Surface Engineering of Graphene Composite Transparent Electrodes

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Surface Engineering of Graphene Composite Transparent Electrodes for High Performance Flexible Triboelectric Nanogenerators and Self-Powered Sensors Jun Yang, Peibo Liu, Xingzhan Wei, Wei Luo, Jin Yang, Hao Jiang, Dapeng Wei, Ruiying Shi, and Haofei Shi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10373 • Publication Date (Web): 22 Sep 2017 Downloaded from http://pubs.acs.org on September 23, 2017

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ACS Applied Materials & Interfaces

Surface Engineering of Graphene Composite Transparent Electrodes for High Performance Flexible Triboelectric Nanogenerators and Self-Powered Sensors Jun Yang1, 2, Peibo Liu1,3, Xingzhan Wei1,2,*, Wei Luo1, Jin Yang4,*,

Hao Jiang1,

Dapeng Wei1, Ruiying Shi3, Haofei Shi1,*

1

Chongqing Key Laboratory of Multi-scale Manufacturing Technology, Chongqing

Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, P.R. China.

2

University of Chinese Academy of Sciences, Beijing, 100049, P.R. China

3

Physics Department, Sichuan University, Chengdu 610064, P.R. China

4

Department of Optoelectronic Engineering, Chongqing University, Chongqing,

40044, P.R. China

KEYWORDS: surface engineering, graphene, triboelectric nanogenerator, composite electrodes, high output, stability, self-powered sensors.

ABSTRACT:

A

high

performance

transparent

and

flexible

triboelectric

nanogenerators (TENGs) based on graphene composite electrodes via surface engineering is proposed and demonstrated. Through modifying the CVD-grown 1 ACS Paragon Plus Environment


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graphene

by

conductive

polymer

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poly(3,4-ethylenedioxy-thiophene):

polystyrenesulfonate (PEDOT:PSS), the composite electrodes with excellent optoelectronic performances were fabricated, which exhibited a high transmittance up to 83.5% and sheet resistance of 85 Ω/□ decreasing from the initial value of 725 Ω/□. As a consequence, the output current density and power of the corresponding TENG were enhanced by 140% to 2.4 μA/cm2 and by 118% to 12 μW, respectively, comparing with the counterpart composed of the pristine graphene electrodes. Furthermore, the composite electrode exhibited an outstanding durability of the physical and electrical characteristics after 10000 cycles’ bends, and can be readily extended to a large area up to 100 cm2. Such flexible, transparent, stable TENGs pave the way for the application of self-powered body sensors due to their unique characteristics, such as portability, wearability and human-compatibility.

INTRODUCTION

Nanogenerator (NG), one kind of cost-effective and simple-structure device that can produce electricity from mechanical energy, has attracted an increasing interest

for

clean

and

renewable

energy

using

nanomaterials

and

nanotechnology1-2. Besides the distinguished capability of serving as a sustainable power source for various electronic devices, the electric signals generated from nanogenerators can also be utilized for self-powered active sensors3-6. Recently, a new kind of triboelectric nanogenerators (TENGs) has been proposed on the basis of triboelectrification, which shows a large number 2 ACS Paragon Plus Environment

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of desirable advantages such as extremely high output and efficiency, excellent versatility in structural design, outstanding stability and robustness, as well as environment

friendly7-9.

Therefore,

TENGs

have

been

emerging

as

incomparable self-powered sensors for any type of external stimuli, including pressure, touch, vibration and body movement. More importantly, the integration of transparency and flexibility properties is of dramatic significance in the development of TENGs10-12, especially for flexible sensors and artificial skins. Studies on fully integrated flexible and transparent TENGs based on indium tin oxide (ITO), carbon nanotube (CNT) and graphene (Gr) have been reported13-17. ITO electrodes are the main choice owing to the high transparency of over 90% at 550 nm and low sheet resistance of 10-100 Ω/□18-19. However, the utilization of ITO electrodes has been greatly suffered from several disadvantages, such as limited flexibility, mechanical brittleness, chemical instability and high-cost process18-20. Graphene, a two-dimensional

