Self-Sterilized Flexible Single-Electrode Triboelectric Nanogenerator

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Self-Sterilized Flexible Single-Electrode Triboelectric Nanogenerator for Energy Harvesting and Dynamic Force Sensing Huijuan Guo,† Tao Li,† Xiaotao Cao,† Jin Xiong,‡ Yang Jie,†,§ Magnus Willander,† Xia Cao,*,†,§ Ning Wang,*,∥ and Zhong Lin Wang*,†,⊥ †

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, National Center for Nanoscience and Technology (NCNST), Beijing 100083, China ‡ Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China § Research Center for Bioengineering and Sensing Technology, University of Science and Technology Beijing, Beijing 100083, China ∥ Center for Green Innovation, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China ⊥ School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States S Supporting Information *

ABSTRACT: Triboelectric nanogenerators (TENGs) offer great opportunities to deploy advanced wearable electronics that integrate a power generator and smart sensor, which eliminates the associated cost and sustainability concerns. Here, an embodiment of such integrated platforms has been presented in a graphene oxide (GO) based single-electrode TENG (S-TENG). The as-designed multifunctional device could not only harvest tiny bits of mechanical energy from ambient movements with a high power density of 3.13 W·m−2 but also enable detecting dynamic force with an excellent sensitivity of about 388 μA·MPa−1. Because of the twodimensional nanostructure and excellent surface properties, the GO-based S-TENG shows sensitive force detection and sound antimicrobial activity in comparison with conventional poly(tetrafluoroethylene) (PTFE) electrodes. This technology offers great applicability prospects in portable/ wearable electronics, micro/nanoelectromechanical devices, and selfpowered sensors. KEYWORDS: triboelectric nanogenerator, graphene oxide, wearable electronics, dynamic force response, sterilizing effect fabricated on flexible substrates via simple integration processes with low cost and high efficiency for moving objects.17−19 Graphene oxide (GO) consists of a hexagonal ring of carbon networks containing both aromatic (sp2) and aliphatic (sp3) domains. The abundance of reactive oxygen functional groups and high specific surface renders it a good candidate for use in energy-related materials, sensors, and biomedical applications.20−23 Very recently, GO has been studied in the context of energy harvest and bactericidal performance.24−27 It has been demonstrated that the electronic structure of GO can be intensively modified, even be transformed from insulator into a semiconductor. These distinct features make it promising

n recent years, sustainable and flexible energy supply/ storage devices have been urgently needed to complement advances currently being made in flexible electronics.1−5 Triboelectric nanogenerators (TENGs) emerged as not only an energy conversion technology6−8 but also a multifunctional platform for developing self-powered sensor systems and flexible/wearable electronics.9−14 On the basis of the triboelectric effect and electrostatic induction, TENGs harvest mechanical energy from the environment and convert it into electricity to power the integrated electronic devices directly.15−17 However, most TENGs are composed of two electrodes, which incorporate two parts of triboelectric materials with different triboelectric polarities for electric induction to form a closed circuit for electron flow.13−16 The single-electrode-based TENG (S-TENG) techniques are preferred for wearable electronics because they can be easily

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© 2017 American Chemical Society

Received: November 3, 2016 Accepted: January 5, 2017 Published: January 5, 2017 856

DOI: 10.1021/acsnano.6b07389 ACS Nano 2017, 11, 856−864

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Figure 1. (a) Schematic diagram and (b) photograph of the GO-enhanced flexible single-electrode TENG. (c) Thickness of the S-TENG. (d− e) Photograph and cross section of the GO film. (f) Weight of the S-TENG.

