二氧化硅层对氧化石墨烯表面生长碳纳米管的影响

目的 通过在氧化石墨烯表面沉积二氧化硅过渡层,实现碳纳米管在其表面的可控生长。方法 在氧化石墨烯(GO)的分散液中,滴入四乙氧基硅烷(TEOS),通过调节分散液的pH,使得TEOS水解,并在氧化石墨烯表面沉积二氧化硅层(SiO2),获得二氧化硅包覆的氧化石墨烯(GO@SiO2);然后,采用浮动催化剂化学气相沉积(CVD)方法,在GO@SiO2表面生长碳纳米管,通过调节沉积时间,获得二氧化硅包覆的氧化石墨烯–碳纳米管杂化材料(GO@SiO2-CNTs);将未沉积二氧化硅层的氧化石墨烯在同样条件下通过CVD得到氧化石墨烯–碳纳米管杂化材料(GO-CNTs);结合SEM、STEM、EDS分析,对比有无二氧化硅层的氧化石墨烯基底对生长碳纳米管的影响。结果 在GO表面直接生长的CNTs不能实现全面均匀的包覆,相比之下,在SiO2包覆的GO表面生长的碳纳米管阵列均匀且致密,呈现典型的“刷”状结构,通过调节反应时间,可以控制碳纳米管的生长密度和长度。结论 二氧化硅层可以有效地促进碳纳米管在氧化石墨烯基底的生长,实现碳纳米管形貌的可控调节。

Effect of Silicon Dioxide Layer on the Growth of Carbon Nanotubes on the Surface of Graphene Oxide

The work aims to achieve the controllable growth of carbon nanotubes (CNTs) on the surface of graphene oxide (GO) by depositing an intermediate layer of silicon dioxide (SiO2) on the surface of GO. A silicon dioxide-coated graphene oxide (GO@SiO2) was obtained through hydrolyzing drops of tetraethoxysilane (TEOS) in the dispersion of GO with adjusted pH. Then, a layer of SiO2 was formed on the surface of GO. Subsequently, several SiO2-coated graphene oxide-carbon nanotubes hybrid materials (GO@SiO2-CNTs) were prepared by growing carbon nanotubes on the surface of GO@SiO2 through the method of floating catalyst chemical vapor deposition (CVD) for different deposition time. For comparison, some graphene oxide-carbon nanotubes hybrid materials (GO-CNTs) were prepared under the same CVD condition with GO without the SiO2 layer. The effect of the SiO2 layer on the growth of carbon nanotubes was further studied by comparing the results of SEM, STEM and EDS of the GO@SiO2-CNTs and GO-CNTs. Also, the possible mechanism of the SiO2 transition layer affecting the growth of CNTs was proposed. The result of EDS analyzation showed that a layer of silicon dioxide was uniformly deposited on the surface of graphene oxide after the hydrolyzing of TEOS at certain pH. SEM pictures indicated that the growth of CNTs on the GO was uneven with only few surfaces being covered by the CNTs, while the surface of GO@SiO2 was uniformly and densely covered with carbon nanotube arrays, forming a typical "brush" structure. Moreover, the density and length of the CNTs on the surface of GO@SiO2 can be modulated by altering the CVD time. The main reason for the growth of CNTs on the substrate is the interaction of the catalyst particles with the substrate material. There are two typical theories of the growth mechanism of CNTs:one is the top growth theory, and the other one is the bottom growth theory, which both involves the interaction of the catalyst particles with the substrate material. About 20 years ago, Professor Dai of Stanford University and his team characterized the surface of SiO2, indicating that there is a hydroxyl group like structure on the surface of SiO2 that makes its particles have a certain interaction with the metal. While in the CVD process of GO, the oxygen-containing functional groups are reduced by the reducing hydrogen atmosphere, leading to the reduction of the interaction with catalyst particles, which finally restrains the growth of CNTs on the surface. According to previously published papers, CNTs prefer to grow on SiO2 substrates rather than the silicon substrates. Thus, when GO is chosen as the substrate material, the morphology of CNTs grown on its surface by CVD is not good. Yet, after the modification with the SiO2 transition layer, the growth condition is much improved. Therefore, compared with GO substrates, CNTs are selectively grown on the surface of SiO2, which further proves that the SiO2 substrates are better than the GO substrates. In conclusion, the growth of CNTs on the GO substrate is greatly improved with the existence of SiO2 layer, which also helps to control the growth of length and density of the CNTs.