Enabling aqueous electrophoretic growth of adherent nanotitania mesoporous films via intrafilm cathodic deposition of hydrous zinc oxide

Graphical abstract: - Abstract: By far the majority of previous electrophoretic deposition (EPD) works employ organic solvent media and additives that impose environmental challenges to large-scale implementation. Here EPD in water is investigated instead. In particular via the adoption of a low DC voltage approach mesoporous films of nanotitania are successfully grown by aqueous EPD of the commercial P25 mixed phase TiO2 product on FTO-glass as substrate. Growth of adherent and robust films is enabled via the use of isopropanol as co-solvent and Zn(NO3)2 as additive, the latter leading to codeposition of hydrous zinc oxide that acts as nanoglue. The optimization of the suspension composition, mechanism of zinc codeposition with TiO2, and the structural and electronic properties of the resultant TiO2–ZnO semiconductor films are described. Under galvanostatic EPD regime, zinc codeposition occurs initially (first 15 s) as Zn(OH)2 via the cathodic (NO3− to NO2− reduction) interfacial generation of OH−. Upon rise of the cathodic potential with increasing film thickness, nanosized metallic zinc codeposits uniformly throughout the film profile (∼2% Zn content). As such Zn(OH)2 and metallic nanozinc that partially oxidizes in situ act as intrafilm nanoglue. After annealing at 450 °C the nanocomposite TiO2–ZnO film exhibits blue-shifting and abundant hydroxyl surface coverage that are desirable in photocatalytic applications.