Materials Research Bulletin 47 (2012) 527–531

Synthesis of monodisperse spherical nanometer ZrO2 (Y2O3) powders via thecoupling route of w/o emulsion with urea homogenous precipitation

Ying Chang a, Shijie Dong b,*, Huihu Wang b, Kuanhe Du a, Qingbiao Zhu a, Ping Luo b

a Department of Materials, College of Chemical and Environment Engineering, Hubei University of Technology, Wuhan, Hubei 430068, Chinab School of Mechanical Engineering, Hubei University of Technology, Wuhan, Hubei 430068, China

A R T I C L E I N F O

Article history:

Received 11 August 2011

Received in revised form 27 November 2011

Accepted 30 December 2011

Available online 8 January 2012

Keywords:

A. Nanostructures

B. Chemical synthesis

C. X-ray diffraction

A B S T R A C T

Using xylol as the oil phase, span-80 as the surfactant, and an aqueous solution containing zirconium

(3 mol% Y2O3) and urea as the water phase, tetragonal phase ZrO2 nano-powders have been prepared via

the coupling route of w/o emulsion with urea homogenous precipitation. The effects of the zirconium

concentration, the reaction temperature and the urea content on the average size of the products have

been examined. The as-prepared ZrO2 powders and the precursor powders were characterized by TGA–

DTA, XRD, TEM and BET. Experimental results indicate that ZrO2 powders prepared via the coupling route

of w/o emulsion with urea homogenous precipitation possess some excellent characteristics, such as

well-rounded spherical shape and excellent dispersing.

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Materials Research Bulletin

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1. Introduction

As well known, powder characteristic has great effect onproperties of the final ceramic products. Ceramic powderconsisting of microparticles (one-tenth to several micrometers)with spherical morphology and narrow size distribution possesseslow sintering temperature, high packing density, and uniformmicrostructure [1–4,10]. Nanoparticles (<0.1 mm) can furtherenhance sinter ability at lower temperatures [5–8], and the as-prepared ceramics possess smaller residual pores and grain sizes ifan appropriate technology process is carried out [9]. As formicroemulsion route, the water droplets wrapped up in surfactantenveloped by oil phase can be used as nanosized reactors for theformation of nanoparticles [10,15]. Therefore, spherical powderwith uniform size distribution and good dispersibility can beobtained. All of them suit for the need of the preparation of ceramicmaterials, so there are lots of papers about the preparationmethods of ZrO2 nanoparticles from microemulsion [10–15].However, for the routes that have been reported, it needs to mixmicroemulsion containing ions to be precipitated with anotherkind of microemulsion, solid or gas containing precipitator [13–15]. During the mixture process, it is inevitable to produceconcentration gradient, which makes the water droplets ofmicroemulsion in different micro-circumstances, changes the sizeand stability of water droplets, leads to unsynchronization ofprecipitate reaction in different location of microemulsion, thus

* Corresponding author. Tel.: +86 027 88032313; fax: +86 027 88032313.

E-mail address: dongsjsj@163.com (S. Dong).

0025-5408/$ – see front matter � 2012 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2011.12.055

the size distribution of particles will become scattering. At thesame time, the ratio of the aqueous phase as reactants inmicroemulsion is too small, and the organic solvent prices arecomparatively high [10,12–15]. As for homogenous precipitationprocess, which is another method for producing nanoparticles, theprecipitate can be uniformly generated on-site by their precursors[16–18]. This route essentially reduces the concentration gradientof reactants, and effectively controls the size distribution ofnanoparticles. There are lots of reports about nanoparticlesprepared via homogenous precipitation route [19–23]. However,as the precipitating reaction takes place in the whole aqueoussolution, it is difficult to restrict the growth and aggregation ofgrains. At the same time, it is difficult to control the shape ofparticles.

