  In the context of two-dimensional metal-free photocatalyst, we investigate
the electronic, optical and excitonic properties of phosphorene derivatives
within first-principles approach. While two-dimensional phosphorene does not
catalyze the complete water splitting reactions, O, S, and N coverages improve
the situation drastically, and become susceptible to catalyze the complete
reaction at certain coverages. We find that for all these dopants, 0.25 -- 0.5
ML coverages are thermodynamically more stable, and does not introduce midgap
defect states and the composite systems remain semiconducting along with
properly aligned valance and conduction bands. Further, within visible light
excitation, the optical absorption remain very high 10$^5$ cm$^{-1}$ in these
composite systems, and the fundamental optical anisotropy of phosphorene
remains intact. We also investigate the effect of layer thickness through
bilayer phosphorene with oxygen coverages. Finally we investigate the excitonic
properties in these composite materials that are conducive to both redox
reactions. The present results will open up new avenues to take advantage of
these metal-free phosphorene derivatives toward its outstanding potential in
photocatalysis.
