Effects of particle size of dielectric fillers on the output performance of piezoelectric and triboelectric nanogenerators

Recently, piezoelectric/triboelectric nanogenerators based on piezoelectric composite materials have been intensively studied to achieve high electrical output performance. In this work, flexible BaTiO3 (BT)/PDMS nanocomposite films with various sizes and concentrations were fabricated and used as the nanogenerators. The influence of dielectric properties on the electrical output of nanogenerators was studied as well as the structure of the composites. The dielectric constant increased from 6.5 to 8 with the concentration of BT nanoparticles and decreased with the frequency from 102 to 106 Hz. Furthermore, the dielectric constant showed 11% decrease with the temperature range from 30 to 180 °C. It was found that the concentration of BT nanoparticles has promoted the electrical output of nanogenerators. The output voltage and current are all enhanced with the BT nanoparticles, which reached 200 V and 0.24 °A in TENG with 40 wt% BT nanoparticles, respectively. The selected device exhibited the power of 0.16 mW and employed to demonstrate its ability to power wearable/portable electronics by lighting the LEDs.


Introduction 
Generating electricity with an effective and environmental way is a hot spot in the current energy field. There are many types of wasted mechanical energy in our daily life and nature, including human motion (shaking hands, walking, running) [1][2][3], vibration [4,5], wind flexible which can be utilized in various applications. For TENG, there are four different modes, contact separation mode [17], lateral sliding mode [18], single electrode mode [19], freestanding triboelectric layer mode [20], which can suit for various applications.
Until now, many researchers focused on improving the output performance of PENG and TENG with a compact structure, a high sensitivity, and a long durability for self-powered sensors [21][22][23][24][25]. To make the nanogenerators suit for more applications, polymer/ elastomer-based nanocomposites which is a polymer matrix reinforced with nanoparticles have been intensively investigated because of their merits of excellent flexibility/stretchability and easy processing. For example, PDMS [26], PMMA [27], and PVDF [28][29][30][31] are mainly used as the matrix. Among them, PDMS as a biocompatible elastomer has advantages of flexible, transparent, and simple fabrication, which has been widely used in flexible nanogenerators [32][33][34][35]. For polymer/elastomer-based PENG and TENG, there are two effective methods to improve the output performance. In one hand, the working areas of the devices have the strong impact on the output performance. To improve output power, nanostructures are fabricated on the surface of the contact layers to increase the working area [36]. In another hand, selecting the inorganic fillers with high dielectric constant which can provide an enhanced charge storage for the devices [37]. For the selection of fillers, a variety of materials have been used to fabricate nanogenerators such as PZT [38], ZnO [39,40], and BaTiO 3 (BT) [41,42]. However, the toxicity of PZT and the low output performance of ZnO limited their applications. BaTiO 3 exhibited a high dielectric constant, stable properties over wide temperature range, and controlled size and shape with simple fabrication, which has been widely used in different flexible electronics [43][44][45][46][47][48][49].
In this work, we focused on investigating the dielectric properties of BT/PDMS with various particle sizes and output performance of nanogenerators based on the composite films. Commercially available BaTiO 3 nanoparticles with uniform size and shape distributions are selected as the filler to guarantee the expected experimental design. PDMS-based nanocomposites with different weights of BT nanoparticles were fabricated, and the different concentrations of nanocomposites were used in PENG and TENG devices. The phase and morphology of the nanocomposites were characterized by X-ray diffraction and scanning electron microscope. In addition, the temperature and frequency dependence of dielectric constant and loss was studied, respectively. Finally, the open-circuit voltage and short-circuit current were measured to show the output performance of the PENG and TENG. To demonstrate the practical and commercial applications, the power generated by the TENG device was utilized to power up various low-power wearable/portable electronics.

