Crafting NiCo2O4@Co9S8 nanotrees on carbon cloth as flexible pressure sensors for effectively monitoring human motion

Hierarchically micro/nano-structural design in material optimization has been considered as a rational strategy for producing high-performance pressure sensors. Herein, we first investigate pressure sensors based on different nanostructured cobalt–nickel compound materials [i.e., one-dimensional (1D) Co9S8 nanoneedle, two-dimensional (2D) NiCo2O4 nanosheet and three-dimensional (3D) NiCo2O4@Co9S8 nanotree films] in situ grown on carbon cloth. The 3D NiCo2O4@Co9S8 electrode just wrapped with polydimethylsiloxane (PDMS) presents capacitive pressure sensing with superior sensitivity and durability as well as applicability to monitor human joint motion, primarily due to its special architecture with Co9S8 nanoneedle trunks decorated by many NiCo2O4 nanosheet branches. Such 3D feature possesses numerous antennas to enhance the capacitance response as the loaded pressure is changed. Furthermore, when combining the NiCo2O4@Co9S8 electrode with an Ag interdigital electrode to further amplify the pressure signal, the corresponding pressure sensor demonstrates resistive sensing with relatively higher sensitivity as compared to the above capacitive one, which then is very sensitive to human pulse and respiratory. This study shows a new way for designing sensing materials with the potential application in wearable real-time healthcare electronics.