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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.
We have synthesized single crystal bismuth nanowires by pressure injecting molten Bi into anodic alumina templates. By varying the template fabrication conditions, nanowires with diameters ranging from 10 to 200nm and lengths of ~50[.proportional]m can be produced. We present a scheme for measuring the resistance of a single Bi nanowire using a 4-point measurement technique. The nanowires are found to have a 7nm thick oxide layer which causes very high contact resistance when electrodes are patterned on top of the nanowires. The oxide is found to be resilient to acid etching, but can be successfully reduced in high temperature hydrogen and ammonia environments. The reformation time of the oxide in air is found to be less than 1 minute. Focused ion beam milling is attempted as an alternate solution to oxide removal.
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.
Substitution of Ca(II) for Y(III) into tetragonal YBa2Cu3O6 has been achieved and a superconducting transition has been observed for a material of nominal composition (Y0.5Ca0.5) Ba2Cu3O6 (Tc (onset) ˜50K) . Observations using high resolution electron microscopy show samples with x < 0.3 consist of a complex mixture, including (Y, Ca) Ba2Cu3O6+δ, YBa2Cu3O6+δ, and BaCuO2. Further, structural refinement using neutron diffraction data provide evidence of a solid solution limit at x ˜ 0.3. A direct analogy can be drawn between superconducting (Y1−xCax)Ba2Cu3O6 (0.1 < x < 0.3) and superconductors of the type (Y1−xCax)Pb2Sr2Cu3O8 and (La2−xM(II)x)CuO4. Substitution of the Ca(II) for Y(III) effectively increases the formal copper oxidation state. The refined structural model is fully consistent with partially oxidized CuO2 sheets separated by linear O-Cu(I)-O units.
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.
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.
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