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Synthesis of γ-Al2O3/CeO2 CoatedNanoparticles and Dispersion Stability ofTheir SuspensionXiaolan Song a , Shutao Huang a , Baiyang Gong a , Rangbin Meng a ,Mingwan Zhang a , Yizhao Yang a , Yishun Zhong a & Nan Jiang aa School of Minerals Processing and Bioengineering , Central SouthUniversity , Changsha , 410083 , ChinaPublished online: 12 Jul 2013.

To cite this article: Xiaolan Song , Shutao Huang , Baiyang Gong , Rangbin Meng , Mingwan Zhang ,Yizhao Yang , Yishun Zhong & Nan Jiang (2013) Synthesis of γ-Al2O3/CeO2 Coated Nanoparticles andDispersion Stability of Their Suspension, Integrated Ferroelectrics: An International Journal, 146:1,76-87, DOI: 10.1080/10584587.2013.789721

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Integrated Ferroelectrics, 146:76–87, 2013Copyright © Taylor & Francis Group, LLCISSN: 1058-4587 print / 1607-8489 onlineDOI: 10.1080/10584587.2013.789721

Synthesis of γ -Al2O3/CeO2 Coated Nanoparticlesand Dispersion Stability of Their Suspension

XIAOLAN SONG,∗ SHUTAO HUANG, BAIYANG GONG,RANGBIN MENG, MINGWAN ZHANG, YIZHAO YANG,YISHUN ZHONG, AND NAN JIANG

School of Minerals Processing and Bioengineering, Central South University,Changsha 410083, China

γ -Al2O3 nanoparticles prepared through a chemical precipitation method were directlycoated with CeO2 by means of a precipitation technique. X-ray diffraction (XRD),infrared (IR) spectra, transmission electron microscopy (TEM) and energy disper-sive analysis of X-ray (EDAX) were used to characterize the γ -Al2O3/CeO2 coatednanoparticles. XRD patterns show that the composite nanoparticles are the cubic crys-tal γ -Al2O3 and the fluorite-structured CeO2. IR results indicate that the coating of theγ -Al2O3 nanoparticles with CeO2 evidently gives rise to the bands at 720 and 600 cm−1

whose vibration intensity becomes strong shift to lower wavenumbers. The IEP of theγ -Al2O3/CeO2 coated nanoparticles is about 7.2, which displays a significant shift to-ward pure CeO2. TEM micrographs reveal that γ -Al2O3/CeO2 coated nanoparticlesshow a spherical morphology with the diameter about 15 nm. Dispersion behaviors ofγ -Al2O3/CeO2 coated nanoparticles under different conditions were studied in watersuspension by investigating the zeta potential and absorbance. The results indicate thatthe surfaces of γ -Al2O3/CeO2 suspension are charged with positive charge under acidcondition and with negative charge under alkaline condition, respectively. Zeta poten-tial is higher when pH value is about 4, corresponding to dispersion stability is better.The dispersion effect of ultrasonic wave on γ -Al2O3/CeO2 nanoparticles is markedlysuperior to mechanical stirring. Only nonionic surfactant can improve the dispersionstability. A suspension with γ -Al2O3/CeO2 at 0.02 wt% is added in 0.05 g/L nonionicsurfactant PEG4000, and its pH value is adjusted to within the range of 3.5–4. In thiscondition, the stable suspension without sedimentation for a long time is successfullyobtained.

Keywords γ -Al2O3/CeO2; coated nanoparticles; precipitation method; zeta potential;absorbance; dispersion stability

1. Introduction

With prompt development of the ultra large scale integrated (ULSI) technique, the rapidadvances in the microelectronics industry demand significant improvements in the chemicalmechanical polishing (CMP), which is the only and best technology known to provide globalplanarization [1–7].

CMP is a process that is influenced to a great extent by numerous slurry parameterssuch as abrasives, dispersion stability of slurry, pH of slurry, surfactants, oxidizers, etc.

