Symposium NM6 : Nanoscale Materials and Devices by High-Temperature Gas-Phase Processes

Flame aerosol reactor (FLAR) synthesis of nanoparticles is widely used to produce a range of nanomaterials. However, the incipient cluster formation by nucleation and vapor condensation is not well understood. This gap in knowledge of incipient particle formation is due to limitations of instruments in the measurement of sub 3 nm particles because of losses by diffusional deposition. Our group has overcome this limitation by use of a high resolution Differential Mobility Analyzer (DMA) and an Atmospheric Pressure Interface-Mass Spectrometer (APi-TOF) to observe incipient cluster formation during flame synthesis. By using these instruments in tandem, differential mobility analysis-mass spectrometry (DMA-MS) allowed the measurement of the size and mass of the incipient particles simultaneously, and the effective density of the sub 3 nm particles was estimated. It was found that the sub 3 nm clusters generated during the synthesis of SiO₂ and TiO₂ have effective densities of 1.42 g/cm³ and 1.75 g/cm³, respectively. These incipient clusters were less dense than the SiO₂ and TiO₂ bulk materials. The APi-TOF further provided the chemical composition of the detected clusters based on highly accurate mass and isotope distributions. Measurements in a blank flame detected a large number of sub 3 nm particles generated from chemical ionization reactions, and the negative ions were dominantly nitrate ions. The blank flame-generated ions played an important role during particle synthesis. The APi-TOF observed the appearance of nitrate ions in clusters containing silicon or titanium (such as Si₂H₆NO₁₀^- and TiN₂O₁₀^-) species. As precursor feed rate increased, particles with larger mass and sizes were formed due to enhanced coagulation and vapor condensation. Results of latest advances in measurement of particle formation and growth in flame aerosol reactors will be presented.

Inline characterization of nanoparticle formation with particle mass spectrometry (PMS) is well established to investigate the evolution of particle size and particle size distribution during nanoparticle synthesis. It is commonly used in the gas-phase synthesis of nanoparticles to determine the influence of process conditions (temperature, precursor concentration, pressure) on the particle growth. The method is based on a molecular beam formed by a vacuum expansion through a nozzle/skimmer system. The vacuum conditions immediately suppress further reactions and particle growth. The system is equipped with a deflection unit with a Faraday cup detector to measure energy-filtered currents of charged particles and a quartz-crystal microbalance to detect the total particle loading of the molecular beam. Due to limitations in pressure drop in the conventional two-stage vacuum system and clogging of the sampling nozzle with particles, conventional PMS systems with an one skimmer/nozzle setup are limited to sampling from low-pressure (< 100 mbar) reactors. There are only few reports on molecular-beam mass spectrometry (MBMS) of particles from atmospheric-pressure systems, mostly used for the investigation of sooting flames [1].
In this study we present a newly developed PMS that can be operated at atmospheric pressure. It consists of a two nozzle/skimmer system integrating an additional pre-expansion chamber downstream the first nozzle operated at about 50 mbar. This allows sampling, dilution of particle concentration and freezing of the particles' properties by adding inert gases. After a very short residence time within this first chamber of about a few µs, the sample is transferred into a particle-laden molecular beam by expansion of the aerosol via the commonly used two-stage nozzle/skimmer system. The additional pre-expansion chamber enables operation of the PMS within a wide pressure range between 5 mbar and atmospheric pressure and the nanoparticle sampling rate can be tuned by adjusting the pressure in the pre-expansion chamber by adding dilution gases or reducing pumping power thus preventing clogging of the second nozzle. As the setup is mounted on a two-way positioning unit, it allows for spatially-resolved investigations of particle formation in gas-phase reactors. Results concerning the investigation of nanoparticles from sooting flames as well as from a spray-flame nanoparticle reactor, both operated at ambient pressure, will be discussed.

References:
H.-H. Grotheer, K. Wolf, K. Hoffmann; Appl. Phys. B 104 ( 011) 367–383

Nanoparticles typically form irregular or fractal-like agglomerates of primary particles (PPs) by ballistic coagulation in volcanic plumes, and manufacture of carbon black or fumed silica as well as an array of nanoparticle compositions at low pressure (e.g. inert gas condensation). Even though the dynamics of coagulating spherical particles, such as self-preserving size distribution (SPSD) and coagulation rate are reasonably well-understood, there is significant uncertainty for fractal-like agglomerates. For the latter, coagulation rates have been proposed [1], their mobility [2] and SPSDs have been determined [3] and even the time needed to reach their asymptotic structure has been estimated [4]. All these have been confined to agglomerates with monodisperse PPs. Realistic agglomerates, however, consist of polydisperse PPs. So little is known for the effect of constituent PP polydispersity on agglomerate structure and, most importantly, coagulation dynamics, which affect the environmental impact of agglomerates (climate forcing or visibility impairment by soot) or performance of gas sensors, catalysts and even biomaterials and nutritional products.
Here coagulation of nanoparticles of varying PP polydispersity (s_g,PP = 1 – 3) in the absence of coalescence, sintering or surface growth is investigated by a discrete element method in the free molecular regime. This is an early stage of particle formation especially at high temperatures followed typically by rapid quenching to facilitate collection of particles and retention of their ramified structure as in manufacture of fumed SiO₂, Ni or TiO₂. As a result, particle dynamics at this stage frequently dominate the end product characteristics. The effect of PP polydispersity on agglomerate size, morphology (fractal dimension, D_f, mass mobility exponent, D_fm, and their prefactors) as well as on the attainment of the well-known asymptotic fractal-like structure (D_f = 1.91 and D_fm = 2.15) and SPSD is investigated. Increasing the polydispersity of the constituent PPs does not affect but only delays the attainment of the asymptotic D_f, D_fm and SPSD of the resulting agglomerates. The crossover agglomerate size and critical number of PPs per agglomerate that mark the transition between D_f = 3 and 1.91 scaling are obtained by various ways and increase with PP polydispersity. Only clusters larger than the crossover size should be considered for drawing conclusions on the formation mechanism as well as their structure. Furthermore, a simple modification in the collision kernel of monodisperse agglomerates is proposed that captures the evolution on the asymptotic collision frequency in the free molecular regime.

[1] Thajudeen T, Gopalakrishnan R, Hogan CJ Jr. (2012) Aerosol Sci. Technol. 46: 1174-1186.
[2] Sorensen CM (2011) Aerosol Sci. Technol. 45: 765-779.
[3] Vemury S, Pratsinis SE (1995) J. Aerosol Sci. 26: 175-185.
[4] Goudeli E, Eggersdorfer ML, Pratsinis SE (2015) Langmuir 31: 1320-1327.

Fractal aggregates in nature grow with a scaling dimensionality less than the spatial dimension. This results in their packing density (θ_f)–defined as actual volume occupied by solid subunits constituting an aggregate relative to total aggregate volume–decreasing with increasing size R_g/a (aggregate radius of gyration normalized by average radius of monomers (repeating subunits)). Fundamental questions remain regarding the scaling laws and physical mechanisms controlling the evolution of θ_f, especially after the onset of gelation. This is in part because of experimental challenges owing to the effects of gravity and thermal fluctuations on the formation and structural stability of aggregates in the large R_g/a limit. Here, we experimentally map the scaling of θ_f for aggregates–made of non-repulsive monomers–across five orders of magnitude of R_g/a. Our experiments reveal three successive growth regimes, namely diffusion-limited cluster aggregation (DLCA) of monomers, percolation of aggregates, and DLCA of percolates, with distinct cross-over points occurring at R_g/a ≈ 4 and 1×10³. Corresponding to these regimes, we show θ_f to decrease in distinct power-law exponents of -1.3, -0.5, and -1.3, respectively. Our work, besides demonstrating the experimental realization of stable aggregation in very large R_g/a limit, redefines the currently held scaling law for θ_f of rigid aggregates, and has implications for synthesis of materials with superlative properties and accurate estimation of climate forcing by carbonaceous aerosols.

Soot impact on health and environment strongly depends on its primary particle size and effective density. Scaling laws based on the projected agglomerate area exponent, D_a, and prefactor, k_a, have been used recently in tandem with mass-mobility measurements to obtain the size of the constituent primary particles of agglomerates and aggregates, as an alternative to off-line microscopy. The D_a and k_a of soot aggregates, however, depend on combustion sources and conditions.
Here, soot dynamics after inception are investigated by a Discrete Element Model (DEM) of agglomeration and surface growth during pyrolysis of acetylene. The initial acetylene molar fraction is varied to attain different final soot volume fractions. For high soot volume fractions, surface growth is dominant over coagulation at the early stages of soot formation and acetylene molecules react on the surface of soot primary particles increasing rapidly their mass and size. At these conditions, the soot mobility size distribution narrows down and its self-preserving geometric standard deviation, σ_g,m, decreases from 2.03 for point-contact agglomerates to 1.8. When acetylene molecules are consumed, soot aggregates continue to grow by coagulation in the absence of surface growth and σ_g,m passes through a minimum of 1.45 ± 0.05 in the transition regime, regardless of the employed soot volume fractions consistent with agglomerate dynamics.
The evolutions of D_a and k_a are quantified in terms of their normalized mobility size and relationships bridging the asymptotic D_a and k_a between small nascent and large mature soot aggregates are presented. The correlation among the number of primary particles, the mobility and the primary particle diameters based on the D_a and k_a derived here is in better agreement with mass-mobility measurements of soot aggregates than prior models for agglomeration of monodisperse and polydisperse primary particles. A relationship for the evolution of soot effective density as function of the normalized mobility size is derived and compared to measurements at different combustion conditions. The asymptotic D_a and k_a for mature soot aggregates are combined with a power law relationship to calculate the primary particle size of diesel and flame soot, achieving relative errors down to 0.2 and 1 %, respectively.

Laser-based diagnostics enable non-intrusive in situ characterization of the gas-phase synthesis flow field, as well as the as-formed nanomaterials themselves during flame synthesis, permitting fundamental understanding of the physical processes and growth mechanisms involved. Well-known techniques, such as laser-induced fluorescence (LIF) and Raman spectroscopy, can be utilized to characterize the gas-phase flow field (e.g. temperature, species concentrations). Moreover, novel developments of existing techniques have been recently used for in situ nanomaterials characterization during synthesis. Specifically, low-intensity phase-selective laser induced breakdown spectroscopy (PS-LIBS), for detection of the formation of nanoparticle phase, and in-situ Raman, for identification of nanoparticle crystallinity, are discussed.
Different from conventional LIBS, in PS-LIBS, the laser power used is much lower, with microplasmas created from breakdown of the nanoparticles but no gas breakdown. Thus, high selectivity is established between emissions from atoms emanating from nanoparticles versus atoms from gas-phase molecules (e.g. precursor species). For TiO₂ nanoparticles, the emission intensity increases as the nanoparticles grow in the synthesis flow field, plateauing as the particles become larger than 6nm. PS-LIBS provides important tracking information of nanoparticle formation from gas-phase precursors to nanoparticles. Furthermore, when the wavelength of the laser is tuned to match the transition line of the excited atoms in the plasma, the signal intensity is greatly enhanced (over 100 times) by secondary resonant excitation from the same laser pulse, remarkably improving the detection threshold for measurements. The enhancement of the resonant excitation is highly sensitive to the excitation laser wavelength, with a narrow excitation spectral window, 14 to 22 pm (FWHM, full width at half maximum) for neutral atomic lines and 45 to 60 pm (FWHM) for the ionic lines. With further evolution of the nanomaterials after formation, in situ Raman spectroscopy is applied to characterize the crystallinity of the nanomaterials in the aerosol. For example, the crystalline transformation of titania from amorphous to anatase to rutile phases has been observed in situ in a flame. This technique serves as a sensitive and reliable way to characterize particle composition and crystallinity and to delineate the phase conversion of nanoparticles, allowing for better understanding of the governing growth and kinetic mechanisms.

We propose a method for on-line monitoring of gas phase synthesis of nanoparticles. It is based on Laser-Induced Breakdown Spectroscopy (LIBS). LIBS is a method of chemical analysis that offers many advantages. It allows remote specific detection of most of the chemical elements in a sample and at very low concentrations. This technique was already used to probe the chemical composition stability of SiC nanoparticles during their production by laser pyrolysis. However, sampling stability control difficulties and interface problems between optical elements and particle flow are a drag to its use as an efficient method in continuous-wave mode on high-throughput production plants. Here we propose a new experimental setup for eliminating these difficulties by performing the laser-particle interaction in vacuum. A small part of the aerosol stream is sampled and driven to an aerodynamic lens system. The latter produces a dense and collimated beam of nanoparticles under vacuum from the atmospheric pressure aerosol flow. The laser-particle interaction takes place at 10^-3 mbars. The photon signal from the plasma is collected by an UV-compatible optical fiber connected to a spectrograph. As the interaction takes place at low pressure, the photons are emitted only from particles. Unlike previous experiments, the background from interaction with the gaseous component is totally eliminated. Moreover, as the nanoparticle beam is highly collimated, the optical interfaces are not obstructed by particle deposition and the system can be kept running for hours. This method can also be adapted to wet chemical synthesis techniques or any particle samples in a stable suspension. In this case, the particle suspension is atomized to bring the sample in the aerosol form. The proof of concept is performed with a collimated beam of silicon nanoparticles. With a 20 kHz fiber-laser focused in order to have at least 10 GW/cm² intensity on the particle beam, exploitable spectra are recorded at a repetition rate of less than one minute, allowing for continuous-wave or at constant time intervals in-process monitoring of particle chemical composition.

A microwave-plasma reactor for nanoparticle synthesis is designed, optimized and investigated by numerical modelling and simulation, which are validated against measured data.
Nanoparticle synthesis from the gas phase is a stable and scalable process for high purity particles with a narrow size distribution. In the case investigated, Si-C particles (for battery applications) are synthesized and coated in a microwave-plasma reactor. In contrast to other high-temperature processes (e.g. hot-wall or flame reactors), the particles from the plasma reactor are small, oxygen-free and non-agglomerated. Silane (SiH₄) is injected with Argon and hydrogen as carrier gases at a low Reynolds number. In a first step, the gaseous precursor SiH₄ is decomposed in the micro-wave plasma and Si nanoparticles are formed. In a second step, downstream of the plasma, the carbon shell is produced from Ethylen (C₂H₄), which is supplied through a set of coating nozzles, decomposed and homogeneously mixed with the dispersed Si nanoparticles, on which the carbon will be formed. In the complete process, predominantly spherical particles with a Si core and a C shell are formed.
To achieve effective mixing between Si nanoparticles and C₂H₄, numerical studies of the reactor have been performed and the mixing of two tracer streams with different temperatures (without plasma) was investigated in order to optimize the design of the coating nozzle. The resulting geometry was produced and used in experiment.
In the succeeding investigation of the synthesis process, the flow, the SiH₄ decomposition and the particle dynamics were modelled to obtain insights into the gas-phase velocities, temperatures, and the resulting particle properties (volume, surface area and particle number concentration). The plasma was modeled as a distributed heat source, and as the reactor is not well accessible for direct measurements of the (wall) temperature, a conjugate heat-transfer model was employed. The evolution of the particles, described by the population balance equation, was solved by a monodisperse moment-model¹ based on particle number-, area-, and volume concentration. The model was implemented in an Euler-Euler formulation for the gas- and particle phase, respectively. Nucleation was described by one-step chemistry from SiH₄ to Si, assuming fast pyrolysis inside the plasma. Particle sizes are compared to experiments and show a plausible agreement.
The authors gratefully acknowledge the support by the German research foundation (DFG) in scope of the research group 2284 “Model-based scalable gas-phase synthesis of complex nanoparticles”.

1. F.E. Kruis, K.A. Kusters, S.E. Pratsinis, B. Scarlett, (1993). A simple model for the evolution of the characteristics of aggregate particles undergoing coagulation and sintering, Aerosol science and technology, 19 (1993) 514-526.

An outstanding issue in the field of nanoparticle science and technology is the prediction of the morphological properties of nanoparticle agglomerates in different media directly from the chemical constitution of the primary particles and their inter-atomic interactions with the medium atoms. Such a problem is not easy to address through a brute-force application of the atomistic molecular dynamics (MD) method because the longest times that can be simulated today even with the most powerful supercomputers and the use of thousands of graphics processing units (GPUs) with such a method are still on the order of a few hundreds of nanoseconds (up to a few microseconds in some cases).
However, if one is not interested in the dynamic properties of the system but only in its final morphological properties, one can resort to a non-dynamic method which is inherently free of any such long time restrictions. Such a method is the Metropolis Monte Carlo (MC) method of importance sampling, based on the generation of a Markov chain of states, i.e., a sequence of states in which the outcome of a trial state depends only on the state that immediately precedes it [1]. The method has been used with exceptional success in simulating the thermodynamic and morphological properties (conformation and self-assembly) of many Soft Matter Physics systems.
We extend it here to the case of nanoparticle (NP) agglomeration using as test primary particles amorphous silica (SiO₂) and fullerenes (C₆₀). Our Monte Carlo algorithm is based on the design and efficient application of a set of powerful synthetic moves that help the systems ample configurational space and be driven quickly to the state of thermodynamic equilibrium. Four different such moves are applied here: a) NP displacement, b) NP rotation, c) agglomerate displacement, and d) agglomerate rotation. The simulations have been executed with two different force fields, a fully atomistic one [2, 3] and a coarse-grained one based on the potential of mean force between a pair of NPs.
Results will be presented from both types of simulations (which are typically completed within a few hours of CPU time) and will be compared with each other. They will also be compared with other simulation methods in the literature [4]. We will see that the new method can provide accurate predictions of the fractal dimensions of the agglomerated nanoparticles in the gas phase starting solely from their chemical structure.

REFERENCES
[1] Allen, M.P.; Tildesley, D.J. Computer Simulation of Liquids, Oxford Univ. Press (1987).
[2] Demiralp, E.; Cagin, T.; Goddard, W.A. Phys. Rev. Lett. 82, 1708−1711 (1999).
[3] Girifalco, L.A; Hodak, M.; Lee, R.S. Phys. Rev. B. 62, 13104−13110 (2000).
[4] Goudeli, E.; Eggersdorfer, M.L.; Pratsinis, S.E. Langmuir. 31, 1320−1327 (2015).

The influence of turbulence on the particle size distribution (PSD) of nanoparticles synthesized in spray flames is investigated by numerical experiments. Simulation results are presented, focusing on the size distribution (SD) for different turbulence levels and two different particle models.
Metal oxide nanoparticles are commonly produced in flame spray pyrolysis (FSP) processes, which enable an effective supply of the precursor into a hot environment, i.e. the reaction zone of the spray flame. The investigated FSP reactor, originally designed by Mädler et al.[1], consists of a spray nozzle and three concentric inlets for the dispersion gas, a premixed pilot flame and a sheath gas. A combination of ethanol/hexamethyldisiloxane, methane/oxygen and pure oxygen are used as solvent/precursor, for the pilot flame and as sheath gas, respectively.
In the simulations, the spray droplets are modelled as Lagrangian particles and the gas-phase and the nanoparticles are described by Eulerian-specifications. The large eddy simulation technique and the flamelet generated manifold approach combined with artificial flame thickening are used as turbulence and combustion models, implemented in the in-house code PsiPhi[2]. Accounting for nucleation and coagulation, the general dynamics equation of the particles is solved by a) a monodisperse moment model[3] and b) a polydisperse sectional model[4]. The moment model solves the transport equations for the particle number-, surface area and volume concentration, from which the particle properties are calculated. The sectional model solves transport equations for the particle number concentrations for several classes, representing different particle diameters. The unresolved turbulent fluxes are modelled by an eddy diffusivity approach[5].
The influence of turbulence on the PSD is discussed based on instantaneous and sample averaged results which are obtained from the monodisperse and polydisperse model. In contrast to the polydisperse sectional model, the SD predicted by the monodisperse moment model is only a result of turbulence. Furthermore, the studies highlight the differences of the particle properties and the particle size distributions predicted by the models and show the validity of the simple monodisperse model, for the investigated test case.

