There are several gas phase synthesis methods available for advanced nanomaterial production. Here we present flame synthesis as a scalable method (Wegner 2011) for mass production of e.g. battery materials and chemical vapor synthesis (CVS) (Backman et al., 2005) for precise synthesis method of materials for high-end applications.
The demand of Li-ion battery electrode materials is increasing drastically as the internet of things, amount of mobile devices and electric vehicles require large amounts of these materials. As anode material lithiumtitanate (LTO) is becoming important due to its inherent safety, stability and performance in extreme conditions. The problem with LTO has been low electric conductivity of bulk LTO. To overcome this problem it has been proposed to decrease the primary particle size and/or dope LTO with conductive agents like Ag or Cu (Karhunen et al. 2011). Flame synthesis was applied to obtain nanosized LTO and it was also doped with Ag to increase the electric conductivity. Silver nanoparticles were nucleated on top of surface of LTO forming a separate phase and thus enhancing the surface conductivity. Good results on specific discharge capacity were obtained especially at high charge/discharge rates.
CVS was used for iron and iron oxide nanoparticle synthesis in a novel porous tube (PRD) aerosol reactor. In this advanced CVS reactor it is possible to mix rapidly the hot reactant gas. Iron pentacarbonyl was used as precursor and nitrogen or nitrogen/oxygen mixture as reactant. In pure nitrogen flow iron was produced and with specified amount of oxygen mixed with nitrogen maghematite or magnetite could be produced. The produced particles showed special magnetic properties and further biomedical applications of these particles are sought as they showed to be non-cytotoxic.
Two different gas phase synthesis methods, flame synthesis and chemical vapor synthesis were utilized to produce advanced nanomaterials suitable for specific applications.
References
K. Wegner, B. Schimmoeller, B., Thiebaut, C. Fernandez, T.N. Rao (2011) Pilot plants for industrial nanoparticle production by flame spray pyrolysis. KONA No. 29, 251-265.
Ulrika Backman, Ari Auvinen and Jorma Jokiniemi (2005) Deposition of nanostructured titania films by particle assisted MOCVD, Surface & Coatings Technology, Vol 192 pp. 81-87.
Tommi Karhunen, Anna Lähde, Jani Leskinen, Robert Büchel, Oliver Waser, Unto Tapper and Jorma Jokiniemi (2011). Transition metal doped lithium titanium oxide nanoparticles made using flame spray pyrolysis. ISRN Nanotechnology Volume 2011, Article ID 180821, 6 pages; doi:10.5402/2011/180821, ISSN: 2090-6072.

The most commonly used flame types for material synthesis include premixed, normal diffusion, inverse diffusion, and co-flow flames. The use of multiple diffusion flames offers several key advantages, such as uniform temperature and chemical species profiles and many of the limitations related to premixed flames such as flashback and flame speed are avoided. Using the multiple-diffusion burner, we have demonstrated the growth of graphene, iron-oxide, carbon-coated titanium dioxide (TiO₂) nanoparticles, TiO₂ doped/coated with different metals (vanadium, silicon, and iron), and SiO₂ doped with carbon.
In lab scale experiments, laminar flames remain the dominant approach for the flame synthesis of nanomaterials. However in industry, turbulent flames are commonly utilized as it enables larger-scale production of nanomaterials. Recently we investigated the use of a novel turbulent burner for the flame synthesis of nanoparticles. It involves the use of an axisymmetric curved-wall jet (CWJ) burner, where the Coanda effect enables flame stability. Key advantages of this method include the enhancement of turbulence due to the radially inward velocity component of the jet. The vaporized precursor mixes with the gases delivered in the axisymmetric curved-wall jet burner. The combined effects enable enhanced mixing of the precursor, which is important in gas-phase synthesis. Results suggest we can effectively control the growth of anatase and rutile particles based on the operating conditions of the flame. Other designs that are currently being investigated incorporate a spray atomizer within the curved-wall jet burner. Such a setup can be used to burn low cost chemical precursors due to the high temperature of the turbulent flame. It is possible this setup can provide new pathways for scalable material synthesis.

Magnetoplasmonic nanoparticles are of interest for cancer theranostics, in which the same agent (here, the nanoparticle) enables both diagnostic and therapeutic modalities. We report the synthesis of multilayer nanoparticles that consist of a superparamagnetic iron oxide core, a thin silica shell that is decorated with gold nanoparticles, and a surface layer of polyethylene glycol (PEG). These particles are synthesized by a continuous-flow sequence of dry gas-phase processes. PEGylating the particles allows them to be collected into stable aqueous suspension for biomedical applications. The sequence of gas-phase processes allows for fewer impurities compared to wet chemical processes, and avoids the need for surfactants between each layer as well as the need to manage and dispose of hazardous solvents. Total residence time is on the order of a seconds, compared to hours or even days for batch wet chemical processes.
Iron oxide particles are produced by a direct-current plasma torch with injected ferrocene vapor and oxygen. The particles produced have mean diameters around 10 nm, and are superparamagetic, with coercivity around 25 Oe and saturation magnetization around 40 emu/g. These particles are carried as an aerosol in inert carrier gas to a photo-induced chemical vapor deposition chamber, where tetraethylorthosilicate vapor is decomposed by a xenon excimer lamp, growing thin (1-2 nm thick) silica shells on the iron oxide particles. Small (< 5 nm) gold nanoparticles are generated by resistively heating a gold-coated platinum wire. The gold particles are scavenged onto the silica surfaces by aerosol mixing. Finally, the nanoparticles are PEGylated in a two-step aerosol process. In the first step, 2-mercaptoethanol vapor is thermally activated, functionalizing the gold nanoparticles by attaching an SH group to the gold surface, with a terminal hydroxyl group at the other end. In the second step, ethylene oxide vapor is introduced, and reacts with the OH group to grow PEG via insertion polymerization.
During these processes, particles are characterized online by tandem differential mobility analysis, and offline by techniques including x-ray diffraction, transmission electron microscopy, energy dispersive x-ray spectroscopy in scanning transmission electron microscopy, x-ray photoelectron microscopy, Fourier transform infrared spectroscopy, and vibrating sample magnetometry.
This work was partially supported by the U.S. National Science Foundation (CBET-1066343). Materials characterization was performed at the Characterization Facility of the University of Minnesota College of Science and Engineering, and at the UMN Institute for Rock Magnetism.

Among the high temperature processes, the Laser Pyrolysis, based on the interaction between a high power CO₂ laser and a gaseous or liquid precursor, appeared since the first experiments as a convenient method for the production of non oxide nanoparticles of high purity with low size dispersion (Si, SiC, Si₃N₄,hellip;). In the last years, great efforts have been done to develop the versatility of this synthesis method for the production of more complex phases (for example doped or co-doped nanoparticles) or original architectures (for example core@shell structures) as illustrated in this presentation.
During the last ten years, TiO₂ has been intensively investigated as a photocatalyst for degradation of water pollutants where a key issue is the synthesis of photocatalysts with good efficiency both in the UV and the visible range. In this context, various TiO₂-based nanoparticles (NPs) were prepared in a one step process. Their chemical composition was adjusted through the choice of the different reagents in the precursor mixture: TiO₂ is obtained from pure TTIP (Titanium tetraisopropoxide), nitrogen doped TiO2 is obtained from TTIP by adding a slight flow of NH₃, Au-TiO₂ NPs are obtained from a mixture of TTIP+HAuCl₄.3H₂O. The photocatalytic activity of the different samples was investigated by following the degradation of formic acid as a model pollutant under UV or visible irradiation. The results prove that both TiO₂ and Au-TiO₂ synthesized from laser pyrolysis are more active than the reference Degussa P25 under UV light, while Au-N co-doped TiO₂ present a significant activity under visible range together with reactivity similar to P25 under UV.
We also developed a reactor composed of two reaction chambers. In the present example, Si@C nanoparticles are synthesized in the two successive reaction zones: silicon cores are synthetized in the first zone from silane decomposition, these Nps are transferred in the second reaction zone by a gas flow where the carbon shell is deposited from decomposition of ethylene. Therefore, the Si core is not exposed to air before shell deposition preventing from SiO₂ formation around silicon. Auger electron spectroscopy (STEM-EELS analysis) and high resolution electron microscopy confirm that Si@C Nps are well covered with carbon. In this configuration, we can control the core diameter in the range 20 to 200 nm, the core organization (amorphous or polycrystalline), the shell thickness and its organization (amorphous or turbostratic). These nanoparticles were tested as active material in the anode of a Li-Ion battery in coin cell configuration. Without the carbon shell, Si materials suffer from volumic expansion and electrolyte decomposition at its surface resulting in rapid capacity fading and low cyclability while Si@C Nps show a capacity higher than 1000 mA.h.g^-1 for more than 500 cycles.

There is a considerable interest in developing magnesium and magnesium alloys in the form of nanoparticles for hydrogen storage purposes. In this study, the possibility of synthesizing Mg-Ni nanoparticles was investigated using inductively coupled R.F. plasma. In this method, starting materials were fed into plasma torch where the temperature is high enough (up to ~10,000 K) for complete vaporization of powder which then condenses into nanoparticles further down in the reactor. Plasma reactor incorporated two injection probes located axially in the torch one from the top and the other from the bottom. Starting materials were either elemental powders or pre-alloyed compounds. Ni upon feeding to plasma torch can easily be processed to sizes less than 100 nm. Special precautions are necessary to produce Mg nanopowders due to its reactivity. The study, to a greater part, has concentrated on the synthesis of Mg₂Ni nanoparticles. The use of pre-alloyed Mg₂Ni powder as precursor leads to its disintegration in the plasma, condensing into separate phases and therefore was not suitable for the synthesis of Mg2Ni nanoparticles. The study further showed that Mg2Ni can be synthesized quite successfully with the use of elemental powders provided that elements are fed into the plasma at carefully controlled positions. While the fraction of Mg2Ni was quite substantial, it co-existed with other phases and therefore additional treatments would be necessary for separation. It was shown that a substantial size reduction was possible with thermal plasma where Mg2Ni could be produced in sizes around 100 nm. The significance of these observations was discussed with regard to the potential of thermal plasma processing in yielding nanoparticles of metallic alloys and compounds.

The continuous and increasing discussion regarding possibilities for energy harvesting, power efficiency, and sustainability has led to an intensive (re)search for further development and optimization of energy conversion and storage. In many cases, utilization of nanomaterials is a promising way to improve efficiency and enhance the fields of application. This covers multiple areas such as photovoltaics, battery systems, and thermoelectrics. All of these applications are dealing with materials that exhibit specific optical and/or electronic properties and commonly, processing either as dry powders or as dispersion is required. As a matter of fact, nanoparticles are particularly suitable as they provide specific properties and can be easily transformed into dispersions for further processing.
The synthesis and processing of group IV nanoparticles from gas-phase plasma synthesis is studied with respect their practicability in the above mentioned fields of application. Microwave plasma synthesis is used as method of choice as it provides steep temperature gradients and electrostatic separation during particle nucleation and growth leading to small particles with narrow size distribution. Moreover, short residence time within the reaction zone enables for kinetic control and surprisingly high dopant concentrations. This leads to the formation of spherical, highly crystalline and soft-agglomerated materials with controlled size and composition. The formation of specific silicon and germanium nanoparticles with adjustable particle size and dopant concentration will be discussed and few examples will be shown concerning their applicability in thermoelectrics, and lithium ion batteries.

