Structure-selection Dynamics of Cobalt Nanoparticles from Solution Synthesis and Their Impact on the Catalytic Functionality

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 struc-ture 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/scattering data with atomistic multiscale simulations to investigate the self-restructuring of self-assembled Co-NPs from solution under dark or photocatalytic water oxidation condi-tions at distinct reaction times and atomic length-scales. Using the atomic range order as a descriptor, we reveal that dissolu-tion of a Co-salt in borate buffer leads to a self-optimization route forming disordered oxyborite Co3BOx-NPs unveiling a high oxygen yield due to the formation of surface oxo/hydroxo adsorbates. Those NPs further self-restructure into distorted Co3O4-NPs, and lastly, into CoOOH-NPs through a rate-limiting step integrating Co3+-states during the course of a representative photocatalytic assay. Self-restructuring does not proceed from amorphous-to-ordered states, but through stochastic fluctua-tions of atomic nanoclusters of 10 Å domain size. Our key insight into the structure-selection dynamics of TMO-NPs from solution offers new routes for tunning their structure-function relationships.