Piezoelectric-Induced Triboelectric Hybrid Nanogenerators Based on

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Piezoelectric Induced Triboelectric (PIT) Hybrid Nanogenerator Based on ZnO Nanowires Layer Decorated on Au/PDMSAl Structure for Enhanced Triboelectric Performance Chaiyanut Jirayupat, Winadda Wongwiriyapan, Panita Kasamechonchung, Tuksadon Wutikhun, Kittipong Tantisantisom, Yossawat Rayanasukha, Thanakorn Jiemsakul, Chookiat Tansarawiput, Monrudee Liangruksa, Paisan Khanchaitit, Mati Horprathum, Supanit Porntheeraphat, and Annop Klamchuen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17314 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 25, 2018

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ACS Applied Materials & Interfaces

Piezoelectric Induced Triboelectric (PIT) Hybrid Nanogenerator Based on ZnO Nanowires

Layer

Decorated

on

Au/PDMS-Al

Structure

for

Enhanced

Triboelectric Performance Chaiyanut Jirayupat1, Winadda Wongwiriyapan1, Panita Kasamechonchung2, Tuksadon Wutikhun2, Kittipong Tantisantisom2, Yossawat Rayanasukha2, Thanakorn Jiemsakul2, Chookiat

Tansarawiput2,

Monrudee

Liangruksa2,

Paisan

Khanchaitit2,

Mati

Horprathum3, Supanit Porntheeraphat3 and Annop Klamchuen2* 1

College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang,

Chalongkrung Rd., Ladkrabang, Bangkok 10520, Thailand 2

National Nanotechnology Center (NANOTEC), NSTDA, 111 Thailand Science

Park,Paholyothin Rd., KlongLuang, Pathum Thani, Thailand 3

National Electronics and Computer Technology Center (NECTEC), NSTDA, 111

Thailand Science Park, Paholyothin Rd., KlongLuang, Pathum Thani,Thailand The e-mail address of the corresponding author: [email protected] Abstract Here we demonstrate a novel device structure design to enhance a converting electrical output of triboelectric device through piezoelectric effect as called piezo-induced triboelectric (PIT) device. By utilizing the piezo-potential of ZnO NWs embedded into PDMS layer attached on the top electrode of conventional triboelectric device (Au/PDMS-Al), the PIT device exhibits the output power density of 50 µW/cm2 which is larger than that of conventional triboelectric

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device up to 100 folds under the external applied force of 8.5 N. We found that the external piezo-potential affected on the top Au electrode of triboelectric device not only enhances the electron transfer from Al electrode to PDMS but also boosts up the internal built-in potential of triboelectric device through external electric field of piezoelectric layer. Furthermore, the 100 LEDs could be lighted up via PIT device whereas the conventional device could illuminate the LED bulbs less than 20 bulbs. Thus, our results highlight that the enhancement of triboelectric output can be achieved by using PIT device structure, which enable us to develop the hybrid nanogenrators for various self-power electronics such as wearable, and mobile devices. Keywords Hybrid nanogenerators, triboelectric, piezoelectric, ZnO nanowires, PDMS, internal built-in potential, surface charge density


