Surface Growth Of Single-Walled Carbon Nanotubes With High Density And Controllable Structure

Single-walled carbon nanotubes(SWNTs)have unique one-dimensional tubular structure,which is coherently related to the SWNT diameter and chirality.According to their different fine structures,SWNTs could exhibit excellent and unique mechanical,thermal,electrical and optical properties,which not only give SWNTs important basic research value,but also render them great potential in many fields,such as composite enhancement,thermal conductivity materials,flexible sensors,and field-effect transistors.Fabrication process determines the structure and properties of SWNTs.As the mainstream method to prepare SWNTs,the product synthesized by chemical vapor deposition usually processes a variety of different chiral structures,and the efficiency of the catalyst is low,which leads to the low density of carbon tubes and binds the practical application.In order to solve these problems,we propose to enhance surface-grown SWNT density and control SWNT structures by using carbon fiber covering and designing novel bimetallic catalysts,respectively.Based on previous theories,the growth mechanism of single-walled carbon nanotubes was analyzed.Experimentally,a layer of carbon fibers was laid on Co nanoparticles uniformly dispersed on a flat SiO2/Si substrate for improving the SWNT growth efficiency.Using such a strategy,high-density SWNTs were successfully grown on the SiO2/Si surface by CO chemical vapor deposition.During the chemical vapor deposition process,carbon fibers form numerous microspaces with the substrate surface,which change the Knudsen number at the growth stage and increase the effective contact between the carbon source gas CO and the Co nanoparticles,ultimately promoting the catalyst reduction and the SWNT growth.Compared with SWNTs grown on a flat substrate without covering carbon fibers,SWNTs grown from the carbon fiber assisted chemical vapor deposition show a much higher density,i.e.～140 SWNTs/μm2.X-ray photoelectron spectroscopy characterizations revealed that the presence of carbon fibers promotes the reduction of cobalt oxide nanoparticles and the possible carbon dissolution,leading to the growth of high density SWNTs.On the basis of increasing the density of SWNTs,a narrow chiral(12,6)carbon tube was prepared.Structure-controlled synthesis of SWNTs is one of the most challenging topics in SWNT synthesis field.During chemical vapor deposition synthesis of SWNTs,the catalyst plays crucial roles in governing SWNT nucleation thermodynamics and growth kinetics.In this work,a NiPt bimetallic catalyst prepared by co-reduction was applied as catalyst for SWNT growth.NiPt bimetallic catalysts uniformly dispersed onto the plat substrate were subjected to chemical vapor deposition process.By tuning growth parameters,an optimal growth temperature of 700℃ was obtained using CO chemical vapor deposition.Raman spectroscopy was applied to characterize SWNTs.Based on the characterization results,(12,6)SWNTs occupy 42%of the total carbon nanotubes,indicating a high chirality selectivity.In addition,ST-cut quartz was used as substrate to prepare horizontal SWNT arrays through crystal lattice-oriented growthThe diameter of SWNTs and catalyst were analyzed,and the growth model of SWNTs was analyzed from kinetic.Transmission electron microscopy characterizations show that the NiPt particles exist as alloy states and could maintain their solid state during the growth process.Therefore,the SWNT growth follows a vapor-solid-solid(VSS)model.From the kinetic point of view,the near(2n,n)SWNTs exposed at the catalyst and carbon tube interface has more active sites,which is conducive to the reception of foreign carbon atoms,thus has a faster growth rate and a tendency of chiral concentration in the final product.