微波辅助水热法Synthesis of Fine Micro-sized BaZrO3 Powders Based on a
Subscriber access provided by SHENYANG INST CHEM TECH
Crystal Growth and Design is published by the American Chemical Society. 1155
Sixteenth Street N.W., Washington, DC 20036
Article
Synthesis of Fine Micro-sized BaZrO Powders Based on a Decaoctahedron Shape by the Microwave-Assisted Hydrothermal Method Ma#rio L. Moreira, Juan Andre#s, Jose# A. Varela, and Elson Longo Cryst. Growth Des.
, 2009, 9 (2), 833-839? DOI: 10.1021/cg800433h ? Publication Date (Web): 15 December 2008
Downloaded from https://www.sodocs.net/doc/cf12730590.html, on March 2, 2009
More About This Article
Additional resources and features associated with this article are available within the HTML version:?Supporting Information
?Access to high resolution figures
?Links to articles and content related to this article
?Copyright permission to reproduce figures and/or text from this article
Synthesis of Fine Micro-sized BaZrO3Powders Based on a
Decaoctahedron Shape by the Microwave-Assisted Hydrothermal
Method
Ma′rio L.Moreira,?Juan Andre′s,?Jose′A.Varela,§and Elson Longo*,§
LIEC,Departamento de Quimica,Uni V ersidade Federal de Sao Carlos,P.O.Box676,
13565-905,Rod.Washington Luis,Km235,Monjolinho,Sao Carlos-SP,Brazil,and LIEC,
Departamento de Fisico-Quimica,Instituto de Quimica,Uni V ersidade Estadual Paulista,
P.O.Box355,14801-907,R.Francisco Degni,s/n,Bairro Quitandinha,Araraquara-SP,Brazil
Recei V ed April26,2008;Re V ised Manuscript Recei V ed No V ember17,2008
ABSTRACT:Micro-sized decaoctahedron BaZrO3powders were synthesized by means of a hydrothermal microwave method at 140°C for40min.The X-ray diffraction,Raman,UV-visible,and inductively coupled plasma atomic emission(ICP-AES) spectroscopy,as well as measurements of photoluminescence(PL)emission,were used for monitoring the formation of a perovskite phase with random polycrystalline distortion in the structure.Two emissions at477and526nm were accompanied by increase in the gap values,indicating the existence of different emission mechanisms.A theoretical model derived from previous?rst principle calculations allow us to discuss the origin of the photoluminescence emission in BaZrO3powders which can be related to the local disorder in the network of both ZrO6octahedral and dodecahedral(BaO12)hence forming the constituent polyhedron of BaZrO3 system.Initial observations of a novel morphology of the BaZrO3perovskite crystal growth based on decaoctahedron shape is demonstrated and indirectly related to photoluminescence emissions.
1.Introduction
The controlled synthesis of desirable composition,size,shape, and crystal structure is of considerable interest because of its potential application to obtaining materials with wide techno-logical applications.The continuing trend toward miniaturization requires techniques to produce materials with particle shape accuracy.1In particular;materials with ABO3perovskite-type structure have been the subject of a major focal point of interest in material science because they are of fundamental interest and technological importance.Recently,different studies have been performed on the barium zirconate,BaZrO3(BZ)ceramics, because of their promising properties2including a high dielectric permittivity of about30and a wide band gap with5.3eV.3BZ can be used as refractory material,as a substrate in the synthesis of superconductors,as high-temperature microwave dielectrics, as well as electro ceramic capacitors applicable in wireless communications.4a-d Because a re?ned control on the morphol-ogy of BZ powders is sought,a variety of processes have been employed.Many sizes and shapes have been obtained for BZ powders and other perovskite compounds,such as nanocubes5 polyhedrons,6a,b nanowires,4d,7a,b and monodispersed particles,8a,b which have attracted vast attention because they exhibit unique optical,magnetic,and electronic properties which are not dependent only on the size but also on the shape of the particles.7a
Zirconates have been conventionally synthesized through solid-state reactions using zircon carbonates,oxides,and/or nitrate elements of group IIa to yield randomly shaped particles with broadly dispersed sizes.Solid state reactions are commonly performed at temperatures of up to1300°C2for the production of crucibles;9however,other synthetic procedures have been employed such as soft chemistry routes(polymeric precursor),10which allowed a systematic study of the evolution of the BZ phase,with diameter of30-40nm if they were synthesized by the sol-gel process11while an urea-induced precipitation process led to BZ nanoparticles with a diameter around90nm.12 The high-energy dry grinding,laser ablation and spray pyrolysis usually requires long synthesis times using expensive and sophisticated equipments.Ultra?ne powders of ZrO2have been converted into MZrO3(M)Ba,Sr,Ca)by both bath hydrothermal13and co-precipitation/calcinations14methods.The hydrothermal synthesis has been developed over the last decades to obtain nanometric BZ powders15a-c under fast and less expensive methods.Recently,microcrystalline(2-5μm)4c and nanometric15c BZ powders were obtained using a hydrothermal synthesis process,working under supercritical conditions. The conventional hydrothermal(CH)routes were normally conducted for several hours at temperatures higher than200°C.16a,b An interesting results was reported by Yun et al.7a who discussed the evolution of the morphology of barium zirconate morphology from truncated dodecahedral to a spherical shape by increasing the ethanol content in the water precursor solu-tion.Moreover,these authors suggested a shape-dependent on photoluminescence(PL)emission with Eu-doped BZ microc-rystals obtained at temperatures of the order of130°C for12, 24,and48h,7a which requires the use of an airtight high-pressure vessel.