monolayer

of carbon atoms arranged in a hexagonal and honeycomb lattice with a sp2 atomic configuration, has attracted great interests due to its extraordinary physical and chemical properties, including ultrahigh electron mobility, excellent optical transparency, prominent mechanical flexibility, outstanding mechanical elasticity, and high thermal stability21-25. It is expected that graphene has a huge potential for applications in the fields of transparent and flexible TENGs26-30. In 2014, Kim et al. reported the first graphene-based TENGs and provided a simple and cost-effective means to fabricate the transparent and 3 ACS Paragon Plus Environment


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flexible TENGs through using CVD-grown graphene electrodes, however, it exhibited a relative low output current density of 0.5 µA/cm2 due to the high sheet resistance of 2.23 kΩ/□14. Even though with the aid of microstructured dielectric layer, Liu’s graphene-based TENGs still yielded a quite low output current density of 0.075 µA/cm2, as the sheet resistance is quite large (around 5.2 kΩ/□)31. Although many methods have proposed to optimize the quality of graphene film, the corresponding conductivity is still worse than the commercial ITO due to the existence of grain boundaries, defects and wrinkles32, which greatly limits the electrical output of corresponding TENG devices. Functionalization of graphene is a viable way to reduce the sheet resistance while maintain the high optical transmission characteristic. A reduction in the sheet resistance of monolayer has been achieved by wet-chemical doping method33-35. However, the problem of instability is inherent for the doped graphene, which is a huge hindrance for long-term service. Besides, the graphene film combines with the substrate by an extremely weak Van der Waals' force, which is disadvantageous for achieving stable performance after repeated friction process32,

36

. Therefore, it is

challenging to achieve a high performance flexible and transparent graphene-based TENGs which can meet the essential requirements of practical applications.

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In this paper, we propose and demonstrate one kind of composite electrodes with high transmittance and excellent conductivity through surface engineering, which is quite beneficial to construct flexible, transparent, and durable TENGs with high output performance. The CVD-grown graphene has been modified by conductive polymer PEDOT:PSS (PH1000), which can effectively gain a sheet resistance of 85 Ω/□, while the transmittance is kept higher than 83.5%. What’s more, the adhesion between graphene and substrate can be greatly strengthened, and the durability of TENG has been greatly enhanced. The output current, voltage and power of the TENG have been improved significantly. Such flexible and transparent TENGs based on graphene composite electrodes can be applied in self-powered wearable sensors. 2. EXPERIMENT 2.1. Synthesis of graphene. The large-area monolayer graphene film was synthesized on copper foils (25 μm thick, Alfa Aesar) via CVD method. In detail, the synthesis experiment was performed using Ar, H2 and CH4 source gases in a vacuum chamber at temperature of 1050 °C and working pressure of 45 Pa. The gas-flow rate of CH4: H2: Ar during the graphene growth was kept at 5:50:200, and the growth time was 15 min. The Cu foils were annealed at 1000 °C in H2 for 30 min beforehand to clean and smooth the surface of Cu foils. 2.2 Transfer and surface engineering of graphene. In order to prepare flexible and transparent electrodes, we chose PMMA (polymethylmethacrylate) 5 ACS Paragon Plus Environment


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-assisted transfer method to transfer graphene onto PET substrate. After spinning PMMA (4% wt, ethyl lactate) on the surface of graphene, the copper foils were etched in 0.5 M aqueous FeCl3 solution for 5 hours. Then the floating graphene was washed in deionized (DI) water for 5 times to remove the residual metal ions, and transferred to PET. Afterwards, the graphene/PET (PET Gr) sample was dried in air and annealed at 130 °C for 30 min. For the surface engineering of graphene, the conductive polymer PEDOT:PSS (Clevious PH1000, Heraeus) was spin-coated on the transferred graphene. To enhance the conductivity and wettability, 4.5 wt% DMSO (99.0% purity, Sigma-Aldrich) and 0.5 wt% Zonyl (FS-300, Fluka) were mixed with 95wt% PH1000 and stirred overnight37, respectively. Graphene film is hydrophobic, and it is difficult to gain a homogeneous coating using pristine PEDOT:PSS. Therefore, we appropriately increase the additive amount (0.5 wt%) of FS-300 to improve the film formation on graphene. The PH1000 mixtures were filtered with 0.45 μm filters and spin-coated with the speed of 500 r/min, 1000 r/min, 2000 r/min, ℃ for