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1. Introduction Nanogenerators based on mechanical energy harvesting have been attractive for building self-power systems, which are applicable for micro-electromechanical system, mobile environment sensors, homeland security and portable/wearable personal electronics.1–18 Triboelectric nanogenerators (TENGs) are an emerging new mechanical energy harvesting technology, which scavenge mechanical energy from our living environment and human body to produce electricity through electrification and electrostatic induction mechanism.2–12,14–16,19–26 Generally, it has been known that the performance of TENG depends on the ability to generate the electrostatic charge on the opposite material side (i.e. surface charge density), which can be achieved by selecting the materials with the largest different triboelectric polarities2–4,6,8,10,11,15,16,18,20,21,23–25 and the modification of surface morphology (e.g. nanopattern and chemical surface functionalization).2–4,8,10,11,16,18–21,24,25 However, the material selection is limited based on triboelectric series, whereas the effective contact area requires sophisticated surface pattern and nanostructure design, which make a complex and high cost fabrication.2,10,19 Recently, several research groups utilized a hybrid effect between piezoelectricity and triboelectricity to enhance the performance of TENG through a composite film impregnated with piezo-materials such as nanoparticles, nanowires and nanofiber, which are available to boost electron transfer and induced extra charges.5,9,15–17,22,27,28 Nevertheless, it is hard to avoid the agglomeration of piezo-materials buried in the composite films, which pull down the emergence of piezo-potential.27 Furthermore, it is necessary to align the polling direction (i.e. polarization) of piezo-materials within a composite film for obtaining the highest piezo-potential.12–14,21,25,29,30 In this study, we design and fabricate a novel device structure called piezo-induced triboelectric (PIT) for



enhancing electrical output by utilizing a piezo-potential of ZnO NWs layer attached on top Au electrode of triboelectric device (Au/PDMS-Al). The schematic diagram for PIT operating mechanism is proposed and confirmed by the different directions of ZnO NWs piezo-potential. Additionally, the output power of PIT device can successfully illuminate 100 LED bulbs through the application of human’s finger force (~8.5 N). This finding can provide an important guide for the PIT device structure design, enabling us to develop the hybrid nanogenerators for various self-power electronics, such as, wearable and mobile devices.


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2. Experimental PIT device fabrication The structure of piezoelectric induced triboelectric (PIT) device was consisted of two major parts: i) a top layer of ZnO NWs embedded on PDMS which was sandwiched by double layers of Au/PDMS and ii) Al electrode. The details of the device fabrication were shown in figure 1 (a). First, ZnO NWs with the length of 2 µm were grown on 1.5×1.5 cm2 of Si (100) substrate via seed assisted hydrothermal technique.31 ZnO NWs grown on Si substrate were then wrapped up with PDMS membrane (thickness ∼ 4 mm) and, afterward were peeled off from Si substrate. Such ZnO NWs embedded into PDMS membrane surface could act as ZnO seed for hydrothermal ZnO NWs growth (Supporting information 1). Noted that the details of seed assisted hydrothermal growth and PDMS membrane fabrications can be seen elsewhere.18,32,33 After ZnO NWs with the length of 2 µm were grown on PDMS membrane, a 40 nm thickness of Au electrode was deposited via RF magnetron sputtering on both sides of device (i.e. PDMS and ZnO NWs surface). It should be noted that the deposition was carried out in an argon gas pressure of 5.0×10-3 mbar and RF power of 100 watts. The thickness of Au film was controlled by using thickness monitor (INICON AQM-160) during deposition. Finally, such Au/PDMS/ZnO NWs/Au layer was sandwiched by PDMS membranes with the thickness of 300 µm. Characterization and measurement of PIT device The morphology and cross-section images of ZnO nanowires grown on Si and PDMS substrate were observed by employing field-emission scanning electron microscope (HITACHI S-3400 microscope). The crystallinity of ZnO nanowires was investigated



by X-ray diffraction measurement (XRD, D8 ADVANCE, BRUKER). The output voltage and current characteristic of PIT and conventional triboelectric device were measured by using an oscilloscope (Tektronix TDS 3032B) under the applied external mechanical force of 0.8 N with the frequency in the range of 1 Hz to 5 Hz (GABO TESTANLAGEN GMBH). Noted that the mechanical force of 0.8 N is the maximum force of the programmable mechanical force system (see in supporting information 2). The output current of PIT and conventional device was measured under the load resistance of 10 MΩ. Finally, the 100 LED bulbs induced by PIT device were exhibited by applying human finger force of 8.5 N, which was calibrated by force sensor (Inter link force tester, No.402). The rectified open-circuit output voltage and current were collected by utilizing bride rectifier circuit.34


