Chemical vapor synthesis (CVS) is a gas phase process to generate nanoparticles via a chemical reaction. In this contribution a review of recent methodological advances is provided using nanocrystalline iron oxides as model system. Novel methods are described to control precursor flow of sublimating metalorganic solids, characterization of electronic and molecular structure of vapors using X-ray absorption spectroscopy (XAS), in situ determination of oxygen partial pressure, variation of the time-temperature profile, in situ characterization of electronic, local, crystal and micro-structure using XAS, wide and small angle scattering (WAXS and SAXS) and finally methods to design the CVS process and materials development.

In case of very small nanocrystals, the surface is the dominant defect, generating a microstructure as deviation from the ideal crystal structure by breaking the translational symmetry of the lattice. This heterogeneous disorder may be observed as a different local structure in the core and shell of the nanocrystals and is highly relevant in all properties and applications for which defects are detrimental or beneficial, for example, charge-carrier traps in (opto-)electronics or active surface sites in heterogeneous catalysis. Very small SnO2 nanocrystals generated by chemical vapor synthesis with coherent diffracting domain sizes between 2 and 13 nm are investigated as a model system since they form phase-pure stannic oxide isostructural to rutile. Local structure, crystal structure, and microstructure are studied using X-ray absorption spectroscopy, wide-angle X-ray scattering, X-ray diffraction, and transmission electron microscopy analyzed using reverse Monte Carlo simulations, Rietveld refinement, and image analysis. Size, surface, and interface effects as well as structural defects are discussed. The fraction of atoms at surfaces is substantial for very small nanoparticles. However, only very subtle structural changes are observed here for very small SnO2 nanocrystals as revealed by detailed quantitative structural analysis. This may be because the particles are highly crystalline even for the ultrasmall 2 nm particles which are close to the molecular cluster regime.

A model flow reactor provides a narrow particle temperature-residence time distribution with well-defined conditions and is mandatory to measure changes of the particle structure precisely. The experimental data of iron and iron oxide agglomerates are used to determine the sintering kinetics considering the temperature-time history of the particles. Thousand particle trajectories are tracked in a validated CFD model at three different furnace temperatures each. Strongly agglomerated particles with a small primary particle size (similar to 4 nm) are synthesized by spark discharge and are size-selected (25-250 nm) before sintering. The structure development is measured simultaneously with different online instrumentations and the structure calculated by means of structure models. A simple sintering model, based on the reduction of surface energy, is numerically quantified with the experimental results. The surface of the particles is strongly dependent on the primary particle size and the agglomerate structure. The chemical phase is analyzed using the offline techniques XANES, XRD, and EELS. It is observed that the addition of hydrogen led to a reduction of iron oxide to iron nanoparticles and to changes of the sintering kinetics. The sintering exponent m = 1 was found to be optimal. For Fe, an activation energy E-a of 59.15 KJ/mol and a pre-exponential factor A(s) of 1.57 10(4) s/m were found, for Fe(3)O(4 )an activation energy E-a of 55.22 kJ/mol and a pre-exponential factor A(s) of 2.54 10(4) s/m.

A novel chemical vapor synthesis reactor design is used to control the pore-particle mesostructure and investigate the pore formation mechanism through the variation of residence time in oxygen. This enables the exploitation of the Kirkendall effect at the nanoscale to generate ultrasmall pores in small nanocrystalline iron oxide particles. Detailed structural characterization and quantitative data analysis of complementary high resolution transmission electron microscopy images, X-ray diffractograms, nitrogen sorption isotherms and X-ray absorption spectra provide a consistent comprehensive picture of the hollow nanoparticles from the local to the microstructure. The pore formation mechanism seems to play a key role for β-Fe2O3 polymorph formation.

We describe a versatile reactor system for chemical vapor synthesis of nanoparticles, which enables in situ investigations of high temperature gas phase particle formation and transformation processes by x-ray scattering and x-ray absorption spectroscopy. The system employs an inductively heated hot wall reactor as the energy source to start nanoparticle formation from a mixture of precursor vapor and oxygen. By use of a modular set of susceptor segments, it is especially possible to change solely the residence time of the gas mixture while keeping all other process parameters (temperature, gas flow, pressure) constant. Corresponding time-temperature profiles are supported by computational fluid dynamics simulations. The operation of the system is demonstrated for two example studies: tin oxide nanoparticle formation studied by small angle x-ray scattering and iron oxide nanoparticle formation by x-ray absorption spectroscopy.