The authors gratefully acknowledge the financial support by the AiF (grant No. 18298N/3) and of the state NRW, as well as the Center for Computational Sciences and Simulation of the University Duisburg-Essen for the computational resources.

[1] L. Mädler, H. K. Kammler, R. Mueller, S. E. Pratsinis, J. Aerosol Sci. 33 369-389 (2002).
[2] A. Rittler, F. Proch, A. M. Kempf, Comb. Flame 162 1575-1598 (2015).
[3] F. E. Kruis, K. A. Kusters, S. E. Pratsinis, B. Scarlett, Aerosol Sci. Technol. 19 514-526 (1993).
[4] A. Prakash, A. P. Bapat, M. R. Zachariah, Aerosol Sci. Technol. 37 892-898 (2003).
[5] J. Loeffler, S. Das, S. C. Garrick, Aerosol Sci. Technol. 45 616-628 (2011).

The scale-up of the production of metallic nanoparticles can be done efficiently by means of arc discharge in an inert carrier gas. The use of many single production units in parallel, which can be thoroughly optimized and tested on a lab scale for a given material, ensures that a highly effective scale-up of the synthesis process in terms of cost and energy consumption is possible. We demonstrated the integration of this technology in the aerotaxy production line for solar cells, direct deposition of nanoparticles on textiles, nanocomposites, deposition of nanoparticles within periodic arrays for photonics, higher heat transfer with nanoparticle dispersions and direct deposition of catalytic nanoparticles on membrane structures.
This work addresses the problems which appear when modeling the particle formation from an atomic vapour, formed by plasma evaporation from a melt as in the case of arc discharge. Here more than only the particle volume is relevant, such as agglomerate structure and particle charge. The modeling of distributed multiple particle properties can be efficiently done by means of population-balance modeling (PBM) in the framework of a Monte-Carlo simulation. It will be shown how the problems with large simulation times can be overcome when the simulations are being done with help of GPUs which allow parallelization of the algorithms, shown e.g. for coagulation (Wei and Kruis, Chem. Eng. Sci. 104, 451–459, 2013). The modeling of particle formation initiated by a physically induced nucleation controlled by the local temperature, is especially challenging due a strong variation of the Kelvin diameter which makes it almost impossible to apply conventional discrete-sectional PBM.
Inclusion of nucleation in a Monte-Carlo approach is challenging as the number of simulation particles which can be used is limited. The application of weighted simulation particles offers here a practical solution. Keeping the number of simulation particles constant requires strategies to discard other simulation particles while keeping the loss of information to a minimum, the newly developed low-weight merging technique shows here the best performance. An extensive validation of Monte-Carlo PBM to the coupled nucleation-coagulation problem will be shown, based on a comparison with a discrete-sectional model. Another challenge is the modeling of condensational growth, here the continuous variation of the Kelvin diameter, induced by e.g. temperature variation or monomer depletion, has to be taken into account. Care has to be taken that particles smaller than the Kelvin diameter effectively evaporate while large ones grow.

Acknowledgment
This work has been financially supported by the European Unions Seventh Framework Program (EU FP7) under Grant Agreement No. 280765 (BUONAPART-E) and the Deutsche Forschungsgemeinschaft in the frame of the priority program SPP 1679.

Detailed simulations of the transport phenomena and the of particle dynamics have been conducted for a hot wall reactor. Simulations are essential for the scaling of laboratory experiments to pilot and industrial plants, as the similarity parameters can rarely be preserved.

Two modeling frameworks for the representation of the dispersed particle phase (Euler-Lagrange and Euler-Euler) are presented. Both models are compared and validated against data measured in a laboratory scale hot-wall reactor producing iron nanoparticles by pyrolysis of iron pentacarbonyl in a nitrogen atmosphere.
A Monodisperse model¹ for the population balance equation of the nanoparticles has been implemented into the open-source library OpenFOAM for both (Eulerian and Lagrangian) representations of the dispersed phase. Thermophoretic transport of the particle phase is described following Li and Wang²; particle diffusion is modeled as a diffusive flux or through a random walk. Concentrations of the gas phase are solved by transport equations, with a finite rate model describing chemical reactions. The Lagrangian and Eulerian implementations were individually tested against generic setups and later applied in simulations of a laboratory scale hot-wall reactor. The laminar flow field, simple reaction kinetics and the known wall temperature profile made the hot-wall reactor an ideal target for validation of the different models describing the physical effects and their impact on the major nanoparticle characteristics. The experimental setup allowed TEM probing and was additionally attached to a scanning mobility particle sizer system in parallel.
The comparison of the experimental data with both simulation approaches shows a good agreement in the mean particle size. Buoyancy effects show a crucial impact on the alignment and recirculation strength of the three dimensional flow field. Thermophoresis influences notably the particle size and distribution in the reactor. Applying diffusive random walk, the Lagrangian approach is capable to model a particle size distribution. Furthermore, rare big particles released from the recirculation zone of the reactor can be captured leading to a more realistic particle distribution. Therefore, it is pointed out that the Euler-Lagrange approach is more suitable for modelling nanoparticle formation in this context. Nevertheless, the Lagrangian model requires a higher computational effort, which makes the Euler-Euler approach attractive for parameter studies.

The work is supported by the European commission in the Horizon 2020 framework, project Nanodome (reference: 646121).

¹ Kruis, F. Einar, et al. A simple model for the evolution of the characteristics of aggregate particles undergoing coagulation and sintering. Aerosol science and technology, 1993, 19. Jg., Nr. 4, S. 514-526.
² LI, Zhigang; WANG, Hai. Thermophoretic force and velocity of nanoparticles in the free molecule regime. Physical Review E, 2004, 70. Jg., Nr. 2, S. 021205.

Bimetallic nanoparticles have gained significant commercial interest because of their superior electronic, chemical and plasmonic properties compared to the monometallic counterparts making them excellent candidates for many biomedical, sensory or catalytic applications. For example, gold nanoparticles that are used extensively in catalysis exhibit reduced electron transfer on Au(111) surface, hindering the adsorption of O₂. However, formation of gold-based bimetallic nanoparticles (e.g. with Ag) can enhance the affinity with O₂ compared to pure gold nanocatalysts. Furthermore, Ag nanoparticles exhibit remarkable antibacterial properties but are highly cytotoxic rendering their use as therapeutic agents challenging. Alloyed Ag-Au is an interesting example in biomedicine with potential in theranostic applications since addition of Au in Ag nanoparticles improves biocompatibility without destroying the antibacterial activity of nano-silver (Sotiriou et al., 2014).

Here, the evolution of surface composition of free-standing but coalescing Ag-Au nanoparticles is investigated for different particle sizes and temperatures by atomistic molecular dynamics (MD) simulations. The MD method is validated by the attainment of the melting point of Ag-Au core-shell nanoparticles that increases with increasing particle size and follows closely the trend of the size-dependent melting temperature of pure Au (Goudeli et al., 2016) and Ag nanoparticles (Buesser et al., 2015). Silver atoms exhibit increased mobility upon coalescence and occupy gradually the surface of the segregated particle, consistent with experiments. When Ag nanoparticles are sufficiently smaller than Au ones, a patchy Ag layer forms at the Au particle surface. Sintering of equally-sized Ag and Au nanoparticles results in the formation of segregated nanoparticles with Ag-enriched surface, consistent with the literature. The initial particle morphology affects the particle crystallinity: the X-ray diffraction patterns of Ag-Au nanoparticles calculated during sintering, reveal that even though core-shell configurations and segregated structures exhibit the characteristic peaks of pure Ag and Au, the alloyed nanoparticles exhibit only the (111) and (200) peaks.

References
Buesser, B., and Pratsinis, S.E. (2015). Morphology and Crystallinity of Coalescing Nanosilver by Molecular Dynamics. J. Phys. Chem. C, 119, 10116-10122.
Goudeli, E., and Pratsinis, S.E. (2016). Crystallinity Dynamics of Gold Nanoparticles During Sintering or Coalescence. AIChE J, 62, 589-598.
Sotiriou G A, Etterlin G D, Spyrogianni A, Krumeich F, Leroux J-C, and Pratsinis S E. (2014). Plasmonic biocompatible silver–gold alloyed nanoparticles. Chem. Commun. 50: 13559-13562.

A Gibbs free energy driven kinetic Monte Carlo model is developed for simulating vapor-phase homogeneous nucleation of metal nanoparticles (NPs). The model effectively accounts for monomer-cluster condensation, cluster-cluster collision, and cluster evaporation processes. The goal here is to develop a robust stochastic model that can capture the ensemble picture behind the rare event of metal NPs nucleating out of random collision events of clusters/monomers during typical high temperature gas-phase synthesis processes. The ability to capture the rarity of the event is made possible through an efficient pseudo sampling technique specifically developed to simulate the process without any stochastic bias. Specifically, we present results from the nucleation study of Aluminum nanoparticles evolving out of different initial temperatures, and cooling rates for the synthesis route. We validate the model by comparing our current KMC results with those from previously established phenomenological models developed within the framework of Gibbs’ free energy for cluster formation derived from Classical Nucleation Theory (CNT). The simulation results indicate the well-known entropic gain for clusters approaching the nucleation barrier, which culminates in the typical drastic entropic loss during first-order phase transitions due to NP formation. Furthermore, it is observed that the steady-state assumptions for nucleation rates of clusters are only valid for cluster sizes ≥10, below which the assumption fails to hold true as the clusters approach the onset of nucleation. The development of such elegant and high-fidelity KMC model with allow for easy incorporation of size-dependent surface tension (non-capillarity approximation), and non-isothermal nucleation into the model in future, thereby allowing the study of nucleation without any apriori assumptions.

INTRODUCTION: Current fabrication methods used to produce thin ceramic films include traditional tape casting, spin casting of precursor solutions or vapor deposition methods. For the most part, tape casting relies on doctor blading slips made from ceramic powders with particle sizes > 0.5 µm. Consequently, sintering to full densities generally leads to final grain sizes of 2-5 µm. This in turn limits the final film thicknesses typically of 40-50 µm simply to avoid having films just a few grains thick, which would make them quite susceptible to brittle failure and impede handling for further processing. In contrast spin-coating sol-gel and ceramic precursors can often provide uniform and sometimes epitaxial films (depending on substrate) but typically at thicknesses of just 1-5 µm and often only by repeated coating because of the very significant volume changes that occur as precursors transform to a dense ceramics. Vapor deposition is often used to process very high quality thin films, especially in the electronics industry but is equipment intensive and again thicknesses beyond about 5 µm are sometimes quite tedious to process. Thus, there is considerable need for rapid, facile and low cost routes to 5-40 µm thick dense (or porous) ceramic films that offer superior mechanical properties but also versatility in the types of ceramic materials that can be made.
We present here, a simple method of making such films using wire-wound roller coating methods to cast thin polymer/ceramic nanopowder composites that can be made as single layers or laminated to make multiply ceramic laminates that on sintering provide dense single oxide thin films, ceramic composite thin films and ceramic/metal composite thin films. Several examples will be discussed including superionic lithium ion conducting electrolytes and cathode materials as well as some novel oxide materials.
MATERIALS AND METHODS: All nanopowders used in these studies were produced using liquid feed flame spray pyrolysis as discussed in detail elsewhere1-3. Thin films of superionic Li1.7Al0.3Ti1.7Si0.4P2.6O12 with 5 % excess lithium (LATSP+5) were produced. Thin Film Preparation: Suspensions were cast using a wire wound rod coater on mylar. Dried green films were peeled off the Mylar substrate, and cut to 2.5 2.5 cm. Sintering: First binder burnout was done at 400°C/air. LATSP was crystallized at 660 °C/5 °C/min. Sintering was done at 1200°, 1180°, or 1140 °C/1 h/60 ml/min air flow.
RESULTS AND DISCUSSION: Multiple film types were prepared using this processing approach. Numerous formats exist for making both single thin films and laminates as will be discussed in the presentation. In particular, it is possible to start with mixtures of nanopowders and sinter thin films where the final composite and chemical composition evolve during densification.
CONCLUSIONS: The use of nanopowders to process thin films and the use of wire wound roller coating methods provide access to a wide variety of novel thin films including those that offer particular utility for producing all solid state batteries.

REFERENCES
1. E. Yi, J. Furgal, J. Azurdia, R. M. Laine,” Roll Your Own – Nano-Nanocomposite Capacitors,” J. Chem. Mater. A. 2014, 2 3766-3775.
2. N. J. Taylor, A. J. Pottebaum, V. Uz, R. M. Laine, “The bottom up approach is not always the best processing method. Dense α-Al2O3/NiAl2O4 composites,” Adv. Functional Mater. 2014, 24, 3392–3398.
3. E. Yi, W. Wang, S. Mohanty, J. Kieffer, R. Tamaki, R. M. Laine, “Materials that can replace liquid electrolytes in Li batteries: Superionic conductivities in Li1.7Al0.3Ti1.7Si0.4P2.6O12. Processing combustion synthesized nanopowders to free standing thin films,“ J. Power Sources 2014, 269 577-588.

In recent years, rare earth zirconates have been rigorously investigated due to their superior thermophysical properties, including low thermal conductivity, high thermal stability, and high melting point [1] and [2]. These materials can effectively protect metal parts from extreme high temperature environment in gas turbines and diesel engines. Among numerous rare earth zirconates, pyrochlore-type La₂Zr₂O₇ is considered as one of the most preferred candidate materials for thermal barrier coatings (TBCs) application. It has a high melting point of 2300 °C, a low thermal conductivity of 1.56 W m^-1 K^-1, and high phase stability [1] and [3]. La₂Zr₂O₇ has been synthesized via a number of methods, such as sol-gel, co-precipitation, solid state reaction, hydrothermal reaction, and solution plasma spray [4], [5], [6], [7] and [8]. In this study, pyrochlore-type La₂Zr₂O₇ nanocrystalline coating is successfully synthesized and deposited via a novel flame based method, namely reactive spray deposition technology (RSDT). This alternative synthesis route of La₂Zr₂O₇ is a single-step continuous process, providing the advantage of low cost, simplicity and scalability. A precursor solution of lanthanum acetylacetonate and zirconium acetylacetonate in organic solvents is feed through an atomization nozzle and the resulting vapor spray is ignited to generate a turbulent flame. The precursors rapidly decompose in the high temperature flame, followed by phase transition to vapor and homogeneous reactions to form oxides. X-ray diffraction (XRD) indicates that a single phase La₂Zr₂O₇ with pyrochlore structure is obtained. The microstructure of as-prepared coating is investigated with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The nanocrystalline La₂Zr₂O₇ coating deposited by RSDT demonstrates a low thermal conductivity.

References:
[1] R. Vassen, X. Cao, F. Tietz, D. Basu, and D. Stöver, 83 (2000), pp.2023-2028.
[2] Z.G. Liu, J.H. Ouyang, Y. Zhou, and X.L. Xia, Electrochim. Acta, 54 (2009), pp.3968-3971.
[3] S. Yugeswaran, A. Kobayashi, P.V. Ananthapadmanabhan, and L. Lusvarghi, Curr. Appl. Phys., 11 (2011), pp. 1394-1400.
[4]D.R. Chen, and R.R. Xu, Mater. Res. Bull., 33 (1998), pp. 409-417.
[5] H.F. Chen, Y.F. Gao, Y. Liu, and H.J. Luo, J. Alloys Compd., 480 (2009), pp. 843-848.
[6] Y.P. Tong, J.W. Zhu, and L.D. Lu, J. Alloys Compd., 465 (2008), pp. 280-284.
[7] J. Wang, S.X. Bai, H. Zhang, and C.R. Zhang, J. Alloys Compd., 476(2008), pp. 89-91.
[8] X.Q. Ma, F. Wu, J. Roth, M. Gell, and H.E. Jordan, Surf. Coat. Technol., 201 (2006), pp. 4447-4452.

Nanostructured materials have the potential to significantly enhance the performance of several devices as recently demonstrated for solar cells, sensors and energy storage technologies. This has resulted in a rush toward novel applications ranging from flexible electronics to wearable nanogenerators. However, integration of nanomaterials in devices is challenging and their assembly in suboptimal morphologies may drastically limit the final performance. Here, we will present the fabrication of highly performing optoelectronic devices by integrated ultraporous nanoparticle networks. We will showcase the use of scalable and low cost flame-synthesis approaches for the wafer-level nanofabrication of tailored and well-reproducible 3D morphologies. The fundamental mechanisms controlling the gas-phase self-assembly of these nanostructures will be discussed with respect to current limitations and future opportunities. In particular, the rapid high-temperature gas-phase fabrication of highly performing photodetectors for quantitative monitoring of UV radiation will be presented [1]. We will demonstrate a rapid approach for the synthesis of tunable band-selective UV photodetectors capable of detecting very low light intensities with record-high signal to noise ratios and very low power-consumptions. These results provide a robust set of guiding principles for the design of wearable electronic devices for personalized medicine and environmental monitoring.

[1] N. Nasiri, R. Bo, F. Wang, L. Fu, A. Tricoli, Ultraporous Electron-Depleted ZnO Nanoparticle Networks for Highly Sensitive Portable Visible-Blind UV Photodetectors, Adv. Mater., 27, 336-4343 (2015)

Lead-free nanogenerators made with BaTiO₃ are attractive for piezoelectric energy harvesting solutions in medical devices. Polycrystalline BaTiO₃ nanoparticles of various sizes with average diameter from 20 to 50 nm were made by hydrogen-driven Flame Spray Pyrolysis (FSP) and embedded into thin poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) films. In these nanoparticles the tetragonal phase and a high c/a lattice ratio were maintained, despite having small crystal sizes of only 10 nm. Piezoelectric nanogenerators were assembled using these nanocomposite films as well as films containing commercial, rather mono-crystalline particles of 64 and 278 nm in diameter. The time-averaged output voltage of nanogenerators containing as-prepared flame-made nanoparticles was 1.4 V, almost twice as that of commercial ones. That output was maintained stable for over 45’000 cycles with each cycle corresponding to a heartbeat of 60 bpm. Most likely, the high performance of these nanogenerators was facilitated by a higher degree of domain orientation upon poling these polycrystalline flame-made nanoparticles.

We propose a method for the elaboration, in a single step and in a confined chamber, of composite coatings made of nanoparticles embedded in a matrix. The process combines a beam of nanoparticles with Physical Vapor Deposition. The association of the two techniques is made possible by routing nanoparticles by aerodynamic means to the substrate, either immediately after their synthesis in the gas phase, or from atomized colloidal suspensions. The simultaneous deposition of the particles and the matrix is performed on the same substrate. The process allows a virtually unlimited selection in the respective chemical compositions of nanoparticles and the matrix, and a moderate temperature of the substrate. Different source types of nanoparticles are possible. A laser-driven pyrolysis reactor can be used for the in-situ synthesis. Laser pyrolysis is an efficient method to synthesize various high purity nanopowders, oxides and non-oxides, in a gas phase bottom-up approach. An atomizer that produces an aerosol from colloidal suspensions of previously synthesized nanoparticles can alternatively be used. The particle stream is formed by means of an aerodynamic lens system. This system is currently used to produce a collimated beam of particles under vacuum for further gas phase characterization or for precision 3D micro printing. It allows for long-term stable and high transmission of particles in a wide range of size and density. We show that it is possible to get an angle-controlled divergent beam of nanoaerosols by optimizing the geometry of a classical lens. In this way, homogenous deposition of nanoparticles is performed on large areas. We demonstrate the adaptation if the technique with pressure environment required for running a classical magnetron sputtering device. The later is used for depositing the material constituting the matrix of the composite film. The deposition of a large variety of materials is conceivable by this means. The possibility to elaborate large and homogenous nanostructured films were investigated with different types of nanoaerosols with different sizes and densities. Numerous application domains are already considered for this kind of nanostructured coatings, including photovoltaic, photocatalysis, aesthetic coatings, hard covering, biomedical and self-healing films. The development of the process is carried out in the frame of the HYMALAYAN project funded by the French Research National Agency (ANR) under Grant No ANR-14-CE07-0036. It is open to new potential applications.

