Triboelectric Nanogenerator Based on the Internal Motion of Powder

When the freestanding powder starts to move upward toward the top electrode, the electric potential ..... circuit (BQ25504) of the types used for the ...
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Triboelectric Nanogenerator Based on the Internal Motion of Powder with a Package Structure Design Daewon Kim, Yura Oh, Byeong-Woon Hwang, Seung-Bae Jeon, Sang-Jae Park, and Yang-Kyu Choi* School of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea S Supporting Information *

ABSTRACT: Harvesting the ambient mechanical energy that is abundant in the living environment is a green technology which can allow us to obtain an ecofriendly and sustainable form of energy. Here, we report a powder-based triboelectric nanogenerator (P-TENG) using polytetrafluoroethylene powder as a freestanding triboelectric layer. By employing powder, which has fluid-like characteristics, the device is able to harvest random vibrational energy from all directions and can be fabricated regardless of the size or shape of its container. Notably, this device shows excellent durability against mechanical friction and immunity against humidity. It is also capable of powering 240 green LEDs and charging a commercial energy-harvesting battery. The P-TENG is expected to be applicable as an energy harvester in self-powered systems for the upcoming Internet-of-Things era. KEYWORDS: powder, triboelectric nanogenerator, three-dimensional vibration, polytetrafluoroethylene, freestanding triboelectric layer, durability, humidity

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freestanding feature. A triboelectric generator that utilizes an internal polymer ball has been reported, with research showing that it is capable of multidirectional energy harvesting.35,36 Here, in this work, we demonstrate for the first time a triboelectric generator which uses a polymer powder as a triboelectric material. In contrast to the previous device that employs a bulk polymer ball as its moving material, the powderbased triboelectric nanogenerator (P-TENG) offers tremendous benefits with fluid-like characteristics of powder, as will be discussed further in the following section. It works in a freestanding mode, in which the powder moves between two electrodes as a freestanding triboelectric layer. The fabricated PTENG can scavenge energy from mechanical vibrations in the x-, y-, and z-directions. The electrical output of the fabricated device was measured, with the result showing an open-circuit voltage as high as 245 V and a short-circuit current of 9.5 μA, corresponding to a peak power density of 1.2 W/m2 in the case of z-direction harvesting. Moreover, immunity to humidity due to the package-typed structural and compositional characteristics of the device is demonstrated. The fabricated device also shows reliable durability because microsized powder is free from abrasion, which is not the case if a solid plate form is used

n recent years, energy-harvesting technology has been extensively developed, thus allowing self-powered electronics such as mobile electronics and human-bodyimplantable devices to be realized, all of which would be impractical if the power sources of these devices must be replaced or recharged regularly. To date, various types of energy-harvesting techniques based on classic scientific phenomena such as piezoelectric,1−4 thermoelectric,5−7 electrostatic,8−10 and electromagnetic effects11,12 have been investigated to generate electricity from the mechanical or thermal energy in our surrounding environment. Of these means of scavenging mechanical energy, triboelectric energy-harvesting technology based on physical principles in conjunction with triboelectrification and electrostatic induction has recently been suggested.13−17 Its manufacturing is compatible with a highly efficient, low-cost, and eco-friendly manufacturing process.18−34 The direction of mechanical movement which can be harvested by a triboelectric energy generator is usually limited to a single direction, such as the direction perpendicular to the contact surface in the contact-separation mode. This limitation stems from the fact that the surface on which triboelectric charging occurs is a solid plate with a fixed form. If a triboelectrically charged material can move freely in any direction within suitably placed electrodes, it will be possible for a triboelectric energy harvester to generate electrical energy regardless of the direction of movement due to its inherent © XXXX American Chemical Society

Received: October 8, 2015 Accepted: December 22, 2015

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Figure 1. (a) Schematic illustration of the fabricated P-TENG. Two aluminum metal plates are fixed at the top and bottom. A rigid acrylic cylinder is placed between the top and bottom metal plates. The PTFE powder is kept within the cylinder. Four pieces of copper tape are placed around the acrylic cylinder. (b) Scanning electron microscope image of the nanostructure on the aluminum electrode. (c) Magnified image of (b). All scale bars are 500 nm. (d) Digital camera snapshot of an actual P-TENG. A digital image of the PTFE powder is shown in the inset of (d). (e) Spectrum captured by XPS of the Al 2p of the Al nanograss (top) and a bare sample as a control (bottom). (f) X-ray diffraction spectra of the Al nanograss (top) and the bare sample as a control (bottom).