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3. Results and Discussion Figure 1 (a) shows a schematic diagram of hybrid device, which consists of two major devices (i) piezoelectric generator based on ZnO NWs (PDMS/Au/PDMS/ZnO NWs/Au) and (ii) triboelectric nanogenerator (Au/PDMS-Al). Both of them have a common Au electrode shared as a bottom electrode of piezoelectric and a top electrode of triboelectric device. The total active area size of hybrid device is 1.5×1.5 cm2 as shown in figure 1 (b). Figure 1(c) exhibits the cross-section image of ZnO NWS with length of ~2 µm embedded into PDMS membrane, which was covered by Au/PDMS layer on the topside of ZnO NWs. Noted that the XRD pattern of ZnO grown on PDMS membrane is provided in supporting information 1, which shows the crystal orientation almost along [0001] of ZnO (JCPDS no. 36-1451). Such piezoelectric layer based on ZnO NWs embedded into insulated PDMS membrane probably allows us to avoid nonuniform force distribution and reduce screening effect.17 To understand working principle of our hybrid device, here we have measured the electrical outputs of the hybrid device compared with a reference triboelectric (RT) device. Noted that the difference between the hybrid device and the RT device is that there is no ZnO NWs grown on PDMS membrane inserted into RT device structure as shown in figure 2. Figure 2 (a) and 2 (d) show the cross-section of hybrid device and RT device, which were applied by vertical force of 0.8 N on the topside. Herein six sub-devices were separately measured in each major device such as Z1, Z2 and Z3 for the hybrid device and T1, T2 and T3 for the RT device. The structures of sub-device Z1, Z2 and Z3 comprise of Au/PDMS/ZnONWs/Au/PDMS-Al, Au/PDMS/ZnONWs/Au and Au/PDMS-Al whereas the structures of sub-device T1, T2 and T3 consist of



Au/PDMS/Au/PDMS-Al, Au/PDMS/Au and Au/PDMS-Al, respectively. Figure 2 (b) and 2 (c) exhibit the open circuit voltage (Voc) and short circuit current (Isc) of subdevice Z1, Z2 and Z3 as a function of time. On the other hand, the Voc and Isc of subdevice T1, T2 and T3 at different time are shown in figure 2 (e) and 2 (f). Noted that the frequency of applied force is 5 Hz. It can be seen that the average value of Voc peaks of Z1, Z2 and Z3 sub-device are 0.11 V, 0.21 V and 0.82 V, whereas the average values of Isc peaks of Z1, Z2 and Z3 sub-device are 12 nA, 10 nA and 45 nA. On the other side, the average values of Voc peaks of T1, T2 and T3 sub-device are 0.11 V, 0 V and 0.42 V, while the average values of Isc peaks of T1, T2 and T3 sub-device are 8 nA, 0 nA and 15 nA. Attributed to the basic mechanism of each sub-device, the electrical output production of Z1, Z3, T1 and T3 should directly come from the induction of triboelectric charge between PDMS surface (negative polarity) and Al surface (positive polarity) based on triboelectric series.2,4,20,23 On the other hand, the electrical converting output of sub-device Z2 relies on piezoelectric effect based on ZnO NWs. The absence of ZnO NWs in sub-device T2 should be the cause of undetectable electrical output due to the absence of piezoelectric layer. Generally, it has been known that the fundamental triboelectric device is a conjugation of contact electrification and electrostatic induction, which inherently rely on the capacitive behaviour.4,6,26,35,36 Therefore, the capability of electrical converting output of our triboelectric devices should be regularly dependent on the ability of charge transfer between two triboelectric material surfaces23 and the electric field inside triboelectric materials (i.e. dielectric strength) that produce the electrostatic charge induction on the surface of electrodes contacted with dielectric materials.26 The former factor strongly relates to a difference in their triboelectric polarity of selected materials,4,37-39 while the later factor correlates with the thickness of


