Photocorrosion Stability of Cd_<i>x</i>Zn_1–<i>x</i>S Yellow-Orange Pigments

Cadmium zinc sulfide pigments (CdxZn1–xS) have been extensively used in art and industry for their bright colors. However, concerns exist over their long-term chemical stability. Due to their semiconductive properties under light exposure, these compounds may trigger photocatalytic processes that can lead to degradation issues in artworks. This study aims to replicate and compare the historical wet and dry synthesis of CdxZn1–xS pigments and investigate their photocatalytic behavior, specifically their reactivity and ion leaching predispositions. Using adapted historical methods, we synthesized a series of CdxZn1–xS pigments and fully characterized them using a range of analytical techniques. Their photocatalytic activity was evaluated against methylene blue dye under simulated sunlight, alongside a concomitant assessment of metal ion leaching. These experiments provide valuable insights into the historical pigments photocatalytic behavior, proposing key indicators of pigment reactivity in real artworks and demonstrating the origin of the inherent instability of historically synthesized pigments, particularly those made via wet methods. Under solar simulation, cubic nanosized pigments with 20% and 40% zinc content exhibit the highest degradation activity. This process is accompanied by the leaching of Cd2+ and Zn2+ ions, which may contribute to the formation of undesirable secondary products. The same pigments exhibited ion leaching even in the dark, although at significantly lower levels.