Highly Surface-Embossed Polydimethylsiloxane-Based Triboelectric

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Energy, Environmental, and Catalysis Applications

Highly surface-embossed polydimethylsiloxane-based triboelectric nanogenerators with hierarchically nanostructured conductive Ni–Cu fabrics. Da Song Choi, Seung Mo Yang, Choong Hyun Lee, Woo Jong Kim, Jae Ho Kim, and Jin-Pyo Hong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10613 • Publication Date (Web): 07 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

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ACS Applied Materials & Interfaces

Highly surface-embossed polydimethylsiloxane-based triboelectric nanogenerators with hierarchically nanostructured conductive Ni–Cu fabrics Dasong Choi†, Seungmo Yang†, Choonghyun Lee†, Woojong Kim‡, Jaeho Kim‡ and Jinpyo Hong*†‡ †Research

Institute of Natural Science, Novel Functional Materials and Device Laboratory,

Department of Physics, Hanyang University, Seoul, 04763, Republic of Korea ‡

Division of Nano-Scale Semiconductor Engineering, Hanyang University, Seoul 04763, South

Korea

E-mail: [email protected]

Keywords: triboelectric nanogenerator, ZnO nanowire, nanoflake, self-powered, Ni–Cu fabric substrate, surface-embossed frames

ACS Paragon Plus Environment

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Abstract

Wearable fabric-based energy harvesters have continued to gain importance for use in portable consumer electronics as an eco-friendly energy source that is independently self-powered by various activities. Herein, we address the output features of highly flexible Ni–Cu fabric-based triboelectric nanogenerators (F-TENG) employing surface-embossed polydimethylsiloxane (SEPDMS) layers, as a crucial approach for enhancing power generation. Such SE-PDMS configurations were achieved via control of the ZnO nanowire (NW) and nanoflake (NF) frames initially prepared on the bare Ni–Cu fabrics by a hydrothermal approach. The wearable SEPMDS and Al-evaporated fabrics respectively served as triboelectric bottom and top materials in F-TENGs. Along with structural analyses of the F-TENGs, the enhanced power generation of the F-TENGs was illustrated via application of a periodic mechanical stress using an adjustable bending machine. The present approach may provide a useful and simple route for developing self-powered, wearable, smart electronics based on fabric substrates.

1. Introduction Energy harvesters continue to be of immense interest for power generation in sustainable systems based on manipulation of the abundant natural ambient energy sources in our daily life.13

Wearable energy harvesters combined with human activities are rapidly emerging as alternative

self-powered energy supplies, enabling the independent operation of small wireless autonomous devices, wearable electronics, and portable sensor networks.4-6 Among the recently developed self-powered energy harvesters, triboelectric nanogenerators (TENGs) that couple the interaction of triboelectric effects with electrostatic induction show promise for various applications.7-11 In this regard, highly flexible conductive fabric-based triboelectric nanogenerators (F-TENGs) that


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serve as a power supply for daily life would offer great potential for the implementation of advanced portable smart electronics with high reliability, as possible replacements for conventional energy scavengers;12-16 that is, diverse human mechanical movements is one of reliable options for the utilization of unlimited and eco-friendly energy resources daily occurring in common life. Much effort has been dedicated towards exploiting self-powered energy harvesters in which wearable or flexible fabrics such as polyethylene terephthalate are used as substrates.17-19 However, the utilization of such materials is still hampered by low output performance. Thus, the implementation of further enhanced output performance of F-TENGs still remains a challenge as one core work. One basic factor for achieving high output performance is the judicious selection of appropriate tribo-materials with large opposite electrical polarities in the triboelectric series.2025

Another alternative approach is to augment the surface charge densities at the interfaces

between the tribo-materials by enhancing the surface contact areas of the triboelectric materials.26-34 As such, this work, first, aims to bring the highly flexible Ni–Cu fabric as a generic substrate for harvesting energy from diverse activities, such as vibration, rotation, and displacement. Ni–Cu fabric was selected owing to its low sheet resistivity, highly flexible elasticity, and cost-effectiveness among the commercially available two-dimensional fabrics. Anisotropic ZnO nano-wire (NW) or nano-flake (NF) frames were initially prepared on the bare Ni–Cu fabrics to generate surface-embossed (SE) configurations of a polydimethylsiloxane (PDMS) layer by a dip-coating approach for the achievement in surface charge densities. We address herein the output features of F-TENGs employing the surface-embossed PDMS (SE-PDMS) and Al layers serving as bottom and top plates, respectively, by utilizing flexible/conductive Ni–Cu fabric substrates. Highly nanostructured SE-PDMS configurations


