Effect of active impurities on the condensation of nanoparticles from supersaturated carbon vapor in the combined laser photolysis of C<Subscript>3</Subscript>O<Subscript>2</Subscript> and H<Subscript>2</Subscript>S

The effect of active H₂S, HS^·, and atomic hydrogen impurities on the condensation of highly supersaturated carbon vapor obtained in the combined laser photolysis of a mixture of C₃O₂ and H₂S diluted with argon was studied. The concentrations of carbon vapor, HS^·, and atomic hydrogen obtained in the laser photolysis of the mixture were determined using the absorption cross sections of C₃O₂ and H₂S molecules measured in this work and the measured amount of absorbed laser radiation. The time profiles of the sizes of growing nanoparticles synthesized in C₃O₂ + Ar and C₃O₂ + H₂S + Ar mixtures were measured using the laser-induced incandescence (LII) method. An improved LII model was developed, which simultaneously took into account the heating and cooling of nanoparticles and the temperature dependence of the thermophysical properties of nanoparticles, as well as the cooling of nanoparticles by evaporation and thermal emission. The size distributions of carbon nanoparticles formed in the presence and absence of active impurities were determined with the use of a transmission electron microscope. The final average size of carbon nanoparticles was found to decrease from 12 to 9 nm upon the addition of H₂S to the system, whereas the rate of nanoparticle growth decreased by a factor of 3, and the properties of nanoparticles changed. In particular, the translational energy accommodation coefficient for Ar molecules at the surface of carbon nanoparticles was found to decrease from 0.44 to 0.30. A comparison of the calculated total carbon balance at the early stage of nanoparticle formation with experimental data demonstrated that the reaction C + H₂S → HCS^· + H, which removes a portion of carbon vapor from the condensation process, has a determining effect on the carbon balance in the system. It was found that HS^· and atomic hydrogen affect the carbon balance in the system only slightly. Thus, the experimentally observed decrease in the rate of nanoparticle growth and in the sizes of nanoparticles can be explained by a decrease in the concentration of free carbon upon the addition of H₂S molecules to the system.     

Formation of Iron-Carbon Nanoparticles behind Shock Waves

An attempt was made to obtain iron-carbon nanoparticles by two-step pyrolysis of Fe(CO)₅- and C₃O₂-containing mixtures behind incident and reflected shock waves in a shock tube. The formation of binary particles was monitored by recording the extinction of He-Ne laser radiation and laser-induced incandescence (LII). The LII method provides particle size estimates if the thermal and optical properties of the constituting material are known. Behind an incident shock wave, at temperatures of 700–1500 K, Fe(CO)₅ decomposes within a short period of time (∼50 µs). The resulting iron atoms combine into particles, which serve as condensation nuclei for carbon vapor resulting from C₃O₂ pyrolysis at 1500–3000 K behind the reflected shock wave. The binary particles thus produced are considerably larger than pure carbon or iron particles. As the mixture temperature behind the reflected shock wave is raised, the diameter of these binary particles decreases.__________Translated from Kinetika i Kataliz, Vol. 46, No. 3, 2005, pp. 333–343.Original Russian Text Copyright © 2005 by Gurentsov, Eremin, Roth, Starke.Based on a report at the VI Russian Conference on Mechanisms of Catalytic Reactions (Moscow, October 1–5, 2002).     

Photolytic cage effect of iodine in gases at high pressures

The photolysis of iodine has been studied in the gas phase using laser flash photolysis at 6943 A. The dependence of the quantum yield on the pressure has been investigated in the range 0.1–1000 atm for several inert gases. For Kr, Xe, O₂, CO₂, CH₄, C₂H₆ and C₃H₈ a decrease of the quantum yield with increasing pressure was obtained; for He, Ne, Ar and H₂ no effect could be observed. These results may correspond to a photolycitc cage effect of iodine in the gas phase analogous to that known from the liquid phase.     

Vapor phase nucleation on neutral and charged nanoparticles: condensation of supersaturated trifluoroethanol on Mg nanoparticles

A new technique is described to study the condensation of supersaturated vapors on nanoparticles under well-defined conditions of vapor supersaturation, temperature, and carrier gas pressure. The method is applied to the condensation of supersaturated trifluoroethanol (TFE) vapor on Mg nanoparticles. The nanoparticles can be activated to act as condensation nuclei at supersaturations significantly lower than those required for homogeneous nucleation. The number of activated nanoparticles increases with increasing the vapor supersaturation. The small difference observed in the number of droplets formed on positively and negatively charged nanoparticles is attributed to the difference in the mobilities of these nanoparticles. Therefore, no significant charge preference is observed for the condensation of TFE vapor on the Mg nanoparticles.     

