Donggeun Lee

Coalescence enhanced synthesis of nanoparticles to control size, morphology and crystalline phase at high concentrations

Size, morphology, and crystalline phase of nanoparticles determine the properties of nanostructured materials. Therefore, the mastery of controlling properties ultimately requires the control of size, morphology, and phase of nanoparticles. From various aerosol methods, highly pure nanoparticles can be produced; however, agglomeration has been considered as almost unavoidable when nanoparticles are generated at high concentrations that are necessary for a practical application. Efforts to control agglomeration have had only limited success. Here we report that the enhancement of coalescence of nanoparticles using laser beam irradiation on aggregates formed in flames can be a solution for this problem and successfully controls the size, morphology, and crystalline phase of high concentration nanoparticles of silica and titania. We demonstrate this principle by not only synthesizing smaller and unagglomerated nanoparticles, but also generating them in high concentrations. In addition, we show that the present method is capable of even controlling the crystalline phase of titania nanoparticles. Surprisingly, stable rutile titania particles have been transformed into metastable anatase and the weight percent of each phase could be controlled.

Controlled formation of nanoparticles utilizing laser irradiation in a flame and their characteristics

Controlled synthesis of nanoparticles has been carried out using laser beam irradiation on aggregates forming in a flame. This coalescence enhanced method transforms titania aggregates into much smaller unagglomerate spherical anatase nanoparticles by reducing the characteristic time of coalescence of nanoparticles. These transformed nanoparticles exhibit not only better thermal stability, but also unusually small grain growth during a simple pressureless sintering, which ultimately lead to a full dense bulk ceramic with 60 nm grains. Such small grain size in full dense ceramics has not been achieved before without employing special sintering techniques. This would be attributed not only to the successful control of size and morphology of nanoparticles, but also to the synthesis of oxygen vacancy free particles, which was confirmed from Raman spectroscopy.

CONTROL OF SIZE AND MORPHOLOGY OF NANO PARTICLES USING CO2 LASER DURING FLAME SYNTHESIS

An attempt has been made to control size and morphology of nanoparticles generating and growing in a co-flow oxy-hydrogen flame combining CO2 laser irradiation. The coalescence characteristic time could be controlled nearly independent of the collision characteristic time of particles by raising particle temperature rapidly with laser beam irradiation during flame synthesis. Silica aggregates generated by hydrolysis of SiCl4 in a flame are irradiated by a high-power CO2 laser beam, and rapidly heated up to temperatures that are high enough to enhance sintering, but lower than evaporation temperature. Sintering of aggregates is rapidly enhanced to change aggregates into more spherical particles. Since spherical particles have much smaller collision cross sections than volume-equivalent aggregates, much slower growth of particles is observed when the flame is irradiated by CO2 laser beam and as a result, the diameter of resulting spherical particles decreases to about 60% of the size observed without CO2 laser beam as the laser power increases. A higher precursor flow rate even resulted in the change of non-spherical particles into smaller spherical particles as CO2 laser power increased. Light-scattering measurement with an Ar-ion laser and TEM observation through a localized thermophoretic sampling were used to confirm the above effects of CO2 laser irradiation in a flame. Depending on the irradiation height of CO2 laser beam in a flame, significantly different mechanisms were found. The radial distributions of scattering intensity and morphological change were also studied. The proposed method controlling the sintering characteristic time of particles using CO2 laser irradiation in a flame seems to be promising to produce smaller and, at the same time, spherical nanoparticles even for high carrier gas flow rate.

Determination of the Size Distribution of Polydisperse Nanoparticles with Single-Particle Mass Spectrometry: The Role of Ion Kinetic Energy

We develop a method to determine size and size distribution (30–150 nm) of polydisperse nanoparticles using a laser ablation/ ionization time-of-flight single-particle mass spectrometer that extends the work first described by Reents and Ge. We found a composition independent “power law” dependence between the total peak area and original particle volume that enables one to determine particle volume directly from a particles mass spectrum. This power-law relationship suggests that some ions ablated and ionized from a particle are selectively lost during transport from the laser ablation/ionization region to the detector. A numerical calculation of ion trajectories shows that ion loss is highly dependent on the initial kinetic energy of ions. We show that the size-dependent energetic ions formed by the laser-particle interaction lead to power-law relationship between the cube root of peak area and particle diameter. The results demonstrate that particle size distributions measured with the mass spectrometer are in good agreement with those measured with a scanning mobility particle sizer.

Crystalline phase reduction of cuprous oxide (Cu2O) nanoparticles accompanied by a morphology change during ethanol-assisted spray pyrolysis.

Metallic copper nanoparticles are produced by spray pyrolysis of copper nitrates with an addition of ethanol as cosolvent at 600 degrees C. Depending on the synthesis temperature, two interesting reaction pathways are found: below 525 degrees C, approximately 10% of hollow Cu(2)O parent particles are oxidized to CuO and then reduced to Cu, but at higher temperature, the remaining Cu(2)O takes a direct path to Cu, accompanied by a morphology change. These interesting reaction regimes are discussed in the aspects of phase-transformation kinetics, gas-phase and solid-phase thermodynamics, force balance, and their possible influences on structural instability. Experimental observations are fairly consistent with the predictions by the present models.

