Combustion processes are advantageous for manufacture of nanomaterials as they do not involve the tedious steps of wet chemistry, have no moving parts and can scaleably produce particles (and films) of high purity (e.g. for optical fibers), unique filamentary morphology and metastable phase composition that can be readily separated from their carrier gases. From a fundamental point of view, transport phenomena (e.g. diffusion) in gases afford more rigorous treatment than in liquids or solids, facilitating process design from first principles[1]. The science focus of this field goes well beyond that of classic combustion science and engineering centered on energy generation by fuel combustion & environmental compliance. There flame-made particles are viewed as nuisance (soot) and the focus is on understanding the onset and minimization of their production. In contrast, in combustion synthesis of materials this is the start as maximization of production is sought! Here, how to optimally control “soot” characteristics (e.g. primary and mobility diameters, phase composition as well as extent of aggregation) is the focus as they largely determine material and eventually product and device performance[2]. Here design and flame spray synthesis of functional materials is presented focusing on a) silica-coated Janus-like, plasmonic Ag, luminescent (green-to-red) Tb/Eu/Y2O3 and Fe2O3 nanoparticles along with their incorporation into multilayer nanocomposites resulting in thin superparamagnetic cantilevers, and b) portable, highly selective sensors consisting of flame-made, metastable epsilon-WO3 for monitoring breath acetone, a tracer for diabetes type-1, along with their early testing with humans. If time permits, multiscale modeling (from molecular dynamics to mesoscale or discrete element modeling and eventually to continuum fluid-particle dynamics)[3] of combustion aerosol processes would be highlighted focusing on extracting, for the first time to my knowledge, the primary particle diameter from a newly developed power law[4] & nearly in-situ mass-mobility measurements of flame-made ZrO2 rather than relying on ex-situ tedious microscopy counting or nitrogen adsorption measurements. 1. Aerosol-based Technologies in Nanoscale Manufacturing: from Functional Materials to Devices through Core Chemical Engineering, AIChE J., 56, 3028-3035 (2010) 2. History of the Manufacture of Fine Particles in High-Temperature Aerosol Reactors, in "Aerosol Science and Technology: History and Reviews", ed. D.S. Ensor & K.N. Lohr, RTI Press, Ch. 18, pp.475-507, 2011. 3. B Buesser, SE Pratsinis, Design of Nanomaterial Synthesis by Aerosol Processes, Annual Rev. Chem. Biomol. Eng., 3: 103-27 (2012). 4. ML Eggersdorfer, D Kadau, HJ Herrmann, SE Pratsinis, Aggregate morphology evolution by sintering: Number and diameter of primary particles, J. Aerosol Sci, 46: 7-19 (2012).

Combustion synthesis methods offer control of important nanoparticle characteristics such as the particle size, size distribution, phase and composition. Many high-value-per-gram applications can benefit from studies that quantify the effects of morphology and composition on device performance. Such efforts can include optimization for performance, as well as understanding and developing theory to understand underlying physis and chemistry important in the materials systems. Synthesis of nanoparticles with targeted features are also well suited for developing, validating and scaling models which span atomistic to micron scale dimensions. In this presentation, several metal oxide systems that have been the focus of recent studies at the University of Michigan will be discussed, including mixed metal tin oxides, cerium oxides, and lithium and manganese oxides. The applications for the nanopowders include solid state gas sensing, fuel additives, health effects, energy storage and catalysts.

Flame synthesis of nanoscale aerosols has produced materials that are essential for studies in environmental health effects and for biomedical applications. Several examples of these applications will be given. An inverse diffusion flame has been used to generate nano particle aerosols of iron oxide with variable oxidation state. These materials have been examined for the potential to adsorb arsenic from water. It was found that the optimum iron oxide offered significantly greater adsorption capacity then the best available commercial adsorbent nano material. Lanthanide-doped nanoparticle oxides have been used to study the translocation and clearance of ultrafine particles from an animal air way. Europium was doped into gadolinium oxide that was generated by flame synthesis. The aerosol was instilled into the airway of a mouse. The fate of the particles was determined by very sensitive ICP-MS. Most particles were found to remain in the lung after 24 hours, a significant amount was found in the gastrointestinal tract, but also measurable amounts were found in the peripheral organs such as the heart, liver, and spleen. A consistent ratio of europium to gadolinium indicated that the dissolution of the particles was unlikely and that transport of intact nano particles was probable. The absence of europium or gadolinium in the bloodstream confirmed this hypothesis. The aerosol generation method will be applied to a full-scale inhalation study that removes uncertainties with regard to the instillation method.

Upconversion nanophosphors are emerging as a new group of luminescent materials in imaging, photodynamic therapy, and energy conversion. We will present the recent progress of synthesis and photodynamic studies of lanthanide ion doped upconversion nanophosphors by using combustion and in-solution thermolysis methods. First, the chemistry and kinetics of hexagonal phase nanophosphor synthesis will be discussed. The strong dependence of phase transition on synthesis temperature and precursor concentration is analyzed. It is shown that the diffusion controlled limit leads to the formation of monodispersed hexagonal phase nanophosphors. The photophysical properties of nanophosphors are studied with near infrared excitation. The correlations between the nanoparticle fluorescence intensity and nonoparticle size, crystal structure, and surface to volume ratio are examined. The results show that there is a linear dependence between the dynamic photoluminescence timescale and the surface to volume ratio and that the rapid luminescence decrease with the decrease of particle size is caused by surface quenching. Finally, applications of upconverting nano-phosphors in photodynamic therapy, nanoprinting, and simultaneous velocity and temperature measurement will be demonstrated.

Flame Spray Pyrolysis (FSP) is a well investigated high temperature aerosol technique for synthesizing nano-sized metal oxide catalysts of almost all metals and transition metals by gas-to-particle conversion [1]. However, specific designs of multi-component catalysts and the controlled dispersion of catalytic material on the support are limited in this one- step approach. Double Flame Spray Pyrolysis (DFSP) offers the potential to overcome this limitation by burning individual metal oxide precursors in two opposing nozzles and intersecting the flames at a defined temperature range [2]. The temperature profile of the intersecting flames is the key parameter for defining the final material composition and is of major interest in research. When the flames intersect at small distances where both precursors are still in vapor phase, a mixture of oxides on the atomic scale and formation of solid solutions are likely comparable to the conventional FSP approach. An increase in the flame intersection distance leads to the formation of individual oxide particles, which are well intermixed. If temperatures at the intersection point are high enough for partial sintering, heterojunctions or surface coated particles are synthesized. Additionally properties such as particle size, shape, and specific surface area can be tuned for each compound individually by adjusting the flames separately. The produced particles are characterized and the influence of the intersection distance on particle configuration is investigated using UV-Vis spectroscopy, XRD, TEM and dispersion measurements. Computational fluid dynamics simulations of two intersecting flames complement the experimental data and give insights into the temperature profiles of the process. The presentation will highlight current examples such as the synthesis of TiO2//SnO2 heterojunction used in photocatalytic water splitting, Co//Al2O3 catalysts for Fischer-Tropsch synthesis, Pd//Si-Al2O3 catalysts for selective hydrogenation, Co//Mo-Al2O3 as hydrotreating catalysts and Pd//SnO2 layers for CO gas sensing. [1] W.Y. Teoh, R. Amal, L. Mädler, Nanoscale, 2 (2010) 1324-1347. [2] R. Strobel, L. Mädler, M. Piacentini, M. Maciejewski, A. Baiker, S.E. Pratsinis, Chemistry of Materials, 18 (2006) 2532-2537.

More than 100 million tonnes of municipal solid waste are incinerated worldwide every year. However, little is known about the fate of nanomaterials during incineration, even though the presence of engineered nanoparticles in waste is expected to grow. In this study we show that flame-made cerium oxide nanoparticles introduced into a full-scale waste incineration plant bind loosely to solid residues from the combustion process and can be efficiently removed using current filter technology. The nanoparticles were introduced either directly onto the waste before incineration or into the gas stream exiting the furnace of an incinerator that processes 200,000 tonnes of waste per year [1]. Nanoparticles that attached to the surface of the solid residues did not become a fixed part of the residues and did not show any physical or chemical changes. Our observations show that while it is possible to incinerate waste without releasing nanoparticles into the atmosphere, the residues to which they bind eventually end up in landfills, which confirms that there is a clear environmental need to develop degradable nanoparticles [2].

