Methods for Synthesis of Nanoparticles

8/11/2019 Methods for Synthesis of Nanoparticles

1/20

c 2007 Institute of Chemistry, Slovak Academy of Sciences

DOI: 10.2478/s11696-007-0014-7

REVIEW

A Review of Methods for Synthesis of Nanostructured Metals

with Emphasis on Iron Compounds

A. TAVAKOLI, M. SOHRABI*, and A. KARGARI

Department of Chemical Engineering, Amirkabir University of Technology, Tehran 15914, Iran

e-mail: [email protected]

Received 28 June 2006; Revised 24 December 2006; Accepted 4 January 2007

Synthesis of metal nanoparticles with specific properties is a newly established research areaattracting a great deal of attention. Several methods have been put forward for synthesis of thesematerials, namely chemical vapor condensation, arc discharge, hydrogen plasmametal reaction,and laser pyrolysis in the vapor phase, microemulsion, hydrothermal, sol-gel, sonochemical, andmicrobial processes taking place in the liquid phase, and ball milling carried out in the solid phase.

The properties of metal nanoparticles depend largely on their synthesis procedures. In this paperthe fundamentals, advantages, and disadvantages of each synthesis method are discussed.

Keywords: synthesis, metal nanoparticles, iron compounds, biomaterials, magnetite, microemul-sions

INTRODUCTION

Nanotechnology manipulates matter at the scaleof one billionth of a meter (109 m). It is more of anapproach to engineering than a science, although itdraws on the scientific knowledge of biology, physics,chemistry, and materials science and is expected tochange these sciences dramatically. Eric Drexler intro-duced the term nanotechnology in Engines of Cre-ation(1986) to describe the manipulation of individ-ual atoms and molecules to build structures to com-plex, atomic specifications and stated that perhapsthe arrival of the concept of nanotechnology cameabout in physicist Richard Feynmans landmark 1959lecture called Theres Plenty of Room at the Bottom[1].

The interest in nanoscale materials stems fromthe fact that new properties are acquired at thislength scale and, equally important, that these prop-erties change with size or shape of nanomaterials [2].Nanoparticles show peculiar optical [3], magnetic [4],and electronic [5] properties that bulk solid or iso-lated molecules do not usually exhibit [6, 7], which

may find important applications in material technolo-

gies like microelectronics, catalytic systems, hydrogenstorage, ferrofluids, and chemical nanosensors [8].

There are two fundamental approaches to fabricat-ing nanomaterials. The bottom-up approach repre-sents the concept of constructing a nanomaterial frombasic building blocks, such as atoms or molecules.This approach illustrates the possibility of creatingexact materials that are designed to have exactly thedesired properties. The second approach, the top-down method, involves restructuring a bulk mate-rial in order to create a nanomaterial [9]. Top-downmethod is usually not very well suited to preparinginformally shaped particles; very small sizes are es-pecially difficult to realize. Bottom-up procedures aremuch better suited to generate uniform particles, of-ten of distinct size, shape, and structure [10].

Preparation of nanomaterials can be classified intophysical and chemical methods. The physical meth-ods are based on subdivision of bulk metals, includingmechanical crushing or pulverization of bulk metal,arc discharge between metal electrodes, etc. Metalnanoparticles thus produced are usually large in sizeand have wide size distribution [11]. Several physi-

cal methods have been reported for the synthesis of

*The author to whom the correspondence should be addressed.

Chem. Pap. 61 (3)151170 (2007) 151


2/20

A. TAVAKOLI, M. SOHRABI, A. KARGARI

Table 1. Classification of Available Methods of Nanoparticles Synthesis

Phase Method

Vapor Chemical vapor condensation, arc discharge, hydrogen plasma, laser pyrolysisLiquid Microemulsion, hydrothermal, sol-gel, sonochemical, microbial

Solid Ball milling

nanosized particles. These include vapor condensa-tion methods, spray pyrolysis, mechanical deforma-tion, thermochemical decomposition of metal-organicprecursors in flame reactor, and other aerosol pro-cesses named after the energy sources applied to pro-vide the high temperature during gas-particle conver-sion [10, 12]. The chemical methods are based on thereduction of metal ions or decomposition of precursorsto form atoms, followed by aggregation of the atoms.

Nanoparticles prepared by chemical methods usuallyhave a narrow size distribution [11]. Increasing inter-est in chemical synthesis of nanoparticles is clearlyindicated by the number of reports and reviews onthis subject [2, 1317]. However, it is notable thatsome methods can be considered as either chemical orphysical routes depending on the media, precursors,and operating conditions such as milling.

The formation of metal nanoparticles by chemicalmethods can be carried out by reduction of metal ionswith chemical reductants or decomposition of metalprecursors with extera energy. The chemical reduc-tants involve molecular hydrogen, alcohol, hydrazine,

NaBH4, LiAlH4, citrate, etc. Energy provided fromthe outside involves photoenergy (ultraviolet and vis-ible light), -ray, electricity, thermal energy (heat),sonochemical energy, etc. In order to produce metalnanoparticles with a narrow size distribution, agentsstabilizing colloidal dispersion of metal nanoparticlesare of vital importance [11].

Mechanochemical synthesis methods involving so-lid-state chemistry reactions have also been investi-gated as an alternative chemical route providing nano-materials. Mechanochemical synthesis involves me-chanical activation of solid-state displacement reac-

tions. It involves the milling of precursor powders toform nanomaterials [10].Various methods of the nanoparticles synthesis can

be classified based on the process media, including va-por, liquid, and solid state processing routes, and com-bined method, such as vaporsolidliquid approach[15]. Table 1 indicates different methods, which willbe considered in this review according to this classifi-cation.

Many of properties associated with nanoparticlesare directly related to the relatively higher energeticstate of atoms and molecules at a surface when com-pared with those in the bulk. In many cases the pro-

duction of nanoparticles involves techniques to hinderthe natural course of thermodynamics through manip-ulation of kinetics. In other cases it is possible to hin-

der the natural growth of phases through the use of di-lution orviaprotection of surfaces using surface-activeagents or by coating and encapsulation of nanoparti-cles in a glassy media such as those used for instancein the case of polymers [18].

The change in the properties at this length scale isnot a result of scaling factors. It results from differentcauses in different materials. As noble metals are re-duced in size to tens of nanometers, a new very strong

absorption is observed resulting from the collective os-cillation of the electrons in the conduction band fromone surface of the particle to the other. In transitionmetal nanoparticles, the decrease in the particle sizeto the nanometer length scale increases the surface-to-volume ratio. This property, together with the presentability to produce nanoparticles in different sizes andshapes, makes the latter potentially useful in the fieldof catalysis [2]. As an example, iron is used as a suit-able catalyst in the FischerTropsch process [19, 20].Recent studies have demonstrated that iron nanopar-ticles may also be applied as a more effective catalystfor this reaction [21, 22].

The synthesis of magnetic nanoparticles has beenan area of study for a long time because of the inter-esting practical applications of such particles in mag-netic recording, magnetic fluids, permanent magnets,etc.[23]. To synthesize such particles several methodsare used. The nanoparticles formed using each methodshow specific properties. The objective of this reviewis to present the recent results on synthesis of metalnanoparticles by different processes and to comparethe latter with other methods. In the first step thefundamentals of each method are discussed and thenthe procedures for the synthesis of metal nanoparticles

are described. The application of iron nanoparticles asa FischerTropsch catalyst is being considered in or-der to determine a correlation between the operatingconditions and catalysts particle size [24].

VAPOR PROCESSING METHODS

Chemical Vapor Condensation (CVC)

In order to fabricate nanoparticles, the vaporiza-tion method has been frequently used, in which thetarget materials are vaporized by heat source and thenrapidly condensed. The vaporization process can be

subdivided into physical and chemical methods de-pending on whether the reaction is present [25]. Ifthe resultant nanoparticles have the same composition

152 Chem. Pap. 61 (3)151170 (2007)


3/20

METHODS FOR SYNTHESIS OF METAL NANOPARTICLES

Fig. 1. A schematic drawing of the chemical vapor condensation (CVC) process (reprinted from Ref. [27] with permission fromElsevier).

with the target materials, they are prepared by physi-cal vapor condensation (PVC) [26]. However, nanopar-ticles having a different composition with the targetare usually obtained by chemical vapor condensation(CVC), because the chemical reaction occurs betweenthe vapor and other system components during thevaporization and condensation. The CVC process has

a merit in selecting composition, whereas the PVCin purity [27]. Chemical vapor condensation (CVC)method has been developed for preparation of manykinds of nanoparticles with a narrow size distribu-tion. Their unique properties and the improved perfor-mances are determined by their particle sizes, surfacestructures, and interparticle interactions [28].

Of particular relevance to this technique is the pre-vious work on the synthesis of nanopowders by (i)thermal decomposition of metal-organic precursors us-ing a focused laser beam, combustion flame, or plasmatorch as heat source, and (ii) evaporation and conden-

sation of volatile species in a reduced pressure environ-ment such as in inert gas condensation (IGC) synthesismethod [27]. The laser process is capable of producinga variety of monodispersed and loosely agglomeratednanopowders on the laboratory scale, but it is not suit-able for the industrial-scale production of such pow-ders. The flame process has been applied successfullyto the production of commercial quantities of carbonblack, TiO2, and SiO2. The plasma process has beenused to produce experimental quantities of nonoxideceramics. However, a feature of the synthesis methodis the highly agglomerated state of the so synthesizednanopowders.

In 1985, a potential solution to the nanoparticleagglomeration problem came with the introduction ofthe IGC synthesis method. Experimental quantities of

high-purity nonagglomerated nanopowders of variousmaterials were synthesized by the IGC process. How-ever, numerous other useful ceramics and low vaporpressure materials cannot be easily produced by thismethod and, therefore, another approach is needed.

