Synthesis and characterization of high yield and quality controlled morphology carbon nanotubes with novel transition metal alloys

Abstract

A novel form of carbon, buckminsterfullerene C60 was discovered in 1985 by Kroto, Smalley, and their colleagues. This invention awarded them the Nobel Prize in chemistry in 1997. Pure carbon atoms bond in the shape of hexagons and pentagons to form the soccer-ball-like molecule C60. Besides well-known forms of carbon atoms such as diamond, C60, and graphite, carbon nanotubes (CNTs) were discovered in 1991 by Iijima. First, he found multiwall carbon nanotubes (MWCNTs) in the carbon soot synthesized by an arc discharge method. Two years later, he observed the single-wall carbon nanotubes (SWCNTs). Since then, nanotubes have attracted the interest of scientists worldwide due to their exceptional physical and chemical characteristics. They have been recognized with higher mechanical, thermal, and electrical properties that have never been seen in a material before. Because of these properties, nanotubes are the ideal material for various applications, including basic science research. Theoretical and experimental studies have shown that CNTs have exceptional mechanical properties, a high aspect ratio (~104), excellent waviness characteristics, high thermal properties (2000-6000 W/m K), good electrical conductivity (106 to 107 S/m), low density (1.3-1.4 g/cm3), excellent hydrogen storage, high corrosion resistance, and unique optical properties. Due to their unique characteristics, CNTs have triggered a big interest among mainstream researchers. They are now considered to be the most promising material used in nano-electronics, energy storage devices, composite materials, the medical field, nano-sensor applications, and so on. Although it has the aforementioned great qualities, the low-quality batch synthesizing of CNTs with smaller diameters and non-selective chirality growth of CNTs remains to restrict the application of CNTs in demanding applications such as electronics, supercapacitors, and sensors. Even though vertically aligned carbon nanotubes (VACNTs) can be grown with transition metals like Fe, Ni, Mo, and Cu, it remains difficult to synthesize fully aligned CNTs with low defect counts, high quality, and homogeneous and controlled chirality specifically for electronic applications. A recent study revealed that bimetallic catalysts offer the potential for size control, quality enhancement, and chirality distribution. Many experimental variables such as carbon source (CO, C2H4, C2H2, CH4, etc.), catalyst particle type (monometallic or bimetallic), the feedstock flux, the support materials (SiO2, Al2O3, etc.) surface treatments (chemical etching, ion bombardment, etc.), reaction time, and temperatures have been suggested as experimental variables for the controlling CNT diameter. The control of the tube diameter, which will directly impact the chiral distribution, is still a hurdle due to the enormous number of variable combinations. In Chapter 3, detailed information about VACNTs synthesized on different catalyst particles was given with the synthesis method and characterization. First, VACNTs were synthesized on pure Fe with standard protocol by catalytic chemical vapor deposition (CCVD). Then, they were characterized by Raman spectroscopy, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmett-Teller (BET) to identify IG/ID ratio with both single point and mapping, the purity and decomposition, the direction of VACNTs, the surface area of VACNTs, the crystallinity, sp2: sp3 ratio and determination of all elements in the structure. For the growth of VACNTs, different catalyst particles, such as Fe-Ni and Fe-Mo, with varying weight fractions were prepared using the mechanical alloying (MA) method. These catalyst particles were then pressed into bulk form and characterized by X-ray powder diffraction (as detailed in Chapter 3). The prepared catalysts were subsequently coated onto silicon wafers with the desired thickness using the electron beam evaporation method. Following this step, VACNTs were synthesized on the coated Si wafer using catalytic chemical vapor deposition (CCVD) with a standard recipe. For each catalyst particle, the structural characterization of VACNTs was conducted using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to determine the alignment of the CNT, innermost and outer diameters, and the number of walls. Furthermore, Raman spectroscopy was employed to analyze CNT chirality, chirality distribution, IG/ID mapping, and defect density. Additionally, thermogravimetric analysis (TGA) was used to assess the purity and decomposition behavior of VACNTs concerning different catalyst particles. Studies have shown that the addition of nickel and molybdenum enhances the quality of VACNTs. Moreover, it has been reported that incorporating nickel and molybdenum into iron alters the chirality of CNTs. Additionally, the water-assisted chemical vapor deposition (WA-CVD) system was established at ITU-ARC (Chapter 3). Extensive experiments were conducted to achieve the maximum length of VACNTs. According to the results, VACNTs synthesized via WA-CVD reached an approximate height of 7.6 mm. These findings hold significant potential for applications in nanocomposite materials and the dry spinning of CNTs. In Chapter 4, FeSi was used as a catalyst for the growth of CNTs. In the CCVD system, CNTs were synthesized directly on FeSi powder. All samples were characterized using Raman spectroscopy, SEM-EDS, and TEM to analyze the IG/ID ratio, chemical composition, presence of CNTs on FeSi particles, tube diameter, and number of walls. This chapter elucidates the growth mechanism of CNTs on FeSi particles based on characterization results. Using different synthesis parameters, CNTs were grown on FeSi powder and characterized by SEM. Raman spectroscopy was employed to compare the full-width at half-maximum (FWHM) values of the G and D peaks. Additionally, the defect distance (Lᴅ) and defect density (nᴅ) were calculated for each sample. The magnetic properties of CNT-doped FeSi samples synthesized with different recipes, as well as FeSi powder, were measured at 10 K and 300 K using a vibrating sample magnetometer (VSM). According to the results, the average magnetic saturation of FeSi powder was measured as 20 emu/g at 10 K. For the FeSi-1000-CNT and FeSi-500-CNT samples, magnetic saturation values at 10 K were 14.8 emu/g and 11.5 emu/g, respectively. At 300 K, the measured average magnetic saturation values were 15.4 emu/g, 12.8 emu/g, and 9 emu/g for FeSi, FeSi-1000-CNT, and FeSi-500-CNT samples, respectively. Raman spectroscopy results indicated that as the CNT content increased, the decomposition of iron within the powder led to a decrease in magnetic saturation. This observation was further supported by SEM-EDS and TEM analyses. In Chapter 5, CNT-reinforced polymer matrix nanocomposites (PNCs) were fabricated using different polymer types and various weight fractions. The dynamic mechanical properties of these PNCs were analyzed based on polymer type, CNT volume fraction, CNT alignment, and temperature effects. The study presents the production of both randomly oriented and aligned CNT-PNCs, examining their dynamic mechanical behavior at different temperatures. The morphological characterizations of these composites are provided and compared to assess the impact of CNT alignment and dispersion on their mechanical performance. This thesis investigates the effects of different catalyst types and concentrations on VACNT properties, a topic not previously explained together in the literature. Additionally, CVD and WACVD systems were established and used for various studies at İTÜ ARC. VACNT production was carried out using different catalysts, and the impact of catalysts on the morphological and chemical properties of VACNTs was reported. Furthermore, CNT-reinforced PNCs were produced using various polymers, and their dynamic mechanical properties were analyzed based on CNT orientation and volume fraction at different temperatures using dynamic mechanical analysis.