  Two-dimensional (2D) materials viz. transition metal dichalcogenides (TMD)
and transition metal oxides (TMO) offer a platform that allows creation of
heterostructures with a variety of properties. The optoelectronic industry has
observed an upheaval in the research arena of MoS$_2$ based van der Waals (vdW)
heterostructures (HTSs) and Janus structures. Therefore, interest towards these
structures are backed by the selectivity in terms of electronic and optical
properties. The present study investigates the photocatalytic ability of
bilayer, MoS$_2$ and Janus (MoSSe) based vdW HTSs viz. MoS$_2$/TMO,
MoS$_2$/TMD, MoSSe/TMO and MoSSe/TMD, by first-principles based approach under
the framework of (hybrid) density functional theory (DFT) and manybody
perturbation theory (GW approximation). We have considered HfS$_2$, ZrS$_2$,
TiS$_2$, WS$_2$ and HfO$_2$, T-SnO$_2$, T-PtO$_2$ from the family of TMD and
TMO, respectively. The photocatalytic properties of these vdW HTSs are
thoroughly investigated and compared with the respective individual monolayers
by visualizing their band edge alignment, electron-hole recombination rate and
optical properties. Strikingly we observe that, despite most of the individual
monolayers do not perform optimally as a photocatalyst, type II band edge
alignment is noticed to vdW HTSs and they appear to be efficient for
photocatalysis via Z-scheme. Moreover, these HTSs have also shown promising
optical response in the visible region. Finally on computing electron-hole
recombination rate we find MoSSe/HfS$_2$, MoSSe/TiS$_2$, MoS$_2$/T-SnO$_2$,
MoS$_2$/ZrS$_2$ and MoSSe/ZrS$_2$ are probable, most efficient Z-scheme
photocatalysts.
