Preparation Of Carbon Nanomaterials By Chemical Vopor Deposition In A Hydrogen-free Atmosphere

Carbon nanotubes and graphene possess a series of surprising microstructures and intrinsic properties.Therefore,a lot of potential applications could be envisioned in molecular electronics,materials science,energy storage and transformation,biomedicine,sensors,and biological detection.As a result,wide attentions from the majority of scientific and technological workers have been paid throughout the world.However,the precondition for realizing their potential applications is to synthesize carbon nanotubes or graphene with controlled properties and structures,which could meet the demands of practical applications.So far,there are many techniques developed for growing carbon nanomaterials.Among them,chemical vapor deposition(CVD)arises as the most promising technique and demonstrate many advantages over other techniques in terms of low costand fair controllability.In most CVD processes,the generally applied carbon sources are mainly hydrocarbons,like CH₄,C₂H₄and C₂H₅OH.However,the generated hydrogen during the decomposition process could be detrimental to nanotube or graphene synthesis.Besides,excessive hydrogen atomscould either etch or kill the growing carbon nucleus.In order to avoid the negative effects of hydrogen on carbon nanomaterial growth,we herein propose to synthesize carbon nanomaterials using CO,a hydrogen-free molecule as the carbon source.Besides,detailed investigations are carried out to clarify the roles of different parameters involved in CVD in growing carbon nanomaterials.The results obtained mainly include the following parts:(1)High-quality multilayer graphene was prepared in hydrogen free atmosphere by using CO as carbon source and nickel foil as catalyst substrate.Systematic studies revealed that high quality graphene with large coverage area could be achieved at 900 ^oC with 200 sccm CO and a reaction time of 10 min with the assistance of fast cooling process.Further transmission electron microscopy(TEM)characterizations on the transferred graphene revealed that most of the graphene have layer number ranging from three to ten.This work investigates the growth of graphene on nickel foil using CO CVD,which not only helps understand the graphene nucleation and growth mechanisms,but also sheds light on the synthesis of multi-layer graphene.(2)Similarly,graphene synthesis using CO as the carbon source was investigated on copper foils pretreated under different annealing conditions.High quality monolayer graphene could be achieved on 1060 ^oC-annealedcopper foil in the presence of low pressure H₂.At a reaction temperature of 900 ^oC,large area coverage graphene was synthesized for a reaction time as short as 30 min.In contrast,800 ^oC-annealedcopper foil can only afford large coverage graphene growth for a reaction time of 60 min.Interestingly,bilayer graphene was synthesized on copper foil pre-annealed at 1000 ^oC at ambient pressure,although the growth time should be extended to 120 min.This work not only highlights the importance of foil calcination in subsequent graphene nucleation and growth,but also paves the way towards commercial applications of graphene.(3)A magnesia supported nickel(Ni/MgO)catalyst was developed for the efficient growth of single-walled carbon nanotubes(SWNTs).The catalyst was prepared by an impregnation technique and the applied metal precursor was nickel(II)bis(acetylacetonate).Prior to CVD growth,the catalyst was annealed in open air at 600 ^oC.After CO CVD at 800 ^oC for 40 min,high quality SWNTs were synthesized.Systematic characterizations onboth the catalyst and the product indicate that the catalyst forms an ideal solid solution upon reduction Ni_xMg_1-xO,which facilitates the generation of uniform Ni nanoparticles for the subsequent synthesis of SWNTs.Further characterizations on the purified SWNTs using transmission electron microscopy and UV-vis-NIR absorption spectrum revealed that the produced SWNTs have small diameters and a relatively narrow chirality distribution.The newly designed Ni/MgO catalyst would ultimately help achieve the chiral-selective synthesis of SWNTs.