Density functional theory calculation were performed to check the possibilities of two-dimensional M 4 X 9 s (M = Ge, Se; X = S, Se, Te) as the platform for photocatalytic water splitting. [Display omitted] • The reported Ge 4 Se 9 was not active toward water splitting because of the unfavorable thermodynamics of HER and OER. • The expanded 2D M 4 X 9 series (M = Ge, Sn; X = S, Se, Te) were found to be stable in thermodynamics, thermotics, and mechanics. • The 2D M 4 X 9 series were also not the ideal candidates for water splitting, although some of them show a certain OER activity. • Ge 4 S 9 and Sn 4 S 9 have negative Poisson's ratios. By means of first-principles calculations, we systematically evaluated the possibilities of M 4 X 9 s (M = Ge, Sn; X = S, Se, Te) as the platform for photocatalytic water splitting by checking their stabilities, electronic and optical properties, carrier properties, and the free energy changes of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). It was found that all the studied monolayers are stable. From the viewpoints of electronic and optical properties, the facts of suitable band gaps and band edge positions as well as good optical absorption meet with the fundamental requirements for a qualified photocatalyst toward overall water splitting. However, from the viewpoints of carrier mobility and carrier location, the electron mobilities are in moderate quantity whereas the hole mobilities are in single digits, and only Te-based materials have the feature of carrier separation, which are indicative of a low application prospect as the photocatalysts. Additionally, thermodynamic calculations clearly show that the external potential provided by the light-induced electrons and holes are insufficient to drive the HER and OER, respectively; thus, the M 4 X 9 s monolayers are not the ideal candidates for water splitting. Our results offer clear information and guidance of these novel M 4 X 9 s for the application in water splitting. [ABSTRACT FROM AUTHOR]