Chemical Vapor Synthesis Research Articles

Two-Step Atmospheric RF-Microplasma Synthesis of Lithium-Ion Battery Cathode

To address the increasing demand for lithium-ion batteries for use in energy storage applications, such as electric vehicles, there is a need to make the cathode synthesis process less energy intensive, as the current method requires a sintering temperature of 650 – 950 °C for up to 12 hours. Radiofrequency (RF) microplasma setups provide an avenue to approach this problem by providing a tunable system that can be operated under atmospheric conditions, with a more efficient transfer of energy in the plasma setup as compared to a thermal furnace. Through an RF-microplasma setup, nickel, manganese, and cobalt (NMC) particles were produced from a metal acetate solution under a plasma assisted chemical vapor synthesis. These produced particles were then sintered for less time and under a lower temperature than traditional cathode synthesis methods, to produce a lithiated NMC type cathode. The product was examined physiochemically and electrochemically to examine its structure and functionality. The produced product was reversibly cycled over 60 times with a peak specific capacity around 150 mAh/g, and with future process refinement could meet the current capacity standards set by more established synthetic routes

Manipulating the Curie point in γ-Fe filled carbon nano-onions: The role of Mn and Gd for enhanced magnetic functionalities

Recent works have proposed the stabilization of γ-Fe nanocrystals inside carbon nano-onions (CNOs) and nanotubes (CNTs) as active ferromagnetic phases for possible applications in hyperthermic dissipative systems. Stabilization of the γ-phase of iron is of particular interest due to the unit-cell dependent ferromagnetic properties and controllable Curie point. Here we investigate a modification of the chemical vapor synthesis (CVS) method for the in-situ manipulation of γ-Fe nanocrystals within CNOs, in presence of bis(isopropyl-cyclopentadienyl)manganese or tris(tetramethyl-cyclopentadienyl)gadolinium(III) as additional growth-precursors. In the presented experiments ferrocene, dichlorobenzene and sulfur mixtures were employed with a fixed ratio. Interestingly, when employing bis(isopropyl-cyclopentadienyl)manganese we demonstrate the stabilization of a modified FCC γ-Fe phase inside the CNOs, identifiable as a γ-Fe23C6 through X-ray diffraction and refinement of the structural model (obtained by employing the Rietveld method), with unit cell parameters a=b=c= 10.5569 Å and space group Fm-3m. The relative amount of Mn content was found to vary from 0.2% to 0.5%. Differently, comparative experiments performed in presence of great excess of tris(tetramethylcyclo-pentadienyl)gadolinium(III) reveal a significant variation of the sample’s morphology, with the nucleation of Gd nanoparticles around γ-Fe filled CNOs. Magnetic characterization with field-dependent zero field cooled (ZFC) and field cooled (FC) acquisitions revealed for the first batch of experiments (method 1) a ferromagnetic behavior comparable to that of γ-Fe, with a T1 Curie point located in proximity of T∼ 400 K and a T2 Curie point in proximity of T∼90 K. Differently, for method-2, an important interplay of ferromagnetic and paramagnetic components was found in the Gd-decorated γ-Fe filled CNOs, with a reduction of the Curie point down to T∼350 K. The presented findings highlight the important role of 1) the γ-Fe23C6 phase in enhancing the saturation magnetization Ms of the filled CNOs and 2) the crystallinity of the Gd-component in allowing for an enhancement of the paramagnetic response in the filled CNOs.

Single-step in situ synthesis of bimetallic catalysts via a gas-phase route: the case of PdZn–ZnO

In this study, we explore the catalytic activity of highly pure PdZn–ZnO nanopowder, synthesized via an innovative metal–organic chemical vapor synthesis (MOCVS) method.

Chemical vapor synthesis of nanocrystalline iron oxides

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.

Synthesis of carbon nano-onions filled with γ-Fe and Gd/GdCl3: A candidate multifunctional system