Received August 13, 2012; in final form February 1, 2013.∗Corresponding Author. E-mail: songxiaolan@csu.edu.cn; xlsong8822@126.com

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[8–16]. The most commonly used abrasives in CMP slurries are alumina (Al2O3) and silica(SiO2), moreover Al2O3 is the most often used abrasive in tungsten slurries. However, thepolish rate of Al2O3 slurry is lower (the polish rates of oxides with different abrasivesdecreases in the order of CeO2 > TiO2 > MnO2 > Al2O3 > SiO2 [17]. Coating thesurface of the particles with harder substance can improve the polish rate and the selectivity[18,19]. The softer particles such as Al2O3 can be coated with a harder surface such asCeO2 to improve the polish rate and the selectivity. Composite abrasive slurry containingAl2O3/CeO2 particles has been developed with a goal to improve the selectivity of theCMP slurry. Certain Al2O3/CeO2 abrasive concentration of composite abrasive slurry hasshown marked increase in selectivity for underlying tantalum, and a different concentrationof CeO2 and Al2O3 has shown excellent polishing for oxide over nitride. The details of theexperiments with these novel mixed abrasive slurries have been published by Jindal et al.[20,21].

Composite abrasive slurries were first proposed by Babu. They filed a patent whichpresented two types of composite abrasive slurry with a title of “Chemical–MechanicalPolishing” to the United State Patent and had its publication No. US 20030047710 [22].One is alumina/ceria slurry for oxide CMP. From the same research group, Jindal et al. [21]presented the CMP study of the Al2O3/CeO2 composite abrasive slurry.

In applications of CMP technology, the dispersion stability of slurry has great influ-ence on polish effect, storage, transportation, and so on. Meanwhile, the performance ofdispersion stability of slurry has a close relation to the property of abrasive particles underdifferent solution.

In this study, γ -Al2O3 abrasive nanoparticles were synthesized by chemical precipita-tion method, the CeO2 coating then coated on the synthesized γ -Al2O3 nanoparticles byprecipitation techniques, and the dispersion stability of γ -Al2O3/CeO2 coated nanoparticleswas studies. The CeO2-coated γ -Al2O3 CMP abrasive nanoparticles were investigated byXRD, IR, TEM and EDAX. By analyzing reaction, some value clues could be provided forpreparing new nanosized composite CMP slurries.

2. Materials and Method

2.1. Chemicals

The chemicals used in the synthesis were Al(NO3)3·9H2O, (NH4)2CO3·H2O,Ce(NO3)3·6H2O, PEG4000 and isopropanolamine.

2.2. Preparation of Materials

2.2.1. Preparation of γ -Al2O3 Nanoparticles/Preparation of γ -Al2O3 Cores. A certainamounts of Al(NO3)3·9H2O and (NH4)2CO3·H2O with a volume ratio of 1.33 were dis-solved in deionized water to form 0.1 mol/ L Al(NO3)3 and 0.16 mol/ L (NH4)2CO3 solution.The Al(NO3)3 solution A was put into glass reactor, added by 0.024% (volume fraction)surfactant PEG600, mechanically stirred at 800 r/min rate and heated to temperature 40◦C.The corresponding (NH4)2CO3 solution B was heated to the same temperature 40◦C, thensolution B was rapidly added into solution A at constant stirring rate, reacting for 15 min.After stabilizing for 24 h, the resulting precursor powders were thoroughly washed withdeionized water and alcohol and then dried in vacuum at 80◦C for 8 h. The final activeγ -Al2O3 nanoparticles were calcined at 800◦C for 1 h in air.

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2.2.2. CeO2 Coating of γ -Al2O3 Nanoparticles/Coating Procedure. The CeO2 coating ofγ -Al2O3 nanoparticles was prepared by chemical precipitation method. γ -Al2O3 nanopar-ticles prepared above used as cores for the coating experiments.

