  Water splitting reactions through photocatalysis is an efficient and
sustainable technique for the generation of green energy. The photocatalyst's
ability to effect simultaneous generation of hydrogen and oxygen, along with
efficiency in utilisation of charged carriers, conversion of solar energy to
hydrogen, fast migration, and low recombination rates of carriers, are the
parameters to decide its suitability in water splitting. In literature,
comprehensive calculation and analysis of all these performance parameters for
a potential photocatalyst are rare. In this work, we have performed
first-principles-based computations to find new efficient photocatalysts from
the family of Janus MXenes and assessed their performance parameters. Strain
engineering has been invoked in search of new materials. Out of 14 studied
materials, we find 5 materials: Sc$_{2}$COS, Zr$_{2}$COS, Hf$_{2}$COS, and
ZrHfCO$_{2}$ under zero or finite tensile strain and Hf$_{2}$COSe at 6\%
tensile strain meeting the requirements of simultaneous reactions to split
water. The computations of various efficiency-related parameters demonstrate
that Zr$_{2}$COS, Hf$_{2}$COS, and Hf$_{2}$COSe have excellent efficiencies,
significantly better than the well-known photocatalysts. The origin of such
performances lies in their electronic and optical properties, which are
analysed systematically.
