Synthesis And Properties Of Carbon-ZnO Composites By Plasma-enhanced Chemical Vapor Deposition

Carbon nanotubes (CNTs) and ZnO nanomaterials have attracted great attention for years because of their superior electronic properties for potential applications in electronics, biology, chemistry, optics, biological medicine and so on. Especially, CNTs/ZnO nanocomposite has been known to be one of the most promising candidates for field emission devices due to their small curvature radius, low work function, high thermal and mechanical stability, and high conductivity. In recent years, in order to improve the optical and electronic properties of CNTs, ZnO nanoparticles have been coated on the surface of CNTs by the methods such as atomic layer deposition, chemical vapor deposition and vapor phase transport. According to these coating processes, the average size of ZnO particles and interparticle distance can be controlled, resulting in the change of surface electronic properties of CNTs/ZnO hybrid materials. However, until now, there have been no reports on CNTs/ZnO grown via a straightforward method by plasma enhanced chemical vapor deposition (PECVD). It is well known that PECVD is a very simple and effective technique for producing well-aligned CNTs with high productivity at relatively low temperatures. Fothermore, it is worth mentioning that when the CNTs are synthesized by PECVD using hydrogen, it is very difficult to remove the influence of discharge hydrogen gas on the surface of CNTs, resulting in defect formation in CNTs. In general, for good field emission (FE) properties of hybrid CNTs/ZnO nanoparticles, it is necessary to employ the CNTs with minor surface defects as the composites because these defects cause a decrease in conductivity owing to the damage of graphite lattice. In addition, for existing coating methods, the ZnO nanoparticles are coated on the walls of as-deposited CNTs. This can cause an increase in surface defects of CNTs due to the interaction between discharge hydrogen or oxygen gas remained in the chamber and C atoms of CNTs since CNTs are maintained at high temperature for a long time. Since the FE properties in CNTs/ZnO materials are significantly influenced by the geometric factors of the structure, it is important to pay an attention to the variation in geometrical shapes of CNTs surface in the coating process. Moreover, until now, many efforts have focused on the enhanced FE properties of CNTs/ZnO materials with increasing coated ZnO nanoparticles, but there have been no reports on the effect of the variation in the geometrical factors for ZnO particles on the FE properties for CNTs. On the other hand, graphene sheets (GSs) are currently of great interests for efficient FE sources because they have unique electronic properties, large surface areas, and sharp edges. In general, the GSs can be synthesized by hot filament chemical vapor deposition, plasma enhanced chemical vapor deposition and a chemical exfoliation method associated with graphite intercalation compounds. However, there is a problem with the field emission from GSs because the existing deposition methods lead to the sheets that have the planar morphological features along the entire substrates, limiting the field enhancement.In this paper, we realized a direct coating of ZnO nanoparticles on CNTs via a straightforward process by radio-frequency PECVD, and studied an effect of the coating time on the spectroscopic properties of CNTs/ZnO composite and an effect of the variation in the size and inter-particle distance for ZnO nanoparticles on the FE properties for CNTs. And, we report that the GSs with pyramid-like morphologies have been grown on the ZnO nanowires coated with Ni catalyst nanoparticles by PECVD, and the greatly improved FE property for the hybrid ZnO/GSs material has been achieved.In Chapter 1, we first give a brief introduction to the unique structures, outstanding properties, applications and synthesis methods of CNTs and ZnO. Second, we give a review on the structures, growth mechanism, preparation methods and applications of CNTs/ZnO composites. At last, we simply describe the purpose and the results of our work.In Chapter 2, we introduce the mechanism, equipment of PECVD, the process of catalyst preparation and CNTs synthesis, and coating process and characterization of CNTs/ZnO nanoparticles. An MgO-supported Ni catalyst obtained by solution method was used for CNT synthesis in PECVD. Furthermore, when CNTs were synthesized for 6-7min, ZnO nanoparticles were coated on the surface of the CNTs. The appropriate synthesis conditions for CNTs/ZnO nanoparticles are as follows: heating at 200 Pa with a hydrogen gas flow of 20 sccm, temperature 800 oC,a CH4:H2 flow ratio of 80:20, air pressure 1000 Pa, plasma power 200 W, current 260 mA,voltage 950 V,CNTs growth tempreture more than 6 min,ZnO coating time 3-25 min,anneal tempreture 400 oC,and anneal time 60 min。