In a conventional inert gas condensation system,the vapor source is used to generate ultrafine par-

ticles and these are convectively transported to andcollected on a cold substrate. In the CVC technique,such a system is adapted for the purpose of synthesiz-ing nanopowders from metal-organic precursors. Es-sentially, the vapor source is replaced with a heatedtubular reactor used to decompose the precursor toform a continuous stream of clusters or nanoparticlesentrained in a carrier gas [27]. Compared with IGC,which has a limitation of scaling up and evaporationproblem of low vapor pressure materials, CVC pro-cess was developed for preparation of almost all kindsof materials [29]. Moreover, chemical vapor condensa-

tion can produce a large amount of nanoparticles in anonagglomerated state [27].CVC process proceeds essentially in two separate

segments (Fig. 1): a reaction chamber that is main-tained at vacuum and a precursor delivery system op-erating at ambient pressure. The segments are con-nected via a needle valve, which continuously mon-itors and controls the flow rate of precursor/carriergas stream from the gas delivery system into the re-action chamber. During the short residence time ofprecursor in the heated tube, individual moleculesstart to decompose and combine to form small clus-ters or nanoparticles. At the outlet of the furnace

tube, rapid expansion of the two-phase gas stream (gas+ nanoparticles) serves to mitigate particle growthand agglomeration. Finally, the nanoparticles con-

Chem. Pap. 61 (3)151170 (2007) 153


4/20


dense out on a rotating liquid nitrogen cooled sub-strate from which the particles can be scraped off andcollected. Heat treatment of the synthesized nanopow-ders in various high-purity gas streams causes compo-sitional and structural modifications, including parti-cle purification and crystallization, as well as transfor-mation to a desirable size, composition, and morphol-ogy [27].

Critical to the success of this method are: 1. lowconcentration of precursor in the carrier gas whichminimizes the collision frequency between the clustersformed during the short residence time (less than 0.1s) in the tubular reactor, 2. rapid expansion of the gasstream through a uniformly heated tubular reactor,and 3. rapid quenching of the gas phase nucleated clus-ters or nanoparticles as they leave the reactor tube.

Other attractive features of the process are the use

of a high-vacuumed chamber for the synthesis andcollection of nanoparticles and a rotating substrateof large diameter relative to the size of the reactortube [27, 30]. Thus, high efficiency of collection of thenanoparticles and a constant quench rate are secured.It should be emphasized that a scraper on the backside of the rotating substrate ensures continuous re-moval of deposited particles, thereby providing a cleanmetallic surface for continuous deposition of particlesat a constant quench rate [27, 30].

Since the properties of nanoparticles are basicallydetermined by their mean size, size distribution, ex-ternal shape, internal structure, chemical composition,

and the other characteristics of powders, their produc-tion has to be controlled in order to obtain nanopar-ticles suitable for specific applications [31, 32]. Theproperties of particles synthesized by the gaseous re-action method depend on the physicochemical char-acteristics of the reaction systems. Their propertiesare also affected by the reactor design, the heatingmethod, the temperature gradient, preheating of thereactive gases, the method of introducing the gas intothe reactor, and other conditions such as decompo-sition temperature of precursors, condensation tem-perature, heating temperature for vaporization of the

precursor, the flow rate of carrier gas, composition ofthe atmosphere, chamber pressure, etc. [3235].It appears that, by appropriate choice of precursor

compounds and carrier gases, the CVC method maybe used to produce nanopowders of metals, oxides,carbides, nitrides, borides, or their composites withthe potential use as semiconductors, superconductors,ferroelectrics, optically active materials, catalysts, andmagnetic materials [27, 28, 30, 34, 36].

It was found that the composition of atmosphere(carrier gases) affected the formation of Co and ironnanoparticles. Co and iron nanoparticles with differentmorphology, shape, saturation magnetization, and co-

ercivity were produced in Ar and He atmosphere [31,37]. The size of particles prepared in helium atmo-sphere was smaller than that obtained in argon due

to the lower atomic mass [37] or higher mobility andthermal conductivity of He causing more rapid coolingof the metal particles compared with Ar environment[31, 36].

Similar to solidification of metals, the condensa-tion process may be absolutely dependent on the cool-ing rate and atmosphere during the CVC process [26].Thus, the smaller particle experiments a rapid conden-sation rate rather than the coarse particles in a forma-tion of the amorphous phase. Variation of the vaporcomposition and activation energy with temperatureduring the CVC process induces different propertiesof the resultant phases [38].

The size of nanoparticle prepared by CVC becamecoarse with the increase of the decomposition temper-ature due to the increase in the metal vapor concentra-tion [26]. However, this idea may be more effective for

the case when an inert gas is used as the carrier gas.At these conditions, no reaction with the metal va-por and, consequently, no stoichiometric change in thegas composition with the decomposition temperaturewere expected [38]. It was proposed that absorptiongrowth mechanism, the particle growth proceedingviaabsorption of separate atoms, might be predominantat lowest decomposition temperatures. The increase ofdecomposition temperature leads to predomination ofcoalescence growth mechanism, the particle size distri-bution becomes lognormal and the mean size of pre-pared particles increases [31]. At higher decompositiontemperature the increasing saturation vapor pressure

can enhance the growth of particle nuclei resulting inthe formation of larger and more asymmetric parti-cles. Also the higher kinetic energy of gas moleculesand so formed particles in the gas phase can lead tothe increasing number of collisions between the par-ticles and, consequently, to the growth of larger ones[36].

For each nucleus size, there is a certain saturationvapor pressure ratio that will exactly maintain suchparticle; too high the ratio and the particle grows;too small and it evaporates. Saturation vapor pres-sure ratio increases with an increase of the decomposi-

tion temperature. It is believed that higher saturationvapor pressure ratio enhances the growth of nucleus,which results in the larger particle formation [32]. Theresidence time in the CVC tube (reactor) decreasedwith increasing temperature [27]. The increase in thegas flow rate decreases particle size and leads to thechanges in particle phase composition [36].

Average particle size of oxide-coated nanoparticlescan affect the lattice constant. This influence can beexplained by the interaction between the metallic coreand its oxide shell if the growth of oxide is assumedto be epitaxial. Fung et al. [39] showed that the epi-taxial growth of oxide shell on the iron nanoparticles

has a lattice misfit of about 3 %. That can lead tocompressive stresses induced in oxide shell and tensilestresses in metallic core, which causes increasing lat-

154 Chem. Pap. 61 (3)151170 (2007)


5/20


tice constant in oxide-coated nanoparticles. Anotherpossible reason for lattice parameter increasing is theinfluence of dissolved interstitial atoms. The admix-tures of interstitial atoms such as carbon or oxygencan be introduced in the lattice of iron particles dur-ing their formation by vapor condensation and thenfixed by subsequent rapid quenching. The second rea-son can cause a general increase of lattice constant,which is independent of the particle size [31, 35, 36,40].

Lee et al. [40] compared the size distribution of ironnanoparticles synthesized from pentacarbonyliron va-porized in the condensation system without or with achilling device, showing that the rapid cooling allowedpreparing smaller iron particles with narrower size dis-tribution. The authors concluded that the slowly air-cooled particles inside the chamber would be coalesced

effectively because they can keep the resident heatenergy even in the smoky state. They showed thatthe thickness of the oxide layer increases relativelywith the decrease of reaction temperature. This con-clusion is considerably different from those reportedby other studies regarding usual CVC process. Typi-cally, the iron nanoparticles produced by CVC couldbe taken out from the chamber only after passiva-tion with an oxygen-containing gas, i.e. the surfaceof the iron nanoparticles should be coated with an ox-ide layer in order to avoid the explosion when exposedto air [40].

By controlling the preparation conditions, Wang

et al. [35] obtained FeCo alloyed nanoparticles withvarious content and magnetic states of the elements.Their core was metallic, while the shell was composedof metal oxides. The authors found that the thick-ness of the oxide layer was about 34 nm, regardlessof the particle size and decrease of the lattice con-stant of the BCC (body center cubic) phase while in-creasing the cobalt content in nanoparticles [35]. Ohet al.[41] investigated the magnetic properties of FeCo nanoparticles synthesized by CVC process. Theyreported that the synthesized particles were nearlyspherical with the surface layer comprising-FeOOH,

-FeOOH, and Fe3

O4

, but not -Fe2

O3

. The varia-tion of average particle size was independent of thecobalt content. On the other hand, increasing cobaltcontent influenced the magnetic properties of preparednanoparticles. The authors also found that if the de-composition temperature and the oxygen content inthe carrier gas (Ar) were increased, the magnetic prop-erties of particles were reduced while decreasing theaverage particle size [41]. Carbon-coated Fe and Conanocapsules have been synthesized by a CVC pro-cess using carbon monoxide as the carrier gas [33,42].

Some iron/iron compounds nanoparticles that have

been synthesized by the CVC method using Fe(CO)5as the precursor in a flowing NH3 atmosphere com-prised nanocrystalline-Fe and-Fe3N [28].

Arc Discharge Method

Most nanocapsules are synthesized by the arc dis-charge method, in which metal precursors are nor-mally packed inside a cave drilled into a graphite elec-trode and then undergo arc vaporization. Metal car-bides can be encapsulated in carbon cages using thismethod. Dravid and coworkers [43] modified the arcdischarge method to successfully produce nanophaseNi encapsulated in graphite shells. Harrisand Tsang[44] prepared carbon-encapsulated metal or metal car-bides using the high-temperature (1800C) treat-ment of microporous carbon materials impregnatedwith metal. One should mention that these methodsinvolve ultrahigh power consumption.

In addition, the previously reported carbon shellsin encapsulated metal nanoparticles normally have a

graphite structure. Recently, Zhang et al. [45] usedthe arc discharge method in a diborane atmosphereto prepare amorphous boron oxide-encapsulated mag-netic nanocapsules.

Thus far, metal or metal carbide nanoparticles en-capsulated in a carbon shell have been prepared us-ing the direct current arc discharge method [43]. Themethod requires the temperature as high as 3500 Cfor the cathode and over 2000C for the anode. As aresult, the metalgraphite composite evaporates andcarbon and metal vapors are deposited on the cath-ode surface to form encapsulation-structured nanopar-ticles [45].

Magnetic metal-filled carbon nanocapsules, rang-ing from 10 nm to 50 nm in diameter have been syn-thesized by using an arc discharge apparatus and puri-fied by concentrated acid treatment [46]. Purificationmethods used for removing various contaminants inarc-discharge products were reported in various refer-ences, e.g. [46, 47].

