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Recent Patents on Nanotechnology 2009, 3, 32-41 3

1872-2105/09 $100.00+.00 © 2009 Bentham Science Publishers Ltd.

Strategies of Large Scale Synthesis of Monodisperse Nanoparticles

Hongtao Cui1,*, Yongmei Feng

2, Wanzhong Ren

1, Tao Zeng

1, Hongying Lv

1 and Yanfei Pan

1

1College of Chemistry and Biology, Yantai University, Yantai 264005, China,

2College of Life Science and Technology,

Beijing University of Chemical Technology, Beijing 100029, PR China

Received: August 21, 2008; Accepted: October 7, 2008; Revised: October 8, 2008

Abstract: Intensive research focuses on the development of nanoparticles, not only for their fundamental scientific

interest, but also for a variety of technological applications. Monodisperse nanoparticles with a size variation of less than

5% exceptionally have been received much attention, due to their novel and high performance induced by the strong

dependence of properties upon the dimension of the nanoparticles. Their unique properties result in a great of potential

applications in the area of ultra-high density magnetic storage media, electronics, biomedical usage, medical diagnosis,

catalyst, etc. This is stimulating a high level of interest in the development of large scale synthesis techniques of

monodisperse nanoparticles. In this article, the recent advance about the relevant aspects of large scale synthesis

approaches for various monodisperse nanoparticles was summarized.

Keywords: Monodisperse, nanoparticle, large scale synthesis.

INTRODUCTION

According to the acquired knowledge, novel propertiesand high performances of nanoparticles are presented notonly due to their small size, but also their uniform sizedistribution. Monodisperse nanoparticles with a sizevariation of less than 5% show unique properties and higher performances as compared with the corresponding poly-disperse nanoparticles. The noted applications of mono-disperse nanoparticles include quantum dots in the areas ofoptics, biology and computation, magnetic materials in theareas of high-density magnetic recording, medicaldiagnostics, magnetic resonance imaging and drug deliveryetc [1-8]. However, to use their excellent properties forhuman benefit, their large scale production processes withhigh product quality are highly required to be developed.

Chemical preparation of monodispersed nanoparticlesinvolves the process of precipitation of a solid phase fromsolution, which includes the nucleation and growth of particles in the solution. Control of nucleation and thefollowing growth are the key factors for the synthesis ofmonodisperse nanoparticles. Uniformity of the particle sizedistribution is only achieved through a short nucleation period that produces all the particles obtained at the end ofreaction. In this article, recent advance is briefly summarizedfor the large scale production approaches of differentmonodisperse nanoparticles, which include metals, metalalloys, metal oxides, multi-metallic oxides quantum dots andother non-oxide compounds.

CHEMICAL STRATEGIES OF PREPARATION

(1) High Temperature Decomposition and ReductionRoutes of Organometallic Compounds and MetalComplexes

Since the decomposition approach of organometalliccompounds was first introduced in the early 1990s to

Address correspondence to this author at the College of Chemistry andBiology, Yantai University, Yantai 264005, China;

E-mail: [email protected]

produce monodisperse quantum dots [9] as shown in TEMimage of Fig. (1), it has been a common way to obtain mono

disperse nanoparticles. Organometallic compounds arecomplexes which feature direct metal-carbon bonds betweenmetal and organic ligand. Other complexes withoucontaining M-C bonds such as iron pentacarbonyl andtetrakis(trifluorophosphine) nickel are still categorized in thiclass, because their properties and reactivity patterns aretypical of organometallic compounds. Due to the unique

Fig. (1). 5.1 nm CdSe monodisperse nanoparticles prepared by

decomposition of organometallic compound [9].



Large Scale Synthesis of Monodisperse Nanoparticles Recent Patents on Nanotechnology 2009 , Vol. 3, No. 1 33

properties, they gained an advantage on the preparation ofmonodisperse nanoparticles. The general scheme of theorganometallic route can be described as two representativesynthetic procedures. The first is that pyrolysis oforganometallic reagents is initiated by their rapid injectioninto a hot high boiling point solvent, providing temporallydiscrete homogeneous nucleation and permitting controlledgrowth of particles. The second is that reagents including

organometallic compounds are mixed at low temperature andthe resulting solution is slowly heated in controlled mannerto generate nuclei. The particle growth occurs by the furtheraddition of resultants, or particle size is increased by aging athigh temperature by Oswalt ripening. The growth of particles can be stopped by the rapid decrease of reactiontemperature.

