Reversing CdS Preparation Order and Its Effects on Photocatalytic Hydrogen Production of CdS/Pt-TiO2 Hybrids Under Visible Light

A facile synthesis of high efficiency semiconductor photocatalyst hybrids is of great importance in making the photocatalytic systems more viable and applicable. This study presents that simply reversing chemical precipitation order of CdS results in significantly different photocatalytic activity in terms of hydrogen production from water under visible light when hybridized with platinized TiO2 particles (Pt-TiO2). It has been found that CdS obtained via dropping an aqueous cadmium cation in aqueous sulfide solution (i.e., Pt-TiO2 suspension with S2-) with equal molar ratios (hereafter CdSR) has a maximum >10-fold greater amount of hydrogen than that obtained by simply reversing the dropping order (i.e., dropping S2- to Pt-TiO2 suspension with Cd2+; hereafter CdRS). Such a high activity of CdSR, however, is very sensitive to photocatalytic running conditions, in particular, kind and concentration of electron donor (Na2S and/or Na2SO3) which largely changes the hydrogen production ratio (R-H) of CdSR to CdRS. Detailed surface analyses indicate that physicochemical properties of CdSR are very different from those of CdRS including larger and red-shifted onset light absorption and altered photoluminescence, S/Cd atomic ratios >1, and hexagonal crystallinity (vs cubic-CdRS), the differences of which were attributed to the primary reasons for higher activity of CdSR. Finally, the photocatalytic hydrogen production mechanism was proposed based on the experimental results.