Origin of the Anomalous Temperature Dependence of the Photochromic Reaction of Cu-Doped ZnS Nanocrystals

Photochromic Color Tuning of Copper-Doped Zinc Sulfide Nanocrystals by Control of Local Dopant Environments

Inorganic photochromic materials offer several advantages over organic compounds, including relatively inexpensive and higher thermal stability. However, tuning their color with the same component has remained a significant challenge. In this study, we demonstrate that the photochromic color of Cu-doped ZnS nanocrystals (NCs), which is initially pale yellow before light irradiation, can be tuned from gray to brown by adjusting the surface stoichiometry of Zn and S, which is controlled through the use of thiol and non-thiol ligands. Several experiments and quantum chemical calculations using model clusters revealed that the color change is determined by the distribution of Cu, which significantly contributes to the coloration, specifically whether it resides on the Zn-rich or S-rich surface. In contrast, particle size and Cu concentration were found to have little effect on the photochromic color. This study expands the diversity of photochromic responses in inorganic NCs and marks an important step toward the development of further advanced photochromic nanomaterials.

Photochromism of colloidal ZnO nanocrystal powders under ambient conditions

Zinc oxide (ZnO) nanocrystals (NCs) exhibit photochromic reactions under specific conditions upon ultraviolet light irradiation. Since the color is originated from the excited electrons at the conduction band of ZnO NCs, the photoinduced absorption is observed only in the solution with hole acceptors under inert conditions. ZnO is earth-abundant and less toxic than many other substances, and has been widely used in various industrial fields. If the photochromic reaction of ZnO can be observed consistently under ambient conditions, the material may pave the way for large-scale photochromic applications such as in pigments, windows, and building materials in addition to conventional photochromic applications. In this study, we synthesize hydrophilic ZnO NCs and observe the solid-state photochromic reactions in the visible to mid-infrared regions even in humid-air conditions. We reveal that the coloration of powders of ZnO NCs under ambient conditions originates mainly from two factors: (1) charge separation induced by hole trapping by water molecules adsorbed on the surface of NCs, and (2) deceleration of the reactions involving the electrons in the conduction band of ZnO NCs with molecular oxygen and the adsorbed water molecules.