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UPFLU-1223; No. of Pages 9

J. of Supercritical Fluids xxx (2006) xxx–xxx

Synthesis of nanoparticulate yttrium aluminum garnet insupercritical water–ethanol mixtures

Albertina Cabanas a,∗, Jun Li b, Paul Blood b, Tadeusz Chudoba c, Witold Lojkowski c,Martyn Poliakoff d, Edward Lester b,∗∗

a Departamento de Quımica-Fısica I, Universidad Complutense de Madrid, E-28040 Madrid, Spainb School of Chemical, Environmental and Mining Engineering, University of Nottingham, Nottingham NG7 2RD, UK

c Institute of High Pressure Physics (UNIPRESS), Warsaw PL-01142, Polandd School of Chemistry, University of Nottingham, Nottingham NG7 2RD, UK

Received 25 May 2006; received in revised form 26 June 2006; accepted 30 June 2006

bstract

Single-phase YAG nanoparticles have been produced by the rapid hydrolysis and dehydration of Al3+ and Y3+ salts in EtOH–H2O mixtures usingontinuous supercritical water hydrothermal synthesis. The effects of the nature of the starting solution, the metal concentration, the EtOH content,he flow rate and the temperature on the particle size were studied. EtOH promotes crystallization and highly crystalline materials are obtained atelatively low temperatures. Furthermore the critical temperature of EtOH–H2O mixtures is lower than that of pure H2O, reducing the operating

emperature. From the variables studied, the most important parameter seems to be the temperature and much smaller particles have been obtainedhen experiments are conducted under supercritical conditions (average 50 nm), in comparison to near critical or subcritical conditions (average50 nm). The method is simple and clean and requires neither addition of a base to the system nor the use of very high reaction temperatures andotentially allows the continuous production of large quantities of material.

2006 Elsevier B.V. All rights reserved.

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eywords: Supercritical water; Yttrium aluminum garnet; Nanoparticles; Wate

. Introduction

The synthesis of yttrium aluminum garnet (YAG or3Al5O12) has received much attention on account of its use

n structural and functional materials [1]. Because of its highreep- and oxidation-resistance at high temperature and loweat conductivity, YAG might also be suitable for high temper-ture structural components, e.g. inside gas turbines [2]. Singlerystal YAG is the most creep-resistant oxide available and haseen used for fiber reinforcement and in ceramic composite cut-ing tool material [3]. Polycrystalline YAG also has considerable

otential as a refractory fiber material [4]. Among its functionalroperties, YAG possesses a high efficiency of energy transfernd is resistant to radiation damage, which therefore makes it

∗ Corresponding author at: Departamento de Quımica-Fısica I, Ciudad Uni-ersitaria s/n, E-28040 Madrid, Spain. Tel.: +34 91 394 4200;ax: +34 91 394 4135.∗∗ Corresponding author.

E-mail addresses: a.cabanas@quim.ucm.es (A. Cabanas),dward.Lester@Nottingham.ac.uk (E. Lester).

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896-8446/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.supflu.2006.06.006

Please cite this article as: Albertina Cabanas et al., Synthesis of nanoparticuJ. of Supercritical Fluids (2006), doi:10.1016/j.supflu.2006.06.006.

nol mixtures; Hydrothermal synthesis; Solvothermal synthesis

deal for laser lenses. The usable transmittance range of YAGxtends from the UV to the mid-IR range, making it very usefulor IR and laser windows, with a very low absorption coefficient5]. Furthermore YAG displays a cubic crystal structure, whichmparts optical isotropy (unlike sapphire). Doping YAG with dif-erent trivalent ions changes the optical properties of the dopedaterials [6,7]. YAG has also been used in phosphors for scan-

ers and contrast-enhanced display applications [8]. Whetheranufacturing YAG for phosphor applications or ceramic fabri-

ation, fine-sized particles with no agglomeration are desirable.YAG powder is conventionally prepared from the solid-state

eaction of Y2O3 and Al2O3 at 1600–1800 ◦C. However the pro-ess is very slow (10–20 h) because YAlO3 (YAP) and Y4Al2O7YAM) phases are formed as reaction intermediates and theynly transform into single-phase YAG at high temperatures [6].iner particles can be prepared using wet chemical routes suchs sol–gel [9], co-precipitation [10], combustion [11] and spray

yrolysis [12]. Sol–gel and co-precipitation techniques requiredditional high temperature heat treatment in order to produceingle-phase crystalline materials. Furthermore aggregation ofhe products during the post-heating treatment is quite common.

late yttrium aluminum garnet in supercritical water–ethanol mixtures,

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UPFLU-1223; No. of Pages 9

A. Cabanas et al. / J. of Super

AG particles produced by combustion or spray pyrolysis areenerally large and appear highly aggregated [12].

