Font Size: a A A

Synthesis And Characterization Of Carbon Nanotubes By Plasma-enhanced Chemical Vapor Deposition

Posted on:2009-02-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:J W LiuFull Text:PDF
GTID:1101360245963291Subject:Materials Physics and Chemistry
Abstract/Summary:PDF Full Text Request
Since carbon nanotubes (CNTs) were discovered by S. Iijima in 1991, their synthesis, growth mechanism and application have become one of the most active issues in the research of nanoscience and nanotechnology. CNTs have widespread applications in microelectronics, composites, hydrogen storage, electron field emission, and electrochemical sensors for their unique structures and novel properties of CNT.Single-walled and multi-walled CNTs can be regarded as seamless tubular structure by rolling up one or more graphitic layers and they are always well-crystalline. However, amorphous carbon nanotubes (ACNTs), composed of randomly packed carbon clusters, can be regarded as long-distance disordered and short-distance ordered structures by rolling up disordered carbon layers. Because of the special amorphous structure, ACNTs have many extraordinary properties compared with crystalline CNTs. For the absence of chirality, the electronic property of ACNTs simply depends on their diameter. The defective structure of ACNTs yields a semiconductor band gap which is larger than that of crystalline CNTs and can be easily tuned by controlling the diameter of ACNTs, thus they have considerably potential applications in electronic devices. ACNTs can find applications in gas adsorbent and catalyst carrier for their porous structure and stability. ACNTs are well-functional for their coarse surfaces and large amount of defects. ACNTs with active surfaces can be filled with various solutions without any treatment, and then nanowires can be obtained through chemical reactions. ACNTs have been successfully synthesized by template method, thermal chemical vapor deposition, solvothermal route, but there are no reports on ACNTs synthesized by plasma-enhanced chemical vapor deposition (PECVD). In this study, ACNYs have been synthesized by PECVD method. The morphology, structure and composition of ACNTs have been characterized and the growth mechanism has been explored. Meanwhile, the influences of experimental conditions, such as catalyst chemical state, catalyst surface morphology after pretreatment, introduction of water vapor and bombardment of plasma on the morphology and structure of CNTs have been investigated.In Chapter 1, we first give a brief introduction to the discovery, unique structures, outstanding properties, applications and syntheses of CNTs. Second, we give a review on the structures, mechanisms, syntheses, properties and applications of ACNTs. At last, we simply describe the purpose and the results of our work.In Chapter 2, we introduce the mechanism, equipment and experimental conditions of the catalyst preparation as well as the mechanism of PECVD, synthesis and characterization of CNTs.In Chapter 3, ACNTs have been successfully synthesized on 20 nm Ni film and Fe-Ni film in CH4/H2 ambient by PECVD method. The obtained ACNTs consist of randomly distributed carbon clusters and show structural characteristics of short-distance order and long-distance disorder. Low catalyst activity, high thermal conductivity of H2 and the sufficient carbon supply are responsible for ACNT growth. ACNTs can be synthesized with a CH4:H2 flow ratio of 50:50 and 80:20 at a temperature of 800 and 900 oC at a pressure from 500 to 1000 Pa.On the basis of ACNT synthesis, we have successfully synthesized two types of novel ACNTs: ACNTs with graphitized branches and Fe2O3 nanowires-filled ACNTs (Fe2O3-NFACNTs).In Chapter 4, ACNTs with graphitized branches have been carefully characterized. ACNTs showing good orientation are about 5μm long and 100-150 nm wide. Many small well-crystalline multi-walled CNTs are located on the sidewalls of ACNTs. The diameter and length of the branches are both less than 100 nm. The structural difference between the ACNTs and the branches is attributed to the difference of catalyst composition. The filling material in the ACNTs is nickel oxide, which is suitable for ACNT growth for the low activity triggered by its low carbon solubility. Nevertheless, the catalyst responsible for crystalline branches is Ni particles, which are ideal catalyst to grow multi-walled CNTs.In Chapter 5, 80% of the obtained ACNTs are filled with Fe2O3 nanowires. This filling rate is rather high for the filled CNTs directly synthesized. The ACNTs are randomly oriented, 5μm in length and 100-150 nm in diameter. The filling material in the ACNTs is Fe2O3 nanowires, which are a few hundred nanometers to a few microns in length and 10-20 nm in diameter. Crystalline Fe2O3 nanowires are entirely enwrapped by amorphous carbon shell. We believe that Fe2O3 nanoparticles are genuine catalyst for ACNTs. At high temperature, Fe2O3 nanoparticles exist like fluid and can deform easily. The liquid Fe2O3 nanoparticles can easily diffuse into the tunnel of the ACNTs after the ACNTs begin to grow. Thus Fe2O3 nanowires-filled ACNTs come into being.In Chapter 6, the influences of catalyst chemical state, catalyst surface morphology after pretreatment, introduction of water vapor and bombardment of plasma on the morphology and structure of CNTs have been investigated.The chemical state of catalyst is controlled by heating atmosphere. Catalyst is oxidized when heated in H2 and results in the growth of ACNTs. Most of the catalyst remains metallic when heated in 10 Pa air ambient and leads to the growth of multi-walled CNTs.The type and structure of the product highly depend on the catalyst morphology after pretreatment. The main product is carbon thin film when catalyst film has not been broken into small islands. With the decrease of particle size after pretreatment, the CNT structure becomes more and more disordered.The partial pressure of water vapor has significant influence on the products. The structure of the product is most disordered when the partial pressure of water is 250 Pa. The CNT structure becomes more ordered with the increase of water pressure. Novel carbon nanostructures, nodule-like CNTs, carbon nano-necklaces and CNT knots have been synthesized under the water partial pressure of 750 Pa. The growth of such structures is promoted by the oxidation effect of water.The effect of plasma bombardment on the morphology and structure of CNTs has been investigated. The weak bombardment results in little structural changes of CNTs when the length of CNTs is less than 2μm, while for the CNTs longer than 3μm, the structure of the CNTs under severe bombardment of plasma has been destroyed. At the same time, some novel structures such as ACNTs with graphtized branches and carbon thorns have been obtained.Above all, two types of novel ACNTs, ACNTs with graphtized branches and Fe2O3-NFACNTs have been successfully synthesized. Their formation greatly depends on the catalyst in oxidized state. These results provide a novel method to synthesize ACNTs and reveal the growth mechanism of ACNTs. In addition, it is helpful for the synthesis of novel carbon nanostructures to investigate the influences of catalyst chemical state, catalyst surface morphology after pretreatment, introduction of water vapor and bombardment of plasma on the morphology and structure of CNTs.
Keywords/Search Tags:PECVD, Amorphous carbon nanotube, Synthesis, Characterization, Structure
PDF Full Text Request
Related items