4 research outputs found
HAuCl<sub>4</sub>: 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<sup>–</sup> ions at low and high concentrations,
respectively, with Au serving as a marker for electron microscopy
analysis. Both the Cl<sup>–</sup> ions and Au atoms could be
introduced through the use of HAuCl<sub>4</sub> as a dual agent. When
HAuCl<sub>4</sub> was added into an aqueous mixture of citrate-free
Ag nanoplates, ascorbic acid (AA), and polyÂ(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<sup>–</sup> 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 AgNO<sub>3</sub>. 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<sup>–</sup> 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 HAuCl<sub>4</sub> into a suspension of Ag cuboctahedra in
the presence of ascorbic acid (AA), NaOH, and polyÂ(vinylpyrrolidone)
(PVP) at room temperature. Initially, the Au atoms derived from the
reduction of Au<sup>3+</sup> 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
H<sub>2</sub>O<sub>2</sub> 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 NaBH<sub>4</sub> 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 H<sub>2</sub>O<sub>2</sub>
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 CO<sub>2</sub> 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 [bPYDH<sub>2</sub>]<sup>+•</sup> 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><sub>int</sub>, of 1.05 ± 0.13 × 10<sup>10</sup> s<sup>–1</sup> between a QD and an adsorbed singly protonated [bPYDH]<sup>+</sup>. Theoretical studies of the adsorption of [bPYDH]<sup>+</sup> 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
[bPYDH<sub>2</sub>]<sup>+•</sup> 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 CO<sub>2</sub>
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