Mechanism of Iron Oxide Formation from Iron Pentacarbonyl‐Doped Low‐Pressure Hydrogen/Oxygen Flames

A chemical reaction mechanism was developed for the formation of iron oxide (Fe₂O₃) from iron pentacarbonyl (Fe(CO)₅) in a low‐pressure hydrogen–oxygen flame reactor. In this paper, we describe an extensive approach for the flame‐precursor chemistry and the development of a novel model for the formation of Fe₂O₃ from the gas phase. The detailed reaction mechanism is reduced for the implementation in two‐dimensional, reacting flow simulations. The comprehensive simulation approach is completed by a model for the formation and growth of the iron oxide nanoparticles. The exhaustive and compact reaction mechanism is validated using experimental data from iron‐atom laser‐induced fluorescence imaging. The particle formation and growth model are verified with new measurements from particle mass spectrometry.     

Vacuum ultraviolet laser‐induced fluorescence kinetic study of the reactions of Cl atoms with fluoroalkenes (CxF2x+1CHCH2, x = 1,2,4, 6, and 8) at low pressures

Pulsed laser photolysis/vacuum ultraviolet laser‐induced fluorescence techniques were used to measure rate coefficients for Cl atom reactions with a series of fluoroalkenes (C_xF_2x+1CH?CH₂, x = 1,2,4,6,8) in 6–10 Torr of CF₄ diluent at 295 ± 2 K. Rate coefficients (units of 10^?11 cm³ molecule^?1s^?1) of 4.49 ± 0.64, 6.58 ± 0.59, 8.91 ± 0.58, 9.27 ± 0.64, and 9.00 ± 0.87 were determined for C_xF_2x+1CH?CH₂ with x = 1,2,4,6, and 8, respectively. In 6–10 Torr of CF₄ diluent, the kinetics of the title reactions are at, or near, the high‐pressure limit for x = 4, 6, and 8, approximately 30% below the high‐pressure limit for x = 2, and approximately 50% below the high‐pressure limit for x = 1. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 328–332, 2007     

Direct determination of the equilibrium constant and thermodynamic parameters in the reaction of pentadienyl radicals with O2

Reaction of pentadienyl radicals (C₅H₇) with O₂ has been studied by a combination of pulsed laser photolysis and photoionization mass spectrometry. These radicals could be generated either by the photolysis of 1,3-pentadiene or by the two-step reaction of carbon tetrachloride photolysis followed by the H-atom abstraction reaction of Cl atom with 1,4-pentadiene. The equilibrium between pentadienyl radicals, O₂ and pentadienylperoxy radicals could be observed over the range 268–308 K. An analysis of the temporal signal of pentadienyl radicals was used to evaluate the equilibrium constant. Third-law analysis was used to evaluate the enthalpy change for the reaction C₅H₇ + O₂ ⇌ C₅H₇O₂. The observed CO bond energy in the C₅H₇O₂ adduct was found to be 56.0 ± 2.2 kJ·mol^–1, which is lower than the values of peroxy radicals formed with allyl and cyclohexenyl radicals which have an allylic resonance structure.     

激光诱导的亚硝酸与二苯醚的反应机理

355 nm光照下利用瞬态吸收光谱技术进行了有氧、无氧条件下二苯醚与亚硝酸体系的反应机理研究, 考察了其中瞬态物种的衰减行为, 并对其光解产物进行了GC-MS分析. 研究表明, HNO₂在355 nm紫外光的照射下产生的OH自由基和二苯醚反应生成C₁₂H₁₀O-OH 加合物, N₂条件下C₁₂H₁₀O-OH衰减的速率常数为(1.86±0.14)×10⁵ s^－1, 在有氧条件下, C₁₂H₁₀O-OH可转化为C₁₂H₁₀O-OHO₂, 衰减的速率常数为(6.6±0.4)×10⁶ s^－1. N₂条件下最终产物为苯酚、2-羟基二苯醚、4-羟基二苯醚、4-硝基二苯醚.     

Chemiluminescence from C2 produced by C3 oxidation

The reaction of molecular carbon vapor with oxygen has been studied in a flowing system. For an equilibrium disitribution of carbon molecular species at 2470 K, the dominant reaction observed was: C₃ + O₂ → C^*₂(d³II, ν′ = 1) + CO + O (or CO₂). Of the product species, only excited C₂ was detected. From these measurements a lower bound on the rate constant has been determined to be k ≥ 2 × 10^?12 cm³/s.     