TENG and GO film is less than 0.6 g (Figure 1f) and 0.05 g (Figure S1c) (4 cm × 4 cm), respectively. Morphology of the as-prepared GO sheets is shown in Figures 2a−c. The selected-area electron diffraction (SAED) pattern taken from the GO sheet (inset of Figure 2a) reveals the original graphene lattice.29 The wrinkled GO sheets are folded together with an interlayer spacing of about 0.2 μm (Supporting Information, Figure S1d). X-ray photoelectron spectroscopy (Figure 2d) shows the C 1s spectrum of the graphene oxide. A considerable degree of oxidation can be observed from peaks at 284.8, 286.5, 287.4, and 288.9 eV, which correspond to CC/C−C, C−O, CO, and OC− O, respectively.34,35 These results are consistent with the corresponding oxygen contents revealed by FTIR (Figure S2). The intense and narrow (002) peak in the XRD spectra (Figure 2f) suggests good crystallization.34 The operation principle of the as-designed S-TENG is schematically depicted in Figure 3. Interestingly, GO film is proven to be positively charged due to its repulsion to a glass rod which has been rubbed against silk. At the original state before the contact of fingers (active object) and the friction layer (GO), there is no charge transferred or electric potential produced (Figure 3a). When the fingers and GO film fully contact each other (Figure 3b), charges are produced and transferred from the surface of GO film to the fingers, generating equivalent negative and positive charges on the two surfaces. With the separation between GO film and fingers (Figure 3c), electrons would transfer from the ground electrode to the Al electrode because of their different potentials. This electric current stops flowing when an electrostatic equilibrium is reached by increasing the distance of two contact surfaces (Figure 3d). As the fingers approach the GO film again, the

interfacial material for TENGs to elevate output voltage and overcome the high leakage currents.28−30 In this paper, the as-fabricated GO-based S-TENG is designed with only one triboelectric layer because the surface of objects can serve as another triboelectric layer. Thus, the most distinctive feature of S-TENG is its simple construction, which can be easily integrated onto ultrathin flexible substrates for application in wearable electronics. By utilizing the ultrathin flexible GO films, the as-fabricated S-TENGs present not only much higher energy harvesting efficiency but also great mechanical durability and low weight. Also, a wide linear range in response to tiny dynamic pressure is also demonstrated with a sensitivity of about 388 μA·MPa−1. Thus, the as-designed single-electrode TENGs can be used as both energy collector/supplier as well as self-powered active force sensor. The sterilizing effect and flexibility also increase its application potential in wearable electronics.

RESULTS AND DISCUSSION The structure of the as-designed S-TENG is shown in Figure 1a. Poly(tetrafluoroethylene) (PTFE) was chosen as the substrate material because of its light weight, good workability, and excellent impact strength,31 with a thickness of 0.07 mm. GO was synthesized from purified natural graphite powder using the modified Hummer method32,33 and acts as both a contact surface and triboelectric layer. The grounding electrode was made of an aluminum film, with thickness of 0.03 mm. Parts a and b of Figure 1 show a schematic diagram and photographs of the GO-based S-TENG. The flexibility by bending around corners is shown in Figure 1d and Figures S1a,b. The thickness of the S-TENG and GO film is ∼0.12 mm (Figure 1c) and 0.01 mm (Figure 1e), and the weight of the S857

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Figure 2. (a) TEM image and SAED pattern (inset) of the GO product. (b, c) SEM image of the GO film. (d) C 1s XPS spectrum of GO and the fitted curves. (e) XRD spectrum of GO sample.

electrons flow from the Al electrode to the ground until the fingers and GO fully contact with each other, resulting in a reversed output signal (Figure 3e). Finite element simulations were employed to verify the electric potential distribution in the TENG and the charge transfer between the Al electrode and ground by modulating the relative distance between the hand and a GO film using COMSOL. The model constructed here is based on a GO film and a grounding Al electrode with the same size of 4 cm × 4 cm. The thickness is of 1 mm for GO film and 2 mm for Al, as illustrated in the inset of Figure 3f. The Al electrode was connected to ground and the triboelectric charge density on GO film was assigned as 50 μC·m−2, uniformly. Figure 3f shows the calculated results of the electric potential distribution with inter hand-GO distances of 0, 20, and 50 mm, respectively. When in contact with each other, the electric potentials on both the hand and GO film approach zero. The electric potential difference increases dramatically with increasing separation distance. With a gap distance of 50 mm, the potential difference between the hand and GO film can approach to 2000 V, as shown in Figure 3f. Detailed electrical characterizations have been carried out to investigate the output performance of the as-designed singleelectrode TENGs. The output voltage and current of the STENG were measured using a digital oscilloscope (Agilent DS0 × 2014A) with its inner impedance of 100 MΩ and a Keithley

