Electropolymerized graphene coatings on TiO₂ nanotube arrays for the photoelectrochemical degradation of organics in water

Waste treatment becomes a major problem in the world and various technologies have been suggested for this purpose. Photoelectrochemical (PEC) oxidation is one of the promising technologies for the treatment of organic pollutants in water since it enables oxidation of organic compounds to CO₂. The common concept of photoanodes preparation is to use planar and transparent electrodes coated with nanostructured photocatalysts. However, the employed coating methods not always provide high efficient surface area. This limits large scale applications in PEC cells.

In the present work porous photoanodes based on TiO₂ nanotube (NT) arrays were prepared via anodization of Ti foils followed by annealing at 500 ̊C. These are characterized by good exposure to light and high surface area for photocatalytic reactions. SEM images revealed TiO₂NTs with a length of 3-10µm and an average tube diameter in the range of 50-150 nm. XRD showed that anatase TiO₂ was formed after annealing the anodized Ti foil.

Graphene Oxide (GO) based coatings were obtained on TiO₂NTs by a new and simple process of electropolymerization. While the previously reported electrochemically reduced GO (erGO) coatings were obtained due to the less soluble nature of reduced GO in comparison to GO, the present coatings were based on the formation of chemically bonded GO sheets. According to SEM observation very thin layers of epGO decorated the NTs’ surface.

The performances of the Ti/TiO₂, TiO₂/epGO and TiO₂/erGO photoanodes were examined in a PEC cell for methanol degradation. The methanol photooxidation onset potential in a solution of 1 M methanol shifted cathodically by 200 mV for TiO₂/epGO as compared to Ti/TiO₂/erGO. Moreover, a much higher potential to reach current saturation was observed for the TiO₂NTs/erGO as compared to that of TiO₂NTs/epGO (+0.7V and -0.2V, respectively), with an increase of 30% of the methanol (1M) saturation photocurrent density for TiO₂/epGO.