Influences of dehydrating process on properties of ATO nano-powders

Vol.14 No. 6Trans. Nonferrous Met. Soc. ChinaDec. 2004Article ID: 1003 - 6326(2004)06 - 1123 - 06Influences of dehydrating process onproperties of ATO nano-powdersW∪Xiang wei(吴湘伟)' , CHEN Zhen-hua( 陈振华)°, HUANG Pei-yun( 黄培云)(1. School of Materials Science and Engineering, Central South University,Changsha 410083, China;2. School of Materials Science and Engineering, Hunan University ,Changsha 410083, China)Abstract: Sb-doped SnO2 (ATO) nanometer powders were synthesized by hydrolysis of alkoxides, using SnCl4●5H2O and SbCs as raw materials. Some dehydrating processes，such as n-butanol/ xylene mixed solvent heterogene-ous azeotropic distillation, organic dehydrating agent and other dehydrating processes, were used to treat the wetcolloids for preparing nonagglomerated ATO nanoparticles. The influences of dehydrating processes on the particlesize, agglomeration and resistance were investigated using X- ray diffraction (XRD), transmission electron microsco-py (TEM) and Brunauer-Emmet- Teller (BET). It is indicated that the dehydrating methods have great influences onthe products properties, and that n-butanol/ xylene mixed solvent heterogeneous azeotropic distillation processingand organic dehydrating agent can effectively remove the residual H2O molecules in wet colloids, and be used to pre-pare powders with high surface areas, about 85. 32 m2 /g, low agglomeration and good conductivity.Key words: ATO; conduetive nanometer -sized powder; agglomeration; dehydrationCLC number: TQ174Document code: A1 INTRODUCTIONying°，supercritical drying[10]， organic solventwashing and heterogeneous azeotropic distilla-Crystalline Sb doped tin oxide (ATO), cassit-ting[". The former two kinds of methods whicherite structure, is a wide band gap n-type semicon-needs specialized equipments， are not used asductor. Because of its optical property ( transpar-widespread as the latter two, which are simple andent for visible light and reflective for IR) and elec-easy to operate. The method of organic solventtroconductibility，good chemical and mechanicalwashing mainly uses ethanol, isopropanol and ace-stability, it has many applications, such as trans-tone to substitute for H2O molecules remained inparent conductive electrodes, photovoltaic devices，the gels, but this substitution is limited and thephotosensors, catalyst, antistatic coatings andagglomeration can not be completely eliminated.electrochromic materialsh-4. A variety of tech-Compared with organic solvent washing method,niques have been used to prepare ATO superfineheterogeneous azeotropic distillation is a more ef-powders, some involve dry processes, others arefective anti-agglomeration method. But， up to .based on wet chemical processes，including chemicalnow, the solvent used in heterogeneous azeotropiccoprecipitation'S) ，sol-gel-related process'l ，emulsiondistillation mainly is n-butanol. No any other sol-method7l. Compared with dry process, wet chem-vents are found to be utilized in the process, andical process is a low-cost method to fabricate super-also the report on the preparation of free agglomer-fine powders. However, the process suffers theate nanometer sized powder with organic dehydra-disadvantage of producing hard agglomerate duringting agent are not found.drying and calcining procedures due to the highIn the present study, nanometer-sized ATOsurface tension stress, physically adsorbed and /orpowders are prepared through hydrolysis of metalchemically coordinated H2O molecule, and hydrox-alkoxides. In order to eliminate the agglomerate,yl group on hydrate particles surface. The key is-an improved heterogeneous azeotropic distillationsue to decrease the agglomeration has been consid-process, n-butanol/ xylene mixed solvent heteroge-ered to decrease the tension stress and to removeneous azeotropic distillation, and a kind of organicthe residual H2 O molecule with maximum limit bydehydrating agent are used in dehydrating proce-proper dehydrating processI7.81. The dehydratingdure.中国煤化工ifect of the two de-methods adopted currently include refrigeration dr-hydrah other processes,.YHCNMHGD Foundation item: Project (50174025) supported by the National Natural Science Foundation of ChinaReceived date: 2004 -02 - 16; Accepted date: 2004-07 - 30Correspondence: WU Xiang wei, PhD candidate; Tel: + 86-731-8830491 ; E mail: wqxf@ mail csu. edu. cn●1124●Trans. Nonferrous Met. Soc. ChinaDec. 