Optimisation of microwave-assisted acid digestion for the purification of supported carbon nanotubes

Experimental optimisation of microwave-assisted acid digestion for supported carbon nanotube (CNT) purification is reported. Process variables including ramp rate, temperature, duration, scalability, acid type, volume and concentration were investigated using thermogravimetric, Fourier transform infra-red and Raman spectroscopy performance metrics. Key factors in purification were temperature, duration and acid. HNO₃ damaged CNTs and introduced carboxyl and C–O functional groups. HCl and H₂SO₄ achieved similar impurity removal but, despite the incorporation of sulphur-based functional groups, H₂SO₄ was preferred due to a more homogeneous product. Concentration and volume variables could be condensed into a single factor – the stoichiometric excess of acid to impurities – for process simplification and to permit direct literature comparison. The scalable optimised process increased CNT purity from ∼22 wt% to > 95 wt% with negligible damage in a single 15 min isothermal treatment at 230 °C with a H₂SO₄ excess of 4.5 times the required stoichiometry (1 M).