Recently,an alternative method using microwave heating has emerged in the?eld of powder preparation,17a-i with both expected and unexpected merits,for example,kinetic enhance-ment,low reaction temperature,time reduction,control of the overall particle size and aggregation process.17a,h,18a,b Rao et al.presented a summary of recent reports on the synthesis of inorganic materials using microwaves heating associated with hydrothermal conditions.17a In1992,Komarmeni et al.17b introduced the hydrothermal microwave(HTMW)processing for synthesis of electro ceramic powders;this is a genuine low temperature and fast reacting method.17c A year later the same authors developed the synthesis of perovskite oxides for BZ based materials,which are usually found to have spherical
*To whom correspondence should be addressed.E-mail:elson@ iq.unesp.br.Phone:+551633615215.Fax:+551633518350.
?Universidade Federal de Sao Carlos.
?On leave from Departamento de Qu?′mica F?′sica y Anal?′tica,Universitat Jaume I,12071Castello′,Spain.
§Universidade Estadual Paulista.
CRYSTAL GROWTH &DESIGN
2009 VOL.9,NO.2 833–839
10.1021/cg800433h CCC:$40.75 2009American Chemical Society
Published on Web12/15/2008
morphologies with a particle size range of about0.1-0.5μm. Recently,Maksimov et al.19prepared?ne BaZrO3and BaHfO3 powders by the HTMW process,and the results indicate that microwave processing during hydrothermal synthesis notably reduces the average particle size of the resulting powder and ensures a narrower particle size distribution in comparison with particles prepared by a CH process.Phani et al.20synthesized the nanocrystalline ZnTiO3perovskite thin?lms by the sol-gel process assisted by microwave irradiation.These authors have emphasized a dramatic change in the morphological properties of the?lms irradiated in microwave compared to the conven-tional treatment.
The HTMW process is a fast and very promising technique for ceramic materials.It offers signi?cant advantages when compared with the CH method such as,reduced processing time, increased nucleation rate,homogeneous heating,hence,leading to the formation of smaller structures.17a-i Our group has signi?cantly expanded the effort to demostrate that the HTMW method is one of the simplest,most versatile,and highly cost-effective approaches available to obtain crystalline,chemically pure,single-phase micro and/or nano scale materials at lower temperature and shorter reaction times with little residual impurities.The intrinsic scalability,?exibility,and facility of this method render it attractive for the fabrication of a wide range of perovskite based materials.18a,21a,b Godinho et al.21b observed the interesting fact that,under HTMW heating,the growth process of the gadolinium-doped cerium oxide system is faster than under hydrothermal electric heating.Thus,they postulated that electromagnetic microwave heating increases the effective collision rates during the process.The very simple, attractive,and novel procedure concentrates microwave radiation into the solution,thereby enhancing the structural and morpho-logical properties.
The present work reports further progress on the synthesis of BZ powders using the HTMW method.In addition,the effects,as well as the merits,of microwave heating on the process and characteristics of the obtained BZ powders are reported.To the best of our knowledge,this is the?rst synthesis of microcrystalline BZ powders presenting a decaoctahedron shape.The corresponding structural properties and morphology were investigated and con?rmed by means of various tech-niques:X-ray diffraction(XRD)including Rietveld re?nements, UV-visible,ICP,and Raman spectroscopy.In addition,pho-toluminescence(PL)measurements at different times were performed at room temperature.This technique is an effective tool that provides important information on physical properties at molecular level defects and gap states.22a,b
2.Experimental Details
The BZ powders were synthesized using ZrCl45H2O(99.9%, Aldrich),BaCl22H2O(99%,Mallinckrodt),and KOH(P.a,Merck). The solutions were prepared as follows:in the?rst one,0.05mol of the ZrCl45H2O was added to125mL of deionized water at room temperature under stirring until it turned into a transparent solution. Similarly,0.05mol of the BaCl22H2O was dissolved in125mL of deionized water.The two precursor solutions were mixed,homogenized, and separated in?ve portions,and50mL of the KOH solution(6M) were added in each solution under constant stirring to act as mineralizer, taking the co-precipitation of the Zr and Ba hydroxides to form the reaction mixture.
The reaction mixture was loaded in a110mL Te?on autoclave reaching90%its volume,hence providing maximum pressure ef-?ciency,23which was sealed and placed in the microwave-hydrothermal system using2.45GHz microwave radiation with maximum power of 800W.24The reaction mixture was heated in less than1min to140°C and kept at this temperature for BaZrO3-Xmin,where X)10(BZ10),20(BZ20),40(BZ40),80(BZ80),and160min(BZ160), without agitation and under constant pressure of2.5bar.After the reaction,the autoclave was naturally cooled to room temperature.Next, the solid product was washed with deionized water several times until neutral pH,and then,dried overnight at80°C.
