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Cu heterostructure for improving the stability of triboelectric nanogenerators
Nano Energy 70 (2020) 104540
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Electron transfer mechanism of graphene/Cu heterostructure for improving the stability of triboelectric nanogenerators Yahui Li a, Wei Zheng a, Haodong Zhang a, Haoqiang Wang b, Han Cai a, Yanxin Zhang a, Zhuoqing Yang a, * a
National Key Laboratory of Science and Technology on Micro/Nano Fabrication, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China State Key Laboratory of Advanced Welding and Joining, School of Materials Science and Engineering, Harbin Institute of Technology (Shenzhen), Shenzhen, 518055, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Triboelectric nanogenerators (TENG) Graphene Metal work function Electrodeposited copper nanostructure Temporal stability
Harvesting energy from the mechanical motion has been demonstrated to be a preferable strategy to satisfy the requirements for self-powered electronics. Here we report a flexible and stable induction electrode based on graphene/Cu heterostructure, which is fabricated by electrodeposition and spin-coating, and is also utilized in energy harvesting for triboelectric nanogenerator (TENG). Typically, graphene dispersion prepared by physical exfoliation is spin-coated on the copper nanostructure based on PDMS. Besides, we provide favorable evidences that electrons transfer from graphene to copper, and ignite the reduced reaction of copper oxides. The function of graphene in avoiding the oxidation of copper nanostructure, thus improving the stability of graphene/Cu het erostructure and achieving its applications in triboelectric nanogenerator, is confirmed. The interactive mech anism is formulated, including energy barrier, metal work function and the electrochemical potential difference between graphene, copper and oxygen. The enhanced stability of graphene/Cu heterostructure is inspected by the chemical state of copper as the function of exposure time, and the applicability is also evaluated by their output electrical performance for TENG applications.
1. Introduction In view of the increasingly exhausted resources and severe energy crises, thus, it’s a challenge to explore renewable and green energy facing the world, which is indispensable and significant for our social development and human survival [1–4]. Harvesting energy from ambient environment brings forward a desirable strategy to enlarge the applications of the renewable and clean energy, such as solar, wind, water and mechanical motion [5–12]. In particular, mechanical motion, which is widely existed in the ambient environment and people’s daily lives, has become a preferable target for energy harvesting in recent years [13–17]. Triboelectric nanogenerator (TENG), coupling the con tact electrification and electrostatic induction, has been proved to be a promising energy harvester due to their excellent reliability, large output power, high efficiency and low cost [18–22]. Noticeably, only one friction surface is required for the single electrode TENG (S-TENG), whereas the other friction surface can be replaced by human skin, which greatly expand the applications of TENG in wearable electronic
equipment [23,24]. Presently, indium-tin-oxide (ITO) or noble metals are the most commonly used electrodes for TENG fabrication [25–27]. However, it’s unrealistic for substantial production and application due to their scarce storage and expensive costs. Therefore, the development of advanced electrode materials and processing technologies to satisfy the reliability, cost-effectiveness, and scalability for TENG is imminent. Metallic copper displays analogous performance in mechanical and electrical properties compared that of noble metals, such as silver. Thus, it’s considered to be an ideal alternative to noble metals, showing great potential in a great many of applications [28]. However, the obvious disadvantage of copper is its chemical instability, that is, it’s susceptible to be oxidized to copper oxides according to the chemical equations (Table S1), where the changes of enthalpy and Gibbs free energy at 298.15 K are both negative, indicating the reaction could be occurred spontaneously at room temperature and the properties of copper could be impaired once the oxidation [29]. Therefore, for purpose of exploring the applications of copper nanostructure in TENG and acquiring the high-performance devices based on copper structure, it is critical to
* Corresponding author. E-mail address: [email protected] (Z. Yang). https://doi.org/10.1016/j.nanoen.2020.104540 Received 5 December 2019; Received in revised form 21 January 2020; Accepted 25 January 2020 2211-2855/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 1. SEM images of graphene dispersions dropped on the silicon wafer: (a) PEG and (b) RGO. Raman spectra of graphene excited at 514.5 nm laser: (c) PEG and (d) RGO, inset in (c) shows the four fitted Lorentzians for 2D peak of PEG. The detailed XPS spectra for PEG and RGO: (e) C 1s and (f) O 1s. (g, h) The SEM images of graphene/Cu heterostructure. (i) The survey XPS spectrum of graphene/Cu heterostructure, and the atomic percentage of individual elements. (j) The surface scanning pictures of graphene/Cu heterostructure, including the element information of carbon, copper and oxygen. k) Typical Raman spectra of G band for physical exfoliation with and without contacting electrodeposited Cu. l) Position shift of binding energies for Cu 2p3/2 before and after doping graphene on the electro deposited Cu substrate. Schematic diagram illustrates the difference of work function between graphene and polycrystalline Cu, and visualizes the electron transfer from graphene to Cu.
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establish novel mechanism for copper protection. As a cost-effective and multifunctional processing technology, electrodeposition has been extensively utilized for the deposition of coatings and films due to their outstanding performance, such as good uniformity, high deposition rate, simplicity for mass manufacturing, controllable thickness and grain orientation [30]. In particular, copper is the most widely studied metal in electrodeposition due to its excellent electrical conductivity and machinability, which establishes its position in industrial applications. Here, we first report the preparation of flexible electrode for TENG based on the electrodeposited Cu nanostructure. Ever since it was discovered, graphene has become a rising star in the field of research by cause of its exceptional electrical, chemical and mechanical properties [31,32]. Additionally, recent works, functional izing metal nanostructure with graphene, have been confirmed to be an efficient approach to enhance the properties of metal nanostructure, such as chemical stability, conductivity, and oxidation resistance, etc. Yan et al. fabricated graphene/AgNW hybrid film, where Ag nanowires were acted as electric network and graphene was served as protective layer. Moreover, the as-prepared hybrid film displayed excellent con ductivity, and good superior oxidation resistance [33]. The improved chemical stability of Ag was also demonstrated by Tang et al., which modified the randomly distributed silver nanoparticles with reduced graphene oxide [34]. Wang et al. achieved the enhanced oxidation resistance and stability of Cu nanowires by decorating with multilayered reduced graphene oxide [35]. However, the above mentioned oxidation resistance and stability exerted by graphene is simply forming a pro tective layer for metal nanostructure, which ignores the discussion about chemical structure and electron transfer information between the graphene-metal heterostructure. Thus interaction mechanism between graphene and metal is exactly what this work emphasizes. In this work, we investigate the internal stress and electron transfer between physical exfoliating graphene and electrodeposited Cu nano structure for purpose of revealing the functioning nature of graphene/ Cu heterostructure, achieving the fundamental goal of avoiding copper oxidation, and realizing its applications in TENG. Graphene/Cu nano structure is fabricated by spin-coating the graphene dispersion on the surface of preferred Cu (111). The information of doping and electron transfer between graphene and copper is directly demonstrated by Raman and XPS, and the chemical state of copper as the function of exposure time is also acquired. In addition, we provide favorable evi dences that electrons could transfer from graphene to copper, and ignite the reduced reaction of copper oxides. Finally, the flexible and conductive electrode based on graphene/Cu/PDMS nanostructure is proposed, and is successfully utilized in the TENG applications.