Engineering less toxic nanomaterials that maintain valuable functional properties is crucial to the sustainability of the nanotech industry. Here, we present a safer formulation concept for flame-generated nanomaterials based on the encapsulation of potentially toxic nanomaterials by a biologically inert nanothin amorphous SiO2 layer. The core-shell particles maintain specific properties of their core material but exhibit surface properties of their SiO2 shell. The SiO2-coating process was performed using a modified flame spray pyrolysis (FSP)-based Versatile Engineered Nanomaterial Generation System (VENGES) in which core ENMs are coated in-flight by the swirl injection of hexamethyldisoloxane (HMDSO). We first demonstrate the versatility of the proposed SiO2-coating process by applying it to several ENMs (CeO2, Fe2O3, ZnO, Ag) marked by their prevalence in consumer products as well as their range in toxicity. We then investigate (1) the effect of the SiO2-coating on core material structure, composition and morphology (XRD, BET, and TEM), (2) the mobility and aggregation of SiO2-coated and uncoated ENMs in DI-water, biological media (DLS) and air (SMPS), and (3) the coating efficiency (XPS, Chemisorption). Finally, we provide valuable toxicological evidence for the safety of this novel formulation concept by evaluating the relative toxicity of SiO2-coated vs. uncoated ENMs. A number of in-vitro cellular assays (MTT, LDH, Live/Dead) and several cell-lines (A549 cancer alveolar epithelial cells and THP-1, macrophages) as well as an animal model were used to assess toxicological outcomes. Our results show that the proposed method can be used to effectively coat flame generated ENMs with a nanothin layer of amorphous SiO2 thereby resulting in a significant reduction of their toxicological profile. Moreover, the proposed method can readily be scaled up and used by NT industry in the production of safer ENMs.

Flame synthesis of nanoparticles has made a tremendous progress in the past decade of intense research and a plethora of new nanomaterials has been synthesized at the laboratory scale [1]. Today&’s challenge is the translation of these achievements into an industrial production environment which poses questions on continuous nanoparticle manufacture, safe handling and packaging [2]. Here, a fully automated pilot plant for nanoparticle manufacture by flame spray pyrolysis [3, 4] is presented that allows continuous production of nanopowders at rates up to 2000 g/h. By the example of zirconia, scale-up from the 20 g/h laboratory reactor is investigated with the help of computational modeling and process operation diagrams, relating synthesis parameters with product nanoparticle properties. It is shown how the increase of the primary particle diameter with precursor feed can be opposed by simultaneous increase of the dispersion gas flow yielding guidelines for further process scale-up. Release of nanoparticles into the workspace during production and packaging is monitored with a condensation particle counter at different locations in the pilot plant in order to assure operator safety and optimize equipment and plant design. Process economics are briefly discussed, showing that economic manufacture even of complex multi-component nanoparticles by flame spray pyrolysis is in reach. [1] Teoh, W.Y., Amal, R. and Mädler, L. (2010), Nanoscale 2, 1324. [2] Wegner, K., Schimmoeller, B., Thiebaut, B., Fernandez, C., Rao, T.N. (2011), KONA Powder and Particle 29, 251. [3] Bickmore, C.R., Waldner, K.F., Treadwell, D.R. and Laine, R.M. (1996), J. Am. Ceram. Soc. 79, 1419. [4] Mädler, L., Kammler, H.K., Mueller, R. and Pratsinis, S.E. (2002, J. Aerosol Sci. 33, 369.

Hybrid metal oxide nanowires (NWs), with small characteristic diameter and large aspect ratio, can have tunable chemical, optical and electrical properties by assigning different functionalities to the individual components, and independently optimizing the chemical compositions and morphologies of them. Such hybrid NWs are promising building blocks in many applications, such as catalysis, sensors, batteries, solar cells and photoelectrochemical (PEC) devices. However, these applications are hindered by the lack of scalable and economic methods for the synthesis of hybrid NWs. Herein, we report a simple, scalable and new sol-flame method to synthesize various hybrid metal oxide NWs. The sol-flame method uniquely combines the merits of the flame process (e.g., high temperature and fast heating rate) with low temperature sol-gel method (e.g., broad material choices and excellent chemical composition control). Firstly, the existing NWs are coated with NPs or dopants precursors prepared by the sol-gel process, and then these precursors are dissociated/oxidized in flame. By varying the NWs, precursors and flame treatment conditions, various hybrid metal oxides were successfully synthesized, including nanoparticle-shell decorated NWs (NP-shell@NW), NP-chain decorated NW (NP-chain@NW) and doped NWs. For both the NP-shell@NW and NP-chain@NW cases, the high temperature flame, compared to furnace, provides much faster heating rate and shorter duration for annealing, which evaporates and burns the precursor solvent rapidly, causing NPs to quickly nucleate around NWs without significant agglomeration. Hence, higher loading density of NPs with smaller sizes is decorated to the NWs, and the formed hybrid NP@NW exhibits significantly higher catalytic activity than that of the furnace-annealed sample. Similarly, for doped NWs, the high temperature flame enables rapid dopant diffusion and short annealing duration that maintains the morphology of the original materials and protects the delicate NW substrates from damage. Given the generality and versatility of the sol-flame method, we believe that the new sol-flame method can be applied to synthesize various 1-D hybrid metal oxide nanostructures, thereby impacting diverse application fields.

Because of its natural high temperature and oxidizing environment, combustion synthesis offers many advantages for the mass-production of 1-D metal oxide nanomaterials, such as atmospheric operation, rapid growth rate, low cost, scalability, broad choices of growth substrate materials and morphologies, and fine control over the morphology and composition of the synthesized products. Here, we present versatile combustion synthesis methods for the growth of various 1-D simple binary and complex metal oxide nanomaterials. For example, single, branched, and flower-like α-MoO3 nanobelt arrays were grown on diverse substrates using flame vapor deposition growth, where the flame oxidizes Mo mesh and evaporates MoOx vapors that further condense onto colder growth substrates in the form of α-MoO3 nanobelts. The growth rate, morphology, and surface coverage density of the α-MoO3 nanobelts were controlled by varying the flame equivalence ratio, the source temperature, the growth substrate temperature, and the material and morphology of the growth substrate. In addition, we developed three new combined combustion synthesis methods: 1) simultaneous vapor-vapor growth, 2) simultaneous solid-vapor growth, and 3) sequential solid-vapor growth, to grow 1-D complex metal oxide nanostructures with well-defined compositions and morphologies. These three methods combine the previously reported flame vapor deposition and solid diffusion growth methods that were separately used to grow 1-D simple binary metal oxide nanostructures, and significantly advance the capabilities of existing combustion synthesis methods for the growth of 1-D nanomaterials. The first method, simultaneous vapor-vapor growth, combines the flame vapor deposition growth of two different metal oxides by oxidizing and evaporating two different metal sources. With this we have successfully grown W-doped MoO3 nanoplates and nanoflowers. In the second method, simultaneous solid-vapor growth, one precursor is again provided by oxidizing and evaporating metal oxide from a metal, while the other precursor diffuses out from a different growth substrate. With this we have successfully grown ternary Cu3Mo2O9 nanowires. The third method, sequential solid-vapor growth, essentially uses the 1-D nanostructures firstly grown by solid diffusion as the substrates for subsequent flame vapor deposition. With this we have successfully grown hierarchical CuO/MoO3 core/shell nanowires and MoO3-branched CuO nanowires. We believe that these combustion synthesis methods will provide a general platform for the synthesis of 1-D simple binary and complex metal oxide nanostructures with tailored properties.

We report the flame synthesis of various tungsten oxide nanostructures and their application for increasing the performance of solar cell power output. The structures are synthesized on the surface of tungsten probes directly in the gas-phase of an opposed flow flame. A high purity tungsten wire inserted into the oxygen-rich region of the flame was used as a material source. The synthesized structures include of WO3/C organic/inorganic octagonal nanoplatelets/nanorods and inorganic W oxide nanorods and platelets. The ideal conditions for the rapid and direct formation of these novel nanostructures are attributed to the synergy of the strong thermal and chemical gradients present in the flame volume. The entire process takes only a few seconds. Additional experiments show that tungsten oxide structures can influence light scattering and can redirect photon flux, increasing the fraction of light absorbed in the thin active layer of silicon solar cells. This research aims to demonstrate that the application of transition metal-oxide nanostructures to the surface of silicon solar panels can enhance the power output of the solar cells. The results demonstrate an increase in power output of up to 5% with nanorod treatment and up to 2.3% with nanoplatelet treatment. A proposed mechanism of the hybridization process of the WO3 nanorods and nanoplatelets to WO3/C is described.

