Bringing Together Laser Treated MXene and Titania Nanotubes Towards Composite Electrode Material Active under Visible Light

For the past 20 years, taking into account as the starting date the discovery of the extraordinary properties of a single layer graphene, finally the materials exhibiting 2D architecture have gained more attention. Among family of chalcogenides and boron nitrides that became so popular, the carbides and nitrides of transition metals became a rising star due to the attractive properties [1]. In MXene flake one can find a layer of transition metal and interlayer of carbon with some termination (O, OH, F or Cl) that are linked also to the metallic part. Most of known MXene synthesis routes involved fluoride-containing compounds, either aqueous or molten salts treatment of the MAX phase. Despite individual sheets are obtained they are still formed in a specific stack of several micrometers providing some problem when the composite with other materials should be achieved.<br/>To overcome this problematic issue we propose laser treatment as an additional step in the fabrication route. The fabrication of Ti₃C₂T_x is performed in a quite typical way including mixing of MAX phase with HF and HCl solution and further delamination supported with NaCl. Microscopic investigation revealed that MXene sheets form agglomerates and additional efforts should be undertaken. Therefore, the MXene powder was frozen using liquid nitrogen and on such frozen material a liquid isopropanol was poured. Then beam of 1064 nm generated by pulsed Nd:YAG laser was irradiating frozen Ti₃C₂T_x. The interaction between laser and this material results in its ablation and its capture in a liquid medium. During the process the solution above the frozen material become grayish and such liquid was collected after different periods of time. Both the resonant peak at 120 cm^-1 and the out-of-plane vibrations at 736 cm^-1 of Ti₃C₂T_x are clearly visible on the Raman spectrum confirming the preservation of the MXene structure.<br/>Finally, the material was used for the modification of surface provided by titania nanotubes obtained via anodization of Ti plates and Ti sputtered layer. The modification was carried out via treatment with TiCl₄/Ti₃C₂T_x solution followed by thermal annealing. The SEM inspection revealed that the initially smooth and thin walls of titania were uniformly overgrown and more porous but any agglomerates clogging the tubular interior were not detected. Cyclic voltammetry curves recorded for the series of modified titania electrodes indicated enhanced photoresponse under both visible and UV-vis light. In a particular case when the electrode was exposed to visible light, almost 20 times higher photocurrent at 0 V vs. Ag/AgCl/0.1M KCl comparing to bare titania was reached.<br/>Apart from the improved photoresponse, the electrochemical activity of modified titania in the cathodic range was significantly changed, namely the cathodic current was much higher than the one registered for bare titania. For example at -1.0 V, bare titania exhibits nearly 0.1 mA/cm² whereas for the modified substrate the recorded current equals 2.6 mA/cm². It should be underlined that when the treatment of titania was carried out with TiCl₄ solution but without addition of laser treated MXene, such electrochemical response was not achieved. It indicated that the presence of MXene is crucial for greatly improved electrochemical and photoelectrochemical properties of the composite electrode material.<br/>Summarizing we would like to show the unique laser treatment of 2D metal carbide material and its possible usage for formation of the composite with inorganic platform exhibiting promising electrochemical and photoelectrochemical properties being of high importance for development in the research area of water splitting and photoconversion.<br/>[1] G. Sha, Z. Ding, Y, Gogotsi, Frontiers of Physics 18 (2023) 13604<br/>Acknowledgements<br/>The authors acknowledge the financial support of the National Science Centre (Poland) via grant no.: 2020/02/Y/ST8/00030