J. of Supercritical Fluids 37 (2006) 1–22

Review

Nanomaterials and supercritical fluids

E. Reverchona,∗, R. Adamia,b

a Dipartimento di Ingegneria Chimica e Alimentare, Universita di Salerno, Via Ponte Don Melillo 1, 84084 Fisciano (SA), Italyb Department of Biochemistry and Food Chemistry, University of Turku, FIN-20014 Turku, Finland

Received 25 February 2005; received in revised form 22 July 2005; accepted 25 August 2005

Abstract

The interest in the preparation and application of nanometer size materials is increasing since they can exhibit properties of great industrialinterest. Several techniques have been proposed to produce nanomaterials using supercritical fluids. These processes, taking advantage of the specificproperties of supercritical fluids, are generally flexible, more simplified and with a reduced enviromental impact. The result is that nanomaterialswith potentially better performances have been obtained.

We propose a critical review of the supercritical based techniques applied to the production of nanoparticles, nanofibers, nanowires, nanotubes,nanofilms and nanostructured materials. The most relevant characteristics of each process and the kind of nanomaterial that can be produced arehighlighted.

here

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66

678

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12131515

161718

© 2005 Elsevier B.V. All rights reserved.

Keywords: Supercritical fluids; Nanoparticles; Nanomaterials; Nanocomposites; Nanofibers; Nanowires; Nanotubes; Nanofilms; Reverse micelles; Nanosps andnanocapsules

Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2. Nanoparticles generation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.1. RESS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.2. SAS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.3. SAA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.4. Sol/gel drying. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.5. Synthesis in SCFs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.5.1. Hydrothermal synthesis in supercritical water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.5.2. Reduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.5.3. Hydrolysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.5.4. Thermal decomposition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.5.5. Dispersion polymerisation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.5.6. Reverse micelles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3. Other nanomaterials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.1. Nanofibers, nanowires and nanotubes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.2. Nanofilms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4. Nanocomposite materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.1. Composite nanoparticles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.2. Composite nanowires and nanotubes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5. Conclusions and perspectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗

Corresponding author. Fax: +39 089964057.E-mail addresses: ereverchon@unisa.it (E. Reverchon), radami@unisa.it (R. Adami).

0896-8446/$ – see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.supflu.2005.08.003

2 E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22

1. Introduction

There is a great interest in the preparation and application ofnanometer size materials since they can exhibit new propertiesof industrial interest. Which are the matter properties that canshow dramatic changes at nanoscale range? Mainly the proper-ties related to the ratio between surface and volume: at nanoscale,surface properties become relevant with respect to volume prop-erties. For example, surface molecules can impart high hardnessto metals and higher energy to propellants and explosives; elec-tronic devices and pharmaceuticals with improved performancecan also be produced.

Different interpretations of the dimensions that set the bound-ary between normal materials and nanomaterials have beenproposed. In this work, we assume that a nanoproduct shouldhave at least one dimension smaller than 200 nm; though, morerestrictive definitions have been proposed that set the upper limitat 100 nm. Nanoparticles, nanofilms and nanowires are nanomet-ric along three, two and one dimension, respectively. In the caseof nanostructured materials, at least one of the components hasnanometric dimensions.

The various processes that have been proposed to obtainnanomaterials follow two main approaches: top down andbottom up. Top-down is characterized by the production ofnanoproducts departing from normal size materials; i.e., reduc-ing the dimensions of the original material; for example, usings hi fromt r-t

edt riticfl ivi-t thep ess.P canb s.

ox-i ticalp ratuH havb uctioa

maia partc wid

range of applications; it will be possible to produce explosiveswith a higher potential; i.e., approaching the ideal detonation;coloring matter with brighter colors; toners with a higher reso-lution; polymers and biopolymers with improved functional andstructural properties. Moreover, pharmaceutical products can bedesigned that have enhanced pharmaceutical activity or that usedifferent delivery routes and/or overcome human body internalbarriers. Metals, metal oxides and ceramic compounds at nan-odimensions can exhibit unusual strength and/or can be usedas fillers in nanostructured materials. Composite nanospheresand nanocapsules can be used, for example, in pharmaceuticalapplications for controlled and sustained release of drugs. Theproduction of nanowires, nanofilms and nanotubes has also beenconsidered in supercritical fluid assisted processes.

Nanostructured polymers can be generated in form ofnanocellular foams and membranes. For example, nanocom-posite polymers can be obtained modificating the host polymerproperties using nanofillers (nanoparticles, nanoclays). How-ever, nanostructured polymers will not be treated in this worksince they have been the subject of a recent excellent review[1].

Several supercritical based techniques have been proposedin the literature for the production of micro and nano materials,since several processes can operate in the micronic or in thenanometric domain depending on the operating conditions andon the process arrangement. For what concerns micrometric andsub-micrometric particles generation by supercritical assistedp

duc-t sses;b casesm fore,i dis-c sed;i sionsl lysisi ega-t terialt

2

ticlesg e rolep n pro-p ia.

nd b

pecial size reduction techniques (Fig. 1). Bottom-up approacs related to the “synthesis” of nanosized materials, startinghe molecular scale (Fig. 1); for example, the formation of paicles by precipitation from a fluid phase.

Supercritical fluids (SCFs) have also been proposed as mo produce nanomaterials. The properties that make supercuids particularly attractive, as a rule, are gas-like diffusies, the continuously tunable solvent power/selectivity andossibility of complete elimination at the end of the procarticularly, the mix of gas-like and liquid-like propertiese useful in many applications related to nanotechnologie

The most widely used supercritical fluid is carbon dide (CO2), that is cheap and non polluting, and whose criarameters are simple to be obtained in an industrial appaowever, ammonia, alcohols, light hydrocarbons and watereen proposed, among the others, for nanomaterials prodt supercritical conditions.

Among all the nanoproducts that can be envisaged, tworeas have been explored using supercritical fluids: nanoles and nanostructured materials. Nanoparticles cover a

Fig. 1. Top-down a

iaal

s.en

ni-e

rocesses, some good reviews are available in literature[2–6].A large number of papers in the literature claims the pro

ion of nanomaterials by supercritical fluids assisted proceut, the dimensions of materials described are in severalore properly in the range of sub-micronic products. There

n this work we performed a first selection of the papers to beussed on the basis of the “nano” definition previously propo.e., papers related to materials with characteristic dimenarger than 200 nm have not been considered. A critical anas proposed that highlights the most relevant positive and nive characteristic of each process and the kind of nanomahat can be produced.

. Nanoparticles generation

A possible general classification of SCF based nanopareneration techniques can be proposed according to thlayed by the SCF in the process. Indeed, SCFs have beeosed as solvents, solutes, anti-solvents and reaction med

ottom-up approaches.

E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22 3

2.1. RESS

The rapid expansion of supercritical solutions (RESS)consists of the saturation of the supercritical fluid with a solidsubstrate; then, the depressurization of the solution through aheated nozzle into a low pressure chamber produces a rapidnucleation of the substrate in form of very small particles thatare collected from the gaseous stream. The morphology ofthe resulting solid material, crystalline or amorphous, dependson the chemical structure of the material and on the RESSparameters (temperature, pressure drop, impact distance of thejet against a surface, nozzle geometry, etc.)[3]. The very fastrelease of the solute in the gaseous medium should assure theproduction of very small particles. This process is particularlyattractive due to the absence of organic solvents.

The authors that first proposed RESS[7,8] patented the pro-cess with respect to the possibility of producing nanoparticles;but, also microparticles and films formation were claimed (Smithet al.[9–11]). However, only several years later in the scientificliterature appeared papers that confirmed this possibility. Forexample, Turk et al.[12] used the RESS process to produce�-sitosterol (an anticholesteremic) nanoparticles of about 200 nmmean diameter. They tested the process in SC-CO2 at differentpre-expansion temperatures and pressures and observed that inthe case of�-sitosterol the variation of pre-expansion conditionsdoes not lead to appreciable variations in nanoparticles diame-t ofulv

nyl-p heri-c from6 estoe 2,5-d rtiesu ticald fromC wasa

apide ent( tioni le toq theR g thn n thl

-c ticals eatef siono lu-t ul-fi nmT( samp olutio nol

solution of Na2S, produced nanoparticles of PbS. The liq-uid solution also in this case contained PVP to stabilize PbSnanoparticles. Nanoparticles with mean diameters of about4 nm were produced[17]. The same research group[18] usedthe RESOLV process to produce ibuprofen (anti-inflammatory)nanoparticles with an average diameter of 40 nm and a parti-cle size distribution with a standard deviation (S.D.) of 8.5 nm.In the same paper, naproxen (anti-inflammatory, analgesic,antipyretic) nanoparticles with an average size of 64 nm anda standard deviation of 10 nm were also produced. In bothcases they used high molecular weight PVP as stabilizer inthe aqueous solution to reduce the agglomeration process andto control the average diameter of nanoparticles. They appliedthe RESOLV process also to polymers[19,20]. They processedpoly(heptadecafluorodecylacrylate) (PHDFDE) that is highlysoluble in SC-CO2, but insoluble in water. Therefore, theyexpanded the supercritical solution (CO2 + PHDFDE) directlyinto an aquesous NaCl solution. However, even in the presenceof NaCl the nanoparticles tended to agglomerate. The problemwas resolved adding a surfactant (sodium dodecyl sulfate) to abasic acqueous solution.

Kropf et al.[21] patented a RESS-like process in which thesupercritical solution is expanded into a gas or liquid thus pro-ducing nanoparticles that can range between 10 and 300 nm.Subsequently, Foerster et al.[22] and Kropf et al.[23] used thisprocess to patent the generation of nanometric particles of chi-t

theu daw di Thefi eanp -t wasa ith ab cen-t 10 nm(

en-s

thet rtic-u tic toc pan-s ratedi edle-l on ofo ectro-s e fastr ed int

moree thatr owth.T ctsp

ers. They also used this process for the production of grisein (an antibiotic) nanoparticles using supercritical CHF3.

Sane et al.[13] used RESS to produce fluorinated tetrapheorphyrin (a photosensitizer for photodynamic therapy) spal, agglomerated nanoparticles with average particle sizes0 to 80 nm, at different pre-expansion temperatures. Pt al. [14,15] used RESS to produce nanoparticles ofistyrylpyrazine (DSP), (a polymer that can change propepon exposure to light or chemicals, with application as opata storage and chemical sensor). They precipitated DSPHClF2 and an increase of photoreactivity of nanoparticleslso observed.

An interesting variation of the RESS process is the rxpansion of a supercritical solution into a liquid solvRESOLV) that consists of spraying the supercritical solunto a liquid. Operating in this manner, it should be possibuench particles growth in the precipitator, thus improvingESS process performance. Moreover, by interaction amonucleating solid particles and the compounds contained i

iquid phase, a chemical reaction step can also be added.For example, Sun and Rollins[16] proposed a RESOLV pro

ess in which the liquid receiving the spray of the supercriolution also contains a reactant for the solute that nuclrom the expanding jet. They performed the rapid expanf Cd(NO3)2 in SC-ammonia into a room-temperature so

ion of Na2S in water or ethanol, producing cadmium sde (CdS) nanoparticles with an average diameter of 3.3he Na2S solution also contained poly(N-vinyl-2-pyrrolidone)PVP) to stabilize the produced nanoparticles. Using therocess arrangement, they produced an homogeneous sf Pb(NO3)2 in SC-ammonia that, expanding in an etha

-

v

ee

s

.

eon

osan and sterols.A further modification of the RESOLV process consists of

se of a water in supercritical CO2 (w/c) microemulsion uses a modified supercritical solvent to dissolve AgNO3 [24]. A/c microemulsion containing AgNO3 was rapidly expande

nto a room temperature solution of sodium borohydride.nal product of the reaction were Ag nanoparticles with a marticle size of 7.8 nm. In a subsequent work[25] the reduc

ive solution at the receiving end of the rapid expansiondjusted to be highly basic. Nanocrystalline Ag particles wimodal distribution were obtained, with the smaller ones

ered around 3.1 nm (S.D. 0.8 nm) and larges ones aroundS.D. 2 nm).

The list of compounds processed down to nanometric dimions by RESS and RESOLV is reported inTable 1.

The potential features of RESS, are very interesting fromheoretical point of view; but, the results have not been palarly good in several cases. It is in many cases problemaontrol the particle size of the precipitates. During the exion, the particles coalesce in the supersonic free jet genen the precipitation vessel and, therefore, in many cases neike particles have been obtained. Sometimes, the formatiriented needles can be explained by the presence of eltatic charges on the surface of the particles, induced by thelative motion between the particles and the gas containhe expansion chamber[26].

RESOLV configuration has been demonstrated to beffective in producing nanoparticles, since the liquideceives the expanding jet, can suppress the particle grhe addition of a stabilizing agent in the liquid also protearticles from agglomeration.

4 E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22

Table 1Compounds produced in nanoparticles by RESS

Material Particle dimensions (nm) Note Reference

Range Mean size

CdS 3.3 RESOLV [16]PbS 2.5–4 RESOLV [17]Ag 7.8 RESOLV + w/c [24]Griseofulvin 200± 50 [12]�-Sitosterol 200± 50 [12]Fluorinated tetraphenylporphyrin 60–80 [13]DSP 100–300 [14]Ibuprofen 40 RESOLV [18]Naproxen 64 RESOLV [18]PHDFDE 41 RESOLV [19,20]Ag 3.1; 10 RESOLV [25]

The major limitation of RESS and RESOLV processes is thatthey are applicable only to products that show a reasonable sol-ubility in the selected supercritical fluid. Unfortunately, manysolid compounds with high molecular weight and polar bonds,that could be candidate to nanoparticles generation, show a verylow or negligible solubility in SC-CO2, that is the most widelySCF used and show a reduced solubility in many other com-pounds that could be good candidates to act as SCF.

RESOLV has also the problem of the recovery of particlesfrom the liquid solution used to improve the process perfor-mance: in this configuration, the process is no more solventless.

2.2. SAS

Supercritical anti-solvent precipitation (SAS) has been pro-posed using various acronyms; but, the process is substantiallythe same in all the cases. A liquid solution contains the solute tobe micronized; at the process conditions, the supercritical fluidshould be completely miscible with the liquid solvent; whereas,the solute should be insoluble in the SCF. Therefore, contact-ing the liquid solution with the SCF induces the formation ofa solution, producing supersaturation and precipitation of thesolute. The formation of the liquid mixture is very fast due to theenhanced mass transfer rates that characterize supercritical fluidsand, as a result, nanoparticles could be produced. This processh rangm ed tt mod[ ita-t tiona finap ASt con-t t orc thel top letst id ant bep ev the

liquid and the SCF also play a relevant role in SAS. Particu-larly, VLEs of the ternary system solute-solvent-SC antisolventand the position of the operating point in SAS processing withrespect to these VLEs, can be decisive for the success of the pro-cess. The formation of a single supercritical phase is the key stepfor the successful production of nanoparticles[31]. The washingstep with pure supercritical antisolvent at the end of the precip-itation process is also fundamental to avoid the condensation ofthe liquid phase that otherwise rains on the precipitate modify-ing its characteristics. As a rule SC-CO2 has been used in thisprocess. A specific indication will be given in this chapter whena different SC-antisolvent has been used.