The most widely used technique for the produc-tion of carbon nanotubes (CNTs) is the direct cur-rent (DC) arc discharge, because it can yield struc-turally excellent high-quality CNTs. By this process,nanotubes containing carbon on the cathode sur-

face [48] (open-edged, single-walled, double-walled,multi-walled, and metal-filled CNTs [49]), nanopar-ticles, fullerenes, nanocapsules, nanowires, nanorods,nanofibers [48], and carbon nanotube knees [50] areproduced. The anode is usually filled with transitionmetal catalysts such as Fe, Co, Ni [48], which are favor-able for the single-walled carbon nanotubes (SWNTs)formation. Bimetallic Y/Ni (1/4) catalyst is one of themost efficient catalysts for generating SWNTs [48].

Fig. 2 shows a schematic drawing of an arc reactorcomprising a chamber cooled with a fluid such as wa-ter. An anode is formed by a graphite rod in which ahole was drilled and filled with a mixture of appropri-

ate catalysts, e.g. Fe [46, 48], metal composite [4850], Mo [51], and carbon source, i.e. graphite powder[48]. The catalyst-filled anode is supported by a holder

Chem. Pap. 61 (3)151170 (2007) 155


6/20


Fig. 2. Set-up of the arc discharge process (reprinted from Ref. [48] with permission from Elsevier).

connected to a translation-motion feed through a cath-ode, a pure graphite rod fixed by a stationary holder.The arc reactor may be filled with an inert gas (He[48, 50], Ar [46]), water [51], or hydrogen [49]. Sanoet al. [51] have shown that an arc discharge betweentwo carbon electrodes submerged in water yields largequantities of carbon bulky onions. With the anodetip gradually being moved toward the cathode, an arc

discharge process was maintained between the tips ofthe anode and cathode. Knowing that at the beginningof the process the current was unstable, the arc gap of12 mm between the electrodes was kept during theexperiment [48].

The electric arc method produces the stiff, nearperfect, and whisker-like multi-walled carbon nano-tubes MWCNT (suggesting superior properties com-pared to catalytically grown carbon nanofibers) al-though some reports suggest that the high temper-ature of the arc could provoke sintering and cross-linking between tubes. However, the arc technique

suffers from drawbacks. Being a batch process, theamount of material that can be produced by batchis limited and the material formed contains substan-tial amounts of nanoparticles with polyhedral shapeand low aspect ratio [47].

Another method proceeding in the vapor phaseis hydrogen plasmametal reaction (HPMR). Thismethod developed by Ohno and Uda [52] is used toproduce nanoparticles by DC thermal plasma in amixture of hydrogen and argon gas at vacuum. In prin-ciple, it is similar to the arc discharge method, how-ever, there are some differences. HPMR, as an aerosolmethod, is a very promising way to produce inter-

metallic compound nanoparticles with the possibilityto control the content of each element in nanoparticlesaccurately. In addition, HPMR is suitable to prepare

ultrafine particles (UFPs) industrially at low cost [7,8]. Nanoparticles of several metals and alloys, such asMg, Ni, Cu, Ag, FeCo, FeNi, and FeCr havebeen prepared by HPMR [7, 53].

Iron aluminide is one of intermetallic compoundssynthesized by HPMR. The interest in intermetalliccompounds arises from their attractive properties suchas good corrosion resistance, high melting tempera-

ture, light weight, and excellent mechanical proper-ties. Since metallic nanoparticles have a large specificsurface area, they actively react with oxygen even atroom temperature. On the other hand, the high ox-idation resistance of intermetallic nanoparticles wasattributed to the aluminum oxide layer [53]. Iron alu-minide nanoparticles have many potential uses, partic-ularly in the field of metallurgy and magnetism for barcoding and magnetic ink applications. Although mi-cropowder nanocrystalline FeAl intermetallics couldbe produced by mechanical alloying, the effort neededmade this procedure impractical for the nanoparticles

synthesis [7].Comparing with mechanical alloying, the HPMRmethod has the advantage to produce more con-venient nanoparticles of intermetallic compounds atlower cost. It is possible to control the mean par-ticle size of Fe3Al by changing the content of hy-drogen in the arcing atmosphere and arc current.The former is the main controlling factor of theprocess. With lower hydrogen content in the arcingatmosphere, Fe and Al are evaporated slowly andthere are fewer particles to collide and coalesce dur-ing the cooling, which leads to smaller mean parti-cle size. However, the production rate of intermetal-

lic particles decreases with decreasing hydrogen con-tent [7]. It is known that also in HPMR the evap-oration rate of metal elements is an important fac-

156 Chem. Pap. 61 (3)151170 (2007)


7/20


Fig. 3. Schematic illustration of the equipment for the production of nanoparticles (reprinted from Ref. [54] with permission from

Elsevier).

tor affecting the rate of generation of metal particles[54].

The experimental equipment for production ofFe3Al nanoparticles consists primarily of an arc-melting chamber and a collecting system [7]. Fig. 3shows schematic illustration of experimental equip-ment used for the production of magnesium UFPs byHPMR [54].

The bulk AlFe ingots were prepared from 99.9 %purity Al and Fe by arc melting in an argon gas at-

mosphere. Arc-melted ingots were flipped over andremelted four times to get a homogeneous composi-tion. Then AlFe nanoparticles were produced by arcmelting of AlFe ingots in a mixture comprising 50 %H2 in Ar at 0.1 MPa. The flow rate of the circulationgas for collection of nanoparticles was 100 L min1.After passivation in Ar containing 5 % of O2 for 24h, the nanoparticles were taken out of the arc-meltingchamber [7, 8].

Laser Pyrolysis

Laser-driven pyrolysis of organometallic precursorsis a general synthetic tool allowing the synthesis ofnanoscale particles ranging from 2 nm to 20 nm atrapid heating and cooling rates (100 000C s1) [55,56].

Laser pyrolysis is based on the resonant interac-tion between laser photons and at least one gaseousspecies, reactants or sensitizer. A sensitizer is an en-ergy transfer agent that is excited by the absorption ofthe CO2laser radiation and transfers, by collision, theabsorbed energy towards the reactants [57]. Negligibleabsorption of radiation by the metal donor gas precur-sors requires the addition of a reaction sensitizer [58],

thus altering the expected reaction path.Dumitrache et al. [57] reported for the first time

the ability of the laser pyrolysis technique to synthe-

size nanotubes entirely in the gas phase (without hot-walls interactions). This process occurred when ther-mal conditions allowed for the appearance of catalyz-ing iron nanoparticles and carbon fragments (issuedfrom ethylene decomposition) [57].

A scheme of an apparatus used for laser synthe-sis of nanosized powder is depicted in Fig. 4. Es-sentially, the CO2 laser radiation orthogonally inter-sects the reactant gas stream admitted to the cen-ter of the reaction cell through a nozzle. The reac-

tant gas is confined to the flow axis by a coaxial Arstream. The nucleated particles formed during the re-action are entrained by the gas stream towards thecell exit where they are collected in a trap, closedwith a microporous filter in the direction of the ro-tary pump. For the synthesis of iron carbides shell-structured nanoparticles the flows of the hydrocar-bon used as a carrier gas for Fe(CO)

5 vapors and

of the sensitizer (SF6) [56] were controlled indepen-dently. The total pressure in the reactor for laser py-rolysis is maintained constant. Studies of Fe(CO)

5

sequential decarbonylation by laser pyrolysis using

sensitized gas mixture revealed fast removal of car-bonyl ligands and formation of metallic iron [5658].

The effects of the process conditions on the struc-tural and magnetic properties of-Fe2O3 nanoparti-cles produced by laser pyrolysis have been studied. Itwas mentioned that the particle size depends on theoxygen proportion in the gas phase and is indepen-dent of the laser power. It can be concluded that thesize and crystallinity of the maghemite nanoparticlescould be changed by the laser power used and theoxygen mole ratio in the carrier gas. The origin of thiseffect could be related to the different temperatures of

synthesis [59].The laser-induced pyrolysis [58] offers the advan-

tages of being a potentially clean process producing

Chem. Pap. 61 (3)151170 (2007) 157


8/20


Fig. 4. A schematic drawing of laser pyrolysis system (reprinted from Ref. [55] with permission from the American Institute ofPhysics).

particles with uniform and controllable size distribu-tion. Particle size is controlled by changing the flowrate of chemicals through the pyrolysis reaction zone[55]. Moreover, the use of CO2 laser and a continu-ous flow reactor is, in principle, scalable to pilot plantdimensions [58].

LIQUID PROCESSING METHODS

Microemulsion Method

The use of an inorganic phase in water-in-oil mi-croemulsions employed for the preparation of uniformand size-controlled metal particles with 550 nm indiameter has received extensive attention [6]. A mi-croemulsion is an isotropic and thermodynamicallystable single phase formed by at least three compo-nents; two of them are nonmiscible, and the thirdone, called surfactant, is characterized by amphiphilicproperties [23]. Depending on the proportion of suit-able components and hydrophiliclipophilic balance

(HLB) value of the surfactant used, the formation ofmicrodroplets can be in the form of oil-swollen mi-celles dispersed in the aqueous phase as the oil/water(O/W) microemulsion or water-swollen micelles dis-persed in oil as the water/oil (W/O) microemulsion(reverse microemulsion). In the intermediate phase re-gion between O/W and W/O microemulsions, theremay exist bicontinuous microemulsions with aqueousand oil domains interconnected randomly in the formof sponge-like microstructures [6].

W/O microemulsion solutions are mostly trans-parent, isotropic liquid media with nanosized waterdroplets that are dispersed in the continuous oil phase

and stabilized by surfactant molecules at the water/oilinterface. These surfactant-covered water pools of-fer a unique microenvironment for the formation of

nanoparticles. They not only act as microreactors forprocessing reactions but also allow the aggregation ofparticles [60].

A stabilizer (emulsifier) is a molecule that pos-sesses both polar and nonpolar moieties. In very di-luted water (or oil) solutions, emulsifier dissolves andexists as monomer, but when its concentration ex-ceeds a certain minimum value, the so-called criticalmicelle concentration (CMC), the molecules of emul-

sifier associate spontaneously to form aggregates micelles. Above the CMC, the physical state of thesurfactant molecules changes dramatically, and addi-tional amount of surfactant exists in the form of ag-gregates or micelles. The bulk properties of the sur-factant, such as osmotic pressure, turbidity, solubiliza-tion, surface tension, conductivity, and self-diffusionchange around the CMC. Micelles are responsible formany processes,e.g.enhancement of the solubilizationof organic compounds in water (oil-in-water (O/W)emulsion) or hydrophilic compounds in the oil phase(water-in-oil (W/O) emulsion).

