Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 296–301

An aqueous process for the production of fully dispersible t-ZrO2 nanocrystals

Chien-Wei Chen, Xiu-Sheng Yang, Anthony S.T. Chiang *

Department of Chemical & Materials Engineering, National Central University, Chung-Li 32054, Taiwan

A R T I C L E I N F O

Article history:

Received 1 August 2008

Received in revised form 22 November 2008

Accepted 1 December 2008

Keywords:

Nanocrystal

ZrO2

Dispersion

Aqueous process

Hydrothermal

A B S T R A C T

The aqueous process currently employed in industry for the production zirconia has been modified to

produce fully dispersible nanocrystals. Compared to the non-aqueous processes, only less expensive raw

materials, lower temperature and simpler equipment were needed. Yet isolated �10 nm ZrO2

nanocrystals of pure tetragonal phase were achieved. After proper surface modification, the produced

nanocrystals could be re-dispersed as 10 wt% transparent aqueous sol. Less transparent but still fully

dispersed sol could also be achieved when modified and re-dispersed in organic solvent such as hexane

or chloroform. Further reduction of crystal size in this aqueous process should be possible after

optimizing the process parameters, so that fully dispersed organic sol with<10 nm zirconia nanocrystals

needed for the preparation of transparent high refractive inorganic–organic can be produced in large

volume.

� 2009 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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1. Introduction

Transparent inorganic–organic nanocomposites have potentialapplications in a wide range of optic applications such aswaveguide, non-linear optical materials, photochromic materialsand in particular, as transparent substrate or flexible functionallayers for optoelectronic devices. These nanocomposites aretypically obtained by the incorporating inorganic particles thatcontribute to the desired optical properties such as a highrefractive index, into a transparent polymer matrix that providesprocessability, flexibility, and transparency. The major challenge inthe development of inorganic–organic nanocomposite is however,the availability of <10 nm nanoparticles in larger quantity atreasonable price and the homogeneous dispersion of them in thepolymer matrix without agglomeration, so that the transparencycan be maintained.

The transparency of a nanocomposite is largely determined bythe scattering of nanoparticles in a matrix, which can be estimatedbased on the following formula (Caseri, 2006);

I

I0¼ exp �4

pnm

l

� �4

d3pf pl

ðn p=nmÞ2 � 1

ðn p=nmÞ2 þ 2

" #28<:

9=; (1)

where np and nm are the refractive index of the particles and thematrix, respectively, dp and fp are the diameter and volume

* Corresponding author.

E-mail address: stchiang@cc.ncu.edu.tw (Anthony S.T. Chiang).

1876-1070/$ – see front matter � 2009 Taiwan Institute of Chemical Engineers. Publis

doi:10.1016/j.jtice.2008.12.009

fraction of the particles in the matrix, and l is the length of lightpath. For inorganic particles whose refractive index is verydifferent from that of polymer, the scattering would be excessive.On the other hand, high n particles are needed to prepare a high n

nanocomposite.Zirconia (ZrO2), with its chemical inertness, thermal stability,

high hardness and high refractive index (�2.2) is an ideal inorganicfor the fabrication of high n organic–inorganic nanocomposites.With respect to the stability of the composites, it is even betterthan titania (n � 2.5) which might cause polymer degradation dueto its photocatalytic activity. For example, a novel ligand moleculehaving positively charged diamine head group and siloxane tailgroup had been developed to enable the dispersion of 20 vol.% ofZrO2 in polydimethylsiloxane (PDMS), and an increase of therefractive index from 1.38 to 1.65, while keeping the transmittanceabove 92% over 300 nm thickness (Lee et al., 2008).

The ZrO2 nanocrystal Lee used was supplied by SumitomoCorp. in the form of 10 wt% transparent toluene sol. With thedensity and refractive index of ZrO2 (6.0 g/mL, n = 2.2) andtoluene (0.867 g/mL, n = 1.496) and assuming transparency meansI/I0 > 90% at 600 nm, we can estimate from Eq. (1) that the sizeof the nanocrystals supplied by Sumitomo should be smallerthan 8 nm. In other word, a supply of <10 nm ZrO2 nanocrystals,fully dispersible as transparent sol in organic solvent afterappropriate ligand capping, is essential to the production of highn nanocomposite.