Yttrium, samarium and neodymium acetates, that are precur-sors of high temperature superconductors, have been micronizedusing dimethyl sulfoxide (DMSO) andN-methyl 2-pyrrolidone(NMP) as liquid solvents. Nanoparticles down of a mean diam-eter of about 100 nm have been obtained[32,33]. A high resolu-tion scanning electron microscope (HR-SEM) image of samar-ium acetate precipitated by SAS at 150 bar, 40◦C is reported inFig. 2as an example.

In Fig. 3 the particle size distribution of Samarium acetatenanoparticles is also reported. All particles are smaller than200 nm with a mean diameter of about 120 nm[32,34].

F MSO,1

as been used by several authors using different process arents; however, the most significant differences are relat

he way the process operates: in batch or semi-continuous2]. In batch operation (GAS: Gas AntiSolvent) the precipion vessel is loaded with a given quantity of the liquid solund, then, the supercritical antisolvent is added until theressure is obtained. In the semi-continuous operation (S

he liquid solution and the supercritical anti-solvent areinuously delivered to the precipitation vessel in co-currenounter-current mode. An important role is also played byiquid solution injection device[27]. The injector is designedroduce liquid jet break-up and the formation of small drop

o produce a large mass transfer surface between the liquhe gaseous phase. Several injector configurations haveroposed in the literature and patented[28–30]. High pressurapor-liquid equilibria (VLEs) and mass transfer between

e-oe

l),

den

ig. 2. Samarium acetate nanoparticles precipitated by SAS from D0 mg/mL, 150 bar, 40◦C.

E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22 5

Fig. 3. Particle size distribution of samarium acetate nanoparticles obtained bySAS at conditions indicated inFig. 2.

Nanoparticles of zinc acetate, a catalyst precursor, have alsobeen produced by SAS. A particle size down to 30 nm witha mean diameter of 50 nm was obtained at the best operatingconditions[35].

Some pigments have been produced in nanoparticles by SAS:Wu et al. [36,37] micronized pigment Red 177 by precipita-tion from DMSO and analysed the influence of several processparameters on particle size. Spherical nanoparticles down to46 nm mean diameter were obtained. Disperse Red 60[38,39]has also been precipitated from DMSO by SAS. In this case,solubility of the pigment in the supercritical solution was notnegligible; therefore, the yield of the precipitation process waslow due to the partitioning of Red 60 in the fluid phase.

Chattopadhyay and Gupta[40] precipitated fullerene (C60)nanoparticles from a toluene solution. The experiments wereperformed operating in a SAS batch mode (injection in staticSCF) and fullerene particles as small as 29–63 nm were obtained(with a standard deviation of 7–25 nm operating at various con-ditions).

Tetracycline, an antibiotic, is an example of pharmaceuticalcompound successfully processed by SAS. It has been obtainedusing NMP as solvent. The mean particle size of precipitatedparticles was about 150 nm[41]. Chattopadhyay et al.[42,43]proposed a batch supercritical antisolvent (GAS) micronizationprocess enhanced with the addition of a vibrating surface inthe precipitation vessel; the technique was named supercriti-c heyp lowa nmN eterop

rti-c ineda heref d bed webs proa f the

Fig. 4. SAS precipitated dextran nanoparticles, from DMSO, 10 mg/mL,150 bar, 40◦C (pilot plant).

nanometric particles. In some cases these structure collapse bysonication.

Nanoparticles of some polymers have also been produced.Dextran (a bio-polymer) has been processed using DMSO. Theparticles produced have a spherical morphology and a mean par-ticle size ranging between 125 and 150 nm[46]. Pilot scale SASexperiments confirmed these results[31,47]and the possibilityto scale-up SAS process.Fig. 4 reports a SEM image of dex-tran nanoparticles produced using the pilot scale SAS apparatuslocated at the University of Salerno (Italy).

Another example of successful production of nanoparticlesusing SAS is the formation of Polylactic acid (PLLA) particles[48]. These authors performed the semicontinuous antisolventprocess injecting the solvent in a jet-swirl nozzle designed toenhance the mixing within a swirl chamber.

Some explosives have also been successfully processed[34].Good results have been obtained in the case of 3-nitro-1,2,4-triazol-5-one (NTO) using methanol as the liquid solvent.Nanoparticles with a mean particle size of 120 nm with a stan-dard deviation of about 30 nm have been obtained.

The list of compounds processed by SAS down to nanometricdimensions is reported inTable 2.

When the injection of the liquid solution is properly per-formed, the limits of the SAS process are in the difficulty ofpredicting VLE modifications induced by the presence of soluteon the binary liquid-SCF system. Very complex phase-behaviorsc xturec rved.I largep andt

ases[ ased cibil-i erentm recip-i ase( s are

al antisolvent with enhanced mass transfer (SAS-EM). Troduced griseofulvin (antifungal, antibiotic) particles ass 130 nm and lysozyme (enzyme) particles of about 190anometric lysozyme particles with a minimum mean diamf 180 nm were also produced by Muhrer et al.[44] using GASrocess.

Snavely et al.[45] produced insulin (antidiabetic) nanopales by SAS with the aid of an ultrasonic nozzle. They obtapowder consisting of physical aggregates of 50 nm sp

orming sponge-like and cob-web-like structures that couleagglomerated in smaller units. The formation of cob-tructures has also been reported by other authors and isbly due to the collision and the random coalescence o

.

s

b-

an be produced. In the simplest case the shift of the miritical point (MCP) towards higher pressures can be obsen the case of a large increase of the MCP pressure, veryressures will be required to obtain a single phase system

he successful production of nanoparticles[49].The formation of very large crystals observed in some c

2] is connected to SAS precipitation from a liquid-rich phue to a modification of the shape and the extent of the mis

ty gap. When two phases are simultaneously present, difforphologies can be observed that can be related to the p

tation from a liquid rich phase (crystals) and a SCF-rich phamorphous particles); the relative quantities of precipitate

6 E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22

Table 2Compounds produced in nanoparticles by SAS

Material Particle dimensions (nm) Note Reference

Range Mean size

AcY 50–150 100 [32]AcSm 80–120 ∼100 [32]AcZn <30 50 [35]Tetracycline 150 [41]HPMA 150 [46]Dextran 125–150 [46,31]Fullerene 29–83 SAS batch [40]Griseofulvin 130 SAS-EM (GAS) [42]Lysozyme 190 SAS-EM (GAS) [43]Lysozyme 180 GAS [44]Insulin 50 [45]PLLA 193± 20 [47]Pigment Red 177 49–74 [37]Disperse Red 60 50–100 [39]

correlated to the partition factor of the solute between the twophases.

2.3. SAA

Supercritical assisted atomization (SAA) is a recent process[50,51] in which the SCF acts as atomizing medium. The pro-cess is based on the solubilization of supercritical CO2 in theliquid solution formed by the solvent and the (solid) solute,and on its subsequent atomization using a thin wall nozzle.Using SAA, PMMA (polymethylmethacrylate) nanoparticleshave been obtained using acetone as solvent. At the concen-tration of 10 mg/mL of PMMA in acetone and at a mixingtemperature of 80◦C and a mixing pressure of 76 bar, particleswith a mean diameter of 120 nm have been produced.

When SAA is properly conducted, two atomization processestake place: the first one is the production of primary droplets atthe exit of the nozzle by pneumatic atomization; the second ondestroys these droplets by the fast release of CO2 from the inter-nal of the droplet (decompressive atomization). Amorphous ocrystalline particles have been produced, depending on the process temperatures and the chemical characteristics of the solsolute[52].

The limit of this process is that the smallest particles produceddepend on the dimensions of the smallest droplets generated (od ectet duri SCFd

2

ce ita idingt covet ep watei d,

Table 3Compounds produced in nanoparticles by SC-drying

Material Particle dimensions(nm)

Note Reference

Cu3B2O6 10–20 SC-CO2 [53]TiO2 10–20 SC-CO2 [54]Be2BO3(OH) 7 SC-CO2 [55]Mg2B2O5–MgB6O10

5H2O–MgB2

10 SC-EtOH [56]

then, the elimination of the organic solution using SC-CO2. Inthis case the process was finalized to the production of copperborate nanoparticles. Hu et al.[54] proposed the formation ofTiO2 nanoparticles by sol preparation and the replacement ofwater in the precipitate with n-butanol and subsequent super-critical drying. Particle sizes ranging between 10 and 20 nmand surface areas up to 166.8 m2/g were obtained. The sameresearch group[55] also performed the supercritical drying of aberyllium borate aerogel to produce nanoparticles. In this casethe replacement of water in the precipitate was obtained with amixture of n-butanol and ligroin; then, supercritical drying wasobtained using CO2. In a subsequent work they[56] used super-critical ethanol as the drying medium to recover magnesiumborate nanoparticles.

A sort of SC-drying process used to produce nanometric par-ticles has also been patented[57]. In this case a liquid colloidalsuspension is first formed, then it is introduced into a supercrit-ical fluid that extracts the liquid solvent.

2.5. Synthesis in SCFs

Powder synthesis in gas phase can be carried out by reactionof precursor gases. Powders produced by the gas to particlesroute can have a sharp particle size distribution and are formed bynon-porous primary particles[58]. In analogy with the classicalm ion inw

2hetic

mr ngesw rox-i

tionm ther tanta of thee ticles.T

etals d. Thep treama tinga tor, iti les.

roplet-one particle process). These dimensions are conno the classical parameters that control droplet dimensionsng atomization: surface tension, viscosity and quantity ofissolved in the liquid.

.4. Sol/gel drying

The use of SCFs is well established in gels drying sinllows the drying process with zero surface tension, avo

he gel collapse. In this process, the SCF is only used to rehe produced nanoparticles (Table 3). Hu et al[53] proposed threparation of gels in aqueous solution, the replacement of

n the precipitate with a mixture ofn-propanol and benzene an

e

r-id

ned-

r

r

ethods, nanoparticles can also be the product of a reacthich a SCF is used as the reaction medium.

.5.1. Hydrothermal synthesis in supercritical waterHydrothermal synthesis (HTS) is used to produce synt

aterials imitating natural geothermal processes[59,60]. Theeaction equilibrium of metal salt aqueous solutions chaith temperature and results in the formation of metal hyd

des or metal oxides.Supercritical water (SCW) provides an excellent reac

edium for hydrothermal synthesis, since it allows to varyeaction rate and equilibrium by shifting the dielectric consnd solvent density with pressure and temperature. Onexpected benefits are higher reaction rates and smaller parhe reaction products have to be not soluble in SCW.

The HTS-SCW process is usually operated as follows: a malt aqueous solution is prepared, pressurized and heateressurized metal salt solution and a supercritical water sre combined in a mixing point, which leads to rapid heand subsequent reaction. After the solution leaves the reac

s rapidly quenched and in-line filters remove larger partic

E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22 7

Fig. 5. A schematic representation of the HTS-SCW process.

Cooling water is directly fed to the reactor to quench the reaction(Fig. 5). Two different process mode have been proposed: thefirst uses a batch reactor and is characterized by a long reactiontime; the second uses a flow reactor that assures continuousoperation.

HTS-SCW process has been extensively explored by Adshiriet al. [61–64], Hakuta et al.[65–68], Li et al. [69] in variousworks to produce nanometric particles of several single andcomplex metal oxides (seeTable 4). This research group usedseveral types of flow apparatus to perform HTS-SCW exper-iments. They evidenced several parameters of the process thatcan control dimension and morphology of the produced nanopar-ticles. First of all, solubility of metallic oxides in SCW has to beconsidered and its dependence on temperature and pH. Also theinitial concentration of the feed and the heating rate can play arelevant role in this process.

Cabanas et al.[70,71] used a small continuous reactor toproduce Ce1−xZnxO2 by HTS-SCW starting from mixtures ofcerium ammonium nitrate and zirconium acetate. According tothese authors, the continuous reactor allows a better control ofthe experimental conditions when compared to the batch pro-cess. Very small nanoparticles have been obtained (seeTable 4).After calcination at 1000◦C for 1 h the material sintered up to145 nm. In a subsequent work[72] they used continuous HTS-SCW to obtain nanoparticles of Fe3O4 and various Fe–Ni–Znmixtures. In all cases, crystalline particles with mean diametersl fp

u que:c ater( work

Table 4Compounds produced in nanoparticles by hydrothermal synthesis in SCW

Product Particle dimensions (nm) Reference

Range Mean size

AlOOH 80–1000 [62,66]�-Fe2O3 50 [61]Fe3O4 50 [61]Co3O4 100 [61]NiO 200 [61]ZrO2 (cubic) 10 [61]TiO2 10–1000 20 [61]TiO2 (anatase) 20 [61]CeO2 20–300 18 [65]BaO–6Fe2O3 50–1000 ∼100 [65]Al5(Y + Tb)3O12 20–600 [66]LiCoO2 40–400 [63]ZrO 10–1000 [63]AlO(OH) 20–200 [63]Ce1−xZrxO2 3–5, 4–7 [70]CeO2 20–200 [64]Fe3O4 + Fe 40–92 [72]CoFe2O4 39–72 [72]NiFe2O4 28–43 [72]NixCo1−xFe2O4 23–42 [72]ZnFe2O4 47–105 [72]AlOOH 13–16 [69]�-Fe2O3 30–60 30–40 [73]

25–170 90 [73]Co3O4 30–60 30–40 [73]

20–30 [73]CoFe2O4 20 [74]ZnO 120–320 [75]

39–251YAG:Tb 14–152 [67]ZrO2 3–5 [76,77]TiO2 7–9 [76,77]1% Pd/ZrO2 3–5 [77]1% Pd/TiO2 7–9 [77]

they also obtained CoFe2O4 nanocrystals using the same processvariations[74].

Viswanathan and Gupta[75] obtained ZnO nanoparticles ina continuous tubular reactor starting from zinc acetate and usingvarious flow rates and feed concentrations. Spherical nanopar-ticles with a mean size down to 39 nm were identified by laserscattering and down to 120 nm by TEM analysis.

HTS-SCW can produce very small nanometric particles;however, this process requires challenging operative conditionssince supercritical point for water is located at 374◦C and220 bar. To operate in SCW, stainless steel with special charac-teristics is required due to the simultaneous application of highpressures and high temperatures. Moreover, SCW is a strongoxidizing agent not only for processed materials, but also forthe elements of the plant which it is in contact with. HTS-SCWcan be used only for compounds that are stable at high temper-atures.

2.5.2. ReductionShah et al.[78], demonstrated that it is possible to stabilize

Ag nanocrystals in SC-CO2 using opportune surfactants (alka-nethiols). The process, called by the authors arrested growth,

ower than 100 nm were obtained (seeTable 4for the details oroduced oxides).

