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Chemical Aerosol Engineering as a Novel Tool forMaterial Science: From Oxides to Salt and MetalNanoparticlesEvagelos K. Athanassiou a , Robert N. Grass a & Wendelin J. Stark aa Institute for Chemical and Bioengineering, ETH Zurich, Zurich, Switzerland

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Aerosol Science and Technology, 44:161–172, 2010Copyright © American Association for Aerosol ResearchISSN: 0278-6826 print / 1521-7388 onlineDOI: 10.1080/02786820903449665

Chemical Aerosol Engineering as a Novel Tool for MaterialScience: From Oxides to Salt and Metal Nanoparticles

Evagelos K. Athanassiou, Robert N. Grass, and Wendelin J. StarkInstitute for Chemical and Bioengineering, ETH Zurich, Zurich, Switzerland

Aerosol nanotechnology has rapidly evolved in the past years.This fascinating technology has resulted in the development offunctional nanomaterials providing novel solutions in industrialapplications. The extensive research on the physical understand-ing of gas phase processes has strongly contributed to the presentindustrial use of single and mixed oxides and the design of indus-trial aerosol reactors. Recent advances have shown that chemicalaerosol engineering can be established on the interface betweenclassical aerosol science and chemical engineering. The emergingnew methods give access to a much broader class of functional ma-terials including salt and metal nanoparticles. The latter impliesthat aerosol production units can now be considered as chemi-cal reactors. The incorporation of thermodynamic considerationsand chemical kinetics in the modelling of gas phase processes willfurther boost the development of aerosol engineering and will pro-vide deeper understanding of the fundamentals of particle forma-tion mechanisms. This will ultimately enable access to new multi-component materials with various structures or morphologies andthe development of more sustainable, energy efficient gas phaseprocesses.

INTRODUCTIONAerosol particle technology has gone through a tremen-

dous development in the past thirty years (Kruis et al. 1998a;Pratsinis 1998; Rosner 2005; Roth 2007; Swihart 2003). Fun-damental studies on particle dynamics, combustion science andinnovative engineering now enable the scalable synthesis ofnanoparticles through a broad variety of processes. The increas-ing amount of research on nanoparticle synthesis has been trig-gered by very distinct properties of such particles, leading tonovel materials with possible new and improved applications(Gleiter 1989). Some of the most advantageous properties were

Received 20 August 2009; accepted 26 October 2009.The authors acknowledge financial support by ETH Zurich in the

form of a TH grant (TH-02 07-3) and the Swiss National Science Foun-dation (SNF 200021-116123).

Address correspondence to Wendelin J. Stark or EvagelosK. Athanassiou, Institute for Chemical and Bioengineering, ETHZurich, Wolfgang-Pauli str. 10, 8093 Zurich, Switzerland. E-mail:wendelin.stark@chem.ethz.ch; athanssiou@chem.ethz.ch

discovered several decades ago and motivated the developmentof new nanoparticles. Carbon black, silica, and titania, are read-ily available from commercial gas-phase-based processes in theorder of several million metric tons per year (Kammler et al.2001; Osterwalder et al. 2006; Stark and Pratsinis 2002). Thewide success of these materials as well as a better understandingof the process dynamics favored the analysis and synthesis of amuch broader class of materials than the traditional metal ox-ides, such as metal salt (Grass and Stark 2005), sulfides (Kruis etal. 1998b) and pure metal nanoparticles (Grass and Stark 2006a)and films (Rosner 2005).

Nanoparticles of such a wide range of materials can be pre-pared by a variety of methods. Nevertheless, gas-phase-basedaerosol processes have dominated over wet-based methods (Parket al. 2004) and top-down approaches, as they provide a scalablemanufacturing of nanomaterials exhibiting high purity and rela-tively narrow particle size distributions. The scope of this reviewis, therefore, to give an overview on the use of aerosol basedprocesses for the production of nanoparticles. We would like tohighlight the evolutionary development of gas phase aerosolprocesses from mostly physical investigations into chemicalprocessing, which has allowed the synthesis of new materialsbeyond oxides. Without the physical understanding of particledynamics and combustion aerosol formation, later inclusion ofchemical control in aerosol processing would have not beenfeasible (Friedlander and Wang 1966; Kruis et al. 1993; Land-grebe and Pratsinis 1990; Zachariah and Carrier 1999). Thedetailed understanding of particle nucleation, coagulation andaggregation provided the fundament to engineer the productionof nanoparticles under controlled combustion conditions. Thesemainly physical studies have enabled the production of titaniaand silica, which today serve as reference studies while thephysical principles can be implemented to any material class(oxides, salts, metals).