Plasmonic nanoparticles play an important role in biomedical applications today as they can serve as superior optically-stable bioimaging agents, be employed in biosensor devices for the early diagnosis of diseases, and exhibit promising results for their employment in vivo as therapeutic agents. For several bioapplications, however, nanoparticles that express more than one functionality are often advantageous. This has led to the synthesis of multifunctional plasmonic nanoparticles that combine the attractive plasmonic properties with other functionalities like magnetism, photoluminescence, dispersibility in aqueous solutions and resistance to degradation. Here, biocompatible, SiO₂-coated, Janus-like multifunctional plasmonic nanoparticles are prepared by one-step, scalable flame aerosol technology. A nanothin SiO₂ shell around these multifunctional nanoparticles leaves intact their morphology, magnetic and plasmonic properties but minimizes their toxicity. Furthermore, this silica shell hinders flocculation and allows for easy dispersion of such nanoparticles in aqueous and biological buffer (PBS) solutions without any extra functionalization step. Their performance as bioimaging agensts was explored by selectively binding them with live tagged Raji and HeLa cells enabling their detection under dark-filed illumination. Finally, their potential in photothermal treatment of cancer cells is investigated. [1] Sotiriou GA, Pratsinis SE. Antibacterial Activity of Nanosilver Ions and Particles. Environ Sci Technol 2010, 44: 5649-5654. [2] Sotiriou GA, Pratsinis SE. Engineering Nanosilver as an Antibacterial, Biosensor and Bioimaging Material. Curr Opin Chem Eng 2011, 1: 3-10. [3] Sotiriou GA, Sannomiya T, Teleki A, Krumeich F, Vörös J, Pratsinis SE. Non-Toxic Dry-Coated Nanosilver for Plasmonic Biosensors. Adv Funct Mater 2010, 20: 4250-4257. [4] Sotiriou GA, Hirt AM, Lozach PY, Teleki A, Krumeich F, Pratsinis SE. Hybrid, Silica-Coated, Janus-Like Plasmonic-Magnetic Nanoparticles. Chem Mater 2011, 23: 1985-1992. [5] Sotiriou GA, Franco D, Poulikakos D, Ferrari A. Optically Stable Biocompatible Flame-Made SiO₂-Coated Y₂O₃:Tb³⁺ Nanophosphors for Cell Imaging. ACS Nano 2012, 6: 3888-3897.

Molybdenum oxide nanocrystals were formed in an opposed flow methane oxy-flame. Molybdenum probes placed in a high temperature oxygen-rich zone of the flame served as material sources. Strong temperature and chemical gradients formed in the opposed flow flame provided the ideal conditions for the rapid and direct formation of unique nanostructures directly in the flame volume. Oxidation of molybdenum in the oxygen-rich zone of the flame was followed by its evaporation in the form of MoO3, transport, and reduction to MoO2 in the lower temperature, fuel-rich zone of the flame. The molybdenum dioxide nanomaterials were formed in this part of the flame. Essential morphological variations of generated nanomaterials were observed depending on flame and probe parameters. In particular, the molybdenum probes with diameters of 0.75 mm and 1 mm were used to achieve two distinct synthesis conditions. The variation of probe diameter affected probe temperature and resulted in different supersaturation levels of molybdenum dioxide vapors. Experiments with 1.0 mm diameter corresponded to a lower supersaturation levels and resulted in the synthesis of well-defined convex polyhedron nanocrystals and nanorods. Higher material etching rates and, hence, supersaturation levels were obtained with 0.75 mm diameter probes. These conditions resulted in synthesis of mainly spherical molybdenum oxide nanomaterials agglomerated in soot-like fractal aggregates. The effect of flame parameters on shape and structure of generated nanomaterials is studied numerically. The underlying mechanisms governing the morphological variation of molybdenum oxide nanocrystals are analyzed using the following steps: monomers formation, nucleation, and growth. The model predictions are in a good qualitative agreement with the experimental data.

Synthesis techniques capable of tightly controlling nanoparticle composition and micro-structure are required to develop better understanding of micro- and macro-level physical and chemical properties. In this experiment, a solid phase precursor, cerium(III) acetate is delivered continuously to a hydrogen-oxygen Henken burner at atmospheric pressure to generate ceria nanoparticles in a well-controlled combustion synthesis environment. The impact of the synthesis environment on the nanoparticle composition and morphology is investigated. The synthesis process is controlled by the reactant composition in conjunction with two time scales, the reaction rate and the residence time. Changes in nanoparticle composition are accomplished by controlling flame stoichiometry. Altering the level of nitrogen diluent varies temperature, as does the use of a chimney. Residence time is estimated using the volumetric flow rates of the gas phase reactants. Using these parameters as control variables, cerium powders are generated at ten conditions, where five conditions are repeated with and without the chimney: oxygen lean conditions with minimal diluent, one condition just above the stoichiometric hydrogen to oxygen ratio, three oxygen rich conditions at increasing levels of dilution (reducing temperature and residence time). The nanoparticles products are analyzed in four ways. 1) A scanning mobility particle sizer (TSI model 3080) is used to monitor the size distribution and continuity of the particle production during a 20-minute synthesis period. 2) In-situ sampling of particles onto transmission electron micrograph (TEM) grids and subsequent imaging and diffraction analysis provides composition and crystallite size of the nanoparticles. 3) Bulk powder is collected in the exhaust plume via thermophoresis and analyzed using x-ray diffraction to determine composition and particle size. 4) Bulk powder is also subject to TEM analysis to compare in-situ and bulk particle size and composition. Preliminary results confirm expected trends. Oxygen lean environments lead to mixed oxides of cerium, while increasing temperatures and/or residence times lead to larger nanoparticle crystallites as indicated by average crystallite size determined using XRD spectra. Current work includes TEM imaging studies to determine the relative sensitivities of particle size to temperature and residence time. Early imaging confirms both in-situ and bulk ceria crystallites are face-centered cubics. The results of the characterization and the combustion synthesis procedure are to be incorporated in an ongoing health and toxicity study of nano-ceria particulates.

α-Fe₂O₃ nanowires (NWs), due to their larger surface area and superior charge transport properties compared to bulk α-Fe₂O₃, have important applications in catalysis, photoelectrochemical water splitting, battery electrodes, and other energy conversion applications. In addition, owing to their anisotropic magnetic properties and biocompatibility, they have potential uses in magnetic imaging and drug delivery. However, typical synthesis routes for α-Fe₂O₃ NWs suffer from various serious problems. For instance, the growth of α-Fe₂O₃ NWs during the thermal oxidation of iron in air is slow, and results in the formation of a thick oxide scale with poor electronic conductivity. As another example, α-Fe₂O₃ NWs grown by solution phase precipitation typically have poor crystallinity. Herein, we report the rapid, atmospheric, scalable flame synthesis of dense arrays of single crystal α-Fe₂O₃ NWs on iron substrates. This flame synthesis method achieves α-Fe₂O₃ NW axial growth rates in excess of 1 µm/minute, which is more than an order of magnitude larger than the highest growth rate reported for thermal oxidation synthesis in air using a conventional tube furnace. In this method, α-Fe₂O₃ NWs are synthesized by inserting iron foils and wire substrates into the hot post-flame region of a fuel lean co-flow CH₄-H₂-Air flame, where they are rapidly oxidized to produce iron oxide layers on their surfaces. Simultaneously, iron diffuses from the bulk of the substrates to the surface through the growing oxide layer. When the iron reaches the surface, it accumulates in the form of α-Fe₂O₃ NWs. This diffusion of iron to the surface was found to be the rate limiting process for NW growth. Unlike conventional tube furnaces, in which the mass flow rate is limited by slow peripheral electrical heating, flames naturally and uniquely produce a large mass flow rate of hot oxidizers because of the exothermic, volumetric combustion reaction. As a result, the atmospheric post-flame region has a high flow velocity and a large concentration of oxidizers such as CO₂ and H₂O. The key reasons for the rapid growth rate of the α-Fe₂O₃ NWs in the flame, compared to conventional thermal oxidation, are 1) the rapid heating rate of the iron substrates in the flame because of the high flow velocity, and 2) the presence of O₂, H₂O and CO₂ oxidizers in the post-flame region, instead of just O₂. The rapid heating rate creates a thinner, more defective oxide layer, through with iron diffusion is faster. This increases the flux of iron to the surface and increases the growth rate of the NWs. H₂O allows the growth of the nanowires to occur at elevated temperatures up to 1000 °C, whereas NW growth occurs only up to 700 °C in the absence of H₂O. Since the diffusion of iron is much faster at higher temperatures, the presence of H₂O causes a much faster growth rate of NWs in the flame. CO₂ also contributes by increasing the porosity of the oxide layer, and minimizing its thickness.

