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Natural Materials Assembled Biodegradable and Transparent Paper-Based Electret Generator Xiang Gao, Liang Huang, Bo Wang, Dingfeng Xu, Junwen Zhong, Zhimi Hu, Lina Zhang, and Jun Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12913 • Publication Date (Web): 01 Dec 2016 Downloaded from http://pubs.acs.org on December 5, 2016
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Natural Materials Assembled Biodegradable and Transparent Paper-
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Based Electret Nanogenerator
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Xiang Gaoa‡, Liang Huanga‡, Bo Wanga‡, Dingfeng Xub, Junwen Zhonga, Zhimi Hua, Lina Zhangb* and
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Jun Zhoua*
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a
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Wuhan 430074, China. Email:
[email protected];
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b
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Email:
[email protected].
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‡These authors contributed equally for this work.
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology,
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China.
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KEYWORDS: : biodegradable, transparent, electret nanogenerators, paper-based, self-powered
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ABSTRACT Developing eco-friendly and low-cost electronics is an effective strategy to address the
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electronic waste issue. In this study, transparent cellulose nanopaper (T-paper) and polylactic acid (PLA)
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electret were used to construct a biodegradable and transparent paper-based electret nanogenerator. The
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nanogenerator could be assembled with paper products to form a self-powered smart packaging system
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without impairing the appearance, due to the high transparency and desirable output performance.
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Furthermore, the self-degradation property in the natural soil of the nanogenerator is demonstrated,
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indicating that the nanogenerator is recycled and will not pollute the environment. We anticipate that this
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study will provide new insights to develop eco-friendly power source and paper-based electronics.
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Introduction
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The rapid development of flexible and portable electronics revolutionizes people’s life.1-5 For instance,
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wearable electronics and smart sensors attract growing interest for their specific attempt in health
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monitoring,6-8 wireless communication9 and smart packaging10. However, the fast upgrading of
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electronics has brought a serious problem, that is, the electronic waste, which will result in a waste of
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money and cause pollution to environment.11 Hence, flexible devices entirely built with biodegradable
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and non-toxic materials receive growing attention.12-15 Paper-based electronics with low cost, eco-friendly
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property, high flexibility and light weight may be an appropriate solution to address above issue.16-20
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Specifically, transparent nanopaper (T-paper) consisting of nano cellulose is a promising substrate for
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flexible electronics, by virtue of its high transparence, smooth surface and excellent mechanical
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strength.10,
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(OLED), thin film transistors (TFTs) and antennas, have been successfully assembled on the T-paper.1, 13,
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17, 25
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invented, establishing self-powered paper-based electronic systems.5,
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piezoelectric,28-29 triboelectric5, 27,30 and electret nanogenerators7, 10, 30 can convert irregular mechanical
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energy from ambient environment and human motions into electricity. However, these paper-based
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nanogenerators are not completely recycled or harmless to the environment, as the piezoelectric or
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dielectric materials in the nanogenerators are not biodegradable.5, 10
21-24
A series of flexible electronics, such as touch screens, organic light-emitting diodes
In order to sustainably supply power for above-mentioned devices, paper-based nanogenerators are 20, 26-30
Normally, paper-based
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In this work, a biodegradable and transparent paper-based electret nanogenerator with a basic working
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mechanism electrostatic induction is presented. We successfully demonstrate the design, fabrication,
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output and applications of the nanogenerator. Our designs possesses the following features and
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advantages: (1) the building elements, T-paper and polylactic acid (PLA) electret, are derived from
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natural materials (Figure 1a); (2) T-paper and PLA are flexible and transparent (Figure 1b); (3) the
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whole nanogenerator is invisible and can generate electricity for powering electronics, with potential 2
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applications in self-powered smart packaging (Figure 1c); (4) the nanogenerator is biodegradable and
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harmless to the environment (Figure 1d). This study opens up a new strategy for developing eco-friendly
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power sources, and promotes the progress of paper-based electronics.
