CO Oxidation on Metal Oxide Supported Single Pt atoms: The Role of the Support

Supported metal single atoms have demonstrated excellent catalytic performance for many chemical transformations. The effects of support on the catalytic performance of supported single metal atoms, however, have not been clearly elucidated. We carried out a systematic investigation of the effects of supports on CO oxidation by single Pt (Pt₁) atoms dispersed on different metal oxides: highly reducible Fe₂O₃, reducible ZnO, and irreducible γ-Al₂O₃. It was found that Pt₁ atoms on three metal oxides are active for CO oxidation, and the chemical properties of supports determine the catalytic performance of Pt₁ single-atom catalysts (SACs). Both the presence of −OH groups on support surfaces and the addition of H₂O significantly modify CO oxidation on three SACs and reduce the effects of supports on their catalytic performances. We conclude that the interaction between single metal atoms and support as well as surface properties of supports control the catalytic behavior of SACs

Facet-Selective Epitaxial Growth of δ‑Bi₂O₃ on ZnO Nanowires

Integration of ZnO nanowires with Bi₂O₃ coating layers can modify their physicochemical properties and efficiently improve the performance of the resulting nanostructured composites for specific applications. The interfacial structures often control the properties of such nanocomposite systems. We report a novel approach to achieving selective epitaxial growth of δ-Bi₂O₃ layers onto the {11–20} nanoscale facets of ZnO nanowires. By detailed atomic scale characterization of the surfaces and interfaces of the Bi₂O₃/ZnO nanocomposites we proposed that growth of the δ-Bi₂O₃ species on ZnO {11–20} surfaces follows the Stranski–Krastanov mechanism with an epitaxial relationship of ZnO [10–10] (11–20)∥δ-Bi₂O₃ [001] (100). An atomic model is proposed to explain the epitaxial relationship, the interfacial atomic structure, and the facet-selective growth

Galvanic Replacement-Free Deposition of Au on Ag for Core–Shell Nanocubes with Enhanced Chemical Stability and SERS Activity

We report a robust synthesis of Ag@Au core–shell nanocubes by directly depositing Au atoms on the surfaces of Ag nanocubes as conformal, ultrathin shells. Our success relies on the introduction of a strong reducing agent to compete with and thereby block the galvanic replacement between Ag and HAuCl₄. An ultrathin Au shell of 0.6 nm thick was able to protect the Ag in the core in an oxidative environment. Significantly, the core–shell nanocubes exhibited surface plasmonic properties essentially identical to those of the original Ag nanocubes, while the SERS activity showed a 5.4-fold further enhancement owing to an improvement in chemical enhancement. The combination of excellent SERS activity and chemical stability may enable a variety of new applications

Syntheses, Plasmonic Properties, and Catalytic Applications of Ag–Rh Core-Frame Nanocubes and Rh Nanoboxes with Highly Porous Walls

We report a simple and general method for the production of Ag–Rh bimetallic nanostructures with a unique integration of the plasmonic and catalytic properties exemplified by these two metals, respectively. When a Rh­(III) precursor is titrated into a polyol suspension of Ag nanocubes held at 110 °C in the presence of ascorbic acid and poly­(vinylpyrrolidone), Rh atoms are generated and deposited on the nanocubes. When the amount of Rh­(III) precursor is relatively low, the Rh atoms tend to nucleate from the edges of the Ag nanocubes and then follow an island growth mode because of the relatively low temperature involved and the high cohesive energy of Rh. The Rh islands can be maintained with an ultrafine size of only several nanometers, presenting an extremely large specific surface area for catalytic applications. As the amount of Rh­(III) precursor is increased, the galvanic replacement reaction between the Rh­(III) and Ag nanocubes will kick in, leading to the formation of increasingly concaved side faces and an increase in surface coverage for the Rh islands. Meanwhile, the resultant Ag⁺ ions are reduced and deposited back onto the nanocubes, but among the Rh islands. By simply controlling the amount of Rh­(III) precursor, we observe the transformation of Ag nanocubes into Ag–Rh core-frame and then Ag–Rh hollow nanocubes with a highly porous surface. Upon selective removal of Ag by wet etching, the hollow nanocubes evolve into Ag–Rh and then Rh nanoboxes with highly porous walls. Although the Ag–Rh core-frame nanocubes show a unique integration of the plasmonic and catalytic properties characteristic of Ag and Rh, respectively, the Rh nanoboxes show remarkable activity toward the catalytic degradation of environmental pollutants such as organic dyes

HAuCl₄: A Dual Agent for Studying the Chloride-Assisted Vertical Growth of Citrate-Free Ag Nanoplates with Au Serving as a Marker

We have investigated the vertical growth of citrate-free Ag nanoplates into truncated right bipyramids and twinned cubes with truncated corners in the presence of Cl^– ions at low and high concentrations, respectively, with Au serving as a marker for electron microscopy analysis. Both the Cl^– ions and Au atoms could be introduced through the use of HAuCl₄ as a dual agent. When HAuCl₄ was added into an aqueous mixture of citrate-free Ag nanoplates, ascorbic acid (AA), and poly­(vinylpyrrolidone), Au would be immediately formed and deposited on the surfaces of the nanoplates due to the reduction by both Ag and AA. The deposited Au could be easily resolved under STEM to reveal the growth patterns of the nanoplates. We found that the presence of Au did not change the growth pattern of the original Ag nanoplates. In contrast, the Cl^– ions could deterministically direct the formation of Ag nanoplates with a triangular or hexagonal shape, followed by their further growth into truncated right bipyramids or twinned cubes with truncated corners upon the introduction of AgNO₃. This work demonstrates, for the first time, that citrate-free Ag nanoplates could be transformed into right bipyramids or twinned cubes by controlling a single experimental parameter: the concentration of Cl^– ions in the growth solution. The mechanistic understanding represents a step forward toward the rational design and shape-controlled synthesis of nanocrystals with desired properties

