Scalable, efficient piezoelectric wood nanogenerators enabled by wood/ZnO nanocomposites

The need for sustainable development creates opportunities for biomass-based materials design toward piezoelectric mechanical energy harvesting. Wood is promising due to its hierarchical, porous structure. Here, piezoelectric nanogenerators (PENGs) were prepared through nanostructure-controlled zinc oxide (ZnO) growth inside the outer wood layers of veneers. Mechanisms for formation of various ZnO nanostructures in wood are analyzed. Controlled morphologies of nanoparticles, nanorods, nanowires, and nanoflakes were realized and characterized by field emission-scanning electron microscopy (FE-SEM) and small angle x-ray scattering (SAXS), allowing tunable piezoelectric output. Nanostructures with higher aspect ratios i.e. nanorods and nanowires resulted in higher voltage during cyclic loading. An optimum voltage of 1.3–1.4 V was obtained with wood/ZnO nanowire or nanorod composites at a force of ≈8 N. The current output is in the range of 0.85–11 nA, which could be scaled up to ∼130 nA with a larger area device. When mounted in shoe soles, these wood/ZnO PENGs generated 1–4 V from walking/jogging motions. The hydrothermal growth method is scalable, which facilitates practical applications.