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Results and discussion
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The device structure of the biodegradable and transparent paper-based electret nanogenerator is given
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in Figure 2a. Specifically, a PLA/conductive T-paper component and a conductive T-paper component
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were assembled together to form an arch-structured nanogenerator, in which the PLA electret was faced
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to the PEDOT:PSS conductive electrode. PEDOT:PSS was covered on the T-paper surface via a simple
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spin coating method with spin rate of 1000 rpm (Figure S1-I and Figure S2), as the smooth surface and
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hydrophilic features of the T-paper endow itself the ability to be modified by the PEDOT:PSS aqueous
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ink.21, 31 Scanning electron microscope (SEM) images and corresponding element mapping images of the
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conductive T-paper surface reveal the uniformly coating of PEDOT:PSS (Figure 2b). Atomic force
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microscopy (AFM) images indicate that the roughness of the conductive T-paper surface was ~83 nm
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(Figure 2c), which has little effect on the transmittance. The transmittance of the T-paper, conductive T-
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paper and the nanogenerator were ~90%, ~ 87% and ~ 81%, respectively (Figure 2d). A digital
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photograph of the nanogenerator (inset in Figure 2d) also reveals its excellent transparency. Moreover,
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deformation tests under different bending angles (from 30o to 90o, Figure S3a) indicate that the sheet
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resistance of the conductive T-paper almost remained at ~ 0.9 MΩ sq-1 (suitable conductivity for electret
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nanogenerators) for the first bending, first recovery and second bending process (Figure 2e). Meanwhile,
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the conductive T-paper was continuously bent under a 60o bending angle for 1000 cycles. The I-V curves
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remained almost the same, revealing a good stability and flexibility (Figure S3b). A tape test also proves
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the outstanding adhesion between PEDOT:PSS and T-paper (Figure S3c).
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The spin rates during the spin coating process will affect the surface morphology, sheet resistance,
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transmittance and mechanical behavior of the conductive T-paper. When the spin rate increased, the 3
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surface roughness reduced, as indicated in Figure S2. With increase of the spin rate from 500 to 1500
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r/min, the sheet resistance increased from ~ 0.24 MΩ sq-1 to ~ 4.7 MΩ sq-1, respectively (Figure S3d).
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Simultaneously, the transmittance reached the maximum value of ~ 90% when the spin rate was 1000
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rpm (Figure S3d). The trend of transmittance is coincident with the fact that the surface of the nanopaper
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is more compact while the PEDOT:PSS coating thickness on the paper surface decreased with increasing
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spin rate. Moreover, the strength was enhanced with the spin rate increasing (Figure S4).
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A PLA electret film with thickness of 55 µm was adhered to a conductive T-paper to form the
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PLA/conductive T-paper component (Figure S1-II and Figure S5a). Fourier transform infrared (FTIR)
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spectroscopy was employed to identify the molecular structure of PLA. As depicted in Figure S5b, the
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spectrum of the electret used in our device is coincident with the standard spectrum of PLA, confirming
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the normal PLA material in the device.32 Compared with traditional electret materials like
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polytetrafluoroethylene (PTFE), polypropylene (PP) and polyethylene (PE), PLA is a biodegradable
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polyester derived from renewable resources, such as trees, corn starch, tapioca roots or sugarcane.32-34 By
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virtue of the rich oxygen functional groups, PLA can be strongly polarized and charged, making it a well-
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suited natural material to construct eco-friendly electret nanogenerators.35-38
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As our nanogenerator is arch-structured, it can be regarded as a parallel capacitor in ideal analysis,
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ignoring the edge effect. In the original state (Figure 3a-I), the PLA electret in the PLA/conductive T-
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paper component was pre-charged via a corona charging process, which was investigated by measuring
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the surface potential. Specifically, the surface potential dropped rapidly for the first two days and
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remained at ~ 0.97 kV for 32 days (Figure 3b). Because of the existence of the surplus charges (σ0) in the
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PLA electret surface, corresponding induced charges will be generated on the top (σ1) and bottom (σ2)
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PEDOT:PSS electrode. Assuming that the charges are uniformly distributed on the materials surface,
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thus:39
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= −( + )
(1)
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Meanwhile, at any equilibrium state of the nanogenerator, the induced charge in the bottom electrode σ2 is given by:20 = − ⁄( + )
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(2)
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where d1 and εrp are thickness and relative permittivity of the PLA, which are constants. d2 is the
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thickness of air gap between the PLA/conductive T-paper component and the conductive T-paper
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component.
