Article pubs.acs.org/cm
Molecularly Engineered Surface Triboelectric Nanogenerator by SelfAssembled Monolayers (METS) Giyoung Song,† Younghoon Kim,‡ Seunggun Yu,† Min-Ook Kim,§ Sang-Hee Park,∥ Suk Man Cho,† Dhinesh Babu Velusamy,† Sung Hwan Cho,† Kang Lib Kim,† Jongbaeg Kim,§ Eunkyoung Kim,‡ and Cheolmin Park*,† †
Department of Materials Science and Engineering, and ‡Department of Chemical and Biomolecular Engineering, Yonsei University, Yonsei-ro, Seodaemun-gu, Seoul, 120-749, Korea § School of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, 120-749, Korea ∥ Department of Chemical and Biological Engineering, Korea University, 5-1 Anam-dong, Seongbuk-gu, Seoul, 136-713, Korea S Supporting Information *
ABSTRACT: Self-powered energy harvesters utilizing triboelectric effect and electrostatic induction have been widely studied, leading in the materials viewpoint to numerous material pairs for facile charge separation upon repetitive contacts with elaborate topological structures. Here, we present a simple but robust triboelectric platform based on a molecularly engineered surface triboelectric nanogenerator by self-assembled monolayers (METS). Triboelectric surface charge density of a substrate was readily controlled by the variation of end-functional groups of self-assembled monolayers (SAMs). In particular, by employing fluorine terminated SAMs, we are able to develop a METS with the maximum open circuit voltage and short circuit current of 105 V and 27 μA, respectively, under relatively gentle mechanical contacts with the 3N vertical force at 1.25 Hz. The power density of the device was 1.8 W/m2 at the load resistance of 10 MΩ more than 60 times greater than that of an unmodified dielectric/Al device. Moreover, our approach with SAMs was extended to various types of surfaces including fabrics of silk, cotton, and poly(ethylene terephthalate) (PET) and a PET film, and the results of singlefriction-surface triboelectric nanogenerators with these materials offers a facile and universal guideline for designing triboelectic materials. performances of 4 and 6 times greater than those with flat films, respectively. Most of the topological structures were, however, fabricated by physical methods usually necessitating highenergy processing (e.g., RIE, ICP etching, electrochemistry anodization, and hydrothermal growth),13−15 which thus significantly restricts material selection. There is, therefore, a great demand to develop a way to conveniently control and enhance the triboelectric effect even without new substrate design and elaborated topographic modification. Considering that the triboelectric effect mainly depends on physical and chemical properties of a top molecular surface in contact with another one, we envisioned that self-assembled monolayers (SAMs), i.e., ordered surface of organic molecules which are covalently linked to various oxide and metal surfaces, are useful to control the interface16 and thus surface charge transfer when appropriately employed to a TENG. Furthermore, our molecularly engineered surface of a few nanometer thick
1. INTRODUCTION The scavenging of abundant mechanical energies such as piezoelectric, electromagnetic, and electrostatic energies has become a central issue for new energy generation.1−3 Recently, a triboelectric nanogenerator (TENG) based on the coupling of triboelectric effect and electrostatic induction has been extensively studied because of its suitability as an energy harvester with high energy conversion efficiency for diverse selfpowered devices such as sensors, transistors, wearable devices, batteries and hybrid power supplies.4−8 Generally, the triboelectrification depends critically on