Stabilization of high-spin high-volume ferromagnetic γ-Fe nanocrystals inside carbon nano-onions (CNOs) has recently attracted an important attention for possible applications in magnetic hyperthermia cancer therapy. Here we report an advanced investigation of the chemical vapour synthesis (CVS) fabrication methodology of the filled CNOs, with focus on 1) the stabilization of the γ-Fe nanocrystals in presence of larger concentration of Cl radicals and 2) the structural functionalization, through simultaneous encapsulation of both ferromagnetic γ-Fe and paramagnetic GdCl3 phases. By employing HRTEM we demonstrate an anomalous structural configuration of the CNOs, involving the coexistence of defect-rich concentrical-like and radially-oriented graphitic layers. XPS and Raman spectroscopy evidence the presence of CS bond-formation and defect nucleation as key parameters for the growth of the radially-oriented graphitic layers. This unusual structural arrangement of the CNO is found to stabilize the γ-Fe phase at room temperature. We highlight the absence of FCC to BCC structural relaxation within timescales of 8 months from sample-fabrication. XRD, Rietveld refinements, together with magnetometry confirm the presence of a ferromagnetic transition arising below T ∼ 70 K within γ-Fe. This interpretation is further corroborated through extended characterization with ESR spectroscopy. Structural manipulation and functionalization of the CNOs is demonstrated by including tris(tetramethyl-cyclopentadienyl)gadolinium(III) as a precursor in the pyrolysis experiments. The functionalized CNO (filled with ferromagnetic γ-Fe/Fe3C and paramagnetic Gd/GdCl3) can be visualized as a multifunctional-container and a candidate for magnetic-hyperthermia-cancer-therapy and MRI contrast applications.

LaCo1–xFexO3 Nanoparticles in Cyclohexene Oxidation

The identification of structure–activity relationships is challenging for complex oxides like lanthanum-based perovskite nanoparticles. The purpose of this study is to examine the structure of LaCo1–xFexO3 nanoparticles and develop a detailed structural model to identify subtle changes of the structure after catalysis. Therefore, small nanoparticles with a significant fraction of surface atoms and varying iron content (x = 0, 0.5, 1) are synthesized in the gas phase by chemical vapor synthesis and tested in cyclohexene oxidation in the liquid phase. The crystal structure is examined by X-ray and selected area electron diffraction. Additionally, the local structure of the cations is probed by X-ray absorption spectroscopy at the Co, Fe, La K, and La L3-edges. For a quantitative analysis of the local structure before and after catalysis, an atomistic model is refined by the available extended X-ray absorption fine structure spectra using Reverse Monte Carlo methods. The produced nanoparticles are small with a coherent diffraction domain size between 5 and 10 nm, highly crystalline, and consist mainly of the perovskite phase with a secondary spinel phase. In cyclohexene oxidation, the cobalt-containing samples exhibit significant catalytic activities. The active samples show structural changes after catalysis accompanied by reconstruction of the local structure surrounding the cations in the perovskite phase, which resembles edge-sharing cobalt octahedra.

Chemical vapour synthesis of carbon nano-onions filled with high-spin ferromagnetic γ-Fe50Ni50 nanocrystals: a structural and magnetic investigation

The recent discovery of an innovative chemical vapour synthesis (CVS) approach for the encapsulation and stabilization of high-spin ferromagnetic γ-Fe nanocrystals inside carbon nano-onions (CNOs) has attracted an important attention....

Possible interplay of tangential and perpendicular modes in the growth of Fe-filled carbon nanotubes

Boundary layer chemical vapour synthesis is a fabrication approach which exploits random fluctuations within the viscous boundary layer established between a laminar flow of pyrolyzed ferrocene and a rough substrate, to yield iron filled radial carbon nanotubes (CNTs).

Sulfur-Mediated Synthesis of Spherical Nickel Nanoparticles in a Chemical Vapor Reactor.

In this study, we investigated the effect of the sulfur content in the NiCl2 precursor on the shape of nickel nanoparticles (Ni-NPs) prepared by chemical vapor synthesis. We obtained spherical Ni-NPs when using anhydrous NiCl2 mixed with NiSO4 or Na2SO4 with a molar ratio of 0.002 as precursors without changing any other process parameters whereas faceted Ni-NPs when using only anhydrous NiCl2 as a precursor. First-principles calculations supported experimental results, which showed that NiSO4-mixed NiCl2 and Na2SO4-mixed NiCl2 precursors favored the growth of spherical NPs.

Flash evaporation of low volatility solid precursors by a scanning infrared laser

