J A C# Soc., 8. (11140-44 (I998J

Preparation of Novel Core-Shell Nanocomposite Particles byControlled Polymer Bridging

Tsung-yuan Chen- and P. Somasundarant

Langmuir Center for Colloids and Interfaces, Department of the Chemical Engineering,Material Science, and Mining Engineering, Columbia University, New York, New York 10027

Ceramics that are composed of nanosized granules showenhanced properties in various applications. A layer-by-layer deposition process is developed in this study to pre-pare core-shell nanocomposite particles, using controlledpolymer adsorption. Submicrometer-sized alumina par-ticles that have been chosen as the starting core particlesare first dispersed by pH control and then modified byadsorption of anionic poly (acrylic acid) to facilitate the sub-sequent deposition of nanoparticles on them. A key step inthe processing involves the removal of excess polymer in thesupernatant after the adsorption by controlled washing, sothat the added nanoparticles do not self-flocculate amongthemselves. A coating of nanosized particles is indicated bythe charge reversal of the micrometer-sized composite par-ticles. The isoelectric point of the composite particle is simi-lar to that of nanosized particles under both low and highcoverage conditions. Scanning electron microscopic obser-vation or coated particles shows full monolayer coverage ofthe nanoparticles on core particles, which confirms the ef-ficiency or the coating scheme that is discussed here. Thepresent study reveals the reasibility of coating nanoparticlesonto micrometer-sized core particles, using controlled poly-mer- bridging. for the preparation of the multilayernanocomposite powders.

Introduction

R ECENT progress in the synthesis of nanosized particles hasopened an avenue for producing new materials with en-

hanced properties.I.2 These ultrafine particles have propertiesthat correspond neither to those of free atoms or molecules thatcompose the clusters nor to those of bulk solids of the samecomposition. Distinct properties of these nanosized particlesresult from their large surface-area-to-volume ratio. with alarge fraction of atoms at the surface or grain-boundary re-gions. Ceramics that are composed of nanosized granules areattractive because of their specific properties and improvedsinterability.3.4 Wet processing methods that use colloidalchemistry techniques provide an efficient means to counteragglomeration problems and achieve superior green micro-structures for the fabrication of ceramics.s Nanosized or sub-micrometer-sized particles have a tendency to agglomerate.because of the nature of their small size. In colloidal process-ing. this problem can be counteracted by creating repulsiveforces between particles using either electrostatic or steric in-

teractions.6-8 The resultant particle suspensions are stable andsuitable for further processing.

The stability of particle suspensions is dependent on theinteraction between particles. The adsorption of surfactantsand/or polymers on the particle surface can modify their sur-face properties and, thus, particle-particle interactions.6-9Polymers can act as dispersants or flocculants for particle sus-pensions, depending on parameters such as polymer molecularweight (Mw) and surface coverage by the polymer.IO Floccu-lation of particles by the polymer can occur either because ofa bridging of particles with each other by polymers or becauseof charge neutralization that is caused by polyelectrolyte ad-sorption.10-13 In this study, polyelectrolyres will be used asbridging aids between nanoshell particles and submicrometer-sized core particles for particle engineering.

Coating powder surfaces by using polymer and/or surfac-tants provides a way to control particle interactions. Coatingpowders by using other particles also provides a means to tailorthe surface properties of powders for specific functions andapplications.14-16 In cerainic processing, powder coating is in-vestigated mostly for improving the structural homogeneity ofthe productS.17.18 For example, the process of coating siliconnitride by using oxide sols as sintering aids, via methods suchas solution precipitationl9-21 and colloidal coating,2z.23 hasbeen studied by various groups to enhance their sintering. Theimprovement that is obtained in homogeneity results in anenhanced sintering of the green compacts and permits the useof less additive for sintering materials to high density. How-ever, despite much of the work that has been performed inpowder coating. none of these earlier studies focused on the useof nanosized clusters for the fabrication of novel nanomateri-also

The purpose of this work is to investigate the feasibility ofpreparing nanostructured composite particles for enhanced ma-terials fabrication using colloidal chemical techniques. The ap-proach that is used involves the production of nanocompositeparticles with a core-shell structural unit (a submicrometer-sized particle as the core and layer(s) of nanoparticles as theshell(s». Such nanocomposite particles can be used as buildingblocks for bulk nanocomposites or composite coating.

