Carbon nanotubes and nanofibers as building blocks for the future: Structure, synthesis, properties, and functionalization perspectives

• Various synthesis methods of CNTs/CNFs and processing parameters discussed. • Functionalization strategies enhancing mechanical, electrical, catalytic properties highlighted. • Applications in energy storage, environmental remediation, sensors explored. • Challenges in scalability, uniformity, and advancements addressed. In this review, we discuss the structure, synthesis, functionalization, and applications of carbon nanotubes (CNTs) and carbon nanofibers (CNFs). We cover a variety of synthesis techniques, including chemical vapor deposition, templating, electrospinning, and microwave-assisted synthesis. Specifically, we discuss the capacity of these methods and their critical processing parameters to control the morphology (straight, helical, or branched structures for CNTs, and hollow or solid architectures for CNFs) and the alignment and diameter of these nanostructures. In addition, we explain the principles, advantages, and limitations of each technique. We also review surface functionalization strategies, such as acid treatment, polymerization, doping and metal nanoparticle decoration, and stress the role of surface functionalization in enhancing the mechanical, electrical, and catalytic properties of CNTs and CNFs. We then highlight key challenges in their synthesis and functionalization, such as achieving uniformity, scalability, and precise structural control, to promote CNT and CNF applications, particularly in energy storage and production, environmental remediation, biomedical systems, and biosensors. Their high mechanical strength, electrical conductivity, and large surface area support their integration across various industries. Overall, this review outlines future opportunities and addresses the main challenges in advancing CNT and CNF development for real-world applications.