Indirect Phase Transformation of CuO to Cu2O on a Nanowire Surface

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• 作者：Fei Wu ; Sriya Banerjee ; Huafang Li ; Yoon Myung ; Parag Banerjee
• 刊名：Langmuir
• 出版年：2016
• 出版时间：May 10, 2016
• 年：2016
• 卷：32
• 期：18
• 页码：4485-4493
• 全文大小：564K
• 年卷期：0
• ISSN：1520-5827

文摘

The reduction of CuO nanowires (NWs) to Cu₂O NWs undergoes an indirect phase transformation on the surface: from single crystalline CuO, to a disordered Cu_2−δO phase, and then to crystalline Cu₂O. A 9–12 nm disordered Cu_2−δO is formed on the NW surface by exposing CuO NWs to CO at 1 Torr, 300 °C for 30 min. After 60 min, this layer decreases to 2–3 nm and is eliminated after 180 min. Energy dispersive X-ray spectroscopy using a scanning tunneling electron microscope and across a single NW reveals the disordered layer to be O-rich with respect to Cu₂O with a maximum at. % Cu:O = 1.8. X-ray photoelectron spectroscopy shows adsorbed CO on the surface as evidence of the reduction reaction. Micro-Raman spectroscopy tracks the transformation in NWs as a function of reduction time. A CO enabled surface reduction reaction coupled to diffusion-limited transport of “nonlattice” O to the surface is proposed as a mechanism for Cu_2−δO formation. The initial buildup of out-diffusing O to the surface appears to aid the formation of the disordered surface layer. The transformation follows Ostwald–Lussac’s law which predicts formation of unstable phases over stable phases, when phase transformation rates are limited by kinetic or diffusional processes. The study provides a generalized approach for facile growth of few nanometer transient layers on multivalent, metal oxide NW surfaces.