Dipole-moment-induced Effect On Contact Electrification .
Nano Research 2014, 7(7): 990–997DOI 10.1007/s12274-014-0461-8Dipole-moment-induced effect on contact electrificationfor triboelectric nanogeneratorsPeng Bai1,2,§, Guang Zhu1,§, Yu Sheng Zhou1,§, Sihong Wang1, Jusheng Ma2, Gong Zhang2, and Zhong LinWang1,3 ( )1School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, USADepartment of Mechanical Engineering, Tsinghua University, Beijing 100084, China3Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, China§Authors with equal contribution and order of authors determined by coin toss.2Received: 13 February 2014ABSTRACTRevised: 22 March 2014Triboelectric nanogenerators (TENGs) have been demonstrated as an effective wayto harvest mechanical energy to drive small electronics. The density of triboelectriccharges generated on contact surfaces between two distinct materials is a criticalfactor for dictating the output power. We demonstrate an approach to effectivelytune the triboelectric properties of materials by taking advantage of the dipolemoment in polarized polyvinylidene fluoride (PVDF), leading to substantialenhancement of the output power density of the TENG. The output voltageranged from 72 V to 215 V under a constant contact force of 50 N. This work notonly provides a new method of enhancing output power of TENGs, but alsooffers an insight into charge transfer in contact electrification by investigatingdipole-moment-induced effects on the electrical output of TENGs.Accepted: 27 March 2014 Tsinghua University Pressand Springer-Verlag BerlinHeidelberg dipole moment1IntroductionThe triboelectric effect, manifest as a type of contactelectrification, is a common phenomenon in whichcharges are transferred from one material to anotherupon contact [1–6]. Although the triboelectric effect isyet to be fully explored due to a limited understandingof the fundamental mechanism [7–10], variousapplications have been developed for purposes suchas photocopying [11], electrophotography [12], selfassembling systems [13, 14], electrostatic separationAddress correspondence to zlwang@gatech.edu[15] and recently developed triboelectric nanogenerators(TENGs) [16–18]. A TENG relies on the couplingbetween the triboelectric effect and electrostaticinduction to convert mechanical energy into electricity.A major factor determining the output power ofTENGs is the density of triboelectric surface charges,which serves as a driving force for induced freeelectrons. Since it is generally believed that the chargetransfer between two distinct materials originatesfrom the difference in their ability to gain electrons,the density of surface charges is primarily determined
991Nano Res. 2014, 7(7): 990–997by the intrinsic properties of specific materials, thoughexternal factors in the environment may have a minoreffect. As a consequence, there is a lack of effectivemeans to enhance the output power of TENGs bymodifying the intrinsic properties of materials incharge transfer [19–27].In this work, we investigated the effect of an intrinsicdipole moment on the electrical output of TENGs.Polarized polyvinylidene fluoride (PVDF) thin filmswere utilized in fabricating a TENG without extraprocessing steps. Compared to a TENG fabricatedusing nonpolarized PVDF thin films, the outputvoltage could be either enhanced to 240% or reducedto 70%, depending on the direction of polarization.It is proposed that the presence of bond chargesgenerated by aligned dipoles can influence the potentialenergy on the surface of PVDF thin films. As a result,the potential energy difference between the polarizedPVDF and aluminum is modified, which leads totunable quantity of charge being transferred uponcontact. This work presents an initial effort to adjustthe electrical output of TENGs over a wide rangethrough changing the intrinsic electrical properties ofa material. By proposing a mechanism that involvespolarization-induced surface potential modification,this work also offers a new insight into contactelectrification, suggesting that charge transfer is likelyto be subject to manipulation by surface potentialengineering.2Results and discussionAs sketched in Fig. 1(a), the TENG has a double-layerstructure. The first layer is a piece of aluminum foilwith uniformly sized and distributed nanopores onthe surface (Fig. 1(b)). Based on previous work, thenanopore-based modification will further increasethe effective contact area, thus enhancing the electricaloutput of TENGs (see the Electronic SupplementaryMaterial (ESM)). The second layer is a piece of PVDFthin film deposited with copper on one side as theback electrode. Acrylic sheets were used as substrates.The two substrates were connected through twoarc-shaped polyimide films that are anchored atedges, maintaining a gap between the substrates. Anas-fabricated TENG is presented in Fig. 1(c). TwoFigure 1 (a) Schematic of the TENG with a double-layer structure.