The morphology and structure of the obtained hybrid CNTs/ZnO nanoparticles were characterized by SEM, TEM and HRTEM. The diameters and the length of the CNTs before coating are aboute 10-40 nm and 4μm, the coated ZnO nanoparticles have bead-shape, and the average size of most particles is about 4 nm. In addition, the size distribution of ZnO particles, interparticle distance and average radius were obtained by the statistical analysis for the size and number of the nanoparticles from the TEM image. For coating less than 9 min, the average interparticle distance is larger than the average diameter, and for coating 9-15 min, the average diameter is bigger than the interparticle distance. For coating more than 15 min, the surface of CNTs are completely covered with the ZnO film layer.In Chapter 3, we carried out the spectroscopic analysis for CNTs/ZnO composite, and studied the effects of coating time on the defect formation in CNTs and ZnO surface. When the substrate is annealed at 400 oC without hydrogen gas flow, sp3-hybridized bond intensity in XPS C1s peaks of the CNTs and the degree of disorder (ID/IG) in Raman peaks apparently decrease with the increase of coating time. We believe that this is because the active oxygen atoms are rapidly absorbed to the coated ZnO nanoparticles before contacting to carbon atoms on CNTs. Additionally, the hydrogen atoms existing in the chamber at annealing temperature of 400 oC causes the additional deformation of C-H bonds on the surface of CNTs. Therefore, a direct coating of ZnO nanoparticles on CNTs has a great effect on preventing an increase in defects due to oxidation and formation of C-H bonds on the walls of CNTs. But, PL spectra for CNTs/ZnO show the apparent increase in the structural defects such as interstitial zinc and oxygen vacancies with the over-increase of coating time.In Chapter 4, we have described the FE principle for metal and semiconductor, measurement system, the effects of ZnO nonoparticle size and interparticle distance on FE property of CNTs, and studied an equi-potential model of the electrostatic field on the wall of CNT/ZnO and a band structure analysis of the emission mechanism. The values of turn-on field defined as the electric field required for extracting a current density of 1μA/cm2 show an apparent decrease with increasing coating time, and are much lower than that (2.1 V/μm) for pure CNTs. Also, the field enhancement factors for CNTs coated ZnO for 3, 6 and 10 min are estimated to be about 2.7×104, 4.3×104 and 3.0×104, respectively, which are much higher than that (8.5×103) for CNTs. We consider that the bead-shaped ZnO nanoparticles in the hybrid CNTs/ZnO can act as additional emission sites because their small sizes and spherical shapes leave many small and sharp tips on CNTs, leading to an enhanced local field at the tip region. The electrons can emit from the top of the ZnO particle to the vacuum, and the emission current will increase with an increase in the number of ZnO particles on the CNTs-wall. However, the value of the threshold field required to produce a current density of 1 mA/cm2 is also decreased with the over-increase of coating time. We believe that too short in the inter-particle distance with increasing coating time, which can correspond to a flat ZnO coating layer on the wall of CNT, will lead to an apparent decrease in the local field strength owing to an electrostatic screening effect, resulting in a damage of FE property. In this work, the most efficient FE property is obtained at coating time of about 6-10 min.In Chapter 5, we have firstly fabricated the pyramid-like GSs on the Ni coated ZnO nanowires by a radio-frequency PECVD, and explained the growth process of ZnO/GSs, characterization and FE property and so on. The average size of Ni nanoparticles attached on ZnO surface is about 15 nm and the GSs synthesized on the nanoparticles via a base growth mechanism have pyramid-like shapes. The surface morphology of the GSs can be controlled by the variation in density of catalyst particles and deposition time. The over-increase of coating time causes a substantial increase in the degree of disorder in GSs. In the current density of 1μA, hybrid ZnO/GSs material has the lower turn-on field of 1.3 V/μm than that (2.5 V/μm) for ZnO nanowires. A number of sharp edges with the small radii of curvature existing in pyramid-like GS can act as independent emitters on the ZnO surface, in addition to ZnO tips, thus causing greatly enhanced FE property than the ZnO wires. The efficient FE properties of ZnO/GSs nanocomposite imply avenues for graphene and ZnO's potential applications.