crystal structure of the aluminum nanograss in a heating process, the X-ray photoelectron microscopy (XPS, Al Kα source) spectrum and X-ray diffraction (XRD) patterns were acquired. As shown in Figure 1e, the Al 2p spectrum indicates that the aluminum was oxidized slightly by the hot water, with subsequent significant changes (Al2O3, 74.1 eV). Moreover, the XRD patterns of the Al nanograss and the bare Al indicate no significant changes other than the oxidization of the aluminum by the hot water, as shown in Figure 1f. Due to inherent property of negative triboelectricity, PTFE powder was used as a triboelectric dielectric material; the diameter of the particles was 20 μm, as shown in the inset of Figure 1d. To investigate the effect of nanostructured aluminum on the output performances of the P-TENG, we prepared two samples to compare, one without any modification and the other with nanostructured aluminum. The performances of the P-TENGs were measured under identical conditions. Both were standardized in terms of the vibration setup, which utilized a periodic press and release motion at a frequency of 3 Hz. As shown in Figure S2 in the Supporting Information, the device without any modification showed lower output with an open-circuit voltage of 100 V. Higher output was achieved with nanostructured aluminum with an open-circuit voltage of 180 V. In addition, COMSOL simulation was conducted to compare the electric potential of the two P-TENGs under the same vibrating frequency and compressive force. The results showed that the triboelectric-induced charge potential is significantly increased with the nanostructured aluminum (see Figure S3 in the Supporting Information). This confirms that, under the same experimental conditions, the increased surface morphology of the contact electrode results in an increased contact area between the two materials, yielding a larger amount of triboelectric charge. It is noteworthy that output performance of the P-TENG is largely enhanced by the hydrothermal

as a triboelectric material. Because the powder has no specific shape, the structure of the triboelectric energy harvester can be designed rather freely, which leads to the possibility of small size applications of the harvester with enhanced scalability. In addition, the electricity generated by the P-TENG can directly power 240 green commercial light-emitting diodes (LEDs) at a vibrating frequency of 3 Hz. Given its remarkable performance and the additional advantageous characteristics that stem from the features of the powder, the P-TENG clearly shows its potential as a small-sized effective energy harvester for selfpowered systems.

RESULTS AND DISCUSSION Figure 1a provides a schematic illustration of the fabricated PTENG. Two aluminum metal plates serve as the top and bottom electrodes, also providing a contact surface with a diameter of 5 cm. A rigid acrylic cylinder is fixed between the two metal plates. Additionally, four rectangular pieces of copper tape are attached onto the outer side wall of the cylinder at the same distances from one another, in a configuration of two pairs facing each other. The cylinder contains polytetrafluoroethylene (PTFE) powder (Sigma-Aldrich, USA) with an average diameter of 2 μm; however, it is not fully occupied. Since the fine PTFE particles can be adhered to the top and bottom electrode, due to its triboelectric charge interaction, a relatively large particle size of the PTFE powder is preferred. In order to induce a greater triboelectric charge density due to the enhancement of the contact surface area, nanostructured aluminum is realized using a hydrothermal process by hot water, as shown in Figure 1b (see the experimental details in the Methods section).37,38 Vertical aluminum nanowires were formed uniformly and densely with a diameter of less than 30 nm, as shown in Figure 1c. To investigate how the hydrothermal process affects the chemical properties and B

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Figure 2. Detailed illustration of the electrical energy generation steps in the (a) vertical vibration mode and (b) horizontal vibration mode of the P-TENG.

Figure 3. Basic electrical output characteristics of the P-TENG in the vertical vibration mode. (a) Open-circuit voltage of the P-TENG with a PTFE volume ratio of 50% at a vibration frequency of 3 Hz. (b) Short-circuit current of the P-TENG with a PTFE volume ratio of 50% at a vibration frequency of 3 Hz. (c) Volume ratio dependency of the P-TENG with vertical force of 100 N and a vibration frequency of 3 Hz. (d) Load resistance dependency of the open-circuit voltage and the short-circuit current with a volume ratio of 50%. (e) Load resistance dependency of the power density of the P-TENG with a volume ratio of 50% with vertical force of 100 N and a vibration frequency of 3 Hz.

