E. V. Gurentsov

Influence of the bath gas on the condensation of supersaturated iron atom vapour at room temperature

The influence of the kind of bath gas and its pressure on the iron nanoparticle formation and growth was investigated experimentally. Iron nanoparticles were synthesized from supersaturated iron vapour generated by ArF excimer laser pulse photolysis of gaseous Fe(CO)5 at room temperature. The particle size was determined by time-resolved laser-induced incandescence (TiRe-LII) as a function of time after photolysis at different experimental conditions. Additionally, final particles were sampled and analysed by transmission electron microscopy and by energy-dispersive x-ray analysis. The particle growth rate and the final particle size depended on the bath-gas composition and pressure. Increasing the argon bath-gas pressure accelerated the iron nanoparticle growth rate. In contrast to argon, no influence of helium on the particle growth rate was observed. The experimental results are compared with numerical simulations of particle surface growth, based on the model developed in previous investigations. The simulations indicate that the observed differences in the influence of the bath gas on the particle formation are caused by the species-dependent quenching probability of the active atom-particle complexes by the bath gas.

Shock wave induced carbon particle formation from CCL4 and C3O2 observed by laser extinction and by laser-induced incandescence (LII)

Abstract The formation of carbonaceous particles from the hydrogen-free precursors CCl 4 and C 3 O 2 , both diluted in argon was studied behind reflected shock waves in the temperature range 1400 K ≤ T ≤3700 K and at pressures 1.3 bar ≤ p ≤ 4.5 bar. The appearance of particles was measured by laser light extinction (LLE) and by laser induced incandescence (LII). Also, some time and spectrally resolved emission measurements were performed. The LLE experiments are sensitive to the optical density of the post-shock gas-particle mixture and show a time-dependent increase, depending on the detailed reaction conditions. The evaluation of the experiments at a reaction time of t = 1 ms results in a double, bell-shaped temperature dependency of the optical density. The LII-experiments, which are sensitive to the particle size, provide particle growth curves determined from several “identical” shock tube experiments with delayed triggering of the LII heat-up laser. Particle sizing experiments at a reaction time of t = 1 ms after shock-induced heat-up of the initial gas mixtures also clearly yield a double, bell-shaped temperature dependency of the particle diameter and confirm the optical density experiments. The shock tube was also equipped with a molecular beam system allowing supersonic beam probing from the shock-heated gases. Particles were collected on TEM grids and visualized by HR-TEM. The sizes of these images more or less confirm the LII sizing.

Formation of carbon nanoparticles by the condensation of supersaturated atomic vapor obtained by the laser photolysis of C3O2

A new technique is suggested for obtaining nanoparticles from highly supersaturated vapor resulting from the laser photolysis of volatile compounds. The growth of carbon nanoparticles resulting from C3O2 photolysis has been studied in detail. Absorbing UV quanta (from an Ar-F excimer laser), C3O2 molecules decompose to yield atomic carbon vapor with precisely known and readily controllable parameters. This is followed by the condensation of supersaturated carbon vapor and the formation of carbon nanoparticles. These processes have been investigated by the laser extinction and laser-induced incandescence (LII) methods in wide ranges of experimental conditions (carbon vapor concentration, nature of the diluent gas, and gas pressure). The current and ultimate particle sizes and the kinetic parameters of particle growth have been determined. The characteristic time of particle growth ranges between 20 and 1000 μs, depending on photolysis conditions. The ultimate particle size determined by electron microscopy is 5–12 nm for all experimental conditions. It increases with increasing total gas pressure and carbon vapor partial pressure and depends on the diluent gas. The translational energy accommodation coefficients for the Ar, He, CO, and C3O2 molecules interacting with the carbon particle surface have been determined by comparing the LII and electron microscopic particle sizes. A simple model has been constructed to describe the condensation of carbon nanoparticles from supersaturated atomic vapor. According to this model, the main process in nanoparticle formation is surface growth through the addition of separate atoms to the nucleation cluster. The nucleus concentrations for various condensation parameters have been determined by comparing experimental and calculated data.

