**5. Conclusions**

The NR-TiO2 TENG for harvesting mechanical energy into electricity was successfully fabricated. The addition of rutile TiO2 nanoparticles at 0.5%wt of NR latex to form NR-TiO2 composite was found to enhance energy conversion efficiency of the TENG. The modification of TiO2 by the ball-milling technique for 24 h prior to mix with NR materials was found to effectively disintegrate TiO2 nanoparticles which consequently helped the dispersion of the nanoparticle fillers in the polymer matrix. Owing to the high dielectric constant of TiO2 fillers, the dielectric constant of the NR-TiO2-B24h film increased with increasing TiO2 concentration. The NR-TiO2-B24h film with improved dielectric constant attributed to the enhancement of TENG electrical output with the highest power density of 237 mW/m2. This work showed the potential applications of NR-TiO2 TENG as an environmentally friendly power source for portable electronic devices.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/polym13132213/s1, Video S1: The demonstrations of TENG to operate a portable calculator.

**Author Contributions:** Conceptualization, V.H.; Methodology, V.H. and W.B.; Performing experiment, W.B.; Validation, V.H., C.C., V.N.H.; Investigation, V.H. and W.B.; Writing—original draft preparation, V.H.; Writing—review and editing, V.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the Basic Research Fund of Khon Kaen University, grant number 1500147, the Research Network NANOTEC (RNN) program of the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Higher Education, Science, Research and Innovation (MHESI) and Khon Kaen University, Thailand, the Thailand Center of Excellence in Physics (ThEP), and Research and Graduate Studies, Khon Kaen University.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available in the article.

**Acknowledgments:** This work was supported by the Research Network NANOTEC (RNN) program of the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Higher Education, Science, Research and Innovation (MHESI) and Khon Kaen University, Thailand, the Thailand Center of Excellence in Physics (ThEP), the Basic Research Fund of Khon Kaen University[Grant No. 1500147], and Research and Graduate Studies, Khon Kaen University. WB would like to thank for the support from Thailand Graduate Institute of Science and Technology (TGIST) (SCA-CO-2563-12216-TH).

**Conflicts of Interest:** The authors declare no conflict of interest.