exfoliation has fewer defects, and more perfect structure. It is well recognized that the intensity of I2D/IG and the shape of 2D peak is closely dependent on the number of graphene layers [37]. As shown in Fig. 1d, the intensity of 2D peak is very weak with the wide shape, indicating the graphene is stacked in layers. In the case of PEG, the intensity of 2D peak is closer to that of G peak with the significant enhancement, and the peak is sharper. Moreover, the four fitted Lorentzians are also acquired with peak positions at 2661 cm 1, 2693 cm 1, 2717 cm 1, and 2739 cm 1, respectively, which is consistent with the bilayer graphene re ported previously [38,39]. In order to study the chemical composition of graphene prepared by PEG and RGO, XPS analysis was applied to detect the element infor mation. As illustrated in Fig. S1, for both types of graphene, they are composed of carbon and oxygen. The difference, however, is the atomic percentage of the two elements. The percentage of carbon and oxygen for RGO is 92% and 8%, respectively, which indicates that some oxygencontaining functional groups are remained in graphene after the chemical reduction. However, the existence of oxygen-containing functional groups would worsen the properties of graphene, such as the conductivity [40]. Contrastively, the content of oxygen in PEG de creases sharply, and the small amount of oxygen atoms may be caused by the formed O–C–O bonds with oxygen during the sample preparation. Fig. 1e displays the detailed C 1s spectrum of RGO and PEG. The C 1s spectrum of RGO can be fitted by a series of components at 284.8 eV, 285.7 eV, and 286.6 eV, which correspond to C-sp2, C–OH, and O–C–O, respectively [41,42]. However, the C 1s detailed spectrum of PEG is mainly fitted by the component of C-sp2 with a very small amount of O–C–O bonds. Correspondingly, the O 1s spectra are also obtained and analyzed (Fig. 1f), and the fitted results are highly coincident with those of C 1s [43]. Therefore, the physical exfoliation graphene was selected to be used in this study based on the above analysis. As we all know, the metal work function varies with crystal plane orientation and has anisotropy, which would directly involve the graphene-Cu interaction in this work [44]. Besides, the structures and properties of electrodeposited Cu are intimately dependent on the selected electrodeposition parameters, especially the current density. Therefore, a series of current density were attempted to control the crystal orientation and morphology of copper nanostructure based on the electrodeposition technology. As shown the SEM images in Fig. S2a–e, it can be seen that when the current density is 2 mA/cm2, the Cu particles are large in size and approximate a disordered polyhedral shape. Moreover, the deposited Cu structures are uneven and have poor compactness. Once increasing the current density, however, the size of electrodeposited Cu particles decreases significantly, and the boundaries between Cu particles become less noticeable. The XRD patterns of Cu deposited at various current density are shown in Fig. S2f, both of which display obvious characteristic peaks. At the lower current density of 2 mA/cm2, one distinguished peak and two frail peaks could be seen at 43.3� , 50.4� and 74.1� , which correspond to the (111), (200) and (220) crystal planes of metallic copper, respectively [35]. Additionally, the texture coefficient of (111) crystal plane decreases with the increasing current density, on the contrary, the texture coefficient of (200) crystal plane increases with the increasing current density (Fig. S2g). Since the required energy for (200) crystal nucleation is higher, thus, increasing the current density is more preferable to the nucleation and growth of particles on the (200) crystal plane. However, the preferred orientation of (111) crystal plane is still dominant. For the transition metal copper of fcc structure, the dense plane (111) has the lowest surface energy and the largest metal work function. In particular, the metal work function corresponding to the (111), (100) and (110) crystal planes of copper are 4.98 eV (5.10 eV), 4.59 eV and 4.48 eV, respectively [45,46]. Accord ingly, the work function for bilayer graphene is 4.69 eV, which is lower than that of the metallic Cu (111) crystal plane. Therefore, electron transfer could be occurred when metal copper is in contact with gra phene, and thus electron transfer is very essential for the interaction between copper and graphene. Additionally, as the current density
2. Results and discussion 2.1. Evidences for doping and electron transfer from graphene In this work, two kinds of graphene obtained by physical exfoliation (PEG) and chemical reduction (RGO) were used for preparing the stable graphene dispersion. As shown in Fig. 1a, the PEG is like a thin layer of yarn, and the physical separation is realized between layers with a lot of edge warping. However, the agglomeration of RGO sheets is serious (Fig. 1b), which probably deteriorates the intrinsic properties of gra phene. Besides, the structural characteristics of PEG and RGO were also investigated by Raman spectroscopy with 514.5 nm laser excitation. For the spectrum of RGO (Fig. 1d), two distinct peaks at 1346 cm 1 and 1584 cm 1 are observed, corresponding to the D and G bands of gra phene, respectively. Generally, the former is due to the disorder caused by the introduction of structural defects, which indicates that there’re a large number of defects between graphene layers after the strong oxidation. The latter characteristics can be attributed to the doubly degenerate phonon (E2g) mode at the Brillouin zone center [36]. Whereas for PEG (Fig. 1c), the intensity of D peak is very weak compared that of G peak, which indicates the graphene prepared by physical 3
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Fig. 2. XPS spectra of the Cu 2p3/2 photoelectron core level for the electrodeposited Cu, graphene/Cu and multiple graphene/Cu on Cr–Cu/Si substrate: (a–c) as prepared soon; (d–f) after 1-week air exposure with 50% R.H.; (g–i) after 1-week air exposure with 40% R.H.
increases, the deposition rate of copper increases remarkably, however, the current efficiency is decreasing (Fig. S2h and i), which may be due to the strong side reaction of cathode at a large current density. Above all, considering both the morphology, structure, preferred orientation, deposition rate and current efficiency, the current density of 20 mA/cm2 was chose to prepare the electrodeposited Cu nanostructures. As shown the SEM images of graphene/Cu heterostructure (Fig. 1g and h), graphene adsorbed on copper can be readily observed at a higher magnification, from which the copper is nano-sized with the even dis tribution, and the graphene is like a thin yarn with wrinkled edges. Besides, the survey XPS spectrum further confirm the existence of car bon, oxygen, and copper, and the atomic percentage of individual ele ments is also displayed (Fig. 1i). Fig. 1j provides details on the elemental surface scanning of graphene/Cu nanostructure. The position adsorbed with graphene corresponds to the carbon-rich region, however, the exposed Cu particles correspond to the carbon-poor region of energy dispersive spectroscopy (EDS). Meanwhile, O atoms are distributed evenly on the surface of sample, which indicates that there is a great possibility of the reaction between Cu and O. Raman spectroscopy is a powerful non-destructive technique for identifying the structure, doping and disorder of graphene [37]. To our knowledge, the G peak position is a sensitive indicator for the doping state and internal structure changes
of graphene [47]. Fig. 1k presents key information on the red shift of G band, concretely, it can be surveyed that the G band moves about 5 cm 1 from 1580 cm 1 (physically exfoliated graphene on silicon) to 1575 cm 1 (Cu-doped graphene), from which the stress of graphene in carbon atoms plane can be deduced [48]. Normally, since the metal work function of Cu (111) plane is significantly higher than that of graphene, thus, the p-type doping of graphene could be formed and cause blue shift of G peak when the graphene is in contact with copper [49]. However, the blue shift of G peak isn’t found in our Raman spectra after doping with copper. It may be explained that the red shift caused by stress on G peak masks the blue shift caused by hole-doping of graphene, therefore, the blue shift of G peak caused by charge transfer is not observed. Fortunately, the direct evidence of electron transfer from graphene to copper is obtained from the Cu 2p3/2 photoelectron spectroscopy (Fig. 1l). After interacting with graphene, the metallic Cu component in Cu 2p3/2 peak shifts to lower binding energy, from 932.8 eV to 932.4 eV. That is, a negative shift of 0.4 eV to lower binding energy is caused by the electron transfer from graphene to copper [50]. Meanwhile, the shift of Dirac point relative to the Fermi level induced by physical doping, and the visualization of electron transfer from graphene to copper are also presented.
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Fig. 3. XPS spectra of the C 1s and O 1s photoelectron core levels for the as-prepared electrodeposited Cu without and with graphene on the top.
