Solar CO2 reduction into hydrocarbons helps to solve the global warming and energy crisis. However, conventional semiconductors usually suffer from low photoactivity and poor photostability. Here, atomically-thin oxide-based semiconductors are proposed as excellent platforms to overcome this drawback. As a prototype, single-unit-cell Bi2WO6 layers are first synthesized by virtue of a lamellar Bi-oleate intermediate. The single-unit-cell thickness allows 3-times larger CO2 adsorption capacity and higher photoabsorption than bulk Bi2WO6. Also, the increased conductivity, verified by density functional theory calculations and temperature-dependent resistivities, favors fast carrier transport. The carrier lifetime increased from 14.7 to 83.2 ns, revealed by time-resolved fluorescence spectroscopy, which accounts for the improved electron-hole separation efficacy. As a result, the single-unit-cell Bi2WO6 layers achieve a methanol formation rate of 75 μmol g(-1) h(-1), 125-times higher than that of bulk Bi2WO6. The catalytic activity of the single-unit-cell layers proceeds without deactivation even after 2 days. This work will shed light on designing efficient and robust photoreduction CO2 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 .

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