Photoassisted Enhancement of the Electrocatalytic Oxidation of Formic Acid on Platinized TiO₂ Nanotubes

A solvothermal method is used to deposit Pt nanoparticles on anodized TiO₂ nanotubes (T_NT). Surface characterization using SEM, EDX, and XRD indicates the formation of polycrystalline TiO₂ nanotubes of 110 ± 10 nm diameter with Pt nanoparticle islands. The application of the T_NT/Pt photoanode has been examined toward simultaneous electrooxidation and photo­(electro)­oxidation of formic acid (HCOOH). Upon UV–vis photoillumination, the T_NT/Pt photoelectrode generates a current density of 72 mA/cm², which is significantly higher (∼39-fold) than that of the T_NT electrode (1.85 mA/cm²). This boosting in the overall current is attributable to the enhanced oxidation of formic acid at the T_NT/Pt-electrolyte interface. Further, a series of cyclic voltammetric (CV) responses, of which each anodic scan is switched to photoillumination at a certain applied bias (i.e., 0.2 V, 0.4 V, etc.), is used to identify the role of T_NT/Pt as a promoter for the photoelectrooxidation of formic acid and understand a carbon monoxide (CO)-free pathway. Chronoamperometric (j/t) measurements demonstrate the evidence of an external bias dependent variation in the time lag during the current stabilization. An analysis of the CV plots and j/t profiles suggests the existence of both the charge-transfer controlled process and the diffusion-controlled process during formic acid photoelectrooxidation.