The effects of oxidative environments on the synthesis of CuO nanowires on Cu substrates

Copper foils are thermally oxidized at 400 and 500 °C under various gaseous environments, including nitrogen, air and oxygen, with and without water vapor. The oxidized samples are examined using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. It is found that the nanowires are formed exclusively from monoclinic CuO crystals under a gaseous atmosphere with a sufficiently high oxygen partial pressure. Thus, no nanowires are found in samples oxidized in nitrogen, with or without water vapor. A high density of uniform nanowires is formed in wet air, while only a small amount is formed in dry air. The CuO nanowires formed in pure oxygen have the highest density, with sizes of 50-100 nm in diameter and up to 15 μm in length. Our experiment shows that a high oxygen partial pressure enhances both the nucleation probability and the growth rate of the nanowires, while the effect of the water vapor is mainly to assist their nucleation. Their formation can be best explained on the basis of a vapor-solid growth mechanism.