The steady and stoichiometric delivery of metal-organic precursor mixtures is essential for the production of complex, functional nanomaterials in the gas phase. Chemical vapor synthesis (CVS) is a corresponding process which enables the production of complex oxide nanoparticles such as perovskites. While there exist a vast number of compositions that form perovskite structures, many technically relevant materials consist of transition metals and lanthanides. Their corresponding metal organic precursors often deviate significantly in their thermal behavior, resulting in a challenging delivery of precursors to the reactor. One suitable method for precursor delivery is flash evaporation by an infrared laser, where a mixture of solid precursors is instantly sublimed. Using flash evaporation, the stoichiometry of the generated vapor corresponds to the composition of precursors in the solid mixture. In this study, we present an alternative flash evaporation system based on a marking laser which rapidly scans a focused infrared beam across a precursor powder bed. By focusing the beam, higher energy densities are reached, compared to existing systems while a large area powder bed is repeatedly scanned and sublimed. Fourier-transform infrared spectroscopy (FTIR) measurements confirm the decomposition-free sublimation of precursor mixtures. Furthermore, we confirm the successful precursor delivery by the synthesis of LaFeO3 nanoparticles with an average crystallite size of 5.3 nm. The structure of the ensemble of nanoparticles is examined using X-ray diffraction (XRD) and Rietveld refinement, transmission electron microscopy (TEM), selected area diffraction (SAED), and extended X-ray absorption fine structure (EXAFS) at the Fe-K edge analyzed by reverse Monte Carlo (RMC) analysis.

A versatile chemical vapor synthesis reactor for in situ x-ray scattering and spectroscopy.

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.

Formation of polymorphs and pores in small nanocrystalline iron oxide particles

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.

Stacking-dependent exciton multiplicity in WSe2 bilayers

Twisted layers of atomically thin two-dimensional materials realize a broad range of quantum materials with engineered optical and transport phenomena arising from spin and valley degrees of freedom and strong electron correlations in hybridized interlayer bands. Here, we report on experimental and theoretical studies of ${\mathrm{WSe}}_{2}$ homobilayers obtained in two stable configurations of $2H$ (${60}^{\ensuremath{\circ}}$ twist) and $3R$ (${0}^{\ensuremath{\circ}}$ twist) stackings by controlled chemical vapor synthesis of high-quality large-area crystals. Using optical absorption and photoluminescence (PL) spectroscopy at cryogenic temperatures, we uncover marked differences in the optical characteristics of $2H$ and $3R$ bilayer ${\mathrm{WSe}}_{2}$ which we explain on the basis of beyond-density functional theory calculations. Our results highlight the role of layer stacking for the spectral multiplicity of momentum-direct intralayer exciton transitions in absorption and relate the multiplicity of phonon sidebands in the PL to momentum-indirect excitons with different spin valley and layer character. Our comprehensive study generalizes to other layered homobilayer and heterobilayer semiconductor systems and highlights the role of crystal symmetry and stacking for interlayer hybrid states.

Synthesis, Crystal Growth, and Annealing of CdSxSe1-x and Cr:CdS0.8Se0.2 Single Crystals.

Mid-infrared laser in the 2-5 μm wavelength region is in the atmospheric transmission window range, and hence, it has important application prospects in the fields of optoelectronic countermeasures, space communication, environmental remote sensing, and molecular spectroscopy. One of the most promising technological approaches to achieve mid-infrared laser output is based on direct lasing of transition-metal (TM)-doped II-VI chalcogenide crystals. In this work, CdSxSe1-x and Cr:CdS0.8Se0.2 polycrystals were synthesized by a chemical vapor synthesis method from a stoichiometric mixture of vacuum-sublimed CdS and CdSe. The structure of the synthesized products was analyzed by X-ray diffraction (XRD). Using these synthesized products, CdSxSe1-x and Cr:CdS0.8Se0.2 single crystals were grown by the physical vapor transport (PVT) method. After annealing, the band gap becomes smaller and the transmission range widens to 17 μm. The composition of the single crystals was determined by energy-dispersive spectrometry (EDS) mapping and XPS, and it was found to be uniform throughout the ingot. In addition, the absorption peak maximum for the Cr2+ ion in the Cr:CdS0.8Se0.2 crystal is at 1.84 μm.

Multi-step chemical vapor synthesis reactor based on a microplasma for structure-controlled synthesis of single-walled carbon nanotubes

To address the need for the high precision synthesis of nanomaterials with the potential for high productivity, we present a novel design of a multi-step chemical vapor synthesis (CVS) reactor, which centers on the use of an atmospheric, continuous flow microplasma. Based on computational fluid dynamic gas flow simulations, the CVS reactor was designed to provide abrupt interaction steps to provide a well-defined and highly controlled start and stop to the nanomaterial (e.g., nanoparticle) growth. By applying this framework to gas-phase single-walled carbon nanotube (SWCNT) synthesis, the precise start and stop of the catalyst nanoparticle nucleation process can be done using the microplasma reactor (start) and carbon reactant gas to suppress further aggregation/promote SWCNT growth (stop), respectively. In so doing, we demonstrated the generation of controlled size of catalyst nanoparticles and SWCNTs despite high particle densities. We envision that our approach demonstrates the feasibility of fine structural control of nanomaterials through a multi-step CVS process with precisely regulating the aggregation time for high density particle flow, which overcomes the obstacle of both structural control and productivity.