Well ordered multidimensional multiscale nano/microstructures could provide a platform for various novel nanodevices including sensors, electronic/magnetic devices and solar/fuel cells. In this presentation, we introduce an aerosol assembly technique for fabricating multidimensional multiscale nano/microstructures and their applications to 3D (three dimensional) gas sensors, 3D SERS(surface enhanced Raman spectroscopy) substrate, 3D nanostuctured solar cells, etc. Ion Assisted Aerosol Lithography (IAAL)¹ or Electric-field Assisted Aerosol Lithography(EAAL) that we developed has been further developed to enable multi-furcation assembly of nanoparticles and even dynamic 3D printing of nanoparticles for manufacturing exotic 3D nanoparticle structures that could not be made by other existing methods. In addition, new ways of increasing the rate of charged nanoparticle generation will be also discussed for scaling-up aerosol assembly process.
[1] Kim, H., Kim. J., Yang, H., Suh, J., Kim, T., Han, B., Kim, S. and Choi, M. (2006) Nat. Nanotech. 1(2), 117-121.

Lung cancer is one of the major health problems of modern society with predicted 1.8 million newly diagnosed cases per year (2012).¹ The early detection of lung cancer could significantly improve medical therapy and thus reduce morbidity and mortality rates. Formaldehyde (FA) is a potential breath marker for lung cancer.² It typically occurs below 100 parts-per-billion (ppb) together with other gases at higher levels (e.g. acetone, NH₃, etc.) posing a sensitivity and selectivity challenge to current sensors. This can be overcome by combining broadly sensitive but differently selective sensors in an electronic noses (E-nose), a bio-inspired approach mimicking the human olfactory system³ with its array of receptors.⁴
Here, we present a highly sensitive, selective and compact E-nose for real-time quantification of FA at realistic conditions.⁵ This E-nose consists of four nanostructured and highly porous Pt-, Si-, Pd- and Ti-doped SnO₂ sensing films directly deposited onto silicon wafer-based microsubstrates by flame spray pyrolysis (FSP). The constituent sensors offer stable responses and detection of FA down to 5 ppb (signal-to-noise ratio > 30) at breath-realistic 90% relative humidity. Each dopant induces different analyte selectivity enabling selective detection of FA in 2- and 4-analyte mixtures by multivariate linear regression. In simulated breath (FA with higher acetone, NH₃ and ethanol concentrations), FA is detected with an average error ≤ 9 ppb using the present E-nose and overcoming selectivity issues of single sensors. That way, exhaled FA levels of lung cancer patients (~80 ppb) can be distinguished from healthy ones (~50 ppb).² This E-nose can be readily incorporated into a portable breath analyzer⁶ that could facilitate easy and inexpensive screening of lung cancer patients.


1. J. Ferlay, I. Soerjomataram, R. Dikshit, S. Eser, C. Mathers, M. Rebelo, D. M. Parkin, D. Forman and F. Bray, Int. J. Cancer, 2015, 136, E359-E386.
2. P. Fuchs, C. Loeseken, J. K. Schubert and W. Miekisch, Int. J. Cancer, 2010, 126, 2663-2670.
3. K. Persaud and G. Dodd, Nature, 1982, 299, 352-355.
4. S. Firestein, Nature, 2001, 413, 211-218.
5. A. T. Güntner, V. Koren, K. Chikkadi, M. Righettoni and S. E. Pratsinis, ACS Sens., 2016, 1, 528-535.
6. M. Righettoni, A. Ragnoni, A. T. Güntner, C. Loccioni, S. E. Pratsinis and T. H. Risby, J. Breath Res., 2015, 9, 047101.

Heterogeneous gas-phase condensation is a promising method of producing hybrid multifunctional nanoparticles and alloys with tailored composition and microstructure but also intrinsically introduces greater complexity to the nucleation process and growth kinetics. Herein, I will talk about the synthesis and growth modeling of multi-metallic nanoparticles using gas-aggregated co-sputtering from different but neighboring elemental source targets. I will demonstrate that we are able to control the size, chemical composition, microstructure and shape of these nanoparticles. The nanoparticles are integrated directly into a smart gas sensing platform for mobile device.

References:
1-Engineering high-performance Pd core-MgO porous shell nanocatalysts via heterogenous gas-phase synthesis .V. Singh, C. Cassidy, F. A.-Pedersen, J. -H. Kim, K. Aranishi, S. Kumar, C. Lal, C. Gspan, W. Grogger, and M. Sowwan Nanoscale 7 (2015) 13387-13392.

2-Heterogeneous Gas-phase Synthesis and Molecular Dynamics Modeling of Janus and Core-satellite Si-Ag Nanoparticles V. Singh, C. Cassidy, P. Grammatikopoulos, F. Djurabekova, K. Nordlund, and M. Sowwan J Phys Chem C 118 (2014) 13869-13875.

3-Steinhauer, S., V. Singh, C. Cassidy, C. Gspan, W. Grogger, M. Sowwan and A. Köck (2015). "Single CuO nanowires decorated with size-selected Pd nanoparticles for CO sensing in humid atmosphere." Nanotechnology 26(17): 175502.

Chemoresistive gas sensors based on metal oxide nanoparticles are capable of analyte detection down to the ppb-level.¹ Such sensors prepared in particular by direct deposition of flame-made nanoparticles exhibit outstanding performance because of, for example, the wide selection of high purity nanomaterials and their assembly into porous, crack-free films.² Nevertheless, the existing fabrication technology is based on a three-step process in which nanoparticle deposition is frequently followed by in situ annealing and calcination. These later two steps compact and stabilize the nanoparticle film² and ensure the measurability of a low-noise resistance-based response. The current method, furthermore, in adequately investigates the influence of deposited material amount on the sensor performance.
This work extends the employed technology by introducing a single-step flame deposition technique of nanoparticle-based gas sensors in which the film thickness is precisely tuned during synthesis. The as deposited SnO₂ nanoparticle films attain a more compact morphology than conventionally obtained³ with this technology. Tuning the film morphology enables a direct correlation with the sensor performance. Furthermore, the in situ resistance,⁴ measured during deposition, gives direct insight into the stage of film formation: Upon forming an interconnected, percolating particle film the resistance drops and then oscillates around a constant value during prolonged deposition. As a consequence the least amount of deposited material, and therefore thinnest percolating nanoparticle film, is easily distinguished. Such thin and minimalistic gas sensors are compared to such prepared with superfluous amounts of deposited material in respect to their performance.


1. Righettoni M, Tricoli A, Gass S, Schmid A, Amann A, Pratsinis SE. Breath acetone monitoring by portable Si:WO₃ gas sensors. Anal. Chim. Acta. 738(0), 69-75 (2012).
2. Tricoli A, Graf M, Mayer F, Kuhne S, Hierlemann A, Pratsinis SE. Micropatterning layers by flame aerosol deposition-annealing. Adv. Mater. 20(16), 3005-3010 (2008).
3. Mädler L, Roessler A, Pratsinis SE, Sahm T, Gurlo A, Barsan N, Weimar U. Direct formation of highly porous gas-sensing films by in-situ thermophoretic deposition of flame-made Pt/SnO₂ nanoparticles. Sensor Actuat. B-Chem. 114(1), 283-295 (2006).
4. Blattmann CO, Pratsinis SE. In situ measurement of conductivity during nanocomposite film deposition. Appl. Surf. Sci. 371, 329-336 (2016).

Flame aerosol reactors (FLAR), which are an industrially successful route to synthesize nanoparticles, provide an alternate choice for designing and fabricating nanostructures in a one-step and scalable process. However, due to the complex interactions among different process parameters, such as the precursor concentration, time-dependent flame temperature profile, chemical reaction rate, particle growth rate, coagulation rate, and sintering rate, it is truly hard to relate process conditions to the nanoparticles’ structures (except particle size) obtained from one-step FLARs. A depth understanding of flame process will be beneficial for the design of multifunctional nanostructures.
Our groups focus on nanostructures synthesized and scale-up via flame process for decades. By adjusting the temperature, concentration, and residence time distribution in the flame, various novel nanomaterials such as TiO₂@SnO₂, SnO₂@TiO₂, Ag@SiO₂ core-shell nanostructures, Fe₂O₃||SiO₂ Janus-like structures has been achieved via a flame process.
In this work, flower-like SiO₂/SnO₂ architectures composed of SiO₂ core and SnO₂ nanorods shell were successfully fabricated by a simple flame synthesis method. The presence of water in precursor and the completely immiscible nature of SiO₂ and SiO₂ in solid phase played a major role in the formation of the 3D architectures. After the SiO₂ was removed by HF, the flower-like SnO₂ with hollow core exhibited high sensitivity and rapid response and recovery toward ethanol gas. We demonstrated a scalable flame approach for the fabrication of 3D SiO₂/SnO₂ architecture which shows potential applications in many fields beside gas sensors and lithium batteries. Through the growth mechanism investigation, many other 3D heterostructures can be synthesized in flame by a similar route, which is now under our studying.

This talk will introduce two projects involving high-temperature aerosol synthesis of nanoparticles for use in on-demand hydrogen generation and in hydrogen purification, respectively. In the area of hydrogen generation, we have recently shown that boron nanoparticles, in the presence of catalytic quantities of an alkali metal or metal hydride, can react with water at room temperature to generate hydrogen. This provides a very high energy density source of hydrogen for use in portable fuel cell applications. In our laboratory, we have used laser pyrolysis of diborane gas in hydrogen to produce these loosely agglomerated amorphous boron nanoparticles of 10 to 15 nm diameter. For commercial application, lower-cost production from less expensive precursors will be necessary. We are therefore currently exploring alternative production methods employing boron powder as a precursor. We will discuss both our current fundamental understanding of the particle production and hydrogen generation processes as well as the commercial potential of these materials for on-demand hydrogen generation. This talk will also introduce our work on production of palladium-silver-copper alloy nanoparticles by the flame-based high-temperature reducing jet process. These palladium alloy nanoparticles are being incorporated into polymer thin-film membranes for hydrogen-carbon dioxide separation. The nanoparticles provide high specificity of hydrogen transport through the nanocomposite (mixed-matrix) membranes. The nanoparticles are produced from aqueous precursors injected into the hot post-combustion gases of a fuel-rich hydrogen flame, within a converging-diverging nozzle. High-temperature aerosol production of these particles allows control of composition in ternary alloys that cannot be achieved by solution-phase methods. It also provides high purity materials without any organic ligands attached to their surface. However, further surface functionalization of the particles is required for incorporation into nanocomposite membranes and for controlling aggregation during membrane fabrication. In both of these projects, high-temperature aerosol synthesis is an enabling technology for producing nanomaterials that are inaccessible by other approaches.

In contrast to materials properties particle characteristics such as size, specific surface area, degree of agglomeration or crystallinity can be directly controlled by the synthesis process and also determined experimentally without further processing. Although they only indirectly influence the (extrinsic) materials properties such as processibility or charge carrier mobility and life time, they are key parameters to optimize device performance in applications.

Titania is a model material for the Chemical Vapor Synthesis (CVS) process as synthesis is possible over a wide range of process parameters and particle characteristics are readily available by experimental and theoretical methods. Process parameters, especially the time-temperature-profile, control the nanoparticle characteristics and can be adjusted in CVS using a multizone induction furnace [1, 2].

We discuss a new method to find paths to optimized nanoparticle characteristics in CVS. As examples, we predict physically realistic, optimal time-temperature profiles for a
- minimized degree of agglomeration at a desired primary particle size and
- maximized degree of crystallinity at a desired specific surface area
integrating a simple model [3] describing particle formation and growth into a Monte Carlo optimization algorithm for the case of TiO2.

[1] R. Djenadic and M. Winterer, chapter 2, Chemical Vapor Synthesis of Nanocrystalline Oxides, in A. Lorke, M. Winterer, R. Schmechel, and C. Schulz, Nanoparticles from the Gas Phase, Springer 2012
[2] R. Djenadic and M. Winterer, Control of nanoparticle agglomeration through variation of the time-temperature-profile in chemical vapor synthesis, submitted 2016
[3] M. Winterer, Nanocrystalline Ceramics - Synthesis and Structure. Springer, Berlin 2002

Palladium subnano-clusters (< 1 nm) on TiO₂ nanoparticles are prepared in one step by flame aerosol technology.¹ Under solar light irradiation, these materials remove NO_x 4 or 9 times faster than commercial TiO₂ (P25, Evonik) with or without photodeposited Pd on it.² X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared fourier transform spectroscopy (DRFTS) reveal that such photodeposited Pd consists of metallic Pd along with several Pd oxidation states. In contrast, flame-made Pd subnano-clusters on TiO₂ dominantly consist of an intermediate Pd oxidation state between metallic Pd and PdO. In that intermediate state, the Pd subnano-clusters are stable up to, at least, 600 ^oC for 2 hours in air. However, a fraction of them is reduced into relatively large (> 1 nm) metallic Pd nanoparticles by annealing in N₂ at 400 ^oC for 2 hours, as elucidated by XPS and scanning transmission electron microscopy. The Pd subnano-clusters interact with oxygen defects on the TiO₂ surface as shown by Raman spectroscopy, analogous to strong metal support interactions (SMSI) of nano-sized noble metals on TiO₂.

1. Fujiwara, K.; Müller, U.; Pratsinis, S. E., Pd subnano-clusters on TiO₂ for solar-light removal of NO. ACS Catal. 2016, 6 (3), 1887-1893.
2. Fujiwara, K.; Pratsinis, S. E., Atomically dispersed Pd on nanostructured TiO₂ for NO removal by solar light. AIChE J (Special issue for ISCRE24) 2016, In review.

Aerosol gels are a novel class of materials with potential to serve various energy and environmental applications. Synthesis of aerosol gel materials in gas-phase could be a cost-effective alternative to the conventional wet sol-gel process. Past studies have shown nanoparticle gelation occurring in a down-fired, buoyancy-opposed methane flame aerosol reactor (mFAR) as a viable technique for scalable production of gel materials. However, this gas-phase technique has yet to be proved capable of synthesizing gels with tunable material properties. In this talk, we present the results of our experiments specifically aimed at controlling the primary particle properties of gels using a mFAR. We were able to optimize the operating temperature conditions of the mFAR between 1250 and 590 °C by adjusting the oxidizer dilution ratio. At these two extreme temperature conditions, we delivered Titanium tetraisopropoxide (precursor) to the FAR for producing carbon-free titanium dioxide (TiO₂) gel particles. Our results show approximately tenfold increase in the primary particle size of the TiO₂ aerosol gel with the increase in the flame temperature. Pure rutlile and anatase phase TiO₂ were formed at the high and low temperature conditions, respectively. Our work demonstrates that non-carbonaceous gel materials can be synthesized through a rapid single-step gas-phase process with high purity and tunable material properties.

Titania is one of the bulk materials produced industrially by flame synthesis for pigment or photocatalytic applications. Therefore, TiCl₄ is evaporated and introduced into premixed or diffusion flame reactors [1, 2]. An alternative production process is flame spray pyrolysis (FSP) that circumvents pre-evaporation of the precursor. Steep temperature gradients and very short particle growth times in the flame make it attractive for synthesis of nanoparticles e.g. for catalytic applications [3].
Here, this process is given a closer look. Detailed X-ray diffraction, transmission electron microscopy and Raman spectroscopy analyses of FSP product powders not only revealed an amorphous fraction but also monoclinic titania TiO₂(B) in addition to the expected anatase and rutile phases. Monoclinic titania was first identified by Marchand et al. [4] in 1980 as a polymorph and has since then received much attention for photocatalysis [5] and lithium ion batteries [6] due to enhanced activity and high Li ion mobility, respectively.
The effect of reactor operating conditions as well as process scale-up on the titania phase composition and especially the monoclinic fraction is investigated here. Photocuring of epoxy is used as a test reaction to demonstrate enhanced performance of powders with monoclinic content. Such nanoparticles are compared to commercial titania P25 (Evonik) under UVA radiation. FSP-made titania with approximately 300 m2/g specific surface area exhibits the highest photocatalytic activity, almost 6 times higher than P25.

[1] Buxbaum G, Pfaff G (2005), Industrial inorganic pigments, 3^rd ed., WileyVCH, Weinheim.
[2] Pratsinis S.E. (1998), Flame aerosol synthesis of ceramic powders, Prog. Energy Combust. Sci. 24, 197-219.
[3] Strobel R, Baiker A, Pratsinis SE (2006), Aerosol flame synthesis of catalysts. Adv. Powder Technol. 17 457-480.
[4] Marchand R, Brohan L, Tournoux M (1980), TiO2(B) a new form of titanium dioxide and the potassium octatitanate K2Ti8O17. Mater. Res. Bull. 15: 1129-1133.
[5] Yang D, Liu H, Zheng Z, Yuan Y, Zhao J, Waclawik E R, Ke X, Zhu H (2009), An efficient photocatalyst structure: TiO2(B) nanofibers with a shell of anatase nanocrystals. J. Am. Chem. Soc. 131: 17885-17893.
[6] Fehse M, Ventosa E (2015), Is TiO2(B) the future of titanium-based battery materials?, Chem. Plus Chem. 80, 785-795.

We review our progress in the area of Sustainable Mobility and Energy Production, that has been enabled exploiting advances in Aerosol Based Nanotechnology (ABN). Highly compact, multifunctional reactors are developed for applications in Automotive Emission Control and the technology is further extended and cross-fertilized in the area of Solar Thermochemical Reactors for the production of Carbon-Neutral Fuels using exclusively renewable/recyclable raw materials. Aerosol Based Synthesis and Deposition occurs in an integrated fashion in a dedicated pilot plant that combines aerosol spray generation, thermal treatment with a variety of sources (electric, burner and plasma) and deposition on structured substrates.
Catalytically coated ceramic monolithic reactors such as wall-flow Diesel Particulate Filters are the most complex component of today’s emission control systems as they need to incorporate different and often conflicting functionalities such as high soot nanoparticle filtration efficiency, low pressure drop behavior, direct catalytic soot oxidation activity, high oxidation activity for Carbon Monoxide, Hydrocarbons and Nitrogen Oxide, as well as ability to reduce Nitrogen Oxides. We show how ABN can address these challenges and demonstrate a multifunctional reactor for Diesel Emission Control that exhibits significantly better performance with respect to state of the art. An important spin-off activity from this research is the concept of a functionalized solar thermochemical reactor for the production of Carbon-Neutral Fuels. These solar fuels, can be synthesized from Hydrogen and Carbon Monoxide produced by solar thermochemical Water and Carbon Dioxide (CO₂) splitting respectively, opening the door to the treatment of CO₂ as a raw material, rather than as a waste to be disposed. The advent of Carbon-Neutral Solar Fuels along with efficient Multifunctional Reactors for emission control is a promising route for the transition to a sustainable and clean future with only minimal changes in the existing fuel infrastructure and automotive technology.

The delivered nanoparticle dose to cells in vitro may depend on nanoparticle sedimentation rate. Here, the conditions under which optical absorption spectroscopy can be used to quantify the deposited nanoparticle dose in vitro are investigated. Nanoparticle cytotoxicity in both upright and inverted cell culture orientations is studied in presence and absence of serum. Dissolvable nanoparticles, such as ZnO, exhibit no difference in upright and inverted cultures due to dissolved Zn2+ ions that dominate cytotoxicity. Insoluble nanoparticles, however, exhibit different sedimentation rates and deposited doses that are linked to the observed cytotoxicity. The combined use of upright-inverted cell orientations and optical absorption spectroscopy can provide a simple experimental approach to interpret in vitro nano-biointeractions.

