Piezoelectricity Enhancement of Nanogenerators Based on PDMS and ZnSnO3 Nanowires through Microstructuration

Abstract

The current trend for smart, self-sustainable, and multifunctional technology demands for the development of energy harvesters based on widely available and environmentally friendly materials. In this context, ZnSnO₃ nanostructures show promising potential because of their high polarization, which can be explored in piezoelectric devices. Nevertheless, a pure phase of ZnSnO₃ is hard to achieve because of its metastability, and obtaining it in the form of nanowires is even more challenging. Although some groups have already reported the mixing of ZnSnO₃ nanostructures with polydimethylsiloxane (PDMS) to produce a nanogenerator, the resultant polymeric film is usually flat and does not take advantage of an enhanced piezoelectric contribution achieved through its microstructuration. Herein, a microstructured composite of nanowires synthesized by a seed-layer free hydrothermal route mixed with PDMS (ZnSnO₃@PDMS) is proposed to produce nanogenerators. PFM measurements show a clear enhancement of d₃₃ for single ZnSnO₃ versus ZnO nanowires (23 ± 4 pm/V vs 9 ± 2 pm/V). The microstructuration introduced herein results in an enhancement of the piezoelectric effect of the ZnSnO₃ nanowires, enabling nanogenerators with an output voltage, current, and instantaneous power density of 120 V, 13 μA, and 230 μW·cm^-2, respectively. Even using an active area smaller than 1 cm², the performance of this nanogenerator enables lighting up multiple LEDs and other small electronic devices, thus proving great potential for wearables and portable electronics.

Keywords: PDMS; ZnSnO3; micro-structuration; nanogenerator; nanowires; piezoelectricity.

Figure 1

(a) XRD patterns of PDMS, ZnSnO₃@PDMS film, and ZnSnO₃ nanowires before mixing with PDMS. The identification was following ICDD card #28-1486 as explained in ref (24). (b) SEM image of ZnSnO₃ nanowires.

Figure 2

Atomic force microscopy to characterize the piezoresponse of individual ZnO and ZnSnO₃ nanowires: (a) nanowire topographies of ZnO and ZnSnO₃ obtained in noncontact mode (the red spot indicates the area that was contacted in piezoresponse measurements); (b) contact mode tip oscillation as a function of tip-bias ac-voltage to extract the effective piezocoefficient d₃₃. (c) Piezoelectric response of ZnSnO₃@PDMS and only PDMS for a pushing force of 10 N. The inset shows the output of ZnSnO₃@PDMS with a greater detail, highlighting the peaks corresponding to the pressing and releasing of the composite.

Figure 3

Fabrication steps of a microstructured nanogenerator. The inset is a photograph of one nanogenerator.

Figure 4

Output (a) voltage and (b) current generated from the ZnSnO₃@PDMS device with different configurations: unstructured films, microstructured films with aligned or misaligned microcones with a gap between cones of 0 or <100 μm. Output (c) voltage and (d) current generated from only PDMS and ZnSnO₃@PDMS microstructured films devices with aligned microcones with a gap between cones of <100 μm. The circles represent average values with standard deviations (of 4–6 measurements) for positive and negative peaks. Abbreviations: A—aligned, M—misaligned, G—gap. (e) Proposed schematics of force deformation for a microstructured and unstructured device and photographs showing the cross section of the devices before and after pushing force. Note that the schemes are not at scale. T (1.2 mm) and T′ (940 μm) and t (782 μm) and t′ (724 μm) are the thicknesses of the microstructured/unstructured ZnSnO₃@PDMS films before and after pushing, respectively.

Figure 5

Output (a,b) voltage and (c,d) current of the nanogenerator with the optimized configuration, generated by applying a human force from 15 to 50 N and over 100 N using a pen (inset) to deliver the stimulus.

Figure 6

Output (a) voltage and (b) current generated from the nanogenerator with the optimized configuration for 12,000 cycles (100 min).

Figure 7

Output (a) voltage and current and (b) instantaneous power generated from the ZnSnO₃@PDMS microstructured films with the optimized configuration under several load resistances. Photographs of a device directly connected to (c) a single white LED and (d) five blue LEDs in series, with the respective insets showing the LED/LEDs off and on (driven by the energy of the nanogenerator). Photographs of (e) an electronic humidity sensor and (f) a digital watch being powered up by a 10 μF capacitor previously charged by the nanogenerator.