2004on the properties of ATO superfine powders. It isTransmission electron microscopy (TEM, Hi-found that the n- butanol/ xylene mixed solvent het-tachi H-800 microscope) was performed to observeerogeneous azeotropic distillation process and or-the particle size and morphology. Specific surfaceganic dehydrating agent can effectively dehydrate,area was measured by the BET method ( Quanta-and obtain high performance nanoscaled ATO pow-chrome Monosorb， USA) using N2 adsorption.ders.The particle size was also obtained by the formulaDgET=6/(6.6●Seer). The resistivity ρ of powders2 EXPERIMENTALwas obtained using a self made equipment and amicrohmmeter ( HZ2520，Bejjing).2.1 Preparation of samplesA requisite amount of SnCl,●5H2O and SbCl3 .3 RESULTS AND DISCUSSION(AR，in the mass ratioof 7 : 1), were dissolved inabsolute ethanol. After refluxing in a flat- bottom3.1 Characteristics of samples and analysisflask equipped with a condenser for 8 h, a whiteFig.1presents the XRD spectra of nanosizedsuspension was produced. When cooled to roomATO powders sintered at 550 C for 2h. All of thetemperature, the suspension was isolated by cen-diffraction patterns were identical to that for tetra-trifugation，and a clear metal alkoxides solutiongonal SnO2 (JCPDS file number 21 - 1250), show-was obtained. Under magnetic stirring, the surfac-ing characteristic of cassiterite structure. It can betant was added into the as prepared alkoxides solu-easily seen from Fig. 1 that the X-ray diffractiontion. Then the solution was heated to 50 - 70 C，peaks of these samples broadened. It is owing toand the hydrolysis was performed as an aqueousboth the microstrain and the particle size diminu-ammonia solution was added in dropwise. Whention of the nanocrystallines.the pH value of the solution reached 1- 3, lightyellow ATO colloid was obtained. After aging at(110) (101)room temperature for 2 h, the colloid was washed(211)with distilled water several times until no white de-(200)posit of AgCl was observed in waste water， astested by AgNO3. The as-washed gel was dividedSample IVinto 5 parts, each part was performed different de-hydration processing. The dehydrating methodsSample Iare given in Table 1. The dehydrated gels weredriedat100Cfor2h,calcinedat550Cfor2h,人。，期Sampleand finally transformed into shallow blue ATOpowders.SampleTable 1 Samples dehydrating methods10507080Dehydrating methods20/(*)Fig.1 XRD patterns of powdersWashing with distilled waterprepared via different dehydrating methodsAzeotropic distillating with n butanol(calcined at 550 C for 2 h, respectively)Washing with absolute ethanol for two timesThe values of the powders BET specific sur-IVAzeotropic distillating with xylene/ n-butanolface area (SEr)，particle size (dxRD and DgEr), re-mixed solventsistivity (p) and green density are listed in TableDehydrating with organic dehydrating agent2. From Table 2, we can see that the particle sizesdetermined with XRD-LB and BET techniques2.2 Characterization of samplesshow obvious difference. According to GAO etX-ray diffraction ( XRD) patterns were ob-a1[12], the value of dxRD can be thought as the origi-tained with a Rigaku D/ Max2550VB+ diffractome-nal crystallite size, and that of DgEr may be the av-ter using Cu K。radiation at 40 kV and 30 mA. Thehard agglomerates,scan rate was 8( )/min and covered the range be-the中国煤化工nen agloeration oftween 15° and 80° (20)。The average crystallite sizethCN M H Glers are heated at low-(dxRD) was deduced from the half-height lineer temperature (< 600飞)，the crystallites growbroadening by applying Scherrer formula and asslowly, and the crystallite sizes are mainly deter-suming Gaussian profiles for experimental and in-mined by the hydrolysis parameters. With thestrumental broadenings.same hydrolysis parameters, the difference of theVol.14 No. 6Influences of dehydrating process on properties of ATO nano powders， 1125 ●original crystallite size, dxRD， is not big. But theof the sample treated by mixed solvent azeotropicagglomerate degree of the particles prepareddistillation process; instead, the small dispersalthrough different dehydrating routes is not equal,aggregates can be seen.so the values of DBET have very great difference.Table 2 Characteristics of ATO powdersprepared via different dehydrating methodsSample SBET /Density(、 dxn/ Der/ D/aNo.(m2●g~)(0*cm)(g°cm-)nmnm53.71 0. 