Ethene is an important feedstock in chemical industry used for the synthesis of polyethylene, ethylene dichloride, ethylene oxide and monoethylene glycol. It is typically produced by steam cracking of naphtha and ethane which requires high temperatures leading to severe coke deposition on the reactor walls, thus requiring periodic cleaning [1]. Catalytic oxidative dehydrogenation of ethane (ODHE) can serve as an alternative to this process offering lower reaction temperature (< 800 °C) and higher ethene selectivity (> 70%). Although the reaction can be performed at much lower temperatures (< 500 °C) using O₂ as an oxidant, ethene selectivity is reduced due to the strong oxidizing nature of this oxidant [2]. Therefore, the use of a milder oxidant (e.g. CO₂) is preferred even though it requires higher reaction temperatures (> 600 °C). In this study, a wide range of single and mixed oxides supported cobalt catalysts were produced using a single step flame method and tested for the ODHE reaction [3]. The performance of the catalysts was strongly affected by the support type which could be attributed to the different structural and chemical properties of the catalysts characterized by various methods. Among all the supported catalysts, SiO₂-, ZrO₂- and TiO₂-ZrO₂-supported cobalt catalysts showed similar ethene yield (~24%). However, in-depth analysis of these catalysts revealed that the performance of SiO₂-supported catalysts is superior (ethene yield ~ 34%, under non-optimized conditions) compared to others. Through material characterization, it was revealed that cobalt is present in different oxidation states depending on the support type indicating occurrence of different reaction pathways. More specifically, a non-redox reaction pathway is expected in SiO₂-supported catalyst where Co²⁺ is embedded in the SiO₂ matrix, while a redox mechanism (over CoO_x species) is proposed to be the dominant pathway over ZrO₂- and TiO₂-ZrO₂-supported catalysts.

References:
1. Ren, T., Patel, M., Blok, K. Energy, 31, 425 (2006).
2. Koirala, R., Buechel, B., Krumeich, F., Pratsinis, S.E., Baiker, A. ACS Catal.,5, 690 (2015).
3. Koirala, R., Buechel, B., Pratsinis, S.E., Baiker, A. In preparation (2016).

Nanostructured materials are gaining widespread use, which requires new approaches to powder synthesis, in particular with respect to increased production while maintaining proper safety procedures. Flame spray pyrolysis (FSP) is an excellent tool for pioneering development of complex nanomaterials for various applications and at the same time being a scalable process already being investigated by commercial powder producers [1]. Such nanomaterials are of interest for electrodes in various energy applications e.g. PEM fuel cells where high conductivity, high surface area, well defined and sustainable pore structure/size distribution, stability and corrosion resistance are required material properties [2, 3].

In the present work flame spray pyrolysis (FSP) is applied to produce and investigate properties of tin-based oxide materials for use as cathode catalyst support in PEM fuel cells, offering high stability and corrosion resistance for this application as compared to carbon [4-6]. SnO₂-based materials are already reported synthesized by FSP in the literature for the use in gas sensor applications [7-9]. In this work it is elaborated on using antimony and niobium as dopants (Sn_1-xM_xO₂, x=0.00-0.15, M=Sb/Nb). Sb is introduced to enhance the electronic conductivity, required for the support material, while Nb is added with the intention of supressing the segregation of Sb to the surface, as observed when in contact with Pt-catalyst particles [10].

Pt-catalyst is applied to the SnO₂-based support material through:
i) Well-established polyol-based Pt deposition route
ii) Single-step FSP synthesis

In addition to thorough physical characterization (XRD, BET, TEM) the materials are investigated with respect to electrical conductivity as well as oxygen reduction reaction (ORR). A general comparison of the two methods for preparing tin oxide supported Pt catalysts is given, in addition to the evaluation of observed effects of Sb and Nb-doping and the interaction of these elements with the Pt-catalyst particles.

Acknowledgement
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 #325327 (SMARTCat project).

References
1. Teoh, W.Y., Nanoscale, 2010. 2(8): p. 1324-1347.
2. Sharma, S., J. Power Sources, 2012. 208: p. 96-119.
3. Rabis, A., Acs Catalysis, 2012. 2(5): p. 864-890.
4. Takasaki, F., J. Electrochem. Soc., 2011. 158(10): p. B1270-B1275.
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Catalytic property of platinum group metal (PGM) nanoparticles can be altered significantly by the nature of facets exposed. For example, Pt (100) plane excels in activity/selectivity compared to other planes in many reactions, including ring-opening hydrogenation of pyrroles, benzene hydrogenation, methanol oxidation (MOR), and electro-oxidation of ammonia (AOR). Pt-Ni (111) surface can exhibit exceptionally high activity towards oxygen reduction reaction (ORR) for polymer electrolyte membrane fuel cells (PEMFCs). The findings have stimulated the exploration of new methods for preparing PGM catalysts with tailored particle morphology, because the usage of PGMs and thus the cost can be largely decreased. However, to date, there have been no feasible methods for cost-effective and mass production of the shaped PGM nanoparticle catalysts.
We realize scalable production of PGM nanoparticle catalysts with tailored particle morphology by developing a green and low manufacturing cost impregnation approach. A variety of shaped PGM nanoparticles, for instance cubic Pt/SiO₂, cubic Pt/C, tetrahedral Pd/C, octahedral Pt-Ni/C, and cubic Pt-Cu/C, have been demonstrated for preparation. The experiments suggest that the formation of shaped PGM nanoparticles is resultant of employing both CO and H₂ gases, wherein H₂ aids transportation and reduction of the metal precursors on support and CO is responsible for the particle morphology formation. Several catalytic reactions, including preferential CO oxidation, hydrazine decomposition, ORR, and AOR, have been studied using the prepared catalysts. The octahedral Pt_1.5Ni/C catalyst exhibits high ORR activities of 3.99 mA/cm² Pt and 1.96 A/mg Pt at 0.90 V vs. RHE, which are about 20 and 10 times the values for commercial Pt/C specifically. The cubic Pt/C catalyst shows 1.44 mA/cm² at 0.6 V vs. RHE in AOR, which is five times that of 0.30 mA/cm² using commercial Pt/C.

Liquid Flame Spray (LFS) method utilizes hydrogen-oxygen flame to synthesize nanoparticles . Liquid precursor is sprayed into a turbulent flame, where the droplets evaporate, vapor decomposes and the reaction product re-condenses into product species. This process generates well-defined nanoparticles which can be collected as a nanopowder or sprayed on surface to fabricate large area functional coatings. In the process, the particle formation ideally follows the gas-to-particle aerosol route. However, due to residues of the sprayed droplets, a fraction of the original material may be lost to form larger particles via droplet-to-particle route. Thus, homogeneity of the product suffers. The residual particles are an issue, especially, with some inexpensive metal nitrate precursors. However, according to the recent literature, the amount of the residual particles can be substantially reduced by adding 2-ethylhexanoic acid (EHA) to the nitrate precursor solvents. Here, we utilize modern aerosol diagnostics to follow on-line the effect of adding EHA to the nitrate based precursor, to find minimum threshold concentration for EHA by which residuals can still be avoided. With e.g. aluminum nitrate, the residual mode dominated the mass size distribution of the particles produced without EHA. However, an EHA volume concentration of 5 % was enough to shift the mass practically entirely to the nanoparticle mode. In the presentation, the overall characteristics of the Liquid Flame Spray method for generating nanopowders and coatings will be presented. The consequences of successfully avoiding the residual particles for the benefit of various applications of the method, are further discussed.

We show that engineered silicone coatings become superhydrophobic after sufficient exposure to heat. The partial thermal degradation of the silicone by high temperatures (>500C, provided by a torch) creates porosity at the surface and beneath, and deposits hydrophobic nanostructured silicone “soot” on the surface. The coatings can be generated and regenerated (a 1.6-mm-thick silicone coating can be regenerated more than 20 times) in situ at a rate of >1m² min^-1 with a propane torch (much faster than self-healing methods), and strongly adhere to flexible, curved, and rough surfaces of diverse compositions. Straightforward and scalable process modifications yield superhydrophobic/superhydrophilic patterns and oleophobicity. The deep surface structuring caused by thermal processing creates a surface that remains porous after moderate wear. Subsequent regeneration of the surface yields a hierarchical texture that displays drastically increased wear resistance and tolerates foot traffic (>1,000 steps by a 75Kg person). By comparison, a state-of-the-art commercial solution (Neverwet®) is shown to be ineffective after 100 steps.
We will further show preliminary work that describes in more detail the process of silicone nanosoot formation that is at the heart of this processing strategy. The work will also establish the limits of the technique in terms of speed of processing and temperature.
This comprehensive strategy to address simultaneously the many challenges towards the implementation of superhydrophobic surfaces in large area outdoor applications, including easy in situ regeneration, resilience to weathering and wear, favorable environmental compatibility, and the possibility to turn the surfaces into SLIPS which avoid irreversible transitions to a Wenzel state and yields oleophobicity. The tunability of the chemical and physical properties of silicones by chemical modification or introduction of fillers, should provide avenues towards self-healing schemes which would make active surfaces such as these even more suitable for large scale outdoor applications.

References: Thermal Processing of Silicone Coatings for Green, Scalable, and Healable Superhydrophobic Surfaces
X. Tian, S. Shaw, K. R. Lind, Ludovico Cademartiri*
Advanced Materials, 2016, 19, 3677-3682

In this work we report an interdisciplinary effort which integrates high-temperature vapor-phase transport deposition of VO2 nanobeam materials with RF/wireless antenna structures towards the development of a prototype frequency-reconfigurable patch-antenna operating at wireless LAN frequencies. Individual VO2 nanobeams which exhibit metal-insulator transition (MIT) properties are used as active electrical switch components to tune the operation frequency of the microstrip aperture-coupled fed antenna.

VO₂ nanobeams are grown by vapor phase transport deposition method using low pressure argon carrier gas and V₂O₅ powder placed in the center of a tube furnace in an alumina crucible at 800 °C. The nanobeams are grown on a c-cut sapphire substrate, elongated along the rutile c-axis. Low adhesion of nanobeams to sapphire substrate allows easy transfer between the metallic pixels of the patch antenna using a micromanipulator. After the placement of crystals, whole antenna structure is heated above the melting point of indium. Pre-drawn fine indium pins are used as a soldering agent to fix and ensure decent electrical contact between the VO2 nanobeams and metallic antenna pixels.

The MIT behavior of individual VO2 beams have been characterized by measuring the resistance between two adjacent metallic pixels. Resistance change due to metal insulator transition is very sharp as indium is soft enough for crystal to change its length upon the MIT while still keeping the electrical contact with the contact pads. Phase transition occurs at a slightly elevated temperature of around 76 °C which is most likely due to the relatively poor and un-optimized thermal contact between the heating stage and the antenna structure. Nevertheless, a resistance change over 3 orders of magnitude has been obtained from individual VO2 nanobeam integrated antenna pixel pair.

Electromagnetic simulations of the fabricated reconfigurable antenna structure has been carried out using full-wave analysis method. By using the experimentally verified resistance values for metallic and insulator states of VO2 nanobeams, the simulations show a promising frequency-tuning ability between 2.35 and 5.4 GHz.

This study shows that high-temperature gas-phase deposited VO2 nanobeams can be used as critical enabling components for smart RF-antenna architectures which might provide a viable technology for next-generation smart wireless communication systems.

Vanadium oxide (VO₂) films grown by gas phase process such as MOCVD offer considerable advantages in terms of ease in scalability, simplicity, rate of growth and cost effectiveness when compared to other synthesis techniques like PVD, Sputtering or MBE. However precise control over the phase of vanadium oxide films is challenging as Vanadium forms many stable oxides like VO₂, V₂O₅, V₂O₃, and V₆O₁₃.

Wafer scale synthesis of high quality VO₂ coatings was attained taking advantage of a one-pot, three step process comprising (a) chemical vapour deposition, (b) an oxidative sintering, and then (c) a vacuum reduction of the films. This is achieved by utilizing a thermodynamically favourable pathway. During the post deposition oxidation, nano crystalline VO₂ is first oxidised to V₂O₅. Pure high quality VO₂ with large grain size is later obtained by reducing V₂O₅ to VO₂ (M).

Structural and optical phase transitions were studied by temperature dependent X-Ray diffraction and IR reflectivity spectroscopy. The semiconductor-metal transition occurs at 67°C from the pure monoclinic, M1, phase to yield a pure rutile (R) phase with an abrupt electrical resistivity change of more than three orders of magnitude. The cyclic transition reveals a narrow hysteresis with a width of 3K. Spatially resolved infrared and Raman analyses evidence the self-assembly of VO₂ disordered metamaterial, compressing monoclinic (M1 and M2) and rutile (R) domains, that occur at the transition temperature region.

An electric switch based on VO₂ operating in this transition region is demonstrated. Short heating or cooling pulses induce the coalescence/confinement of the metallic domains within the otherwise stable metamaterial. The degree of coalescence was thermally triggered with high precision to provide a reliable electrical switching with adjustable amplitude and profile.

Fullerenes, particularly C₆₀ and C₇₀, single-walled carbon nanotubes (SWCNT), their chemical derivatives and dispersions are important building blocks for many nanoscale devices such as organic photovoltaics and photodetectors, non-volatile memory and optical, chemical or biological sensors.
As the availability at industrial scale of high quality fullerenes and SWCNT with consistent characteristics is essential for the emergence of commercial applications, suitable manufacturing technology needed to be developed. Being exothermic, scalable and allowing for the selective formation of either fullerenes or SWCNT, premixed hydrocarbon combustion is a particularly appealing approach and has been developed at Nano-C to a mature industrial process. While fuel-rich combustion of aromatic hydrocarbons at reduced pressure enables the targeted synthesis of fullerenes, low-pressure combustion below the sooting threshold of aliphatic and other hydrocarbons 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 analytical techniques will be discussed. Scale-up strategies allowing to meet the quickly increasing demand will be presented.
An important aspect of the manufacturing of materials for which applications are still under development, such as fullerenes and SWCNT, consists in their processing after synthesis and collection in order to allow for optimized performance in targeted devices. In most cases, the identification of the desired characteristics is addressed in the context of close interaction between materials manufacturers and device developers. 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.
SWCNT applications require in nearly all cases control of their length, diameter distribution and amorphous carbon content during manufacturing. Advances made in this regard along with purification procedures allowing for the efficient removal of residual catalyst metals and amorphous carbon while keeping nearly defect-free SWCNT will be presented. Enabling applications further necessitates the dispersion of purified SWCNT in acceptable solvent systems that are free of surfactants. Application of combustion-synthesized SWCNT in a wide range of areas such as non-volatile memory, transparent conductors, fuel cells, printed electronics and sensors will be highlighted.

Current application of graphite as the anode of Li-ion battery is limited by its low theoretical capacity (372 mAh).¹ To mitigate this, various anodes have been proposed. Among them, anodes based on Sn seem promising for the high theoretical capacity (993 mAh/g, close to three times as that of graphite).¹ However, the large volume change during the lithiation/delithiation processes of Sn based anodes, which induces the pulverization or aggregation of the Sn particles, is believed to be the main cause for the decrease of battery capacity. To promote the further applications of Sn-based anodes, Sn/C composites have attracted much attention due to their excellent electric contact and mechanical integrity. Various methods have been researched to generate well-designed structures of Sn/C composite anodes, such as repeated reduction by strong reducing agents, freeze-drying, and spray pyrolysis.^1-3

Spray pyrolysis is a promising method to produce anode materials because of its scalable high-rate of production, facile equipment requirement, and simple post-treatment procedures. Spray pyrolysis requires a stable or monophasic precursor, without aggregation and precipitation in the precursor, for a uniform droplet-to-droplet composition.⁴ However, reported Sn-anode fabrication methods via spray pyrolysis all use Sn salts as the Sn source. Sn salts hydrolyze and form Sn oxide colloids with uncontrollable sizes in polarized solvents even at room temperature. The formation of uncontrollable Sn oxide aggregates in the precursor could result in a composition bias among the final particles because of the random Sn amount in the generated droplets.

We will present a new spray process to generate Sn/C anodes. Our method has successfully reduced the SnO₂ colloids in the precursor to metallic Sn in the final powders without introducing H₂ in the carrier gas. Our method integrates the features of spray drying (multiphase precursor) and spray pyrolysis (chemical reactions). It is termed as spray dryolysis. By applying spray dryolysis, various structures of Sn/C anodes have been fabricated. C provides the elastic matrix and acts as a conductive agent, which increases the anti-pulverization ability. Our well designed Sn/C anodes exhibited high initial coulombic efficiency (~ 80 %) and high capability retention (1000 cycles). This work demonstrates a promising method to fabricate Li-ion battery anodes with high power density and reliability at industrial adaptable scale.

(1) Xu, Y.; Liu, Q.; Zhu, Y.; Liu, Y.; Langrock, A.; Zachariah, M. R.; Wang, C. Nano Letters 2013, 13, 470.
(2) Qin, J.; He, C.; Zhao, N.; Wang, Z.; Shi, C.; Liu, E.-Z.; Li, J. ACS Nano 2014, 8, 1728.
(3) Eom, K.; Jung, J.; Lee, J. T.; Lair, V.; Joshi, T.; Lee, S. W.; Lin, Z.; Fuller, T. F. Nano Energy 2015, 12, 314.
(4) Liang, Y.; Felix, R.; Glicksman, H.; Ehrman, S. H. To be submitted.

Although the Li-ion battery (LIB) currently offers the most suitable balance between power and autonomy for consumer electronics and electric vehicle applications, there continues to be a demand for increased energy capacity. One strategy to increase LiB’s energy density is to replace graphite (372 mAh.g^-1) as the anode active material by higher specific capacity materials. Silicon appears as an attractive alternative material thanks to its high theoretical specific capacity (3579 mAh.g^-1 for the Li_3,75Si phase) and its low discharge potential. Despite being the focus of scientific activity for over 10 years, the use of silicon based anodes have not yet been realized because the performance of these materials degrades rapidly during cycling. Silicon nanostructuration together with association of carbon to Si greatly enhance the performances in terms of both cyclability and capacity. In particular, core-shell silicon-carbon Si@C nanoparticles are attractive candidates as active material to increase the capacity of Li-ion batteries while mitigating the detrimental effects of volume expansion upon lithiation processes.
The innovative solution proposed here is to use at the anode nanoparticles of Si@C synthesized in a single step by a scalable continuous gas phase method particularly interesting for industrial production, i.e. the laser pyrolysis method. Moreover, thanks to the control of experimental parameters, this method allows producing an amorphous core of silicon (a-Si) as well as a crystalline one (c-Si); Indeed using a-Si as core material, instead of c-Si, is an considered option not often considered but it appears promising to enhance cyclability because a-Si is not subject to the drastic crystalline state alteration upon its first lithiation. In order to cumulate all the benefits cited above, active material should be a composite of an a-Si core covered with a carbon shell
We report the synthesis, in a single-step process, of amorphous silicon nanoparticles coated with a carbon shell (a-Si@C), via a two-stage laser pyrolysis where decomposition of silane and ethylene are conducted in two successive reaction zones. Auger electron spectroscopy and scanning transmission electron microscopy show a carbon shell about 1 nm in thickness which prevents detrimental oxidation of the a-Si cores. The advantages of the a-Si@C material will be emphasized by comparison with c-Si@C material used as active materials. In particular, cyclic voltammetry demonstrates that the amorphous core-shell composite reaches its maximal lithiation during the first sweep, which is attributed to the amorphous core. After 500 charge/discharge cycles, it retains a capacity of 1250 mAh.g^-1 at a C/5 rate and 800 mAh.g^-1 at 2C, with an outstanding coulombic efficiency of 99.95 %. Moreover, post-mortem observations show an electrode expansion of less than 20% in volume where the nanostructuration is preserved.