3000 r/min,5000 r/min for 60 s, followed by an annealing process at 120 20 min in ambient air. The graphene composites after surface engineering with different speed of PH1000 spin-coated (500 r/min, 1000 r/min, 2000 r/min, 3000 r/min and 5000 r/min) were abbreviated as PET Gr/P-500, PET Gr/P-1k, PET Gr/P-2k, PET Gr/P-3k and PET Gr/P-5k, respectively. 2.3 Fabrication of TENGs based on modified graphene. The preparation process of TENGs can be described as follows: we firstly pasted the Ag pads at 6 ACS Paragon Plus Environment

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end of the modified graphene/PET film as external electrodes to characterize the device. And then, the modified graphene/PET films were covered with PI paste acting as triboelectric layer. Lastly, the composite film and another graphene/PET were placed face to face to form an air gap by placing two spacers at the edge. The dimension of the device herein is 15 mm×15 mm and the thickness is around 0.6 mm. 2.4 Characterization and measurement of the device Surface morphology of modified graphene was analyzed by scanning electron microscopy (JEOL JSM-7800F) and atomic force microscopy (Bruker, Dimension Edge). The carbon derivatives were verified by confocal Raman microspectroscopy (Renishaw inVia Reflex) with a laser excitation wavelength of 532 nm. The thickness of the PH1000 film with different speed was measured via an Alpha-step IQ system. The optical transmission of the modified graphene was tested by Ultraviolet-Visible spectrophotometer (Lambda 35). The electrical property of the composite film was measured by four-point probe method (RTS-9). The output of the modified graphene based NG was measured by a low-noise current preamplifier (Stanford SR570) and Data Acquisition Card (NI PCI-6259), with a Modal vibrator (JZK-5) driven by function signal generator (Agilent 3522A) and power amplifier (YE5871) to produce continuous periodic stress.

3. RESULTS AND DISCUSSION



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Figure 1. Structural and morphology characterization of PET Gr/P. (a) Schematic view of the surface engineering process for obtaining the graphene composite transparent electrodes. (b) Schematic of the conductive channel of graphene electrodes before and after surface engineering. (c) Raman spectrum of pristine graphene. (d) Raman spectrum of graphene modified by PH1000. (e)-(h) Morphology characterization of graphene before and after modified. (e) and (f) SEM and AFM image of pristine graphene transferred to PET substrate. (g) and (h) SEM and AFM image of PET/graphene modified by PH1000.


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Figure 1a shows the preparation process of the graphene composite transparent electrodes. The conductivity of graphene can be improved by constructing a facile conductive channel via coating conductive polymer (PH1000), especially at the grain boundaries caused by graphene growth (as shown in Figure 1b). Raman spectroscopy was employed to identify the quality of graphene electrodes. Figure 1c shows the Raman spectroscopy of the pristine graphene before modification, which shows strong G band peak at 1580 cm-1, 2D band peak at 2700 cm-1 and weak D peak at 1350 cm-1. The 2D peak intensity is about twice of G peak intensity, which proves that the graphene sample is single-layer. In addition, D peak intensity is weak, indicating that little edges and defects exist in the graphene. Figure 1d shows Raman spectroscopy of the graphene after modification (PET Gr/P-1k). Compared with Figure 1c, after surface engineering of graphene, D peak and G peak are concealed by the characteristic peaks of PH1000 (Figure S1c), however the 2D peak is still clearly visible, which indicates that PH1000 has modified the graphene successfully. With the increasing conductive polymer, the intensity of 2D peak decreases significantly as shown from the Raman results of Gr/P-1k, Gr/P-3k and Gr/P-5k in Figure S1a-b. The morphology of graphene after the surface engineering process was investigated using the SEM and AFM tools. Figure 1e-f shows the typical SEM and AFM images of graphene on PET substrate, which clearly reveals the graphene wrinkles and grain boundaries, caused by inhomogeneous interactions 9 ACS Paragon Plus Environment