(b) SEM image of nanopores on aluminum foil. (c) Photographof a TENG.categories of PVDF films were utilized, namelypolarized films and nonpolarized films. Dependingon the direction of the polarity, TENGs fabricatedfrom polarized PVDF films are further categorizedinto two types. For clear reference, the ones withintrinsic dipole moment pointing to the back electrodeare referred to as forward-polarized TENGs, whilethose with the opposite dipole moment are categorizedas reverse-polarized TENGs (see the ESM). Furtherdetailed fabrication specifications are discussed inthe Experimental Section.To characterize the electrical output, short-circuitcurrent density (Jsc) and open-circuit voltage (Voc) ofTENGs fabricated using different types of PVDFfilms were measured under a periodic compressiveforce around 50 N applied by an electric shaker at afrequency of 4 Hz (Fig. S2 in the ESM). As shownin Fig. 2(a), a Jsc of 8.34 μA/cm2 was achieved by aforward-polarized TENG, corresponding to an increaseof 36.3% when compared with a nonpolarized TENGthat produces a Jsc of 6.13 μA/cm2. To further demonstrate the influence of dipole moment on the TENGs’output, Jsc of a reverse-polarized TENG was measuredunder the same conditions. A Jsc of 4.83 μA/cm2 wasobtained, which is a decrease of about 21% incomparison to the nonpolarized TENG. Likewise, theobtained Voc of the three types of TENGs showssimilar results, as illustrated in Fig. 2(b). The largestVoc of 215 V and smallest Voc of 72 V correspond tothe forward-polarized and reverse-polarized TENG,respectively. The differentiated electrical output fromwww.theNanoResearch.com www.Springer.com/journal/12274 NanoResearch
992Nano Res. 2014, 7(7): 990–997the three categories of TENGs can be furtherevidenced by charge output, as shown in Fig. 2(c).Through a diode bridge, alternating electrons that flowbetween electrodes can be rectified, which leads toaccumulative induced charges (accumulative Q). Theforward-polarized TENG generates a charge outputof 9.05 μC in 5 seconds, corresponding to inducedcharges of 0.51 μC per cycle. For the nonpolarizedand reverse-polarized TENG, charges output reaches4.72 and 2.50 μC in 5 seconds, in accordance to 0.25and 0.14 μC per cycle, respectively. To eliminaterandom errors that may be caused by fabrication andmeasurement, five batches of devices were tested foreach category of TENGs under the same conditions.As shown in Fig. 2(d), average Jsc values of 7.55 μA/cm2,5.40 μA /cm2, and 4.28 μA/cm2 were obtained fromforward-polarized, nonpolarized, and reverse-polarizedTENGs, respectively. The same trend also applies tothe average amplitude of Voc and equivalent directcurrent density (equivalent Jsc) which is defined as theaverage quantity of positive charges flowing throughthe circuit per second after being rectified (see theESM), as shown in Figs. 2(e) and 2(f), respectively.Taking the average amplitude of Voc for example, itshows over three-fold enhancement between theforward-polarized and reverse-polarized TENGs.Resistors were utilized as external loads to investigatethe output power of different types of TENGs undera compressive force of around 50 N. As shown inFig. 2(g), a peak power of 3.74 mW was achieved at aload resistance of 40 MΩ for the forward-polarizedTENG, and a peak power of 1.19 mW was achievedFigure 2 (a) Jsc of TENGs fabricated using different types of PVDF films under a periodic compressive force around 50 N applied byan electric shaker at a frequency of 4 Hz. (b) Voc of TENGs fabricated using different types of PVDF films. (c) Accumulative Qgenerated by TENGs fabricated using different types of PVDF films. (d) Average Jsc of TENGs fabricated using different types of PVDFfilms. (e) Average Voc of TENGs fabricated using different types of PVDF films. (f) Average equivalent Jsc of TENGs fabricated usingdifferent types of PVDF films. (g) Dependence of the peak power output of the forward-polarized TENG on the resistance of theexternal load. (h) Dependence of the peak power output of the nonpolarized TENG on the resistance of the external load. (i) Dependenceof the peak power output of the reverse-polarized TENG on the resistance of the external load. The curve is a fitted result. www.editorialmanager.com/nare/default.asp
993Nano Res. 2014, 7(7): 990–997at 40 MΩ for the nonpolarized TENG (Fig. 2(h)) whilea smaller peak power of 0.91 mW was achieved at20 MΩ for the reverse-polarized TENG (Fig. 2(i)).Though a variety of factors could have an influenceon the electrical output of TENGs, considering theconstancy of the other parameters—materials selection,fabrication process, device dimension, and measurement conditions—the differences in the electricaloutput of the three types of TENGs most likelyoriginate from the variations in the surface chargedensity. Therefore, it is proposed that the intrinsicdipole moment alters the surface potential level of thePVDF film, thus modulating surface charge transferbetween the PVDF and the aluminum electrode.Abundant experimental evidence has shown thatcharge transfer between metals and dielectrics occursvia tunneling, which results from unequal effectivework functions (Φ) of two materials [1, 2, 28]. Asillustrated by a simplified band diagram in Fig. 3(a),the surface energy state of nonpolarized PVDF can berepresented by