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

Ag@Au Concave Cuboctahedra: A Unique Probe for Monitoring Au-Catalyzed Reduction and Oxidation Reactions by Surface-Enhanced Raman Spectroscopy

We report a facile synthesis of Ag@Au concave cuboctahedra by titrating aqueous HAuCl₄ into a suspension of Ag cuboctahedra in the presence of ascorbic acid (AA), NaOH, and poly­(vinylpyrrolidone) (PVP) at room temperature. Initially, the Au atoms derived from the reduction of Au³⁺ by AA are conformally deposited on the entire surface of a Ag cuboctahedron. Upon the formation of a complete Au shell, however, the subsequently formed Au atoms are preferentially deposited onto the Au{100} facets, resulting in the formation of a Ag@Au cuboctahedron with concave structures at the sites of {111} facets. The concave cuboctahedra embrace excellent SERS activity that is more than 70-fold stronger than that of the original Ag cuboctahedra at an excitation wavelength of 785 nm. The concave cuboctahedra also exhibit remarkable stability in the presence of an oxidant such as H₂O₂ because of the protection by a complete Au shell. These two unique attributes enable <i>in situ</i> SERS monitoring of the reduction of 4-nitrothiophenol (4-NTP) to 4-aminothiophenol (4-ATP) by NaBH₄ through a 4,4′-dimercaptoazobenzene (<i>trans</i>-DMAB) intermediate and the subsequent oxidation of 4-ATP back to <i>trans</i>-DMAB upon the introduction of H₂O₂

Ultrafast Photoinduced Interfacial Proton Coupled Electron Transfer from CdSe Quantum Dots to 4,4′-Bipyridine

Pyridine and derivatives have been reported as efficient and selective catalysts for the electrochemical and photoelectrochemical reduction of CO₂ to methanol. Although the catalytic mechanism remains a subject of considerable recent debate, most proposed models involve interfacial proton coupled electron transfer (PCET) to electrode-bound catalysts. We report a combined experimental and theoretical study of the photoreduction of 4,4′-bipyridium (bPYD) using CdSe quantum dots (QDs) as a model system for interfacial PCET. We observed ultrafast photoinduced PCET from CdSe QDs to form doubly protonated [bPYDH₂]^+• radical cations at low pH (4–6). Through studies of the dependence of PCET rate on isotopic substitution, pH and bPYD concentration, the radical formation mechanism was identified to be a sequential interfacial electron and proton transfer (ET/PT) process with a rate-limiting pH independent electron transfer rate constant, <i>k</i>_int, of 1.05 ± 0.13 × 10¹⁰ s^–1 between a QD and an adsorbed singly protonated [bPYDH]⁺. Theoretical studies of the adsorption of [bPYDH]⁺ and methylviologen on QD surfaces revealed important effects of hydrogen bonding with the capping ligand (3-mercaptopropionic acid) on binding geometry and interfacial PCET. In the presence of sacrificial electron donors, this system was shown to be capable of generating [bPYDH₂]^+• radical cations under continuous illumination at 405 nm with a steady-state photoreduction quantum yield of 1.1 ± 0.1% at pH 4. The mechanism of bPYD photoreduction reported in this work may provide useful insights into the catalytic roles of pyridine and pyridine derivatives in the electrochemical and photoelectrochemical reduction of CO₂

Constructing Two-Dimensional Nanoparticle Arrays on Layered Materials Inspired by Atomic Epitaxial Growth

Constructing nanoparticles into well-defined structures at mesoscale and larger to create novel functional materials remains a challenge. Inspired by atomic epitaxial growth, we propose an “epitaxial assembly” method to form two-dimensional nanoparticle arrays (2D NAs) directly onto desired materials. As an illustration, we employ a series of surfactant-capped nanoparticles as the “artificial atoms” and layered hybrid perovskite (LHP) materials as the substrates and obtain 2D NAs in a large area with few defects. This method is universal for nanoparticles with different shapes, sizes, and compositions and for LHP substrates with different metallic cores. Raman spectroscopic and X-ray diffraction data support our hypothesis of epitaxial assembly. The novel method offers new insights into the controllable assembly of complex functional materials and may push the development of materials science at the mesoscale