process despite the fact that aluminum oxide loses electrons less easily than bare aluminum during the contact-electrification process. Actually, the bare aluminum also has a native oxide layer known as alumina (i.e., aluminum oxide). Although the hydrothermal process oxidizes some extra extent of aluminum, it seems that this very thin additional alumina does not seriously influence the amount of triboelectric charge and output performance. The nanostructure, which was formed by the hydrothermal process, is critical to enhance the output performance of the P-TENG due to the largely enhanced contact area even though it was made of the aluminum oxide. The fabricated P-TENG operates in a freestanding mode in which the powder serves as a freestanding dielectric layer and

the metals serve as the counter triboelectric material and as the electrodes. The working mode of the P-TENG can be divided into two types: the vertical vibrating mode (Figure 2a) and the horizontal vibrating mode (Figure 2b). The process of generating electricity when vertically vibrating force is applied to the device is depicted in Figure 2a. Driven by the vertical force, the freestanding powder will make contact and separate from the top and the bottom Al electrode periodically and thus generate alternating current in the load. In the original state, when the powder stays at the bottom due to gravity and comes into contact with the bottom Al electrode, triboelectric charges are generated between the powder and the Al electrode due to the triboelectric effect. According to the triboelectric polarity C

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Figure 4. (a) Electrical output performance of the P-TENG upon three-dimensional vibration. Short-circuit current of the P-TENG with mechanical vibration according to (left) the x-axis, (center) y-axis, and (right) z-axis. (b) Humidity dependency of the normalized output voltage of the P-TENG with a wide range of relative humidity levels from 20 to 85%. (c) Schematics of the small-scale P-TENG. (d) Schematics of the operation principle of the small-scale P-TENG. (e) Digital snapshot of the small-scale P-TENG. (f) Short-circuit current of the small-scale P-TENG, (left) e-1 and (right) e-2.

open-circuit voltage and short-circuit current of the P-TENG were found to be 245 V and 10 μA, respectively, at a volume ratio of 50%, that is, the ratio of the volume of the total powder to the volume of the cylindrical acrylic container, in Figure 3a,b. To find the optimized volume ratio of the powder, different amounts of powder were put into the cylinder. As shown in Figure 3c, at a volume ratio of 50%, the P-TENG generates the largest electrical output. To be more specific, as the volume ratio is increased from 20 to 80%, the open-circuit voltage increases to its maximum value of 245 V at the volume ratio of 50%, past which it decreases as the volume ratio increases further, as shown in Figure 3c. As more powder is placed into the cylinder, the contact force applied by PTFE powder to Al electrodes increases. As reported in previous works, greater contact force increases the contact area, yielding an increase in the triboelectric charge and in the open-circuit voltage.13−17,20 However, with an increase in the amount of powder, the distance between the negatively charged powder and the electrode that the powder approaches during the vibrating cycle decreases. This decreased distance between the two triboelectric materialsthe powder and the Al electrode strengthens Coulomb force and results in an increase in the amount of induced positive triboelectric charge on the electrode. This, in turn, decreases the potential difference between the two electrodes on which the powder vibrates, decreasing the open-circuit voltage, as well. Thus, the optimal volume ratio of 50% is attained, with the trade-offs between the two factors leading to the maximum output. The short-circuit current also increases to its maximum value of 9.5 μA with an increase in the volume ratio, but as the volume ratio increases further, it decreases slightly and tends to become saturated at a

table, Al is more triboelectrically positive than PTFE, giving the powder and the Al electrode negative and positive charges, respectively. When the freestanding powder starts to move upward toward the top electrode, the electric potential difference generated between them drives the electrical flow through the external loads from the bottom to the top electrode. When the powder finally reaches the top electrode, a state of electrostatic equilibrium is reached, stopping the electrical flow. Subsequently, the freestanding powder moves downward to the bottom electrode, breaking the former electrostatic equilibrium. As a consequence, it drives the electrical flow from the top to the bottom electrode, reducing the amount of induced charge. When the freestanding powder and the bottom Al electrode come into contact again, all of the induced charges are neutralized, resulting in electrostatic equilibrium again. The powder again separates from the bottom electrode until it reaches the top Al electrode. This is one full cycle of the electricity generation process of the PTENG in the vertically vibrating mode. The typical working mechanism for the P-TENG when vibrating horizontally is depicted in Figure 2b. As horizontal vibrating motion is applied to the device, the powder comes into contact with one side of the cylinder, inducing positive charges and negative charges on the Cu tape and on the powder, respectively. Subsequently, the powder moves toward the opposite side of the cylinder wall, inducing triboelectric charges on the powder and the Cu tape, placing the cylinder wall between them. The basic characteristics of the P-TENG are shown in Figure 3. For a quantitative analysis, the electrical output measurement of the P-TENG was carried out under vibrating motion with a frequency of 3 Hz generated by an electrodynamic shaker. The D