First, 1.02 g γ -Al2O3 cores and 0.868 g Ce(NO3)3·6H2O were ultrasonically dissolvedinto 200ml deionized water containing 0.11ml surfactant isopropanolamine. The solutionA was ultrasonically dispersed for 30 min after its pH was adjusted to about 4 by usinga certain concentration HCl solution. The other solution B prepared by dissolving 0.342g (NH4)2CO3·H2O in 50 ml deionized water was added, drop-by-drop, into the solutionA with a proper agitation. Afterward, the mixed solution was stirred for 1 h at 50◦C andaged for 3 h. The final product was collected by filtration, washed with deionized waterfor several times, and then dried overnight at 60◦C in vacuum and calcined at 800◦C for1 h.

2.3. Characterization

The crystalline phase and the component of the sample were examined by XRD (RigakuDmax 350) with a 2θ range of 10◦∼90◦. The IR spectrum analysis of precursor was carriedout in IR measurement system (NICOLET-210, USA). The microstructure and shape of theparticle were investigated by TEM (Tecnai G220, Philips). A small amount of the samplewas dispersed in ethanol via ultrasonic. The suspension was then taken onto copper grids forTEM measurement. The element was determined using a GENESIS 60E EDAX analyserwith TEM measurement. Zeta potential measurement of the sample was determined usingthe American DELSA 440SX Zetaplus Particle apparatus.

2.4. Dispersion Stability Experiment of γ -Al2O3/CeO2 Suspension

The effect of dispersion stability in γ -Al2O3/CeO2 suspension under different pH valueswas investigated by the method of absorbance and zeta potential. According to Reylenghequation:

A = k·n

where A is the absorbance of suspension, k is the absorbance constant, n is the solidparticle number of suspension per unit volume, the absorbance is directly proportional tothe concentration of solid particle in suspension. It means that the higher absorbance is, thebetter dispersion stability in suspension is. The absorbance was measured by Chinese 756PUV-vis spectrophotometer. The zeta potential of γ -Al2O3/CeO2 nanoparticles dispersedin deionized water, as a function of pH, was measured with American DELSA 440SXZetaplus Particle apparatus. Zeta potential measurements can be conducted on very dilutesuspensions. The 0.02 wt% γ -Al2O3/CeO2 suspensions using deionized water as dispersingmedium were prepared by ultrasonicating 15 min and stirring 5 min. The suspensions wereultrasonicated 15 min prior to the measurement to ensure that only the mobility of thesingle particles was measured. The pH value of suspensions was adjusted with a givenconcentration HCl and NaOH solution.

The effect of dispersion stability in γ -Al2O3/CeO2 suspension under different me-chanical stirring velocity and ultrasonic wave time were studied by testing the absorbance.The suspensions with γ -Al2O3/CeO2 at 0.02 wt% had been prepared. The pH values ofthe suspensions was adjusted to ranging from 3.5 to 4 with a certain concentration HClsolution, and then a little solution from a certain height of the suspensions was used to do

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Figure 1. XRD patterns of γ -Al2O3 nanoparticles and γ -Al2O3/CeO2 coated nanoparticles. (Figureavailable in color online)

the absorbance experiment after the suspensions were dispersed for 10 min under differentmechanical stirring velocity or dispersed under different ultrasonic wave time.

The effect of dispersion stability in γ -Al2O3/CeO2 suspension under different surfac-tant types and contents was studied by testing the zeta potential. PEG4000, SDBS andCTAB surfactant was used as dispersant. The 0.02 wt% γ -Al2O3/CeO2 suspensions usingdeionized water as dispersing medium with different surfactant amounts were prepared byultrasonicating 15 min and stirring 5 min after the pH was adjusted to ranging from 3.5to 4 by using a certain concentration HCl solution. The suspensions were ultrasonicated15 min prior to the measurement to ensure that only the mobility of the single particles wasmeasured.

3. Results and Discussion

3.1. γ -Al2O3/CeO2 Nanoparticles

3.1.1. XRD. Figure 1 shows the XRD patterns of γ -Al2O3 nanoparticles, γ -Al2O3/CeO2

coated nanoparticles obtained from calcining the precursor at 800◦C for 1 h in the air. Itis indicated that the main crystal phases of the γ -Al2O3/CeO2 coated samples are cubiccrystal γ -Al2O3 (�) whose space group is O7

H FD3M and fluorite-structured CeO2 (·). Itmeans that CeO2 is present inγ -Al2O3/CeO2 coated samples, which is confirmed by EDAXanalysis (see Fig 4).

