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Introduction

Nanomaterials and/or nanoparticles areused in a broad spectrum of applications.Today they are contained in many prod-ucts and used in various technologies. Mostnanoproducts produced on an industrialscale are nanoparticles, although they al-so arise as byproducts in the manufactureof other materials1.

Most applications require a precisely de-fined, narrow range of particle sizes (mo-nodispersity). Specific synthesis processesare employed to produce the various nano-particles, coatings, dispersions or compos-ites. Defined production and reaction con-ditions are crucial in obtaining such size-dependent particle features. Particle size,chemical composition, crystallinity andshape can be controlled by temperature,pH-value, concentration, chemical compo-sition, surface modifications and processcontrol.

Two basic strategies are used to producenanoparticles: “top-down” and “bottom-up”. The term “top-down” refers here to themechanical crushing of source material us-ing a milling process. In the “bottom-up”strategy, structures are built up by chemi-cal processes (Figure 1). The selection ofthe respective process depends on thechemical composition and the desiredfeatures specified for the nanoparticles.

Summary

Materials in the nanometer range havebeen produced for several decades.Carbon Black, for example, has beenused in tires since 1930. Today, the pro-duction capabilities for specially de-signed nanomaterials have increasedconsiderably. Most synthetically pro-duced nanomaterials are nanoparticles(80 %). The various applications requireprecisely defined nanoparticle charac-teristics. A number of production process-es have been developed to meet the de-sired shapes, compositions and size dis-tributions. This dossier describes themost common production processes suchas milling, gas phase and liquid phasetechnologies.

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* Corresponding author

Production of nanoparticles andnanomaterials

Christina Raab, Myrtill Simkó*,Ulrich Fiedeler, MichaelNentwich, André Gazsó

Figure 1:Methods of nanoparticle production:

top-down and bottom-up2

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Top-Down/mechanical-physicalproduction processes

“Top-down” refers to mechanical-physicalparticle production processes based on prin-ciples of microsystem technology. Thetraditional mechanical-physical crushingmethods for producing nanoparticles involvevarious milling techniques (Figure 2).

Milling processes

The mechanical production approach usesmilling to crush microparticles. This ap-proach is applied in producing metallic andceramic nanomaterials. For metallic nano-particles, for example, traditional source ma-terials (such as metal oxides) are pulverizedusing high-energy ball mills. Such mills areequipped with grinding media composed ofwolfram carbide or steel.

Milling involves thermal stress and is ener-gy intensive. Lengthier processing can po-tentially abrade the grinding media, contam-inating the particles. Purely mechanical millingcan be accompanied by reactive milling:here, a chemical or chemo-physical reac-tion accompanies the milling process.

Compared to the chemo-physical produc-tion processes (see below), using mills tocrush particles yields product powders witha relatively broad particle-size ranges. Thismethod does not allow full control of parti-cle shape.

Bottom-up/Chemo-physicalproduction processes

Bottom-up methods are based on physico-chemical principles of molecular or atomicself-organization. This approach producesselected, more complex structures fromatoms or molecules, better controlling sizes,shapes and size ranges. It includes aerosolprocesses, precipitation reactions and sol-gel processes (Figure 3).

Gas phase processes (aerosol processes)1

Gas phase processes are among the mostcommon industrial-scale technologies forproducing nanomaterials in powder or filmform.

Nanoparticles are created from the gasphase by producing a vapor of the productmaterial using chemical or physical means.The production of the initial nanoparticles,which can be in a liquid or solid state, takesplace via homogeneous nucleation. De-pending on the process, further particlegrowth involves condensation (transitionfrom gaseous into liquid aggregate state),chemical reaction(s) on the particle surfaceand/or coagulation processes (adhesion oftwo or more particles), as well as coalescenceprocesses (particle fusion). Examples includeprocesses in flame-, plasma-, laser- and hotwall reactors, yielding products such asfullerenes and carbon nanotubes:

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Figure 2:Overview of mechanical-physical

nanoparticle production processes

Figure 3: Chemo-physical processes in nanoparticle production

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• In flame reactors, nanoparticles are for-med by the decomposition of source mo-lecules in the flame at relatively high tem-peratures (about 1200–2200°C). Flamereactors are used today for the industrial-scale production of soot, pigment-titaniumdioxide and silicon dioxide particles.

