Rapid Synthesis of Non-Metal Doped TiO2 Nanotube Arrays for Wastewater Treatment

Electrochemical Advanced Oxidation Processes (EAOPs) are attractive for recalcitrant waters since they rely on the in-situ formation of highly reactive, non-selective radicals (e.g., •OH) to degrade organic pollutants which challenge most traditional treatment technologies (e.g., pharmaceuticals, pesticides). Specifically, anodic oxidation (AO) technologies oxidize organic pollutants via direct electron transfer and/or indirectly through generated reactive species. AO requires efficient and robust anode materials, the most popular of which is currently boron-doped diamond (BDD). However, the practicality of BDD is severely limited by prohibitively high manufacturing cost, motivating interest in possible alternatives.

Doped- and sub-stoichiometric TiO₂ based materials – such as “blue” and “black” nanotube arrays (NTAs) - have been reported to exhibit comparable conductivity and electrochemical oxidation activity to BDD. NTAs (grown directly on titanium plates) have a major advantage over particulate and layered anodes, since they can be utilized directly without additional adhesive substrates, interlayers, and/or organic binders. However, synthesis of even moderate length (i.e., 7 µm) NTA materials typically takes on the order of hours to complete, and loss of catalytic activity during AO is typically observed after a few hours due to surface passivation. In this work, both of these shortcomings are addressed. First, a versatile anodization method employing electrolyte conductivity as the central design variable has been developed which can generate tunable NTAs with steady growth rates >1 µm∙min^-1. Second, we have investigated a host of non-toxic dopants (e.g., N, P, S) to prevent surface passivation and stabilize the catalytically active oxygen vacancies in the lattice structure. Together, these advancements in NTA synthesis techniques offer a promising strategy to sustainably generate an inexpensive alternative to BDD and provide the water treatment sector with a viable method to target high-priority organic contaminants.