For realizing the size dependent properties of nanoparticles, synthesis of nanoparticle having controllable size and narrow size distribution is one of the important prerequisites. In this presentation, synthesis and applications of size selected nanoparticles prepared by an integrated nanoparticle synthesis set up will be described. The synthesis set up comprises of a spark generator for forming metal agglomerates, a radioactive charger for charging, differential mobility analyzer for selecting the size and in-flight sintering for converting the agglomerates to compact, crystalline and spherical nanoparticles. The integrated gas phase deposition method has been used to grow Pd, Cu, Ag, Pd-Cu, Pd-Ag and Si-Sn alloy nanoparticles. A novel modification in the synthesis set up has been carried out for growing metal-graphene core-shell nanoparticles. Depending upon the solid solubility of carbon in the metal and relative surface energy values, thickness and nature of the graphene shell can be controlled by varying the sintering temperature. Utilizing the ability of controlling the nanoparticle size and composition, the effect of alloying on Pd 4d valence band position and thus Pd-H interactions has been investigated in Pd alloy nanoparticles. Si-Sn nanoparticles dispersed in SiO₂ thin films having optical and electronic properties suitable for new generation solar cell devices have been synthesized.

This work investigates the production of metal nanoparticles by atmospheric plasma synthesis, from the electrical discharge between two electrodes in an inert gas atmosphere. This physical synthesis process has the advantage that is does not require expensive precursors and also allows the recycling of the inert gas. The main aim is to find to optimal synthesis conditions, not only with respect to particle properties but also in view of the potential for upscaling as well as minimal energy consumption. For a plasma synthesis process, an essential process property is the electricity consumption, expressed as kWh/g nanoparticles produced. Different discharge regimes were investigated, with successively increasing power ranging from spark synthesis over glow discharge to arc synthesis. The synthesis is performed in an extremely versatile reactor which allows the rapid optimization on the basis of application aimed at, which ranges from nanofluids, catalytic reactors, functional textiles, solar cells, nanocomposites to photonic sensors. It also allows a direct adaptation to for these applications optimized processing and deposition methods. A variety of diagnostic methods has been used, ranging from online aerosol instrumentation to high-voltage probes and in-situ optical instrumentation. This allows to investigate the effects of changing the process parameters, such as interelectrode spacing, gas flow rates and power input.
As gas phase synthesis processes are always strongly influenced by Brownian collisions, one way to obtain much more narrowly distributed product nanoparticles is to apply size-selection as post-processing step, which allows to obtain a monodisperse product. This is demonstrated here with help of differential mobility analysis, which is scaled up from its original application as size measurement technique to a postprocessing technique. As Brownian coagulation cannot be avoided, one strategy for obtaining a larger yield is to increase the total carrier gas flow rate. We show here a specially designed size selection instrument, which will be commercially available, allowing the processing of larger quantities of narrowly distributed aerosols. Even at product flow rates of a few hundreds l/min, we show that geometric standard deviations below 1.10 can be obtained.
Acknowledgment
This work has been financially supported by the European Union's Seventh Framework Program (EU FP7) under Grant Agreement No. 280765 (BUONAPART-E).

A series of Si nanomaterials, i.e. silicon nanosheets (Si NSs), silicon nanoparticles (Si NPs) and silicon nanoribbons(Si NRs) had been fabricated by the DC arc-discharge method under diverse atmospheres (inert gases, hydrogen or the mixture). As a consumable raw matter, bulk silicon acted as the anode while a tungsten rod served as the cathode. Arc current was set at 90A and the voltage was maintained at about 20V. All in situ prepared Si nanopowders were collected from the water-cooled wall of the evaporation chamber after a passivation process. The morphologies, crystal structures, surface characters and optical properties of the Si nanopowders were analyzed by using of Transmission electron microscopy (TEM), X-ray diffraction (XRD), Atomic Force Microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS), BET, Raman apectra, UV-Visible absorption and photoluminescence spectrum. Influences of the preparation conditions on the strucutures and properties, as well as the formation mechanisms of these Si nanopowders were compared and investigated. It was shown that the as-prepared Si nanosheets (Si NSs) were about ~40 nm in width and ~3 nm in thinkness, the mean size of silicon nanoparticles (Si NPs) was approximate 30 nm, the silicon nanoribbons(Si NRs) actually consisted of fine grains and can be long as 100 nm in length. It was interesting that the anisotropic or isotropic growth of Si matter can be induced by the high energic atoms of inert gas or active hydrogen. A visible red-shift of Raman spectra were found among the Si nanopowder samples. It was also indicated that the oxidation layers on Si nanomatreials significantly affected the band gap transition and may have applications in the optoelectronics and electronic devices.

Fifty years ago, although millions of tons of flame-produced nanoparticles were being made annually, little was known about molecular-scale particle formation and growth. I was at Cabot Corporation then; working with Cab-O-Sil, their fumed silica. Curiosity about microscopic particle precipitation and growth prompted me to postulate and develop a theory thereof.
This interest continued to fuel my research activity while professor of chemical engineering at the University of New Hampshire. Meanwhile, Friedlander and others were exploring similar phenomena tied to the formation of smog and other aerosol pollutants.
Research has since proliferated through exploding interest in nanoparticle materials synthesis (as evidenced by the papers at this and past MRS meetings). Yet, many current investigators seem to ignore or overlook some universal fundamentals of nanoparticle formation and growth.
This presentation will review the theory as I understand it today, the tortuous path that led to it, and fundamentals that I believe deserve more attention.

ZnO rod-shape nanoparticles, i.e. quasi one-dimensional primary nanoparticles with high aspect ratios, have promising applications in small scale electric conductors and optical waveguides. Flame synthesis has been proven to be capable to produce a variety of nano-sized particles of high purity in a one-step process [1]. Although flame synthesis usually generates spherical particles due to the high temperatures, it was recently shown that ZnO nano-rods can be made in large scale quantities without any dopants [2]. However, the mechanisms leading to ZnO nano-rod growth in flames are still vague, complicating the production of ZnO nano-rods with controlled aspect ratio.
In the present work, we studied a bottom-to-top analysis from single droplet experiments [3] to large scale flame synthesis of ZnO nano-rods. TEM analysis of nanoparticles from single droplet experiments show comparable characteristics as flame-made lab-scale and pilot-scale nanoparticles, suggesting similar formation and growth mechanisms. Single droplet experiments with different different ZnO precursors, solvents and concentrations lead to different shaped ZnO nanoparticles. For example, single droplet experiments with Zn Nitrate Hexahydrate as the precursor show droplet extinction with certain solvents, leading both to residues of dried Zn Nitrate Hexahydrate (EDX) and large ZnO nanoparticles (TEM), suggesting droplet-to-particle formation. Different solvent/precursor combinations and precursor concentrations lead to different disruptive burning characteristics and different amounts of spheroidal and rod-shaped ZnO nanoparticles. Lab-scale and pilot-scale experiments with identical solvent/precursor combinations and precursor concentrations resulted in ZnO nanoparticles with similar sizes and shapes respectively. For example, in single droplet experiments the Zn Nitrate Hexahydrate precursor in combination with certain solvents resulted in several large particles, emphasizing that the observed droplet extinction on single droplet experiments and the subsequent droplet-to-particle formation is taking place.
[1] W.Y. Teoh, A. Rose, L. Mädler, Nanoscale 2 (2010) p. 1324-1347
[2] K. Hembram, D. Sivaprakasam, T.N. Rao, K. Wegner, Journal of Nanoparticle Research 15:1461 (2013)
[3] C. D. Rosebrock, N. Riefler, T. Wriedt, S. D. Tse, L. Mädler, AIChE Journal 59:12 (2013) p. 4553-4566

Synthesis of functional nanoparticles requires more and more refined production processes. Enclosing the scalable flame spray pyrolysis (FSP) process facilitates synthesis of a number of sophisticated nanomaterials¹. That way, for example, it is possible to make metallic nanoparticles², coat nanoparticles with thin silica³ or carbon layers⁴ and control the FeO/Fe₃O₄/Fe₂O₃ particle phase composition⁵. Enclosed FSP typically yields substantially larger primary particles than open FSP under identical reactant flows and composition⁶ since the enclosing tube hinders the natural air entrainment into the flame. This decreases the heat losses by convection and radiation but also increases the aerosol concentration leading to larger primary particles through enhanced coagulation and sintering.
Here the natural air entrainment flow rate drawn into a lab-scale FSP flame is determined by tracer gas analysis and its effect during tube-enclosed FSP on product particle dynamics is investigated by microscopy, scanning mobility particle sizing (SMPS) and N₂ adsorption while the aerosol tube exit temperature is measured by Fourier transform infrared (FTIR) spectroscopy. Furthermore, this air entrainment flow rate is harnessed to control the primary particle diameter of CuO nanoparticles from 10 to 42 nm in tube-enclosed FSP. This is achieved by controlling air entrainment from 0 to 250 L/min by gradually lifting off the enclosing tube from the FSP burner surface. Optimal air entrainment during tube-enclosed FSP facilitates rapid gas-to-particle conversion and high process yields by minimizing vortex recirculation as well as particle deposition on the enclosing tube walls and burner surface.
1. R. Strobel and S.E. Pratsinis: Flame aerosol synthesis of smart nanostructured materials J. Mater. Chem. 17(45), 4743 (2007).
2. E.K. Athanassiou, R.N. Grass and W.J. Stark: Large-scale production of carbon-coated copper nanoparticles for sensor applications Nanotechnology. 17(6), 1668 (2006).
3. A. Teleki, M.C. Heine, F. Krumeich, M.K. Akhtar and S.E. Pratsinis: In situ coating of flame-made TiO₂ particles with nanothin SiO₂ films Langmuir. 24(21), 12553 (2008).
4. O. Waser, R. Buchel, A. Hintennach, P. Novak and S.E. Pratsinis: Continuous flame aerosol synthesis of carbon-coated nano-LiFePO₄ for Li-ion batteries J. Aerosol Sci. 42(10), 657 (2011).
5. R. Strobel and S.E. Pratsinis: Direct synthesis of maghemite, magnetite and wustite nanoparticles by flame spray pyrolysis Adv. Powder Technol. 20(2), 190 (2009).
6. O. Waser, M. Hess, A. Guntner, P. Novak and S.E. Pratsinis: Size controlled CuO nanoparticles for Li-ion batteries J. Power Sources. 241, 415 (2013).

