To explore the photocatalytic performance of mono-elemental semiconductors, the electronic structure and optical properties of red and black phosphorous were investigated using first-principles calculations. Interestingly, although red phosphorous (rP) in the bulk form is a typical indirect semiconductor, it transforms into a direct semiconductor when thinned to a monolayer. The increased band gap still spans the redox potential levels of water with stronger oxidizing capacity. Additionally, the lighter charge carrier mobility is hardly affected by the smaller electrostatic potential in the plane, which favors photocatalysis. Black phosphorous (bP) in the bulk form is a narrow band gap semiconductor with high electronic mobility. Its band gap can be tuned as the number of layers is reduced and the interlayer distance is widened. In monolayer bP, the high efficiency of charge carrier mobility is retained, and its band gap increases to 1.67 eV, which indicates an opportune response to visible light irradiation. The redox potentials of the valence band and conduction band edges are suitable for the catalysis of the water splitting reaction.