Cote et al.[73] produced nanocrystals of�-Fe2O3 and Co3O4sing two variations of the continuous hydrothermal techniold mixing and hot mixing of the reactants. Compressed wsub-critical) was used in the experiments. In a subsequent

8 E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22

Table 5Compounds produced in nanoparticles by SC assisted reactions

Material Reaction Particle dimensions (nm) Reference

Range Mean size

Ag Reduction 3–12 [82]Pd Reduction 3–6 [82]Pd Reduction 5–15 [83]Ag Reduction 3 [83]Ag Reduction 2–10 [81]Ag, In, Pt Arrested growth/reduction 2–12 [79]Ag Arrested growth/reduction 2–4 [80]Cu + Cu2O Hydrolysis 10–35 [84]Ti(OH)4 Hydrolysis 70–110 [85]Ti(OH)4 Hydrolysis 20–800 [86]Si Thermal decomposition 1.5–4 [89]Si Thermal decomposition 2–10 [81]FeN4–Fe2O3 mixture Thermal decomposition 50 [87]Aromatic polycarbonate Dispersion polymerization 30–140 [93]

consists of the reduction with H2 in a batch reactor of SC-CO2soluble organo-metallic precursors in the presence of a stabiliz-ing perfluoro-octanediol ligand that binds to the surface of metalnanoparticles and arrests particle growth. The key characteristicsof this process are: (a) precursors soluble in SC-CO2 (b) polarproducts not soluble in SC-CO2. Using the same process[79],they synthesized stabilized nanocrystals of Ag, In and Pt withdiameters ranging between about 2 and 12 nm. Moreover, ana-lyzing the arrested precipitation of Ag nanocrystals in SC-CO2,they studied the influence of the process parameters on particlesdiameter and polydispersity[80], concluding that CO2 densityis the major parameter affecting particle size and distribution inthis process. At higher solvent densities they obtained crystalsof about 2 nm in diameter due to the strong barrier formed by thesurfactant; whereas, at lower CO2 densities, larger Ag crystals ofabout 4 nm were obtained with higher polydispersity, since par-ticles grew to a larger size before the coverage of surfactant wassufficient to prevent their further coagulation. Precursor concen-tration, thiol/precursor ratio and reaction time do not appreciablyaffect the crystals size, though they can affect their polydisper-sity. Korgel et al.[81] synthesized by arrested precipitation Agnanoparticles in SC-CO2 by reduction of silver acetilacetonatein presence of organic ligands that acted as stabilizers of thenanoparticles. Using the same technique silicon nanocrystalsranging between 2 and 20 nm were also synthesyzed in SC-hexane.

byr te ins IN)w ot e ine actow wee3 anP

ndA lp g

agent and a stabilizer used to limit the growth of nanoparticles.Ag particles with a mean diameter of 3 nm and a distributionranging between 1.5 and 9 nm were obtained. Results obtainedusing the reduction process are grouped inTable 5.

2.5.3. HydrolysisZiegler et al.[84] synthesized copper oxide (Cu2O) nanopar-

ticles from copper nitrate by hydrolysis in SCW. They performedthe reaction with and without ligands. Cu2O polydisperse par-ticles with diameters ranging from 10 to 35 nm were obtainedby hydrolysis when they did not use alkanethiol ligands; when1-hexanethiol was added in the reactor, Cu nanocrystals of about7 nm in diameter were obtained. The alkanethiol ligand stabi-lized the synthesis of nanocrystals and controlled their oxidationby reduction to Cu nanoparticles. Ligands that bind on thenanoparticles surface can block the growth of nanoparticles,with a stabilization process analogous to the arrested growthdiscussed in the previous chapter[79]. The authors also stud-ied different precursors, obtaining particles with different mor-phologies.

Titanium hydroxide nanoparticles were produced by hydrol-ysis of titanium tetra-isopropoxide (TTIP) in SC-CO2 using acontinuous large scale plant (5 dm3 reactor volume) schemat-ically reported inFig. 6, operated at mild pressure conditions(80–140 bar) and in the temperature range between 40 and 60◦C[85]. Two solutions of TTIP and water in SC-COwere pre-p IPa tionswp e-t rfaceaw

isb su somee alp een

Kameo et al.[82] produced Ag and Pd nanoparticleseduction of silver acetylacetonate and palladium acetaupercritical CO2. The precursors and a surfactant (FOMBLere loaded into small fixed volume vessel; CO2 was added t

he reactor, forming a solution. Then, dimethylamine boranthanol solution was injected into the solution and the reas stirred. Nanoparticles with mean diameters ranging betand 12 nm and between 3 and 6 nm were obtained for Agd, respectively.McLeod et al.[83] produced metallic nanoparticles of Pd a

g, spraying a SC-CO2 solution containing CO2-soluble metarecursors into a second CO2 solution containing a reducin

rnd

2ared by flowing SC-CO2 in two contactors, containing TTnd water, respectively. Then, the two supercritical soluere injected into the reactor and by hydrolysis Ti(OH)4 wasroduced. Ti(OH)4 spherical nanoparticles with mean diam

ers ranging between 70 and 110 nm were produced, with sureas larger than 300 m2/g. Calcination of the product to TiO2as also performed.Stallings and Lamb[86] also performed TTIP hydrolys

ut they injected pure TTIP into water in SC-CO2 dispersionsing a batch reactor. A surfactant was also added inxperiments to stabilize the water-SC-CO2 dispersion. Sphericarticles with a broad particle size distribution ranging betw

E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22 9

Fig. 6. Schematic representation of the Pilot Plant used for the TTIP hydrolysis in SC-CO2. V1 = CO2 storage vessel; D = CO2 dryer; V2 = CO2-TTIP contactor;V3 = CO2–H2O contactor; R = reaction vessel; LS = liquid separator[85].

20 and 800 nm were obtained, independently from the use ofthe surfactant. According to the authors, they were formed byagglomerates of primary particles of about 20 nm in diameter.The results of thermal decomposition process in SCFs are alsosummarized inTable 5.

2.5.4. Thermal decompositionThe thermal decomposition of a precursor is a classical pro-

cess used to induce high supersaturations in a fluid phase whichleads to the nucleation of nanoparticles. A classical thermaldecomposition process is the spray pyrolisis. The precursormaterial is atomized and carried by a gas into a high temper-ature zone, the solvent evaporates from the droplets and theporous dried particles obtained sinter to form dense particles.The use of the same process in a supercritical medium can lead tohigher nucleation rates because of the very high supersaturationof metal atoms. Nucleation generates metal clusters composedby few atoms. These clusters grow by binary contact and coa-lescence to give nanoparticles. Depending on the main processparameters (pressure, temperature, residence time, metal pre-cursor concentration), these nanoparticles can grow to largercrystalline particles or aggregates to produce nanostructured par-ticles.

Cansell et al.[87] proposed the thermal degradation of metal-lic precursors dissolved in a supercritical fluid (ammonia). Theyused as precursors Cu and Fe acetylacetonates that show ag per-a thep s ann plei subu -q ducn , Co

Cu, Ni, Al, Ti and Ga using supercritical ammonia-methanolmixtures.

Holmes et al.[89] prepared organic-passivated Si nanocrys-tals by thermally degrading diphenylsilane in mixtures of super-critical octanol and hexane. The nanocrystals were relativelymonodisperse and sterically stabilized. The smallest nanocrys-tals exhibited also a discrete optical transition, characteristic ofquantum confinement effects for crystalline nanocrystals with anarrow size distribution.

2.5.5. Dispersion polymerisationDispersion polymerisation in SC-CO2 is a process that has

been used by various authors to produce microparticles of poly-mers[90–92]. A stable dispersion in SC-CO2 is produced usinga co-polymer with CO2-phobic and CO2-philic groups and eachcell of the dispersion acts as a nanoreactor in which the polymeri-sation takes place. Lee et al.[93,94]used dispersion polymeriza-tion in presence of a triblock copolymer stabilizer (surfactant)in SC-CO2 to produce aromatic polycarbonate (PC) nanoparti-cles. Spherical particles ranging between 30 and 140 nm wereobtained depending on the process conditions.

2.5.6. Reverse micellesWater-in-oil (w/o) microemulsions are thermodynamically

stable aggregates formed by a nanometric sized water core inan apolar continuous phase. They are generated by amphiphilics thew apo-l ma tact.M ylin-d il andw ellesa

ood solubility in ammonia; then, they increased the temture of the reactor thus inducing the decomposition ofrecursors and the precipitation of the corresponding oxideitrites. Aggregates of about 50 nm were obtained, for exam

n the case of Fe compounds, consisting of very smallnits <10 nm in diameter. The same research group[88] subseuently proposed the thermal decomposition process to proanostructured particles of several other compounds: Cr

d,-

e,

urfactants with an hydrophilic head group surroundingater core and a hydrophobic tail that extends into the

ar continuous phase[95]. The oil-surfactant interactions forlarge variety of structures to avoid the direct water/oil conicelles are the simplest structure: they are spherical or crical objects formed by surfactant molecules separating oater. Drops of oil in water are called micelles, reverse micre drops of water in oil.

10 E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22

Reverse micelles are widely used as nanoreactors to syn-thesize organic and inorganic nanoparticles. A reactant is, as arule, contained in the aqueous core and the other in the continu-ous organic phase. As the reaction is confined in the water core,whose dimensions are controlled by thermodynamic conditions,the diameter of the produced nanoparticles can be controlled bythe core size. Traditional recovery methods have a relevant effecton the increase of particle size and particle size distribution, dueto aggregation phenomena. A large amount of the surfactantremains on the nanoparticles.

Supercritical CO2 has a large affinity with many organicsolvents; therefore, it can be used in the recovery step as anantisolvent, or solvent-catcher, to extract all the liquid solvent(and most of surfactant) producing dry non-coalescing nanopar-ticles in a single process step.

Using selected surfactants that can stabilize water droplets innear critical or supercritical CO2, an elegant process, is also pos-sible in which water-in-CO2 (w/c) microemulsions are formed.In this case no organic solvents are required for the continu-ous phase and particles collection can be performed by simpledecompression.

Han and coworkers[96] synthesized Ag nanoparticles inwater-in-isooctane continuous phase using w/o reverse micellesand sodium bis(2-ethylhexil)succinate (AOT) as surfactant.

Tetraethylene-glycol-dodecyl-ether (C12E4) was added as co-surfactanct. The reactants, AgNO3 and KBH4, were loadedseparately in two micellar solutions and, then, mixed. Super-critical CO2 was used to eliminate the organic phase and mostof surfactants by solubilization of the continuous phase. TEManalysis of the collected Ag nanoparticles indicated particle sizesranging between 2 and 5 nm (minimum size) and 6 and 20 nm(maximum size) varying pressure, the reactants molar ratio andthe molar ratio water to surfactant (w). The same research group[97] used this process to synthesize ZnS nanoparticles. In a sub-sequent work[98], they also analyzed the influence of someprocess parameters on ZnS particle size. They studied the effectof temperature, pressure, surfactant concentration,w and stirringrate, concluding that is possible to control ZnS particle size inthe range 1–100 nm using this technique. The same group alsoproduced TiO2 nanoparticles using the reverse micelles, usingagain SC-CO2 as solvent-catcher to eliminate the organic phase[99]. Particles approximately ranging between 10 and 20 nmwere observed.

Chattopadhyay and Gupta[100] used w/o microemulsionsin a different process. They injected the w/o microemulsioninto a batch reactor containing SC-CO2 that acted either as thesolvent-catcher eliminating the organic solvent and either as areactant. They tested this process in the precipitation of silica

Fig. 7. Water-in-CO2 (w/c) double microemulsio

n reaction as describerd by Ohde et al.[103].

E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22 11

Table 6Nanoparticles produced in reverse micelles processes assisted by SCF

Material Process Particle dimensions (nm) Surfactant Reference

Range Mean size

ZnS w/o 1–8 AOT [97]Ag w/o 2–20 AOT [96]ZnS w/o 1–100 AOT [98]TiO2 w/o 10–20 AOT [99]SiO2 w/o 20–50 AOT [100]CdS w/c 0.9–1.8 PFPE-NH4 [101]Ag w/c 5–15 Surfactant with fluorinated tails NaBH(OAc)3 [102]Ag w/c 3.8 AOT in propane or SC-ethane [111]Cu w/c 9Cu w/c <20 AOT in SC-ethane [110]AgI w/c 3–15 AOT [103]AgBr w/c 3 AOT [103]Ag w/c 5–15 4 Mixture AOT/PFPE-PO4 [104]Cu w/c 5–15 Mixture AOT/PFPE-PO4 [104]CdS w/c 5–10 Aggregates 50–100 Fluorinated AOT [105]ZnS w/c 5–15 Aggregates 50–100 Fluorinated AOT [105]TiO2 w/c 8–19 PFPE-NH4, PFPECOO-NH4, PDMAEMA-b-PFOMA [108,109]Rh w/c 3–5 AOT/PFPE-PO4 [106]TiO2 w/c 8–18 PDMAEMA-b-PFOMA [107]Ag w/c 2–10 3.8 PFPE-NH4 [113]

nanoparticles obtaining nanoparticles with narrow particle sizedistributions, changing pressure, surfactant (AOT) concentrationand water/surfactant ratio.

The formation of w/c reverse micelles was explored byHolmes et al.[101] using as surfactant ammonium perfluo-ropolyether (PFPE-NH4) for the synthesis of CdS nanoparticles.Particles with mean diameters ranging between 0.9 and 1.8 nmwere obtained. Ji et al.[102] used w/c reverse micelles to syn-thesize Ag nanoparticles with average sizes ranging between 5and 15 nm. Surfactants with fluorinated tails were used by theseauthors.

Ohde et al.[103] formed two w/c reverse micelles contain-ing Ag+ and X− ions separately; the surfactant used was AOT.Then, they put in contact the two solutions to start the reaction(Fig. 7). This process is based on the consideration that a reversemicelle is a dynamic environment; therefore, when two micro-droplets with different content collide, exchange of contentstakes place. The process was successful and AgX nanoparticles(AgI and AgBr) were obtained. Ohde et al.[104–106]usingthis process also synthesized nanoparticles of other materials(seeTable 6) concluding that the diffusion between the micel-lar core and supercritical CO2 can be the rate determining stepfor the formation of nanoparticles. They also found discrepan-cies between particle diameter evaluated by spectroscopy in themicellar solution and the diameter measured by TEM on thedry particles. The authors attributed to aggregation phenomena,g le sizo

dh db sizei ctanr n

of TiO2 nanoparticles starting from TTIP. These authors usedreverse micelles w/c stabilized by low molecular weight andpolymeric surfactants. In particular, they analyzed micelles sta-bility depending on the kind of surfactant used andw parameter.After calcination they obtained TiO2 nanoparticles whose meandiameter ranged from 8 to 19 nm.

Cason and Roberts[110] studied the formation of nanosizedCu particles in w/c reverse micelles using SC-ethane. They usedAOT as the surfactant and small quantities of isooctane as co-solvent. The authors compared the particle growth rate in liquidsolvents and in SCF showing that the process is faster in SCF dueto the enhanced transport properties. The same research groupalso used reverse micelles (w/c) in compressed propane or super-critical ethane, stabilized using AOT, to produce nanoparticlesof Ag and Cu by reduction of the corresponding precursors (Agmean diameter 3.8 nm, S.D. 2.4 nm; Cu mean diameter 9 nm)[111].