the intrinsic property of materials, which involves the triboelectric charge transfer between the two contact materials, arising from the difference of the materials in losing and gaining charges. It is indeed crucial to make a proper combination of two materials which gives rise to the maximum output performance with mechanical stability upon repetitive contacts. Either periodic or random topological nano/micron scale features have been commonly introduced to further enhance the contact-electrostatic induction. Micropatterned pyramid arrays9,10 and nanoporous/nanowire structural films11,12 exhibited their output © XXXX American Chemical Society
Received: April 23, 2015 Revised: June 14, 2015
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DOI: 10.1021/acs.chemmater.5b01507 Chem. Mater. XXXX, XXX, XXX−XXX
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plasma treated (50 W, 2 min) samples were dipped into the silane solution, followed by heating at 80 °C for 3 h. 2.4. Characterization. The formations of SAMs on the PDMS surface were confirmed by a contact angle meter (CAM101 model, KSV Instruments Ltd., Finland) and X-ray photoelectron spectra (XPS) with a K-alpha (Thermo VG, UK) at room temperature using a monochromatic Al X-ray source at 12 kV and 3 mA. The sample analysis chamber of the XPS instrument was maintained at a pressure of 2.9 × 10−9 mb. The surface roughness and morphology were observed by a tapping mode atomic force microscope (AFM) (Nanoscope IVa Digital Instruments) with height and phase contrast. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was employed with an ION-TOF (ToF.SIMS5, Münster, Germany) ata current of 25 keV, 1 pA for image mapping of the F− ion. ToF-SIMS images were collected after 30 scans at 4 points/pixel across the area of 200 × 200 μm2. Surface zeta-potential values were recorded by an electrophoretic light scattering spectrophotometer equipped with a zeta flow cell (ELSZ-1000, OTSUKA Electronics Co. Ltd., Japan). All samples were measured using monitoring particles dispersed in 10 mM NaCl solution at room temperature. The electrical outputs of METS were measured by a Keithley 6485 picoammeter, a Tektronix DPO2024 oscilloscope, and a Keithley 6517B electrometer for lownoise and precise current/voltage measurements.
SAMs allows for facile modification of chemical characteristics by introducing diverse end-functional groups to the surface without changing either metal or polymer composing a TENG, which also offers a useful platform to examine the functional group dependent triboelectric effect. Here, we present the significantly enhanced triboelectric properties by a molecularly engineered surface triboelectric nanogenerator by self-assembled monolayers (METS). The triboelectric output performance was significantly varied, dependent upon end-functional groups of SAMs uniformly formed on a substrate by either solution or vapor deposition. For instance, the fluorosilaned METS in a size of 2 × 3 cm2 delivers the maximum open-circuit voltage and the maximum short-circuit current of 105 V and 27 μA, respectively, under relatively gentle mechanical contacts with a 3 N vertical force at 1.25 Hz. The power density of the device was 1.8 W/m2 at the load resistance of 10 MΩ more than 60 times greater than that of an unmodified dielectric/Al device. The improved triboelectric effect by SAMs is systematically examined by various surface characterization analysis; thus, the relationship between the surface specific chemical molecules and triboelectrical properties can be established. Furthermore, SAMs were successfully employed on a variety of the substrates including silk, cotton, poly(ethylene terephthalate) (PET) fabrics, and a PET film, resulting in apparent change in the average opencircuit voltage values due to the modification of triboelectric charge difference between two contact surfaces by the SAMs in single-friction-surface triboelectric nanogenerator (STEG) mode.17