In the past few years, chemical aerosol engineering estab-lished itself on the interface between classical aerosol scienceand the discipline of chemical engineering. This implies thataerosol production units can be considered as chemical re-actors, where ultra-fast chemical reactions take place duringparticle formation or, in other words, next to physical (coagu-lation, sintering, agglomeration) chemical processes (reaction,

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reduction, solid-state chemistry) contribute to the particle for-mation process. First indications show that simple thermody-namic considerations and calculations follow very accuratelythe experimental findings. This indicates that the gas compo-sition and temperature of the aerosol comprising gas streamafter the combustion can be used for the further developmentof aerosol processes to prepare new classes of materials such asmetals, salts, sulfide and other compositions of nanoparticles.Nevertheless, it remains a challenge to incorporate these chemi-cal thermodynamic considerations in the modeling of gas phaseprocesses and to analyze the competition of different chemicalreactions (reduction vs. oxidation), the influence of the concen-tration of combustion gases (H2O, H2, CO2, CO) and the char-acteristic time of each chemical reaction step in comparison tothe particle dynamics. Another important factor is the validityof thermodynamics concerning the turbulent or diffuse condi-tions of the produced aerosol. This will provide a deeper un-derstanding of nanomaterial synthesis from gas phase processesand further contribute to controlled scale-up of reactors for themanufacturing of more complicated materials than oxides. Thereview gives an overview on the different classes of nanomate-rials derived from aerosols and highlights how chemical aerosolengineering triggered this rapid expansion of aerosol derivednanoparticle products. It further analyzes some of the key futurechallenges for aerosol engineering, which arise at the interface ofthis established technology with other engineering disciplines.

GAS PHASE PROCESSESWhile top-down approaches, such as ball milling (Schiotz

et al. 1998; Suryanarayana 2001) are generally straightforwardand simple, they are of very low energy efficiency (Oster-walder et al. 2006) and mostly result in products with inho-mogeneous particle sizes. Bottom-up synthesis methods resultin well-controlled nanoparticles, which are produced by reac-tion of a metal precursor, followed by a controlled aggregation.The “bottom-up” methods are divided in liquid or gas phasemethods. Compared with the gas phase, reactions in liquid me-dia are easily conducted in a laboratory setup, usually involvemoderate temperatures but often require complex chemicals atmoderate prices. Due to the smaller diffusion coefficient in liq-uids the chemical reactions are much slower, resulting in lowproduction rates (Osterwalder et al. 2006). Separation of theproduced nanomaterials is also a major issue in comparisonto the more easily powder filtration of an aerosol gas phaseprocess. Nevertheless the slower chemical reaction rates allowbetter control of the experimental conditions and parameters de-termining the physical formation processes such as nucleation,growth, and aggregation of nanoparticles in contrast to the oftenturbulent conditions of gas phase processes. This has as an effectthat wet phase methods result in the preparation of functionalnanomaterials with uniform (monodisperse) sizes and size dis-tributions and also enable the easier access to nanomaterials withvarious anisotropic shapes or structures comparing to the poly-

disperse, usually spherical nanomaterials derived by gas phaseprocesses.

Compared to the liquid phase, the gas phase manufacturingof nanoparticles excels in terms of high rate and low volumerequirements. Reaction and residence times are generally in theregion of milliseconds and rarely exceed one second for thewhole process from nanoparticle formation to separation. Mostaerosol gas phase processes are characterized by the use of hightemperatures, often above 1000◦C. The common denominator ofthe individual processes is the formation of a molecular nuclei,either from condensation or from chemical reaction at high tem-peratures. The nuclei then quickly collide by Brownian motionand thermophoresis and grow by coalescence in high tempera-ture regions (physical description). In order to limit the growthand aggregation of the nanoparticles, the residence time needsto be kept as short as possible or the process needs to exhibitextremely fast cooling rates (>1000 K s−1).

Aerosol gas phase processes vary in their energy or heat sup-ply and nucleation source. The main, industrially relevant, gas-phase processes exhibiting high temperatures in combinationwith high cooling rates are plasma and laser-based processingwith electricity as an energy source and combustion processeswhich rely on fossil energy.

Laser-Based Aerosol ProductionThese aerosol reactors involve a laser source to vaporize

chemical precursors followed by condensation and/or chem-ical reaction to form nanoparticles. Kato (1976) showed thepreparation of several oxides such as SiO2, Fe3O4, CaTiO3,and Mg2SiO4 by laser ablation and the use of a continuous-wave CO2 laser. A more suitable large-scale synthesis methodis laser-ablation of microspheres. This process proceeds by thetransport of suitable microspheres (e.g., metals, ceramics, ox-ides, semiconductors) to the reaction zone where the spheresare irradiated with a laser beam. If the energy of the laser issufficiently high, the particles vaporize (ablate) without goingthrough the liquid state and nanoparticles are formed from thecooling vapors (Hergenroder 2006).

Illumination of gaseous precursors by a laser results in a rapidtemperature increase triggering the precursor decomposition.Examples include silicon nanoparticles from SiH4 gas (Cannonet al. 1982) and iron or iron oxide nanoparticles from gaseousFe(CO)5 (Majima et al. 1994; Majima et al. 1993; Martelliet al. 2000). Advanced process reactor systems have been de-scribed by Gardner et al. (2003) or Bi and Kambe (2001) andallow the use of liquid, vapor and aerosol precursor systems en-abling the fabrication of nanomaterials of various compositionsand relatively low cost. This approach, therefore, enables themanufacture of nanoparticles at high production scales (Fried-lander and Wang 1966; Kambe 2001). In spite of the steadyprogress in laser technology, lasers are still expensive and pro-cess efficiency remains low, constraining the commercial ap-plication of laser-based aerosol manufacturing methods in anindustrial scale.

