Evaluation of the Role of Au in Improving Catalytic Activity of Ni Nanoparticles for the Formation of One-Dimensional Carbon Nanostructures

Abstract

In situ dynamic imaging, using an environmental transmission electron microscope, was employed to evaluate the catalytic activity of Au/SiO(2), Ni/SiO(2), and Au-Ni/SiO(2) nanoparticles for the formation of one-dimensional (1-D) carbon nanostructures such as carbon nanofibers (CNFs) and nanotubes (CNTs). While pure-Au thin-film samples were inactive for carbon deposition at 520 °C in 0.4 Pa of C(2)H(2), multiwalled CNTs formed from Ni thin films samples under these conditions. The number of nanoparticles active for CNF and CNT formation increased for thin films containing 0.1 mol fraction and 0.2 mol fraction of Au but decreased as the overall Au content in thin films was increased above 0.5 mol fraction. Multiwalled CNTs formed with a root growth mechanism for pure Ni samples, while with the addition of 0.1 mol fraction or 0.2 mol fraction of Au, CNFs were formed via a tip growth mechanism at 520 °C. Single-walled CNTs formed at temperatures above 600 °C in samples doped with less than 0.2 mol fraction of Au. Ex situ analysis via high-resolution scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) revealed that catalytically active particles exhibit a heterogeneous distribution of Au and Ni, where only a small fraction of the overall Au content was found in the portion of each particle actively involved in the nucleation of graphitic layers. Instead, the majority of the Au was found to be segregated to an inactive capping structure at one the end of the particles. Using density-functional theory calculations, we show that the activation energy for bulk diffusion of carbon in Ni reduces from ≈1.62 eV for pure Ni to 0.07 eV with the addition of small amounts (≈0.06 mol fraction) of Au. This suggests that the enhancement of C diffusion through the bulk of the particles may be responsible for improving the number of particles active for nucleating the 1-D carbon nanostructures and thereby the yield.

Dynamic Processes on Gold-Based Catalysts Followed by Environmental Microscopies

“…Different Au-based alloys were studied by TEM or E-TEM; these studies could be extended to their behaviour under catalytic conditions. These alloys are Au-Ni [282], Au-Cu alloys [283][284][285], Au-Pd, Au-Ag and Au-Pt nanorods [286].…”

Metal‐Free Carbonaceous Materials as Promising Heterogeneous Catalysts

“…prepared carbon nanofibers on activated carbons (ACs) produced from agricultural waste, while iron already present in the ash content of the agricultural waste was employed as a natural catalyst 51. The most preferred carbon source in CVD are hydrocarbons such as methane,52 acetylene,53 xylene,54 their mixture,55 or ethanol 56. The residual catalyst impurities in the obtained MWCNTs are little, with purity close to 99.5 % carbon, which is sufficient for most applications.…”

In Situ Microscopy and Spectroscopy Applied to Surfaces at Work

“…Moreover, owing to the ≈0.2 nm spatial resolution, they identified the catalytically active facets on the Cu particles. The time and spatial resolution of in situ TEM has also led to a series of work on the growth of graphite and carbon nanotubes over metal particles . Also of interest is the work by Helveg et al.…”