Some papers also faced the study of w/c reverse micellesfrom the point of view of the understanding of the processesinvolved in their formation and stabilization. Particularly, Shahet al. [112] focused their attention on the tuning of the size ofthe resulting particles by modulating the SCF solvent density.SC-ethane was used in this study.

McLeod et al.[113] produced Ag nanoparticles in reversemicelles w/c stabilized by ammonium perfluoropolyether(PFPE-NH4). These authors concluded that stabilization isf PE-N Os

ousp st coa-l ingle

enerated during the decompression, the increase of particbserved by TEM.

Hong et al.[107] prepared TiO2 nanoparticles by controlleydrolysis of TTIP in water-in-CO2 reverse micelles stabilizey polymeric surfactants. They found that the mean particle

ncreases from 8 to 18 nm increasing the water-to-surfaatio (w). Lim et al. [108,109]also worked at the productio

e

t

acilitated by the specific environment resulting by PFH4 surfactant rather than any effect arising from C2olvent.

In the case of w/o reverse micelles in which the continuhase is eliminated by SC-CO2 the use of the SCF confirm

he advantage of the elimination/reduction of nanoparticlesescence, the elimination of most of surfactant and the s

12 E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22

step operation. Using w/c reverse micelles no organic solventsare used and the presence of the SCF adds further flexibilityto the process since the variation of density with pressure andtemperature allows to use it as a tunable medium for the reac-tion and separation processes. The limits of this process fromthe point of view of an industrial application could be the useof batch reactors and the quantity of nanomaterial produced ineach operation.

Studies performed using reverse micelles to produce nanopar-ticles are summarized inTable 6.

3. Other nanomaterials

3.1. Nanofibers, nanowires and nanotubes

Nanowires can have a particular relevance as building blocksfor the realization of nanoscale structures, for example, in themicroelectronic industry in which they can act as both devicesand electrical contact. For this reason, the production of semi-conductor nanowires has been particularly investigated. Severaltechniques have been proposed, including laser ablation, liq-uid crystals templating methods and vapor–liquid–solid growthprocesses[114]. These routes, typically produce disorderednanowires that could be difficult to manipulate into structureddevices. In some other cases, deposition of nanowires within thepores of mesoporous templates (silica) has been proposed, thatc s nos

usint twot –sol( -p

gon nal Sa f thed diss tion,t . Fore eterw ngthT press niquu ired ageo hno-l sultt tor iwI mert tratea cond aso

o-p eth-o ue to

mass transfer limitations. The relatively high diffusion coeffi-cients and the reduced surface tension of SCF based materialsallow to overcome this difficulty. Holmes et al.[117] workedat the preparation of three-dimensional arrays of nanowires ofmetals and semi-conductors within the pores of mesoporous sil-ica formed by pores of 3 and 5 nm in diameter. They obtained,for example, Ge nanowires with an average diameter of 6.4 nmwith lengths of the order of 500 nm[122]. At higher precursorsconcentration they observed the formation of silica whiskers pro-truding from the mesoporous silica surface (larger in diameter50–100 nm with respect of the size of the mesopores)[123]. Inthis case the mesopores act as sites for surface whisker growth.At lower concentrations nanorods formed within the pores ofthe silica matrix. Therefore, the formation of nanowires possi-bly occurs through the initial formation of nanoparticles withinthe mesopores and their subsequent growth into nanorods and,then, nanowires.

The RESOLV process, previously illustrated for the produc-tion of nanoparticles[16,17,19] has also been proposed forthe production of polymeric nanofibers of poly(heptadecaflu-orodecyl acrylate) (PHDFDA), PMMA and PLA[124].PHDFDA is a polymer soluble in SC-CO2, its rapid expan-sion in an aqueous NaCl solution produce nanoparticles[20]or nanofibers with the diameter smaller than 100 nm dependingon the polymer concentration in the expanding solution. Whenneat water was used, the authors observed a more severe fibera

pro-c singlefi asa solu-b dedt bersw

wires.H TO)n pro-c , thea difiest ocessa iam-e hisw ingH ersd larlya ereo cleary

n ofs ater.T per-a itstd rtherv n ofn nd al

an produce well defined architectures, though pore filling iimple.

The production of nanowires has also been proposedechniques based on the use of supercritical fluids. Mainlyechniques have been used: a SCF based vapor–liquidSFLS) formation technique[115,116]and a mesoporous temlate filling SCF aided technique[117,118].

The SFLS synthesis process uses alkanethiol-cappedanocrystals as seed particles to direct the one dimensiond Ge crystallization in supercritical hexane. Instead oeposition on the surface, the semiconductor preferentiallyolves into the gold droplet until saturation. Upon saturahe semiconductor exits from the droplet forming a whiskerxample, Ge nanowires ranging from 4 to 30 nm in diamere obtained, that were grown several micrometers in lehe crystal structure can be modified changing reactionure and temperature. The distinct advantage of this techpon non-SC ones is the possibility of producing small wiameters[119]. According to the authors another advantf this technique is the scalability of the synthesis to tec

ogical significant quantities on nanowires. To obtain this rehey converted the batch process to a continuous flow reachich lower precursors concentrations are also required[120].

n an alternative approach to reduce seed particles aggloion [121], Au nanocrystals were tethered to a silicon subss seeds for Si nanowires growth. Under optimized processitions, a significant quantity of high-quality Si nanowires wbtained.

The second process is the SCF-CO2 based variation of mesorous template filling with nanowires. Non supercritical mds were unable to achieve the complete filling of pores d

t

g

id

ldi

-

.-e

n

a-

-

ggregation.However, as shown by high resolution SEM images, the

ess tends to produce arrays of nanofibers more thanbers. The production of nanofibers of PMMA and PLA wlso proposed, but, since these polymers show negligibleilities in SC-CO2, a cosolvent (ethyl alcohol at 10%) was ad

o increase their solubility in the SCF. Again arrays of nanofiere produced.HTS-SCW has also been proposed to synthesize nano

akuta et al.[68] obtained potassium hexapentanate (Kanowires in a flow reactor under SCW conditions. Thisess is known to produce KTO nanoparticles; thereforeuthors hypothesized that the use of the flow reactor mo

he characteristing heating and residence times of the prnd can produce KTO nanowires. KTO nanowires 10 nm dter and about 1�m length were consistently obtained in tork. Ohara et al.[125] also synthesized ZnO whiskers usTS-SCW. In this work they obtained ZnO particles or whiskepending on the concentration of the precursors. Particut low concentrations ZnO highly crystalline whiskers wbtained. The mechanism of morphology change is notet.

Chang et al.[126] reported the spontaneous organizatioilver nanoparticles in nanowires in sub- and supercritical whis organization is promoted by variations in pressure, temture and time of treatment. Ag2O was used as precursor and

hermal decomposition at temperatures higher than 227◦C pro-uced Ag nanoparticles ranging between 2 and 20 nm. Fuariations in the conditions in SCW induced the organizatioanowires, with a maximum average diameter of 70 nm a

ength of several microns.

E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22 13

Fig. 8. Cellulose acetate nanofibers precipitated by SAS from acetone, at120 bar, 40◦C.

Louvier-Hernandez et al.[127] produced chitin microfibersby dissolving it in hexafluoroisopropanol and processing thesolution by SAS using SC-CO2 as antisolvent. SEM analysis ofthe fibers revealed that they were formed by a web of nanofibersof about 84 nm in diameter.

Nanofibers were also obtained for cellulose acetate precipi-tated from acetone by SAS (Reverchon et al.[128]) as shown inFig. 8.

A technique called nanoscale casting (NC-SCF) has beedeveloped by Wakayama et al.[129–137]and Fukushima et al.[138] to produce nanomaterials (mainly fibers) using templatesIt mainly consists of soluble precursors dissolution in SC-CO2and their subsequent deposition on activated carbon (covered ban organic matrix) templates. Then, activated carbon is elimi-nated (for example by calcination) and the deposited materiareplicates not only the macroscopic shape, but also the poroustructure on the nanometric scale. This last characteristic is thtrue novelty of this technique, since traditional casting tech-niques in metal or plastic casting can replicate the macroscopiaspect of the template, but not its nanometric characteristicsThe replicating materials cannot penetrate very small spacesif a liquid phase is used its high viscosity prevents penetra-tion; even when a gas phase is used, it can capillary condensain the fine spaces. When SC-CO2 is used these problems canbe overcome. Using NC-SCF, Wakayama et al. obtained replicates with nanoporous characteristics using silica[131,132,138],p[

ntiod s ant pplic robm torad s anf

d anc micam erg

consumption, but, can produce lower quality carbon nanotubes.Carbon nanotubes production has also been proposed using SCFbased processes.

Calderon-Moreno et al.[139,140]reported a HTS-SCW pro-cess to produce CNTs from amorphous carbon; no catalysts wereused. The nanotubes observed had diameters in the range oftens and the length in the range of hundreds of nanometers.Gogotsi et al.[141] proposed polymers like polyethylene ascarbon-containing precursors that were pyrolyzed before thehydrothermal synthesis. In a subsequent paper Motiei et al.[142], proposed the reaction of CO2 with Mg at 1000◦C toproduce MgO and carbon nanotubes. In this process CO2 is atsupercritical conditions since at 1000◦C in a closed system thecalculated pressure is approximately 10 kbar.

Operating in SCW Chang et al.[143] also observed in pres-ence and absence of oxygen the opening and thinning of mul-tiwall CNTs. It is known that the opening of CNTs starts atthe ends where defective patches are present. These authorsproposed a possible oxidation mechanisms and analysed theinfluence of several process parameters on the opening process.

Lee et al.[144] proposed the synthesis of CNTs in SC-toluene; catalysts like Fe or Ferrocene were also used. In thisprocess toluene serves as both the carbon source for nanotubesand as the solvent for the reaction.

Using SCF based techniques it is also possible to obtainpolymeric nanotubes. Zhang et al.[145] proposed a methodt clesf ubesu ntin-ua tion.W iam-e plied,a s areo ollidea ounda ateso ime,n essingP

3

ionsa emicalv d top ces,b recur-s nto ad ursora m ands

an bed

••

latinum[129,131], titania[131,133], alumina[133] and Pt-Ru137].

Carbon nanotubes (CNT) have attracted very great atteue to their structural, mechanical and electronic propertie

he related technological applications. They have potential aations in semiconductor devices, field emitters, scanning picroscopes, quantum wires, hydrogen and other gases sevices. Other possible uses include materials for batterie

uel cells, capacitors and chemical filters.Several methods of CNT preparation have been propose

an be classified in physical and chemical methods. Cheethods seem to be more efficient in terms of yield and en

n

.

y

lse

c.:

te

-

nd-eged

dl

y

o produce polystyrene (PS) and PMMA hollow nanopartirom ciclohexane solution and their arrangement in nanotsing GAS (that in this paper has been defined as a discoous SAS) with the application of ultrasounds (US). CO2 actss antisolvent for the polymer thus producing its precipitahen ultrasounds are not applied, PS nanoparticles with d

ters around 40 nm are obtained. When ultrasounds are apt the same operating conditions, hollow PS nanoparticlebserved. At the higher PS concentrations nanoparticles cnd fuse end-to-end to form short fibres. In the case of ultraspplication a tube-like polymer structure is formed by aggregf hollow nanoparticles. Increasing ultrasound application tanotubes with regular surfaces have been obtained procMMA.

.2. Nanofilms

Thin films can be used in various technological applicats catalysts, gas sensors, permselective membranes. Chapour deposition (CVD) is the technique traditionally useroduce metal and semiconductor thin films on solid surfaut requires high temperatures and high vapor pressure pors. In this technique a volatile precursor is transported ieposition chamber by means of a carrier gas. The precdsorbs on a heated surface and reacts to yield a metal atourface-bound ligand decomposition product.

Supercritical assisted techniques to produce thin films civided in two groups:

physical deposition processes;chemical deposition processes.

14 E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22

Physical film deposition processes are related to RESS pro-cess and its modifications. Indeed, the authors that first proposedthis process[8] also proposed its application to thin films depo-sition in three patents[9–11]. The concept is: when RESSproducts are sprayed onto a solid substrate, films can be formed.Petersen et al.[7] using this technique obtained the formation ofamorphous powders and thin films from organic and inorganicmaterials. Uniform films of SiO2 with a thickness of about 1�mwere deposited on a Millipore filter.

Another physical deposition process has been proposed byMurthy et al.[146]. In this case the compound (metal or poly-mer) is solubilized in the SCF, a substrate is put in a high pressureautoclave and the deposition onto the substrate is induced byvariation of temperature and/or pressure of the supercritical solu-tion. The formation of films with thicknesses of about 5 nm isclaimed by the authors.

A film deposition process by chemical reaction in SCFshas been proposed by Sievers and coworkers[147–149]. It issubstantially a RESS process in which, when the supercriticalsolution is expanded, a chemical reaction is induced; thereforea film of the desired material, that results from the chemicalreaction, is deposited on the substrated surface. No particularindication is given about film thickness obtainable by this tech-nique; however, controlling the exposition time it is possible tocontrol this parameter.

Louchev et al.[150] induced the decomposition of the pre-c stiveo thinfi uenw f as thinI

uct-i RESp hee gene tratU me-t rticlec l reso

thea nte[ kinse ons of am lutioo es-s 0 nmc on thsn iont ferso lmsdfi thaa

synthesized Pd films at controlled depths within porous alluminadisks.

Cu nanofilms have also been prepared[160,161]using var-ious precursors. Particularly, Cu films with thickness between100 and 400 nm were obtained varying precursors and operat-ing conditions. The authors obtained the CFD of Cu films ontoplanar and etched silicon substrates by H2-assisted reductionof copper-hexafluoro-acetyl-acetonates. Supercritical CO2 andC2F6 were successfully used. Using CFD technique also Rh, Au,Co and Ni thin nanometric films have been obtained[162–164].

Thin films deposited by decomposition of organometalliccomplexes in SCFs are also the object of a patent submittedby Morita et al.[165]. The authors also claimed the efficientremoval of carbon residues deriving from decomposition of theorganic part of the molecule.

Another film deposition process assisted by SC-CO2 hasbeen called by the authors supercritical fluid immersion depo-sition (SFID)[166–168]. It is a variation of a classical galvanicdisplacement process traditionally carried out in aqueous HFsolutions. It consists of the formation of thin films (from 10 to50 nm thick) of Pd, Cu, Ag and other metals on Si-based sub-strates in supercritical CO2 solutions. The process starts withthe oxidation of elemental silicon to SiF4 or H2SiF6 using HF,thus causing the reduction of a metal chelate precursor (that wasdissolved in SC-CO2) to the metallic form. SC-CO2 processflexibility enhances the performance of the traditional processa strates

osi-t genr te).T filmst hick-n and1

rt nalh elec-t COa ctantw First,t exanea im-i entsu nd as usingt ibutedb dur-i heS obsta-c rfacet imper-f

-b n oft -olt in

ursor compound, previously dissolved in a SCF, by resir laser heating. Operating in this manner, they producedlms of Cu with thickness as small as 50 nm. In a subseqork Popov et al.[151] by rapid expansion and heating oupercritical solution containing two precursors, producednP films.

