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Available online at www.sciencedirect.com

J. of Supercritical Fluids 44 (2008) 85–91

Continuous hydrothermal synthesis of iron oxide andPVA-protected iron oxide nanoparticles

Chunbao Xu, Amyn S. Teja ∗School of Chemical & Biomolecular Engineering, Georgia Institute of Technology,

Atlanta, GA 30332-0100, United States

Received 3 August 2007; received in revised form 20 September 2007; accepted 21 September 2007

bstract

Factors that affect the size, size distribution, and morphology of �-Fe2O3 nanoparticles obtained via continuous hydrothermal synthesis haveeen investigated in this work. The presence of polyvinyl alcohol (PVA) during synthesis is shown to limit the aggregation of particles and toroduce narrow particle size distributions than in the case when PVA is absent. Narrow particle size distributions are also obtained with an increasen PVA concentration, beyond a minimum concentration required to cover all particles with a polymer layer. The average particle size is shown to

ncrease with temperature and residence time, and is accompanied by morphology changes in some cases. Both modeling and experimental studiesndicate that equilibrium species distributions do not affect the particle morphology, so that changes in morphology in this work are probably dueo kinetic phenomena.

2007 Elsevier B.V. All rights reserved.

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eywords: Continuous hydrothermal method; Iron oxide; Polymer coating; PV

. Introduction

Nanoparticles of metal oxides often exhibit enhanced chem-cal, thermal, optical, electrical, or magnetic properties, which

ake them useful for applications in catalysis, pigments, coat-ngs, and inks, as well as in electronic and biomedical devices1]. These applications often depend on specific physical char-cteristics of the nanoparticles such as size, morphology, andrystallinity that are influenced by the method employed toroduce the particles [1–3]. Among such methods, continuousydrothermal processing (CHP) offers a relatively simple routeo make metal oxide nanoparticles of specific size and morphol-gy [4]. The method is environmentally benign and amenableo scale-up, although the mechanisms of particle formation haveet to be determined. In particular, the role of several process-ng variables on particle characteristics has not been determinedonclusively. For example, Hao and Teja [5] have reported that

emperature has no apparent effect on the size of iron oxideanoparticles obtained in the CHP method, whereas Matson etl. [6] note that increasing temperature leads to an increase in

∗ Corresponding author. Tel.: +1 404 894 3098; fax: +1 404 894 2866.E-mail address: amyn.teja@chbe.gatech.edu (A.S. Teja).

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896-8446/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.supflu.2007.09.033

rticle aggregation

he size of iron oxide nanoparticles in their implementation ofhe CHP method. An increase in particle size with temperatureas also reported by Adschiri et al. [7] in the case of boehemitearticles. On the other hand, Hakuta et al. [8] showed that theize of ceria particles decreased with temperature in their CHPethod. In addition to these reported discrepancies, aggregation

f particles during CHP is also of concern because of the inher-ntly high specific surface energy of nanoparticles. It has beenuggested [9] that aggregation can be minimized with a “protec-ive polymer” that limits the action of attractive forces betweenarticles during processing.

In the present work, we have investigated the synthesis of ironxide (�-Fe2O3) nanoparticles using the CHP method with andithout polyvinyl alcohol (PVA) to minimize aggregation. The

ffects of experimental variables such as PVA concentration andemperature on particle size, size distribution, and crystallinityere also investigated. Hematite (�-Fe2O3) was used as a modelxide because it is easily obtained via hydrolysis and dehydra-ion of ferric nitrate, and also because it has a wide field ofechnological applications in catalysis, inorganic pigments, and

agnetic recording media [1]. PVA was chosen as a protectiveolymer because it has the desired solution properties in waternd it contains many isolated hydroxyl functional groups, whichan adsorb and complex with metal ions [10].

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6 C. Xu, A.S. Teja / J. of Supe

. Experimental

.1. Materials

Iron(III) nitrate nonanhydrate (Fe(NO3)3·9H2O, ACSeagent) of minimum purity 98 wt% and polyvinyl alcoholMw = 89,000–98,000) were purchased from Sigma–AldrichWI, USA) and were used as received.