There is a vast collection of literatures on the synthesis ofzirconia nanocrystals. Many among them, particularly in theceramics field, claimed to produce ‘‘non-agglomerated’’ nanocrys-

hed by Elsevier B.V. All rights reserved.

C.-W. Chen et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 296–301 297

tals, but usually supported only by TEM pictures. It is very difficultto judge the isolation of particles from a TEM picture unless thepicture showed just a monolayer of nanoparticles. The more directproof of isolation would be the optical transparency of a dispersedsol in an appropriate solvent as the Sumitomo product.

Except for the work of Vollath et al. (2004), where 3 nmnanocrystals coated with poly(methyl methacrylate) (PMMA)were produce by a microwave plasma and made tetrahydrofuran(THF) dispersible, most of the researches on dispersible zirconiananocrystals were limited to solution processes, and in particularthe non-aqueous solvent route. For example, multi-gram synthesisof well isolated�4 nm t-ZrO2 nanocrystals that could be dispersedin toluene and hexane was reported by Joo et al. (2003), where thereaction of zirconium isopropoxide and zirconium chloride wasconducted at 340 8C with trioctylphosphine oxide (TOPO) as boththe solvent and the capping ligand. Mizuno et al. (2006), on theother hand, used dioctyl ether as the non-aqueous solvent tocrystallize pre-synthesized oleic acid capped nano-precursor at280 8C. Shi and Verweij (2005) used decane as the continuousphase in a modified emulsion method to produce decanedispersible ZrO2 after the removal of excess surfactant.

Niederberger’s group (Garnweitner et al., 2007; Zhou et al.,2007) was able to produce �3 nm cubic zirconia nanocrystalsusing benzyl alcohol as both the solvent and the reactant tohydrolyze zirconium alkoxide at 210 8C (slightly above benzylalcohol’s normal boiling point) for 2 days. The product formed aclear dispersion in chloroform and selected organic monomersafter modified with long chain fatty acid, and became dispersible inTHF and in vinyl monomers such as styrene and methylmethacrylate (MMA) if modified with vinyl group-containingligands.

There were also several other groups that took a multi-phaseroute to prepare colloidal nanocrystals including zirconia. Forexample, Wang et al. (2005) developed a general liquid–solid-solution (LSS) process involving the heterogeneous reaction andphase transfer between water, solvent, linoleic acid and metallinoleate. Although details specific to zirconia production were notgiven, they showed a picture of clear cyclohexane sol with 2%zirconia. Another multi-phase process was reported by Zhou et al.(2007) where a stagnant two phases system, with zirconium n-propoxide and fatty acid surfactants in toluene phase andbutylamine in water phase, was placed in an autoclave at 120–180 8C for 3 days without agitation. Tang et al. (2008) went further byseparating the toluene phase containing zirconium alkoxide and thewater phase in separated compartments of the autoclave and reactedat 100–240 8C. The product appeared as fine white powder in thetoluene phase, but they claimed to have excellent dispersibility.

Noticed that in the above non-aqueous processes, a multi-gramproduction was claimed as ‘‘large scale process’’ (Garnweitneret al., 2007; Joo et al., 2003). This could due to the relative lowconcentration of the alkoxide in the organic solution, and therequirement of high temperature and high pressure (except thecase of benzyl alcohol) in the process.

Although it is less popular to academia in recent year (Chenet al., 2001; Deshpande et al., 2004; Rajh et al., 2002; Tahir et al.,2007), industry has been producing fine zirconia particles usingaqueous process for years (Burgard et al., 1999; Kolb and Chien,2002). We believe that an aqueous process starting with lessexpensive inorganic salt would be more economic and environ-mental friendly for the production of dispersible nanocrystals inlarge scale, as long as the chemistry and process conditions arecarefully adjusted. Here, we will present such efforts and show thatfully dispersible t-ZrO2 nanocrystals (�10 nm) could indeed beproduced with no more than 110 8C of heating and withoutexpensive operations such as ultra-centrifugation, dialysis and thelikes.