Through the careful control of reaction conditions suchas time, temperature and the concentration of reagents andstabilizing surfactants, uniformity of the particle sizedistribution can be achieved. A typical example of the firstsynthetic procedure is the preparation of CdSe [9].Dimethylcadmium and bis(trimethylsily)selenium dissolvedin trioctylphosphine were rapidly injected to a 300°C solventof tri-n-octylphosphine oxide to allow the decom-position oforganometallic compounds. The final nano-particles wereobtained after reaction at 230-260°C for a few hours. In thiscategory of preparation, it was found that the sizedistribution of semiconductor particles can be focusedthrough the concentration control of reactants. At the sametime, their shape was controlled through the manipulation ofgrowth kinetics such as anisotropic growth [1,10]. For thesecond synthetic approach, the synthesis of monodisperseiron nanoparticles [11,12] is exampled. Under inert atmosphere, iron pentacarbonyl was added to a solution at atemperature of higher than 100°C which containeddehydrated octyl ether as high boiling point solvent and oleicacid as surfactant, forming metal surfactant complex. Then,the obtained complex was heated to reflux temperature andwas aged at this temperature for 1 hour to allow the completedecomposition of metal complex. The resulting mixturesolution was cooled and centrifuged to separate the ironnanoparticles. Usually, high decomposition temperatureresults in the formation of metal oxide on the surface ofmetal nanoparticles. The problem can be resolved by thelowering of temperature. For example, during the preparation of iron nanopart icles through the decompositionof pentacarbonyl iron, the iron precursor solution wasinjected to kerosene(solvent) instead of octyl ether at amoderate temperature of 180°C, producing 2g ironmonodisperse nanoparticles without the formation of iron

oxide on the particle surface [13]. The group of Chaudret[14] described another low temperature decompositionapproach of organometallic compounds to produce metal andalloy monodisperse nanoparticles. Its process can begeneralized as following: carboxylic acid (preferably oleicacid) and amine (preferably oleylamine) were dissolved in asolvent (preferably ether). The obtained solution was thenmixed with organometallic metal to form a reactant solution.The solution was heated at 150°C under pressure of 3 Barsof H2 to allow the decomposition of metal precursor for48 hours to obtain shape controlled nanoparticles.

Most of transition metal naoparticles can be prepared bythe simple decomposition of metal complexes oorganometallic precursors, including cobalt, nickel, ironcobalt alloy and so on [11,15]. Metal oxides [16,17] can besynthesized through the oxidation of metal nanoparticles produced from the decomposition of metal complex oorganometallic precursors. For instance, iron nanoparticles[16] were formed first at a refluxing temperature by the

decomposition of iron complex which was synthesized a100°C in octyl ether with oleic acid as surfactant. Theresulting iron particle containing solution was cooled toroom temperature, following by the addition of dehydratedtrimethylamine- N -oxide as oxidizing agent, and then theoxidation of iron to iron oxide nanoparticles was carried ouat 130°C (Fig. 2). The oxidation process does not alwayfollow the mode of metal formation and its oxidation. Theorganometallic compounds or the intermediates can beoxidized directly to metal oxides by any oxidizing agentsuch as air, water vapor, the organic oxidizing agents and theother non-organic oxidizing agents [18].

Fig. (2). Low resolution and high resolution TEM image of 13 nm

-Fe2O3 nanoparticles prepared by the oxidation of iron nanparticles

obtained from the decomposition of iron complex [16].

It is quite difficult for using decomposition of organometallic compounds to prepare some metal alloys. Oneimportant reason is the unavailability of some organometallic precursors due to their complicated and harsh preparation procedures. Therefore, metal salts are consideredto be used for the preparation of alloys. In a successfu production of monodispersed FePt nanoparticles, anorganometallic iron (iron pentacarbonyl) was used as iron

source precursor. At the same time, a platinum salt (platinumacetylacetonate) was used as platinum precursor. Iron pentacarbonyl was added to a 100 °C solution containing the platinum acetylacetonate and the reducing agent. Thesynthesis of FePt nanoparticles was realized by the simultaneous thermal decomposition of iron pentacarbonyl andreduction of Pt salt in the present of surfactant [19,20]However, the stoichiometry of FePt is difficult to control dueto the fact that reaction temperature is higher than boiling point of iron pentacarbonyl which is evaporated before itincorporation into the alloy. Therefore, iron salts such as ironacetylacetonate instead of organometallic iron are used as the