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ACS Nano value of 8.5 μA, that is, less than the maximum value. The two factors to govern behaviors of the open-circuit voltage similarly affect those of the short-circuit current. However, the decreased distance between the electrode and freestanding triboelectric layer largely increases the short-circuit current, as reported by the previous study.39 On the other hand, the shortened distance strengthens Coulomb force, hence it increases the induced positive triboelectric charges on the electrode. Thereby, the potential difference between the two electrodes decreases, and the short-circuit current decreases accordingly. This counteracts the increment of the short-circuit current beyond the optimal volume ratio, thus the short-circuit current tends to be saturated as the volume ratio of the powder increases further. All of the following measurements were then fixed at this optimal volume ratio of 50%. The output power of the P-TENG was investigated using an external load resistor. The frequency of the shaker remained fixed at 3 Hz. As shown in Figure 3d, the output voltage maintained a constant level at a low resistance level, after which it increases as the resistance increases from 106 to 5 × 108 Ω. The output current follows an opposite trend compared to the output voltage, decreasing from the initial constant value as the load resistance increases from 107 to 109 Ω, owing to the ohmic loss. Consequently, the instantaneous peak power (Pd = Ipeak2 × R) is maximized with a value of 1.2 W/m2 at a load resistance of 108 Ω. This indicates that the internal resistance of the fabricated P-TENG is 108 Ω. Thus, when the internal resistance is identical to the load resistance, the P-TENG yields the largest amount of power, as shown in Figure 3e. Figure 4a shows the output voltage and current signals of the P-TENG when vibrating along different coordinate axes (the x-, y-, and z-axes). When the external vibrational force acts on the P-TENG along the x- and y-axes (in the planar direction), it shows the same output current of 4.5 μA. These identical output values of two directions are attributed to the fact that the P-TENG is structurally symmetric in the x- and ydirections. However, when force acts along the z-axis (in the vertical direction), the P-TENG shows greater electrical output, as shown in Figure 4a. The open-circuit voltage of the P-TENG is shown in Figure S5. This occurs because the contact area of the electrode when the P-TENG is operated along the z-axis is larger than that of P-TENG when it acts along the x- and ydirections; hence, more charges accumulate on the interfacial surface. As a result, the P-TENG yields enhanced electrical output. Moreover, when the external force acts along the z-axis, direct contact between the electrode and powder occurs. In contrast, when it is applied along the x- and y-direction, the copper plate and powder do not directly come into contact because the wall of the acrylic cylinder blocks any direct contact between the two materials. This results in an increased distance between the two triboelectric materials. The increased distance weakens the Coulomb force, which leads to electrostatic induction. Thus, fewer electrons flow through the electrode, yielding less electricity compared to the case of direct contact. Moreover, while the x- and y-axis vibrations create symmetrical current amplitudes, the z-direction generates asymmetrical current amplitude with a larger value, as the motion of the shaker is affected by gravity when it is operated along the z-axis. Gravity accelerates the downward motion of the shaker and retards its upward motion. Because greater force is applied during the downward motion, increased triboelectric charges arise, as do larger current peaks, whereas the opposite case arises for the upward motion, resulting in asymmetry of the

