Chirality-selective growth of single-walled carbon nanotubes from layered double hydroxide supported monometallic catalysts
Single-walled carbon nanotubes(SWNTs)display exceptional electrical and optical properties,closely tied to chiral indices(n,m)that define their structures.To fully exploit these distinctive characteristics and push forward their applications,there is a significant demand for preparing SWNTs with specific chirality and controllable properties.In order to facilitate the separation of single-chirality SWNTs by postgrowth approaches,it is essential to synthesize bulk SWNTs with a narrow chirality distribution.This can be achieved through cost-effective techniques like chemical vapor deposition(CVD)by using solid supported catalysts,which is characterized by a well dispersed active catalyst component on a mesoporous support with a high surface area.However,it is important to address the challenge of nonuniform impregnation during the catalyst preparation process.This issue can lead to the formation of nanoparticles with varying diameters,which hinders the growth of uniform SWNTs.To overcome this hurdle,the development of novel supported catalysts specifically designed for chirality-selective SWNT synthesis becomes imperative.Hereby,we prepared two types of layered double hydroxides(LDHs),which respectively consist of Co/Mg/Al and Ni/Mg/Al,as the catalyst precursors for catalytic growth of carbon nanotubes by CVD.As a structured catalyst,LDHs consist of positively charged brucite-like layers,which are built from metal hydroxide octahedra with hydroxyl groups(OH-)at the corners.Through the process of calcination,the LDHs undergo transformations into mixed oxides.Importantly,the resulting mixed oxides retain a plate-like morphology and uniformly distributed metal cations.Upon reduction under the reaction temperature,metal cations,like Co2+and Ni2+,could be reduced and migrate into uniform metal clusters anchored by the underlying host materials for subsequent growth of carbon nanotubes.In this work,we systematically investigated the catalytic performances of calcined LDHs in which the respective catalyst components are Co and Ni.Notably,the annealed Co/Mg/Al variant could readily synthesize SWNTs at a reaction temperature of 600℃,which afforded the selective synthesis of small diameter SWNTs with dominant(6,5)species.The remarkable chirality selection observed can be attributed to the intricate interplay between the catalyst composition and the reaction parameters.On one hand,the Co/Mg/Al LDHs exhibited effective reduction at 600℃,rendering them active at this reaction temperature.Coupled with the utilization of CO as the carbon source,the relatively low reaction temperature facilitated the creation of dissociated carbon atoms with elevated chemical potentials on the Co catalyst surfaces.This environment was conducive to the nucleation of subnanometer-sized SWNTs in a perpendicular growth mode.On the other hand,the reduced Co nanoparticles from the reduction of calcined Co/Mg/Al LDHs had strong metal-support interactions with the oxide support,which inhibited the formation of large diameter Co particles and subsequently prevented the growth of large diameter SWNTs.Similar to previously reported Fe/Mg/Al LDHs,pre-annealed Ni/Mg/Al LDHs could only be activated at temperatures exceeding 700℃.This higher activation temperature potentially led to the coalescence of reduced Ni on the support surface,giving rise to the formation of large diameter particles,as revealed by transmission electron microscopy characterizations.Consequently,the chirality distribution of SWNTs determined by optical characterizations exhibited a broad diameter distribution.Furthermore,the yield of SWNTs obtained from the Ni/Mg/Al LDHs was found to be lower compared to the yield from Co/Mg/Al LDHs,highlighting the importance of the anchoring effect of the support materials in chirality-selective synthesis of SWNTs with a narrow chirality distribution.This study not only broadens the application of monometallic LDHs in the context of chirality-selective synthesis of SWNTs,but also provides valuable insights into the mechanisms governing the activation of catalysts for SWNT growth.
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