Condensation of supersaturated n-butanol vapor on charged/neutral nanoparticles of D-mannose and L-rhamnose

The effects of size, charge, and solubility on the condensation of supersaturated n-butanol vapor on monodisperse nanoparticles of D-mannose and L-rhamnose are investigated in a flow cloud chamber. The dependence of the critical supersaturation S(cr) on particle size in the range from 30 to 90 nm is determined experimentally. The results show that the experimental S(cr) decreases with increasing particle size and solubility, qualitatively in agreement with the prediction by the Volmer theory of nucleation on soluble particles and by the Kohler theory, but quantitatively smaller than both theoretical predictions. The condensation of supersaturated vapor on singly positive/negative charged particles with diameters of 30, 60, and 90 nm is examined, and no obvious charge effect and sign preference are observed.     

Kinetic measurements of the gas phase HO2+CH3O2 cross-disproportionation reaction at 298 K

Flash photolysis kinetic absorption spectroscopy was used to investigate the gas phase reaction between hydroperoxy (HO₂) and methylperoxy (CH₃O₂) radicals at 298 K. Due to the large difference between the self-reactivities of the two radicals, first- or second-order kinetic conditions could not be maintained for either species. Thus, the rate constant for the cross reaction was determined from computer-modeled fits of the radical absorption decay curves, at wavelengths between 215 and 280 nm. This procedure yielded k = 2.9 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ independent of total pressure (using N₂) between 25 and 600 Torr, and of the partial pressure of water vapor (up to 11.6 Torr). There was also no effect of water vapor on the rate constant for the self-reaction of methylperoxy radicals.     

The rate of the reactions of C2O radicals with H and H2

The rate constants for the reactions C₂O + H → products (1) and C₂O + H₂ → products (2) have been determined at room temperature by means of laser-induced fluorescence detection of C₂O radicals, generated either by the KrF excimer laser photolysis Of C₃O₂, or by the reaction of C₃O₂ with O atoms. Values of k₁ = (3.7 ± 1.0) × 10^?11 cm³ s^?1 and k₂ = (7 ± 3) × 10^?13 cm³ s^?1 were obtained.     

Self-Organization and Dynamics of Nanoparticles in Chemically Active Plasmas for Low-Temperature Deposition of Silicon and Carbon-Based Nanostructured Films

Self-organization and dynamic processes of nano/micron-sized solid particles grown in low-temperature chemically active plasmas as well as the associated physico-chemical processes are reviewed. Three specific reactive plasma chemistries, namely, of silane (SiH₄), acetylene (C₂H₂), and octafluorocyclobutane (c—C₄F₈) RF plasma discharges for plasma enhanced chemical vapor deposition of amorphous hydrogenated silicon, hydrogenated and fluorinated carbon films, are considered. It is shown that the particle growth mechanisms and specific self-organization processes in the complex reactive plasma systems are related to the chemical organization and size of the nanoparticles. Correlation between the nanoparticle origin and self-organization in the ionized gas phase and improved thin film properties is reported. Self-organization and dynamic phenomena in relevant reactive plasma environments are studied for equivalent model systems comprising inert buffer gas and mono-dispersed organic particulate powders. Growth kinetics and dynamic properties of the plasma-assembled nanoparticles can be critical for the process quality in microelectronics as well as a number of other industrial applications including production of fine metal or ceramic powders, nanoparticle-unit thin film deposition, nanostructuring of substrates, nucleating agents in polymer and plastics synthesis, drug delivery systems, inorganic additives for sunscreens and UV-absorbers, and several others. Several unique properties of the chemically active plasma-nanoparticle systems are discussed as well.     

Formation of formic and acetic acids by low-temperature condensation of a mixture of methane and water vapor dissociated in MW discharge

Formic and acetic acids are formed by the low-temperature (77 K) condensation of a mixture of methane and water vapor dissociated by MW discharge at a low pressure. The effect of experimental conditions on the yield of HCOOH and AcOH was studied under different experimental conditions. The yields of H, OH, and O₂ from MW discharge in the CH₄+H₂O mixture were determined by ESR in the gas phase under the experimental conditions used to synthesize HCOOH and AcOH. The kinetics of the gas phase reactions in the connecting channel was simulated. The mechanism of formation of HCOOH and AcOH through the interaction of active species from the gas phase on the condensate surface was suggested. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 379–382, February, 2000.     

Silicon transitions, 3p 2→→3p n f (n=4, 5) and 3p 2→→3p 6p,detected in the multiphoton dissociation of phenylsilane

Fragmentation to neutral atoms has been observed in the laser photolysis of phenylsilane, C₆H₅SiH₃. A tunable dye laser (UV region) was used both to photolyze the molecule and to probe the resulting fragments. Atomic silicon and carbon lines (two photon resonances) are prominent in the multiphoton dissociation/ionization spectrum of phenylsilane.     

