Kuk Cho

Sintering Rates for Pristine and Doped Titanium Dioxide Determined Using a Tandem Differential Mobility Analyzer System

Sintering rates of pristine and V-doped TiO 2 were obtained using a tandem DMA system. A range of experiments were conducted to first map out the variation of mobility size of a monodisperse (by mobility) agglomerate with time at three fixed temperatures. Using relationships of the surface area to the mobility size, the sintering equation was solved to determine the activation energy and pre-exponential factor. The value of the activation energy was 236 (± 46) kJ/mol for pristine TiO 2 and 363 (± 1) kJ/mol for V-doped TiO 2 . The corresponding pre-exponential factors were 7.22 × 10 19 and 2.22 × 10 12 s/m 4 K, respectively. These values were then used to predict changes in mobility diameter at different temperatures, and good agreement was obtained with measurements. Possible reasons for faster sintering rates of V-TiO 2 relative to pristine TiO 2 were conjectured.

Co-Assembly of Nanoparticles in Evaporating Aerosol Droplets: Preparation of Nanoporous Pt/TiO2 Composite Particles

Nanoporous Pt/TiO2 micro-particles were synthesized via an aerosol assisted co-assembly (AACA) route. Aerosol droplets were produced from a colloidal mixture of 5 nm Pt and 20 nm TiO2 nanoparticles, which formed spherical micro-aggregates of Pt and TiO2 with average diameter of around 1.2 μm. The resulting composite micro-particles have very open structure with pore sizes ranging from 20 to 200 nm. Pt nanoparticles were found to be well dispersed on the surface of the supporting TiO2 particles. Electrocatalytic application of the nanoporous Pt/TiO2 composites was examined through methanol oxidation reaction. The performance of 20 wt% Pt/TiO2 particles was found to be comparable to that of commercial 20 wt% Pt/carbon black catalyst.

Coagulation Coefficient of Agglomerates with Different Fractal Dimensions

A coagulation coefficient of agglomerates with different fractal dimensions has not been considered in the past, even though there is a possibility of variations in the fractal dimension of agglomerates at any instant. In this study, a Brownian dynamics simulation was performed with simultaneous collision and sintering, and variations in the fractal dimension of agglomerates were observed. A coagulation coefficient expression for agglomerates with two different fractal dimensions was proposed. The coagulation coefficient based on the different fractal dimensions was at most 140% higher than that based on the average fractal dimension. To determine an accurate coagulation coefficient of agglomerates, the fractal dimension of each agglomerate has to be considered.

Fringe-Covariance "Fingerprinting" of Nanoparticle Lattice Images

Individual diffraction patterns only provide 2nd-moment (e.g. atom-atom correlation) data on specimens, while HRTEM and z-contrast images provide information on higher order correlations as well. Variable coherence-width microscopy [1], and high-tilt lattice-parameter determination [2], involve ways to quantitatively characterize higher order (e.g. pair-pair) correlation by examining data from more than one image or diffraction pattern. Here we discuss a simple strategy for quantifying information of this sort found in a single lattice-fringe image of multiple nano-particles. The theory we discuss addresses the kinds of correlations expected if the nanoparticles are randomly oriented. On the experimental end, the strategy is simple. Find all nanoparticles showing cross-fringes in the image, and draw one or more pairs of xs on a plot of interspot angle versus lattice spacing. The angles and spacings might be measured directly, or perhaps better from a small power spectrum superposed on each particle of interest. Each cross-fringe pair will result in marks at two spacings (one for each spot in the power spectrum) and at a common interspot angle value. Examples of such patterns, for 10 nm particles in a WC1-x plasma enhanced CVD film (Fig. 1) and TiO2 aerosol catalyst particles wet deposited on a holey carbon film (Fig. 2). The data for the foregoing figures was obtained manually. Nonetheless, the figures offer a scatter diagram characteristic of both the primary zones expected for randomly-oriented particles of each crystal type, and the range of spacing errors resulting from both foreshortened projection (more significant for smaller particles) and measurement error. Automated analysis may become practical in the future, for example by tiling the image on a size scale characteristic of the grain size of interest, and then using power spectrum peak analysis to determine what to plot where. What if we now wish to generate a theoretical fingerprint for a given crystal type? To this end, we consider maps of angular covariances, i.e. the average product of power spectrum intensity at two reciprocal spacings, separated in the three dimensional reciprocal-lattice by an interspot angle of ∆α. Suitable normalization, with means and standard deviations, allows one to thereby map a kind of angular correlation coefficient across the space of scattering probabilities. A plot of such correlations in the (hk0) plane of a simple cubic lattice is shown in Fig. 3. The maps for comparison with a given set of data will depend on both crystallite size (e.g. fringe foreshortening effects are more prevalent in smaller crystals) and the microscopes instrument response function (e.g. even single zone axis patterns will show more fringe pairs with better contrast transfer). Links to angular covariance maps for some commonly encountered structures will made available on the web [3].

Flame Synthesis of Metal Oxide Nanoparticles from Sprayed Droplets of Precursor Solution using a Two Fluid Nozzle

ABSTRACT Flame synthesis was applied to produce metal oxide nanoparticles such as titanium dioxide (TiO2) and lithium manganese oxide (LiMn2O4) from sprayed droplets of precursor solution. A high enough flame temperature was used for the complete thermal decomposition of the aerosol precursor. Molar concentration of the precursor solution and the flow rate of combustion gas such as hydrogen were varied to control the particle size, the size distribution and the crystal structure. Crystalline TiO2 and LiMn2O4 nanoparticles with the average particle diameter ranging from 9 to 45 nm and from 12 to 19 nm were synthesized, respectively.