Hittorf's violet phosphorene as a promising candidate for optoelectronic and photocatalytic applications: first-principles characterization.

Utilizing density functional theory, we investigate the structural stabilities, electronic structures, and optical properties of monolayer violet phosphorene, i.e., Hittorfene, under an external vertical electric field and upon in-layer biaxial strain control. We find that compared with monolayer black phosphorene, monolayer violet phosphorene has a significantly larger direct band gap of 2.50 eV, and it is sensitive to an external vertical electric field, under which it undergoes an intriguing direct-indirect and insulator-metal transition. By applying an in-layer biaxial strain, the semiconductor characteristic of monolayer violet phosphorene is found to be robust and stable over a wide range of strains (-10 to 10%), with a minimum bulk gap still being up to 0.90 eV at a tensile strain of 10%. This demonstrates that the band edges of monolayer violet phosphorene not only can straddle water redox potentials in the equilibrium state but can also be available within the strain range of -7 to 7% for facilitating photocatalytic water splitting. In particular, the suitable band edges and intensive absorption of visible light suggest that a strain ratio of -7% would be the more favorable condition for water splitting under visible light.