Assisted Enhanced Photocatalysis by the Addition of a Ferroelectric Component to Copper-Doped TiO₂ Nanotubes

Introduction<br/> <br/>Titanium dioxide is the most used semiconductor in photocatalysis due to its good chemical stability and suitable band edge potentials for H₂ production. However, its large band gap (3.2 eV for anatase) restrains its use to photoexcitation under UV illumination. TiO₂ also presents a high recombination rate of electron-hole pairs limiting its intrinsic activity. Different strategies have been envisaged to decrease recombination including change of morphology, formation of heterojunctions, doping with cations or combination with carbon allotropes. Even if successful, these strategies rely on the consumption of part of the excitons to keep others available for reaction. In this respect, ferroelectric polarization can modify band structures and charge transport performance in adjacent semiconductors without consumption of part of the excitons. Among ferroelectrics, SrTiO₃ offers favourable conditions for photocatalysis with a conduction band only 200 meV more negative than TiO₂ while it is also a metastable paraelectric compound becoming ferroelectric under optical excitation. Therefore, herein, the interest of using SrTiO₃ with Cu-doped TiO₂ nanotubes was here considered. <br/>. <br/>Materials and Methods<br/> <br/>TiO₂ nanotubes (TNT) were synthesized using a hydrothermal procedure under strong alkaline conditions. TiO₂ nanotubes were impregnated with Cu(NO₃)₂.3H₂O (0.5 wt% Cu) before drying and calcination. The resulting material was then doped with strontium (0.2 to 1.0 wt %). Photocatalytic tests were performed under UV and visible irradiations for the formic acid photodegradation. <br/> <br/>Results and Discussion<br/> <br/>Deep structural characterization was performed. First, UV-vis DRS emphasizes the role of plasmonic properties following the Cu addition making these materials active under visible irradiation. Second, Raman shows the formation of surface Sr-O-Ti entities up to 0.8 wt% Sr.<br/>XPS analysis also reveals a net modification of the Ti, O and Sr species with increasing addition of strontium resulting from the progressive replacement of surface oxygen vacancies by Sr-O-Ti entities. PL and EIS also show that incorporating strontium decreases the recombination rate of electron-hole pairs and stabilizes photogenerated charges increasing their lifetime by 75%.<br/>Combined together, limited recombination and higher stabilization of photogenerated charges leads to an increase of the photocatalytic activity under UV irradiation by 50% at the optimum Sr loading for in situ SrTiO₃ formation (0.8 wt% Sr) and by an impressive higher activity under visible irradiation (initial rate: 0.6 µmol.L^-1.min^-1 without Sr, 109 µmol.L^-1.min^-1 at optimum Sr loading).<br/>Extension of this study was also made to Cu-doped systems comprising graphene oxide. Once again, the Sr addition results in a strong increase of the photocatalytic response. Modelization of the kinetics of photodegradation including diffusion phenomena was performed and compared to dielectric properties. A perfect correlation was observed between the increase of activity and the propensity to charge polarization induced by Sr addition confirming the role played by the ferroelectricity induced by in situ SrTiO₃ entities on the enhanced photocatalytic properties. <br/> <br/>Conclusion<br/> <br/>Assisted photocatalysis by addition of a ferroelectric component to semiconductor systems was herein demonstrated as a promising alternative for enhancing the photocatalytic performance of commonly used semiconductors like TiO₂ through stabilization of photogenerated charges while limiting recombination processes. Application made using Sr addition to Cu-doped TiO₂ nanotubes can be extended widely to other semiconductor systems allowing better yields in pollutant abatement or in green energy production.