Continuous supercritical water hydrothermal synthesisSWHS) is an emerging technique, which has been recently usedo produce nanoparticulate metal oxides [13–19]. As water iseated towards its critical temperature and pressure (Tc = 374 ◦C,c = 221 bar) [20], it changes from a polar liquid to a fluidith a low dielectric constant and low pH. The self-dissociation

onstant of water also increases up to 300 ◦C, giving rise toorrespondingly increased concentrations of H+ and OH− [21].urthermore, like any other supercritical fluid, the density ofupercritical water (scH2O) and its related properties can beanipulated by relatively small changes in pressure and tem-

erature. The lower dielectric constant in scH2O induces super-aturation and precipitation of small particles of metal hydrox-des, which then undergo rapid dehydration.

Hydrothermal synthesis of YAG powder has been previouslyeported in the literature. Hakuta et al. have produced parti-les of YAG doped with Tb (YAG:Tb) from nitrate solutions incH2O using a flow reactor different to the one used in thisork [22,23]. In their first paper [22], a solution of yttrium

nd aluminum nitrates in the correct stoichiometry was pre-ixed with a KOH solution and brought into contact with a

tream of scH2O. The solution passed through a tube reactornd was cooled externally downstream of the reaction zone. Ann-line filter collected the particles. Pressure was maintainedt 300 bar by a Tescom back-pressure regulator. By control-ing the KOH concentration, single-phase YAG (ca. 100 nm)as obtained between 400 and 420 ◦C and 300 bar; particles

xhibited the emission spectra characteristic of YAG:Tb. A sec-nd paper [23] showed the effects of pH and the presence ofoexisting ions (KOH and KNO3) on the hydrolysis of theetal nitrate solution prior to hydrothermal treatment. By adjust-

ng the metal solution to pH 9, single-phase YAG could bebtained at 400 ◦C and 300 bar. The YAG:Tb particles wereube-like and not aggregated with a particle size distributionn the range 40–140 nm with an average of 81 nm. Slurriesroduced from the precipitation of a mixture of nitrates withOH were found to produce particles as small as 17 nm. Alter-

ng the composition of the precursor was shown to produceifferent particle sizes, exhibiting different photoluminescenceroperties.

Other papers have described YAG manufacture usingolvothermal methods [24–28]. In particular, alcohols have beenroved to be efficient in the crystallization of YAG. Li et al. haveecently shown the production of YAG crystalline powders fromhe treatment of precipitates of aluminum and yttrium nitratesith ammonium hydrogen carbonate in EtOH–H2O mixtures at90 ◦C and 100 bar using a batch reactor [24]. By using a 2:1tOH–H2O mixture, YAG was manufactured around the criticaloint of the mixture, 283 ◦C and 92 bar [24] (compared to H2Ot 374 ◦C and 220 bar). Particles obtained with this method werepherical, with a mean grain size of 60 nm and relatively narrow

article size distribution for those samples treated for 1.5 h. Sim-larly YAG:Ce and YAG:Tb of ca. 60 and 80 nm, respectively,ere produced from EtOH–H2O mixture at 300 ◦C and 100 bar

25,26]. Inoue et al. have also reported [28] the solvothermal syn-

va0t

Please cite this article as: Albertina Cabanas et al., Synthesis of nanoparticuJ. of Supercritical Fluids (2006), doi:10.1016/j.supflu.2006.06.006.