3.Characterizations
The samples were characterized by XRD(Rigaku DMax 2500PC)using Cu K R(λ)1.5406?)radiation.The data were collected from10°to120°in the2θrange with0.5°divergence slit,0.3mm receiving slit,in?xed-time mode with0.02°step size and Is/point.Crystal structures were identi?ed and re?ned by the Rietveld method25using the GSAS26software.This software is specially designed to re?ne simultaneously both the structural and the microstructural parameters through a least-squares method.The peak pro?le function was modeled using the convolution of the Thompson-Cox-Hastings pseudo-Voigt (pV-TCH)with the asymmetry function described by Finger et al.27The background of each pattern was?tted by a polynomial function.
An inductively coupled plasma atomic emission spectrometer ICP-AES Simultaneous CCD-VISTA-MPX(Varian,Mul-grave,Australia)with radial con?guration was used for chemical analyses.The operating conditions are the following:40Mhz frequency,100kW power,in a radial plasma view con?guration. Background signal correction was carried out by the instruments operating software.Microstructural and morphological charac-terization were performed by?eld emission scanning electron microscopy(FE-SEM,Zeiss Supra35).
Raman spectra were recorded on a RFS/100/S Bruker Fourier transform(FT-Raman)spectrometer,with Nd:YAG laser excita-tion light at1064nm in a spectral resolution of4cm-1.UV-vis absorption of the optical absorbance for BaZrO3decaoctahedrons was taken using Varian Cary5G equipment.Photoluminescence (PL)spectra were collected with a Thermal Jarrel-Ash Monospec 27monochromator and a Hamamatsu R446photomultiplier.The 350.7nm(3.52eV)exciting wavelength of a krypton ion laser (Coherent Innova)was used,with output power of the laser kept at200mW.All measurements were taken at room tem-perature.
4.Results and Discussion
XRD patterns of the samples treated from10to160min, corresponding until BZ10to the BZ160samples,respectively, are presented in Figure1,which also reports the BZ unitary cell with the main diffraction plane(110),indicating that this diffraction is related to the periodic ordination of dodecahedral (BaO12)and octahedral(ZrO6)sites.One can observe a pattern of a crystalline material composed by BZ cubic structure from JCPDS card No.06-0399with Pm3m space group and a weak amount of orthorhombic witherite BaCO3phase from JCPDS card No.05-0378with the Pmcn space group especially noticed for the BZ10sample.The low crystallinity and the small amount of amorphous material observed in the XRD of the BZ10sample indicates that the crystallization process remain incomplete up to10min.After20min,the cubic BZ phase can be clearly identi?ed in Figure1by the aforementioned JCPDS card. However,for40min(BZ40)during the HTMW process,it renders the existence of intense diffractions peaks marked as BZ phase together with a very low intensity peak at2θ)26°associated with the most intense diffraction of the witherite phase.For samples BZ80and BZ160,Figure1illustrates the high crystalline BZ powders,without background and with smallest amount of BaCO3phase(<0.6%).
834Crystal Growth&Design,Vol.9,No.2,2009Moreira et al.
Rietveld re?nements were recorded for samples BZ40,BZ80,and BZ160to obtain a deeper insight into the evolution of the barium zirconate phase.The BZ10and BZ20samples were not re?ned,as they do not correspond to the barium zirconate phase as indicated by the Raman,X-ray,and low Ba/Zr ratio obtained from ICP-AES measurements.The re?nement continues until convergence is reached with the value of the quality factor, 2,approaching 1,which con?rms the goodness of the re?nement.28In this case,the BZ160sample reports the more ordered structure among the samples,due to great 2)1.9and R wp )6%parameter values besides the smallest value for R-Bragg.Furthermore,the re?ned lattice parameters reported in the Table 1for BZ160are the closest to the JCPDS card No.06-0399,(a )b )c )4.18?).Another important factor is related to the 1.6%amount of barium carbonate phase for the BZ40sample and the 0.6%for the BZ160sample.The increasing values of R-Bragg for the BZ160sample occur because there is only one indexed peak for the BaCO 3phase,undermining the adjustment of the deleterious phase as depicted in the Figure 2,which represent a typical Rietveld re?nement analysis output for assessing the quality of ?tting.
Figure 3illustrates all the typically Raman modes of the BaCO 3phase.29The wide and weak vibrational modes at 381and 544cm -1are related to the m-zirconia phase.However,the others Raman modes of the m -zirconia were not observed because of the superposition of the vibrational modes of the BaCO 3.30a -e Thus,after 10min of synthesis,the orthorhombic barium carbonate and monoclinic ZrO 2phase were favored instead of the cubic BZ phase.An analysis of the micrographies presented in Figure 4a point out that this sample is associated to nearly spherical or equi-axed agglomerates generated by the smaller particles self-assembly.
The Raman spectrum of BZ20sample depicted in Figure 3supplies the same information observed for the BZ10sample.
