Ti₃C₂T_x MXene as Effective Conductive Material for Enhancing the Activity of Cu-Doped TiO₂ Nanotube Photocatalysts – Application in Photooxidation and in H₂ Production

Photocatalysis is one of the most promising approach for photon energy conversion and environmental remediation. In this respect, TiO₂ is the most used semiconductor due to its high photocatalytic response. However, TiO₂ also presents a high propensity for electron-hole recombination. To solve this issue, several options were found to limit recombination: cationic doping, change of the semiconductor morphology or addition of C allotropes.<br/>Previous studies performed in our group have shown that 1D TiO₂ nanotubes can help in enhancing the separation of electron-hole pairs resulting in higher photooxidation while doping by copper allows increasing the response under visible light. Another option is to add a conductive material which can enhance the transport of photoelectrons from TiO₂ to Cu. Previous work performed using graphene oxide showed the validity of this approach but also the instability of this material under irradiation. An alternative solution is to use the emerging class of MXene compounds which were found to exhibit very high conductivity.<br/>The objective of this work will then be to synthesize Cu/Ti₃C₂T_x/TiO₂ systems in which the Ti₃C₂T_x MXene will be evaluated for its propensity in enhancing the transport of photoelectrons, improving in this way the separation of electron-hole pairs. Comparison will be performed between 1D TiO₂ nanotubes and a P25 reference while Cu/Ti₃C₂T_x/TiO₂ will be applied for both photooxidation (formic acid photodegradation) and for H₂ production.<br/><br/>Materials and Methods<br/><br/>Ti₃C₂T_x MXene layers were obtained by HF etching of Ti₃AlC₂ MAX precursors to remove Al. TiO₂ nanotubes were obtained under hydrothermal alkaline conditions. Two series of impregnation were performed: first, GO or MXene were impregnated onto P25 or TiO₂ nanotubes while Cu(NO₃)₂.3H₂O was then impregnated onto the resulting materials.<br/><br/>Results and Discussion<br/><br/>Systematic comparisons between P25 and nanotubes were done during the construction of various Cu/TiO₂, MXene/TiO₂ and Cu/MXene/TiO₂ configurations. Results emphasized the important role played by surface oxygen vacancies in the enhancement of the photooxidation activity. While the addition of Cu onto P25 induces the creation of new oxygen vacancies onto P25 improving its photooxidation efficiency, on TiO₂ nanotubes, copper interacts mainly through the consumption of these surface oxygen vacancies initially present on the titanium oxide surface. Beneficial effects were the observed only at low Cu loadings. Photocurrent analysis also emphasized the strong increase in conductivity resulting from the addition of Ti₃C₂T_x. This high conductivity enhancement leads to a substantial increase of the photocatalytic response in photooxidation with an activity of Cu/Ti₃C₂T_x/P25 150 % higher than for P25 and 50 % higher than for Cu/P25 while the highest activity was reached using Cu/Ti₃C₂T_x/TiO₂ nanotubes.<br/>In H₂ production, surface oxygen vacancies play a more indirect role since their consumption through copper addition favors a higher Cu-Ti interaction and a better transfer of electrons from TiO₂ to Cu. This leads to very active Cu/TiO₂ nanotube systems for H₂ production. Surprisingly, the addition of Ti₃C₂T_x leads to an apparent lower H₂ evolution activity. H₂ chemisorption analysis reveals in fact that Ti₃C₂T_x plays the role of an excellent trap for the hydrogen produced which is then stored inside our materials revealing in this way the ability for H₂ storage of Ti₃C₂T_x MXene.<br/><br/>Conclusion<br/><br/>The utilization of Ti₃C₂T_x MXenes as conductive materials enhancing the transport of photoelectrons from a semiconductor, TiO₂ to an electron acceptor, Cu has been demonstrated in photooxidation. Results also emphasized the important role played by surface oxygen vacancies while Ti₃C₂T_xMXene was found to present quite interesting properties as H₂storage material.