Formation of Triboelectric Series via Atomic-Level Surface

May 30, 2017 - When two oppositely charged surfaces move toward each other, the induced electrons at the bottom electrode start to flow toward the top...
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Formation of Triboelectric Series via AtomicLevel Surface Functionalization for Triboelectric Energy Harvesting Sung-Ho Shin,† Young Eun Bae,‡ Hyun Kyung Moon,∥ Jungkil Kim,§ Suk-Ho Choi,§ Yongho Kim,*,‡ Hyo Jae Yoon,*,∥ Min Hyung Lee,*,‡ and Junghyo Nah*,† †

Department of Electrical Engineering, Chungnam National University, Daejeon 34134, Korea Department of Applied Chemistry and §Department of Applied Physics, Kyung Hee University, Yongin, Gyeonggi 17104, Korea ∥ Department of Chemistry, Korea University, Seoul 02841, Korea ‡

S Supporting Information *

ABSTRACT: Triboelectric charging involves frictional contact of two different materials, and their contact electrification usually relies on polarity difference in the triboelectric series. This limits the choices of materials for triboelectric contact pairs, hindering research and development of energy harvest devices utilizing triboelectric effect. A progressive approach to resolve this issue involves modification of chemical structures of materials for effectively engineering their triboelectric properties. Here, we describe a facile method to change triboelectric property of a polymeric surface via atomic-level chemical functionalizations using a series of halogens and amines, which allows a wide spectrum of triboelectric series over single material. Using this method, tunable triboelectric output power density is demonstrated in triboelectric generators. Furthermore, molecular-scale calculation using density functional theory unveils that electrons transferred through electrification are occupying the PET group rather than the surface functional group. The work introduced here would open the ability to tune triboelectric property of materials by chemical modification of surface and facilitate the development of energy harvesting devices and sensors exploiting triboelectric effect. KEYWORDS: triboelectric series, chemical surface functionalization, triboelectric generators, solution-based process, kinetic energy harvesting

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A simple chemical-functionalization method to effectively modify triboelectric properties of contact surface has been previously reported.21 It has greatly improved the output performance of TEGs and can be easily adopted for functionalization of diverse material surfaces irrespective of their original triboelectric properties. Recently, more chemical functional groups have been introduced, expanding the choices for surface functionalization.22,23 However, the introduced method is insufficient to provide a wide spectrum of triboelectric series and contact pairs for TEGs, and understanding of detailed mechanism of triboelectric charging behavior at the chemically modified surface remains uncertain. Here we report triboelectric series prepared by atomic-level chemical functionalization of a polymeric surface using a series of halogens and amines, and investigate triboelectric charging

nergy harvest from various physical movements existing in our living environment has attracted great interest to develop independent power sources for sharply increasing personal, wireless, and wearable electronic devices.1−4 To this end, electromagnetic,5,6 piezoelectric,7−9 and triboelectric energy conversion schemes10−12 have been extensively investigated. Among them, energy harvest using triboelectric generators (TEGs) has been highlighted as a viable route to build self-powered electronic devices thanks to their relatively high output power, low-cost fabrication process, and easy integration of the device.13,14 Furthermore, the output performance of TEGs has been continuously improved, using various methods such as control of surface topography,15,16 chemical functionalization of surface,17,18 and modulation of permittivity.19,20 In TEGs, however, output performance is still determined largely by the choices of triboelectric contact pairs, necessitating two different materials far apart in the triboelectric series. Thus, only limited triboelectric contact pairs apart in the triboelectric series are available for TEG fabrication. © 2017 American Chemical Society

Received: March 29, 2017 Accepted: May 30, 2017 Published: May 30, 2017 6131

DOI: 10.1021/acsnano.7b02156 ACS Nano 2017, 11, 6131−6138

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Figure 1. Schematic representations of surface functionalized negative and positive PETs with adopted molecules. The PET/ITO substrates were treated by O2 plasma to form hydroxyl (−OH) groups on the PET surface for strong hydrogen bonding with target molecules. (a) PET surfaces were functionalized with halogen (Br, F, and Cl)-terminated phenyl derivatives for negatively charged surfaces. (b) Aminated molecules, such as linear polyethylenimine (PEI(l)), hexyltrimethoxysilane (HTMS), poly-L-lysine (PLL), 3-aminopropyltrimethoxysilane (APTES), and branched polyethylenimine (PEI(b)), were functionalized on the O2 plasma treated PET surfaces.

