Piezoelectric nanogenerators – a review of nanostructured piezoelectric energy harvesters

A huge number of piezoelectric materials have been demonstrated since the discovery of piezoelectricity over a century ago. The use of nanostructured piezoelectrics, however, is a relatively recent development, and the understanding of the nanoscale size effects on ferro- and piezoelectricity is still being formulated. Despite this, the application of piezoelectric nanostructures to energy harvesting has expanded rapidly in the last decade leading to a huge range of reported devices [96], [97]. Most studies focus on zinc oxide, as its nanostructures are formed relatively easily using low temperature methods [98], unlike many ferroelectrics which require high temperature processing. In addition, the nanostructures are crystallographically aligned and non-ferroelectric, and therefore do not require poling. However, more recently other well-known materials have been investigated for nanostructured energy harvesters including lead zirconate titanate (PZT) and barium titanate, with the potential for higher power outputs due to their higher piezoelectric coefficients. This review summarises the work to date on nanostructured piezoelectric energy harvesters, commonly called nanogenerators, starting with early reports of piezoelectric output from single strained ZnO nanorods, and moving through the use of nanorod arrays, flexible substrates and alternative materials and nanostructures. Applications that have been demonstrated for devices are summarised, and the future prospects for this field are considered. Before embarking on the development of any nanostructured energy harvesters a thorough understanding of piezoelectric and ferroelectric materials – and the broader energy harvesting field – is essential, for which there are a number of suitable reviews and textbooks [1], [2], [3], [4].