Nanoceramics by chemical vapour synthesis

Abstract The synthesis of nanoparticles and nanopowders is in principle not difficult. Consequently, various methods on a laboratory and industrial scale have been developed in the last century. However, in the last decades it became obvious that the control of synthesis and processing is required to develop nanocrystalline materials with novel functional properties leading to specific applications. The potential of nanostructures was recognized through the pioneering work of Gleiter in the early 80’s [1]. In this review, we show that chemical vapour synthesis allows the production of nanostructured materials with tailored microstructures and properties.

Synthesis of Hexagonal Structured GaS Nanosheets for Robust Femtosecond Pulse Generation.

Gallium sulfide (GaS), with a hexagonal structure, has received extensive attention due to its graphene-like structure and derived optical properties. Here, high-quality GaS was obtained via chemical vapor synthesis and then prepared as a saturable absorber by the stamp-assisted localization-transfer technique onto fiber end face. The stability of the material and the laser damage threshold are maintained due to the optimized thickness and the cavity integration form. The potential of the GaS for nonlinear optics is explored by constructing a GaS-based Erbium-doped mode-locked fiber laser. Stable femtosecond (~448 fs) mode-locking operation of the single pulse train is achieved, and the robust mode-locked operation (>30 days) was recorded. Experimental results show the potential of GaS for multi-functional ultrafast high-power lasers and promote continuous research on graphene-like materials in nonlinear optics and photonics.

Controlling the quantity of γ-Fe inside multiwall carbon nano-onions: the key role of sulfur.

One of the most interesting structural features of multiwall carbon onions (MWCNOs) and nanotubes (MWCNTs) is the excellent chemical stability, which allows in situ encapsulation of chosen magnetic materials of interest and multifunctional applications. In this letter, we present an innovative chemical vapour synthesis (CVS) approach, in which the inclusion of small quantities of sulfur during the pyrolysis of ferrocene/dichlorobenzene mixtures allows for an important control in the relative abundance of FCC γ-Fe, up to a maximum value of ∼86.5% (structural- and phase-control). The variation in the relative percentage of the encapsulated Fe-based phases was estimated by employing X-ray diffraction (XRD) and Rietveld refinement analyses. The magnetic characterization was achieved by employing superconducting quantum interference device (SQUID) magnetometry, with zero field cooled (ZFC) and field cooled (FC) curves acquired at applied fields of 300 Oe and ∼50 000 Oe.

Chemical Vapor Synthesis of Nonagglomerated Nickel Nanoparticles by In-Flight Coating.

Nickel (Ni) nanoparticles (NPs) prepared through vapor-phase synthesis (VPS) are preferred for multilayer ceramic capacitor electrodes due to their high purity and crystallinity advantages. Agglomerated Ni NPs are usually generated using VPS but are undesirable because they cause various problems such as low packing density and electrical shorts. This study proposes the use of coating-assisted chemical vapor synthesis (CVS) for agglomerate inhibition using NaCl or KCl as a coating agent. We have found that the agglomeration ratio, 34.40%, for conventional CVS, can be reduced to 4.80% in the proposed method by in-flight coating with KCl at 900 °C by image analysis using field-emission scanning electron microscopy. Furthermore, the X-ray diffraction and X-ray fluorescence analyses confirm that the NaCl and KCl coating agent can be removed by washing with distilled water. We believe that this coating process can be used to inhibit the formation of agglomerates during the CVS of Ni NPs.

Always cubes: A comparative evaluation of gas phase synthesis methods and precursor selection for the production of MgO nanoparticles

MgO nanocubes are well-suited for the study of defects and interfaces inside metal oxide powders and ceramics. We evaluated the synthesis of MgO nanocubes via flame spray pyrolysis (FSP) and compared the structural and spectroscopic particle properties with materials obtained by chemical vapor synthesis (CVS). Characterization of the as-synthesized and vacuum annealed powder samples revealed slightly larger particle sizes for the FSP material that is absolutely comparable to CVS powders in terms of the particles structural definition, morphology and crystallinity. Moreover, optical absorption and photoluminescence emission studies show fingerprints, which are characteristic of MgO nanocrystals with a high abundance of grain corners, kinks and edges. The opportunity to synthesize MgO nanoparticles of high structural definition via FSP at significantly larger production rates and lower costs compared to CVS carries great potential to upscale powder production to generate MgO based functional oxide nanocomposites for ceramics and catalysis.