Binary iron–carbon nanoparticle synthesis in photolysis of Fe(CO)5 with methane and acetylene

The experimental investigation of iron-carbon nanoparticles synthesis by joint laser photolysis of iron pentacarbonyl in the mixture with methane or acetylene has been carried out. The radiation source used for photo-dissociation of precursors was a pulsed Nd:Yag laser operated at a wavelength of 266 nm. Under uv radiation the molecules of Fe(CO)5 decomposed, forming atomic iron vapor and unsaturated carbonyls at well-known and readily controllable parameters. The subsequent condensation of supersaturated metal vapor resulted in small iron clusters and nanoparticles formation. It was assumed that the active catalytic surface of metal nanoparticles could activate the hydrocarbon molecules up to carbon layer formation on their surface. The growth process of the nanoparticles was observed by a method of laser light extinction. Additionally nanoparticle samples were investigated by a transmission electron microscope. The particle sizes were measured by microphotographs treatment. The sizes of synthesized particles from methane-iron-pentacarbonyl mixture were found to be in a range of 4-16 nm with a count median diameter of 8.9 nm and standard deviation of 1.13. These particles consisted of iron oxide without any carbon content. The particles formed in photolysis of acetylene-iron-pentacarbonyl mixture had the sizes of 3-7 nm with count median diameter of 4 nm and standard deviation of 1.28 and contained the essential amount of carbon. The iron cores were surrounded with a carbon shell.

UV laser synthesis of nanoparticles in the gas phase

This review deals with the UV laser photodissociation of metal carbonyls, ferrocene, carbon suboxide, and other precursors. The formation of supersaturated atomic vapors followed by the formation of carbon, metal, and metal–carbon nanoparticles is discussed. Application of UV laser synthesis to preparation of catalytic nanomaterials is considered.

Synthesis of Small Carbon Nanoparticles in a Microwave Plasma Flow Reactor

Abstract Unusually small carbon nanoparticles were synthesized in a microwave plasma flow-reactor by pyrolysis of 0.3–1.2% CH4, C2H4, and C2H2 with 0.3–3.6% addition of molecular hydrogen in argon. Final particle sizes were analyzed by in-line particle-mass spectrometry (PMS) and by transmission electron microscopy (TEM). TEM measurements of primary particle sizes were found to be in a good agreement with PMS data. The carbon particles formed in the plasma generated by a 2.45 GHz magnetron with an applied power of 180 W and a total pressure of 13 mbar have diameters of 4–6 nm. The type of hydrocarbon precursor and 0.3–3.6% of hydrogen addition did not noticeably influence the final particle sizes. The formation of such small particles is attributed to the low pressure and the comparably low operation power. This method of small carbon nanoparticles synthesis could be useful for the production of carbon black material, where large surface area is important.

Analysis of the production and clusterization of iron atoms under pulsed laser photolysis of Fe(CO)5

Atomic-resonance absorption spectroscopy is used to study the production and loss of iron atoms under dissociation of the Fe(CO)5 vapor in a quartz reactor that is induced by the pulses of the KrF excimer laser. Iron atoms populate the ground state owing to the quenching of the excited states generated in the course of the laser photolysis and are detected using the resonance absorption at a wavelength of 385.99 nm. The effective quenching rates are in good agreement with the known rates of the quenching of metastable iron atoms by the Fe(CO)5 molecules. It is demonstrated that a loss of iron atoms is related to the recombination with dimer and trimer formation and the secondary atomic reactions with the Fe(CO)5, CO, and FeCO molecules. The rates of the main elementary reactions responsible for the loss of iron atoms are determined using the comparison of the experimental results and kinetic simulation data.

Effect of active impurities on the condensation of nanoparticles from supersaturated carbon vapor in the combined laser photolysis of C3O2 and H2S

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.

Study of thermodynamic properties of carbon nanoparticles by the laser heating method

A new experimental approach to the analysis of thermodynamic properties of amorphous carbon nanoparticles synthesized via hydrocarbon pyrolysis behind shock waves is discussed. The proposed approach is based on the analysis of thermal radiation of nanoparticles heated by a laser pulse. The sublimation temperature of the carbon nanoparticles might be determined by the two-colour pyrometry; their sizes, by laserinduced incandescence; and the volume fraction of the sublimated substance, by the laser extinction method. The sublimation temperature depends on both the particle size and the temperature conditions of their formation. The value of surface energy for amorphous carbon nanoparticles was estimated.

Nanoparticle formation from supersaturated carbon vapour generated by laser photolysis of carbon suboxide

Particle formation and growth from condensation of supersatd. carbon vapor was investigated. At. carbon vapor was generated under well-controlled conditions from UV-laser pulse photolysis of C3O2 at 193 nm. Particle formation and growth were studied in a wide range of conditions with varying carbon vapor concn., bath gas compn., and pressure. The formation of particulate matter was obsd. as a function of time by laser light extinction. Particle sizes were detd. in situ by time-resolved laser-induced incandescence and ex situ by transmission electronic microscopy. The characteristic time of particle growth was 20-1000 micro s. The final particle size was 5-12 nm, increased with pressure, and depends on bath gas compn. We propose a simple model for the description of carbon vapor condensation that assumes condensation of individual atoms on the cluster surface as the main growth mechanism. The comparison of expts. and simulations provides information about the initial concn. of carbon clusters for the different mixt. conditions.