Lead-free piezoelectric nanowire-nanocellulose hybrids for flexible energy harvesters – NanoFlex

A large number of small-scale electronic devices are increasingly integrated in our everyday lives. Future requirements for these devices are to be sustainable, maintenance-free, and self-powered (free of batteries). In this regard, nanowire (NW) based piezoelectric energy harvesters have emerged as one of the attractive solutions. Thanks to the direct piezoelectric effect, such nanogenerators can drive low-power electronics by harvesting the mechanical energy from their ambient. The mechanical flexibility given by the use of NWs enable device integration on various soft surfaces, which is highly relevant for future biomedical applications. This prospect further drives research efforts to find new piezoelectric materials showing enhanced performances in terms of stretchability and conformability. In addition, these materials should show biocompatibility, biodegradability and a high electromechanical efficiency.
NanoFlex will synthesize a novel inorganic-organic flexible piezoelectric hybrid film to be used as an alternative building block for the next-generation of piezo-harvesters. We will explore the combination of three unique nanomaterials: highly piezoelectric nitride NWs, plant-based nanocelluloses, and graphene. The rationale for the choice of materials is to minimize the environmental footprint of the flexible energy harvester, while ensuring high flexibility and maintaining an appreciable electromechanical efficiency. We set three goals for the project, which are described in the following.
(1) Here, the inorganic nitride NWs will be used for piezo-charge generation. We will explore various options for boosting their piezoelectric performances to surpass ZnO and other biocompatible alternatives. We will start with single crystalline AlN NWs grown by molecular beam epitaxy, since AlN is biocompatible and shows the highest piezocoefficients in the family of group-III nitrides. We will next enter the field of the “new nitrides” by investigating the alloying of AlN NWs with rare-earth elements like Scandium (Sc). The lattice frustration induced by the presence of Sc results in an extraordinary enhancement of the piezoelectric coefficients. Yet, the synthesis of single crystalline (Al,Sc)N in the shape of NWs remains to date an uncharted territory. We will further tailor the piezo-response of the nitride NWs by a strain engineering, to benefit from nonlinearities in the piezoelectric coefficients and to profit from flexoelectricity. Our results will help identifying generic routes to optimize the piezoelectric properties of nitride and other wurtzite materials like ZnO.
(2) We aim at replacing eco-unfavourable components of conventional flexible devices (e.g., synthetic polymers, plastic substrates and metallic contacts) by green materials. The target is to develop a complete process that combine for the first-time nitride NWs with bio-sourced nanocelluloses and graphene. The nanocelluloses (nanofibrils or nanocrystals) will be used as an encapsulating matrix for the NWs, while the nanopapers synthesized by casting these nanocelluloses will be used as a flexible substrate. Graphene, a highly conductive two-dimensional material with excellent mechanical strength and flexibility, will be used as electrical contact to couple the generated signal to the external circuit. Our goal is to produce inorganic-organic piezo-films having a total thickness down to 10 µm, with a high mechanical conformality, stretchability, and stability.
(3) The functionality of the hybrid films as flexible piezo-harvesters in nanogenerators and self-sustained mechanical sensors will be evaluated with respect to ZnO and other lead-free competitors. We will systematically analyze the overall device performances (piezo-output, flexibility, and robustness), correlating with the properties of each components characterized at the nanoscale and with choices made in the process optimization and in the device structure.