  Using density functional theory (DFT), we investigated the structural,
electronic and optical properties of functionalized and doped MXenes such as
M$_1$M$_2$CO$_2$ (M$_1$ = Zr,Hf; M$_2$ = Hf,Ti,Sc). This study aimed to find a
suitable photocatalyst that would work well in the water splitting process.
Among the calculated nanostructures, MXenes ZrHfCO$_2$ and ZrTiCO$_2$ were
chosen as the suitable photocatalysts for the water splitting process. The
calculated value of the band gaps with the GGA-PBE functional was 1.08(0.79) eV
for the ZrHfCO$_2$ (ZrTiCO$_2$) monolayer. Also, the band gaps for these
monolayers with the HSE06 hybrid functional were 1.86 and 1.57 eV,
respectively. These MXenes' optical properties, such as complex dielectric
function, refractive index, extinction coefficient, and reflectivity, were also
investigated. The results showed that these monolayers had good absorption in
the visible and ultraviolet regions. Additionally, we discovered that
ZrHfCO$_2$ and ZrTiCO$_2$ MXenes could be used for the water splitting process
by calculating the photocatalytic properties. Meanwhile, the results showed
that the monolayers of M$_1$M$_2$CO$_2$ could be promising candidates for
photocatalytic, solar energy, and optoelectronic applications.