an characteristic energy level with aneffective work function around 5.0 eV [29, 30], whichis higher than that of aluminum (4.2 eV). As a consequence, when aluminum and PVDF are broughtinto contact, electrons tunnel from the Fermi level ofaluminum (EF) to the characteristic energy level ofPVDF (E0), making the PVDF surface negativelycharged. The transfer of electrons continues until thetwo energy levels are lined up. Therefore, it is apparentthat the density of surface charge is directly related tothe potential difference between the two energy levels,which is primarily determined by materials’ properties,such as chemical composition [1, 2, 31]. Depending onthe relative magnitudes of EF and E0, electrons tend toflow out of the filled Fermi level of aluminum intothe empty surface states of the PVDF. The probabilityof charge transfer is a function of the potentialdifference between the two adjacent materials [32].As shown in Fig. 3(b), the electric field developed bycharge transfer raises the energy of the electrons inPVDF’s surface states by δE(Q) relative to EF, and thepotential difference between aluminum and PVDFconsequently becomes smaller. When the quantity ofcharge reaches its saturation value, the transfer ofcharge will cease. The center of positive charges andthe center of negative charges do not coincidence inevery molecule of PVDF in its polar β-phase, but thebond charge is zero when the β-phase forms naturallybecause of the random arrangement of dipoles. Afterbeing polarized along the thickness, all the dipolemoments are oriented along the same direction,resulting in bond charges on the surfaces of the PVDFthin film (see the ESM) [33–36]. Although molecules orspace charges will be adsorbed onto the surfaces ofPVDF, the bond charges cannot be always completelycompensated, and partially compensated surfaces arelikely to be the usual state in air [37–39]. If the filmis forward-polarized with positive bond charges onthe surface that is contact with the aluminum whilethe other side of its surface is covered by the backelectrode, the characteristic energy level of PVDF willbe reduced to E0’ on the surface as shown in Fig. 3(c).Such a shift of energy level leads to an enlargedpotential difference with the Fermi level of aluminum,which will in turn increase the probability of chargetransfer by tunneling upon contact. As a result, anenhanced density of surface triboelectric charge canbe expected. In contrast, the negative bond chargeson the surface of reverse-polarized PVDF will raise thecharacteristic energy level to E0’’ as shown in Fig. 3(d),Figure 3 (a) Schematic energy band diagram illustrating theprocess when electrons tunnel between EF and E0. (b) The electricfield developed by charge transfer raises the energy of the electrosin the PVDF’s surface states by δE(Q). (c) The positive bondcharges of the forward-polarized PVDF reduce the characteristicenergy level of PVDF to E0'. (d) The negative bond charges of thereverse-polarized PVDF increase the characteristic energy levelof PVDF to E0".www.theNanoResearch.com www.Springer.com/journal/12274 NanoResearch
994Nano Res. 2014, 7(7): 990–997and thus reduce the density of triboelectric charge incomparison with the nonpolarized PVDF film. Basedon previous studies, the surface triboelectric chargesare the direct driving force for transport of inducedelectrons between electrodes in a TENG [18, 20]. Thesurface charge density thus determines the amountof induced electrons that flow to screen electric fieldfrom the triboelectric charges. Therefore, forwardpolarized PVDF films afford enhanced electrical output,while reverse-polarized PVDF does the opposite. Adetailed description of the electricity generation of aTENG is sketched in Fig. S6 (in the ESM).To validate the above proposed mechanism, wefurther investigated the dependence of the equivalentJsc from different types of TENGs on the interactiveforce during contact. As shown in Fig. 4(a), theenhanced equivalent Jsc from nonpolarized TENGs asa function of the contact force is attributed to theincreased contact area between the PVDF and thealuminum, which has been discussed in previousreports [18, 26]. Also in accordance with literature,such dependence has a linear behavior in the rangeof small contact force [20, 26]. It is noteworthy thatthe equivalent Jsc from TENGs based on forwardpolarized PVDF presents an enhanced growth ratewith a larger slope in Fig. 4(a). This is due to theincreased surface charge density that results from theintrinsic dipole moment. It is to be noted that the effectof the dipole moment can play a role only when thetwo materials come into contact. As a result, such aneffect can be assumed to be also linearly related to thecontact force. Therefore, it is expected on the basis ofour proposed mechanism that the ratio between thetwo types of TENGs remains at a constant valuethat is independent of the contact force. A detailedexamination of the experimental data in Table S1 (inthe ESM) confirms our hypothesis and thus validatesthe proposed mechanism. Likewise, the equivalent Jscobtained from reverse-polarized TENGs also closelyfollows our expectation but shows the opposite effectfrom the intrinsic dipole moment. As shown in Fig. 4(b),to illustrate the different electrical output of differenttypes of TENGs under the same conditions, weconnected the TENGs and resistances as external loadsFigure 4 (a) Equivalent Jsc of TENGs fabricated using different types of PVDF under different contact force. Lines are the fitting results.(b) Circuit diagrams and the different performances among TENGs fabricated using different types of PVDF when they were used as directpower source for LED bulbs. (c) Jsc and (d) Voc of piezoelectric output from PVDF thin films under a contact force of around 50 N. www.editorialmanager.com/nare/default.asp
