Microwave-assisted acid digestion of alumina-supported carbon nanotubes

Microwave-assisted acid digestion was assessed for the purification of carbon nanotubes (CNTs), synthesised in a fluidised-bed by chemical vapour deposition (CVD) on alumina-supported catalysts. The effects of (i) digestion time (10, 20 or 30 min), (ii) temperature (120, 180 or 230 °C) and (iii) acid type (HNO3, 12.8 mol/L or H2SO4, 4.4 mol/L) were investigated. Thermogravimetric analysis (TGA) results show that CNT purity increases with increasing digestion time and temperature, from 2.6 wt% in the as-synthesised bed material to 67.9 wt% for samples treated at 230 °C in H2SO4 for 30 min. Raman spectra show bands corresponding to disordered and sp2-bonded crystalline graphitic carbon at approximately 1350 and 1580 cm−1 respectively. These Raman results further suggest that microwave-assisted purification is effective at removing amorphous carbon and defects from the outer CNT tube walls. TEM and HRTEM images suggest that CNTs are able to exist in contact with strong oxidizing acids at high temperatures in a microwave, and in particular, that it causes no damage to the CNT walls. Microwave-assisted acid digestion is able to achieve comparatively high purity from low-quality feedstocks in less than 30 min.