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Plasma-Based Aerosol ProductionSimilar to nanoparticle formation by laser, plasma synthesis

can use various types of precursor materials. Molecular nucleiare formed by electrically heating the precursor up to 104◦C,decomposing the reactants into partially ionic species. Whenexiting the plasma zone the temperature of the gas drops andnanoparticles are formed. Again, steep homogeneous coolinggradients are necessary for the fabrication of materials with anarrow particle size distribution. Thermal plasmas applicablefor nanoparticle synthesis include direct current plasma jets, di-rect current arc plasmas and radio frequency induction plasmas.Large production rates have been achieved for nickel nanopar-ticles (70 g h−1) using an anodic arc plasma with a power con-sumption of above 5 kW and, a very recent application, witha DC thermal plasma for the synthesis of ZnO from commer-cial zinc powders at production rates exceeding 1 kg h−1 (Koet al. 2006). The high energy consumption of up to 70 kW andthe required inert gas for the plasma formation may limit thecommercial applicability of this process. A potentially moreenergy efficient process is the pulsed wire discharge synthesis.This rather flexible and simple system has been used for thesynthesis of metal oxides (Kinemuchi et al. 2002; Kinemuchiet al. 2001; Sangurai et al. 2001; Suzuki et al. 2001), metals(Jiang and Yatsui 1998), and metal nitrides (Lee et al. 2006)and offers aerosol gas phase nanoparticle synthesis at an energyconsumption of down to 20 MJ kg−1 (Jiang and Yatsui 1998).

Flame SynthesisFlame synthesis has its roots in the formation of carbon black

(soot). For nearly a century this method has been used for thelarge-scale preparation of carbon black of over 8 million metrictons per year, mostly covering applications in the production oftires and rubber (Stark and Pratsinis 2002). The flame technol-ogy was later adapted for the production of silica and titaniaby the oxidation of chlorides (SiCl4/TiCl4) in high temperatureflames (Osterwalder et al. 2006).

TiCl4 + O2 → TiO2 + 2Cl2SiCl4 + O2 → SiO2 + 2Cl2

Today this is the major synthesis route for the manufacturing ofpigmentary titania (≈5 Mt/year) and fumed silica nanoparticles(≈4 Mt/year) (SRI 2001), which are used as powder flowingaids and in cosmetics and in the fabrication of optical fibersfor telecommunication. However, most nanoparticles cannot bederived from the metal chloride oxidation process. This aerosol-based process only works for a limited set of elements andrequires volatile metal chlorides, which can be fed to a flame re-actor and reacted exothermally with oxygen. At applicable tem-peratures this is only possible for silicon-, aluminum-, titanium-,vanadium- (Stark et al. 2001), and zirconium-chloride, all cur-rently used for the large scale synthesis of nanoparticles in flameprocesses. In order to manufacture other interesting nanomate-

rials (there are more than 100 chemical elements) another al-ternative metal feed system for flame reactors was required. Assolids are usually not easily combustible, liquid systems arepreferred. Water based precursor systems were introduced byZachariah and Huzarewicz (1991a) as well as Matsoukas andFriedlander (1991), where an aqueous precursor was atomized,dried and then fed to a burning flame, resulting in the decompo-sition of the precursor and the formation of nanoparticles. Usingthis technology, complex mixed oxide nanoparticles, such as su-perconducting yttrium-barium-copper oxides, could be formed.The use of aqueous starting materials resulted in a broader win-dow of materials, but the often larger and less homogeneousparticles suggest the preferred use of organic and combustibleprecursors. Optimized metal-organic precursors consist of an or-ganic liquid of high metal loading (5–30 wt%; achieve high massproduction), which is combustible at a high gross calorific value.This allows the complete combustion of the organic constituentsto water and carbon dioxide and the nucleation of the metalconstituents in the hot-zone of the flames. Various precursorsystems have been applied, including metal alkoxides (Muelleret al. 2004a), acetates (Tani et al. 2003), triethanolamines (Kimet al. 2004; Marchal et al. 2004), and metal carboxylates (Starket al. 2003; Stark et al. 2004; Stark and Pratsinis 2004). Themetal carboxylate system appears promising, as nearly all ele-ments are available in low-cost napthenate-metal salts. Further,these precursors are generally stable in air and against humid-ity, allowing the fabrication of mixed-oxide nanoparticles withexcellent chemical homogeneity and relatively narrow particlesize distributions.

PHYSICAL UNDERSTANDINGIn spite of its apparent simplicity the long history of aerosol

process development has been characterized by evolutionarytrial and error research. This experimental early knowledge haslater been supported by an in-depth understanding of the pro-cess dynamics. A typical sequence of the basic steps illustratingparticle formation in a gas stream is given in Figure 1. A metalloaded precursor is injected as a gas or liquid spray into theflow. The sprayed precursor is evaporated due to rapid heatingthrough surrounding gases depending on the process (source ofthe thermal energy: laser, plasma, or flame). The additionallygenerated heat of combustion (flames) favors the evaporation ofthe solvent and precursor, ideally resulting in the formation of ahomogeneous gas phase monomer. Consumption of the gaseousprecursors proceeds either by gas phase and or by surface re-actions (Spicer et al. 2002). This is typically followed by theformation of some first nuclei clusters. The thermal stability ofthese nuclei strongly influences the evolving particle formationprocess. According to classical nucleation theory based on bulkmaterial properties, the critical cluster size is often smaller thanthe dimension of the idealized gas phase monomers. The clusterfurther grows either by collision with additional monomers andparticles with sticking coefficients often assumed to be one or