The approach that is used in this study involves a two-stageprocess for coating nanosized particles onto micrometer-sizedparticles. A polymer with a strong affinity for both core andshell particles is used as a bridging aid, to enhance the integrityof the nanocomposite particles. First, submicrometer-~ized par-ticles are dispersed in aqueous solution by adjusting the pHaway from the point of zero charge (the isoelectric point, iep).An ionic polymer is then adsorbed on these particles, to maketheir surfaces suitable for the deposition of nanoparticles. Theexcess polymer in the supernatant is removed through con-trolled washing: this is the critical step for the success of nano-particle deposition, because the presence of free polymer in thesuspension can cause self-flocculation of nanoparticles bypolymer bridging. Such self-flocculation among nanoparticleswill make it impossible to obtain a uniform monolayer coatingof nanoparticles on the core particles. Therefore. removal ofexcess polymer from the suspension supernatant must be

C. F. Zukowski-<onlributiog editor

Manuscript No. 191905 Received April 18. 1996; approved May 9. 1997Supponed by the Engineering Researdl Center (ERC) for Particle Science and

Technology at tlte University of Ronda (Gainesville. FL). National Science Foun-dalioo (NSI-) Granl No. EEC-94-O2989. and the Industrial Partners of the ERc.

'Member. American Ceramic ~iely.t Author to whom all correspondence should be add~ssed.

40

141Preparation of Novel Core-Shell Nanocomposite Particles by Controlled polymer BridgingJanuary 1998

achieved carefully. After this step, the nanoparticle suspensionis mixed with the core-particle suspension to yield the nano-particle coating. By repeating the above-described polyrner-adsorption-particle-deposition sequence, multilayer compositeparticles can be prepared as desired. A schematic drawing ofthe multilayer nanocomposite particles is given in Fig. 1. Coreand shell particles may be the same or different materials, asindicated by the different gray shading.

In the current study, submicrometer-sized alumina that wasused as core particles was coated with nanosized alumina par-ticles, using poly (acrylic acid) (PAA) as a bridging aid betweenthem. The feasibility of the process and the effects of variousprocessing parameters are discussed.

II. Experimental Procedure

used as received. Sodium chloride (NaCI) (Fisher Scientific,Pittsburgh, PA) was used for the ionic strength control of thecolloidal suspensions. Hydrochloric acid (HCI) and sodium hy-droxide (NaOH) (Fisher Scientific) were used for pH adjust-ment All the reagents were of ACS grade and were used asreceived. The water that was u..,ed was triply distilled and hada conductivity of 10-6 mhos (I mhos = I/O).

Samples for the adsorption experiment were prepared byultrasonicating (Model Labsonic, Lab-line Instruments, Mel-rose Park., IL) I g of alumina in 10 mL of a 0.03M salt (NaCl)solution at 40 W of power for I min. The pH was then adjusted,and the suspension was mixed by magnetic stirring for 3 h. APAA solution that was adjusted to the desired pH and the sameionic strength as the solid suspension was mixed with the solidsuspension for 15 h. and the supernatant that was separated bycentrifugation was analyzed for residual polymer by a totalorganic carbon analyzer (Model DC-90. Dohrmann, SantaClara, CA). The polymer adsorption density at the solid/liquidinterface was determined from the difference between the ini-tial and residual polymer concentrations.

Tests to determine whether the polymers will desorb duringthe washing (to remove excess polymer from the solution) wereconducted in a manner that was similar to the adsorption ex-periment. Therefore, the samples were diluted and the polymerconcentration after the desorption tests was determined by thetotal organic carbon analyzer.