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with the substrate and the residual PMMA resist during the transfer process. The black reunion parts are the remains of PMMA in transfer process, and the white crack lines are the grain boundaries induced by the recrystallization of copper foils. The Cu annealing temperature (1000 ℃) is far above the Cu recrystallization temperature of ~227 ℃ 38, resulting in the formation of Cu grain boundaries (Figure S2). During the subsequent CVD growth process, the Cu grain boundaries can be conformally covered by graphene film, forming the graphene grain boundaries and cracks after transferred to PET, which directly affect the conductivity behaviour. In comparison, Figure 1g-h show the SEM and AFM images of composite film after modified by conductive polymer of PH1000, in which the entire surface is fully covered by continuous PH1000 films. The problems about grain boundaries, wrinkles, and cracks have been solved by the PH1000 (even with the thinnest Gr/P-5k, as shown in Figure S3), indicating that the surface engineering of graphene with PH1000 would gain a highly conductive surface. The roughness of the surface was further characterized by AFM with a measure area of 3 μm×3 μm. As shown in Figure 1e-h, the surface of pristine graphene is relatively flat with a small root-mean-square roughness (Rrms) of 3 nm. By contrast, after modified by PH1000 the surface of composite film becomes rougher, and Rrms reaches 8 nm (Gr/P-1k). We can find that the thickness of conductive polymer film has a great influence on the roughness. The Gr/P-1k film owns the maximum surface topography (Rmax=35.6 nm). 10 ACS Paragon Plus Environment

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Compared with pristine graphene, the Gr/P composite electrodes have a higher roughness due to the formation of large domains or grains with the additive DMSO concentration. For the TENGs, Gr/P composite films worked as both electrodes and friction layers, and the large roughness is beneficial for increasing the electric output.

Figure 2. Optical and electrical properties of graphene before and after modified. (a) Optical transmittance as a function of wavelength for different transparent electrodes. (b) Sheet resistance of graphene before and after surface engineering. (c) Optical transmittance versus the corresponding sheet resistance for our transparent electrode samples on PET at the wavelength λ=550 nm. (d) Photograph of the fabricated graphene/polymer hybrid transparent electrode.



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The transparency of graphene/conductive polymer electrodes was identified by UV-Vis spectrum, as shown in Figure 2a. The pristine graphene on PET film owns a high transparency about 87.6% at the wavelength of 550 nm. After modified by PH1000, the transmittance (T) of the composite electrodes declines slightly, but still possesses a high transparency above 83.5%, when the spin-coating speeds of PH1000 are 1000 r/min, 2000 r/min, 3000 r/min and 5000 r/min. However, for the thickest Gr/P-500 film, the relevant transmittance dramatically decreases to 77.9%, which is unsuitable for transparent flexible electronics. The average transmittance of the samples declines with the increase of PH1000 thickness. This phenomenon is due to that the conductive polymer of PH1000 layer owns a strong absorption at long wavelength band. The sheet resistance (Rsh) of the graphene composite transparent electrodes was characterized by Four-point probe method, as shown in Figure 2b. The average sheet resistance of the pristine graphene is about 725 Ω/□, whereas the composite electrode modified by PH1000 is below 85 Ω/□ (Gr/P-1k), which is almost 8 times lower than the pristine graphene film. It should be noted that the result herein is 2 orders better than that of previously reported graphene TENG14,

