A direct route to activated two-dimensional cobalt oxide nanosheets for electrochemical energy storage, catalytic and environmental applications.

Two-dimensional CoO nanosheets have emerged as attractive materials for use in a number of relevant technological applications. To exhibit a competitive performance in such uses, however, their structure needs to be activated, which is frequently accomplished via post-synthesis reduction strategies that introduce oxygen vacancies and increase the number of active Co(II) sites. Here, we investigate a direct route for the synthesis of activated CoO nanosheets that avoids reduction post-treatments, yielding materials with a high potential towards energy- and environment-related applications. The synthesis relied on an interim amorphous cobalt oxide material with nanosheet morphology, which upon calcination afforded CoO nanosheets having Co(II) sites in quantities similar to those usually found for CoO nanostructures activated by reduction post-treatments. When tested as electrodes for charge storage, the nanosheets demonstrated a competitive behavior in terms of both capacity and rate capability, e.g., a gravimetric capacity of ∼293 mAh g at 1 A g with 57% retention at 60 A g was measured for nanosheets calcined at 350 °C. The materials were shown to be efficient catalysts for the reduction of nitroarenes (4-nitrophenol and 4-nitroaniline), outperforming other CoO nanostructures, as well as effective adsorbents for the removal of organic dyes (methyl orange, methylene blue) from water.