The electrochemical reactivity of tailor-made Cu 2 O or CuO powders prepared according to the polyol process was tested in rechargeable Li cells. To our surprise, we demonstrated that CuO, a material well known for primary Li cells, and Cu 2 O could reversibly react with 1.1 Li and 2 Li ions per formula unit, respectively, leading to reversible capacities as high as 400 mAh/g in the 3-0.02 V range. The ability of copper oxide-based Li cells to retain their capacity upon numerous cycles was found to be strongly dependent on the particle size, and the best results ͑100% of the total capacity up to 70 cycles͒ were obtained with 1 m Cu 2 O and CuO particles. Ex situ transmission electron microscopy data and in situ X-ray experiments show that the reduction mechanism of Cu 2 O by Li first involved the formation of Cu nanograins dispersed into a lithia (Li 2 O) matrix, followed by the growth of an organic coating that partially dissolved upon the subsequent charge while Cu converted back to Cu 2 O nanograins. We believe that the key to the reversible reactivity mechanism of copper oxides or other transition metal oxides toward Li is the electrochemically driven formation of highly reactive metallic nanograins during the first discharge, which enables the formationdecomposition of Li 2 O upon subsequent cycles.

Modification of the polyol process has enabled the synthesis of nanometre-sized silver particles with a narrow size distribution. Dispersions of colloidal silver have been prepared by the reduction of silver nitrate in ethylene glycol in the presence of a polymeric protective agent, namely polyvinylpyrrolidone (PVP). Ageing these dispersions at 120 "C for 1 h yields systems with average particle sizes ranging from 15 to 36 nm. The amount of both precursor and protective agent was found to control the final particle size and size distribution of the colloidal silver, which exhibits a well crystallized structure.

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We have synthesized silver nanoparticles from silver nitrate solutions using extracts of Rumex hymenosepalus, a plant widely found in a large region in North America, as reducing agent. This plant is known to be rich in antioxidant molecules which we use as reducing agents. Silver nanoparticles grow in a single-step method, at room temperature, and with no addition of external energy. The nanoparticles have been characterized by ultraviolet-visible spectroscopy and transmission electron microscopy, as a function of the ratio of silver ions to reducing agent molecules. The nanoparticle diameters are in the range of 2 to 40 nm. High-resolution transmission electron microscopy and fast Fourier transform analysis show that two kinds of crystal structures are obtained: face-centered cubic and hexagonal.

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