Salt-responsive ZnO microcapsules loaded with nitrogen-doped carbon dots for enhancement of corrosion durability

Capsules containing corrosion inhibitors have drawn considerable attention in the field of self-healing corrosion protection because of their appropriate size and high efficiency. ZnO has become a type of promising encapsulants due to its high structural tunability and unique surface chemical property. Meanwhile, carbon dots (CDs) have huge application potential to replace the existing toxic corrosion inhibitors. Herein, a corrosion-resistant microcapsule with nitrogen-doped carbon dots (NCDs) as the core material and ZnO as the wall material (NCD@ZnO MCs) was developed to exert perfectly the advantages of the two materials. The NCDs were first synthesized by solvothermal method, and the NCD@ZnO MCs were then fabricated just by stirring at room temperature. The morphology, microstructure and composition of NCDs and NCD@ZnO MCs were characterized, respectively. The superiority of NCD@ZnO MCs in improving the anti-corrosion resistance of waterborne coatings was also discussed. The results showed that the obtained NCDs had a size distribution of 2–5 nm and the N-doping content was 6.56%. The size distribution and shell thickness of the prepared NCD@ZnO MCs were 100–200 nm and 30–90 nm, respectively. The encapsulation efficiency of NCDs was 34.69 ± 3.04%. The release behavior of NCD@ZnO MCs exhibited a dependence on salt concentration. Electrochemical impedance spectroscopy (EIS) measurements showed that NCD@ZnO MCs as fillers significantly improved the durability of the coating in terms of anti-corrosion resistance in all solutions with different salt concentrations. This smart filler is a promising candidate for application in anti-corrosive coatings, due to its low toxicity, facile synthetic route, near nanoscale size and sensitive environmental responsiveness.