Synergistic Effects in Bimetallic Nanoparticles for Low Temperature Carbon Nanotube Growth

Abstract

Carbon nanotubes (CNTs) have been extensively studied in recent years for potential applications spanning from electronics to nanocomposites. Current synthesis approaches rely on transition metal catalysts to provide structural control, which has led to considerable efforts to understand and improve catalytic activity. While the most commonly used catalysts for CNT growth are Ni and Fe, experiments with metal alloys such as NiFe, NiCo, CoMo, and FeMo have been empirically observed to produce higher growth rates and lower growth temperatures. Recent molecular simulation results suggest that the unique catalytic properties of bimetallic nanoparticles may be a result of the role different metals play during critical steps in CNT growth. Therefore, dimensional and compositional tuning of catalyst nanoparticles is critical to precise control of CNT growth. The majority of efforts to study catalytic efficacy have been hampered by large variations in particle size and composition. Catalyst nanoparticles are often prepared in solution-phase routes where stabilizing molecules or oxidation on the particle surface are a source of contamination during CNT growth. Here, we report on a clean, gas-phase approach to synthesize tunable compositions of bimetallic nanoparticles at constant particle size. A continuous-flow, atmospheric pressure microplasma reactor is used to decompose organometallic precursors and prepare narrow dispersions of bimetallic nanoparticles in a single step. CNT growth is achieved by mixing the particle-laden flow exiting the microplasma reactor with acetylene and hydrogen gas and heating in a tubular flow furnace. To relate the catalytic properties of bimetallic nanoparticles to CNT growth, the flow system is integrated with aerosol instrumentation to ‘‘watch’’ CNT nucleation and growth. Kinetic studies are performed to obtain activation energies as a function of particle composition. We demonstrate that nanotubes can be grown at temperatures as low as 300 8C using Ni0.67Fe0.33 catalysts, which is significantly lower than temperatures normally required for thermal chemical vapor deposition (CVD) growth. Metal nanoparticles of varying chemical composition were synthesized at atmospheric pressure using a continuous-flow