The low crystallinity,evidenced by XRD,indicates that the crystallization process remains incomplete up to 20min,while in Figure 4b the morphologies start turning into a multifaceted shape;however,they are still badly de?ned and in the same manner as described for the BZ10sample.For the BZ40sample the monoclinic zircon phase is described as a natural reaction in aqueous medium;however,Raman characteristic modes of m -ZrO 2could not be all observed.31,32On the other hand,after 40min of the process,the BZ phase becomes stable and the decaoctahedron shapes become evident (Figure 4c).The deca-octahedron shape exhibits 18faces,6square and 12hexagonal faces.However,a discharge super?cial porosity can be observed (inset Figure 4c),in addition to badly de?ned face contours.The micrographs in Figure 4d,e show highly de?ned deca-octahedron shapes,with low face porosity together with the well de?ned face contour.Thus,the sample processed for 160min (BZ160)presents a more regular morphology.Local and dynamic symmetry of the Raman spectra for samples BZ40,BZ80,and BZ160report a new weak mode at 758cm -1.This mode can be associated to the presence of a slightly distorted ZrO 6octahedron.33Therefore,this fact points out that after 40min of synthesis,the disordered cubic barium zirconate phase became more favored than the barium and zirconium individual phases.Lee et al.34reported that OH -ions are important in the nucleation of BaTiO 3powders under hydrothermal conditions,and these ions also seem to act as catalysts to accelerate
the
Figure 1.XRD of BaZrO 3powders obtained for (a)10(BZ10),(b)20(BZ20),(c)40(BZ40),(d)80(BZ80),and (e)160(BZ160)at 140°C by the HTMW method.Also shown is the BZ unitary cell with the main diffraction plane (110).
Table 1.Rietveld Parameters of BaZrO 3Powders According to
HTMW Time Treatment
parameters
sample a )b )c (?) 2R-Bragg BZ (%)R-Bragg BaCO 3(%)BaCO 3(%)R wp (%)BZ40 4.211584 2.1 2.08.1 1.6 6.33BZ80 4.208129 1.7 1.5 6.3 1.2 5.48BZ160
4.206291
1.9
1.3
11.5
0.6
6.0
Figure 2.Rietveld re?nement of the BaZrO 3powders synthesized by the HTMW method at 140
°for 160min.
Figure 3.Raman spectra of the BaZrO 3powders obtained from 10up to 160min at 140°C.
Synthesis of Fine Micro-sized BaZrO 3Powders Crystal Growth &Design,Vol.9,No.2,2009835
transition of Ba-OH bonds to BaTiO 3crystals.In a similar way,the KOH concentration in the sheath acts on the formation of the BZ powders.This non-thermal effect yields an increase of the diffusion rate and a decrease of the activation energy by polarization molecules.24
On the other hand,the viscosity of the water decreases with increasing temperature.Under hydrothermal conditions at 500°C and 10bar,the water has only 10%of its viscosity under ambient conditions.Even under milder conditions,the viscosity is still low,35and thus it may be conceived that the mobility of dissolved ions and molecules is higher under hydrothermal conditions than at ambient pressure and temperature,taking into account the increase of the effective collision rate in the solution that occurs when particles collide,hence producing irreversibly oriented attachments during the HTMW processing.36a Besides,the use of the microwaves energy acts directly on the rotational barriers of the water employing uniform ratings 36a,b and are enhanced by one or two orders of magnitude of the crystalliza-tion kinetic behavior 17b,c because of a direct interaction of radiation with matter.These conditions offer a favorable medium for the formation of barium zirconate oxide at low temperatures and short annealing times.
The agglomeration process can be related to the van der Waals forces;then,to reduce surface energy,the primary particles have a basic tendency to form self-assembled agglomerations,orig-inating nearly spherical or equi-axed agglomerates with a minimum surface to volume ratio;hence,minimum surface free energy can be achieved.37After this process is observed for samples BZ10and BZ20,the agglomerated particles start the growth of multifaceted shapes to form a stable decaoctahedron shape for the BZ40,BZ80,and BZ160samples as recorded in Figure 4.
XRD,Raman,and ICP-AE spectroscopy recordings are used to carry out an analysis of the crystal morphology evolution along the synthetic process.The results seem to indicate for samples BZ10and BZ20that the dissolution of the barium
carbonate and m -zirconia takes place,supplying the initial formation of the barium zirconate phase.After 40min of processing,the BZ phase can indeed be signaled as preferential and sitable crystalline cubic phase with well de?ned decaocta-hedron shapes (Figure 4c -e),besides a better Ba/Zr ratio as follows.
ICP-AES spectroscopy was used to determine the Ba/Zr ratio,as well as the purity degree of the samples.To estimate the accuracy of the determinations in the BZ solutions with lower salt concentrations,two series of 1:10and 1:100dilutions were prepared from the original digested samples.The dissolution procedure was carried out using 10mL of HCl (37%m/m)and 3mL of HNO 3(68%m/m)in closed vessels at room temperature for 2h.Analytical blanks were prepared following the same acid digestion procedure.Table 2shows the nominal composi-tions of the powders,and the concentration of impurities (Ca +Sr +Mg)lesser than 1ppm is achieved.
The performance of the material (optical,electric,and structural properties)is not only sensitive to pore sizes (1to 100nm)but also to access to the pore network that depends on particle size (1to 100mm)and morphology;38then,dense particles are desirable specially for electrical applications.The decaoctahedron shape is an important factor regarding the degree of disorder that leads to an incomplete circumference (the decaoctahedron circumference is less than 360°)hence
causing
Figure 4.FE-SEM image of nearly spherical BaZrO 3powders obtained for (a)10and (b)20min,and of decaoctahedral BaZrO 3powders obtained for (c)40,(d)80,and (e)160min.