A one-step continuous synthesis of carbon-supported Pt catalysts using a flame for the preparation of the fuel electrode.

We introduce a novel single-step method capable of a continuous production of Pt catalysts supported on carbon agglomerates for the preparation of the fuel electrode. An acetylene-air diffusion flame is employed as heat and carbon sources, and Pt(acac)(2)-containing xylene droplets are injected into the flame. A sampling height from the burner top, initial concentration of Pt(acac)(2), and acetylene flow rate are demonstrated as efficient systematic parameters to control the size (from 2 to 7 nm) and surface coverage (or content up to 60 wt %) of spherical Pt particles. A variety of characterization methods manifest that the Pt exists mostly in a metallic form and carbon agglomerates are in good crystalline order. Finally, the electrochemical activity is confirmed to be higher than 74.9 m(2) g(-1) Pt, more efficient than an equivalent commercial (E-TEK 10 wt % Pt) catalyst.

Three-dimensional off-lattice Monte Carlo simulations on a direct relation between experimental process parameters and fractal dimension of colloidal aggregates

It has been a big challenge to explore a direct relation of experimental parameters such as pH, electrolyte concentration, particle size, and temperature with the final structures of aggregates, because Monte Carlo simulations have been performed on the basis of arbitrarily chosen sticking probability. We attempted to incorporate colloidal theory to Monte Carlo simulations for two model systems of CuO- and SiO(2)-water systems, so as to resolve this difficulty. Conducting three-dimensional off-lattice MC simulations at various pHs for both systems, we investigated effects of pH on fractal structures of aggregates, encompassing the whole aggregation regime from diffusion-limited cluster-cluster aggregation to reaction-limited cluster-cluster aggregation. Moreover, developing a functional analysis, we found an explicit correlation between experimental parameters, sticking probability, and the fractal dimension of aggregates for both systems.

Microstructure-controlled aerosol-gel synthesis of ZnO quantum dots dispersed in SiO2 nanospheres.

ZnO quantum dots dispersed in a silica matrix were synthesized from a TEOS:Zn(NO(3))(2) solution by a one-step aerosol-gel method. It was demonstrated that the molar concentration ratio of Zn to Si (Zn/Si) in the aqueous solution was an efficient parameter with which to control the size, the degree of agglomeration, and the microstructure of ZnO quantum dots (QDs) in the SiO(2) matrix. When Zn/Si ≤ 0.5, unaggregated quantum dots as small as 2 nm were distributed preferentially inside SiO(2) spheres. When Zn/Si ≥ 1.0, however, ZnO QDs of ∼7 nm were agglomerated and reached the SiO(2) surface. When decreasing the ratio of the Zn/Si, a blue shift in the band gap of ZnO was observed from the UV/Visible absorption spectra, representing the quantum size effect. The photoluminescence emission spectra at room temperature denoted two wide peaks of deep-level defect-related emissions at 2.2-2.8 eV. When decreasing Zn/Si, the first peak at ∼2.3 eV was blue-shifted in keeping with the decrease in the size of the QDs. Interestingly, the second visible peak at 2.8 eV disappeared in the surface-exposed ZnO QDs when Zn/Si ≥ 1.0.

Transient ion ejection during nanocomposite thermite reactions

We observe an intense ion pulse from nanocomposite thermite reactions, which we temporally probe using a recently developed temperature jump/time of flight mass spectrometer. These ion pulses are observed to be much shorter in duration than the overall thermite reaction time. Ion ejection appears in stages as positive ions are ejected prior to nanocomposite thermite ignition, and ignition of the thermite mixtures leads to a second ionization step which is primarily dominated by negative species. The positive species are identified from mass spectrometric measurements and the results show that the positive ion species are comprised of Na ions with minor species of Al and K ions. This observation can be explained by a diffusion based ion-current mechanism, in which strong Al ion diffusion flux formed through the oxide shell, and the surface Na and K ions from salt contaminations are ejected by the strong electrostatic repulsion. The fact that the negative ionization step occurs during the ignition event sug...

Numerical Simulations on Aerodynamic Focusing of Particles in a Wide Size Range of 30 nm–10 μm

Previous designs of conventional aerodynamic lenses have the limitation of a narrow range of focusable particle size, e.g., just one order of magnitude such as 30–300 nm or 3–30 nm. To enlarge the focusable size range to two orders of magnitude (30–3,000 nm), it is necessary to focus small particles and at the same time not to loose the large ones. From numerical simulations of size-resolved particle trajectories, we confirmed that the traveling losses of such large particles could be avoided only when the radial positions of particles approaching the orifice lenses were near the axes of the lenses. Hence, we designed a lens system consisting of seven orifices to fulfill the requirement. In particular, the orifices were aligned in such a way that their diameters would descend and ascend downstream. As a result, 30–2500 nm particles could be focused to produce particle beams with radii of 0.2 mm or less with a transmission efficiency of above 90% 40 mm downstream of the aerodynamic lens exit. Even 10 μm particles could be focused with a transmission efficiency of 80%. Copyright 2013 American Association for Aerosol Research