2. Experimental

Industry-grade zirconyl chloride octahydrate (ZrOCl2�8H2O)was used as starting materials. Other chemicals including sodiumhydroxide, ammonia chloride, ammonium bicarbonate(NH4HCO3), hydrochloric acid (37 wt%), tartaric acid (C4H6O6,TA), citric acid (C6H8O7, CA) and oleic acid (C18H34O2, OA) werefrom Merck. Methacrylic acid (C4H6O2, MA) and triethanolamine(C6H15O3N, TEA) were from Aldrich and TEDIA, respectively.Deionized water was used through out.

A 2 M zirconyl stock solution was first prepared by dissolving644 g of zirconyl chloride octahydrate, 21 g of MA (corresponds toabout 1 mmol MA/g ZrO2 equivalence) to 580 g of deionized water.This acidic solution was heated to 110 8C for 24 h and stored inroom temperature for further use.

In a typical process, the acidic stock solution was neutralized by4 M NaOH in an pH 10 aqueous buffer containing ammoniumbicarbonate and TEA. The neutralization was carried out under theagitation of a emulsifier with metered influx of the acid andalkaline, so that the pH could be maintained at 10 during the wholeprocess. The amount of TEA used corresponds to �1.5 mmol/g ofZrO2 equivalence. A translucent mixture containing about 2.5 wt%ZrO2 equivalence was obtained when the forced hydrolysis iscompleted, but turned into a grayish transparent sol when aged atroom temperature for about 18 h.

The aged mixture could be flocculated by adjusting the pH valueto about 5 with hydrochloric acid. The flocculants were filtered andwashed several time until the pH value of the filtrate reached about7.6. The wet cake was diluted to �17 wt% ZrO2 equivalence and apH value of 11.5 with water and NaOH, then sealed in a large PPbottle and heated to 110 8C for 15 h. A white colloid was obtained,which was subjected to surface modification discussed later.Instead of flocculation and wash before hydrothermal, one couldalso heat the 2.5 wt% transparent sol directly. In this case, a whiteslurry was obtained.

The surface modification was performed under acidic condi-tions. The hydrothermal products was filtered and washed withdeionized water. Tartaric acid (TA) or citric acid (CA), at the amountof�2 mmol/g ZrO2 was added, and the pH was adjusted to 1.6 withhydrochloric acid to obtain a white paste after ultra-sonification.Modified nanocrystals were filtered out, and re-dispersed in wateras neutral sol having 10 wt% zirconia for later characterizations.The modification of oleic acid followed similar procedures but aheating at 90 8C to 1 h was needed. The pH value was at about 6 andthe amount of OA added was 6 mmol/g ZrO2.

Shimadzu XRD-6000 X-ray diffractometer was used to collectthe XRD patterns with a scanning rate of 0.758/min and a step sizeof 0.028 to confirm the crystal structure of the nanocrystals.Differential scanning calorimeter (DSC, PerkinElmer DSC-7) wasconducted with a heating rate of 10 8C/min from 50 to 600 8C. Theexothermic peak at 430–470 8C was identified as due to thecrystallization of amorphous part of zirconia. The heat flux in thistemperature range could thus be used to calculate the degree ofcrystallinity of the sample. Visible Raman spectroscopy (JobinYvon T64000 spectrophotometer, excited with 515 nm laser) wasused to confirm the phase purity of the obtained nanocrystals.

UV–vis spectra of the stocked solution as well as thetransmittance of the re-dispersed sol were measured with UV–vis spectrometer (UV/Vis, Jasco V-530). The particle size of thenanocrystals in the re-dispersed sol was also examined withtransmission electron microscope (TEM, JEOL JEM-2000FXa). Theobservation was performed on a drop of diluted sol deposited oncopper grids. FTIR measurement (Jasco FT/IR-410) was made byKBr pellet method. Surface area measurements were by theadsorption of nitrogen at 77 K (Micromeritics ASAP 2010) afterdegassed in vacuo at 200 8C.

Fig. 1. (a) Spectra of zirconyl solutions containing MA after heated at 110 8C for

various times. (b) Spectra of MA solution, fresh zirconyl solution without MA, and

that after heated at 110 8C for 24 h.

C.-W. Chen et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 296–301298

3. Results and discussion

The preparation of zirconia colloidal by the force hydrolysis ofzirconyl solution, and the subsequent hydrothermal crystallizationof the hydrous zirconia into nanocrystals are well known inindustrial practice (Chuah and Jaenicke, 1997; Denkewicz et al.,1990; Hu et al., 1998, 1999; Kaga et al., 1997; Stefanic et al., 1997;Tyagi et al., 2006). What we had done differently here is a finercontrol of solution chemistry so that the colloid size of the hydrouszirconia produced in the first step could be reduced, and thesubsequent crystallization could be proceeded without particleagglomeration.