The process of coating the nanoparticles was conducted byadding various amounts of Nanotek. alumina suspensions at arate of 5 rnUmin into stirred. PAA-adsorbed AKP suspensions.The mixed suspension was stirred using a magnetic stirrer for30 min. The coating was monitored by measuring zeta poten-tials of the core and composite particles in the suspension(LaserZee Meter, Model 501, Pen Kern, Bedford Hills, NY).

Two types of alumina powder (AKP-15 and AKP-50, Sumi-tomo Chemical, Tokyo, Japan) were used as the core particles.Both AKP powders were a-alumina. had mean particle sizes of0.7 and 0.2 ~m, respectively. and Brunauer-Emmett- Teller(BET) specific surface areas (Quantasorb. Quantachrome,Syosset, NY) (using nitrogen) of 3.6 and 10.9 m2/g, respec-tively. Nanosized alumina powder (Nanotek, Nanophase Tech-nologies, Darien. IL) with a mean particle size of 36 nm andspecific surface area of 47 m2/g was used as shell particles. andit consisted mainly of -v-alumina.

PAA of two different molecular weights-Mw = 2<XX> (Al-drich Chemical Co.. Milwaukee. WI) and Mw = 150000(Polysciences. Warrington. PA)-were used as bridging aids.The purity of both polymers was confirmed via Fourier trans-fonn infrared (FTlR) spectroscopy (FT -IR Spectrometer Model1800. Perkin-Elmer. Norwalk. CT), and the polymers were

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142 Jollmal of 1M American uram;c Sociel)'-Chen and Somasllndaran Vol. 81, No.1

The zeta potential of particles was measured by monitoring themovements of their scattering dots that were generated by la-ser-light scattering.

The morphology of composite particles after the coating wasobserved using scanning electron microscopy (SEM) (Stereo-scope MK II, Cambridge Instruments. Cambridge, U.K.).Sample suspensions were diluted to a low concentration of -50ppm and then dropped on top of the sample mount and dried inair. A thin layer of gold was sputter coated onto the samplesprior to SEM observation.

III. Results and Discus.~ion

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Fig. 3. Adsorption fsothenns ofPAA on Sumitomo AKP-15 aluminapanicles (pH 4. ionic strength of O.O3M NaCI. and solid: liquid ratio of5 (in Wt%».

ciaJ potential and. hence. their interaction with the PAA-covered AKP alumina particles. More importantly. the adsorp-tion of free polymers on the Nanotek particles will also causeself-flocculation of these nanoparticles before they have anopportUnity to coat individually on the AKP particle surface.Therefore. it is critical that any nonadsorbed PAA polymer beremoved before the addition of Nanotek particles into the sys-tem. It is equally important to accomplish the removal of theresidual bulk PAA polymer in the suspension without causingany desorption of the polymer from the sUrface of the corealumina particles. To test the feasibility of this coocept, poly-mer-adsorbed AKP particles were washed several times witha salt solution of the same ionic strength and pH as the originalsupernatant. The effects of stepwise washing on the residualpolymer concentration is shown in Fig. 5. For both PAAs(Mw = 2000 and 15CXXXJ). the residual concentrations in thesupernatant after three washes were approximately zero. Dif-ferent polymer adsorption densities on alumina, as indicated in

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(I) Surface ModifICation of Core Alumina byAdsorption of Poly(acrylic acid)

The suspension properties of oxide powders are dominatedby the particle surface charge that is generated by surface hy-droxylatioD reactions}4 The zeta potential of the alumina pow-ders is shown in Fig. 2 as a function of pH; the isoelectric point(iep) of AKP alumina is pH 8.9 and that of Nanotek alumina ispH 9.3. AKP and Nanotek alumina bom have a zeta potentialof approximately +55 mV at pH 4. In Fig. 3, adsorption iso-thenns of the two different-molecular-weight PA.~s on theAKP-15 alumina at pH 4 are shown. Both isotherms are thehigh-affinity type that is nonnally observed for polyelectrolyteadsorption on charged surfaces, which suggests strong interac-tion between the adsorbate and the adsorbent The maximumadsorption density of the 2(XX) Mw PAA is estimated to be 1.0mg/m2 and that of the 15(XXX) Mw PAA is estimated to be 1.4mg/m2.

