Cu3N/Cu2O core–shell nanowires: growth and properties

Abstract

CuO nanowires with diameters between 100 and 200 nm, lengths up to ∼10 μm and a uniform distribution have been grown at 600 °C under 100 mL min⁻¹ O₂ on 15 mm × 30 mm Cu foils. The CuO nanowires have a monoclinic crystal structure, grow by a vapor–solid mechanism and can be reduced to Cu under H₂ at 300 °C but they are shortened, contain residual Cu₂O and are eliminated above 400 °C. We develop a strategy to preserve their integrity via the deposition of Cu over the CuO in order to convert them into Cu₃N under NH₃:H₂. The Cu₃N nanowires obtained in this way are curly and have a cubic anti-ReO₃ crystal structure but are surrounded by a surface shell of Cu₂O with a thickness of a few tens of nm as shown by transmission electron microscopy. We find that the CuO NWs coated with Cu having a thickness greater than 200 nm are not fully converted into Cu₃N and have an inner core of CuO. The Cu₃N nanowires exhibited four maxima in differential transmission at 2.41, 2.17, 1.9 and 1.8 eV, using ultrafast absorption-transmission spectroscopy, corresponding to the M and R direct energy band gaps of Cu₃N in good agreement with theory but we find no evidence for quantization. In addition, we observed two minor peaks at 1.69 and 1.67 eV that may be related to transitions between states in the Cu₂O shell or Cu₃N under compression. Despite the fact that Cu₃N has no mid-gap states the photogenerated carriers have lifetimes less than 100 ps, so its potential as a defect tolerant semiconductor for energy conversion is discussed along with its perspective for energy storage.