Pure and highly ordered graphene can be rapidly and continuously produced through the substrate-free gas-phase synthesis method. The single-step process involves delivering an aerosol consisting of argon gas and liquid ethanol droplets directly into an atmospheric-pressure microwave-generated argon plasma. Experimental results have indicated that the technique produces species that react to form aromatic nuclei, which then rapidly grow into graphene in the high-temperature plasma afterglow region. Here, we will present the most recent advances in the gas-phase synthesis of graphene. Precursors and synthesis conditions that have increased the rate of graphene production will be discussed. Furthermore, we will show how changes to the delivery method and precursor composition can result in the formation of other carbonaceous nanomaterials. Our experimental results elucidate the nucleation and growth processes of graphene in the gas phase. We will also report the performance of gas-phase-synthesized graphene in a number of applications, such as a filler in polymer-matrix nanocomposites.

Particle surface properties strongly influence the processing and application of nanoparticles. Post-processing by particle coating, embedding, or surface functionalization is often required to adjust the materials’ properties with respect to their utilization in functional structures. Single-step processes for direct surface treatment are highly attractive, especially for incorporation into industrial products. We report on a scalable and continuous gas-phase synthesis with subsequent inline coating of silicon nanoparticles by a high temperature single-step plasma process. Depending on process conditions, the coating step produces either a SiC-, polyethylene- or a more carbon-like shell. The generated materials are of specific interest, e.g. as ceramics or as components of anodes in lithium ion batteries.
The process requires a two-stage supply of particle precursor (SiH₄) and the addition of a coating agent into the particle-laden gas flow. Silicon nanoparticles were synthesized by thermal decomposition of gaseous silane (SiH₄) in a high-temperature H₂/Ar plasma and directly coated downstream of the nucleation zone by heterogeneous nucleation/decomposition of gaseous ethylene (C₂H₄) on the particle surface. To facilitate a homogeneous intermixing of C₂H₄ and the hot, nanoparticle-laden exhaust gas stream of the plasma zone, laminar flow simulations were performed for the complete reactor to design and optimize the geometry of a special coating nozzle (number, direction angle and diameter of the bores). A ring nozzle with 16 bores and an outlet velocity of 15 m/s was found to provide suitable mixing conditions.
Online measurements with quadrupole mass spectrometry confirmed an almost quantitative conversion of silane. Measurements by X-ray diffraction, Raman- and FTIR spectroscopy have revealed that with increasing distance of the coating nozzle from the plasma either a SiC-, a carbon- or a polyethylene-like shell is produced. XPS measurements indicated a high number of Si-C bonds for materials coated close to the plasma zone forming a crystalline silicon carbide (SiC) phase. We attribute this to the free radicals, ions and electrons in the plasma zone and to the high reactivity and temperature of the pristine nanoparticles. For all process conditions, spherical, non-aggregated particles of about 40 nm in diameter with a highly crystalline silicon core were observed as indicated by TEM investigations.
The authors gratefully acknowledge the support by the German research foundation (DFG) in scope of the research group 2284 “Model-based scalable gas-phase synthesis of complex nanoparticles”.

Flame is an ideal reactor for the production of well-controlled nanostructured materials. By changing combustion parameters, i.e., fuel composition, the mixture of fuel and oxidizer, the inert diluents, pressure and residence time, it is possible to control the gas-to-particle conversion, thus producing a large variety of nanostructures having different chemical characteristics and morphologies.
In this study, we focus on the process of carbonaceous particle formation via the mechanism of homogeneous nucleation of aromatic molecules followed by agglomeration/growth owning to the competing mechanism of collision-coalescence among nascent particles and gas-phase compounds.
It is demonstrated that, starting from gaseous and pre-vaporized hydrocarbons, gas-phase aromatic compounds and atomically-thin disk-like structures composed of aromatic compounds connected by non aromatic bonding are formed during hydrocarbon oxidation in fuel-rich conditions. The magnitude of the p-electron extension in the nano-disks and the possible inclusion of heteroatom bonding may be controlled by flame operating conditions. Multi-layered nano-disks can be also obtained; their formation precedes the formation of nanoparticles containing stacked aromatic layers. All of these nanoscale carbonaceous materials possess interesting optical and electronic properties that make these materials potentially interesting for new applications, as in the field of the organic electronic, both in the form of powder and thin nano-coatings.
A variety of diagnostic techniques are used to follow and control their formation/evolution process. They include in-situ laser induced emission spectroscopy with ultrafast laser sources in the UV (fluorescence and incandescence), laser light scattering and broadband light absorption, and ex-situ on-line measurements with differential mobility analyzers. Off-line atomic force microscopy, operated in both semi-contact and force spectroscopy mode, HR-TEM, Raman spectroscopy, X-ray and ultraviolet photoemission spectroscopy complement the on-line measurements. A molecular dynamics approach coupled to a detailed model of hydrocarbon oxidation and pyrolysis is also used to explore the flame operating conditions which allow the obtaining of the desired morphology of the particles.

Carbon nanotubes (CNTs) have been widely studied for the last couple of decades and it has been recognized for nearly as long that they should have a profound impact on the mechanical properties of composite materials. The extreme aspect ratios, strong sp² carbon bonds, and chemical stability all contribute to making CNTs ideal reinforcement fillers. However, the problems associated with dispersing these materials in the composite's matrix has hindered the realization of this impact, as the CNTs tend to aggregate into bundles making them effectively much larger particles. Various strategies, such as chemical functionalization, have been attempted to aid with the dispersion of bulk CNTs in a composite matrix during processing with some success. Catalytic chemical vapor deposition (CVD) differs from other CNT synthesis methods in that the CNTs can be synthesized directly on a substrate, thereby immobilizing them and potentially preventing them from bundling after synthesis. In this work, we investigate the use of this strategy to prepare ceramic composite structures with unbundled CNTs. CNTs are synthesized as conformal coatings on various ceramic materials, including alumina fibre mats, foams, and powders following a conformal catalyst deposition. The CNT deposition is carried out in a large volume CVD reactor. The conformal catalyst is deposited by ALD / CVD deposition of ultra-thin nickel or iron films, or via deposition from an iron nitrate / poly vinyl alcohol solution. The resulting hybrid materials are characterized and the processing optimized to improve the CNT coatings. The optimized CNT-ceramic hybrids are sintered into composite materials and characterized mechanically to inform on the potential applications [M Bolduc, et al., Proceedings of Personal Armour Systems Symposium, Amsterdam, 2016; J Lo, et al., Proceedings of the 39th international Conference and Exposition on Advanced Ceramics and Composites, Daytona Beach, 2015]. Scaling up of the fabrication process is discussed.

Ceramic carbides materials such as SiC, due to their refractory nature and their low neutron absorption are believed to be promising candidates for high temperature nuclear or aerospace applications.
However, SiC brittleness has limited its structural application. In order to overcome this drawback, a reduction of grain size below 100 nm is expected to enhance mechanical properties. On the other hand, the grain downsizing should result in a strong decrease of the thermal conductivity because of the enhanced phonon scattering at the grain boundaries. In order to counteract this effect, multiwall carbon nanotubes (MWCNTs) could be of great interest because of their interesting thermal properties. Moreover, MWCNTs show a strong toughness which should also help to enhance the mechanical properties as reviewed by several authors. We report here the study of the elaboration of such nanoSiC / MWCNTs composites using gas-phase synthesized nano-objects together with the related thermal, electric and mechanical properties.
For this study, the starting nanoscale building blocks (nanoparticles and nanotubes) were synthesized by gas phases processes. β-SiC nanopowders with a mean particle smaller than 20 nm were obtained by a laser assisted CVD flow process, namely laser pyrolysis, using a CO2 laser to decompose the gaseous precursors (silane and acetylene). MWCNTs several hundred microns in length were grown as carpets on substrates by continuous catalytic CVD using an aerosol of toluene and ferrocene used as carbon and catalytic iron precursors, respectively. Dispersion of SiC nanopowders was obtained in an aqueous medium under magnetic stirring with dedicated dispersing agent. Several samples were prepared, differing in surface composition (C or Si excess) and sintering additives content (from 0 to few wt%). MWCNTs were dispersed by means of an ultrasonic probe and subsequently mixed with SiC slurries with different concentrations. Green bodies were then prepared by slip-casting. In order to avoid grain growth while keeping satisfying densification, spark plasma sintering (SPS) was used for this study. Thanks to this fast sintering process, SiC matrix grain size was kept under 100 nm while final densities were higher than 96%. Finally, samples with different chemical (Si, O, C) compositions and MWCNTs contents were subjected to mechanical characterization (hardness, toughness), resistivity, and thermal conductivity measurements with the aim of correlating the final microstructures to the mechanical, electric and thermal behavior.

Organometallic complexes are important catalysts, and are interesting for other applications as well. Inducing porosity in solid organometallic crystals is critical for applications where a high surface area is required. However, unlike for metal organic frameworks (MOFs), fabrication of porous organometallic crystals remains a significant challenge. Here we demonstrate a simple method to modulate porosity using ferroin perchlorate, a model system that combines a common ionic complex with a very reactive counter-ion. Using thermal chemical vapor deposition (CVD), we show that by annealing ferroin perchlorate crystals at 350 °C under a flow of ethylene, hydrogen, argon, and oxygen, we induced pores in the crystal. We demonstrate that small amounts of oxygen that may combine with hydrogen to form water are essential to pore formation. We also demonstrate that pore size and density can be easily controlled by varying the ethylene flow. Upon raising the annealing temperature to 500 °C, we observed a second transition in which carbon nanofibers (CNFs) grew from the porous crystal. This approach represents a simple and effective method for the synthesis of porous materials with good control over pore size and density. It also enables the synthesis of complex networks of nanostructures (in our case CNFs) by simply varying process parameters such as temperature and gas flows. This represents an important advance for the fabrication of porous organometallic complex crystals that may open new doors to the application of these compounds in catalysis.

Hollow carbon nanospheres with controlled morphologies were synthesized via copper-carbon direct current arc discharge method by alternating the concentrations of methane in the reactant gas mixture. A self-healing process to keep the structural integrity of encapsulated graphitic shells was evolved gradually with the adding of methane gas from 0% to 20%. The outer part of coated layers expanded and hollow nanospheres grew to be large fluffy ones further with high methane concentrations from 30% to 50%. A self-repairing function by the reattachment of broken graphitic layers initiated from near-electrode space to the distant was also distinctly exhibited. By comparing several comparable metals (e.g., copper, silver, gold, zinc, iron and nickel)-carbon arc discharge products, a catalytic carbon-encapsulation mechanism combined with a core-escaping process has been proposed. Specifically, on the basis of the experimental results, copper could be applied as a unique model for both the catalysis of graphitic encapsulation and the adequate template for the formation of hollow nanostructure.

The number of cancer-related mortalities according to WHO is projected to increase by 45% from 2007 to 2030 partly influenced through increasing global population. The annual budget spent by European Union in cancer care is almost €126 billion. Hence, novel, high efficacy and cost-efficient strategies in this direction are immensely required.¹ Cancer cells have unique but largely varying characteristics making the treatment difficult using classical therapeutics and calls for new and selective options.² Advanced materials in the nanoscale have shown to display biological effects as a function of their chemical composition where the nature of these effects vary between different biological environments. ^3,4 To contribute in this direction, pure and 1-10%Fe doped ZnO nanoparticles (NPs) were designed using flame spray pyrolysis (FSP). These highly pure and single crystalline particles were exposed to normal (MSC and Beas-2B) and cancer (HeLa, KLN 205) cell types. The increasing level of iron doping in ZnO enabled fine tuning of the dissolution rate resulting in significant differences in their biological behavior on cancer or normal cells. Using 2% Fe-doped ZnO NMs, the kinetics of Zn²⁺ release and induction of oxidative stress by the NPs was tailored for selective cancer cell apoptosis and reduced metastasis in both cultured cells and syngeneic tumor models. The data reveal that tuning the dissolution kinetics of NPs can be used to tailor rather cheap therapeutic anticancer agent and under precise conditions can selectively target cancer cells. These results show the potential of FSP in tuning NP chemical composition at high temperature (~2500°C), could be utilized for the production of such particles for selective anticancer therapy.

References
1. Ferlay J, Soerjomataram I, Dikshit R, et al. Cancer incidence and mortality worldwide: sources, methods and major patterns in GLOBOCAN 2012, Int J Cancer, 2015, 136(5), E359-86.
2. Meacham, C. E., Morrison, S. J. Tumor heterogeneity and cancer cell plasticity. Nature, 2013, 501, 328-337.
Zhang, Y. et al. Tuning the autophagy-inducing activity of lanthanide-based nanocrystals through specific surface-coating peptides. Nature Mater. 2012, 11, 817-826.
Pradines, B., Lievin-Le Moal, V., Vauthier, C., Ponchel, G., Loiseau, P. M. & Bouchemal, K. Cell line-dependent cytotoxicity of poly(isobutylcyanoacrylate) nanoparticles coated with chitosan and thiolated chitosan: Insights from cultured human epithelial HeLa, Caco2/TC7 and HT-29/MTX cells. Int. J. Pharm. 2015, 491, 17-20.

Multimodal contrast agents are of great interest as biomedical diagnostic systems as in advance it is frequently unclear which imaging method will be the most conclusive one. Here, Zn_0.4Fe_2.6O₄ core Gd₂O₃ shell nanoparticles as bimodal T1 / T2 contrast agents for magnetic resonance imaging (MRI) are investigated. These nanostructures are produced in a single process step via scalable and dry flame spray pyrolysis (FSP). To this end, a novel dual nozzle setup was utilized to allow the chemical bonding of Zn_0.4Fe_2.6O₄ to Gd₂O₃ nanostructures without the undesirable elemental mixing of Zn/Fe and Gd. The efficiency of the production method is closely investigated. The performance of the produced nanoparticles as T1 and T2 MRI contrast agent of the nanoparticle system is analyzed on a 4.7-T small animal MRI unit and optimized by varying the amount of Gd₂O₃. Furthermore, SiO₂ is added as a spacing material reducing the undesirable magnetic interaction between the Zn_0.4Fe_2.6O₄ and Gd₂O₃ material and therefore increasing the MRI contrast effect.

ABSTRACT BODY:
Abstract Body: Cerium oxide nanoparticles have received a lot of attention recenly due to their antioxidant enzyme-like (nanozyme) properties [1]. Here, uncoated europium doped cerium oxide (CeO₂:Eu³⁺) nanoparticles with well-defined size (d_XRD: 4.4 – 16 nm) were prepared by flame aerosol technology and characterized in regards to H₂O₂ sensor response in physiologically-relevant solutions. Temporal stability was compared to a commercially available fluorescent dye in a peroxidase coupled reaction. While largest CeO₂:Eu³⁺particles showed highest signal intensity in the absence of substrate, smallest nanoceria (d_XRD: 4.4 nm) showed highest sensor response and were therefore selected for subsequent H₂O₂sensing. A clear substrate concentration signal dependency could be observed. A limit of detectionof 150 nM in PBS (pH 7.4) was observed with linearity up to 5 µM. When the concentration and sensor response reciprocals were plotted, a broader linear range could be observed.

[1] S. Das et al (2013) Nanomedicine 8(9): 1483-1508.
[2] A. Pratsinis et al (2013) Small 9(15): 2576-2584.

This study reports the development of a one-step method for the production of antimicrobial protective coatings for aluminum surfaces with titania nanoparticles. An aerosol flame synthesis system was used to produce monodisperse, ultra-fine TiO₂ nanoparticles, which were directly deposited by thermophoresis onto plates of aluminum alloy by means of a rotating disc. Fuel-lean reactor conditions were used to synthesize pure anatase nanoparticles of 3.5 nm in diameter. Substrates were mounted onto the rotating disc that repetitively passes through the flame. Convection due to the rotational motion cooled the substrates, on which particles were deposited as films by thermophoresis. Such a system allowed to obtain submicron coatings of different thickness and porosity, by varying the total time of deposition (t_des=10 s, 30 s and 60 s). Atomic Force Microscopy was used to evaluate topological properties of the film, such as thickness, surface topography and roughness.
The antimicrobial activity of TiO₂ coatings was tested against the dimorphic fungus Candida albicans (ATCC SC5314), the Gram positive bacteria Staphylococcus aureus (ATCC 6538), and the Gram negative bacteria Salmonella Typhimurium. To determine the inhibition of biofilms formation, microbes were plated on TiO₂ coatings and a semi-quantitative colorimetric assay was performed using crystal violet. The tests showed that the TiO₂ coating obtained with t_des=10 s completely inhibits Candida biofilm formation, and inhibits up to 70-80% Staphylococcus aureus biofilm formation. The inhibition of biofilms formation was confirmed by means of Scanning Electron Microscopy observation. The results of the present work are promising for using titania films as protective coatings for applications where an antimicrobial activity is required.

One dimensional nanostructures have the prospect to change the properties of materials used in contemporary devices. Physical properties change with dimension and size. Ceramics, semiconductor and carbon materials are easily synthesized as one dimensional structures with typical diameters of several nanometers and length-diameter ratios of 1000:1. However, metals are difficult to fabricate in similar geometries. Recently we adopted a recipe from 1574 to grow 1D nanostructures under UHV conditions by physical vapor deposition. Several fcc , bcc and hcp metals can be synthesized by this method as nanowires (NWs). Typical diameters of the nanowhiskers are 100 nm and lengths of up to 50 µm are observed.
Scanning and Transmission Electron Microscopy analysis on the NW microstructure for fcc NWs yielded the following results: (i) The NWs are freestanding and prismatic with no taper along the NW axis; (ii) the surface facets are the low energy surface planes of the corresponding metals, typical four {111} and two {100} planes; (iii) the cross-section is close to the predicted Wulff shape; (iv) the NWs axis is parallel to the <110> crystal direction; (v) the NWs are perfect single-crystals, no flaws, dislocations nor grain boundaries are typically observed in the bulk of the NWs. At the present, no other technique is able to yield such unique microstructures.
Investigations of physical properties of the NWs focused on the mechanical strength and the onset of the plastic deformation. Mechanical test were performed on the grown elemental NWs. The NWs performed remarkably well; due to the perfect crystal structure, the NWs exhibit tensile and bending strengths close to the predicted theoretical strength. Post mortem and in-situ TEM revealed that the deformation is carried by partial dislocation nucleation and propagation, therefore by formation of stacking faults. Nucleation of further partial dislocations on adjunct planes leads consequently to twin formation. The twin boundary is a perfect Σ3{111}<110> symmetrical tilt grain boundary.
Correlating with the low defect density in the NWs is the low conductivity. No size effect is observed but a constant conductivity, again, close to the theoretical limits. The magnetic domain structure was studied by electron holography. Only one single domain is present in ferromagnetic metallic NWs, showing they are perfect bar magnets.

(1) G. Richter, et al., Ultrahigh Strength Single Crystalline Nanowhiskers Grown by Physical Vapor Deposition, Nano Lett. 9, 3048-3052 (2009)
(2) M. Schamel, et al., The filamentary growth of metals. IJMR 102, (2011) 828
(3) C. Schopf, et al, Cyclic cantilever bending of copper nanowhiskers. Adv. Eng. Mater. 14, (2012) 975
(4) L. Y. Chen, et al, Measuring surface dislocation nucleation in defect-scarce nanostructures. Nature Materials 14 (2015) 707
(5) C. Leclere, et al. In situ bending of an Au nanowire monitored by micro Laue diffraction. J. Appl. Crystallogr. 48, (2015) 291

Controlling of the nanostructure synthesis is crucial to obtaining the extraordinary physical, chemical, and mechanical properties in nanomaterials. These efforts to realize the controlled synthesis are especially highlighted by the large improvement in the nanomaterials based devices over recent decades. Among these efforts, manipulating the crystal facets has attracted much attention due to the distinguishing reaction activities on different facets. In principle, crystals tend to exist with the most stable facets exposed. Delicate control over the growth kinetics and thermodynamic reduction of surface energy via selective adsorption of capping agents, have been reported to obtain nanocrystals with high energy facets through solution based methods. While the vapor based method has many advantages against the solution based method, like the high crystal quality, however, it is still a challenge to control the specific facet growth of the crystal.

