**3. Results**

The electrical output of the NR-TiO2 at 0.1–0.5%wt were measured under a vertical contact-separation mode with a single electrode configuration, as presented in Figure 1. PTFE was used as a contact material with negative triboelectric polarity. The electrical voltage and current were generated by the physical contact-separation of the NR-TiO2 film and PTFE surfaces. When the surfaces are in contact, the electrification effect causes electrons to be transferred between the two materials, resulting in the formation of positive and negative charges on surfaces of NR-TiO2 film and PTFE, respectively. When the two surfaces were separated, electrostatic induction of triboelectric charges allowed free electrons in the electrical contact to flow, neutralizing triboelectric charges on the surface. Under the repeated contact-separation, the alternative current was generated.

**Figure 1.** Schematic diagram of the device configuration for measuring energy conversion performance with working mechanism of the fabricated TENG under a vertical-contact separation mode with single electrode configuration.

The generated voltage and current of the fabricated NR-TiO2 TENGs using as-received TiO2 powders are presented in Figure 2a,b, respectively. The electrical outputs of the NR-TiO2 TENGs with as-received TiO2 increased with increasing TiO2 content and were at the highest in the NR-TiO2 0.5%wt TENG, which were 78.4 V and 7.0 μA, respectively. However, the improvement of electrical output was not significant. It was suspected that the as-received TiO2 nanoparticles were agglomerated, giving rise to the poor dispersion in the NR matrix.

**Figure 2.** (**a**) Electrical output voltage and (**b**) current of the NR-TiO2 0.1–0.5%wt.

In order to improve the dispersion in the NR matrix, TiO2 nanoparticles were ballmilled for 6, 12, and 24 h periods, prior to mixing with the NR latex to form composite materials. The ball-milled TiO2 at 0.1–0.5%wt (same as above experiment) were added to NR latex. Electrical output voltage and current of all the ball-milled NR-TiO2 TENG are displayed in Figure 3 and are summarized in Figure 4. It was found that ball-milled TiO2 helped to improve the electrical outputs of NR-TiO2 TENG, which increased with ball-milling time. The dependence of electrical output on TiO2 concentration of the ballmilled TiO2 TENG exhibited the same trend, as electrical output increased with increasing TiO2 concentration. The addition of the 24-h-ball-milled TiO2 nanoparticles into NR significantly improved TENG performance, and the highest output voltage of 113 V and current of 9.8 μA was achieved from the NR-TiO2-B24h-0.5%wt TENG. The enhancement of TENG performance was attributed to the disintegration of TiO2 nanoparticles at long ball-milling times, producing the well-dispersion in the NR polymer matrix. The role of TiO2 nanoparticles on TENG performance will be further discussed in the dielectric properties in the following section.

**Figure 3.** *Cont.*

**Figure 3.** Electrical voltage and current of (**a**,**b**) NR-TiO2-B6h TENG, (**c**,**d**) NR-TiO2-B12h, (**e**,**f**) NR-TiO2-B24h 0.1–0.5%.

μ

**Figure 4.** Electrical voltage and current of NR TENG and the NR-TiO2-composite TENGs fabricated from ball-milled TiO2 at 6, 12, and 24 h at 0.1–0.5%wt concentration.

The SEM images of the plain NR film, NR-TiO2, NR-TiO2-B6h, NR-TiO2-B12h, and NR-TiO2-B24h composite films at TiO2 0.5%wt are displayed with the insets of their TiO2 nanoparticle fillers in Figure 5. Clearly, the dispersion of TiO2 without the ball-milling treatment was poor, as evidenced by the large agglomeration size of particles observed in SEM images of TiO2 powders and NR composite film. The agglomeration of TiO2

nanoparticles was less observed in the ball-milled TiO2 powders, which was reduced with increasing ball-milling times, contributing to the better dispersion in the NR composite films accordingly. The physical appearances of the NR and NR-TiO2-B24h 0.1–0.5%wt composite films are presented in Figure 6. The transparency of the pure NR film decreased as the TiO2 content increased.

**Figure 5.** SEM images of the pristine NR film, NR-TiO2, NR-TiO2-B6h, NR-TiO2-B12h and NR-TiO2-B24h composite films at 0.5%wt with the insets of their TiO2 particles fillers.

**Figure 6.** Digital photograph of the NR and NR-TiO2-B24h 0.1–0.5%wt.

The rutile phase of as-received and ball-milled TiO2 at 6, 12, and 24 h samples were confirmed by the XRD patterns as shown in Figure 7a (JCPDS No. 21–1276). This suggested that the ball-milling process did not change the crystal structure of the TiO2 nanoparticles. In this study, ball-milling was employed to break up the agglomerated particles and rutile phase is the most stable structure of TiO2; therefore, it should not cause the microstructural change of the particles. FTIR analysis of the NR and NR-TiO2-B24h 0.5%wt was performed and presented in Figure 7b. FTIR spectra of the NR and NR-TiO2-B24h 0.5%wt film are relatively similar, consisting of C-H stretching at 2850–2960 cm−<sup>1</sup> and 1300–1400 cm−<sup>1</sup> and C=C stretching at 839 cm−<sup>1</sup> of polyisoprene molecules [30], and some C-O hydroxyl groups from non-rubber components in latex such as inorganic substances, proteins, phospholipids, carbohydrates, and fatty acids [16,31]. This suggested that no chemical bond was formed between TiO2 and NR polymer.