To overcome the disadvantages of both aforementioned routesand take the full of advantage of them, the coupling route of w/oemulsion with urea homogenous precipitation are used to preparethe nanometer ZrO2 (3 mol% Y2O3) powders, in which xylol wasused as the oil phase, span-80 as the surfactant, and an aqueoussolution containing Zr(NO3)4, Y(NO3)3 and urea as the water phase.During synthesizing Zr(OH)4 via homogenous precipitation, ureaand Zr(NO3)4 needs to be resolved in water to get mixed solution.At room temperature, because of the long inducing time ofhydrolysis of urea and the low NH3�H2O concentration, it is difficultfor the mixed solution of urea and Zr(NO3)4 to form Zr(OH)4

precipitation. But the inducing time of hydrolysis of urea would begreatly shortened with the increase of temperature of solution, andOH� via ionizing NH3�H2O and NH3�H2O would be producedhomogenously in solution [24–26]. The Zr(OH)4 particles wouldhomogenously and synchronously precipitate, so it is possible to

Fig. 1. Typical DTA/TG curves of precursor powders.

Y. Chang et al. / Materials Research Bulletin 47 (2012) 527–531528

control the size distribution of Zr(OH)4 particles. But it is difficultfor homogenous precipitation to restrict the growth degree and theaggregation of particles [16–27], thus it is hard to attain smallZr(OH)4 particles by this method.

In this paper, by adding appropriate amount of span-80 andxylol into mixed solution of urea and zirconium nitrate, theuniform w/o emulsion were firstly prepared. The water droplets ofemulsion would be employed as the microreactors [10–13], withinwhich the homogenous precipitation of urea with zirconiumnitrate takes place. At the same time, in the water droplets ofemulsion, when the stable nucleus of Zr(OH)4 was formed, itenlarged through the growth and aggregation of primary particles.As the particle size reached the interface of water droplets, thesurfactants would cover the particles surface and hinder furtherparticle growth [11–15], which also restricted the size of particles.On the other hand, since the water droplets contained the identicalsolution, located in the similar surroundings, and went through thesame reactions, the size and the stability of water droplets and theparticles in them were near the same, which resulted in producingthe Zr(OH)4 particles with narrow size distribution [18,20,23–26].It can be known that this coupling route not only eliminated thegradient of precipitation concentration, but also confined the spaceof precipitating reaction, which is neither similar to the ordinarymicroemulsion route, from which the mixing process of micro-emulsion with another reactant was often necessary [11,13,15],nor to the conventional homogenous precipitation route, fromwhich the precipitates were deposited in the whole aqueoussolution [17,20–27]. So it is able to synthesis the particles withsmall size and narrow size distribution by this coupling route. Inaddition, it should be pointed out that because the temperaturegradient exists in reaction solution, it would affect the synchro-nism of hydrolysis reaction and precipitate reaction in waterdroplets. As a result, the ranges of size distribution of particleswould broaden.

2. Experiments

At room temperature, Zr(NO3)4�2H2O, Y(NO3)3�6H2O andCO(NH2)2 (AR, all of them were produced by Sinopharm ChemicalReagent Co., Shanghai, China) were first confected into four kindsof precursor solutions in which the mol ratio of Y(NO3)3 to Zr(NO3)4

was 3:97, and the mol ratios of CO(NH2)2 to Zr(NO3)4 respectivelywere 2:1, 2.5:1, 3:1 and 3.5:1, then the solutions respectively wereadded into the xylol (AR, Tianli Reagent Co., Tianjin, China) solutioncontaining surfactant span-80 (CP, Xilong Chemical Reagent Co.,Guangzhou, China), and the volume ratio of span-80 to xylol was1:5. During the process of adding, kept stirring with a magneticstirrer (DF-101S, Dongxi Refrigeration equipment Co., China). Thenmixed solutions were carried on dispersion with supersonicequipment (KS-180D, Haishu Kesheng Ultrasonic Equipment Co.,China) for 20 min to form emulsions. The emulsions were addedinto high pressure container (100 ml, Yuhua Instruments Corpo-ration, China) to produce gels at 130 8C water bath for 3 h. After thegels were treated with azeotropic distillation method, and most ofthe water and oil were removed when temperature increases tonear the boiling point of oxlyl, which is about 144 8C. The distilledgels were filtered, and repeatedly washed with deionized water toremove the residual surfactants and oil phase. At last, thedeionized water in gels would be got rid of through washing withethyl-alcohol, then the gels were dried at 80 8C vacuum for 24 h.Finally, the dried gels were calcinated under different tempera-tures (400 8C, 450 8C, 500 8C, 550 8C, 600 8C, 700 8C and 800 8C) toproduce ultrafine zirconia powders.