88-0.6 3. 775.8 16.9 2564.14 1.8-1.34 2. 667.114.1 849.93 0.88-0.6 3. 776.718.2 2CIV72. 191. 822. 626.4 12.5 785.32 2.3-1.93 2. 765.510.6 7Since ethanol washing for hydrous particles125 nmcan partly substitute for the H2O molecules in the800 12582 10020902gel, the agglomeration degree of particles is lowerthan that washed only with distilled water. But theformer particle size is bigger than the latter, andthe value of SBET is lower. This phenomenon wasalso observed in preparing TiO2 superfine powdersby hydrolysis of metal alkoxide12,13].In the case of typical azeotropic distillationprocess, the hydrous oxides are mixed thoroughlywith appropriate amount of n-butanol, then care-fully heated to the azeotropic point (93 C) of wa-ter-n-butanol system, at which water content rea-ches 44. 5% in the vapor of azeotropic composite.As the distillation process goes on, water in thesuspension is gradually removed up to the boiling20 125338020902point of n-butanol (117 C). During distillation,the following reaction between the hydroxyl on theFig.2 TEM micrographs of ATO wet colloids(a)- Sample I ; (b)- Sample IVsurface of particles and n-butanol may take place:M- OH+C.H,- OH =→M -0C,H。+ H2Oi. e. the surface OH groups of particles are substi-Although overwhelming majority H2O mole-tuted by一OC, Hg groups. Consequently, the pos-cules on the surface of colloid particles can be re-sibility for the particles to get close as well as themoved by aezotropic distillation process， the crys-formation of chemical bonds is greatly eliminated.tallization water inside the colloid particles cannotFor all that,at the end of the process, a lttle a-be replaced, for the volume of the organic solventmount of H2O molecules will still remain in themolecules is much bigger than that of H2O mole-system, which will lead to the crystallite growingcules(12]. On the other hand， the dehydrationand forming hard agglomeration to some extent.process of azeotropic distillation is carried on grad-However, by using a high boiling point hydropho-ually. Thus the particles may aggregate and growbic organic solvent, xylene, to mix with n-butanolup during the process, which will lead to biggerfor azeotropic distillation process, the H2O mole-grain size. When the wet colloid is treated with or-cules remained in the wet colloid are removed moreganic dehydrating agent, which can more easily re-thoroughly to get lower degree of agglomerate.act with adsorbed water and OH group and graftFig. 2 shows the morphologies of wet colloidson t' 中国煤化iles, well dispersedtreated with distilled water washing and xylene/n-andned. Among all ofbutanol mixed solvent azeotropic distillationTHC N M H Grganic dehydratingprocess. From Fig. 2, it is seen that the colloidagent has the biggest specitic surface areas (SB:T=particles, only washed by distilled water, connect85. 32 m2 /g), the smallest crystallite size (dxuB =to each other to form a net-chain structure. The5.5 nm) and mean agglomerate (D*/d=7).similar structure cannot be found in the micrographFig. 3 shows the TEM micrographs of pow-Trans. Nonferrous Met. Soc. ChinaDec.2004hydrating agent is discernable and uniform but ag-gregated together. Combining the specific surfaceareas based on BET measurement, the aggregationmay be a porous cluster, which has something todo with the function of organic dehydrating agent.3.2 Crystallite growingOrdinarily, the average crystallite size of nan-oparticles increases with the calcining temperaturerising. When the temperature is below 600 C，thecrystallizing process carrys on and the averagecrystallite size increases slowly. However， thegrain size accelerates rapidly as the calcining tem-perature is above 600 C[14,15].102。130004R020902In present work, the crystallizing behavior ofATO particles was investigated at 550 - 950 C.6The results are shown in Fig. 4. It is indicated thatthe crystallizing process can be obviously dividedinto two periods. In the first period, from 550 Cto 850 C，the crystallite size increased slowly.When the temperature was above 850 C, the crys-tallite size grew up quickly. But the increasing rateof the sample V was apparently slower than oth-ers, even calcined at 950 C for 2 h, the crystallitesize was only 11. 1 nm, and that of others reached30 nm. It may have relation with the porous clusterstructure of the sample. This kind of structuremay have the function of limiting the grain to growup.:08 12586 1月020902(C)30 t- - Sample|▲- Sample IV25下●一Sample V20 F5-5 10-500600700009001 000Temperature/'C1月020902100 nmFig. 