Understanding how molecules can restructure the surfaces of heterogeneous catalysts under reaction conditions requires methods that can visualize atoms in real space and time. We applied a newly developed aberration-corrected environmental transmission electron microscopy to show that adsorbed carbon monoxide (CO) molecules caused the {100} facets of a gold nanoparticle to reconstruct during CO oxidation at room temperature. The CO molecules adsorbed at the on-top sites of gold atoms in the reconstructed surface, and the energetic favorability of this reconstructed structure was confirmed by ab initio calculations and image simulations. This atomic-scale visualizing method can be applied to help elucidate reaction mechanisms in heterogeneous catalysis.
The
effects of RbF postdeposition treatment (RbF-PDT) on Cu(In,Ga)Se2, CuInSe2, and CuGaSe2 thin films and
solar cell devices are comparatively studied. Similar to the effect
of the KF postdeposition treatment (KF-PDT), Cu(In,Ga)Se2 and CuInSe2 film surfaces show significant pore formation
resulting in a rough surface morphology with RbF-PDT, whereas this
is not the case for In-free CuGaSe2. The device properties
of the In-containing and In-free Cu(In,Ga)Se2 solar cells
also show contrasting results, namely, Cu(In,Ga)Se2 or
CuInSe2 devices show an increase in the open circuit voltage
(V
oc) and fill factor (FF) values and
almost constant or a slight decrease in the short-circuit current
density (J
sc) values with RbF-PDT, whereas
CuGaSe2 devices show no significant improvements in the V
oc and FF values but a substantial increase
in the J
sc values. These results suggest
that the alkali effects on the Cu(In,Ga)Se2 film and device
properties strongly depend on the group III elemental composition
in the Cu(In,Ga)Se2 films as well as alkali-metal species.
We present first-principle calculations on symmetric tilt grain boundaries (GBs) in bcc Fe. Using density functional theory (DFT), we studied the structural, electronic and magnetic properties of Σ3(111) and Σ11(332) GBs formed by rotation around the [110] axis. The optimized structures, GB energies and GB excess free volumes are consistent with previous DFT and classical simulation studies. The GB configurations can be interpreted by the structural unit model as given by Nakashima and Takeuchi (2000 ISIJ 86 357). Both the GBs are composed of similar structural units of three- and five-membered rings with different densities at the interface according to the rotation angle. The interface atoms with larger atomic volumes reveal higher magnetic moments than the bulk value, while the interface atoms with shorter bond lengths have reduced magnetic moments in each GB. The charge density and local density of states reveal that the interface bonds with short bond lengths have more covalent nature, where minority-spin electrons play a dominant role as the typical nature of ferromagnetic Fe. In order to understand the structural stability of these GBs, we calculated the local energy and local stress for each atomic region using the scheme of Shiihara et al (2010 Phys. Rev. B 81 075441). In each GB, the interface atoms with larger atomic volumes and enhanced magnetic moments reveal larger local energy increase and tensile stress. The interface atoms constituting more covalent-like bonds with reduced magnetic moments have lower local energy increase, contributing to the stabilization, while compressive stress is generated at these atoms. The relative stability between the two GBs can be understood by the local energies at the structural units. The local energy and local stress analysis is a powerful tool to investigate the structural properties of GBs based on the behavior of valence electrons.
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