First Principles Study of Photocatalytic Water Splitting by M$_1$M$_2$CO$_2$ (M$_1$ = Zr,Hf; M$_2$ = Hf,Ti,Sc) MXenes

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.