Model 6514 system electrometer, respectively. Unless otherwise specified, sizes of the S-TENGs used in this work are all 4 cm × 4 cm. A short circuit current (Isc) of 55 μA and an opencircuit voltage (Voc) of 1100 V were obtained with gloved hand patting (Figures 4a,b). Here, the GO-based electrode is connected to the positive probe of the electrometer, and the vertical compressive force of approximately 125 kPa was monitored by a dynamic force sensor. The peak value of Isc (55 μA) corresponds to the half cycle of pressing, and the positive voltage of 1100 V is generated because of the immediate charge separation. The voltage holds at a plateau until the subsequent pressing deformation in the second half cycle. Here, the current density is about 3.5 μA·cm−2 (Figure 4c) and the amount of transferred charges (Q) is about 0.16 μC (Figure 4d). Usually, the effective output power of the TENG depends on the match with the loading resistance. Figure 4e shows the resistance dependence of the short circuit current using an external load resistor varying from 10 Ω to 1 GΩ. As the resistance increases, the current amplitude decreases. The instantaneous power that is delivered onto the load by the TENG is shown in Figure 4f. With the resistance below 0.1 MΩ, the instantaneous power is near 0 and then increases in the resistance region from 0.1 to 10 MΩ. The instantaneous power exhibits a maximum value of 5 mW at the resistance load of 10 MΩ, and the corresponding power density of the TENG is about 3.13 W·m−2. The instantaneous power decreases with 858

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Figure 3. (a) Schematic diagram and (b−e) operation principal of the as-designed flexible S-TENG. (f) Finite element simulation of the potential distribution in the TENG for different gap distances between hand and a GO film (inset: model for the calculation).

material. The GO-enhanced S-TENG gives much higher output currents than the frequently used PTFE, indicating that the GO film contributes significantly to the performance enhancement. On one hand, GO film is positive charged due to the trapping cations in the large dielectric area and the protonation of carboxylate groups, which is apt to gain electrons easily and produces triboelectric current.28,29 On the other hand, due to condensed chemical groups bearing on the GO sheet, the strong space−charge effect, and the large specific surface, the induced electric field should be much higher than other ordinary triboelectrically negative materials.30,37,38 To optimize the output performance and achieve the best matching, the GO-enhanced S-TENG was further measured by patting with bare palm, linear motor with latex glove, and palm with latex glove (F = 125 kPa). The short currents are 6, 48, and 56 μA, respectively (Figure 5c). Because of grease and moisture, the S-TENG powered by bare palm produces the lowest short-circuit current, while the S-TENG powered by the palm in latex glove generates the highest. Meanwhile, the STENG powered by a gentle finger tapping produces a shortcircuit current of 4 μA and can light up 10 light-emitting diodes

loading resistance above 100 MΩ. Those results demonstrate the GO-based single-electrode TENG has excellent output performance in comparison with previous reports.17,19,36 In addition, the output stability of the as-prepared S-TENGs was evaluated, which is significant for their long-term use. Figure S3 shows that the short-circuit current of the TENGs was measured after different work times under the same conditions. This result did not show any obvious decrease after work for 7 days (Supporting Information). To obtain a stable and optimized S-TENG design, different materials are systematically screened for the two triboelectric layers. The as-designed S-TENG was mechanically triggered by a linear motor, which provided dynamic impact with controlled force (F = 37.5 kPa) at a frequency of 5 Hz monitored by a force sensor (Figure S4). Figure 5a shows the S-TENG output performance when contact with different materials of active objects. In another test, a PTFE/Al/PTFE S-TENG was fabricated with the same procedure except that GO was replaced by PTFE (Figure S5), and the electrical performance is shown in Figure S6. Figure 5b demonstrates that the STENG could be significantly affected by the friction layer 859

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Figure 4. Output performance of the as-designed S-TENGs: (a) open-circuit voltage, (b) short-circuit current, (c) transferred charges, and (d) current density. (e) Dependence of the current peak on the external loading resistance. (f) Plot of the instantaneous power versus the loading resistance.

