Polyol synthesis of polycrystalline cuprous oxide nanoribbons and their growth chemistry

A facile organic-solution method was developed for the synthesis of two-dimensional cuprous nanostructures. Ribbons as thin as 50 nm were successfully prepared by dissolving CuCl in ethylene glycol before raising the solution temperature to 150°C in air. Transmission electron microscopic studies revealed that the ribbon nanostructures obtained were polycrystalline, with nanocrystals present in the structures mostly less than 25 nm. Selective-area electron diffraction patterns taken from the ribbon nanostructures indicated that the chemical composition of the nanocrystals was Cu₂O, though X-ray photoelectron spectrometric analysis showed that the nanostructures also contained the Cu²⁺ phase. Growth factors including the molecular structure of the solvent and the counter-ion of copper in the precursor that may affect the formation of polycrystalline nanoribbons were examined. More importantly, the detail of chemistry involved in the step-by-step, dimensional growth of copper-based nanostructures in ethylene glycol is presented at the molecular level for the first time using the growth of the Cu₂O nanoribbon as an example. Ethylene glycol chelates Cu²⁺, which is produced from Cu⁺ undergoing disproportionation reactions, to form tetragonally elongated glycolates. A sequence of nucleophilic substitutions then takes place to bond glycolates together to yield stripe-like polymers before the polymers aggregate via van der Waals force into ribbon nanostructures. The Cu⁰ produced from the disproportionation reaction is crystallized out within the polymers and oxidized at elevated temperature by the dissolved O₂ in the solution to form Cu₂O nanocrystals.