readily accessible, inexpensive, and solution-processable. Furthermore, hydroxyl surface groups (−OH) can be easily formed on a PET surface by oxygen plasma treatment,24,25 providing strong covalent bonds between the PET surface and molecules having appropriate anchoring groups (e.g., trialkoxy-, or trichloro-silanes). To negatively functionalize the surfaces, halogen-terminated “aryl”-silane derivatives [Figure S1], triethoxy(p-halophenethyl)silane (p-XC6H4)CH2CH2Si(OEt)3, were synthesized: triethoxy(phenethyl)silane (X = H), triethoxy(4-bromophenethyl)silane (X = Br), triethoxy(4fluorophenethyl)silane (X = F), and triethoxy(4chlorophenethyl)silane (X = Cl). For simplicity, we named the H-, Br-, F-, and Cl-terminated PET surfaces as H-PET, BrPET, F-PET, and Cl-PET, respectively [Figure 1]. The detailed synthesis process is described in the Methods section. These molecules provide four advantages for this study. First, halogens attached to sp2 carbons usually do not undergo nucleophilic substitution reactions in ambient conditions, thus exhibiting stability higher than that of alkyl halides during preparation of self-assembled monolayer (SAM). They are also inert against any functional groups on the surface of the counter material in the course of contact electrification. Second, adding a flexible ethyl spacer between an anchoring group (triethoxysilane) and a phenyl terminal group could lead to a more ordered structure of SAM than the SAM composed of the analogous molecule without the spacer.26−29 Third, the ethyl spacer unit adjacent to the p-halophenyl group isolates it electronically from the anchoring group and the substrate and thereby allows us to reliably examine the trend of triboelectric charging behavior of the materials. Last, the triethoxysilane anchoring group effectively forms strong covalent bonds with surface hydroxyl groups of O2-treated PET substrate. For positive functionalization of the surfaces, several commercially available aminated materials were chosen. These forms strong hydrogen bonds with the surface −OH

behaviors for different functional groups. This work is physicalorganic in nature, that is, the chemical structure of surface functional group was varied while keeping other components constant, which allowed us to relate them to the performance of the corresponding TEGs. Such a physical-organic approach made it possible to investigate the mechanism of triboelectric process on chemically functionalized surfaces. To induce triboelectrically negative property on PET surface, we functionalized it with arylsilane terminated with electron-accepting elements, halogens. For triboelectrically positive side, the surface was functionalized using several aminated molecules. Using the prepared contact pairs, triboelectric charging behaviors between the functionalized surfaces were investigated by employing scanning Kelvin probe force microscope (KPFM) and electrometer to unveil electrostatic surface potentials (Φ) and charge densities (σ) of the electrified contact surfaces. Besides, calculation on a molecular scale was performed to unveil triboelectric charging behavior at the chemically functionalized surface. Our results show that a wide spectrum of triboelectric series can be achieved by appropriate chemical functionalization on a material’s surface. Specifically, the surfaces functionalized with triethoxy(4-chlorophenethyl)silane and branched polyethylenimine were revealed as the most triboelectrically negative and positive ones, respectively. Calculations further show that energetically accessible frontier orbitals, HOMO and LUMO, for halogen-functionalized PET substrates are present in the PET region and the halogensubstituted phenyl group, respectively. Therefore, upon contact electrification, an electron transferred to the negatively functionalized surface is occupied at the PET group rather than the halogen-substituted phenyl group.

