Synthesis of antimony-doped tin oxide (ATO)nanoparticles
by the nitrate–citrate combustion method
Jianrong Zhang,Lian Gao *
State Key Lab of High Performance Ceramics and Superfine Microstructure,Shanghai Institute of Ceramics,
许冠杰个人资料
Chine Academy of Sciences,Shanghai 200050,PR China
Received 9June 2003;accepted 16August 2003
Abstract
Antimony-doped tin oxide (ATO)nanoparticles having rutile structure have been synthesized by the combustion method using citric acid (CA)as fuel and nitrate as an oxidant,the metal sources were granulated tin and Sb 2O 3.The influence of citric acid (fuel)to metal ratio on the average crystallite size,specific surface area and morphology of the nanoparticles has been investigated.X-ray diffraction showed the tin ions were reduced to elemental tin during combustion reaction.The average ATO crystallite size incread with the increa of citric acid (fuel).Powder morphology and the comparison of crystallite size and grain size shows that the degree of agglomeration of the powd
er decread with an increa of the ratio.The highest specific surface area was 37.5m 2/g when the citric acid to tin ratio was about 6.
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Keywords:A.Oxides;B.Chemical synthesis;C.X-ray diffraction
1.Introduction
Tin oxide (SnO 2)is a typical wide band gap n-type miconductor (3.8eV),its n-type conductivity is generally caud by nonstoichiometry,associated with oxygen vacancies in the SnO 2lattice [1].The conductivity of undoped SnO 2is limited to 102À103V À1cm À1and unstable during its operation becau of the reaction of oxygen vacancies with ambient oxygen [2].Doping SnO 2with proper elements,such as
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Materials Rearch Bulletin 39(2004)2249–2255
夏景*Corresponding author.Tel.:+862152412718;fax:+862152413122.
E-mail address:Liangaoc@online.sh (L.Gao).
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热闹造句doi:10.1016/j.materresbull.2003.08.009
F,Sb is the best choice to increa the conductivity and the conductivity can be controlled by adjusting the amount of dopants instead of the nonstoichiometry [1–3].
As a member of transparent conductive oxides (TCOs)that have received great attention for their applications in electrochemical,electrochromic devices,window coatings [4],antimony-doped tin oxide (ATO)has been the focus of intensi fied study for its chemical,mechanical and environmental stabilities,even at high temperature [5].At low antimony doping level,ATO is transparent throughout the visible region,while re flects infrared light.The enable ATO to be ud as transparent electrode
s for electrochemical devices and displays,heat mirrors and energy storage devices [1,6].When ud as antistatic agents,ATO shows better performance over the currently ud carbon blacks,metallic pigments and organic polymer binders [7].Heavily doped ATO is a good catalyst for ole fin oxidation,dehydrogeneration and ammoxidation of alkenes [8].ATO nanoparticles have been ud as electro-chromic material for the production of printed,interdigitated electrochromic displays [9].Tin antimonite has good resistance to decomposition at elevated temperatures and ionizing radiation,and a high ion exchange lectivity for certain elements,the features render that the material has great application in nuclear waste management [10].
ATO particles have been synthesized by the traditional solid state reaction [11]and wet chemical methods,such as coprecipitation [1,9,10]and Pechini method [11].It is well known that the solid state reaction needs a high calcination temperature and leads to uncontrollable large grain growth,gregation of dopant oxides and possible loss of stoichiometry due to volatilization of reactants at high temperature.The coprecipitation method improves reactivity of the components,but the incomplete precipitation results in an alteration of stoichiometry and the chemical homogeneity cannot be readily achieved owing to differences in the solubility or complexation between various chemical species.The Pechini method can reduce gregation of the metal elements and improve the chemical homogeneity during the decomposition of the polymer at high temperatures.
