Natural Materials Assembled, Biodegradable, and Transparent Paper

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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

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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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Liu, Z.; Ameer, G. A.; Huang, Y.; Rogers, J. A., Biodegradable Elastomers and Silicon

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Nanomembranes/Nanoribbons for Stretchable, Transient Electronics, and Biosensors. Nano Lett. 2015,

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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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Future Paper Based Printed Circuit Boards for Green Electronics: Fabrication and Life Cycle Assessment.

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(17) Bettinger, C. J.; Bao, Z., Organic Thin-film Transistors Fabricated on Resorbable Biomaterial Substrates. Adv. Mater. 2010, 22, 651-655.

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(19) Yuan, L.; Yao, B.; Hu, B.; Huo, K.; Chen, W.; Zhou, J., Polypyrrole-Coated Paper for Flexible

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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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Bloking, J. T.; McGehee, M. D.; Wågberg, L.; Cui, Y., Transparent and Conductive Paper from

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Nanocellulose Fibers. Energy Environ. Sci. 2013, 6, 513-518.

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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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Mater. 2016, 28, 106-110.

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Nano 2015, 9, 4236-4243.

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Adv. 2016, 2, e1501478.

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Surface Charge Self-recovering Electret Flm for Wearable Energy Conversion in A Harsh Environment.

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Energy Environ. Sci. 2016. DOI: 10.1039/C6EE02135B.

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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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J.; Park, D. W.; Jiang, H.; Lee, J.; Qiu, Y.; Zhou, W.; Cai, Z.; Gong, S.; Ma, Z., High-performance Green

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Flexible Electronics Based on Biodegradable Cellulose Nanofibril Paper. Nat. commun. 2015, 6, 7170.

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Mater. 2007, 19, 821-825.

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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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