Precursor Chemical Composition Research Articles

Elemental carbon and hydrogen concentrations as the main factors in gas-phase graphene synthesis: Quantitative fourier-transform infrared spectroscopy study

Gas-phase microwave plasma-assisted synthesis of freestanding graphene is a promising route to produce high-quality graphene flakes. However, a comprehensive understanding of the gas-phase kinetics that is required to advance production rates and yields is still lacking. Here, we use line-of-sight Fourier-transform infrared (FTIR) absorption spectroscopy as an in situ diagnostic to measure gaseous species formed during graphene synthesis, thereby elucidating the chemical kinetics mechanism. Different carbonaceous precursors that lead to the formation of either few-layer graphene flakes or soot-like particles are examined, and the results are compared with numerical simulations and mass spectrometry measurements. Quantitative FTIR measurements show a correlation between concentration of atomic carbon and hydrogen in the post-plasma region and particle morphology. Lower carbon and higher hydrogen concentrations lead to graphene formation, while higher carbon and lower hydrogen concentrations shift the reaction toward soot-like particles. We also investigate how the precursor chemical composition, precursor flowrate, and delivered microwave power affect the carbon concentration in the post-plasma region, thus enabling to control particle morphology.

Effect of H2O Activity on Zeolite Formation

In an effort to understand the effects of H2O activity on zeolite formation, we have synthesized LTA zeolite using a combination of freezing processes and varying drying temperatures. Sodium aluminate and sodium silicate were used to form LTA zeolite, according to the IZA (International Zeolite Association) protocol. The synthesis steps were modified by adding the precursor frozen process by a rapid liquid nitrogen (−196 °C) treatment or slow conventional freezer treatment (−20 °C). The samples were subsequently sonicated and then dried at 80 °C or 40 °C. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were performed on the samples immediately after the drying process as well as after 2 weeks and 1 month of aging the solid products. The results indicated that LTA zeolite does not form. The silica-alumina precursor after both freezing processes and after being dried at 80 °C showed the presence of sodalite displaying stable behavior over time. Both sets of samples dried at 40 °C and did not show the presence of zeolite immediately after the drying process. However, after 2 weeks, the liquid nitrogen–frozen precursor was characterized by the presence of EMT whereas zeolites never formed in the −20 °C samples. These results suggest that freezing processes differently control the H2O activity during the drying and aging processes in the solid state. Thus, although the precursor chemical composition is the same, the type of zeolite formed is different.

Precursors for direct-write nanofabrication with electrons

The influence of precursor's chemical composition and side reactions on materials obtained by direct nanoprinting using FEBID techniques is discussed.

Electrospun SiOC ceramic fiber mats as freestanding electrodes for electrochemical energy storage applications

Polymer-derived-ceramics (PDCs) and PDC-based fibers are being studied as potential high capacity electrode materials for electrochemical energy storage applications. Among these, silicon oxycarbide (SiOC) has shown most promise, especially as electrodes in lithium ion batteries (LIB). The study of PDCs for supercapacitor electrodes is, however, very limited. This work investigates the fabrication of SiOC-based PDC fiber mats via electrospinning and pyrolysis of three commercial pre-ceramic silicone resins (MK, H44, RSN) of varying composition, and their use as supercapacitor and LIB electrodes. Electron microscopy, Fourier transform infrared spectroscopy and Raman spectroscopy was performed to study the effect of precursor chemical composition, pyrolysis time and temperature on the morphology, polymer to ceramic transformation and free carbon content. As supercapacitor electrodes, the H44 (a methyl-siloxane) derived SiOC showed the best performance with maximum specific capacitance of 50 F g-1 with capacity retention of ~100% after 2000 cycles. Similarly, as LIB electrodes, H44 derived SiOC fiber mat showed highest reversible capacity of 578 mAh g-1 at a current density of 50 mA g-1.

