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

[Display omitted] •A topotactic transformation was proposed via rapid calcination at 300 °C in air for only 5 min to transform the P123-encapsulated ultrathin CoOxHy nanosheets.•This facile post-synthetic treatment allows the formation of surface-clean, hierarchical polycrystalline Co3O4-VO nanosheets with enriched oxygen vacancies.•Co3O4-VO outperforms the as-synthesized CoOxHy nanosheets and the conventionally calcined Co3O4 in OER and particularly in HMF electrooxidation to FDCA. Surfactant-free, freestanding, and hierarchical two-dimensional (2D) polycrystalline cobalt oxide (Co3O4) nanosheets with enriched oxygen vacancies (Co3O4-VO) were synthesized by a topotactic conversion via rapid calcination of the solvothermally synthesized ultrathin cobalt oxide hydrate (CoOxHy) nanosheets. The topochemically transformed Co3O4-VO outperforms the as-synthesized P123-encapsulated CoOxHy nanosheets and their conventionally calcined Co3O4 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 Co3O4 nanocrystallines are also proposed to be responsible for the improved electrooxidation performance. Furthermore, Co3O4-VO exhibits remarkable long-term stability during the chronoamperometric test in 1 M KOH.