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 S^2-) with equal molar ratios (hereafter CdS_R) has a maximum >10-fold greater amount of hydrogen than that obtained by simply reversing the dropping order (i.e., dropping S^2- to Pt-TiO₂ suspension with Cd ²⁺; hereafter Cd_RS). Such a high activity of CdS _R, 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 CdS_R to Cd_RS. Detailed surface analyses indicate that physicochemical properties of CdS_R are very different from those of Cd_RS including larger and red-shifted onset light absorption and altered photoluminescence, S/Cd atomic ratios >1, and hexagonal crystallinity (vs cubic-Cd_RS), the differences of which were attributed to the primary reasons for higher activity of CdS_R. Finally, the photocatalytic hydrogen production mechanism was proposed based on the experimental results.