al Fluids xxx (2006) xxx–xxx

hesis of YAG from aluminum isopropoxide and yttrium acetaten 1,4-butanediol at 300 ◦C and 50 bar in a batch reactor to pro-uce spherical particles of ca. 30 nm, but the particles showedome aggregation. By comparison, hydrothermal synthesis ofhe same starting materials required more severe conditions thanhe solvothermal method. In contrast to batch systems, flow reac-ors allow a better control of the experimental conditions andield continuous production of the materials in a much fasteray.We have recently carried out a detailed study of a new design

f flow reactor. The mixing of a cold metal solution and acH2O stream was simulated for different nozzle geometriessing light adsorption imaging (LAI) and computational fluidynamics (CFD) [18]. This study revealed the importance ofhe mixing geometry in order to get uniform particle size distri-ution and allowed us to implement an original nozzle designhich promotes mixing, avoids possible clogging of the reac-

or during operation and yields fairly homogeneous particle sizeistribution materials, making our high pressure flow reactornique [19]. With this nozzle, highly turbulent macro-mixingas observed at the mixing point, due to the large density differ-

nce existing between the metal salt solution and scH2O streamshich induces large natural convection.This paper reports an investigation of YAG production in

ixtures of EtOH and H2O to promote crystallization of YAGsing our new flow reactor. As it will be discussed later, whenetal solutions in EtOH or EtOH–H2O mixtures are used, the

olume and the heat of mixing with the scH2O stream must beonsidered in order to control particle size distribution.

. Experimental

Aluminum acetylacetonate [Al(acac)3] (99% purity),ttrium(III) acetate hydrate [Y(ac)3·xH2O] (99.9% purity),luminum nitrate nonahydrate [Al(NO3)3·9H2O] (98% purity)nd yttrium nitrate hexahydrate [Y(NO3)3·6H2O] (99% purity)ere purchased from Aldrich and were used without furtherurification. EtOH (99.8% purity) and ammonium hydroxideH4OH (35 wt.%) from Fisher and nanopure water were used

n these experiments.Two different starting solutions were employed: Solution A,

repared using crystalline Al(acac)3 and Y(ac)3·xH2O and Solu-ion B, prepared from mixtures of “Al and Y acetates” generatedrom the corresponding nitrates.

Solution A was prepared by dissolving independently(ac)3·xH2O in a small amount of H2O and Al(acac)3 in EtOH.he solutions of Y(ac)3·xH2O and Al(acac)3 were mixed in aolumetric flask and were made up to the final volume by addingtOH. Al(acac)3 exhibits low solubility in H2O but it is solu-le in EtOH. Y(ac)3·xH2O is soluble in H2O but decomposesn pure EtOH. The mixture of Y(ac)3·xH2O and Al(acac)3 intOH–H2O was stable. The EtOH content in the solutions is

eported as vol.% of EtOH in the metal salt solution and was

aried from 60 to 90%. Solutions 0.05 and 0.025 M in Al, withY:Al molar ratio 3:5 were prepared to yield YAG suspensions.01 and 0.005 M, respectively (without considering mixing withhe water stream).

late yttrium aluminum garnet in supercritical water–ethanol mixtures,

SUPFLU-1223; No. of Pages 9

A. Cabanas et al. / J. of Supercritic

Fig. 1. Schematic of the supercritical reactor. scH2O (FW) is pump through apre-heater (PH) using a Gilson pump (G) and enters the reactor (R) from the top,through a 1/8 in. piece of tubing. Reactor volume is ca. 4 mL. Simultaneouslya metal salt solution (FS) is pumped through the bottom of the reactor into a3/8 in. piece of tubing. Mixing of both streams using this arrangement is veryeffective, as it has been previously shown [19]. Good mixing can be attributedto buoyancy forces existing between the two fluids (scH2O stream and metalsalt solution) of very different density. Hydrolysis and dehydration of the metalsalts take place in the reactor to yield metal oxides nanoparticles. The reactivemixture is rapidly cooled down by a water cooler (WC). A computer controlledbHt

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ack-pressure regulator (BPR) keeps the pressure above the critical pressure of

2O. A 1 �m stainless steel filter, placed after the cooler, protects the BPR andraps large particulate material.

Solution B was prepared using mixtures of “Al and Ycetates” generated from the corresponding nitrates. A mix-ure of 6.00 g of Y(NO3)3·6H2O (0.016 mol) and 9.76 g ofl(NO3)3·9H2O (0.026 mol) was dissolved in 100 mL H2O

Y:Al molar ratio 3:5). NH4OH (35 wt.%) was added drop-wiseo the solution until precipitation was complete. The precipi-ate was then washed with water and centrifuged several timeso remove excess base. After the washing, the precipitate wasedissolved in 10 mL of glacial acetic acid and diluted in H2O upo a final volume of 200 mL. The maximum final concentrationsf Al and Y in the solution are 0.13 and 0.08 M, respectively.his aqueous solution was further diluted with EtOH to concen-

rations of Al and Y equal to 0.05 and 0.03 M, respectively. Thenal EtOH content in the sample was ca. 60 vol.%.