31

. Consequentially, in the graphene/polymer composite system, the

Gr/P-500 film deserves the lowest sheet resistance of 65 Ω/□. This excellent performance is because that the defect problems in pristine graphene film introduced during the growth and transfer process have been delicately handled, 12 ACS Paragon Plus Environment

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as shown in Figure 1g. After modified by the continuous conductive polymer film, the winkles and cracks of graphene film are covered by PH1000, forming a continuous conductive channel. More excitingly, as shown in Figure S4, the Gr/P composite electrode endows a much better conductivity than both pristine graphene and PH1000 with different thickness of PH1000 due to the synergetic effects of surface engineering. The mechanism of conductivity enhancement of Gr/P composite electrodes can be understood by using an equivalent resistance circuit diagram39 to qualitatively model conduction through the Gr/P composite electrodes, as shown in Figure S5. Figure S5a shows the schematic diagram of the underneath graphene film separated by grain boundaries or cracks and the upper PH1000 film. Within the graphene film, there are a mount of grain boundaries (RGB) or cracks (RC), which dominates the large sheet resistance of graphene film. Within the homogeneous and continuous PH1000 film, the sheet resistance is dominated by pristine resistance of PH1000. In the composite films, the two layers are coupled by the graphene-PH1000 contact resistance (RG-P). For this study, we assumed that the solution coating and annealing process established an ideal ohmic contact and deserved a low RG-P. Defects and high resistance boundaries of graphene film can be repaired via the covered PH1000, which constructs various low resistance transport channels and forms a shunt resistance circuit. Therefore, the equivalent resistance of the shunt resistance circuit will be lower than that of both pristine graphene and the individual PH1000 film. 13 ACS Paragon Plus Environment


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The trade-off between conductivity (Rsh) and optical transmittance (T) is evaluated with the figure of merit (FoM)40. The FoM (F) is defined as the ratio of DC conductivity and optical conductivity (F=σdc/σopt), which is related to Rsh and T as follows41  188.5  T = 1+  Rsh F  

-2

(1)

The FoM has been widely used to judge the overall performance of transparent electrodes (TE), and the larger FoM corresponds to a more excellent TE with lower Rsh and higher T. Here the optical transmittance at 550 nm is defined as T. Typically a FoM value greater than 35 is considered to be feasible for commercial application. The high FoM of Gr/P composite TE shows the tremendous potential in the transparent TENGs application. Figure 2c summarizes the experimental and reference data, by plotting the T (λ=550 nm) versus the Rsh. Such Gr/P composite TEs show an increasing FoM value as the increasement of PH1000 ranging from 12 to 56. While for the Gr/P-500 composite, as the thickness of PH1000 increases, the sheet resistance slightly deceases to 65 Ω/□ but with an unsatisfactory transmittance of 77.9%, which results in a reduced FoM of 35. Obviously, the Gr/P-1K composite electrodes possess the relative high comprehensive performance, because it is with lower Rsh and higher T which results in the best FoM. Encouragingly, this Gr/P electrode could be readily expanded to a conductive sheet with a large area up to 100 cm2 (10 cm×10 cm), which still maintains the comparable performances of small-size counterpart. Thus, the architecture and 14 ACS Paragon Plus Environment

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manufacture procedures for Gr/P electrode are scalable, providing an excellent opportunity to realize large-area flexible electronic devices.

Figure 3. Flexible and transparent TENGs device based on surface engineered graphene composite electrodes. (a)Structure of the transparent flexible TENGs device. The inset shows the photographs of the TENGs device illustrating its transparency and flexibility. (b) The transmittance of the TENGs device with graphene (Gr) electrodes and graphene composite electrodes (Gr/P-1k), respectively. (c) Output current of TENGs (d) output voltage of TENGs.

To explore the performance of the graphene composite electrodes in flexible transparent TENGs device, the graphene-based TENGs were fabricated, and the corresponding schematic view is shown in Figure 3a. The upper layer consists of PET/graphene/PH1000 (PET Gr/P), where the graphene/PH1000 composite film acts as the electrode as well as the friction layer of TENGs. Note that the composite film has larger roughness than pristine graphene, which 15 ACS Paragon Plus Environment


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make TENGs more suitable for obtaining high-output voltage, due to the significant amounts of friction and surface charges created by the triboelectric effect.