current curves. The result of this three-dimensional energyharvesting method proves the capability of the P-TENG to harvest energy from mechanical vibration regardless of the vibrating direction. This is due to the freely moving characteristic of the powder, given that the powder possesses a fluid-like characteristic and can occupy any empty space created inside the cylinder. Thus, when vibrating motion occurs, the powder readily vibrates along the same direction as the cylinder, whereas such behavior would not be possible for a hard, solid plate made of PTFE film. This is a well-known limitation of a triboelectric generator composed of a solid polymer plate. Thus, the P-TENG is an effective energy harvester due to the unique feature of being able to scavenge energy from any direction in the ambient environment. To investigate the relationships between the electric output of the P-TENG and the humidity of the surrounding air, a humidity test was performed at different relative humidity (RH) levels. Previous reports usually showed humidity dependency of the electrical outputs. However, as shown in Figure 4b, even when varying the RH from 20 to 85%, the output open-circuit voltage of the P-TENG remained nearly constant. Generally, when water penetrates into the triboelectric layer, it forms a water layer on the surface. When such a water layer is created, the induced triboelectric charges are discharged by the increased surface conductivity. However, the fabricated P-TENG is completely encapsulated by the acrylic cylinder; thus, the water in the surrounding air cannot penetrate into the device. Moreover, the PTFE itself harnesses hydrophobic characteristic, improving the humidity immunity further. Thereby, the P-TENG can maintain its constant electrical output even under a high RH condition. The fluid-like characteristic of the powder removes restrictions on the geometric design of the container, making it possible for the device to be fabricated at an arbitrarily small size. To demonstrate this capability, a straw with a diameter of 6 mm was cut into two cylinders. The heights of the two parts are 2.4 and 4.6 cm, as shown in Figure 4c. Powder was added and filled each cylinder at the optimal volume ratio of 50%. As shown in Figure 4c, pieces of copper tape, which serve as the electrodes, were also attached around the top and bottom sides of the straw cylinder wall, as shown in Figure 4c. Figure 4d illustrates a cross-sectional view of the straw cylinder. When vertical motion is applied to the cylinder, the powder in it periodically comes into contact with and separates from the top and bottom electrodes, driving the current flow between them. At a vertical vibrating frequency of 3 Hz, each cylinder (e-1 and e-2 in Figure 4e) showed a current peak of 0.1 and 0.4 μA, respectively, in Figure 4f. As mentioned earlier, asymmetrical current peaks of each cylinder arise due to gravity, which is in parallel with the vertical motion. Considering the small volume of the straw cylinder, the generated current peaks are fairly large, corresponding to a power density of 0.5 W/cm3. Although a triboelectric nanogenerator utilizing liquid instead of powder also allows small-sized fabrication, it shows comparatively low electric output. In this regard, the fabricated P-TENG with a small size is advantageous for portable devices.40,41 A frequency response is an important factor in the mechanical vibration-based energy harvester. The P-TENG with the volume ratio of 50% was tested with various vibration frequencies from 1 to 300 Hz in the vertical vibration mode, as shown in Figure S6. Both output voltage and current show the resonance approximately at the condition of from 5 to 20 Hz. E

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ACS Nano The open-circuit voltage and short-circuit current show relatively wide bandwidth (full width at half-maximum) of 100 and 220 Hz, respectively. The P-TENG can be practically appropriate for harvesting mechanical energy from the vibration that exists in daily life, such as human motion (sub-30 Hz). Related to this issue, the energy-harvesting capability at an arbitrary direction is also another advantageous feature of the proposed device. The output performance, which is dependent on a tilted vibration angle, is shown in Figure S7. As the tilted angle increases, the effective contact area between the PTFE and the top and bottom electrode is accordingly decreased. Therefore, output voltage and current are decreased as expected. To investigate the stability of the P-TENG, we continuously ran the P-TENG over 1 week, corresponding to 7 × 106 cycles, at a vibrating frequency of 10 Hz. As shown in Figure 5a, the generated output current of 10 μA did not show any degradation after 7 × 106 cycles. The fabricated device showed this reliable durability since the microsized powder does not undergo abrasion, which is not the case if a solid plate is used as triboelectric material. This excellent durability makes the PTENG advantageous as a sustainable power source. To further demonstrate the capability of the fabricated P-TENG as a sustainable power source, it was connected to green commercial LEDs. When the P-TENG was shaken at a vibration frequency of 3 Hz, 240 green LED bulbs were lit instantly (Supporting Information Video S1). Figure 5b shows a photograph of the commercial LEDs. Even when the light of the laboratory room was not switched off, the light emitted from the LEDs was easy to see. The inset of Figure 5b shows the LEDs when the room light was switched off. To demonstrate the charging behavior by the fabricated PTENG, a 22 μF capacitor was used for electric energy storage. The charging curve of the capacitor charged by the power generated from the P-TENG is shown in Figure S6 in the Supporting Information. The curve confirms that the P-TENG requires 10 s to charge the 22 μF capacitor to 3 V. The PTENG was also able to power up a management circuit (BQ25504) of the types used for the operation of commercial smart phones, as shown in Figure 5c,d. The feasibility of the PTENG to be commercialized is demonstrated by this result.

