In situ X-ray diffraction patterns obtained at different temperatures for Cu0.7Zn0.3 foil during the heating process in air with a heating rate of 20 °C/min. The scan for each XRD pattern takes ∼15 min to complete.
Typical SEM images of the brass substrates oxidized at different temperatures under the oxygen pressure of 300 mbar. (a) 300 °C; (b) 350 °C; (c)400 °C; (d) 450 °C; (e) 500 °C; and (f) 600 °C. Insets are corresponding SEM images showing the low-magnification overview of the surface.
Room temperature XRD patterns obtained from the brass substrates oxidized for 1 h at the different temperatures under the oxygen pressure of 300 mbar.
Cross-sectional SEM images of the brass substrates oxidized for 1 h under the oxygen pressure of 300 mbar and the oxidation temperature of (a)300 °C; (b) 350 °C; (c) 400 °C, (d) 450 °C; (e) 500 °C; and (f) 600 °C.
Dependence of the surface density of ZnO nanowires and thickness of the underlying ZnO layer on the oxidation temperature.
(a) Cross-sectional BF TEM image showing three-layered structure: from top to bottom are ZnO nanowires, ZnO oxide layer, and Cu-Zn substrate; (b) HRTEM image of the ZnO/Cu-Zn interface corresponding to the marked rectangle area (1) in (a), white arrows show the locations of misfit dislocations, slight lattice distortion in the interface region can be also discerned; (c) HRTEM image obtained from the marked rectangle area (2) in (a), inset is a SAED pattern from the Cu-Zn layer, which shows a  zone axis; (d) HRTEM image recorded from the rectangle area (3) of a ZnO nanowire in (a) showing the existence of a twin boundary along the length direction of the nanowire.
(a) SEM micrograph of a brass substrate oxidized at 400 °C for ∼20 min, showing the initial growth morphology of the ZnO nanowires; (b)–(d): SEM images of the different growth stages of individual ZnO nanowires; (e) cross-sectional BF TEM image showing the nanowire root region and the underlying ZnO grain.
Schematic illustration of the growth process of a single ZnO nanowire: Zn ions diffuse outward from the ZnO/Cu-Zn interface to the free surface via grain boundary diffusion driven by the ZnO/Cu-Zn interfacial reaction, followed by surface diffusion to the nanowire root and then to the nanowire tip driven by the concentration gradient.
Surface and interface SEM images of brass foil oxidized in oxygen at 300 °C for 3 h.
(a) Surface morphology of the brass oxidized in oxygen at 500 °C for 5 min; (b)–(e) Cross-sectional SEM images of the brass oxidized for 5 min, 15 min, 30 min, and 45 min, respectively; (f) oxide thickness vs. square root of oxidation time.
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