Piezoelectricity in monolayer MXene for nanogenerators and piezotronics

Highlights: • The first study on the piezoelectricity of monolayer Ti₃C₂Tx MXene is reported, showing the efficient piezoelectric outputs. • The functional groups on the MXene break the inversion symmetry of lattice structures to possess piezoelectric properties. • The response current of Ti₃C₂Tx MXene is 0.3 nA, the power density is 6.5 mW/m² and the conversion efficiency is 11.15%. • The discovery of piezo-MXene can lay the foundation for understanding and applying Ti₃C₂Tx MXene in self-powered nanodevices. The piezoelectric properties of two-dimensional (2D) materials have been widely studied due to their broad application prospects. MXene, one of the well-known 2D material members, is predicted to be a highly directional piezoelectric material with a non-centrosymmetric lattice structure. Here, the first experimental study of piezoelectric responses of the monolayer Ti₃C₂Tx MXene is reported, showing that the cyclic strain excites stable oscillating piezoelectric voltage and current outputs. The functional groups on the surface of the MXene break the inversion symmetry of lattice structures to possess piezoelectric properties. The piezoelectricity in the armchair direction of the Ti₃C₂Tx MXene sheet exhibits an intrinsic current output of 0.3 nA at 1.08% tensile strain, corresponding to the 6.5 mW/m² power density and the 11.15% conversion efficiency, which are all higher than that of previous reported 2D materials. Further, theoretical calculations of the MXene have explained the origin of piezoelectric polarizations among the multi-atomic structure and the surface functional groups. The discovery of the piezo-MXene can lead to the foundation for the understanding and applications of the Ti₃C₂Tx MXene in powering nanodevices and stretchable electronics.