RESULT AND DISCUSSION Figures 1 and S1 show materials used for this study. PET surfaces were functionalized with different molecules. PET is 6132

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ACS Nano groups30−32 [Figure 1b]. Specifically, the following chemicals were adopted to render the surface positively: linear polyethylenimine (PEI(l)), hexyltrimethoxysilane (HTMS), poly-L-lysine (PLL), 3-aminopropyltrimethoxysilane (APTES), and branched polyethylenimine (PEI(b)). Each functionalized surface is named PEI(l)-PET, HTMS-PET, PLL-PET, APTESPET, and PEI(b)-PET, respectively; the detailed chemical structures of these molecules are shown in the Figure S1. Chemical surface functionalization of PET substrate was achieved by treating solutions containing triethoxysilane derivatives or aminated molecules onto thoroughly rinsed and O2-treated PET substrates (see detailed procedures in the Methods section). The functionalized PET substrates were then characterized by X-ray photoelectron spectroscopy (XPS) [Figure S2]. For all the halogen-functionalized PETs, the relative atomic % of Si of silane anchoring group was almost identical (7.6, 8.6, and 8.1% for F-PET, Cl-PET, and Br-PET, respectively), indicating similar halogen density in the unit area. To determine how the surface functionalization influences the surface potential, the functionalized surfaces were examined using KPFM. Before KPFM scanning, each functionalized PET surface was contact-electrified against the bare PET surface for 500 cycles (the applied force of 0.15 MPa) using the custombuilt push machine [Figure S3]. Figure 2 summarizes the

electrons in 2p orbital (F) being bigger than those in 3p orbital when accepting an additional electron. Figure 3 shows the experimentally measured transferred charge density (σtr) of all the electrified surfaces using the

Figure 3. (a) Charge density measurement of negatively functionalized PET surfaces: H-, Br-, F-, and Cl-PETs. All the surfaces were electrified by contacting with a bare PET surface at the applied pressure of 0.15 MPa for 500 cycles. (b) Charge density measurement of the positively functionalized PET surfaces: APTES-, HTMS-, PEI(b)-, PEI(I)-, and PLL-PET.

electrometer. The detailed charge transfer process and characterization of charge density are described in the Note S1. Before measurements of surface charge density, both halogenated- and aminated-PET surfaces were electrified by contacting them against bare PET substrate for 500 cycles at the applied force of 0.15 MPa. While H-PET, Br-PET, F-PET, and Cl-PET were negatively charged, PEI(l)-PET, HTMS-PET, PLL-PET, APTES-PET, and PEI(b)-PET were positively charged when they are contact-electrified against a bare PET surface. As expected, negative and positive σtr values were measured for halogenated-PETs [Figure 3a] and aminatedPETs [Figure 3b], respectively. In details, the magnitudes of the transferred charge density, |σtr |, were 76 μ C m−2 (Cl-PET), 60 μ C m−2 (F-PET), 25 μ C m−2 (Br-PET), and 12 μ C m−2 (HPET) for negatively charged surfaces, whereas 52 μ C m−2 (PEI(b)-PET), 32 μ C m−2 (APTES-PET), 31 μ C m−2 (PLLPET), 27 μ C m−2 (HTMS-PET), and 17 μ C m−2 (PEI(l)PET) for positively charged surfaces. For negatively charged surfaces, transferred charge density was arranged in the order of electron affinity, consistent with the results in Figure 2. For positively charged surface, an obviously high charge density was observed on the PEI(b)-PET surface, which originates from high surface charge density due to the branched structure of PEI(b) with primary, secondary, and tertiary amines. On the basis of XPS survey peak analysis, the N 1s percentage of PEI(l)-PET (15%) was slightly higher than that of PEI(b)-PET (12%). However, PEI(l)-PET showed the lowest charge density, which was attributed to the absence of exposed amine at the surface that directly contact to X-PET to transfer electrons. The higher positive charge density of PEI(b) than PEI(l) is consistent to the other report.34 The PLL-PET exhibited similar positive charge density compared to APTESPET. Using the charge density data in Figure 3, we defined the