.2. Apparatus and procedure

The apparatus used in our experiments is shown in Fig. 1nd is similar in principle to that used in previous work [5,11].n aqueous solution of ferric nitrate was prepared by adding00 ml of deionized water to a known mass of Fe(NO3)3·9H2Orystals and stirring until a homogeneous solution resulted. Aecond 500 ml aqueous solution of ferric nitrate and PVA wasrepared by adding a known mass of PVA to about 300 ml ofater in a flask, heating the flask while stirring the contentsntil all PVA had dissolved, and then adding a known amountf Fe(NO3)3·9H2O crystals to the homogeneous PVA solution,ollowed by the addition of water to make 500 ml of the finalolution. Nitrogen was bubbled through each flask to degas theolutions. During each experimental run, a micrometering pumpModel 2396, Milton Roy Co., Ivyland, PA) was used to deliverne of the precursor (ferric nitrate with or without PVA) solu-ions to a mixing tee (Swagelock SS-400-4), where the solutionas contacted with hot compressed water. The water was heatedy pumping it through two stainless steel coils (1/8-in. O.D.) andelivered to the mixing tee by two high-pressure syringe pumpsModel 260D and Model 100DX, Isco Inc., Lincoln, NE) oper-ted such that one was filling while the other emptying. Theumps also maintained a constant ratio of the flow rate of water tohat of the precursor solution at 3:2 throughout each run. The two

oils were heated by cartridge heaters (Omegalux CIR Series,mega Engineering Inc., Stamford, CT) controlled by variable

utotransformers (Powerstat and Staco Energy Products), anddditionally by heating tape (Omegalux SRFE Series) controlled

2

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Fig. 1. Schematic diagram

al Fluids 44 (2008) 85–91

ith a feedback controller (CN9000A, Omega Engineering,tamford, CT) connected to a thermocouple (KTSS-116G-12,mega Engineering Inc., Stamford, CT). The temperature of

he mixed stream was measured at the tee with a thermocou-le (KTSS-116G-12, Omega Engineering, Stamford, CT) andhe pressure was measured with a digital pressure gauge (Model01A, Heise® Pressure Instruments, Stratford, CT). The mixedtream, consisting of precipitated particles and other products ofhe hydrolysis and dehydration reactions between ferric nitratend hot compressed water, then flowed through a cooling coil,tubular crystallizer, a condenser, a filter, and a backpressure

egulator, to a collection vessel. The backpressure regulatorModel 26-1722-24, Tescom Corporation, Elk River, MN) wassed to maintain constant pressure in the system. The tempera-ure was regulated by three feedback controllers (Omega ModelN9000A, Omega Engineering Inc., Stamford, CT) connected

o the thermocouples and heating tapes. The experiment waserminated after approximately 4 h, or after at least 100 ml ofhe precursor solution had been contacted with the hot water.he system temperature and pressure were recorded periodi-ally and the pressure was maintained at 21–23 MPa to suppressormation of a vapor phase during the experiment. After 4 h,he precursor solution was exchanged with deionized water tonsure that no salt would remain in the tubing between the pumpnd the mixing tee. When the pH of the exit solution returnedo neutral, the heaters were turned off and water was pumpedhrough the system until the temperature decreased below 375 K.he pressure was then reduced to ambient and the flow of watertopped. Finally, the filter was removed from the system andhe deposited particles were collected and added to the effluentn the collection vessel. Reaction conditions (including averageemperatures and pressures during each run) are summarized inable 1.

.3. Sample analysis

A small drop of the solution in the collection vessel waslaced on a carbon-coated copper grid and examined using a

of the apparatus.

C. Xu, A.S. Teja / J. of Supercritical Fluids 44 (2008) 85–91 87

Table 1Summary of experimental conditions

Experiment no. Feed concentration (M) Temperature ofwater (K)

Temperature atmixing point (K)

Pressure (MPa) Tube length(cm)

[Fe3+]:[VA]a

S1 0.03 625 ± 5 573 ± 2 22.9 ± 0.1 5 b

S2 0.06 625 ± 4 574 ± 2 23.0 ± 0.1 5 b

S3 0.03 623 ± 3 569 ± 2 22.9 ± 0.1 10 b

S4 0.03 623 ± 5 572 ± 2 22.9 ± 0.1 20 b

S5 0.03 623 ± 1 487 ± 3 22.6 ± 0.1 5 1:1S6 0.03 621 ± 4 573 ± 5 22.0 ± 0.1 5 1:1S7 0.03 621 ± 4 573 ± 3 22.0 ± 0.1 5 1:2S8 0.03 663 ± 1 648 ± 4 22.0 ± 0.1 20 1:1S9 0.03 622 ± 1 574 ± 5 21.8 ± 0.1 20 2:1S10 0.03 622 ± 1 572 ± 4 21.9 ± 0.1 20 1:1S 73 ±

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a Mole ratio of Fe3+ to vinyl alcohol monomer units.b No PVA added.