We report an observation of the unique facet evolution phenomenon at the NW tip at different deposition supersaturation within a narrow vapor deposition window. Electron microscopy characterization revealed that at certain conditions, high-energy crystal facets, including the {103} and {112} facets could be stably exposed at the NW tip. The relative area of these crystal facets continuously changed following the supersaturation. The facet evolution path was identified as starting with facet (0001), gradually transited to facets {103}, subsequently to facets {112}, and finally back to facet (0001) as the supersaturation continuously decreased. The facets evolution and exposure of high-energy facets were attributed to the fluctuation of the energy barriers for the formation of different crystal facets during the layer-by-layer growth of the NW tip. An empirical law for the NW tip facet formation was proposed in analogy to the Ostwald-Lussac Law of phase transformation. That is, at appropriate deposition conditions, exposure of the crystal facets at NW growth front is not merely determined by the surface energy. Instead, the NW may choose to expose the facets with minimal formation energy barrier, which can be determined by the ES barrier variation as the supersaturation changes. Based on the surface area difference among different crystal facets, the energy barrier differences were quantified as a function of supersaturation. This study provides a new platform for understanding the one-dimensional or two-dimensional crystal growth kinetics, which will contribute a new insight to the controlled growth of nanostructures.

Mo₁₇O₄₇ nanowire-arrays are promising active materials and electrically-conductive supports for batteries and other devices. While high surface area resulting from long, thin, densely packed nanowires generally leads to improved performance in a wide variety of applications, the Mo₁₇O₄₇ nanowire-arrays synthesized previously by electrically-heated chemical vapor deposition under vacuum conditions were relatively thick and short. Here, we demonstrate a method to grow significantly thinner and longer, densely packed, high-purity Mo₁₇O₄₇ nanowire-arrays with diameters of 20 - 60 nm and lengths of 4 - 6 mm on metal foil substrates using rapid atmospheric flame vapor deposition without any chamber or walls. The atmospheric pressure and 1000 °C evaporation temperature resulted in smaller diameters, longer lengths and order-of-magnitude faster growth rate than previously demonstrated. As explained by kinetic and thermodynamic calculations, the selective synthesis of high-purity Mo₁₇O₄₇ nanowires is achieved due to low oxygen partial pressure in the flame products as a result of the high ratio of fuel to oxidizer supplied to the flame, which enables the correct ratio of MoO₂ and MoO₃ vapor concentrations for the growth of Mo₁₇O₄₇. This flame synthesis method is therefore a promising route for the growth of composition-controlled one-dimensional metal oxide nanomaterials for many applications.

P. Allen, L. Cai, L. Zhou, C. Zhao and P. M. Rao, “Rapid Synthesis of Thin and Long Mo₁₇O₄₇ Nanowire-Arrays in an Oxygen Deficient Flame”, Scientific Reports, 6, 27832, 2016.

The effective synthesis of atomically thin molybdenum disulfides (MoS₂) with high quality and large area uniformity is essential for their applications in electronic and optical devices. In this work, we synthesize high quality and large area uniform MoS₂ using chemical vapor deposition (CVD) with volatile S organic compound and alkali salt catalysts. Interestingly, in the catalytic CVD MoS₂ process, the alkali salts significantly enhance growth rate (5 min for synthesis of 1L MoS₂) and eliminate carbon contamination in synthesized MoS₂. The optical microscopy, Raman, X-ray photoemission spectroscopy, photoluminescence and transmission electron microscopy measurements indicate that the catalytic CVD MoS₂ have large grain size (tens of µm), clear Raman shift, strong photoluminescence, good stoichiometry and 6-fold coordination symmetry. Moreover, we demonstrate that the high electron mobility (over 10 cm²/Vs) and on/off current ratio (3 × 10⁷) of 1L MoS₂ measured using a field-effect transistor are comparable to that of CVD grown MoS₂. We expect that similar growth behavior is feasible for other 2D TMDCs materials using the chalcogen organic compounds for chalcogen source.

A new research paradigm has been pioneered by the discovery of single MoS₂ layer from bulk 2D-MoS₂ van der Waals solid. MoS₂ structure control is important to optimize performances in various applications and understand the reaction mechanism of materials. However, it still remains a challenge to tune MoS₂ structure and morphology because only stacking number control is partially possible and its range is limited to single or few layers. Here, we demonstrated an innovative fabrication methodology to simultaneously control MoS₂ structure (length, stacking number, distribution, alignment) and related morphology. In the system where multi-atomic elements are involved, we studied how the interplay of gas-phase reaction induced competitive redox and atomic diffusion can induce a large scale fabrication of MoS₂. We think that this study can be extended to rational design rule of nanomaterial synthesis.
Our system is composed of Mo-S-C-O quaternary elements. We established it by the calcination of electrospun ammonium tetrathiomolybdate ((NH₄)₂MoS₄) + polyacrylonitrile ((C₃H₃N)_n) nanofibers under O₂ gas. Formable compounds by redox reactions between them are Mo_xS_y, Mo_xO_y, Mo_xC_y, C_xS_y, C_xO_y, S_xO_y. Among them, only Mo sulfidation + C oxidation should be induced with precluding others. To induce this selective reaction, specific processing parameters (calcination pressure and temperature) and material compositions were derived by Ellingham diagram and ternary phase diagrams, calculated by thermochemical database software (Factsage^TM). This novel approach enabled to control MoS₂ lateral growth and vertical stacking by the degree of oxidation based C combustion. With increasing oxygen partial pressure, C nanofibers started to be decomposed and its porosity increased. Accordingly, MoS₂ layers were aligned parallel and their length, stacking number increased. According to the processing parameters, MoS₂ average length and stacking number were modulated by the range of 3.137~33.01 nm and 1.1~16.5 layers inside nanofiber. Also, they were distributed toward the surface edge of nanofibers. Finally, at the condition of maximum C decomposition, MoS₂ nanotubes with high aspect-ratio were formed by the spontaneous MoS₂ assembly. This phenomenon has not been observed in previous researches and can be compared to the formation of carbon nanotube by growing and rolling the graphene nanoflakes. In this work, precise control of MoS₂ lateral growth and vertical stacking was realized for the first time, by predictive synthesis with thermodynamic theoretical background. We applied MoS₂/C nanofibers to hydrogen evolution reaction (HER) catalyst and obtained superior performance of E vs. RHE less than -0.2V at -10 mA/cm². Intimate relationship between MoS₂ structures and electrochemical performances is discussed.

Metal-doped titanium dioxide nanoparticles can be employed in various applications, including dye-sensitized solar cells and gas sensors. Metal-doping is able to enhance the performance of titanium dioxide as photocatalysts by the dispersion of metal ions into the TiO₂ matrix. In this work, tungsten-doped titanium dioxide nanoparticles are synthesized by a multiple diffusion flames burner using TTIP as the precursor (for titania) and tungsten mesh as the metal source (for doping). This novel method of using a metal mesh as precursor for doping is especially advantageous for low-vapor-pressure precursors, and the entire nanoparticle synthesis process is still gas-phase based. The effect of varying the tungsten loading rate is studied for synthesizing doped titanium dioxide with different doped-tungsten amount. The results show that high loading rate of tungsten can trigger homogenous nucleation of WO₃ prior to reaching the TTIP precursor loaded region, thereby leaving less tungsten ions to be doped into TiO₂, when compared to the relatively lower tungsten loading rate configuration. Heat treatment at 973 K in Ar atmosphere releases some of the tungsten out of the TiO₂ structure, thus making a new WO_x-TiO₂ solid solution, while tungsten ions are reduced to lower oxidation states. Moreover the annealing process also increases the unit cell volume of W-doped TiO₂, making the value closer to that of the un-doped TiO₂. XRD analyses show the different shifted angles between the doped-TiO₂ and pure TiO₂. EDS results give the tungsten weight ratio in the as-synthesis particles. XPS analyses clearly show the new Ti-O and W-O bonds after heat treatment. SEM and TEM figures present the morphology and size of the as-synthesis particles. UV Vis results show tungsten doping and heat treatment improve the absorbing ability of titanium dioxide in the visible light wavelength range significantly.

Rare-earth doped nanocrystals are promising materials for applications in biomedical imaging, display technology and photovoltaic cells due to their characteristic luminescence from the visible to the near-infrared (NIR). Several functional crystal host matrices with different dopants have been investigated. However, literature still lacks a wide-range comparison of these different host matrices, as typically only dopant effects are analyzed. Here, a variety of different flame-made nanocrystals (oxides, phosphates, vanadates) doped with neodymium are systematically investigated as optical contrast agents for biological imaging. Nd³⁺ is the optimal dopant for potential in vivo applications, since it has excitation and emission wavelengths between 700-900 nm (near-infrared window), where absorption and scattering by human tissue could be significantly reduced. Through close control of nanocrystal size, the resulting photoluminescence properties under 808 nm excitation are investigated. The relative increases in phosphorescence as a function of crystal size are compared and a clear material dependency is derived. Their application as optical imaging nano-agents in the near-infrared window is demonstrated ex vivo with chicken breast tissue. Under harmless laser power density (0.2 W cm^-2), particles could be detected at an injection depth of up to 20 mm by a commercially available camera.

Nowadays, obesity is one of the major health challenges of modern society. Online monitoring of the fat burn rate offers key advantages for efficient weight loss as physical exercise and diet can be tailored individually and monitored in real-time. Breath acetone detection could provide a non-invasive method to monitor the fat metabolism,¹ as it is a by-product of the lipolysis.² Chemo-resistive metal-oxide gas sensors are quite attractive featuring low fabrication cost,³ simple application and they can be easily integrated into portable devices⁴ due to their small size.⁵
Here, we present a breath analyzer comprising a sampler tube and a sensor for online monitoring of acetone during exercise. The sampler tube enables reproducible and monitored (flow and CO₂) exhalations. As sensing material, flame-made nanoparticles are directly deposited onto compact substrates. This breath analyzer can detect breath acetone concentrations accurately in humans and display the acetone dynamics during exercise in real-time. Simultaneous cross-validation was performed with state-of-the-art mass spectrometry. As a result, a portable prototype is presented that could facilitate easy monitoring of fat burn, for example, during exercise in the gym.


(1) Kundu, S. K.; Bruzek, J. A.; Nair, R.; Judilla, A. M., Clin. Chem. 1993, 39 (1), 87-92.
(2) Anderson, J. C., Obesity 2015, 23 (12), 2327-2334.
(3) Hagleitner, C.; Hierlemann, A.; Lange, D.; Kummer, A.; Kerness, N.; Brand, O.; Baltes, H., Nature 2001, 414 (6861), 293-296.
(4) Righettoni, M.; Ragnoni, A.; Güntner, A. T.; Loccioni, C.; Pratsinis, S. E.; Risby, T. H., J. Breath Res. 2015, 9 (4), 047101.
(5) Güntner, A. T.; Koren, V.; Chikkadi, K.; Righettoni, M.; Pratsinis, S. E., ACS Sens. 2016, 1 (5), 528-535.

Nanoparticles with compact structure are attractive for biomedical applications, such as drug delivery and theranostics, due to enhanced bioavailability and light absorption in the near infrared spectrum. Flame-made nanoparticles, however, typically are ramified agglomerates of primary particles with open structure and mass-mobility exponent, D_fm, of about 2.17. Such agglomerates can restructure in the presence of humidity to smaller and more compact entities, as has been observed for soot. Here, this effect of agglomerate restructuring is investigated for flame-made silica aiming at small and compact nanoparticles with well-defined size distributions.
Water vapor produced by an evaporator is introduced into a diluted aerosol stream sampled above the flame, resulting in controlled saturation ratios, S, of 1.15 to 1.85. Water initially condenses on the particle surface and then is largely removed, as the humid aerosol flow passes through a series of two diffusion dryers.
Combined Differential Mobility Analyser (DMA) and Aerosol Particle Mass (APM) measurements are employed to determine particle structure and anisotropy by the D_fm and the mass-mobility prefactor, k_fm, respectively. Silica nanoparticles restructure and become almost spherical with increasing S. More specifically, D_fm increases from 2.1 ± 0.06 for ambient conditions to 2.65 ± 0.15 for S = 1.85, while k_fm, decreases from 1.77 ± 0.28 to 0.63 ± 0.24. The effective density of silica agglomerates increases and practically does not depend on the mobility diameter at higher S. The mobility size distribution obtained at ambient conditions shifts up to 20 % to smaller mobility diameters and narrows by 20 % for S = 1.85. Thus, particle processing with humidity is a promising method to finely tune their morphology and size distribution.

Nascent soot particles of mobility size 1 – 10 nm are the building blocks of diesel soot, a major environmental pollutant, but also carbon black, a valuable commodity material used in reinforcing tires and other industrial rubber products. Recently, however, major concerns have been raised, since microscopy and mass-mobility measurements have revealed the existence of ultrafine aggregates during nascent soot formation. So, their impact on climate, health and nanomaterials manufacturing needs to be determined accurately.
Here, Discrete Element Model (DEM) simulations are used to investigate the dynamics of nascent soot particle growth after nucleation accounting for simultaneous surface growth and agglomeration in the absence of soot oxidation. The model is validated with theoretical expressions for pure agglomeration as well as with surface growth with and without coagulation at full coalescence. The evolution of nascent soot structure is benchmarked against previous numerical studies.
Nascent soot growth by agglomeration with or without acetylene molecule reaction (pyrolysis) is compared to that by full coalescence. Neglecting the non-spherical or fractal-like nature of soot underestimates its aggregate polydispersity up to 40 %. The DEM-derived size distributions of soot growing by agglomeration with surface growth are in good agreement with microscopy and mass-mobility measurements of nascent soot produced in ethylene flames, indicating that surface growth narrows down soot size distributions at low heights above the burner, consistent with literature. The evolution of nascent soot structure from spheres to aggregates quantified by the mass fractal dimension and mass-mobility exponent is also in excellent agreement with experiments. The effect of soot volume fraction on nascent soot morphology is elucidated. Based on the aggregate projected area, a scaling law is derived for the number and size of the nascent soot primary particles from mass-mobility measurements.

Si-rich transition metal (M) silicide films are non-equilibrium semiconductors with tunable energy gaps and controllable carrier types, when the films are composed of Si_n clusters, each of which encapsulates a M atom [1]. Actually, the film composed of W-atom-encapsulated Si clusters was inserted between electrical contact metals and Si or Ge to reduce the Schottky barrier height by eliminating the interface states and the Fermi-level pinning [2, 3]. For mass-scale production, an efficient method for film deposition is needed to be developed.
In this work, we demonstrate the deposition of the amorphous film composed of compositionally controlled WSi_n clusters in a hot-wall reactor using a gas-phase reaction of WF₆ and SiH₄. The hydrogenated WSi_nH_x clusters with reduced F concentration were synthesized in a gas phase heated to 50–400 °C and subsequently deposited on a substrate heated to 350–420 °C, where they dehydrogenated and coalesced into the film. Under a gas pressure of SiH₄ high enough for the WSi_nH_x reactant to collide a sufficient number of times with SiH₄ molecules before reaching the substrate, the resulting film was composed of WSi_n clusters with a uniform n, which was determined by the gas temperature. These films had an optical gap of ~0.8–1.5 eV, which increased as n increased from 8 to 12.
We investigated the thermal stability of the film structure by Raman measurements with annealing at 500–800 °C. The film composed of WSi_n clusters with a uniform n = ~8 or ~12 remained in the same amorphous Si network after annealing at 800 °C. In contrast, when the film was composed of WSi_n clusters in which a W atom was not fully encapsulated in Si cages, the annealing caused aggregation of Si atoms even with an average composition n = ~10. From calculations based on density-functional theory, the binding energy of a Si atom was estimated to be 3.27 eV in the film composed of WSi_n clusters with uniform n = 10, in which the first- and second-neighbor atoms of a W atom are all Si atoms. In contrast, it was estimated to be 1.87 eV in the film composed of unencapsulated WSi_n clusters, in which some W atoms are present as the second neighbor of a W atom. This binding energy was close to that in an amorphous Si film. Therefore, the film composed of homogeneous WSi_n clusters has specifically strong W–Si–Si–W bonds, resulting in the higher thermal stability despite the non-equilibrium composition.
In conclusion, the amorphous film composed of WSi_n clusters with n ≤ 12 is formed in a thermal reactor using gas-phase reactions of WF₆ and SiH₄. The film composed of homogeneous WSi_n clusters with n ≥ 8 has an optical gap of ~0.8–1.5 eV, and has an excellent structural stability against annealing up to 800 °C.
[1]N. Uchida et. al., APEX 1, 121502 (2008). [2]N. Okada et. al., APL 101, 212103 (2012). [3]N. Okada et. al., APL 104, 062105 (2014).

Scalability of a production method determines the industrial success of nanomaterial-based devices. In case of gas sensors, silicon wafer micromachining enables their cost-effective large-scale production for low power (< 50 mW)¹ and compact electronic devices.² Metal-oxide sensing films can be patterned on such Si substrates by dry deposition methods offering key advantages such as low cost, rapid processing, high reproducibility and purity. Among these methods, flame spray pyrolysis enables highly porous nanoparticle films to be directly³ and precisely deposited using micropatterned⁴ shadow masks. Subsequent rapid in-situ annealing with a particle-free flame mechanically stabilizes the deposited loose particle network through the formation of sinter necks while maintaining the high film porosity.⁴ The FSP-made gas sensors can be carefully tuned in respect to their nanomaterial composition, crystal/grain/neck size as well as the total film thickness, which is essential for efficient gas sensing. In combination with micromachined substrates up to 720 SnO₂-based microsensors can be fabricated on a single 4 inch Si wafer in one step.⁵ This concept is implemented successfully only for SnO₂ sensing films and therefore needs to be tailored for other, e.g., thermally sensitive, nanomaterials.
Here, fabrication parameters are optimized for Si:MoO₃, a promising material for breath ammonia detection.⁶ The low melting point of this thermally sensitive material is challenging, as evaporation of MoO₃ nanoparticles could be observed already at 790° C.⁷ In this work, we show that by carefully tuning deposition time and in-situ annealing flame conditions, the morphology of highly porous Si:MoO₃ films can be controlled on Si wafers. The influence of layer microstructure (e.g., porosity, sinter neck size, film thickness) on the gas sensing properties (e.g., film resistance, sensitivity, analyte selectivity) is demonstrated. This study paves a path for further optimization of gas sensors also composed of different nanomaterials by finely tuning their synthesis conditions. In the long run, these microsensors can be combined into highly sensitive arrays and integrated into portable electronic devices. (e.g. smart phones or portable breath samplers²)

1. Simon I, Bârsan N, Bauer M, Weimar U. Sens Actuators B. 2001;73:1-26.
2. Righettoni M, Ragnoni A, Güntner AT, Loccioni C, Pratsinis SE, Risby TH. J Breath Res. 2015;9:047101.
3. Mädler L, Roessler A, Pratsinis SE. Sens Actuators B. 2006;114:283-295.
4. Tricoli A, Graf M, Mayer F, Kühne S, Hierlemann A, Pratsinis SE. Adv Mater. 2008;20:3005-3010.
5. Güntner AT, Koren V, Chikkadi K, Righettoni M, Pratsinis SE. ACS Sensors. 2016;1:528-535.
6. Güntner AT, Righettoni M, Pratsinis SE. Sens Actuators B. 2016;223:266-273.
7. Azurdia JA, McCrum A, Laine RM. J Mater Chem. 2008;18:3249.