CONCLUSION In summary, we presented a triboelectric nanogenerator that employs powder as a triboelectric material for the first time. The electrical characteristics of the fabricated device were investigated. Due to the fluid-like characteristics of the powder, the proposed device is capable of effectively scavenging mechanical vibrating motion regardless of the direction, and it delivers a maximum voltage of 245 V, a maximum current of 9.5 μA, and maximum power of 1.2 W/m2. In addition, the PTENG shows immunity against humidity and excellent durability against mechanical friction. Moreover, since it can be fabricated into a small size with a large power density, the TENG is applicable as an energy supplier for portable electronics and as an energy harvester for self-powered systems.

Figure 5. (a,b) Digital snapshot of the powering of 240 green LEDs via vertical vibration under a dimly lit and (inset) dark environment. (c) Commercial energy-harvesting circuit for an energy-harvesting device with an output voltage of 3.5 V (BQ25504, Texas Instruments). (d) Plot of temporal charging by the P-TENG with a commercial energy-harvesting circuit.

METHODS Fabrication of the Nanostructure on an Aluminum Plate for Electrodes. Round aluminum plates (diameter = 5 cm, thickness = 1 mm) were cleaned with isopropyl alcohol, acetone, and deionized water with mild ultrasonication. The Al plates were then dried with nitrogen gas using a blow gun. The deionized water on a hot plate was

heated to 80 °C. Afterward, the prepared Al plates were immersed in the hot water for 2 h (see Figure S1 in the Supporting Information). F

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ACS Nano After being washed with deionized water at room temperature, the nanostructures on aluminum plates were formed. Characterization. Morphology of the nanostructured aluminum electrode was observed using a scanning electron microscope (Magellan 400) at an operation voltage of 10 kV. Chemical compositions of nanostructured aluminum electrode were analyzed by XPS (Thermo VG Scientific). Electrical Measurement. The electrodynamic shaker converts electric signals into the mechanical motion of a rigid cylinder, which enables the contact and separation behavior of the internal polymer powder. The voltage and current generated by the fabricated powdertype triboelectric nanogenerator were measured by a Keithley 6514 system electrometer and a SR570 low-noise current amplifier from Stanford Research Systems.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b06329. Fabrication procedure of nanostructured aluminum by hot water, effect of nanostructured aluminum on output performance, COMSOL simulation for visualization, switching test, and output voltage in the horizontal vibration mode (PDF) Movie clip showing the real-time green 240 LED operation with serial connection by the P-TENG (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was partially supported by Open Innovation Lab Project from the National NanoFab Center (NNFC) and the End Run Project funded by the Ministry of Science, ICT & Future Planning. It was also supported by the Center for Integrated Smart Sensors funded by the Ministry of Science, ICT & Future Planning as part of the Global Frontier Project (CISS-2011-0031848). REFERENCES (1) Wang, Z. L.; Song, J. Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays. Science 2006, 312, 242−246. (2) Lin, Y. F.; Song, J.; Ding, Y.; Lu, S. Y.; Wang, Z. L. Piezoelectric Nanogenerator Using CdS Nanowires. Appl. Phys. Lett. 2008, 92, 022105. (3) Cha, S. N.; Seo, J.−S.; Kim, S. M.; Kim, H. J.; Park, Y. J.; Kim, S.−W.; Kim, J.−M. Sound-Driven Piezoelectric Nanowire-Based Nanogenerators. Adv. Mater. 2010, 22, 4726−4730. (4) Francioso, L.; De Pascali, C.; Farella, I.; Martucci, C.; Creti, P.; Siciliano, P.; Perrone, A. Flexible Thermoelectric Generator for Ambient Assisted Living Wearable Biometric Sensors. J. Power Sources 2011, 196, 3239−3243. (5) Kim, S. J.; We, J. H.; Cho, B. J. A Wearable Thermoelectric Generator Fabricated on a Glass Fabric. Energy Environ. Sci. 2014, 7, 1959−1965. (6) Kim, S. J.; We, J. H.; Kim, J. S.; Kim, G. S.; Cho, B. J. Thermoelectric Properties of P-type Sb2Te3 Thick Film Processed by a Screen-Printing Technique and a Subsequent Annealing Process. J. Alloys Compd. 2014, 582, 177−180. G

DOI: 10.1021/acsnano.5b06329 ACS Nano XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsnano.5b06329 ACS Nano XXXX, XXX, XXX−XXX