Sun Xianhu, Zhu Wenhui, Wu Dongxiang, Liu Zhenyu, Chen Xiaobo, Yuan Lu, Wang Guofeng, Sharma Renu, Zhou Guangwen
Department of Mechanical Engineering & Materials Science and Engineering Program, State University of New York, Binghamton, NY 13902, USA.
Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA.
Adv Funct Mater. 2019;30(4). doi: https://doi.org/10.1002/adfm.201906504.
A fundamental knowledge of the unidirectional growth mechanisms is required for precise control on size, shape, and thereby functionalities of nanostructures. The oxidation of many metals results in oxide nanowire growth with a bicrystal grain boundary along the axial direction. Using transmission electron microscopy that spatially and temporally resolves CuO nanowire growth during the oxidation of copper, here we provide direct evidence of the correlation between unidirectional crystal growth and bicrystal grain boundary diffusion. Based on atomic scale observations of the upward growth at the nanowire tip, oscillatory downward growth of atomic layers on the nanowire sidewall and the parabolic kinetics of lengthening, bicrystal grain boundary diffusion is the mechanism by which Cu ions are delivered from the nanowire root to the tip. Together with density-functional theory calculations, we further show that the asymmetry in the corner-crossing barriers promotes the unidirectional oxide growth by hindering the transport of Cu ions from the nanowire tip to the sidewall facets. We expect the broader applicability of these results in manipulating the growth of nanostructured oxides by controlling the bicrystal grain boundary structure that favors anisotropic diffusion for unidirectional, one-dimensional crystal growth for nanowires or isotropic diffusion for two-dimensional platelet growth.
要精确控制纳米结构的尺寸、形状及其功能，就需要对单向生长机制有基本的了解。许多金属的氧化会导致沿轴向形成具有双晶晶界的氧化物纳米线生长。利用能在空间和时间上解析铜氧化过程中CuO纳米线生长的透射电子显微镜，我们在此提供了单向晶体生长与双晶晶界扩散之间相关性的直接证据。基于对纳米线尖端向上生长、纳米线侧壁上原子层的振荡向下生长以及伸长的抛物线动力学的原子尺度观察，双晶晶界扩散是铜离子从纳米线根部传输到尖端的机制。结合密度泛函理论计算，我们进一步表明，拐角穿越势垒的不对称性通过阻碍铜离子从纳米线尖端传输到侧壁面来促进单向氧化物生长。我们期望这些结果在通过控制双晶晶界结构来操纵纳米结构氧化物的生长方面具有更广泛的适用性，这种双晶晶界结构有利于各向异性扩散以实现纳米线的单向一维晶体生长，或有利于各向同性扩散以实现二维片状生长。