The effect of N precursors in Fe-N/C type catalysts based on activated silicon carbide derived carbon for oxygen reduction activity at various pH values

Simple and modified synthesis route for preparation of Fe-N/C type oxygen reduction reaction (ORR) catalysts has been worked out and discussed. The significant effect of the N precursor chemical composition on the ORR activity in 0.1 M KOH and 0.1 M HClO4 electrolyte solutions has been demonstrated. The half-wave potential values increased in the following order: C(SiC) < Fe + Urea/C < Fe + EDTA/C < Fe + DIPO/C ≤ Fe + Phen/C ≤ Fe + Bipyr/C in both electrolyte solutions studied. Molar ratio of Fe:N precursor and Fe content change in the Fe + Bipyr/C catalyst was found to be negligible. Noticeable change in ORR activity has been observed after acid treatment step of Fe + Bipyr/C. The 150-h durability tests demonstrated that in terms of the activity/stability ratio the Fe + Phen/C is the best ORR catalyst.

Supramolecular Assembly of Single-Source Metal–Chalcogenide Nanocrystal Precursors

In this feature article, we discuss our recent work in the synthesis of novel supramolecular precursors for semiconductor nanocrystals. Metal chalcogenolates that adopt liquid-crystalline phases are employed as single-source precursors that template the growth of shaped solid-state nanocrystals. Supramolecular assembly is programmed by both precursor chemical composition and molecular parameters such as the alkyl chain length, steric bulk, and the intercalation of halide ions. Here, we explore the various design principles that enable the rational synthesis of these single-source precursors, their liquid-crystalline phases, and the various semiconductor nanocrystal products that can be generated by thermolysis, ranging from highly anisotropic two-dimensional nanosheets and nanodisks to spheres.

Temperature-dependence of electrical properties for the ceramic composites based on potassium polytitanates of different chemical composition

The samples of ceramic materials based on potassium polytitanate (PPT) characterized with various TiO2/K2O molar ratio, are produced by calcination at 900 °C and investigated. AC conductivity (σ ac ) of the obtained ceramics is measured at different temperatures between 200 and 800 °C in frequency range of 0.1 Hz–1 MHz. The method of combined impedance and modulus spectroscopy is used to analyze the obtained results. The activation energies of DC conductivity, bulk and grain-boundary conductivity as well as relaxation frequency for studied composites are estimated. Using the correlated barrier hopping (CBH) model, the energies of potential barrier between neighboring defect sites for all kinds of investigated materials are presented. The bulk and grain boundary parameters of the produced ceramic materials based on potassium polytitanates are calculated. The mechanism of different vacancies formation in the investigated ceramic system is discussed. The influence of precursor chemical composition on electrical properties for the ceramic composites based on potassium polytitanates is studied.

Influence of the precursor chemical composition on heavy metal adsorption properties of hemp (Cannabis Sativa) fibers based biocarbon

Waste hemp (Cannabis sativa) fibers were used as sustainable and renewable raw materials for production of low-cost biocarbon sorbent for heavy metals removal. Carbon precursors of different chemical composition were obtained by oxidative and alkaline treatments of hemp fibers. Influence of lignocellulosic precursor chemical composition on hemp fibers-based biocarbon (HFB) characteristics was examined by BET surface area measurement, scanning electron microscopy and mass titration. It was found that lignin content and polymorphic transformation of cellulose increase the SBET of microporous HFBs, while hemicelluloses induce more homogeneous distribution of adsorption active sites. Heavy metal ions adsorption onto HFBs is primarily influenced by the amount of surface oxygen groups, while specific surface area plays a secondary role. Equilibrium data obtained for lead ions adsorption were analyzed by different nonlinear adsorption isotherms, and the best fitting model was chosen using standard deviation and Akaike information criterion (AICC). The maximum adsorption capacities of HFBs ranged from 103.1 to 116.3 mg Pb/g. Thermodynamic parameters showed that Pb2+ adsorption onto HFBs is a spontaneous and complex endothermic process, suggesting the coexistence of physisorption and chemisorption mechanisms.