which the linearity cannot be maintained. If the force is further increased, it maybe damage the surface structure of GO. Figure 6d shows the short-circuit current of the as-fabricated GObased S-TENGs rises up with a sharper slope. The slope is 0.24 for the GO-enhanced S-TENG and 0.08 for the PTFE-based STENG. The correlation coefficients (R2) are of 0.999 and 0.039, demonstrating both the good accuracy and potential for tiny dynamic force detection. Recently, it has been reported that GO has superior inhibition ability for a wide range of bacteria,26,27,39 such as Escherichia coli,26 Staphylococcus aureus,40 airborne bacteria,26 and other bacteria, including Pseudomonas aeruginosa,41 Bacillus subtilis,42 and Salmonella typhimurium.42 Three predominant mechanisms were proposed: nanoknives derived from the action of sharp edges, oxidative stress, and wrapping or trapping of bacterial membranes derived from the flexible thin-film structure of GO.41,43 Hence GO-enhanced S-TENG also endows wearable electronics with particular benefits over the “weaving in” approach and its antibacterial property. In this work, E. coli CMCC (B) 44102 was chosen as the experimental target to test the antibacterial effect of GO-enhanced S-TENGs. Sterilization performance was evaluated by the colony-counting

(LEDs) (Figure 5d). Similarly, parts e and f of Figure 5 confirm that the plate-type GO-based S-TENGs directly drive 40 green LEDs under hand patting. The process of driving LEDs by hand patting and finger tapping is shown in movies 1−3 (see the Supporting Information). Electromechanical performance was further tested to explore the potential applications of the GO-enhanced S-TENG for force sensing, and the current of S-TENGs shown in Figure 6 rises linearly with increasing applied force and electrode surface. This is mainly because the two plates are not completely flat, and a stronger applied force contributes to the large contact area. As shown in Figures 6a,b, the S-TENG exhibits high sensitivity for dynamic force regardless of size. Because the asdesigned TENG is highly extensible, a much higher energy output and a diversified dynamic force sensor can surely be expected. The currents under dynamic force of 200 and 60 N from the 2 cm × 2 cm are 25 and 11 μA, while the currents reach 110 and 25 μA for the 10 cm × 10 cm unit. In the subsequent tests, the current of S-TENGs also rises linearly with increasing applied force. Figure 6c shows the short-circuit currents generated by dynamic force rising from 2.5 to 125 kPa with an excellent sensitivity of about 388 μA·MPa−1, beyond 860

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Figure 5. Short-circuit current produced by different contact materials (F = 60 N, 4 cm × 4 cm): (a) GO-enhanced S-TENG; (b) comparison of GO- and PTFE-based S-TENG. (c) Comparison by patting with bare palm, linear motor with latex glove, and hand with latex glove. (d) Short-circuit current of the GO-enhanced flexible TENG by a gentle finger tapping. (e, f) Photographs of green LEDs powered by the asfabricated S-TENG with hand patting.

method. Parts a and b of Figure 7 show that the E. coli was treated for 60 min by GO film, PTFE-based S-TENG, and GObased S-TENG, respectively. It is obvious that E. coli density treated by GO-based S-TENG decreased much more remarkably (Figures 7a,b, IV) than that treated by pure GO film (Figures 7a,b, III), while only a slight decrease can be observed from that treated by PTFE-based S-TENG (Figures 7a,b, II). The qualitative analysis indicates GO-based TENG has very good antibacterial performance mainly caused by GO. It can be proven that besides the sterilizing effect of GO,26,27,39,44 the as-generated high voltage can also help to kill the bacteria in the energized area, as demonstrated by the bacterial death in movie files 4 and 5 (Supporting Information). Figure 7c is a schematic diagram and equivalent to the electric circuit of the models of GO-enhanced S-TENG for sterilization. Here, RB and CB are the body resistance and the body capacitance, respectively. Considering GO-enhanced S-TENG as wearable electronics for energy harvesting, instantaneous force sensing and bacteriostasis, we conceived GO-enhanced STENG as a bactericidal deodorant insole. Figure 7d depicts photographs of a shined shoe driven by S-TENG. Moreover, the lab-gown (right) was decorated with the flexible S-TENG

to prove that the S-TENG for wearable electronics harvests energy, as shown in Figure S7. These results demonstrate that this technique could create opportunities in fashion and consumer technology, such as incorporating LED lighting into clothing or having touch-screens on shirt sleeves.

