Sustainable microwave-assisted synthesis of high-quality carbon nanotubes from rice husk biochar

The conventional synthesis of carbon nanotubes (CNTs) often involves high temperatures, inert atmospheres, non-renewable precursors, and elevated costs. This study explores rice husk, a ligno-cellulose-rich agricultural waste, as a sustainable precursor for CNT synthesis via a two-stage process. First, rice husk was slow pyrolyzed at 300ºC, 400ºC, and 500ºC to produce biochar. The biochar was then reacted with a ferrocene catalyst in varying ratios (1:1, 1:2, and 1:3 ferrocene to biochar, g/g) under microwave irradiation. FESEM analysis revealed highly elongated CNTs with 55 and 78 nm average diameters for biochars pyrolyzed at 400ºC and 500ºC, respectively. Smaller diameters were observed at higher pyrolysis temperatures and lower ferrocene-to-biochar ratios. Raman spectroscopy confirmed high-quality CNTs (I D /I G < 1), with the highest degree of wall graphitization (I D /I G = 0.75) for Rice Husk CNT 500 1:3. Optical studies showed maximum absorbance in the 208–226 nm range, consistent with previous findings. XRD analysis highlighted the influence of amorphous silica in limiting CNT crystallinity at lower pyrolysis temperatures. TEM analysis revealed multiple graphene sheets stacked one above the other, similar to that of Multi-walled carbon nanotubes. This study demonstrates the viability of rice husk as a low-cost, renewable precursor for the synthesis of carbon nanotubes. The findings provide insights into optimizing process parameters for better structural and graphitic properties. By converting agricultural waste into value-added nanomaterials, this work offers a sustainable synthesis of carbon nanotubes using rice husk biochar. • Rice husk biochar is used as a sustainable precursor for carbon nanotube synthesis. • The microwave-assisted process enhances CNT quality with diameters between 47 and 95 nm. • Optimal CNT synthesis achieved at 500°C pyrolysis and 1:3 ferrocene to biochar ratio. • High-quality CNTs with excellent graphitization (ID/IG = 0.75) confirmed by Raman spectroscopy. • The study highlights sustainable CNT production and potential industrial applications.