The

lower

layer

is

transparent

PI

film

transferred

on

to

PET/graphene/PH1000 layer, which acts as a dielectric friction layer. A 0.3 mm space is constructed by PI tape, which increases the strength of dipole moments and the capacitance of the TENGs in the mechanical deformation process. Remarkably, the flexible and transparent nature of both PET/graphene/PH1000 electrode and PI dielectric layer enables the ideal fabrication of transparent and flexible TENGs. As shown in Figure 3b, the fabricated graphene/PH1000 TENGs possess a relatively excellent transparency about above 55%. The electrical power outputs of TENGs based on graphene modified by PH1000 were measured under the same conditions of periodic external stress, as shown in Figure 3c-d. The output current and voltage exhibit an obvious increasement with more amount of PH1000. The average output current density (Jout) values of 1.0, 1.3, 1.8 and 2.4 μA/cm2 and average output voltage (Vout) values of 26, 41, 46 and 52 V were observed for the pristine graphene, Gr/P-5k, Gr/P-3k, and Gr/P-1k TENGs, respectively. Although the Gr/P-500 based TENGs possess the highest open-circuit voltage of 54 V and current density of 2.5 μA/cm2, the increasement of electric output is just improved by 3.7%, compared with the case of Gr/P-1k. Meanwhile, the transmittance of Gr/P-1k is below 80%, which is unsatisfactory for the application requiring high transparence. Table 1 resumes the values of the performance of electrodes and 16 ACS Paragon Plus Environment

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TENGs based on different electrodes, from which the Gr/P-1k electrode is ideal for practical applications. Table 1. Performance of TENGs based on Different Electrodes Electrodes

TENGs Device

Electrode Thickness of

Rsh

T

FoM

Rrms

Jout

Vout

(%)

(σdc/σopt)

(nm)

(μA/cm2)

(V)

Structure PH1000 (nm) (Ω/□) PET Gr

0

725

87.6

12

3.0

1.0

26

PET Gr/P-5k

45

305

85.2

25

5.8

1.3

41

PET Gr/P-3k

60

240

84.7

30

7.1

1.8

46

PET Gr/P-1k

120

85

83.5

56

8.0

2.4

52

PET Gr/P-500

170

68

77.9

35

8.3

2.5

54

It is clear that Gr/P-1k TENGs produce an excellent output current and voltage. For the pristine graphene, PMMA residue acts an insulator on the surface of graphene, which can block the charge transporting through contact electrification/triboelectric effect. After modified, the surface graphene is covered by PH1000 and the charge can transport efficiently, thus the TENG based on modified graphene shows high output. What’s more, the surface of modified graphene becomes coarser, as shown in Figure 1 and Figure S3, which means that it owns larger surface area and can restore more surface charges. In addition, surface engineering of graphene via conductive polymer can increase work-function difference and result in an enhanced TENGs output. Generally, the large difference in work function values can significantly change the surface-charge density due to the triboelectric effect, which further enhances the output voltage and current. The contact potential difference (V) is given as 17 ACS Paragon Plus Environment


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V~-(Φf -Φd)/e, where Φf is the effective work function of the friction electrode, Φd is the work function of the dielectric layer, and e is the elementary charge14, 42. The work-function value of graphene via traditional PMMA transfer method is about 4.90 eV43, and the value of PH1000 doping with DMSO is about 5.12 eV44. The surface engineering via PH1000 is helpful for enhancing the effective electric output through increasing the work function difference. It also should be noted that the increasement of open-circuit voltage is not only due to the work function difference ∆Φ, but also has a relationship with the sheet resistance of the composite electrode. For the thinnest PH1000 composite electrodes (Gr/P-5k, sheet resistance of 305 Ω/□), the output voltage was greatly enhanced from 26 V (pristine graphene TENGs) to 41 V (Gr/P-5k TENGs). While the Gr/P-3k TENGs only possessed an improvement of open-circuit voltage from 41 V to 46 V, and further to 52 V for Gr/P-1k TENGs. This is because, for the cases of Gr/P-3k and Gr/P-1k, the work functions are similar to that of Gr/P-5k, therefore ∆Φ exhibit little change, which indicates that the increase of the electric output is mainly benefitted from the low sheet resistance for the cases with PH1000.


