Understanding\nIon-Exchange Processes in the Synthesis\nof ZnS<sub><i>x</i></sub>@ZnO<sub><i>1–x</i></sub> Heterostructures from Controlled Sulfidation of ZnO Nanocrystals

Semiconducting heterostructures are considered promising\ncandidates\nfor meeting specific environmental challenges, such as greener or\ndecarbonated production of energy. However, optimizing the performance\nof these hybrid systems largely depends on the fine understanding\nof the mechanisms by which they are formed in relation to their mode\nof preparation. We report herein the synthesis of nanosized semiconducting\nheterostructures of ZnS@ZnO shell-core nature; this is from well-controlled\npreformed ZnO nanoparticles (NPs) modified via anion exchange process\nusing (TMS)<sub>2</sub>S. The formation of these ZnS@ZnO heterostructures\nhas been investigated in depth, shedding light specifically on the\nsulfidation mechanism and its dynamics. Our study reveals the dynamic\nevolution of the nanomaterial in the sulfidation process, evidencing\nthat it is both driven by the initial presence of oxygen vacanciesacting\nas gateways for sulfur atomsand also by the action in the\nmedium of (TMS)<sub>2</sub>S, which as a sulfurizing agent behaves\nalso as an oxygen atom extractor. The structural modification of the\npreformed monocrystalline ZnO nanomaterial into a polycrystalline\nZnS hollow nanostructure occurs via amorphization–crystallization\nsteps, which clearly depends on the amount of (TMS)<sub>2</sub>S in\nthe reaction. This morphological transition to a hollow structure\nhas been followed by multinuclear NMR spectroscopy (<sup>1</sup>H, <sup>13</sup>C, <sup>17</sup>O), and notably oxygen atoms at the interfaces\nof ZnS@ZnO heterostructures have been identified and quantified. Consistently,\nour study clearly establishes the link between the preparation mode\nof the ZnS@ZnO heterostructures and the modification of their optical\nband gaps as a function of their composition. The variation in optical\nproperties, and the bowing of the band gap, depends on the sulfidation\nlevel, and this mode of sulfidation is clarified step-by-step by a\nDFT computational approach of surface and interface processes that\nis fully supported by the experimental characterization (XRD, WAXS,\nEDX line-analysis, HRTEM, STEM-HAADF) of these materials.