The structure-property relationship of single-wall carbon nanotubes (SWCNTs) necessitates their controlled synthesis for various applications, including nanoelectronics, energy storage, composites, sensors, and biomedicine. Depending on the geometry, SWCNTs can be either metallic (m-SWCNTs) or semiconducting (s-SWCNTs). The bandgap of s-SWCNTs varies inversely with the SWCNT diameter, with well-defined band gaps (>1 eV) requiring a SWCNT diameter of less than 1 nm. Unfortunately, SWCNT forests are characterized by large tube diameters (mostly > 1nm), which makes the fraction of s-SWCNTs essentially semi-metallic due to their small band gaps, thereby limiting their applications in areas where s-SWCNTs are required. Therefore, there is a need to design catalysts with high selectivity toward small-diameter SWCNTs (<1 nm), which are crucial for applications requiring substantial band gaps (>1 eV). However, conventional catalysts (e.g., Fe, Co) used in chemical vapor deposition (CVD) experience nanoparticle sintering, resulting in broad diameter distributions (0.7-3 nm), which limits their effectiveness. To address this issue, Fe-based alloys with enhanced thermal stability are being investigated as catalysts for the growth of SWCNTs. Alloying Fe with a high-melting-point metal, such as Ru, can enhance thermal stability, reduce sintering, and improve selectivity for small-diameter SWCNTs. The Fe-Ru catalyst alloy was synthesized via wet impregnation with Fe catalyst for comparison. SWCNTs were grown at different temperatures, ranging from 700 to 800°C, using the Fe-Ru catalyst alloy. The feedstock used was a gaseous mixture from the Fischer-Tropsch synthesis process (FTS- GP). Raman spectroscopy was employed to analyze the as-grown SWCNTs. The Raman spectra indicated that SWCNT growth on the Fe-Ru catalyst alloy at 800°C resulted in smaller diameter nanotubes, whereas the Fe catalyst produced larger diameter SWCNTs at 750°C. This study demonstrates that alloying a conventional catalyst (Fe) with a high-melting-point metal (Ru) facilitates the growth of SWCNTs, resulting in improved small-diameter selectivity and a narrower diameter distribution. Moreover, the incorporation of Ru enables Fe to selectively produce high-density SWCNTs across the temperature range (700 - 800°C). This indicates that Ru impacts the catalytic activity of Fe and stabilizes smaller catalyst nanoparticles at elevated temperatures, promoting uniform and controlled growth of SWCNT.
