Water-solid surface contact electrification and its use for harvesting liquid-wave energy.

Contact electrification, also called triboelectrification, is an old but well-known phenomenon in which surface charge transfer occurs when two materials are brought into contact. Although some of the fundamental mechanisms about triboelectrification are still under discussion, such as what subjects (electrons, ions, or small amounts of material) are transferred during the contact and separation process to produce the charged surface, and why surface charge transfer occurs even between identical materials, triboelectrification does exist and it has some practical applications together with many negative consequences. Recently, contact electrification has been demonstrated for some potential applications, such as energy harvesting, chemical sensors, 6] electrostatic charge patterning, metal-ion reduction, and laser printing. The triboelectric nanogenerator (TENG), which is the first invention utilizing contact electrification to efficiently convert mechanical energy into electricity, has been systematically studied to instantaneously drive hundreds of lightemitting diodes (LEDs) and charge a lithium-ion battery for powering a wireless sensor and a commercial cell phone. Recently, the research has been broadened to collect energy from environment, such as wind and human motion, under which the TENG works in relatively dry conditions, because the surface triboelectrification would be greatly decreased if not totally eliminated by the presence of water. However, water vapor and liquid water are abundant and the most obvious example is ocean waves and tides that have huge amounts of mechanical energy, which is inexhaustible and not largely dictated by daytime, season, weather and climate, in contrast to solar energy. Until now, TENG is designed to work between solid materials and works best under dry conditions. 13–16] However, tribolelectricity does exist when liquids are flowing through insulating tubes. For example, a voltage variation rising up to 300 mV is observed when deionized water flows through a 1 m-long rubber tube. Or a surface charge density of 4.5 mC m 2 is measured on each water droplet pipetted from a polytetrafluoroethylene (PTFE) tip. Therefore, herein we explore the opportunity to use water contact as one type of “material” choice for TENG. We demonstrate that the contact electrification between water and insulating polymer films can also be useful for TENG, which can derive a new application of TENG especially in liquid environments for sensing. Polydimethylsiloxane (PDMS) and PTFE are chosen in this study for their hydrophobic properties and high negativity in the triboelectric series. To further investigate this effect, TENGs using tap water, deionized water, and deionized water with a high concentration of NaCl are also compared. Under periodic contacting deionized water by a linear motor, PDMS film with patterned pyramid array can provide an open-circuit voltage (Voc) of 52 V and short-circuit current density (Jsc) exceeding 2.45 mAm 2 with a peak power density of nearly 0.13 W m , which is large enough to light up 60 commercial LEDs. The incubation shaker and platform rocker are used to stimulate different wave motions in the environment and the water–TENG successfully harvests these types of mechanical energy into electricity. Moreover, the water–TENG also has the capability to act as a chemical and temperature sensor. Figure 1 shows the fabrication process of the TENG and how the water contact electrification is included in the action unit. The TENG fabrication starts from the design of a PDMS film with patterned pyramid array (Figure 1a). The Si wafer mold was made by photolithography and then etched by a dry etching process. Liquid PDMS elastomer and cross-linker were mixed, degassed, and uniformly spin-coated on the Si wafer mold. After thermal incubation, a uniform PDMS film with a patterned pyramid array was formed. For the other part of the construct, thin films of Cu (100 nm) were deposited on two poly(methyl methacrylate) (PMMA) substrates by a RF magnetron sputtering deposition system. PMMA is selected as the substrate material because it provides a flat surface, light weight, and high strength. The PDMS film with the pyramid array pattern was peeled off the Si wafer mold and then placed on one of the Cu thin-film-deposited PMMA substrates with uncured PDMS mixture on top. Finally, the structure was incubated again to obtain a fully developed device for use in the next step. To investigate the contact electrification between water and the PDMS film, the second Cu thin-film-deposited PMMA substrate was placed on the bottom of an insulating tank (Figure 1b), acting as the conducting electrode for the water. The dimensions of the tank were 11 cm 7 cm. After the tank was filled with water, the device consisting of the [*] Dr. Z. -H. Lin, Dr. G. Cheng, L. Lin, Dr. S. Lee, Prof. Z. L. Wang School of Material Science and Engineering Georgia Institute of Technology Atlanta, GA 30332-0245 (USA) E-mail: zlwang@gatech.edu