Structure-Selection Dynamics of Cobalt Nanoparticles from Solution Synthesis and Their Impact on the Oxygen Evolution Reaction.

Resolving the three-dimensional structure of transition metal oxide nanoparticles (TMO-NPs), upon self-restructuring from solution, is crucial for tuning their structure-functionality. Yet, this remains challenging as this process entails complex structure fluctuations, which are difficult to track experimentally and, hence, hinder the knowledge-driven optimization of TMO-NPs. Herein, we combine high-energy synchrotron X-ray absorption and X-ray total scattering experiments with atomistic multiscale simulations to investigate the self-restructuring of self-assembled Co-NPs from solution under dark or photocatalytic water oxidation conditions at distinct reaction times and atomic length-scales. Using the atomic range order as a descriptor, we reveal that dissolution of a Co-salt in BO buffer leads to a self-optimization route forming disordered oxyborate CoBO-NPs unveiling a high oxygen yield due to the formation of surface oxo/hydroxo adsorbates. Those CoBO-NPs further self-restructure into distorted CoO-NPs and, lastly, into distorted CoOOH-NPs through a rate-limiting step integrating Co-states during the course of a representative photocatalytic assay. Self-restructuring does not proceed from amorphous-to-ordered states but through stochastic fluctuations of atomic nanoclusters of ≈10 Å domain size. Our key insight into the structure-selection dynamics of TMO-NPs from solution offers a route for tuning their structure-function relationships for wide-ranging emergent technologies.