Formation and growth of PbO2 inside TiO2 nanotubes for environmental applications

• The formation and growth of PbO2 inside TiO2 nanotubes can be performed controlling the electrolysis time at constant current density.
• TiO2 NTs::PbO2 (x s) anodes have higher photocurrent density values than TiO2 NTs as well as better photoconversion efficiencies.
• PbO2 promotes the electronic transfer of photoelectrons through the TiO2::PbO2 interface.
• The TiO2 NTs::PbO2 (5 s) anode exhibits the best photoelectrocatalytic performance in the bleaching process of a methyl red dye solution.
• The combination of TiO2 and PbO2 in a nanostructured array produces an important photoelectro-synergistic effect.

The electrodeposition of lead dioxide (PbO2) in a Ti/TiO2 nanotube (TiO2 NT) array was achieved using a galvanostatic method at room temperature. The formation and growth of PbO2 inside the TiO2 NTs was followed as a function of the deposition time (td). The TiO2 NT::PbO2 (x s) samples, as well as the TiO2 NTs previously prepared by anodization of Ti sheets, were characterized by scanning electronic microscopy (SEM) and X-ray diffraction (XRD) techniques. It was found that PbO2 formation starts at the bottom of TiO2 nanotubes and grows inside them until a well-dispersed deposit is achieved (td ≤ 5 s) or until formation of PbO2 crystalline deposits or cauliflower-like clusters outside the TiO2 NTs at td ≥ 10 s. In any case, the PbO2 deposit always partially covered the TiO2 NT arrays. Photochemical activity results showed that TiO2 NT::PbO2 (x s) samples had higher photocurrent density values than TiO2 NTs. They also had better photoconversion efficiencies, which strongly suggest the occurrence of a photoelectro-synergistic effect due to electronic transfer assisted by the close contact between TiO2 and PbO2. This assumption was supported by the catalytic bleaching of methyl red (MR) dye solutions. Electrocatalytic (EC) and photoelectrocatalytic (PEC) results showed that, although the discoloration process is not entirely PEC dependent, its contribution is greater than the sum of the individual photocatalytic (PC) and EC processes, which confirms the synergic effect produced by the combination of TiO2 and PbO2 in a nanostructured array.

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