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2 Y. Yu, X. Wang / Extreme Mechanics Letters ( ) – environmental concerns [1]. Based on this regard, self- powered charging system would be a desired power source for modern electronics. Ambient environmental mechan- ical energy is considered abundant and the most acces- sible energy source for powering small electronics [2]. Among many endeavors of harvesting mechanical energy effectively, the nanogenerator was invented using piezo- electric nanostructures as the functional building blocks, which offers high sensitivity, long life time, and poten- tial superior efficiency [3]. During the last decade, nu- merous piezoelectric nanogenerators (PENGs) were de- veloped to harvest energy from physical deflection [4,5], acoustic waves [6,7], air and fluid flow [8–10], and body movements (e.g. walking, talking and breathing) [11–13]. In 2012, triboelectric nanogenerator (TENG) was first re- ported on the basis of the traditional electrostatic en- ergy harvesting principles [14]. Owing to its simple de- sign and scalable/integratable nature, the area and vol- ume power density of TENGs rapid reached 500 W/m2 and 15 MW/m3, respectively within a few years [15]. An in- stantaneous conversion efficiency of 70.6% and a total con- version efficiency of 85% were achieved [16,17]. The high power output and large energy conversion efficiency made TENGs feasible in driving various commercial electronics, such as cell phones [18,19], light bulbs [15,19], alarms [20, 21], pacemakers [22], and a large number of sensors [23– 28] and light emitting diodes (LEDs) [29–34]. According to the device configuration, TENGs can be categorized into four modes: vertical contact-separation mode, contact- sliding mode, single-electrode mode, and freestanding triboelectric-layer mode [35], which cover the functions of TENGs in converting various energy forms including wa- ter waves [36–38], wind [39,40], rain droplets [30,41], ve- hicle [32,42] and human movement [17,43,44]. These two advantages promise TENGs in not only developing practical self-powered energy systems but also generating electric- ity at large scale. The development and broad applications of TENGs were extensively summarized in a number of re- cent reviews [1,2,35,45–48]. The operation of TENG is on the basis of two com- bined effect of triboelectrification and electrostatic induc- tion. The triboelectrification is the phenomenon that a material becomes electrically charged after intimate con- tact with a different object. When two dissimilar surfaces with distinct triboelectric polarity (i.e. tendency to gain or lose electrons) touch each other, electrons, holes or other charged ions/molecules transfer across the interface to compensate the difference of surface potentials. This charge separation is the primary driving force of mechan- ical to electric energy conversion in TENGs. Triboelectric materials (mostly polymer) are usually attached to metal electrodes, where electrostatic charges are induced. Once the device capacitance changes by displacement, current flow occurs between the conductive electrodes and thus outputs electric power. Therefore, controlling the charge density on the triboelectric surfaces is the most fundamen- tal strategy for improving the performance of TENG. In gen- eral, the charge density is determined by the amount of charge carrying locations and the tendency of these sites to gain or lose electron. These two factors can be regulated by morphologically and chemically modifying the triboelec- tric polymers. The number of charge carriers can be sim- ply increased by enlarging the surface roughness factor. For instance, morphologies like nanowires [49–51], nanopar- ticles [31,52] or other nanoscale patterns [53–59] are fre- quently adopted to raise the surface area of triboelectric materials and thus enable more charge carrying site. On the other hand, the tendency of these sites can be controlled by altering their chemical properties. The objective of this article is to give a review of most recent advances in chem- ically engineering polymer surface and to discuss the con- tribution of four different modification techniques to en- hance TENG performances. 2. Fluorinated surfaces The triboelectrification process exists in various ma- terial pairs. To maximize the charge transfer density, ef- ficient TENGs are built on two surfaces with the most distinguished ability to gain or refuse electrons. Different surfaces’ tendency of receiving and losing electrons were listed in several triboelectric series [1,29,60], which are the guidance of material selection in TENG designing. Fun- damentally, this ability/tendency is closely related to the electron affinity of the surface atoms. For instance, most electron-attracting polymers contain elements with strong electron affinity, such as fluorine (F). Therefore, one intu- itively expected approach to increase the charge density is introducing extra F atoms onto the polymer surface by chemical modifications, so called fluorinated surfaces. In principle, surface fluorination is achieved by chemically grafting F-containing unit, such as CF3 group through ei- ther solution reactions or vapor treatments. Shin et al. manipulated the triboelectric polarities of polyethylene terephthalate (PET) film by functionalizing its surface with positively charged amino group (–NH3) and negatively charged –CF3 group using poly-L-lysine solution and trichloro(1H,1H,2H,2H-perfluorooctyl)silane (FOTS) vapor, respectively [61], The modification process started from the cleaning of PET substrates. After that, the PET substrates were treated in oxygen plasma for 100 s, resulting in the formation of reactive hydroxyl groups (–OH) on the surface. At last, two pieces of plasma-treated PETs were individually soaked in poly-L-lysine solution for 5 min and exposed in FOTs vapor environment at 95 °C for 1 h. During this process, target molecules (i.e. poly- L-lysine and FOTs) were anchored on the PET surface by covalently bonding with the reactive hydroxyl groups. To verify the surface functionalization, water contact angles of the pristine and treated samples were analyzed through static sessile drop experiments. The contact angles of poly-L-lysine-treated PET (P-PET), pristine PET, and FOTs- treated PET (F-PET) were 44°, 81° and 123°, indicating the hydrophilic and hydrophobic properties, respectively. The hydrophilic characteristic of P-PET was attributed to the positively charged amino groups that actively interact with the polar hydrogen bonds of water molecules. The hydrophobic feature of F-PET was a consequence of the –CF3 group in the graphed FOTs molecules, which has low surface energy to interact water molecules. 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