EOL 100C transmission electron microscope (Japan Electronptics Laboratory Co. Ltd., Tokyo, Japan). Several TEM imagesf each sample were obtained, and particle characteristics werestimated from measurements of the diameters of at least 100articles in the images. In the case of nonspherical particles, theiameter of each particle was estimated from the average of theong and short axes of the particle. The JEOL 100C TEM waslso used to get electron diffraction patterns of some samples.ue to the limited number of particles used in TEM analyses,

he statistics only provide reliable estimates of the size and sizeistribution of the nanoparticles for comparative purposes [12].he majority of the solution in the collection vessel was allowed

o settle, and then filtered to separate the solid product. This wasried overnight in an oven at 325 K for subsequent analysis usingScintag XRD (X-ray diffractometer) equipped with a Cu K�

adiation source (Scintag Instruments). Great care was taken tomploy the same procedure and approximately the same amountf material in all XRD analyses.

.4. Residence time calculation

Since only dilute solutions were employed in our experi-ents, the solvent density at the measured temperature and

ressure was assumed to be that of pure water and an averageesidence time τ determined as follows:

= V

v(ρ25 ◦C/ρT )

here ρ25 ◦C is the density of water at 25 ◦C, ρT the density ofater at the crystallizer temperature and pressure, V the volumef the crystallizer, and v is the volumetric flow rate.

. Solubility modeling

It has been reported [5,11,13] that species distribution playsn important role in determining the characteristics of metal

xides particles obtained in the CHP method. The synthesiseactions to obtain metal oxides generally involve hydrolysis ofetal salts followed by dehydration of the resulting hydroxides

6]. However, a large number of redox reactions are also possible

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3 21.7 ± 0.2 20 1:2

n this system, and hydroxide networks can be formed duringydrolysis [14]. We have modeled our system at the crystallizeremperature and pressure using OLI software (OLI Systems, NJ)o obtain equilibrium species distributions and hence elucidatehe role that species distribution may play in determining parti-le characteristics in our system. The OLI software employs theelgeson–Kirkham–Flowers–Tanger (HKFT) equation of state,

s well as the Bromley–Meissner and Pitzer activity coefficientodels to obtain species concentrations. Although the upper

imits of applicability of this software are 575 K and 150 MPa,e have shown previously that the results may be acceptableeyond those limits [11]. In the reaction of ferric nitrate with hotater, our calculations show that soluble iron species in solution

nclude Fe(OH)2, Fe(OH)3, Fe(OH)2+, Fe(OH)3

−, Fe(OH)4−,

e(OH)42−, Fe2+, Fe3+, Fe2(OH)2

4+, Fe(NO3)2+, Fe(OH)+ ande(OH)2+. However, only the amounts of Fe(OH)2

+, Fe2+, Fe3+,nd Fe(OH)2+ are significant at the conditions of our experi-ents. Moreover, the percentage of Fe2+ in the total soluble

ron species was over 90% at all feed concentrations. It is there-ore clear that morphology changes observed in our experimentsre not due to the presence of different species, since the speciesistributions were approximately the same in all experiments.

. Results and discussion

Nanoparticles were produced in all our experiments and X-ay diffraction analysis confirmed that the particles were mostly-Fe2O3. Average particle sizes and standard deviations inxperiments labeled S1–S11 are presented in Table 2 (corre-ponding to experimental conditions presented in Table 1).

.1. Effect of ferric nitrate concentration

The effect of changing the precursor (ferric nitrate) con-entration from 0.03 to 0.06 M (with all other variables keptonstant) is shown in Fig. 2, which shows TEM images of par-

icles obtained in experiments S1 and S2. In experiment S1,he precursor concentration was 0.03 M and small, sphericalarticles with an average particle radius of 15.6 ± 4.0 nm werebtained. A few larger rhombic particles were also obtained.