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Figure 4. Plots of output versus external loads. (a) Plots of output voltage of NG based on graphene versus and current external loads. (b) Plots of output power of NG based on graphene versus the external loads. (c) Plots of output voltage of NG based on modified graphene versus current external loads. (d) Plots of output power of NG based on modified graphene versus the external loads.

We further tested the output power of TENG based on graphene under a frequency of 1 Hz. Figure 4 shows the output voltage, current, and power versus resistance of an external load. The maximum output power (Pmax) of TENG based on pristine graphene electrode is about 5.5 μW with an external load of 3×107 Ω. In comparison, the maximum output power of TENG based on modified graphene electrode (Gr/P-1k) is about 12 μW with an external load of 1.2×107 Ω. The output power of TENG based on surface engineered graphene is twice of that of TENG based on pristine graphene. It is well known that the TENG produces the largest output power only under the condition that internal 19 ACS Paragon Plus Environment


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resistance is equal to the external load. Thus, the internal resistance of Gr-TENG and Gr/P-TENG is 3×107 Ω and 1.2×107 Ω, respectively, which is consistent with the sheet resistance after surface engineering.

Figure 5.

Durability of Gr TENGs and Gr/P-1k TENGs. (a) The change of output voltage

over 1300 cycles. (b) SEM image of the Gr/P-1000 electrode after 1300 cycles. (c-d) SEM image of the Gr electrode after 1300 cycles with low and high magnification, respectively.

The durability test was also carried out to confirm the mechanical stability of the grapheme-based TENGs. As shown in Figure 5a, the Gr/P-1k TENGs owned an excellent stability over 1000 cycles, and the output voltage slightly decreased about 5.1%. While the Gr TENGs dramatically decreased by 56.3% after 1300 cycles, the details can be found in Figure S6-S7. To explain this phenomenon, we employed SEM to observe the morphology evolution of the conductive film after 1300 cycles. Obviously, there was no distinct abrasion on the Gr/P-1k conductive 20 ACS Paragon Plus Environment

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surface (Figure 5b). However, cracks and defects sprang up on the pristine graphene surface after the repeated friction (Figure 5c-d). The graphene film adheres on PET substrate via the relative weak Van der Waals' force, and the surface engineering of PH1000 constructs a stronger adhesion to PET substrate, which greatly improves the lifetime of graphene composite electrodes. The change of sheet resistance for all electrodes before and after cycling is shown in Figure S8. After 1000 cycles, the sheet resistance of pristine graphene increased dramatically over 3500 Ω/□. While for the Gr/P-1k TENGs, the sheet resistance of Gr/P-1k slightly decreased to 93 Ω/□, and the output voltage slightly decreased about 5.1%. Besides, after 1000 cycle frictional contacts, the roughness of pristine graphene increased by 53% (3 nm to 4.6 nm, Figure S9a) and the Gr/P-1k electrode only changed from 8 nm to 8.2 nm (Figure S9b), which verifies that the Gr/P composite electrodes are durable for cyclic frictional contacts. For further investigating the durability, we have carried out a 10000 cyclic measurement for Gr/P composite electrodes and related TNEGs, as shown in Figure S10. From the output voltage and SEM morphologies under 1~10000 cycles, the composite electrodes and TENGs possess an excellent cyclic stability.



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Figure 6.