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that increased the surface contact areas between the triboelectric materials were achieved by means of the ZnO NW and NFs initially prepared on the bare fabric by a hydrothermal approach. An Al thin layer, working as a top plate, was also thermally evaporated on the same bare fabric. The electrical features of the resulting F-TENGs were systematically evaluated by using a mechanical pusher tester that provides a uniformly constant pressure during the contactseparation process, along with structural observations of both the top and bottom plates used in the F-TENGs.

2. Results and Discussion Figure 1 illustrates a simple schematic for the fabrication step of the wearable F-TENG employing the SE-PDMS and Al-coated fabrics as the bottom and top plates, respectively, where the PDMS and Al materials were chosen due to the effective charge transfer induced by the large polarity difference between these materials upon contact, according to the triboelectric series. The top and bottom plates started from the highly conductive Ni–Cu fabrics composed of artificial woven fibers with naturally formed microscale roughness, as seen in the optical image in Figure 1. First, the 150 nm thick Al layer, as a top plate, was thermally evaporated onto the bare fabric under a pressure of 10−7 Torr at room temperature with a deposition rate of 0.2 Å/s. As a bottom plate, the SE-PDMS layer was prepared on the ZnO NW and NF nanostructured fabric by a dip-coating approach (detailed information is presented in the Experimental section). Thus, to ensure that the required configuration was achieved, the initial growth of the ZnO NFs on the bare bottom Ni–Cu fabric by the hydrothermal approach was crucial in this work, where a 30 nm thick Al layer (serving as a seed layer) was selected to induce the desired growth of the ZnO NFs. Note that, frequently, some parts of the ZnO NFs or Al seed layer were lifted off


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during the measurements when the proper annealing process was not done. Therefore, postannealing treatment at ~200 °C was carefully employed to initially enhance the adhesion of the Al seed layer on the bare Ni–Cu fabric after growth of the NFs. The output responses were evaluated via the iterative touch process using a pushing tester with the assembled bottom and top plates with a rigid spacer of 0.5 cm. More information on the wearable/flexible bare Ni–Cu fabric is presented in the Supporting Information (Figure S1, Supporting Information). Figures 2a−d present the representative FE-SEM images of the ZnO NWs and NFs prepared on the bare Ni–Cu fabric by the hydrothermal approach, where the ZnO and Al films were employed as the seed layers for growth of the ZnO NW and NFs, respectively. As seen in Figures 2a and c, the ZnO NWs were uniformly well-coated over the entire bare fabric that has an initially microscale surface roughness. The grown ZnO NWs had a diameter of about 100−150 nm and a length of 2.5 µm. In general, it is widely believed that the use of ZnO seed layers with naturally hexagonal configurations leads to the growth of hexagonal columnar ZnO NWs along the wurtzite crystal structure of the ZnO seed layer during the hydrothermal process; that is, the Zn2+ complexes used in the hydrothermal solution induce the formation of ZnO nuclei with a high tendency towards growth along the [0001] polar orientation 25. However, as mentioned above, a suitable Al seed layer was required for growth of the ZnO NFs; that is, the Al seed layer surface is easily oxidized to Al3+, followed by the formation of an aluminum hydroxide (AlO2−) capping layer. The negatively charged AlO2− ions were initially absorbed on the (0001) plane of ZnO terminated with positively charged Zn2+ ions, thereby facilitating relatively slow growth of the c-axis in the ZnO NWs, compared to the growth of the ZnO NWs grown by the absence of Al seed layer. Thus, suitable control of the Al seed layer herein generated a slow supply of Zn2+ complexes by efficiently activating growth in various facets of