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When pressing the nanogenerator (Figure 3a-II), d2 reduces. According to Equation 2, more σ2 will
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be induced in the bottom electrode. Current flows from the top to the bottom electrode, generating a
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positive current peak (left in Figure 3c). In the releasing process (Figure 3a-III), the d2 variation
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tendency is opposite to the pressing process, thus, a negative current peak can be observed (left in Figure
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3c, black curve). In general, the variation of d2 breaks the electrical potential equilibrium between the
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two electrodes, driving electrons to flow alternately. As a result, alternating electricity is generated by the
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nanogenerator. Switching polarity tests were also carried out to confirm that the measured output signals
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were generated from the nanogenerator rather than from the measurement system (right in Figure 3c,
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blue curve).
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Significantly, the PLA electret determines the output property of the nanogenerator. Under the same
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stimulating conditions, the peak current of the device with PLA reached ~ 1.33 µA, while the device
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without PLA was only ~ 4 nA (Figure 3d).
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The output performances of a biodegradable and transparent paper-based electret nanogenerator with
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effective area of 2×2 cm2 was carefully studied by periodically pressing and releasing with controlled
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amplitude and frequency. Under stimulating amplitude and frequency of 2 mm and 5 Hz, the
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nanogenerator obtained a maximal power of ~ 84.4 µW, with a corresponding load peak current and load 5
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resistance of ~ 0.92 µA and 100 MΩ, respectively (Figure 4a). For a given frequency of 5 Hz, both of the
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load peak current and corresponding transferred charges (△Q) increased step by step with increasing the
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amplitude, from ~ 0.17 µA and ~ 7.11 nC at 0.5 mm to ~ 1.47 µA and ~ 19.29 nC at 2.5 mm, respectively
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(Figure S6a and S6b). On the other hand, for a given amplitude of 2.5 mm, only the load peak current
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increased gradually with increasing the frequency, while the corresponding △Q remained at ~ 19.4 nC
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(Figure S6c and S6d). Moreover, by connecting two nanogenerators in parallel, the output performance
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would be enhanced (Figure S7).
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The excellent stability performance of the nanogenerator is an important factor to ensure its
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applications. In this work, the nanogenerator was continuously stimulated for 54000 cycles, with an
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amplitude and frequency of 2.5 mm and 5 Hz, respectively (Figure S8). For different stimulated cycles,
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the peak current and corresponding △Q remained almost consistent at ~ 1.47 µA and ~ 19.3 nC,
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respectively (Figure 4b). This outstanding stability performance may be attributed to the good charge
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maintaining ability of PLA electret and the robust mechanical property of the nanogenerator.
