Nano Energy 58 (2019) 227–233 Contents lists available at ScienceDirect Nano Energy journal homepage: www.elsevier.com/locate/nanoen Full paper Metal nanowire–polymer matrix hybrid layer for triboelectric nanogenerator T Hyungseok Kanga,1, Hyoung Taek Kimb,1, Hwi Je Wooa, Han Kimb, Do Hwan Kimc, Sungjoo Leea, ⁎⁎ ⁎ SeongMin Kimb, Young Jae Songa, Sang-Woo Kima,b, , Jeong Ho Chod, a SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea b School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea c Department of Chemical Engineering, Hanyang University, Seoul 04763, Republic of Korea d Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 03722, Republic of Korea ARTICLE INFO ABSTRACT Keywords: In this work, we studied the surface potential of a metal–polymer hybrid layer and its effect on the performance Triboelectric nanogenerator of a triboelectric nanogenerator (TENG). Ag nanowires (AgNWs) separately embedded in two different poly- Surface potential mers–one with a positive tribopotential and the other with a negative tribopotential–were prepared as model Kelvin probe force microscopy hybrid systems. The surface potentials of the hybrid system were systematically investigated by Kelvin probe Silver nanowire force microscopy. The results demonstrated that each component of the hybrid layer affected the other com- Hybrid ponent because of the difference in their work functions. The following two important findings were obtained. First, the surface potential of each polymer shifted drastically toward that of Ag and the surface potential of Ag shifted toward that of each polymer. Second, higher density of AgNWs led to higher Ag-induced charge density in the polymer, which consequently resulted in larger shift in the surface potential of the polymer. TENG per- formance measurements revealed that the tribopotential difference between the contact surfaces of the AgNW–polymer hybrid layer and the perfluoroalkoxy alkane (or Nylon) used as the top triboelectric layer governed the TENG performance. Our systematic investigation of the surface potential of a hybrid surface consisting of two materials with different surface potentials provides insight into the design of triboelectric layers for high-performance TENGs. 1. Introduction enhance the density of induced charges. The first factor is the effective contact area between the two triboelectric layers [25–30]. Considerable Environmental energy harvesting is a promising approach for ad- efforts have been devoted to developing one-dimensional (1D) and two- dressing global energy issues and for realizing self-powered operation dimensional (2D) micro/nanopatterned surface reliefs via various pat- of various electronic devices such as flexible displays, elastic circuits, terning techniques such as photolithography, soft lithography, e-beam and e-skin sensors [1–8]. Technologies for the conversion of environ- lithography, nanoimprinting, and nanoparticle deposition. The second mental energy into electricity through mechanical sources such as factor is the triboelectric potential difference between the two tribo- wind, water flow, vibration, and human body motions have been de- electric layers [31–39]. The TENG performance is governed by the veloped [9–18]. Recently, triboelectric nanogenerators (TENGs) have choice of materials in the triboelectric contact pair, where the selection undergone rapid development as a technology for harvesting electricity of two different materials far apart in the triboelectric series is neces- through contact triboelectrification and electrostatic induction sary for achieving a high performance. To date, various fluorinated [19–24]. The use of TENGs in practical applications necessitates that dielectric materials such as polyvinylidene difluoride (PVDF), poly- their output performance be as high as possible; their output perfor- tetrafluoroethylene (PTFE), and perfluoroalkoxy alkane (PFA) have mance is critically dependent on the density of charge induced on the been utilized as negative triboelectric layers, whereas metals such as surface of the triboelectric layer. Two factors can primarily be tuned to aluminum and copper have been widely employed as positive ⁎ Corresponding author at: Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 03722, Republic of Korea. ⁎⁎ Corresponding author at: SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea. E-mail addresses: [email protected] (S.-W. Kim), [email protected] (J.H. Cho). 