Thin, flexible and conductive films are an essential component in many modern electronics which demand small size, light weight and high shape-conformability. These properties are frequently obtained by coating metals in intricate shapes onto polymeric substrates or combining conductive filler nanoparticles with polymers. Unfortunately not all proposed preparation methods are suitable for large scale fabrication. Inevitably, this has been leading researchers to evaluate roll-to-roll alternative such as inkjet printing and stamping.
In this work, the flame synthesis of silver nanoparticles¹ and their facile incorporation into polymers² is presented for conductive silver-polymer nanocomposite films.^1,3 The silver aerosol growth is shown to occur even 25 cm above the burner to about 10 nm in diameter.¹ Much larger nanosilver, on the other hand, is be obtained when directly depositing these aerosols at 15 cm onto polymer-coated substrates due to rapid coalescence of the incoming nanoparticles with the substrate bound ones.^1,3 Sinternecks form between these growing nanosilver particles leading to a percolating, and therefore conductive, nanoparticle network. The formation of this network in dependence on silver loading can be tracked by determining their sheet resistance by conventional four-point measurements. Here, a more precise in situ resistance measurement¹ is proposed for determining this threshold, which even circumvents the fracturing of fragile sinternecks. In situ resistance measurements, furthermore, revealed a substrate dependent formation of conductive nanosilver films. This detail becomes especially important for the rapid preparation of sub-micrometer thick films on diverse polymers which retain their high electrical conductivity even during hundreds of bending cycles.³


1. Blattmann CO, Pratsinis SE. In situ measurement of conductivity during nanocomposite film deposition. Appl. Surf. Sci. 371, 329-336 (2016).
2. Sotiriou GA, Blattmann CO, Pratsinis SE. Flexible, multifunctional, magnetically actuated nanocomposite films. Adv. Funct. Mater. 23(1), 34-41 (2013).
3. Blattmann CO, Sotiriou GA, Pratsinis SE. Rapid synthesis of flexible conductive polymer nanocomposite films. Nanotechnology. 26(12), 125601 (2015).

Core/shell structured nanoparticles have received much interest owing to their utmost potential in permanent magnetic applications. In the present work, AlNiCo permanent magnet powders were synthesized by ball milling and a core/shell nanostructure was obtained using hydrogen plasma reactor. The effects of particle size and nanoshell structure on the magnetic properties were investigated in details. The coercivity of AlNiCo powders was found to increase with decreasing particle size, exclusively nanopowders encapsulated with Fe3O4 shell showed the highest coercivity values. The shell structure obtained with hydrogen plasma reaction not only contributed to the magnetic properties but also found to act as a resistant layer against oxidation of metallic nanoparticles as observed in the scanning transmission electron microscopy (STEM) analyses.

Particles synthesis methods that involve plasma have been widely studied over the last decade. The variety of plasma based processes is linked to the advantages of such techniques in controlling in-situ the specific particle's properties, namely size, shape, composition, etc.
In this work we have combined the advantages of pulsed plasma with the ones offered by atmospheric pressure processes in order to obtain nano or micrometer size metallic particles. As a metallic source for particle generation the RF powered electrode of a plasma jet was used [1]. The radio-frequency (RF) plasma jet operates at atmospheric pressure. It must be highlighted that the plasma jet works in inert gaseous atmosphere (argon) and in pulsed mode between 1000 and 10000 Hz. The plasma is modulated with a square-wave on/off cycle of varying period aiming to control the particle's sizes and morphologies. The material is collected on a substrate downstream the discharge.
Optical and Scanning Electron Microscopy (SEM) analyses reveal that spherical nano and micro-particles were deposited. By Energy Dispersive X-ray Spectroscopy (EDS) it was found that copper particles were obtained. The particles' size can be controlled by modifying the pulse duty cycle or pulse repetition frequency, hence, we obtained particles ranging from tens on nm up to 1-3 microns. Moreover, the applied RF power is an additional important factor in adjusting the particles properties. In the end, a correlation between the plasma parameters (gas temperature, species, etc) and the particles characteristics was made.

Acknowledgements: This work was supported by a grant of the Romanian National Authority for Scientific Research and Innovation, CNCS – UEFISCDI, project number PN-II-RU-TE-2014-4-2035.

References:
[1] A. Lazea-Stoyanova, A. Vlad, A. M. Vlaicu, V. S. Teodorescu, G. Dinescu, Synthesis of copper particles by non-thermal atmospheric pressure plasma jet, Plasma Processes and Polymers, Vol. 12, Issue 8, 836, 2015.

Reducing the particle size of noble metals on ceramic supports can maximize noble metal performance and minimize its use. Here Pd subnano-clusters onto nanostructured TiO₂ particles are prepared in one step by scalable flame aerosol technology while controlling the Pd cluster size from a few nanometers to that of single atoms.^1,2 Annealing such materials at appropriate temperatures leads to solar photocatalytic NO_x removal in a standard ISO reactor up to10 times faster than that of commercial TiO₂ (P25, Evonik). Such superior performance can be attained by only 0.1 wt% Pd loading on TiO₂. Annealing these flame-made powders in air up to 600 ^oC decreases the amorphous TiO₂ fraction and increases its crystal and particle sizes as observed by X-ray diffraction (XRD) and N₂ adsorption. The growth of single Pd atoms to Pd clusters on TiO₂ prepared at different Pd loading and annealing conditions was investigated by scanning transmission electron microscopy (STEM) and XRD.
The single Pd atoms and clusters on TiO₂ are stable up to, at least, 600 ^oC for 2 hours in air but at 800 ^oC they grow into PdO nanoparticles whose fraction is comparable with the nominal Pd loading. So most of Pd atoms are on the TiO₂ surface where at 800 ^oC they diffuse and coalesce. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reveals NO adsorption on single, double, 3- and 4-fold coordinated Pd atoms depending on their synthesis and annealing conditions. The peak intensity of NO adsorption sites involving multiple Pd atoms is substantially lower in TiO₂ containing 0.1 wt% than 1 wt% Pd but that intensity from single Pd atoms is comparable. This indicates the dominance of isolated Pd atoms compared to clusters in Pd/TiO₂ containing 0.1 wt% Pd that match or exceed the photocatalytic NO_x removal of Pd/TiO₂ of higher Pd contents.

1. Fujiwara, K.; Müller, U.; Pratsinis, S. E., Pd subnano-clusters on TiO₂ for solar-light removal of NO. ACS Catal. 2016, 6 (3), 1887-1893.
2. Fujiwara, K.; Pratsinis, S. E., Atomically dispersed Pd on nanostructured TiO₂ for NO removal by solar light. AIChE J (Special issue for ISCRE24) 2016, In review.

Synthesis of nanoparticles in flame reactors is a reliable method to produce functional materials in a nano-scale size. Nanoparticles are of great interest for a wide field of applications, e.g. in medicine, due to the unique physical and magnetic properties of such particles. The properties of flame-synthesized nanoparticles strongly depend on the particle size, which is influenced by the temperature of the synthesis and precursor concentration. The network of chemical reactions of the precursor decomposition and small species involved in particle growth interacts with the flame chemistry. Consequently, the fundamental understanding of the decomposition of the iron based precursor to intermediate species and stable products and the inception of particles during flame synthesis is a great challenge. Time-of-flight mass spectrometric in-situ measurements of iron pentacarbonyl doped model-flames can identify which species occur in the synthesis flames in large concentrations. This information is the first step in generating a model of the chemical processes in flame synthesis. In all flames small concentrations of ionic species are present which are formed by chemi-ionization processes. Because their concentration are small they are not believed to have a pronounced impact on the overall reaction network in the flame, but they may be used as easily measurable indicators for neutral species chemistry. To analyse a gas sample it needs to be transferred from the synthesis flame to the high-vacuum of the mass analyser. During the sampling process, condensing material can block the nozzle and reduce the sampling efficiency. Consequently, in nanoparticle synthesis particles in a certain mass range are hard to detect. Sampling charged species improves sampling efficiency for small nanoparticles and allows measurement of species, which often elude detection in molecular-beam-sampling systems. The evaluation of the experimental data reveals protonated species in a wide mass range. The mass spectra obtained from iron oxide nanoparticle synthesis in hydrogen flames show highly convoluted signals of protonated and non-protonated iron oxide species of the structure FexOyHn. Due to the high sensitivity of the experimental setup isotope patterns of the species can be determined and enable an accurate characterization of predominant charged decomposition species in a small to intermediate size range. Small iron oxide particle precursors differ by a FeOH2 group. This observation is a first experimental hint that FeOH2 could be a main growth unit for these particles. In addition, concentration profiles as function of flame position have been obtained to aid the development of a chemistry model for iron oxide nanoparticle formation in hydrogen flames.

Recent advances in nanotechnology have led to a continuously increasing number of engineered nanoparticles, such as SiO₂, TiO₂ and Ni, with plethora of high-performance applications in catalysis, electronics, medicine and food industries, to name a few. Interactions of nanoparticles with other nanoparticles, physiological media or complex biological systems alter their transport properties, thus they need to be accounted for in mesoscale models to assess their effect on coagulation and settling rate, mobility and even cellular uptake that affects cytotoxicity (Demokritou et al., 2013). The complexity of these interactions and the fact that they typically take place at very disparate time and length scales renders their description through an analytical model a very formidable task.
In this work, we propose using molecular dynamics (MD) simulations together with an atomistic forcefield to estimate the potential of mean force (PMF) between two equally-sized amorphous SiO₂ nanoparticles with diameters ranging from 2 to 10 nm and then to perform large-scale coarse-grained simulations with the derived PMF to study their dynamics and coagulation. In our simulation, the SiO₂ nanoparticles are coarse-grained to the level of entire particles interacting with each other in the simulation cell based on their MD-derived PMF. Continuous collisions between pairs of nanoparticles are observed to lead initially to the formation of small clusters of nanoparticles that gradually grow into larger ones to form eventually agglomerates. The dynamics of collisions is quantified by the collision time, the relative particle velocity and the collision angle. Criteria for successful particle collisions are proposed based on the mean collision time. Derived results for the collision frequency are in excellent agreement with the kinetic theory of gases. We also find that the collision efficiency of spherical SiO₂ nanoparticles (defined as the ratio of successful collisions over all possible collisions) decreases with decreasing particle size.
Our method can be extended to investigate the collision frequency of agglomerates without any assumptions on their sticking probability. The extracted coagulation rates can then be employed in simplified population balance equation models (e.g., Kruis et al., 1993) coupled with fluid dynamics to facilitate the design and operation of aerosol reactors.

References
Demokritou P., Pyrgiotakis G., DeLoid G., Cohen J. Nanotoxicology, 2013; 7:417.
Kruis E. F., Kusters K. A., Pratsinis S. E., Scarlett B. Aerosol Science and Technology, 1993; 19:514.

Atmospheric plasma, especially pencil-type plasma jet operated in open air (open air plasma jet, OAPJ) , allows us to easily handle plasma because of easy-to-use apparatus without large equipment. Thus it has been tried to simply utilize for various plasma enhanced material processing.

In the nanoparticle synthesis using the OAPJ, several methods have been reported and they are largely classified into two types: the ways using downstream region in ambient air, and upstream region inside the discharge tube. Representative in the former is the electrolysis of metal salt solution for NP source, by the OAPJ irradiation. On the other hand in the later, NP source, typically such as gas and mist of solution, is supplied from upstream region into the discharge tube. Most of the NPs are formed in the stream of gaseous plasma via CVD process, and gashed out in the ambient air. This allows synthesis of NPs and the subsequent deposition on supported material by one-step in open air. Thus, the application to, such as easy coating with functional NPs in open air, can be expected. However, toxic and hazardous raw materials such as metal chloride, required for metal and the compound synthesis, cannot be handled in open air, and variation of synthesizing NPs is limited.

In order to enable synthesis and deposition of a wide variety of NPs in open air, the authors have been investigating the method employing metal wire as NP source. The metal wire, placed in the discharge tube, is etched with the plasma, and the resultant reactive species collide with each other to form NPs in the plasma flowing. In this paper, we report the application of this technique to gold (Au) nanoparticle synthesis and the subsequent deposition in open air.

In synthesis of AuNPs, pulsing of the OAPJ generation is indispensable. The pulsing can prevent the meltdown of the Au wire source, which happens in constant generation of the OAPJ, due to lowering the gas temperature time-averagely. Typical experimental conditions were as follows. Au wire source material was inserted into an alumina capillary (plasma discharge tube). Ar gas mixed with H₂ (below lower explosion limit concentration of H₂ (4 vol.%) in air) was fed into the capillary at a given flow rate, and a pulse-modulated UHF (450 MHz) was applied to electrode wound around the discharge tube to generate the OAPJ.

Under above conditions, uniform-sized and well-crystallized AuNPs were effectively synthesized. As an example of the results, AuNPs with mean diameter of 7.9 nm and standard deviation of 1.2 nm were formed. Additionally, we can directly deposit them on non-heat resistant materials such as paper without apparent damage, because the time-averaged gas temperature of the OAPJ was so low we could directly touch it with our fingers, due to the pulsed plasma generation.

We report on the progress towards the application of a new laser diagnostic for the in situ characterization of gaseous clusters. Additionally, we aim to use this diagnostic in order to detect nanoparticle formation in an arc discharge environment. Already succesfully demonstrated in atomic and molecular gaseous environments, this four wave mixing diagnostic technique exploits the strong interaction between a particle’s polarizability with intense laser fields. Through this interaction, we can detect the temperature, pressure, relative density and speed of sound of the gas particle. In nanoparticles, we aim to validate their presence, differentiate between the different types of nanoparticles (nanotubes, fullerenes etc) and also provide with relative density measurements of nanoparticles in the arc discharge environment. The anticipated spatial resolution of this technique is ~150 μm while the spectral acquisition time is in the order of ~200 ns, rendering the technique ideal for fast changing environments, such as arc discharges, combustion and transient flow environments.

We are synthesizing binary and ternary palladium alloy nanoparticles in a flame driven High Temperature Reducing Jet (HTRJ) aerosol reactor. In such a system, an aqueous precursor solution evaporates and decomposes, leading to nucleation of particles in a hydrogen-rich environment. These multicomponent aerosol nanopowders are “bare” with no ligands attached to their surface. The primary particle diameter of such gas phase synthesized nanoparticles is under 50 nm. This aerosol synthesis approach allows production of alloy nanoparticles that cannot be generated in solution, with precise composition control and high precursor utilization. In this contribution, we focus on the post synthesis surface functionalization of “bare” palladium alloy nanopowders synthesized using HTRJ. We report the surface passivation and functionalization of binary and ternary alloys of palladium, copper and silver of controlled composition. We treated the aerosol-synthesized nanoparticles with long chain organic ligands, such as oleylamine and oleic acid, which are typically used in a wet chemistry synthesis of nanoparticles. These surface passivated nanoparticles are then dispersible in common solvents such as toluene, chloroform, and hexane. Because the synthesis and surface functionalization steps are decoupled, this enables us to choose from a wide library of organic ligands for surface modification. We also demonstrate the use of more volatile short chain organic compounds to stabilize these palladium alloy nanoparticles. The targeted application of the stable palladium alloy nanoink is in the synthesis of polymer nanocomposite membranes for H₂/CO₂ separation. The palladium based nanoparticles facilitate selective H₂ transport thereby improving the H₂ permeance and H₂ / CO₂ selectivity of the membrane.

In-situ and operando microscopy techniques help in understanding how materials react to different stimuli by correlating the application of the stimulus with the material reaction as it happens. With environmental cell transmission electron microscopy (TEM), the materials’ structural and chemical state can be monitored with high spatial resolution—down to the atomic level—as well as with high temporal resolution, as changes are induced by different agents.

Here we present an in-situ environmental cell study of catalytic material subjected to pressures > 1bar and temperature > RT in a TEM.

Two stacked microfabricated windowed chips spaced apart to form a flow channel acts as both the observation cell—since the films or windows are transparent to the electron beam—and the reaction cell. Different gases can be flown into the cell and a thin-film micro-heater is used to locally heat the sample inside the gas cell. By flowing hydrogen and oxygen at different temperatures we show that it’s possible to observe reduction and oxidation reactions of ferrihydrites in a TEM. Fast-frame image acquisition allow us to follow the reaction dynamics with detail.

This type of studies are relevant in fields where the direct tracking of the reaction morphology and chemical structure in the TEM is instrumental to understand and develop catalytic materials.

Combustion synthesis offers a facile route to scalable synthesis of iron oxide nanoparticles with tailored properties. Detailed understanding of the particle evolution history along the gas flow coordinate is important for synthesis of nanomaterials with desired functionalities since the resulting deposit strongly depends on the stage of particle formation upon arrival at the substrate.
Metal oxide NPs formation in flame assisted synthesis in general and iron oxide NPs in particular was traditionally postulated to occur via precursor evaporation and decomposition leading to gas phase metal atoms formation with subsequent oxidation followed by subsequent condensation and agglomeration leading to formation of nano-sculpted metal oxides far away from the burner surface. However, recent studies suggest that formation route including intermediate stage of metal particle formation in close proximity to the burner can play an important role.
To monitor the temperature and both gas phase and particle concentration field evolution versus the distance from the burner (DFB), which quantitatively represents formation history stage in the synthesis reactor, we employ combination of molecular beam sampling and laser based diagnostics methods. Partcicles mass concentration is monitored using quartz crystal microbalance detection, while charged particles mass distribution is monitored using particle mass spectrometry. Gas phase species (e.g. FeO, Fe and OH, allowing to monitor the temperature field) are detected using intracavity laser absorption spectroscopy, cavity ringdown spectroscopy and laser induced fluorescence. Using the information retrieved from the consolidated use of laser based and molecular beam sampling methods we show how the varying the flame parameters, such as gas flow rate, fuel/oxygen ratio, precursor concentration and fuel type influence the history of charged and uncharged iron oxide nanoparticle formation.

Silicene, monolayer of silicon, is predicted to demonstrate exotic properties similar to graphene, such as a Dirac cone in the band-structure and Fermi velocities comparable to graphene, quantum spin Hall effect, chiral superconductivity and giant magnetoresistance. Furthermore, it has the benefit of being easy to integrate into current industrial device engineering processes. Silicene monolayers have been grown on Ag (111) and Ir (111) substrates and recently a working field-effect transistor has been demonstrated at room temperature. While it is possible to characterize the final monolayer structure, it is impossible to probe experimentally the evolving atomic structure during growth. We use large-scale molecular dynamics (MD) simulations to study the growth of silicene monolayers on Ag (111) and Ir (111) surfaces. Initially, individual Si atoms aggregate to form small clusters containing 3-6 Si atoms with different ring geometries. Over time, these rings merge to form larger clusters, but containing extended line defects and grain structures. Interestingly, structures grown on the Ag surface are prone to more defects, in particular extended regions of sub-surface growth, while the structures grown on Ir are relatively defect free. We also explore the effect of different growth conditions such as temperature and deposition flux on the structure of the monolayers. Finally, we study the mechanical and thermal properties of these in-silico grown monolayers and compare their performance to previously reported data for ideal structures. The processing-structure-property correlations we draw will enable experimental processes to be tailored to obtain target device properties.