Table 2.Nominal and Experimental Compositions of BaZrO 3
composition,in %molar fraction nominal
analyzed a ceramic denomination
x Ba/Zr x Ba/Zr BZ1010.97(0.006BZ2010.98(0.002BZ4010.99(0.003BZ8010.99(0.003BZ160
1
0.99(0.002
a
n )5(n )measures).
836Crystal Growth &Design,Vol.9,No.2,2009Moreira et al.
network tensions similar to that shown for the decahedron by Wang et al.39subsequently,tensions will surely be induced in the particles.
Meng et al.40proposed that the PL emission band increases the intensity with decrease of the grain size,which is closely correlated with interfaces of nanocrystallites (<100nm)and d-surface states in the forbidden gap by the distortion of the TiO 6octahedra.In our case,(Figure 5)the samples present a micro-sized decaoctahedron shapes;therefore,the PL emissions cannot be attributed only to the size of the crystals (nanosized).Furthermore,Cavalcante et al.10developed a consistent discus-sion regarding the evolution of the structural order of barium zirconate,based on the existence of zircon complex clusters in the BZ matrix,such as:
[ZrO 6]x +[ZrO 5V 0x ]f [ZrO 6]
+[ZrO 5V 0·]
[ZrO 6]x +[ZrO 5V 0·]f [ZrO 6] +[ZrO 5V 0··]
[ZrO 5V 0·]+1
2
O 2f [ZrO 6]
When [Z rO 6]x .[Z rO 5V ?0]and/or [ZrO 5V ?0])0,the PL emission is quickly reduced.For these octahedral complex clusters the authors attributed the formation of localized levels within the band gap,for disordered samples thermally treated at temperatures between 350and 600°C.Therefore,no PL emission was observed for samples heat treated at 700°C by 2h in a conventional furnace,which was described as the more ordered structure (structurally sample [ZrO 5V ?0])0).In the other samples heat treated at lower temperatures (<700°C)the existence of a signi?cant relationship between [ZrO 6]x and [ZrO 5V ?0]clusters allowed the polarization system.
Leite et al.41a reports the typical PL emission of disordered barium zirconate powders at room temperature around 585nm,and Cavalcante et al.reports the PL emission for the same material at around 540nm,10using the same wavelength excitation.Therefore,the PL emission of BZ40,BZ80,and BZ160presents more ordered structures than observed by them,because of a large blue-shift emission centered at 526nm (Figure 5).However,a certain disorder degree must persist in the ZrO 6octahedral site which allows the formation of the intermediate levels within the barium zirconate band gap.41a,b
Thus,the PL emission can be attributed to the freezing of structural defects inside the zirconium octahedrons,which are related to the use of the microwave energy acting directly on the rotational barriers of the water providing a high heating rate
(around 120°/min)and increase the self-assembled pressure,hence forcing the fast formation of distorted BZ (quasi-crystalline)powders.With the increase of the thermal treatment time,it can be perceived from the Rietveld re?nement data that the unitary cell tends to relax the structural tensions approach-ing the system with more ordered structure.
The structural properties associated with both distorted octahedral (ZrO 6)and dodecahedral (BaO 12)clusters forming the constituent polyhedrons of BaZrO 3system are crucial for the PL property of this system.This seems to be general for the perovskite-based materials.The slight distortions on the ZrO 6octahedron are corroborated by Raman spectra due to the existence of the weak signal at 758cm -1,33which are responsible for the polarization of the structure,just as proposed by Meng et al.10,41b,42On the other hand,non-substantial distortions were observed on the dodecahedral BaO 12sites.Besides,the PL emission at 477nm corresponds to a non-uniform structure of the BZ phase,associated with m -zirconia structure as previously discussed in the Rietveld re?nements section.36a,41b
In our proposed model,the wide-band model 22b,42for PL emission of the barium zirconate samples,the most important events occur before excitation,that is,before the arrival of the photon.The short and intermediate range structural defects generate localized states within the band gap and a non-homogeneous charge distribution in the cell thereby allowing electrons to become trapped.The localized levels are energeti-cally distributed,so that various energies are able to excite the trapped electrons 22b and the recombination of the excited trapped electrons gives a PL emission.
The same type of PL emission behavior can be observed in the UV -visible measurements in Figure 6,which reveal a typical absorbance for crystalline materials in the high energy region and two other absorbancies in the low energy region,suggesting large non-uniform distribution states within the band gap.The optical energy band gap is related to the absorbance and the photon energy by the following equation according to Wood and Tauc:
h VR ∝(h V-E g opt )2
(2)
where R is the absorbance,h is the Planck constant,V is the frequency,and (E g opt )is the optical band gap.43
The equi-axed shape for samples BZ10and BZ20results in optical gap values of 4.78and 4.89eV,respectively,which are smaller than the gap of highly structural BZ powders (5.3eV).
3
Figure 5.Photoluminescence spectra of equi-axed and decaoctahed-rons BaZrO 3
samples.