34 Recent Patents on Nanotechnology 2009 , Vol. 3, No. 1 Cui et a

iron precursors [1]. Other disadvantages of organometallic precursors, such as iron pentacarbonyl, are their highly toxicand flammable characteristics, which are quite unsuitable forthe industrial production. Therefore, in the recent progress,organometallic compounds were replaced by metal salts to produce the metal oxides [21] by the decomposition of metalsalts, or metal by the decomposition and reduction of metalsalts [5,22,23] in the hot solvent. TEM image [23] in Fig. (3)

shows the 7 nm Pd monodisperse nanparticles synthesized by decomposition and reduction of Pd surfactant-complex.At first, Pd-surfactant complex was prepared by reactingPd(acac)2 and trioctylphosphine in an argon atmosphere. Pd particles were then formed by the decomposition of thecomplex in a solvent of trioctylphosphine at 300°C andreduction by CO molecules generated in situ from thethermal decomposition of acetylacetonate.

Fig. (3). 7 nm Pd monodisperse nanoparticles synthesized by

decomposition and reduction of Pd surfactant-complex [23].

Some reductants used for the reducing of metal salt, suchas superhydride (LiB(Et)3H) [24], are flammable andcorrosive. As substitution, polyol is widely used to reducethe metal salts to metal particles, which is named polyol process. Polyol process, in which polyol (1,2-propanediol,

1,2-docecanediol, 1,2-hexanedecanediol, 1,2-octanediol etc.)act as a mild reducing agent, is a popular route for thesynthesize metals and metal alloy monodisperse nanoparticles [25-27]. It allows an accurate and reproduciblecontrol of the particles in a broad size range from a fewnanometers to a few micrometers. At the same time, high boiling point of polyols can also be used as solvent as well asreducing agents. In a case of this type of polyol process [28], platinum acetylacetonate and iron (III) acetylacetonate weredissolved in tetraethylene glycol, following by the heating to300°C under irradiation of microwaves and nitrogen gas bubbling. FePt monodisperse nanoparticles were then

synthesized by the decomposition and the reduction byglycol after aging at this temperature for 50 min. A modified polyol process [29] was proposed using a polyme(polyvinylpyrrolidone) to control the size and the dispersityof the silver nanoparticles (Fig. 4). In a typical experimentethylene glycol solution of silver nitrate and polyvinylpyrrolidone was heated to 120°C and maintained at thistemperature for several hours. After cooling and dilution

with water, the reaction mixture afforded silver particleshaving a mean particle size of 21 nm. However, when thi process is scaled up, the size and shape of the particle became non-uniform and the formation of large chunksneedle-like particles and the like was observed. To resolvethis problem, an improved procedure was proposed to avoidthe local concentration gradients and inhomogeneoureaction conditions, through the rapid combination o preheated solutions of polymer and silver precursor [30].

Fig. (4). TEM images of silver nanoparticles whose dispersity andsize were controlled by the concentration of silver precursor and

polymer [29].

Recently, the decomposition approach of metal salts isextended to the preparation of metal oxides and multimetallic oxides, where they are obtained by the thermadecomposition of metal complex salts in hot solvent withhigh boiling point. In a typical synthesis of magnetitenanoparticles [31,32], iron(III) acetylacetonate, 1,2-hexadecanediol, oleic acid, oleyl amine were dissolved in dioctyether and heated to reflux for 30 minutes to allow theformation of magnetite nanoparticles. The particles in theobtained mixture solution were precipitated and separated by

centrifugation. In a similar way, by changing the cobalt salto other metal salts, various MFe2O4 nanoparticles can bemade, in which M=Zn, Cu, Ni, Co, Mn, Cr, V, Ti, Mg, orBa. The particle size can be controlled by changing thestabilizer/iron salt ratio or the reaction temperature.

Another strategy, developed by Hyeon’s group, exhibit potential future in industrial application. This could bespecialized with the preparation of iron oxide of high qualityas shown by a typical TEM image in Fig. (5). These monodisperse nanoparticles [7] were prepared as large as 40g in a




Fig. (5). 12 nm magnetite monodisperse nanoparticles. Inset is a

photograph showing a Petri dish containing 40 g of the mon-

odisperse magnetite nanoparticles [7].