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the ZnO layer, thereby ensuring the growth of the ZnO NFs during the hydrothermal process (Figure S2, Supporting Information). As evident in Figure 2d, the use of a 30 nm thick Al seed layer led to the formation of flake-shaped two-dimensional (2D) ZnO NFs with an average length of 4−5 µm and a width of 30−40 nm. It should be noted that the ZnO NWs and NFs used as initial nanostructured frames were selected owing to their simple growth at low temperature, without any significant impact on or damage to the bare Ni–Cu fabric upon fabrication and thermal annealing, even though the hydrothermal process may not currently be effective for large area growth. Figures 2e−f show the representative high-resolution X-ray diffraction (HR-XRD) patterns of the ZnO NWs and ZnO NFs. Both figures illustrate the presence of predominantly oriented wurtzite phases at 34.35° and 34.6°, corresponding to the (002) diffraction peaks of ZnO along the c-axis. The peaks at 43.35° and 50.55° correspond to the diffraction peaks of Ni and Cu, respectively, where these two species are the constituents of the fabric substrate. The peak at 38.8° in Figure 2f corresponds to the Al (111) diffraction signal, supporting the assistance of Al seed layer for the growth of the ZnO NFs. Figures 2g−h exhibit the representative photoluminescence (PL) profiles of the ZnO NWs and ZnO NFs monitored at room-temperature. As seen in this figure, the near-band-edge (NBE) electron peak at 372.59 nm and broad deep level emission (DLE) profiles are clearly observed, thereby ensuring the efficient growth of the ZnO NWs and NFs, as evident from the XRD findings.35 In particular, it is widely believed that the DLE profiles are closely associated with structural defects, oxygen vacancies, etc.36, 37 Closer investigation also revealed a slight suppression in the DLE intensity of the ZnO NFs that may represent a reduction in the number of pre-existing vacancy-related defects (that is, the presence of low defects) in the crystalline structure. This may improve transport of the generated tribocharges without electrical loss,38, 39 compared with that of the ZnO NWs without Al atoms (not


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shown in this figure). In addition, the use of the Al seed layer for growth of the ZnO NFs may also facilitate the impregnation of Al atoms into the ZnO NFs (Figure S3, Supporting Information), corresponding to the increased charge transport, given that previous theoretical calculations predicted that relatively low formation energy of vacancy in n-type ZnO as a vacancy nature can allow for the out-diffusion of the Al atoms or ions towards crystalline solids through a vacancy mechanism.40 Thus, the impregnated Al atoms or ions inside the ZnO NF layer leads to the formation of Al–O bonds with the oxygen ions since Al has the greater reduction potential than that of Zn.41 Thus, the generated Al–O bonds may permit higher charge mobility, reflecting the improved tribo-charge transport. One of the crucial factors considered in this study is the formation of surface-embossed PDMS configurations along the ZnO NW or NF geometries to promote charge separation during the contact process. Thus, significant refinement to achieve precise adjustment of the thickness of the PDMS layer was required, and could be accomplished by introducing a suitable amount of hexane into the PDMS precursor solution. Thus, the hexane/PDMS volume ratio (0:1 (without hexane), 1:1, and 2:1) was varied in the dipping approach (Figure S4, Supporting Information). Various surface configurations of SE-PDMS layers by the introduction of different hexane solutions are also given in the Figure S5 of Supporting Information. Figures 3a−b show the typical top and cross-sectional SEM images of the pure PDMS layers coated on the bare fabric without the use of ZnO nano-frames, where the 2:1 volume ratio was used during the dipping process. As seen in these figures, the uniformly distributed PDMS layer fully covered the entirety of the bare fabric areas, along with the self-patterned fabric surface, indicating the usefulness of the dip-coating process when the proper hexane mixture was used. Figures 3c−d show the magnified top SEM images of the SE-PDMS layers, demonstrating the presence of the


