Two-dimensional MXene nanosheets on nano-scale fibrils in hierarchical porous structure to achieve ultra-high sensitivity

The complex hybrid nanostructure combining a two-dimensional (2D) conductive material and a hierarchical nanoscale skeleton plays an important role to enhance its piezoresistive sensitivity. To construct such a novel hybrid nanostructure, a piezoresistive sensor was designed with the following strategy to take the full advantages of 2D MXene and nanoscale fibrils: ethylene oxide propylene oxide random copolymer (EOPO) was grafted to ethylene-vinyl alcohol (EVOH) molecular chains and was foamed by an environmentally-friendly supercritical CO₂ (scCO₂) foaming technology to fabricate abundant nanoscale EVOH fibrils surrounding micropores; MXene featured as a 2D structure of nanoscale size that strongly interacted with this hierarchical nanoscale skeleton, and MXene not only convolved on nanoscale fibrils to generate bumps but also MXene covered the end of broken fibrils to build spots, and furthermore, MXene adhered on the soft EOPO embedded EVOH fibrils to form wrinkles, in which these bumps, spots and wrinkles assembled by highly conductive 2D MXene offered sufficient contacts when the hierarchical nanoscale skeleton was compressed (these contacts would then destruct when the skeleton recovered). Such an elaborated hybrid nanostructural design exploits the full potential of 2D MXene and hence achieves an ultra-high sensitivity of 6895.0 kPa^-1 for this fabricated MXene piezoresistive sensor.