88 C. Xu, A.S. Teja / J. of Supercritic

Table 2Characteristics of particles obtained in experiments

Experiment no. Residencetime (s)

Average diameter,d (nm)

Standarddeviation,a σ (nm)

S1 2.6 15.6 4.0S2 2.6 27.4 7.0S3 5.2 20.6 4.7S4 10.4 22.2 5.9S5 2.6 7.7b 1.6S6 2.6 10.1b 2.2S7 2.6 7.2b 1.5S8 10.4 22.4 5.0S9 10.4 22.9 4.4S10 10.4 22.2 4.2S11 10.4 22.1 3.7

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σ = (1/N)i=1(di − d) .

b Actual average particle sizes may be smaller because of the large number ofltra-fine particles produced in these experiments.

n experiment S2, the ferric nitrate concentration increased to.06 M, and average particle size increased to 27.4 ± 7.0 nm.owever, the particles were now mostly rhombic, and thereere few, if any, smaller spherical particles. As noted above,

ig. 2. TEM images of iron oxide nanoparticles obtained in (a) experiment S1100,000×) and (b) experiment S2 (140,000×).

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al Fluids 44 (2008) 85–91

quilibrium species distributions did not change much at theseonditions, and therefore changes in the morphology cannot bettributed to speciation changes in our experiments.

.2. Effect of residence time

It is possible to change the residence time in our experimentsy changing the length of the crystallizer and keeping other vari-bles unchanged. The crystallizer length was therefore variedrom 5 cm (in experiment S1) to 10 cm (in experiment S3) and0 cm (in experiment S4) with corresponding residence timesn the crystallizer of 2.6, 5.2, and 10.4 s, respectively. Spheri-al crystalline particles were produced in all these experiments,nd average particle sizes increased with residence time (seeable 2). The average particle size was 15.6 nm in experiment1 (residence time 2.6 s), 20.6 nm in experiment S3 (residence

ime 5.2 s), and 22.2 nm in experiment S4 (residence time 10.4 s).he same trends were observed when PVA was present in therecursor solution (experiments S7 and S11). Again, the particleize increased with the residence time. However, peak intensitiesn XRD patterns also increased with residence time, suggestinghat crystallinity of the particles increased with residence timeFig. 3).

.3. Effect of temperature

The effect of temperature was investigated at a feed concen-ration of 0.03 M in the presence of PVA. Average particle sizesncreased from 7.7 to 10.1 nm when the temperature increased

rom 487 K (experiment S5) to 573 K (experiment S6) as shownn Table 2. However, the particle size remained approximatelyonstant at temperatures greater than 573 K at the highest res-dence time studied (10.2 s in experiments S8 and S10). Small

ig. 3. XRD patterns of iron oxide nanoparticles obtained in experiments S7nd S11.

rcritical Fluids 44 (2008) 85–91 89

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C. Xu, A.S. Teja / J. of Supe

ifferences in the width of the size distribution were observed athe higher temperatures (experiments S8 and S10), but this wasrobably due to PVA decomposition at higher temperatures. Theigh reaction rates associated with high temperatures generallyead to high supersaturations of inorganic species because ofheir low solubility in supercritical water. This would result in

any small particles being obtained in the experiments. How-ver, the diffusivity of the solute is also higher at supercriticalonditions, resulting in increased growth rates. Relative ratesf these competing phenomena determine the eventual parti-le size obtained in the CHP method. The temperature alsoffects the residence time, with increasing residence time favor-ng particle growth by diffusion and aggregation. Since averagearticle size in our experiments did not change when the resi-ence time changed from 5.2 to 10.4 s, this suggests that particlerowth reaches a constant value after about 5 s. Any additionalncrease in size with residence time must therefore result fromggregation.

.4. Effect of PVA concentration

The ratio of Fe3+ to [vinyl alcohol] in the precursor solu-ion was adjusted from 2:1, to 1:1 and 1:2 when the residenceime was 2.6 s (experiments S6 and S7) or 10.4 s (experiments9, S10 and S11), with all other variables kept the same. When

he residence time was 2.6 s, the particle size decreased withn increase of the PVA concentration as shown in Table 2. Theverage particle size decreased from 15.6 nm in experiment S1o 10.1 nm in experiment S6 and 7.2 nm in experiment S7 ashe ratio of the moles of the vinyl alcohol monomer unit toFe3+) increased from 0:1 in experiment S1 to 1:1 in experiment6 and 2:1 in experiment S7. However, the average particleize (∼22 nm) was nearly independent of PVA concentrationn the range of PVA concentrations studied in our work whenhe residence time was 10.4 s. Note, however, that the size dis-ribution (standard deviation) decreased with increasing PVAoncentrations at both residence times. This is shown in Fig. 4

or particles obtained at a residence time of 10.4 s. Representa-ive TEM images of particles obtained in experiments S9 and11 are shown in Figs. 5 and 6. The particles are mostly spher-

cal, and more aggregated at lower PVA concentrations when

owfa

Fig. 5. (a) TEM image of nanoparticles obtained in experiment S9 (

Fig. 4. Particle size distributions in experiments S4, S9, and S11.