The effect of P AA adsorption on the zeta potential of AKPalumina ~cles is shown in Fig. 4. With the adsorption ofP AA on the surface, the surface charge of the alumina particlereversed from positive to negative at pH <9, with the iep of thealumina shifting from pH 9.1 to pH <3. The adsorption of PAAhas modified the surface charge of the AKP alumina particlesso that when the Nanotek particles are mixed into the AKPsuspension, there will be strong electrostatic attraction betweenthe AKP and Nanotek particles.

The modification of the AKP alumina surface charge makesthe process of coating Nanotek particles on the alumina surfacefeasible. However, as mentioned earlier, any residual polymerin the suspension after the adsorption will hinder the coatingprocess in many ways. First. the adsorption of any free polymeron the surface of Nanotek particles will decrease their interfa-

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Preparation of Novel Core-SheU Nanocomposite Particles by ControUed Polymer Bridging 143

system. The polymer-covered AKP-50 alumina particles wereinitiall>: negatively charged; when dte Nanotek particles were~ Into the suspension. the zeta potential of the AKP-50PanIcles reversed from a negative to a positive value. whichs.uggests gradual coverage of the AKP-50 particles by the posi-t1~ely charged Nanotek nanoparticles. For AKP-50 ~cleswIth adsorbed PAA of either high or low molecular weIght. thefinal zeta potential of all the composite particles attained thesame value. which suggests that the coating coverage in bothsystems is the same. A similar coating experiment that wasperformed with AKP-15 alumina as the core particles showedthat the flDal composite particles can attain die zeta potential ofNanotek particles (+50 mV at pH 4) when the Nanotek additionwas sufficient to yield a coverage that resulted in a surface arearatio of 3.6.

The effect of polymer adsorption on the particle coating isalso shown in Fig. 6. The core particles with only half of themaximum polymer adsorption density were slightly chargedinitially; however. their zeta potentials increased as theNanotek particles were added into the suspension. The finalzeta potentials of die composite particles that started with either

rig. 5. .Remova.1 of oonadsorbed free polymer from the suspension .full. or half polymer adsorption coverages were the same.. whi~h~y ~lepWIs.e washIng. P AA adsorption densities for each sample set are indIcates the same surface coverage of Nanotek particles InIndICated m the legend (pH 4. ionic sb"ength of O.O3M NaCl. and both cases.solid:liquid ratio of 5 (in ~». The detection of a nanoparticle coating on AKP particles via

zeta potential measurement is more sensitive at low surfacecoverages than at high surface coverages. The sharp increase inzeta potential of the coated AKP particles at low coveragescould be due to the addition of positive charges on AKP alu-mina by Nanotek particles and the shift of the shear plane. Theattachment of Nanotek particles on the AKP particle surfaccwill shift the shear plane outward by -36 nm, which is muchlarger than the double-layer thickness in a O.O3M salt solution.Such an outward shift of the shear plane will make the zetapotential of the composite particles similar to that of the coatednanoparticles alone. even at low coverages.

The iep of a particle is dependent on its surface compositionand is related to the ratio of various components on the particlesurface?4 The relationship between the Nanotek addition andthe zeta potential variation of the coated particles is shown inFig. 7. The iep of the Nanotek alumina is 9.3 and that of thePAA-adsorbed AKP-15 alumina is 2. Even with small addi-tions of Nanotek particles. the iep of the resultant composite

d)e figure, did not affect the dilution results. The decrease inthe polymer adsorption densities in all cases was <2% of theoriginal adsorption densities, which suggests that the adsorp-tion of PAA on alumina is essentially an irreversible processwhen the suspension pH is kept constant.

