**1. Introduction**

Energy harvesting technologies have attracted great attention because of the significance in producing sustainable energy sources to overcome energy crisis and climate change. In addition, the rapidly increasing number of personal electronic devices and other components for the Internet of Things (IoT) platform leads to the increasing demand for energy. Triboelectric nanogenerator (TENG) is a mechanical energy harvesting device based on the combination of contact electrification and electrostatic induction effects [1]. TENG has gained much interest due to its high energy conversion efficiency with high power output, straightforward fabrication process, and low cost [2]. Apart from energy harvesting applications, TENGs also have the potential to be used for many self-powered sensor applications, including physical, chemical, gas, and liquid sensors [3–6].

A wide range of materials can be used to fabricate TENG; most of them are polymeric materials [7]. The common known materials are polytetrafluoroethylene (PTFE) [8,9], polydimethylsiloxane (PDMS) [10,11], polyvinylidenefluoride (PVDF) [12,13], and polymethyl methacrylate (PMMA) [14,15]. Natural rubber (NR) or polyisoprene is one of the natural polymers with good flexibility and strength employed in a wide range of applications [16]. Most of NR products, such as car tires, gloves, shoe insoles, and mattresses, involve the applications in direct contact with mechanical energy sources. NR is one of the triboelectric materials located in the triboelectric series possessing slightly negative polarity [7]. In this

**Citation:** Bunriw, W.; Harnchana, V.; Chanthad, C.; Huynh, V.N. Natural Rubber-TiO2 Nanocomposite Film for Triboelectric Nanogenerator Application. *Polymers* **2021**, *13*, 2213. https://doi.org/10.3390/ polym13132213

Academic Editor: Jung-Chang Wang

Received: 8 June 2021 Accepted: 1 July 2021 Published: 5 July 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

regard, the fabrication of NR-based TENGs would be beneficial for boosting power output to realize practical applications of the TENG.

Generally, the output performance of TENG is a function of triboelectric charges on triboelectric materials which depend on electrification between two triboelectric materials, surface area, and ability of surface to hold charges [17,18]. In order to improve triboelectric charge density on the surface, many approaches have been proposed, including surface patterning with nanostructures [19,20] and improving dielectric properties of triboelectric materials [21–24]. For the latter case, filling nanomaterials, such as SiO2, TiO2, BaTiO3, and SrTiO3 in polymer triboelectric materials, were reported to improve dielectric constant and TENG performance [23]. Among these filler materials, TiO2 is an extensively used material for a wide range of applications due to many excellent physical and chemical properties, including optical-electronics [25], photocatalytic properties[26], chemically stability, nontoxicity, as well as low cost. TiO2 exists in three main polymorph phases including anatase, brookite, and rutile [27]. Among them, rutile-TiO2 exists as the most thermodynamically stable phase and exhibits a high dielectric constant [28,29].

In this work, rutile-TiO2 nanoparticles were incorporated into NR material forming NR-TiO2 composite film which was then used as a triboelectric material to convert mechanical energy into electricity. However, the NR-TiO2 composite fabricated by mixing the TiO2 nanoparticles directly with NR latex did not greatly improve the TENG output, possibly due to the agglomeration of as-received TiO2 nanoparticles. In the present work, the ball-milling method is proposed as an effective and straightforward method to alleviate the agglomeration of TiO2 nanoparticles prior to mixing with NR latex, thereby improving the dispersion of nanoparticles in the NR matrix. The effects of milling times and the concentration of TiO2 nanoparticles in the NR film on dielectric properties and TENG output performance were investigated. The performance of the NR-TiO2 TENG was probed under a vertical contact-separation mode. The morphologies and dielectric properties of the composite films were examined using a scanning electron microscope (SEM) and an impedance analyzer, respectively.