Strong Coupling between ZnO Excitons and Localized Surface Plasmons of Silver Nanoparticles Studied by STEM-EELS

We investigated the strong coupling between the excitons of ZnO nanowires (NWs) and the localized surface plasmons (LSPs) of individual Ag nanoparticles (NPs) by monochromated electron energy loss spectroscopy (EELS) in an aberration-corrected scanning transmission electron microscopy (STEM) instrument. The EELS results confirmed that the hybridization of the ZnO exciton with the LSPs of the Ag NP created two plexcitons: the lower branch plexcitons (LPs) with a symmetrical dipole distribution and the upper branch plexcitons (UPs) with an antisymmetrical dipole distribution. The spatial maps of the LP and UP excitations reveal the nature of the LSP–exciton interactions. With decreasing size of the Ag NP the peak energies of the LPs and UPs showed a blue-shift and an anticrossing behavior at the ZnO exciton energy was observed. The coupled oscillator model explains the dispersion curve of the plexcitons and a Rabi splitting energy of ∼170 meV was deduced. The high spatial and energy resolution STEM-EELS approach demonstrated in this work is general and can be extended to study the various coupling interactions of a plethora of metal–semiconductor nanocomposite systems

Use of Reduction Rate as a Quantitative Knob for Controlling the Twin Structure and Shape of Palladium Nanocrystals

Kinetic control is a powerful means for maneuvering the twin structure and shape of metal nanocrystals and thus optimizing their performance in a variety of applications. However, there is only a vague understanding of the explicit roles played by reaction kinetics due to the lack of quantitative information about the kinetic parameters. With Pd as an example, here we demonstrate that kinetic parameters, including rate constant and activation energy, can be derived from spectroscopic measurements and then used to calculate the initial reduction rate and further have this parameter quantitatively correlated with the twin structure of a seed and nanocrystal. On a quantitative basis, we were able to determine the ranges of initial reduction rates required for the formation of nanocrystals with a specific twin structure, including single-crystal, multiply twinned, and stacking fault-lined. This work represents a major step forward toward the deterministic syntheses of colloidal noble-metal nanocrystals with specific twin structures and shapes

Self-Assembly of Atomically Thin and Unusual Face-Centered Cubic Re Nanowires within Carbon Nanotubes

Rhenium (Re), a high-performance engineering material with a hexagonal close-packed (hcp) structure, remains stable even under pressures of up to 250 GPa and at temperatures up to its melting point (3453 K). We observed here that Re atoms self-assembled, within the confined space of carbon nanotubes (CNTs) with a diameter of <1.5 nm, into ultrathin nanowires stacking with an unusual face-centered cubic (fcc) structure along the CNTs. In contrast, only Re nanoparticles of hcp structure formed on an open surface of graphite and carbon black. Aberration-corrected electron microscopy unambiguously showed the atomic arrangements of the Re nanowires and their confinement within the CNTs, ∼80% exhibiting a four-atom and 15% a nine-atom configuration. Density functional theory calculations confirmed that the formation of unusual fcc-stacking Re nanowires is largely facilitated by the strong interaction between Re atoms and CNTs and the spatial restriction within the CNTs. The use of CNTs as nanoscale reactors to create novel structures not only is fundamentally interesting but also may find unique applications in catalysis, sensing, and nanoelectronics

Shape-Controlled Synthesis of Palladium Nanocrystals: A Mechanistic Understanding of the Evolution from Octahedrons to Tetrahedrons

Palladium octahedrons and tetrahedrons enclosed by eight and four {111} facets have been synthesized from cuboctahedral Pd seeds by using Na₂PdCl₄ and Pd­(acac)₂, respectively, as the precursors. Our mechanistic studies indicate that the cuboctahedral seeds were directed to grow into octahedrons, truncated tetrahedrons, and then tetrahedrons when Pd­(acac)₂ was used as a precursor. In contrast, the same batch of seeds only evolved into octahedrons with increasing sizes when the precursor was switched to Na₂PdCl₄. The difference in growth pattern could be attributed to the different reduction rates of these two precursors. The fast reduction of Pd­(acac)₂ led to a quick drop in concentration for the precursor in the very early stage of a synthesis, forcing the growth into a kinetically controlled mode. In comparison, the slow reduction of Na₂PdCl₄ could maintain this precursor at a relatively high concentration to ensure thermodynamically controlled growth. This work not only advances our understanding of the growth mechanism of tetrahedrons but also offers a new approach to controlling the shape of metal nanocrystals

Facile Synthesis of Gold Wavy Nanowires and Investigation of Their Growth Mechanism

We describe a synthesis of Au wavy nanowires in an aqueous solution in the presence of cetyltrimethylammonium bromide (CTAB). The resultant Au nanowires automatically separated from the solution and floated at the air/water interface. We investigated the formation mechanism by characterizing the samples obtained at different stages of the synthesis. Both particle attachment and cold welding were found to be involved in the formation of such nanowires. Based on X-ray photoelectron spectroscopy and thermogravimetric analysis, the CTAB molecules adsorbed on the surface of a Au nanostructure went through a change in structure from a bilayer to a monolayer, converting the Au surface from hydrophilic to hydrophobic. As a result, the Au wavy nanowires were driven to the air/water interface during the synthesis. This growth mechanism is potentially extendable to many other systems involving small surfactant molecules