Arrays of high aspect ratio WO₃ nanowires (NWs) are theoretically superior to planar WO₃ films for photoelectrochemical (PEC) water-splitting, because they orthogonalize the directions of light absorption (along the long axis) and charge transport (across the short radius), leading to both efficient light absorption and charge carrier collection. However, a typical shortcoming of previously demonstrated WO₃ NWs grown by hydrothermal methods or thermal vapor deposition has been low NW packing density, which allows light to pass through the NWs without being absorbed, and undermines the benefits of the NW geometry. Herein, we have synthesized arrays of densely packed WO₃ NWs on transparent conducting oxide (TCO) substrates by a rapid, atmospheric and scalable flame vapor deposition method, and demonstrated their excellent performance as photoanodes for PEC water-splitting. Briefly, the hot combustion products from a co-flow pre-mixed CH₄-Air flame are used to heat, oxidize and evaporate tungsten filaments, generating tungsten oxide vapors that condense onto a downstream water-cooled substrate in the form of single crystal W₁₈O₄₉ NWs. The NWs are then oxidized to WO₃ by heating in air, and tested for PEC water-splitting. Flames naturally and uniquely produce a large mass flow rate of hot oxidizers because of the exothermic, volumetric combustion reaction. As a result, the atmospheric post-flame region has a high flow velocity and a large concentration of oxidizers such as CO₂ and H₂O, which allow the rapid generation of tungsten oxide vapors by oxidative evaporation. This fast evaporation rate creates a much higher concentration of tungsten oxide vapors at the growth substrate than is possible from thermal evaporation in conventional vacuum furnaces, in which the mass flow rate is limited by slow peripheral electrical heating. Due to the large vapor concentration of tungsten oxide, we observe a dramatic change in the morphology of the condensed product from sparsely packed NWs at high substrate temperatures, to NWs densely packed into single and concentric tubes at low substrate temperatures, whereas only sparsely packed NWs are observed in typical thermal vapor deposition, regardless of substrate temperature. Furthermore, the low substrate temperatures (500-600 °C) allow the use of temperature-sensitive TCO substrates. The high NW packing density and TCO substrates are both particularly important for practical PEC water-splitting devices, in which the high packing density allows strong light absorption, and the transparent substrate allows for tandem operation of the WO₃ photoanode with a photocathode. Thus, this flame synthesis method solves a major challenge for the growth of WO₃ NWs, and enables their application as a PEC water-splitting material.

Advancement in the understanding of state of charge and efficiency requires better coupling of battery level properties with the micro-structure of the constituents. This work supports ongoing experimental and modeling efforts to characterize the macro-level performance of lithium ion batteries given detailed knowledge of the micro-structure properties. In this experiment, one or both solid phase precursors, lithium acetate-hydrate and manganse acetate-hydrate, are aerosolized and delivered to a hydrogen-oxygen Henken burner at atmospheric pressure. The composition of the target synthesis material, lithium manganese oxide (LiMn2O4, or LMO for short) is known to impact lithium ion battery properties. Following this motivation, our aim is to demonstrate control over the microstructure and compositional properties of LMO using parameters of the combustion synthesis environment. The synthesis process is controlled by the characteristic time scales for reaction and flow. Changes in nanoparticle composition are accomplished by controlling reactant concentrations. Flow rates control residence times and synthesis temperatures. First lithium oxide (Li2O) and manganese oxide (MnO2) powders are generated independently from the corresponding acetate precursors to assess particle crystallite sizes in the synthesis environment. Following that, several mixtures of lithium and manganese acetate precursor trials are conducted and the resulting material properties are investigated. Five conditions varying the oxygen concentration in the synthesis environment are studied. The effects of dilution are also considered. The nanoparticle products are analyzed to determine average particle size and size distribution via scanning mobility particle sizer, transmission electron microscopy (TEM), and x-ray diffractrometry (XRD). Particle phase and composition are also evaluated using XRD and TEM. Bulk as-produced materials and in-situ samples are considered and compared. The well-characterized materials will be incorporated in an experimental battery development cycle and the performance of these batteries will be compared.

Flame spray pyrolysis (FSP) is an effective, continuous and scalable route to manufacture high purity metal oxides, metal salts, and even pure metals with great capability to control particle characteristics such as crystal phase and size over a wide range [1]. Additionally, FSP&’s high-temperature nature allows co-formation of highly dispersed acetylene-black enhancing rate and cycle stability of battery materials [2]. Copper oxide nanoparticles as so called conversion reaction battery material offers a high active to passive mass ratio resulting in specific capacities up to 5 times larger than those of commercially used intercalation-based materials. Furthermore, nano-structuring of this conversion reaction active material plays a key role regarding electrochemical performance. Here, cooper oxide nanoparticles of various sizes were made by spraying 0.25 M Cu solution in 2-ethylhexanoic acid and xylene. Controlled increase in particle size was obtained by raising the ratio between precursor to dispersion flow rate, hence increasing the enthalpy density and Cu concentration within the flame. Shielding the flame with a tube and introducing a variable lift-off height (gap between tube and FSP nozzle) allowed a controlled dilution of the aerosol by ambient air entrainment. Controlling this ambient air entrainment offers an additional parameter for size control and extends the obtainable primary particle size towards larger particles without sacrificing product yield or quality. Particle characterization was done by BET, TEM, XRD and electrochemical performance analysis. Phase pure CuO particles were produced within a crystal size ranging from 3 to 40 nm and specific surface area (SSA) from 164 to 19 m^2/g corresponding to BET equivalent diameters of 5 to 50 nm. The impact of CuO particle size on electrochemical performance was investigated in test cells, showing clear benefit in specific capacity of using flame made nanoparticles compared to commercial micron sized particles. [1] R. Strobel, S.E. Pratsinis, Flame aerosol synthesis of smart nanostructured materials, J. Mater. Chem. 17 (2007) 4743-4756. [2] O. Waser, R. Buchel, A. Hintennach, P. Novák, S.E. Pratsinis, Continuous flame aerosol synthesis of carbon-coated nano-LiFePO4 for Li-ion batteries, J. Aerosol. Sci. 42 (2011) 657-667.

Combustion synthesis of nanoparticles allows to generate high purity materials with well-controlled properties in continuous flow situations that provide a chance for scale-up to industrial scale. Nanoparticles with well-controlled composition and narrow size distributions are of interest for a wide variety of applications from coatings to electronics to functional materials, e.g., for energy conversion and storage. For the synthesis of materials with desired properties, however, the reaction conditions must be well controlled and the undernot;lying processes understood. The decomposition kinetics of vaporized metal organic comnot;pounds, the ignition properties of the mixture of these materials with oxidizing environments as well as the reaction mechanisms of the decomposition, cluster formation and the potential interaction with flame chemistry is a prerequisite for a targeted synthesis of materials. Kinetics experiments are carried out in shock tube reactors with optical and mass spectronot;metric detection of intermediate and product species, in flow reactors with laser-based detection of temperature and species concentration as well as in particle-generating systems connected to molecular-beam particle mass spectrometers. At the same time, reaction conditions such as temperature, intermediate species concen-tration and particle size must be determined in situ in lab-scale nanoparticle reactors as well as in pilot-plant-scale reactors to provide input and validation data for numerical simulation. In this presentation these aspects will be introduced based on four systems, iron-penta-carbonyl-based synthesis of iron-oxide particles [1], the decomposition and reaction of Ga-based metal organic compounds [2], the formation of particles with tuned stoichiometry [3, 4] as well as the ignition, reaction and particle formation from silicon-based precursors for the synthesis of silicon dioxide particles [5]. Detailed mechanisms are derived from these measurements that than can be used for the simulation of reactive flows that then also allow to transfer the knowledge gained from small-scale laboratory experiments to the design of production plants that can be used to generate significant amounts of nanomaterials with well-defined properties. References 1. S. Staude; C. Hecht; I. Wlokas; C. Schulz; B. Atakan, Z. Phys. Chem. 223 (2009) 639-649 2. M. Fikri; A. Makeich; G. Rollmann; C. Schulz ; P. Entel, J. Phys. Chem. A 112 (2008) 6330-6337 3. P. Ifeacho; T. Huelser; H. Wiggers; C. Schulz; P. Roth, Proc. Combust. Inst. 31 (2007) 1805-1812 4. A. Gupta; P. Ifeacho; C. Schulz; H. Wiggers, Proc. Combust. Inst. 33 (2011) 1883-1890 10.1016/j.proci.2010.06.162. 5. A. Abdali; B. Moritz; A. Gupta; H. Wiggers; C. Schulz, Journal of Optoelectronics and Advanced Materials 12 (2010) 440-444