The morphology and size of the resulting particles weredetermined by transmission electron microscopy (JEM-200CX, JEOLCorporation, Japan). Dried powders were thermally decomposed

by carrying out thermogravimetric and differential thermal analyses(TG/DTA7200, Seiko Instruments Inc-SII, Japan) heating at a rate of10 8C/min up to the temperature of 1000 8C. The crystalline phases ofcalcined powders were identified by X-ray diffractometer(RINT1100, Rigaku Corporation, Japan) with Cu Ka radiationoperating at 40 kV, 40 mA, and scanning at a rate of 18 min�1. Thespecific surface area was measured by N2 adsorption–desorption,BET method (BELSORP-miniII, Ankersmid Corporation, Holland).The size distributions of particles were determined by means of alaser particle size analyzer (ZetaPlus, Brukerhave InstrumentsCorporation, USA).

3. Results and discussion

3.1. TG/DTA and XRD results

Fig. 1 shows the TG/DTA curves of ZrO2 precursor powderheated in air up to 1000 8C. In the TG curve, the precursor powdershows a total weight loss of 22.5% during heat treatment, and thisvalue is similar to 22.61%, the theoretical weight loss of Zr(OH)4 orZrO2�2H2O. However, a broad endothermic peak and a large weightloss appear between 20 8C and 130 8C, corresponding to the loss ofphysically adsorbed water. On the other hand, two exothermicpeaks respectively appear at about 330 8C and 520 8C; the former isattributed to the oxidation and the decomposition of organicresidue, and the latter is a sharp exothermic peak, which is relatedto the crystallization of amorphous zirconia [28].

Fig. 2 shows XRD patterns of calcined powders under differenttemperature. There is no obvious diffraction peaks, this resultshows that the powders maintained their amorphous states orpolycrystalline at 400 8C. Some small diffraction peaks at 500 8Cindicates that some crystal phase has come into being. The shape ofdiffraction peaks at 600 8C shows that crystallization has beenfinished and 100% tetragonal phase is obtained [29], so the 600 8Ccalcination temperature of precursor is reasonable according to theXRD. As the calcinations temperature rises, the width of diffractionpeaks becomes narrower but the crystal phase remains stable,which indicates that the powder crystal grain grows gradually.

3.2. The effect of reaction conditions upon the particle sizes

3.2.1. The effect of concentration of zirconium nitrate solution on

particle sizes

The variation of the average size of ZrO2 particles withconcentration of Zr(NO3)4 is shown in Fig. 3. The reaction

Fig. 4. The particle sizes at various reaction temperatures.Fig. 2. X-ray diffraction patterns of powders at different calcination temperatures in

air for 1 h.

Y. Chang et al. / Materials Research Bulletin 47 (2012) 527–531 529

conditions are as follows: reaction temperature was 130 8C,reaction time was 3 h, and the mol ratio of urea to zirconiumnitrate was 3.5:1. Average particle size was calculated with theequation:

d ¼ 6

sr(1)

where s is the specific surface area (m2 g�1), d is the averageparticle size (um), and r is the theoretical density (6.111 g/cm2) ofthe sample (g cm�3), the particles were assumed to be spheres[14]. It can be seen from Fig. 3 that with the increase of theconcentration of Zr(NO3)4, the particle size of powders graduallyenlarges. The reason is, within a certain range, the weaker theconcentration of solution, the slower the growth velocity ofnucleus, and the smaller the particle size [30]. With the increase ofthe solution concentration, the growth velocity of nucleusbecomes fast, which would further the possibility of aggregationof primary particles. Therefore, the size of the result particleswould be large. But in this route, xylol is used as the oil phase, andspan-80 as the surfactant, which block the aggregation betweengel particles [12,31,32].

Fig. 3. Variation of the average size of ZrO2 particles with concentration of Zr(NO3)4.