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the triboelectric materials.7 Since the top plate of sub-device Z1 and T1 are comprised of multilayer of Au/PDMS/ZnONWs/Au/PDMS and Au/PDMS/Au/PDMS which are thicker than that of sub-device Z3 and T3 (i.e. Au/PDMS), the lower in electrical output value of sub-device Z1 and T1 compared to sub-device Z3 and T3 should be from the thickness effect. In other words, the thicker device layer results in lower the electric field strength in PDMS, which reduces the electrostatic induction within device.28 In addition, our results demonstrate that the insertion of ZnO NWs embedded on PDMS membrane dose not significantly affect on the electrical converting output of triboelectric in sub-device Z1, resulting that the average values of Voc and Isc peaks of sub-device Z1 and T1 are close to each other. Here we question that why the Voc and Isc peaks of sub-device Z3 are higher than that of sub-device T3 for 2 and 3 folds, even though the triboelectric charges of both devices are generated from same couple of materials surface and structure of device (Au/PDMS-Al). Since the only distinction between sub-device Z3 and sub-device T3 is that the top Au electrode of sub-device Z3 connected to ZnO NWs (sub-device Z2) whereas top Au electrode of sub-device T3 contacted with PDMS layer (sub-device T2), the enhancement of electrical converting output in sub-device Z3 might be caused by piezo-potential of ZnO NWs affected on the top Au electrode.13,14,23 Noted that there are no any surface modifications of PDMS layer and Al electrode on both devices. To understand how piezoelectric effect of ZnO NWs can raise the electrical converting output of triboelectric sub-device Z3, we have collected the Voc output signal of triboelectric sub-device Z3 in single press-and-release compared to the Voc of piezoelectric sub-device Z2 and the Voc of triboelectric subdevice T3 as shown in figure 3.



Figure 3 (a) shows the Voc characterization of triboelectric sub-device Z3 (red line), piezoelectric sub-device Z2 (blue line) and triboelectric sub-device T3 (black line) in single press-and-release as a function of time. The results exhibit that the Voc output signal of triboelectric sub-device Z3 comprises of a major peak and a shoulder at the pressing state whereas at the releasing state, a small pit and a major valley are observed. On the other hand, the Voc output signal of piezoelectric sub-device Z2 and triboelectric sub-device T3 compose of one major peak at the press state and a major valley at the release state. Interestingly, the occurrence period of the Voc peak and valley of piezoelectric sub-device Z2 cover the range of the occurrence period of the Voc of triboelectric sub-device Z3. On the other hand, the occurrence period of the Voc peak and valley of triboelectric sub-device T3 are consistent to the occurrence period of the major Voc peak at pressing stage and the major Voc valley at release state of triboelectric sub-device Z3. These imply that the triboelectric event of sub-device Z3 simultaneously appears with the period of piezoelectric event of sub-device Z2. Within the framework of triboelectric device based on metal-insulator contact, the surface charge on insulator surface (σinsulator) strongly affect to the electrical converting output of triboelectric device. A larger value of σinsulator can induce a higher surface charge density on back contact electrode (σelectrode), leading to an enhancement of the electrical converting output.21,40 Recently, J. Peng et al. reported that the increment of the transferred surface charge density on PDMS surface could be achieved through the positive charge CNCFS impregnated in PDMS, which boosted the potential difference with respect to the Fermi level of the contact metal.23 Therefore, the increase in Voc and Isc of sub-device Z3 might be attributed to the synergetic effect between piezoelectricity and triboelectricity by enhancing the numbers of electron transfer and accumulation on the surface of