2.2. Confirmation of the oxidation resistance and copper oxides reduction
for the samples adsorbed with graphene, part of Cu is converted into Cu (OH)2 rather than CuO, but the content of unoxidized Cu is still higher than that of oxidized Cu. In order to further explore the evolution of graphene-Cu interaction, the identical as-prepared samples were exposed to air for 1-week at a lower humidity, e.g. 40% R.H. The experimental data shows the chemical state of Cu 2p3/2 for pure elec trodeposited Cu has not changed substantially, that is, most of the metallic Cu is completely oxidized to CuO. What’s interesting, though, is that the chemical state of Cu 2p3/2 for the graphene/Cu heterostructure changes noticeably. Specifically, the proportion of Cu(OH)2 decreases from 37% to 11% for the graphene/Cu/Si, and a decrease from 30% to 10% for the multiple graphene/Cu/Si. Fig. S4 shows the Cu 2p electron spectra of pure Cu, graphene/Cu, and multiple graphene/Cu, from which pure Cu shows obvious satellite peaks about 9 eV apart from the main Cu 2p peaks, indicating the presence of copper oxides. Whereas for graphene/Cu heterostructure, the corresponding satellite peaks are very small, and shows no satellite structure at all for multiple graphene/Cu, indicating the almost absence of copper oxides, which is consistent with the discussed results of Fig. 2. Fig. 3 shows the C 1s and O 1s spectra of Cu with and without gra phene. Before graphene contacting, the C 1s spectrum appears fit-
For purpose of ascertaining the information about the chemical state of graphene and copper, samples were analyzed by X-ray photoelectron spectroscopy (XPS), survey XPS spectra indicate that carbon, oxygen and copper are detected (Fig. S3). Fig. 2 presents the Cu 2p3/2 spectra of electrodeposited Cu with and without graphene. For the Cu 2p3/2 spectra of as-prepared samples, obvious differences could be revealed between the each sample. Typically, the Cu/Si is fitted by Cu with a small amount of CuO, and the CuO is probably resulting from the rapid oxidation of Cu during the sample preparation and measurement [51]. After spin-coating the graphene, the content of metallic Cu is still dominant, but there is a small amount of Cu(OH)2, which is ascribed to the chemical reaction between Cu2O with O2 and H2O in the air [52]. Besides, the sample with spin-coated graphene for twice was also pre pared, which was marked as multiple graphene/Cu/Si. As shown in Fig. 2c, only metallic Cu is detected, indicating the energy barrier formed by graphene could effectively avoid the oxidation of electro deposited Cu [53]. After 1-week air exposure at 50% R.H., the content of CuO is much higher than that of Cu for the deposited pure Cu nano structure, indicating the serious oxidation of Cu nanostructure. Whereas 5
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increasing number of spin coating graphene improves the coverage of graphene on the copper surface. In addition, the electron transfer caused by difference in metal work function between graphene and copper is essential in such graphene/Cu heterostructure. For the preferred orientation of Cu (111) crystal plane, 4.98 eV and 5.10 eV have been reported, which is higher than that of the bilayer graphene, i.e., 4.69 eV. Therefore, electron transfer occurs naturally during the formation of graphene/Cu heterostructure, and results in a downward shift of Fermi level and p-type doping of graphene, which is conforming to the XPS investigation. The graphene/Cu heterostructure exposed to moist air (50% R.H.) for 1-week shows considerable proportion of Cu(OH)2 (Fig. 2e and f), whereas the identical sample exposed to low relative humidity (40% R.H.) for 1-week reveals no obvious changes (Fig. 2h and i) compared that of the sample without exposure (Fig. 2b and c). This suggests the moisture in the air affects the formation of Cu(OH)2, as shown by the chemical reaction (Table S2), that is, as the moisture content decreases, the formation of Cu(OH)2 is suppressed [52]. How ever, there is no fitting component of Cu2O in our experimental data, indicating the involvement of deoxidation process for copper oxides. In addition, as shown the reaction equation of CuO in Table S2, the enthalpy is negative and the entropy is positive, so the reaction could be proceed spontaneously under any conditions. Thus, we speculate that in addition to Cu(OH)2, the copper oxides is mainly existed in the form of Cu2O and is placed in a dynamic state for graphene/Cu heterostructure. In order to explain the deoxidation case that the copper oxides are significantly reduced in exposure conditions with low relative humidity, a chemical potential is introduced (Fig. 4b). That is, the reduction po tential of CuO (Cu2þþ2e →Cu; E0 ¼ þ0.34 V) and Cu2O (Cuþþe →Cu; E0 ¼ þ0.52 V) is higher than that of graphene (þ0.22 V) and the con version of O2 to superoxide involving only individual electron (O2þe →O2 ; E0 ¼ 0.16 V) [57,58]. Since the reduction potential of Cuþ is higher than that of Cu2þ, therefore, it’s more likely to react with graphene and cause electron transfer. Specifically, the electron is injected into copper from graphene and triggers the following reactions (Scheme 1), where graphene is marked as Cgr and serves as medium for the electron transfer and copper deoxidation, and Cgr*O2 implies the molecular O2 anion chemisorbed on graphene. The mechanism we speculate is rest on the appreciation that graphene performs as an electron transfer medium and catalyst. The electron is implanted into Cu by cause of the metal work function difference, conversely, the trans ferred electron initiates the reduction of Cu according to the chemical reduction potential, and generates chemisorbed O2 on the graphene, which is released finally. The mechanism proposed in this paper is distinct from the antioxidant function reported previously, which is simply forming a protective layer for copper [33,35,54].
Fig. 4. (a) The reasonable mechanism proposed in this paper about the oxidation resistance and deoxidation for the graphene/Cu heterostructure. (b) The involved electron transfer and chemical reactions on thus mechanism.
components at 285.0 eV and 288.5 eV, which is due to the C-sp3 from adventitious carbon contamination and the C–O adsorbed on Cu from air [54]. After Cu is in contact with graphene, new C 1s components, at 284.8 eV owing to C-sp2 of graphene, at 286.6 eV owing to O–C–O, and – O, emerge [42,55]. Accordingly, the O 1s at 288.2 eV owing to C– photoelectron level of pure deposited Cu plays a dominant composition at 531.7 eV, attributed to the chemisorbed O2, and is followed by another composition at 530.6 eV due to the CuO. Once the contacting with graphene, the chemical state of O 1s changes greatly, from the chemisorbed O2 to functional groups bonded with carbon, and the hy droxide formed at surface. In detail, the fitted components at 531.4 eV, – O, OH , 531.7 eV, 532.2 eV and 533.2 eV are corresponding to the C– chemisorbed O2 and O–C–O, respectively [35,52]. Moreover, the gra phene/Cu and multiple graphene/Cu are consistent in fitting results for C 1s and O 1s.
2.4. Graphene/electrodeposited Cu heterostructure for TENG applications
2.3. Interpretation of the oxidation resistance and copper oxides reduction
As with conventional triboelectric nanogenerators, the working principle of S-TENG is also involved in contact electrification and elec trostatic induction (Fig. S5a) [9]. Specifically, once the contact between PDMS and finger, electrons could be transferred from finger to PDMS due to the different triboelectric coefficient between them, which results in a negatively charged PDMS surface and a positively charged finger. When the finger and PDMS are moved and separated, the positive charge on finger and the negative charge on PDMS cannot be completely neutralized due to the presence of air layer. In order to achieve elec trostatic equilibrium, the graphene/Cu electrode induces positive charge that is opposite to the electrical polarity remaining on the PDMS
The mechanism of oxidation resistance and deoxidation for copper can be explained in the following three aspects, as illustrated in Fig. 4a. That is, the energy barrier formed by graphene, the difference in metal work function between graphene and copper, and the electrochemical potential of graphene compared that of copper oxides. Clearly, the spincoated graphene on the surface of electrodeposited Cu structure divides space into two domains, each domain has different physical or chemical properties. Since the impermeability of graphene film has been demonstrated previously, i.e., it can hinder atoms or molecules from the external atmosphere [56]. Therefore, graphene acts as an energy barrier on the surface of Cu structure, so as to reduce the touch probability of deposited Cu with air in a certain degree. As displayed in Fig. 2b, after spin-coating graphene for once, some areas of deposited Cu are not adsorbed by graphene, thus for graphene/Cu, there is presence of Cu (OH)2. For the sample spin-coated graphene for twice, the results of Cu 2p3/2 are basically free of copper oxidation, which is mainly due to the
Scheme 1. The chemical reaction equation involved in copper ox ides reduction. 6
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Fig. 5. (a) The schematic diagram and optical picture of skin-based flexible single-electrode triboelectric nanogenerator. The output characteristics of TENG based on graphene/Cu/PDMS: (b) output voltage; (c) transfer charge quantity. (d) The bending performance of flexible electrode based on graphene/Cu/PDMS. (e) The variation of output voltage, current and power density with the external load resistance. (f) LED application: blue LED array powered by TENG. (g–i) Flexible applications of skin-based single-electrode triboelectric nanogenerator.
and causes the migration of free electrons towards to the ground through an external circuit. Conversely, when finger is closed to PDMS, electrons are also transferred between the electrode and ground through elec trostatic induction. Besides, the experimental data that verify the working principle is also presented (Fig. S5b). The device structure of flexible single-electrode TENG developed in this work is illustrated in Fig. 5a, which is composed of graphene/Cu heterostructure induction electrode and PDMS films. As shown in Fig. 5b and c, the peak value of output voltage and transfer charge quantity based on graphene/Cu/PDMS are ~60 V and ~2.5 nC, respectively, which is equivalent to or better than the previously reported TENGs based on pure PDMS [59–61]. In order to verify its reliability in flexible application scenarios, the TENG was bent and folded for 10000 times. As shown in Fig. S6, the micro-morphological characteristic of flexible electrode without and with bending 10000 times shows no significant changes, in addition, the electrodeposited Cu nanostructure shows no obvious cracks in a large scanning range (12*9 μm). The bending ex amination shows the sheet resistance of flexible electrode (Fig. 5d) and the electrical output characteristics of TENG are not significantly dete riorated compared to that of the devices without bending, indicating the
flexibility and reliability of such TENG based on graphene/Cu/PDMS. Additionally, the output power density exhibited by graphe ne/Cu/PDMS based TENG under different external load resistance is evaluated, which is determined to the power generation efficiency. The output voltage, current and power density, as a function of the applied external load, are shown in Fig. 5e. Typically, the output voltage in creases with the increasing load resistance, while the current gradually decreases, which results in a balanced value of power density due to the ohmic loss. Notably, the peak value of power density is reached at 91.9 mW/m2 when the external load is 100 MΩ, which is superior to that of the reported works based on pure PDMS [62]. Based on the mechanism of graphene/Cu heterostructure proposed in this work to enhance the stability of TENG, the stability of such device was verified. In order to be consistent with the XPS analysis that verify the stability of graphene/Cu heterostructure for 1-week, the prepared inducing electrodes (graphene/Cu/PDMS and Cu/PDMS) were also placed in air for 1-week to inspect their performance changes with and without exposure. As seen in Fig. S7, after exposure in air for 1-week, the performance of graphene/Cu heterostructure-based TENG doesn’t change markedly (Fig. S7a). However, for Cu-based TENG, its 7
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performance deteriorates significantly as the expected (Fig. S7b), which is mainly due to the instability of Cu and its severe corrosion behavior in the air. As a result, the stability and electrical property of inducing electrode is worsen, and the performance of TENG is also deteriorated. Finally, the stability experiment based on TENG performance is consis tent with that based on XPS analysis, indicating the excellent stability of graphene/Cu heterostructure. For purpose of detecting the response time of such TENG, the electrical output at different duration of the first contact between skin and PDMS was compared (Fig. S8). The opencircuit voltage shows no significant changes with the increasing dura tion time (from 1 s to 10 s), indicating the TENG has favorable inducing capability. Besides, the response sensitivity of the releasing and approaching process of TENG at working status was also studied. The result shows that the response time of the releasing and approaching process is about 50 ms, indicating the TENG has excellent response time. For purpose of investigating the practical applications of such pro posed TENG, it was directly linked to light-emitting diodes (LEDs) associated in series, and 22 blue lights could be powered by fingers typing (Fig. 5f). Moreover, it was also fitted with skin and explored the flexible applications, showing that certain electrical signals could be generated in the knuckles, elbows and knees (Fig. 5g–i). Undoubtedly, the TENG proposed in this work will act as a reliable supplier and pro vide an ideal strategy for power electronics.