Gas-phase synthesis of metal oxide (e.g. TiO2) nanomaterials by stagnation flames (e.g. divergence, burner-stabilized and swirl-stabilized stagnation flames) exhibits multiple advantages including controllable short residence time in high temperature regime and one-step synthesis of nano-films by in-situ deposition. For the coagulation at nanoscale, the interaction between particles becomes crucial. Our molecular dynamics (MD) simulation on charge-neutral TiO2 nanoparticles shows that, besides the conventional van der Waals (vdW) force, the dipolar force that arises from the asymmetrical ions distribution in the particle surface is a significant contribution to the inter-particle forces. The dipole can be as large as 60D for 3nm TiO2 particles at room temperature and decay at higher temperatures due to the thermal fluctuation. Further investigation found that the dipolar force could enhance the Brownian collision rate by 8-10 times since the force enlarges the capture radius, suggesting that nanocrystals may collide much faster due to these attractive forces. MD simulation also shows that the crystalline structures of TiO2 nanoparticles are greatly influenced by the surface curvature and thus size-dependent. For nanoparticles above a certain transition diameter (around 2.1nm for TiO2) e.g. 3nm, the nanoparticle consists of a crystalline core and amorphous shell at the surface (4-6Å). Below the transition diameter e.g. 2nm, the entire particle is amorphous as the surface amorphous layer penetrates the whole particle. Considering that such grain-structure characteristics may lead to different dynamic behaviors, the coalescence between pairs of 2nm-2nm and 3nm-3nm nanoparticles are investigated. For 2nm-2nm nanoparticle coalescence, the process is independent of initial temperature and seemingly viscosity-controlled. For 3nm-3nm nanoparticle coalescence, the process is sensitive to initial temperature. Above the melting temperature, the dynamics are similar to the 2nm-2nm amorphous case. Just below the melting point, coalescence consists of melting of the crystalline cores with subsequent large increase in temperature due to recrystallization. Then, in the view of macroscopic scale at flow-field length, the collision-coalescence determines the size and morphology of primary particles while the deposition controls the properties of the nano-films. Experimental and theoretical analysis shows that, with respect to the nanoparticle transport, the boundary layer of stagnation flames can be divided into three layers by comparing gas phase velocity and thermophoretic velocity: a convection-controlled regime where particle concentration scales with gas density; a transition regime in the middle; and a thermophoresis-controlled regime attached to the wall where particle transport is dominated by thermophoresis and Brownian diffusion.

Nanoparticles of TiO2 have been used in a wide array of applications, including the conductometric sensing of CO. In recent years, our group developed a novel technique to synthesize single-crystal TiO2 nanoparticles with narrow size distributions using a premixed stagnation flame called Flame Stabilized on a Rotating Surface (FSRS). The unburned gas mixture is doped with an organometallic precursor (Titanium tetra-Isopropoxide), impinging on a rotating Titanium disc. Gas sensing sensors were prepared by directly growing nanoporous films of TiO2 on inter-digitated electrodes while the nanoparticle median diameter was controlled at around 9nm. The as-deposited sensors show a CO sensitivity of an order of magnitude higher compared to the commercially used powder with a median particle size of 25nm. Further, a gas-surface model was used to analyze the sensor&’s kinetic and equilibrium behaviors. It is observed that even though the nature of the gas-surface reactions remains unchanged between the two samples, the smaller particle size and therefore larger surface area of the gas sensing films made by FSRS lead to a more electrically sensitive conduit.

Nanocrystalline TiO2 films are materials used in dye sensitized solar cells. Synthesis of TiO2 in flame allows one-step production of a mesoporous film of TiO2 nanocrystals on a substrate. The purpose of this work was to study formation of nanocrystalline TiO2 films from premixed H2/O2/Ar flames doped with Ti(OC3H7)4 and Fe(CO)5 on a cooled substrate, to manufacture samples of solar cells on the basis of films of nanocrystal particles of TiO2, and to study the effect of iron oxide additive in TiO2 nanoparticles on the efficiency of solar cells. Mesoporous nanocrystalline TiO2 films were synthesized in a premixed H2/O2/Ar (12.9/14.4/72.7 vol %) flame doped with 0.1 vol. % of Ti(OC3H7)4 and also of Fe(CO)5. The content of Fe additive (mTi/mFe) in the film specimens obtained was 0, 0.002, 0.005, 0.01 and 0.05. The specimens of films of nanocrystalline TiO2 particles obtained were studied by high-resolution scanning electron microscopy and X-ray diffraction. Deposited on a glass substrate with a transparent conducting layer (FTO TEC15 glass), the films of nanoparticles of TiO2 were used to manufacture samples of solar cells. It was shown that in the H2/O2/Ar flame, TiO2 in the anatase modification with a characteristic particle size 5-35 nm and a lognormal distribution 1.45 wide was formed, with the film growth rate being 1.4 mu;m/s. The proposed method of synthesis of nanocrystal particles of TiO2 and deposition of mesoporous films of TiO2 on a cooled substrate from H2/O2/Ar flame allows TiO2 to be doped with additives containing Fe and elements. The materials obtained by this method can be used for the manufacture of solar cells and in photocatalytic reactors. Acknowledgements This study was supported by the Russian Foundation of Basic Research (Grant No. 10-03-00442).

A sectional method for determining particle size distributions has been implemented within the particle tracking module included with CHEMKIN-PRO [1] . The module is available for use with many types of reactor models, ranging from 0-D batch reactors to laminar flame simulations. Coupled with the Burner-stabilized Stagnation Flame (BSSF) Model, the sectional model offers a high-fidelity, robust, and efficient computational framework for simulating flame synthesis of particles in a laminar, premixed stagnation flame environment. The CHEMKIN-PRO coupling allows inclusion of detailed gas-phase chemistry that determines key particle-formation precursors, along with physical processes such as nucleation and coagulation of particles. These capabilities are demonstrated for two flame-particle systems of practical importance, viz. nano-crystalline titania synthesis and soot formation. The results are compared with experimental data obtained at the University of Southern California (USC) flame facility. Computed particle size distributions show good agreement with experimental data. Simulations have led to exploration of the parameter space for particle production and particle-size influences. -------------- [1] Reaction Design, CHEMKIN-PRO 2012, San Diego.

Carbon-doped TiO2 nanoparticles of different phases are synthesized using a low pressure (20 torr) premixed flame in stagnation-point geometry. Hydrogen/ethylene mixture (1:1) serves as fuel, reacting stoichiometrically with oxygen, with nitrogen dilution. Titanium Tetra Iso-Propoxide (TTIP) precursor is injected to the flame. The flow field is simulated with detailed chemistry and transport. Particle growth models (monodisperse and sectional) then predict particle dynamics and characteristics that are compared with the experimental results. Parametrically, temperature, residence time, and pressure are investigated for their effects on carbon doping and nanoparticle crystallinity. Results from XRD and Raman spectroscopy indicate different phases of TiO2 such as rutile, anatase, and TiO2-II, for different processing conditions. TEM and SEM show that the primary particle sizes range from 3-7nm in diameter, with loosely-agglomerated sizes of ~1 micron. XPS reveals that nanoparticles are carbon doped with Ti-O-C chemical structure. UV-VIS spectroscopy shows photocatalytic degradation of phenol (comparable to P25), with Tauc direct bandgaps of 2.9, 2.5, and 2.2eV for carbon-doped anatase, TiO2-II, and rutile, respectively.

Dye sensitized solar cells (DSSC) offer great promise as an inexpensive alternative to conventional pn junction solar cells. DSSCs typically consist of an organometallic dye molecule coated onto a mesoporous thin film of TiO2 nanoparticles, submerged in an iodide/tri-iodide electrolyte. The Titania layer is a critical component of the cell, since it acts as the diffusion media for the electrons, and allows for the electrolyte to diffuse throughout the film to reduce the oxidized dye molecules. Hence, particle and film properties of the Titania layer directly affect the performance of the DSSC. Despite low manufacturing costs and relatively high efficiencies, the time consuming Sol-Gel technique used for particle synthesis and the subsequent film coating by screen printing is one of the main barriers to large scale commercialization of this technology. The authors have developed a premixed stagnation flame synthesis technique to combine particle synthesis and film deposition in a single step, while maintaining high control over particle size and crystal phase. Thin films were fabricated using the Flame Stabilized on a Rotating Surface (FSRS) technique and were characterized for size, crystal phase and porosity. Furthermore, the films were assembled into DSSCs and the cells were tested for photoconversion efficiency, and recombination behavior. The results indicate that FSRS can indeed produce high efficiency DSSCs, while reducing fabrication time substantially, and maintaining high control over particle and film properties.

