Ultrathin polycrystalline Co3O4 nanosheets with enriched oxygen vacancies for efficient electrochemical oxygen evolution and 5-hydroxymethylfurfural oxidation

Surfactant-free, freestanding, and hierarchical two-dimensional (2D) polycrystalline cobalt oxide (CoO) nanosheets with enriched oxygen vacancies (CoO-V) were synthesized by a topotactic conversion via rapid calcination of the solvothermally synthesized ultrathin cobalt oxide hydrate (CoOH) nanosheets. The topochemically transformed CoO-V outperforms the as-synthesized P123-encapsulated CoOH nanosheets and their conventionally calcined CoO counterpart for both electrochemical oxygen evolution and 5-hydroxymethylfurfural (HMF) oxidation to 2,5-furandicarboxylic acid (FDCA), owing to their largely preserved 2D structure and elimination of P123 for abundant exposed surface active sites. More importantly, the strain-induced oxygen vacancies at grain boundaries of CoO nanocrystallines are also proposed to be responsible for the improved electrooxidation performance. Furthermore, CoO-V exhibits remarkable long-term stability during the chronoamperometric test in 1 M KOH.