/journalsProgressinNaturalScienceVol.15No.72005cle's distribution was narrow. The molar ratio of 2 Results and discussioncopper oxide to cerium oxide equaled 5%,12%15%,and 20%. The ethylene glycol solution of cop- 2. 1 Characterization of granularityper nitrate and cerium nitrate was mixed in a beaker,and after 1 h of stirring, the ph of the mixture wasIt is well recognized that the sensor characterisadjusted to a value by adding nitric acid. The trans- tics are greatly affected by the grain size and distribuparent sol was poured into a teflon beaker and allowed tion of nanocomposites. The main challenge in their. Then the gel was hydrolyzed. The gel synthesis of nanoparticles is the attainment of well-was dried in vacuum oven by slowly raising the tem- defined particle size and morphology The TEM imperature to 95C for 4 h. The xerogel was calcined age of CuO/CeO, nanocrystal is shown in Fig.2and kept for 2 h at 400C, 600C, 800C, and 1000 which shows very small spherelike crystallinesC. The resulting nanoparticles were successfully sup- about 15 nm in diameter. CuO/CeO crystal, s grantpressed in grain growth, yielding materials with ex- larity distribution was analyzed, mainly within 20ceptional sintering resistancnm-50 nm. No particle aggregation was observed1.2 Choice of binder and sensitivity measurementThe choice of binder is an important factor forthe fabrication and sensitivity of sensor. Some binderssuch as pva, teflon and latex were introduced in literature[5 which would easily decompose at elevatedtemperature, so the composite will break off from thesensingWe investigated a novel ibinder that comprises adhesive, solidifying reagentand stuffing bond, with good stickability and less so-100nmlidifying contractibility in the process of fabricatedgas-sensing element. For gas sensitivity studies, theFig. 2. The TEM photograph of Cuo/CeO, crystalcalcined nanocomposite powders were coated on theelectrical lead with the binder, the element wasmounted in a quartz tube, which was heated to 50- 2.2 Characterization of the specific surface300C in a furnace. The schematic of the measurement setup is shown in Fig. 1. Prior to sensitivityThe surface area of composites powders was detesting, the element was heated to 300 C, target gas termined from nitrogen adsorption analysis by theand air flows were then introduced by flow conBrunauer emmett teller bet)method. The bettrollers. A constant current was applied to the electrispecific surface areas of various composition nanocomcal lead and the gas concentration was varied. theposites calcined to 600 C and to different temperaresistance was measured in air (R.) and intures are presented in Fig. 3. The results show thatthe presence of the target gas in air (Rg), the sensi- the bet specific surface areas are not only relatetivity was presented by R, /Rwith Cuo content but also related with the calcinationtemperature. The specific surface decreases when theIcination temperature is higher than 800C due to2 Gas outlet3 Sensor elementthe particles congregated, but when the calcination62K1 sUpply voltagetemperature is lower than 600 C, composite samples5 Voltage dropwill not form the crvstal, and the specific surface willacross sensorredul中国煤化工 that thesistanceCNMHG Cu has the highestsurface area of 61 m/g. The secondary component(Ceo,) acted as a diffusion barrier for the advancinggrain boundaries of the CuO component, and effectively prevented grain growth and loss in surface areaFig,1 ic of the gas-sensing measurement setupin the resulting nanocomposites 6]ProgressinNaturalScienceVol.15No.72005www.tandf.co.uk/journals6711)5%Cuo/CeO(4)Ceo,Cuo(%)400℃600℃Fig. 4. XRD patterns of different CuO content composites calat300℃0.8&()uO(%)Fig. 3. Specific area of the composites with different CuO contents calcined at 600C (a) and at different temperatures(b)26(°2.3 Characterization of phase statesFig. 5. XRD patterns of 12% Cuo composite calcined at differenttemperature.In addition to enhancing surface area and suppressing grain growth, CeO incorporation affected2. 