Seung Min Kim

Role of water in super growth of single-walled carbon nanotube carpets.

The Ostwald ripening behavior of Fe catalyst films deposited on thin alumina supporting layers is demonstrated as a function of thermal annealing in H2 and H2/H2O. The addition of H2O in super growth of single-walled carbon nanotube carpets is observed to inhibit Ostwald ripening due to the ability of oxygen and hydroxyl species to reduce diffusion rates of catalyst atoms. This work shows the impact of typical carpet growth environments on catalyst film evolution and the role Ostwald ripening may play in the termination of carpet growth.

Size and Support Effects for the Water–Gas Shift Catalysis over Gold Nanoparticles Supported on Model Al2O3 and TiO2

The water-gas shift (WGS) reaction rate per total mole of Au under 7% CO, 8.5% CO(2), 22% H(2)O, and 37% H(2) at 1 atm for Au/Al(2)O(3) catalysts at 180 °C and Au/TiO(2) catalysts at 120 °C varies with the number average Au particle size (d) as d(-2.2±0.2) and d(-2.7±0.1), respectively. The use of nonporous and crystalline, model Al(2)O(3) and TiO(2) supports allowed the imaging of the active catalyst and enabled a precise determination of the Au particle size distribution and particle shape using transmission electron microscopy (TEM). Further, the apparent reaction orders and the stretching frequency of CO adsorbed on Au(0) (near 2100 cm(-1)) determined by diffuse reflectance infrared spectroscopy (DRIFTS) depend on d. Because of the changes in reaction rates, kinetics, and the CO stretching frequency with number average Au particle size, it is determined that the dominant active sites are the low coordinated corner Au sites, which are 3 and 7 times more active than the perimeter Au sites for Au/Al(2)O(3) and Au/TiO(2) catalysts, respectively, and 10 times more active for Au on TiO(2) versus Al(2)O(3). From operando Fourier transform infrared spectroscopy (FTIR) experiments, it is determined that the active Au sites are metallic in nature. In addition, Au/Al(2)O(3) catalysts have a higher apparent H(2)O order (0.63) and lower apparent activation energy (9 kJ mol(-1)) than Au/TiO(2) catalysts with apparent H(2)O order of -0.42 to -0.21 and activation energy of 45-60 kJ mol(-1) at near 120 °C. From these data, we conclude that the support directly participates by activating H(2)O molecules.

Influence of alumina type on the evolution and activity of alumina-supported Fe catalysts in single-walled carbon nanotube carpet growth.

We have studied the lifetime, activity, and evolution of Fe catalysts supported on different types of alumina: (a) sputter deposited alumina films (sputtered/Fe), (b) electron-beam deposited alumina films (e-beam/Fe), (c) annealed e-beam deposited alumina films (annealed e-beam/Fe), (d) alumina films deposited by atomic layer deposition (ALD/Fe), and (e) c-cut sapphire (sapphire/Fe). We show that the catalytic behavior, Ostwald ripening, and subsurface diffusion rates of Fe catalyst supported on alumina during water-assisted growth or supergrowth of single-walled carbon nanotube (SWNT) carpets are strongly influenced by the porosity of the alumina support. The catalytic activity increases in the following order: sapphire/Fe < annealed e-beam/Fe < ALD/Fe < e-beam/Fe < sputtered/Fe. With a combination of microscopic and spectroscopic characterization, we further show that the Ostwald ripening rates of the catalysts and the porosity of the alumina support correlate with the lifetime and activity of the catalysts. Specifically, our results reveal that SWNT carpet growth is maximized by very low Ostwald ripening rates, mild subsurface diffusion rates, and high porosity, which is best achieved in the sputtered/Fe catalyst. These results not only emphasize the connection between catalytic activity and particle stability during growth, but guide current efforts aimed at rational design of catalysts for enhanced and controlled SWNT carpet growth.

Metallic Corner Atoms in Gold Clusters Supported on Rutile Are the Dominant Active Site during Water−Gas Shift Catalysis

Au/TiO(2) catalysts used in the water-gas shift (WGS) reaction at 120 °C, 7% CO, 22% H(2)O, 9% CO(2), and 37% H(2) had rates up to 0.1 moles of CO converted per mole of Au per second. However, the rate per mole of Au depends strongly on the Au particle size. The use of a nonporous, model support allowed for imaging of the active catalyst and a precise determination of the gold size distribution using transmission electron microscopy (TEM) because all the gold is exposed on the surface. A physical model of Au/TiO(2) is used to show that corner atoms with fewer than seven neighboring gold atoms are the dominant active sites. The number of corner sites does not vary as particle size increases above 1 nm, giving the surprising result that the rate per gold cluster is independent of size.

Double-Walled Boron Nitride Nanotubes Grown by Floating Catalyst Chemical Vapor Deposition

One-dimensional nanostructures exhibit quantum confinement which leads to unique electronic properties, making them attractive as the active elements for nanoscale electronic devices. Boron nitride nanotubes are of particular interest since, unlike carbon nanotubes, all chiralities are semiconducting. Here, we report a synthesis based on the use of low pressures of the molecular precursor borazine in conjunction with a floating nickelocene catalyst that resulted in the formation of double-walled boron nitride nanotubes. As has been shown for carbon nanotube production, the floating catalyst chemical vapor deposition method has the potential for creating high quality boron nitride nanostructures with high production volumes.

