  Converting solar energy into chemical energy by splitting water is a
promising means to generate a sustainable and renewable solution without
detrimental environmental impact. The two-dimensional semiconductors serve as
potential catalysts in this regard, and here we combine Janus transition-metal
dichalcogenides (MoXY, X/Y = S, Se, Te) and graphitic carbon nitride in a van
der Waals heterostructure. Within the first-principles calculations, we
investigate the electronic, optical and excitonic properties that determine the
photocatalytic activity. Due to the internal electric field, the photogenerated
electrons and holes are separated in the MoXY layers, and also generates high
overpotentials for the redox reactions. The high optical absorptions span
throughout the entire visible and near ultraviolet regime in these
heterostructure nanocomposites. Further, the lower exciton binding, calculated
within the two-dimensional hydrogenic model, indicates efficient charge
separation. Enormous tunability of photocatalytic properties in such
heterostructures should attract considerable theoretical and experimental
attention in future.