In the case of reverse micelles, there is no obvi-ous CMC because the number of aggregates is usuallysmall and they are not sensitive to the surfactant con-centration [2, 61].

In both cases, the micelles present only a smallamount of solubilized hydrophobic or hydrophilicmaterial. If the concentration of surfactant is in-creased, the solubilization process can be enhanced.The droplet size can increase to a dimension that ismuch larger than the monolayer thickness of the sur-factant because the inside pool of water or oil is en-larged. As the surfactant concentration increases fur-ther, micelles can be deformed and can change into

different shapes [2, 62].The shape of micellar aggregates and the formation

of microemulsion can be controlled and understood

158 Chem. Pap. 61 (3)151170 (2007)


9/20


Fig. 5. Proposed mechanism for the formation of metal particles by the microemulsion approach (reprinted from Ref. [6] withpermission from Elsevier).

from the packing parameter of emulsifier molecule inthe micellar assembly v/al, where v is the emulsifierhydrocarbon volume, athe polar head area, and lthefully extended chain length of the emulsifier [2, 6].Formation of O/W microemulsions for v/al < 1 orW/O microemulsions for v/al > 1 may be observed[2, 6].

The general method of using reverse micelles tosynthesize nanoparticles can be divided into two cases.The first case involves mixing of two reverse micellescarrying the appropriate reactants in order to obtainthe desired particles. A schematic picture of this pro-cess is presented in Fig. 5. The reaction then takesplace inside the droplets (nucleation and growth), thuscontrolling the final size of the particles. Once the par-ticles attain their final size, the surfactant moleculesare attached to the surface of particles, thus stabiliz-ing and protecting them against further growth. Thesecond case involves mixing of one reactant that is sol-

ubilized in the reverse micelles with another reactantthat is dissolved in water. The reaction can take placeby coalescence or aqueous phase exchange between thetwo reverse micelles [2, 6].

The dynamic exchange of reactants such as metal-lic salts and reducing agents between dropletsviathecontinuous oil phase is strongly depressed due to therestricted solubility of inorganic salts in the oil phase.This is a reason why the attractive interactions (per-colation) between droplets play a dominant role inthe particle nucleation and growth in the W/O mi-croemulsion reaction medium.

Generally, the chemical reactions of metallic salt

and reducing agent within the microdroplet are veryfast and, therefore, the rate-determining step inthe overall reaction will be the initial communica-

tion step of the microdroplets with different reac-tants.

The supply of metal salt must be controlled and, ifsmall particles are needed, then the particle growthmust be stopped at an appropriate size by cuttingoff the supply of reagent. For this reason, very lowconcentrations are used and a stabilizing agent must

be added to preserve monodispersity [6].The average size of the nanoparticles synthesized

by the microemulsion method depends on the size ofthe microemulsion droplet, which is determined by thewater-to-surfactant ratio W. The final size, however,does not depend, in general, only on the size of micro-droplets, but it may be influenced by the other factors,such as concentration of reactants (especially surfac-tant), flexibility of the surfactant film, etc.It is knownthat the flexibility of the surfactant films, presenceof additional stabilizing agents, and concentration ofreactants influence the final size of the product par-

ticles irrespective of the size of the microdroplets [6].The results of previous studies regarding the synthesisof-Fe showed that the preparation conditions (e.g.pH, solution concentration, and mixing procedure)strongly influenced the chemical composition, parti-cle size, particle morphology, crystal structure, and,consequently, the magnetic properties of the productsobtained [63].

One of disadvantages of this method lies in itshigh expense due to the large amounts of surfactant(as much as 2030 %) added to the system. An-other drawback is that the surfactant ensuring col-loidal stability is adsorbed on the surface of nanopar-

ticles, thereby decreasing their usability. The disad-vantages may be circumvented by the application ofmicellar synthesis, in the course of which the desired

Chem. Pap. 61 (3)151170 (2007) 159


10/20


reaction takes place in the interior of micelles. Themost obvious way to circumvent the above-mentionedproblems is to decrease the amount of surfactant orevent to avoid the use of surfactants at all. Anotherproblem of using W/O microemulsions for nanopar-ticle synthesis is the separation and removal of some(high-boiling point) solvents from products [6].

Precipitation of metal particles in the reverse mi-cellar system seems to be the simplest method usedfor the nanoparticles production. After their prepara-tion, nanoparticles need to be recovered from the re-verse micelles and immobilized onto stable supports.One of the most attractive procedures employed forthe processing of nanoparticles is direct recovery andimmobilization by using thiol-modified supports viachemical bonding [6].

The synthesized metal nanoparticles can be col-

lected by several other methods. Ji et al. [64] usedthe RESS (rapid expansion of supercritical solution)method to collect the silver nanoparticles. Ohde et al.[65] used an in situ deposition method by reducingthe pressure of the system to the cloud point of themicroemulsion.

For the purpose of metal nanoparticles preparationin the reverse micelles by reduction of metal salts,strong reduction agents such as NaBH4, N2H4, andsometimes hydrogen were used. FeNi has been syn-thesized using this method [2]. Metal oxide nanoparti-cles can be prepared inside the reverse micelles by hy-drolysis of metal alkoxides dissolved in oil with water

inside the droplets. -Fe2O3 nanoparticles have beenprepared in this fashion. Metal sulfates, carbonates,oxides, and silver halides can also be produced by theprecipitation reaction between the reactants in reversemicelles [2].

The water-in-oil microemulsion has been widelyused to synthesize iron/iron compound nanoparticlesof various kinds, including metallic iron nanoparticles[63, 66], iron oxide [2, 6, 23], iron boride [6], gold-coated iron nanoparticles [6], FeNi alloys [2, 6], mag-netic polymeric particles [67, 68], iron oxide-dopedalumina nanoparticles [69], and silica-coated iron ox-

ide nanoparticles [70].A variety of surfactants can be used when prepar-ing the nanoparticle materials by the microemul-sion method, such as bis(2-ethylhexyl)sulfosuccinate(AOT), sodium dodecyl sulfate (SDS), cetyltrimethyl-ammonium bromide (CTAB), polyvinylpyrrolidone(PVP), diethyl sulfosuccinate (DES), Igepal CO-520, Brij-97, Triton-X, pentadecaoxyethylenenonylphenyl ether (TNP-35), decaoxyethylenenonyl phenylether (TNT-10), poly(oxyethylene)5nonyl phenyl ether(NP5), etc. [2, 6, 10, 23, 63, 69, 70]. Some cosur-factants used in this method are aliphatic alcoholswith a chain length of C6C8. Organic solvents

used for reverse micelle formation are usually alka-nes or cycloalkanes with six to eight carbon atoms[2].

Hydrothermal Method

Hydrothermal processing where aqueous solutions,vapors, and/or fluids react with solid materials at hightemperature and high pressure, is a well-known pro-cess in mineralogy and geology fields for formation,alteration, or deposit of minerals, ores, or rocks [71].This method exploits the solubility of almost all inor-ganic substances in water at elevated temperaturesand pressures and subsequent crystallization of thedissolved material from the fluid. Water at elevatedtemperatures plays an essential role in the precursormaterial transformation because the vapor pressure ismuch higher and the structure of water at elevatedtemperatures is different from that at room tempera-ture. The properties of the reactants, including theirsolubility and reactivity, also change at high tempera-

tures. The solvent is not limited to water but includesalso other polar or nonpolar solvents, such as benzene,and the process is more appropriately called solvother-mal synthesis in different solvents [2].

High temperature-high pressure solutions, vapors,and/or fluids can act on materials as a) transfermedium of pressure, temperature, and mechanical en-ergy, b) adsorbate, which plays a role of catalystor reaction accelerator, c) solvent, which dissolvesand allows to reprecipitate the solid materials, d)reagent, which forms hydroxides, oxides, oxyhydrox-ides, and/or salts, i.e. the substances acting as b)and/or c) are called mineralizers. These actions

can also be used in processing of inorganic materials:preparation, formation, alteration, sintering, etching,etc. Particularly, the hydrothermal processing is suit-able for the preparation of powders in the form ofnanoparticles or even single crystals [71].

Recent trends in preparing the starting powders forhydrothermal or solvothermal synthesis of nanoparti-cles are directed toward more dispersed systems usingsolutions (wet systems) and/or gases (dry systems)rather than traditional solid state systems. In the solidstate systems, the homogeneity of composition, struc-ture, and microstructure cannot be assured exceeding

the range of the solid particle size, whereas the gasor solution systems can manipulate much finer par-ticles on molecular or atomic size. Thus, the gas- orsolvent-dispersed systems can assure much better pro-cess control. Pressurized and heated gas and solutionsystems are transferred to hydrothermal equipment,so that the hydrothermal systems can be regarded asdeveloped gas and/or solution systems [71].

During hydrothermal treatment metal cations ini-tially precipitate in the form of polymeric hydroxides.Over time, these hydroxides undergo dehydration toform the metal oxide crystal structures. It was foundthat the presence of the second metal cation was ben-

eficial in controlling the particle formation processprobably by preventing the formation of complex hy-droxides when the base was added to the cold metal

160 Chem. Pap. 61 (3)151170 (2007)


11/20


Fig. 6. Schematic diagram of the experimental apparatuses forthe flow experiments with cold (a) and hot (b) mixingconfiguration (reprinted from Ref. [72] with permissionfrom Elsevier).

salt solution [72]. Addition of oxidizer can suppressagglomeration between primary particles compared tothe situation when particles are formed in the absenceof an oxidizer [73].

Two variations of the continuous hydrothermaltechnique were examined, namely cold mixing and hot

mixing. Schematic diagram of the experimental ap-paratuses employed for flow experiments is shown inFig. 6. The product formed by the cold mixing com-prises fewer impurities than that prepared by the hotmixing technique.

The cold mixing configuration was successful inproducing uniform nanoparticles of CoFe2O4. A mech-anism of particle formation was postulated involvingthe precipitation of metal hydroxides at ambient con-ditions, dissolution of the hydroxides as temperaturewas increased, followed by rapid precipitation of metaloxides at elevated temperatures. The hot mixing ex-periments, on the other hand, simply involved the pre-

cipitation of metal oxides due to the addition of hothydroxide solution [72].