3.2.2. The effect of reaction temperature on particle sizes

Particle sizes at various reaction temperatures are shown inFig. 4. The reaction conditions are as follows: the concentration ofZr(NO3)4 was 0.4 mol/l, reaction time was 3 h, the mol ratio of urea tozirconium nitrate was 3.5:1. The result indicates that with thereaction temperature rising, the particle size reduces before itincreases. The reason of this phenomenon is that the process ofdynamic equilibrium of precipitation-solution would become fastwith the increase of temperature, which makes Zr(OH)4 particlesdisperse more homogenously. Besides, with the rise of the reactiontemperature, the growth velocity of nucleus becomes fast, thequantity of nucleus increases and the particle size becomes small.But if the reaction temperature is too high, the reaction velocitywould become too fast, and the particle size distribution is difficultto control. Therefore, too high temperature would result in bigparticle size via forming unstable emulsion. At the same time, it canbe seen from Fig. 4 that the suitable reaction temperature is 130 8C.

3.2.3. The effect of urea content on particle sizes

Theoretically, during the reaction process, the most appropriatemol ratio of urea to zirconium nitrate was 2:1. But the quantity ofurea should be in excess of zirconium nitrate, because it isimpossible for urea to hydrolyze completely [26]. The variation ofthe average size of ZrO2 particles with mol ratio of CO(NH2)2 to

Fig. 5. The particle sizes at different ratio of CO(NH2)2 to Zr(NO3)4.

Fig. 6. TEM micrographs of zirconia powders at different ratio of CO(NH2)2 to Zr(NO3)4.

Fig. 7. Size distribution of zirconium dioxide powders with ultrasonic

deagglomeration for 10 min (the reaction conditions are as follows: reaction

temperature was 130 8C, reaction time was 3 h, and the mol ratio of methyl oxalate

to zirconium nitrate was 3.5:1).

Y. Chang et al. / Materials Research Bulletin 47 (2012) 527–531530

Zr(NO3)4 is shown in Fig. 5. Fig. 6 shows TEM of ZrO2 powders, inwhich the mol ratio of urea to zirconium nitrate varied from 2:1 to3.5:1. It can be seen from Figs. 5 and 6 that the bigger the mol ratioof urea to zirconium nitrate, the smaller the particle size. Thereason is that with the rise of the quantity of urea, theconcentration of OH� become large, and the degree of supersatu-ration increases, which is beneficial to form small particleprecipitation [33,34]. The main reaction principle can be expressedas follows (Eqs. (2)–(4))

COðNH2Þ2þ 3H2O ! 2NH3�H2O þ CO2 (2)

NH3�H2O ! NH4þ þ OH� (3)

4OH� þ Zr4þ ! ZrðOHÞ4 # (4)

Besides, it can be seen from Fig. 6 that the shape of powders is awell-rounded sphere, at the same time, according to the data oflaser particle size analyzer, Fig. 7 shows the size distribution ofzirconium dioxide powders, it indicates that the as-preparedpowders possess narrow particle size distribution and excellentmonodispersity [35]. Therefore, the result of Figs. 6 and 7 indicatesthat the coupling route can control the shape and the sizedistribution of particles.

4. Conclusions

1. Tetragonal phase ZrO2 nanopowders containing 3 mol% Y2O3

have been prepared via the coupling route of w/o emulsion with

Y. Chang et al. / Materials Research Bulletin 47 (2012) 527–531 531

urea homogenous precipitation, in which xylol was used as theoil phase, span-80 as the surfactant, and an aqueous solutioncontaining zirconium and urea as the water phase. The powderspossess some excellent characteristics, such as spherical shapeand excellent dispersing.

2. During preparation process, it has been found that theconcentration of zirconium nitrate, reaction temperature ofwater bath and the quantity of urea effect regularly on theaverage particle size of products.

Acknowledgements

The authors gratefully acknowledge the financial support forthis work from State Key Development Program for Basic Researchof China (No. 2010CB635107), The National Natural ScienceFoundation of China (Nos. 51004046, 51075129). The NationalNatural Science Foundation of Hubei province of China (No.2010CDB05806).

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