Preparation and Gas-Sensing Properties of NdFeO3 Nanocrystalline

JOURNAL OF RARE EARTHSVol.21,N.6,Dec,2003,p.630Preparation and Gas-Sensing Properties of NdFeo3 NanocrystallineNiu xinshu(牛新书), Du Weimin(杜卫民), Du Weiping(杜卫平), Jiang Kai(蒋凯)College of Chemistry Environmental Science, Henan Normal University, Key Laboratory of Environmental Science and Engineering Education Commission of Henan Xinxiang 453002, ChinaAbstract: With the Nd2O3, Fd NO3 ) 9H20, nitric acid 1: I vloume fraction )for the starting materials rare earth com-posite oxide NdFeO3 with the structure of perovskite type was synthesized by sol-gel method in the system of citric acidStructural characteristics were characterized by XRD and TEM which indicate that the sample is nanocrystallite with uform grain size distribution and the average grain size is about 28 nm. Moreover the gas sensing properties of the materialwere tested. The results show that NdFeO3 sensors have high sensitivity excellent selectivity and quick response and recovery behavior to H2s, hence this gas sensing material has a better prospects in industrial practiceKey words: gas-sensing material i NdFeO3' nanocrystalline i rare earthsCLC number: TP212 Document code: A Article ID: 1002-0721(2003 06-0630-03H2S is a kind of deadly poisonous gas which ex- mixture was stirred and sol formed finally. The sol wasists extensively in petroleum and chemical engineering evaporated on a water bath at 80 C and then dried atproduction line. Its mass concentration in the air is re103 C in baking oven until xerogel formed which wasstricted below 0. 01 mg dm-3in industry(=7x 10-4 ground in agate mortar and turned into powder.Thevolume fraction ). A certain quantity of H2S in the powder was slowly heated to 400 C and pre-calcinedPG liquefied petroleum gas )results in the container for 4 h to make the organic matter decompose cominvalidating easily National standard is that the con- pletely then calcined at 950C for 6 h in muffle furtent of H2S in the LPG can not exceed 340 x 10-6 vol- nace. The product was cooled to room temperature andume fraction )2. The content of H2 S in some mineral the nanocrystalline was obtaineddeposits is in connection with the type of mineral de1.2 Characterization of NdFeO3 nanocrystalso it can be used as a mark in exploration oflitesome mineral depositsThe structure and crystal state of the powder were1 ExperimentalBluker D8-Advance ) with a CuKa radiation ( wave1.1 Preparation of NdFeO3 nanocrystallitelength A =0. 15406 nm )operating at 20 mA and40 kv. The shape and size were analyzed with the helpFe( No3b'9H20( Shuanghuan Reagent Plantof JEM-100SX Transimission ElectronBeijing , A.R. ) Nd20 Yuelong Chemical Engineer- 1.3 Manufacturing of gas sensors and the mea-ing plant, Shanghai,A.R.), nitric acid( ChemicalReagent Plant, Xinyang ,AR.), citric acid( Chemisuring principle of gas sensing propertiecal Reagent Department Medical Station Tianjin AAppropriate quantity of organic solvent was addedR.)were used as the starting materials. Appropriatermquantity of Fe NO3 9H20 was dissolved in de-ion- paste obtained was coated onto ceramic tubes whichized water and Nd2O3 in nitric acid 1: I volume frac- have four platinum wires. Electrical contacts weretion), respectively. Citric acid was dissolved in demade with four platinum wires attached to the elecionized water(0. 2 mol. dm-3). Stoichiometric trodes. The material on the ceramic tubes was an-erethoroughly mixednealed at600℃ for 1 h and aged at320℃for240then a certain amount of citric acid(0. 2 mot dm -3h. The sensors were set up in a glass test chamber andwas introduced into the above mixture. Ultimately the keflow of fresh air for 10 minmole ratio of Fe3+ plus Nd+ to citric acid is 1: 2.Thebefo中国煤化CNMHG12 revised date: 2003-07-30Foundation item: Project supported by the Natural Science Foundation of Henan Province(004030300 )and the Natural Science Foundation ofHenan Educational Committee( 20021500007)Xinshu( 1954-), Male, Professor Speciality: Inorganic nanometer materialshor(E-mailduweimin0081@163.com)Niu X s etPreparation and Gas-Sensing Properties of NdFeO3 Nanocrystalline631en amount of gases were injected into the chamber andmixed by a fan for 30 s. The measuring principle ofgas sensing properties was shown in Fig. 1. The heating voltage Vh)was supplied to either of the coils forheating the sensors and theit voltage( Ve)wassupplied across the sensors and the load resisto( RL)connected in series. The signal voltage( Vout )acrossthe load, which changed with sort and concentration ofgas was measured. The resistance of the load resistor200nmwas alternative and the heating voltage can be adjustedFig 3 TEM photo of nanometer NdFeO( X 50000)within a wide range. The sensitivity for gases S, isdefined as S=RgRa, where R and Ra are the resis-The sensitivity changes of NdFeO3 sensorstance of the sensors in the testing gases/air mixtureby 50 x10 volume fraction H2s gas at dif-and in the air, respectivelyferent temperatures were tested. As shown in Fig 4.2 Results and discussion280C was assumed to be the optimum operating tem-H2s gas which was attributed to the2. 