The main role of a co-catalyst in photocatalytic water splitting is that the photoexcited electrons in the conduction band need to be smoothly transferred to the hydrogen-evolution

cocatalyst particles without releasing energy. As one of the requirements of a co-catalyst like Pt should have catalytic activity for water reduction and being a metal it has smaller barrier for migration of electrons from the semiconductor photocatalyst to the cocatalyst

co-catalyst are the active sites on main catalyst site that used to separate the photo-carriers (photoelectrons and photoholes) where Pt produce active sites for accumulation of the photoelectrons that used for reduction of the species (in case of water; hydrogen is produced).