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FIG. 1. In high temperature gas phase processes nanoparticles grow from monomers or nuclei by fast agglomeration forming fractal aggregates (transitionregion). Sintering and coalescence then leads to the formation of spherical particles that continue to collide by Brownian motion. If the temperature is insufficientfor full coalescence to spheres, hard agglomerates are formed. In the cold zone of the process and during filtration soft agglomerates are formed, which are onlyinterconnected by Van-der Waals forces. Chemical aerosol engineering allowed considering flame spray synthesis as chemical reactors. The product compositioncan be controlled by the combustion and reaction conditions, resulting in a wide variety of nanoparticles (present example: iron).

by the addition of gas phase monomers on the cluster’s sur-face. The coalescence of cluster ensembles is normally veryfast, resulting in compact, dense and nearly spherical particles.The characteristic time of the gas-phase reactions and the co-alescence is usually much shorter than the characteristic timefor subsequent evolution of the particle shape (i.e., sintering).When typical particle diameters become large enough (severalnanometers), the further development of a particle is often de-termined by surface growth and by the interdependence of coag-ulation and coalescence (Tsantilis and Pratsinis 2000). Fractalstructures start to form by Brownian motion and coagulation andcan merge (collapse) again into spheres by coalescence (Mat-soukas and Friedlander 1991). This finally results in the forma-tion of fractal aggregates. These groups of particles are called“soft” if the primary particles are interconnected (Figure 1) byvan-der-Walls forces and “hard,” if sinter necks exist (Grass etal. 2006; Tsantilis and Pratsinis 2004). Physical material prop-erties, residence time of the particles, core and temperature ofthe gas stream determines the final morphology and crystallinityof the agglomerates.

This understanding of the particle/aggregate formation inan aerosol process was initiated with the observation of self-preserving particle size distributions (Friedlander and Wang1966). It was later discovered (Schaefer and Hurd 1990; Xiongand Pratsinis 1993) that flame-made particles had a fractal

structure with a fractal dimension of 1.7–1.9, representing avery open aggregate structure. To get real experimental in-sight into the dynamics of the flame process Arabi-Katbi et al.(2001) used thermophoretic sampling to remove relevant sam-ples from different positions in the flame. Analysis by electronmicroscopy confirmed that the earlier derived physical modelsof the flame process indeed correctly predicted the nanopar-ticle growth process. While more complex and exact popula-tion balance models are now available in literature (Koch andFriedlander 1990; Tsantilis et al. 2002; Xiong et al. 1993; Xiongand Pratsinis 1993), a simple model based on particle nucle-ation, agglomeration by Brownian motion and coalescence hasbeen proposed by Kruis et al. (1993). Despite the often roughassumptions made, the model offers a good quantitative un-derstanding of many flame processes and is routinely used incombination with computational fluid dynamics (CFD) pack-ages for the improved design of flame reactors (Johannessenet al. 2000, 2001; Muhlenweg et al. 2002), yielding nanoparti-cles with a desired size and morphology (Grass et al. 2006; Heineand Pratsinis 2006; Tsantilis and Pratsinis 2004). This detailedunderstanding has allowed the scale-up, design and engineeringof industrial aerosol reactors for nanoparticle synthesis. Labora-tory set-ups of flame-spray reactors usually operate now in theregion of 10–100 g/hr and small pilot plants have been shownto produce silica, zirconia, and yttrium stabilized zirconia at

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production rates of up to 1 kg/hr (Jossen et al. 2005; Mueller etal. 2004b; Mueller et al. 2003).

CHEMISTRY IN AEROSOL PROCESSING OF MATERIALS

Single OxidesThe understanding of the particle formation dynamics and

combustion science did not only facilitate a better reactor designbut it also resulted in the synthesis of more complex materials.Besides silica, many other single oxides have been synthesizedincluding TiO2, Al2O3 GeO2, V2O5 , SnO2, ZrO2, CeO2, ZnO,and WO2. The most important parameters determining particlemorphology are the concentration of the precursor, residencetime and temperature next to flame gas composition. The con-trolled adjustment of high temperature residence time and thehigh cooling rates have given access to the formation of metaloxides with various crystallinities and particle morphologies.The controlled formation of maghemite (Fe2O3) (Grimm et al.1997), rutile or anatase TiO2 (Pratsinis and Vemury 1994), mon-oclinic or cubic Y2O3 (Camenzind et al. 2005), and tetragonalZrO2 (Karthikeyan et al. 1997) are examples of some high tem-perature phases (crystal polymorphs) obtained by aerosol flametechnology. These are often materials which are difficult to pre-pare by conventional wet phase (low temperature preparationmethod in water) and subsequent slow bulk calcination (largesintered area, loss of nanoparticle size). In a further extensionof flame spray synthesis Laine et al. (2006) has also shown thetransformation of suspended γ -Al2O3 particles into α-Al2O3 bya short additional heat treatment in a flame aerosol process.

