Flexible Nanogenerators Based on Graphene Oxide Films for Acoustic Energy Harvesting

Abstract

Harvesting ubiquitous mechanical energy, an important energy resource, directly from the environment has been proposed as an effective approach to powering nanodevices. [1, 2] Numerous piezoelectric nanogenerators, such as zinc oxide (ZnO) nanowires, [3] indium nitride (InN) nanowires, [4] lead zirconate titanate nanofibers, [5] poly(vinylidene fluoride) nanofibers, [6] and cadmium sulfide (CdS) nanowires [7] have been explored for converting mechanical energy into electricity. For practical applications, low-cost, high-efficiency nanogenerators are demanded that can be fabricated on flexible substrates and require only simple integration processes.Graphene oxide (GO), a derivative of graphene, consists of a hexagonal ring of carbon network having both sp 2 -and sp 3 -hybridized carbon atoms bearing hydroxyl and epoxide functional groups on basal planes, as well as carbonyl and carboxyl groups at the edges of the sheet. [8,9] Those functional groups can extensively modify the electronic structure and chemical properties of GO, [10][11][12] enabling various applications. [14][15][16][17][18][19][20][21][22][23][24] Here, we report the exciting application of GO as a flexible, high-efficiency nanogenerator, realized through oxygen-containing functional groups which enable GO to store charges and harvest acoustic energy.In this study, GO exfoliated from a modified Hummer method [13,14] was used to fabricate a nanogenerator, which could convert acoustic energy to electricity at a high conversion efficiency of 12.1 %. The induced mean current is sensitively dependent on the pH values of the suspensions used to prepare the GO films. The findings reveal the exciting potential of GO for fabricating nanogenerators for energy harvesting, as well as a novel avenue for nanoelectronic applications.GO was synthesized from expandable graphitic flakes using the modified Hummer method. [13,14] Graphene was purchased from Sigma-Aldrich and used as received. The crystal structure of the samples was characterized with transmission electron microscopy (TEM, FEI Tecnai F20, 200 kV). Raman spectra were recorded using a confocal microprobe Raman system (HR800, Jobin Yvon) using a 633 nm HeNe laser. The electronic structure of carbon was characterized with X-ray photoemission spectroscopy (XPS, Kratos Axis UltraDLD, monochromatized Al K a source) operated at a base pressure of 5 10 À10 Torr. Atomic force microscopy (AFM) measurements were conducted with a Mutilmode V AFM system (Veeco). All the experimental processes are listed in the Supporting Information.The as-prepared GO suspensions from different pH solutions (adjusted with HCl and NaOH) and different concentrations were dropped onto a Teflon tape, followed by drying in a convection oven at 60 8C for 30 min to form GO films. The morphology of the GO films was characterized by scanning electron microscopy (SEM) with a FEI Quanta 200F SEM spectrometer. Microattenuated total-reflection (ATR)-Fourier transform infrared (FTIR) measurements were performed in air using a Bruker FTIR spec...

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