batch by decomposition of iron-oleate salt precursors, whichis described in an overall preparation scheme Fig. (6). Thesolution containing iron chloride and sodium oleate washeated to 70°C and kept at the same temperature for 4 hoursto obtain an iron-oleate complex. The obtained complex wasadded into a dehydrated octadecene solution with oleic acidas surfactant under inert atmosphere at room temperature.The resultant mixture was heated to 320°C for thedecomposition of the complex to metal oxide. The obtained

solution containing the nanoparticles was cooled to the roomtemperature, and then separated by centrifuging. The particlesize can be controlled by using various solvents withdifferent boiling points. This approach is extended readily tothe preparation of other transitional metal oxides and multi-metallic oxides by using different metal salts. The corresponding metal nanoparticles can be prepared by the selfreduction of the resultants at higher reaction temperature.This route is evaluated as a general process for the industrial production of metals, metal oxides and multi-metallic oxideswithout further size-selection step due to its facile,

economical and low toxic characteristics [33-35]. For the preparation of rare earth oxide monodisperse nanoparticlesthe transformation of rare earth oleate to oxide can not becompletely finished until 500°C. However, by the catalysiof the base of oleylamine, the decomposition temperature orare earth complex can be lowered to 310°C [6].

(2) Solvothermal and Hydrothermal Routes

Solvothermal and hydrothermal approaches utilizesolvent under elevated pressures and temperature above or below its critical point to increase the solubility of a solidand to speed up reaction between precursors. Undesupercritical conditions, solvent exhibit characteristics o both a liquid and a gas, where the interfaces of solids andsolvent lack surface tension, yet the solvent shows highviscosities and easily dissolves chemical compounds thawould otherwise exhibit very low solubility under ambienconditions. Some processes of solvothermal and hydrothermal approaches simply take advantage of the increasedsolubility and reactivity of metal salts and complexes aelevated temperatures and pressures without bringing thesolvent to its critical point [36].

Solvothermal and hydrothermal approaches feature hightemperature and pressure in a sealed reaction vessel. Usuallytheir systems consist of three components: precursorssurfactants and solvents, and the precursors include organometallic compounds, metal complexes or inorganic speciesSolvothermal and hydrothermal processes have recently beenextensively applied in the fundamental researches for thesynthesis of monodisperse nanoparticles with new structureand properties. This is due to the well control of growthdynamics and agglomeration of the nanoparticles [37-40].

No matter what well quality control of the route induced by harsh reaction conditions of solvothermal and hydrothermal approaches, it is quite difficult for them to be

applied in industrial synthesis application due to theielevated pressure and temperature during reaction, which ithe reason of a few related documents in patents. Howeverthe description for them in patents maybe provides a way ocomprehension for the well synthetic control preparation omonodisperse nanoparticles in large scale. The simplessolvothermal procedure, exampled with ZnO nanoparticle preparation [41], uses the high pressure reaction omethylzinc isopropoxide in hexadecane at 200 °C for 10hours, yielding 1.2g ZnO particles of less than 10 nm. Fothe further improvement of nanoparticle size uniformity, an

Fig. (6). The overall preparation scheme of monodisperse nanoparticles by decomposition of metal oleate precursors [7].




invert micelle solvothermal approach was adopted in thesynthesis of Ge particles [42]. Invert micelle solution wasformed by mixing of GeCl4, phenyl-GeCl3, hexane (invertmicelle solvent) and pentaethylene glycol ether (cappingagent and shape controlling agent). The micelle solution washeated in a Parr reactor to 280 °C for 72 hours, reduced by Na, producing quite monodispersed Ge nanoparticles. The particle size and dispersity was controlled by the formed

invert micelles. Solvothermal approach has been found thatit is quite unique for the synthesis of nitrides nanoparticles.Some nitrides, such as GaN of rocksalt-phase, weretraditionally prepared at high pressure and temperature.Qian’s group [43] adopted a thermal reaction of Li3 N andGaCl3 in which benzene was used as the solvent under pressure of 5 Mpa at 280 °C, yielding 30 nm mainlyhexagonal-phase GaN with a small fraction of rocksalt-phaseGaN.

Hydrothermal process using water as reaction mediumfollows the similar chemistry of solvothermal approach to prepare monodisperse nanoparticles. Synthesis of metaloxides by this route usually uses pre-prepared hydroxide as precursors before the hydrothermal treatment [44-46]. In atypical synthesis of cerium-titanium composite oxidenanoparticles [44], a suspension of pre-prepared (Ce,Ti)(OH)4 was acidized by nitric acid. Then, the mixture was placed in a closed vessel for a hydrothermal treatment at300 °C. The obtained oxide slurry was cooled to roomtemperature and excess water was decanted, following the pH adjustment to 4 by NH4OH. The final oxide particleswere collected by the repeated washing and the followingfiltration.