Figure 2. Electrostatic surface potentials (Φ) of functionalized surfaces, measured using KPFM. The measured surface potentials (left, red circles) of H-PET, Br-PET, F-PET, and Cl-PET were −102 mV (ΦrmsCl) < −60 mV (ΦrmsF) < −47 mV (ΦrmsBr) < −40 mV (ΦrmsH) and surface potentials (right, blue diamonds) of PEI(l)-PET, HTMS-PET, PLL-PET, APTES-PET, and PEI(b)-PET were 93 mV (ΦrmsPEI(b)) > 69 mV (ΦrmsAPTES) > 66 mV (ΦrmsPLL) > 62 mV (ΦrmsHTMS) > 41 mV (rmsPEI(l)). We note that each functionalized PET surface was contact-electrified with the normal PET surface for 500 cycles under the identical condition using the custom-built push machine.

results. The measured surface potentials (red circles in Figure 2) of H-PET, Br-PET, F-PET, and Cl-PET were −102 mV (ΦrmsCl) < −60 mV (ΦrmsF) < −47 mV (ΦrmsBr) < −40 mV (ΦrmsH). The measured surface potentials (blue diamonds in Figure 2) of PEI(l)-PET, HTMS-PET, PLL-PET, APTES-PET, and PEI(b)-PET were 93 mV (ΦrmsPEI(b)) > 69 mV (ΦrmsAPTES) > 66 mV (ΦrmsPLL) > 62 mV (ΦrmsHTMS) > 41 mV (rmsPEI(l)). We note that the surface potentials are the root-mean-square (rms) values of multiple scanning locations of the measured PET samples as shown in Figure S4a−i, and the error bar for each sample is marked in Figure 2. The tendency of electrostatic potentials of halogenated-PET surfaces is consistent with the sequence of electron affinity of halogen atoms in gas-phase, Cl > F > Br,33 where higher electron affinity of Cl than F results from the repulsive force experienced from the 6133

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ACS Nano Table 1. Normalized Charge Density (σN) and Figure of Merit (FOM) of the Functionalized Surfaces

a

FOM = σN2 = σ2material:PET/σ2CI:PET, where σN is the normalized charge density and the reference material is PET.

Figure 4. (a) Optimized structure of the Cl-PET model. (b) Lowest unoccupied molecular orbital (LUMO). (c) Highest occupied orbitals (HOMO) calculated at the M11-L/6-31G(d) level. (d) Electrostatic potential surface of the anion form of Cl-PET. Red area represents relative electronegative regions on the van der Waals surface of the molecule.

dimensionless figure-of-merit (FOM) to evaluate the triboelectric charging performance as the following: FOM = σN 2 =

triboelectric charging occurred by the exchange of electrons between the two functionalized surfaces and investigated where the electrons are located if the electron transfer is made during contact friction process. Model compounds of the form 2-(((4halophenethyl)-dimethoxysilyl)oxy)ethyl methyl terephthalate were used for calculation. In the predicted structure, phalophenethyl group lay on the PET surface, as depicted in Figure 4a, due to the π−π stacking. The lowest unoccupied and the highest occupied molecular orbitals (LUMO and HOMO) are also shown in Figure 4b,c, where the former and the latter are distributed only on the PET and the p-halophenyl group, respectively, i.e., donor−acceptor (D-A) type molecule. When an electron is transferred to the Cl-PET molecule through electrification, it is occupied in the LUMO of Cl-PET, thereby increasing the electron density of PET group. Indeed, electrostatic surface potential of charged Cl-PET described in Figure 4d shows negative charge distribution on the PET group rather than the chlorophenyl group: The red part represents relative electronegative regions on the van der Waals surface of the molecule. These results suggest that the functionalized molecule (p-halophenyl) on the PET surface is not an actual electron acceptor. The role of p-halophenyl group is to stabilize the anion developed on the PET group rather than to carry an electron. Besides, due to charge accumulation at the PET surface, more effective electrostatic induction on the facing electrode is expected. For positively functionalized surface, we

