Oxygen vacancy and lattice distortion of Ni-Fe oxide catalyst for CNT growth
LEE J. 1, HA J. 2, SHIN H. 2, KIM J. 3, LEE H. 1, LEE S. 1
1 School of Materials science and engineering, Pusan national university, Busan, Korea (Republic of); 2 System & Energy Division, Korea Testing Laboratory, Seoul, Korea (Republic of); 3 Department of research and development, HyMAR corporation, Busan, Korea (Republic of)
Carbon nanotube (CNT) has studied across in variety of fields (e.g., catalysts, sensors, and electrodes) owing to their unique electronic, mechanical, and structural properties. Multi-walled carbon nanotubes (MWCNTs) are generally grown using transition metal-based catalysts such as nickel (Ni), cobalt (Co), iron (Fe) or their alloys. The controlling surface properties of the transition metal oxide catalysts is critical in CNT growth as it influences the decomposition and bonding of carbon atoms on the catalyst surface. Defect control on the oxide catalyst surface has been reported to affect surface adsorption energy and diffusion with carbon atoms. Lattice strain resulting from oxygen vacancies acts as active site for chemisorption of carbon sources. Therefore, controlling defects such as oxygen vacancies on the catalyst surface is essential for improving the growth efficiency and characteristics of MWCNTs.
We studied the change in chemical bonding state and lattice distortion of Ni-Fe oxide catalysts, and the structural properties of MWCNTs. Nano-sized Ni-Fe oxide catalysts were synthesized, and these catalysts were used to grow MWCNTs through thermal chemical vapor deposition (CVD) method. The crystal structure and the particle size analysis were conducted according to Ni:Fe ratio. The intensity and width of NiO peaks were decreased and broaden, respectively, and the lattice strain of catalyst particles was also gradually increased as the Fe content increased. EPR analysis was used to analyze the changes in the formation of oxygen vacancies. EPR signal (g) and peak intensity were increased, which showed the increase of the oxygen vacancies formation. These results indicate that the lattice distortion occurred as the formation of oxygen vacancies in NiO increased by Fe. XPS analysis was used to identify the changes in the valence states and bonding state on Ni-Fe oxide catalyst surface. As a result of structural properties for MWCNT using TEM and Raman, ID/IG ratio increased as incresing Fe content, representing the high graphitization on the MWCNT surface. Average distance between defects (LD) of MWCNTs also decreased with the addition of Fe, and MWCNT growth mechanism by lattice distortion will be discussed.