3.1.2. IR. IR measurement was conducted for γ -Al2O3 Nan particles, CeO2 nanoparticles,γ - Al2O3/CeO2 coated nanoparticles as shown in Figure 2. In Fig. 2, all the powdersexhibited –OH stretching vibration in the frequency range of 3350–3450 cm−1 and thebending vibrations of associated water in the frequency range of 1550–1650 cm−1. Carefullyexamine the spectra from 400 to 1000 cm−1 corresponding to the location of the bandsassociated with the γ - Al2O3 networks. The spectrum of the γ - Al2O3 particle shows twomain maximum related to Al–O vibration at about 720 and 600 cm−1. As indicated, thebands at 720 and 600 cm−1 shift to lower wavenumbers and the vibration intensity of the

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Figure 2. IR spectra of γ -Al2O3 nanoparticles, CeO2 nanoparticles, γ -Al2O3/CeO2 coated nanopar-ticles . (Figure available in color online)

bands at 600 cm−1 becomes strong after coating. It is because that the addition of CeO2 hasinfluence on the structure of γ - Al2O3.

3.1.3. TEM and EDAX. Figure 3 shows the TEM images of the γ -Al2O3/CeO2 coatednanoparticles. The TEM images reveal that the particle size of γ -Al2O3/CeO2 is about15 nm, and the shape is spherical. The core-shell structure of composite particles can notbe clearly observed in TEM micrographs, and can be observed only in a few regions.However, EDAX (Fig. 4) for (Fig. 3a) shows that both aluminum and cerium are present inthe microspheres, and copper mainly comes from copper grid for loading samples.

Figure 3. TEM images of the γ -Al2O3/CeO2 coated nanoparticles.

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Figure 4. EDAX spectrum of the nanoparticles shown in Fig 3a. (Figure available in color online)

3.1.4. Zeta Potential. Figure 5 indicates the zeta potential of γ -Al2O3 nanoparticles, CeO2

nanoparticles, γ -Al2O3/CeO2 coated nanoparticles at different pH values. The isoelectricpoint (IEP) of γ -Al2O3, CeO2 and γ -Al2O3/CeO2 is around pH = 9.23, 6.8 and 7.2,respectively, which displays a significant shift of the IEP from γ -Al2O3 cores toward pureCeO2. These data indicate that either the shell is incomplete, or that γ -Al2O3 may still exertan effect on the zeta potentials of the very small CeO2 particles comprising the shell.

3.2. Dispersion Stability Studies

3.2.1. The Effect of pH Values on Dispersion Stability of γ -Al2O3/CeO2 Suspension. Thecurves of the change in zeta potential and absorbance of γ -Al2O3/CeO2 suspension withthe pH value are shown in Fig. 6. The results indicate that the dispersion stability of γ -Al2O3/CeO2 nanoparticles is affected obviously by pH value of suspension and is/keeps

Figure 5. The zeta potential of γ -Al2O3 nanoparticles, CeO2 nanoparticles, γ -Al2O3/CeO2 coatednanoparticles at different pH. (Figure available in color online)

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Figure 6. Zeta potential and absorbance of γ -Al2O3/CeO2 suspension.

