Using the electron and hole effective masses 0.82 m e and 0.44 m e , respectively, and the calculated static dielectric constant of 30, the predicted exciton binding energy of Y 2 Ti 2 O 5 S 2 is 1 meV. For photocatalysts for water splitting, the binding energy should be less than thermal energy (k B T ), which is around 25 meV at room temperature. Evidently this crude estimate shows efficient separation of the charge carriers is possible at low temperatures.