Experiments were conducted in the supercritical reactorystem developed at the University of Nottingham [19]. Achematic of the reactor is shown in Fig. 1. At supercriticalnd near critical conditions, the hydrolysis and dehydration ofhe metal salts take place and nanoparticulate metal oxides areroduced. In most cases, the particles produced exhibit sizes inhe nanometer range, pass through the filter and BPR and areollected at the end of the line in the form of a suspension.

Flow rates for scH2O (FW) were 10–20 mL/min. Flow ratesf the metal salts (FS) were one-half of FW. scH2O was injectedrom the top at 415 ◦C (or lower) while temperature of the metalolution introduced from the bottom of the reactor was kept

aatu

Please cite this article as: Albertina Cabanas et al., Synthesis of nanoparticuJ. of Supercritical Fluids (2006), doi:10.1016/j.supflu.2006.06.006.

al Fluids xxx (2006) xxx–xxx 3

elow 30 ◦C. In most experiments reactor temperature (R) was85 ◦C, but experiments at 355 and 260 ◦C were also conducted.t these temperatures, decomposition of EtOH does not takelace [29]. Pressure was kept at 240 bar. The mean residenceime was estimated from the reactor volume, the total flow ratend the density of the reaction mixture at the reaction temper-ture and pressure (approximated to that of pure H2O [30]).esidence times vary between ca. 3 and 13 s for experimentsonducted at 385 and 262 ◦C, respectively. It is very difficulto estimate the time needed for the reaction mixture to equili-rate after mixing with the scH2O stream, but due to the highiffusivity at supercritical conditions, the process will be veryast. Considering the flow rates and the densities of the flu-ds [30], the mole fractions of EtOH (x) in the reactor at the

ixing point for the mixtures 90 and 60 vol.% EtOH in H2Ore ca. 0.13 and 0.08, respectively. The critical parameters forhese mixtures are 335 ◦C and 180 bar for x = 0.13 and 347 ◦Cnd 200 bar for x = 0.08 (interpolated using data from literature31]).

All products were characterised by powder X-ray diffrac-ion (XRD) and, in selected cases, by transmission electron

icroscopy (TEM) and scanning electron microscopy (SEM).RD data were collected using a Phillips EXPERT θ–2θ diffrac-

ometer with a Cu K� radiation. The sample was loaded onto anndented glass plate and scans were typically taken for 1 h over2θ range of 20–80◦. The Jade® 5 program was used to assessurity of the powders by comparison to the JCPDS databasecard numbers in brackets). Particles size was estimated using thecherrer equation on the XRD line-widths. TEM pictures werebtained using a Jeol JEM-2000FXII TEM electron microscopeperating at 200 KeV by mixing the aqueous powder suspen-ion with EtOH and placing a drop of this mixture on a 3.05 mmiameter copper mesh (300 lines per inch mesh), which was thenried in air. SEM pictures were taken on a Jeol-6400 electronicroscope working at 20 kV. Samples were gold coated prior

o analysis. Particle size distributions were obtained from thenalysis of the TEM pictures with an image analysis programsing MATLab®.

. Results and discussion

Continuous supercritical water hydrothermal synthesis wasarried out using Solutions A and B in the reactor shown in Fig. 1t 260–385 ◦C and 240 bar. In every case, white-yellowish mate-ials were obtained. XRD patterns of all the samples matchedlosely the JCPDS [33–0040] pattern, cubic YAG. In some sam-les, very weak peaks at 2θ values ca. 28 and 38 could indicatehe presence of some bohemite AlOOH [21–1307].

Previous authors have suggested that using alcohols mayeduce the crystallization temperature [24–26,28] possiblyecause the critical parameters of the mixtures of EtOH and2O are lower than those of pure H2O. The solvent propertiesf the mixture (density, dielectric constant, viscosity, . . .) will

lso be different to those of EtOH or H2O [32], and size, shapend aggregation of the particles may be strongly influenced byhe EtOH content. Furthermore the heat absorbed or releasedpon mixing scH2O and an EtOH–H2O solution and the volume

late yttrium aluminum garnet in supercritical water–ethanol mixtures,

SUPFLU-1223; No. of Pages 9

4 A. Cabanas et al. / J. of Supercritical Fluids xxx (2006) xxx–xxx

F ◦C a9

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ig. 2. XRD patterns of YAG samples produced by SWHS of Solution A at 3850% (as synthesized).

