Colloidal Hexagonal Tungsten Oxide Nanorods: Synthesis, Characterization, and Proton-Coupled Electron Transfer

Tungsten oxide (WO₃) is a well-studied material with rich structural chemistry and redox properties. Herein, we report the synthesis, characterization, and redox reactivity of colloidal oleylamine-capped hexagonal tungsten oxide nanorods (NRs). Our adapted synthesis gave slightly reduced, anisotropic NRs (∼3 × 3 × 20 nm), which formed stable colloids in aprotic, nonaqueous media such as THF. The as-prepared NRs could be oxidized to essentially colorless h-WO₃ and reduced to hydrogen tungsten bronzes, to a maximum of 0.3H⁺+e^- per W (h-H_0.3WO₃). The redox reactions used photolysis, simple solution PCET reagents, or e^-/H⁺ reagent pairs: electron-donors + acids or electron-acceptors + bases. Reactions proceeded in the absence of water or noble metal catalysts. Spectroscopic and titration studies showed that the redox transformations have an obligate 1:1H⁺:e^- stoichiometry, and were strongly dependent on the solution proton activity and protonation of the oleylamine ligand. The nanorod structure was stable through redox cycling. At the same overall stoichiometries, all of the paths reach the same equilibrium state, showing the generality of the PCET perspective. This work expands the understanding of PCET processes for WO₃ materials.