The predominantly physical aspects of morphology reflectsits dependence on the above mentioned physical process pa-rameters. The first chemical modification investigated was theuse of dopants. The addition of specific elements (mostly met-als) may change the surface energy resulting of nanoparticlesand consequently influence the product morphology. The typicalpolyhedral CeO2 nanoparticles from flame processing becomespherical upon doping with TiO2 and Al2O3 (Feng et al. 2006).In an even more astonishing example, the addition of indiumduring ZnO formation promoted one-directional crystal growthand allowed aerosol synthesis of elongated particles (Heightet al. 2006).

Single metal oxides have found broad applications in thefield of catalysis and photocatalysis since the 1970s. Severalexcellent reviews (Astruc et al. 2005; Daniel and Astruc 2004;Strobel et al. 2006a) describe these fascinating applications ofderived aerosol single oxides both as catalytic material or asupport. One of the first commercial materials from a flame pro-cess has been a specific TiO2 grade (Degussa P25) for catalystsubstrates and rapidly became the benchmark material in pho-tocatalysis. More recently single oxides have found numerousapplications as sensing materials (Madler et al. 2006; Sahm etal. 2004). Among them, TiO2 and SnO2 are widely applied assemiconducting gas sensors. Tricoli et al. (2008) even demon-strated a one-step preparation of dense SiO2 doped SnO2 film on

wired silicon substrates with the ability to quickly sense ethanolvapor.

Mixed Oxides and Oxide GlassesA decisive advantage of many flame spray methods is the use

of low-cost metal precursors with high processing and handlingstability. This array of starting materials can often be com-bined at will and results in an unlimited number of precursorcombinations and stoichiometric ratios, enabling the prepara-tion of complicated mixed oxides. As in the case of singlemetal oxides, the literature has become very broad and onlyselected examples will be highlighted. General control of thecomposition (stoichiometric ratio) can be illustrated in the caseof solid solutions of CeO2-ZrO2 ceramics for automotive cat-alysts (Stark et al. 2003). Still one of the most complicatedclasses of mixed metal oxides are perovskites for catalysis or forcathode (Gd-doped ceria (Jud et al. 2006), Sm0.5Sr0.5CoO3−x-Sm0.1Ce0.9O3−x (Liu et al. 2004)) and anode materials in solidoxide fuel cells. The high thermal stability of flame derivedperovskites such as NiMnO3 (Kriegel et al. 1994) and BaTiO3

(Brewster and Kodas 1996) enabled maintaining a very highcatalytic activity and deactivation-resistance at harsher condi-tions than conventional materials (cold preparation and sinter-ing). Additionally, mixed oxides have been investigated for elec-troceramics. Superconducting YBa2Cu3O7−x films (Zachariahand Huzarewicz 1991b) and lithium-based spinels (Ernst et al.2007) have been prepared by flame spray synthesis. The lat-ter exhibited high initial discharge capacity even after severalcharge/discharge cycles. The ability of metal carboxylates to ho-mogeneously mix and combust even allows materials with fourto five different elements and enables the preparation of amor-phous, bioactive glasses (Brunner et al. 2006). Different compo-sitions of SiO2-CaO-P2O5-Na2O-F nanoparticles were preparedin flames and yielded a highly bioactive material. The combi-nation of bioactivity, biocompatibility, and reactivity motivatedfor the application of bioglass nanoparticles for remineralizationof dentin (the basal tissue for teeth) (Vollenweider et al. 2007)or as a dental filler for root canal treatments (Waltimo et al.2009).

From Oxides to Salt NanoparticlesClassical aerosol based processes were used for the manu-

facturing of metal oxides. More recently, Loher et al. (2005)demonstrated how a flame can be considered a chemical reactor(Figure 1). Chemical reactions, usually preformed in solid-statesynthesis, can be readily adapted for the aerosol preparation ofsalt nanoparticles opening the number of accessible compoundsby several orders of magnitude. With the aim of producingcalcium phosphate nanoparticles for biomedical applications(bone surgery) a flame spray set-up was fed with a mixtureof calcium carboxylate and a combustible phosphate source,

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FIG. 2. Salt nanoparticles can be highly biocompatible and reactive. (a) Transmission electron microscopy image of as-prepared amorphous tricalcium phosphate(Ca3(PO4)2). The biodegradable nanoparticles can be incorporated in a medically used polymer (PLGA) and processed into millimeter-sized long fibers byelectrospinning. (b) Such nanocomposites are highly bioactive, promoting hydroxyapatite formation on the fiber surface after immersion in simulating bodyfluid.This biomineralization enables use of such composites in bone surgery. (c) The morphology of the fibrous PLGA/TCP scaffold is similar to cotton wool, whichgreatly facilitates clinical handling.

tributylphosphate:

Ca(C8H15O2)2 + 22O2 → (CaO)np + 16CO2 + 15H2O2(C4H9)3PO4 + 36O2 → (P2O5)g + 24CO2 + 27H2O3 (CaO)np + (P2O5)g → (Ca3(PO4)2)np

The calcium oxide formed in the hot zone of the flame reactedwith the simultaneously forming phosphate precursor, affordinga dry synthesis of calcium phosphate nanoparticles with particlesizes below 50 nm. By varying the calcium to phosphate ratioin the precursor, a broad range of calcium phosphate phases, in-cluding the major biomaterials tricalciumphosphate (Figure 2a)and hydroxapatite, were accessible (Loher et al. 2006). By im-plementing amorphous tricalcium phosphate (TCP) particles inpoly(lactide-co-glucolide), a flexible and cotton-wool like bio-material was produced by electrospinning (Figure 2b) (Schnei-der et al. 2008). This novel composite conserved the very highbioactivity and absence of cytotoxic effects of TCP nanoparti-cles and is being studied as bone substitute material for complexbone defects (Figure 2c).