Fig. (7). 13 nm PbSe monodisperse nanoparticles prepared by

solvothermal route [40].

(3) Micelle Routes

When concentration of surfactant exceeds the criticalmicelle concentration in water, micelles are formed asaggregates of surfactant molecules. Normal micelles areformed by the orientation of hydrophobic hydrocarbonchains of surfactants toward the interior of the micelle, andthe contact of hydrophilic groups of the surfactants with the

surrounding aqueous solution. Reverse micelles are formed by the directing of hydrophilic head groups toward theaqueous micelles core with outward hydrophobic groups toorganic medium [47]. Since cetyltrimethylammonium bromide (CTAB) was used as surfactant to prepare the firsmicelles by Hoar and Schulman [48] in 1943, CTAB [49]was investigated thoroughly for the synthesis of micelleBesides the CTAB, sodium bis(2-ethylhexyl)sulfosuccinate

(Aerosol OT or AOT) [50] is the most notably anionsurfactant during the recent years. Other less commonly usedsurfactants are nonionic, most based on polyethylene ethersuch as pentaethylene glycol dodecyl ether [51]CH3(CH2)11-O-(CH2-CH2-O-)5-H [52] or Triton-X [53].

Micelles, which are thermodynamically stable, acts as anano-reactor to limit occurrence of reactions inside the wateor oil pools, where the size and dispersity of particle arecontrolled by the droplet size and the surfactants[54].

The formation of reverse micelles is through the mixingof surfactant, cosurfactant, organic solvent and water. The pre-parations of nanoparticles through this way arecategorized into two approaches. The first one is to mix two

different reverse micelles, and reaction occurs by thecoalescence and materials exchange between the micellesThe second is by the coalescence and reaction between onereactant in oil phase and another reactant in the water pool oreverse micelles.

Monodisperse metal nanoparticles as shown in Fig. (8can be prepared by reducing of metal salts in the reversemicelles. For example, FePt nanoparticles were synthesized by mixing of two micelles of metal precursor and reducingagents [55]. The metal precursor micelle was obtained bydispersing of triammonium iron trioxalate, potassiumchloroplatinate aqueous solution in surfactant containingdecane. The reducing agent (NaBH4) containing micelle waobtained in the same way. The FePt monodispersenanoparticles were formed by the mixing of the two reversemicelles and the heat treatment at 50 °C for 1 hour. The fina

Fig. (8). TEM image of Ag2Se nanoparticles prepared by

hydrothermal process [46].




Fig. (9). TEM image of Cu nanoparticles synthesized by reverse

micelle approach [54].

product was collected through destroying of particlecontaining reverse micelles, repeated washing and particlesettling. During the reaction, the dispersity of the particleswas controlled by the ratio of water and surfactant.

Fig. (10). Preparation scheme of FePt monodisperse nanoparticles

by reverse micelle route [55].

Metal oxides, metal carbonate, metal sulfide and othernon-metal nanoparticles were synthesized inside the reversemicelles by hydrolysis and precipitation procedures. In anexample of metal oxide nanoparticle preparation, maghemite[56] was produced through hydrolysis of FeCl3.6H2O inwater pool. Reverse micelles were formed by the mixing ofmetal salt solution and dibenzylether using oleic acid assurfactant. The hydrolysis of FeCl3 and the followingcondensation to form iron hydroxide were initiated and promoted by propylene oxide (proton scavenger) which wasdissolved in oil phase. Yielding of maghemite nanoparticleswas realized by the refluxing of the obtained hydroxide intetralin solvent at above 200 °C. By adjusting molar ratio ofdistilled water and metal salt, or molar ratio of water andsurfactant, the particle size can be tuned. Non-metal oxidesare usually synthesized through precipitation occurringinside micelles. In a typical synthesis of ZnCO3 mono-disperse nanoparticles [57], two 8 nm reverse micelles were prepared by using zinc nitrate and ammonium carbonate as precursors respectively, n-Octane as solvent, cetyltrimethylammonium bromide and n-butanol as surfactant andcosurfactant. The two micelles were mixed together,allowing the exchange of the solutes which resulted in the precipitation of ZnCO3.

