Insight into the existent state of nitrogen-doped carbon dots in titanate nanotubes and their roles played toward simultaneous removal of coexisted Cu2+ and norfloxacin in water.

In this work, nitrogen-doped carbon dots (NCDs) were introduced in different existent sites of titanate nanotubes (TNTs) by a facile synthesis, and their effects on surface potential, photoelectrochemical properties and simultaneous removal of coexisted Cu2+ and norfloxacin (NOR) performance in water were systematically investigated. Constructed NCDs-TNTs composite displayed superior performance towards the adsorption (ion exchange/coordination) of Cu2+ and adsorption-oxidization of NOR over the two individuals, mainly benefiting from the collaboration of NCDs in different existent states. The existence of TiNH chemical linkage was identified between TNTs and NCDs-OT (NCDs on the outer surface of TNTs), which not only modulates the surface potential to favor the external diffusion of Cu2+ or NOR+ from aqueous solution to the negatively charged NCDs-TNTs, but also facilitates the intraparticle transfer of contaminants to the reactive sites. In addition, the up-conversion light property of NCDs-OT and the interstitial NCDs-IT (NCDs on the inner surface of TNTs) doping in TNTs interact together to enable NCDs-IT-TNTs to fully absorb and utilize all visible light. The photoexcited electrons were further trapped by NCDs-OT to promote the photogenerated carrier separation. Adsorbed Cu2+ could also improve the performance of NCDs-TNTs toward NOR oxidization, mainly owing to the self-synchronous doping of adsorbed Cu2+ broadening light absorption area and acting as mediators for delivering electrons. This work provides unique insights into the structural design of composite materials for such combined contamination remediation in water.