3. RESULTS AND DISCUSSION Our METS is based on conventional dielectric/metal contact in a capacitor, and SAMs were readily developed on a polydimethylsiloxane (PDMS) layer as schematically shown in Figure 1a. To demonstrate the effectiveness of SAMs on the triboelectric properties, we employed four representative SAMs
2. EXPERIMENTAL SECTION 2.1. Materials. 3-Aminopropyltriethoxysilane (APTES), 3-glycidoxypropyltriethoxysilane (GPTES), 1H,1H,2H,2H-perfluorooctyltrichlorosilane (FOTS), trichloro (3,3,3-trifluoropropyl) silane (TFPS), and a PET/ITO film were purchased from Aldrich (Korea). Polydimethylsiloxane (PDMS) prepolymer (Sylgard 184) and a curing agent were purchased from Dow Corning. Commercially available fabrics such as silk, cotton, and PET were used as received unless otherwise mentioned. 2.2. Fabrication of METS. Our approach is based on utilizing molecularly engineered surfaces by self-assembled monolayers (SAMs) to control the surface potential. SAMs employed in the work were interacted spontaneously with a hydroxyl group on dielectric surface, giving rise to nanometer thick monolayers with highly ordered molecular domains over a large area. For preparing dielectric substrates with hydroxyl functional groups, a mixture of PDMS precursor and curing agent (5:1 by weight) was prepared, and subsequent spin coating at a spin rate of 1500 rpm produced a flat film with the thickness of approximately 20 μm on a p-doped Si substrate. After the PDMS sample was cured at 60 °C for 6 h, the film was treated with oxygen (O2) plasma (40 W) for 2 min for developing hydroxyl groups on the surface, and SAMs were subsequently prepared on the PDMS by either liquid−phase or vapor-phase deposition method. The FOTS and TFPS samples were stored in a desiccator for 2 h with guided vacuum flow, and APTES and GPTES-films were immersed in 2 vol % of ethanol-based solution for 1 h. METS was made of two films of a flat-aluminum 70 nm-thick Al electrode (deposited by thermal evaporation under a vacuum of 10−6 Torr) and a SAM-modified PDMS in size of 2 × 3 cm2 as schematically depicted in Figure 1a. 2.3. Preparing of METS-STEG. The bleached fabrics (100% of silk, cotton, and PET (Dacron)) and PET film were cleaned by soaking in ethanol and dried before treatment. To modify the samples with fluorosilane, 5 vol % of 1H,1H,2H,2H-fluorooctyl triethoxysilane was added to a mixture of ethanol, H2O and 2 M HCl (10:3:1 volume ratio), and the solution was heated to 40 °C for 2 h. The oxygen
Figure 1. (a) Scheme of a molecularly engineered triboelectric nanogenerator by self-assembled monolayers (METS). (b) Schematics of various silane-based SAMs anchored on the O2 plasma-treated PDMS substrate and characterizations of surface functionalities by XPS (the middle row), height-mode AFM analysis (the bottom row), and diwater contact angle measurement (insets). The root-meansquares roughness (RRMS) of FOTS, TFPS, APTES, and GPTES are 0.232, 0.450, 0.151, and 0.147 nm and the contact angles are 107°, 98°, 67.9°, and 49.9°, respectively. B
DOI: 10.1021/acs.chemmater.5b01507 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 2. Triboelectric properties of (a) the maximum open circuit voltages and (b) the maximum short circuit currents of METS measured at the frequency of 1.25 Hz with the vertical contact force of 3 N. (c) Zeta-potential values obtained, dependent upon SAMs in aqueous NaCl.
