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Research Letter

Array pattern effects on the voltage output of vertically aligned BaTiO3 nanotubular flexible piezoelectric nanogenerator Camelle Kaye A. Aleman, James Albert B. Narvaez, Gabriel Drix B. Lopez, and Candy C. Mercado, Department of Mining, Metallurgical, and Materials Engineering, University of the Philippines Diliman, Quezon City 1100, Philippines Address all correspondence to Candy C. Mercado at [email protected] (Received 30 October 2019; accepted 23 June 2020)

Abstract The advancement of lead-free piezoelectric nanogenerators (PENGs) for flexible electronics necessitates designing more efficient systems for improved energy storage capacity. In this light, the effects of patterning BaTiO3 nanotubes within PENG on the electromechanical properties of the device were investigated. The PENGs comprised a sandwich structure of Ti–BaTiO3–graphite–Ti encapsulated in polydimethylsiloxane. Four patterns of vertically aligned BaTiO3 nanotubes were synthesized via the hydrothermal conversion of selectively-anodized TiO2 nanotubes. The highest output voltage reached up to 1.9 V. Decreasing the nanotube array spacing and pattern diameter increased the lateral displacement of BaTiO3 therefore, increasing the output voltage of the device.

Introduction Energy-harvesting materials allow the storage of varying energy forms which are, thereafter, transformed into electrical energy.[1] Materials that are scoped within the nanoscale inherently providing the energy-harvesting capacity are referred to as nanogenerators. Specifically, nanomaterials that collect energy from mechanical vibration are collectively denoted as piezoelectric nanogenerators (PENGs).[2] Several approaches to improve the electromechanical response of PENGs include compositional engineering where component fractions are adjusted to obtain favorable material characteristic(s), and structural engineering wherein the micro and/or nanoscale structures are altered to change the overall material properties. Significant developments in electromechanical properties have been achieved by developing various structural engineering techniques including templated grain growth,[3] grain size optimization,[4] domain engineering,[5] and piezoelectric nanogenerator design.[6] In designing the PENG device, one-dimensional perovskites, particularly the nanotube structure, are known to be advantageous for nanogenerator applications due to their enhanced piezoelectric coupling coefficient along the anisotropic direction—a crucial element in enhancing the output power of PENG devices.[7,8] More importantly, PENGs capitalize on the desirable mechanical and electrical properties of ferroelectric materials such as perovskite-structured metal oxides.[9] At present, piezoelectric devices in commercial use are commonly made up of lead zirconate titanate (Pb-Zr2O3-TiO2 or PZT). The use of PZT, however, is limited due to environmental and health concerns.[2] Lessening pollution by avoiding the

use of Pb-based substances has bolstered the present initiative to research and develop