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SE-PDMS patterns along the ZnO NWs and NFs, respectively. These technologically nanostructured configurations of SE-PDMS formed with the assistance of the ZnO nano-frames initially prepared on the bare fabric are expected to contribute to enhancing the surface charge density by enlarging the surface contact areas, thereby providing an enhancement in electrostatic induction between the two triboelectric materials. Figure 3e shows the representative SEM images of the thermally evaporated Al top layer on the bare fabric, serving as a top plate, along with the enlarged top SEM image (bottom figure). As seen in this figure, the Al top layer exhibited nanoscale surface roughness, compared with the microscale surface roughness inherently created by the bare fabric itself. Figure 4 illustrates the principle of operation of the F-TENGs employing the Al-coated Ni–Cu fabric (top electrode) and the SE-PDMS fabric (bottom electrode). It is widely believed that charge generation is the consequence of the inevitable triboelectric effects and electrostatic induction during periodic contact and separation.10, 11 When the SE-PDMS bottom layer contacts with the Al top layer, the former will become negatively charged due to a higher electron attractive features according to the tribo-series. In the initial state before contact, no electrical potential difference is established, thus there is no charge transfer in either tribo-material. However, in the first compressed position where an external stress is applied, the surfaces of the SE-PDMS and Al layers make direct contact with each other, thereby allowing for charge transfer from the Al layer to the SE-PDMS fabric; that is, the SE-PDMS electrode accumulates the electrons, reflected by the generation of positive charges on the side of the Al top electrode (Figure 4a). When the F-TENG comes back to the initial state (Figure 4b), a net electric field is generated by the previous charges accumulated in each triboelectric layer. As the separation becomes larger, a gradual increase in dipole moment between the two tribo-materials provides a


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reversed output signal. However, even complete separation of two tribo-electric layers might not experience the entire annihilation of the previously accumulated charges. Instead, the insulating properties of the SE-PDMS surface would maintain the accumulated charges for a long period.42 that is, electron transfer and accumulation on the SE-PDMS surface will last until the generated charge reaches a state of saturation due to repeated impact. When the interfaces of the SE-PDMS layer and Al-layer are completely separated, both positive and negative charges are neutralized, corresponding to the absence of electrical signal through the external load (Figure 4c). The second time the two tribo-materials are brought into contact, the accumulated positive charges flow from the SE-PDMS surface to the Al top layer for the compensation of the electrical potential differences, thereby generating the forward output signal (Figure 4d). This operation will provide an alternating output potential and current in F-TENGs. To clarify the detailed influence of the SE-PDMS configuration on the electrical responses, three different PDMS surface configurations working as negative triboelectric layers were prepared for comparison: flat PDMS on the bare fabric (pure bare PDMS), SE- PDMS on the ZnO NWs (SE-PDMS on NW), and SE-PDMS on the ZnO NFs (SE-PDMS on NF). In addition, the bare and Al layer-coated fabrics were also used as top layers for the measurements, along with three different SE-PDMS layers, where the bare and Al layer-coated fabrics had intrinsic microscale (Fig. S1a) and nanoscale (Fig. 3e) surface roughness, respectively, as mentioned before. Note that the measurement variables, such as the number of times the surfaces were brought into contact and isolated, were carefully controlled for equivalent analysis of all the samples. For example, the representative output voltages of the F-TENG increased periodically for the first consecutive 80–100 contacts and remained unchanged after reaching the maximum value during evaluation of the mechanical stability (Fig. S6a−b, Supporting Information). This


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corresponds to the charge transfer/accumulation and charge saturation states during consecutive measurements. In this work, the mechanical robustness and durability of the devices were confirmed from consecutive cyclic impacts of more than 10ଷ cycles, where no significant degradation was observed. Figure S6c reveals forward and reverse output signals for the FTENG recorded upon pressing and releasing states, respectively. As seen in this figure, the presence of more irregular and less uniform signal at releasing state is mainly relevant to the relatively weak interaction and friction force between the SE-PDMS and the Al fabrics, when compared to the forward output signal, as depicted in the operation principle section above. In addition, the F-TENG revealed the polarity dependent output performance (Figure S6a−b, Supporting Information). The measurements were conducted with repeated motion to eliminate any other external impacts. The resulting polarity-dependent output feature was induced by triboelectrification and charge induction from the F-TENGs. The observed electrical responses for all samples were compared after 1000 contacts, as depicted above. Figures 5a−b show the representative output voltages and current densities of the F-TENGs with three different bottom and two top plates, i.e., bare-flat PDMS, SE-PDMS on NW, and SE-PDMS on NF as negative triboelectric plates and bare/Al-coated fabric as top plates. Herein, the power density or current density per unit area; however, the actual surface areas of the nanostructured configurations is expected to be larger than those of the bare flat fabric. With the bare top layer (left in Fig 5a−b), the bare flat PDMS-based F-TENG (black) provided an initial output voltage and current density of 51.4 V and 0.6 µA cm-2, respectively. These outputs may arise from the self-patterned surfaces of the flat PDMS bottom layer itself and bare fabric layer with microscale roughness, in the absence of any SE-configurations. The FTENG employing the SE-PDMS on NW (red) gave rise to an output voltage and current density


