The long-term stability of single-atom catalysts is a major factor affecting their large-scale commercial application. How to evaluate the dynamic stability of single-atom catalysts under working conditions is still lacking. Here, taking a single copper atom embedded in N-doped graphene as an example, the "constant-potential hybrid-solvation dynamic model" is used to evaluate the reversible transformation between copper single atoms and clusters under realistic reaction conditions. It is revealed that the adsorption of H is a vital driving force for the leaching of the Cu single atom from the catalyst surface. The more negative the electrode potential, the stronger the adsorption of H. As a result, the competitive hydrogen evolution reaction is inhibited, and Cu−N bonds are weakened, resulting in some Cu atoms being tethered on the catalyst surface and some being dissolved in the aqueous solution. The collision of the Cu atoms in the two states forms a transient Cu cluster structure as a true catalytic active site to promote CO 2 reduction to ethanol. As the applied potential is released or switched to a positive value, hydroxyl radicals (OH • ) play a dominant role in the oxidation process of the Cu cluster, and then Cu returns to the initial atomic dispersion state by redeposition, completing the reconstruction cycle of the copper catalyst. Our work provides a fundamental understanding of the dynamic stability of Cu single-atom catalysts under working conditions at the atomic level and calls for a reassessment of the stability of currently reported single-atom catalysts considering realistic reaction conditions.
Photoluminescence
(PL) of organometal halide perovskite has been
broadly investigated as a fundamental signal to understand the photophysics
of these materials. Complicated PL behaviors have been reported reflecting
complex mechanisms including effects from crystal defects/traps whose
nature still remains unclear. Here in this work we observed, besides
the PL enhancement, a surprising PL decline phenomenon in methylammonium
lead triiodide (CH3NH3PbI3) perovskite
showing a high initial PL intensity followed by a fast decline in
time scale of milliseconds to seconds. The similarity between the
PL enhancement and PL decline suggests both processes are due to PL
quenching traps in the material. Combining experimental and theoretical
results, two interstitial defects of iodide and lead were identified
to be responsible for the PL enhancement and PL decline, respectively.
Both traps can be switched between active and inactive states, leading
to a reversible process of PL enhancement and PL decline. The identification
of the chemical nature of the PL quenching traps is an important
step toward fully understanding the crystals defects in these materials.
Two-dimensional
(2D) perovskites are attracting broad attention
for their stability and wavelength tunability. However, random crystallization
of sample preparation makes it difficult to obtain 2D perovskites
with pure structure, especially when the number of layers is large.
Herein, we prepared 2D perovskite (C8H17NH3)2(MA)
n−1Pb
n
I3n+1 with different
layers (n = 1–10). For the first time, we
experimentally identified the band gap energy E
g
of 2D perovskite (C8H17NH3)2(MA)
n−1Pb
n
I3n+1 with
layers up to 10 by investigating specific pieces of crystal with pure
emission spectra using fluorescence microscopy. Intriguingly, the
relationship between E
g
and n perfectly fits an exponential function rather
than the pure quantum confinement effect in good agreement with the
theoretical calculation based on first principles. Our results suggest
that the band gap of the 2D perovskite is determined not only by quantum
confinement effect, but other factors including chemical components
also give significant contribution.
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