Exploring the role of Ti₃C₂T_xMXene in photoelectrochemical water splitting over solution-processed BiVO₄ photoanode

Transition metal carbides, nitrides, or carbonitrides known as MXenes commonly improve the efficiency of photo(electro)catalysts, but their role is still not fully explored. Herein, we developed a novel and facile solution-processing method to fabricate BiVO₄ and Ti₃C₂T_x-modified BiVO₄ (Ti₃C₂T_x/BiVO₄) photoanodes to explore the role of Ti₃C₂T_x MXene in photoelectrochemical water splitting. The charge transfer and recombination kinetics were studied by transient open-circuit potential, transient photocurrent, photoelectrochemical impedance spectroscopy, and intensity-modulated photocurrent spectroscopy. It was demonstrated that Ti₃C₂T_x forms a Schottky-like junction at the BiVO₄/Ti₃C₂T_x interface and that it acts as a hole transport/storage layer. The Ti₃C₂T_x layer creates a built-in electric field at the surface that increases the band bending and space charge layer width, eventually enhancing the charge separation efficiency. Modification of Ti₃C₂T_x/BiVO₄ with a water oxidation catalyst such as cobalt phosphate (Co-Pi) was essential to deliver the maximum photocurrent. For instance, the Co-Pi/Ti₃C₂T_x/BiVO₄ photoanode delivered 4.72 mA/cm⁻² at 1.23 V_RHE, which is 2- and 11-fold higher than those of Ti₃C₂T_x/BiVO₄ and pristine BiVO₄ photoanodes, respectively. This study provides a practical protocol for fabricating Ti₃C₂Tx/BiVO₄ and reveals the role of MXene in photoelectrochemical water splitting.