sputtering. Afterwards, the UV laser processing system was used to divide the integral substrate into sample wafers with certain size for the subsequent usage, e.g. 20*20 mm, 5*5 mm. Then, the preparation of electrodeposited Cu layer on the conductive substrate was completed by using the Cu2þ solution configured by our team. Meanwhile, the mois ture removal was very essential after accomplishing the fabrication of Cu layer by electrodeposition, therefore, the adsorbed moisture on Cu film was removed by high-speed rotation and heating. Finally, the gra phene dispersion was modified on the deposited Cu film by spin coating and dried in a vacuum oven to form a thin film. 4.3. Fabrication of the flexible electrode of graphene/Cu/PDMS Fig. S9 shows the schematic illustration of the fabrication process of flexible conductive electrode based on graphene/Cu/PDMS. Firstly, a layer of 10 μm thick positive photoresist (PR) was spin-coated on the pretreated glass substrate, and then placed in an oven at 110 � C to be cured by heating for 2 h. After that, the pre-treated PDMS mixture was spin-coated onto the cured photoresist and placed on a hot plate at 120 � C for 1 h. After the completion of PDMS curing, the PDMS/PR/Glass substrate was immersed in acetone to remove the photoresist layer, resulting in a stress-free, and flat PDMS film. Subsequently, the peeled PDMS film was ultrasonically washed in sodium hydroxide and alcohol to obtain a smooth and transparent PDMS film. Then, the PDMS film was placed on the surface of glass substrate and dried in an oven at 60 � C. Afterwards, the preparation strategy of structured graphene/Cu/PDMS/ Glass was followed by process of graphene/Cu film aforementioned. Finally, the PDMS was stripped from glass substrate to complete the preparation of flexible electrode, i.e., graphene/Cu/PDMS.
3. Conclusions In conclusion, constructing the nanostructure of graphene/Cu has been confirmed to be an effective strategy for enhancing the stability of copper. Such graphene/Cu heterostructure is fabricated by spin-coating the graphene dispersion on the surface of preferred Cu (111). As stated in the results of Raman and XPS, the internal stress and electron transfer could be occurred naturally, which is coupled with the electrochemical potential difference between graphene, copper and oxygen, and an interactive mechanism is proposed for explaining the improved stability of graphene/Cu heterostructure. Additionally, electrodeposition tech nology is utilized to fabricate copper nanostructure and is demonstrated to be an ideal approach for the preparation of flexible induction elec trodes utilized in TENG. Finally, a skin-based S-TENG based on gra phene/Cu/PDMS nanostructure is fabricated, which acts as a reliable supplier and shows superior performance and promising potential in TENG applications and power electronics.
4.4. Fabrication of the triboelectric nanogenerator (TENG) with single electrode The prepared graphene/Cu/PDMS flexible electrode was cleaned with deionized water and dried with nitrogen to remove the adsorbed impurities on the surface. Subsequently, wires were bonded at the edge of electrode by using silver paste to lead electrons, and were cured at 60 � C for 20 min. Then the completed device was preserved for the subse quent examinations. 4.5. Electrical performance assessment
4. Experimental section
The Keithley 6514 electrometer was used to examine the electrical performance of a single-electrode triboelectric nanogenerator fabricated in this paper, including the open circuit voltage, electrostatic current, and transfer charge. The noise of electrometer was less than 1 fA, the input impedance in the voltage measurement was greater than 200 TΩ, and the transfer charge measurement range was 10 fC~20 μC.
4.1. Graphene dispersion fabrication Reduced graphene oxide (RGO) was obtained by reducing graphene oxide (GO) with ascorbic acid, and the GO was synthesized from natural graphite powder (325 mesh, Aladdin Reagent Co.) by an improved Hummers’ method [63]. The physically exfoliated graphene was sup plied by Tanfeng Graphene Technology Co. Ltd. As for graphene dispersion, the weighed 50 mg graphene powder was slowly added to 50 ml deionized water and dispersed in an ultrasonic cleaner for 1 h. Then, a certain amount of sodium dodecyl sulfonate was added into the graphene dispersion and sonicated again for 20 h with the power of 450 W. Meanwhile, the solution was stirred with glass rod every hours of sonication for purpose of obtaining the uniform and stable graphene dispersion.
4.6. Characterization Phase analysis was accomplished on a D/max 2500PC highresolution X-ray diffractometer (Rigaku, Japan) with Kα radiation. The electrodeposited Cu film with different current density was examined by XRD, and the effects of current density on the texture coefficient of Cu crystal planes were also analyzed. The micro-morphological character istics and elemental information were obtained on a JEOL JSM-7800F field-emission scanning electron microscope (JEOL, Japan). Specif ically, the morphology of graphene and graphene/Cu heterostructure, the influence of current density on the morphology of electrodeposited Cu, and the surface element distribution of graphene/Cu structure were confirmed. Microstructural analysis was carried on a Renishow Invia Reflex micro-confocal spectrometer (Renishaw, Britain) with 514.5 nm laser excitation. Raman spectroscopy is a powerful non-destructive technique for identifying the structure, doping and disorder of gra phene. The characteristic peaks of graphene, such as D, G and 2D peaks,
4.2. Construction of the structured graphene/Cu film Before sputtering, silicon wafer with 250 μm thickness was treated in a plasma cleaner for purpose of removing the surface contaminants and promoting the adhesion of cadmium/copper (Cr–Cu) layer to silicon substrate. After that, the adhesive layer of Cr and the conductive layer of Cu were deposited on the cleaned silicon substrate by magnetron 8
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were detected, and the G peak shift of graphene caused by stress was also analyzed. Chemical analysis was recorded by X-ray photoelectron spectroscopy using Sigma probe XPS (Thermo VG Scientific), and all XPS measurements were proceeded at room temperature. The survey spectra of Cu, graphene, and graphene/Cu structure were obtained, and the chemical states of the elements, such as copper, oxygen and carbon, were also analyzed. Additionally, the comparative information of sam ples before and after exposure in air was also completed by XPS.
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Nano Energy 70 (2020) 104540 Yahui Li received his M.S. degree in materials physics and chemistry from Harbin Institute of Technology, and is currently a Ph.D. candidate of electronic science and technology at Shanghai Jiao Tong University. His research interests focus on the energy harvesting and MEMS sensors.
Han Cai received her M.S. degree in mechanical engineering from Xiamen University, and is currently a Ph.D. candidate of electronic science and technology at Shanghai Jiao Tong Uni versity. Her research interests focus on the micro-nano flexible sensing technology and the design of microchannel heat-sink.
Wei Zheng received his B.S. degree in mechatronics engineer ing from Northwestern Polytechnical University, and is currently a M.S. degree candidate of electronic and communi cations engineering at Shanghai Jiao Tong University. His research interests focus on the flexible electronic skin and MEMS sensors.
Yanxin Zhang received his M.S. degree in materials processing engineering from Harbin Institute of Technology, and is currently a Ph.D. candidate of electronic science and technol ogy at Shanghai Jiao Tong University. His research interests focus on the electronic packaging integration technology and its application.
Haodong Zhang received his B.S. degree in optoelectronic in formation science and engineering from Sun Yat-sen University, and is currently a Ph.D. candidate of electronic science and technology at Shanghai Jiao Tong University. His research in terests focus on the curved lithography technology and its applications.
Prof. Zhuoqing Yang received his B.S. and M.S. degree in electromechanical engineering from Harbin Engineering Uni versity, respectively, and the Ph.D. degree in microelectronics and solid state electronics from Shanghai Jiao Tong University. In 2011, he was awarded a prestigious JSPS post-doctoral fellowship and worked at the National Institute of Advanced Industrial Science and Technology in Tsukuba. Currently, he is a full professor with the National Key Laboratory of Science and Technology on Micro/Nano Fabrication in SJTU. He is also an editorial board member of the International Journal of Micro and Nanosystems. His research interests include the design, simulation, and fabrication of MEMS/NEMS and the flexible sensing devices.
Haoqiang Wang received his M.S. degree in materials engi neering from Harbin Institute of Technology, and is currently a Ph.D. candidate of materials science and engineering at Harbin Institute of Technology. His research interests focus on the design and applications of sodium-ion hybrid capacitors.