This work presents the development of a standardized burner for the investigation of spray-flame nanoparticle synthesis through laser-based experiments and numerical modeling.
Spray-flame synthesis is commonly used for generating metal oxide and ceramic nanoshy;particles. Commercially available burner concepts[1] are used by various laboratories, but these burners were not designed for detailed in situ investigations. As a result, both in situ measurements and detailed simulations are overcomplicated, diverting the focus from measuring and modeling the relevant physics and chemistry. At the same time, the large number of different burner concepts makes it hard to obtain a complete dataset for a burner from the different laboratories and modeling groups. These problems can be overcome by standard flames that are designed for model validation and in-situ measurements. Such standardized flames that can be easily reproduced have long been used for analyzing and modeling soot chemistry[2] and turbulent combustion[3].
The predecessor of the standard burner was applied for the synthesis of titanium dioxide nanoparticles in a spray flame[4]. The nanoparticle properties are affected by a) the break-up of the liquid jet from the spray nozzle, b) the combustion of the spray and the pilot flame and c) the formation and growth of the nanoparticles. The primary breakup of the injected liquid was modeled by a simplified volume of fluid calculation and validated by shadowgraph imaging to obtain the size distribution and the mean droplet velocity. The spray angle was determined from a side illuminated long exposure image of the spray. The resulting spray properties served as inlet boundary conditions for the downstream combustion simulations. The gas and spray were simulated with an Euler-Lagrange approach, turbulent stresses and fluxes were modeled by the RNG k-epsilon model, and turbulent combustion was described as a partially stirred reactor. The formation and growth of the nanoparticles was obtained from a monodisperse model. The present work discusses the findings from experiment and simulation in terms of flow field, species concentration, temperature, and nanoparticle formation inside the reactor.
Based on the experience with the analysis for the complete burner, a new burner was designed that has the potential to serve as a cost effective, easy to measure and model configuration that works for pressures from 75 mbar to several bar. The burner design avoids fine geometrical features to reduce the multi-scale problem involved and will be made available to all interested research groups.
[1] L. Mädler, H. K. Kammler, R. Mueller, S. E. Pratsinis, J. Aerosol Sci.33 369-389 (2002).
[2] D. R. Snelling, K. A. Thomson, G. J. Smallwood, Ö. L. Gülder, Appl. Opt.38 2478-2485 (1999).
[3] Turbulent non-premixed flames workshop series www.sandia.gov/TNF/abstract
[4] C. Weise, J. Menser, S. A. Kaiser, A. Kempf, I. Wlokas, Proc. Combust. Inst. 35, in press (2015).

Novel low-intensity phase-selective resonant laser induced breakdown spectroscopy (LIBS) is employed in the in-situ study of flame synthesis of TiO₂ nanoparticles. Excitation from the third harmonic (354.71 nm) of an injection-seeded Nd:YAG laser breaks down flame-synthesized titanium-dioxide nanoparticles into their elements and then excites the titanium electrons resonantly. With low-intensity laser excitation (~35 mJ/pulse or 70 J/cm²), only the titanium in the particle phase is selectively broken down, without any macroscopically-visible plasma. The induced emission at 497.534 nm related to the resonant excitation is markedly stronger than other emissions. Compared to 532 nm excitation, with saturation intensity ~20 mJ/pulse, the emission at 498.173 nm from 354.71 nm excitation also saturates ~20 mJ/pulse; however, the 497.534 nm emission intensity continues to increase beyond 20 mJ/pulse, until gas-phase breakdown. Temporal evolution of the emissions is studied, and the results show that the 497.534 nm emission peaks earlier, but does not last as long compared to the other emissions. Emission peak splitting and the dependence on excitation laser linewidth are observed and investigated, respectively. Our technique has the potential to make elemental measurements of nanoparticles at very-low detection limits.

In-situ non-intrusive diagnostics of nano-aerosols is highly desired for both laboratory research and industrial process to well understand and control the formation of nanoparticles during flame synthesis. A phase-selective laser induced breakdown spectroscopy (PS-LIBS) has been developed for characterizing the volume fraction and gas-to-particle conversion, which overcomes the shortage of laser induced incandescence (LII) and extends from soot to metal oxides nanoparticles. Different from the traditional laser-induced breakdown spectroscopy (LIBS), which ionizes all the matters in the measuring volume by millimeter-sized plasmas, here in PS-LIBS only matters in particle phase are ionized by selecting the excitation laser power between breakdown thresholds of gas and particles phase, during which the laser-induced nano-plasmas are formed and confined around nanoparticles.
As a demonstration of PS-LIBS, TiO₂ nanoparticles are synthesized by a Bunsen flame which is stabilized by a multi-diffusion flat flame. When the laser energy increases, the intensity of Ti atomic spectra first increases and saturates around 20 J/cm². The saturation signals show perfect linear relationship with the particulate volume fraction showing the feasibility of particulate volume fraction measurement after the calibration. Benefiting the nano-sized plasma confined around every nanoparticle, a two-dimensional imaging of particulate distribution is realized, in which the quick transition from gas phase precursor to TiO₂ nanoparticles across the flame sheet can be clearly identified. Finally, in the V-doped TiO₂ synthesis environment, PS-LIBS can trace the gas-to-particle conversion of both V and Ti simultaneously. The atomic spectrums of two elements reveal that V nucleates earlier than Ti and triggers the gas-to-particle conversion of Ti.

In the gas-phase synthesis of nanoparticles, e.g., in plasma or combustion processes, in situ diagnostics for the determination of particle size are rare. It is known that soot particle sizes can be measured by time-resolved laser-induced incandescence (TiRe-LII). LII shows a large potential for particle-size measurements especially for elemental nanomaterials with high boiling points.
Silicon nanoparticles synthesized from SiH₄ in a microwave plasma reactor at 120 mbar were investigated with LII downstream of the plasma zone. The synthesis conditions lead to spherical, single-crystalline silicon nanoparticles in the size regime of a few ten nanometers depending on precursor concentration. LII measurements were performed using a Nd:YAG laser at 1064 nm to heat the silicon particles above ambient temperature. Incandescence was measured at 442 and 716 nm simultaneously and the particle temperature was determined via two-color pyrometry. The time-resolved temperature profile was then used to determine the particle size of the silicon nanoparticles. Based on a heat transfer model taking into account nanoparticle cooling by evaporation, radiation, and heat conduction, a mean particle size around 30 nm could be calculated from the temperature decay. This is in almost perfect accordance with the diameters of particles scavenged from the gas flow determined from electron microscopy and from the specific surface area via nitrogen adsorption (BET) assuming monodisperse spheres.

A novel double-slit curved wall-jet (DS-CWJ) burner was designed for flame synthesis. The burner comprises of an inner annular slit for precursor delivery and an outer annular slit for supplying premixed fuel/air mixture. This setup enables rapid mixing between the precursor and the supporting flame. The effect of using different fuels on the phase and particle growth of titanium dioxide (TiO₂) nanoparticles has been investigated. Methane (CH₄), ethylene (C₂H₄), and propane (C₃H₈) fuels were used with air as the supporting flame. The burner body was heated to prevent precursor condensation. Titanium tetraisopropoxide (TTIP) was used as the precursor for the TiO₂ nanoparticles.
Measurements were made to investigate the flow field, mixing, and flame structure using particle image velocimetry (PIV), acetone planner laser induced fluorescence (acetone-PLIF), and OH-PLIF, respectively. The nanoparticles were characterized using high-resolution transmission electron microscopy (HRTEM), x-ray diffraction (XRD), Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), and nitrogen adsorption BET for surface area analysis.
DS-CWJ burner enables enhanced mixing characteristics between the precursor and combustion gases, with a slight increase in the axial velocity due to the precursor injection. As a result of improved mixing between the precursor and the flame gases, the burner produces a uniform distribution of 13-18 nm particles with a high BET surface area (>100 m²/g). Phase of the TiO₂ nanoparticles was mainly dependent on the equivalence ratio and fuel type, which impacts flame height, heat release rate and high temperature residence time of the precursor vapor. For methane and ethylene flames, the anatase content is proportional to the equivalence ratio, whereas it is inversely proportional in the case of propane flame. The synthesized TiO₂ nanoparticles exhibited high crystallinity and the anatase phase was dominant at high equivalence ratios (Phi; >1.6) for CH₄ and C₂H₄, and at low equivalence ratios (Phi; <1.3) for the C₃H₈ flame.

Flame-assisted synthesis offers a convenient route for production of nano-shaped metal oxide materials - in large quantities as bulk nano-powders, or, when combined with molecular beam sampling and size-selected deposition, as supported nanoparticle (NP) samples with well-defined size distribution, coverage and spatial uniformity. Iron oxide NPs are used in diverse applications, including optical magnetic recording, catalysis, gas sensors, targeted drug delivery, magnetic resonance imaging, and hyperthermic malignant cell therapy. The NP properties are largely dependent on their characteristics, such as, size distribution, phase and morphology.
Detailed understanding of the mechanism governing the particle formation in flames is a necessary pre-requisite for flame synthesis of NPs with tailored functionalities. Detailed mechanism validation requires quantitative (and desirably in-situ) diagnostics of both gas-phase molecular precursors (e.g. FeO) and nascent solid iron oxide NPs. The high temperature and particle-laden flame synthesis environment imposes consderable challenges on in-situ diagnostics.
Here we describe experiments capable of delivering quantitative information regarding gas-phase intermediates and solid phase particulates. The most powerful experimental tools at our disposal comprise combination of mass-spectrometric and laser-based techniques. These include the newly developed combined Particle Mass Spectrometry-Quartz Crystal Microbalance (PMS-QCM) technique, which delivers the particle m/z distribution, ionization efficiency, as well as mass and number concentration (at least in relative units). These measurements are complemented by monitoring the increase of the resonance frequency of the quartz crystal induced by laser irradiation (LID-QCM method). The magnitude of the frequency detuning is proportional to the energy absorbed on one of the quartz crystal electrodes. When the crystal is covered by flame-generated nanoparticles (deposited by molecular beam sampling), the amount of the laser energy absorbed, and the consequent frequency detuning are reflecting the magnitude of the absorption coefficient of the NP layer. The monitoring of gas-phase intermediates (e.g. FeO) in particle-laden environment with standard methods, such as laser induced fluorescence, is hampered due to light scattering from solid particulates and luminosity of particle-generating flames. In this work these interferences were circumvented by deploying Intracavity Laser Absorption Spectroscopy (ICLAS). While the sensitivity to narrowband absorption of the gas-phase FeO is largely enhanced, ICLAS is not sensitive to the broadband absorption by the NPs, allowing to monitor FeO in particle-laden environment. These methods are applied for different flame configurations, including low pressure flat flame and a hybrid, two-stage flame. With such data in hand we can draw conclusions regarding the possible mechanisms of NP formation and consumption.

Molecular-beam sampling methods enable the inline investigation of high temperature gas-phase nanoparticle synthesis processes. This sampling method immediately interrupts all gas-phase processes and thus allows investigation of the gas phase and the particle-phase composition at various locations in a reactor corresponding to different stages of the formation process. The size distribution of charged particles can be measured by particle-mass spectrometry (PMS). This technique covers the mass range of particles having diameters in the nanometer regime (m/z > 1000 u). The aim of our study is to quantify the ratio between charged and neutral particles for various experimental conditions observed in a laminar low-pressure H₂/O₂ flame where vaporized precursors are added to the feed gases to initiate particle formation. A quartz-microbalance (QMB) was synchronized with the PMS measurements because this combination of techniques has shown that it can provide information about the fraction of charged particles in a molecular beam [1, 2]. The QMB measurement sensitivity (asymp; 0.02 ng/s) was determined by measuring the frequency shift performing control experiments without the use of a precursor.
Iron oxide nanoparticles are synthesized through the decomposition of iron pentacarbonyl Fe(CO)₅ in a premixed H₂/O₂ flame. An almost one-dimensional, low-pressure flat flame burning from bottom to top operated at 30 mbar is used to investigate the spatially extended reaction zone. A small sample of the aerosol is expanded through a nozzle and a skimmer into high vacuum to form a particle-laden molecular beam. The progress from the initial decomposition of the precursor to the formation and growth of nanoparticles can be studied by varying the distance between the burner head and the nozzle position. The distance between the sampling nozzle and the burner, the so-called height above burner (HAB), represents the residence time within the reactor and thus gives an insight into various stages within the formation process. We present results for the particle-size distribution and mass deposition rate as a function of HAB and precursor concentration.
References:
[1] I.-K. Lee, M. Winterer, Review of Scientific Instruments, 76 (2005) 095104-095108.
[2] A. Hevroni, H. Golan, A. Fialkov, I. Rahinov, V. Tsionsky, G. Markovich, S. Cheskis, Measurement Science and Technology, 22 (2011) 115102.