This early study indicated that otherwise chemical reactions(typical solid-state synthesis take hours to react) can take placein a very hot zone within milliseconds, mainly ignoring the on-going physical processes (growth, coalescence, and agglomer-ation), and paved the way to other salt nanomaterials. Grassand Stark (2005) introduced combustible fluoride and chlo-ride sources (chloro- and fluorobenzene) to nanoparticle form-ing flames and obtained the corresponding barium-, calcium-,strontium- fluorides as well as sodium chloride, the commonkitchen salt. The particle size distribution of these nanoparti-cles was preserved and could be described as log normal with ageometric standard deviation of σ = 1.33. This value is consis-tent with free molecular regime, diffusion-limited coagulationof non-fractal particles as shown from modelling results for

the traditional metal oxide synthesis (Dekkers and Friedlander2002; Vemury and Pratsinis 1995). This observation confirmedthat the physical process remained unaffected even if the chem-ical changes to the system were large. Besides access to phos-phates and halides the concept of exchanging the anion (oxidevs. salt) produced carbonates, such as nano-limestone (Huber etal. 2005), sulfates, nano-gypsum (Osterwalder et al. 2007), andbarium carbonates (Strobel et al. 2006b).

Reducing Flames: From Oxides to Metal NanoparticlesThe possibility to produce complex salt nanoparticles has

showed us that flame reactors can well be considered as chemi-cal reactors operating at higher temperatures. This observationobviously suggested to not stop at anion exchange but funda-mentally altering the chemistry of these reactors (Figure 1).Depending on the reaction and combustion conditions in theflame a complex gas mixture can be altered from CO2/H2O (ox-idizing, traditional flame) to CO/H2/H2O (reducing conditions).Noble metal nanoparticles such as Pt, Au, Ag, or their alloys canbe obtained readily in oxygen-rich flames but the production ofnon-noble metals (i.e., most technically important metals) re-quires reducing conditions. Forsman et al. (2008) showed thatcobalt and nickel nanoparticles can be accessed by hydrogenreduction from cobalt or nickel chloride precursor vapor in ni-trogen carrier gas. The combustion of cobalt or bismuth organicprecursors (cobalt(II)-, bismuth(III)-2-ethylhexanoate) in con-trolled atmosphere (O2 < 100 ppm) with a high fuel to oxygenratio (Figure 3) allows the rapid preparation of pure Co and Bimetal nanoparticles by simple extension of the traditional flameprocess (Grass and Stark 2006a; b). The reducing conditionsprovoke the full consumption of the available oxygen by H2 andCO and induce the reduction of oxides to metals (Figure 3). Afterremoval from the protective atmosphere, the particles covered

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FIG. 3. Classical (left) and reducing flame synthesis lab unit (right) for theproduction of oxide or metal nanoparticles up to 20 g per hour, respectively.The flame spray pyrolysis is surrounded by a porous tube and placed into aglove box with an inert atmosphere. Controlling the gas flow rates allows highlyreducing conditions (O2 < 100 ppm). While traditional reactors mostly yieldoxides, this modified process allows the one-step synthesis of metal or alloy orcarbon coated metal nanoparticles, or metal/ceramic composites.

themselves with a very thin protective oxide layer enabling fur-ther processing of the particles. The combustion of carboxylateprecursors under inert atmospheres has shown that the particlesize distribution was preserved as in the case of metal oxideand salt nanoparticles and could be described as log-normal re-sulting in spherical particles with standard geometric deviationsof σ = 1.5 (Grass and Stark 2006a) indicating that the inert(low oxygen concentrations) conditions do not affect the physi-cal processes of nucleation, growth and aggregation. Latter hasbeen further confirmed by the experimental observation of Grassand Stark (2006b) where the combustion of a bismuth precursorunder various precursor flow rates and constant fuel to oxygenratio resulted in metal nanoparticles with unchanged sizes sim-ilar to previous studies on the formation of metal oxides. Onlydilution of the precursor resulted in a significant decrease of thepowder mean particle size due to smaller particle concentrationin the flame.