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Full paper
Electron transfer mechanism of graphene/Cu heterostructure for improving the stability of triboelectric nanogenerators Yahui Li a, Wei Zheng a, Haodong Zhang a, Haoqiang Wang b, Han Cai a, Yanxin Zhang a, Zhuoqing Yang a, * a
National Key Laboratory of Science and Technology on Micro/Nano Fabrication, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China State Key Laboratory of Advanced Welding and Joining, School of Materials Science and Engineering, Harbin Institute of Technology (Shenzhen), Shenzhen, 518055, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Triboelectric nanogenerators (TENG) Graphene Metal work function Electrodeposited copper nanostructure Temporal stability
Harvesting energy from the mechanical motion has been demonstrated to be a preferable strategy to satisfy the requirements for self-powered electronics. Here we report a flexible and stable induction electrode based on graphene/Cu heterostructure, which is fabricated by electrodeposition and spin-coating, and is also utilized in energy harvesting for triboelectric nanogenerator (TENG). Typically, graphene dispersion prepared by physical exfoliation is spin-coated on the copper nanostructure based on PDMS. Besides, we provide favorable evidences that electrons transfer from graphene to copper, and ignite the reduced reaction of copper oxides. The function of graphene in avoiding the oxidation of copper nanostructure, thus improving the stability of graphene/Cu het erostructure and achieving its applications in triboelectric nanogenerator, is confirmed. The interactive mech anism is formulated, including energy barrier, metal work function and the electrochemical potential difference between graphene, copper and oxygen. The enhanced stability of graphene/Cu heterostructure is inspected by the chemical state of copper as the function of exposure time, and the applicability is also evaluated by their output electrical performance for TENG applications.
1. Introduction In view of the increasingly exhausted resources and severe energy crises, thus, it’s a challenge to explore renewable and green energy facing the world, which is indispensable and significant for our social development and human survival [1–4]. Harvesting energy from ambient environment brings forward a desirable strategy to enlarge the applications of the renewable and clean energy, such as solar, wind, water and mechanical motion [5–12]. In particular, mechanical motion, which is widely existed in the ambient environment and people’s daily lives, has become a preferable target for energy harvesting in recent years [13–17]. Triboelectric nanogenerator (TENG), coupling the con tact electrification and electrostatic induction, has been proved to be a promising energy harvester due to their excellent reliability, large output power, high efficiency and low cost [18–22]. Noticeably, only one friction surface is required for the single electrode TENG (S-TENG), whereas the other friction surface can be replaced by human skin, which greatly expand the applications of TENG in wearable electronic
equipment [23,24]. Presently, indium-tin-oxide (ITO) or noble metals are the most commonly used electrodes for TENG fabrication [25–27]. However, it’s unrealistic for substantial production and application due to their scarce storage and expensive costs. Therefore, the development of advanced electrode materials and processing technologies to satisfy the reliability, cost-effectiveness, and scalability for TENG is imminent. Metallic copper displays analogous performance in mechanical and electrical properties compared that of noble metals, such as silver. Thus, it’s considered to be an ideal alternative to noble metals, showing great potential in a great many of applications [28]. However, the obvious disadvantage of copper is its chemical instability, that is, it’s susceptible to be oxidized to copper oxides according to the chemical equations (Table S1), where the changes of enthalpy and Gibbs free energy at 298.15 K are both negative, indicating the reaction could be occurred spontaneously at room temperature and the properties of copper could be impaired once the oxidation [29]. Therefore, for purpose of exploring the applications of copper nanostructure in TENG and acquiring the high-performance devices based on copper structure, it is critical to
* Corresponding author. E-mail address: [email protected] (Z. Yang). https://doi.org/10.1016/j.nanoen.2020.104540 Received 5 December 2019; Received in revised form 21 January 2020; Accepted 25 January 2020 2211-2855/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 1. SEM images of graphene dispersions dropped on the silicon wafer: (a) PEG and (b) RGO. Raman spectra of graphene excited at 514.5 nm laser: (c) PEG and (d) RGO, inset in (c) shows the four fitted Lorentzians for 2D peak of PEG. The detailed XPS spectra for PEG and RGO: (e) C 1s and (f) O 1s. (g, h) The SEM images of graphene/Cu heterostructure. (i) The survey XPS spectrum of graphene/Cu heterostructure, and the atomic percentage of individual elements. (j) The surface scanning pictures of graphene/Cu heterostructure, including the element information of carbon, copper and oxygen. k) Typical Raman spectra of G band for physical exfoliation with and without contacting electrodeposited Cu. l) Position shift of binding energies for Cu 2p3/2 before and after doping graphene on the electro deposited Cu substrate. Schematic diagram illustrates the difference of work function between graphene and polycrystalline Cu, and visualizes the electron transfer from graphene to Cu.
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establish novel mechanism for copper protection. As a cost-effective and multifunctional processing technology, electrodeposition has been extensively utilized for the deposition of coatings and films due to their outstanding performance, such as good uniformity, high deposition rate, simplicity for mass manufacturing, controllable thickness and grain orientation [30]. In particular, copper is the most widely studied metal in electrodeposition due to its excellent electrical conductivity and machinability, which establishes its position in industrial applications. Here, we first report the preparation of flexible electrode for TENG based on the electrodeposited Cu nanostructure. Ever since it was discovered, graphene has become a rising star in the field of research by cause of its exceptional electrical, chemical and mechanical properties [31,32]. Additionally, recent works, functional izing metal nanostructure with graphene, have been confirmed to be an efficient approach to enhance the properties of metal nanostructure, such as chemical stability, conductivity, and oxidation resistance, etc. Yan et al. fabricated graphene/AgNW hybrid film, where Ag nanowires were acted as electric network and graphene was served as protective layer. Moreover, the as-prepared hybrid film displayed excellent con ductivity, and good superior oxidation resistance [33]. The improved chemical stability of Ag was also demonstrated by Tang et al., which modified the randomly distributed silver nanoparticles with reduced graphene oxide [34]. Wang et al. achieved the enhanced oxidation resistance and stability of Cu nanowires by decorating with multilayered reduced graphene oxide [35]. However, the above mentioned oxidation resistance and stability exerted by graphene is simply forming a pro tective layer for metal nanostructure, which ignores the discussion about chemical structure and electron transfer information between the graphene-metal heterostructure. Thus interaction mechanism between graphene and metal is exactly what this work emphasizes. In this work, we investigate the internal stress and electron transfer between physical exfoliating graphene and electrodeposited Cu nano structure for purpose of revealing the functioning nature of graphene/ Cu heterostructure, achieving the fundamental goal of avoiding copper oxidation, and realizing its applications in TENG. Graphene/Cu nano structure is fabricated by spin-coating the graphene dispersion on the surface of preferred Cu (111). The information of doping and electron transfer between graphene and copper is directly demonstrated by Raman and XPS, and the chemical state of copper as the function of exposure time is also acquired. In addition, we provide favorable evi dences that electrons could transfer from graphene to copper, and ignite the reduced reaction of copper oxides. Finally, the flexible and conductive electrode based on graphene/Cu/PDMS nanostructure is proposed, and is successfully utilized in the TENG applications.