(2) Coating of Poly(acrylk acid) CoHnd AKP Particles byNano/~k Alumina Nanoparticles

Secause the zeta potential of a colloidal particle is deter-mined by its surface composition, d)e coating of Nanotek par-ticles on PAA-covered AKP alumina particles was monitoredby measuring the zeta potential variation of AKP particles afterd)e coating process. The zeta potential change of PAA-coveredAKP alumina panicles, when mixed with Nanotek oanopar-ticles, is illustrated in Fig. 6. The amount of Nanotek particlesthat are added to the AKP suspension is presented as the ratioof the surface area of shell to that of the core particles in the

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SurfKe area ratio (Nanotek I AKP-SO)Fig. 7. Effect of NaIIO(ek coverage on the isoelectric point (iep) ofcoated panicles; the amount of Nanotek that is added is presented asthe surface area (SA) ratio of Nanotek to AKP alumina particles in thesystem (PM molecular weight of ISCXXXJ. adSOrption pH 4. coatingpH 4. ionic strength of 0.03.41 NaCl. and solid:liquid ratio of 5 (inWt%».

Fig. 6. Zeta potential variation of PM-adsorbed AKP-50 aluminawith a coating of Nanotek nanoparticles. Polymer adSOrption densitieson AKP surface are indicated in the legend (coating pH 4, ionicstrength of O.O3M NaCl and solid:liquid rati() of 5 (in wt%».

Vol. 81. No.Journal of the American Ceramic Society-Chen and Somasundaran144

processes has been shown to provide a promising route toobtain tailored nanoclusters for composite designs.

.\

particles has shifted markedly away from that of the PAA-adsorbed AKP particles. Generally. the iep of the compositeparticles shifts closer to that of the Nanotek particles as theaddition of Nanotek particles into the system increases; how-ever. the shift is not proportional to the quantity of Nanotekparticles that have been added. The iep shifts more at lowNanotek additions and less at medium and high Nanotek ad-ditions; this makes accurate determination of the saturationcoating density difficult. As indicated earlier, the nonlinearresponse of the iep shift to the Nanotek coverage is attributc:dto the outward shifting of the shear plane by nanoparticles onthe surface. As mentioned earlier. such a shift of the shearplane will lower the contribution of the adsorbed polymer tothe zeta potential value. because the new shear plane is fartheraway from the original plane. The nonlinear change in the iepof the composite particle with shell particle coverage must beconsidered when using the iep to monitor surface coverage. Inthe present study. the iep has served as a good measure ofcoverage to show that good coating of the coarse core particlesby the smaller shell particles can be achieved through con-trolled polymer adsorption/desorption.

The morphology of the nanocomposite particles with AKP-15 particles as core particles and Nanotek particles coating thesurface was monitored using SEM. The AKP-15 particle sur-face was fully covered by the Nanotek nanoparticles. In con-trast. a AKP-15 and Nanotek particle mixture with no PAApresent in the system showed that AKP-15 particles exhibitedsmooth surfaces with no Nanotek particles on them. The SEMobservations support the conclusion that was derived from thezeta potential results. which was that the desired nanoparticlecoating had been achieved.

IV. Conclusions

Coating nanoparticles on micrometer-sized core particles toproduce a multilayer nanocomposite powder has been achievedusing controlled polymer bridging. Poly(acrylic acid} adsorp-tion caused the alumina surface to undergo charge reversal,which made it possible for the alumina shell particles to coatthe coarser alumina core particles. Polymer adsorption on alu-mina was essentially irreversible under constant pH conditions;this made it possible to remove the nonadsorbed free polymerfrom the solution by washing without detaching the polymerthat had been adsorbed on the core particles. Undesirable self-flocculation of the fine shell particles was prevented by theremoval of free polymer from the solution prior to their addi-tion. The process of coating nanosized particles on the microm-eter-sized core particles was monitored by measuring the zetapotential variation of the composite particles. The nonlinearshift of the isoelectric point (iep) of the composite particleswhen the Nanotek particles were added suggests a complexmechanism for the surface charge of the coated powders thatinvolves both shear plane shift and charge neutralization. Thefinal iep and zeta potential of the composite powder ap-proached the values of the Nanotek particles. which indicatedan almost-saturated surface coverage of the core particles bythe finer shell particles. Scanning electron microscopic obser-vation of the coated particles further confirmed tbe successfulnanoparticle coating. Preparation of core-shell nanocompositeparticles through controlled polymer adsorption/desorption

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