hange associated to this process should be also considered. Theixing of EtOH and H2O shows a strong temperature depen-

ence changing from exothermic at near ambient conditions totrongly endothermic at elevated temperature [33,34]. Pressureependence, however, seems to be very weak. In particular, at50 ◦C and 200 bar, the maximum heat evolved upon mixingtOH and water reaches 1690 J/mol at 0.459 EtOH molar frac-

ion [35]. Although there is much less information about theolumes of mixing EtOH and H2O at high temperature and pres-ure, a similar temperature and pressure dependence is expected

36]. As the starting metal solutions (Solutions A and B) con-ain large quantities of EtOH, and this solution is suddenly mixedith scH2O, interesting thermal phenomena may take place at

he mixing point.

fcpo

Fig. 3. TEM of YAG sample produced by SWHS of Solution A with 90 vol.% E

Please cite this article as: Albertina Cabanas et al., Synthesis of nanoparticuJ. of Supercritical Fluids (2006), doi:10.1016/j.supflu.2006.06.006.

nd 240 bar at different vol.% EtOH in H2O: (a) 60%, (b) 70%, (c) 80% and (d)

To assess the role of EtOH in the starting solution, we con-ucted a series of experiments changing the initial concentrationf EtOH in H2O. XRD of samples synthesized at 385 ◦C and40 bar using Solution A (Al(acac)3 and Y(ac)3·xH2O) 0.005 Mnd a FW:FS 20:10 mL/min at different EtOH–H2O concentra-ions (90, 80, 70 and 60 vol.% EtOH in H2O) are compared inig. 2. The broader peaks obtained for the sample synthesizedith the lowest EtOH content (60 vol.% EtOH) suggest a smallerarticle size. Analysis of the width of the peaks using the Scher-er equation gave particles sizes of ca. 40 nm for all cases apart

rom the 60 vol.% EtOH sample, ca. 20 nm. The samples werealcined in air at 10 ◦C/min up to 1000 ◦C and kept at this tem-erature for 3 h. XRD analysis of the calcined samples showednly YAG.

tOH in H2O at 385 ◦C and 240 bar. Scale bars: (a) 200 nm and (b) 100 nm.

late yttrium aluminum garnet in supercritical water–ethanol mixtures,

SUPFLU-1223; No. of Pages 9

A. Cabanas et al. / J. of Supercritical Fluids xxx (2006) xxx–xxx 5

l.% E

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Fig. 4. TEM of YAG sample produced by SWHS of Solution A with 60 vo

TEM images of YAG particles produced with the high-st and lowest EtOH content, 90 and 60 vol.% EtOH in H2OFigs. 3 and 4, respectively) show clear differences between theamples. The material produced with the highest EtOH content90 vol.% EtOH; Fig. 3) is composed of highly regular cube-likearticles suggesting high crystallinity. The particle size distri-ution analysis based on Fig. 3a is shown in Fig. 5, along withhe analysis of other samples produced under different condi-ions. The size distribution is quite broad with particle between5 and 135 nm, with an average particle size ca. 90 nm. Themallest particles observed in the TEM images in Fig. 3a weregnored in the particle size analysis. By contrast, particles pro-uced with the lowest EtOH content (60 vol.% EtOH; Fig. 4)re much smaller but more irregular in shape, with a size dis-

ribution based on Fig. 4a that is quite narrow (average particleize ca. 50 nm). The particle sizes estimated using the Sherrerquation (40 and 20 nm, respectively) are systematically lowerhan the sizes derived from the TEM images, because large par-

ig. 5. Particle size distributions analysis of YAG particles obtained from TEMmage analysis (see text for details).

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Please cite this article as: Albertina Cabanas et al., Synthesis of nanoparticuJ. of Supercritical Fluids (2006), doi:10.1016/j.supflu.2006.06.006.

tOH in H2O at 385 ◦C and 240 bar. Scale bars: (a) 200 nm and (b) 50 nm.

icles have a negligible effect on the line width. In both cases,articles were not aggregated.