exfoliation has fewer defects, and more perfect structure. It is well recognized that the intensity of I2D/IG and the shape of 2D peak is closely dependent on the number of graphene layers [37]. As shown in Fig. 1d, the intensity of 2D peak is very weak with the wide shape, indicating the graphene is stacked in layers. In the case of PEG, the intensity of 2D peak is closer to that of G peak with the significant enhancement, and the peak is sharper. Moreover, the four fitted Lorentzians are also acquired with peak positions at 2661 cm 1, 2693 cm 1, 2717 cm 1, and 2739 cm 1, respectively, which is consistent with the bilayer graphene re ported previously [38,39]. In order to study the chemical composition of graphene prepared by PEG and RGO, XPS analysis was applied to detect the element infor mation. As illustrated in Fig. S1, for both types of graphene, they are composed of carbon and oxygen. The difference, however, is the atomic percentage of the two elements. The percentage of carbon and oxygen for RGO is 92% and 8%, respectively, which indicates that some oxygencontaining functional groups are remained in graphene after the chemical reduction. However, the existence of oxygen-containing functional groups would worsen the properties of graphene, such as the conductivity [40]. Contrastively, the content of oxygen in PEG de creases sharply, and the small amount of oxygen atoms may be caused by the formed O–C–O bonds with oxygen during the sample preparation. Fig. 1e displays the detailed C 1s spectrum of RGO and PEG. The C 1s spectrum of RGO can be fitted by a series of components at 284.8 eV, 285.7 eV, and 286.6 eV, which correspond to C-sp2, C–OH, and O–C–O, respectively [41,42]. However, the C 1s detailed spectrum of PEG is mainly fitted by the component of C-sp2 with a very small amount of O–C–O bonds. Correspondingly, the O 1s spectra are also obtained and analyzed (Fig. 1f), and the fitted results are highly coincident with those of C 1s [43]. Therefore, the physical exfoliation graphene was selected to be used in this study based on the above analysis. As we all know, the metal work function varies with crystal plane orientation and has anisotropy, which would directly involve the graphene-Cu interaction in this work [44]. Besides, the structures and properties of electrodeposited Cu are intimately dependent on the selected electrodeposition parameters, especially the current density. Therefore, a series of current density were attempted to control the crystal orientation and morphology of copper nanostructure based on the electrodeposition technology. As shown the SEM images in Fig. S2a–e, it can be seen that when the current density is 2 mA/cm2, the Cu particles are large in size and approximate a disordered polyhedral shape. Moreover, the deposited Cu structures are uneven and have poor compactness. Once increasing the current density, however, the size of electrodeposited Cu particles decreases significantly, and the boundaries between Cu particles become less noticeable. The XRD patterns of Cu deposited at various current density are shown in Fig. S2f, both of which display obvious characteristic peaks. At the lower current density of 2 mA/cm2, one distinguished peak and two frail peaks could be seen at 43.3� , 50.4� and 74.1� , which correspond to the (111), (200) and (220) crystal planes of metallic copper, respectively [35]. Additionally, the texture coefficient of (111) crystal plane decreases with the increasing current density, on the contrary, the texture coefficient of (200) crystal plane increases with the increasing current density (Fig. S2g). Since the required energy for (200) crystal nucleation is higher, thus, increasing the current density is more preferable to the nucleation and growth of particles on the (200) crystal plane. However, the preferred orientation of (111) crystal plane is still dominant. For the transition metal copper of fcc structure, the dense plane (111) has the lowest surface energy and the largest metal work function. In particular, the metal work function corresponding to the (111), (100) and (110) crystal planes of copper are 4.98 eV (5.10 eV), 4.59 eV and 4.48 eV, respectively [45,46]. Accord ingly, the work function for bilayer graphene is 4.69 eV, which is lower than that of the metallic Cu (111) crystal plane. Therefore, electron transfer could be occurred when metal copper is in contact with gra phene, and thus electron transfer is very essential for the interaction between copper and graphene. Additionally, as the current density
2. Results and discussion 2.1. Evidences for doping and electron transfer from graphene In this work, two kinds of graphene obtained by physical exfoliation (PEG) and chemical reduction (RGO) were used for preparing the stable graphene dispersion. As shown in Fig. 1a, the PEG is like a thin layer of yarn, and the physical separation is realized between layers with a lot of edge warping. However, the agglomeration of RGO sheets is serious (Fig. 1b), which probably deteriorates the intrinsic properties of gra phene. Besides, the structural characteristics of PEG and RGO were also investigated by Raman spectroscopy with 514.5 nm laser excitation. For the spectrum of RGO (Fig. 1d), two distinct peaks at 1346 cm 1 and 1584 cm 1 are observed, corresponding to the D and G bands of gra phene, respectively. Generally, the former is due to the disorder caused by the introduction of structural defects, which indicates that there’re a large number of defects between graphene layers after the strong oxidation. The latter characteristics can be attributed to the doubly degenerate phonon (E2g) mode at the Brillouin zone center [36]. Whereas for PEG (Fig. 1c), the intensity of D peak is very weak compared that of G peak, which indicates the graphene prepared by physical 3
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Fig. 2. XPS spectra of the Cu 2p3/2 photoelectron core level for the electrodeposited Cu, graphene/Cu and multiple graphene/Cu on Cr–Cu/Si substrate: (a–c) as prepared soon; (d–f) after 1-week air exposure with 50% R.H.; (g–i) after 1-week air exposure with 40% R.H.
increases, the deposition rate of copper increases remarkably, however, the current efficiency is decreasing (Fig. S2h and i), which may be due to the strong side reaction of cathode at a large current density. Above all, considering both the morphology, structure, preferred orientation, deposition rate and current efficiency, the current density of 20 mA/cm2 was chose to prepare the electrodeposited Cu nanostructures. As shown the SEM images of graphene/Cu heterostructure (Fig. 1g and h), graphene adsorbed on copper can be readily observed at a higher magnification, from which the copper is nano-sized with the even dis tribution, and the graphene is like a thin yarn with wrinkled edges. Besides, the survey XPS spectrum further confirm the existence of car bon, oxygen, and copper, and the atomic percentage of individual ele ments is also displayed (Fig. 1i). Fig. 1j provides details on the elemental surface scanning of graphene/Cu nanostructure. The position adsorbed with graphene corresponds to the carbon-rich region, however, the exposed Cu particles correspond to the carbon-poor region of energy dispersive spectroscopy (EDS). Meanwhile, O atoms are distributed evenly on the surface of sample, which indicates that there is a great possibility of the reaction between Cu and O. Raman spectroscopy is a powerful non-destructive technique for identifying the structure, doping and disorder of graphene [37]. To our knowledge, the G peak position is a sensitive indicator for the doping state and internal structure changes
of graphene [47]. Fig. 1k presents key information on the red shift of G band, concretely, it can be surveyed that the G band moves about 5 cm 1 from 1580 cm 1 (physically exfoliated graphene on silicon) to 1575 cm 1 (Cu-doped graphene), from which the stress of graphene in carbon atoms plane can be deduced [48]. Normally, since the metal work function of Cu (111) plane is significantly higher than that of graphene, thus, the p-type doping of graphene could be formed and cause blue shift of G peak when the graphene is in contact with copper [49]. However, the blue shift of G peak isn’t found in our Raman spectra after doping with copper. It may be explained that the red shift caused by stress on G peak masks the blue shift caused by hole-doping of graphene, therefore, the blue shift of G peak caused by charge transfer is not observed. Fortunately, the direct evidence of electron transfer from graphene to copper is obtained from the Cu 2p3/2 photoelectron spectroscopy (Fig. 1l). After interacting with graphene, the metallic Cu component in Cu 2p3/2 peak shifts to lower binding energy, from 932.8 eV to 932.4 eV. That is, a negative shift of 0.4 eV to lower binding energy is caused by the electron transfer from graphene to copper [50]. Meanwhile, the shift of Dirac point relative to the Fermi level induced by physical doping, and the visualization of electron transfer from graphene to copper are also presented.
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Fig. 3. XPS spectra of the C 1s and O 1s photoelectron core levels for the as-prepared electrodeposited Cu without and with graphene on the top.
2.2. Confirmation of the oxidation resistance and copper oxides reduction
for the samples adsorbed with graphene, part of Cu is converted into Cu (OH)2 rather than CuO, but the content of unoxidized Cu is still higher than that of oxidized Cu. In order to further explore the evolution of graphene-Cu interaction, the identical as-prepared samples were exposed to air for 1-week at a lower humidity, e.g. 40% R.H. The experimental data shows the chemical state of Cu 2p3/2 for pure elec trodeposited Cu has not changed substantially, that is, most of the metallic Cu is completely oxidized to CuO. What’s interesting, though, is that the chemical state of Cu 2p3/2 for the graphene/Cu heterostructure changes noticeably. Specifically, the proportion of Cu(OH)2 decreases from 37% to 11% for the graphene/Cu/Si, and a decrease from 30% to 10% for the multiple graphene/Cu/Si. Fig. S4 shows the Cu 2p electron spectra of pure Cu, graphene/Cu, and multiple graphene/Cu, from which pure Cu shows obvious satellite peaks about 9 eV apart from the main Cu 2p peaks, indicating the presence of copper oxides. Whereas for graphene/Cu heterostructure, the corresponding satellite peaks are very small, and shows no satellite structure at all for multiple graphene/Cu, indicating the almost absence of copper oxides, which is consistent with the discussed results of Fig. 2. Fig. 3 shows the C 1s and O 1s spectra of Cu with and without gra phene. Before graphene contacting, the C 1s spectrum appears fit-
For purpose of ascertaining the information about the chemical state of graphene and copper, samples were analyzed by X-ray photoelectron spectroscopy (XPS), survey XPS spectra indicate that carbon, oxygen and copper are detected (Fig. S3). Fig. 2 presents the Cu 2p3/2 spectra of electrodeposited Cu with and without graphene. For the Cu 2p3/2 spectra of as-prepared samples, obvious differences could be revealed between the each sample. Typically, the Cu/Si is fitted by Cu with a small amount of CuO, and the CuO is probably resulting from the rapid oxidation of Cu during the sample preparation and measurement [51]. After spin-coating the graphene, the content of metallic Cu is still dominant, but there is a small amount of Cu(OH)2, which is ascribed to the chemical reaction between Cu2O with O2 and H2O in the air [52]. Besides, the sample with spin-coated graphene for twice was also pre pared, which was marked as multiple graphene/Cu/Si. As shown in Fig. 2c, only metallic Cu is detected, indicating the energy barrier formed by graphene could effectively avoid the oxidation of electro deposited Cu [53]. After 1-week air exposure at 50% R.H., the content of CuO is much higher than that of Cu for the deposited pure Cu nano structure, indicating the serious oxidation of Cu nanostructure. Whereas 5