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of 65.2 V and 0.76 µA cm-2, respectively. The output voltage and current density of the F-TENG with the SE-PDMS on NF (blue) were 88 V and 1.01 µA cm-2, respectively. The typical observations indicated that for the same SE-PDMS configuration, the Al layer-coated top fabric induced a slight enhancement in the electrical performance compared with that of the bare top fabric with microscale roughness. The additional surface modification of the fabric by the deposited Al layer that leads to nanoscale roughness (as shown above) is one plausible factor leading to these results. Another factor is the relatively higher conductance of the Al material that enables improved transport of the generated tribo-charges with low electric loss, compared with that of the bare Cu–Ni fabric. With the Al-coated top fabric (right in Fig. 5a−b), the bare flat PDMS-based F-TENG (black) provided an output voltage and current density of 74 V and 0.9 µA cm-2, respectively. The FTENG employing SE-PDMS on NW (pink) gave an output voltage and current density of 89.6 V and 1.07 µA cm-2, respectively. The output voltage and current density of the F-TENG employing SE-PDMS on NF (green) were 130 V and 1.45 µA cm-2, respectively. In general, all of the F-TENGs with the SE-PDMS layers exhibited enhanced electrical output due to the introduction of the nanostructured SE-PDMS, as evident in Fig. 5. Thus, it should note that the NF configurations (green) generally produced a slight enhancement in the electrical performance of the F-TENG. One possible factor underlying the observed enhancement seems to be the additional effective friction achieved through the NF surfaces during device operation, compared with that of the NWs (pink); that is, the much higher NF surface configurations would lead to the relatively larger contact areas, thus facilitating more effective charge generation relative to that achieved with the NWs. The relatively low number of defects in the crystalline structure and impregnation of Al atoms or ions inside the ZnO NF layer may also be contributing factors, as


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previously illustrated in Fig. 2; however, further work is still required to definitively clarify the origin of the enhanced electric performance when the NF configuration is used. Figure 6a−b displays the average electrical output of all the F-TENGs. The observed maximum voltage and the current density of the F-TENG with the SE-PDMS NFs and Al-coated fabric were 130 V and 1.45 µA cm-2, respectively, leading to a corresponding power density of 188.54 µW cm-2. To further optimize the output performance of the F-TENGs for various applications, the external load dependency of the F-TENG was examined using a variable resistor. Hereafter, only the F-TENG with the SE-PDMS NFs and Al-coated fabric, which provided the maximum output performance, is discussed and is denoted as Sample A for convenience. Figure 6c shows the output voltage and current of Sample A verse load resistance, providing a maximum voltage of 166.6 V and current of 113.3 µA, along with a maximum output power of 229.28 µW under an external resistance of 10 MΩ (Fig. 6d). However, during operation, the SE-PDMS with NWs was frequently damaged and peeled off due to an issue with adhesion of the NWs, even though post-annealing was conducted at several annealing temperatures after the growth of the NWs. However, it should be noted that the consecutive operation in our work permitted the nanostructured SE-PDMS surfaces to remain almost undistorted, except under extremely high pressing forces. Such high pressing forces introduced peeled fragments to the opposite Al layer. This is attributed to the peel-off phenomenon of the SE-PDMS surfaces from the fabric, induced by the possibly adhesive features of SE-PDMS surfaces under high pressing forces. Thus, the utilization of high pressing forces in our work has the disadvantage of degrading the properties of the presented F-TENGs so that more additional works is highly necessary for the application of high pressing forces in the future by means of the choice of suitable seed layer or annealing process. The mechanical stability performance for