As the problem of oxidation is inevitable, this limits the useof highly pyrophoric materials consisting of elements with ahigh negative reduction potential such as silicon, aluminum.Therefore, different strategies have been developed to prepareair-stable core/shell metal nanoparticles and composites. Themost common techniques are to passivate the surface or to en-capsulate it with carbon. Aizawa and Buriak (2007) have incor-porated copper nanoparticles in a diblock polymer in a two stepprocess to avoid oxidation, whereas Forsman et al. (2008) sug-gested that a thin HCl layer on the metal nanoparticles protectsthem from oxidation. Athanassiou et al. (2006) have shown thatthe combustion of copper(II)-2-ethylhexanoate under highly re-ducing conditions resulted in the production of metallic coppernanoparticles covered with thin (∼1 nm) carbon layers. These

core-shell materials showed an excellent air and acid stabilityand non-traditional electronic properties. The thickness of thecarbon layers (2–3 layers of carbon, about 1 nm or particleswith 6–10 layers, about 3 nm) could be adjusted by the simulta-neous feed of acetylene during combustion. When pressed intopills by uniaxial compression this core-shell material exhibiteda pronounced temperature and pressure dependent conductivity.The temperature dependence, given by the negative tempera-ture coefficient was similar or even enhanced when comparedto commercial piezoelectric materials (Athanassiou et al. 2006).The tunneling based conduction mechanism (Figure 4) showedthat core/shell conductive/insulator materials offered a new andsimple production route of highly sensitive pressure and tem-perature sensors (Athanassiou et al. 2008; Athanassiou et al.2007b).

Cu(C8H15O2)2 + 22O2 → (CuO)np + 16CO2 + 15H2O(CuO)np + H2 → (Cu)np + H2O

(Cu)npCO/H2→ (C/Cu)np

Luechinger et al. (2007) implemented carbon/copper nanopar-ticles in water based dispersions. These aerosol derived metalinks could be used either as humidity sensors (Luechinger et al.2007) or printed onto flexible polymer substrate by conventionalink-jet printing (Luechinger et al. 2008a). The in-situ carbon en-capsulation of the metal nanoparticles within an aerosol processgives access to numerous functional materials. The possibility tolink organic chemistry to nanoparticles makes aerosol-derivedparticles amenable to the atom-precision control of syntheticchemistry. The carbon serves as a platform, where covalent sur-face functionalization with different organic molecular groupsvia radical chemistry is possible (Grass et al. 2007b; Herrmannet al. 2009). Carbon coated cobalt nanoparticles have been cor-respondingly functionalized resulting in the preparation of mag-netic catalysts (Schatz et al. 2008), magnetic chelating agentsfor water purification from heavy metal contamination (Koehleret al. 2009) or magnetic actuators (Fuhrer et al. 2009) withmuscle-like flexibility and magnetic melt-processable polymers(Luechinger et al. 2008b).

The successful synthesis of non-noble metal nanoparticles ina flame aerosol based process motivates the use of thermody-namics as a guiding principle to predict possible materials be-fore attempting their preparation by an aerosol route. Assumingequilibrium conditions is reasonable in most cases as chemicalgas-phase reactions are exceptionally fast, the gas compositioncan then be calculated from known thermodynamic properties(Figure 5). The use of straightforward tools, such as the Elling-ham diagram, confirms these recent experimental findings andhelps rationalizing how flame synthesis could be extended to re-ducing or reactive conditions, ultimately enabling the formationof metal alloys such as nickel-molybdenum superalloy nanopar-ticles with enhanced mechanical properties (Athanassiou et al.2007a).

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FIG. 4. An example of a non-traditional aerosol derived nanomaterial. Two core/shell metal/insulator (carbon/coated) nanoparticles in close vicinity can beviewed as a classical tunneling barrier. The energy gap arises from the different Fermi levels in the metal cores and the shell material (graphene bi- or trilayers,right). Electron transport through such an assembly follows multiple parallel paths along (virtual) particle chains. The macroscopic conductivity is a result ofa large number of parallel and serial tunneling gaps along the conduction path. Such non-traditional materials mimic the properties of traditional piezoelectricmaterials (spinels).

Simultaneous Synthesis of Oxides and MetalNanoparticles Resolves the Mixing Gap Problem inNanocomposite Preparation

The thermodynamic concept outlined above (Athanassiouet al. 2007a) has paved the way to further experimental inves-tigations combining bismuth and cerium containing precursors.This resulted in the simultaneous formation of metal and ce-ramic nanoparticles that could be compacted to non-traditionalcomposites. The possibility of preparing a homogeneous mix-ture of metallic bismuth and cerium oxide nanoparticles in asingle process step circumvents the otherwise difficult or im-possible dry mixing of non-wetting nanoparticles. Pressed pills

of the as-prepared powder resulted in bulk samples with ceramicloadings as high as 25 vol %. For comparison, classical oxidereinforced steel contains 0.5–1 wt% oxides. These high-oxideloaded nanocomposites combine metallic properties (electricalconductivity, high gloss) and ceramic properties (Vickers hard-ness comparable to steel, coarse grained steel ≈100–120 HV)while pure bismuth (no oxide) can be indented with a finger nail(Grass et al. 2007a).

The successful preparation of salt and metal alloy nanopar-ticles indicate that aerosol processes can indeed be used asa versatile chemical reactor (Figure 5). Naturally, the basicprinciples of particle dynamics dominate the morphology and

FIG. 5. Thermodynamic calculations in the form of an Ellingham diagram allow discussions on accessible alloys and metals. The lines which lie below theflame process operating line indicate accessible metals at the corresponding temperature (copper, cobalt, nickel, and iron above 1500◦C). Others, such as cerium,manganese are not accessible in the current CO/H2 system.