Normal micelles are oil droplets in water, where thelength of the surfactant akyl chain controls the size of thedroplets. For example, SiO2 nanoparticles were preparedthrough this approach [58], where normal micelles were firs produced by the mixing of AOT and n-butanol in largeamount of water, following with the second step otriethoxyvinylsilane dissolving into oily micelles. Thehydrolysis and condensation of Si precursor was initiated by

aqueous ammonia which was added into the water phase.A two phase procedure developed by Brust [59

represents a modified micelle route, which focuses on the preparation of Au and other metal monodispersed nano particles. This route is involved in reduction of metal salts inthe interface of oil and water in the micelles under vigoroustirring as exampled in the preparation of Au monodispersenanoparticles [60]. Starting from an aqueous solution oAuCl4

-, the tetrachloroaurate ions were transferred to anorganic phase by vigorously mixing the aqueous solutionwith a toluene solution of tetraoctylammonium bromide(TOAB) (TOAB is a well known phase-transfer catalyst)After adding C10H21SH to the organic phase, an aqueoussolution of NaBH4 was subsequently introduced into themixture to form micelles under rapid stirring. Colloidal goldwas formed in the interface of micelles and was subsequently isolated by vacuum evaporation and following precipitation with methanol.

(4) Sol-Gel Routes

Sol-gel process is a widely used wet chemistry synthesistechnique for the preparation of various oxide materialsTraditionally, it refers to the hydrolysis and condensation oalkoxide-based precursors. Recently, its meaning has beenextended to any kinds of condensation procedures occurring between precursors to form M-O-M bridges. The precursorinclude the conventional alkoxides, metal complexes and

metal salts. The conventional sol-gel processes for the preparation of metal oxides are divided to synthesis stepwhere an amorphous phase of the desired oxide is formedand an annealing step where the obtained precursor is heatedto a high temperature to crystallize the solid. This usuallycauses the aggregation of the produced nanoparticles anddoes not allow the formation of uniform-sized particlesTherefore, it is very difficult to obtain monodispersenanoparticles through hydrolytic sol-gel route. Howevermonodispersed nanoparticles can still be prepared by amodified hydrolytic sol-gel route, where the hydrolysiscondensation, and crystallization occur in a high boiling point solvent under the protection of surfactants [61-63]. In atypical synthesis of TixSn(1-x)O2 [61], titanium and tin

alkoxides were mixed with oleic acid under a nitrogen gasatmosphere to form a reaction solution. Trimethylamineoxide as reaction initiator was dissolved in water to form acatalyst solution. The two solution was mixed together andheated to 100 °C to initiate reaction of hydrolysis, condensation and crystallization to produce TixSn(1-x)O2 nano particles. The particle size can be controlled by appropriateselection of the type of alkoxy group in the alkoxides.

Since a non-hydrolytic sol-gel approach [64] for the preparation of metal oxides and multi-metallic oxides wasestablished in the early 1990s, it recently became a new route




Fig. (11). Preparation scheme of TixSn(1-x)O2 by a sol-gel route [61].

for the uniform-sized nanoparticles. In conventionalhydrolytic sol-gel processes, sol and gel are formed due tothe formation of M-O-M bridges through hydrolysis andcondensation reactions. However, in the non-hydrolytic sol-gel route, M-O-M bridges formation results from the non-hydrolytic condensation of metal precursors. Non-hydrolyticsol-gel processes are classified into two categories: non-hydrolytic hydroxylation reaction and aprotic condensationreactions [65]. The latter is the frequently used reaction for

the synthesis of nanoparticles. In this process, M-O-M bridges are formed by the condensation reaction betweentwo metal centers with functional groups through removing asmall organic molecule through ether elimination, esterelimination and alky halide elimination. Through the non-hydrolytic sol-gel approach, monodispersed or nearlymonodispersed nanoparticles can be produced [62,66].

Fig. (12). TEM image of zirconia monodisperse nanoparticles

prepared by non-hydrolytic sol-gel route [66].

(5) Chemical Precipitation Routes

Chemical precipitation is a widely used approach for the preparation of nanoparticles. During the process of precipitation, nucleation is a key step of the precipitation process and a large number of small particles are generated.In the following, secondary process such as Ostwaldripening and aggregation dramatically affect the size of the particles. Due to its simultaneous occurrence of nucleation,growth, coarsening and agglomeration processes, carefulcontrol of the reaction conditions is required for the

generation of monodisperse nanoparticles. Its essentiacontrolling factors include styles of precipitation reaction pH, concentration, temperature, surfactants etc. All thesefactors must focus on the nucleation with only one bursformation of nuclei during the whole precipitation reactions.