Biotemplated Aqueous-Phase Palladium Crystallization in the Absence of External Reducing Agents

A new synthetic strategy enabling highly controlled aqueous-phase palladium crystallization on the tobacco mosaic virus (TMV) is demonstrated without the addition of external reducing agents. This low cost, solution processing method yields continuous and uniform coatings of polycrystalline palladium on TMV, creating highly uniform palladium nanowires of tens of nanometers in thickness and hundreds of nanometers in length. Our approach utilizes a palladium chloride precursor to produce metallic Pd coatings on TMV without the need for an external reducing agent. X-ray photoelectron spectroscopy and in situ transmission electron microscopy were used to confirm the reduction of the surface palladium oxide layer on the palladium metal wires during room temperature hydrogenation. This leads to metallic palladium nanowires with surfaces free of residual organics, making these structures suitable for applications in nanoscale electronics.

Biomagnetic glasses: preparation, characterization, and biosensor applications.

In this work, a novel avenue to create a generic approach for the fabrication of biofunctional materials with magnetic capabilities to be used in the design of highly stable, magnetically separable enzyme-based systems was explored. As a model system, immobilization of acetylcholinesterase (AChE) was investigated using biomagnetic glasses composed of a magnetic core with a size tunable porous silica shell. The efficiency of the immobilization was determined by analyzing the biosensing capability of these biomagnetic glasses for the detection of the organophosphorous pesticide paraoxon. Screen printed electrodes with the AChE-biomagnetic glasses showed higher current response and stability than for the free enzyme. The detection limit of the paraoxon biosensor was in the nanomolar range.

Quantitative study of Au(III) and Pd(II) ion biosorption on genetically engineered Tobacco mosaic virus.

One major obstacle in the mineralization of metal onto biologically derived templates is the lack of fundamental information pertaining to the relationship between metal ion loading and overall metal deposition onto the biotemplate. This study focuses on Au(III) and Pd(II) biosorption on the genetically-modified model biological template Tobacco mosaic virus (TMV1Cys). Metal ion (Au(III) or Pd(II)) loading on the TMV1Cys template was measured as a function of the equilibrium concentration of Au(III) or Pd(II) ions in solution at several temperatures. In addition, the Pd(II) loading on the TMV-wild (wild-type TMV) and TMV1Cys were compared to estimate the improvement of metal ion loading by genetic modification of the biotemplate. The gold or palladium coatings on the TMV1Cys were prepared using various metal ion loadings. Results show, for a range of metal ion loadings, a positive correlation existed between the concentration of the metal ions and the coating density of the metals deposited on the virus surface.

Investigating Local Degradation and Thermal Stability of Charged Nickel-Based Cathode Materials through Real-Time Electron Microscopy

In this work, we take advantage of in situ transmission electron microscopy (TEM) to investigate thermally induced decomposition of the surface of Li(x)Ni(0.8)Co(0.15)Al(0.05)O2 (NCA) cathode materials that have been subjected to different states of charge (SOC). While uncharged NCA is stable up to 400 °C, significant changes occur in charged NCA with increasing temperature. These include the development of surface porosity and changes in the oxygen K-edge electron energy loss spectra, with pre-edge peaks shifting to higher energy losses. These changes are closely related to O2 gas released from the structure, as well as to phase changes of NCA from the layered structure to the disordered spinel structure, and finally to the rock-salt structure. Although the temperatures where these changes initiate depend strongly on the state of charge, there also exist significant variations among particles with the same state of charge. Notably, when NCA is charged to x = 0.33 (the charge state that is the practical upper limit voltage in most applications), the surfaces of some particles undergo morphological and oxygen K-edge changes even at temperatures below 100 °C, a temperature that electronic devices containing lithium ion batteries (LIB) can possibly see during normal operation. Those particles that experience these changes are likely to be extremely unstable and may trigger thermal runaway at much lower temperatures than would be usually expected. These results demonstrate that in situ heating experiments are a unique tool not only to study the general thermal behavior of cathode materials but also to explore particle-to-particle variations, which are sometimes of critical importance in understanding the performance of the overall system.

Catalyst and catalyst support morphology evolution in single-walled carbon nanotube supergrowth: Growth deceleration and termination

Detailed understanding of growth termination in vertically aligned single-walled carbon nanotubes (SWNTs) made via supergrowth, or water-assisted growth, remains critical to achieving the ultralong SWNTs necessary for next-generation applications. We describe the irreversible catalyst morphology evolution that occurs during growth, and which limits the lifetime of surface supported catalysts. Growth termination is strongly dependent on growth temperature, but not sensitive to C 2 H 2 :H 2 O ratio. In addition to both planar Ostwald ripening of small (sub-5 nm) Fe catalyst particles and diffusion of metal atoms into the alumina support, other features that contribute to growth termination or deceleration are described, including center-of-mass particle motions and coalescence of smaller species of surface supported Fe nanoparticles. Additionally, a temperature-induced structural transition in the alumina catalyst support is found to be coincident with abrupt growth termination at temperatures of 800 °C and higher. In situ electron microscopy observations are used to directly support these observations.