surfaces show the root-mean-squares roughness (RRMS) values of 0.232, 0.450, 0.151, and 0.147 nm, respectively, and no difference in RRMS from bare surfaces. Water contact angle measurement further confirmed the formation of SAMs as shown in Figure 1b. The FOTS and TFPS modified films containing end-fluorine atoms on the surface displayed contact angles of 107° and 98°, respectively. The lower contact angle with TFPS may arise from the lower surface density of fluorine atoms. Upon APTES and GPTES treatment, the contact angles decreased to 67.9° and 49.9° as compared to that of the pristine PDMS (∼98°). The results from various experimental tools of AFM, XPS, and contact angle together indicate the formation of smooth, chemically homogeneous, relatively condensed SAMs on PDMS substrates. Interestingly, triboelectric properties were significantly altered, depending upon the type of SAMs as shown in Figure 2. All METS were measured at a frequency of 1.25 Hz and with a vertical pressure of 3 N, and the data were collected after saturation of the properties over 5,000 cycles. While an
of 3-aminopropyltriethoxysilane (APTES), 3-glycidoxypropyltriethoxysilane (GPTES), 1H,1H,2H,2H-perfluorooctyltrichlorosilane (FOTS), and trichloro (3,3,3-trifluoropropyl) silane (TFPS) whose chemical structures are shown in Figure 1b. Hydroxyl groups on the PDMS surface resulting from oxygen plasma treatment are covalently linked with triethoxysilane groups of SAMs. The SAM modified PDMS surfaces were first characterized by X-ray photoelectric spectroscopy (XPS). The layer formation from FOTS and TFPS abundant with fluorine atoms was verified by the appearance of the characteristic F 1s electron peak at 690 eV. In addition, the strong electronemission spectra at 400 and 533 eV were observed on the surface of APTES and GPTES, corresponding to the N 1s electron peak of APTES and the O 1s electron peak of GPTES, respectively. The common atomic components of oxygen, carbon, and silane on PDMS surface are also apparent as shown in Figure S1 of Supporting Information.18,19 The atomic force microscope (AFM) images in height contrast of FOTS, TFPS, APTES, and GPTES modified C
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Figure 3. (a) Voltage (black) and current (blue) and (b) output power as a function of load resistance of a FOTS-METS. (c) Maximum power density values of METS fabricated by different SAMs. (d) Photograph of 76 LEDs connected in series turned on by a FOTS-METS. (e) Short circuit current (black) and open circuit voltage (blue) of a FOTS-METS as a function of vertical contact cycles with the frequency and force of 1.25 Hz and 3 N, respectively. (f) ToF-SIMS image mapping F− ions of a FOTS-modified PDMS surface (200 × 200 μm2) after 20,000 cycles. The inset shows a water droplet on the surface with the contact angle of 104°.
value of −11.74 mV, similar to the reference PDMS. While FOTS and TFPS modified films showed more negative zeta potentials of −34.72 and −31.79 mV, respectively, the APTES surface with amine molecules was nearly neutral with its potential of −0.45 mV, which is qualitatively consistent with the surface properties of our triboelectrified METS in Figure 2a and b. The zeta potential results display that the anions of either chlorines or hydroxide ions from the aqueous NaCl are preferentially accumulated in all of the SAM modified surfaces except for the APTES one, which suggests a clue of how the charges are transferred from one material to another during triboelectric measurements. In particular, in the case of FOTS and TFPS modified surfaces, hydroxide ions preferentially interacted with fluorine atoms as noted by previous work.20,21 The strong affinity to negative ions or charges of both FOTS and TFPS surfaces can occur during triboelectric contact with the Al electrode in which large amounts of electrons are transferred to the SAM surface, giving rise to the improved VOC and ISC (Figure S2, Supporting Information). However, amines in APTES SAMs are Lewis-base and tend to attract proton ions in solution. The triboelectrification of APTES-METS can be, therefore, accompanied by proton ion transfer from the Al electrode (Figure S3, Supporting Information). To confirm the enhanced surface charges on a FOTS surface resulting from the
unmodified PDMS/Al device shows the maximum open circuit voltage (VOC) and the maximum short circuit current (ISC) of 17.3 V and 6.23 μA, respectively, our FOTS-METS exhibits the maximum VOC (105 V) and ISC (27.2 μA) as shown in Figure 2a and b. The significant enhancement of more than 6 and 4 times in VOC and ISC, respectively, indicates the effectiveness of SAMs modification. When TFPS was employed, both the maximum VOC and ISC decreased, compared with the values with FOTS due to its lower surface density of fluorine atoms as confirmed by water contact angle measurement. When APTES and GPTES were used, the maximum values of VOC (−25.8 V), ISC (−6.43 μA) for APTES, and VOC (−9.9 V) and ISC (−2.51 μA) for GPTES were obtained as shown in Figure 2a and b. Both negative VOC and ISC values for APTES and GPTES clearly suggest that the surface of PDMS with these SAMs is positively charged, while the surface of bare PDMS and one treated with either TFPS or FOTS are negatively charged with respect to the Al electrode. To further elucidate the surface characteristics of SAM modified PDMS substrates, we measured the zeta-potential of the samples which could quantify the magnitude of the surface charges. The values are largely varied with the types of SAMs as shown in Figure 2c. The bare PDMS displayed −11.04 mV, and GPTES consisting of epoxy functional groups gave rise to the D