Nanofilms of a fluoropolymer onto a metal or semicondng substrate have also been obtained using a variation ofrocess[152,153]. Fulton et al. applied a high voltage to txpansion nozzle and the electric charged nanoparticlesrated by RESS where thus collected onto a metallic subsniform coating with thicknesses ranging from tens of nano

ers to some microns were obtained. The charged nanopaan be deposited on the conducting medium with a spatialution better than 50 nm.

A supercritical fluids alternative to CVD, named byuthors chemical fluid deposition (CFD), has been pate

154,155]and published in various scientific papers. Watt al.[156] described the formation of platinum metal filmsilicon wafers and polymer substrates via hydrogenolysisetallorganic precursor. The process is based on the dissof the precursor in SC-CO2; then, hydrogen is added into the vel and reduction takes place. Platinum in form of 80–10rystals was released from the fluid phase and depositedolid support forming a film of approximately 1.3�m thick-ess. In a subsequent paper they proposed the format

hinner palladium films using the CFD technique on Si war polymide[157] and in another paper copper and nickel fieposited onto planar and etched silicon substrates[158]. Theselms can be deposited also within patterned silicon wafersre 100–120 nm in width and 1�m deep. Fernandes et al.[159]

t

S

-e.

s-

d

n

e

of

t

nd the thickness of the metal film deposited on the suburface depends on the immersion time.

Ohde et al.[169] performed a low-temperature SCF depion of conformal copper with palladium-catalyzed hydroeduction of Cu(hfa)2 (copper(II) hexafluoroacetylacetonahe bottom-up SCF deposition mechanism allowed to Cu

o fill up small features patterned on Si wafer. The average tess of the Cu films were extimated to be between 10050 nm.

Yan et al.[170]and Yoshida et al.[171,172]proposed anothehin film deposition technique that is a variation of the traditioexane emulsion electroplating method. The supercritical

roplating reaction was performed in an emulsion of dense2,metal salt solution (the electroplating solution) and a surfaith agitation in a high pressure cell operated at 100 bar.

he authors compared the electrochemical properties of hnd dense CO2 emulsions and found that they are very s

lar. Then, they performed the electro-deposition experimsing SC-CO2, and obtained a higher uniformity, hardness amoother formation of nickel nanostructured surfaces thanhe liquid process. The absence of surface defects was attry the authors to the high solubility of hydrogen (produced

ng the reaction) in SC-CO2 and to the reduced viscosity of tC medium. These characteristics assure the absence ofles and a fast contact of the deposition material on the suo be covered that greatly reduces the presence of surfaceections.

Brasseur-Tilmant et al.[173,174]modified inorganic memranes (the solid support) using the hydrolytic decompositio

itanium tetraisopropoxide (TTIP) in supercritical propan-2o produce TiO2 nanoparticles. Nanoparticles of about 30 nm

E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22 15

Table 7Other nanomaterials

Material Technique Dimension Reference

Diameter (nm) Length (�m)

Nanowires and nanofibersSi Thermal degradation in SC-hexane 4–5 >5 [115]Si Precipitation in SC-cyclohexane 5–30 >1 [121]Ge Precipitation in SC-cyclohexane 10–150 >1 [116]Ge Template filling 6.4 0.5 [122]Si Template filling 4.5–7.3 >1 [175]Ag SCW thermal decomposition 60 >0.1 [126]Si, Ge SFLS synthesis 4–30 4–10 [119]KTO Hydrothermal synthesis 10 0.5–1 [68]Chitin SAS 84 [127]Pt NC-SCF 20–80 [129,131]TiO2 NC-SCF 5–15 [131]SiO2 NC-SCF Porosity 3.8–4.8 nm [131]Pt-Ru NC-SCF 10–20 [137]PHDFDA RESOLV in aqueous NaCL solution <100 [124]

Width (nm) Length (�m)

NanotubesCNT Hydrothermal synthesis 10–50 0.1 [139,140]CNT Hydrothermal synthesis In: 160, out: 30–200 >100 [141]CNT Reaction in SC-CO2 30–40 0.5–0.6 [142]PS GAS 40 [145]PMMA GAS [145]

Structural elements (nm) Thickness (�m)

NanofilmsCu SFCD <0.05 [150]InP SFCD 0.4 [151]Pt CFD 80–100 1.3 [156]Pd CFD 30–70 0.1–0.2 [157]Cu, Ni CFD 2.5 [158]Cu CFD 0.1–0.4 [160,161]Au CFD 40–60 0.1–1 [163]Rh, Au CFD 0.18 [162]Co CFD 50–100 0.15–1 [164]Ni CFD 80–190 0.1–0.3 [164]Pd OR-CFD 2–80 [159]Pd, Cu, Ag SFID 0.01–0.05 [167,168]TiO2 SC-propan-2-ol 1–4 [173]TiO2 SC- propan-2-ol 30 1–3 [174]PFOA RESS deposition <100 0.077 [153]Cu Catalytic reduction 0.1–0.15 [169]

size deposited as a thin film on the surface and in the pores ofalumina supports. TiO2 films with thickness ranging between1 and 3�m were produced. The penetration depth was around20–30�m. The results in producing nanowires, nanofibers, nan-otubes and nanofilms using SCFs based processes are summa-rized inTable 7.

4. Nanocomposite materials

The production of composite nanomaterials can be of interestin several applications: particularly for the controlled release ofpharmaceuticals, medical devices, semiconductors and super-conductors, microelectronic applications and barrier materials(gas barriers, oxygen barriers, food packaging).

4.1. Composite nanoparticles

From a general point of view, composite nanoparticles canbe classified as nanospheres and nanocapsules. Nanospheres areformed by the random dispersion of two or more compounds(for example, a polymer and a drug); nanocapsules, instead, areformed by a shell of one component and a core of the activecompound. Nanocomposite materials in form of nanofibers, nan-otubes plus fillers and nanoparticles on nanowires have also beenproposed for specific applications.

Conventional techniques used to produce composite nanopar-ticles are generally an extension of the processes used for thegeneration of single compound nanoparticles. The same consid-eration is valid for supercritical fluids based processes that are,

16 E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22

as a rule, an extension of the techniques discussed in the previ-ous chapters. Therefore, RESOLV, SAS, w/c microemulsions,SC water, SC based reactions, etc., have also been adapted toproduce composite materials. RESOLV has been used by Sunet al. [176] to produce polymer protected nickel, cobalt andiron nanoparticles precipitated from SC-ethanol. These authorsused the RESOLV plus chemical reaction process adding in thereceiving solution PVP at concentrations large enough to coverthe nanoparticles formed in the first part of the process. Using thesame technique Meziani et al.[177] produced nanocrystallineAg particles by precipitation from SC-ammonia. As coveringmaterials they selected PVP and bovine serum albumin (BSA)that were dissolved in the receiving liquid solution that alsocontained the reducing agent (an aqueous solution for BSAand an ethanolic solution for PVP). Particles with mean diam-eters ranging between 4.8 and 43 nm were obtained dependingon the coating agent and on the composition of the receivingsolution. The RESOLV plus reaction process was also usedfor the preparation of BSA protein-conjugated silver sulfidenanoparticles[178]. The aim of the authors was the produc-tion of core-shell nanocapsules in which albumin acts as thecoating agent. To obtain this result they dissolved AgNO3 in SC-ammonia and the supercritical solution was RESS-sprayed intoan aqueous solution containing BSA. TEM analysis confirmedthat particles with an average diameter of 6.3 nm (and a standarddeviation of 1.6 nm) were produced. AFM images showed thatw rialw

andE d byW asp ticlew silicarh opat inald d theo ver,d dis-t d tot ticul eoup riesi onlym t int f thc stant rgelydc ore,S

i rmedi ip -c ec esi

was added, the Fe3O4 crystallite size was constant. Accordingto the authors, this result indicates that Ni coating was fast andprobably inhibited further Fe3O4 particle growth. They obtainedparticles as small as 59 nm with a standard deviation of 19 nm.

The use of HTS-SCW to produce composite nanoparticleshas also been proposed[182] using two SCW reactors operatedin series. In the first one ZnO nanoparticles were produced, then,ZnO nanoparticles were immediately sent to the second reactorwere TiO2 was produced obtaining the composite nanoparticles.The composite particles are not in the core-shell form and thetwo metals are not mixed: the composite particle is formed bythe random coalescence of nanoparticles of the two oxides.

Composite nanoparticles have also been produced by a vari-ation of the w/o reverse micelles process. In a first work Hanand coworkers[183] proposed the formation of two micel-lar solutions containing precursor compounds that were, then,mixed forming ZnS/CdS composite nanoparticles in the reversemicelles due to the exchange of contents. Supercritical CO2 wasused as solvent-catcher to eliminate the liquid solvent and thesurfactant. The same research group[184] formed Ag nanopar-ticles in a w/o reverse micelle and added PS in the liquidsolution. Then, SC-CO2 was added as solvent-catcher to elim-inate the liquid solvent (ciclohexane) and the surfactant and toinduce the precipitation of the polymer on the Ag nanoparti-cles trapped or wrapped in the polymer. The reduction of thesize of the composite nanoparticles with increasing pressure andt l coa-l o usedt pos-im cur-s wasa eri-s MAw ningC sec-o medo terics par-t

meoe theS

4

Oa ncap-s t-m opar-t e thef orga-n tion).P endo iono d int pos-

ell-dispersed Ag2S nanoparticles surrounded by soft mateere obtained with an overall diameter of 20–30 nm.The production of nanocapsules formed by silica

udragit (a polymer) was the scope of the work proposeang et al.[179]using the SAS process. A polymer solution w

repared by dissolving Eudragit in acetone. Silica nanoparere suspended in this solution at the desired polymer-

atio. The suspension was SAS treated with SC-CO2 causingeterogeneous polymer nucleation with spherical silica nan

icles acting as nuclei for the growth of the polymer. The origiameter of silica nanoparticles ranged from 16 to 30 nm anbtained nanocapsules ranged from 50 to 100 nm. Howeifferent morphology was obtained, due to the non uniform

ribution of the polymer on the surface of the particles anhe aggregation of the composite nanoparticles in multiparate systems contained in a polymer matrix. When simultanrecipitation is attempted, the loading efficiency by SAS va

n a wide range whereas the process parameters haveinor effect on loading capacity; this is due to the fact tha

his case the nanoparticles are formed by coprecipitation oompounds from their solutions. Therefore, they are subially solvent induced physical blending processes and laepend on the similarity of compounds involved[180]. Theseonditions are particularly difficult to be obtained: therefAS coprecipitation is rarely successful.Sue et al.[181]precipitated Ni nanoparticles on Fe3O4 nuclei

n a continuous reactor. The composite particles were fon two steps: formation of Fe3O4 nuclei by HTS and, then, Nrecipitation on the surface of nuclei by H2 reduction of a preursor in an aqueous solution. Although in Fe3O4 synthesis thrystallite size increased with temperature, when Ni synth

s

r-

a

-s

a

e-

s

emperature was explained by the reduction of the physicaescence of the nanoparticles. The same process was also produce CdS/PMMA and PVP-Prussian blue (PB) comte nanoparticles[185,186]. In the first work water/AOT/MMA

icelles were produced; CdS was produced from its preor by chemical reaction; then a polymerisation initiatordded and the solution was put in an oven to start polymation. Controlling the precipitation pressure, CdS and PMere precipitated simultaneously from the solution, obtaidS nanoparticles distributed in the polymer matrix. In and work water/AOT/isooctane reverse micelles were forbtaining PB particles. PVP contained in water acted by stabilizer of the PB particles. Therefore, PVP protected PBicles were obtained.

Combined Ag-Pd nanoparticles were also obtained by Kat al. [82] by reduction of their precursors, as described inection2.5.2.

.2. Composite nanowires and nanotubes

The reverse micelles w/o process and the use of SC-C2 asntisolvent have also been proposed to produce polymer eulated silver nanofibers[187] in combination with US treaent. The reverse micelles were used to produce Ag nan

icles; then, ultrasound treatment was performed to inducormation of nanorods and nanofibers by coalescence andization of these nanoparticles (see also nanowires formaolystyrene was also solubilized in the solution and at thef the process SC-CO2 was added to induce the precipitatf polystyrene on Ag nanoparticles. The process resulte

he coating of the polymer on the silver nanofibers. Com

E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22 17

Table 8Composite materials produced by SCF based processes

Technique Material Dimensions Reference

Core (�m) Shell (�m)

NanocapsulesRESOLV + reaction in SC-EtOH Ni, Co, Fe covered by PVP 5.8–9.8 [176]RESOLV + reaction in SC-NH3 Ag covered by PVP 4.8 [177]RESOLV + reaction in SC-NH3 Ag covered by BSA 43 [177]RESOLV + reaction Ag2S covered by BSA 6.3 20–30 [178]SAS SiO2 covered by Eudragit 50–100 [179]HS-SCW Ni precipitated on Fe3O4 59 [181]Reverse micelles + SAS PS-Ag nanospheres 3 40–65 [184]

Core (�m) Shell (�m)

NanospheresReverse micelles + SAS ZnS/CdS 2–6 [183]Reduction AgPd 4–5 [82]HS-SCW ZnO/TiO2 200–300 [182]Water/AOT/MMA micelles CdS/PMMA 10–14 [186]Water/AOT/isoctane micelles PVP/Prussian blue 20–27 [186]

Width (nm) Length (�m)

Composite nanowires and nanotubesHydrogen reduction Pd on carbon nanotubes 5–10 [189]w/o reverse micelles Si (nanofibers) 35 >1 [187]

Polystyrene in CNT 40–50 2–3 [188]

ite nanofibers of about 35 nm diameter and length up to severalmicrons were obtained.

Encapsulation of polystyrene within carbon nanotubes withthe aid of SC-CO2 was also attempted by Han and coworkers[188]. In the first step CNTs were ultrasonically dispersed instyrene monomer; then, SC-CO2 was added to allow monomerpenetration inside the CNTs. In the third step CO2 was releasedand the vessel was transferred in an oven at 100◦C to startpolymerisation. A composite material in which CNTs are con-sistently filled by PS was obtained.

It has been suggested that carbon nanotubes (CNT) couldbe used as templates for confining and directing the growthof nanowires. Ye et al.[189] proposed metal deposition insideCNTs using a SC-CO2 based process. A metal chelate precursorwas dissolved in SC-CO2 and the solution was contacted withCNTs. Then, temperature was increased and in presence of H2a reduction reaction took place. This process produced the for-mation of nanowires of Pd, Cu or Ni inside the CNTs. Somenanoparticles of these metals were also found on the externalwalls of CNTs.