• In plasma reactors, plasma (ionized gas)provides the energy for the vaporizationand for initializing the decomposition re-actions.

• In laser reactors, lasers selectively heatthe gaseous source material, utilizing itsabsorption wavelength, and decomposeit to the desired product.

• In hot wall reactors, vaporization andcondensation are applied. The source ma-terial is vaporized in an inert gas underlow pressures (ca. 1 mbar). This removesthe enriched gas phase from the hot zone. The particles created by the rapidcooling are collected on filters. Techni-cally, hot wall reactors are used for exam-ple in producing nanoscale nickel- andiron powders.

• The chemical gas phase deposition proc-ess is used to directly deposit nanoparti-cles from the gas phase onto surfaces. Here, the source material is vaporized ina vacuum and condensed on a heatedsurface by a chemical reaction, i.e. de-posited from the gas phase into the so-lid final product.

Droplet formationcontaining particles

Particles can also be produced from dropletsusing centrifugal forces, compressed air, son-ic waves, ultrasound, vibrations, electrostat-ics and other methods. The droplets aretransformed into a powder either through di-rect pyrolysis (thermal cleavage of chemicalcompounds) or via direct reactions with an-other gas. In spray pyrolysis, droplets of thesource material are transported through ahigh-temperature field (flame, oven), whichrapidly vaporizes the readily volatile com-ponents or leads to decomposition reactions.The formed particles are collected on filters.

Liquid phase processes3

The wet-chemical synthesis of nanomateri-als typically takes place at lower tempera-tures than gas phase synthesis. The most important liquid phase processes in nano-material production are precipitation, sol-gel-processes and hydrothermal syntheses(see Figure 3).

Precipitation processes

The precipitation of solids from a metal ion-containing solution is one of the most fre-quently employed production processes fornanomaterials. Metal oxides as well asnon-oxides or metallic nanoparticles can beproduced by this approach. The process isbased on reactions of salts in solvents. A pre-cipitating agent is added to yield the desiredparticle precipitation, and the precipitate isfiltered out and thermally post-treated.

In precipitation processes, particle size andsize distribution, crystallinity and morphol-ogy (shape) are determined by reactionkinetics (reaction speed). The influencingfactors include, beyond the concentration ofthe source material, the temperature, pH value of the solution, the sequence in whichthe source materials are added, and mix-ing processes.

A good size control can be achieved by us-ing self-assembled membranes, which inturn serve as nanoreactors for particle pro-duction. Such nanoreactors include micro-emulsions, bubbles, micelles and liposomes.They are composed of a polar group and anon-polar hydrocarbon chain. Micro-emul-sions, for example, consist of two liquids thatcannot be mixed with one another in the con-centrations used, usually water and oil alongwith at least one tenside (substance that re-duces the surface tension of liquids). In cer-tain solvents this gives rise to small reactorsin which nucleation and controlled particlegrowth take place. Particle size is determinedby the size of the nanoreactors and, at thesame time, particle agglomeration is pre-vented. Micro-emulsion processes are of-ten used to produce nanoparticles for phar-maceutical and cosmetics applications.

An additional process that is based on self-organized growth with templates and coat-ings is hydrothermal synthesis. Zeolites (mi-croporous aluminum-silicon compounds)are produced from aqueous superheated so-lutions in autoclaves (airtight pressure cham-bers). The partial vaporization of the solventcreates pressure in the autoclaves (severalbars), triggering chemical reactions that dif-

fer from those under standard conditions, forexample by altering the solubility. Nanopar-ticle formation and cavity shape can be con-trolled by adding templates. Templates areparticles with bonds that enable the forma-tion of certain forms and sizes.

Sol-gel processes

Sol-gel syntheses (production of a gel frompowder-shaped materials) are wet-chemicalprocesses for producing porous nanomate-rials, ceramic nanostructured polymers aswell as oxide nanoparticles. The synthesistakes place under relatively mild conditionsand low temperatures. The term sol refers todispersions of solid particles in the 1-100 nmsize range, which are finely distributed in wa-ter or organic solvents. In sol-gel process-es, material production or deposition takesplace from a liquid sol state, which is con-verted into a solid gel state via a sol-geltransformation. The sol-gel transformationinvolves a three-dimensional cross-linkingof the nanoparticles in the solvent, where-by the gel takes on bulk properties. A con-trolled heat treatment in air can transformgels into a ceramic oxide material.

