Reduced TiO₂ nanotube arrays as environmental catalysts that enable advanced oxidation processes: A mini review

Advanced oxidation processes (AOPs) that utilize heterogeneous catalysts are an engaging and compelling technology for treating emerging contaminants in water and wastewater. Substantial efforts have been devoted to developing cost-effective and environmentally benign materials for application in AOPs. Reduced TiO₂ nanotube arrays (r-TNAs) have recently emerged as promising environmental materials for efficiently mitigating persistent organic pollutants (POPs) resistant to conventional biological treatment. r-TNAs possess distinct electrical, optical, and catalytic features, including superior conductivity and solar light absorption due to structural defects involving oxygen vacancies and Ti³⁺ states. Moreover, their well-organized open channel structures provide a substantial surface area, facilitating the efficient mass transfer of reactants and products. Notably, r-TNAs can be grown directly on the substrate without using organic adhesives or binders, which improves their mechanical and chemical durability and facilitates their repeated use. This article represents the pioneering comprehensive review of r-TNAs in AOP applications. We introduce an electrochemical anodization technique to fabricate TNAs and summarizes the reduction methods employed in r-TNA synthesis. We also elucidate the appealing attributes of r-TNAs and explore their applications in emerging AOPs, including persulfate activation, electrocatalysis, and photo(electro)catalysis. Finally, we outline critical challenges and provide insights into future prospects for the development of AOP systems based on r-TNAs.