consistent with zeta potential. The surfaces of γ -Al2O3/CeO2 suspension are charged withpositive charge under acid condition and with negative charge under alkaline condition,respectively. When the pH value is less than 4 or more than 11, the zeta potential of theγ -Al2O3/CeO2 suspension is higher, corresponding to the dispersion stability is better.However, when the IEP is about 7.2, the zeta potential is zero, and the dispersion stabilityis the worst. This means that static repulsion interaction of double electrical layer has effecton the dispersion of γ -Al2O3/CeO2 suspension. Lower or higher pH value of CMP slurrymakes polish process hard to achieve, because γ -Al2O3 and CeO2 are amphoteric metaloxides and react with acid and alkali materials. Experiment results show that it is difficultto control the pH value of CMP slurry less than 3. Moreover, the CMP slurry is very easyto bring bulk-like sediments if the pH value is comparative high (pH = 11). In addition,polished material surface may corrosively react with CMP slurry if the pH value is higher.Those mentioned above must affect the CMP important properties, such as polish rate andpolish effect. Through the comparison for various factors, the pH value of γ -Al2O3/CeO2

CMP slurry should be controlled in 3.5–4.

3.2.2. The Effect of Mechanical Stirring on Dispersion Stability of γ -Al2O3/CeO2 Suspen-sion. Mechanical stirring dispersion is a simple physical dispersion method which makesaggregated particles deaggregated compulsively by means of outside shear stress. Fig. 7shows the relation between dispersion of γ -Al2O3/CeO2 suspension and mechanical stirringvelocity when stirring time is 10 min. The dispersion stability of γ -Al2O3/CeO2 suspen-sion is improved as the mechanical stirring velocity increases because the increase of shearstress leads to the enhancement of turbulent flow movement. When the mechanical stirringvelocity reaches 800 r/min, the dispersion stability of γ -Al2O3/CeO2 suspension tends tobe relative stable. It is found that when shear stress reaches a certain value, it will makeγ -Al2O3/CeO2 particles collide and aggregate, and make dispersion and coagulation reachequilibrium state. Higher mechanical stirring velocity will make the suspension splash.

3.2.3. The Effect of Ultrasonic Wave Time on Dispersion Stability of γ -Al2O3/CeO2 Sus-pension. The relation between dispersion of γ -Al2O3/CeO2 suspension and ultrasonicwave time is shown in Fig. 8. Comparing Fig. 7 with Fig. 8, the effect of ultrasonic wave

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Figure 7. Relation between dispersion of γ -Al2O3/CeO2 suspension and mechanical stirring velocity(pH≈4, 10 min).

dispersion is more effective than that of mechanical stirring dispersion. As indicated inFig. 8, the dispersion stability of γ -Al2O3/CeO2 suspension dispersed for 2 min by ultra-sonic wave can be obviously improved, and the dispersion stability can be improved withincrease of ultrasonic wave time. When the ultrasonic wave time is longer than 6 min, thedispersion stability almost does not change and tends to be relative stable.

3.2.4. The Effect of Surfactant on Dispersion Stability of γ -Al2O3/CeO2 Suspension. Ascan be seen in Figure 9, three kinds of surfactants can remarkably affect the dispersionstability of suspension, but only nonionic surfactants can enhance the dispersion stability.The dispersion ability of all kinds of surfactants is different. There is an optimum value forthe concentration of suspension, and the dispersion stability is the best under this condition.

Figure 8. Relation between dispersion of γ -Al2O3/CeO2 suspension and ultrasonic wave time(pH≈4, 20 kHz).

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Figure 9. Relation between zeta potential of γ -Al2O3/CeO2 suspension and dispersant concentration(pH≈4) (a) PEG4000; (b) SDBS; (c) CTAB.

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In Fig. 9a, γ -Al2O3/CeO2 particle surface keeps positive zeta potential, and the zetapotential of suspension increases firstly and then decreases as the concentration of nonionicsurfactant PEG4000 increases. When the concentration amounts to 0.05 g/L, the maximumzeta potential reaches to 61.3 mV, and the dispersion stability is the best. Because nonionicsurfactant PEG4000 existing as molecule does not ionize in water and is hydrophilicity,hydrated film existing around PEG4000 molecule gives rises to steric stabilization. Withaddition of PEG4000 at the beginning, γ -Al2O3/CeO2 particle surface absorbs surfactantmolecule. When the concentration increases, γ -Al2O3/CeO2 particle surface absorbingmore and more surfactant molecule results in particle surface is coated with surfactantmolecule, the effect of steric stabilization is gradually enhanced, the zeta potential increases,and the dispersion stability is improved. When the concentration amounts to 0.1 g/L, thezeta potential reaches to maximum. When the concentration further increases, the zetapotential decreases. It is because that superfluous surfactant molecules that can bridgeamong particles leads to aggregation and makes the dispersion stability become poor athigher concentration.

