Porous FTO thin layers created with a facile one-step Sn4+-based anodic deposition process and their potential applications in ion sensing

Abstract

Porous fluorine-doped tin oxide (FTO) thin layers were created from commercial FTO glass with a novel, facile, one-step Sn⁴⁺-based anodic deposition process. The Sn²⁺ necessary for the anodic deposition of SnO₂ was generated in situ from the starting Sn⁴⁺ such that a comparable competition between deposition and acid-etching of SnO₂ became possible, leading to a rich morphological evolution of the deposit. The fluoride ions released from the acid-etching of the starting FTO layer were later incorporated into the newly deposited SnO₂ to make it fluorine-doped, resulting in the formation of a conductive, porous FTO layer. This unique FTO product may find a wide range of applications in processes requiring large surface areas for the accommodation of active sites to host functional interfacial events and excellent electric conductivities for charge transfer/transport involved in the interfacial events. To demonstrate its applicability, this conductive porous FTO thin layer was applied to the sensing of nitrate ions, which play a fundamental role in a wide range of environmental and biological processes. A simple device was developed for the sensing of nitrate ions based on surface charge-modulated adsorption of nitrate ions and measurements of consequent currents. At 0.5 M HNO₃, the porous FTO showed a 55 fold sensitivity improvement over the commercial FTO. In buffer solutions of a specific pH value, the cation effects on ion sensitivities were also discussed. A low detection limit of 10⁻⁸ M was achieved for nitrate ion sensing with the present porous FTO layer.