Water-Regulated Formation of Asymmetric Janus-Like CeO₂ Nanoparticles with Increased Specific Surface Area

The intermediating chemistry and accessible surface area of CeO₂ nanoparticles (CNPs) are strongly dictated by their mesoscale textural features and morphology rather than their bulk particle size. It remains crucial to develop facile synthetic strategies for the continuous tuning of these architectural parameters as well as their surface valence states. We report a water-regulated solvothermal synthesis in ethylene glycol that gives rise to phase-pure CeO₂ nanoparticles in a variety of architectures which transit from asymmetric, defect-perforated (Janus-like) to symmetric, sea-urchin-like. Through the systematic variation of the deionized water content, it is possible to directly modulate the effective solvation and supersaturation equilibria of the system. Electron microscopy shows that a continuous morphological evolution occurs. At high effective precursor concentrations, asymmetric Janus-like holes are kinetically preferred. Then, upon further dilution, smaller uniform sea-urchin-like particles appear, which finally promotes aggregation. It is confirmed with structural characterizations that all samples exhibit a face-centered cubic (fcc) lattice. Moreover, the crystallinity increases with a decrease in the effective concentration. Through comparative analysis using two size-matched representative samples, the surface Ce³⁺ fractions of the two systems are comparable. Interestingly, the defect-rich Janus-like structure has a Brunauer-Emmett-Teller (BET) specific surface area that is higher by ∼75.9% than that of the sea-urchin-like counterpart. We suggest that the topologically asymmetric Janus-like behavior is a result of the combination of Ostwald ripening and shell collapse.