DOI: 10.1021/acs.chemmater.5b01507 Chem. Mater. XXXX, XXX, XXX−XXX
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The preservation of the FOTS layer after multiple contact cycles was further confirmed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging in which no difference in the distribution of fluorine ions was observed before and after 20,000 cycles across the area of 200 × 200 μm2 as shown in Figure 3f (also Figure S8, Supporting Information). A watercontact angle value of 104° on the FOTS surface was also maintained after the reciprocating contacts as shown in the inset of Figure 3f. To demonstrate the further applicability of our METS, we modified the four different commercial products with another fluorine terminated SAM, 1H,1H,2H,2H-perfluorotriethoxysilane by a simple solution dipping process: the fabric-based products such as silk, cotton, and PET and film-based PET that can be suitable for flexible and wearable triboelectric devices. They are ranked in order of positively charged materials in triboelectric series as schematically shown in Figure 4a. The
facile electron transfer from the Al electrode, we protected the Al surface with APTES. The triboelectric properties of the device was significantly degraded, compared with one without APTES, and the results imply that nanometer thick insulating SAMs on the Al surface hindered electron transfer from Al to fluorine atoms on PDMS during triboelectric contact (Figure S4, Supporting Information). On the basis of the theoretical equations of the contact-mode TENG model,22 the enhanced transferred charges between the SAM modified PDMS and Al arose from the increase of charge density due to fluorine atoms on the surface, which implies that our molecularly engineered surface can be an alternative for high performance triboelectric energy harvesting. The effect of ligands of SAMs on device performance was also revealed by APTES and TFPS in which trifluoromethyl (−CF3) and amine (−NH2) groups are connected to an alkyl chain (−CH2CH2−). The significant differences in output results (VOC, ISC, and power) as depicted in Figure 2 and Figure S5 (Supporting Information) suggest that the ligand is one of the most important factors for determining device performance. To obtain the information on output power of our METS, we performed triboelectric measurement of voltage and current as a function of load resistance, and the results of FOTS-METS are shown in Figure 3a. As expected, the current decreases with resistance, while the voltage increases with resistance. Output power was determined by voltage × current at each resistance, and a plot of the output power as a function of resistance shows the maximum power of approximately 1.08 mW at the resistance of 10 MΩ as shown in Figure 3b. The output power plots of our METS with other SAMs as a function of load resistance were obtained from the voltage and current behavior of the devices with load resistance (Figure S5, Supporting Information). We also examined an unmodified PDMS/Al device for comparison. The maximum output power values of all the devices examined are plotted in Figure 3c. Our devices are very reliable with the small standard deviations of power densities of 0.013, 0.14, 0.038, 0.028, and 0.057 corresponding to devices with bare PDMS, FOTS, TFPS, APTES, and GPTES, respectively. Notably, the power density of our FOTS-METS (1.8 W/m2) is approximately 60 times as high as that of the unmodified PDMS (0.03 W/m2). The power harvested from relatively gentle physical motion with the vertical force of 3 N at the frequency of 1.25 Hz and was sufficiently high to illuminate 76 LEDs connected in series as shown in Figure 3d (Video 1, Supporting Information). SAMs covalently bonded to the PDMS surface by silane interaction provide excellent mechanical stability upon repetitive contacts. As shown in Figure 3e, a FOTS-METS exhibited multiple cycle durability in which the output voltage and the current values were slightly increased without film damages and morphological alteration (Figure S6, Supporting Information). To clarify the issue, we examined the cycle performance of a nanogenerator treated with TFPS, which also contains fluorine terminal groups (Figure S7, Supporting Information). While the TFPS treated device exhibits early saturation of VOC at the cycle of approximately 1 × 104, one with FOTS still shows gradual increase of VOC with cycles. Considering that FOTS is longer than TFPS with the same ligand, the slight increase in VOC with cycles in the device with FOTS may be due to the unsaturated charges on the surface arising from the abundant fluorine atoms consisting of the molecules. It should be noted that a control device with a bare PDMS rarely shows the variation in VOC with the cycle.