Tetramethylsilane is a precursor often used for the production of flame-synthesized silica nanoparticles or coatings. This study investigates the chemical reaction mechanism of tetramethylsilane in a series of H-2/O-2/Ar low-pressure (p = 30 mbar) flames from fuel-lean to slightly fuel-rich flame conditions (phi= 0.8, 1.0 and 1.2). Mole fraction profiles are obtained by molecular-beam mass spectrometry. The experimental data are compared to simulations using a recently published reaction mechanism. The present study reveals the influence of the flame composition on the depletion of the precursor TMS, the formation of its main carbon-containing products (e.g. CO2 and CO) and the main silicon-containing intermediates (e.g. Si(CH3)(3)(CH2)OO), Si(OH)(4), SiO2, Si4O10H4) appearing along the routes of particle formation. TEM images of synthesized particles reveal that the nanoparticles obtained from the gas-phase synthesis are spheres with a low degree of agglomeration. The particle size distribution appears to be dependent on the equivalence ratio of the synthesis flames and the changes can tentatively be traced to different particle formation pathways. The data set provided in this work can serve a basis for improvements to the reaction mechanisms of the Si/C/H/O system that are urgently needed to improve particle synthesis processes. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Technologically relevant gas phase processes rely on reactants in vapor form for the production of thin films and nanoparticles. An instrument is described which enables the investigation of such vapors by x-ray absorption spectroscopy. Corresponding in situ studies provide information about gas phase precursor chemistry and optimized synthesis processes. The setup consists of a sealed vapor container heated by a hot air bath. Inert gas filling and temperature monitoring are implemented. Fluid dynamic simulations reveal a homogeneous temperature distribution without hot or cold spots. Temperature stability better than 1 K for at least 190 min allows time-dependent measurements or improved signal to noise ratios by averaging of datasets. Iron acetylacetonate is studied as a model system. X-ray absorption spectra measured by fluorescence are of high quality, allowing a detailed analysis of X-ray Absorption Near Edge Structure (XANES) and extended x-ray absorption fine structure. A molecular structure transformation is observed in XANES spectra of iron acetylacetonate vapor above 480 K probably due to the loss of one ligand. The setup allows the investigation of low volatility compounds with vapor pressures above 2 kPa at temperatures up to 520 K.

Controlling the oxidation state of iron and the crystal structure of iron containing compounds is the key to improved materials such as iron oxide nanoparticles for cancer treatment or heterogeneous catalysis. Iron oxides contain iron in different oxidation states and form different phases for one valence state (α-Fe^3+ _2O^2- _3, β- Fe^3+ _2O^-3 _2, etc.). Chemical vapor synthesis (CVS) allows the reproducible production of pure nanocrystals with narrow size distribution where particle formation and growth take place in the gas phase. Through the controlled variation of synthesis parameters CVS enables the synthesis of diverse iron oxide phases. In this study the energy for the CVS process is supplied by a hot wall furnace and a microwave plasma. The advantage of an plasma reactor as the first CVS stage is the fast and complete precursor decomposition at low temperatures. This results in a larger process window for the hot wall reactor in the second stage. The nanoparticles are examined regarding their structure, surface and valence by XRD and TEM.

Platinum and iridium are rare and expensive noble metals that are used as catalysts for different sectors including in heterogeneous chemical automotive emission catalysis and electrochemical energy conversion. Nickel and its alloys are promising materials to substitute noble metals. Nickel based materials are cost-effective with good availability and show comparable catalytic performances. The nickel-molybdenum system is a very interesting alternative to platinum in water electrolysis. We produced ligand-free nickel-molybdenum nanoparticles by laser ablation in water and acetone. Our results show that segregated particles were formed in water due to the oxidation of the metals. X-ray diffraction shows a significant change in the lattice parameter due to a diffusion of molybdenum atoms into the nickel lattice with increasing activity in the electrochemical oxygen evolution reaction. Even though the solubility of molecular oxygen in acetone is higher than in water, there were no oxides and a more homogeneous metal distribution in the particles in acetone as seen by TEM-EDX. This showed that dissolved molecular oxygen does not control oxide formation. Overall, the laser ablation of pressed micro particulate mixtures in liquids offers a combinational synthesis approach that allows the screening of alloy nanoparticles for catalytic testing and can convert micro-mixtures into nano-alloys.