Ye et al.[190] proposed the precipitation of Pd nanoparticleson carbon nanotubes by reduction of a Pd precursor in SC-CO2.Well-dispersed spherical nanometric particles ranging between5 and 10 nm were anchored on the external walls of nanotubes.The same research group using a reduction reaction in SCF alsodeposited catalysts (Pd, Rh, Ru) nanoparticles on carbon nan-o ther i-cn sedT

5. Conclusions and perspectives

A large quantity of SCF based processes that were success-ful in producing nanomaterials has been found in the literatureat laboratory scale. They can be divided in new processes and(more commonly) adaptation of existing processes to the useof SCFs. In all cases, using SCFs, more flexible and/or sim-plified processes have produced and with a reduction of theenvironmental impact. The final result is that nanomaterialswith potentially better performances have been obtained usingimproved processes. Probably not all the SCF based processesthat have been proposed will find an application; a “natu-ral” selection is expected among them and only some will beused.

The application to a large variety of nanomaterials has beenproposed and we can expect that different materials will betested in the future. However, extensive scientific research isstill required on the large majority of the process proposed toevaluate:

• the extent of their applicability;• their scalability outside the scientific laboratories.

Moreover, a problem shared by all nanomaterials processedis the nanomaterial collection after its generation.

Among all the processes reviewed, at the best of our knowl-e pilots it ont tagew asedp

tubes[191]. Using the same technique, they also inducededuction of metal precursors in SC-CO2 producing nanopartles deposited onto SiO2 nanowires[192]. A list of compositeanomaterials produced in SCFs assisted process is propoable 8.

in

dge, only SAS has been successfully proposed on thecale yet, and some efforts are in progress to develophe industrial scale. We expect that this development sill be reached in the next future by several other SCF brocesses.

18 E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22

References

[1] D.L. Tomasko, X. Han, D. Liu, W. Gao, Supercritical fluid applica-tions in polymer nanocomposites, Curr. Opin. Solid State Mater. Sci.7 (2003) 407.

[2] E. Reverchon, Supercritical antisolvent precipitation of micro- andnano-particles, J. Supercrit. Fluids 15 (1999) 1.

[3] J. Jung, M. Perrut, Particle design using supercritical fluids: Literatureand patent survey, J. Supercrit. Fluids 20 (2001) 179.

[4] Z. Knez, E. Weidner, Particles formation and particle design usingsupercritical fluids, Curr. Opin. Solid State Mater. Sci. 7 (2003) 353.

[5] A. Shariati, C.J. Peters, Recent developments in particle design usingsupercritical fluids, Curr. Curr. Opin. Solid State Mater. Sci. 7 (2003)371.

[6] Y. Hakuta, H. Hayashi, K. Arai, Fine particle formation usingsupercritical fluids, Curr. Opin. Solid State Mater. Sci. 7 (2003)341.

[7] R.C. Petersen, D.W. Matson, R.D. Smith, Rapid precipitation of lowvapor pressure solids from supercritical fluid solutions: the formationof thin films and powders, J. Am. Chem. Soc. 108 (1986) 2100.

[8] D.W. Matson, J.L. Fulton, R.C. Petersen, R.D. Smith, Rapid expansionof supercritical fluid solutions: solute formation of powders, thin films,and fibers, Ind. Eng. Chem. Res. 26 (1987) 2298.

[9] R.D. Smith, Supercritical fluid molecular spray film deposition andpowder formation, US A 4582731 (19860415).

[10] R.D. Smith, Method of making supercritical fluid molecular sprayfilms, powder and fibers, US 4734227 (19880329).

[11] R.D. Smith, Supercritical fluid molecular spray thin films and finepowders, US A 4734451 (19880329).

[12] M. Turk, P. Hils, B. Helfgen, K. Schaber, H.-J. Martin, M.A. Wahl,Micronization of pharmaceutical substances by the rapid expansion

rovercrit.

on of0.2,5-rcrit-

Ofpandr, I.ym-3, p.

con-fluid

scalelution3.ticles842.

, Y.-lyme

icles. 11

r, P.n of

caleeutica

se of47177

[24] Y.-P. Sun, P. Atorngitjawat, M.J. Meziani, Preparation of sil-ver nanoparticles via rapid expansion of water in carbon dioxidemicroemulsion into reductant solution, Langmuir 17 (2001) 5707.

[25] M.J. Meziani, P. Pathak, F. Beacham, L.F. Allard, Y.-P. Sun, Nanopar-ticle formation in rapid expansion of water-in-supercritical carbondioxide microemulsion into liquid solution, J. Supercrit. Fluids 34(2005) 91.

[26] E. Reverchon, G. Della Porta, R. Taddeo, P. Pallado, A. Stassi, Sol-ubility and micronization of griseofulvin in supercritical CHF3, Ind.Eng. Chem. Res. 34 (1995) 4087.

[27] F. Dehghani, N.R. Foster, Dense gas anti-solvent processes for phar-maceutical formulation, Curr. Opin. Solid State Mater. Sci. 7 (2003)363.

[28] P. York, M. Hanna, Salmeterol xinafoate with controlled particle size,WO A1 9501324 (19950112).

[29] R.B. Gupta, P. Chattopadhyay, Method of forming nanoparticles andmicroparticles of controllable size using supercritical fluids and ultra-sound, US A1 2002000681 (20020103).

[30] B. Subramaniam, S. Said, R.A. Rajewski, V. Stella, Methods andapparatus for particle precipitation and coating using near-critical andsupercritical antisolvents, WO A1 9731691 (19970904).

[31] E. Reverchon, G. Caputo, I. De Marco, Role of phase behavior andatomization in the supercritical antisolvent precipitation, Ind. Eng.Chem. Res. 42 (2003) 6406.

[32] E. Reverchon, G. Della Porta, A. Di Trolio, S. Pace, Supercriticalantisolvent precipitation of nanoparticles of superconductor precursors,Ind. Eng. Chem. Res. 37 (1998) 952.

[33] E. Reverchon, I. De Marco, G. Della Porta, Tailoring of nano- andmicro-particles of some superconductor precursors by supercriticalantisolvent precipitation, J. Supercrit. Fluids 23 (2002) 81.

[34] E. Reverchon, K. Hartmut, T. Ulrich, Crystallization with compressedim,

iticalrsor,

entPro-ocess

redercrit.

Dis-iques,on-ting

Dis-iques,

f

ano-. 106

ticlesInt.

n byJ. 48

romnol.

pis,per-2026.

of supercritical solutions (RESS): a promising method to impbioavailability of poorly soluble pharmaceutical agents, J. SupeFluids 22 (2002) 75.

[13] A. Sane, S. Taylor, Y.P. Sun, M.C. Thies, RESS for the preparatifluorinated porphyrin nanoparticles, Chem. Commun. (2003) 272

[14] D. Pestov, N. Levit, R. Kessick, G. Tepper, Photosensitivedistyrylpyrazine particles produced from rapid expansion of supeical solutions, Polymer 44 (2003) 3177.

[15] D. Pestov, N. Levit, O. Guney-Altay, G. Tepper, Developmentencapsulated microspheres from a multiple-component, rapidly exing supercritical solution for sensing applications, in: G. BrunneKikic, M. Perrut (Eds.), Proceedings of the sixth International Sposium on Supercritical Fluids, tome 3, Versailles, France, 2001911.

[16] Y.-P. Sun, H.W. Rollins, Preparation of polymer-protected semiductor nanoparticles through the rapid expansion of supercriticalsolution, Chem. Phys. Lett. 288 (1998) 585.

[17] Y.-P. Sun, R. Guduru, F. Lin, T. Whiteside, Preparation of nanosemiconductors through the rapid expansion of supercritical so(RESS) into liquid solution, Ind. Eng. Chem. Res. 39 (2000) 466

[18] P. Pathak, M.J. Meziani, T. Desai, Y.-P. Sun, Nanosizing drug parin supercritical fluid processing, J. Am. Chem. Soc. 126 (2004) 10

[19] M.J. Meziani, P. Pathak, R. Hurezeanu, M.C. Thies, R.M. EnickP. Sun, Supercritical-fluid processing technique for nanoscale poparticles, Angew. Chem. Int. Ed. 43 (2004) 704.

[20] Y.-P. Sun, M.J. Meziani, P. Pathak, L. Qu, Polymeric nanopartfrom rapid expansion of supercritical fluid solution, Chem. Eur. J(2005) 1366.

[21] C. Kropf, H. Dolhaine, T. Forster, K. Schaber, M. Turk, S. CihlaChristophliemk, Method for producing nanoparticles by expansiosupercritical solutions, WO A1 2000015329 (20000323).

[22] T. Foerster, B. Fabry, M. Hollenbrock, C. Kropf, Use of nanossterols and sterol esters for producing cosmetic and/or pharmacpreparations, WO A1 2000021490 (20000420).

[23] C. Kropf, B. Fabry, T. Foerster, R. Wachter, S. Reil, C. Panzer, Unanoscale chitosans and/or chitosan derivatives, WO A1 20000(20000817).

-

r

l

gases, in: T. Ulrich (Ed.), Energetic Materials, Wiley-VCH, Weinhe2004, p. 159.

[35] E. Reverchon, G. Della Porta, D. Sannino, P. Ciambelli, Supercrantisolvent precipitation of nanoparticles of a zinc oxide precuPowder Technol. 102 (1999) 127.

[36] H.-T. Wu, M.-J. Lee, H. Lin, Nano-particles formation for pigmred 177 via a continuous supercritical anti-solvent process, in:ceedings of the 16th International Congress of Chemical and PrEngineering, Praha, Czech Republic, 2004, C7.3.

[37] H.-T. Wu, M.-J. Lee, H. Lin, Nano-particles formation for pigment177 via a continuous supercritical anti-solvent process, J. SupFluids 33 (2005) 173.

[38] E. Reverchon, R. Adami, I. De marco, C.G. Laudani, A. Spada,perse red 60 micronisation using supercritical fluid based technin: I. Kikic, M. Perrut (Eds.), Proceedings of the Seventh Italian Cference on Supercritical Fluids and their Applications/Ninth Meeon Supercritical Fluids, Trieste, Italy, 2004 (Md30).

[39] E. Reverchon, R. Adami, I. De marco, C.G. Laudani, A. Spada,perse red 60 micronisation using supercritical fluid based technJ. Supercrit. Fluids 35 (2005) 76.

[40] P. Chattopadhyay, R.B. Gupta, Supercritical CO2-Based Production oFullerene Nanoparticles, Ind. Eng. Chem. Res. 39 (2000) 2281.

[41] E. Reverchon, G. Della Porta, Production of antibiotic micro- and nparticles by supercritical antisolvent precipitation, Powder Technol(1999) 23.

[42] P. Chattopadhyay, R.B. Gupta, Production of griseofulvin nanoparusing supercritical CO2 antisolvent with enhanced mass transfer,J. Pharm. 228 (2001) 19.

[43] P. Chattopadhyay, R.B. Gupta, Protein nanoparticles formatiosupercritical antisolvent with enhanced mass transfer, AIChE(2002) 235.

[44] G. Muhrer, M. Mazzotti, Precipitation of lysozyme nanoparticles fdimethyl sulfoxide using carbon dioxide as antisolvent, BiotechProg. 19 (2003) 549.

[45] W.K. Snavely, B. Subramaniam, R.A. Rajewski, M.R. DefelipMicronization of insulin from halogenated alcohol solution using sucritical carbon dioxide as an antisolvent, J. Pharm. Sci. 91 (2002)

E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22 19

[46] E. Reverchon, G. Della Porta, I. De Rosa, P. Subra, D. Letourneur,Supercritical antisolvent micronization of some biopolymers, J. Super-crit. Fluids 18 (2000) 239.

[47] E. Reverchon, I. De Marco, G. Caputo, G. Della Porta, Pilot scalemicronization of amoxicillin by supercritical antisolvent precipitation,J. Supercrit. Fluids 26 (2003) 1.

[48] D.J. Jarmer, C.S. Lengsfeld, T.W. Randolph, Manipulation of particlesize distribution of poly(L-lactic acid) nanoparticles with a jet-swirlnozzle during precipitation with a compressed antisolvent, J. Supercrit.Fluids 27 (2003) 317.

[49] E. Reverchon, I. De Marco, Supercritical antisolvent micronization ofCefonicid: thermodynamic interpretation of results, J. Supercrit. Fluids31 (2004) 207.

[50] E. Reverchon, Supercritical-assisted atomization to produce micro-and/or nanoparticles of controlled size and distribution, Ind. Eng.Chem. Res. 41 (2002) 2405.

[51] E. Reverchon, Process for the production of micro and/or nano parti-cles, WO A1 2003004142 (20030116).

[52] E. Reverchon, A. Spada, Crystalline microparticles of controlled sizeproduced by supercritical-assisted atomization, Ind. Eng. Chem. Res.43 (2004) 1460.

[53] Z.S. Hu, J.X. Dong, G.X. Chen, F. Lou, Preparation of nanometer cop-per borate with supercritical carbon dioxide drying, Powder Technol.102 (1999) 171.

[54] Z.S. Hu, J.X. Dong, G.X. Chen, Preparation of nanometer titaniumoxide with n-butanol supercritical drying, Powder Technol. 101 (1999)205.

[55] Z.S. Hu, L.G. Wang, Z.W. Ou, L. Huang, R. Lai, T. He, J.X.Dong, G.X. Chen, Preparation of aerogel beryllium borate withsupercritical carbon dioxide drying, Powder Technol. 114 (2001)163.

ng,orat

rcrit-

wder

ders

ersedium,118

rmalAm.

rmalater,

talRes.

ofrticle

n ofritical

ro-esis

pro-esis

cat-itions

[69] G. Li, R.L. Smith, H. Inomata, K. Arai, Synthesis and thermaldecomposition of nitrate-free boehmite nanocrystals by supercriticalhydrothermal conditions, Mater. Lett. 53 (2002) 175.

[70] A. Cabanas, J.A. Darr, E. Lester, M. Poliakoff, Continuous hydrother-mal synthesis of inorganic materials in a near-critical water flow reac-tor; the one-step synthesis of nano-particulate Ce1−xZrxO2 (x = 0–1)solid solutions, J. Mater. Chem. 11 (2001) 561.

[71] A. Cabanas, J.A. Darr, M. Poliakoff, E. Lester, A continuous and cleanone-step synthesis of nano-particulate Ce1−xZrxO2 solid solutions innear-critical water, Chem. Commun. (2000) 901.

[72] A. Cabanas, M. Poliakoff, The continuous hydrothermal synthesisof nano-particulate ferrites in near critical and supercritical water, J.Mater. Chem. 11 (2001) 1408.

[73] L.J. Cote, A.S. Teja, A.P. Wilkinson, Z.J. Zhang, Continuous hydrother-mal synthesis and crystallization of magnetic oxide nanoparticles, J.Mater. Res. 17 (2002) 2410.

[74] L.J. Cote, A.S. Teja, A.P. Wilkinson, Z.J. Zhang, Continuous hydrother-mal synthesis of CoFe2O4 nanoparticles, Fluid Phase Equilib. 210(2003) 307.

[75] R. Viswanathan, R.B. Gupta, Formation of zinc oxide nanoparticles insupercritical water, J. Supercrit. Fluids 27 (2003) 187.

[76] A.A. Galkin, B.G. Kostyuk, N.N. Kuznetsova, A.O. Turakulova, V.V.Lunin, M. Polyakov, Unusual approaches to the preparation of hetero-geneous catalysts and supports using water in subcritical and super-critical states, Kinet. Catal. 42 (2001) 154.