σ 2 test material σ 2Cl

where the σN is the normalized transferred charge density of each functionalized surface with respect to transferred charge density of Cl-PET (σCl). Here, σN has a negative value if σtest material and σCl have different signs. The σN of all the functionalized surfaces tested in this study is summarized in Table 1, and triboelectric tendency of a certain material can be quantitatively estimated by referring Table 1. For instance, if σN < 0, then the material can be positively charged when contactelectrified with an untreated PET surface. On the other hand, the material can be negatively charged in case of 0 < σN < 1. These results explicitly show that diverse triboelectric series and contact pairs can be constructed simply by functionalizing the surface of a material with various molecules. To understand molecular-level charging behavior of the functionalized surface, accessible energy levels of halogenated PET surfaces were calculated by density functional theory (DFT) at the M11-L/6-31G(d) level.35 We note that there exist different theories and experimental results which make it difficult to establish the origin of triboelectric charging between polymeric surfaces.36 Among them, we assumed here that 6134

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generating an electric current pulse of a half cycle. Next, as two contacted surfaces are separated from each other, the potential difference appears again, and the electrons will flow from the top electrode to the bottom electrode through an external load, generating an electrical current pulse in the reverse direction. This process iterates during the cyclic pressing/releasing motions, generating alternating current (ac). Figure 5b,c shows the output voltages of the TEGs with aminated-PET:PET pair and halogenated-PET:PET pair, respectively. For comparison, the connection configuration to the measurement setup was kept the same during the measurement, where the bottom and the top electrodes were connected to the positive and the negative inputs of oscilloscope. There was a significant influence of surface functionalization on the output signals in TEGs. Positive polarity signals were measured from the TEG with the aminated PETs and PET pair [Figure 5b]. In contrast, negative polarity signals were measured from the TEGs with halogenated surfaces [Figure 5c]. The magnitudes of output voltages from each TEG were consistent with the results in Figures 2 and 3. Specifically, the output voltage peaks from ClPET:PET and PEI(b)-PET:PET contact pairs exceeded 300 and 200 V, respectively. We note that the sum of these output voltages is similar to the output voltage generated with the ClPET and PEI(b)-PET contact pair. Lastly, each surface functionalized with halogen molecules was electrified against five different aminated PET surfaces. The output voltage and output current density of each TEG were examined for different triboelectric contact pairs as demonstrated in Figure 6. All the measurement data obtained from 20 different TEGs were included in Figure S6. For fair comparison, all the contact pairs were tested under the same applied force of 0.15 MPa at the same frequency of 2 Hz. The results show that the maximum output voltage and current density were obtained from Cl-PET:PEI(b)-PET contact pair, generating ∼520 V and ∼110 mA/m2, respectively. These values were much higher than those obtained from the F-PET:PEI(b)-PET contact pair, resulting in approximately 2-fold higher output power density [Figure 6e]. This result agrees well with the results in Figure 3 since triboelectric output performance is directly affected by surface charge density (σ) as indicated in Note S2. Figure 6e summarizes the power density measured from each TEG. The maximum power density of ∼55 W m−2 was achieved in the TEG with the Cl-PET:PEI(b)-PET contact pair, which is more than 2 orders of magnitude higher the value of the HPET:PEI(l) contact pair. This finding indicates that a wide power density spectrum of TEG can be obtained by simple surface functionalization. Finally, the reliability of TEG was tested over 5000 cyclic measurements, assuring the superior mechanical durability and stability of the functionalized surfaces [Figure S7].

performed a similar calculation and found consistent results [Figure S5]. Using these chemically functionalized surfaces, the output voltage (V) and current density (J, mA m−2) of the TEGs were measured. The detailed power generation mechanism of TEG is described in Figure 5a. After cyclic contacts between two

Figure 5. Power generation mechanism of TEG and polarities of functionalized surfaces. (a) When the two differently charged surfaces move toward each other, the electrons at the bottom electrode start to flow toward the top electrode through an external load until the potential difference reaches zero. Next, as two contacted surfaces are separated from each other, the potential difference appears again, and the electrons will flow from the top electrode to the bottom electrode. (Inset) Scale bar is 2 cm. (b) Output voltage pulses generated by cyclic contacts between PET:APTES, PET:HTMS, PET:PEI(b), PET:PEI(I), and PET:PLL pairs. Positive polarity signals were generated since the functionalized surfaces are triboelectrically positive compared to PET. (c) Output voltage pulses generated by cyclic contacts between PET:H, PET:Br, PET:F, and PET:Cl pairs. Negative polarity signals were generated since the functionalized surfaces are triboelectrically negative by comparison to PET.

