Metallic single-unit-cell orthorhombic cobalt diselenide atomic layers: robust water-electrolysis catalysts.

The bottleneck in water electrolysis lies in the kinetically sluggish oxygen evolution reaction (OER). Herein, conceptually new metallic non-metal atomic layers are proposed to overcome this drawback. Metallic single-unit-cell CoSe2 sheets with an orthorhombic phase are synthesized by thermally exfoliating a lamellar CoSe2 -DETA hybrid. The metallic character of orthorhombic CoSe2 atomic layers, verified by DFT calculations and temperature-dependent resistivities, allows fast oxygen evolution kinetics with a lowered overpotential of 0.27 V. The single-unit-cell thickness means 66.7 % of the Co(2+) ions are exposed on the surface and serve as the catalytically active sites. The lowered Co(2+) coordination number down to 1.3 and 2.6, gives a lower Tafel slope of 64 mV dec(-1) and higher turnover frequency of 745 h(-1) . Thus, the single-unit-cell CoSe2 sheets have around 2 and 4.5 times higher catalytic activity compared with the lamellar CoSe2 -DETA hybrid and bulk CoSe2 .