Figure 6.UV -visible gap dependence of BaZrO 3phase and growth of the particles.
Synthesis of Fine Micro-sized BaZrO 3Powders Crystal Growth &Design,Vol.9,No.2,2009837
When the structures of BZ becomes predominant in the samples BZ40,BZ80,and BZ160,a well de?ned decaoctahedron shape is observed;then,the band gap values tend to the values of structurally ordered barium zirconate powders(5.3eV)3and higher than for more ordered samples obtained by Cavalcante et al.10On the other hand,structural distortions at octahedral sites provoke the PL emission shift to the blue region in the visible spectra and actually present a distorted structure.The band gap energy is much higher than the excitation energy used to record of PL spectra(3.54eV).This fact seems to indicate the probability that certain localized levels exist within the band gap because the direct electron transition between the valence band and the conduction band is not allowed.36a
The exponential optical absorption edge and hence the optical band gap are controlled by the structural order-disorder degree on the BZ lattice.The decrease of the band gap can be directly associated with the increase of defects in the BZ lattice (octahedral distortion),which raises the intermediary levels within the band gap region,supplying a fundamental condition for the existence of polarizations and PL emission.Thus,the reduction to4.99eV of the band gap(for more ordered HTMW samples)can be related to the distortions on the ZrO6octahedral clusters in a cubic barium zirconate phase,41a-c evidenced by the Raman spectra as a weak signal at758cm-1.33
5.Conclusion
In summary,micro-sized decaoctahedron shaped BZ powders were successfully synthesized using the HTMW method.This technique offers clean,low cost-effective,energy ef?cient,quite faster,and convenient method of heating,which results in higher yields and shorter reaction times in comparison with CH, complex polymerization,and solid state reactions because of the increase of the effective collision rate and the consequent favorable growth of BZ crystals by successive attachments.We believe that this mechanism is able to promote the formation of defects and/or distortions on the BZ polyhedrons.According to XRD,Raman spectra,UV-visible absorbancies,ICP-AES spectroscopy,and FE-SEM microscopy,the powders after40 min in the HTMW system present a predominance of the barium zirconate phase that composes novel micro-sized decaoctahedron shapes.The presence of two PL emissions centered at477and 526nm,as well as the time evolution of the band gap energies, seems to indicate the existence of two different emission mechanism for the processed material at different times.A theoretical model derived from previous?rst principle calcula-tions allows us to discuss the origin of PL emission in BaZrO3 powders.This behavior can indeed be assigned to the local disorder in the network of both ZrO6octahedral and dodeca-hedral(BaO12),hence forming the constituent polyhedron of the BZ system,owing to the fact that if one of the polyhedrons is distorted it should have consequent distortions on the other, as they share the same crystalline net.In this work,we have shown that by means of the HTMW synthetic method,reliable size,shape,and composition control can be achieved to obtain micro-sized decaoctahedron BZ powders,opening the door for advanced materials as promising candidates for future catalysis applications.
Acknowledgment.The authors acknowledge the support of the Brazilian agencies CAPES,CNPq,and FAPESP/CEPID98/ 14324-8.J.A.acknowledges by research funds provided by the Ministerio de Educacio′n y Cultura of the Spanish Government to carry out a research stay.We thank Diogo Volanti,Rorivaldo Camargo,and Madalena Tursi for their technical contributions.
References
(1)Mao,Y.;Wong,S.S.Ad V.Mater.2005,17,2194–2199.
(2)Zhou,H.;Mao,Y.;Wong,S.S.Chem.Mater.2007,19,5238–5249.
(3)Robertson,J.J.Vac.Sci.Technol.B.2000,18,1785–1791.
(4)(a)Erb,A.;Traulsen,T.;Muller-Vogt,G.J.Cryst.Growth1994,137,
487–492.(b)Macmanus-Driscoll,J.L.;Foltyn,S.R.;Jia,Q.X.;Wang,
H.;Serquis,A.;Civale,L.;Maiorov,B.;Hawley,M.E.;Maley,M.P.;
Peterson,D.E.Nat.Mater.2003,3,439–443.(c)Kolen’ko,Y.V.;
Burukhin,A.A.;Churagulov,B.R.;Oleinikov,N.N.;Vanetsev,A.S.
Inorg.Mater.2002,38,252–255.(d)Bennett,J.W.;Grinberg,L.;
Andrew,M.R.Phy.Re V.B2006,73180102(R),1–4.
(5)Mao,Y.B.;Banerjee,S.;Wong,S.S.J.Am.Chem.Soc.2003,125,
15718–1519.
(6)(a)Dias,A.;Ciminelli,V.S.T.Chem.Mater.2003,15,1344–1349.
(b)Lu,Z.;Tang,Y.;Chen,L.;Yadong,L.J.Cryst.Growth2004,
266,539–544.
(7)(a)Urban,J.J.;Spanier,J.R.;Ouyang,L.;Yun,W.S.;Park,H.Ad V.
Mater.2003,15,423–426.(b)Yun,W.S.;Urban,J.J.;Gu,Q.;Park,
H.Nano Lett.2002,2,447–450.
