Direct Observation of Radical-Mediated Oxide-to-Alloy Transformation in Solution Under Pulsed Laser Irradiation

초록

Pulsed laser irradiation (PLI) is widely used to synthesize metal-based functional materials in solution, yet the molecular-level mechanism by which a laser pulse reduces a metal-oxide precursor has remained unresolved. Here we combine time-resolved in situ x-ray absorption spectroscopy (TR-XAS) at both the Co and Ni K-edges with density-functional tight-binding molecular dynamics (DFTB-MD) simulations to investigate the laser-driven transformation of NiCo2O4 into a metallic NiCo alloy in ethanol. Considering the results from TR-XAS and DFTB-MD simulations, the reduction is initiated by hydrogen atom transfer from the alpha-C & horbar;H bond of ethanol to a surface lattice oxygen, generating an alpha-hydroxyethyl radical; the oxygen is subsequently released as water and the alcohol is dehydrogenated to acetaldehyde. Even though the initial hydrogen transfer is facile, the release of lattice oxygen is entropy-driven and becomes accessible only at temperatures of order 103 K, so that the extent of reduction is governed by the transient local temperature and hence by the laser fluence: complete reduction occurs at fluences >= 100 mJ/pulse, yielding a metallic surface that achieves a Faradaic efficiency of 86.6% for NH3 formation in electrochemical nitrate reduction. These findings establish a mechanistic framework for the rational control of PLI-synthesized metal-based electrocatalysts.

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