The continuously growing energy demand asks for clean technologies. The sun is a very promising renewable energy source, and light conversion continues to challenge materials sciences. Printable low-cost solar cells are the aim of research and industry. Next to organic dye-sensitized TiO₂ solar cells and pure polymer cells, quantum-dot (e.g. PbS, CdS, CdSe) sensitized TiO₂ solar cells gained continuously in importance. But the synthesis of the quantum dots and mainly the heterojunctions involves time-consuming, repetitive steps and wet-chemical based processing. Furthermore, a direct interface between the two semiconducting particles is favorable for good electron transport properties. We present [1] a reduced flame spray synthesis as a single-step and easily scalable tool to produce PbS (quantum dot) - TiO₂ heterojunction nanoparticles. Nanopowders with different lead sulfide to titanium dioxide ratios were produced and characterized. Lead sulfide quantum dots (2 nm, bandgap ~ 1.5 - 2 eV) directly supported on TiO₂ nanoparticles (10 nm) could be synthesized with production rates up to 15 g/hour on a lab-scale set-up. Additionally, thermodynamic equilibrium calculations of the gaseous environment during the combustion show the process robustness regarding usual environmental changes or fluctuations. We further show how this approach allows to vary structure and size of the PbS-TiO₂ heterojunction particles, as long as an excess of sulfur species (S/Pb = 2.5) is applied during processing. Implementing this material as an active layer into a quantum-dot sensitized solar cell may promote a further step towards low-cost solar cells. [1] S.B. Bubenhofer, C.M. Schumacher, F.M. Koehler, N.A. Luechinger, R.N. Grass, W. J. Stark, J. Phys. Chem. C, (under review).

Single-walled carbon nanotubes (SWCNTs) have been synthesized at high catalyst yield (sim; 90%) and rapid rates (>100 microns/s) using a composite iron/silicon-oxide nanoparticle catalyst in a gas-phase diffusion flame environment. An oxy-fuel ethylene inverse diffusion flame is employed to provide a soot-free, carbon-rich environment for growth of high-purity nanotubes. Iron and silicon precursors are added to the fuel stream for nucleation of iron/silicon/oxygen catalyst particles, with the amount of particle oxidation determined by the amount of oxygen-enrichment and fuel dilution at a given temperature. Under optimum conditions, nearly 90% of the catalyst particles produce single-walled carbon nanotubes as compared to less than 10% when the catalyst consists of only iron and oxygen. The effect of silicon addition is investigated through variation of the iron/silicon ratio and measurement of nanotube growth rates. Silicon is shown to primarily affect SWCNT inception with minimal influence on growth rate. Since catalyst yields without silicon are less than 10%, the role of silicon in improving catalyst yield is studied. Car- Parrinello molecular dynamics simulations are employed to investigate the structure of Fe/Si and Fe/Si/O nanoparticle catalysts at synthesis temperatures (1300 K). The simulations show that silicon is uniformly dispersed on the iron surface when oxygen is not present, but covers only one hemisphere of the particle surface when oxygen is present to form a silica “cap”. The structure of the catalyst particle when oxygen and silicon are present thus facilitates the preferential decomposition of a carbon precursor on the Fe-rich side of the particle. On the basis of this finding, SWCNTs will nucleate preferentially on Fe/Si/O with segregated phases compared to catalyst particles with a uniform surface composition that typically become encapsulated in carbon before nucleation can occur. High catalyst yields are also demonstrated on Fe/Al/O catalysts, which indicate that high yields are not specific to the presence of silicon in the particle.

First, it isn't the usual form of soot. It is very challenging to produce large quantities of useful graphitic carbon nanostructures through homogeneous gas phase combustion. Next, a catalyst is required to order the carbon atoms into graphene sheets. Third, we've generally been trying to understand the flame synthesis of carbon nanotubes empirically. If the combustion synthesis of graphitic carbon nanostructures is to become more commercially viable than other methods, then we require robust theories and a detailed understanding of the mechanisms of their formation. Here, we examine the synthesis of carbon nanotubes on a fundamental theoretical level and show how the growth of these nano structures depends upon various various interdependent physical and chemical factors.The overall mechanism of the formation of these graphitic nanostructures considers (a) the impingement of carbon atoms from the predominant carbon-containing species in the ambient, generally CO, (b) their adsorption and desorption at the catalyst-gaseous hydrocarbon interface, (c) surface and bulk diffusion, (d) nucleation, and (e) separation of solid undissolved carbon in nanostructured form. Even with such a model, validation targets are sparse since there is a paucity of accurate experimental growth rate data for carbon nanotubes produced through flame synthesis. Hence, despite the richness of the phenomena associate with the now quite familiar material, there is much to learn about the flame synthesis of carbon nanotubes on a fundamental level. Once we obtain this knowledge, it could form the basis of a significant materials processing and manufacturing industry.

Since its discovery in 1991 and 2004, selective combustion synthesis of either fullerenes (C60, C70, hellip;, C84, hellip;) or single-walled carbon nanotubes (SWCNT) has been developed to a robust industrial process. While fuel-rich combustion of aromatic hydrocarbons at reduced pressure allows for the targeted synthesis of fullerenes, low-pressure combustion of aliphatic and other hydrocarbons below the sooting threshold in presence of a continuously supplied catalyst precursor results in the efficient formation of SWCNT. Correlations between reactor design, operating parameters such as pressure, fuel-to-oxygen ratio or residence time and characteristics of the generated fullerenes or SWCNT determined by means of a range of techniques including high pressure liquid chromatography (HPLC), Raman as well as UV-vis spectroscopy, thermogravimetric analysis (TGA), scanning and transmission electron microscopy will be discussed. Strategies to grow SWCNT of increased length taking advantage of the in situ formation of hydrogen and water, both minimizing catalyst deactivation, will be presented. Post-processing steps, necessary for most applications will be outlined. The extraction and purification of fullerenes will be shown and the use of chemical functionalization for the achievement of solubility in specific solvents presented. The development of fullerene derivatives with electronic structures suitable for their optimized use as electron acceptor phase in the active layer of organic photovoltaic applications and photodetectors in combination with selected electron donor materials will be discussed. Purification procedures allowing for the close to quantitative removal of metal and amorphous carbon residues while keeping nearly defect-free SWCNT will be presented. The dispersion of SWCNT in aqueous and organic solutions using non-ionic dispersal aids and the application of such solutions for the preparation of transparent conducting films allowing for the deposition on flexible substrates will be described. Combining improved formulations (“inks”) with the capability of the combustion process to produce particularly long SWCNT, resistivity and transparency characteristics consistent with commercial applications are expected to be met soon. Scalable new technology addressing the separation between metallic and semi-conducting SWCNT but avoiding the need of centrifugation, will be presented and the use of the resulting products, e.g., for the fabrication of thin film transistors (TFT) in the case of semi-conducting SWCNT, described.

Few-layer graphene (FLG) nanostructures and multi-walled and single-walled carbon nanotubes (MWNTs and SWNTs) are grown directly on spinel solid solutions using flame synthesis. Carbon nanotube (CNT) and graphene growth occurs through decomposition of flame-generated carbon precursors (e.g. CH4, CO and C2H2) over nanoparticles (i.e. Cu, Ni, Co, and Fe) reduced from the solid oxide. Our setup demonstrates the ability to produce FLG and its composites in open and ambient conditions, thus providing advantages of scalability for large-area surface coverage, increased growth rates, continuous processing, and reduced costs due to efficient use of fuel as both heat source and reagent. The growth of CNTs is investigated on NiAl2O4, CoAl2O4 and ZnFe2O4 using counterflow diffusion flame (CDF) and multiple inverse-diffusion flames, while the growth of graphene is investigated on CuFe2O4 using multiple inverse-diffusion flames. As shown by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and x-ray diffraction (XRD) studies, substrate temperature and spinel composition play a critical role in the growth of both CNTs and graphene.