There were four key parameters we had modified to finallyachieve the production of un-agglomerated nanocrystals, specifi-cally, the stabilization of zirconyl solution with MA, the choice ofpH for the force hydrolysis, the choice of chelating agent tostabilize the amorphous zirconia produced by force hydrolysis andthe selection of pH and temperature for hydrothermal reaction. Inwhat followed, we will discuss the reason behind each develop-ment before showing the final results.

3.1. Stabilization of zirconyl solution with MA

Aqueous zirconium, although less expensive than alkoxide, isnot a stable precursor for the synthesis of zirconia. The aqueouschemistry of zirconium ion has been the subject of many earlierresearches. When dissolved in water, zirconium ions immediatelyhydrolyzed to tetramer [Zr4(OH)8(OH2)16]8+ (Bleier and Cannon,1986). Upon heating or aging, the tetrameric ion polymerize via –OH bridging and finally form ordered three-dimensional particleswith the general formula [ZrOx(OH)4�2x�yH2O]n (Clearfield et al.,1994). Excessive aging or low temperature heating of zirconylchloride solution would lead to either oligomers or crystal nuclei.Primary crystallite of �3 nm in size would then grown oragglomerated into secondary particles (Cannon and Bleier, 1982).Therefore, it is very important to control the hydrolysis of zirconylchloride to avoid uncontrolled formation of crystal nuclei andagglomeration or growth. In here, we have used the addition ofmethacrylic acid to change the chemistry.

The formation of zirconium–methacrylate complex has beenstudied rather extensively by Schubert’s group (Gross et al., 2002;Schubert et al., 1992, 2001). In most cases, however, thesecomplexes were prepared by the reaction of zirconium alkoxideand MA. Depending on the molar ratio of MA and zirconium ion,cluster of various sizes with MA on the surface could be formed. Inour case, the MA to zirconium molar ratio is about 0.12. We expectthe formation of cluster about a few nanometers protected by theorganic acid, thus avoid the further hydrolysis and polymerization.As indicated by the UV spectra in Fig. 1, a small absorption peak at340 nm appeared after a pure zirconyl solution was heated at110 8C for 24 h. However, if MA was added, this peak became muchlarger, indication the fast formation of stable oligomers. Theamorphous hydrous zirconia nanoparticles produced after neu-tralization of the MA modified zirconyl solution showed a lowercrystallization temperature by DSC analysis (data not showed)compared to that without adding MA.

The addition of MA in the zirconyl solution was an essential stepfor the production of isolated t-ZrO2 in our process. Without thisstep, all our trials failed to produce non-agglomerated nanocrystals.

3.2. The choice of pH and chelating agent during force hydrolysis

The degree of hydrolysis for zirconium ion increases with pH. Inorder to increase the degree of hydrolysis, the pH of the zirconylsolution must be increased. However, an uncontrolled hydrolysisof zirconium ion would lead to the formation of large and

interconnected clusters or aggregates, until the repulsive interac-tion of the negative surface charge takes over. To reduce thepossibility of forming large particles, the common wisdom is to usechelating agent. TEA is known to form strong bond with zirconia,and change it isoelectric point from pH 4.5 to above 9 (Millesimeet al., 1996). In fact, Kirsch and Tolbert (2003) had preparedisolated hydrous amorphous zirconia nanoparticle about 19 nm insize by the hydrolysis of zirconia chloride directly with TEA.However, the amorphous nanoparticles they produced were latercrystallized by solvent thermal process in ethanol at 275 8C insteadof an aqueous process.