**Figure 7.** (**a**) XRD spectra of as-received TiO2 and ball-milled TiO2 at 6, 12, and 24 h. (**b**) FTIR spectra of the NR film and NR-TiO2-B24h 0.5%wt composite film.

TENG electrical output is essentially a function of triboelectric charge density (*σ*) that forms upon contact electrification. For the contact mode TENG under open-circuit (OC) condition, the open-circuit voltage (*Voc*) is expressed by [32]

$$V\_{\text{oc}} = \frac{\sigma \chi(t)}{\varepsilon\_0} \tag{1}$$

and short circuit current (*Isc*) is given by

$$I\_{\rm sc} = \frac{S \sigma d\_0 v(t)}{\left(d\_0 + \varkappa(t)\right)^2} \tag{2}$$

where *ε*0, *S*, *d*0, *x*(*t*), and *v(t)* are electrical permittivity of free space, contact area size, effective thickness constant, separation distance, and contact electrode velocity, respectively.

Triboelectric charge density depends on the material contact couple, contact area, as well as the charge storing ability of the surface. In the latter case, it refers to the dielectric constant of the material. For a contact-separation mode TENG which can be considered by a capacitive model, triboelectric charge is proportional to the capacitance of the device, which is given by [18]

$$\mathcal{C} = \frac{\varepsilon\_0 \varepsilon\_r S}{d} \tag{3}$$

where *ε<sup>r</sup>* is dielectric constant and *d* is thickness of triboelectric material.

Dielectric constants of the NR-TiO2-B24h 0.1–0.5%wt films measured at the frequencies ranging from 102–108 Hz is presented in Figure 8. The dielectric constant at 1 kHz of the NR-TiO2-B24h was found to increase with TiO2 concentration. The improvement of dielectric constant in the NR-TiO2-B24h films with increasing TiO2 concentration was ascribed to the fact that TiO2 has a greater dielectric constant than NR. The addition of increasing TiO2 filler concentration to NR polymer matrix gave rise to the increasing dielectric constant of the composites. The dielectric constant contributed to the charge capacitance at the

surfaces of triboelectric materials, which intensified triboelectric charges that attributed to the increased electrical output of the TENG.

**Figure 8.** Dielectric constant of the NR-TiO2-B24h 0.1–0.5%wt.

The dependence of the output performance on the contact-separation frequency were also studied. The voltage and current outputs of the NR-TiO2-B24h 0.5%wt TENG were measured at operation frequencies ranging from 2–10 Hz, as presented in Figure 9a,b, respectively. It was found that electrical outputs depended on working frequency, and that the highest peak-to-peak voltage and current were 204 V and 13 μA, respectively, at a working frequency of 10 Hz. The increased electrical output was caused by charge retention on the surface due to a short contact-separation cycle at high frequencies.

**Figure 9.** The frequency dependence of (**a**) electrical voltage and (**b**) current of the NR-TiO2-B24h 0.5%wt TENG.

The delivered power density of the NR-TiO2 TENG was also studied by measuring voltage and current at different load resistances ranging from 1–100 MΩ. The plot of voltage and current versus load resistances is shown in Figure 10a. The working power density of 200–237 mW/m<sup>2</sup> was achieved at load resistances ranging from 3–20 MΩ and the maximum power density of 237 mW/m<sup>2</sup> was achieved at a matched load resistance of 7 MΩ (Figure 10b), which was 3.6 times larger than that of pristine NR TENG (66 mW/m2). This electrical output was enough to charge up a 10, 22, and 47 μF capacitors, as presented in a voltage profile in Figure 10c, and was able to charge a 99 μF to operate a portable calculator and light up 60 green LEDs, as demonstrated in Figure 10d and Video S1 in the Supplementary Materials. In addition, a TENG device was fabricated which was able to

light up 21 green LEDs by hand pressing, as demonstrated with the inset showing the schematic diagram of device components in Figure 10e.

(**c**) (**d**)

(**e**)

**Figure 10.** (**a**) The plot of voltage and current output versus load resistance and (**b**) Power density of the NR-TiO2-B24h 0.5%wt TENG compared to NR TENG. (**c**) Voltage profile of TENG to charge up the 10, 22, and 47 μF capacitors. (**d**) The demonstrations of TENG to operate a portable calculator (top) and to light up 60 green LEDs (bottom). (**e**) The fabricated TENG device able to light up 21 green LEDs by hand pressing with the inset showing a schematic diagram of device components.
