Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Nanoscale. 2018 Jan 18;10(3):1015-1022. doi: 10.1039/c7nr05853e.
Growth mechanisms of graphitic nanostructures on metal oxides by chemical vapor deposition (CVD) are observed at 750 °C, using titania nanowire aerogel (NWAG) as a three-dimensional substrate and without metal catalysts. We temporally observed catalytic transformation of amorphous carbon into few-layer graphene on the surface of 5-10 nm diameter titania nanowires. The graphitization spontaneously terminates when the titania nanowires are encapsulated by a shell of approximately three graphene layers. Extended CVD time beyond the termination point (>1125 seconds) yields only additional amorphous carbon deposits on top of the few-layer graphene. Furthermore, it was discovered that the islands of amorphous carbon do not graphitize unless they catalytically grow beyond a threshold size of 5-7 nm along the nanowire length, even after an extended thermal treatment. The electrical conductivity of the NWAG increased by four orders of magnitude, indicating that the graphene shell mediated by titania nanowires yielded a network of graphene throughout the three-dimensional nanostructure of the aerogel. Our results help us understand the growth mechanisms of few-layer graphene on nanostructured metal oxides, and inspire facile and controllable processing of metal oxide-nanocarbon fiber-shell composites.
在 750°C 下，通过化学气相沉积（CVD）观察到在金属氧化物上生长石墨纳米结构的生长机制，使用二氧化钛纳米线气凝胶（NWAG）作为三维基底，并且没有金属催化剂。我们在 5-10nm 直径的二氧化钛纳米线表面上临时观察到无定形碳向少层石墨烯的催化转化。当二氧化钛纳米线被大约三层石墨烯壳包裹时，石墨化自发终止。超过终止点的延长 CVD 时间（>1125 秒）只会在少层石墨烯的顶部产生额外的无定形碳沉积物。此外，我们发现除非无定形碳岛沿着纳米线长度催化生长超过 5-7nm 的阈值尺寸，否则它们不会石墨化，即使经过延长的热处理也是如此。NWAG 的电导率增加了四个数量级，表明通过二氧化钛纳米线介导的石墨烯壳在气凝胶的三维纳米结构中产生了整个石墨烯网络。我们的结果有助于我们理解在纳米结构金属氧化物上生长少层石墨烯的生长机制，并启发了金属氧化物-纳米碳纤维-壳复合材料的简便和可控处理。