Fig. 2 compares the DSC and FTIR analysis of the amorphouszirconia collected when the force hydrolysis was performed in thepresence of just ammonium bicarbonate, with extra diethylamineor with extra triethanolamine. All these samples had beencollected by flocculation and washed. Clearly, only in the case ofTEA did we observed an exothermic peak at �300 8C for theburning of organics, as well as the existence of R3N group, indicatedby the absorption at around 1100/cm, in the washed solid. Thissuggested first that most of the MA had been removed from thezirconia after hydrolysis, and replaced by TEA if it existed in thebuffer. Secondly, the bonding of TEA was strong enough towithstand the flocculation and wash while diethylamine orinorganic cation did not bond to the zirconia strong enough. Infact, as we will see, TEA was bonded to the particle surface evenafter later hydrothermal crystallization reaction. The mostinteresting fact was however that a transparent sol could beobtained after aging when TEA was incorporated in the buffer. Thisis a direct evidence of the nanometer size of the amorphouszirconia particles formed during the force hydrolysis step. Noticethat we had used only about 1.5 mmol TEA/g ZrO2. This is muchless than what Hirsch and Tolbert had used as well as that used byMillesime et al. (1996).

3.3. Hydrothermal conditions

The crystallization of zirconia under hydrothermal condition iswell known, but the crystalline phases produced vary among

Fig. 2. DSC results and FTIR spectra of the washed precipitates obtained from the

neutralization of zirconyl/MA with NaOH at pH 10 in buffer containing: (a) just

ammonium bicarbonate, (b) additional diethylamine and (c) additional

triethanolamine.

Fig. 3. XRD and DSC of sample: (a) before hydrothermal and (b–h) samples

produced after 95 8C/10 h hydrothermal reaction under different pH conditions. (b)

pH 4.0, (c) 5.5, (d) 7.5, (e) 9.0, (f) 9.76, (g) 11, and (h) 12.13.

Fig. 4. XRD and DSC of samples before and after hydrothermal reaction at 110 8C/

15 h under pH of 11.5. All samples had been filtrated and washed.

C.-W. Chen et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 296–301 299

authors. For example, Cheng et al. (1996) found a mixture of m-ZrO2 and t-ZrO2 independent of the pH as long as KOH was used asthe mineralizer, while Stefanic et al. (1997) also obtained m-ZrO2

and t-ZrO2 mixed phase at pH 7 using NaOH as mineralizer, butamorphous zirconia was produced at pH 9. Dell’Agli et al. (1999,2000) claimed that tetragonal phase would be gradually sub-stituted by cubic phase upon increasing concentration of alkalinemineralizers (MOH, M = Li, Na, and K). The variation of theexperimental result is believed to relate to the precursor subjectedto the hydrothermal crystallization and cannot be generalized.

Before we settled on the particular choice of hydrothermalcondition, a large set of experiments were performed. As indicatedin Fig. 3, m- and t-ZrO2 mixed phase appeared if the hydrothermalreaction was done at 95 8C for 10 h under a very high pH value (pH12.13). In all lower pH conditions, only t-ZrO2 was found, but withvary degree of crystallization. The degree of crystallization couldbe determined by the size of the exothermic peak at 450 8C, whichappeared due to the crystallization of the amorphous part of theproduct. As the pH increased, the peak area decreased untildisappeared completely when above pH 11.

To avoid the formation of mixed phase and to ensure thecomplete crystallization, the hydrothermal conditions were finallysettled to be 110 8C for 15 h under pH 11.5, which was able toproduce fully crystallized pure t-ZrO2 nanocrystals as demon-strated in Fig. 4. The domain size of the produce crystals was�11 nm as estimated by the peak width with Scherrer equation.There was no exothermic peak at �460 8C, assuring the absence ofamorphous phase. Also notice is the existence of �300 8Cexothermic peak in all cases. This is different from Fig. 3 wherethe precipitates were prepared without the addition of TEA.

However, the XRD patterns of tetragonal and cubic zirconia arerather similar and there were still some doubt about the phasepurity of our product. A further analysis was therefore made by

visible Raman spectroscopy. From Fig. 5, we found Raman bandsappear at 146, 265, 316, 460 and 640/cm, which has been assignedto the Raman-active modes for the tetragonal phase of ZrO2 (Liet al., 2001). Also given in the same figure was the Raman spectrumof a sample heated to 350 8C. Again, only the characteristic bands oft-ZrO2 at 146, 265, 316, 460 and 640/cm were observed butstronger. Therefore, we were sure that the product was pure t-ZrO2. The reason for 350 8C calcination was to remove the TEA thatwas still bonded to the surface. The BET surface area of 350 8Ccalcined sample was 177 m2/g. It reduced to 58 m2/g if thecalcination temperature was increased to 500 8C and <2 m2/gwhen heated to above 1200 8C.