1 H. Kang and H. T. Kim contributed equally this work. https://doi.org/10.1016/j.nanoen.2019.01.046 Received 28 September 2018; Received in revised form 1 January 2019; Accepted 15 January 2019 Available online 15 January 2019 2211-2855/ © 2019 Published by Elsevier Ltd. H. Kang et al. Nano Energy 58 (2019) 227–233 triboelectric layers [40–43]. Kelvin probe force microscopy (KPFM), completely embedded in the polymer matrix (Figs. 1b and 1c). The which measures the contact potential difference between the probe tip height profile obtained from the AFM images indicated that the surface and the sample surface, has been demonstrated to be a powerful tech- of the AgNW-embedded polymer film was significantly flattened after nique for fundamental analysis of the electrostatic potential properties embedding: the root-mean-square roughness (Rrms) decreased from during contact electrification [44–51]. The relationship between the 25.4 nm to 3.1 nm. The prepared model system of a hybrid film com- electrostatic surface potential and triboelectric charges can be under- prising two materials with different tribopotentials was used for sys- stood by comparing the surface potential values obtained by KPFM tematically investigating the surface potentials and their contribution measurements. to the TENG performance. Fig. 1d shows the schematic device structure Ag nanowires (AgNWs) have attracted much attention as conductive of contact-separation-mode TENGs. AgNWs embedded in either of two electrodes owing to their remarkable flexibility and stretchability that different polymers (PVC or PMMA) were utilized as the bottom tribo- result from the formation of a percolation network. In addition, AgNWs electric layer in the TENGs. Nylon (with a strong positive triboelectric can be simply deposited by solution-coating methods such as spin potential) attached to an Al electrode or PFA (with a strong negative coating, dip coating, spray coating, and Meyer-rod coating [52–54]. triboelectric potential) attached to an Al electrode was utilized as the However, the weak adhesion between AgNWs and the substrate results top triboelectric layer in the TENGs. in delamination of the AgNWs from the substrate during TENG opera- In order to understand the electronic influence of the AgNWs in the tion. Very recently, our group proposed an embedded structure of polymer matrix, 2D mapping of the surface potentials of the AgNWs in a polymer matrix for the fabrication of mechanically stable AgNW–polymer hybrid surface was performed via noncontact-mode TENGs [55]. In the present work, we systematically studied a KPFM. For all the KPFM measurements, the same Pt tip was utilized to AgNW–polymer hybrid structure by KPFM and evaluated the relation- ensure that the measured values were referenced to a common energy ship between the electrostatic surface potential of the hybrid surface level. Six films were prepared in total: a bare PVC film, AgNW–PVC and the TENG performance. As model hybrid systems, AgNWs with two films with two different AgNW densities, AgNW–PMMA films with two different areal factors were embedded in polymer matrixes of poly- different AgNW densities, and a bare PMMA film. Fig. 2a shows vinylchloride (PVC), having a negative surface potential, and poly 2 μm×2μm maps of the surface potential difference of the AgNW–- (methyl methacrylate) (PMMA), having a positive surface potential. polymer hybrid films, which was referenced to Pt. The bare PVC film KPFM results showed that each of the two components of the hybrid showed a negative surface potential, whereas the bare PMMA film system affected the other component because of the difference in their showed a positive surface potential. These results indicated that the work functions. For example, the surface potentials of the polymer surface potential of the Pt tip was located between those of PVC and matrixes shifted toward that of Ag (that is, the shift directions of the PMMA. The hybrid films showed a distinct color contrast between the surface potentials of PVC and PMMA relative to the surface potential of AgNW region and the polymer region. The brighter region in the images Ag were positive and negative, respectively). Additionally, in each corresponds to a location with a more positive surface potential in the hybrid system, the surface potential of Ag shifted toward that of the film, whereas the darker region corresponds to that with a more ne- polymer matrix. In the hybrid system with a larger number of AgNWs, gative potential. In particular, in the AgNW–PVC film, the AgNW region more charges were transferred from the AgNWs to the polymer matrix, was brighter than the PVC region, which indicated that the surface which resulted in a larger shift in the surface potential of the polymer potential of Ag was higher than that of PVC. In contrast, in the matrix toward that of Ag. The observed directions of surface potentials AgNW–PMMA film, the AgNW region