1 Materials
BaTiO 3 nanoparticles (99.9%, diameter of ∼30 nm) were purchased from the Alfa Aesar, USA. The PDMS (Sylgard® 184 silicone elastomer base and silicone elastomer curing agent) was purchased from the Dow Corning Corporation, USA. Indium tin oxide coated PET (surface resistively 60 Ω/sq, 1 ft × 1 ft × 5 mil) was purchased from ALDRICH. Bi 2 O 3 , TiO 2 , and BaCO 3 were used as raw materials and employed by Alpha Chemicals. The NaCl and KCl were purchased from Sinopharm Chemical Reagent Co Ltd. The copper tape and Indium tin oxide coated PET were bought from Sigma-Aldrich, Co., Ltd., USA.

2 Synthesis of micro-scale BaTiO 3 flake
The micro-scale BT flake was synthesized by the combination of the molten salt technique and topological synthesis method, and the process was divided into three steps. Firstly, the Bi 4 Ti 3 O 12 platelet was synthesized by a molten salt method. Secondly, BaBi 4 Ti 4 O 15 was synthesized through a molten salt method by using reactants of BaBi 4 Ti 4 O 12 , and some of Bi 3+ converted into Ba 2+ . Thirdly, BT platelets were obtained by a topochemical reaction between BaBi 4 Ti 4 O 15 and BaCO 3 . Figure 1 shows the fabrication process of BT/PDMS films and PENG and TENG devices. The silicone elastomer base and silicone elastomer curing agent were put into a beaker with the ratio of 10:1, then the solution was mixed thoroughly, and bubbles was degassed. Next, the solution with substrate was placed on the spin coating machine with a speed of 800 rpm for 30 s to get a uniform PDMS film, and then cured it at 80 ℃ for 2 h. The two solutions of PDMS were put into a beaker with a ratio of 10:1, and then the BT nanoparticles were dispersed into the PDMS at various concentrations of 5, 10, 20, 30, and 40 wt%, respectively. The following steps are the same as the fabrication of pure PDMS film. Finally, different BT/PDMS films with uniform filler distribution and excellent flexibility can be obtained, as shown in Figs. 2(a) and 2(b). For the PENG, the different concentrations of BT/ PDMS composite film acted as the dielectric film. The copper tape was attached to both surfaces of the film, as shown in Fig. 2(c). For the TENG, the composite film was placed into the two commercial ITO-PET films. And then four spacers which built a gap between the composite film and ITO-PET were put on the four corners of the composite film. The Cu tapes were attached to the ITO layer to connect the load, as shown in Fig. 2(d).

4 Characterization and measurements
The morphologies of the composite surfaces were studied by field emission scanning electron microscopy (Phenom LE). The X-ray diffraction (XRD) analysis was carried out by using an Ultima IV machine.   The mechanical performance is critical to the flexible/ wearable devices. The relationship between the strain and stress was studied and shown in Fig. 3(a). It can be observed that before adding BaTiO 3 , the pure PDMS has a lower strength but a higher elongation at break. After adding BaTiO 3 , the strength becomes decreasing and a lower elongation at break. The results show a good agreement with other reported works [50,51]. The relationship between the filler concentration and fracture strength was studied and shown in Fig. 3(b). It can be found that the fracture strength decreases with adding the BT filler. The decrease in mechanical properties is caused by the poor matrix-filler interaction between the PDMS and BaTiO 3 particles.