[77] A.A. Galkin, A.O. Turakulova, A.F. Sadykov, N.N. Kuznetsova, V.V.Lunin, Nanocrystalline supported catalysts prepared in sub- and super-critical water: catalytic activity in oxidation, hydrogenation and iso-merization reactionsdkjdot, in: G. Brunner, I. Kikic, M. Perrut (Eds.),Proceedings of the Sixth International Symposium on SupercriticalFluids, tome 2, Versailles, France, 2003, p. 1289.

teric-

rested105

sta-itical

tabi-d.),

Tech-, p.

etal215

duc-r 20

ofater,

niumpilot

derside,

n, V.ess-999)

l, S.terialshem.

[56] Z.S. Hu, R. Lai, F. Lou, L.G. Wang, Z.L. Chen, G.X. Chen, J.X. DoPreparation and tribological properties of nanometer magnesium bas lubricating oil additive, Wear 252 (2002) 370.

[57] A. Weber, Production of nanoscale powders from sols using supeical fluids, DE A1 19904990 (20000427).

[58] S.E. Pratsinis, S. Vemury, Particle formation in gases: a review, PoTechnol. 88 (1996) 267.

[59] W.J. Dawson, Hydrothermal synthesis of advanced ceramic powAm. Ceram. Soc. Bull. 67 (1988) 1673.

[60] E. Matijevic, W.P. Hsu, Preparation and properties of monodispcolloidal particles of lanthanide compounds. I. Gadolinium, europterbium, samarium, and cerium(III), J. Colloid Interface Sci.(1987) 506.

[61] T. Adschiri, K. Kanazawa, K. Arai, Rapid and continuous hydrothecrystallization of metal oxide particles in supercritical water, J.Ceram. Soc. 75 (1992) 1019.

[62] T. Adschiri, K. Kanazawa, K. Arai, Rapid and continuous hydrothesynthesis of boehmite particles in subcritical and supercritical wJ. Am. Ceram. Soc. 75 (1992) 2615.

[63] T. Adschiri, Y. Hakuta, K. Arai, Hydrothermal synthesis of meoxide fine particles at supercritical conditions, Ind. Eng. Chem.39 (2000) 4901.

[64] T. Adschiri, Y. Hakuta, K. Sue, K. Arai, Hydrothermal synthesismetal oxide nanoparticles at supercritical conditions, J. NanopaRes. 3 (2001) 227.

[65] Y. Hakuta, S. Onai, S. Terayama, T. Adschiri, K. Arai, Productioultra-fine ceria particles by hydrothermal synthesis under supercconditions, J. Mater. Sci. Lett. 17 (1998) 1211.

[66] Y. Hakuta, K. Seino, H. Ura, T. Adschiri, H. Takizawa, K. Arai, Pduction of phosphor (YAG:Tb) fine particles by hydrothermal synthin supercritical water, J. Mater. Chem. 9 (1999) 2671.

[67] Y. Hakuta, T. Haganuma, K. Sue, T. Adschiri, K. Arai, Continuousduction of phosphor YAG:Tb nanoparticles by hydrothermal synthin supercritical water, Mater. Res. Bull. 38 (2003) 1257.

[68] Y. Hakuta, H. Hayashi, K. Arai, Hydrothermal synthesis of photoalyst potassium hexatitanate nanowires under supercritical condJ. Mater. Sci. 39 (2004) 4977.

e

,

,

[78] P.S. Shah, J.D. Holmes, R.C. Doty, K.P. Johnston, B.A. Korgel, Sstabilization of nanocrystals in supercritical CO2 using fluorinated ligands, J. Am. Chem. Soc. 122 (2000) 4245.

[79] P.S. Shah, S. Husain, K.P. Johnston, B.A. Korgel, Nanocrystal arprecipitation in supercritical carbon dioxide, J. Phys. Chem. B(2001) 9433.

[80] P.S. Shah, S. Husain, K.P. Johnston, B.A. Korgel, Role of stericbilization on the arrested growth of silver nanocrystals in supercrcarbon dioxide, J. Phys. Chem. B 106 (2002) 12178.

[81] B.A. Korgel, P.S. Shah, L.E. Pell, K.P. Johnston, Nanocrystal slization and synthesis in supercritical solvents, in: A. Bertucco (EProceedings Fourth International Symposium on High Pressurenology and Chemical Engineering, vol. 2, Venice, Italy, 2002633.

[82] A. Kameo, T. Yoshimura, K. Esumi, Preparation of noble mnanoparticles in supercritical carbon dioxide, Colloids Surf. A(2003) 181.

[83] M.C. McLeod, W.F. Gale, C.B. Roberts, Metallic nanoparticle protion utilizing a supercritical carbon dioxide flow process, Langmui(2004) 7078.

[84] K.J. Ziegler, R.C. Doty, K.P. Johnston, B.A. Korgel, Synthesisorganic monolayer-stabilized copper nanocrystals in supercritical wJ. Am. Chem. Soc. 123 (2001) 7797.

[85] E. Reverchon, G. Caputo, S. Correra, P. Cesti, Synthesis of titahydroxide nanoparticles in supercritical carbon dioxide on thescale, J. Supercrit. Fluids 26 (2003) 253.

[86] W.E. Stallings, H.H. Lamb, Synthesis of nanostructured titania powvia hydrolysis of titanium isopropoxide in supercritical carbon dioxLangmuir 19 (2003) 2989.

[87] F. Cansell, B. Chevalier, A. Demourgues, J. Etourneau, C. EvePessey, S. Petit, A. Tressaud, F. Weill, Supercritical fluid procing: a new route for materials synthesis, J. Mater. Chem. 9 (167.

[88] S. Desmoulins-Krawiec, C. Aymonier, A. Loppinet-Serani, F. WeilGorsse, J. Etourneau, F. Cansell, Synthesis of nanostructured main supercritical ammonia: nitrides, metals and oxides, J. Mater. C14 (2004) 228.

20 E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22

[89] J.D. Holmes, K.J. Ziegler, R.C. Doty, L.E. Pell, K.P. Johnston, B.A.Korgel, Highly luminescent silicon nanocrystals with discrete opticaltransitions, J. Am. Chem. Soc. 123 (2001) 3743.

[90] J.M. DeSimone, E.E. Maury, Y.Z. Menceloglu, J.B. McClain, T.J.Romack, J.R. Combes, Dispersion polymerizations in supercritical car-bon dioxide, Science 265 (1994) 356.

[91] D.A. Canelas, D.E. Betts, J.M. DeSimone, Dispersion polymerizationof styrene in supercritical carbon dioxide: importance of effective sur-factants, Macromolecules 29 (1996) 2818.

[92] G. Filardo, G. Caputo, A. Galia, E. Calderaro, G. Spadaro, Polymeriza-tion of methyl methacrylate through ionizing radiation in CO2-baseddense systems, Macromolecules 33 (2000) 278.

[93] J.-Y. Lee, C.-H. Song, J.-H. Kim, Preparation and characterization ofaromatic polycarbonate nanospheres via high pressure process technol-ogy, in: A. Bertucco (Ed.), Proceedings Fourth International Sympo-sium on High Pressure Technology and Chemical Engineering, vol. 2,Venice, Italy, 2002, p. 627.

[94] J.-Y. Lee, C.-H. Song, J.-I. Kim, J.-H. Kim, Preparation of aro-matic polycarbonate nanoparticles using supercritical carbon dioxide,J. Nanoparticle Res. 4 (2002) 53.

[95] K.P. Johnston, G.B. Jacobson, C.T. Lee, C. Meredith, S.R.P. Da Rocha,Z.Y. Matt, J. DeGrazia, Microemulsions, emulsions and latexes, in:P.G. Jessop, W. Leitner (Eds.), Chemical Synthesis Using SupercriticalFluids, Wiley-VCH, Weinheim, 1999, p. 127.

[96] J. Zhang, B. Han, J. Liu, X. Zhang, J. He, Z. Liu, T. Jiang, G. Yang,Recovery of silver nanoparticles synthesized in AOT/C12E4 mixedreverse micelles by antisolvent CO2, Chem. Eur. J. 8 (2002) 3879.

[97] J. Zhang, B. Han, J. Liu, X. Zhang, Z. Liu, J. He, A new methodto recover the nanoparticles from reverse micelles: recovery of ZnSnanoparticles synthesized in reverse micelles by compressed CO2,Chem. Commun. (2001) 2724.

ringred b

verylvent

fhem.

is of-

sil-mul-

lverater-

opar-em.

in-ZnS

with,

. Ju,ul-

003)

ston,,

iO

[110] J.P. Cason, C.B. Roberts, Metallic copper nanoparticle synthesis inAOT reverse micelles in compressed propane and supercritical ethanesolutions, J. Phys. Chem. B 104 (2000) 1217.

[111] J.P. Cason, K. Khambaswadkar, C.B. Roberts, Supercritical fluid andcompressed solvent effects on metallic nanoparticle synthesis in reversemicelles, Ind. Eng. Chem. Res. 39 (2000) 4749.

[112] P.S. Shah, J.D. Holmes, K.P. Johnston, B.A. Korgel, Size-selective dis-persion of dodecanethiol-coated nanocrystals in liquid and supercriticalethane by density tuning, J. Phys. Chem. B 106 (2002) 2545.

[113] M.C. McLeod, R.S. McHenry, E.J. Beckman, C.B. Roberts, Synthesisand stabilization of silver metallic nanoparticles and premetallic inter-mediates in perfluoropolyether/CO2 reverse micelle systems, J. Phys.Chem. B 107 (2003) 2693.

[114] Y. Wu, P. Yang, Direct observation of vapor–liquid–solid nanowiregrowth, J. Am. Chem. Soc. 123 (2001) 3165.

[115] J.D. Holmes, K.P. Johnston, R.C. Doty, B.A. Korgel, Control of thick-ness and orientation of solution-grown silicon nanowires, Science 287(2000) 1471.

[116] T. Hanrath, B.A. Korgel, Nucleation and growth of germaniumnanowires seeded by organic monolayer-coated gold nanocrystals, J.Am. Chem. Soc. 124 (2002) 1424.

[117] J.D. Holmes, D.M. Lyons, K.J. Ziegler, Supercritical fluid synthesis ofmetal and semiconductor nanomaterials, Chem. Eur. J. 9 (2003) 2144.

[118] N.R.B. Coleman, M.A. Morris, T.R. Spalding, J.D. Holmes, The for-mation of dimensionally ordered silicon nanowires within mesoporoussilica, J. Am. Chem. Soc. 123 (2001) 187.

[119] T. Hanrath, B.A. Korgel, Supercritical fluid-liquid-solid (SFLS) syn-thesis of Si and Ge nanowires seeded by colloidal metal nanocrystals,Adv. Mater. 15 (2003) 437.

[120] P.S. Shah, T. Hanrath, K.P. Johnston, B.A. Korgel, Nanocrystal andnanowire synthesis and dispersibility in supercritical fluids, J. Phys.

stalicles

or-ires

A.char-ithin

oly-Ind.

i, Y.cles.lveranners

upta,ed.

using

aving

r2000)

acti-)

ima,g pro-

[98] J. Zhang, B. Han, J. Liu, X. Zhang, G. Yang, H. Zhao, Size tailoof ZnS nanoparticles synthesized in reverse micelles and recovecompressed CO2, J. Supercrit. Fluids 30 (2004) 89.

[99] D. Liu, J. Zhang, B. Han, J. Chen, Z. Li, D. Shen, G. Yang, Recoof TiO2 nanoparticles synthesized in reverse micelles by antisoCO2, Colloids Surf. A 227 (2003) 45.

[100] P. Chattopadhyay, R.B. Gupta, Supercritical CO2-based formation osilica nanoparticles using water-in-oil microemulsions, Ind. Eng. CRes. 42 (2003) 465.

[101] J.D. Holmes, P.A. Bhargava, B.A. Korgel, K.P. Johnston, Synthescadmium sulfide Q particles in water-in-CO2 microemulsions, Langmuir 15 (1999) 6613.

[102] M. Ji, X. Chen, C.M. Wai, J.L. Fulton, Synthesizing and dispersingver nanoparticles in a water-in-supercritical carbon dioxide microesion, J. Am. Chem. Soc. 121 (1999) 2631.

[103] H. Ohde, X.-R. Ye, C.M. Wai, J.M. Rodriguez, Synthesizing sihalide nanoparticles in supercritical carbon dioxide utilizing a win-CO2 microemulsion, Chem. Commun. (2000) 2353.

[104] H. Ohde, F. Hunt, C.M. Wai, Synthesis of silver and copper nanticles in a water-in-supercritical-carbon dioxide microemulsion, ChMater. 13 (2001) 4130.

[105] H. Ohde, M. Ohde, F. Bailey, H. Kim, C.M. Wai, Water-CO2 microemulsions as nanoreactors for synthesizing CdS andnanoparticles in supercritical CO2, Nano Lett. 2 (2002) 721.

[106] M. Ohde, H. Ohde, C.M. Wai, Catalytic hydrogenation of arenesrhodium nanoparticles in a water-in-supercritical CO2 microemulsionChem. Commun. (2002) 2388.

[107] S.-S. Hong, M.S. Lee, H.-S. Hwang, K.-T. Lim, S.S. Park, C.-SG.-D. Lee, Preparation of titanium dioxides in the w/c microemsions and their photocatalytic activity, Sol. Energy Mater. 80 (2273.

[108] K.T. Lim, H.S. Hwang, M.S. Lee, G.D. Lee, S.-S. Hong, K.P. JohnFormation of TiO2 nanoparticles in water-in-CO2 microemulsionsChem. Commun. (2002) 1528.

[109] K.T. Lim, H.S. Hwang, W. Ryoo, K.P. Johnston, Synthesis of T2nanoparticles utilizing hydrated reverse micelles in CO2, Langmuir 20(2004) 2466.

yChem. B 108 (2004) 9574.

[121] X. Lu, T. Hanrath, K.P. Johnston, B.A. Korgel, Growth of single-crysilicon nanowires in supercritical solution from tethered gold parton a silicon substrate, Nano Lett. 3 (2003) 93.

[122] N.R.B. Coleman, K.M. Ryan, T.R. Spalding, J.D. Holmes, M.A. Mris, The formation of dimensionally ordered germanium nanowwithin mesoporous silica, Chem. Phys. Lett. 343 (2001) 1.

[123] N.R.B. Coleman, N. O’Sullivan, K.M. Ryan, T.A. Crowley, M.Morris, T.R. Spalding, D.C. Steytler, J.D. Holmes, Synthesis andacterization of dimensionally ordered semiconductor nanowires wmesoporous silica, J. Am. Chem. Soc. 123 (2001) 7010.

[124] M.J. Meziani, P. Pathak, W. Wang, T. Desai, A. Patil, Y.-P. Sun, Pmeric nanofibers from rapid expansion of supercritical solution,Eng. Chem. Res. 44 (2005) 4594.