CONCLUSIONS In summary, a portable, flexible, and multifunctional S-TENG has been demonstrated. The high-concentration of surface groups of GO and its microstructure empower the as-fabricated S-TENGs to display much better output performance such as sensitive force detection and antibacterial properties. In view of the excellent energy-conversion performance, dynamic force response, and sterilizing effect, this technique has wide applications in wearable electronics such as physiological information examination, data capture and feedback for individuals, as well as performance monitoring in sports. METHODS Synthesis of Graphene Oxide and Preparation of GO Films. Graphene oxide was synthesized from purified natural graphite power (SP-1, Bay Carbon) by the modified Hummers method.1,2 The GO 861

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Figure 6. (a, b) Short-circuit current of the S-TENGs with different sizes of GO film under dynamic forces of 200 and 60 N. (c) Short-circuit current of the fabricated GO-enhanced S-TENGs (2 cm × 2 cm) under different applied forces. (d) Fitting curve of short-circuit current (the flexible GO-enhanced S-TENGs and PTFE-based S-TENGs) responding to dynamic force in the range of 2.5−125 kPa.

Figure 7. (a) Density measurements of solutions incubated with various treated and untreated substrates after inoculation with E. coli and overnight incubation. (I, untreated; II, treated with PTFE-based S-TENG for 60 min; III, treated with GO for 60 min; IV, treated with GObased S-TENG for 60 min.) (b) Photographs of agar plates inoculated with E. coli solution treated with GO-enhanced S-TENGs and overnight incubation. (c) Installation diagram of GO-enhanced S-TENG for sterilization. (d) Photographs of shined shoe driven by S-TENG and bactericidal deodorant insole made of GO-enhanced S-TENG.

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ACS Nano colloidal dispersion (in aqueous solution) with a concentration of 10 mg·mL−1 were prepared with the aid of ultrasound (Fisher Scientific FS60 ultrasonic cleaning bath) in 40 mL batches and dropped into a Petri dish with diameter of 10 cm followed by drying in a vacuum oven at 60 °C for 12 h to form GO films. The thickness and size of each GO film sample was controlled by adjusting the volume of the colloidal suspension and the diameter of the Petri dish, respectively. Fabrication of the Single-Electrode Triboelectric Nanogenerator. The GO film was transferred to the Al electrode by double-sided tape, and a comparative experiment was carried out to eliminate interference of the double-sided tape. The Al electrode was connected with a load resistor, and the other end of the loading resistor was connected with the ground. The Al electrode was then fixed onto the PTFE substrate to make the TENG stable. In the control experiment, a PTFE film was used to replace the GO film for fabricating the single-electrode TENG. Bacterial Culture and Antibacterial Test. E. coli CMCC (B) 44102 (supplied by Shang-hai Guangyu Biotechnology Co., Ltd., China) was inoculated into liquid medium and overnight incubation. Next, E. coli was treated for 60 min by GO film, powered PTFE-based S-TENG, and GO-based S-TENG. Untreated E. coli was the control experiment. Treated and untreated E. coli were washed and diluted 6− 9 times. Finally, the E. coli solution was inoculated with agar plates followed by overnight incubation. Characterization. The morphology of the prepared GO films was measured by SEM (Hitachi 8020) and TEM (Tecnai F20 200 kV). The XRD pattern was measured using a PANalytical X’Pert PRO diffractometer. The XPS spectra were measured using an Escalab 250Xi (thermo Scientific) spectrometer. The FTIR spectrum was measured using an IRTracer-100. Photos and videos were taken using a Canon digital camera. The output voltage and current of the STENG were measured using a digital oscilloscope (Agilent DS0X2014A) with an inner impedance of 100 MΩ and a Keithley model 6514 system electrometer, respectively. The periodic motion of the TENG was driven by a LinMot-Talk 6 at a frequency of 5 Hz, and the dynamic force was measured by a dynamic force sensor (501F02).

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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.6b07389. Movies showing the process of driving LEDs by hand patting and finger tapping and bacterial death (ZIP) Additional information and figures (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ning Wang: 0000-0002-7863-8683 Zhong Lin Wang: 0000-0002-5530-0380 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge financial support from the National Key R and D project from the Minister of Science and Technology, China (2016YFA0202702), the National Natural Science Foundation of China (NSFC Nos. 21275102, 51272011, 21575009, and 51432005), and the “Thousands Talents” program for a pioneer researcher and his innovation team, China. 863

DOI: 10.1021/acsnano.6b07389 ACS Nano 2017, 11, 856−864

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DOI: 10.1021/acsnano.6b07389 ACS Nano 2017, 11, 856−864