The reaction temperature required to obtain pure YAG usingtOH–H2O mixtures is lower than the temperature needed inther SWHS experiments using metal aqueous solutions [22,23].his is consistent with the reduction in the crystallization tem-erature observed previously with EtOH or EtOH mixtures inatch experiments [24,28]. In our experiments, the sample pro-uced from the solution with the highest EtOH content (90 vol.%tOH) gave large and highly crystalline particles, while theample with 60 vol.% EtOH yielded smaller particles. Condi-ions of 385 ◦C and 240 bar are above the critical point for thetOH–H2O mixtures in the reactor. However, the distance to theritical point is larger for the solution with 90 vol.% EtOH (lowerritical temperature, Tc) and, in reduced units (T/Tc), the tem-erature is higher than that for the solution with 60 vol.% EtOH.ccording to this, particle size seems to increase with relative

emperature at supercritical conditions. On the other hand, mix-ng EtOH and H2O produces thermal and volumetric effects,hich may alter the size and morphology of the particles. The

elatively broad particle size distribution observed in the sam-les produced with the highest EtOH content may reflect localuctuations in temperature, volume and concentration duringixing, which unfortunately are very difficult to quantify. In

act, if the mixing of H2O and EtOH were endothermic at highemperature and pressure [35], one would expect a lower localemperature at the mixing point in the samples produced withhe highest EtOH content. Understanding the effects of mix-ng the EtOH–H2O metal solution and the scH2O stream is verymportant to be able to control the particle size distribution. CFDimulations could be very useful in estimating the temperaturend density fluctuations across the system.

Further experiments were conducted using Solution A with60 vol.% EtOH at different metal concentrations (0.01 and

.005 M [YAG]) and different flow rates (FW:FS 20:10 and0:5 mL/min). XRD patterns of the resulting materials were very

late yttrium aluminum garnet in supercritical water–ethanol mixtures,

SUPFLU-1223; No. of Pages 9

6 A. Cabanas et al. / J. of Supercritical Fluids xxx (2006) xxx–xxx

l.% E

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dlltdSdihsiwhile different by products were obtained at the other pH condi-tions. The variation of the particle size with the pH was explainedby reaction equilibrium theory [38], assuming that the reactionproceeds through a dissolving-recrystallization mechanism of

Fig. 6. TEM of YAG sample produced by SWHS of Solution B with 60 vo

imilar to those shown in Fig. 2; analysis with the Scherrer equa-ion indicated that changes in concentration and flow rates, inhe range studied, did not alter the particle size substantially.amples synthesised at the lowest flow rates (10:5) gave rise toshoulder in the XRD pattern at 2θ ca. 28, possibly indicating

he formation of some AlOOH [21–1307].Decomposition of solutions of pure Al(acac)3 and

(ac)3·xH2O was also attempted under similar conditions.xperiments with Al(acac) in pure EtOH did not yield any solid

esidue, while hydrothermal reaction of Y(ac)3·xH2O in H2Oielded white powders, most probably Y2O3 or YO(OH), buthe solid quickly redissolved in the acid medium produced inhe reaction.

AlO(OH) is an impurity commonly found in YAG exper-ments generated in scH2O,[23] but since Al(acac)3 does notecompose readily into AlO(OH) under these conditions in oureactor, AlO(OH) it is unlikely to appear as a major impurity.RD patterns of all samples show that most Al3+ reacts to formAG.

Several experiments were also conducted using Solution Bmixture of “Al and Y acetates”). In this case, however, the EtOHontent was kept constant at 60 vol.%. XRD patterns showed thatolution B produced pure YAG with an average particle size

arger than from Solution A. TEM images of samples synthe-ized by SWHS of Solution B, 0.01 M at 385 ◦C and 240 bar withW:FS 20:10 mL/min are shown in Fig. 6, revealing particlesounder than those found from Solution A. Particle size distri-ution analysis of Fig. 6a is shown in Fig. 5, with a broad sizeistribution, with most particles between 140 and 300 nm and anverage particle size of ca. 225 nm. Few cube-like particles arebserved, but most of them are round. Further experiments were

onducted using Solution B at different concentrations (0.01 and.005 M [YAG]) and flow rates (FW:FS 20:10 and 10:5 mL/min)ut little dependence of the particle size with these variables wasound.

F21

Please cite this article as: Albertina Cabanas et al., Synthesis of nanoparticuJ. of Supercritical Fluids (2006), doi:10.1016/j.supflu.2006.06.006.

tOH in H2O at 385 ◦C and 240 bar. Scale bars: (a) 500 nm and (b) 200 nm.