The starting materials in wet chemical methods are all from metal chlorides such as SnCl 4,SnCl 2,SbCl 3,SbCl 5.As is well known,the chlorine ions adsorbed on the tin hydroxide are very dif ficult to be rind off.The residual chlorine ions affect the surface and electrical properties,introducing a random n-type doping in the material,causing agglomeration among particles,leading to both volatile antimony and tin compounds and sintering to higher temperature [12–15].So,it is of great importance to synthesize ATO particles free from chlorides.Metal alkoxides have been ud to synthesize ATO films and can also be ud to synthesize particles [16].But the metal alkoxides are particularly moisture nsitive and have to be procesd under a dry and inert atmosphere,the resulting OXO-polymers that constitute the sol are generally polydisper in size and composition.Additionally,the metal alkoxides are comparatively expensive and cannot be ud in large scale [17].
In this paper,we successfully synthesized the ATO nanoparticles by combustion method for the first time.The metal sources are granulated tin and Sb 2O 3,all free from chlorides.We cho citric acid as fuel and nitrate as an oxidizer.So,the metallic ion stoichiometry can be improved and the lective precipitation can be prevented [18].This method should be a good choice to take the place of the wet chemical methods using metal chlorides as starting materials to synthesize ATO nanoparticles.
2.Experimental procedure
砌块砖In this gel-combustion method,the starting materials were granulated tin,Sb 2O 3,HNO 3,citric acid (C 6H 8O 7CA).All the chemical reagents were of analytical grade.Granulated tin was first carefully
J.Zhang,L.Gao /Materials Rearch Bulletin 39(2004)2249–2255
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dissolved in8mol LÀ1HNO3followed byfiltering to remove the impurity that cannot be dissolved. Sb2O3was dissolved in melted citric acid.Then,the two solutions were mixed and stirred at RT for2h and ammonium hydroxide(25%)was added to the solution until a pH value about7was reached.This produced a very stable transparent solution containing Sn-and Sb-CA complexes.Adjusting pH of the mixed solution is of great importance,becau the dissociation of citric acid and the complexing reaction between metal ions and citric acid are highly dependent upon the solution pH.Only when the pH is above 6.4,the complexing reaction can occur completely[19].The obtained solution was gradually condend at1008C to remove excess water and convert into a‘‘gel’’.During this process,the solution became more and more viscous and changed from colorless to green.Then,the‘‘gel’’was heated on an electric heater, swelling into a form
童鞋品牌and undergoing a strong lf-propagating combustion reaction with the evolution of large volume of gas.The entire combustion reaction lasted only a few conds and black ashes were obtained.The color of the ashes changed from gray to black with the increa of citric acid and revealed an incomplete combustion.The ashes were referred to as‘‘precursor’’.The precursors were ground in a mortal then sieved to a particle sizefiner than200m m and calcined at6008C for2h in a muffle furnace to convert to crystalline ATO.In all cas,the antimony doping level was5at%with respect to Sn,the citric acid to metal ratios were1.4:1,2:1,3:1,4:1,6:1,8:1(samples A,B,C,D,E,F,respectively).
The nanocrystalline ATO particles were characterized by X-ray diffraction(XRD),X-ray diffraction patterns were obtained with a diffractometer(Rigaku D/max-rB),using nickel-filtered Cu K a radiation. The average crystallite size(D)was calculated using Scherrer’s formula[1].
D¼
0:9l b cos u
where l is the wavelength of X-rays,b the corrected half-width that is obtained using the(111)line of the pure silicon as the standard.The specific surface area of the powder was measured with a Micromeritics ASAP2010analyzer using the multipoint Brunauer,Emmett,and Teller(BET)adsorption t
echnique.From the data of specific surface areas,the average grain size was calculated using the equation[19].
d¼
6 r A
where d is the average grain size of spherical particle,A the surface area of the powder and r the theoretical density of SnO2.Thermogravimetry differential scanning calorimetry(TG-DSC,Netzsch STA449, Bayern,Germany)was carried out to follow the decomposition and crystallization of precursors at a heating rate of108C minÀ1in static air.The main grain size and morphology of the particles were obrved by transmission electron microscopy(JEM-200CX TEM).
