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

Abstract

ABSTRACT 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 NiCo 2 O 4 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 α ‐C─H bond of ethanol to a surface lattice oxygen, generating an α ‐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 10 3 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 NH 3 formation in electrochemical nitrate reduction. These findings establish a mechanistic framework for the rational control of PLI‐synthesized metal‐based electrocatalysts.