Fe3+]:[VA] = 2:1 (experiment S9). Few aggregates formed atigher PVA concentrations when [Fe3+]:[VA] = 1:2 (experiment11). An examination of Fig. 6(a) shows that nearly all particlesbtained in experiment S11 were surrounded by a PVA layer.ote that in Fig. 6(a), the dark colored regions represent ironxide particles, and the light color region surrounding each par-icle represents the PVA coating. This coating is approximately0 nm thick and helps to “protect” the particle from growing tooarge. The crystalline nature of the particles can be confirmedrom electron diffraction patterns shown in Figs. 5(b) and 6(b).

A possible mechanism for nanoparticle formation in the pres-nce of PVA is depicted schematically in Fig. 7. Upon additionf ferric nitrate to a PVA solution, a chelating reaction occursetween Fe3+ ions and hydroxyl groups in the polymer. This cane confirmed visually by color changes that occur when ferricitrate is added to water and to an aqueous solution of PVA, event ambient conditions. In the absence of PVA, ferric nitrate isxidized by ambient O2 and the solution becomes turbid afterperiod of time. However, no color change is observed when

erric nitrate is added to a PVA solution, indicating that Fe3+

ations are sterically entrapped in the entangled network of the

rganic polymer [15]. When such a network comes into contactith supercritical water, very small particles of �-Fe2O3 are

ormed inside the network. The coexistence of PVA moleculest the moment of precipitation hinders aggregation of the newly

140,000×) and (b) corresponding electron diffraction pattern.

90 C. Xu, A.S. Teja / J. of Supercritical Fluids 44 (2008) 85–91

Fig. 6. (a) TEM image of nanoparticles obtained in experiment S11 (140,000×) and (b) corresponding electron diffraction pattern.

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reated particles. As a result, fine particles of relatively uniformize are produced. At short residence times (2.6 s), the coatedarticles are not able to grow or undergo secondary changes.his is especially true at the higher PVA concentrations becauseore PVA chains are present to hinder growth. Particle growth

eaches a limiting value at higher residence times (e.g. 10.4 s)nd average particle size becomes nearly independent of the PVAoncentration when the residence time is 10.4 s or higher. Notehat the PVA concentration must be sufficient to obtain a uniformistribution of cations, and hence isolate the particles by a poly-er layer [16]. Weak hydrogen bonding in the hydrated PVA

lso plays a role in promoting homogeneous physical entrap-ent between the (–OH) hydroxyl groups and cations which are

olvated by water molecules [15], thereby hindering aggregationnd resulting in a relatively uniform particle size distribution.

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noparticle formation in PVA.

. Conclusions

We have synthesized iron oxide and PVA-coated iron oxideanoparticles by a continuous hydrothermal method, and shownhat the presence of PVA during particle formation is an effectiveay to obtain narrow particle size distributions. The presence ofVA during synthesis hinders particle aggregation, and therebyesults in uniform particles and narrow particle size distribu-ions. In addition, our results show that the average particle sizeecreases with increasing PVA concentration when the residenceime is of the order of 2 s, and becomes nearly independent of

VA concentration when the residence time is 10 s or higher.

Particle size also increases with temperature and residenceime, and is sometimes accompanied by a change in morphologyf the particles. Both modeling and experimental studies indicate

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C. Xu, A.S. Teja / J. of Supe

hat equilibrium species distribution has no effect on particleorphology. It is likely, therefore, that the change in morphology

s due to kinetic phenomena.

cknowledgements

Partial financial support for this work was provided by theeorgia FOODPAC program. We thank Dr. Angus Wilkinson

or help with X-ray diffraction analysis, and the Georgia Techlectron Microscopy Center for providing the facilities for TEMnalysis.

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[

[

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[9] R.V. Kumar, Y. Koltypin, Y.S. Cohen, Y. Cohen, D. Aurback, O. Palchik,I. Felner, A. Gedanken, Preparation of amorphous magnetic nanoparticlesembedded in polyvinyl alcohol using ultrasonic radiation, J. Mater. Chem.10 (2000) 1125–1129.

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