Optical and photocatalytic properties of two-dimensional MoS2

The electronic structure and optical spectrum of monolayer MoS2 are calculated using both the modified Becke-Johnson (mBJ) approximation and Bethe-Salpeter equation. Bulk MoS2 is an indirect band gap semiconductor, but thinned to a monolayer it converts to a direct band gap semiconductor with increased gap. The calculated mBJ band gaps of MoS2 amount to 1.15 eV for the bulk and 1.90 eV for the monolayer, in excellent agreement with experiment. The experimental excitonic peaks of monolayer MoS2 at 1.88 eV and 2.06 eV are reproduced by the calculations. The high photoluminescence yield can be attributed to a high binding energy of the excitons and is not due to a splitting of the valence bands, as is commonly assumed. We also show that monolayer MoS2 has the ability to oxidize H2O and produce O-2 as well as to reduce H+ to H-2.