Phase evolution of C-(N)-A-S-H/N-A-S-H gel blends investigated via alkali-activation of synthetic calcium aluminosilicate precursors

Stoichiometrically-controlled alkali-activated pastes containing calcium-(sodium) aluminosilicate hydrate (C-(N)-A-S-H) and sodium aluminosilicate hydrate (N-A-S-H) gels are produced by alkali-activation of high-purity synthetic calcium aluminosilicate powders. These powders are chemically comparable to the glass in granulated blast furnace slag, but without interference from minor constituents. The physiochemical characteristics of these gels depend on precursor chemical composition. Increased Ca content of the precursor promotes formation of low-Al, high-Ca C-(N)-A-S-H with lower mean chain length as determined by quantification of solid state nuclear magnetic resonance spectra, and less formation of calcium carboaluminate ‘Alumino-ferrite mono’ (AFm) phases. Increased Al content promotes Al inclusion and reduced crosslinking within C-(N)-A-S-H, increased formation of calcium carboaluminate AFm phases, and formation of an additional N-A-S-H gel. Small changes in precursor composition can induce significant changes in phase evolution, nanostructure and physical properties, providing a novel route to understand microstructural development in alkali-activated binders and address key related durability issues.

Corn-like indium tin oxide nanostructures: fabrication, characterization and formation mechanism

Electrospinning is a simple but efficient procedure enabling the parallel fabrication of a multitude of inorganic fibers. But the precise control of the fiber’s morphology, which seriously affects the electrical, optical and other important properties of such electrospun materials, is still less developed. The creation of nanoscale indium tin oxide fibers with corn-like geometry (corn-like ITO NFs) by our group has provided a good example to show how to modify the morphologies and properties of nanofibers by means of tailoring the fiber’s compositions. Here we show that in the fabrication of corn-like ITO NFs, the usage of different solvents N,N-dimethylformamide (DMF) and deionized water, as well as the calcination temperature, can also lead to dramatic morphology changes, from ribbon-like to cylindrical and then to corn-like. The resultant nanoribbons and nanoscale corn-like fibers exhibit different photoluminescence properties. We find that the morphology of the as-spun fibers is closely related to the vapor pressure of the solvent we used, and the generation of ITO crystals sensitively depends on the calcination temperature, which both are critical for the morphology and properties of the final products. Thus, we demonstrate that the formation of this unprecedented nanostructure is determined by the combined effect of the precursor chemical composition, solvent and calcination temperature.

Effect of precursor chemical composition on the formation and stability of G-quadruplex core supramolecular star polymers

A homologous series of guanosine end-functional poly(butadiene)s has been prepared. The potassium cation-templated assembly of these guanosine functionalised precursors then furnished supramolecular star polymers with a G-quadruplex core. Comparison with the previously reported poly(ethylene glycol)-based supramolecular star polymers revealed that in designing supramolecular star polymers, chemically non-polar assembly precursors – that do not interfere with the supramolecular interactions of the core – are essential for the preparation of high stability and high molecular weight supramolecular branched architectures. In addition, in comparison to star polymers composed of chemically polar polymer chains, the non-polar supramolecular ensembles show chain-length independent properties.

Influence of substrate and precursor film composition on morphology and superconducting transition of Tl-2212 thin films characterized by microwaves

An influence of various substrates (MgO, LaAlO 3, SrTiO 3, YSZ, CeO 2/r-sapphire) and Ba–Ca–Cu precursor chemical composition (212, 223) upon morphology and microwave quality of Tl 2Ba 2CaCu 2O 8 (Tl-2212) thin films has been studied. Under the same thalliation conditions, Tl-2212 phase has been synthesized in all films independently of the initial precursor film chemical composition (212, 223) with almost the same T c values. However, the film surface morphology and, thus, the microwave surface impedance, both were found to be strongly dependent from the type of substrate used as well as from the precursor chemical composition. Films with sharp transition to the superconducting state were prepared when 212 precursors were deposited on the MgO and/or LaAlO 3 substrates.