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Benefited from the high transparency and desirable output performance, our biodegradable and
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transparent paper-based electret nanogenerator can be assembled with paper products, demonstrating its
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typical application in smart packaging. Herein, a nanogenerator was used to power one red light emitting
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diode (LED) in the mouth and two blue LEDs in the eyes of the paper toy, and the specific equivalent
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circuit is given in Figure 4c. As the nanogenerator was invisible, it could be stacked on the surface
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without impairing its appearance (left in Figure 4d). If people pressed the paper toy, the nanogenerator
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was stimulated, generating electricity for lighting up the three LEDs (right in Figure 4d). When the
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nanogenerator was pressed, the voltage across the LEDs was ~ 7.5 V (Figure 4e), which matched the
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threshold voltage of the LEDs (Figure S9). Meanwhile, the peak current through the LEDs reached ~ 18
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µA (Figure 4f). When the nanogenerator was released, the open-circuit voltage generated by the
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nanogenerator was recorded as high as ~ 75 V (Figure S10). Besides, the nanogenerator has the ability to 6
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power other low-power consumption electronics, such as liquid crystal displays (LCD) (Figure 1c). The
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above-mentioned smart packaging system is battery-free, low-cost and simple-structured, and will endow
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paper products with newfangled and unique functions, such as attractive decoration, advertisement, anti-
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theft or anti-fake.
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Note that since our nanogenerator is mainly composed of natural materials, the nanogenerator should
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have the ability to be degraded. In the degradation test, nanogenerators were buried in the natural soil
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without any other treatment. From the digital pictures it can be seen that the degradation process of the T-
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paper was more easy than for the PLA electret (Figure 5a). A porous structure with fungal mycelia on the
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T-paper surface indicates that the microorganisms in the soil directly decompose the T-paper via
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penetrating into the cellulose to obtain nutrients (Figure 5b).40 Digital pictures and weight loss of the
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nanogenerators during the degradation process indicate that T-paper was almost completely degraded
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after 40 days (Figure 5c). After 210 days, a porous structure was observed on the surface of PLA electret
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(Figure 5d). Moreover, tensile strength measurement was employed to investigate the degradation
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property of the PLA electret. The mechanical strength of the PLA electret decreased after 90 days,
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demonstrating the start of structural damage (Figure 5e).41 The above results indicate that the
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biodegradable and transparent paper-based electret nanogenerator can be recycled and will not cause
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pollution in the environment.
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Conclusions
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In summary, we report a biodegradable and transparent paper-based electret nanogenerator based on
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natural materials of T-paper and PLA electret. By virtue of the high transparency and desirable output
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performance, the nanogenerators were assembled with paper products to power low-power consumption 7
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electronics without impairing their appearance, obtaining a self-powered smart packaging system.
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Moreover, the nanogenerator was biodegradable when buried in the natural soil, revealing that it recycled
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and can be harmless to the environment. This study provides a significant approach for developing eco-
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friendly power sources to solve the electronic waste problem, simultaneously expanding the applications
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of paper-based electronics.
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Experimental Section
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Fabrication of T-paper. Cellulose was dissolved in 7 wt% NaOH / 12 wt% urea solution and pre-cooled
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to -12 °C through using a previously reported method to form a 4 wt% transparent cellulose solutions
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within 3 min.42 The solution was then subjected to centrifuging to degas at 7200 rpm for 10 min at 5 oC.
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The transparent solution was immediately cast on a glass plate to give a solution layer with 0.5 mm
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thickness, and then immersed into a coagulation bath of ethanol to give a regenerated cellulose hydrogel.
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The cellulose gels were thoroughly rinsed with deionized water, and then dried in air to give the cellulose
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film. Cellulose T-paper was made by cutting the film into a suitable size.
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Fabrication of conductive T-paper. The transparent conductive nanopaper was prepared via spin
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coating of poly(3,4-ethylenedioxythiophene) polystryrene sulfonate (PEDOT:PSS) on the surface of T-
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paper. The conductive inks was diluted to 10 % with DI water and then spin coatted onto the prepared
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cellulose film with different spinning rates for 2 min. Subsequently, the prepared modified cellulose films
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were dried in air.
