Do two-dimensional group IV-VI M4X9 monolayers have photocatalytic activity toward overall water splitting? A comprehensive theoretical investigation

By means of first-principles calculations, we systematically evaluated the possibilities of M4X9s (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 M4X9s monolayers are not the ideal candidates for water splitting. Our results offer clear information and guidance of these novel M4X9s for the application in water splitting.