A method for online characterization of particle growth during gas-phase nanoparticle production is presented. By combining a differential mobility analyzer, an aerosol particle mass analyzer and a condensation particle counter the average agglomerate size and structure as well as the number and size of primary particles can be determined [1, 2]. Here, this approach was applied to study primary particle and agglomerate growth during production of zirconia nanoparticles by flame spray pyrolysis at rates of 30 g/h [3]. Results were compared to thermophoretic sampling/electron microscopy allowing to elucidate the differences between measured projected-area equivalent diameter, diameter of gyration and mobility diameter. Particles were sampled from the flame at up to 2000 K and 10¹⁸ particles/m³ using a probe allowing continuous aerosol extraction and rapid dilution with quench air that effectively suppressed coagulation. Primary particle growth was complete at ~10 cm above the burner at about 1500 K as the primary particle size did not change radially and axially downstream. As expected, the average agglomerate size increased with axial distance from the burner. Larger agglomerates, however, were observed at the edges of the aerosol plume attributed to prolonged residence time due to lower gas velocities there. The developed online diagnostic method is not only well suited for validation of aerosol dynamics models but can also assist in the control of nanoparticle manufacturing processes or be used for online monitoring of product particle quality.
References:
[1] Park, K., Cao, F., Kittelson, D.B., McMurry, P.H. (2003), Environ. Sci. Technol. 37, 577.
[2] Eggersdorfer, M.L., Gröhn, A.J., Sorensen, C.M., McMurry, P.H. and Pratsinis, S.E. (2012), J. Colloid. Interface Sci. 387, 12.
[3] Gröhn, A.J., Eggersdorfer, M.L., Pratsinis, S.E., and Wegner, K. (2014), J. Aerosol Sci. 73, 1.

We introduce about the recently developed flame transport synthesis (FTS) approach^[1] which enables versatile fabrication of various complex shaped nano- and microstructures from different metal oxides and their interconnected three dimensional (3D) macroscopic networks. A large variety of metal oxide structures such as, nanorods, nanowires, nanotubes, nanobelts, nanoplates, nanonails, nanotetrapods, nanohammers etc. can be efficiently synthesized in a reproducible and cost-effective manner.^[1] The FTS method offers versatile synthesis in terms of integrating ZnO nano- and microneedles in the trenches or ZnO nanotetrapods bridging network on the chip in direct growth process which can be efficiently used for photocatalytic or sensing (UV/gas) applications.^[2,3] Use of nanoscale materials in biomedical applications is now-a-days very important aspect and the FTS grown ZnO nanoseaurchins and tetrapods have shown unique potentials against herpes simplex viruses (type 1 and 2).^[4,5] The slightly large size and unique shape of these nano- and microscale ZnO tetrapods further offer better features in terms of low cytotoxicity and better handling etc. and they have already been utilized in interesting applications, e.g., fabricating self-reporting composites^[6] and as linkers for joining the un-joinable polymers.^[7] Fabrication of porous, flexible and conducting 3D ceramic network is very important aspect for next generation technologies and FTS method offers desired synthesis or such networks.^[1] Synthesis, growth mechanisms and properties of large and highly porous three-dimensional (3D) interconnected networks of metal oxides (ZnO, SnO₂, Fe₂O₃) nano-microstructures including carbon based Aerographite material^[8] using FTS approach will be discussed along with their possible applications.
References:
[1] Particle & Particle Systems Characterization 30, 2013, 775-783
[2] ACS Applied Materials & Interfaces 6, 2014, 7806minus;7815
[3] Advanced Materials 26, 2014, 1541-1550
[4] Antiviral Research 92, 2011, 305-312
[5] Antiviral Research 96, 2012, 363-375
[6] Advanced Materials 25, 2013, 1342-1347
[7] Advanced Materials 24, 2012, 5676-5680
[8] Advanced Materials 24, 2012, 3486-3490

Single crystalline nanowires of metal oxides, formed by vapor-liquid-solid (VLS) method, hold great promise for various nanoscale device applications. Manipulating doping is of utmost importance for any kind of application. However, VLS doping is still far from comprehensive understanding due to the intrinsic difficulties in controlling complex material transports of VLS process. Here, taking indium-tin oxide nanowire as an example, we found an unbalanced nucleation of indium and tin at the liquid-solid (LS) interface. In spite of the fact that the vapor pressure of indium is higher than that of tin, the tin concentration within Sn:In₂O₃ (ITO) nanowires was always lower than the nominal composition, implying an emergence of preferential indium nucleation at LS interface. The resistivity of ITO nanowires can be tuned from 2.1×10^-1 Omega;cm down to 9.0×10^-5 Omega;cm, via increasing intentionally tin concentration.^[1,2] Furthermore, we demonstrated that the crystal phase of metal oxide could be selectively stabilized by manipulating the nucleation competition of indium and tin at LS interface. Oxide nanowires with complete different crystal phases of rutile (SnO₂), metastable fluorite (In_xSn_yO_3.5, so-called ISO phase) and bixbyite (Sn:In₂O₃, ITO) could be sequentially obtained by solely increasing supplied metal flux, with all the other experimental parameters maintained.^[3] These results highlight understanding nucleation competition of metal elements at LS interface is essential for designing functional metal oxide nanowires.
References:
1. Meng et al.J. Am. Chem. Soc. 135 (2013) 7033minus;7038
2. Meng et al.Adv. Mater. 25 (2013) 5893-5897
3. Meng et al.Nanoscale 6 (2014) 7033-7038

One-dimensional (1D) molybdenum trioxide (MoO₃) nanostructures (nanowires, nanoflakes, nanotubes) with interesting electronic, catalytic and chromic properties, have been synthesized by many different approaches, such as hydrothermal synthesis, hot plate synthesis, and electrochemical methods. However, many of these synthesis methods require the use of catalysts and vacuum conditions, and typically have small growth rates, hindering the scale-up of 1D MoO₃ production. Here, we report a flame synthesis technique for growing 1D α-MoO₃ nanobelts, branched α-MoO₃ nanobelts and flower-like α-MoO₃ on diverse substrates. This scalable and economical flame synthesis method has the advantages of atmospheric growth, no need for catalysts, rapid growth rate, high coverage density, controllability of morphology, and broad choices of substrate materials and substrate morphologies. Experimentally, the Mo metal is oxidized in the high temperature region of a flame, forming MoO₃ vapor that is subsequently deposited downstream at a cooler growth substrate. Typically, after only 5 minutes of growth, densely packed rectangular nanobelts with an average width of 4mu;m, thickness of 200 nm, and length of 30 mu;m are formed perpendicularly on the Si growth substrate. XRD and TEM characterizations show that the as-grown nanobelts are orthorhombic α-MoO₃ and the growth direction of the nanobelts is along [001]. The growth rate, morphology, and surface coverage density of the α-MoO₃ nanobelts can be controlled by varying the flame stoichiometry, the source temperature, growth substrate temperature, and the material of the growth substrate. Branched α-MoO₃ nanobelts are formed by performing the synthesis sequentially under two different conditions. Flower-like aggregates of α-MoO₃ nanobelts are formed by lowering the MoO₃ vapor concentration and lowering the nucleation energy barrier on the growth substrate by adding Au nanoparticles. We believe this flame synthesis technique is a promising, alternative way to synthesize 1D metal oxide nanostructures in general.

Nanoscience and technology is not limited to hypothesis and laboratory these days, but a recent trend is production of large scale nanomaterials which could translate into high performance devices and also economical viability. ARCI is equipped with a custom made flame spray pyrolyzer (FSP) which is capable of a large scale production of nanopowders >3kg/h. In this presentation, morphology control of ZnO nanopowder by simple changing type of solvents and equipment operating parameters will be discussed. The powders obtained have been thoroughly characterized by XRD, FE-SEM, TEM and HR-TEM and BET surface measurement to understand the structure and morphology of ZnO powders. Base on evidence, a new growth mechanism for ZnO nanorod in FSP is proposed.
Using optimized process parameters of FSP, doped ZnO nanopowders were synthesized and characterized for varistor applications. The resulting powders were consolidated into more than 98% dense pellets by conventional sintering technique. Electrical properties of sintered pellets at different techniques were studied. Current-voltage (I-V) properties and frequency dependent electrical behavior of sintered varistor pellets were investigated. Best varistor properties, breakdown voltage of 14±0.5 kV/cm, coefficient of non-linearity of 90±2 and leakage current density of 0.22±0.05 µA/cm² were obtained for optimum ZnO grain size and density of the pellets. AC electrical data was analyzed using impedance formalism to understand electrical equivalent circuit of the sintered ZnO varistor for morphologic seen in the SEM.

Fabrication of three dimensional (3D) networks build from interconnected ZnO nano- and microstructured tetrapods using flame transport synthesis (FTS) approach^[1] will be demonstrated here. The FTS method enables controlled synthesis of various metal oxide nano- and microstructures which can be used for various applications. By compressing and re-heating of as-synthesized ZnO tetrapods, adjacent tetrapod-arms interpenetrate each other resulting in formation of porous 3D networks. The porosity of the interconnected networks can be adjusted by controlling the initial ZnO tetrapod concentration in a well-defined volume and networks with porosities up to 98% have been made. Mechanical strength and electrical conductivity of these 3D networks were studied and a structure-property correlation has been understood. These porous 3D networks made from nano-microscale elements enable the combination of material properties with various properties arising from nanoscale dimensions. The ZnO 3D networks, we introduce here, combine the properties of a classical semiconductor with a high mechanical flexibility arising from nano-microstructures. By varying the porosity of ZnO 3D networks, their Young&’s modulus can be tailored from low kPa region to several MPa. As an application example, these 3D interconnected ZnO networks were used as templates for synthesizing the new Aerographite^[2] which is a carbon based 3D interconnected material with outstanding properties.
References:
[1] Particle & Particle Systems Characterization 30, 2013, 775-783
[2] Advanced Materials 24, 2012, 3486-3490