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increasing number of spin coating graphene improves the coverage of graphene on the copper surface. In addition, the electron transfer caused by difference in metal work function between graphene and copper is essential in such graphene/Cu heterostructure. For the preferred orientation of Cu (111) crystal plane, 4.98 eV and 5.10 eV have been reported, which is higher than that of the bilayer graphene, i.e., 4.69 eV. Therefore, electron transfer occurs naturally during the formation of graphene/Cu heterostructure, and results in a downward shift of Fermi level and p-type doping of graphene, which is conforming to the XPS investigation. The graphene/Cu heterostructure exposed to moist air (50% R.H.) for 1-week shows considerable proportion of Cu(OH)2 (Fig. 2e and f), whereas the identical sample exposed to low relative humidity (40% R.H.) for 1-week reveals no obvious changes (Fig. 2h and i) compared that of the sample without exposure (Fig. 2b and c). This suggests the moisture in the air affects the formation of Cu(OH)2, as shown by the chemical reaction (Table S2), that is, as the moisture content decreases, the formation of Cu(OH)2 is suppressed [52]. How ever, there is no fitting component of Cu2O in our experimental data, indicating the involvement of deoxidation process for copper oxides. In addition, as shown the reaction equation of CuO in Table S2, the enthalpy is negative and the entropy is positive, so the reaction could be proceed spontaneously under any conditions. Thus, we speculate that in addition to Cu(OH)2, the copper oxides is mainly existed in the form of Cu2O and is placed in a dynamic state for graphene/Cu heterostructure. In order to explain the deoxidation case that the copper oxides are significantly reduced in exposure conditions with low relative humidity, a chemical potential is introduced (Fig. 4b). That is, the reduction po tential of CuO (Cu2þþ2e →Cu; E0 ¼ þ0.34 V) and Cu2O (Cuþþe →Cu; E0 ¼ þ0.52 V) is higher than that of graphene (þ0.22 V) and the con version of O2 to superoxide involving only individual electron (O2þe →O2 ; E0 ¼ 0.16 V) [57,58]. Since the reduction potential of Cuþ is higher than that of Cu2þ, therefore, it’s more likely to react with graphene and cause electron transfer. Specifically, the electron is injected into copper from graphene and triggers the following reactions (Scheme 1), where graphene is marked as Cgr and serves as medium for the electron transfer and copper deoxidation, and Cgr*O2 implies the molecular O2 anion chemisorbed on graphene. The mechanism we speculate is rest on the appreciation that graphene performs as an electron transfer medium and catalyst. The electron is implanted into Cu by cause of the metal work function difference, conversely, the trans ferred electron initiates the reduction of Cu according to the chemical reduction potential, and generates chemisorbed O2 on the graphene, which is released finally. The mechanism proposed in this paper is distinct from the antioxidant function reported previously, which is simply forming a protective layer for copper [33,35,54].
Fig. 4. (a) The reasonable mechanism proposed in this paper about the oxidation resistance and deoxidation for the graphene/Cu heterostructure. (b) The involved electron transfer and chemical reactions on thus mechanism.
components at 285.0 eV and 288.5 eV, which is due to the C-sp3 from adventitious carbon contamination and the C–O adsorbed on Cu from air [54]. After Cu is in contact with graphene, new C 1s components, at 284.8 eV owing to C-sp2 of graphene, at 286.6 eV owing to O–C–O, and – O, emerge [42,55]. Accordingly, the O 1s at 288.2 eV owing to C– photoelectron level of pure deposited Cu plays a dominant composition at 531.7 eV, attributed to the chemisorbed O2, and is followed by another composition at 530.6 eV due to the CuO. Once the contacting with graphene, the chemical state of O 1s changes greatly, from the chemisorbed O2 to functional groups bonded with carbon, and the hy droxide formed at surface. In detail, the fitted components at 531.4 eV, – O, OH , 531.7 eV, 532.2 eV and 533.2 eV are corresponding to the C– chemisorbed O2 and O–C–O, respectively [35,52]. Moreover, the gra phene/Cu and multiple graphene/Cu are consistent in fitting results for C 1s and O 1s.
2.4. Graphene/electrodeposited Cu heterostructure for TENG applications
2.3. Interpretation of the oxidation resistance and copper oxides reduction
As with conventional triboelectric nanogenerators, the working principle of S-TENG is also involved in contact electrification and elec trostatic induction (Fig. S5a) [9]. Specifically, once the contact between PDMS and finger, electrons could be transferred from finger to PDMS due to the different triboelectric coefficient between them, which results in a negatively charged PDMS surface and a positively charged finger. When the finger and PDMS are moved and separated, the positive charge on finger and the negative charge on PDMS cannot be completely neutralized due to the presence of air layer. In order to achieve elec trostatic equilibrium, the graphene/Cu electrode induces positive charge that is opposite to the electrical polarity remaining on the PDMS
The mechanism of oxidation resistance and deoxidation for copper can be explained in the following three aspects, as illustrated in Fig. 4a. That is, the energy barrier formed by graphene, the difference in metal work function between graphene and copper, and the electrochemical potential of graphene compared that of copper oxides. Clearly, the spincoated graphene on the surface of electrodeposited Cu structure divides space into two domains, each domain has different physical or chemical properties. Since the impermeability of graphene film has been demonstrated previously, i.e., it can hinder atoms or molecules from the external atmosphere [56]. Therefore, graphene acts as an energy barrier on the surface of Cu structure, so as to reduce the touch probability of deposited Cu with air in a certain degree. As displayed in Fig. 2b, after spin-coating graphene for once, some areas of deposited Cu are not adsorbed by graphene, thus for graphene/Cu, there is presence of Cu (OH)2. For the sample spin-coated graphene for twice, the results of Cu 2p3/2 are basically free of copper oxidation, which is mainly due to the
Scheme 1. The chemical reaction equation involved in copper ox ides reduction. 6
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Fig. 5. (a) The schematic diagram and optical picture of skin-based flexible single-electrode triboelectric nanogenerator. The output characteristics of TENG based on graphene/Cu/PDMS: (b) output voltage; (c) transfer charge quantity. (d) The bending performance of flexible electrode based on graphene/Cu/PDMS. (e) The variation of output voltage, current and power density with the external load resistance. (f) LED application: blue LED array powered by TENG. (g–i) Flexible applications of skin-based single-electrode triboelectric nanogenerator.
and causes the migration of free electrons towards to the ground through an external circuit. Conversely, when finger is closed to PDMS, electrons are also transferred between the electrode and ground through elec trostatic induction. Besides, the experimental data that verify the working principle is also presented (Fig. S5b). The device structure of flexible single-electrode TENG developed in this work is illustrated in Fig. 5a, which is composed of graphene/Cu heterostructure induction electrode and PDMS films. As shown in Fig. 5b and c, the peak value of output voltage and transfer charge quantity based on graphene/Cu/PDMS are ~60 V and ~2.5 nC, respectively, which is equivalent to or better than the previously reported TENGs based on pure PDMS [59–61]. In order to verify its reliability in flexible application scenarios, the TENG was bent and folded for 10000 times. As shown in Fig. S6, the micro-morphological characteristic of flexible electrode without and with bending 10000 times shows no significant changes, in addition, the electrodeposited Cu nanostructure shows no obvious cracks in a large scanning range (12*9 μm). The bending ex amination shows the sheet resistance of flexible electrode (Fig. 5d) and the electrical output characteristics of TENG are not significantly dete riorated compared to that of the devices without bending, indicating the
flexibility and reliability of such TENG based on graphene/Cu/PDMS. Additionally, the output power density exhibited by graphe ne/Cu/PDMS based TENG under different external load resistance is evaluated, which is determined to the power generation efficiency. The output voltage, current and power density, as a function of the applied external load, are shown in Fig. 5e. Typically, the output voltage in creases with the increasing load resistance, while the current gradually decreases, which results in a balanced value of power density due to the ohmic loss. Notably, the peak value of power density is reached at 91.9 mW/m2 when the external load is 100 MΩ, which is superior to that of the reported works based on pure PDMS [62]. Based on the mechanism of graphene/Cu heterostructure proposed in this work to enhance the stability of TENG, the stability of such device was verified. In order to be consistent with the XPS analysis that verify the stability of graphene/Cu heterostructure for 1-week, the prepared inducing electrodes (graphene/Cu/PDMS and Cu/PDMS) were also placed in air for 1-week to inspect their performance changes with and without exposure. As seen in Fig. S7, after exposure in air for 1-week, the performance of graphene/Cu heterostructure-based TENG doesn’t change markedly (Fig. S7a). However, for Cu-based TENG, its 7
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performance deteriorates significantly as the expected (Fig. S7b), which is mainly due to the instability of Cu and its severe corrosion behavior in the air. As a result, the stability and electrical property of inducing electrode is worsen, and the performance of TENG is also deteriorated. Finally, the stability experiment based on TENG performance is consis tent with that based on XPS analysis, indicating the excellent stability of graphene/Cu heterostructure. For purpose of detecting the response time of such TENG, the electrical output at different duration of the first contact between skin and PDMS was compared (Fig. S8). The opencircuit voltage shows no significant changes with the increasing dura tion time (from 1 s to 10 s), indicating the TENG has favorable inducing capability. Besides, the response sensitivity of the releasing and approaching process of TENG at working status was also studied. The result shows that the response time of the releasing and approaching process is about 50 ms, indicating the TENG has excellent response time. For purpose of investigating the practical applications of such pro posed TENG, it was directly linked to light-emitting diodes (LEDs) associated in series, and 22 blue lights could be powered by fingers typing (Fig. 5f). Moreover, it was also fitted with skin and explored the flexible applications, showing that certain electrical signals could be generated in the knuckles, elbows and knees (Fig. 5g–i). Undoubtedly, the TENG proposed in this work will act as a reliable supplier and pro vide an ideal strategy for power electronics.