While containing the same concentration of metal ions, theifferences between the Solutions A and B are: (i) the muchower pH of Solution B and (ii) the presence of the ligand acety-acetone (acac) in Solution A. Solution A has a higher pH andhe presence of the large (acac) ligand. pH of Solution A is ca. 9ue to the hydrolysis of Y(acac)2, whereas excess acetic acid inolution B lowers the pH to ca. 4.5. Hakuta et al. have studied inetail the effect of the pH in the hydrolysis of Al nitrates [37] andn mixtures of Y and Al nitrates [22,23] using a flow reactor, andave found that particle size decreases as the pH of the startingolution increases, in agreement with our results. In their exper-ments, Hakuta et al. produced pure YAG only at pH 9.1 [22],

ig. 7. XRD patterns of YAG samples produced by SWHS of Solution A at40 bar and different temperatures: (a) 385 ◦C, (b) 355 ◦C and (c) 260 ◦C (FW:FS0:5 mL/min).

late yttrium aluminum garnet in supercritical water–ethanol mixtures,

SUPFLU-1223; No. of Pages 9

A. Cabanas et al. / J. of Supercritical Fluids xxx (2006) xxx–xxx 7

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ig. 8. TEM of YAG samples produced by SWHS of Solution A with 60 vol.%nd (b) 200 nm.

he single metal hydroxides. As the solubility of AlO(OH) andO(OH) increases with the pH [37], a higher super-saturation

s attained at the highest pH and smaller particles are obtainedsing Solution A. At the same time, the presence of the stronglyoordinating ligand (acac) in Solution A may prevent particleggregation. Mousavand et al. have used organic reagents toodify the surface of the metal oxides produced from SWHS

nd have shown that particle morphology and crystal structurean be changed [39]. In our synthesis, the ligand (acac) adsorbso the YAG particle surface (as shown by IR of the dry powders)nd may stabilize the particles against aggregation. The changen the particle shape is also significant as particles produced fromydrolysis of Solution B are rounder; resembling those obtainedy Li et al. from the solvothermal synthesis in a batch reactor24].

The effect of temperature was studied for the SWHS ofolution A (Al(acac)3 and Y(ac)3·xH2O) 0.01 M in [YAG] and

0 vol.% EtOH in H2O. Fig. 7 compares the XRD patterns ofaterials obtained at 385, 355 and 260 ◦C. In all cases, YAG

s the major phase. At 260 ◦C, XRD pattern shows very broadeatures corresponding to YAG, but the yield at this temperature

cp

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ig. 9. SEM of YAG samples produced by SWHS of Solution A with 60 vol.% H2Ohe flow rates were FW:FS 20:10 mL/min.

Please cite this article as: Albertina Cabanas et al., Synthesis of nanoparticuJ. of Supercritical Fluids (2006), doi:10.1016/j.supflu.2006.06.006.

in H2O at 355 ◦C and 240 bar (FW:FS 10:5 mL/min). Scale bars: (a) 500 nm

as quite low. At 355 and 385 ◦C, the peaks are more intensend better defined, with a minor peak at 2θ ca. 28 (AlOOH).

TEM images of the sample synthesized at 355 ◦C are shown inig. 8. Particles are similar to those shown in Fig. 3 for the sam-le obtained from Solution A (90 vol.% EtOH) at 385 ◦C. Thearticles have cube-like shape and exhibit high crystallinity. Theize distribution is broad (Fig. 5), with most particles between00 and 180 nm (average ca. 150 nm), with a few as large as00 nm.

The product obtained from the SWHS of Solution AAl(acac)3 and Y(ac)3·xH2O) at 355 ◦C and 240 bar was studiedy SEM, Fig. 9. SEM images of the powder obtained after dryingFig. 9a) and after washing and centrifuging the powder in waterFig. 9b) show highly regular cube-like particles, 100–150 nmong, consistent with the distribution shown in Fig. 5. The crystalabit reflects the cubic structure of the materials. Some smallerarticles were observed before washing but, after washing and

entrifuging, only the large particles are recovered (the smallarticles will not settle).