Chemical routes such as the hydrothermal reaction

method, sol-gel process, chemical co-precipitation,etc.usually involve synthesis of a precursor gel of iron fol-lowed by the decomposition of the gel or precursorinto the designed crystalline iron oxide phase at an el-evated temperature [74]. -Fe2O3 nanoparticles havebeen synthesized by a new hydrothermal method. Itwas found that the size and nature of the prepared-Fe2O3 nanoparticles strongly depended on the pH,temperature, and residence time [72], as well as onthe concentration of the complexing agent (oxalicacid), cationic surfactant (CTAB), and alkali source(tetramethylammonium hydroxide, TMAOH, used tomaintain the pH of the medium). By changing thesevariables, it was possible to optimize the crystallinity,size, and size distribution of the prepared nanoparti-cles [74].

A strong base could shift the equilibrium toward

the formation of metal hydroxides. The concentra-tion of alkali source was kept in excess relative to themetal salt concentration to ensure precipitation of allmetal ions present in the solution [72]. This processwas further complicated by the possibility of reduc-tion/oxidation of the metal cations. The choice of basereagent was important to obtain the desired phase un-der the synthesis conditions [75]. Therefore, the solu-tion environment had to be adjustedviapH and tem-perature to favor the desired species. In addition, theability of supercritical water to provide a good en-vironment for oxidation in the presence of dissolvedoxygen should be overcome [72].

The hydrothermal precipitation-calcination routefor synthesis of nanosized barium hexaferrite has beenstudied byMishra et al. and Ataie et al. [76, 77]. Theauthors found that the particle size, morphology, andmagnetic properties of the hydrothermally preparedbarium hexaferrites were strongly dependent on thepreparation conditions and on the nature of precursorsused.

Hydrothermal synthesis of nanoceria (CeO2) pow-ders has been studied by Leeand Choi[73]. By usingH2O2 as the oxidizer and NH4OH as the mineralizerthe authors observed that the particle size was de-

creased with increasing concentration of oxidizer. Theprepared CeO2 particles were spherical in shape andrelatively uniform.

Interestingly, similar hydrothermal crystallizationwas observed at the solid/gas interface when the gasphase was saturated with water. This reaction, there-fore, allows direct conversion of solid precursor intocrystalline films [78].

Fine powders can be prepared by 1. the breakdown(size reduction) and 2. build-up (size increase) meth-ods. Hydrothermal processing uses both these proce-dures; hydrothermal crystallization is one of the build-up methods, such as hydrothermal crystallization of

zirconia [79], while hydrothermal oxidation representsthe breakdown methods,e.g. production of fine pow-der of-Al2O3 [71].

Chem. Pap. 61 (3)151170 (2007) 161


12/20


Table 2. Comparison of the Advanced Oxide Powder Processes [80]

Process Conventional Sol-gel Coprecipitation Hydrothermal

Cost Low-moderate High Moderate ModerateState of development Commercial R&D Commercial/demonstration Demonstration

Composition control Poor Excellent Good Good/excellentMorphology control Poor Moderate Moderate GoodPowder reactivity Poor Good Good GoodPurity/% < 99.5 > 99.9 > 99.5 > 99.5Calcination step Yes Yes Yes NoMilling step Yes Yes Yes No

Hydrothermally prepared powders are generallywell reacted and crystallized because the surround-ing aqueous solutions accelerate these processes [71,72]. These features facilitate the fabrication of finecrystals, which are homogeneous in size, shape, and

composition. In particular, multi-component crystalsare not always easy to fabricate by conventional so-lution methods, because the processes of solvent re-moval or solvent separation frequently cause inhomo-geneities even if the starting solution is homogeneous.The crystals formed by hydrothermal treatment typ-ically have high density with no porosity and are ho-mogeneous in composition. Furthermore, crystals withcontrolled shape and size are almost nonaggregated,because their surfaces might be characterized by rel-atively low surface energies. The main advantages ofhydrothermal synthesis (Table 2) are related to homo-geneous nucleation processes, very low grain sizes, nar-

row particle size distribution, single phase, controlledparticle morphology, and high-purity powders due toelimination of the calcination step [73, 75, 80].

Sol-Gel Method

Sol-gel is a useful technique for the production ofnanomaterials made of particles in an insulating ma-trix with interesting magnetic or optical properties[81]. The sol-gel method is based on inorganic poly-merization reaction including hydrolysis, polyconden-sation, drying, and thermal decomposition. Precursors

of the metal or nonmetal alkoxides hydrolyze with wa-ter or alcohols according to the reaction scheme

M(OR)x+mH2O M(OR)xm(OH)m+mROH (A)

If m x, the reaction represents total hydrolysisthat is followed by either water

2M(OR)xm(OH)m (OH)m1(OR)xm-M-O-M(OR)xm(OH)m1+

+ H2O (B)

or alcohol condensation

2M(OR)xm(OH)m

(OH)m1(OR)xm-M-O-M(OR)xm1+ROH(C)

The total reaction can be expressed as

M(OR)x+x/2H2O MOx/2+xROH (D)

In addition to water and alcohol, an acid or a basecan also help to hydrolyze the precursor. In the case ofan acid, a reaction takes place between alkoxide andthe acid.

-M-OR + AOH -M-O-A + ROH (E)

The rates of hydrolysis and condensation are im-portant parameters that affect the properties of finalproducts. Slower and more controlled hydrolysis typ-ically leads to smaller particle size and more uniqueproperties.

After the solution condensation to a gel, the sol-vent should be removed. Typically, higher calcinationtemperature is needed to decompose the organic pre-cursor [2].

The size of the sol particles depends on the solutioncomposition, pH, and temperature [2, 82]. By control-ling these factors, one can tune the size of the preparedparticles. This method has been successfully used tosynthesize numerous metal oxide nanoparticles suchas TiO2, UO2, TnO2, ZrO2, CeO2, SnO2, SiO2, CuO,SnO2, ZnO, Al2O3, Sc2O3, ZnTiO3, SrTiO3, BaZrO3,CaSnO3 [2, 83, 84], and other nanomaterials [82, 85

87].Fe2O3SiO2 nanocomposites have been preparedby a very simple sol-gel method. Commercial precur-sors (TEOS and iron(III) nitrate) were dissolved in analcoholic aqueous medium, and the gels formed after afew days were heated giving the final materials with-out further manipulation. Since the decomposition ofthis iron salt can lead to the formation of various formsof oxides with different properties, special attentionwas devoted to the characterization of the materialobtained under the adopted experimental conditionsand after treatments at elevated temperatures [86, 87].

Synthesis of alumina network containing iron by

the sol-gel method has been investigated discussingthe structural and magnetic properties of the pre-pared product, -Fe/Al

2O3. The samples were ob-

162 Chem. Pap. 61 (3)151170 (2007)


13/20


Fig. 7. Sonochemical synthesis of various forms of nanostructured materials (reprinted from Ref. [14] with permission from AnnualReviews).

tained starting from aqueous solutions of precursorsAl(NO

3)39H2O and FeSO4 7H2O at 5C. The re-

sulting solution was neutralized with NH4OH (25mass %) and gelatinous precipitates were adequatelywashed and dried at 80C [85].

Magnetic ordering in the sol-gel system dependsnot only on the formed phases and particles volume

fraction, but it is also particularly sensitive to the sizedistribution and special dispersion of the particles. Inthe case of nanocomposites derived from gels, thesestructural parameters and material porosity, which arecorrelated, are determined by the rate of hydrolysisand condensation of the gel precursor (generally analkoxide) and of other oxidation-reduction reactionsoccurring during the gelling stage and subsequent heattreatment [81].

Sonochemical Method

Sonochemistry is the research area, in whichmolecules undergo chemical reactions due to the appli-cation of powerful ultrasound radiation (20 kHz to 10MHz) [88]. The physical phenomenon responsible forthe sonochemical process is acoustic cavitation. Thismethod, initially proposed for the synthesis of ironnanoparticles [89], is nowadays used to synthesize dif-ferent metal oxides [90]. Concentration of sonochemi-cally produced iron nanoparticles was reported to bevery small and the particles tend to agglomerate dueto their high reactivity [90].

The sonochemical method for nanoparticles prepa-ration is simple and it is operated at ambient con-

ditions. It is also easy to control the particle size ofthe product by varying the concentration of the pre-cursors in the solution [91]. Ultrasound power effects

the occurring chemical changes due to cavitations phe-nomena involving the formation, growth, and collapseof bubbles in liquid [91, 92].

The sonolysis technique involves passing soundwaves of fixed frequency through a slurry or solutionof carefully selected metal complex precursors. In asolvent with vapor pressure of a certain threshold, the

alternating waves of expansion and compression causecavities to form, grow, and implode [90].

Sonochemical reactions of volatile organometallicshave been exploited as a general approach to the syn-thesis of various nanophase materials by changing thereaction medium, as shown in Fig. 7 [14].

A number of theories have been developed in or-der to explain how 20 kHz sonic radiation can breakchemical bonds. They all agree that the main eventin sonochemistry is the creation, growth, and collapseof a bubble that is formed in the liquid [93]. The hotspot mechanism is one of the theories that explain

why, upon the collapse of a bubble, chemical bondsare broken. This theory claims that very high temper-atures (500025000 K) are obtained upon the col-lapse of the bubble [88]. Since this collapse occurs inless than a nanosecond, very high cooling rates, in ex-cess of 1011 K s1, are also obtained. This high cool-ing rate hinders the organization and crystallizationof the products. For this reason, in all cases dealingwith volatile precursors where gas phase reactions arepredominant, amorphous nanoparticles are obtained.While the explanation for the creation of amorphousproducts is well understood, the reason for the nano-structured products is not clear. One explanation is

that the fast kinetics does not permit the growth ofthe nuclei and in each collapsing bubble a few nucle-ation centers are formed the growth of which is limited

Chem. Pap. 61 (3)151170 (2007) 163


14/20


by the short collapse. If, on the other hand, the precur-sor is a nonvolatile compound, the reaction occurs ina 200 nm ring surrounding the collapsing bubble. Thetemperature in this ring is lower than that inside thecollapsing bubble, but higher than the temperature ofthe bulk [93].

