We have undertaken first-principles electronic structure calculations to show that the chemical functionalization of two-dimensional hydrogenated silicene (silicane) and germanene (germanane) can become a powerful tool to increase the photocatalytic water-splitting activity. Spin-polarized density functional theory within the GGA-PBE and HSE06 types of exchange correlation functionals has been used to obtain the structural, electronic, and optical properties of silicane and germanane functionalized with a series of nonmetals (N, P, and S), alkali metals (Li, Na, and K) and alkaline-earth metals (Mg and Ca). The surface-adsorbate interaction between the functionalized systems with H-2 and O-2 molecules that leads to envisaged hydrogen and oxygen evolution reaction activity has been determined.