2 Dielectric properties
The plots of dielectric constant and dielectric loss against frequency for different concentrations of BT/PDMS composite films are displayed in Figs. 4(a) and 4(b), respectively. The size of this BT particle is about 50-80 nm. It was found that the dielectric constant was increasing with the concentration of BT because of the introduction of inorganic fillers with high dielectric constant [52,53]. In addition, the dielectric constant of BT/PDMS composite decreases first at a low frequency (< 600 Hz) and then becomes steady. The possible reason can be explained by the relatively high loss value at the low frequency range and the similar trend.   30 wt% of fillers with different filler sizes: 5-25 μm, 25-60 μm, and 50-80 nm. As shown in the result, the dielectric constant increases with the size of BT filler and a tiny decrease with frequency. The composite with nano-sized filler exhibits lower dielectric constant and higher loss from 1 kHz to high frequency range, which is like the previous report in polymer-based composites [54].
The temperature dependence of dielectric constant and loss of BT/PDMS composite films are shown in Figs. 5(a)-5(d). The details of the concentration of BT and frequency are labeled in the figures. When the temperature was rising, the dielectric constant was decreasing slightly, and the dielectric loss was approximate steady. From the value of the dielectric properties, it can be obtained that the dielectric loss was decreasing with the frequency. More specifically, under 1 kHz, the dielectric constant of 40 wt% BT/PDMS nearly reaches 8. When the frequency was 100 kHz, the dielectric constant dramatically dropped to 7.5. Figures 5(e)-5(h) show the relationship of the temperature between the dielectric constant of 30 wt% BT/PDMS composite films. The four various frequencies were merged in a diagram. The value of dielectric constant decreased slightly with temperature. Compared with the two figures, the larger size of BT filler has a better dielectric constant. In addition, it also confirmed that the dielectric constant was decreasing with the frequency.  Figure 6 shows the electrical output performance of the fabricated PENG and TENG devices with different concentrations of BT/PDMS composite films (5, 10, 20, 30, 40 wt%) and micro-scale and nano-scale BT. The output voltage and current were measured under the same situation (frequency, external force, etc.). It was observed that both output voltage and current were increased significantly with the concentration of BT nanoparticle filler. It can be explained that the dielectric constant was increasing with the concentration of BT, and a higher dielectric constant of nanocomposite films has a good capability of storage charge, which can lead to a better performance of nanogenerators. It was demonstrated that the output voltage and current of PENG with 40 wt% BT were nearly three times higher than PENG with 5 wt% BT. The average output voltage and current of PENG with 40 wt% BT reached about 18 V and 0.24 μA, respectively. Similar results can be found on the electrical output performance of TENG in Figs. 6(d) and 6(e). The output performance of TENG is much more excellent than PENG which has been studied widely. From Figs. 6(a) and 6(d), the output voltage of TENG is ten times than PENG. The output voltage and current of 40 wt% BT/PDMS TENG reach 200 V and 0.4 μA, respectively. For the different sizes of BT, it can be observed that the output performance of nanogenerators with nano-scale is enhancing significantly than micro-scale. The reason is that the electrical performance is associated with the resistance of material, and the resistance of material is governed by the sintering temperature. Generally, a higher sintering temperature can lead to an increase in resistance of material. The micro-scale flakes need a higher sintering temperature than nano-scale. Therefore, the micro-scale flakes have a relatively high resistance than nano-scale. The electric output performance of nanogenerators with nano-scale is better than microscale. On the other hand, the shape and distribution of the micro-scale flakes are various and random, and the shape and size of nanoparticles are almost the same and show good arrangement. Figures 6(c) and 6(f) show the compared results of the output performance of PENG and TENG with different concentrations and sizes of BT, respectively. For the various concentrations of BT nanoparticles, the output voltage and current are increasing with the concentration. The output performance of devices based on different sizes of BT composite films was compared, too. The PENG-based micro-scale BT nanocomposite film shows better performance than the nano-scale BT under the same concentration. More specifically, the output performance of PENG-based nano-scale BT nanocomposite film enhanced approximately 30% than micro-scale BT nanocomposite film. The same trend results of TENG are shown in Fig.  6(f). It can also be confirmed that the output performance of TENG is better than PENG. Figure 6(g) shows the comparison of the output performance of NGs operating with the piezoelectric and triboelectric. It can be found that both the output current and voltage of triboelectric nanogenerator are much higher than piezoelectric nanogenerator. More specifically, the output performance of TENG is two times than that of PENG. Figure 6(h) shows the output power of 40 wt% BT/PDMS-based PENG and TENG with the external load resistance ranging from 10 2 to 10 9 Ω. The peak output power of TENG reaches up to 0.16 mW and it can be employed in many applications. In addition, as shown in Fig. 6(i), the output performance of nanogenerators based on BT/PDMS particles is still stable after 1000 cycles. Figure 7(a) shows the working principle of TENG. At the initial position, there are no charges on the two surfaces and no potential difference between the two electrodes. With an external physical force pressing on the device, the two layers are going to access and contact with each other. At this time, the opposite charge is occurred on the two surfaces because of the triboelectric effect the opposite charges have occurred on the electrodes. The electrons in one electrode would transfer to the other electrode to balance the electrostatic field. When the TENG is released to the origin state, the output voltage will keep raising to the maximum value. The cycled contact and separation process of the two contact layers drives the induced electrons in the circuit to flow back and forth to build an AC output. Compared to PENG, the piezoelectric-based TENG combines both piezoelectric and triboelectric effect together, leading to an enhanced output performance. The studies shown in Fig. 6 provide several understandings on the piezoelectric filler/PMDS-based PENG/TENG. First, the output performance of both devices enhances with the increase of the content of piezoelectric filler because more piezoelectric fillers are in the matrix which provide more charge during the pressing/releasing. Second, the micro-sized filler shows lower output power then nano-sized filler when the devices have the  same concentration. The reason may raise from two aspects: i) the large distribution of size and shape of the BT flakes may result in an inhomogeneous microstructure and non-uniform charge distribution, ii) the same weight percent leads to the less particles of BT flakes in the matrix compared to the nanoparticles, and iii) the possible flakes are broken during the pressing/releasing process. Third, it clearly shows that the TENG exhibits higher value of both output current and voltage compared to the PENG, which can be ascribed to the synergetic effect of both the piezoelectric and triboelectric effects: i) the triboelectric friction between two layers could generate positive and negative charges across their surfaces, and ii) the compression on the piezoelectric composites generates the piezoelectric potential caused by the resultant electric dipole moments.