CONCLUSIONS In this work, we have demonstrated a simple and efficient method to change triboelectric property of a polymeric surface via atomic-level chemical functionalization and adopted it to unveil trends of triboelectric charging behaviors as a function of surface functional groups. Functionalization of the PET surface with the molecules containing halogen (Br, F, and Cl)substituted phenyl moieties or the aminated molecules successfully yielded a wide spectrum of triboelectric property on its surface. The results described in this work indicate that surface, rather than bulk, modification of material can be

surfaces with different electron affinities, two surfaces are charged with the opposite polarities: The surface attracting electron is negatively charged, while the one losing an electron is positively charged. When two oppositely charged surfaces move toward each other, the induced electrons at the bottom electrode start to flow toward the top electrode through an external load until the potential difference reaches zero, 6135

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Figure 6. Output voltage (V), current density (J), and power density of the TEGs with different contact pairs. Output voltage (left axis) and current (right axis) of TEGs generated during contact electrification of each negatively functionalized surface (H-PET (a), Br-PET (b), F-PET (c), and Cl-PET (d), with positively functionalized surfaces (APTES-, HTMS-, PEI(b)-, PEI(l)-, and PLL-PET). (e) Power density plot of TEGs measured for 20 different contact pairs. We note that the TEG with the Cl-PET:PEI (b)-PET contact pair shows the best output performance. Synthesis of (p-XC6H4)CH2CH2Si(OEt)3 Compounds. The molecules used in this work have been synthesized from styrene derivatives and triethoxysilane by modifying the method previously reported37,38 (see the detailed procedures below). Triethoxysilane. Triethoxysilane was synthesized according to the literature procedure.39 Triethoxy(phenethyl)silane. The title compound was synthesized by following the method A described in Figure S8. Triethoxysilane (0.92 mL, 5 mmol) was added slowly to the mixture of styrene (0.58 mL, 5 mmol) and PtO2 (1.3 mg, 5 μmol) at 0 °C. After stirring for 6 h at 0 °C followed by stirring for additional 12 h at room temperature and filtration over a polytetrafluoroethylene syringe filter, the crude product was obtained as a light brown colored liquid. The color results from colloidal Pt(0) particles, which could be removed by the addition of activated charcoal. The crude product was purified by passing through a silica-gel column chromatography (eluent: a mixture of hexane and DCM, 4:1, v/v) to give the title compound in 39% isolated yield (0.53 g). 1H NMR (CDCl3) δ 7.02− 7.39 (m, 5 H), 3.81 (t, J = 6.9 Hz, 2H), 2.72 (t, J = 8.6 Hz, 2H), 1.22 (t, J = 6.9 Hz, 4H), 0.98 (t, J = 8.6 Hz, 2H). 13C NMR and mass spectral data of the compound were consistent with those reported in literature.40,41 Triethoxy(4-fluorophenethyl)silane. The title compound was synthesized by following the method B described in Figure S8. The

sufficient for controlling or enhancing the performance of TEGs. DFT calculations further revealed that the halobenzene surface groups play a role in stabilizing the anionic PET surface, rather than holding transferred electron resulting from contact electrification. The study of structure−performance relations in TEGs reported herein is a step toward well-defined physical-organic approach to disclose detailed mechanism of triboelectric effect and demonstrates that TEGs can be fabricated on any polymeric surface that is originally inefficient for triboelectric energy harvest, unlocking the limit of triboelectric materials for designing TEGs. Indeed, the ability to introduce a variety of functional groups onto materials via straightforward surface chemical functionalization is a powerful and potentially general way of creating TEG materials with finely tunable triboelectric property.