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Fabrication of biodegradable and transparent paper-based electret nanogenerator. Firstly, a
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commercial PLA electret film (Changzhou Rong Sunny Advaced Materials Technology company, China)
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was adhered to a conductive T-paper to form a PLA/conductive T-paper component. Then, the PLA film
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was polarized in the direction perpendicular to the film via the corona method under a high voltage of -15
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kV for 5 min. A PLA/conductive T-paper component and a conductive T-paper component were 8
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assembled to fabricate the nanogenerator. Specifically, the PLA side of the PLA/conductive T-paper
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component was faced to the PEDOT:PSS side of the conductive T-paper component, and then two edges
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of the two components were adhered together to obtain an arch-structured nanogenerator with effective
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area of 2×2 cm2.
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Biodegradation tests. Nanogenerators with similar size of which the initial weight was recorded were
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buried about 20 cm in the natural soil. The average values of the temperature and moisture of the soil
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were about 25 oC and 25%, respectively. The tests were observed gradually during 210 days. At different
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time intervals, triplicate specimens for each sample condition were removed from the soil. The samples
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were first rinsed several times with distilled water and dried at room temperature for 2 days under
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vacuum. The degree of degradation could directly be detected follow the change of the weight,
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morphology and mechanical strength.
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Characterization. The morphology of the sample was probed by a field emission scanning electron
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microscope (SEM. FEI Nova Nano450) and an atomic force microscope (AFM, Dimension 3100 Veeco).
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Fourier transform infrared spectra was recorded by a Bruker, Equinox 55 spectrometer. The tensile
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strength measurements were carried out by an electronic universal testing machine (M-4005T, Shenzhen).
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The conductance of the samples was studied by Keithley 2400 sourcemeter. The transmittance of the
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samples were measured by a UV 2550 spectrophotometer (Shimadzu). The corona charging method was
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carried out with high voltage source (DW-N503-4ACDE, Tianjin). The surface potential of the sample
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was detected by an electrometer (EST102, Huajing Beijing, China). In the electrical measurements, the
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periodically pressing-releasing process of the samples was provided by a resonator (JZK, Sinocera,
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China) which was controlled by a swept signal nanogenerator (YE 1311-D, Sinocera, China). The output
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current of the samples were measured by a Stanford low-noise current preamplifier (Model SR570) and
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NI PCI-6259. The output voltage of the samples were measured by a Keithley 6514 electrometer. It
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should be noted that all the electrical measurements were carried out after the surface potential of the
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PLA electret had reached stable states.
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Supporting Information. More detailed information about detailed device fabrication process and
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supporting figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENT
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This work was financially supported by the National Natural Science Foundation of China (51322210,
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51602115, 61501215, 61434001, 20874079, and 81171480), the Major Program of National Natural
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Science Foundation of China (21334005) and the Director Fund of WNLO. The authors thank to the
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National Program for Support of Top-Notch Young Professionals, the facility support of the Center for
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Nanoscale Characterization & Devices, WNLO-HUST and the Analysis and Testing Center of Huazhong
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University of Science and Technology.
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(16) Liu, J.; Yang, C.; Wu, H.; Lin, Z.; Zhang, Z.; Wang, R.; Li, B.; Kang, F.; Shi, L.; Wong, C. P.,
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Solid-State Energy Storage. Energy Environ. Sci. 2013, 6, 470-476. (20) Zhong, Q.; Zhong, J.; Hu, B.; Hu, Q.; Zhou, J.; Wang, Z. L., A Paper-based Nanogenerator as A Power Source and Active Sensor. Energy Environ. Sci. 2013, 6, 1779-1784.