1 Structural characteristics of NdFeosemiconductivity of NdFeO3 and the characters of H2s3 It is well known that NdFeO, has the structure of perte type. The same neodymium atoms at the verFig 2 demonstrates that the structure of NdFeO3tIcefter calcining at 950C for 6 h belongs to the structemperature and left neodymium vacancies. The conture of perovskite type( JCPDS: 82-2421). According ductivity of NdFeO3 was due to the ionizing ofto the scherrer formula, the calculated diameter of theneodymium vacancies5101:VM→∨"Md+3h:.hnpowder is 21 nm or so. The TEM photo of NdFeO3 low-temperature range NdFeO3 showed lower conduc-Fig 3 )shows that the sample particles are uniformtivity because the energy of the system was less thanspherical grains. The average grain size calculated inits band gap energy. With the temperature increasingproportion to the photo is 28 nm which is consistentNdFeO3 attained enough energy to overcome its bandwith the results of xrd these results show thatgap energy. Therefore, the concentration of carrier innanometer powder with uniform grain size distribution creased swiftly and the conductivity enhanced quicklysynthesized byThe sensors took on characters of the P-type semicon2.2 Gas-sensing properties of NdFeOjsensors the sensors exposed to the air, O2 adsorbed on NdFeO32.2. 1 Influence of temperature on the sensitivity ofsurface would trap electrons from the body of NdFeO3due to the strong eleity of theand produced adsorbed oxygen Ox ads ) So the concen-Sensortration of holes in valence band increased and the resistance of material decreased due to the increasingconcentration of available carrier. Introduction of reduike H2sbetween H2S and Owhich had a relative strongactivation on the surface of sensors: H2S+ 3/20x ads)3/2ne] electrons released fromFig 1 Schematic representation of measuring principlethe reaction would annihilate the holes:h'+e-oHence, the resistance of material increased and conare applicable for sensing H2S gas. If the temperatureISH中国煤化工 rption of the adsorbedthe concentration ofcheCNMHGed, the sensitivity off(")sensors declined. On the other hand the high temperature may be good for enhancing the speed of oxidationFig 2 XRD pattern of NdFeO3 powdersreaction but it prevented the diffusion of reductive gas. Itwas the less concentration of the reductive gas which adsorbed on the63JOURNAL OF RARE EARTHS, Vol 21, No 6, Dec 2003Working temperature. 280C I10020406080100120140Concentration ofH.S/10-6Working temperature/C4 Gas sensitivity as a function of working temperatureig. 6 Gas sensitivity as a function of H2s concentrationHSWorking temperature/280 CAcetone gasolineLPG200250300Fig 5 Sensitivity of sensors to different tested gasesFig 7 Response and recovery characteristic of NdFeO3 sensorssensor surface that make the sensitivity decreaseto50×10-6 volume fraction h2Sat280℃2.2. 2 Relationship between gas selectivity and senmeaned the time that the signal voltage changed fromsitivity of sensors and the concentration of H,su. to U。-10%(U。-Ux) when isolated from F2SThe NdFeO3 sensors was tested for their sensitivity to Tres and Trey were 7 and 17 s, respectivelydifferent gases at the concentration of 50 x 10- vol-ume fraction with the optimum operating temperature3 Conclusionsof 280C. As shown in Fig. 5, Sensors based on ndThe nanocrystalline rare earthcomposIteKideFeO3 nanocrystalline had high sensitivity to H2S, mod- NdFeO, can be prepared by sol-gel method in the sys-erate sensitivity to gasoline and acetone and low sensitem of citric acid. This method is more simple, lesstivity to H2 and LPG. As shown in Fig. 6, when the pollution and high productivity compared with the othconcentration of h,s is below 50x 10-6 volume frac-ers. 2. The structure of NdFeO3 belongs to the pertion, the sensitivity increased with the concentration of ovskite type with average grain size of 28 nm. TheH2S enhancing. While the sensitivity almost remaines sensors based on NdFeO3 nanometer material exhibitunchanged when the concentration of H2S is beyond high sensitivity excellent selectivity and quick re50 x 10-6 volume fraction. This suggests that H2S can sponse behavior to low-concentration H2s gasbe detected in a broad concentration rangeReferences2. 2.3 Response and recovery characteristicsII Xu J Q, Yan D L, Wang G Q, et al. StudFig. 7 shows response-recovery curve to 50 x 10-6vol-based HSng materials[ J ]. J.Chinume fraction H2S in 26%Rh at 18C for NdFeO, senic Soc.,1999,275):591sors. After introducing H2S the resistance of the sen[2 Fang G J, Liu Z L, Ji X D, et al. Research progresssors increases and the signal voltage reduces. The res gas sensing material[J].J. Inorganic materialsmediately. On theth中国煤化工istance of the sensors decreases and the signal voltageCN Ban gold field, Hainan /o?TZ, et al. Fluid inclusputs up when the sensors are isolated from H2s. Theland, south China [J ] J. Chinese Science Bulletinrecovery was obviously observed. The response time1993,3824):2273Tres is defined as the time when the signal voltage [4 Ge X T, Chu X F, Liu X Qchanges from u。toU。-90%U。-Ux) after introducing h2方数据 ilarly, the recovery time,Tn[].cproperties of Mg2+-doped lFeO3 solid solutionChinese J, Chemical Physics 2000, 133): 350