The YAG particles produced at 355 ◦C are larger than thosebtained at slightly higher temperature, 385 ◦C. Even though

in EtOH at 355 ◦C and 240 bar. The metal solution was 0.01 M in [YAG] and

late yttrium aluminum garnet in supercritical water–ethanol mixtures,

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UPFLU-1223; No. of Pages 9

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he critical temperature of the EtOH–H2O mixture at the mix-ng point for the 60 vol.% EtOH in H2O is ca. 347 ◦C [31]n the experiments conducted at 355 ◦C, the local temperatureould be subcritical due to the endothermic heat of mixing [35].he different particle size obtained from different experimentsan be rationalized considering reaction at supercritical or sub-ritical conditions. Under supercritical conditions (385 ◦C), theower density and dielectric constant of the medium causes aarge super-saturation resulting in the formation of many smalluclei, which yield smaller particles. At subcritical or near criti-al temperatures, however, the self-dissociation constants of theedium is higher, favoring hydrolysis, but the dielectric con-

tant and density are higher so that the super-saturation is lowernd fewer nuclei are produced, which can then grow larger fromolution. In addition, the density at the mixing point is larger atubcritical than at supercritical conditions and accordingly theesidence time is larger at 355 ◦C, which will allow particles torow larger.

. Conclusions

Highly crystalline YAG particles have been produced by con-inuous SWHS of metal solutions containing Y3+ and Al3+ in

2O–EtOH mixtures. Rapid hydrolysis and dehydration of theetal solutions yield YAG nanoparticles in a single step. Our

ozzle design [19] favors the mixing of the two fluids andncreases the reproducibility and reliability of the process. Inomparison to previous reports on the continuous hydrothermalynthesis of YAG [22,23], our method is simpler as it does notequire the addition of a base prior to the hydrothermal treat-ent or the use of slurries, which are more difficult to pump.urthermore, the operating temperature can be reduced because

he critical temperature of EtOH–H2O mixtures is lower thanhat of pure H2O. However, the use of EtOH introduces a newariable, as there are temperature and volume variations whentOH and H2O are mixed. This may create local fluctuations in

emperature at the mixing point, which deserve further study.Two different starting solutions have been employed: Solu-

ion A (mixture of Al(acac)3 and Y(ac)3·xH2O) and Solution Bmixture of “Al and Y acetates”). At the same conditions, YAGarticles obtained using Solution A were comparatively smallernd highly crystalline, while particles obtained using Solution

were larger and slightly rounder. The higher pH in Solutionfavors nucleation versus particle growth and yields smaller

articles. Furthermore, the ligand (acac) in Solution A adsorbso the particle surface and may prevent particle aggregation. Forolution A, the effect of the EtOH content was further studied.t 385 ◦C (above the Tc of the 90–60 vol.% EtOH in H2O mix-

ures), larger particles were obtained for the solution with theargest EtOH content (lower Tc). The larger the temperature (ineduce units), the larger the particle size. The effects of metaloncentration, flow rate and temperature in the morphology andarticle size were studied for Solution A with 60 vol.% EtOH.

ithin the limits of this study, the metal concentration and the

ow rate do not affect significantly the particle size. The mostmportant parameter seems to be temperature and large differ-nces in particle size have been found when experiments are

[

Please cite this article as: Albertina Cabanas et al., Synthesis of nanopmixtures, J. of Supercritical Fluids (2006), doi:10.1016/j.supflu.2006.06

al Fluids xxx (2006) xxx–xxx

onducted at 385 ◦C (supercritical) and 355 ◦C (near critical orubcritical) conditions. At supercritical conditions, YAG parti-les 50 nm in average with a narrow particle size distributionere produced, whereas at subcritical or near critical condi-

ions, particles 150 nm in average with a wider size range werebtained. At supercritical conditions many small nuclei are pro-uced and the particles are smaller. At subcritical or near criticalonditions, particles are larger. Although the particles producedn this work are still larger than those reported by Hakuta etl. from the hydrolysis of slurries at 400 ◦C and 300 bar [23],ur method appears to be more straightforward and we envis-ge that the particle size could be further reduced by optimizinghe reaction temperature and the ethanol content in the metalolution.

In contrast to other solvothermal processes in a batch reactor,ur method is much faster (a few seconds versus hours) and athe concentrations and flow rates employed in these experiments,llows the continuous production of up to 3.6 g YAG/h.

cknowledgements

We gratefully acknowledge the financial support of EU COSTetwork D30 and ICI Strategic Technology Group. A.C. thanksEC (Spain) for its support through a “Ramon y Cajal” contract.e thank Messrs P. Fields, R. Wilson and M. Guyler for technical

ssistance.

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