The study, published in the journal Industrial Crops and Products by authors Sai Parameshwar, Siddharth Jain, Uday Bhan, and Varun Pratap Singh, presents a transformative approach to managing agricultural waste. Globally, nearly one billion tonnes of agricultural waste are produced annually, and the common practice of burning these residues contributes significantly to greenhouse gas emissions and hazardous smog. To address this environmental challenge, the researchers utilized rice husk, a material rich in both organic carbon and silica, as a sustainable precursor for creating carbon nanotubes. These nanotubes are highly valued across various industries, including energy storage, water filtration, and biomedical drug delivery, due to their exceptional electromechanical properties and chemical inertness. By employing a two-stage process that combines slow pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More with microwave-assisted synthesis, the team successfully converted low-value farm waste into high-value nanomaterials.

The findings indicate that the quality and structure of the resulting carbon nanotubes are heavily influenced by the initial temperature at which the rice husks are processed into biochar. When the husks were pyrolyzed at higher temperatures, specifically 400 and 500 degrees Celsius, the resulting biochar led to the formation of elongated and tubular structures. In contrast, biochar produced at a lower temperature of 300 degrees Celsius resulted in spherical and agglomerated structures with incomplete growth. This difference is primarily due to the presence of amorphous silica in the rice husk, which gradually crystallizes as temperatures rise. The nanotubes synthesized from the 500-degree biochar exhibited average diameters ranging from 55 to 61 nanometers, while those from the 400-degree biochar ranged from 71 to 78 nanometers. These measurements fall within the typical range for multi-walled carbon nanotubes, confirming the success of the biomass-to-nanomaterial conversion.

Structural analysis through Raman spectroscopy highlighted the superior quality of the nanotubes produced at the highest settings. A critical measure of quality in carbon materials is the ratio between structural defects and graphitic order. The researchers found that all synthesized samples maintained a high degree of graphitization, with the highest quality reaching a value of 0.75. This indicates a very high level of wall graphitization, which is vital for the material’s performance in high-tech applications. Furthermore, the study explored the effect of varying the ratio of ferrocene catalyst to biochar. It was observed that a higher proportion of biochar relative to the catalyst generally led to smaller diameters but could eventually reduce the overall yield if the catalyst became insufficient to accommodate the carbon source. The highest yield achieved was 0.34 grams of nanotubes per gram of biochar, occurring at the highest pyrolysis temperature and the most balanced catalyst ratio.

Energy efficiency was a central focus of the research, as traditional methods for creating these materials often require extreme heat and unsustainable precursors. The microwave-assisted method proved to be a time-saving and energy-efficient alternative because it works through localized and volumetric heating, interacting with particles at the molecular level. The total energy consumption for this laboratory-scale process ranged from 1.437 to 1.456 kilowatt-hours per gram of synthesized nanotubes. This energy demand is comparable to, and in many instances lower than, conventional commercial synthesis methods like chemical vapor deposition. By reducing the energy footprint and utilizing renewable agricultural waste, this two-step process offers a sustainable pathway for industrial-scale production.

Ultimately, the study confirms that rice husk biochar is an effective and environmentally friendly precursor for the synthesis of multi-walled carbon nanotubes. The resulting materials exhibit favorable optical and structural properties, including maximum ultraviolet absorbance between 208 and 226 nanometers. This research not only provides a solution for solid waste disposal in the agricultural sector but also creates a domestic, cost-effective source for advanced nanomaterials. The ability to fine-tune the diameter and quality of the nanotubes by adjusting processing temperatures and catalyst ratios paves the way for optimized, large-scale manufacturing that supports both the economy and the environment.

Source: Parameshwar, S., Jain, S., Bhan, U., & Singh, V. P. (2026). Sustainable microwave-assisted synthesis of high-quality carbon nanotubes from rice husk biochar. Industrial Crops and Products, 241, 122768.