4 Effects of calcination temperature on sensitivitythe phase stabilities of the resulting nanocompositesPhase identification was achieved using powder X-rayThe sensing materials require calcinations at apdiffraction (XRD). The addition of a secondary com- propriate temperature to achieve crystallization andponent also had an effect on the phase composition of structural evolution. A sufficient degree of crystallinthe Cuo-based nanocompsitety is required to attain desired electronic propertiesnecessary for gas sensor application. The effects ofigure 4 shows a single CeO2 phase. For material composition, calcination temperature and op-nanocomposites containing CuO lower than 15%, erating temperature on the sensing properties of theCuO dispersed on the CeO2 surface in an amorphous nanocomposites were examined. However, ultrafineform. For the nanocomposites containing Cuo higher grain sizes should be maintained to achieve highlythan 15%, phase segregation occurred and both Ceosensitive materials. Figure 6 illustrates that at higherand Cuo phases can be observed in the XRd patterncalcination temperature of800℃,12%CuTo further elucidate the thermal and phase stabilitiesnanocomposite underwent substantial grain growth orof the nanocomposite, the materials containing 12%agglomerate, which results in a large decrease in COCuO were calcined at different temperatures as shown sensitivity. In contrast, the calcination temperature isin Fig. 5. The Cuo phase did not appear until to 800 too low to satisfy the requirement of sensing elementC, only the change of mean crystallite size of CeOresistance, because nanocomposite did not achieve aoccurs with the increase of calcinations temperature. sufff crystally until600℃TheThis confirmed that CeO addition greatly enhanced中国煤化工600℃, hich was ththe thermal stability of Cuo against grain growth atopCNMHGe for the maximum COelevated temperature. The composite calcined atsensitivity of 5. 4 at the operating temperature of 270 C600C was mostly thermally stable. The result obined by XRd analysis was in good agreement with(Fig. 6). Calcination of 600 C ensured high CuOit obtained by BEt analysiscrystallinity in the nanocomposites without significantgrain growth, which can thereby maximize the sensi672www.tandf.co.uk/journalsProgressinNaturalScienceVol.15No.72005tween CuO and CeO400℃enhanced CO adsorption giving rise to excellent CO600℃sensitivity800℃Table 1. XPS analysis of core electron bindiCe3ds/olsRefStandard compound Cu2p3933.6530.8,531.9[8200250300350400[7]Operating temperature(C)CeO,6. Sensitivity curves of sensing element at different calcination temperatures for 0.5%CO.3 Conclution2. 5 Effects of Cuo content on sensitivity(1) The sol-gel matrix-mediated synthesis maybe employed to obtain a single-phase non-agglomeratThe effects of Cuo content and operating temed material with controlled nanoparticle size and morperature on the Co sensitivity were examined for phology to attain superb surface area and sintering resistance,so as to effectively suppress grain growththat an increase of the Cuo loading from 5% to 15%provokes a marked increase of the CO sensitivity. A(2)The enhanced gas sensing properties and longmong the three composite oxides tested, the term stability of the CuO/CeO nanocomposites were at-nanocomposite sample containing 12%CuO has the tributed to the ultrafine grain size and tailored compositiohighhrough careful control of the sol-gel processin(3)Structural characterization techniques andthe effects of composition of composites, calcination-5% CuO→-12%CuOtemperature and operating temperature on gas sensi学15%CuOtivity were examined, the 12% CuO content, calmined to600℃ and operating at270℃ are the optional gas sensing conditions for CO(4) Incorporation of the secondary oxide pro5030050400vides electronic interaction with Cuo and enhancesOperating lemperature(C)CO adsorption on the surface of nanocomposite, eviFig. 7. Sensitivity curves of different Cuo contents sensing eleIncreament calcined at 600C for 0. 5% coing with Cu 2p binding energy decreasing2.6 Surface electronic states analysis of nanocom-References1 Ying, J.Y. 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