sputtering. Afterwards, the UV laser processing system was used to divide the integral substrate into sample wafers with certain size for the subsequent usage, e.g. 20*20 mm, 5*5 mm. Then, the preparation of electrodeposited Cu layer on the conductive substrate was completed by using the Cu2þ solution configured by our team. Meanwhile, the mois ture removal was very essential after accomplishing the fabrication of Cu layer by electrodeposition, therefore, the adsorbed moisture on Cu film was removed by high-speed rotation and heating. Finally, the gra phene dispersion was modified on the deposited Cu film by spin coating and dried in a vacuum oven to form a thin film. 4.3. Fabrication of the flexible electrode of graphene/Cu/PDMS Fig. S9 shows the schematic illustration of the fabrication process of flexible conductive electrode based on graphene/Cu/PDMS. Firstly, a layer of 10 μm thick positive photoresist (PR) was spin-coated on the pretreated glass substrate, and then placed in an oven at 110 � C to be cured by heating for 2 h. After that, the pre-treated PDMS mixture was spin-coated onto the cured photoresist and placed on a hot plate at 120 � C for 1 h. After the completion of PDMS curing, the PDMS/PR/Glass substrate was immersed in acetone to remove the photoresist layer, resulting in a stress-free, and flat PDMS film. Subsequently, the peeled PDMS film was ultrasonically washed in sodium hydroxide and alcohol to obtain a smooth and transparent PDMS film. Then, the PDMS film was placed on the surface of glass substrate and dried in an oven at 60 � C. Afterwards, the preparation strategy of structured graphene/Cu/PDMS/ Glass was followed by process of graphene/Cu film aforementioned. Finally, the PDMS was stripped from glass substrate to complete the preparation of flexible electrode, i.e., graphene/Cu/PDMS.
3. Conclusions In conclusion, constructing the nanostructure of graphene/Cu has been confirmed to be an effective strategy for enhancing the stability of copper. Such graphene/Cu heterostructure is fabricated by spin-coating the graphene dispersion on the surface of preferred Cu (111). As stated in the results of Raman and XPS, the internal stress and electron transfer could be occurred naturally, which is coupled with the electrochemical potential difference between graphene, copper and oxygen, and an interactive mechanism is proposed for explaining the improved stability of graphene/Cu heterostructure. Additionally, electrodeposition tech nology is utilized to fabricate copper nanostructure and is demonstrated to be an ideal approach for the preparation of flexible induction elec trodes utilized in TENG. Finally, a skin-based S-TENG based on gra phene/Cu/PDMS nanostructure is fabricated, which acts as a reliable supplier and shows superior performance and promising potential in TENG applications and power electronics.
4.4. Fabrication of the triboelectric nanogenerator (TENG) with single electrode The prepared graphene/Cu/PDMS flexible electrode was cleaned with deionized water and dried with nitrogen to remove the adsorbed impurities on the surface. Subsequently, wires were bonded at the edge of electrode by using silver paste to lead electrons, and were cured at 60 � C for 20 min. Then the completed device was preserved for the subse quent examinations. 4.5. Electrical performance assessment
4. Experimental section
The Keithley 6514 electrometer was used to examine the electrical performance of a single-electrode triboelectric nanogenerator fabricated in this paper, including the open circuit voltage, electrostatic current, and transfer charge. The noise of electrometer was less than 1 fA, the input impedance in the voltage measurement was greater than 200 TΩ, and the transfer charge measurement range was 10 fC~20 μC.
4.1. Graphene dispersion fabrication Reduced graphene oxide (RGO) was obtained by reducing graphene oxide (GO) with ascorbic acid, and the GO was synthesized from natural graphite powder (325 mesh, Aladdin Reagent Co.) by an improved Hummers’ method [63]. The physically exfoliated graphene was sup plied by Tanfeng Graphene Technology Co. Ltd. As for graphene dispersion, the weighed 50 mg graphene powder was slowly added to 50 ml deionized water and dispersed in an ultrasonic cleaner for 1 h. Then, a certain amount of sodium dodecyl sulfonate was added into the graphene dispersion and sonicated again for 20 h with the power of 450 W. Meanwhile, the solution was stirred with glass rod every hours of sonication for purpose of obtaining the uniform and stable graphene dispersion.
4.6. Characterization Phase analysis was accomplished on a D/max 2500PC highresolution X-ray diffractometer (Rigaku, Japan) with Kα radiation. The electrodeposited Cu film with different current density was examined by XRD, and the effects of current density on the texture coefficient of Cu crystal planes were also analyzed. The micro-morphological character istics and elemental information were obtained on a JEOL JSM-7800F field-emission scanning electron microscope (JEOL, Japan). Specif ically, the morphology of graphene and graphene/Cu heterostructure, the influence of current density on the morphology of electrodeposited Cu, and the surface element distribution of graphene/Cu structure were confirmed. Microstructural analysis was carried on a Renishow Invia Reflex micro-confocal spectrometer (Renishaw, Britain) with 514.5 nm laser excitation. Raman spectroscopy is a powerful non-destructive technique for identifying the structure, doping and disorder of gra phene. The characteristic peaks of graphene, such as D, G and 2D peaks,
4.2. Construction of the structured graphene/Cu film Before sputtering, silicon wafer with 250 μm thickness was treated in a plasma cleaner for purpose of removing the surface contaminants and promoting the adhesion of cadmium/copper (Cr–Cu) layer to silicon substrate. After that, the adhesive layer of Cr and the conductive layer of Cu were deposited on the cleaned silicon substrate by magnetron 8
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were detected, and the G peak shift of graphene caused by stress was also analyzed. Chemical analysis was recorded by X-ray photoelectron spectroscopy using Sigma probe XPS (Thermo VG Scientific), and all XPS measurements were proceeded at room temperature. The survey spectra of Cu, graphene, and graphene/Cu structure were obtained, and the chemical states of the elements, such as copper, oxygen and carbon, were also analyzed. Additionally, the comparative information of sam ples before and after exposure in air was also completed by XPS.
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Nano Energy 70 (2020) 104540 Yahui Li received his M.S. degree in materials physics and chemistry from Harbin Institute of Technology, and is currently a Ph.D. candidate of electronic science and technology at Shanghai Jiao Tong University. His research interests focus on the energy harvesting and MEMS sensors.
Han Cai received her M.S. degree in mechanical engineering from Xiamen University, and is currently a Ph.D. candidate of electronic science and technology at Shanghai Jiao Tong Uni versity. Her research interests focus on the micro-nano flexible sensing technology and the design of microchannel heat-sink.
Wei Zheng received his B.S. degree in mechatronics engineer ing from Northwestern Polytechnical University, and is currently a M.S. degree candidate of electronic and communi cations engineering at Shanghai Jiao Tong University. His research interests focus on the flexible electronic skin and MEMS sensors.
Yanxin Zhang received his M.S. degree in materials processing engineering from Harbin Institute of Technology, and is currently a Ph.D. candidate of electronic science and technol ogy at Shanghai Jiao Tong University. His research interests focus on the electronic packaging integration technology and its application.
Haodong Zhang received his B.S. degree in optoelectronic in formation science and engineering from Sun Yat-sen University, and is currently a Ph.D. candidate of electronic science and technology at Shanghai Jiao Tong University. His research in terests focus on the curved lithography technology and its applications.
Prof. Zhuoqing Yang received his B.S. and M.S. degree in electromechanical engineering from Harbin Engineering Uni versity, respectively, and the Ph.D. degree in microelectronics and solid state electronics from Shanghai Jiao Tong University. In 2011, he was awarded a prestigious JSPS post-doctoral fellowship and worked at the National Institute of Advanced Industrial Science and Technology in Tsukuba. Currently, he is a full professor with the National Key Laboratory of Science and Technology on Micro/Nano Fabrication in SJTU. He is also an editorial board member of the International Journal of Micro and Nanosystems. His research interests include the design, simulation, and fabrication of MEMS/NEMS and the flexible sensing devices.
Haoqiang Wang received his M.S. degree in materials engi neering from Harbin Institute of Technology, and is currently a Ph.D. candidate of materials science and engineering at Harbin Institute of Technology. His research interests focus on the design and applications of sodium-ion hybrid capacitors.
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