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our device upon a low pressing force was also given in Figure S8 of Supporting Information, confirming no critical degradation in mechanical endurance test during consecutive 2000 cyclic impacts. In addition, the transferred charges for the bare and SE-PDMS surfaces are carefully compared (Figure S9) since the effective surface contact is highly linked with the generated charges. As evident in this figure, the transferred charges increases proportionally to cyclic impacts. Furthermore, influence of high and low pressing forces on the surface geometrics of FTENG as one damaged example was also provided in Figure S10 of Supporting Information. Based on the output data, measurements were also carried out to exploit the possible piezoelectric contribution to the output responses, resulting from the ZnO NWs or NFs themselves used in the F-TENGs. Two samples with different device configurations were prepared (Figure S11a−b, Supporting Information), where the first was a bare flat PDMS bottom layer with the Al-coated top fabric (Figure S11a, Sample B) and the second was the SE-PDMS bottom layer employing the NF configuration with the Al-coated top fabric (Figure S11b, Sample C). The respective samples are denoted as Samples B and C for convenience, where the SE-PDMS bottom layer with the NW configuration is excluded for convenience. As seen in this figure, the respective layers in both samples were brought into direct contact with each other without any gap (spacer) between them in order to provide no triboelectric influence in these schematics. The measurements were carried out by applying direct mechanical pressure to the samples. As seen in Figure S11c−d, the output voltage and current of Sample C were about 1.0 V and 0.2 µA, respectively, whereas no distinct output responses were observed in Sample B. Thus, the possible piezoelectric contribution of the ZnO NFs and PDMS themselves to the output performance could be excluded, implying that the main output of the devices was derived from the triboelectric effect.


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To further validate the applicability of the F-TENGs as possible wearable devices, simple sandwiched multi-stacked F-TENGs were also tested. Figure S12a shows typical geometric photographs and 3D-sketches of the single and double-stacked F-TENGs, which were composed of the SE-PDMS on NF and Al top fabrics with a spacer (0.5 cm) between them. The doublestacked F-TENG had two SE-PDMS on NF layers on the outer sides and one Al-coated fabric in the center, where both sides of the center fabric were coated with thermally evaporated Al. For simple comparison, the measurements were conducted with the application of a relatively low force to the single and double-stack samples relative to those used for the samples in Figure 5. As evident in Figures S12b−c, the double-stacked sample exhibited stacking number-dependent output performance. The resulting voltage and current of the double-stacked F-TENG were consistently higher (3.73 and 10.4 times, respectively) than those of the single-stack F-TENG. The much higher performance (3.73 and 10.4 times) of the double-stacked F-TENG than the roughly expected outputs (2.5−3 times) may be the consequence of the more uniform and instantaneously larger contact areas when the external low forces were applied to both sides of the double-stacked F-TENG. In order to further verify the suitability of the F-TENG as a flexible/wearable device, random twisting/bending evaluations were also performed by manually applying stress with hand fingers. Figures S12d−e show the photographs of the F-TENGs and the corresponding output voltage/currents of about 3 V and 0.5 µA, respectively. However, the relatively low output performance is likely due to the irregular and random external forces exerted on the sample manually, versus with the bending machine. The experimental findings exhibit the clear surface contact area-dependent outputs when the surface modification is systematically done. Thus, employing surface embossing configurations through diverse surface-


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embossed frames could be a generalized approach to various types of future 2D/3D energy harvesting fabric networks.

3. Conclusion In summary, we have demonstrated the output features of F-TENGs with all-wearable SEPDMS bottom and Al-coated top plates by employing a conductive Ni–Cu fabric as a substrate, for the development of practical energy harvesters. Suitable SE-PDMS configurations were constructed via deposition of ZnO NWs or NFs on the bare fabrics by the hydrothermal process, clearly verifying the ability of these systems to convert mechanical energy to electricity without significant degradation during repetitive operation. The morphology of the SE-PDMS layer was strongly affected by the precise composition of the hexane solution during the dip-coating process. Enhancement of the electrical features of the F-TENGs was primarily due to the increase in the surface contact areas and consequently enhanced surface charge transfer derived from the nanostructured SE configurations. Thus, we anticipate that the exploration of conductive/flexible fabric substrates may provide new opportunities for their implementation as wearable electrical devices, combined with the use of surface embossed configurations.