The chemical reactions driven by intense ultra-sonic waves that are strong enough to produce cav-itations are oxidation, reduction, dissolution, and de-composition. Other reactions such as promotion ofpolymerization have also been reported to be in-duced by ultrasound. It is assumed that three dif-ferent regions are formed during the aqueous sono-chemical process: a) the inner environment (gas phase)of the collapsing bubble, where elevated tempera-tures (several thousands of degrees) and pressure(hundreds of atmospheres) are produced, causing wa-

ter to vaporize and further to pyrolyze into H andOH radicals; b) the interfacial liquid region betweenthe cavitation bubbles and bulk solution; the tem-perature in this region is lower than that of thebubble interior, however, it is still high enough forthe thermal decomposition of solutes to take place,in addition, higher local hydroxyl radical concentra-tions in this region have been reported; and c) thebulk solution, which is at ambient temperatures andwhere the reaction between the reactant moleculesand OH or H takes place. It appears that the sono-chemical reactions occur within the interfacial region[92].

The advantage of sonochemistry is that one can ob-tain atomic level mixing of the constituent ions in theamorphous phase so that the crystalline phase can beobtained by annealing at relatively low temperatures.The cavitation is a quenching process, and hence thecomposition of particles formed is identical to the com-position of the vapor in bubbles, without phase sep-aration. This becomes important in the preparationof crystalline ferrite or other mixed oxide materialswhere the conventional ceramic method requires heat-ing at high temperatures, which can cause an increasein the particle size and aggregation. Sonochemistry

has been used to prepare various kinds of amorphousmagnetic nanomaterials of metal, metal alloy, oxide,ferrite, and nitride and has been extended to producecore-shell-type materials [91, 94]. The advantage of thesonolysis technique is the absence of many reactantsthat remain as contaminants and require more chem-icals or solvents to be removed [90]. It is well knownthat the amount of contaminants depends on the irra-diation time. The shorter the sonication, the smalleris the amount of contaminants [92].

There are four topics related to materials sci-ence and nanotechnology, in which the sonochemicalmethod is superior to other techniques, i.e. 1. prepa-

ration of amorphous products, e.g. metal oxides, sul-fides, or other chalcogenides, by sonication does notneed any additives, while the cold quenching of bulk

metals requires the addition of glass-former materials,2. insertion of nanoparticles into mesoporous materi-als in the form of a smooth layer on the inner meso-pores walls, without blocking them, 3. deposition ofsmooth homogeneous layer of nanoparticles coveringceramic and polymeric surfaces, and 4. the formationof proteinaceous micro- and nanospheres. It has beendemonstrated recently that any protein can be con-verted into a sphere upon sonication [93].

Pure nanometer-sized Fe3O4particles were synthe-sized using iron(II) acetate dissolved in double dis-tilled deoxygenated water by irradiation with a high-intensity ultrasonic horn (Tihorn, 20 kHz) for 3 h at1.5 atm and 25C under argon atmosphere. The prod-uct was washed thoroughly with double distilled de-oxygenated water and finally with dry pentane in aninert glove box (less than 1 ppm of O2), and dried in

vacuum [92].

Microbial Synthesis

The use of bacteria as a novel biotechnology tofacilitate the production of nanoparticles is in itsinfancy. Bacterially mediated electrochemical pro-cess was used to synthesize metal (Co, Cr, or Ni)-substituted magnetic powder employing iron(III)-reducing bacteria under anaerobic conditions. The ob-tained results suggested that the bacteria might beviewed as a nonspecific source of electrons at a poten-tial that can be calculated or surmised, based on the

underlying thermodynamics [95].In contrast to purely chemical procedures for the

manufacture of magnetite particles, microbial reac-tions are characterized by their selectivity and pre-cision for magnetite formation. In most bioprocesses,it is assumed that highly specific structures capableto drive highly specific interactions with the culturemedium exist on the bacteria membrane (enzymes,proteins, etc.) [95].

Bacteria may be seen as a catalyst supplying elec-trons for the crucial step of splitting the substrate (re-duction of Fe(III)). In contrast to classical catalysts,

however, the bacteria extract some of the electrochem-ical energy in order to live, while the electrons are shedto their surroundings at a potential that is sufficientto precipitate magnetite. It is perhaps the most ap-propriate to think of the bacteria as an electrode thatis substantially indifferent to the exact mix of metalions present in the surrounding medium [95].

The possible advantages of this method of nano-particles production may include 1. biologically facil-itated production of magnetite that does not requirethe addition of exogenous electron carrier substancessuch as humic acids; 2. particle growth to a size thatwould not be feasible inside the cell; 3. multiple use

while the bacteria do not need to die in order to har-vest the product; and 4. control of the particle dis-lodging when it reached a desired size by the process

164 Chem. Pap. 61 (3)151170 (2007)


15/20


variables,e.g. agitation, fluid flow, or magnetic forces[95].

Biomaterials have been used in the incorporationof foreign bodies into fullerene structures. Tsang etal.[96] described the formation of iron-filled sphericalcarbon nanocapsules of a very narrow size distribu-tion in macroscopic quantities by the controlled pyrol-ysis of ferritin molecules. The protein cage of ferritinmolecule can also provide internal sites for other metalions exchange and inorganic oxide crystals nucleation(Fe, Mn, U, etc.) where the spatially constrained cav-ity for their accommodation is strictly defined. Theauthors believed that the controlled carbonization ofthese biomaterials or similar organic supramolecularand biological assemblies (with inorganic material in-clusion) could open up a new approach for preparationof filled carbon nanocapsules [96].

SOLID PROCESSING METHOD

Ball Milling Method

Mechanical processing, in particular high-energyball milling, is a convenient way to produce nano-sized powders [97]. It is the most common methodreported in the literature for the synthesis of inter-metallic nanoparticles [98]. This method is a mechan-ical process that requires high energy for the synthesisof various glassy, metastable, and amorphous materi-als [95]. It is noted that mechanical alloying or ball

milling has been widely used to produce amorphousalloys in various systems, such as metalmetal, tran-sition metalmetalloid, and even metalcarbon sys-tems [99].

Fine alloying particles may be formed from ele-mental coarse powdersviamechanochemical reactionsduring the high-energy ball milling. Before a mechan-ical milling is started, powder(s) is loaded togetherwith several heavy balls (steel or tungsten carbide) ina container. By vigorously shaking or high-speed rota-tion, a high mechanical energy is applied on the pow-ders because of collision with heavy balls. The milling

process embraces a complex mixture of fracturing,grinding, high-speed plastic deformation, cold weld-ing, thermal shock, and intimate mixing. The millingprocess will promote the diffusion of particles. Hence,an alloying phase may be formed at low temperature(mechanochemical process) [100].

Thermal energy generated during mechanical al-loying or reactive milling [99] has been found to facili-tate chemical reactions leading to the metallothermicreduction and/or resulting in the formation of com-pounds. The redox reactions in powder mixtures of ox-ide and pure metal during reactive milling allow boththe refining of metals and the direct production of al-

loys from their respective oxides, thus attracting muchattention. Most of the oxides were reduced by the solidmetallic reducing agents through an unstable combus-

tion reaction, which is similar in nature to the ther-mally ignited self-propagating high-temperature syn-thesis (SHS) technique. The critical adiabatic temper-atures for the displacement redox reactions induced byreactive milling, Tad < 1300 K, are much lower thanthose achieved during SHS. The repeated fracture andrewelding of reacting powders ensures high reactioninterface areas. In addition, the high defect densitiesas well as the nanocrystalline particles induced in thereactive milling enhance diffusion rates through theproduct phases. This combination of factors decreasesthe value of Tad. However, the reaction does not al-ways come to completion just by combustion and willproceed gradually in the following ball milling stage.Regarding the reactions carried out adiabatically athigh temperatures, other reaction mechanisms havealso been reported [101].

It was found that a small fraction of the reactants isconverted gradually before the combustion is ignited.In addition, the reaction proceeds in a controlled man-ner without the occurrence of spontaneous combustionprovided that diluents, such as process control agents,are added to the powder mixture to prevent powderagglomeration. The role of diluents is to reduce localtemperatures and inhibit the particle welding process,thus preventing the occurrence of ignition conditions.Moreover, it should be emphasized that the key tofacilitate the low-temperature reactions during the re-active milling is to minimize their kinetic dependenceon diffusion rates. This is achieved by the dynamically

maintained high reaction interface areas, as well as theshort-circuit diffusion path provided by the large num-ber of defects such as dislocations and grain bound-aries induced during the ball milling [101].

In spite of simplicity and efficiency of ball millingin synthesis of nanoparticles of metallic alloys, thereare some problems and limitations of this method. Themicrostructure of the milling products is very sensitiveto the grinding conditions and may be unpredictablyaffected by unwanted contamination from the millingmedia and atmosphere. In addition, extensive long pe-riods of milling time may be required to obtain parti-

cles smaller than 20 nm [98].Nanosized particles formed by condensation tech-niques are relatively free of lattice defects and pos-sess almost no residual strain. In contrast, large por-tion of the nanopowder grains formed by mechanicalmilling is produced due to the generation of disloca-tions. The resulting grains are highly strained and theprepared particles contain numerous defects. Duringmilling, the powder particles are cold-worked result-ing in the multiplication of dislocations. Eventuallythe grain size can be reduced to the point that theycannot sustain dislocations within the grain. In addi-tion to grains formed by dislocations, during milling,

grain boundaries are also formed by repeated particlefragmentation and cold welding [102].

With decreasing grain size, dislocations migrate

Chem. Pap. 61 (3)151170 (2007) 165


16/20


Table 3. Various Techniques for Synthesis of Nanostructured Materials

Synthesis method Nanostructured materialsynthesized

Process advantages Process disadvantages

Chemical vapor condensation.

During the short residencetime of the precursor in theheated tube, the precursormolecules start to decom-pose. This gas stream thenexpands rapidly to mitigateparticle growth. Finally, thenanoparticles condense out ona cooled substrate, they arescraped off and collected.

Metals (Fe, Cu [105], Co [32,

37]), metal oxides (MgO,TiO2[105, 106]), carbides(tungsten carbide [107]), ni-trides (Fe/N [34, 36, 38],-Fe3N [28]), borides andtheir composites, FeCoalloys [35, 41], Fe(C) andCo(C) nanocapsules [33, 42],SiCxNy, ZrOxCy, SiC, andSi3N4 nanopowders [27].

Composition may be selected

[38], preparation of differentkinds of nanoparticles [2729], narrow size distribution[28], large amount of nanopar-ticles with a nonagglomeratedstate [27], high purity [105108].

