Mechanically robust, humidity-responsive nanofilms for self-oscillating actuators and energy harvesting

Abstract

Developing sustainable energy harvesting systems is paramount to address the escalating global energy crisis. This study reports the synthesis of free-standing, ultrathin nanofilms by using rigid 4,4′,4′′-((1,3,5-triazine-2,4,6-triyl)tris(oxy))tribenzaldehyde and flexible hydrazide-poly(ethylene glycol)₆₀₀-hydrazide at the air/liquid interface. The nanofilm exhibits high mechanical strength with an elastic modulus of 887.9 MPa that is attributed to the reversible acyl-hydrazone bonds, physical entanglement of PEG flexible chains and supramolecular hydrogen bonding networks. A high adhesion force of 38.2 nN is observed between the nanofilm and a Si probe. The rapid water absorption/desorption ability and extraordinary mechanical strength of the nanofilm enable its fast reversible deformation in response to humidity changes. The nanofilm based humidity-driven actuator can serve as a smart switch and an intelligent dehumidifier. The most intriguing property of the nanofilm actuator is its self-oscillating behavior, driven by fluctuations in ambient humidity. Integrating the actuator with a piezoelectric film yields a self-sustaining power generator, achieving a maximum output voltage of 1.1 V and a high power density of 98.8 µW kg⁻¹ at ambient humidity. This study provides new insights into designing soft robotics and self-sustaining energy harvesting systems using responsive thin films, paving the way for battery-free devices and environmental sensing.