  The highly efficient photocatalytic water splitting to produce clean energy
requires novel semiconductor materials to achieve high solar-to-hydrogen energy
conversion efficiency. Herein, the photocatalytic properties of anisotropic
$\beta$-PtX$_2$ (X=S, Se) and Janus $\beta$-PtSSe monolayers are investigated
based on density functional theory. Small cleavage energy for \{beta}-PtS2
(0.44 J/m2) and $\beta$-PtSe$_2$ (0.40 J/m$^2$) endorses the possibility of
their mechanical exfoliation from respective layered bulk material. The
calculated results find \{beta}-PtX2 monolayers to have an appropriate bandgap
(~1.8-2.6 eV) enclosing the water redox potential, light absorption
coefficients (~104 cm$^{-1}$), and excitons binding energy (~0.5-0.7 eV), which
facilitates excellent visible-light driven photocatalytic performance.
Remarkably, an inherent structural anisotropy leads to the anisotropic and high
carrier mobility (up to ~5 x 10$^3$ cm$^2$ V$^{-1}$ S$^{-1}$) leading to fast
transport of photogenerated carriers. Notably, the small required external
potential to derive hydrogen evolution reaction and oxygen evolution reaction
processes with an excellent solar-to-hydrogen energy conversion efficiency of
$\beta$-PtSe$_2$ (~16%) and $\beta$-PtSSe (~18%) makes them promising
candidates for solar water splitting applications.