[125] S. Ohara, T. Mousavand, M. Umetsu, S. Takami, T. AdschirKuroki, M. Takata, Hydrothermal synthesis of fine zinc oxide partiunder supercritical conditions, Solid State Ionics 172 (2004) 261

[126] J.-Y. Chang, J.-J. Chang, B. Lo, S.-H. Tzing, Y.-C. Ling, Sinanoparticles spontaneous organize into nanowires and nanobin supercritical water, Chem. Phys. Lett. 379 (2003) 261.

[127] J.F. Louvier-Hernandez, G. Luna-Barcenas, R. Thakur, R.B. GFormation of Chitin nanofibers by supercritical antisolvent, J. BiomNanotech. 1 (2005) 1.

[128] E. Reverchon, unpublished results.[129] H. Wakayama, Y. Fukushima, Porous platinum fibers synthesized

supercritical fluid, Chem. Commun. (1999) 391.[130] H. Wakayama, S. Inagaki, Y. Fukushima, Porous metal structure h

high specific surface area, JP A2 2000119770 (20000425).[131] H. Wakayama, Y. Fukushima, Supercritical CO2 as a solvent fo

synthesis of nanoporous materials, Ind. Eng. Chem. Res. 39 (4641.

[132] H. Wakayama, Y. Fukushima, Nanoporous silica prepared withvated carbon molds using supercritical CO2, Chem. Mater. 12 (2000756.

[133] H. Wakayama, H. Itahara, N. Tatsuda, S. Inagaki, Y. FukushNanoporous metal oxides synthesized by the nanoscale castincess using supercritical fluids, Chem. Mater. 13 (2001) 2392.

E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22 21

[134] H. Wakayama, S. Inagaki, Y. Fukushima, Nanoporous titania synthe-sized by a nanoscale casting process in supercritical carbon dioxide,J. Am. Ceram. Soc. 85 (2002) 161.

[135] H. Wakayama, Y. Fukushima, Nanoporous materials synthesized usingsupercritical fluid solvents, Recent Res. Dev. Phys. Chem. 6 (2002)241.

[136] H. Wakayama, Y. Goto, Y. Fukushima, A novel method for tailoringporous structures of nanoporous materials using supercritical solvents,Phys. Chem. Chem. Phys. 5 (2003) 3784.

[137] H. Wakayama, T. Hatanaka, Y. Fukushima, Synthesis of Pt-Runanoporous fibers by the nanoscale casting process using supercriti-cal CO2 for electrocatalytic applications, Chem. Lett. 33 (2004) 658.

[138] Y. Fukushima, H. Wakayama, Nanoscale casting using supercriticalfluid, J. Phys. Chem. B 103 (1999) 3062.

[139] J.M. Calderon-Moreno, S.S. Swamy, T. Fujino, M. Yoshimura, Carbonnanocells and nanotubes grown in hydrothermal fluids, Chem. Phys.Lett. 329 (2000) 317.

[140] J.M. Calderon-Moreno, M. Yoshimura, Hydrothermal processing ofhigh-quality multiwall nanotubes from amorphous carbon, J. Am.Chem. Soc. 123 (2001) 741.

[141] Y. Gogotsi, J.A. Libera, M. Yoshimura, Hydrothermal synthesis ofmultiwall carbon nanotubes, J. Mater. Res. 15 (2000) 2591.

[142] M. Motiei, Y.R. Hacohen, J. Calderon-Moreno, A. Gedanken, Prepar-ing carbon nanotubes and nested fullerenes from supercritical CO(2)by a chemical reaction, J. Am. Chem. Soc. 123 (2001) 8624.

[143] J.-Y. Chang, A. Ghule, J.-J. Chang, S.-H. Tzing, Y.-C. Ling, Open-ing and thinning of multiwall carbon nanotubes in supercritical water,Chem. Phys. Lett. 363 (2002) 583.

[144] D.C. Lee, F.V. Mikulec, B.A. Korgel, Carbon nanotube synthesis insupercritical toluene, J. Am. Chem. Soc. 126 (2004) 4951.

[145] J. Zhang, Z. Liu, B. Han, Z. Li, D. Sun, J. Chen, Ultrasound-inducedCO

ng

ritical

itionical

criti-992)

ilityint,

ki,ew

from017

ung,romelec-

erial

fluidub-O A2

ition:tion,

nfor-em.

[158] J.M. Blackburn, D.P. Long, A. Cabanas, J.J. Watkins, Deposition ofconformal copper and nickel films from supercritical carbon dioxide,Science 294 (2001) 141.

[159] N.E. Fernandes, S.M. Fisher, J.C. Poshusta, D.G. Vlachos, M. Tsapat-sis, J.J. Watkins, Reactive deposition of metal thin films within poroussupports from supercritical fluids, Chem. Mater. 13 (2001) 2023.

[160] A. Cabanas, J.M. Blackburn, J.J. Watkins, Deposition of Cu films fromsupercritical fluids using Cu(I) b-diketonate precursors, Microelectron.Eng. 64 (2002) 53.

[161] A. Cabanas, X. Shan, J.J. Watkins, Alcohol-assisted deposition of cop-per films from supercritical carbon dioxide, Chem. Mater. 15 (2003)2910.

[162] D.P. Long, J.M. Blackburn, J.J. Watkins, Chemical fluid deposition:a hybrid technique for low-temperature metallization, Adv. Mater. 12(2000) 913.

[163] A. Cabanas, D.P. Long, J.J. Watkins, Deposition of gold films andnanostructures from supercritical carbon dioxide, Chem. Mater. 16(2004) 2028.

[164] E.T. Hunde, J.J. Watkins, Reactive Deposition of Cobalt and NickelFilms from Their Metallocenes in Supercritical Carbon Dioxide Solu-tion, Chem. Mater. 16 (2004) 498.

[165] K. Morita, T. Ohtsuka, M. Ueda, Organometallic deposition of dielec-tric layers using supercritical fluids such as CO2 in semiconductordevice fabrication, EP A2 1024524 (20000802).

[166] C.M. Wai, H. Ohde, S. Kramer, Method of forming metal-containingfilms over surfaces of semiconductor substrates, US A1 2003183938(20031002).

[167] X.R. Ye, C.M. Wai, D. Zhang, Y. Kranov, D.N. McIlroy, Y. Lin, M.Engelhard, Immersion deposition of metal films on silicon and ger-manium substrates in supercritical carbon dioxide, Chem. Mater. 15(2003) 83.

icaln of) 25.ition

n2004)

, S.

fac-

, S.per-086.ao,

d oftingolid

em-999)

orteds. 64

heLett.

, andfluid9) 7.tedque-

rapidAm.

formation of polymer capsules by precipitation with compressed2,Eur. Polym. J. 40 (2004) 1349.

[146] A.K.S. Murthy, A.Y. Bekker, K.M. Patel, Deposition of thin films usisupercritical fluids, USA 4737384 (19880412).

[147] R.E. Sievers, B.N. Hansen, Chemical deposition using supercsolution, US A 4970093 (19901113).

[148] B.M. Hybertson, B.N. Hansen, R.M. Barkley, R.E. Sievers, Deposof palladium films by a novel supercritical fluid transport-chemdeposition process, Mater. Res. Bull. 26 (1991) 1127.

[149] B.N. Hansen, B.M. Hybertson, R.M. Barkley, R.E. Sievers, Supercal fluid transport-chemical deposition of films, Chem. Mater. 4 (1749.

[150] O.A. Louchev, V.K. Popov, E.N. Antonov, The morphological stabin supercritical fluid chemical deposition of films near the critical poJ. Cryst. Growth 155 (1995) 276.

[151] V.K. Popov, V.N. Bagratashvili, E.N. Antonov, D.A. LemenovsSupercritical fluid chemical deposition of thin InP films. A napproach and precursors, Thin Solid Films 279 (1996) 66.

[152] J.L. Fulton, G. Deverman, Electrostatic deposition of particlesrapid expansion of supercritical fluid solutions, US A1 2003222(20031204).

[153] J.L. Fulton, G.S. Deverman, C.R. Yonker, J.W. Grate, J. De YoJ.B. McClain, Thin fluoropolymer films and nanoparticle coatings fthe rapid expansion of supercritical carbon dioxide solutions withtrostatic collection, Polymer 44 (2003) 3627.

[154] J.J. Watkins, T.J. Mccarthy, Method of chemically depositing matonto a substrate, US A 5789027 (19980804).

[155] J.J. Watkins, J.M. Blackburn, D.P. Long, J.L. Lazorcik, Chemical-deposition of metal or alloy films on the optionally patterned sstrates, typically as semiconductor wafers or porous ceramics, W2001032951 (20010510).

[156] J.J. Watkins, J.M. Blackburn, T.J. McCarthy, Chemical fluid deposreactive deposition of platinum metal from carbon dioxide soluChem. Mater. 11 (1999) 213.

[157] J.M. Blackburn, D.P. Long, J.J. Watkins, Reactive deposition of comal palladium films from supercritical carbon dioxide solution, ChMater. 12 (2000) 2625.

[168] X.R. Ye, C.M. Wai, Y. Lin, J.S. Young, M.H. Engelhard, Supercritfluid immersion deposition: a new process for selective depositiometal films on silicon substrates, Surf. Coat. Technol. 190 (2005

[169] H. Ohde, S. Kramer, S. Moore, C.M. Wai, Low-temperature deposof conformal copper films in supercritical CO2 by catalytic hydrogereduction of copper hexafluoroacetylacetonate, Chem. Mater. 16 (4028.

[170] H. Yan, M. Sone, A. Mizushima, T. Nagai, K. Abe, S. IchiharaMiyata, Electroplating in CO2-in-water and water-in-CO2 emulsionsusing a nickel electroplating solution with anionic fluorinated surtant, Surf. Coat. Technol. 187 (2004) 86.

[171] H. Yoshida, M. Sone, A. Mizushima, K. Abe, X.T. Tao, S. IchiharaMiyata, Electroplating of nanostructured nickel in emulsion of sucritical carbon dioxide in electrolyte solution, Chem. Lett. (2002) 1

[172] H. Yoshida, M. Sone, H. Wakabayashi, H. Yan, K. Abe, X.T. TA. Mizushima, S. Ichihara, S. Miyata, New electroplating methonickel in emulsion of supercritical carbon dioxide and electroplasolution to enhance uniformity and hardness of plated film, Thin SFilms 446 (2004) 194.

[173] J. Brasseur-Tilmant, K. Chhor, P. Jestin, C. Pommier, Ceramic mbrane elaboration using supercritical fluid, Mater. Res. Bull. 34 (12013.

[174] J. Brasseur-Tilmant, C. Pommier, K. Chhor, Synthesis of suppTiO2 membranes using supercritical alcohol, Mater. Chem. Phy(2000) 156.

[175] D.M. Lyons, K.M. Ryan, M.A. Morris, J.D. Holmes, Tailoring toptical properties of silicon nanowire arrays through strain, Nano2 (2002) 811.

[176] Y.-P. Sun, H.W. Rollins, R. Guduru, Preparations of nickel, cobaltiron nanoparticles through the rapid expansion of supercriticalsolutions (RESS) and chemical reduction, Chem. Mater. 11 (199

[177] M.J. Meziani, H.W. Rollins, L.F. Allard, Y.-P. Sun, Protein-protecnanoparticles from rapid expansion of supercritical solution into aous solution, J. Phys. Chem. B 106 (2002) 11178.

[178] M.J. Meziani, Y.-P. Sun, Protein-conjugated nanoparticles fromexpansion of supercritical fluid solution into aqueous solution, J.Chem. Soc. 125 (2003) 8015.

22 E. Reverchon, R. Adami / J. of Supercritical Fluids 37 (2006) 1–22

[179] Y. Wang, R.N. Dave, R. Pfeffer, Polymer coating/encapsulation ofnanoparticles using a supercritical anti-solvent process, J. Supercrit.Fluids 28 (2004) 85.

[180] S.-D. Yeo, E. Kiran, Formation of polymer particles with supercriticalfluids: a review, J. Supercrit. Fluids 34 (2005) 287.

[181] K. Sue, N. Kakinuma, T. Adschiri, K. Arai, Continuous production ofnickel fine particles by hydrogen reduction in near-critical water, Ind.Eng. Chem. Res. 43 (2004) 2073.

[182] R. Viswanathan, G.D. Lilly, W.F. Gale, R.B. Gupta, Formation of zincoxide-titanium dioxide composite nanoparticles in supercritical water,Ind. Eng. Chem. Res. 42 (2003) 5535.

[183] J. Zhang, M. Xiao, Z. Liu, B. Han, T. Jiang, J. He, G. Yang, Preparationof ZnS/CdS composite nanoparticles by coprecipitation from reversemicelles using CO2 as antisolvent, J. Colloid Interface Sci. 273 (2004)160.

[184] J. Zhang, Z. Liu, B. Han, D. Liu, J. Chen, J. He, T. Jiang, A novelmethod to synthesize polystyrene nanospheres immobilized with sil-ver nanoparticles by using compressed CO2, Chem. Eur. J. 10 (2004)3531.

[185] Z. Li, J. Zhang, J. Du, T. Mu, Z. Liu, J. Chen, B. Han, Preparationof cadmium sulfide/poly(methyl methacrylate) composites by precipi-tation with compressed CO2, J. Appl. Polym. Sci. 94 (2004) 1643.

[186] Z. Li, J. Zhang, T. Mu, J. Du, Z. Liu, B. Han, J. Chen, Prepara-tion of polyvinylpyrrolidone-protected Prussian blue nanocompositesin microemulsion, Colloids Surf. A 243 (2004) 63.

[187] J. Zhang, Z. Liu, B. Han, T. Jiang, W. Wu, J. Chen, Z. Li, D. Liu,Preparation of polystyrene-encapsulated silver nanorods and nanofibersby combination of reverse micelles, gas antisolvent, and ultrasoundtechniques, J. Phys. Chem. B 108 (2004) 2200.

[188] Z. Liu, X. Dai, J. Xu, B. Han, J. Zhang, Y. Wang, Y. Huang, G. Yang,Encapsulation of polystyrene within carbon nanotubes with the aid ofsupercritical CO2, Carbon 42 (2004) 458.

[189] X.-R. Ye, Y. Lin, C. Wang, C.M. Wai, Supercritical fluid fabricationof metal nanowires and nanorods templated by multiwalled carbonnanotubes, Adv. Mater. 15 (2003) 316.

[190] X.R. Ye, Y. Lin, C.M. Wai, Decorating catalytic palladium nanopar-ticles on carbon nanotubes in supercritical carbon dioxide, Chem.Commun. (2003) 642.

[191] X.-R. Ye, Y. Lin, C. Wang, M.H. Engelhard, Y. Wang, C.M. Wai,Supercritical fluid synthesis and characterization of catalytic metalnanoparticles on carbon nanotubes, J. Mater. Chem. 14 (2004) 908.

[192] X.-R. Ye, H.-F. Zhang, Y. Lin, L.-S. Wang, C.M. Wai, Modificationof SiO2 nanowires with metallic nanocrystals from supercritical CO2,J. Nanosci. Nanotechnol. 4 (2004) 82.