First principles study of photocatalytic activity in ZnO-Janus van der Waals heterostructures.

The design of type-II van der Waals (vdW) heterostructures is regarded as a promising route to produce green hydrogen photocatalytic water splitting. To this aim, we propose novel vertically stacked vdW heterostructures based on ZnO and Janus VXY (X = Br, Cl, Y = Se, and Te) phases, and investigate their optoelectronic properties and photocatalytic performance by means of density functional theory simulations. The thermal stability of the heterostructures is confirmed by molecular dynamics simulations at 300 K. The HSE06 calculated band structures show that a specific stacking of ZnO-VBrSe and ZnO-VClSe exhibits an indirect band gap with type-II band alignment, while all other stackings exhibit a direct band gap with type-I band alignment. The type-II band alignment, along with the difference in the work function and the electrostatic potential between the ZnO and VXY monolayer, will result in a built-in electric field direct from the ZnO monolayer to the VXY monolayer which is crucial for photogenerated charge separation, and prevents the charge recombinations. The optical absorption coefficient of all the considered ZnO-VXY heterostructures displays the first excitonic peak in the energy range required for photocatalysis applications. Based on the band edge potential analysis, all the studied systems are capable of starting an oxygen evolution reaction spontaneously, while some external stimuli will be required to initiate the hydrogen evolution reaction. The reported results suggest that the proposed ZnO-VXY vdW heterostructures have great potential for photocatalysis and optoelectronic device applications.