The preparation of metal oxides by precipitation cangenerally be classified into two categories: one is that the precursors obtained by precipitation reaction are required to

be heat treated at high temperature for the crystallizationanother is the direct formation of metal oxide duringreaction. During the preparation of oxides, the products o precipitation, particularly those performed at or near roomtemperature, are usually amorphous. In those cases wherehydroxides or carbonates of mixed metals are precipitatedfrom solution and subjected to a calcination or postannealing process, some aggregation is unavoidable due to the heatreatment at high temperature. Therefore, there is littlechance of the particles being monodispersed, which can beexampled by the synthesis of ZnO [67] through precipitationreaction. The zinc hydroxide was prepared with the reaction between zinc ions and KOH in methanol solution, followed by the drying of obtained hydroxide gel to form thecrystallized ZnO. The obtained ZnO shows a wide particlesize distribution induced by aggregation. The kind oaggregation can be decreased by using of aggregationinhibitor during the heat treatment. In a synthesis of yttriumstabilized zirconia (YSZ) nanoparticles [68], hydrous YSZ precursor was prepared by the precipitation reaction of metasalts with NH4OH. The obtained YSZ precursor was mixedwith SrCO3 nanoparticles, and then followed by thecalcination of the mixture at 600 °C. The nearly monodispersed YSZ nanoparticles were obtained by avoiding theaggregation between YSZ particles through the physicaisolation of SrCO3 nanoparticles. The SrCO3 as impurity wadissolved by the washing with 10% HNO3.

Fig. (13). TEM image of nearly monodispered YSZ nanoparticle

prepared by precipitation route with a post calcination step [68].




On the other hand, direct formation of metal oxide duringreaction gains better chance for the goal of monodispersestate than the first approach through careful control ofnucleation and avoiding of the heat-treatment inducedaggregation. A typical illustration is the preparation ofanatase TiO2 [69]. The precipitation and crystallization ofTiO2 were initiated by the dropewise addition of aqueousacidic solution of hydrazine to an acidic solution of titanium

tetrachloride. The nearly monodispersed nanoparticles of the product were collected through the filtration, washing anddrying step. For the example of mixed metal oxides such as(CoxMn1x)3O4 [70], its crystalline phase was formed by theautoxidation of Mn(OH)2. In the synthesis, aqueous solutionof Co(NO3)2 and Mn(NO3)3 was added dropwise to a LiOHsolution, where LiOH was used as precipitating agent. Thenearly monodisperse (CoxMn1x)3O4 nanoparticles were produced by the in situ autoxidation of Mn(OH)2 in thesolution. LiOH was applied as precipitating agent in stead ofthe widely utilized NH4OH due to the ammonium ion’sretardation on the autoxidation of Mn(OH)2.

Fig. (14). TEM image of nearly monodispersed (CoxMn1x)3O4

nanoparticles synthesized by precipitation at room temperature

[70].

(6) Size Selection for Monodisperse Nanoparticles

The definition of monodisperse nanoparticle is strictlyreferred to the specified particles with a size variation of lessthan 5%. Although careful control of nucleation and particlegrowth, avoiding occurrence of secondary nucleation duringthe particle growth, can lead to the formation of mono-disperse nanoparticles, sometimes, it is inevitable to exceedthis standard size deviation of particles induced by thereaction conditions and preparation methods. In this case,size selection is necessary for the synthesis of monodispersenanoparticles, especially for the large scale production.

The physicochemical properties of semiconductornanoparticles appear in dependence on particle size due tothe quantum size effect. Accordingly, it is necessary toaccurately conduct a particle-size selection from thesemiconductor nanoparticles immediately after preparationthat have a wide particle size distribution, using a chemical

technique, so that only semiconductor nanoparticles of aspecific particle size can be isolated and extracted to achievemonodispersion. There exists a conventional and subtle sizeselection technique for metal chalcogenide quantum dotsnamely the size-selective photoetching method [71-73]. Thimethod utilizes the fact that the energy gap increases due tothe quantum size effect as the particle size of thesemiconductor nanoparticles decreases, and that the meta

chalcogenide semiconductor undergoes oxidizing melting ait is irradiated with light in the presence of dissolved oxygenSpecifically, semiconductor nanoparticles with a wide particle size distribution are irradiated with monochromaticlight of a wavelength shorter than the wavelength of theabsorption edge of the particles, so that semiconductonanoparticles with larger particle sizes are selectivelyoptically excited and dissolved, thereby obtaining smallesemiconductor nanoparticles with uniform particle size. In atypical process of size selection of metal chalcogenide, thesize selective photoetching was described as following [71]First, bubbling is performed using nitrogen gas in polydispersed semiconductor nanoparticles containing solutionwhich are stabilized by hexametaphosphate. Anothe

