ABSTRACT
The origin of photoluminescence in copper indium sulfide (CIS) quantum dots (Qdots) has previously been ascribed to a donor-acceptor pair (DAP) recombination, with a crystal lattice defect implicated as the origin of the donor state. In this study, electron energy-loss spectroscopy (EELS) was used to observe defect-rich compositional domains within individual CIS Qdots, supporting a model of defect-state-mediated photoluminescence for these particles, and identifying them as an ideal model system for future study of lattice defects on Qdot properties.
ABSTRACT
In this work we address the challenge of furthering our understanding of the driving forces responsible for the metal-metal interactions in industrially relevant bimetallic nanocatalysts, by taking a comparative approach to the atomic scale characterization of two core-shell nanorod systems (AuPd and AuRh). Using aberration-corrected scanning transmission electron microscopy, we show the existence of a randomly mixed alloy layer some 4-5 atomic layers thick between completely bulk immiscible Au and Rh, which facilitates fully epitaxial overgrowth for the first few atomic layers. In marked contrast in AuPd nanorods, we find atomically sharp segregation resulting in a quasi-epitaxial, strained interface between bulk miscible metals. By comparing the two systems, including molecular dynamics simulations, we are able to gain insights into the factors that may have influenced their structure and chemical ordering, which cannot be explained by the key structural and energetic parameters of either system in isolation, thus demonstrating the advantage of taking a comparative approach to the characterization of complex binary systems. This work highlights the importance of achieving a fundamental understanding of reaction kinetics in realizing the atomically controlled synthesis of bimetallic nanocatalysts.
ABSTRACT
Gold catalysts supported on titania (Au-TiO(2)) and alumina (Au-Al(2)O(3)) were prepared by deposition-precipitation with urea and then activated before characterisation and reaction in CO oxidation, either by calcination in air at 500 °C or reduction under H(2) at 300 °C. Gold nanoparticles with average size in the range 2-4 nm were obtained, with calcination leading to larger gold nanoparticles than reduction. For Au-TiO(2), high activity was observed in CO oxidation at room temperature, independent of the activation treatment. This high activity could not be correlated to the presence of sub-nanometer gold clusters as reported in the literature, since they could not be detected by atomic-resolution high-angle annular dark-field scanning-transmission electron microscopy (HAADF-STEM). In the case of Au-Al(2)O(3), the performance in CO oxidation was found to strongly depend on the water content in the reaction gas feed and on the activation conditions, with calcination resulting in a poorly active catalyst whereas reduction gave activity of the same order as Au-TiO(2). A comparative study of Au-TiO(2) and Au-Al(2)O(3) by electron microscopy did not reveal distinct differences in the shapes of the Au nanoparticles, which are mostly flattened through interaction with the substrate in both samples, with side profile shapes varying from rounded hemispherical to well faceted truncated cubo-octahedra. More faceting is found for the samples calcined at 500 °C than reduced at 300 °C. Various possible parameters affecting the catalytic properties of gold in CO oxidation are discussed in the context of the relevant literature.
ABSTRACT
This study focuses on the deposition and growth mode of rhodium (Rh) on gold (Au) seed nanorods (NRs). Using a combination of scanning transmission electron microscopy imaging, energy-dispersive X-ray spectroscopy, and UV-visible absorption spectroscopy, we show that Rh deposition results in an uneven overlayer morphology on the Au NR seeds, with a tendency for Rh deposition to occur preferentially on the Au NR ends. The results suggest that complex and kinetically driven metal-metal interactions take place in this system.