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PDMS. Likewise, we theoretically investigate the increase of Voc through the surface charge density generated by piezoelectric (σP). Based on the existing theoretical model of a conductor-to-dielectric contact-mode triboelectric41 (see the calculation details in Supporting Information 3), the surface charge density by PIT (σPIT) and triboelectric (σTENG) are computed and shown in Fig. S3. The difference between σPIT and σTENG stems from σP where its maximum reaches nearly 2 times of the maximum σTENG. This σP simultaneously leads to piezo-potential as the driving force for the charge transfer and accumulation on the electrode. Such change of surface charge density exhibits the same trend as that of voltage (Fig. 3) since voltage is a function of the amount of induced charge transferred and accumulated. The possible schematic diagram for operating mechanism of hybrid sub-device Z3 that refers from the measurement of force profile in a single press-and-release as a function of time (see in Supporting information 2) is proposed as shown in figure 3 (b). (i) At the initial state before contact of the two materials (i.e. PDMS and Al), neither triboelectric nor piezoelectric potential is occurred. (ii) When an external force applied on the top layer side (i.e. Au/PDMS/ZnONWs/Au/PDMS) to be in contact with Al surface, a physical contact between PDMS and Al electrode causes the charge transfer. Since the characteristic energy level of PDMS is lower than the work function of Al, the electrons trend to flow from the filled Fermi level (EF) of Al to surface of PDMS, resulting in the positive charges on Al surface.23 At the same time, the tensile strength is occurred via the bending of PDMS layer in sub-device Z2, which produces the positive piezo-potential at the tip of ZnO NWs due to stretched event.1,42 Such positive piezopotential on Au electrode not only drives electrons to flow across external circuit in order to compensate the potential difference between Au electrode and Al electrode17, 43



(see supporting information 4) but also simultaneously produces a net electric field along the direction from PDMS to Al electrode, resulting that more electron transfer from Al to the surface of PDMS owing to larger difference with respect to the Fermi level of contacted metal.14,23,44 (iii) Once the external force is released (PDMS still contacts with Al), the piezoelectric electrons (ep) flow back to the Al electrode due to the reduction of piezo-potential. The small pit is then observed. (iv) During residual strain in ZnO NWs is released, the PDMS suddenly separates from Al surface. Accordingly, the triboelectric electrons begin to transfer to the Al electrode through the external circuit to neutralize the positive triboelectric charges. Since the amount of surface charges density on PDMS are generated from the synergetic effect between triboelectricity and piezoelectricity as called piezo-induced triboelectric (PIT), the total number of electrons (epit) flowed from Au electrode to Al electrode during electrostatic charge induction process is much higher than that of conventional triboelectric devices (sub-device T3). (v) The triboelectric electrons transfer from Au electrode to Al electrode until reaching an electrostatic equilibrium. On the other hand, the piezoelectric electrons (ep) still flow to Al electrode until no longer piezo-potential. This is the first half of hybrid device cycle. (vi) When the hybrid device is compressed again, the electrostatic equilibrium in triboelectric device is broken, resulting that electrons flow back from Al electrode to Au electrode. At the same time, the positive piezo-potential abruptly occurs at the tip of ZnO NWs contacted with the topside of Au electrode of sub-device Z3. Such positive piezo-potential not only drives piezo-electron from Al electrode to Au electrode but also increases the built-in potential of triboelectric device,17, 44 resulting that the electrical converting output of sub-device Z3 is elevated. Although the electrostatic equilibrium in triboelectric device seems to be reached when


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the surface of PDMS and Al are contacted together (i.e. no longer tribo-electrons), the piezo potential persistently appears due to the softness of PDMS layer.17,

