Sustainable synthesis and multifunctional applications of biowaste-derived carbon nanomaterials and metal oxide composites: A review

The conversion of biowaste into carbon nanomaterials (CNMs) and their metal oxide composites has emerged as a sustainable strategy to address environmental challenges while advancing nanotechnology. This comprehensive review critically examines recent advancements in the green synthesis, properties, and applications of biowaste-derived CNMs, including carbon nanotubes, graphene, and carbon quantum dots, alongside their metal oxide hybrids. We highlight eco-friendly synthesis techniques such as hydrothermal carbonization, chemical vapour deposition, and microwave-assisted methods, which enable the scalable production of high-performance nanomaterials with tailored properties. These materials exhibit exceptional characteristics, including high surface area, tunable porosity, superior electrical conductivity, and biocompatibility, making them ideal for energy storage, environmental remediation, catalysis, and biomedical applications. The integration of metal oxides (e.g., ZnO, TiO₂, Fe₃O₄) with CNMs enhances their functional performance through synergistic effects, improving photocatalytic activity, electrochemical stability, and sensing capabilities. We discuss their role in supercapacitors, biosensors, antimicrobial agents, and pollutant degradation, emphasizing mechanistic insights and structure-property relationships. Despite significant progress, challenges such as scalable synthesis, long-term stability, and toxicity assessment remain unresolved. Future research should focus on optimizing synthesis protocols, exploring novel composite designs, and assessing environmental impacts to facilitate commercialization. This review provides a forward-looking perspective on the potential of biowaste-derived nanomaterials to drive sustainable technological innovations, aligning with global circular economy goals.