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(21) Hu, L.; Zheng, G.; Yao, J.; Liu, N.; Weil, B.; Eskilsson, M.; Karabulut, E.; Ruan, Z.; Fan, S.;
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(22) Fang, Z.; Zhu, H.; Bao, W.; Preston, C.; Liu, Z.; Dai, J.; Li, Y.; Hu, L., Highly Transparent Paper with Tunable Haze for Green Electronics. Energy Environ. Sci. 2014, 7, 3313-3319. (23) Zhu, H.; Fang, Z.; Preston, C.; Li, Y.; Hu, L., Transparent Paper: Fabrications, Properties, and Device Applications. Energy Environ. Sci. 2014, 7, 269-287. (24) Jin, J.; Lee, D.; Im, H. G.; Han, Y. C.; Jeong, E. G.; Rolandi, M.; Choi, K. C.; Bae, B. S., Chitin Nanofiber Transparent Paper for Flexible Green Electronics. Adv. Mater. 2016, 28, 5169-5175. (25) Zhu, H.; Xiao, Z.; Liu, D.; Li, Y.; Weadock, N. J.; Fang, Z.; Huang, J.; Hu, L., Biodegradable Transparent Substrates for Flexible Organic-light-emitting Diodes. Energy Environ. Sci. 2013, 6, 2105. (26) Hu, Q.; Wang, B.; Zhong, Q.; Zhong, J.; Hu, B.; Zhang, X.; Zhou, J., Metal-free and Non-fluorine Paper-based Generator. Nano Energy 2015, 14, 236-244.
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(27) Li, J.; Zhang, C.; Duan, L.; Zhang, L. M.; Wang, L. D.; Dong, G. F.; Wang, Z. L., Flexible
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Organic Tribotronic Transistor Memory for a Visible and Wearable Touch Monitoring System. Adv.
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(40) Jung, Y. H.; Chang, T. H.; Zhang, H.; Yao, C.; Zheng, Q.; Yang, V. W.; Mi, H.; Kim, M.; Cho, S.
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Figure 1. Features and advantages of the biodegradable and transparent electret nanogenerator. (a) PLA
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and cellulose transparent paper (T-paper) were derived from the nature; (b) These transparent "green
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materials" can be assembled to an invisible electret nanogenerator; (c) The nanogenerator can generate
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electricity for powering electronics and be degraded in the soil; (d) The degraded nanogenerator is finally
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cycled back to the nature without detrimental environmental effects.
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Figure 2. Fabrication of the biodegradable and transparent electret nanogenerator. (a) Device structure of
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the nanogenerator; (b) SEM image and corresponding elements mapping image of the conductive T-paper
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surface; (c) AFM image of the conductive T-paper surface; (d) The transmittance of T-paper, conductive
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T-paper and nanogenerator, respectively. Insert shows the digital picture of the nanogenerator; (e) the
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sheet resistance of the conductive T-paper for the first bending, first recovery and second bending process,
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respectively. Inset shows the digital picture for T-paper.
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Figure 3. Working mechanism of the biodegradable and transparent electret nanogenerator. (a) Schematic
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diagram indicating the working process of a nanogenerator, when it is at (I) original state, (II) pressing
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state and (III) releasing state; (b) Surface potential decay of the PLA electret of the PLA/conductive T-
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paper component; (c) Corresponding current-time curves for a nanogenerator when it is forward-
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connected and reverse-connected to the measurement system; (d) Output currents for the devices with and
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without PLA electret.
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Figure 4. Output performances and the biodegradable and transparent electret nanogenerator and
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its application in smart packaging. (a) Load peak currents and corresponding peak power curves
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as a function of the load resistance, under stimulating amplitude and frequency of 2 mm and 5
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Hz; (b) Output stability measurement for the nanogenerator, under stimulating amplitude and
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frequency of 2 mm and 5 Hz; (c) Equivalent circuit for the smart packaging system; (d) Digital
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pictures of smart packaging system; (e) Voltage across and (f) current went through the LEDs,
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when they were lit up.
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Figure 5. Degradation test. (a) Digital pictures indicating the degradation process; (b) SEM
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image of the T-paper surface after degrading for 21 days; (c) Weight loss of the nanogenerator
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depended on degradation time in soil; (d) SEM image of the PLA electret surface after degrading
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for 210 days; (e) Tensile strength measurement of the PLA depended on degradation time in soil.
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