Large-Scale Green Electrochemical Synthesis of Smart Titanium Dioxide Nanomaterials: Controlled Morphology and Rotatable Surface Ligands Via Tuning Electrolyte Structures

Titanium dioxide (TiO2) nanomaterials have been comprehensively studied in recent decades because of their possible practical applications in diverse fields, including photocatalysis, energy storage, sensing and biomedicine. Numerous strategies have been reported on preparing TiO2 nanostructures with regulated sizes, shapes and surface ligands; however, developing a simple, eco-friendly, cost-effective and scalable procedure for the fabrication of TiO2 nanomaterials while controlling those properties is still challenging. Herein, titanium dioxide nanostructures, e-TiO2, are prepared using a scalable green electrochemical approach, in which Ti foils act as precursors. Electrolysis is carried out in an electrolyte solution containing only a surfactant in water. This approach reduces the use of toxic chemicals, and thus it is eco-friendly and cost-effective. Moreover, the electrochemical synthesis of colloidal e-TiO2 can be successfully scaled up to 5 L. Three cationic surfactants, tetrapropylammonium bromide (TPAB), tetrabutylammonium bromide (TBAB) and cetyltrimethylammonium bromide (CTAB), are employed as electrolytes to fabricate e-TiO2, resulting in the formation of three types of nanostructures: negatively charged e-TiO2/TPAB nanospheres, negatively charged e-TiO2/TBAB nano-ellipses and positively charged e-TiO2/CTAB nanorods. They are characterized to clarify the roles of surfactants in controlling sizes, shape and surface properties of e-TiO2 nanostructures. Dispersibility of three types of e-TiO2 is investigated in aqueous and nonpolar solvents. e-TiO2/TPAB and e-TiO2/TBAB are only well-dispersed in aqueous media. In particular, e-TiO2/CTAB can be dissolved in both kinds of solvents. Thanks to the unique rotatable property of the coating agent (CTAB), a “smart” TiO2 nanomaterial (e-TiO2/CTAB nanorod) has been fabricated, which can migrate from an aqueous medium to a nonpolar one and vice versa.