22, 44

Consequently, the piezo-electrons and the residual tribo-electrons boosted up through piezo-potential continually flow from Au electrode to Al electrode (i.e. small shoulder is found). This is a second half of hybrid device cycle. To confirm the effect of positive piezo-potential on the topside of Au electrode that enriches the electrical converting output of sub-device Z3, we have investigated the electrical converting output of subdevice Z3 which is applied by the external force from different direction as a function of frequency as shown in figure 4. Figure 4 (a) and 4 (b) demonstrate the Voc and Isc value of sub-device Z3 applied by external force from topside (red line) and bottom side (blue line) at various force frequency. Noted that the black dash line in figure 4 (a) and 4 (b) are the Voc and Isc values of the reference conventional triboelectric device (T3). The external force is 0.8 N. The results show that the Voc value of sub-device Z3 compressed from the topside increases from 0.15 V to 0.83 V when the frequency is varied from 1 Hz to 5Hz while the Isc value increase from 14 nA to 45 nA. This indicates that the external electrons flow to reach equilibrium in a shorter time, which will lead to larger electrical converting output.15,19,24 When the external force applied from the bottom side of subdevice Z3, on the other hand, the Voc values slightly increase from 0.06 to 0.11V whereas the Isc value slightly increases from 7 nA to 9 nA with increasing the force frequency. Intriguingly, such Voc and Isc values obtained from the bottom side force are not only lower than that of the Voc and Isc values of the force applied from topside direction in sub-device 3 but also lower than that of the Voc and Isc values of the conventional triboelectric device (Voc = 0.06 to 0.45 V and Isc = 8 nA to 14 nA),



implying that the different directions of applied force compressed on sub-device Z3 strongly affect on the electrical converting output. Since the opposite applied force direction can be attributed to the opposite piezo-potential direction along c-axis of ZnO NWs,30 the piezo-potential on the top Au electrode compressed through the bottom side (Al electrode) should be the negative potential which will produce a net electric field along the direction from Al to PDMS.30 Therefore, the net electric field created via negative potential might decrease the potential difference with respect to Fermi level of contacted metal (Al) and suppress electrons to transfer to PDMS in electrification process, resulting that the tiny signal of electrical converting output could be observed. Conversely, the positive piezo-potential on the top Au electrode compressed through the topside could enhance the electrical converting output (figure 2 and figure 4). Noted that the electrical converting output of conventional triboelectric sub-device T3 does not change, even though it was applied by different force directions. In addition, we found that utilizing the stronger piezo-potential of Ag-doped ZnO NWs instead of the piezopotential of ZnO NWs in sub-device Z3 allows us to produce the higher electrical converting output (see Supporting information 5). Thus, our results highlight that the enrichment of the electrical converting output of triboelectric device through boosting up the electron transfer and induced extra surface charge density could be achieved by utilizing piezo-potential of ZnO NWs. Figure 5 (a) and 5 (b) exhibits the output voltage and current at various resistances of sub-device Z3 and conventional triboelectric sub-device T3, respectively. Noted that the external applied force and frequency are 0.8 N and 5 Hz. As the load resistance increases, the amplitude of output current peaks is observed to drop whereas the voltage output peaks increase and then seems to be saturate. The peak power


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densities corresponding to load resistances are calculated as the product of corresponding current and voltage25 as shown in figure 5 (b) and 5 (d). It can be seen that the maximum power density of sub-device Z3 is 8.74 nW/cm2 at the matched load of 22 MΩ. On the other hand, the maximum power density of sub-device T3 is 0.86 nW/cm2 at the matched load of 7 MΩ. This indicates that the hybrid sub-device Z3 could generate the output power density higher than that of conventional triboelectric sub-device T3 for 10 folds. Finally, we have directly illuminated LEDs in the series connected to our Z3 device as shown in figure 6 (a) and 6 (b). In this experiment, we utilized the finger force of 8.5 N calibrated via force sensor (Inter link force tester, No.402) to apply on our device. The results show that the output power from device Z3 without any external load resister could light up to 100 LEDs (Video 1). The details of Voc and current density output value during applied finger force are shown in figure 6 (c) and 6 (d). The average value of Voc peak is ~60 V while the average value of Isc peak is ~2.0 µA/cm2. The achieved maximum power density of a device connected through circuit loads is 50 µW/cm2. On the other hand, the conventional triboelectric sub-device T3 could illuminate the LED bulbs less than 20 bulbs. The average value of Voc peak and the average value of Isc peak are ~30 V and ~0.8 µA/cm2 whereas the maximum power density is 0.42 µW/cm2. Generally, it has been known that the increment of surface charge density can be achieved by increasing applied force due to the enrichment of surface contact area during pressing state.20 Since the electrical converting output of the hybrid sub-device Z3 comes from the synergetic effect between piezoelectric and triboelectric, the increase in applied force not only increases surface contact area (i.e. increase surface charge density)20 but also intensely increases the potential difference between Au electrode and Al electrode from 0.15 V to 25 V when



