Composite photocatalysts based on Cd_1- Zn S and TiO₂ for hydrogen production under visible light: effect of platinum...

Ternary composite photocatalysts based on titania and solid solutions of CdS and ZnS were prepared and studied by a set of physicochemical methods including XRD, XPS, HRTEM, UV-vis spectroscopy, and electrochemical tests. Two synthetic techniques of platinization of Cd_1- Zn S/TiO₂ were compared. In the first case, platinum was deposited on the surface of synthesized Cd_1- Zn S (x = 0.2-0.3)/TiO₂ P25; in the second one, Cd_1- Zn S (x = 0.2-0.3) was deposited on the surface of Pt/TiO₂ P25. The photocatalytic properties of the obtained samples were compared in the hydrogen evolution from TEOA aqueous solution under visible light (λ = 425 nm). The Cd_1- Zn S (10-50 wt%; x = 0.2-0.3)/Pt (1 wt%)/TiO₂ photocatalysts demonstrated much higher photocatalytic activity than the Pt (1 wt%)/Cd_1- Zn S (10-50 wt%; x = 0.2-0.3)/TiO₂ ones. It turned out that the arrangement of platinum nanoparticles precisely on the titanium dioxide surface in a composite photocatalyst makes it possible to achieve efficient charge separation according to the type II heterojunctions and, accordingly, a high rate of hydrogen formation. The highest photocatalytic activity was demonstrated by 20% Cd_0.8Zn_0.2S/1% Pt/TiO₂ in the amount of 26 mmol g^-1 h^-1 (apparent quantum efficiency was 7.7%) that exceeds recently published values for this class of photocatalysts.