Ultrathin transition metal oxychalcogenide catalysts for oxygen evolution in acidic media

Two-dimensional transition metal dichalcogenides (TMDs) exfoliated from bulk layered materials possess interesting properties. Most transition metal oxides are not layered and therefore cannot be exfoliated. Here we report the synthesis of a family of ultrathin materials-transition metal oxychalcogenides (TMOCs)-and demonstrate their unique properties. Two-dimensional TMOCs (MX _x O _y , M = group IV or V transition metal, X = chalcogen, O = oxygen; x, y = 0-2) from bulk transition metal dichalcogenides (MX₂) have been fabricated using tetrabutylammonium intercalation. The stoichiometry of TMOCs can be adjusted, which enables control of their optical bandgaps and tunability of electrical conductivity by more than eight orders of magnitude. By tuning the chalcogen-to-oxygen ratio along with local atomic structure in TMOCs, it is possible to impart unexpected properties. For example, in contrast to conventional TMDs, the hybrid structure of TMOCs renders them surprisingly stable and electrochemically active in strong acids, allowing them to be used as proof-of-concept catalysts for the oxygen evolution reaction at pH ≈ 0. The HfS_0.52O_1.09 catalyst shows high mass activity (103,000 A g^-1 at an overpotential of 0.5 V) and exhibits durability in proton exchange membrane water electrolysers.