Zinc oxide (ZnO) is a wide-bandgap metal oxide semiconductor that finds applications in a variety of fields and products including paints, pigments, batteries, photocatalysis, foods, to name a few. When in nanometer size range, it may be used in polymer nanocomposites and sunscreens as an efficient UV-filter with high transparency in the visible wavelength range. However, the photocatalytic activity of ZnO may cause polymer degradation rendering it unsuitable for long-term employment in many applications. Furthermore, ZnO nanoparticles exhibit cytotoxicity and may pose risks to environmental and human health. Here, ZnO nanorods are made by scalable flame synthesis and are in-situ encapsulated by an amorphous nanothin SiO₂ layer [1]. The as-prepared nanoparticles are characterized by electron microscopy (EM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and N₂ adsorption. The hermetic nature of the SiO₂ coating is confirmed by monitoring the decomposition of methylene blue dye caused by the photocatalytic activity of ZnO under UV irradiation and by monitoring the Zn ion release in aqueous solutions. It was also demonstrated that the core ZnO nanorods exhibit the characteristic optical properties as determined by UV/vis, while the SiO₂ coating minimizes the cytoxicity [2] and genotoxicity [3]. Therefore, this “safer by design” formulation approach can be used to generate flame-made and SiO₂ hermetically coated ZnO nanorods which could be used in polymer UV-filter nanocomposites and sunscreen applications. These core-shell ZnO nanoparticles exhibit superior performance while at the same time possess a minimum toxicological footprint.
References
[1] G.A. Sotiriou, A. Hirt, P.-Y. Lozach, A. Teleki, F. Krumeich, S.E. Pratsinis, “Hybrid Silica-coated, Janus-like Plasmonic-Magnetic Nanoparticles”, Chem. Mater.23, 1985-1992 (2011).
[2] S. Gass, J. Cohen, G. Pyrgiotakis, G.A. Sotiriou, S.E. Pratsinis, P. Demokritou, “Safer Formulation Concept for Flame-Generated Engineered Nanomaterials”, ACS Sustainable Chem. Eng.1, 843-857 (2013).
[3] G.A. Sotiriou, C. Watson, K.M. Murdaugh, T.H. Darrah, G. Pyrgiotakis, A. Elder, J.D. Brain, P. Demokritou. “Engineering Safer-by-Design, Transparent, Silica-coated ZnO Nanorods with Reduced DNA Damage Potential”, Environ. Sci.: Nano1, 144-153 (2014).

Combustion as a method of material synthesis has its rich history. Examples include soot or carbon black that found its uses dating back to the prehistoric time. More recently, carbon nanoparticles have found wide ranging applications, from rechargeable batteries and pseudocapacitors to photovoltaics. Condensed-phase materials other than carbon can be produced in gaseous flames by introducing elements beyond the common constituents of hydrocarbon flames. Metals or metal precursors added to the flame produce metal oxide nanoparticles or other structures. New advances include specialty nanoparticles and the films produced from them for applications in catalysis, solar cells and biomedical devices.
Beyond their common origin in flames, flame soot formation and functional nanomaterial synthesis by flames share many common characteristics. Both involve the formation of condensed-phase materials from gases starting with vapor-phase nucleation, followed by mass and size growth through coalescence, coagulation, surface reactions and condensation of vapor species, and finally by aggregation into fractal structures, all of which occur over very short periods of time, typically a few milliseconds. Similar diagnostic and computational tools are employed to study the formation of both condensed-phase materials.
This talk will discuss features common to the formation of soot and metal oxide particles in flames. In that context, several unresolved questions about the mechanism of soot formation will be presented. In the second part, we discuss how a long history of fundamental flame studies helped us to advance useful concepts and applications for flame synthesis of functional nanomaterials. Finally, the applications of material synthesis in several interesting devices will be presented with the goal to demonstrate that gaseous flame synthesis offer unique advantages over traditional methods of material processing.

For energy applications including storage, generation and conversion, physical structure over a range of length scales is important. Carbon based materials and energy applications are highly synergistic. Nanostructured carbon, in the form of nanotubes, onions, nanocages, graphene and fullerenes has advanced performance in each of these applications considerably.
Three energy processes are applied to carbon include electron beam irradiation, pulsed laser light and high temperature, thermal conductive heating, for comparison. Each process adds energy to the carbon material, albeit in different forms and interestingly, with different outcomes. Interest in such processes stems from environmental and energy applications. Each area relies upon carbon materials featuring high surface energy yet with physical integrity for macroscale use. The carbon allotropes of fullerenes, nanotubes and graphene offer different geometrical dimensions by which to fabricate macroscale materials using nanoscale components as building blocks. Bonding these allotropes is required to create architectures with useful scale. Understanding how different forms of energy interact with these different allotropes is the first step in process design for their integrated fabrication.
Our results thus far confirm the following three hypotheses.
1. Chemistry controls the growth of nanostructure - to be tested by using varied allotropes with dopant levels of sp3 hybridization, O- and H- atoms.
2. Initial nanostructure sets the course of nanostructure evolution to be tested using carbon spheres and MWNTs with varied nanostructures including amorphous, fullerenic and graphitic nanocarbons.
3. Morphology provides geometrical constraints to nanostructure spatial expansion - to be tested using varied nanocarbon shapes including carbon spheres (possessing different nanostructure), CNTs and graphene.
Results will be shown for each method illustrating these tenets, as illustrated by the 3x3 matrix of carbon allotropes and energy addition method. HRTEM and XPS are the primary characterization tools. Implications for the observed structural changes will be discussed. Comparative results from flame formed carbon, catalyzed or uncatalyzed will be shown as time permits.

Catalytic chemical vapor deposition (CVD) is a popular method to synthesize carbon nanotubes (CNTs). At the presence of catalysts (usually trasition metals), the hydrocarbon feedstock decomposes controllably at elevated temperatures and can form tubular structures. It has been suggested that trace amounts of weak gas-phase oxidants, such as CO₂, can enhance the CNT synthesis by extending the catatlyst life. It is not clear, however, how these additives affect the CVD reaction environment. In this study, ethylene gas was introduced to a preheated furnace/CVD reactor where meshes of stainless steel were introduced. Therein ethylene was thermally decomposed in nitrogen mixed with different amounts of carbon dioxide. The meshes served as catalytic substrates for CNT growth. The compositions of the ethylene pyrolyzates were were analysed both with and without the presence of catalysts, to explore the possible contributions of CO₂ addition to the CNT formation. These compositions were compared with kinetic model predictions. Results indicated that 1,3-butadiene (C₄H₆) was the most abundant hydrocarbon species of ethylene decomposition (at 800 °C) and that decomposition was inhibitted at the presence of CO₂ with a commesurate effect on CNT formation.

Carbon nanomaterials with different structures were synthesized and mixed for an electric double layer capacitor (EDLC) electrode. We used two kinds of carbon nanomaterial: arc black (AcB) and a carbon nanoballoon (CNB). AcB is an amorphous carbon nanoparticles and synthesized by an arc discharge between graphite rod electrodes in N₂ gas [1,2]. We developed a twin-torch arc apparatus that can synthesize AcB continuously by pushing out the rod electrodes using a motor. AcB is mainly composed of cocoon-shaped carbon nanoparticles with a lot of amorphous ingredients. Carbon nanoballoon (CNB) is a hollow graphitic nanoparticles with a high electrical conductivity and obtained by heating AcB in a Tammann oven in Ar gas above 2000°C for 2 h. The number of graphitic layers of CNB increases and its shell becomes thick with increasing the heating temperature [1,2]. In this research, CNB was prepared at 2600°C.
We used a typical two-electrode cell with two symmetric coin-type electrodes for the electrochemical measurement of the EDLCs. Activated carbon (AC; YP80F, provided by Kuraray Co. Ltd.) was used as a comparison material. First, 10 mg of polytetrafluoroethylene (PTFE) dispersion liquid was dropped onto 90 mg of AcB and CNB. AC was mixed with Ketjen black (KB) and PTFE by weight ratio of 80:10:10. Then each of them was mixed for 15 min by an automatic mortar. 100 mg of the mixed material was put into the jig (inner diameter: 15 mm). The jig was pressed by 14 MPa for 30 min at room temperature. The materials were put into a two-electrode cell in the following order: collector electrode (Nirako SUS 304 foil), prepared electrode, separator (Nippon Kodoshi MPF0830), prepared electrode, and collector electrode. 1 M H₂SO₄ were used as electrolysis solution.
To utilize their characteristics, AcB and CNB were used as the main materials of the EDLC electrode in weight ratios of 1 : 1, 2 : 1, and 1 : 2. An electrochemical measurement system (Hokuto Denko HZ-5000) was used for the electrochemical measurement of the EDLC. As a result, by mixing AcB and CNB, both the power and energy densities became higher than those of AcB or CNB alone. The EDLC mixed in 1 : 1 weight ratio of AcB and CNB showed the highest performance, with a higher electric power density than AC. Very recently, we have found that oxidized CNBs (Ox-CNB) had a higher specific capacitance than CNB and AC at a high scan rates (ge; 500 mVs^-1) [3].
References
[1] Toshiyuki Sato, Yoshiyuki Suda, Hikaru Uruno, Hirofumi Takikawa, Hideto Tanoue, Hitoshi Ue, Nobuyoshi Aoyagi, Takashi Okawa, Kazuki Shimizu, Journal of Physics: Conference Series, 352 (2012) 012032.
[2] Yuta Okabe, Harutaka Izumi, Yoshiyuki Suda, Hideto Tanoue, Hirofumi Takikawa, Hitoshi Ue, Kazuki Shimizu, Japanese Journal of Applied Physics, 52 (2013) 11NM05.
[3] Yuta Okabe, Yoshiyuki Suda, Hideto Tanoue, Hirofumi Takikawa, Hitoshi Ue, Kazuki Shimizu, Electrochimica Acta, 131 (2014) 207-213.

Nanoscale zinc oxide is an extremely versatile material that finds application e.g. in electronics, optics, solar cells, catalysis, sensors or polymer nanocomposites. Special properties have been reported for one dimensional nanostructured materials such as nanorods or -wires that can lead to improved performance.
Here, conditions for flame synthesis of spherical and rod-shaped ZnO nanoparticles are investigated. Therefore, reactant flow rates are varied and resulting flame temperature profiles are determined by Fourier-transform infrared spectroscopy (FTIR). Particle samples are taken at different heights along the center axis of the flame by thermophoretic sampling allowing to track the evolution of particle size and morphology. Based on this, operation windows for the synthesis of sphere- or rod-like ZnO nanoparticles are defined.
The surface chemistry of the as-synthesized particles is characterized to reveal shape-dependent differences that could affect the performance of the powders in catalytic hydrogen formation, the envisioned application of the nanoparticles.

A fabrication process of highly porous nanoparticle layers form dry high temperature aerosol techniques (flames, hot wall reactors etc.) on temperature sensitive substrates will be presented. Through the use of aerosol deposition techniques the reachable deposition efficiencies can be raised up to almost 100%. The porous layers are transferred on various substrates with a two roll laminator that can be up scaled to a roll-to-roll process. This has high cost reduction potential in comparison to deposition methods used today.
Besides layer transfer, the results show that restructuring and mechanical stabilization of the nanoparticle layer is a result of the applied pressure during the lamination step. The restructuring affects mostly the larger pores leading to homogenization of the pore structure. Additionally, it is possible to create nanoparticle layers with adjustable porosities for different applications. For example, in gas sensing treatments the porosity can be optimized leading to enhanced electrical conductivity with constant surface areas and still high gas diffusion into the layers. The mechanical stabilization opens possibilities of using the aerosol technique for applications, where the nanoparticle layer has to be abrasion resistant or has to withstand liquid environments. Examples for such applications are super-hydrophilic coatings, dye-sensitized solar cells, immobilized catalysts for photocatalytic water treatments and water splitting as well as polymeric/inorganic nanocomposites. Examples based on advanced materials (metastable metal oxides, metal oxide heterojunctions) will be presented.