4 Relationship between crystallite size ofFig.3 TEM micrographs of ATO powdersATO powders and calcining temperature(calcined at 550 C for 2 h, respectively)(a)- Sample I ; (b)- Sample INV ;3.3 Green density(c) - Sample VFig. 5(a) shows the green density of powdersfimon of calcining tem-ders sintered at 550 C for 2h. It can be seen thatper I中国煤化工of samples I and mthe ATO particles are dispersive and uniform forCNMHGkeptconstantasthexylene/ n-butanol mixed solvent azeotropic distil-calnng temperature lcreases. The reason is thatlation，but heavily agglomerated when simplyhe particles of the two samples have formed hardwashing with distilled water was performed. Theagglomeration, as temperature increases, the parmorphology of particles treated with organic de-ticles inside the aggregate stuck to each other andVol.14 No. 6Influences of dehydrating process on properties of ATO nano powdersgrew up, but the whole physical volume of the ag-last two items on total resistance(R) is. The resis-gregate did not change obviously.tivity arrangement of samples (calcined at 550 C4.“(a)for 2h) in ascending sequence isp1≈pn<ρn<ρn .<ρv ，which is consistent with specific surface are-4.0as arrangement.The variation of electrical conductivity withheat treatment temperature is shown in Fig. 5(b).i 3.650●一SampleIt can be seen that the resistivity of most samples●- -SSample i会3.2-Sample IIIdecreases lttle at first, but starts rising at higher-Sample IVtemperature. However, the resistivity of the sam--Sampleples Iand IV increases all through. The possiblereason is that the particles performed with azeo-tropic distillation tend to volatilize the antimonyoxide (the melting point of Sb2O, is 655 C) and500600700800900 1000reduce the concentration of Sb5+ more easily due toTemperature/Cdispersive and nonagglomerate nanoparticles, and60(b)thus the carrier concentration decrease, ZR。in-creases.The volatilization of antimony oxide is一Sample 1supported by electron energy spectrum analysis.-Sample 可4 CONCLUSION30ATO nanoparticles were obtained through the20hydrolysis of metal alkoxides, followed by differ-ent dehydrating processes with wet colloid. Crys-10Ftallite size determination based on the Scherrer0叶method shows that all samples are in the nanome-600 700 ”ter range. The dehydrating process has marked in-fluence on the properties of the powders. Xylene/n butanol mixed solvent azeotropic disillation andFig.5. Effects of heating temperature onorganic dehydrating agent have strong dehydratinggreen density (a), and resistivity (b) forability, and can be used to prepare nonagglomeratepowders prepared via differentnanoparticles. The dehydrating ability of differentdehydrating methodsdehydrating method in descending order is: organicdehydrating agent, xylene/n- butanol mixed solventThe green density of the sample V is lowerazeotropic distillation, n butanol azeotropic distil-than those of the former two. As temperaturelation, ethanol washing, dstilled water washing.rises, it tends to increase, but the variation is not :The lower the agglomerate is, the bigger the spe-big，which also verifies that the porous clustercific surface areas and the resistivity are. As calci-structure of the sample can prevent the grain fromning temperature increases (form 550 to 950 C),growing up. The green densities of samples II andthe crystallite growing and the resistivity variationIV are very small when calcined at 550 C for 2 h.of the powders prepared via different debydratingHowever, as the temperature stepped up, it in-routes have different behaviors. As for powderscreased quickly.prepared through organic dehydrating agent trea-ting, the speifie surface areas reach 85. 32 m'/g,3.4 Electrical conductivitycrystallite size is 5.5 nm(at 550 C for2 h), and atThree parts decide the resistance of powders,higher calcining temperature, the crysallite growsnamelyslowly, the resistivity is small and varies little.R= 2R.+ ER.+ ERwhereR gis the intrinsic resistance of pow-中国煤化工ders, ZR。is the direct contact resistance, anc.DH.CNMHGrino-Class M, et al.nonr natoparucre u ox10e ior microhotplate gas2 jR.is the band contact resistance. The last twosensors [J]. Sensors and Actuators B, 2001, 77: 145 .- 154. .items are very important for powders. 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