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Heterogeneous gas–solid catalyst reactions occur at the atomic level, and understanding and controlling complex catalytic reactions at this level is crucial for the development of improved processes and materials. There are postulations that single atoms and very small clusters can act as primary active sites in chemical reactions. Early applications of our novel aberration-corrected (AC) environmental (scanning) transmission electron microscope (E(S)TEM) with single-atom resolution are described. This instrument combines, for the first time, controlled operating temperatures and a continuous gas environment around the sample with full AC STEM capabilities for real-time in situ analysis and visualization of single atoms and clusters in nanoparticle catalysis. ESTEM imaging and analysis in controlled gas and temperature environments can provide unique insights into catalytic reaction pathways that may involve metastable intermediate states. Benefits include new knowledge and more environmentally friendly technological processes for health care and renewable energy as well as improved or replacement mainstream technologies in the chemical and energy industries.
Various porous titania photocatalysts are analyzed three-dimensionally in real space by electron tomography. Shapes and three-dimensional (3D) distributions of fine pores and silver (Ag) particles (2 nm in diameter) within the pores are successfully reconstructed from the 3D data. Electron tomography is applied for measuring the specific surface area of the porous structures including open and closed porosity. Calculated specific surface areas of 22.8 m2/g for a conventional sol-gel TiO2 sample and 366 m2/g for a highly porous TiO2 sample prepared using the Pluronic P-123 self-assembly process are compared with those measured by the general BET method. The real-space surface measurement indicates that the highly porous TiO2 produced by the present method using block copolymers has a greater number of effective reaction sites for the degradation of methylene blue. Electron tomography shows a great potential to contribute considerably to the nanostructural analysis and design of such catalyst materials for photocatalysis.
We review the development of time-resolved, high-resolution environmental scanning/ transmission electron microscopy [E(S)TEM] for directly probing dynamic gas–solid, liquid–solid, and gas–liquid–solid interactions at the atomic level. Unlike a regular TEM, such a microscope allows us to use high gas pressures (up to 40 mbars) in the sample region. The unique information available from experiments performed using E(S)TEM has enabled visualization of the dynamic nature of nanostructures during reactions. Such information can be directly applied to the development of advanced nanomaterials such as carbon nanotubes, silicon nanowires and processes, including the design of novel routes to polymers synthesis, and has aided in the identification of important phenomena during catalysis, chemical vapor deposition, and electrochemical deposition.
Advances in atomic-resolution environmental transmission electron microscopy (ETEM) and related techniques for probing gas–solid reactions in situ are described. The capabilities of ETEM allow the dynamic nanostructure of heterogeneous catalysts in their functioning states to be directly monitored in real time. Applications of ETEM in catalysis are outlined, and they illustrate significant new insights into the dynamic nanostructure of the catalyst materials and their modes of operation.
Highlights of our pioneering development of atomic resolution in situ environmental transmission electron microscope (ETEM) for direct probing of gas-solid reactions and in situ nanosynthesis are described. Dynamic studies on supported nanoparticle catalysts and carbon nanotubes under different reaction environments have been carried out. The results provide deep insights into the dynamic nanostructure of the materials and their mode of operation.
Extended abstract of a paper presented at the Pre-Meeting Congress: Materials Research in an Aberration-Free Environment, at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, July 31 and August 1, 2004.
Many dynamic processes do not occur in nature, science or industry in a typical TEM high (and sometimes not so high) vacuum environment. Dynamic in-situ data related to the real world need to be obtained under Controlled conditions of gas/vapor/liquid environment and temperature. In the ETEM (Environmental Transmission Electron Microscope), the specimen - but nothing much else - is shared between the chemical reactor on the horizontal axis and the vertical microscope column (Fig.1). The original Philips CM30 column is highly modified with pressure limiting apertures around the beam and multiple stages of differential pumping (Fig.2).
Nanowires of bismuth with diameters ranging from 10 to 200
nm and lengths of 50 μm have been synthesized by a pressure
injection method. Nanostructural and chemical compositional
studies using environmental and high resolution transmission
electron microscopy with electron stimulated energy dispersive
X-ray spectroscopy have revealed essentially single crystal
nanowires. The high resolution studies have shown that the
nanowires contain amorphous Bi-oxide layers of a few nanometers
on the surface. In situ environmental high resolution transmission
electron microscopy (environmental-HRTEM) studies at the atomic
level, in controlled hydrogen and other reducing gas environments
at high temperatures demonstrate that gas reduction can be
successfully applied to remove the oxide nanolayers and to maintain
the dimensional and structural uniformity of the nanowires,
which is key to attaining low electrical contact resistance.
We present the development of in situ wet environmental
transmission electron microscopy (Wet-ETEM) for direct probing
of controlled liquid–catalyst reactions at operating
temperatures on the nanoscale. The first nanoscale imaging and
electron diffraction of dynamic liquid hydrogenation and
polymerization reactions in the manufacture of polyamides reported
here opens up new opportunities for high resolution studies
of a wide range of solution–solid and
solution–gas–solid reactions in the chemical and
Nanowires have potential applications in future generations of nanoscale electronics. Our motivation of studying Bi nanowires is based on the unique properties of bulk Bi. These include very small effective masses, a long mean-free path and the low melting point (271°C). Calculations of the transport properties predict that the Bi nanowires have a very high thermoelectric efficiency. The effects of quantum confinement are pronounced in Bi because of its small effective masses. The resulting change in the band structure of bulk Bi is shown in Fig. 1. The broken curves depict the band structure in which the T-point valence band overlaps with the L-point conduction band by 38mev making bulk Bi a semimetal. Due to quantum effects, the band edges split into subbands shown by solid curves. As the wire diameter decreases, eventually the lowest conduction subband and the highest valence subband no longer overlap and the material becomes an indirect-gap semiconductor (at ∼50nm diameter).
Silica and titania based ceramics and their analogs are some of the most fundamental in crystal chemistry and ceramic science Our interests include applications of nanostructures and chemical composites of the ceramics in nanoelectronics, chemical processes and as scaffolds in biotechnologies. Finely divided titania is used in a vast array of products including paper, paint, food and clothing. Novel microscopy methods including dynamic environmental-high resolution transmission EM (EHREM) at the atomic level, FESEM and cathodoluminescence are leading to striking progress in the development of the ceramic nanotechnologies.
Phase transformations in the cristobalite form of silica, from the tetragonal a phase (low or room temperature form) to the cubic β phase (high temperature, (270°C) form) result in discontinuous thermal expansion and are not conducive to nanotechnology. Here we report fundamental in situatomic resolution studies of the phase transformations using EHREM and have used the results to design a number of stable, single-phase structures at room temperature (RT).