Nanoparticulate oxides are low cost and promising materials for electronic and optical devices. For example zinc oxide, as a wide band gap semiconductor, is utilized as basic material in printable electronics. To cover the high demand of these raw materials, an economic, energy efficient and continuous synthesis route has to be established.
This can be achieved by a flame spray pyrolysis (FSP) process, which is one of the most versatile, cost-effective, scalable and manageable gas phase techniques for the synthesis of complex and functional nanoparticles at a commercial scale. The pyrolysis reaction in the flame leads to high supersaturation, particle nucleation with subsequent growth, agglomeration and sintering. Due to the highly turbulent and multiphase character of the spray flame it is very important to have a detailed understanding of the interplay between these different particle formation steps and the relevant locations in the flame. To achieve this goal, it is advantageous to combine various noninvasive optical measurement techniques for an in situ measurement.
We report on our results regarding ZnO and SiO₂ formation. By Mie-scattering imaging we get information about the spatial droplet distribution in the flame [1]. Additionally the droplet size and velocity can be determined by Phase-Doppler Anemometry (PDA) and related to the particle size distribution (PSD). Chemiluminescence imaging allows us to acquire information about the location of the nucleation regime and therefore the particle formation mechanism of ZnO and SiO₂. To this end, we investigated the influence of different precursor concentrations on the spatial position and expansion of the nucleation regime. CARS measurements were carried out to obtain temperature information at different flame heights in order to localize the sintering zone associated with high temperatures in the flame [1,2].
In order to support the results from optical measurements, thermophoretic sampling in different flame regimes and TEM analysis is favourably used. Mean particle sizes were also obtained by BET. For ZnO the particle size varies from 5 to 15 nm and for SiO₂ from 8 to 26 nm depending on the concentration of the precursor.
Optical analysis methods like Mie-Scattering, PDA and chemiluminescence imaging will be discussed and conclusions regarding growth mechanism and PSD will be drawn. Furthermore the particle size of the material systems will be demonstrated by gas adsorption measurements.
Financial support by the Cluster of Excellence “Engineering of Advanced Materials” funded by DFG within the German Excellence Initiative is gratefully acknowledged.
References:
[1] Kilian, D., Engel, S., Borsdorf, B., Gao, Y., Kögler, A.F., Kobler, S., Seeger, T., Will, S., Leipertz, A., Peukert, W. J. Aerosol Sci, 69, 82-97 (2014).
[2] Engel, S.R., Koegler, A.F., Gao, Y., Kilian, D., Voigt, M., Seeger, T., Peukert, W., Leipertz, A. Appl. Opt., 51, 6063-6075 (2012).

Indium tin oxide (ITO) conductive nanowires (NWs) are regarded as one of the most promising structures for use as the embedded electrodes of electronic devices such as organic solar cells, light-emitting devices and various gas/liquid sensors. The authors have developed a fabrication technology for ITO NWs using a conventional magnetron sputtering system (USP-66F, Universal Systems Co.) ^{[1, 2]}. The key factor is the deposition of ITO in Ar sputtering gas, but in the absence of O₂. In this report, we present details on the electronic and microstructural properties of the fabricated ITO NWs. The resistivity of each ITO NW was determined from current-voltage characteristics obtained using a semiconductor device analyzer and four nanoprobes embedded in a field-emission scanning electron microscope (nanoEBAC NE4000, Hitachi High-Technologies Co.). The resistivities of ITO NWs prepared using a 7.0 wt%-SnO₂ ITO sputtering target were 0.13-0.60 mu;Omega; m. Electron diffraction of a sample using a transmission electron microscope (TEM; H9500, Hitachi High-technologies Co.) indicated that the ITO NW was a single crystal, of which the crystal growth direction was <100>. It is considered that the single crystal structure of the ITO NWs results in a resistivity that is lower than that of ITO transparent films formed in Ar plasma containing traces of oxygen. The resistivities of the ITO NWs obtained using a 30 wt%-SnO₂ ITO sputtering target were 0.43-2.20 mu;Omega; m. The resistivity of the ITO NWs increased with the Sn content. The In, Sn and O contents of the ITO NWs corresponded directly to the elemental compositions of the ITO sputtering targets. The elemental compositions of the ITO NWs are thus controlled according to the ITO target used with the intended SnO₂ content. The main processing conditions required for the growth of ITO NWs by sputtering are (1) a substrate temperature higher than ca. 175 °C, which is close to the melting point of indium (156 °C), and (2) a SnO₂ content in the ITO target in the range from ca. 5 to 30 wt%. ITO NWs formed using an ITO target with a SnO₂ content of ca. 5.0-12.0 wt% had circular cross-sections, whereas those sputtered using an ITO target with a SnO₂ of ca. 12.0-30.0 wt% had square cross-sections.
The authors would like to thank Mr. Teruho Shimotsu and Mr. Junichi Fuse at the Hitachi High-Tech Manufacturing & Service Co. for conducting electrical measurements with a nanoprobe system and TEM observations of the ITO NW microstructures. This work was supported by Sekisui Chemical Co., Ltd. References: [1] Naoki Yamamoto, et al., ECS Solid State Letters, Vol. 3, (7), p84-p86, (2014). [2] Naoki Yamamoto, et al., 2013 MRS Fall Meeting, SS13.70, Dec. 4, 2013, Boston.

Strontium aluminates hosts are attractive because of their excellent phosphorescence properties and good stability. Nowadays synthesis routes producing large particle sizes, 20 to 100 mu;m require temperatures in the range1300-15000C. New long lasting phosphors having particles size <10 µm are demanded for printing industries.In the present work, SrAl₂O_4: Eu²⁺, Dy³⁺ powders have been synthesized by an original method based on molten salts. The goal is to produce sub-micronic particles with a high degree of crystallinity at relatively low temperatures (900-1000 0C).The eutectic mixture, NaCl-KCl has been selected (m.p., 6590C) as the solvent, the starting materials were homogenized with the salt by dry milling. Different kinds of Al₂O₃ (0.1 µm, 0.5 µm, 6 µm) have been employed to evaluate the role of the size of this precursor in the kinetics of reaction. To shed some light on the formation of SrAl₂O₄ in melt flux, the system was thermally treated in early stage of the reaction. The hexagonal SrAl₂O₄ phase starts to appear at 670°C, at lower temperature than other methods. SrAl₂O₄ has two polymorphs:a monoclinic symmetry stable below 6500C and hexagonal stable above this temperature. Monoclinic phase shows luminescent properties;there is no clear agreement about the emission of the hexagonal structure.These phases have similar diffraction patterns: the monoclinic patterns overlap the hexagonal one; therefore it is not possible to make an accurate phase assignment. Raman studies have been carried out to elucidate the monoclinic and hexagonal coexistence.SrAl₂O_4: Eu²⁺, Dy³⁺ phosphors have been successfully obtained at 10000C during 2 h in reducing atmosphere (N₂-H₂). The morphology of the particles was analyzed by SEM and TEM. Particles synthesized from 6-Al₂O₃ have SrAl₂O_4-submicron particles growing on the surface of the Al₂O₃.The particles retain the shape of the original Al₂O_3, this formation process can be attributed to the Kirkendall effect. Ex-situ RT-MAS ²⁷Al solid state NMR results exhibit one AlO₄ (max.75ppm ) and one AlO₆ resonances (max. 12ppm), indicating the presence of SrAl₂O₄ and Al₂O₃ in the system, in accordance with the unreacted Al₂O₃ in the core.However, SrAl₂O₄ synthesized using 0.1Al₂O₃ and 0.5 Al₂O₃ have spherical morphology and particle size le;0.5mu;m. This can be attributed to a higher reactivity.TEM observation on the products indicates a high degree of crystallinity.Upon the excitation of 380 nm, it is observed that the PL spectrum consists of a single green emitting broad band with a maximum at 514 nm. The decay curves are fitted by bi-exponential decay function; the decay times for exponential components being, tau;₁(s) and tau;₂(s), equal to 22 and 96, 21 and 110 and 21 and 116 for powders synthesized with 0.1-Al₂O₃, 0.5 Al₂O₃, 6 Al₂O₃, respectively.Therefore, the molten salt method appears to be a feasible possibility in preparing phosphorescent particles with different shapes and sizes using low processing temperature.

The Boron Nitride material is known for its excellent thermal and chemical stability, furthermore, high thermal and low electrical conductivities at high temperature. In addition, the nano boron-nitride (BN) could be an analogy of carbon-carbon but with heteronuclear other than homonuclear characteristics, which can potentially provide greater inter donor-acceptor functionality. Indeed, BN nano-tubules and nano-sheet have been developed in contrast to CNT and graphene in carbon-carbon family.
As described, the optimal integration of intrinsic material properties of BN with nano size and shape tunability can facilitate many functionality improvements and new intriguing explorations.
In this study, we intend to analyze the products formation of a simple source compound, ammonia borane (H3NBH3) as function of various syntheses routes, including thermal pyrolysis, nano catalyst-assisted (i.e. Ni, Co,Cu), and photon assisted techniques. Through a comparison analysis with respect to yield, types of product, shape and size among these synthesis routes, an optimal processing receipt for a desired nano Boron Nitride materials will be discussed and formulated.

Gold nanoparticles find a score of applications in catalysis, plasmonic biosensing, target-specific drug delivery, nanolithography and ion detection. Molecular Dynamics (MD) simulations provide useful physical insight in understanding phenomena such as sintering and crystal structure changes on atomistic scale and complement experiments especially for very small nanoparticles (smaller than 10 nm). For example, it was recently shown by MD that during aerosol synthesis of TiO₂ nanoparticles surface diffusion rather than grain boundary diffusion dominate their coalescence and growth rates [1]. Here, detailed atomistic MD simulations are used to systematically investigate the sintering up to full coalescence of gold nanoparticles of different size at various temperatures.
The method is validated by obtaining basic material properties like the melting temperature which increases with increasing particle size and gradually approaches the bulk melting point, in excellent agreement with phenomenological models. The characteristic sintering time of pairs of Au particles is determined by tracing their surface area evolution. The crystallinity along with the particle size can affect the final product performance. Here the attainment of crystallinity is theoretically quantified by calculating the deviation from a perfect face cubic centered crystal for each gold atom providing an indication of the system&’s degree of disorder.
Buesser, B., Gröhn, A.J., and Pratsinis S.E. (2011) J. Phys. Chem. C 112, 11030-11035.

Silicon carbide (SiC) thin #64257;lms were deposited on carbon substrate by low-pressure chemical vapor deposition (LPCVD) technique using SiCl₄, CH₄ and H₂ gas precursors at different deposition temperatures (1130#8451;-1330#8451;). The films properties were correlated with the growth conditions, including deposition temperature, deposition partial pressure and flow rate of SiCl₄. The properties of the films such as crystallinity, chemical composition and mechanical properties of the 3C-SiC films were investigated. The experimental XRD results showed that the 3C-SiC films preferred c-axis orientation along the (111) plane. The average crystallite size as determined by X-ray diffraction line broadening ranged from about 30.1 to 39.9 nm, and increased with increasing substrate temperature. The experimental results have shown that the deposition temperature had strong influence on the hardness and deposition rate of SiC coatings. It has been observed that when the deposition temperature was raised from 1130 #8451; to1330#8451;, the hardness increased from 10.7 GPa to 21.2 GPa, and the deposition rate of the SiC coatings measured by SEM-EDS increased from 7.6mu;m/hr to 33.5 mu;m/hr accordingly.

