**4. Discussion**

In the present work, the development of NR TENG with enhanced performance was demonstrated by the incorporation of TiO2 nanoparticles. The improved TENG performance was attributed to the enhanced triboelectric charge density by enhancing the dielectric constant of materials, as discussed in the previous section. TiO2 nanoparticles were employed as an effective filler for improving dielectric constant of NR composite film due to the high dielectric constant of TiO2. However, the agglomeration of nanoparticles suppressed the dispersion of nanoparticles in the NR matrix leading to an insignificant improvement of TENG performance, as presented in Figure 2. In this work, the simple and efficient approach to reduce the agglomeration of TiO2 nanoparticles using ball-milling was proposed. TiO2 nanoparticles were ball-milled prior to mixing with NR latex, which was found to effectively reduce the agglomeration of nanoparticles, as evidenced by SEM images (Figure 5), which then consequently produced the well-dispersion of TiO2 in NR-TiO2 composite films. In this work, the milling time of 24 h was found to efficiently reduce the agglomeration and produce the uniformly dispersed TiO2 in the NR films. The power output enhancement of the NR-TiO2-B24h was attributed to the improved dielectric constant due to the good dispersion of TiO2 nanoparticles. This suggested that ball-milling was an effective treatment to alleviate the agglomeration of TiO2 nanoparticles, which magnified the TENG electrical output to about 1.5 times higher than the untreated TiO2 composite TENG.

Comparing to other previous reports, the fabricated TENG showed a superior performance than the PDMS-Kapton-implanted TENG with a power density of 8.44 mW/m2 [33], the 2D woven wearable TENG fabricated from nylon and polyester threads with a power density of 2.33 mW/m2 [34], and approaching a propeller TENG made of PTFE and Al triboelectric materials with a power density of 283.95 mW/m<sup>2</sup> [35]. In addition, comparing to the NR-based TENG, the NR-TiO2 TENG exhibited the comparable output power to the NR-Ag TENG in our previous report which was 262.4 mW/m2 [36]. The slightly lower TENG electrical output of the NR-TiO2 composite than that of the NR-Ag one was attributed to the lower dielectric constant of the NR-TiO2. The conductive Ag filler produced stronger interfacial polarization than the TiO2 semiconductor filler in the NR insulating matrix [37]. Therefore, the main contribution for the improved dielectric constant of NR-TiO2 was from the intrinsic dielectric property of TiO2, as described earlier.

One of the most attractive aspects for employing NR as triboelectric material is the ability to scale up the production for large-area energy harvesting, owing to its low fabrication cost and feasibility to form composite with other materials. Comparing to other triboelectric polymers mentioned above, the costs of NR and TiO2 are much lower. In addition, the fabrication process of NR-TiO2 composite in the present work is straightforward, low cost and effective, which is promising for the development of large-scale energy harvesting device.