The temperature dependence of the HO2 + HO2 reaction

The temperature dependence of the rate constant for the reaction HO₂ + HO₂ → H₂O₂ + O₂ (2k₁) has been determined using flash photolysis techniques, over the temperature range 298–510 K, in a nitrogen diluent at a total pressure of 700 Torr. The overall second order state constant is given by k₁ = (4.14 ± 1.15) × 10^?13 exp[(630 ± 115)/T] cm³ molecule^?1 s^?1, where the quoted errors refer to one standard deviation. This result is compared with previous findings and the negative activation energy is shown to be consistent with the observation that the rate constant is pressure dependent at 700 Torr.     

Thermal behavior of carbon nanotubes decorated with gold nanoparticles

Three different forms of carbon, i.e., multi-walled carbon nanotubes (CNTs), single-walled CNTs, and soot, were decorated with gold nanoparticles by a new method. In this method C₁₀H₈ ⁻ ions transfer electrons to the CNTs or soot. These electrons on the carbon surface can then reduce Au³⁺ species to form supported Au nanoparticles with a narrow particle size distribution. Thermogravimetric/differential thermal analyses (TG/DTA), XRD, Raman, and TEM show that naphthalene molecules remain trapped inside the Au nanoparticles and can only be removed by treatment at ca. 300 °C. Remarkable effect of the Au nanoparticles on the oxidation of carbon by O₂ is also observed by TG/DTA, i.e., on-set oxidation temperature and activation energy (E _a). It is shown that as the Au particle size decreases from 25 to 2 nm a linear decrease of the oxidation temperature is observed. Au particles larger than 25 nm do not produce any significant effect on carbon oxidation. These results are discussed in terms of spillover catalytic effect where Au nanoparticles activate O₂ molecules to produce active oxygen species which oxidize the different carbon supports.     

Excited state dynamics of C2O(A3Πi)

The fluorescence decay and bimolecular electronic quenching behavior of C₂O ($\text{A}$³Π_i) is reported. C₂O($\text{X}$³Σ^?) is produced by laser photolysis of C₃O₂ at 266 nm and is subsequently excited by a tunable flashlamp pumped dye laser. The fluorescence decay is highly nonexponential and dominated by both short (≈ 15 μs) and long (50–250 μs) decay components. The long-lived emission, itself, is nonexponential. The fluorescence decay is modeled as the sum of three exponential components. The short-lived emission is quenched by C₃O₂ at higher than the gas kinetic rate while the long-lived fluorescence is quenched much less efficiently. Fluorescence quenching measurements are also reported for collisions with Ar, N₂ and O₂.     

Rate constant and a detailed error analysis for C2H3 + H reaction

The production and reactions of vinyl radicals and hydrogen atoms from the photolysis of vinyl iodide (C₂H₃I) at 193 nm have been examined employing laser photolysis coupled to kinetic-absorption spectroscopic and gas chromatographic product analysis techniques. The time history of vinyl radicals in the presence of hydrogen atoms was monitored using the 1,3-butadiene (the vinyl radical combination product) absorption at 210 nm. By employing kinetic modeling procedures a rate constant of 1.8 × 10^?10 cm² molecule^?1 s^?1 for the reaction C₂H₃ + H has been determined at 298 K and 27 KPa (200 torr) pressure. A detailed error analysis for determination of the C₂H₃ + H reaction rate constant, the initial C₂H₃ and H concentrations are performed. A combined uncertainty of ±0.43 × 10^?10 cm² molecule^?1 s^?1 for the above measured rate constant has been evaluated by combining the contribution of the random errors and the systematic errors (biases) due to uncertainties of each known parameter used in the modeling. © 1995 John Wiley & Sons, Inc.     

Impact of Chemical and Physical Properties of Organic Solvents on the Gas and Hydrogen Formation during Laser Synthesis of Gold Nanoparticles

Laser ablation in liquids has been established as a scalable preparation method of nanoparticles for various applications. Particularly for materials prone to oxidation, it is established to suppress oxidation by using organic solvents as a liquid medium. While this often functionalizes the nanoparticles with a carbon shell, the related chemical processes that result from laser-induced decomposition reactions of the organic solvents remain uncertain. Using a systematic series of C₆ solvents complemented by n-pentane and n-heptane during the nanosecond laser ablation of gold, the present study focuses on the solvent-dependent influence on gas formation rates, nanoparticle productivity, and gas composition. Both the permanent gas and hydrogen formation was found to be linearly correlated with ablation rate, ΔH_vap, and pyrolysis activation energy. Based on this, a decomposition pathway linked to pyrolysis is proposed allowing the deduction of first selection rules for solvents that influence the formation of carbon or permanent gases.