Low production rates, difficult

to control size and particlesize distribution [105].

Arc discharge. Metal precur-sors, usually packed inside acave drilled into a graphiteelectrode, undergo arc va-

porization employing an-other electrode furnishedwith a mixture of appropri-ate catalysts.

Metal carbides [45], mostnanocapsules [45, 48], boronoxide-encapsulated magneticnanocapsules [45], carbon

nanotubes [48, 49], fullerenes[48], inorganic fullerenes (IF)MoS2 [51], carbon nanofibers[49].

Excellent high-quality CNTsstructure, submerged arc dis-charge allows to produce largequantities of IF nanoparti-

cles in a cost-effective manner[51], stiff, near perfect, andwhisker-like MWCNT produc-tion [47].

Batch process, limited pro-duction [47].

Hydrogen plasma-metal reac-tion. Nanoparticles are pro-duced by DC thermal plasmain a mixture of hydrogen andargon gas at vacuum [53].

Intermetallic compoundnanoparticles, metals, andalloys such as Mg, Ni, Cu,Ag, FeCo, FeNi, FeCr[7, 53, 54], iron aluminide [7],TiFe [8].

Industrial preparation of ul-trafine particles at low cost [7,8], high yield of nanoparticles[53].

The oxidation resistance ofnanoparticles (Fe3Al) is lowerthan that of bulk intermetallic[53].

Laser pyrolysis. The methodis based on the resonant in-teraction between laser pho-tons and at least one gaseousspecies.

Nanotubes [57], iron carbidesshell-structured nanoparti-cles [5658], -Fe2O3 [59],AlN, MnO2, TiO2, Ti [105],FeC, carbides (WCx), oxycar-bides (Mo2CxOy), oxynitrides(Mo2NxOy), sulfides (MoS2,CoS2) [55].

Potentially clean process, uni-form and controllable parti-cle size distributions [58, 27],scalable to pilot plant dimen-sions [58].

Low production rate, highenergy consumption, highlyuneconomical [105].

Microemulsion. W/O mi-croemulsion solutions arenanosized water dropletsdispersed in the continuousoil phase and stabilized bysurfactant molecules. Thesesurfactant-covered water poolsoffer a unique microenviron-ment for the formation ofnanoparticles [6].

Metal nanoparticles (Cd,Ag, Au, Cu, Co, Pt, Rh,Pd, Ir, Ni, Fe [2, 6]), metaloxides (ZrO2, TiO2, SiO2,-Fe2O3 [2]), metal sulfates(BaSO4 [2]), metal carbonates(BaCO3, CaCO3, SrCO3 [2]),iron boride [6], FeNi alloys,gold-coated iron nanoparti-cles [2, 6], magnetic polymeric

particles [67, 68], iron oxide-doped alumina nanoparticles[69], silica-coated iron oxide[70], CdS, ZnS, Cd1yMnyS,Cd1yZnyS, CdTe [10].

Simple method [6], powderswith well-defined and con-trolled properties [10], uni-form and size controllablenanoparticles [6, 66], homo-geneous nanopowders [69].

Expensive, surfactant ad-sorbed on the surface ofnanoparticles, separation andremoval of some solvents fromproducts [6], low productionyield, use of a large amount ofliquids [10].

Hydrothermal. Hightemperature-high pressureaqueous solutions, vapors,and/or fluids react with solidmaterials.

Iron oxide [72, 74],CoFe2O4[72], barium hexa-ferrite [76], ceria powders[73], zirconia [71], -Al2O3[71], Co3O4 [72], CoAl2O4[75], TiO2, LaCrO3, ZrO2,BiTiO3, SrTiO3, Y2Si2O7,Sb2O3, CrN, -SnS2, PbS,Ni2P nanotubes, Bi2S3

nanorods, SiC nanowires [2].

Desired size and shape [73],well crystallized powders [71,72], homogeneous in size,shape, and composition, high-density powders, very lowgrain sizes, narrow particlesize distribution, single phase,controlled particle morphol-ogy, high-purity powders [71,

73, 75], nanocrystals withhigh crystallinity [2].

Difficult to control process,problems of reliability andreproducibility [109].

166 Chem. Pap. 61 (3)151170 (2007)


17/20


Table 3. (Continued)

Synthesis method Nanostructured materialsynthesized

Process advantages Process disadvantages

Sol-gel. This method is basedon inorganic polymerizationreactions. It includes foursteps: hydrolysis, polyconden-sation, drying, and thermaldecomposition [2].

Metal oxides (TiO2, UO2,TnO2, ZrO2, CeO2, SnO2,SiO2, CuO, ZnO, Al2O3,Sc2O3, ZnTiO3, BaZrO3,CaSnO3 [2, 10, 83, 84]),nanocomposites (Fe2O3SiO2 [86, 87], alumina inor-ganic network containing iron[85].

Excellent composition control[71], ultrafine porous pow-ders, homogeneity of product,strong promise for employ-ment industrially on a largescale [10], use of matrix sup-port which can, in principle,modify the properties of nano-materials [87].

High cost [71].

Sonochemical. During thisprocedure, molecules undergoa chemical reaction due tothe application of powerfulultrasound radiation.

Iron nanoparticles [14, 90],metal oxides [14, 90, 92, 94],alloy nanoparticles [93, 94],coating for the surface ofvarious particles [91, 93, 94,

110], nitride [94], ferrite [94],chalcogenides (ZnS, Sb2S3,HgSe, SnS2, CdS, CdSe, PbS,PbSe, PbTe, CuS), ZnO,ZrO2, MnOx [93].

Simple method, operating atambient conditions [91], easycontrol of the particle size[91], atomic level mixing ofconstituent ions in the amor-

phous phase [94], absence ofmany reactants [90], prepara-tion of amorphous products,inserting of nanoparticles intomesoporous materials, depo-sition of nanoparticles on ce-ramic and polymeric surfaces,formation of proteinaceousmicro- and nanospheres [93].

Very small concentration ofthe prepared nanoparticles,particle agglomeration [90].

Microbial. Bacteria are usedto facilitate the production ofnanoparticles [95].

Metal (Co, Cr, or Ni)-substituted magnetic pow-ders [95], iron-filled carbonnanocapsules [96].

Selectivity and precision fornanoparticle formation doesnot require the addition of ex-ogenous electron carrier sub-stances, controllable size, bac-

teria do not need to die [95],narrow size distribution [96].

Little knowledge.

Ball milling. Powder(s) isloaded together with severalheavy balls in a container. Byvigorously shaking or high-speed rotation, a high me-chanical energy will be ap-plied on the powders [100].

Intermetallic nanoparticles[98], FeS2 [100], magneticnanoparticles [97], Fe, Co,Ni, NiAl, TiAl, FeSn [105],Al2O3, ZrO2 [10].

Formation of alloying phase atlow temperature [100], simpleand efficient method [98].

High energy requirement[95], extensive long periodof milling time, very sensi-tive microstructure to grind-ing conditions [98], highlystrained grains, numerousdefects in resulting particles[102], powder contaminationdue to WC or steel balls, de-pending on the material andsize of mill [105].

or are forced to the grain boundary leaving thenanograin interior essentially defects-free. The milledgrain boundaries have a disordered structure approx-imately 1 to 2 lattices wide. The grain boundary dis-order expands the local grains atomic lattice, which,as the grain size is reduced, also produces a uniformlattice strain. Mechanical alloying with lattice substi-tution atoms such as aluminum has little effect on thegrain boundary strain. Mechanical alloying with inter-stitial alloys such as nitrogen strongly enhances grain

boundary strain [102].The canted spin structure could arise from the

weakening of the super-exchange interaction on me-

chanical milling which causes structural damages anddefects. The canted spin structure is linked to the ef-fective anisotropy, which increases because of the lat-tice strains introduced on milling and also due to thesurface spin effects of the small particles. The reduc-tion in the magnitude of the hyperfine fields on millingis due to the relaxation effects and also to the surfacespins of the small particles [103].

The time of milling affects the size, size distribu-tion, coercivity, magnetization, chemical composition,

and morphology of nanoparticles [97, 103].Nanoparticles can be obtained, using mechani-

cal milling together with a solid-dispersing medium.

Chem. Pap. 61 (3)151170 (2007) 167


18/20


Fig. 8. A schematic diagram of the ball mill equipment(reprinted from Ref. [104] with permission from Else-vier).

Chin and coworkers [100] used NaCl as the disper-sion medium. NaCl could result in well-dispersed FeS2nanoparticles and promote the formation of fine par-ticles. NaCl did not react with FeS2 [100].

It was demonstrated that the synthesis of maghe-mite nanoparticles by ball milling of iron powder inwater is possible. During the synthesis, water was re-duced by iron and hydrogen was formed. The in situproduction of hydrogen hinders the hematite forma-tion during the grinding [97]. A simple schematic di-

agram of the ball mill equipment is shown in Fig. 8[104].For readers convenience, the fundamentals, ad-

vantages/disadvantages of each method, and compo-sitions of different nanomaterials produced in variousstudies are given in Table 3.

Acknowledgements. This project was supported by the Re-search and Technology Division of National PetrochemicalCompany of Iran (Grant No. 84153).

REFERENCES

1. Glenn, J. C., Technol. Forecast. Soc. 73, 128 (2006).

2. Burda, C., Chen, X., Narayanan, R., and El-Sayed, M.A.,Chem. Rev. 105, 1025 (2005).3. Huang, W. C. and Lue, J. T., J. Phys. Chem. Solids

58, 1529 (1997).4. Huang, W. C. and Lue, J. T.,Phys. Rev. B: Condens.

Matter 59, 69 (1999).5. Lue, J. T., Huang, W. C., and Ma, S. K., Phys. Rev.

B: Condens. Matter 51, 14570 (1995).6. Capek, I.,Adv. Colloid Interface Sci. 110, 49 (2004).7. Liu, T., Leng, Y. H., and Li, X. G.,Solid State Com-

mun. 125, 391 (2003).8. Liu, T., Shao, H. Y., and Li, X. G.,J. Phys.: Condens.

Matter 15, 2507 (2003).9. Lane, R., Craig, B., and Babcock, W., AMPTIAC 6,

31 (2002).10. Nanoscale Materials in Chemistry (Klabunde, K. J.,

Editor). Chapter 4. Wiley, New York, 2001.