applied force is varied from 0.8 N to 8.5 N (see supporting information 4). Consequently, a large number of electrons are available to transfer from Al to the surface of PDMS owing to a lager of piezo-potential. On the other side, the increment of electrical converting outputs of conventional triboelectric sub-device T3 is only based on the increase of surface contact area. Therefore it is expected on the basis of our results that the power output difference about 100 folds between the hybrid sub-device Z3 and conventional triboelectric sub-device T3 should be from the increase of surface charge density and additional built-in potential of triboelectric device due to the enrichment of triboelectric surface charges and piezoelectric potential which are a function of external applied force.20 Compared with the previous works as to utilizing dual effect of the piezoelectricity and triboelectricity to enhance the electrical converting output in triboelectric device9,14,20,25,29,45,46 most of previous works used a composite film based on PDMS impregnated with piezo-materials such as nanoparticles, nanowires and nanofiber to modify the dielectric electric strength inside triboelectric material for boosting up the amount of charge transfer (i.e. triboelectrification) and inducing extra charges (i.e. electrostatic charge induction) through the enhanced internal electric field within the triboelectric materials.9– 11,14,17,18,20,28,46,47

This is different from the major feature of our study, which is an

intentional decoration of piezoelectric ZnO NWs embedded into PDMS layer on the top electrode of triboelectric device (i.e. Au electrode). Such piezoelectric layer is attributed to the external source of electric field influenced along the direction from insulator layer (PDMS) to metal electrode (Al), which not only enables us to modulate the surface charge transfer between triboelectric materials and induces the extra charges (supporting information 3) but also elevates the built-in potential of triboelectric device. Such


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enrichment of triboelectric performance as to the enhanced built-in potential could be exhibited through the increase of positive piezo-potential occurred at the top electrode layer of triboelectric device as a function of applied force (supporting information 4 and figure 5). In order to achieve higher output device performance, there are several methods to be done based on our PIT device structure design, such as, decreasing the internal resistance of device by dispersing conductive materials into PDMS layer,9– 11,14,17,18,20,28,46,47

modifying surface of PDMS and Al,30 using stronger piezoelectric

layer,18,48 combining dual signal of piezoelectric and triboelectric via external circuits 9,14,20,25,29,45,46

and so on. The further investigation is in the progress. This discovery

might eventually lead to high efficient nanogenerators applicable for various self-power electronics such as wearable, and mobile devices.



4. Conclusions In this work, we have provided a novel design of hybrid device structure as called piezo-induced triboelectric (PIT) device. The piezoelectric ZnO NWs embedded into PDMS layer is intentionally designed to decorate on the top electrode of triboelectric device (Au/PDMS-Al). Compared with the conventional triboelectric device, the positive piezo-potential of ZnO NWs affected on the top electrode of conventional device induce an extra external electric field along the direction from the insulator surface to metal electrode surface. Such extra external electric field not only allows us to enhance the electron transfer from Al electrode to PDMS but also enriches the internal built-in potential of triboelectric device. We find that the PIT device exhibits the output power density of 50 µW/cm2, which larger than that of conventional triboelectric device up to 100 folds under an external applied force of 8.5 N. The enhance power of PIT device is capable of instantaneously lighting up 100 LED bulbs. Our concept based on PIT device structure can be applied to broad application prospects in the field of self-power electronics.

Acknowledgement This work was partially supported by the Research and Development of White Light Emitting Diode based on Zinc Oxide Optoelectronic Material Project (P1350230) from the National Science and Technology Development Agency, Thailand.