3.Results and discussion
Fig.1illustrates the TG and DSC curves of the precursor from sample C.The TG curve indicates a nearly vertical weight loss of21.16%at about2248C and another heavy weight loss of14.78%extending up to4808C,beyond that temperature the weight remained constant.Therefore,it is concluded that all of the reactions in the weight-loss process werefinished at4808C.The DSC scan
of the precursor shows two large exothermic features corresponding well to the two large weight loss obrved by TG.Thefirst weight loss is mostly due to the oxidation of the residual carbon,which gave off lots of heat consistent J.Zhang,L.Gao/Materials Rearch Bulletin39(2004)2249–22552251
with the first exothermic feature.The cond large weight loss at 4808C can be ascribed to decom-position of organics in the precursor,corresponding to the cond exothermic feature.The ATO particles were formed above 4808C.
Some large particles,much larger than 200m m in precursor from sample E,were chon for X-ray diffraction.Fig.2shows the XRD pattern.Two phas were detected,Sn (JCPDS 4-673)and cassiterite SnO 2(JCPDS 21-1250),the intensity of Sn pha is larger than SnO 2pha.In the combustion method,combustion reaction lasts only a few conds.In oxidant-of fuel-rich precursors,the excess of fuel (reduction agent)does not participate or be oxidized to some intermediate products,such as carbon,just as shown by the experimental result.Thus,a reducing condition was formed and the Sn ions were reduced to element Sn.
Fig.3shows the powder X-ray diffraction patterns of the ATO nanoparticles prepared with different citr
ic acid to tin ratios.In all cas,the peak positions agree well with the re flections of bulk cassiterite SnO 2.No other pha was detected in the X-ray diffraction patterns,indicating that all antimony ions came into the crystal lattice of bulk SnO 2to substitute for tin ions [1].The width of the re flection is considerably broadened due to the small crystalline domain size.The average crystallite size,as calculated using Scherrer ’s formula are prented in Table 1.The crystallite size incread with the increa of citric acid to tin ratio.It is known that temperature is critical in controlling the nanocrystalline particle size.In the combustion reaction,the more amount of citric acid (fuel)ud,the more reaction
J.Zhang,L.Gao /Materials Rearch Bulletin 39(2004)2249–2255
护肤的重要性2252Fig.1.TG and DSC curves of the precursor from sample
C.
Fig.2.XRD spectra of some big particles in precursor from sample E.
enthalpy evolved,therefore,the higher local temperature obtained and leads to a bigger crystalline size
[20].Comparing with the ATO nanoparticles synthesized by coprecipitation method,the crystallite synthesized by the combustion method is bigger,the reason could also be a higher temperature evolved during the combustion reaction.
Fig.4shows variation in the speci fic surface area of the ATO powder as a function of the citric acid to metal ratio,which shows a parabola shape.When ratio was about 6,the highest speci fic surface area,37.5m 2/g was obtained.In the combustion reaction,the number of moles of gas and carbon evolved increa with the increa of fuel to metal ratio,which prevent agglomeration among particles and result in highly porous structure of the product,increasing the speci fic surface area.Taking the temperature
J.Zhang,L.Gao /Materials Rearch Bulletin 39(2004)2249–2255
文明礼仪伴我行主题班会
2253
Fig.3.XRD spectra of ATO powder obtained with different citric acid to tin ratios.
Table 1
Average crystallite size and grain size of ATO powder synthesized by the nitrate –citrate combustion method
Sample
平方的进率是多少A
B C D E F Citric acid to metal ratio
1.423468Grain size (nm)
36.133.831.028.322.827.8Crystallite size (nm)9.111.614.218.721.3
26.7Fig.4.Speci fic surface area of ATO powder as a function of citric acid to tin ratios.