11. Nanoscale Materials (Liz-Marzan, L. M. and Kamat,P. V., Editors), p. 81. Kluwer Academic Publishers,Boston, 2003.

12. Olah, G. A. and Laureate, N., in Handbook of Nano-structured Materials and Nanotechnology (Nalwa, H.

S., Editor), Vol. 1, p. 3. Academic Press, San Diego,2000.

13. Gonsalves, K. E., Li, H., Perez, R., Santiago, P., andJose-Yacaman, M.,Coord. Chem. Rev. 206, 607 (2000).

14. Suslick, K. S. and Price, G. J.,Annu. Rev. Mater. Sci.29, 295 (1999).

15. Tjong, S. C. and Chen, H., Mater. Sci. Eng., R 45, 1(2004).

16. Huber, D. L.,Small 1, 482 (2005).17. Daniel, M. C. and Astruc, D., Chem. Rev. 104, 293

(2004).18. Nano-Powders: Organization of the Disordered/Nano-

cluster Nucleation, Chapter 1, http://www.eng.uc.edu/gbeaucag/Classes/Nanopowders/Chapter 1 html/Chapter 1.html

19. Marvast, M. A., Sohrabi, M., Zarrinpashne, S., andBaghmisheh, G., Chem. Eng. Technol. 28, 78 (2005).

20. Marvast, M. A., Sohrabi, M., Zarrinpashne, S., andBaghmisheh, G., Gasoline Production from Syngas:Fixed Bed FT Reactor Study, CHEMCA 2004, Sydney,2004.

21. Mahajan, D., Gutlich, P., Ensling, J., Pandya, K.,Stumm, U., and Vijayaraghavan, P., Energy Fuels 17,1210 (2003).

22. Mahajan, D., Gutlich, P., and Stumm, U.,Catal. Com-mun. 4, 101 (2003).

23. Lopez-Perez, J. A., Lopez-Quintela, M. A., Mira, J.,

Rivas, J., and Charles, S. W., J. Phys. Chem. B 101

,8045 (1997).24. Tavakoli, A., Sohrabi, M., and Kargari, A., Prepara-

tion of Iron Nanoparticles and Study on their CatalyticProperties in FischerTropsch Process, Report No.61/160. Amirkabir University of Technology, Tehran,2005.

25. Champion, Y., Guerin-Mailly, S., Bonnentien, J. L.,and Langlois, P., Scr. Mater. 44, 1609 (2001).

26. Sanders, P. G., Eastman, J. A., and Weertman, J. R.,Acta Mater. 45, 4019 (1997).

27. Chang, W., Skandan, G., Danforth, S. C., Kear, B. H.,and Hahn, H., Nanostruct. Mater. 4, 507 (1994).

28. Li, D., Choi, C. J., Yu, J. H., Kim, B. K., and Zhang,

Z. D., J. Magn. Magn. Mater. 283, 8 (2004).29. Wang, Z. H., Choi, C. J., Kim, B. K., Kim, J. C., and

Zhang, Z. D., J. Alloys Compd. 351, 319 (2003).30. Chang, W., Skandan, G., Hahn, H., Danforth, S. C.,

and Kear, B. H., Nanostruct. Mater. 4, 345 (1994).31. Choi, C. J., Tolochko, O., and Kim, B. K.,Mater. Lett.

56, 289 (2002).32. Choi, C. J., Dong, X. L., and Kim, B. K.,Scr. Mater.

44, 2225 (2001).33. Wang, Z. H., Choi, C. J., Kim, B. K., Kim, J. C., and

Zhang, Z. D., Carbon 41, 1751 (2003).34. Li, D., Choi, C. J., Kim, B. K., and Zhang, Z. D.,J.

Magn. Magn. Mater. 277, 64 (2004).35. Wang, Z. H., Choi, C. J., Kim, J. C., Kim, B. K., and

Zhang, Z. D., Mater. Lett. 57, 3560 (2003).36. Choi, C. J., Kim, B. K., Tolochko, O., and Da, L.,Rev.

Adv. Mater. Sci. 5, 487 (2003).

168 Chem. Pap. 61 (3)151170 (2007)


19/20


37. Dong, X. L., Choi, C. J., and Kim, B. K.,Scr. Mater.47, 857 (2002).

38. Kim, T. S., Sun, W., Choi, C. J., and Lee, B. T., Rev.Adv. Mater. Sci. 5, 481 (2003).

39. Fung, K. K., Qin, B., and Zhang, X. X., Mater. Sci.

Eng., A 286, 135 (2000).40. Lee, D. W., Yu, J. H., Jang, T. S., and Kim, B. K.,

Mater. Lett. 59, 2124 (2005).41. Oh, S. J., Choi, C. J., Kwon, S. J., Jin, S. H., Kim, B.

K., and Park, J. S., J. Magn. Magn. Mater. 280, 147(2004).

42. Wang, Z. H., Zhang, Z. D., Choi, C. J., and Kim, B.K.,J. Alloys Compd. 361, 289 (2003).

43. Dravid, V. P., Host, J. J., Teng, M. H., Elliott, B.,Hwang, J. H., Johnson, D. L., Mason, T. O., and Weert-man, J. R., Nature 374, 602 (1995).

44. Harris, P. J. F. and Tsang, S. C., Carbon 36, 1859(1998).

45. Wu, W. Z., Zhu, Z. P., Liu, Z. Y., Xie, Y. I., Zhang, J.,and Hu, T. D., Carbon 41, 317 (2003).46. Chen, C. P., Chang, T. H., and Wang T. F., Ceram.

Int. 28, 925 (2002).47. Ajayan, P. M., Chem. Rev. 99, 1787 (1999).48. Wang, Y. H., Chiu, S. C., Lin, K. M., and Li, Y. Y.,

Carbon 42, 2535 (2004).49. Kajiura, H., Huang, H. J., Tsutsui, S., Murakami, Y.,

and Miyakoshi, M., Carbon 40, 2423 (2002).50. Osvath, Z., Koos, A. A., Horvath, Z. E., Gyulai, J.,

Benito, A. M., Martinez, M. T., Maser, W., and Biro,L. P., Mater. Sci. Eng., C 23, 561 (2003).

51. Sano, N., Wang, H. L., Chhowalla, M., Alexandrou,I., Amaratunga, G. A. J., Naito, M., and Kanki, T.,

Chem. Phys. Lett. 368, 331 (2003).52. Ohno, S. and Uda, M., Trans. Jpn. Inst. Met. 48, 640

(1984).53. Liu, T., Shao, H. Y., and Li, X. G., Nanotechnology14,

L1 (2003).54. Shao, H. Y., Wang, Y. T., Xu, H. R., and Li, X. G.,

Mater. Sci. Eng., B 110, 221 (2004).55. Grimes, C. A., Qian, D., Dickey, E. C., Allen, J. L.,

and Eklund, P. C., J. Appl. Phys. 87, 5642 (2000).56. David, B., Pizurova, N., Schneeweiss, O., Bezdicka, P.,

Morjan, I., and Alexandrescu, R., J. Alloys Compd.378, 112 (2004).

57. Dumitrache, F., Morjan, I., Alexandrescu, R., Ciupina,

V., Prodan, G., Voicu, I., Fleaca, C., Albu, L., Savoiu,M., Sandu, I., Popovici, E., and Soare, I., Appl. Surf.Sci. 247, 25 (2005).

58. Martelli, S., Mancini, A., Giorgi, R., Alexandrescu, R.,Cojocaru, S., Crunteanu, A., Voicu, I., Balu, M., andMorjan, I., Appl. Surf. Sci. 154, 353 (2000).

59. Veintemillas-Verdaguer, S., Bomati-Miguel, O., andMorales, M. P., Scr. Mater. 47, 589 (2002).

60. Paul, B. K. and Moulik, S. P.,J. Dispersion Sci. Tech-nol. 18, 301 (1997).

61. Gutmann, H. and Kertes, A. S., J. Colloid InterfaceSci. 51, 406 (1973).

62. Liu, J., Kim, A. Y., Wang, L. Q., Palmer, B. J., Chen,Y. L., Bruinsma, P., Bunker, B. C., Exarhos, G. J.,

Graff, G. L., Rieke, P. C., Fryxell, G. E., Virden, J.W., Tarasevich, B. J., and Chick, L. A., Adv. ColloidInterface Sci. 69, 131 (1996).

63. Wang, C. Y., Jiqng, W. Q., Zhou, Y., Wang, Y. N., andChen, Z. Y., Mater. Res. Bull. 35, 53 (2000).

64. Ji, M., Chen, X., Wai, C. M., and Fulton, J. L.,J. Am.Chem. Soc. 121, 2631 (1999).

65. Ohde, H., Hunt, F., and Wai, C. M., Chem. Mater. 13,

4130 (2001).66. Li, F., Vipulanandan, C., and Mohanty, K. K.,Colloids

Surf., A 223, 103 (2003).67. Xu, Z. Z., Wang, C. C., Yang, W. L., Deng, Y. H., and

Fu, S. K., J. Magn. Magn. Mater. 277, 136 (2004).68. Deng, Y., Wang, L., Yang, W., Fu, S., and Elaissari,

A.,J. Magn. Magn. Mater. 257, 69 (2003).69. Tartaj, P. and Tartaj, J.,Chem. Mater. 14, 536 (2002).70. Santra, S., Tapec, R., Theodoropoulou, N., Dobson, J.,

Hebard, A., and Tan, W. H.,Langmuir17, 2900 (2001).71. Yoshimura, M. and Somiya, S., Mater. Chem. Phys.

61, 1 (1999).72. Cote, L. J., Teja, A. S., Wilkinson, A. P., and Zhang,

Z. J., Fluid Phase Equilib. 210

, 307 (2003).73. Lee, J. S. and Choi, S. C.,Mater. Lett. 58, 390 (2004).74. Giri, S., Samanta, S., Maji, S., Ganguli, S., and Bhau-

mik, A., J. Magn. Magn. Mater. 285, 296 (2005).75. Chen, Z. Z., Shi, E. W., Li, W. J., Zheng, Y. Q., and

Zhong, W. Z., Mater. Lett. 55, 281 (2002).76. Mishra, D., Anand, S., Panda, R. K., and Das, R. P.,

Documents

Methods for Synthesis of Nanoparticles