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

Two-dimensional Co3O4 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 Co3O4 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 Co3O4 nanosheets having Co(II) sites in quantities similar to those usually found for Co3O4 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−1 at 1 A g−1 with 57% retention at 60 A g−1 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 Co3O4 nanostructures, as well as effective adsorbents for the removal of organic dyes (methyl orange, methylene blue) from water.Funding by the Spanish Ministerio de Economía y Competitividad (MINECO) and the European Regional Development Fund (ERDF) through project MAT2015-69844-R is gratefully acknowledged. We are also grateful to Plan de Ciencia, Tecnología e Innovación, Spain 2013-2017 del Principado de Asturias and the ERDF, European Union for partial funding through project IDI/2018/000233. J.M.M. and S.G-D. are grateful to the Spanish Ministerio de Educación, Cultura y Deporte (MECD) and MINECO, respectively, for their pre-doctoral contracts (FPU14/00792 and BES/2016 077830, respectively).Peer reviewe