Flexible plastic substrates have received attention as components in next-generation optoelectronic devices such as organic light-emitting diodes (OLEDs) and organic solar cells because of lightweight, inexpensive, and enable to roll-to-roll mass production. To improve the performance of devices based on flexible substrates, nanostructuring technology has become indispensable, because planar structure causes unwanted surface reflection and total internal reflection. Recently, there have been tremendous efforts to fabricate nanostructures on rigid substrates; photolithography and bottom-up synthesis of semiconductors. In spite of impressive progress in fabrication of nanostructures, most of them are not suitable for fabricating nanostructure on flexible substrates. Because the lithography techniques use photoresists; to remove them the methods use solvents that can damage the polymer film. Bottom-up synthesis of semiconductor nanorods requires growth temperatures > 500 °C, whereas most plastic substrates cannot tolerate temperature > 150 °C.
Here, single-crystal AgCl nanorods are grown on any kinds of substrates by using a Cl₂ plasma source in which Cl atoms are consistently produced to react with Ag. At the Ag/AgCl interfaces, large volume expansion between Ag and AgCl causes accumulated strain energy, which guides nanorod growth with (200)-preferred orientation to minimize the lattice mismatch. Periodicity of nanorods is scalable from sub-wavelength scale (<400 nm) to wavelength scale (>400 nm) by adjusting the thickness of the Ag film. OLEDs with sub-wavelength scale AgCl nanostructures enhance luminance efficiency (from 50.3 cd/A to 54.8 cd/A at 1,000 cd/m²), but the haze maintains to 0.23%, similar to that of as-received PI film. Such enhancement in the luminance efficiency is mainly due to graded refractive index. Wavelength scale nanorods increases the efficiency (66.9 cd/A) of OLEDs up to 33% with haze of 100%. The nanorods on polyimide extracts the confined wave guided electromagnetic wave in the substrate, confirmed by finite-difference time-domain simulations.

Pulsed laser annealing of nanocarbons, with focus upon soot was performed using a Nd:YAG laser operating at 1064 nm and laser fluence of 250 mJ/cm². Carbon materials on this scale are heated uniformly by the laser allowing for comparisons based on morphology, nanostructure and chemical content between different carbon nanomaterials such as nanotubes, nanospheres and even bulk carbons with micro-scale dimensions. A gated intensified CCD captured the spectra of the carbon incandescence, fitting the spectral profile yielded a temperature of 2800 K for this applied laser fluence. To resolve detailed morphological and nanostructural changes induced by the high intensity laser light, high resolution transmission electron microscopy (HRTEM) is used to examine the laser-heated nanocarbons. While the annealing process enables lamellae graphitization, the spatial organization of lamellae is quite different across the varied carbons examined. Laser annealing occurs on timescales ~ 10¹⁰ faster than traditional thermal methods using high-temperature furnaces. Initial nanostructure in conjunction with the chemistry of construction governs the carbon transformation under pulsed laser annealing on these short timescales. As consequence, the kinetics of the graphitization path can be explored in experiment and compared to reactive simulations. Here experimental HRTEM observations are compared to atomistic simulations using ReaxFF force fields. Initial comparisons between ReaxFF and TEM-structures in response to sequential laser pulses show excellent agreement between simulation and experiment.

The production of inorganic nanoparticles from the gas phase is a standard method to manufacture a variety of bulk chemicals amounting to millions of tons a year. They are used industrially as reinforcements, pigments, stabilizers, catalysts or catalyst supports, flowing aids, and for multiple other applications. As gas phase processes enable for a kinetic control of nanoparticle formation, this method is basically favored for the production of metastable materials as doped nanoparticles and nanocomposites. Due to an increased understanding of size- and composition-dependent nanoparticle properties and probable applications thereof, an increasing demand in understanding synthesis steps towards the formation of specific nanoparticles with tuned properties has evolved.

The principles of high temperature reactive particle formation in flames are characterized by a sequence of partly interacting rate processes in the gas flow, while the necessary energy is delivered by the exothermic combustion reaction heating the flow to high temperatures. A complete description of the precursor decomposition kinetics and the subsequent oxidation/hydrolysis reactions is rarely obtained, while the properties of the products manufactured like size, morphology, phase composition, and crystallography are decisively influenced by these parameters. Therefore, a precise understanding and control of the initial steps is required to open the ability for tuning particle properties.

Within the last years, our low pressure premixed H₂/O₂ burner has evolved into a standard experiment for the investigation of intermediates occurring between precursor decomposition in the flame zone and particle nucleation downstream. Due to the spatial expansion of the almost one-dimensional low-pressure flame allowing for an increased time resolution, multiple experimental methods mostly based on online laser diagnostics and inline mass spectrometry enable a precise measurement of the emergence and disappearance of intermediate chemical compounds. This has extremely broadened the experimental database for the development and validation of simulations covering combustion and precursor decomposition as well as nanoparticle formation and growth. Examples will be presented for the growth processes of iron oxide and tungsten oxide nanoparticles.

As flame synthesis in a low pressure premixed reactor is limited to vaporizable precursors, only a finite number of materials is accessible via this technique while spray flame synthesis of metal oxide nanoparticles covers an almost unlimited number of chemical compounds. Therefore, an outlook will be presented concerning strategies towards standardized experiments for the spray flame synthesis of nanoparticles that allow both, in-depth determination of experimentally accessible data as well as a detailed simulation of the nanoparticle production process.

Spray-flame synthesis is a highly promising route for industrial production of highly-specific nanoparticles, since this approach allows for the use of low-cost raw materials and scalable processes. Industrially-established combustion-based routes generate nanomaterials in the regime of 100 tons per year, but the generation of size- and phase-selected materials is not yet possible. Though we demonstrated the synthesis of highly-specific materials on the lab scale before, the transfer into industry fails due to fundamental chemical and physical up-scaling challenges like particle formation in the flame and material deposition inside the reactor.
In this work we demonstrate the synthesis of highly-specific phase-selected ferromagnetic iron-oxide nanoparticles. It is expected that the O₂-concentration in the reaction zone directly influences the phase formation, therefore, experiments under various amounts of oxygen are performed on the pilot-plant scale.
The synthesis was performed in a reactor operating at up to atmospheric pressure. The flow direction is from top to bottom to prevent particles from contaminating the gas and liquid supply. The nozzle consists of coaxial tubes, while the outer tubes carry oxygen/air-mixtures, the center tube delivers the precursor solution into the spray nozzle. A quartz inliner tube is inserted into the reactor to confine the gas flow keeping the flow tube walls at moderate temperatures and avoiding deposits on the wall.
For the synthesis, Fe(NO₃)₃ 9H₂O dissolved in ETOH (0.5 molar) was used as precursor and fed into the burner with a constant flow rate of 500 g/h from a 2.5 l reservoir. The process conditions (precursor concentration, liquid and gas flow rates) were kept constant, while the over-all fuel/air equivalence ratio φ was varied from 0.51 to 0.42 at 900 mbar. The resulting material was characterized using transmission electron microscopy (TEM), nitrogen adsorption (BET), X-ray diffraction (XRD) and SQUID magnetometry. TEM measurements show an agglomerated structure of small particles for all samples. BET measurements with particles generated at φ = 0.42 show a higher specific surface area (SSA) compared to particles synthesized at higher φ values. Ferromagnetic phases γ-Fe₂O₃/Fe₃O₄ as well as the antiferromagnetic phase α-Fe₂O₃ were found by XRD measurements with a ferromagnetic/anti-ferromagnetic ratio of 79/21 for φ = 0.42 and 85/15 for φ = 0.51, respectively. The lower α-Fe₂O₃ phase content can be explained by the favored formation of Fe₃O₄ under process conditions with lower oxygen content (φ = 0.51). Furthermore, magnetic measurements show a saturation magnetization M_s at 5 K of 55 and 64 emu/g for the materials synthesized at φ = 0.42 and 0.51, respectively. Since ferromagnetic nanoparticles exhibit a decreasing M_s with increasing SSA and the anti-ferromagnetic oxide does not contribute to the overall saturation magnetization, these results are in good agreement with BET and XRD measurements.

Within the last years, numerous research results have indicated that nano-sized silicon can be used for a multitude of different applications. Especially its size-dependent properties such as limited phonon transport (in thermoelectric devices), high mechanical stability (for battery applications) or quantum-confined optical properties (for optical applications) are of high interest. Lab-scale microwave plasma reactors working at 2.54 GHz have been shown to fulfill the demands regarding high-quality materials for the specific applications.

To further investigate the properties and applicability of this sustainable material, the demand for larger amounts of silicon nanoparticles increases. Gas phase synthesis is a well-suited method to provide nanoparticles in reproducible and high quality also in large amounts.

However upscaling of well-studied laboratory-scale syntheses is still a challenge due to different scaling laws. Increasing the geometry directly affects temperature gradients, the time-temperature profile and thus the residence time within the reaction zone leading to altered growth conditions. As a result, the produced nanoparticles may be different in morphology, type of agglomeration and size compared to the lab-scale produced particles.

Here, we present our results on transferring the synthesis of silicon nanoparticles from laboratory scale to pilot-plant scale. Emphasis is on investigating process conditions to obtain specific material properties known from lab-scale synthesis such as adjustable particle size, high crystallinity, and prevention of partial sintering and aggregation. A 915 MHz microwave plasma reactor providing up to 75 kW microwave power is used to transfer the technical expertise from lab to pilot scale. Based on supporting simulations, an inlet nozzle generating a radial swirl flow that stabilizes the gas flow of the central axial inlet and prevents the deposition of particles on the reactor walls was designed and successfully tested.
Highly crystalline and soft-agglomerated silicon nanoparticles could be synthesized at production rates of about 100 g/h. Systematic investigations of process conditions such as reactor pressure and precursor concentration indicate similar effects as observed for the lab-scale reactors. However, quantitative results differ in that way that nominally similar conditions in respect of pressure, precursor concentration, and specific microwave power per volume lead to bigger particles. This effect is most probably due to differences in the time-temperature profile and have to be further investigated.

The authors gratefully acknowledge the support by the German research foundation (DFG) in scope of the research group 2284 “Model-based scalable gas-phase synthesis of complex nanoparticles”.

Polymers have taken over the way we view materials in modern times. Polymers have excellent formability and chemical stability and this makes them viable materials for a wide range of applications in engineering and consumer products. Their biggest drawback is their aversion to natural decomposition in a reasonably finite time period. Among others, polymer waste is proving to be a big concern to human race. Efforts are being made to promote the recycling of polymer waste. In research conducted at Northeastern University post-consumer polymers have been converted (recycled thermally) by pyrolytic gasification to nanocarbons and gaseous fuel. To achieve this conversion, a laboratory-scale pyrolytic gasifier was fed with post-consumer pelletized polymers (polyethylene, polypropylene, etc., and combinations of the same) with a purposely designed feeding mechanism. The pellets were gasified in an electrically heated reactor at 800 C, in an inert N₂ atmosphere. Under these conditions the polymer pyrolyzed into a gaseous mixture of hydrocarbons and hydrogen. The pyrolyzate mixture was then conducted into a separate reactor where it was used as a carbon precursor for chemical vapor deposition process to synthesize carbon nanoparticles. The catalytic substrate used for the nanoparticle growth is a T304 stainless steel mesh. T304 was selected due to its favorable composition. The excess (unreacted) pyrolyzate mixture, due to its inherent composition of light hydrocarbons and hydrogen, was subsequently premixed with air and was combusted effectively with minimal soot generation.

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 method has been used to grow Pd, Cu, Ag, Pd-Cu and Pd-Ag alloy nanoparticles. A novel modification in the synthesis set up has been carried out by exposing the metal agglomerates to carbon precursor gases followed by sintering for growing metal-graphene core-shell nanoparticles. Based on the amount of adsorption of the carbon precursor gas onto nanoparticle surface, solid solubility of carbon in the metal and relative surface energy values, thickness and nature of the graphene shell can be conrolled. The effect of nanoparticle size, alloy formation, and the presence of carbon shell on the Pd 4d centroid position and thus Pd-H interaction has been investigated in detail. In an other study, SnOX-C core shell nanoparticles have been prepared by using Sn and C electrodes in the spark generator. By growing and sintering the nanoparticles in the oxidizing or reducing conditions, the stoichiometry value of the oxide shell (x-value) can be changed and thus the gas sensing response of naonparticle to different gases can be modified. Finally, Si-Sn core shell nanoparticle having optical absorption and phonon density of states properties suitable for hot carrier solar cell applications have been prepared.

A novel rapid radially-mixed, swirl tubular flame with a central jet flow is used to synthesize both pure TiO₂ and Pd-doped TiO₂ nanopowders. The synthesis conditions, i.e., the tubular flame field, are investigated under different inert-dilution ratios, equivalent ratios, and flow rates of the central jet flow. The flame topographies were monitored by a digital camera and the flame temperatures were measured by a thermocouple and three-color thin filament pyrometry. Different feeding methods, correspond to different temperature histories of the formed nanoparticles, result in different sizes of nanoparticles. It is based on a fact that the duration time of the collision-coalescence process of nanoparticles in the high-temperature flame zones determines the final sizes and morphologies of as-synthesized nanoparticles. On the one hand, feeding precursors through tangential inlets by the fuel and oxidant gases gives rise to uniform small nanoparticles, because the particles do not pass the recirculation zone of the tubular flame and thus have almost the same residence time in the high temperature regions. On the other hand, feeding precursors through the central jet flow gives rise to non-uniform nanoparticles. In this case, some of particles pass the recirculation zone of the swirl and spend more time in high temperature regions. These particles have relatively much larger sizes than those particles that do not pass the recirculation zone, which cause the non-uniformity of the particle size. This analysis is further supported by a monodisperse population balance model based on the flame field obtained from a CFD simulation. Moreover, the central jet flow is able to carry much more precursors than that carried by the tangential premixed gas, and thus can be used in scale-up productions of nanoparticles. As for the doping synthesis, two kinds of precursors were fed through the two different routes. The thermal-driven Pd precursors enter the tubular flame field through the central jet for longer residence time in the high temperature region, while the radical-driven TiO₂ precursors are carried by the tangential flow premixed gas to pass through the reactive zone of the flame surface. The two kinds of nanoparticles are then well-mixed in the flame field and formed highly-dispersed nanoparticles before deposited on the stagnation plate.

Transition metal nitrides have recently been studied extensively as alternative plasmonic materials for use in a variety of nanophotonics applications ^[1]. Titanium nitride (TiN) in particular attracted attention due to its wide use in several industries. The refractory nature of the material combined with the gold-like optical properties in the visible and near-infrared regions of the electromagnetic spectrum provides a unique set of properties promising solutions to long-standing challenges in the fields of plasmonics and metamaterials ^[2,3]. In this talk, we will present results from characterization of nanostructured plasmonic TiN samples fabricated via DC magnetron sputtering and direct nitridation of TiO₂ at elevated temperatures. Optical characterization is supported with XPS, XRD, and EFTEM measurements for a better understanding of the relation between material quality and plasmonic performance.

Lithographically patterned large scale surfaces made of TiN enable precise control of light with plasmonic layers that have refractory properties. Well established fabrication of TiO₂ allows complex nano-architectures for plasmonic TiN components obtained via nitridation. The performance of the nanostructures will be presented with an emphasis on the optical properties. Material properties that meet application-specific requirements will be highlighted and the room for improvement will be discussed. Recent progress and challenges in large scale fabrication of nanostructured refractory plasmonic materials will be reviewed. The use of plasmonic metal nitrides in photocatalysis ^[4], photothermal therapy ^[5] and high temperature applications such as thermophotovoltaics ^[6] will be introduced.

[1] G. V Naik, J. Kim, A. Boltasseva, Opt. Mater. Express 2011, 1, 1090.
[2] U. Guler, A. Boltasseva, V. M. Shalaev, Science 2014, 344, 263.
[3] U. Guler, V. M. Shalaev, A. Boltasseva, Mater. Today 2015, 18, 227.
[4] A. Naldoni, U. Guler, Z. Wang, M. Marelli, F. Malara, X. Meng, A. V. Kildishev, A. Boltasseva, V. M. Shalaev, to be submitted.
[5] U. Guler, S. Suslov, A. V Kildishev, A. Boltasseva, V. M. Shalaev, Nanophotonics 2015, 4, 269.
[6] W. Li, U. Guler, N. Kinsey, G. V Naik, A. Boltasseva, J. Guan, V. M. Shalaev, A. V Kildishev, Adv. Mater. 2014, 26, 7959.

Plasmas have been used in various configurations for the synthesis of nanoscale materials. A common method uses thermal plasma to evaporate a solid feedstock. The high temperature vapor can then be cooled to nucleate nanoparticles. Using thermal plasma, growth by coagulation is inevitable, and the product is typically comprised of polydispersed hard agglomerates [1].
In the last decade, non-thermal plasmas were found to have unique features for the synthesis of nanoparticles [2]. Inherent advantages of using cold plasma as a synthesis medium are particle charging and ion bombardment. When the particle number density is smaller than the ion density, it is believed that the particles experience unipolar charging. The negative charge acquired by the particles suppresses coagulation and leads to higher ion bombardment rates. Ion bombardment is known to selectively heat particles up to temperatures of 1000K, despite the low gas temperature near 323 K. Thus, crystalline particles can be synthesized with minimal heating of the surrounding gas [3,4]. Selective heating results in a system that is not in equilibrium, since the gas is cool but the particles are hot, which leads to an energy efficient process. However, our understanding of ion-particle interactions in these systems remains incomplete. Does the ion bombardment cause any other unexpected phenomena?
The hypothesis we explore in this work is that ion bombardment of nanoparticles suspended in non-thermal plasma can cause them to vaporize. Vaporization could proceed by two different mechanisms: thermal evaporation and in-flight sputtering. We expect vaporization to be most pronounced in materials with low surface binding energy and high equilibrium vapor pressure. In this experimental work, controlled aerosols that consisted of argon and metal nanoparticles, with no other intentionally introduced chemical species, were sent into a radio frequency capacitively coupled plasma. We explored particles comprised of 3 different metals: Zn, Bi and Sb. Optical emission spectroscopy was used to assess the amount of metal vapor generated in the plasma. TEM analysis of particles captured after plasma treatment revealed that the plasma leads to significant surface restructuring and vaporization, such that the particles coming out of the plasma were more monodispersed and spherical compared to the inlet condition. The findings are well represented with a model that involves vaporization mechanisms of thermal evaporation and in-flight sputtering.
References:
[1] A. Gurav, T. Kodas, T. Pluym, and Y. Xiong, Aerosol Sci. Technol., vol. 19, no. 4, pp. 411–452, Jan. 1993.
[2] U. Kortshagen, Plasma Chem. Plasma Process., vol. 36, no. 1, pp. 73–84, Sep. 2015.
[3] L. Mangolini, E. Thimsen, and U. Kortshagen, Nano Lett., vol. 5, no. 4, pp. 655–659, Apr. 2005.
[4] L. Mangolini and U. Kortshagen, Phys. Rev. E, vol. 79, no. 2, p. 026405, Feb. 2009

We have developed unique synthetic capabilities for the high-pressure infiltration of semiconductors^1-3 into diverse 3D nanotemplates to create new materials in which electronic, magnetic, and vibrational degrees of freedom interact with well-ordered nanometer-scale 3D structural modulations. We call these materials metalattices.^4,5 Ordered, electrically continuous 3D structural modulations of quantum confinement and interfacial physics may define a new physical regime for electronic, optical, magnetic, and thermal response. The greatly altered palette of physical properties thereby made available in well-developed semiconductor platforms such as Si could enable practical application in diverse areas such as solar cells, near-IR photonics, light emitting devices, and improved thermoelectrics.

Using high pressure chemical vapor deposition silicon and germanium metalattices with periodicities from 14 nm to 60 nm have been synthesized, highlighting the possibility of continuous tuning of properties. The smallest periodicity metalattices have semiconductor regions only a few nm across such that quantum confinement may arise, yet electrical current can flow throughout them. We will present characterization of metalattices by means of TEM, SEM, electrical conductivity, thermal conductivity, Raman spectroscopy, photoluminescence, and positron annihilation spectroscopy techniques.

This work was supported by the National Sciences Foundation Materials Research Science and Engineering Centers (MRSEC) under award DMR-1420620.

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