Poly(ferrocenylsilane) Hydrogels as a Foundry for Metal Nanoparticle Synthesis by Direct Reduction of Electrolytes via a Catalytic Route

Abstract

Poly(ferrocenylsilane)s (PFSs) as redox-responsive organometallic polymers have been used in redox reactions in noble metal ion-containing electrolyte solutions and in hydrogels swollen by electrolytes for successful preparation of metal nanoparticles (NPs) by in situ direct reduction. We report a surprising imbalance of stoichiometric ratios in redox reactions with transition metals between metal cations and PFS observed in hydrogels. Due to the redox-active ferrocene center in PFS backbones, well-dispersed noble metal NPs are directly formed in situ in the hydrogel matrix depending on their redox potential. By careful analysis, an extra-stoichiometric amount of gold NPs was found in the reaction when following the principle of exergonic electron transfer from ferrocene to gold(III) with the reduction of AuIII to Au0, and the iron center’s oxidation state increases from II to III. This surprising result is targeted to account for a stoichiometric balance of the electrons in the redox reaction, that is, we tackle the question “where do the electrons come from?” First, HAuCl4 was allowed to react with an oxidized PFS hydrogel to determine the source of the unaccounted electrons. Surprisingly, essentially regardless of the redox state of PFS, similar amounts of gold NPs were formed. The formation of H+ ions in the reaction indicates that H2O could be a possible electron source. A comparison with the stoichiometric relations in PtNP formation further confirms this hypothesis.