Flame synthesis of CNTs provides unique features not realized in current synthetic methods utilizing an arc discharge or high-temperature furnace. Combustion of a portion of the hydrocarbon gas provides the elevated temperatures required with the remaining fuel quite naturally serving as the hydrocarbon reagent. Hence the flame constitutes an efficient source of energy and hydrocarbon reactant. Furthermore, since flame synthesis has a demonstrable history of scalability for high-volume commercial synthesis, the method holds promise in this regard. Perhaps the most significant aspect of the flame synthesis approach are the very short residence times realized for catalyst inception and nanotube growth. Reported results from the high-temperature furnace approach suggest reaction times on the order of seconds. In contrast, the flame synthesis method realizes residence times on the order of tens of milliseconds, an attribute that underscores the potential of flame synthesis for large-scale production of nanotubes. As with other processes demonstrated to date, the critical steps for optimum nanotube yield and growth are catalyst particle formation and their proper entrainment into a high-temperature hydrocarbon environment where nanotube growth can commence. Illustrated in this talk will be the use of premixed and diffusion flames as reaction environments for carbon nanotube synthesis. We have tested both systems using catalysts as aerosols and supported upon substrates. Highlights will be shown demonstrating success and further challenges. For illustration, this abstract illustrates two key parameter spaces, namely gas composition and catalyst size and composition in premixed and diffusion flames, respectively.

Flame synthesis has the potential to provide higher throughput at a lower cost for bulk synthesis of nano materials as compared to chemical vapor deposition (CVD) and plasma CVD processes. Spectroscopic studies have clearly established the formation of multi-walled and single-walled CNTs in flames. Flames provide the chemical species and the thermal energy necessary for driving the synthesis process. Various geometries, configurations, fuel/oxidizer combinations, process parameters involving pressure, temperature and composition have been used clearly establishing the potential for flame synthesis of CNTs. Metal catalysts used in the form of either a substrate or a particle cloud include Fe, Ni, La, Co and their oxides and nitrates often on Ca and Mg substrates. Opposed flow and porous burner stabilized premixed, partially premixed and non-premixed flat flames have resulted in CNT formation. Laminar jet flames, co-flow laminar jet flames, piloted laminar jet flames have also been used to synthesize carbon nano structures. Fuel oxidizer mixtures resulting in nanostructure formation have included CH4/air, C2H2/air, C2H4/air and CH4/O2. The pressures and temperatures of CNT synthesis have spanned the continuum and rarefied gas regimes. The gas phase environment leading to CNT formation is often far from chemical equilibrium. Identification of the optimum ingredients, fuel and oxidizer, for the development of industrial processes is essential for further commercialization of CNT. Measurements of carbon nanotube growth rates under well-defined micro-scale gas phase environment are scarce. Development of the reacting boundary layer around the catalyst surface and its effect on the gas species composition around the active particle need to be carefully examined. Detailed models are required for describing multiscale mass and energy transfer during the inception and growth processes. Spectroscopic diagnostic techniques at the micro-scale need to be applied near the particle surface to obtain in-situ measurement of species concentrations and temperature. These measurements can provide the missing link of data between the large scale and the nano-scale processes occurring simultaneously during the flame synthesis of CNTs. It has been observed that careful control of the synthesis process is essential to preserve the properties of the CNTs and to avoid impurities. However, flame environment is characterized by presence of large number of chemical species and a varying temperature field. Thus advanced control strategies are necessary for reliable process design. Applications of multi-scale, multi-domain computational, experimental and engineering techniques are anticipated at a fast rate. Engineering control of the bulk gas phase composition and systematic inclusion of key gaseous compounds such as silane (SiH4), acetylene (C2H2) and hydrogen can provide designer materials for advanced application of flame synthesis.

Graphene is now broadly known to be outstanding with respect to its unusual mechanical and electronic properties. Recent progress in the synthesis of graphene has stimulated a wide range of research for next generation device applications. Among all the synthesis methods, chemical vapor deposition is considered promising as it is able to produce graphene in large area (up to several inches) with controllable layer numbers and atomic doping, but it is indispensable to well-sealed chambers and complex gas handling equipment which is expensive and difficult to continuous synthesis. The flame synthesis is a self-propagating high-temperature synthesis technique, which involves only a single-step process, resulting in a large-scale, rapid and low-cost approach for preparing diverse materials including low-dimension carbon structures such as fullerenes and CNTs. Here, we introduce a facile yet time- and energy-saving flame method to produce graphene films. High quality graphene films can be obtained in high rate (only 10s). In detail, dual flame setup was applied to effectively generate high quality graphene films directly on the surface of nickel foils in ambient conditions. A butane torch burner functioned as heating flame was used to meet the temperature requirement for carbon decomposition and diffusion on nickel. A second flame normally fueled by ethanol was lit up by the torch burner and surrounded the nickel substrate with its inner flame during the whole process. The second flame provided carbon stock for oversaturated the nickel with carbon and served as protection flame avoiding oxidation during high temperature synthesis process. The heating flame was switched off after sufficient carbon intake in nickel. The protection flame was capped off in follow to force cooling of the nickel foils facilitating graphene aggregation on the nickel surface. The concept combines the CVD growth mechanism with the advantages of the flame method. Thickness, purity and even atomic dopant of graphene films can be readily tuned by choosing the organic fuel of the flame. The graphene film was identified to be single or few-layered with high quality by SEM, HRTEM and Raman charaterization. The quality evaluation of the graphene was also performed by making Schottky solar cells from the graphene films with n-Si. The solar cells show conversion efficiencies of 2.88% for pristine cells and 4.35% upon HNO3 treatment. The results confirmed that the flame-synthesized graphene may rival their CVD-derived counterparts. The flame method offers the advantages of simplicity, high-efficiency, energy-saving, low-cost and the ability to easily extend to continuous and mass production of graphene, and will significantly contribute to the fundamental research and practical applications of graphene.

Functional nanomaterials are being produced by many different types of fabrication methodologies, of which combustion synthesis is one approach. The focus of the present paper is not on the particular synthesis method, but on the usage of such nanomaterials in reactive systems where the nanoscale structure and organization are used to control reaction and response characteristics of materials. Two examples include the usage of functionalized graphene sheets and nanoporous silicon. Graphene sheets, because of their large surface areas, are excellent carriers of nanoparticles that may be functionalized to act as energetic materials or catalysts. Nanoporous silicon, again because of its high surface area and well studied chemistries, allows for controllable feature design and organization to study the influence of multiscale characteristics on the ignition and propagation of reactive materials. Examples of various systems are given in the paper.

Development of controlled microstructure, to produce materials with unique thermal, mechanical, electromagnetic, and other properties, is a primary goal in reaction synthesis. For example, certain alloys exhibit unique corrosion, strength, and radiation resistance when prepared in an amorphous state. Unfortunately, the range of amorphous materials is limited due to traditional cooling methods used in their processing, with quenching rates of 10^5 - 10^6 K/s. However, shock wave consolidation and reaction can produce cooling rates as high as 10^10 K/s upon release. Such processing leads to a crystal/amorphous transformation due to material melting in a shock wave region that is then followed by rapid cooling due to the rarefaction wave. In order to characterize the compaction dynamics and reaction of the materials, the Asay shear impact experiment was used. The amount of shear and compression can be tailored by varying the geometry of the plunger impacting face and the material is confined to be nearly two-dimensional. The gas gun used in these experiments fires a 25.4 mm diameter projectile at velocities of up to 1.2 km/s. Digital image correlation software is used to track objects on the surface of the sample and to calculate displacement and strain fields. In addition to visible light imaging, a high-speed infrared system to study the temperature evolution in impacted samples was utilized. In this presentation we report the experiments and characterization of the thermo-mechanical and chemical response of heterogeneous reactive materials to dynamical loading and heating. This research was focused on reactive composites produced by high-energy ball milling, a promising avenue to control the degree and character of the intermixing between various components and, consequently, tune the resulting properties. The structures of such materials exhibit complexity at various scales, from tens of microns to a few nanometers, which confer unique properties to these composites. Our primary focus is to examine primarily the intermetallic Ni-Al system, but the approach is applicable to many other compositions. It is important that thus synthesized materials are contrasted to non-shock combustion synthesized materials.