In Fig. 9b, the initial suspension has 51.3 mV zeta potential value in acid system (pH =4), but the suspension becomes more flocculated after addition of anionic surfactant SDBS.As the concentration of SDBS increases, the zeta potential shows a steep of decreasefrom positive to negative, passing the points of zero charge, but shows plateaus at theconcentration of about 2 g/L. Evidently, γ -Al2O3/CeO2 particle surface with positivecharge in acid condition can adsorb DBSAS anionic groups. γ -Al2O3/CeO2 nanoparticlesurface absorbs more and more anionic groups to neutralize its positive charge with additionof SDBS results in the zeta potential in absolute value decreases firstly and then increases.However, the dispersion stability of suspension becomes poor/worse after addition of SDBS.

In Fig 9c, addition of small amounts of cationic surfactant CTAB affects the dispersionstability of suspension causing decrease of the zeta potential. The zeta potential remarkablychanges from 51.3 mV to38.4 mV with increasing the concentration of CTAB from 0 to0.2 g/L, and stabilizes in the range from 38 to 39 mV within extensive range of concentration,which makes the suspension exhibit very poor dispersion stability.

Based on the effect of dispersion and the amount of dispersant, nonionic surfactantPEG4000 should be selected to use as the dispersant of γ -Al2O3/CeO2 CMP slurry.

4. Conclusions

In this study, the coating of CeO2 on the γ -Al2O3 particle surface was performed usingprecipitation method. XRD study clarifies that the core region of the composite particlesis cubic crystal γ -Al2O3, and the shell region is fluorite-structured CeO2. IR spectra showthat the coating of the γ -Al2O3/CeO2 coated nanoparticles evidently induces the bands at720 and 600 cm−1 whose vibration intensity becomes strong shift to lower wavenumbers.The IEP of the γ -Al2O3/CeO2 coated nanoparticles displays a significant shift toward pureCeO2. TEM micrographs reveal that γ -Al2O3/CeO2 nanoparticles possess spherical shapeand the particle size is about 15 nm.

The results about dispersion stability study indicate that the dispersion stability of γ -Al2O3/CeO2 suspension is affected obviously by pH value and keeps a coincident relationwith zeta potential. The dispersion mechanism is static repulsion interaction of doubleelectrical layer. The IEP of γ -Al2O3/CeO2 nanoparticles is about 7.2. The surfaces ofγ -Al2O3/CeO2 nanoparticles are charged with positive charge in acid water suspensionand with negative charge in alkaline water suspension, respectively. When the pH valueis less than 4 or more than 11, zeta potential is higher, and the dispersion stability is

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better. The dispersion stability is improved as mechanical stirring velocity and ultrasonicwave time increase. The dispersion effect of ultrasonic wave is more effective than that ofmechanical stirring. Three kinds of surfactants have great effect on the dispersion stabilityof suspension, but only nonionic surfactants can improve the dispersion stability. The zetapotential grows up evidently with increasing the concentration of PEG4000, and tends to bestable as the concentration of PEG4000 reaches 0.05 g/L. The content of the γ -Al2O3/CeO2

coated nanoparticles in the suspension is 0.02 wt% and the pH value is 3.5–4. The resultsof pH, zeta potential and absorbance show that the suspension is stable by using 0.05 g/Lnonionic surfactant PEG4000 as dispersant. Stabilized suspension of γ -Al2O3/CeO2 coatednanoparticles could be obtained.

Acknowledgments

This study was supported by the National Nature Science Foundation (50774095) and theUndergraduate Innovation Experimentation Plan of Central South University (CL12111).The authors would like to express their appreciation to these financial supports.

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