| Summary: | Hybrid nanoparticles involving 10-nm silver nanoparticles (AgNPs) nucleated on unmodified rod-like cellulose nanocrystals (CNCs) were prepared by chemical reduction. H<sub>2</sub>O<sub>2</sub> used as a post-treatment induced a size-shape transition following a redox mechanism, passing from 10-nm spherical AgNPs to 300-nm triangular or prismatic NPs (AgNPrisms), where CNCs are the only stabilizers for AgNPs and AgNPrisms. We investigated the role of the H<sub>2</sub>O<sub>2</sub>/AgNP mass ratio (α) on AgNPs. At α values above 0.20, the large amount of H<sub>2</sub>O<sub>2</sub> led to extensive oxidation that produced numerous nucleation points for AgNPrisms on CNCs. On the contrary, for α below 0.20, primary AgNPs are only partially oxidized, releasing a reduced amount of Ag<sup>+</sup> ions and thus preventing the formation of AgNPrisms and reforming spherical AgNPs. While XRD and EXAFS reveal that the AgNP fcc crystal structure is unaffected by the H<sub>2</sub>O<sub>2</sub> treatment, the XANES spectra proved that the AgNP–AgNPrism transition is always associated with an increase in the metallic Ag fraction (Ag<sub>0</sub>). In contrast, the formation of new 15-nm spherical AgNPs keeps the initial Ag<sub>0</sub>/Ag<sup>+</sup> ratio unmodified. For the first time, we introduce a complete guide map for the fully-controlled preparation of aqueous dispersed AgNPs using CNC as a template.
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