Nanostructured metal oxide materials have become important research topics in nanotechnology for their novel properties and wide potential applications in nanophotonic, nanoeclectronic, and nanobiotechnology. More recently, considerable attention has been directed to oxide nanostructures formation by thermal oxidation of metals in oxygen atmosphere, largely due to its technical simplicity, low cost and large-scale growth capability. The formation of metal oxide nanostructures by high-temperature oxidation of metals is a long-established phenomenon dating back to the 1950s; however, a satisfactory growth mechanism for the spontaneous formation of these oxide structures has not yet been established. In addition, a significant challenge is the growth control of these nanostructures (morphology, growth direction, size, and structure) due to the various influencing parameters, such as temperature, pressure, oxidation time, and surface conditions. In this work we present a detailed study of CuO, α-Fe₂O₃, and ZnO nanostructures (nanowires, nanobelts and nanoblades) formation during the oxidation of copper, iron, zinc and copper-zinc alloy as model systems to understand the mechanisms of oxidation-induced oxide nanostructures growth. Meanwhile, our interest in these systems also stems from the prospective broad applications of nanostructured CuO, α-Fe₂O₃ and ZnO. Our results show that the compressive stresses generated by the volume change associated with the interfacial reaction accompanying the layered oxide growth stimulate oxide nanostructures formation and the oxide nanostructures growth kinetics is controlled by surface diffusion of cations supplied by outward grain boundary diffusion through the oxide layers. In addition to addressing the growth mechanism of the oxide nanostructures, we also find that the CuO oxide nanowire formation can be effectively promoted by surface bending tensile stress or surface roughening via sandblasting. It is also found that the oxidation of iron at low oxygen gas pressures (~ 0.1 Torr) is dominated by the growth of hematite nanobelts while at the high pressure (~ 200 Torr) is dominated by the growth of hematite nanowires. Detailed transmission electron microscopy study shows that both the nanobelts and nanowires grow along the [1120] direction with a bicrystal structure. It is shown that nanowires are rooted on Fe₂O₃ grains while nanobelts are originated from the boundaries of Fe₂O₃ grains. We also demonstrate that surface roughness of irons can be employed to tune the α-Fe₂O₃ oxide growth morphologies from nanowires to nanoblades.

A rapid and facile method to prepare nickel foam from nickel nitrate and glycine using a conventional microwave oven is presented. The foam, characterized by SEM, XRD-Rietveld, TG, magnetization measurements and BET contains mostly nickel metal (80%) and Nickel oxide (20%); it exhibits pores in the submicrometric and nanometric scale and consists of particles with an average diameter of ca 45-47 nm and BET surface of 15.9 g/m2. This microwave-assisted combustion synthesis is then applied to the infiltration of porous scaffolds with nickel metal as a potential method to accelerate the fabrication of electrodes in solid oxide fuel cells and electrolyzers. After repeated impregnation, the scaffolds of gadolinia doped ceria, saffil (high temperature insulating brick), La0.2Sr0.7TiO3 and BaCe0.5Zr0.3Y0.16Zn0.04O3-d were black, exhibited electrical continuity and were easily lifted with a magnet. A comparative SEM study of the microstructure of the porus scaffolds with and without Nickel is presented. The authors would like to thank Sasol for funding.

Laser pyrolysis is a gas phase process which is currently used for the synthesis of various kinds of nanoparticles. It is based on the resonance between the emission of a laser and the absorption by a gaseous or liquid precursor molecule. In this thermal process, the CO2 laser is used as a spatially well-defined and easily adjustable heat source allowing collision-induced decomposition. The versatility of this method allows synthesizing of a great variety of nanopowders in terms of material, size, phase and stoichiometry. Two examples are given. First, we present how the experimental parameters can be tuned to synthesize crystalline nanoparticles of titanium oxide (TiOx, x=1 to 2) or doped nanoparticles, for example N-TiO2. These nanoparticles exhibit original optical properties including an efficient absorption of light in the visible range. This opens the way to the development of applications in the field of hybrid photovoltaic cells and photocatalysis. First results are presented. Second, it was possible to synthesize very small nanoparticles such as silicon quantum dots with a diameter as low as 4 nm with a narrow size distribution (± 15%) and a production rate in the range of 300 mg.h-1. The size control allows adjusting the band gap, so these nanoparticles can be considered as building blocks of all-silicon tandem solar cells. A very precise control of the particle size in the 3-5 nm range is achieved by adjusting the laser timing parameters. A composite nanomaterials is elaborated in-situ by association of a supersonic beam of Si nanoparticles and simultaneous magnetron sputtering of a matrix material. This original method allows a secured single step process for layers elaboration composed of nanoparticles embedded in a matrix without any limitation in both materials.

Combustion synthesis (CS) is an attractive technique for production of different materials. In a conventional scheme, a heterogeneous exothermic mixture of solid powders is pressed into a pellet and ignited at one end, generating a high-temperature combustion wave front that propagates through the reactive media, converting the precursors to the desired product. The unique characteristics of combustion synthesis include: (i) short (seconds) synthesis duration; (ii) great energy efficiency, since the internal system chemical energy is primarily used for materials production; (iii) ability to produce high purity products with unique properties and high production rate, since the extremely high-temperature conditions (up to 4000 K), which takes place in the combustion wave, burns off most of the impurities. Bulk processing of porous silicon nanoparticles (nSi) of 50minus;200 nm size and surface area of 50minus;200 m2/g by using a reduction-type salt-assisted CS method in an inert (argon) atmosphere is reported. More specifically, the production of nSi is achieved by combustion synthesis with an exothermic mixture of silica, magnesium and sodium chloride (SiO2 + βMg + αNaCl → Si + γMgO + αNaCl). Hydrometallurgical treatment of the combustion product is performed with different acids (i.e., HCl, HF, etc). The characteristics of as-synthesized nSi are analyzed by different material science techniques including XRD, BET, FESEM, and STEM. The photo-activity of the CS-nSi, which is elucidated by transient absorption spectroscopy and photo-electrochemical techniques, is investigated and discussed. Photoelectrochemical experiments reveal a prompt n-type semiconductor response and measureable photocurrent and photovoltage under both AM1.5 and visible (>400 nm) illumination. Transient absorption spectroscopy measurements also show prompt bleaching resulting from photo-induced charge separation. The obtained results show that CS-nSi is a highly crystalline photoactive material that has the potential to lower the cost of silicon-based thin-film solar cells.

The sintering of tantalum carbide is a challenge due to its covalent bonding and high vapor pressure. Grain growth is often cited as a primary obstacle to obtaining fully dense tantalum carbide. It is typically favorable to use a smaller particle size to improve densification, but smaller tantalum carbide particle sizes result in higher oxygen contents that can facilitate grain growth. The goal of this research is to investigate the grain growth of reactive sintered tantalum carbide nanopowders produced by a hybrid combustion synthesis technique. Details of ignition, process control, post-synthesis, and sintering will be discussed. We will show that grain growth can be directly correlated to strain introduced into the tantalum carbide lattice. Strain can be introduced by loss of carbon from the tantalum carbide crystal structure or by forming solid solutions with other compounds. The additions of WC, HfC, and ZrC can introduce tensile or compressive strains on the tantalum carbide lattice, having a marked effect on the grain growth during sintering. We estimate that the lattice can be strained approximately 2-3%.

Self-propagating High-temperature Synthesis (SHS) was applied for the first time to prepare Chevrel phases, MxMo6T8 (M = metal, T = S, Se). We used electron microscopy and X-ray powder diffraction to characterize the chemical reactions in the Cu-Mo-S system. We showed that the replacement of the frontal combustion by thermal explosion increased the Cu2Mo6S8 yield from 86 to 96 %, while the synthesis remained ultra-fast: 10-20 min in a hot furnace (1000 °C), as compared to at least 17 hours of heating for the conventional solid state technique. The electrochemical activity of the synthesized material was similar to that of the Chevrel-based cathodes for Mg battery cathodes produced using conventional techniques. The high speed and high yield make this fabrication technique suitable for industrial fabrication.

Anatase TiO2 is still a strong candidate material for dye-sensitized solar cells (DSSCs), due to the low cost of processing, relative to that of single and amorphous Si-based materials. Moreover, due to its wide bandgap (3.2 eV), it displays excellent photoelectric activity with UV light absorption. Hence, much effort is being directed to shift its absorbance edge toward the visible wavelength region, in order to further modify the bandgap and improve efficiency. Various processing methods have been considered, including doping with nitrogen and carbon. The role of these elements is not fully understood, and further work is needed to understand mechanisms that control efficiency. In this paper, we describe the flame synthesis of C-doped TiO2 utilizing a flat-flame burner of novel design, using titanium-isopropoxide as the precursor material. The resulting nanopowders were collected on a chilled substrate. Carrier gas flow rate was adjusted to control the amount of dopant in the TiO2 nanoparticles. The synthesized C-doped TiO2 was used to make the photoanode of DSSCs. Photo catalytic activity of the fabricated DSSCs using standard Pt-FTO as counter electrode, ruthenium dye, and iodide/tri-iodidebased electrolyte was measured. Structural and phase characterizations were determined using analytical electron microscopy techniques. EELS was used for chemical analysis, and elemental distribution. Initial results show ~17% increase in the photocurrent density, and ~23% overall increase in the efficiency with respect to photoanodes made from commercially available anatase, Degussa P-25 TiO2