Carbon Nanotubes from Renewable Precursors: Green Synthesis, Functional Properties, and Environmental Applications - A Comprehensive Review

Carbon nanotubes (CNTs), particularly multi-walled carbon nanotubes (MWCNTs), have emerged as one of the most versatile nanomaterials of the twenty-first century owing to their exceptional mechanical, thermal, electrical, and chemical properties. This review consolidates two decades of research progress on the synthesis of CNTs using bio-derived, renewable precursors via chemical vapour deposition (CVD) and spray pyrolysis techniques. Special emphasis is placed on natural oil precursors such as Madhuca longifolia, Brassica juncea, Oryza sativa methyl esters, and Zingiber officinale-derived hydrocarbons, as well as the role of transition-metal catalysts in morphology control. The paper further examines the integration of MWCNTs in wastewater treatment for heavy metal removal, dye adsorption, and air stripping, alongside energy applications including dye-sensitised solar cells (DSSCs). Emerging areas—biopolymer nanocomposites, silver nanoparticle synthesis, and pH-responsive food packaging—are also reviewed. Design-of-experiment approaches such as Box–Behnken design for synthesis optimisation are critically evaluated. The review identifies prevailing challenges in scalability, surface functionalisation, and toxicity, and outlines promising future directions toward sustainable nanomanufacturing and environmental remediation.