EXPERIMENTAL METHODS Materials. All reagents were used as supplied unless otherwise specified. Starting compounds for synthesis were purchased from Sigma-Aldrich, TCI, and ACROS. 6136

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ACS Nano mixture of 4-fluorosytrene (0.6 mL, 5 mmol), triethoxysilane (0.92 mL, 5 mmol), and PtO2 (1.3 mg, 5 μmol) was stirred and heated at 85 °C for 21 h. After being cooled to room temperature, colloidal Pt(0) particles in the crude product was removed by the addition of activated charcoal and washed with ethanol. The reaction solution was concentrated in vacuo, and the crude product was purified by passing through a silica-gel column chromatography (eluent: a mixture of hexane and DCM, 3:1, v/v) to give the title compound in 30% isolated yield (0.40 g). 1H NMR (CDCl3) δ 7.12−7.19 (m, 2H), 6.95 (t, J = 8.8 Hz, 2H), 3.82 (q, J = 7.0 Hz, 6H), 2.66−2.75 (m, 2H), 1.23 (t, J = 7.0 Hz, 9H), 0.91−1.00 (m, 2H). 13C NMR and mass spectral data of the compound were consistent with those reported in literature.42 Triethoxy(4-chlorophenethyl)silane. The title compound was synthesized by following the method B. Isolated yield: 32% (0.49 g). 1 H NMR (CDCl3) δ 7.27−7.32 (m, 2H), 7.15−7.20 (m, 2H), 3.86 (q, J = 7.0 Hz, 6H), 2.70−2.78 (m, 2H), 1.27 (t, J = 7.0 Hz, 9H), 0.95− 1.03 (m, 2H). 13C NMR and mass spectral data of the compound were consistent with those reported in literature.43,44 Triethoxy(4-bromophenethyl)silane. The title compound was synthesized by following the method B. Isolated yield: 17% (0.30 g). 1 H NMR (CDCl3) δ 7.38 (m, J = 8.3 Hz, 2H), 7.08 (m, J = 8.3 Hz, 2H), 3.82 (q, J = 7.0 Hz, 6H), 2.63−2.73 (m, 2H), 1.23 (t, J = 7.0 Hz, 9H), 0.89−1.00 (m, 2H). 13C NMR and mass spectral data of the compound were consistent with those reported in literature.45 Surface Functionalization. Polyethylene terephthalate films were functionalized with arylsilane terminated with halogens and aminated molecules to induce negative and positive charges on TEG, respectively. Before surface functionalization, PET films were rinsed with ethanol followed by N2 blow drying; then, PET films were treated with O2 plasma (100 W, 1 min, 50 sccm O2 flow) to form hydroxyl terminated surface on PET. Triethoxy(p-halophenyl-ethyl)silane or phenethyl triethoxysilane in methanol (5 wt %) were dropped on OHterminated PET, and the PETs were immediately placed in vacuum chamber for 4 h. Next, the samples were put on hot plate at 100 °C for 20 min. To prepare positively charged PET surfaces, HTMS, APTES, PLL, PEI(b), and PEI(l) were functionalized on PETs. Solutions of PLL, PEI(b), and PEI(l) (0.1(w/v)%) were prepared, and OHterminated PETs were soaked in the solutions for 5 min, followed by N2 blow-drying. For HTMS and APTES coating on PETs, 5 wt % HTMS and APTES solutions in toluene were dropped on OHterminated PETs, and the samples stayed under vacuum conditions for 3 h. Next, solvents were perfectly dried by placing the PETs on hot plate at 100 °C for 20 min. Characterization. NMR. 1H NMR spectra were recorded on Varian Mercury 300 using CDCl3 as a solvent and residual solvent as an internal standard. Chemical shifts are expressed in parts per million (ppm) related to internal TMS and coupling constant (J) are in Hertz. XPS. For qualitative estimation of halogenated silane SAMs on PET, Si 2p signal was monitored with XPS (Thermo Electron K-Alpha). KPFM. KPFM and atomic force microscopy (AFM) images were taken by using an AFM system (XE-100, Park Systems) at room temperature under dark condition. The bias applied to the KPFM tip was 2.0 V for all samples. Electrical Measurement. The output voltage (V) and current density (J) were measured by using an oscilloscope (LeCroy, LT354) and current preamplifier (Stanford Research Systems, SR 570), respectively. The cyclic contact/release motions were performed with a custom-built push machine setup.

samples, derivation of V-σ relationship, reliability test of Cl-PET:PEI(b)-PET, schemes for preparation of triethoxysilane derivatives (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

[email protected]. [email protected]. [email protected]. [email protected].