(8)(a)OBrien,S.;Bras,L.;Murray,C.B.J.Am.Chem.Soc.2001,123,
12085–12086.(b)Liu,C.;Zou,B.S.;Rondinone,A.J.;Zhang,Z.J.
J.Am.Chem.Soc.2001,123,4344–4345.
(9)Kirby,N.M.;Trang,A.;Riessen,V.A.;Buckley,C.E.;Wittorff,
V.W.;Cooper,J.R.;Panagopoulos,C.Supercond.Sci.Technol.2005, 18,648–657.
(10)Cavalcante,L.S.;Longo,V.M.;Zampieri,M.;Espinosa,J.W.M.;
Pizani,P.S.;Sambrano,J.R.;Varela,J.A.;Longo,E.;Simo?es,M.L.;
Paskocimas,C.A.J.Appl.Phy.2008,103(063527),1–8.
(11)Veith,M.;Mathur,M.;Lecerf,N.;Huch,V.;Decker,T.;Beck,H.P.;
Eiser,W.;Haberkorn,R.J.Sol-Gel Sci.Technol.2000,17,145–158.
(12)Boschini,F.;Robertz,B.;Rulmont,A.;Cloots,R.J.Eur.Ceram.
Soc.2003,23,3035–3042.
(13)Kutty,T.R.N.;Vivekanandan,R.;Philip,S.J.Mater.Sci.1990,25
(8),3649–3658.
(14)Brzezinska-Miecznik,J.;Haberko,K.;Bucko,M.M.Mater.Lett.2002,
56(3),273–278.
(15)(a)Gopalakrishnan,J.Chem.Mater.1995,7,1265–1275.(b)Hirano,
M.Recent Res.De V.Mater.Sci.2002,3,563–577.(c)Aimable,A.;
Xin,B.;Millot,N.;Aymes,D.J.Solid State Chem.2008,181,183–189.
(16)(a)Fuenzalida,V.M.;Pilleux,M.E.J.Mater.Res.1995,10,2749–
2754.(b)Pilleux,M.E.;Grahmann,C.R.;Fuenzalida,V.M.Appl.
Surf.Sci.1993,65,283–288.
(17)(a)Rao,K.J.;Vaidhyanathan,B.;Ganguli,M.;Ramakrishnan,P.A.
Chem.Mater.1999,11,882–895.(b)Komarneni,S.;Roy,R.;Li,Q.
Mater.Res.Bull.1992,27,1393–1405.(c)Komarneni,S.;Li,Q.;
Stefansson,K.M.;Roy,R.J.Mater.Res.1993,8,3176–3183.(d) Abothu,I.R.;Liu,S.F.;Komarneni,S.;Li,Q.H.Mater.Res.Bull.
1999,34,1411–1419.(e)Bondioli,F.;Ferrari,A.M.;Leonelli,C.;
Siligardi,C.;Pellacani,G.C.J.Am.Ceram.Soc.2001,84,2728–2730.(f)Corradi,A.B.;Bondioli,F.;Focher,B.;Ferrari,A.M.;
Grippo,C.;Mariani,E.;Villa,C.J.Am.Ceram.Soc.2005,88,2639–2641.(g)Sun,W.;Liu,W.Li.J.Powder Technol.2006,166,55–59.
(h)Clark,D.E.;Sutton,W.H.Annu.Re V.Mater.Sci.1996,26,299–
331.(i)Booske,J.H.;Cooper,R.F.;Freeman,S.A.Mat.Res.Inno V at.
1997,1,77–84.
(18)(a)Volanti,D.P.;Keyson,D.;Cavalcante,L.S.;Simo?es,A.Z.;Joya,
M.R.;Longo,E.;Varela,J.A.;Pizani,P.S.;Souza,A.G.J.Alloys Compd.2008,459,537–542.(b)Shi,S.;Hwang,J.Y.J.Miner.Mater.
Charact.Eng.2003,2,101–110.
(19)Maksimov,V.D.;Meskin,P.E.;Churagulov,B.R.Inorg.Mater.
2007,47,988–993.
(20)Phani,A.R.;Passacantando,M.;Santucci,A.J.Phys.Chem.Solids
2007,68,317–323.
(21)(a)Keyson,D.;Volanti,D.P.;Cavalcante,L.S.;Simo?es,A.Z.;Varela,
J.A.;Longo.,E.Mater.Res.Bull.2008,43,771–775.(b)Godinho, M.;Ribeiro,C.;Longo,E.;Leite,E.R.Cryst.Growth Des.2008,8, 384–386.
(22)(a)Schroder,D.K.Semiconductor Mat.De V ise Char.;John-Wiley:
New York,1990.(b)Longo,V.M.;Cavalcante,L.S.;Erlo,R.;
Mastelaro,V.R.;de Figueiredo,A.T.;Sambrano,J.R.;de La′zaro, S.;Freitas,A.Z.;Gomes,L.;Vieira,N.D.,Jr.;Varela,J.A.;Longo,
E.Acta Mater.2008,56,2191–2202.
(23)Richard,I.Walton.Chem.Soc.Re V.2002,31,230–238.
(24)Volanti,D.P.;Cavalcante,L.S.;Keyson,D.;Lima,R.C.;de Moura,
A.P.;Moreira,M.L.;Macario,L.R.;Godinho,M.Met.Mater.2007,
63,351.