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Supporting Information S1. SEM images and XRD pattern of ZnO NWs grown by seed-assisted hydrothermal technique on Si and PDMS substrates S2. Demonstration of testing device design based on programmable mechanical force system and force profile during press and release cycle for a single press-and-release cycle S3. Simulation on surface charge density of PIT S4. Electrical converting output of sub-device Z3 without any contact between PDMS and Al electrode S5. Electrical converting output of hybrid device based on Ag doped ZnO NWs layer attached on Au/PDMS-Al triboelectric structure



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FIGURE CAPTIONS

Fig. 1. (a) Schematic diagram and fabrication process of the hybrid nanogenerator. (b) Optical photography of hybrid device. (c) Cross-sectional SEM image of ZnO NWs embedded into PDMS membrane.

Fig. 2. The electrical converting output peaks of hybrid nanogenerator (sub-device Z1, Z2 and Z3) and reference triboelectric (RT) device (sub-device T1, T2 and T3) under external applied force of 0.8 N. (a) Cross-section image of hybrid nanogenerator. (b) Open circuit voltage (Voc) peak of sub-device Z1, Z2 and Z3 as a function of time. (c) Short circuit current (Isc) peak of sub-device Z1, Z2 and Z3 as a function of time. (d) Cross-section image of (RT) device. (e) Voc peak of sub-device T1, T2 and T3 at a function of time. (f) Isc peak of sub-device Z1, Z2 and Z3 as a function of time.

Fig. 3. (a) Voc characterization of triboelectric sub-device Z3 (red line), piezoelectric sub-device Z2 (blue line) and triboelectric sub-device T3 (black line) that are simultaneously measured in a single press-and-release cycle. (b) Working mechanism of piezoelectric induced triboelectric (PIT) hybrid device (sub-device Z3) in a press and release cycle.

Fig. 4. (a) Dependence of force frequency on Voc of sub-device 3 applied by topside force direction (red) and bottom side force direction (blue). (b) Isc value of sub-device Z3 applied by external force from topside (red line) and bottom side (blue line) at various force frequencies. Noted that the black dash line in figure 4 (a) and 4 (b) are the Voc and Isc values of the reference conventional triboelectric device (T3).



Fig. 5. Dependence of resistance on output voltage and current of (a) sub-device Z3 and (b) triboelectric sub-device T3. The close-circuit power density under varied external loading resistance of (c) sub-device Z3 and (d) triboelectric sub-device T3

Fig. 6. Snapshots of the 100 LED bulbs configured in series (a) before and (b) after the moment of being lit up. (c) Rectified voltage of PIT device during the moment of being lit up as a function of time. (d) Current density of PIT device during the moment of being lit up as a function of time.


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Table of Content Graphic

1

Press state

VV+

Release state

-

-

-

-

-

-

+

+

+

+

+

+

e-P

- - - - - - - - - - - - - -+ + + + + + + + ++ + + + + +

PDMS Au

0.5 Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


V- -

-

-

-

-

-

V+ +

+

+

+

+

+

e-P

- - - - - - - - - - - - - -++ ++ ++ ++ ++ ++ ++ +

(iii) Relaxation

(ii) Full contact/PIT

PDMS

0

ZnO NWs

VV+

Au PDMS

PIT device Piezoelectric device Triboelectric device

-0.5

d1

V V

d2

Al

0.06

0.12 Time (s)

0.18

-

-

-

-

-

+ + + + + + +

(iv) Separating

(i) Non-contact

-1 0

-

+ + + + + + + + + + + + + - - - - - - - - - - - - - --

0.24

VV+

-

-

-

-

-

-

+ + + + + + + + + + + + + - - - - - - - - - - - - - --

d2

e-PIT e-

P

+ + + + + + + + + + + + + ++ - - - - - - - - - - - - - -d1

+ + + + + + +

(vi) Pressing


(v) Released

e-PIT e-

P

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


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Piezoelectric-Induced Triboelectric Hybrid Nanogenerators Based on

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