Figure 4. Characterization of METS-STEG with the various fluorosilane-modified materials. (a) Photographs of the hydrophobic surfaces of the silk, cotton, and PET (Dacron) fabrics after SAMs modification, which are arranged in triboelectric series in the order of relative polarity of the contact charge created. (b) Average open circuit voltage values of before (open black diamonds) and after SAMs treatment (solid red diamonds) of the materials obtained in STEG mode. The inset shows the device structure of METS-STEG in which RB presents the body resistance, and RL is the road resistance of 100 MΩ.
fluorinated SAM treatment successfully made the surfaces of the materials hydrophobic as shown in the photographs of Figure 4a. To effectively characterize triboelectric output performance of these four SAM modified materials, we introduced another device architecture of STEG17 consisting of the modified samples as an objective film, PET/ITO film for a friction surface and Al tape for a reference electrode as shown in the inset of Figure 4b. STEG results of the SAM modified fabrics and film show the significant change in average output voltage compared with those of unmodified ones as shown in Figure 4b. In the cases of silk and cotton with a size of 2 × 3 cm2 upon contact to the flat PET/ITO film by hand, the average VOC decreased from 57.4 and 35.7 V to 23.1 and 20.6 V, respectively, with surface modification, which indicates that the tribocharge difference between a SAM treated surface and PET on ITO becomes smaller than that between an E
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unmodified surface and PET in both cases. However, the values of the PET fabric and PET film increased from 25.7 and 52.3 V to 42 and 113 V, respectively, due to the enlarged tribocharge gap in triboelectric series after the modification (Also Figure S10, Supporting Information). Our METS is indeed useful to control the relative tribocharge difference between two contact materials and thus enhance the energy harvesting performance without complicated fabrication processes. Notably, even two identical surfaces, e.g., PET films, were successfully utilized as a triboelectric nanogenerator with SAM modification.
4. SUMMARY In summary, we developed a simple but robust route for harvesting triboelectric energy by employing molecularly engineered triboelectric surfaces modified by conventional SAMs. The systematic investigation of SAM treated PDMS substrates based on zeta potential as well as triboelectric measurement revealed that the surface charge was varied, dependent upon the end-functional groups of the SAMs, giving rise to the facile control of the triboelectric properties of our METS. In particular, a FOTS-METS exhibited the excellent output performance of the maximum VOC and ISC of 105 V and 27 μA, respectively. The power density of the device was 1.8 W/m2 at the load resistance of 10 MΩ more than 60 times greater than that of an unmodified PDMS/Al device and sufficient to turn on the 76 LED bulbs in series. Moreover, our approach with SAMs was extended to various types of surfaces including fabrics of silk, cotton, and PET and a flexible PET film, and the results of single-friction-surface triboelectric nanogenerators with these materials confirmed the controllability of triboelectric performance with SAMs. Versatility of SAMs with numerous end-functional surface groups as well as surface anchoring groups will trigger a new research field for triboelectric self-powered energy harvesters with molecularly engineered surfaces.
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ASSOCIATED CONTENT
S Supporting Information *
Chemical and electrical characterizations, the mechanism of charge-transfer, and stability of METS. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b01507.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2014R1A2A1A01005046) and the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (2010-0019313). This research was also supported by the third stage of Brain Korea 21 Plus Project in 2014.
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REFERENCES
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