3 Energy harvesting performance
For practical applications, the effect of external load resistance on the output performance of the TENG device was also tested and the result is showed in Fig.  7(b). As the external load was increasing from 10 4 to 10 8 Ω, the peak-peak voltage increased significantly. Figure 7(c) shows the measurement system, which consists of an electrometer and a computer. The schematic diagram of a full rectifying diode bridge is attached on the top right corner. It can be connected to the load to convert AC to DC to use to charge energy storage devices such as batteries or supercapacitors. Figures 7(d) and 7(e) exhibit the photographic images of a logo of XATU, which made of many commercial light-emitting diodes (LEDs). The LEDs are brightened instantly by the power generated from the TENG through the rectifier when the force was releasing and pressing, respectively. The results indicate that the BT/PDMS TENG device can be utilized as a selfpowered source for wearable/portable electronics.

Conclusions
The BT/PDMS composite films are systematically investigated and utilized for energy harvesting applications. The composites were fabricated by the spincoating method with different concentrations of nanosized BT particles from 5 to 40 wt% and micro-sized BT flakes. The dependence of dielectric constant and dielectric loss of the BT/PDMS composite films on frequency and temperature was investigated. It is found that the dielectric constant and loss were increasing significantly with the concentration of the nanoparticles and decreasing with the frequency. The value of the dielectric loss of 40 wt% BT/PDMS was still in a low value (~0.12). The dielectric constant was influenced by temperature and the dielectric loss was almost steady with the temperature. The output performance of PENG and TENG is enhancing by the concentration of BT/PDMS composite films. The output voltage and current of 40 wt% BT/PDMS TENG reach 200 V and 0.4 μA, respectively, which are more than two times higher than that with 5 wt% of BT. It clearly shows that the TENG exhibits higher value of both output current and voltage compared to the PENG, which can be ascribed to the synergetic effect of both the piezoelectric and triboelectric effects. The stable and high electrical output power generated by the TENG was utilized to light commercial LEDs, which suggests that the device has the potential to the energy harvesting in biomedical application.