2D Shuttle Ti₃C₂T_x MXene Systems for Enhancing the Performance of Composite Cu₂O/TiO₂ Photocatalysts – Comparison with Graphene Oxide

Introduction<br/> <br/>The most common semiconductor used in photocatalysis is titanium dioxide due to its high photocatalytic response under UV irradiation. However, the use of TiO₂ remains limited due to its high recombination rate of electron-hole pairs. Different strategies were envisaged to improve its photocatalytic response like cationic or anionic doping, coupling with C allotropes or changing semiconductor morphology. In this respect, previous studies made by our group showed that 1D TiO₂ nanotubes (TNT) provides a better separation of photogenerated charges while the coupling with graphene oxide (GO) allows transfer of photoelectrons to GO making more holes available for photooxidation reaction. However, GO was also found to be unstable when combined with TiO₂ under irradiation through overreduction of oxygen functional groups contained in graphene layers, leading to a complete loss of beneficial effect of adding GO after long photocatalytic runs.<br/>The solution envisaged here is to use a different 2D conductive material, MXene sheets (Ti₃C₂T_x), to separate physically the semiconductor (TNT) producing photoelectrons from an electron-acceptor component (herein, copper) to improve both the photocatalytic response and the stability under irradiation conditions. Applications are envisaged for both photooxidation reactions and H₂ production.<br/> <br/>Materials and Methods<br/> <br/>Ti₃C₂T_x MXene layers were obtained by HF etching of Ti₃AlC₂ MAX precursors to remove aluminium. GO was synthesized according to a modified version of the Hummers method. TNT was obtained under hydrothermal alkaline conditions. Two series of impregnation are performed: first, GO or MXene were impregnated onto TNT while Cu(NO₃)₂.3H₂O was then impregnated onto the resulting materials. Photocatalytic tests were performed for the photodegradation of formic acid (FA) and for the H₂ production.<br/> <br/>Results and Discussion<br/> <br/>To define the role played by Ti₃C₂T_x MXene layers, several systems combining the TiO₂ semiconductor (TNT as compared to the P25 reference) with Cu and/or MXene were considered.<br/>In the case of Cu doping of TiO₂, the formation of Cu₂O species with well-defined plasmonic properties was evidenced after adequate post-calcination treatment leading to improved photocatalytic activity for both photooxidation and H₂ production.<br/>Adding Ti₃C₂T_x MXene to TiO₂ semiconductors (TNT or TiO₂ P25) was also studied considering several parameters (the MXene loading, the HF concentration during etching or the size of the MXene sheets) showing that an optimized choice of Ti₃C₂T_x MXene layers with adequate sheet size and appropriate etching can lead to high beneficial gain in activity.<br/>In a last step, combining copper to the hybrid Ti₃C₂T_x MXene/TiO₂ system allows an even more efficient separation of charges by beneficiating from the electric conductivity of the MXene layers to transport photoelectrons from the semiconductor to the Cu component with an activity twice higher than without MXene. This last result confirms the beneficial role played by Ti₃C₂T_x MXene as a physical separator between donor and acceptor of electrons allowing a better separation of photogenerated charges.<br/>Finally, comparison to the use of GO instead of Ti₃C₂T_x MXene layers confirms the interest of both systems with their own advantages: higher stability of MXene layers under photocatalytic conditions but better flexibility of GO favoring a higher interaction with each other component.<br/> <br/>Conclusion<br/> <br/>The interest of using a physical separator between donors and acceptors of photoelectrons has been demonstrated using Ti₃C₂T_x MXene layers as 2D shuttle conductive materials. The physical separation achieved between holes and electrons leads to more active photocatalytic systems for both photooxidation reactions and H₂ production opening the possibility to form a vast family of new, more efficient and more stable photocatalytic hybrid systems.