Flame spray pyrolysis (FSP) is an effective way of fabricating nanoparticles with sophisticated properties¹. Enclosing such a reactor is of special interest as this facilitates in-situ coating of particles with SiO₂² and carbon black³ or product particle phase control⁴. Furthermore, it allows control of the ambient air entrainment (AE) mass flow that is drawn into the enclosure by the high velocity dispersion O₂⁵. This AE dilutes the precursor, reduces the temperature within the enclosure and alters stream conditions, consequently it can be used to control the primary particle diameter⁵. Tube enclosing, however, also induces challenges: for example the formation of recirculation zones⁶ inside the enclosing tube may lead to unwanted wide residence time distributions (RTD) and associated broad particle size distribution. Here we want to gain insight into these processes to optimize the tube-enclosed FSP.
Commercial software (ANSYS Fluent 12.1) is complemented with user-defined code⁷ and is subjected to a 2D axisymmetric mesh. Basic fluid dynamics, global chemical reactions and multiphase droplet evaporation are calculated as a function of AE from 0 - 300 l/min⁵. Tracking of an inert tracer gas, mixed to the sheath gas was used to determine the RTD as function of AE and enclosing tube length. The simulations are validated by tube exit temperature and RTD measurements.
Preliminary results show substantial vortex formation at low AE while no vortices and significantly lower tube exit temperatures are seen at higher AE. This correspond with theory for confined jets⁶ and FTIR temperature measurements⁵. The obtained insights will facilitate advanced enclosure designs to prevent vortex formation allowing closer control over particle&’s high temperature residence time that potentially narrows the primary particle size distribution of enclosed FSP.
Bibliography:
1. R. Strobel and S.E. Pratsinis: Flame aerosol synthesis of smart nanostructured materials J. Mater. Chem. 17(45), 4743 (2007).
2. A. Teleki, M.C. Heine, F. Krumeich, M.K. Akhtar and S.E. Pratsinis: In situ coating of flame-made TiO₂ particles with nanothin SiO₂ films Langmuir. 24(21), 12553 (2008).
3. O. Waser, R. Buchel, A. Hintennach, P. Novak and S.E. Pratsinis: Continuous flame aerosol synthesis of carbon-coated nano-LiFePO₄ for Li-ion batteries J. Aerosol Sci. 42(10), 657 (2011).
4. R. Strobel and S.E. Pratsinis: Direct synthesis of maghemite, magnetite and wustite nanoparticles by flame spray pyrolysis Adv. Powder Technol. 20(2), 190 (2009).
5. O. Waser, A.J. Grohn, M.L. Eggersdorfer and S.E. Pratsinis: Air entrainment during flame aerosol synthesis of nanoparticles Aerosol Sci. Tech. (in preparation).
6. J. Beer and N. Chigier: Combustion Aerodynamics Applied Science Publ. London. (1972).
7. A.J. Grohn, S.E. Pratsinis and K. Wegner: Fluid-particle dynamics during combustion spray aerosol synthesis of ZrO₂Chem. Eng. J. 191, 491 (2012).

Cellulose-based PLA/PBAT polymer blends can potentially be a promising class of biodegradable nanocomposites. We show here that this blend can be rendered thermal resistant while maintaining its impact resistance by the formation of an unusually hard surface shell when exposed to flame or high temperature front. In this study, we show that resorcinol diphenyl phosphates (RDP) is easily absorbed by cellulose particles, modifying their surface energy and allowing them to disperse easily in a PLA/PBAT(commercially named Ecoflex) biodegradable polymer blend. Mechanical testing shows that the IZOD impact is decreased by only 15% at a cellulose concentration of 10% since the crack propagation was inhibited by the structure of cellulose. When exposed to flame a hard shell forms on the surface, which, when analyzed with Fourier Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopy, points out that the surface is enhanced in PLA and it is rich in phosphate moieties. The Nanomechanical measurements indicate that the modulus of this shell is nearly 100x higher than that of the unexposed nanocomposites. Subjecting these samples to the UL-94 test indicates that they satisfy the stringent UL94-V0 condition and rapidly self-extinguish without dripping. We utilized Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS) to examine the surface segregation of different components as a function of flame exposure time in order to understand the chemistry of the shell layer. The material properties of this blend can be varied such that it can replace conventional polymers such as PS, PE to explore numerous applications for packaging, disposable cutlery, drug delivery etc. and it is a demonstration that flame retardant, high impact and environmentally responsible materials can be engineered.

There has been a considerable interest in the synthesis of iron oxide nanoparticles , especially magnetite and maghemite, due to their biocampatability and useful properties in a number of application areas, e.g. in hyperthermia for cancer treatment . Although the particles can be synthesized through a variety of techniques, spark-discharge synthesis, a vapour phase synthesis method is particularly suitable as it allows a clean synthesis environment and reproducible particle size and distribution. In the present work, iron oxide nanoparticles were produced in a spark discharge with the use of iron electrodes under argon flow. The distance between the electrodes are adjusted automatically such that spark occurs at a constant energy and the same breakdown voltage ensuring uniformity in the particle size. Aerosol generated by spark-discharge is mixed with an auxiliary gas; hydrogen or oxygen, and fed to a tube furnace and then to a filter to collect the powders. The purpose of the tube furnace is two-fold. One is, via annealing at elevated temperature, to coarsen particle size and the other with the use of auxiliary gas to oxidize or reduce the resulting particles so as to control the particle chemistry. Initial experiments have yielded particles which had 3.3 nm in size on average and when passed through the furnace at 575^oC have coarsened to 4.2 nm. Particles as characterized by VSM had superparamagnetic behavior. Currently parametric study is underway to obtain magnetite particles with sizes in the range 10-20 nm superparamagnetic with as high a saturation magnetization as possible.

Materials used for catalysis, separation and energy conversion technologies, as well as several raw materials used in traditional metallurgical industry, have particular requirements when it comes to particle morphology such as size, shape and porosity, as well as mechanical strength and flow properties. Nanostructured materials are gaining widespread use, which requires new approaches to powder synthesis, in particular with respect to increased production while maintaining proper HSE procedures (to avoid exposure and potential health risk). Flame spray pyrolysis (FSP) is an excellent tool for pioneering development of complex novel nanomaterials for various applications and at the same time being a scalable process already implemented by commercial powder producers.
Electrode materials for various energy applications e.g. PEM fuel cells and electrolyzers or super capacitors have strong requirements with respect to their properties, and require powders with high conductivity, high surface area, well defined and sustainable pore structure/size distribution, stability and corrosion resistance. FSP is applied to investigate properties of tin-based and titanium-based oxide powders used as cathode catalyst support materials for PEM fuel cells and PEM electrolyzer, respectively. Single phase powders without any additional heat treatment and significantly higher surface area (from BET analysis) is demonstrated for the FSP powders (100-120 m²/g) as compared to commercially provided powders and powders synthesized through alternative routes (30-50 m²/g). The pore size distribution in the FSP powders, with a significant amount of micropores, is not ideal for efficient gas flow in the catalyst layers. Current work on the catalyst support powders focuses on optimization of the FSP procedure, e.g. changes of precursor/solvent system and flow rates, in order to tailor pore size distribution and enhance the electronic conductivity. For super capacitors electrode materials based on manganese oxides (for red-ox super capacitors) are prepared by FSP, again demonstrating high surface area. In addition to tailoring the oxide powder properties by varying the precursor/solvent system, an interesting approach for the super capacitors is to coat/impregnate carbon fibers/foams with the manganese oxide or synthesize hybrid composite electrodes, which is also attempted using FSP.
The present work will give an overview of synthesis procedures, and thorough characterization of microstructure and electrochemical properties, of selected material systems for the mentioned energy applications - all based on powders produced by FSP.
The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n° 325327 - SMARTCat and n° 303484 - NOVEL, as well as from the NANO2021 program of the Norwegian Research Council, grant n° 233019/O70.

Acetone is one of the many volatile organic compounds (VOCs), “signaling gases”, that exist in our breath and monitoring of its concentration may lead to the control of disease or metabolic malfunctions. Thus, acetone is a breath biomarker. Our group was the first to develop the only selective acetone biomarker detectors, based on the ferroelectric polymorph of WO₃. The material though was doped WO₃ and this had potential implications to the long term stability of the nanosensor breathalyzer. In this study the rapid solidification process of Flame spray pyrolysis (FSP) has been modified so as to produce pure, undoped, ε-WO₃ nanoparticles that are fully crystalline which maintain phase stability at high temperatures.
References:
1. P.I. Gouma, A. K. Prasad, and K.K. Iyer, “Selective Nanoprobes for `Signaling Gases`”, Nanotechnology, 17, pp. S48-S53, 2006.
2. Lisheng Wang, “Tailored Synthesis and Characterization of Selective Metabolite-detecting anoprobes for Handheld Breath Analysis”, Ph.D. thesis, SUNY Stony Brook, Dec 2008
3. P. Gouma, Nanomaterials for Chemical Sensors and Biotechnology, Pan Stanford Publishing, 2009.
4. P. Gouma, “Revolutionizing Personalized Medicine with Nanosensor Technology”, Personalized Medicine, 8(1), pp. 16-16, 2011
5. P. Gouma, “Nanoceramic Sensors for Medical Applications”, Am. Ceram. Soc. Bulletin, 91 (7), pp. 26-31, September 2012.

The structural optimization of electrocatalytic nanomaterials plays a vital role in achieving the performance/cost goal set for commercialization of fuel cells. It is essential to study the relation between morphology of electroccatalyst and its catalytic activity in order to effectively reduce the loading. Reactive Spray Deposition Technology (RSDT) offers a convenient solution to catalyst design by allowing independent control of catalyst, support, and ionomer on fly and direct deposition on membrane. It has the capability to tailor the properties of the catalyst by controlling the processing conditions. The size, phase, morphology, and composition of particles can be controlled via processing parameters, including the reactant concentration in the solvent, the heat enthalpy of solvent, the flow rate of precursor solution, and the flow rate of oxidant gases. In our study, RSDT, an one-step flame based deposition method in ambient environment, is used to synthesize platinum nanoparticles. Pt is often selected due to its high activity for electrochemical reactions (e.g., oxygen reduction and evolution reactions). The flow rate of oxidant has a significant influence on the rate of combustion, length of the flame, and the particles residence time in the hot zone of the flame. A higher oxidant flow rate produces smaller Pt particle due to a reduced residence time in the high temperature zone, faster reaction rate, and diluted aerosol concentration. A short time of particles staying in the high temperature region can greatly limit their growth. In the meantime, the precursor flow rate also has a strong impact on the morphology of the catalyst. When the liquid feed is increased, the enthalpy of the flame is also increased, which leads to a higher flame temperature for particle sintering or coalescence, and results in formation of larger particles. A higher solution feed rate leads to a longer flame, which corresponds to a longer residence time of the particles. Rapid quenching of particles after nucleation by air stops particle growth and provides a narrow particle size distribution. The crystallinity of synthesized Pt nanoparticles is confirmed by X-ray diffraction. Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) are used to investigate the shape, size, and surface structure of the obtained Pt nanoparticles.