4. Experimental section Growth of ZnO NWs and NFs on the bare Ni–Cu fabric: Highly flexible/conductive Ni–Cu fabrics were employed as a starting substrate for development of the F-TENGs, where the bare fabrics were first washed with the following standard series: acetone, ethanol, and deionized (DI) water to remove possible residual prior to device fabrication. A 30 nm thick ZnO film was first


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deposited as a seed layer for growth of the ZnO NWs on the bare Ni–Cu fabric by using a dipping approach with a solution of zinc acetate dihydrate (30 mM; Zn(CH3COO)2·2H2O, Sigma–Aldrich) and variable amounts of ethanol. The ZnO layer-coated fabric was then heated at 200 °C for 15 min. As the second step, a 30 nm thick Al film, as a seed layer for growth of the ZnO NFs, was deposited on the bare fabric via thermal evaporation, where the deposited Al layer was post annealed (~200 °C) to enhance its adhesion to the substrate. The ZnO and Al layers deposited on the bare fabric were inserted into DI water. In particular, different molar concentrations of zinc nitrate hexahydrate (Zn(NO)3·6H2O, Sigma–Aldrich) were chosen for the growth of ZnO NWs (0.25 M) or ZnO NFs (0.01 M) with the same molar concentration of hexamethylenetetramine (HMTA, Sigma–Aldrich), where the 1:1 volume ratio between zinc nitrate and HMTA was taken. In general, the coalescence of the ZnO nuclei during growth is strongly affected by the growth parameters in the hydrothermal process.35, 43 Thus, the growth of ZnO NFs was conducted with a relatively small amount of source flux (10 mM), compared to that used for the ZnO NWs (25 mM); that is, an efficient growth of the ZnO NFs was obtained with the decreased source flux, reflecting the severely slow growth of the ZnO NWs 44 in the presence of the Al layer that plays the catalytic role. Both ZnO NWs and NFs were synthesized in an aqueous solution in a convection oven at a constant temperature of 90 °C over the course of 6 h and 4 h, respectively Preparation of SE- PDMS layers on the ZnO nano frames: Various SE-PDMS configurations were achieved on the initially grown ZnO NWs and NFs by a dip-coating approach. First, for a PDMS solution, the weight ratio (1:10) between the cross-linker (curing agent) and a Sylgard 184 (Dow Corning Corp.) base elastomer was initially taken and then followed by a degassing procedure for 10 min at room temperature to remove the bubbles. Thermal curing treatment was


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conducted at 90 ℃ for 2 h in a convection oven after the insertion of the fabrics into the PDMS solution. The hexane solution was carefully introduced into the PDMS solution to achieve control of the SE-PDMS configurations on the fabrics initially prepared with the ZnO NWs and NFs, where the thickness of the PDMS layer was carefully manipulated by controlling the amount of hexane added. Electrical responses of F-TENGs and structural features of SE-PDMS: The electrical responses of the F-TENGs were monitored by using an adjustable pushing instrument (Z-tech Corp.), an oscilloscope (Lecroy, WavePro 735Zi), and a low-noise current amplifier (Standard Research System, SR570), where the F-TENG had dimensions of 3 cm × 3 cm with a space of 0.5 cm between both plates. Field-effect scanning electron microscopy (FE-SEM), high-resolution X-ray diffraction (HR-XRD), and room temperature photoluminescence (RT-PL) analyses were performed to evaluate the surface structural and optical features of both plates used in the STENGs.

Supporting Information FE-SEM images of bare Ni-Cu fabric and EDS information including the composition ratio of the bare fabric (Figure S1); Schematic of chemical reaction steps for growth of the ZnO NFs on Al seed layer during hydrothermal process (Figure S2); EDS data for 2D ZnO NFs (Figure S3); Cross-sectional SEM images of bare fabric substrate coated with PDMS layer without the use of hexane solution (Figure S4); Top SEM images of only bare PDMS layer confirming flat/smooth surfaces, and SE-PDMSs prepared on the ZnO NWs and ZnO NFs (Figure S5); Charge transfer curves of the F-TENGs with SE-PDMS on the ZnO NFs (Figure S6); polarity-dependence of the