bubbling is conducted using oxygen for 10 min. Thenmethylviologen is added to the solution and irradiation omonochromatic light is conducted to optically dissolve thesemiconductor nanoparticles while stirring. When thesemiconductor nanoparticles with a size deviation of morethan 15% are irradiated with light with wavelength 476.5nm, the resulted particles exhibit a very narrow particle sizedistribution with a standard deviation of 6%. The sizeselective photoetching procedure is not only used in sizeselective photoetching of semiconductor nanoparticles bualso in that of other nanoparticles to which size-selective photoetching can be applied. Examples of the othernanoparticles are those of Ag, Au, other metals, and singlecrystal carbon [72].

Other size selection procedures, such as electrophoreticseparation method [74] that takes advantage of the variationof surface charge of the nanoparticle depending on particlesize, the exclusion chromatography [75] that takes advantageof the difference in retention time, are quite unsuitable forlarge scale size selection due to their high operation costSize selective precipitation [76-79] is a frequently usedtechnique for the narrowing of the particle size distributionThis procedure takes advantage of the difference indispersibility into an organic solvent depending on particlesize. The dispersibility of particles with different size can bemanipulated by solvent composition of the nanocrystallitesuspension. A typical size selective precipitation may be

illustrated by an example of CdSe [76]. The CdSenanocrystallites are stabilized in solution by the formation oa lyophilic coating of alkyl groups on the crystallite outersurface. The alkyl groups are provided by the coordinatingsolvent (butanol) which is used during the growth periodThe interparticle repulsive force introduced by the lyophiliccoating prevents aggregation of the particles in solution. Theeffectiveness of the stabilization is strongly dependent uponthe interaction of the alkyl groups with the solvent. Graduaaddition of non-solvent (methanol) will lead to the sizedependent flocculation of the nanocrystallites, resulting infirst flocculating of the largest particles.




In a process described by group of Roberts [80], size-selective precipitation of semiconductor nanoparticles isachieved by finely tuning the solvent strength of theCO2/hexane medium by simply adjusting the applied CO2 pressure. These subtle changes affect the balance betweenosmotic repulsive and van der Waals attractive forces,thereby allowing fractionation of the nanocrystals intomultiple narrow size populations. In another process [81],

through the size dependent interfacial interaction betweenthe nanoparticles, the particle-particle separation distancewas modified by selecting a suitable surfactant type and/or

conditions. Smaller particles were stabilized against agglo-meration and larger particles were agglomerated and settled.

CURRENT & FUTURE DEVELOPMENTS

With the great development of nanotechnology, mono-disperse nanoparticles are showing their promising appli-cation in different areas. Among all the available approachesfor their synthesis, some of them are quite capable for thecontrol of particles size and size distribution, providing highquality products. For example, micelle route can be used tosynthesize various monodisperse nano-particles including

metal, metal oxides and quantum dots. However, micelleroute can not be applied in the large scale industrial production due to its inherent disadvantages such as lowconcentration of reactant, large mount of surfactant, repeatedwashing for removal of surfactant, impurities etc. Hightemperature decomposition of metal complexes is a widelyused route, not only owing to its well control of nucleationand particle growth, but also its typical charac-teristics forthe large scale synthesis. Hereinto, the route developed byHyeon group [7, 33-35] is the most promising generalapproach. It can produce monodisperse nanoparticles ofmetals, metal alloys, metal oxides and multi-metallic oxideswith very high quality in large scale. The most important point is the use of low cost precursors and mild reaction

condition.

The trend for large scale production of monodispersenanoparticles is the use of low cost precursors, simple process, low consumption of energy, environment friendlyand no further process of size selection. The existingapproaches do not fully follow this trend in every aspect. Amuch simpler route is still highly required. Recently, anepoxide assisted sol-gel route was developed, showing potential future in large scale synthesis of monodispersemetal oxide nanoparticles. The particles can be obtained byone step reaction which is performed by the simply boilingof ethanol solution of metal salts after the addition ofepoxide. The epoxide acts as an acid scavenger that con-

sumes protons from the metal aquo complexes [M(H2O)x]

n+

,which promotes the hydrolysis and condensation of thecomplexes resulting in the formation of sol and gel. Thisroute has the typical characteristics of the developing trendfor the large scale preparation of uniform-sized nanoparticles. Although the work is still under the way, it has been proven a general route for the synthesis of iron basedspinel type monodisperse nanoparticles [82].

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