Decorating ZnS photocatalyst with noble metal species - Strengthening or weakening the activity?

Decorating semiconductors with metals like platinum is a common practice to enhance their photocatalytic activity. Drawbacks of such modifications are seldom reported, which may explain their widespread use as a seemingly universal method for improving activity. In this context, it seems interesting to investigate whether the presence of these modifiers will translate to the improvement of the observed photocatalytic activity of zinc sulfide in the reduction of carbon dioxide and water. In this study, we present an in-depth investigation of ZnS modified through the deposition of noble metal species of Ag, Au, Ru, and Pt. A series of materials was prepared by the reduction of metal precursors with NaBH4. The structural and morphological properties of the materials were discussed. A thorough analysis of the photophysical properties of the samples was enabled by photoelectrochemical, spectroelectrochemical, and spectroscopic measurements. Noble metal modifiers entailed the formation of deep electron traps, decreasing the driving force of the reduction reactions and improving the recombination processes. Surprisingly, the chemical properties of ZnS prevented noble metals from complete reduction, leading to the creation of non-zero valent metals, inter alia Pt(II), which shows enhanced photocatalytic CO2 reduction due to its catalytic properties. Our results provoke further studies to elucidate the Pt(II) role in improving ZnS photoactivity.