Interpretation of size evolution is an essential part of nanocluster transformation processes for unraveling the mechanism at an atom-precision level. Here we report the transformation of a nonsuperatomic Au 23 to a superatomic Au 36 nanocluster via Au 28 cluster formation, activated by the bulky 4-tert-butylbenzenethiol ligand. Time-dependent matrix-assisted laser desorption ionization mass spectrometry data revealed that the conversion proceeds through ligand exchange followed by the size focusing method, ultimately leading to size growth. We also validated this transformation through time-dependent ultraviolet−visible data. Density functional theory calculations predicted that the kernel of the Au 28 cluster evolved through a linear combination of molecular orbitals of the fragment of 2e − units (Au 4 2+ and Au 3 + ) from the kernel of the Au 23 cluster. Periodic growth of gold cores through continuous growth of Au 4 tetrahedral unit leads to the formation of the Au 36 cluster from the Au 28 cluster. These results reinforce the plausibility of size evolution through the growth mechanism during the transformation process. Differential pulse voltammetry studies showed that the highest occupied molecular orbital−lowest unoccupied molecular orbital gap inversely varies with the kernel size of these clusters. Photophysical experiments support the molecular-like intersystem crossing rather than core−shell relaxation to these clusters. The trends of photoluminescence lifetime were found to be the reverse of those of the energy gap law. The increment of lifetimes for the larger cluster can be mainly due to the contribution of both hot carriers and band-edge carriers.
In this work, we demonstrate morphology‐controlled synthesis of flower‐like cobalt phosphide decorated on nitrogen doped reduced graphene oxide (NrGO), which act as bifunctional electrocatalysts toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The as‐synthesized CoP‐NrGO, formed via hydrothermal reaction followed by gas‐solid reaction, attains an OER current density of 10 mA cm−2 at an overpotential of 0.38 V, which is on par with that of its Co(OH)2‐NrGO counterpart and the benchmarked IrO2 catalyst. On the other hand, the conversion of Co(OH)2 to CoP is seen to improve its HER performance drastically. CoP‐NrGO reached a current density of 10 mA cm−2 at an overpotential of 0.184 V which is 0.204 V anodic to Co(OH)2‐NrGO. The improved performance could be attributed to reduced charge transfer resistance following the phosphidation process. From the comparison of the electrocatalytic performance of the catalysts, it can be inferred that phosphidation has a considerable effect only on the cathodic HER.
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