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output voltage and the current of the F-TENGs during several measurement cycles (Figure S7); Long term stability features for the F-TENGs (Figure S8); Transferred charge curves upon cyclic impact for Bare PDMS and SE-PDMS layers with NFs (Figure S9); Comparison of surface morphologies for SE-PDMS layers under high and low pressing forces, ensuring the damaged and undistorted SE-PDMS surfaces during a high pressure operation (Figure S10); Output voltage and current for the piezoelectric device with only flat PDMS layer and SE-PDMS layers (Figure S11); Typical output voltage and current observed from the single and multi-stacked FENGs (Figure S12);

Acknowledgements This research was supported by Korea Electric Power Corporation. (Grant Number : R18XA0608).


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Figures & captions

Figure 1. Simple flowchart for fabrication of top and bottom plates of wearable F-TENGs as a self-powered system. The starting point is the use of commercially available bare Ni–Cu fabric (left), where the SE-PDMS layers on both ZnO NW and ZnO NF frames, and the bare Ni–Cu fabric/Al-coated Ni–Cu fabric, respectively serving as bottom and top plates, were separately prepared. Output features were evaluated by a pushing test upon repeated contact.


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Figure 2. Representative SEM views of (a) ZnO NWs and (b) NFs uniformly and regularly distributed over the entire fabric area via the hydrothermal process, where two different seed layers were employed for growth of the ZnO NW and NFs, respectively, as shown in Fig.1. Magnified SEM images of (c) ZnO NWs and (d) ZnO NFs. Typical HR-XRD pattern of (e) ZnO NWs and (f) ZnO NFs, clearly illustrating the presence of ZnO wurtzite phases oriented along the c-axis and Al used for growth of the ZnO NFs. Representative room temperature PL spectra of (g) ZnO NWs and (h) 2D ZnO NWs.


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Figure 3. (a) Top and (b) cross-sectional SEM images of flat PDMS layers that fully covered the bare fabric substrate when deposited by a dipping approach prior to growth of the ZnO nanoframes. Enlarged top SEM images of SE-PDMS layers formed through the assistance of (c) ZnO NWs and (d) ZnO NFs initially prepared on the bare fabric, where the inset shows the side SEM image of SE-PDMS growth on the ZnO NFs. Both layers work as bottom electrodes in this study. (e) Top SEM image of Al-coated fabric serving as a top plate, where the magnified top SEM image of the Al-evaporated fabric (top) shows the initially rough surfaces of the bare fabric. The fabric was woven from microfibers with microscale roughness.


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Figure 4. Illustration of principle of operation of F-TENGs under open-circuit conditions during repeated contact−separation. (a) Charge transfer on both plates during the first pressing state, thereby allowing for accumulation of electrons on the SE-PDMS surface. (b) Current flow from the top Al layer to the SE-PDMS fabric through the external load, generating a reversed signal when the top plate returns to the initial state prior to contact. (c) No signal at the electrical equilibrium state, and (d) accumulated positive charge flow from the SE-PDMS fabric to the top Al plate in the 2nd pressing state to compensate the electrical potential differences.


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Figure 5. Electrical output responses of F-TENGs, where bare fabric (left) and Al-coated fabric (right) plates are used as top electrodes for comparison. (a) Output voltage and (b) current density recorded from the samples with three different bottom plates: bare flat PDMS (black), SE-PDMS on the ZnO NWs (red), SE-PDMS on the ZnO NFs (blue) for the bare top fabric electrode and bare flat PDMS (gray), SE-PDMS on the ZnO NWs (pink), and SE-PDMS on the ZnO NFs (green) for the Al-coated fabric electrode. Maximum output performance of F-TENGs was achieved when the SE-PDMS on the ZnO NFs and Al-coated fabric were used as top and bottom electrodes, respectively.


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Figure 6. (a) Average output performance (output voltage/current density) of F-TENGs and (b) power density generated by energy harvesting process for each sample. (c) Output voltage/current and (d) power as function of difference in resistance of F-TENGs with SE-PDMS on the ZnO NFs and Al-coated fabric electrodes as bottom and top electrodes.


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Highly Surface-Embossed Polydimethylsiloxane-Based Triboelectric

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