To start with, adding organic substances inthe sol-gel process produces an organo-metallic compound from a solution contain-ing an alcoxide (metallic compound of analcohol, for example with silicon, titaniumor aluminum). The pH value of the solutionis adjusted with an acid or a base which, asa catalyst, also triggers the transformationof the alcoxide. The subsequent reactions arehydrolysis (splitting of a chemical bond bywater) followed by condensation and poly-merization (reaction giving rise to many- orlong-chained compounds from single-chained ones). The particles or the polymeroxide grow as the reaction continues, untila gel is formed. Due to the high porosity ofthe network, the particles typically have alarge surface area, i.e. several hundredsquare meters per gram. The course of hy-drolysis and the polycondensation reactiondepend on many factors: the compositionof the initial solution, the type and amountof catalyst, temperature as well as the reac-tor- and mixing geometry.

For coatings, the alcoxide initial solution ofthe sol-gel process can be applied on sur-faces of any geometry. After the wetting, thebuild-up of the porous network takes placethrough gel formation, yielding thicknessesof 50-500 nm. Thicker layers, suitable asmembranes for example, are created by re-peated wetting and drying. The sol-gel proc-ess can also be used to produce fibers. In

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all cases, gel formation is followed by a dry-ing step. Figure 4 illustrates the different re-action and processing steps of the sol-gelprocess.

A distinct advantage of the sol-gel processlies in the processability of the sols and gels,depending on processing step, into powders,fibers, ceramics and coatings. Moreover,highly porous nanomaterials can be pro-duced. Composites can be created by fill-ing in these pores during or after gel pro-duction. The low process temperature alsoenables substances to be embedded into thegel during the synthesis step; these can thenbe stored or released in a controlled man-ner. The disadvantage of the sol-gel processlies in the difficult-to-control synthesis anddrying steps, which complicate scaling up theprocess. Moreover, organic contaminantscan remain in the gel. The resulting neces-sary cleaning steps, drying and thermal post-treatment makes this production processmore complex than gas phase synthesis.

The disadvantage of the wet-chemical syn-thesis of nanomaterials is that the desiredcrystalline shapes often cannot be configuredand that the thermal stability of the productpowder is lower. This requires thermal post-treatment with repeated reduction of theparticle surface. The advantage is that theliquid phase enables highly porous materi-als to be produced; this would normally notbe possible in gas phase reactors due to thehigh temperatures. With a few exceptions,gas phase processes also do not allow theproduction of organic nanoparticles. Liquidphase processes are particularly suited forthe targeted production of monodisperseproduct powder (with uniform particle size).

Notes and References1 Rössler A, Georgios Skillas Sotiris E. Pratsinis,

2001, Nanopartikel – Materialien der Zukunft:Maßgeschneiderte Werkstoffe, Chemie in un-serer Zeit 35(1), 32-41.

2 lmn.web.psi.ch.3 Stefan, E., 2004, Chemische Technik. Prozesse

und Produkte, 5. Aufl. Herausgegeben vonRoland Dittmeyer, Wilhelm Keim, GerhardKreysa und Alfred Oberholz, AngewandteChemie 116(42), 5687-5788.

4 www.uni-ulm.de/uni/fak/natwis/anorgchem.

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Explanatory notes:

Dispersion (dispersed system): a mixturecomposed of two or more phases in whichone component is very finely distributed inanother.

Emulsion: dispersed system in which liquiddroplets are distributed in another liquid.

Suspension: dispersed system in which in-soluble solid particles are distributed in aliquid.

Figure 4: Reaction and processing steps in the sol-gel process4

MASTHEAD:

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Mode of publication: The NanoTrust Dossiers are published irregularly and contain the research results of the Institute of Technology Assessment in the framework of its research project NanoTrust. The Dossiers are made available to the public exclusively via the Internet portal “epub.oeaw” :epub.oeaw.ac.at/ita/nanotrust-dossiers

NanoTrust-Dossier No. 006en, February 2011: epub.oeaw.ac.at/ita/nanotrust-dossiers/dossier006en.pdf

ISSN: 1998-7293

This Dossier is published under the Creative Commons (Attribution-NonCommercial-NoDerivs 2.0 Austria) licence: creativecommons.org/licenses/by-nc-nd/2.0/at/deed.en