Comparing Ultralong Carbon Nanotube Growth from Methane over Mono- and Bi-Metallic Iron Chloride Catalysts

Tim Yick; Varun Shenoy Gangoli; Alvin Orbaek White

doi:10.3390/nano13152172

Comparing Ultralong Carbon Nanotube Growth from Methane over Mono- and Bi-Metallic Iron Chloride Catalysts

Nanomaterials (Basel). 2023 Jul 26;13(15):2172. doi: 10.3390/nano13152172.

Authors

Tim Yick¹, Varun Shenoy Gangoli¹, Alvin Orbaek White^{1

2

3}

Affiliations

¹ Energy Safety Research Institute, Swansea University, Bay Campus, Swansea SA1 8EN, UK.
² Department of Chemical Engineering, Faculty of Science and Engineering, Swansea University, Bay Campus, Fabian Way, Swansea SA1 8EN, UK.
³ TrimTabs Ltd., 63 St Christophers Ct, Swansea SA1 1UA, UK.

Abstract

This research endeavours to study the growth of ultralong carbon nanotubes (UL-CNTs) from methane using diverse catalysts, namely FeCl₃, bi-metallic Fe-Cu, Fe-Ni, and Fe-Co chlorides. Aqueous catalyst solutions were evenly dispersed on silica substrates and grown at 950 °C in the presence of hydrogen via a horizontal chemical vapour deposition (CVD) furnace. The samples underwent characterisation by Raman spectroscopy, scanning electron microscopy (SEM), and optical microscopy to identify the quality of CNTs and enumerate individual UL-CNTs. Our findings revealed that FeCl₃, as a mono-metallic catalyst, generated the longest UL-CNTs, which measured 1.32 cm, followed by Fe-Cu (0.85 cm), Fe-Co (0.7 cm), and Fe-Ni (0.6 cm), respectively. The G/D ratio (graphene to defects) from the Raman spectroscopy was the highest with the FeCl₃ catalyst (3.09), followed by Fe-Cu (2.79), Fe-Co catalyst (2.13), and Fe-Ni (2.52). It indicates that the mono-iron-based catalyst also produces the highest purity CNTs. Moreover, this study scrutinises the vapour-liquid-solid (VLS) model for CNT growth and the impact of carbide formation as a precursor to CNT growth. Our research findings indicate that forming iron carbide (Fe₃C) is a crucial transition phase for amorphous carbon transformation to CNTs. Notably, the iron catalyst generated the longest and densest CNTs relative to other iron-based bi-metallic catalysts, which is consistent with the temperature of carbide formation in the mono-metallic system. From correlations made using the phase diagram with carbon, we conclude that CNT growth is favoured because of increased carbon solubility within the mono-metallic catalyst compared to the bi-metallic catalysts.

Keywords: carbon nanotube; iron chloride; ultralong carbon nanotube; vapour-liquid-solid.

Grants and funding

A.O.W. was funded through the Sêr Cymru II Fellowship by the Welsh Government and the European Regional Development Fund (ERDF). A.O.W. and T.Y. acknowledge funding from the Welsh Government Circular Economy Capital Fund for FY 2020–21. V.S.G. is funded thanks to Salts Healthcare Ltd. and TRIMTABS Ltd.