Tungsten-oxide nanowires are synthesized directly on the surface of tungsten substrates via a catalyst-free flame synthesis technique at various gas phase temperature, i.e. 1280 K, 1500 K and 1720 K. Two types of quasi-1D counter-flow diffusion flame (methane flame—50% N₂/50% CH₄ and hydrogen flame—63.5% N₂/36.5% H₂) with identical temperature profiles have been applied to investigate the temperature, O₂, H₂O and CO₂ oxidizing routes on the growth of tungsten-oxide nanostructures. Specially, the H₂O oxidizing route is found to play a very important role at the hydrocarbon-rich fuel-side of both CH₄ flame and H₂ at elevated temperature due to the formation of volatile compound of WO₂(OH)₂. Single-crystalline, well-vertically-aligned and dense WO_2.9 nanowires (diameters of 20-50 nm, lengths of >10 µm and growth direction of [110]) are obtained at 1720 K from the air-side of CH₄ flame, where the CO₂ route is presumed to seed the growth of vertically-aligned nanowires at the nucleation stage.

The fabrication of functional thin films and devices by direct deposition of nanoparticles from the gas phase is a promising approach enabling, for instance, the integration of complex analytical and sensing capabilities on microfabricated platforms [1]. Aerosol-based techniques ensure large-scale nanoparticle production and they are potentially suited for this goal. However, they are not adequate in terms of fine control over the lateral resolution of the coatings, mild processing conditions (avoiding high temperature and aggressive chemicals), low contamination and compatibility with microfabrication processes. Here we present and discuss the high-rate and efficient production of functional nanostructured films by nanoparticle assembling obtained by the combination of flame spray pyrolysis and supersonic expansion [2]. Our approach merges the advantages of flame spray pyrolysis for bulk nanopowders such as process stability and wide material library availability with those of supersonic cluster beam deposition in terms of lateral resolution and of direct integration of nanomaterials on devices [3]. We efficiently produced nanostructured films and devices (such as gas sensors) using metal oxide, pure noble metal and oxide-supported noble metal nanoparticles. Photoactive membranes coated with TiO₂ and Pt/TiO₂ nanostructured thin films were also produced and tested in hydrogen production by photo-steam reforming of ethanol [4].
[1] E. Barborini et al., J. Micromech. Microeng. 18, 055015 (2008)
[2] K. Wegner et al., Nanotechnology 23, 185603 (2012)
[3] E Tolstosheeva et al., J. Micromech. Microeng. 24, 015001 (2014)
[4] F. Della Foglia et al., Int. J. Hydrog. Ener., in press (2014)

Liquid Flame Spray (LFS) method utilizes hydrogen-oxygen flame to synthesize nanoparticles (Mäkelä et al., 2011; Aromaa et al., 2012). Liquid precursor is sprayed into a turbulent, high-velocity flame, where the droplets evaporate. The vapor decomposes and the reaction product re-condenses into product species. This process generates well-defined nanoparticles which can be sprayed on a surface or collected as a powder. With moderately high production rate (~ g/min), the method is applicable for thin aerosol coating of large area surfaces. LFS-made nanocoating can be synthesized e.g. on paper, board or polymer film in roll-to-roll process. The degree of particle agglomeration is governed by both physicochemical properties of the particle material and residence time in aerosol phase prior to deposition. By adjusting the speed of the substrate, even heat sensitive materials can be coated. In this study, nanoparticles were deposited directly on a moving paperboard with line speeds 50-300 m/min.
Functional properties of the nanocoating can be varied by changing nanoparticle material; e.g. TiO₂ and SiO₂ are used for changing the surface wetting properties (Teisala et al., 2013). Precursors are usually organic liquids or inorganic metal salts dissolved in water, xylene or isopropanol. If the precursors are dissolved in one solution, synthesis of multi-component nanoparticle coatings is possible in a one phase process (Haapanen et al., 2014).
We present an overall analysis of the properties of LFS-fabricated nanocoatings on paperboard. Deposited superhydrophobic and superhydrophilic nanocoatings have been analysed by water contact angle (WCA) measurement, SEM, TEM, XPS, and TOF-SIMS. Wear resistance of the LFS-made nanocoating has been studied with Taber wheel -type instrument (Stepien et al., 2013a) and compressibility of the thin nanoparticle film using a traditional calendering technique (Stepien et al., 2013b).
References
Aromaa et al. (2012) J. Aerosol Sci. 52, pp. 57-68
Haapanen et al. (2014) Materials Chemistry and Physics, submitted
Mäkelä et al. (2011) Aerosol. Sci. Technol. 45, pp. 817-827
Stepien et al. (2013a) Wear 307 pp. 112-118
Stepien et al. (2013b) Nanoscale Research Letters 8 (1) , pp. 1-6
Teisala et al. (2013) Cellulose 20, pp. 391-408

Nanostructured mesoporous metal-oxide films can be used in various applications, including dye-sensitized solar cells based on titania. Optimization of the properties of these films is crucial in improving their efficiency. Nanostructured TiO₂ films with high uniformity and porosity are grown in a stagnation swirl flame setup under an applied electric field. The effects of external electric-field magnitude and polarity are studied for different substrate temperatures and precursor loading concentrations. The results show considerable differences in film characteristics, for differing electric fields, with more columnar structures and higher porosities under low electric fields up to (-/+) 400 V/m. The films have higher packing density at higher electric fields of (-/+) 800 V/m. At low substrate temperatures, the morphology and structure are more prominent owing to less on-substrate sintering of the nanoparticles. At low voltages, oppositely-charged particles will be attracted to the substrate increasing the electrophoretic velocity but decreasing the in-flame agglomeration; while at high voltages, the particles will be repelled and stay in the flame longer, thus increasing the in-flame agglomeration. A simple model is proposed which predicts the trend for deposition of particles and formation of nanostructured TiO₂ films of a given morphology by balancing the effects of thermophoresis, electrophoresis, and Brownian motion of the particles. The model&’s trend for packing density agrees with the experiments.

Metal oxide semiconductors such as n-type WO₃ has remarkable sensing properties for analytes including NO_x, NH₃, acetone, ethanol, H_2, and O₃. However, sensing function of a particular analyte is highly dependent on the particle size, adhesiveness, porosity, phase, and the structure of the WO₃ film.
In this work, Reactive Spray Deposition Technology (RSDT) has been explored as a single step flame based open atmosphere direct deposition process for synthesizing 6-15 nm of SiO₂ doped WO₃ particles of metastable ε phase directly on Pt or Au interdigitated electrodes. By using RSDT, the synthesis of ε-WO₃ was made possible by 1. Increasing the length of the reaction zone in which the particles of WO₃ are formed by enclosing the flame in a quartz shroud, 2.Rapidly quenching the particles from 700 #730;C to 30 #730;C in 7 s, by a ring of cold air flowing at 1-10 L/min. 3. Doping WO₃ with 5-10 mol% amorphous SiO₂ to prevent the formation of stable γ-WO₃.
Quenching the particles as soon as they are formed serves two roles. First, it assists in the formation of acentric ε-WO₃ by freezing the WO₃ lattice in its thermodynamically unstable state by rapid reduction of temperature from 700 #730;C to 30 #730;C in 7 s. Second, it prevents sintering of the particles. The role of the length of the reaction zone will be discussed .The length of the reaction zone is chosen such that no sintering is observed.
The role of amorphous SiO₂ in stabilizing the ε phase at temperatures of 400-500 #730;C will be described in detail. The position of SiO₂ with respect to WO₃ was investigated using High resolution Transmission Electron Microscopy (HRTEM) Energy Dispersive Spectroscopy (EDS) maps. The microstructural properties of the SiO₂ doped WO₃ was determined by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area, and Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS).
Authors would like to acknowledge support from the University of Connecticut School of Engineering for funding this work.

α-MoO₃ has been reported as a selective ammonia sensor[1-3]. The sensitivity of the sensors is dependent on the processing of MoO₃[1] and the surface: volume ratio of the MoO₃ particles[3]. In this study, we have synthesized α-MoO₃ through flame spray pyrolysis. The MoO₃ particles obtained were uniform nanospheres. Sensors were prepared through drop coating the powders onto alumina substrates coated with platinum electrodes. Sensing tests were conducted for the sensors with ammonia in ppm and ppb ranges. The sensors showed higher sensitivity than MoO₃ sensors reported earlier with the resolution of 500ppb.
[1] D. J. K. A.K. Prasada, P.I. Goumaa, "Comparison of sol-gel and ion beam deposited MoO3 thin film gas sensors for selective ammonia detection," Sensors and Actuators B: Chemical, vol. 93, pp. 25-30, 2003.
[2] P. I. G. A.K. Prasada, D.J. Kubinskib, J.H. Visserb, R.E. Soltisb, P.J. Schmitzb, "Reactively sputtered MoO3 films for ammonia sensing," Thin Solid Films, vol. 436, pp. 46-51, 2003.
[3] K. K. a. A. B. P. Gouma, "Electrospun single-crystal MoO3 nanowires for biochemistry sensing probes," Journal of Materials Research, vol. 21, pp. 2904-2910, 2006.

Silver nanoparticles have been the focus in a multitude of research investigations such as electronics¹, sensors² and antibacterial materials³ because of their high electrical conductivity¹, plasmonic light interaction² and Ag⁺ ion release³. Many times such applications require the use of films in order, for example, to integrate in miniaturized devices.⁴ Correspondingly Ag nanoparticle have in the past been attached to surfaces by drop-coating², in-situ nucleation and growth⁵ or lithography⁶ to name a few.
Flame-spray pyrolysis (FSP) enables the flexible and controllable synthesis of nanoparticles, which can be easily deposited onto substrates during particle production.⁷ The aerosol direct deposition of flame-made nanosilver is the focus of this research. Silver particle size, phase and film morphology are of key interest. These were altered by varying process parameters such as deposition height, duration and temperature as well as substrate material. Silver nanoparticles on the substrate increased in crystal size throughout the deposition duration. Depending on the particle mobility on the substrate, a tendency either towards sinter-neck formation or full coalescence could be observed. This resulted in substrate dependent growth. The widely tunable properties of such Ag nanoparticle films open doors for facile, rapid and application specific fabrication. Electronics, sensors and polymer nanocomposites are anticipated to be promising applications.
1. Magdassi S, Grouchko M, Berezin O, Kamyshny A. Triggering the sintering of silver nanoparticles at room temperature. ACS Nano. 4(4), 1943-1948. (2010)
2. Sotiriou GA, Sannomiya T, Teleki A, Krumeich F, Voros J, Pratsinis SE. Non-toxic dry-coated nanosilver for plasmonic biosensors. Adv. Funct. Mater. 20(24), 4250-4257. (2010)
3. Sotiriou GA, Pratsinis SE. Antibacterial activity of nanosilver ions and particles. Environ. Sci. Technol. 44(14), 5649-5654. (2010)
4. Marelli M, Divitini G, Collini C, et al. Flexible and biocompatible microelectrode arrays fabricated by supersonic cluster beam deposition on SU-8. J. Micromech. Microeng. 21(4), (2011)
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Compliant electrodes are fundamental ingredients for the fabrication of devices and systems based on Electromechanically Active Polymers (EAPs), such as smart actuators and artificial muscles. The integration of EAPs on micro-devices requires the fabrication of compliant electrodes on polymeric