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crystallinity of the nanoparticles. Since the characteristic timescale of chemical reactions is often faster, the traditional as-sumption of constant sintering kinetic parameters may loose itsvalidity in complex chemical transformations. The Arrheniusrelationship implies that the chemical reactions proceed veryfast at these typically high temperatures. The composition ofthe combustion gases in the different temperature regimes in-fluences the final outcome of the combustion. In addition, theusually (but not always) applied turbulent conditions favor agood mixing of the gas-phase “reactants.” For all these reasonsthe problem of considering chemical aerosol engineering in themodeling of gas-phase particle processes is very complicated.A deeper understanding of what exactly happens during particleformation will ultimately provide access to more complex multi-component functional materials and will facilitate the up scalingof gas-phase aerosol reactors for the fabrication of more com-plicated nanomaterials beyond oxides such as titania or silica.

In order to incorporate chemical processes in modeling andup scaling it is important to implement or develop adequateanalytical techniques. Such instrumentation needs to be moreaccurate than existing methods determining the physical prop-erties of the formed nanoparticles, such as their diameter. Ideallythey have to exhibit a high resolution in order to analyze chem-ical reactions in the micro-millisecond range. Unfortunately,this is not straightforward. Current methods such as FourierTransform Infrared Spectroscopy have already been applied tomeasure in situ the temperature during aerosol production. Ifsuch methods could be used at better resolution or combinedwith other analytical methods based on X-ray spectroscopy ordiffraction, we could get more phase/chemical information. Insitu applications of such methods have already been success-fully applied in heterogeneous catalysis where they assist theinvestigation of complex chemical mechanisms. A space re-solved measurement of the gas-phase composition via massspectroscopy in a gas-phase reactor would enable understand-ing the gas phase kinetics, which determine the final chemicalcomposition of nanoparticles. In combination with the tempera-ture profile of the reactor zone this will give a great insight in thecomplex chemical and physical aspects of aerosol processing,allowing the aerosol community to model and predict the ade-quate combustion conditions (flow rate of dispersion gas, flowrate of precursor, temperature profile of the reactor zone), forthe production of novel nanomaterials with predefined physicaland chemical properties (particle size distribution, mean particlesize, crystallinity, surface area, composition, morphology).

SUMMARY AND OUTLOOKAerosol engineering has contributed significantly to the

tremendous success of nanomaterials during the past decades.Today, our discipline offers reliable and scalable gas phase meth-ods and some of them have found their way to industrial im-plementation. The deep and valuable understanding of physicalparticle dynamics and combustion engineering has enabled the

production of novel and functional materials. The field con-tinues to expand particularly in the direction of functionaliza-tion and chemistry, where we find plenty of room for morecomplex materials. This may ultimately require reconsideringfundamental process assumptions, such as constant sinteringtime or separation of physical and chemical processes. Chemi-cal reaction kinetics will greatly influence the final design andoperation of next generation aerosol reactors. To achieve thatadequate, reliable measurement techniques need to be found oreven developed.

The wealth of now available nanoparticle-forming aerosolprocesses have provoked novel, technical nanosolutions. Atpresent many of them demand large amounts of energy, ei-ther in the form of electrical energy, fossil sources or by theinvolvement of large amounts of solvents and chemicals, whichhave to be purified for reuse or discarded. Therefore, new andimproved processes for the synthesis of nanoparticles need tobe engineered. Besides obvious properties such as quality of theproducts, additional focus should be placed on overall energy, re-source and recycling if possible (Osterwalder et al. 2006) as wellas powder processing. More energy efficient processes directlyreduce the costs of nanoparticles and enable more widespreadapplications. Again, a deeper understanding of the (physicaland chemical) fundamentals during nanoparticle formation willprovoke optimization of current production methods.

Chemical and aerosol engineering still initially provide drypowders. In most technical applications these raw products can-not be directly applied in a final product. Further processingand derivatization is required. To achieve an optimal engineer-ing solution fundamental interactions of nanoparticles with theirsurroundings or at interfaces must be investigated at more de-tail. Most applications require particles dispersed in solvents,polymers or other materials, which typically demand functionalmodification of the particles surface. The design of such second-step processes relies on the understanding of the interactionsand forces at the nanoscale and how they are influenced bycrystallinity, morphology, size, chemical surface properties, andsolubility. An optimum engineering approach considers nano-materials as functional building blocks. The demand for postprocessing and derivatization results in rapidly growing appli-cations and initiates the need for understanding the potentialhealth risks related to these novel nanosolutions. Therefore, thephysico-chemical interactions of the nanoparticles with livingorganisms and the environment need to be extensively stud-ied. This will provide the key to the design of inherently saferand sustainable handling and implementation of nanoparticles(Limbach et al. 2009).

In summary, the striving for an improved understanding andthe combination of physical and chemical engineering in aerosolprocesses and unit operations will provide access to:

• more complex multicomponent functional materials,• more efficient scale-up and development of energy-

efficient nanoparticle reactor designs,

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• development of analytical methods to characterize in-situ chemical kinetics during aerosol formation,

• a better understanding of the influence of physico-chemical parameters on the interface and its interactionwith the surrounding material or organism.

The challenges and the opportunities for producing these fasci-nating materials are still huge, in spite of the respectable devel-opment during the past thirty years. Most of the future work,however, will happen in areas where the concepts of aerosol ma-terial science meet with other engineering disciplines and evenwith medicine.

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