ORCID

Hyo Jae Yoon: 0000-0002-2501-0251 Min Hyung Lee: 0000-0001-8313-9857 Junghyo Nah: 0000-0001-9975-239X Present Address

J.K.: Department of Physics, Korea University, Seoul 02841, Korea. Author Contributions

S.-H.S., Y.E.B., and H.K.M. contributed equally to this work. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF-2015R1A1A1A05027235) and by Korea Ministry of Environment as “Environmental industry advanced technology development program. M.H.L. acknowledges support by the NRF (NRF-2017R1A2B4007641). H.J.Y. acknowledges support by the NRF (NRF-2016R1D1A1A02937504). REFERENCES (1) Hudson, M. The Final Energy Crisis. Environ. Pollut. 2006, 15, 677−679. (2) Wang, Z. L.; Wu, W. Nanotechnology-Enabled Energy Harvesting for Self-Powered Micro-/Nanosystems. Angew. Chem., Int. Ed. 2012, 51, 11700−11721. (3) Aricò, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.-M.; van Schalkwijk, W. Nanostructured Materials for Advanced Energy Conversion and Storage Devices. Nat. Mater. 2005, 4, 366−377. (4) Rome, L. C.; Flynn, L.; Goldman, E. M.; Yoo, T. D. Generating Electricity While Walking with Loads. Science 2005, 309, 1725−1728. (5) Williams, C. B.; Shearwood, C.; Harradine, M. A.; Mellor, P. H.; Birch, T. S.; Yates, R. B. Development of An Electromagnetic MicroGenerator. Proc. IEEE Circ. Dev. Syst. 2001, 148, 337−342. (6) Miller, M. M.; Prinz, G. A.; Lubitz, P.; Hoines, L.; Krebs, J. J.; Cheng, S. F.; Parsons, F. G. Novel Absolute Linear Displacement Sensor Utilizing Giant Magnetoresistance Elements. J. Appl. Phys. 1997, 81, 4284−4286. (7) Shin, S.-H.; Kim, Y.-H.; Lee, M. H.; Jung, J.-Y.; Nah, J. Hemispherically Aggregated BaTiO3 Nanoparticle Composite Thin Film for High-Performance Flexible Piezoelectric Nanogenerator. ACS Nano 2014, 8, 2766−2773. (8) Wang, Z. L.; Song, J. Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays. Science 2006, 312, 242−246. (9) Yang, R. S.; Qin, Y.; Dai, L. M.; Wang, Z. L. Power Generation with Laterally Packaged Piezoelectric Fine Wires. Nat. Nanotechnol. 2009, 4, 34−39. (10) Shin, S.-H.; Kim, Y.-H.; Lee, M. H.; Jung, J.-Y.; Seol, J. H.; Nah, J. Lithium-Doped Zinc Oxide Nanowires Polymer Composite for High Performance Flexible Piezoelectric Nanogenerator. ACS Nano 2014, 8, 10844−10850. (11) Xu, S.; Qin, Y.; Xu, C.; Wei, Y. G.; Yang, R. S.; Wang, Z. L. SelfPowered Nanowire Devices. Nat. Nanotechnol. 2010, 5, 366−373.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b02156. Chemical structures of functionalized molecules, additional XPS data, photograph of experimental push machine, additional KPFM data, DFT calculation of APTES-PET, description of charge transferring process, measured output voltage and current density of 20 6137

DOI: 10.1021/acsnano.7b02156 ACS Nano 2017, 11, 6131−6138

Article

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