Noticed that we could have conducted the hydrothermalreaction either with the�2.5 wt% zirconia transparent sol obtainedafter aging the hydrolyzed product, or the more concentrated�17 wt% re-dispersed sol after flocculation and wash. The 2.5 wt%zirconia sol contained about the same amount of NaCl salt. Asindicated by the similarity of the XRD and DSC results given in

Fig. 5. Raman spectrum of the hydrothermal product (A) and that after heated to

350 8C (B).

Fig. 6. Transparency of the re-dispersed sol measured with 1 cm quartz UV cell. The

dashed lines were calculated from Eq. (1) assuming different ZrO2 particle sizes at

the given concentration.

C.-W. Chen et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 296–301300

Fig. 4, the excess electrolytes in the 2.5 wt% sol did not interfer withthe hydrothermal crystallization process. Therefore, the removal ofelectrolyte prior to hydrothermal reaction may be unnecessary.However, after the flocculation and filtration procedures, a moreconcentrated hydroxide sol was obtained, which would largelyreduce the size of the hydrothermal reactor needed.

3.4. Dispersibility

The target product of our process was a fully dispersed, hopefullytransparent sol. Without surface modification, the hydrothermalproduct was not dispersible even in neutral pH water. The dispersionin aqueous phase could be accomplished via the modification witheither citric acid or tartaric acid, while oleic acid was used for organicsolvent dispersion. The hydrothermal products were filtered andwashed with deionized water. Tartaric acid or citric acid was added

Fig. 7. TEM picture of the re-dispersed ZrO2 nanocrystals. From left to right are for (a) no

inserts were the SAED results.

and the pH was adjusted to 1.6 with hydrochloric acid to obtain awhite paste after ultra-sonification. Modified nanocrystals werefiltered out, and re-dispersed in water as neutral sol having 10 wt%zirconia. The transmittance of the dispersed sol was higher than 70%at 600 nm over a 1 cm quartz UC cell (Fig. 6). Based on Eq. (1) wecould estimate that the size of the zirconia particles should be about11 nm, which is consistent with the crystal domain size estimatedfrom the XRD data. In other word, the surface modified nanocrystalindeed existed as isolated particle. This was further substantiated bythe TEM picture given in Fig. 7.

However, it would be more important to modify the surface sothat it is dispersible in organic solvent. To do so, the hydrothermalproduct was mixed with a ethanol solution of oleic acid. Theamount of OA added corresponded to 6 mmol/g ZrO2. The pH wasthen adjusted to 6.0 and heated to 90 8C to 1 h. A greasy precipitatewas found at the bottom of the vial, which could be recovered andrinsed with small quantity of ethanol for several times to removeexcess oleic acid. This precipitate was fully dispersible in hexaneand chloroform. Unfortunately, the re-dispersed sol was not as

n-modified sample, (b) TA modified and (c) CA modified samples, respectively. The

C.-W. Chen et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 296–301 301

clear as in the aqueous case. To measure the transmittance, wehave to dilute it to less than 5 wt%. As shown in Fig. 6, the measuredtransmittance for both 5 wt% and 1 wt% could be roughly predictedby Eq. (1) assuming the particles were 22 nm in size. Thus, the OAmodification did not result in completely isolated particles.

4. Conclusion

With the addition of a small quantity of appropriate chelatingagents at different stages of the conventional aqueous precipita-tion and hydrothermal process, and the tuning of the processconditions, we were able to produce �10 nm isolated nanocrystalsof phase pure t-ZrO2. The process involved only low temperature(<110 8C) heating and used no expensive chemicals. Flocculationand filtration (with filter paper) were all that needed for therecovery of nanocrystal. After modified with citric acid and re-dispersed in water, the high transparency of the aqueous sol was astrong indication that the nanocrystals were indeed isolated.However, we were less successful in oleic acid modification, whichlead to a small degree of agglomeration when re-dispersed inorganic solvent. We believe that with further adjusting of thesurface modification procedure, a transparent organic sol shouldbe achievable. In other word, the existing aqueous processesproducing finer zirconia could be retrofitted to producednanocrystals in large volume, and enable a faster developmentof high refractive index nanocomposite.

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