(25)Rietveld,H.M.J.Appl.Crystallogr.1969,2,65–71.
838Crystal Growth&Design,Vol.9,No.2,2009Moreira et al.
(26)Larson,A.C.;Von Dreele,R.B.Los Alamos National Laboratory
Report https://www.sodocs.net/doc/cf12730590.html,UR86-748,2004.
(27)Finger,L.W.;Cox,D.E.;Jephcoat,A.P.J.Appl.Crystallogr.1994,
27,892–900.
(28)Moreira,M.L.;Pianaro,S.A.;Andrade,A.V.C.;Zara,A.Mater.
Charact.2006,57,193–198.
(29)Edwin,A.;Schauble,J.;Ghosh,P.;Eiler,J.M.Geochim.Cosmochim.
Acta2006,70,2510–2529.
(30)(a)Phillippi,C.M.;Mazdiyasni,K.S.J.Am.Ceram.Soc.1971,54,
254–258.(b)Feinberg,A.;Perry,C.H.J.Phys.Chem.Solids1981, 42,513–518.(c)Perry,C.H.;Liu,D.W.;Ingel,R.P.J.Am.Ceram.
Soc.1985,68,184–187.(d)Kim,B.K.;Hahn,J.W.;Han,K.R.J.
Mater.Sci.Lett.1997,16,669–671.(e)Ishigame,M.;Sakurai,T.
J.Am.Ceram.Soc.1977,60,367–369.
(31)Zyuzin,D.A.;Svetlana,V.C.;Moroz,E.M.;Burgina,E.B.;Sadykov,
V.A.;Kostrovskii,V.G.;Matyshak,V.A.J.Solid State Chem.2006, 179,2965–2971.
(32)Xie,S.;Iglesia,E.;Bell,A.T.Chem.Mater.2000,12,2442–2447.
(33)Dobal,P.S.;Dixit,A.;Katiyara,R.S.;Yu,Z.;Guo,R.;Bhalla,A.S.
J.Appl.Phys.2001,89,8085–8091.
(34)Lee,S.K.;Park,T.J.;Choi,G.J.Mater.Chem.Phys.2003,82,
742–749.
(35)Rabeneau,A.Angew.Chem.,Int.Ed.Engl.1985,24,1026–1040.
(36)(a)Moreira,M.L.;Mambrini,G.P.;Volanti,D.P.;Leite,E.R.;
Orlandi,M.O.;Pizani,P.S.;Mastelaro,V.R.;Paiva-Santos,C.O.;
Longo,E.;Varela,J.A.Chem.Mater.2008,20(16),5381–5387.(b)
Wilson,G.J.;Matijasevich,A.S.;Mitchell,D.R.G.;Schulz,J.C.;
Will,https://www.sodocs.net/doc/cf12730590.html,ngmuir.2006,22,2016–2027.
(37)Khollam,Y. B.;Deshpande, A.S.;Patil, A.J.;Potdar,H.S.;
Deshpande,S.B.;Date,S.K.Mater.Chem.Phys.2001,71,304–308.
(38)Billinge,S.L.J.;Levin,I.Science2007,316,516–519.
(39)Wang,Z.L.J.Phys.Chem.B.2000,104,1153–1175.
(40)Meng,J.;Huang,Y.;Zhang,W.;Du,Z.;Zhu,Z.;Zou,G.Phys Lett
A1995,205,72–76.
(41)(a)Leite,E.R.;Pontes,F.M.;Lee,E.J.H.;Aguiar,R.;Longo,E.;
Pontes,D.S.L.;Nunes,M.S.J.;Pizani,P.S.,Jr.;Boschi,T.M.;
Varela,J.A.;Paskocimas,C.A.;Pinheiro,C.D.;Taft,C.A.J.Mol.
Struct.(Theochem).2004,668,87–91.(b)Longo,V.M.;Cavalcante, L.S.;de Figueiredo,A.T.;Santos,L.P.S.;Longo,E.;Varela,J.A.;
Sambrano,J.R.;Parkocimas,C.A.;De Vicente,F.S.;Hernades,A.C.
Appl.Phys.Lett.2007,90(091906),1–3.(c)Paris,E.C.;Espinosa, J.W.M.;de Lazaro,S.;Lima,R.C.;Joya,M.R.;Pizani,P.S.;Leite,
E.R.;Souza,A.G.;Varela,J.A.;Longo,E.Chem.Phys.2007,335,
7–14.
(42)Anicete-Santos,M.;Cavalcante,L.S.;Orhan,E.;Paris,E.C.;Simoes,
L.G.P.;Joya,M.R.;Rosa,I.L.V.;de Lucena,P.R.;Santos, M.R.M.C.;Santos-Junior,L.S.;Pizani,P.S.;Leite,E.R.;Varela, J.A.;Longo,E.Chem.Phys.2005,316,260–266.
(43)Wood,D.L.;Tauc,J.Phys.Re V.B Solid State1972,5,3144–3151.
CG800433H
Synthesis of Fine Micro-sized BaZrO3Powders Crystal Growth&Design,Vol.9,No.2,2009839