Monitoring motion signals of different joints. (a) Variation of electric voltage

for different bending angle of a finger. The inset shows the typical voltage signals at the angle of 30°; (b) Open-circuit voltage of the TENGs at different frequencies (bending angle: 30°~90°; frequency: 0.1 Hz~1 Hz).

The TENGs device can also be operated by human body joints movement, showing its potential as a wearable device to accumulate energy from human movements and as self-powered gesture sensors. To investigate the capacity of our TENGs in monitoring human body gestures, such as the strain from bending motion of finger, we fabricated a 1 cm×3 cm TENGs gesture sensor. The TENGs gesture sensor was fixed on the joints, and the electric voltage variation rate with the joints angle was measured using the electrochemical workstation, as shown in Figure 6. The TENGs gesture sensor suffered different strain with the finger bending motion. In this process, the voltage exhibited an obvious increasement as the bending angle increases, as shown in Figure 6a, which is attributed to the fact that the larger bending angle results in an increased contact area and higher surface charge density. Subsequently, to further explore the reliability and service behavior of the TENGs for angle 22 ACS Paragon Plus Environment

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sensing application, the voltages of the self-powered sensor at different frequencies were investigated with bending angle ranging from 30° to 90° (Figure 6b). When the bending angle changes from 30° to 90°, the output voltage generated by the Gr/P TENGs increases from 0.25V to 4.3 V (Fig. 5d and e). The fitting curve in Fiugre S6 shows that output voltage and bending angle (or curve radian) have a linear relationship with a correlation coefficient of 0.992, and the maximum linear sensitivity of 15.4 rad−1, which is superior to the previous reports36. Particularly, the output voltage almost remains constant with the variation of the bending speed or frequencies. Zhang et. al. systematically investigated that the bending speed affected the output current and with little influence on the voltage37. It should be noted that the Gr/P TENGs gesture sensor can detect continuous angle variation of finger joints, and the largest bending angle can exceed 90°.

3. CONCLUSION In this work, we have successfully demonstrated the application of the surface engineered graphene electrodes for flexible and transparent TENGs. In order to improve the output of TENG, we modified graphene by using conductive polymer of PH1000 film. After the surface engineering, the transmittance of the graphene composite electrode declines slightly, and the conductivity is improved greatly. The graphene electrodes modified by PH1000 can enhance the output current density from 1.0 to 2.4 μA/cm2，the output 23 ACS Paragon Plus Environment


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voltage from 26 V to 52 V, and output power from 5.5 to 12 μW. Furthermore, the surface engineering distinctly enhances the durability of graphene electrodes, which plays a critical role for practical application. This study provides a method for harvesting mechanical energy using transparent TENGs. The flexible, transparent, stable TENGs with human-compatibility shows high portability and wearability for self-powered sensors. The graphene-based TENGs were shown to monitor human joints movements, such as fingers gesture sensor. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.xxxxxxx.

Raman spectra of Gr/P-3k, Gr/P-5k, and pristine PH1000; The low and high magnification SEM image of copper surface after CVD graphene growth, in which the Cu grain boundaries and wrinkles can be clearly observed; SEM and AFM image of PET/graphene modified by PH1000 (Gr/P-5k and Gr/P-3k); Sheet resistance of PH1000 and Gr/P with different thickness of PH1000; Schematic of equivalent circuit of Gr/P composite electrodes; The durability test results of the pristine graphene TENGs and the composite Gr/P-1000 TENGs; Changes of sheet resistance for graphene-based electrodes after cyclic friction; Changes of morphologies for graphene-based electrodes after cyclic friction.


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AUTHOR INFORMATION Corresponding Author * Prof. Xingzhan Wei, E-mail address: [email protected] * Prof. Jin Yang, E-mail address: [email protected] * Prof. Haofei Shi, E-mail address: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (NSFC 61504148, 11574308), the Basic Science and Frontier Technology Research Program of Chongqing (cstc2016jcyjA0315, cstc2017shmsA1471) and HundredTalent Program of Chinese Academy of Sciences.

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Table of Contents


Surface Engineering of Graphene Composite Transparent Electrodes

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