Insights into MXene-based materials for environmental applications: Performance, mechanisms, and future directions

MXenes are 2D-layered transition metal carbides, carbonitrides and nitrides that have received considerable attention as promising materials for water purification systems. MXenes' unique layered structure allows for boosted performance, e. g., huge surface area and porosity, enhanced light harvesting, improved charge separation, tunable band gap energies, and the presence of abundant functional groups (i.e., hydroxyl, oxygen, fluorine, etc.) on layers' surfaces and terminals that make them perfect adsorbents and photocatalysts. However, the performance of native materials is hampered owing to hydrogen bonding and Van der Waals force, which cause the layers of pure MXenes to stack. In contrast, MXene-based heterostructures can hinder layer restacking and exhibit enhanced synergistic benefits. This review provides a comprehensive overview of MXene-based composite materials for adsorption and photocatalytic processes. It discusses the synthesis methods of MXenes and their composites, as well as the role of various additives and dopants in enhancing their performance. Furthermore, it explores the underlying mechanisms governing the adsorption and photocatalytic activity of MXene-based composites, including the impact of surface defects, functional groups, band structure engineering, and interfacial charge transfer processes. The review also addresses the challenges and prospects of MXene-based composites, along with potential strategies for improving their performance.