995Nano Res. 2014, 7(7): 990–997in series to drive LED bulbs. TENGs worked under acontact force around 50 N, and the performance ofeach type of TENGs can be observed by the LEDbulb. The LED bulb can be completely lit up whenthe forward-polarized TENG was used as the powersource, while the LED bulb was comparatively lessbright when we use the nonpolarized TENG as thepower source. In addition, because of the minimumelectrical output, reverse-polarized TENGs cannoteven drive the LED bulb.It is recognized that polarized PVDF is also knownas a type of piezoelectric material that generatesadditional dipole moments along the poling directionin response to induced strain as a result of externallyapplied pressure [33]. The piezoelectric output ofpolarized PVDF thin films was measured independently under the same conditions as for the TENGs.A layer of copper was deposited on the back of thepolarized PVDF thin films as a back electrode while alayer of aluminum was deposited as the other electrode.When a compressive force is applied onto the top ofthe aluminum electrode, a potential difference will bedeveloped with the negative polarity at the aluminumside for the forward-polarized PVDF thin films andthe positive one for the reverse-polarized PVDF thinfilms, resulting in a voltage signal (Fig. S7, in the ESM).The piezoelectric effect can be ruled out as a possiblereason for the modified electrical output of the TENGsbased on the following four reasons: (1) Both the shortcircuit current density (Fig. 4(c)) and open-circuitvoltage (Fig. 4(d)) from the piezoelectric signal areonly about 10% of the triboelectric output under thesame contact force of 50 N (a Jsc around 0.5 μA/cm2and a Voc around 4.5 V). Such a big difference in themagnitude of triboelectric and piezoelectric outputmeans that the influence of piezoelectricity on theelectrical output of TENGs is negligible. It needs to benoted that the polarity of piezoelectric output fromforward-polarized PVDF thin films is opposite to thatfrom the reverse- polarized PVDF thin films, which isbecause the directions of piezoelectric fields establishedin forward-polarized and reverse-polarized PVDF thinfilms are opposite to each other as we mentionedbefore; (2) for the forward-polarized TENG, the polarityof piezoelectric output is opposite to that of thetriboelectric output. According to the electricity-generation principle of TENGs, a positive electricalsignal will be generated when a periodic compressiveforce is applied onto the top of the aluminum electrodefor the forward-polarized TENG, but the correspondingpiezoelectric output is negative as shown in Figs. 4(c)and 4(d). If the piezoelectric and triboelectric outputcan be added up, the electrical output of forwardpolarized TENGs will be smaller than that of thereverse-polarized TENGs, which is not consistent withthe experimental results. Therefore, the assumptionof superposition between the piezoelectric output andtriboelectric output does not hold; (3) the electricaloutput from the TENG and the piezoelectric effectare not synchronized processes. For the triboelectricoutput, the electrical signal can be only generatedeither when the two contacting materials approach orwhen they start to separate from each other. The TENGdoes not produce electricity when the two surfaces stayin contact, while piezoelectric effect becomes effectiveonly when the contact is achieved with a pressure inbetween, which is consistent with the fact that thecurrent signal of the triboelectric output lasts longerthan the one of piezoelectric output (Fig. S8, in theESM). Therefore, the triboelectric charge density thatactually determines the electrical output of the TENGthus becomes independent of the piezoelectric effect;(4) we investigated the dependence of t
dipole-moment-induced effects on the electrical output of TENGs. 1 Introduction The triboelectric effect, manifest as a type of contact electrification, is a common phenomenon in which charges are transferred from one material to another upon contact [1–
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Vol.10, No.8, 2018 3 Annual Book of ASTM Standards (1986), “Standard Test Method for Static Modulus of Elasticity and Poissons’s Ratio of Concrete in Compression”, ASTM C 469-83, Volume 04.02, 305-309. Table 1. Dimensions of a typical concrete block units used in the construction of the prisms Construction Method a (mm) b
